The resistivity of graphite electrode is an important physical performance index, which is usually measured by the voltage drop method. The resistivity can be used to measure the graphitization degree of graphite electrode. The lower the resistivity of graphite electrode is, the higher its thermal conductivity is, and the better its oxidation resistance is.
Allow current density of graphite electrode used related to its resistivity and the electrode diameter, the lower the resistivity of graphite electrode, allow the current density increase, but allow and electrode current density is inversely proportional to the size of the diameter, this is because the electrode diameter, the greater the electrode in the cross-section near the center and surface temperature increases, the resulting thermal stress increase will cause the electrode produce crack or surface spalling, so the increase of current density is limited.
Resistivity is a property of resistance to current flowing through a conductor, which is numerically equal to the resistance of a conductor of 1m in length and 1m2 in cross-sectional area at a certain temperature. Conductor resistance depends on material properties, length and cross-sectional area. For a conductor of the same kind, the resistance is proportional to the length of the conductor and inversely proportional to its cross-sectional area.
The resistivity calculation formula is as follows:
Rho = UA/IL (1)
Type of rho as the resistivity of a conductor, Ω m.; L is the distance between two probes of the voltage tester, m; A is the cross-sectional area of the conductor, m2; U is the voltage drop between the two probes of the voltage meter, V; I is the intensity of the current flowing through the conductor, A.
According to the above principle, the whole electrode resistivity and the sample resistivity were measured respectively in the carbon products, and the powder resistivity was measured in the carbon raw materials.
► Determination of the overall resistivity of graphite electrodes
Used for the determination of resistivity of graphitized blank electrode and finished electrode. The picture 1 is a schematic diagram of the overall electrode resistivity measurement. A pressure head leading the current at both ends of the electrode and a pair of probes on the surface of the contact electrode are installed on the insulated measuring rod, which are connected to the ammeter and voltmeter respectively. After being energized (dc power supply), the current intensity passing through the electrode and the voltage drop between the two probes are measured. Under the condition that the specified current intensity, probe spacing and electrode cross-sectional area of the sample electrode are fixed, the resistivity of the graphite electrode can be calculated by measuring the voltage drop between the probes multiplied by the coefficient under the above fixed conditions.
Picture 1 Measurement of overall electrode resistivity
1 -- Ammeter; 2 - DC power supply; 3 - Potentiometer; 4 -- Measured electrode
► Determination of resistivity of graphite electrode sample
After the sample is taken at the specified position of the electrode, it is processed into a 40mm±0.1mm square or 45mm×40 mm(±0.1mm) cylindrical sample. The side length or diameter is measured with caliper and the cross-sectional area of the cylindrical sample is calculated. Place the sample in the center of 49 ~ 59kN material testing table and press the specified compression direction. On the two end faces of the sample, a copper plate (or a copper net) with an insulating rubber pad, whose area is slightly larger than the end face of the sample, connects the positive and negative terminals of the DC power supply to the copper plate respectively. Start the material testing machine, slowly pressurize to 3.92mpa, make the steel plate and the sample in close contact, and adjust the input current to 5A. Two 20mm spaced probes connected to a 0.5 dc millivolt meter were pressed tightly on the sample along the direction of voltage drop. Three points were measured at different locations. The average value was taken and the resistivity was calculated according to formula (1). For rod and tubular samples, the resistivity was measured by microohmmeter, and the resistivity was calculated according to formula (2).
Rho = RA/L (2)
Type of rho for resistivity, Ω m.; R for microhm meter readings, Ω; A is the cross-sectional area of the sample; M2; L is the distance between the two probes of the voltage detector, m.
► Determination of electric resistivity of carbon powder
The sample was crushed to 0.3 ~ 0.5mm and then loaded into the insulated sleeve of the powder resistivity measuring instrument with an inner diameter of 16.3mm. Then the sample was loaded into the sleeve. There is a cylinder on each side of the tube, inserted into the tube, the section is slightly smaller than the inside diameter of the sample tube. The lower cylinder is fixed on the base of the instrument, and the upper cylinder can be moved up and down in the tube as a pillar for pressurizing the sample. Sample is located in between the upper and lower cylinder bushing tube, galvanometer and millivoltmeter is negative respectively to the two columns, rotating measuring sample height drum, driven by screw stabilizer bar lift, to the level of balance beam, the specimen under the pressure of 3.92 MPa, and open the current switch, adjust the current 300 mA, going into the sample and open millivoltmeter switch, read the millivolt values and the height of the specimen respectively, according to the formula (1) calculate the resistivity. The powder resistivity tester is shown in Picture 2.
Picture 2 Powder resistivity tester
1, 2 - Leverage; 3 - hanging ring; 4 - Measuring drum wheel; 5 -- Lifting lead screw; 6 - On the live column; 7 -- Sample tube; 8 - Directional ring; Bakelite casing; 9 -- Bakelite casing; 10 - sample; 11 -- Insulation board;12 -- Down the live column; 13 - base; 14 - Horizontal adjustment filaments; 15 - the handle; 16, 17 - Heavy hammer; 18 - Horizontal alignment; 19 -- Current voltage terminal