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Seebeck coefficient (thermoelectric power) measurements

The ProboStat can be used to measure Seebeck coefficient in many ways. The standard method is to suspend a bar sample between two thermocouple tips and measure temperatures at both ends and then the sample voltage between the + or - leads of the thermocouples, or use separate electrodes for sample voltage measurement as in the picture.

Temperature gradient over the sample is either naturally occurring furnace gradient or one induced by small internal heater placed above the spring load top plate. This method can also be used in combination with 4-point conductivity measurement.

This method can be be done from ambient temperature up to 1600ºC, from vacuum to high pressure (high pressure option), with any type of gas atmosphere.

Best results are achieved with bar sample of 3-4 cm length. Sample may be of arbitary shape but it is easier to mount if the sample is bar shaped with longer length than cross section.Disc and thin film samples are not mounted as easily as bars and the electrode contacts may have to be glued (with Pt ink) to the side (since the film only covers the side and not the ends of the sample).

There is no clear limit how small samples can be measured, but as the sizes go smaller the mounting gets harder and more importantly the ratio between signal and error gets less favourable.

This method involves reading two thermocouple voltages and the sample voltage, converting the thermocouple voltages to temperatures, calculating the temperature difference and plotting the sample voltage against the temperature difference. This can be performaed by the user or automatically with the Omega software. NORECS can provide with full system (including ProboStat sample holder, furnace, Omega software, multimeter and power supply for the internal heater) that can automatically perform all the tasks needed for determining Seebeck coefficient at various temperatures.

ProboStat and Omega offer well proven and repeatable way of measuring Seebeck coefficients but also allows user great freedom to alter the setup to match for the specific qualities and challenges each individual sample has. This freedom and control over every aspect is the key to reach accurate result164 and is missing in 'turn-key' systems.

It is possible to combine this method with four point resistance measurement by adding two additional electrodes on the sample, see combined Seebeck coefficient and conductivity measurement

Seebsys - Combined Seebeck coefficient and resistance measurement system

 

These articles refer to ProboStat or other NORECS products, filtered with keywords: 'Seebeck, Thermoelectricity, Thermoelectric '  
ID=709

Experimental application of a laser-based manufacturingprocess to develop a free customizable, scalablethermoelectric generator demonstrated on a hot shaft

Authors Marvin Abt, Katharina Kruppa, Mario Wolf, Armin Feldhoff, Ludger Overmeyer
Source
Engineering Reports
Time of Publication: 2022
Abstract Geometry, design, and processing in addition to the thermoelectric materialproperties have a significant influence on the economic efficiency and perfor-manceofthermoelectricgenerators(TEGs).WhileconventionalBULKTEGsareelaborate to manufacture and allow only limited variations in geometry, printedTEGs are often restricted in their application and processing temperature due totheuseoforganicmaterials.Inthiswork,aproof-of-conceptforfabricatingmod-ular, customizable, and temperature-stable TEGs is demonstrated by applyingan alternative laser process. For this purpose, low temperature cofired ceram-ics substrates were coated over a large area, freely structured and cut withoutmasks by a laser and sintered to a solid structure in a single optimized thermalpost-processing.Ascalabledesignwithcomplexgeometryandlargecoolingsur-face for application on a hot shaft was realized to prove feasibility. Investigationson sintering characteristics up to a peak temperature of 1173K, thermoelec-tric material properties and temperature distribution were carried out for aCa3Co4O9/Ag-based prototype and evaluated using profilometer, XRD, and IRmeasurements. For a combined post-processing, an optimal sintering profilecould be determined at 1073K peak temperature with a 20min holding time.Temperaturegradientsofupto100Kcouldbeachievedalongathermocouple.Asingle TEG module consisting of 12 thermocouples achieved a maximum powerof0.224μWandopen-circuitvoltageof134.41mVatanaveragehot-sidetemper-ature of 413.6 K and temperature difference of 106.7 K. Three of these modulescombined into a common TEG with a total of 36 thermocouples reached a maxi-mumpowerof0.58Kandopen-circuitvoltageof319.28mVwithalesseraveragehot-side temperature of 387.8 K and temperature difference of 83.4 K.
Remark https://doi.org/10.1002/eng2.12590
Link
ID=699

Electrospun Ca3Co4−xO9+δ nanofibers and nanoribbons: Microstructure and thermoelectric properties

Authors Katharina Kruppa, Itzhak I. Maor, Frank Steinbach, Vadim Beilin, Meirav Mann-Lahav, Mario Wolf, Gideon S. Grader, Armin Feldhoff
Source
J Am Ceram Soc.
Volume: 106, Pages: 1170–1181
Time of Publication: 2023
Abstract Oxide-based ceramics offer promising thermoelectric (TE) materials for recy- cling high-temperature waste heat, generated extensively from industrial sources. To further improve the functional performance of TE materials, their power factor should be increased. This can be achieved by nanostructuring and texturing the oxide-based ceramics creating multiple interphases and nanopores, which simultaneously increase the electrical conductivity and the Seebeck coef- ficient. The aim of this work is to achieve this goal by compacting electrospun nanofibers of calcium cobaltite Ca3 Co 4−xO 9+δ, known to be a promising p-type TE material with good functional properties and thermal stability up to 1200 K in air. For this purpose, polycrystalline Ca3 Co 4−xO 9+δ nanofibers and nanorib- bons were fabricated by sol–gel electrospinning and calcination at intermediate temperatures to obtain small primary particle sizes. Bulk ceramics were formed by sintering pressed compacts of calcined nanofibers during TE measurements. The bulk nanofiber sample pre-calcined at 973 K exhibited an improved Seebeck coefficient of 176.5 S cm−1 and a power factor of 2.47 μW cm−1 K−2 similar to an electrospun nanofiber-derived ceramic compacted by spark plasma sintering.
Remark DOI: 10.1111/jace.18842
Link
ID=697

Tuning the Thermoelectric Performance of CaMnO3-Based Ceramics by Controlled Exsolution and Microstructuring

Authors Nikola Kanas, Benjamin A. D. Williamson, Frank Steinbach, Richard Hinterding, Mari-Ann Einarsrud, Sverre M. Selbach, Armin Feldhoff, and Kjell Wiik
Source
CS Appl. Energy Mater.
Volume: 5, Issue: 10, Pages: 12396–12407
Time of Publication: 2022
Abstract The thermoelectric properties of CaMnO3−δ/CaMn2O4 composites were tuned via microstructuring and compositional adjustment. Single-phase rock-salt-structured CaO–MnO materials with Ca:Mn ratios larger than unity were produced in reducing atmosphere and subsequently densified by spark plasma sintering in vacuum. Annealing in air at 1340 °C between 1 and 24 h activated redox-driven exsolution and resulted in a variation in microstructure and CaMnO3−δ materials with 10 and 15 vol % CaMn2O4, respectively. The nature of the CaMnO3−δ/CaMn2O4 grain boundary was analyzed by transmission electron microscopy on short- and long-term annealed samples, and a sharp interface with no secondary phase formation was indicated in both cases. This was further complemented by density functional theory (DFT) calculations, which confirmed that the CaMnO3−δ indeed is a line compound. DFT calculations predict segregation of oxygen vacancies from the bulk of CaMnO3−δ to the interface between CaMnO3−δ and CaMn2O4, resulting in an enhanced electronic conductivity of the CaMnO3−δ phase. Samples with 15 vol % CaMn2O4 annealed for 24 h reached the highest electrical conductivity of 73 S·cm–1 at 900 °C. The lowest thermal conductivity was obtained for composites with 10 vol % CaMn2O4 annealed for 8 h, reaching 0.56 W·m–1K–1 at 700 °C. However, the highest thermoelectric figure-of-merit, zT, was obtained for samples with 15 vol % CaMn2O4 reaching 0.11 at temperatures between 800 and 900 °C, due to the enhanced power factor above 700 °C. This work represents an approach to boost the thermoelectric performance of CaMnO3−δ based composites.
Remark https://doi.org/10.1021/acsaem.2c02012
Link
ID=689

Synthesis of a Novel Nanoparticle BaCoO2.6 through Sol-Gel Method and Elucidation of Its Structure and Electrical Properties

Authors Fareenpoornima Rafiq, Parthipan Govindsamy, and Selvakumar Periyasamy
Source
Journal of Nanomaterials
Time of Publication: 2022
Abstract The physical properties of cobalt oxide with varied oxidation states, and coordination numbers, in the transition series, have numerous applications. The present study explores the physical properties of BaCoO2.6 nanoparticles synthesized through the sol-gel method. The X-ray diffraction figure exhibits a 25 nm crystallite size hexagonal phase. The observational data shows the reduction in the real part of impedance (), dielectric constant (), dielectric loss (), and a raise in ac conductivity of mixed type of conduction with an elevation in frequency analyzed through impedance spectroscopy. The conductivity due to grain and grain boundaries is shown foremost in the complex impedance analysis. The plot of (Seebeck coefficient) in the low-temperature range indicates p-type behavior and the metal-insulator transition in the as-synthesized sample. The sample characteristics suggest applications in optical and switching devices. The Seebeck coefficient is the generation of potential difference when subjected to temperature difference. Thermoelectric materials are associated with the concept of high electrical conductivity like crystals and low thermal conductivity to that of glass. Nanothermoelectric materials can decrease further the thermal conductivity through phonon scattering. Electrical characterization suggests the presence of both NTCR and PTCR behavior in the sample, and hence, it explores the application in thermistor/resistance temperature detector’s (RTD) and low dielectric constant and loss to electro-optical and higher conversion efficiency to storage devices. Additionally, impedance spectroscopy helps in the study of electrochemical systems and solid-state devices wherein the transition of metal-insulator is an add-on to the research.
Remark https://doi.org/10.1155/2022/3877879
Link
ID=668

Lanthanum strontium cobaltite as interconnect in oxide thermoelectric generators

Authors Reshma K.Madathil, TrulsNorby
Source
Solid State Sciences
Volume: 124, Pages: 106801
Time of Publication: 2022
Abstract Issues related to use of metallic interconnects in oxide thermoelectric generators (TEGs) need to be addressed to secure performance and durability. Metal interconnects suffer from high cost of noble metals or chemical instability and contact resistance of non-noble metals, arising from oxidation, evaporation, and delamination in the oxidising conditions of ambient air at high operating temperatures. This work introduces the use of a stable and highly conducting ceramic oxide, in our case p-type lanthanum strontium cobaltite (La0.6Sr0.4CoO3, LSC) as interconnect. We verified the thermochemical stability of LSC in contact with p-type Ni0.98Li0.02O (Li–NiO) and n-type Zn0.98Al0.02O (Al–ZnO) and examined the electrical characteristics. An area specific contact resistance (ASRc) of ∼1800 Ω cm2 for a direct p-n junction was reduced to ∼400 mΩ cm2 for a p-LSC-n junction at a temperature of 300 °C, validating the concept. The use of a screen-printed LSC/Al–ZnO composite as a thin interconnect layer was found to decrease the contact resistance of the junction further to ∼260 mΩ cm2 at 300 °C, attributed to increased effective area of the LSC/Al–ZnO p-n junction.
Keywords Thermoelectric generator; All-oxide; Thermoelectric materials; Oxides; Interconnect; Oxide; p-n-junction; Ohmic; LaCoO3; Sr-substituted; La0.6Sr0.4CoO3
Remark Link
ID=660

Glass-ceramic composites as insulation material for thermoelectric oxide multilayer generators

Authors Sophie Bresch, Björn Mieller, Paul Mrkwitschka, Ralf Moos, Torsten Rabe
Source
Time of Publication: 2021
Abstract Thermoelectric generators can be used as energy harvesters for sensor applications. Adapting the ceramic multilayer technology, their production can be highly automated. In such multilayer thermoelectric generators, the electrical insulation material, which separates the thermoelectric legs, is crucial for the performance of the device. The insulation material should be adapted to the thermoelectric regarding its averaged coefficient of thermal expansion α and its sintering temperature while maintaining a high resistivity. In this study, starting from theoretical calculations, a glass-ceramic composite material adapted for multilayer generators from calcium manganate and calcium cobaltite is developed. The material is optimized towards an α of 11 × 10−6 K−1 (20–500°C), a sintering temperature of 900°C, and a high resistivity up to 800°C. Calculated and measured α are in good agreement. The chosen glass-ceramic composite with 45 vol.% quartz has a resistivity of 1 × 107 Ωcm and an open porosity of <3%. Sintered multilayer samples from tape-cast thermoelectric oxides and screen-printed insulation show only small reaction layers. It can be concluded that glass-ceramic composites are a well-suited material class for insulation layers as their physical properties can be tuned by varying glass composition or dispersion phases.
Remark https://doi.org/10.1111/jace.18235
Link
ID=631

Influence of Doping on the Transport Properties of Y1−xLnxMnO3+δ (Ln: Pr, Nd)

Author Kacper Cichy and Konrad Swierczek
Source
Crystals
Volume: 11, Pages: 510
Time of Publication: 2021
Abstract It has been documented that the total electrical conductivity of the hexagonal rare-earth manganites Y0.95Pr0.05MnO3+δ and Y0.95Nd0.05MnO3+δ, as well as the undoped YMnO3+δ, is largely dependent on the oxygen excess δ, which increases considerably at temperatures below ca. 300 ◦C in air or O2. Improvement for samples maintaining the same P63cm crystal structure can exceed 3 orders of magnitude below 200 ◦C and is related to the amount of the intercalated oxygen. At the same time, doping with Nd3+ or Pr3+ affects the ability of the materials to incorporate O2, and therefore indirectly influences the conductivity as well. At high temperatures (700–1000 ◦C) and in different atmospheres of Ar, air, and O2, all materials are nearly oxygen-stoichiometric, showing very similar total conduction with the activation energy values of 0.8–0.9 eV. At low temperatures in Ar (δ ≈ 0), the mean ionic radius of Y1−xLnx appears to influence the electrical conductivity, with the highest values observed for the parent YMnO3. For Y0.95Pr0.05MnO3+δ oxide, showing the largest oxygen content changes, the recorded dependence of the Seebeck coefficient on the temperature in different atmospheres exhibits complex behavior, reflecting oxygen content variations, and change of the dominant charge carriers at elevated temperatures in Ar (from electronic holes to electrons). Supplementary cathodic polarization resistance studies of the Y0.95Pr0.05MnO3+δ electrode document different behavior at higher and lower temperatures in air, corresponding to the total conduction characteristics.
Remark Link
ID=623

Versatile four-leg thermoelectric module test setup adapted to a commercial sample holder system for high temperatures and controlled atmospheres

Authors Raphael Schuler, Reshma K. Madathil, and Truls Norby
Source
Review of Scientific Instruments
Volume: 92, Pages: 043902
Time of Publication: 2021
Abstract A high temperature thermoelectric test setup for the NORECS ProboStat™ sample holder cell has been designed, constructed, and tested. It holds four thermoelectric legs of up to 5 × 5 mm2 area each and flexible height, allows various interconnects to be tested, and utilizes the spring-load system of the ProboStat for fixation and contact. A custom stainless steel support tube flushed with water provides the cold sink, enabling large temperature gradients. Thermocouples and electrodes as well as the gas supply and outer tube use standard ProboStat base unit feedthroughs and dimensions. The setup allows for testing in controlled atmospheres with the hot side temperature of up to around 1000 °C and a temperature gradient of up to 600 °C. We demonstrate the test setup on a four-leg Li–NiO/Al–ZnO module with gold interconnects. The comparison between the predicted performance based on individual material parameters and the experimentally obtained module performance underlines the necessity for testing materials in combination, including interconnects. The four-leg setup allows versatile match-screening, performance evaluation, and long-term stability studies of thermoelectric materials in combination with hot and cold side interconnects under realistic operational conditions.
Remark Link
ID=612

Improved environmental stability of thermoelectric ceramics based on intergrowths of Ca3Co4O9–Na0.75CoO2

Authors Damjan Vengust, Bostjan Jancar, Tilen Sever, Andreja Šestan, Vid Bobnar, Zdravko Kutnjak, Nina Daneu, Danilo Suvorov, Matjaz Spreitzer
Source
Ceramics International
Volume: 47, Issue: 8, Pages: 11687-11693
Time of Publication: 2021
Abstract Ceramics based on calcium and sodium cobaltates are promising high-temperature thermoelectric oxide materials with complementary advantages. Ca3Co4O9 is stable at high temperatures, whereas Na0.75CoO2 can be easily processed as a textured ceramic with excellent thermoelectric properties. Na0.75CoO2, however, lacks long-term stability and degrades in even a relatively mild humid environment. In this work, we present a novel approach to the synthesis of complex composite materials based on intergrowths of sodium and calcium cobaltates that have excellent thermoelectric performance and improved stability. We synthesized samples with the mixed composition (3-x)Ca3Co4O9–4x(Na0.75CoO2) in an over-pressured oxygen atmosphere. Samples with the mixed Ca–Na composition developed textured microstructures composed of intergrowths of both end-members, as revealed by transmission electron microscopy. We also examined the thermoelectric performance of the investigated materials after exposure to high humidity and found that the composition with x = 0.8 (Ca:Na = 2.75) has long-term stability.
Keywords Composite materials; Microstructure; Transmission electron microscopy; Thermoelectric
Remark Link
ID=611

Time-Enhanced Performance of Oxide Thermoelectric Modules Based on a Hybrid p–n Junction

Authors Nikola Kanas, Rasmus Bjørk, Kristin Høydalsvik Wells, Raphael Schuler, Mari-Ann Einarsrud, Nini Pryds, and Kjell Wiik
Source
ACS Omega
Volume: 6, Issue: 1, Pages: 197–205
Time of Publication: 2020
Abstract The present challenge with all-oxide thermoelectric modules is their poor durability at high temperatures caused by the instability of the metal-oxide interfaces at the hot side. This work explains a new module concept based on a hybrid p–n junction, fabricated in one step by spark plasma co-sintering of Ca3Co4–xO9+δ (CCO, p-type) and CaMnO3−δ/CaMn2O4 (CMO, n-type). Different module (unicouple) designs were studied to obtain a thorough understanding of the role of the in situ formed hybrid p–n junction of Ca3CoMnO6 (CCMO, p-type) and Co-oxide rich phases (p-type) at the p–n junction (>700 °C) in the module performance. A time-enhanced performance of the modules attributed to this p–n junction formation was observed due to the unique electrical properties of the hybrid p–n junction being sufficiently conductive at high temperatures (>700 °C) and nonconductive at moderate and low temperatures. The alteration of module design resulted in a variation of the power density from 12.4 (3.1) to 28.9 mW/cm2 (7.2 mW) at ΔT ∼ 650 °C after 2 days of isothermal hold (900 °C hot side). This new concept provides a facile method for the fabrication of easily processable, cheap, and high-performance high-temperature modules.
Remark Link
ID=576

Defects and polaronic electron transport in Fe2WO6

Authors Raphael Schuler, Truls Norby, Helmer Fjellvåg
Source
Physical Chemistry Chemical Physics
Issue: 27 Time of Publication: 2020
Abstract We report the synthesis of phase pure Fe2WO6 and its structural characterization by high quality synchrotron X-ray powder diffraction, followed by studies of electric and thermoelectric properties as a function of temperature (200–950 °C) and pO2 (1–10−3 bar). The results are shown to be in accordance with a defect chemical model comprising formation of oxygen vacancies and charge compensating electrons at high temperatures. The standard enthalpy and entropy of formation of an oxygen vacancy and two electrons in Fe2WO6 are found to be 113(5) kJ mol−1 and 41(5) J mol−1 K−1, respectively. Electrons residing as Fe2+ in the Fe3+ host structure act as charge carriers in a small polaron conducting manner. A freezing-in of oxygen vacancies below approximately 650 °C results in a region of constant charge carrier concentration, corresponding to an iron site fraction of XFe2+ ≅ 0.03. By decoupling of mobility from conductivity, we find a polaron hopping activation energy of 0.34(1) eV and a charge mobility pre-exponential u0 = 400(50) cm2 kV−1 s−1. We report thermal conductivity for the first time for Fe2WO6. The relatively high conductivity, large negative Seebeck coefficient and low thermal conductivity make Fe2WO6 an interesting candidate as an n-type thermoelectric in air, for which we report a maximum zT of 0.027 at 900 °C.
Remark Link
ID=547

Tuning of Mg content to enhance the thermoelectric properties in binary Mg2+δ Si (δ = 0, 0.1, 0.15, 0.2)

Authors Priyadarshini Balasubramanian, Manjusha Battabyal, Dhruba Das, Arumugam Chandra Bose and Raghavan Gopalan
Source
Materials Research Express
Volume: 6 Time of Publication: 2019
Abstract We report the enhanced thermoelectric properties of binary Mg2Si by tuning the Mg content. Polycrystalline Mg2+δ Si (where δ is the excess Mg content in the starting composition of the samples and δ = 0, 0.1, 0.15, 0.2) samples were processed by solid-state synthesis route using ball milling followed by rapid spark plasma sintering in order to minimize the Mg loss during processing. Microstructural and x-ray diffraction analysis revealed that, Mg content (δ) of 0.1–0.15 is required to get the binary Mg2Si phase without any elemental Mg/Si phase. Hall effect measurement and Fourier Transform Infrared Spectroscopy analysis show that, the excess Mg content helps to enhance the carrier concentration and charge carrier effective mass due to the occupancy of Mg at the interstitial site in Mg2Si structure. The influence of Mg content on thermoelectric properties, viz., electrical resistivity, Seebeck coefficient and thermal conductivity is investigated from 300 K to 780 K. A marked enhancement in thermoelectric power factor (~1.6 mW m−2K−2) is obtained for Mg2.15Si sample at 780 K. The occupancy of excess Mg at interstitial sites reduces the lattice thermal conductivity by lowering lattice symmetry. A maximum figure of merit (ZT) ~ 0.39 ± 0.03 at 780 K has been achieved in Mg2.15Si sample, the highest among that reported in n-type binary Mg2Si system. This suggests that excess Mg content in the starting composition of Mg2+δ Si helps in stabilizing the phase as well as improves the thermoelectric properties of the Mg2Si.
Remark https://doi.org/10.1088/2053-1591/ab58fb
Link
ID=514

Tuning the optical and thermoelectric properties of SrTi0.8−x Sn0.2FexO3

Authors Keerthana Muthamilselvam, M Mayarani, G Mohan Muralikrishna, Manjusha Battabyal, and Raghavan Gopalan
Source
Materials Research Express
Volume: 6, Issue: 4 Time of Publication: 2019
Abstract Effect of Fe doping on the structure, optical and thermoelectric properties of SrTi0.8Sn0.2O3 sample has been investigated. The SrTi0.8−xSn0.2FexO3 (x = 0, 0.1, 0.3) samples are fabricated using solid-state synthesis route. It is observed that Fe doping helps in reducing the densification temperature of SrTi0.8Sn0.2O3 during spark plasma sintering. Precipitation of Sn has been observed in SrTi0.8−xSn0.2FexO3 (x = 0, 0.1) samples while the SrTi0.8−xSn0.2FexO3 (x = 0.3) sample is of purely single cubic perovskite phase. All the samples consist of nanocrystalline grains and the grain size varies between 150 to 200 nm. Fourier transform infrared spectroscopy (FTIR) analysis reveals the distortion of TiO6 octahedra due to the increase in Fe content. Raman spectroscopy analysis has shown that perovskite cubic structure is stable from room temperature to 873 K. From thermophysical measurements, it is shown that the Fermi band gap reduces from 2.87 to 0.66 eV with increase in Fe in the investigated samples. The Seebeck co-efficient is found to change the sign from n –type to p-type with the increase of Fe concentration in SrTi0.8Sn0.2O3, which is an interesting observation to obtain p-type SrTiO3 based thermoelectric materials. The optical and thermoelectric properties show that Fe doping improves the thermoelectric properties of SrTi0.8Sn0.2O3 ceramics by altering the Seebeck co-efficient and thermal conductivity.
Remark Link
ID=504

A comprehensive study on improved power materials for high-temperature thermoelectric generators

Authors
Source
Journal of Power Sources
Volume: 410-411, Pages: 143-151
Time of Publication: 2019
Abstract Dense Ca3Co4O9-NaxCoO2-Bi2Ca2Co2O9 (CCO-NCO-BCCO) nanocomposites were produced from sol-gel derived Ca2.25Na0.3Bi0.35Tb0.1Co4O9 powder by four methods: Hot-pressing (HP), spark plasma sintering (SPS) and pressureless sintering in air or O2 atmosphere. Nanocomposites from HP and SPS revealed nanosized grains and showed a thermoelectric power factor of 4.8 and 6.6 μW cm−1 K−2, respectively, at 1073 K in air. A dense 2D nanocomposite with structures on multiple length scales and enhanced thermoelectric properties was obtained from pressureless sintering in O2 atmosphere. The resulting 2D nanocomposite enabled the simultaneous increase in isothermal electrical conductivity σ and Seebeck coefficient α, and showed a thermoelectric power factor of 8.2 μW cm−1 K−2 at 1073 K in air. The impact of materials with enhanced electrical conductivity and power factor on the electrical power output of thermoelectric generators was verified in prototypes. A high electrical power output and power density of 22.7 mW and 113.5 mW cm−2, respectively, were obtained, when a hot-side temperature of 1073 K and a temperature difference of 251 K were applied. Different p- and n-type materials were used to verify the effect of the thermoelectric figure-of-merit and power factor on the performance of thermoelectric generators.
Remark Link
ID=494

Computational Prediction and Experimental Realization of p-Type Carriers in the Wide-Band-Gap Oxide SrZn1–xLixO2

Authors Christos A. Tzitzeklis, Jyoti K. Gupta, Matthew S. Dyer, Troy D. Manning, Michael J. Pitcher, Hongjun J. Niu, Stanislav Savvin, Jonathan Alaria, George R. Darling, John B. Claridge, and Matthew J. Rosseinsky
Source
Inorg. Chem.
Volume: 57, Issue: 19, Pages: 11874-11883
Time of Publication: 2018
Abstract It is challenging to achieve p-type doping of zinc oxides (ZnO), which are of interest as transparent conductors in optoelectronics. A ZnO-related ternary compound, SrZnO2, was investigated as a potential host for p-type conductivity. First-principles investigations were used to select from a range of candidate dopants the substitution of Li+ for Zn2+ as a stable, potentially p-type, doping mechanism in SrZnO2. Subsequently, single-phase bulk samples of a new p-type-doped oxide, SrZn1–xLixO2 (0 < x < 0.06), were prepared. The structural, compositional, and physical properties of both the parent SrZnO2 and SrZn1–xLixO2 were experimentally verified. The band gap of SrZnO2 was calculated using HSE06 at 3.80 eV and experimentally measured at 4.27 eV, which confirmed the optical transparency of the material. Powder X-ray diffraction and inductively coupled plasma analysis were combined to show that single-phase ceramic samples can be accessed in the compositional range x < 0.06. A positive Seebeck coefficient of 353(4) μV K–1 for SrZn1–xLixO2, where x = 0.021, confirmed that the compound is a p-type conductor, which is consistent with the pO2 dependence of the electrical conductivity observed in all SrZn1–xLixO2 samples. The conductivity of SrZn1–xLixO2 is up to 15 times greater than that of undoped SrZnO2 (for x = 0.028 σ = 2.53 μS cm–1 at 600 °C and 1 atm of O2).
Remark Link
ID=493

Microstructure and doping effect on the enhancement of the thermoelectric properties of Ni doped Dy filled CoSb3 skutterudites

Authors Vikrant Trivedi, Manjusha Battabyal, Priyadarshini Balasubramanian, G. Mohan Muralikrishna, Pawan Kumar Jain and Raghavan Gopalan
Source
Sustainable Energy Fuels
Volume: 2, Pages: 2687-2697
Time of Publication: 2018
Abstract The thermoelectric properties of nanostructured Ni doped Dy filled CoSb3 skutterudites (Dy0.4Co4−xNixSb12 (x = 0, 0.4, and 0.8)) have been reported. The samples are processed using a solid-state synthesis route. The structural analysis of the samples using X-ray diffraction reveals the existence of a single skutterudite phase in Ni doped samples irrespective of the Ni concentration. Microstructure studies using transmission electron microscopy and scanning electron microscopy show the existence of nanometer (∼60 nm) size equiaxed grains in the investigated samples. A few recrystallized elongated grains (∼200 nm) are observed in the Dy0.4Co3.2Ni0.8Sb12 sample. The power factor of the Dy0.4Co3.2Ni0.8Sb12 sample is enhanced to 5.2 mW mK−2, which is the highest power factor for the doped ternary skutterudites reported so far. The enhancement of the power factor is due to the substantial reduction in electrical resistivity with an increase in Ni concentration at higher temperature. The lattice thermal conductivity is drastically reduced to 0.3 W mK−1 at 773 K in the Dy0.4Co3.2Ni0.8Sb12 sample due to the enhanced phonon scattering from Ni induced point defects and grain boundaries. As a result, a huge increase in the figure of merit (ZT ∼ 1.4 ± 0.14) at 773 K is observed in the Dy0.4Co3.2Ni0.8Sb12 sample, the highest among those of the single element filled CoSb3 skutterudites reported so far at this temperature. Hence, Ni doping could enhance the thermoelectric efficiency of Dy filled CoSb3 skutterudites. This can be taken as a reference to synthesize CoSb3 skutterudite thermoelectric materials having a higher figure of merit.
Remark DOI: 10.1039/C8SE00395E
Link
ID=492

Computational Prediction and Experimental Realization of p-Type Carriers in the Wide-Band-Gap Oxide SrZn1–xLixO2

Authors Christos A. Tzitzeklis, Jyoti K. Gupta, Matthew S. Dyer, Troy D. Manning, Michael J. Pitcher, Hongjun J. Niu, Stanislav Savvin, Jonathan Alaria, George R. Darling, John B. Claridge, and Matthew J. Rosseinsky
Source
Inorg. Chem.
Time of Publication: 2018
Abstract It is challenging to achieve p-type doping of zinc oxides (ZnO), which are of interest as transparent conductors in optoelectronics. A ZnO-related ternary compound, SrZnO2, was investigated as a potential host for p-type conductivity. First-principles investigations were used to select from a range of candidate dopants the substitution of Li+ for Zn2+ as a stable, potentially p-type, doping mechanism in SrZnO2. Subsequently, single-phase bulk samples of a new p-type-doped oxide, SrZn1–xLixO2 (0 < x < 0.06), were prepared. The structural, compositional, and physical properties of both the parent SrZnO2 and SrZn1–xLixO2 were experimentally verified. The band gap of SrZnO2 was calculated using HSE06 at 3.80 eV and experimentally measured at 4.27 eV, which confirmed the optical transparency of the material. Powder X-ray diffraction and inductively coupled plasma analysis were combined to show that single-phase ceramic samples can be accessed in the compositional range x < 0.06. A positive Seebeck coefficient of 353(4) μV K–1 for SrZn1–xLixO2, where x = 0.021, confirmed that the compound is a p-type conductor, which is consistent with the pO2 dependence of the electrical conductivity observed in all SrZn1–xLixO2 samples. The conductivity of SrZn1–xLixO2 is up to 15 times greater than that of undoped SrZnO2 (for x = 0.028 σ = 2.53 μS cm–1 at 600 °C and 1 atm of O2).
Remark DOI: 10.1021/acs.inorgchem.8b00697
Link
ID=488

All-Oxide Thermoelectric Module with in Situ Formed Non-Rectifying Complex p–p–n Junction and Transverse Thermoelectric Effect

Authors Nikola Kanas, Michael Bittner, Temesgen Debelo Desissa, Sathya Prakash Singh, Truls Norby, Armin Feldhoff, Tor Grande, Kjell Wiik, and Mari-Ann Einarsrud
Source
ACS Omega
Volume: 3, Issue: 8, Pages: 9899–9906
Time of Publication: 2018
Abstract All-oxide thermoelectric modules for energy harvesting are attractive because of high-temperature stability, low cost, and the potential to use nonscarce and nontoxic elements. Thermoelectric modules are mostly fabricated in the conventional π-design, associated with the challenge of unstable metallic interconnects at high temperature. Here, we report on a novel approach for fabrication of a thermoelectric module with an in situ formed p–p–n junction made of state-of-the-art oxides Ca3Co4–xO9+δ (p-type) and CaMnO3–CaMn2O4 composite (n-type). The module was fabricated by spark plasma co-sintering of p- and n-type powders partly separated by insulating LaAlO3. Where the n- and p-type materials originally were in contact, a layer of p-type Ca3CoMnO6 was formed in situ. The hence formed p–p–n junction exhibited Ohmic behavior and a transverse thermoelectric effect, boosting the open-circuit voltage of the module. The performance of the module was characterized at 700–900 °C, with the highest power output of 5.7 mW (around 23 mW/cm2) at 900 °C and a temperature difference of 160 K. The thermoelectric properties of the p- and n-type materials were measured in the temperature range 100–900 °C, where the highest zT of 0.39 and 0.05 were obtained at 700 and 800 °C, respectively, for Ca3Co4–xO9+δ and the CaMnO3–CaMn2O4 composite.
Remark DOI: 10.1021/acsomega.8b01357
ID=475

Thermoelectric properties of (1-x)LaCoO3.xLa0.7Sr0.3MnO3 composite

Authors Ashutosh Kumar, Karuna Kumari, B. Jayachandran, D. Sivaprahasam, Ajay D.Thakur
Source
Journal of Alloys and Compounds
Volume: 749, Pages: 1092-1097
Time of Publication: 2018
Abstract We report the thermoelectric (TE) properties of (1-x)LaCoO3.xLa0.7Sr0.3MnO3 (0 < x < 0.50) composite in a temperature range 320–800 K. Addition of La0.7Sr0.3MnO3 to LaCoO3 in small amount (5 weight %) improves the overall Seebeck coefficient (α) at higher temperatures. The electrical conductivity however decreases due to a decrease in carrier concentration of the composite. The decrease in electrical conductivity of the composite at high temperature may be attributed to the insulating nature of the LSMO above room temperature. Thermal conductivity (κ) of all the samples increases with an increase in the temperature, but decreases with increasing LSMO content. We also report the local variation of Seebeck coefficient across the composite samples measured using a precision Seebeck measurement system. A maximum value of 0.09 for the figure of merit (ZT) is obtained for 0.95LaCoO3.0.05La0.7Sr0.3MnO3 at 620 K which is significantly higher than the ZT of either of LaCoO3 or La0.7Sr0.3MnO3 at 620 K. This suggests the potential for enhancement of operating temperatures of hitherto well known low temperature thermoelectric materials through suitable compositing approach.
Keywords Thermal conductivity, Electrical conductivity, Perovskites, Manganites, Cobaltate, Composite
Remark https://doi.org/10.1016/j.jallcom.2018.03.347
Link
ID=468

Inter-diffusion across a direct p-n heterojunction of Li-doped NiO and Al-doped ZnO

Authors Temesgen D. Desissa, Reidar Haugsrud, Kjell Wiik, Truls Norby
Source
Solid State Ionics
Volume: 320, Pages: 215-220
Time of Publication: 2018
Abstract We herein report inter-diffusion across the interface between p-type Ni0.98Li0.02O and n-type Zn0.98Al0.02O for various applications including p-n-heterojunction diodes and oxide thermoelectrics. Diffusion couples were made of polished surfaces of ceramic samples pre-sintered at 1250 and 1350 °C for Ni0.98Li0.02O and Zn0.98Al0.02O, respectively. The inter-diffusion couples were annealed at 900–1200 °C for 160 h in ambient air. Electron Probe Micro Analysis (EPMA) was used to acquire diffusion profiles, followed by fitting to Ficks second law and Whipple–Le Claires models for bulk and grain-boundary diffusion calculation, respectively. Zn2+ diffused into Ni0.98Li0.02O mainly by bulk diffusion with an activation energy of 250 ± 10 kJ/mol, whereas Ni2+ diffused into Zn0.98Al0.02O by both bulk and enhanced grain boundary diffusion with activation energies of 320 ± 120 kJ/mol and 245 ± 50 kJ/mol, respectively. The amount of Al3+ diffused from the Al-doped ZnO into the NiO phase was too small for a corresponding diffusion coefficient to be calculated. Li-ion distribution and diffusivity were not determined due to lack of analyzer sensitivity for Li. The bulk and effective diffusivities of Zn2+ and Ni2+ into NiO and ZnO enable prediction of inter-diffusion lengths as a function of time and temperature, allowing estimates of device performance, stability, and lifetimes at different operation temperatures.
Keywords NiO, ZnO, Cation diffusion, Grain-boundary diffusion, p-n junction
Remark https://doi.org/10.1016/j.ssi.2018.03.011
Link
ID=446

Enhanced Flexible Thermoelectric Generators Based on Oxide–Metal Composite Materials

Authors Benjamin Geppert, Artur Brittner, Lailah Helmich, Michael Bittner, Armin Feldhoff
Source
Journal of Electronic Materials
Volume: 46, Issue: 4, Pages: 2356–2365
Time of Publication: 2017
Abstract The thermoelectric performance of flexible thermoelectric generator stripes was investigated in terms of different material combinations. The thermoelectric generators were constructed using Cu-Ni-Mn alloy as n-type legs while varying the p-type leg material by including a metallic silver phase and an oxidic copper phase. For the synthesis of Ca3Co4O9/CuO/Ag ceramic-based composite materials, silver and the copper were added to the sol–gel batches in the form of nitrates. For both additional elements, the isothermal specific electronic conductivity increases with increasing amounts of Ag and CuO in the samples. The amounts for Ag and Cu were 0 mol.%, 2 mol.%, 5 mol.%, 10 mol.%, and 20 mol.%. The phases were confirmed by x-ray diffraction. Furthermore, secondary electron microscopy including energy dispersive x-ray spectroscopy were processed in the scanning electron microscope and the transmission electron microscope. For each p-type material, the data for the thermoelectric parameters, isothermal specific electronic conductivity σ and the Seebeck coefficient α, were determined. The p-type material with a content of 5 mol.% Ag and Cu exhibited a local maximum of the power factor and led to the generator with the highest electric power output Pel.
Remark Link
ID=427

Influence of processing on stability, microstructure and thermoelectric properties of Ca3Co4 − xO9 + δ

Authors Nikola Kanasac Sathy, Prakash Singh, Magnus Rotan, Mohsin Saleemi, Michael Bittner, Armin Feldhoff, Truls Norby, Kjell Wiika, Tor Grande, Mari-Ann Einarsrud
Source
Journal of the European Ceramic Society
Time of Publication: 2017
Abstract Due to high figure of merit, Ca3Co4 − xO9 + δ (CCO) has potential as p-type material for high-temperature thermoelectrics. Here, the influence of processing including solid state sintering, spark plasma sintering and post-calcination on stability, microstructure and thermoelectric properties is reported. By a new post-calcination approach, single-phase materials were obtained from precursors to final dense ceramics in one step. The highest zT of 0.11 was recorded at 800 °C for CCO with 98 and 72% relative densities. In situ high-temperature X-ray diffraction in air and oxygen revealed a higher stability of CCO in oxygen (∼970 °C) than in air (∼930 °C), with formation of Ca3Co2O6 which also showed high stability in oxygen, even at 1125 °C. Since achievement of phase pure high density CCO by post-calcination method in air is challenging, the phase stability of CCO in oxygen is important for understanding and further improvement of the method.
Keywords Ca3Co4 &#8722; xO9 + &#948;, Post calcination, Phase stability, Microstructure, Thermoelectric performance
Remark Available online 6 November 2017, https://doi.org/10.1016/j.jeurceramsoc.2017.11.011
Link
ID=421

Defect chemistry and electrical properties of BiFeO3

Authors
Source
Journal of Materials Chemistry C
Issue: 38 Time of Publication: 2017
Abstract BiFeO3 attracts considerable attention for its rich functional properties, including room temperature coexistence of magnetic order and ferroelectricity and more recently, the discovery of conduction pathways along ferroelectric domain walls. Here, insights into the defect chemistry and electrical properties of BiFeO3 are obtained by in situ measurements of electrical conductivity, σ, and Seebeck coefficient, α, of undoped, cation-stoichiometric BiFeO3 and acceptor-doped Bi1−xCaxFeO3−δ ceramics as a function of temperature and oxygen partial pressure pO2. Bi1−xCaxFeO3−δ exhibits p-type conduction; the dependencies of σ and α on pO2 show that Ca dopants are compensated mainly by oxygen vacancies. By contrast, undoped BiFeO3 shows a simultaneous increase of σ and α with increasing pO2, indicating intrinsic behavior with electrons and holes as the main defect species in almost equal concentrations. The pO2-dependency of σ and α cannot be described by a single point defect model but instead, is quantitatively described by a combination of intrinsic and acceptor-doped characteristics attributable to parallel conduction pathways through undoped grains and defect-containing domain walls; both contribute to the total charge transport in BiFeO3. Based on this model, we discuss the charge transport mechanism and carrier mobilities of BiFeO3 and show that several previous experimental findings can readily be explained within the proposed model.
Remark Link
ID=417

On the formation of phases and their influence on the thermal stability and thermoelectric properties of nanostructured zinc antimonide

Authors Priyadarshini Balasubramanian, Manjusha Battabyal, Duraiswamy Sivaprahasam and Raghavan Gopalan
Source
Journal of Physics D: Applied Physics
Volume: 50, Issue: 1 Time of Publication: 2016-11
Abstract To investigate the thermal reliability of the structure and thermoelectric properties of the zinc antimony compounds, undoped (Zn4Sb3) and doped (Zn4Sb2.95Sn0.05 and Co0.05Zn3.95Sb3) zinc antimonide samples were processed using the powder metallurgy route. It was observed that the as-prepared undoped sample contains a pure β-Zn4Sb3 phase, whereas the doped samples consist of Ω-ZnSb as the major phase and β-Zn4Sb3 as the minor phase. Differential scanning calorimetry analysis confirms the stability of the β-Zn4Sb3 phase up to 600 K. X-ray diffraction data of the undoped and doped samples show that the nanocrystallinity of the as-prepared samples is retained after one thermal cycle. The thermal bandgap, thermopower and thermal conductivity are not affected by the thermal cycle for the doped samples. A maximum power factor of 0.6 mW m−1 K−2 was achieved in the Sn-doped sample (Zn4Sb2.95Sn0.05). This is enhanced to 0.72 mW m−1 K−2 after one thermal cycle at 650 K under Ar atmosphere and slightly decreases after the third thermal cycle. In the case of the Co-doped sample (Co0.05Zn3.95Sb3), the power factor increases from 0.4 mW m−1 K−2 to 0.7 mW m−1 K−2 after the third thermal cycle. A figure of merit of ~0.3 is achieved at 573 K in the Zn4Sb2.95Sn0.05 sample. The results from the nanoindentation experiment show that Youngs modulus of the Sn-doped sample (Zn4Sb2.95Sn0.05) after the thermal cycle is enhanced (96 GPa) compared to the as-prepared sample (~76 GPa). These important findings on the thermal stability of the thermoelectric and mechanical properties of Sn-doped samples (Zn4Sb2.95Sn0.05) confirm that Sn-doped zinc antimonide samples can be used as efficient thermoelectric materials for device applications.
Keywords Seebsys
Remark Link
ID=416

The effect of Cu2O nanoparticle dispersion on the thermoelectric properties of n-type skutterudites

Authors M Battabyal, B Priyadarshini, D Sivaprahasam, N S Karthiselva, R Gopalan
Source
Journal of Physics D: Applied Physics
Volume: 48, Issue: 45 Publisher: IOP Publishing Ltd, Time of Publication: 2015-11
Abstract We report the thermoelectric properties of Ba0.4Co4Sb12 and Sn0.4Ba0.4Co4Sb12 skutterudites dispersed with Cu2O nanoparticles. The samples were synthesized by ball milling and consolidated by spark plasma sintering. Dispersion of Cu2O is found to significantly influence the electrical resistivity and thermopower at high temperatures with a more pronounced effect on the electrical resistivity due to the energy filtering effect at the interface between Cu2O nanoparticles and a Ba0.4Co4Sb12 and Sn0.4Ba0.4Co4Sb12 matrix. At 573 K, the electrical resistivity of Ba0.4Co4Sb12 decreases from 5.01  ×  10−5 Ohmm to 2.98  ×  10−5 Ohmm upon dispersion of Cu2O. The dispersion of Cu2O reduces the thermal conductivity of the samples from 300 K and above by increasing the phonon scattering. The lowest observed thermal conductivity at 573 K is found to be 2.001 W mK−1 in Cu2O dispersed Ba0.4Co4Sb12 while it is 2.91 W mK−1 in the Ba0.4Co4Sb12 sample without Cu2O dispersion. Hence Cu2O dispersion plays a significant role in the thermoelectric properties and a maximum figure of merit (ZT ) ~ 0.92 is achieved in Cu2O dispersed Ba0.4Co4Sb12 at 573 K which is more than 200% compared to the pure Ba0.4Co4Sb12 sample. The results from nanoindentation experiments show that the Cu2O dispersed sample (Cu2O  +  Sn0.4Ba0.4Co4Sb11.6) has a higher reduced Youngs modulus (~139 GPa) than the pure Sn0.4Ba0.4Co4Sb11.6 sample (~128 GPa).
Keywords Seebsys
Remark Link
ID=410

Relating defect chemistry and electronic transport in the double perovskite Ba1−xGd0.8La0.2+xCo2O6−δ (BGLC)

Authors
Source
Journal of Materials Chemistry A
Volume: 5, Pages: 15743-15751
Time of Publication: 2017
Abstract Rare earth double perovskites comprise a class of functional oxides with interesting physiochemical properties both for low- and high-temperature applications. However, little can be found relating electrical properties with equilibrium thermodynamics of non-stoichiometry and defects. In the present work, a comprehensive and generally applicable defect chemical model is developed to form the link between the defect chemistry and electronic structure of partially substituted BGLC (Ba1−xGd0.8La0.2+xCo2O6−δ, 0 ≤ x ≤ 0.5). The equilibrium oxygen content of 4 different compositions is determined as a function of pO2 and temperature by thermogravimetric analysis, and combined with defect chemical modelling to obtain defect concentrations and thermodynamic parameters. Oxidation enthalpies determined by TG-DSC become increasingly exothermic (−50 to −120 kJ mol−1) with increased temperature and oxygen non-stoichiometry for all compositions, in excellent agreement with the thermodynamic parameters obtained from the defect chemical model. All compositions display high electrical conductivities (500 to 1000 S cm−1) with shallow pO2-dependencies and small and positive Seebeck coefficients (3 to 15 μV K−1), indicating high degree of degeneracy of the electronic charge carriers. The complex electrical properties of BGLC at elevated temperatures is rationalized by a two-band conduction model where highly mobile p-type charge carriers are transported within the valence band, whereas less mobile “n-type” charge carriers are located in narrow Co 3d band.
Remark DOI: 10.1039/C7TA02659E
Link
ID=331

The effect of Cu2O nanoparticle dispersion on the thermoelectric properties of n-type skutterudites

Authors M Battabyal, B Priyadarshini, D Sivaprahasam, N S Karthiselva and R Gopalan
Source
Journal of Physics D: Applied Physics
Volume: 48, Issue: 45 Time of Publication: 2015
Abstract We report the thermoelectric properties of Ba0.4Co4Sb12 and Sn0.4Ba0.4Co4Sb12 skutterudites dispersed with Cu2O nanoparticles. The samples were synthesized by ball milling and consolidated by spark plasma sintering. Dispersion of Cu2O is found to significantly influence the electrical resistivity and thermopower at high temperatures with a more pronounced effect on the electrical resistivity due to the energy filtering effect at the interface between Cu2O nanoparticles and a Ba0.4Co4Sb12 and Sn0.4Ba0.4Co4Sb12 matrix. At 573 K, the electrical resistivity of Ba0.4Co4Sb12 decreases from 5.01  ×  10−5 Ωm to 2.98  ×  10−5 Ωm upon dispersion of Cu2O. The dispersion of Cu2O reduces the thermal conductivity of the samples from 300 K and above by increasing the phonon scattering. The lowest observed thermal conductivity at 573 K is found to be 2.001 W mK−1 in Cu2O dispersed Ba0.4Co4Sb12 while it is 2.91 W mK−1 in the Ba0.4Co4Sb12 sample without Cu2O dispersion. Hence Cu2O dispersion plays a significant role in the thermoelectric properties and a maximum figure of merit (ZT ) ~ 0.92 is achieved in Cu2O dispersed Ba0.4Co4Sb12 at 573 K which is more than 200% compared to the pure Ba0.4Co4Sb12 sample. The results from nanoindentation experiments show that the Cu2O dispersed sample (Cu2O  +  Sn0.4Ba0.4Co4Sb11.6) has a higher reduced Youngs modulus (~139 GPa) than the pure Sn0.4Ba0.4Co4Sb11.6 sample (~128 GPa).
Remark Link
ID=319

Tetragonal tungsten bronzes Nb8−xW9+xO47−δ: optimization strategies and transport properties of a new n-type thermoelectric oxide

Authors Christophe P. Heinrich, Matthias Schrade, Giacomo Cerretti, Ingo Lieberwirth, Patrick Leidich, Andreas Schmitz, Harald Fjeld, Eckhard Mueller, Terje G. Finstad, Truls Norby and Wolfgang Tremel
Source
Materials Horizons
Issue: 5, Pages: 519-527
Time of Publication: 2015
Abstract Engineering of nanoscaled structures may help controlling the electrical and thermal transport in solids, in particular for thermoelectric applications that require the combination of low thermal conductivity and low electrical resistivity. The tetragonal tungsten bronzes Nb8−xW9+xO47 (TTB) allow a continuous variation of the charge carrier concentration while fulfilling at the same time the concept of a “phonon-glass electron-crystal” through a layered nanostructure defined by intrinsic crystallographic shear planes. The thermoelectric properties of the tetragonal tungsten bronzes Nb8−xW9+xO47−δ (0 < x < 2) were studied in the temperature range from 373 to 973 K. Structural defects and the thermal stability under various oxygen partial pressure pO2 were investigated by means of thermogravimetry, HR-TEM, and XRD. Nb8W9O47−δ was found stable at 973 K and a pO2 of ≈10−15 atm. The oxygen nonstoichiometry δ can reach up to 0.3, depending on the applied atmosphere. By increasing the substitution level x, the electrical resistivity ρ and the Seebeck coefficient S decreased. For x = 2, ρ reached 20 mΩ cm at 973 K, combined with a Seebeck coefficient of approximately −120 μV K−1. The thermal conductivity was low for all samples, ranging from 1.6 to 2.0 W K−1 m−1, attributed to the complex crystal structure. The best thermoelectric figure of merit zT of the investigated samples was 0.043, obtained for x = 2 at 973 K, but it is expected to increase significantly upon a further increase of x. The control of the oxygen non-stoichiometry δ opens a second independent optimization strategy for tetragonal tungsten bronzes.
Remark DOI: 10.1039/C5MH00033E
Link
ID=300

Electrical conductivity and thermopower of (1 − x) BiFeO3 – xBi0.5K0.5TiO3 (x = 0.1, 0.2) ceramics near the ferroelectric to paraelectric phase transition

Authors E. T. Wefring, M.-A. Einarsrud and T. Grande
Source
Physical Chemistry Chemical Physics
Volume: 17, Issue: 14, Pages: 9420-9428
Time of Publication: 2015
Abstract Ferroelectric BiFeO3 has attractive properties such as high strain and polarization, but a wide range of applications of bulk BiFeO3 are hindered due to high leakage currents and a high coercive electric field. Here, we report on the thermal behaviour of the electrical conductivity and thermopower of BiFeO3 substituted with 10 and 20 mol% Bi0.5K0.5TiO3. A change from p-type to n-type conductivity in these semi-conducting materials was demonstrated by the change in the sign of the Seebeck coefficient and the change in the slope of the isothermal conductivity versus partial pressure of O. A minimum in the isothermal conductivity was observed at [similar]10−2 bar O2 partial pressure for both solid solutions. The strong dependence of the conductivity on the partial pressure of O2 was rationalized by a point defect model describing qualitatively the conductivity involving oxidation/reduction of Fe3+, the dominating oxidation state of Fe in stoichiometric BiFeO3. The ferroelectric to paraelectric phase transition of 80 and 90 mol% BiFeO3 was observed at 648 ± 15 and 723 ± 15 °C respectively by differential thermal analysis and confirmed by dielectric spectroscopy and high temperature powder X-ray diffraction.
Remark DOI: 10.1039/C5CP00266D
Link
norecs.com

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