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1 DOI: j v q œ w z» w e gqd w ½ k*á x Áw x w»,» Influence of Metal-Coating Layer on an Electrical Resistivity of Thick-Film-Type Thermoelectric Modules Fabricated by a Screen Printing Process Kyung Tae Kim*, Hye Young Koo, and Gook Hyun Ha Powder Technology Research Group, Korea Institute of Materials Science, 797 Changwon-daero, Changwon-si, Gyeongnam , Korea (Received July 1, 2011; Revised July 23, 2011; Accepted August 13, 2011) Abstract Thermoelectric-thick films were fabricated by using a screen printing process of n and p-type bismuth-telluride-based pastes. The screen-printed thick films have approximately 30 µm in thickness and show rough surfaces yielding an empty gap between an electrode and the thick film. The gap might result in an increase of an electrical resistivity of the fabricated thick-film-type thermoelectric module. In this study, we suggest a conductive metal coating onto the surfaces of the screen-printed paste in order to reduce the contact resistance in the module. As a result, the electrical resistivity of the thermoelectric module having a gold coating layer was significantly reduced up to 30% compared to that of a module without any metal coating. This result indicates that an introduction of conductive metal layers is effective to decrease the contact resistivity of a thick-film-typed thermoelectric module processed by screen printing. Keywords: Bismuth telluride, Thick film, Thermoelectric module, Metal coating, Electrical resistivity 1. y w š, w y š. p» ƒ wš» ƒw j y y d j š [1-3]. xk Ás y w w w»» þƒ e š.» yz ùküš» 423 (Seebeck Coefficient),» w ƒƒ α, ρ, κ t w (1) tx (figure of merit:z) w sƒ [4,5]. (Z)ƒ y p ùkü. Z α2 = ρκ (1) wy w» w w w. wš. ƒ mm j, µm, µm~500 µm z x [6-8].» Ás w *Corresponding Author : [Tel : ; ktkim@kims.re.kr, ktkim1796@gmail.com]

2 424 ½ ká x Áw x y ù CPU x þƒ e y j š. x t, ƒ x w»q w w w.» ü ü š w y w 30 µm ƒ sw z x šwš [6-8]. z x w» w ƒ j v q œ š [9]., px nx r pyw»q v qw z px, nx ƒ ƒ gq»q ww z x x w» œ w ƒ gq x w. ù, j v q z» rl ù yw» œ w t ƒ j l j r. j t v q w w j» ù ³ w yw w. j t w w z w»q w ƒ w. š w ü» w j ƒ g ª yz k.» w» w»q w z w w wš» w j» w, j v q w z» ƒ w gqw» y jš w. 2. x x 99.99% š Bi, Te, Se š Sb Bi 0.4 w w œ wš, w px w w š Bi 2 ƒ nx w w. x w 250 o C 3 y w w y w [3]. w px w œ y z nx y z ƒ» y z qj v (Welltech Co., WL-5-400, Korea)œ w nx w p sƒw. 350 o C 150 o C, 10 3 torr œ» ƒ 50 MPa. nx y z y w» w œ ù y e š w œ w p w. y px nx» ƒƒ 1:10, 1:9 yw yww z 3 œ ü ³ w r p w. p x nx r p 5.5 cm 5.5 cm ql Au j v q. j v q z 350 w. w z gqd» w e w y w» w rl (sputtering)œ w g. px nx ƒ gq»q š wœ w ww z w. z w sƒ x (FE-SEM; Hitachi S-4900) m w y w. X-ray z mw y w. nx w p» w q wš y (Hall coefficient) d e w d w š, (Seebeck coefficient) w 4-probe y w sƒ (( ) Seepel ) w. Ÿ (Netzch Model No. LFA 447 NanoFlash) w y (Thermal diffusivity) d w z (Differential Scanning Calorimetry: DSC) w (Cp) w m sƒ w. w d bi-couple w d w w gqd ü w e w y w.

3 j v q œ w z» w e gqd w š» œ mw w w Bi- Sb-Te 3 nx px w ƒƒ w š r pyw. 1(a) Bi 0.4 ƒ px t ù kü SEM [3]. 1(b) ùkù XRD y w JCPDS e y w w Bi 0.4 y w. w px yw» 3 e w» ³ yww r p. nx 1(c) SEM µmj» x p ƒ. x» yw ³ w x w ³ yw ƒ w. nx 1(d) XRD JCPDS e y w w Bi 2 ƒ ƒ y w [10]. nx px» yww r p. r p y w» w Fig. 1. (a) SEM image and (b) XRD pattern of p-type Bi 0.4 powders, (c) SEM image, and (d) XRD patterns of synthesized n-type Bi 2 powders. Table 1. Comparison of room-temperature thermoelectric properties such as electrical resistivity (ρ), Carrier Concentration (Nb), Carrier Mobility (u) and Seebeck coefficient (α) and thermal conductivity (κ) of n-and p-type bismuth telluride sintered bodies P-type powders [Ref. 3] N-type powders [Ref. 10] Electrical resistivity Carrier concentration Carrier mobility Seebeck coefficient Thermal conductivity ρ(ωm) Nb(/cm 3 ) u(cm 2 /Vs) α(µv/k) κ(w/m ½ K) Before reduction After reduction Before reduction After reduction

4 426 ½ ká x Áw x Fig. 2. (a) n- and p-type electrodes designed on the Si substrates, schematic diagram fabricating thick-film type thermoelectric modules: (b) screen printed n- and p-type pastes (c) the coating of Au onto the paste by the sputtering process, and (d) the bonding process between n-and p-type substrate. y w» w px nx w ƒƒ y œ w. t 1 ùkü px nx w w» w(ρ) 5% [3, 10].» w û ü w w û w p» q wš. px (+) d š, nx ( ) d» p x nx yw y w. y p x 180 µv/k w n x -102 µv/k -112 µv/k 10% w y w. nx y z ƒ j yƒ y ù px y z ƒ 20%ƒ y w. y w 2(a) g (Si)»q 5.5 mm 14 mmj» px nx px nx r p j v qw z x jš w. 2(b), (c) š (d) px nx r p j v q Fig. 3. (a) schematic cross-sectional illustration of thickfilm module, (b) conceptual depiction showing a poor contact between an electrode and a thermoelectric thick film, and (c) cross-sectional SEM image displaying empty gaps between an electrode and a thick film.

5 j v q œ w z» w e gqd w 427 œ ùkü., z x px j v qœ»q x k. ƒ nx j v q œ»q x k. p x nx z œ w ƒ gqw. ƒƒ ƒ gq»q w v ww x w. w œ» z œ z w j, v q r p z Au gq œ ƒw. 3(a) gq œ v w 3(b) š. px nx r pƒ g q, z t ƒ w. 3(a). j v q œ v q w r p w w» q v q r p w ƒ w. t 3(b) v q r pƒ t ƒ ww w ƒ w. 3(c) y w w œ š. / ü» w ƒw» ª. w w» w ƒ w (Au) gqw, 3(b) ƒ ùkù Au gqd w y w» ƒ ù wš, ƒ gqd w w w š w. 4 2 Bi-couple z ùkü. 4(a) 4(b) ƒƒ px nx r Fig. 4. (a) a photo of p-type paste-coated substrate, (b) a photo of n-type paste coated substrate, (c) surface morphology of Au coating layer as an electrode, and (d) csem image of screen-printed p-type paste on Au electrodeu

6 428 ½ ká x Áw x pƒ sw»q ùkü (c) (Au) r pƒ ƒƒ v q. 4(d) v q z SEM v q px r p ̃ sky t š SEM y ü k. px nx r pƒ v q»q š» w š j» 2 cm 2 cm z. wœ š œ. w š œ» 30 µm z 20 µm ƒ û. z w ùkú x œ w w gq w w. 5(a) z gq w x ùkü., gq r p ³ yw w z ù ƒ, ƒ x d» ƒ w x w j z ƒ. 5(b) gq» j v q z FE-SEM 5(c) gq z z SEM ƒƒ ùkü. gq w t e»ƒ sky š y w. e d w, w Au gqd Fig. 5. (a) schematic illustration of metal coating layer-introduced thermoelectric thick-fim-type module, (b) the cross-sectional SEM image before Au coating and (c) the cross-sectional SEM image exhibiting a planarized surface by Au coating.

7 j v q œ w z» w e gqd w 429 ƒ gqd e t sky j z ƒ š,» x» 2 pair w j k y w.» p w ZT w ª y z ƒ k., mw wz j v q œ w l z x w gqd ƒw œ š w y w. Fig. 6. Comparison of the electrical resistivity of thickfilm-type thermoelectric module with the presence of gold coating layer. j v q z t e»ƒ j y w. 6» w w gq w» w 30% y w. ü w w w yw e d w ù, gq z y w» w w y w. gq w» ƒ» ü w 6.8 ohm 1.9 ohm j w y w. q wz m (Au), gqd ü w w (ZT)d w w wš w. 4.» œ mw w Bi 0.4 px Bi 2 nx r pyw z w, ü w z» j z gqd ƒ wš. l z w» 2011» ( y: K ) w. š x [1] R. Venkatasubramanian, E. Silvola, T. Colpitts and B. O Quinn: Science, 413 (2001) 297. [2] M. Toprak, Y. Zhang and M. Muhammed: Mat. Lett., 57 (2003) [3] K. T. Kim, K. M. Jan and G. H. Ha: J. Korean Powder Metall. Inst., 17 (2010) 136 (Korean). [4] K. T. Kim, D. W. Kim and G. H. Ha: Res. Chem. Intermed., 36 (2010) 835 [5] D. M. Rowe: CRC Handbook of Thermoelectrics, CRC Press, Inc., New York (1995). [6] K. T. Kim and G. H. Ha: J. of Nanosci. and Nanotech., 11 (2011) to be published as the title of Fabrication and Characterization of Thermoelectric Thick Film Prepared from p-type Bismuth Telluride Nanopowders. [7] H. Böttner, J. Nurnus, J. A. Gavriko, G. Kühner, M. Jägle, C. Kunzel, D. Eberhard, G. Plescher, A. Schubert and K. H. Schlereth: J. Microelectromech. Syst., 13 (2004) 414. [8] R. Venkatasubramanian, T. Colpitts, E. Watko, M. Lamvik and N. ElMasry: J. Cryst. Growth, 170 (1997) 817. [9] C. Navone, M. Soulier, M. Plissonnier and A. L. Seiler: J. Elect. Mater., 39 (2010) [10] K. T. Kim, K. M. Jang and G. H. Ha: Rev. Adv. Mater. Sci., 28 (2011) 14.