Suspension high velocity oxy-fuel spraying of TiO2: A quantitative approach to phase composition

A range of coatings from a water based suspension of anatase has been prepared by suspension high velocity oxy-fuel spraying with the aim to study effects of heat power of the flame on phase composition, microstructure and surface topography. Three most commonly used approaches of quantitative phase analysis have been scrutinized with respect to their applicability and as some of the coatings showed presence of preferred orientation and it was argued that quantitative Rietveld refinement is the most accurate method for phase composition determination. Coatings had a layered duplex anatase/rutile microstructure with fraction of rutile increasing exponentially with heat power. Spraying at the lower heat power led to a lower surface roughness and higher power resulted in surfaces with pronounced humps, which were distributed homogeneously on the surface. The emergence of humps is related to an increase in macroscopic surface area of up to 30% with respect to the flat coating.     

Measurement and optimization of atmospheric plasma sprayed CoMoCrSi coatings parameters on Ti-6Al-4V substrates affecting microstructural and properties using hybrid abductor induction mechanism

A hybrid artificial intelligence method (AIM) is used to optimize the atmospheric plasma spraying (APS) processing parameters for CoMoCrSi coatings deposited on Ti-6Al-4V substrates. In the proposed approach, the Taguchi design method is used to obtain an initial solution for the optimal set of APS parameters. The true optimal values of the spray distance, chamber pressure, current, argon gas flow rate and hydrogen gas flow rate are then obtained using an artificial neural network (ANN) and genetic algorithm (GA). The coatings deposited using the processing parameters determined using the Taguchi design method alone have a porosity of 8.5%. However, following the ANN/GA optimization procedure, the porosity reduces to just 5.6%. The XRD analysis results show that the as-sprayed coatings contain Cr₃Si phase as a result of the high-temperature plasma flame. In addition, it is shown that the structure of the Ti phase in the coating transforms from a α-Ti crystal structure (i.e., HCP) to a β-Ti crystal structure (i.e., BCC) during thermal treatment at temperatures to 1200 °C. Finally, it is shown that the hardness of the annealed coatings increases with both an increasing annealing time and an increasing annealing temperature. The higher hardness under higher temperature conditions is attributed to the precipitation of Cr₃Si phase. Overall, the coatings obtained in the present study using the optimal spray-coating parameters have a low porosity, a high hardness and a good thermal stability at high temperatures.     

On the fabrication of functional graded 3Y-PSZ/316L materials by SPS: Process optimization and characterization of the obtained products

Dense and crack free six-layered functional graded materials were successfully produced by Spark Plasma Sintering by combining 3 mol% Y₂O₃-partially stabilized ZrO₂ (3Y-PSZ) and 316L stainless steel. All the sintered products consisted of a steel free layer on one side and a cermet composite containing 50 vol% of both constituents on the opposite side. Conversely, the stainless steel concentration in the interlayers was progressively changed following diverse spatial profiles.It was found that the temperature interval from 1080 to 1180 °C required for the full consolidation from the 50 vol% composite layer to the 3Y-PSZ one, respectively, can be reached when adopting a specific die configuration where the cross section was varied from 30 to 28 mm, respectively. Correspondingly, the densification level of each layer, as well as the related hardness and fracture toughness properties, were highly enhanced with respect to the standard cylindrical die. In addition, a significant improvement of the material toughness was obtained when the material concentration exponent was decreased from 2 to 1, whereas this effect tends to vanish when such parameter was further reduced to 0.5.     

Excilamps and their applications

The design and practical applications of excilamps having a dielectric barrier and capacitive structure and driven by nanosecond, high voltage pulses are described. A simple design lamp having forced cooling and an excitation generator was demonstrated to have a lifetime of more than one thousand hours. The results of an experimental study of capacitive and high voltage nanosecond discharge-driven excilamps are presented. The influence of the excitation regime and operating gas mixture pressure on the dielectric barrier discharge (DBD) driven excilamps' efficiency is discussed. The discharge formation and output radiation dynamics of DBD driven excilamps was experimentally studied for the first time.The practical applications of several lamps designed in the High Current Electronics Institute, Tomsk, Russia to photochemistry, photobiology and photomedicine are discussed.     

Core–shell polypyrrole nanoparticles obtained by atmospheric pressure plasma polymerization

Polypyrrole hollow nanoparticles were prepared by atmospheric pressure plasma polymerization. The structure of the nanoparticles was studied using Fourier transform infrared and X‐ray photoelectron spectroscopies, thermogravimetric analysis, scanning and transmission electron microscopies and atomic force microscopy. In contrast to low‐pressure plasma polymerization of pyrrole, which can produce films and solid nanoparticles, we obtained two types of hollow nanoparticles: a fraction with single spherical core and another with a core composed of small bubbles. Thermal characterization allowed us to determine that the nanoparticles are composed of highly crosslinked polymer. A mechanism that explains the formation of both types of hollow nanoparticles as well as solid nanoparticles is proposed. Chemical characterization shows that, in addition to the expected chemical structures due to pyrrole polymerization, the high energy of the plasma at atmospheric pressure produces intense dehydrogenation and oxidation processes. The fluorescence spectrum of the nanoparticles, however, shows a peak at 482 nm indicating that some degree of π‐conjugation is present in the material. © 2014 Society of Chemical Industry     

Influence of Al content and post-annealing on synthesis and mechanical properties of plasma sprayed Cr–Al–C composite coatings

Due to ultra-high temperature and short reaction time, it was very challenging to produce high purity MAX phase by plasma spraying. In this study, Cr–Al-graphite agglomerated powders with different Al additions (x = 0.2–1.5) was used to prepare Cr–Al–C composite coatings by atmospheric plasma spraying followed with annealing. Results showed that the as-sprayed coatings displayed typical lamellar structure, mainly composed of Cr–C binary carbides (Cr₇C₃ and Cr₂₃C₆) and residual Al. After annealing at 700 °C, the newly formed Cr₂AlC phase increased significantly in the coatings. The higher addition of Al, the more Cr₂AlC phase formed after annealing. The enhanced atomic diffusion, sufficient Al source and existence of (Cr, Al)C_x contributed to the formation of Cr₂AlC under annealing. Annealing treatment improved the hardness of the coating, but with the increase of Cr₂AlC phase content, the hardness decreased slightly. The Al content and post-annealing had a synergistic effect on the formation of Cr₂AlC phase in the sprayed coatings. This provided an effective route to control the Cr₂AlC content in sprayed Cr–Al–C composite coatings.     

A high-Nb TiAl alloy with highly refined microstructure and excellent mechanical properties fabricated by electromagnetic continuous casting

In the present research, microstructure refinement of a high-Nb TiAl alloy(Ti-48Al-8Nb-0.15B) was realized by means of the electromagnetic continuous casting(EMCC) technique. The microstructure of an ingot obtained by EMCC was analyzed using scanning electron microscopy(SEM). As compared with the raw as-cast al oy, the obtained EMCC alloy presented a much finer microstructure with lamellar colonies with a mean size of about 50-70 μm because the electromagnetic stirring broke initial dendrites and enhanced the heterogeneous nucleation. As the grains were refined, the properties of the TiAl alloy were improved significantly. This implies that the EMCC technique could offer the possibility of application for high-Nb TiAl alloys with a refined microstructure and excel ent properties to be used as a structural material.     

EPR and Raman study of silicon layers obtained by gas detonation spraying

We demonstrate availability of gas detonation deposition spraying (GDS) to obtain silicon layers that can be used for production of solar cells. Silicon powder remaining as secondary raw material of silicon and/or silicon production is used during GDS. To study defects and structural perfection of initial powders and obtained layers, electron paramagnetic resonance (EPR) and Raman spectroscopy are used. It is shown that one part of EPR spectra displays resonances originating from different nearest-neighbor configurations of silicon dangling bonds, whereas an increase of the total number of paramagnetic defects in GDS silicon layer is related to the rise of conduction electrons or electrons filled band tail states. Thermal annealing of layers in hydrogen ambience further reduces the number of silicon dangling bonds owing to their passivation. Based on the results of X-ray diffraction, EPR and Raman spectroscopy it is assumed that the GDS Si layers are composed of randomly oriented and partially oxidized monocrystalline silicon grains. It is found that optical and photoelectric properties of the layers obtained indicate a possibility to apply them for solar cells production.