Versatile apparatus for thermoelectric characterization of oxides at high temperatures Matthias Schrade, Harald Fjeld, Truls Norby, and Terje G. Finstad Citation: Review of Scientific Instruments 85, 103906 (2014); doi: 10.1063/1.4897489 View online: http://dx.doi.org/10.1063/1.4897489 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High temperature transport properties of thermoelectric CaMnO3−δ — Indication of strongly interacting small polarons J. Appl. Phys. 115, 103705 (2014); 10.1063/1.4868321 A temperature dependent screening tool for high throughput thermoelectric characterization of combinatorial films Rev. Sci. Instrum. 84, 115110 (2013); 10.1063/1.4830295 Thermoelectric properties and microstructure of modified novel complex cobalt oxides Sr3RECo4O10.5 (RE = Y, Gd) AIP Conf. Proc. 1449, 339 (2012); 10.1063/1.4731566 Phase compatibility and thermoelectric properties of compounds in the Sr–Ca–Co–O system J. Appl. Phys. 107, 033508 (2010); 10.1063/1.3276158 Thermoelectric properties of highly grain-aligned and densified Co-based oxide ceramics J. Appl. Phys. 93, 2653 (2003); 10.1063/1.1542942

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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 103906 (2014)

Versatile apparatus for thermoelectric characterization of oxides at high temperatures Matthias Schrade,a) Harald Fjeld, Truls Norby, and Terje G. Finstad Centre for Materials Science and Nanotechnology, University of Oslo, Gaustadalléen 21, 0349 Oslo, Norway

(Received 27 March 2014; accepted 28 September 2014; published online 15 October 2014) An apparatus for measuring the Seebeck coefficient and electrical conductivity is presented and characterized. The device can be used in a wide temperature range from room temperature to 1050 ◦ C and in all common atmospheres, including oxidizing, reducing, humid, and inert. The apparatus is suitable for samples with different geometries (disk-, bar-shaped), allowing a complete thermoelectric characterization (including thermal conductivity) on a single sample. The Seebeck coefficient α can be measured in both sample directions (in-plane and cross-plane) simultaneously. Electrical conductivity is measured via the van der Pauw method. Perovskite-type CaMnO3 and the misfit cobalt oxide (Ca2 CoO3 )q (CoO2 ) are studied to demonstrate the temperature range and to investigate the variation of the electrical properties as a function of the measurement atmosphere. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4897489] I. INTRODUCTION

The electronic properties of a material determine its functionality and potential applicability, and the measurement of the electronic transport parameters is therefore a central task in experimental materials science. Most instruments for electrical characterization routinely measure electrical conductivity, whereas the determination of the Seebeck coefficient requires more specialized instrumentation. The Seebeck coefficient α is sensitive to, e.g., carrier concentration and scattering processes, and it may, in combination with the conductivity, provide complementary information on the charge transport of the studied material. In particular, to measure the Seebeck coefficient is necessary when evaluating the performance of a thermoelectric material. It can be shown that the theoretical conversion efficiency of a certain material scales with the dimensionless figure of merit zT (e.g., Ref. 1): σ α2 · T, (1) κ where σ is the electrical conductivity, κ the total thermal conductivity and T the absolute temperature. The numerator term σ α 2 is often referred to as the power factor. The search for new thermoelectric materials applicable at high temperatures underlines the need for scientific instrumentation to measure the thermoelectric parameters in a wide temperature range with high precision and reliability. The different properties in Eq. (1) are highly interrelated and sensitive to small sample variations, e.g., grain size and (unintended) impurities. It is therefore desirable to measure all parameters on one sample and under the same experimental conditions, ideally simultaneously. In the field of thermoelectrics, the laser-flash diffusivity (LFA) technique is commonly used to assess the thermal conductivity. On the other hand, the Seebeck coefficient and electrical conductivity (and zT =

a) Electronic mail: [email protected]

0034-6748/2014/85(10)/103906/8/$30.00

thus the power factor) are usually measured simultaneously in the same instrument. Only a few instruments for characterization of the power factor of thermoelectric materials at elevated temperatures are commercially available (e.g., ZEM-3, Ulvac, Japan, IPM-SRX-900 K, Fraunhofer, Germany and LSR3, Linseis, Germany). In addition, a number of custom-made alternatives are reported in the literature,2–7 but often limited to temperatures below 900 ◦ C and vacuum/inert atmospheres. The materials showing the highest zT values are often based on scarce, expensive or toxic elements like Pb, Te, and As.8, 9 In air, the most promising thermoelectric materials are further kinetically stable only at rather low temperatures, so that encapsulation is required to prevent oxidation at the targeted elevated application temperatures, thereby complicating module fabrication. It is for these reasons that different classes of oxide materials have been investigated for thermoelectric applications after Terasaki et al.10 reported the unusual combination of a high Seebeck coefficient and a relatively low resistivity in Nax CoO2 . Several related cobaltites and delafossites have been shown to be among the most promising oxide classes with respect to their thermoelectric performance.11 However, due to their layered crystal structure, it is important to measure all of the parameters in Eq. (1) along the same geometrical orientation of the sample to obtain zT in a meaningful way. This is also relevant for polycrystalline samples, as these may exhibit anisotropic properties due to possible grain alignment during the compacting process. In the proposed high application temperature range, many oxides exhibit, depending on the surrounding atmosphere, a significant variation of their oxygen content. This oxygen nonstoichiometry influences the charge carrier concentration and thereby the thermoelectric properties. Still, due to a lack of proper instrumentation and awareness, most work on high temperature thermoelectric oxides has been carried out under conditions where the sample is not in chemical

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equilibrium with its surrounding atmosphere. This makes the thermoelectric properties dependent on the thermal history of the sample, and a significant scatter in reported thermoelectric data can, indeed, be observed on nominally identical samples (see Ref. 12 for an extensive review). In this paper, we therefore present an apparatus for characterization of materials – like oxides – whose thermoelectrical properties depend on the atmospheric composition during the measurement. It can be used in a large temperature range and has been tested from room temperature to 1050 ◦ C. The apparatus is compatible with samples of different geometry (disk- and bar-shaped). While conductivity is measured via the in-plane van-der-Pauw method, the Seebeck coefficient can be simultaneously measured both in cross- and in-plane configurations, allowing the convenient characterization of anisotropic samples. In Sec. II we describe the different components and the design principles of the apparatus, followed by the description of a typical measurement cycle in Sec. III. In Sec. IV, the instrument performance and accuracy is demonstrated using different materials, and sources of measurement errors are discussed. Finally, the main features of the presented apparatus are summarized in Sec. V. II. APPARATUS DESCRIPTION

In this section we describe our apparatus in more detail and motivate the respective design principles. The complete apparatus is shown in Figs. 1(a) and 1(b). It is designed to be compatible with a commercially available measurement cell (Probostat, NorECs, Norway), implying certain restrictions on size and geometry, but also allowing easy modifications of the setup as well as replacement of individual parts. The measurement cell is placed in a vertical tubular furnace of 50 cm length – equipped with a temperature controller – providing the base temperature of the measurement. We have successfully tested the setup up to 1050 ◦ C, limited by the melting point of gold. Most of the common thermoelectric materials are either unstable above 1050 ◦ C, or the peak in zT is observed at lower temperatures, so that the present temperature range is sufficient for most applications. However, all instrumental components could well be used at even higher temperatures, e.g., up to 1600 ◦ C, if the Au pellet and wires are replaced by a metal of higher melting point. We note that tungsten – as a typical material for high temperature applications – is avoided inside the measurement chamber to prevent sample and chamber contamination due to evaporation in oxidizing atmospheres. After mounting the sample, the cell is gas-tightly closed by a quartz tube – allowing a visual inspection of the mounted sample in the cell – placed in the vertical tube furnace and flushed with the chosen gas composition. The total pressure in the measurement chamber is 1 atm. The desired gas composition in the cell is controlled by an in-house built gas mixer.13 The gas mixer is based on several rotameters (Sho-Rate 1355, Brooks Instrument) coupled in pairs to allow repeated dilution of the active gas (e.g., O2 ) by an inert gas (in our case Ar). Under oxidizing conditions, the oxygen partial pressure is simply set by the dilution by Ar, and the lower limit is in

Rev. Sci. Instrum. 85, 103906 (2014)

this case determined by O2 residuals in Ar and the amount of leakage of air into the system, resulting typically in an oxygen partial pressure of around 10−5 atm. Under reducing conditions, the oxygen partial pressure is determined by the equilibrium of the chemical reaction H2 (g) + 12 O2 (g) = H2 O(g) or CO(g) + 12 O2 (g) = CO2 (g), depending on the chosen reducing gas species. The partial pressure of the relevant gases is calculated from the flowrates of the different rotameters and thermodynamic data using custom made software. The apparatus is optimized to measure disk-shaped samples, which usually are easiest to manufacture, considering the brittleness of many thermoelectric materials. In addition, the same sample can be used in a subsequent (or prior) LFA experiment. However, it is also possible to mount samples of other geometries (e.g., bar shaped) into the cell, given they fulfill certain requirements on geometry and mechanical strength. Different designs and placements of the thermocouples (TCs) have their respective advantages and disadvantages. In the ideal case, both temperature and voltage should be obtained at the exact same position. In realistic designs, however, this is not possible to achieve due to, e.g., finite wire diameters and thermal resistances across the TC-sample interface. In this apparatus, we chose the TCs to be in direct contact with the sample, acting as both temperature and voltage probe. Compared to setups with TCs embedded in heat source and heat sink blocks, the acquired voltage and temperature readings with this placement have not to be corrected for thermal and electrical resistances in the respective blocks, but give the voltage and temperature of the surface of the sample directly. To minimize the so-called “cold-finger effect,” that is the erroneous temperature reading resulting from the heat transported along the TC wires, we have used thin thermocouple wires with a diameter of 0.2 mm. A 4-bore TC design was adapted from Iwanaga et al.,6 and offers a well defined spot for the temperature and voltage readings due to the welding-free contact between the two wires (Fig. 1(c)). On the other hand, the small contact area between the sample and TC results in a non-zero thermal resistance across the interface, leading to a systematic overestimation of the temperature difference T. Thus, the measured U/T will be systematically lower than the actual absolute value of the Seebeck coefficient. In Sec. IV, we estimate the influence of this effect for different samples. In total, three identical TCs (cf. Fig. 1(c)) are attached to the surface of the sample and kept in position by a springload system. The springs, made of stainless steel, are located at the bottom of the measurement chamber and not exposed to temperatures significantly higher than room temperature. One TC (TC1) connects to the bottom side of the sample, another TC (TC2) is placed on the top side of the sample just above TC1, while the third one (TC3) is also on the top side in a distance of ∼10 mm to TC2. We use S-type TCs, with the Pt-wire being in direct contact with the surface of the sample. The apparatus is equipped with two small Pt10Rh-coils with a room temperature resistance of ∼5 placed below the sample next to TC1 and behind TC2, respectively, acting as internal heaters to vary the temperature difference across the sample between the respective thermocouples. Either TC1 or

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(a)

(b)

(c)

FIG. 1. (a) The apparatus mounted into the cell and connected to peripherical instrumentation. (b) Schematic of disk-shaped sample placed in the heating zone of the tube furnace. (c) Design principle of a welding-free TC.

TC3 (depending on the chosen measurement geometry (inplane or cross-plane)) is used to control the external tube furnace, providing the base temperature of the measurement. To reduce the direct heating from the internal heaters on the thermocouples without heating the sample, both heaters are shielded by alumina tubes. Further, a small Au-pellet is placed between heater 2 and the sample, acting – due to its high thermal conductivity – as a thermal diffusor and leading to a homogeneous temperature distribution. Finally, four Pt-wires are attached to the rim of the sample and held in position by a spring-load system, acting as voltage-/current probes for the resistivity measurement employing the van der Pauw method. III. MEASUREMENT PROCEDURE

Voltage and temperature are measured with an Agilent 34970A multichannel multimeter and the current for the resistivity measurement and the resistive heating of the Pt10Rh coils is set and measured by an Agilent E3642A power source. All measurements are controlled and recorded by in-house made LabVIEW software. Prior to measuring ρ and α – as will be described in detail below – the thermodynamic equilibrium after a change in temperature and/or gas atmosphere is assessed by monitoring the voltage signal across two electrodes when a constant current is applied across the two other electrodes.

Two different approaches have been presented:14 The integral and the differential method, where the latter is presented here in more detail. In principle, it is – according to Eq. (2) – sufficient to measure only one pair of temperatures and a voltage to obtain the Seebeck coefficient. In most set-ups however, the temperature difference T is changed within a certain range and the corresponding voltage is recorded. This removes erroneous contributions and offsets from non-identical thermocouples and increases the statistical significance of the obtained data. Both in-plane and cross-plane measurements of the Seebeck coefficient follow the same measurement routine, differing only by the employed heating coil and the recording pair of thermocouples. The heating output power of the internal heater is increased linearly, leading to a variation in T as a function of time (Fig. 2). The value of T at the beginning and end of a Seebeck measurement is indicative for the

A. Seebeck coefficient measurements

In practical terms, the Seebeck coefficient α is determined by measuring the open circuit voltage U that is created due to a temperature difference T across the sample: α=

U . T

(2)

FIG. 2. Data recorded during a typical Seebeck coefficient measurement cycle.

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Recording time [min] 0

5

10

15

200

225

250

132 -1

temperature distribution within the cell: The equilibrium temperature difference across a sample of thickness ≈ 2 mm is at 500 ◦ C typically in the order of TCross = TC2 − TC1 = 10 K and reduces slightly with increasing temperature. The in-plane temperature gradient is much smaller, giving TIn-Plane = TC2 − TC3 ≈ −1 K. Both temperatures and the voltage across the platinum leads of the thermocouples are recorded using one voltmeter shortly after each other, giving rise to an error due to thermal drift.15 To minimize the influence of thermal drift during the acquisition of one temperature-voltage data point (typically around 12 s), we measure both temperatures before and after the voltage reading at the time tVolt , and interpolate the respective temperatures to the time tVolt . As an example, we can estimate the respective error for an (hypothetical) experiment without thermal drift correction, recording sequently T1 , U12 , and T2 . In that case, the acquisition time for one T1 , U12 , T2 data point would be reduced to 8 s. If T is varied by 5 K within 12 min, the corresponding drift in T within the acquisition of one temperature voltage data point is 0.06 K. The voltage reading in between is thus done at a time where the actual Treal is 0.03 K different from the measured Tmeas = T1 − T2 . In the second half of the cycle, where T increases again, the thermal drift leads to a misinterpretation of T in the opposite direction, so that the U vs. T plot shows a slight loop rather than a straight line (inset of Fig. 3). For the given parameters and a Seebeck coefficient of 150 μVK−1 , the loop width UDrift can be estimated to be ≈10 μV. A typical dataset for a given experimental condition consists of ∼100 data points (T1 , T2 , and U12 ) with the temperature difference T varying by around 5 K. The recorded voltage is corrected for the contribution of the platinum leads: UCorr = URaw − T · α Pt (Tav ) (Fig. 3). Ideally, the average temperature of the sample should be kept constant during a measurement cycle. The observed increase of the

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Seebeck coefficient α [μVK ]

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Chosen cycle length

128

126

124 0

20

40

60

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Recorded data points FIG. 4. Seebeck coefficient with varying measurement length. High precision data is obtained for a cycle length above 15 min. Error bars indicate the statistical error obtained from the linear regression.

average temperature in a typical cycle (Fig. 2) can be minimized by increasing the total time of one measurement cycle, allowing the external furnace to react on the effect of the internal heater. In any case, as the variation in Seebeck dα · (Tav,Max − Tav,Min ) coefficient α(Tav ) usually is small ( dT  α(Tav ), a variation of the average temperature by a few degrees will not lead to significant errors in the obtained Seebeck coefficient. Assuming an unlikely and disadvantageous dα = 1 μV K−2 and a relatively high variation of Tav value for dT of 10 K, the possible error in the Seebeck coefficient due to a non-constant average temperature during the cycle is in the order of 5 μVK−1 . Realistic values for the variation of α with T and Tav during one cycle are each by a factor 5 lower, so that this effect can be usually neglected. Long measurement cycles, where the average sample temperature is practically kept constant are therefore only necessary when α changes steeply with temperature, for example across a phase transition. Compromising precision and reasonable data acquisition times, a typical cycle to measure the Seebeck coefficient at a given condition is set to around 20 min (Fig. 4). In the classification scheme given by Martin et al.,2 our setup thus corresponds to a quasi steady-state differential method. B. Resistivity measurements

The in-plane conductivity of a disk-shaped sample is measured via the van der Pauw method.16 The influence of an additional voltage due to the Peltier effect is eliminated by changing the polarity of the current. In the case of a bar sample, the two thermocouples at the rear ends serve as the current electrodes, while two additional platinum wires have to be attached to the sample to act as voltage probes. FIG. 3. The data recorded during the measurement cycle in Fig. 2 in a U vs. T plot. A linear regression yields the Seebeck coefficient. The difference between the slope of the uncorrected raw and the Pt-lead corrected voltage is equal to the Seebeck coefficient of platinum at the average temperature of the measurement cycle. The inset shows an exaggerated Seebeck cycle when the data is not corrected for thermal drift.

IV. RESULTS

The presented apparatus is suitable for most sample materials, but one of its most prominent features, the precise

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Rev. Sci. Instrum. 85, 103906 (2014)

αIn−Plane ρ

80

-200 60 -250 40 -300

20 0

250

500

750

1000

Temperature [°C] FIG. 5. The Seebeck coefficient α and resistivity ρ of CaMnO3 as a function of temperature. α is measured along two orthogonal sample directions and differs by less than 10 μVK−1 in the complete temperature range, even when remounting the sample.

control of the surrounding atmosphere, is optimally demonstrated on oxide materials which can accommodate significant amounts of oxygen vacancies while keeping their crystal structure. We chose the perovskite-type material CaMnO3−δ (CMO) to demonstrate the instrument performance and reproducibility as a function of temperature and geometry. The crystal structure of CMO is cubic at high temperatures and orthorhombic at lower temperatures.17 We therefore expect to measure the same Seebeck coefficient in both geometrical directions, and identify the observed differences as indicative of the possible anisotropic resolution of the apparatus. In Fig. 5 we show the Seebeck coefficient and resistivity of CMO as a function of temperature. Both the magnitude of α and ρ are in good agreement with values reported in the literature.17–19 At temperatures below 500 ◦ C, the obtained Seebeck coefficient of the in-plane and cross-plane geometry scatters somewhat, but follow the same temperature trend. Still, the difference is always below 10 μVK−1 or 3%. At higher temperatures, the difference decreases and α is measured in both directions with a difference of less than 1%. We tested the reproducibility of the obtained Seebeck coefficient when remounting the sample. In each step, the thermocouples are attached to different spots of the sample, so that the thermal and electrical contact may be different, and the internal heaters have a different position relative to the sample. We found the variation of the Seebeck coefficient between each mounting to be in the same order as the observed differences between α In-plane and α Cross-plane for a single mounting, i.e., the observed difference in α after re-mounting the sample was not distinguishable from the scatter in inplane and cross-plane measurements: At temperatures below 500 ◦ C, all measurements differed by less than 10 μVK−1 , with no observable systematic trend, while at higher temperatures, the difference reduced to ≤2 μVK−1 . We think that there are two reasons for the seemingly better performance at higher temperatures: At lower T, the voltage reading of the thermocouples is smaller, so that the statistical error of the multimeter reading has a more significant contribution. Secondly, the electronic properties of CMO at

low temperatures are dominated by unintended impurities, so that the Seebeck coefficient will be different, if the impurity concentration of the grains along the different measurement directions is different. On the other hand at higher temperatures, intrinsic excitation of charge carriers dominates the electronic properties and both in- and cross-plane measurements yield the same results. We further chose misfit calcium cobalt oxide (Ca2 CoO3 )0.62 (CoO2 ) (CCO) as a material for investigating the dependency of Seebeck coefficient and conductivity on the surrounding atmosphere. CCO is a p-type material and shows the highest reproduced zT values among oxides for both single crystal and ceramic samples.12 At high temperatures, it is well established that the material can contain a significant amount of oxygen vacancies without altering its overall structure.20, 21 In a simplified description using the Kröger-Vink-notation,22 the creation of oxygen vacancies v•• O decreases the concentration of hole-type charge carriers h• via the chemical reaction 1 OxO + 2h•

v•• O + 2 O2

(3)

with the equilibrium constant K as  ••   pO v K =  O   22 . x OO h•

(4)

By changing the oxygen partial pressure, the equilibrium state   of Eq. (3) and thereby the hole concentration h• can be varied. In Fig. 6 we show how such relaxation towards thermodynamic equilibrium is monitored. A constant current is applied between two of the electrodes while the voltage across the two other electrodes is recorded. When the concentration of charge carriers changes, the voltage signal varies as well, reflecting the change in the sample’s resistivity. In the example shown in Fig. 6, we first wait until the voltage (and thus the sample) reaches thermodynamic equilibrium in an atmosphere with an oxygen partial pressure of pO2 ≈ 0.1 atm. Once equilibrium was reached (t = 25 h), the atmosphere was quickly switched to pure oxygen by changing the flowrate of 812 800°C

11.6

pO2: 0.1 atm → 1 atm 810

11.4

808

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Voltage Temperature 0

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Resistivity ρ [mΩ cm]

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Seebeck coefficient α [μVK ]

αCross−plane

Voltage [mV]

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Time [h] FIG. 6. Voltage with a constant current applied across the sample. The furnace temperature is held constant, while the atmosphere is changed. The relaxation process reflects the solid state chemical diffusion of oxygen during equilibration of the sample when the atmosphere is changed. The TC controlling the furnace (TC1 in this case) is kept at constant 800 ◦ C, while TC2 is recorded. Small differences in TC2 are due to Joule-heating of the sample.

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Conductivity σ [S cm ]

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Oxygen partial pressure pO2 [atm] FIG. 7. The electrical conductivity of polycrystalline (Ca2 CoO3−δ )0.62 CoO2 as a function of oxygen partial pressure.

the respective gases. Considering typical flowrates and the volume of the measurement cell, we expect that the atmospheric composition is constant within 20 min, so that the observed slow relaxation seen in Fig. 6 can be attributed to the solid-state in- and out-diffusion of oxygen-ions within the sample. The recorded voltage reduces, reflecting the increase in conductivity by an increased charge carrier concentration via Eq. (3). The temperature of the recorded thermocouple (TC2 in the present case) follows roughly the voltage signal and varies by 2 K. We note that the thermocouple TC1 underneath the sample – regulating the external furnace – is kept constant at 800 ◦ C. The variation in TC2 thus reflects the effect of Joule heating of the sample, which changes when the resistivity changes. The obtained electrical conductivity and Seebeck coefficient of CCO for different oxygen partial pressures pO2 and temperatures T are shown in Figs. 7 and 8, respectively. All measurements are taken after the sample has reached chemical equilibrium with the chosen atmospheric composition. The T- and pO2 -range studied here reflect the

FIG. 8. The cross-plane Seebeck coefficient of polycrystalline (Ca2 CoO3−δ )0.62 CoO2 as a function of oxygen partial pressure.

chemical stability of CCO.20 The oxygen partial pressure was changed in random order, to check for consistency and reproducibility. The Seebeck coefficient α decreases with increasing pO2 , in agreement with an increase in hole-type carrier concentration via Eq. (3). Analogously, the electrical conductivity σ increases with increasing pO2 . It can be seen that the transport coefficients σ and α of CCO can be reversibly changed by a variation of the surrounding atmosphere and that the changes are reliably resolved by the presented apparatus. We further investigate the anisotropy of the Seebeck coefficient at temperatures below 500 ◦ C, where the structure is fully oxidized. The crystal structure of CCO is built up by alternating layers of trigonal units of CoO2 and rocksalt-type units of Ca2 CoO3 leading to anisotropic transport properties in single crystals.23 Due to pronounced preferred orientation of the grains during compacting (commonly done by a hot isostatic press or spark plasma sintering), the transport properties of polycrystalline samples of CCO are expected to show a directional dependency, too. This has been shown for the electrical conductivity24 and for the thermal conductivity,25 but we are not aware of any reported results on the anisotropy of the Seebeck coefficient in polycrystalline CCO samples. We used a bar-shaped CCO-sample of 3 × 4 × 12 mm3 in dimension. We first mounted the sample in an upward orientation with the long side between TC1 and TC2 to have a reference data set for the Seebeck coefficient perpendicular to the compacting direction (inset A in Fig. 9). In a second step, we mounted the sample with the long side orientated along TC2 and TC3. Indeed, both measurements gave very similar results for the Seebeck coefficient perpendicular to the compacting direction, while the Seebeck coefficient parallel to the compacting direction is slightly lower by ≈5 μVK−1 for the entire temperature range (Fig. 9). Still, this observed degree of anisotropy of the Seebeck coefficient for CCO is in the range of the instrument resolution at lower temperatures as concluded from the measurement on CMO (Fig. 5). However, we note, that our results (assuming a systematically higher α In-plane than α Cross-plane ) show a similar trend as in a single crystal,26 where the Seebeck coefficient along the ab-plane is

FIG. 9. Anisotropy of Seebeck coefficient of a ceramic CCO sample. The reproducibility of the in-plane Seebeck coefficient is investigated by measuring the sample in two different orientations (Insets A and B).

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higher than along the c-axis. Still, the degree of anisotropy is much weaker for the polycrystalline sample compared to the single crystal. Unfortunately, there has not been established a high temperature standard for the absolute scale of the Seebeck coefficient yet, though promising results have been obtained for FeSi2 .4 Still, the accuracy of most instruments in the literature is validated using nickel, as the Seebeck coefficient shows a pronounced drop by 6 μVK−1 between 200 ◦ C and 380 ◦ C due to a ferromagnetic transition.3, 7, 27 The thermal conductivity of nickel is of the same order of magnitude as that of platinum, so that we have to take the temperature drop across the thermocouple into account. We can estimate the temperature drop across the sample compared to the measured temperature difference by TSample TTotal

dSample

=

V. SUMMARY

κSample dSample κSample

+

dPt κPt

(5)

,

where di is the thickness and κ i the thermal conductivity of the sample and thermocouple wire, respectively. In the present case, the thickness of the Pt-wires of both thermocouples is 2 × 0.2 = 0.4 mm and the thickness of the Ni-sample is 2 mm. Our results for nickel (99.999% purity, MRC Toulouse) obtained in a reducing atmosphere (3% H2 , 97% Ar) to avoid oxidation, compared with reference data set are shown in Fig. 10. The overall temperature dependency of the Seebeck coefficient is well reproduced, but the absolute value is significantly smaller than the reference data in the entire temperature range. We attribute this to a thermal contact resistance across the thermocouple-sample interface.15 At higher temperatures, the thermal contact between thermocouple and sample improves and the obtained Seebeck coefficient approaches the reference dataset. The relative deviation between our obtained results and the reference changes from 25% at room temperature to 5% at 500 ◦ C. The thermal contact resistance depends on parameters like surface roughness, wire ductility, and contacting pressure and is therefore difficult to estimate. However, as we have not observed significant differences in α of CMO for both directions as well as for repeated measurements, we conclude the

-15

-1

Seebeck coefficient α [μV K ]

thermal interface resistance to be of minor importance for a sample with a low thermal conductivity. If measuring on a sample with a low thermal conductivity compared to Pt (for thermoelectric materials, typically κ Sample ≈ 2–5 Wm−1 K−1  70 Wm−1 K−1 = κ Pt ), the influence of the temperature drop across the thermocouple according to Eq. (5) is ≈1% of the total T and therefore negligible. With the presented apparatus it is now possible to not only determine the Seebeck coefficient as a function of temperature and oxygen partial pressure, but also to monitor the change in two different sample directions simultaneously, thus opening an additional dimension in the experimentally accessible parameter space.

We have built and characterized a new apparatus that allows reliable determination of the Seebeck coefficient and electrical conductivity in a large experimental parameter space. The apparatus has the following main features:

r Temperature range from room temperature up to

r

r r r

1050 ◦ C, therefore, being suitable for most materials relevant for thermoelectric applications at elevated temperatures. Precise control of the surrounding atmosphere and its influence on the thermoelectric properties of the material under investigation. The apparatus can be used in most atmospheres, including oxygen, hydrogen, water vapour or inert species. Compatible with different sample geometries (diskand bar-shaped). Simple and relatively cheap replacement of apparatus components in case of failure or ageing. The Seebeck coefficient can be measured along two orthogonal directions of the sample, allowing the investigation of the anisotropy of α without moving the sample.

The performance of the apparatus was tested on two different ceramic oxide samples: Perovskite-type CaMnO3 and misfit calcium cobaltite (Ca2 CoO3 )q (CoO2 ). Metallic nickel was chosen to test the set-up’s capabilities when measuring a highly heat conductive sample. Small changes in Seebeck coefficient (≤1 μV K−1 ) as a function of temperature and/or atmosphere could be reliably resolved.

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ACKNOWLEDGMENTS -25 This s tudy Foiles Domenicali -30

0

100

200

300

400

500

Temperature [°C] FIG. 10. The Seebeck coefficient of nickel measured with the presented apparatus and compared with literature data.28, 29

The authors gratefully acknowledge funding by the Research Council of Norway within the THERMEL project (Project No. 143386). 1 H. J. Goldsmid, Introduction to Thermoelectricity (Springer-Verlag, Berlin,

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