Understanding Graphite Electrical Resistivity

Graphite electrical resistivity is an amount of a physical quantity, indicating its conductivity. This method is the technique for assessing conductivity in different graphite materials effectively. The graphite electric resistivity pertains to the size of the resistance per unit length and is generally expressed in ohm-meters, short as Ω·m.

Graphite Electrical Resistivity Measurement

Four-probe method

Four-probe method is the most commonly used in the laboratory with high precision measurement method. Four probes, press the current on the sample to calculate the resistivity by measuring the voltage difference. The advantage of four-probe method requires avoiding the influence of contact resistance, the measurement accuracy is very high, and can be applied to the samples in the form of bulk and thin film.

Two-probe method

The two-probe method directly applies the current to both ends of the sample and measures voltage. Calculate the total resistance. The advantage of the two-probe method is that it is simple and easy to operate. In addition, the two-probe method also adapts to fast and low-demand measurement scenes.

Hot Wire Method

The hot wire method uses the current to heat the hot wire or coil inside the sample in order to measure characteristics of resistance changing by temperature. This method also studies the conductivity under a high-temperature condition of different materials.

High-temperature Four-probe Method

The resistivity under the conditions of a high-temperature environment can be tested by this method. The high-temperature four-probe method is combining the high-temperature furnace with a four-probe device for high-temperature performance evaluation.

Contact Resistance Method

The contact resistance method is mostly used for close laboratory studies. One of the accurate measurements used to test the resistance of a sample is the Wheatstone bridge balanced measurement circuit.

Temperature dependence analysis

Temperature dependence analysis can study the rule of changing resistance with temperature in graphite material, gain the stable and related conductive property of graphite, and offer data support for applying materials in high temperatures.

The following table shows the resistance of different graphite materials

Factors affecting graphite electrical resistivity

Material purity

The less impurities a material contains the lower is its resistivity

Grain size and orientation

Grain size

Graphite electrical resistance depends significantly on grain size and orientation. Larger-sized grains reduce the grain boundary scattering effect and allow more continuity of the conductive path, reducing the resistance;  conversely, smaller grains have increased grain boundaries resulting in more frequent electron scattering and thereby increasing the resistance.

Orientation

Graphite is an anisotropic material, with low resistance to the flow of electrons along the layer plane (a-b plane) and along low resistance. In contrast, its resistance increases significantly due to the van der Waals force acting perpendicular to the layer plane (c-axis). Therefore, larger the grain and the more close the grain orientation is to the current flowing direction, the lower would be the resistance of graphite.

Structural defects

Lattice defects

Point defects in graphite, like vacancies and impurity atoms, will destroy the complete π bond system of carbon atoms, block the free movement of electrons within a layer, and thus increase resistivity.

Grain boundary defects

The presence of grain boundaries increases electron scattering, hindering the flow of electrons across grains and leading to enhanced resistivity. Moreover, the larger the number of grain boundaries or the smaller the grain size, the more pronounced this effect becomes.

Interlayer defects

This dislocation, wrinkle, or gap between layers would reduce conductivity between graphite layers, making electrons flow more difficultly along the c-axis direction, thereby vertically increasing its resistivity considerably.

Porosity and cracks

Pores and cracks in the product make the effective conductive area of graphite lower, the path of current longer, leading to enhanced resistivity.

Temperature effects

It can be observed that as the calcination or graphitisation temperature increases, the specific resistivity of the product gradually decreases. The reasons for decreasing are different, however. During the roasting stage, the drop of the specific resistivity is mainly caused by releasing volatiles, coking of binders, and continuous shrinking of the product. During the graphitization stage, the drop in the specific resistivity was due to the transformation of amorphous carbon into graphite crystal structure.

External pressure

The external pressure increases the densification of the material by compressing the pores of graphite structure. The pressure also influences the arrangement of graphite crystal layers and decreases the resistivity in c-axis direction. In general, the external pressure is reflected in reducing porosity, improving grain connection, and enhancing interlayer arrangement.

Comparison of the electrical conductivity of graphite and copper

Under normal temperature applications, copper has higher conductivity than graphite; however, under high temperature applications, graphite still maintains a higher conductivity than copper.

Conclusion

Resistivity had a great effect on graphite electrical property. Electrical resistivity is one of the critical factors that determines the electrical property of graphite. The smaller the conductivity of graphite, the better its conductivity and the lower the energy consumption.