Physical and chemical indicators of carbon materials ：Electrical and magnetic properties

Electrical and magnetic properties of carbon graphite materials:

A. Electrical properties of carbon graphite materials:

Graphite is the best conductor in non-metallic materials, and its conductivity is close to that of some metals. This is related to the microscopic arrangement of the atoms of the graphite crystal. The conductivity of graphite has obvious anisotropy, and the anisotropy ratio of natural flake graphite and pyrolytic graphite can be as high as 10,000. The anisotropy ratio of artificial graphite products is only 1.2-1.4. The conductivity of various carbon graphite materials is different, the degree of graphitization is high, the layer arrangement is nearly parallel, and the crystal defects are few, which is favorable for the flow of free electrons, so the resistivity is low.

The electrical resistivity of the graphite material can be measured by grinding the product into small samples or directly measured by the entire product.

When measuring the entire product directly, follow the following two formulas:

R=U/I R=ρL/S

Then U/I=ρL/S ρ =US/IL

R is the resistance in ohms (Ω); U is the voltage in volts (V); I is the current in amps (A); ρ is the resistivity in microohms per meter (μΩ·m); S is the cross-sectional area of ​​the material in meters (m2); L is the length of the material in meters (m).

1Ω=1000mΩ=1000000μΩ

1 V = 1000m V = 1000000μV

1 A = 1000m A = 1000000μA

In the process of measuring the whole product, the cross-sectional area of ​​the material is constant, the output current of the resistivity meter is constant, and the length of the material, that is, the length of the voltage drop is fixed, then S/IL is a constant, as long as it is read. When the voltage U value is output, the resistivity ρ can be calculated.

B. Magnetic properties of carbon graphite materials:

The magnetic properties of carbon graphite materials mainly include magnetic susceptibility, average diamagnetic susceptibility, and magnetoresistance. The magnetic field strength generated by magnetization of the carbon graphite material is opposite to the direction of the magnetic field strength, so it is a diamagnetic material and its magnetic susceptibility (X) is a negative value. The magnetic susceptibility of most carbon graphite materials exhibits significant anisotropy. The average diamagnetic susceptibility of various carbon graphite materials at different temperatures is related to the crystallite size. Therefore, determining the diamagnetic susceptibility of graphite materials is a method to study the degree of development of graphite crystals. The magnetic resistance is closely related to the heat treatment temperature of the carbon graphite material. When the heat treatment temperature is below 2400 °C, the magnetic resistance of the carbon graphite material is negative. When the heat treatment temperature is above 2400 °C, the magnetic resistance of the carbon graphite material increases linearly. Therefore, magnetoresistance is one of the most sensitive indicators for evaluating the degree of graphitization.