Synthesis And Thermoelectric Properties Of Typical Oxide And Clathrate Thermoelectric Materials

Thermoelectric materials, which can convert heat and electricity into each other, have great prospect to be applied as environmental friendly functional materials. In this paper, three different material systems, including p-type oxide semiconductor Ca3Co4O9+δ, p-type oxide semiconductor CaMnO3 and I-type clathrate Ba8Cu4Si42, were investigated. The main purpose of this study is to improve thermoelectric properties of the materials by microstructural optimization and doping modification. The phase compositions, microstructures and thermoelectric properties of the materials were characterized, and their relationships were discussed. The main achievements are listed as follows:Ca3Co4O9+δ flak powders were prepared by the solid state reaction method. The powders possess good morphology and uniform size distribution with a diameter of about 2 μm and thickness of about 200 nm. The Ca3Co4O9+δ samples were fabricated by hot pressing, with a relative density higher than 90%. The samples are anisotropic in grain orientation, with a highly preferred orientation of (010). The degree of texture increases with increasing hot-pressing time. Lotgering factor of the sample obtained with a pressure of 80 MPa at 850 ℃for 40 min is 0.84. Both electrical transport and thermal transport show anisotropic characters. The electric conductivity, thermal conductivity and ZT value are higher in the direction perpendicular to the pressing direction. At 600℃, ZT value of the hot-pressed sample in the direction perpendicular to the pressing direction reaches 0.180.A sol-gel method combined with a calcined process was used to synthesize Ca3Co4O9+δ flak powders. The obtained powder, about 500 nm in diameter and 100 nm in thickness, has a uniform size distribution. The effects of doping ions with various valences in Ca-site on thermoelectric properties were studied. The increase in hole carrier concentration due to the Na+ ions doped in the Ca3Co4O9+δ enhances the electric conductivity and decreases the Seebeck coefficient. Compared with Ca2+, Sr2+ has a larger radius, stronger activity and higher molar conductivity. As a result, the electrical conductivity of (Ca1-xSrx)3Co4O9+δ increases with the doping content of Sr2+. Carrier migration mechanism is determined by the change of Co-Co bond length which is influenced by the Sr2+ doping ions. There is no obvious change in carrier concentration in (Ca1-xSrx)3Co4O9+δ, so there is little change in the Seebeck coefficient. The replacement of divalent Ca2+ bytrivalent Eu3+ in (Ca1-xEux)3Co4O9+δ results in a reduction in hole carrier concentration, which decreases electric conductivity and increases Seebeck coefficient. Thermal conductivity of the Ca3Co4O9+δ was reduced due to the increment in phonon scattering ability by M ions doping in Ca-site. At 600℃, the ZT values of (Ca0.93Na0.07)3Co4O9+δ, (Ca0.95Sr0.05)3Co4O9+δ and (Ca0.93Eu0.07)3Co4O9+δ are 0.086,0.087 and 0.090, respectively.CaMnO3 powders with a particle size below 50 nm were synthesized by a co-precipitation method using ammonium hydrocarbonate as the precipitant. With the increase of Sm3+ doping concentration, the size of Ca1-xSmxMnO3 powders increases and particle dispersion improves. The relative density of the sample sintered at 1300℃ reaches 96%. DTA-TG curves of the precursor were measured at four different heating rates in oxygen atmosphere. The apparent activation energies corresponding to the three steps was calculated using both Doyle-Ozawa and Kissinger methods, which are 20.305,71.124 and 361.068 kJ·mol-1, respectively. The effects of doping ions in Ca-site on thermoelectric properties were studied. Sm3+ ions doped in the CaMnO3 change the length of Mn-O bond lengths and Mn-O-Mn bond angles, widening the bandwidth of the conduction band and leading to an increase in electrical conductivity. The absolute value of Seebeck coefficient of Ca1-xSmxMnO3 decreases with increasing the carrier concentration. Moreover, thermal conductivity of Ca1-xSmxMnO3 decreases with the Sm3+ doping. Sr2+ doping in the Cao.95Smo.o5Mn03 increases the electrical conductivity and thermal conductivity in different extent and decreases the absolute value of Seebeck coefficient. At 600℃, the ZT values of Cao.95Sm0.05MnO3 and Ca0.86Sr0.09Sm0.05MnO3 are 0.080 and 0.098, respectively.Porous Ca3Co4O9+δ ceramics with parallel sheet shaped pores were prepared by a template sacrifice method using epispastic polystyrend (EPS) hollow spheres as the templates. The porosity was controlled by changing the mass ratio of EPS spheres to Ca3Co4O9 powders. The existence of sheet shaped pores raises the ratio of electrical conductivity to thermoelectric conductivity, and thus improves the ZT value consequently. At 500℃, the ZT value of the porous sample obtained with 10% of the ratio of EPS spheres to Ca3Co4O9+δ powders is 0.049. The preparation of porous thermoelectric ceramics with parallel sheet shaped pores was realized for the first time by this study, which provide a new idea for increasing the rate of electrical conductivity to thermal conductivity of the porous thermoelectric materials.The Ba8Cu4Si42 clathrate material was prepared by a melting method in an electric arc furnace. Detectable levels of impurity phases (Si phase or/and LaFe9Si4 phase) exist in the powder samples obtained according to different proportions. The sample was prepared by annealing, ball milling and hot pressing, with a relativity density of 0.95 and a microstructure of uniform dispersion of tiny second phase in the clathrates phase. Different processing conditions resulted in different microstructure of the samples, affecting the thermoelectric merit value in different aspects. The ZT value of the sample is 0.257, which is of the best ZT value compared with that of the sample obtained through other processes.