Vijay Shukla

Nanopowder deposition by supersonic rectangular jet impingement

With a view toward developing the next generation of coatings using nanopowders, a cold gas dynamic spray (CGDS) technique has been investigated. In this method, a powder feeder is used to inject nanopowder agglomerates into a supersonic rectangular jet, with a design Mach number of 3.2. The powder particles gain speeds of up to 700 m/s through momentum transfer from the jet and bond to the substrate surface due to kinetic energy dissipation. Coatings of copper and nano-WC/10% Co on steel and aluminum substrates (3 to 5 µm in thickness) have been produced. The benefit of this process is that the material does not undergo any chemical changes during coating formation. To improve the quality of the coatings produced, the flapping motions produced by supersonic jet impingement were studied. Powder particle velocities and the jet impingement flow field were quantified using particle image velocimetry (PIV).

Hyperkinetic deposition of nanopowders by supersonic rectangular jet impingement

Abstract In a relatively new coating deposition process, Cold Gas Dynamic Spray (CGDS), a supersonic jet is used to propel the feedstock powder onto a substrate to form coating. Pure Titanium (Ti) powder was sprayed by the CGDS process to establish conditions for the buildup of dense and porous Ti coatings on Ti substrates. In order to better understand the mechanism of CGDS coating of titanium on titanium, we performed a detailed study of the initial stages of coating buildup, where we examined initial attachment of Ti particles to the Ti substrate and the subsequent evolution of their morphology. Use of Ti powder as a one of the components of a composite powder in which the other component is biocompatible nanohydroxyapatite powder (to promote bone growth) for use in coating titanium orthopedic implants is currently under investigation.

Recent Progress in the Development of Neodymium-Doped Ceramic Yttria

Solid-state lasers play a significant role in providing the technology necessary for active remote sensing of the atmosphere. Neodymium-doped yttria (Nd:Y2O3) is considered to be an attractive material due to its possible lasing wavelengths of ~914 and ~946 nm for ozone profiling. These wavelengths, when frequency tripled, can generate ultraviolet (UV) light at ~305 and ~315 nm, which is particularly useful for ozone sensing using differential absorption light detection and ranging (LIDAR) technique. For practical realization of space-based UV transmitter technology, ceramic Nd:Y2O3 material is considered to possess a great potential. A plasma melting and quenching method has been developed to produce Nd3+-doped powders for consolidation into Nd: Y2O3 ceramic laser materials. This far-from-equilibrium processing methodology allows higher levels of rare earth doping than can be achieved by equilibrium methods. The method comprises two main steps: 1) plasma melting and quenching to generate dense, and homogeneous doped metastable powders and 2) pressure-assisted consolidation of these powders by hot isostatic pressing to make dense nanocomposite ceramics. Using this process, several in 1times1 ceramic cylinders have been produced. The infrared transmission of a 2-mm-thick undoped Y2O3 ceramic was as high as ~75% without antireflection coating. In the case of Nd:Y2O3, ceramics infrared transmission values of ~50% were achieved for a similar sample thickness. Furthermore, Nd:Y2O3 samples with dopant concentrations of up to ~2 at.% were prepared without significant emission quenching.

Submicron-grained transparent yttria composites

New materials with improved mechanical properties and high optical transmission in the full 3-5 micron MWIR region wavelength are required. Commercially available polycrystalline transparent Yttria, with >100 micron average grain size, does not perform satisfactorily in demanding applications because of its modest strength. One way to improve strength is to develop an ultra-fine grained material with acceptable optical transmission properties. To realize fine grains it is necessary to use other phases to inhibit grain growth during fabrication. A promising processing method comprises: (a) synthesis of an extended metastable solid solution by plasma melting and quenching, and (b) consolidation of the metastable ceramic powder to form dense submicron-grained (<1 micron) composites. Two ceramic composites containing 20 and 50 vol% of second phase are evaluated in this study. Optical transmission, hardness, and indentation fracture toughness are measured and correlated with structure.

Parametric study of zirconia nanoparticle synthesis in low pressure flames

Center for Nanomaterials Research, Rutgers University, Piscataway, NJ 08855, USA(Received August 21, 2000)(Accepted in revised form December 18, 2000)Keywords: Cubic zirconia; Thermophoresis; Experimental synthesis; Computer modelingIntroductionOver the last decades a number of studies have been dedicated to understanding and controlling particleformation and growth in flames. Most of these studies used counterflow diffusion flames (1–3) andatmospheric pressure reactors. Few studies treated low pressure flames synthesis of particles (4,5) althoughthe degree of agglomeration of nanoparticles in this type of flames is greatly reduced. The properties of boththe primary particles and agglomerates highly depend on the time/temperature history. This information isessential for modeling the particle growth, especially in non-isothermal environments such as flames. A lowpressure hydrogen/oxygen/helium/zirconium n-butoxide flat flame reactor in a Hiemenz flow geometry isused in this study for zirconia nanoparticles synthesis. In this simple axisymmetric velocity flowfield, inwhich thermal and chemical profiles are one-dimensional, the computational modeling is greatly simplified.Different temperature-time profiles are obtained by varying the total pressure, the axial dimension (burner-substrate distance), the total volumetric flow, dilution and stoichiometry of the flame. Results from thesynthesis experiments are compared with the computer model predictions for each case.ExperimentalThe experimental setup is described in greater detail in a study on silica nanopowder synthesis (5). For thisstudy on the zirconia system, the liquid precursor (zirconium n-butoxide 80% by volume in n-butanol) isplaced inside an evaporator and heated. After ignition is induced, the precursor vapor temperature and reactorpressure are brought to operating conditions (e.g. 325°C and 20 torr, respectively). The precursor vapor isentrained in the premixed gases flow and carried towards the flat flame, where it pyrolizes. Table 1 showsthe flame conditions used in this study. The concentration (partial pressure) of the precursor vapor iscontrolled by changing its temperature. The substrate is water cooled and maintained at constant temperature,which is monitored using a K-type thermocouple. The nanopowder deposited on the substrate is analyzedusing X-Ray Diffraction, BET and TEM to yield particle size and morphology measurements. X-raydiffraction measurements were performed on a Siemens Diffraktometer machine. BET specific surface areameasurements were performed on a Micromeritics DeSorb 2300A. A TOPCON 002B TransmissionElectron Microscope is used to examine the morphology and size of the nanopowders.

Development of solid state laser material using ceramic yttria

High power solid state tunable lasers have played an important role in providing the technology necessary for active remote sensing and would be very useful for space exploration. Many recent studies on diode-pumped solid state lasers have focused on polycrystalline ceramic lasers. We present our initial results on the material, optical, and spectroscopic properties of a solid-state ceramic laser material using neodymium doped Yttria (Nd:Y2O3). Using a proprietary scalable production method, spherical non agglomerated and monodisperse ceramic powders of Nd:Y2O3 are made that can be used to fabricate polycrystalline ceramic material disks with sintered grain size in a suitable range. Initially, we produced translucent material with good emission properties. In further studies we have successfully prepared transparent Nd:Yttria ceramic material. Polycrystalline ceramic lasers have enormous potential commercial applications, which include remote sensing, chemical detection and space exploration research. Furthermore, the cost to produce ceramic laser materials is potentially much lower than that for single crystal materials because of the shorter time it takes to fabricate the material and also because of the possibility of mass production. The polycrystalline ceramic material that we have produced will be characterized for its suitability as a diode pumped solid state laser. Different laser designs will be discussed including end-pumping schemes and the thin-disk laser configuration.

Far-from-Equilibrium Processing of Multiphase Ceramic Nanocomposites

A new far-from-equilibrium processing methodology has been developed to produce multiphase nanocomposite ceramics. The proposed method uses the following steps: (a) spray drying of commercially available powders to make flowable multicomponent feed powder aggregates, (b) plasma melting and quenching to generate dense and homogeneous metastable powders, and (c) pressure assisted consolidation of these powders to make dense nanocomposite ceramics. By controlling of process parameters, inherent superplasticity of these powders can be used to aid in densification process. In this article, results on multiphase zirconia- and alumina-based ceramics will be presented.