Silicate-rich Phases Research Articles

Unveiling the effect of phase separation on crystallization of calcium borosilicate glass-ceramics

The CaO–B2O3–SiO2(CBS) glass-ceramics with low sintering temperature and good microwave dielectric properties have been applied on the famous commercial low temperature co-fired ceramics (LTCC) Tape for passivation substrate materials for 5G/6G technology. However, the phase separation and its relationship with the crystallization is rarely studied but extremely important for understanding and designing the high-performance CBS glass-ceramics. Herein, we find that the phase separation type of CBS glass-ceramics changes from metastable immiscibility to stable immiscibility with the increase of B2O3 content. In the metastable immiscible glass-ceramics, three phases are observed, including continuous borate-rich and silicate-rich phases as well as spherical silicate-rich phases. In the stable immiscible glass-ceramics, the silicate-rich phases transform into petal-liked shapes. β-CaSiO3 nuclei are formed in fractions with BO3/SiO4 equaling to 0.56. Notably the β-CaSiO3 nucleus do not grow after heat treatment at 850 °C, but grow at higher temperatures. Phase separation hinders the precipitation of β-CaSiO3 and promoted the formation of CaB2O4 and SiO2. In addition, interfacial defects caused by phase separation alter the crystal barrier and promote the formation of α-CaSiO3. However, α-CaSiO3 dissolves at 1000 °C and may provide Ca2+ cations together with the borate-rich phase for the regrowth of β-CaSiO3. Our work provides another perspective for understanding the effects of phase separation on crystallization in CBS glass-ceramics, contributing to improving the microwave dielectric performance by structural design.

Effects of Elevated Temperature and Activation Solution Content on Microstructural and Mechanical Properties of Fly Ash-based Geopolymer

This paper examined the effects of sodium silicate to sodium hydroxide (SS/SH) and fly ash (FA) to alkaline solution (FA/AS) ratios on the thermal and mechanical properties of FA-based geopolymers. To tackle this, two factors were varied including the SS/SH (1.5, 2.5, and 3.5) and the FA to the AS (1.5, 2.5 and 3.5). The elaborated nine specimens were characterized employing several techniques including FT-IR, XRD, 29Si and 27Al MAS NMR, and SEM analysis. The thermogravimetric analysis (TGA) was utilized to study the thermal stability of geopolymer specimens. The fire resistance of the geopolymer specimen was evaluated by weight change and residual compressive strength before and after being subjected to fire testing (up to 1,000°C). A decrease in the resistance recorded when heated at 200 and 400°C was related to the vaporization of free water was observed. Meanwhile, a gain in strength was achieved for the geopolymer samples after being subjected to 600 and 800°C which was attributed to the viscous flow of silicate-rich phases that eventually bridged the fissures or defects occurring during its solidification. Samples with a high FA/AS ratio showed good fire resistance after high-temperature exposure.

Effect of Al2O3 content on amorphous phase-separation and self-limited crystallization of phosphosilcate glasses

Amorphous phase-separation and crystallization behavior of phosphosilicate glasses with different Al2O3 contents were systematically investigated. We demonstrate that nanophase-separation spontaneously occurs during cooling of all the glass melts. The two separated phases are found to be the phosphate-rich droplet phase with high ion movability and the rigid silicate-rich matrix. With increasing Al2O3 content, nucleation rate decreases whereas growth rate is enhanced in terms of nucleation-growth type phase-separation. In addition, for the compositions with 0 and 2 mol% Al2O3, sodium phosphate nanocrystals simultaneously form in the droplets in the cooling process, this is further confirmed from the micro-morphologies the samples heat-treated at 1.2 Tg for various durations. In this case, the growth of sodium phosphate is self-limited by the surrounding silicate-rich phase. Our results show that the addition of Al2O3 tends to inhibit nucleation of sodium phosphate rather than crystal growth. This work indicates that phase-separation can be used as a route to the preparation of amorphous/nanocrystal composites by adjusting Al2O3 content in phosphosilcate glass system.

Self-limited growth of nanocrystals in phosphosilicate melts during cooling

In this Letter, we demonstrate that the spontaneous nanophase-separation can greatly enhance the heterogeneous nucleation in the investigated phosphosilicate melts. The two separated phases are found to be the phosphate-rich phase as the floppy domain and the silicate-rich phases as rigid phase. We found that sodium phosphate nanocrystals form in the phosphate-rich phase during melt cooling. The growth of these nanocrystals are self-limited, i.e., limited by the surrounding silicate-rich phase with higher viscosity, and hence lower ionic diffusion compared to the phosphate-rich phase. Our results show that the substitution of B2O3 or Al2O3 for partial Na2O enhances the spontaneous nucleation, although the viscosity of silicate-rich matrix phase is increased by such substitution. This implies that the compositional substitution enhances nanophase separation and thereby lowers the activation energy for non-isothermal crystallization. This work indicates that nanophase separation is crucial for fabrication of transparent glass-ceramics from phosphosilicate melts.

Determination of Three-Dimensional Morphology and Inner Structure of Second-Phase Inclusions in Metals by Non-Aqueous Solution Electrolytic and Room Temperature Organic Methods

The secondary-phase particles in metals, particularly those composed of non-metallic materials, are often detrimental to the mechanical properties of metals; thus, it is crucial to control inclusion formation and growth. One of the challenges is determining the three-dimensional morphology and inner structures of such inclusions. In this study, a non-aqueous solution electrolytic method and a room-temperature organic technique were developed based on the principle of electrochemistry to determine the three-dimensional morphologies and inner structures of non-metallic inclusions in Al-killed steel, Si-killed steel, and ductile cast iron. The inclusions were first extracted without any damage to the inclusions, and then the collected inclusions were wrapped and cut through Cu ion deposition. The results revealed that the inclusions in Al-killed steel had an irregular morphology, that those in the Si-killed steel were mainly spherical, and that almost all the spheroidal graphite in the ductile cast iron featured a uniform globular morphology. In addition, nucleation was not observed in the inner structures of the inclusions in the Al-killed steel, while some dendritic or rod-like MnS phase precipitates appeared on the silicate inclusion surfaces, and some silicate-rich phases were detected in their inner matrix. For spheroidal graphite, rare-earth oxides (one particle or more) were observed as nuclei in the center of almost every graphite particle. The formation and evolution of inclusions in these types of metals can be better understood by means of the two developed methods.

Impact of chemical variability of ground granulated blast-furnace slag on the phase formation in alkali-activated slag pastes

The influence of ground granulated blast-furnace slag (GGBS) chemical variability on phase formation in sodium hydroxide-activated GGBS pastes has been investigated using X-ray total scattering and subsequent pair distribution function (PDF) analysis. Crystalline phase identification based on reciprocal space analysis reveals that despite large chemical variations in the neat GGBSs the secondary reaction products are quite similar, with the majority of pastes containing a hydrotalcite-like phase. However, PDF analysis reveals considerable differences in short range atomic ordering of the main calcium-sodium aluminosilicate hydrate (C-(N)-A-S-H) gel phase in the pastes. Quantitative analysis of these local structural differences in conjunction with published PDF data identifies the important role calcium plays in dictating the atomic structure of disordered silicate-rich phases in cementitious materials. This study serves as a crucial step forward in linking GGBS chemistry with phase formation in alkali-activated GGBS pastes, revealing key information on the local structure of highly-disordered cementitious materials.

Surface characterisation of fine inert gas and water atomised stainless steel 316L powders: formation of thermodynamically unstable surface oxide phases

New insights are presented on the speciation of surface oxide phases on fine inert gas atomised (GA, <45 and <4 μm) and water atomised (WA, <45 μm) stainless steel AISI 316L powders. X-ray photoelectron and Auger electron spectroscopy, scanning electron microscopy, Raman spectroscopy, and cyclic voltammetry were applied for the characterisation. Oxidised manganese was strongly enriched in the outermost surface oxide of the GA powders (13 and 47 wt-%), an effect increasing with reduced particle size. Manganese and sulphur were enriched in oxide nanoparticles on the surface. Oxidised silicon (59 wt-%) was enriched on the WA powder surface. Tri- or tetravalent manganese oxides were observed on the GA particles in addition to α-Fe2O3, and Cr2O3. The oxide of the WA powder revealed in addition the likely presence of a silicate rich phase, mainly consisting of tetravalent Si, di- and/or trivalent Fe, and hexavalent Cr, which was confirmed not present as chromate.

Simulating the Formation of Surfactant-Templated Mesoporous Silica Materials: A Model with Both Surfactant Self-Assembly and Silica Polymerization

We have used Monte Carlo simulations to study the formation of the MCM-41 mesoporous silica material, with a new lattice model featuring explicit representations of both silicic acid condensation and surfactant self-assembly. Inspired by experimental syntheses, we have adopted the following two-step "synthesis" during our simulations: (i) high pH and low temperature allowing the initial onset of mesostructures with long-range order; (ii) lower pH and higher temperature promoting irreversible silica condensation. During step (i), the precursor solution was found to spontaneously separate into a surfactant-silicate-rich phase in equilibrium with a solvent-rich phase. Lamellar and hexagonal ordering emerged for the surfactant-silicate-rich mesosphases under different synthesis conditions, consistent with experimental observations. Under conditions where silica polymerization can be neglected, our simulations were found to transform reversibly between hexagonal and lamellar phases by changing temperature. During step (ii), silica polymerization was simulated at lower pH using reaction ensemble Monte Carlo to treat the pH dependence of silica deprotonation equilibria. Monte Carlo simulations produced silica-surfactant mesostructures with hexagonal arrays of pores and amorphous silica walls, exhibiting Q(n) distributions in reasonable agreement with (29)Si NMR experiments on MCM-41. Compared with bulk amorphous silica, the wall domains of these simulated MCM-41 materials are characterized by even less order, larger fractions of 3- and 4-membered rings, and wider ring-size distributions.

Transparent phosphosilicate glasses containing crystals formed during cooling of melts

Effect of P 2O 5–SiO 2 substitution on spontaneous crystallization of SiO 2–Al 2O 3–P 2O 5–Na 2O–MgO melts during cooling was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and rotation viscometry. Results show that addition of P 2O 5 leads to amorphous phase separation (APS), i.e., phosphate- and silicate-rich phases. It is due to the tendency of Mg 2+ to form [MgO 4] linking with [SiO 4]. Molar substitution of P 2O 5 for SiO 2 enhances the network polymerization of silicate-rich phase in the melts, and thereby the spontaneous crystallization of cubic Na 2MgSiO 4 is also enhanced during cooling of the melts. In addition, the sizes of the local crystalline and separated glassy domains are smaller than the wavelength of the visible light, and this leads to the transparency of the obtained glasses containing crystals.

Transparent Glass-Ceramics Derived from Spontaneously Crystallized Phosphosilicte Glasses

Transparent phosphosilicate glass-ceramics were prepared by heating the spontaneously crystallized precursor glasses in which crystals have formed during cooling of the melts. And the effects of compositional variation on the crystalline phases, degree of crystallization and transparency were investigated by using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). It is found that Mg2+ tends to exist in silicate-rich phase rather than phosphate-rich phase, such MgO is more beneficial to the transparency of phosphosilicate glass-ceremics compared to CaO. In addition, the substitution of P2O5 for SiO2 enhances the tendency of phase separation, and hence the precipitation of Na2MgSiO4 crystals for the precursor glasses as well as glass-ceramics with good transparency.

Synthesis and characterization of ordered mesoporous silicas with high loadings of methyl groups

Ordered silicas with high loadings of pendant methyl groups were synthesized via cocondensation of tetraethyl orthosilicate and triethoxymethylsilane under basic conditions in the presence of cetyltrimethylammonium surfactant. The resulting materials were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), 29Si and 13C NMR spectroscopy, nitrogen adsorption and thermogravimetry. For lower loadings of the organosilane in the synthesis mixture, XRD patterns were characteristic of a 2-dimensional hexagonal structure. As seen from XRD, the degree of structural ordering diminished as the loading increased, but an XRD pattern with a single pronounced peak and TEM images characteristic of a 2-dimensional hexagonal structure were recorded even for a methyl-functionalized silica with about 50% of silicon atoms carrying the organic groups. The extent of organosilane incorporation was found to be nearly quantitative. The methyl groups were stable during surfactant removal, both via solvent extraction and pyrolysis at 573 K under nitrogen. The (100) interplanar spacing and the pore diameter were found to decrease from 3.45 to 3.12 nm, and from 2.9 to 2.3 nm, respectively, as the loading of the organosilane increased up to 43%, but no further decrease was observed when the loading increased to 50%. The BET specific surface area was relatively constant (720–800 m2 g−1) for loadings of 20–50%, whereas the primary pore volume decreased as the loading of organics increased. Nitrogen adsorption provided some evidence of phase separation into organosilicate-rich and silicate-rich phases for methyl group loadings of 33% or higher and particularly for the 50% loading. Thermogravimetric data suggested that the methyl groups are highly thermally stable in both nitrogen and air atmospheres.

Ephemeral carbonate melts in the upper mantle: carbonate-silicate immiscibility in microveins and inclusions within spinel peridotite xenoliths, La Gomera, Canary Islands

Carbonate droplets containing mafic silicate glass ± CO 2 occur within mineral inclusions and late microveins in spinel-bearing ultramafic xenoliths (dunite, dunite-werhlite and pyroxenite) from La Gomera, Canary Islands. Primary carbonates are Mg-calcite (X Ca 0.89–0.93) and dolomite (X Ca 0.46–0.54), with low Na 2 O (≤ 0.1 wt.%) and variable MnO (0.2–8 wt.%). The mafic glass has high MgO (24–35 wt.%), FeO (1–18 wt.%) and SiO 2 (40–55 wt.%), with low Al 2 O 3 , TiO 2 , CaO and alkalis. Mafic glass contains high amounts (> 10 wt %) of volatiles ( i.e. , H 2 O). The composite carbonate droplets represent a quenched liquid, resulting from unmixing within the inclusions of a carbonate-rich melt into separate carbonate- and silicate-rich phases. Modelling of initial bulk compositions suggests dolomitic melts, with high silica (≈ 10 wt.%) and H 2 O, but low alkali contents. If not protected within inclusions, these melts are ephemeral, unstable in the P-T field of spinel peridotites (10–18 kbar; 900–1000°C). Mafic glass remnants in microveins represent a residual degassed hydrous mafic silicate fraction after decarbonation.

The effects of mineralogy on the biological leaching of nickel laterite ores

The amenability of limonite ore, the iron rich nickel laterite phase, was observed to be less than saprolite, the silicate rich phase, when subjected to biological leaching with heterotrophic microorganisms. To understand this difference, controlled leaching of the various nickel laterite minerals was conducted using citric acid to mimic the chemical action of the organisms and subsequent characterisation of the leached residues. The raw ore and the leached mineral residues were examined using high-resolution synchrotron based X-ray diffraction and optical microscopy. The possible contribution of electrosorption and solid porosity was also considered. The results suggest that although electrosorption contributes to nickel and cobalt losses, it is the structural changes induced by the acid in the minerals which effects the recovery of the metals. Citric acid was found to dehydroxylate saprolite minerals resulting in an unstable nickel oxide and thus making it susceptible to acid attack. This behaviour was not observed in the limonite ore, which was found to be poorly leached by citric acid.