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MoSi2 Heater Property

Molybdenum Disilicide (MoSi2) Heating Elements Tube

1.Introduction:

Molybdenum disilicide (MoSi2,or molybdenum silicide), an intermetallic compound, a silicide of molybdenum, is are fractory ceramic with primary use in heating elements. It has moderate density, melting point 2030 °C, and is electrically conductive. At high temperatures it forms a passivation layer of silicon dioxide, protecting it from further oxidation. It is a gray metallic-looking material with tetragonal crystal structure.

Molybdenum Disilicide Heating Elements Type MOSI2 heater is an electric element principally containing molybdenum disilicide for use in industrial and laboratory furnaces/ovens/kilns.

Molibdenumdisilicideheatingtube

Elements fabricated of this unique material support high power densities on their surface (e.g. 20 W/cm² of element surface area at an element temperature of 1700°C, 14 W/cm² at  1800°C and 5 W/cm² at 1900°C), can be cycled rapidly, and  provide up to 10KVA / sq. ft. power density in rapid cycle and continuous furnace applications. 4 grades are available with maximum element temperatures of 1700°C, 1800°C,1850 °Cand 1900°C in air.

The element’s “U” shaped heating MoSi2 are supported by larger non-heating terminals. Aluminized ends are provided for positive connection to electrical straps.

Made of molybdenum disilicide based cermets MOSI2 heating elements withstand oxidation at very high temperatures. Their surface is  protected by of a thin layer of silica glass that forms when they are exposed to oxygen at elevated temperatures. Although the base cermet is consumed in the formation of the silica “skin” the rate of decomposition is minimal. They provide long performance at elevated temperatures while delivering high power.

2.Physical Properties

  Volume Density

    Bending Strength   Vickers Hardness Porosity    Water Absorption Heating Elongation 

5.5g/cm³

15-25kg/cm² (HV)570kg/mm² 7.40% 1.20% 4%

3.Chemical Properties

Mosi2 heating element is used in the high temperature(800℃-1900℃) under oxidizing atmosphere. It will form the silicon dioxide(SiO2) protecting film when the element was heated to a high temperature. The Mosi2 heating element must not be used in the temperature between 400℃ and 700℃ for a long time; otherwise the element will be cremated under the strong oxidizing function in the low temperature.

Below are the application temperature under different atmosphere:

                 Atmosphere                                             The Max Temperature under using
1700 Grade  1800 Grade

  NO2,CO2,O2,Air

1700℃

1800℃

 He,Ar,Ne

1650℃

1750℃

 SO2

1600℃

1700℃

 CO,H2

1500℃

1600℃

 Wet H2

1400℃

1500℃

 Dry H2 1350℃

1450℃

4.Electrical Properties

(1)Resistance

  • The resistivity of MoSi2 heating elements rises with increase in temperature (shown in the graph below). When the temperature of the heating stage is below 300°C, less than 35% of the total output power should be applied to avoid stressing the elements with excessive current.
  • Generally the resistance of heating elements will not be affected by aging under normal operating conditions, therefore old elements can be used together with new ones.

mosi2-resistivity

(2)Surface Load

According to the structure, a correct choice of MoSi2 heating elements should based on the temperature and atmosphere,what is most important is the surface load which is the key point to elements’life-time. The picture below  shows the relationship among the surface temperature, the element temperature and surface load under the circumstance that the thermal radiation components is uncrossed. The shaded part is for common surface load.

MOSi2 surface laod