Glossary

"Anti pest"

Heating elements of molybdenum disilicide (MoSi₂) are usually used at application temperatures starting at 1200°C. Depending on the production process of a product, however, hold times at low temperature ranges up to approx. 700 C are necessary, where the heating element may disintegrate into a powder. This effect is called „MoSi₂-Pest“. We have developed a special heating element quality to prevent this reaction. MolyCom^®-Hyper 1800AP provides an "anti-pest" heating element that is oxidation-resistant in the temperature range of 200°C to 700°C.

Application atmospheres

Electrical high-temperature furnaces with MoSi₂ heating element may be operated under different process atmospheres such as air, nitrogen (N₂), argon (Ar), helium (He) or hydroxide (H₂). The highest possible application temperature can be reached under air atmosphere. MoSi₂ heating elements are also used successfully in the other atmospheres named, under consideration of the recommended maximum element temperatures, however.

The insulation of polycrystalline mullite/alumina wool (PCW) can be used in other atmospheres as well. Furthermore, process temperature control rings PTCR can be used deviating from the standard atmosphere (air) under vacuum or reducing conditions such as N₂/H₂-mixtures. For this, we recommend pre-firing the PTCR, except for types MTH and HTH, at 600°C for 2 hours, in order to evaporate the organic binder. We will gladly perform pre-firing for you if you wish.

Application temperature

The term of application temperature is often replaced by furnace temperature or operating temperature in practice. The application temperature is the temperature at which our products, components and systems can be operated permanently under consideration of operational influences. It considers influences that occur in practice, such as atmospheres, hold times and the maximum temperature at which the furnace is operated. This is countered by the classification temperature for insulation material and the element temperature for heating elements.

Classification temperature

The classification temperature is defined as the temperature where a product has a certain linear shrinkage after 24 hours of heat application in the electrically heated laboratory furnace and an oxidising atmosphere. Depending on the type of product, the values vary from 2% in insulation boards and shapes to 4% in needled blankets.

The classification temperature is indicated in steps of 50°C. In contrast to insulation products of amorphous fibres (ASW/RCF), products of polycrystalline mullite/alumina wool (PCW) can be used permanently up to the indicated classification temperature. For ASW/RCF, the permanent application is by about 100°C to 150°C below the indicated classification temperature.

Corrosion in cone and/or in the heating zone (Le)

Heating elements can be operated in different maximum application temperatures depending on their quality. A corrosion in the cone and/or in the heating zone (Le) happen, when the maximum application temperature prescribed for the heating element is regularly exceeded and thus it is operated at excessively elevated temperatures.

Element temperature

The element temperature is the surface temperature of the heating element. The surface load is one important criterion for the surface temperature.

Heating element

A heating element is a resistance heater that converts electrical energy into heat. It has diverse uses - from household appliances such as hair dryers, dishwashers or baking ovens to industrial application areas in the electrical high-temperature furnace. One example of metallic/ceramic materials for use in high temperatures is molybdenum disilicide (MoSi₂).

Heating element geometries

Heating elements of molybdenum disilicide can be produced in many different shapes or geometries in individual sizes. Typical shapes include the U-shape, W-shape, L-shape, which can be bent both in the cold zone or the heating zone. In addition to these standard designs, individual special geometries can be implemented as well. This includes panorama, block and coil shapes, as well as rods.

Heating element sizes

Determination of a heating element size requires some information. The information Lu (cold zone), Le (heating zone), a (shank distance), c (diameter of the cold zone) and d (diameter of the heating zone) is needed for a U element, for example.

A W element additionally requires the values B (height of the heating zone from the upper to the lower edge) and S (number of the shanks a1, a2, a3). An L element requires indication of where the bend is to be and at which angle the bend is to be applied. The dimensions are usually indicated in millimetres.

Heating elements become round bodied

It is also a normal reaction, which is due to electric fields in terms of attraction and repulsive forces. A slightly round bodied deformation does not generally effect the performance or function. It is advisable to check the circuit (series or parallel circuit) and the connections.

Heating elements crack (mechanical crack)

This phenomenon happens in various ways. One possibility results in an user error and occurs, when too much force is applied to fix the heating elements with the contact straps or the double holders. Finesse is needed in the truest sense of word. Furthermore it is necessary that the heating elements are really straight installed through the furnace roof into the furnace chamber. A slightly tilting can already lead in a mechanical crack.

Heating elements lengthen

This is a normal reaction. As a rule, the lengthening involves rather larger heating elements with a heating zone (Le) of approx. >500mm. In general the phenomenon does not lead to any problem during operation. As soon as the elements threaten to touch the furnace bottom, they should be replaced, otherwise the insulation will be damaged.

Molybdenum disilicide (MoSi2)

Molybdenum disilicide is an intermetallic compound of molybdenum and silicon (Mo + 2 Si -> MoSi₂). It is a material that is extruded to heating elements in different shapes or geometries by the powder-metallurgic method. Heating elements of MoSi₂ are resistance heating elements.

The intermetallic compound is a suitable material for high-temperature applications due to its high melting temperature of 2030°C and its outstanding oxidation resistance. MoSi₂ also has a high hardness and corrosion resistance and is situated between metallic super-alloys and ceramic materials in terms of structural applications in high-temperature technology.

MoSi₂ is corrosion resistant up to 1800°C, mostly due to the formation of an SiO₂-protective layer. It forms at above 1000°C and is just a few µm thick. As compared to the ceramic materials, the silicides are characterised by high heat conductivity and electrical conductivity. The high temperature change resilience permits faster heating up and cooling down. Due to the barely existing wear of the heating elements, a long heating element service life can be achieved.

"MoSi2-pest"

"MoSi₂-pest" is an effect occurring in the temperature range between approx. 300°C and 700°C. In this temperature range, heating elements of molybdenum disilicide (MoSi₂) sometimes show strong oxidation with powdery disintegration of the material. One possible cause of this is in the intercrystalline disintegration that is facilitated by porosity and the structure. This disintegration may be prevented by a high density and a very slow porosity. In processes where longer hold times at low temperatures are needed, a heating element must fulfil particular demands. MolyCom^®-Hyper 1800AP (Anti-Pest) does so.

Polycrystalline mullite/alumina wool (PCW)

Polycrystalline mullite/alumina wool (PCW) is made up of fibres composed of alumina (Al₂O₃) and silica (SiO₂). The Al₂O₃ content is between 72% and 99%. At approx. 72% Al₂O₃ and approx. 28% SiO₂, we speak of a mixed-oxide fibre. This is also called a mullite fibre or structure. The fibres are produced in a "sol-gel procedure". Polycrystalline mullite/alumina wool (PCW) is used at application temperatures above 1250°C and in applications that require a very high chemical resilience.

Refractory Ceramic Fibre (RCF)

Aluminosilicate wool (ASW), also known as refractory ceramic fibre (RCF), is made up of amorphous fibres that are produced in a melting process of, among others, alumina (Al₂O₃) and silica (SiO₂). The Al₂O₃ content is between 45% and 55%. The material is usually used at application temperatures of 600°C to 1400°C.

Regeneration firing

MoSi₂ heating elements form a protective SiO₂ layer on their surfaces, which prevents oxidation of the basic material. If the SiO₂ layer bursts off, regeneration firing at temperatures above approx. 1450°C for several hours can be applied. There should be no products in the furnace for this. Please contact us if you have any questions about regeneration firing.

Shrinkage

Shrinkage means the reduction of volume of a material or workpieces. It happens when drying or cooling off, as well as during sintering/firing processes without application of pressure.

Spalling on the heating elements

Spalling often occurs in reducing atmospheres. This is caused by the fact that the heating elements cannot form a new protective SiO₂ layer. You have two solutions, if you notice that phenomena:

• 1) A regeneration firing in oxidizing atmosphere at temperature of 1450°C for several hours without a product inside the furnace;
• 2) You will use an element with a thicker protective SiO₂

We would be glad to advise you on both solutions.

Surface load

One of the most important indices for dimensioning electric heating elements is the surface load or area-related performance. The surface load is indicated in Watt per cm². It measures stress and service life of a heating element.

Thermal shock resistance

Thermal shock resistance describes the resistance of a material or workpieces to quick, shock-like temperature changes. Mechanical tensions result since the heat can be transferred more quickly at the surface of a workpiece than inside of it. If the resulting tension exceeds a critical value, the material will take damage.

Vacuum-forming

A slurry of polycrystalline mullite/alumina wool (PCW) and organic binders is prepared. The water is extracted from the slurry by vacuum technology in a shape. This kind of shaping permits implementation of complex parts for application temperatures up to 1800°C.