Supercooled liquid metal soldering material with a protective shell

Abstract

A droplet comprises a core comprising an alloy containing most of a first metal element and a small portion of a second element, and the core is in a liquid state below the solidus temperature of the alloy. A shell is arranged to surround the core and comprises an outer surface containing most of a second element and a small portion of a first metal element, and the shell is in a solid state below the solidus temperature of the alloy. The alloy may comprise a solder material that can be used to form a solder connection below the solidus temperature of the alloy.

Description

Technology Field

[0001] Cross-reference with related applications

[0002] This application claims the benefit and priority to U.S. Provisional Application No. 63 / 056,448, filed July 24, 2020, with the title "Undercoled Liquid Metallic Droplets Having a Protective Shell," and to U.S. Provisional Application No. 17 / 383,150, filed July 22, 2021, with the title "Undercoled Liquid Metallic Droplets Having a Protective Shell." The disclosures of each of the aforementioned applications are incorporated herein by reference in their entirety. Background Technology

[0003] There currently exist a wide variety of electronic devices containing low-temperature materials that are unsuitable for forming soldered connections due to the high processing temperatures required for soldering. New low-temperature soldering materials and processes are needed for electronic devices containing low-temperature materials.

[0004] Some embodiments of the present disclosure relate to a supercooled liquid metal droplet surrounded by a protective shell that maintains the core in a liquid state below the solidus temperature of the core material. The protective shell may have relatively few nucleation sites and may impose a thermodynamic barrier that prevents the core from transitioning to a solid state below the solidus temperature of the core. The shell may comprise one or more layers arranged according to the reduction potential of each element. The shell may be coated with a fluid that protects the exterior of the shell from oxidation and decomposition. In some embodiments, the liquid metal droplet may be used to solder materials at a temperature below the solidus temperature of the droplet.

[0005] In some embodiments, the droplet comprises a core containing a majority of a first metal element and a minority of a second element, and the core is in a liquid state below the solidus temperature of the first metal element. A shell is arranged to surround the core and comprises an outer surface containing a majority of a second element and a minority of the first metal element, and the shell is in a solid state below the solidus temperature of the first metal element. In various embodiments, the second element is a metal. In some embodiments, the second element is a metalloid. In various embodiments, the shell comprises a concentration gradient transitioning from a first concentration of the second element on the inner surface of the shell to a second concentration on the outer surface, where the first concentration is lower than the second concentration.

[0006] In some embodiments, the droplet further comprises a fluid disposed on an outer surface. In various embodiments, the outer surface of the shell comprises a third element having a concentration lower than that of the first metal element and the second element. In some embodiments, the shell comprises a concentration gradient of at least the first metal element and the second element that prevents the core from transitioning to a solid state. In various embodiments, the first metal element and the second element on the outer surface are oxides of the first metal element and oxides of the second element. In some embodiments, the first metal element has a lower E than the second element o It has. In various embodiments, the second element has a higher E° than the first metal element.

[0007] In some embodiments, the droplet comprises a core containing at least one metal element in a liquid phase and a shell in a solid phase arranged to completely surround the core. The shell comprises a concentration gradient of at least one metal element that varies through the thickness of the shell, and the shell prevents the metal element from transitioning to a solid phase at a temperature below the solidus temperature of at least one metal element. In various embodiments, the shell has at least two layered regions, each of the at least two layered regions being defined by a unique element having a dominant concentration within each layered region.

[0008] In some embodiments, at least one metal element is a first metal element, and the core comprises a second metal element having a lower concentration than the first metal element. In various embodiments, the first metal element forms a dominant percentage of the inner region of the shell, and the second metal element forms a dominant percentage of the outer region of the shell. In some embodiments, the concentration of the first metal element decreases from the inner region of the shell to the outer region of the shell, and the concentration of the second metal element increases from the inner region of the shell to the outer region.

[0009] In some embodiments, a change in the concentrations of the first and second metal elements generates a thermodynamic shift in activation energy, causing the core to transition from the liquid phase to the solid phase. In various embodiments, the inner region of the shell contains oxides and suboxides of the first metal element, and the outer region of the shell contains oxides and suboxides of the second metal element. In some embodiments, the second metal element has a lower E° than the first metal element.

[0010] In some embodiments, the droplet comprises a supercooled liquid core comprising a first element, a second element, and a third element, wherein the first element has a higher concentration than the second and third elements. The solid shell comprises the liquid core and includes a concentration gradient of the first, second, and third elements. In various embodiments, the first, second, and third elements are arranged in layers to form the shell.

[0011] In some embodiments, a method for forming a droplet comprises the steps of forming a liquid core of the droplet from an alloy comprising a first element, a second element, and a third element, and forming a solid shell around the liquid core. The solid shell comprises an innermost layer having one of the first element, the second element, or the third element at a dominant concentration and an outermost layer having a different one of the first element, the second element, or the third element at a dominant concentration. The method further comprises the step of cooling the liquid core and the solid shell to a temperature below the solidus temperature of the alloy while maintaining the core in a liquid state.

[0012] In some embodiments, the solid shell comprises three layers, the innermost layer having a dominant concentration of the first element, the middle layer having a dominant concentration of the second element, and the outermost layer having a dominant concentration of the third element. In various embodiments, the shell is formed in an oxidizing environment. In some embodiments, the oxidizing environment is controlled by varying the partial pressure of oxygen in the oxidizing environment. In various embodiments, the innermost layer has a higher E than the middle layer. o The middle layer has a higher E° than the outermost layer. In some embodiments, the thickness of one or more of the three layers of the solid shell is determined by the exposure time to an oxidizing environment.

[0013] In some embodiments, the shell is formed in a reducing environment. In various embodiments, the innermost layer has a lower E° than the intermediate layer, and the intermediate layer has a lower E° than the outermost layer. In some embodiments, the thickness of one or more of the three layers of the solid shell is determined by the exposure time to the reducing environment. In various embodiments, the method further comprises the step of exposing the solid shell to one or more chelating agents to remove at least a portion of the outermost layer. In various embodiments, one or more chelating agents may be used to polish the outer surface of the shell to improve activation, droplet packing, and droplet flow, which are all characteristics capable of improving the performance of droplets (e.g., a combination of droplets and flux) within the solder paste material. In some embodiments, one or more chelating agents comprise at least one of a carboxylate, an amide, an alkoxide, an amine, a thiol, or a phosphate. In various embodiments, the method further comprises an etching process for polishing the inner layer of the solid shell.

[0014] A number of advantages are achieved by the present invention over the prior art. For example, an embodiment of the present invention provides the ability to solder a material at a temperature below the solidus temperature of the core element. Brief explanation of the drawing

[0015] FIG. 1 illustrates a simplified partial cross-sectional view of a droplet having a supercooled liquid metal core surrounded by a solid metal shell according to an embodiment of the present disclosure. FIG. 2a illustrates a partial cross-sectional view of an example of a droplet having a two-layer shell formed in an oxidizing environment according to an embodiment of the present disclosure. FIG. 2b illustrates an exemplary concentration gradient graph of the relative concentrations of each element (e.g., Sn and Bi) and their oxides for a cross-section of the droplet shown in FIG. 2a. FIG. 2c illustrates a partial cross-sectional view of an example of a droplet having a three-layer shell formed in an oxidizing environment according to an embodiment of the present disclosure. FIG. 2d illustrates an exemplary concentration gradient graph of the relative concentrations of each element (e.g., Bi, Sn, and In) and their oxides for a cross-section of the droplet shown in FIG. 2c. FIG. 3 illustrates steps associated with a method for forming a supercooled droplet comprising a liquid metal core surrounded by a solid shell comprising at least two layers, according to an embodiment of the present disclosure. FIG. 4 illustrates steps associated with a method for forming a supercooled droplet comprising a liquid metal core surrounded by a solid shell having an outer surface having a composition different from that of the liquid metal core, according to an embodiment of the present disclosure. FIG. 5a illustrates a partial cross-sectional view of an example of a droplet having a two-layer shell formed in a reducing environment according to an embodiment of the present disclosure. FIG. 5b illustrates an exemplary concentration gradient graph of the relative concentrations of each element (e.g., Sn and Bi) and their oxides for a cross-section of the droplet shown in FIG. 5a. FIG. 5c illustrates a partial cross-sectional view of an example of a droplet having a three-layer shell formed in a reducing environment according to an embodiment of the present disclosure. FIG. 5d illustrates an exemplary concentration gradient graph of the relative concentrations of each element (e.g., Bi, Sn, and In) and their oxides for a cross-section of the droplet shown in FIG. 5c. FIG. 6a illustrates, according to an embodiment of the present disclosure, a Darken-Gurry plot being overlaid and compared with a redox-indicated analog. FIG. 6b is a plot of FIG. 6a with E° and vapor pressure introduced according to an embodiment of the present disclosure. FIG. 7a is a schematic diagram of a surface-tunable hindrance of the solidification of core-shell metal particles according to an embodiment of the present disclosure. FIG. 7b is a high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) image of a shell according to an embodiment of the present disclosure. FIG. 8 illustrates the stability of supercooled field metal particles according to an embodiment of the present disclosure. FIG. 9a illustrates a Sobel filter-processed HAADF STEM image of metal particles highlighting the oxide thickness of the shell, according to an embodiment of the present disclosure. Figure 9b illustrates the analysis of the oxide thickness of the shell of the metal particles shown in Figure 9a. Fig. 9c illustrates an SEM image of the metal particles shown in Fig. 9a. FIG. 9d illustrates a chart showing the change in supercooling with respect to the number of heating cycles according to an embodiment of the present disclosure. FIG. 9e illustrates a chart showing the change in the surface area to volume ratio for supercooling according to an embodiment of the present disclosure. FIG. 10 illustrates a chart of the change in ΔT versus the change in yield for various alloys according to an embodiment of the present disclosure. FIG. 11a illustrates a table of organic ligands of various properties investigated according to an embodiment of the present disclosure. FIG. 11b illustrates a table of changes in the properties of an oxide shell according to an embodiment of the present disclosure. FIG. 12a illustrates the correlation between changes in composition and supercooling according to an embodiment of the present disclosure. FIG. 12b illustrates the correlation between changes in enthalpy and supercooling according to an embodiment of the present disclosure. FIG. 13 illustrates a table of various alloys evaluated and their results according to an embodiment of the present disclosure. FIG. 14a illustrates a DSC trace of supercooled SAC305 particles according to an embodiment of the present disclosure. FIG. 14b illustrates the theoretical ratio of particle radii to surface work according to an embodiment of the present disclosure. FIG. 15 illustrates the particle size distribution of the synthesized particles as they are, according to an embodiment of the present disclosure. FIG. 16 illustrates a table containing element constants used in some embodiments according to an embodiment of the present disclosure. FIG. 17 illustrates a table plotting the supercooling level and yield before and after re-flow for various alloys according to some embodiments of the present disclosure. FIG. 18 is an enlarged view of a hypothetical surface oxide architecture for a field metal according to an embodiment of the present disclosure. Specific details for implementing the invention

[0016] The technology disclosed herein generally relates to a supercooled liquid metal droplet surrounded by a solid shell. More specifically, the technology disclosed herein relates to a metal droplet surrounded by a shell having one or more solid metal and / or organic layers, which is in a stable liquid state below its solidus temperature. In some embodiments, the shell may be designed to hinder (e.g., prevent) the phase transition of the droplet from liquid to solid, as described in more detail below. Various embodiments of the invention, including methods, processes, systems, configurations, etc., are described herein.

[0017] For example, in some embodiments, a droplet is formed by being enclosed by a shell having two or more layers, each layer comprising a concentration gradient and defined by a dominant element. In other examples, a droplet is formed by being enclosed by a shell having an outer composition in which an element different from the dominant element of the core is dominant. In some embodiments, the droplet may be formed in an oxidizing atmosphere to promote the formation of a specific outer layer or an arrangement of layers, whereas in other embodiments, the droplet may be formed in a reducing atmosphere to promote the formation of a different specific outer layer or an arrangement of layers.

[0018] To better understand the features and aspects of a supercooled liquid metal droplet surrounded by a solid metal shell according to the present disclosure, additional context regarding the present disclosure is provided in the following section by discussing some specific configurations of the supercooled metal droplet according to embodiments of the present disclosure. These embodiments are, for example, most suitable, and other embodiments may have different configurations using different elements, fluid materials, gases, layer organization, etc.

[0019] FIG. 1 illustrates a simplified partial cross-sectional view of a droplet (100) having a supercooled liquid metal core (105) surrounded by a solid metal shell (110) according to an embodiment of the present disclosure. In some embodiments, the shell (110) may prevent the supercooled liquid metal core (105) from transitioning to a solid when the droplet (100) is exposed to a temperature below the solidus temperature of the liquid metal core by providing an internal surface without nucleation sites, as described in more detail below, and / or by generating a "thermodynamic tension" that increases the energy threshold at which a liquid-solid phase transition occurs. As illustrated in FIG. 1, the shell (110) comprises two layers (115, 120) which may each have different compositions, as described in more detail below. In some embodiments, the shell (110) may be manufactured in one, two, three, or more layers, each layer may be defined by a dominant concentration of a different element. In additional embodiments, the shell (110) may be terminated with a ligand (125) or other liquid, as described in more detail below.

[0020] More specifically, as defined herein, a layer (e.g., 115, 125) is a region of the shell (110) having a specific element at a dominant concentration. The approximate boundaries of each layer (e.g., innermost starting point and outermost ending point) are defined at locations where the specific element is no longer dominant and another element has a dominant concentration. Accordingly, each layer may also be referred to as an enrichment region, each enrichment region may be defined by the dominance of a specific element, and each enrichment region may have a concentration gradient of two or more elements. In some embodiments, the concentration gradient may be manipulated to generate a thermodynamic tension that hinders the phase transformation of the core, as described in more detail below.

[0021] In some embodiments having multiple layers, each layer may be arranged according to the reduction potential E° of the dominant element in that layer relative to the reduction potential of the dominant element in another layer. More specifically, in some embodiments, the element having the lowest E° (e.g., having the greatest propensity to form oxides in an oxidizing environment) may form the outermost layer of the shell, and the element having the highest E° (e.g., having the lowest propensity to form oxides) may form the innermost layer of the shell. Conversely, when formed under reducing conditions, the shell may be formed in the opposite order. In additional embodiments, the arrangement of each layer may depend on other factors, such as the propensity of each element to react with the gas or fluid forming the droplet. In some embodiments, the shell (110) may be at least partially terminated with a fluid, which may be a ligand or other composition that improves the stability of the shell, as described in more detail below.

[0022] Two-layer shell in an oxidizing environment

[0023] FIG. 2a illustrates a partial cross-sectional view of an example of a droplet having a two-layer shell formed in an oxidizing environment according to an embodiment of the present disclosure. As shown in FIG. 2a, the liquid metal core (205) of the droplet (200) is predominantly element bismuth (Bi) and has residual element tin (Sn). In the embodiment shown in FIG. 2a, the core (205) comprises 58 wt% Bi and 42 wt% Sn, but in other embodiments, these elements may have any other suitable ratio. For example, in one embodiment, the core (205) comprises 95 wt% Bi and 5 wt% Sn. In some embodiments, the liquid metal core (205) is a single metal element, whereas in other embodiments, it may be an alloy of a plurality of metal elements, or a combination of metal elements, metalloid elements, metalloid elements and / or non-metal elements, as described in more detail below. As additionally illustrated in FIG. 2a, the core (205) comprises a shell (210) having two layers, wherein layer 1 (215) is the innermost layer and layer 2 (220) is the outermost layer. In the example illustrated in FIG. 2a, layer 1 (225) is predominantly bismuth oxide (Bi a O b It is composed of (where a and b are any rational numbers), and layer 2 (220) is predominantly tin oxide (Sn c O d It consists of (where c and d are any rational numbers). In some embodiments, each layer (215, 220) may have a concentration gradient of a number of metal oxides and may be limited by a dominant metal oxide (e.g., layer 1 (215) is predominantly bismuth oxide and layer 2 (220) is predominantly tin oxide), as described in more detail below.

[0024] The inner surface (225) of layer 1 (215) may have a relatively smooth surface without nucleation sites, so that the nucleation and growth (i.e., phase transformation) of the liquid metal core (205) can be prevented from transitioning to a solid at a temperature below the solidus temperature of the core material (e.g., below 138°C in the case of eutectic 58Bi42Sn). In some embodiments, the concentration gradient within the shell (210) creates a thermodynamic tension that generates an increased energy barrier for liquid-solid phase transformation, thereby increasing the stability of the liquid metal core (205) in a liquid state at a temperature much lower than its solidus temperature.

[0025] FIG. 2b illustrates an exemplary concentration gradient graph of the relative concentrations of each element (e.g., Sn and Bi) and their oxides for a cross-section of the droplet (200) shown in FIG. 2a. This concentration gradient is, for example, most appropriate, and other embodiments may have different elements and / or different concentration gradients. As shown in the concentration graph (245) of FIG. 2b, starting from the left portion of the graph within the liquid core (205), the concentration of element Sn is relatively constant near 42%, and the concentration of element Bi is also relatively constant near 58%.

[0026] First, as you proceed into layer 1 (215) (i.e., toward the right in the concentration graph (245) in the case of FIG. 2b), the elements Sn and Bi form oxides (e.g., Bi a O b and Sn c O d) exists as such, and the concentrations of the two elements (e.g., their oxides) change rapidly in the middle part of layer 1 (215), with bismuth oxide increasing to approximately 90% while tin oxide decreases to approximately 10%. Toward the right side of layer 1 (215), the concentration of bismuth oxide decreases and the concentration of tin oxide increases so that the concentrations of tin oxide and bismuth oxide are equal at the interface between layer 1 (215) and layer 2 (220). Thus, since the concentration of Bi within layer 1 is higher than that of Sn, layer 1 (215) can be identified as having most of Bi and a small portion of Sn.

[0027] Now, proceeding into layer 2 (220), the composition of bismuth oxide continues to decrease to approximately 10%, while the concentration of tin oxide reaches a maximum of approximately 90%. Thus, on the outer surface (235) of the shell (210), tin oxide has a higher concentration than bismuth oxide. In some embodiments, during the formation of the shell (210), the environment may be a so-called "oxidation environment" containing one or more forms of oxygen that promote the formation of oxides (e.g., tin oxide and bismuth oxide). In some embodiments, the outer surface (235) may be covered with a fluid (230) that stabilizes the outer surface from oxidation and / or decomposition, as described in more detail below.

[0028] A concentration gradient within the shell (210) can generate thermodynamic tensions that hinder the phase transition of the liquid core (205) into a solid. More specifically, the concentration gradient can generate relatively large surface dipoles that create a relatively large Laplace pressure jump state, which implies that the core is at a relatively high pressure to maintain this high pressure jump state. For solidification, a critical nucleation agent size is required, implying that there is diffusion from these tensions that further increases the free energy of the entire droplet (200). However, the free energy of the droplet (200) must decrease so that this becomes a spontaneous process. This situation creates an increased barrier to solidification. As evidence of these conditions, when a high-melting-point alloy is cooled, it forms a glass-like (e.g., amorphous) structure in contrast to the crystalline structure preferred by the metal under normal conditions. This change to an amorphous structure shows that the liquid-solid transition can occur from an incomplete hypereutectic or hypoeutectic composition.

[0029] In the example illustrated in FIGS. 2a and 2b, the liquid metal core (205) is predominantly elemental Bi and the shell (210) comprises two layers, wherein the outer surface is a metal oxide different from the metal core (e.g., Sn c O d ) is dominant. More specifically, the core (205) is predominantly composed of Bi, and the outer surface of the droplet is mainly composed of Sn.

[0030] In some embodiments, the core (205) may contain a relatively small percentage of another element that may include, for example, the dominant element on the outer surface. As described herein, a fractional percentage may be smaller than a majority percentage of comparison. In additional embodiments, the outer surface may contain, for example, a relatively small percentage of another element that may include the dominant element in the core (e.g., less than 50%, less than 10%, less than 1%).

[0031] In some embodiments, the shell (210) may have a layer in which the dominant element on the outer surface of the shell is different from the dominant element of the core. For example, in one embodiment, the dominant element of the liquid metal core is tin and the dominant element of the shell is indium. That is, the core may be predominantly tin with a small portion of indium, and the outer surface of the shell may be predominantly indium (e.g., indium oxide) with a small portion of tin (e.g., tin oxide).

[0032] As used herein, the terms oxide (e.g., tin oxide) and chemical expression (e.g., Sn) c O d ) represents all possible oxides of tin where c and d are rational numbers. Additionally, specific oxides (e.g., rational numbers c and d) may vary throughout a specific droplet where one form of tin oxide may exist in layer 1 (215) and another form may exist in layer 2 (220).

[0033] In some embodiments, the fluid (230) may provide a physical barrier to physisorption and chemisorption to the shell (210). In one embodiment, the fluid is a ligand or other solution, e.g., but not limited to, an ammonia-based solution, a sulfur-based solution, a carboxylic acid, any organic acid, any inorganic acid, phosphotungstic acid, hexafluorophosphate, trichloroacetic acid, tribromoacetic acid, chloroacetic acid, zwitterionic species (e.g., glutamic acid, serine, etc.), dicarboxylic acid (e.g., glutaric acid, malonic acid, fumaric acid, oxalic acid, pimeloacetic acid, etc.), anhydrides, aldehydes, or other functional groups that convert into reactive species such as dicarboxylates, acetals, ketals, hemiacetals, etc. in the system. Those skilled in the art having the benefit of the present disclosure will understand that the above list of fluids is not exhaustive and that other organic and non-organic fluids may be used and are within the scope of the present disclosure. In additional embodiments, after formation, the outer surface of the shell (210) may be in contact with a gas, such as nitrogen, for example, to prevent oxidation and / or decomposition of the shell (210). Also in additional embodiments, the outer surface of the shell (210) may be terminated with a relatively inert metal, such as gold, silver, nickel, or platinum, for example. In one embodiment, the outer surface may be stable without fluid, gas, or termination.

[0034] In some embodiments, during the formation of the shell (210), the process may be performed in an oxidizing environment. In one embodiment, one or more forms of oxygen (e.g., O, O) 2 , O 3(etc.) may be injected in gaseous form into the solution in which the droplet (200) is formed to promote the rapid oxide formation and growth of the shell (210). In some embodiments, this change may be referred to as changing the partial pressure of oxygen. Additionally, the oxidizing environment may promote the order of shell formation and the order of shell layers, that is, the element having the highest potential for oxide formation may be dominant in the shell and may form the outer surface of the shell. The exposure time to the oxidizing environment and / or the concentration of oxygen may change the thickness of one or more layers of the shell. Similarly, a change in the environment to a reducing atmosphere in which oxidation is prevented by the removal of oxygen and other oxidizing gases may change the order of the shell layers. The reducing atmosphere may be formed through reducing gases, such as hydrogen, carbon monoxide, and hydrogen sulfide which is oxidized by any present oxygen.

[0035] Three-layer shell in an oxidizing environment

[0036] FIG. 2c illustrates a partial cross-sectional view of an example of a droplet having a three-layer shell formed in an oxidizing environment according to an embodiment of the present disclosure. As shown in FIG. 2c, the liquid metal core (255) of the droplet (250) is predominantly the element bismuth (Bi) and has smaller amounts of the elements indium (In) and tin (Sn). In the embodiment shown in FIG. 2c, the core (255) comprises 57 wt% Bi, 25 wt% In, and 17 wt% Sn, but in other embodiments, these elements may have any other suitable ratio. In some embodiments, the liquid metal core (255) is a single metal element, whereas in other embodiments, it may be an alloy of multiple metal elements, or a combination of metal elements, metalloid elements, metalloid elements, and / or non-metal elements, as described in more detail below. As additionally illustrated in FIG. 2c, the core (255) comprises a shell (260) having three layers, wherein layer 1 (265) is the innermost layer, layer 2 (270) is the middle layer, and layer 3 (275) is the outermost layer. In the example illustrated in FIG. 2c, layer 1 (265) is predominantly bismuth oxide (Bi a O b It is composed of (where a and b are any rational numbers), and layer 2 (270) is predominantly tin oxide (Sn c O d It is composed of (where c and d are any rational numbers), and layer 3 (275) is predominantly indium oxide (In e O f It consists of (where e and f are any rational numbers). In some embodiments, each layer (265, 270, 275) may have a concentration gradient of a number of metal oxides and may be limited by a dominant metal oxide (e.g., layer 1 (265) is predominantly bismuth oxide, layer 2 (270) is predominantly tin oxide, and layer 3 (275) is predominantly indium oxide), as described in more detail below.

[0037] The inner surface (285) of layer 1 (265) may have a relatively smooth surface without nucleation sites, so that the nucleation and growth (i.e., phase transformation) of the liquid metal core (255) can be prevented from transitioning to a solid at a temperature below the solidus temperature of the core material (e.g., below 62°C in the case of 57Bi26In17Sn). In some embodiments, the concentration gradient within the shell (260) creates a thermodynamic tension that generates an increased energy barrier for liquid-solid phase transformation, thereby increasing the stability of the liquid metal core (255) in a liquid state at a temperature much lower than its solidus temperature.

[0038] FIG. 2d illustrates an exemplary concentration gradient graph of the relative concentrations of each element (e.g., Bi, Sn, and In) and their oxides for a cross-section of the droplet (250) shown in FIG. 2c. This concentration gradient is, for example, most appropriate, and other embodiments may have different elements and / or different concentration gradients. As shown in the concentration graph (290) of FIG. 2d, starting from the left portion of the graph within the liquid core (285), the concentration of element Bi is relatively constant near 57%, the concentration of element In is relatively constant near 26%, and the concentration of element Sn is also relatively constant near 17%.

[0039] First, as you proceed into layer 1 (265) (i.e., toward the right in the concentration graph (290) in the case of FIG. 2d), the elements Bi, Sn, and In are oxides (e.g., Bi a O b , Sn c O d and In e O f) exists as such, and the concentrations of each element (e.g., their oxides) change such that bismuth oxide becomes dominant and increases, while tin oxide increases at a slower rate and indium oxide decreases. Toward the right of layer 1 (265), the concentration of bismuth oxide is higher than the concentrations of tin oxide and indium oxide, so layer 1 (265) can be identified as having most of Bi.

[0040] Now, proceeding into layer 2 (270), the composition of bismuth oxide decreases, while the concentration of tin oxide increases to become dominant in layer 2. Thus, layer 2 (270) can be identified as having most of the tin. Now, proceeding into layer 3 (275), the composition of tin oxide and bismuth oxide decreases, while indium continues to increase and becomes dominant in layer 3. Thus, layer 3 (275) can be identified as having most of the indium. Therefore, on the outer surface (293) of the shell (260), indium oxide has a higher concentration than bismuth oxide or tin oxide. In some embodiments, during the formation of the shell (210), the environment may be a so-called "oxidation environment" containing one or more forms of oxygen that promote the formation of oxides (e.g., indium oxide, tin oxide, and bismuth oxide). In some embodiments, the outer surface (293) may be covered with a fluid (280) that stabilizes the outer surface from oxidation and / or decomposition, as described in more detail below.

[0041] A concentration gradient within the shell (260) can generate a thermodynamic tension that hinders the phase transformation of the liquid core (255) into a solid. In the example illustrated in FIGS. 2c and 2d, the liquid metal core (255) is predominantly elemental Bi and the shell (260) comprises three layers, wherein the outer surface (293) is a metal oxide different from the metal core (e.g., In e O f) is dominant. More specifically, the core (255) is predominantly composed of Bi, and the outer surface of the droplet is mainly composed of In.

[0042] In some embodiments, the core (255) may contain a relatively small percentage of another element that may include, for example, the dominant element on the outer surface. As described herein, a fractional percentage may be smaller than a majority percentage of comparison. In additional embodiments, the outer surface may contain, for example, a relatively small percentage of another element that may include the dominant element in the core (e.g., less than 50%, less than 10%, less than 1%).

[0043] In some embodiments, the shell (260) may have an outer layer in which the dominant element of the outer surface of the shell is different from the dominant element of the core. For example, in one embodiment, the dominant element of the liquid metal core is bismuth and the dominant element of the shell is indium. That is, in some embodiments, the core may be predominantly bismuth with a small portion of indium, and the outer surface of the shell may be predominantly indium (e.g., indium oxide) with a small portion of bismuth (e.g., bismuth oxide).

[0044] In some embodiments, as described in more detail above, the fluid (230) may provide a physical barrier to physisorption and chemisorption to the shell (260). Also in additional embodiments, the outer surface (293) of the shell (260) may be terminated with a relatively inert metal, such as gold, silver, nickel, or platinum. In one embodiment, the outer surface may be stable without fluid, gas, or termination.

[0045] In some embodiments, during the formation of the shell (210), the process may be performed in an oxidizing environment. In one embodiment, one or more forms of oxygen (e.g., O, O) 2 , O 3(etc.) may be injected in gaseous form into the solution in which the droplet (250) is formed to promote the rapid oxide formation and growth of the shell (260). Additionally, the oxidizing environment may promote the order of shell formation and the order of shell layers, that is, the element having the highest potential for oxide formation may be dominant in the shell and may form the outer surface of the shell. Thus, a change in the environment to a reducing atmosphere, in which oxidation is prevented by the removal of oxygen and other oxidizing gases, may change the order of the shell layers. The reducing atmosphere may be formed through reducing gases, such as hydrogen, carbon monoxide, and hydrogen sulfide which is oxidized by any present oxygen. Similarly, changing the concentration of the reducing gas in the environment and the exposure time thereto may change the thickness of one or more layers of the shell. In a further embodiment, the environment may be dynamically changed between a reducing environment and an oxidizing environment during the growth of the shell to promote the formation of specific shell layers in a specific order.

[0046] The relative concentrations of Sn, In, Bi, and related oxides shown in FIGS. 2a through 2d are for exemplary purposes only. Those skilled in the art who benefit from the present disclosure will understand that other embodiments may have different relative concentrations and / or different elements.

[0047] manufacturing process

[0048] FIG. 3 illustrates steps associated with a method (300) for forming a supercooled droplet comprising a liquid metal core surrounded by a solid shell comprising at least two layers, according to an embodiment of the present disclosure. As described in FIG. 3, in step (305), a molten solution comprising most of element A and a small portion of element B is prepared. In some embodiments, the molten solution may be formed by heating element A and adding element B. In other embodiments, element A and element B may already be mixed but may exist as a solid that can be simply heated to form a molten solution.

[0049] In step (310), the molten solution from step (305) is immersed in a fluid. In some embodiments, the fluid comprises a conjugated acid-base pair in which an acid component is configured to polish the shell in situ and a base is configured to stabilize the shell against physisorption and / or chemisorption. More specifically, in one embodiment, the acid component smooths the inner surface of the shell to minimize nucleation sites that could cause the liquid metal core to solidify when cooled below the solidus temperature. In other embodiments, a different type of fluid may be used. In additional embodiments, one or more gases, such as but not limited to oxygen to promote the formation of oxides or a reducing gas to delay the formation of oxides, may be added to the fluid. Exemplary gases are described in more detail herein and may be used to promote the formation of one or more layers of the shell in a specific order.

[0050] In step (315), the molten solution may be separated into droplets while immersed in a fluid and / or gas. In some embodiments, the molten solution may be separated using a mechanical shearing device, such as a high-speed blade immersed in a fluid. During shearing, each droplet is surrounded by a fluid and / or a gas that may be introduced into the fluid. The fluid and / or gas may be selected and / or changed during the shearing process to manipulate the properties of the shell and / or the order of the shell layers as the droplets are formed. That is, the formation of the shell may be performed in a chemically dynamic environment (e.g., changing the gas flow rate, gas composition, gas partial pressure, temperature, fluid composition) and a mechanically dynamic environment (e.g., changing the shear rate, shear stress, etc.) that can be optimized to form a specific shell composition.

[0051] In step (320), a shell may be formed around each droplet. In some embodiments, the shell may comprise more than one layer, wherein each layer may be defined by a different element of a dominant concentration. During the formation of the shell, the bulk-dispersed element may be stabilized on the surface of the core due to its potential to be oxidized and / or its stronger affinity for the fluid. As discussed in more detail below, the internal migration and diffusion of elements within the core bring them to the surface, and reaction and stabilization at the surface cause them to remain on the surface to form the shell.

[0052] In one embodiment, the fluid and one or more elements may be selected to have a propensity for bonding higher than the propensity for one or more elements to form oxides. That is, the fluid and elements may be manipulated to react preferentially. Thus, the element with the highest propensity to react with the fluid will form the outermost layer, the element with the next highest propensity to react with the fluid will form the middle layer, and the element with the next highest propensity to react with the fluid will form the innermost layer. In one example, an ammonia-based fluid may be selected to react preferentially with transition metals. In another example, a sulfur-based fluid may be selected to react preferentially with gold, and a carboxylic acid-based fluid may be selected to react preferentially with indium, while a phosphate may be selected to react preferentially with gallium.

[0053] In various embodiments, the solid shell may be exposed to one or more chelating agents to remove at least a portion of the outermost layer. In some embodiments, one or more chelating agents include at least one of a carboxylate, amide, alkoxide, amine, thiol, or phosphate, but other suitable chelating agents may be used.

[0054] In another embodiment, one or more elements may be selected to have a propensity to form oxides higher than the propensity of the element reacting with the fluid. Thus, each layer may be arranged according to the reduction potential E° of the dominant element within that layer relative to the reduction potential of the dominant element within another layer. More specifically, cohesive energy density and partial miscibility may be used to "select" lower E° elements, which are then pushed to the surface of the core to form a shell during processing. These lower E° elements may be completely removed from the bulk of the core, substantially removed from the bulk of the core, or partially removed from the bulk of the core, depending on the desired final concentration of these elements within the core and shell. This can be controlled by selecting fluid properties, oxidant concentration, partial pressure, and / or processing parameters, such as temperature, time, etc.

[0055] In additional embodiments, the shell may be formed using the competition between the reaction potential with the fluid and the reduction potential of one or more elements to manipulate the multilayer shell with specific arrangements of the layer compositions and shell layers. More specifically, the highest potential for reaction may be for element A to react with the fluid, the next highest potential may be for element B to react with oxygen to form an oxide, the next highest potential may be for element C to react with the fluid, and so on. Thus, any organization and composition of the layers within the shell may be manipulated by selecting appropriate elements, fluids, and / or gases.

[0056] In some embodiments, a specific amount of an element (e.g., element B) may be contained in a molten solution (e.g., the molten solution contains elements A and B) such that element B remains in or is almost non-existent in the liquid core of each droplet, thereby preferentially forming a portion of the shell such that the core is substantially 100% element A. In some embodiments, the elements may be miscible (e.g., become solution together with each other), whereas in other embodiments, one or more of the elements may be immiscible and form an interstitial material that does not become solution with the base metal. In embodiments having immiscible elements, removing all or almost all of the immiscible elements from the core (i.e., moving them out of the core to form the shell) may increase the amount of supercooling in embodiments where a specific element may precipitate from the solution in the core, causing nucleation of the core and subsequent solidification. To form a shell by removing element B from the core, the fluid and / or gas may be selected to have a high potential for reaction with element B and a low potential for reaction with element A so that the shell is preferentially formed from element B. In additional embodiments, time and temperature during shell formation may be adjusted so that a number or all of element B can diffuse into the shell. In additional embodiments, a portion or substantially all of element B may be leached from the bulk.

[0057] In some embodiments, miscibility and reduction potential are used to construct an alloy in which the element having the highest oxidation / reduction potential under these dynamic processing conditions enriches the outer surface of the shell. If this component is not highly soluble in other alloy components (e.g., In in Sn versus Ge, or Bi in In versus Ge), the propensity to be on the surface may be increased. In some embodiments, a modified Darker-Garth plot and / or the Hume-Rothery rule may be used to predict solubility in the bulk and to correct these solubility parameters by the probability of being divided on the surface to predict the final alloy composition.

[0058] In additional embodiments, the specific compounds formed can be controlled by controlling the time and temperature during shell formation. For example, when forming tin oxide, 2+ tin oxide is preferentially formed using a relatively short time and / or low temperature, and 4+ tin oxide is preferentially formed using a relatively longer time and / or higher temperature.

[0059] In step (325), droplets each containing a shell are cooled to a temperature below the solidus temperature of the core material. That is, the composition of the core has a specific solidus temperature after the formation of the shell, and due to the absence of nucleation sites on the inner surface of the shell and / or the concentration gradient of the shell, the liquid metal core can be cooled to a temperature below its solidus temperature without causing the core to transition to a solid state. In some embodiments, the droplets are coated with a fluid that protects the shell from decomposition. In various embodiments, the droplets have a diameter of approximately 1 micrometer, but in other embodiments, the droplets have a diameter of 0.5 micrometers to 10 micrometers, and in other embodiments, 0.25 micrometers to 100 micrometers.

[0060] It will be understood that the method (300) is exemplary and variations and modifications are possible. The steps described sequentially may be executed in parallel, the order of the steps may be changed, and the steps may be modified, combined, added, or omitted.

[0061] FIG. 4 illustrates steps associated with a method (400) for forming a supercooled droplet comprising a liquid metal core surrounded by a solid shell having an outer surface having a composition different from that of the liquid metal core, according to an embodiment of the present disclosure. As described in FIG. 4, in step (405), a molten solution comprising most of element A and a small portion of element B is prepared. In some embodiments, the molten solution may be formed by heating element A and adding element B. In other embodiments, element A and element B may already be mixed but may exist as a solid that can be simply heated to form a molten solution.

[0062] In step (410), the molten solution from step (505) is immersed in a fluid. In some embodiments, the fluid comprises a conjugated acid-base pair configured such that an acid component polishes the shell in situ and a base stabilizes the shell against physisorption and chemisorption. More specifically, in one embodiment, the acid component smooths the inner surface of the shell to minimize nucleation sites that could cause the liquid metal core to solidify when cooled below the solidus temperature. In other embodiments, a different type of fluid may be used. In additional embodiments, one or more gases, such as but not limited to oxygen, may be added to the fluid.

[0063] In step (415), the molten solution may be separated into droplets while immersed in a fluid and / or gas. In some embodiments, the molten solution may be separated using a mechanical shearing device, such as a high-speed blade immersed in the fluid. During shearing, each droplet is surrounded by the fluid and / or gas. The fluid and / or gas may be selected to manipulate the properties of the shell and / or droplet, as described in more detail below.

[0064] In step (420), a shell may be formed around each droplet. In some embodiments, the shell may comprise more than one layer, wherein each layer may be defined by a different element of dominant concentration. In some embodiments, the liquid metal core comprises most of element A and a small portion of element B, while the outer surface of the shell comprises most of element B and a small portion of element A. As described above in step (320), various methods may be used to selectively manipulate the composition and arrangement of each layer.

[0065] In step (425), droplets each containing a shell are cooled to a temperature below the solidus temperature of the core material. That is, the composition of the core has a specific solidus temperature after the formation of the shell, and due to the absence of nucleation sites on the inner surface of the shell and / or the concentration gradient of the shell, the liquid metal core can be cooled to a temperature below its solidus temperature without causing the core to transition to a solid state. In some embodiments, the droplets are coated with a fluid that protects the shell from decomposition. In various embodiments, the droplets have a diameter of approximately 1 micrometer, but in other embodiments, the droplets have a diameter of 0.5 micrometers to 10 micrometers, and in other embodiments, 0.25 micrometers to 100 micrometers.

[0066] In one embodiment, a solder alloy known as SAC305 comprising tin, silver, and copper is doped with germanium so that the core of the droplet is substantially SAC305 and the outer shell is predominantly germanium. In another embodiment, an alloy of bismuth and tin is doped with germanium, and the outer shell is predominantly germanium.

[0067] In an additional embodiment, after the droplet is formed, the droplet may undergo a temperature to increase the amount of supercooling the droplet can withstand, as described in more detail below.

[0068] It will be understood that the method (400) is exemplary and variations and modifications are possible. The steps described sequentially may be executed in parallel, the order of the steps may be changed, and the steps may be modified, combined, added, or omitted.

[0069] Two-layer shell in a reducing environment

[0070] FIG. 5a illustrates a partial cross-sectional view of an example of a droplet having a two-layer shell formed in a reducing environment according to an embodiment of the present disclosure. The droplet (500) of FIG. 5a is similar to the droplet (200) of FIG. 2a, in which the core contains the same constituent elements at similar concentrations, but the droplet (500) is formed in a reducing environment, so that instead of forming bismuth oxide on the outer surface as in the droplet (200), the droplet (500) forms tin oxide on the outer surface.

[0071] As illustrated in FIG. 5a, the liquid metal core (505) of the droplet (200) is predominantly element bismuth (Bi) and has residual element tin (Sn). In the embodiment illustrated in FIG. 5a, the core (505) contains 58 wt% Bi and 42 wt% Sn, but in other embodiments, these elements may have any other suitable ratio. For example, in one embodiment, the core (505) contains 95 wt% Bi and 5 wt% Sn. In some embodiments, the liquid metal core (505) is a single metal element, whereas in other embodiments, it may be an alloy of multiple metal elements, or a combination of metal elements, metalloid elements, metalloid elements and / or non-metal elements, as described in more detail below. As additionally illustrated in FIG. 5a, the core (505) comprises a shell (510) having two layers, wherein layer 1 (515) is the innermost layer and layer 2 (520) is the outermost layer. In the example illustrated in FIG. 5a, layer 1 (525) is predominantly tin oxide (Sn c O d It is composed of (where c and d are any rational numbers), and layer 2 (220) is predominantly bismuth oxide (Bi a O b It consists of (where a and b are any rational numbers). In some embodiments, each layer (515, 520) may have a concentration gradient of a number of metal oxides and may be limited by a dominant metal oxide (e.g., layer 1 (515) is predominantly tin oxide and layer 2 (520) is predominantly bismuth oxide), as described in more detail below.

[0072] The inner surface (525) of layer 1 (515) may have a relatively smooth surface without nucleation sites, so that nucleation and growth (i.e., phase transformation) of the liquid metal core (505) can be prevented from transitioning to a solid at a temperature below the solidus temperature of the core material (e.g., below 138°C in the case of eutectic 58Bi42Sn). In some embodiments, the concentration gradient within the shell (510) creates a thermodynamic tension that generates an increased energy barrier for liquid-solid phase transformation, thereby increasing the stability of the liquid metal core (505) in a liquid state at a temperature much lower than its solidus temperature.

[0073] FIG. 5b illustrates an exemplary concentration gradient graph of the relative concentrations of each element (e.g., Sn and Bi) and their oxides for a cross-section of the droplet (500) shown in FIG. 5a. This concentration gradient is, for example, most appropriate, and other embodiments may have different elements and / or different concentration gradients. As shown in the concentration graph (545) of FIG. 5b, starting from the left portion of the graph within the liquid core (505), the concentration of element Sn is relatively constant near 42%, and the concentration of element Bi is also relatively constant near 58%.

[0074] First, as you proceed into layer 1 (515) (i.e., toward the right in the concentration graph (545) of FIG. 5b), the elements Sn and Bi form oxides (e.g., Sn a O b and Bi c O d ) exists as such, and the concentrations of the two elements (e.g., their oxides) change rapidly in the middle part of layer 1 (515), with tin oxide increasing to approximately 90% while bismuth oxide decreasing to approximately 10%. Thus, the concentration of Sn within layer 1 (515) is higher than that of Bi, so layer 1 can be identified as having most Sn and a small portion of Bi.

[0075] Now, proceeding into layer 2 (520), the composition of tin oxide continues to decrease to approximately 10%, while the concentration of bismuth oxide reaches a maximum of approximately 90%. Thus, on the outer surface (535) of the shell (510), bismuth oxide has a higher concentration than tin oxide. In some embodiments, during the formation of the shell (510), the environment may be a so-called "reducing environment" that promotes the formation of bismuth oxide on the outer surface (535) and may include one or more forms of reducing gases, such as hydrogen, carbon monoxide, and any present oxygen, which are oxidized by hydrogen, carbon monoxide, and any present oxygen. In some embodiments, the outer surface (535) may be covered with a fluid (530) that stabilizes the outer surface from oxidation and / or decomposition, as described in more detail herein.

[0076] A concentration gradient within the shell (510) can generate a thermodynamic tension that hinders the phase transition of the liquid core (505) into a solid. In some embodiments, the core (505) may contain a relatively small percentage of other elements that may include, for example, the dominant element on the outer surface. As described herein, a fractional percentage may be smaller than a majority percentage of comparison. In additional embodiments, the outer surface may contain, for example, a relatively small percentage of other elements that may include the dominant element in the core (e.g., less than 50%, less than 10%, less than 1%).

[0077] In some embodiments, the fluid (530) may provide a physical barrier to physical adsorption and chemical adsorption to the shell (510). In additional embodiments, the outer surface of the shell (510) may be terminated with a relatively inert metal, such as gold, silver, nickel, or platinum. In one embodiment, the outer surface may be stable without fluid, gas, or termination.

[0078] Three-layer shell in an oxidizing environment

[0079] FIG. 5c illustrates a partial cross-sectional view of an example of a droplet having a three-layer shell formed in a reducing environment according to an embodiment of the present disclosure. The droplet (550) of FIG. 5c is similar to the droplet (250) of FIG. 2c, in which the core contains the same constituent elements at similar concentrations, but the droplet (550) is formed in a reducing environment, so that instead of forming indium oxide on the outer surface as in the droplet (250), the droplet (550) forms bismuth oxide on the outer surface.

[0080] As illustrated in FIG. 5c, the liquid metal core (555) of the droplet (550) is predominantly the element bismuth (Bi) and contains smaller amounts of the elements indium (In) and tin (Sn). In the embodiment illustrated in FIG. 5c, the core (555) contains 57 wt% Bi, 26 wt% In, and 17 wt% Sn, but in other embodiments, these elements may have any other suitable ratio. In some embodiments, the liquid metal core (555) is a single metal element, whereas in other embodiments, it may be an alloy of multiple metal elements, or a combination of metal elements, metalloid elements, metalloid elements, and / or non-metal elements, as described in more detail below. As additionally illustrated in FIG. 5c, the core (555) comprises a shell (560) having three layers, wherein layer 1 (565) is the innermost layer, layer 2 (570) is the middle layer, and layer 3 (575) is the outermost layer. In the example illustrated in FIG. 5c, layer 1 (565) is predominantly indium oxide (In a O b It is composed of (where a and b are any rational numbers), and layer 2 (570) is predominantly tin oxide (Sn c O d It is composed of (where c and d are any rational numbers), and layer 3 (575) is predominantly bismuth oxide (Bi e O fIt consists of (where e and f are any rational numbers). In some embodiments, each layer (565, 570, 575) may have a concentration gradient of a number of metal oxides and may be limited by a dominant metal oxide (e.g., layer 1 (565) is predominantly indium oxide, layer 2 (570) is predominantly tin oxide, and layer 3 (575) is predominantly bismuth oxide), as described in more detail below.

[0081] The inner surface (585) of layer 1 (565) may have a relatively smooth surface without nucleation sites, so that the nucleation and growth (i.e., phase transformation) of the liquid metal core (555) can be prevented from transitioning to a solid at a temperature below the solidus temperature of the core material (e.g., below 62°C in the case of 57Bi26In17Sn). In some embodiments, the concentration gradient within the shell (560) creates a thermodynamic tension that generates an increased energy barrier for liquid-solid phase transformation, thereby increasing the stability of the liquid metal core (555) in a liquid state at a temperature much lower than its solidus temperature.

[0082] FIG. 5d illustrates an exemplary concentration gradient graph of the relative concentrations of each element (e.g., Bi, Sn, and In) and their oxides for a cross-section of the droplet (550) illustrated in FIG. 5c. This concentration gradient is, for example, most appropriate, and other embodiments may have different elements and / or different concentration gradients. As illustrated in the concentration graph (590) of FIG. 5d, starting from the left portion of the graph within the liquid core (55), the concentration of element Bi is relatively constant near 57%, the concentration of element In is relatively constant near 26%, and the concentration of element Sn is also relatively constant near 17%.

[0083] First, as you proceed into layer 1 (565) (i.e., toward the right in the concentration graph (590) in the case of FIG. 5d), the elements Bi, Sn, and In are oxides (e.g., Ina O b , Sn c O d and Bi e O f The concentrations of each element (e.g., their oxides) are as follows: indium oxide increases to become dominant, while tin oxide increases at a slower rate and bismuth oxide decreases. Toward the right of layer 1 (565), the concentration of indium oxide is higher than that of tin oxide and indium oxide, so layer 1 (565) can be identified as having most of the In.

[0084] Now, proceeding into layer 2 (570), the composition of indium oxide decreases while the concentration of tin oxide increases to become dominant in layer 2. Thus, layer 2 (570) can be identified as having most of the tin. Now, proceeding into layer 3 (575), the composition of tin oxide and indium oxide decreases while bismuth continues to increase and becomes dominant in layer 3. Thus, layer 3 (575) can be identified as having most of the bismuth. Therefore, on the outer surface (593) of the shell (560), bismuth oxide has a higher concentration than indium oxide or tin oxide. In some embodiments, during the formation of the shell (510), the environment may be a so-called "reducing environment" that promotes the formation of bismuth oxide on the outer surface (593) and may include one or more forms of reducing gases, such as hydrogen, carbon monoxide, and any present oxygen, which are oxidized by hydrogen, carbon monoxide, and any present oxygen. In some embodiments, the outer surface (593) may be covered with a fluid (580) that stabilizes the outer surface from oxidation and / or decomposition, as described in more detail in this specification.

[0085] A concentration gradient within the shell (560) can generate a thermodynamic tension that hinders the phase transition of the liquid core (555) into a solid. In some embodiments, the core (555) may contain a relatively small percentage of other elements that may include, for example, the dominant element on the outer surface. As described herein, a fractional percentage may be smaller than a majority percentage of comparison. In additional embodiments, the outer surface may contain, for example, a relatively small percentage of other elements that may include the dominant element in the core (e.g., less than 50%, less than 10%, less than 1%).

[0086] In some embodiments, as described in more detail above, the fluid (580) may provide a physical barrier to physisorption and chemisorption to the shell (560). Also in additional embodiments, the outer surface (593) of the shell (560) may be terminated with a relatively inert metal, such as gold, silver, nickel, or platinum. In one embodiment, the outer surface may be stable without fluid, gas, or termination.

[0087] The relative concentrations of Sn, In, Bi, and related oxides shown in FIGS. 5a through 5d are for exemplary purposes only. Those skilled in the art who benefit from the present disclosure will understand that other embodiments may have different relative concentrations and / or different elements.

[0088] Incomplete phase transition through surface asymmetry

[0089] Surface speciation and autonomous differentiation in metals can be influenced by miscibility, reactivity, and the environment. Under non-reactive environments, surface speciation can be influenced by flux, cohesive energy density, and surface energy minimization. Under oxidizing (e.g., ambient) conditions, reduction potential, curvature, and surface plasticity play a role and influence surface texture and stoichiometry. Such speciation can alter the energy domain of the material through surface asymmetry. This complex surface architecture provides an active platform based on asymmetry in the surface structure and its induced influence on solubility, which can be utilized to disrupt liquid-solid transitions. Induced interfacial ordering impairs Cahn-Hilliard type diffusion, thereby hindering homogeneous nucleation. In-situ formed surfaces that passivate oxides with accompanying size reduction trap molten metal in a 'containerless' state (since oxides are a continuum from the bulk) while establishing a physical barrier against heterogeneous nucleating agent(s). Manipulating the distribution of components across metal particles and their surfaces can affect the degree of supercooling and provide a generalized approach to hindering supercooling.

[0090] Homogeneous material mixing can be understood through discipline-specific rules. In metals, solid solutions (where entropy is dominant) can be understood through the Hume-Lordery law, which is often grasped in two-dimensional dark-distance plots (Fig. 6A). In metals, while a focus on solid solutions may be driven by the structural applications of these materials, a molten liquid phase is often required for ease of processing. This necessity has been exacerbated by advancements in flexibility, wearables, and bioelectronics, even at reduced temperatures, as most substrates are incompatible at temperatures above 100°C. Liquid-solid (LS) phase transitions can be considered, particularly when enabling new applications of metals in hybrid / mixed material systems. However, such transitions depend on fundamental kinetics and thermodynamics. Dynamically, diffusion can be important for nucleation and growth for this reason, and rapid quenching is often used to form metastable states such as glass or supercooled liquid.

[0091] Thermodynamically, nucleating agents (extrinsic or homogeneous) lower the activation barriers supporting LS phase transitions. If the nucleation barrier is high or Khan-Hilliad type diffusion is poor, the LS transition can be thermodynamically hindered. Heterogeneic (extrinsic source) nucleation can be eliminated via containerless approaches or through surface barriers such as passivating oxides. High entropy is favorable for the liquid phase and significantly reduces the probability of homogeneous nucleation. High entropy can be achieved through composition or through dimergence in the distribution of microstates occupied by the alloy components. For example, a high surface area-to-volume ratio in nanoparticles can lead to restricted homogeneous nucleation. Speciation across passivating oxides and the appropriate selection of dimensions bound to the surface texture can induce differences in the density of energy states (microstates) that the alloy components can occupy. These differences in energy states can lead to fluctuations in the energy regions of the bulk (similar to nanoparticles) and can therefore be used to tune diffusion and equilibrium state(s). The oxidation process can be an essential component in understanding the miscibility and LS phase transitions of ambient metal powders.

[0092] Redefining the dark-distance plot in an oxidizing environment involves the valence as the standard reduction potential (E 0This may involve substitution with ) and cohesive energy density (CED). For illustrative purposes only, considering the example of Bi, In, and Sn, the Darker-Distance plot (Fig. 6a) is overlaid and compared with redox-indicated analogs (Fig. 6b). In Fig. 6b, the introduction of E° and vapor pressure (instead of CED) enables the prediction of surface speciation. By the modified Darker-Distance plot, the so-called preferential interaction parameter (PIP) is defined by the region within the three-dimensional plot where the overlapping area represents solubility. E 0 The composition is predominant at the surface. For example, in the field metal (BiInSn), a shell predominantly composed of In2O3 should form, but it contains significant incorporation of Sn suboxides; therefore, interfacial Bi must be separated beneath the oxide shell. However, this separated Bi occupies a trapped state because it is energetically costly to dissolve into the bulk, as it pushes the mixture out of the eutectic. It is immiscible like Te but has a significantly lower E. 0 The introduction of elements may result in TeO2 shells with induced surface speciation, although there may be small variations in the eutectic composition (Fig. 6a). This speciation leads to increased thermodynamic stress, and thus is more likely to hinder the LS phase transition.

[0093] Conversely, high E such as Au 0 The introduction of the component (Fig. 6b) limits it to the bulk, where E 0Significant differences can lead to the formation of intermetallic material supporting homogeneous nucleation, and consequently poor supercooling. Surface-driven thermodynamic tuning of the LS transition can be achieved by i) establishing a smooth passivation oxide shell on the surface of the molten metal, ii) manipulating this oxide shell through the appropriate selection of alloy composition, processing temperature (to control thickness), and surface ligands, and iii) achieving this organization while the metal is superheated, followed by ambient cooling followed by rapid cooling to near the alloy's melting point under mechanical stress. The appropriate selection of processing conditions and management of the relaxation energy region can lead to incomplete solidification. Similarly, activation energy can be degraded, leading to tunable solidification based on differences (asymmetry) in the surface microstate distribution.

[0094] FIG. 7a is a schematic diagram of the surface-tunable disturbance of solidification of core-shell metal particles (700). FIG. 7b is a high-angle annular field scanning transmission electron microscope (HAADF-STEM) Sobel filter analysis of the oxide thickness of the shell (710). Since surface characteristics may not be dynamic, the resulting metastable state must remain stable for a long period and be elastic to variable handling conditions as long as the oxide layer is not destroyed. FIG. 8 illustrates the stability of supercooled field metal particles produced by this approach (under conditions of approximately 2 years of storage and 50 thermal cycles). FIG. 8 illustrates differential scanning calorimetry (DSC) traces of field metal particles under different conditions.

[0095] Background Technology

[0096] The supercooled liquid is a metastable phase due to an incomplete liquid-solid transition. The change in free energy during the LS phase transition can be defined by the following:

[0097] (1)

[0098] Here, ΔHf is the enthalpy of fusion, Tm is the melting temperature, ΔT is the temperature difference between the melting and freezing points (ΔT / Tm is the degree of supercooling), Cp is the heat capacity, Γi is the interfacial excess, and μi is the chemical potential of surface component i. The first part of this equation (classical form) identifies the bulk enthalpy-entropy balance as the driving force of the phase transition. The second part of this equation (correction for surface anisotropy) identifies surface work similar to the Gibbs-Duhem equation, even considering that passivation oxides in metal alloys are compositionally anisotropic and form flux-differentiated / specimenized assemblies (due to redox, cohesive energy density, and atomic radius). Therefore, the contribution of this self-sorting interfacial layer to the total energy of the material can be understood as the sum of the contributions of each component of the thin (approx. 0.7 to 4 nm) oxide layer to the change in chemical potential, Δμ, across the entire surface. Until recently, the entropy limit of the surface (too thin compared to the bulk) typically implied that the surface could be neglected. However, energetically, interfacial metastables can determine the energy range of the material by inducing significant tensions that can be overcome during phase transitions. However, this Δμ varies with temperature as the material approaches the phase transition point. Ambiently, the composition of the oxide layer undergoes irreversible temperature-dependent changes that make the approach to the phase transition point asymmetric (e.g., the total energy depends on the direction of the LS transition). A significant increase in total surface work (e.g., through a steep concentration gradient) is the change in Gibbs free energy (Δg LS Make ) positive, thereby tuning the energy barrier associated with the LS transition.

[0099] In liquid droplets, the Laplace pressure jump state (ΔP = 2γ / r; where γ = surface tension, r = radius) can lead to mechanical equilibrium but can induce asymmetry in the chemical potential beneath these oxide interfaces. By definition, the surface tension term (γ) is the product of the interfacial excess (Γ) and the difference in chemical potential (Δμ), as follows:

[0100] (2)

[0101] However, liquid metals and associated oxides are non-volatile, and therefore Considering complex oxides of approximately 1 to 4 nm for liquid metals, steep concentration gradients make surface terms significant contributors to pressure jump states and the associated thermodynamic potentials of the bulk liquid metal. This complexity of interfacial excess and ground Δμ implies that such surfaces can be utilized to manipulate bulk PV-work under appropriate selection of processing conditions and alloy compositions. By tuning the surface architecture of molten metal droplets, along with concomitant changes in bulk composition, the solid-liquid phase transition can be thawed by: i) utilizing self-organizing surface oxides as "containers" to eliminate heterogeneous nucleation, and ii) utilizing chemical potential gradients due to the complex oxide structures created to hinder homogeneous nucleation by generating interface-driven asymmetric energy tensions. These tensions must be overcome for critical nucleation growth. As with entropy, these surface-driven tensions

[0102] It increases the total free energy, thereby further hindering solidification (see Equation 1). Considering that the composition and dimensions of surface oxides evolve with time, stimulus, temperature, and diffusivity, the surface term evolves based on treatment conditions and alloy composition (e.g., component reactivity). Therefore, under a potentially large number of treatment conditions, ΔG LS The surface effect on is It can be understood as.

[0103] Consequently, as surface and interface complexity increases, the ability to hinder homogeneous nucleation (e.g., high activation energy, ΔE) a The same applies to ). Metastable interfaces can disrupt phase transition kinetics and can be the basis of landscape inversion phase transition theory (LIPT). A similar full-energy-domain inversion is used here to synthesize and stabilize metastable supercooled liquid metal droplets.

[0104] result

[0105] Supercooled core-shell metal particles were synthesized using the SLICE (Shearing Liquid into Complex Particles) method. In most of these cases, field metal (32.5% Bi, 16.5% Sn, and 51% In, Tm) 335 K) and eutectic bismuth-tin (58% Bi, 42% Sn, Tm Although 411 K) was used as the base alloy, this method can be used for other alloys. The molten ingot was sheared in the presence of a conjugated acid-base pair to form supercooled core-shell particles with a diameter of about 1 μm. The acid-base pair polishes in situ (acid) and stabilizes a thin oxide shell (about 4 nm, Fig. 8a, Fig. 8b). Fig. 9a illustrates a Sobel filter-processed HAADF STEM image of the metal particles highlighting the oxide thickness of the shell (910). Fig. 9b illustrates an analysis of the oxide thickness of the shell (910) of the metal particles shown in Fig. 9a. The supercooling degree, yield, and purity were evaluated by analyzing the synthesized particles as is using DSC. The particles (900) were characterized using other microscopy and spectroscopic methods (e.g., SEM in Fig. 9c). For field metals, the yield of supercooled particles is (> 98%), while the degree of supercooling is ΔT / Tm for newly manufactured particles. It was 0.34 (Fig. 8).

[0106] To evaluate the stability of the synthesized supercooled particles, accelerated aging and ambient aging experiments were performed on the supercooled field metal particles. A sample (15 g) was stored in ethyl acetate under ambient conditions (benchtop) for over 2 years, resulting in a total loss of 43% of the supercooled particles after 8 months and 57% over 2 years. Figure 9d illustrates a chart showing the change in supercooling with respect to the number of heating cycles. Figure 9e illustrates a chart showing the change in the surface area to volume ratio with respect to supercooling. ΔT / Tm changed from 0.34 to 0.14 over 2 years, but there was no loss of yield. The change in ΔT / Tm is likely due to the continued growth of the oxide shell and / or the loss of protective ligands. These data suggest that the supercooled metal particles are stable against accidental ambient fluctuations over the long term. To further support this, the second sample was accelerated aging through weekly thermal cycling (200 K to 373 K) over 70 cycles. For the accelerated-aging sample, a significant change in ΔT / Tm is observed (Fig. 9e). The freezing point of these supercooled particles gradually increases but approaches an asymptote at approximately 273 K with an exponential decay trend (maximum Δ(ΔT / Tm) = 0.15, Fig. 9e). Heat-driven growth in the oxide shell, loss of surface ligands, and associated changes in surface morphology due to repeated expansion and contraction support the decrease in ΔT / Tm. When the particles were heated to 473 K (where total loss of supercooling is observed), a gradual increase in particle size was observed from the original approximately 1 μm. This change also directly affects ΔT / Tm. Powder X-ray diffraction confirms the change in overall crystallinity with temperature. The amorphous supercooled material is heated to 473 K to exhibit solidification and then becomes a completely crystalline state.This is further confirmed by combined TGA-IR-MS, where loss of surface ligands is observed at 475 K to 573 K, followed by a gradual increase in mass with increased oxidation and sintering.

[0107] The preservation of the organic layer can be a critical factor in maintaining the stability of these core-shell metal particles. Organic ligands can act as physical barriers (limited physisorption and chemisorption) against the thin surface oxide shell. While changes in surface morphology were observed after repeated heating cycles, the control sample (maintained in ambient conditions) remained spherical and smooth. Surface defects can serve as precursors for further oxide growth, which reduces the interfacial chemical potential gradient (Δμ tension) per unit volume and ultimately ΔT / Tm. The slow oxygen diffusion process at low temperatures is the reason for the trend in freezing point changes. The asymptote at ΔT / Tm (Fig. 9d) occurs when the oxide reaches a critical thickness, resulting in significantly slower oxygen diffusion. From these studies, the surface of the particles can play a role in supercooling. While maintaining a smooth, ligand-stabilized surface may be a critical factor for stable supercooling, other factors may also play an important role. FIG. 10 shows a chart of the change in ΔT versus the change in yield for various alloys, but this chart is intended for exemplary purposes only, and any of the described alloys or other alloys may be suitable for manufacturing core-shell metal particles.

[0108] Interface-driven supercooling of various alloys

[0109] From Equation 1, interface-driven changes in chemical potential can alter the kinetics of the LS phase transition. To explain the role of the surface beyond the observed stabilization, the role of the chemical properties of the dominant surface component was investigated. First, all (a total of 4) moietyes with different properties—organic ligands, acetic acid analogs, and other moiety having better binding to passivation oxides—were investigated (Fig. 11). To understand the influence of surface oxides, various lower and higher standard reduction potentials (E 0 ) components were introduced into the BiSn-based alloy.

[0110] Influence of oxide shell and interfacial properties

[0111] FIG. 12a illustrates the correlation between changes in composition and supercooling, and FIG. 12b illustrates the correlation between changes in enthalpy and supercooling. More detailed information for each alloy is shown in Table (1700) of FIG. 17. These data are from "Stabilization of Undercooled Metals via Passivating Oxide Layers" Angew. Chem. Int. Ed. This is described in more detail in the published literature titled “2021, 60, 5928-5935” and in the supporting related information titled “Stabilization of Undercooled Metals via Passivating Oxide Layers” published by Andrew Martin et al. in Angewandte Chemie, which are incorporated herein by reference for all purposes. FIG. 13 shows a table illustrating the change in supercooling degree and yield of BiSn-based alloys having different additives that form a eutectic phase or simply add a small amount (imp.).

[0112] The field metal (eutectic BiInSn alloy) has a surface predominantly indium oxide with significant supercooling (lowest E 0It forms ) (ΔT / Tm = 0.34). To evaluate the role of the oxide shell for supercooling, particles were prepared from eutectic BiSn (yield = 89%, ΔT / Tm = 0.36, Figs. 12a and 13). Binary BiSn was given a slightly lower yield of supercooled particles, but a slightly higher ΔT / Tm compared to ternary BiInSn. The effect of additives on BiSn-based alloys was evaluated (by forming eutectic alloys of 3, 4, or 5 components or by adding small "impurities"). First, other non-indium ternary alloys were evaluated to confirm that the change in yield was not a result of a change in compositional entropy (based solely on the number of components making up the alloy). When In (in the field metal) was substituted with Pb (E 0 = -0.13V, Rose metal, BiSnPb), quantitative yield (100%) was obtained, but the degree of supercooling decreased slightly (T / Tm = 0.30, Fig. 12b). Compositional entropy BiSnPbCd(Wood metal, Cd E 0 Increasing to (= -0.4V) led to a decrease in both yield (95%) and supercooling (T / Tm = 0.21, Fig. 13). In(E for manufacturing BiSnPbCdIn alloy 0 The reintroduction of (V = -0.34V) led to the recovery of quantitative yield (100%) but with associated losses due to supercooling (T / Tm = 0.13). The compilation of these data suggests that an increase in compositional entropy does not always correlate with an increase in metastability (Fig. 13). While the formed alloys typically yield higher quantities, ΔT / Tm remains constant or decreases with increasing composition. The majority of the added components forming these alloys are Sn (E 0 Lower E compared to = -0.14V)(dominant component of oxide shell in BiSn particles) 0 It has. Therefore, lower E 0The addition of components has the potential to reconfigure the surface architecture and alter supercooling behavior. To better understand this, gallium(E 0 A homologue (BiSnGa) containing (V = -0.51V) was quantitatively supercooled (yield = 73.4%, ΔT / Tm = 0.29, Fig. 13). The addition of Ga altered both surface and bulk behavior due to strong Ga-Sn interactions, causing a significant shift in thermal behavior (identified by the large broadening of the solidification and melting point peaks in the thermogram).

[0113] (E for oxide shell 0 To determine the role of the dominant component (from the perspective) and the resulting chemical potential gradient, a small amount (e.g., ≤ 1%) of "impurities" Sn(E 0 E higher or lower than = -0.14V) 0 It was introduced into the eutectic BiSn. Figure 13 summarizes the results based on DSC traces from each sample of the supercooled alloy. Generally, higher E 0 The addition of impurities does not alter the dominant composition in the oxide shell, even with variations in surface suboxide and bulk cohesive energy densities. The latter, in better cases, will lead to a decrease in metastability due to a high propensity to form critical nucleating agents. Ge(E 0 The addition of = 0.1V, 1%) led to a similar ΔT / Tm with a slightly lower yield. An increase in the amount of Ge (approx. 15%) led to a decrease in ΔT / Tm. Lower E 0 In the case of impurities, additives in the oxide shell are possibly predominant. A small amount of Sb(E 0 = -0.51 V, 1%) and Te(E 0The addition of = -0.90 V, 1%) led to ΔT / Tm = 0.31 (63% yield) and ΔT / Tm = 0.35 (81% yield), respectively. Both additives cause a decrease in supercooling and yield, which is expected in such systems because both cause surface modification.

[0114] In addition to changes in surface oxides and cohesive energy density, the formation of nucleating agents can depend on the flux. In viscous media or solids, the flux can be proportional to the atomic radius. Ho(E 0 = -2.33 V) and Au(E 0 The addition of larger atoms, such as (= 1.83 V), led to relatively significant changes in both ΔT / Tm (0.26 and 0.33) and yield (82.3% and 65.11%, respectively). From these data, ΔT / Tm decreases with changes in oxide shell structure or cohesive energy density. Specific manufacturing methods can lead to statistical enrichment of minor components in some particles, altering yield and supercooling. The dark-distance plot and TC-YM plot show that solid solubility can be limited when the Pauling electronegativity difference is >0.4, even when atomic sizes are comparable.

[0115] Understanding Compositional Entropy

[0116] To evaluate the causes of differences in yield and supercooling, the correlation between the number of components and ΔT and ΔT / Tm was assessed for all fabricated BiSn homologues. While an increase in the number of components forming the eutectic alloy generally led to a decrease in Tm and ΔT / Tm, it was observed that other additives could have different effects. Adding a relatively large amount of impurities (see Ge in Fig. 13) leads to a similar decrease. Consequently, an increase in the number and amount of components within the alloy can interfere with the ability to hinder solidification, which may be due to the formation of intermetallic compounds upon cooling. An increase in favorable interactions can correlate with the enthalpy of the system. Thus, while compositional entropy increases, an increase in cohesive energy density weakens the entropy advantage, thereby hindering solidification. For example, the addition of Ga confirms this behavior. Although Ga is theoretically dominant in oxides, its strong interaction with Sn aids in bulk relaxation. To verify this reasoning, the correlation between ΔHf and the transition points was evaluated. All three parameters (Tm, ΔT, and ΔT / Tm) decrease as the fusion enthalpy increases, although at different rates. Therefore, alloys with stronger bulk interactions may be difficult to supercool.

[0117] Fig. 14a shows the DSC traces of supercooled SAC305 particles. Fig. 14b shows the theoretical amount of surface work versus particle radius. When correlated with solubility parameters, similar to the dark-distance plot, appropriate size and E 0Differences in impurity elements tend to impart the highest ΔT / Tm. To test this hypothesis, a 3-component alloy (SAC305 solder) was supercooled, resulting in ΔT / Tm = 0.3 (95.2% yield, Fig. 14b). SAC305 has a significantly higher ΔHf compared to any BiSn-based alloy and therefore requires a greater amount of surface work to achieve a comparable degree of supercooling.

[0118] Predictive analysis of supercooling behavior and simplification

[0119] In some embodiments, based on accumulated data for various alloys, maximum supercooling may be achievable within a 2 to 3-component alloy with an appropriate ΔHf. The correlation between ΔHf from each additive and the average minimum supercooling indicates a critical point in an alloy of approximately 3 components. A full surface map is generated when the number of components in the alloy and ΔHf are compared with ΔT / Tm. From the trends of these plots, in some embodiments, the maximum is found in a window of a 2 to 3-component alloy with an appropriate ΔHf. This implies that the enthalpy-entropy balance may be important in achieving high levels of supercooling by tuning the surface work. Based on Equation 1, when the enthalpy contribution and the entropy contribution in the system are balanced, the two terms are eliminated, leading to a simplified description of the associated free energy as follows:

[0120] (3)

[0121] Equation 3 is ΔHf When ΔG is 0, it indicates that ΔG depends on surface work. Therefore, the surface composition of the liquid metal core-shell particles exhibits an LS phase transition. This simplified equation also suggests that alloys with high ΔHf, such as SAC305, may require more work to reach the same supercooling level as BiSn. The theoretical amount of surface work can be enhanced based on curvature (increase in Laplace pressure jump), and thus, as expected, the degree of supercooling should increase with decreasing particle diameter. Since the particle size distribution is tunable using SLICE, the supercooling behavior of the synthesized particles can be predicted.

[0122] This demonstrates a new method of hindering the LS phase transition by tuning the interfacial surface tension of metal core-shell particles.

[0123] i. Supercooled particles can be stabilized using an organic core-shell architecture. Organic ligands improve stability by limiting chemisorption and physisorption on smooth passivation oxides. Under ambient conditions, most particles remain supercooled for storage of >2 years.

[0124] ii. The appropriate selection of alloying components enhances the enthalpy-composition entropy balance, which can be important for maximizing surface work and consequent supercooling. A balance of barrier and miscibility between the surface and the bulk enables tunable supercooling and stability.

[0125] iv. Solubility is redefined by introducing a preferential interaction parameter (PIP) that identifies the adjusted miscibility due to surface reactions. In addition to predicting solubility, this parameter predicts surface speciation and the associated differences in interfacial tension. Consequently, good liquid miscibility with the accompanying separation of less miscible components to the surface predicts improved supercooling (meta-stability).

[0126] Experimental method

[0127] Bismuth-tin alloying: A tin ingot was melted and then transferred to a crucible. The mass of tin in the crucible was recorded as 42% of the total alloy weight, and 58 wt% of bismuth shots were calculated based on the mass of tin required to achieve a eutectic composition. Other metals of different compositional percentages were deposited into the molten tin pool and mixed until completely dissolved. Then, bismuth shots were added to the mixture and mixed until completely dissolved.

[0128] Synthesis of BiSn-based Alloy Particles: Various supercooled metal particles having a bismuth-tin base alloy were synthesized according to the following method. 1 gram of trichloroacetic acid was mixed with 200 ml of diethylene glycol in a beaker. Approximately 5 g of metal pellets were added to the solution and heated to 433 K while stirring on a hot plate. A high-speed rotary blade was used for the shearing process, variable heating tape was wrapped around the perimeter of the device, and the device was sealed using an aramid blanket to limit heat dissipation during the shearing process. The diethylene glycol solution was transferred to the device and then sheared at approximately 27,000 rpm for 4 minutes with the shearing blade raised to one side to create an angle of approximately 10°. Once completed, the solution was extracted and washed with ethanol and ethyl acetate while cooling under ambient conditions. The solution was filtered using a Buchner filter in conjunction with a Whatman GF / F paper filter. The filtered particles were washed, harvested, and stored in ethyl acetate.

[0129] Synthesis of SAC 305 particles: 0.5 ml paraffin oil, 0.2 g trichloroacetic acid, 0.2 g poly(acrylic acid), and 0.5 g SAC 305 alloy were placed in a beaker of approximately 5 ml and heated in an oil bath. The solution was heated to 533 K and maintained at that temperature to keep the metal in a liquid form. The particles were sheared using a rotary blade at approximately 27,000 rpm for 6 minutes. Subsequently, the particles were extracted, quenched in an ethanol bath, and then cooled to room temperature. Once at room temperature, the particles were decanted and washed with ethyl acetate to remove trace oil inclusions.

[0130] Differential Scanning Calorimetry (DSC) Analysis: To measure the level of supercooling, a DSC (Model Q2000, TA Instruments) equipped with a liquid nitrogen cooling unit was employed. Particles stored in an ethyl acetate solution were transferred to an aluminum pan, and after evaporating the ethyl acetate content under ambient conditions, the sample was sealed with an aluminum lid. The sample was heated from an ambient temperature of 313 K to 573 K (varying depending on the alloy) at a heating rate of 10 K / min, and then cooled to 203 K at the same rate. Several alloys were circulated through the heating and cooling cycles to demonstrate reflow and recirculation behavior. Data analysis was performed using TA TRIOS software.

[0131] Scanning Electron Microscope (SEM) Characterization: Metal particles stored in an ethyl acetate solution were transferred onto a silicon wafer using a pipette and then characterized using a scanning electron microscope (FEI Quanta 250 FEI-SEM). The samples were mounted on a standard SEM mount (Ted Pella Inc.) secured with copper tape. The SEM was operated under high vacuum at a voltage of 10 to 15 kV with a spot size of 3 at a working distance of 10 mm. Microscopic images were taken at various magnifications using an Everhart-Thorley secondary electron detector and a backscatter detector.

[0132] High-Angle-Field Scanning Transmission Electron Microscope (HAADF-STEM) Characterization: Synthesized metal particles were drop-cast onto a copper TEM grid (Ted Pella Inc.) and mounted on a double-tilt TEM sample holder. Images were acquired using an aberration-corrected FEI Titan Themis 300 probe-calibrated TEM operating at 200 kV. EDS analysis was performed on the same instrument using a Super-X EDX detector. Oxide shell thickness approximation was performed using the "Find edges" function in the J. Sobel filter, taking into account pixel contrast intensity calculations along drawn lines where higher intensity corresponds to brighter colors (white) and a drop in intensity corresponds to the oxide layer (black).

[0133] Thermogravimetric Analysis (TGA)-Infrared (IR)-Mass Spectroscopy (MS) Analysis: A combined TGA-IR-MS instrument (Netzsch STA449F1) was used to analyze the emitted gases and mass changes during the heat treatment of the particles. Samples were deposited and dried in an alumina crucible alongside a matching reference crucible. Simulated dry air (80% oxygen, 20% nitrogen) was used as the purge gas. Subsequently, samples were loaded and passed through a heating ramp stage at 10°C / min. The acquired data were analyzed using Proteus and Opus software.

[0134] Free Energy and Symmetry

[0135] Gibbs free energy (G) is a thermodynamic potential that can be used as a means to determine whether an event is favorable, considering constant temperature and pressure. If the change in G (ΔG) from one state to another causes a lower energy state (i.e., ΔG < 0), the thermodynamic system will undergo a favorable transition. The principal components of the Gibbs free energy equation involve the balance between enthalpy (H) and entropy (S), which is expressed as follows: G = H - TS Enthalpy is a state function that accommodates internal energy and PV work in a system ( H = U + PV ) but, here U is U = TdS ― PdV + δ It is the internal energy of the system that can be further extended, such as w' (δw' is non-pv or non-mechanical work in the system). If the G term is extended into a more general derivative form, it eventually becomes the familiar Δ G = Δ H ― T Δ S + δIt becomes w'. In some calculations, δw' is ignored by assuming that no non-pv work is performed in the system. Subsequently, the equation will depend on whether the process will be enthalpy-dominated (increasing order) or entropy-dominated (increasing symmetry), which can be respectively represented by solidification (low temperature, atomic ordering, exothermic, enthalpy-dominated) or melting (high temperature, atomic disordering, endothermic, entropy-dominated).

[0136] Landau's theory of phase transitions expresses thermodynamic potential as a function of an order parameter (θ), providing a completely new perspective on phase transition events. In Landau's theory, an ordering event will cause θ to approach 0, where nearest-neighbor interactions force order in one direction and thus U is dominant (this is the same principle as an enthalpy-dominated event or solidification, but in this case, ordering is addressing self-order, M). At the opposite end, disorder or high entropy will cause θ to approach 1. Extending Landau, Ising's theory [includes] the coordination number (q) and the Boltzmann constant (K B It provides better quantification parameters for the equilibrium between symmetric orders in the form of ) and the J-commutation constant (a parameter that directly deals with nearest neighbor interactions). In Ising theory, order-symmetric equilibrium = 1 or Jq = K B T It reaches a critical point. This can be thought of as a balance between enthalpy and entropy, once transferred back into Gibbs free energy.

[0137] Correlation between thermodynamic potentials, as hypothesized by Gibbs, Landor, and Ising, provides a means to control phase transitions, specifically liquid-solid phase transitions that are important for supercooling.

[0138] Nucleation

[0139] Supercooling is a process that occurs within the liquid-solid transition window, also known as solidification or nucleation. Solidification is an event initiated by nucleation resulting from the energy minimization of the system, wherein the free energy difference between the liquid and solid states is negative (ΔG ls It is advantageous in the case < 0). Although different free energy terms can be used to describe this phenomenon, the Gibbs free energy (ΔG) assuming constant temperature and pressure can be used to describe this event. There exist numerous nucleation initiation mechanisms that can be classified into two main categories: homogeneous and heterogeneous. Homogeneous nucleation can be described by the following:

[0140] (4)

[0141] Here, ΔG corresponds to the free energy of the particle / droplet. ΔG is governed by volume and interface terms based on radius (r), volume free energy (Δg), and interface energy (γ). Due to the ratio of these volume and interface terms, there are regions where the interface term is dominant (low r) and regions where the volume term is dominant (high r). At a given radius, the transition point between these two regions is the nuclear critical size (r * It appears as a maximum value indicated by ). The activation energy to reach this critical size is Δ G * = 16 πr 3 / 3Δ g 2 It is defined by, and this can be used to define the initiation of the solidification process. Therefore, achieving high activation energy to reach high supercooling can be utilized.

[0142] The definition of homogeneous nucleation is primarily governed by intrinsic processes and characteristics. On the other hand, heterogeneous nucleation is an exogenous process that depends on nucleation seeds existing within the boundaries of the system. The formation of heterogeneous nucleating agents can be described using the following interfacial tension equilibrium equation:

[0143] (5)

[0144] Here, γ ls , γ sv and γ lv are the liquid-solid, solid-vapor, and liquid-vapor interfacial tensions, respectively, and θ θ is the wetting angle. Considering this relationship, the volume of the heterogeneous nucleating agent is r * The volume reached is much smaller than that of uniform nucleation and depends on the wetting angle. The ratio of volume reduction can be described by the following expression: f ( θ ) = 1 / 4(2 ― 3 cosθ + cos 3 θ ). Here, now f ( θ ) is the ratio between the critical heterogeneous nucleation energy and the homogeneous nucleation energy There are multiple heterogeneous nucleation modes, and from this relationship, it can be seen that the critical energy for heterogeneous nucleation is lower than that for homogeneous nucleation. For this reason, heterogeneous nucleation is eliminated in various supercooling studies by using a containerless approach, thereby eliminating any surface contacts. Another approach to this is achieved by achieving complete non-wetting. Preventing both nucleation modes by introducing a high activation energy barrier ultimately leads to high levels of supercooling in metallic systems.

[0145] Supercooling thermodynamics

[0146] The free energy of a thermodynamic system under constant temperature and pressure is, in general, Δ G = ΔH - TΔS It can be defined by the Gibbs free energy expressed as . Here, H and S are the enthalpy and entropy of the system, respectively. The free energy of the system depends on the balance of these two terms, and phase transitions can be driven by enthalpy or entropy forces. The process is considered favorable when ΔG < 0, and therefore, in the case of supercooling (liquid-solid transition), G S ―G L < 0. The free energy term additionally includes specific heat (C p It can be derived into a term of ), where and Δ S = Δ Sf - and Δ Hf = T Δ Sf is. Subsequently, this provides the following to the relationship shown as Equation 1 in the text:

[0147] (6)

[0148] Here, ΔH f is the enthalpy of fusion, and T m is the melting temperature, ΔT is the temperature difference between the melting point and the freezing point, and the term ΔT / T m It is used to determine the degree of supercooling in the system. C under different assumptions leading to an increase in ΔG p Various studies have been conducted to evaluate the increase. These changes ultimately increase the degree of supercooling. Some models introduce what is known as the maximum supercooling limit. Studies of the liquid-solid transition focus on enthalpy and entropy terms (PV-based), often neglecting the existence of interface and surface terms previously discussed in nucleation theory, as they often have a small or negligible effect on the overall system. However, surface terms can be dominant in the bulk when considering different processing conditions that force high interfacial tension toward the bulk.

[0149] FIG. 15 illustrates the particle size distribution of the synthesized particles as is, which is in the range of 0.1 to 1.5 micrometers in this embodiment, but other embodiments may have particle sizes in different ranges.

[0150] Supercooling of various BiSn-based alloys

[0151] FIG. 16 illustrates a table containing the elemental constants used in some embodiments. FIG. 17 illustrates a table (1700) plotting the supercooling level and yield before and after reflow for various alloys according to some embodiments of the present disclosure. T m From T f ΔT was calculated by subtracting . The yield was calculated by taking the ratio of the areas under the melting curve and the freezing curve.

[0152] (7)

[0153] Calculations for ΔT and yield were performed for the synthesized samples as they were and for the reflowed samples. The pure yield was recorded from the reflowed samples as the reflowing process removed any solid inclusions that emerged during the synthesis process.

[0154] Effects of organic ligands

[0155] ΔT / T when the inductive properties of the organic shell are diagonally opposite m The comparison shows that the inferred surface chemical potential gradient varies across positive or negatively induced surface moiety. This may be due to inductive effects when the electron density across the bonding moiety is perturbed, or due to the characteristics of the ligand-oxide bond. By introducing these inductive effects into the synthesis of SAC305 particles, the ΔT / T of these particles mIt was improved from 0.21 to 0.3, which can push the freezing point to < 100°C. The stabilization of these particles due to the presence of organic ligands was also confirmed, as these particles can withstand multiple thermal cycles without undergoing a large shift in supercooling.

[0156] Figure 17 illustrates tabulated data showing the effects of different ligands on the synthesis of BiSn particles. The influence of organic ligands used to synthesize core-shell particles is investigated by using various acids, thereby altering the inductive effect due to changes in the electronegative properties of the ligands. The most common experiments performed for this purpose are conducted using trichloroacetic acid, which has a high electron-attracting (i.e., high electronegativity). This behavior induces a negative inductive effect on the surface to which the ligand is attached, creating an electron tension (negative dipole moment) at the interface. When an acid with opposite electronegative behavior is used (e.g., phosphotungstic acid), a positive inductive effect is applied, and thus electron compression is generated. Both the negative and positive inductive effects can produce high supercooling (e.g., >0.34), although with varying yields.

[0157] By investigating the effects of induction, ligands with lower electronegativity are used (tribromoacetic acid and chloroacetic acid). Both acids used result in lower supercooling and lower yield. These results further suggest that the effect of surface induction from the organic shell itself plays a role in driving the supercooling behavior in core-shell particles.

[0158] Liquid metal surface oxide

[0159] This part of the disclosure describes the opportunity for passivation oxides of liquid metal particles. The complexity and order beneath the surface hinder homogeneous nucleation and provide an opportunity to enable enhanced supercooling. The plasticity of the underlying liquid metal surface provides a subsurface that corrects autonomously, and thus the lowest E° component is dominant at the surface unless it is stoichiometrically restricted. This plasticity provides an opportunity to synthesize organometallic polymers that self-assemble in situ into high aspect ratio nanomaterials. Induced surface speciation implies that oxide thickness and composition can be tuned under appropriate oxidizing agent tension, leading to temperature-dependent compositional inversion and so-called chameleon metals.

[0160] FIG. 18 is an enlarged view of a hypothetical surface oxide architecture for a field metal. In this case, the surface is assumed to be massless and volumeless (e.g., the Gibbs Dividing plane (GDP)). In other cases, the surface constitutes a continuum of bulk (varying only by the number densities of the components) occupying this region (the Gibbs-Duhem Interface (GDI)). Thermodynamically, the surface may include the horizontal lines of mass and energy dissipation boundaries of any system. Therefore, defining the surface solely in terms of mass distribution may be insufficient. Considering energy distribution, the GDP requires duality in that a transition from one phase to another constitutes a point in space where two energy values ​​are feasible and an instantaneous energy jump exists. This scenario nullifies any semblance of equilibrium and, consequently, may be improbable. On the other hand, GDI can explain material systems with low or high cohesive energy density (vapor pressure) depending on the decreasing concentration gradient. In crystalline materials, the lattice planes are clearly delimited, and accordingly, it can be argued that GDP is appropriate in this scenario. Flat crystalline metal systems (pressure jump = 0, vapor pressure Considering 0), and considering the characteristics of metallic bonding, a 'sea of ​​electrons' must occupy the surface. Given the electron duality and the uncertainty principle, limiting the surface trajectory nullifies the surface electron flux (velocity), and vice versa. Assuming an energy gradient (GDI) near the surface of a mixed material in equilibrium, autonomous speciation may have been driven by curvature in the Lowengrub-Voigt model or in the thermal oxidation compositional inversion.

[0161] In some embodiments, the following governing rules drive speciation. In the case of reactive components, the preferential formation of bonds can lead to order and organization over relatively short distances (1 to 2 nm), as in hydrocarbon self-assembled monolayers. In more stochastic systems, such as the formation of passivation oxides on metal alloys, redox-driven speciation can occur over several nanometers. Consequently, the surface of the material can be a complex part due to its size (nm) energy profile composition, structure, and reactivity.

[0162] Liquid metal particles:

[0163] Most metals oxidize rapidly in air to form a thin layer of oxide. Although a thin passivation oxide layer constitutes the particle surface, since oxides are not metals, they can be referred to as thermodynamically distinct components. In this regard, defining the surface involves finding a set of components that are not part of the object under consideration. An interfacial metal layer that is not similar but energetically dissimilar to the passivation oxide constitutes the surface.

[0164] Passivated oxides may not resemble self-assembled monolayers (SAMs) on coinage metals, where organic and metallic elements are clearly distinct entities. Unlike monolayer systems with final connection points, for example in Au-S bonds, the passivated oxide is a dynamic continuum resulting from the bulk and equilibrating system. This relationship leads to tensor properties of surface tension, in contrast to the scalar properties of SAMs. The governing rules for its establishment may depend on the environment (temperature, reactive species, pressure, etc.), the reactivity of the alloy components, cohesive energy density (how well the alloy components like each other), diffusivity (consequently atomic radius), and the thermodynamic state of the bulk. The high vapor pressures of both the metal and the oxide can rule out the possibility of concentration gradients, which in turn imposes energy duality at the metal-oxide interface, suggesting the existence of energy jumps across the plane (GDP). Consequently, as the metal oxide layer approaches from the oxide or metal side of the interface, there may be some gradient in composition or energy state. Thus, regardless of the adopted definition, the surface is a metastable region of the material where the energy state can be averaged solely from the difference in energy states across each point on the surface. In the surroundings, the diffusion layer of the speciated surface material (σ) mirrors the energy gradient between the system and its surroundings, and this gradient can be dynamic and sensitive to small fluctuations. An equilibrium state is established based on the standard reduction potential of the underlying component, its propensity for flux, and interactions with other alloying components. Fig. 19 illustrates this behavior for BiInSn (field metal). Therefore, understanding such a surface depends, among other characteristics, on the observation length scale, time, and its complexity. Passivation oxides are typically larger than most SAMs (e.g., oxides of eutectic gallium indium). While 2 nm, decanthiol SAM 1 nm).

[0165] analogy

[0166] SAM is a specific example of a thin (nanometer) layer on a metal surface that significantly alters material properties, particularly work function, frictional / wetness, conductivity, and plasmonic activity. SAMs are formed through thermodynamically driven self-assembly processes, enabling the creation of highly ordered structures. The deposited thin material layers offer significant opportunities in more fundamental areas, most notably in structure-property relationships and interfacial phenomena. A SAM system can be modeled as two interfaces surrounding a bulk material (often hydrocarbons), where each of these three components can be investigated separately by tuning the basic components, the molecules. Due to the small size of the molecules and their dependence on molecular orientation, any small change at the surface can alter the entire system. However, under well-controlled conditions, SAMs can be analyzed. Applications of SAMs can be divided into two types, some of which directly utilize the structure-property relationships of the SAM molecules. For example, the monomolecular nature of SAMs leads to their applications in molecular electronics as well as their potential as tunable hydrophobic coatings. On the other hand, the highly tunable nature of SAMs makes them a great candidate platform for building or immobilizing other components on metal surfaces.

[0167] Studies on passivation oxide layers in liquid metal particles have been significantly limited, partly due to issues with characterization techniques. These difficulties stem primarily from the compositional complexity within the underlying metal-oxide interface over very short distances. However, when properly formed and / or manipulated, these passivation oxide layers offer various benefits to the material. In non-reactive liquid droplets, the curved surfaces of micro- to nano-sized particles possess abrupt energy and compositional gradients, which are primarily characterized by interfacial excess, Γi, and Laplace pressure jump states (ΔP = 2γ / r, where γ is surface tension and r is the particle radius). By definition, these abrupt gradients serve the purpose of establishing both energy and mechanical equilibrium between the particle and its surroundings. However, in the case of metal droplets, exposure to ambient conditions leads to the rapid formation of a passivation oxide layer. In metal alloys, differences in redox potential and diffusivity result in competitive oxidation occurring at the time of exposure to air, t = 0, followed by the lowest standard reduction potential (E 0 This implies that it leads to ), where the most abundant and mostly diffusible components are dominant on the surface of the formed oxide. However, over time, kinetically resolved self-sorting and speciation often occur, causing the provision of monomolecular metal oxides on the surface of the metal alloy. This sorting / organization is non-limiting E 0 It is governed by stoichiometry, atomic size, cohesive energy density, atomic flux, oxidizing agent diffusivity, temperature, and pressure. When a certain thickness is reached, oxidation becomes infinitely slow and equilibrium is established.

[0168] In an oxidizing environment, all elements within the alloy have an equal probability of oxidation at t = 0. This is merely perturbed by their stoichiometry and surface occupancy tendencies. Kinetically, this results in the type having the most "good" element dominant on the outer surface of the oxide layer. In EGaIn, this element is gallium. Over time, as the oxidizing agent flux slowly decreases, the oxide reaches d_c^p, leading to the formation of a sub-oxide derived from less reactive components that are kinetically restricted to complete oxidation. In the case of eutectics, the selective reduction of some alloy components generates an energetically unfavorable sub-eutectic, which leads to the enrichment of unreacted components at the metal-oxide interface mirroring across the oxide shell. However, a steep compositional gradient provides an interface with a large chemical potential gradient (Δμ), resulting in a metastable surface. Δμ coupled to ΔP provides a difference in stress proportional to the grain size. This gradient influences the grain properties.

[0169] Based on the asymmetric energy distribution across surface oxides, it may be possible to induce relaxation by disrupting bulk energy dissipation and tuning surface stress. Considering that molten metals possess high symmetry (i.e., lack of order as defined in Landor's phase transition theory), the formation of passivation oxides and the underlying enrichment required to maintain equilibrium (e.g., eutectic composition) must introduce some pressure on free diffusion. The growth of nucleating agents must overcome surface tension. Even after the nucleating agent seed is formed, a competition follows between growth (decrease in bulk energy) versus contraction (increase in surface energy), at which point the process shifts toward growth as the size increases. It can be noted that the magnitude of the nucleation rate is sensitive to the value of interfacial energy, and that fluctuations of only a small percentage of σls (solid-liquid interfacial free energy) can alter the magnitude of the predicted rate by several times. The second non-dynamic solid-liquid interface, and the associated order and free energy transitions, can also be mentioned as existing beneath the passivation oxide layer, leading to two solid-liquid interface free energy fluctuations that must be overcome for successful nucleation growth—namely, the nucleating agent interface and the oxide interface.

[0170] Unlike dynamic nucleating agent seed interfaces, where the shrinkage of the nucleating agent is overcome by growth, the structure of the passivation oxide is fixed and cannot be disturbed by growth. Consequently, manipulating the surface oxide regarding the miscibility of the bulk (liquid) component can have a significant effect on the growth of the nucleating agent, particularly with small (<10 μm) particles. Thus, the passivation oxide must lead to significant hindrance of the liquid-solid phase transition and consequently enhanced supercooling. Previously, supercooling was achieved, for example, by removing heterogeneous nucleating agent(s) via a containerless approach. However, the containerless approach does not remove homogeneous nucleating agent(s)—a process attributed to structural variations in the liquid—and consequently does not utilize non-dynamic surface oxide interfacial tension.

[0171] The smaller the particle size, the higher the likelihood of supercooling. This size effect is due to the large surface area-to-volume ratio, which limits homogeneous nucleation. Considering that Gibbs free energy (ΔG) is expressed as ΔG = ΔH - TΔS + δw' under the enthalpy-entropy compensation condition on the right (where ΔH and ΔS are the changes in enthalpy and entropy, respectively, T = temperature, and δw' = non-PV work), surface work (δw') can be dominant on ΔG. Although the aforementioned method relies heavily on tilting the bulk enthalpy-entropy balance to manipulate phase transitions, δw' and surface contributions are often assumed to be negligible due to entropy constraints. However, by definition, a curved surface is metastable and is therefore a source of free energy stress capable of altering the energy domain of the entire material. Therefore, manipulating surface oxides can lead to the extension of this size dependence (surface area to volume ratio) beyond the nanoscale to the microscale.

[0172] The surface work term defines the amount of energy required to maintain the surface per unit area, δw^'=γdA. By definition, surface tension is the product of interfacial excess and the chemical potential difference. The aforementioned complex compositional gradient generates a large curvature-dependent Δμ gradient, and thus, in small particles with a large surface area, the amount of δw' can overcome the enthalpy-entropy balance and can hinder the liquid-solid phase transition (ΔGLS > 0) even when such fluctuations are small. The structural complexity of the appropriately tuned oxide can be sufficient, as evidenced by the long-term stability of these fabricated particles.

[0173] In addition to its effect on free energy, the formation of a uniformly smooth passivation oxide layer creates a physical barrier against heterogeneous nucleating agent(s) and consequently improves the stability of the supercooled state. This understanding has enabled the synthesis of stable undercooled liquid metal core-shell (ULMCS) particles, which allows for heat-free solder and the overuse of other ambient or low-temperature metal processing. Since the surface is the primary driving force for metastability, the fracture of the oxide shell leads to instantaneous flow, aggregation, and solidification. Recent developments have enabled the supercooling of commercially available lead-free solder, SAC 305, making low-temperature surface mounting and electronic packaging possible. Such low-temperature sintering has enabled the integration of conductive traces or circuits onto other temperature-sensitive substrates (i.e., organics and polymers). Understanding surface oxides presents a Braess-type paradox. Surface speciation can induce reverse organization for liquid metals.

[0174] Although the droplet (100) (see FIG. 1) is described and exemplified with one specific composition and configuration, embodiments of the present disclosure are suitable for use with a number of compositions and configurations. Any elements and combinations thereof may be used in the core and shell. The shell may have any number of layers that are inorganic and / or organic.

[0175] In the foregoing specification, embodiments of the present disclosure have been described with reference to a number of specific details that may vary from embodiment to embodiment. Accordingly, the specification and drawings are to be regarded as illustrative rather than restrictive. The sole and exclusive indicator of the scope of the present disclosure, and what is intended by the applicant to be the scope of the present disclosure, is the literal and equivalent scope of the set of claims presented in this application, in the specific form presented by the claims, including any subsequent amendments. Specific details of specific embodiments may be combined in any suitable manner without departing from the spirit and scope of the embodiments of the present disclosure.

[0176] Furthermore, spatially relative terms such as “bottom” or “top” may be used to describe the relationship of an element and / or feature(s) to another element(s) and / or feature(s), for example, as illustrated in the drawings. It will be understood that spatially relative terms are intended to include different orientations of the device in use and / or operation in addition to the orientations depicted in the drawings. For example, if the device in the drawings is inverted, the element described as the “bottom” surface may be oriented “up” to another element or feature. The device may be oriented otherwise (e.g., rotated 90 degrees or in a different orientation), and the spatially relative terms used herein may be interpreted accordingly.

[0177] The terms “and,” “or,” and “and / or” as used herein may include various meanings that are also expected to depend at least partially on the context in which these terms are used. Typically, “or” when used in association with a list such as A, B, or C is intended to mean A, B, and C as used herein in an inclusive sense, as well as A, B, or C as used herein in an exclusive sense. Furthermore, the term “one or more” as used herein may be used to describe any feature, structure, or characteristic in the singular, or to describe some combination of a feature, structure, or characteristic. However, it should be noted that this is merely an exemplary example and the claimed subject matter is not limited to such examples. Additionally, the term “at least one” when used in association with a list such as A, B, or C may be interpreted to mean any combination of A, B, and / or C, such as A, B, C, AB, AC, BC, AA, AAB, ABC, AABBCCC, etc.

[0178] Throughout this specification, references to “an example,” “one example,” “a specific example,” or “an exemplary embodiment” mean that a specific feature, structure, or characteristic described in relation to a feature and / or example may be included in at least one feature and / or example of the claimed subject matter. Accordingly, the appearance of phrases “in an example,” “an example,” “in a specific example,” “in a specific embodiment,” or other similar phrases in various places throughout this specification does not necessarily refer to the same feature, example, and / or limitation. Additionally, a specific feature, structure, or characteristic may be combined in one or more examples and / or features.

[0179] In some embodiments, operation or processing may involve the physical manipulation of physical quantities. Typically, but not necessarily, such quantities may take the form of electrical or magnetic signals that can be stored, transmitted, combined, compared, or otherwise manipulated. For reasons of common use, it has often proven convenient to refer to such signals as bits, data, values, elements, symbols, characters, terms, numbers, numerals, etc. However, it should be understood that all of these or similar terms are to be associated with appropriate physical quantities and are merely convenient labels. Unless otherwise specifically stated, discussions throughout this specification using terms such as “processing,” “computing,” “calculation,” “decision,” etc., are understood to refer to the operation or process of a specific device, such as a special-purpose computer, a special-purpose computing device, or a similar special-purpose electronic computing device. Accordingly, in the context of this specification, a special-purpose computer or a similar special-purpose electronic computing device may manipulate or convert signals typically represented as physical electronic or magnetic quantities within the memory, registers, or other information storage devices, transmission devices, or display devices of the special-purpose computer or similar special-purpose electronic computing device.

[0180] In the preceding detailed description, many specific details have been presented to provide a complete understanding of the claimed subject matter. However, those skilled in the art will understand that the claimed subject matter may be practiced without these specific details. In other cases, methods and apparatus known to those skilled in the art have not been described in detail so as not to obscure the claimed subject matter. Accordingly, the claimed subject matter is not limited to the specific examples disclosed, but is intended to include all aspects and equivalents that fall within the scope of the appended claims.

Claims

Claim 1 A soldering material comprising: a core comprising an alloy comprising a majority of a first metal element and a minority of a second element, wherein the core is in a liquid state below the solidus temperature of the alloy; and a shell arranged to surround the core and comprising an outer surface comprising a majority of the second element and a minority of the first metal element, wherein the shell is in a solid state below the solidus temperature of the alloy. Claim 2 A soldering material according to claim 1, wherein the second element is a metal. Claim 3 A soldering material according to claim 1, wherein the second element is a metalloid. Claim 4 A soldering material according to claim 1, wherein the shell comprises an innermost layer having a dominant concentration of the first metal element and an outermost layer having a dominant concentration of the second element. Claim 5 In paragraph 4, the innermost layer is higher than the outermost layer E o Soldering material having Claim 6 In paragraph 4, the innermost layer has a lower E than the outermost layer. o Soldering material having Claim 7 A soldering material according to claim 4, wherein the innermost layer and the outermost layer comprise an oxide of the first metal element and an oxide of the second element. Claim 8 A soldering material according to claim 1, further comprising a ligand coating on the outer surface. Claim 9 A method for forming a soldering material, comprising: forming a liquid core of the soldering material from an alloy comprising a first element, a second element, and a third element; forming a solid shell around the liquid core, wherein the solid shell comprises an innermost layer having one of the first element, the second element, or the third element at a dominant concentration and an outermost layer having a different one of the first element, the second element, or the third element at a dominant concentration; and cooling the liquid core and the solid shell to a temperature below the solidus temperature of the alloy while maintaining the core in a liquid state. Claim 10 A method according to claim 9, wherein the solid shell comprises three layers, the innermost layer having the first element at a dominant concentration, the middle layer having the second element at a dominant concentration, and the outermost layer having the third element at a dominant concentration. Claim 11 In claim 10, the method wherein the shell is formed in an oxidizing environment. Claim 12 In claim 11, the oxidation environment is controlled by changing the partial pressure of oxygen in the oxidation environment. Claim 13 In Clause 11, the innermost layer is higher than the intermediate layer E o Having, the above intermediate layer is higher E than the above outermost layer o A method having. Claim 14 In claim 11, the method wherein the thickness of one or more of the three layers of the solid shell is determined by the exposure time to the oxidizing environment. Claim 15 In paragraph 10, the method wherein the shell is formed in a reducing environment. Claim 16 In paragraph 15, the innermost layer has a lower E than the intermediate layer. o Having, the above intermediate layer has an E lower than the above outermost layer o A method having. Claim 17 In claim 15, the method wherein the thickness of one or more of the three layers of the solid shell is determined by the exposure time to the reducing environment. Claim 18 A method according to claim 10, further comprising the step of exposing the solid shell to one or more chelating agents to remove at least a portion of the outermost layer. Claim 19 In claim 18, the method comprises one or more chelating agents comprising at least one of a carboxylate, an amide, an alkoxide, an amine, a thiol, or a phosphate. Claim 20 A method according to claim 10, further comprising an etching process for polishing an inner layer of the solid shell.

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