[Trade Journal]
Publication: Journal of the American Institute of Electrical Engineers
New York, NY, United States
vol. 42, no. 6, p. 631-635, col. 1-2
The Production of Porcelain for Electrical Insulation-III
BY FRANK H. RIDDLE
Associate. A. I. E. E.
Champion Porcelain Company. Jeffery-Dewitt Insulator Company
Review of the Subject.—The raw materials should be carefully tested before using. China and ball clays should be tested for fired color, porosity at the regular burning temperature, fineness of grain, etc. Ball clays should be tested for raw physical strength.
The grain size of quartz is of particular importance and the fine grain sizes should be determined by elutriation or water separation.
Feldspar should fuse to a glass with the fusion of pyrometric cone No. 8, 1280 deg. cent. (2336 deg. fahr.), and its color. degree of glassiness, amount of deformation, etc. noted.
The composition and formation of porcelain in firing are briefly described. The limits of composition are wide but the quality of the final product will vary with the composition. Special porcelains for use as spark plug cores are made by eliminating feldspar and quartz and substituting synthetic calcines.
During firing the mechanical water is first expelled. Chemically combined water is driven off al 500 deg. cent. (932 deg. fahr.). Alpha quartz assumes the beta form at 5;5 deg. cent. (106; deg. fahr.) with a similar expansion in volume. Shrinkage and condensation of the volume of the clay substance takes place at 900 deg. cent. (1652 deg. fahr.). Continued firing contracts the porcelain and decreases the porosity, the feldspar finally melting and gradually taking the more refractory clay and quartz into solution or assisting in converting it into other materials, particularly the clay which breaks up into sillimanite and free silica. The solution of the quartz depends upon grain size and heat treatment. On account of volume change of the quartz grains, it is evident that the fired porcelain, in which the quartz grains are in intimate contact with the glassy groundmass will be placed in a condition of stress after cooling down as the quartz contracts more rapidly than the rest of the porcelain. Naturally the greatest strain occurs around the largest quartz grains and clearly demonstrates the necessity for fine grinding.
The properties and testing of porcelain are described. Ordinary porcelain has a tensile strength of from 3000 to 6000 pounds per square inch and a coefficient of lineal thermal expansion of from 4 to 9 X 10-6 per degrees centigrade, while special porcelains have a strength as high as 12,000 pounds and an expansion as low as 2.7 X 10-6.
TESTS OF RAW MATERIALS
IN the manufacture of porcelain it is necessary that the raw materials used be subjected to tests in order to check up their properties. Of these chemical analysis is one of the most important as it a- sures constancy of composition.
The china clays or kaolins are tested for color upon firing, for porosity at the kiln temperature employed and for fineness by passing the material through a 200-mesh sieve. In the latter test the amount of residue is determined and its color upon firing noted. The presence of impurities in the clay thus becomes readily apparent.
The ball clays are likewise tested for firing color, for porosity at the kiln temperature to note the occurrence of overfiring, for their residue on the 200-mesh sieve and for their strength in the dried state. The last test usually expresses also in an indirect way the plasticity of the material. The strength is expressed by the modulus of rupture of bars made from 50 per cent of the sample and 50 per cent of flint, which are dried up to 110 deg. cent., and kept in desiccators over sulphuric acid to keep them dry until ready for the test. The bars are usually 6 inches long with a cross-section of 1 inch. by 1 inch. The load is applied at the center of the specimen which is supported by movable bearings, a fixed distance apart. From the breaking load, the length, depth and width of the bars, the modulus of rupture is computed. For good ball clays, under the conditions of the test, the modulus may vary from 300 to 450 pounds per square inch.
The ground quartz, or flint as it is commonly called, is tested for fineness by determining the more accurate determination of the fineness the so-called elutriation test is applied in which a weighed sample of flint, usually 50 grams, is placed in a cylindrical vessel, conical at the bottom, through which a current of water is caused to ascend and which carries away all finer particles, below a given size. The latter value depends upon the velocity of the water current, which must be kept constant. The velocities employed range from 0.8 to 1.5 mm. per second according to the kind of porcelain which is to be made. The elutriation is continued until the water runs off clear, showing that the washing separation is completed. The residue left, in the conical part of the apparatus is then dried and weighed. It is evident that such a separation is much to be preferred to the sieve test, especially in the case of high grade porcelains where it is imperative that the flint be as fine as possible and its content of flour-like material as high as can be obtained. The importance of the fineness of flint for this industry cannot be emphasized too much and every precaution must be taken to insure the constancy of this factor.
Feldspar is tested for fineness in the same way as flint and in addition its degree of fusion at the temperature of pyrometric cone No. 8, about 1280 deg. cent. (2336 deg. fair.), is noted, by observing the degree of glassiness which has been reached, the color and the degree of deformation. In addition the composition is obtained by chemical analysis. The accessory raw materials used are subjected to analytical tests.
THE COMPOSITION AND THE FORMATION OF PORCELAIN IN FIRING
It has been shown that the composition of porcelain may vary between wide limits. According to the proportions of the constituents, clay, feldspar and ground quartz (flint), the required firing temperature and the resulting properties of the product will vary decidedly. This is well illustrated in the triaxial diagram of Fig. 7, according to Klinefelter of the Westinghouse Electric Company, in which the relation Graphs of this character are used extensively in the study of ceramic problems as they are convenient for judging areas of compositions. In studying the triaxial diagram it is evident that if a composition was made up of two of the three ingredients used it would lie on the line or leg of the triangle somewhere between the two angles which represent 100 per cent of the two particular ingredients. A mixture of 50 per cent clay and 50 per cent flint would be represented by a dot on the leg and half way between 100 per cent clay and 100 per cent flint. It is also evident that a dot right in the center of the triaxial would represent a body containing 33.3 per cent of each of the three ingredients, flint, feldspar and clay.
Again, take a body of the composition, 25 per cent feldspar, 25 per cent flint and 50 per cent clay. Make a dot on this point on the triaxial and it will be noted that it lies in the field between bodies having the best high dielectric strength, mechanical strength and resistance to temperature change. This brings out the point that it is very difficult to get a body composition having all three properties at their best, it usually being necessary to sacrifice some of each making a body of about 85 per cent perfect in all respects or perhaps 100 per cent perfect in two respects and less perfect in the third respect. The firing, grinding, kinds of raw materials used and other variables also have an effect on these characteristics; however, the example is given to illustrate the general method of study.
While roughly speaking many high-tension porcelains contain about 50 per cent of clay substance, 27 to 30 per cent of feldspar and 20 to 23 per cent of flint, there are some in which the feldspar is as high as 45 per cent, the clay 40 to 45 per cent and the flint only 10 to 15 per cent. In some European porcelains which are matured at about cone 15 (approximately 1400 deg. cent., 2552 deg. fahr.), the clay content is 50 per cent, that of feldspar 25 per cent and of flint 25 per cent.
As has already been indicated above there are being produced for special and severe service as in spark plugs, special porcelains in which both feldspar and quartz have been practically eliminated and replaced by synthetic calcines. Thus feldspar has been replaced by a flux, corresponding to the formula, MgO. Al2 O3 4 Si O2, composed of 56 per cent of kaolin, 18.2 per cent of magnesite and 25.8 per cent of flint. Again, the flint is replaced by another calcine, sintered at a high temperature, composed of 70.2 per cent of kaolin, 27.8 per cent of anhydrous alumina and 2 per cent of boric acid. The whole spark plug composition would then be, kaolin 30 per cent, ball clay 10 per cent, fluxing calcine (as given above) 20 per cent, and refractory or sillimanite calcine (as above) 40 per cent. In this case the refractory calcine may, of course, be replaced by one of the natural minerals which yields sillimanite. It is evident that the important stage in the production of porcelain is the firing and what happens there determines once and for all its physical properties. It is also not difficult to see that different kinds of heat treatment may produce different structures from exactly the same composition. It might be well to consider briefly the changes in structure which occur during the firing process, as told us by the petrographic microscope.
During the earliest part of the firing the mechanical and hygroscopic water is expelled and at above 500 deg. cent. (932 deg. fahr.) chemically combined water is lost. The clay substance shows a drop in density from 2.55 to 2.47, approximately, between 500 deg. cent. and 600 deg. cent. (932 deg. to 1112 deg. fahr.) and hence an increase in molecular volume. The average expansion for different clays at this point is about 6 per cent, at 575 deg. cent. (1067 deg. fahr.) the alpha quartz assumes the beta form with a similar expansion in volume. At about 900 deg. cent. (1652 deg. fahr.) a condensation in the volume of the clay substance takes place coincident with an exothermic change. With further rise in temperature the porcelain begins to contract in volume and to decrease in porosity, slowly at first and then faster as the temperature increases commensurate with the amount of fluxes present. With the progress of vitrification the true density of the porcelain decreases, in spite of the contraction in external volume, and in fact, the progress of the molecular changes due to vitrification is admirably shown by the drop in true density with temperature. The increase in molecular volume with progressive vitrification and fusion is characteristic of most silicates. As the temperature rises the most fusible eutectics of the porcelain soften, followed later by the fusion of feldspar. The fluidity of the feldspar increases rapidly and with it its solvent effect upon the more refractory constituents, clay and quartz. The latter is attacked vigorously, the fine particles being dissolved entirely and the edges of the larger ones being rounded off. If the quartz is sufficiently fine grained all of it will be brought into solution ultimately, provided the heat treatment is continued long enough. On the other hand if the quartz is coarse the large particles remain as such suspended in the glassy matrix, thus causing the structure of the porcelain to remain heterogeneous. That such coarser quartz grains are detrimental to the best development of the porcelain cannot be doubted. Since they are subject to the crystalline inversions of quartz and the accompanying expansion and contraction in volume it is evident that upon cooling these grains which in intimate contact with the glassy groundmass must necessarily cause a condition of stress to be produced. On passing through 575 deg. cent. (1067 deg. fahr.) the contraction in volume from the beta to the alpha quartz is certain to react upon the surrounding material in causing a strained state. The larger the quartz grains the greater must be the stress produced. The importance of having the quartz as finely ground as possible is hence obvious.
It is a curious fact that the quartz remains largely as such and is not converted to cristobalite. It has been suggested by Klein that the solution of the quartz progresses faster than its inversion to cristobalite. However, some European investigators report the presence of some cristobalite in hard fired porcelains.
With the complete vitrification of the porcelain and the enrichment of the feldspathic glass with dissolved quartz and some of the clay substance, the bulk of the latter undergoes dissociation into sillimanite. This mineral constituent at first assumes the amorphous and crypto-crystalline form and later develops as needlelike crystals. According to the length and final temperature of the heat treatment this crystallization is more or less pronounced. In the presence of small amounts of fluxes like magnesia the crystal development becomes very marked and may become excessive, due to the growth of coarse crystals which actually reduce the mechanical strength of the porcelain. The solution of the quartz grains and the development of sillimanite are shown in the microphotograph of a porcelain in Fig. 8.
If the temperature continues to rise after complete vitrification has been reached the porcelain may become overfired, that is, the fluidity of the mass may become increased to such an extent that an evolution of gases takes place. This results in the formation of a vesicular structure which is worthless for the purposes of electrical insulation.
The progress of the vitrification of a porcelain and its subsequent overfiring are shown in the diagram of Fig. 9, in which both the porosity and the shrinkage of a number of porcelains are plotted against temperature.
From what has been said it is evident that there is a wide latitude in the production of widely different porcelain structures and that the heat treatment must be carefully regulated for each special type. The translucency of porcelain increases with vitrification since more light is transmitted in the presence of a larger volume of glassy matrix.
THE PROPERTIES AND THE TESTING OF ELECTRICAL PORCELAIN
The chief properties of the porcelain with which we are concerned are their imperviousness to the absorption of liquids, their dielectric resistance and their mechanical strength.
It is evident that the first characteristic must be developed to the highest extent since any absorption of water by high-voltage porcelain must be fatal. The test commonly employed for this purpose is the immersion of the porcelain in fuchsine dye dissolved in alcohol under a pressure of 200 pounds per square inch. The appearance of the dye penetration is a measure of the thoroughness of the vitrification.
The dielectric resistance of these porcelains has been the subject of many discussions and need not be gone into at this time. This test is carried out at every plant making high-tension porcelain. In the case of special porcelains such as for spark plugs, still other severe requirements must be met. The porcelain must not only be a good insu1ator at atmospheric temperatures but must resist moderate voltages at temperatures around 815 deg. cent. (1500 deg. fahr.) at which the average feldspathic porcelain breaks down completely. This electro-thermic resistance is expressed by the so-called Te value, which represents the temperature at which a cubic centimeter of the material still shows a resistance of one megohm. While for the usual type of porcelain this value is approximately 400 deg. cent. (752 deg. fahr.) some of the special porcelains reach figures varying from 650 deg. to 800 deg. cent. (1202 deg. to 1472 deg. fahr.)
The mechanical strength of porcelains differ widely. The resistance to compression may vary from 45,000 to 65,000 pounds per square inch and the tensile strength from 3000 to 12,000 pounds per square inch.
The feldspathic porcelains show a resistance to tensile stress of from 3000 to 6000 pounds per square inch and some of the modern special porcelains from 9000 to 12,000 pounds per square inch. The latter values cannot be reached with feldspathic porcelains containing much undissolved quartz. Of the less obvious properties of porcelain the thermal expansion deserves some consideration. It is easy to see that this property is likewise the result of the internal structure of the material. In this connection it is interesting to compare the total expansion of different silica materials with those of porcelain and quartz glass which are shown in the diagram of Fig. 10. The thermal expansion of porcelains as a rule decreases with the firing temperature. The coefficient of thermal expansion of some special porcelains between 16 deg. and 250 deg. cent. (59 deg. and 482 deg. fahr.) is as low as 0.00000269 to 0.000004, while clay-feldspar-quartz porcelains of ordinary low fire type are as high as 0.000009. The more homogeneous the structure of the porcelain and the lower the content of the alkalies the lower should we expect the thermal expansion to be.
It has been noted that the importance of a properly developed porcelain structure is of paramount importance and the realization of this fact will lead ultimately to the use of petrographic inspection as a criterion of the quality of porcelain. Just as in the metallurgical arts the microstructure of metals has been studied so successfully, so for electrical porcelain the petrographic microscope will aid in the detection of inferior structures.
GLAZE
The subject of porcelain cannot be dismissed without some reference to the glazes which are applied to the surface. These are invariably alkali-lime-alumina silicates and form typical glasses. These must be adjusted to the body, particularly with reference to the coefficient of thermal expansion. When properly fitted the glazes increase the mechanical strength of the porcelain decidedly. On the other hand, poorly adjusted glazes weaken the product markedly. The composition of porcelain glazes may be expressed by means of empirical chemical formulas according to which it may vary from-1 R O, 0.5 Al2 O3 4Si O2 to 1 R O, 1.2 Al2 O3, 12 Si O2, depending upon the maturing temperature of the porcelain. In these expressions R O represents the sum of the molecular equivalents of the alkalies and lime. The alkalies may fluctuate between 0.2 to 0.3 and the lime between 0.8 to 0.7 molecular equivalents. In the case of colored glazes metallic oxides such as those of iron, manganese and chromium may be introduced. The first is usually brought in by means of iron carrying, fusible clays, called slip clays.
There is usually an intimate contact produced between the glaze and the body which is shown very strikingly in the photomicrograph of Fig. 14 where crystals of sillimanite have grown out of the body.
