Effects of additives on sintering and some properties of calcium phosphates with various Ca/P ratios

The effects were studied of additives on the sintering and properties such as the compressive strength and chemical durability of ceramics of calcium pyrophosphate, tricalcium phosphate and hydroxyapatite. In improving the compressive strength of these calcium phosphates, the effect of additives was found to be in the order Na₂OCaF₂>MgOgAl₂O₃, SiO₂ for the case of single-doping, and was most marked in the combination of Na₂O-MgO-Al₂O₃ for the complex addition. It was also found that the chemical durability of the calcium phosphates was improved greatly by using a single or complex additives.     

Characterization and thermal behavior of calcium deficient hydroxyapatite whiskers with various Ca/P ratios

Calcium deficient hydroxyapatite (Ca-def HA) whiskers with various Ca/P ratios were synthesized by a homogeneous hydrothermal precipitation method using acetamide. They had high crystallinity for Ca/P > 1.54, with a mean length of 79–113 μm and an aspect ratio of 85–103. No whisker entanglement and aggregation were found in the products. Thermal analyses revealed that the Ca/P ratio and crystallinity of the products showed significant effects on the crystalline phase, constitution and morphological stability of the whiskers at an elevated temperature. The whiskers with Ca/P > 1.54 were morphologically stable at temperatures below 1200 °C, with minor TCP appearance after heat-treatment at 800 °C; no further decomposition of the whiskers was found with increasing temperature. Such whiskers are possibly one promising reinforcing materials for preparing highly reliable apatite ceramics and composites either in dense or porous block materials with high mechanical properties and good enhanced biological performance.     

<Emphasis Type="Italic">In vitro</Emphasis> synthesis and characterization of amorphous calcium phosphates with various Ca/P atomic ratios

Amorphous calcium phosphates (ACP) were synthesized utilizing poly(ethylene glycol) as stabilizing additive at low temperature. Effects of aging time, pH value, reactant and initial Ca/P atomic ratio on the phase and chemical composition of calcium phosphate precipitates were investigated by powder X-ray diffraction and induced coupled plasma atomic spectroscopy. It was found that ACP could be stabilized by poly(ethylene glycol) in the mother solution for more than 18 h at 5 °C, and Ca/P atomic ratios of ACP precipitates could be adjusted from 1.33 to 1.50 by controlling pH values and initial Ca/P atomic ratios. ACP precipitates were characterized by thermal gravity analysis, Fourier transform infrared spectroscopy, scanning electron microscopy and energy dispersive spectrum. The results show that there is 4 wt% poly(ethylene glycol) in ACP powders without any contaminated ions, and the spherical particle size of ACP powders is 60∼70 nm in the diameter with uniform size distribution which endows it as a potential precursor to prepare crystalline calcium phosphate phases. ACP has potential to be used as biodegradable and/or bioresorbable biomaterials and tissue engineering scaffold.     

Heat treatment-induced phase transformations of materials in a system of calcium phosphates and magnesium phosphates with (Ca + Mg)/P = 2

We have studied the effect of heat treatment in a wide temperature range (from 300 to 1500°C) on the phase composition, heat effects and weight loss of powder materials in a system of calcium phosphates and magnesium phosphates with (Ca + Mg)/P = 2. The results demonstrate that crystalline magnesium-substituted whitlockite phases begin to form at temperatures above 600°C. Raising the heat treatment temperature reduces the degree of magnesium substitution for calcium in the structure of the magnesium-substituted whitlockite. Tetracalcium phosphate, a high-temperature phase, is formed through apatite phase recrystallization.     

Electrical conductivity of calcium phosphate ceramics with various Ca/P ratios

Calcium orthophosphate powders with various Ca/P ratios were prepared by a wet process, employing CaCO₃ and H₃PO₄ as starting materials. After they were calcined and pressed to form pellets, they were fired at various temperatures ranging from 800 to 1200 °C. The samples at various stages were examined by X-ray diffraction and SEM, The a.c. electrical conductivity was measured for a series of samples. For some of the samples, the d.c. electrical conductivity and electromotive force were also measured. The samples showed relatively high conductivity (4×10^–5 S cm^–1 at 800 °C). With respect to the tricalcium orthophosphate with nearly stoichiometric composition, the predominant charge carrier at 800 °C was presumed to be an ion although it is not identified at the moment.     

Synthesis and cellular biocompatibility of two nanophase hydroxyapatite with different Ca/P ratio

The cellular biocompatibility of two types of nanophase hydroxyapatites including nanophase standard hydroxyapatite (n-HA) and nanophase calcium deficient hydroxyapatite (n-CDHA) synthesized by a wet chemical method were assessed using primary cultured osteoblasts. Cytotoxicity of both materials was investigated with L929 cell line. The MTT method was used to evaluate the proliferation of osteoblasts on the third day and ALP activity assay was carried out on the fifth day. SEM was used to observe the morphology of the osteoblasts on the third day. Two types of nanophase hydroxyapatite both showed no cytotoxicity. Higher cell proliferation was observed on n-CDHA than n-HA. At the same time, cells spread more actively on the n-CDHA group. The ALP level of n-CDHA was also significantly higher on the former. Our results show that the n-CDHA is more suitable for osteoblasts growth and is also helpful for ALP synthesis.     

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.     

Effect of the Ca/P ratio on the dielectric properties of nanoscaled substoichiometric hydroxyapatite

Nanoscaled hydroxyapatite (n-HAp) was prepared by a wet chemical precipitation method, pressed to pellets and sintered at various temperatures between 900 and 1200°C. With input stoichiometries of Ca/P ratios between 1.4 and 2.0, compositions in the substoichiometric range of Ca/P between 1.45(1) and 1.62(3) were determined after preparation. After sintering, final values of the Ca/P ratio between 1.45(8) and 1.66(4) were found. Capacitances and dielectric losses were determined in the frequency range between 20 Hz and 1 MHz and dielectric constants calculated from the capacitances. Dependencies of the dielectric properties on the composition, as well as on sintering temperature and frequencies were investigated. The dielectric constants generally tend to increase with increasing Ca-content. Different behaviour was observed for low frequencies (below 10³ Hz) and for compositions far from the stoichiometric point of hydroxyapatite (Ca/P: 1.67). Comparable results were found for dielectric losses.     

Effect of Ca/P ratio and milling material on the mechanochemical preparation of hydroxyapaptite

The mechanochemical transformation of Ca(OH)₂–(NH₄)₂HPO₄ with different Ca/P ratios 1; 1.5; 1.67 and 1.75 was carried out for different periods of time from 10 min to 24 h in a horizontal vibration mill using steel and agate vials and balls. The phase transformations obtained at each milling stage were characterized by X-ray diffraction, infrared spectroscopy and transmission electron microscopy. Complete transformation to hydroxyapatite took place during the first 5 h of milling, for Ca/P ratios 1.5 to 1.7, when milling was carried out with steel vials and balls. The contamination was not significant for the periods of milling studied for both milling media.     

Peculiarities of the thermal activation of carriers in CdSe/ZnSe QD structures

The results of experimental and theoretical investigations of thermal quenching of quantum dot (QD) photoluminescence (PL) in CdSe/ZnSe heterostructures are presented. It is found that at low temperatures, when carriers are strongly localized in QDs, QD PL intensity depends linearly on excitation power, but at higher temperatures, when carriers are thermally excited to the wetting layer, this dependence becomes superlinear. The activation energy of thermal quenching of QD PL intensity is found to be smaller than the sum of QD electron and hole potential depths. It is shown that these facts are explained satisfactorily by the model of independent electron and hole capture (escape) in QDs.     

Effects of nano-surface properties on initial osteoblast adhesion and Ca/P adsorption ability for titanium alloys

Titanium alloys (Ti6Al4V), while subjected to high temperature surface treatment, experience altered nano-surface characteristics. The effects of such surface treatments are examined, including the initial adhesion force experienced by osteoblasts, the Ca/P adsorption capability, and the nano-surface properties, including the amounts of amphoteric Ti-OH groups, surface topography, and surface roughness. The initial adhesion force is considered a quantitative indicator of cyto-compatibility in?vitro. Previously, a cyto-detacher was applied in a pioneer attempt measuring the initial adhesion force of fibroblasts on a metal surface. Presently, the cyto-detacher is further applied to evaluate the initial adhesion force of osteoblasts. Results reveal that (1)?titanium alloys subjected to heat treatment could promote the adsorption capability of Ca and P; (2)?titanium alloys subjected to heat treatment could have higher initial osteoblast adhesion forces; (3)?the adhesion strength of osteoblasts, ranging from 38.5 to 58.9?nN (nanonewtons), appears stronger for rougher surfaces. It is concluded that the heat treatment could have impacted the biocompatibility in terms of the initial osteoblast adhesion force and Ca/P adsorption capability.     

The crystallization of calcium phosphate glass with the ratio [CaO]/[P2O5] < 1

The phase evolution in a calcium phosphate glass with the molar ratio [CaO]/[P₂O₅] < 1, to which 6.4 mol% TiO₂ and 10 mol% Al₂O₃ were added as nucleation agents, was studied. The results indicate that the primary phase -Ca₂P₂O₇ is formed during surface nucleation and crystallization which are dominant at T < 930°C. No presence of metastable calcium phosphates was registered during the formation of the primary phase. The growth of -Ca₂P₂O₇ crystals occurs on the faceted crystal/glass interface with dendritic morphology at a crystal growth rate that is independent of time. The kinetics of -Ca₂P₂O₇ crystal growth are characterised by a growth activation energy of E _a = 426 ± 15 kJ/mol. The parameters of the unit cell of the -Ca₂P₂O₇ phase decrease with increasing temperature. The secondary TiP₂O₇ and AlPO₄ phases are formed by volume nucleation and crystallization. The temperature of the maximum nucleation rate was determined to be T _n = 690°C, and it is higher than the transformation temperature T _g. In the temperature interval T < 930°C secondary phases appear during long annealing times. At T > 930°C glass volume crystallization is dominant.     

Decomposition of Ca : Cu = 1 : 1 nitrate powder: thermal analysis and structural studies

Thermal decomposition processes in mixtures of Ca : Cu = 1 : 1 nitrate powders produced by a conventional technique and by freeze drying method have been investigated by thermal analysis and X-ray diffraction. Results have been compared with literature values and data obtained by our investigations of individual calcium and copper nitrates. Temperatures at which decomposition processes occur change with by ±30°C and depend also on the level of mixing of the nitrates. An exemption is the temperature for CuO phase formation, which was determined for the Cu-nitrate and for both 1 : 1-nitrate powders to be at 266°C. The domains of stability for different phases are shown to be variable. Also, for 1 : 1-powders, -type and -type Ca(NO₃)₂ · 2H₂O phases coexist, while in Ca-nitrate only the -type phase was observed.     

Constant thrust programming for a multistage rocket flown in vacuum with a given initial/final thrust-to-weight ratio in an arbitrary stage

The burn time and burnout velocity of a multistage rocket flown vertically in vacuum with constant thrust tangential to the flight path and a prescribed initial/final thrust-to-weight ratio in an arbitrary stage have been determined. The present paper also deals with optimal staging under given conditions of flight.     

Development of non-flammable high strength extruded Mg-Al-Ca-Mn alloys with high Ca/Al ratio

A series of non-flammable high strength Mg-Al-Ca-Mn alloys with high Ca/Al ratio were fabricated by water-cooled casting and hot extrusion. Microstructure and mechanical properties were investigated to study the effect of hot extrusion. The experimental results showed that hot extrusion significantly improved the mechanical properties by grain refinement and precipitates. Ignition temperature was measured by furnace test, and the highest temperature is up to approximately 1040?°C due to the composite oxide layer consisting of CaO and MgO. In addition, a laboratory-scale flame test was conducted to evaluate the flammability of smaller specimens. These alloys exhibited marvelous flame resistance attributed to the protective effect of dense and stable oxide film.     

Influence of temperature, [Ca2+], Ca/P ratio and ultrasonic power on the crystallinity and morphology of hydroxyapatite nanoparticles prepared with a novel ultrasonic precipitation method

Nanocrystalline hydroxyapatite was prepared by a precipitation method with the aid of ultrasonic irradiation using Ca(NO₃)₂ and NH₄H₂PO₄ as source material and carbamide (NH₂CONH₂) as precipitator. The influence of Ca/P molar ratio, precipitation temperature, concentration of Ca²⁺ ([Ca²⁺]) and ultrasonic power on the crystallinity of the nanopowder were systematically investigated by XRD analysis. The size of the as-prepared particles was analyzed using TEM and XRD methods. The results revealed that the monophase hydroxyapatite could be obtained at the following technological conditions: [Ca²⁺] = 0.01–0.1 mol/L, ultrasonic power = 300 W, Ca/P (mol) = 1.2–2.5 and T = 313–353 K. In addition, the acicular and spherical particles could be prepared at different ultrasonic powers of 300 and 200 W, respectively.     

Behaviour of Composite Ca/P Bioceramics in Simulated Body Fluid

Behavior of Ca/P ceramics enhanced by MgAl2O4 in simulated body fluid(SBF) was studied.Results show that, biological apatite can precipitate on the sudece of Ca/P composite ceramics,however, dissolution of the surface of Ca/P composite ceramics also occurs due to a kind of amorphous material which is produced through reaction of MgAl2O4 with Ca/P ceramics at elevated temperature, therefore, the behavior of Ca/P composite ceramics in SBF mainly depends on those two processes. Analyses of joint influence of those two processes show that low content of MgAl2O4 favors the formation of biological apatite, therefore, brings about higher bioactivity     

Synthesis of lamellar mesostructured calcium phosphates using n-alkylamines as structure-directing agents in alcohol/water mixed solvent systems

Lamellar mesostructured calcium phosphates constructed by ionic bonds were prepared by using n-alkylamines (n-C _n H_2n+1NH₂, n = 8–18) at room temperature in the mixed solvent systems of aliphatic alcohol (C _n H_2n+1OH, n = 1–4) and water, and the synthetic conditions were investigated in detail. The mixed solvent systems suppressed the formation of crystalline calcium phosphates like brushite (CaHPO₄·2H₂O) and monetite (CaHPO₄) at low temperatures, successfully affording pure lamellar mesostructured calcium phosphates. Other crystalline phases such as hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) were not formed under the conditions with the Ca/P molar ratios in the range of 0.7–1.0 in the starting mixtures. The Ca/P molar ratio of the lamellar mesostructured calcium phosphates was ca. 1.0, calculated by ICP and ³¹P MAS NMR data. Interestingly, the kind of alcohols strongly influenced the solubilities of calcium phosphate species and n-alkylamines, and then lamellar mesostructured phases were obtained with some morphological variation.     

Transformations in a metal/insulator/semiconductor structure with an amorphous insulator film caused by contacts

The thermodynamic criteria for and structural consequences of interphase interactions at the boundaries of adjacent layers in an Nb/a-Nb₂O₅/MnO₂ (where a indicates amorphous) metal/insulator/semiconductor (MIS) structure were analysed and studied as an example of the behaviour of similar MIS structures. It was shown experimentally that the interaction in the thin insulator film induced a permanent electrochemical breakdown of the MIS structure which was followed by a step-like electrothermal breakdown under certain conditions. The relation between transformations in the ionic and electronic subsystems of the MIS structure at both stages of its relaxation into a state of thermodynamic equilibrium was investigated. A mechanism was proposed for the electroforming of the film, and investigations of the current-controlled negative resistance and bistable memory switching were performed.