Improving thermoelectric technology performance and durability with aerogel

The recent interest in thermoelectric technology has spurred the development of advanced materials and device technology. In particular, current work at the Jet Propulsion Laboratory has identified aerogel as an effective sublimation barrier for a wide range of thermoelectric technologies based on SiGe, novel Skutterudites, TAGS, and PbTe. Aerogel is typically known as an extremely porous (>99% porous) silicon dioxide, which has very low thermal and electrical conductivity. Aerogel has interconnected pores, which are generally in the range of angstroms to tens of nanometers. As such, the path required for metal vapor to permeate aerogel is extremely tortuous, thus significantly decreasing sublimation rates. Another added benefit to using aerogel as a coating is that it can also serve as thermal insulation. Since aerogel is made through liquid synthesis it can be cast in or around thermoelectric devices, thus providing intimate contact between the device and the insulation to assist in channeling heat through the thermoelectric legs and eliminated lateral heat loss. Altogether, aerogel coatings could significantly improve a variety of thermoelectric technologies by enhancing durability and performance by acting as an effective sublimation barrier as well as effective thermal insulation, respectively. In this work, a comprehensive assessment of aerogel sublimation barriers for Skutterudite-based power generators is reported. INTRODUCTION The recent interest in thermoelectric technology has spurred the development of advanced materials and device technology. In particular, current work at the Jet Propulsion Laboratory (JPL) involves the integration of aerogel into thermoelectric technology, to improve both performance and durability. The basic concept involves casting aerogel into or around a thermoelectric device such that intimate contact is made between the thermoelectric elements and the aerogel (Figure 1). Since aerogel is synthesized using sol-gel chemistry it can be cast in place, thus filling all free void space in a device. Once in place, the inherently low thermal conductivity of aerogel (a few mW/m-K) channels heat through the thermoelectric elements, thus minimizing parasitic heat loss. A more intriguing effect was recently discovered, however, and involves the use of aerogel as a sublimation suppression barrier (Figure 2). The inherently micro-meso porous structure of aerogel combined with the cm-scale mean free path of heavy metal vapors significantly slows the sublimation rates of practically all thermoelectric materials relevant to thermoelectric power generation (Skutterudite, PbTe, TAGS and SiGe). Altogether, the integration of aerogel into thermoelectric technology could have a significant impact on how thermoelectric generators, and to a lesser extent coolers, are designed and fabricated. Intense efforts supported by NASA’s Science and Mission Directorate, Project Prometheus and a DOE sponsored project to improve fuel efficiency of vehicles using thermoelectrics are considering aerogel for use in the next generation thermoelectric generator technology. Efficient thermal-to-electric conversion efficiency using thermoelectric materials is primarily determined by the ZT of the thermoelectric materials and the maximum hot-side (Thot) operating temperature. The latter parameter, Thot, is often limited by the sublimation of volatile metal species, which is considered one of the primary modes of degradation in thermoelectric generators. Current efforts at JPL are focusing on Skutterudite-based thermoelectric technology, thus Antimony (Sb) is the volatile, subliming species. Essentially, sublimation reduces the effective cross-section of the thermoelectric elements, thus resulting in increased electrical resistance and thermal impedance (Figure 3). As Sb leaves n-type Skutterudite, the reaction is as follow: CoSb3 CoSb2 + CoSb + Sb (vapor). Simultaneous decomposition into lower cobalt antimonides occurs such that CoSb forms on the outer most regions followed by CoSb2 as shown in an n-type Skutterudite coupon heated in vacuum (10 torr for) 72 hours at 700C (Figure 3). The p-type Skutterudite (CeFe3Ru1Sb12) decomposes into Ce, Fe and Ru diantimonides as Sb sublimes and a similar depletion band forms along the perimeter of coupons or legs. Thermoelectric technology is known for reliability, which typically results from the simplicity of operation, i.e. no moving parts, static thermal environment, etc. Thus, sublimation must be suppressed to ensure reliable operation over a decade or decades of operation. In the past, Radioisotope Thermoelectric Generators (RTG) employed SiO2 and Si3N4 coatings and or a cover gas of Argon to suppress sublimation for SiGe and TAGS technology, respectively. During the course of the development these schemes were characterized using simple mass loss experiments of coupons and power output of prototypical devices. In the end, a value for acceptable sublimation rate is determined and corresponds to a projected power reduction based on the abovementioned increase in electrical and thermal resistance associated with sublimation. Using the procedures developed under previous programs, several goals have been established for effectively suppressing sublimation in Skutterudite-based technology: 1) Sublimation of Sb must not result in more than a 5% reduction in cross-section over 10 years of operation, which will require a Sb sublimation (flux) rate of ~1 x10 g/cm hours or ~ 10 cm/hour, 2) The method for suppressing sublimation should also prevent sublimed species from condensing on the cold-side circuitry, which will likely result in short circuiting and 3) The method for suppressing sublimation should not have a negative impact on the system performance, e.g. if a coating is considered it must have low thermal and electrical conductivity to prevent thermal and electrical shorting. Recent investigations have determined that Silica-based aerogel could potentially meet all three criteria. Aerogel is a made through Sol-Gel chemistry in which liquid precursors are combined to form a gel, which is then supercritically dried in an autoclave (Figure 4). The resulting structure is comprised of an interconnected network of pores ranging from the micro to meso scale and porosity ranging from 90 to 99% (density from 30 to 300 mg/cc). The nanometer scale porosity combined with the homogeneous dispersion of pores creates a highly tortuous path for vapor to permeate. It is for this reason that aerogel is currently being considered as a sublimation barrier (Figure 2). The mean free path of most heavy metal vapors, like Sb, are in the range of centimeters, thus it is expected that the permeation rate of Sb vapor through aerogel should be low. Additionally, since aerogel has low thermal and electrical conductivity it should not impose a negative impact on the device efficiency and it is likely that the device performance can be improved. In SiGe technology, significant parasitic heat loss resulted from gaps between the SiGe unicouples and the multi-foil insulation used to prevent the hot-side components from heating the cold-side components. Since aerogel is cast in place, intimate contact can be made between the thermoelectric elements and the thermal insulation, thus reducing parasitic heat loss. Although the benefits of using aerogel as thermal insulation are clear, the emphasis of this report is on the use of aerogel for sublimation suppression. In this report, detailed analysis of aerogel sublimation suppression coatings for Skutterudite-based technology is presented. Specifically, the sublimation rates of aerogel-coated coupons in various environments, temperatures and time periods along with detailed analysis of aerogel stability at elevated temperature under various atmospheres will be covered.