High-entropy ceramics, a novel class of single-phase ceramic solid solutions consisting of near-equimolar multielement species, are recently attracting tremendous attentions. Especially, the transition metal non-oxide high-entropy ceramics, such as transition metal carbide and boride high-entropy ceramics, have been proposed for potential applications in aerospace, nuclear energy, high-speed machining and many other extreme environments, owing to their excellent physical and chemical properties including super-high hardness, low thermal conductivity, good oxidation resistance and corrosion/erosion resistance. Recently, the research of high-entropy ceramics is only focused on composition design, fabrication methods, single-phase stability and mechanical properties, but the design criterion and theoretical analysis are rarely reported. Based on the researches of high-entropy alloy, the fabrication, characterization and theoretical study of several transition metal non-oxide high-entropy ceramics are summarized, along with some related results of high-entropy film. The prospects for the future developments of high-entropy ceramics are also discussed.

Boron nitride aerogel is a kind of new nanomaterials with three-dimensional porous network structure, which takes solid as the framework and gas as the dispersion medium. It has high specific surface area, high porosity, low density and other excellent properties. In addition, compared with graphene aerogels, it exhibits better insulation, oxidation resistance, thermal stability and chemical stability. These outstanding properties make it promising application in the fields of gas adsorption, catalysis, sewage purification, thermal insulation/conduction. This article systematically reviewed the preparation methods of boron nitride aerogels including the hard template method, soft template method, low-dimensional boron nitride assembly method, and template-free method in the light of domestic and foreign research status. Moreover, the important applications of boron nitride aerogels in key fields are summarized, and the future development direction is prospected.

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.

To study the respective growths of micro-arc oxidation (MAO) ceramic coatings under three phase duty cycles of 40%-50%-60%, 50%-60%-40%, and 60%-50%-40%, and to thereby improve the compactness and hydrogen permeation resistance of the MAO ceramic coatings on the surface of zirconium hydride, the zirconium hydride matrix went through MAO treatment under constant voltage in the phosphate electrolyte system. On the one hand, the morphology, phase structure, and thickness of the ceramic coatings were analyzed by using scanning electron microscope (SEM), X-ray diffractometer (XRD), and film thickness meter. On the other hand, the hydrogen permeation resistance of the ceramic coatings under the different phase duty cycle was obtained through a vacuum dehydrogenation experiment. The research results indicate that under three phase duty cycle, the accumulated thickness of zirconium oxide ceramic coatings on the surface of ZrH_1.8 are 162.6, 175.9, and 158.7 μm, respectively; all of the produced MAO ceramic coatings consist of three phases, namely M-ZrO₂, T-ZrO₂, and Zr_0.95Ce_0.05O₂. Phase duty cycle has no significant effect on the phase composition of ceramic coatings. Under the phase duty cycle of 40%-50%-60%, the ceramic coating achieves on the surface of zirconium hydride has the thickness of 162.6 μm and the Permeation Reduction Factor (PRF) value of 12.5, indicating a relatively satisfactory hydrogen permeation resistance.

In this work, an efficient approach was presented to produce stable zirconium carbide (ZrC) fibers by centrifugal spinning technique. Zirconium acetate and sucrose were used as the zirconium source and carbon source respectively. And polyvinyl pyrrolidone (PVP) was used as the spinning aid. After pyrolysis and carbothermal reduction at 1600 ℃, the as-spun green fibers can be converted to homogeneous ZrC fibers composed of uniform nano-sized ZrC crystals. Moreover, it is revealed that the as-fabricated ZrC fibers can sustain superior microstructure stability at an ultra-high temperature up to 2000 ℃, benefiting from a limited amount of residual carbon. The reaction mechanisms and the resulted ZrC fibers were also investigated.

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.

Chemical vapor deposition (CVD) is an effective method for preparing large-size and high-quality graphene materials. The properties of the metal catalysts are direcly related to the quality of the prepared graphene films, so the surface pretreatment of the metal catalysts is required. In this study, the effects of different pretreatment methods on copper substrates are investigated, and the combination of passivation paste pickling and electrochemical polishing is proposed to be an effective method to modify the surface morphology of copper catalyst. The electrochemical polishing parameters (such as voltage, time) and the copper substrate annealing parameters (such as annealing temperature, time) are systematically studied. This study demonstrates that high electrochemical polishing voltage and long polishing time easily lead to the excessive polishing. It is appropriate to set the polishing voltage and polishing time to 8 V and 8 min, respectively. It is found that the annealing temperature and time have significant effects on the grain size of the copper catalyst. After annealing at 1000 ℃ for 30 min, the grain is larger and more uniform. In addition, the structure characterization of graphene prepared by CVD is also performed. According to the SEM image and Raman spectrum, the few-layer, high-quality graphene film is successfully prepared.

Mixed anion compounds can generate the emergence of novel properties that differ from those with mono-type anion due to the difference of electronegativities, ionic radii, polarizabilities, and oxidation states between unlike anions. Abundant research has been conducted on metallic mixed-anion materials with potential application in electronics, detectors of moisture, gas sensors, electrodes for solar batteries, etc. The flux method has been widely applied for mixed-anion crystal growth, which based on metathetical reaction with appropriate metal-salts ?ux under mild conditions. It is meaningful to synthesize the mixed anion compounds by the flux method. Single crystals of tungsten oxychloride Li₂₃CuW₁₀O₄₀Cl₅ were prepared via CuCl₂ flux-growth method by two steps, which using high quality and phase-pure polycrystalline Li₄WO₅ as precursor. The crystal structure was determined by single-crystal X-ray diffraction analysis, which indicates that Li₂₃CuW₁₀O₄₀Cl₅ crystallizes in P6₃/mcm space group (a= 1.02846(3) nm, c=1.98768(9) nm, V=1.82076(11) nm ³, and Z=2). There are crystallographically independent five Li, two W, one Cu, two Cl, and five O atoms in the unit cell, where W(1) atoms are coordinated with one Cl and five O atoms in a distorted octahedra geometry, while W(2) atoms are connected with four O atoms in a tetrahedral coordination. The Cu atoms are connected with six O atoms forming [CuO₆] octahedra. Thus, the crystal structure of the titled compound consists of [CuO₆] and [W(1)O₅Cl] octahedra, and [W(2)O₄] tetrahedra. The successful synthesis of tungsten oxychloride Li₂₃CuW₁₀O₄₀Cl₅ through flux-growth method is meaningful for explore new mixed anion compounds in future.

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 ℃.

Hollow fiber ceramic membranes have been widely accepted in membrane separation due to their advantages of high packing density, low transfer resistance and long-period operation. Fabrication of highly-asymmetric hollow fiber membrane is helpful to achieve high flux as well as high rejection simultaneously. In this work, dual-layer hollow fiber ceramic composite membranes were prepared by the co-extrusion method. The inner and outer suspensions were doped with α-Al₂O₃ powders with average particle sizes of 1 μm and 300 nm respectively. Effects of TiO₂ content in inner suspension, Al₂O₃/polyether sulfone (PESf) mass ratio of outer suspension and calcination temperature on structure and properties of membrane were investigated extensively. When the TiO₂ content was 2wt% in inner suspension, the Al₂O₃/PESf mass ratio was 5.60 in outer suspension and the sintering temperature was 1350 ℃, the hollow fiber membrane got the optimum performance, with fracture load of 24 N, average pore size of 0.15 μm, and oil rejection of 97.5%.

The creep properties of 2D-SiC_f/SiC composites prepared by chemical vapor infiltration were studied. The creep temperatures were 1200, 1300 and 1400 ℃, and the stress levels ranged from 100 MPa to 140 MPa. Scanning electron microscope was used to observe the fracture morphology, and their microstructure was analyzed by high resolution transmission electron microscope. The results show that the creep damage modes of 2D-SiC_f/SiC composites mainly include the generation of matrix crack, interfacial debonding and fiber creep. The creep of bridging fibers leads to increase of the opening distance of the matrix cracks and further creep rupture of the composite. The microstructural stability of the SiC fiber plays a critical role in the creep properties of 2D-SiC_f/SiC composites. SiC grains in the fibers of 2D-SiC_f/SiC composites do not grow when it is crept at 1200 ℃/100 MPa. However, the grains grow significantly when the creep temperature increases to 1400 ℃. The creep rupture time decreases to 8.6 h from above 200 h, and the steady-state creep rate increases by three orders of magnitude.

In order to investigate the influence of precursors on impregnation behaviors of C/SiC composites, C/SiC composites (C/SiC-0,C/SiC-Ⅰand C/SiC-Ⅱ) prepared with three different precursors (solid polycarbosilane, PCS(s)), liquid polycarbosilane PCS-Ⅰ(l) and PCS-Ⅱ(l)) were prepared via precursor infiltration and pyrolysis (PIP) method. Impregnation behaviors of different precursors were studied to mainly focuse on the combination of mechanical properties as well as microstructures of C/SiC composites with different PIP cycles. Results showed that C/SiC-Ⅰ composites exhibited the highest flexural strength of 336 MPa. The microstructures of C/SiC composites showed that the internal pores of C/SiC-0 composites were distributed between carbon fiber bundles, C/SiC-I composites were dense and the pores were evenly distributed. The pores of C/SiC-II composites were inside carbon fiber bundles and SiC matrix. Gel permeation chromatography (GPC) results showed that due to the difference of molecular weights of the impregnating solution, the macromolecules cannot impregnated into the carbon fiber bundles, which resulted in the lack of SiC matrix and degradation of mechanical properties for the composites after PIP cycles.

BN and BN/SiC interphases were deposited on the surface of SiC fibers by CVI process, and the mechanical properties of the as-received and coated fibers were evaluated. SiC_f/SiC minicomposites were prepared by PIP using the as-received, BN-coated and BN/SiC coated fiber bundles as reinforcements. The effects of interphases on the mechanical properties of the composites were studied. The results show that the interphases prepared by CVI process are uniform and compact. The deposited BN interphase contains hexagonal phases with small crystal size (1.76 nm). The deposited SiC interphase has better crystallinity and larger grain size (18.73 nm) than BN interphase. The elastic modulus of coated SiC fibers shows basically no change, but the tensile strength decreases. The maximum tensile load and fracture strain of SiC_f/ BN/SiC and SiC_f/(BN/SiC)/SiC minicomposites are significantly increased, in comparison to SiC_f/SiC minicomposites. It can be seen from the cross-sections of SiC_f/BN/SiC and SiC_f/(BN/SiC)/SiC mini-composites that the fibers with interphases pull out obviously relative to SiC_f/SiC mini-composites, and the BN interphases played a reinforcing role in the tensile fracture process of the composites. The composites with interphases exhibit obvious fiber pull-out resulting in more energy consumption during the fracture, so that the composite can endure more load.

To obtain high performance needled C/C composites, a series of needled non-woven carbon fiber felt with different characteristics was prepared. The needled C/C composites were prepared by means of high-pressure impregnation-carbonization, and their microstructure features, mechanical properties and thermo-physical properties of needled C/C composites were characterized. The investigation results show that the types of preform structure have obvious effects on the mechanical and thermo-physical properties of the C/C composites. When the preform is produced with key characteristics of needling depth at 12 mm, needling density at 22 pin/cm ² and fiber web/weft less ply at 1.0:4.8, the needle C/C composites shows excellent comprehensive performance, with tensile strength, compression strength, flexural strength, in-plane shear strength, and interlaminar shear strength of 207, 228, 285, 54 and 28 MPa, respectively.

Significant matrix cracks and residual pores may form within ceramic matrix composite which was prepared by polymer infiltration and pyrolysis (PIP) method. Explore the matrix cracking mechanism and the crack evolution behavior would benefit the design and performance optimization of the composite. A spinning PIP process in vacuum was adopted to prepare mini C/SiCN composite without weak interphase. The tensile properties and the matrix crack evolution phenomena were analyzed. The influences of PIP cycles and heat-treatment temperatures were discussed. Results showed that the compositions are essentially the same for the samples heat-treated at 1000-1400 ℃. While the heat-treatment temperature rises to 1600 ℃, the precursor-derived SiCN matrix decomposes, with the carbon content decreased and SiC content increased significantly. As PIP cycles increased from 1 to 4, the average tensile strength of the composite increased by 14.19%, 38.83%, and 63.47%, respectively. The matrix crack spacing and crack opening distance gradually decreased, and the bonding between matrix and fiber was enhanced, leading to little fiber pull-out. When the heat-treatment temperature increased from 1000 ℃ to 1400 ℃, the tensile strength of the composite changed slightly. In contrast, when heat-treatment temperature roses to 1600 ℃, the SiCN matrix was transformed from amorphous SiC_xN_4-x tetrahedron structural units to SiC crystals. Then the matrix and the fiber debonded, resulting in their bonding strength weakened. As a result, the tensile strength of the C/SiCN composite decreased by 30.0%, due to a combined effect of interfacial debonding and fiber damage.

The effects of impact energy on damage characteristics and tensile properties of 2D C/SiC-ZrC composites were studied by low-speed impact, post-impact tensile tests and the CT scanning methods. The results indicated that C/SiC-ZrC composites had a high impact damage tolerance. The damage state of the composites in the energy range of 15-24 J was mainly shown as penetrating damage. When the impact energy increased within the range of 15-24 J, the nominal tensile strength of C/SiC-ZrC composites reduced slowly, with a maximum decrease of around 25%. The impact mainly caused lamination and fiber fracture damage near the impact areas. However, the impact damage was not observed out of the impact areas.

Ceramic matrix composites (CMCs) are promising candidates for application in aeroengine, aerospace aircraft thermal protection systems, nuclear power system, and other fields. At present, CMCs are developing from structural bearing materials to multi-functional composites. MAX phases are a group of layered ternary ceramics with excellent plastic deformation capacity, high electrical conductivity, good irradiation resistance and ablation resistance. Besides strengthening and toughening CMCs, the introducing MAX phases into CMCs can effectively improve the anti-irradiation, anti-ablation and electromagnetic interference shielding performance, meeting requirements of multi-functional CMCs. This paper reviewed the progress on MAX phases modified CMCs, design mechanism and application prospect.

Mo₂Ga₂C, a double-A-layer MAX, is reported to be films or powders. This paper researched the sintering properties of M₂Ga₂C powders to make dense bulk samples by vacuum hot pressing. It was found that 750 ℃ was a suitable sintering temperature, while higher temperature (850 ℃) resulted in decomposition of Mo₂Ga₂C yielding to main product of Mo₂C. During sintering process at 750 ℃, its grain size did not increase obviously with sintering time, meanwhile the size of pores decreased markedly and the relative density increased significantly with the increasing sintering time. Additionally, the hot-pressed samples had obvious texture. Due to layering, some grains changed their orientations during sintering, of which most of the (00l) planes in the hot-pressed samples preferred to be perpendicular to the direction of hot press. Almost fully densed Mo₂Ga₂C bulk (relative density: 98.8%) was obtained by hot pressing at 750 ℃ for 8 h. This advantage of the method suggested that it can serve as a promising preparation for Mo₂Ga₂C, a double-A-Layer MAX.

Equal channel angular pressing (ECAP) followed by heat treatment was carried out to prepare Ag/Ti₃AlC₂ composites. Effects of heat treatment on the electrical resistivities and mechanical properties of the Ag/Ti₃AlC₂ composites were investigated. Results show that ECAP effectively densifies the Ag/Ti₃AlC₂ compacts, and layered Ti₃AlC₂ particles are delaminated and aligned due to shearing effect during ECAP. Alignment of Ti₃AlC₂ particles resulted in anisotropy of electrical and mechanical properties of the composites. Perpendicular to the alignment of Ti₃AlC₂ particles displayed high resistivity and compressive strength. Moreover, resistivity and compressive strength increased with following heat treatment, yielding the maximum at 800 ℃. These increments are attributed to the enhanced interfacial reactions between Ag and Ti₃AlC₂ at high temperatures. Findings in this study indicate that densification and microstructural control of Ag/MAX composites can be achieved simultaneously by ECAP, while the following heat treatment can tailor their properties.

Cr₂AlC is a representative material in MAX phase family due to its combination of metallic and ceramic properties such as high electrical conductivity, high thermal conductivity, resistance to corrosion, good oxidation resistance. To further improve performance of Cr₂AlC, ZrC as a reinforcement was selected to reinforce Cr₂AlC matrix composites by hot pressing technique. Influence of ZrC content on the mechanical property of ZrC/Cr₂AlC composites has been investigated. The results showed that 10vol% ZrC/Cr₂AlC composite improved flexural strength (715 MPa) and Vickers hardness (7 GPa) by 80% and 106%, respectively, as compared with those of pure Cr₂AlC material. Date from this study indicate that Cr₂AlC MAX possesses broaden application potential.

Four groups of Cr_1-xAl_xN films with different Al contents were deposited on ultrafine cemented carbide substrates by arc ion plating, their morphology, composition, phase structure and mechanical property were investigated via field emission scanning electron microscope, X-ray photo electron spectrometry, grazing incidence X-ray diffraction, nanoindenter and scratch tests. The results revealed that the thickness of the films is 1.28, 1.42, 1.64 and 1.79 μm and the aluminum concentration x is 0.41, 0.53, 0.64 and 0.73, respectively, increased with incremental current of Al target. There is a close relationship between phase structure and composition. Films performed single centered cubic B1 structure when x is 0.41, however, demostrated mixed structure of c-(Cr,Al)N and hcp-AlN when x≥0.53. The dimensions of grains obtained a minimum value of 8.9 nm near x=0.64, as Al content increasing, while the variation in hardness followed an increase-decrease pattern, reaching peak values of 35.3 GPa at x=0.64. The films performed pretty adhesion strength and their critical loads were all over 60 N. Based on all test results, the Cr_1-xAl_xN films exhibited the best combination properties including high hardness of 34.7 GPa, the highest elastic recovery coefficient of 57.4% and the best toughness while x is 0.53.

The micro-arc oxidation (MAO) ceramic coating on AZ31 magnesium alloy was treated by hydrothermal treatment in NaOH solution to study the effect of the concentration of NaOH solution on the microstructure and corrosion resistance of the obtained Mg(OH)₂/MAO composite coating, the formation mechanism and the corrosion mechanism of the hydrothermal film was annalysed at the same time. The results showed that MgO in the surface of MAO ceramic layer was partially dissolved during hydrothermal reaction. And the released Mg ²⁺ was combined with OH ^- in hydrothermal solution to form Mg(OH)₂ nanosheets which deposited on the surface of ceramic layer and inner wall of the pores. With increasing of NaOH concentration of the hydrothermal solution, more Mg(OH)₂ formed during hydrothermal reaction and sealed the inherent defects such as pores and cracks in the MAO ceramic layer, which improved the compactness of the coating. In addition, the electrochemical test results demonstrated that the Mg(OH)₂/MAO composite coating prepared by MAO and hydrothermal treatment achieved better corrosion resistance than the single MAO ceramic layer, and the corrosion resistance of Mg(OH)₂/MAO composite coating increased with the increasing concentration of the hydrothermal solution. And the immersion test showed that Mg(OH)₂/MAO composite coating could provide long-term corrosion protection for magnesium alloy.