WO1989002647A1

WO1989002647A1 - Electrical resistance element of molybdenum disilicide type

Info

Publication number: WO1989002647A1
Application number: PCT/SE1988/000468
Authority: WO
Inventors: Venanzio Bizzarri
Original assignee: KANTAHL AB
Current assignee: KANTAHL AB
Priority date: 1987-09-14
Filing date: 1988-09-13
Publication date: 1989-03-23
Anticipated expiration: 1990-03-14
Also published as: SE8703542D0; SE458646B

Abstract

The invention is for an electrical resistance element of the molybdenum disilicide type. It refers to elements which mainly are composed of silicide compounds, where molybdenum is the main part of the metal component. The element according to the invention can be used at higher operation temperatures than previously known elements of this kind. This means about 1800°C furnace temperature. According to the invention this is achieved by substituting part of the molybdenum in the molybdenum silicide structure for tungsten. When different properties of the material are taken into account it is concluded that 25-50 atomic % molybdenum shall be substituted for by tungsten. The resistance elements are produced according to previously known methods.

Description

ELECTRICAL RESISTANCE ELEMENT OF MOLYBDENUM DISILICIDE TYPE

The present invention relates to an electric resistance element of molybdenum disilicide type, where the element essentially consists of suicides and where molybdenum forms the main part of the metal component in the suicides.

Electric resistance elements of molybdenum disilicide type have been known a long time. They are mainly used in so called high temperature applications which in the first place refers to industrial furnaces operating at temperatures up to about 1700°C.

Pure molybdenum disilicide is not suitable as material for the direct manufacture of resistance elements. To obtain satisfactory properties of the elements, e.g. an oxide com¬ ponent is added where the predominant part could consist of silicon dioxide. The oxide component part can be up to 10% by weight or more. The manufacturing methods and the compo¬ sition of material have been described before in the US patents 3 395 029 and 3 640 907. Elements manufactured according to these patents can be used for continuous operation at element temperatures up to about 1820 C. The present invention relates to electrical resistance elements of molybdenum disilicide type with higher maximum service temperature than previously known elements. Elements according to the invention can be used for continuous opera¬ tions up to 1880 - 1900 C element temperature, which means about 1800 C furnace temperature. According to the invention this is achieved by substituting part of the molybdenum in the molybdenum suicide structure for tungsten. and Mo are isomorphus and can be substituted for each other in the same crystalline structure. (The crystallographic designation of MoSi is C116).

Two characteristics of the material are crucial for the operation temperature that can be allowed. These characteris- tics are the quality of the enamel formed on the surface of the elements and also the state of equilibrium of the chemical reactions in the elements which determines the partial pressure of the gases formed by the chemical reac- tions. When the partial pressure equals the atmospheric pressure, bubbles appear on the surface of the elements. Further increase of temperature leads to a total disintegra tion of the material. This is called over-heating.

There are no standardized and generally agreed upon methods to test these two characteristics. The enamel quali is judged by optical inspection, both ocular and by means o a microscope. Essential properties are smoothness, thicknes adherence to the substrate, the colour of the enamel etc.

The formation of bubbles in the material and on its surface is probably mainly caused by the following chemical reaction:

= MoSi₂ + 7 Si0₂ Mo₂Si₃+ 14 SiO(g)

The equilibrium of this reaction is among others depending of the temperature and the formation of bubbles and can therefore be shown in graph as a function of the temperatur To determine a certain bubble temperature (Tb) a method was developped, where a test sample of the resistance material is fixed between water cold terminals. Electric current of increasing strength controlled e.g. by an adjustable transformer is then conducted through the test sample. The temperature of the sample is measured by means of an optic pyrometer. When the temperature approaches the expected Tb the increase of the current is reduced to a very low rate. Eventually bubbles are formed on the surface of the test sample and the temperature reading at that moment is calle the bubble temperature, Tb. Therefore Tb is the temperatur where the gas pressure in the material exceeds the atmosph ric pressure, whereby bubbles are formed under the viscous enamel. In a certain sense this can be regarded as a measu of the refractory properties of the material. Surprisingly it has now been found that if molybdenum to a certain extent is substituted for by tungsten in the suicide phase a material with higher Tb and maintained enamel quality is obtained. Resistance elements of this material can therefore have a higher continuous service temperature than previously known elements of molybdenum suicide type. It appears then that Tb increases by increasing contents of tungsten in the structure. On the other hand, the enamel quality deteriorates by higher contents of tungsten. This relation is illustrated in principle by diagram 1, where on the x-axis the material composition in terms of weight % of MoSi„ calculated on the total amount of MoSi„ and WSi„ is indicated. The y-axis marks two characteristics, the bubble temperature Tb expressed in C whose dependency of the material composition is shown by the graph marked Tb and the enamel quality given as a quality index G, whose dependency of the material composition is shown by the graph marked G. When consolidating these characteristics it is found that up to 50 atomic % of the input molybdenum should be substituted for by tungsten. 25 - 50 atomic % of molybdenum would be suitable to be substituted for by tungsten. At higher contents of tungsten the enamel quality will be too poor and at lower contents the bubble temperature decreases. Otherwise the resistance elements are manufactured essentially according to known methods as mentioned above. This means that they also contain an oxide component amounting to about 10 % by weight.

The Composition of the material of a resistance element according to the invention can be comprehended by the formula Mo_x Wι__x Si2, where x ≥0,5, preferably 0,50 <x <0,75. It is essential for the invention that the resistance element com¬ prises a homogeneous material with the formula given above. A mixture of materials consisting partly of molybdenum sui¬ cides and partly of tungsten suicides does not have the desired properties. Resistance elements generally have three parts, i.e. two terminals and between these a hot zone. The resistance element as described above refers to the hot zone. Such a zone according to the invention can be combined with termi- nals made from other material, e.g. known molybdenum disili¬ cide material. In certain special designs the elements can have several hot zones and between these intermediate zones operating at lower temperatures. These intermediate zones could then also be made from known molybdenum disilicide materials.

Claims

1) Electric resistance element of molybdenum disilicide type comprising at least one hot zone and at least two terminals, c h a r a c t e r i z e d i n, that the hot zone comprises a molybdenum disilicide type material, wherein molybdenum partly is substituted for by tungsten, so that the formula for the suicide component is Mox Wl-χ Si2.

2) Resistance elements according to claim 1, c h a r a c ¬ t e r i z e d i n, that 25 to 50 atomic % of molybdenum is substituted for by tungsten, so that x is between 0,50 and 0,75.

3) Resistance elements according to any of the above claims, c h a r a c t e r i z e d i n, that the terminals consist of a molybdenum suicide material essentially comprising MoSi₂.