3.5 Conductivity
Carbon fiber and graphite are both resistive loss absorbing materials which mainly rely on their electrical conductivity and dielectric constant, and absorb electromagnetic waves by forming a conductive path inside. The conductivity of cement-based materials without conductive fillers is negligible, and its microwave absorbing performance is also poor. There is a percolation threshold for the content of conductive filler in cement-based materials. When the content reaches the threshold, the conductivity of the material is significantly improved. When the content exceeds the threshold, the conductivity does not increase significantly and the increase speed gradually slows down to zero[36]. In order to explore the effects of different contents of carbon fiber and graphite on the conductivity of foam concrete and to provide reference for subsequent electromagnetic parameter tests, the resistivity and conductivity of foam concrete are tested and calculated. Figure 12 shows the conductivity and resistivity of carbon fiber foam concrete.
As demonstrated in Figure 12, group O is the least conductive (only 0.00275 S/m). The conductivity of group C1 with 0.3wt.% carbon fiber increases to 0.00353 S/m, which is due to the tunnel conduction effect[37]. At this time, the conductive effect of carbon fiber gradually improves the internal conductive network of the specimen. As the carbon fiber content rises, the resistivity of group C2 drops to 169.73 Ω∙m. The conductivity is 0.00590 S/m which is 114.55% higher than that of group O. Since then, the change in carbon fiber content has little effect on the conductivity, and the conductivity of group C3 with 1.2wt.% carbon fiber is 7.29% higher than that of group C2. It can be assumed that the percolation threshold is reached at 0.6 wt.% of carbon fiber doping.
The resistivity and conductivity of carbon fiber-graphite foam concrete are displayed in Figure 13. The conductivity of foam concrete increases slowly as the graphite content increases. The conductivity of groups C2S1, C2S2, and C2S3 rises 122.91%, 135.64%, and 143.64%, respectively, compared with that of group O. Therefore, the change in graphite content has little effect on conductivity during the adding of carbon fiber and graphite. And the percolation threshold is reached for the groups with the two absorbents when the carbon fiber content is 0.6wt.% and the graphite content is 2wt.%. From the above discussion, it is known that the conductivity of group C2 increases by 114.55% compared to that of group O. Therefore, adding 0 to 10wt.% graphite will not significantly increase the resistivity of carbon fiber foam concrete.