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Journal of Microwave Power
and Electromagnetic Energy (JMPEE) |
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FDTD Simulation of Microwave Sintering in Large (500/40000 Liter) Multimode Cavities [PDF] |
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M. Subirats, M.F. Iskander, M.J. White and J.O. Kiggans, Jr. 1997 32 3 161-170 |
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Abstract The recently developed multigrid 3D Finite-Difference Time-Domain (FDTD) code and the 3D Finite-Difference Heat-Transfer (FDHT) code were integrated and used to simulate realistic material processing (drying and sintering) in microwave furnaces. The objective is to use results from these numerical simulations to help develop large-scale microwave-sintering processes and to explore the feasibility of the commercial utilization of the microwave processing of materials technology. This paper presents results obtained from the simulation of realistic sintering experiments carried out in both 500- and 4000- liter multimode microwave cavities operating at 2.45 GHz The ceramic ware being sintered was placed inside a cubical/cylindrical crucible box made of ceramic materials that have higher electrical conductivities than the ceramic samples. A higher conductivity crucible helps increase the amount of microwave power absorption and hence stimulates the microwave heating process, particularly at lower temperatures. The dimensions of the crucible box were made sufficiently large to accommodate up to 5 layers of ceramic samples with 16 to 20 cup-like samples per layer. Simulation results provided guidelines regarding selection of crucible-box materials, crucible-box geometry, and the possibility of using rings of highly conducting materials instead of the crucible box to improve the efficiency and uniformity of heating. The effect of the material type to be used as shelves between the layers of the ceramic samples, the fraction volume of the load vs. that of the furnace, and the effect of the insulation electrical conductivity on the efficiency and uniformity of heating were also simulated. Simulation results illustrating
the trade offs involved in these rather complex sintering processes are
discussed and compared graphically. For the simulation cases reported in this
paper, it is shown that: (1) ceramics processed in BN crucibles present an
increase of 44% in uniformity and 52% in the average microwave power absorbed
with respect to those processed with the SiC crucibles; (2) crucibles
containing SiC may present a non-uniform heating pattern due to the excessive
heating in the crucible walls and the microwave shielding effect of the SiC;
(3) a cylindrical crucible box creates more uniform and higher (20%)
microwave power absorption in the ceramic samples than cubical ones; (4)
shelves composed of SiC allow 25% less electric-:field penetration and
decrease the uniformity of heating in the ceramic load by 56% compared to
shelves composed of BN; (5) increasing the number of layers results in lower
fields and poor uniformity in each layer although the overall efficiency
increases; and (6) while an increase in the conductivity of the insulation
may stimulate the sintering process, excessively large values of the
conductivity of the insulation beyond Results from these simulations
and the corresponding analysis, including those related to increasing the
uniformity and the efficiency when SiC crucibles are used, with help in
identifying important trends in optimizing the design of large-scale
microwave-sintering systems. Several solutions are proposed to increase the
uniformity and efficiency of the heating when SiC crucible boxes are used.
The first suggests the use of a lower weight percentage of SiC in the
material of the crucible box. Another suggestion is to reduce the thickness
of the SiC crucible wall to a maximum of Key Words: Microwave furnace, multimode microwave cavity, FDTD, sintering, ceramics, heat transfer, drying, large-scale microwave sintering |
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