A. Presz

Morphology and luminescence properties of zinc oxide nanopowders doped with aluminum ions obtained by hydrothermal and vapor condensation methods

In this paper, we analyze the influence of Al doping on microstructure and light emission efficiency of ZnO nanoparticles obtained by the hydrothermal method and after vapor condensation method, where vaporization of the precursor powders was caused by high solar energy and subsequent deposition. We report an increase of lattice parameters with increasing level of doping as well as large emission enhancement, which we relate to surface passivation of recombination mechanisms and/or plasmon mechanism of photoluminescence stimulation.

Size-dependent density of zirconia nanoparticles

Summary The correlation between density and specific surface area of ZrO2 nanoparticles (NPs) was studied. The NPs were produced using a hydrothermal process involving microwave heating. The material was annealed at 1100 °C which resulted in an increase in the average grain size of the ZrO2 NPs from 11 to 78 nm and a decrease in the specific surface area from 97 to 15 m2/g. At the same time, the density increased from 5.22 g/m3 to 5.87 g/m3. This effect was interpreted to be the result of the presence of a hydroxide monolayer on the NP surface. A smaller ZrO2 grain size was correlated with a larger contribution of the low density surface layer to the average density. To prove the existence of such a layer, the material was synthesized using 50% heavy water. Fourier transform infrared spectroscopy (FTIR) permitted the identification of the –OD groups created during synthesis. It was found that the –OD groups persisted on the ZrO2 surface even after annealing at 1100 °C. This hydroxide layer is responsible for the decrease in the average density of the NPs as their size decreases. This study of the correlation between particle size and density may be used to assess the quality of the NPs. In most cases, the technological aim is to avoid an amorphous layer and to obtain fully crystalline nanoparticles with the highest density possible. However, due to the effect of the surface layers, there is a maximum density which can be achieved for a given average NP diameter. The effect of the surface layer on the NP density becomes particularly evident for NPs smaller than 50 nm, and thus, the density of nanoparticles is size dependent.

Structure, Morphology and Luminescence Properties of Pr-Doped Nanocrystalline ZrO2 Obtained by Hydrothermal Method

The synthesis, physical characterisation and photoluminescence of n an crystalline ZrO2:Pr 3+ powders are presented. ZrO 2 powders with crystallite sizes in the range 5-12 nm were prepared by a hydrothermal technique. A high-pressure microwave r e cto was used to produce the system zirconia-praseodymium (Pr content 0.05-2% mol). The emission s pectra and luminescence decays were recorded. The concentration effect on Pr 3+ emission quenching was investigated. The size effect on emission properties of Pr :ZrO2 nanocrystallites is discussed. Introduction In recent years, nanocrystalline materials have attracted much interest because of their unique physical properties, such as the superior plasticity of nanoceram ics [1] or enhanced optical properties [2,3] . There are many techniques for producing nanometric oxide material s such as solgel [4], hydrothermal [5], co-precipitation [6], combustion [7] or gas -phase condensation methods [8]. One of the novel techniques for preparing nanopowders with a narrow distr ibut on of grain-size is a microwave (MW) hydrothermal method [9]. At room temperature pure ZrO 2 has a monoclinic structure. Most physical properties of doped zirconia are related to the presence of oxygen vacancies assoc iated with the charge compensation of dopants. Stabilised by foreign ions, usually trivalent yttrium ions, c ubic phase zirconia is an important anion-conducting ceramic. However, the tetragonal phase ha s been found at room temperature when grains of pure ZrO 2 are of nanometer size [10]. The influence of grain-size on the structure of zirconia has been reported by many authors for diffe ent preparation methods [11-13]. It is also well known that ZrO 2 is an excellent host material for Tb 3+ [14] and Eu [15,16] ions for green and red luminophors, respectively. Recently, we have describe d the photoluminescent of Tbdoped Al 2O3-ZrO2 sol-gel derived nanostructural material as well as its cat hodoluminescence characteristics [17]. In this work, the size-effect on the lumine scence properties of nanocrystalline ZrO2, with different contents of Pr 3+ ions prepared by the MW hydrothermal method, was reported. Experimental The synthesised powders were analysed with a computer-assiste d X-ray (CuKα) powder diffractometer (Siemens D 5000). X-ray diffraction patterns were coll cted in a 2 θ range of 20-90 ° at room temperature, with a scanning rate of 0.005 °/s and step size of 0.02 °. The analysis of the powder’s surface area was made by the BET method (Gemini 2360) usi ng nitrogen as an adsorbate. Photoluminescence spectra were measured at room temperature wit h a OceanOptics Spectrometer SD2000 with a resolution of 0.3 nm. Emission decays were measured usi ng Jobin-Yvon TRW 1000 spectrophotometer and a photomultiplier (Hamamatsu R928) by means of a Tektronics TDS 380 oscilloscope. The 308 nm line of excimer laser (LPX100)was used as the excitat ion source. Solid State Phenomena Online: 2003-06-20 ISSN: 1662-9779, Vol. 94, pp 141-144 doi:10.4028/www.scientific.net/SSP.94.141

Synthesis of Nano-sized Yttrium-Aluminum Garnet in a Continuous-Flow Reactor in Supercritical Fluids

The conditions for obtaining pure yttrium-aluminum garnet (YAG) in a one-step process starting from components soluble in supercritical (SCR) liquids in a flow reactor have been studied. The powders were characterized using XRD, BET, SEM and helium pycnometry. Favorable conditions for YAG production were achieved using acetates (and acetylacetonates) in aqueous-alcoholic solutions as starting materials. Aqueous and alcoholic solutions of nitrates and aqueous solutions of acetates were found not appropriate as precursors for YAG production. The powders obtained had the shape of nano-sized cubes with diameters in the range 80 - 120 nm. After annealing at 600 °C the powders have comparable density as those made via a conventional precipitation-calcination route but differ from the calcinated products form soft agglomerates.

The Luminescence Properties of ZnO nanopowders

Pure and Al3+ doped ZnO nanopowders were studied by means of time-resolved luminescence spectroscopy. The powders were synthesized by hydrothermal and plasma methods. These powders were used as a raw material for vaporization-condensation process inside the Solar reactor. The commercially available ZnO nanopowder was studied for a comparison. Exciton to defect band luminescence intensity ratio was estimated in different types of ZnO nanopowders. It was found that nanopowders with whiskers morphology show superlinear luminescence intensity depending on excitation density. The observed effect depends on the average nanoparticle size and on the powder morphology.

Investigation of the Microstructure of SiC-Zn Nanocomposites by Microscopic Methods: SEM, AFM and TEM

SiC-Zn nanocomposites with about 20% volume fraction of metal were fabricated by infiltration process under the pressure of 2-8 GPa and at the temperature of 400_1000oC. SiC nanopowders used in the experiments formed loosely agglomerated chains of single crystal nanoparticles. The dimension of the agglomerates was in the micrometer range, the mean grain size was up to tens of nanometers. Microstructural investigations of the nanocomposites were performed at a different resolution levels using scanning, transmission electron microscopy and atomic force microscopy techniques (SEM, TEM, AFM, respectively). SEM observations indicate a presence of nano-dispersed, uniform (on the micrometer scale) mixture of two phases. TEM observations show that distribution of SiC and Zn nanocrystallites is uniform on the nanometer scale. High-resolution TEM images demonstrate an existence of thin (on the order of tens of Angstroms) Zn layers separating SiC grains. AFM images of the mechanically polished samples show a smooth surface with the roughness on the order of the SiC grain size (10-30 nm). After ion etching of some samples the AFM topographs show surface irregularities: periodically spaced hillocks 50-100 nm in height. The size of the SiC grains remains equal to that of the initial powder crystallites. The size of the Zn grains varies in the range of 20-100 nm depending on the initial SiC grain size and the composite fabrication conditions.

Study of Grain Size Distribution in Nanocrystalline Iron Oxides Synthesized by Hydrothermal Method

A novel microwave reactor was used to hydrothermally synthesiz e nanopowders of iron oxides under a pressure of up to 10 MPa and temperatures of up to 250 C. At reactions time as short as 5 minutes it was possible to synthesize powders with gr a n size in the range 10 – 100 nm. Proper selections of the substrates and their concentration enabled the grain size and phase composition to be controlled to some extent. Pure hematite, maghemi te and magnetite were obtained. The grain size and distribution were determined by image analysis of t he p wders. Introduction Small particles of iron oxides with a narrow size distribution ha ve ttracted attention due to their potential applications as magnetic pigments, high density magnetic r ecording media, ferrofluids, in catalysts, and in medicine [1,2,3]. The properties of the iron oxides depend strongly on shape, grain size and phase composition [4]. Therefore it is important to de velop various methods of synthesis in which the microstructure of the powders can be control led. The present study aimed at the possible use of a microwave reactor for hydrothermal synthe sis of nanopowders. The objective was to control the size distribution and phase composition by appropriate adjustment of the process procedure. Nano-crystalline iron oxides were synthesized by the decomposition of ferric a nd ferrous salts in aqueous solutions and precipitation under microwave irradiation [3] . Experimental Iron oxide nano-powders were prepared from ferric chloride (FeCl 3 6H2O), ferrous sulphate (FeSO4 7H2O) and urea (CO(NH 2)2) solution. All experiments were carried out in microwave reactor (2.45 GHz model UniClever Plazmatronica Corp.) in which the power, pressure and synthesis time could be controlled. The pressure during reaction was set at 45 atmospheres. Synthesis of the iron oxide was carried out for different concent rations of Fe and Fe ions (Table 1). When the reaction was completed the powder and solution were filte d and washed with distilled water. Dry powders were characterized by X-Ray di ffraction (XRD) using a SIMENS D5000 diffractometer operating with CuK α radiation, a LEO 1530 Scanning Electron Microscopy, IDEAS VSM Vibrating Sample Magnetometer. The surface area m surements were conducted by the BET method using a Gemini 2360, Micromeritics. Three methods were used to evaluate the grain size. The nitroge adsorption method (BET) provides the powder surface area S BET. Assuming that the particles are spherical (Equ. 1) the grain diameter dBET is obtained from simple geometrical consideration. However, it is pos ible to obtain the characteristic dimension of the powders without any assumption of their sha pe. Equation 2 gives the expression for the average chord <l> [5]. Based on analysis o f the SEM image and proper computer codes, it is possible to determine particles size distr ibution and calculate their average diameter as well as the coefficient of variation (Cv), which show s the degree of non-uniformity in Solid State Phenomena Online: 2003-06-20 ISSN: 1662-9779, Vol. 94, pp 239-244 doi:10.4028/www.scientific.net/SSP.94.239