Ceramic nanoparticles for thermal spraying

1. Ceramic nanoparticles for thermal spraying KE-31.5530 Nanoparticles Maria Oksa

2. Contents • Background • Thermal spraying • Materials • Synthesis methods for ceramic NPs • Drawbacks • Analysis and characterization • Case studies of ceramic nanoparticle synthesis • Summary • References

3. Substrate Gas and particle stream Powder Energy Melted particles form lamella structured coating Thermal spraying shortly • Plasma, HVOF and CJS spraying • Metals, ceramics, cermets in powder form • Wear and corrosion resistance, hardness, electrical properties etc.

4. Ceramics for thermal spraying • Oxides: Al2O3 and TiO2 • Carbides: WC and SiC • Nanosized titania TiO2 • Unique structural, electrical, optical, magnetic and chemical properties • Use as white pigments, in photo catalysis, solar cells, water and air purification, etc. • Nanosized alumina Al2O3 • High strength and toughness, electrical resistance • Use e.g. for electronics and high temperature applications • Silicon carbide SiC • High melting point, hardness, wear and chemical resistance, electrical properties • Used in electrical industry, high temperature applications, reinforcement for ceramic composites • Tungsten carbide WC • High melting point, hardness, oxidation resistance, electrical conductivity • Applications e.g. cutting tools and wear-resistant parts

5. Synthesis methods of ceramic NPs • WC • Direct carburization of W powder • Solid state metathesis • Reductions-caraburization • Mechanical / reaction milling • Polymeric precursor routes using metal alkoxides • SiC • Si metal direct carbonization • CVD (chemical vapor deposition) • Thermal plasma synthesis • Carbothermal reduction of silicon dioxide • Sol-gel TiO2 • Wet-chemical synthesis by precipitation of hydroxides from salts • Sol–gel processes • Microemulsion-mediated methods • Gas phase (aerosol) synthesis Al2O3 • Mechanical synthesis (milling) • Vapor phase reaction • Precipitation • Hydrothermal method • Combustion • Sol–gel

6. Examples of synthesis methods Sol-gel method • Use of precursor, solvent, catalyst, surfactant • Solution fabrication & evaporation  amorphous gel  drying  possible calcination • High purity, high chemical activity Thermal plasma synthesis • Vapor-phase precursors with plasma  rapid quenching  homogeneous nucleation • High-purity particles, suitable especially for carbides and nitrides Flame aerosol synthesis • Oxidation of vapor in atmospheric pressure reactor ( metal oxides, e.g. TiO2) • Safe and flexible, high purity particles with different sizes and phase composition,commercial scale Mirjalili 2010 Tong 2006

7. Drawbacks in NPs synthesis methods • Strong tendency to agglomerate during synthesis and/or subsequent processing • Expensive • Raw material • Complex technique • High temperature and pressure • Time consuming • Low efficiency • Impurities to produced particles

8. Analysis and characterization • X-ray diffraction • Phase structure • Rheometry analysis • Viscosity measurement • Thermal analysis (TGA, DTA) • Evaporation, reactions and phase transformations • Electron microscopes SEM, TEM • Microstructure, size and shape • Surface area analyser • Dynamic laser light scattering method • Particle size

9. Case 1Spray pyrolysis for titania synthesis • Low-pressure spray pyrolysis (LPSP) • Controlled composition and morphology • Good crystallinity • Uniform size distribution • One-step method • Technique: Precursor solution is atomized and droplets poured into glass filter. Aerosol is heated and solvent evaporates in the reactor. Anatase-titania particles with nominal size of about 10 nm Wang 2004

10. Case 2Sol-gel method for alumina powder • High purity solid particles with high specific surface area • High cost of alumiun alkoxides (e.g. Al isopopoxide) • Aqueous sol-gel method • Low cost Al and AlCl36H2O powders and HCl • Stirring at 95C for 4 hours  transparent solution (sol) • Drying at 85C for 48 hours (gel) • Grinding and calcination at high temperature (600…1200 C)  Spherical 32-100 nm -alumina particles Shojaie 2008

11. Case 3Solid-state synthesis for WC • Solid-state carbothermic reduction of tungsten oxide • Calcining mechanically activated mixtures of WO3 and graphite • Planetary ball mill, Ar, 10 h • Reduction by heating at 1215C in vacuum • Mechanical milling increased homogeneity and enabled production by decreasing the diffusion path  WC particles via formation of intermediates, Magneli phases WO2.72 and WO2 Ma 2010

12. Case 4Sol-gel method for SiC powder • Benefits: high purity, high chemical activity, improvement of powder sinterability, possibility for particles mixing at molecular scale • Materials: Tetraethyl orthosilicate (TEOS), chlorocidric acid and NaOH (catalysts solutions), phenolic resin, ethanol, acetone (resol solvent), distilled water and ammonium polycarboxylate (APC) (dispersant agent) • Method: Solution  homogenisation  hydrolysis reactions and gelation  heating and drying  pyrolyzation 700C 1 h (Ar)  heat treatment 1500C 1 h  cubic –SiC semi-spherical particles (agglomerates less than 100 nm) Najafi 2010

13. Summary and conclusions • Large variety of different methods for different materials • Differences consist e.g. of • Temperature (TR… 1500C), pressure • Wet, solid or sol-gel type • Wide amount of different raw materials, precursors, surfactants etc. • One- or several steps • Need for post treatment (calcination) • Synthesis time • Produced particle size, homogeneity, size distribution, purity  Influence on efficiency, cost and application • As a conclusion: The possibilities for synthesizing ceramic nanoparticles is in practice countless. Therefore thorough data acquisition and comparison is needed for finding the correct method for certain material and application need.

14. References • Sahil Sahni, et al., Influence of process parameters on the synthesis of nano-titania by sol–gel route. Materials Science and Engineering A 452–453 (2007) 758–762 • Kranthi K. Akurati, Andri Vital, Ulrich E. Klotz, Bastian Bommer, Thomas Graule, Markus Winterer, Synthesis of non-aggregated titania nanoparticles in atmospheric pressure diffusion flames. Powder Technology 165 (2006) 73–82 • K.M. Parida, et al., Synthesis and characterization of nano-sized porous gamma-alumina by control precipitation method. Materials Chemistry and Physics 113 (2009) 244–248 • M. Shojaie-Bahaabad, E. Taheri-Nassaj, Economical synthesis of nano alumina powder using an aqueous sol–gel method. Materials Letters 62 (2008) 3364–3366 • F. Mirjalili, et al., Size-controlled synthesis of nano a-alumina particles through the sol–gel method. Ceramics International 36 (2010) 1253–1257 • Lirong Tong, Ramana G. Reddy, Thermal plasma synthesis of SiC nano-powders/nano-fibers. Materials Research Bulletin 41 (2006) 2303–2310 • J. Ma , S.G. Zhu, Direct solid-state synthesis of tungsten carbide nanoparticles from mechanically activated tungsten oxide and graphite. Int. Journal of Refractory Metals and Hard Materials 28 (2010) 623–627 • A. Najafi, et al., Effect of APC addition on stability of nanosize precursors in sol–gel processing of SiC nanopowder. Journal of Alloys and Compounds 505 (2010) 692–697 • Wei-Ning Wang, et al., One-step synthesis of titanium oxide nanoparticles by spray pyrolysis of organic precursors. Materials Science and Engineering B 123 (2005) 194–202 • Wei-Ning Wang, Yoshifumi Itoh, I. Wuled Lenggoro, Kikuo Okuyama, Nickel and nickel oxide nanoparticles prepared from nickel nitrate hexahydrate by a low pressure spray pyrolysis. Materials Science and Engineering B 111 (2004) 69–76