Preparation And Representation Of PbI₂ Doped PbTe_1+x Under High Pressure And High Temperature

Thermoelectric (TE) materials are a functional material which can convert heat and electricity directly. TE devices based on TE materials could convert heat to electricity or use electricity to pump heat from cold to hot, both without any moving parts or bulk fluids. They are lightweight, small, portable, inexpensive, quiet performance and the ability for localized'spot'cooling. Thus, thermoelectric materials are not wildly used.Presently, the use of TE devices is limited by their low efficiencies. The efficiency of a TE device depends on the TE material. The efficiency of a TE materials can be defined by the dimensionless thermoelectric figure of merit, ZT, where T is the absolute temperature and Z =σS²/κ(S is the thermopower,σis the electrical conductivity andκis the thermal conductivity). In theory, the ZT value can attain infinite. However, the best TE materials available today such as Bi₂Te₃,PbTe,SiGe for devices have a ZT of about 1, which is just about 10% of Carnot efficiency. 30% of Carnot efficiency (comparable to home refrigeration) could be reached by a device with a ZT of 4. Increasing ZT by a factor of 4 is a formidable challenge.There are now several good reasons to renew the quest for superior TE materials, with worsening environment and energy crisis. Several approaches have been adopted in the research for the improved thermoelectric materials, ranging from the synthesis of new bulk materials and quantum-well structures that may exhibit improved ZT to combinatorial synthesis techniques that rapidly screen materials for desirable thermoelectric properties. Pressure tuning and high pressure and high temperature (HPHT) synthesized may offer a new means.About high pressure and high temperature synthesized TE materials, Doctor Zhu first prepared TE materials PbTe used cubic multi-anvil high pressure apparatus. PbTe samples which have NaCl construction were synthesized under pressure ranging 3.0-5.0GPa and temperature 900-1000℃and time ranging 10-30 minute. His results show that the thermoelectric properties of PbTe prepared by HPHT are higher dramatically than that synthesis at normal press. He contributed the enhancing to the ETT for PbTe which changing at high press. Doctor Guozhong Ren validated the result through the In-situ measurement at HPHT.There are several reports on the doping for PbTe under normal pressure. It could optimize the carrier concentration of PbTe and enhances its thermoelectric properties. According to Godwal, ETT could be induced by alloy as well as pressure tuning. In order to study the effect of doping at high press for PbTe, PbI₂ are used to dope for PbTe.Firstly, we prepare PbTe_1+x samples doped with PbI₂ using the HPHT method, at P=2.0GPa,2.5GPa,3.0GPa,3.5 GPa,4.0GPa,4.5GPa, and 1200K. All the samples are single phase with NaCl structure through the test of X-Ray.We test the electrical resistivity and Seebeck coefficient of PbTe1.04 samples at room temperature. The result shows that the Seebeck coefficients of all the samples are negative which indicate they are N type semiconductor. The Seebeck coefficient and electrical resistivity of PbTe_1+x firstly increase and then decrease with the synthetic pressure increasing. Moreover, the Seebeck coefficient and electrical resistivity of PbTe_1+x decreases at the lower pressure. However, the variety of both parameters is more complex at the higher pressure. PbTe1.04 has the most optimal electrical transport property comparing with the other ratios of Pb and Te at P=3.5GPa.On the base of above research, we select the PbTe1.04 as basic sample, and dope the PbI₂ in order to obtain the more enhanced performance thermoelectrical materials at 3.5 GPa. The electrical resistivity and the Seebeck coefficients decreases dramatically first and then slowly with an increase of the PbI₂ content. The carrier concentration and hall mobility of PbTe1.04 are calculated using the test results of Hall coefficient. The carrier concentration increases with an increase of PbI₂ content. The hall mobility increase first and then decrease with the increasing of doping content. Especially the carrier concentration and hall mobility of most samples satisfy to the condition of good thermoelectric material reported by G. D. Mahan.The electrical properties of PbTe prepared under high press are more sensitive to dopants compare with the samples prepared at normal press, which can reflected by the resistivity and Seebeck coefficients and carrier concentration. For example with the same carrier concentration 2.097×1025 m-3, 0.065 mol% PbI₂ need to be doped for PbTe at high press, while the same doped sample prepared at ambient pressure, is corresponding to 0.2 mol% PbI₂ at least.We calculate the figure of merit for doped PbTe samples from the measured quantities, Seebeck coefficience and electrical resistance. The maximum value is 24.2μW·cm-1·K-2 for PbTe doped with PbI₂, which is twice higher than that PbTe doped PbI₂ prepared by hot press. These results indicate that HPHT technique combining with doping may be helpful to prepare thermoelectric materials with enhanced thermoelectric properties.In conclusion, the electrical properties for PbTe are more sensitive to dopants at high pressure and the doping of heavy atoms can reduce the phonon thermal conductivity effectively. The most significant conclusion is the HPHT method has the effect of doped, namely, the trace quantity dopant can dramatically influence the electrical transport property of PbTe-based thermoelectrical materials.