| Phosphorous-containing Zintl phases are of great interest due to their diverse structure types as well as abundant physical properties.Especially for their complex crystal and electronic structures,which correspond to good electrical transport properties and low lattice thermal conductivity,they are particularly of potential in the thermoelectric applications.In this thesis,a series of defected phosphorous-containing Zintl phases were synthesized and studied.Their crystal structures,electronic band structures and various doping strategies aiming at optimizing the thermoelectric properties were discussed.The contents are summarized as follows:(1)Ca9Zn4.5-δSb9(0<8<0.5)single crystals were synthesized and a series of Sr-doped materials Ca9-xSrxZn4.5-δSb9(0<δ<0.5)were prepared.These crystals belong to the orthogonal crystal system and Pbam(No.55)space group,which bear substantial interstitial Zn atoms in the anionic structure.As seen in Ca8.42(2)Sr0.58(2)Zn4.47(2)Sb9,which exhibits the highest Sr-doping contents,strongly supported that Sr can be doped into the Ca sites.The incorporation of Sr can greatly improve the electrical conductivity and meanwhile,lead to the decreased lattice thermal conductivity.Consequently,the sample Ca8.2Sr0.8Zn4.5Sb9 approached an optimized maximum ZT-0.73 at 873K.On the basis of above results,Eu-doping was tried with similar strategies as well.The metal flux method was used to synthesize Ca7.38(3)Eu1.62(3)Zn4.48(2)Sb9 single crystals,corresponds to the cell parameters a＝12.7783(12)A,b=21.839(2)A,c=4.5650(4)A,which proved that Eu can also be doped into the Ca9Zn4.5-δSb9 crystal structure.By powder X-ray diffraction,pure samples of various Eu-doped Ca9-xEuxZn4.5-δSb9(0<δ<0.5)were prepared and their thermoelectric properties were characterized.In order to better understand the electrical properties of the material,the effective mass and lorentz number were calculated.Combined with the experimental data of various properties mentioned above,the composition Ca8.2Eu0.8Zn4.5-δSb9 found a maximum ZT～0.81 at 873K,exhibited the better effects on the optimization than the Sr-doping.(2)A novel Mg-containing 9-4-9 series,Sr9Mg4.45(1)Bi9 and Sr9Mg4.42(1)Sb9,were discovered by using the metal flux reactions.Both crystals belong to the orthomorphic system and Pnma(No.62)space group.However,compared to the conventional 9-4-9 structure,Sr9Mg4.45(1)Bi9 and SrgMg4.42(1)Sb9 bear obviously the larger unit cells with parameters of a=13.3156(13)/13.0962(11)A,b=4.9089(5)/4.8193(4)A,c=46.993(5)/46.259(4)A.In addition,Mg defects or disorders exist within the crystal structure,which is beneficial for the extremely low thermal conductivity.Polycrystalline samples Sr9Mg4.45Bi9 and Sr9Mg4.42Sb9 were both prepared by induction furnace and the phase purity was verified by the powder X-ray diffraction.However,Sr9Mg4.42Sb9 significantly decomposed during the SPS experiment and thus only Sr9Mg4.45Bi9 was chosen for later property studies.Sr9Mg4.45Bi9 has extremely low lattice thermal conductivity(close to the glassy limit),whereas the layer structure still enables good electrical transport properties,which already resulted in a high ZT value of 0.57 at 773K without any optimization.These results indicate great potential of these novel materials as new thermoelectric candidates.A series of Sr9Mg4.45Bi9-xSbx(x=0.01,0.02,0.05,0.10,0.15)materials were synthesized by doping Sb at the Bi position.The purpose of increasing Seebeck coefficient and decreasing thermal conductivity is achieved(3)A series of Zn/Cu mixed Zintl phases AZn(i-x)/2CuxSb(A=Ca,Eu,Sr;0<x<1)were synthesized by the metal flux reactions.These compounds belong to hexagonal LiGaGe structure type and P63mc(No.186)space group.The a/c Cell parameters of CaZn0.36(1)Cu0.28(1)Sb,EuZn0.25(3)Cu0.50(3)Sb and SrZn0.17(3)Cu0.65(3)Sb are 4.5234(3)/7.9562(10)A,4.5705(6)/8.431(2)A and 4.5863(9)/8.698(3)A,respectively,which correspond to our previously reported analogues CaZn1-xAg1-ySb(0<x<1,0<y<1),which exhibit a broad phase width.The powder polycrystalline materials of AZn0.25Cu0.5Sb(A＝Ca,Eu,Sr)were successfully formed by solid phase on the the ratio of EuZn0.25(3)Cu0.50(3)Sb single crystal.The thermoelectric properties of the sample were determined after SPS molding process.The thermoelectric conductivity of SrZn0.25Cu0.5Sb were nearly 0.5 W/(m*K)and the corresponding ZT value(～0.34)was achieved.Subsequently,the roles of the transition metals on the formation of crystal structures and governing related thermoelectric properties were carefully discussed.By replacing the Cu position with Ag on the basis of SrZn(1-x)/2CuxSb,the SrZn0.30(1)Ag0.40(1)Sb single crystal with high ratio of Zn was obtained by the flux method,which crystallizes in space group P63mc(No.186)and a/c cell parameters are a＝4.6996(2)A,c＝8.5002(6)A.Due to the large phase width of this type of materials,a series of SrZn(1-x)/2AgxSb(x＝0.2,0.3,0.4,0.6)materials were prepared,in which the ratio of the best thermoelectric value was SrZn0.35Ag0.3Sb and reached the maximum value of 0.48 at 873K.Because the 1-1-1 system is highly flexible and adjustable in structure and has a wide component threshold,the performance optimization can be achieved by further solution at the cationic position on this basis. |
PDF Full Text Request