Quantum-Chemical Justification of the Structure of Fluorozirconate Glasses1

A number of fluorozirconate clusters of the general formula [Zr _m F _n ]^{(4m– n)}and barium fluorozirconate clusters [BaZr₂F _n ]^{(10 – n)}are studied by the quantum-chemical discrete variational X method. It is shown that the mean energies of fluorine ion addition for all structural units observed in real fluorozirconate crystals lies in a narrow range. The vibrational spectra are analyzed within the energy approach. It is demonstrated that the coordination number of zirconium in a number of barium fluorozirconate glasses is equal to seven or eight. The possible mechanism of structural transformations in binary fluorozirconate glass with a variation in its composition is proposed. The rigidity of different bonds in fluorozirconate clusters is investigated qualitatively. It is established that the rigidities of zirconium bonds with bridging and terminal fluoride ions level off in polymers, which is a prerequisite for the glass formation in fluorozirconate systems.     

Generalized Flory–Huggins model for heat‐of‐mixing and phase‐behavior calculations of polymer–polymer mixtures

An extended and generalized Flory–Huggins model for calculating the heats of mixing and predicting the phase stability and spinodal diagrams of binary polymer–polymer mixtures is presented. In this model, the interaction parameter is considered to be a function of both temperature and composition. It is qualitatively shown that the proposed model can calculate the heats‐of‐mixing curves containing exothermic, endothermic, and S‐shaped or sigmoidal types and predict the spinodals, including the upper and lower critical solution temperatures, and closed‐loop miscibility regions. Using experimental results of analog calorimetry for four polymer mixtures of polystyrene/poly(vinyl chloride) (PS/PVC), polycarbonate (PC)/poly(ethylene adipate) (PEA), polystyrene/poly(vinyl acetate) (PS/PVAc), and ethylene vinyl acetate copolymer (EVA Co)/chlorinated polyethylene (CPE), the capabilities of the proposed functionality for the interaction parameter was studied. It is shown that this function can be used satisfactorily for the heat‐of‐mixing calculations and phase‐behavior predictions. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1328–1340, 2000     

Thermochemistry of Sodium Borosilicate Glasses

Glasses synthesized in the system Na²0-B ²0₃−SiO₂ were studied by high-temperature (974 K) solution calorimetry. The sign of the heats of mixing along four joins in the ternary reflects the presence or absence of glass-glass immiscibility. Calculations of binary phase diagrams using thermochemical data and a configurational entropy model give good agreement with experimental determinations, but complexities arise due to the simplicity of the entropy model used.     

Heats of Fusion of Fluorozirconate Systems

The heat of crystallization of a glass obtained by an isothermal DSC experiment does not represent the true value of the heat of fusion. A procedure is described to evaluate the heat of fusion from measured values of heats of crystallization. The correction term is estimated to be as important as the measured values of heats of crystallization in the case of fluorozirconate glasses.     

Inhibiting effect of heterogeneous cations aggregation enhanced by oxygen deficiency on glass formation of BaTi2O5 melts

Although remarkable development of titanate-based glasses has been achieved, challenge remains to elucidate the correlation between structure and glass-forming properties in these systems due to their complex structure that is inconsistent with the classic Zachariasen's model. In this work, we aim to correlate the structural evolution of titanate melts to their glass-forming ability (GFA). The prototypical material barium dititanate (BaTi₂O₅, BT2) melts with different GFA were rendered by controlled melting atmospheres, and the corresponding structural changes were determined using in situ high-energy synchrotron X-ray diffraction combined with empirical potential structure refinement and ab initio molecular dynamics. The results show that BT2 melt in reducing atmosphere shows poor GFA but that in oxidizing atmosphere presents good GFA. Structural analysis demonstrates the mean coordination number of [TiO_m] polyhedra is analogous in the melt under two different atmospheres but an enhanced heterogeneous cations aggregation takes place in the melt under reducing atmosphere, which is closely related to oxygen-deficiencies. Furthermore, we reveal that the enhanced heterogeneous cations aggregation promotes crystallization (and therefore hinders glass formation) through disordering the distribution of [TiO_m] and [BaO_n] polyhedra, changing the connectivity between these polyhedra, creating more crystal-like Ti-Ti clusters, and decreasing topological disorder of BT2 melt. Our work provides a new viewpoint to understand the GFA of titanates melt from structural heterogeneity beyond the previous perspectives that only focus on [TiO_m] polyhedra.     

Thermochemistry of Glasses in the Y2O3-Al2O3-SiO2 System

Enthalpies of drop solution in molten 2PbO-B₂O₃ at 1078 K were measured for glasses along the 2YAlO₃-3SiO₂ and return ½Y₃Al₅O₁₂-3SiO₂ joins. The onset glass transition temperature increases slightly with increasing silica content and Y/Al. Enthalpies of mixing were calculated on the basis of amorphous end members. Strongly negative heats of mixing support the absence of miscibility gaps except possibly for very high silica content, consistent with experimental phase analyses, which indicate much narrower miscibility gaps compared with the phase diagrams calculated on the basis of previous data and the CALPHAD formalism.     

Effect of the thermal treatment on the phase composition of the surface of coatings based on Si–B–ZrB<Subscript>2</Subscript> and modified with ZrO<Subscript>2</Subscript> fibers

Coatings on graphite that are stable to oxidation and based on the silicon–boron–zirconium boride composite containing from 5 to 50% of fibrous zirconium dioxide as a modifying dopant have been produced by the suspension–annealing method. A nonporous layer is formed at the zirconium dioxide content ranging from 5 to 15%, while a porous layer is formed at its 50% content. A glass-forming melt, as well as zirconium dioxide and silicate, is formed during thermal treatment as a result of the chemical reactions with the oxygen in air. The zirconium silicate content increases, along with the modifier’s concentration and temperature.     

Activities in Borosilicate Melts: II, Some Mixtures of Calcium Borates-Calcium Silicates

Simple expressions of activities, as functions of composition, were developed from structural models for some borosilicate melts. Their applicability was tested with data from the liquidus curves of phase diagrams. If the heat of fusion is known, a value of the activity-composition function can be calculated for each temperature of the liquidus curve. These values are compared with activities computed from the structural model. Calorimetric heats of fusion were available for the calcium borates studied. Experimental data from published phase diagrams for the systems Ca₃(BO₃)₂–Ca₂SiO₄, Ca₂B₂O₆–Ca₂SiO₄, Ca₃(BO₃)₂–CaSiO₃, Ca(BO₂)–CaSiO₃, Ca₂B₂O₅–CaSiO₃, CaSiO₃–BaSiO₃, and CaSiO₃–K₂SiO₃ were used. Simple ionic models seemed to be applicable in most cases, except for melts of highly polymerized mixed ions (metaborate-metasilicate), for which the so-called "group" model appeared to be useful. A value for the heat of fusion of CaSiO₃ was calculated from this work (13,200 cal. per mole).     

Time–Temperature–Transformation Diagrams for Borosilicate Glasses and Preparation of Chemically Durable Porous Glasses

To clarify the effects of glass composition and heat-treatment conditions on phase separation and crystallization, time–temperature–transformation (TTT) diagrams for the system (65 − x )SiO₂·25B₂O₃·(10 − y )Na₂O· y CaO· x ZrO₂, which is used for fabricating alkali-durable porous glasses, were determined. The microstructures and properties, such as resistance to alkaline attack as a function of ZrO₂ content, of the porous glasses fabricated on the basis of the developed TTT diagrams were examined. CaO and ZrO₂ additions were shown to affect the locations of the noses in the TTT curves for phase separation and crystallization. The substituting of CaO for Na₂O was effective for retaining ZrO₂ in the skeleton of the porous glasses. The alkali resistance of ZrO₂-containing porous glasses was 8–10 times superior to that of the porous glass without ZrO₂. The results demonstrate that TTT diagrams can be used as guides for heat-treatment scheduling in processing or predicting of glass-forming ability and the onset of phase separation and crystallization in glass-forming systems.     

Problems of spectrophotometry of glass-forming melts: II. A technique for measuring the absorption spectra of glasses and melts in the red and near-infrared ranges at temperatures from 20 to 1500°C

A technique for measuring the absorption spectra of glasses and glass-forming melts in the red visible and near IR ranges, the experimental setup, and the measuring cell intended for use over a wide range of temperatures are described. The main problems concerning the acquisition of reliable data on the absorption spectra of glass-forming melts at high temperatures and the basic sources of errors in measurements of the spectra are considered.     

Investigation of the Phase Diagram of the Na2O–B2O3–SiO2 System by the Small-Angle X-ray Scattering Technique

Sodium borosilicate melts containing 60 mol % SiO₂ are investigated in the temperature range T _g–1100°C by the small-angle X-ray scattering (SAXS) technique. The temperature dependences of the SAXS intensity for the studied melts consist of two linear portions. A change in the slope of these portions is observed at temperatures that coincide, to within the accuracy of the SAXS experiment (±5–10 K), with the liquidus temperatures, which are usually determined to approximately identical accuracy. The change in the slope is associated with the change in the temperature coefficients of the isothermal compressibility upon transition from a liquid state to a supercooled liquid state. Similar results were obtained earlier for binary melts in the sodium borate system and a number of other systems. On this basis, it is assumed that the transition from a liquid state to a supercooled liquid state for melts in any glass-forming system can be revealed from the temperature dependences of the SAXS intensity, and the SAXS technique can be used to determine the liquidus temperatures, primarily for glass-forming melts with a low crystallization ability.     

Influence of the method of mixing on homogeneity and crystallization kinetics of blends of linear and branched polyethylene

The influence of mixing method—solution and melt mixing—on the homogeneity and crystallization kinetics of a series of blends of single‐site materials of linear polyethylene and ethyl‐branched polyethylene was studied by differential scanning calorimetry. Data obtained for heats of melting and crystallization, melting and crystallization peak temperatures, and melting and crystallization temperature profiles were essentially the same for the samples obtained by the two mixing methods. The results obtained can be interpreted as indicating that melt mixing is capable of producing homogeneous melts of these relatively low molar mass polymers, given that solution mixing is considered to give perfectly homogeneous blends. The heat associated with the high temperature melting peak after crystallization at 125°C of the blend samples, obtained by the two preparation methods, was higher than that of the linear polyethylene included in the blends, suggesting that a part of the branched polyethylene crystallized at 125°C. The unblended branched polyethylene showed no crystallization at 125°C. Samples obtained by powder mixing showed independent crystallization and melting of the linear and branched polyethylene components. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1730–1736, 2004     

Activities in Borosilicate Melts: I, Some Melts in the System CaO–B2O3

Several types of structural models were evaluated for some calcium borate melts by calculating melt activities. The calculated activities were compared with values determined from calorimetric heats of fusion. An ionic model was the most valid for the melts studied. The presence of (BO₂)₃^3– rings in melts of calcium metaborate was indicated.     

Polymer blends containing poly(vinylidene fluoride). Part IV: Thermodynamic interpretations

A summary of the transitional behavior of blends containing poly(vinylidene fluoride), PVF₂, and various oxygen-containing polymers is presented. These data are used to establish the presence of a single miscible amorphous phase. The depression of the PVF₂ melting point in those blends judged to be miscible is analyzed using standard thermodynamic arguments to determine the heats of mixing between the amorphous diluents and the PVF₂. These heats of mixing are exothermic indicating the presence of strong interactions between the binary pairs. A comparison between the observed interaction strengths and the dipole moments of the various diluents suggests that the exothermic heats of mixing are the result of strong dipole-dipole interactions. Nearly all the miscible blends with PVF₂ show Lower Critical Solution Temperature (LCST) behavior. The direct correlation between the temperature location of this phase instability and the observed interaction strength suggests that the instability is more the result of enthalpic considerations than entropic ones.     

Temperature Dependence of the Viscosity of Chalcogenide Glass-Forming Melts

A procedure is proposed for extrapolating (up to the boiling point) the temperature dependences of the viscosity of chalcogenide glass-forming melts, which are measured in the softening temperature range. It is shown that the calculated and experimental data obtained for a series of stoichiometric glass-forming melts are in satisfactory agreement. The viscosity is calculated for the GeS₂ melt, for which experimental data on viscosities of less than 10¹⁶ P are not available in the literature.     

Influence of Chemical and Phase Equilibria on the Medium-Range Order and Physicochemical Properties of Chalcogenide Glasses1

The problem associated with the influence of chemical and phase equilibria in melts on the properties of formed glasses is analyzed experimentally and theoretically by the example of chalcogenide glasses prone to glass formation. It is proved that inorganic glasses should be treated as microinhomogeneous systems formed by a set of nanometer-sized pseudophases that differ in character. The equivalentometry method has revealed that, in inorganic glasses, there exist at least two types of the pseudophases which have different chemical compositions and are responsible for microinhomogeneous structure of vitreous materials. It is established that the pseudophases in glasses are fragments composed of elementary substances and are also stable and metastable compounds. The concept of a variable number of components in vitrifying melts is introduced for the first time. This concept and the eutectoidal model of the vitreous state of matter allow one to reveal the types of chemical and phase equilibria whose freezing in glasses results in the formation of a set of the pseudophases responsible for glass microinhomogeneity. The regularities revealed make it possible to put forward the principles of analysis of composition–property diagrams for vitreous systems.     

双流体配位状态方程在共聚物及共混物体系中的应用

采用双流体配位模型状态方程,对三个共聚物体系P(VAC/E)、P(S/MMA)及P(EP),两个共混物体系PVME/PS、PPO/PS进行了关联,精度较好.提出了共聚物的共混物假设的新设想.在关联中发现共聚物及共混物体系的二元模型参数具有一定的加和性,本文采用一个较为简单的混合规则,用均聚物的模型参数估算二元模型参数,进行了pVT数据预测,并与实验数据比较,具有良好的预测效果.     

Phase diagrams of ABC linear triblock copolymers under nanopore confinements

The phase diagrams of triblock copolymers in cylindrical nanopores are investigated using the real-space self-consistent field theory in a two-dimensional space. We concentrate on pores with neutral surfaces and invariable diameters, whose rich variety of phases are considered to originate from pure geometric frustration. A series of triangular phase diagrams are constructed by varying the volume fractions for several sets of interaction parameters. These diagrams are sorted into three categories according to their interaction parameters. The confined phase diagrams exhibit several characteristic features that differ from those observed in the bulk. First, a rich variety of geometric frustration phases with strong symmetries, such as cylindrical and square, are observed in the triangular phase space because of the symmetry constraint in the geometric boundary. Second, the frustrated phases present some novel and complex features compared with those in the bulk, demonstrating that the confined morphologies much more sensitively depend on the subtle variation in the binary interaction parameters than those in the bulk. Meanwhile, the entropic energies of the symmetric melts with equal end block volume fractions are investigated to further understand the geometric frustration phase behaviors in the triangular phase diagrams. The reasonable formation mechanisms of the frustration phases are also discussed.     

Thermodynamic assessment within the Zr–B–C–O quaternary system

Thermodynamic modeling of Zr–B–C–O quaternary system is conducted within the CALPHAD framework by employing data obtained from first-principle calculations and literature. The lower order binary B–O is assessed in this work by estimating the thermodynamic properties of stable solid phases of B₂O₃ and B₆O and by estimating the gas and liquid phases. First-principle calculations, in conjunction with special quasirandom structure were used to predict enthalpies of mixing for the ternary solid-solution phase of FCC-Zr(C, O). The calculated results were used to optimize the model parameters pertaining to the cubic phase, which is described by a two-sublattice model. The modeled Zr–C–O ternary phase diagrams calculated at 1923 and 2273 K under ambient pressure and 4 Pa, respectively, are in agreement with experimental phase diagrams.