A Simple and Portable Setup for Thermopower Measurements - ACS

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A Simple and Portable Setup for Thermopower Measurements RAFIQ MULLA, and M. K. Rabinal ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.5b00128 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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ACS Combinatorial Science

A Simple and Portable Setup for Thermopower Measurements Rafiq Mulla and M. K. Rabinal* Department of Physics, Karnatak University Dharwad, Karnataka State, India-580003 *[email protected]

Graphical Abstract

Soft Adjustable Pressure

HOT

COLD

ACS Paragon Plus Environment


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A Simple and Portable Setup for Thermopower Measurements Rafiq Mulla and M. K. Rabinal* Department of Physics, Karnatak University Dharwad, Karnataka State, India-580003 Abstract Thermoelectric energy conversion technology has received much attention due to its promising applications and environmentally clean nature. The innovative design of efficient thermoelectric materials is assisted by simple and reliable techniques for fast and accurate testing. Here, a simple approach for rapid measurement of Seebeck coefficient is described, using commonly available materials based on hot and cold probes, both the probes are heated by built-in micro-heaters. A spring loaded sample mounting arrangement provides easy sample loading/unloading. The setup is suitable for measurements on a wide range of materials, such as pellets, films and even soft surfaces without damage. Several known thermoelectric materials such as alumel, bismuth and silicon yielded values close to reported ones. The setup is very compact, simple, fast, low cost and reliable to develop as a laboratory characterization tool.

Keywords: Thermoelectric setup, Seebeck coefficient, low cost, portable, films and soft surfaces *Corresponding author Tel.: +91-836-2215316; Fax: +91-836-2472444 e-mail address: [email protected] (M. K. Rabinal)


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1.

Introduction

Thermoelectric materials are important for the conversion of waste heat into useful electricity. While the efficiency of thermoelectric devices is usually poor on an absolute scale, they are attractive because they are simple, noise-free, and do not contain any moving parts.1 The most promising materials developed for practical application are alloys such as Bi2Te3 and PbTe but the development of more efficient, cost-effective and non-toxic materials remains an important goal. Simple, fast and reliable technique for thermoelectric characterization must therefore be developed to help study a wide variety of materials, including nanostructured and soft materials. The Seebeck coefficient represents the ability of a material to produce voltage under an applied temperature gradient, so its measurement is of fundamental importance in the development of new thermoelectric materials.1 This parameter is usually determined in two ways, the integral method and the differential method.2 Although the integral method is simpler, it requires sample be in the form of long wire, and so is not used in most cases.2 The differential method can accommodate various apparatus designs for different kinds of samples,2 and so many such setups have been described, with each apparatus tailored for a particular sample shapes.3-19 Several experimental setups based on hot probe techniques have been recently reported, useful only with samples of particular shapes3-6 or requiring samples in bulk form.7-12 Only a few are suitable for both bulk materials and films.4,13-15 In most cases, Seebeck coefficients of alloy samples are measured by drilling holes to insert thermocouples for accurate measurements of temperature and voltage,20 which is unsuitable for materials which are brittle, films, small in size, or soft in nature. For the screening of thermoelectric materials, several rapid techniques of Seebeck coefficient measurements have been suggested.4,5,16-19,21 Fujimoto et al. designed a probe for



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uneven powders and films that is capable of very rapid measurements (about 5 seconds),16 but is not suitable for temperature-dependent measurements. A similar instrument has been successfully developed for a rapid method by Watanabe et al., requiring the deposition of each sample on a custom alumina substrate.17 Since the technique is based on thermal imaging to read the temperature, each sample surface must also be coated with blackbody paint. A similar dynamic method has been suggested by Zhou et al. for fast measurements,18 providing Seebeck coefficient from room temperature to 473 K within 12 minutes. However, this setup is not convenient for very thin and fragile samples. While the above reported setups/methods are useful, we were motivated to search for a more general approach. Here we present a setup based on a hot probe technique which minimizes many of the above issues. The design is simple, low-cost, and reliable, providing fast and convenient measurements of Seebeck coefficient that can be easily adopted in most laboratories. We therefore believe it to be especially suitable for preliminary screening of many thermoelectric samples in a short time. 2.

Experimental Setup

As shown in Figure 1, the apparatus for Seebeck coefficient measurement resembles one reported by us for electrical conductivity measurement of polymeric films.22 It starts with a single-sided non-etched copper printed circuit board (PCB) of 1.5 mm thick as a platform, cut to the desired dimensions of 1.5 cm width and 10 cm length. From its centre, two rectangular copper film electrodes (1.5 cm x 4 mm width) were made by leaving 3 mm separation in between (Figure 1b). This gap can be further reduced to a smaller size by etching the unwanted copper using concentrated ferric chloride solution. The resulting copper contacts were washed well and were polished to remove the surface oxide, the final thickness was about 200 µm. The resulting electrodes were coated with a thin layer of gold by a standard electroless deposition process23 to prevent the copper oxidation.


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The PCB was fixed on a stable platform having vertical bolts (4 cm height and 2 mm diameter) with two parallel brass plates, nuts, and a spring as shown in Figure 1a. To make a measurement, the sample was placed upside down on the electrode pattern and pressed to a desired load by compressing the brass plates loaded with springs and nuts. A small thick insulating teflon was attached on the sample contact area of the bottom brass plate in order to eliminate electrical contact between the sample and the brass plate and also to reduce thermal loss (Figure 1a). On both electrode patterns, small, thin resistive micro-heaters were attached using thermal glue, leaving enough space for sample mounting. The surface area of the each heater was about 5x4 mm2 with thickness of 1 mm. These were retrieved from a commercially available well sealed resistor network (9 pin, 100 Ω, Model-A101G, Shenzhen Yuzhouxin Electric Co. Ltd., China) which was cut into a segment having 2 pins to make a two-probe micro-heater. The resistance of each heater was 100 Ω and the maximum temperature obtained was approximately 120 ºC for a dc voltage of 16 V. Two K-type thermocouples were fixed using thermal glue on both electrodes near the sample mounting place. All the electrical contacts, thermocouples and heater wires were connected to jacks fixed on the base platform for easy access. The voltage was measured by Keithley 617 electrometer, temperature by digital thermometers, and regulated dc power supplies were used as voltage sources for each heater. For thermopower measurements, a temperature difference across the sample is required, which can be made by applying dc voltage to one of the heaters. Once the hot side temperature became stable for a given voltage applied to the heater, the Seebeck voltage (∆V), the temperatures of the hot end (THot) and the cold end (TCold) were measured simultaneously. The heater power was increased stepwise to increase the temperature of the hot electrode. In each step enough time was given to reach a stable temperature at the hot end before making voltage and temperature measurements at both ends. For temperature



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dependent Seebeck coefficients, a small temperature difference (∆T) of 4-5 ºC was maintained by slowly increasing the temperatures of both the ends. The ∆T and Seebeck voltage (∆V) for each step of heater power were used to calculate the Seebeck coefficient (S), given by equation (1).2 Usually, Seebeck coefficients do not change much at different values of ∆T.24 ∆୚

ܵ = −ቀ ቁ ∆୘

------------------------

(1)

The absolute Seebeck coefficient of a sample is obtained by subtracting the contribution of gold electrodes that is given by equation (2).2 ∆୚

ܵ௦ = − ቀ ቁ + ܵ஺௨ - - - - - - - - - - - - - - - - - - - - - - (2) ∆୘

where Ss and SAu are the absolute Seebeck coefficients of the sample and gold, respectively. From the literature SAu is taken as 2.08-2.55 µV K-1 in the temperature range 300-400 K.25 3.

Results and discussion

The time-response of present setup was studied by recording temperature as a function of time for progressive heating of the hot probe. Figure 2a shows the variation of temperature with time for an alumel sample for a step-wise increase in voltage of the hot probe. It can be seen that for each step, a stable temperature value was reached in less than a minute, making quick data recording possible with good temperature stability (Figure 2a, inset). A similar measurement was carried out for direct heating of the probe to about 80 ºC without sample loading (Figure 2b), showing a rise of the cold end temperature by only 11 ºC when the hot end is heated to 80 ºC, even after relatively long thermal equilibrium (about 15 minutes) and without a heat sink. This is due to the low thermal conductivity (0.343 W m-1 K-1)26 of the back-support material, usually fabricated from composites of fibreglass and epoxy resin.


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There is also no lateral heat flow from hot to cold end since the electrodes are quite small (200 µm thick), and so are almost embedded on PCB surface. Hence, the small amount of heat that flows from hot to cold end is mainly due to the sample, and the heat loss by the measuring system is quite negligible. This allows the acquisition of multiple data points of Seebeck voltage over a sufficient range of ∆T. In figure 2b, it can be seen that even for a direct heating to reach highest temperature (80 ºC), the stabilization was achieved within 2.5 minutes, in contrast to other reported cases.27 In order to confirm proper working and reliability of the setup, three standard materials were tested: Alumel, bismuth and silicon. Alumel and bismuth were in the form of films and silicon as a single crystal wafer (0.001 Ω-cm resistivity, p+-type, 500 µm thick, boron doped and one side mirror polished, supplied by Semiconductor Wafer, Inc., Taiwan). All the measurements were performed from room temperature to 75 ºC in air, inside an enclosure to minimize air flow over the sample. One heater was turned on with a small dc voltage to get a small increase in temperature at the hot end of the sample and enough time was given (nearly 2 min) to reach constant temperature. After recording ∆V, THot and TCold, the cycle of temperature increase, stabilization, and data acquisition was repeated. Temperature differences (∆T) from 0.5 to 30 ºC and corresponding voltage differences (∆V) were noted. The Seebeck voltage produced for different ∆T is shown in Figure 3a for alumel, bismuth and silicon. Measurements were repeated 4-5 times for each sample and measurements are quite reproducible, and average of this is shown in Figure 3a. The slope of linear curve is taken as Seebeck coefficient (S) of sample with respect to reference material or differential value is obtained by just taking single point ∆V/∆T. The obtained absolute Seebeck coefficient of alumel is -20.1 ± 1.1 µV K-1, that is close to literature value -18 µV K1 28,29

.

Similarly, for bismuth it is -76.5 ± 1.2 µV K-1 where as its reported value is -78 µV K-



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1 29

.

For silicon it is +314.6 ± 1.4 µV K-1 with p-type behaviour. The value is in agreement

with the reported one that is 300-320 µV K-1 for ~1020 doping density.30 Measured and reported Seebeck coefficients are given in Table 1 with typical errors. Figure 3b shows the temperature dependent absolute Seebeck coefficients of the above materials. Further, our setup is also used to measure Seebeck coefficient of soft surface that is morphologically higher structures of Cu2S nanoparticles formed on copper coated epoxy by a simple chemical route. Figure 4a shows the SEM picture of Cu2S film of thickness 80 µm. These are highly oriented two-dimensional nanosheets with an average size of 20 µm. The setup is tested for these soft films to measure Seebeck coefficient without damage, the data is plotted in Figure 4b. These films exhibit a good Seebeck coefficient close to 110 µV K-1 in the measured range of temperature. The detailed studies on synthesis, morphological structure and thermo-electrical properties of these films will be published soon. The proposed setup is quite suitable, simple and easily adoptable to measure Seebeck coefficient of soft surfaces. Due to planar contacts, the technique is suitable for both bulk as well as thin films. High temperature measurements are not possible in the present setup, but it is perfectly convenient for measurements up to 100 ºC, which is suitable temperature for soft and nanomaterials. Its proper modifications would definitely take the measurements to still a higher temperature. 4.

Conclusions

A Seebeck measurement setup has been designed and fabricated successfully using simply available materials. The setup is very convenient to use without any complexity hence fast and reliable measurements can be carried out. The measurements on standard samples match well with their literature values. Further, the setup is convenient for pellets, films and also for


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soft surfaces. Since the setup is simple, portable and reliable, it can be easily adopted in research laboratories as a testing tool for thermoelectric materials.



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References (1) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105-114. (2) Rowe, D. M. Thermoelectrics Handbook: Macro to Nano, CRC Press, Taylor & Francis Group: Boca Raton, 2006; pp 22(2-12). (3) Kumar, S. R. S.; Kasiviswanathan, S. A Hot Probe Setup for the Measurement of Seebeck Coefficient of Thin Wires and Thin Films using Integral Method. Rev. Sci. Instrum. 2008, 79, 024302(1-4). (4) Canadas, J. G.; Min, G. Multifunctional Probes for High-Throughput Measurement of Seebeck Coefficient and Electrical Conductivity at Room Temperature. Rev. Sci. Instrum. 2014, 85, 043906(1-4). (5) Cowles, L. E. J.; Dauncey, L. A. Apparatus for the Rapid Scanning of the Seebeck Coefficient of Semiconductors. J. Sci. Instrum. 1962, 39, 16-18. (6) Goldsmid, H. J. A Simple Technique for Determining the Seebeck Coefficient of Thermoelectric Materials. J. Phys. E: J. Sci. Instrum. 1986, 19, 921-922. (7) Kallaher, R. L.; Latham, C. A.; Sharifi, F. An Apparatus for Concurrent Measurement of Thermoelectric Material Parameters. Rev. Sci. Instrum. 2013, 84, 013907(1-7). (8) Soni, A.; Okram, G. S. Resistivity and Thermopower Measurement Setups in the Temperature Range of 5–325 K. Rev. Sci. Instrum. 2008, 79, 125103(1-4). (9) Ponnambalam, V.; Lindsey, S.; Hickman, N. S.; Tritt, T. M. Sample Probe to Measure Resistivity and Thermopower in the Temperature Range of 300–1000 K. Rev. Sci. Instrum. 2006, 77, 073904(1-5). (10) Chandra, L. S. S.; Lakhani, A.; Jain, D.; Pandya, S.; Vishwakarma, P. N.; Gangrade, M.; Ganesan, V. Simple and Precise Thermoelectric Power Measurement Setup for Different Environments. Rev. Sci. Instrum. 2008, 79, 103907(1-5).


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(11) Paul, B.; Simple Apparatus for the Multipurpose Measurements of Different Thermoelectric Parameters. Measurement 2012, 45, 133-139. (12) Mishra, A.; Bhattacharjee, S.; Anwar, S. Simple Apparatus to Measure Seebeck Coefficient up to 900 K. Measurement 2015, 68, 295-301. (13) Boffoue, O.; Jacquot, A.; Dauscher, A.; Lenoir, B.; Stolzer, M. Experimental Setup for the Measurement of the Electrical Resistivity and Thermopower of Thin Films and Bulk Materials. Rev. Sci. Instrum. 2005, 76, 053907(1-4). (14) Ravichandran, J.; Kardel, J. T.; Scullin, M. L.; Bahk, J. H.; Heijmerikx, H.; Bowers, J. E.; Majumdar, A. An Apparatus for Simultaneous Measurement of Electrical Conductivity and Thermopower of Thin Films in the Temperature Range of 300–750 K. Rev. Sci. Instrum. 2011, 82, 015108(1-4). (15) Tripathi, T. S.; Bala, M.; Asokan, K. An Experimental Setup for the Simultaneous Measurement of Thermoelectric Power of Two Samples from 77 K to 500 K. Rev. Sci. Instrum. 2014, 85, 085115(1-7). (16) Fujimoto, K.; Taguchi, T.; Yoshida, S.; Ito, S. Design of Seebeck Coefficient Measurement Probe for Powder Library. ACS Comb. Sci. 2014, 16, 66-70. (17) Watanabe, M.; Kita, T.; Fukumura, T.; Ohtomo, A.; Ueno, K.; Kawasaki, M. HighThroughput Screening for Combinatorial Thin-Film Library of Thermoelectric Materials. J. Comb. Chem. 2008, 10, 175–178. (18) Zhou, Y.; Yang, D.; Li, L.; Li, F.; Li, J. F. Fast Seebeck Coefficient Measurement Based on Dynamic Method. Rev. Sci. Instrum. 2014, 85, 054904(1-5). (19) Funahashi, R.; Urata, S.; Kitawaki, M. Exploration of n-Type Oxides by High Throughput Screening. Appl. Surf. Sci. 2004, 223, 44-48.



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(20) Kim, Y. G.; Kang, K. H.; Gam, K. S.; Kim, J. C.; Kim, J. H. Measurement of the Seebeck Coefficients of Binary Cu–Ni Alloys. Meas. Sci. Technol. 2004, 15, 12661270. (21) Otani, M.; Itaka, K.; Wong-Ng, W.; Schenck, P. K.; Koinuma, H. Development of a High-Throughput Thermoelectric Screening Tool for Combinatorial Thin Film Libraries. Appl. Surf. Sci. 2007, 254, 765-767. (22) Hiremath, R. K.; Rabinal, M. K.; Mulimani, B. G. Simple Setup to Measure Electrical Properties of Polymeric Films. Rev. Sci. Instrum. 2006, 77, 126106(1-3). (23) Okinaka, Y.; Sard, R.; Wolowodiuk, C.; Craft, W. H.; Retajczyk, T. F. Some Practical Aspects of Electroless Gold Plating. J. Electrochem. Soc. 1974, 121, 56-62. (24) Borup, K. A.; de Boor, J.; Wang, H.; Drymiotis, F.; Gascoin, F.; Shi, X.; Chen, L.; Fedorov, M. I.; Muller, E.; Iversen, B. B.; Snyder, G. J. Measuring Thermoelectric Transport Properties of Materials. Energy Environ. Sci. 2015, 8, 423-435. (25) Roberts, R. B. The Absolute Scale of Thermoelectricity II. Philos. Mag. B 1981, 43, 1125-1135. (26) Sarvar, F.; Poole, N. J.; Witting, P. A. PCB Glass-Fibre Laminates: Thermal Conductivity Measurements and their Effect on Simulation. J. Electron. Mater. 1990, 19, 1345-1350. (27) Bottger, P. H. M.; Flage-Larsen, E.; Karlsen, O. B.; Finstad, T. G. High Temperature Seebeck Coefficient and Resistance Measurement System for Thermoelectric Materials in the Thin Disk Geometry. Rev. Sci. Instrum. 2012, 83, 025101(1-6). (28) Robin, E. B. Handbook of Temperature Measurement: Theory and Practice of Thermoelectric Thermometry, Vol. 3; Springer-Verlag Pte. Ltd.: Singapore, 1998; pp 7-9.


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(29) Cardarelli, F. Materials Handbook: A Concise Desktop Reference, 2nd ed.; SpringerVerlag: London, 2008; pp 543-544. (30) Neophytou, N.; Zianni, X.; Kosina, H.; Frabboni, S.; Lorenzi, B.; Narducci, D. Simultaneous Increase in Electrical Conductivity and Seebeck Coefficient in Highly Boron-Doped Nanocrystalline Si. Nanotechnology 2013, 24, 205402(1-11).



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Figure captions Figure 1. (a) Homemade setup for Seebeck coefficient measurement and (b) the configuration of copper electrodes made on PCB used in the setup. Figure 2. (a) Variation of hot end temperature as a function of time with alumel sample, under the application of stepwise increase in voltage to the heater (inset shows enlarged plot). (b) The temperature variation of hot and cold end as a function of time immediately after the power given to the heater. These measurements are carried out without loading the sample. Figure 3. (a) Seebeck voltage (∆V) produced under different temperature differences (∆T) for alumel, bismuth and silicon (b) Temperature dependent Seebeck coefficients of alumel, bismuth and silicon. Figure 4. (a) The surface morphology of the nanostructured copper sulfide film. (b) Temperature dependent Seebeck coefficient of nanostructured copper sulfide film.


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Figure 1

(a)

Teflon

Heater

Sample

TCold

THot V

(b) 10 cm 1.5 cm

PCB 1.5 cm

4 mm Heater

3 mm



Figure 2 33

(a) 32

o

THot ( C)

70

60

31

o

THot ( C)

30 29

50

12

14 16 Time (min)

18

40

30 0

10

90

20

30 Time (min)

40

50

60

(b)

Without Sample

THOT

80 70

o

Temperature ( C)

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o

60

∆ T = 44 C

50 40

TCOLD

30 20 0

5

10 Time (min)


15

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Figure 3

6

Seebeck voltage (∆ V) (mV)

(a) 4

Silicon

2

Alumel

0

-2

Bismuth

0

5

10

15

20

25

30

35

o

Temperature difference (∆T) ( C)

(b) 300

Silicon

-1

Seebeck coefficient (S) (µ V K )

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200

100

Alumel

0

Bismuth

-100 25

30

35

40

45

50

55 o

Temperature ( C)


60

65

70

75


Figure 4

(a)

150

(b)

-1

Seebeck coefficient (S) (µV K )

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100

50

0 25

30

35

40

45

50

55 o

Temperature ( C) ACS Paragon Plus Environment

60

65

70

75

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Table 1

Table 1.

Compares the numerical values of measured and reported room temperature Seebeck coefficient for various materials.

Sample

Measured Reported Seebeck coefficient Seebeck coefficient (µV K-1) (µV K-1)

Alumel

-20.1 ± 1.1

-18

Bismuth

-76.5 ± 1.2

-78

p-Silicon

314.6 ± 1.4

300-320


A Simple and Portable Setup for Thermopower Measurements - ACS

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