Microwave Sintering of Oxides
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MICROWAVE SINTERING OF OXIDES M. ALIOUAT, L. MAZO AND G. DESGARDIN CRISMAT - ISMRA 14032 CAEN CEDEX France ABSTRACT: Microwave sintering of oxides has been studied, owing to its great flexibility for thermal treatments, which may influence in particular the microstructure and the electrical properties of the so sintered material. In order to point out the influence of the structure and electrical properties we studied the microwave sintering : i) of insulating complexe oxides with the perovskite structure, ii) of close packed structure such as spinels and iii) of high Tc superconductors. The differents behaviours of those materials in a microwave field are compared and discussed. Introduction : Microwave sintering of oxides is a very attractive method for the elaboration of ceramics [12] owing to its great flexibility for thermal treatments, which may influence in particular the microstructure of the material. Among the different factors which are susceptible to govern the microwave sintering, one important issue concerns the role of the structure, in connection with the chemical bond. In this respect, we studied at first the sintering of insulating complex oxides with the perovskite structure and characterized by high dielectric constants. We extend later the study to the sintering of oxides with a close packed structure such as spinels, especially ferrites, which are well known for their magnetic applications, and to the sintering of high Tc superconductors whose structure is derived from the perovskite, but which are characterized by a large electrical conductivity even above their critical temperature. We report here and discuss our results concerning the microwave sintering of these different oxides in a monomode cavity (TE 103 mode) working at 2.45 GHz. Experimental procedure: a) Powder synthesis: The starting materials consist of oxides or carbonates mixted in adequate molar ratio in alcohol using a shaker mill, then dryed with an infrared radiator and then eventually precalcined using a conventional furnace at temperatures ranging from 800 to 11000. This thermal treatment leads to the decomposition of the carbonates and to the synthesis of the expected phases. After a sintering agent,(lithium salt) has been eventually added, the samples are isostatically pressed (P=3000bars) into the form of cylinders (0 = 7-8 mm).The length of the bars allows that each sample to cross the cavity from top to bottom. b) Sintering conditions : The so pressed samples are introduced in a resonant applicator linked to a 1KW - 2,45GHz microwave generator as shown on fig la which represents the assembly line. Shol"-circuit weton
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Fig 1: a) Assembly line : AL adapted load; C : coupling iris ; Ci: circulator;G : microwave generator; M : piston monitor ; P : pyrometer ; S : sample ; WI,W2 : power meters. b) Monomode cavity used to sinter the oxides Mat. Res. Soc. Symp. Proc. Vol. 189. 01991 Materials Research Society
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