Choice of additives in the technology of corundum ceramics with a diminished sintering temperature

The special features of the choice of additives in the technology of corundum ceramics with a diminished sintering temperature are considered. It is shown that this choice should provide a structure of the liquid phase that affects considerably the sintering process. The main principles that should be taken into account in choosing additives for fabricating a high-quality ceramic material at a relatively low firing temperature are formulated.     

Corundum ceramics with a low sintering temperature

A corundum ceramics with a high ultimate bending strength up to 550 MPa and an average crystal size of 3 µm is obtained. The initial material was highly active -Al₂O₃ powder with a particle size of less than 1 µm prepared from commercial aluminum hydroxide. The introduction of an addition based on Al₂O₃-ZrO₂ lowered the sintering temperature to 1500°C and gave a virtually poreless uniform fine-crystal structure.Translated from Ogneupory, No. 10, pp. 7–9, October, 1994.     

Development of a corundum ceramic having a low sintering temperature

Conclusions We obtained a vacuum-tight MKh-1 type corundum material (96% Al₂O₃) having a low (up to 1460°C) sintering temperature. The material possesses high levels of electrophysical and physicotechnological properties, meets the specifications [GOST 20419-83 (ST SÉV 3567-82), group 786.1], and it can be used for producing the insulators of the metal-ceramic assemblies.Translated from Ogneupory, No. 1, pp. 15–18, January, 1986.     

Effect of the initial kind of alumina on the properties of corundum ceramics with diminished sintering temperature

The effect of the initial kind of A1₂O₃ powder on the properties of corundum ceramics with a diminished sintering temperature is investigated. It is shown that A1₂O₃ fabricated by calcinating industrial powder of aluminum hydroxide has the best prospects. This material with an addition of partially stabilized ZrO₂ has been used for fabricating a ceramics with an ultimate strength in three point bending equal to 500±40 MPa and a density of 4.19 g/cm³. Additives containing MnO are shown to provide strong ceramics with a diminished sintering temperature.     

氧化铝陶瓷低温烧结的研究现状和发展前景

本文综述了氧化铝陶瓷低温烧结的研究现状,包括粉体的制备,处理,成型及烧结工艺,并对此领域做了相应的设想和展望。     

A benzoate coprecipitation route for synthesizing nanocrystalline GDC powder with lowered sintering temperature

Electrolyte powders with low sintering temperature and high-ionic conductivity can considerably facilitate the fabrication and performance of solid oxide fuel cells (SOFCs). Gadolinia-doped ceria (GDC) is a promising electrolyte for developing intermediate- and low-temperature (IT and LT) SOFCs. However, the conventional sintering temperature for GDC is usually above 1200 °C unless additives are used. In this work, a nanocrystalline powder of GDC, (10 mol% Gd dopant, Gd_0.1Ce_0.9O_1.95) with low-sintering temperature has been synthesized using ammonium benzoate as a novel, environmentally friendly and cost-effective precursor/precipitant. The synthesized benzoate powders (termed washed- and non-washed samples) were calcined at a relatively low temperature of 500 °C for 6 h. Physicochemical characteristics were determined using thermal analysis (TG/DTA), Raman spectroscopy, FT-IR, SEM/EDX, XRD, nitrogen absorptiometry, and dilatometry. Dilatometry showed that the newly synthesized GDC samples (washed and non-washed routes) start to shrink at temperatures of 500 and 600 °C (respectively), reaching their maximum sintering rate at 650 and 750 °C. Sintering of pelletized electrolyte substrates at the sintering onset temperature for commercial GDC powder (950 °C) for 6 h, showed densification of washed- and non-washed samples, obtaining 97.48 and 98.43% respectively, relative to theoretical density. The electrochemical impedance spectroscopy (EIS) analysis for the electrolyte pellets sintered at 950 °C showed a total electrical conductivity of 3.83 × 10^?2 and 5.90 × 10^?2 S cm^?1 (under air atmosphere at 750 °C) for washed- and non-washed samples, respectively. This is the first report of a GDC synthesis, where a considerable improvement in sinterability and electrical conductivity of the product GDC is observed at 950 °C without additives addition.     

Regulation of sintering temperature of zirconia-based ceramics

The results on regulation of the sintering temperature of ceramics by treating the surface of powder particles with organic liquids are presented. It is established that in some cases the sintering temperature is reduced by 350°C.Translated from Steklo i Keramika, No. 10, pp. 25 – 29, October, 1995.     

High-strength TiB2-B4C composite ceramics sintered by spark plasma sintering

Spark plasma sintering (SPS) is an advanced sintering technique because of its fast sintering speed and short dwelling time. In this study, TiB₂, Y₂O₃, Al₂O₃, and different contents of B₄C were used as the raw materials to synthesize TiB₂-B₄C composites ceramics at 1850°C under a uniaxial loading of 48 MPa for 10 min via SPS in vacuum. The influence of different B₄C content on the microstructure and mechanical properties of TiB₂-B₄C composites ceramics are explored. The experimental results show that TiB₂-B₄C composite ceramic achieves relatively good comprehensive properties and exceptionally excellent flexural strength when the addition amount of B₄C reaches 10 wt.%. Its relative density, Vickers hardness, fracture toughness, and flexural strength reach to 99.20%, 24.65 ± .66 GPa, 3.16 MPa·m^1/2, 730.65 ± 74.11 MPa, respectively.     

TiO2和CaO-Al2O3-SiO2复合烧结助剂对刚玉陶瓷烧结的影响

在以α-Al2O3为原料的刚玉陶瓷配料中,分别添加0、0.15%、0.30%、0.45%、0.60%的TiO2和0、0.5%、1.0%、2.0%、4.0%的CaO-Al2O3-SiO2(CAS)玻璃,共组合出25组试验配方,以150MPa压力干压成型为45mm×5mm×4mm的试样,烘干后以5℃.min-1的升温速率升至1550℃保温2h烧成,然后检测烧后试样的相对密度、常温抗折强度和断裂韧性,并用扫描电镜观察烧后试样的显微结构。结果表明:引入TiO2能显著提高刚玉陶瓷的烧结致密化速率,但是引起晶粒的异向生长和异常长大,并使其断裂方式以沿晶断裂为主,试样力学性能也普遍较低;引入少量的CaO-Al2O3-SiO2易引起样品反致密化,只有CaO-Al2O3-SiO2的引入量大于1.3%才能有效地促进刚玉陶瓷的烧结,并抑制由0.6%的TiO2引起的严重的晶粒异向生长和异常长大,使材料的断裂方式向穿晶断裂转变,并使其力学性能逐渐回升。     

Ultralight-weight sol-gel bonded corundum ceramics

The possibility of making a thermostable corundum ceramic material of ultralight-weight, low shrinkage, high strength at high temperatures, and low thermal conductivity was demonstrated. The transformations occurring in mixtures of corundum microgranules and modified ethyl silicate bonding agents were investigated and the structure of the resulting material was examined. Translated from Steklo i Keramika, No. 5, pp. 15–18, May, 1997.     

Recrystallization in disperse corundum ceramics

A technique is proposed whereby recrystallization can be investigated quantitatively in a ceramic material composed of grains widely ranging in size (due to localization of sintering) and containing grain-boundary barriers (such as impurities and micropores) that vary in spatial distribution and quantity. It is shown that only primary recrystallization takes place in alumina sintered at 1550–1700°C and that the grain size distribution is about the same as in a barrier-free material thus implying that the recrystallization proceeds in an effectively homogeneous medium. An addition of 2% Nb₂O₅ to activate sintering leaves the general pattern of recrystallization unchanged and only quantitative differences are noted. These are an increase in the average particle size (because a liquid phase appears and causes a change in the properties of the boundaries and because structure defects are formed when Nb₂O₅ dissolves in Al₂O₃, which accelerates bulk diffusion) and an increase in the number of grain sizes (related to the increased inhomogeneity of contacts between alumina particles). Translated from Ogneupory i Tekhnicheskaya Keramika, No. 1, pp. 13–18, January, 1998.     

Analysis of corundum ceramics strength

Based on 50 compact and porous corundum materials with sintering additives, the dependence of the material strength on the strength of the interparticle contactsP, the interlayer thicknessh, the particle sized, and the amount of additive is analyzed. A linear dependence ofP ond for porous materials and a quadratic dependence ofP ond ² for compact materials are established. It is shown that the parameterP is more sensitive to the structural parameters of the material than the material strength. Translated from Steklo i Keramika, No. 5, pp. 26 – 29, May, 1999.