MXene compounds having novel crystal forms and a process for producing MAX phase compounds by synthesizing said MXene compounds

Abstract

The present invention provides a process for producing MXene compounds having a novel crystalline morphology and a MAX phase type compound for synthesizing said MXene compounds. The present invention relates firstly to an MXene compound having the advantage of having a predominantly plate-like crystalline morphology, which can be obtained from a MAX phase precursor obtained by a spark plasma sintering process of insulating powder mixtures, and a process for producing said MXene compounds. The present invention also relates to a MAX phase type compound obtained by a spark plasma sintering process of insulating powder mixtures. The present invention also relates to a process for synthesizing MXene compounds from said precursors, and to an MXene compound obtained by this process having the advantage of having a predominantly plate-like crystalline morphology.

Description

[Technical Field]

[0001] The present invention relates to lamellar MXene compounds synthesized from precursors called MAX phases.

[0002] The present invention relates to a process for producing MAX phase type compounds, MAX phase type compounds obtained by said process, an apparatus for carrying out said process, and a novel type of MXene compound and related manufacturing process. [Background technology]

[0003] The MXene compound was discovered in 2011 (Non-Patent Document 1). This compound is n+1 X n wherein n is 1, 2 or 3, M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta, and X is C or N; alternatively, M n+1 X n T x where T corresponds to a terminal group selected from O, OH, F or any other halogen group, and S or any other chalcogen group. This compound is described and characterized in U.S. Pat. No. 5,699,499.

[0004] MXene is a lamellar compound, i.e., a two-dimensional material (2D), that can be exfoliated to form single sheets with approximately 1 nm thick repeat units, represented by MXM, MXMXM, or MXMXMX when n is 1, 2, or 3, respectively. -M It has the following structure.

[0005] These materials have many applications, particularly due to their lamellar structure, electrical conductivity, and / or ability to insert and desorb species in the interlayer space. The most studied MXene for this purpose is Ti3C2T x It has the formula:

[0006] While most literature focuses on MXene after delamination, several papers, including [2], have used scanning electron microscopy (SEM) to clarify the shape of MXene particles before delamination. These particles are bulk particles without a defined geometric shape. They also exhibit a wide dispersion in both size and shape. The structure of these particles may limit their conductive properties and the intercalation rate of other species within the crystals. For example, it can be assumed that the interconnectivity between crystals is low when assembled to create thin layers useful for various applications. Furthermore, the large dispersion in particle size and shape makes it difficult to easily obtain thin sheets with a uniform size distribution after delamination (which is useful for some applications), necessitating additional size selection procedures.

[0007] In known methods, these MXene compounds are synthesized from precursors commonly referred to as "MAX phase type compounds" or "MAX phase compounds." These MAX phase compounds are n+1 AX n where n is 1, 2 or 3, M and X are the same as in MXene, and A is selected from Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl and Pb. MAX phase compounds have a structure similar to MXene, but with the addition of M n+1 X n They further contain element A present in the form of layers between the repeating units. To produce MXenes, element A is chemically removed from the MAX phase type precursor. In most cases, MAX phase precursors in which A is aluminum are preferred, and a related method consists of chemically attacking the MAX precursor with an aqueous acid containing hydrofluoric acid. This results in M n+1 X n Aluminum is removed from the space between the repeating units, forming the terminal group T xIn most cases, T is -F, -OH, or =O, and is bonded to the element M located outside the repeat unit. To ensure a good yield of the chemical attack, the MAX precursor is pre-milled. After the chemical attack, the product is washed and centrifuged several times to remove excess reagents used in the attack and soluble by-products such as AlF3. If the goal is to obtain single sheets (2D) of MXene instead of three-dimensional particles, an additional delamination step may be performed. One variation of the attack process with hydrofluoric acid solution is the addition of a fluoride salt, typically Li, as described in the aforementioned non-patent document 2. F and a strong acid, typically HCl.

[0008] Regarding the synthesis of MAX phase precursors, the most widespread process for producing MXene, especially of the formula Ti3AlC2, is to add a mixture of carbide or nitride powder of metal M (e.g., MC or MN), metal A (e.g., Al), and metal M to a tube furnace under a flow of inert gas. n+1 AX n The key is to provide the elements in the amounts necessary to reach the stoichiometric amount. In the specific case of Ti3AlC2, the reaction mixture consists of TiC, Al, and Ti powders. The aluminum in the starting mixture may be in slight excess of the stoichiometry. Mixing of these three elements (e.g., Ti, Al, and C) is usually avoided because the reaction is highly exothermic and there is a risk of explosion. The use of a tube furnace under inert gas flow requires a temperature ramp up to temperatures on the order of 1300°C to 1500°C over several hours, with a temperature hold at the maximum temperature for several hours. Other synthesis processes exist, relying on higher pressures and / or more difficult-to-control conditions and / or using other reaction mixtures.

[0009] Prior to the first synthesis of MXene, the authors in Non-Patent Document 3 synthesized a MAX phase precursor by spark plasma sintering (SPS) using a mixture of TiC, Ti, Al, and approximately 20% Si relative to the Al. The mixture was fed into a graphite sample holder, typical of SPS equipment, and the product was obtained in the form of a highly compact pellet with no porosity. This lack of porosity makes the synthesis of MXene from the MAX phase precursor described above difficult and time-consuming. This is because, in the prior art, a reasonable synthesis rate for MXene compounds requires the MAX precursor to have a submillimeter-sized powder morphology, and obtaining this powder from a compact pellet is difficult. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] International Publication No. 2021 / 177712 [Non-patent literature]

[0011] [Non-Patent Document 1] Naguib et al. Adv. Master. 23, 4248, 2011 [Non-patent document 2] Alhabeb et al. Chem. Mater. 29, 7633, 2017 [Non-patent document 3] Zhou et al., J. Mater. Scie. 40, 2099, 2005 Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention therefore aims to produce MXene compounds with a smaller dispersion in particle shape and size than the prior art, which not only allows for an increased seed insertion rate within each crystal and a better interconnectivity between these crystals during the assembly of the crystal layers, but also makes it possible to obtain, via delamination, sheets with a more uniform particle size distribution than those reported in the prior art.

[0013] The present invention also relates to the preparation of MAX phase precursors and related manufacturing processes that ensure in particular the obtaining of the above-mentioned MXene compounds. [Means for solving the problem]

[0014] To this end, the present invention provides n+1 X n T x wherein n is 1, 2 or 3, M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta, X is C or N, and T corresponds to an end group selected from O, OH, F or any other halogen group, and S or any other chalcogen group, predominantly in the form of a crystal having a general plate shape, characterized in that the plate has two opposing parallel surfaces of a defined length L spaced apart by a defined height H; and The length of the plate-shaped crystal is 1 μm to 15 μm, The height H of the plate-shaped crystal is 0.2 μm to 1 μm, and The compound is characterized in that the flat aspect ratio of the plate-shaped crystal, defined by the ratio of the length L to the height H, is 5 to 50.

[0015] The compounds of the invention may also comprise any of the following characteristics, considered individually or according to all possible combinations of the technique: Ti3C2T x It has the formula: M n+1 AX nwherein n is 1, 2 or 3, M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta, A is selected from Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl and Pb, and X is C or N; and in the form of pellets with a porosity of more than 30%, preferably more than 40%, and having a stoichiometric element ratio M / X within a relative error of less than ±0.5%; Obtained from a MAX phase type precursor compound. The MAX phase type precursor compound has the formula Ti3AlC2. Most of the above MAX phase type precursor compounds have morphologies that exhibit the following two types of grain cross sections. a rectangular flat cross section having a length of 1 μm to 20 μm and a height of 0.5 μm to 2 μm, and / or Rounded corner flat cross section with diagonal length of 1μm to 20μm.

[0016] The present invention also provides the above-mentioned M n+1 X n T x The present invention relates to a process for producing a compound having the general formula:

[0017] Furthermore, the present invention provides the above-mentioned M obtained from a MAX phase type precursor compound. n+1 X n T x 1. A process for producing a compound of the general formula: wherein the MAX phase type precursor compound is obtained by a spark plasma sintering process in a spark plasma sintering apparatus comprising a graphite die defining a hollow chamber and two graphite punches, the spark plasma sintering process comprising: mixing precursor powders; placing the powder mixed in the above step into a closed container made of insulating ceramic material housed in the hollow chamber; performing a spark plasma sintering operation; obtaining pellets of the MAX phase type compound; The present invention relates to a process characterized by comprising at least the following:

[0018] The process of the invention may also include the following optional features, considered individually or according to all possible technical combinations: The container is made of alumina. The powder is not in contact with the two punches, is not directly exposed to the applied current, and is insulated from the applied pressure, which is applied during the spark plasma sintering operation. The spark plasma sintering operation includes at least one thermal cycle applying a heating rate of more than 60° C. / min. The above thermal cycle is A first temperature ramp is performed at a rate of greater than 60°C / min to a temperature of 550°C to 700°C and held for 2 to 15 minutes, followed by a second temperature increase in which the temperature is increased to 1400°C to 1500°C at a rate of more than 60°C / min and maintained for 5 to 15 minutes; Includes: The process involves two steps of chemically attacking the MAX phase precursor compound with aqueous acid.

[0019] The present invention also provides the above-mentioned M n+1 X n T x a spark plasma sintering apparatus for carrying out a process for producing a MAX precursor compound for use in a process for producing a compound of the general formula:

[0020] Furthermore, the present invention provides a spark plasma sintering apparatus having a graphite die and two graphite punches that define a hollow chamber, and n+1AX n wherein n is 1, 2 or 3; M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta; A is selected from Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl and Pb; and X is C or N, comprising: mixing the powders; placing the powder mixed in the above step into a closed container made of insulating ceramic material housed in the hollow chamber; performing a spark plasma sintering operation; obtaining pellets of the MAX phase type compound; The present invention relates to a process characterized by comprising at least the following:

[0021] The spark plasma sintering process of the present invention may also include the following optional features, considered individually or according to all possible technical combinations: The container is made of alumina. The powder is not in contact with the two punches, is not directly exposed to the applied current, and is insulated from the applied pressure, which is applied during the spark plasma sintering operation. The spark plasma sintering operation includes at least one thermal cycle applying a heating rate of more than 60° C. / min. The above thermal cycle is A first temperature ramp is performed at a rate of greater than 60°C / min to a temperature of 550°C to 700°C and held for 2 to 15 minutes, followed by a second temperature increase in which the temperature is increased to 1400°C to 1500°C at a rate of more than 60°C / min and maintained for 5 to 15 minutes; Includes.

[0022] The present invention also provides M n+1 AX nwherein n is 1, 2 or 3, M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta, A is selected from Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl and Pb, and X is C or N, which is obtainable by the spark plasma sintering process as described above, in the form of pellets with a porosity of more than 30%, preferably more than 40%, and which has a stoichiometric element ratio M / X within a relative error of less than ±0.5%.

[0023] The MAX phase type compounds of the present invention may also comprise any of the following characteristics, considered individually or according to all possible technical combinations: It has the formula Ti3AlC2. Most of them have morphologies showing the following two types of particle cross sections. a rectangular flat cross section having a length of 1 μm to 20 μm and a height of 0.5 μm to 2 μm, and / or Rounded corner flat cross section with diagonal length of 1μm to 20μm.

[0024] The present invention further relates to a spark plasma sintering apparatus for carrying out the spark plasma sintering process of the present invention described above, comprising a graphite die and two graphite punches defining a hollow chamber, the spark plasma sintering apparatus also comprising an enclosure made of an insulating ceramic material, for example alumina, located within the hollow chamber and capable of receiving a powder mixture during application of the spark plasma sintering process and maintaining the powder mixture insulated from applied current and pressure, wherein the current and pressure are applied during the spark plasma sintering operation.

[0025] Furthermore, the present invention provides M n+1 X nwherein n is 1, 2 or 3, M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta, and X is C or N, characterized in that it is produced from the MAX phase type compound of the present invention as described above and / or is obtained by the spark plasma sintering process of the present invention as described above.

[0026] Advantageously, M n+1 X n T x wherein n is 1, 2 or 3, M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta, X is C or N, and T corresponds to an end group selected from O, OH, F or any other halogen group, and S or any other chalcogen group, in the form of crystals having a general plate shape, the plate having two opposing parallel surfaces of a defined length L spaced apart by a defined height H, and The length of the plate-shaped crystal is 1 μm to 15 μm, The height H of the plate-shaped crystal is 0.2 μm to 1 μm, and The flat aspect ratio of the plate-shaped crystal, which is defined by the ratio of the length L to the height H, is 5-50.

[0027] More advantageously, the compound is Ti3C2T x It has the formula:

[0028] Finally, the present invention provides n+1 X n The present invention relates to a process for producing such compounds having the general formula:

[0029] Advantageously, the MAX phase type precursor compound of the invention is as described above and / or is obtained by the inventive spark plasma sintering process as described above.

[0030] Other characteristics and advantages of the invention will become apparent from the following non-limiting description, given by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0031] [Figure 1] 1 is a schematic diagram of an apparatus of the present invention used to carry out a spark plasma sintering process (SPS) of the present invention to produce a MAX phase type precursor of the present invention. [Figure 2A] 1 is a scanning electron microscope (SEM) photograph taken at 600x magnification of a cross section of a pellet of the MAX phase precursor of the present invention obtained by the spark plasma sintering process of the present invention, the pellet having been coarsely crushed. [Figure 2B] 1 is a scanning electron microscope (SEM) photograph taken at 2500x magnification of a cross section of a pellet of the MAX phase precursor of the present invention obtained by the spark plasma sintering process of the present invention, the pellet having been coarsely crushed. [Figure 3A] 1 is a scanning electron microscope (SEM) photograph of a prior art MAX phase precursor taken at 600x magnification. [Figure 3B] 1 is a scanning electron microscope (SEM) photograph of a prior art MAX phase precursor taken at 2500x magnification. [Figure 4A] 1 is a scanning electron microscope (SEM) photograph taken at 600x magnification of an MXene compound of the present invention obtained from a MAX phase precursor of the present invention by a chemical attack process of the present invention. The precursor in pellet form was coarsely ground. [Figure 4B] 1 is a scanning electron microscope (SEM) photograph taken at 2500x magnification of an MXene compound of the present invention obtained from a MAX phase precursor of the present invention by a chemical attack process of the present invention. The precursor in pellet form was coarsely ground. [Figure 4C] 1 is a scanning electron microscope (SEM) photograph taken at 4500x magnification of an MXene compound of the present invention obtained from a MAX phase precursor of the present invention by a chemical attack process of the present invention. The precursor in pellet form was coarsely ground. [Figure 5A]1 is a scanning electron microscope (SEM) photograph taken at 600x magnification of an MXene compound of the present invention having the formula Ti3C2Tx obtained from a MAX phase-type precursor of the present invention by a chemical attack process of the present invention. The precursor in pellet form was pulverized. [Figure 5B] 1 is a scanning electron microscope (SEM) photograph taken at 2500x magnification of an MXene compound of the present invention having the formula Ti3C2Tx obtained from a MAX phase-type precursor of the present invention by a chemical attack process of the present invention. The precursor in pellet form was pulverized. [Figure 5C] 1 is a scanning electron microscope (SEM) photograph taken at 4500x magnification of an MXene compound of the present invention having the formula Ti3C2Tx obtained from a MAX phase-type precursor of the present invention by a chemical attack process of the present invention. The precursor in pellet form was pulverized. [Figure 6] 1 is an X-ray diffraction diagram of an MXene compound of the present invention having the formula Ti3C2Tx, where the boxed areas correspond to broadening of low intensity peaks. [Figure 7A] 3A and 3B are scanning electron microscope (SEM) photographs taken at 600x magnification of MXene-type compounds obtained by applying two chemical attacks according to the present invention to the prior art MAX phase precursors of FIGS. 3A and 3B. [Figure 7B] 3A and 3B are scanning electron microscope (SEM) photographs taken at 2500x magnification of MXene-type compounds obtained by applying two chemical attacks according to the present invention to the prior art MAX phase precursors of FIGS. 3A and 3B. [Figure 7C] 3A and 3B are scanning electron microscope (SEM) photographs taken at 4500x magnification of MXene-type compounds obtained by applying two chemical attacks according to the present invention to the prior art MAX phase precursors of FIGS. 3A and 3B. [Figure 8] 1 is an energy dispersive X-ray (EDX) spectrum of an MXene compound of the present invention having the formula Ti3C2Tx. [Figure 9] 1 is an energy dispersive X-ray (EDX) spectrum of an MXene compound obtained from a prior art MAX phase precursor. [Figure 10] 2A and 2B are X-ray diffraction diagrams of the MAX phase compounds of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] The present invention relates primarily to MXene compounds obtainable from the MAX phase precursors of the present invention, as well as to processes for producing MXene compounds, including the inventive process and associated apparatus for producing the MAX phase precursors.

[0033] The present invention also relates to a first innovative process and associated apparatus for producing a MAX phase precursor, which is subsequently used to produce the MXene compounds of the present invention according to a second innovative manufacturing process.

[0034] The process for producing MAX phase precursors, particularly but not exclusively, having the formula Ti3AlC2, is based on the known spark plasma sintering (SPS) technique.

[0035] 1, and in a known manner, this process involves the use of an apparatus 1 comprising a graphite die 2 and lower 3a and upper 3b punches defining a hollow chamber 4. This single configuration corresponds to SPS equipment widely sold and used to perform this sintering technique.

[0036] In the present invention, the apparatus 1 further includes a closed hollow vessel 5 housed within the hollow chamber 4. The closed vessel 5 is made of a hollow cylinder 6 made of a high-temperature resistant, chemically neutral insulating material, and is composed of disk-shaped lower and upper covers 7a and 7b made of the same material and attached to both ends of the hollow cylinder 6. For example, the hollow cylinder 6 and covers 7a and 7b can be made of alumina or another insulating ceramic material. The powder mixture used to synthesize the MAX phase precursor is placed in the alumina hollow vessel 5, followed by the known procedures of applying a vacuum and increasing the temperature. After cooling, a cylindrical pellet is obtained.

[0037] To synthesize a MAX phase precursor with the composition Ti3AlC2, commercially available titanium, aluminum, and titanium carbide powders are mixed in an agate mortar. To more finely mix the three components, the mixture is passed through a ball mill. The Ti:Al:TiC mixture can be stoichiometric (1:1:2 elemental ratio, respectively, within a relative error of less than ±0.5%), but a slight excess of aluminum is preferred, particularly a 1:1.1:2 elemental ratio, respectively. That is, the elemental ratio Ti / C is stoichiometric within a relative error of less than ±0.5%.

[0038] With the hollow cylinder 6 and its lower cover 7a already in place, the powder mixture is loaded into the hollow alumina container 5 of the apparatus of FIG. 1. The powder is compacted within the hollow container 5, after which the upper cover 7b of the container 5 and the upper graphite punch 3b are placed in place. After applying a vacuum, a thermal cycle is performed that includes a rapid temperature ramp (approximately 15-30 minutes) to a temperature of approximately 1400°C-1600°C, more specifically 1450°C, over a period of 5-15 minutes. More typically, the ramp rate is greater than 60°C / min. More specifically, the thermal cycle includes a first ramp to a temperature of 550°C-700°C at a rate greater than 60°C / min and a hold time of 2-15 minutes, followed by a second ramp to a temperature of 1400°C-1500°C at a rate greater than 60°C / min and a hold time of 5-15 minutes.

[0039] A cylindrical porous pellet is obtained, and as shown below, this porosity is essential for the synthesis of MXene compounds.

[0040] As detailed in Comparative Example 1, MAX phase precursor synthesis trials were conducted using conventional SPS techniques (without an alumina hollow container). In these trials, the powder mixture was loaded into an assembly formed by a graphite die and punch. XRD analysis showed that these trials yielded a MAX phase mixture with other peaks of unidentified crystalline phases. The strong presence of titanium carbide (TiC) was also observed. The pellets were also very dense, compact, and not porous. They were difficult to crush and difficult to chemically attack.

[0041] These differences between the MAX phase precursors obtained by known SPS techniques and those obtained by the process and apparatus of the present invention may be due to several factors. Using conventional SPS techniques, additional reactions between the powder mixture and the carbon of the graphite assembly may occur. Additionally or concomitantly, the powder mixture is subjected to high pressures similar to those experienced by the graphite punch. Also concomitantly, an electric current is passed through the powder reaction mixture, which may cause electromigration of one of the components.

[0042] Therefore, in contrast to the usual use of SPS equipment, in the present invention the powder mixture is not in contact with graphite, is not directly exposed to applied current, and is insulated from applied pressure, which allows the synthesis of innovative MAX phase precursors in the form of porous pellets, as shown below, and allows the synthesis of MXene compounds with specific crystalline configurations.

[0043] The second process of the present invention is a process for synthesizing MXene compounds by chemically attacking the MAX phase precursor obtained by the process of the present invention described above. The chemical attack is carried out using an aqueous hydrofluoric acid solution. More specifically, an in situ formed aqueous hydrofluoric acid solution, more specifically, a mixture of a fluoride salt such as lithium fluoride and a strong acid such as hydrochloric acid, is used, with a fluorine species concentration of less than 5M.

[0044] In the present invention, at least two chemical attacks are carried out, each additional attack step consisting of recovering the product and a new attack with a hydrofluoric acid solution.

[0045] Most of the crystals have a morphology consisting of flat crystals, especially Ti3C2T xThe resulting MXene compound has the formula: These plates have two parallel planes, and the flattened aspect ratio, defined as the ratio of the plate's length L to its height H, is between 5 and 50, with an average of about 10. The crystal size distribution is relatively uniform, with the majority of crystals having lengths between 1 and 15 microns and heights H between 0.2 and 1 micron. This distribution and flattened aspect ratio are evaluated by counting the distance measured in scanning electron microscope (SEM) images and by measurements on the SEM images, respectively.

[0046] This two-step chemical attack process for preparing MXene compounds was applied to coarsely ground MAX phase precursors of the present invention, as well as to finely ground MAX phase precursors of the present invention. As shown in detail below, in both cases, when the MAX phase precursors are milled prior to chemical attack, an improved particle size distribution is observed, while the bulk morphology of the crystals remains the same, being platelet-like as defined above.

[0047] For comparison, the same process for preparing the MXene compound by two chemical attacks was also applied to a commercially available MAX phase precursor. The morphology of the resulting MXene compound was completely different from that of the present invention (Comparative Example 2 and Figures 7A, 7B, and 7C). These results demonstrate that the plate-shaped MXene compound of the present invention is obtained through the specificity of the MAX phase precursor of the present invention. This specificity is believed to induce the special crystalline morphology of the MXene compound of the present invention, and subsequent two chemical attacks do not degrade this crystalline morphology.

[0048] Morphological features are revealed by visual observation and measurement of multiple crystals in several samples in scanning electron microscope (SEM) images, performed at several points on each sample. To our knowledge, to date, there are no reliable quantitative techniques to replace imaging for measuring the oblate aspect ratio and particle size distribution when particles cannot be approximated as spherical particles and when their large size makes them difficult to place in suspension.

[0049] Examples of the crystalline forms of the MXene compounds of the present invention are shown in Figures 4A, 4B, and 4C. These are Ti3C2T of the present invention having the above-mentioned crystalline forms. x SEM images of the compound are shown at several magnifications.

[0050] The compounds obtained by the process of the present invention are of the "MXene" type, in particular Ti3C2T x The fact that this corresponds to a compound with the composition of was confirmed by two techniques: analysis of the crystalline phase by X-ray powder diffraction (XRD) using a Cu Kα radiation source (Figure 6), and energy dispersive X-ray spectroscopy (EDX) using an EDX detector integrated into the SEM (Figure 8).

[0051] As shown in Figure 6, the X-ray diffraction pattern of the MXene compound of the present invention obtained by the above-mentioned process is characterized by a large peak designated by the symbol 9, located at an angle 2θ of 6.5° to 6.9°. This corresponds to the (hkl) = (002) plane and is the origin of Ti3C2T x The peaks at higher angles are also observed at lower densities. These peaks correspond primarily to replicas of the (002) plane, and therefore correspond to multiples of the angle of the main peak.

[0052] Ti3C2T x Elemental analysis of the MXene compound of the present invention having the formula was performed by energy dispersive X-ray (EDX) spectroscopy. Referring to Figure 8, the EDX reveals a composition of Ti and C, with no Al present (no peak at approximately 1.5 keV, in contrast to the EDX of the MXene compound obtained from the prior art MAX phase precursor shown in Figure 9). This indicates that the main structural unit of the MXene compound has a composition of Ti3C2, and the T corresponding to -OH or =O, -F, and -Cl groups. xThe atomic ratios of the elements forming the end groups to Ti, i.e., Cl / Ti, F / Ti, and O / Ti, are 0.05, 0.5, and 0.4, respectively, on average, according to quantitative EDX analysis of a sample obtained according to the procedure of Example 2 described below.

[0053] By changing the synthesis or post-treatment conditions, the terminal group T x The composition of the MXene may be different, or one type of end group may be absent, but even in such cases the resulting compound will still be of the MXene type, especially Ti3C2T x It should be noted that in the MXene compound, aluminum is absent. This is in contrast to the case where the chemical attack involves only one step, even if the attack continues for a long time (more than 48 hours), and excess F relative to the strongly acidic medium and MAX phase precursor is still present at the end of the attack step. In this case, a single chemical attack was observed to result in an Al / Ti ratio of approximately 0.01, which corresponds to the Al / Ti ratio of 0.003 to 0.01 in the aforementioned non-patent document 2. [Example]

[0054] Example 1: Synthesis of MAX phase precursor with composition Ti3AlC2 A spark plasma sintering apparatus was used, which is commercially available from Fuji Electric Industrial Co., Ltd. under the name of Dr. Sinter Lab. Jr. (Model: SPS-211Lx). This apparatus was modified in accordance with the present invention as described with reference to FIG. 1, by retaining the graphite mold 2 and two graphite punches 3a and 3b, but adding a hollow alumina cylinder 6 and an upper 7 forming a hollow alumina container 5. b and a lower 7a alumina disk were added to the hollow chamber 4. The powder mixture described below was placed in the container 5 and compressed, keeping it electrically isolated from the current passing through the graphite punch and die. The mixture was also kept isolated from the pressure exerted by the punch and was kept out of contact with the graphite throughout the operation.

[0055] Commercially available titanium, aluminum, and titanium carbide powders were mixed in an agate mortar in an elemental ratio of 1:1.1:2, respectively, for a total powder mixture weight of approximately 5 g. The mixture was placed in a ball mill equipped with a tungsten carbide bowl and balls and ground at 300 rpm for 1 hour and 15 minutes. With the lower cover 7a already in place within the apparatus, 1 g of ground powder was placed into the hollow cylinder 6. powder The upper cover 7b was placed on top of the hollow cylinder 6, thereby forming the hollow container 5. With the graphite upper punch 3b in place, the entire assembly 1 was placed in an SPS apparatus and a vacuum was applied. The following thermal cycle was applied: a 6-minute ramp to 580°C, a 5-minute hold at 580°C, a 12-minute ramp to 1450°C, and an 8-minute hold at 1450°C. Upon completion of the thermal cycle, the temperature rapidly decreased to below 580°C over approximately 5-10 minutes. A cylindrical porous pellet was obtained. The pellet porosity was greater than 30%, most often greater than 40%, and most often about or greater than 50%. The porosity was assessed by the difference between the measured volume of the pellet and the volume of the compressed material calculated from the pellet's theoretical density and weight.

[0056] Figures 2A and 2B show the morphology of porous pellets of the MAX phase precursor obtained according to the present invention after coarse grinding. These images are compared with images of a commercially available powder of the MAX phase compound in Figures 3A and 3B. A clear difference is observed between the two compounds. In contrast to the particles bonded together in the precursor of the present invention, the MAX phase compounds of the prior art are characterized by clusters of particles with a large dispersion of particle sizes (tens of nanometers to tens of micrometers). The images in Figures 3A and 3B are similar to the MAX phase compounds obtained and published in the above-mentioned literature.

[0057] Furthermore, as shown in Figure 2B, the MAX phase of the present invention exhibits joined particles with cross sections resembling the particle shape of MXene obtained after chemical attack. In particular, many particle cross sections appear to have either rectangular flat cross sections with lengths of 1 μm to 20 μm and heights of 0.5 μm to 2 μm, or rounded flat cross sections with rounded outlines with diagonal lengths of 1 μm to 20 μm, depending on the orientation of the cross sections in the crushed section. Because the particles of this MAX phase are not isolated but joined together, these dimensions are rough estimates.

[0058] 10, the X-ray diffraction pattern obtained for the MAX phase compound of the present invention using a Cu Kα radiation source corresponds to the crystalline phase of the TiAlC compound with the addition of a low intensity peak at a 2θ angle of 36.0° attributed to a titanium carbide phase contained in the initial powder. This titanium carbide phase is estimated to be less than 5% by volume of the crystalline phase of the MAX phase compound.

[0059] The EDX spectrum of the MAX phase compound of the present invention reveals the presence of three elements in the composition of this MAX phase, namely titanium, aluminum and carbon, and the absence of impurities such as oxygen or any other element.

[0060] Comparative Example 1: Trial production of MAX phase precursor with a composition of Ti3AlC2 using a conventional SPS apparatus The powder mixture described and prepared in Example 1 was loaded into a commercially available spark plasma sintering apparatus as in Example 1. In contrast to the apparatus used in Example 1, this apparatus was not modified; the powder was placed in a hollow graphite chamber exposed to electric current and pressure. Several heating parameters were tested, including maximum holding temperatures (1150°C to 1450°C), temperature holding times (8 minutes to 24 minutes), and the addition of an intermediate hold at 650°C. All of these tests resulted in highly compact pellets without porosity. Due to their extensive compactness, these pellets were unsuitable for one or more chemical attacks to produce MXene compounds.

[0061] Furthermore, the composition of the pellets, as determined by X-ray diffraction analysis, showed a high content of titanium carbide, to the detriment of the desired Ti3AlC2.

[0062] Further tests were carried out by adding 20 atomic % Si relative to Al in the powder mixture (following the example of Non-Patent Document 3). The pellets obtained here were also very compact with no porosity. Furthermore, analysis by X-ray diffraction showed that the product was clearly Ti3(Al) with an additional unidentified phase. 1-x Si x )C2.

[0063] Therefore, prior art sintering techniques by spark plasma sintering do not allow for obtaining MAX phase compounds that would subsequently allow the synthesis of MXene compounds.

[0064] Example 2: Ti3C2T from the coarsely ground MAX phase precursor of Example 1 x Preparation of MXene compounds having the formula The porous pellets of Ti3AlC2 obtained in Example 1 were coarsely crushed in an agate mortar, resulting in clusters of approximately 1 mm to 2 mm in size. A first chemical attack was performed using an in situ-formed hydrofluoric acid solution. To this end, 0.5 g of the MAX phase compound having the formula Ti3AlC2 obtained in Example 1 was placed in a Teflon centrifuge tube with a capacity of approximately 50 mL, to which 10 mL of a pre-prepared aqueous solution containing 2 M lithium fluoride and 6 M hydrochloric acid was added. The mixture was left stirring at ambient temperature for 5 minutes using a magnetic stirrer. The tube was then placed in a temperature-controlled bath at 35 °C for approximately 72 hours, with continued stirring.

[0065] After this first attack, deoxygenated water was added to a level of approximately 35 mL, centrifuged at 9000 rpm for 10-15 minutes, and then the supernatant was discarded to retain the precipitate. A second chemical attack was then performed by pouring 10-20 mL of the same aqueous solution containing 2 M lithium fluoride and 6 M hydrochloric acid onto the precipitate and stirring in a temperature-controlled bath at 35°C for approximately 72 hours.

[0066] Upon completion of this second chemical attack, a rinse step was performed by adding deoxygenated water (typically 200 mL) with stirring for 5 minutes, followed by centrifugation at 9000 rpm for 10-15 minutes, discarding the supernatant, and retaining the precipitate. This procedure was repeated approximately five times until the pH of the supernatant reached 4.5 or higher. The precipitate was then washed with ethanol in two steps: rinsing with ethanol and centrifugation, and then stored covered with ethanol. Alternatively, the precipitate can be dried by vacuum heating at temperatures between 40°C and 120°C.

[0067] In this way, Ti3C2T x The resulting compound has the formula Ti3C2, or more commonly Ti3C2. Figures 4A, 4B, and 4C show the crystalline morphology of this compound. It can be seen that the compound is composed of crystals separated from one another in the form of plates with two parallel planes. The oblate aspect ratio (the ratio of the crystal length L to the crystal height H) ranges from 5 to 50, averaging approximately 10. The crystal size distribution is relatively uniform, with the crystal length L ranging from 1 micron to 15 microns and the crystal height ranging from 0.2 microns to 1 micron.

[0068] As mentioned above, the X-ray diffraction pattern in Figure 6 is characterized by a large peak, designated by the symbol 9, located at an angle 2θ of 6.5° to 6.9°. This corresponds to the (hkl) = (002) plane and is the origin of Ti3C2T x It corresponds to the interplanar distance between lamellar surfaces consisting of units.

[0069] The EDX spectrum of this same compound (Figure 8) reveals the presence of two major elements in the composition of this MXene-type compound, namely titanium and carbon, as well as the presence of the terminal group T x The EDX spectrum reveals the presence of fluorine, oxygen, and chlorine, which may correspond to -F, -Cl, =O, or -OH groups, among others. As mentioned above, the EDX spectrum also reveals the absence of elemental Al. This means that the MAX phase precursor has been completely attacked, leaving no aluminum-containing by-products, such as the oxide Al2O3.

[0070] Example 3: Ti3C2T from finely ground MAX phase precursor of Example 1 x Preparation of MXene compounds having the formula The Ti3AlC2 porous pellets obtained in Example 1 were crushed in an agate mortar and then finely ground. The powder was passed through a sieve with a mesh size of 50 μm, resulting in a powder with a particle size of less than 0.50 μm. The same two chemical attack and rinsing procedures as described in Example 2 were applied.

[0071] Figures 5A, 5B, and 5C show the crystalline morphology of this compound. It is confirmed that this compound is also composed of crystals separated from each other in the form of plates with two parallel planes. The flat aspect ratio (the ratio of the crystal length L to the height H) is also 5 to 50, with an average of approximately 10. The crystal size distribution is relatively uniform, with the crystal length L ranging from 1 micron to 15 microns and the crystal height H ranging from 0.2 microns to 1 micron. Nevertheless, a strong presence of crystals with a size of approximately 1 micron or less is confirmed. These crystals are often bulk-shaped (with a height H on the same order as the length L) rather than plate-shaped, but most crystals are plate-shaped.

[0072] The X-ray diffraction pattern and EDX spectrum are the same as those described in Example 2.

[0073] Comparative Example 2: Ti3C2T from prior art MAX phase precursor Ti3AlC2 x Preparation of MXene compounds having the formula Two batches of a commercially available MAX phase precursor sold under the name Ti3AlC2 were used. Both had the appearance of a fine powder with particle sizes less than about 50 microns. SEM images of these compounds (Figures 3A and 3B) show a heterogeneous distribution of particle sizes ranging from tens of microns to tens of nanometers.

[0074] The same two-step chemical attack and rinse procedure described in Example 2 was applied to each batch.

[0075] Figures 7A, 7B, and 7C show the crystalline morphology of the resulting MXene compound. The compound is composed primarily of bulk-shaped particles (length on the same order as height) and particle clusters. Only a small number of these particles have a plate-like morphology. The particle size distribution is highly heterogeneous, ranging from tens of microns to tens of nanometers. This morphology was the same for both batches of the MAX phase precursor used. This morphology is consistent with that reported in the literature.

[0076] The X-ray diffraction patterns of the two MXene compounds obtained are similar to those of the compound of the present invention obtained in Example 2. The EDX spectrum (FIG. 9) shows that, as in the case of the compound obtained in Example 2, in addition to the presence of two main elements, namely titanium and carbon, in the composition of this MXene-type compound, the terminal group T x It can be seen that there are fluorines, oxygens and chlorines present which may correspond in particular to -F, -Cl, =O or -OH groups.

[0077] On the other hand, these spectra also reveal the presence of aluminum as an impurity, in contrast to the inventive MXene compounds of Examples 2 and 3 (Figure 8). The atomic ratio Al / Ti is estimated to be 0.03-0.04 from EDX spectra acquired on images containing many particles (such as the spectrum in Figure 9). EDX analysis of individual particles reveals that the aluminum is present mainly as submicrometer particles of aluminum oxide, which is undoubtedly the compound Al2O3.

Claims

1. M ｎ＋１ Xn, wherein n is 1, 2 or 3, M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta, and X is C or N, in the form of crystals having the shape of a plate, characterized in that the plate has two opposite parallel surfaces having a defined length (L) spaced apart by a defined height (H); The length of the plate-shaped crystals is 1 μm to 15 μm; The height (H) of the plate-shaped crystal is 0.2 μm to 1 μm, and A compound characterized in that the flat aspect ratio of the plate-shaped crystals, defined by the ratio of the length (L) to the height (H), is 5 to 50.

2. Ti ３ The compound of claim 1 having the formula: C2

3. M ｎ＋１ AX ｎ wherein n is 1, 2 or 3; M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta; A is selected from Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl and Pb; and X is C or N; in the form of pellets with a porosity of more than 30%, preferably more than 40%, and having a stoichiometric element ratio M / X within a relative error of less than ±0.5%; 3. The compound of claim 1 or 2.

4. The MAX phase type precursor compound is Ti ３ AlC ２ 4. The compound of claim 3 having the formula:

5. The compound of claim 3, wherein the MAX phase precursor compound has a morphology exhibiting the following two types of particle cross sections: a rectangular flat cross section with a length of 1 μm to 20 μm and a height of 0.5 μm to 2 μm, and / or A rounded flat cross section with a diagonal length of 1 μm to 20 μm.

6. 10. A process for producing the compound of claim 1, comprising two chemical attack steps in which an aqueous acid is contacted with a MAX phase precursor compound.

7. 4. A process for producing a compound according to claim 3, in which a MAX phase type precursor compound is obtained by a spark plasma sintering process in a spark plasma sintering apparatus (1) comprising a graphite die (2) defining a hollow chamber (4) and two graphite punches (3a, 3b), said spark plasma sintering process comprising: mixing precursor powders; placing the powder mixed in the previous step into a closed container (5) made of insulating ceramic material housed in the hollow chamber (4); performing a spark plasma sintering operation; obtaining pellets of a MAX phase type compound; A process comprising at least

8. 8. The process according to claim 7, wherein the vessel (5) is alumina-based.

9. 8. The process according to claim 7, wherein the powder is not in contact with the two punches (3a, 3b), is not directly exposed to applied current, and is insulated from applied pressure, wherein the current and pressure are applied during the spark plasma sintering operation.

10. 8. The process of claim 7, wherein the spark plasma sintering operation comprises at least one thermal cycle applying a heating rate of greater than 60°C / min.

11. The thermal cycle a first ramp to a temperature of 550°C to 700°C at a rate of greater than 60°C / min and hold for 2 to 15 minutes; a second temperature increase at a rate of greater than 60°C / min to a temperature of 1400°C to 1500°C and hold for 5 to 15 minutes; The process of claim 10, comprising:

12. 8. The process of claim 7, comprising two chemical attack steps of contacting the MAX phase precursor compound with an aqueous acid.

13. 10. A spark plasma sintering apparatus for carrying out a process for producing the MAX phase type precursor compound used in the process according to claim 7, comprising a graphite die (2) and two graphite punches (3a, 3b) defining a hollow chamber (4), characterized in that it also comprises an enclosure made of insulating ceramic material located within the hollow chamber (4) for receiving a powder mixture during the application of the spark plasma sintering process and capable of keeping the powder mixture isolated from direct exposure to applied current and from applied pressure, wherein the current and the pressure are applied during the spark plasma sintering operation.

14. In a spark plasma sintering apparatus (1) equipped with a graphite die (2) defining a hollow chamber (4) and two graphite punches (3a, 3b), M ｎ＋１ AX ｎ wherein n is 1, 2 or 3, M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta, A is selected from Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl and Pb, and X is C or N, mixing the powders; placing the powder mixed in the previous step into a closed container (5) made of insulating ceramic material housed in the hollow chamber (4); performing a spark plasma sintering operation; obtaining pellets of the MAX phase type compound; A process comprising at least

15. 15. The process according to claim 14, wherein the vessel (5) is alumina-based.

16. 16. The process according to claim 14 or 15, wherein the powder is not in contact with the two punches (3a, 3b), is not directly exposed to applied current and is insulated from applied pressure, wherein the current and pressure are applied during the spark plasma sintering operation.

17. 15. The process of claim 14, wherein the spark plasma sintering operation comprises at least one thermal cycle applying a heating rate of greater than 60°C / min.

18. The thermal cycle a first ramp to a temperature of 550°C to 700°C at a rate of greater than 60°C / min and hold for 2 to 15 minutes; a second temperature increase at a rate of greater than 60°C / min to a temperature of 1400°C to 1500°C and hold for 5 to 15 minutes; 20. The process of claim 17, comprising:

19. 15. An apparatus for performing the process according to claim 14, comprising a graphite die (2) and two graphite punches (3a, 3b) defining a hollow chamber (4), and also comprising an enclosure (5) made of insulating ceramic material located within said hollow chamber (4) and adapted to receive a powder mixture during application of the spark plasma sintering process and to maintain said powder mixture isolated from direct exposure to applied current and from applied pressure, said current and said pressure being applied during the spark plasma sintering operation.

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