Growth and characterization of nano‐crystalline hydroxyapatite at physiological conditions

Pure, stable stoichimetric nano crystalline hydroxyapatite material was crystallized by double diffusion technique at physiological conditions, temperature at 37°C and pH at 7.4. The sample was sintered at 400°C, 750°C and 1200°C with equal interval of time. They were characterized by X‐ray diffraction studies, Fourier Transformation Infra‐Red analysis, Thermogravimetric analysis, Scanning Electron Microscopic studies and Atomic Force Microscopic studies. The X‐ray analysis confirmed that the grown crystals are to be the pure form of hydroxyapatite. Infra‐red studies confirmed CO free hydroxyapatite. Thermogravimetric studies showed the thermal stability of the hydroxyapatite crystals even at 1200°C. The presence of pores in the sintered sample was traced by scanning electron microscopy. Atomic force microscopy revealed the presence of nano crystalline HAP of size 0.958 nanometer in the samples grown using this technique. At higher temperature the deagglomeration of bulk phases and agglomeration of nano phases leads to the nano crystalline HAP were observed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of copper aluminum borate whiskers via flux method

Single‐crystal and uniform copper aluminum borate whiskers have been synthesized by heating a mixture of boric acid, copper sulfate and aluminum sulfate with potassium sulfate as flux at 870 °C for 4 h. The synthesized whiskers exhibit a well‐crystallized, one‐dimensional structure with diameters ranging from 100 nm to 5 μm, lengths from 5 to 100 μm. Heating temperature and flux addition affect the aspect ratio and morphology of the copper aluminum borate whiskers. A possible growth mechanism of the whiskers is proposed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An X‐ray powder diffraction study of the microstructural evolution on heating 3:2 and 2:1 mullite single‐phase gels

Single‐phase gels with compositions 3Al₂O₃·2SiO₂ and 2Al₂O₃·SiO₂ were prepared by gelling mixtures of aluminium nitrate and tetraethylorthosilicate. Gels were fast heated at different temperatures between 900°C and 1600°C. The phase transformation and microstructural changes of both mullite precursor gels over the temperature range were followed by X‐ray powder diffraction (XRD), lattice parameter determination (LP), and scanning and transmission electron microscopies (SEM and TEM). The distribution of crystallite sizes and strains were determined by linewidth refinements of X‐ray diffraction patterns using the integral breadth method of Langford and the Warren‐Averbach analysis. XRD of both heated gels showed the formation of crystalline mullite single phase. Some amount of glassy phase coexisted with mullites at low temperatures, i. e. below 900°C. The compositional range of mullites formed on heating gels at temperatures between 900°C and 1600°C was dependent on the starting nominal composition of gels. SEM and TEM micrographs of both heated gels below 1200°C showed the formation of small, discrete, prismatic, well‐shaped nanocrystals in a very ordered arrangement. The size of these nanocrystals was dependant on the nominal composition of gels and increased on rising the heating temperature of gel precursors. The microstructural features obtained from linewidth refinement results of X‐ray diffraction patterns also allowed to suggest the formation of prismatic a little elongated nanocrystals at temperatures below 1200°C. Microstrain values were small and only displayed a relatively significant value for mullites processed at 900°C. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of boron nitride films on w‐AlN (0001), 4° off‐cut 4H‐SiC (0001), W (110) and Cr (110) substrates by Chemical Vapor Deposition

Boron Nitride is a promising group 13–group 15 compound material that exhibits various interesting properties like wide band gap, chemical stability, attractive mechanical properties and other. The growth behavior of this material has not been investigated in sufficient details to tailor properties of the resulting films. In this work we present the results on the growth of turbostratic boron nitride (t‐BN) thin films at a relatively high growth rate of 3 μm/h with the aim to investigate the potential use of boron trichloride in combination with ammonia as precursors for growth. Deposition experiments were conducted in a vertical cold wall high temperature chemical vapor deposition reactor in the temperature range 1000°C–1700°C depending on the substrate used. Templates of w‐AlN (0001), 4° off‐cut 4H‐SiC (0001), Cr (110) and W (110) were employed as substrates for the BN growth. As‐grown BN layers were characterized by Scanning Electron Microscopy, X‐Ray Diffraction, Electron Diffraction and Raman Spectroscopy. The results indicate that temperature and N/B ratio have a great influence on the crystallinity of the deposited films. For AlN and SiC substrates, a temperature of 1600°C and N/B ratio in range between 3 and 7.5 were identified as the best parameters for the growth of a 2 μm thick t‐BN layer with a spacing between basal planes of about 3.36 Å compare to the 3.33 Å spacing between basal planes of hexagonal or rhombohedral BN (h‐BN or r‐BN). For Cr and W substrates which have a lower mismatch with h‐BN (1 and 8.8 %), layers of t‐BN were deposited at much lower temperature (1000°C–1150°C) with a spacing between basal planes of 3.5 Å and morphology similar to that observed on SiC substrates. We obtained t‐BN layers with in plane strong disorder but out of plane orientation (c‐axis normal to the surface).     

Preparation and characterization of bioceramics produced from calcium phosphate cements

The present work reports a method for preparing calcium phosphate ceramics by calcination of calcium phosphate cements composed mainly of calcium deficient hydroxyapatite (CDHA). It was found that hardened cements calcinied at temperatures from to 600 to 1300 °C were transformed to tricalcium phosphates. Moreover the compressive strength was determined and porosity was estimated as a function of the calcination temperature. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dielectric and thermal behaviour of holmium tartrate trihydrate crystals

Measurements on dielectric constant of holmium tartrate trihydrate crystals at frequencies of the applied a.c. in the range 1 kHz to 1 MHz and at temperature in the range 30°C to 330°C are reported. The dielectric constant ε′ increases with temperature at all frequencies, attains a peak near 250°C, and then decreases as the temperature goes beyond 250°C. The anomalous dielectric behaviour at near about 250°C is attributed to be as a result of crystallographic/polymorphic phase transition brought about in the material. The results on the dielectric behaviour of the material are supplemented by results of thermal analysis viz., TG and DTA. Thermogravimetric and differential thermal analytic techniques have been used to study thermal behaviour of the material. It is shown that the material is thermally stable up to 220°C beyond which it decomposes through three stages till the formation of holmium oxide at 1200°C. The non‐isothermal kinetic parameters e.g., activation energy and the frequency factor have been evaluated for first two stages of thermal decomposition by using the integral method of Coats and Redfern. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sintering behavior of ZnO:Al ceramics fabricated by sol‐gel derived nanocrystalline powders

ZnO:Al ceramics (Zn:Al, 0.95:0.05) were prepared by using sol‐gel derived nanocrystalline powders. XRD patterns of the doped ceramics revealed the existence of both zincite (JCPDS 36‐1451) and gahnite (JCPDS 5‐0669) phases. Gahnite phase (ZnAl₂O₄) was segregated along the ZnO grain boundaries. At the sintering temperature of 1200 °C, relative density of the undoped and Al doped ceramics were measured as 0.695 and 0.628, respectively. Both grain size and relative density of the ceramics decreased with Al doping. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some features of thermo‐activated structural transformation of biogenic and synthetic Mg‐containing apatite with β‐tricalcium‐magnesium phosphate formation

Temperature transformation of biogenic and synthetic Mg‐containing apatite with β‐tricalcium‐magnesium phosphate (β‐TCMP) formation was investigated by X‐ray diffraction. Samples were annealed in air at the temperature range from 600 to 1200 °C at intervals 100 °C and cooled down to room temperature. The appearance of β‐TCMP was ascertained in samples annealed at 800 °C. As revealed, the relative amount of β‐TCMP increases and Mg concentration in this phase decreases as the annealing temperature is raised. While this, the replacement degree of Ca by Mg in the β‐TCMP lattice is the annealing temperature function and does not depend either on sample origin (biogenic/synthetic) or on initial Mg concentration. The results of present work together with other investigation data testify to a high thermally activated mobility of Mg both in structure of Mg‐containing apatites and in the structure of β‐TCMP formed after thermal decomposition. Obtained data can be used for new biomaterials design with varied prolongation of Mg released into the living biological tissue. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations of phase relations and eutectic directional solidification on the NiAl-V join

The NiAl-V join was investigated out to 45 at % vanaduim and the two-phase nature of the alloys formed indicates that a true binary exist with a eutectic at 40 ± 2% vanadium. The eutectic temperature was determined by optical methods to be 1350°C (1360°C was indicated by DTA). Directional solidification of the NiAl-V eutectic composition produced a structure which is lamellar, with interlamellar sapcing of λ = 4.2 μm at a solidification rate R = 1.5 cm/h.     

Growth of lead bromide polycrystalline films

Lead bromide polycrystalline films were grown by the physical vapor deposition method (PVD). Glass 1″x1″ in size, uncoated, and coated with Indium Tin Oxide (ITO), was used as substrate and rear contact. The starting material was evaporated at temperatures from 395°C to 530°C under high vacuum atmosphere (6 x 10^‐3 Pa) and during 8 days. The substrate temperature was prefixed from 190°C to 220°C. Film thickness yielded values from 40 to 90 μm. Optical microscopy and scanning electron microscopy (SEM) were performed on the films. Grain size resulted to be from 1.0 to 3.5 μm. SEM and X‐ray diffraction indicate that films grow with a preferred orientation with the (0 0 l) planes parallel to the substrate. The Texture Coefficient (TC) related to the plane (0 0 6) was 7.3. Resistivity values in the order of 10¹² Ωcm were obtained for the oriented samples, but a strong polarization indicates severe charge transport problems in the films. Film properties were correlated with the growth temperature and with previous results for films of other halides. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Influence of the Purity and Thickness of Niobium Foils on its Mechanical Properties

The content of contaminants, microstructures, surface condition, electrical resistivity and mechanical properties of niobium foils and bands (13–1000 μm thick) after cold working (90–99.9%), surface etching and heat treatment under vacuum (0.001 Pa at 700–1200 °C for 1–5 hrs) are studied. The effect of the previous working and different environmental conditions including initial and acquired surface contamination with oxygen, carbon, nitrogen and iron increases noticeably with the rise of the working degree and decrease of the foil thickness. The mechanical properties of foils and bands (≧ 100 μm) practically don't differ from massive standard samples. At the foil thickness of ≦ 40 μm the dimensional effects and the influence of the surface layer are especially noticeably. The optimal combination of the whole complex of surface and volume properties at the minimum dimensional effects is observed on the 50 μm thick foil. The improvement of foil properties can be achieved due to etching its surface layer up to the depth not less than 4 μm (on each side) after the cold working and following annealing at 1000–1100 °C (1–3 hrs).     

HRTEM simulation in determination of thickness and grain misorientation for hydroxyapatite crystals

High-resolution transmission electron microscopy (HRTEM) and HRTEM simulation by the Bloch wave method (JEMS) are used to determine the structure and thickness of micro-and nanocrystals of biominerals and hydroxyapatite grown from aqueous solutions. It is established that thin (from one to several lattice parameters) crystals, including hydroxyapatite in mineralized biological tissues, are usually formed in low-temperature (up to 40°C) solutions. Relatively thick (up to several tens of lattice parameters) crystals grow only in high-temperature (~95°C) aqueous solutions. HRTEM simulation showed that crystals with a thickness exceeding one lattice parameter consist of nanograins misoriented with respect to one another along various directions within an angle of 0.7°.     

Synthesis and characterization of Cu2ZnSnSe4 nanocrystals prepared by one pot route

Quaternary compound Cu₂ZnSnSe₄ (CZTSe) is one of the most promising absorber layer materials for thin film solar cells. In present work, the CZTSe nanocrystals were successfully synthesized via one pot route, and the influences of reaction temperature on the structural, compositional, morphological and optical properties of as‐synthesized CZTSe nanocrystals were investigated in detail via X‐ray powder diffraction (XRD), energy dispersive X‐ray spectrometry (EDS), transmission electron microscopy (TEM) and UV‐Vis spectrophotometry, respectively. The characterization results of as‐synthesized nanocrystals, under optimal synthesis condition (250 °C, 1 h), indicated that the nanocrystals was monodispersed with polycrystalline, the size was in the range of 10–15 nm, and the band gap energy was around 1.44 eV which is very closed to the best band gap energy for the solar cell. All results suggested that the as‐synthesized CZTSe nanocrystals were good light absorber layer material for thin film solar cell.     

The relations of sintering conditions and microstructures of [Bi0.5(Na1‐x‐yKxLiy)0.5]TiO3 piezoelectric ceramics

Bismuth sodium titanate (abbreviated as BNT) based solid solution, [Bi_0.5(Na_1‐x‐yK_xLi_y)_0.5]TiO₃ (0 < x + y < 1) ceramics, was invented in our group. These ceramics, which are considered as new candidates for lead‐free piezoelectric materials, were prepared by conventional ceramic sintering technique. The results of X‐ray diffraction show that the ceramics possess a single perovskite phase. The relations of the sintering conditions and the microstructures of the ceramics were studied. It was found that the optimized sintering condition is at 1100‐1150 °C for 2‐3 h, the grains of the ceramics have very regular shape, and the grain size of the ceramics is in the range of 1.3‐2.2 μm. These ceramics with the compositions of high amount of K⁺ and low amount of Li⁺ have relatively large piezoelectric charge constant (d₃₃), and can be put into practical applications. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Precipitation of nesquehonite from homogeneous supersaturated solutions

The homogeneous (unseeded) precipitation of nesquehonite (MgCO₃·3H₂O) was studied over the temperature range of 10‐40 °C. Precipitation was triggered by the supersaturation created by mixing MgCl₂ solution (0.5‐1.5 M) with Na₂CO₃ solution in the same concentration range. The Meissner's method was adopted in the calculation of supersaturations during the MgCl₂‐Na₂CO₃ reaction to monitor the precipitation. Solids were identified using X‐ray diffraction (XRD) analysis and scanning electron microscope (SEM) images. In the temperature range of 10‐40 °C, MgCO₃·3H₂O with needle‐like or gel‐like morphology was precipitated. It was seen that the length, width and surface smoothness of the particles changed with reaction temperature and supersaturation. The supersaturation (S) was in the range of 1.09‐58.68 during titration of Na₂CO₃ solution. The dimension of the crystals increased with longer addition time (or lower initial concentration of reactant) at the same temperature. Slower addition via titration of 2 h followed by 2 h of equilibration at 40 °C proved successful in producing well developed needle‐like MgCO₃·3H₂O crystals of 30‐50 μm long and 3‐6 μm wide. MgCO₃·3H₂O obtained were calcined to produce highly pure magnesium oxide (MgO) at 800 °C. The morphology of MgO was similar to that of their corresponding precursors. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nucleation of GaN on sapphire substrates at intermediate temperatures by Hydride Vapor Phase Epitaxy

A novel approach to deposit GaN layers directly on a sapphire substrate by Hydride Vapor Phase Epitaxy is presented. The two‐step deposition process includes the growth of GaN nucleation layers at intermediate temperatures in the range of 750 – 900 °C and subsequent high‐temperature overgrowth at about 1040 °C. Closed and non‐closed nucleation layers with a thickness of up to 2 μm were produced and characterized by scanning and transmission electron microscopy, micro‐Raman spectroscopy and X‐ray diffraction. A growth temperature of 780 °C is found to be optimal with respect to density and size distribution of nucleation islands. Raman measurements performed on the nucleation layers reveal nearly zero residual stress indicating effective stress relaxation on cooling down from growth temperature. The results of first overgrowth experiments demonstrate the possibility to grow 10 μm thick, crack‐free GaN layers of high crystalline quality on the nucleation layers. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of annealing on nanocrystal formation in Ni amorphous alloy

An amorphous Ni alloy has been obtained as a ribbon by plasma-arc melt spinning in a copper water-cooled lining slag crucible. Ribbon samples have been annealed in a special cylindrical furnace at temperatures from 200 to 400°C for 1 h in Ar. It is established that nanocrystals in the alloy samples increase in size with increasing annealing temperature. The rise of resistivity upon annealing at temperatures below 300°C and its significant decrease at higher temperatures is consistent with the change in the amorphous alloy microstructure.     

Experimental investigation of different configurations for the seeded growth of SiC crystals via a VLS mechanism

The growth of SiC crystals or epilayers from the liquid phase has already been reported for many years. Even if the resulting material can be of very high structural quality and the possibility to close micropipes was demonstrated, handling the liquid phase still is a challenge. Moreover, it is highly difficult to stabilize the C dissolution front and then to stabilize the growth front over a long growth time. Based on the Vapour‐Liquid‐Solid (VLS) mechanism, we present a new configuration for the growth of SiC single crystal which should allow first to simplify the liquid handling at high temperature and second to precisely control the crystal growth front. The process consists in a modified top and bottom seeded solution growth method, in which the liquid is held under electromagnetic levitation and fed from the gas phase. 3C‐SiC crystals exhibiting well‐faceted morphology were successfully obtained at 1100‐1200 °C with exceptional growth rates, varying from 1 to 1.5 mm/h in Ti‐Si melt. It was shown that the nucleation density decreases simultaneously with increasing propane partial pressure. At 1200‐1400 °C, thick homoepitaxial 6H‐SiC layers were successfully obtained in Co‐Si and Ti‐Si melts, with growth rate up to 200 µm/h. Large terraces with smooth surfaces are observed suggesting a layer by layer growth mode, and the influence of the system pressure was demonstrated. It was shown that the terrace size decrease simultaneously with increasing propane partial pressure which suggests the beginning of a two dimensional to three dimensional growth mode transition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microwave assisted synthesis of hydroxyapatite nano strips

Synthesis of hydroxyapatite (HAP) nano strips was carried out by chemical precipitation method followed by microwave irradiation. The microwave assisted reactions proceed at fast rates. It is found that the presence of the complex reagent EDTA plays an important role in the morphological changes of nanostructure hydroxyapatite. EDTA acts as a hexadentate unit by wrapping itself around the Ca²⁺ metal ion with, four oxygen and two nitrogen atoms and forms several five member chelate rings. The relative specific surface energies associated with the facets of the crystal determines the shape of the crystal. Scanning electron microscopy revealed the presence of hydroxyapatite nano strips with the range 50‐100 nm in EDTA influenced HAP powders. Fourier transform‐infrared spectroscopy (FT‐IR) result combined with the X‐ray diffraction (XRD) indicates the presence of amorphous hydroxyapatite (HAP) in the as‐prepared material. X‐ray patterns collected on the powder after heat‐treatment at 1100 °C for 2 h in air exhibits single phase of HAP. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High rate epitaxial growth of ZnO films on sapphire by planar magnetron rf sputtering system

Single crystal films of (11$\text{2}$0)ZnO are obtained on (01$\text{1}$2) sapphire at a deposition rate of 2 μm/h at a substrate temperature of 210–260°C by using a planar magnetron type high rate and low substrate temperature rf sputtering system. The surface of the film is so smooth that it can be used as an optical waveguide without post-deposition treatment. However, the films so far obtained have considerably higher optical waveguide loss than that of chemically vapor deposited films on the same face of sapphire.