Enthalpy driving force and chemical bond weakening: The solid-solution formation mechanism and densification behavior of high-entropy diborides (Hf1−x/4Zr1−x/4Nb1−x/4Ta1−x/4Scx)B2

(Hf_0.2Zr_0.2Nb_0.2Ta_0.2Sc_0.2)B₂ was designed to improve the densification and solid-solution formation of high-entropy transition metal diborides, and its phase stability was predicted using the energy distribution of the local mixing enthalpy of all possible configurations. It was found that (Hf_0.2Zr_0.2Nb_0.2Ta_0.2Sc_0.2)B₂ are enthalpy-stabilized materials. The two-component metal diborides formed by transition metal diborides (HfB₂, ZrB₂, TaB₂ and NbB₂) with ScB₂ are thermodynamically favorable, based on the mixing enthalpy. Therefore, the introduction of ScB₂ in high-entropy metal diborides is beneficial to reduce the mixing Gibbs free energy during the boro/carbothermal reduction process, which enables the formation of single-phase solid solution at low temperatures. Even high-entropy metal diboride powders with large particle sizes, 25–57 µm, can achieve sintered density up to ~97% due to the introduction of ScB₂ in high-entropy metal diborides, owing to its weakening action on the TM d - B p and the TM dd bonding.     

MgO assisted densification of highly transparent YAG ceramics and their microstructural evolution

In current study, various amounts of MgO single dopant was adopted to fabricated high quality transparent YAG ceramics, by utilizing a simple one-step solid state reaction sintering method in vacuum. At a MgO doping amount of only 0.03 wt.%, YAG transparent ceramics with a transmittance of 84.5% at 1064 nm could be obtained, after sintering at 1820 °C for 8 h. The microstructure evolution and optical property of as-fabricated YAG ceramics as a function of MgO doping concentration were systematically investigated. MgO dopant could effectively promote densification of YAG ceramics when the sintering temperature was lower than 1660 °C, and dramatically accelerate its grain growth between 1540 °C and 1660 °C. Further increase the doping amount of MgO would not benefit to the optical quality of YAG ceramics, and the intragranular pores as well as the Mg-riched secondary phase were observed from the MgO heavily doped ceramics.     

Fine-grained dual-phase high-entropy ceramics derived from boro/carbothermal reduction

In the current work, fine-grained dual-phase, high-entropy ceramics (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂-(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C with different phase ratios were prepared from powders synthesized via a boro/carbothermal reduction approach, by adjusting the content of B₄C and C in the precursor powders. Phase compositions, densification, microstructure, and mechanical properties were investigated and correlated. Due to the combination of pinning effect and the boro/carbothermal reduction approach, the average grain size (～0.5?1.5 μm) of the dual-phase high-entropy ceramics was roughly one order of magnitude smaller than previously reported literature. The dual-phase high-entropy ceramics had residual porosity ranging from 0.3 to 3.2 % upon sintering by SPS and the material with about 18 vol% boride phase exhibited the highest Vickers hardness (24.2±0.3 GPa) and fracture toughness (3.19±0.24 MPam^1/2).     

Effect of dual liquid phase sintering aids on the densification and microstructure of low temperature sintered alumina ceramics

Alumina ceramics was prepared by pressureless sintering technology in which a CuO–TiO₂–Bi₂O₃ mixture (0–4.0 wt% Bi₂O₃ and 4.0 wt% CuO and TiO₂) was added as dual liquid phase sintering aids. The phase compositions, microstructural feature, and sintering behaviour of the alumina ceramics were analyzed. The results showed that adding 2.5 wt% Bi₂O₃ to alumina ceramics can increase the contribution rate of initial stage of sintering to the sintering process. The relative density of the sample reached 97.63% after sintering at 1200 °C for 90 min. Measurements from differential scanning calorimetry, with the addition of CuO–TiO₂–Bi₂O_3, demonstrated the formation of two liquid phase points, 827.4 and 936.8 °C. Notably, the solid solution temperature of TiO₂ and Al₂O₃ ceramics diminished thanks to the dual liquid phase sintering aids, and at the same time the activation energy required also dropped from 368.96 to 137.31 kJ/mol. Research indicates that the combined action of dual liquid phase sintering and solid-state reaction sintering has promoted the densification of alumina ceramics during the sintering process while at the same time inhibiting the growth of abnormal grains so that a homogeneous microstructure can be formed.     

Effect of bimodal pores on sintering and residual stresses of YSZ membranes prepared by non-solvent induced phase separation method

Densification mismatch and residual stresses of tri-layered yttria-stabilized zirconia (YSZ) membranes prepared by a non-solvent induced phase separation (NIPS) method were investigated. The tri-layered membrane consisted of sponge-like structures and finger-like voids in macroscale. The densification of the two structures were characterized to elaborate their contribution to the densification mismatch, which led to residual stresses of hundreds of megapascal retained in the sintered membranes. The profile of residual stress suggested that it was related to the strain rate mismatch within the NIPS membranes, which was further quantified with an in-situ monitored camber evolution.     

Theoretical analysis of experimental densification kinetics in final sintering stage of nano-sized zirconia

The experimental densification kinetics of 7.8 mol% Y₂O₃-stabilized zirconia was analyzed theoretically during isothermal sintering in the final stage. By taking concurrent grain growth into account, a possible value of the grain-size exponent n was examined. The Coble’s corner-pore model recognized widely was found not to be applicable for explaining the densification kinetics. The corner-pore model of n = 4 shows a significant divergence in the kinetics at different temperatures. Microstructural observation shows that most pores are not located at grain corners and have a size comparable to the surrounding grains. The observed pore structure is similar to the diffusive model where single pore is surrounded by dense body. The diffusive model combined with theoretical sintering stress predicts n = 1 or n = 2, which shows a good consistence to the measured densification kinetics. During sintering of nano-sized powder, it is found that the densification kinetics can be explained distinctively by the diffusive single-pore model.     

Influence of rare earth Tb4O7 addition on the densification,abrasion resistance and microstructure of alumina ceramics

This study aimed to improve the purity and performance of alumina ceramics used as ball milling media. High-alumina ceramics (>?96?wt% Al₂O₃), with high densification and excellent abrasion resistance, were fabricated by the cold isostatic pressing method. The effects of adding the rare earth Tb₄O₇ on the densification, abrasion resistance, crystalline phase, micro-morphology and grain size of the ceramics were studied. The experiment results showed that the densification and abrasion resistance of the samples increased with Tb₄O₇ addition. The sample with 0.8?wt% Tb₄O₇ sintered at 1625?°C exhibited the best performance, with a linear shrinkage, relative density and abrasion rate of 22.28%, 95.70% and 0.103‰^, respectively. The abrasion resistance improved by 27.5% compared with the sample without Tb₄O₇. X-ray diffraction analysis indicated that the primary phases of the samples were corundum, spinel, CaAl₁₂O₁₉ and α-quartz, and a small quantity of Tb₃Al₅O₁₂ was generated when more than 0.4?wt% Tb₄O₇ was added. Furthermore, some Mg²⁺ and Ca²⁺ ions in the liquid phases dissolved into Tb₃Al₅O₁₂ crystalline grains during the sintering process, which enhanced the grain boundary cohesion of the materials. Scanning electron microscopy indicated that the existence of Tb₃Al₅O₁₂ at grain boundaries reduced the average size of the corundum grains. This helped transform inter-granular fractures into trans-granular fractures, thereby improving the abrasion resistance of the ceramic materials.     

Fabrication and mechanisms of densification of ZrB2-based ultra high temperature ceramics by reactive hot pressing

Dense ZrB₂–SiC (25–30 vol%) composites have been produced by reactive hot pressing using stoichiometric Zr, B₄C, C and Si powder mixtures with and without Ni addition at 40 MPa, 1600 °C for 60 min. Nickel, a common additive to promote densification, is shown not to be essential; the presence of an ultra-fine microstructure containing a transient plastic ZrC phase is suggested to play a key role at low temperatures, while a transient liquid phase may be responsible at temperatures above 1350 °C. Hot Pressing of non-stoichiometric mixture of Zr, B₄C and Si at 40 MPa, 1600 °C for 30 min resulted in ZrB₂–ZrC_x–SiC (15 vol%) composites of 98% RD.     

不同粒径α-Al_2O_3对CABS玻璃-陶瓷的性能影响

初步探索了陶瓷粉粒度对钙铝硅硼玻璃(CABS)-陶瓷烧结性能和介电性能的影响。从而理解致密化机制以及玻璃对不同粒径Al2O3润湿情况。玻璃与不同粒径的氧化铝粉混合均匀,流延成型。结果表明:当陶瓷粉D50为0.83um时,玻璃-陶瓷的介电常数较小,介电损耗较大;当Al2O3粉D50为3.26um时,样品的介电性能显著提升,且介电常数为7.9,介电损耗小于2×10-3。     

微波烧结SiC颗粒增强铝基复合材料

采用微波烧结的方法,在烧结温度分别为680℃,710℃,740℃,770℃,800℃制备了15％的SiCp／Al复合材料。探讨温度对材料的致密度和力学性能的影响。结果表明：致密度和材料硬度及冲击韧性随温度变化呈马鞍形,在770℃样品的密度和硬度及冲击韧性达到最佳值,分别为2．62g／cm3,42．6MPa,40J／cm2。结论：用微波烧结SiCp／Al复合材料可在短时间内使样品达到烧结致密化,缩短烧结时间,节约能源。