Year: 1989
Type: dissertation
Abstract: This study is divided into two parts;the first part deals with effects of particle size distribution on sintering in the commercial Bayer Alumina system, whereas the second part deals with the effects of particle size distribution o n sintering of mod e l glass powder compacts.In the first part of the study two commercially available Bayer Aluminas (99.5 -99.1% pure) are centrifugally classified to produce three powders of differing particle size distribution from each of the two starting powders.It is found that impurities and phases are segregated to certain particle size fractions; for example sodium in the form of Na|3M ^2^3 > anc* SiC>2 , are segregated to the finer particle size fractions in one of the two starting materials studied.It is found that the sintering response of the powder compacts is dominated by impurity effects with the of particle size distribution playing a secondary role.T h e onset of anomalous or exaggerated grain growth in the aluminas studied is linked to the non-uniform distribution of soda and silica in the sintering compact.Possible explanations for anomalous grain growth based upo n the experimental observations are proposed and then tested by controlled doping experiments o n a high purity alumina ( > 9 9 .9 9 5 % pure).In addition to probing the anomalous grain growth effects, the controlled doping experiments reveal the pfo*tfUi\d effect that silica has at the 0 .05wt%level on the retardation of densification and grain growth, and on the w a y in which the soda and silica impurities interact and affect grain growth kinetics.A correlation-between grain growth inhibition and late stage densification kinetics is observed and explained using the pore closure model developed by Kingery and Francois^), Lange(52), an£j Kellet and Lange (^9).Part 2 of the study attempts to eliminate the chemical inhomogenieties that masked the effects of particle size distribution in part 1.In addition the experiment designed in part 2 is further simplified so that it can be relatively easily and m o r e accurately modelled.T h e experiment involves the sintering of large («500jim), chemically homogeneo u s glass spheres which are packed densely and uniformly into compacts.Particle size distribution can be m o r e readily defined by using spherical particles and gross packing defects that can have a profound effect o n sintering can be eliminated by making the particles large.T h e experiment is limited to the study of particle size distributions in the 425-850jim particle size range; at wider particle size ranges, particle size segregation effects in the powder compacts becomes significant.T h e experiment finds that densification kinetics and pore closure behaviour are independant of particle size distribution within the bounds of experimental error in the powder compacts of the study.Large variations in densification kinetics between compacts of a given powder packed to similar initial compact densities to within + 0.26 -0 .3 1 % are observed.T h e large variations in densification kinetics are discussed in detail and are attributed to either an u n k n o w n chemical effect, and/or a particle rearrangement phenomena.T h e influence of pQfS ClPSWe behaviour u p o n densification kinetics in powder compacts is also discussed.
Source: Spiral (Imperial College London)
Author: Nigel Ian Steward
Language: English
Subfield: Ceramics and Composites
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2759765893