Small plate-shaped nanoparticles, their compositions, and their formation

Abstract

The metal nanoparticles are platelet nanoparticles and can be grown under conditions that promote the formation of platelet nanoparticles and that have a surfactant coating thereon. Such conditions can include slow metal salt reduction and slow cooling after the formation of the metal nanoparticles. The platelet nanoparticles have a melting temperature significantly below the melting point of the corresponding bulk metal and form a robust structure when consolidated with each other. The composition comprises a plurality of metal nanoparticles that have a surfactant coating thereon, at least about 20% of the metal nanoparticles being platelet nanoparticles, and the surfactant coating comprising at least one surfactant. The composition can further comprise various amounts of substantially spherical metal nanoparticles. The metal nanoparticles can be incorporated into nanoparticle paste compositions, sprayable formulations, and inks that can assist in the dispensing and consolidation of the metal nanoparticles.

Description

Technical Field

[0001] Lead has traditionally been used in many industrial applications, but current regulations mandate the elimination and / or phased discontinuation of lead in most commercial products. In particular, soldering applications in the electronics and vehicle manufacturing industries have been significantly affected by the lead ban. A number of alternatives to traditional lead-based solders have been developed, and the Sn / Ag / Cu (Sn / Ag / Cu, SAC) system is among the most widely used, but many exhibit drawbacks that can render them unsuitable for use in certain applications, such as excessive cost (each weight percent of silver added approximately doubles the cost, and tin is similarly expensive), a high eutectic melting point, and potential whisker growth in tin-based solders containing a high proportion of tin.

[0002] Compositions containing metal nanoparticles have begun to be used as alternatives to traditional soldering materials. Such compositions are increasingly being referred to as sintered metal systems. Metal nanoparticles with a size of about 100 nm or less, particularly those with a size of about 20 nm or less, can exhibit a significant melting point depression that is superior to the melting point depression of the corresponding bulk metal, thereby pseudo-liquefying the metal nanoparticles and enabling densification at temperatures comparable to traditional soldering materials. For example, spherical copper nanoparticles have been widely studied as alternative soldering materials due to the benefits of this metal's high thermal and electrical conductivity, as well as the relatively low cost of copper. When densification of the metal occurs above the melting temperature, the melting point of the resulting metal matrix returns to a value close to the melting point of the corresponding bulk metal, thereby enabling suitable operating conditions for the densified metal nanoparticles to be based on the melting point of the bulk metal instead of the much lower melting temperature of the metal nanoparticles.

[0003] Typically, many processes have been developed to produce substantially spherical metal nanoparticles within a target size range having a narrow particle size distribution by utilizing surfactants to control the nucleation and growth rate of the metal nanoparticles. Compositions containing substantially spherical metal nanoparticles can be suitable for many applications, but in particular, when the metal matrix needs to support mechanical loads, for example, or is exposed to mechanical stresses, considerable care may be required to produce a robust metal matrix during densification. Without being bound by theory or mechanism, the void volume in a close-packed or nearly close-packed metal sphere can lead to excessive porosity when producing a bulk metal matrix during densification of substantially spherical metal nanoparticles. The void volume can further arise as the metal matrix gradually hardens as densification of the metal nanoparticles occurs to form the bulk metal state. Thus, the looser the initial packing state of the metal nanoparticles, the higher the porosity of the resulting bulk metal state. Using substantially spherical metal nanoparticles having a bimodal particle size distribution can address this difficulty to some extent by facilitating a higher packing density before the metal nanoparticles are subjected to densification.

[0004] In addition to soldering applications, the use of metal nanoparticles has been proposed in several other fields including, but not limited to, communications, electronic devices, and medical uses. Silver nanoparticles and gold nanoparticles are widely used for these purposes. Effective densification of substantially spherical metal nanoparticles similarly remains a challenge in these and many other fields. For example, silver nanoparticles may require heating for up to 1 hour to facilitate effective densification and pressure may need to be applied to achieve acceptable density, electrical conductivity, and thermal conductivity in the resulting bulk metal. For high-temperature applications in an electric field, silver migration can be a problem. In addition, the high material cost of noble metal systems similarly remains a challenge. For example, gold nanoparticles are extremely expensive for most applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The following figures are included to illustrate certain aspects of the present disclosure and should not be regarded as exclusive embodiments. The disclosed subject matter is capable of substantial modification, alteration, combination, and equivalents in form and function, as would occur to one of ordinary skill in the art and to those who benefit from the present disclosure.

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DETAILED DESCRIPTION OF THE INVENTION

[0006] The present disclosure generally relates to metal nanoparticles, more specifically, metal nanoparticle compositions containing platelet nanoparticles, and their formation and use.

[0007] As discussed above, it is possible to consolidate substantially spherical metal nanoparticles, such as copper nanoparticles, to form a bulk metal matrix. However, when consolidating substantially spherical copper nanoparticles, in some cases, considerable care may be required, including applying pressure to achieve a suitable density, to form a robust bulk metal matrix.

[0008] The present disclosure provides a composition comprising metal nanoparticles in which at least a portion of the metal nanoparticles are platelet nanoparticles (i.e., metal nanoparticles having a shape that is not substantially spherical but rather a platelet morphology). At least about 20%, and often more, of the metal nanoparticles in the composition can comprise the platelet nanoparticles described herein. Surprisingly, some simultaneous modifications of the synthetic conditions used to produce substantially spherical metal nanoparticles can, instead, result in a composition containing a significant amount of platelet nanoparticles, as further discussed herein. Substantially spherical metal nanoparticles can result from the rapid introduction of a reducing agent into a solution containing a metal salt and one or more surfactants (e.g., an addition time of 1 - 2 minutes), but adding the reducing agent to the solution more slowly and maintaining the reaction medium at a controlled temperature, sometimes without applying additional heat thereto, can result in at least partial formation of platelet nanoparticles. Slow cooling of the reaction medium can further assist in promoting the formation of platelet nanoparticles. The platelet nanoparticles may, in various instances, be formed in combination with substantially spherical metal nanoparticles, and the platelet nanoparticles may be present in an amount greater than or less than that of the substantially spherical metal nanoparticles, depending on the synthetic conditions. The ratio of substantially spherical metal nanoparticles to platelet nanoparticles can be further adjusted through the modulation of additional components present in the reaction medium in which the metal nanoparticle formation is carried out. For example, the type and amount of surfactant in the organic solvent in which the metal nanoparticle formation is carried out can further affect the type of metal nanoparticles formed.

[0009] There is a natural limit to how closely spheres can be packed in three-dimensional space. For single-sized spheres, theory predicts that an ideal cubic close-packed or hexagonal close-packed structure is filled with about 74% efficiency by volume (26% void volume). When packing metal nanoparticles with two different sizes where the diameter ratio of the larger metal nanoparticles to the smaller ones is about 10:1, the packing density can increase up to about 87% (13% void volume). Random packing arrays result in a considerably large void volume (about 36%), which is often the arrangement seen when depositing metal nanoparticles on a surface. As a result of these characteristics, it can be a problem to form a robust bulk metal matrix from substantially spherical metal nanoparticles.

[0010] Compositions containing platelet nanoparticles, preferably compositions containing platelet nanoparticles as at least the majority of the metal nanoparticles in the composition, can result in a much more robust bulk metal matrix when subjected to metal nanoparticle consolidation. It is possible to achieve a packing efficiency higher than 90% (void volume less than 10%) and produce a metal matrix of a much higher density (e.g., within thin films, solder joints, injection molded parts, etc.) than is possible when consolidating substantially spherical metal nanoparticles alone. The packing efficiency may further increase during aging, thermal shock, and / or thermal cycling. Without being bound by theory or mechanism, platelet nanoparticles are thought to result in a much higher density packing prior to the consolidation of the metal nanoparticles, and the higher density packing is facilitated by the stacking of platelet layers, which in turn leads to a lower porosity (higher density) bulk metal matrix after consolidation. Thus, a higher degree of long-term integrity can be achieved in the bulk metal matrix obtained from metal nanoparticle consolidation. These benefits can be realized even when consolidation occurs with little or no external pressure applied, in contrast to the behavior of substantially spherical metal nanoparticles. The electrical conductivity values achieved upon consolidation of metal nanoparticles containing a significant amount of platelet nanoparticles can approach the electrical conductivity values of bulk metal structures produced by techniques such as casting or plating. At least, the electrical conductivity achieved upon consolidation of platelet nanoparticles can exceed the electrical conductivity of bulk metal structures produced upon consolidation of substantially spherical metal nanoparticles alone.

[0011] In addition to the benefits provided by the platelet nanoparticles themselves, compositions containing a significant amount of platelet nanoparticles can further facilitate the easy consolidation of the metal nanoparticles as a high-density bulk metal matrix, and can be formulated as nanoparticle pastes that can easily distribute the metal nanoparticles. Sprayable formulations and inks containing platelet nanoparticles can also be prepared and can provide similar benefits. Additionally, since solvents and other volatiles are removed from the composition containing the platelet nanoparticles, as the platelet nanoparticles undergo consolidation, the platelet nanoparticles are further drawn together, thereby further increasing the packing efficiency.

[0012] When the metal nanoparticles are processed into a bulk metal matrix, the bulk metal matrix can remain stable up to a temperature close to the melting point of the corresponding bulk metal. Thus, the metal nanoparticles, and nanoparticle pastes containing the metal nanoparticles, enable initial processing to be carried out at relatively low temperatures (about 180 - 240 °C or less, depending on the metal, the size of the metal nanoparticles, and the ratio of platelet nanoparticles to substantially spherical metal nanoparticles), and then can facilitate use at much higher operating temperatures. The low initial processing conditions advantageously match the range of substrate materials and processing conditions used to form integrated circuits and other electronic materials, which can constitute one non-limiting type of use for the metal nanoparticles described herein. Such processing conditions can be similar to those used in traditional soldering applications.

[0013] Copper can be a desirable metal for forming metal nanoparticles such as metal nanoparticles containing a significant amount of platelet nanoparticles as described herein, due to the low cost of this metal, as well as the high values of electrical and thermal conductivity. Additional disclosure regarding copper nanoparticles and their synthesis is provided below.

[0014] Before delving further into the embodiments of the present disclosure, a brief description of metal nanoparticles and metal nanoparticle pastes is first provided, as copper nanoparticles are representative examples of such metal nanoparticles, enabling a better understanding of the remaining disclosure. Metal nanoparticles exhibit several properties that can be significantly different from those of the corresponding bulk metal. One property of metal nanoparticles that can be particularly important is nanoparticle fusion or densification that occurs at the melting temperature of the metal nanoparticles. As used herein, the term "melting temperature" refers to the temperature at which the metal nanoparticles appear to be liquefied, thereby giving the appearance of melting. As used herein, the terms "fusion" and "densification" are used synonymously to refer to the coalescence or partial coalescence of metal nanoparticles above the melting temperature to form a larger mass (sintered mass) such as a bulk metal matrix like a bulk copper matrix. The bulk metal matrix can take various forms such as a thin film, an electrical or thermal connection between two surfaces, an interconnect, a solder joint, or a larger bulk metal block. The form of the bulk metal matrix can be influenced by the amount of platelet nanoparticles present in combination with substantially spherical metal nanoparticles, as described in more detail herein.

[0015] In the case of substantially spherical metal nanoparticles, as the size decreases, especially when the equivalent spherical diameter is less than about 20 nm, the temperature at which the metal nanoparticles appear to liquefy drops dramatically from the liquefaction temperature of the corresponding bulk metal. For example, substantially spherical copper nanoparticles within a suitable size range can have a melting temperature of about 240 °C or lower, or about 220 °C or lower, or about 200 °C or lower, as compared to the melting point of bulk copper of 1084 °C. Both substantially spherical metal nanoparticles and platelet nanoparticles can exhibit this type of reduced melting temperature. For example, platelet nanoparticles containing copper and having the particle sizes described herein can exhibit a melting temperature of about 180 °C to about 240 °C, or about 200 °C to about 240 °C, or about 220 °C to about 240 °C, which can only be slightly different from the melting temperature of substantially spherical metal nanoparticles. Without being bound by theory, it is believed that platelet nanoparticles, as a result of the lower thermodynamic stability of platelet nanoparticles, as further discussed below, can exhibit a lower melting temperature at a larger particle size than substantially spherical metal nanoparticles. When the metal nanoparticles are consolidated above the melting temperature, a bulk metal matrix can be fabricated at a processing temperature significantly lower than when directly processing the bulk metal itself as the starting material. When the bulk metal matrix is formed from the metal nanoparticles, the melting point approaches the melting point of the bulk metal itself, and the bulk metal matrix contains a plurality of grain boundaries.

[0016] The platelet nanoparticles have a higher surface area compared to spherical nanoparticles of equivalent size, thereby providing higher contact between other platelet nanoparticles to facilitate densification into the bulk metal matrix. Advantageously, the platelet nanoparticles of the present disclosure can lead to a reduction in the formation of such grain boundaries within the bulk metal matrix. When the platelet nanoparticles are consolidated together, the reduction in grain boundary formation can contribute to improved electrical and thermal performance. The reduction in grain boundary formation can result from the tendency of the stacked platelet nanoparticles to coalesce into larger crystal phases, rather than multiple contact points occurring and generating numerous grain boundaries as in the case of substantially spherical metal nanoparticles. The tendency of the platelet nanoparticles to coalesce into larger crystal phases is thought to result from the atomically flat surfaces of the platelet nanoparticles and the ease with which the crystal lattices therein can align when the platelet nanoparticles stack on top of one another. In contrast, substantially spherical metal nanoparticles can undergo energetically unfavorable rearrangements to form a polycrystalline phase with multiple grain boundaries. The platelet nanoparticles can be considered atomically flat, for example, when at least a portion of their upper or lower surfaces appears substantially flat when viewed in an SEM image. In some cases, the thickness of the platelet nanoparticles can vary like a staircase, with individual regions of the platelet nanoparticles being atomically flat and then transitioning abruptly to another atomically flat region. That is, the platelet nanoparticles can, in some cases, have different through-plane thicknesses at various positions thereon.

[0017] Another aspect regarding the atomically flat surfaces of the platelet nanoparticles is their higher reactivity compared to metal nanoparticles of equivalent size that are substantially spherical in shape. Without being bound by theory or mechanism, the higher reactivity is thought to result from the lower thermodynamic stability of the platelet nanoparticles compared to substantially spherical metal nanoparticles. Thus, the platelet nanoparticles can exhibit characteristic metal nanoparticle properties (e.g., a low melting temperature) that exceed the particle size threshold at which these properties begin to disappear in substantially spherical metal nanoparticles.

[0018] The relatively high reactivity of platelet nanoparticles may be desirable in many cases, but due to the lower thermodynamic stability of platelet nanoparticles, it is quite difficult to substantially preferentially produce platelet nanoparticles over spherical metal nanoparticles. The present disclosure overcomes this problem and provides a composition containing a significant amount of platelet nanoparticles.

[0019] As used herein, the term "metal nanoparticle" refers to metal particles having a size of about 150 nm or less in one or more dimensions, particularly about 100 nm or less in one or more dimensions. In the case of substantially spherical metal nanoparticles, the aforementioned value may represent the diameter of the sphere, while in the case of platelet nanoparticles, the aforementioned value may represent the lateral dimension or the through-plane dimension (longitudinal thickness) of the metal nanoparticles. Some of the platelet nanoparticles of the present disclosure have a lateral dimension of up to about 400 nm, but still can be classified as nanoparticles because they have a longitudinal thickness of about 150 nm or less. As used herein, the term "copper nanoparticle" refers to metal nanoparticles made of copper or mainly copper.

[0020] As used herein, the term "micron-scale metal particle" refers to metal particles larger than metal nanoparticles and having a size in the range of up to about 1000 μm, such as about 1 μm to about 1000 μm, or about 5 μm to about 500 μm. The micron-scale metal particles may have a substantially spherical shape or may have a non-spherical shape such as dendritic or rod-shaped.

[0021] The terms "densify", "densification" and other variations are used interchangeably herein with the terms "fuse", "fusion" and other variations.

[0022] As used herein, the terms "partially fused", "partial fusion", and other derivatives and grammatical synonyms thereof refer to the partial agglomeration of metal nanoparticles. Completely fused metal nanoparticles retain little of the structural form of the original unfused metal nanoparticles (i.e., they resemble bulk metals with minimal grain boundaries), while partially fused metal nanoparticles retain at least a portion of the structural form of the original unfused metal nanoparticles. The properties of the partially fused metal nanoparticles can be intermediate between the properties of the corresponding bulk metal and the properties of the original unfused metal nanoparticles.

[0023] Numerous large-scale realizable processes have been developed for producing large quantities of metal nanoparticles within a target size range, preferably substantially spherical metal nanoparticles within the target size range. Particularly facile metal nanoparticle fabrication techniques and their use for producing substantially spherical metal nanoparticles are described, for example, in U.S. Patent Nos. 7,736,414, 8,105,414, 8,192,866, 8,486,305, 8,834,747, 9,005,483, 9,095,898, 9,700,940, 9,797,032, 9,881,895, and 9,976,042, each of which is hereby incorporated by reference in its entirety. Such processes for producing substantially spherical metal nanoparticles are carried out by reducing a metal precursor (metal salt) in solution and in the presence of a surfactant system containing one or more surfactants. Small platelet nanoparticles can be synthesized through a similar process by modifying various reaction conditions as further described herein. Without being bound by any theory or mechanism, the surfactant system is thought to be able to mediate the nucleation and growth of metal nanoparticles, limit the surface oxidation of metal nanoparticles, and / or inhibit the extensive aggregation of metal nanoparticles with each other before they at least partially fuse together. The metal nanoparticles are then isolated and purified from the reaction mixture by common isolation techniques and can be processed into a nanoparticle paste as needed.

[0024] Figures 1 and 2 are diagrams of the presumed structure of substantially spherical metal nanoparticles having a surfactant coating thereon. Although Figures 1 and 2 show round or spherical metal nanoparticles, the concepts shown therein are applicable to platelet nanoparticles having other geometric shapes. As shown in Figure 1, the metal nanoparticle 10 includes a metal core 12 and a surfactant layer 14 that overcoats the metal core 12. The surfactant layer 14 can contain any combination of surfactants, as described in more detail below. The metal nanoparticle 20 shown in Figure 2 is similar to that depicted in Figure 1, except that the metal core 12 grows around the nucleus 21, and the nucleus 21 can be the same metal as the metal of the metal core 12 or a different metal. Since the nucleus 21 is deeply embedded within the metal core 12 in the metal nanoparticle 20, it is not considered to significantly affect the overall nanoparticle properties. Figure 3 is a diagram of the presumed structure of platelet nanoparticles 50 having a surfactant layer 54 thereon. Although depicted as a disk shape in Figure 3, other geometric shapes are possible, as further discussed below. The platelet nanoparticles 50 generally include substantially flat surfaces 52a and 52b that can be atomically flat, and a longitudinal surface 54 that extends between the substantially flat surfaces 52a and 52b. The substantially flat surfaces 52a and 52b can be substantially parallel to each other. The surfactant layer 56 covers the substantially flat surfaces 52a and 52b and the longitudinal surface 54. In some cases, different surfactants may be present on the substantially flat surfaces 52a and 52b and the longitudinal surface 54.

[0025] The metal nanoparticles can be single-crystalline, polycrystalline, and / or amorphous. Small platelet nanoparticles and substantially spherical metal nanoparticles can have different morphologies even if they are co-generated during a given metal nanoparticle synthesis. For example, small platelet nanoparticles can be single-crystalline and may have no grain boundaries or limited grain boundaries when consolidated, while substantially spherical metal nanoparticles can be amorphous or polycrystalline and may exhibit multiple grain boundaries when consolidated. Substantially spherical metal nanoparticles having a size of about 10 nm or less can be significantly more amorphous in properties due to the energetic disadvantage of maintaining a crystalline phase in this particle size range.

[0026] Without being bound by theory or mechanism, the difference in crystallinity is thought to result from the mechanisms by which substantially spherical metal nanoparticles and small platelet nanoparticles are formed and grow. Specifically, substantially spherical metal nanoparticles are thought to grow through Ostwald ripening, while small platelet nanoparticles do not grow. Ostwald ripening leads to the consolidation of multiple small particles in substantially spherical metal nanoparticles, thereby leading to polycrystallinity. The single-crystallinity of small platelet nanoparticles leads to the alignment of their crystal lattices during stacking, thereby promoting the consolidation and formation of minimal low-energy grain boundaries.

[0027] Furthermore, without being bound by theory or mechanism, the growth of small platelet nanoparticles is thought to occur under kinetic growth conditions, while substantially spherical metal nanoparticles can be formed under thermodynamic growth conditions due to their higher thermodynamic stability. Specifically, small platelet nanoparticles can be effectively formed if the nanoparticle growth can be slowed down sufficiently so that kinetic growth conditions become effective. Factors that affect kinetic growth conditions versus thermodynamic growth conditions can include, for example, temperature, growth time, type and amount of surfactant used, and the like.

[0028] Kinetically-driven metal nanoparticle growth versus thermodynamically-driven metal nanoparticle growth is thought to be influenced by how and where surfactants attach to the growing metal nanoparticles. Amorphous spherical nanoparticles can be formed first due to their greater thermodynamic stability, thereby avoiding the isolated corner or edge atoms required to form crystalline particles. As the nanoparticles begin to grow larger, the crystal structure can become more stable. When certain crystal planes begin to develop, surfactants with specific geometries or shapes can preferentially adhere to certain crystal planes over others, thereby being able to act more favorably in the formation of the crystal phase. Thus, if the kinetic growth conditions can be induced by slowing down the nanoparticle formation process (e.g., via control of temperature, reduction rate, and cooling), the growth morphology can be altered to favor the production of platelet nanoparticles with a crystalline phase. The selected surfactants and their concentrations can further assist this process. The designated surfactants can preferentially adhere to specific crystal planes and can block growth in one direction preferentially over the other.

[0029] A further advantage of slowing the growth rate in accordance with the foregoing is that higher metal salt concentrations can be utilized without affecting the quality of the product produced, including the size and size distribution of the platelet nanoparticles produced. An increase in metal salt concentration can facilitate an increase in the production yield per run. When producing platelet nanoparticles, for example, metal salt concentrations in the range of about 20% to about 60% higher than comparable syntheses that result in substantially spherical metal nanoparticles can be utilized.

[0030] Suitable metal salts for producing metal nanoparticles can include those that are soluble in the selected organic solvent. Non-limiting examples of suitable metal salts include, but are not limited to, metal halides, metal carboxylates, metal nitrates, etc. For example, cupric chloride anhydrous can be utilized in the disclosure herein to form copper nanoparticles including platelet nanoparticles.

[0031] Suitable organic solvents for solubilizing metal salts and forming metal nanoparticles include, for example, formamide, N,N-dimethylformamide, dimethyl sulfoxide, dimethylpropyleneurea, hexamethylphosphoramide, tetrahydrofuran, glyme, diglyme, triglyme, and tetraglyme. The concentration of the metal salt in the selected organic solvent may vary over a wide range and may be determined, for example, by the dissolution characteristics of the metal salt. Suitable reducing agents for reducing the metal salt and promoting the formation of metal nanoparticles include, for example, alkali metals (such as lithium naphthalide, sodium naphthalide, or potassium naphthalide) or borohydride reducing agents (such as sodium borohydride, lithium borohydride, potassium borohydride, or tetraalkylammonium borohydride) in the presence of a suitable catalyst.

[0032] The reaction temperature used to produce metal nanoparticles containing platelet nanoparticles can range from room temperature (25 °C), or even lower, up to a maximum of about 40 °C, or a maximum of about 50 °C, or a maximum of about 55 °C, or a maximum of about 60 °C, or a maximum of about 65 °C, or a maximum of about 70 °C. The aforementioned temperature represents the highest temperature that the reaction can reach during the formation of the metal nanoparticles. The highest temperature can be adjusted by the addition rate of the reducing agent, as will be discussed later. The reaction medium may be heated externally while the reducing agent is being added, and / or the reducing agent may be heated, provided that the maximum temperature of the reaction remains below the aforementioned values. In some cases, no external heating is applied to the reaction medium in which the platelet nanoparticles are formed, and the temperature increase in the reaction medium can result from the exotherm of the reduction of the metal salt by the reducing agent.

[0033] The rate of addition of the reducing agent to the solution containing the metal salt can affect the extent to which the exothermic heating during reduction raises the temperature of the reaction medium. Surprisingly, a suitably slow rate of addition can further promote the formation of platelet nanoparticles as well. To help maintain the temperature of the reaction medium below a maximum temperature of about 50 °C, or about 55 °C, or about 60 °C, or up to about 65 °C, or up to about 70 °C, the rate of addition of the reducing agent can be maintained at a slow rate to limit the extent to which the exothermic heating overheats the reaction mixture and reduces the production of platelet nanoparticles. In a non-limiting example, the reducing agent is added to the solution containing the metal salt and a suitable surfactant system such that the reducing agent is added over about 5 minutes or more, or about 6 minutes or more, or about 7 minutes or more, or about 8 minutes or more, or about 9 minutes or more, or about 10 minutes or more, or about 15 minutes or more, or about 20 minutes or more, or about 25 minutes or more, or about 30 minutes or more, or about 40 minutes or more, or about 50 minutes or more, or about 1 hour or more, or about 2 hours or more, and as any closed sub-range within any of the foregoing values, so as to be fully combined. For example, in a non-limiting example, the reducing agent can be added to the solution containing the metal salt over about 5 minutes to about 30 minutes, or about 10 minutes to about 40 minutes, or about 6 minutes to about 15 minutes, or about 8 minutes to about 20 minutes, or about 10 minutes to about 25 minutes, or about 12 minutes to about 24 minutes, or about 16 minutes to about 32 minutes, or about 18 minutes to about 36 minutes.

[0034] The addition rate of the reducing agent can be selected to bring about a maximum temperature rise of the reaction medium within a desired range. In non-limiting examples, the addition rate of the reducing agent can be selected to promote a temperature rise of up to about 30 °C, or up to about 25 °C, or up to about 20 °C, or up to about 15 °C, or up to about 10 °C, or up to about 5 °C. As a non-limiting example, the reducing agent may be added to a room temperature solution of the metal salt at a rate sufficient to promote a temperature rise of up to about 20 °C or up to about 25 °C and a maximum temperature of about 45 °C, or the reducing agent may be added to a 30 °C solution of the metal salt at a rate sufficient to promote a temperature rise of up to about 10 °C and a maximum temperature of about 40 °C. In still other examples, the reducing agent may be added to a solution of the metal salt having a temperature of up to about 35 °C, about 45 °C, or about 55 °C to bring about a temperature rise of up to about 5 °C, or the reducing agent may be added to a solution of the metal salt having a temperature of up to about 25 °C, about 35 °C, or about 45 °C to bring about a temperature rise of up to about 10 °C or up to about 15 °C, or the reducing agent may be added to a solution of the metal salt having a temperature of up to about 20 °C, about 30 °C, or about 40 °C to bring about a temperature rise of up to about 20 °C. In still other cases, it may be desirable to add the reducing agent to the solution containing the metal salt while maintaining the solution at a temperature below room temperature, such as in the range of about -10 °C to about 15 °C or about 0 °C to 15 °C.

[0035] In still other non-limiting examples, the reducing agent is added to a solution of the metal salt at a rate sufficient to maintain the solution at a temperature in the range of about 30 °C to about 70 °C, or about 30 °C to about 65 °C, or about 30 °C to about 60 °C, or about 40 °C to about 70 °C, or about 40 °C to about 60 °C, or about 50 °C to about 70 °C, or about 50 °C to about 60 °C, or about 35 °C to about 50 °C, or about 35 °C to about 60 °C, or about 35 °C to about 70 °C, or about 40 °C to about 70 °C, or about 40 °C to about 60 °C, or about 45 °C to about 70 °C, or about 45 °C to about 60 °C while forming the metal nanoparticles, and at least a portion of the metal nanoparticles includes platelet nanoparticles, preferably at least about 20% of the metal nanoparticles includes platelet nanoparticles.

[0036] When metal nanoparticles are formed upon addition of a reducing agent, the reaction medium may be maintained at that temperature for a desired period of time or cooled to a lower temperature, such as room temperature, at which nanoparticle isolation can occur. In non-limiting examples, the reaction medium can be cooled from the heating temperature to room temperature over about 30 minutes, or over about 1 hour, or over about 2 hours, or over about 3 hours, or over about 4 hours, or over about 6 hours, or over about 10 hours. Slow cooling can, similarly, promote the formation of platelet nanoparticles.

[0037] As discussed above, the surfactant system present on the surface of the metal nanoparticles can include one or more surfactants. The properties of the metal nanoparticles may be adjusted using the different properties of various surfactants. Factors that can be taken into account when selecting a surfactant or combination of surfactants for inclusion on the metal nanoparticles include, for example, the ease of dissipation of the surfactant from the metal nanoparticles during or prior to nanoparticle fusion, the nucleation rate and growth rate of the metal nanoparticles, the metal component of the metal nanoparticles, and the like. Forming a surfactant coating on them during the synthesis of the metal nanoparticles desirably limits the ability of the metal nanoparticles to fuse with each other before heating above the melting temperature, limits the aggregation of the metal nanoparticles, and can promote the formation of a population of metal nanoparticles having a narrow size distribution. The surfactant coating contains at least one surfactant that was present during the formation of the metal nanoparticles. If two or more types of surfactants are used during the formation of the metal nanoparticles, each type of surfactant or less than each type of surfactant may be positioned within the surfactant coating. Again, the particular surfactant incorporated as the surfactant coating on the platelet nanoparticles can depend on the particular surfactant used and their ability to coordinate to the particular face of the platelet nanoparticles. The surfactant coating generally is lost during the densification of the metal nanoparticles when heated above the melting temperature, although for low boiling point surfactants and depending on how strongly they are bound to the metal nanoparticles, some surfactant loss can occur even below the melting temperature. In various embodiments, the surfactant coating can inherently be non-polymeric.

[0038] In some embodiments, an amine surfactant or a combination of amine surfactants, particularly aliphatic amines, can be present on the metal nanoparticles. The amine surfactant may be particularly desirable, for example, when used in combination with copper nanoparticles or metal nanoparticles containing alternative transition metals. In some embodiments, two amine surfactants can be used in combination with each other. In other embodiments, three amine surfactants can be used in combination with each other. In some cases, four amine surfactants may also be used in combination with each other. In more specific embodiments, a primary amine, a secondary amine, and a diamine may be used in combination with each other when forming the metal nanoparticles. In even more specific embodiments, the three amine surfactants can include a long-chain primary amine having a linear or branched alkyl group, a secondary amine having a linear or branched alkyl group, and a diamine having at least one tertiary alkyl group substituent on the nitrogen atom. Thus, at least a portion (including one, two or more, or all) of at least one amine surfactant may include a branched alkyl chain. Further disclosure regarding suitable amine surfactants follows below.

[0039] In some embodiments, the surfactant system can include a primary alkylamine. In some embodiments, the primary alkylamine can be a C2-C 18 alkylamine, and the alkyl group can be linear or branched. In some embodiments, the primary alkylamine is a C6-C 10It can be an alkylamine, and the alkyl group can be linear or branched. In some embodiments, a C5-C6 primary alkylamine can be used, and the alkyl group can be linear or branched. Without being bound by any theory or mechanism, the exact size of the primary alkylamine is long enough to provide an effective inverse micelle structure during the synthesis of metal nanoparticles, has high volatility, and / or can be easily handled during nanoparticle consolidation. For example, primary alkylamines having more than 18 carbons may also be suitable for use in the present disclosure, but they may be more difficult to handle due to their wax-like properties. In particular, C6-C 10 The primary alkylamine can represent a good balance of desired properties for ease of use.

[0040] Suitable C2-C 18Examples of primary alkylamines include, for example, n-hexylamine, n-heptylamine, n-octylamine, n-nonylamine, or n-decylamine. All of these are linear primary alkylamines, but in other embodiments, branched primary alkylamines can also be used. For example, branched primary alkylamines such as 7-methyloctylamine, 2-methyloctylamine, or 7-methylnonylamine can be used. In some embodiments, such branched primary alkylamines may be sterically hindered when they are attached to the nitrogen atom of the amine. Non-limiting examples of such sterically hindered primary alkylamines include, for example, t-octylamine, 2-methylpentan-2-amine, 2-methylhexan-2-amine, 2-methylheptan-2-amine, 3-ethyloctan-3-amine, 3-ethylheptan-3-amine, 3-ethylhexan-3-amine, etc. Additional branching can also be present. Without being bound by any theory or mechanism, it is believed that primary alkylamines can act as ligands in the metal coordination sphere but can readily dissociate from the metal coordination sphere during the consolidation of metal nanoparticles. In some cases, at least a portion of the primary alkylamine may dissipate from the surface of the metal nanoparticles below its melting temperature during consolidation to form a metal matrix.

[0041] In some embodiments, the surfactant system can include a secondary amine. Suitable secondary amines for forming metal nanoparticles include linear, branched, or cyclic C3-C 14 , or C3-C8, or C4-C 12, or a C4-C8 alkyl group. The two alkyl groups may be the same or different. In some embodiments, the branching can occur on a carbon atom bonded to the nitrogen atom of the amine in one or more of the alkyl groups, thereby causing significant steric hindrance at the nitrogen atom of the amine. Suitable secondary amines include, but are not limited to, dihexylamine, diisobutylamine, di-t-butylamine, dineopentylamine, di-(2-ethylhexyl)amine, di-t-pentylamine, dicyclopentylamine, dicyclohexylamine, and the like. C4-C 12 Secondary amines outside the range of C4-C8 can also be used, but such secondary amines may have undesirable physical properties such as low boiling points or waxy viscosities that can complicate their handling. In some cases, at least a portion of the secondary alkylamine can dissipate from the surface of the metal nanoparticles below its melting temperature during consolidation to form a metal matrix.

[0042] In some embodiments, the surfactant system can include a diamine. In some embodiments, one or both of the nitrogen atoms of the diamine can be substituted with one or two alkyl groups. When two alkyl groups are present on the same nitrogen atom of the diamine, the alkyl groups may be the same or different. Further, when both nitrogen atoms are substituted, the same alkyl group or different alkyl groups may be present. In some embodiments, the alkyl group in the diamine can be a C1-C6 alkyl group. In other embodiments, the alkyl group in the diamine can be a C1-C4 alkyl group or a C3-C6 alkyl group. In some embodiments, the alkyl group having 3 or more carbon atoms in the diamine may be linear or branched. The alkyl group having 3 or more carbon atoms in the diamine can be cyclic. Without being bound by any theory or mechanism, it is believed that the diamine can facilitate the formation of metal nanoparticles by promoting the nucleation of the nanoparticles.

[0043] Suitable diamines include N,N'-dialkylethylenediamines, especially C1-C4 N,N'-dialkylethylenediamines. Corresponding methylenediamine derivatives, propylenediamine derivatives, butylenediamine derivatives, pentylenediamine derivatives or hexylenediamine derivatives can also be used. The alkyl groups in the diamine may be the same or different. Examples of C1-C4 alkyl groups that can be present include, for example, methyl group, ethyl group, propyl group, and butyl group, or branched alkyl groups such as isopropyl group, isobutyl group, s-butyl group, and t-butyl group. Exemplary N,N'-dialkylethylenediamines that may be suitable for generating metal nanoparticles according to the disclosure herein include, for example, N,N'-di-t-butylethylenediamine, N,N'-diisopropylethylenediamine, and the like.

[0044] In some embodiments, suitable diamines include N,N,N',N'-tetraalkyl ethylenediamines, especially C1-C4 N,N,N',N'-tetraalkyl ethylenediamines. Corresponding methylenediamine derivatives, propylenediamine derivatives, butylenediamine derivatives, pentylenediamine derivatives or hexylenediamine derivatives can also be used. The alkyl groups may similarly be the same or different and examples thereof include those described above. Exemplary N,N,N',N'-tetraalkyl ethylenediamines that may be suitable for use in forming metal nanoparticles include, for example, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetraethylethylenediamine, and the like.

[0045] In some examples, the one or more surfactants (surfactant system) used to produce a composition containing platelet nanoparticles can include at least one amine surfactant, more preferably two or more amine surfactants, and even more preferably three or more amine surfactants such as a combination of primary amines, secondary amines, and diamines. One or more or all of the amine surfactants can include a branched alkyl group. Thus, in some embodiments, the primary amine, secondary amine, and diamine may be used in combination with the formation of platelet nanoparticles, and the primary amine, secondary amine, and diamine each contain a branched alkyl group. Suitable examples of the primary amine, secondary amine, and diamine include those listed above. When the surfactant system contains a combination of primary amine, secondary amine, and diamine, the ratios of the various surfactants to each other in the metal salt solution can be adjusted to promote the formation of platelet nanoparticles. Similarly, the ratios of these surfactants to the metal salt can be adjusted to promote the formation of platelet nanoparticles, such as by facilitating the formation of platelet nanoparticles under kinetic growth conditions.

[0046] In some examples, the secondary amine may be present in a molar amount higher than the total amount of the primary amine and the diamine. In some or other examples, the primary amine may be present in a molar amount higher than the diamine. In non-limiting examples, the molar ratio of the primary amine to the diamine may range from about 0.9 to about 3.0, or from about 1.0 to about 1.5, or from about 1.5 to about 2.0, or from about 2.0 to about 2.5, or from about 2.5 to about 3.0, or from about 1.6 to about 2.2, or from about 2.1 to about 2.6, and the molar ratio of the secondary amine to the diamine may range from about 2.5 to about 6.5, or from about 2.5 to about 3.0, or from about 3.0 to about 3.5, or from about 3.5 to about 4.0, or from about 4.0 to about 4.5, or from about 4.5 to about 4.0, or from about 5.0 to about 5.5, or from about 5.5 to about 6.0, or from about 6.0 to about 6.5, or from about 3.1 to about 3.7, or from about 3.7 to about 4.3, or from about 4.3 to about 4.8. In some or other examples, the molar ratio of the primary amine to the metal salt may range from about 1.5 to about 2.5, or from about 1.8 to about 2.3, or from about 1.6 to about 2.1, or from about 2.1 to about 2.4. The molar ratio of the secondary amine to the metal salt may range from about 4.0 to about 5.2, or from about 4.0 to about 4.6, or from about 4.6 to about 5.2, or from about 4.4 to about 5.0, or from about 4.6 to about 4.9, and the molar ratio of the diamine to the metal salt may range from about 0.8 to about 1.6, or from about 0.8 to about 1.3, or from about 0.9 to about 1.2, or from about 1.2 to about 1.6.

[0047] Surfactants other than aliphatic amines can also be present in the surfactant system. In this regard, suitable surfactants can include, for example, pyridine, aromatic amines, phosphines, thiols, or any combination thereof. These surfactants can be used in combination with aliphatic amines including the above, or can be used in a surfactant system in which there is no aliphatic amine, or one or more of the primary aliphatic amine, secondary aliphatic amine, or diamine are omitted. Further disclosure regarding suitable pyridines, aromatic amines, phosphines, and thiols follows.

[0048] Suitable aromatic amines are ArNR 1 R2 can have the formula, wherein Ar is a substituted or unsubstituted aryl group, and R 1 and R 2 are the same or different. R 1 and R 2 can independently be selected from H or an alkyl or aryl group containing from 1 to about 16 carbon atoms. Exemplary aromatic amines that may be suitable for use in forming the metal nanoparticles include, for example, aniline, toluidine, anisidine, N,N-dimethylaniline, N,N-diethylaniline, and the like. Other aromatic amines that can be used in combination with the metal nanoparticles can be envisioned by those skilled in the art.

[0049] Suitable pyridines can include both pyridine and its derivatives. Exemplary pyridines that may be suitable for promoting the formation of the metal nanoparticles include, for example, pyridine, 2-methylpyridine, 2,6-dimethylpyridine, collidine, pyridazine, and the like. Chelating pyridines such as bipyridyl chelating agents may also be used. Other pyridines that can be used in combination with the formation of the metal nanoparticles can be envisioned by those skilled in the art.

[0050] Suitable phosphines can have the formula PR3, where R is an alkyl or aryl group containing from 1 to about 16 carbon atoms. The alkyl or aryl groups bonded to the phosphorus center may be the same or different. Exemplary phosphines that can be present when forming metal nanoparticles include, for example, trimethylphosphine, triethylphosphine, tributylphosphine, tri-t-butylphosphine, trioctylphosphine, triphenylphosphine, and the like. Phosphine oxides can likewise be used. In some embodiments, surfactants containing two or more phosphine groups configured to form a chelate ring can also be used. Exemplary chelating phosphines include, for example, bisphosphines such as 1,2-bisphosphine, 1,3-bisphosphine, and BINAP. Other phosphines that may be potentially useful in conjunction with the formation of metal nanoparticles can also be envisioned by those skilled in the art.

[0051] Suitable thiols can have the formula RSH, where R is an alkyl or aryl group having from about 4 to about 16 carbon atoms. Exemplary thiols that can be used in combination with the formation of metal nanoparticles include, for example, butanethiol, 2-methyl-2-propanethiol, hexanethiol, octanethiol, benzenethiol, and the like. In some embodiments, surfactants containing two or more thiol groups configured to form a chelate ring can also be used. Exemplary chelating thiols include, for example, 1,2-dithiols (e.g., 1,2-ethanedithiol) and 1,3-dithiols (e.g., 1,3-propanedithiol). Other thiols that may be potentially useful in conjunction with the formation of metal nanoparticles can also be envisioned by those skilled in the art.

[0052] Copper can be a particularly desirable metal for incorporation into metal nanoparticles due to its low cost, strength, and excellent electrical and thermal conductivity values, as well as additional advantages further addressed herein. Aliphatic amine surfactants, including those described above, can also readily form metal-ligand bonds with copper and promote the formation of copper nanoparticles. It should be understood that while copper nanoparticles can be advantageous for the reasons described above, other types of metal nanoparticles may be suitable for use in some instances. Other metal nanoparticles that can be formed in accordance with the disclosure herein include, for example, aluminum nanoparticles, palladium nanoparticles, silver nanoparticles, gold nanoparticles, iron nanoparticles, cobalt nanoparticles, nickel nanoparticles, titanium nanoparticles, zirconium nanoparticles, hafnium nanoparticles, tantalum nanoparticles, and the like. Any of the foregoing metal nanoparticles may preferably contain platelet nanoparticles in the disclosure herein in an amount exceeding 20% based on all nanoparticles. Micron-sized particles of these metals can similarly be present in the nanoparticle paste containing the metal nanoparticles and, in some cases, can provide processing advantages.

[0053] Suitable alloys of copper can be formed in situ by co-reduction / precipitation during the initial process for forming metal nanoparticles. In one configuration, additional metal salts can be co-reduced with the copper salt. In another configuration, a metal-organic compound that decomposes can be present during the reduction for forming copper nanoparticles. In yet another process configuration, the compound can be reduced or decomposed before forming copper nanoparticles within the same reaction layer. In this configuration, nanoparticle nucleation seeds can be generated to promote the growth of metal nanoparticles around the nuclei. At least a portion of the metal nanoparticles produced in accordance with any of the foregoing can be platelet nanoparticles.

[0054] Accordingly, the present disclosure provides a composition comprising a plurality of metal nanoparticles having a surfactant coating thereon, wherein at least a portion of the metal nanoparticles are platelet nanoparticles, and the surfactant coating comprises at least one surfactant. More specifically, the composition comprises a plurality of metal nanoparticles having a surfactant coating thereon, wherein at least about 20% of the metal nanoparticles are platelet nanoparticles, and the surfactant coating comprises at least one surfactant. The metal nanoparticles can, in various instances, comprise or consist essentially of copper nanoparticles. In some embodiments, the platelet nanoparticles can constitute at least a majority of the metal nanoparticles.

[0055] The platelet nanoparticles can constitute from about 20% to about 100% of the metal nanoparticles in the composition by volume. For example, the platelet nanoparticles can constitute more than about 20%, or more than about 30%, or more than about 40%, or more than about 50%, or more than about 60%, or more than about 70%, or more than about 80%, or more than about 90%, or more than about 95% of the plurality of metal nanoparticles.

[0056] In addition to the platelet nanoparticles, the plurality of metal nanoparticles can further comprise a plurality of substantially spherical metal nanoparticles. The substantially spherical metal nanoparticles, when present, can have a diameter of about 150 nm or less, or about 70 nm or less, or about 20 nm or less, or about 10 nm or less, such as from about 5 nm to about 20 nm, or from about 10 nm to about 30 nm, or from about 20 nm to about 50 nm, or from about 50 nm to about 70 nm, or any combination thereof. When present, the substantially spherical metal nanoparticles can constitute from about 80% to about 10% of the plurality of metal nanoparticles by volume. For example, the substantially spherical metal nanoparticles can constitute less than about 20%, or less than about 30%, or less than about 40%, or less than about 50%, or less than about 60%, or less than about 70%, or less than about 80% of the plurality of metal nanoparticles. In some embodiments, a plurality of substantially spherical metal nanoparticles having a bimodal particle size distribution can be present in combination with the platelet nanoparticles of the present disclosure.

[0057] Substantially spherical metal nanoparticles can have a circularity of about 0.8 or more, such as about 0.8 to about 1, or about 0.80 to about 0.95, or about 0.90 to about 1.0, or about 0.93 to about 0.99, or about 0.95 to about 0.99, or about 0.97 to about 0.99, or about 0.98 to about 1.0. To determine the circularity of a given particle, the perimeter (P) and area (A) of the particle can be evaluated from an optical image. The circularity can then be determined from the relationship CEA / P, where CEA is the circumference of a circle having an area equal to the area (A) of the actual particle. Thus, the circularity represents the ratio of the perimeter of the nanoparticle compared to the perimeter of a perfect sphere having the same radius. In contrast, platelet nanoparticles can have a circularity of 0.8 or less, such as 0.5 or less, or about 0.4 or less, or about 0.3 or less, or about 0.2 or less, provided that the platelet nanoparticles are not substantially round (disk-shaped). Disk-shaped platelet nanoparticles can have a circularity similar to that of substantially spherical metal nanoparticles.

[0058] In non-limiting examples, the platelet nanoparticles described herein can have a longitudinal thickness (i.e., through the plane of the platelet and thus representing the through-plane thickness) in the range of about 5 nm to about 40 nm, or about 5 nm to about 10 nm, or about 5 nm to about 20 nm, or about 10 nm to about 20 nm, or about 15 nm to about 30 nm, or about 20 nm to about 40 nm.

[0059] In non-limiting examples, the platelet nanoparticles described herein can have a maximum dimension (including any dimension through the plane of the platelet) in the range of about 10 nm to about 400 nm, or about 10 nm to about 100 nm, or about 50 nm to about 200 nm, or about 10 nm to about 50 nm, or about 100 nm to about 200 nm, or about 200 nm to about 400 nm.

[0060] In a non-limiting example, the platelet nanoparticles described herein can have an aspect ratio in the range of about 1.5 to about 30, or about 1.5 to about 3, or about 2 to about 4, or about 3 to about 5, or about 5 to about 8, or about 8 to about 12, or about 10 to about 15, or about 12 to about 20, or about 15 to about 25, or about 15 to about 30, or about 20 to about 25, or about 25 to about 30 (the ratio of the length to the width of the platelet surface, not including the through-thickness of the platelet surface).

[0061] In a non-limiting example, the platelet nanoparticles described herein can have a longitudinal aspect ratio in the range of about 1.5 to about 100, or about 1.5 to about 3, or about 2 to about 5, or about 3 to about 10, or about 5 to about 8, or about 8 to about 12, or about 10 to about 15, or about 12 to about 20, or about 15 to about 25, or about 15 to about 30, or about 20 to about 25, or about 25 to about 30, or about 20 to about 50, or about 30 to about 75, or about 35 to about 100 (the ratio of the length or width to the length or width in the longitudinal or through-thickness direction).

[0062] In addition to the above dimensions, the shape of the platelet nanoparticles is considered to be not particularly limited. In non-limiting examples, the platelet nanoparticles can have shapes such as, for example, triangular, rectangular, square, disc-shaped (including oval and circular discs), pentagonal, hexagonal, etc., or any combination thereof.

[0063] For small platelet nanoparticles that are not oval or circular (i.e., polygonal small platelet nanoparticles), the length of each edge may range from about 5 nm to about 400 nm, or from about 5 nm to about 300 nm, or from about 5 nm to about 200 nm, or from about 10 nm to about 200 nm, or from about 5 nm to about 50 nm, or from about 5 nm to about 25 nm, or from about 50 nm to about 100 nm. In non-limiting examples, triangular small platelet nanoparticles may have edges in the range of about 5 nm to about 200 nm in size, and hexagonal small platelet nanoparticles may have edges in the range of about 5 nm to about 150 nm in size. The foregoing values represent the edge length, not the maximum dimension (apothem) of the small platelet nanoparticles. The length of each edge in the small platelet nanoparticles may be the same or different. Thus, the polygonal small platelet nanoparticles may have a regular or irregular shape. The vertices of the polygonal small platelet nanoparticles may, in some cases, be rounded.

[0064] The compositions containing the small platelet nanoparticles described above herein can be further incorporated into various nanoparticle formulations, which can facilitate the distribution and consolidation of the small platelet nanoparticles. Suitable formulations can include nanoparticle pastes, sprayable formulations, inks, and the like. An illustrative disclosure directed to such nanoparticle pastes and similar formulations follows. Again, copper nanoparticles (e.g., copper nanoparticles having a platelet morphology, optionally combined with substantially spherical copper nanoparticles) represent only one type of metal nanoparticles that can be suitably incorporated into the nanoparticle paste and further consolidated in accordance with the disclosure herein.

[0065] The nanoparticle paste can be prepared by dispersing as-generated or isolated metal nanoparticles in an organic matrix containing one or more organic solvents and various other optional components. As used herein, the terms "nanoparticle paste formulation" and "nanoparticle paste" are used interchangeably and synonymously refer to a fluid composition containing dispersed metal nanoparticles that is suitable for dispensing using a desired technique. Suitable pastes can include fluid dispersions that do not flow freely due to their viscosity, while fluid dispersions including inks can flow freely. The use of the term "paste" does not necessarily imply an adhesive function of the paste alone. Through judicious selection of the organic solvents and other additives in the nanoparticle paste, the introduction of metal nanoparticles, such as, can be facilitated and the distribution of the metal nanoparticles at a desired location can be promoted.

[0066] Particularly when substantially spherical metal nanoparticles are used, cracking and shrinkage can sometimes occur during the consolidation of the metal nanoparticles. In contrast, platelet nanoparticles overlap each other much more effectively than substantially spherical metal nanoparticles, thereby making it difficult for cracks to develop and propagate. FIG. 4 is a diagram showing substantially spherical metal nanoparticles 301 in a closest-packed configuration 300. As shown, even in the closest-packed configuration 300, after its formation, a path for crack propagation 302 through the consolidated grain boundaries 304 is provided. FIG. 5 is a diagram showing platelet nanoparticles 401 in an overlapping and stacked packed configuration 400. As shown, in the case of the stacked platelet nanoparticles 401, there is no continuous path through which crack propagation 402 can easily proceed.

[0067] One way in which nanoparticle pastes containing platelet nanoparticles can further promote the degree of crack generation and reduction of void formation after metal nanoparticle densification is by maintaining a high solids content. Thus, the nanoparticle pastes disclosed herein can contain at least about 30 wt% metal nanoparticles, particularly about 30 wt% to about 98 wt% metal nanoparticles of the nanoparticle paste, or about 50 wt% to about 98 wt% metal nanoparticles of the nanoparticle paste, or about 70 wt% to about 98 wt% metal nanoparticles of the nanoparticle paste. Further, in some embodiments, in addition to the metal nanoparticles, small amounts (e.g., about 0.01 wt% to about 15 wt%, or about 35 wt% to about 70 wt%, or about 10 wt% to about 35 wt% of the paste composition) of micron-scale metal particles can be present. Such micron-scale metal particles can desirably facilitate the fusion of the metal nanoparticles into the bulk metal matrix and further reduce the incidence of crack generation, shrinkage, and overall porosity. For example, the shrinkage during the formation of fused copper nanoparticles can be reduced to about 5 vol% or less in the presence of micron-scale particles compared to a shrinkage rate of about 20 - 40 vol% when no micron-scale particles are present. The reduced shrinkage can be achieved even when platelet nanoparticles are present in the nanoparticle paste and similar formulations and in the absence of micron-scale metal particles, thereby allowing micron-scale metal nanoparticles to be used in lesser amounts or not at all in the nanoparticle pastes disclosed herein. The reduction in shrinkage can result from improved lamination, since the platelet nanoparticles are closer together initially. Instead of being liquefied and undergoing direct densification, micron-scale metal particles, when present, can simply join together when they come into contact with metal nanoparticles that have been raised above their melting temperatures. Further, platelet nanoparticles can provide better contact between two or more micron-scale metal particles (in some cases further assisted by the flexibility of the platelet nanoparticles), thereby cross-linking the micron-scale metal particles together when raised above the melting temperature.Due to these factors, it is possible to reduce the porosity that occurs after fusing together platelet nanoparticles and micron-scale metal particles. The micron-scale metal particles can contain the same or different metals as the metal nanoparticles. For example, micron-scale copper particles can be used in combination with copper nanoparticles containing at least some platelet nanoparticles. Suitable metals for the micron-scale metal particles include, for example, copper, silver, gold, aluminum, tin, and the like. Micron-scale graphite particles can also be included as another type of micron-scale particle. Carbon nanotubes and / or graphene can be included as yet another type of micron-scale particle. In still other cases, carbon black and / or nanocarbon can be included. For example, other additives such as diamond particles, AlN, and cubic BN (boron nitride) can similarly be included. Suitable forms for additional additives include, for example, milled fibers having a length of about 50 microns to about 350 microns and a diameter of about 5 microns to about 25 microns.

[0068] The reduction of crack generation and void formation during the densification of metal nanoparticles can also be facilitated by a judicious selection of the solvent that forms the organic matrix present in the nanoparticle paste or similar formulations. An adjusted combination of organic solvents can desirably reduce the crack generation rate and void formation. More specifically, an organic matrix containing one or more hydrocarbons (saturated, monounsaturated, polyunsaturated (two or more double bonds) or aromatic), one or more alcohols, one or more amines, and optionally, one or more organic acids, may be particularly effective for this purpose. In some embodiments, in addition to, or in place of, other solvent components of the nanoparticle paste, one or more esters, ethers, ketones, aldehydes, and / or one or more anhydrides may be included. Without being bound by any theory or mechanism, this combination of organic solvents is thought to facilitate the removal and sequestration of surfactant molecules surrounding the metal nanoparticles during densification, such that the metal nanoparticles can more readily fuse together. More specifically, hydrocarbon and alcohol solvents are thought to passively solubilize surfactant molecules liberated from the metal nanoparticles by Brownian motion, reducing the ability of the surfactant molecules to reattach to the metal nanoparticles. In cooperation with the passive solubilization of surfactant molecules, amine and organic acid solvents can actively sequester surfactant molecules via chemical interactions such that the surfactant molecules are no longer available for recombination with the metal nanoparticles.

[0069] To reduce the rapid volume contraction that occurs during surfactant removal and metal nanoparticle consolidation, further adjustment of the solvent composition can be carried out. Specifically, two or more members of each class of organic solvents (i.e., hydrocarbons, alcohols, amines, and optionally organic acids) can be present in the organic matrix, and the members of each class have boiling points that are separated from each other by a set temperature. For example, in some embodiments, the various members of each class can have boiling points that are separated from each other by about 20°C to about 50°C. By using such a solvent mixture, the various components of the solvent mixture can be gradually removed over a wide range of boiling points (e.g., about 50°C to about 250°C), so that the rapid volume change due to the rapid loss of the solvent during the consolidation of the metal nanoparticles can be minimized.

[0070] In some embodiments, at least a portion of the one or more organic solvents can have a boiling point of about 100°C or higher. In some embodiments, at least a portion of the one or more organic solvents can have a boiling point of about 200°C or higher or about 300°C or higher. In some embodiments, the one or more organic solvents can have boiling points in the range of about 50°C to about 350°C, or about 50°C to about 200°C, or about 100°C to about 200°C, or about 150°C to about 350°C. The use of high-boiling organic solvents can desirably increase the pot life of the nanoparticle paste and limit the rapid loss of the solvent that can lead to crack formation and void formation during nanoparticle consolidation. In some embodiments, at least one of the organic solvents can have a boiling point higher than the boiling point of the surfactant that associates with the metal nanoparticles. Thus, the surfactant can be removed from the metal nanoparticles by evaporation before the removal of the organic solvent, and in some examples, at least a portion of the surfactant can be removed below the melting temperature of the metal nanoparticles.

[0071] In some embodiments, the organic matrix can contain one or more alcohols. In various embodiments, the alcohol can include a monohydric alcohol, a diol, a triol, a glycol ether (e.g., diethylene glycol and triethylene glycol), an alkanolamine (e.g., ethanolamine, triethanolamine, etc.), or any combination thereof. In some embodiments, one or more hydrocarbons can be present in combination with one or more alcohols. As discussed above, it is believed that the alcohol and hydrocarbon solvents can passively promote the solubilization of the surfactant since they are removed from the metal nanoparticles by Brownian motion and the re-association with the metal nanoparticles is limited. Further, since the hydrocarbon solvent and the alcohol solvent only weakly coordinate to the metal nanoparticles, they do not simply replace the surfactant substituted in the nanoparticle coordination sphere. Illustrative but non-limiting examples of alcohol solvents and hydrocarbon solvents that can be present include, for example, light aromatic petroleum distillate (CAS 64742-95-6), hydrotreated light petroleum distillate (CAS 64742-47-8), tripropylene glycol methyl ether, ligroin (CAS 68551-17-7, C 10 ~C 13 mixture of alkanes), diisopropylene glycol monomethyl ether, diethylene glycol diethyl ether, 2-propanol, 2-butanol, t-butanol, 1-hexanol, 2-(2-butoxyethoxy)ethanol, and terpineol. In some embodiments, a polyketone solvent can be used as well.

[0072] In some embodiments, the organic matrix can contain one or more amines and optionally one or more organic acids. In some embodiments, one or more amines and one or more organic acids can be present in an organic matrix that also contains one or more hydrocarbons and one or more alcohols. As discussed above, amines and organic acids can actively block surfactants that are passively solubilized by hydrocarbon and alcohol solvents, thereby rendering the surfactants unavailable for re-association with metal nanoparticles. Thus, an organic solvent containing a combination of one or more hydrocarbons, one or more alcohols, one or more amines, and one or more organic acids can provide a synergistic benefit for promoting the consolidation of metal nanoparticles. Illustrative but non-limiting examples of amine solvents that can be present include, for example, tallow amine (CAS 61790-33-8), alkyl (C8-C 18 ) unsaturated amine (CAS 68037-94-5), di(hydrogenated tallow) amine (CAS 61789-79-5), dialkyl (C8-C 20 ) amine (CAS 68526-63-6), alkyl (C 10 -C 16 ) dimethylamine (CAS 67700-98-5), alkyl (C 14 -C 18 ) dimethylamine (CAS 68037-93-4), dihydrogenated tallow methylamine (CAS 61788-63-4), and trialkyl (C6-C 12 ) amine (CAS 68038-01-7). Illustrative but non-limiting examples of organic acid solvents that can be present in the nanoparticle paste include, for example, octanoic acid, nonanoic acid, decanoic acid, caprylic acid, pelargonic acid, undecylic acid, lauric acid, tridecylic acid, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, nonadecylic acid, α-linolenic acid, stearidonic acid, oleic acid, and linoleic acid.

[0073] In some embodiments, the organic matrix can include two or more hydrocarbons, two or more alcohols, two or more amines, and two or more optional organic acids. For example, in some embodiments, each class of organic solvents can have two or more members, or three or more members, or four or more members, or five or more members, or six or more members, or seven or more members, or eight or more members, or nine or more members, or ten or more members. Further, the number of members in each class of organic solvents can be the same or different. The specific benefits of using multiple members of each class of organic solvents are described below.

[0074] One particular advantage of using multiple members within each class of organic solvents is the ability to provide a wide range of boiling points in the nanoparticle paste. By providing a wide range of boiling points, as the temperature increases, the organic solvent can be gradually removed while affecting the densification of the metal nanoparticles, thereby limiting volume shrinkage and acting unfavorably on crack generation. As a result, greater structural integrity of the connection can be achieved. By gradually removing the organic solvent in this way, less temperature control may be required to facilitate slow solvent removal than when using a single solvent with a narrow boiling range. In some embodiments, the members within each class of organic solvents can have a boiling point window in the range of about 50°C to about 200°C, or about 50°C to about 250°C, or about 100°C to about 200°C, or about 100°C to about 250°C. In some cases, a boiling point of up to about 350°C may be suitable. In more specific embodiments, the various members of each class of organic solvents can each have boiling points that are at least about 20°C, specifically about 20°C to about 50°C apart from each other. More specifically, in some embodiments, each hydrocarbon can have a boiling point that is about 20°C to about 50°C different from other hydrocarbons in the organic matrix, each alcohol can have a boiling point that is about 20°C to about 50°C different from other alcohols in the organic matrix, each amine can have a boiling point that is about 20°C to about 50°C different from other amines in the organic matrix, and each optional organic acid can have a boiling point that is about 20°C to about 50°C different from other organic acids in the organic matrix. The greater the number of members of each class of organic solvents present, the smaller the difference between the boiling points. By making the difference between the boiling points smaller, solvent removal can be carried out more continuously, thereby limiting the degree of volume shrinkage occurring at each stage. When there are four or more, for example, four or more hydrocarbons, four or more alcohols, four or more amines, and four or more organic acids; or five or more hydrocarbons, five or more alcohols, five or more amines, and five or more organic acids, each having boiling points that are separated from each other within the above ranges, a reduction in the degree of crack generation may occur.

[0075] In addition to the metal nanoparticles and the organic solvent, other additives, including micron-scale metal particles or other micron-scale particles as described above, can also be present in the nanoparticle paste. Such additional additives can include, for example, rheology control aids, thickeners, micron-scale conductive additives, nano-scale conductive additives, and any combination thereof. Chemical additives can also be present. As will be discussed below, including micron-scale conductive additives such as micron-scale metal particles can be particularly advantageous. In some cases, it may be desirable to include nano-scale or micron-scale diamond or other thermal conductive additives.

[0076] In some embodiments, the nanoparticle paste can contain micron-scale metal particles in an amount of about 0.01 wt% to about 15 wt%, or about 1 wt% to about 10 wt%, or about 1 wt% to about 5 wt%, or about 0.1 wt% to about 35 wt%, or about 10 wt% to about 35 wt%, or about 35 wt% to about 70 wt%. By including micron-scale metal particles in the nanoparticle paste, the incidence of cracks that occur during the densification of metal nanoparticles due to shrinkage can be desirably reduced. Here too, the use of platelet nanoparticles can reduce the shrinkage and the incidence of cracks compared to substantially spherical metal nanoparticles, even when micron-scale metal particles are omitted or used in a smaller amount. Without being bound by any theory or mechanism, it is believed that the metal nanoparticles are liquefied and form a temporary liquid coating on the micron-scale metal particles, filling the voids between them, so that the micron-scale metal particles can be partially densified. In essence, the metal nanoparticles function as an "adhesive" that binds the micron-scale particles together. In some embodiments, the micron-scale metal particles can range in size from about 500 nm to about 100 microns in at least one dimension, or from about 500 nm to about 10 microns in at least one dimension, or from about 100 nm to about 5 microns in at least one dimension, or from about 100 nm to about 10 microns in at least one dimension, or from about 100 nm to about 1 micron in at least one dimension, or from about 1 micron to about 10 microns in at least one dimension, or from about 5 microns to about 10 microns in at least one dimension, or from about 1 micron to about 100 microns in at least one dimension. The micron-sized metal particles can contain the same metal as the metal nanoparticles or can contain a different metal. Thereby, a metal alloy can be fabricated by including micron-sized metal particles in a nanoparticle paste having a metal different from that of the metal nanoparticles.Suitable micron-scale metal particles include, for example, Cu particles, Ni particles, Al particles, Fe particles, Co particles, Mo particles, Ag particles, Zn particles, Sn particles, Au particles, Pd particles, Pt particles, Ru particles, Mn particles, Cr particles, Ti particles, V particles, Mg particles, or Ca particles. Borides, carbides, phosphides, nitrides, and silicides of these metals, as well as combinations thereof, can similarly be used. For example, non-metal particles such as Si micron-scale particles and B micron-scale particles can be similarly used, including their borides, carbides, phosphides, nitrides, and silicides. Specific examples of particles that can be present in the nanoparticle paste include SiC, AlN, SiN, BN, etc. In some embodiments, the micron-scale metal particles can be in the form of metal flakes such as, for example, high aspect ratio copper flakes. That is, in some embodiments, the nanoparticle paste described herein can contain a mixture of copper nanoparticles and high aspect ratio copper flakes. Specifically, in some embodiments, the nanoparticle paste can contain from about 30 wt% to about 98 wt% of copper nanoparticles and from about 0.01 wt% to about 15 wt% of high aspect ratio copper flakes, or from about 0.1 wt% to about 35 wt% of high aspect ratio copper flakes, or from about 1 wt% to about 35 wt% of high aspect ratio copper flakes, or from about 35 wt% to about 70 wt% of high aspect ratio copper flakes.

[0077] Other micron-scale metal particles that can be used equivalently to the high aspect ratio metal flakes include, for example, metal nanowires and other high aspect ratio particles that can have a length of up to about 300 microns or about 500 microns. The ratio of metal nanoparticles to metal nanowires can be in the range of about 10:1 to about 40:1 according to various embodiments. Suitable nanowires can have, for example, a length of about 5 microns to about 50 microns or about 100 microns, and a diameter of about 100 nm to about 200 nm. Mild fibers (e.g., carbon fibers, ceramic fibers, metal fibers, and similar fibers having high thermal conductivity) can be used similarly. In non-limiting examples, suitable mild fibers can have a diameter of about 5 microns to about 25 microns and a length of about 50 microns to about 500 microns.

[0078] In some embodiments, nanoscale conductive additives can also be present in the nanoparticle paste. These additives can desirably provide further structural reinforcement and reduce shrinkage during metal nanoparticle densification. Further, including nanoscale conductive additives can increase the electrical conductivity value and thermal conductivity value approaching or even exceeding the corresponding bulk metal after nanoparticle densification. In some embodiments, the nanoscale conductive additives can have a size in the range of about 1 micron to about 100 microns, or in the range of about 1 micron to about 300 microns in at least one dimension. Suitable nanoscale conductive additives can include, for example, carbon nanotubes, graphene, etc. Carbon fibers at the nanoscale can also be used similarly. When present, the nanoparticle paste can contain about 1 wt% to about 15 wt% of the nanoscale conductive additive, or about 1 wt% to about 10 wt% of the nanoscale conductive additive, or about 1 wt% to about 5 wt% of the nanoscale conductive additive.

[0079] Additional substances that can optionally be present include, for example, flame retardants, UV protectants, antioxidants, carbon black, graphite, fiber materials (e.g., chopped carbon fiber materials), diamond, and the like.

[0080] In addition to the nanoparticle paste, the metal nanoparticles disclosed herein can be incorporated into sprayable formulations and inks that can be dispensed by alternative procedures such as inkjet printing, stencil printing, gravure printing, aerosol spraying, painting, dip coating, 3D printing, and the like. The sprayable formulations and inks can similarly include metal nanoparticles dispersed in a suitable solvent, but can have a lower viscosity than the nanoparticle paste described above. Suitable sprayable formulations have a viscosity of from about 1 cP to about 500 cP or from about 1 cP to about 100 cP and can contain a loading of metal nanoparticles in the range of from about 1 wt% to about 35 wt%, or from about 10 wt% to about 25 wt%, or from about 1 wt% to about 10 wt%, or from about 10 wt% to about 15 wt%, with at least about 20% being platelet nanoparticles. Sprayable formulations and inks containing metal nanoparticles can include one or more organic solvents and optionally water, where the metal nanoparticles can be dispersed prior to dispensing onto a substrate. Additional additives can optionally be present in the sprayable formulations and inks as well. The sprayable formulations can be dispensed, as non-limiting examples, using an aerosol propellant, forced pressure, suction, or mechanical pump pressure.

[0081] A method for consolidating metal nanoparticles comprising platelet nanoparticles can include depositing the metal nanoparticles onto a substrate and consolidating the metal nanoparticles to form a bulk metal matrix on the substrate. Suitable substrates are not thought to be particularly limited. In various embodiments, consolidating the metal nanoparticles can include heating the metal nanoparticles above their melting temperature. Solvent removal from the metal nanoparticle paste can be performed during consolidation, which can further facilitate packing of the platelet nanoparticles, as shown in FIG. 6.

[0082] FIG. 6 is a diagram showing how small platelet nanoparticles can be stacked and consolidated. As shown in FIG. 6, the small platelet nanoparticles 500 can first be randomly dispersed in the solvent of the nanoparticle paste 502. As the solvent gradually dissipates from the surface of the substrate, the small platelet nanoparticles 500 can become organized into a stacked configuration 510, and the small platelet nanoparticles 500 can be layered on top of each other either in an overlapping manner or a non-overlapping manner. Following the consolidation of the small platelet nanoparticles 500 at a temperature above the melting temperature, a bulk metal matrix 520 can be formed.

[0083] The embodiments disclosed herein include the following. A. A small platelet nanoparticle composition. The composition is a plurality of metal nanoparticles having a surfactant coating thereon, at least a portion of the metal nanoparticles being small platelet nanoparticles, and the surfactant coating including at least one surfactant, the composition including a plurality of metal nanoparticles. A1. A nanoparticle paste including the composition of A. A2. A sprayable formulation including the composition of A. A3. An ink including the composition of A. B. A method for forming a bulk metal. The method includes depositing the metal nanoparticles of A on a substrate and consolidating the metal nanoparticles to form a bulk metal matrix. C. A method for forming small platelet nanoparticles. The method includes providing a solution including a metal salt dissolved in an organic solvent and at least one surfactant, and adding a reducing agent to the solution at a rate sufficient to form a plurality of metal nanoparticles, at least a portion of the metal nanoparticles being small platelet nanoparticles, and the metal nanoparticles having a surfactant coating including at least one surfactant thereon, the reducing agent being added at a rate sufficient to maintain the solution at a temperature of about 60° C. or less while the metal nanoparticles are being formed.

[0084] Embodiments A - C can optionally have one or more of the following additional elements in any combination. Element 1: The platelet nanoparticles have a thickness in the range of about 5 nm to about 40 nm. Element 2: The platelet nanoparticles have a longitudinal aspect ratio in the range of about 1 to about 100. Element 3: The platelet nanoparticles have a maximum dimension in the range of about 10 nm to about 400 nm. Element 4: The plurality of metal nanoparticles further includes a plurality of substantially spherical metal nanoparticles. Element 5: The metal nanoparticles include copper nanoparticles. Element 6: The at least one surfactant includes at least one amine surfactant. Element 7: The at least one amine surfactant includes two or more amine surfactants, and at least one of the two or more amine surfactants is N,N'-dialkylethylenediamine. Element 8: The platelet nanoparticles are atomically flat. Element 9: Consolidating the metal nanoparticles includes heating the nanoparticle paste above the melting temperature of the metal nanoparticles. Element 10: The reducing agent is added to the solution over a period of about 10 minutes or more. Element 11: The temperature of the solution rises by about 10 °C to about 15 °C while the reducing agent is being added. Element 12: The reducing agent is added at a rate sufficient to maintain the solution at a temperature of about 40 °C to about 60 °C while the metal nanoparticles are being formed.

[0085] As a non-limiting example, exemplary combinations applicable to A to C include 1, 2, and / or 3, and 4; 1, 2, and / or 3, and 5; 1, 2, and / or 3, and 6; 1, 2, and / or 3, and 7; 1, 2, and / or 3, and 8; 1 and 2; 1 and 3; 1 and 4; 1 and 5; 1 and 6; 1 and 7; 1 and 8; 2 and 3; 2 and 4; 2 and 5; 2 and 6; 2 and 7; 2 and 8; 3 and 4; 3 and 5; 3 and 6; 3 and 7; 3 and 8; 4 and 5; 4 and 6; 4 and 7; 4 and 8; 5 and 6; 5 and 7; 5 and 8; 6 and 7; 6 and 8; and 7 and 8, but are not limited thereto. Any of the foregoing may be further combined with one or more of 9, 10, 11, and / or 12. Additional exemplary combinations applicable to C include 10 and 11; 10 and 12; 11 and 12; and 10 to 12, but are not limited thereto.

[0086] Additional embodiments disclosed herein include the following. Embodiment 1. A composition comprising a plurality of metal nanoparticles having a surfactant coating thereon, wherein at least about 20% of the metal nanoparticles are platelet nanoparticles, and the surfactant coating comprises at least one surfactant, the composition comprising a plurality of metal nanoparticles. Embodiment 2. The composition according to Embodiment 1, wherein at least a majority of the metal nanoparticles are platelet nanoparticles. Embodiment 3. The composition according to Embodiment 1, or the composition according to Embodiment 1 or 2, wherein at least about 80% of the metal nanoparticles are platelet nanoparticles. Embodiment 4. The composition according to Embodiment 1, or the composition according to any one of Embodiments 1 to 3, wherein the platelet nanoparticles have a longitudinal thickness in the range of about 5 nm to about 40 nm. Embodiment 5. The composition according to Embodiment 1, or the composition according to any one of Embodiments 1 to 4, wherein the platelet nanoparticles have a longitudinal aspect ratio in the range of about 1 to about 100. Embodiment 6. The composition according to Embodiment 1, or the composition according to any one of Embodiments 1 to 5, wherein the platelet nanoparticles have a maximum dimension in the range of about 10 nm to about 400 nm. Embodiment 7. The composition according to any one of Embodiments 1 to 6, wherein the plurality of metal nanoparticles further comprises a plurality of substantially spherical metal nanoparticles. Embodiment 8. The composition according to any one of Embodiments 1 to 6, wherein the metal nanoparticles comprise copper nanoparticles, or the composition according to any one of Embodiments 1 to 7. Embodiment 9. The composition according to any one of Embodiments 1 to 6, wherein at least one surfactant comprises at least one amine surfactant, or the composition according to any one of Embodiments 1 to 8. Embodiment 10. The composition according to Embodiment 9, wherein at least one amine surfactant comprises two or more amine surfactants. Embodiment 11. The composition according to Embodiment 9, wherein at least one amine surfactant comprises one or more branched amines, or the composition according to Embodiment 9 or 10. Embodiment 12. The composition according to any one of Embodiments 1 to 6, wherein the platelet nanoparticles have a melting temperature in the range of about 180°C to about 240°C, or the composition according to any one of Embodiments 1 to 11. Embodiment 13. A nanoparticle paste comprising the composition according to any one of Embodiments 1 to 6, or the composition according to any one of Embodiments 1 to 12. Embodiment 14. A sprayable formulation comprising the composition according to any one of Embodiments 1 to 6, or the composition according to any one of Embodiments 1 to 12. Embodiment 15. An ink comprising the composition according to any one of Embodiments 1 to 6, or the composition according to any one of Embodiments 1 to 12. Embodiment 16. A method comprising depositing the composition according to any one of Embodiments 1 to 6, or the composition according to any one of Embodiments 1 to 13, onto a substrate, and consolidating the metal nanoparticles to form a bulk metal matrix on the substrate. Embodiment 17. The method according to Embodiment 16, wherein consolidating the metal nanoparticles comprises heating the composition above the melting temperature of the metal nanoparticles. Embodiment 18. The method according to Embodiment 16, or the method according to Embodiment 16 or 17, wherein the platelet nanoparticles have a melting temperature in the range of about 180°C to about 240°C. Embodiment 19. The method according to Embodiment 16, or the method according to any one of Embodiments 16 to 18, wherein the plurality of metal nanoparticles further comprises a plurality of substantially spherical metal nanoparticles. Embodiment 20. The method according to Embodiment 16, or the method according to any one of Embodiments 16 to 19, wherein the metal nanoparticles comprise copper nanoparticles. Embodiment 21. The method according to Embodiment 16, or the method according to any one of Embodiments 16 to 20, wherein the at least one surfactant comprises at least one amine surfactant. Embodiment 22. The method according to Embodiment 21, wherein the at least one amine surfactant comprises two or more amine surfactants. Embodiment 23. The method according to Embodiment 21, or the method according to Embodiment 21 or 22, wherein the at least one amine surfactant comprises one or more branched amines. Embodiment 24. The method according to Embodiment 21, or the method according to any one of Embodiments 21 to 23, wherein at least a portion of the at least one amine surfactant is removed from the metal nanoparticles at a temperature below the melting temperature of the metal nanoparticles when consolidating the metal nanoparticles. Embodiment 25. A method comprising providing a solution comprising a metal salt dissolved in an organic solvent and at least one surfactant, and adding a reducing agent to the solution at a rate sufficient to form a plurality of metal nanoparticles, wherein at least about 20% of the metal nanoparticles are platelet nanoparticles and the metal nanoparticles have a surfactant coating comprising at least a portion of the at least one surfactant, and wherein the reducing agent is added at a rate sufficient to maintain the solution at a temperature of about 70°C or less while the metal nanoparticles are being formed. Embodiment 26. The method according to Embodiment 25, wherein the at least one surfactant comprises at least one amine surfactant. Embodiment 27. The method according to Embodiment 26, or the method according to Embodiment 25 or 26, wherein at least one amine surfactant comprises two or more amine surfactants. Embodiment 28. The method according to Embodiment 26, or the method according to Embodiment 26 or 27, wherein at least one amine surfactant comprises a primary amine, a secondary amine, and a diamine. Embodiment 29. The method according to Embodiment 28, wherein the secondary amine is present in a molar amount higher than the total amount of the primary amine and the diamine. Embodiment 30. The method according to Embodiment 28, wherein the primary amine is present in a molar amount higher than the diamine. Embodiment 31. The method according to Embodiment 26, or the method according to any one of Embodiments 26 to 30, wherein at least one amine surfactant comprises one or more branched amines. Embodiment 32. The method according to Embodiment 25, or the method according to any one of Embodiments 25 to 31, further comprising cooling the solution to room temperature over at least about 30 minutes after adding the reducing agent. Embodiment 33. The method according to any one of Embodiments 25 to 32, wherein at least most of the metal nanoparticles are platelet nanoparticles. Embodiment 34. The method according to any one of Embodiments 25 to 32, or the method according to any one of Embodiments 25 to 33, wherein at least about 80% of the metal nanoparticles are platelet nanoparticles. Embodiment 35. The method according to any one of Embodiments 25 to 32, or the method according to any one of Embodiments 25 to 34, wherein the plurality of metal nanoparticles further comprises a plurality of substantially spherical metal nanoparticles. Embodiment 36. The method according to any one of Embodiments 25 to 32, or the method according to any one of Embodiments 25 to 35, wherein the metal nanoparticles comprise copper nanoparticles. Embodiment 37. The method according to any one of Embodiments 25 to 32, or the method according to any one of Embodiments 25 to 36, wherein the platelet nanoparticles have a longitudinal thickness in the range of about 5 nm to about 40 nm. Embodiment 38. The method according to any one of Embodiments 25 to 32, or the method according to any one of Embodiments 25 to 37, wherein the platelet nanoparticles have a longitudinal aspect ratio in the range of about 1 to about 100. Embodiment 39. The method according to any one of Embodiments 25 to 32, or the method according to any one of Embodiments 25 to 38, wherein the platelet nanoparticles have a maximum dimension in the range of about 10 nm to about 400 nm. Embodiment 40. The method according to any one of Embodiments 25 to 32, or the method according to any one of Embodiments 25 to 39, wherein the platelet nanoparticles have a melting temperature in the range of about 180 °C to about 240 °C. Embodiment 41. The method according to any one of Embodiments 25 to 32, or the method according to any one of Embodiments 25 to 40, wherein the reducing agent is added at a rate sufficient to maintain the solution at a temperature of about 40 °C to about 70 °C while forming the metal nanoparticles. Embodiment 42. The method according to any one of Embodiments 25 to 32, or the method according to any one of Embodiments 25 to 41, wherein the temperature of the solution rises by about 10 °C to about 15 °C while adding the reducing agent. Embodiment 43. The method according to any one of Embodiments 25 to 32, or the method according to any one of Embodiments 25 to 42, wherein external heating is not applied to the solution while forming the metal nanoparticles.

[0087] For easier understanding of the present disclosure, the following examples of preferred or representative embodiments are given. By no means should the following examples be read to limit or define the scope of the present invention.

Examples

[0088] General synthesis conditions for producing platelet nanoparticles. Copper platelet nanoparticles are prepared by slowly adding a reducing agent to reduce a copper salt while maintaining the reaction at a temperature at which the platelet nanoparticles are formed. Briefly, a copper metal precursor (copper salt), such as copper(II) chloride, copper(II) bromide, or copper(II) sulfate, is dispersed in a glyme solvent at a concentration of 0.25 - 11 wt% based on the dissolved metal ions from the salt. Next, a surfactant mixture containing 1, 2, 3, or 4 surfactants is added at a molar ratio of total surfactant to copper metal precursor of at least 4.1:1. After dissolving the copper salt and mixing thoroughly for 2 - 3 hours, a sufficient amount of reducing agent to form copper platelet nanoparticles is slowly added over 7 - 45 minutes such that the temperature exceeds 35 °C. The reaction mixture is then cooled to room temperature over 20 - 90 minutes, and the platelet nanoparticles are isolated by centrifugation.

[0089] Specific synthesis conditions (experiment) for producing platelet nanoparticles. Copper platelet nanoparticles were prepared by slowly reducing copper(II) chloride in glyme using NaBH4. Specifically, the copper salt was dispersed in a glyme solvent at a concentration of at least 0.95 wt% based on the dissolved metal ions, and a surfactant mixture containing a bidentate amine, a primary amine, and a secondary amine was added at an overall molar ratio of amine surfactant to copper of at least 5.7. The individual amine surfactants were present such that the bidentate amine was present in the lowest molar amount and the secondary amine was present in the highest molar amount. After mixing thoroughly, approximately 1.0 - 1.5 equivalents of NaBH4 relative to the dissolved metal ions was slowly added at a constant rate over at least 7 minutes. The addition rate was controlled such that the temperature exceeded 35 °C. Thereafter, the reaction mixture was slowly cooled to room temperature over at least 20 minutes, and the copper platelet nanoparticles were isolated via centrifugation. The isolated copper platelet nanoparticles were washed with water and stored while still wet. The still-wet product can be safely stored at room temperature for several years in a well-sealed container.

[0090] Specific synthesis conditions for generating spherical nanoparticles (comparison). Substantially spherical copper nanoparticles were prepared by rapidly reducing copper(II) chloride in glyme using NaBH4. Specifically, the copper salt was dispersed in the glyme solvent at a concentration of 0.40 - 0.50 wt% based on the dissolved metal ions from the copper salt, and a surfactant mixture containing a bidentate amine, a primary amine, and a secondary amine was added at an overall molar ratio of amine surfactant to copper of at least 4.1. The individual amine surfactants were present such that the bidentate amine and the primary amine were present in approximately equal molar amounts and the secondary amine was present in the maximum molar amount. After thorough mixing, approximately 1.0 equivalent of NaBH4 with respect to the dissolved metal ions was rapidly added over a time period of less than 3 minutes. At the rapid addition rate, the temperature peaked at around 45 °C. Thereafter, the reaction mixture was rapidly cooled to room temperature over a time period not exceeding 10 minutes, and substantially spherical copper nanoparticles were isolated via centrifugation. The isolated copper platelet nanoparticles were washed with water and stored while still wet. The still-wet product can be safely stored at room temperature for several years in a well-sealed container.

[0091] Figure 7 is a histogram of the particle size distribution obtained from a representative copper nanoparticle synthesis in which platelet nanoparticles are produced. Figures 8A - 8D are exemplary SEM images of copper nanoparticles generated at various reducing agent introduction rates. As shown, the platelet nanoparticles may vary considerably in size, and a variable amount of substantially spherical copper nanoparticles may also be present in combination with the platelet nanoparticles. In addition, the platelet nanoparticles can assume a range of semi-regular polygonal shapes. Figure 8D shows that the platelet nanoparticles can readily stack on top of each other even in the as-generated form.

[0092] Copper nanoparticles containing platelet nanoparticles were sintered to produce a free-standing film containing a bulk copper matrix. Briefly, reflow (sintering) was carried out in an inert gas environment at a peak temperature of 235 °C for 4 - 6 minutes in a standard commercially available convection reflow oven (e.g., by Heller Industries), a BTU reflow oven, a Sikama oven, etc. A comparative copper nanoparticle sample containing mainly substantially spherical metal nanoparticles, or a 1:1 mixture of substantially spherical copper nanoparticles and platelet nanoparticles was sintered under the same conditions for comparison. Figure 9 is an illustrative SEM image of bulk copper mainly formed from platelet copper nanoparticles after sintering. As shown, there are few traces of porosity in the film, indicating the high-density packing brought about by the platelet nanoparticles. Figure 10A is a photograph of a sintered thin film produced from copper nanoparticles containing mainly platelet nanoparticles. Figure 10B is a photograph of a sintered thin film produced from a 1:1 mixture of substantially spherical copper nanoparticles and platelet copper nanoparticles. Figure 10C is a photograph of a comparative sintered thin film produced from copper nanoparticles containing substantially spherical metal nanoparticles. Each of the sintered thin films had a thickness of 25 μm. The sintered thin film mainly produced from platelet copper nanoparticles (Figure 10A) was free-standing, light pink in color, and appeared to contain a substantially continuous metal layer. The film quality decreased substantially as the input amount of substantially spherical copper nanoparticles increased (Figure 10B). The comparative sintered thin film produced from substantially spherical copper nanoparticles (Figure 10C) was more faded and more in line with the bronze color of metallic copper, but had significantly more cracks and was more brittle than the sintered thin film produced from platelet nanoparticles alone. The comparative thin film contained multiple melted copper regions, but its long-term integrity was limited. The electrical resistance of the comparative sintered thin film was about 4 - 30 times higher than that of the sintered thin film produced from copper nanoparticles mainly having a platelet morphology. Depending on the thickness, the sheet resistance value for the thin film produced from platelet nanoparticles was below about 1 - 3 mOhm / square, while when using substantially spherical metal nanoparticles, the sheet resistance value was in the range of 5 - 10 mOhm / square or more.

[0093] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and the like used in this specification and the appended claims are to be understood as being modified in all instances by the term "about." Accordingly, unless otherwise indicated, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the embodiments of the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the reported number of significant digits and by applying ordinary rounding techniques.

[0094] One or more exemplary embodiments incorporating the features of the present disclosure are presented herein. For clarity, not all features of a physical implementation are described or illustrated in this application. It is to be understood that in developing a physical implementation incorporating the present disclosure, numerous implementation-specific decisions must be made to achieve the developer's goals, such as and varying with the implementation, compliance with system-related, business-related, government-related, and other constraints. While the developer's efforts may require substantial time, such efforts would be routine for those of ordinary skill in the art and beneficiaries of this disclosure.

[0095] Accordingly, the present disclosure is well adapted to attain the ends and advantages mentioned as well as those inherent therein. Since the present disclosure may be modified and embodied in different but equivalent manners apparent to those skilled in the art who will enjoy the benefits of the teachings herein, the specific embodiments disclosed above are only illustrative. Furthermore, no limitation is intended with respect to the details of construction or design shown herein other than as described in the following claims. Accordingly, the specific illustrative embodiments disclosed above may be varied, combined, or modified, and it is evident that all such variations are within the scope and spirit of the invention. The disclosure herein is capable of being suitably practiced without the presence of any element and / or any optional element disclosed herein that is not specifically disclosed. Compositions and methods are described using the terms "comprising," "containing," or "including" various components or steps, but the compositions and methods may also "consist essentially of" or "consist of" various components and steps. All of the numbers and ranges disclosed above may vary by some amount. Whenever a numerical range is disclosed using a lower limit and an upper limit, any number and any range falling within the range are specifically disclosed. In particular, all ranges of values (in the form of "from about a to about b," or equivalently "from approximately a to b," or equivalently "from approximately a-b") disclosed herein are to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain ordinary meaning unless expressly and clearly defined by the patentee. Further, the indefinite articles "a" or "an" used in the claims are defined herein to mean one or more of the elements that they introduce.

Claims

**Claim 1** A composition comprising a plurality of metal nanoparticles having a surfactant coating thereon, wherein at least about 20% of said metal nanoparticles are platelet nanoparticles, and said surfactant coating comprises at least one surfactant, the composition comprising a plurality of metal nanoparticles. **Claim 2** The composition according to claim 1, wherein at least a majority of said metal nanoparticles are platelet nanoparticles. **Claim 3** The composition according to claim 1, wherein at least about 80% of said metal nanoparticles are platelet nanoparticles. **Claim 4** The composition according to claim 1, wherein said platelet nanoparticles have a longitudinal thickness in the range of about 5 nm to about 40 nm. **Claim 5** The composition according to claim 1, wherein said platelet nanoparticles have a longitudinal aspect ratio in the range of about 1 to about 100. **Claim 6** The composition according to claim 1, wherein said platelet nanoparticles have a maximum dimension in the range of about 10 nm to about 400 nm. **Claim 7** The composition according to any one of claims 1 to 6, wherein said plurality of metal nanoparticles further comprises a plurality of substantially spherical metal nanoparticles. **Claim 8** The composition according to any one of claims 1 to 6, wherein said metal nanoparticles comprise copper nanoparticles. **Claim 9** The composition according to any one of claims 1 to 6, wherein said at least one surfactant comprises at least one amine surfactant. **Claim 10** The composition according to claim 9, wherein said at least one amine surfactant comprises two or more amine surfactants. **Claim 11** The composition according to claim 9, wherein said at least one amine surfactant comprises one or more branched amines. **Claim 12** The composition according to any one of claims 1 to 6, wherein said platelet nanoparticles have a melting temperature in the range of about 180 °C to about 240 °C. **Claim 13** A nanoparticle paste comprising the composition according to any one of claims 1 to 6. **Claim 14** A sprayable formulation comprising the composition according to any one of claims 1 to 6. **Claim 15** An ink comprising the composition according to any one of claims 1 to 6. **Claim 16** A method comprising depositing the composition according to any one of claims 1 to 6 on a substrate, and consolidating said metal nanoparticles to form a bulk metal matrix on said substrate. **Claim 17** The method according to claim 16, wherein consolidating the metal nanoparticles comprises heating the composition above the melting temperature of the metal nanoparticles.

18. The method according to claim 16, wherein the platelet nanoparticles have a melting temperature in the range of about 180 °C to about 240 °C.

19. The method according to claim 16, wherein the plurality of metal nanoparticles further comprises a plurality of substantially spherical metal nanoparticles.

20. The method according to claim 16, wherein the metal nanoparticles comprise copper nanoparticles.

21. The method according to claim 16, wherein the at least one surfactant comprises at least one amine surfactant.

22. The method according to claim 21, wherein the at least one amine surfactant comprises two or more amine surfactants.

23. The method according to claim 21, wherein the at least one amine surfactant comprises one or more branched amines.

24. The method according to claim 21, wherein at least a portion of the at least one amine surfactant is removed from the metal nanoparticles below the melting temperature of the metal nanoparticles when consolidating the metal nanoparticles.

25. A method comprising: providing a solution comprising a metal salt and at least one surfactant dissolved in an organic solvent; adding a reducing agent to the solution at a rate sufficient to form a plurality of metal nanoparticles, wherein at least about 20% of the metal nanoparticles are platelet nanoparticles and the metal nanoparticles have a surfactant coating comprising at least a portion of the at least one surfactant; The method, wherein the reducing agent is added at a rate sufficient to maintain the solution at a temperature of about 70 °C or lower while the metal nanoparticles are being formed.

26. The method according to claim 25, wherein the at least one surfactant comprises at least one amine surfactant.

27. The method according to claim 26, wherein the at least one amine surfactant comprises two or more amine surfactants.

28. The method according to claim 26, wherein the at least one amine surfactant comprises a primary amine, a secondary amine, and a diamine.

29. The method according to claim 28, wherein the secondary amine is present in a molar amount higher than the total amount of the primary amine and the diamine.

30. The method according to claim 28, wherein the primary amine is present in a molar amount higher than that of the diamine.

31. The method according to claim 26, wherein the at least one amine surfactant comprises one or more branched amines.

32. The method according to claim 25, further comprising cooling the solution to room temperature over at least about 30 minutes after adding the reducing agent.

33. The method according to any one of claims 25 to 32, wherein at least most of the metal nanoparticles are platelet nanoparticles.

34. The method according to any one of claims 25 to 32, wherein at least about 80% of the metal nanoparticles are platelet nanoparticles.

35. The method according to any one of claims 25 to 32, wherein the plurality of metal nanoparticles further comprises a plurality of substantially spherical metal nanoparticles.

36. The method according to any one of claims 25 to 32, wherein the metal nanoparticles comprise copper nanoparticles.

37. The method according to any one of claims 25 to 32, wherein the platelet nanoparticles have a longitudinal thickness in the range of about 5 nm to about 40 nm.

38. The method according to any one of claims 25 to 32, wherein the platelet nanoparticles have a longitudinal aspect ratio in the range of about 1 to about 100.

39. The method according to any one of claims 25 to 32, wherein the platelet nanoparticles have a maximum dimension in the range of about 10 nm to about 400 nm.

40. The method according to any one of claims 25 to 32, wherein the platelet nanoparticles have a melting temperature in the range of about 180 °C to about 240 °C.

41. The method according to any one of claims 25 to 32, wherein the reducing agent is added at a rate sufficient to maintain the solution at a temperature of about 40 °C to about 70 °C while forming the metal nanoparticles.

42. The method according to any one of claims 25 to 32, wherein the temperature of the solution rises by about 10 °C to about 15 °C while adding the reducing agent.

43. The method according to any one of claims 25 to 32, wherein external heating is not applied to the solution while forming the metal nanoparticles.

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