Improving the breakdown strength of rutile capacitor by gelcasting

Abstract

Gelcasting, one of the colloidal in-situ forming techniques, has been highly concerned and utilized in the forming of many sorts of ceramic material systems [1–3], for homogeneous microstructures of bodies can be assured by this low-cost and near-net-shape molding process [4–8]. Thus the parts exhibit good mechanical performances and high reliability [9, 10]. Because of high power and large breakdown strength, rutile capacitors have been widely used in high frequency appliances such as broadcast emitters, radars, high frequency welding machines and smelting furnaces. There are more than one hundred kinds of such products with various sizes and complex shapes. In the past decades, rutile capacitors with complex shape and large size were made by extrusion forming and then machining. First, the rutile mixture with a definite composition obtained by ball-milling, was filtered one day, aged for one month, and mixed in vacuum. Second, after extruded, cylindric green bodies were rolled, and dried for more than two weeks. Next, by means of machining, the green bodies possessed a desired complex shape. Finally, through sintering and some sequent process, the high power rutile capacitor can be prepared. The whole procedure needed two and a half months, and the machining of dried green bodies resulted in a great deal of environment pollution and waste of starting materials. Furthermore, in the traditional process of high power rutile capacitors, a bottleneck, which results from the large size, complex shape and many kinds of components in raw materials, greatly impairs their functional properties, so the breakdown strength by this process generally reached only 15 KV/mm. A new route for preparing rutile capacitors by gelcasting, as indicated in Fig. 1, has been developed. By this route, not only can the production cycle be decreased to three weeks, but also the dust pollution caused by machining can be eliminated due to the insitu forming. Thus the production cost can be reduced and the environment can also be improved. And particularly, the breakdown voltage is improved from original 15 KV/mm to more than 22 KV/mm. The composition of rutile capacitors is shown in Table I. The commercial rutile powder and additives are of industrial purity, from Shanghai, China. The following are used in our work: deionized water with conductivity of 1.02 μS·cm−1, acrylamide (AM) as monomer, methylenebisacrylamide (MBAM) as crosslinker, (NH4)2S2O8 as initiator, N,N,N′,N′tetraethylmethylenediamine (TEMED) as catalyst and PMAA-NH4 as dispersant. The rutile mixture was pre-calcined at 900 ◦C and then ball-milled for 50 h. The particle size distribution measured by an X-ray centrifugal sedimentation technique (Instrument type: BI-XDC, made in Brookhaven Instrument Co., USA) is shown in Fig. 2. The volume moment mean diameter was 1.8 μm. A concentrated 53 vol% rutile suspension with low viscosity was successfully prepared. The apparent viscosity was measured by rotating viscometer (model NSX-11, Chengdu Instrument Plant, People’s Republic of China). Microstructures of sintered bodies were observed by S450 SEM from Hitachi of Japan. The rutile mixture pre-calcined was well dispersed in premixed solution made up of acrylamide, deionized water, and N,N′-methylenbisacrylamide in the aid of a PMAA-NH4, after ball-milling 50 h. It can be seen from Fig. 3 that the apparent viscosity strikingly decreased