A two-stage procedure of preparation of dense strontium zirconate ceramics capable of being used as electrically insulating material in MHD generators is described. A small addition of TiO2 was introduced at the first stage to favour development of the properties, especially low apparent porosity, required for MHD purposes. The effect of TiO2 on sintering temperature, phase composition and microstructure of the bodies is described. Properties of the resulting ceramics i.e. porosity, cold crushing strength, microhardness as well as electrical conductivity over the temperature range of 20–2000°C were measured. The results of successful testing of these ceramics in the channel of an experimental MHD generator are presented.
Kinetic studies have been performed on primary and secondary (minimal) creep of reaction-bonded silicon nitride in 4-pt-bending tests up to 1500°C. The creep deformation depends strongly on the extent of internal oxidation. In spite of the very marked dependence of the creep rate on material and pretreatment parameters, the stress exponents (n = 1.7–1.8) and activation energies (360–390 kJ/mole) are hardly influenced, suggesting similar creep mechanism. Creep deformation is provided by relative motion and separation of grain boundaries. Oxidation and deformation lead to remarkable changes of the pore size distribution; the creep processes are accompanied by deformation of the pores. The creep rupture strain is very limited in highly creep resistant materials and vice versa. Methods for the determination of oxidation products and oxide profiles along sample cross section have been developed and the chemical changes which the material can undergo during creep are outlined.
Possible relationships between Young's modulus at room temperature of (1–20) wt% CeO2-fluxed HPSN and some microstructural features such as amount of the intergranular phase, porosity, α/β ratio and crystallographic β texture have been investigated. The dependence of Young's modulus on amount of integranular phase expressed in terms of the Cohen-Ishai model resulted in the relation: E = 98.0 [1 + VN(1.45 − VN13)] GN/m2. A quadratic relationship, E = 290.0 (1 — AP + BP2), proved the most adequate to account for the effect of porosity on Young's modulus over the 0–36% porosity range. αβ ratio and β texture did not seem to have an appreciable effect.
The kinetics of the solid state high-temperature transformation of kyanite (Al2SiO5Al2O3·SiO2) powders (≤40 μm) to 3:2-mullite (3Al2O3·2SiO2) and silica (SiO2 were investigated by means of quantitative X-ray diffraction techniques. The transformation interval was found to lie between about 1150 and 1350°C. The reaction law best fitting the kinetic data is: 1-α = kta. The transformation is believed to be reconstructive, with decomposition of the kyanite structure, solid-state atom diffusion, and (epitactic) rearrangement of mullite and cristobalite. Cristobalite represents part of the ⪡free⪢ silica, the rest being present as a glassy phase. Addition of Fe2O3 and TiO2 to the starting material exerts a marked decrease of the transformation temperature, with TiO2 having a somewhat stronger influence than Fe2O3. The reason may be an oxide-catalyzed reaction; the decomposition begins at nuclei formed at the surfaces of the kyanite particles, which are coated with thin layers of hematite and rutile respectively.
The induced damage effects of 14.3 Mev neutron fluxes on quartz-based porcelain and alumina-based porcelain were studied using X-ray diffraction analysis and electric, resistivity measurements. The results showed that both the degree of crystallinity and the electric resistivity of the samples decreased upon irradiation. Resistivity values plotted as a function of neutron flux proved to fit semi-empirical formulae that could be used for measuring neutron fluxes (1010−2 × 1011 n/cm2) in the presence of gamma radiation (up to 5 megarads). Moreover, plots of the area under the X-ray peaks as a function of neutron flux were found to fit empirical formulae which could be used for the same purpose. The effect of storage at temperatures up to 50°C and for periods of up to 2 weeks on the radiation induced changes in the porcelain were studied.
Non-metallic nitrides and alloys based on them are used as refractory and electric insulating materials in modern high temperature technology. The properties of these materials depend on powder production techniques and on technological industrial processes. With efficient control of production conditions and chemical composition, materials of given property levels can be obtained.
Results of an investigation of ceramics based on yttria (Y2O3), scandia (Sc2O3) and aluminum nitride (AIN) are given. Test procedures used for evaluating their mechanical behaviour as well as their thermal shock resistance on quenching and monotonic heating are described. Special emphasis was placed on yttria from which one- and two-phase materials were fabricated. It was found that, of the materials studied, ceramics of AIN possessed the highest strength and thermal shock resistance. The analysis involved also the use of data from fractographic studies.
Results of experiments to study the production of dolomite clinker having a density close to the theoretical density of pure sintered dolomite are discussed. Density values are reported for samples obtained by the following process: decarbonization of dolomite, hot briquetting at 300–700°C, firing the semi-product at 1500°C. A similar procedure was also developed using salt-doped dolomite. The technology used leads to especially good results when dealing with ‘hard to sinter’ dolomites. It also avoids the necessity to use extremely high temperatures for the sintering process, thus making the process more economical from the point of view of energy consumption.
Reaction-sintered silicon nitride was investigated to determine the effect of its pore size on thermal stress resistance to fracture initiation. Samples of controlled pore structure were prepared by using an organic component to incorporate pores in the green silicon compact as well as by using silicon starting powders with different particle size. Critical temperature differences ΔTc after water quenching is discussed in relation to changes in most important variables affecting thermal shock, such as fracture strength, Young's modulus of elasticity and thermal conductivity. The results show that when total porosity as well as other microstructural parameters are held constant, an increase in pore size leads to a decrease in ΔTc. Moreover, the results indicate that thermal conductivity plays a significant role in the interpretation of the thermal shock behaviour of reaction-sintered Si3N4.
The flexural strength, elastic modulus, fracture toughness (Ktc) and grain size were determined for a partially stabilized calcia-zirconia alloy (Ca-PSZ) which was progressively aged at 1300°C. Data for the same properties were obtained also for a fully stabilized cubic magnesia-zirconia alloy (Mg-CSZ) which was used as a reference material. The growth of the zirconia precipitate phase in the Ca-PSZ material was monitored. The flexural strength and fracture toughness increased smoothly to peak values of 645 MPa and 9.6 Mpa m12, respectively, at a critical value of the ageing time and thereafter declined rapidly. The precipitate phase coarsened during ageing. Its structure was tetragonal up until the critical ageing time and thereafter the majority of the particles transformed to monoclinic. The peak strength increased three times relative to the cubic stabilized material. The grain size and elastic modulus showed only a slight dependence on ageing time. The study confirmed the hypothesis that the enhanced strength of transformation toughened zirconia alloys arises from an increase in the fracture energy. This increase is brought about by the presence of tetragonal particles, metastable at room temperature, which can be transformed by stress.
Controlled Nucleation Thermochemical Deposition (CNTD) has emerged from classical chemical deposition (CVD) technology. This paper describes the techniques of thermochemical grain refinement. The effects of such refinement on mechanical properties of materials at room temperature and at elevated temperatures are outlined. Emphasis is given to high temperature structural ceramic materials such as SiC, Si3N4, AiN, and TiB2 and ZrB2. An example of grain refinement accompanied by improvements in mechanical properties is SiC. Grain sizes of 500 to 1000 Å have been observed in CNTD SiC with room temperature MOR of 1380 to 2070 MPa (4 pt bending) and MOR of 3450 to 4140 MPa (4 pt bending) at 1350°C. Various applications of these materials to the solution of high temperature structural problems are described.
Using a variety of surface analytical tools, the interfaces of several metal-ceramic composites have been characterized. Three processes that lead to the bonding of a metallizing to a ceramic substrate are illustrated. When a pure refractory metallizing is deposited onto a 94% Al2O3, bonding is achieved by glass migrating from the ceramic into the metallizing during firing. During cooling, the glass forms a mechanical-chemical bond between the ceramic and metallizing. In order to achieve bonding to a 99+% Al2O3 or 99+% BeO, the metallizing itself must contain a sufficient quantity of glass for wetting the ceramic, or be capable of forming a direct chemical bond to the ceramic.
Similar to pressed compacts, slip cast alumina can be sintered to near theoretical density if minor additions of MgO or NiO are made. Sintering kinetics of alumina were analyzed in terms of the Wong and Pask model which was shown to give the more realistic representation of powder compacts. The rate controlling step with and without additions is the migration from the neck along the pore surface rather than diffusion along the grain boundaries to the neck. Both MgO and NiO retard mass transport probably due to changes in surface energy. However, they allow sintering to near complete densification. Although grain growth limitation via second phase inclusions or solute segregation is expected, the final density improvement was shown to result from modifying pore kinetics such that the pores remain attached to the grain boundaries during grain growth until complete densification.
Fine powders (particle size smaller than 0.1 μm) are often used for the fabrication of modern fine-grained ceramics. During die compaction of such fine powders, special phenomena at the powder-wall boundary can be expected because the size of the particles is of the same order as the size of the wall asperities. In the present paper, the principles of powder mechanics are applied to experimental results obtained with a fine ferric oxide powder. The appearance of a dense boundary layer on the surface of compacts could be related to the occurrence of powder failure during compaction at the die wall.
It is well known that at high temperatures basalt glass wool looses its elasticity; crystalline phases appear and the fibres eventually disintegrate. Experiments reported here show that one of the factors responsible for this processes is oxidation of Fe2+ to Fe3+. Other possible physicochemical processes, which can effect the behaviour of basalt glass fibres at high temperatures were also analysed.
Rice husk ash, an agricultural waste material, is available in large quantitaties in the rice paddy growing countries of the world at little or no cost. This ash is highly porous, mostly silica and possesses refractory and thermal insulation properties. It is therefore an attractive starting raw material for the manufacture of low to moderate cost thermal insulations for dryers, ovens, kilns and furnaces, including those employed in the ceramic industry. This paper deals with manufacture, properties and usage of a spectrum of low to high temperature thermal insulations and insulating refractories that can be made from rice husk ash, namely: (i) Calcium ferrite bonded porous silica refractory: (ii) Sodium silicate bonded porous silica refractory; (iii) Fired and chemically bonded forsterite insulating refractory; (iv) Hydraulic setting calcium silicate/silica thermal insulation.
