Structural variation in palladium nanoparticles of the C-Pd system when dissolving hydrogen in them

The structure of nanocomposites of the C-Pd system has been studied by X-ray diffraction analysis and transmission electron microscopy. Variations in the lattice period of nanosized palladium, the average amount of hydrogen dissolved in it, and the size distribution of palladium nanoparticles have been analyzed as functions of the nanocomposite fabrication temperature. Based on the structural data, the solubility of hydrogen in nanosized palladium has been estimated.     

Study of polycrystalline diamond layers deposited from gas phase

Crystallite size in polycrystalline diamond layers with a grain size exceeding 3 μm are determined by the X-ray topography method with the use of a divergent beam from a point source. For layers with thicknesses in the range 80–700 μm deposited in SHF plasma and 1–40 μm obtained by the method of a hot filament, the size distribution of crystallites is obtained. Asterism of some spots on X-ray diffraction patterns from the diamond layers with thicknesses exceeding 100 μm showed plastic deformation of individual crystallites. The parameters of deep levels in the band gap of undoped high-resistance diamond layers and the acceptor-type defects with an activation energy higher than 1 eV are determined by the method of charge relaxation spectroscopy.     

Nucleation and growth of diamond

Nucleation and growth of diamond in the catalyst alloy Ni₇₀Mn₂₅Co₅, wt% have been observed at high temperature and pressure.     

Structure of autoepitaxial diamond films

A reflection high energy electron diffraction (RHEED) method has been used to investigate the structural peculiarities of synthetic diamond autoepitaxial films, deposited from a gaseous phase onto natural diamond seed crystal. The diamond films on the (111) and (110) faces usually have internal stresses, which cause the (111) plane microtwinning process at thicknesses of about 1000 Å. The thickness of twin lamellas is about 0.01?1μm. On the (100) face the diamond films do not undergo twinning and have a high structural perfection.     

Study of the absorption edge of SnO2 nanoparticles embedded in silica films

Silica thin films with embedded SnO₂ nanoparticles have been grown on transparent substrates by the sol–gel method. Tin dioxide crystals with cassiterite structure are semiconductors with a wide band gap of ~ 3.6 eV. Optical absorption spectroscopy in the near ultraviolet–visible range has been exploited to probe nanostructuring features of such nanocrystals. The results show that the sintering conditions modify crystallite mean size and enable the occurrence of quantum confinement effects. The outcome is in accordance with transmission electron microscopy data conducted on analogous bulk samples.     

Changes of Auger parameter in doped SnO2 powders

Doped SnO₂ powders were studied using X‐ray photoelectron spectroscopy (XPS). The Auger parameters (AP) for Sn and different doping elements were determined. A dependence of the Auger parameter on the doping element and doping concentration was observed. The detected shift could be correlated with the electrical properties of the studied samples as well as with the formation of a second phase and a segregation layer at the surface. Bulk doping with elements having a valency greater than 4 (Sb), leads to an increase of the electron density connected with a decrease of resistivity. In this case we can observe the shift of the AP to higher values. This means that the relaxation of the electron holes is energetically more favourable for Sb doped than for pure tin oxide. On the other side, doping with elements the valency of which is less than 4 (In) on tin sites leads to a decrease in electron density and causes an increase of resistivity of the material. We observe in this case a shift of the AP to lower values due to lower relaxation energy. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of mechanical properties of concrete with low concentration of magnetic nanoparticles

Concrete samples containing small amounts (0.6 wt%) of agglomerated magnetic nanoparticles of cobalt (Co/C) or iron carbide (Fe₃C/C) coated with carbon have been synthesized. The time dependence of the compression, bending and contraction strength of samples with and without magnetic nanoparticles have been investigated. Twelve different samples have been prepared and tested during 28 days to detect variation in compression, bending and contraction properties. The temporal change of the bending strength for concrete with and without nanoparticles has showed measurable differences. During the initial stage (first 3 days) of hardening all samples with nanoparticles were less resistant to bending than samples without nanoparticles. After 1 week samples with iron carbide agglomerates and cobalt nanoparticles showed an increase in resistance to bending in comparison with undoped concrete, 3% and 6%, respectively. After 28 days in both cases the bending strength slowly decreased for samples containing magnetic nanoparticles. The compression strength after 28 days decreased by about 9% for samples containing cobalt and 12% with iron carbide. The contraction increased by about 38% for sample with cobalt ions, however no change was observed for samples containing iron carbide. The FMR investigation has shown that the resonance line has shifted more for samples with cobalt and it has been suggested that this process could be connected with contraction caused by decreasing temperature after freezing [P.C. Aitcin, Cement Concrete Res. 30 (2000) 1349]. This study can be very important for magnetic shielding effects and the selection of optimal conditions for building materials with magnetic nanoparticles. It can also be useful for new technological solutions aimed at increasing the functionality of these materials.     

A characterization study of xerogel silicapropylaniline powders

Silicapropylaniline nanometric materials with varying organic content were obtained using a sol-gel synthesis. By increasing the organic load, the scanning electron microscopy technique shows a slight increase in the average size of aggregated particles. N₂ isotherms and positron annihilation lifetime spectroscopy measurements show that the average pore size decreases accompanied by a surface area reduction. FTIR thermal analysis was used to estimate the thermal stability of the organic phase and also to detect the presence of trapped organic groups in closed pores. From the organic coverage and surface area measurements the surface density of the immobilized organic molecules as well as the average intermolecular distance between them could be estimated.     

Structural evolution in mechanical alloying of Co and B powders

X-ray diffraction and scanning electron microscopy were used to monitor the mechanical alloying of four different mixtures of cobalt and boron with atomic compositions Co₅₀B₅₀, Co₆₇B₃₃, Co₇₅B₂₅ and Co₈₀B₂₀, respectively. The process induces amorphization reactions depending on the boron content; an almost complete amorphization was reached for the Co₆₇B₃₃ and Co₈₀B₂₀ samples, where only minor traces of unreacted cobalt are present. Extended X-ray absorption fine structure data collected on the most amorphous sample confirmed the diffraction results. In all the samples, the formation of t-Co₂B was detected.     

Particle size of powders under hydrothermal conditions

Various non‐oxide (CuI, AgI, AgCl, PbS, CuS and ZnS) and oxide (ZnO, TiO₂, SnO₂, CeO₂ and ZrO₂) powders were prepared under hydrothermal conditions to investigate the effects of temperature, pH and precursors on the particle size of powders. It was found that the particle sizes of PbS, CuS and ZnS powders were much smaller than that of CuI, AgI and AgCl powders prepared under the same conditions. The particle sizes of TiO₂, SnO₂, CeO₂ and ZrO₂ powders are much smaller than that of ZnO powders prepared under the same conditions. It is concluded that the solution conditions have a certain effect on the particle size of powders under the hydrothermal conditions. The particle size of powders increased with the rising of temperature. Additional factors affecting the particle size were uncovered through studying the nucleation mechanism. The particle size was mainly related to the Madelung constant and the electric charge number of ions. Powders with smaller particle size resulted from systems that possessed the larger Madelung constant and ionic charge number. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fabrication of PZT materials from nanostructured powders

The nanostructured PbZr_0.52Ti_0.48O₃ powders (PZT) were prepared by urea hydrolysis and subsequent thermal treatment. Phase pure PZT is obtained after calcination of the gel at 873 K for 4h in air atmosphere. The XRD pattern showed the tetragonal symmetry with lattice parameters: a_t = 3.97 Å and c_t = 4.15 Å. The XRD crystallite size of the PZT material heat-treated at 873 K for 4h was 6.0 nm. TEM analysis showed that the powder consisted of small clusters of crystallites of 30 to 40 nm in size. PZT materials fired at 1373 K showed nearly 99% density with equiaxed grains of 2–3 μm. The good sinterability relatively at lower temperatures is thought to result from enhanced diffusivities in nanosized particles.     

Nucleation of diamond powder particles in an RF methane plasma

The author examined the structure of powder particles obtained in the gas phase by decomposition of methane in an RF electric field. The powders are not nucleated on the substrates. They are formed when the process duration is long enough (i.e. dozens of minutes) at a large flow of methane gas. By means of TEM it was found that the powder obtained consisted of superfine crystallites of a diamond structure. The author presumes that the powders were formed in the gas phase. It seems that superfine diamond crystallites present in the powder particles may also have formed directly in the gas phase since the growth conditions are suitable for sp³ electron hybridization, and the Laplace pressure (the surface energy effect) is of the order GPa, especially in crystallites of a nanometer size.     

Dichroism and birefringence of natural violet diamond crystals

Investigation of the optical properties of natural violet diamonds from the Yakutian kimberlites is performed. A red shift of the absorption edge is revealed in the absorption spectra of these crystals. This shift is indicative of the presence of a high concentration of nitrogen in the diamonds studied. Along with the strong band at 0.550 μm, weaker bands at 0.390, 0.456 and 0.496 μm are revealed. It is shown that violet diamond crystals have birefringence and dichroism of about 10^?5 and 10^?6, respectively. When a light beam propagates perpendicularly to colored lamellas, the dichroism is much larger and the birefringence is smaller than in the case where the beam direction is parallel to lamellas.     

Characterization of homoepitaxial diamond for ionizing radiation detectors

Synthetic diamond has been proven as an important material for advanced electronic applications, such as those encountered in high energy physics and astrophysics. In fact, diamond transparency to visible light, its high carrier mobility, high breakdown field and strong resistance to chemical attack and radiation damage have suggested the potential application of this material for ‘solar-blind’ UV detectors which have to operate in extreme environments. To avoid the possible problems connected with the presence of grain boundaries in polycrystalline diamond films, a great effort has been devoted to the optimization of the growth process leading to high-quality single-crystal diamond on diamond substrates (homoepitaxy). In this view, characterization studies play a crucial role, because they provide the feedback for the optimization of the deposition process, in order to obtain the best quality material. In this work, a characterization study of homoepitaxial diamond grown by chemical vapor deposition (CVD) on synthetic diamond substrates is presented. The samples have been deposited in a CVD tubular reactor using a CH₄–H₂ gas mixture (1–7%) at approximately 560 °C substrate temperature. The growth rate ranged between 0.9 μm/h and 2.2 μm/h, microwave powers between 520 W and 720 W. The crystalline quality of the diamond layer has been studied by means of Raman spectroscopy. Photoluminescence has been used to study the nature and the distribution of impurities, having energy levels in the diamond band gap, which influence negatively the electronic quality of the material. The results have been compared with electro-optical characterization of UV detectors for astrophysics based on the analyzed diamond samples. The growth parameters which guarantees both high material quality and optimal device response have been determined.     

Formation of Co nanoparticles in metal-carbon composites

The effect of the IR-pyrolysis intensity on the phase formation and structure of Co nanoparticles in a matrix of IR-pyrolyzed polyacrylonitrile has been investigated.     

Preparation of nanocrystalline lithium niobate powders at low temperature

A facile route to prepare lithium niobate (LiNbO₃) powders was proposed by an alternative solid‐state method. Stoichiometric Li₂C₂O₄ and ammonium niobium oxalate were mixed with small amounts of water and then dried at room temperature. It was demonstrated that Li[NbO(C₂O₄)₂]·n H₂O intermediate was produced by an ion‐exchange reaction. Pure LiNbO₃ powders were successfully synthesized by heating the intermediate at 500, 600 and 700 °C for 3 h. X‐ray diffraction (XRD), scanning electron microscopy (SEM), Fourier‐transform infrared (FTIR) spectroscopy, UV‐Vis diffuse reflectance (UV‐Vis) spectroscopy and thermogravimetric (TG) analysis were used to characterize the precursor compound and as‐prepared samples. XRD results reveal that all the products are identified as hexagonal structure with high relative crystallinity (>87%). The particle size is found to be about 40 nm for the mixture calcined at 500 °C according to XRD data, which is in good agreement with SEM data. The as‐prepared LiNbO₃ powders by this method are high quality according to FTIR spectra. (Li_0.996Nb_0.005)Nb_0.999O₃ phase was formed when the calcination temperature was raised to 800 °C. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

MOVPE growth of single-crystal hexagonal AlN on cubic diamond

We have obtained single-crystal aluminum nitride (AlN) layers on diamond (1 1 1) substrates by metalorganic vapor-phase epitaxy (MOVPE). When the thermal cleaning temperature of the substrate and growth temperature of the AlN layer were below 1100 °C, the AlN layer had multi-domain structures mainly consisting of rotated domains. An interface layer, consisting of amorphous carbon and poly-crystal AlN, was formed between the AlN layer and the diamond substrate. On the other hand, when the thermal cleaning temperature and growth temperature were above 1200 °C, a single-crystal AlN layer was grown and no interface layer was formed. Therefore, we attribute the multi-domain structures to the interface layer. Even at the growth temperature of 1100 °C, by performing the thermal cleaning at 1200 °C, the single-crystal AlN layer was obtained, indicating that the thermal cleaning temperature of the substrate is a critical factor for the formation of the interface layer. The epitaxial relationship between the single-crystal AlN layer and the diamond (1 1 1) substrate was determined to be [0 0 0 1]_AlN∥[1 1 1]_diamond and [1 0 1¯ 0]_AlN∥[1 1¯ 0]_diamond. The AlN surface had Al polarity and no inversion domains were observed in the AlN layer.     

Raman amplification in nanoparticles doped glasses

Resonant Raman effects are studied for CdS_xSe_1−x nanoparticles in a silicate glass matrix in order to explore new possibilities of Raman amplification for telecommunication fibers. Nanoparticles with diameters ranging from less than 2 nm up to 6 nm are excited with the 458 nm, 488 nm and 633 nm laser lines corresponding both to resonance and off-resonance conditions. Due to confinement effects the resonance conditions are achieved at a given frequency by varying the size of the nanoparticles. In resonance no enhancement is observed for the silicate glass matrix but the (1LO-CdS), (2LO-CdS), (1LO CdSe + 1 LO CdS) modes are strongly enhanced showing the possibility of resonant Raman amplification of the discrete frequencies of the LO modes and of their overtones.