Chabazite (CHA) zeolite membranes exhibit superior performances in light gas separations owing to the eight-membered ring channel structure with small pore size (0.38 nm), adjustable surface characteristics, and high material stability and preparation reproducibility, and have gradually become one of the hot spots of zeolite membrane research in recent years. This review article first introduces the basic characteristics, the two typical CHA zeolite membranes (SAPO-34 and SSZ-13 membranes), then compares the synthesis and preparation methods of CHA zeolite membranes (in-situ synthesis, secondary growth synthesis, microwave heating methods) and analyzes their advantages and disadvantages in application status. The influences of their key synthesis conditions of the secondary growth as the mainstream synthesis method on the qualities of SSZ-13 and SAPO-34 membranes have been elaborated in detail, mainly including 1) the seeding conditions, such as carrier type, seeding crystal and seeding approach; 2) the hydrothermal synthesis conditions, such as crystallization time and temperature, water content, silica-to-alumina ratio, structure directing agent, and cation type; 3) the calcination approaches, such as conventional calcination, staged calcination, and rapid heating treatments. After comparative analysis, the preferred synthesis of the above two typical CHA zeolite membranes are proposed. Furthermore, the modulation of membrane surface chemistry is discussed for the enhancement in gas separation, such as silica-to-alumina ratio adjustment, ion exchange, heteroatom substitution, amino-group functionalization, and surface modification. The detailed characteristics of gas separation in various gas mixture systems and the permeation properties of different single gases on CHA zeolite membranes are analyzed and summarized as well. Finally, the future development of CHA zeolite membranes is prospected.

A thin and dense high-performance T-type zeolite membrane was successfully prepared by a two-step seed crystal induction plus two-step temperature-varied hydrothermal synthesis on inexpensive and macroporous α-Al₂O₃ support. This method can fully perform nucleation of seed crystal, regulate the epitaxial growth and crystal growth direction by changing the hydrothermal crystallization temperature and time during the two-stage. Finally a continuous and defect-free a&b oriented zeolite T membrane was obtained. Effects of crystallization temperature and crystallization time of the first-stage and crystallization temperature of the second-stage on the surface structure and properties of zeolite membranes were investigated. The T-type zeolite membrane prepared under the optimal two-step crystallization condition displayed high pervaporation performance with flux over 3.84 kg·m ^-2·h ^-1 and separation factor higher than 10000 for separation of 90wt% isopropanol/water at 75 ℃.

Amorphous SiBCN powders were prepared by high-energy ball milling-mechanical alloying method. The SiBCN/HfC ceramic composites were consolidated by spark plasma sintering (SPS). The influence of high temperature heat treatment on the microstructural evolution and phase composition was investigated. The results showed that the oxygen was introduced during the mechanical alloying process, leading to the oxidation of BN to form B₂O_3. HfO₂ was formed in the sintering process result from HfC oxydation and reduced to HfB₂ by carbothermal reduction reaction after heat-treatment at 1600 ℃ for 1 h. Both HfO₂ and HfC were reduced to HfB₂ during the heat-treatment at 1650 ℃ and 1800 ℃ by reaction of HfC + C + B₂O₃ → HfB₂ + CO. Introduction of oxygen causes phase transformation of the SiBCN/HfC ceramic composites after heat treatment at high temperatures, during which ceramic matrix becomes loose and porous due to volatilization of the gaseous byproducts. Therefore, control of oxygen content is the key to the real applications of SiBCN/HfC ceramic composites at high temperatures.

Si₃N₄-ZrO₂-La₂O₃ ternary system were prepared via solid-state reaction at 1500 ℃ for 1 h with N₂ protection, yielding coexistence phase of ZrN and lanthanum-based compounds, such as La_4.67Si₃O₁₃, La₅Si₃NO₁₂, La₄Si₂N₂O₇, LaSiNO₂, and La₂Zr₂O₇. Since the generated ZrN and lanthanum-based compounds are not located on the triangle plane of Si₃N₄-ZrO₂-La₂O₃ system, it is necessary to add SiO₂ to extend the ternary into quinary system of Si₃N₄-SiO₂-La₂O₃-ZrO₂-ZrN. After the phase diagram of this quinary system is confirmed and presented, the phase diagram of La₂O₃-SiO₂-ZrO₂ ternary subsystem at 1570 ℃ is further proposed for the first time. In addition, La₂O₃ in the Si₃N₄-ZrO₂-La₂O₃ ternary system can help to simulate the substitution reaction between Si₃N₄ and ZrO₂ to produce ZrN.

Phase diagrams, also known as equilibrium phase diagrams, serve as a road map for materials design. However, preparation process of coatings (such as Physical Vapor Deposition, PVD) is generally far from equilibrium and results in metastable phases. Thus, the CALPHAD (Calculation of Phase Diagrams) approach faces a challenge in calculating the metastable phase diagrams for PVD coating materials. Here we summarized the development of the modeling methodology for the metastable phase diagrams, where the model with critical surface diffusion distance established in recent years were highlighted. The CALPHAD approach, first-principles calculations coupled with high-throughput magnetron sputtering experiments were used to model the atomic surface diffusion, while only one key combinatorial experiment was performed to obtain the basic data for the computation, and the calculated metastable phase diagrams were confirmed by further experiments. Therefore, the database of the stable and metastable phase diagrams can be established, which will be used to guide the design of the ceramic coating materials by the relationship of composition, processing, microstructure, and performance. This model can also help to achieve the goal to shorten the time and reduce the costs of materials research and development.