Thermal diffusivity measurement system applied to ...

Thermal diffusivity measurement system applied to polymers B. Abad, P. Díaz-Chao, A. Almarza, D. Amantia, S. Vázquez-Campos, Y. Isoda, Y. Shinohara, F. Briones, and M. S. Martín-González Citation: AIP Conference Proceedings 1449, 354 (2012); doi: 10.1063/1.4731570 View online: http://dx.doi.org/10.1063/1.4731570 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1449?ver=pdfcov Published by the AIP Publishing

This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 83.40.103.63 On: Sat, 30 Nov 2013 09:45:40

Thermoelectric Materials

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Thermal Diffusivity Measurement System Applied To Polymers B. Abad1, P. Díaz-Chao1, A. Almarza2, D. Amantia2, S. Vázquez-Campos2, Y. Isoda3, Y. Shinohara3, F. Briones1, M.S. Martín-González1 1

IMM-Instituto de Microelectrónica de Madrid (CNM CSIC), Isaac Newton 8, PTM, E-28760 Tres Cantos, Madrid, Spain 2 Centro Tecnológico LEITAT C/ de la Innovació, 2 08225 Terrassa, Barcelona, Spain 3 NIMS-National Institute for Materials Science, 1-2-1 Sengen, Tsukuba-city Ibaraki 305-0047 Japan

Abstract. In the search for cleaner energy sources, the improvement of the efficiency of the actual ones appears as a primary objective. In this way, thermoelectric materials, which are able to convert wasted heat into electricity, are reveal as an interesting way to improve efficiency of car engines, for example. Costeffective energy harvesting from thermoelectric devices requires materials with high electrical conductivities and Seebeck coefficient, but low thermal conductivity. Conductive polymers can fulfil these conditions if they are doped appropriately. One of the most promising polymers is Polyaniline. In this work, the thermal conductivity of the polyaniline and mixtures of polyaniline with nanoclays has been studied, using a new experimental set-up developed in the lab. The novel system is based on the steady-state method and it is used to obtain the thermal diffusivity of the polymers and the nanocomposites. Keywords: thermal conductivity, diffusivity, polyaniline, sepiolite, montmorillonite PACS: 65.90.+i, 07.20.-n

INTRODUCTION These days many efforts are being dedicated to the research and development of new clean energy sources motivated by the climate change. However, an improvement in the efficiency of the energy generation is also a relevant factor to be taken into account. In this point, the thermoelectric materials have an important role to play converting the wasted heat of the conventional energy sources into electricity. The efficiency of this kind of materials is given by the figure of merit, ZT = (S2ıț 7 where S is the thermoelectric power (Seebeck coefficient), ı is the electrical conductivity, ț is the thermal conductivity and T is the absolute temperature [1]. In order to optimize the figure of merit, new materials are nowadays under study. Among the most promising ones are nanocomposites, which are made up of polymers and inorganic materials. Since 1990 many researchers have focused their efforts on this sort of materials. Even though a low percentage of inorganic material is present, its properties can be transferred to the polymer matrix improving the physical properties of the polymer such as electrical conductivity, stiffness, hardness, thermal stability, etc. In particular, polyaniline (PANI) is a conductive polymer with electrical conductivity values up to 101 S·cm-1 and a low thermal conductivity (ț of ~ 0.4 W/m·K, that makes it interesting to be used in near room temperature thermoelectric devices [2] and electronic devices [3]. In fact, LEDs [4], biosensors [5], pH sensors [6] based on PANI are being developed. The electrical properties of polyaniline, however, depends on the oxidation state, being the fully oxidized (pernigraniline) and the fully reduced (leucoemeraldine) states non-conductive polymers, and the half-reduced/half oxidized state (emeraldine) the conductive one. In this work, the design and development of an experimental system to measure the thermal properties of prepared emeraldine based polyaniline samples are described. The measurements performed on a labdeveloped steady-state system of these samples are shown. Polyaniline/nanoclays composites to achieve possible further reduction of the thermal properties have also been prepared and their properties shown.

EXPERIMENTAL SET-UP In general, the procedures to measure thermal properties can be divided into two types: the steadystate method and transient method. The steady-state method is the one chosen in this work because the experimental set-up is considerably simpler than in the other case. The experimental system developed in this work is based in the measurement of the heat going thought the sample by conduction. Since heat can 9th European Conference on Thermoelectrics AIP Conf. Proc. 1449, 354-357 (2012); doi: 10.1063/1.4731570 © 2012 American Institute of Physics 978-0-7354-1048-0/$30.00

European Conference ontoThermoelectrics This article is copyrighted as indicated in the abstract.9th Reuse of AIP content is354 subject the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 83.40.103.63 On: Sat, 30 Nov 2013 09:45:40

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be transferred by conduction, convection and radiation, the system must be designed so that the contributions of the convection and radiation are minimized. The experimental set-up developed at the lab (see figure 1) consists of two copper blocks with the sample placed between them. The temperatures of the blocks are measured via two thermocouples placed inside them, very near the contact surface with the sample. The upper block acts as a heat source, thanks to two heaters located inside it (see figure 1b). The lower block acts as a heat sink and has cylindrical shape to fit the shape of the sample. The whole system is placed inside a vacuum chamber. A stringmagnet system is attached to the hot block via 4 Nylon® screws placed in each corner (see Fig. 1a) to control externally the position of this block during the experiment. The copper blocks are coated by a CuO!" Au=0.02). COMSOL® simulations were carried out at two different pressures (103 mbar and 0.01 mbar) to study the convection influence inside the chamber. It can be seen in Fig. 2 that the reduction of the pressure down to 0.01mbar reduces the convection considerably, therefore, the whole system was maintained at P = 10-1 mbar to reduce convection. Moreover, the simulation at P = 103 mbar shows the importance of placing the hot block on top of the set-up to avoid any heating of the cold block.

FIGURE 1. a) Experimental set-up sketch and b) copper blocks and sample final configuration

FIGURE 2. COMSOL simulations of convection: a) P = 103 mbar and b) P = 0.01 mbar. In this simulation both blocks are in contact

However, one of the main challenges of the development of the systems that measure thermal properties is the improvement of the thermal contact resistance. The contact surfaces are usually nonconforming and rough, which makes the real contact area considerably smaller than the real nominal one and, consequently, the heat transfer across the interfaces takes place through micro-contacts and through air-filled micro-gaps giving rise to an important thermal contact resistance [7]. In our case, to minimize this term, a thin molten layer gallium has been employed as thermal interface material. Gallium has been chosen because of its high thermal conductivity (40 W/m·K) and its low melting point (29 ºC), which makes it easy to be spread on the surfaces allowing a good thermal contact. To improve the wetting of the gallium to the copper blocks a 100nm thick titanium layer was evaporated onto the contact surfaces. The measurement protocol starts heating the hot copper block. When the steady-state is reached, this block is lowered and placed in contact with the sample by the string-magnet system. Once a steady-state is reached again across the sample, the power supply is switched off and the cooling rate is measured.

THEORETICAL BASIS The heat flux across the sample is given by the Fourier´s Law, Eq. (1), ·

P

Q

AN

dT dx

(1)

However, in this expression, P corresponds to the effective power supplied by the heaters to the sample, being necessary to quantify the losses of the system. In the steady state power losses are equal to



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heating power, so that if the heating power is removed the power losses, and hence the effective power, can be calculated from the cooling rate of the sample:

Q

mCe

wT wt

(2)

Where m is the copper block mass, T the temperature, and Tr is the room temperature. Eq. 1 and 2 can be combined to obtain a new formula for the thermal diffusivity. In order to reduce the experimental error, the thermal diffusivity of the samples is to be referenced to a known sample (stainless steel AISI 304L), so that the following equation must be used:

DS

§ wT · ¨ ¸ 2 © wt ¹ S 'TR LS D R § wT · 'TS L2R ¨ ¸ © wt ¹ R

(3)

Where subscripts S and R refer to the sample to be measured and the reference sample, respectively. In this way, remaining losses due to radiation as well as the surrounding atmosphere can be neglected since they are essentially the same in each experiment, regardless of the measured sample.

RESULTS AND DISCUSSION Firstly, thermal diffusivity for polyaniline samples was studied. The preparation procedure was the same for all the samples, except the final purification process (Table 1). As a result, it can be seen in Table 1 that there is a clear difference between the various processes. This variation of the thermal diffusivity is in agreement with previous results reported by Hu Yang et al. [8], who showed a broad variation of the thermal diffusivity of PANI samples depending on the preparation method Į = (0.7 – 4.7)10-7 m2/s. When polyaniline is washed with strong acids like HCl instead of water, thermal diffusivity increases in a factor of two due to changes in the protonation in the polymeric chains (See Fig. 4). Albuquerque et al. [9] observed a similar increase in the thermal diffusivity of polyaniline when HCl is used in the purification process of the samples. Moreover, the value obtained with the system designed in this work is very similar to Albuquerque´s value, who obtained Į = 2.28 10-7 m2/s. TABLE 1. Thermal diffusivity values of polyaniline samples depending on the purification step. Purification process 7KHUPDOGLIIXVLYLW\Į-7(m2/s) H2O-MeOH 2.42 HCl-MeOH 2.77 HCl-EtOH 3.93 HCl-H2O-EtOH 4.70

Sample A B C D

2.8x10-7

2.8x10-7

2.4x10-7

2.4x10-7

2.0x10-7

2.0x10-7

Dm2/s)

Dm2/s)

Secondly, in order to achieve further reduction of the thermal diffusivity, nanocomposites of PANI and nanoclays were prepared. The purification process with water and methanol was chosen because, as shown before, this method gave lower values of the thermal diffusivity (see Table 1).

-7

1.6x10

1.2x10-7 -8

1.2x10-7 8.0x10-8

8.0x10

-8

4.0x10

1.6x10-7

0

10

20

30

Content SEP (%)

40

50

4.0x10-8

0

10

20

30

40

50

Content MMT (%)

FIGURE 3. Thermal diffusivity of the polyaniline samples mixed with a) sepiolite (SEP) and b) montmorillonite (MMT)

As it can be seen in Fig. 3, when the nanoclays content increases, thermal diffusivity decreases. This result can be understood taking into account that nanoclays are not only great thermal insulators, but also the nanoclays dispersed in the polymer matrix (sepiolite can form needles of 650 nm length and 24 nm


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diameter whereas montmorillonite is broken in plates of 100 nm distributed inside the polymer) can act as scattered phonon centres. As mentioned above, thermal conductivity can be obtained from thermal diffusivity by measuring density and specific heat. These last measurements have been carried out by Differential Scanning Calorimetry (DSC). Examples of the obtained results are shown in Table 2. When polyaniline is mixed with nanoclays, the thermal conductivity is approximately half, as it could be inferred from the results of thermal diffusivity.

Sample HCl-MeOH MMT SEP

TABLE 2. Specific heat and density of some PANIs/PANIs nanoclay %MMT/SEP Į-7(m2/s) Density (g/cm3) Specific heat (J/gºC) 0 2.77 1.3 1.02 30 1.00 1.5 1.32 30 0.90 1.5 1.33

Thermal conductivity (W/m·ºC) 0.37 0.20 0.17

In order to corroborate the reliability of our system, the specific heat, density and thermal diffusivity and conductivity of a reference polyaniline samples was measured in the Laboratory of Thermoelectric Materials at the National Institute for Materials Science (NIMS) of Japan by the laser flash method [10, 11]. Comparison between this measurement and the result obtained with the experimental set-up designed in our lab are in good agreement within the estimated error of both techniques, as shown in Table 3. Moreover, our system presents the advantage of being completely non-destructive, whereas the laser flash measurement of the sample resulted in the surface damage of the PANI. TABLE 3. Comparison of Thermal diffusivity measured in two different labs Į-7(m2/s) Sample 1.40 ± 0.07 H2O-acetone (NIMS) 1.25 ± 0.13 H2O-acetone (IMM)

CONCLUSIONS A new experimental set-up based on the steady-state method has been developed. This system has been used to characterize PANI samples, which are very promising materials for future room temperature thermoelectric devices. Therefore, it has been shown the influence of the polymer cleaning procedure in the thermal diffusivity. Finally measurements of thermal diffusivity of PANI with different contents of nanoclays composites have been made, and it could be seen that thermal diffusivity decreases when content of nanoclay increases, making them even more suitable for thermoelectric applications. In order to verify the accuracy of the steady-state method developed for this study, thermal conductivity of one sample was measured by commercial equipment, and the result shows that our technique is reliable.

ACKNOWLEDGMENTS Authors would like to thanks the MICINN for financial support from project FCCI 2009 ACI PLAN E (JAPON) Ref.: PLE2009-0073 and CONSOLIDER-Ingenio 2010, under contract number CSD201000044. PDC would like to thank the ERC-2009-StG-20081029 for financial support.

REFERENCES 1. 2. 3.

CRC Thermoelectrics Handbook: Macro to Nano, (D. M. Rowe, Editor), CRC Press, 2005 C. Meng, C. Liu, S. Fan; Adv. Mater., 22, 535-539 (2010) S. Angappane, R. N. Kini, T. S. Natarajan, G. Rangarajan, B. Wessling; Thin Solid Films, 417, 202-205, (2002) 4. G. Gustafsson et al., Synthetic Metals, 55-57, 4123-4127, (1993) 5. Y.C. Luo, J. S. Do; Biosens. Bioelectron., 20, 15-23 (2004) 6. Z. Jin, Y. Su, Y. Duan; Sensor. Actuat. B-Chem. 71, 118-122, (2000) 7. M. Grujicic, Z.L Zhao, E.C. Dusel, Applied Surface Science, 246, 290–302, (2005) 8. Hu Yan et al. Journal of Thermal Analysis and Calorimetry, 69, 881-887, (2002) 9. Albuquerque et al. Review of Scientific Instruments, 74, 306-308, (2003) 10. W. Nunes dos Santo, P. Mummeryb, A. Wallwork, Polymer Testing, 24, 628–634, (2005) 11. Z.Y. Deng, J.M.F. Ferreira, Y. Tanaka b, Y. Isoda, Acta Materialia 55, 3663–3669, (2007)
