As solid-state deposition technique avoiding oxidation, cold gas spraying is capable of retaining feedstock material properties in the coatings, but typically fails to build up coatings of brittle materials. Ceramic MAX phases show partial deformability in particular lattice directions and may thus successfully deposit in cold spraying. However, deformation mechanisms under high strain rate, as necessary for cohesion and adhesion, are not fully clear yet. A MAX-phase deposit only builds up, if the specific mechanical properties of the MAX phase allow for, and if suitable spray parameter sets get realized. To investigate the influence of material properties and deposition conditions on coating microstructure and quality, three MAX phases, Ti3SiC2, Ti2AlC and Cr2AlC, were selected. Up to ten passes under different spray parameters yielded Ti2AlC and Cr2AlC coatings with thicknesses of about 200-500 µm. In contrast, Ti3SiC2 only forms a monolayer, exhibiting brittle laminar failure of the impacting particles. In all cases, the crystallographic structure of the MAX-phase powders was retained in the coatings. Thicker coatings show rather low porosities (< 2%), but some laminar cracks. The deposition behavior is correlated with individual mechanical properties of the different MAX-phase compositions and is discussed regarding the particular, highly anisotropic deformation mechanisms.
For the present study on Aerosol Deposition of MAX-phase materials, Ti3SiC2 was chosen as model system due to the availability of property data and commercial powder. The as-received powder was milled to different nominal sizes. For revealing details on coating formation and possible bonding mechanisms, Aerosol Deposition experiments were performed for different particle size batches and process gas pressures. Microstructural analyses reveal that coating formation preferably occurs for particle sizes smaller two microns. Using such small particle sizes, crack-free, dense layers can be obtained. The individual deposition efficiencies for the different particle sizes, particularly the critical size below which deposition gets prominent, vary with process gas flows and associated pressures. Detailed microstructural analyses of coatings by high-resolution scanning electron microscopy reveal plastic deformation and fracture, both attributing to shape adaption to previous spray layers and probably bonding. In correlation to coating thickness or deposition efficiencies, respective results give indications for possible bonding mechanisms and a tentative window of Aerosol Deposition for Ti3SiC2 MAX-phases as spray material.
Hydrogen generation from renewable energy sources will play a key role in the concerted endeavor to constrain climate change. One environmentally friendly route, powered by sunlight, is the photoelectrochemical water splitting cell (PEC). This technology employs electrodes coated with thin films of semiconductor materials to capture light and generate charge carriers that directly drive the water splitting reaction. Bismuth vanadate is a promising metal oxide semiconductor, as it absorbs visible light, and is abundant, non-toxic and cost-effective. The present study investigates the formation of bismuth vanadate thin films by the aerosol deposition (AD) method. Operating with layer formation at room temperature, AD offers advantages over other routes for the fabrication of photoactive thin film coatings, as no binders or sintering processes need to be applied. Furthermore, compared to traditional cold spraying, micrometer-sized particles can be used, resulting in coatings with thicknesses below 1 µm. Additionally, the lower kinetic energy of the feedstock powder particles enables the use of delicate substrates, such as FTO-coated glass, expanding the range of possible PEC device configurations. The process parameters explored in this study had considerable influence on the resulting coating microstructure, which in turn showed a significant impact on the photoelectrochemical performance.
Bismuth vanadate (BiVO4) offers high photon efficiencies in solar photo-anodes, due to its suitable semiconductor band gap energies and associated visible light absorption. In well-tuned conditions, such anodes enable green hydrogen generation in photoelectrochemical water splitting cells. Bismuth vanadate films have to ensure high efficiencies in electron/hole pair generation and sufficiently high rates of charge transfer to the conducting substrate and the electrolyte, respectively. Thus, the tuning of coating properties has to aim for high phase purity, good layer integrity as well as optimum diffusion path lengths. In order to explore the potential of aerosol deposition to produce BiVO4 films with high photoelectrochemical activity and to elucidate influences on microstructure and application properties, powder sizes and spraying parameters had to be tailored. By ball milling over durations of up to 20 min, particles sizes in the range from 8.3 down to 0.6 µm were obtained. With respect to spray conditions, the process gas pressure was varied from 1.0 to 2.1 bar corresponding to gas flow rates of 10-40 l/min. The wide range of powder sizes and parameters in aerosol deposition allowed for developing a window of deposition in order to derive the most promising combinations for layer build-up. Optimum parameter sets in application on stainless steel substrates were transferred to FTO-coated glass substrates for backlit cell layouts. The thickness and conductivity of the layers were adjusted to a layer thickness range of 200-500 nm in order to achieve maximum photocurrents. The production of homogeneous, large-scale prototypes demonstrates that aerosol deposition is suitable for processing layers for solar energy harvesting with high photo current densities of up to 3.55 mA/cm2.
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