Effect of Pressure on Synthesis of Pr-Doped Zirconia Powders Produced by Microwave-Driven Hydrothermal Reaction

A high-pressure microwave reactor was used to study the hydrothermal synthesis of zirconia powders doped with 1 mol % Pr. The synthesis was performed in the pressure range from 2 to 8 MPa corresponding to a temperature range from 215C∘ to 305C∘. This technology permits a synthesis of nanopowders in short time not limited by thermal inertia of the vessel. Microwave heating permits to avoid contact of the reactants with heating elements, and is thus particularly well suited for synthesis of doped nanopowders in high purity conditions. A mixture of ZrO2 particles with tetragonal and monoclinic crystalline phases, about 15 nm in size, was obtained. The p/T threshold of about 5-6 MPa/265–280C∘ was necessary to obtain good quality of zirconia powder. A new method for quantitative description of grain-size distribution was applied, which is based on analysis of the fine structure of the X-ray diffraction line profiles. It permitted to follow separately the effect of synthesis conditions on the grain-size distribution of the monoclinic and tetragonal phases

Effect of pressure on synthesis of Pr-doped zirconia powders produced by microwave-driven hydrothermal reaction

A high-pressure microwave reactor was used to study the hydrothermal synthesis of zirconia powders doped with 1 mol % Pr.The synthesis was performed in the pressure range from 2 to 8MPa corresponding to a temperature range from 215◦C to 305◦C.This technology permits a synthesis of nanopowders in short time not limited by thermal inertia of the vessel. Microwave heatingpermits to avoid contact of the reactants with heating elements, and is thus particularly well suited for synthesis of dopednanopowders in high purity conditions. A mixture of ZrO2 particles with tetragonal and monoclinic crystalline phases, about15nm in size, was obtained. The p/T threshold of about 5-6MPa/265–280◦C was necessary to obtain good quality of zirconiapowder. A new method for quantitative description of grain-size distribution was applied, which is based on analysis of the finestructure of the X-ray diffraction line profiles. It permitted to follow separately the effect of synthesis conditions on the grain-size distribution of the monoclinic and tetragonal phases

Microwave technique applied to the hydrothermal synthesis and sintering of calcia stabilized zirconia nanoparticles

This study is focused on the synthesis of zirconia nanopowders stabilized by 6%mol calcia prepared under hydrothermal conditions using microwave technology. Sodium hydroxide-based hydrolysis of zirconyl chloride solution containing calcium nitrate followed by microwave irradiation at the temperature of 220 \ub0C for 30 min was sufficient to obtain white powders of crystalline calcia stabilized zirconia. By means of X-ray diffraction and transmission electron microscopy, it was shown that tetragonal zirconia nanocrystallites with a size of ca 7 nm and diameter/standard deviation ratio of 0.10 were formed. The effects of the [Ca2+] and [NaOH] as well as temperature and time of microwave irradiation on the density and specific surface area were evaluated. Sintering test of the tetragonal nanopowders at 1,300 \ub0C in air was performed in a monomode microwave applicator. The sample was sintered to the density of 95% and the grain size, analyzed by field emission scanning electron microscopy, was in the range from 90 to 170 nm. \ua9 2009 Springer Science+Business Media B.V

Grain size and grain size distibution of zro<>2<>:Pr ceramics nanopowders determined by different methods

A range of characterization methods were used to measure size and distribution of sizes of the ZrO2 nanocrystals synthesized hydrothermally in a microwave heated high pressure reactor [1-4]. The aim of the work is to compare size readings of the same ceramics nanopowders as reported by different characterization methods. We describe briefly capabilities of TEM, XRD and BET characterization techniques, such as size vs. size distribution output, crystalline phase resolution, ability of error estimation of the size as well as dispersion of sizes readings (error bars: yes/no)

High-pressure high-temperature in situ diffraction studies of nanocrystalline ceramic materials at HASYLAB

High-pressure in situ diffraction studies were performed up to 8 GPa in a cubic anvil cell MAX80 (Station F2.1) and up to 45 GPa in a Diamond Anvil Cell (DAC-Station F3 at HASYLAB, Hamburg). A series of nanocrystals of SiC with grain sizes ranging from 2 nm to several μm were examined in non-hydrostatic conditions by pressing pure powders. A new method of evaluation of powder diffraction data measured at high pressures is presented. This method is based on quantitative evaluation of asymmetry of Bragg reflections where each peak is described as a combination of two reflections of two similar crystallographic phases having different compressibilities. The measured changes of the lattice parameters calculated for split Bragg reflections were used for determination of the pressure gradient which occurs across the grain boundaries in the compressed materials. A model of the strain induced in compacts of pure powders under high pressures is proposed. The model accounts for the presence of two phases: a volume phase corresponds to cores of individual grains which are surrounded by a surface phase which is formed of free surfaces in loose powders and of grain boundaries in solids. Due to extreme hardening of the boundaries under non-hydrostatic pressure conditions, the effective pressure in the interior of the grains is much lower than the applied external pressure. It is suggested that additional ‘hardening’ of the grain boundaries results from the presence of dislocations which are generated at the surface of the grains. The actual gradient of the pressure depends on the size of the grains, and also on the method of synthesis of the materials

Generation and Relaxation of Microstrains in GaN Nanocrystals under Extreme Pressures

Nanocrystalline powders of GaN with grain sizes ranging from 2 to 30 nm were examined under high external pressures by in situ diffraction techniques in a diamond anvil cell at DESY (HASYLAB, Station F3). The experiments on densification of pure powders under high pressure were performed without a pressure medium. The mechanism of generation and relaxation of internal strains and their distribution in nanoparticles was deduced from the Bragg reflections recorded in situ under high pressures at room temperature. The microstrain was calculated from the full-width at half-maximum (FWHM) values of the Bragg lines. It was found that microstrains in GaN crystallites are generated and subsequently relaxed by two mechanisms: generation of stacking faults and change of the size and shape of the grains occurring under external stress

Generation and Relaxation of Microstrains in GaN Nanocrystals under Extreme Pressures

Nanocrystalline powders of GaN with grain sizes ranging from 2 to 30 nm were examined under high external pressures by in situ diffraction techniques in a diamond anvil cell at DESY (HASYLAB, Station F3). The experiments on densification of pure powders under high pressure were performed without a pressure medium. The mechanism of generation and relaxation of internal strains and their distribution in nanoparticles was deduced from the Bragg reflections recorded in situ under high pressures at room temperature. The microstrain was calculated from the full-width at half-maximum (FWHM) values of the Bragg lines. It was found that microstrains in GaN crystallites are generated and subsequently relaxed by two mechanisms: generation of stacking faults and change of the size and shape of the grains occurring under external stress

Diffraction Studies of Nanocrystals: Theory and Experiment

Based on theoretical calculations of powder diffraction data it is shown that the assumption of the infinite crystal lattice for small particles is not justified, leads to significant changes of the diffraction patterns, and may lead to erroneous interpretation of the experimental results. An alternate evaluation of diffraction data of nanoparticles, based on the so-called "apparent lattice parameter", alp, is proposed

Diffraction Studies of Nanocrystals: Theory and Experiment

Based on theoretical calculations of powder diffraction data it is shown that the assumption of the infinite crystal lattice for small particles is not justified, leads to significant changes of the diffraction patterns, and may lead to erroneous interpretation of the experimental results. An alternate evaluation of diffraction data of nanoparticles, based on the so-called "apparent lattice parameter", alp, is proposed