Ganesh Skandan

Phase characterization and stabilization due to grain size effects of nanostructured Y2O3

Abstract X-ray diffraction and micro-Raman scattering studies of nanostructured Y2O3 prepared by gas phase synthesis are reported. The as-prepared material has the monoclinic structure of the high pressure polymorph, γ-Y2O3. The high-pressure phase is stable in the nanostructured grains at ambient conditions resulting from the additional hydrostatic pressure component due to the Gibbs-Thomson effect. At temperatures exceeding 600°C monoclinic γ-transforms to cubic α-Y2O3. The dependence of the phase transition temperature on the state of compaction of the nanostructured powders indicates that the stability of the high pressure phase at ambient conditions results from the nanostructured particle size.

Processing of nanostructured zirconia ceramics

The inert gas condensation (IGC) technique was employed to synthesize nonagglomerated nanoparticles of ZrO 2 2 and Y 2 O 3 with different average particle sizes ranging from 4 to 14 nm. High pressure phases were found in the as-prepared powders of both n-Y 2 O 3 and n-ZrO 2 . This has been explained on the basis of Gibbs-Thomson effect. The sintering behaviors (in air and vacuum) of single phase n-ZrO 2 (monoclinic crystal structure) and Y 2 O 3 -ZrO 2 mixture (Y-TZP) were studied in terms of densification rate and final sintering temperature. There was a strong correlation between densification characteristics and properties of the starting powder compacts, such as average particle size, particle and pore size distributions. n-ZrO 2 was sintered to full density in air at temperatures as low as 1125°C (0.47 T m ) and in vacuum at 975°C (0.42 T m ). Although the grain sizes in the fully sintered samples were well below 100 nm, the grains had grown by a factor of at least 10, as compared to the initial particle size. Therefore, pressure-assisted sintering techniques, such as sinter-forging and hot pressing, were employed to further reduce the densification temperature and final grain size. Threshold effects in these processes are also discussed

Synthesis of oxide nanoparticles in low pressure flames

A method is described for the production of oxide nanopowders by controlled thermochemical decomposition of metalorganic precursor/carrier gas mixtures in low pressure flames. A specially designed burner is used to achieve a flat flame, extending a few millimeters out of the burner, which ensures that the temperature distribution and gas phase residence time are identical over the entire burner surface. Metalorganic precursors introduced along with the fuel/air mixture, therefore, experience completely uniform decomposition, thereby yielding a uniform nanopowder product. The substantial heat release in the flame allows the burner to support a high precursor flow rate. In this paper, we describe the progress made in flame synthesis of nanophase SiO2 (from Hexamethyldisilazane), TiO2 (Titanium ethoxide) and Al2O3 (Aluminum tri-sec butoxide) powders.

Factors controlling decarburization in HVOF sprayed nano-WC/Co hardcoatings

Abstract The mechanism of decarburization in HVOF sprayed nano-WC/Co coatings has been investigated. The controlling factor is heterogeneous melting and localized superheating of the high surface area feed powder during spraying. This has the effect of causing extensive dissolution of the WC nanoparticles in the liquid Co, accompanied by rapid reaction of the dissolved C with oxygen in the flame environment. Upon cooling down from the peak temperature in the flame, the Co-rich liquid, now deficient in C, forms W 2 C and/or W phases, depending on the loss of carbon by gasification. Thus, it is concluded that HVOF sprayed high surface area nano-WC/Co powder is more susceptible to decarburization than conventional WC/Co powder, which is in agreement with observation.

Chemical vapor processing and applications for nanostructured ceramic powders and whiskers

Abstract A new chemical synthetic process for producing non-agglomerated nanostructured ceramic (n-ceramic) powders from metalorganic precursors is described. The process combines rapid thermal decomposition of a precursor/carrier gas stream in a hot tubular reactor with rapid condensation of the product particle species on a cold substrate under a reduced inert gas pressure of 1–50 mbar. A wide variety of metalorganic precursors is available commercially, all of which can be utilized in this process to produce n-ceramic powders, including single phase, multiphase and multicomponent systems. The process has been used to synthesize nonagglomerated n-SiC x N y and n-ZrO x C y powders, with controlled particle size in the range 2–20 nm. Heat treatment of the as-synthesized n-powders in various high purity gas streams causes compositional and structural modifications, including particle purification and crystallization, as well as transformation to a whisker-like morphology. Non-agglomerated n-ceramic powders form uniformly dense powder compacts by cold pressing, which can be sintered to theoretical density at temperatures as low as 0.5 T m .

Plasma-sprayed nanostructured Al2O3/TiO2 powders and coatings

Air plasma spray has been used to produce metastable oxide-ceramic powders and coatings, starting with commercially available Al2O3/13TiO2 powder feed. The feed material undergoes rapid melting and homogenization in the high-temperature zone of the plasma jet. A metastablex-Al2O3·TiO2 phase is formed when the molten droplets are quenched on a chilled substrate. The metastable phase has a defect spinel structure and a nanocrystalline grain size. When heated, it decomposes into an equilibrium two-phase structure, consisting ofα-Al2O3 andβ-Al2O3·TiO2. Both types of ceramic materials have potential as hard, wear-resistant coatings.

Chemical vapor condensation of nanostructured ceramic powders

Abstract This paper describes a modification of the conventional gas condensation processing apparatus in which the usual evaporative source is replaced by a chemical source. The new system combines rapid thermal decomposition and expansion of a precursor gas stream with rapid condensation of the product species on a cold substrate. We have demonstrated the feasibility of synthesizing loosely agglomerated nanoparticles (6 to 10nm) of SiCxNy, starting from hexamethyldisilazane as precursor compound. Critical to the success of this new chemical vapor condensation method (CVC) are: (1) low concentration of precursor in the carrier gas, (2) rapid expansion of the gas stream through an uniformly heated tubular reactor, and (3) rapid quenching of the gas phase nucleated clusters or nanoparticles as they exit from the reactor tube.

Hrtem study of nanocrystalline zirconia powders

Abstract Zirconia powders with grain sizes between 4 and 35 nm were synthesized by the gas condensation technique. Different qualities (grain size, grain size distribution) of powders have been observed and can be attributed to different synthesis conditions (mainly temperature gradients) within the UHV chamber. The existence of necks between single particles has been observed. These necks are due to a first sintering process during the oxidation of the metal/ suboxide clusters condensed on the walls in the synthesis chamber. Several stages of this sintering process are documented by high-resolution micrographs. Furthermore, some particles exhibit a shell-like structure with monoclinic ZrO2 in the outer region and tetragonal ZrO2 in the inner region. This is an indication of (i) the martensitic phase transition character of zirconia in general, and (ii) the existence of a critical particle size in n-ZrO2 responsible for the metastability of a tetragonal polymorph in nanosized zirconia powder. Lattice distortions in some directions can be concluded from image reconstruction of the digitized micrograph of a single nanosized ZrO2 particle.

The effect of applied stress on densification of nanostructured zirconia during sinter-forging

Abstract Nanoparticles of ZrO2 (n-ZrO2) synthesized by the inert gas condensation technique were consolidated by sinter-forging, which is a pressure-assisted sintering process. It was found that there is a threshold stress below which there is only a small contribution to densification by the applied pressure. The value of the threshold stress depends on grain size and the effect is explained on the basis of driving forces for sintering. The grain size in the fully dense n-ZrO2 samples was 45 nm.

Fabrication of translucent MgO ceramics using nanopowders

Abstract Translucent MgO ceramics have been fabricated by hot-pressing using nanopowder of MgO containing 2–4% LiF as a fugitive additive. Thin circular discs were pressed uniaxially with the LiF-containing nanopowders. The hot-pressing was carried out either in Ar or in vacuum, holding at 1100 °C for 30 to 60 min at either 24 or 45 MPa. The samples were then annealed in air by either conventional or microwave processing. The annealed samples were highly dense and optically translucent.