US2280517A - Electrical insulation of modified - Google Patents

Description

P" 1942- R. R. RIDGWAY HAL 2 5 ELECTRICAL INSULATION OF MODIFIED MAGNESIA AND A METHOD OF MAKING THE SAME Filed 001:. 27, 1939 Envcntor and Raymond R. Ridq urcLg Bu Archibald H.B0L1l (1rd wilne'ss; 7

(Ittorncu Hen-bar}. 51love Patented Apr. 21, 1942 smc'rmcar. INSULATION or MODIFIED mcrmsm AND METHOD or MAKING ms SAME Raymond R. Ridgway, Niagara Falls, N. Y., and Archibald H. Ballard, Niagara Falls, Ontario, Canada, assignors to Norton Company, Worcester, Mass., a corporation of Massachusetts Application October 27, 1939, Serial No. 301,644

14 Claims Thisinvention relates to an electrical insulat crystalline magnesia is ordinarily derived from magneslte ore, and this natural source of material ordinarily contains calcium, silicon and iron oxides or other compounds on the crystallized magnesia.

In accordance with the disclosure set forth in our copending application Serial No. 301,642 filed on even date herewith, we have determined that the iron impurity content has a marked effect upon the resistance of the material, and

that it is desirable to have the iron present as ferric oxide and in amount less than 0.1% by weight, if along life of useful service is to be obtained. We have also found, as set forth in our copending application Serial No. 301,643 filed on even date herewith, that if crystalline magnesia containing iron oxide as an impurity is, heated under oxidizing conditions for a considerable time to insure that the iron oxide is absorbed into the magnesia and then quenched rapidly to prevent the crystallization of the iron oxide as a separate external phase, the magnesia insulation will show an increased resistance and a longer life when used in an electrical heating unit.

We have further found that a standard commercial electrical grade of periclase or crystallized magnesia shows a gradual and continued deterioration when used in a heating unit under prolonged heating and high temperature conditins,'whereby the resistance of the insulation in time falls to a point where a breakdown or short circuit occurs and the heating unit reaches the end of its life. It is highly desirable that such an insulating material possess not only a high initial resistance at high temperatures but also maintain or attain such a high and substantially constant resistivity, after it has been used in a heating unit under standard high temperature conditions for a long period of time, that the possibility of breakdown of insulation due to deterioration thereof is minimized. It is also desirable that the insulating material be an excellent conductor of heat while maintaining its electrical resistance at elevated temperatures.

The primary objects of this invention are to satisfy such requirements and to provide an electrical insulating and heat conducting material containing magnesia which will have high initial electrical insulating characteristics without impairing its high heat conductivity at a required high temperature of usage, and which will possess such stability that its electrical resistance will not decrease materially while its heat conductivity is. maintained, and a heating unit embodying the same .will give a long life of useful service.

A further object is to provide a method of beneflciating a magnesia insulating material and of making a modified product suitable for use as an electrical insulation at a high temperature throughout a long period of time. Other objects will be apparent in the following disclosure.

We have found that the standard commercial electrical grade of crystalline magnesia is materially beneficiated by the addition thereto of various substantially pure refractory metal oxides and particularly the oxides of aluminum and chromium or such metal oxides as will form refractory spinels with magnesia and otherwise serve the purposes .described herein. Of these metal oxides, we preferably utilize alumina, and particularly because the addition of a small .amount, such as 10%, to the magnesia results in a substantial increase in the initial specific electrical resistance thereof and provides a product which attains a substantially constant resistivity during use and thus insures'a long life of useful service at a high temperature. This composition has other desirable properties, such as being less critical as regards both the initial and the long time resistance behavior of heating units made from this product. Also, the permissible amount of iron oxide which can be allowed in the final product is greatly increased by this addition of alumina.

As as exampleillustrating this aspect of our invention, we may make a high temperature electrical insulating material, from. a commercialmagnesite ore which contains at'ieast maglize the latter.

nesia, from 1 to 3% of silica and from 1 to 2% of lime and ordinarily not over 1.0% of ferric oxide. We preferably employ purified magnesia and particularly one which has less than 0.2% of iron oxide, so that the final product will have only a small content of impurities and there will be little danger of the iron oxide giving any detrimental results. I

This ore may be treated in accordance with standard practice to convert the magnesium carbonate into the oxide and to melt and to crystal- The magnesium oxide may be melted in a standard Higgins type electric arc furnace which is operated in accordance with standard procedure. It is preferable, however, that a high power input be employed so as to insure a temperature above 2600 C. at which metallic iron will boil off to some extent and thus be eliminated, as well as otherwise to produce asatisfactory and stable product. Either before, during 01' after this melting operation, we propose to add one of the refractory metal oxides, and particularly alumina, within such percentage limits as give the physical characteristics of the final product that may be desired. It is ordinarily preferable to have the magnesia constitute over 50% of the material. For example, vwe may add from 5 to 30% and preferably less than 15%, such as by weight of alumina, and we thus obtain an insulating material which is highly satisfactory for a high temperature stove unit. It is desirable that the alumina be substantially chemically pure, in that it should not contain material amounts of iron oxide, silica and lime, and it should be substantially free from other alkaline earth metal compounds and the alkali metal compounds. This crystalline alumina may be obtained by various methods, such as that set forth in the reissue patent to Ridgeway No. 20,547 dated November 2, 1937.

After this mixture of granular magnesia and alumina has been fused, the mass is allowed to cool slowly and crystallize. Then it is crushed to suitable grit sizes, such as will pass through a screen of 40 meshes and be retained on a screen of 325 meshes per linear inch. Care is to be taken to avoid adding iron during these various steps, and it is desirable to pass the crushed material through a high intensity magnetic separator toremove any available metallic iron and magnetic oxide of iron.

The product is made up of crystalline magnesia or periclase containing crystalline alumina in solid solution or in combination with the magnesia or otherwise absorbed therein. This product shows no free crystalline alumina phase. Examination under the microscope shows that the alumina has combined with the magnesia to form a spinel which is isomorphous with the magnesia crystal and that the spinel isin solid solution in the MgO or interspersed or intergrown with the magnesia crystals as a heterogenous multiphase mass.

The addition of alumina or other refractory metal oxide makes it possible to increase greatly the permissible amount of iron which may be allowed in the final product and yet obtain a satisfactory insulating material. Likewise the influence of other impurities, such as lime and silica, appears to be minimized. It is our belief, although we do not wish to be limit-ed to any particular theory in the matter, that the periclase modified by the alumina, or the other metal oxides, has a superior ability to absorb iron oxide and so prevents the iron oxide from appearing in the final cooled crystalline product as a separate and distinct phase of iron oxide crystals which are external of the magnesia and spinel crystals. In other words, the iron oxide is found absorbed or diffused in or combined with the alumina spinel or the magnesia and so remains within the crystal structure and does not appear as a separate external phase where it may become highly detrimental to the life of the unit. When the standard, old type magnesia is used in an electric heating unit, such as one comprising a metal Wire embedded in and surrounded by the refractory insulating material and enclosed within a metal sheath, it has been found that, due possibly to an electrolytic decomposition of absorbed water, any iron oxide crystals that are present in an external phase tend to be reduced to a conductive metal form which serves to break down the insulation and short circuit the element. If the iron oxide is absorbed within the crystal structure, then it is not in such a position and condition that the reducing action may take place. Therefore, it is possible, in accordance with this invention, to utilize an ore which has a comparatively high content of iron and which may be as high as from 0.5 to 1% by weight and yet provide a satisfactory insulating material, because" of this ability of the alumina and magnesia to absorb iron oxide and hold it in a condition where it cannot be readily reduced to the metallic form.

A further beneficial result involved in the use of the crystalline alumina or other modifying refractory metal oxide lies in the fact that any metallic vapors or oxides coming from the resistor wire or the sheath will be soaked up into the spinel crystal rather than be permitted to form a conductive layer in the interstices of the crystal structure through which a fault may develop. It is noteworthy that the usual metals ordinarily employed in the production of both the sheath and the wire, such as iron, nickel, chromium, aluminum and the like, are all capable of forming spinels which are isomorphous with the spinel in the insulating powder. Hence, the vapors arising from such metals, if oxidized in an open heating unit, will be absorbed into the refractory powder in much the same way as the original iron oxide impurity is absorbed. This insures a very desirable life characteristic for the heating unit and the maintenance of a good insulating quality over a longer period of time than is the case with insulating powders made from periclase which do not contain significant amounts of alumina or other refractory metal oxide dissolved therein.

We may also make use of the procedure set forth in our application Serial No. 301,643 in ac- .cordance with which the crystallized refractory material is crushed to a powder and then heated, preferably in an oxidizing atmosphere, to a temperature of 1200 to 1400 C., and preferably above 1250 C., and maintained at that temperature for tion in a heating unit. By using this roasting and quenching step, wemake sure that the iron impurity is present as the higher or ferric oxide and that it remain absorbed within the crystal.

This heating step causes a partial recrystalliza- I operation-may be materially decreased over that required if the alumina or other refractory metal oxide is not added. That is, we may roast the magnesia alumina material for only 2 hours or so and then quench it rapidly. We presume that in this case the iron oxide is already present within the alumina magnesia crystal structure because of the presenceof alumina and that the quenching operation insures itsbeing stabilized in that position and condition. That is, the addition of the alumina has aided the distribution and absorption of the objectionable residual impurities throughout thepericlase crystal.

It is a further feature of this invention that the alumina or other refractory metal oxide used need not be introduced into-the furnace containing the molten magnesia. It is feasible to melt the magnesiaby itself and crystallize the same and then crush itto a suitable powdered condition, such as above defined. This crushed magnesia containing its various impurities may be then mixed with crystalline alumina in a finely divided form, and the alumina magnesia spinel will be formed by heating the mixture for a long period of time at a high temperature. For example, a mixture of of crystalline alumina, which is free from soda, with 90% by weight of the magnesia may be calcined for: a period of about 48 hours at a temperature approximating Orton cone 16; This heating step may also be utilized'in connection with the above mentioned quenching step whereby the heated material is suddenly cooled to a temperature at which the iron oxide is rendered harmless and is not permitted to crystallize out as a distinct and separate phase. thus formed existsjpartly in solid solution and partly as separate crystals in intimate association or inter-mixture with theunchanged magnesia crystals.

It is also possible to obtain some beneficial effects, if aluminum oxideis distributed over the surfaces of the crystalline magnesia or periclase crystal as a coating, so that at the high temperature of use of the insulation in an electric stove unit this coating forms an absorber or carrier to take up such iron oxide as may be available therefor. When the heating unit is used The alumina magnesia spinel as' voltage and amperage required for standard usage thereof may be applied, such as when the heating unit is being used for its intended purposes. The crystalline magnesia alone has an initially high resistance. As the heating operation goes on, the tendency for the magnesia to deteriorate will be overcome or minimized by the alumina entering into the crystal structure. It, of course, will be understood that the magnesia with its alumina coating may be heated to form the spinel either before or after the insulating powder has been assembled with a resistance wire parison of the resistances of two electrical heatin its intended and final location. That is, the coated magnesia may be heated in a kiln or other type of furnace to accomplish this purpose. It

is preferred that, if the material is heated independently of the resistance wire, it be quenched or cooled rapidly so as to minimize the formation of a separate phase of ferric oxide crystals.

The following table gives data showing a coming stove units when subjected to a destructive life testing operation, in which the first unit was made up of magnesia of the standard commercial electrical grade of periclase and the second unit was an electrical grade of periclase containing 10% of alumina, as made according to thepresent process. The table gives the insulation resistance in ohms between the resistance wire and the sheath or ground of a 1300 watt stove unit heated to .850 C. or 1562 F.

According to the data, the standard but unimproved periclase, or crystalline magnesia, when subjectedto a destructive life test, shows a gradual deterioration of the unit because of a' steady lowering of the resistivity of the ins'ula-- tion. The resistance falls gradually to a point at which breakdown may occur and the life of the unit is ended. The second column shows, on

the other hand, that our MgO and A120: product very soon assumes an almost constant resistance which is well above the point at which a breakdown can occur under normal usage. Hence, our process has greatly increased the dependability and length of life of the electrical heating appliance.

An electrical heating unit of enhanced quality maybe made by combining the insulating maunder standard conditions, suflicient heat may which may have a metal sheath surrounding the insulation. The crystalline alumina, if used as a separate phase, should be a powder of very fine grit size to avoid abrading the electrical resistance wire when the insulating material is packed around the wire in the metal sheath.

After the unit has been assembled in its stove or other desired location, an electric current of terial of this invention with a conventional resistance wire and a protecting sheath of heat resistant metal. Such assemblies are described in the U. S. patent to Sharp No. 2,034,539.

The accompanying drawing is a view, partly broken away, of a simple form of heating unit embodying this invention. The structure comprises a helically coiled metal resistance wire I0 located centrally within a metal sheath or casing H and supportedv by the special insulating material I2 as above described. An electric terminal I3 may be suitably secured to the resistance wire at each end thereof and similarly insulated from the casing by the material I2. Any suitable structure may, however, be emplcyed.

In practice it has been found objectionable in certain cases to use protecting sheaths made of iron alloys containing nickel and chromium such v as stainless steels, and the like, because the detrimental presence of the iron in the alloys resulted in deterioration of the insulating material.

Because of the peculiar property of the in sulating material described in this invention for absorbing detrimental iron oxide, it is now possible to use as a protecting sheath a wide range of oxidation resistant alloys containing iron. Moreover, we may reduce the thickness of the refractory electrical insulation between the inner heating wire and the outer protecting sheath because of the desirable properties incorporated in the insulation material by the procedure of this invention.

This layer of modified magnesia may be notover inch thick and ordinarily about 3 3' inch thick or less and yet be capable of insulating a resistance wire for use in a stove unit when the sheath temperature is 1560 F. for a long period of time, such as 1000 hours. The material is preferably crushed to a fine powder and then densely packed in the sheathing around the resistance wire. This may be done on a jolting machine which serves to orient the grains into a very dense packing, so that the material has substantially the characteristics of large magnesia crystals. That is, the material is almost transparent, and radiant energy passes readily through it. Consequently, the magnesia has a high heat conductivity in that it serves for both conduction and radiation of heat, while it is, on the other hand, an excellent electrical insulator. Hence, the temperature gradient between the wire and the sheath is small.

For example, an electric stove unit made with the magnesia-alumina insulation as above described, was designed to permit an external sheath temperature as high as 860 C. when operated at high power density. A measurement was made of the internal resistor heater wire temperature under these conditions of extreme service. The internal wire temperature was found to be 1050 C. or only 190 higher than the external sheath temperature. Since the temperature of the internal wire is below the limiting value for operation of the ordinary resistor wires with long life, it is possible to have a high external temperature unit of concentrated form because of the use of the insulating oxide of this invention. The high sheath temperature of the unit and its concentrated form makes for rapid transfer of heat to cooking vessels and efiicient use of the heat. Heretofore-this desirable condition of high external sheath temperature could not be achieved because of the resultant temperatures on the internal resistance wire in excess of permissible operating limits, making for a short life of the unit. It is a feature of the novel product of this invention that it retains the high specific insulation value at these high temperature and is not subject to deterioration at a rapid rate under the conditions specified. This permits the construction of a reliable unit having long life and high efiiciency in operation.

We may also coat the crushed magnesia crystals with a spinel of alumina and magnesia. That is, we may fuse molecular proportions of 72 parts of alumina and 28 parts of magnesiafand thus make a spinel of the formula MgO.Al2 O3;

or any other spinel composition may be made..

This fusion is cooled and crystallized and then crushed to a fine powder for coating the previously crushed magnesia crystals. The spinel is used in such amount as is needed to give the de- 7 spinel crystals.

sired alumina content, such as the 10% above specified.

It may also be noted that crystalline alumina used alone does not have the high resistance to alternating current possessed by magnesia. Also, its electrical conductivity is materially raised and even to a detrimental point when the material is heated to the higher temperatures required for stove units. That is one reason why the insulation herein described should possess the characteristics of magnesia and not those of crystal- .line alumina; and the latter is used only in sufficient amount to give the required characteristics to the magnesia but without increasing its electrical conductivity to any material extent within the required temperature range. The electrical resistance to alternating currents of magnesia is substantially 40 times that of crystal-,

line alumina at 850 C.

While we have attempted to explain the phenomena attending this invention in the light of our present knowledge, it is to be understood that the claims are not .to be considered as limited to any particular theories and that the terms thereof are to be interpreted broadly as covering the general process above set forth and the product produced thereby, as well as such equivalent steps and compositions as will now be apparent to one skilled in the art in the light of the above disclosure. Also, since many variations may be made in the methods of procedure and in the composition of the final product, the above disclosure is to be considered as illustrating the general principles and certain preferred features of the invention and not as limitations thereon, except as the invention is defined by the claims appended hereto.

We claim:

1. A refractory, heat conductive, electrical resistance heating element embedding and elec-- trically insulating material consisting of a granular crystalline mass comprising magnesia constituting the major portion intimately associated with a lesser amount of a spinel comprising magnesia and another spinel forming non-ferrous refractory metal oxide which is capable of absorbing iron oxide.

2. A refractory, heat conductive, electrical resistance heating element embedding and electrically insulating material composed of a granular crystalline mass comprising magnesia and a spinel composed of magnesia and another spinel forming non-ferrous refractory metal oxide which is capable of absorbing iron oxide, together with an iron compound which constitutes not over 1.0% of the mass, calculated as ferric oxide, any iron compound present being substantially wholly absorbed within the magnesia and 3. A refractory, heat conductive, electrical resistance heating element embedding and electrically insulating material composed of a granular, heterogenous, multiphase, crystalline mass comprising magnesia crystals forming the major portion and a lesser amount of crystalline particles of a spinel comprising magnesia and alumina intimately associated with not over 1.0% of iron compounds, calculated as ferric oxide, any iron compound present being substantially wholly absorbed within the crystals of magnesia and the spinel.

4. A refractory, heat conductive, electrical resistance heating element embedding and electrically insulating material composed of a granular crystalline mass comprising magnesia forming amounts of not over 3% of silica, not over 2% the major portion intimately associated with a lesser amount of a spinel of magnesia and another spinel forming non-ferrous refractory metal oxide, and not over 02% of iron com-' pounds, calculated as ferric oxide, any iron compound present being substantially wholly absorbed within said crystalline mass.

5. A refractory, heat conductive, electrical re sistance heating element embedding and electrically insulating material composed of a granular crystalline mass comprising magnesia constituting the major portion of the mass which is intimately associated with a lesser amount of a spinel of magnesia and chromium oxide, together with not over 1.0% of iron compounds, calculated as ferric oxide, any iron compound present being absorbed substantially wholly within the magnesia and spinel crystals.

6. A refractory, heat conductive, electrical resistance heating element embedding and electrically insulating material composed of a granular, heterogenous, multiphase crystalline mass comprising a partially recrystallized magnesia which constitutes the major portion of the mass and a lesser amount of a spinel of magnesia and alumina intimately associated therewith, together with iron compounds constituting not over 1.0% of the mass, calculated as ferric oxide, the iron content being substantially wholly absorbed within said magnesia and spinel crystals.

'7. A refractory, heat conductive, electrical resistance heating element embedding and electrically insulating material composed of a granular crystalline mass comprising magnesia and a spinel composed primarily of magnesia and alumina, said material containing a substantial amount of not over 1.0% by weight of iron compounds, calculated as ferric oxide, in which substantially all of the iron content is absorbed within the magnesia and spinel crystals, the alumina content of the material being from to 30% of the total.

8. A refractory, heat conductive, electrical resistance heating element embedding and electrically insulating material composed of a granular crystalline mass comprising magnesia forming the major portion of the material and a lesser amount of aspinel ef magnesia and alumina, said material containing a substantial amount of not over 3% of silica, not over 2% of calcium oxide and not over 1% of iron compounds, calcu-. lated as ferric oxide, any of the latter present being substantially wholly absorbed within said ,magnesia and spinel crystals.

,terial and a spinel of magnesia and alumina, the

alumina content being from 5 to 30% of the total mass, and said mass containing substantial of calcium oxide and not over 1% of iron compounds c'alculated asferric oxide, any of the iron content present being substantially wholly absorbed within the magnesia and spinel crystals.

10. The method of making an electrical heating element insulating material of magnesia contaminated with an iron compound. comprising the'steps of incorporating therewith a spinel of magnesia and a refractory spinel forming nonferrous metal oxide and causing the iron compound to be absorbed therein at a temperature above 1200 C. and providing a granular crystalline mass of magnesia and said spinel.

11.. The method of making an electrical heating element insulating material of magnesia contaminated with not over 1% of iron oxide comprising the steps of incorporating therewith-a lesser amount of alum na and heating the material to a temperature of at least 1200 C. and forming a crystalline mass, comprising a magnesia alumina spinel, which contains substantially the total iron content as an iron compound absorbed withinthe crystals.

. 12. The method of making an electrical heating element insulating material of. crystalline magnesia which contains iron oxide as an impurity comprising the steps of mixing the crystalline magnesia in granular condition with from 5 to 30% by weight of granular alumina, heating the mass under oxidizing conditions to a temperature above 1200 C. but below the melting points thereof and causing the iron oxide to be absorbed therein, and then cooling the material, and thus providing an intermixture of crystals ofmagnesia and a spinel of magnesia and alumina which contains the iron content largely as an internal phase of an iron compound absorbed therein.

13. The method of making an electrical heating element insulating material comprising the steps of melting magnesia and cooling it to form a crystalline mass, adding a lesser amount of alumina to the magnesia at any stage and forming an alumina magnesia spinel, and subsequently heating the composite material to a temperature above 1200 C. and then cooling the same.

14. The method of making an electrical heating element insulating material of magnesia contaminated with not over 1% of iron compounds, calculated as ferric oxide, comprising the steps of melting and crystallizing the impure magnesia, mixing from 5 to 30% of alumina with the crystalline magnesia, heating said mixture as a granular crystaline mass at a temperature above 1200 C. for at least 2 hours and subsequently cooling the same, and thereby providing a material in which substantally all of the iron content is present as an iron compound absorbed within the magnesia and spinel crystals.

RAYMOND R. RIDGWAY. ARCHIBALD H. BALLARD.

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