Calcium Silicate Glasses Research Articles

Effect of Glass Phase on the Defect Formation during the Dissolution of Hydroxyapatite

Particle separations occurred frequently in sintered hydroxyapatite when immersed in distilled water or simulated body fluid. This dissolution initiated at grain boundary creating nano-size defects such as small pores and grew up to micro scale by increasing immersion time. The dissolution, probably due to the appearance of secondary phases in grain boundary, resulted in grain separation at the surfaces and finally in degradation and fracture. And the dissolution concentrated on those grains adjacent to pores rather than those in the dense region. Hydroxyapatite ceramics incorporated with calcium silicate glass were prepared by slip casting to enhance the sinterability as well as to reduce dissolution. Glass phase was incorporated into hydroxyapatite to act as sintering aids followed by crystallization in order to improve the mechanical properties without reducing biocompatibility. From dissolution test, significant damage was reduced even more than 7 days and the dissolution pattern somewhat changed than pure hydroxyapatite. X-ray diffraction and SEM showed no decomposition of secondary phases in grain boundary and fracture toughness somewhat increased.

Continuous compositional changes of crystal and liquid during crystallization of a sodium calcium silicate glass

This paper deals with a systematic study of crystal nucleation and growth kinetics in a 14.6Na 2O–34.0CaO–51.4SiO 2 mol% glass, which is close to the CaO · SiO 2–Na 2O · SiO 2 pseudo-binary section, just left of the stoichiometric Na 2O · 2CaO · 3SiO 2 (N 1C 2S 3) compound. We show that crystallization begins with nucleation of a Na 4+2 x Ca 4− x [Si 6O 18] (0 < x < 1) solid solution that is enriched in sodium as compared with both parent glass and the N 1C 2S 3 compound; while a fully crystallized sample is composed only by a solid solution that is stable at very high temperatures, but is metastable in the temperatures under investigation. We thus confirm a continuous compositional change of the crystals during the course of crystallization.

Contribution of 43Ca MAS NMR for probing the structural configuration of calcium in glass

The 43Ca MAS NMR spectra obtained for three calcium silicate glasses containing aluminum and/or sodium are analyzed. The 43Ca NMR parameters are shown to be highly sensitive to variations in its chemical environment. The transition from a charge-compensating role of [AlO 4] − groups to a network-modifying role near nonbridging oxygen atoms results in an increase of both the isotropic chemical shift and the quadrupolar coupling constant of 43Ca. This effect can be related to the first coordination shell of calcium (through the Ca–O bond distance) which is affected by the chemical environment beyond the first neighbors of calcium.

The use of advanced diffraction methods in the study of the structure of a bioactive calcia: silica sol-gel glass

Sol-gel derived calcium silicate glasses may be useful for the regeneration of damaged bone. The mechanism of bioactivity is as yet only partially understood but has been strongly linked to calcium dissolution from the glass matrix. In addition to the usual laboratory-based characterisation methods, we have used neutron diffraction with isotopic substitution to gain new insights into the nature of the atomic-scale calcium environment in bioactive sol-gel glasses, and have also used high energy X-ray total diffraction to probe the nature of the processes initiated when bioactive glass is immersed in vitro in simulated body fluid. The data obtained point to a complex calcium environment in which calcium is loosely bound within the glass network and may therefore be regarded as facile. Complex multi-stage dissolution and mineral growth phases were observed as a function of reaction time between 1 min and 30 days, leading eventually, via octacalcium phosphate, to the formation of a disordered hydroxyapatite (HA) layer on the glass surface. This methodology provides insight into the structure of key sites in these materials and key stages involved in their reactions, and thereby more generally into the behaviour of bone-regenerative materials that may facilitate improvements in tissue engineering applications.

A molecular dynamics study of densification mechanisms in calcium silicate glasses CaSi2O5 and CaSiO3 at pressures of 5 and 10GPa

Molecular dynamics is used to obtain models of (CaO)(x)(SiO(2))(1-x) glasses, with compositions CaSi(2)O(5) (x=0.33) and CaSiO(3) (x=0.50), at pressures of 5 and 10 GPa. At 5 GPa there are increases in Ca and Si coordinations for x=0.33, whereas for x=0.50 there is distortion of CaO(N) polyhedra but no substantial change in coordination. At 10 GPa the Ca coordination increases by approximately 20% for x=0.33 and by approximately 10% for x=0.50. This increase is due to increased Ca bonds to bridging oxygens (O(b)), since nonbridging oxygens (O(nb)) are already highly bonded to Ca, and the proportion of O(nb) is decreasing due to changes in the silica network. At 10 GPa there are approximately 20% of fivefold and a few percent of sixfold coordinated Si. Since the new Si-O bonds involve the conversion of O(nb) to O(b), there is a corresponding increase in the network connectivity. The x=0.50 glass is more resistant to deformation because there is less possibility to convert O(nb) to O(b) due to lower Si content. The changes in Ca-O, Si-Ca, and Ca-Ca correlations are predicted to produce changes in the x-ray diffraction structure factor S(Q), including a shift of the first sharp diffraction peak to higher Q values.

Optical and structural properties of calcium silicate glasses

A detailed analysis of the reflectance and transmittance spectra of a set of glasses allowed retrieval, besides the optical functions, of quantitative information on the structural changes occurring in the calcium silicate glass system. The occurrence of a threshold around 46% of CaO content is confirmed and is connected to the appearance in the glass structure of a new configuration of calcium cations. It was also shown that the calcium cations that exceeds the compositional threshold does not act as modifiers of the silicate network since they form Ca–O–Ca bonds in the glass structure. Obvious changes occurring at the threshold in the physical or chemical properties can be understood in the light of the structural changes observed in the system.

Disorder and the extent of polymerization in calcium silicate and aluminosilicate glasses: O-17 NMR results and quantum chemical molecular orbital calculations

Estimation of the framework connectivity and the atomic structure of depolymerized silicate melts and glasses (NBO/T > 0) remains a difficult question in high-temperature geochemistry relevant to magmatic processes and glass science. Here, we explore the extent of disorder and the nature of polymerization in binary Ca-silicate and ternary Ca-aluminosilicate glasses with varying NBO/T (from 0 to 2.67) using O-17 NMR at two different magnetic fields of 9.4 and 14.1 T in conjunction with quantum chemical calculations. Non-random distributions among framework cations (Si and Al) are demonstrated in the variation of relative populations of oxygen sites with NBO/T. The proportion of non-bridging oxygen (NBO, Ca–O–Si) in the binary and ternary aluminosilicate glasses increases with NBO/T. While the trend is consistent with predictions from composition, the detailed fractions apparently deviate from the predicted values, suggesting further complications in the nature of polymerization. The proportion of each bridging oxygen in the glasses also varies with NBO/T. The fractions of Al–O–Si and Al–O–Al increase with increasing polymerization as CaO is replaced with Al 2O 3, while that of Si–O–Si seems to decrease, implying that activity of silica may decrease from calcium silicate to polymerized aluminosilicates ( X SiO 2 = constant ) . Quantum chemical molecular orbital calculations based on density functional theory show that a silicate chain with Al–NBO (Ca–O–Al) has an energy penalty (calculated cluster energy difference) of about 108 kJ/mol compared with the cluster with Ca–O–Si, consistent with preferential depolymerization of Si-networks, reported in an earlier O-17 NMR study [Allwardt, J., Lee, S.K., Stebbins, J.F., 2003. Bonding preferences of non-bridging oxygens in calcium aluminosilicate glass: Evidence from O-17 MAS and 3QMAS NMR on calcium aluminate glass. Am. Mineral. 88, 949–954]. These prominent types of non-randomness in the distributions suggest significant chemical order in silicate glasses that leads to a decrease in silica activity coefficient and will be useful in modeling transport properties of melts.

On the rigid to floppy transitions in calcium silicate glasses from Raman scattering and cluster constraint analysis

Calcium silicate glasses xCaO − (1 − x)SiO2 exhibit a threshold in Raman line-shapes which can be related, on the basis of Maxwell constraint counting, to the onset of network rigidity as the concentration of calcium oxide x is decreased. The present results are more deeply characterized by a size-increasing cluster approximation that allows to perform Maxwell mechanical constraint counting beyond the usual meanfield treatment. This permits to discuss under which structural conditions an elastic intermediate phase should be obtained in the future.

The structure of a bioactive calcia–silica sol–gel glass

We have used neutron diffraction with isotopic substitution to gain new insights into the nature of the atomic scale calcium environment in bioactive sol–gel glasses, and also used high energy X-ray total diffraction to probe the nature of the processes initiated when bioactive glass is immersed in vitro in simulated body fluid (SBF). Recent work has highlighted the potential of sol–gel derived calcium silicate glasses for the regeneration or replacement of damaged bone tissue. The mechanism of bioactivity and the requirements for optimisation of the properties of these materials are as yet only partially understood but have been strongly linked to calcium dissolution from the glass matrix. The data obtained point to a complex calcium environment in which calcium is loosely bound within the glass network and may therefore be regarded as facile. Complex multi-stage dissolution and mineral growth phases were observed as a function of reaction time between 1 min and 30 days, leading eventually to the formation of a disordered hydroxyapatite (HA) layer on the glass surface, which is similar to the polycrystalline bone mineral hydroxyapatite. This methodology provides insight into the structure of key sites in these materials and key stages involved in their reactions, and thereby more generally into the behaviour of bone-regenerative materials that may facilitate improvements in tissue engineering applications.

Cation diffusion and ionic conductivity in soda-lime silicate glasses

We have studied the mobilities of calcium and sodium ions in silicate glasses of compositions xNa2O (3 - x)CaO x 4SiO2 with x = 0.0, 0.1, 0.3, 1.0 and 3.0 by means of radiotracer diffusion, electrical conductivity measurements, and dynamic mechanical thermal analyses. In glasses containing sodium oxide, the Na+ ions are much more mobile than the Ca2+ ions, and are, therefore, governing the electrical conductivity. In the pure calcium silicate glass, the activation energy of Ca2+ diffusion is higher than the activation energy of the electrical conductivity. This provides strong evidence that the electrical conductivity of this glass is not determined by the migration of Ca2+ ions, but by impurity charge carriers, which are most likely Na+ ions. We sketch the composition-dependent mobilities of Na+ and Ca2+ ions in soda-lime silicate glasses with variable Na2O and CaO content. Our results indicate that the coordination environment of Ca2+ ions remains unchanged when CaO is replaced by Na2O which is consistent with recent results of molecular dynamic simulations. Moreover, our results confirm the formation of dissimilar Na-Ca pairs which lead to a non-random mixing of the cations in the glass. The formation of such pairs was recently deduced from nuclear magnetic resonance spectra of soda-lime silicate glasses.

Kinetics of crystal nucleation of Na2O � 2CaO � 3SiO2-based solid solutions in glasses of the Na2SiO3?CaSiO3 pseudobinary join

The kinetics of crystal nucleation is investigated in sodium calcium silicate glasses of two compositions (22.4 and 24.4 mol % Na2O), which belong to the Na2SiO3—CaSiO3 pseudobinary join and, according to the phase diagram, lie in the region of the formation of solid solutions between the compositions Na2O · 2CaO · 3SiO2 and Na2O · CaO · 2SiO2. The stationary rate of crystal nucleation of Na2O · 2CaO · 3SiO2-based solid solutions is measured as a function of temperature. It is shown that the maximum stationary rate of nucleation increases with an increase in the sodium oxide content in the initial glasses. The experimental data are analyzed in the framework of the classical nucleation theory.

Structural studies of bioactivity in sol-gel-derived glasses by X-ray spectroscopy.

Extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure, X-ray fluorescence spectroscopy, and X-ray powder diffraction have been used to study the local calcium environment in four sol-gel-derived bioactive calcium silicate glasses of the general formula (CaO)(x)(SiO(2))(1-x). The formation of a hydroxyapatite layer on the composition with the highest bioactivity (x = 0.3) with time has been studied, in an in vitro environment, by immersion in simulated body fluid (SBF) at 37 degrees C. The calcium oxygen environment in the four compositions has been shown to be six-coordinate in character. Both the extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure show a gradual increase in coordination number and Ca--O bond distance with longer exposure to SBF. X-ray fluorescence show that calcium is quickly lost from the samples on exposure to SBF and the calcium concentration then recovers with time. There is clear evidence that the recovery of calcium content is due to the formation of a CaO-P(2)O(5)-rich layer. Annealing of samples at 650 degrees C shows the presence of what, on the length scales probed by X-ray diffraction, appears to be noncrystalline calcium phosphate after 1 h of exposure to an SBF solution, which becomes more crystalline on longer exposure.

Kinetics and Mechanism of the Interaction of Alkali Calcium Silicate Glasses with Salt Melts in the KNO3–Pb(NO3)2System

The interaction of alkali calcium silicate glasses with salt melts in the KN03-Pb(N03)2 system is investigated at temperatures of 420-520°C. The chemical composition of crystalline coatings formed upon treatment contains both components of the initial glass (Si02, 9-12 wt %; CaO, 0.8-1.2 wt %) and components of the salt melt (PbO, 82-89 wt %). The treatment temperature is the main factor affecting the structure of the modified surface layer. The mechanism of the interaction of alkali calcium silicate glasses with salt melts is ana- lyzed. According to this mechanism, the interaction involves the ion exchange (with the participation of Na+, Kf, Ca2+, and pb2+ ions), crystallization of modified surface layers, and incorporation of PbxO, nanoparticles (formed in the salt meltj into the coating structure.

Design of bioactive organic-inorganic hybrids on the basis of the bonding mechanism of calcium silicate glass with living bone

Les verres et les vitroceramiques bioactifs sont des materiaux interessants pour la substitution osseuse car ils forment spontanement des liaisons avec l'os vivant lorsqu'ils sont implantes dans un defaut osseux. Cependant, ils sont beaucoup plus fragiles et beaucoup moins souples que l'os naturel. L'hybridation des constituants essentials des verres et des vitroceramiques avec un polymere flexible peut resoudre ce probleme. La presente contribution est consacree a la conception d'hybrides organiques-inorganiques susceptibles de se lier a l'os vivant. Des etudes anterieures ont montre que la condition pour qu'il y ait une liaison directe entre l'os vivant et les verres et vitroceramiques bioactifs est la formation sur leur surface d'une couche d'apatite analogue a l'os, apres exposition aux liquides corporels. Le meme type de formation d'apatite s'observe meme dans le liquide corporel simule (SBF) propose par Kokubo et coll.. La formation de l'apatite de surface est induite a la fois par la dissolution des ions calcium du verre bioactif, et par les groupes silanol (Si-OH) du gel de silice hydratee forme a la surface. Ces resultats suggerent que l'incorporation d'ions calcium (Ca 2+ ) et de groupes Si-OH dans des substances organiques conduit a un hybride bioactif. Il a ete confirme que des hybrides organiques-inorganiques contenant un sel de calcium, synthetises a partir de vinyltrimethoxysilane ou de 3-methacryloxypropyltrimethoxysilane (MPS) presentent la formation d'apatite dans le SBF. Ce type d'hybride organique-inorganique peut etre melange avec un polymere organique. Un hybride organique-inorganique a ete synthetise a partir de MPS et de 2-hydroxymethacrylate de methyle (HEMA) a la composition MPS:HEMA=0,1:0,9. L'hybride MPS-HEMA forme aussi une couche d'apatite quand du chlorure de calcium lui est incorpore. Ces resultats indiquent que de nouveaux materiaux bioactifs peuvent etre developpes a partir d'hybrides organiques-inorganiques obtenus par incorporation d'ions calcium et de certains types specifiques de groupes fonctionnels tels que Si-OH.

Biocompatibility tests with fibroblasts of CaO rich calcium silicate glasses

Biocompatibility tests with fibroblasts of CaO rich calcium silicate glasses

Collagen-hydroxyapatite composite prepared by biomimetic process.

A novel bone graft substitute comprising a porous, collagenous scaffold was biomimetically coated with hydroxyapatite using a simulated body fluid solution chemistry method. The scaffold had a porosity of approximately 85%, with pore sizes between 30 microm and 100 microm. Glutaraldehyde vapor was used to stabilize the collagenous scaffold, giving a significantly increased thermal stability over an unstabilized scaffold, as shown by differential scanning calorimetry. A thin layer (<10 microm) of crystalline hydroxyapatite was deposited onto the stabilized collagenous scaffold by soaking the collagenous construct in simulated body fluid in the presence of calcium silicate glass. The presence of crystalline hydroxyapatite was confirmed by X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. In vitro cytotoxicity testing of the composite construct using L-929 fibroblasts (ISO 10993-5) and rabbit periosteal cells revealed a cytocompatible material that supported cellular attachment and proliferation.

Molecular Dynamics Simulations of Calcium Silicate Intergranular Films between Silicon Nitride Crystals

Molecular dynamics simulations were used to study the structure of calcium silicate intergranular films (IGFs) formed between the basal planes of silicon nitride crystals. A multibody potential was used to describe the interactions between ions. Samples with different film thickness and CaO contents were studied. Epitaxial adsorption of Si and O atoms from the intergranular films onto N and Si, respectively, in the crystal surface was observed. This epitaxial order induced a structural order into the nominally amorphous IGF that decreased as a function of distance from the IGF/crystal interface. A higher concentration of strained siloxane bonds was observed at the IGF/crystal interface in comparison to the amorphous interior of the IGF. While Ca ions were observed to segregate to the IGF/crystal interface in simulations of calcium silicate glass IGFs between alumina crystals, no segregation of calcium to the first adsorbed layer on the nitride was found in these simulations using silicon nitride crystals. Planar alignment of Ca ions parallel to the IGF/crystal interface occurred with either the largest concentrations of CaO or with the thinnest IGFs studied here. This alignment creates localized nonbridging oxygen that would affect the stability of the IGF/crystal system.

フッ化物イオンを添加したカルシウムケイ酸塩ガラスの構造と骨石灰化

The in vitro bioactivity, in terms of apatite-forming ability, of calcium silicate glasses containing 10mol% fluoride ions in the NaF-CaO-SiO2 system was examined by soaking in Kokubo's simulated body fluid (SBF) at 36.5°C for 7d. The surface structure of the glasses was examined by thin-film X-ray diffraction (TF-XRD) and Fourier transform infrared reflection spectroscopy (FT-IR) in order to confirm the apatite deposition. The structure of the calcium silicate glasses was examined by 29Si magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR) and X-ray photoelectron spectroscopy (XPS). Three calcium silicate glasses containing 10mol% fluoride ions had different apatite-forming abilities. In the case of CaO/SiO2=1, the apatite-forming ability of 10NaF·45CaO·45SiO2 was lower than that of 50CaO·50SiO2. This indicated that the addition of fluoride ions suppressed the apatite-forming ability, irrespective of the structural similarity of the silicate network based on the ratio of CaO/SiO2 between the glasses.

Specific Heat Measurement of High Temperature Thermal Insulations by Drop Calorimeter Method

By applying the drop calorimeter method, a specific heat measuring apparatus that can be used in the temperature range from 100 to 1000°C has been developed. It is generally difficult to measure the specific heat of thermal insulation, because the insulation material is porous, and has low thermal conductivity and small heat capacity. In the present apparatus, the specific heat of thermal insulation is simply measured by dropping a heated specimen into water. The specific heat of the specimen obtained by the apparatus is the “mean” specific heat between the initial specimen temperature before dropping and the equilibrium water temperature after dropping. This apparatus was used to measure the mean specific heat of standard specimen SRM 720 Synthetic Sapphire (α-Al2O3) whose reference values are certified by the National Institute of Standards and Technology (NIST). The measured values agreed well with the reference values within an error of ±10%. The specific heats of SiC refractory material, rock wool, alumina silica fiber, alumina silica board, calcium silicate, and SiO2 glass measured with this apparatus are also presented.

Formation of Metallic Nanoparticles in Silicate Glass through Ion Implantation

The composite layers based on sodium calcium silicate glass that contains metallic nanoparticles synthesized by implantation with Ag+ ions are studied. The ion implantation conditions are as follows: the energy is 60 keV, the doses are (2.0–4.0) × 1016 ions/cm2, and the temperatures of the irradiated glass are 20, 35, 50, and 60°C. The data on the distribution of implanted silver and the nucleation and growth of metallic nanoparticles over the depth as functions of the temperature and dose are obtained using the Rutherford backscattering technique and the reflectance optical spectra. It is demonstrated that small changes in the temperature of the irradiated glass substrate lead to considerable differences in the specific features of the nanoparticle formation in the bulk of samples. At implantation temperatures of 50 and 60°C, the reflectance spectrum contains overlapping bands with two selective maxima. This suggests the possible formation of two layers that are located at different depths and involve nanoparticles differing in size, whereas one layer composed of nanoparticles is formed at 20°C.