Towards new zero-thermal-expansion materials: Li-free quartz solid solutions stuffed with transition metal cations

Stuffed aluminosilicate quartz solid solutions (Qss) represent the main functional component of state-of-the-art zero-thermal-expansion glass-ceramics. For the first time, we present the synthesis of Li-, Mg- and Zn-free Qss solely stuffed with transition metal cations (Fe²⁺, Ni²⁺, Co²⁺); partial Li⁺ co-doping enables also significant Mn²⁺ incorporation. They were obtained by glass powder crystallization; Qss crystals exhibit compositionally tunable coefficients of thermal expansion (CTEs), from ? 2 × 10^?6 K^?1 to ～10 × 10^?6 K^?1 in the range 30–300 °C. Co²⁺-bearing crystals exhibit the closest-to-zero CTE value (0.2 ×10^?6 K^?1) and the most isotropic behavior in the whole Qss family, opening up new perspectives for the development of Li-free low-expansion materials. From a structural point of view, we identified the unit cell volume (and not the pseudo-hexagonality of the aluminosilicate framework) as the key structural parameter leading to low or negative CTEs in Qss, with a linear correlation extending to non-stuffed non-silicate quartz-like phases.     

Inversion of quartz solid solutions at cryogenic temperatures

Quartz solid solution crystals of six different compositions were obtained from crystallization of glass powders belonging to the Li₂O–Al₂O₃-SiO₂ (LAS) system. They were analyzed in situ by laboratory-based X-ray diffraction down to cryogenic temperatures (−190°C). Temperature-resolved analysis of their lattice parameters allowed determination of the critical inversion temperature T_c in these materials, marking the displacive phase transition from a high-quartz- to a low-quartz-like lattice. Integrating available data from other literature sources, an updated phase diagram for the occurrence of high and low quartz solid solution phases is provided for the LAS system; these data are expected to support future development of functional materials relying on these crystalline phases.     

Kinetics of the CaO  quartz  H2O reaction at 120° to 180°C in suspensions

Kinetics of hydrothermal reactions have been studied for mixtures of CaO and quartz (<10 μm 10–20 μm) with Ca/Si = 0.8 and 1.0 in stirred suspensions at 120 – 180°C. Reaction proceeds through the sequence: Ca(OH)₂ + SiO₂ → Ca-rich C-S-H + SiO₂ (at 120°C) → poorly crystalline tobermorite (at 140°C)→ highly crystalline tobermorite (at 180°C) → xonotlite at 180°C and Ca/Si = 1.0 and 180°C and Ca/Si = 0.8 if 10–20 μm quartz is used. Reaction is controlled by dissolution of the quartz. For both Ca/Si ratios the radius of the 10–20 μm quartz decreases at a constant rate, viz 0.85 μm/h at 180°C, 0.13 μm/h at 140°C, 0.04 μm/h at 120°C.     

Effect of Cu and Zr Co-doped SiO<Subscript>2</Subscript> Nanoparticles on the Stability of Phases (Quartz-Tridymite-Cristobalite) and Degradation of Methyl Orange at High Temperature

SiO₂ nanoparticles doped by 10 mol% Zr and 10 mol% Cu were prepared via the sol-gel method in a controled process. The effects of doping and calcination temperature on the structural and photo-catalytic properties of SiO₂ nanopowders were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-Vis absorption spectroscopy. The phases of cristobalite, quartz and tridymite were found at a calcinations temperature range of 800 to 1000 ^°C and only cristobalite phase was formed at a temperature of 1200 ^°C. The degradation of methyl orange was examined under visible light radiation indicating that the effect of doped elements (Zr, Cu) on SiO₂ reduces the band gap effectively.     

Plasma resistance of quartz with a glass coating layer by aerosol deposition

ABSTRACT

To improve the plasma resistance behaviour, glass frits of SiO₂–Al₂O₃–Y₂O₃ with various powder sizes were coated onto quartz substrates by the aerosol deposition (AD) method. The thickness and microstructure of the coating layers were observed using a surface profiler and scanning electron microscopy. Plasma resistance was measured via the quartz substrate, after exposure to an inductively coupled plasma etcher. The coating layers were densely formed on the quartz substrates without additional heat treatment, and the layer thickness changed for the glass frit size distribution and AD process conditions. The SiO₂–Al₂O₃–Y₂O₃ glass coating layer showed a higher plasma resistance than quartz. Furthermore, the AD coating layer was evenly etched after plasma exposure. This study improves the lifetime of plasma chamber components in the semiconductor industry.     

Formation of scawtite from mixtures of CaO,dolomite and quartz under hydrothermal conditions

Water suspension of mixtures of CaO, and quartz (CaO/SiO₂ = 1,2; 1,6 and 2,0) in which 10–50 wt % CaO were substituted by dolomite, were hydrothermally treated at 200°C. X-ray phase analysis of reaction products showed that chiefly scawtite and partly calcium- and magnesium silicate hydrates were formed. The scanning electron micrographs showed variations in morphology of scawtite particles in dependence on the molar ratio of CaO/SiO₂.     

Effect of in situ synthesis of Si2N2O on microstructure and the mechanical properties of fused quartz ceramics

Si/SiO₂ composite billets were prepared using a low-toxicity gel system, and the resulting billets were sintered at high temperature in nitrogen to synthesize Si₂N₂O in the central position of the fused silica ceramic matrix. The influences of in situ synthesized Si₂N₂O on the microstructure and mechanical properties of fused silica ceramics were studied. The results show that Si/SiO₂ composite billets can be used to synthesize spike-like and fibrous Si₂N₂O in situ in nitrogen at 1450 °C. Si₂N₂O synthesized in situ can improve the mechanical properties and microstructure of quartz ceramics. When the Si/SiO₂ composite billet is sintered in nitrogen at 1450 °C for 2 h, the volume density and bending strength of the quartz ceramics can reach 2.36 g/cm³ and 114.37 MPa, respectively.     

Temperature Stability and Photocatalytic Activity of Nanocrystalline Cristobalite Powders with Cu Dopant

The SiO₂ nanoparticles doped by 10 % mol Cu were prepared via a sol-gel method under process control. The effects of doping and calcination temperature on the structural and photo-catalytic properties of SiO₂ nanopowders have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-Vis absorption spectroscopy. Cristobalite and tridymite crystalline phases were found at a calcinations temperature range of 900～1200 °C and amorphous phase was formed at a temperature of 800 °C for doped SiO₂. The photocatalyst activity was evaluated by photocatalytic degradation kinetics of aqueous methyl orange (MO) under visible radiation. The results show that the photocatalytic activity of the 10 % mol Cu doped SiO₂ nanopowders have a larger degradation efficiency than pure SiO₂ under visible light at 900 °C temperature.     

Effects of particle size in silica sol on the mechanical and thermal properties of SiO2f/SiO2 composites

In this paper, quartz fiber-reinforced silica matrix SiO_2f/SiO₂ composites were prepared by the precursor impregnation-heat treatment method using quartz fiber needle felt as the reinforcement and silica sol as the precursor. The effects of particle size in silica sol (10, 50, and 100 nm) on the density, apparent porosity, mechanical properties, and thermal properties of SiO_2f/SiO₂ composites were investigated. The phase composition and microstructure of the composites were characterized by X-ray diffraction and scanning electron microscopy, respectively. The thermal expansion coefficient and thermal conductivity of composites were measured by a push rod method and the laser method. The results show that the density, apparent porosity, and mechanical strength of the specimens firstly increase and then decrease with the increase in the particle size in silica sol. The sample using silica sol with particle size 50 nm has the optimum overall performances (i.e., the flexural strength of 13.7 MPa and the compressive strength of 59.8 MPa), and shows a ductile fracture behavior. At 300°C–700°C, the average thermal expansion coefficient of the optimal sample is .783 × 10⁻⁶/°C. And the thermal conductivity of the samples increases with the increase in temperature, and it reached the highest value of .810 W/(m·K) at 700°C. The SiO_2f/SiO₂ composites show obvious advantages in the application of load-bearing and thermal insulation integration, and they are expected to meet the demanding requirements of hot-pressing sintering and non-ferrous metallurgy industries.     

Precipitous weakening of quartz at the α–β phase inversion

Crystalline quartz has long been identified as among the weakest of abundant crustal minerals. This weakness is particularly evident around the α–β phase inversion at 573°C, in which Si–O bonds undergo a displacive structural transformation from trigonal to hexagonal symmetry. Here we present data using indentation testing methodologies that highlight the precipitous extent of the transformational weakening. Although the indentations are localized over relatively small specimen contact areas, the data quantify the essential deformation and fracture properties of quartz in a predominantly (but not exclusively) compressive stress field, at temperatures and pressures pertinent to conditions in the earth's crust.     

High temperature single crystal properties of mullite

Extensive neutron diffraction and Rietveld studies of dense, hot pressed mullite (3Al₂O₃·2SiO₂) have been conducted up to 1650 °C in air, yielding a complete set of lattice parameters and axial thermal expansion coefficients. Unconstrained powders of the same stoichiometric composition were also analyzed by X-ray diffraction and Rietveld techniques up to 900 °C in air, from which lattice parameters and thermal expansion coefficients were obtained. An earlier reported structural discontinuity was confirmed by XRD to lie in the temperature range 425 to 450 °C. Single-crystalline mullite fibers of composition 2·5Al₂O₃·SiO₂ were grown from the melt by a laser-heated, float zone method. A partial set of the single-crystal elastic moduli were determined from various sections of fiber, by Brillouin spectroscopy, from room temperature up to 1400 °C. They indicated a roughly 10% reduction in stiffness over that temperature range.     

Formation of scawtite from mixtures of CaO,magnesite and quartz under hydrothermal conditions

The phase composition of samples prepared under hydrothermal conditions at 200°C from a suspension of mixtures of CaO, quartz and magnesite has been studied. When 10, 20, 30 and 50 wt % of CaO were substituted by magnesite, the mole ratio CaO/SiO₂ being 0,83 and 1,20 - chiefly tobermorite, gyrolite and calcite or scawtite and tobermorite were formed, and the strength of the tested materials was observed to decrease.     

Influences of quartz and muscovite on the formation of mullite from kaolinite

A kaolin containing muscovite and quartz (K-SZ) and a pure kaolin (K-SX) with the addition of potassium feldspar, K₂SO₄ and quartz, respectively, were used to investigate the influences of muscovite and quartz on the formation of mullite from kaolinite in the temperature range 1000–1500 °C. In K-SZ formation of mullite began at 1100 °C, and in K-SX at 1000 °C. In K-SZ quartz accelerated the formation of cristobalite and restrained the reaction of mullite and silica. Muscovite in K-SZ acted as a fluxing agent for silica and mullite before 1400 °C and accelerated the formation of cristobalite. The FTIR band at 896.8 cm^− 1 was used to monitor the formation of orthorhombic mullite.     

Hydrothermal Dissolution of Technical Grade Vitreous Silica in NaOH Solutions to Liquid Water Glasses with Higher SiO2:Na2O Ratios

Technical grade vitreous silica was dissolved hydrothermally in NaOH lyes. Liquid sodium water glasses with silica concentrations of up to 26.4 wt % and molar SiO₂:Na₂O ratios of up to 3.5 were obtained. The dissolution behavior showed two temperature regimes: up to 130 °C the SiO₂ content approached a final solubility limit controlled by cristobalite crystallization. Above 130 °C, crystallization of low quartz restricted solubility. The dissolution kinetics were governed by surface area reduction and saturation control according to the Noyce-Whitney law.     

High-temperature mechanical properties of NextelTM 610 fiber reinforced silica matrix composites

Novel Nextel? 610 fiber reinforced silica (N610f/SiO₂) composites were fabricated via sol-gel process at a sintering temperature range of 800–1200?°C. The sintering-temperature dependent microstructures and mechanical properties of the N610f/SiO₂ composites were investigated comprehensively by X-ray diffraction, nanoindentation, three-point bending etc. The results suggested a thermally stable Nextel? 610 fiber whose properties were barely degraded after the harsh sol-gel process. A phase transition in the silica matrix was observed at a critical sintering temperature of 1200?°C, which led to a significant increase in the Young's modulus and hardness. Due to the weak fiber/matrix interfacial interaction, the 800?°C and 1000?°C fabricated N610f/SiO₂ composites exhibited quasi-ductile fracture behaviors. Specially, the latter possessed the highest flexural strength of ≈ 164.5?MPa among current SiO₂-matrix composites reinforced by fibers. The higher sintering-temperature at 1200?°C intensified the SiO₂ matrix, but strengthened the interface, thus resulting in a brittle fracture behavior of the N610f/SiO₂ composite. Finally, the mechanical properties of this novel composite presented good thermal stability at high temperatures up to 1000?°C.     

High temperature transformation of pure calcium germanates

The polymorphic transformation of pure 2CaO·GeO₂ and 3CaO·GeO₂ which are the nearest crystal chemical analogous with main cement minerals 2CaO·SiO₂ and 3CaO·SiO₂ has been studied by X-ray diffraction. X-ray patterns of the polymorphic forms of the 3CaO·GeO₂ were obtained at ambient temperature, 820°C and 1410°C. The fragments of X-ray pattern of unstable modification were obtained at 1060°C. X-ray diffraction patterns of the modification of the 2CaO·GeO₂ were received at ambient temperature and 1480°C. The lattices constants of the modifications of pure 3CaO·GeO₂ and 2CaO·GeO₂ are given.     

High‐Pressure Investigation in the System SiO2–GeO2: Mutual Solubility of Si and Ge in Quartz,Coesite and Rutile Phases

Mutual solubilities in crystalline phases in the system SiO₂–GeO₂ have been investigated up to 10 GPa pressure and 1500°C temperature, using a bulk composition of 50 mol% GeO₂. Solid solution of up to 40 mol% GeO₂ into the mineral quartz has been confirmed as well as solubility of Si into GeO₂ rutile (argutite) and Ge into SiO₂ rutile (stishovite) phases and limited Ge into coesite. Solubility of Ge in quartz is very high, and decreases with pressure, with the univariant quartz‐out reaction occurring near 3.4 GPa at 1200°C. The solubility of GeO₂ in coesite is highest at 3.4 GPa (about 8 mol%) and decreases with increasing pressure. Significantly more extensive solubility than previously reported for the rutile phases has been found and measured in detail as a function of pressure and temperature. Extensive solubility of SiO₂ in GeO₂ is found in argutite at 1200°C, increasing strongly with pressure and reaching a maximum of 25.2 mol% SiO₂ in GeO₂ at 9 GPa. At this point coesite (ss) plus argutite (ss) react to form a stishovite phase with 18 mol% GeO₂, and the mutual solubility in both phases decreases above this pressure. At 1500°C, similar solubilities are observed but the maximum SiO₂ solubility in argutite of just over 25 mol% occurs near 10 GPa. All these solid solutions can be recovered to ambient temperature and pressure. Phase diagrams and unit cell information of the phases are presented here. Based on these results, a useful and industrially relevant, application for accurately measuring high pressure is suggested.     

Sealing Glass‐Ceramics with Near‐Linear Thermal Strain,Part II: Sequence of Crystallization and Phase Stability

The sequence of crystallization in a recrystallizable lithium silicate sealing glass‐ceramic Li₂O–SiO₂–Al₂O₃–K₂O–B₂O₃–P₂O₅–ZnO was analyzed by in situ high‐temperature X‐ray diffraction (HTXRD). Glass‐ceramic specimens have been subjected to a two‐stage heat‐treatment schedule, including rapid cooling from sealing temperature to a first hold temperature 650°C, followed by heating to a second hold temperature of 810°C. Notable growth and saturation of Quartz was observed at 650°C (first hold). Cristobalite crystallized at the second hold temperature of 810°C, growing from the residual glass rather than converting from the Quartz. The coexistence of quartz and cristobalite resulted in a glass‐ceramic having a near‐linear thermal strain, as opposed to the highly nonlinear glass‐ceramic where the cristobalite is the dominant silica crystalline phase. HTXRD was also performed to analyze the inversion and phase stability of the two types of fully crystallized glass‐ceramics. While the inversion in cristobalite resembles the character of a first‐order displacive phase transformation, i.e., step changes in lattice parameters and thermal hysteresis in the transition temperature, the inversion in quartz appears more diffuse and occurs over a much broader temperature range. Localized tensile stresses on quartz and possible solid‐solution effects have been attributed to the transition behavior of quartz crystals embedded in the glass‐ceramics.     

Experimental phase equilibria studies of the PbO–SiO2 system

Phase equilibria of the PbO–SiO₂ system have been established for a wide range of compositions: (i) liquid in equilibrium with silica polymorphs (quartz, tridymite, and cristobalite) between 740°C and 1580°C, at 60‐90 mol% SiO₂; (ii) with lead silicates (PbSiO₃, Pb₂SiO₄, and Pb₁₁Si₃O₁₇) and lead oxide (PbO) between 700°C and 810°C. A high‐temperature equilibration/quenching/electron probe X‐ray microanalysis (EPMA) technique has been used to accurately determine the compositions of the phases in equilibrium in the system. Significantly, no liquid immiscibility has been found in the high‐silica range, and the liquidus in this high‐silica region has been accurately measured. The phase equilibria information in the PbO–SiO₂ system is of practical importance for the improvement of the existing thermodynamic database of lead‐containing slag systems (Pb–Zn–Fe–Cu–Si–Ca–Al–Mg–O).     

Diffusion mechanisms between La2SiO5 and SiO2 during formation of textured lanthanum silicate oxyapatite crystals

Lanthanum silicate oxyapatite (LSO) is a promising alternative electrolyte for solid oxide fuel cell (SOFC) applications. They exhibit anisotropic oxide-ionic conduction; therefore, a preferential crystal orientation along the c-axis is necessary to achieve optimal conductivity. This study is performed to understand the reaction mechanisms involved in the formation of an LSO layer at the interface of the La₂SiO₅/SiO₂ diffusion pair during reactive sintering at high temperatures. Different La₂SiO₅/SiO₂ bilayers were fabricated in sandwich-type structures using silica-type quartz or La₂SiO₅ supports obtained via electrophoresis and uniaxial pressing, respectively. These supports were pre-sintered and then coated with a suspension of the opposite component. The diffusion pairs were isothermally heated at 1500 °C at different holding times (10, 20, and 40 h) and at 1600 °C for 10 h. Their surfaces and cross-sections were investigated via X-ray diffraction, scanning electron microscopy (SEM) observations, energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy.Experimental results show that two different driving forces were involved in the nucleation and growth of apatite crystals. The first is the chemical potential gradient in lanthanum between the components of the layers, and the second is the electric field created to preserve the electroneutrality of the system. The predominant diffusing species in this bilayer system were La^+III and O^-II, and their diffusion is enhanced by the formation of a glassy phase on the silica grains through the transformation of silica-type quartz into cristobalite. The formation of a La₂Si₂O₇ intermediate phase was detected, which is vital to the interdiffusion of species and the nucleation and growth of oriented apatite crystals. The porosity gradient observed in the cross-section after reactive diffusion suggests the possibility of achieving an SOFC half-cell device via this process.