Glass-ceramics in the CaO–MgO–Al2O3–SiO2 system as potential dental restorative materials

This paper reports on development of novel alumina-containing glass-ceramics (GCs) with a high content of Al₂O₃ (12.5 wt.%) in the CaO–MgO–Al₂O₃–SiO₂ system aimed for dental restorations. The thermal properties of the parent glasses, the microstructure and the mechanical properties of the produced sintered and crystallized GCs along with bio-inertia performance were experimentally studied. Dense, white, and bio-inert GCs, comprised of melilite, either as a single-phase or with diopside, were produced. The values of flexural strength ranged between 120 and 171 MPa, the modulus of elasticity varied between 28 and 42 GPa, while the values of the hardness and the fracture toughness (measured by the indentation–Niihara equation) ranged from 6.3 to 7.0 GPa, and from 2.6 to 2.8 MPa m^0.5, respectively. The mechanical properties of the produced GCs, after being meticulously compared with the mechanical properties of GCs of various compositions reported in literature, including commercial ones, are a good match to the properties of dental hard tissues, and satisfy the requirements of the ISO 6872 “Dentistry-Ceramic Materials”.     

Phase equilibria studies in the CaO–SiO2–Al2O3–MgO system with CaO/SiO2 ratio of 0.9

Phase equilibria and liquidus temperatures in the CaO–SiO₂–Al₂O₃–MgO system at a CaO/SiO₂ weight ratio of 0.9 in the liquid phase have been experimentally determined employing high-temperature equilibration and quenching technique followed by electron probe X-ray microanalysis. Isotherms at 1573, 1623, 1673, and 1773 K were determined and the primary phase fields of wollastonite, melilite, olivine, periclase, spinel, and corundum have been located. Compositions of the olivine and melilite solid solutions were analyzed and discussed. Comparisons between the newly constructed diagram, existing data, and FactSage predicted phase diagrams were performed and differences were discussed. The present study will be useful for guidance of industrial practices and further development of thermodynamic modeling.     

1250°C liquidus for the CaO–MgO–SiO2–Al2O3–TiO2 system in air

Ti-bearing blast furnace slags have been regarded as an important secondary material in modern society, and the efficient recycling of Ti oxides from it is of key interest. For this reason, more thermodynamic data is needed regarding the phase relations in different composition ranges and sections. Therefore, the equilibrium phase relations of CaO–MgO–SiO₂–Al₂O₃–TiO₂ system in a low w(CaO)/w(SiO₂) ratio of 0.6–0.8 at 1250 °C in air and fixed concentrations of MgO and Al₂O₃, were investigated experimentally using a high temperature equilibration and quenching method followed by SEM-EDS (Scanning Electron Microscope and Energy Dispersive X-ray Spectrometer) analyses. The equilibrium solid phases of perovskite (CaO·TiO₂), a pseudo-brookite solid solution (MgO·2TiO₂, Al₂O₃·TiO₂)_ss, and anorthite (CaO·Al₂O₃·2SiO₂) were found to coexist with the liquid phase at 1250 °C. The calculated results of Factsage and MTDATA were used for comparisons, and significant discrepancies were found between predictions and the experimental results. The 1250 °C isotherm has been constructed and projected on the CaO–SiO₂–TiO₂-8 wt.% MgO-14 wt% Al₂O₃ quasi-ternary plane of the phase diagram. The obtained results provide new fundamental data for Ti-bearing slag recycling processes, and they add new experimental features for thermodynamic modeling of the high-order titanium oxide-containing systems.     

Flame-spraying synthesis of aluminate glasses in the Al2O3–La2O3 system

The paper deals with the synthesis and characterisation of binary aluminate glasses in the La₂O₃–Al₂O₃ system with Al₂O₃ contents changing between 74.6 and 86.9 mol% (48–65 wt.%), and of ternary glasses with 75.7 mol% Al₂O₃ doped with 1 mol% of Nd₂O₃ or Er₂O₃. Six binary and two ternary compositions were prepared. Flame synthesis facilitated the preparation of X-ray amorphous microspheres in the systems with 58 wt.% Al₂O₃, and with eutectic composition in the pseudobinary LaAlO₃–LaAl₁₁O₁₈ system doped with Er. Other systems contained low fractions of crystalline LaAlO₃ perovskite, regardless of the composition. The diameter of prepared microspheres ranged between 2 and 10 μm. They were transparent for visible light, as well as in the IR wavenumber range from 1300 to 4000 cm^?1.     

Thermal evolution of Al2O3–CaO–Cr2O3 castables in different atmospheres

Al₂O₃–CaO–Cr₂O₃ castables are required for various furnaces linings due to their excellent corrosion resistance. However, toxic and water-soluble Cr(VI) could be generated in these linings during service. In this study Al₂O₃–CaO–Cr₂O₃ castables were prepared and heated at 300–1500 °C in air and coke bed to simulate actual service conditions. The formations of various phases were investigated by XRD and SEM-EDS. The Cr(VI) compounds CaCrO₄ and Ca₄Al₆CrO₁₆ formed in air at 300–900 °C and 900–1300 °C respectively, while C₁₂A₇ and CA₂ were generated rather than forming Cr(VI) compounds in coke bed at 700–1300 °C. However, at 1500 °C, nearly all the chromium existed in the form of (Al_1-xCr_x)₂O₃ solid solution in both atmosphere. As a result, the specimens treated in air contained 185.0–1697.8 mg/kg of Cr(VI) at 500–1300 °C but only 17.2 mg/kg of Cr(VI) at 1500 °C, whereas specimens treated in coke bed exhibited extremely low Cr(VI) concentration in the whole temperature range studied. Moreover, in coke bed, the mutual diffusion between Cr₂O₃ and Al₂O₃ was suppressed and a trace of Cr₂O₃ would even be reduced to form chromium-containing carbides on its surface, which would hindered the sintering process and hence lower the density as well as strength of the castables.     

Stable garnets in the Er2O3–Sc2O3–Al2O3 oxide system for optical ceramics application

Erbium-scandium-aluminum garnets (ErSAG) with the maximum possible concentrations of scandium cations in the dodecahedral and octahedral positions of the crystal lattice have been synthesized. It has been established that in ErSAG at a temperature of 1600 °C the maximum possible concentration of scandium in the dodecahedral and octahedral position reaches 64.7 ± 2.0 at. % and 92.0 ± 2.0 at. %, respectively. The maximum possible concentrations of scandium cations in the dodecahedral position and the set of range of possible ErSAG compositions decrease at increasing of calcination temperature. It was shown that ceramics technique allow to produce the optical ceramics with compositions {Er_1.5Sc_1.5}[Sc_0.2Al_1.8](Al₃)O₁₂ (SAG: Er 50 at%) and {Y_1.57Er_0.03Sc_1.4}[Sc_0.2Al_1.8](Al₃)O₁₂ (YSAG: Er 1 at%), which can be considered as analogs of yttrium-aluminum garnet single crystals (YAG: Er 50 at% and YAG: Er 1 at%).     

Characterization and sintering of gels in the system MgO–Al2O3–SiO2

Cordierite aerogels, made by supercritical drying, and xerogels, formed by ambient pressure drying, have been prepared by combining two different recipes. The chemical composition of the gels varied from stoichiometric cordierite 2MgO·Al₂O₃·5SiO₂ to 0·5MgO·1·4Al₂O₃·5SiO₂ due to different procedures for washing of the gels. The crystallization of nearly stoichiometric cordierite gels was shown to be relatively complex involving the formation of several metastable phases such as μ-cordierite (Mg₂Al₄Si₅O₁₈), spinel (Al₆Si₂O₁₃) and sapphirine (Mg₄Al₈Si₂O₂₀) before the equilibrium phase composition was obtained at around 1350°C. On the other hand, during crystallization of gels with stoichiometry close to 0·5MgO·1·4Al₂O₃·5SiO₂ the equilibrium phases mullite, cristobalite and α-cordierite were the major phases formed during heat treatment. A lower densification rate was observed for aerogels compared to xerogels due to a larger pore size. A lower crystallization temperature in aerogels probably due to heterogeneous nucleation reduced the densification. For gels with a composition near 0·5MgO·1·4Al₂O₃·5SiO₂ nucleation and densification occur simultaneously and large differences in the densification behavior was observed. ©     

A new ternary phase in the system CaO–Al2O3–Cr2O3: Crystal structure and thermal expansion of CaAl2Cr2O7

Polycrystalline material of a novel phase in the system CaO–Al₂O₃–Cr₂O₃ has been obtained by solid-state reactions. Chemical analysis indicated the composition CaAl₂Cr₂O₇. Single-crystal growth of the new compound using borax as a mineralizer was successful. Diffraction experiments at ambient conditions on a crystal with composition CaAl_2.13Cr_1.87O₇ yielded the following basic crystallographic data: space group P 3, a = 7.7690(5) Å, c = 7.6463(5) Å, V = 399.68(6) ų, Z = 3. Structure determination and subsequent least-squares refinements resulted in a residual of R(|F|) = 2.3% for 1440 independent observed reflections and 113 parameters. To the best of our knowledge, the structure of CaAl_2.13Cr_1.87O₇ or CaAl₂Cr₂O₇ represents a new structure type. It belongs to the group of double layer structures where individual double layers contain octahedrally and tetrahedrally coordinated cation positions. Linkage between neighboring sheet packages is provided by additional calcium cations. Furthermore, thermal expansion has been studied in the interval between 29 and 790°C using in situ high-temperature single-crystal diffraction. No indications for a structural phase transition were observed. From the evolution of the lattice parameters the thermal expansion tensor has been obtained. A pronounced anisotropy is evident. The response of structural building units to variable temperature has been discussed.     

Microstructure and phase transformations in the AlN–Al2O3 pseudo-binary system

The microstructure and phase transformations in the AlN–Al₂O₃ pseudo-binary system of samples having an AlN content in mol% ranging between 44 and about 0 are reported as a function of the thermal treatments. The nature of the phase equilibria, temperature and composition range and coherence degree of the different phases were studied by using various complementary experimental techniques.     

The impact of nano-quartz on the structure of glass-ceramic glazes from the SiO2–Al2O3–CaO–MgO–Na2O–K2O system

The present study reports the impact of the introduction of nano-grained quartz (SA = 325 m²/g) into the composition on the structure and properties of the ceramic glaze of the CMAS-Na₂O–K₂O system. The results were compared to the glaze which had an identical oxide composition, with a difference that quartz was introduced in the form of quartz powder with a much smaller specific surface area SA = 1.41 m²/g. Both glazes are characterized by a large part of the glassy phase, above 90% by volume. The results obtained show a higher arrangement of the continuous glassy phase structure in the glaze with the addition of nano-quartz. This glaze also shows significantly higher values for all measured mechanical properties. It seems that if, in the near future, new cheaper methods for the production of nano-quartz are developed, it will be a new interesting direction of research aimed at improving the parameters of glazes and glass-ceramic materials of CMAS type.     

Phase compositions and equilibria in the CaO–Al2O3–Fe2O3–SO3 system,for assemblages containing ye'elimite and ferrite Ca2(Al,Fe)O5

Phase equilibria in the CaO–Al₂O₃–Fe₂O₃–SO₃ system have been studied, mainly at 1325 °C. In particular the solid solution compositions of ye'elimite Ca₄(Al₆O₁₂)SO₄ and brownmillerite (C₂(A,F)) phases have been analysed and used to estimate the composition boundaries of these phases in their co-existence with liquid at 1325 °C. Sulfate and iron oxide facilitate the formation of a liquid phase, whose amount increases with overall Fe₂O₃ content. However, the solubility of sulfate in this liquid phase is very low and liquid formation tends to lead to the volatilisation of sulfate in unsealed samples.     

Preparation of glass–ceramic glazes in the SiO2–Al2O3–CaO–MgO–K2O–Na2O–ZnO system by variable content of ZnO

This paper is focused on glass–ceramic glazes from the SiO₂–Al₂O₃–CaO–MgO–K₂O–Na₂O system with ZnO additions (2.5, 5, 10, 15, 15, 20 and 25 wt%). The compositions were designed based on constant molar ratio of SiO₂/Al₂O₃. In the resulting glazes diopside (CaMg[Si₂O₆]), willemite (Zn₂SiO₄) and vitreous phase were identified by X-ray diffraction. Morphological and structural date of these glazes were supplementary determined by EPMA, FTIR and Raman Spectroscopy. DSC analysis was carried out to characterize thermal properties of the materials.     

Stability of the Y2O3–SiO2 system in high-temperature,high-velocity water vapor

High-temperature, high-velocity water vapor (steam-jet) exposures were conducted on Y₂O₃, Y₂SiO₅, Y₂Si₂O₇, and SiO₂ for 60 hours at 1400°C. Volatility of Y₂O₃ was not observed. Phase-pure Y₂SiO₅ exhibited SiO₂ loss forming Y₂O₃ and porosity. A mixed porous and dense Y₂SiO₅ layer formed on the surface of Y₂Si₂O₇ due to SiO₂ depletion. The mechanisms and kinetics of the reaction between SiO₂ and H₂O(g) to form Si(OH)₄(g) from Y₂SiO₅, Y₂Si₂O₇, and SiO₂ are discussed.     

Effect of BaO ratio on the structure of glass–ceramic composite materials from the SiO2–Al2O3–Na2O–K2O–CaO system

This article focused on effect of the content of barium oxide on microstructure of the glass–ceramic materials based on the system SiO₂–Al₂O₃–Na₂O–K₂O–CaO. The following characterisation techniques have been used: X-ray diffraction (XRD), scanning electron microscopy with micro-analyser (SEM–EDS), mid-infrared analysis (MIR), far-infrared analysis (FIR) and Raman Spectroscopy. Significant differences were observed in microstructure of silica–alumina network of glassy phase and phase composition related to changes in the amount of the barium oxide additive. Discussed results are part of a larger project implemented under the PBS Applied Research Programme, in order to determine the compositions of glass–ceramic materials with potential application as a chemically resistant hard coatings or/and resistant to thermal shock or as construction materials.     

Preparation of glass–ceramic glazes for fast firing applications by CaF2 substitution with B2O3 in the CaO–CaF2–Al2O3–SiO2 system

The present work aims to obtain glass–ceramic glazes for floor tile applications. In this regard, CaF₂ was gradually replaced by B₂O₃ in the glass compositions belonging to the CaO–CaF₂–Al₂O₃–SiO₂ system. This substitution led to a noticeable decrease of crystallization peak temperatures and to an alteration of the crystallization trend. In the B₂O₃ bearing glazes, anorthite and gehlenite were identified as the major and minor crystalline phases, respectively. During concurrent crystallization and sintering based on the fast firing program, glass–ceramic glazes containing 9 weight parts of fluorine and 12 weight parts of boron oxide showed the most desirable sinterability. The optimized glass–ceramic glazes offered acceptable micro-hardness, whiteness and thermal expansion behavior after fast firing heat treatment.     

Effect of ZnO content on the physical,mechanical and chemical properties of glass-ceramics in the CaO–SiO2–Al2O3 system

The objective of this study was to evaluate the effect of ZnO content on the physical, mechanical and chemical properties of CaO–Al₂O₃–SiO₂ (CAS) glass-ceramics produced from Colombian wastes, such as fly ash, granulated blast furnace slag and glass cullet. The CaO/SiO₂ molar ratio of the mixtures was held constant (0.36). ZnO was added to the mixtures in proportions of 4, 7 and 10 wt%. The glass-ceramics were produced by the controlled crystallization of a parent glass. The values of crystallization temperature (T_p) show a fall up to 7 wt% and then shoots up with 10 wt% concentration of ZnO, but in general, ZnO addition lowers the temperature required for the formation of crystalline phases. In general, anorthite (CaAl₂Si₂O₈) is the main phase observed in all heat treated samples, in addition to albite (Na(AlSi₃O₈)) and labradorite (Na_0.45 Ca_0.55 Al_1.55 Si_2.45 O₈). The crystalline phases hardystonite (Ca₂ZnSi₂O₇) and willemite (Zn₂SiO₄) were also identified in the samples with 7 and 10 wt% ZnO. The densities of the glass-ceramics were between 2658 and 2848 kg/m³, and it was found that ZnO helps to increase the density of glass-ceramics. The elastic modulus was in the 100–105 GPa range, the fracture toughness was between 0.45 and 0.64 MPa m^1/2, and the Vickers microhardness was between 632 and 653 MPa. With regards to the durability, the weight loss of the glass-ceramics immersed in alkaline solution (NaOH) did not exceed 1.5 wt% after immersion for 6 h at 80 °C. The results of this study confirm that the vitrification process is a favorable option to utilize these industrial wastes.     

Phase evaluation during high temperature long heat treatments in the Y2O3–Al2O3–SiO2 system

The effect of high temperature long heat treatments on the microstructure of the eutectic glass composition in the Y₂O₃–Al₂O₃–SiO₂ system was examined. The qualitative and quantitative phase analyses were conducted by using scanning electron microscopy combined with energy dispersive analysis and electron microprobe. Simultaneous thermal analyses were carried out to determine the transition temperatures and enthalpies. The crystallization behavior of these glasses was monitored with X-ray powder diffraction. A needle like X-phase was observed in the structure of this eutectic composition after long heat treatment at 1350 °C.     

Structural studies on Ca3Al4MgO10 (C3A2M)—A ternary phase in the system CaO–Al2O3–MgO

In a sequence of temperature-dependent solid-state reactions in the system CaO–Al₂O₃–MgO the formation of the ternary phase Ca₃Al₄MgO₁₀ or C₃A₂M has been studied. Whereas the compound could not be prepared at 1200°C, a yield of 85 wt.-% of Ca₃Al₄MgO₁₀ was obtained at 1320°C (incongruent melting point: 1330°C). Powder diffraction data compare well with results of previous investigations from the 1960s. Single crystals of Ca₃Al₄MgO₁₀ could be retrieved from the sinter-pellets. Basic crystallographic data are as follows: orthorhombic symmetry, space group Pbcm, a = 5.14073(8), b = 16.7576(2), c = 10.70977(16) Å, V = 922.61(2) ų, Z = 4. Using synchrotron diffraction data it was possible to solve the crystal structure. Least-squares refinements resulted in a residual of R(|F|) = 0.021 for 1000 independent observed reflections with I > 2σ(I) and 97 parameters. The structure contains [TO₄]-tetrahedra (T=Al,Mg) forming a three-dimensional (3-D) framework whose topological characteristics have been determined. Al-Mg distributions on the different T-sites have been studied. The calcium cations are located in voids of the network. More than 50 years after its first observation our investigation clarifies the crystal structure of a compound belonging to a system that is of relevance for several fields of materials science.