Hans-Peter Martin

TiO x -Based Thermoelectric Modules: Manufacturing, Properties, and Operational Behavior

Substoichiometric titanium oxides are attractive thermoelectric materials for high-temperature modules. Their advantages are availability, economy, and safety. This paper gives results of thermoelectric data on scale-up titanium suboxides, manufacturing technologies of TiOx modules, glass coating as an option for oxidation protection, and test results of TiOx modules. The thermoelectric efficiency of TiOx is low compared with established thermoelectric materials. However, TiOx is very attractive for economic reasons, and there are still expectations for efficiency rise by modification of the material’s microstructure. TiOx can be produced in large quantities of several tens of kilograms. A manufacturing process for TiOx-based unileg n-type modules has been established, including all technological steps. The design of the TiOx-based modules was optimized for the thermoelectric conversion process and thermal robustness. A test device was used for experimental analysis with maximum temperature of 600°C at the hot side and 100°C at the cold side. This initial test revealed similar power output and internal resistance of all fabricated modules. Furthermore, thermal cycles with increasing and decreasing temperatures at the hot side were realized to characterize the reliability and stability of the modules. Additionally, modules were tested in a hot gas test rig to simulate the thermal stresses during power generation in the exhaust line of a passenger car.

Pressureless sintering of ZrC with variable stoichiometry

This paper presents the experiments on the synthesis of zirconium carbide (ZrC) using carbothermal reduction of zirconia (ZrO2). The ratio of ZrO2:C is used to adapt ZrCxOy with x < 1 or ZrC + C. The modification of ZrCxOy and the total carbon amount allows the use of pressureless sintering method in combination with sintering temperatures ≤ 2000 °C. Fully densified ZrC products are obtained. The relevant details of ZrC formation are investigated by X-ray diffraction (XRD). The sintered products are characterized by XRD, field emission scanning electron microscopy (FESEM), as well as mechanical and electrical methods. XRD and FESEM investigations show that ZrCxOy is formed during the manufacturing process. The grain size and additional zirconia or carbon are related to the ZrO2:C ratio of the starting powder mixture. Bending strength up to 300 MPa, Young’s modulus up to 400 GPa, fracture toughness up to 4.1 MPa·m1/2, and electrical resistance at room temperature around 10−4 Ω·cm are reached by the pressureless sintered ZrC.