AB2X4 compounds and other new thermoelectric materials studied at JPL

Thermoelectric QW device

S. Ghamaty, N. Elsner

Hi-Z Technology, Inc., San Diego, CA 92126-4210

The electronic and thermal properties of bulk materials are altered when they are incorporated into quantum wells. Two-dimensional quantum wells have been synthesized by alternating layers of B₄C and B₉C in one system and alternating layers of Si and Si_0.8Ge_0.2 in another system. Such nanostructures are being investigated as candidate thermoelectric materials for high figures of merit (Z). The predicted enhancement is attributed to the confined motion of charge carriers and phonons in the two dimensions and separating them from the ion scattering centers.

Sputtering techniques have been used to prepare these multilayer films with thickness 10mm. Films have been deposited on single-crystal silicon substrates. The a and l properties of these films have been determined over a broad range of temperatures from 4.2K to l200K and were previously reported. The a²/ l values for these P type B-C and N type SiGe films were more than a factor of 10 to 30 times higher than bulk P type B-C and N type SiGe.

Hi-Z has recently measured power and efficiency demonstrating a QW couple conversion efficiency of 14%. These measurements were made recently on a small couple that combined a multilayer QW of P type B₄C/B₉C with a QW of N type Si/SiGe. This couple operated between 70°C and 250°C and was fabricated on a Si substrate with ~11mm QW film thickness. The 14% efficiency was calculated by dividing the power out of the couple by the power in. The 14% efficiency was obtained with no correction for any extraneous heat losses, such as through the Si substrate and the heater wires. The experimental set up also confirmed a known efficiency of ~5.5% for Bi₂Te₃ bulk alloys, assuring the data accuracy. The experimental data point and the predicted values agree quite well. A confirmation that these QW materials exhibit a much higher figure of merit than bulk alloys is that the maximum efficiency was achieved at a ratio of load resistance to QW couple resistance of 3 yielding a ZT of 4.1 at T~250°C. The Bi₂Te₃ bulk alloys meet their maximum efficiency at a resistance ratio of ~1.2 when their ZT value is close to 1.

In another separate experiment, the B₄C/B₉C film was used as a cooler creating a maximum temperature difference of ~45°C. This temperature difference gives ZT~3 for T~25°C. For this experiment, the P-type B₄C/B₉C was joined to small Cu wire. The QW film was the same material and thickness as used in the couple mention above for the power generation.