HPHT Synthesis And Thermoelectric Properties Of Nonstoichiometric Titanium Dioxides

The thermoelectric material able to realize the direct conversion between heat and electricity has been attracting increasing attention worldwide as an effective mean to alleviate the global energy crisis.As is well known,the performance of a thermoelectric material is gauged by the dimensionless figure of merit,zT=S²?T/?_tot,where S is the Seebeck coefficient,?the electrical conductivity,T the absolute temperature,and?_totthe total thermal conductivity.A high performance of a thermoelectric material is assured by the high power factor（S²?）and low total thermal conductivity(?_tot).These thermoelectric parameters,however,are so closely related to each other that it is knotty to integrate high electrical properties and low thermal conductivity in a single material.A high Seebeck coefficient is usually resulted in by a low carrier concentration,which inevitably brings about poor electrical conductivity;while the increase of the electrical conductivity is often accompanied by an increase in the thermal conductivity.The exploration of potential thermoelectric materials with high performance is a perpetual topic and great achievements have been achieved in such materials as Bi₂Te₃,PbTe,SiGe,skutterudite and so on.Most of the conventional alloy-based thermoelectric materials,however,are of toxicity and cost intensive and poor thermal stability,hindering their practical applications.Being an alternative,thermoelectric oxides distinguish themselves（generally speaking）because of their better addressing these concerns.Among which is titanium dioxide with versatile properties and applications.As such,the relatively greater freedom of compositional manipulation with the x value of TiO_2-x-x ranging from 0 to 1 and the concomitant tunable semiconducting properties are the heralds of possibly great thermoelectric potential.Along with the reduction of TiO₂ to TiO_2-x（x≠0）,the so-called crystallographic shear structures are developed,which are desired for thermoelectric materials for its featured with the scattering centers for phonons but not for carriers.Their characteristics including adjustable wide band gap,unique crystallographic shear structures and the consequent electron-transport behaviors and phonon-transport properties have been attracting continual attentions from the viewpoint of thermoelectricity.The utilizing of advanced synthesis technologies is an important leverage in exploringhigh-performancethermoelectricmaterials.High-pressureand high-temperature（HPHT）hasdemonstratedeffectivenessinoptimizing thermoelectric performances,characterized by simultaneous modulations by pressure and temperature,simplifying procedures,shortening the process period,tailoring intrinsic material parameters and thus inducing novel physical phenomena and properties.The good properties obtained under high pressure can be preserved to ambient conditions by virtue of high-pressure quench.In this paper,a series of nonstoichiometric titanium dioxides were prepared by HPHT.The effects of synthesis pressure,synthesis temperature and oxygen vacancy on their thermoelectric properties were studied.New physical phenomena introduced by HPHT was also revealed.The main contents are as following.1.A series of nonstoichiometric titanium dioxides(TiO_1.76,TiO_1.80,TiO_1.84)were successfully prepared by HPHT at 3.0GPa and 1080°C.The effects of oxygen vacancy on their thermoelectric properties were systematically studied.It was found that all samples were composed of Magnèli phases(Ti₄O₇,Ti₇O₁₃)and rutile TiO₂.The nonstoichiometric titanium dioxides synthesized by HPHT were of multi-scale hierarchical structures and ample lattice defects,beneficial for decreasing the thermal conductivity,resulted from the effect of high pressure.The resistivity and the absolute value of the Seebeck coefficient of the sample are inversely related to the oxygen vacancy,and the thermal conductivity also decreases with the oxygen vacancy.All samples were semiconductors and the higher the test temperature was,the better the thermoelectric performance got.In TiO_1.76,we got the lowest electrical resistivity of0.004?cm@700℃,the lowest thermal conductivity of 1.60Wm^-1K^-1@700℃,and the highest zT value of 0.33@700℃.2.Nonstoichiometric titanium dioxide TiO_1.80.80 was synthesized by HPHT under different pressures（2.0GPa,2.5GPa,4.0GPa,5.0GPa）and effects of the pressure on their thermoelectric properties were studied.XRD,SEM,TEM and UV spectrum measurements demonstrated that the crystal structures and microstructures were strongly modulated by HPHT.All samples were composed of phases of rutile TiO₂and Ti₉O₁₇.With the increasing of the pressure,the peaks of XRD shifted to higher angles,both the lattic parameter and the grain size became smaller and the band gap was narrowed.Also,the electrical conductivity was improved.Additionally,HPHT had introduced ample lattice defects and multi-scale hierarchical structures in the samples,resulting in full phonon spectral scattering and low thermal conductivity.Consequently,a high dimensionless figure of merit（zT）of 0.36 was obtained at 700℃in the sample fabricated at 5.0GPa,higher than all results of nonstoichiometric titanium dioxide by means of ambient pressure and an enhancement of 57%of the ever best result.HPHT offers us a promising alternative for the optimization of thermoelectric properties.3.The effects of synthesis temperature on the thermoelectric properties of nonstoichiometric titanium dioxide TiO_1.76.76 was investigated under 5.0GPa.The phase conditions,morphologies and microstructures were all strongly modulated by the synthesis temperature of HPHT.All samples contained Magnèli phase Ti_nO_2n-1(Ti₆O₁₁,Ti₂O₃)and rutile TiO₂,and the relative content of each phase also changed with the change of synthesis temperature.Compared with the samples synthesized by the atmospheric pressure method,the electrical properties of the samples by HPHT were improved and the thermal conductivity was suppressed.As a result,a zT of 0.35 was peaked in the sample prepared at 1080℃at the test temperature of 700℃.Optimized pressure and temperature are the key to the preparation of high performance nonstoichiometric TiO_1.76.4.Pressure-induced thermoelectric properties of nonstoichiometric titanium oxides.A series of nonstoichiometric titanium oxides(TiO_1.16,TiO_1.18,TiO_1.20,TiO_1.21,TiO_1.25)were successfully synthesized by HPHT at 5.0GPa.All samples were composed of two phases,TiO and Ti₂O₃.From TiO_1.25.25 to TiO_1.16,the ratio of phase TiO to phase Ti₂O₃ increased.In all samples,the particle size range was relatively large and HPHT introduced abundant lattice defects into the samples.HPHT was found to induce phase transitions,bring about unique morphologies and microstructures and thus novel properties for nonstoichiometric titanium oxides..The electrical resistivity of samples by HPHT was suppressed,and the change of conducting type of some samples with measurement-temperature was novel.In summary,a series of nonstoichiometric titanium dioxide thermoelectric materials were successfully prepared by HPHT.Their electrical and thermal properties were simultaneously optimized by regulations of pressure,temperature,oxygen vacancy,phase compositions and microstructures.By virtue of high-pressure quench,their thermoelectric properties were improved.This work points the way for us to further use HPHT to prepare green and efficient titanium dioxide based thermoelectric materials.