A Complete Solid Solution with Rutile‐Type Structure in SiO2–GeO2 System at 12 GPa and 1600°C

High pressure and temperature synthesis of compositions made of (Si_1?x,Ge_x)O₂ where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7–12 GPa and 1200–1600°C using a Kawai‐type high‐pressure apparatus. At 12 GPa and 1600°C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile‐type phases in the SiO₂–GeO₂ system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600°C, but not at 1200°C. At lower pressures (7 and 9 GPa) and 1600°C, we observed the decomposition of (Si_0.5,Ge_0.5)O₂ into SiO₂‐rich coesite and GeO₂‐rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite–stishovite phase transition pressure in SiO₂.     

Hardness of polycrystalline SiO2 coesite

We measured elastic moduli and hardness of polycrystalline SiO₂ coesite. Translucent polycrystalline bulk coesite with a grain size of about 10 micrometers was fabricated at 8 GPa and 1600°C using a Kawai-type multianvil apparatus. The obtained bulk and shear moduli are 94(1) and 60.2(3) GPa, respectively. The resulting Vickers and Knoop hardness values are 10.9(7) and 9.6(4) GPa, respectively, at an indentation load of 4.9 N. Coesite is as hard as other fourfold coordinated silica materials such as quartz and densified silica glasses. The hardness values of coesite and the fourfold coordinated silica materials are about one-third of those of sixfold coordinated silica materials, stishovite, and seifertite, which are the hardest known oxides.     

Nonlinear Property of (Nb2O5, SrCO3, Ge,GeO2)‐Codoped TiO2‐Based Varistor Ceramics

This study investigated the influence of codoping with Ge and GeO₂ on the nonlinear coefficient α and the breakdown voltage E_B of TiO₂–Nb₂O₅–SrCO₃ varistor ceramics. Ge–GeO₂ codoping notably changed the microstructure of the TiO₂–Nb₂O₅–SrCO₃ ceramics, increased α, and decreased E_B. When the doping contents of Nb₂O₅ and SrCO₃ were 0.1 and 0.2 mol%, respectively, the optimum doping content of 0.25 mol% Ge and 0.75 mol% GeO₂ exhibited high α (11.6), low E_B (13.8 V/mm), and high grain‐boundary barrier Φ_B (0.96 eV). These results are superior to previous findings. In addition, Ge and GeO₂, which function as sintering aids, reduced the sintering temperature caused by the low melting point. The optimal sintering temperature was 1260°C for the TiO₂–Nb₂O₅–SrCO₃ ceramics doped with Ge and GeO₂.     

Synthesis of Al2O3/SiO2 nano‐nano composite ceramics under high pressure and its inverse Hall–Petch behavior

We report the synthesis of alumina/stishovite nano‐nano composite ceramics through a pressure‐induced dissociation in Al₂SiO₅ at a pressure of 15.6 GPa and temperatures of 1300°C‐1900°C. Stishovite is a high‐pressure polymorph of silica and the hardest known oxide at ambient conditions. The grain size of the composites increases with synthesis temperature from ~15 to ~750 nm. The composite is harder than alumina and the hardness increases with reducing grain size down to ~80 nm following a Hall–Petch relation. The maximum hardness with grain size of 81 nm is 23 ± 1 GPa. A softening with reducing grain size was observed below this grain size down to ~15 nm, which is known as inverse Hall–Petch behavior. The grain size dependence of the hardness might be explained by a composite model with a softer grain‐boundary phase.     

Effect of Germanium Oxide (GeO2) Additive on the Anatase-to-Rutile Phase Transition

The effect of germanium oxide (GeO₂) addition on the anatase-to-rutile phase transition was investigated by differential thermal analysis (DTA), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). TiO₂ xerogels containing up to 20 mol% GeO₂ were prepared by refluxing and hydrolyzing titanium tetraisopropoxide (TTIP) and germanium butoxide (GB) using nitric acid as a catalyst. The following occurred with increasing amounts of GeO₂ in the xerogels: (i) the crystallization temperature of anatase increased from 410° to 565°C and the DTA temperature of the anatase-to-rutile phase transition increased from 676° to 977°C in 20 mol% GeO₂-containing xerogel; (ii) the crystallite size of anatase became smaller; (iii) the lattice a-parameter of the anatase showed little change, but the c-parameter decreased up to 20 mol% GeO₂; (iv) both the lattice a- and c-parameters of the rutile decreased monotonically. From these results, the added GeO₂ is considered to become incorporated into the anatase structure. The following occurred with increasing anatase heating temperature: (i) the lattice c-parameter of the anatase increased gradually and approached the value of pure anatase; and (ii) the chemical composition of the xerogel surfaces, measured by XPS, showed an increase in GeO₂ content, indicating the expulsion of GeO₂ from the anatase to form an amorphous surface layer. The formation of this amorphous GeO₂ surface layer is thought to play an important role in retarding the anatase-to-rutile phase transition by suppressing diffusion between the anatase particles in direct contact, and limiting their ability to act as surface nucleation sites for rutile, as in the case of SiO₂ additions. However, GeO₂ addition is less effective than SiO₂ in retarding the phase transition.     

Femtosecond laser direct writing in SiO2-Al2O3 binary glasses and thermal stability of Type II permanent modifications

We investigate the potential of fabricating thermally stable refractive index contrasts using femtosecond (fs) near-infrared (IR) radiation in aluminosilicate glasses. A set of pure SiO₂-Al₂O₃ glasses are manufactured, characterized (density and Raman), and investigated after being irradiated by fs laser within the Type II regime. The formation of nanogratings is identified and studied using quantitative birefringence measurements. Their thermal stability is then investigated through 30 minutes step isochronal annealing (up to 1250°C). For both SiO₂ and 50SiO₂-50Al₂O₃ compositions, the normalized birefringence does not decrease when tested up to 1100°C, while for the 4,6 mol% GeO₂-SiO₂ erased for 20% at 1000°C.     

Mg-bearing quartz solid solutions as structural intermediates between low and high quartz

Glass powder samples of cordierite composition (doped with 8 mol% TiO₂) were heat-treated to produce a series of increasingly SiO₂-enriched Mg-bearing quartz solid solutions (Qss). The obtained materials were then analyzed by X-ray diffraction: Rietveld structural refinements revealed that Mg-bearing Qss phases possess trigonal symmetry and a compositionally dependent intermediate structural arrangement between those of low and high quartz. High-temperature diffraction measurements were performed up to 700°C to characterize the thermal expansion behavior of the crystals. At SiO₂-rich compositions, a reversible high-to-intermediate inversion of the quartz structure is observed, which shifts with increasing stuffing to lower temperatures than the conventional 573°C for pure quartz. Similarities and differences to the better-established Li-bearing Qss are discussed in the text.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Influence of Al2O3/SiO2 ratio in multicomponent LNAS glasses and glass-ceramics on the crystallization behavior,microstructure and mechanical performance

Transparent glass-ceramics containing eucryptite and nepheline crystalline phases were prepared from alkali (Li, Na) aluminosilicate glasses with various mole substitutions of Al₂O₃ for SiO₂. The relationships between glass network structure and crystallization behavior of Li₂O–Na₂O–Al₂O₃–SiO₂ (LNAS) glasses were investigated. It was found that the crystallization of the eucryptite and nepheline in LNAS glasses significantly depended on the concentration of Al₂O₃. LNAS glasses with the addition of Al₂O₃ from 16 to 18 mol% exhibited increasing Q⁴ (mAl) structural units confirmed by NMR and Raman spectroscopy, which promoted the formation of eucryptite and nepheline crystalline phases. With the Al₂O₃ content increasing to 19–20 mol%, the formation of highly disordered (Li, Na)₃PO₄ phase which can serve as nucleation sites was inhibited and the crystallization mechanism of glass became surface crystallization. Glass-ceramics containing 18 mol% Al₂O₃ showed high transparency ～84% at 550 nm. Moreover, the microhardness, elastic modulus and fracture toughness are 8.56 GPa, 95.7 GPa and 0.78 MPa m^1/2 respectively. The transparent glass-ceramics with good mechanical properties show high potential in the applications of protective cover of displays.     

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.     

Experimental Phase Diagram Determination and Thermodynamic Assessment of the CeO2‐Gd2O3‐CoO System

New phase diagram data and a thermodynamic assessment of the CeO‐Gd₂O₃‐CoO system using the CALPHAD approach are presented. This information is needed to understand the surprisingly low sintering temperature (950°C–1050°C) of CeO₂‐based materials doped with small amounts of transition metal oxide (e.g., CoO). Experimental phase equilibria between 1100°C and 1300°C are reported based on the analysis of annealed and molten samples. No isolated compound exists in the ternary. At 1300°C the Co solubility in the ternary compounds Ce_1?x?yGd_xCo_yO_2?x/2?y (fluorite) is 2.7 mol% and is less than 1 mol% in the Gd_2?xCe_xO_3+x/2 (bixbyite). The Ce solubility in the perovskite GdCoO_3?δ was found to be 1 mol%. The lowest temperature eutectic melt in the ternary has a composition of 57.2 mol% Co and 41.1 mol% Gd melting at an onset temperature of 1303 ± 5°C, which is close to the binary eutectic in the Gd₂O₃‐CoO system at 60 ± 2 mol% Co and 1348 ± 1°C.     

Preparation of α-cordierite through mechanochemical activation of MgO–Al2O3–SiO2 ternary system

Samples of the ternary system MgO–Al₂O₃–SiO₂ with stoichiometric composition in relation to α-cordierite (Mg₂A_l4Si₅O₁₈), consisting of 22.2 mol% MgO, 22.2 mol% Al₂O₃, and 55.6 mol% SiO₂, were activated in a low energy mill with a constant speed of 100 rpm, in an aqueous medium. The precursors used were corundum (Al₂O₃), silica gel HF254 type 60 (SiO₂), and periclase (MgO). The objective of the present study was to evaluate the effect of mechanochemical activation on the solid-state synthesis of α-cordierite, using a low energy ball mill. Another objective was to shed light on the effect of mechanochemical activation on the steps of α-cordierite formation. For this end several grinding conditions were evaluated, varying the time and mass ratio of precursors/grinding elements, as well as calcination at different temperatures between 950 °C and 1350 °C for 2 h. The samples were analyzed for the determination of the formed phases by Infrared (IR) and X-ray Diffraction (XRD). The phases identified in uncalcined samples were brucite (Mg(OH)₂), forsterite (Mg₂SiO₄), enstatite (MgSiO₃), spinel (MgAl₂O₃), amorphous silica (SiO₂), corundum (α-Al₂O₃), and zirconia (monoclinic and tetragonal ZrO₂). The lowest temperature corresponding to the formation of α-cordierite (α-Mg₂Al₄Si₅O₁₈) was 1150 °C and a considerable amount of this phase (16.2%) was observed at this temperature, for the sample with the higher mechanochemical activation. In a solid-state reaction, α-cordierite is normally obtained at around 1400 °C, therefore, the formation of this phase at 1150 °C confirms that the mechanochemical activation method, using a low-cost ball mill, is efficient in reducing the solid-state reaction temperature.     

Phase development in the catalytic system V2O5/TiO2 under oxidising conditions

The target of this work was to investigate phase development in the catalyst system consisting of TiO₂ (anatase) and V₂O₅ (Shcherbinaite). Thus, a set of V₂O₅/TiO₂ specimens was prepared by ball milling and exposed to subsequent annealing in air in the temperature range from 400 to 700 °C. The XRD-results showed the presence of anatase and shcherbinaite as the only phases up to 525 °C. For temperatures above 525 °C the peak intensities were diminishing and rutile as a new TiO₂-phase occurred. Peak intensities and positions were shifted. No loss of oxygen or vanadium was detected. The reaction involves the formation of a rutile solid solution containing VO_x species. XPS studies showed an oxidation state of 4.75 for V in the rutile solid solution as compared to 4.65 in the shcherbinaite. A rutile solid solution once formed could not be re-transformed.The rutile solid solution was first found at 525 °C < T < 550 °C for compositions of 3 mol% < V₂O₅ < 5 mol%. The phase field for rutile solid solutions extends to 10 mol% < V₂O₅ < 12.5 mol% at 675 °C. For very high V₂O₅ concentrations (95 mol% V₂O₅) a eutectic reaction was found at 631 °C. The DTA runs showed a widened endothermic melting peak and a very sharp crystallization peak on cooling. A shcherbinaite structure remained with shifted peak intensities and positions due to the alloying of Ti-ions.SEM inspections showed that the rutile formation and the eutectic reaction both cause a substantial grain growth and a loss of surface area. The catalytic activity is entirely lost when the rutile formation occurs. The knowledge of phase relations helps to find the appropriate processing conditions and to understand the aging phenomena of catalysts.     

Systematic Control of Structural Changes in GeO2 Glass Induced by Femtosecond Laser Direct Writing

We report the structural changes inside germania glass induced by femtosecond laser pulses. Inspection by polarization microscopy and secondary electron microscopy indicate that the periodic nanostructures consist of oxygen defects such as ODCs (oxygen deficient centers) and NBOHCs (nonbridging oxygen hole centers) for laser pulse energy less than 0.2 μJ. However, the glass network was dissociated and O₂ molecules were generated for laser pulse energy greater than 0.4 μJ. Two different structural changes, form‐birefringence and dissociation, were induced in GeO₂ glass, depending on the laser pulse energy. The form‐birefringence exhibited by the nanogratings in GeO₂ glass is larger than that in SiO₂ glass for pulse energy less than 0.2 μJ, as the density of nanovoids enclosed by ODCs in GeO₂ glass is higher than that in SiO₂ glass. Arrhenius plots of the phase retardation caused by the nanogratings in GeO₂ and SiO₂ indicate that the oxygen defects are relaxed at a temperature 100°C above the glass‐transition temperature.     

Effect of substitution of ZrO2 by SnO2 on crystallization and properties of environment-friendly Li2O–Al2O3–SiO2 system (LAS) glass-ceramics

In this study, a transparent and environmentally friendly Li₂O–Al₂O₃–SiO₂ (LAS) glass-ceramic was prepared by melt-quenching and two-step heat treatment. The influence of the substitution amount of ZrO₂ by SnO₂ on the crystallization, microstructure, transparency, and mechanical properties of LAS glass and glass-ceramics was investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Ultraviolet–visible Spectrophotometer, three-point bending strength test, and microhardness test. The results indicate that the main crystalline phase of LAS glass ceramics was a β-quartz solid solution when heat treated at 780 °C for 2 h and 870 °C for 1.5 h. When the substitution amount of ZrO₂–SnO₂ increased from 0.4 mol% to 2.5 mol%, the grain size and thermal expansion coefficient of LAS glass-ceramics first decreased and then increased, and the crystallinity first increased and then decreased. When the substitution amount of ZrO₂–SnO₂ was 0.8 mol%, the transparency of the LAS glass-ceramics was maximum, the bending strength was 96 MPa, and the Vickers hardness was 10.9 GPa.     

Phase equilibrium in binary La2O3-Dy2O3 and ternary CeO2-La2O3-Dy2O3 systems

Cerium oxide doped with oxides of rare earth elements is a multifunctional material, a wide range of uses which is associated with its unique physicochemical properties. Phase diagrams of multicomponent systems are the physicochemical basis for the creation of new materials with improved characteristics.In this work, phase equilibria in ternary CeO₂–La₂O₃–Dy₂O₃ and binary La₂O₃–Dy₂O₃ systems in the whole concentration range were studied. No new phases have been identified in these systems. An isothermal section of the phase diagram of the CeO₂–La₂O₃–Dy₂O₃ system at a temperature of 1500 °С is constructed. No new phases have been detected in the system. It was found that in the studied ternary system solid solutions are formed on the basis of (F) modification of CeO₂ with structure of fluorite type, monoclinic (B), cubic (C) and hexagonal (A) modifications of Ln₂O₃.In the La₂O₃–Dy₂O₃ binary system (1500–1100 °С) three types of solid solutions are formed: based on hexagonal modification A-La₂O₃, monoclinic modification B-Dy₂O₃ and cubic modification C-Dy₂O₃ separated by two-phase fields (A+B) and (B+C), respectively. The boundaries of the regions of homogeneity of solid solutions based on A-La₂O₃ are determined by compositions containing 35–40, 20–25, 15–20 mol% Dy₂O₃ at 1500, 1250, 1100 °C, respectively. From the obtained data it follows that the solubility of Dy₂O₃ in the hexagonal modification of lanthanum oxide is 39 mol% at 1500 °C, 23 mol. % at 1250 °C and 16 mol% at 1100 °C. The limits of existence of solid solutions based on monoclinic B-modification are determined by compositions containing 30–35, 65–60 (1250 °С), 35–40, 55–60 (1100 °С) 40–45, 70–75 (1500 °C) mol% Dy₂O₃.In the studied system, with a decrease in temperature from 1500° to 1100°C, there is a decrease in the solubility of La₂O₃ in the crystal lattice of cubic solid solutions of C-type from 16 to 10 mol%.     

High-pressure torsion of SiO2 quartz sand: Phase transformation,optical properties,and significance in geology

Phase transformation and optical properties of silica (silicon dioxide, SiO₂) quartz sand under high pressure/temperature has been of interest in geology and optical physics for many years. In this study, besides high pressure/temperature, high plastic strain is simultaneously applied to the quartz sand by high-pressure torsion (HPT) processing. The material shows oxygen vacancy formation and transformation to (a) a denser nanocrystalline quartz phase, (b) a high-temperature amorphous phase and (c) a high-pressure coesite phase. These structural and microstructural changes lead to light absorbance, electron spin resonance, photoluminscence and photocatalytic activity, while these changes are enhanced by increasing strain. This study introduces a possible pressure-temperature-strain-based mechanism for the formation of naturally observed vacancies and coesite phase in SiO₂-based minerals and sands.     

Formation and crystal growth of needle-like rutile in glass- ceramics

Glass-ceramic, which has negligible dielectric loss, high mechanical strength, excellent drop resistance, low CTE, and low density for lightweight design, is the best option for the back cover of mobile devices in the 5 G era. Herein, the effect of P₂O₅ on the phase separation and crystallization of MgO–Al₂O₃–SiO₂-TiO₂ glass-ceramics is studied. The incorporation of P₂O₅ in the glass structure leads to phase separation, in which the P and Mg-enriched phase was formed in the glass matrix, and promotes the increase of T_g. With the increase of P₂O₅ content, the precipitated crystals change significantly. First, the silicate crystals (Mg₂SiO₄) disappear, whereas the phosphate crystals (LiMgPO₄) emerge when 2 mol% P₂O₅ is introduced. Second, titanate crystal (MgTi₂O₅) can not be observed when 4 mol% P₂O₅ is introduced. The Ti₅O₉ crystals appear simultaneously with LiMgPO₄ crystals and transform to rutile TiO₂ crystals at high temperature. Interestingly, the needle-like rutile TiO₂ crystals, which is 300 nm long and 20 nm wide, have been found in a glass with 4 mol% P₂O₅. The large L/D ratio of needle-like crystals increases the hardness significantly from 6.08 GPa to 7.14 GPa. Similar to other fiber reinforced composites, this needle-like crystals provide a new strategy to improve the mechanical properties of glass ceramics.     

On the influence of silica type on the structural integrity of dense La9.33Si2Ge4O26 electrolytes for SOFCs

Apatite-type rare earth based oxides, such as R-doped lanthanum oxides of general formula La_9.33(RO₄)₆O₂ with R = Ge, Si, exhibit high ionic conductivity and low activation energy at moderate temperatures, when compared to the yttria-stabilized zirconia electrolyte making them potential materials to be used in the range 500–700 °C, for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, dense oxyapatite-based La_9.33Si₂Ge₄O₂₆ electrolytes have been successfully prepared either by electrical sintering at 1400 °C or microwave hybrid sintering at 1350 °C for 1 h from La₂O₃, SiO₂ and GeO₂ powders dry milled at 350 rpm for 15 h in a planetary ball mill. The densification behaviour of the apatite-type phase synthesized by mechanical alloying was found to be dependent on the grade of SiO₂ used: either pre-milled quartz powder or amorphous nanosized fumed silica. The influence of the silica type on the La_9.33Si₂Ge₄O₂₆ integrity was assessed by dynamic Young's modulus, microhardness and indentation fracture toughness measurements. A good correlation between the degree of densification (as observed by SEM/EDS) and the resulting mechanical properties could be established. Pre-milling of quartz powder has favoured higher densification rates to be attained suggesting that both Fe content, resulting from the dry milling (as determined by PIXE analyses) and crystallinity of SiO₂ do promote densification of these electrolytes thereby improving their structural integrity.     

Microstructure,mechanical properties,and high-temperature oxidation resistance of AlN/SiO<Subscript>2</Subscript> nanomultilayer coatings

The microstructure, mechanical properties, and high-temperature oxidation resistance of AlN/SiO₂ nanomultilayer coatings with various SiO₂ layer thicknesses were investigated using X-ray diffractometry, high-resolution transmission electron microscopy, scanning electron microscopy, and nanoindentation. The results revealed that SiO₂ formed wurtzite-typed hexagonal pseudo-crystal structures and grew epitaxially with AlN when its thickness was less than 0.6 nm. Meanwhile, the multilayer coatings yielded superhardness effect with a maximum hardness of 29.0 GPa. A minute increase in SiO₂ thickness from 0.6 to 0.9 nm would decrease the hardness of the nanomultilayer coatings due to the formation of amorphous SiO₂ and destruction of epitaxial structure. The high hardness of superhard coatings was sustained after high-temperature annealing treatment of up to 800°C. However, a further increase in annealing temperature to 900°C caused severe oxidation of AlN and thus degradation of coating’s hardness.