Study of the pressureless sintering preparation of large-size MAX phases

[Background and purpose] Ti₃AlC₂ is a ternary transition metal carbide that exhibits both metallic and ceramic properties, and has a wide range of potential applications in various fields. In recent years, with the continuous development of preparation technologies, researchers have successfully prepared Ti₃AlC₂ materials using various methods, such as hot-press sintering, hot isostatic pressing, and spark plasma sintering. However, these methods are generally associated with high preparation costs and complex processes, small sizes, which significantly limit the large-scale production and practical application of Ti₃AlC₂ materials. In this study, a simple and low-cost pressureless sintering process was used to successfully prepare large-sized Ti₃AlC₂ materials. Through systematic experiments, the key process parameters were optimized, and the effects of different processing conditions on the microstructure and properties of the materials were investigated. [Methods] The precursor powders with controlled stoichiometric ratios were precisely weighed based on the predetermined chemical composition (molar ratio) and homogenized through mechanical mixing. The blended powders were subsequently loaded into an alumina crucible and positioned in the central heating zone of a horizontal tube furnace. Prior to thermal treatment, the reaction chamber was purged with argon gas (99.999%) for 30 minutes at ambient temperature to achieve an oxygen concentration below 10 ppm. The furnace temperature was linearly elevated to the target range of 1200–1500 ℃ at a constant heating rate of 3 ℃·min⁻¹ under continuous argon flow (100 sccm). Isothermal holding was maintained for a defined duration, followed by controlled cooling at an identical rate of 3 ℃·min⁻¹, with the protective gas atmosphere sustained throughout the entire thermal cycle. The phase composition of pressureless-sintered Ti₃AlC₂ was characterized by X-ray diffraction (XRD, Rigaku D/max-RP) using Cu K_α radiation (λ=1.5406 Å). The diffraction patterns were acquired over a 2θ range of 5°–80° with a step size of 0.02° and a counting time of 2 s/step. Microstructural evolution was investigated using field-emission scanning electron microscopy (FE-SEM, Zeiss Ultra Plus) operated at 20 kV accelerating voltage. [Results] In this study, the effects of carbon source type, sintering temperature and holding time on the synthesis of Ti₃AlC₂ MAX phase were systematically investigated. By comparing the synthesis effects of graphite and TiC carbon sources, it was found that when graphite was used as the carbon source, the XRD pattern (Fig. 1) showed a significant (002) crystallographic peak at 2θ=9.1°, corresponding to the main phase of high purity Ti₃AlC₂, with a narrower half-peak width, indicating a high degree of order in the crystal structure and a good degree of crystallinity, whereas when TiC was used as the carbon source, the intensity of the (002) peak was significantly reduced and the half-peak width increased, indicating a poor degree of crystallinity of the product. The SEM analysis (Fig. 2) further shows that the Ti₃AlC₂ crystals synthesized by the graphite carbon source can reach about 100 μm in size, with ordered lamellae stacking and few heterogeneous phases, while the TiC carbon source products have an uneven size distribution and contain a large number of heterogeneous phases. The analyses showed that the carbon vacancies in TiC hindered the formation of titanium-carbon covalent bonds, leading to the limitation of crystal growth.Based on the optimized carbon source (graphite), the sintering temperature experiments (Fig. 3) show that the product contains a small amount of TiC and Ti₃Al impurity phases at 1450 ℃; when the temperature is increased to 1600 ℃, the impurity phase disappears and the purity of the Ti₃AlC₂ phase is significantly increased, suggesting that the high temperature helps to overcome the reaction potential barrier. The holding time study (Fig. 4) shows that when the holding time is extended from 3 h to 9 h at 1600 ℃, the intensity of the Ti₃AlC₂ characteristic peaks is significantly enhanced and the crystal size increases, but the content of heterogeneous phases does not increase significantly, indicating that the extension of the holding time can improve the quality of the crystals. Finally, large domain size and high purity Ti₃AlC₂ crystals were successfully prepared under the optimal process (graphite carbon source, 1600 ℃, 9 h) (Fig. 5), with an average lateral size of 64 μm and a maximum size of 130 μm. SEM showed that the crystals had a regular lamellar structure with very few heterogeneous phases. [Conclusion] In this study, the controlled preparation of Ti₃AlC₂ MAX phase ceramic materials was achieved by a pressureless sintering process, which has the advantages of low equipment requirements and low cost. Based on a multi-parameter synergistic optimization strategy, the effects of raw material composition (different carbon sources: graphite, TiC), sintering temperature gradient (1450–1600 ℃) and holding time variables (3/9 h) on the material composition and microstructure were systematically investigated. The experimental results show that Ti₃AlC₂ ceramic materials with typical layered features were successfully prepared using graphite as the carbon source under the optimised conditions of isothermal sintering at 1600 ℃ for 9 h. The maximum transverse size was up to 130 μm, and the average transverse size was up to 64 μm. The present study can provide a good academic support for the synthesis of high quality, large domain size MAX and subsequent MXene, which will enhance MXene's excellent intrinsic physical properties.