Sintering behaviour and translucency of dense Eu2O3 ceramics

Eu₂O₃ ceramics have been obtained at sintering temperatures of between 1000 °C and 1550 °C. X-ray diffraction and scanning electron microscopy, in combination with dilatometry experiments, allowed understanding the sintering behaviour. Moderate grain growth followed an efficient densification process between 1400 °C and 1550 °C, which yielded high-density ceramics with an average grain size of 4 μm. The ceramics had Young modulus of 125 GPa, in agreement with the previously published data. The dense Eu₂O₃ ceramics were translucent (35.1% transmittance at 800 nm of 0.8 mm thick discs), showing in addition a slightly pink colour. We propose that the combination of high density and an average grain size of 4 μm is responsible for this novel functionality.     

Ultra-high heating rate densification of nanocrystalline magnesia at high pressure and investigation on densification mechanisms

The pressure-assisted densification method based on combustion reaction heating was applied to prepare dense nanocrystalline ceramics. The densification process of magnesia compact with a particle size of 50 nm was investigated, under the pressure range of 0–170 MPa, and the temperature range of 1620–1880 K with ultra-high heating rate (above 1600 K/min). The pressure was found to have an effect on enhancing densification while suppressing grain growth, and the higher sintering temperature lead to the larger grain size and lower density of the compact. Pure magnesia nanocrystalline ceramics with a relative density of 99.1% was obtained at 1620 K and 170 MPa, and the concurrent grain growth was almost completely restrained. Furthermore, the investigation on the pressure-dependent densification mechanisms including plastic flow, diffusion and power-law creep was also carried out. The result indicated the rate-controlling mechanism was the plastic flow accommodated by grain-boundary diffusion creep.     

Effect of geometric work-hardening and matrix work-hardening on workability and densification of aluminium–3.5% alumina composite during cold upsetting

The densification is a measure of deformation in upset forming of Powder Metallurgy (P/M) processes. A complete experimental investigation on the deformation behaviour of aluminium–3.5% alumina powder composite has been discussed for the case of triaxial stress state condition. Cold upsetting of aluminium–3.5% alumina composite with and with no annealing having different initial preform relative density and with different aspect ratio was carried out and the densification behaviour of the preform under triaxial stress state condition was determined. A new true strain considering the effect of bulging was taken into account for the case of determination of the hoop strain. Plots made for different preforms were analyzed for the densification behaviour of preforms considering the effect of both geometrical work-hardening (GWH) and the matrix work-hardening (MWH).     

Microstructures, densification and mechanical properties of TiC–Al2O3–Al composite by field-activated combustion synthesis

Dense TiC–Al₂O₃–Al composite was prepared with Al, C and TiO₂ powders by means of electric field-activated combustion synthesis and infiltration of the molten metal (here Al) into the synthesized TiC–Al₂O₃ ceramic. An external electric field can effectively improve the adiabatic combustion temperature of the reactive system and overcome the thermodynamic limitation of reaction with x < 10 mol. Thereby, it can induce a self-sustaining combustion synthesis process. During the formation of Al₂O₃–TiC–Al composite, Al is molten first, and reacted with TiO₂ to form Al₂O₃, followed by the formation of TiC through the reaction between the displaced Ti and C. Highly dense TiC–Al₂O₃–Al with relative density of up to 92.5% was directly fabricated with the application of a 14 mol excess Al content and a 25 V cm⁻¹ field strength, in which TiC and Al₂O₃ particles possess fine-structured sizes of 0.2–1.0 μm, with uniform distribution in metal Al. The hardness, bending strength and fracture toughness of the synthesized TiC–Al₂O₃–Al composite are 56.5 GPa, 531 MPa and 10.96 MPa m^1/2, respectively.     

原位聚合增密次数对C/C复合材料性能的影响

以萘为基体碳源,Lewis酸为催化剂,通过对芳烃小分子的催化缩聚建立了一种新型原位聚合C/C复合材料的增密方法。分别对两种不同密度的炭纤维增强C/C复合材料预制体进行致密化处理,研究了原位增密次数对材料体积密度、电阻率、弯曲强度和断面形貌的影响。结果表明:经过五次致密化循环,样品密度分别由原来的1.05 g/cm3和1.68g/cm3提高到1.52g/cm3和1.83g/cm3,电阻率由4.44mΩ.cm和0.84mΩ.cm降至1.09mΩ.cm和0.28mΩ.cm,弯曲强度由26MPa和86MPa增至95MPa和211MPa,说明原位聚合增密方法非常有利于快速提高复合材料的密度和其他物理性能,是一种有前途的增密新途径。     

用于烧结碳化硼陶瓷的低温快速直流烧结设备

碳化硼新型陶瓷具有低密度、高硬度、高模量等优良特性,被广泛用来制造军工防弹装甲、航天核能材料。本文总结与分析了目前烧结碳化硼陶瓷的主要烧结方法,并就本公司研发的用于快速低温烧结碳化硼陶瓷的DCS烧结炉主要结构、参数与控制系统进行了说明与介绍,并成功探索出致密度大于99.5%的碳化硼烧结工艺,烧结温度与烧结时间均大幅下降,降低了碳化硼陶瓷烧结的时间与成本。     

Vacancy ordering in zirconium carbide with different carbon contents

Zirconium carbide (ZrC_x) ceramics with different carbon contents were prepared by reactive hot-pressing. The rock-salt structure of ZrC_x was the only phase detected by x-ray diffraction of the hot pressed ceramics. The relative densities of ZrC_x decreased as carbon content increased, in general. The actual carbon contents were measured by completely oxidizing the ZrC_x ceramics to ZrO₂. For most compositions, the actual carbon contents were higher than nominal batched compositions, presumably due to carbon uptake from the graphite furnace and hot press dies. Selected area electron diffraction and neutron powder diffraction revealed the presence of carbon vacancy ordering in ZrC_x for 0.6 < x < 0.75. Rietveld refinement of the neutron diffraction patterns determined that the crystal structure of the ordered phase was hexagonal, and the carbon site occupancies were higher than nominal batched carbon stoichiometry.     

Cr3C2 assisted ultrafast high-temperature sintering of TiC

The strong covalent bonding of TiC renders its densification through conventional sintering difficult. Here, we propose a method involving liquid-phase-assisted ultrafast high-temperature sintering (UHS) for obtaining nearly full density of TiC ceramics by the addition of Cr₃C₂. The samples were heated at a rate of 600 °C/min to 2200 °C and held at this temperature for 1 min. The effects of sintering parameters and the Cr₃C₂ content on the relative density and microstructure of the sintered samples were investigated. The main causes of rapid densification were particle rearrangement associated with the Cr₃C₂ liquid phase, dissolution-reprecipitation associated with the solid solution, and the weak evaporation of Cr formed during UHS. In particular, the addition of Cr₃C₂ helped increase the hardness and elastic modulus of TiC significantly. This paper presents an effective and extensible method involving UHS for rapidly obtaining dense ceramics.     

Densification,microstructure and properties of ultra-high temperature HfB2 ceramics by the spark plasma sintering without any additives

Ultra-high temperature HfB₂ ceramic with nearly full densification is achieved by using gradient sintering process of SPS without any additives. The effect of the sintering temperature on the densification behavior, relative density, microstructure, mechanical and thermionic properties is systematically investigated. The results show that the fast densification of HfB₂ ceramic occurs at the heating stage, and the highest relative density of 96.75% is obtained at T =1950 °C, P = 60 MPa and t =10min. As the temperature is increased from 1800 to 1950 °C, the grain size of HfB₂ increases from 6.12 ±1.33 to 10.99 ± 2.25 μm, and refined microstructure gives the excellently mechanical properties. The highest hardness of 26.34 ±2.1GPa, fracture toughness of 7.12 ± 1.33 MPa m^1/2 and bending strength of 501 ±10MPa belong to the HfB₂ ceramic obtained at T =1950°C. Moreover, both the Vickers hardness and fracture toughness obey the normal indentation size effect. HfB₂ ceramic also exhibits the thermionic emission characterization with the highest current density of 6.12 A/cm² and the lowest work function of 2.92 eV.