Effect of Materials Design on Properties of Porcelain Insulators

Transcription

1 Effect of Materials Design on Properties of Porcelain Insulators Substitution of alumina for silica improved the mechanical properties of high-voltage porcelain insulators but did not have a significant effect on their electrical properties. Fig. 1 Examples of insulators. N. Riahi Noori, R. Sarraf Mamoory and S. Mehraeen High-strength electrical porcelain is needed by the power industry. This need is met by a specific composition of raw materials, which consists of clays and feldspars. These two raw materials, depending on geological and geophysical characteristics, have various compositions and specifications. Three main constituents of electrical porcelain bodies (Fig. 1) are clays (as plastic materials), fillers (such as silica and alumina) and refractories (such as feldspars and talc). 1 When an electrical porcelain body that contains silica is fired at high temperatures, the corners of the quartz particles are dissolved into a feldspar-containing corrosive liquid. In the critical case, these particles are completely dissolved in the liquid phase. The decrease of solid quartz content in the system leads to remarkable decrease in the mechanical strength of the body. Thus, firing time and temperature of bodies that contain quartz must be controlled carefully. To achieve the highest strength, quartz particles must be fine, and the bodies must be fired carefully. Long periods of firing, higher firing temperatures and annealing processes completely vitrify the body. This leads to decreases in strength. 2,3 Another imperfection in the electrical properties of porcelain bodies that contain quartz results from the difference between thermal expansion coefficients of the quartz grains and the body matrix around them. This difference results in tensile stress in the quartz grains and compressive stress in the surrounding matrix. When the temperature changes, these stresses intensify. This leads to development of microcracks that are created in the grinding step of quartz grains and in the α- to β-phase transformation of the quartz. These microcracks act as stress localization centers. Therefore, stress near the cracks is remarkably greater than the applied stress. 4 Decrease of strength under load (when the insulator is suspended) usually results in abrupt fracture of the insulator because of the growth of these microcracks. This problem can be solved by decreasing the size or content of quartz grains in the body matrix or by substituting a portion of the quartz with other refractory materials. 4 Decrease in silica grain size leads to higher strength, lower workability and higher firing to drying shrinkage. On the other hand, when the content of fine grains increases, fracture toughness decreases. The only proper substitution for quartz in industrial production of porcelain insulators is alumina, which can replace a large portion or total content of quartz. 4 6 American Ceramic Society Bulletin, Vol. 86, No

2 Porcelain Insulators Bending strength (MPa) Fig. 2 Effect of silica replacement with alumina on bending strength of porcelain insulators ( ) 1250, ( ) 1300 and ( ) 1350 C). Density (g/cm 3 ) Fig. 3 Effect of silica replacement with alumina on density of porcelain insulators ( ) 1250, ( ) 1300 and ( ) 1350 C). The Experiment To study the effect of alumina on the mechanical and electrical properties of electrical porcelain bodies, six body groups were selected (Table 1). Group 1 had the common composition of electrical insulator bodies. In groups 2 6, silica content decreased gradually and alumina content increased gradually. In group 6, silica was totally replaced by alumina. The wet method was used for porcelain body preparation. Kaolin, ball clay, feldspar and silica were used as raw materials. All of the groups had the same amount of kaolin, ball clay, and feldspar. Alumina and silica were smaller in size than 120 µm. The materials were wet milled with wt% water and then cast into plaster molds to be formed. The samples were dried at room temperature for 48 h and kept at 110 C for 5 h. The temperature then was increased to maximum in 4 h. The samples were fired at this temperature for 2 h and cooled inside the furnace at its natural rate. The maximum firing temperatures selected in this study were 1250, 1300, and 1350 C. Bending strength (Table 2), thermal shock resistance, density (Table 2) and porosity were determined in accordance with ASTM standards. In each case, several samples were tested. Relative permeability factor (ε r ) and dielectric loss tangent (tan δ) at 60 Hz and 1 MHz were measured. The measurement accuracy for ε r was 0.5% and for tan δ was 3%. Strength The effect of silica replacement by alumina on bending strength of electrical porcelain bodies was determined (Fig. 2). In the main composition, the body that contained 25% silica without alumina had the lowest strength. When the alumina content was increased and the silica content decreased, the strength of the samples increased. On the other hand, as temperature increased, strength increased because dissolution of quartz (silica) in system increased. Moreover, when firing temperature increased, alumina was partially dissolved into system, but the rate of dissolution was much less than that of quartz. When the alumina content in the body and the firing temperature increased, mullite content (Al 2 O 3 2SiO 2 ) increased, which acted as an agent for strength increase. Although alumina content in the body was <8%, the formation of mullite phase was possible at temperature as high as 1340 C. Density and Porosity Body composition was important to final density (Fig. 3). When silica was replaced with alumina in the main composition of the body, the density of the fired bodies increased. In other words, as the silica content of the body decreased, the density of the bodies increased because of decreased porosity induced from microcracks around silica grains. Open porosity content (available pores in bodies) versus increased alumina content in the system was determined (Fig. 4) American Ceramic Society Bulletin, Vol. 86, No. 3

3 Porcelain Insulators The microstructures of two samples, one with 30% alumina and another with 30% silica, were observed using SEM. The porosity in the silica bodies was studied in particular (Fig. 5). Table 1 Composition of Prepared Bodies Water Absorption The water absorption of bodies decreased with increased firing temperature because pores and open porosities of the bodies decreased and there was increased glassy phase content in the body. In addition, as the silica content in the body decreased (and alumina increased), water absorption of the bodies decreased because of decreased microcracks and pores. Water absorption of high-voltage insulators must be zero. 7 Thermal Shock Thermal shock resistance was determined by heating samples to 100 C for 1 h and then immersing them in 0 C ice water to be cooled completely. This cycle was repeated n times. The number of cycles before crack creation is the criteria for thermal shock resistance. For samples prepared in this study, n = 30. The literature requires n 20 for electrical ceramics in a similar condition. Group Feldspar Kaolin Ball clay Alumina Silica number (wt%) (wt%) (wt%) (wt%) (wt%) Table 3 Loss Tangent Group Dielectric loss No. tangent ( 10 3 ) Dielectric Properties Dielectric loss tangent was measured at 60 Hz and 20 C (Table 3). The dielectric loss tangent was similar for the various bodies (~ ). The dielectric loss tangent of bodies at 1 MHz decreased to that again was similar for all bodies. The relative permeability factor for all bodies at 60 Hz was 6 7. About the Authors N. Riahi Noori is a Researcher at the Niroo Research Institute, Tehran, Iran, and is a Ph.D. student at Tarbiar Modares University, Tehran. R. Sarraf Mamoory is Associate Professor, Tarbiat Modares University. S. Mehraeen is a Researcher in the Ceramic and Polymer Group, Niroo Research Institute. References 1 S.I. Warshaw and R. Seider, Comparison of Strength of Triaxial Porcelains Containing Alumina and Silica, J. Am. Ceram. Soc., 50, 337 (1967). 2 R.S. Gorour, et al., Outdoor Insulators, Ravi S. Gorur Inc., Ariz., 1999; pp J.E. Schroeder, Inexpensive High-Strength Electrical Porcelain, Am. Ceram. Soc. Bull., 57, 526 (1978). 4 P. Johnson and W.G. Robinson, Development of Pottery Bodies Electrical Porcelain ; presented at the 22nd Meeting of the Pottery Section (Trentham Gardens, Stoke-on-Trent, U.K., 1947), pp A.M. Bisha, B.H F. Al-Khayat and F.A. Awni, Dielectric and Physicomechanical Properties of Electrical Porcelain Bodies, Am. Ceram. Soc. Bull., 64, 598 (1985). 6 J. Liebermann, Reliability of Materials for High-Voltage Insulators, Am. Ceram. Soc. Bull., 42 [5] 670 (2000). 7 M. Nemamcha, S. Rouaiguia, A. Belbah and B. Belfarhi, Dielectric Properties of Alumino-Silicate Ceramic Materials, IEEE, 22, 413 (1995). American Ceramic Society Bulletin, Vol. 86, No

4 Table 2 Properties of Bodies Group Sintering Bending No. temperature ( C) Density (g/cm 3 ) strength (MPa) Porosity (%) Fig. 4 Effect of silica replacement with alumina on water absorption of porcelain insulator ( ) 1250, ( ) 1300 and ( ) 1350 C).

5 (a) (b) Fig. 5 SEM photographs of body with (a) 30% silica and (b) 30% alumina.