US2106598A - Ceramic compositions and methods of making the same - Google Patents

Description

v Patented Jan. 25, 1938 UNITED STATES PATENT OFFICE CERAMIC COMPOSITIONS AND OF MAKING THE SAME METHODS p ratlori of Delaware No Drawing. Application June 6, 1934,

Serial No. 729,344

19 Claims.

' the electrical strain on the insulator is increased.

With increased engine compression the combustion chamber temperatures increase. The effect of increased temperatures is to lower the insulating properties of the insulator. To meet the new conditions the plug must therefore have higher electrical insulating properties and must retain these properties at high temperatures. This last-named combination of properties is known as hot dielectric strength, and measurements of it are known as thermo-electric or T. E. value.

The insulator. must also be capable of withstanding the'eflect of rapid increase of temperature from its normal cold state to the very high temperatures attained timing the operation of the engine. This quality is known as resistance 'on the insulator to heat shock and if the insulator does not possess the necessary resistance to heat shock it will breal; in the engine and in some cases parts of the insulator willdrop into the combustion space and may interfere with the operation of the engine. Naturally the heat shock is greater the higher the operating'temperature of the engine. Consequently the insulator must have increased capacity. for resisting heat shock. o

Another essential quality in'the insulator is mechanical strength for the mechanical strain V increases with increased compression.

Porcelains are characterized by vitrification. Vitrification, as the term is used in connection with porcelains. consists essentially of partial fusion, that is, the glass and other low melting 'gredients, in'other words, the ingredients subject point materials are heated to the point where they fuse and bind together the non-plastic into little if any fusion at the firing temperatures. This partial fusion must be accomplished without deforming the body. If fusion is carried too far, the body will lose its shape. If it is not carried far enough the body will be porous. One of the vdimcult' problems in the design of a porcelain insulator consists in the selection of materials such that the range of temperatures in which partial fusion-takes place is sumciently great so that slight difierences' in firing conditions from one batch to another will not result in aninferior product.

Perhaps the greatest problem of all arises from the fact that the fired porcelain body is both physically and chemically diiferent from the unfired batch. Firing consequently often results in the production of products in the final conglomerate called the porcelain which lower some of the physical properties to a pointwhich is objectionable. In' most cases it is not possible to predict from a new mixture of materials what the properties of the fired body will be, nor is it possible, without experimentation, to determine the temperatures at which vitrification will take place. As a consequence past experience can serve but in a somewhat general way as a guide of sacrifice of other properties, but of course such porcelains are impractical for use as spark plug insulators whatever may be their utility in I other fields. I

There is here disclosed an improved porcelain insulator possessing to a superior degree the properties necessary for successful use in internal combustion engines, together with a method of manufacturing such insulators. Perhaps the most important feature of the new porcelain body is the nonplastic material used in its composition and this new material together with themethod of manufacturing it constitutes important features of this invention. The non-plastic material consists of a mixture of the artificial minerals mullite and. cordierite, with preferably some glass, the three constitutents being formed simultaneously from a completely molten aluminum silicate containing a proportion of the alkaline earth, magnesium oxide.

The method of manufacturing this material consists essentially in charging into an electric arc furnace a composition of material containing I alumina and silica in substantially the mullite ing it to cool at such a rate that the proper sizes and distribution of crystals are obtained.

The principal merit of the end product appears to be in the fact that it is composed of a mixture of mullite crystals and the mineral cordierite, 2MgO.2A12Oa.-5Si0z. To the best of my knowledge this is the only method bywhich it v is possible to obtain such a mixture commercially without the introduction of high-silica glass or of other undesirable constituents. Natural mullite is too rare for commercial use, sothat it is not practicable to make a mixture of the natural minerals; neither is it feasible to make the two minerals separately, for either a high-silica glass or corundum is almost certain to be introduced with the mullite, both of which are undesirable in this case. As will be explained later, the same results are not obtained by calcining, both on account of the size of crystals developed, which is an essential quality of my product, but also because it is not possible to get complete reaction between the silica and alumina at ordinary calcination temperatures for complete fusion is necessary to obtain the degree of equilibrium required. i

The raw ingredients used in the production of the new non-plastic material may be assembled in various ways. My method consists in the employment of natural kyanite (AlzOsSiOz) of suitable analysis, to which is added alumina to bring the proportion of alumina to silica up to the mullite ratio, 3 to 2, and the addition of the requisite proportion of magnesia. This material is then charged into an electric arc furnace in sufficient quantity to insure comparatively slow cooling after complete fusion. This slow cooling is highly essential, since the rate of cooling through the higher temperature regions determines the sizes of the crystals obtained as well as the distribution of the final phases. That is, the quickly cooled melt will possess small mullite needles, similar to calcined products, with intimate association of glass or other residual phases that might appear. But the slowly cooled melt will have large mullite prisms with large interstitial areas of glass or later crystallizing phases. In the case of the quickly cooled melt no reasonable amount of grinding will release the mullite from the other constituents, and since the flux, in this case, MgO, is found in the glass or interstitial phases, it can not be effective in developing a bond or fluxing the clays in a porcelain composition for it is distributed and held interstitially in mullite fibers. In the case of the slowly cooled melts the mullite crystals are readily separated from the otherconstituents by ordinary grinding. The flux-containing constituents, thus released from the mullite, may be ground finer because of the greater friability of the phases in which they occur and are then capable of coming intimately into contact with the clay as well as with the mullite fragments of the electric furnace melt, and serve most eifective as a flux or vitrifying agent. As mentioned above the proper conditions of cooling and crystallization can be met if the size of the ingot is controlled. A small batch would cool too quickly to produce the proper size in the primary mullite, and one too large would cool needlessly slowly even if complete crystallization were desired. The completeness of crystallization is of course also influenced by the composition of the melt. That is, with a given alumina-silica ratio, the nature and amount of fiuxing oxide crystallization proceeds in ingots of the same size.

composition given above ence might have are determine the extent to which' I have found that in the case of magnesia as flux I get the best results with an ingot weighing about seven tons, and having a magnesia content of about 2%. This composition yields -85% mullite crystals in a matrix, about half of which has devitrified to the mineral cordierlte. I have found that cordierite in this condition imparts to the porcelain in which it is incorporated certain desirable properties, which will be discussed later. While most of the-benefit seems to be derived from the cordierite, I have learned that there is some advantage in arresting crystallization after only part of the cordierite has come out, in order to leave some glass. 7 This is to prevent the impurities, which it is not commercially practicable to remove, from crystallizing out in some undesirable form. As long as there is some glass left most of them remain in solution there. A typical chemical analysis of the final product is substantially the following:

Per cent Silica (S102) 30.62 Alumina (AhOa) 66.36 Ferric oxide (Fe-203) .44 Titania (T103) .30 Lime (CaO) .24 Magnesia (MgO) 2.1! Alkalies .22

If it is desirable to increase the cordierite content it may be done by cooling the ingot more slowly or by increasing the magnesia content. If the latter course is'chosen, spinel (MgOAlzOa) crystallizes out in 'more appreciable quantities, which may or may not be detrimental. The lies very close to the borderline, on one side of which spinel is developed, on the other cristobalite, (S102) The composition of a residual glass phase in a melt having higher percentages of magnesia would contain considerably less silica than one with less MgO. Thus within the range of compositions covered here it is possible to vary the properties of the porcelain in which it is used by varying the M1 50 content, in order to obtain in addition to the essential mullite and cordierite'a glass high in silica or one comparatively low in that oxide. Compositions higher in magnesia than 10% contain so much spinel at the expense of the mullite that the material does not greatly benefit a porcelain with respect to the properties required of a spark plug insulator. Up to about 10% MgO, however. the spinel exists in such tiny crystals that its effect is not felt, and any'disadvantages its preseifectually offset by the beneficial effect of the cordierlte.

When used in the manufacture of porcelains the above described material is finely ground and mixed with clays in the proper proportions to oughiy mixed, pugged and worked in the usual manner and formed into the desired shape, in-

accordance with the usual practices, or if desired the method of working the material described and claimed in my copending application S. N. 741,089, filed August 23, 1934 may be used,- this method having the working of the body by previous plastic processes. The mullite grains resulting from the crushing of the large mullite crystals of the elec- The material is thereafter thoradvantage of decreasing the 1 amount of plastic material necessary for propertrically fused product. act as the non-plastic.

while the cordierite and the glassy interstitial phases, which contain the magnesia as well as the fiuxing impurities, such as lime and alkalies, act as the flux. Since they are softer and more friable than the mullite, they have reached, for the most part, a state of finer subdivision and are hence in an optimum condition for fiuxing the other constituents, being fine enough to come into intimate contact with a maximum of clay and mullite particles, thus" yielding a'high degree .of homogeneity.

The formed bodies are then fired in suitable kilns to a temperature of approximately 1500 degrees C. A glaze is then applied to the plugs, and this is followed by the usual glost firing.

Insulators made as above described willbe found to have markedly superior hot dielectric strength, resistance to heat shock, and mechanical strength. The coefiiclent of expansion is somewhat increased and this is desirable as it tends to improve the seal with the shell.

The proportions of ingredients and the methods of manufacturing my improved porcelain are susceptible of considerable variation in practice. it is well known in the ceramic art that the emperature depends on the amount of flux employed since usually the greater the amount of temperature. Hence flux the lower the firing where firing temperature or amount of flux are mentioned it will be understood that the amounts are not absolute but are subject to -mutual change. In making the fused material minerals other than kyanite may be employed, or if prefer-red alumina and silica may be used as such, or

, a part ofthese materials may be added in, the .form of clay. The silica content may be introduced in any of its various allotropic forms. The formula given is especially adapted for use with wet mixingmethods but it is obvious that' the non-plastic material as well as the raw batch may he varied considerably in composition where dry forming, casting or other methods are employed.

I claim:

1. The method of making a non-plastic ceramic comp sition consisting chiefly of mullite and cordierite which consists in preparing a mixture of alumina and silica in substantially the mulliteratio," together with magnesia, fusing the mixture and permitting it to cool slowly, so as to produce mullite andcordierite in a glassy matrix.

2. The method of making a non-plastic ceramic composition consisting chiefly of mullite and cordierite for use in porcelain body batches and the like, which consists in preparing a mixture of alumina and silica in substantially the mullite ratio, together with up to magnesia, fusing the mixture, and allowing the fused material to cool slowly .so as to form mullite and cordierite in a of alumina d silica in substantially the mullite .ratio, together with n the order of 2% magnesia,-

fusing the mixture and allowing the fusedmaterial to cool slowly so as to form mullite and cordierite in a glassy matrix and permit ready separation of the mullite crystals from the remaining constituents.

4. A porcelain body made by firing to vitrification a ceramic composition comprlsingnon-plastic material in the form of a fused mixture comprising mullite and cordierite as the principal ingredients, together with plastic material in suillcient amount to form a workable body.

1 5. A porcelain body made by firing tovitrification a ceramic composition comprising non-plastic material in the form of a fused mixture consisting principally of mullite and cordierite together with a flux-containing glass, and plastic material in sufilcient amount to form a workable body.

6. A porcelain body made by firing to vitrification a ceramic composition comprising non-plastic material in the form of a fused mixture consisting principally of mullite and cordierite together with a classy matrix containing a magnesium compound, and plastic material in sufficient amount to form a workable q y.

7. Non-plastic material for use in the manufacture of-ceramic wares consisting of a fused mixture of on the order of 80% mullite crystals in a glassy matrix containing a substantial tion of cordierite.

8. The method of making non-plastic ceramic material for use in porcelain. body batches and the like which consists in fusing a mixture of alumina and silica in substantially the mullite ratio together with up to 10% magnesia, and casting it in large ingots on the order of seven weight, and allowing it to cool slowly to produce large mullite crystals, together with a substantial proportion of cordierite.

9. A porcelain body made by firing to vitrification a ceramic composition consisting of an intimate mixture of clay and non-plastic material comprising chiefly mullite, together with a substantial proportion. of cordierite and a glassy matrix, the cordierite and glassy matrix acting as a flux in the production of the glass phase in the fired body.

10. A porcelain body made by firing to vitrifica-J tion a ceramic composition consisting of an intimate mixture of clay and non-plastic material; comprising on the order of 80% mullite crystals together with a substantial proportion of cordierite anda glassy matrix, the cordierite and. glassy matrix acting as a fiux in the production of the glass phase in the fired body.

proportons in.

11. The processwhich -.consists in fusing a .mixture consisting of alumina and silica in sub-' stantially the""mullite ratio and magnesia flux:

in an amount up to but not exceeding 10% magnesium oxide, allowing the fused material to cool:

slowly until-the major portion takes the form of -mullite crystals and the remainder consists of cordierite in a glassy matrix and then arresting lite crystals and the remainder consists of ap-' proximately equal quantities of cordierite and a glassy matrix and then arresting further crystallization.

13. Fused non-plastic material for use in the manufacture of ceramic compositions composed principally of mullite together with a substantial proportion of cordierite. I I

14. Fused non-plastic materialfor use in the manufacture of ceramic compositions consisting principally of mullite, together with a substantial proportion of cordierite and a flux-containin: class.

15. Fused non-plastic material for use in the manufacture of ceramic compositions consisting principally of mullite together with a substantial proportion of cordierite and a glassy matrix containing a magnesium compound.

16. Non-plastic material for use in the manufacture of ceramic ware consisting of a mixture of on the order of 80 to 85% mullite in a glassy matrix of which substantially one-half consists of cordierite.

17. The method of producing a non-plastic ceramic material consisting chiefly of mullite and cordierite which consists in adding sumcient alumina to kyanite to bring the proportion of alumina to silica in the resultant mixture up to the mullite ratio, adding magnesium flux to the mixture, fusing the mixture and permitting it to cool slowly so as to produce mullite and cordierite in a glassy matrix.

18. The method of producing a non-plastic 'nesium flux in an ceramic material consisting chiefly of mullite and cordierite which consists in adding sufflcient alumina to kyanite to bring the proportion of alumina to silica in the resultant mixture up to the mullite ratio, adding to the mixture magamount up to but not exceeding 10% magnesium oxide, fusing the mixture and permitting it to cool slowly so as to produce mullite and cordierite in a glassy matrix.

19. The method of making a non-plastic ceramic composition consisting chiefly of mullite and cordierite for use in porcelain body batches and the like which consists in adding sufficient alumina to kyanite to bring the proportion of alumina and silica in the resultant mixture up to the mullite ratio, adding on the order of 2% magnesia to the mixture, fusing the mixture and permitting it to cool slowly so as to produce mullite and cordierite in a glassy matrix.

ALBRA H. FESSLER.