From the pioneering works of McPherson in 1973 who identified nanometre-sized features in thermal spray conventional alumina coatings (using sprayed particles in the tens of micrometres size range) to the most recent and most advanced work aimed at manufacturing nanostructured coatings from nanometre-sized feedstock particles, the thermal spray community has been involved with nanometre-sized features and feedstock for more than 30 years.Both the development of feedstock (especially through cryo-milling, and processes able to manufacture coatings structured at the sub-micrometre or nanometre sizes, such as micrometre-sized agglomerates made of nanometre-sized particles for feedstock) and the emergence of thermal spray processes such as suspension and liquid precursor thermal spray techniques have been driven by the need to manufacture coatings with enhanced properties. These techniques result in two different types of coatings: on the one hand, those with a so-called bimodal structure having nanometre-sized zones embedded within micrometre ones, for which the spray process is similar to that of conventional coatings and on the other hand, sub-micrometre or nanostructured coatings achieved by suspension or solution spraying. Compared with suspension spraying, solution precursor spraying uses molecularly mixed precursors as liquids, avoiding a separate processing route for the preparation of powders and enabling the synthesis of a wide range of oxide powders and coatings. Such coatings are intended for use in various applications ranging from improved thermal barrier layers and wear-resistant surfaces to thin solid electrolytes for solid oxide fuel cell systems, among other numerous applications.Meanwhile these processes are more complex to operate since they are more sensitive to parameter variations compared with conventional thermal spray processes. Progress in this area has resulted from the unique combination of modelling activities, the evolution of diagnostic tools and strategies, and experimental advances that have enabled the development of a wide range of coating structures exhibiting in numerous cases unique properties. Several examples are detailed. In this paper the following aspects are presented successively (i) the two spray techniques used for manufacturing such coatings: thermal plasma and HVOF, (ii) sensors developed for in-flight diagnostics of micrometre-sized particles and the interaction of a liquid and hot gas flow, (iii) three spray processes: conventional spraying using micrometre-sized agglomerates of nanometre-sized particles, suspension spraying and solution spraying and (iv) the emerging issues resulting from the specific structures of these materials, particularly the characterization of these coatings and (v) the potential industrial applications.Further advances require the scientific and industrial communities to undertake new research and development activities to address, understand and control the complex mechanisms occurring, in particular, thermal flow?liquid drops or stream interactions when considering suspension and liquid precursor thermal spray techniques. Work is still needed to develop new measurement devices to diagnose in-flight droplets or particles below 2??m average diameter and to validate that the assumptions made for liquid?hot gas interactions.Efforts are also required to further develop some of the characterization protocols suitable to address the specificities of such nanostructured coatings, as some existing 'conventional' protocols usually implemented on thermal spray coatings are not suitable anymore, in particular to address the void network architectures from which numerous coatings properties are derived.

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Engineering a new class of thermal spray nano-based microstructures from agglomerated nanostructured particles, suspensions and solutions: an invited review

Component distribution in plasma-deposited ultrafine powders of binary (Cr2O3, Fe2O3, SnO2)–Al2O3 systems

With the recent interest in application of nanoscale metal and metal oxide particles, there is now interest in manufacturing these materials in larger quantities. A number of techniques have evolved for the synthesis of nanopowders. For metal and metal oxide nanoparticles where coalescence is the dominating growth mechanism, concentration and the time–temperature history of the nanoparticles resulting from the reactor design determines the final particle average diameter. In this paper, we present the results of controlled synthesis of alumina nanopowder in a three-jet plasmachemical reactor, where temperature and velocity distributions are controlled by the angle of impingement of the plasma jets. The flow and temperature fields for various impingement angles were measured by enthalpy probe. The products were characterized by means of electron microscopy, XRD analysis, and BET analysis. The dependences of particle size on the operation parameters (injection velocity of raw material, plasma jet enthalpy, solid/gas mass flow rates ratio, and impingement angle of plasma jets) are discussed. In comparison with the single jet reactor, the novel reactor can increase production rate several times to manufacture the product with the same required surface area.

Controlled Synthesis of Alumina Nanoparticles in a Reactor with Self-Impinging Plasma Jets

Bulk and supported catalysts that are active in the CO oxidation reaction were obtained by the reaction of a low-temperature plasma with various catalyst materials Treating oxide catalysts with an HF discharge plasma results in an increase of their activity.

M. S. J. Gani

Papers

Al2O3-SnO2 composite plasma-synthesized powders