Peter R. Strutt

Chemical processing and applications for nanostructured materials

Abstract This paper presents an overview of recent research performed at Rutgers University and the University of Connecticut on the synthesis and processing of nanostructured materials. Highlights of this collaborative research program include: (1) synthesis of carbide strengthened steel and hard cermet powders from aqueous solution precursors, (2) synthesis of ceramic powders and ceramic matrix composites from metalorganic precursors, (3) densification of powder compacts by liquid phase sintering, (4) formation of high quality coatings by thermal spraying, and (6) demonstration of superior hardness and wear resistance in bulk materials and coatings.

Laser surface melting of high speed tool steels

Abstract Laser surface melting experiments have been carried out on the refractory-element-rich alloy M2 and the chromium-rich alloy 440-C. In the laser-melted zone of M2 the microstructure contains both δ-ferrite and austenite (γ) with residual amounts of carbides. A predominant feature is the formation of austenite (γ) by peritectic reaction of δ-ferrite with liquid. The extent to which the reaction proceeds depends upon position within the melted zone. Thus nearly pure γ is formed near the maximum melt depth and nearly pure δ at the external surface. In contrast to the rather complex microstructures in M2, laser melting produces a simple coupled growth δ-ferrite/Cr7C3 carbide eutectic structure in 440-C.

Synthesis of Si(C, N) nanoparticles by rapid laser polycondensation/crosslinking reactions of an organosilazane precursor

A study has been made of basic mechanisms involved in the rapid synthesis of pre-ceramic and ceramic nanoparticle powders. In this process an aerosol, formed from an ultrasonically atomized liquid organosilazane monomer, [CH3SiHNH]n, is injected into the beam of an industrial CW CO2 laser. One critical feature examined was the rapid condensation of molecular species from the laser plume, in a process involving three-dimensional crosslinking polycondensation reactions. In accompanying studies, a model has been formulated to determine the laser plume temperature, the cooling rate of condensing species and the particle diameter. These are obtained by analytical solution of heat conduction, momentum and mass conservation equations that consider heat loss by gas conduction, radiation, evaporation and convection.

Observation of Co nanoparticle dispersions in WC nanograins in WC-Co cermets consolidated from chemically synthesized powders

Abstract High resolution, and analytical electron microscopy has been used to analyze morphological features in consolidated specimens of nanostructured tungsten carbide-cobalt cermet materials prepared from chemically synthesized nanostructured WC-Co powder. The investigation includes studying materials produced by consolidation (i) above, and (ii) below the eutectic temperature by thermo-mechanical working. In the case of liquidphase sintered material, the study includes examining the influence of a vanadium carbide additive on morphological features, as well as its effect in inhibiting grain growth. Electron microscope examination of all consolidated materials reveals a dispersion of nanoprecipitates within the WC nanograins of the WC-Co cermet. Micro-diffraction and analytical studies show that these nanoprecipitates are f.c.c. cobalt. This novel discovery is consistent with the concept that the nanoprecipitates nucleate from cobalt retained within the WC nanograins, which is a consequence of the intimate intermixing of tungsten and cobalt in the original chemical synthesis process.

Synthesis of Si(N,C) nanostructured powders from an organometallic aerosol using a hot-wall reactor

Current studies show that nanostructured Si(N,C) powders are readily synthesized by rapid condensation of a pyrolytically decomposed silazane precursor, namely [CH3SiHNH]n,n−3 or 4. Basically, the process involves ultrasonic conversion of the liquid-phase precursor to an aerosol, followed by thermal decomposition in a hot reactor. This was followed by the rapid condensation of the gaseous product exiting the reactor, to form ceramic particles of nanoscale dimension. Thermal decomposition was performed at a temperature of 1000 °C, near ambient pressure with a flow rate of ∼ 150 standard cm3 min−1 for NH3. One critical feature examined in this process was the rapidity of the powder synthesis, in a reaction which involves (i) elimination of ligand groups, (ii) formation of ceramic species, and (iii) condensation of ceramic species into ultrafine ceramic particles. These features have been studied using Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray photo-electron and nuclear magnetic resonance spectroscopies. Additionally, a model is formulated to determine the effect of process parameters on particle size.

Microstructural study of hollandite-type MnO2 nano-fibers

A new morphology of manganese oxide in the form of nano-fibers was synthesized using a novel synthetic route, and structurally characterized using high-resolution transmission electron microscopy. The obtained manganese oxide, doped with potassium, adopts a hollandite-type structure with “few useful defects” which provide an open tunneled structure in three-dimensions. This characteristic is actually believed to improve, to a high extent, the ionic conductivity of the material and exhibits the doped hollandite-type manganese oxide as a highly promising candidate for electrochemical applications such as rechargeable batteries.

Nanometer‐size fiber composite synthesis by laser‐induced reactions

A new approach in ultrafine composite synthesis involves rapid condensation of metallic and nonmetallic species, produced by laser‐induced evaporation. A heated tungsten filament is simultaneously employed for codeposition of W via a chemical transport mechanism. This process occurs in a reducing environment of hydrogen gas, where evaporated species were produced by a laser‐induced plume. Composite layers were formed on a Ni alloy substrate surface, at a rate of about 1 μm/s. The matrix of the composite films was either Al or W, and the dispersed phase was amorphous silica fibers. The diameter of the fibers was between 25 and 120 nm, depending on the laser‐beam materials‐interaction time. Various analytical techniques have been employed to characterize the as‐synthesized layers. Experimental evidence does not support the vapor‐liquid‐solid model for fiber growth. An alternative fiber growth mechanism is proposed: ultrafine silica particles (10–25 nm diam) form by rapid condensation from the laser plume, a...

Microstructural characteristics of chemically processed manganese oxide nanofibres

Nanostructured fibrous manganese oxide cryptomelane-type hollandite powders were prepared via a chemical route. Powders were obtained by the chemical reaction of the aqueous solutions of potassium permanganate and manganese sulfate in the presence of a strong acid, followed by crystalline growth at elevated temperatures. These powders were characterized by powder X-ray diffraction (XRD), Rietveld refinement, analytical scanning electron microscopy (SEM), transmission electron microscopy (TEM) combined with micro-micro diffraction (μμd), and high resolution transmission electron microscopy (HRTEM). The crystalline structure, morphology, crystallite size, fibre growth direction, and structural defects are discussed.

STUDIES OF CARBIDES IN A RAPIDLY SOLIDIFIED HIGH-SPEED STEEL

Rapid solidification by electron beam surface melting of a Mo-base high-speed steel (M7) has produced microstructural features different from those observed in the conventionally processed material. As a result of rapid solidification, the volume percent of the carbide phases formed has decreased sharply and has resulted in the formation of M2C and M23C6 carbide phases, while in the conventionally processed material, M6C and MC carbides were present. Microanalysis of the extracted carbides formed by electron beam melting has yielded an intriguing finding. M23C6 is found to be unusually rich in molybdenum, tungsten, and vanadium; the concentration of (Mo + W), for instance, is approximately 60 wt pct. The corresponding values for Fe and Cr are surprisingly low (6 wt pct Cr and 1 wt pct Fe). This is in marked contrast with carbides encountered in the conventionally processed high-speed steel, where Cr and Fe are the major constituents. The shift in composition of the carbide phases could be attributed to the accelerated evaporation of chromium during surface melting as compared to the evaporation of Mo, W, and V.

Synthesis of nanostructured NiCr and Ni-Cr3C2 powders by an organic solution reaction method

Nanostructured nickel/chromium (NiCr) and nickel-chromium carbide (NiCr3C2) powders were prepared via a chemical route. The technique involves reductive decomposition of an organic solution of metal chloride precursors using sodium triethylborohydride. After co-precipitation of a nanoscale mixed metal (Ni+Cr) powder, low temperature annealing is used to form nanostructured NiCr alloy powder, or low temperature gas phase carburization to form nanostructured Ni-Cr3C2 cermet powder. These powders were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and chemical analysis. The mechanisms of halide reduction and phase formation are discussed.