FDTD SIMULATION OF MICROWAVE SINTERING IN LARGE (500 4000 LITER) MULTIMODE CAVITIES/

The recently developed multigrid 3D Finite-Difference Time-Domain (FDT D) code and the 3D Finite-Difference Heat-Transfer (FDHT) code were in tegrated and used to simulate realistic material processing (drying an d sintering) in microwave furnaces. The objective ir to use results fr om these numerical simulations to help develop large-scale microwave-s intering processes and to explore the feasibility of the commercial ut ilization of the microwave processing of materials technology. This pa per presents results obtained from the simulation of realistic sinteri ng experiments carried out in both 500- and 4000-liter multimode micro wave cavities operating at 2.45 GHz. The ceramic ware being sintered w as placed inside a cubical/cylindrical crucible box made of ceramic ma terials that have higher electrical conductivities than the ceramic sa mples. A higher conductivity crucible helps increase the amount of mic rowave power absorption and hence stimulates the microwave heating pro cess, particularly at lower temperatures. The dimensions of the crucib le box were made sufficiently large to accommodate up to 5 layers of c eramic samples with 16 to 20 cup-like samples per layer. Simulation re sults provided guidelines regarding selection of crucible-box material s, crucible-box geometry, and the possibility of using rings of highly conducting materials instead of the crucible box to improve the effic iency and uniformity of heating. The effect of the material type to be used as shelves between the layers of the ceramic samples, the fracti on volume of the load vs. that of the furnace, and the effect of the i nsulation electrical conductivity on the efficiency and uniformity of healing were also simulated. Simulation results illustrating the trade offs involved in these rather complex sintering processes are discuss ed and compared graphically. For the simulation cases reported in this paper, it is shown that: (1) ceramics processed in BN crucibles prese nt an increase of 44% in uniformity and 52% in the average microwave p ower 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 mo re 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 t he ceramic load by 56% compared to shelves composed of BN; (5) increas ing the number of layers results in lower fields and poor uniformity i n each layer although the overall efficiency increases; and (6) while an increase in the conductivity of the insulation may stimulate the si ntering process, excessively large values of the conductivity of the i nsulation beyond sigma = 10(4) S/m decreases the penetration of the el ectromagnetic (EM) fields to the sample and hence reduces the uniformi ty of the heating. Results from these simulations and the correspondin g analysis, including those related to increasing the uniformity and t he efficiency when SiC crucibles are used, will help in identifying im portant trends in optimizing the design of large-scale microwave-sinte ring systems Several solutions are proposed to increase the uniformity and efficiency of the heating when SiC crucible boxes are used The fi rst suggests the use of a lower weight percentage of SiC in the materi al of the crucible box Another suggestion is to reduce the thickness o f the SiC crucible wall to a maximum of 0.25 cm Microwave power deposi tion and uniformity may also be improved by using a non-solid crucible , created from highly electrically conducting rings, rather than solid crucible plates or cylinders. increasing the diameter of the rings (r elative to wavelength) also results in higher and more uniform pattern s in the ceramic ware. A diameter of four times the wavelength is sugg ested Detailed results validating these findings are presented in the following sections.