Thomas Stöcker

An Overview of the Aerosol Deposition Method: Process Fundamentals and New Trends in Materials Applications

Ceramic materials typically have to be sintered at high temperatures, often above 1000 °C. This precludes the coating of lower-melting substrate materials, such as metals, glasses and polymers, with dense and robust thin or thick ceramic films. In addition, decomposition or uncontrolled volatilization of the ceramic components can occur at elevated temperatures. As an alternative, the Aerosol Deposition (AD) method is a spray coating process to produce dense and nanocrystalline ceramic films at room temperature directly from an initial bulk powder on almost any substrate material with no need for sintering. This great potential attracts the attention of a growing number of research groups as reflected by a rapidly growing number of publications. The objective of this review is to give a holistic overview of the AD science and technology. It describes typical process equipment and parameters and starting powder and resulting film characteristics. Special attention is given to Al2O3, TiO 2, BaTiO3 and Pb(Zr,Ti)O3, as they represent a few of the most frequently used ceramics in AD. Aerosol Deposition of many other materials are also described to demonstrate the versatility of this new technology, its ability to realize novel combinations of materials and microstructures, and its suitability for future applications. Also discussed is the current state of understanding of aerosol deposition behavior and the experimental and modeling approaches used to explain the primary aerosol deposition mechanism(s).

Influence of Oxygen Partial Pressure during Processing on the Thermoelectric Properties of Aerosol-Deposited CuFeO2

In the field of thermoelectric energy conversion, oxide materials show promising potential due to their good stability in oxidizing environments. Hence, the influence of oxygen partial pressure during synthesis on the thermoelectric properties of Cu-Delafossites at high temperatures was investigated in this study. For these purposes, CuFeO2 powders were synthetized using a conventional mixed-oxide technique. X-ray diffraction (XRD) studies were conducted to determine the crystal structures of the delafossites associated with the oxygen content during the synthesis. Out of these powders, films with a thickness of about 25 µm were prepared by the relatively new aerosol-deposition (AD) coating technique. It is based on a room temperature impact consolidation process (RTIC) to deposit dense solid films of ceramic materials on various substrates without using a high-temperature step during the coating process. On these dense CuFeO2 films deposited on alumina substrates with electrode structures, the Seebeck coefficient and the electrical conductivity were measured as a function of temperature and oxygen partial pressure. We compared the thermoelectric properties of both standard processed and aerosol deposited CuFeO2 up to 900 °C and investigated the influence of oxygen partial pressure on the electrical conductivity, on the Seebeck coefficient and on the high temperature stability of CuFeO2. These studies may not only help to improve the thermoelectric material in the high-temperature case, but may also serve as an initial basis to establish a defect chemical model.

Superconducting Properties of Thick Films on Hastelloy Prepared by the Aerosol Deposition Method With Ex Situ MgB2 Powder

We present a new high-deposition-rate coating technique that allows us to produce at room temperature long thick films of MgB₂ on flexible steel substrates. Such a technique might give rise to new tapes with higher filling factors compared to the standard processed tapes. With the so-called aerosol deposition technique, MgB₂ films were prepared on Hastelloy substrates with commercially available <italic>ex situ</italic>-prepared MgB ₂ powder (<italic>T</italic>_C,onset of 38 K, <italic>J</italic>_C of 3.8·10³ A/cm² at 4 K and 1 T). Microscopic analyses yield nanocrystalline dense films with high film stresses. The as-deposited films have so far a superconducting transition temperature <italic>T</italic>_C0 of 18.1 K and a critical current density <italic>J</italic>_C up to 5·10³ A/cm² at 4 K and self-field obtained.

Influence of the calcination procedure on the thermoelectric properties of calcium cobaltite Ca3Co4O9

Calcium cobaltite is one of the most promising oxide p-type thermoelectric materials. The solid-state reaction (or calcination, respectively), which is well known for large-scale powder synthesis of functional materials, can also be used for the synthesis of thermoelectric oxides. There are various calcination routines in literature for Ca3Co4O9 powder synthesis, but no systematic study has been done on the influence of calcination procedure on thermoelectric properties. Therefore, the influence of calcination conditions on the Seebeck coefficient and the electrical conductivity was studied by modifying calcination temperature, dwell time, particle size of raw materials and number of calcination cycles. This study shows that elevated temperatures, longer dwell times, or repeated calcinations during powder synthesis do not improve but deteriorate the thermoelectric properties of calcium cobaltite. Diffusion during calcination leads to idiomorphic grain growth, which lowers the driving force for sintering of the calcined powder. A lower driving force for sintering reduces the densification. The electrical conductivity increases linearly with densification. The calcination procedure barely influences the Seebeck coefficient. The calcination procedure has no influence on the phase formation of the sintered specimens.

Effect of Oxygen Partial Pressure on the Phase Stability of Copper–Iron Delafossites at Elevated Temperatures

Oxide-based materials are promising candidates for use in high temperature thermoelectric generators. While their thermoelectric performance is inferior to commonly used thermoelectrics, oxides are environmentally friendly and cost-effective. In this study, Cu-based delafossites (CuFeO2), a material class with promising thermoelectric properties at high temperatures, were investigated. This work focuses on the phase stability of CuFeO2 with respect to the temperature and the oxygen partial pressure. For this reason, classical material characterization methods, such as scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction, were combined in order to elucidate the phase composition of delafossites at 900 °C at various oxygen partial pressures. The experimentally obtained results are supported by the theoretical calculation of the Ellingham diagram of the copper–oxygen system. In addition, hot-stage X-ray diffraction and long-term annealing tests of CuFeO2 were performed in order to obtain a holistic review of the phase stability of delafossites at high temperatures and varying oxygen partial pressure. The results support the thermoelectric measurements in previous publications and provide a process window for the use of CuFeO2 in thermoelectric generators.