MOSIL (MoSi2) range of Molybdenum Disilicide Heater

Molybdenum disilicide with a self-forming silica dioxide glazing makes up the high-density material known as the MoSi2 heating element. Up to 1800°F furnace temperature is its operating range. These components last a very long time.

The unique material known as MoSi2 Heating Element combines the best qualities of metallic and ceramic elements. It has a low thermal expansion and like ceramic materials, is resistant to oxidation and corrosion. Like metallic materials, it also has strong electrical and thermal conductivity. Because of its robustness, the element is immune to thermal shock and can function as a heating element for many years.

Resistivity

As the temperature rises, MOSIL (MoSi2) becomes more resistive. When the elements are coupled to a steady voltage, they can provide high power at lower temperatures and progressively lose power as the temperature rises. This speeds up the furnace’s process of reaching the operational temperature.

Nevertheless, the likelihood of an element overheating decreases together with a decrease in element power.

Even when operating at high temperatures, the MOSIL (MOSI2 Heating element) resistance does not deteriorate. During the first term, there is just a minor (≈ 5%) decrease in resistance.

These characteristics make it simple to replace a failing component without compromising the functionality of other components connected in series.

Density	5.6 h/cm³
Porosity	~1%
Thermal Conductivity 20 – 6000 °C 601 – 12000°C	30W/- °K 15W/m-°K
Coefficient of linear expansion	7.5 x 10-⁶ °C
Specific heat capacity at 20°C	420J/Kg- °K
Emissivity	0.7 – 0.8

Chemical  Resistance

MOSIL (MoSi2) has a remarkable chemical resistance and may thus be employed in the majority of furnace atmospheres. MOSIL (MoSi2) elements perform best in oxidising atmospheres such as carbon dioxide, air, and water vapour, but they also perform well in carburizing, reducing, and neutral atmospheres.

At temperatures about 500°C , oxidation of silicon and molybdenum on the surface of the components occurs. MoO3, a yellowish powder is generated as the oxidised result. This generated product norm.

Carburizing furnaces commonly use MOSIL (MoSi2) components. The components that are not affected by the atmosphere are usually nitrogen or an endogas with small amounts of a carburizing gas like methanol or propane.

The temperature of the component is normally controlled below 1400°C  in this sort of furnace. Carbon buildup in the furnace as a result of high carbon potential might cause element failure. It is recommended that the furnace be fired on a regular basis under oxidising conditions to remove carbon accumulation.

Ceramic nitration takes place at temperatures ranging from 1250 to 1550°C. When the protective layer of glaze is consumed at such high temperatures, the silicon in the component’s silicide may react with the nitrogen, producing silicon nitride (Si3N4). This silicon nitride formation causes harmful scaling on the component. To reduce nitrogen penetration, the components used for this purpose must be thoroughly heat-treated with Molybdenum Di Silicide. This treatment is mostly recommended when the operation takes place in a nitrogen atmosphere with a low dew point.

Generally has no negative impact on the operation of Molybdenum Di Silicide components.

Since they are inert by nature, noble gases such as argon and helium do not react with molybdenum di silicate. Nonetheless, the component’s chemical equilibrium will be upset if there is a gas flow surrounding it. The protective glaze (layer) is eaten and reacts with at high temperatures. It is advised to regenerate a fresh layer of glaze while using such gasses, rather than waiting until the previous coating has entirely worn off. It is thus advised to use a heating solution containing molybdenum di silicate.

The MoSi2 heating element disintegrates in a dry hydrogen environment, generating low-silicon silicon and gaseous silicides. Both the temperature and the H2 gas’s capacity for reduction affect this chemical process. By raising the dew point, the maximum temperature that can be achieved is increased.

Generally speaking, hydrogen is the mixture of nitrogen and hydrogen. Although N2 decreases H2’s reactivity, hydrogen still has a significant impact, particularly when operating for extended periods of time. The gas mixture’s dew point and gas velocity are always crucial. Certain heat treatment techniques can improve the performance. Therefore, it is advised to use a MoSi2 heating solution.

Metals

All metals that come into contact with MoSi2 produce silicides. At high temperatures, molten metal vapours (bronze, zinc, and copper) tend to react with the components. Metal oxide dust particles in the furnace also react with the protective glaze. It is critical that the components be shielded against molten metal splashes. Metals with melting points below 1300°C  can be melted in this type of furnace with proper precautions.

Ceramics

Because the operating temperature of Molybdenum Di Silicide is typically high, reactions occur easily between the silica layer on the component surface and most salts and oxides. This is especially important when the components are supported by ceramics. Ceramics in such cases must be made of stable compounds that are non-reactive with silica, such as silicates. Sillimanite and mullite are two suitable ceramics. Reactions can no longer occur when component temperatures exceed 1600°C.

Elements and tubes

Two-shank The most common design is a U-shaped element. The heating zone is connected to terminals via welding. These terminals are typically twice the diameter of the heating zone.

Two-shank elements can be bent 45° or 90° in the heating zone or at the terminals, whereas four-shank elements are only used horizontally.

Standard Dimensions	Temperature
MOSIL Heaters	1700	1800	1900
3	X	✔	✔
4	X	✔	✔
6	✔	✔	✔
9	✔	✔	✔
12	✔	✔	X

MOSIL (MoSi2 Heating Element) Shapes

For further information on MOSIL…

Download the MOSIL Range of Molybdenum Di-Silicide Heater Brochure