Year 2013, Volume 33

Abstract

The processing of nanocrystalline yttria doped zirconia powder via dry forming routes has been investigated via the granulation of the powder using spray freeze drying (SFD). Free-flowing and crushable powders suitable for either die or isosatic pressing have been achieved via the combination of SFD with additions of up to 2 vol% of Freon 11; the latter reducing the strength of the granules whilst not affecting the powder flowability into the die. The approach has allowed relic-free green bodies of up to 55% of theoretical density to be produced using pressures as low as 250 MPa.

Abstract

New synthesis process to prepare nano-particles of lithium niobate, sodium niobate and potassium niobate by thermal decomposition of the constituent double metal alkoxides was developed. Single crystals of such double-metal alkoxides as Na-Nb, Li-Nb and K-Nb ethoxides were newly synthesized from a mixed solution of the constituent metal ethoxides. The doublemetal alkoxides of the Li-Nb, Na-Nb and K-Nb systems decomposed at low temperatures below
673 K to form nano-particles of LiNbO₃, NaNbO₃ and LiNbO3. The lattice constants and crystallite size of the obtained LiNbO₃, NaNbO₃ and LiNbO₃ particles were elucidated. It was shown that this new synthesis process was useful for preparation of niobate nano-particles at low temperatures.

Abstract

A modified complex sol-gel process was developed to synthesize LiMn₂O₄ and LiNixCo₁-xO₂ powders. Sols were prepared from 1.5 M of Mn²⁺ or 1 M of xNi₂+ and (1-x)Co²⁺acetates plus ascorbic acid aqueous solutions by alkalizing them with LiOH and NH₄OH. Ascorbic acid was added to prevent precipitation in the precursor solutions. When the sols were concentrated to one-third of their initial volume under reduced pressure and then gelled by drying for 10 days, heat treatment to ≥700 ºC was accompanied by foaming, violent self-ignition, and formation of carbonates in the Ni-containing species. Significant improvements to the process and resulting powders were gained by modifying the gelation step. When gelation and drying were carried out under reduced total pressure, subsequent heating produced self-ignition, but no foaming, for all heating rates. The resulting products were determined to be nearly phase pure and carbonate free by thermogravimetric, differential thermal, X-ray, and infrared spectroscopy methods.

Abstract

In this study, our objective is to deposit an alumina or an aluminum nitride layer on a turbostratic carbon substrate. The coatings are synthesized via a sol-gel route followed by a heat treatment in order to obtain α-alumina or hexagonal aluminum nitride by carbothermal nitridation of alumina. The synthesis of such layers on carbon substrates is not reported in literature.
Several slurries were elaborated using various solvents and catalysts, and aluminum-tri-secbutoxide as an aluminum precursor. The coating is obtained by dropping the substrate in the sol. After drying and pyrolysis, the amorphous alumina layer obtained has a thickness ranging from 500 nm to 1 μm. The material is finally heat treated. Several treatment conditions were evaluated.
A thermodynamic study of the Al-C-O-N system will be drawn, and the composition of the synthesized sol-gel and heat treatment parameters will be detailed. Then, the layer’s morphology and structure will be characterized thanks to Scanning Electron Microscopy and X-Ray Diffraction analyses. The impact of heat treatment parameters will be discussed and experimental results will be compared to the theoretical thermodynamic results.

Abstract

Monodispersed plate-like CeO₂ particles were successfully synthesized by a mild solution process followed by calcination in air at 400oC. During the solution processing, monodispersed orthorhombic single-crystal of plate-like Ce₂(CO₃)₃.8H₂O was obtained by using Ce(NO₃)₃.6H₂O as cerium source, and NaHCO₃ aqueous solution as precipitation reagent and carbon source. The concentration of NaHCO₃ solution affects the morphologies and particle size of the cerium carbonate compounds. High concentration of NaHCO₃ led to a decrease in the particle size of Ce₂(CO₃)₃.8H₂O effectively. The CeO₂ particles synthesized by the decomposition of carbonate precursors possessed similar morphologies and slightly smaller particle size. The plate-like ceria particles possessed good UV-shielding properties and low oxidation catalytic activities, indicating the potential application as cosmetics materials

Abstract

Different valences of metal ions such as Fe³⁺ and Nb⁵⁺ were co-doped with nitrogen ion into titanium dioxide by hydrothermal method using metal chlorides and hexamethylenetetramine as the sources of metal ions and nitrogen ion, respectively. The co-doping of low-content metal ion showed no noticeable influence on the crystalline phases and specific surface area (S.S.A.) of the samples. Doping with Fe ion could significantly enhance the absorption in visible light region, but doping with Nb ion showed almost no effect. The photocatalytic activities of the samples were determined for the oxidative destruction of NO gas under various wavelengths. Co-doping with Nb ion improved the deNO_x ability, but co-doping with Fe ion depressed it, indicating that co-doping with higher valence metal ion was effective in reducing the vacancy in the lattice which acts as the recombination center of the photo-induced electrons and holes, and achieving higher photocatalytic activity.

Abstract

In this work, the sol-gel technology is used to produce silica based xerogels and aerogels suitable for insulation applications in Space. The properties of the obtained materials are tailored varying the precursor – Methyltrimethoxysilane (MTMS) or Methyltriethoxysilane (MTES), and the solvent – methanol or ethanol. A two-step acid-base catalyzed synthesis is used, being the obtained gels dried at atmospheric pressure, in the case of xerogels, and in supercritical conditions, for aerogels. Density and thermal conductivity must be made as low as possible for the sought application and only highly porous materials can fulfill this requirement. The obtained xerogels and aerogels, either with MTMS or MTES, show very promising properties for thermal insulation in Space, when methanol is used as solvent. The more suitable materials are obtained with MTMS and exhibit very low density (80-100 kg/m³), very high surface area (~ 400 m²/g) and small pore size (~ 30-40 Å). They also show moderate flexibility and a remarkable hydrophobic character (~ 150º).

Abstract

Fabrication of advanced electronic components requires high-quality powders. In this work, nano-powders of Li or Na niobates are synthesized from (Li or Na)-Nb ethoxide by a sol-crystal method. A single crystal of (Li or Na)-Nb ethoxide is decomposed to an amorphous matrix below 473 K. Next, small crystals are grown by heating at the appropriate temperature for each specimen. The sol-crystal method provides homogeneous quality and fine grains by heating at lower temperature. Structural analysis of the powders is performed by a transmission electron microscope (TEM) and X-ray diffraction. As a result, LiNbO₃ turns to dense-powders, but NaNbO₃ forms nano-porous powders. In order to understand this difference, we try to observe in-situ the crystallization and grain growth processes by high-temperature TEM. We successfully observe in-situ this processing and discuss the structural change and formation mechanism of LiNbO3, comparing these features with those of NaNbO₃.

Abstract

Niobium and nitrogen co-doped SrTiO₃ possessing excellent visible light responsive photocatalytic activity was successfully synthesized by microwave-assisted solvothermal reaction using SrCl₂.6H₂O, Ti(OC₃H₇)₄, NbCl₅ and hexamethylenetetramine in KOH aqueous solution. The photocatalytic activity was determined by DeNOx ability using LED lamps with the wavelengths of 627 nm (red), 530 nm (green), 445 nm (blue) and 390 nm (UV). The photocatalytic activity of SrTiO₃ for DeNO_x ability in visible light region could be improved by co-doping Nb⁵⁺ and N^3-. The excellent visible light photocatalytic activity of this substance may be due to the generation of a new band gap that enables to absorb visible light and decrease in the lattice defects which acts as a recombination center of photoinduced electrons and holes.

Abstract

A macro-scale model of spark plasma sintering (SPS) that couples electrical, thermal, stress-strain and densification components is presented. The continuum theory of sintering is incorporated enabling the evolution of the densification based on local conditions, thus a true spatial density distribution could be obtained. Specimen behavior is described through a non-linear viscous constitutive relation. The simulation is based on an FEM computer code. Several examples are shown and results are compared with experimental data available.

Abstract

The densification mechanism during the park-plasma-sintering (SPS) processing was examined in high purity MgAl₂O₄ spinel. As the density ρ_t increases, that is, as the effective stress σ_eff decreases, stress exponent n evaluated from σ_eff dependence of densification rate varies from n ≥ 4 in the low ρ_t region, n ≈ 2 in the intermediate ρt region to n ≈ 1 in the high ρt region. TEM observation shows that significant stacking faults caused by partial dislocations are observed in the low ρ_t region, but limited in the high ρ_t region. The ρ_t dependent densification behavior and microstructure suggest that the predominant densification mechanism during the SPS processing changes with ρ_t from plastic flow by partial dislocation motion for the low ρ_t region (n ≥ 4) to diffusion-related creep for the high ρ_t region (n ≈ 1).

Abstract

This study was carried out to produce and characterize B₄C - TiB₂ ceramics by spark plasma sintering. Initial B₄C and TiB₂ powders were mixed in ethanol solution with YSZ balls for 1 hour. Sintering was performed with 5, 10, 15 and 20 volume % TiB₂ containing mixtures respectively in disc-shaped graphite dies. The applied pressure was 40 MPa at 1760 °C for 5 minute sintering duration. The improvement of low sinterability and low strength of B₄C was investigated as well as strength of flexibility, hardness and fracture  toughness. The obtained products were characterized by X-ray diffraction and SEM imaging. The hardness values were measured under 1000 g load and the density values were measured with Archimedes' principle. The 3-point bending tests and modulus of elasticity measurements were also conducted.