Highly Porous Max Phase Precursor For Mxene

Abstract

A method, comprising: forming a porous MAX-phase material. A MAX-phase material, the MAX-phase material made according to the present disclosure. A MXene material, the MXene material formed by removal of the A-group element of a MAX-phase material made according to the present disclosure.

Claims

1. A method, comprising: forming a porous MAX-phase material.

2. The method of claim 1, wherein the method comprises processing (i) a carbide and/or nitride that comprises the metal M of the porous MAX-phase material, (ii) an amount of the A-group element of the MAX-phase material, and (iii) an amount of the metal of the porous MAX-phase material, the amount of the metal of the porous MAX-phase material optionally being in powdered or porous form.

3. The method of claim 2, wherein the porous form is characterized as a sponge.

4. The method of claim 2, wherein (i), (ii), and (iii) are present such that there is an excess of the A-group element of the porous MAX-phase material.

5. The method of claim 1, wherein the processing comprises milling.

6. The method of claim 1, wherein the processing comprises sintering.

7. The method of claim 1, further comprising reducing the porous MAX-phase material to particulate form.

8. The method of claim 7, wherein reducing the porous MAX-phase material to particulate form comprising crushing the porous MAX-phase material.

9. The method of claim 1, further comprising forming a MXene material from the porous MAX-phase material.

10. A MAX-phase material, the MAX-phase material made according to claim 1.

11. A MXene material, the MXene material formed by removal of the A-group element of a MAX-phase material made according to claim 1.

12. A porous MAX-phase material, the porous MAX-phase material optionally reduceable to particulate form by hand, the porous MAX phase material optionally having a porosity of up to about 60%.

13. The porous MAX-phase material of claim 12, wherein the porous MAX-phase material comprises Ti3AlC2.

14. A method, comprising forming a MXene from a porous MAX-phase material.

15. The method of claim 14, wherein the MXene comprises Ti.sub.3C.sub.2T.sub.x.

16. The method of any one of claim 14, wherein the MXene comprises a plurality of flakes.

17. The method of claim 16, wherein a flake has a cross-sectional dimension of from about 4.5 to about 6.0 m.

18. The method of claim 14, further comprising forming a free-standing film from the MXene.

19. The method of claim 14, wherein the MXene has a conductivity in the range of from about 12,000 to about 17,000 S/cm.

20. The method of claim 19, wherein the MXene has a conductivity in the range of from about 13,000 to about 15,000 S/cm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various aspects discussed in the present document. In the drawings:

[0012] FIG. 1 Process flow chart and raw materials: (a) fine Al, (b) coarse Al, (c) sponge Ti, (d) fine Ti.

[0013] FIG. 2. Pictures of sintered bodies after MAX synthesis. (a) Fine Ti and fine Al, (b) sponge Ti and fine Al, (c) sponge Ti (fraction <75 m) and fine Al, (d) fine Ti and coarse Al.

[0014] FIG. 3. Characterization of MAX phases. (a) XRD patterns of samples produced with fine and coarse Ti sponge, SEM images of MAX phases produced with (b) fine Ti and (c) coarse Ti sponge.

[0015] FIG. 4. Schematic of MXene synthesis (a), AFM images of and flake size distributions of delaminated Ti.sub.3C.sub.2T.sub.x produced with fine Ti (b, c) and coarse Ti sponge (d, e). AFM statistical analysis was performed on 100 individual flakes of delaminated Ti.sub.3C.sub.2T.sub.x, produced from (b, c) fine Ti and (d, e) coarse Ti sponge.

[0016] FIG. 5. (a) XRD patterns of delaminated Ti.sub.3C.sub.2T.sub.x. Insets show Ti.sub.3C.sub.2T.sub.x films (4 cm diameter) produced by vacuum-assisted filtration. (b) UV-vis spectra of delaminated Ti.sub.3C.sub.2T.sub.x produced from MAX phases obtained using fine and coarse Ti powders.

[0017] FIG. 6. SEM image (a) and XRD pattern (b) of coarse Ti sponge.

[0018] FIG. 7. Photograph of a MAX phase sample made with coarse Ti sponge and sintered at 1380 C.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0019] The present disclosure may be understood more readily by reference to the following detailed description of desired embodiments and the examples included therein.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict, the present document, including definitions, will control. Preferred methods and materials are described below, although methods and materials similar or equivalent to those described herein can be used in practice or testing. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety.

[0021] The materials, methods, and examples disclosed herein are illustrative only and not intended to be limiting.

[0022] The singular forms a, an, and the include plural referents unless the context clearly dictates otherwise.

[0023] As used in the specification and in the claims, the term comprising can include the embodiments consisting of and consisting essentially of. The terms comprise(s), include(s), having, has, can, contain(s), and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that require the presence of the named ingredients/steps and permit the presence of other ingredients/steps. However, such description should be construed as also describing compositions or processes as consisting of and consisting essentially of the enumerated ingredients/steps, which allows the presence of only the named ingredients/steps, along with any impurities that might result therefrom, and excludes other ingredients/steps.

[0024] As used herein, the terms about and at or about mean that the amount or value in question can be the value designated some other value approximately or about the same. It is generally understood, as used herein, that it is the nominal value indicated 10% variation unless otherwise indicated or inferred. The term is intended to convey that similar values promote equivalent results or effects recited in the claims. That is, it is understood that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but can be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. In general, an amount, size, formulation, parameter or other quantity or characteristic is about or approximate whether or not expressly stated to be such. It is understood that where about is used before a quantitative value, the parameter also includes the specific quantitative value itself, unless specifically stated otherwise.

[0025] Unless indicated to the contrary, the numerical values should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.

[0026] All ranges disclosed herein are inclusive of the recited endpoint and independently of the endpoints. The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value; they are sufficiently imprecise to include values approximating these ranges and/or values.

[0027] As used herein, approximating language can be applied to modify any quantitative representation that can vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as about and substantially, may not be limited to the precise value specified, in some cases. In at least some instances, the approximating language can correspond to the precision of an instrument for measuring the value. The modifier about should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression from about 2 to about 4 also discloses the range from 2 to 4. The term about can refer to plus or minus 10% of the indicated number. For example, about 10% can indicate a range of 9% to 11%, and about 1 can mean from 0.9-1.1. Other meanings of about can be apparent from the context, such as rounding off, so, for example about 1 can also mean from 0.5 to 1.4.

[0028] Further, the term comprising should be understood as having its open-ended meaning of including, but the term also includes the closed meaning of the term consisting. For example, a composition that comprises components A and B can be a composition that includes A, B, and other components, but can also be a composition made of A and B only. Any documents cited herein are incorporated by reference in their entireties for any and all purposes.

[0029] Any embodiment or aspect provided herein is illustrative only and does not limit the scope of the present disclosure or the appended claims. Any part or parts of any one or more embodiments or aspects can be combined with any part or parts of any one or more other embodiments or aspects.

[0030] MXenes, a large family of 2D carbides and/or nitrides, are among the most studied materials worldwide due to their great diversity of structures and compositions. Their unique properties are finding use in several applications, ranging from printable electronics and communication to energy storage and healthcare. Typically, they are manufactured by selective wet-chemical etching of layered MAX phase ceramics, which are produced nowadays primarily for MXene synthesis. However, the synthesis of MAX phases has not been changed since the time of their use in structural and high-temperature applications, and it has not been optimized for MXene manufacturing. The main purpose of this study is to develop a porous Ti.sub.3AlC.sub.2 MAX phase that can be easily ground into individual grains without time-consuming, harsh, and tedious crushing and milling steps. Moreover, we also demonstrate the synthesis of highly porous Ti.sub.3AlC.sub.2 from an inexpensive titanium sponge instead of a highly pure and fine titanium powder, as well as explaining the mechanisms of reaction sintering and formation of porous MAX phase. The developed Ti.sub.3AlC.sub.2 synthesis protocol is commercially relevant as it saves time, decreases the costs, minimizes the content of secondary phases in the reaction products, and leads to high-quality Ti.sub.3C.sub.2T.sub.x MXene. In fact, MXenes obtained from this MAX phase show larger flake size and higher electrical conductivity in thin films, compared to the materials produced from the costly fine titanium powder. The proposed approach may apply to the synthesis of other MAX phases as well.

[0031] In this invention, we obtain a highly porous MAX phase, such as Ti.sub.3AlC.sub.2, by using much less expensive raw material (e.g., titanium sponge) instead of the metal or hydride or carbide powder, aluminum and carbon for preparing MAX phase.

[0032] The produced porous material can be easily crushed into powder, which leads to a significant cost reduction by elimination of hard crushing/milling and high-energy ball milling.

[0033] Decreased wear of milling equipment and milling bodies not only saves costs, but also minimizes impurities in MAX and the subsequent MXenes.

[0034] Decreased distortion and damage of MAX due to avoiding high-energy milling leads to a lower concentration of defects in the MAX crystal lattice and MXene, increasing the properties (strength, conductivity, environmental stability) of the end-product MXene produced by selectively etching the MAX phase and its flake size.

INTRODUCTION

[0035] Ternary layered carbides and/or nitrides, known as MAX phases, represent a large class of materials with a general formula M.sub.n+1AX.sub.n, where M is a transition metal, A is an element from the p-block, mostly groups 13 and 14 of the periodic table, and X is C and/or N, with n=from 1 to 5..sup.1,2 They possess a unique combination of physicochemical properties, exhibiting some characteristics of ceramics and others of metals. In this regard, MAX phases have been considered for several different applications in aerospace and automotive industries as high-temperature elements, friction parts, and good electrical and thermal conductors. For those applications, high density, phase purity, and mechanical strength are required. The MAX phase can be fabricated via powder-metallurgical routes, including reactive hot-pressing or hot-isostatic pressing (HIP).sup.3.4, pressureless sintering (PS) under an inert environment.sup.5, self-propagating high-temperature synthesis (SHS).sup.6, spark plasma sintering (SPS).sup.7,8, and other methods. Most of those methods, except for SHS, lead to hard sintered bodies that require crushing and high-energy milling to produce powders.

[0036] Since their discovery in 2011, two-dimensional (2D) carbides and/or nitrides, known as MXenes.sup.9, for which MAX phases serve as precursors, captured the interest of the scientific and industrial communities. Nowadays, MAX phases are produced primarily for the MXene manufacturing..sup.10 Ti.sub.3C.sub.2T.sub.x (T.sub.x stands for surface terminations, mainly including O, OH, and F) is the most studied and important MXene, with more than half of all MXene publications dedicated to this material..sup.11 Therefore, Ti.sub.3AlC.sub.2 is the most important and widely manufactured MAX phase, as it is used as the main precursor for Ti.sub.3C.sub.2T.sub.x. However, the requirements for MAX phases used to make MXenes differ drastically from those of structural MAX materials. In particular, powders or porous, loosely bonded sintered bodies are needed. Some unreacted metals, intermetallic or oxides are less critical in MXene precursors compared to structural MAX phases. Although they can strongly affect the mechanical properties for structural applications, they can be either dissolved in acids (e.g., HCl or HF) during MXene synthesis or removed during MXene delamination. At the same time, Ti:C stoichiometry in MAX phases, as well as the oxygen presence in the carbon sublattice, which was usually ignored when producing MAX phases, play a critical role in determining the electrical conductivity and environmental stability of MXenes..sup.12

[0037] A mono-transition MXene can take the form of M.sub.2C, M.sub.3C.sub.2, and M.sub.4C.sub.3. A MXene can also be in double-transition form, which can be M.sub.2MC.sub.2 or M.sub.2M.sub.2C.sub.3 where M and M are different transition metals. A solid-solution MXene can take the general formula of: (M.sub.2-yM.sub.y)C, (M.sub.3-yM.sub.y)C.sub.2, (M.sub.4-yM.sub.y)C.sub.3, or (M.sub.5-yM.sub.y)C.sub.4.

[0038] In certain embodiments, the MXene composition is any of the compositions described in at least one of U.S. patent application Ser. No. 14/094,966 (filed Dec. 3, 2013), 62/055,155 (filed Sep. 25, 2014), 62/214,380 (filed Sep. 4, 2015), 62/149,890 (filed Apr. 20, 2015), 62/127,907 (filed Mar. 4, 2015) or International Applications PCT/US2012/043273 (filed Jun. 20, 2012), PCT/US2013/072733 (filed Dec. 3, 2013), PCT/US2015/051588 (filed Sep. 23, 2015), PCT/US2016/020216 (filed Mar. 1, 2016), or PCT/US2016/028,354 (filed Apr. 20, 2016), preferably where the MXene composition comprises titanium and carbon (e.g., Ti3C2, Ti2C, etc.). Each of these compositions is considered independent embodiment.

[0039] Similarly, MXene carbides, nitrides, and carbonitrides are also considered independent embodiments. Various MXene compositions are described elsewhere herein, and these and other compositions, including coatings, stacks, laminates, molded forms, and other structures, described in the above-mentioned references are all considered within the scope of the present disclosure.

[0040] A porous and low-density MAX phase that can be crushed into powders with minimal force and without extensive milling steps is highly desired for MXene synthesis. This would not only decrease the cost of MXene manufacturing by eliminating drilling, crushing, and extensive ball-milling, but also prevent lattice distortion of the MAX phase and produce higher quality MXenes with larger flake size and lower concentration of defects. A traditional approach to manufacturing porous materials in the TiAlC systems involves the use of salt additives, such as NaCl.sup.13 or NH.sub.4HCO.sub.3.sup.14, that prevent complete sintering and can be removed during or after processing. However, their addition may lead to contamination of the MAX phases and MXenes, as well as affecting the stoichiometry, purity, and properties of the resulting materials. That is why the direct synthesis of the low-density MAX phase is preferable.

[0041] The composition and porosity of MAX phases are determined not only by the synthesis process, but also by the selection of their precursors. Conventionally, Ti.sub.3AlC.sub.2 is synthesized via high-temperature reactive sintering of pure Ti, Al, and C powders.sup.15-17 or TiC, Al, and Ti powders..sup.18,19 Since pure Ti and other transition metal powders are quite expensive, low-cost raw materials are a high priority for industrial manufacturing of MXenes and their MAX precursors. Therefore, inexpensive raw materials, such as TiO.sub.2.sup.4, TiH.sub.2.sup.20, and Al.sub.4C.sub.3.sup.4,21 have been explored. Different carbon sources can also be used, such as graphite, carbides, or carbon fibers.sup.19,22. A study of the effects of three carbon sourcesgraphite, lampblack, and titanium carbideon the structure and properties of Ti.sub.3AlC.sub.2 MAX phases, along with their derived Ti.sub.3C.sub.2T.sub.x MXenes.sup.19, showed that the MAX synthesis protocol and particle morphology dramatically influence the synthesis, quality, and properties of the resulting MXenes.

[0042] Another crucial aspect is represented by the stoichiometric ratio of the reactants. For the typical synthesis of Ti.sub.3AlC.sub.2, the stoichiometric ratio of precursors is 2TiC:1Ti:1Al..sup.23 However, it was shown that an excess of aluminum in the TiC:Al:Ti mixture leads to the stoichiometric Ti.sub.3AlC.sub.2 MAX phases with no detectable oxygen impurities..sup.24

[0043] The main purpose of this study is to fabricate a porous Ti.sub.3AlC.sub.2 MAX phase that can be easily crushed into individual crystals/particles without utilizing any pore-forming additives. Moreover, we demonstrate the synthesis of highly porous Ti.sub.3AlC.sub.2 from inexpensive coarse titanium sponge, instead of the highly pure commercial titanium powders, and explain the mechanisms of reaction sintering and porous MAX phase formation. Titanium sponge is used as a raw material for manufacturing pure and ductile iodide titanium and titanium alloys. It is at least an order of magnitude less expensive compared to the commercial Ti powder. Our Ti.sub.3AlC.sub.2 phase synthesis process is commercially relevant, as it minimizes the amounts of secondary phases, such as TiC or Ti.sub.2AlC, in the reaction products and leads to high-quality as well as highly conductive MXenes. The proposed approach may apply to the synthesis of other Ti-based and beyond-Ti MAX phases.

EXPERIMENTAL

Ti.SUB.3.AlC.SUB.2 .MAX Phase Synthesis

[0044] TiC powder (99.5%, 325 mesh, Alfa Aesar, USA) and aluminum powder (99.5%, 325 mesh, Alfa Aesar, USA) were mixed with titanium powder (99.5%, 325 mesh, Alfa Acsar, USA) or coarse titanium sponge (60-325 mesh, Pyro Chemical Source LLC) in molar ratios 2TiC:2.2Al:1.25Ti, providing an excess of aluminum (AlTi.sub.3AlC.sub.2 MAX)..sup.24 To prove our hypothesis about the sintering mechanism, we also prepared samples using course atomized aluminum powder PA-1 (450-600 m, Ukraine) instead of Alfa Acsar Al. The mixing was performed in a ball mill using zirconia beads at 70 rpm for 18 h. A 2:1 mass ratio of zirconia milling beads to the precursor powder mixture was used. The homogeneous mixture was cold pressed in a stainless-steel mold with a pressure of 1000 psi to form a pellet with a diameter of 28 mm.

[0045] The sintering of pellets was carried out in a GSL-1700X tube furnace (MTI Corporation, USA) at 1380 C., 1400 C., and 1420 C. for 2 h under an argon flow for a mixture with titanium sponge. Only 1380 and 1400 C. reactions were carried out with fine titanium powders, which react faster compared to the sponge. Synthesis of Ti.sub.3AlC.sub.2 from TiC, Al, and fine Ti was adopted from ref.sup.24 suggesting sintering at 1380-1400 C. to obtain pure Ti.sub.3AlC.sub.2. The heating and cooling rates were both 3 C./min. The sintered pellets of Ti.sub.3AlC.sub.2were crushed into powders manually and with a planetary ball mill (for dense samples). For removing impurities, such as intermetallics and oxides, the produced powder was sieved to less than 38 m particle size and washed in 9 M HCl for 20 h at room temperature while stirring. HCl washing reduced the product mass by 20-25%, due to the dissolution of excess Al and TiAl.sub.3 intermetallic. The washed and dried Ti.sub.3AlC.sub.2 was used to produce Ti.sub.3C.sub.2T.sub.X.

MXene Synthesis

[0046] Ti.sub.3C.sub.2T.sub.x was produced by selective wet-chemical etching following the previously described protocol..sup.25 One gram of Ti.sub.3AlC.sub.2 powder was slowly added to 20 mL of etchant and stirred at 300 rpm at 35 C. for 24 h. The etchant was a 6:3:1 mixture (by volume) of 12 M HCl, DI water, and 50 wt. % HF (Acros Organics, Fair Lawn, NJ, USA). Multilayered Ti.sub.3C.sub.2T.sub.x MXene was intercalated with LiCl (using 1 g of LiCl per gram of Ti.sub.3AlC.sub.2 MAX) dissolved in 50 mL of DI water and stirred at 300 rpm at room temperatures for 24 h. The resulting solution was washed with DI water and centrifuged at 3500 rpm for 5 min. The supernatant was discarded, and the delaminated MXene was redispersed by manual shaking. The washing procedure was repeated until the pH of the mixture was higher than 6. Then, the colloidal solution was centrifuged at 3500 rpm for 60 min, and the supernatant containing delaminated Ti.sub.3C.sub.2T.sub.x was collected. The MXene free-standing films were prepared from delaminated Ti.sub.3C.sub.2T.sub.x by vacuum-assisted filtration.

Characterization

[0047] The phase analysis of MAX and MXene films was carried out by X-ray diffraction (XRD; Miniflex, Rigaku Corporation, Tokyo, Japan) using Cu K.sub. radiation at 40 kV and 15 mA. Step-scan data (with step size equal to) 0.02 were recorded over a range 3-90 (2). Conductivity measurements were performed using a four-point probe (Jandel Engineering Ltd., Bedfordshire, UK) on freestanding MXene films. SEM analysis was performed using a Zeiss Supra 50VP electron microscope. UV-Vis spectra were collected using an Evolution 201 spectrometer (Thermo Scientific, MA, USA) with a 10 mm optical length cuvette and scanning from 200 to 1000 nm. AFM images were taken with a Bruker Dimension Icon microscope under ambient conditions, operating in Tapping Mode and using TESPA-V2 tips with spring constant, k=42 N/m. Images were captured at scan rate of 1 Hz with 1024 lines per image. The statistical analysis was performed on 100 individual Ti.sub.3C.sub.2T.sub.x flakes for each sample. The results were fitted by using a log-normal distribution.

RESULTS AND DISCUSSIONS

Analysis of MAX Produced Using Different Metal Sources

[0048] Pressed and sintered Ti.sub.3AlC.sub.2 MAX samples were produced from a mixture of TiC, Al, and Ti powders according to the flow chart shown in FIG. 1. We used the same TiC powder for all samples, but the titanium sources were different. To investigate the effect of particle size on the porosity of sintered pellets of MAX, two grades of titanium powder with particle sizes less than 325 mesh (fine Ti sponge) and 60-325 mesh (coarse Ti sponge) were used. We used fine commercial Ti and coarse Ti sponge powders (Table 3). SEM images of Ti precursors (FIG. 1) show that Ti sponge has a specific spherical shape and a significantly larger particle size than fine titanium. An example Ti sponge XRD pattern in FIG. 6 exhibits five diffraction peaks at 2=35 (100), 38.5 (002), 40.2 (101), 53 (102) and 63 (110), which correspond to -Ti with lattice parameters c=4.6802 , a=2.49481. No secondary phases were found.

[0049] The morphology of samples sintered at 1400 C. is shown in FIG. 2. Samples made from coarse powders increased in volume during the reaction sintering, while shrinkage was observed for samples made with fine powders at all sintering temperatures. The porosity of the sample with coarse Ti after sintering was 73%, compared to 16% for the sample produced using fine Ti. All samples with coarse Ti powders grew after sintering, regardless of temperature. They could be easily crushed by hand into powders, which led to a cost reduction via the elimination of hard crushing/milling and high-energy ball milling. The density and porosity measurements before and after sintering, as well as the shrinkage of the samples, are summarized in Table 4. At 1380 C., pellets with Ti sponge fell apart. In addition, XRD analysis showed TiC presence in this sample (Table 1), demonstrating an incomplete reaction. The temperature of 1380 C. was not sufficient for TiC to react completely with the Ti sponge and Al. Lattice parameters and compositions for all experimental samples are reported in Table 1. For samples with fine and coarse Ti sintered at 1400 C., the c-lattice parameter was the largest, suggesting that they contained less oxygen in the MAX phase lattice..sup.26 TiAl.sub.3 and Al.sub.2O.sub.3 impurities present in the samples were removed during HCl washing, HCl/HF etching of MAX phases, and delamination of MXenes.

TABLE-US-00001 TABLE 1 Composition and lattice parameters of two different MAX phases produced by sintering at 1400 C. using fine titanium powder and coarse titanium sponge. Lattice Lattice Lattice 1380 C., parameters, 1400 C., parameters, 1420 C., parameters, Mixture mass % mass % mass % TiC, Al, Ti.sub.3AlC.sub.2 - a: 3.0794 Ti.sub.3AlC.sub.2 - a: 3.0838 Fine Ti 98.32 c: 18.5957 98.97 c: 18.6342 Al.sub.2O.sub.3- a: 4.7591 Al.sub.3Ti - a: 3.8546 0.57 c: 12.9309 1.03 c: 8.6037 Al.sub.3Ti - a: 3.8546 1.11 c: 8.6037 TiC, Al, Ti.sub.3AlC.sub.2 - a: 3.0565 Ti.sub.3AlC.sub.2 - a: 3.0770 Ti.sub.3AlC.sub.2 - a: 3.0765 Coarse 97.06 c: 18.5458 97.5 c: 18.5875 99.38 c: 18.5754 sponge Al.sub.2O.sub.3 - a: 4.6600 Al.sub.2O.sub.3 - a: 4.7625 Al.sub.2O.sub.3 - a: 4.7467 Ti 0.21 c: 13.0930 1.34 c: 12.9883 0.62 c: 13.0703 Al.sub.3Ti- a: 3.7631 Al.sub.3Ti- a: 3.8546 0.44 c: 8.5037 1.13 c: 8.6037 TiC - a: 4.3314 2.29

[0050] To figure out how porosity depends on the particle size of powders, we crushed coarse Ti with a ball mill and a fraction less than 75 m was added to TiCAl, pressed, and sintered following the same procedure. This sample also showed some growth (4%), but not as significant as with the coarse Ti sponge. Pictures of sintered bodies are shown in FIGS. 2c and 2d. This experiment suggested that it is not the composition, but the size of the Ti particles that was responsible for the differences in the sintering process. To verify this hypothesis, we produced samples with coarse Al powders. SEM images of coarse Al in FIG. 1b show large spherical particles, which contain more oxygen than fine Al. The porosity of pellets with coarse Al grew up to 50%. Thus, the expansion during the reactive sintering can be explained by the particle size of the metal powders. It is known that nucleation and growth of MAX occur on the carbide..sup.27 The presence of a coarse Ti sponge or Al separated from TiC particles hinders diffusion and reaction with the TiC. During sintering at temperatures 640-670 C., aluminum melts and goes into the liquid phase, forming voids at the place of large metal inclusions. In addition, for the TiAl system, the mutual solubility of the components varies greatly. Aluminum dissolves well in both liquid and solid titanium, but the solubility of titanium in aluminum is very low. This fact contributes to the Kirkendall effect (formation of voids) at the TiAl interface..sup.28,29 This, together with the volume expansion when TiC reacts with Al and Ti, transforming to Ti.sub.3AlC.sub.2, explains the swelling of samples during the sintering and formation of the highly porous MAX phase.

[0051] The success of the MAX synthesis was confirmed by the XRD. FIG. 3 shows XRD patterns and SEM images of Ti.sub.3AlC.sub.2 produced at 1400 C. with commercial fine Ti and coarse Ti. For both samples, all peaks expected for the p63/mmc MAX phase structure are observed. The positions of the peaks were almost identical, with non-significant shifts. The only difference was observed in the intensities of the (002), (101), (104) and (110) peaks. For samples with coarse Ti sponge, the peak at 2=9.57 (002) is more intense. No peaks related to the Ti.sub.2AlC phase or TiC were observed for both samples. Only the intermetallic compound, TiAl.sub.3, which can be easily removed by HCl washing at room temperature, was present in both samples. The samples with a coarse Ti sponge also had about 1.3% Al.sub.2O.sub.3. SEM images of MAX phase samples (FIG. 3) suggest that layered Ti.sub.3AlC.sub.2 MAX phases with coarse Ti had larger particles than the ones with fine Ti.

Analysis of MXenes

[0052] Although the MAX phase purity and crystal size are important, only MXene composition and properties can verify the quality of the MAX precursors. One of the important characteristics of MXenes is their flake size. FIG. 4 shows the AFM statistical analysis of delaminated Ti.sub.3C.sub.2T.sub.x samples along with their flake size distribution obtained by mapping over 100 individual flakes. The average lateral size of Ti.sub.3C.sub.2T.sub.x flakes from coarse-Ti MAX is larger (5.6 m, FIG. 4) than flakes from fine-Ti MAX (4.5 m).

[0053] Vacuum-assisted filtration of delaminated Ti.sub.3C.sub.2T.sub.x flakes produced shiny, dark-purple, free-standing films with good flexibility (FIG. 5). The XRD measurements show that after etching in HF/HCl/H.sub.2O mixture the Al layer was completely removed from the MAX phases and this led to the disappearance of all the Ti.sub.3AlC.sub.2 related peaks. The (002) peak was shifted toward lower values, from 2=9.57 to 7.13, highlighting the successful formation of MXene. All other peaks for both samples are at the same 2 values. The peak (002) for the sample with Ti sponge is broader and more intense. Both films have comparable d-spacing values (12.399 for fine Ti and 12.356 for Ti sponge), indicating that all films have similar quantities of water trapped in between the MXene flakes after drying in air.

[0054] Stability studies were performed after synthesis, by diluting each delaminated MXene solution. The UV-visible spectroscopy measurements (FIG. 5) showed a broad absorption peak at 756 nm (fine Ti) and at 760 nm (Ti sponge), which belongs to Ti.sub.3C.sub.2T.sub.x, confirming the good stability of both samples.

[0055] Four-point probe measurements of vacuum-filtered films showed that the electrical conductivity of Ti.sub.3C.sub.2T.sub.x obtained from Ti sponge MAX/MXene reached 16,500 S/cm, whereas the MXene films obtained from fine-Ti MAX showed lower conductivity of 11,500 S/cm (Table 2).

TABLE-US-00002 TABLE 2 Electrical conductivity of free-standing Ti.sub.3C.sub.2T.sub.X films produced from two different titanium sources. Average Concentration, Film mass, thickness, Conductivity, Mixture mg/ml mg m S/cm Fine Ti 1.25 16.3 5.66 11,500 Coarse Ti 1.28 18.0 4.80 16,500

[0056] This can be explained by the larger flake size for Ti.sub.3C.sub.2 T.sub.x from the Ti sponge MAX phases, even though a decreased concentration of defects and other factors may also contribute to MXene electrical properties. Additional in-depth characterizations are needed to fully evaluate the characteristics of MXenes produced using the Ti sponge.

CONCLUSIONS

[0057] In this study, we have optimized the synthesis of MAX phases for MXene manufacturing. The results of this work show that the use of a coarse Ti sponge as a precursor allows the synthesis of pure Ti.sub.3AlC.sub.2 MAX phase with high porosity (about 70%) by using a significantly less expensive raw material. Experiments with different particle sizes of titanium and aluminum powders suggest that the swelling of the pellets during the reaction sintering was mainly due to the large particle size of the metal. The produced MAX phases can be easily crushed by hand, eliminating the need for drilling and intense ball-milling before MXene synthesis. This is advantageous from the standpoint of MXene large-scale synthesis, as well as decreasing the cost and improving the quality of the final materials. MXenes obtained from these MAX phases showed excellent electrical conductivity in thin films and larger flake size compared to the materials produced from the fine Ti powders under the same conditions.

TABLE-US-00003 TABLE 3 Characteristics of raw materials used for the synthesis of MAX phases. Raw material Vendor Purity and particle size Titanium carbide Alfa Aesar Thermo 99.5% (metals basis) (TiC) Fisher Scientific 2 m Chemicals, Inc. Fine aluminum Alfa Aesar Thermo 99.5% powders (Al) Fisher Scientific 325 mesh (45 m) Chemicals, Inc. Fine titanium Alfa Aesar Thermo 99.5% powders (Ti) Fisher Scientific 325 mesh (45 m) Chemicals, Inc. Coarse titanium Pyro Chemical Powder 60-325 mesh sponge (Ti Sponge) Source LLC (45-250 m)

TABLE-US-00004 TABLE 4 Characteristics of MAX phase samples produced using different Ti sources. Sample .sub.1, g/cm.sup.3 .sub.2, g/cm.sup.3 P.sub.1, % P.sub.2, % V/V, % Fine Ti 2.60 3.52 38 16.3 28 Coarse Ti sponge 2.74 1.12 34.8 73.4 132 Coarse Al 2.50 2.09 40.7 50.2 16 Ti sponge <75 m 2.60 2.67 38.8 36.4 4 .sub.1, .sub.2density of green and sintered pellets; P.sub.1, P.sub.2porosity of green and sintered pellets, V/Vshrinkage/growth of pellets

REFERENCES

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Aspects

[0087] The following Aspects are illustrative only and do not limit the scope of the present disclosure or the appended claims. Any part or parts of any one or more Aspects can be combined with any part or parts of any one or more other Aspects. [0088] Aspect 1. A method, comprising: forming a porous MAX-phase material. [0089] Aspect 2. The method of Aspect 1, wherein the method comprises processing (i) a carbide and/or nitride that comprises the metal M of the MAX-phase material, (ii) an amount of the A-group element of the MAX-phase material, and (iii) an amount of the metal of the MAX-phase material, the amount of the metal of the MAX-phase material optionally being in powdered or porous form. The carbide and/or nitride can be in powder form. The A-group element can be present in powder form.

[0090] The amount of the metal of the MAX-phase material can be in powdered form. The amount of the metal of the MAX-phase material can also be in porous form, such as a sponge. Titanium sponge is considered a particularly suitable form. [0091] Aspect 3. The method of Aspect 2, wherein the porous form is characterized as a sponge. A sponge can be in comparatively coarse form, for example from about 40 microns to about 250 microns (i.e., 60-325 mesh). Powder can have a particle size of less than 325 mesh. [0092] Aspect 4. The method of any one of Aspects 2-3, wherein (i), (ii), and (iii) are present such that there is an excess of the A-group element of the MAX-phase material. [0093] Aspect 5. The method of any one of Aspects 1-4, wherein the processing comprises milling. [0094] Aspect 6. The method of any one of Aspects 1-5, wherein the processing comprises sintering. [0095] Aspect 7. The method of any one of Aspects 1-6, further comprising reducing the porous MAX-phase material to particulate form. [0096] Aspect 8. The method of Aspect 7, wherein reducing the porous MAX-phase material to particulate form comprising crushing the porous MAX-phase material. The crushing can be free of milling and/or ball milling. [0097] Aspect 9. The method of any one of Aspects 1-8, further comprising forming a MXene material from the porous MAX-phase material. [0098] Aspect 10. A MAX-phase material, the MAX-phase material made according to any one of Aspects 1-9. [0099] Aspect 11. A MXene material, the MXene material formed by removal of the A-group element of a MAX-phase material made according to any one of Aspects 1-10. [0100] Aspect 12. A porous MAX-phase material, the MAX-phase material optionally reduceable to particulate form by hand. The porous MAX phase material can have a porosity of up to about 30%, 40%, 50%, or even 60% in some embodiments. The porous MAX phase material can have a porosity of up to about 50%, up to about 45%, up to about 40%, up to about 35%, up to about 30%, up to about 25%, up to about 20%, up to about 15%, up to about 10%, or even up to about 5%. [0101] Aspect 13. The porous MAX-phase material of Aspect 12, wherein the porous MAX-phase material comprises Ti.sub.3AlC.sub.2. [0102] Aspect 14. A method, comprising forming a MXene from a porous MAX-phase material. The MXene can be formed by way of wet-chemical etching, for example by using HCl as an etchant. [0103] Aspect 15. The method of claim 14, wherein the MXene comprises Ti.sub.3C.sub.2T.sub.x. [0104] Aspect 16. The method of any one of Aspects 14-15, wherein the MXene comprises a plurality of flakes. [0105] Aspect 17. The method of Aspect 16, wherein a flake has a cross-sectional dimension of from about 4.5 to about 6.0 m. The cross-sectional dimension can be in the range of from about 4.7 to about 6 m, or from about 4.9 to about 5.8 m, or from about 5.1 to about 5.7 m, or from about 5.2 to about 5.6 m. [0106] Aspect 18. The method of Aspect 14, further comprising forming a free-standing film from the MXene.

[0107] Aspect 19. The method of Aspect 14, wherein the MXene has a conductivity in the range of from about 12,000 to about 17,000 S/cm.

[0108] Aspect 20. The method of Aspect 19, wherein the MXene has a conductivity in the range of from about 13,000 to about 15,000 S/cm.