Rory M. Wilson

Rietveld refinement of the crystallographic structure of human dental enamel apatites

Abstract Rietveld refinements using 12 sets of X-ray diffraction powder data from milligram samples of human dental enamel provide detailed information about the structure and composition of enamel apatite. The principal difference in atomic parameters between enamel apatite and Holly Springs hydroxylapatite is in O2, which is reflected in a reduction in the P-O2 bond length of 0.085 Å and PO4 volume by 3.6%. Modeling the hexad axis scattering with a single OH⁻ ion gives a 0.089 Å shift of the ion further away from the mirror plane at z = 1⁄4. The known distributed electron density along the hexad axis in enamel has been confirmed by direct comparison with synthetic hydroxylapatite. Although the CO32- ion position could not be determined directly, evidence for partial replacement of PO43- by CO32- ions came from an 8% diminution of the P site occupancy compared with that in stoichiometric hydroxylapatite. The observed reduction in the P-O2 bond length and PO4 volume in enamel is also consistent with this substitution. The loss of negative charge caused by CO32- replacing PO43- ions and loss of OH⁻ ions is compensated by loss of Ca2+ ions from Ca2 sites. The calculated density from the X-ray results is 3.021 g/cm3, in agreement with deductions from previous chemical analyses.

Preparation and characterisation of monoclinic hydroxyapatite and its precipitated carbonate apatite intermediate

Five 100 g batches of a carbonate apatite (the intermediate) were produced by heating an aqueous slurry of CaCO3 and CaHPO4 with an overall Ca/P mole ratio of 5/3 with vigorous stirring. Each intermediate produced by boiling off water was heated in vacuum at 1100 degrees C to remove carbonate, then steamed at 900 degrees C to ensure complete hydroxylation. Comparison of calculated and observed X-ray diffraction patterns showed final products containing 50-100 wt% monoclinic hydroxyapatite (remainder hexagonal). Rietveld refinements in P6(3)/m gave structures similar to several hydroxyapatite standards, including NIST SRM 2910, although there was no evidence from X-ray diffraction that the latter was in the monoclinic form. Refinements from standards and final products were slightly different from published single crystal data for Holly Springs hydroxyapatite. This is attributed to known impurities in mineral hydroxyapatite and indicates that parameters from the Rietveld refinements are closer to the true values for pure hydroxyapatite. Rietveld refinements for intermediates showed small, but significant differences from the final product, the largest being in O1x, O2x and O(H)z. All P-O bond lengths were shorter than in the final product, resulting in a 3.2% lower PO4 tetrahedron volume. The occupancies of P and Ca(2) were reduced. These differences are attributed to partial replacement of PO4(3) by CO3(2-) ions.

Synchrotron X-Ray Microtomographic Investigation of Mineral Concentrations at Micrometre Scale in Sound and Carious Enamel

Synchrotron X-ray microtomography (XMT) was used to measure the linear attenuation coefficient (LAC) for 1.9-µm sidelength voxels within approximal brown spot lesions and sound human enamel. XMT demonstrated three-dimensional features, notably sheets with ∼30 µm periodicity having low LAC, identified as regions of demineralization corresponding to Retzius lines. Quantitative three-dimensional measurements of mineral concentration, derived from LAC with assumption of a single model composition, were consistent with previous measurements of sound and carious enamel from microradiographic projections. The uncertainty in measurements of mineral concentration and mineral fraction volume was investigated by modelling enamel with a range of composition and component densities. This analysis showed that, although mineral concentration can be determined from LAC with an error of <0.2 g cm–3, the variation in pore fraction volume within caries lesions cannot be reliably determined from X-ray attenuation measurements alone.

Reversibility in electric field-induced transitions and energy storage properties of bismuth-based perovskite ceramics

The effects of temperature and electric field-induced structural modifications on the energy storage properties of 0.95[0.94Bi0.5Na0.5TiO3–0.06BaTiO3]–0.05K0.5Na0.5NbO3 (BNT–BT–5KNN) ceramics were investigated. X-ray diffraction performed on unpoled and poled ceramics in the temperature range 25–500 °C suggested an increment in the rhombohedral phase intensity peaks and in the tetragonal distortion after electrical poling. The rhombohedral phase content reduced with increasing temperature in both unpoled and poled ceramics. In the unpoled ceramic, the rhombohedral phase eventually disappeared, while it survived in the poled specimen up to 500 °C. The stabilization of the rhombohedral ferroelectric phase by dc poling produced remarkable differences in the temperature dependence of permittivity, loss, current–polarization–electric field loops and energy density. As a consequence of a reversible transition induced by an alternating electric field, competitive energy densities (0.39–0.51 J cm−3 in the range 25–175 °C) with those of lead-based and lead-free bulk ceramics recently developed was obtained, indicating bismuth-based perovskites as potential lead-free systems for energy storage applications.

Effects of sodium hypochlorite solution on root dentine composition

Sodium hypochlorite (NaOCl) solution,≤5% w/v available chlorine (abbreviated subsequently to %), is widely used as an irrigant in root canal treatment of teeth, so its effects on dentine are of clinical importance. The effects of ∼0.5%, 3% and 5% NaOCl solution on the composition of root dentine were studied at ambient temperature. For dentine powder treated for 30 min, depletion of the organic phase was confirmed by infrared spectroscopy. Apatite lattice parameters showed no significant change, but NaCl was also detected by X-ray powder diffraction. The low solubility of apatite mineral in the NaOCl solutions was demonstrated by the constant weight of bulk enamel specimens immersed for seven days. The stability of the mineral phase was confirmed by scanning microradiography (SMR), an X-ray attenuation method employing photon counting. Repeated SMR measurements of the local mineral content of bulk samples of root dentine and a synthetic hydroxyapatite aggregate during exposure to pumped NaOCl solutions for 100 h showed no mineral loss. As predicted from apatite chemistry, reaction of NaOCl with the mineral phase can be excluded as a primary factor in changes in mechanical properties of treated dentine. Effects of retention of NaCl on endodontic sealants requires further investigation.

Solid‐State Processing of Organic Semiconductors

By Mohammed A. Baklar , elix F Koch , Avinesh Kumar , Ester Buchaca Domingo , Mariano Campoy-Quiles , Kirill eldman , F Liyang u , Y Paul Wobkenberg , James Ball , Rory M. Wilson , Iain McCulloch , Theo Kreouzis , Martin Heeney , Thomas Anthopoulos , Paul Smith ,

Crystallization and flexural strength optimization of fine-grained leucite glass-ceramics for dentistry

OBJECTIVES Leucite glass-ceramics with fine-grained leucite crystals promote improved mechanical strength and increased translucency. The objectives of the study were to optimize the microstructure of a fine-grained leucite glass-ceramic in order to increase its flexural strength and reliability as measured by its Weibull modulus. METHODS Glass was prepared by a melt-derived method and ground into a powder (M1A). The glass crystallization kinetics were investigated using high temperature XRD and DSC. A series of two-step heat treatments with different nucleation/crystal growth temperatures and holds were carried out to establish the optimized crystallization heat treatment. Glass-ceramics were characterized using XRD, SEM and dilatometry. The glass-ceramic heat treated at the optimized crystallization parameters (M1A(opt)) was both sintered (SM1A(opt)) and heat extruded (EM1A(opt)) into discs and tested using the biaxial flexural strength (BFS) test. RESULTS High temperature XRD suggested leucite and sanidine crystallization at different temperatures. Optimized crystallization resulted in an even distribution of fine leucite crystals (0.15 (0.09) μm(2)) in the glassy matrix, with no signs of microcracking. Glass-ceramic M1A(opt) showed BFS values of [mean (SD), MPa]: SM1A(opt)=252.4 (38.7); and EM1A(opt)=245.0 (24.3). Weibull results were: SM1A(opt); m=8.7 (C.I.=7.5-10.1) and EM1A(opt); m=11.9 (C.I.=9.3-15.1). Both experimental groups had a significantly higher BFS and characteristic strength than the IPS Empress Esthetic glass-ceramic, with a higher m value for the EM1A(opt) material (p<0.05). SIGNIFICANCE A processable fine-grained leucite glass-ceramic with high flexural strength and improved reliability was the outcome of this study.

Bioactivity of Sodium Free Fluoride Containing Glasses and Glass-Ceramics

The bioactivity of a series of fluoride-containing sodium-free calcium and strontium phosphosilicate glasses has been tested in vitro. Glasses with high fluoride content were partially crystallised to apatite and other fluoride-containing phases. The bioactivity study was carried out in Tris and SBF buffers, and apatite formation was monitored by XRD, FTIR and solid state NMR. Ion release in solutions has been measured using ICP-OES and fluoride-ion selective electrode. The results show that glasses with low amounts of fluoride that were initially amorphous degraded rapidly in Tris buffer and formed apatite as early as 3 h after immersion. The apatite was identified as fluorapatite by 19F MAS-NMR after 6 h of immersion. Glass degradation and apatite formation was significantly slower in SBF solution compared to Tris. On immersion of the partially crystallised glasses, the fraction of apatite increased at 3 h compared to the amount of apatite prior to the treatment. Thus, partial crystallisation of the glasses has not affected bioactivity significantly. Fast dissolution of the amorphous phase was also indicated. There was no difference in kinetics between Tris and SBF studies when the glass was partially crystallised to apatite before immersion. Two different mechanisms of apatite formation for amorphous or partially crystallised glasses are discussed.

Influence of cell culture medium composition on in vitro dissolution behavior of a fluoride-containing bioactive glass.

Bioactive glasses are used clinically for bone regeneration, and their bioactivity and cell compatibility are often characterized in vitro, using physiologically relevant test solutions. The aim of this study was to show the influence of varying medium characteristics (pH, composition, presence of proteins) on glass dissolution and apatite formation. The dissolution behavior of a fluoride-containing bioactive glass (BG) was investigated over a period of one week in Eagles Minimal Essential Medium with Earles Salts (MEM), supplemented with either, (a) acetate buffer, (b) 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer, (c) HEPES + carbonate, or (d) HEPES + carbonate + fetal bovine serum. Results show pronounced differences in pH, ion release, and apatite formation over 1 week: Despite its acidic pH (pH 5.8 after BG immersion, as compared to pH 7.4-8.3 for HEPES-containing media), apatite formation was fastest in acetate buffered (HEPES-free) MEM. Presence of carbonate resulted in formation of calcite (calcium carbonate). Presence of serum proteins, on the other hand, delayed apatite formation significantly. These results confirm that the composition and properties of a tissue culture medium are important factors during in vitro experiments and need to be taken into consideration when interpreting results from dissolution or cell culture studies.

Crystallization of High-strength Fine-sized Leucite Glass-ceramics

Manufacturing of leucite glass-ceramics often leads to materials with inhomogeneous microstructures. Crystal-glass thermal mismatches which produce microcracking around larger crystals-agglomerates are associated with reduced mechanical properties. The hypotheses were that fine (< 1 µm) crystal size and uniform microstructure in a thermally matched glass would increase the biaxial flexural strength (BFS). Glass was synthesized, attritor-milled, and heat-treated. Glasses and glass-ceramics were characterized by XRD, SEM, and Dilatometry. Experimental (A, M1A and M2A) and commercial glass-ceramics were tested by the BFS test. Experimental glass-ceramics showed an increased leucite crystal number and decreased crystal size with glass particle size reduction. Leucite glass-ceramics (< 1 µm) showed minimal matrix microcracking and BFS values of [mean (SD) MPa]: M1A = 253.8 (53.3); and M2A = 219.5 (54.1). Glass-ceramics M1A and M2A had higher mean BFS and characteristic strength than the IPS Empress Esthetic glass-ceramic (p < 0.05). Fine-grained, translucent leucite glass-ceramics were synthesized and produced high mean BFS.