Shrinkage anisotropy of glass powder compacts sintered in dilatometers
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Shrinkage anisotropy of glass powder compacts sintered in dilatometers Aldo R. Boccaccinia) Institute for Mechanics and Materials, University of California, San Diego, La Jolla, California 92093-0404 (Received 14 April 1997; accepted 26 January 1998)
Dilatometer push rods exert uniaxial compressive stresses on powder compacts undergoing sintering. In this study the effect of such stresses on the shrinkage anisotropy behavior of glass powder compacts is considered. A shrinkage anisotropy factor (k), defined by the ratio between axial and radial strain, is used to quantify the anisotropy in cylindrical compacts (k 1 represents isotropic shrinkage). The available experimental data in the literature indicate that the effect of the applied dilatometer stress is to increase the value of k over that of free sintering conditions. For the small stresses exerted by the dilatometer push rods, it is shown that k is always .1. The common practice of calculating the sintered density from uniaxial dilatometer data, ignoring the existence of shrinkage anisotropy, is shown to lead to significant errors, even for values of k only slightly different from k 1.
I. INTRODUCTION
Probably the most popular technique to study in situ the kinetic of densification during sintering of powder compacts is standard dilatometry. In this technique, the change of one dimension of the sintering sample as densification proceeds (for cylindrical compacts usually the axial contraction) is recorded by the movement of a push rod in contact with one of the extremes of the sample.1 A certain load is required to maintain contact between the push rod and the compact, and thus a certain applied compressive stress on the sample results. In standard vertical devices this load is usually constant during the experiment and may be simply the weight of the push rod. In more special dilatometers, e.g., loading dilatometers, a spring is used to maintain contact between the push rod and the sample and the load can be controlled to a desired level.2 In this case the load is a function of the spring length which, in turn, is a function of the instantaneous length of the sintering compact.3 Standard dilatometers do not allow for a simultaneous monitoring of the transversal dimension during densification, e.g., the radial contraction, and therefore shrinkage anisotropy effects cannot be measured. In order to determine the instantaneous radial shrinkage, and thus to study shrinkage anisotropy, a separate set of experiments must be conducted, known as interrupted sintering experiments. In these experiments, the sintering is interrupted at several times (or temperatures) before completion, and the radius of the
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Present address: TU Ilmenau, Fachgebiet Werkstofftechnik, PF 100565, D-98684 Ilmenau, Germany. J. Mater. Res., Vol. 13, No. 6, Jun 1998
sample is measured with a micrometer.3 However, this practice is usually not followed (it is certainly time and sample consuming) and isotropic shrinkage behavior is assumed. Th
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