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Thermal Conductivity And Linear Expansion

Release time:2019-11-04 From: Zhengnai Industry in China

Two methods for determining thermal conductivity

There are various methods for determining the thermal conductivity of a graphite electrode, such as a steady state heat flow method and a laser pulse method. The medium-temperature thermal conductivity of the graphite electrode measured at 100~800 °C was determined by direct energization longitudinal steady-state heat flow method.


 

Principle of measurement by steady state heat flow method

The principle of measurement is that when the cylindrical sample passes direct current, the heat generated is mainly transmitted along the longitudinal axis of the sample. After reaching the thermal stability state, the sample is a one-dimensional longitudinal heat flow, which corrects the heat exchange between the sample and the side environment.
 

Method for measuring laser pulse method

The laser pulse method uses a pulsed laser as a heat source to direct it to one side of a thermally insulated sheet sample at a certain temperature, so that the exposed surface is instantaneously heated, heat is transferred from the heated surface to the other side, and the infrared receiver receives the back surface temperature of the sample. The change is performed by using a high-speed photosensitive paper to record the time-temperature dynamic curve when the laser pulse is emitted to the highest temperature on the back side of the sample, and the thermal diffusivity of the sample at the temperature is calculated from the time-temperature dynamic curve.

 

Size of  UHP Graphite electrode

Item Unit UHP
300-500mm 550-600mm 700-800mm
Electric resistivity Electrode μΩm 4.6-6.2 4.5-6.0 4.5-5.8
Nipple 3.5-5.0 3.4-4.5 3.3-4.3
Bending strength Electrode Mpa ≥10.5 ≥10.0 ≥10.0
Nipple ≥20.0 ≥22.0 ≥23.0
Elastic modulus Electrode Gpa ≤14.0 ≤14.0 ≤14.0
Nipple ≤20.0 ≤22.0 ≤22.0
Bulk density Electrode g/cm3 1.66-1.73 1.68-1.74
Nipple 1.75-1.82 1.78-1.85
CTE(100-600℃) Electrode 10-6/℃ 1.20-1.50 1.10-1.50
Nipple 1.10-1.40 1.00-1.30
Ash % ≤0.3

 

Three factors affecting the thermal conductivity of graphite electrodes

The thermal conductivity of graphite electrodes increases with increasing density.
The thermal conductivity of the graphite electrode decreases as the porosity of the material increases.
The thermal conductivity of graphite electrode has a certain correspondence with its electrical resistivity.
 
 

Definition of coefficient of linear expansion

The coefficient of linear expansion refers to a measure of the degree of expansion of a material after it has been heated. That is, when the temperature rises by 1 ° C, the expansion ratio constant of the solid material sample per unit length in a certain direction is called the linear expansion coefficient along the direction.
 

Relationship between linear expansion coefficient and measured temperature

The linear expansion coefficient of graphite materials increases with the increase of the measured temperature, but there are certain regularities, some adopt 30~100 °C, and some adopt 30~130 °C.


 

4 factors affecting the coefficient of linear expansion

The lower the linear expansion coefficient of the raw material, the smaller the linear expansion coefficient of the graphite electrode prepared under the same process conditions.
The linear expansion coefficient of the finished graphite electrode is related to the particle size composition of the formulation.
It is related to whether it is subjected to immersion treatment.
The graphite electrode with a higher degree of graphitization has a smaller linear expansion coefficient.
 

The direction of extrusion is related to the coefficient of linear expansion

The particle size range of the coke particles decreases from large to large, and the linear expansion coefficient parallel to the extrusion direction decreases, but the linear expansion coefficient perpendicular to the extrusion direction increases.


 

Graphite electrode with low coefficient of linear expansion

A graphite electrode using needle coke as a raw material, a high-quality graphite electrode produced using needle coke as a raw material, has a linear expansion coefficient much lower than that of a conventional power graphite electrode using ordinary petroleum coke as a raw material.
 

A sharp change in temperature causes damage to the graphite electrode

The existence of linear expansion property is the root cause of the destruction of graphite electrode under the violent fluctuation of temperature. When the temperature is sharply cooled, the electrode surface will generate tensile stress due to shrinkage. When the generated stress exceeds the ultimate mechanical strength of the electrode body or nipple, It can cause cracking or breaking, which is the essence of the destruction of the graphite electrode when the temperature changes drastically.
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