A method for preparing MXene by staged vacuum annealing of MAX phase precursor

Abstract

The application discloses a staged vacuum annealing treatment method for MAX phase precursor for preparing MXene. A MAX phase precursor thin layer is loaded into a high-temperature-resistant crucible with a cover, and after vacuumizing, low-temperature degassing treatment, medium-temperature stress release treatment and high-temperature crystallization annealing treatment are sequentially performed; wherein the high-temperature crystallization annealing temperature is lower than the initial temperature of thermal decomposition of the MAX phase under the corresponding furnace pressure by 50-200 DEG C, and then controlled cooling is performed. By limiting the high-temperature annealing temperature, holding time and powder layer thickness, and by using the high-temperature-resistant crucible with a cover to reduce the volatilization of A-site elements, the generation of defects, micro-strain release and sintering adhesion can be reduced while promoting defect recovery and micro-strain release. The method is suitable for improving the structural integrity and batch stability of the MAX phase precursor for MXene.

Description

Technical Field

[0001] This invention relates to precursor processing technology for two-dimensional transition metal carbides, nitrides or carbonitride materials, specifically to a staged vacuum annealing method for preparing MAX phase precursors for MXene. Background Technology

[0002] The MAX phase can usually be represented as M n ₊1AX n In this MAX phase, M represents an early transition metal, A is primarily a group 13 or 14 element, X represents carbon and / or nitrogen, and n is an integer from 1 to 4. MXene is typically obtained by selectively removing the A-site element from the MAX phase and can be represented as M n ₊1X n T x T x This indicates a surface termination group. The phase purity, stoichiometry, crystal defects, microstrain, and particle state of the MAX phase precursor will affect the uniformity of subsequent etching, the integrity of MXene sheets, and material properties.

[0003] Existing MAX phase powders can be obtained through solid-state reactions, mechanical alloying followed by annealing, molten salt methods, or chemical vapor deposition. Powders obtained by some methods may contain residual stress, dislocations, microcracks, surface oxide layers, or non-equilibrium defects. Appropriate heat treatment of the prepared MAX phase powders can help remove adsorbates, recover defects, and release microstrain.

[0004] However, the A-site element in the MAX phase may undergo selective volatilization under low-pressure, high-temperature conditions. Taking Ti3AlC2 as an example, excessively high temperatures or prolonged holding times under high vacuum conditions can lead to Al loss and potentially the formation of TiC. x The MAX phase is a secondary phase. Therefore, simply increasing the annealing temperature cannot guarantee improved crystallinity of the MAX phase; the annealing temperature needs to be limited below the thermal decomposition initiation temperature, depending on the specific MAX phase, furnace pressure, and holding time. Meanwhile, powder thickness, crucible structure, and cooling regime also affect sintering adhesion and compositional stability.

[0005] Existing technologies disclose a method for preparing Ti3AlC2 raw materials by mechanical alloying followed by annealing at 850–1100°C under vacuum or argon protection. However, this method primarily focuses on the reaction of the raw materials to generate Ti3AlC2 and does not address the synergistic control of low-temperature degassing, mid-temperature stress release, high-temperature phase stability window, and compatibility with subsequent MXene preparation for the synthesized MAX phase powder. Therefore, it is still necessary to provide a staged annealing method that balances defect recovery, A-site element retention, and powder dispersibility. Summary of the Invention

[0006] This invention aims to solve the problems of insufficient degassing, uneven release of residual stress, volatilization of A-site elements at high temperature, formation of secondary phases, and powder sintering and adhesion that exist in the heat treatment process of aluminum-containing MAX phase precursors. It provides a staged vacuum annealing method based on thermal decomposition temperature constraints.

[0007] To solve the above-mentioned technical problems, the present invention adopts the technical solution described in claims 1-8. The thermal decomposition initiation temperature T in step S4... d , refers to the initial temperature at which, under the same or equivalent furnace pressure and atmosphere conditions as step S4, in-situ high-temperature X-ray diffraction, in-situ neutron diffraction, thermogravimetric-mass spectrometry, or verified equivalent methods, the content of the MAX main phase continuously decreases, the volatilization signal of A-site elements increases significantly, or the decomposition secondary phase begins to form.

[0008] Preferably, when determining the process window for a specific material, a small-scale gradient annealing experiment is first conducted on the MAX phase to be treated, and the T value is determined jointly by the secondary phase change, A / M atomic ratio, annealing mass loss, and peak shape parameters obtained from Rietveld refinement. d The full width at half maximum (FWHM) of a single diffraction peak is not used as the basis for judgment.

[0009] Step S2 is used to remove adsorbed water, adsorbed gas and low-boiling-point impurities; Step S3 is used to promote point defect migration, micro-strain relaxation and mechanical stress release; Step S4 promotes crystallographic defect recovery within a temperature window that does not cause significant A-site element loss; Step S5 is used to reduce cooling thermal stress and the risk of local powder adhesion.

[0010] Compared to direct one-step annealing, this invention uses staged temperature control to ensure that low-temperature degassing, medium-temperature stress release, and high-temperature defect recovery occur within suitable temperature ranges. By limiting the high-temperature crystallization annealing temperature below the corresponding MAX phase thermal decomposition initiation temperature, the risk of A-site element volatilization and decomposition secondary phase formation is reduced. Furthermore, the use of a covered crucible, thin-layer loading, and controlled-rate cooling reduces the risk of powder sintering and adhesion. Experimental verification shows that this invention improves the structural integrity and batch stability of the MAX phase precursor and provides a stable precursor for subsequent MXene preparation. Attached Figure Description

[0011] Figure 1 Scanning electron microscope (SEM) image of the MAX precursor after matrix annealing. Detailed Implementation

[0012] The following examples are for illustrative purposes only and are not intended to limit the scope of protection of this invention. Unless otherwise stated, the temperature is the actual measured temperature in the furnace homogenization zone, and the pressure is the absolute pressure of the furnace. The following data are for reference only; the actual experimental results should prevail in the formal application. Example

[0013] Ti3AlC2 powder from the same batch was selected, with a purity of 98.0% and a median particle size D. 50 The particle size is 27.4 μm. Weigh 10 g of powder and spread it evenly in a covered graphite crucible, with a powder layer thickness of 4-6 mm. Place the crucible in the homogenization zone of a vacuum annealing furnace and evacuate the furnace to 2 × 10⁻² Pa. Increase the temperature to 300 °C at 5 °C / min and hold for 1 h; continue to increase the temperature to 650 °C at 4 °C / min and hold for 1 h; continue to increase the temperature to 950 °C at 2 °C / min and hold for 2 h. After holding, decrease the temperature to 750 °C at 2 °C / min, then decrease it to below 500 °C at 4 °C / min, and then allow it to cool naturally to room temperature.

[0014] The contents of the Ti3AlC2 main phase before and after annealing were 96.8 wt% and 96.9 wt%, respectively, while the contents of the TiC secondary phase were 3.2 wt% and 3.1 wt%, respectively. The Al / Ti atomic ratios were 0.322 and 0.320, respectively. The mass loss rate after annealing was 0.06%.

[0015] After annealing, the coherent diffraction domain size of Ti3AlC2 increased from 62 nm to 76 nm, the microstrain decreased from 0.21% to 0.17%, and the proportion of particles with obvious sintering necks was 1.2%. The particle size of the powder changed little before and after annealing, and no obvious hard agglomeration was observed.

[0016] Ti3AlC2 before and after annealing was etched and stripped under the same conditions to obtain Ti3C2T x Table 1 shows the various properties before and after annealing.

[0017] Table 1 Comparison of precursor properties before and after annealing MXene yield 33.1% 36.8% Median film diameter 3.6μm 4.2μm Single-layer film ratio 67.2% 73.6% Thin film conductivity 5.9×10³ S / cm 7.4 × 10³ S / cm The results show that staged vacuum annealing can reduce lattice defects and residual strain without significantly changing the phase composition of Ti3AlC2, and improve the yield, sheet size, monolayer ratio and thin film conductivity of subsequent MXene. Example

[0018] Except for the high-temperature crystallization stage temperatures being set at 850℃, 900℃, 950℃, 1000℃, and 1050℃ respectively, the other conditions were the same as in Example 1. Table 2 shows the test results for each group.

[0019] Table 2 Performance parameters at various temperatures 850℃ 0.02% 0.322 3.2wt% 65nm 0.20% 0.4% 900℃ 0.03% 0.321 3.2wt% 70nm 0.18% 0.7% 950℃ 0.06% 0.320 3.1wt% 76nm 0.17% 1.2% 1000℃ 0.10% 0.319 3.3wt% 80nm 0.16% 2.6% 1050℃ 0.18% 0.316 3.8wt% 82nm 0.15% 5.8% As the annealing temperature increases, the size of the coherent diffraction domain gradually increases, while the microstrain gradually decreases. When the temperature exceeds 950℃, Al loss and particle sintering degree gradually increase; at 1050℃, the TiC secondary phase content and particle connection ratio increase significantly.

[0020] Taking into account crystal integrity, Al element retention rate, secondary phase content and particle dispersibility, the optimal high-temperature crystallization annealing temperature for the Ti3AlC2 powder under 2×10⁻² Pa conditions was determined to be 950℃.

[0021] Using the same Ti3AlC2 powder and charging method as in Example 1, the furnace was evacuated to 2×10⁻² Pa, and then directly heated to 950℃ at 10℃ / min and held for 2 hours. Subsequently, the temperature was lowered according to the cooling regime of Example 1.

[0022] After one-step annealing, the content of the Ti3AlC2 main phase was 96.6 wt%, the content of the TiC secondary phase was 3.4 wt%, the Al / Ti atomic ratio was 0.318, and the mass loss rate was 0.10%. The coherent diffraction domain size was 69 nm, the microstrain was 0.19%, and the proportion of sintered neck particles was 2.8%.

[0023] The obtained MXene had a mass yield of 34.5%, a median wafer diameter of 3.8 μm, a monolayer ratio of 69.5%, and a film conductivity of 6.4 × 10³ S / cm.

[0024] Compared with Example 1, the lattice defect repair of the powder obtained by one-step annealing is lower, the particle sintering and Al loss are more obvious, and the overall performance of the MXene obtained subsequently is lower than that of the staged vacuum annealing group.

[0025] Using the same Ti3AlC2 powder as in Example 1, after low-temperature degassing and medium-temperature stress release under 2×10⁻² Pa conditions, the temperature was increased to 1175℃ at 2℃ / min and held for 30min, and then cooled down according to the cooling regime of Example 1.

[0026] After annealing, the sample mass loss rate was 0.72%, the content of Ti3AlC2 main phase decreased to 91.8wt%, the content of TiC secondary phase increased to 8.2wt%, the Al / Ti atomic ratio decreased to 0.306, and the proportion of sintered neck particles reached 12.4%.

[0027] The obtained MXene had a mass yield of 27.6%, a median wafer diameter of 2.7 μm, a monolayer ratio of 55.8%, and a film conductivity of 4.1 × 10³ S / cm.

[0028] The results show that under the vacuum pressure and heat preservation conditions, 1175℃ exceeds the safe phase stability window of this batch of Ti3AlC2, causing Al element loss, partial decomposition of Ti3AlC2 and an increase in TiC secondary phase, and reducing the preparation quality of subsequent MXene. Example

[0029] The thickness of the Ti3AlC2 powder layer was set to 2 mm, 5 mm, 8 mm, and 12 mm, respectively, with all other conditions remaining the same as in Example 1. The test results are as follows: Table 3 Performance Comparison at Different Thicknesses 2mm 2.0℃ 96.9wt% 0.319 74nm 0.9% 5mm 3.2℃ 96.9wt% 0.320 76nm 1.2% 8mm 6.1℃ 96.7wt% 0.321 72nm 2.9% 12mm 12.8℃ 96.4wt% 0.322 65nm 6.7% When the powder layer thickness is 2-8 mm, the internal temperature of the powder layer is relatively uniform, the powder crystallization effect is good, and the degree of particle sintering is low. When the powder layer thickness increases to 12 mm, the internal temperature difference of the powder layer increases significantly, the central area is not heated enough, and more obvious particle adhesion occurs.

[0030] Therefore, under the crucible size and loading conditions used in this embodiment, the thickness of the Ti3AlC2 powder layer is preferably 2-8 mm, and more preferably 4-6 mm.

[0031] Those skilled in the art can adjust the temperature, holding time, pressure, and heating / cooling rates at each stage according to the specific thermal stability, particle size, loading method, and target MXene preparation requirements of the MAX phase. The temperature of the high-temperature crystallization stage should be lower than the thermal decomposition initiation temperature of the corresponding MAX phase under actual pressure and holding conditions to avoid reducing the quality of subsequent MXene preparation due to the volatilization of A-site elements or the decomposition of the MAX phase.

Claims

1. A method for staged vacuum annealing of MAX phase precursors for preparing MXene, characterized in that, Includes the following steps: S1: The aluminum-containing MAX phase precursor is loaded into a covered high-temperature resistant crucible, so that the loose powder layer thickness of the aluminum-containing MAX phase precursor is no more than 8mm. The high-temperature resistant crucible is placed in the uniform temperature zone of a vacuum annealing furnace, and the furnace chamber is evacuated to an absolute pressure of 1×10⁻³-1Pa. S2: Heat to 200-400℃ at a heating rate of 2-8℃ / min, hold for 0.5-2h, and perform low-temperature degassing treatment; S3: Continue heating at a rate of 2-6℃ / min to 500-750℃, hold for 0.5-3 hours, and perform medium-temperature stress release treatment; S4: Continue heating at a rate of 1-5℃ / min until the high-temperature crystallization annealing temperature T is reached. a Hold at this temperature for 0.5-4 hours; the high-temperature crystallization annealing temperature T a The temperature is 800-1100℃, and it is higher than the thermal decomposition initiation temperature T of the aluminum-containing MAX phase precursor under the furnace pressure in step S4. d 50-200℃ lower; S5: After the heat preservation is completed, the temperature is reduced to below 500℃ at a cooling rate of 1-5℃ / min, and then naturally cooled to room temperature to obtain the annealed aluminum-containing MAX phase precursor.

2. The processing method according to claim 1, characterized in that, The aluminum-containing MAX phase precursor is one of Ti3AlC2, Ti2AlC, V2AlC or Nb2AlC.

3. The processing method according to claim 1 or 2, characterized in that, When the aluminum-containing MAX phase precursor is Ti3AlC2, the high-temperature crystallization annealing temperature in step S4 is 850-1050℃, and the holding time is 1-3h.

4. The processing method according to claim 1, characterized in that, In step S4, the absolute pressure in the furnace is 1×10⁻²-10 Pa.

5. The processing method according to claim 1, characterized in that, The covered high-temperature resistant crucible is a covered graphite crucible or a covered boron nitride crucible; a non-sealed gap is maintained between the crucible lid and the crucible body for gas escape.

6. The processing method according to claim 1, characterized in that, In step S5, the temperature is first reduced to 700-800℃ at a cooling rate of 1-3℃ / min, and then reduced to below 500℃ at a cooling rate of 3-5℃ / min.

7. The processing method according to claim 1, characterized in that, The median particle size D of the aluminum-containing MAX phase precursor 50 The thickness is 2-50 μm, and the loose powder layer thickness is 1-6 mm.

8. A method for staged vacuum annealing of MAX phase precursors for preparing MXene, characterized in that, The method includes obtaining an aluminum-containing MAX phase precursor by means of any one of claims 1-7, and performing A-site element-selective etching and stripping treatment on the aluminum-containing MAX phase precursor.

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