Thermoelectric Properties Of Carbon Fiber Reinforced Cement Matrix Composites

Carbon fiber reinforced cement matrix composites(CFRC), which acted as a new intelligent construction material, has a broad prospect of applications in the collection of urban and industrial thermal energy conversion, health monitoring sensor for important architectural structure and heat melt snow removal with low power consumption, etc. Now, the Seebeck coefficient of CFRC is still low, its thermoelectric performance is not high enough, which became the main factors restricting its application. So, in this paper, aimed at how to improve the Seebeck coefficient of CFRC, optimization approaches were explored by doping metal oxide, adjusting metal oxide/substrate interface, adjusting carbon fiber/matrix interface and ordering Cu fibers. Finally, the strengthening mechanism of CFRC-Seebeck effect was revealed.(1) The effect of metal oxide and matrix content on CFRC-Seebeck coefficient was investigated. Bi2O3 and Fe2O3 contents were 0.5wt%, 1.0wt%, 2.5wt%, 5.0wt%, 7.5wt%, 10.0wt%, 12.5wt% and 15.0wt%. The results showed that: when the doping content was in the range of 0.5-5.0wt%, with the increase of oxide content, the measured thermoelectric power of CFRC was increased. The Seebeck coefficient of CFRC was improved by 4-5 times compared to the original sample. When the content of Bi2O3 was 5.0wt%, the maximum absolute value of CFRC-Seebeck coefficient could reach +100.28μV/℃. When the doping content was in the range of 7.5 wt%-15.0 wt%, the thermopower increased with content firstly, then tended to stable.(2) The effect of metal oxide/substrate interface density(characterized by oxide particle size) and the air/substrate interface density(characterized by water-cement ratio) on CFRC-Seebeck coefficient and electrical conductivity was investigated. The selected Fe2O3 particle sizes were(50-200nm),(200-500nm) and(1-10μm), Fe3O4 particle sizes were(50-150nm),(150-250nm) and(250-500nm). The results showed that: when the Fe2O3 particle size was in the range of 50-200 nm and 200-500 nm, with the increase of particle size, the measured conductivity of CFRC was increased.However, its Seebeck coefficient was decreased, the power factor was almost constant. When the Fe3O4 particle sizes were 50-150 nm, 150-250 nm, and 250-500 nm, with the increase of particle size, the measured value of CFRC-Seebeck coefficient and power factor were decreased. Furthermore, at low temperatures, the CFRC conductivity was relatively high with larger particle size; at high temperatures, the CFRC conductivity was relatively high with smaller particle size.When the cement ratios were 2.7:10 and 2:5, Fe2O3 and Fe3O4 particles were added, the results showed that: Compared with the cement ratio of 2.7:10, the value of CFRC-Seebeck coefficient and electrical conductivity of sample with the cement ratio of 2:5 were much smaller(an order of magnitude difference between the conductivities). Small cement ratio was beneficial to improve CFRC-Seebeck coefficient and electrical conductivity.(3) The effect of carbon fiber/matrix interface density(characterized by a carbon fiber surface roughness) on CFRC-Seebeck coefficient and electrical conductivity was investigated. Carbon fiber was impregnated in phenolic resin with the concentrations of 0.5wt%, 1.0wt%, 3.0wt% and 5.0wt%. With the increase of impregnated concentration, the surface roughness was increased. The results showed that: when the dipping concentration was within 1.0-5.0wt% range, with the increase of carbon fiber surface roughness, the electrical conductivity and Seebeck coefficient of CFRC were increased; In the Fe3O4-containing CFRC samples, the maximum CFRC-Seebeck coefficient, which was + 123.5m V/℃, was attained with the largest surface roughness(immersion concentration of 5.0wt%). With the same surface roughness,the conductivity and Seebeck coefficient of CFRC added Fe3O4 were greater than samples containing Fe2O3.(4) The effect of Cu fiber/matrix interface density of CFRC-Seebeck coefficient was investigated. The content of carbon fiber was 0.5wt% and 1.0wt% in CFRC samples, Cu fiber with diameter of 0.5mm were evenly arranged with an aligned orientation/The volume fraction of fiber were 0.125vol%, 0.25vol% and 0.5vol%, respectively. The results showed that: In the same carbon fiber content, the electrical conductivity and Seebeck coefficient of CFRC samples with 0.25vol% Cu fiber were almost twice as that of CFRC samples with 0.125 vol %and 0.5vol% Cu fiber.When the carbon fiber was 1.0wt%, and the copper fiber content was 0.25vol%, the CFRC conductivity and Seebeck coefficient was maximized. This showed that with the increase of the content of Cu fiber, CFRC conductivity and Seebeck coefficient were significantly increased. In addition, with the same content of Cu fiber, the conductivity and Seebeck coefficient of CFRC samples with 1.0wt% carbon fiber were higher than that of samples with 0.5wt% carbon fiber, which indicated that when the carbon fiber content was in the range of 0.5-1.0wt%, with the increase of carbon fiber content, the conductivity and Seebeck coefficient of CFRC were also increased.