Optimization Of The Thermoelectric Properties Of Al-doped Zinc Oxide-based Materials Using HPHT Synthesis Technology

With the development of society,advances in science and technology have gradually improved the drive and ability of humans to obtain energy from nature.Continuous changes in energy storage technology are very important for promoting social progress.However,with the rapid development of humanity,it is necessary to continuously obtain energy from nature and transform it into a form that can be used.In this process,large amounts of pollution and garbage are unloaded into the environment,which has an irreversible and enormous impact on the ecological environment humans depend upon.Sustainable development should be a long-term strategy that people adhere to.As such,the development of new energy technologies has received increasing attention.There is a growing demand for alternative traditional"green"energy to replace existing environmentally unfriendly energy.In this context,promising thermoelectric materials and related technologies can be implemented.However,these materials are receiving less attention than other energy conversion technologies,such as solar cells.Devices based on thermoelectric materials can directly convert electrical and thermal energy without polluting the environment.For example,a thermoelectric converter does not generate noise because it does not require any mechanically movable parts.In addition,devices based on thermoelectric materials are environmentally safe,light,compact,and flexible,and have less energy loss than other energy generators.At present,thermoelectric materials are widely used in cooling technology,energy-saving motors in the automobile industry,biomedicine(such as biothermal batteries used to power pacemakers),various military and aerospace technologies,and other applications.The most important aspect of thermoelectric materials is the dimensionless figure of merit(z T=S²?T/?).Scholars are exploring zinc oxide(ZnO)-based thermoelectric materials because these materials are pollution-free,easy to obtain,and exhibit excellent high-temperature stability.Due to the simple hexagonal wurtzite lattice structure of ZnO,the intrinsic material has a higher lattice thermal conductivity(33.3Wm^-1K^-1 at 303 K).On the one hand,because ZnO has a wide bandgap(3.37 e V),the conductivity of the intrinsic material is relatively low(0.7 Sm^-1 at 303 K).Poor performance is necessary.In this paper,high-pressure high-temperature(HPHT)technology was used to synthesize ZnO-based thermoelectric materials,and their thermoelectric properties were systematically studied.This work focuses on improving the dimensionless figure of merit of ZnO by adding the Al element in the form of a solid solution to the lattice of ZnO.Moreover,graphene is introduced into ZnO to construct a new type of heterostructure to reduce the lattice thermal conductivity of ZnO.The main research results are as follows:1.The relatively suitable cavity temperature for sintering ZnO was explored,and the best synthesis environment for ZnO-based thermoelectric materials was determined.The test results of X-ray diffraction(XRD),Scanning electron microscope(SEM),Transmission electron microscope(TEM)show that HPHT has a strong control effect on the crystal structure and microstructure of the sample.The highest power factor(PF)was achieved with the help of high temperature and high pressure,and it was as high as 6.42?W·cm^-1K^-2 at 973 K.The electrical conductivity of the sample is as high as 6×10⁴ Sm^-1 at 373 K.According to related literature,this result is better than the previously reported PF.It can be concluded that HPHT is more conducive to the synthesis of thermoelectric materials compared with conventional synthesis methods.At the same time,the energy band structure of Al-doped ZnO was obtained by using density functional theory,and the essence of improving the electrical properties of ZnO was qualitatively analyzed.2.In the present study,HPHT technology was implemented and XRD,SEM,TEM,and Ultraviolet and visible spectrum(UV)spectrum test results were combined with theoretical methods,such as density functional theory and finite element simulation,to improve the thermoelectric performance of ZnO.By taking the idea of combining theory and experimentation as the guideline,the results show that HPHT has a strong control effect on the crystal structure and microstructure of the sample.Increasing the sintering pressure can optimize its electrical and thermal properties.With the increase in sintering pressure,the change in the ZnO energy band structure leads to the decrease in the forbidden bandwidth and the improvement of electrical properties.Moreover,HPHT's adjustment of the microstructure and the scattering of the full-spectrum phonon spectrum result in the sample having a relatively low thermal conductivity.Therefore,the sample prepared at a pressure of 4.5 GPa has a z T value of 0.165 at 973 K.Accordingly,HPHT provides a promising method for improving thermoelectric performance,which is worthy of promotion and application.3.The HPHT technology is used to adjust the lattice parameters of ZnO to adjust the optical bandgap of ZnO,thereby improving the samples'electrical properties.Under high pressure,it shows an optimal doping ratio different from normal pressure,bringing about a new method to further improve the thermoelectric properties of ZnO.On the other hand,high pressure can promote the refinement of grains,which is conducive to the construction of the multi-scale hierarchical structure of the sample.Finally,the Zn_0.96Al_0.04O obtained by sintering at a pressure of 3 GPa and a temperature of 1053 K for 30 min reached the highest PF(7.8?Wcm^-1K²)and the highest z T(0.16)at 970 K.4.To further understand the micro-physical mechanism that improves the thermoelectric performance of ZnO in a higher-pressure environment,the microscopic morphology and z T of?and?crystal Al₂O₃(?-Al₂O₃,?-Al₂O₃)-doped ZnO have been systematically studied.Zn_1-xAl_xO samples(?-x,?-x;x=0.02,0.04,0.06,and 0.08)were synthesized by HPHT technology.The test results show that the HPHT synthesis method effectively improves the solid solubility of?-Al₂O₃ and?-Al₂O₃ in ZnO,and?-Al₂O₃ presents more advantages.Under the same doping ratio,the conductivity of the sample synthesized with?-Al₂O₃ is higher than that of the sample synthesized with?-Al₂O₃.In addition,with the increase in the amount of Al doping and the generation of ZnAl₂O₄,although the generation of ZnAl₂O₄ does not utilize the samples'electrical properties,a small amount of ZnAl₂O₄ is evenly dispersed in the ZnO sample,which can inhibit the growth of crystal grains.Among them,the effect of?-Al₂O₃ is more obvious.The final high-pressure sintering temperature is 1123 K,and the z T value of?-0.04 at 973 K is 0.16.5.Graphene is used in various fields with its unique properties brought about by its 2D structure.The heterostructure(C-ZnO)formed by graphene and ZnO has never been studied in the field of thermoelectric conversion.In this research,a C-ZnO energy material with a new microstructure was prepared for the first time using HPHT rapid sintering.Observation through a microscope revealed that many graphene nanowires are connected between the ZnO grains with the increase of graphene doping.Additionally,with the excessive introduction of graphene,the morphology of the ZnO grains changes less.The new microstructure of the sample 0.1C-ZnO produces an ultra-high conductivity of 2.8×10⁴ Sm^-1,and the lattice thermal conductivity of the sample 0.17C-ZnO is at least 3.4 Wm^-1K^-1 at 973 K.Finally,the z T value of the sample 0.1C-ZnO at 973 K is 129 times higher than ZnO.Finally,the possible heterostructures are obtained by first-principles,and their band structures and phonon spectral structures are qualitatively analyzed.In summary,this work implements HPHT methods to explore the thermoelectric properties of?-Al₂O₃-and?-Al₂O₃-doped ZnO in the pressure range of 1–5 GPa.The main role of pressure in the synthesis process is summarized by comparing the microscopic morphology and thermoelectric properties of ZnO under different pressures.Moreover,the quality factor of ZnO was optimized under the appropriate synthesis environment.The analysis of the results showed that the optimal z T value of?-Al₂O₃-doped ZnO reached 0.17 at 973 K,which was well improved at the same temperature.Furthermore,the thermoelectric properties of graphene-ZnO composite materials have been studied to solve ZnO's relatively high thermal conductivity.Observed under an electron microscope,graphene can change the shape of ZnO crystal grains and inhibit the growth of ZnO crystal grains,thereby reducing the thermal conductivity of ZnO.This provides a new direction for further developing the thermoelectric properties of ZnO.