Performance Optimization And Preparation Of P-type Bismuth Telluride Based Materials For Industrial Using

Thermoelectric materials can directly convert electrical and thermal energy.Thermoelectric devices manufactured by them have broad application prospects in the fields of waste heat recovery,micro-zone temperature control,and micro-area power generation.Bi Sb Te alloy is the only p-type thermoelectric material that has been built into wide commercial application at present.Most thermoelectric devices are composed of p/n-type Bi2Te3 particles.Strategic industries such as 5G communications,large-scale integrated circuits,and wearable electronic products have put forward higher requirements for the yield and performance of Bi2Te3-based micro thermoelectric devices,therefore,how to realize the performance regulation and industrialized preparation of p-type Bi2Te3-based materials has become the focus of current research.The traditional zone melting method（ZM）can realize the batch preparation of high-oriented Bi Sb Te single crystals.However,ZM samples have coarse grains and are prone to cleavage,so they can hardly be used in the field of relatively advanced micro-thermoelectric device preparation.The Bi Sb Te samples prepared by structural nanostructure have excellent mechanical properties,but the grain refinement leads to the weakening of the orientation of the samples,resulting in lower electrical conductivity and power factor.In addition,the present Bi Sb Te material still face the difficulties of failing to meet the needs of industrial production and the intrinsic excitation to deteriorate the thermoelectric properties.Therefore,it is of great significance to study how to prepare Bi Sb Te materials with uniform composition,high orientation,wide operating temperature range,excellent thermoelectric properties and mechanical properties for the commercial production of thermoelectric microdevices.The main conclusions obtained are as follows:（1）Effects of Ge,Sn and Pb doping and selective scattering of minority carriers by microstructure on the thermoelectric properties of Bi Sb Te compounds.The different effects of Ge,Sn and Pb doping on the phase composition,microstructure and thermoelectric properties of Bi Sb Te compounds were systematically investigated.The Seebeck coefficient of the Sn-doped sample increases due to the introduction of the resonance level at low doping amount.The sample with the Sn doping amount of0.002 can obtain the highest power factor of 3.8×10^-3 W m^-1 K^-2 at 320 K,which is 12%higher than the undoped sample.Pb doping significantly optimizes the thermoelectric properties of the samples,and the sample with Pb content of 0.005 achieves the highest ZT value of 1.07 at 377 K.Melting spinning was employed on the Pb-doped samples with the best performance to further improve the thermoelectric properties of the materials.It is found that the microstructures in the range of 150-350 nm introduced by the MS process can selectively scatter the electrons and holes in Bi Sb Te compounds,suppressing intrinsic excitation by selectively reducing the mobility of minority carriers.When the Pb doping amount is 0.005,the bipolar thermal conductivityκb of the MS samples is reduced from 0.65 W m^-1 K^-1 by the traditional melting method to 0.4 W m^-1 K^-1.As a result,the sample with Pb doping content of 0.002 achieved a maximum ZT value of 1.31 at 380 K,and the average ZT value from 300 K to 500 K could reach 1.17,which was 24%higher than the samples without MS process.（2）Bi Sb Te sample preparation and uniformity research.The influence of Peltier effect on the uniformity of Bi Sb Te ingot in PAS process was studied.Under the direct current,the thermoelectric material absorbs and releases heat at both ends of the sample and forms a temperature difference of 10 K due to the Peltier effect.This temperature difference affects the mass transfer process and causes the unevenness of the sintered ingot.In order to obtain more uniform samples,the uniformity of raw materials before PAS and the sintering time was prolonged.By using the glass tube with a smaller diameter to melt the sample,the uniformity of the raw materials before PAS can be improved,making the homogenization process easier.Therefore,a uniform sintered ingot can be obtained after only 1 minute of PAS process.The standard deviation of the Seebeck coefficient statistical distribution at room temperature of the sintered ingot obtained by using aΦ10 mm glass tube is only 4.5μV K^-1,which is much lower than the 9μV K^-1 of the ingot prepared usingΦ36 mm glass tube.Apart from that,by extending the sintering time,the mass transfer process can be ensured to fully proceed.Based on the PAS raw material obtained by using theΦ36 mm glass tube,the standard deviation of the Seebeck coefficient statistical distribution of the ingot at room temperature is further reduced to 3.7μV K^-1 with the sintering time of12 min.（3）Study on the optimization of hot extrusion die and its effect on the properties of Bi Sb Te compounds prepared in large quantities.The effect of different extrusion die extrusion ratio and extrusion angle on the microstructure,mechanical properties and thermoelectric properties of Bi Sb Te compounds was systematically studied,and the uniformity and stability of the extruded samples were explored.The results show that the extrusion process can significantly optimize the sample orientation while maintaining great mechanical properties,and can meet the needs of industrial production.The sample with an extrusion ratio of A4 and an extrusion angle of B2 can obtain an impact factor of up to 0.43,benefiting from the high mobility brought by the high orientation,the conductivity of the sample at room temperature is 16×10⁴ S m^-1,and can get the highest power factor of 5.6 m W m^-1 K^-2at 318 K and the highest ZT value of 1.23 at 365 K,which is suitable for room temperature refrigeration and power generation devices.When the extrusion ratio is fixed as A2,for the sample with an extrusion angle of B5,the orientation factor is 0.37,and with the decrease of the extrusion angle,for the extrusion angle B1,B2,B3 and B4samples,the orientation factor of the samples gradually decreased.The compressive strength of all samples increases with the reduction of the orientation factor,while the bending strength decreases with the reduction of the orientation factor.（4）The effect of hot extrusion process optimization on the properties of Bi Sb Te compounds prepared in large quantities.The effect of extrusion temperature and holding time on the microstructure,mechanical properties and thermoelectric properties of Bi Sb Te compound in the extrusion process was systematically studied,and microdevices were fabricated in practice to prove the properties of the extruded materials.The study found that the orientation of the extruded samples increased with growing extrusion temperature.The samples with the extrusion temperature of C1 and the holding time of D1 can obtain an orientation factor up to 0.5.With the decrease of the extrusion temperature,the orientation factors of the extruded samples with the extrusion temperature of C2 and C3 are 0.43 and 0.4,respectively.Benefiting from the high mobility brought by the high orientation,the samples with the extrusion temperature of C1 and the holding time of D1 can attain the electrical conductivity of17×10⁴ S m^-1 at room temperature,and the highest power factor of 5.7 m W m^-1 K^-2 can be obtained at 320 K.The orientation of the extruded sample increases with the extension of the holding time.The orientation factor of the extruded sample with the extrusion temperature of C2 and the holding time of D3 is only 0.24,while the orientation factor of the sample with the holding time of D1 is 0.31.Due to its excellent mechanical properties,the processed micro-sheets possess good mechanical processing properties,and 4.7 mm×4.9 mm×1.18 mm particles suitable for micro-devices can be cut out.The size of the micro device in this experiment is 4.7 mm×4.9 mm×1.18 mm,and is composed of 28 pairs of p and n-type particles.Benefiting from the high conductivity brought by high orientation,the internal resistance of the manufactured micro device is only 1.6Ω.The maximum cooling temperature difference is 54 K,and the open circuit voltage is 265 mV.