US3477058A - Magnesia insulated heating elements and methods of production - Google Patents

Description

Nov. 4, 1969 w. VEDDER ETAL MAGNESIA INSULATED HEATING ELEMENTS AND METHODS OF PRODUCTION Filed Feb. 1, 1968 Fig.

M60 1 2 Pyrophy/li/e Tempera/ure "f Inventor VV/llem l edaer; Jo/v .Sc/w fz, Jr. 7 S U Afiorn United States Patent U.S. Cl. 338238 8 Claims ABSTRACT OF THE DISCLOSUREj Compacted, granular, fused magnesia used as thermallyconducting electrical insulation in tubular, electrical resistance elements is substantially improved both in compaction density and electrical resistivity through the addition of 0.1 to 5.0 percent of any of a variety of substances of layer-structure crystal form such as pyrophyllites.

The present invention relates generally to tubular, electrical-resistance, heating elements and is more particularly concerned with novel sheathed elements having superior performance characteristics, With a method of making these novel elements, and with a new magnesia-base composition having special utility as thermally-conducting, electrically-insulating, packing material in these elements.

Heating elements of the type comprising an inner, electrically-resistive conductor, a surrounding layer of magnesia, electrical insulation and an outermost protective jacket are widely used in many industrial heating devices as well as in devices such as domestic ranges, dishwashers and water heaters. This type of heating element is much more durable than, for example, exposed resistance wire. Structurally, it usually includes: (1) a coiled resistance wire composed of alloys such as those made up of 20 percent chromium and 80 percent nickel; (2) compacted magnesia powder containing minor amounts of impurities surrounding the resistance coil as an insulator; and (3) an outer protective metal jacket.

Over the long period in which such elements have been in general use, they have been developed and improved to a state of good performance and service life, meeting high safety standards and competing with consistent success with gas and high-frequency current heating devices. At the same time, however, it has long been recognized that a substantial increase in the thermal conductivity of the magnesia insulation employed in these elements would be desirable, and that a sizable increase in the electrical resistivity of this material would be even more important. Each of these objectives, however, would have to be realized without incurring any substantial offsetting disadvantage of cost of production or operation or impairment of efliciency of these elements. To the best of our knowledge, no one heretofore has achieved either of these objectives.

In accordance with this invention based upon our discovery subsequently to be described, tubular heating elements having superior operating characteristics attributable to our achievement of the foregoing objectives can be produced consistently. Moreover, no substantial modification of the principal operations involved in commercial production is required in the manufacture of these elements.

This invention in its method, article and composition aspects is predicated upon our discovery that certain materials in particulate form, when added in amounts as small as 0.1 percent to granular, fused magnesia, increase electrical resistivity in accordance with the foregoing objectives and improve thermal conductance. We have also found that these additive substances, which preferably will be used in amounts of approximately 2.0 percent, but may. be used in amounts up to 5.0 percent, have in common the characteristic that they exist in layer-structure crystal form. Thus non-swelling layer silicates such as pyrophyllites, talcs and non-silicate layer-structure materials such as boron nitride are useful as additives in accordance with this invention, except that those having impurities, such as iron or alkali metals, in significant quantities (generally of the order of more than 5.0 percent in the aggregate) are unsuited for this purpose because of the appreciable electronic or ionic electrical conductivity which they would impart to the resulting magnesia mixture.

As disclosed and claimed in a copending application filed: of even date herewith in the name of Louis Balint and assigned to the assignee hereof the foregoing new results and advantages can be obtained through the use of layer-structure clay minerals and, alternatively, through the use of quartz in comparatively small amounts.

Compaction densities approximating those of the compositions of this invention can also be obtained without the use of any of the foregoing additives by surface hydration of the magnesia powder before compacting it in a heating element in the usual manner. This additional discovery or invention is disclosed and claimed in copending application Ser. No. 809,149, filed Mar. 21, 1969 in the name of Willem Vedder and assigned to the assignee hereof and entitled Tubular Heating Elements and Magnesia Insulation Therefor and Method of Production.

In some way, not totally understood, the additives of the present invention function to increase the electrical resistivity of magnesia powder used as packing in tubular heating elements, having apparently a physical-chemical effect at high temperature manifesting itself in the form of substantially increased electrical resistivity of the magnesia insulation. This increase is suprising in that the resistivity of the combined materials is substantially greater than that of either material alone. Additionally, these platey powder additives apparently act as lubricants in the compaction operation, thereby functioning to increase the compaction density and the thermal conductivity of the magnesia insulation.

This invention in its composition aspect accordingly in general comprises a uniform powder mixture of granular, fused magnesia and from 0.1 percent to 5.0 percent of an electrically non-conducting additive of layer-structure crystal form. This composition is further characterized in that at about percent of theoretical density, it has a specific impedance of at least 50 megohm-in. at 830 C.

More in detail, the composition of this invention, as indicated above, will preferably contain about 2.0 percent of a non-swelling layer-structure silicate additive such as a pyrophyllite or a talc. Alternatively, a layer structure non-silicate such as boron nitride may be the additive in part or whole. Further, the mixture may include a wide variety of particle sizes both of magnesia and the additive material, the magnesia preferably, however; being a mixture of particle sizes from 40-mesh to below 325-mesh (US. Standard screen sizes). The add tive'particulate material is suitably of a size or a mix ture of sizes within that range. In any case, the additive material preferably will not be of particle size larger than that of the largest magnesia particles of the mixture at the outset of the compaction operation. Also, as indicated above, a mixture of additives may be employed providing they meet the foregoing requirements and providing further that the aggregate amount of the additives is within the range stated above. We have discovered, in fact, that mixtures of pyrophyllite and boron nitride are especially effective additives for the purposes of this invention.

In its article aspect, this invention, generally described,

comprises a tubular heating element including a metal sheath, a coaxial coiled resistor in the sheath and a compacted, polycrystalline mass of a magnesia composition of this invention filling the space between the resistor and the sheath. Those skilled in the art will understand that this description of the article applies to the article at the intermediate stage of its production when the composition of this invention has been introduced into the sheath but prior to the time when the article has been thermally cycled to the extent that the identity of the additive may no longer be readily detected.

Finally, in its method aspect, this invention, described broadly, involves the use of the novel composition described above in the production of a tubular heating element including particularly the step of filling the metal sheath with that novel material. Thus, this method centers in a use concept which in itself has novelty independently of the uniqueness of the composition per se.

Referring to the drawings accompanying and forming a part of this specification:

FIGURE 1 is an enlarged, side-elevational view of the heating element of this invention, portions being broken away for purposes of illustration; and,

FIGURE 2 is a chart bearing curves comparing the specific impedance of typical magnesia insulation with magnesia insulation of this invention, impedance being plotted on a semi-logarithmic scale as a function of temperature.

The heating element of FIGURE 1 resembles the heretofore conventional tubular heaters in that it is made up of three principal parts. Thus, a coiled resistance wire 1 is disposed within an outer protective metal jacket 2 and is embedded in and spaced from the jacket by compacted magnesia powder 3 which serves both as a thermal conductor and electrical insulator. In contrast to the prior devices, however, the heating element of FIGURE 1 incorporates magnesia powder which has uniquely high electrical resistivity and may also have superior thermal conductivity because of the presence in it of a minor amount of a layer-structure susbtance such as pyrophyllite.

The FIGURE 1 element is suitably fabricated in accordance with the usual practice in the art, the parts being assembled and the element being conditioned at elevated temperature.

Thus, essentially the only significant departure from prior practice in terms of the fabrication operation consists in the use of the new magnesia compositions of this invention, these being substituted for magnesia used in accordance with the prior art practices in order to obtain the special new results and advantages stated above.

Three pyrophyllites and a tale preferred for use in this invention have analyses as follows:

PYROPHYLLITE A 4 PYROPHYLLITE C Percent SiO, 62.9 A1 0 23.8 CaO 3.0 MgO n 0.8 F6203 0.7 Ign. loss -a 5.1

Total 96.3

TALC D Percent Si0 51.0 A1 0 7.3 F6 0 1.4 MgO 32.5 CaO 0.2 Ign. loss 7.3

Total 99.7

Those skilled in the art will understand that tales and pyrophyllites in their natural forms are hydro-silicates which can be dehydrated upon heating. When used in natural form in preparing the mixtures of this invention, they are dehydrated during the normal annealing or heat-treating operation after fabrication of the heating element or possibly during initial operation of the finished unit if such a preliminary heating operation is not involved. Alternatively, the additives can be dehydrated by heating prior to loading the insulation mixture into the heating unit or even prior to the time that these materials are mixed with magnesia. The eifects obtained as described above and the special advantages of this invention are realized independently of how and when this dehydration step is carried out.

Those skilled in the art will also recognize that although the plate-like powder additives of this invention can act as compaction aids during forming operations in the course of fabricating heating elements and thus result in improved density of the insulation, the principal benefits described above can be achieved in certain instances without effecting a substantial increase in compaction density of the material.

The following illustrative, but not limiting, examples are offered in the interest of insuring a full and clear understanding of this invention by those skilled in the art and enabling their practice of it without the necessity for experiment to obtain the new results and advantages stated above:

EXAMPLE I' To 100 grams of magnesia of minus 40-mesh particle size are added two grams of pyrophyllite A of minus 200- mesh particle size. A portion of the resulting powder mixture is introduced into a nickel-chromealloy sheath containing a nickel-chrome electrical resistance element, and the powder is compacted therein to a density of 3.05 grams per centimeter, i.e. about percent theoretical density. The resulting element is then annealed at about 1,970 F. for from 10 to 15 minutes, at which time it is ready for test. Results of insulation impedance and thermal conductance tests on this element and on an element which differs only in that the magnesia powder contains no additive are set out as the first and third items in Table I below.

EXAMPLE II Another portion of the mixture prepared in accordance with the description'in Example I is mixed with an additional amount of minus ZOO-mesh pyrophyllite 'A to bring the pyrophyllite content to approximately four percent. On test, a heating element made with this mixture as described in Example I yields insulation impedance and thermal conductance values set out as the fourth item in Table I.

5 EXAMPLE 111 To another 100-gram portion of minus 40-mesh magnesia is added 0.10 gram of 325-mesh boron nitride. A heating element test specimen prepared as described 6 EXAMPLE IX Pyrophyllite B mixed together with magnesia and used to provide a heating unit as described above yields the test results stated in the ninth entry in Table I. Again above through the use of the resulting mixture yields test 5 the materials are of the powder sizes stated in Example results as set forth in the sixth entry in Table I.

I for both the magnesia and the additive.

TABLE I Insulation Thermal Impedance Conductance (1,700 F.), (1,625 F. megohms mean), B.t.u. in.

MgO, no additive 0. 45 11.0 MgO plus 0.5% pyrophyllite A... 1. 85 11. 2 MgO plus 2% pyrophyllite A- 1. 60 1. 20 MgO plus 4% pyrophyllite A... 0. 65 14. MgO plus 2% talc 0. 60 13. 5 MgO plus 0.1% boron nitride 0. 65 12. 0 MgO plus 3% boron nitride. 0. B7 20. 0 MgO plus 2% Dyroplryllite 0-- 0. 65 12. 9 MgO plus 4% pyrophylhte B 0. 61 12. 7 MgO plus 2% pyrophyllite B plus 0.1 boron nitride 1. 10 12. 7

EXAMPLE IV Boron nitride of minus 325-mesh is added to magnesia to produce a uniform powder mixture containing three percent boron nitride as described in Example I. A test heating element prepared as described in Example I using this mixture is tested with results stated in the seventh entry in Table I.

EXAMPLE V In still another operation, magnesia and pyrophyllite B and boron nitride powders are mixed together as stated in the foregoing examples to provide a composition containing 97.9 percent MgO particle size of minus -mesh pyrophyllite B (minus ZOO-mesh) 2.0 percent and 0.1 percent boron nitride (minus 325-mesh).

Again, on test of a heating unit prepared as described in Example I, it is found that the thermal conductivity of this mixture is superior to that of the standard magnesia and that the insulation resistance and the current leakage resistance of this mixture are far superior to those properties of the standard magnesia. These test results appear as the final entry in Table 1.

EXAMPLE VI A magnesia (minus 40-mesh)-0.5 percent pyrophyllite A powder mixture (minus ZOO-mesh) prepared as described in Example I is tested in a heating element test specimen produced as also described in Example I. As indicated by the second entry in Table I, the insulation impedance of this mixture is substantially better than that of the magnesia powder alone and thermal conductance is slightly improved.

EXAMPLE VII Talc D of minus 325-mesh particle size is mixed with magnesia of minus 40-mesh particle size to provide a heating unit magnesia mixture containing about two percent talc. Upon test in a heating unit made as described above, this mixture is found to have insulation impedance greater than standard magnesia alone and a thermal conductance comparing favorably with the compositions of Examples I-IV as shown by the fifth entry of Table I.

EXAMPLE VIII A magnesia mixture prepared by mixing together 100 grams of magnesia of -40-mesh particle size and two grams of 200-mesh pyrophyllite C is used to produce a heating unit as described above. Upon tests this product shows substantial improvement in both insulation impedance and thermal conductance as reported in the eighth entry in Table I.

As illustrated in FIGURE 2, specific impedance of a magnesia insulation containing 2. 0 percent of pyrophyllite A" compared very favorably with the same magnesia containing no additive over the temperature range from about 1600 to about 1800 F. Thus, at each specific temperature over that range, the specific impedance of the pyrophyllite magnesia mixture additive approached an order of magnitude greater than that of the magnesia containing no such additive and consisting essentially of magnesia powder.

Wherever in this specification and in the appended claims reference is made to percentages or proportions, reference is had to the weight basis rather than the volume basis unless otherwise specifically stated.

By the term non-swelling as used herein and in the appended claims is meant the property of layer silicates like micas of maintaining the distance between layers of the layer structure in the presence of pure water.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be Within the purview and scope of the invention and the appended claims.

What We claim as new and desire to secure by Letters Patent of the United States is:

1. In a tubular heating element including a metal sheath and a coaxial coiled resistor enclosed in the sheath, the combination of a compacted polycrystalline electrical insulating mass filling the space in the sheath between the resistor and the sheath and comprising fused magnesia and from 0.1 percent to 5.0 percent of an electrically nonconducting additive of layer-structure crystal form, said polycrystalline mass having a density of at least percent of theoretical density of pure magnesia and a specific impedance of at least 50 megohm-in. at 830 C.

2. In the method of making a tubular heating element including the step of positioning a coiled resistor coaxially within a metal sheath, the combination of the step of filling the metal sheath and embedding the coiled resistor with a polycrystalline, electrically-insulating mixture of magnesia and from 0.1 percent to 5.0 percent of an electrically non conducting additive of layer-structure crystal form, said mixture at 85 percent of theoretical density having a specific impedance of at least 50 megohm-in. at 830 C.

3. The heating element of claim 1 in which the additive is present in the amount of about two percent.

4. The method of claim 2 in which the amount of the additive is about two percent.

7 5. The heating element of claim 1 in which the additive is a non-swelling layer silicate which is present in the amount of about two percent.

6. The method of claim 2 in which the additive is a pyrophyllite.

7. The method of claim 2 in which the additive is a talc. 5

8. The heating element of claim 1 in which the additive consists of 2.0 percent of a pyrophyllite and 0.1 percent of boron nitride.

8 References Cited UNITED STATES PATENTS 2,280,517 4/1942 Ridgway 338-238 X 3,201,738 8/1965 Mitofi 338238 E. A. GOLDBERG, Primary Examiner US. Cl. X.R.

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