The effects of temperature on the behaviour of an apatitic calcium phosphate cement

An apatitic calcium phosphate cement is obtained by mixing -tricalcium phosphate (-TCP) and precipitated hydroxyapatite into a cement powder, and by then mixing this powder with an aqueous solution of Na₂HPO₄ as an accelerator. Setting times were reduced by about 30% by increasing the temperature from 22 to 37°C. Compressive strength reached higher intermediate and final values at 37 °C. Degrees of transformation of the -TCP in the resulting calcium-deficient hydroxyapatite (CDHA) were much higher at 37 °C after 24 h of storage in Ringer's solution according to X-ray diffraction. Differential scanning calorimetry indicated that the rate of reaction increased by a factor of about 5 when the temperature was increased from 25 to 37 °C. Scanning electron microscopy showed that the microstructure was more homogeneous and that a more tight entanglement of the precipitated CDHA crystals occurred after storage at 37 °C than at room temperature.     

Hydroxyapatite crystal growth on calcium hydroxyapatite ceramics

Calcium hydroxyapatite (Ca-HAP) ceramics containing tricalcium phosphate (TCP) were soaked in three solutions: phosphate buffer, tris buffer, and simulated body fluid (SBF). Petal-like crystals of Ca-HAP were deposited on the Ca-HAP ceramics when (i) Ca-HAP ceramics contained -TCP, (ii) the soaking solution contained phosphate ion and (iii) the pH of soaking solution was higher than 7.3. These conditions facilitate the presence of HPO ₄ ^2– and Ca²⁺ ions, the latter from dissolution of -TCP. A well-defined X-ray diffraction pattern for the deposited Ca-HAP crystals indicates preferred growth of {002} planes. Slower crystal growth of Ca-HAP was found for SBF (pH=7.5) than in the phosphate buffer, due possibly to the lower phosphate ion content in SBF.     

Interfacial bond strength of electrophoretically deposited hydroxyapatite coatings on metals

Hydroxyapatite (HAp) coatings were deposited onto substrates of metal biomaterials (Ti, Ti6Al4V, and 316L stainless steel) by electrophoretic deposition (EPD). Only ultra-high surface area HAp powder, prepared by the metathesis method 10Ca(NO₃)₂ + 6(NH₄)₂HPO₄ + 8NH₄OH), could produce dense coatings when sintered at 875–1000°C. Single EPD coatings cracked during sintering owing to the 15–18% sintering shrinkage, but the HAp did not decompose. The use of dual coatings (coat, sinter, coat, sinter) resolved the cracking problem. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) inspection revealed that the second coating filled in the valleys in the cracks of the first coating. The interfacial shear strength of the dual coatings was found, by ASTM F1044-87, to be 12 MPa on a titanium substrate and 22 MPa on 316L stainless steel, comparing quite favorably with the 34 MPa benchmark (the shear strength of bovine cortical bone was found to be 34 MPa). Stainless steel gave the better result since -316L (20.5 m mK^-1) > -HAp (14 m mK^-1), resulting in residual compressive stresses in the coating, whereas -titanium (10.3 m mK^-1) < -HAp, resulting in residual tensile stresses in the coating. © 1999 Kluwer Academic Publishers     

Injectable magnesium-doped brushite cement for controlled drug release application

Dicalcium phosphate dihydrate (CaHPO₄·2H₂O), also known as brushite, is one of the important bioceramics for bone regeneration. However, fast setting of the brushite cement under physiological conditions has limited its clinical use. Furthermore, brushite cement without any additives normally has poor injectability due to the liquid–solid phase separation. In the present study, magnesium-doped β-tricalcium phosphate (Mg-β-TCP) with chemical formula of β-Ca_2.96?x Mg _x (PO₄)₂ was used to prepare injectable brushite cements with improved physicochemical properties. β-TCP containing different amounts of Mg²⁺ ions were reacted with monocalcium phosphate monohydrate [Ca(H₂PO₄)₂·H₂O, MCPM] in the presence of water to furnish corresponding brushite cement. The samples were characterized using X-ray diffractometry, Fourier transform infrared spectroscopy and field emission scanning electron microscopy. The effect of magnesium ions on the structural, mechanical, and setting properties of the cements is reported. Our results indicate that the presence of Mg²⁺ ions increases the degree of injectability, setting time, and mechanical properties of the brushite cement. The compressive strength of brushite cement was substantially increased upon incorporation of Mg²⁺ ions. Furthermore, the setting times of the brushite cement were significantly improved. Gentamicin sulfate, amoxicillin and ampicillin trihydrate were incorporated into the Mg-brushite cement, and their release profiles showed a sustained drug release over 14 days. Cumulative releases of 99.3, 87, and 79 % were observed for gentamicin sulfate, amoxicillin, and ampicillin trihydrate, respectively.     

On dual solutions occurring in mixed convection in a porous medium

Summary The dual solutions to an equation, which arose previously in mixed convection in a porous medium, occuring for the parameter in the range 0 < < ₀ are considered. It is shown that the lower branch of solutions terminates at =0 with an essential singularity. It is also shown that both branches of solutions bifurcate out of the single solution at =₀ with an amplitude proportional to (₀-)^1/2. Then, by considering a simple time-dependent problem, it is shown that the upper branch of solutions is stable and the lower branch unstable, with the change in temporal stability at =₀ being equivalent to the bifurcation at that point.     

Effect of temperature and immersion on the setting of some calcium phosphate cements

Calcium phosphate cements based on powders containing -Ca₃(PO₄)₂ and aqueous solutions containing Na₂HPO₄ as accelerator were used to determine the effects of accelerator concentration, temperature and immersion on the setting time. Increases in accelerator concentration and temperature increased the rate of setting, but immersion had a retarding effect. These results were used to design a method whereby a syringe filled with cement paste can be kept ready for injection of the paste into the implantation site for a long time, whereas setting of the cement paste in the body takes place in a short time.     

Production and characterization of new calcium phosphate bone cements in the CaHPOv4–α-Ca3(PO4)2 system: pH, workability and setting times

The initial setting properties of calcium phosphate cements in the CaHPOv₄–-Ca₃(PO₄)₂ (DCP–-TCP) system have been investigated. Interest was focused on the pH, workability, cohesion time and initial and final setting times. The addition of CaCO₃ modified the structure of the cement reaction product such that it became more similar to the apatite phase in bone mineral. The addition of 10% w/w of CaCO₃ reduced the viscosity of the cement pastes resulting in an increase in initial and final setting times and improved injectability. © 1999 Kluwer Academic Publishers     

Seeding effects on crystallization temperatures of cordierite glass powder

The role of solid state epitaxy in the crystallization of nanocomposite cordierite glass to glass ceramic was investigated. The use of isostructural (-cordierite) seeds in cordierite glass led to a lowering in the crystallization temperature to form glass ceramic by about 50 °C compared to the unseeded glass. The use of non-isostructural seeds such as ZrO₂ and TiO₂ did not lower the crystallization temperature of cordierite glass to glass ceramic, and in the case of the TiO₂-seeded glass the crystallization temperature increased by about 50 °C compared to the unseeded-cordierite glass. The lowering in crystallization temperature by-cordierite seeding can be attributed to the nucleation and epitaxial growth mechanism.     

The effect of additives on the crystallization and sintering of 2MgO-2Al2O3-5SiO2 glass-ceramics

Crystallization and sintering behaviour of three cordierite (2MgO-2Al₂O₃-5SiO₂) glasses containing different amount of additives were investigated and compared by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and the Archimedes method. The stoichiometric 2MgO-2Al₂O₃-5SiO₂ (MAS) glass and the 2MgO-2Al₂O₃-5SiO₂ glass containing 3 wt% of B₂O₃ and 3 wt% of P₂O₅ (MASBP) showed two exotherms (one for -cordierite formation from a glass and the other for -cordierite formation from the -cordierite phase), whereas the 2MgO-2Al₂O₃-5SiO₂ glass containing 2 wt % of B₂O₃, 2 wt% of P₂O₅, and 2 wt % of TiO₂ (MASBPT) showed only a single exotherm representing -cordierite formation. By using Kissinger, Augis-Bennett, Ozawa, and modified Kissinger methods, the activation energy values for -cordierite formation in the MASBP and MASBPT glasses were determined as 310±6 and 326±13 kJ mol^–1, respectively, whereas that in the MAS glass was determined as 868±5 kJ mol^–1. The MASBPT glass showed the lowest peak temperature value for -cordierite formation (980 °C) amongst the three glasses. Both the MASBP and MASBPT glasses showed excellent sintering behaviour (> 99.7% of theoretical density).     

Phase diagram and some properties of Cu2−xSe(2.01≥2−x≥1.75)

The temperatures of phase transformation from low-temperature -phase to high-temperature -phase were measured for copper selenide Cu_2–x Se in the composition range 2.012–x1.75. The -phase was found to be unstable in air, and further experiments should be performed in a good vacuum. The phase diagram of Cu_2–x Se was redetermined by the X-ray parametric method and the possible reasons why the phase diagrams reported by several authors do not coincide with each other are discussed.     

Mechanical property and in vitro biocompatibility of brushite cement modified by polyethylene glycol

Brushite (dicalcium phosphate dihydrate, DCPD) cement, owing to its high solubility in physiological condition and ability to guide new bone formation, is widely used to treat bone defects. In the present study, we have evaluated the effects of poly ethylene glycol (PEG) addition on the setting time, compressive strength and in vitro biocompatibility of brushite cement. The brushite cements were prepared by mixing β-tricalcium phosphate [β-TCP, Ca₃(PO₄)₂] and monocalcium phosphate monohydrate [MCPM, Ca(H₂PO₄)₂ ? H₂O]. PEG was introduced at 2.0 and 5.0 wt% with the liquid. Introduction of PEG resulted in marginal increase in both initial and final setting time, however, significantly affected the compressive strength. Effects of PEG incorporation on in vitro biocompatibility of brushite cements were studied by using human fetal osteoblast cells (hFOB) cells. Field emission scanning electron microscope (FESEM) images and immunohistochemical analysis indicated that pure and PEG incorporated brushite cement facilitates cell adhesion, proliferation and differentiation. Fewer cells expressed vinculin protein with increased PEG content in the cement. Cell proliferation was found to decrease with increased PEG concentration while the cell differentiation increased with PEG content. Our results provide a better understanding of in vitro biocompatibility of PEG added brushite cements that can be used to customize the cement compositions based on application need.     

Preparation of tricalcium phosphate/alumina composite nanoparticles and self-reinforcing composites by simultaneous precipitation

The composite nanoparticles and corresponding self-reinforcing composites comprising tricalcium phosphate (TCP) and alumina (Al₂O₃) were synthesized by simultaneous precipitation from the CaCl₂, AlCl₃ and (NH₄)₂HPO₄ aqueous solutions, using aqueous NH₄OH as precipitant. Influences of the precipitating media pH and the Ca/P atomic ratios on phase composition and morphology of the composites were investigated. Results showed that except for the major phases -TCP and -Al₂O₃, there was always a third minor phase in the calcined composites coprecipitated either in neutral or alkaline condition. Formation of -TCP is, however, favored at pH 9.2, whereas more of the third phase, mainly AlPO₄, is formed under neutral condition. High Ca/P ratios suppress the formation of -Al₂O₃ phase under alkaline precipitating condition, but the effect is less significant in neutral condition. TEM observation showed that the as prepared composite particles are nano-sized but interconnected to form a network-like morphology. They were changed to a core-shell-like structure after calcination, while their nano-scale dimension was retained. FEGSEM analysis revealed that the -Al₂O₃ phase in the sintered composite compacts was in the form of fibrils dispersed in the phosphate phases. These in situ formed fibrils impart a unique role in self-reinforcement of the sintered composites. Mechanical measurements showed that the incorporation of alumina reinforced -TCP effectively: the flexural strength increased from 15 MPa of the pure -TCP to 84 MPa of the composite with 40 wt% of -Al₂O₃.     

Influence of the high temperature treatment of zinc phosphate conversion coatings on the corrosion protection of steel

An anhydrous -Zn₃(PO₄)₂ phase converted by the dehydration of hydrous zinc phosphate, Zn₃(PO₄)₂·2H₂O, crystal coatings in air at a temperature of approximately 300 °C, significantly enhances the corrosion resistance of steel, and also reduces the susceptibility of the crystals to alkaline dissolution. A subsequent phase transition at approximately 500 °C results in a poor protection behaviour, because of the formation of numerous microcracks on the crystal faces.     

Reactivity of α-tricalcium phosphate

The reactivity of -tricalcium phosphate (-TCP) in forming hydroxyapatite (HAp) at 37°C was investigated. The effects of synthesis route, HAp seeding and the presence of calcium salts on the mechanism and extent of HAp formation were examined by pH measurements and/or isothermal calorimetric analyses. A synthesis temperature at the lower end in the temperature range of 1100–1300°C and the reaction of -TCP with a high specific surface area greatly improved rate and extent of HAp formation. The time for complete reaction decreased from 18 h to 14 h, when the reaction was carried out in the presence of 1 wt% of HAp seeds; the hydrolysis mechanism did not change. At HAp seeds proportion of 5 wt% and 10 wt%, transformation occurred without a nucleation period. The calcium salt additives studied were anhydrous and dihydrate form of dicalcium phosphate (CaHPO₄ and CaHPO₄ · 2H₂O), calcium carbonate (CaCO₃), and calcium sulfate hemihydrate (CaSO₄ · 1/2H₂O). All the additives delayed HAp formation as determined by the isothermal calorimetric analyses. Their retarding effects in decreasing order are CaCO₃, CaSO₄ · 1/2H₂O, DCPD, DCP. CaCO₃ almost completely retarded HAp formation. After 24 h, hydrolysis was complete only for pure -TCP and for the -TCP-DCP blend. Reaction was complete in other formulations before 48 h except for the CaCO₃-containing blend. In all mixtures conversion to HAp occurred without forming any intermediates. However gypsum formed in the mixture containing CaSO₄ · 1/2H₂O. All the -TCP-additive mixtures, excluding -TCP-CaCO₃, reached nominally the same strength value after 24 h of reaction as governed by the transformation of -TCP to HAp. For phase-pure -TCP, the average tensile strength changed from 0.36 ± 0.03 MPa to 7.26 ± 0.6 MPa. Upon hydrolysis only the CaSO₄ · 1/2H₂O-containing mixture exhibited slightly higher strength averaging 8.36 ± 0.9 MPa.     

Spectroscopic characterization of passivated titanium in a physiologic solution

Titanium (Ti) has been used for many biomedical applications. Surface characteristics of titanium devices are critical to their success. In this study, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to analyse Ti surfaces prior to immersion in alpha-modification of Eagle's medium (-MEM). The ionic constituents deposited onto Ti surfaces after in vitro exposure to -MEM were investigated using XPS and Fourier transform infrared spectroscopy (FTIR). Surface studies revealed an amorphous oxide layer on the Ti surface, with a chemistry similar to TiO₂. However, after exposure to the physiologic solution for 12 days, dynamic changes in surface chemistry were observed. Ions such as phosphorus (P) and calcium (Ca) were increasingly deposited as amorphous fine crystalline calcium-phosphate (Ca–P) compounds, having a Ca/P ratio of 1.2 and a chemistry similar to brushite.     

Preparation of micaceous iron oxide by the oxidation of iron with pressurized oxygen in concentrated sodium hydroxide solution at elevated temperatures

Iron powders were oxidized in NaOH solutions of 5–25 mol kg^–1 at 403–563 K and 5 MPa of oxygen partial pressure. Various types and morphologies of iron compounds such as fine particles of Fe₃O₄, micaceous -Fe₂O₃, and coagulated particles of -NaFeO₂ were formed depending on the experimental conditions. The observed critical concentrations of NaOH above which -NaFeO₂ was formed was in good agreement with those thermodynamically calculated for the hydrolysis equilibrium of -NaFeO₂.     

Analytical electron microscopy of Al2O3 implanted with iron ions

Single crystals of -Al₂O₃ were implanted with iron ions at room temperature to fluences ranging from 4×10¹⁶ Fe cm^–2 to 1×10¹⁷ Fe cm^–2. The microstructure and composition in the implanted region were examined using analytical electron microscopy techniques. Special emphasis was placed on monitoring the microstructural changes which take place during post-implantation annealing. Clusters of metallic -Fe were identified in the specimen after implantation to a dose of 1×10¹⁷ Fe cm^–2. Analytical electron microscopy of implanted specimens annealed in oxygen revealed the redistribution of the implanted iron and the formation of surface precipitates of -Fe₂O₃, subsurface precipitates of various forms of spinel, and, in some cases, subsurface precipitates of iron, depending on the annealing temperature. Examination of implanted specimens annealed under reducing conditions revealed the presence of precipitates of -Fe.     

Anharmonic effects in librational motion of N2-type crystals

The dynamics of librational motion in N₂-type crystals (-N₂, -CO, N₂O, CO₂) is treated by taking into account both anharmonic and correlation effects. The method used is similar to Tyablikov's method in the theory of magnetism. The main thermodynamic characteristics of the librational subsystem are calculated: the order parameter, rms librational angle, librational mode frequencies and corresponding Grüneisen parameters, librational heat capacity, and internal and free energies. The librational isotope effects for -¹⁴N₂ and -¹⁵N₂ are considered. An explanation of the anomalous isotope effects in the heat capacity is proposed. A theory of the phase transition into the orientationally disordered state is developed.     

Factors influencing the crystallization of ultrafine plasma-synthesized silicon nitride as a single powder and in composite SiC-Si3N4 powder

A study of various parameters affecting the crystallization process of amorphous silicon nitride produced by plasma gas-phase reaction was undertaken to determine the conditions under which whiskers are formed. This process is influenced by the ammonium chloride content of the starting powder and the presence of nitrogen in the furnace atmosphere. This last parameter is also influential on the / phase ratio, along with other factors like the silica content, temperature and duration of the thermal treatment. Heat treatment at 1500°C for 30 min under argon produced well-defined -Si₃N₄ crystals with a hexagonal cross section, a mean length around 0.8 m, and no sign of agglomeration. Under the same conditions, crystallization of silicon nitride in SiC-Si₃N₄ composite did not give crystals, but Si₃N₄ whiskers. Therefore silicon carbide plays a major role in their formation.     

Ultrasonic velocities and thermal expansion coefficients of amorphous Se80Te20 and Se90Te10 alloys near glass transitions

Precise measurements of the ultrasonic velocities and thermal expansivities of amorphous Se₈₀Te₂₀ and Se₉₀Te₁₀ alloys are reported near the glass transition. The samples are produced by liquid quenching. The longitudinal and transverse velocities are measured at 10 MHz frequency using the McSkimin pulse superposition technique. The thermal expansivities,, are measured using a three-terminal capacitance bridge. The-values show a sharp maximum near the glass transition temperature,T _g. The ultrasonic velocities also show a large temperature derivative, dV/dT nearT _g. The data are discussed in terms of existing theories of the glass transition. The continuous change in shows that the glass transition is not a first-order transition, as suggested by some theories. The samples are found to be deformed by small loads nearT _g. The ultrasonic velocities and dV/dT have contributions arising from this deformation.