Alumina Paste Layer as a Sublimation Suppression Barrier for Yb<sub>14</sub>MnSb<sub>11</sub>
- Sunday, 01 August 2010
This material can be applied to any thermoelectric couples requiring sublimation suppression.
Sublimation is a major cause of degradation of thermoelectric power generation systems. Most thermoelectric materials tend to have peak values at the temperature where sublimation occurs. A sublimation barrier is needed that is stable at operating temperatures, inert against thermoelectric materials, and able to withstand thermal cycling stress.
Sublimation Rate Measurement with Yb14MnSb11 coupons with an alumina paste layer. The sublimation rate at the beginning of life is rather high, but the rate decreases steadily and after 1,500 hours, the rate reaches below the goal and the filling of the pore during sublimation is believed to be the reason for the rate decrease with time." class="caption" align="left">A porous alumina paste layer is suitable as a sublimation barrier for Yb14MnSb11. It can accommodate stress generated by the thermal expansion discrepancy between the suppression layer and thermoelectric materials. Sublimation suppression is achieved by filling pores naturally with YbO2, a natural byproduct of sublimation. YbO2 generated during the sublimation of Yb14MnSb11 fills the porous structure of the alumina paste, causing sublimation to decrease with time as the pores become filled. During testing, it was found that application of this paste caused an initial ten-fold decrease in sublimation, but this factor increased with time. At 1,500 hours of burnout time at 1,273 K, the decrease in sublimation was measured as much as 1,000 times lower.
A commercial alumina paste was applied to Yb14MnSb11. Both elements were polished to remove oxidation, then the paste was applied to the Yb14MnSb11. The Yb14MnSb11 exhibited 2 × 10–6 to 3 × 10–6 g/cm2 sublimation rate at 1,000 °C after initial burnout. With this rate, the sublimation barrier becomes suitable for 14-year operation, with less than 10 percent cross-section reduction at the hot side junction.
Using scanning electron microscope imaging, the alumina layer was found to be converted into a denser composite of alumina and ytterbia. This clogged, dense layer makes an effective sublimation barrier.
This work was done by Jong-Ah Paik and Thierry Caillat of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Materials category.
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Refer to NPO-46845, volume and number of this NASA Tech Briefs issue, and the page number.
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