Microwave sintering of multiple aritcles Page: 8 of 18
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believed that the rate of heating below 5000C is less
than 480C per minute. After the hold period, temperature
was slowly lowered to about 13500C by reducing the
microwave power, at which point the power was turned off
05 and the articles allowed to cool at a rate determined by
the physical surroundings. It is believed that the maximum
cooling rate should be no more than about 300C per minute
in order to avoid cracking of the articles. It should be
noted that the rate of cooling is established in part by
10 the design of the sintering container, as the insulation
material and the zirconia, which also functions as
insulation, slow the rate of cooling. Theoretical density
of sintered alumina articles was about 98 to 99% of
theoretical density and the dimensions were about 0.5 inch
15 diameter and about 1.025 inch high. The microstructure of
sintered alumina articles was quite uniform, having only
isolated porosity, and grain size did not vary with
location in an article. The grain size was about 5 to 50
microns. Of 320 articles sintered in 20 article batches,
20 none showed visible cracks and three were rejected because
of the effects of reaction with the insulating material.
The insulating material used in the experimentation was
Type SALI alumina insulation from Zircar Products, Inc. of
Florida, NY, which the manufacturer specifies as having a
25 composition of about 80 wt% of A1203 and about 20 wt%
SiO2 and a density of about 0.48 gram per cubic
centimeter. Other materials which are suitable for high
temperature use and have sufficiently low thermal
conductivity to provide an insulating function may be used;
30 examples of such materials are magnesium oxide, boron
nitride, and yttrium oxide. Another requirement for an
insulating material is that it be relatively transparent to
microwaves, so that it does not heat to any significant
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Blake, R. D. & Katz, J. D. Microwave sintering of multiple aritcles, patent, December 31, 1992; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1389885/m1/8/: accessed May 27, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.