Crystalline Phase Assemblage Research Articles

Long-term sulfate resistance of synthesized cement systems with variable C3A/C4AF ratio at low temperature or ambient conditions: Insights into the crystalline and amorphous phase assemblage

Synthesized cement systems made with variable C3A/C4AF ratios, containing C3S, gypsum and, optionally, calcite, were stored long-term at humid conditions at 5 or 20 °C, without any protection against atmospheric carbonation. Analytical techniques able to assess both the crystalline and amorphous phases were used. Experimental results were compared with thermodynamic simulations. The systems with C3A/C4AF < 1 better preserved the soundness of the CSH phase, which hosted iron, and prevented thaumasite formation. The addition of calcite in these systems inhibited carbonation. When occurred (mixtures without calcite), the carbonation was significantly more intense at ambient temperature. In the systems that underwent extensive deterioration, cross-linking of silicate structures, AFt decomposition, and iron release from the deteriorating CSH, occurred, while Al-incorporating amorphous silica, calcium carbonate polymorphs and hydrous iron oxide formed. The presence of unreacted C3A in the systems with C3A/C4AF = 1, suggested that CSH decomposition was contributed by available sulfates.

Fabrication and biological evaluation of three-dimensional (3D) Mg substituted bi-phasic calcium phosphate porous scaffolds for hard tissue engineering.

This work reports on the fabrication of three-dimensional (3D) magnesium substituted bi-phasic calcium phosphate (Mg-BCP) scaffolds by gel-casting, their structural and physico-chemical characterization, and on the assessment of their in vitro and in vivo performances. The crystalline phase assemblage, chemical functional groups and porous morphology features of the scaffolds were evaluated by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR) and field emission scanning electron microscopy (FE-SEM), respectively. The sintered scaffolds revealed an interconnected porosity with pore sizes ranging from 4.3 to 7.28 μm. The scaffolds exhibited good biomineralization activity upon immersion in simulated body fluid (SBF), while an in vitro study using MG-63 cell line cultures confirmed their improved biocompatibility, cell proliferation and bioactivity. Bone grafting of 3D scaffolds was performed in non-load bearing bone defects surgically created in tibia of rabbits, used as animal model. Histological and radiological observations indicated the successful restoration of bone defects. The overall results confirmed the suitability of the scaffolds to be further tested as synthetic bone grafts in bone regeneration surgeries and in bone tissue engineering applications.

A Reappraisal of Peridotite Solidus Phase Equilibria from 6 to 14 GPa in the System CaO–MgO–Al2O3–SiO2

AbstractExperimentally determined isobaric invariant melting phase relations from 6 to 14 GPa in the system CaO–MgO–Al2O3–SiO2 (CMAS), involving the crystalline phases forsterite + orthopyroxene + clinopyroxene + garnet and liquid, are reported. Experiments were conducted using a multianvil device with stepped lanthanum chromite heaters in the pressure cells. At a fixed pressure, the five-phase assemblage identified above can exist only at a single temperature. As such, these isobaric invariant points correspond to the solidus of model garnet peridotite in this part of the composition space in the studied system, as is the case at lower pressures in some previous studies. The solidus of model peridotite is univariant in pressure–temperature space, has a positive Clapeyron slope, and the isobaric invariant solidus temperatures, at 6, 8, 10, 12, and 14 GPa, are 1965 °C, 2090 °C, 2200 °C, 2280 °C, and 2320 °C, respectively. Over the investigated pressure range, orthopyroxene is in reaction relation with the liquid, with the fusion reaction taking the form forsterite + clinopyroxene + garnet = orthopyroxene + liquid. The compositions of liquids reported here do not seem to depend on orthopyroxene being present in the experiments. Compositionally, liquids here are fairly magnesian and siliceous, and have lower alumina and lime concentrations than at lower pressures with the identical crystalline phase assemblage in the system CMAS. In contrast to some previous studies, in this study, there is no evidence of maximum and minimum normative forsterite concentration of the isobaric invariant liquid at around 8 and 12 GPa, respectively, nor of a substantial curvature in the track of liquid compositions, when such liquids coexist with the above-mentioned four-phase crystalline phase assemblage. Instead, here, with increasing pressure from 6 to 14 GPa, liquids at the isobaric invariant points (defining the univariant solidus) become progressively (quasi-linearly) enstatite-normative. This experimental observation on liquid compositions from the present study might be important for future work aimed at investigating the chemistry of liquids derived from partial fusion of anhydrous peridotite at pressures, and corresponding depths in Earth, greater than investigated here.

Structural changes in aluminosilicate glasses up to 164 GPa and the role of alkali, alkaline earth cations and alumina in the densification mechanism

Pressure induced structural changes in silicate melts have a great impact on their physico-chemical properties and hence on their behaviour in the deep Earth's interior. In order to gain a deeper understanding we have studied the densification mechanism in multicomponent aluminosilicate glasses (albitic and albit-diopside composition) by means of extended X-ray absorption fine structure spectroscopy coupled to a diamond anvil cell up to 164 GPa. We have monitored the structural modifications from the network-former Ge as well as the network-modifier Sr. Notably, we tracked the evolution of Ge-O and Sr-O bond lengths (RGe-O, RSr-O) and their coordination number with pressure. We show that RGe-O increases strongly up to about 32 GPa, whereas RSr-O increases only slightly up to ~26 GPa. We assign these extensions to the increase of the coordination number from 4 to 6 (Ge) and from ~6 to at least 9 (Sr). Upon further compression RGe-O and RSr-O exhibit a continuous decrease to the highest probed pressure. These bond contractions, notably of RGe-O, that are continuous and exceed the one observed in pure SiO2 and GeO2, reflect a higher structural flexibility of multi-component glasses compared to those simple systems. Particularly, the high fraction of non-bridging oxygen atoms due to the presence of Na, Sr, Ca, Mg in the studied glasses, favours the simple compression of the highly-coordinated polyhedra of Si and Ge at pressure greater than 30 GPa. This is in strong contrast to pure oxides where cation polyhedral distortions govern the densification mechanism of the glass. The results of this study demonstrate that low field-strength alkali and alkaline earth cations, ubiquitous in deep Earth's melts, have a profound influence on the densification mechanism of glasses. Our results provide important constrains for interpreting the observed low velocity anomalies at the Earth's core-mantle boundary that have been, beyond others, referred to the presence of high-density melts. The hypothesis that non-buoyant melts at the Earth's core-mantle boundary can be formed by peculiar structural transformations in melts leading to higher coordination numbers compared to their crystalline equivalents is not supported from the present observations. The present results rather suggest that if velocity anomalies are to be explained by melts, these likely have considerable differences in chemical composition to the surrounding crystalline phase assemblage.

Effect of crystalline phase assemblage on reliability of 3Y-TZP

Statement of problemStrengthening mechanisms of zirconia ceramics stabilized with 3 mol% yttria (3Y-TZP) are complex and dictated by the crystalline phase assemblage. Although their clinical performance for dental restorations has been excellent, there is evidence that framework fractures do occur and have been underreported. Meanwhile, the relationship between phase assemblage and reliability of 3Y-TZP is not properly understood. PurposeThe purpose of this in vitro study was to elucidate the relationship between crystalline phase assemblage and the reliability of 3Y-TZP and to calculate the associated probabilities of survival. Material and methodsDisks of 3Y-TZP were prepared from cylindrical blanks and randomly assigned to 12 experimental groups (n=20 per group). Different crystalline phase assemblages were produced by either varying the sintering temperature from 1350 °C to 1600 °C and/or treating the surface by airborne-particle abrasion with 50-mm alumina particles at a pressure of 0.2 MPa for 1 minute with or without subsequent heat treatment. Crystalline phases were analyzed by standard and grazing incidence X-ray diffraction (GIXRD). The relationship between phase assemblage and reliability was determined by measuring the biaxial flexural strength (BFS) according to ISO standard 6872 and by using Weibull statistics to calculate the Weibull modulus (m), probability of survival, and maximum allowable stresses. XRD results were analyzed by ANOVA to detect statistically significant differences between groups. Adjustment for all pairwise group comparisons was made using the Tukey method (α=.05). ResultsStandard incidence XRD confirmed the presence of a small amount of cubic phase after sintering at 1350 °C. A cubic-derived nontransformable tetragonal t’-phase was observed at sintering temperatures of 1450 °C and above, the amount of which increased linearly. GIXRD revealed that airborne-particle abraded groups sintered at 1350 °C and 1600 °C had the highest variability in monoclinic phase fraction as a function of depth. These groups were also associated with the lowest reliability. Groups as-sintered at 1350 °C and 1600 °C had the lowest modulus (m=8.1 [0.5] and 7.0 [0.8], respectively) and probability of survival (Ps) for a maximum allowable stress of 700 MPa, while treated groups sintered at 1450 °C and 1550 °C were associated with the highest modulus (from 15.0 [1.4] to 20.9 [1.4]) and Ps (≥0.9999). The lower strength and reliability of groups sintered at 1600 °C was consistent with the presence of a significant amount of nontransformable t’-phase. The pattern of BFS results indicated that ferro-elastic domain switching was a dominant strengthening mechanism in 3Y-TZP. ConclusionsThe present study first reported on the detrimental effect of the cubic-derived nontransformable t’-phase on the mechanical properties of 3Y-TZP. It was demonstrated that phase assemblage determined reliability and was directly linked to the probability of survival.

Fabrication and electrical characteristics of flash-sintered SiO2-doped ZnO-Bi2O3-MnO2 varistors

The dense ZnO-Bi2O3-MnO2-xSiO2 (ZBMS) varistors for x = 0, 1, 2, 3 wt% were fabricated by flash sintering method under the low temperature of 850 °C within 2 min. The sample temperature was estimated by a black body radiation model in the flash sintering process. The crystalline phase assemblage, density, microstructure, and electrical characteristics of the flash-sintered ZBMS varistors with different SiO2-doped content were investigated. According to the XRD analysis, many secondary phases were detected due to the SiO2 doping. Meanwhile, the average grain size decrease with increasing SiO2-doped content. The improved nonlinear characteristics were obtained in SiO2-doped samples, which can be attributed to the ion migration and oxygen absorption induced by the doped SiO2. The flash-sintered ZBMS varistor ceramics for x = 2 wt% exhibited excellent comprehensive electrical properties, with the nonlinear coefficient of 24.5, the threshold voltage and leakage current of 385 V·mm−1 and 11.8 µA, respectively.

THE EFFECT OF REPEATED HEAT-PRESSING ON THE BIAXIAL FLEXURAL STRENGTH AND SURFACE ROUGHNESS OF LITHIUM DISLILICATE GLASS-CERAMICS

Statement of problem: During heat-pressing of lithium dislilicate glass-ceramics, It is more economical to press several restorations from one ingot at the same time. However, this is often not possible and may result in a considerable amount of leftover material. It has been reported that the leftover materials after heat-pressing are reused in some dental laboratories. Sufficient knowledge about the consequences of such procedure is not available. The issue is thereby raised whether the leftover material should be discarded or reused.Purpose: The aim of the present study was to evaluate the effect of repeated heat-pressing on biaxial flexural strength and surface roughness of lithium dislilicate pressable glass-ceramics (IPS e.max Press). As well as to describe the microstructural features present in fresh-pressed, and repressed material using Xray diffraction, EDAX and SEM Materials and methods: Twenty pressed and repressed samples were prepared following the manufacturers’ recommendations measuring 15 mm × 1 mm per material. Biaxial flexure (piston on 3-ball method) was used to assess strength. X-ray diffraction was performed to identify the crystalline phases, and a scanning electron microscope was used to disclose microstructural features. Also surface roughness was evaluatedResults: BFS for pressed and repressed respectively; (375.8± 4.55) and (389.4±12.12) MPa; no significant difference was found between the Pressed and Repressed groups. Surface roughness for pressed glazed and unglazed respectively (0.27±0.05, and 1.14±0.16) – for repressed glazed and unglazed respectively (0.21±0.06 and 1.33±0.20) - for both groups, significant difference was found between Glazed and Un-Glazed ceramics. No statistical significant difference was found between Pressed and Repressed groups. Xray diffraction revealed the material is predominantly crystalline structure; lithium disilicate was identified to be the main crystalline phase, peaks after pressing and repeating pressing are similar, the crystalline phase assemblage did not change; however their radiation intensities (height) has, the dominant peak (highest peak) for the repressed sample is smaller compared to the pressed. EDAX results showed no change in composition between pressed and repressed samples. Microstuructural features SEM displayed a noted increase in crystal dimension after repressing Conclusions. IPS e.max Press left over pressed buttons can be safely reused since repressing does not display consequent negative effects on Biaxial flexural strength and surface roughness, as well as microstructure.

Thermo-mechanical and high-temperature dielectric properties of cordierite-mullite-alumina ceramics

Heterogeneous ceramics made of cordierite (55–56wt%), mullite (22–33wt%) and alumina (23–11wt%) were prepared by sintering non-standard raw materials containing corundum, talc, α-quartz, K-feldspar, kaolinite and mullite with small amounts of calcite, cristobalite and glass phases. The green specimens prepared by PVA assisted dry-pressing were sintered within the temperature range of 950–1500°C for different dwelling times (2–8h). The effects of sintering schedule on crystalline phase assemblage and thermomechanical properties were investigated. The sintered ceramics exhibited low coefficients of thermal expansion (CTE) (3.2–4.2×10−6°C−1), high flexural strength (90−120MPa and high Young modulus (100GPa). The specimens sintered at 1250°C exhibited the best thermal shock resistance (∆T~350°C). The thermal expansion coefficients and thermal shock resistance were studied using Schapery model, the modelling results implying the occurrence of non-negligible mechanical interactions between the phases in bulk. The dielectric properties characterized from room to high temperature (RT– HT, up to 600°C) revealed: (i) noticeable effects of sintering schedule on dielectric constant (5–10) and dielectric loss factor (~0.02–0.04); (ii) stable dielectric properties until the failure of the electrode material. The thermomechanical properties coupled with desirable dielectric properties make the materials suitable for high density integrated circuitry or high temperature low-dielectric materials engineering.

Direct Ink Writing of a Preceramic Polymer and Fillers to Produce Hardystonite (Ca2ZnSi2O7) Bioceramic Scaffolds

The direct ink writing of an ink composed of a preceramic polymer and fillers was used to produce hardystonite (Ca2ZnSi2O7) bioceramic scaffolds. Suitable formulations were developed for the extrusion of fine filaments (350 μm diameter) through a nozzle. The preceramic polymer was employed with the double purpose of contributing to the rheology of the ink by increasing its viscosity and of forming the hardystonite phase upon heat treatment by reacting with the fillers. A control of the rheology is essential when spanning features have to be produced, and therefore the main rheological characteristics of the inks were measured (flow curves, dynamic oscillation tests, viscosity recovery tests) and compared to models reported in the literature. Highly porous scaffolds (up to 80% total porosity) were produced and heat treated in air or in nitrogen atmosphere. The influence of the heat‐treatment atmosphere on the morphology, crystalline phase assemblage, and compressive strength of the scaffolds was investigated.

Influence of Al2O3 and B2O3 on Sintering and Crystallization of Lithium Silicate Glass System

This article reports on the effect of Al2O3 and B2O3 added as dopants on the preparation of glass‐ceramics (GCs) belonging to the lithium silicate glass system. The GCs are prepared by sintering route using glass powders. The reasons for the crystallization of the metastable crystalline phase lithium metasilicate (LS) are discussed and the impact of the dopants on the thermodynamics and kinetics of crystallization is investigated. The addition of dopants modifies the thermodynamic equilibrium of the system and this change is mainly entropy driven and also slowdown the kinetics of crystallization. Differential thermal analysis and hot‐stage microscopy are employed to investigate the glass‐forming ability, sintering, and crystallization behavior of the studied glasses. The crystalline phase assemblage studied under nonisothermal heating conditions in the temperature range of 800°C–900°C in air. Well sintered and dense glass‐ceramics are obtained after sintering of glass powders at 850°C–900°C for 1 h featuring crystalline phase assemblage dominated by lithium disilicate (LS2).

Alkali-free bioactive diopside–tricalcium phosphate glass-ceramics for scaffold fabrication: Sintering and crystallization behaviours

The sintering and crystallization behaviours of alkali-free bioactive glass compositions designed in the diopside (CaO·MgO·2SiO2)–tri-calcium phosphate (3CaO·P2O5) binary join by varying the Di/TCP ratios were investigated by hot-stage microscopy and differential thermal analysis, respectively. The effects of non-isothermal heating for 1h at 900, 1000 and 1200°C on the structural changes were assessed by solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) and crystalline phase assemblage. The influence of the heat treatment temperature on 3 point bending strength was also investigated. Diopside and hydroxyapatite formed as primary crystalline phases, accompanied by calcium silicate as secondary phase, in samples are sintered at 900°C and 1000°C. With further increase of the heat treatment temperature to 1200°C, whitlockite that was stated appearing in some compositions at 1000°C, appeared as main crystalline phase after diopside as a result of hydroxyapatite transformation. The data gathered was highly relevant towards setting the experimental conditions required for preparing well sintered and mechanically strong glass/glass-ceramic scaffolds for bone regeneration and tissue engineering applications.

Influence of Strontium Oxide on Structural Transformations in Diopside-Based Glass-Ceramics Assessed by Diverse Structural Tools

A full understanding of the structural glass variations and of the nucleation and growth of crystalline phases in glass-ceramics (GCs) induced by compositional changes is highly relevant for applications such as sealants for solid oxide fuel cells, bio- and dental materials, and photonics. Systematic substitution of Ca with Sr in diopside (Di-CaMgSi2O6) has a significant impact on the crystalline-phase assemblage after heat treating at 850 °C for 500 h. A high percentage of Di crystallized from Sr-free glass; however, with increasing Sr content the tendency toward crystallization decreased, and Sr-akermanite (Sr-Ak) was preferentially crystallized. In this study, the structural transformations in the Di-based-GC system upon the substitution of calcium with strontium have been examined by X-ray powder diffraction (XRD), 29Si and 27Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and Raman spectroscopies. XRD Rietveld refinement results indicate that up to a maximum of ∼25 at. % of Sr occupie...

Crystallization of pyroxene phases and physico-chemical properties of glass-ceramics based on Li2O–Cr2O3–SiO2 eutectic glass system

The crystallization characteristics, crystalline phase assemblages and solid solution phases developed due to thermally crystallized glasses based on the Li2SiO3–Li2Si2O5–LiCrSi2O6 (1028 ± 3 °C) eutectic glass system by replacing some trivalent oxides instead of Cr2O3 were investigated. The microhardness and chemical durability of the glass-ceramics were also determined. Lithium meta and disilicate (Li2SiO3 and Li2Si2O5), lithium gallium silicate (LiGaSiO4), and varieties of pyroxene phases, including Cr-pyroxene phase, i.e. lithium-kosmochlor (LiCrSi2O6), lithium aluminum silicate (LiAlSi2O6), lithium indium silicate (LiInSi2O6) and pyroxene solid solution of Li-aegerine type [Li (Fe0.5, Cr0.5) Si2O6] were the main crystalline phases formed in the crystallized glasses. There is no evidence for the formation of solid solution or liquid immiscibility gaps between LiAlSi2O6 or LiInSi2O6 phases and LiCrSi2O6 phase. However, LiCrSi2O6 and LiFeSi2O6 components were accommodated in the pyroxene structure under favorable conditions of crystallization to form monomineralic pyroxene solid solution phase of the probably formula [Li (Fe0.5, Cr0.5) Si2O6]. The type and compatibility of the crystallized phases are discussed in relation to the compositional variation of the glasses and heat-treatment applied. The microhardness values of the crystalline materials ranged between 5282 and 6419 MPa while, the results showed that the chemical stability of the glass-ceramics was better in alkaline than in acidic media.

Melting phase equilibria of model carbonated peridotite from 8 to 12 GPa in the system CaO-MgO-Al2O3-SiO2-CO2 and kimberlitic liquids in the Earth's upper mantle

Pressure-temperature divariant melting phase relations of model carbonated peridotite in the system CaO-MgO-Al2O3-SiO2-CO2 from 8 to 12 GPa are reported. From 8 to 12 GPa, melting temperatures on the studied pressure-temperature divariant surface rise quite rapidly. Liquids, in equilibrium with forsterite, orthopyroxene, clinopyroxene, and garnet, on the low-temperature side of the divariant surface are magnesiocarbonatitic in composition. With increasing temperature along an isobar on the pressuretemperature divariant surface, and with the same crystalline phase assemblage, liquids gradually become kimberlitic in their composition. Given the model system data reported here, Group IB kimberlites and perhaps some kimberlites from Greenland, Canada, and South Africa could be generated from direct melting of carbonated peridotite in the pressure range of approximately 6-8 GPa. Some kimberlites from Canada and Russia could have formed by partial melting of carbonated mantle peridotite at pressures of about 10-12 GPa. From 8 to 12 GPa, liquid compositions on the studied divariant surface show a limited compositional range, which implies that the divariant surface is essentially flat. The implied flatness of the divariant surface, and so long as the liquid is in equilibrium with the crystalline assemblage of forsterite + orthopyroxene + clinopyroxene + garnet on the divariant surface, indicate that kimberlites belonging to Group IA and those more magnesian than Group IA cannot have their origin in only being a melting product of carbonated mantle peridotite. The absence of topography of the studied pressure-temperature divariant surface in all likelihood limits the generation of kimberlites in the Earth’s upper mantle only.

Melting phase relations of model carbonated peridotite from 2 to 3 GPa in the system CaO‐MgO‐Al2O3‐SiO2‐CO2 and further indication of possible unmixing between carbonatite and silicate liquids

AbstractMelting phase relations of model carbonated peridotite in the system CaO‐MgO‐Al2O3‐SiO2‐CO2 from 2 to 3 GPa are reported. Experimentally produced melts, which are model carbonatites, with approximately 36–40 wt % CaO, 12–17 wt % MgO, 0.2–1.5 wt % Al2O3, 1–4 wt % SiO2, and 40–42 wt % CO2 (carbon dioxide) are present at all pressures investigated. At 2.8 and 3 GPa, carbonatitic melts are seen experimentally at temperatures that are very close to the vapor‐free (CO2) peridotite solidus and are found in equilibrium with forsterite, orthopyroxene, clinopyroxene, and garnet. Solidus phase relations with isobaric and pressure‐temperature invariant points, defining the so‐called carbonated peridotite solidus ledge, are also reported from 2.1 to 3 GPa. A divariant region exists from 2 to 2.6 GPa wherein two, compositionally different melts are present. In this region, these two melts, carbonatitic and silicate in composition, coexist with crystalline phase assemblage and free vapor. The silicate liquid has approximately 30–48 wt % SiO2 and approximately 6 to 20 wt % of dissolved CO2. The presence of carbonatitic and silicate liquids is interpreted to be due to liquid immiscibility. On the basis of melting phase relations reported here, we conclude that (a) the ledge is a feature along which model carbonatitic liquids are produced by reaction of silicates and CO2 vapor and (b) alkali‐free carbonatites and silicate melts can form through melt unmixing at depths of ~60–80 km in the Earth's mantle.

Silicate liquid‐carbonatite liquid transition along the melting curve of model, vapor‐saturated peridotite in the system CaO‐MgO‐Al2O3‐SiO2‐CO2 from 1.1 to 2 GPa

AbstractSolidus phase relations of carbon dioxide‐saturated (CO2 vapor) model peridotite in the system CaO‐MgO‐Al2O3‐SiO2‐CO2 in the 1.1–2.1 GPa pressure range are reported. The solidus has a positive slope in pressure‐temperature (PT) space from 1.1 to 2 GPa. Between 2 and 2.1 GPa, the melting curve changes to a negative slope. From 1.1 to 1.9 GPa, the liquid, best described as CO2‐bearing silicate liquid, is in equilibrium with forsterite, orthopyroxene, clinopyroxene, spinel, and vapor. At 2 GPa, the same crystalline phase assemblage plus vapor is in equilibrium with two liquids, which are silicate and carbonatitic in composition, making the solidus at 2 GPa PT invariant. The presence of two liquids is interpreted as being due to liquid immiscibility. Melting reactions written over 1.1–1.9 GPa are peritectic, with forsterite being produced upon melting, and the liquid is silicate in composition. Upon melting at 2.1 GPa, orthopyroxene is produced, and the liquid is carbonatitic in composition. Hence, the invariance between 1.9 and 2.1 GPa is not only the reason for the dramatic change in the liquid composition over an interval of 0.2 GPa, but the carbonated peridotite solidus ledge itself most likely appears because of this PT invariance. It is suggested that because carbonatitic liquid is produced at the highest solidus temperature at 2 GPa in PT space in the system studied, such liquids, in principle, can erupt through liquid immiscibility, as near‐primary magmas from depths of approximately 60 km.

Thermo-mechanical behaviour of alkali free bioactive glass-ceramics co-doped with strontium and zinc

We report on the influence of SrO and ZnO co-doping on thermo-mechanical behaviour of alkali free bioactive glass-ceramics in system: (mol%) (36.07−x) CaO–x SrO–(19.24−y) MgO–y ZnO–5.61 P2O5–38.49 SiO2–0.59 CaF2 (x=2–10, y=2–10). Hot-stage microscopy and differential thermal analysis revealed that densification of all the glass powders occurs before the onset of crystallization, resulting in well sintered and mechanically strong glasses/glass-ceramics after heat treating for 1h under non-isothermal conditions at 800, 850 and 900°C. The crystalline phase assemblage in the sintered compacts included diopside (CaMgSi2O6) and fluorapatite [Ca5(PO4)3F] with partial replacement of Ca for Sr with increasing strontium contents. The effect of heat treatment temperature on mechanical reliability was assessed by Weibull statistical analysis applied to the flexural strength data. The results obtained revealed that a better balance of properties was achieved for the samples sintered at 850°C. These results provide an insight into the choice of final material/experimental conditions for the development of three-dimensional porous scaffolds for bone tissue engineering.

Influence of nitrogen overpressure on the nitridation, densification and formation of β-SiAlONs produced by silicothermal reduction

Abstract β-SiAlON materials (with z = 1) with 2 mol% DyAG have been synthesised by silicothermal reduction under different nitrogen pressures. The possibility of rapidly nitriding silicon and forming the SiAlON phase has been investigated through its reaction sequence of formation under different pressures. β-SiAlONs were fully nitrided (degree of nitridation greater than 93%) and formed after a 1-h hold at 1400 °C, or after a straight ramp to 1500 °C under 0.7 MPa of nitrogen. This was not achievable under static nitrogen at atmospheric pressure where the degree of nitridation was only 43% and β-SiAlON phase represented only about 60% of the crystalline phase assemblage at the same temperature. The formation of β-SiAlON depended on the formation of Si 3 N 4 whose reaction rate was enhanced by nitrogen overpressures.

Effect of calcium fluoride on sintering behavior of SiO2-CaO-Na2O-MgO glass-ceramic system

The crystallization characteristics of glasses based on the SiO2-CaO-Na2O-MgO (SCNM) system containing calcium fluoride (CaF2) have been investigated by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The partial replacement of CaO by CaF2 in the studied glass-ceramics led to the development of different crystalline phase assemblages, including wollastonite and diopside using various heat-treatment processes. With the increase of CaF2 content, the crystallization temperature of the glass and the strength of the crystallization peak temperature decreases. Addition of CaF2 up to 6.0 mol%, as expected, improved the sinterability. This sample reached to maximum density by sintering at 950 ?C.

Sintering behavior of lanthanide-containing glass-ceramic sealants for solid oxide fuel cells

This article reports on the influence of different lanthanides (La, Nd, Gd and Yb) on sintering behavior of alkaline-earth aluminosilicate glass-ceramic sealants for their application in solid oxide fuel cells (SOFCs). All the glasses have been prepared by the melt–quench technique. The in situ follow up of sintering behavior of glass powders has been done by a high temperature-environmental scanning electron microscope (HT-ESEM) and a hot-stage microscope (HSM) while the crystalline phase evolution and assemblage have been analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). All the glass compositions exhibit a glass-in-glass phase separation followed by two stage sintering resulting in well sintered glass powder compacts after heat treatment at 850 °C for 1 h. Diopside (CaMgSi2O6) based phases constituted the major crystalline part in glass-ceramics followed by some minor phases. The increase in lanthanide content in glasses suppressed their tendency towards devitrification, thus resulting in glass-ceramics with a high amount of residual glassy phase (50–96 wt%) which is expected to facilitate their self-healing behavior during SOFC operation. The electrical conductivity of the investigated glass-ceramics varied between (1.19 and 7.33) × 10−7 S cm−1 (750–800 °C) while the coefficient of thermal expansion (CTE) varied between (9.4 and 11.2) × 10−6 K−1 (200–700 °C). Further experimentation with respect to the long term thermal and chemical stability of residual glassy phase under SOFC operation conditions along with high temperature viscosity measurements will be required in order to elucidate the potential of these glass-ceramics as self-healing sealants.