- Email: [email protected]
Simple apparatus to measure Seebeck coefficient up to 900 K
Accepted Manuscript Simple apparatus to measure Seebeck coefficient upto 900 K Avinna Mishra, Sarama Bhattacharjee, Shahid Anwar PII: DOI: Reference:
S0263-2241(15)00141-4 http://dx.doi.org/10.1016/j.measurement.2015.03.005 MEASUR 3309
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
3 December 2014 28 February 2015 5 March 2015
Please cite this article as: A. Mishra, S. Bhattacharjee, S. Anwar, Simple apparatus to measure Seebeck coefficient upto 900 K, Measurement (2015), doi: http://dx.doi.org/10.1016/j.measurement.2015.03.005
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Simple apparatus to measure Seebeck coefficient upto 900 K Avinna Mishra1,2, Sarama Bhattacharjee1,2, Shahid Anwar1,2 * 1
2
Academy of Scientific and Innovative Research, New Delhi 110001, India Colloids & Materials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar 751013, Odisha, India
Abstract An apparatus for measuring Seebeck coefficient (S) has been designed that allows measurement of S from room temperature to 900 K. It is constructed from readily available equipment and instrumentation with parts that can be easily fabricated. The details of instrument fabrication, sources of errors, method of calibration, typical measurement in test sample are described. We report the Seebeck coefficient measurement of Ca-cobaltite (Ca3Co4O9) a p-type thermoelectric material. The obtained results from the fabricated set up are well matched with the reported and standard instrument data with standard deviation of ±3%.
Keywords: Thermoelectric; Seebeck; Calcium Cobaltite; Setup * Corresponding author: M. Shahid Anwar Colloids & Materials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar, India Email: [email protected]
1
Introduction The current technologies for energy production that rely on fossil fuels have a negative impact on the environment and on our living conditions. On the other hand, the global energy demand is increasing day by day. Alternative and environmentally benign sources of primary energy do exist. However, existing technologies for converting these alternative energy sources into desired energy forms are far from adequately competitive with conventional technologies. In addition to alternative sources, improvement in the efficiency of energyconversion of conventional technologies is considered to be a viable alternative for reducing negative impact on our society. For example, during conversion of heat to electricity by conventional technologies, a major part of the primary energy is lost to the environment as waste heat. Therefore, technologies for harnessing the dissipating heat component as useful energy are highly desired to improve overall efficiency of the process. Thermoelectric generator (TEG), that converts directly the waste heat to electrical energy without any intermediate step is seen as one of the more promising strategies.
Thermoelectric (TE) materials are known to human kind from early 19th century with discovery of Seebeck, Peltier and Thomson effects, however not much development was reported due to low thermoelectric efficiency. Renewed interest, however, resurfaced in mid 1990s when theoretical predictions [1] suggested that thermoelectric efficiency could be greatly enhanced through nanostructure engineering. Vigorous research efforts led to the development of different complex materials [2, 3] with high efficiency. The efficiency of TE materials is measured in terms of a dimensionless quantity, ZT, the figure of merit and which is defined by the following relation: ZT = S2σT/κ, where S is the Seebeck co-efficient, σ is the electrical conductivity, T is the absolute temperature and κ is the thermal conductivity. Recently theoretical calculations [4, 5, 6, 7] predicted high thermoelectric efficiency
2
materials such as Nowotny–Juza NaZnX (X = P, As and Sb) compounds, Srn+1TinO3n+1 homologues series, carbon nitride, H2S absorbed graphene compounds.
High temperature Seebeck coefficient measurement is often critical due to lack of standardized guidelines and hence often is resulted in irreproducibility and inconsistency in experimental data. In practical cases, the measurement geometry deviates from ideal measurement geometry that introduces errors in the experimental data. There are many earlier reports [8, 17] on fabrication of instrument set-up for measuring the TE properties of bulk material. Martin et al [18] has reviewed different techniques and apparatus designs to address the uncertainty in Seebeck co-efficient measurement.
Several recent reports [19, 22] on the fabrication of set up to measure the S for both bulk and thin film samples relies on complex sample holder design to reduce the uncertainties in the data. Iwanaga et al [19] reported a set up which can measure the Seebeck coefficient upto 1000 K but the main complexity lies in the designing of the sample holder and placing the probes in it through drilling. Paul [20] reported a set up for both Seebeck coefficient and electrical resistivity measurement but both the set-up are applicable till 600 K. Budngam et al [21] reported a set-up for measuring very low Seebeck co-efficient for metals for a wide range of temperature (85-1200 K). Gunes et al [22] reported a set up that also features complicated sample probe design with custom made temperature and voltage probes.
In this paper we report a simple set-up developed from available instrumentation with sample holder part that can be easily fabricated. We employ steady state differential method and measurements are carried out in air from room temperature to 900 K. We keep the design simple and try to probe the extent of errors introduced due to deviation from ideal geometry.
3
We describe in detail the instrument fabrication, sources of errors, method of calibration, and typical measurement in test sample
Principle for determining Seebeck coefficient by differential method: Seebeck coefficient is the e.m.f, ∆V, measured between two points of a sample having temperature T1 and T2 at the respective points, under the condition that no current flows through the sample during measurement. Then S is given by S≅
∆V ∆T
, where ∆T = ( T2 – T1) →0
Though the concept of measuring S is simple, in reality it is complicated due to spurious emf generated in the measuring circuit and also the practical difficulty in measuring voltage and temperature at the same point of the sample.
Two distinct approaches namely, the integral and the differential method are utlized for measuring S. Two methods differ in the magnitude of the temperature gradient that has to be maintained across the sample. In the integral method, one end of the sample is kept at a fixed temperature while the other end is heated slowly. Using the gradient of voltage versus temperature, the S can be found for any given temperature. In the differential method, the entire sample is heated in steps to successively higher temperatures and for each step small temperature differentials are created between the ends. The linear slope of voltage drop ∆V versus the temperature difference ∆T then gives the S for the specific temperature step. This linearity is dependent upon the value of S and ∂S/∂T. These values also determine ∆T that should be chosen for the measurement to give the most accurate results. Usually for a small ∆T (~ 5 K), the relation remains linear.
4
One drawback of the integral method is the difficulty of maintaining a large temperature gradient in small samples, or in samples with high thermal conductivity. Also large temperature difference may introduce thermal stresses in the sample that can cause the sample to break. Further, there is no clear-cut way to mathematically evaluate the slope of a noisy signal and the associated error. These drawbacks are not present in the differential method, and also good for thin samples and is used in preference nowadays. We employ here differential method.
Experimental Apparatus Description The complete schematic block diagram for the setup is shown in the Fig. 1 which consists of mainly four major components : sample holder, primary heater (heater 1), secondary heater (heater 2) and its measuring components like multimeter and temperature controller.
The primary heater basically consists of a alumina tube with one end closed having inner diameter 7 cm and length 32 cm and is vertically placed in a cubical box made of mild steel of dimension 45x45x35cm3 with suitable insulation. High resistive wire is wounded on the alumina tube to have a hot zone of length 10 cm and can operate from room temperature to 900 K in a control mode.
The longitudinal 3D view of the sample holder is shown in the Fig.1b with an enlarged view of the sample holding section. The holder is made up of inconel with overall height of 30 cm and consists of two electrodes (1,2) with a spring loading arrangement, secondary heater and all other supporting components. The electrodes are basically two stainless steel (SS) cylindrical blocks of dimension of 2 cm in length and 1.5 cm in diameter. The sample is sandwiched between these two electrodes. The lower electrode block is fixed with the base 5
(10) while the upper electrode block is movable through a distance of 1.5 cm in vertical direction. To generate a temperature difference (2-10 K) between two surfaces of the sample, a secondary heater (6) is directly attached to the top electrode. All sides except the bottom of the secondary heater are coated with ceramic cement to reduce the heat loss. The top electrode assembly (along with the secondary heater) is attached to a solid screw (8) of 15 cm length and 1 cm diameter. To establish good thermal contact between the sample and the electrodes, pressure is applied through the screw on the sample surface which is spring loaded. The spring is fixed outside the primary heater in order to avoid the loss of the spring elasticity on heating. Whole top electrode assembly is supported by a circular steel disc with 4.5 cm diameter and 0.4 cm thick (11) and steel frame (9). The circular disc additionally helps in shielding the radiation heat loss from the secondary heater. The complete sample holder is kept firm in place with the help of a steel frame consists of three solid stainless rods of length 20 cm and having 0.8 cm diameter. The rods are screwed to the base (10) and fixed to the upper circular disc.
To measure the sample surface temperatures two K-type thermocouples (3,4) are silver brazed to the respective electrodes close to the sample surfaces. Thin metal wires, 0.05 cm thick (5) are silver brazed to each electrodes close to thermocouples junctions and used as voltage leads. The voltage leads also contribute to the Seebeck voltage, since their one ends are at room temperature. However their contribution is insignificant since the wire used has small S (≈1-2 µV/K) value, while the set up is designed for thermoelectric materials of having S>100 µV/K. The thermocouples and all the wires for electrical measurement are electrically insulated by alumina beads.
Temperatures of both the heaters are controlled by a Eurotherm 2416 temperature controller with resolution of 0.1 K. The temperature difference (∆T) between the sample ends is 6
maintained at ~5-10 K over the complete temperature range up to 900 K. Temperature stability is maintained in the tubular chamber at every operational temperature and found to be not exceeding more than ten degrees even at high temperature. Keithley 2010 multimeter with resolution 0.01 µV is connected to the voltage jacks of the sample holder in order to measure the voltage difference (∆V).
Measurement of the test sample To measure S, a bar shaped pellet of diameter of ~1.25 cm and thickness of ~ 0.2 cm is mounted between the two SS block electrodes. Heater 1 is programmed at a rate of 2.5-3 K/min to desired temperature. Once the temperature is reached, heater 2 is started in order to develop the temperature difference of ~5-10 K between the two surfaces of the samples. Once stabilised, the voltage difference ∆V is noted down.
At each temperature, the
measurement is repeated five times within short interval. Also the measurement is made during heating and cooling also.
Result and discussion: Test sample measurement: Sintered pellet of calcium cobaltite (Ca3Co4O9) is used as test sample. The sample is prepared by following solid state synthesis route staring from corresponding oxides and by established Pechini method [23]. To check the durability and repeatability of the system the measurement are performed several times on the test samples. Figure 2a shows plot of S versus temperature for Ca3Co 4O9 test sample for three runs and the standard deviation. The obtained S values of Ca3Co4O9 in all three trials are found to increase monotonically with the temperature. At 878 K, S is found to vary within the range of 185 to 190 µV/K .The standard deviation of S is
7
within ∆S ±5 µV/K ( Fig. 2b) in the total temperature range studied. The obtained data are well compared with the published result for Ca3Co4O9 from different literatures [24, 26].
Estimation of errors: To determine the offset voltage arising due to spurious emf in the measuring circuit, an experiment is designed where an insulating block is placed between the two electrodes and voltage between the electrodesis measured in the complete temperature range. The variation of voltage is listed in Table -1. The variation of voltage recorded is varied within nanovolt and hence its effect may be neglected. In the fabricated set-up since temperature and voltages measurement locations are not exactly in touch with the sample surfaces, the major errors in voltage and temperature values arise due to thermal and electrical contact resistance.
Electrical contact resistance measurement: Electrical contact resistance refers to the contribution to the total resistance of a material/setup which comes from all electrical and mechanical connections come in the path of current flow. It is setup specific and different in different conditions. Contact resistance is independent of the measurement method.
The contact resistance of the fabricated setup is measured using four probe arrangement that consists of a current injecting channels (+I, -I) through Keithley2400 source meter and a voltage sensing channel (+V, -V)( measured with Keithley-2010 multimeter).
Fig 3a shows the measurement arrangement for evaluation of contact electrical resistance. The two external points (Fig.3a) where generated thermoelectric voltage is measured is taken as two end of the system, since between these two points it includes various electrical and mechanical contacts that can contribute to contact resistance. The two Keithley meter then accurately measures the current injection while simultaneously senses the voltage drop across 8
the setup, the resistance is then calculated. The four probe arrangement eliminates the error induced by test lead resistances. Measurement is carried out by sweeping the total temperature range.
Fig. 3b shows the variation of resistance measured from room temperature to 900 K. Measured average resistance (5.01 Ω) with a standard deviation of ± 0.1 Ω in the total temperature region is nominal and within error limit.
Thermal contact resistance measurement: To determine the thermal contact resistance, an experiment is designed where two extra thermocouples are placed by digging a shallow channel on both the surfaces of the sample pellet (Fig. 4). Both the heaters (primary & secondary) are swept through the total temperature range. The temperatures measured with the four thermocouples (Fig. 4a) are noted down, Fig.4b shows the top view of the sample during measurement. The experiment is repeated three times. It is observed that there is not much temperature lag between the lower thermocouple and the thermocouple in touch with lower surface. It is understandable since the primary heater is programmed to run sufficiently slower rate to reach thermal equilibrium, however, the upper surface exhibits temperature lag between the upper thermocouple and the thermocouple in touch with the sample upper surface. It is found that there is an error of ~ 25 K up to 600 K which increases upto 10-15 K at 900 K in measuring temperature gradient. Hence in fabricated set up the major error in the S value is coming from the error introduced in measuring the temperature of (hot side) of the sample.
Comparison of data collected in the setup with Calibrated equipment: To determine experimentally the deviation of S values measured in the fabricated set up, the S values of the same sample is measured in a commercially available (Linseis, LSR – 3) instrument which is calibrated against standard compound by the manufacturer. In the 9
commercial set-up, the readings are taken in the temperature range from 340 to 1000 K with temperature interval of 50 K. The generated potential difference (∆V) is measured in every 10 K rise of temperature gradient (∆T) of 10 K. The comparative results of commercial system along with data obtained from developed system are shown in Fig. 5a. A standard deviation plot of obtained data has been plotted (Fig 5b) with respect to data obtained from commercial equipment of same sample. At low temperature (upto 600 K) the deviation is nominal (~2-3%). Deviation seems to be higher at higher temperature (~ 5-7%). This is because the error introduced in measuring hot side temperature increases with temperature as discussed above. It is important to note here that we compare our raw data without any temperature or lead corrections.
Conclusions A simple apparatus is fabricated for measurement of Seebeck co-efficient upto temperature of 900 K using differential concept. The instrument works in air and good for measuring S for small samples. Accuracy in measuring temperature gradient is most crucial in determining the Seebeck coefficient and we observe that in our fabricated set up there is an error in determining ∆T ~ 2-5 K up to 600 K which increases upto 10-15 K at 900 K . It introduces an error in S value of ~ 2-3% at lower temperature and increases to ~5-7% at higher temperature. The data obtained in a commercial set-up and the fabricated set up, when compared also shows similar deviation in S values. Measurement of S of calcium cobaltite test sample shows fair reproducibility with standard deviation of ∆S ±5 µV/K. Finally, we report here a simple apparatus for measuring without any tailor-made sample probe, current and voltage probes design.
10
Acknowledgement The authors gratefully acknowledge Director, CSIR-IMMT, Bhubaneswar for his encouragement. The authors also record their sincere thanks to Dr. Shovit Bhattacharya in carrying out Seebeck coefficient measurement at Bhaba Atomic Research Centre, Mumbai, India. This research is partially supported by Board of Research in Nuclear Sciences, Department of Atomic Energy, Mumbai, India.
11
References: [1]
L. D. Hicks and M. S. Dresselhaus, Effect of quantum well structures on the thermoelectric figure of merit, Phys. Rev.B, 47 (1993)12727-12731.
[2]
Terry M. Tritt and M.A. Subramanian, Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View, MRS BULLETIN 31 (2006) 188-229.
[3]
G. Jeffrey Snyder and Eric S. Toberer, Complex thermoelectric materials, Nature materials 7 (2008) 105-114.
[4]
A .H. Reshek, S. Auluck, Thermoelectric properties of Nowotny–Juza NaZnX (X = P, As and Sb) compounds, Computational Materials Science 96 A (2015) 90–95.
[5]
A .H. Reshek, Thermoelectric properties of Srn+1TinO3n+1 (n=1, 2, 3, ∞) Ruddlesden– Popper Homologous Series, Renewable Energy 76 (2015) 36–44.
[6]
A .H. Reshek, Thermoelectric properties for AA- and AB-stacking of a carbon nitride polymorph (C3N4), RSC Adv 4 (2014) 63137-63142.
[7]
A. H. Reshak, Saleem Ayaz Khan and S. Auluck, Thermoelectric properties of a single graphene sheet and its derivatives, J. Mater. Chem. C 2 (2014) 2346-2352.
[8]
Z.H.Zhou and C. Uher, Apparatus for Seebeck coefficientand electrical resistivity measurements of bulk thermoelectric materials at high temperature, Rev. Sci. Instrum. 76 (2005) 023901.
[9]
A. T. Burkov, A.Heinrich, P. P. Konstantinov, T. Nakama and K. Yagasaki, Experimental set-up for thermopower and resistivity measurements at 100–1300 K, Meas. Sci. Technol. 12 (2001) 264–272.
[10] C. Wood, A. Chmielewski and D. Zoltan, Measurement of Seebeck coefficient using a large thermal-gradient, Rev. Sci. Instrum. 59 (1988) 951–954. [11] V. Ponnambalam, S. Lindsey, N. S. Hickman and T. M. Tritt, Sample probe to measure resistivity and thermopower in the temperature range of 300–1000 K, Rev. Sci. Instrum. 77 (2006) 073904. [12] S. Y. Loo, K. F. Hsu, M. G. Kanatzidis and T. P. Hogan, High temperature power factor measurement system for thermoelectric materials, Proc. Electrochem. Soc. 27 (2004) 247–256. [13] G. T. Kim, J G Park, J. Y. Lee, H. Y. Yu, E. S. Choi, D. S. Suh, Y. S. Ha and Y. W. Park, Simple technique for the simultaneous measurements of the four-probe resistivity and the thermoelectric power, Rev. Sci. Instrum. 69 (1998) 3705–3706.
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[14] S. Fujimoto, H. Kaibe, S. Sano and T. Kajitani, Development of transient measurement method for investigating thermoelectric properties in high temperature region, Japan. J. Appl. Phys. 45 (2006) 8805–8809. [15] T. Hogan, S. Loo, F. Guo and J. Short, Measurement techniques for thermoelectric materials and modules Thermoelectric Materials 2003: Research and Applications (MRS Symposium Proceedings Series vol 793) (Cambridge: Cambridge University Press) (2004) pp 405–411. [16] G. Min and D. M. Rowe, A novel principle allowing rapid and accurate measurement of a dimension less thermoelectric figure of merit, Meas. Sci. Technol. 12 (2001) 1261– 1262. [17] T. Dasgupta and A. M. Umarji, Apparatus to measure high-temperature thermal conductivity and thermoelectric power of small specimens, Rev. Sci. Instrum. 76 (2005) 09490. [18] J. Martin, T. Tritt, and C. Uher, High temperature Seebeck coefficient metrology, J. Appl. Phys. 108 (2010) 121101-12. [19] Shiho Iwanaga, Eric S. Toberer, Aaron LaLonde, and G. Jeffrey Snyder, A high temperature apparatus for measurement of the Seebeck coefficient, Review of scientific instruments 82 (2011) 063905. [20] Biplab Paul, Simple apparatus for the multipurpose measurements of different thermoelectric parameters, Measurement 45 (2012) 133–139. [21] Sopon Budngam, Areewichainchai, Saichol Pimmongkol and Udom Tipparach, Designing Apparatus for Highly Precise Measurement of Electrical Conductivity and Seebeck Coefficient from 85 K to 1200 K, Advanced Materials Research 740 (2013) 426-432. [22] Murat Gunes, Mehmet Parlak, and macit Ozenbas, An instrument for the high temperature measurement of the seebeck coefficient and electrical resistivity, Meas. Sci. Technol. 25 (2014) 055901-10. [23] Hoa Tran, Tejas Mehta, Matthias Zeller ,Richard H. Jarman, Synthesis and characterization of mixed phases in the Ca-Co-O system using the pechni method, Materials Research Bulletin 48 (2013) 2450-2456. [24] N.V. Nong, Chia-Jyi Liu, M. Ohtaki, High-temperature thermoelectric properties of late rare earth-doped Ca3Co 4O9+ δ , Journal of Alloys and Compounds 509 (2011) 977–981.
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[25] David Moser, Lassi Karvonen, Sascha Populoh, Matthias Trottmann, Anke Weidenkaff, Influence of the oxygen content on thermoelectric properties of Ca3xBixCo 4O9+δsystem,
Solid State Sciences 13 (2011) 2160-2164.
[26] Ngo Van Nong , Nini Pryds , Søren Linderoth and Michitaka Ohtaki, Enhancement of the Thermoelectric Performance of p –Type Layered Oxide Ca3Co4O9+
δ
Through
Heavy Doping and Metallic Nano inclusions, Adv. Mater. 2011, 23, 2484–2490.
14
Figure Captions: Fig. 1 (a) Schematic block diagram of seebeck set up. (b): 3D view of the sample holder. Fig. 2 (a) Variation of Seebeck coefficient of Ca3Co 4O9 with temperature for three different trials (b) Standard deviation of the data from three trials. Fig. 3 (a) Experimental arrangement to measure electrical contact resistance. (b) Variation of resistance with temperature. Fig. 4 (a) Experimental arrangement to measure thermal contact resistance. (b) Top view of the sample with channel to place thermocouple TCs1. Fig. 5 (a) Comparison of Seebeck coefficient (S) values with temperature (T) from the commercial instrument and the fabricated set up. (b) Variation in percentage error in Seebeck coefficient values with temperature between the data measured in commercial instrument and the fabricated set up.
Table Caption: List of offset voltage generated with increasing temperature.
15
Tables:
Table - 1 List of offset voltage generated with increasing temperature Temperature (K)
373
473
573
673
773
873
Voltage (µV)
0.015
0.017
0.007
0.011
0.009
0.013
16
17
18
19
20
21
22
Highlights
• • • • •
Simple Seebeck (S) coefficient measurement setup without any tailor made probes. Able to measure S from room temperature to 900 K in air. The measured S values has fair reproducibility with standard deviation ∆S ± 5 µV/K. Error in S values of the order of 2-3% till 600 K, increases to 5-7 % at 900 K. User friendly setup for measurement of S values for small bulk samples.
23
Graphical abstract
24
S0263-2241(15)00141-4 http://dx.doi.org/10.1016/j.measurement.2015.03.005 MEASUR 3309
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
3 December 2014 28 February 2015 5 March 2015
Please cite this article as: A. Mishra, S. Bhattacharjee, S. Anwar, Simple apparatus to measure Seebeck coefficient upto 900 K, Measurement (2015), doi: http://dx.doi.org/10.1016/j.measurement.2015.03.005
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Simple apparatus to measure Seebeck coefficient upto 900 K Avinna Mishra1,2, Sarama Bhattacharjee1,2, Shahid Anwar1,2 * 1
2
Academy of Scientific and Innovative Research, New Delhi 110001, India Colloids & Materials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar 751013, Odisha, India
Abstract An apparatus for measuring Seebeck coefficient (S) has been designed that allows measurement of S from room temperature to 900 K. It is constructed from readily available equipment and instrumentation with parts that can be easily fabricated. The details of instrument fabrication, sources of errors, method of calibration, typical measurement in test sample are described. We report the Seebeck coefficient measurement of Ca-cobaltite (Ca3Co4O9) a p-type thermoelectric material. The obtained results from the fabricated set up are well matched with the reported and standard instrument data with standard deviation of ±3%.
Keywords: Thermoelectric; Seebeck; Calcium Cobaltite; Setup * Corresponding author: M. Shahid Anwar Colloids & Materials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar, India Email: [email protected]
1
Introduction The current technologies for energy production that rely on fossil fuels have a negative impact on the environment and on our living conditions. On the other hand, the global energy demand is increasing day by day. Alternative and environmentally benign sources of primary energy do exist. However, existing technologies for converting these alternative energy sources into desired energy forms are far from adequately competitive with conventional technologies. In addition to alternative sources, improvement in the efficiency of energyconversion of conventional technologies is considered to be a viable alternative for reducing negative impact on our society. For example, during conversion of heat to electricity by conventional technologies, a major part of the primary energy is lost to the environment as waste heat. Therefore, technologies for harnessing the dissipating heat component as useful energy are highly desired to improve overall efficiency of the process. Thermoelectric generator (TEG), that converts directly the waste heat to electrical energy without any intermediate step is seen as one of the more promising strategies.
Thermoelectric (TE) materials are known to human kind from early 19th century with discovery of Seebeck, Peltier and Thomson effects, however not much development was reported due to low thermoelectric efficiency. Renewed interest, however, resurfaced in mid 1990s when theoretical predictions [1] suggested that thermoelectric efficiency could be greatly enhanced through nanostructure engineering. Vigorous research efforts led to the development of different complex materials [2, 3] with high efficiency. The efficiency of TE materials is measured in terms of a dimensionless quantity, ZT, the figure of merit and which is defined by the following relation: ZT = S2σT/κ, where S is the Seebeck co-efficient, σ is the electrical conductivity, T is the absolute temperature and κ is the thermal conductivity. Recently theoretical calculations [4, 5, 6, 7] predicted high thermoelectric efficiency
2
materials such as Nowotny–Juza NaZnX (X = P, As and Sb) compounds, Srn+1TinO3n+1 homologues series, carbon nitride, H2S absorbed graphene compounds.
High temperature Seebeck coefficient measurement is often critical due to lack of standardized guidelines and hence often is resulted in irreproducibility and inconsistency in experimental data. In practical cases, the measurement geometry deviates from ideal measurement geometry that introduces errors in the experimental data. There are many earlier reports [8, 17] on fabrication of instrument set-up for measuring the TE properties of bulk material. Martin et al [18] has reviewed different techniques and apparatus designs to address the uncertainty in Seebeck co-efficient measurement.
Several recent reports [19, 22] on the fabrication of set up to measure the S for both bulk and thin film samples relies on complex sample holder design to reduce the uncertainties in the data. Iwanaga et al [19] reported a set up which can measure the Seebeck coefficient upto 1000 K but the main complexity lies in the designing of the sample holder and placing the probes in it through drilling. Paul [20] reported a set up for both Seebeck coefficient and electrical resistivity measurement but both the set-up are applicable till 600 K. Budngam et al [21] reported a set-up for measuring very low Seebeck co-efficient for metals for a wide range of temperature (85-1200 K). Gunes et al [22] reported a set up that also features complicated sample probe design with custom made temperature and voltage probes.
In this paper we report a simple set-up developed from available instrumentation with sample holder part that can be easily fabricated. We employ steady state differential method and measurements are carried out in air from room temperature to 900 K. We keep the design simple and try to probe the extent of errors introduced due to deviation from ideal geometry.
3
We describe in detail the instrument fabrication, sources of errors, method of calibration, and typical measurement in test sample
Principle for determining Seebeck coefficient by differential method: Seebeck coefficient is the e.m.f, ∆V, measured between two points of a sample having temperature T1 and T2 at the respective points, under the condition that no current flows through the sample during measurement. Then S is given by S≅
∆V ∆T
, where ∆T = ( T2 – T1) →0
Though the concept of measuring S is simple, in reality it is complicated due to spurious emf generated in the measuring circuit and also the practical difficulty in measuring voltage and temperature at the same point of the sample.
Two distinct approaches namely, the integral and the differential method are utlized for measuring S. Two methods differ in the magnitude of the temperature gradient that has to be maintained across the sample. In the integral method, one end of the sample is kept at a fixed temperature while the other end is heated slowly. Using the gradient of voltage versus temperature, the S can be found for any given temperature. In the differential method, the entire sample is heated in steps to successively higher temperatures and for each step small temperature differentials are created between the ends. The linear slope of voltage drop ∆V versus the temperature difference ∆T then gives the S for the specific temperature step. This linearity is dependent upon the value of S and ∂S/∂T. These values also determine ∆T that should be chosen for the measurement to give the most accurate results. Usually for a small ∆T (~ 5 K), the relation remains linear.
4
One drawback of the integral method is the difficulty of maintaining a large temperature gradient in small samples, or in samples with high thermal conductivity. Also large temperature difference may introduce thermal stresses in the sample that can cause the sample to break. Further, there is no clear-cut way to mathematically evaluate the slope of a noisy signal and the associated error. These drawbacks are not present in the differential method, and also good for thin samples and is used in preference nowadays. We employ here differential method.
Experimental Apparatus Description The complete schematic block diagram for the setup is shown in the Fig. 1 which consists of mainly four major components : sample holder, primary heater (heater 1), secondary heater (heater 2) and its measuring components like multimeter and temperature controller.
The primary heater basically consists of a alumina tube with one end closed having inner diameter 7 cm and length 32 cm and is vertically placed in a cubical box made of mild steel of dimension 45x45x35cm3 with suitable insulation. High resistive wire is wounded on the alumina tube to have a hot zone of length 10 cm and can operate from room temperature to 900 K in a control mode.
The longitudinal 3D view of the sample holder is shown in the Fig.1b with an enlarged view of the sample holding section. The holder is made up of inconel with overall height of 30 cm and consists of two electrodes (1,2) with a spring loading arrangement, secondary heater and all other supporting components. The electrodes are basically two stainless steel (SS) cylindrical blocks of dimension of 2 cm in length and 1.5 cm in diameter. The sample is sandwiched between these two electrodes. The lower electrode block is fixed with the base 5
(10) while the upper electrode block is movable through a distance of 1.5 cm in vertical direction. To generate a temperature difference (2-10 K) between two surfaces of the sample, a secondary heater (6) is directly attached to the top electrode. All sides except the bottom of the secondary heater are coated with ceramic cement to reduce the heat loss. The top electrode assembly (along with the secondary heater) is attached to a solid screw (8) of 15 cm length and 1 cm diameter. To establish good thermal contact between the sample and the electrodes, pressure is applied through the screw on the sample surface which is spring loaded. The spring is fixed outside the primary heater in order to avoid the loss of the spring elasticity on heating. Whole top electrode assembly is supported by a circular steel disc with 4.5 cm diameter and 0.4 cm thick (11) and steel frame (9). The circular disc additionally helps in shielding the radiation heat loss from the secondary heater. The complete sample holder is kept firm in place with the help of a steel frame consists of three solid stainless rods of length 20 cm and having 0.8 cm diameter. The rods are screwed to the base (10) and fixed to the upper circular disc.
To measure the sample surface temperatures two K-type thermocouples (3,4) are silver brazed to the respective electrodes close to the sample surfaces. Thin metal wires, 0.05 cm thick (5) are silver brazed to each electrodes close to thermocouples junctions and used as voltage leads. The voltage leads also contribute to the Seebeck voltage, since their one ends are at room temperature. However their contribution is insignificant since the wire used has small S (≈1-2 µV/K) value, while the set up is designed for thermoelectric materials of having S>100 µV/K. The thermocouples and all the wires for electrical measurement are electrically insulated by alumina beads.
Temperatures of both the heaters are controlled by a Eurotherm 2416 temperature controller with resolution of 0.1 K. The temperature difference (∆T) between the sample ends is 6
maintained at ~5-10 K over the complete temperature range up to 900 K. Temperature stability is maintained in the tubular chamber at every operational temperature and found to be not exceeding more than ten degrees even at high temperature. Keithley 2010 multimeter with resolution 0.01 µV is connected to the voltage jacks of the sample holder in order to measure the voltage difference (∆V).
Measurement of the test sample To measure S, a bar shaped pellet of diameter of ~1.25 cm and thickness of ~ 0.2 cm is mounted between the two SS block electrodes. Heater 1 is programmed at a rate of 2.5-3 K/min to desired temperature. Once the temperature is reached, heater 2 is started in order to develop the temperature difference of ~5-10 K between the two surfaces of the samples. Once stabilised, the voltage difference ∆V is noted down.
At each temperature, the
measurement is repeated five times within short interval. Also the measurement is made during heating and cooling also.
Result and discussion: Test sample measurement: Sintered pellet of calcium cobaltite (Ca3Co4O9) is used as test sample. The sample is prepared by following solid state synthesis route staring from corresponding oxides and by established Pechini method [23]. To check the durability and repeatability of the system the measurement are performed several times on the test samples. Figure 2a shows plot of S versus temperature for Ca3Co 4O9 test sample for three runs and the standard deviation. The obtained S values of Ca3Co4O9 in all three trials are found to increase monotonically with the temperature. At 878 K, S is found to vary within the range of 185 to 190 µV/K .The standard deviation of S is
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within ∆S ±5 µV/K ( Fig. 2b) in the total temperature range studied. The obtained data are well compared with the published result for Ca3Co4O9 from different literatures [24, 26].
Estimation of errors: To determine the offset voltage arising due to spurious emf in the measuring circuit, an experiment is designed where an insulating block is placed between the two electrodes and voltage between the electrodesis measured in the complete temperature range. The variation of voltage is listed in Table -1. The variation of voltage recorded is varied within nanovolt and hence its effect may be neglected. In the fabricated set-up since temperature and voltages measurement locations are not exactly in touch with the sample surfaces, the major errors in voltage and temperature values arise due to thermal and electrical contact resistance.
Electrical contact resistance measurement: Electrical contact resistance refers to the contribution to the total resistance of a material/setup which comes from all electrical and mechanical connections come in the path of current flow. It is setup specific and different in different conditions. Contact resistance is independent of the measurement method.
The contact resistance of the fabricated setup is measured using four probe arrangement that consists of a current injecting channels (+I, -I) through Keithley2400 source meter and a voltage sensing channel (+V, -V)( measured with Keithley-2010 multimeter).
Fig 3a shows the measurement arrangement for evaluation of contact electrical resistance. The two external points (Fig.3a) where generated thermoelectric voltage is measured is taken as two end of the system, since between these two points it includes various electrical and mechanical contacts that can contribute to contact resistance. The two Keithley meter then accurately measures the current injection while simultaneously senses the voltage drop across 8
the setup, the resistance is then calculated. The four probe arrangement eliminates the error induced by test lead resistances. Measurement is carried out by sweeping the total temperature range.
Fig. 3b shows the variation of resistance measured from room temperature to 900 K. Measured average resistance (5.01 Ω) with a standard deviation of ± 0.1 Ω in the total temperature region is nominal and within error limit.
Thermal contact resistance measurement: To determine the thermal contact resistance, an experiment is designed where two extra thermocouples are placed by digging a shallow channel on both the surfaces of the sample pellet (Fig. 4). Both the heaters (primary & secondary) are swept through the total temperature range. The temperatures measured with the four thermocouples (Fig. 4a) are noted down, Fig.4b shows the top view of the sample during measurement. The experiment is repeated three times. It is observed that there is not much temperature lag between the lower thermocouple and the thermocouple in touch with lower surface. It is understandable since the primary heater is programmed to run sufficiently slower rate to reach thermal equilibrium, however, the upper surface exhibits temperature lag between the upper thermocouple and the thermocouple in touch with the sample upper surface. It is found that there is an error of ~ 25 K up to 600 K which increases upto 10-15 K at 900 K in measuring temperature gradient. Hence in fabricated set up the major error in the S value is coming from the error introduced in measuring the temperature of (hot side) of the sample.
Comparison of data collected in the setup with Calibrated equipment: To determine experimentally the deviation of S values measured in the fabricated set up, the S values of the same sample is measured in a commercially available (Linseis, LSR – 3) instrument which is calibrated against standard compound by the manufacturer. In the 9
commercial set-up, the readings are taken in the temperature range from 340 to 1000 K with temperature interval of 50 K. The generated potential difference (∆V) is measured in every 10 K rise of temperature gradient (∆T) of 10 K. The comparative results of commercial system along with data obtained from developed system are shown in Fig. 5a. A standard deviation plot of obtained data has been plotted (Fig 5b) with respect to data obtained from commercial equipment of same sample. At low temperature (upto 600 K) the deviation is nominal (~2-3%). Deviation seems to be higher at higher temperature (~ 5-7%). This is because the error introduced in measuring hot side temperature increases with temperature as discussed above. It is important to note here that we compare our raw data without any temperature or lead corrections.
Conclusions A simple apparatus is fabricated for measurement of Seebeck co-efficient upto temperature of 900 K using differential concept. The instrument works in air and good for measuring S for small samples. Accuracy in measuring temperature gradient is most crucial in determining the Seebeck coefficient and we observe that in our fabricated set up there is an error in determining ∆T ~ 2-5 K up to 600 K which increases upto 10-15 K at 900 K . It introduces an error in S value of ~ 2-3% at lower temperature and increases to ~5-7% at higher temperature. The data obtained in a commercial set-up and the fabricated set up, when compared also shows similar deviation in S values. Measurement of S of calcium cobaltite test sample shows fair reproducibility with standard deviation of ∆S ±5 µV/K. Finally, we report here a simple apparatus for measuring without any tailor-made sample probe, current and voltage probes design.
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Acknowledgement The authors gratefully acknowledge Director, CSIR-IMMT, Bhubaneswar for his encouragement. The authors also record their sincere thanks to Dr. Shovit Bhattacharya in carrying out Seebeck coefficient measurement at Bhaba Atomic Research Centre, Mumbai, India. This research is partially supported by Board of Research in Nuclear Sciences, Department of Atomic Energy, Mumbai, India.
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References: [1]
L. D. Hicks and M. S. Dresselhaus, Effect of quantum well structures on the thermoelectric figure of merit, Phys. Rev.B, 47 (1993)12727-12731.
[2]
Terry M. Tritt and M.A. Subramanian, Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View, MRS BULLETIN 31 (2006) 188-229.
[3]
G. Jeffrey Snyder and Eric S. Toberer, Complex thermoelectric materials, Nature materials 7 (2008) 105-114.
[4]
A .H. Reshek, S. Auluck, Thermoelectric properties of Nowotny–Juza NaZnX (X = P, As and Sb) compounds, Computational Materials Science 96 A (2015) 90–95.
[5]
A .H. Reshek, Thermoelectric properties of Srn+1TinO3n+1 (n=1, 2, 3, ∞) Ruddlesden– Popper Homologous Series, Renewable Energy 76 (2015) 36–44.
[6]
A .H. Reshek, Thermoelectric properties for AA- and AB-stacking of a carbon nitride polymorph (C3N4), RSC Adv 4 (2014) 63137-63142.
[7]
A. H. Reshak, Saleem Ayaz Khan and S. Auluck, Thermoelectric properties of a single graphene sheet and its derivatives, J. Mater. Chem. C 2 (2014) 2346-2352.
[8]
Z.H.Zhou and C. Uher, Apparatus for Seebeck coefficientand electrical resistivity measurements of bulk thermoelectric materials at high temperature, Rev. Sci. Instrum. 76 (2005) 023901.
[9]
A. T. Burkov, A.Heinrich, P. P. Konstantinov, T. Nakama and K. Yagasaki, Experimental set-up for thermopower and resistivity measurements at 100–1300 K, Meas. Sci. Technol. 12 (2001) 264–272.
[10] C. Wood, A. Chmielewski and D. Zoltan, Measurement of Seebeck coefficient using a large thermal-gradient, Rev. Sci. Instrum. 59 (1988) 951–954. [11] V. Ponnambalam, S. Lindsey, N. S. Hickman and T. M. Tritt, Sample probe to measure resistivity and thermopower in the temperature range of 300–1000 K, Rev. Sci. Instrum. 77 (2006) 073904. [12] S. Y. Loo, K. F. Hsu, M. G. Kanatzidis and T. P. Hogan, High temperature power factor measurement system for thermoelectric materials, Proc. Electrochem. Soc. 27 (2004) 247–256. [13] G. T. Kim, J G Park, J. Y. Lee, H. Y. Yu, E. S. Choi, D. S. Suh, Y. S. Ha and Y. W. Park, Simple technique for the simultaneous measurements of the four-probe resistivity and the thermoelectric power, Rev. Sci. Instrum. 69 (1998) 3705–3706.
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[14] S. Fujimoto, H. Kaibe, S. Sano and T. Kajitani, Development of transient measurement method for investigating thermoelectric properties in high temperature region, Japan. J. Appl. Phys. 45 (2006) 8805–8809. [15] T. Hogan, S. Loo, F. Guo and J. Short, Measurement techniques for thermoelectric materials and modules Thermoelectric Materials 2003: Research and Applications (MRS Symposium Proceedings Series vol 793) (Cambridge: Cambridge University Press) (2004) pp 405–411. [16] G. Min and D. M. Rowe, A novel principle allowing rapid and accurate measurement of a dimension less thermoelectric figure of merit, Meas. Sci. Technol. 12 (2001) 1261– 1262. [17] T. Dasgupta and A. M. Umarji, Apparatus to measure high-temperature thermal conductivity and thermoelectric power of small specimens, Rev. Sci. Instrum. 76 (2005) 09490. [18] J. Martin, T. Tritt, and C. Uher, High temperature Seebeck coefficient metrology, J. Appl. Phys. 108 (2010) 121101-12. [19] Shiho Iwanaga, Eric S. Toberer, Aaron LaLonde, and G. Jeffrey Snyder, A high temperature apparatus for measurement of the Seebeck coefficient, Review of scientific instruments 82 (2011) 063905. [20] Biplab Paul, Simple apparatus for the multipurpose measurements of different thermoelectric parameters, Measurement 45 (2012) 133–139. [21] Sopon Budngam, Areewichainchai, Saichol Pimmongkol and Udom Tipparach, Designing Apparatus for Highly Precise Measurement of Electrical Conductivity and Seebeck Coefficient from 85 K to 1200 K, Advanced Materials Research 740 (2013) 426-432. [22] Murat Gunes, Mehmet Parlak, and macit Ozenbas, An instrument for the high temperature measurement of the seebeck coefficient and electrical resistivity, Meas. Sci. Technol. 25 (2014) 055901-10. [23] Hoa Tran, Tejas Mehta, Matthias Zeller ,Richard H. Jarman, Synthesis and characterization of mixed phases in the Ca-Co-O system using the pechni method, Materials Research Bulletin 48 (2013) 2450-2456. [24] N.V. Nong, Chia-Jyi Liu, M. Ohtaki, High-temperature thermoelectric properties of late rare earth-doped Ca3Co 4O9+ δ , Journal of Alloys and Compounds 509 (2011) 977–981.
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[25] David Moser, Lassi Karvonen, Sascha Populoh, Matthias Trottmann, Anke Weidenkaff, Influence of the oxygen content on thermoelectric properties of Ca3xBixCo 4O9+δsystem,
Solid State Sciences 13 (2011) 2160-2164.
[26] Ngo Van Nong , Nini Pryds , Søren Linderoth and Michitaka Ohtaki, Enhancement of the Thermoelectric Performance of p –Type Layered Oxide Ca3Co4O9+
δ
Through
Heavy Doping and Metallic Nano inclusions, Adv. Mater. 2011, 23, 2484–2490.
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Figure Captions: Fig. 1 (a) Schematic block diagram of seebeck set up. (b): 3D view of the sample holder. Fig. 2 (a) Variation of Seebeck coefficient of Ca3Co 4O9 with temperature for three different trials (b) Standard deviation of the data from three trials. Fig. 3 (a) Experimental arrangement to measure electrical contact resistance. (b) Variation of resistance with temperature. Fig. 4 (a) Experimental arrangement to measure thermal contact resistance. (b) Top view of the sample with channel to place thermocouple TCs1. Fig. 5 (a) Comparison of Seebeck coefficient (S) values with temperature (T) from the commercial instrument and the fabricated set up. (b) Variation in percentage error in Seebeck coefficient values with temperature between the data measured in commercial instrument and the fabricated set up.
Table Caption: List of offset voltage generated with increasing temperature.
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Tables:
Table - 1 List of offset voltage generated with increasing temperature Temperature (K)
373
473
573
673
773
873
Voltage (µV)
0.015
0.017
0.007
0.011
0.009
0.013
16
17
18
19
20
21
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Highlights
• • • • •
Simple Seebeck (S) coefficient measurement setup without any tailor made probes. Able to measure S from room temperature to 900 K in air. The measured S values has fair reproducibility with standard deviation ∆S ± 5 µV/K. Error in S values of the order of 2-3% till 600 K, increases to 5-7 % at 900 K. User friendly setup for measurement of S values for small bulk samples.
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Graphical abstract
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