REV. CHIM. (Bucureºti) ♦ 58 ♦ Nr. 2 ♦ 2007 146 Study of the Distribution and Shape of the Pores in Silica Porcelain MARIA GOREA*, FERENC KRISTALY Babes-Bolyai University, 1 M. Kogalniceanu Str., Cluj Napoca, 400084, Cluj, Romania Two industrial chemically-similar compositions of electric silica porcelain, containing variable amounts of kaolin, feldspar and quartz, have been tested for their microstructure. The crystalline and glassy phases, and the pores in the studied samples were investigated by optical and electron microscopy. The number of measured pores in identical area sections in the two samples differs in relationship with the type and amount of raw materials used. Sample R4, with a higher concentration of clay minerals and a lower quartz amount presented twice as many closed, larger pores as sample R3, which was obtained from higher amounts of feldspar and lower clayey materials. On the contrary, the roundness of pores in sample R4 is higher than that of pores in sample R3. SEM pictures of the investigated ceramic surfaces illustrate the study. Keywords: silica porcelain, microstructure, pores Porcelain is one of the oldest materials used as electric insulator due to some of its properties, i.e.: mechanical strength, high-power dielectric strength and corrosion resistance. The most frequently used porcelain insulators are rich in silica or alumina, being classified as groups C- 110 and C-120 according to the IEC 6672-3 standard. The silica porcelain consists of a vitreous phase (matrix) and some dispersed/embedded crystalline phases. The dominant crystalline phases are represented by mullite and quartz, sometimes cristobalite, the crystals having various sizes according to the type and amount of raw materials and to the thermal treatment applied. The porcelain’s electric and mechanical characteristics depend on both the properties of the crystalline phases (crystal size and distribution, some isomorphous substitutions in the crystalline network), and those of the vitreous phase (presence of some ions, especially Na + and K + from the feldspar melt) and of the pores (shape, size, distribution). It was shown that porcelain strength decreases with the increase of the amount, size and asymmetric size distribution of mullite crystals, but increases with the increase of quartz, cristobalite and vitreous phase [1]. The characterization and identification of mineral phases and the presence of amorphous one in X-ray patterns, the dependency of mechanical properties of silica- and alumina-rich porcelain on the original composition and the thermal treatment was long time ago emphasized, but it is still an actual research topic [2,3]. The large quartz grains unevenly distributed in the glassy matrix cause the occurrence of micro cracks, thus contribute to the strength decrease of the silica porcelain. Higher homogeneity of quartz crystals size distribution and average sizes between 10-20 µm limit the negative effects due to the occurrence of fissures in the matrix [4]. The microstructure of standard silica porcelain consists of α quartz and mullite crystals in a glassy matrix, while alumina-rich porcelain also contains corundum. The mullite crystals growth from the clay minerals-feldspar interface on feldspar relics was observed, indicating the transformation of primary mullite into secondary mullite [5, 6]. At optimum firing temperature for each raw materials mixture (receipt), the amount of melt has to provide the * email: mgorea@chem.ubbcluj.ro crystalline phases and to react with them in a corresponding time interval according to the firing diagram. A lower firing temperature determines the formation of open pores in the ceramic body, while a higher temperature leads to increasing porosity, especially amount of closed pores, as a result of oxygen release during Fe 2 O 3 decomposition and gas expansion in the pores [7]. The aim of this paper is to characterize the pores from the glassy matrix of two silica electric porcelain types, of different compositions. The pores size, shape and distribution, and their correlation with the chemical composition of the glassy phase are investigated. Experimental part The tested materials are two masses of silica porcelain of different compositions (R3 and R4) obtained industrially for being used as ceramic components of low voltage electrical devices. Compositionally, the concentration of clay raw materials is 43 % in the R3 ceramic mass and 47.5 % in the R4; the feldspar (F) to sand (N) ratio is 1.28 (corresponding to an F/N ratio of 32:25 for R3, and 29.5:23 for R4). The characteristics of the raw materials and the correlation with the microstructure of the corresponding ceramics were presented in a previous publication [8]. The processing of the raw materials mixture was performed by wet grinding in a ball mill. The slurry was dried by pulverisation at about 7 % humidity and the powder was then semidried pressed in metal dies. The pressed and dried samples were submitted to thermal treatment at 1300 o C for 10 h. The bulk chemical composition of the R3 and R4 silica porcelain samples was determined by the industrial producer by following the standardized wet procedure. For defining the chemical composition of the vitreous matrix in the two samples, energy dispersive measu- rements (EDX) were performed at the Department of Mineralogy and Petrology, University of Miskolc on a JEOL JXA-8600 Superprobe micro-beam electron microscope (acceleration voltage 15 kV, probe current 12 nA, beam diameter 10 µm, live time 90 s). It must be emphasized that during the measurements, the crystals – mainly mullite and quartz, visible at the electron microscope resolution were avoided. For each sample two by two measurement