Amorphous Nanopowders Research Articles

Synthesis and sintering of YAG:Eu nanopowder

Abstract YAG:Eu nanopowder was synthesized through a sol–gel method. A master sintering curve was used as a practical approach to analyze the sintering behavior of the synthesized powder. The effect of MgO doping on sintering of the synthesized nanopowders was evaluated. An amorphous nanopowder was synthesized and crystallized to YAG after heat-treatment via a solid-state reaction. MgO improved the sintering rate of the YAG nanopowders and suppressed grain boundary mobility. The activation energy for sintering decreased from 917 to 837 kJ/mol by adding MgO to the nanopowders. The results of this study can be used to predict the densification of YAG:Eu nanopowder.

Production of ZnO and Zn-ZnO nanopowders using evaporation by a pulsed electron beam in low-pressure gas

This paper describes a method for the production of ZnO and Zn-ZnO nanopowders (NPs) using the evaporation of a target by a pulsed electron beam in a low-pressure gas and condensation on a cold crystallizer. Crystalline and amorphous NPs of pure ZnO and a Zn-ZnO mixture have been synthesized. These powders have a specific surface of up to 40 m2/g and are produced at a rate as high as 5 g/hour. It has been found that the pressure and the composition of gases (air, oxygen, and argon) in the evaporation chamber of the installation influenced the composition and the morphology of the powders. The effect that the target composition has on the production rate and characteristics of the powders has been studied. All the synthesized powders possessed marked magnetic properties.

Compressibility and sinterability of Al 2O 3–YAG nanocomposite powder synthesized by an aqueous sol–gel method

In the present work, the compressibility and sinterability of Al 2O 3–YAG nanocomposite powder prepared by an aqueous sol–gel method have been investigated. An amorphous nanopowder was synthesized, and it was crystallized to Al 2O 3–YAG, with an average particle crystallite size of 50 nm, after heat-treatment via a solid-state reaction. It was shown that the compressibility curve of this powder is characterized by two linear parts at low and middle pressures. The agglomeration strength of the powder was about 142 MPa. The densification of composites started at 1000 °C. The relative density and grain size of sintered Al 2O 3–0.75 mol Y 2O 3 were measured to be 96% and 650 nm at 1500 °C, respectively. TEM results showed a sub-micronic/nano microstructure is formed. The amount of Y 2O 3 had a strong effect on the sintering behavior and final microstructure of nanocomposite powders and it inhibited alumina grain growth and suppressed densification process.

Effect of cobalt oxide on the sintering and grain growth of Al2O3-YAG composite nanopowder

In the present study, the effect of cobalt oxide on the densification of Al2O3-YAG composite nanopowder was investigated. An amorphous nanopowder was synthesized and crystallized to Al2O3-YAG after heat-treatment via a solid-state reaction. The average particle size of heat-treated powder at 800?C was about 80 nm. Cobalt oxide improved the sintering rate of Al2O3-YAG composite nanopowders and promoted grain boundary mobility. Cobalt oxide doping increased the ratio of densification/grain growth rate. The activation energy for grain growth decreased from 590 to 485 kJ/mol by adding cobalt oxide to the composite nanopowder.

Preparation of Silver Nanopowder by Freeze-Drying Procedure

A freeze-drying procedure is developed for manufacturing silver nanopowder with regular shape and uniform diameter. The procedure includes four steps: preparing the precursor solution, freezing the solution, drying the congelation, and calcining the drying product. The starting substances are silver nitrate and caustic soda and the precursor is diammine silver solution. Three freezing styles including direct freezing, vacuum evaporation freezing, and spray freezing are adopted to freeze the precursor solution. In the drying stage, two heating styles for the frozen solution are compared. X-ray diffraction (XRD) and energy disperse spectroscopy (EDS) spectrograms show that the products of freeze drying are pure amorphous silver nanopowder without hard agglomerations. Silver cubic nanocrystals are subsequently obtained by calcination. The chemical fundamentals and the drying curves of freeze drying are given. The processes of preparing silver nanopowder are described in detail. The effect of freezing and heating on characteristics of the powder are discussed. The mechanism of grain forming and growing up is analyzed according to the observation results. The characteristics of powder freeze drying and its feasibility for volume production are presented.

Preparation of bioactive glass ceramic nanoparticles by combination of sol–gel and coprecipitation method

SiO2-CaO-P2O5 ternary bioactive glass ceramic nanoparticles were prepared via the combination of sol-gel and coprecipitation processes. Precursors of silicon and calcium were hydrolyzed in acidic solution and gelated in alkaline condition together with ammonium dibasic phosphate. Gel particles were separated by centrifugation, followed by freeze drying, and calcination procedure to obtain the bioactive glass ceramic nanoparticles. The investigation of the influence of synthesis temperature on the nanopartilce's properties showed that the reaction temperature played an important role in the crystallinity of nanoparticle. The glass ceramic particles synthesized at 55 degrees C included about 15% crystalline phase, while at 25 degrees C and 40 degrees C the entire amorphous nanopowder could be obtained.

Preparation of nanodisperse solid solutions based on ZrO2 and HfO2 from hydroperoxides

A procedure for preparing solid solutions based on zirconium and hafnium hydroperoxides is developed. This procedure is based on the coprecipitation of components in the presence of hydrogen peroxide. Amorphous nanopowders (S sp = 80–90 m2/g, d mean = 2–3 nm) without any indication of the agglomeration of particles are synthesized. After the decomposition of complex hydroperoxides at a temperature of 500°C, the compacted powders undergo an active sintering at a temperature of 1450°C. The particle size after heat treatment does not exceed 50 nm. As follows from the electrical and physical characteristics obtained for these materials, they can be recommended for the use as solid electrolytes and sensors of oxygen partial pressure.

Catalytic Efficiency of Iron(III) Oxides in Decomposition of Hydrogen Peroxide: Competition between the Surface Area and Crystallinity of Nanoparticles

Various iron(III) oxide catalysts were prepared by controlled decomposition of a narrow layer (ca. 1 mm) of iron(II) oxalate dihydrate, FeC(2)O(4).2H(2)O, in air at the minimum conversion temperature of 175 degrees C. This thermally induced solid-state process allows for simple synthesis of amorphous Fe(2)O(3) nanoparticles and their controlled one-step crystallization to hematite (alpha-Fe(2)O(3)). Thus, nanopowders differing in surface area and particle crystallinity can be produced depending on the reaction time. The phase composition of iron(III) oxides was monitored by XRD and (57)Fe Mössbauer spectroscopy including in-field measurements, providing information on the relative contents of amorphous and crystalline phases. The gradual changes in particle size and surface area accompanying crystallization were evaluated by HRTEM and BET analysis, respectively. The catalytic efficiency of the synthesized nanoparticles was tested by tracking the decomposition of hydrogen peroxide. The obtained kinetic data gave an unconventional nonmonotone dependence of the rate constant on the surface area of the samples. The amorphous nanopowder with the largest surface area of 401 m(2) g(-1) revealed the lowest catalytic efficiency, while the highest efficiency was achieved with the sample having a significantly lower surface area, 337 m(2) g(-1), exhibiting a prevailing content of crystalline alpha-Fe(2)O(3) phase. The obtained rate constant, 26.4 x 10(-3) min(-1) (g/L)(-1), is currently the highest value published. The observed rare catalytic phenomenon, where the particle crystallinity prevails over the surface area effects, is discussed with respect to other processes of heterogeneous catalysis.

Glass and bioglass nanopowders by flame synthesis

The preparation of amorphous nanopowders by flame synthesis opens access to common soda-lime, metal-doped glasses or bioglasses in the range of 20-80 nm and offers an alternative to conventional wet-phase preparation, solid state reactions or melting.

Nanostructure of Er3+ doped silicates

We demonstrate nanostructural evolution resulting in highly increased photoluminescence in silicates doped with Er3+ ions. High-resolution transmission electron microscopy (HRTEM) imaging, nano-energy dispersed X-ray (NEDX) spectroscopy, X-ray diffraction (XRD) and photoluminescence analysis confirm the local composition and structure changes of the Er3+ ions upon thermal annealing. We studied two types of amorphous nanopowder: the first is of the composition SiO2/18Al2O3/2Er2O3 (SAE), synthesized by combustion flame-chemical vapor condensation, and the second is with a composition of SiO2/8Y2O3/2Er2O3 (SYE), synthesized by sol-gel synthesis (composition in mol%). Electron diffraction and HRTEM imaging clearly show the formation of nanocrystallites with an average diameter of approximately 8 nm in SAE samples annealed at 1000 degrees C and SYE samples annealed at 1200 degrees C. The volume fraction of the nanocrystalline phase increased with each heat treatment, eventually leading to complete devitrification at 1400 degrees C. Further XRD and NEDX analysis indicates that the nanocrystalline phase has the pyrochlore structure with the formula Er(x)Al(2-x)Si2O7 or Er(x)Y(2-x)Si2O7 and a surrounding silica matrix.

Magnetism of amorphous Fe 2 O 3 nanopowders synthesized by solid‐state reactions

Monodispersed amorphous Fe2O3 nanoparticles with the size ranging from 1 to 4 nm have been prepared by thermally induced oxidative decomposition of Prussian Blue, Fe4[Fe(CN)6]3, in air. Amorphous nanopowders exhibit specific magnetic behaviour strongly affected by interparticle interactions and extremely low particle size with a high contribution of the surface anisotropy as illustrated by temperature dependent and external field Mossbauer spectroscopy and magnetization measurements (4–300 K). Some of the properties are in close similarity to those of spin glasses including the fast thermal variation of superparamagnetic fraction in the Mossbauer spectra, or the particle size independence of the blocking temperature, and non-saturation of magnetization even at 5 K and magnetic field of 10 T. The reduced value of internal magnetic hyperfine field (∼49 T at 5 K) and no indications of the tetrahedral Fe3+ sites in the 5T/5K external field Mossbauer spectra are the principal found differences in comparison with the magnetic behaviour of nanocrystalline γ-Fe2O3. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication and Creep Behavior of SiCN(O) Nanocomposites

A nanometric silicon oxycarbonitride powder was synthesised by CO 2 laser pyrolysis from a mixture of an aerosol of a liquid precursor (hexamethyldisilazane, HMDS), a gaseous precursor (silane, SiH 4 ) and ammonia. The amorphous nanopowder and the sintering aids (6 wt% yttria and 3 wt% alumina) were mixed together in ethanol and de-agglomerated by milling with Si 3 N 4 balls. The mixture was then dried and sintered to nearly full density by hot-pressing. The nature and amount of the crystalline phases in the as-sintered materials appeared to be very sensitive to the drying stage. When drying was performed in a confined atmosphere, a noticeable amount of Si 2 N 2 O was detected in addition to β-Si 3 N 4 , whereas the crystalline phases were α- and β-Si 3 N 4 , when the slip was dried under vacuum. The creep behaviour of the two grades was assessed under compressive loading in air. The creep deformation of the Si 2 N 2 O containing material was about twice that of the other material under the same test conditions.

Amorphous calcium phosphate nano-powders made from radio frequency (RF) plasma spraying

AbstractPredominantly amorphous nano powders of calcium phosphate were produced by radio frequency (RF) plasma and subsequently characterised by SEM, TEM and XRD. Quantitative phase analysis was carried out by the Rietveld method. The starting feedstock was comprised of hydroxyapatite (HA) powders spray dried from a suspension. The input parameters were varied to produce predominantly amorphous calcium phosphate powders. Results showed that as the plate power was increased and the flow rate of feedstock was decreased the as-sprayed powders produced were predominantly amorphous. The average particle size of the powders was about 20-40 nm, depending on process parameters, as confirmed by TEM observation. These powders are thought to have excellent bio-resorbability and are targetted for use in gene and drug delivery technologies. These nano-sized powders are also envisioned to enhance the mechanical properties of bulk products sintered by appropriate techniques.

Sol-Gel Synthesis of Complex Titanates in Micro-Emulsions

Amorphous precursor nanopowders of zirconolite (CaZrTi 2 O 7 ) were prepared using a water-in-oil micro-emulsion synthesis, with Ti(O i C 3 H 7 ) 4 , Zr(O n C 3 H 7 ) 4 and either Ca(OC 2 H 5 ) 2 or an aqueous Ca(NO 3 ) 2 solution as precursors. The stoichiometry and structural evolution of the nanoparticles were investigated using a range of techniques, including ICP-MS, TEM, gas adsorption and vibrational spectroscopy. The use of Ca(OC 2 H 5 ) 2 led to slightly sub-stoichiometric nanopowders (Ca x ZrTi 2 O 6+x , x = 0.8), following washing. Lower values of x were obtained in the washed nanopowders when using Ca(NO 3 ) 2 , with x = 0.004, 0.022 or 0.52 at pH 2, 7 or 13, respectively. In the latter system, a comparison between the washed and unwashed nanopowders revealed that 25% of the calcium was lost during washing. The Ca(NO 3 ) 2 was found to form unstable tetraamine complexes within the reverse micelles at high pH, which transformed into Ca(OH) 2 on ageing. The Ca(OH) 2 can then interact with the hydrolysed titanium/zirconium alkoxides, and thus be integrated chemically into the final nanoparticle structure.

Properties of PZT Nano-Powder Doped Silica Films Prepared by Sol-Gel Process

Silica films doped with amorphous PZT nanopowder were prepared by a modified TEOS sol-gel process. The films were annealed by conventional furnace or rapid thermal annealing methods at 700 ¯ respectively, and were examined by field emission scanning electron microscopy and spectro-ellipsometer. Most of the PZT particles were detached from the film after annealing. To avoid this phenomenon, a hydrothermal treatment process was utilized to crystalize the sol-gel derived amorphous PZT nanopowders before they were added into the silica solution. Silica films doped with the crystallized PZT powder were then annealed and revealed good embedded adhesion, typical perovskite phase, low extinction coefficient, and obvious refractive index change.

Properties of nanocrystalline ferroelectric PZT ceramics

Abstract Amorphous nanopowders of Pb(Zr0.5Ti0.5)O3 (PZT) solid solution were obtained by sol-gel processing. As-obtained nanopowders ( r ≈30×10−9 m) underwent consolidation by conventional ceramic sintering, hot pressing, and rapid thermal annealing. As result of such technological methods PZT ferroelectric ceramic samples were obtained. They exhibited lack of voids, density close to the theoretical X-ray density, homogeneity from both chemical and physical point of view and stoichiometric chemical composition. It was found that each ceramic sample was a conglomerate of grains exhibiting mean dimension r =0.3–3×10−6 m depending on sintering conditions. The nanocrystalline structure of the ceramics grains has been revealed. Dimension of the crystallites ( D =20–75×10−9 m) were found to depend on temperature (TS) and the rate of sintering (νT) of the amorphous nanopowders. Dielectric and piezoelectric properties of nanocrystalline PZT ceramics depended on both sol–gel processing conditions and sintering conditions.

Early stages of the pyrolytic crystallization in amorphous nanopowders of silicon carbonitrides SixCy,Nzby combined wide-angle X-ray and neutron diffractometries

Abstract The transition from the amorphous to the crystalline state has been followed by coupled X-ray and neutron wide-angle diffraction in silicon carbonitrides powders prepared by laser pyrolysis of hexamethyldisilazane. The short-range order (SRO) and the medium-range order (MRO) modifications under heating have been observed using the drastic contrast effect of the scattering factors (or lengths) of N and Si for X-ray and neutrons respectively. In the amorphous state, the fit by models of the first neighbours peak (SRO) of the radial distribution function reveals the existence of C-N bonds in addition to the Si-C and Si-N bonds existing in mixed tetrahedra Si[C1−x N x ]4 with 0>x>1. Extended annealing for 24h at 1400°C modifies the MRO through a solid-state diffusion process increasing the chemical composition fluctuations in Si x C y N z puckered layers, by changing their stacking mode from the α to the β sequences encountered in the Si3N4 polytypes. The destruction of the mixed tetrahedra yields nanocrystals of SiC and Si3N4 at 1550°C. The amorphous-crystalline transition is simulated by molecular dynamic calculations. The starting models of 200–400 atoms are the crystalline α and β-Si3N4 poly types in which C atoms are substituted for certain Si and N atoms. The comparison with experiments confirm the anisotropic layer-like character of the amorphous state and the α → β transition at 1400°C.

Early stages of the pyrolytic crystallization in amorphous nanopowders of silicon carbonitrides SixCyNz by combined wide-angle X-ray and neutron diffractometries

Abstract The transition from the amorphous to the crystalline state has been followed by coupled X-ray and neutron wide-angle diffraction in silicon carbonitrides powders prepared by laser pyrolysis of hexamethyldisilazane. The short-range order (SRO) and the medium-range order (MRO) modifications under heating have been observed using the drastic contrast effect of the scattering factors (or lengths) of N and Si for X-ray and neutrons respectively. In the amorphous state, the fit by models of the first neighbours peak (SRO) of the radial distribution function reveals the existence of C-N bonds in addition to the Si-C and Si-N bonds existing in mixed tetrahedra Si[C1−x N x ]4 with 0>x>1. Extended annealing for 24h at 1400°C modifies the MRO through a solid-state diffusion process increasing the chemical composition fluctuations in Si x C y N z puckered layers, by changing their stacking mode from the α to the β sequences encountered in the Si3N4 polytypes. The destruction of the mixed tetrahedra yields nanocrystals of SiC and Si3N4 at 1550°C. The amorphous-crystalline transition is simulated by molecular dynamic calculations. The starting models of 200–400 atoms are the crystalline α and β-Si3N4 poly types in which C atoms are substituted for certain Si and N atoms. The comparison with experiments confirm the anisotropic layer-like character of the amorphous state and the α → β transition at 1400°C.

Pressure dependence of photoluminescence in amorphous silicon nanopowder produced by plasma enhanced chemical vapour deposition

The photoluminescence emission in silicon powder grown by plasma enhanced chemical vapour deposition has a very unusual dependence on pressure. Its intensity diminishes exponentially, I PL I 0 exp(- P/P 0 ), where P 0 has a value of several pascals. Consequently it is detectable only under vacuum. This dependence is analysed within the framework ofa multistep multiphoton excitation process. It is shown that an exponential dependence on pressure of the dynamic constants (lifetimes and optical cross-sections involved in the model) can account qualitatively for all the experimental results. MST/3288

Synthesis and Characterization of Nanostructured M50 Steel

AbstractThe synthesis of nanostructured multicomponent (Fe-Mo-Cr-V-C) M50 type steel has been carried out by two chemical routes: the sonochemical decomposition of organometallic precursors and the co-reduction of the respective metal halides. Both methods produce amorphous nanopowders, but the co-reduction method requires an additional heat treatment in vacuum at 700 °C to remove the by-product LiCl. In this paper we describe the synthetic methods and the characterization of the powders by XRD and SEM-EDAX. The powders exhibit a composition very close to that of M50 steel with an average grain size in the nanometer regime. In both cases the powders have been compacted in a vacuum hot press after they were heat treated in hydrogen. The characterization of the compacted specimens by XRD and SEM-EDAX is also discussed. The TEM characterization is presented for the sonochemically produced powders and compacts. A similar characterization of the co-reduced powders and the compacts is in progress. The consolidated specimens are homogeneous and dense, with an average grain size of ∼30 nm. Their hardness is higher as compared with that of the conventional M50 alloy.