The present invention relates to a method of manufacturing a heating'element of the molybdenum silicide type and also to a heating element.

An electrical resistance element of the molybdenum silicide type is described in Swedish Patent Specifications 0003512-1 and 0004329-9. According to patent specification 0003512-1 the resistance material of the heating element includes Mo (Sil-xAlx) 2 which is caused to contain aluminium to an ex- tent at which the formation of pest is essentially prevented.

It has been found that when such material is operated in a temperature range of 400-600°C no pest, or only a slight amount of pest, is formed. Pest is formed by virtue of the formation of Mo03 from MoSi2 and 02.

The reason why the formation of pest is significantly reduced or is eliminated is due to the formation of A1203 on the sur- face of the element.

According to one preferred embodiment x is caused to lie in the range of 0.2-0. 6.

The other patent specification 0004329-9 teaches a method of increasing the useful life span of heating elements that con- sist chiefly of molybdenum silicide and alloys of this basic material where the element operates at high temperatures.

According to this patent specification, the heating element is caused to contain aluminium to an extent which is suffici- ent to maintain a stable, slowly growing layer of aluminium oxide on the surface of the heating element.

According to a preferred embodiment the heating element mate- rial is caused to contain Mo (Sil-xAlx) 2 where x lies in the range of 0.2-0. 6.

A material of the molybdenum silicide type that contains alu- minium has been found to possess improved corrosion properti- es at both low and high temperatures.

Such material is often produced by mixing MoSi2 powder with oxidic raw material, such as aluminosilicates. When the raw material is bentonite clay, there is obtained a relatively low melting point which contributes towards so-called smelt phase sintering which results in a dense material that con- tains MoSi2 and a proportion of aluminium silicate correspon- ding to 15-20 percent by volume.

Bentonite clay has different compositions. Some bentonites include 60% by weight Si02 while some contain somewhat more than 70% by weight Si02. Although the A1203 content varies, it normally lies between 13-20% by weight. The melting point varies between about 1200-1400°C.

Bentonite clay that contains chiefly Si02 can be used in the production of heating elements containing Mo (Sil-xAlx) 2. When sintering with an Al-alloyed silicide there takes place a chemical exchange reaction in which the greater affinity of the oxygen to Al than to Si results in Si leaving the alumi- nium silicate and entering the silicide as a result of Al leaving the silicide and being sucked up by the oxide phase.

This exchange reaction also contributes towards improved sin- tering properties of the composite material. The final mate- rial contains Mo (Sil-xAlx) 2 that is substantially deplete of Al, where the oxide phase contains A1203 in all essentials.

The standard procedure of manufacture involves mixing molyb- denum, silicon and aluminium in powder form and firing the powder mix normally under a shielding gas atmosphere. This results in a cake of the material Mo (Sil-yAly) 2, where y is

larger than x in the above formula as a result of said ex- change reaction. The reaction is exothermic. The cake-is then crushed and ground down to a fine particle size normally in the order of 1-20Am. This powder is mixed'with bentonite clay such as to form a wet ceramic material. The material is extruded and dried to a rod form whose diameter corresponds to the diameter of the. subsequent element. The material is then sintered at a temperature that exceeds the melting tem- perature of the including components.

However, there is a drawback with an element of this kind.

The problem is that the oxide that forms on the surface of the element, namely A1203, sometimes peels away or flakes off, i. e. loosens from the surface of the element in the case of cyclic operation.

A peeling oxide gives poorer protection against continued ox- idation of aluminium which becomes impoverished in the outer surface of the element more quickly. Moreover, a peeling ox- ide can contaminate the oven in which the element is fitted, with the risk that performance and the appearance of products heat treated in ovens that have such elements will be signi- ficantly impaired. This restricts the use of such elements in heating processes.

This problem is solved by the present invention.

The present invention thus relates to a method of producing a heating element substantially comprising of the molybdenum silicide type and alloys of this basic material, and is cha- racterized by producing a material that substantially con- tains Mo (Sil-xAlx) 2 and A1203 by mixing a molybdenum alumi- nosilicide Mo (Sil-yAly) 2 with Si02 wherein Si02 has a pu- rity of at least 98%.

Further, the invention relates to a heating element of the kind and with the main features as indicated in claim 5.

The invention will now be described in more detail in the following.

In accordance with the invention a heating'element that con- sists chiefly of molybdenum silicide type and alloys of this basic material is produced by mixing a powder that chiefly contains Mo (Sil-yAly) 2 with highly pure Si02. Pure silicon dioxide has a melting temperature of about 1700oC. When using Si02, however, said exchange reaction between Si in the oxide and Al in the silicide results in a high density sintered product.

The mentioned Si02 can be present as pure Si02 or as an alu- minium silicate of high purity. However, Si02 can be-inclu- ded in silicates in which other substances in the silicate have properties which prevent the molybdenum silicide from being alloyed with the substance or substances concerned and with which the symmetry of crystal lattice of the molybdenum silicide will be retained. Mullite and sillimanite are examp- les of conceivable material in this regard.

The present invention thus replaces the bentonite clay with silicon dioxide, therewith excluding the transfer of impuri- ties in the bentonite clay, such as Mg, Ca, Fe, Na and K, to the heating element, thus eliminating the negative effects of such impurities on the function of said element.

It is possible to partly substitute molybdenum with Re or W in the material Mo (Sil-xAlx) 2 without changing the symmetry of the crystal lattice.

It has been found surprisingly that there is obtained with low contaminant contents an oxide which does not peel after cyclic operation between room temperature and high temperatu- res, for instance 1500°C.

According to one embodiment x is caused to lie in the range of 0.4-0. 6.

According to one preferred embodiment x is caused to lie in the range of 0.45-0. 55.

The present invention thus solves the problem mentioned in the introduction and enables the present element to be used beneficially in ovens without detriment to the material trea- ted in the oven.

The present invention shall not be considered to be limited to the aforedescribed embodiments, since variations can be made within the scope of the accompanying claims.