Investigation On The Synthesis And Thermoelectric Properties Of Ruddlesden-popper Phase Sr_n+1Ti_nO_3n+1

ABO₃-type perovskite composite oxide is one kind of the important inorganiccompounds and is also a versatile functional material. Among the perovskite compounds,SrTiO₃is an important material for electronics caused it has the advantages of high dielectricconstant, low dielectric loss and good thermal stability. It is widely used to make degausscomponents and self-regulation heating elements, ceramic sensors, microwave ceramiccomponents and so on. Especially, high quality SrTiO₃powder could be used to productboundary layer capacitors, PTC thermistor components, which have high performance, highreliability, small size and some other merits. Furthermore, SrTiO₃powder can also be used tomake paints, enamel, insulation and heat-resistant materials etc. Currently, SrTiO₃powder isalso used as a catalyst for pyrolysis gasoline. At present, the methods of producing SrTiO₃powder mainly include solid state reaction and chemical precipitation. The disadvantage ofthose methods is that they need to take long time to grind the raw material and need to sinterat high temperature normally larger than1200°C. Therefore it is meaningful to produce thefine SrTiO₃powder with shorter process time and lower sintering temperature. Since SrTiO₃is one member of the Sr_n+1Ti_nO_3n+1Ruddlesden-Popper （RP） homologous series, it isnecessary to expand the investigation of SrTiO₃powder to the Sr_n+1Ti_nO_3n+1RP phasepowders. The Sr_n+1Ti_nO_3n+1RP phases have a cubic layered perovskite structure. It consists ofSrO layers and TiO₂layers alternatively stacked[3]. In this paper we focused on the RP phaseswhich molecular formula can be expressed as Sr_n+1Ti_nO_3n+1（n=1,2,3,∞）. For n=1, themolecular formula is Sr₂TiO₄[（SrTiO₃）SrO]. It has a square structure which consists ofdouble SrO layers and TiO₂layer alternatively stacked. For n=2, the molecular formula isSr₄Ti₃O₇[（SrTiO₃）2SrO]. It has a square structure which consists of tripled SrO layers anddouble TiO₂layers alternatively stacked. For n=3, the molecular formula is Sr₄Ti₃O₁₀[（SrTiO₃）3SrO]. It has a square structure which consists of fourfold SrO layers and tripledTiO₂layers alternatively stacked. For n=∞, the molecular formula is SrTiO₃, which consists ofsingle SrO and TiO₂layers alternatively stacked.A series of high pure RP phase Sr_n+1Ti_nO_3n+1（n=1,2,3,∞） nanoscale powders have been produced by sol-gel method. The sintering temperatures of SrTiO₃, Sr₂TiO₄, Sr₄Ti₃O₇andSr₄Ti₃O₁₀determined by TG-DSC analysis are800,1050,950and1050℃, respectively. Thesingle phase structures of the four powders are identified by x-ray diffraction analysis. Thedifferent molecular structures and the different vibration bands are studied by Fouriertransform infrared absorption spectroscopy and Raman spectroscopy. It is measured that thedielectric constants of SrTiO₃, Sr₄Ti₃O₁₀, Sr₄Ti₃O₇and Sr₂TiO₄are114,54,41and29,respectively at100kHz. The dielectric losses for the four powders show a larger change in theregion of low frequency10²-10⁴Hz, and a smaller change in the region of high frequency10⁵-10⁷Hz. All of the dielectric losses for the four powders are less than4×10-3at100kHz.The RP phase Sr_n+1Ti_nO_3n+1powders were synthesized by sol-gel method and pressedinto target. High quality Sr_n+1Ti_nO_3n+1epitaxial films have grown on the （001） face of LaAlO₃single crystal substrate by pulsed laser deposition （PLD）. Resistivity and Seebeck coefficientof these thin films were measured by Physical Properties Measurement System （PPMS） over175-375K. The results show that the SrTiO_3-δfilm grown under4×10^-4Pa has the highestpower factor among the RP phase Sr_n+1Ti_nO_3n+1in this work. At300K, the power factor ofthis film approaches5.94mW/m K2. This value is similar to S(r_0.97La_0.03TiO₃single crystal andSrTi_0.8Nb_0.2O₃epitaxial film, and it can be compared with the traditional alloy thermoelectricmaterials Bi₂Te₃. The ZT value is estimated to be reached0.2. Consequently, the SrTiO₃–δfilms have high application potential as thermoelectric material at room temperature and lowtemperatures.