HPHT Synthesis And Thermoelectric Transport Mechanism Of Bi₂Te₃-based Bulk Materials

Thermoelectric materials can directly interconvert temperature gradient and electricity via the charge carriers and phonon.The thermoelectric devices are usually applied in the power generation from heat and solid state cooling.Compared with traditional energy utilization routes,the thermoelectric technique shows advantages in utilizing the flexible,scattered,low grade heat sources.The applications are particularly irreplaceable in such fields as space probes,remote regions,and severe environments.Meanwhile,the thermoelectric cooling devices are pollution-free and noiseless.However,the low conversion efficiency still restricts the large-scale application of the thermoelectric devices.Among all the thermoelectric materials,commercial available Bi₂Te₃-based alloys show the highest thermoelectric performances at near room temperature.Besides,Bi₂Te₃-based alloys are most deeply investigated and most widely applied so far.N/P type?Bi,Sb?2?Te,Se?3 compounds with ZT value of about 1 can be fabricated by traditional method such as zone melting.The routes to optimize thermoelectric properties are band structure engineering and low dimensional nanostructuring.The common synthesis methods include hydrothermal synthesis,ball milling,melt spinning and then the obtain particles are compacted by spark plasma sintering and hot pressing to obtain bulks.The in situ measurements under high pressure reveal that the thermoelectric performances are distinctly enhanced by pressure.Unfortunately,the enhanced properties cannot be obtained at ambient conditions due to severe reversibility after the pressure is released.In this paper,a simple and rapid highpressure-and-high-temperature?HPHT?method is introduced not only to synthesize bulk materials directly from elements,but also to tune the thermoelectric performances simultaneously.After pressure-quenching is involved,the high pressure effects can be partly retained to ambient pressure,including band structure and microstructures.In this paper,HPHT method is applied to synthesize Bi₂Te₃-based samples.The effects of high pressure tuning,element substitution,and nanostructure composition on thermoelectric properties are systematically investigated.The obtained results are listed as following.1.The variations of preferred orientation,anisotropy,and electrical properties with synthetic pressure for the Bi₂Te₃ samples by HPHT.N type pure Bi₂Te₃ bulk samples are successfully synthesized in the pressure range of 0.5⁴.5GPa by HPHT.Meanwhile,these samples exhibit lamellar morphology of large fine grains with size of 10¹00?m,which results from the layered crystal structure of Bi₂Te₃ and the two-dimensional nucleation/growth.When the synthetic pressure comes to 4.5 GPa,both the?0 0 l?preferred orientation and the lamellar grains disappear.As the synthetic pressure increases,the electrical resistivity decreases,while the Seebeck coefficient shows non-monotonic tendency with two peaks at 1 and 4 GPa.The maximum power factor for Bi₂Te₃ at room temperature is 15?10-4Wm-1K-2 at 4 GPa.Compared to standard XRD patterns,the surfaces of samples by HPHT from 1 to 4 GPa reveal more enhanced?0 0 l?preferred orientation than that of the internal sections.As for the samples by HPHT,the Seebeck coefficient is almost isotropic,but the electrical resistivity reveals anisotropy.The treatments of smashing,milling and high pressure sintering can eliminate the preferred orientation and anisotropy.2.The effects of Sb-Cu substitution for Bi sites on thermoelectric performances of Bi₂Te₃.The CuxBi0.5Sb1.5-xTe3?x=0,0.005,0.01,0.05?samples are successfully prepared by HPHT and high pressure sintering under 2 GPa.After the treatments of smashing,milling and high pressure sintering,the preferred orientation disappears;though the grain size is reduced to the magnitude of 1?m,the lamellar character is retained.The Sb substitution for Bi sites yields P-type Bi0.5Sb1.5Te3 from N-type Bi₂Te₃.The increased Cu content results in the decreased Seebeck coefficient and electrical resistivity and an increased thermal conductivity.Both the carrier concentration and band gap are enlarged after Cu is substituted,which suppresses the onset of bipolar effect and moves the extreme value of thermoelectric properties to higher temperature.Compared with the pure Bi₂Te₃?the best performance is at room temperature?,the temperature of peak ZT value for CuxBi0.5Sb1.5-xTe3?x=0.005?sample is 473 K,and the corresponding maximum ZT value is 1.2.3.The effects of high pressure synthesis?2,3,4GPa?on microstructures and thermoelectric properties of carbon nanotubes?CNTs?composited Bi_0.4Sb_1.6Te₃.The CNTs?0.1wt%?composited Bi_0.4Sb_1.6Te₃ samples are successfully synthesized by HPHT method and high pressure sintering under 2,3,and 4GPa.The CNTs are attached on the surfaces of lamellar grains.Multiple and multiscale microstructures are obtained by pressure-quenching and nanostructure composition,including nanocrystals,amorphous regions,lattice distortions,boundaries and CNTs,which can scatter the phonon with corresponding mean free path and thus reduce lattice thermal conductivity.More microstructures are generated by quenching from a higher synthetic pressure,which yields more intense low energy carrier filtering effect and full-spectrum phonon scattering.As the synthetic pressure increases,both of the Seebeck coefficient and electrical resistivity increase,while the thermal conductivity is decreased.Compared with the thermal conductivity values of Bi₂Te₃-based samples by other prepared methods,the result in this paper is relatively low.The minimum thermal conductivity of 0.74 Wm-1K-1@373 K and the maximum ZT value of 1.42@373 K is obtained for Bi_0.4Sb_1.6Te₃+CNTs?0.1wt%?sample at 4GPa.4.The influence of grapheme content?x wt%,x=0,0.02,0.05,0.1?on the thermoelectric properties of Bi_0.4Sb_1.6Te₃ composited samples.The Bi_0.4Sb_1.6Te₃ samples composited with different grapheme content?x wt%,x=0,0.02,0.05,0.1?are fabricated via HPHT synthesis and high pressure sintering under 4 GPa.After HPHT synthesis,the samples show lamellar morphology with grain size larger than 10?m.The preferred grain orientation is eliminated via high pressure sintering.Meanwhile,the grain size is distinctly reduced to the magnitude of 1?m,while the lamellar structure is still retained.With the increasing graphene content,the thermoelectric properties demonstrate nonmonotonic variations.At 323 K,the Seebeck coefficient,electrical resistivity,and lattice thermal conductivity decrease after graphene incorporation as x?0.05wt%.However,this trend is reversed when the graphene content continuously increases.Namely,the Seebeck coefficient,electrical resistivity,and lattice thermal conductivity show increase tendency as x=0.1.The lattice thermal conductivity of sample?x=0.1?is still smaller than that of pure Bi_0.4Sb_1.6Te₃ sample.Compared to the maximum ZT value of 1.09@423K for pure Bi_0.4Sb_1.6Te₃ sample,the thermoelectric performances are enhanced in the temperature range of 323-500 K,and a maximum ZT value of 1.26@423 K is achieved after graphene dispersion?x=0.05?.In conclusion,high pressure and high temperature technique is a simple and rapid route to synthesize Bi₂Te₃-based thermoelectric bulk materials.High pressure,element doping,nanoparticles composition can effectively tune the band structure and microstructures,which can further optimize the thermoelectric performances of Bi₂Te₃-based alloys.