Silicon nitride (Si₃N₄) ceramics have a wide potential application in industrial processes, but high fracture toughness and hardness are always difficult to be handled, thus affecting the application of Si₃N₄ ceramics. In this paper, silicon nitride ceramics were fabricated via spark plasma sintering at 1 600 ℃ with high-entropy boride phase (Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ as sintering aids. The effect of (Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ on the phase assemblage, densification, microstructure and mechanical properties was investigated. Compared with (Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂-free Si₃N₄ ceramic, the addition of only 1.0% (in volume fraction) (Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ in the ceramic can increase the mass fraction of β-Si₃N₄ from 38% to 53% at 1 600 ℃, showing a bimodal microstructure. As a consequence, the fracture toughness of Si₃N₄-based ceramic increases from (5.4±0.3) MPa·m^1/2 to (6.9±0.2) MPa·m^1/2, and the hardness maintains (20.2±0.2) GPa. The relative density of Si₃N₄-based ceramics decreases and the phase transformation enhances with increasing the high-entropy boride fraction to 2.5% and 5.0%, thus leading to a decrease in the hardness.

Transition metal carbide ceramics are typical representatives of ultra-high temperature ceramics with extremely high melting point and hardness. However, the toughness and wear resistance need to be improved. Based on the high-entropy theory, a multi-carbides solid solution, i.e., high-entropy carbides, has a higher melting point, a good toughness and superior friction/wear characteristics. In this paper, (Ti_1/6V_1/6Nb_1/6Ta_1/6Mo_1/3)C high-entropy ceramics were prepared via spark plasma sintering (SPS) at 1 600-2 100 ℃, and the densification behavior, phase composition, microstructure, mechanical properties and wear resistance were investigated. The results show that (Ti_1/6V_1/6Nb_1/6Ta_1/6Mo_1/3)C high-entropy ceramics with a single-phase face-centered cubic structure can be obtained by sintering at 1 700 ℃. The relative density of the high-entropy ceramics is greater than 98% at ≥1 900 ℃. The grain size increases and the elemental distribution becomes uniform as the sintering temperature increases from 1 700 ℃ to 2 100 ℃. The grain boundary sliding and oxide impurities (TiO₂) aggregation occur at grain boundaries. The dislocations appear in grains. The optimal mechanical properties of the high-entropy ceramics prepared by sintering at 2 100 ℃ can be obtained, i.e., the Vickers hardness of 20 GPa, elastic modulus of 431 GPa and fracture toughness of 4.46 MPa·m^1/2, respectively, and the average specific wear rate is 5×10^-7 mm³/(N·m), showing a superior wear resistance.

As a kind of high-entropy ceramics, high-entropy boride ceramics have attracted more and more attention because of their excellent high temperature comprehensive mechanical properties and stability. However, there is no review on the research of high entropy boride ceramics. Therefore, the definition of high-entropy ceramics and the preparation methods of high-entropy borides were introduced. The application of first-principles calculations in the study of high entropy borides, the prediction of synthesis of high-entropy borides and the prediction and understanding of properties are summarized. The advantages and disadvantages of various preparation methods of high-entropy boride ceramic powders and bulks were comprehensively evaluated. Taking mechanical properties as the focus, various physical and chemical properties of high-entropy boride ceramics and their influencing factors and mechanisms were analyzed. At the same time, the shortcomings of high-entropy borides in theoretical calculation, preparation and exploration of material properties are summarized, and the possible future research directions were analyzed and prospected.

Low-temperature hot pressed Si₃N₄ ceramics show a greater Vickers hardness and a lower fracture toughness, while high-temperature hot pressed Si₃N₄ ceramics show a lower Vickers hardness and a higher fracture toughness. To prepare Si₃N₄ ceramics with a great Vickers hardness and a high fracture toughness, a low-temperature hot pressed Si₃N₄ ceramic was prepared via sintering at 1 500 ℃ with 20% SiC whisker (SiC_w, in volume fraction) and 2.5% ZrB₂. The phase composition, relative density, microstructure and mechanical properties of low-temperature hot pressed Si₃N₄ ceramic were investigated, compared with high-temperature hot pressed Si₃N₄ ceramics at 1 800 ℃. The results show that the addition of SiC_w inhibits the densification of Si₃N₄ ceramics sintered at 1 500 ℃, and the relative density decreases from 97.9% to 92.9%, the Vickers hardness decreases from 20.5 GPa to 16.4 GPa, and the fracture toughness increases from 2.9 MPa·m^1/2 to 3.4 MPa·m^1/2. However, the addition of SiC_w and ZrB₂ promotes the phase transition from α-Si₃N₄ to β-Si₃N₄. The relative density of Si₃N₄-SiC_w-ZrB₂ reaches 97.5%, and the Vickers hardness and fracture toughness are 20.7 GPa and 5.1 MPa·m^1/2, respectively. Compared with low-temperature hot pressed Si₃N₄ ceramic, the fracture toughness of Si₃N₄-SiC_w-ZrB₂ ceramic is improved without reducing the Vickers hardness. Compared with high-temperature hot pressed Si₃N₄ ceramic, the hardness is greatly improved. The Si₃N₄-based ceramic with high hardness and toughness prepared via a low-temperature hot pressing with SiC_w and ZrB₂ can have a promising application in ceramic structural parts.

(1-x)BiFeO₃-xBaTiO₃(BF-xBT, x=0.28, 0.29, 0.30,0.31) piezoelectric ceramics were prepared by a conventional sintering technology. The relationship between the phase structure and dielectric, ferroelectric and piezoelectric properties of the ceramics near the phase boundary was investigated. Based on the results by X-ray diffraction, all the samples present a pure perovskite phase structure without any impurity phase, and all the ceramics are located around the rhombohedral-pseudocubic phase boundary. The phase structure gradually transforms from rhombohedral to pseudocubic phase as x increases, and the content of R and pC phase tends to be equal when x=0.30, showing the optimum electrical properties (i.e., piezoelectric coefficient (d₃₃) =165 pC/N, remanent polarization (P_r) = 26.80 μC/cm², and the Curie temperature (T_C) = 465 ℃). In addition, the changes of Raman vibration mode and diffuseness factor of the samples after quenching indicate that the quenching process can effectively improve the long range order degree and the temperature stability of ferroelectric phase of the samples, resulting in a further increase in the Curie temperature of BF-xBT ceramics. For a ceramic BF-0.30BT, the enhanced dielectric and ferroelectric properties are obtained due to its more proper R/pC phase ratio and quenching process. Also, the relationship between the phase structure evolution and piezoelectric properties of BF-xBT ceramics was revealed, and the electrical properties of BF-xBT ceramics were further optimized by quenching process, so as to provide a reference for broadening the application temperature range of the BF-BT system materials.

A series of Ca²⁺-doped cordierite ((Mg_1-xCa_x)₂Al₄Si₅O₁₈) ceramic powders were prepared by a sol-gel method. The effect of the substitution of Ca²⁺ for Mg²⁺ of cordierite ceramics on the sintering properties, structural characteristics, microwave dielectric properties and fracture toughness was investigated. The results show that the introduction of Ca²⁺ can improve the sintering activity of the powder, thus improving the density of the sample and the sintering densification. The single α-cordierite phase is obtained in the range of 0<x≤0.1 because Ca²⁺ is doped into the cordierite structure to form the solid solution. When the x value is larger than 0.1, an anorthite phase appears. The dielectric constant of cordierite ceramic decreases, and the quality factor, the temperature coefficient of frequency and fracture toughness improve when Mg²⁺ substitutes for Ca²⁺. The optimum performance of (Mg_1-xCa_x)₂Al₄Si₅O₁₈ ceramic is obtained when the x value is 0.1, i.e., the dielectric constant of 4.61, the quality factor of 9949GHz and the temperature coefficient of frequency of -26×10^-6/℃, and the fracture toughness of 4.76MPa·m^1/2.

Although engineering ceramics are widely used in industry, its high brittleness is one of the main factors restricting its popularization. This paper was to synergistically improve the strength and toughness of engineering structural ceramics. A wood tree ring-like Si₃N₄/Ni composite was prepared with micron-sized powder of Si₃N₄ and high purity nickel wire via hot pressing sintering. The interface bonding between Si₃N₄ matrix and Ni wire in Si₃N₄/Ni composite was investigated, and the physical and mechanical properties of the composite were characterized. The results show that the interface between Si₃N₄ matrix and Ni wire in the composite is well bonded, and the wood tree ring-like material structure contributes to the strengthening and toughening of ceramic materials. The bending strength and fracture toughness of the composite are (989±87) MPa and (8.12±0.8) MPa·m^1/2, respectively. Compared with single-phase Si₃N₄, the physical and mechanical properties of Si₃N₄ ceramics are improved.

The rich transition metal elements in stainless steelmaking dust, such as Fe, Cr, Ni, and Mn, can be used to prepare black ceramic pigments. The stainless steelmaking dust can be ultilized to prepare cobalt-free black ceramic pigments. According to the color mechanism of Fe-Cr-Ni-Mn black pigment, an appropriate amount of chemical reagents Fe₂O₃, Cr₂O₃, MnO and NiO was added into stainless steel making dust in a mixed mass ratio of 6∶1∶1∶1. The chromaticity value vibration rules of black ceramic pigments prepared under different conditions were investigated, and the coloring mechanism of the pigments was discussed as well. The results show that a high firing temperature or a long holding time causes the decomposition of Cr-containing spinel phase, adversely affecting the coloring properties of the pigment. The optimum firing process of the pigment is the firing temperature of 1 175 ℃, the holding time of 30 min, and the furnace cooling method. The L*, a* and b* values of the prepared pigment are 37.20, 0.13 and 0.02, respectively, and the main phases in the pigment are Fe₂O₃ and mixed spinels. The absorbance of the pigment to each visible light wavelength band is basically the same, and the bandgap of the pigment is 0.88 eV. The color of the pigment is black. The average grain size of spinels is only 26.795 nm, and the pigment has a superior thermal stability at 20-1 100 ℃. The ceramic has a good color performance when the pigment is used to prepare the ceramic. It is indicated that the prepared black ceramic pigment with the superior coloring property has a promising application potential.

The base application of porous ceramic membrane is ceramic support. For conventional ceramic supports (such as alumina), the expensive raw materials and high sintering cost both restrict their applications. Therefore, the low-cost preparation of ceramic support with natural minerals has attracted more attention. In this paper, a series of porous ceramic supports were fabricated with kaolin clay, talc and calcium carbonate as raw materials. The sintering characteristic of the green body was analyzed by thermal expansion instrument. The phase composition, micro-structure, pore size distribution and bending strength were characterized by X-ray diffractometer, scanning electron microscope, mercury porosimeter and universal testing machine. The acid and alkali corrosion resistances of the supports were also investigated. The results indicate that the obtained green body exhibits a superior low-temperature sintering characteristic. The porous supports were prepared via chemical reaction sintering. The suitable sintering temperatures of the supports are in the range of 1 000-1 200 ℃. The obtained supports have a homogeneous micro-structure and a unimodal pore size distribution. The porous supports prepared at 1 000-1 200 ℃ have the open porosities of 49.8%-49.4%, the average pore sizes of 1.09-1.83 μm, the bending strengths of 40.57-28.85 MPa, the nitrogen gas flux of 119-340 m³·m^-2·h^-1 under a trans-membrane pressure of 0.1 MPa and a superior alkali resistance as well.

Silicon carbide ceramic membranes can be used in various water and wastewater treatments due to their high hydrophility, and superior chemical and thermal resistances. However, their applications are restricted due to some corresponding problems like high sintering temperature and then the high production cost. In this work, a low-temperature aid (LTA) bonded SiC ceramic symmetric membrane was sintered at 1 000 ℃. The results show that LTA bonded SiC particles to form a skeleton of the symmetric membrane. The porosity of the membrane decreases from 42% to 35% and the average pore size decreases from 3.5 μm to 2.1 μm with the increase of LTA content from 10%(mass fraction) to 30%. Based on the results of the separation test on oil-in-water emulsion, the oil rejection and flow flux are 93.3% and 123 L/(m²·h) at the trans-membrane pressure of 0.2 bar, respectively. This study provides a low sintering temperature and energy saving strategy for the preparation of SiC ceramic membrane.