Powder Synthesization, Gelcasting And Sintering Of BeO

Beryllium oxide (BeO) is an incomparable material which possesses a combination of unique properties such as high thermal conductivity, high electrical resistivity, low dielectric constant, good mechanical strength, excellent chemical stability and high thermal shock resistance. These properties make BeO a suitable material for applications in electronical packaging, thermal protection and nuclear industry. BeO ceramic with high property is obtained at abroad. The dissertation has studied treating and synthesizing powder, gelcasting and sintering of BeO.Firstly, BeO powders were treated by calcinations to improve the solids loading of its suspensions. The results show that the powder should be calcinated at higher temperatures (1300?1500?) for shorter times (1-2h) to obtain high solids loading suspension.And then, BeO nanopowders were synthesized by polyacrylamide gel method. The mechanisms of forming gel network and that of thermal decomposition of precursor gel were analysed, crystal nucleus formation and crystalline mechanism of BeO nanograin were studied, and the effects of monomer concentration, salt concentration and the ratio of monomer to crosslinker on the synthesis process and properties of powder were studied.BeO suspension preparation and its rheology properties were also investigated systematically. The isoelectric points of micro- and nano- grade BeO in aqueous suspension were determined, and they declined dramatically with the addition of NH4PAA. For the suspension with micro- grade BeO, the influence of pH value, dispersant style and amount, solids loading on rheological properties of suspensions were investigated in detail, and the relationship of solids loading and viscosity was compared with five various models especially. For the suspension with nano- grade BeO, the influence of dispersant amount and solids loading on the rheological properties of suspensions were studied, and the relationship of solids loading and yield stress, fractal dimension and maximum solids loading were determined to be?y=2094.7?4.697, Df=2.14 and?m=0.266, respectively.The reasons for rheological behaviors of BeO suspensions (shear thinning and shear thickening) were also analyzed from the microstructure.Further, the gelation kinetics, homogenization and strength of acrylamide gel system for gelcasting were investigated and the optimal process conditions were obtained. As the concentrations of initiator, catalyst or monomer increase and the reaction temperature raises, the induction period and reaction period of gelation are shortened. However, there is little effect of AM/MBAM ratio on the gelation. The induction period is shortened while the reaction period is prolonged with increasing the solids loading of suspensions. When the concentrations of initiator, catalyst or monomer are too high or too low, the uniformity of gels will decrease. The strengths of pure gel and green body increase with the monomer concentration. As the AM/MBAM ratio decreases, the gels become less homogeneous or even inhomogeneous, but the strength of green body increases. The strength of green body reduces but the homogenization increase with solids loading.Additionally, the green body of gelcasting was dried by the liquid desiccant method, the drying process was discussed and a model was developed to analyse the stress generated during drying. The influence factors are liquid desiccant kind, liquid desiccant concentration, solids loading and size. In the drying process, the stress inside the green body is from compression to tension, and vice versa outside.Besides, the sintering and mechanisms of micro- and nano- grade BeO powders, microstructure and thermal conductivity of the ceramic were studied. The relative density and thermal conductivity of BeO ceramic (with Pr6O11), sintered at 1650?for 2h, is the largest. The sintering temperature of the compact sample with nano-grade powders is down to 1600?, which is around 200?lower than that with micro-grade powders. The suitable sintering technique for nano-grade powders was the two-step sintering. More suitable process is 1450?, lmin plus 1400?,25 h. The relative density and thermal conductivity of the ceramic achieved with this process are 96.6% and 237 W·m-1·K-1, respectively.Finally, an improved forming process- stimuli-responsive gelcasting was proposed and testified. The graft chains with freely mobile end as water releasing channel were introduced into the gelcasting systems to solve the problem of inefficient drying within the conventional gelcasting procedure and to improve the relative density of green body prepared with low solids loading.