US3269806A

US3269806A - Sintered resistance body, preferably for use as heating element

Info

Publication number: US3269806A
Application number: US235640A
Authority: US
Inventors: Fitzer Erich; Rubisch Ottmar
Original assignee: Siemens Plania Werke AG
Current assignee: Siemens Plania Werke AG
Priority date: 1961-11-09
Filing date: 1962-11-06
Publication date: 1966-08-30
Anticipated expiration: 1983-08-30
Also published as: CH479221A; SE305404B; DE1243078B; GB993499A

Description

Aug. 30, 1966 E. FITZER ETAL SINTERED RESISTANCE BODY, PREFERABLY FOR USE AS HEATING ELEMENT Filed Nov. 6, 1962 Fig. 1

United States. Patent 29 Claims. (Cl. 29-491) Our invention relates to sintered electric resistance bodies, such as resistance heating elements, of molybdenum silicade and is described herein with reference to the accompanying drawings in which FIGS. 1, 2 and 3 show three different resistance heating elements whose sintered silicide body comprises a high-temperature portion 1 for working temperatures above 1300 C. joined with two end portions 3 :for lower temperatures which carry respective terminal caps 4 of contact metal.

The sintered silicide material of such resistance bodies may also contain some aluminum in an amount of 0.1 to 5.0% by weight so as to form a molybdenum aluminosilicide, and the silicide material may also contain minor additions of high-melting oxides, for example one or more of zirconium oxide, aluminum oxide, beryllium oxide, silicon oxide. The term moylbdenum silicide as used herein is understood to include such and similar variants of the sintered material.

It is known that sintered bodies of molybdenum silicide are resistant to oxidation, burn-off or scaling at high temperatures up to 1700 C. It is also known that they are destroyed at low temperatures of less than 700 C. due to occurrence of intercrystal-line oxidation. When employing a molybdenum silicide heating element, such as in (form of elongated rods as exemplified by the drawing, the high-temperature portion is exposed, as a rule, to operating temperatures between 1300 and 1700 C., whereas the cold ends of the other portions are subjected only to temperatures between 200 and 700 C.

Due to the reactivity of the silicide materials relative to all known metallic and electrically good conducting substances, it is not feasible to supply the electric current through a silicide free oxidation-resistant material up to within the high-temperature portion of the molybdenum silicide element. This makes it necessary to have the silicide material extend out of the high-temperature zone proper into zones of lower temperatures, for exam- .ple normal room temperature, or at least into a zone in which temperatures of a few 100 C. obtain. Only atthese relatively low temperatures can the silicide material be contactedvwith the conventional metallic conductors, for example with aluminum of which the terminal caps 4 according to the drawing may consist, without the danger of causing a reaction resulting in silicide formation.

Molybdenum silicide has the 'i iiroperjty of protecting itself from further oxidation or sealingin an oxidizing atmosphere at temperatures above 1300} C. by forming on its surface a coating of silicon dioxide glass which constitutes a firmly anchored surfia-ce glaze. This protective sheath of glaze is self-healing. That is, it renews itself in use above 1300 C. in the event it has become damaged or destroyed, for example by mechanical attack, alternating temperature stresses or chemical reaction.

3,269,806 Patented August 30, 1966 However, such a selfhealing protective sheath does not form itself on the portion of the heating element that during its operation remains at temperatures below 1300 C. It has become known, therefore, to subject such element portions, prior to using the element, to an annealing treatment at high temperatures in an oxidizing atmosphere in order to form the above-mentioned protective glaze coating on these portions also. However, there remains the difliculty that such coating is not self-healing so that if the coating in the end portions of the silicide body is mechanically damaged, these end portions may rapidly become subjected to further damage and destruction due to the low operating temperature. It has further been found that during operation of a silicide element for a few hundred hours, there occurs in the lowttemperature portion a crystallization of the roentgen-amorphous glaze sheath. That is, the glazed end portions become gradually de-glazed. Such de-glazing is accelerated by contact of the siO -glaze with insulating parts of ceramic and by reaction of the silicon oxide with the :glaze coating or with basic oxides. Such dc-glazing is accompanied by iormat-ion of fissures, cracks and new points of attack for oxygen upon the underlying body of molybdenum silicide. Particularly endangered are the contacting locations of the silicide material with the metallic contact material as exemplified by the contact locations between the end portions 3 and the terminal caps 4 of the heating elements illustrated on the drawing.

For securing a good electric contact, the previously produced oxidic coating must be removed at the contacting locality. Very often, however, such removal results in a fissures and cracks that permit ingress of oxygen. This takes place particularly at the merger location between the silicide-free contact material and the glazecoated end portion of the heater element because the metallic contact material is not bonded with the ceramic coating. The contacting locations, therefore, are only mechanically enveloped and thus are not protected from penetration of oxygen. The oxygen of the ambient air can thus enter between the contact material and the silicide end portions from which the coating is removed, and this oxygen results in a progressing destruction of the contacted ends ultimately leading to the interruption of the current passage between contact material and silicide body.

It has been proposed to prevent such low-temperature oxidation by depositing a protective layer of silicon. This has the disadvantage of a high electrical resistance in comparison with the metallically conducting silicides, The impregnating treatment required for producing such a silicon coating is also difficult to perform in practice and in many cases has increased the rather limited sensitivity to mechanical shock of the molybdenum silicide sinter bodies.

If an attempt is made to employ molybdenumafree, silicon-containing metallic materials for the abovernentioned end portions of the resistance bodies, then a sintered or welded junction between these materials and the silicide of the high-temperature portion becomes located in a working-temperature range above 700 C. However, at 1000 to 1100 C. such metallic materials fusion-bonded with the molybdenum silicide tend to enter into conversion reactions which may be accompanied by considerable changes in structure that cause mechanical 3 tensions in the body and thus render it more susceptible to fissure fonmation. The use of molybdenum silicidetfree materials for the end portions of resistance heating elements is therefore limited to special types of furnaces Where the just mentioned temperature conditions are reliably excluded.

It is an object of our invention to devise a molydenum silicide sintered resistance body, preferably for use as a heating element, that eliminates all of the above-mentioned deficiencies and difliculties of those heretofore known and is reliably applicable for operating temperatures above 1300 C. of its high-temperature portion without the danger of causing damage to the end portions of the body regardless of the temperature conditions to which they may be subjected in a particular use.

According to our invention, those portions of the molybdenum silicide resistance body (sintered) whose normal operating temperature does not exceed 1300 C., preferably those portions that are subjected to operating temperatures below 1000 C., are given an additional oxide constituent in an amount of 0.1 to 20%, preferably 3 to 12%, this oxide addition having a melting point below l400 C. and being solidified from the molten condition so as to coat the silicide particles in the pores and at the surface of the sintered molybdenum silicide body.

The percentages just stated, as Well as all others mentioned in this specification, are by weight.

By virtue of the invention, the molybdenum silicide sinter body has its end portion and its contacted terminal ends, which become subjected to low operating temperatures, coated on the inner and outer surfaces by the oxide addition with the result that a low-temperature oxidation is prevented without requiring the provision of a glaze coating produced at high temperature. The above-mentioned formation of an SiO coating by oxidizing annealing of the particular molybdenum silicide portion for protection from low-temperature oxidation thus is no longer necessary, although such a coating, if desired, may also be applied in conjunction with the present invention. For example, such additional SiO coating is of advantage in cases where it is necessary to envelop the low-temperature molybdenum silicide portion of the body in ceramic material. In such cases, the formation of an SiO coating has been found favorable for excluding any reaction of furnace masonry with the material of the heating element. However, if the SiO coating becomes de-glazed or fissured, the permanence of the underlying body according to the present invention remains preserved.

The trouble heretofore encountered at the contacting location of the silicide bodies with the metallic conductors, for example between the silicide body and its terminals of aluminum, is likewise at once fully eliminated by virtue of the invention. We have found it to be particularly advantageous if the additional oxide constituent of the molybdenum silicide material has a linear coefiicient of thermal expansion from 5-10 to 8- 10* per degree C. within the temperature range of 20 to 1400 C. It has been ascertained that with such oxide additions, no mechanical destruction of the silicide body is encountered even with repeated alternating temperature stresses regardless of an abrupt application and change of such stresses.

The objective of the invention is achieved with particular reliability if the additional oxide constituent in the portion of the sinter body for working temperatures below 1300 C. is distributed over the entire mass of this portion. Preferably the additional oxide constituent is quantitatively distributed uniformly over each cross section so as to secure constant area density of the constituent. However, we have also found it to be preferable to distribute the quantity of the additional oxide constituent within the just-mentioned portion of the silicide body so that the constituent exhibits a concentration gradient, namely so that the oxide proportion increases with decreasing operating temperature. That is, the end portions of the sintered resistance body preferably have a r C. as required by our invention.

Percent A1 0 27.4 SiO 64.6 Li O 8 A1 0 25 SiO 67 K 0 3 CHFZ 5 It has been discovered, surprisingly, that the molybdenum silicide sinter bodies containing oxide having a melting point below 1400 C. are very insensitive to impurities. In the course of the investigations, we have found that many metallic impurities, for example zirconium, on the one hand, and non-metallic impurities, for example carbon or nitrogen, on the other hand, are extremely troublesome in the natural protective action mechanism of molybdenum silicide under SiO glaze formation. Minute quantities of impurities suffice to cause the formation of voluminous reaction products in the interior that causes bursting of the sinter body. Such destruction also occurs if high-melting oxides are added to the molybdenum silicide sinter body. In no case, however, could it be ascertained that such bursting, due to impurities in the sinter body, took place in cases where this body was given an addition of oxide melt-ing below 1400 A reliable explanation of this phenomenon is not yet available. It is surmised that the oxide melting below 1400 C. is capable of dissolving from the impurities the reaction products that cause the bursting effect and to convert them into a softening, plastically deformable oxide phase which prevents the explosive action of such crystalline derivatives of the impurities.

It seems that in this respect the low softening point of the oxide phase is essential, which oxide phase apparently absorbs the derivative products already in the temperature range commencing at about 1000 C., and hence at a time at which the rate of oxidation-product formation from the impurities is so great as to be capable of resulting in the formation of dangerous crystalline oxidation products.

In principle, such oxides, melting below 1400 C. and applicable for the purposes of the invention, can be given any desired composition. It is only required that they have acidic character, i.e. that acid-forming oxides are preponderant in the molar ratio.

Favorably employed, for example, has been the following oxide substance which melts at about 1380 C. and commences to soften already at considerably lower temperatures:

Percent A1 0 14.1 Si0 81.8 Li O 4.1

Further compositions will be presented hereinafter particularly with reference to Examples Nos. 1 to 5. (As mentioned, all percentages are by weight.)

Prior to melting, the meltable oxides pass through a softening phase. For the operation of a heating element according to this invention, it is favorable to have the operating temperature approach this softening phase or to remain within the range of the softening phase because then, on the one hand, the chemical protection (prevention of dissociation at low temperature) is better and, on the other hand, the end portion or connecting end of the heating element becomes elastic and thus loses or reduces its sensitivity to impact. This is particularly important with respect to alternating temperature stresses as applied to heating elements which in operation are subjected to frequent electric energizations and deenergization. On the other hand, it has been found that oxides whose melting point is too low may cause smearing or gluing of the enveloping ceramic and may also ooze out in furnace zones that are to hot. Best applicable as oxides for the end portions of the silicide bodies have been found those that have melting temperatures of 1250 to 1350 C. An example of such an oxide substance, melting at about 1280 C., is the following:

In heating elements according to the invention, the silicide material in the high-temperature portion, as exemplified by the portions 1 on the accompanying drawing, is preferably made of the ternary system molybdenum-silicon-aluminum. It is known that by partially substituting silicon by aluminum in molybdenum silicide, the high-temperature resistance and the specific electric resistance of high-temperature portion are increased. For that reason, such ternary silicide compositions are preferred for heating conductors to be employed at highest temperatures, but the material may also contain oxidic or non-metallic additions. On the other hand, it has been found that such molybdenum-silicon-aluminum compounds are particularly susceptible to low-temperature dissociation under the effect of oxygen.

For that reason it is particularly preferable to employ for the purposes of our invention a combination of a ternary MoSi-Al high-temperature portion with end portions that consist of molybdenum silicide and contain oxides melting below 1400 C. It has also been found that oxides melting below 1400 C., if added to Mo-Si-Al alloys, also successfully prevent low-temperature dissociation by oxidation of the ternary compound. Consequently a resistance body according to the invention may consist in its high-temperature portion of an M0- Si-Al or Mo-silicicle alloy with or without high-melting oxides (melting point above 1400 C.) and whose lowtemperature or end portion contains an oxygen addition which melts below 1400 C.

The end portions of heating elements according to the invention can be produced in various ways. For example, the pulverulent mixture of the molybdenum silicide base mass with the additional oxide constituent can in molten condition. This method has been used to advantage in cases where very low melting oxide mixtures employed for impregnation, for example oxlidic substances that contain boric acid. However, when impregnating an oxidic sludge into the pores of a pre-sintered silicide body, it is necessary to subject the impregnated body to annealing treatment for melting the sludge.

Generally, however, it is recommended to mix intimately the molybdenum silicide powder and the powder and the powder of the additional oxide constituents, to shape the powder mixture, and to sinter the shaped body. In this manner the desired homogeneous distribution of the additional oxide substance over the entire cross section is most reliably secured. It has been found, however, that oxidic additions do not always promote the sintering of molybdenum silicide. It is known that oxides available in form of a sol or gel, for example hydratized silicic acid, may promote the sintering of silicides. The pulverulent oxide components, however, may constitute an impendiment during sintering operation. We have discovered that such impairment of the sintering ability can be obviated by slight additions of iron-group metals (iron, cobalt, nickel) to the silicide material. Well applicable in this respect, for example, have been additions of 0.01 to 1% of such iron-group metals. Nickel has been discovered to be particularly favorable for promotion or acceleration of the sintering operation. It might be expected that such iron-group metal additions reduce the oxidation resistance of molybdenum silicide. With respect to the molybdenum silicide-oxide composition of resistance bodies according to the invention, it has been unexpectedly determined that the iron-group addition has the opposite effect, namely of increasing the resistance of oxidation. This is probably due to the accelerating effect upon the sintering process which obviates any inherent promotion of oxidation. Protection from oxidation is furthermore secured by the additional oxide constituent even without the formation of the above-mentioned SiO protective coating.

By virtue of the just-mentioned acceleration effect upon the sintering of the silicide, we have succeeded, despite the additional oxide cons-titutent, in attaining in the end portions of the resistance body a specific resistance smaller on the average than 0.4 ohm-mm. /m. Presented in the following are examples which will elucidate this by comparing resistance bodies with and without additional oxide constituent, and with and without addition of nickel.

It will be seen from the following table that the combination of the additional oxide constituent and the mentioned metal constituent according to the invention has the further effect that the desired results to a large extent become independent of the choice of the grain sizes and their distribution.

be jointly shaped as a plastic mass by the extrusion method, and the extruded body can then be sintered. Good products have been made in this manner, as will be described more in detail with reference to the following Examples Nos. 1 to 5. Another way of production is to impregnate a porous molybdenum silicide sinter body with the additional oxide. This can be done by producing a sludge from the oxide in a readily evaporat- The specified substances in pulverulent form were g, g W g liq employing the impregnation mixed intensively for four hours in dry condition. Thereafter 9% by weight of water, relative to the total weight of the powder mixture, was added thereto and the mass was masticated until it was completely plastic. The mass was then extruded to rod shape in the conventional manner with the aid of a piston press. The moist and readily deformable strands were first dried at room temperature for 24 hours and there-after heated in a drying cabinet within 10 hours from 20 to 105 C. and were thereafter kept for 12 hours at 105 C.

Such pre-treated green bodies already had a relatively high strength. They were now sintered. Used for this purpose was a sintering furnace heated with molybdenum wire. The sintering was performed for 1 hour at 1350 C. under hydrogen. Thereafter the sintered rods were annealed at 1350 C. for 1 hour in an oxidizing atmosphere (air). Thereafter the rods were very densely sintered and coated by a glaze. The coating was extremely resistant to changes in temperature and free of fissures. The rods had a volumetric weight of 5.2 g./cm. a bending strength of 1800 kg/cm. and a specific resistance of 0.39 ohm-mm. /m.

Example 2 Constituents: Percent Molybdenum aluminum silicide 35,11. composed of 63% Mo, 36% Si, 1% Al 92.5 Kaolin 5.0 Feldspar 2.0 Fluorspar 0.5

The pulverulent constituents were homogenized in dry condition for 24 hours. Thereafter a aqueous polyvinyl alcohol solution was added in individual portions and up to a total weight of 12% of the powder mixture. The mixture was oalendered until it was completely moist throughout and ready to be extruded. The mass was extruded into rods in the conventional manner and these were dried as described in Example 1. After drying, the bodies were sintered in hydrogen for 30 minutes at 1300 C.

The rods thus produced were already found to glaze at 1100 C. in air and were oxidation-resistant at operating temperatures up to 1200 C. Their volumetric weight was 5.2 g./cm. their bending strength 1800 kg./cm. and their specific resistance 0.4 ohm-mmP/m.

Example 3 Percent MoSi 40p 94 SiO -Ca 2 SiO 1 A1 O -gel 3 The pulveru-lent constituents were processed in the same manner as described in Example 1. The Al O -gel served as binding and plastifying agent. The sintered bodies were found to have a volumetric weight of 5.5 g./cm. a bending strength of 2400 kg./cm. and a specific resistance of 0.31 ohm-mmP/m.

Example 4 Percent Mo-silicide powder, 40 91.7 Fe-powder 0.3 Mica, 4 Quartz, 10,u. 4

The powder was first homogenized in dry condition. Polyvinyl alcohol and water were added in small portions, and the mixture was plastified in a masticating and mixing machine. Thereafter the doughy mass was extruded by means of a vacuum extrusion press. The extruded rods were dried in magazines for 24 hours in air at room temperature and thereafter at 24 hours at 125 C. The rods were then presintered in reducing atmosphere at 1400 C. for 6 hours. Thereafter the rods were sintered in oxidizing atmosphere (air) for 2 hours at 1350 C.

The volumetric weight was found to be 5.6 g./cm.

the bending strength 2500 kg./cm. and the specific resistance 0.28 ohm-mm. /m.

Mixture of 47.5% kaolin+47.5% feldspar+5% fluorspar 5.0

The dry mixture was plastified with aqueous alginate solution. Pressing, drying and reducing sintering was effected in accordance with Example 4. In some cases an oxidizing annealing was effected by thereafter heating the rods with the aid of electric current passing directly through the rods.

The volumetric weight was found to be approximately 5.7 g./crn. the bending strength 2800 to 3000 kg./cm. and the specific resistance was approximately 0.26 ohmmm. /m.

Since the additional oxide constituents are intended only for the end portion of the silicide sinter body subjected to operating temperatures below 1300 C., it is necessary to provide for a suitable junction of the end portion or portions with the high-temperature portion consisting of molybdenum silicide and, as the case may be, also of higher melting oxide additions. Such as junction can be made by sintering or fusing each end portion together with the high-temperature portion. Preferably, however, we provide between the end portion and the high-temperature portion a disc of pressed molybdenum silicide powder which is inserted prior to the sintering or fusing operation and which, after this operation is performed, joins the two portions together by a fusion bond so that they form a seal integral structure. We have found that these junction discs can be given the same or approximately the same diameter as the end portion and the high-temperature portion and that the thickness of the discs is preferably made approximately equal to A to 1 of their diameter.

It is particularly advantageous to dimension the junction discs so that their diameter, under consideration of their radial expansion during the joining operation, is somewhat larger than the diameter of the adjacent body portion so that the individual disc at the junction location results in a bulge relative to the body portions and thus envelops the adjacent ends of these portions. If desired, the bulge of the junction disc can subsequently be ground away.

It has further been ascertained as preferable to give the silicide material of the junction discs a similar oxide addition as the connecting portions of the resistance body. Described in the following are two examples involving junctions made by the disc method just mentioned.

Example 6 Two MoSi rods of circular cross section were used. One of them contained the additional amount of oxide melting below 1400 C. to serve as end portion, whereas the other was to serve as high-temperature portion. Both rods had a diameter of 5.5 mm. and were ground to planar shape at the ends to be joined with each other. These ends were clamped into a welding apparatus so that the two planar surfaces were directly in contact with each other. The two butt surfaces were then slightly moved away from each other and a disc or tablet of 6 .mm. diameter and 1 mm. height was inserted. Thereafter the rods were pressed toward each other and against the tablet in air and without the use of any protective gas. Thereafter, electric current was passed longitudinally through the rods so that they became heated by the passage of current until the welding location attained a temperature of 1750 C. Thereafter the pressure against the rods was reduced to zero and the welding transformer of the apparatus gradually switched to lower current values. The butt welding operation per junction was perwide.

formed within 5 to 15 minutes depending upon the crosssectional area.

The welding tablet was produced as follows:

Prepared was a powder mixture of Percent MoSi in a grain size 40/.L 95.0 Si +4.5 A1 0 +0.5

The powder mixture was moistened with a 5% polyvinyl alcohol solution and then pressed in a die to the shape of tablets having the above-mentioned dimensions. Thereafter the tablets were dried for 4 hours at 50 C. and were then ready for use.

The butt-welded rods had a bending strength of 1600 to 1700 kg./cm. and a specific resistance of 0.24 to 0.25 ohm-mm. /m.

Example 7 Used were two rods of round cross section having a diameter of 12 mm. One rod consisted of MoSi to serve as high-temperature portion. The other rod consisted of MoSi with an addition of oxide having a melting point below 1400 C. to serve as end portion. The butt faces of the rods were ground to planar shape and then clamped into a welding apparatus vertically one above the other. Prior to pressing the rods against each other, a tablet as described above having a diameter of 12 mm. and a height of 1.5 mm. was inserted in unsintered condition. The welding location was then heated by highfrequency current up to 1600" C. without protective gas. Thereafter the temperature was reduced to normal room temperature (20 C) within a few minutes. The junction thus produced was found to be extremely fast at high as well as at low temperatures. The bending strength at room temperature was about 1800 to 2000 kg./cm. and the specific resistance was 0.24 to 0.26 ohm-mm. /m.

For welding oxide-free together with oxide-containing MoSi rods a tablet composition has been found to be well applicable as described above with reference to the production of the oxide-containing end portions in Examples 1 to 5. The production of the tablets is as described in Example 6.

In the following examples reference will be made to the three structural embodiments of heating elements on molybdenum silicide base, as illustrated in FIGS. 1, 2 and 3 of the accompanying drawing.

Example 8 (FIG. I)

The high-temperature portion 1, according to FIG. 1,

constituting the heating element proper, consists of 90% MoSi and 10% of a mixture composed of 75% SiO and 25% A1 0 This portion has a tubular cross section with an outer diameter of 6 mm. and an interior diameter of 3 mm. The tubular structure is provided with slits extending longitudinally and being spaced 50 mm. from each other, each slit being 2 mm. long and 1 mm. Each connecting or end portion 3 of the heating element, subjected to operating temperatures below 1300 C. when in operation, consists of approximately 94.75% MoSi +0.25% Ni+0.4% Li O+1.37% A1 O +3.23% SiO The end portions 3 have a cross section 'of 12mm. diameter and a length of 150 mm. The

portions

1 and 3 are joined with each other by a sintered junction 2 analyzed as follows:

95% MoSi +l% Al O +3.9% SiO +0.1% Fe The terminal caps 4 consist of pure aluminum.

Example 9 (FIG. 2)

The M-shaped high-temperature portion 1 to operate at incandescent temperatures has a full cross section of 5 mm. diameter. Its composition is approximately 63% Mo, 35% Si and 2% Al. The shape of the high-temperature portion 1 is such that the distance between the two parallel end portions 3 is smaller than the width of the heating element in its high-temperature portion 1. Sintercd upon the ends of the high-temperature portion 1 are sleeves 5 of pure MoSi The end portions 3 operating at temperatures below 1300 C. are composed as follows:

Percent MoSi 94 Fe 0.3 SiO 4 A1 0 1 CaO 0.7

The junction between high temperature portion 1 with sleeves 5 and the end portions 3 is made by a sintered composition of:

Percent MoSi 95 Si0 3.5 A1 0 0.5 CaO 0.5 Co 0.5

Example 10 (FIG. 3)

The high-temperature portion according to FIG. 3 comprises three parallel tubes having an outer diameter of 10 mm. and an inner diameter of 5 mm. The tubes consist The upper ends of the tubular parts are connected by a bridge piece 6 of pure sintered MoSi The bridge 6 has respective openings at the junctions with the tubes 1 so that the interior of the tubes is in communication with the interior of the furnace in which the heating element is to be used. This prevents bur-sting of the components by the formation of silicon monoxide inside of the tubes. The junction of the bridge 6 with the tubular parts and also the junction between the tubular parts and the end portions 3, the latter having a diameter of 18 mm., is made by a sinter junction of pure MoSi The length of the end portions 3 depends upon the dimensions of the furnace insulation and, as a rule, is about 200 to 550 mm. The end portions consist of:

Percent MoSi 92 SiO 5 A1 0 1 CaO 1.50 BaO 0.25 Ni 0.25

It will be understood, of course, that our invention is not limited to any particular shape or dimensions of the heating elements or other resistance bodies.

We claim:

1. A sintered electric resistance body having a hightemperature portion suitable for operating temperatures above 1300 C. and end portions for temperatures below 1300 C., said body consisting essentially of molybdenum silicide and containing in said entire end portions 0.1 to 20% by weight of additional oxide substance that is a mixture of oxides selected from the group consisting of oxides of Li, Na, K, Be, Ca, Ba, Al, B and Si and having a melting point below 1400 C., said oxide substance -being in the form of a coating on the silicide particles in the pores and at the surface of the sintered body.

2. A sintered electric resistance body having a hightemperature portion suitable for operating temperatures above 1300 C. and end portions for temperatures below 1300 C., said body consisting essentially of molybdenum silicide with an addition of at least one high-melting oxide, and containing through-out said end portions 3 to 12% by weight of an additional oxide substance that is a mixture 1 1 of oxides selected from the group consisting of oxides of Li, Na, K, Be, Ca, Ba, Al, B and Si and having a melting point below 1400 C., said oxide substance being in the form of a coating on the silicide particles in the pores and at the surface of the sintered body.

3. A sintered electric resistance body having a hightemperature portion suitable for operating temperatures above 1300 C. and end portions for temperatures below 1300 C., said body consisting essentially of molydenum silicide and containing in said end portions 0.1 to 20% by weight of additional oxide substance consisting of at least one oxide of acidic character selected from the group consisting of oxides of Li, Na, K, Be, Ca, Ba, Al, B and Si, having a melting point below 1400 C., said oxide substance being in the form of a coating on the silicide particles of the sintered body.

4. A sintered electric resistance body having a hightemperature portion suitable for operating temperatures above 1300 C. and end portions for temperatures below 1300 C., said body consisting essentially of molybdenum silicide with minor inclusions of high-melting oxides and containing in said end portions 0.1 to 20% by weight of additional oxide substance that is a mixture of oxides selected from the group consisting of oxides of Li, Na, K, Be, Ca, Ba, Al, B and Si and having a melting point below 1400 C. and forming a glaze upon the silicide particles of the body, said oxide substance having a linear coefficient of thermal expansion between 5- l to 8- 10- rnm./ C.

5. In a sintered resistance body according to claim 1, said additional oxide substance being uniformly distributed for constant area density in each cross section of said end portions of the body.

6. In a sintered resistance body according to claim 1, said additional oxide substance having a quantitative distribution in said end portions that possess a concentration gradient from said high-temperature portion along said end portions so that the percentage of said oxide substance increases with decreasing operating temperature along said end portions.

7. In a sintered resistance body according to claim 1, said additional oxide substance having a melting point between 1250 and 1350.

8. In a sintered resistance body according to claim 1, said additional oxide substance having a melting point between 1250 and 1350 C., and having, when molten, a surface tension of approximately 400 (:20) dyn./cm.

9. In a sintered resistance body according to claim 1, said additional oxide substance consisting at least in part of natural silicate.

10. In a sintered resistance body according to claim 1, said additional oxide substance comprising a lithiumaluminum silicate of the composition Li O-Al O -2SiO to Li O-Al O -SiO with 4 to 12% by weight of Li O.

11. A sintered electric resistance body having a hightemperature portion suitable for operating temperatures above 1300 C. and end portions for temperatures below 1300 C., said body consisting essentially of molybdenum silicide with minor inclusions of high-melting oxides and containing in said end portions 0.1 to by weight of additional oxide substance that is a mixture of oxides selected from the group consisting of oxides of Li, Na, K,

Be, Ca, Ba, Al, B and Si and having a melting point below 1400 C. and forming a glaze upon the silicide particles of the body, said high-temperature portion and said end portions being fusion-bonded and forming jointly an integral molydenurn-silicide structure.

12. A resistance body according to claim 1, comprising respective junction discs located between said high-temperature portion and said respective end portions of said sintered molybdenum-silicide body, said discs consisting also of sintered molybdenum silicide and being sinterbonded with said portions.

13. In a resistance body according to claim 12, each of said junction discs having an axial height of to 5 of its diameter.

14. In a resistance body according to claim 12, each of said junction discs of sintered molybdenum silicide containing likewise additional oxide substances as specified for said end portions of said body.

15. In a resistance body according to claim 12, each of said junction discs having a larger diameter than at least one of the two portions of the body joined by said disc and enveloping the adjacent end of said portion.

16. In a sintered molybdenum-silicide resistance body according to claim 1, said end portions for temperatures below 1300 C. containing 0.01 to 1.0% by weight of irongroup metal (Fe, Co, Ni).

17. In a sintered molymdenum-silicide resistance body according to claim 1, said end portions for temperatures below 1300 C. containing 0.01 to 1.0% by weight of nickel.

18. In a sintered molybdenum-silicide resistance body according to claim 1, said end portions for temperatures below 1300" C. containing a resistance reducing addition of not more than 1.0% by weight and having a specific electric resistance below 0.4 ohm-mm. /m. at normal room temperature.

19. In a sintered resistance body according to claim 1, said high-temperature portion consisting substantially of a ternary molybdenum-silicon-aluminum sinter alloy, and said end portions for temperatures below 1300 C. consisting of oxide-containing molybdenum silicide free of non-oxidically bonded aluminum.

20. In a sintered resistance body according to claim 1, said silicide being formed of a molybdenum-silicon-aluminum alloy which contains an oxide constituent having a melting point below 1400 C.

References Cited by the Examiner UNITED STATES PATENTS 1,745,173 l/193O Leonard 29-191 2,258,327 10/1941 Kramer 29191 2,992,959 7/1961 Schrewelius 33 8330 3,001,871 9/1961 Thein-Chi et a1. 211 3,067,032 12/1962 Reed et al. 75211 DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, BENJAMIN HENKIN,

Examiners. R. O. DEAN, Assistant Examiner.

Claims

1. A SINTERED ELECTRIC RESISTANCE BODY HAVING A HIGHTEMPERATURE PORTION SUITABLE FOR OPENING TEMPERATURES ABOVE 1300*C. AND AND PORTIONNS FOR TEMPERATURES BELOW 1300*C., SAID BODY CONSISTING ESSENTIALLY OF MOLYBDENUM SILICIDE AND CONTAINING IN SAID ENTIRE END PORTIONS 0.1 TO 20% BY WEIGHT OF ADDITIONAL OXIDE SUBSTANCE THAT IS A MIXTURE OF OXIDES SELECTED FROM THE GROUP CONSISTING OF OXIDES OF LI, NA, K, BE, CA, BA, AL, B AND SI AND HAVING A MELTING POINT BELOW 1400*C., SAID OXIDE SUBSTANCE BEING IN THE FORM OF A COATING ON THE SILICIDE PARTICLES IN THE PORES AND AT THE SURFACE OF THE SINTERED BODY.