Process of producing high strength porcelains for use in insulators

Abstract: High strength alumina porcelains for use in insulators are disclosed herein, in which the maximum particle diameter of quartz grains remaining in the porcelains is not more than 40 µm. A process for producing such high strength alumina porcelains for use in insulators comprises the steps of mixing the alumina of a controlled grain size which is decomposed in a single crystalline state and in which the 50% average particle diameter is in a range from 4 to 14 µm, with raw materials ground finely which are selected from the group consisting essentially of feldspar, silica sand, and china stone, and clay, shaping the thus obtained mixture, and drying and firing the shaped body.

Description: The present invention relates to a process for producing high strength porcelains which may especially be used for insulators necessitating high strength. There have formerly been demands to increase strength of insulators, and thus development of high strength porcelains has been sought. In order to obtain high strength porcelains, it is known that the amount of alumina in porcelains should be increased or the grain size of bodies made finer. However, among the above conventional techniques for producing high strength porcelains, increasing the amount of alumina in porcelains must inevitably use a great amount of expensive alumina, which raises production cost of insulators. Further, although strength is increased, formability is deteriorated. On the other hand, in the technique of making the grain size of bodies finer, the bodies are likely to crack due to shrinkage during forming, drying or firing of insulators. US-A-4 183 760 describes production of quartz-containing alumina porcelains in which the mean particle size is not greater than 20 µm and the largest alumina agglomerate size is less than 20 µm. It is an object of the present invention to obviate the above-mentioned drawbacks, and to provide high strength porcelains for use in insulators. Porcelains having higher strength can be obtained by specifying the mixed composition of the insulator porcelains and the grain size of raw materials constituting them, even with the same amount of alumina as is conventionally employed. The process for producing high strength porcelains for use in insulators according to the present invention is set out in claim 1. By the process of the invention, porcelains having higher strength can be obtained with the same amount of alumina as is conventionally employed without losing sintering and drying properties, and electric performances, by controlling the grain size of alumina, which is decomposed in the single crystalline state, and controlling the grain size of the other raw materials suitably while the grain size of bodies may be equal to that of conventional porcelains. While the maximum particle diameter of quartz grains remaining in the conventional porcelains is in a range from about 60 to 100 µm, that of quartz grains remaining in the porcelains produced according to the above-mentioned process of the present invention may be not more than 40 µm. This porcelain construction itself greatly differs from conventional porcelains. In the present invention, high strength porcelains are obtained by thus restricting the maximum particle diameter of the quartz grains and employing the alumina with a controlled grain size in the porcelains. These and other optional features, and advantages of the present invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be made by the skilled person in the art to which the invention pertains. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a graph showing the relation among the average particle diameter of alumina, the amount of alumina, and bending strength of porcelains; Fig. 2 is a graph showing the relation between the 50% average particle diameter of alumina and the maximum diameter of quartz grains remaining in the porcelains; Figs. 3(a) and (b) are SEM ( S canning E lectron M icroscope) photographs showing the crystalline structure of a conventional porcelain; Figs. 4(a) and (b) are SEM photographs showing the crystalline structure of a porcelain made according to the present invention; and Fig. 5 is a graph showing the relation between the mixed amount of the alumina of the controlled grain size of not less than 44 µm in particle diameter and bending strength of porcelains. First, reasons for limitations given in the present invention will be explained as follows. The reason why the 50% average particle diameter of alumina is defined to a range from 4 to 14 µm in the present invention is to specify the amount of the alumina of a controlled grain size used in porcelains. If it is less than 4 µm or more than 14 µm, desired porcelain strength cannot be obtained. If coarse alumina grains having a particle diameter of 44 µm or more exceeds 1% by weight, desired porcelain strength is difficult to attain. Thus, the amount of coarse alumina grains of not less than 44 µm is not more than 1% by weight. Further, the amount of alumina used in the present invention is from 20 to 60%, because desired porcelain strength is difficult to obtain if it is less than 20% by weight or more than 60% by weight. In the following, examples of the present invention are given, and are not limitative of the invention. Example 1 First, the relation among the amount of alumina, the grain size of alumina, and various characteristics of porcelains were examined. In order to obtain test porcelains, alumina, feldspar, silica sand, china stone and clay were wet mixed containing 50% by weight of water. The average grain size of alumina had preliminarily been adjusted to a given value by grinding and classifying. The slurry obtained by the above process was dewatered by a filter press to have the water content of from 20 to 25% by weight, and was extruded by a vacuum pug mill, thereby obtaining a formed body in a desired form. The formed body was then dried at 110°C inside an electrically heating drier to have the water content of 1% by weight or less. The dried shaped body was fired in air according to such a firing schedule that it was maintained for 1 hour at the maximum temperature of 1,300°C. With respect to the obtained test porcelains, porcelain bending strength, porcelain dielectric strength, sintering, and drying properties were examined. The bending strength of porcelain was measured by an electron type load tester (the maximum test load: 500 kg, manufactured by Tokyo Shiken Kabushiki Kaisha) according to JIS-C2141 (Testing method of ceramic insulators for electrical and electronic applications) with respect to rods (10 mmΦ × 120 mmℓ). The dielectric strength of porcelain was measured by a dielectric strength tester (manufactured by Toyo Denki Seizo Kabushiki Kaisha) according to JIS-C2110 (Testing method of dielectric strength of solid materials for electric insulation) with respect to circular test pieces having a diameter of 50 mm and a thickness of 2 mm. The sintering temperature of porcelain was tested according to the moisture-absorbing method in JIS-C3801 (insulator-testing method) by using the same test pieces as used in the measurement of the bending strength of porcelain. Results are indicated by ○ and X which mean test pieces sintered when being fired at 1,300°C for 1 hour and those not sintered by the same firing process, respectively. The drying property was evaluated by visually examining whether cracks occurred or not with respect to test pieces having a diameter of 80 mm and a length of 200 mm which had been obtained by extrusion-forming a body with a vacuum pug mill, drying the formed bodies at a temperature of 40°C under a relative humidity of 80% in a thermostat-humidistat vessel for 24 hours, and then drying them at 90°C for 24 hours in an electrically heating drier. Results are indicated in Fig. 1 by ○ and X which denote test pieces free from cracks and those cracked, respectively. Since Comparative Example Nos. 12-15 containing 70% by weight of alumina in Table 1 could not be formed, bending strength or dielectric strength could not be measured. In Table 1 (and also tables 2 and 3) the percentage of alumina having not less than 44 µm in particle diameter is % by weight based on the total weight of alumina. As is clear from the results in Table 1, it was confirmed that when the 50% average particle diameter of alumina was in a range from 4 to 14 µm, porcelains having excellent characteristics and high strength could be obtained irrespective of the amount of alumina used. Among the porcelains fitting this requirement, it was found that those having the amount of the alumina of from 20% by weight to 60% by weight were preferable, because bending strength of not less than 1.4 x 10 8 Pa (1,400 kg/cm 2 ) could be obtained and other characteristics of porcelain were satisfied in this range. From the results obtained, the relation among the 50% average particle diameter of alumina, the amount of alumina used, and the porcelain bending strength is shown in Fig. 1. With respect to the porcelains according to the present invention as obtained above and conventional porcelains without the alumina being controlled in the grain size, the maximum particle diameters of quartz grains remaining in the porcelains in which the amount of alumina in porcelain was 30% were determined from microscopic photographs thereof. The relation between the average particle diameter of alumina and the maximum particle diameter of the quartz grains remaining in the porcelains is shown in Fig. 2. From Fig. 2, it was confirmed that the maximum particle diameter of the quartz grains in the porcelains according to the present invention was smaller than in the conventional porcelains, and was not more than 40 µm. In Figs. 3 and 4 are shown SEM photographs of a comparative porcelain (alumina content: 30% by weight, alumina average particle diameter: 6 µm, average particle size of body: 10 µm) and a porcelain made according to the present invention (alumina content: 30% by weight, alumina average particle diameter: 6 µm, average particle diameter of body: 7 µm), respectively. Figs. 3(a) and (b) are x100 and x1,000 SEM photographs of the comparative porcelain, respectively. It is characteristics that the conventional porcelain contains a lot of large size quartz and small size alumina. Figs. 4(a) and (b) are x100 and x1,000 SEM photographs of the porcelain according to the present invention, respectively. It is characteristics that the invention porcelain contains few of large size quartz and small size alumina. In Figs. 3 and 4, black portions, gray portions and whitish portions are pores, residual quartz, and alumina, respectively. Example 2 Test porcelains having grain sizes and compositions shown in Table 2 were prepared in the same manner as in Example 1. Then, the same characteristics as in Example 1 were measured, and the relation between the 50% average particle diameter of the bodies and the characteristics measured was examined. Results are shown in Table 2. From the results in Table 2, it is seen that porcelains in which the 50% average particle diameter of the body was in a range from 3 to 10 µm had more excellent sintering and drying properties. Since Comparative Examples 16, 17, and 18 had poor sintering and drying properties, their bending strength or dielectric strength could not be measured. Example 3 Test porcelains having grain sizes and compositions shown in Table 3 were prepared in the same manner as in Example 1, and bending strength and dielectric strength were measured by the same ways as in Example 1. Further, the relation between the mixed rate of alumina coarse grains of not less than 44 µm and the characteristics was examined. Results are shown in Table 3. Present invention 36 37 38 39 40 41 42 43 Mixing ratio (wt%) Alumina 30 Feldspar 30 Silica sand 2 China stone 3 Clay 35 Grain size Average particle diameter of alumina (µm) 6 8 12 Average particle diameter of entire body (µm) 7 Amount of alumina having not less than 44 µm in particle diameter (%) 0.6 0.8 1.0 0.5 0.8 1.0 0.7 0.9 Characteristics Bending strength (kg/cm 2 ) 1580 1520 1450 1600 1520 1450 1520 1450 Dielectric strength (kV/mm) 15.1 14.5 13.9 15.0 14.7 14.2 14.6 13.6 Comparative example Conventional product 19 20 21 22 23 6 7 Mixing ratio (wt%) Alumina 30 30 30 Feldspar 30 30 30 Silica sand 2 2 2 China stone 3 3 3 Clay 35 35 35 Grain size Average particle diameter of alumina (µm) 6 8 12 16 2 2 Average particle diameter of entire body (µm) 7 7 7 Amount of alumina having not less than 44 µm in particle diameter (%) 1.3 1.2 1.1 0.9 1.2 0.5 0.8 Characteristics Bending strength (kg/cm 2 ) 1350 1390 1380 1360 1280 1300 1270 Dielectric strength (kV/mm) 13.4 13.6 13.0 14.0 13.4 14.6 14.2 From the results, it is seen that porcelains in which the average particle diameter of alumina fell in the range defined in the present invention and the amount of coarse grains of alumina of not less than 44 µm was not more than 1% by weight (relative to total weight of alumina) are preferable, because their bending strength was not less than 1.4 x 10 8 Pa (1,400 kg/cm 2 ). From the obtained results, the relation between the amount (% by weight based on total alumina) of coarse alumina grains of not less than 44 µm in particle diameter and the porcelain bending strength is shown in Fig. 5. Example 4 Rod-formed insulators having a barrel diameter of 105 mm and an entire length of 770 mm were prepared from porcelains having grain sizes and compositions shown in Table 4, and their bending strengths were measured in the state that one end was supported, whereas a load was applied to the other end. Results are shown in Table 4 together with the porcelain bending strength of test pieces having the grain sizes and compositions. Conventional porcelain Invention porcelain Amount of alumina (wt%) 30 30 Average particle diameter of alumina (µm) 2 6 Average particle diameter of entire body (µm) 7 7 Average bending strength of product (kg/cm 2 ) 1120 δ = 54.8 1288 δ = 25.9 Porcelain bending strength of test piece (kg/cm 2 ) 1280 1570 From the results in Table 4, although the average particle diameter of the bodies was the same between the conventional porcelains and the invention porcelains, the grain size of the raw materials (feldspar, china stone, silica sand) except alumina was greater in the conventional porcelain. Thus, it is seen that variations in strength of the conventional porcelain is greater and the average strength value is lower. As is clear from the above-detailed explanation, according to the process of the present invention, high strength porcelains which have satisfactory characteristics as insulators can be obtained by controlling the larger grain size of alumina, which is decomposed in a single crystalline state coarse and making the other raw materials finer, even if the grain size of bodies is made equal to that of conventional ones.

Claims: A process of producing high strength porcelain for use in insulators, comprising mixing (a) 20-60 wt % alumina powder having a particle size distribution (i) such that the 50% average particle diameter is in the range 4-14 µm and (ii) extending up to above 44 µm with particles of diameter more than 44 µm present in an amount of less than 1% by weight relative to the total weight of alumina with (b) at least one finely ground raw material selected from feldspar, silica sand, china stone and clay, such that the resultant mixture of these components has a 50% average particle diameter in the range 3-10 µm; shaping the resultant mixture to form a shaped body; drying the shaped body; and firing the dried, shaped body; the method being performed so that quartz grains having a maximum diameter of not greater than 40 µm are present in the porcelain. A process according to claim 1, wherein the 50% average particle diameter of said mixture is in the range 3 to 8 µm.

Inventors: HARADA HIROSHI, ISHIKAWA JUNICHI, OGURI NORIYASU, SEIKE SHOJI, HARADA, HIROSHI, ISHIKAWA, JUNICHI, OGURI, NORIYASU, SEIKE, SHOJI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1988
Main classification: H01B3
IPC codes: C04B35, H01B17, H01B3, C04B33, H01B19