Enhanced dielectric constant and structural transformation in Fe-doped hydroxyapatite synthesized by wet chemical method

We report the synthesis of single-phase Fe-doped hydroxyapatite (HAp) [Ca_10?xFe _x (PO₄)₆(OH)₂ (0.0?≤?x?≤?0.3)] and enhanced dielectric constant of HAp with Fe doping. Rietveld analysis shows the change in x-axis-oriented lattice constant a in Fe-doped x?=?0.1 and 0.3 compositions in comparison with parent HAp, while z-axis-oriented lattice constant c does not show any considerable change. Analysis of absorbance data shows two new symmetric stretching peaks for Fe-doped x?=?0.1 and x?=?0.3 compositions, which are not present in parent HAp. Magnetic measurements show paramagnetic behaviour of all Fe-doped samples at 300 K. Fe-doped Ca_9.9Fe_0.1(PO₄)₆(OH)₂ composition shows increase in impedance in the presence of 500 Oersted (Oe) applied magnetic field in comparison with impedance in the absence of magnetic field. Ca_9.9Fe_0.1(PO₄)₆(OH)₂ composition shows increase in dielectric constant in comparison with parent HAp in frequency range 5–35 MHz. Fe-doped Ca_9.9Fe_0.1(PO₄)₆(OH)₂ composition shows?~?970% colossal magnetoimpedance at 100 Hz and?~?200% at 20 MHz frequency.     

Preparation and Thermal Expansion of Calcium Iron Zirconium Phosphates with the NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> Structure

Ca_{0.5(1 + x)}Zr_2–xFe _x (PO₄)₃ phosphates have been synthesized by a sol–gel process. The individual compounds and solid solutions obtained crystallize in the NaZr₂(PO₄)₃ structure (trigonal symmetry, sp. gr. R\(\bar 3\)). Using high-temperature X-ray diffraction, we have determined their thermal expansion parameters in the temperature range from 25 to 800°C. With increasing x, the magnitudes of their linear thermal expansion coefficients and thermal expansion anisotropy decrease. Most of the synthesized phosphates can be rated as low-thermal-expansion compounds and can be regarded as materials capable of withstanding thermal “stress.”     

Crystallization from high-temperature solutions in the K<Subscript>2</Subscript>O-P<Subscript>2</Subscript>O<Subscript>5</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> system

We have studied general trends of crystallization from high-temperature solutions in the K₂O-P₂O₅-V₂O₅-Bi₂O₃ system at P/V = 0.5?2.0, K/(P + V) = 0.7?1.4, and Bi₂O₃ contents from 25 to 50 wt % and identified the stability regions of BiPO₄, K₃Bi₅(PO₄)₆, K₂Bi₃O(PO₄)₃, and K₃Bi₂(PO₄)_{3 ? x} (VO₄) _x (x = 0?3) solid solutions. The synthesized compounds have been characterized by X-ray powder diffraction and IR spectroscopy, and the structure of two solid solutions has been determined by single-crystal X-ray diffraction (sp. gr. C ₂/c): K₃Bi₂(PO₄)₂(VO₄), a = 13.8857(8), b = 13.5432(5), c = 6.8679(4) Å, β = 114.031(7)°; K₃Bi₂(PO₄)_1.25(VO₄)_1.75, a = 13.907(4), b = 13.615(2), c = 6.956(2) Å, β = 113.52(4)°.     

Tunable luminescence properties and energy transfer of single-phase Ca<Subscript>4</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>O: Dy<Superscript>3+</Superscript>, Eu<Superscript>2+</Superscript> multi-color phosphors for warm white light

The novel Ca_4?x(PO₄)₂O: xDy³⁺ and Ca_4?x?y(PO₄)₂O: xDy³⁺, yEu²⁺ multi-color phosphors were synthesized by traditional solid-state reaction. The crystal structure, particle morphology, photoluminescence properties and energy transfer process were investigated in detail. The X-ray diffraction (XRD) results demonstrate that the products showed pure monoclinic phase of Ca₄(PO₄)₂O when x < 0.1. The scanning electron microscopy (SEM) indicated that the phosphors were grain-like morphologies with diameters of ~ 3.7–7.0 μm. Under excitation of 345 nm, Dy³⁺-doped Ca₄(PO₄)₂O phosphors showed multi-color emission bands at 410, 481 and 580 nm originated from oxygen vacancies and Dy³⁺. Interestingly, Ca₄(PO₄)₂O: Dy³⁺, Eu²⁺ phosphors exhibited blue emission band at 481 nm and broad emission band from 530 to 670 nm covering green to red regions. The energy transfer process from Dy³⁺ to Eu²⁺ was observed for the co-doped samples, and the energy transfer efficiency reached to 60% when Eu²⁺ molar concentration was 8%. In particular, warm/cool/day white light with adjustable CCT (2800–6700 K) and high CRI (R_a > 85) can be obtained by changing the Eu²⁺ co-doping contents in Ca₄(PO₄)₂O: Dy³⁺, Eu²⁺ phosphors. The optimized Ca_3.952(PO₄)₂O: 0.04Dy³⁺, 0.008Eu²⁺ phosphor can achieve the typical white light with CCT of 4735 K and CRI of 87.     

Influence of dicalcium phosphate dihydrate coating on the <Emphasis Type="Italic">in vitro</Emphasis> degradation of Mg-Zn alloy

To reduce the degradation rate and further to improve the biocompatibility of magnesium alloy, dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) has been fabricated on a kind of magnesium-zinc alloy by way of electrodeposition method. The experimental results of XRD, SEM and EDS showed that the electrodeposited coating on the Mg-Zn alloy mainly contains the flake-like DCPD, along with some octacalcium phosphate (Ca₈(HPO₄)₂(PO₄)₄·4H₂O, OCP). After the in vitro degradation of the coated alloy in modified-simulated body fluid (m-SBF), it was proved that the coating could reduce the degradation rate effectively, and the samples were covered by calcium phosphate salts with a higher Ca/P ratio. Therefore, it indicates that compared with the bare alloy, the DCPD coating rendered a more biocompatible surface, and is a promising coating candidate for biomedical magnesium materials.     

Electrical Conductivity and Electrochemical Characteristics of Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>-Based NASICON-Type Materials

NASICON-type materials with the compositions Na₃V_2–xAl_x(PO₄)₃, Na₃V_{2 - x}Fe_x(PO₄)₃, Na_{3 + x}V_2–xNi_x(PO₄)₃, and Na₃V_{2 - x}Cr_x(PO₄)₃ (x = 0, 0.03, 0.05, and 0.1) have been prepared and characterized by X-ray diffraction analysis, electron microscopy, and impedance spectroscopy. The results demonstrate that the highest electrical conductivity among the samples studied is offered by the material doped with 5% Fe: Na₃V_1.9Fe_0.1(PO₄)₃. The activation energy for low-temperature conduction in the doped materials decreases from 84 ± 2 to 54 ± 1 kJ/mol and that for high-temperature conduction is ~33 kJ/mol. The discharge capacity of Na₃V_1.9Fe_0.1(PO₄)₃/C under typical working conditions of cathodes of sodium ion batteries has been shown to exceed that of Na₃V₂(PO₄)₃/C. The capacity of the more porous material prepared by the Pechini process (Na₃V_1.9Fe_0.1(PO₄)₃/C-{II}) approaches the theoretical one at a low charge–discharge rate and retains its high level as the charge rate is raised (its discharge capacity was 117.6, 108.8, and 82.6 mAh/g at a discharge rate of 0.1C, 2C, and 8C, respectively).     

Hierarchical flower-like titanium phosphate derived from H-titanate nanotubes for photocatalysis

Nanosheet-assembled hierarchical flower-like titanium phosphate (TiP) is synthesized via hydrothermal treatment of H-titanate nanotubes (Ti-NT) at the optimized conditions of 0.1 M of H₃PO₄ and hydrothermal temperature of 130 °C. A possible formation mechanism for the TiP flowers, involving the disintegration of Ti-NT and the growth and assembling of TiP nanosheets, is proposed. The main compositions of the uncalcined TiP flowers are titanium hydrogen phosphate hydrates (Ti(HPO₄)₂·xH₂O), which can be transformed to titanium phosphate (TiP₂O₇) after high temperature calcination. The photocatalytic activity of the prepared TiP flowers is increased with the increased calcination temperature, which may be attributed to the better photocatalytic activity of TiP₂O₇ than Ti(HPO₄)₂·xH₂O and the increased crystallization of TiP₂O₇.     

Heat capacity of the MVO<Subscript>4</Subscript> (M = Al,Ga, In,Tl) orthovanadates

The heat capacity of InVO₄ has been determined by differential scanning calorimetry in the temperature range 339–1089 K. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of indium orthovanadate: enthalpy increment H°(T)–H°(339 K), entropy change S°(T)–S°(339 K), and reduced Gibbs energy Ф°(Т). The specific heats of GaVO₄ and TlVO₄ have been evaluated.     

High-temperature heat capacity and thermodynamic properties of Tb<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Tb₂Sn₂O₇ has been prepared by solid-state reaction in air at 1473 K over a period of 200 h and its isobaric heat capacity has been studied experimentally in the range 350–1073 K. The C _p(T) data for this compound have no extrema and are well represented by the classic Maier–Kelley equation. The experimental C _p(T) data have been used to evaluate the thermodynamic properties of terbium stannate (pyrochlore structure): enthalpy increment H°(T)–H°(350 K), entropy change S°(T)–S°(350 K), and reduced Gibbs energy Ф°(Т).     

Synthesis and high-temperature heat capacity of Gd<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Gd₂Sn₂O₇ gadolinium stannate with the pyrochlore structure has been prepared by solid-state reaction and its high-temperature heat capacity has been determined by differential scanning calorimetry in the temperature range 350–1020 K. The C_p(T) data are shown to be well represented by the classic Maier–Kelley equation. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of gadolinium stannate: enthalpy increment H°(T)–H°(339 K), entropy change S°(T)–S°(339 K), and reduced Gibbs energy Ф°(Т).     

Mechanochemical Synthesis of Sr-Substituted Hydroxyapatite

We demonstrate that a nanoparticulate Sr-substituted carbonate hydroxyapatite can be prepared by fast mechanochemical synthesis, without further annealing of the material. The synthesis process takes 30 min and yields single-phase products. We have obtained a series of Ca1_0–xSr_x(PO₄)₆(OH)₂ samples with x = 0–2.     

Effects of Iron Contents on the Vortex State in Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>0.5</Subscript>Te<Subscript>0.5</Subscript>

We investigate the effects of iron content on the upper critical field (H _c2) and the activation energy U(T) in thermally activated flux flow in Fe _x Se_0.5Te_0.5 near the superconducting transition temperature T _c . The variations in H _c2(T) with temperature are analyzed using Ginzburg-Landau (GL), Werthamer-Helfand-Hohenberg (WHH) models along with the empirical relation (ER). The obtained values of H _c2(0) depend strongly on the model and the criteria used to determine the transition temperature. However, the general trend is that that H _c2(0) increases with the increasing Fe content. The activation energy U(T) is maximum for x =? 1 and rapidly suppressed by excess or deficiency of iron. The low values of U(T) (～10 meV) reflect the low vortex-pinning nature (due to defects, vacancies, etc.) in the Fe _x Se_0.5Te_0.5 superconductor.     

Thermodynamic properties of (TeO<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>(MoO<Subscript>3</Subscript>)<Subscript>1–<Emphasis Type="Italic">n</Emphasis></Subscript> glasses

The thermal behavior of (TeO₂) _n (MoO₃)_1–n (n = 0.75, 0.85, 0.90) tellurite glasses has been studied by differential scanning calorimetry in the range from T = 300 to T = 850 K and heat capacity has been measured in the temperature range. The thermodynamic characteristics of the devitrification process and glassy state have been determined. The experimental data obtained have been used to evaluate the standard thermodynamic functions of the system in glassy and supercooled liquid states: heat capacity C _p °(T), enthalpy H°(T)–H°(320), entropy S°(T)–S°(320), and Gibbs function G°(T)–G°(320) in the temperature range 320–630 K. The composition dependences of the glass transition temperature and thermodynamic functions for the glasses have been obtained. The thermal and thermodynamic properties of the tellurite glasses have been compared to those of previously studied (TeO₂) _n (WO₃)_1–n and (TeO₂) _n (ZnO)_1–n glasses.     

Microstructure and properties of the Ba<Subscript>0.75−x</Subscript>Ca<Subscript>x</Subscript>La<Subscript>0.25</Subscript>Fe<Subscript>11.6</Subscript>Co<Subscript>0.25</Subscript>Ti<Subscript>0.15</Subscript>O<Subscript>19</Subscript> hexaferrites

In the case of Ti⁴⁺ remain unchanged, the Ca²⁺ substituted Ba_0.75?xCa_xLa_0.25Fe_11.6Co_0.25Ti_0.15O₁₉ (0?≤?x?≤?0.05) were prepared by conventional solid-state reaction method at temperature of 1280 °C. A ball-to-power weight ratio of 10:1. Their crystal structure and magnetic properties were mainly investigated. The results show that the single magnetoplumbite phase structure transformed into the multiphase structure. Meanwhile, the small amount of α-Fe₂O₃ phase existed in M-type phase. The micrographs were observed by a field emission scanning electron microscopy (SEM). Vibrating sample magnetometer (VSM) was used to analyze the magnetic properties. The saturation magnetization (M _s ) first increases then decreases when x from 0 to 0.03. But, when x from 0.03 to 0.05, the saturation magnetization (M _s ) first increases then decreases too. The maximum value is at x?=?0.04 (M _s ?=?70.73 emu/g). The value of coercivity (H _c ) first increases then decreases when x from 0 to 0.04. But, the value increased when x from 0.04 to 0.05. The maximum value is at x?=?0.02 (H _c ?=?1691 Oe).     

Synthesis and High-Temperature Heat Capacity of Neodymium Titanate

Neodymium dititanate, Nd₂Ti2O₇ (monoclinic structure, sp. gr. P2₁), has been prepared by solid-state reaction in air at temperatures from 1673 to 1773 K using the Nd₂O₃ and TiO₂ oxides as starting materials. The high-temperature heat capacity of the resultant polycrystalline Nd₂Ti₂O₇ samples has been determined by differential scanning calorimetry. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of neodymium dititanate (enthalpy increment H°(T)–H°(320 K), entropy change S°(T)–S°(320 K), and reduced Gibbs energy Ф°(T)) in the temperature range 320–1053 K.     

Magnetic Susceptibility and High Field Magneto-transport of Silver-Added Bi-2223 Superconductor: a Revisit

The effect of Ag addition on weak link behaviour of a Bi-2223 (Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O₁₀) polycrystalline sample has been investigated in terms of AC susceptibility, critical current density (J _c), electrical resistivity ρ(T)H and upper critical field (H _c2). A series of phase pure Bi-2223-Ag _x (x = 0.0–0.3) samples are prepared by the solid-state synthesis route. The phase purity, crystal structures and surface morphology are being studied using the X-ray diffraction and scanning electron microscopy (SEM) technique, respectively. The effect of Ag addition on inter- and intragranular coupling has been investigated by means of AC susceptibility and magneto-transport ρ(T)H measurements, and the results are compared with the pure Bi-2223 sample. Enhancement in granular coupling between the grains of the 20 wt% Ag-added Bi-2223 sample has been witnessed. Critical current density (J _c) has been estimated using the AC susceptibility technique, and the results are interpreted in terms of inter- and intragranular coupling of the investigated samples. The high field magneto-transport technique has been used to estimate the upper critical field (H _c2) and thermally activation flux flow (TAFF) activation energy. The ensuing results revealed that H _c2 increases for the 20 wt% Ag-added sample along with enhancement in grain alignment and intergrain connectivity.     

Resorption of Ca<Subscript>3 – <Emphasis Type="Italic">x</Emphasis></Subscript>M<Subscript>2<Emphasis Type="Italic">x</Emphasis></Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript> (M = Na,K) Calcium Phosphate Bioceramics in Model Solutions

The resorbability of bioceramics in the Ca₃(PO₄)₂–CaNaPO₄–CaKPO₄ system is evaluated in an approach involving thermodynamic assessment of solubility and investigation of the dissolution kinetics in model media, in particular in citric acid solutions. Thermodynamic calculation indicates high solubility of the Ca₅Na₂(PO₄)₄, α-CaМPO₄, β-CaKPO₄, and β-СаK_0.6Na_0.4PO₄ phases. Investigation of the dissolution kinetics of ceramics has made it possible to identify two distinct types of behavior of resorbable materials in weakly acidic solutions: with fast resorption kinetics in the case of the phases based on nagelschmidtite solid solutions and α-CaМPO₄ disordered high-temperature solid solutions, and with a nearly constant, relatively low dissolution rate and a high solubility limit in the case of β-СaK_{1 – x}Na _x -based phases.     

Synthesis and High-Temperature Heat Capacity of Dy<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript> and Ho<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates have been prepared by solid-state reactions in several sequential firing steps in the temperature range 1237–1473 K using stoichiometric mixtures of Dy₂O₃ (or Ho₂O₃) and GeO₂. The heat capacity of the synthesized germanates has been determined as a function of temperature by differential scanning calorimetry in the range 350–1000 K. The experimentally determined C _p (T) curves of the dysprosium and holmium germanates have no anomalies and are well represented by the Maier–Kelley equation. The experimental C _p (T) data have been used to evaluate the thermodynamic functions of the Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates: enthalpy increment H°(T)–H°(350 K), entropy change S°(T)–S°(350 K), and reduced Gibbs energy Ф°(T).     

New NaSrPO<Subscript>4</Subscript>:Sm<Superscript>3+</Superscript> phosphor as orange-red emitting material

Sm³⁺-activated NaSrPO₄ phosphors could be efficiently excited at 403 nm, and exhibited a bright red emission mainly including four wavelength peaks of 565, 600, 646 and 710 nm. The highest emission intensity was found for NaSr _1?x PO₄: xSm³⁺ with a composition of x = 0.007. Concentration quenching was observed as the composition of x exceeds 0.007. The decay time values of NaSr_1?x PO ₄ : xSm³⁺ phosphors range from around 2.55 to 3.49 ms. NaSr_1?x PO₄: xSm³⁺ phosphor shows a higher thermally stable luminescence and its thermal quenching temperature T ₅₀ was found to be 350°C, which is higher than that of commercial YAG:Ce³⁺ phosphor and ZnS:(Al, Ag) phosphor. Because NaSr_1?x PO₄: xSm³⁺ phosphor features a high colour-rendering index and chemical stability, it is potentially useful as a new scintillation material for white light-emitting diodes.     

High-temperature heat capacity and vibrational spectra of Eu<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Eu₂Sn₂O₇ compound has been prepared by solid-state reaction (by sequentially firing a stoichiometric mixture of Eu₂O₃ and SnO₂ in air at 1273 and 1473 K) and its heat capacity has been determined by differential scanning calorimetry in the temperature range 370–1000 K. The heat capacity data have been used to evaluate the thermodynamic properties of europium stannate: enthalpy increment H°(T)–H°(370 K), entropy change S°(T)–S°(370 K), and reduced Gibbs energy Ф°(T). Raman spectra of Eu₂Sn₂O₇ polycrystals with the pyrochlore structure have been measured in the range 200–1200 cm^–1.