Molecular Dynamics Simulations of the Structure and Properties of Low Silica Yttrium Aluminosilicate Glasses

Yttrium aluminosilicate glasses are of technological importance in photonics, nuclear waste disposal, and as a delivery vehicle for radiation therapy. Their structures are also of great interest in glass science to elucidate the principles of glass formation and structures. We provide classical molecular dynamics simulation results of a series of yttrium aluminosilicate glasses with low silica concentration. Detailed structure analyses including coordination number, pair and bond angle distributions, Si–O and Al–O network structures, and primitive ring statistics are reported. It is found that the average aluminum coordination number decreases from 4.7 to 4.2 with increasing silica content from 5 to 20 mol%, while maintaining alumina at 55 mol%. Four-coordinated aluminum ions increase from 40% to 84% in the series and the fraction of edge-sharing alumino-oxygen polyhedra decreases, indicating an increased network former role of aluminum ions with increasing silica content. Physical properties such as elastic constant, bulk modulus, Young's modulus, and Possion's ratio were calculated. The results show a decrease of bulk, Young's, and shear moduli with increasing silica and decreasing yttria contents that is in good agreement with experimental data.     

Dehydration and Crystallization Kinetics of Zirconia-Yttria Gels

Zirconia and zirconia-yttria gels containing 4 and 8 mol % yttria were obtained by coprecipitation and drying at 373 K. The dehydration and crystallization behavior of the dried gels was studied by DSC, TG, and XRD. The gels undergo elimination of water over a wide temperature range of 373–673 K. The peak temperature of the endotherm corresponding to dehydration and the kinetic constants for the process were not influenced by the yttria content of the gel. The enthalpy of dehydration observed was in good agreement with the heat of vaporization data. The dehydration was followed by a sharp exothermic crystallization process. The peak temperature of the exotherm and the activation energy of the process increased with an increase in yttria content, while the enthalpy of crystallization showed a decrease. The "glow effect" reduced with increasing yttria content. Pure zirconia crystallizes in the tetragonal form while the zirconia containing 4 and 8 mol% yttria appears to crystallize in the cubic form.     

Tougher zirconia nanoceramics with less yttria

ABSTRACT

Reducing the grain size in zirconia ceramics has shown to decrease its toughness by size-dependent stabilisation of the tetragonal phase that, in turn, hinders the stress-induced phase transformation from tetragonal to monoclinic. The stability of the tetragonal phase increases with the decrease of grain size but decreases with the reduction of the amount of yttria added, implying the need for adjustment of the yttria content when a nano-grained structure is of concern. In this study, low-yttria compositions were investigated. The ceramics were prepared with two sintering methods namely spark plasma sintering (SPS) and pressureless sintering. A clear tendency was noted for the indentation toughness increase with the reduction of yttria content, and a higher toughness achieved in as-SPSed samples in comparison with the annealed samples. The origins of the increased toughness were discussed in terms of yttria content, carbon contamination and increased oxygen vacancies after sintering at reducing atmosphere in SPS.     

Effect of Yttria on the Thermal Conductivity of Aluminum Nitride

The effect of yttria additions up to 10 wt% on the thermal conductivity of pressureless-sintered aluminum nitride was investigated. Additions of up to 3 wt% increased the thermal conductivity to values around 160 W/(m·K). Additions higher than 5 wt% slowly decreased the thermal conductivity because of the increasing volume fraction of the low-conductivity yttrium aluminate phases. In all cases samples containing the same yttria content with the binder burned out in nitrogen had higher thermal conductivity than those with the binder burned out in air. Comparisons are made to previous studies containing additives of silica and calcia to aluminum nitride.     

Bi2O3-Ga2O3-CdO系统玻璃的Raman光谱和X射线光电子能谱研究

用X射线光电子能谱和Raman光谱研究了Bi2O3-Ga2O3-CdO系统玻璃的结构，Raman光谱曲线被分离成6个谱带，4条谱带分属于不同键长的Bi-O振动，一条谱带属于Ga-O振动，Bi2O3-Ga2O3二元系统玻璃的Raman散射最强峰位于400－420cm^-1,当Ga^3 被Cd^2 离子取代后，Raman散射最强峰移向595－630cm^-1，随着Ga2O3含量的增加，位于高波数属于Bi-O振动的2条谱带强度降低并朝低波数移动，位于低波数属于Bi-O振动的2条谱带强度增加并朝高波数移动，添加CdO则出现相反的效应，X射线光电子能谱显示出非常低的O1s电子结合能，甚至低于碱硅酸盐玻璃中非桥氧的O1s电子结合能，并且不可能分为桥氧和非桥氧，O1s和Bi4f的电子结合能都随Ga2O3和C dO含量增加而增加。     

Glass Formation and Properties in the System Calcia-Gallia-Germania

The critical cooling rate for glass formation, R^c, was measured for four compositions in the system calcia-gallia-germania. The activation energy, E, and frequency factor, u, for the crystallization process were determined by reheating the glasses at varied constant heating rates and measuring the temperature of crystallization. Both E and v increased, with increasing germania content of the glass, whereas R^c decreased. The density, refractive index, and Abbe number were also measured; all decreased with increasing GeO² content. These results are compared with those for calcia-gallia-silica glasses of comparable compositions.     

Properties of Soda Aluminosilicate Glasses IV, Relative Acidities and Some Thermodynamic Properties

Electrochemical cells using the oxygen electrode reactions of Förland and Tashiro were constructed from soda aluminosilicate glasses (containing up to 34.8 wt% alumina) and sodium silicate glasses. The emf developed by these cells was measured as a function of temperature. The relative acidities of the glasses, as indicated by the activities of the soda, increased with increasing alumina content, the rate of change increasing when the Al/Na ratio reached 1. The entropy change for the cell reaction went through a maximum. These properties are correlated with the measurements of, and the structural model used in, the previous three parts of this series. Measurements were made to show that the emf values obey an additive relation.     

Physical and Mechanical Properties of Barium Alkali Silicate Glasses for SOFC Sealing Applications

The physical and mechanical properties of two barium alkali silicate glasses were determined as a function of temperature. Their Young's modulus and Poisson's ratio were determined by resonant ultrasound spectroscopy; their viscosity, thermal expansion, and glass transition temperature were determined using a thermomechanical analyzer. The wetting behavior of the two glasses on alumina and 8 mol% yttria stabilized zirconia (8YSZ) substrates was determined by measuring contact angles in air as a function of temperature and time. Values of Young's modulus for both glasses were in good agreement with those predicted by the Makishima and MacKenzie model. The physical and mechanical properties of these glasses are discussed in the context of their potential use for sealing applications in solid-oxide fuel cells.     

Pressureless sintering of yttria-gadolinia co-stabilized zirconia

Zirconia ceramics partially stabilized with yttria and gadolinia [(3-x)Y,xGd-TZP; x = 0–2] were manufactured by intensive co-milling of zirconia and stabilizer oxides, spray granulation, axial pressing and pressureless sintering at 1300–1400 °C. Microstructure, phase composition and mechanical properties were determined. Stabilizer re-distribution in the materials at higher sintering temperatures was monitored by XRD. Substitution of yttria for gadolinia reduces the stabilizer content in the tetragonal phase, increases the tetragonality of the materials and induces higher transformability and toughness. The hardness is thereby reduced. In accordance with thermodynamic considerations the cubic content increases with increasing gadolinia content.     

Oxygen Diffusion in Yttria-Stabilized Zirconia: A New Simulation Model

We present a multiscale modeling approach to study oxygen diffusion in cubic yttria-stabilized zirconia. In this approach, we employ density functional theory methods to calculate activation energies for oxygen migration in different cation environments. These are used in a kinetic Monte Carlo framework to calculate long-time oxygen diffusivities. Simulation results show that the oxygen diffusivity attains a maximum value at around 0.1 mole fraction yttria. This variation in the oxygen diffusivity with yttria mole fraction and the calculated values for the diffusivity agree well with experiment. The competing effects of increased oxygen vacancy concentration and increasing activation energy and correlation effects for oxygen diffusion with increasing yttria mole fraction are responsible for the observed dopant content dependence of the oxygen diffusivity. We provide a detailed analysis of cation-dopant-induced correlation effects in support of the above explanation.     

Formation and Properties of Ln-Si-O-N Glasses (Ln = Lanthanides or Y)

Homogeneous Y-Si-O-N glasses containing 15 or 20 eq% nitrogen (N) were prepared from compositions with Y/Si ratios in the vicinity of that of the lowest eutectic point on the Y₂O₃–SiO₂ phase diagram. The liquidus on the phase diagram shifted toward lower temperatures by incorporation of N. The density, the elastic moduli, and the glass transition temperature of the Y-Si-O-N glasses increased with incorporation of N. This is due to the closer packing of atoms in the glasses by the substitution of N, which is in three-fold coordination with Si, for O which is in two-fold coordination, and the stronger covalent nature of the Si–N bond compared with the Si–O bond. The coefficient of thermal expansion of the Y-Si-O-N glasses increased with increasing Y content, because the discontinuity of the glass network developed with increasing nonbridging anions by the introduction of Y. In contrast, the glass transition temperature and the elastic moduli increased with Y content due to the high coordination of Y for O, and the relatively high cationic field strength of Y. Furthermore, the effect of cationic field strength on properties of Ln-Si-O-N glasses (Ln = lanthanides or Y) is discussed.     

Structure and property of multicomponent germanate glass containing Y2O3

Abstract

CaO–BaO–Al₂O₃–SiO₂–GeO₂ glasses doped with 0·00–16·67 wt-% Y₂O₃ have been prepared by conventional melt quenching method. The influence of Y₂O₃ addition on the properties and structure of the glasses has been investigated. The results show that, with the introducing of Y₂O₃, the density, glass transition temperature and thermal expansion coefficient of the glass increase but the chemical durability declines. In addition, compared with the glass without Y₂O₃, the bend strength of the glass including 4·76 wt-% Y₂O₃ increased from 54 to 110 Mpa. The possible mechanism is that yttria acts as a network former in the structure and makes the island shape network unit repolymerisation by forming Ge–O–Y bond. The absorption strength caused by hydroxyl vibration decreased up to completely disappearance as the Y₂O₃ content increased continuously. The introducing of yttria in the composition causes the conversion from four coordination to higher coordination germanium, which decreases non-bridge oxygen (NBO) and weakens hydroxyl characteristic absorption.     

Effect of yttria on crystallization and microstructure of an alumina–YAG fiber prepared by aqueous sol–gel process

Continuous fiber development is needed for high performance and high temperature composites. Various methods have been used to make ceramic fibers. In this research, composite fibers (yttrium aluminum garnet (YAG)/Al₂O₃) were prepared by a sol–gel method using aqueous solution. They were synthesized from aluminum salt, aluminum metal, yttrium oxide and water used as solvent. Transparent gel fibers were obtained by immersing a thin wire into the viscous sol, then pulling it out by hand. The obtained fibers contained very fine grains with diameter ranging from 10 to 80 μm after heat treatment. When yttria content was increased, the crystallization of YAG shifted to a lower temperature, whereas the transformation temperature to α-Al₂O₃ shifted to a higher temperature. Nevertheless, the fibers with different amounts of yttria contained alumina and YAG after heat treatment at 1400 °C. The composite fibers had vermicular structure and were denser than alumina fibers. The yttria percent concerning the limits of this study (≤10 wt%) effected on fiber diameter. As the yttria content was increased, the fiber diameter increased, whereas grain size and densification of the composite fibers decreased.     

Internal Friction of Simple Alkali Silicate Glasses Containing Alkaline-Earth Oxides: I, Experimental Results

The internal friction of simple alkali silicate glasses to which systematic alkaline-earth oxide additions were made was investigated. These additions produced two significant results: (1) The two peaks previously reported in alkali silicate glasses were shifted to higher temperatures and decreased in height and (2) a third peak was found in some glasses which became more pronounced with increasing alkaline-earth oxide content. Internal friction measurements of alkali-free silicate and phosphate glasses also are described. In these glasses, peaks were present which closely resembled those found in glasses containing alkali.     

Optical Absorption Characteristics of Ni2+ in Mixed-Alkali Borate Glasses

Nickel-containing mixed-alkali borate glasses were prepared and their spectral absorption in the visible and near-ir were measured and used to determine the ligand field strength, Racah parameter, and state of coordination of Ni²⁺ ions. The change in these parameters with alkali content is discussed in terms of mixed-alkali effect. Nickel ions were found to occupy octahedral sites in the 20 mol% (Li₂O+Na₂O) glasses, whereas in the 30 mol% (Na₂O+K₂O) glasses Ni^Z+ ions are believed to occupy both octahedral and tetrahedral sites. In most cases slight deviations were observed which increased with increasing alkali content. The results obtained suggest that the substitution of one alkali for another in alkali borate glasses did not produce gross alterations of the glass network sufficient to induce deviations from linearity in property-composition relations.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

ZnO-P2O5-Sb2O3-B2O3系统玻璃的形成与性能优化研究

采用熔体冷却的方法制备了xZnO-yP2O5-zSb2O3-20B2O3(x=10～30 mol%,y=10～45 mol%,z=10～50 mol%)系统无铅玻璃,研究了该体系玻璃的形成区。采用热膨胀仪、失重法等研究了xZnO-yP2O5-zSb2O3-20B2O3系统玻璃的性能和含有MnO2的20ZnO-40Sb2O3-20P2O5-20B2O3体系玻璃的性能。结果表明,随着Sb2O3含量的增加,xZnO-yP2O5-zSb2O3-20B2O3系统玻璃的转变温度降低。xZnO-yP2O5-zSb2O3-20B2O3系统玻璃的失重随P2O5含量的增加而增加。MnO2的引入降低了20ZnO-40Sb2O3-20P2O5-20B2O3系统玻璃的热膨胀系数和失重。     

Nucleation of Perovskite Nanocrystals in a Levitating Liquid

Containerless liquid-phase processing methods were used to synthesize two glasses, both containing alumina (A), yttria (Y), and lanthana (L), and one containing ∼20 mole% silica (S) as well. In both materials, denoted AYL and AYLS, alumina is the dominant component (>50 mole%), and the lanthana:yttria ratio is ∼2:1. A variety of experimental techniques, e.g. X-ray diffraction, high-field ²⁷Al magic angle spinning-nuclear magnetic resonance (MAS-NMR), differential scanning calorimetry, and neutron diffraction, were used to analyze both the structure of the amorphous phases resulting from liquid-phase processing and subsequent thermal treatments, and the thermodynamic properties of the as-made and heat-treated materials. The results of the experiments on these glasses and liquids have elucidated the nature of the transitions occurring between the glass transition and crystalline regimes, and have enabled the synthesis of a novel class of alumina-based nanomaterials.     

Color and Spectral Transmittance of Amber Bottle Glass

Because of increased interest in color uniformity among iron-sulfur amber bottle glasses, a study was undertaken of their color, their spectral transmittance, and the relationship of these properties. It was found that the psychophysical attributes of color (C.I.E. system) as computed from complete light-transmittance spectra can actually be estimated for iron-sulfur amber glasses with good accuracy from transmittance at only two suitable wave lengths, 550 and 650 mp. Transmittance at 550 mμ is closely correlated with brightness (C.I.E.) and thus is sufficient to characterize color intensity, which is the most important variable of color quality among commercial amber glasses. There is also fair correlation between transmittance at 550 mμ and transmittance in the ultraviolet. The latter is important in the avoidance of photochemical action on bottled products. Incidental to this work, the transmittances of a large number of glasses were compared with their iron and sulfur contents. Broadly speaking, it was found that transmittance at 550 mp decreases with increasing ferrous iron or with increasing sulfide content. However, this transmittance is affected substantially by other factors, such as gross composition, minor constituents like fluorine, and thermal history, of which no account was taken in the present work. Transmittance at 1050 mp can be correlated closely with ferrous iron content (as determined by chemical analysis) and absorbancy per unit concentration of ferrous iron in carbon-sulfur ambers is about the same as in green glasses containing no sulfur.     

Chlorapatite Glass-Ceramics

Apatite glass-ceramics are attractive for medical and dental applications, and fluorapatite glass-ceramics based on aluminosilicate glasses have been extensively studied. This study is the first study of chlorapatite glass-ceramics based on calcium chloride-containing Q² bioactive phosphosilicate glasses. The crystallization behavior of oxychloride glasses is examined and compared with mixed oxychloride/fluoride and oxyfluoride glasses. The glass transition temperature decreased for all three series with increasing halogen content. On increasing the halogen content, there was an increasing tendency of the glasses to crystallize. The halogen-free glass surface crystallized to pseudowollastonite and an apatite. On incorporating a halide, the glasses exhibited largely bulk crystallization to a haloapatite. In the case of chloride, the glasses crystallized to chlorapatite. This is the first time to our knowledge that chlorapatite has been shown to crystallize from a glass. Chlorapatite is very attractive for medical applications because it converts to hydroxyapatite the mineral phase of tooth and bone on immersion in water.