1.The general description
MoSi2 heating elements are resistive heating elements based on molybdenum disilicide. The molybdenum disilicide heating elements prevents the inner core from oxidation by forming a compact quartz glass coating at the surface under high temperature. The temperature generated by MoSi2 goes as high as 1700~1800â in oxidation atmosphere. They can be widely used in such applications as sintering and heat treatment on electronic ceramics, structural ceramics, magnetic materials, flameproof materials, glass, metallurgy, etc. The resistance of the heating elements is independent on their mission time. The MoSi2 heating elements hardly age, which allows the mixed use of new and old packages.
Packaging is available in U shape, W shape, Straight shape (X shape) and bend shape (L shape),etc. The dimension ranges from 3/6, 4/9, 5/10, 6/10, 6/12, 6/14, 7/14, 8/16, 9/18 to 12/24mm. Special request from customers is welcome.
Having the same mechanical character as other ceramic products. The Yuhao MoSi2 Heating Elements belong to the brittleness material so that they are easy to rupture at the normal temperature ,which brings some difficulty to transport and install, but it may be avoided if they were installed and used correctly.
2. Dimension marks of elements
diameter:D1/D2,mm/mm
Hot zone length: Le, mm
Cold end length: Lu, mm
Shank spacing: A, mm
Examples:
ï¼1ï¼U shape,
D1=3mm,D2=6mm,Le=140mm,
LU=125mm,A=25mm
specify as :
YuhaoU Shape,3/6×140×125×25
ï¼2ï¼W Shape
D1=6mm,D2=12mm,
Le=300mm,Lu=250mm,A=50m
specify as :
Yuhao W Shape 6/12×300×250×50
3.Physical and chemical properties of elements
1) Physical property
Volume density
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5.5ï½5.6kg/cm3
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Bend strength
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515ï½25kg/cm2
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Vickers-hadness
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(HV)570kg/mm2
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Porosity rate
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7.4%
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Water absortion
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1.2%
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Hot extensiblity
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4%
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2) Chemical property
Oxygen-resistance under high temperature: in oxidizing atmosphere,on the surface of element owing to the high-temperature combustion a layer of compact quartz(sio2)protective film is formed to prevent Mosi2 from continuously oxidizing.When the element temperature is higher than 1700â,the sio2 protective film, Whose fusing point is 1710â, is fused,and the sio2 is fused into molten drops owing to the action of its surface extension, so that loses its protective ability,In the oxidizing atmosphere,when the element is continuously used,again the protective film forms.
It should be pointed out that element cannot be used for rather long time in 400-700âï¼ot it will be powdered owing to the strong oxidizing action in low temperature.
4ï¼Electric property of elements
1) Resistance property
The resistivity of element rapidly rises as the temperature rises,under normal operating conditions ,generally the element resistance doesn't change with the service time changing.So the old and new elements can be used mixed.
2) Surface load
The key factor to the optimum servivice life of the element is to select the surface load of the element correctly according to the furnace construction,atmosphere and temperature.Right figure shows the relation between the furnace and the element surface load under the condition that the element radiation isn't obstructed.The shadow part in the figure shows the surface load-temperature range in common use.The figure still shows the reference current.
3) Recommend surface load:
Furnace Tempï¼âï¼
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1400
|
1500
|
1600
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1650
|
1700
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Surface load of hot zone(W/cm2)
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<18
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<15
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<12
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<10
|
<8
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5. Installation of elements
1ï¼vertically hanging
Under normal temperature, MoSi, element is very brittleness,while under high temperature it is plasticity.So,the better way for installation if the UShape element is to hang it vertically,to the furnace top through the support clamping chuck H.The aim of such way is to avoid putting the mechanical stress directly to the element heat-generating end ,or the element will easily be broken.
2ï¼support clamp
ã Support clamp are applied to &9/18 and &6/12 two kinds of elements respectively. the support clamp supports the whole weight of the element and the position of the element is also determined by it .therefore ,it must be installed carefully to assure that the element it veryically hung.in order to prevent the element from being over heated locally,the taper part of the element lower end must put into the furnace chamber.
3ï¼wire clip
ãThe wire connection clamp connecting MoSi2 element is made of aluminum woven wire or multilayer aluninum foil.the steel plate outside it only acts as a clamp and isn't used for electric conduction.For&6/12 element,single rowing wire is used ,and for &9/18element,double-rowing wire is used ,the end of the lead wire should be a little larger than the linear distance between the element and bus.
when the element is installed,the thread fixing the wire clip shouldn't be screwed too much at one time,it can be tightened when the element rises to high temperature,as the element has some plastic and isn't easily broken .the temperature of the wire clip generally shouldn't be higher than 200 C .therefor,the contact voltage between the clip wire and element should be lowered to 0.1V,in order to avoid that the radiation heat is conducted to the clip,the distance between the lower end of the clip and upper surface of the through brick should not less then 50mm.Generally to ^&6/12 element ,170A should not be used for a long time and for &9/18 element, 300A shouldn't be used.
6. Operation of MOsi2 furnace
1ï¼ãDrying of the furnace
The new built frunace or the furnace that haven't been used for a long time should be dried before use.The drying temperature generally is 100-200â. The elemnt that has been used for a long period time under low temperature will cause low-temperature oxidation. For the small-sized furnace,as its drying time is short, several hours will affect the element little, but for the large-sized one,as its drying is long. it should be dried carefully.You'd better open the furnace gate to make it ventilated. The gate may be half-ipened with the rising of temperature and fully closed when the temperature rises above 1000â.
2ï¼ãStarting of the furnace
If the furnace has been dried or needn't to be dried,then it may be started to raise temperature. In order to avoid that it is impacted by over current and the electric device is overloaded, the following steps should be adapted:
Small furnace (power<100KW)
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Large furnace (power100-500100KW)
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Furnace temp.
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Voltage
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Furnace temp.
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Voltage
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20-150â
|
1/3Working Volage
|
20-300â
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1/3Working Volage
|
150-500â
|
2/3 Working Volage
|
300-700â
|
2/3 Working Volage
|
500âWorking temp.
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Full Working Volage
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700â-Working temp.
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Full Working Volage
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3ï¼Replacing of element
If it is found that one element is damaged during iperating,firstly,you should determine where it is,at the same time prepare a made up one.Then loosen the thread which links the lead wire of the damaged element and the bus,clear out the ceramic cotton and pull out the through-brick.Afterward,insert the new wlwment from the furnace top,link the lead wire, block the gap with ceramic cotton and start raising temperature.