[show abstract][hide abstract] ABSTRACT: Nanostructured neodymium doped yttrium aluminum garnet (Nd:YAG) powders were synthesized at low temperature by a gel combustion method with citric acid as fuel and nitrate as oxidizer. The method involves exothermic decomposition of an aqueous citrate–nitrate gel. The decomposition is based on a thermally induced anionic redox reaction. A variety of 1.0 at% Nd:YAG powders with different agglomerate structures were obtained by altering the citrate-to-nitrate ratio γ. The gel with γ = 0.277 yielded nanocrystalline Nd:YAG at 800 °C without the formation of any intermediate phase. For other gels nanostructured Nd:YAG powders were obtained at 850 °C. The gel with γ = 0.1 yielded nanostructured Nd:YAG powder with an average particle size of ∼40 nm. The stoichiometric citrate-to-nitrate ratio (γ = 0.277) gave the lowest amount of agglomeration. The decomposition of the gel was investigated by TG–DSC and FTIR. The produced ashes and calcined powders were characterized by XRD, BET and FETEM analysis.
[show abstract][hide abstract] ABSTRACT: Nanosized Nd:YAG powders with different doping concentrations were synthesized at a significantly low temperature by a gel combustion method with citric acid as fuel and nitrate as oxidizer. It is found that the precursor is composed of hydroxycarbonate and dehydrates at below 500 °C to form carbonate. Mono-phase Nd:YAG crystallites can be formed without any intermediate phase at 850 °C. The value of crystallite size of the 850 °C calcined 1.0 at% Nd:YAG powder is 59 nm and the average particle size is 86 nm. The doping of neodymium into YAG garnets increases the lattice constant and the fluorescent intensity decreases drastically when the neodymium concentration is higher than 3 at% because of the fluorescent quenching effect.
[show abstract][hide abstract] ABSTRACT: Superfine α-Al2O3 synthesized by a precipitation method and commercial α-Al2O3 were used to prepare bimodal alumina. Densification of bimodal alumina with MgO, SiO2 as additives was investigated from 1400 to 1550°C. The addition of superfine α-Al2O3 is not effective to enhance the densification at the sintering temperature of 1400°C. With further increase of the sintering temperature, the densities of alumina ceramics from bimodal alumina with different contents of superfine α-Al2O3 are similar in values. The microstructure evolution, as well as mechanical properties of the alumina ceramics was also studied. For the 1450°C sintered Al2O3 ceramics, fine grains located in the skeletal structure of the coarse grains and the increasing fine α-Al2O3 led to fine-grain structure. Spinel and mullite were observed in the fine-grain regions. For the 1500°C sintered Al2O3 ceramic, there was significant grain growth. Mullite, spinel and MAS glass formed in the triple-grain junctions. Both 1450 and 1500°C sintered Al2O3 ceramics have no liquid phase in grain boundaries. The addition of superfine α-Al2O3 can distinctly enhance the bending strength of alumina ceramics. The best mechanical properties were gained when the temperature was 1450°C and then deteriorated with further increase of sintering temperature.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing - MATER SCI ENG A-STRUCT MATER. 01/2006; 435:611-619.
[show abstract][hide abstract] ABSTRACT: Transparent ytterbium and chromium codoped yttrium aluminum garnet (Yb,Cr:YAG) ceramics, containing Ca as charge counter element and TEOS as sintering aid, were fabricated by a solid-state reaction method using high-purity Y2O3, Al2O3, Cr2O3 and Yb2O3 powders as raw materials. The mixed powder compacts were sintered at 1770 °C for 10 h under vacuum and annealed at 1450 °C for 20 h in air. The Yb,Cr:YAG ceramics exhibit a pore free structure with an average grain size of about 40 μm. The optical transmittance of 5 at% Yb, 0.025 at% Cr:YAG reaches 75% at 1100 nm. The strong absorptions of Yb3+ at 940 and 968 nm are suitable for InGaAs diode laser pumping; an absorption band at 1030 nm is suitable for passive Q-switch laser output at 1030 nm. Transparent Yb,Cr:YAG ceramics may be a potential material for compact, efficient, high-stability diode-laser-pumped passive Q-switched solid-state lasers.
[show abstract][hide abstract] ABSTRACT: A novel method has been developed for the fabrication of silicon nitride reticulated porous ceramics (RPCs) with regulated structure, using polyurethane sponge as the template. This technique includes two centrifuging steps. In the first stage, the polyurethane sponges are coated with thin slurry through high-speed centrifuging. After the slurry on sponges dried, a high-strength green body without filled cells is obtained. In the second stage, because of improved adhesion between the body and the slurry after the first coating, the body with uniformly thick strut is produced by recoating the as-prepared green body with thixotropic slurry and centrifuging again. The loading content of slurry and the pore diameter of the RPCs body greatly depend on the slurry viscosity in the process of recoating. The strut diameter and pore diameter of the RPCs are effectively optimized by modulating the slurry viscosity.
Journal of the American Ceramic Society 03/2005; 87(7):1392 - 1394. · 2.11 Impact Factor
[show abstract][hide abstract] ABSTRACT: In an attempt to synthesize amorphous silicon nitride (a-SiNx) thin films with minimal incoporation of impurities, a novel liquid precursor, tris(diethylamino)chlorosilane (TDEACS), was synthesized and proven to be an ideal candidate as a silicon and nitrogen source for depositing of high-quality a-SiNx thin films. a-SiNx films with low carbon and hydrogen contents were prepared from a TDEACS–NH3–N2 system by the LPCVD technique. The films were characterized by X-ray photoelectron spectroscopy, Auger depth profile, Fourier transform infrared spectroscopy, elastic recoil detection, and atomic force microscopy, respectively. Carbide-containing a-SiNx films were obtained at lower NH3/TDEACS ratios while all deposits were essentially stoichiometric at higher NH3/TDEACS ratios. Both carbon and hydrogen contents of the as-prepared a-SiNx films were markedly lower than of those prepared from other organic precursors previously reported. The surface topography of the as-prepared film was smooth and uniform with a root-mean-square roughness of 0.53 nm.
Materials Letters - MATER LETT. 01/2005; 59(1):11-14.
[show abstract][hide abstract] ABSTRACT: Yttrium aluminum garnet (Y3Al5O12, YAG) powders were synthesized by a sol–gel combustion process from a mixed solution containing aluminum and yttrium nitrates. The thermal behavior of the precursor was determined by DSC–TG–MS techniques. FTIR and XRD techniques were used to characterize the processed precursor. It was found that excess citric acid and carboxylate mixture decomposed at about 400 °C to form carbonates. The YAG phase was supposed to nucleate directly from the amorphous precursor at about 800 °C. Mono-phase YAG crystallites could be formed without the formation of any intermediate phase at 900 °C.