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Volume resistivity test method
The test uses surface probe electrodes to measure the current and voltage through the sample to obtain its resistance value or directly measure its resistance value, and calculate the volume resistivity of the sample according to formula (3).
Laboratory environment requirements:
a) Ambient temperature: 20℃±2℃;
b) The relative humidity of the environment: not more than 65%.
The test equipment includes an electrode device, resistance measuring equipment and a load block that provides appropriate electrode contact pressure.
The schematic diagram of the electrode device is shown in Figure 4.
The electrode device contains two insulating parts (high modulus rubber, polyethylene, polystyrene, etc.), and its volume resistivity should be greater than 1.0×1013Ω·m. Install a pair of current electrodes and a pair of voltage electrodes on one of the insulating parts. The electrodes should be parallel to each other, and the tops of all electrodes should be on a horizontal plane. Install another pair of current electrodes on another piece of insulating material at positions corresponding to the current electrodes, so that the two pairs of current electrodes can be superimposed on the same position of the sample. The length of the current electrode should be at least 10.0mm and greater than the width of the sample; the width should be between 5.0mm and 8.0mm; the height should be between 10.0mm and 15.0mm, and the height deviation of the four electrodes should not exceed 0.05mm. The voltage electrode has the same length and height as the current electrode, but the top surface is tapered to a rounded corner with a radius of no more than 0.5mm. The distance between voltage electrodes is 10.0mm~66.0mm, and the accuracy is ±2%. The current electrodes are symmetrically distributed outside the voltage electrodes, and the distance between the current electrodes and the voltage electrodes is not less than 20.0mm. The electrodes are made of non-corrosive materials, such as brass, nickel, stainless steel, etc. The insulation resistance between the electrodes should be greater than 1.0×1012Ω·m.
A—The load block applied to the contact surface of the current electrode and the sample (300N/m times the width of the sample);
B—The load block applied to the contact surface of the voltage electrode and the sample (60N/m times the width of the sample);
F—The distance between voltage electrode and current electrode (≥20.0mm)
G—voltage electrode spacing;
H—Current electrode width (5.0mm~8.0mm);
Figure 4 Schematic diagram of electrode device
Resistance test equipment
The resistance can be measured by any method of measuring the current between the current electrodes and the voltage between the voltage electrodes, and the measurement accuracy should not be less than ±2%.
A resistance meter with an accuracy of 1 milliohm is preferred to measure resistance.
You can also choose to measure the voltage and current separately to calculate the resistance.
Current measurement can use: (1) a precision milliampere meter; (2) or by measuring the voltage of the reference resistance (the difference between the resistance value and the current electrode resistance value is within 2%) to determine the current.
Voltage measurement can use: (1) a galvanometer with a sensitivity of 1μA (or a galvanometer with a sensitivity lower than the minimum scale in the zero voltage loop); (2) an electrostatic voltmeter with a DC resistance greater than 19TΩ; or (3) An electrometer (such as a multi-range voltmeter with a DC input impedance greater than 0.1TΩ) can also be used.
In any case, the current value through the voltage electrode cannot be greater than 1% of the current value through the current electrode during measurement. The test should use a stable and adjustable DC power supply to ensure that the energy loss of the sample between the voltage electrodes is about 0.1W.
Load block that provides electrode contact pressure
Choose an appropriate load block so that the sample has a uniform contact pressure with the electrode over the entire width. The contact pressure between the current electrode and the sample is 300N/m, and the contact pressure between the voltage electrode and the sample is 60N/m.
Three specimens were used for the test.
The sample size is: length 76.2mm±0.2mm, width 12.7mm±0.2mm, thickness (1.40~3.05) mm±0.25mm.
The surface of the sample should be smooth, flat and free of burrs.
The test shall be adjusted for at least 16h in an environment with a humidity of not more than 65% at 20℃±2℃.
Connect the circuit according to the principle of Figure 5.
Figure 5 Schematic diagram of circuit principle
Place the electrode on the sample to be tested. The sample to be tested should be in full contact with the electrode across the entire width to ensure that the probe weight is evenly distributed on the sample to be tested. After powering on for 30s, read and record the resistance or voltage and current values of the milliohmmeter.
The volume resistivity is calculated according to formula (3):
—Volume resistivity, the unit is ohm?cm (Ω?cm);
R—The measured resistance value in ohm (Ω);
A—The cross-sectional area of the sample, in square centimeters (cm2);
L—The distance between electrodes, in centimeters (cm).
The resistance value can be measured directly or calculated by the ratio of the voltage and current measured in the experiment.
The test probe should be the two-point probe shown in Figure 5. If the sample to be tested is too small to be tested with the above-mentioned electrodes, electrodes with an electrode spacing of 12.7mm should be used for measurement. However, when calculating the volume resistivity, The value of L should be 12.7mm. For the smaller part, the width and spacing of the electrodes should be reduced accordingly, and the arc length should be selected when calculating the volume resistivity. Each electrode should be in contact with the sample. If it is difficult or impossible to test by the above method due to the cross-sectional area of the sample to be tested, a sample with a width of 12.7mm must be produced under the same process conditions for testing.
Take the median of the test values of the three samples as the test result.