The crystallinity of calcium phosphate powders influenced by the conditions of neutralized procedure with citric acid additions

Calcium phosphate powders with nano-sized crystallinity were synthesized by neutralization using calcium hydroxide and orthophosphoric acid with the assistance of citric acid. The influence of processing parameters, such as free or additive citric acid, synthetic temperature and ripening time, on the crystallinity of hydroxyapatite were investigated. The results of X-ray diffraction and microstructure observations showed that the crystallinity and morphology of nano-sized hydroxyapatite particles were influenced by the presence or absence of citric acid. It was found that the crystallinities and crystallite sizes of hydroxyapatite powders prepared with the additive citric acid increased with increasing synthetic temperature and ripening time. Especially, the crystallinities of (h k 0) planes were raised and more homogeneously grown particles were obtained with increasing synthetic temperature.     

Influence of Mg-substitution on the physicochemical properties of calcium phosphate powders

Tricalcium phosphate based ceramics (TCP) are bioresorbable and thereby considered to be promising bone replacement materials. The differences in crystal structure between α and β-TCP phases gives rise for different dissolution rates in vitro and in vivo, which may alter the bioresorbable behavior of TCP ceramics. It is suggested that the addition of magnesium ions, which are also present in biological tissues, stabilizes β-phase to higher temperatures and thus enables the sintering of β-TCP at elevated temperatures compared to Mg free TCP.In this paper, Mg-substituted TCP, with the general formula (Ca_1−xMg_x)₃(PO₄)₂ and 0.01 ≤ x ≤ 0.045, were produced by wet chemical synthesis from Ca(OH)₂, H₃PO₄ and MgO, after calcinations at three different temperatures between 750 and 1050 °C. The influence of different amounts of Mg substitution on the physical properties, microstructure, and sintering behavior of calcium phosphate powders was evaluated. Thermal analytical techniques, together with X-ray diffraction analysis, were successfully combined in order to characterize the occurring phase transformations during annealing of the powders. The results show that the addition of small amounts of Mg (up to 1.5 mol%) are adequate to postpone the β-α TCP phase transformation to 1330 °C and to accelerate the densification process during sintering of β-TCP ceramics.     

Variable temperature in situ X-ray diffraction study of mechanically activated synthesis of calcium titanate, CaTiO3

The effect of mechanical activation on formation of calcium titanate (CaTiO₃) from calcined mixtures of CaCO₃ and TiO₂ was studied by monitoring the course of this solid-state reaction by variable temperature in situ X-ray diffraction (XRD) experiments, scanning electron microscopy (SEM) and dilatometry. Two equimolar mixtures of powdered CaCO₃ and TiO₂ were prepared: one was mechanically activated by grinding in a high energy vibro-mill. A total of 32 X-ray diffraction data sets for each sample, collected between 30 and 1100 °C, were analyzed by multiphase Rietveld refinement. Quantitative phase analysis and microstructure analysis obtained from X-ray diffraction are correlated to results of scanning electron microscopy and dilatometry. In the non-activated sample, small quantities of the reactants remain in the product until 1100 °C. In the activated sample, the reaction results in pure CaTiO₃ at 920 °C.     

Phase transformation of calcium phenyl phosphate in calcium hydroxyapatite using alkaline phosphatase at body temperature

Layered calcium phenyl phosphate ((C₆H₅PO₄)_0.92(HPO₄)_0.08Ca·1.3H₂O: CaPP), which is composed of a multilayer alternating bimolecular layer of phenyl groups and amorphous calcium phosphate phase, was treated in aqueous media including different amounts of enzyme such as alkaline phosphatase (ALP) at pH = 9.6 and 37 °C for 48 h. Treating the CaPP in the absence of ALP took place only the dissolution of this material. When the CaPP particles were treated in the presence ALP, calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: Hap) was formed. The yielded Hap contained no phenyl group and the molar ratio Ca/P of this material was 1.66, almost corresponding to 1.67 of the stoichiometric Ca/P ratio of Hap. TEM observation revealed that the irregular-shaped CaPP particles disappeared and rod-shaped Hap nano-particles were generated. The particle length and crystallite size of Hap were slightly increased on increasing the additive amount of ALP. These facts allow us to infer that the CaPP particles are dissolved, hydrolyzed and recrystallized to Hap by treating with ALP in aqueous media at body temperature of 37 °C and that the ALP plays as a catalyst for hydrolysis of phenyl phosphate ions. This phase transformation of CaPP in Hap in the presence of ALP resembles to the formation mechanism of Hap in animal organism.     

High temperature X-ray studies on barium and strontium zirconium phosphate based low thermal expansion materials

Stoichiometric compounds of the chemical formula Ba_1.5−xSr_xZr₄P₅SiO₂₄, x being 0, 1.0, 1.25, and 1.5 have been synthesized by the ceramic procedure. The compounds show low thermal expansion behavior with x=1.0 and 1.25 showing almost 0 thermal expansion up to 800°C. High temperature X-ray diffraction studies have been carried out on these samples and the axial thermal expansion coefficients calculated at each temperature by using the lattice parameters. The thermal expansion behavior is attributed to the unique crystal structure and the ionic substitutions of Ba and Sr in [M] site.     

Formation of TiB2 by volume combustion and mechanochemical process

Titanium diboride was produced both by volume combustion synthesis (VCS) and by mechanochemical synthesis (MCP) through the reaction of TiO₂, B₂O₃ and Mg. VCS products, expected to be composed of TiB₂ and MgO, were found to contain also side products such as Mg₂TiO₄, Mg₃B₂O₆, MgB₂ and TiN. HCl leaching was applied to the reaction products with the objective of removing MgO and the side products. Formation of TiN could be prevented by conducting the VCS under an argon atmosphere. Mg₂TiO₄ did not form when 40% excess Mg was used. Wet ball milling of the products before leaching was found to be effective in removal of Mg₃B₂O₆ during leaching in 1 M HCl. When stoichiometric starting mixtures were used, all of the side products could be removed after wet ball milling in ethanol and leaching in 5 M HCl when pure TiB₂ was obtained with a molar yield of 30%. Pure TiB₂ could also be obtained at a molar yield of 45.6% by hot leaching of VCS products at 75 °C in 5 M HCl, omitting the wet ball milling step. By MCP, products containing only TiB₂ and MgO were obtained after 15 h of ball milling. Leaching in 0.5 M HCl for 3 min was found to be sufficient for elimination of MgO. Molar yield of TiB₂ was 89.6%, much higher than that of VCS. According to scanning electron microscope analyses, the TiB₂ produced had average grain size of 0.27 ± 0.08 μm.     

Phase transformation of calcium phenyl phosphate in calcium hydroxyapatite

Calcium phenyl phosphate (CaPP) was synthesized from a mixture of Ca(OH)₂ and phenyl phosphate (C₆H₅PO₄H₂) in an aqueous media. XRD pattern of CaPP exhibited five diffraction peaks at 2θ = 6.6, 13.3, 20.0, 26.8 and 33.7°. The d-spacing ratio of these peaks was ca. 1:1/2:1/3:1/4:1/5. The molar ratios of Ca/P and phenyl/P of CaPP were 1.0 and 0.92, respectively, and the chemical formula of the material was expressed as (C₆H₅PO₄)_0.92(HPO₄)_0.08Ca·1.3H₂O, similar to that of dicalcium phosphate dihydrate (CaHPO₄·2H₂O: DCPD). These results allowed us to infer that CaPP is composed of a multilayer alternating bilayer of phenyl groups of the phosphates and DCPD-like phase. The structure of the material was essentially not altered after aging at pH 9.0-11.0 and 85 °C in an aqueous media. While, after aging at pH ≤8.0, the diffraction peaks of CaPP were suddenly weakened and disappeared at pH 7.0. Besides, new peaks due to calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: Hap) appeared and their intensity was strengthened with decreasing the solution pH. TEM observation revealed that the Hap particles formed at pH 6.0 are fibrous with ca. 1.5 μm in length and ca. 0.2 μm in width. From these results, it is presumed that the layered CaPP was dissolved, hydrolyzed and reprecipitated to fibrous Hap particles at pH ≤8.0 and 85 °C in aqueous media. This phase transformation of CaPP in Hap resembled to the formation mechanism of Hap in animal organism.     

Microwave-hydrothermal process for the synthesis of rutile

The synthesis of pure rutile titanium dioxide is not an easy achievement, as the crystallization process generally leads to mixtures of two or even three phases; moreover the synthetic processes normally used by industry require harsh reaction conditions. We carried out the synthesis of titanium dioxide from an aqueous titanium tetrachloride solution under microwave irradiation in the reaction time range of 5-120 min. We mostly obtained mixtures of rutile and anatase, but obtained single-phase rutile after a 2-h treatment at 160 °C; transmission electron micrographs revealed well-dispersed spherical nanoparticles. We also investigated the effects of dilution and addition of a dispersant (polyvinylpyrrolidone) on phase crystallization and particle shape.     

Synthesis of alumina powders by the glycine-nitrate combustion process

The combustion synthesis technique using glycine as fuel and aluminum nitrate as an oxidizer is able to produce alumina powders. Thermodynamic modeling of the combustion reaction shows that as the fuel-to-oxidant ratio increases, the amount of gases produced and adiabatic flame temperatures also increases. X-ray diffractions showed the amorphous structure for as-synthesized powder and presence of well-crystallized α-Al₂O₃ after calcination at 1100 °C during soaking time of 1 h. Alumina's largest measured specific surface area was 15 m²/g with BET method and 0.51 glycine-to-nitrate ratio.     

Synthesis of BaZrO3 by a solid-state reaction technique using nitrate precursors

High phase purity barium metazirconate powders have been synthesized from a modified solid-state reaction. Reactive powders consisting of submicron particles and narrow particle size distribution were obtained by heating a 1:1 molar mixture of barium nitrate and zirconyl nitrate at 800°C up to 8 h. Simultaneous thermal analysis (TG-DTA) assisted in elucidating the probable reaction pathways leading to the formation of the target compound in the BaO-ZrO₂ system. Systematic structural and microstructural characterization on the green powders and the compacts sintered up to 1700°C were carried out. A two-stage sintering schedule consisting of a 6 h soak at 1600°C followed by slow heating up to 1700°C with no dwell, led to highly dense microstructural features.     

Effect of HCl concentration on TiB2 separation from a self-propagating high-temperature synthesis (SHS) product

This paper presents a synthesis of TiB₂ powder via self-propagating high-temperature synthesis (SHS) method using a mixture of TiO₂, B₂O₃ and Mg followed by acid leaching. In the acid leaching step, the MgO content in the SHS product was leached in different HCl concentrations. X-ray diffraction (XRD) analysis showed that when 9.3 M HCl was used, the leached SHS product was found similar to that of the commercial TiB₂ powder. However, scanning electron microscopy (SEM) and BET surface area analysis revealed that the leached product was agglomerated and exhibited very high surface area.     

The chemical stability and conductivity of BaCe0.9−xYxNb0.1O3−σ proton-conductive electrolyte for SOFC

BaCe_0.8Y_0.2O_3−δ and BaCe_0.9−xY_xNb_0.1O_3−δ (x = 0.1, 0.15, 0.2, 0.25, 0.3, 0.35) were prepared by a solid-state reactions. It was found that the BaCe_0.8Y_0.2O_3−δ samples decomposed into CeO₂ and BaCO₃ after being exposed in the atmosphere (3% CO₂ + 3% H₂O + 94% N₂) at 700 °C for 10 h. However, samples containing Nb remains unchanged in the same conditions, demonstrating a better stability in the presence of CO₂ and H₂O. The conductivity of BaCe_0.9−xY_xNb_0.1O_3−δ increased with the increase of Y content (x ≤ 0.30), and the highest value was observed at x = 0.30 where a significant decrease in conductivity took place at x = 0.35. The conductivity of BaCe_0.6Y_0.3Nb_0.1O_3−δ reaches 0.01 S/cm in humid hydrogen at 700 °C, slight lower than BaCe_0.8Y_0.2O_3−δ, 0.012 S/cm in the same conditions. Fuel cell with BaCe_0.6Y_0.3Nb_0.1O_3−δ as-prepared was successfully prepared and humidified hydrogen was supplied as fuels in evaluating the fuel cell performance. The open circuit voltage, peak power density and interfacial resistance at 700 °C were 1.02 V, 345 mW/cm² and 0.27 Ω cm², respectively.     

Malayaite ceramic pigments: A combined optical spectroscopy and neutron/X-ray diffraction study

Ceramic pigments based on the Cr-doped malayaite structure were synthesized by solid state reaction and characterized by optical spectroscopy and combined X-ray and neutron powder diffraction in order to elucidate the still unclear chromium substitution mechanisms. The results show that coloration is actually due to simultaneous occurrence of Cr⁴⁺ and Cr³⁺ ions in the crystal lattice. Spectroscopy data confirm that Cr⁴⁺ is replacing Sn⁴⁺ in the octahedral site and, in minor amount, Si⁴⁺ in the tetrahedral site. In addition, neutron powder diffraction data suggest that Cr³⁺ substitution for octahedral Sn⁴⁺ is charge balanced by the formation of oxygen vacancies with no preference over the different oxygen sites. Upon incorporation of Cr ion, the SnO₆ octahedra exhibit an off-centre displacement of central cation which in turn induces a rearrangement of both the octahedral and tetrahedral coordination shells.     

The growth of lanthanum phosphate (rhabdophane) nanofibers via the hydrothermal method

Hexagonal LaPO₄ (rhabdophane) nanofibers are synthesized and characterized. TEM images show that the synthesized LaPO₄ nanofibers are about 5-50 nm in diameter and up to several micrometers long. The morphologies of the product are strongly dependent on the pH value of the reaction solution.     

Adsorption of vitamin E on mesoporous titania nanocrystals

Tri-block nonionic surfactant and titanium chloride were used as starting materials for the synthesis of mesoporous titania nanocrystallite powders. The main objective of the present study was to examine the synthesis of mesoporous titania nanocrystals and the adsorption of vitamin E on those nanocrystals using X-ray diffraction (XRD), transmission electron microscopy, and nitrogen adsorption and desorption isotherms. When the calcination temperature was increased to 300 °C, the reflection peaks in the XRD pattern indicated the presence of an anatase phase. The crystallinity of the nanocrystallites increased from 80% to 98.6% with increasing calcination temperature from 465 °C to 500 °C. The N₂ adsorption data and XRD data taken after vitamin E adsorption revealed that the vitamin E molecules were adsorbed in the mesopores of the titania nanocrystals.     

Synthesis and densification of lutetium pyrosilicate from lutetia and silica

Cerium-doped lutetium pyrosilicate (Lu₂Si₂O₇:Ce) powder was synthesized by solid state reaction of Lu₂O₃ and SiO₂. Stoichiometric mixtures of the starting materials were heat treated at various different temperatures and their phase contents were measured by XRD technique. It was found that the first step in the formation of Lu₂Si₂O₇ (LPS) is the appearance of Lu₂SiO₅ (LSO). This takes place at 1100 °C, fully 300 °C below the first appearance of LPS. Between 1400 and 1500 °C both LSO and LPS coexist in the calcined batch, but by 1550 °C all LSO is completely converted to LPS. LPS formation temperature does not have appreciable effect on the density of the hot pressed samples. Hot pressed samples obtained from powder synthesized at 1650 °C are nearly transparent, although the particle size of the starting powder is higher than that of the powder formed at lower temperatures.     

Synthesis, crystal structure, dielectric properties, and potential use of nanocrystalline complex perovskite ceramic oxide Ba2ErZrO5.5

A new member belongs to Ba₂REZrO_5.5 (RE = Rare-Earth) perovskites, viz. Ba₂ErZrO_5.5, is synthesized as nanocrystals using a combustion process. Unlike the other Ba₂REZrO_5.5 perovskites, which are cubic, Ba₂ErZrO_5.5 crystallizes in tetragonal structure having space group P4/mnc (#128). Phase purity and ultrafine morphology of Ba₂ErZrO_5.5 powders were examined using X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), Fourier transform of infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) specific surface area measurements, and transmission electron microscopy (TEM). Nanocrystals of Ba₂ErZrO_5.5 was sintered at ∼1500 °C for 4 h; whereas coarse-grained powders synthesized through solid state reaction could not be sintered even at 1700 °C for prolonged duration. XRD pattern of Ba₂ErZrO_5.5 was refined for atomic coordinates, lattice parameters, occupancies, and thermal factors using Rietveld analysis of XRD. Dielectric constant (?′) of Ba₂ErZrO_5.5 at 10 MHz is 21.62 ± 2 and dielectric loss (tan δ) is 5 × 10⁻³ at room temperature. Similar to the other Ba₂REZrO_5.5 perovskites, Ba₂ErZrO_5.5 is also showed chemically stability with YBa₂Cu₃O_7−δ (YBCO) and Bi₂Sr₂Ca₁Cu₂O_x (Bi-2212) superconductors. Ba₂ErZrO_5.5 can possibly be used as a substrate for high temperature superconductor (HTS) films, or be used as an insulator in the active superconductor-insulator-superconductor (SIS) structures.     

Synthesis and structural refinement of polycrystalline ceramic powder Pr0.1Ca0.9TiO3

Perovskite structure-based ceramic precursors have a characteristic property of substitution in the ‘A’ site of the ABO₃ structure. This makes them a potential material for nuclear waste management in synthetic rock (Synroc) technology. In order to simulate the mechanism of rare earth fixation in perovskite, Pr_xCa_1−xTiO₃ (where x = 0.1) has been synthesized through ceramic route by taking calculated quantities of oxides of Ca, Ti and Pr as starting materials. The ceramic phase has been characterized by its powder diffraction pattern. The Rietveld analysis of the X-ray diffraction data has been carried out using GSAS software to achieve the convergence which gives the R_p = 5.74% and R_wp = 8.17%. The (h, k, l) values for different lattice planes have been calculated. The praseodymium substituted perovskite crystallizes in orthorhombic symmetry with space group: Pbnm, Z = 4. The unit cell parameters at room temperature are a = 5.39609(31) Å, b = 5.44869(30) Å and c = 7.6565(5) Å. The calculated and observed values of the corresponding intensities, 2θ and density of the polycrystalline powder show good agreement. GSAS-based calculation for bond distances TiO, CaO and bond angles OTiO, OCaO has been reported.     

Synthesis of nanocrystalline magnesium titanate by an auto-igniting combustion technique and its structural, spectroscopic and dielectric properties

Nanocrystalline magnesium titanate was synthesized through an auto-ignited combustion method. The phase purity of the powder was examined using X-ray diffraction, thermo gravimetric analysis, differential thermal analysis, Fourier transform infrared spectroscopy and Raman spectroscopy. The transmission electron microscopy study showed that the particle size of the as-prepared powder was in between 20 and 40 nm. The nanopowder could be sintered to 98% of the theoretical density at 1200 °C for 3 h. The microstructure of the sintered surface was examined using scanning electron microscopy. The dielectric constant (?_r) of 16.7 and loss factor (tan δ) of the order of 10⁻⁴ were obtained at 5 MHz when measured using LCR meter. The quality factor (Q_u × f) 73,700 and temperature coefficient of resonant frequency (τ_f) −44.3 ppm/°C, at 6.5 GHz are the best reported values for sintered pellets obtained from phase pure nanocrystalline MgTiO₃ powder.     

Synthesis of cerium hydroxycarbonate powders via a hydrothermal technique

CeOHCO₃ powders have been directly synthesized using a hydrothermal process at temperatures as low as 160°C. The well-dispersed powders are obtained in a short period of reaction time during hydrothermal reaction via the hydrolysis of urea. For synthesizing CeOHCO₃, the concentration of urea is found to be a crucial determinant, which has significant effects on the morphology of the derived powders. When low urea concentrations are provided, the formed particles are rhomboidal platelets. On the other hand, the high urea concentrations cause the shape of the powders to become prismatic. Increasing the concentration of urea tends to increase the particle size as well as the aspect ratio of CeOHCO₃ powders. After further heating at 500°C, a phase transformation from orthorhombic CeOHCO₃ to cubic CeO₂ takes place. The crystallinity and size of CeO₂ strongly depend on the particle size of CeOHCO₃.