Size dependent photoluminescence of SiC nanocrystals

The paper presents the results of porous SiC study using photoluminescence and scanning electronic microscopy. It is shown that the intensity of defect-related PL bands (2.08, 2.27, 2.44 and 2.63 eV) increases monotonically with the rise of PSiC thickness from 2.1 up to 12.0 μm. These luminescence centers are assigned to surface defects which appear at the PSiC etching process. Photoluminescence intensity stimulation for surface defects is attributed to rise of defect concentrations with increasing of porous layer thickness and to realization of the hot carrier ballistic mechanism at surface defect excitation. Intensity enhancement for exciton-related PL bands (2.79, 2.98 and 3.26 eV ) is attributed to increasing the exciton recombination rate as result of exciton weak confinement in big size SiC NCs of different polytypes (6H–PSiC with inclusions of 15R- and 4H–PSiC).     

A SAXS study of kinetics of aggregation of silica sols

SAXS in situ experiments on the evolution of TMOS solutions during hydrolysis and polycondensation lead to power laws with scaling exponents ≈ 2. It is suggested that this could be the result of the polydispersity of the samples and that only an apparent fractal dimension can be obtained in this way. Kinetic studies tend to indicate that agglomeration in the sol is the result of a diffusion-controlled process.     

Size effect on surface structural relaxation kinetics of silica glass sample

It is known that surface structural relaxation takes place more rapidly than bulk structural relaxation, especially in the presence of water vapor. The surface structural relaxation kinetics of the silica glass fiber and plates was compared at 950 °C and the surface structural relaxation kinetics of silica glass fiber was found to be faster than that of the silica glass plate, even though the composition and initial fictive temperatures of the samples were the same. The observed difference of the surface structural relaxation kinetics between silica glass fiber and silica glass plate can be accounted for using a diffusion equation with time-dependent surface concentration. The analysis indicates that there is a general size effect on the surface structural relaxation kinetics, with smaller sized samples exhibiting faster relaxation kinetics.     

An electron microscopy study of the crystal growth of schwertmannite needles through oriented aggregation of goethite nanocrystals

The formation of the iron oxyhydroxide schwertmannite has been monitored by time-resolved transmission electron microscopy (TEM) studies. Schwertmannite aggregates are found to form initially as spherical agglomerates of ferrihydrite crystallites, which then begin to grow characteristic needles on their surfaces. High-resolution images of the needles show that they are initially comprised of aligned goethite nanocrystals, which subsequently coarsen to form crystallographically coherent needles of goethite. Thus, needle formation on schwertmannite aggregates can be considered as the first stage in the phase transformation from schwertmannite to goethite. The results of this study suggest that schwertmannite is not a distinct mineral phase, but may be a mixture of ferrihydrite and poorly crystalline goethite with a distinctive morphology directed by the presence of surface-adsorbed sulfate anions.     

Residence-time dependent kinetics of CVD growth of SiC in the MTS/H2 system

In most chemical vapor deposition (CVD) experiments in flow reactors carried out until now, growth conditions were chosen which yield growth rates independent or linearly dependent on the total gas flow rate, so that the residence time (t) of the gases in the hot zone of the reactor should not play any role in the growth rate. We have performed CVD experiments in the system MTS/H₂, under conditions of low decomposition of MTS. We have found a region, where the growth rate and its derivatives depend strongly on the operating conditions, in particular, where the growth rate of SiC increases strongly with an increase of t. For lower or higher (but yet incomplete) decomposition of MTS, the growth rate becomes again independent of t, and its apparent energy of activation becomes 200 kJ/mol.     

Aging of oriented polymer glasses

Dimensional (D) and enthalpy relaxation (ΔH) of oriented polymer glasses (PS and PC) have been studied as function of temperature, between T_g and T_g−20 °C, and aging time t, ranging to several weeks. The dimensional relaxation (shrinkage) and enthalpy relaxation curves verify the logarithm law D(t) ∼ H(t) ∼ log t, between an incubation τ_i and a final relaxation time τ_f. The time τ_f to reach the equilibrium (D and ΔH) follows the Vogel–Tamann–Fulcher (VFT) law. Enthalpy relaxation and shrinkage exhibit important differences. Enthalpy relaxation of oriented and isotropic polymers follows the same logarithm law, independent of the draw ratio λ and the mode of deformation, the relaxation time τ_f coincides with the relaxation time of the α segmental motions. Shrinkage depends on λ and the mode of deformation, the relaxation time τ_f is attributed to the normal mode, the relaxation time of the whole chain. Finally the shrinkages of PS and PC show some differences. PC at short aging times presents another type of dimensional relaxation which would be due to the β motions. This would be in close connection with the ductile (PC) and fragile (PS) behavior of these two polymers far below T_g.     

Properties of oriented TTF-TCNQ thin films

Vapour deposited thin films of the organic molecular complex tetrathiafulvaliniumtetracyanoquinodimethane (TTF-TCNQ) are investigated by electrical and electronmicroscopical methods. Morphology studies by means of electron micrographs show, that the thin films up to thicknesses of 300 nm consist of small crystals of a size of 1.2 · 0.2 μm² (deposited on cleaved (100) faces of NaCl) or 15 · 2 μm² (on glass substrate). Dependend on deposition conditions and on material of the substrate thin films are produced with strong or statistic orientation of this crystals. Strong oriented thin films exhibit conductivities up to σ = 65 ω⁻¹ cm⁻¹ and activation energies of W_A = 0.02 eV. The found dependences of the conductivity on electrical field strength. temperature, and size of microcrystals are explained by a linear hopping model.     

Mechanical properties of oriented mica glass ceramic

The mechanical properties of extrusion oriented mica glass ceramic were studied. Anisotropy caused by the alignment of the basal planes of phlogopite crystals along the extrusion axis was observed. Fracture toughness varied in direction by about 350%, Knoop-microhardness by about 30% and Young's modulus by about 15%. By comparison with the randomly oriented mica glass ceramic, an enhancement of the mechanical properties was measured in the direction perpendicular to the extrusion axis. The influence of the crystal alignment on mechanical properties and machineability is discussed.     

Growth of large oriented metal single crystal plates

A method of growing large oriented metal single crystal plates is presented. Crystal plates without any mechanical treatment are suitable for neutron diffraction work. The mosaic spread of these crystals is smaller than that of cylindrical single crystals.     

Control of nanoparticle aggregation in aerogel hosts

Plasmonic aerogel containing 50 nm gold nanoparticles is made using a modified 2-step method that maintains control over the gel time while preventing nanoparticle aggregation. Strong narrow surface plasmon resonances verify that the nanoparticles are well dispersed within the silica matrix, and enable applications in sensing, SERS, nonlinear optics or plasmonic gain. Discrepancies between measured and simulated resonance wavelengths are attributed to the breakdown of the effective index approximation, due to the short-scale penetration of the resonance electric field into the host medium.     

Study of the oriented overgrowth of sodium nitrate crystals

Formation of overgrowths of NaNO₃ on crystalline and noncrystalline substrates has been investigated employing the solvent evaporation method. Calcite, Gypsum, Sodium chloride, Mica, Glass and Polystyrene films formed on glass have been employed as substrate materials. The solvents employed are water, liquid ammonia, methyl alcohol and n-butyl alcohol. Random orientation of the overgrowths is observed on the substrates except in the case of calcite and mica. It is found that the morphology of the overgrowths manifests in the form of rhombic-, triangular- and hexagonal platelets. The occurrence of (001), (110) and (111) twins is observed. The effect of supersaturation and nature of the substrate on the formation of overgrowths is considered.     

粉体粒径对氧化铝陶瓷膜孔径的影响及控制

将三种不同粒径的α-Al2O3(平均粒径为1μm、3μm、5μm)混合后配置稳定悬浮液,利用颗粒级配理论,建立粉体比例、粉体粒径及其分布和膜层厚度对无机膜孔径及其分布的影响,实现利用粉体粒径分布控制膜层孔径分布,获得多选择的无机膜.研究结果表明,无机微滤膜的孔径取决于粉体的粒径分布,其孔径变化的上下限分别为由最粗和最细粉体所组成陶瓷膜的膜孔径.粉体的混合会在一定程度上扩大无机微滤膜的孔径分布,但有助于减少最大膜孔.膜层的厚度的增加在一定程度上降低无机微滤膜的最大孔和平均孔径.该实验结果有助于实现利用粉体粒径控制无机微滤膜的控制,促使无机微滤膜的孔径的多样化.     

Growth of oriented GaN nanowires by controlling nucleation conditions

GaN nanowires (NWs) were grown on GaN(0001) coated sapphire substrate with Ni/Au catalyst by using metalorganic chemical vapor deposition. Nucleation conditions were investigated for improving the growth orientation of NWs. With decreasing catalyst thickness from 5nm/5nm to 2nm/2nm, the NW orientation was improved and the NW morphology was changed from taper to cylindrical, due to the varying of growth mode. Vertical alignment of NWs can be further improved by inserting an additional high‐temperature (850 ℃) nucleation step with an optimum V/III flow ratio of 20.     

Size Distribution Analysis of Needle-Shaped Crystals

Two methods of measurement of the crystal size distribution of needle-shaped carystals are compared: direct measurement of a number of crystals on a microphotograph, which is rather tedious and time consuming, and the photosedimetric method which is quicker, easy to perform but not so suitable for not isometric crystals. A methods is described that brings results of both of these methods into agreement. The method has been verified on strongly anisometric particles of calcium sulphate.     

Nucleation kinetics of sodium disilicate

The kinetics of crystal nucleation in Na₂O · 2SiO₂ have been determined over the range of undercoolings between 173 and 373°C. The plot of log(I_v?) versus $\text{1}\text{ΔT}{}^2{}_{\mathrm{r}}\text{T}{}^3{}_{\mathrm{r}}$ is a straight line of negative slope over some 13 orders of magnitude in I_v. The slope of this relation indicates a nucleation barrier of about 45 kT at ΔT_r = 0.2, and the intercept at $\text{1}\text{ΔT}{}^2{}_{\mathrm{r}}\text{T}{}^3{}_{\mathrm{r}}\text{= 0}$ is 10²⁶ cm^-3 sec^-1. poise. The results are in good agreement with predictions of the theory of homogeneous nucleation, even in the pre-exponential factor.     

Modern preparation methods of oriented thick films of superconducting cuprates

Modern trends in the development of methods of manufacturing biaxially textured large-grained coatings ～ 100 μm thick based on superconducting rare-earth-barium cuprate elements, as well as bismuth-containing high-temperature superconductors, are considered. It is shown that the formation of a symmetric artificial surface relief makes it possible to control the mutual orientation of crystallites on polycrystalline metal substrates with high efficiency. In this case, the fraction of superconductor crystallites with an angle of mutual misorientation <5% may be as high as 75–90%. The high efficiency of this method of forming oriented films of multicomponent oxide phases indicates that it can be used to fabricate new-generation superconducting composite materials.     

Size effects in oxide mechanochemistry

A new size effect has been experimentally found: the mechanochemical synthesis of previously mechanically activated powder mixtures of MgO-Al₂O₃ oxides with a block size of about 80 nm leads to the formation of MgAl₂O₄ spinel at room temperature. A model is proposed according to which the mechanism of mechanochemical solid-phase synthesis changes from dislocation transgranular embrittlement to grainboundary sliding, which is accompanied by the generation of dynamic vacancies and low-temperature phase formation via the diffusion-induced migration of grain boundaries.     

Size effect in displacive ferroelectrics

It is ascertained that the size effect in thin ferroelectric films is related to the spatial correlation of ferroelectric polarization, depending on boundary conditions. The influence of the size effect on the permittivity of displacive ferroelectrics is described. It is shown that the size effect manifests itself differently for different orientations of the polarization vector with respect to the boundaries of a ferroelectric layer.     

Mineralization of oriented nano hydroxyapatite in photopolymerized polyacrylamide gel matrix

Polyacrylamide hydrogel is a biomaterial and nondegradable water based polymer which is used as tissue filler. Mineralization of hydroxyapatite (HAp) in a UV polymerized acrylamide gel matrix was investigated by varying the concentration of precursors and pH in the range 8, 9 and 10. During polymerization, diammonium hydrogen phosphate ions were impregnated in the gel matrix and subsequently, immersed in the calcium nitrate solution. Thin laminated macroporous structures, embedded with nanospheres and ribbons of HAp were mineralized. The HAp was found to be oriented along c‐axis, which could lead to the preferential binding of the acidic proteins on it's surface. In addition, there was an enlargement of pore sizes with an increase in pH. The laminated structures showed resorbable nature whereas, flake like structures obtained at higher concentrations were found to be bioactive. This composite could be an alternative to the use of silicone gel, to avoid long term risk of fibrosis and migration when implanted. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)