Effect of mixing ratio and pH on the reaction between Ca4(PO4)2O and CaHPO4

The reaction of _Ca(PO₄)₂O (TTCP) and CaHPO₄ (DCPA) in an aqueous solution has been shown to be responsible for the hardening of a calcium phosphate cement. This reaction was investigated by monitoring pH changes and composition of solid phases. In the first set of experiments (no attempt to control pH), 2.5 g each of mixtures of TTCP/DCPA, molar ratio from 0.25 to 2, was placed in 12.5 mL of 0.15 mol/L KCl solution, at initial pH about 7, and the pH was allowed to drift for 24 h. Results show that at any time up to 24 h, the pHs were higher for slurries with higher TTCP/DCPA molar ratios. For the slurries with TTCP/DCPA molar ratio of 0.83, the 24 h pHs of the slurries were 9 to 11, whereas for those with TTCP/DCPA of 0.67, the pHs were between 5.3 and 7. The slurries with TTCP/DCPA molar ratios between 0.5 and 1 (Ca/P molar ratio=1.5 to 1.67) reacted completely within 24 h to form hydroxyapatite(OHAp), Ca₅(PO₄)₃OH. In the second set of experiments, 2 g of an equimolar TTCP and DCPA mixture was placed in 20 mL of 0.15 mol/L KCl solution. The pH values were kept constant (6, 8 or 10) by using H₃PO₄ and Ca(OH)₂ or HCl and KOH as titrant solutions. At pH 8, DCPA and TTCP dissolved at about the same rate, whereas at pH 10, DCPA was consumed more rapidly than TTCP. At both pHs, OHAp was the only product formed. However, at pH 6, the composition of reaction products depended on the types of the titrants used. Specifically when H₃PO₄ and _Ca(OH)₂ were used, hydrolysis of TTCP was the predominant reaction and both octacalcium phosphate and OHAp were formed. But, when HCl and KOH were used, only OHAp was formed. In this case hydrolysis of TTCP and DCPA appeared to proceed independently with TTCP hydrolysis beginning immediately and progressing slowly through 48 h while the DCPA hydrolysis began several hours after the reaction started but was completed in 24 h. ©2000 Kluwer Academic Publishers     

Observation of fluorapatite formation under hydrolysis of tetracalcium phosphate in the presence of KF by means of soft X-ray emission and absorption spectroscopy

The effect of fluoride on the hydrolysis of tetracalcium phosphate (TTCP; Ca₄(PO₄)₂O) in 0.1 mol/l KH₂PO₄ containing 62–83 mmol/l KF was studied with the help of X-ray fluorescence measurements. Fluorine X-ray emission and absorption spectra of the final product of hydrolysis and reference samples (CaF₂ and Ca₅(PO₄)₃F) were measured at Beamline BL-2C of Photon Factory (PF, Tsukuba). Based on these measurements we concluded that hydrolysis of TTCP in the presence of KF converts it into fluorapatite. Formation of CaF₂, which is often found in the hydrolysis of hydroxyapatite at high fluoride concentration, was not observed.     

Solubility characteristics of synthetic silicate sulphate apatites

Isomorphic substitution of (Si, S) for phosphorus in apatite (Ca₅(PO)₃F) was investigated in the solid solution series from P-apatite to (Si, S)-apatite utilizing compositions of Ca₅(PO₄)_3–x (SiO₄) _x/2(SO₄)_x/2F with x=0, 0.75, 1.5, 2, 2.25, 3. Study of the solubility characteristics of these apatites showed an increased weight loss in water with progression from the P-apatite end member to the (Si, S)-apatite end member. Experimentally determined pK _sp decreased from 60.62 for the fluorapatite (Ca₅(PO₄)₃F) to 31.65 for fluorellestadite (Ca₅(SiO₄)_1.5(SO₄)_1.5F). The free energy of formation calculated from the solubility product became less negative from –6755 kJ mol ^–1 for fluorapatite to –5760 kJ mol ^–1 for fluorellestadite.     

Single phase cool white light emitting novel Ca9Al(PO4)7:Dy3+ nanophosphor under NUV excitation

A series of novel single-phase white light-emitting Dy³⁺-doped Ca₉Al(PO₄)₇ nanophosphors was successfully synthesized at 1100 °C via solution combustion route. X-ray diffraction (XRD) and Rietveld refinement analysis of Ca₉Dy_0.03Al_0.97(PO₄)₇ sample confirmed that this phosphor had a trigonal crystal structure with space group R3c(161). Meanwhile, as-observed from the transmission electron microscopy (TEM) study; particles of Ca₉Dy_0.03Al_0.97(PO₄)₇ sample were found to have a quadrilateral shape with crystallite sizes around 40–60 nm which were also confirmed by the Debye Scherrer equation. Under near-ultraviolet (NUV) excitation at 350 nm, photoluminescence (PL) emission spectra of nanocrystalline Ca₉Al(PO₄)₇:Dy³⁺ phosphors showed two peaks at 481 nm and 572 nm corresponding to ⁴F_9/2?→?⁶H_15/2 and ⁴F_9/2?→?⁶H_13/2 transitions, respectively. The optimum concentration was found to be x?=?0.03 mol. The critical energy transfer distance was calculated to be 20 Å and further Huang analysis concluded the exact mechanism, i.e. dipole–dipole interactions responsible for concentration quenching in Ca₉Dy_xAl_(1?x)(PO₄)₇ samples. Furthermore, the Commission Internationale de I’Eclairage (CIE) chromaticity coordinates of Ca₉Dy_0.03Al_0.97(PO₄)₇ nanophosphor was calculated to be (0.260, 0.297) and this nanophosphor had correlated color temperature (CCT) of 11,332 K which is located in a cool white area. Existing results indicate that Ca₉Dy_0.03Al_0.97(PO₄)₇ nanophosphor may be considered as a favorable candidate in NUV-based single-phase cool white light-emitting diodes (WLEDs).

     

Thermodynamic properties of NaZr2(PO4)3

The heat capacity of crystalline NaZr₂(PO₄)₃ was measured between 7 and 340 K by adiabatic calorimetry. The results were used to calculate the thermodynamic functionsC _p ⁰ ,H ⁰(T) -H ⁰(0),S ⁰(T), andG ⁰(T) -H ⁰(0) in the range 0-340 K. The absolute entropy was found to be S⁰NaZr₂(PO₄)₃, cr, 298.15 K) = 327.1 ±1.0 J/(mol K), and the standard entropy of formation Δ_fS⁰(NaZr₂(PO₄)₃, cr, 298.15 K) = -1101±1 J/(mol K). Solution calorimetry was used to determine the standard enthalpy of formation, Δ_f H ⁰(NaZr₂(PO₄)₃, cr, 298.15 K) = -5236 ±5 kJ/mol. By combining the data obtained by the two techniques, the standard Gibbs energy of formation was determined to be Δ_fG⁰(NaZr₂(PO₄)₃, cr, 298.15 K) = -4908 ±5 kJ/mol.     

The kinetics of calcium deficient and stoichiometric hydroxyapatite formation from CaHPO4·2H2O and Ca4(PO4)2O

Isothermal calorimetry was performed on intimate mixtures of CaHPO₄·2H₂O and Ca₄(PO₄)₂O constituted at Ca/P molar ratios of 1.50 and 1.67 to form the hydroxyapatite compositions Ca₉HPO₄(PO₄)₅OH and Ca₁₀(PO₄)₆(OH)₂, respectively, at complete reaction. The temperature range investigated was 15–70°C. The effects of the reaction temperature on the rates of heat evolution during hydroxyapatite formation were determined. Reactions were carried out utilizing a liquid-to-solids weight ratio of 1.0. A two-stage reaction mechanism was observed regardless of the Ca/P ratio as indicated by the presence of two reaction peaks in the plots of the rates of heat evolution against time. An Arrhenius relationship was found between the rate and temperature for each reaction stage for both compositions. Apparent activation energies of 120 and 90 kJ/mol (Ca/P=1.67) and 118 and 83 kJ/mol (Ca/P=1.50), respectively, were calculated for the first and second reaction peaks. An Arrhenius relationship was also found between the time of maximum rate and temperature. The following qualitative reaction mechanism is proposed for each of the two reaction stages for both compositions studied. The first stage involves the complete consumption of CaHPO₄·2H₂O and the partial consumption of Ca₄(PO₄)₂O to form a noncrystalline calcium phosphate and nanocrystalline hydroxyapatite. During the second stage the remaining Ca₄(PO₄)₂O reacts with the noncrystalline calcium phosphate to form the final product, stoichiometric or calcium deficient hydroxyapatite.     

Synthesis of new apatite phases by spray pyrolysis and their characterization

This paper describes the synthesis of some apatite compounds, called britholites, by an aerosol pyrolysis technique, which compared to conventional methods of solid-state chemistry, presents the advantages of reducing noticeably the temperature and the duration of the treatment. Britholites are minerals based on Ca_{10 – x} Ln _x (PO₄)_{6 – x} (SiO₄) _x X ₂ formula (with X = F^–, OH^–, X ₂ = O^2–); they are promising materials for nuclear waste storage. In a second part we have synthesized and characterized a new apatite phase where the substitution of calcium by barium in britholite leads to a monophased solid solution Ca_{9 – x} Ba _x Nd(PO₄)_{6 – x} (SiO₄)F₂ for 6 < x < 9. The possible interest of this new material is discussed.     

The effect of modification of KH2PO4 hardening liquid with H3PO4 and chitosan on setting reactions and compressive strength of calcium phosphate cement

The setting processes and mechanical properties of tetracalcium phosphate (TTCP) -monetite cements with H₃PO₄ and chitosan modified KH₂PO₄ hardening liquid have been studied. From XRD phase analysis, it was found that brushite was created in the first stage of the setting process by the interaction between phosphate compounds in hardening liquids and TTCP. Calcium deficient hydroxyapatite was the final product of the hardening process. The orthophosphoric acid addition to KH₂PO₄ hardening liquid caused lowering resistance to disintegration and an increase in the setting times of cements which allows the possibility for their control. Cement with pure KH₂PO₄ hardening liquid was resistant to wash-out immediately after mixing. Chitosan addition to KH₂PO₄ + H₃PO₄ hardening liquid of an amount around 1 wt.% did not affect change of setting time or improvement of disintegration behaviour of cement. Compressive strengths were around 80–100 MPa in cements soaked in SBF without an chitosan addition and chitosan caused an approximately 15% decrease in compressive strength. The compressive strength of calcium phosphate cements is more influenced by the hydroxyapatite particle morphology than their crystallinity.     

Abnormal blue-shift of Eu2+-activated calcium zirconium phosphates CaZr4(PO4)6

The electronic structure of CaZr₄(PO₄)₆ was calculated using the CASTEP code and the band gap for CaZr₄(PO₄)₆ can reach up to 4.30 eV. Ca_1−xEu_xZr₄(PO₄)₆ (0.01 ⩽ x ⩽ 1) samples were prepared by a high temperature solid-state reaction method. XRD analysis shows that Eu²⁺ ion can be totally incorporated into CaZr₄(PO₄)₆ forming complete solid solutions with trigonal lattice. Ca_1−xEu_xZr₄(PO₄)₆ (0.01 ⩽ x ⩽ 1) shows typical broad band emission in wavelength range from 400 to 650 nm for both under ultraviolet (UV) light and X-ray excitation, originating from the 4f⁶5d¹ → 4f⁷5d⁰ transition of Eu²⁺ ions. With increasing Eu²⁺ concentration, there is abnormal blue-shift of the emission peaks for Ca_1−xEu_xZr₄(PO₄)₆ due to the decreasing crystal field strength and Stokes shift. With increasing temperature in CaZr₄(PO₄)₆: Eu²⁺, its emission bands show the anomalous blue-shift with decreasing intensity. The overall scintillation efficiency of Ca_0.9Eu_0.1Zr₄(PO₄)₆ is 1.7 times of that of Bi₄Ge₃O₁₂ (BGO) powder under the same conditions. In addition, its predominant decay time is about 50 ns at room temperature. The potential application of Eu²⁺-doped CaZr₄(PO₄)₆ has been pointed out.     

Location of Cu2+ ions in some protoned Nasicon-type phosphates

Nasicon-type phosphates Cu_1–xH_xZr₂(PO₄)₃ (0 < x <1) and Cu_1–xH_2x–1Zr₂(PO₄)₃ (0.5 < x <1) have been investigated by magnetic susceptibility and electron paramagnetic resonance (EPR). Room-temperature EPR spectra at X-band (9.5 GHz) exhibit relatively different local information about paramagnetic environments in the two sets. Analysis by computer simulations of Cu_1–xH_xZr₂(PO₄)₃ spectra reveals that Cu²⁺ ion is located in an axially distorted octahedron, which can be assigned to the M(1) site. However, in the case of Cu_1–xH_2x–1Zr₂(PO₄)₃, EPR parameters suggest that Cu²⁺ ions are distributed in two types of sites with axial and lower than axial symmetries; these latter can be attributed to M(1) and M(2) sites respectively. g and A components are related to structural properties using molecular orbital method. Data are obtained on variations of the bond covalence with the composition.     

Determination of the Ca/P ratio in calcium-deficient hydroxyapatite using X-ray diffraction analysis

The determination of the calcium to phosphate ratio (Ca/P) in Ca-deficient hydroxyapatite [d-HAP; Ca_10–x (HPO₄) _x (PO₄)_6–x (OH)_2–x ] using X-ray diffraction analysis (XRD) is reported. At temperatures above 700°C HPO₄ ^2- groups are transformed to PO₄ ^3- groups, thereby producing -tricalcium phosphate [-TCP; Ca₃(PO₄)₂]. Thus, the deviation from stoichiometry, x, can be calculated from the mass fraction of -TCP, which in turn can be determined from quantitative XRD analyses. In this study d-HAP powders with various Ca/P ratios were prepared following several procedures. It is shown that the Ca/P ratio determined by quantitative XRD correlates well with that measured by chemical analyses.     

Sol-gel-derived hydroxyapatite powders and coatings

Hydroxyapatite (HAP) and tri-calcium phosphate (TCP) powders and coatings with a Ca/P molar ratio from 1.56 to 1.77 were prepared by the sol-gel technique using calcium 2-ethylhexanoate (Ca(O₂C₈H₁₅)₂) and 2-ethyl-hexyl-phosphate as calcium and phosphorus precursors, respectively. The structural evolution and phase formation mechanisms of HAP and tri-calcium phosphate in calcined powders and coatings on Si wafer and Ti-alloy substrates (Ti-30Nb-3Al and Ti-5Al-2.5Fe) were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The elimination of organics was studied by differential thermal analysis (DTA) and thermogravimetry (TGA). Two different formation mechanisms of crystallization are proposed. In sols with Ca/P 1.67, -tricalcium phosphate is formed as the major phase and hydroxyapatite as a minor phase by calcination at 700°C. At 900°C these phases react to form AB-type carbonated hydroxyapatite (Ca_10–2x/3[(PO₄)_6–x (CO₃) _x ][(OH)_2–x/3–2y (CO₃) _y ]). A release of CO₂ substituting PO₄ ^3– occurs between 900°C and 1100°C yielding carbonate apatite, Ca₁₀(PO₄)₆[(OH)_2–2y (CO₃) _y ], whereas CO₂ substituting OH^– groups in the apatite structure is released above 1200°C. In sols with Ca/P 1.70, rather than carbonate apatite, B-carbonated hydroxyapatite Ca_10–2x/3[(PO₄)_6–x (CO₃) _x ](OH)₂ is formed, which subsequently decomposes into HAP and CaO above 1200°C. The optimum sintering conditions for coatings on Ti-alloys are found to be 600°C for 10 minutes, since, at higher temperature, oxidation of titanium and the formation of rutile (TiO₂) occur. Dip coating and sintering in two cycles yielded a homogeneous and dense coated film with a thickness of 250 nm.     

Immobilization of cesium by crystalline zirconium phosphate

HZr₂(PO₄)₃ was prepared by the thermal decomposition of NH₄Zr₂(PO₄)₃ which was synthesized in advance by a hydrothermal reaction from a mixed solution of ZrOCl₂, H₃PO₄ and H₂C₂O₄. Mixtures of HZr₂(PO₄)₃ with various amounts of CsNO₃ were treated at 700–1200°C, in order to investigate the immobilization of Cs ion. When a mixture of CsNO₃/HZr₂(PO₄)₃ in a molar ratio of 0.36 was treated at 700°C, the main product was suggested to be CsZr₂(PO₄)₃ from XRD measurements. The leaching rate of Cs ion from this product was less than 10^–10 g · cm^–2 · day^–1 in 0.1 mol · 1^–1 HCl solution at 100°C, indicating that HZr₂(PO₄)₃ reacts with CsNO₃ to give a stable Cs-immobilized product.     

Electrochemical Investigation of Calcium Substituted Monoclinic Li3V2(PO4)3 Negative Electrode Materials for Sodium- and Potassium-Ion Batteries

Herein, the electrochemical properties and reaction mechanism of Li_3‒2xCa_xV₂(PO₄)₃/C (x = 0, 0.5, 1, and 1.5) as negative electrode materials for sodium-ion/potassium-ion batteries (SIBs/PIBs) are investigated. All samples undergo a mixed contribution of diffusion-controlled and pseudocapacitive-type processes in SIBs and PIBs via Trasatti Differentiation Method, while the latter increases with Ca content increase. Among them, Li₃V₂(PO₄)₃/C exhibits the highest reversible capacity in SIBs and PIBs, while Ca_1.5V₂(PO₄)₃/C shows the best rate performance with a capacity retention of 46% at 20 C in SIBs and 47% at 10 C in PIBs. This study demonstrates that the specific capacity of this type of material in SIBs and PIBs does not increase with the Ca-content as previously observed in lithium-ion system, but the stability and performance at a high C-rate can be improved by replacing Li⁺ with Ca²⁺. This indicates that the insertion of different monovalent cations (Na⁺/K⁺) can strongly influence the redox reaction and structure evolution of the host materials, due to the larger ion size of Na⁺ and K⁺ and their different kinetic properties with respect to Li⁺. Furthermore, the working mechanism of both LVP/C and Ca_1.5V₂(PO₄)₃/C in SIBs are elucidated via in operando synchrotron diffraction and in operando X-ray absorption spectroscopy.     

Effects of pH and initial Ca2+?H2PO4? concentration on fibroin mineralization

In the present study, the effects of pH and initial Ca²⁺−H₂PO₄ ⁻ (Ca-P) concentration on fibroin mineralization were studied. The crystal growth of calcium phosphates was regulated by regenerated silk fibroin for 8 h (at pH 4.0, 7.0 and 10.0, respectively). Meanwhile, different concentrations of Ca²⁺ were employed at a certain pH value, keeping the initial Ca-P molar ratio constant at 1.67, i.e., the stoichiometry of hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂, HAP]. The products were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The results demonstrated that, compared to pH 4.0 and 10.0, pH 7.0 promoted the transformation of brushite (CaHPO₄·2H₂O, DCPD) to HAP. In the composites of mineralized fibroin, DCPD is the main inorganic phase at both relatively low and high pH, while HAP is the main inorganic phase at pH 7.0. Additionally, the initial Ca-P concentration does not affect the kind of inorganic phase in the synthesized mineralized fibroin, but induce to different contents of inorganic mineral and different morphology of DCPD at pH 4.0 and pH 10.0.     

Sintering of (Ca1−x,Mg x )Zr4(PO4)6 ceramics

Densification and grain growth of (Ca_1–x , Mg _x )Zr₄(PO₄)₆ (CMZP; where x=0.0, 0.1, and 0.4) ceramics at the intermediate stage of sintering have been investigated. A lattice-diffusion model proposed by Coble with tetrakaidecahedron as a grain shape was employed and appears to be applicable to the sintering of CMZP. The apparent diffusion coefficient of the CMZP ceramics for x=0.0 is lower by approximately three and one orders of magnitude than for x=0.1 and x=0.4, respectively. The grain growth in CMZP at the intermediate stage of sintering follows a third-power kinetics, i.e. n=3. A modified expression using the lattice-diffusion model associated with grain-growth kinetics has been derived which makes the microstructures of CMZP controllable.     

Crystal structure of the substituted apatites — deviation from Vegard's Law

The substituted apatites Ca₁₀(PO₄)_6(1–x)(VO₄)_6x (OH)₂ and Pb₁₀(PO₄)_6(1–x)(VO₄)_6x (OH)₂, where x is the fraction of vanadate, were prepared in the powdered microcrystalline nature. The X-ray diffraction patterns and the infrared spectra of these substituted apatites were recorded. The analysis of the measurements established that the unit cell parameters did not vary smoothly with the degree of substitution as expected by the Vegard's Law. Two different types of linear behaviour in the two regions 0 to 50% and 50 to 100% were present. Hence, this implies the inadequacy of the Vegard's Law when applied to the whole range of substitutions.     

Ultrasonic study of liquid-quenched (AgI) x (Ag3PO4)1−x compared with (AgI) x (Ag4P2O7)1−x

The use of ultrasonic velocity-attenuation in the transition region from glassy to supercooled liquid state of the (AgI) _x (Ag₄P₂O₇)_1–x system is extended to the (AgI) _x (Ag₃PO₄)_1–x system. Comparison is made between both systems. The latter system is more liable to crystallize on cooling from the melt during sample preparation, possibly due to the more spherical shape of the Ag₃PO₄ molecule compared to the Ag₄P₂O₇ molecule (allowing easier rearrangement of the molecule for crystallization with AgI). By deliberate quenching one can escape crystallization, but only for the x=0.75 composition. Ultrasonics for this glass reveal structural features which are similar to those observed by differential scanning calorimetry.     

Investigation of Cs(H2PO4)1 − x (HSO4) x (x = 0.15–0.3) superprotonic phase stability

A detailed investigation of the highly conductive Cs(H₂PO₄)_1?x (HSO₄) _x (x = 0.15–0.3) proton electrolyte, its structural properties, and ageing behavior was carried out using X-ray diffraction, DSC, and impedance and NMR spectroscopy. The high conductivity of electrolytes (～2 × 10^?2 S/cm) remains stable during long-term ageing at 180–200°C due to stabilization of the high temperature phase to lower temperatures. The room temperature ¹H MAS NMR spectrum of (CsH₂PO₄)_1?x (CsHSO₄) _x demonstrates the predominantly highly mobile protons present in these materials with the residual low-mobile protons, which agrees with the XRD data. According to XRD and ¹H NMR data, the cubic phase of Cs(H₂PO₄)_{1 ? x} (HSO₄) _x (x = 0.15–0.3) that stabilizes at room temperature gradually transforms to a low-temperature monoclinic one. The kinetics of the phase transformation for mixed salt depends markedly on the relative air humidity. A possible stabilization mechanism of the Cs(H₂PO₄)_{1 ? x} (HSO₄) _x superionic phase with high proton mobility at low temperatures is discussed.     

Luminescence of Eu3+-Activated Potassium Scandium and Potassium Yttrium Phosphate Vanadates

The photoluminescence spectra of potassium rare-earth phosphate vanadates K₃R_{1 – y} Eu _y (PO₄) _x (VO₄)_{2 – x} (R = Sc, Y) were measured in the range 450–800 nm under excitation at 337 nm. The energies of the Stark components of the ⁷ F _j(j= 0, 1, 2) multiplet were determined, and the crystal-field parameters were evaluated. The effects of the Eu³⁺concentration and PO₄ ^3–/VO₄ ^3–ratio on the luminescence of the materials studied and the concentration quenching of luminescence were analyzed. The materials with high Eu³⁺concentrations are shown to be potentially attractive as photo- and cathodoluminophors.