Preparation Of N-type Bi₂Te₃ Thick Films And Design And Evaluation Of Thermoelectric Devices Based On The Thick Films

Thermoelectric?TE?device,which can convert thermal energy into electric energy or vice versa,is a structure and function integration device.With the development of computertechnology,spacetechnologyandmicroelectronicstechnology,high-efficiency micro TE devices are urgently required.Compared to bulk device and thin film micro device,thick film micro device has attracted more attention for its advantages of smaller-size and lower-cost.Traditional methods such as inkjet printing,screen printing,dispenser printing and electrochemical deposition have been extensively reported to prepare Bi₂Te₃-based thick film materials.However,these methods have been some problems such as expensive equipment,complex preparation process and low efficiency.In the master thesis,a facile and rapid brush printing method to prepare thick film TE materials has been developed to solve these problems.The n-type Bi₂Te_2.7Se_0.3.3 thick films were prepared with the brush printing.The effects of the proportion of inks,chemical composition and annealing treatment on the thermoelectric properties of the thick films were also studied.A thick film prototype device was designed and fabricated successfully.The cooling performance of the prototype device was evaluated with the temperature difference??T?obtained by theoretical calculation and experimental measurement.The main results are listed as follows:A series of n-type Bi₂Te_2.7Se_0.3.3 thick films were successfully prepared by the brush printing technique.The effects of annealing temperature and annealing time on phase composition,chemical composition,microstructure and the TE properties of the thick films were investigated.The results indicated that all the samples annealed at 573-773K for 4 h were composed of single-phase Bi₂Te_2.7Se_0.3.The Bi₂Te_2.7Se_0.3.3 particles were randomly distributed in the annealed samples at 573-623 K,however,predominantly oriented along the?00l?plane and formed layered textures in the annealed samples at 673-723 K.When the annealing temperature increased above 773K,lots of pores and cracks formed and the layered textures were destroyed.The TE transport measurements demonstrated that both?and?increased with increasing annealing temperature in the range of 573-673 K,and decreased in the range of673-773 K.The optimum annealing temperature was about 673 K.All the samples annealed for 2-8 h at 673 K were composed of single-phase Bi₂Te_2.7Se_0.3.For the samples annealed in the range of 2-4 h,the Bi₂Te_2.7Se_0.3.3 particles were sintered and the density increased gradually.Above 4 h,a large amount of pores and micro cracks appeared.The content of Se had no significant deviation from the stoichiometry of the samples annealed in the range of 2-4 h,while serious lack of Se was detected in the range of 6-8 h.?and?increased with increasing annealing temperature in the samples annealed in the range of 2-4 h,and the inverse behaviors of?and?were observed in the samples annealed in the range of 4-8 h.The optimum annealing temperature is about 4 h.The largest?²?reached 0.24 mW?K^-2?m^-11 at 320 K for the sample annealed for 4 h at 673 K.Inks with the proportion of Bi₂Te_2.7Se_0.3+x wt%Se?6?x?16,?x=2?were adjusted and prepared by adding additional Se to optimize the chemical composition.The effects of Se addition on the density and thermoelectric properties of the thick films were also investigated.The results show that the main phase of all thick films was Bi₂Te_2.7Se_0.3,a single-phase Bi₂Te_2.7Se_0.3.3 was obtained in the thick films with 6?x?12,and Se impurity was detected in the films with x?14.The increased density of the films is attributed to the liquid phase of Se,which promoting the process of mass transfer and filled some holes.The thick film with x=10 had the optimum stoichiometric ratio of Bi₂Te_2.69Se_0.30.The TE transport measurements demonstrated that the?increased and the?decreased with increasing the x.The largest?²?reached0.29 mW?K^-2?m^-11 at 320 K for the sample with x=10,increased by 21%as compared to that of the films without Se addition.A thick film prototype device was designed and fabricated successfully.To evaluate the cooling performance of the prototype device,a theoretical calculation was performed on the basis of thermodynamic cycle model and an experimentally measuremant was carried out by a designed testing system by myself.The theoretical temperature difference??T?is 9.6 K.The experimental temperature difference is only about 2 K.