Fabrication of porous alumina–zirconia ceramics by gel-casting and infiltration methods

Porous alumina–zirconia ceramics were obtained by infiltrating porous alumina ceramics, which were prepared by tert-butyl alcohol (TBA)-based gel-casting method. Back scattering images of the fracture surface and energy dispersive spectroscopy were performed to obtain composition profiles on the fracture surface and across sections of the sintered composites. The porosity, pore size distribution and compressive strength were also investigated. The results show that the content of zirconia can be adjusted effectively by infiltration times and it decreases with increasing distance from the surface of the samples. The porosity and compressive strength can also be controlled by the infiltration times. With increases of the infiltration times from 1 to 3 cycles, the open porosity decreases slightly from 62.43% to 56.62%, while the compressive strength of the porous alumina–zirconia ceramics increases from 13.57 ± 1.21 to 26.87 ± 2.01 MPa, indicating that the porous ceramics with high porosity and high strength can be prepared by TBA-based gel-casting method combined with the infiltration process.     

Preparation of c-axis textured SiC ceramics by a strong magnetic field of 6 T assisted gel-casting process

C-axis textured SiC ceramics were prepared by a strong magnetic field of 6 T assisted gel-casting and subsequent pressureless sintering. The optimal suspension parameters for gel-casting were determined by analyzing the influences of pH value and dispersant content on the stability and dispersibility of suspensions. The effect of sintering conditions on the texture development and properties of SiC ceramics was discussed. It was found that the increasing sintering temperature or holding time promoted the densification process of SiC ceramics. The c-axis of SiC grain was aligned parallel to the magnetic field by applying a strong magnetic field of 6 T. The degree of texture of SiC ceramics showed a slightly increasing trend with the increase of sintering temperature or holding time. When the samples were sintered at 1950 °C for 4 h or 6 h, the large elongated grains were formed in the samples, leading to the extremely evident anisotropic microstructure on different planes. Textured SiC ceramics exhibited the anisotropic bending strength.     

Electromagnetic wave-transparent porous silicon nitride ceramic prepared by gel-casting combined with in-situ nitridation reaction

To achieve the balance between mechanical properties and electromagnetic wave-transparent properties of porous silicon nitride (Si₃N₄), the key is to form an interlocking microstructure constituted by columnar β-Si₃N₄ crystals. This structure can be realized by liquid-phase sintering. However, grain boundaries which affect high temperature properties and volume shrinkage during sintering are inevitable. We proposed a strategy to realize this structure by gel-casting of β-Si₃N₄ whisker (Si₃N_4w) and Si powder followed by in-situ nitridation of Si. To achieve chemically-stable slurry containing micro-sized Si with low viscosity, a novel formulation was developed. Two key structural parameters of the interlocking Si₃N_4w network, i.e., density of the Si₃N_4w skeleton and inter-whisker bonding mode, were adjusted by composition of raw materials and nitridation temperature. The flexural strength, dielectric constant and loss of the porous ceramics are 44.9 MPa, 2.7 and 2 × 10⁻³, when the volume fraction of Si₃N_4w/Si is 5 and the nitriding temperature is 1400 °C.     

原位生长柱状晶氧化铝陶瓷材料的制备

按96瓷配比,在α-Al2O3中加入烧结助剂(氟化钙-高岭土),通过水基凝胶注模成型出氧化铝陶瓷坯片.在1570℃,2 h条件下无压烧结,制备出原位生长棒状晶自增韧氧化铝陶瓷.研究结果表明:1570℃烧结时完全生成棒状晶,棒晶呈现三维网络状交织在一起,样品的体积密度达到了3.77 g/cm3.而不含氟化物添加剂的样品在1500～1600℃下烧结时,不能生成氧化铝棒晶,基本呈椭球状或球状.加入少量的氟化物,经过高温烧结,即可在组织内部原位生成棒状晶,这样棒状晶就起到了纤维或晶须的强韧化作用,大大提高了96瓷氧化铝陶瓷的力学性能,断裂韧性达到了5.25 MPa·m1/2,比传统96瓷氧化铝韧性提高了60%.     

ZrO2表面改性及凝胶注模研究

凝胶注模成型工艺能够制备出高强度,高致密性,且形状复杂的氧化锆陶瓷.粉体团聚对凝胶注模成型工艺存在不良影响,粉体团聚导致陶瓷内部产生缺陷,导致致密性、均匀性变差,力学性能降低.本文通过对氧化锆粉体表面改性,改善粉体团聚,将改性粉体用于凝胶注模成型,制备出低粘度浆料.通过SEM、粘度仪、显微镜等仪器,对强度、粘度及粉体分...     

NiO／YSZ凝胶注模工艺流变特性研究

用水基凝胶注法制备同体氧化物燃料电池（SOFC）阳极材料NiO／YSZ是目前的研究热点之一。本文研究了凝胶注模工艺中固相含量、分散剂和pH值对NiO／YSZ陶瓷料浆流变性质的影响。结果表明,Ni0／YSZ水基料浆为假塑性流体;当pH=9、分散剂用量为0．012g／ml、固相含量为45％（体积比）的NiO／YSZ水基料浆,稳定性好,满足浇注成型工艺要求,适合成型SOFC阳极材料NiO／YSZ。     

Gelcasting of dense and porous ceramics by using a natural gelatine

An improved gel-casting procedure was successfully exploited to produce porous ceramic bodies having controlled porosity features in terms of mean pore size, total pore volume as well as pore geometry. The gel-casting process in which a natural gelatine for food industry is used as gelling agent was firstly set-up to prepare dense alumina and zirconia components. Then, commercial PE spheres, sieved to select proper dimensional ranges, were added to the starting slurries as pore-forming agent. Both alumina and zirconia porous bodies were then produced, having a porosity ranging between 40 and 50 vol%. The fired components were characterised by spherical pores surrounded by highly dense ceramic walls and struts, having a homogeneous and fine microstructure. Their mean pore size was directly dependent on the sieved fraction of the starting PE spheres selected as pore-forming phase.     

Fabrication of ZrB2-SiC ceramic composites by optimized gel-casting method

The ZrB₂-SiC ceramics with homogenous microstructures were successfully fabricated by the optimized gel-casting method and pressureless sintering. The effects of the processing parameters, including the monomer content, cross-linker/monomer ratios, initiator addition, catalyst concentration and polymerization temperature, on the final gel properties were systematically investigated. The rheological behavior and stability of ZrB₂-SiC suspensions were evaluated, and the microstructures and mechanical properties of sintered ceramics were also analyzed. The homogenous gel networks containing low monomer content (4 wt%) and without catalysts had been successfully obtained, which could be mainly attributed to the homogeneous distribution of crosslinking points and temperature-induced gelation. The crack-free and complex-shaped ZrB₂-SiC ceramic composites were achieved by the optimized gel-casting, which could reach the highest relative density of 97.2% and the flexural strength of 402 ± 57 MPa, respectively. This study provides an optimized gel-casting process for fabricating ZrB₂-SiC ceramics with excellent properties by low colloidal additives and without catalysts.     

Gel-casting of transparent magnesium aluminate spinel ceramics fabricated by spark plasma sintering (SPS)

In this paper, a transparent magnesium aluminate spinel ceramic was fabricated through the newest colloidal gel casting method, using a synthetic powder with the average particle size of 90 nm and Isobutylene-Maleic Anhydride (ISOBAM) additive. ISOBAM served as both a dispersant and a gelation agent to achieve a dense body. Also, the suspension rheological behavior was optimized by the solid loading of 85 wt%, the additive content of 0.7 wt%, and the gelation time of 350 s. This led to a green body with a density equal to 65% of theoretical density and the green strength of 14.48 MPa. The results revealed that the reduction of porosity and the uniform distribution of pores in the green body (smaller than half of the initial powder particle size, 35 nm), as accompanied by spark plasma sintering (SPS), resulted in the final body density of 99.97%, as well as the high in-line transmittance of 86.7% at the wavelength of 1100 nm.     

High efficiency synthesis of Nd:YAG powder by a spray co-precipitation method for transparent ceramics

A spray co-precipitation method was developed to efficiently synthesize Nd:YAG nano-powders. The effects of spray speeds and solution concentrations on the crystallization processes of calcined precursors have been studied. The results indicated that the pure phase of YAG could be obtained by three different crystallization processes owing to different homogeneity levels of Y and Al mixing. Pure YAG powder was obtained at 850?°C and the phase purity persisted to 1600?°C. Using the obtained powders, transparent ceramics with the in-line transmittance up to 80.2%@400 nm and 83.1%@1064?nm were fabricated by gel-casting method and hot isostatic pressing sintering. Furthermore, the microstructure and laser properties of the transparent ceramics have been measured. The maximum laser output of 7.015?W has been obtained with an oscillation threshold and a slope efficiency of 0.235?W and 59.4%, respectively.