Fabrication Of Bi₂Te₃-Based Thermoelectric Thin Films And Study On The Performances

Thermoelectric materials interconvert heat and electricity directly. They are used in power generation and cooling. Bi₂Te₃ based materials are the best thermoelectric materials at room temperature. And PbTe based materials are the better thermoelectric materials from 300 to 900K. But the figure of merits ZT of Bi₂Te₃ and PbTe are about 1 for decades. In recent years, the research of thermoelectric material has made new progress by nanotechnology. Low dimensional and nanostructured materials have a large amount of boundaries that will strongly scatter the phonons and carriers. Therefore the Seebeck coefficient is improved and the thermal conductivity reduced. In this paper, structures of (Bi₂Te₃/PbTe)n multilayer films were designed according to the theoretical calculation from Boltzmann equation and various nanostructured p type Bi₂Te₃ films and composite films of Bi₂Te₃ and PbTe were fabricated by magnetron sputtering. The structures and properties are systematically investigated.In this paper, rough interface of Bi₂Te₃/PbTe and quantum tunnel effect are induced to analyse the ZT of (Bi₂Te₃/PbTe)n multilayer quantum well from Boltzmann equation for the first time. The result shows that increases rapidly when the thickness of PbTe barrier is under 3 nm. Rough interface of Bi₂Te₃/PbTe scatters the carriers strongly and ZT of (Bi₂Te₃/PbTe)n multilayer quantum well decreases rapidly because of the rough interface. If the thickness of PbTe barrier is restricted to 1 nm (the tunnel transmission coefficient is 0.15), the ZT of Bi₂Te₃/PbTe multilayer quantum well is much lower than the ZT of ideal superlattice even if the specularity parameter p is 1. The calculational result also shows that the ZT decreases with increasing of the Bi₂Te₃ sub-layer. If the p is 0.5, the ZT of Bi₂Te₃/PbTe multilayer quantum well is lower than the ZT of bulk Bi₂Te₃ when the thickness of Bi₂Te₃ sub-layer is over 2nm. The calculational results are consistent to the experimental results of (Bi₂Te₃/PbTe)n nultilayer films with low power factors.The influences of magnetron sputtering technics on the structures of Bi₂Te₃, (Bi₂Te₃/PbTe)n multilayer and (PbTe)np/Bi₂Te₃ nanocomposite films are systematically studied in this paper. The results show that amorphous Bi₂Te₃ deposit on the glabrous surfaces (mica or cool polishing quartz glass) perfectly with low sputtering power 25W. Amorphous Bi₂Te₃ will become nano-grain with the increasing of continuous sputtering time, and constitute tree structure vertical the substrate. Amorphous Bi₂Te₃ films can be acquired by the method of intermittent depositing. PbTe deposits on the surfaces with nano-particles magnetron sputtering. When the depositing time is over 3s, PbTe nano-particles will be continuous films with tight contact among the particles. Otherwise, PbTe nano-particles will be nano-island structure on the surfaces. So it can be acquired that (Bi₂Te₃/PbTe)n nano-multilayer and (PbTe)np/Bi₂Te₃ nanocomposite films of PbTe nano-particles dispersing in Bi₂Te₃ substrate. In the multilayer, the thicknesses of PbTe sub-layer are over 6 nm. The size of PbTe nano-particles are 2⁵nm in the Bi₂Te₃ substrate without contact.The amorphous Bi₂Te₃ films crystallize completely after annealing under the condition of 300℃and 3 hours and the grain size is under 20nm. The carriers are confined in the confined states of amorphous Bi₂Te₃ films. So the conductivities of amorphous Bi₂Te₃ films are less than the conductivities in crystalline Bi₂Te₃ films. The conductivities are 300⁷00 S/cm in crystalline Bi₂Te₃ films with over Bi. The conductivities are low because the hole is scattered strongly by the plentiful defects and interface. Seebeck coefficients are 80 to 160μV/K in the films. And the best power factor is only 8×10-4WK-2m-1. In the (Bi₂Te₃/PbTe)n nano-multilayer films, the sub-layer of PbTe is crystalline and the sub-layer of Bi₂Te₃ amorphous. According to the model of rough interface, the specularity parameter p of the Bi₂Te₃/PbTe interface is about 0.3⁰.4. And the value is higher than the p of the Bi₂Te₃/PbTe interface after annealing. Because Bi₂Te₃ deposits on the surface of PbTe sub-layer, but the coherent deposition is broken after annealing. The carriers are scattered strongly by the interface when they transmit in the Bi₂Te₃ sub-layer. So the Seebeck coefficients increase and Seebeck coefficient of p type Bi₂Te₃/PbTe multilayer films is 250μV/K at 100℃. But the conductivities are low because of scattering to the carriers and the power factors are low. In the (PbTe)np/Bi₂Te₃ nanocomposite films, Bi₂Te₃ deposits along the nucleuses of nano-grain and that is benefit to form coherent interface between Bi₂Te₃ and PbTe. The carriers are scattered weakly by the interface. But the coherent interfaces are destroyed after annealing as the same of (Bi₂Te₃/PbTe)n multilayer films. So the conductivities of the (PbTe)np/Bi₂Te₃ nanocomposite films as grown are higher than the conductivities after annealing. In verse, the Seebeck coefficients are higher after annealing because of the interface strongly scattering to carriers. The power factors are higher to the (PbTe)np/Bi₂Te₃ nanocomposite films than that to the (Bi₂Te₃/PbTe)n multilayer films.The 3ωthermal conductivity measurement technique was introduced to measure the thermal conductivities of the (PbTe)np/Bi₂Te₃ nanocomposite films. The results show that the lattice thermal conductivities increase with the increasing of temperatures in the temperature zone 300 to 360K. That is, the phonons are scattered by the interfaces when they transmit in the films in the temperature zone. For interpreting the regular of phonon thermal conductivities depending on fraction of PbTe nano-particles, a model is introduced to integrate Effective Medium Theory, interfacial thermal resistance and nano-grain size effect in the substrates. The interfacial thermal resistances of Bi₂Te₃/PbTe are about 0.94 to 2.48×10-9m2K/W in this paper. For the (PbTe)np/Bi₂Te₃ nanocomposite films after annealing, Bi₂Te₃ crystallize along on the PbTe and so the coherent interface is better in the higher fraction of PbTe nano-particles when the fraction is small. But the coherent interface is destroyed when the fraction of PbTe nano-particles is large enough. That exhibits that the interfacial thermal resistances increase with the increasing of PbTe nano-particles at first. But the interfacial thermal resistances decrease with the increasing of PbTe nano-particles at last when the volume of PbTe nano-particles is large enough.For the need to thermal infrared stealth of high temperature surfaces of martial equipments, two models are designed to resolve the difficulty with the effects to giving power and refrigeration of thermoelectric materials for the first time. The results show that the effect to giving power can't to decrease the external temperature effectively. And the effect to refrigeration can decrease the external temperature effectively and rapidly and control the external temperature by varying the electric current of the thermoelectric module. That is imaginable to achieve intelligent thermal infrared stealth.