Investigation of Reduced (SrxBa1-x)Nb2O6 as a Ferroelectric-based Thermoelectric

Open Access
Author:
Bock, Jonathan Anton
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 18, 2016
Committee Members:
  • Clive Randall And Susan Trolier Mc Kinstry, Dissertation Advisor
Keywords:
  • Thermoelectric
  • Ferroelectrics
  • Tungsten-Bronze
Abstract:
A comprehensive study of a novel type of thermoelectric – a heavily doped material from a ferroelectric base composition – is presented. Due to the low-lying optic modes and scattering of phonons at domain walls, ferroelectrics make interesting candidates for thermoelectrics. The example of (Srx,Ba1-x)Nb2O6-δ (SBN) is explored in detail due to a report of an impressive thermoelectric figure of merit in single crystals. The goal of this research is to understand the source of the large figure of merit in SBN. In attempts to do this, the electron transport mechanism, the coupling between electron transport and ferroelectricity, the phase equilibria, and the single crystalline thermoelectric properties were investigated under various reduction conditions. It was found that the electron transport properties of a normal ferroelectric SBN can be well explained by activation of electrons into the conduction band from a localized impurity band. SBN can be shifted between a normal and relaxor ferroelectric by changing the Sr:Ba ratio. This property of SBN was utilized to study the effect of relaxor ferroelectricity on electron transport. Within the relaxor ferroelectric regime, a change in the activation energy for electronic conduction and an abnormal temperature dependence of the Seebeck coefficient were found. These properties are attributed to Anderson localization caused by the relaxor ferroelectricity. This is not thought to be the cause of the large thermoelectric figure of merit. The electron transport-ferroelectric coupling was also studied in oxygen deficient (Bax,Sr1-x)TiO3-δ (BST). A metallic-like to nonmetallic transition occurs at the ferroelectric transition, and the temperature of the metallic-like to nonmetallic transition can be shifted via Sr doping. The temperature shift on Sr doping is equivalent to the shift in the paraelectriciv ferroelectric transition temperature in unreduced samples, showing that the ferroelectric transition is the cause of the metallic-like to nonmetallic transition. The presence of an impurity band in SBN points toward a large carrier concentration which is difficult to justify with oxygen vacancies alone, and this large carrier concentration is thought to provide the enhanced thermoelectric properties. To investigate the cause of the enhanced carrier concentration in reduced SBN, the defect chemistry and phase equilibria of the system under low oxygen partial pressure conditions was studied. A secondary phase of NbO2 was identified upon reduction, and was found to correlate with an increase in the (Sr+Ba):Nb ratio of the SBN matrix. It will be shown that if the assumption is made that the excess Sr and Ba remain in the SBN lattice, then the (Sr+Ba):Nb ratio of the SBN matrix can accurately predict the amount of NbO2 which forms. Additionally, the amount of NbO2 secondary phase decreases on increasing the Sr and Ba concentration of samples. Eventually a phase pure composition is formed when the (Sr+Ba):Nb ratio corresponds to a Sr and Ba site occupancy of 1, (Sr0.6,Ba0.4)1.2Nb2O6. Above this, a tertiary phase forms. These findings are consistent with the Sr and Ba sites being filled as a function of reduction. This A-site filling process is thought to be the source of the large thermoelectric figure of merit. To investigate the air-stability, thermogravimetric analysis was used to measure the oxygen absorption upon heating in air. The amount of oxygen which is absorbed was equivalent to the necessary amount to fully compensate the excess Sr and Ba. The resulting samples are insulating. The thermoelectric properties of and the NbO2 formation in reduced SBN single crystals were also investigated. The formation of NbO2 was hindered due to the lack of grain boundaries which act as heterogeneous nucleation sites. The homogeneous nucleation energy prevented bulk NbO2 formation until 10-15 atm pO2 at 1300°C, two orders of magnitude below the oxygen partial pressure necessary in ceramics. This was overcome using a pre-anneal at 10-16 atm pO2 followed by a re-equilibration at lower pO2. The highest power factor measured at 600°C was 8.5 μW/cmK2 resulting in a zT of 0.38 if polycrystalline values of the thermal conductivity are assumed. The previously mentioned low-temperature Seebeck anomaly caused by Anderson localization was found to exist along the polar c-axis, but not the non-polar a-axis. This further strengthens the idea that perturbations to the electron transport are caused by the ferroelectric polarization. These results show that the thermoelectric properties found in SBN upon reduction are due to a change from (Srx,Ba1-x)Nb2O6-δ toward (Srx,Ba1-x)1.2Nb2O6-δ and the resulting carrier concentration associated with the additional Sr2+ and Ba2+ cations on the A-site. Relaxor ferroelectricity perturbs the electron transport, but is not a cause of enhanced thermoelectric properties. This points toward A-site doped tungsten bronze materials in general as interesting thermoelectric materials. Future work revolving around decreasing the octahedral tilt angle, increasing the d-orbital overlap, and determining the necessity of ferroelectric-thermoelectric coupling in relation to thermal conductivity could result in further optimization within this new interesting family of thermoelectric oxides.