Auger Spectroscopic Analysis of Bioglass Corrosion Films

Auger spectroscopy and ion beam milling were used to determine surface compositional profiles on a series of bioglass implant materials after exposure to simulated body conditions. Four glasses were examined, a soda-lime-silica glass and 3 compositions produced by adding 3, 6, and 12 wt% P₂O₅ to the ternary glass. An SiO₂-rich layer is formed on the surface of all glasses investigated. As P₂O₅ is added to the bulk composition, a second film rich in Ca and P forms at the SiO₂-rich film-water interface. The rate of formation of the Ca-P film increases as the P₂O₅ content of the bulk glass increases. When glasses are corroded under identical conditions, the thickness of the Ca-P layer increases as the P₂O₅ content of the bulk composition is increased.     

Crystallization of MgO-CaO-SiO2-P2O5 Glass

The crystallization behavior of a glass with a composition of 40 wt% 3CaO · P₂O₅−60 wt% CaO · MgO · 2SiO₂ was investigated. The primary crystalline phase was apatite with a dendritic form and ellipsoidal shape. β-(3CaO · P₂O₅) and CaO · MgO · 2SiO₂ were crystallized as samples heated to 990°C, and a three-layer structure was obtained. The development and morphology of this construction were explained by both the surface crystallization of the apatite and CaO · MgO · 2SiO₂ and the bulk crystallization of apatite and the CaO · MgO · 2SiO₂-β-(3CaO · P₂O₅) composite.     

Porous Glass-Ceramic in the CαO–TiO2–P2O5 System

A porous glass-ceramic in the CaO–TiO₂—P₂O₅ system has been prepared by crystallization and subsequent chemical leaching of the corresponding glass. By applying a two-step heat treatment to 45CaO · 25TiO₂· 30P₂O₅ glasses containing a few mol% of Na₂O, volume crystallization results in the formation of dense glass-ceramics composed of CaTi₄(PO₄)₆ and β-Ca₃(PO₄)₂ phases. By leaching the resultant glass ceramics with HCI, β-Ca₃(PO₄)₂ is selectively dissolved out, leaving a crystalline CaTi₄(PO₄)₆ skeleton. The surface area and mean pore radius of the porous glass-ceramics were approximately 40 m²/g and 13 nm, respectively.     

Crystallization Behavior of CaO–P2O5 Glass with TiO2, SiO2, and Al2O3 Additions

TiO₂ above 4 mol% is an effective nucleating agent for CaO–P₂O₅ glass which also contains substantial SiO₂ and Al₂O₃ additions. Glass ceramics can be made from this glass using a single slow heating ramp with no need for a nucleating heat treatment step. Powder of this composition crystallizes rapidly to β-Ca₂P₂O₇, whereas bulk glass crystallizes from diphasic nuclei consisting of a central cubic Ca-P-Ti-Si-Al oxide phase surrounded by impure AlPO₄ dendrites. Metastable calcium phosphate grows on the AlPO₄ dendrites and later transforms to β-Ca₂P₂O₇.     

Journal. Effects of Composition Changes on the Crystallization Behavior of MgO-CaO-SiO2-P2O5 Glass-Ceramics

The effects of composition changes on the crystallization behavior of apatite-containing glass-ceramics in the system MgO-CaO–SiO₂–P₂O₅ were studied. The eutectic composition of 40 wt% 3CaO-P₂O₅–60 wt% CaO MgO 2SiO₂ was selected as the base glass. Several parameters were quantitatively or qualitatively derived from thermal analysis, X-ray diffraction, and microstructural observations. These parameters can be divided into two groups according to their variation with glass composition. The parameters in the first group depend mainly on the apatite nucleation rate, which increases with an increase in CaO or PiO₅, and with a decrease in MgO or SiO₂. The parameters in the second group are closely related to the extent of deviation of the glass composition from the eutectic. The classical theory for nucleation employed to explain the observed phenomena is discussed.     

Mullite Transformation Kinetics in P2O5-, TiO2-, and B2O3-Doped Aluminosilicate Gels

Mullite transformation kinetics of sol-gel-derived diphasic mullite gels doped with P₂O₅, TiO₂, and B₂O₃ were studied using quantitative X-ray diffraction and differential thermal analysis (DTA). The mullite transformation temperature initially increased with P₂O₅ doping because of phase separation and formation of α-alumina and cristobalite. In TiO₂-doped samples, the mullite transformation temperature decreased with TiO₂ doping, and the transformation rate increased with decreasing TiO₂ particle size. Kinetic studies showed that titania reduced the activation energy for both nucleation and growth relative to pure diphasic mullite gels by lowering the glass viscosity and/or enhancing the solid-state mass transport through lattice defects. B₂O₃ doping decreased the mullite transformation temperature and lowered the activation energy for both nucleation and growth but especially affected the mullite nucleation process, as indicated by the much smaller grain size.     

Proton Conductivity in Zr4+-Ion-Doped P2O5–SiO2 Porous Glasses

The effect of zirconium ions on glass structure and proton conductivity was investigated for sol-gel-derived P₂O₅–SiO₂ glasses. Porous glasses were prepared through hydrolysis of PO(OCH₃)₃, Zr(OC₄H₉)₄, and Si(OC₂H₅)₄. Chemical bonding of the P⁵⁺ ions was characterized using ³¹P-NMR spectra. The phosphorous ions, occurring as PO(OH)₃ in the ZrO₂-free glass, were polymerized with one or two bridging oxygen ions per PO₄ unit with increased ZrO₂ content. The chemical stability of these glasses was increased significantly on the addition of ZrO₂, but the conductivity gradually decreased from 26 to 12 mS/cm at room temperature for 10P₂O₅·7ZrO₂·83SiO₂ glass. A fuel cell was constructed using 10P₂O₅·5ZrO₂·85SiO₂ glass as the electrolyte; a power of ∼4.5 mW/cm² was attained.     

Glass-Ceramic Formation in the ZnO-P2O5 System and the Effect of Silica as a Nucleating Agent

Crystallization of a series of ZnO-P₂O₅ based glasses was investigated. ZnO-P₂O₅-CaO glasses could be converted most readily to glass-ceramics and crystallization of these led to formation of alpha-Zn₂P₂O₇, alpha-CaZn₂(PO₄)₂, and ß-CaZn₂(PO₄)₂ phases. A further phase has been tentatively identified as monoclinic (Zn,Ca)₂P₂O₇. The most promising glass-ceramic composition Z15 (59.4ZnO·33P₂O₅·6.6CaO·1SiO₂) crystallized to alpha-Zn₂P₂O₇ and ß-CaZn₂(PO₄)₂, the latter phase being stabilized by the presence of SiO₂ which also encouraged volume nucleation giving a fine-scale (submicrometer) microstructure.     

Influence of Fluorine Content in Apatite–Mullite Glass-Ceramics

The nucleation and crystallization of a series of glasses based on 4.5SiO₂·3Al₂O₃·1.5P₂O₅·(5 − z )CaO · z CaF₂ with a Ca:P ratio corresponding to apatite were studied. In these glasses, the objective was to investigate the influence of fluorine content and z was varied from 3 to 0. All the glasses studied crystallized to fluorapatite (FAP) and mullite with the exception of the glass containing no fluorine, which crystallized to β-tricalcium phosphate (Ca₃(PO₄)₂) and anorthite (CaAl₂Si₂O₈). Glasses that contained sufficient fluorine to form FAP bulk nucleated to give FAP without a nucleation hold. Thermal gravimetric analysis demonstrated a significant weight loss corresponding to the crystallization of mullite, which increased with the fluorine content of the glass and also with decreased particle size. The loss was attributed to volatile SiF₄. The glass transition temperature decreased with increased fluorine content of the glass.     

Reactions in the System TiO2-P2O5

The system TiO₂-P₂O₅ was investigated in the compositional range TiO₂.P₂O₅ to 100% TiO₂. Two compounds exist, TiO₂.P₂O₅ and 5TiO₂.-2P₂O₅. TiO₂.P₂O₅ begins to lose P₂O₅ at 1400°C. and both fusion and vaporization proceed rapidly at 1500°C. 5TiO₂.2P₂O₆ melts congruently at 1260°± 3°C. to a glass which can be retained in substantial quantities at room temperature. Physical properties of certain compositions are described.     

Nucleation and Crystallization of a Lithium Aluminosilicate Glass

An aluminosilicate glass of composition 61SiO₂6Al₂O₃10MgO6ZnO·12Li₂O·5TiO₂ (mol%) has been prepared by a melting process and investigated as far as crystallization is concerned. Glass-ceramic is easily obtained because glass shows a high tendency to crystallize starting from 700°C. The crystalline phases evolve with temperature, showing the aluminosilicates to be the main phase up to 1050°C, followed by metasilicates and silicates, some of which have lower melting points. The titanates of Mg and Zn develop from the phase-separated glass, soon after T _g, and grow to form nucleation centers for the other crystalline phases. The evolution from phase-separated glass to glass-ceramic has been followed by many thermal, diffractometric, spectroscopic, and microscopic techniques.     

Apatite Formation on Calcium Phosphate Invert Glasses in Simulated Body Fluid

Calcium phosphate invert glasses, which contain P₂O₇²⁻ and PO₄³⁻ ions, have been prepared via the addition of a small amount of TiO₂. The formation of bonelike calcium phosphate apatite on the surface of the phosphate invert glasses was examined in simulated body fluid (SBF) at a temperature of 37°C. Soaking for 20 d resulted in the deposition of leaflike apatite particles on 6CaO·3P₂O₅·TiO₂ invert glass (based on molar ratio). The glass had much-greater chemical durability against SBF, in comparison with a metaphosphate glass; P ions were not dissolved excessively from the 6CaO·3P₂O₅·TiO₂ glass, so the apatite formation was not suppressed.     

Preparation of Void-Free Calcium Phosphate Glass-Ceramics

Voids frequently form on crystallization of calcium phosphate glasses because of the density difference between the original glass and the resulting glass-ceramic. However, the calcium phosphate glass-ceramic with a composition of 40CaO · 50P₂O₅· 10Al₂O₃ (mol%) has a density nearly equal to that of the original glass; this is caused by the formation of AlPO₄, which has a lower density than Ca(PO₃)₂. The glass-ceramic, being void-free, shows improved mechanical properties.     

Crystallization of a Rare Earth-Rich Aluminoborosilicate Glass With Varying CaO/Na2O Ratio

The effect of varying R =[CaO]/([CaO]+[Na₂O]) ratio on the crystallization of a rare earth-rich aluminoborosilicate glass (16 wt% RE₂O₃, RE=Nd or La) is investigated. The crystallization of a silicate apatite with Ca_{2+ x} RE_{8− x} (SiO₄)₆O_{2−0.5 x} composition ( x ≈0.4–0.7), is responsible for a drop of the rare earth solubility in the melt. When successive nucleation and growth stages are performed, crystallization processes change across the glass series as a consequence of glass-in-glass phase separation. An exotic phase of composition close to Ca₁₀Nd₇Si_20.75O₆₂ grows at the expense of silicate apatite.     

The System SiO2–P2O2

Phase relations in the binary system between SiO₂-P₂O₅ and SiO₂ were investigated by the quenching method using sealed platinum tubes to prevent the loss of P₂O₅. The compound Si0₂-P₂O₅ exists in two forms, the low-temperature β form inverting sluggishly but reversibly to the high-temperature β form at 1030°C. The β form melts congruently at 1290°C. The compound 2SiO₂-P₂O₅ melts incongruently at 1120°C to a silica-rich liquid and SiO_a-P₂O₅. In the region between 5 and 25 mole % PO₂, reactions were so sluggish that no data could be obtained by quenching.     

Bioactivity of Na2O-CaO-SiO2 Glasses

Bioactivities of Na₂O-CaO-SiO₂ glasses were evaluated by examining the formation of bonelike apatite, which is responsible for their bonding to living bone, on their surfaces in a simulated body fluid, using thin-film X-ray diffraction and Fourier-transform infrared reflection spectroscopy. It was found that glasses in a wide compositional region in the P₂O₅-free Na₂O-CaO-SiO₂ system can show bioactivity, as those in the P₂O₅-containing system. The rate of apatite formation on the surfaces of glasses varied largely with the composition of the glasses. Under a constant SiO₂ content of 50 mol%, a glass containing equimole of Na₂O and CaO showed the highest rate of the apatite formation. Variation in the rate of apatite formation with the glass composition corresponded well with the rate of increase in the degree of the supersaturation of the simulated body fluid with respect to the apatite due to dissolution of sodium and calcium ions from the glasses. Little difference was observed in the rates of ion dissolution and of apatite formation between P₂O₅-containing Bioglass 45S5-type and a corresponding P₂O₅-free Na₂O-CaO-SiO₂ glass. It is believed that P₂O_s-free Na₂O-CaO-SiO₂ glasses also show bioactivity as high as that of Bioglass.     

Microstructural Evolution in Fast-Heated Cordierite-Based Glass-Ceramic Glazes for Ceramic Tile

The crystallization mechanism of α-cordierite from a B₂O₃- and TiO₂-containing glass submitted to fast heating in the cordierite primary phase field of the CaO–MgO–Al₂O₃–SiO₂ quaternary system was investigated. Addition of B₂O₃ to a SiO₂-rich glass suppressed the formation of μ-cordierite. This suppression facilitated densification by viscous flow before crystallization. Powder X-ray diffractometry, field-emission electron scanning microscopy, and energy-dispersive X-ray analysis revealed that α-cordierite nucleated directly from glass on the boundaries of original particles and was probably favored by TiO₂, which acted as a nucleant. The growth kinetics of α-cordierite crystals was fast, and the crystals seemed to grow by consuming glass directly from the growing interphase. The estimated amount of α-cordierite in the glass heated at 1160°C was 69.5 wt%, as determined using the Alegre method. The final microstructure consisted of an arrangement of well-shaped hexagonal prisms, with sizes <3 μm, immersed in a residual glassy phase. The feasibility to develop new glass-ceramic glazes using the present and previous works is considered.     

Calculated Thermodynamic Data and Metastable Immiscibility in the System SiO2-Al2O3

Thermodynamic data on activities, activity coefficients, and free energies of mixing in SiO₂-Al₂O₃ solutions were calculated from the phase diagram. Positive deviations from ideal mixing in the thermodynamic data suggest a tendency for liquid immiscibility in both SiO₂- and Al₂O₃-rich compositions. The calculated data were used to estimate regions of liquid-liquid immiscibility. A calculated metastable liquid miscibility gap with a consolute temperature of ∼1540^°C at a critical composition of ∼36 mol% Al₂O₃ was considered to be thermodynamically most probable; the gap extended from ∼11 to °49 mol% Al₂O₃ at 1100^°C. SiO₂-rich glass compositions showed evidence of glass-in-glass phase separation when examined by direct transmission electron microscopy.     

Electrical Conductivity of TiO2-V2O5-P205 Glasses

The dc conductivities (σ) of V₂O₅-P₂O₅ glasses containing up to 30 mol% TiO₂ were measured at T=100° to ∼10°C below the glass-transition temperature. Dielectric constants from 30 to 10⁶ Hz, densities, and the fraction of reduced V ion were measured at room temperature. The conduction mechanism was considered to be small polaron hopping between V ions, as previously reported for V₂O₅-P₂O₅ glass. The temperature dependence of σ was exponential with σ = σ₀ exp(-W/kT ) in the high-temperature range. When part of the P₂O₅ was replaced by TiO_2,σ increased and W decreased. The hopping energy depended on the reciprocal dielectric constant which, in this case, increased with increasing TiO₂ content.     

Effects of Glass Additions on (Zr,Sn)TiO4 for Microwave Applications

The effects of glass additions on the properties of (Zr,Sn)TiO₄ as a microwave dielectric material were investigated. The (Zr,Sn)TiO₄ ceramics with no glass addition sintered at 1360°C gave Q = 4900 and K = 37 at 7.9 GH_z. Several glasses, including SiO₂, B₂O₃, 5ZnO–2B₂O₃, and nine commercial glasses, were tested during this study. Among these glasses, (Zr,Sn)TiO₄ sintered with ZnO-B₂O₃–SiO₂ (Corning 7574) showed more than 20% higher density than that of pure (Zr,Sn)TiO₄ sintered at the same temperature. A 5-wt% addition of SiO₂, to (Zr,Sn)TiO₄, when sintered at 1200°C, gave the best Q : Q = 2700 at 9 GHz. Results of XRD analysis and scanning electron microscopy and the effect of glass content are also presented.