Abstract
The energy crisis and environmental problem are drawing more attention to renewable energy research in recent decades. Thermoelectric materials can convert heat directly into electricity with the advantages of long lifespan, maintenance-free operation, and excellent reliability. Si-Ge has been used as the high-temperature thermoelectric material in radioisotope thermoelectric generators (RTGs) by NASA in deep space missions and Moon landings. However, the relatively high cost and low conversion efficiency limit the application of the thermoelectric materials for energy conversion. In this work, laser processing is expected to provide an effective solution to the efficiency and cost problems of thermoelectric materials.
In order to increase the conversion efficiency, thermoelectric materials should have high electrical conductivity, high Seebeck coefficient, and low thermal conductivity, as per the definition of the figure of merit. Laser sintering could minimize grain growth, and thus reduce thermal conductivity via phonon scattering at the grain boundaries. The figure of merit is expected to be enhanced by the reduced thermal conductivity without an appreciable loss in electrical conductivity. In addition, the conversion efficiency is also related to the temperature difference between the hot side and the cold side of the thermoelectric device. The thin-film thermoelectric device has the advantage of high-temperature differences, which also improves conversion efficiency. In terms of the fabrication cost, laser processing has the advantages of low cost, high efficiency, high throughput, and easy operation, which is suitable for large scale manufacturing. Therefore, the laser processing is expected to fabricate thermoelectric devices with high performance and low cost, which is more suitable for commercialization and mass production.
In this work, plasma-synthesis and ball-milling were investigated as the synthesis methods of as-deposited thin films, followed by laser sintering using a quasi-continues-wave infrared laser. The plasma-synthesized-laser-sintered (PSLS) method resulted in a grain size of 68 nm, which reduced the thermal conductivity to ~1.35 W/m∙K from room temperature to 573 K. The Seebeck coefficient increased from 144.9 μV/K at room temperature to 390.1 μV/K at 873 K. The electrical conductivity increased from 16.1 S/cm at room temperature to 62.1 S/cm at 873 K. The figure of merit of the PSLS Si80Ge20 was calculated to be 0.60 at 873 K, which is comparable to a value of ~1 for bulk nanostructured materials. The ball-milled-laser-sintered (BMLS) method produced well-alloyed Si80Ge20 with an average grain size of 50 nm. The thermal conductivity was found to be ~1.5 W/m∙K from room temperature to 573 K. The Seebeck coefficient was 120.2 µV/K at room temperature to 301.5 µV/K at 873 K. The electrical conductivity was measured as 80.9 S/cm at room temperature and 118.5 S/cm at 873 K. Therefore, the figure of merit of the BMLS Si80Ge20 was found to be 0.63 at 873 K.
In addition, a device consists of 3 pairs of n-type phosphorus-doped Si80Ge20 legs and p-type boron-doped Si80Ge20 legs were fabricated with the BMLS method. The maximum temperature difference of 200 K was achieved when the hot-side temperature was 873 K, and the cold side was kept at room temperature in the air (no water cooling). The corresponding maximum thermovoltage and output-power were 311.6 mV and 15.85 μW. The effective power density was calculated as 8.8 mW/cm2.
In summary, a new method for thermoelectric thin film fabrication using laser sintering of Si and Ge nanoparticles has been demonstrated with a thermal-to-electric conversion efficiency approaching the bulk value. Also, a fundamental understanding of laser-sintered Si80Ge20 thermoelectric materials has been provided, and a novel and viable concept of laser processing has been demonstrated for high-efficiency and low-cost thermoelectric device fabrication.
