Measurement of the initial sintering shrinkage of CaAl2O4 at temperatures of 1300, 1325 and 1350 °C are reported. The particle sizes chosen were −53 + 45, −63 + 53 and −75 + 63 microns and the soaking periods were from 15 to 360 min. A time dependence of the shrinkage has shown that volume diffusion is the dominant mechanism of sintering. At any given time and temperature, the per cent shrinkage was found to be a decreasing function of particle size. The activation energy for the sintering of CaAl2O4 was found to be 766.38 KJ mol−1.
The corrosion rate and changes in the microstructure and fracture strength of sintered silicon nitride and hot-pressed silicon nitride were studied in 25 m NaOH solutions at 150-degrees-C to 200-degrees-C, where m = mol . (kg-H2O)-1. Both types of silicon nitride ceramic were susceptible to attack by 25 m NaOH solutions above 150-degrees-C but the corrosion resistance of hot-pressed silicon nitride was superior to that of sintered nitride. The corrosion rate was found to be second order with respect to NaOH concentration and was controlled by a surface chemical reaction with an apparent activation energy of 164 kJ . mol-1 in the temperature range 150-degrees-C to 175-degrees-C. The corrosion resulted in surface roughening of the silicon nitride specimens. The bending strength of sintered silicon nitride was reduced from 1050 MPa to ca. 500 MPa up to 2% of weight loss and then remained almost constant up to 60% of weight loss, while that of hot-pressed silicon nitride decreased from 950 MPa to 150 MPa up to 15.7% of weight loss.
The corrosion rate and changes in the microstructure and fracture strength of sintered alpha- and beta-silicon carbides were studied in 0 to 25 m NaOH solutions and oxygen partial pressures of 0 to 10 MPa at 200-degrees-C to 300-degrees-C where m = mol(kg - H2O)-1. Silicon carbide ceramics were stable in 25 m NaOH solutions even at 300-degrees-C in the absence of oxygen, but susceptible to corrosive attack above 250-degrees-C in the presence of oxygen. The corrosion rate of alpha-silicon carbide was slightly higher than that of beta-silicon carbide. The corrosion data could be adequately described by a surface chemical reaction controlled shrinking core model. The corrosion rate increased linearly with increasing NaOH concentration up to 0.35 m, but thereafter decreased strongly. The corrosion rate in 0.2 m NaOH increased linearly with increasing oxygen partial pressure up to 3 MPa and then remained almost constant, while in 25 m NaOH solutions it increased linearly up to 10 MPa. The apparent activation energy was 170 and 32 kJ . mol-1 in 0.2 m and 25 m NaOH solution, respectively. The corrosion resulted in surface roughening of the silicon carbide specimens and reduced the bend strength from ca. 500 MPa to ca. 250 MPa in the initial stage of corrosion up to 20% weight loss, the bend strength then remained almost constant up to 60% weight loss.
The concept of the CE Mark is that it is a product's passport to the European Community, so it certainly can be claimed to be a matter of importance for the single European market intended to be in place by the end of 1992. All products complying with European legislation, which is concerned that products placed on the market are safe and fit for their intended use, are entitled to carry the CE Mark and they are then able to cross borders without hindrance within the European community. However, the necessary legislation and admininstrative arrangements will not be in place by the end of 1992 and therefore, no building products will initially be entitled to carry this mark. There is also some doubt as to whether it is compulsory or voluntary. This paper attempts to address these issues and indicate the likely outcome for the use and application of the CE Mark.
Beta SiC powders with various surface areas were pressureless sintered in the presence of B and C to densities of over 95% TD. The highest density (> 98% TD) was obtained with deagglomerated powders having high surface area and intermediate green density. The as-received, low surface area powders could also be sintered to high density (approximately 95% TD) provided that they had good pressing characteristics. However, larger quantities of particles above approximately 1-mu-m increased the diffusion distances and prevented the attainment of densities above 95% TD.
The relationship between the magnetic detector properties and the superconductive properties has been studied for a Bi system superconductor containing a small amount of Sb2O3. The formation of the high Tc phase was promoted and the critical temperature was decreased by the addition of Sb2O3. However, the sensitivity to a magnetic field was improved because of the segregation of secondary phase particles at the grain boundaries which operated as a weak link.
Raising slip temperature during the slip casting of clay-based ceramics is believed to influence casting rate via a decrease in slip viscosity and by affecting the state of dispersion of the slip [1]. Little work has been performed on the effect of temperature on the slip casting of technical ceramics however. In the work presented in this paper, alumina-based slips of varying solid content have been slip cast at two temperatures, 20-degrees-C and 60-degrees-C, to determine the effect of raising temperature on the slip casting mechanism and on the properties of the bodies produced.
In the work reported in this paper, two pyrophyllite samples (Hillsboro and Purified pyrophyllite) were studied by X-ray diffraction after mechanical treatment by dry grinding. X-ray variance analysis of the (131) diffraction profiles was used to separate the effects of crystallite size (coherently diffracting domains) and lattice distortion on experimental line broadening. The results show that mechanical treatment produces a diminution of crystallite size towards a limit when the structural alteration is maximum, coinciding with a peak in specific surface area after grinding for 30 minutes. Simultaneously, the microstrains increase with grinding time and are clearly related to the variations in crystallite size.
