Solid State Phenomena Vol. 128

Abstract: Nano-crystalline YAG:Ce (Y3Al5O12:Ce) with an average particle size of 10-50 nm was synthesized by the vapour phase reaction (CVR) in a hot-wall reactor. This nano-crystalline luminescent material has the advantage of a high light-conversion efficiency associated with superior light-scattering properties. Phases and compositions were analyzed by XRD and EDX, respectively. Photoluminescence was used to investigate the optical properties of the YAG host lattice doped with Cerium. As a first step the conditions for the in-situ production of the YAG phase were established. On the basis of previous DSC investigations of YAG phase formation, the DSC results were analyzed according to the Kissinger theory for thermally activated transformations. From the results it is predicted that, with sufficient heat transfer during the residence time of the particles in the hot zone of the reactor, YAG can be obtained in-situ during the CVR process. By modifying the parameters and the CVR setup it was possible to achieve conditions, which allow the production of in-situ YAG. Samples with Ce doping concentrations ranging from 0 to 2.65 at.% Ce were prepared to investigate the influence of the doping concentration on the luminescence of YAG:Ce.

Abstract: In this work the laboratory–scale installation for synthesising powder materials with nano-size grains is presented. Nano-sized zinc and tin oxides were obtained using the spraypyrolysis (SP) method. The zinc oxide was derived from zinc acetate at 300oC, 430oC, 580 oC, 800oC and 950oC. The tin oxide was processed by the thermal decomposition of tin tartrate at 500oC. The SP synthesized zinc oxide was used for manufacturing a methane sensor which was characterized as possessing a much wider range of operating and substantially higher sensitivity, 10 times higher, to methane gas compared to a sensor made out of commercially available zinc oxide.

Abstract: In this work attempts to prepare strontium-doped lanthanum manganites La1-xSrxMnO3 using microwave-assisted hydrothermal synthesis were undertaken from a mixture of lanthanum nitrate, strontium nitrate, manganese(II) nitrate, potassium permanganate and potassium hydroxide as a mineralizer. For x = 0.3, and x = 0.5, the perovskite obtained is not defined since both La0.7Sr0.3MnO3 and La0.5Sr0.5MnO3 phases are consistent with XRD spectra. While with x = 1.0, for the first time, hexagonal strontium manganite was prepared as blade-shaped crystallites with a narrow particle length distribution (range 3.75-7.75 μm) at 210°C using a treatment time of only 1 hour. Conventional hydrothermal synthetic routes require at least 24 hrs treatment time.

Abstract: Nanocomposites (nanocrystals) of KY(WO4)2 and KY(WO4)2+1% mol Yb were synthesized using a Complex Sol-Gel Process (CSGP). A chemical treatment with concentrated nitric acid and hydrogen peroxide was used to reduce the decarbonisation temperature. The expected monoclinic phase C2/c of the KYW of the nanocomposite powder was confirmed using XDR. From the X-ray diffraction measurements, the unit cell parameters and the size of nanoparticles was determined. Electron spin resonance studies in the X band were carried out on KYW and KYW:Yb nanocrystals. The sintered samples were made with using the high pressure technique at temperatures up to 600oC. In addition chemical analysis, X-ray diffraction measurements and ESR investigations were carried out on the sintered samples.

Abstract: Yb3+ doped SnO2 nanocrystals were synthesized, using the hydrolytic route in the presence of starch as the size stabilizer. Starting from salt precursors, the nanopowders obtained with various Yb3+ loads are thermally stable up to 600 °C. They were characterized by XRD, solid state NMR, TEM, ICP and TG-MS techniques. The nanocrystal Cassiterite structure, with a size of a few nanometers (<6 nm), was identified by XRD, NMR and Raman measurements confirming a homogeneous dispersion of Yb3+ ions in the lattice. The preliminary assessment of the spectroscopic features of nanocrystals was performed by absorption and photoluminescence spectroscopy. The typical Yb3+ absorption peak, centred at 977 nm, and an intense 2F5/2→ 2F7/2 Yb3+ emission band were observed.

Abstract: The agglomeration of YAG (Yttrium-Aluminum Garnet Y3Al5O12) nanopowders doped with various rare earth ions obtained by the coprecipitation and calcination route is a major problem if it is wished to exploit the nano-size properties such as the transparency of dispersions of the powders or low temperature sintering. Investigations to optimize the preparation process of powders for High Pressure-Low temperature Sintering (HPLS) of semi-translucent pellets was undertaken. The appropriate milling time to decrease the agglomeration was evaluated with the help of ZETA- potential measurement. The dependence of the agglomerate size against the time of hand milling was used to optimize the process.

Abstract: In this work oxidized and oxide-free amorphous boron (a-B) powder and elemental Mg were used in an attempt to directly synthesize the Mg(BH4)2 complex hydride by controlled reactive mechanical alloying (CRMA) under hydrogen in a magneto-mill up to 200h. The particle size was refined to the 100-200nm range. Nanocrystalline MgH2 (~6nm crystallite size) was formed within the particles when an oxidized a-B is used. In contrast, a mixture of MgB2 and an amorphous hydride MgHx was formed when an oxide-free a-B was used. Differential scanning calorimetry (DSC) test up to 500°C produced a single endothermic heat event at 357.7°C due to hydrogen desorption. In addition, desorption conducted in a Sieverts-type apparatus revealed ~1.4wt.% of hydrogen release. The X-ray diffraction pattern after DSC test of the 200h milled sample made with oxide-free boron showed the presence of MgB2.

Abstract: Manganese-doped ZnS nanophosphors were synthesized by two different methods: hydrothermal and low-temperature precipitation methods. The nanophosphors prepared by the hydrothermal precipitation and low-temperature methods had the size of 20 nm and 4 nm, respectively, and both were the cubic phase. The emission intensity of the nanophosphor prepared by the hydrothermal method was optimized at 10 mole % of Mn2+ concentration whereas that by low-temperature precipitation method was optimized at 3 mole % of Mn2+. The precipitationprepared nanophosphor, of size 4nm, showed a blueshift in the excitation spectrum and a redshift in the emission spectrum compared to the hydrothermal-prepared 20 nm nanophosphor. These phenomena can be explained in terms of the quantum confinement effect. The decay times especially were lengthened with decreasing particle size. This can be explained in terms of the variation in the transition probability induced from the quantum confinement effect.

Abstract: The synthesis behaviour and characterisation of nanocrystalline materials is presented. The materials synthesised are ZnO and InP doped with shallow donors and acceptors, respectively. Characterisation was performed with radioactive isotopes using the perturbed γγ angular correlation technique (PAC), thereby yielding local information on an atomic scale. The characterisation was supplemented by X-ray diffraction, transmission electron microscopy, UV/VIS absorption spectroscopy, photoluminescence spectroscopy, and extended X-ray absorption fine structure spectroscopy. It was shown that the successful incorporation of dopants in nanocrystalline ZnO and InP requires annealing at temperatures at which the growth of the nanocrystals in the sample becomes a significant process.