Thermomechanical and Thermal Contact Characteristics of Bismuth Telluride Films Electrodeposited on Carbon Nanotube Arrays

Adv. Mater. 2009, 21, 1–4 2009 WILEY-VCH Verlag Gmb Aminiaturized thermoelectric (TE) coolermodule is composed of a large number of TE legs connected electrically in series and thermally in parallel. TE devices operating near room temperature typically create a temperature difference of 30–50 8C, and the TE film thickness for such devices ranges from 10 to 100mm. From manufacturing and reliability perspectives, the design of TE cooler modules is often constrained by the shear stresses that result from differential thermal expansion, both during steady-state operation and during on/off cycling. In bulk systems, various strategies, such as spring-loaded systems, tension bolts, and welding, have been proposed to enhance compliance during device operation. However, none of these strategies can be directly applied to thin-film based miniaturized TE devices. Additionally, for TE devices operating at higher temperatures, a pressing need exists for stable, compliant, and low thermal resistance interface materials between the TE element and the metallic interconnects. With this motivation, we report here a scalable electrodeposition process to integrate thick-film TE materials with carbon nanotubes (CNT) arrays. Recently, CNT arrays have been reported to exhibit excellent fatigue strength under cyclic compressive loading and as interface materials, they have been shown to achieve low thermal and electrical interface resistances at moderate contact pressures. Further, CNTs are amenable to heterogeneous integration with other materials. These attributes suggest that compliant CNT arrays can be integrated with minimal parasitic additions to the total electrical and thermal resistances of a TE device. The common substrate for this work was 5 6mm Ni(200 nm)/Ti (800m)/SiO2(1mm)/Si(300mm). For CNT array synthesis, some of these substrates were coated with a metal tri-layer structure of Fe (2.5 nm)/Al(10 nm)/Ti(30 nm) by electron-beam evaporation. Multi-walled CNT arrays were synthesized on these substrates by microwave-plasma chemical vapor deposition (MPCVD). The feed gases for the reaction were methane at 10 standard cubic centimeter per minute (sccm), and hydrogen at 50 sccm, The reaction temperature and pressure were 1173K and 10Torr (1 Torr1⁄4 133.32 Pa), respectively. Further details of the CNTsynthesis by the MPCVD technique have been reported. The remaining substrates, without CNTs, served as control samples. CNTarrays were imaged using a Hitachi S-4800 field-emission scanning electron microscope (FESEM) (Fig. 1). The average length and diameter of the CNTs were 25mm and 40nm, respectively. Bismuth telluride (Bi2Te3) was chosen as a representative TE material for TE/CNT integration by electrodeposition, as it is the parent compound of the alloys used in commercial Peltier devices optimized for cooling at temperatures near 300K. Using a Bio-Analytical Systems (BAS) Epsilon Electrochemical System, Bi2Te3 was electrodeposited potentiostatically on the CNT array-coated substrates, and these films were compared with Bi2Te3 films electrodeposited directly on bare metallized substrates. A three-electrode setup was used with a CNT array-coated working electrode, platinum mesh counter electrode, and an Ag/AgCl (3 M NaCl, 0.175V versus NHE) reference electrode. The deposition bath was composed of Bi3þ (0.75 10 2 M) and HTeO2 (1 10 2 M) (both Alfa Aesar, 99.999% pure) and HNO3 (1 M) at 298K. [2,9] Continuous one-minute pulses at 50mV were applied between the working and the reference electrodes for about 10 h for electrodeposition