Liu Xiaoyan 1, Ma Wenli 1, Zhang Yumei 1, Yuan Jiao 1, Li Tianyu 1, Zuo Junqing 2,3
(1. School of mechanics and materials, Hehai University, Nanjing 210098; 2. Shanghai Construction Engineering Group Co., Ltd., 200080; 3. Shanghai high performance concrete engineering technology research center, 201114)
Abstract: the influence of bismuth telluride (Bi2Te3) incorporation on the thermoelectric properties of carbon fiber (CF) cement-based composite was studied, and the morphology and distribution of the reinforced components of Bi2Te3 and CF were observed. The results show that the thermoelectric properties of CF cement-based composite can be improved by adding Bi2Te3. The effect of gradient mixing is better than that of adding Bi2Te3, and the effect of gradient layer mixing is the best. When the content of Bi2Te3 is 0.5%, the Seebeck coefficient of gradient layer mixing reaches the optimal value of 32.7 μ V / ℃.
Keywords: bismuth telluride; carbon fiber cement-based composite; blending; gradient blending; gradient layer blending; thermoelectric properties
0 foreword
As a kind of green multi-functional material, carbon fiber (hereinafter referred to as CF) is widely used in intelligent concrete due to its excellent physical and mechanical properties, temperature sensitivity and conductivity, but it has limitations in thermoelectric properties. Bismuth telluride (hereinafter referred to as Bi2Te3) is the earliest and most perfect thermoelectric material developed, and also the material with the highest thermoelectric merit value at present. Its compound is a hexahedral layered structure, which can be used to improve the thermoelectric performance of the material.
The good thermoelectric property of the material means that its excellent value coefficient ZT is high. Because of the complex composition of cement-based materials, it is difficult to increase its ZT value. In order to improve the thermoelectric properties of cement-based materials, Yaowu et al. Added Bi2Te3 as coating into CF cement-based materials, which increased the thermoelectric potential rate by 260%; Han et al. Used Bi2Te3 powder as interlayer material of epoxy resin sheet, and added Bi2Te3 powder into continuous CF epoxy resin matrix, which significantly improved the thermoelectric potential rate of interlayer interface and thickness direction.
In this paper, Bi2Te3 is added into CF cement-based composite as a thermoelectric reinforcement component, and bi2te3-cf cement-based composite is prepared by the way of blending, gradient blending and gradient layer blending. The influence of Bi2Te3 content and blending mode on its conductivity and thermoelectric performance is studied, in order to provide reference for improving the thermoelectric performance of cement-based materials.
1 test overview
1.1 raw materials and sample preparation
PAN based 5 m m short cut CF is used, cellulose methyl ether is used as CF dispersant; P · o 42.5 grade cement is used; Bi2Te3 powder size is 40-100 μ m, purity is 99.98%; test water is deionized water; silica fume is provided by Sichuan langtian resources comprehensive utilization Co., Ltd.
11 groups of specimens were made. The size of the specimens was 50 m m × 50 mm × 50 mm. The mixture ratio was m (cement): m (silica fume): m (water): 1:0.1:0.46. The content of CF and CME was 0.4% of the cement quality.
Due to the different effects of different mixing methods on the thermoelectric properties, three thermoelectric models are established as shown in Figure 1: ① the content of Bi2Te3 in the whole mixed samples W1, W2, W3, W4 and W5 is 0.05%, 0.15%, 0.25%, 0.45% and 0.55% respectively; ② the content of Bi2Te3 in each layer of CF cement paste decreases from the hot end to the cold end gradient; ③ When GL1 (3 layers), Gl2 (4 layers) and GL3 (5 layers) are added in the gradient layer, the content of Bi2Te3 in the interlayer of CF cement paste decreases from the hot end to the cold end. See Table 1 for the distribution of Bi2Te3 content in the gradient blending and gradient layer blending. M / N in the table refers to the interlayer between the M and N layers of CF cement paste. Bi2Te3 is mixed in the interlayer, and the layer height of each layer of cement paste is the same. Refer to the literature for the sample preparation process. 　
1.2 test method
The thermoelectric conversion efficiency of the material mainly depends on the optimal coefficient ZT, ZT = S2 κ T / λ, where s is the Seebeck coefficient (SS is the Seebeck coefficient when the temperature rises, SJ is the Seebeck coefficient when the temperature falls), K conductivity, t is the absolute temperature, λ is the thermal conductivity, S2 κ is the power factor.
Determination of Seebeck coefficient: put one end of the test piece on the temperature controlled water bath heating pot for heating, and put the other end at room temperature, and measure the temperature difference at both ends with K-type temperature thermocouple. The temperature difference electromotive force is measured by fluck289 precision digital display multimeter. The calculation formula of Seebeck coefficient is shown in formula (1).
Where: Δ ETB is the electromotive force of temperature difference; Δ t is the temperature difference, which is recorded every 2 ℃ and terminated when the test reaches 18 ℃.
The absolute thermoelectric rate of the specimen is the sum of Seebeck coefficient of the specimen and the thermoelectric rate of copper (+ 1.83 μ V / ℃).
Resistivity test: quadrupole method is adopted for the whole blending test piece, and two pole method is adopted for the gradient whole blending test piece and gradient layer blending test piece. The power supply is rps-3003db type power supply. In the quadrupole method, a fluck289 precision digital display multimeter is used to measure the voltage UBC at both ends of the inner electrode of the test piece, and another multimeter is connected with the outer electrode to test the current I of the test piece. The calculation formula of resistivity ρ quadrupole method is as follows:
In the two pole method, adjust the multimeter to the resistance gear, and directly measure the resistance R of the external electrode b1c1. The calculation formula of resistivity ρ two pole method is as follows:
Where: A is the cross-sectional area perpendicular to the current passing direction; LBC (lb1c1) is the distance between B (B1) electrode and C (C1) electrode.
2 results and discussion
2.1 microstructure of bi2te3-cf cement-based composite
Fig. 2 is a three-dimensional video microscope (3-dvm) image of three kinds of CF cement-based composite materials mixed with Bi2Te3. The sequin particle indicated by the arrow in the figure is Bi2Te3. From Figure 2 (a), it can be seen that Bi2Te3 is densely distributed in the whole mixing test piece, and the long rod CF and Bi2Te3 are overlapped with each other and evenly distributed in the cement matrix. It can be seen from Fig. 2 (b) that the number of Bi2Te3 particles measured under the interlayer boundary is significantly more than that measured on the upper side, the long rod CF is indistinctly visible at the interlayer interface, and the CF is overlapped at the interlayer. It can be seen from Fig. 2 (c) that Bi2Te3 particles are obviously concentrated in the coating layer, and long rod CF and Bi2Te3 particles overlap each other. In conclusion, among the three kinds of Bi2Te3 incorporation modes, Bi2Te3 and CF can form a multi-scale conductive network in space, which can effectively improve the conductivity of the composite; in the gradient layer blending and gradient blending test pieces, the CF at the interlayer interface enhances the overall connectivity of the matrix, so that it does not reduce the conductivity due to delamination.
2.2 electrical conductivity and thermoelectric effect of bi2te3-cf cement-based composite
Table 2 shows Seebeck coefficient, absolute temperature difference EMF and resistivity of each group of test pieces. It can be seen from table 2 that the absolute temperature difference EMF of each group of test pieces is positive, indicating that the CF cement-based composite with Bi2Te3 is a p-type thermoelectric material with hole as the main carrier. The Seebeck coefficient increases with the increase of the content of Bi2Te3 and the resistivity decreases with the increase of the content of Bi2Te3. When the content of Bi2Te3 is 0.45%, the SS of Gl2 is 39% higher than that of G1 and 122% higher than that of W4. The results show that increasing the content of Bi2Te3 can improve the Seebeck coefficient and reduce the resistivity of the specimen in the blending mode; among the three blending modes of Bi2Te3, the gradient layer blending has the most significant effect on the Seebeck coefficient of the specimen, the gradient blending takes the second place, and the blending is the weakest.
Fig. 3 shows the relationship between thermoelectromotive force and temperature difference of different Bi2Te3 content in the whole mixing sample. It can be seen from the figure that the temperature difference electromotive force of the test piece changes in the same direction with the change of the temperature difference. The Seebeck coefficient of the sample increases with the increase of the content of Bi2Te3, because the increase of the content of Bi2Te3 increases the concentration of the hole carrier, which makes up for the deficiency of the single enhancement system of CF network. At the same time, with the increase of the content of Bi2Te3, the relationship curve between the thermoelectric force and the temperature difference is more and more close, which shows that the increase of the content of Bi2Te3 can enhance the consistency of the relationship curve between the thermoelectric force and the temperature difference, because the increase of the content of Bi2Te3 can increase the concentration of the hole carrier, reduce the blocking energy of the hole carrier transition, and make it more Easy to conduct.
Figure 4 shows the relationship between the temperature difference electromotive force and the temperature difference of the gradient blending test piece. It can be seen from the figure that the Seebeck coefficient of the test piece increases with the increase of the number of layers and the content of Bi2Te3, and the SS ratio of G3 increases by 55% compared with G1; the curves of temperature difference EMF and temperature difference in G1 rising and cooling stage are staggered, while the curves of G3 are approximately parallel, and the curves of temperature difference EMF and temperature difference in G3 rising and falling stage are closest. It is shown that increasing the content of Bi2Te3 and the number of layers is not only beneficial to the improvement of Seebeck coefficient, but also to the reversibility of the relationship between thermoelectromotive force and temperature difference.
Fig. 5 shows the relationship between the temperature difference electromotive force and the temperature of the sample with gradient layer. It can be seen from the figure that with the increase of Bi2Te3 content and layers, the Seebeck coefficient of the test piece increases. When the content of Bi2Te3 is 0.5% (GL3), SJ reaches the optimal value of 32.7 μ V / ℃; the curve of EMF and temperature difference of the temperature rise and fall stage of the test piece also fits with the increase of Bi2Te3 content and layers. It is shown that increasing the content of Bi2Te3 and the number of layers is not only beneficial to the improvement of Seebeck coefficient, but also to the reversibility of the relationship curve between thermoelectromotive force and temperature difference.
Fig. 6 shows the influence of Bi2Te3 mixing mode on thermoelectric power ratio of bi2te3-cf cement-based composite. It can be seen from the figure that: ① the seekeck coefficient of each group of test pieces increases with the increase of the content of Bi2Te3, and the relationship between them is basically linear; ② when the content of Bi2Te3 (0.45%) is the same, comparing G1 and W4, it can be found that the SS and SJ of G1 are higher than the latter, which shows that the thermoelectric performance of gradient blending test piece is better than that of blending test piece, because the content of Bi2Te3 between layers decreases from high temperature end to low temperature end In the case of small and high content of Bi2Te3, more hole carriers are generated under the action of large temperature difference, which optimizes the thermoelectric effect and carries the conduction carriers directly by the tunnel effect or cf. therefore, the thermoelectric performance of gradient blending is better than that of blending under the same content of Bi2Te3. Comparing Gl2, G1 and W4, we can find that the Seebeck coefficient of Gl2 is the highest, followed by G1, and W4 is the last, At the end of W4, it shows that the reversibility of the curve is the best, followed by G1, W4 is weak, which shows that the gradient layer doping has the best effect on improving the thermoelectric properties of the samples, and the gradient layer doping is the second, and the whole doping is the worst. The reason is that when the content of Bi2Te3 decreases from the hot end to the cold end gradient, and the intercalation is spread, the dimension of Bi2Te3 decreases from three-dimensional to two-dimensional, which enhances the response speed of thermoelectric components, and the two-dimensional plane structure has the quantum well effect on the transmission of Bi2Te3 carriers, which increases the scattering factor Finally, improve the thermoelectric potential rate.
In conclusion, there is a positive correlation between the content of Bi2Te3 and the thermoelectric strengthening effect of CF cement-based composite; the order of Bi2Te3 adding mode to the thermoelectric strengthening effect of CF cement-based composite is: gradient layer mixing > Gradient integral mixing > integral mixing.
Table 3 shows the power factors of each group of test pieces during temperature rise and fall. It can be seen from table 3 that with the increase of Bi2Te3 content, the power factor increases; when the same Bi2Te3 content, the power factor of the gradient layer is the largest, the gradient integral is the second, and the integral is the smallest. The reason is that increasing the content of Bi2Te3 is beneficial to increase the concentration of hole carrier, decrease the resistivity, increase the seekeck coefficient, and then increase the power factor. Compared with the whole doping, the gradient whole doping improves the power factor of the material by optimizing the utilization efficiency of Bi2Te3; the gradient layer doping improves the thermoelectric conversion efficiency by reducing the dimension of Bi2Te3, and increases the seekeck coefficient through the quantum well effect, thus improving the material