[Trade Journal]
Publication: Journal of the American Institute of Electrical Engineers
New York, NY, United States
vol. 42, no. 5, p. 540-543, col. 1-2
The Production of Porcelain for Electrical Insulation—II.
BY FRANK H. RIDDLE
Associate, A. I. E. E.
Champion Porcelain Company
Jeffery-Dewitt Insulator Company
Review of the Subject.—This article deals with ceramic raw materials, chiefly clays, quartz and feldspar.
CLAYS. The physical properties of clays are of interest. Plasticity is one of the principal properties of clays. The cause of plasticity is probably to be sought in the dispersed nature of the fine clay particles which have the properties of a suspension colloid. Small amounts of alkalies, when added to clay suspensions, cause a marked increase in fluidity. I n other words a mass of plastic clay, by adding these alkalies, can actually be converted into a fluid capable of being poured into molds. This application is of great importance commercially since it enables us to form clay articles of a very homogeneous structure without the use of excessive amounts of water and without the high drying shrinkage which would accompany the use of much water.
The reactions which take place in firing clay are described. Firing ranges of clays can be determined by the drop in porosity and increase in shrinkage as the firing temperature increases. Continued firing of a clay after it has vitrified produces softening of the mass and produces internal changes including the formation of sillimanite (A1203, Si02) crystals. Continued firing releases gases which produce a porous or vesicular structure known as overfiring. The longer the firing range of a clay the better it is suited for commercial purposes.
QUARTZ. Ground quartz which is practically pure silica, (Si02) in porcelain, serves as a refractory skeleton which increases the stiffness of the mass during firing. Quartz undergoes molecular changes at different temperatures. There are eight forms. Each change is accompanied by a definite volume change. The specific volume starting with the lowest form, increases continuously and abruptly at the inversion points until a final total increase in volume of about 20 per cent is reached when the quartz is fused.
FELDSPARS. Used for porcelain contain potash and soda as their fluxing elements. They fuse to glasses at about 1230 deg. cent. (2246 deg. fakir.) Their chemical compositions approximate Na KO Al, O3 6Si O2. They are used as glass forming fluxes in porcelain to cement and unite the other ingredients.
Other minerals are also used:
1. Those which replace feldspar such as alkaline earths.
2. Those which replace quartz and which are preferably constant in volume such as sillimanite, zircon, etc.
Porcelains are now being used in which natural and synthetic sillimanite replace quartz, and alkaline earths replace alkalies.
PHYSICAL PROPERTIES OF CLAYS
THE most outstanding quality of clay is its plasticity, that is, the ability to be molded and to change its shape under pressure without breaking. It is found that clay when forced from a cylinder through an orifice requires a definite pressure before flow is started. It has been attempted to employ the value of this initial pressure applied under standardized conditions as a means of estimating plasticity, since materials vary greatly in this respect. This probably is the most promising method chosen from many others, most of which are faulty or are only indirect. Of all the physical properties of materials, plasticity is one of the most difficult to define and to measure.
The cause of plasticity is probably to be sought in the dispersed nature of the fine clay particles, which have the properties of a suspension colloid.(1,2) This view is plausible because many phenomena observed on clays are those exhibited by colloidal substances. Among these may be noted the power of clay to absorb salts both organic and inorganic from their solutions, the hygroscopic nature of dried clay, the migration of the particles in a dilute suspension to the positive electrode and the profound effect of small quantities of electrolytes(3) upon the fluidity(4) of a clay-water system. These properties may be illustrated by the absorption of dyes like malachite green, which has been suggested by Ashley(5) as a means of estimating plasticity, by the power of dried clays of removing water from sulphuric acid solutions, by the Schwerin process in which clay is removed electrically from suspension and deposited on a revolving metallic electrode and by the fact that heavy, thick suspensions may be instantly made very fluid through the addition of a minute amount of alkali. The phenomenon last mentioned embraces also the action of acids and salts upon clay suspensions according to which these may be thickened by such additions. We have then the fact that alkalies cause a marked increase in fluidity of a clay suspension while acids and salts bring about a thickening. These phases correspond to the deflocculation and coagulation(6) of colloidal systems and it is interesting to note in this connection that the coagulating or deflocculating effect depends upon the nature and valency(7) of the electrolyte. These facts are made use of industrially, especially in the casting process where a heavy suspension of a density as high as 1.8 can be produced with a water content not appreciably greater than that of the plastic mass through the addition of a small amount of sodium silicate and sodium carbonate. In other words a mass of plastic clay, by adding these alkalies, can actually be converted into a fluid capable of being poured into molds. This application is of great importance commercially since it enables us to form clay articles of a very homogeneous structure without the use of an excessive amount of water and without the high drying shrinkage which would accompany the use of much water. What actually happens is not known with certainty but the prompt and effective action of the alkalies suggests a direct chemical combination. In casting we produce the deflocculated state in which the clay sets free part of the water which it must have held in something like solid solution or absorption. Anything that interferes with the deflocculation, like the presence of acids or salts prevents the proper application of the casting process.
EFFECT OF HEAT
Upon heating clay to 110 deg. cent. (230 deg. fahr.) its hygroscopic water is driven off. At higher temperatures above 500 deg. cent. (932 deg. fahr.) it loses its chemical water and the molecule is broken up in an endothermic process which absorbs about 10.8 calories per gram molecule or 258 grams of clay. This brings about a profound change which is indicated by the expansion of the mass shown by the drop in density and the loss of plasticity. At this point clay shows its greatest porosity since there is left not only the volume left vacant by the pore water but also the pore space due to the water of combination. Mechanically the mass is weak and friable(8) As the temperature rises above 800 deg. cent. (1472 deg. fahr.) an exothermic change occurs which by some is thought to be due to polymerization and by others to the formation of sillimanite. At still higher temperatures the clay begins to contract in exterior volume and the porosity decreases. The point at which this action begins and the rate at which it continues vary with the nature of the clay. In pure clays this temperature is higher than it is for the more impure ones and the rate of porosity drop is much slower. The greater the amount of fluxes, such as the alkalies, lime, magnesia and iron oxide, the more rapid are the progress of shrink-age and decrease in porosity. The cause of the contraction is to be sought in the incipient fusion of the fluxing constituents which progresses as the temperature rises. These fluxes dissolve more clay and quartz and consequently the amount of material which softens becomes greater and greater. The result must be that the rigidity or viscosity of the system is decreased and with it the resistance to the surface tension which acts inward from the surfaces of the clay tending to contract it. This powerful tension at a sufficiently high temperature finally closes the entire pore system and we say then that the clay is vitrified.
The drop in porosity and the increasing shrinkage, coordinated with the rising temperature, are often shown graphically and such a diagram expresses the rate of vitrification. In this manner the course of the vitrification process in its various stages is shown clearly and from the graph we can predict with certainty the heat behavior of the clay. The slope of the curve or its. tangent, dy /dx, then expresses the rate of porosity drop, characteristic of the material in question. The shrinkage curve is the converse of the porosity graph, it rises as the latter drops. Such a diagram is shown in Fig. 4. In this manner it is quite easy to differentiate a slowly vitrifying, refractory clay from one which is higher in fluxes and shows a rapid drop in porosity. After a clay has vitrified and the temperature continues to rise, softening of the mass becomes more and more marked and internal changes in structure appear. It has been found that the clay substance crystallizes at the higher temperature and is decomposed according to the reaction.
Al2 03 2 Si 02 = Al2 03 Si 02. Si 02.
The crystalline material produced is sillimanite of the composition A1203 Si02 which usually appears as needle-like crystals. This material, of course, may exist in the amorphous or the crypto-crystalline form long before the visible crystals are developed. At some stage of the high temperature treatment many clays release gases previously held in the absorbed or dissolved condition and such oxides as those of iron are decomposed yielding oxygen. These gases produce a porous structure very similar to the rising of bread dough, which causes a decided loss in the strength of the material. Clays in this state are said to be overtired.
The more readily they are subject to the formation of the vesicular structure the less suited they are for commercial utilization. This increase in porosity is plainly shown in the porosity-temperature graph. Certain clays are so prone to produce this structure that they give rise to it even before they are fully vitrified. Such materials are usually of but little industrial value.
Finally, if the temperature is raised high enough the clay gradually fuses until it has melted into a pool of a slaglike mass. Clays have no definite melting point but progress more or less slowly towards the stage of fusion. The final temperature at which complete deformation has taken place varies widely according to the nature and composition of the materials. Pure kaolins melt at the fusion temperature of platinum, 1755 deg. cent. (3191 deg. fahr.) 1. E., white heat.
In the study of the effect of heat on clays and in industrial practise temperature measurement becomes a very important factor. This is accomplished either with the use of platinum, platinum rhodium thermo-couples and indicating instruments, or the use of the Seger cones, small tetrahedra made of mix¬tures of kaolin, feldspar, calcium carbonate and quartz which deform by bending at fairly definite temperatures and thus give evidence that a certain point has been reached. The interval between the cone numbers is about 20 deg. cent. (36 deg. fahr.). While these pyro-scopes do not indicate the temperatures with accuracy they do show the effect of heat work, that is, the joint effect of temperature and time, which is often more useful than the indication of temperature alone. In all ceramic work the influence of the time of heat treatment is exceedingly important since the same result may be accomplished, within limits, by increasing the temperature more slowly to a lower point as by raising it faster to a higher point.
QUARTZ
The function of ground quartz in porcelain is to serve as a refractory skeleton which increases the stiffness of the mass during the firing process and thus prevent excessive deformation. The mineral is, of course, non-plastic. Its composition is nearly 100 per cent of silica. Quartz possesses the peculiar property of undergoing molecular changes at different temperatures, evidenced by changes in the character of crystallization and in density. Some of these inversions are rapidly and others sluggishly reversible. Alpha quartz is taken to be the stable form at ordinary temperatures which at 575 deg. cent. (1067 deg. fahr.), i. e., low red heat, inverts to the beta quartz. Upon cooling the beta quartz reverses rapidly to alpha quartz. At about 1470 deg. cent. (2678 deg. fahr.) beta quartz undergoes another inversion and becomes beta cristo-balite. This crystalline form likewise has an alpha modification whose inversion temperature is about 230 deg. cent.(446 deg. fahr.). Theoretically the beta quartz should invert to still another form of crystalline silica at 870 deg. cent. (1598 deg. fahr.) namely, 02 tridymite, but practically owing to the sluggish transformation beta cristobalite results at 1470 deg. cent. (2678 deg. fahr.). But on long continued heating very silicious compositions like silica brick do finally become largely tridymite. There are three forms of the latter, alpha, 0i and 02 tridymite. Finally, at very high temperatures the silica fuses and we have what is known as quartz glass. We might say then that there are eight forms of silica, namely, alpha and beta quartz, alpha and beta cris-tobalite, the three modifications of tridymite and glass. The specific volume, starting with alpha quartz, in-creases continuously and abruptly at the inversion points until we finally observe a total increase in volume of about 20 per cent. These changes are shown in the diagram of McDowell,(9) Fig. 5.
Quartz occurs in nature in various forms, in the form of massive crystals, as quartz schists, sand-stones, sand and quartzite. There are also crypto-crystalline forms such as flint, chert and chalcedony. These differ widely as regards their rates of inversion. There should be mentioned also the forms of silica which represent the residue of marine organisms, such as tripoli, randannite and kieselguhr and finally, the colloidal forms of silica, like opal, hyalite, geyserite, etc. In these materials we have represented a series of minerals and rocks from those in which the crystalline character is well developed to materials which are crypto-crystalline or entirely amorphous. The source of the free silica introduced in the body is hence a matter of considerable importance since both its solution by feldspar and its rate of inversion are dependent upon the original structure.
FELDSPAR
The feldspathic minerals with which we are concerned are the potash and soda feldspars, orthoclase, micro-cline and albite. The first two are crystalline modifications of the same composition, K20 Al 20 3 6Si02, while the last corresponds to the formula Na20 A1203 6Si02. Owing to their alkali content the feldspars are comparatively fusible minerals, the deformation or softening point being approximately 1230 deg. cent. (2246 deg. fahr.) for the potash and 1200 deg. cent. (2192 deg. fahr.) for the soda feldspar. These minerals cannot be said to have definite melting points but change gradually from the crystalline to the isotropic state. They sluggishly form a viscous glass which may be transparent as in the case of the pure potash feldspar or more often white and opaque. It is evident that the molecular friction of the fusing mass is very high. There is a distinct difference between the orthoclase and the albite with reference to their viscosities in the fused state. The latter attains a much greater fluidity and it also appears that there might be a viscosity minimum somewhere near the eutectic between potash and soda feldspar. The gradual fusion of the feldspar and its high viscosity are desirable properties inasmuch as this permits the firing of a porcelain body to gradual vitrification without having to fear undue deformation of the mass under the heat treatment.
Like most silicates fused feldspar shows an increase in molecular volume over its volume in the crystalline state. It likewise shows in the molten state electrolytic conductivity. Its power to dissolve silica and silicates when fused has already been referred to.
In nature neither of the feldspars occurs in the pure state but practically all potash spars contain soda and the soda minerals potash. For the purposes of porcelain making the potash feldspars are preferred since they possess a higher viscosity at the kiln temperatures than the soda mineral and it is believed result also in porcelains of higher mechanical strength, although this point has not as yet been proved conclusively. It has been observed that porcelains made with soda feldspar give, when struck, a duller sound than when potash as a glass forming flux, the purpose of which is first to unite and cement the more refractory constituents and finally, to start to dissolve and take them into solution or combine with them.
MISCELLANEOUS MATERIALS
Besides the raw materials already mentioned there may be used two types of constituents: First, substances which are fluxes and hence replace the feldspar, and second, components which are substituted for the quartz.
Among the former there may be mentioned the alkaline earths represented by calcium carbonate, magnesite, dolomite and barium carbonate. Occasionally these constituents are not introduced in the form of the carbonates but may be previously calcined with kaolin and quartz to form synthetic silicates, approaching somewhat the structure of the feldspars but without their alkali content. The use of such artificial fluxes is especially desirable for electrical porcelains subjected in use to higher temperatures, where the electrolytic conductivity of the feldspar would be objectionable.10
The replacement of quartz may likewise be desirable on account of the several crystalline transformations and volume changes which it undergoes and which will always impair the development of the greatest possible strength and resistance to thermal shock. It is obvious that the replacing substances should be free from the crystalline inversions of quartz and should be AS constant in volume as possible. A number of materials is available for this purpose of which there may be mentioned sintered or fused alumina, natural or artificial sillimanite, zircon, zirconium oxide, etc. When previously calcined or fused to constant volume these substances are practically free from crystalline inversions and porcelains made from them should likewise be practically volume constant and should possess a steady coefficient of thermal expansion. Porcelains are now being produced in which synthetic and natural sillimanite replaces quartz and to some extent the mineral zircon, Zr02 Si02, has been applied successfully for the same purpose. Synthetic sillimanite is produced by the calcination to a high temperature of a mixture of 258 parts by weight of kaolin and 102 parts of anhydrous alumina. This product shows no heat effect, has a density of about 3.00 and yields little or no solid solution. The natural sources of sillimanite are andalusite, cyanite and sillimanite. The mineral zircon is used in its natural state.
1. Colloids are dispersed systems of very finely divided particles; the diameter of the particles in typical cases lies between one ten-thousandth and one one-millionth of a millimeter. These particles pass through filters readily, cannot be seen through the microscope and show the Tyndall cone effect; solutions of electrolytes either disperse or coagulate them. There is no reason why every substance may not be produced in the colloidal state. Colloidal silver may be produced by arcing fine silver wires under water.
2. A colloidal system of solid dispersed in liquid is a suspension colloid system; one of liquid in liquid, emulsoid.
3. An electrolyte is a compound which in water solution dissociates into positively and negatively charged molecules termed ions.
K Cl + water 4 K+ + C1—
B a Cl2 + water -- B a++ + 2Cl—
Such a solution conducts electricity. Nearly all salts, strong acids and bases are electrolytes.
4. Fluidity is the measure of the tendency of substances to flow; viscosity the measure of the resistance to flow.
5. United States Geological Survey, Bulletin No. 388.
6. Clay held in suspension by alkali in a fine grained and jellylike mass is in the state of deflocculation; acids and salts operate to collect (coagulate) the clay into larger masses. These settle leaving a clear supernatant solution; the clay is in the state of flocculation.
7. Valency is expressed by the number of charges the ion carries.
K+ — valency is one
B a++ — valency is two
A 1+++ — valency is three
8. Electrical Porcelain, E. E. F. Creighton, A. I. E. E. TRANS. Feb. 1915.
9. American Institute of Mining Engineers, Bulletin 119.
10. Journal American Ceramic Society, Vol. 2, No. 7, 1919.
