Preparation And Properties Of Strontium Titanate-Based Thermoelectric Composites

While the traditional fossil energy is increasingly exhausted,a large amount of energy is wasted in the form of waste heat.If the waste heat can be recycled through appropriate methods,the utilization rate of energy will be greatly improved.Thermoelectric material is a kind of material that can directly convert heat energy and electric energy into each other.Thermoelectric devices made of thermoelectric materials can be applied to waste heat recovery and the development of new energy.They have the advantages of small volume,no noise,simple structure,and so on,and thus receive more and more attention from people.At present,alloy materials such as Bismuth telluride(Bi₂Te₃),lead telluride(Pb Te)and tin selenide(Sn Se)are the most mature materials in the field of thermoelectric materials.Although these materials have a high thermoelectric conversion rate,their raw materials are scarce and expensive,and contain heavy metals,which are easy to cause environmental pollution.It is also easy to be oxidized and lead to functional failure in long-term high temperature working environment.Strontium titanate(SrTiO₃)is considered as a potential high-temperature thermoelectric oxide material due to its rich source,low price,non-toxic,stable chemical structure,oxidation resistance and high Seebeck coefficient.It is expected to be applied to waste heat recovery and power generation in high temperature environment.However,single-phase SrTiO₃ material has the disadvantages of extremely low electrical conductivity and high thermal conductivity,which leads to its poor thermoelectric performance and seriously limits the application and development in the field of high-temperature thermoelectric.Therefore,appropriate methods to enhance the electrical conductivity of SrTiO₃ based materials and reduce their thermal conductivity are of great research significance to improve the thermoelectric properties of SrTiO₃ based materials.Based on the above reasons,this paper takes SrTiO₃ based oxide thermoelectric material as the research object.Firstly,the preparation process of SrTiO₃ based bulk material is optimized by adjusting the doping amount of rare earth elements(La/Nb),reduction duration of powder,sintering temperature of spark plasma sintering(SPS)and other factors.The effect of high temperature annealing on the thermoelectric properties of La and Nb co-doped SrTiO₃(SLNT)bulk samples in reductive atmosphere was investigated.Finally,carbon nanotubes(CNTs)and molybdenum disulfide(MoS₂)nanoflakes were introduced to prepare the composite materials.The effects of CNTs and MoS₂ nanoflakes on the microstructure,thermoelectric properties and mechanical properties of SLNT were systematically studied.The main contents of this paper are as follows:(1)La/Nb element doping and preparation process optimization.In order to improve the conductivity of SrTiO₃,the high-valence elements are firstly selected for doping,and then the powder is anneal at high temperature in a 30%H₂/Ar reduction atmosphere.After high temperature reduction annealing treatment,the doping reaction is not only promoted,but also leads to lattice oxygen loss,which can improve the conductivity of bulk materials.The effects of reduction time,sintering temperature,doping amount and other parameters on SrTiO₃thermoelectric materials were systematically studied.The results show that the conductivity of 10%Nb doped SrTiO₃increases with the increase of powder reduction time,and the doping effect is basically completed when the conductivity of the sample does not increase after 10h reduction.The conductivity and thermal conductivity of 10%Nb doped SrTiO₃ increase with the increase of SPS sintering temperature,while the Seebeck coefficient is the opposite.The thermal property of doped SrTiO₃ prepared at 1400?is the best,and the maximum ZT value obtained at 800 K is 0.13.Increasing the total doping amount of La/Nb can increase the electrical conductivity of SrTiO₃ base materials and reduce the Seebeck coefficient and thermal conductivity.Sr and Ti double doping is more advantageous than single doping.The maximum ZT value of 10%La and 10%Nb co-doped samples at 800K is 0.25.(2)Effect of bulk reduction on thermoelectric properties of doped SrTiO₃.10%La and 10%Nb co-doped SrTiO₃ powders were prepared by solid state reaction.After annealing and reduction,SLNT bulk materials were prepared by SPS sintering at1400?.Subsequently,the bulk materials were annealed and reduced in a mixture atmosphere of 30%H₂/Ar and at 1100?.The results show that the reduction treatment reduces the number of strontium vacancies near the grain boundary of SLNT bulk,and then reduces the double Schottky barrier height composed of strontium and oxygen vacancies,and greatly improves the mobility of carriers.The maximum conductivity of samples increases from?600 Scm^-1 to?1200 Scm^-1.In addition,Nb elements can be precipitated from the SLNT matrix to form nanoparticles in situ after reduction treatment.These nanoparticles,acting as the scattering center of phonons,can significantly reduce the lattice thermal conductivity of the sample.After 50h reduction treatment,the maximum ZT value of the sample reaches 0.32(800 K),which is?34%higher than that of the non-reduced sample.(3)MoS₂ nanoflakes were prepared by mechanical stripping and the thermoelectric properties of MoS₂/SLNT composites were studied.Based on the above work,the MoS₂nanoflakes/doped strontium titanate composite was prepared.Firstly,the liquid phase assisted mechanical stripping method was used to prepare MoS₂ nanoflakes.The peeling effect of MoS₂ nanoflakes at different rotational speeds was studied.The results showed that when the rotational speed was 800rpm,the peeling effect and yield of MoS₂nanoflakes were the best.The MoS₂/SLNT composites were prepared by sintering different content MoS₂ nanosheets with SLNT powder at 1300?with SPS.The experimental results show that MoS₂ nanoflakes are still stable in the matrix after SPS sintering at high temperature,and energy filtering effect is produced at the interface between MoS₂ nanoflakes and SLNT matrix due to band bending,and the Seebeck coefficient of the composite material is greatly improved without significantly reducing the electrical conductivity.At 600 K,1.5vol%MoS₂/SLNT composite has the highest power factor of 1.2 m Wm^-1K^-2,which is 20%higher than the matrix.The MoS₂/SLNT interface also enhanced the phonon scattering and reduced the thermal conductivity of the composites.When the content of MoS₂ nanoflakes is 1.5vol%,the maximum ZT value of the composite is 0.24 at 800 K,which is obviously improved compared with the matrix.(4)Thermoelectric properties of CNTs(BCNTs)/SLNT composites.In order to achieve uniform dispersion of CNTs in the matrix,heterogeneous deposition method was proposed to composite CNTs with SLNT powder with different contents,and then the CNTs/SLNT composite was prepared by SPS technology at 1400?.It is found that CNTs dispersed in the matrix can serve as reducing agent to promote the lattice oxygen loss of SrTiO₃ matrix,increase carrier concentration and serve as transport channel to improve mobility,inhibit grain growth and reduce the thermal conductivity of composite materials.When the volume content of CNTs was 1.5%,the ZT value of the composite reached 0.35 at 800 K,1.4 times that of the matrix.However,when CNTs content was too much,local agglomeration occurred,leading to the decrease of the conductivity of the sample and the rise of the thermal conductivity.In order to further improve the dispersion of CNTs and thermal conductivity of composites,5 at%boron(B)doped CNTs(BCNTs)were prepared by thermal diffusion method on the basis of larger diameter carbon tubes,and the same heterogeneous deposition method and SPS sintering process was adopted.BCNTs/SLNT composites with different volume contents of BCNTs were prepared.The results showed that BCNTs with larger diameters had better dispersion in the SLNT matrix,and the thermal conductivity of BCNTs/SLNT composites continued to decrease with the increase of the addition amount of BCNTs.When the volume content of BCNTs was 1.0%,the composite material had a maximum ZT value of 0.32 at 800 K.Based on the thermoelectric properties,the mechanical properties were investigated.The results show that the MSP strength of SrTiO₃ material increases first and then decreases with the increase of carbon nanotubes content,and the maximum MSP strength is 175±8 MPa,which is?1.6 times that of the matrix material.Due to agglomeration of CNTs in excess,the hardness of CNTs/SLNT first increased and then decreased with the content of CNTs,and the fine grain strengthening effect caused by more evenly dispersed BCNTs made the hardness of BCNTs/SLNT increase with the content of BCNTs,while the fracture toughness of all samples increased with the increase of carbon tube content.