Microstructural development of ZnO using a rate-controlled sintering dilatometer
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Microstructural development of ZnO using a rate-controlled sintering dilatometer Gaurav Agarwal and Robert F. Speyer School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
Wesley S. Hackenberger Center for Dielectric Studies, Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 17 March 1995; accepted 23 October 1995)
Rate-controlled sintering (RCS) of isostatically pressed particulate compacts of ZnO showed lower average grain sizes and intragranular pore densities than constant heating rate temperature controlled sintering. Valid comparisons of this form could only be made after corrections to hardware and software which reduced specimen creep under dilatometer pushrod load, nonuniform pushrod expansion, reproducible specimen temperature determination, thermal expansion during sintering, and instantaneous termination of sintering at the specified end of RCS. The improved microstructures from RCS were attributed to maximized efficiency of densification, optimizing the time and temperatures permitted for grain growth.
I. INTRODUCTION
The diffusional processes associated with heat treatment of powder compacts can be divided into two events. Initially, necks form between grains, replacing high energy solid/gas interfaces with grain boundaries, and smoothing out low radius of curvature contact points. Atomic diffusion for this process is predominantly along the particle surfaces, and there is no associated densification (particle centers do not approach). After these bridges form between grains, and if the temperature is adequately high, thermally activated vacancy formation and motion fosters volume and grain boundary ionic diffusion, where pores are decreased in size, while particle centers approach. Grain boundary diffusion is often considered dominant by virtue of the highest concentration of the defects which in turn facilitate atomic diffusion. Grain boundaries tend to adjust their curvature through atomic diffusion so that they meet at 120± at triple point intersections. As a result, smaller grains (e.g., less than six sides in a two-dimensional case) have boundaries with curvature concave to the grain center, while the opposite is true of larger grains. In an effort to minimize the interfacial energy associated with grain boundaries, the interfaces migrate toward their center of curvature. Thus, large grains tend at the expense of smaller grains (grain coarsening). This process occurs simultaneously with densification, and can foster effects that inhibit complete densification: With grain coarsening, the average diffusion distance for matter flow from grain boundary to pores increases, causing a reduction in the densification rate. If grain growth is too rapid, rather than pores migrating with J. Mater. Res., Vol. 11, No. 3, Mar 1996
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the grain boundary, the grain boundary tends to break away, leaving th
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