固体氧化物燃料电池密封材料的研究进展

本文对固体氧化物燃料电池(SOFC)密封材料的研究进展作了介绍,分别对硅酸盐体系、硼酸盐体系、磷酸盐体系以及云母玻璃密封材料的研究进展情况给予阐述,对每种密封材料的特点和存在的问题进行了详细的论述,并指出通过控制玻璃的析出行为制备微晶玻璃密封材料是今后SOFC密封材料的主要研究方向.     

Interrelation between shear modulus and the molecular parameters of viscous flow for glass forming liquids

The relationship between the free energy of viscous flow activation, the instantaneous modulus and the molar volume of kinetic units overcoming the barrier was derived by the author in the simplest form, ΔG^≠ = F_∞V¹, thirty-eight years ago. It was the result of the common solution for most general equations for the viscosity coefficient, the rate constant for shear relaxation in accordance with Maxwell’s relation. Here it is shown that this equation may be derived in the framework of the theory of elasticity and/or hydrodynamics. In this equation, V¹ = 8(r₀)³N_A, where V¹/N_A in the theory corresponds to the cube volume containing an inscribed molecule (atom) of radius r₀. The experimental proof of the equation shows that the atomic radii found from the viscous and elastic parameters match those obtained from direct structural investigations (X-ray and neutron scattering) with an average accuracy not worse than ∼5% (oxide, fluoride and chalcogenide glass melts). The theory needs development for molecular liquids with more complex structure. It is obvious that Maxwell’s relation is valid for modeling of viscous flow at the molecular level in supercooled liquids.     

On the role of fluctuations at the boundary of Earth’s solid core

The fluctuation effects at the boundary between the internal solid and external liquid regions of the Earth’s core are considered within the vitrification model. If the internal core is characterized by a relatively small static shear modulus and shear acoustic oscillations, this phase transition becomes slightly stepwise and can be accompanied by critical phenomena. The corresponding fluctuation corrections to the thermodynamic derivatives are calculated with arbitrary critical indices. The propagation and absorption of seismic waves, which become weakly anisotropic due to the anisotropic “liquid” inclusions (which arise in the hysteretic boundary region due to the Earth’s rotation and viscous stress effect), are considered. The resulting estimates are compared with the existing geophysical data.     

Evaluation of activation energy of viscous flow of sol–gel derived phenyl-modified silica glass

The temperature-induced softening behavior in sol–gel derived phenyl-modified low-melting glass (phenyl glass) was investigated in terms of the activation energy for the viscous flow. The temperature dependence of the relative viscous flow was measured from the falling rate of a needle loaded with a constant weight. The activation energy for the viscous flow of phenyl-modified silica glass was found to be irrespective of the time of drying the sample phenyl-modified silica glass, which directly affects the extent of polymerization. Furthermore, the obtained activation energy was in considerably good agreement with that for the viscous flow of potassium alkali glass, and approximately twice larger than that of linear amorphous polymer (polystyrene). This result suggests the common microstructural feature of glassy materials interspaced by additive substances like Na/K or covalently bonded chemical functions such as phenyl groups.     

The review of possible interrelations between ionic conductivity, internal friction and the viscosity of glasses and glass forming melts within the framework of Maxwell equations

The review contains the results of application of Maxwell equations for mechanical relaxation and electrical conductivity, to the systematization of large amount of experimental data related to mechanical, viscous, and electrical properties of inorganic glasses and glass forming melts. The generalization of internal friction results shows the existence of characteristic values for the ratios of temperatures, responsible for α-, β- and ionic relaxations; they are independent on the frequency and chemical composition. This is the evidence for the main role of elastic deformations at various corpuscular processes and the existence of characteristic scale of activation barriers predetermined by local volumes of deformation. It is shown the possibility of very precise calculation of the temperature of “ionic” internal friction maximum for one-alkali oxide glasses directly from Maxwell equation and d.c. conductivity experimental data. The volumes of particles overcoming the potential barrier at viscous flow practically coincide with the results of direct structural determinations. The existence of universal relation between d.c. conductivity and viscosity for the extremely wide temperature intervals (Littleton relation) is proved for silicate and phosphate melts. The theory of this dependence is proposed. The results show the effectiveness of the attempts to unite the continual and discrete approaches within the framework of Maxwell equations to obtain the simplest understanding the mechanisms of different types of relaxation. The review comprises many Russian papers unknown in English scientific literature.     

Tensile strength of boric oxide glass in vacuum and at very low humidities

The tensile strength of a glass fiber of pure boric oxide was measured at low humidities and in vacuum. The strength increased from 60 kg/mm² in a relative humidity (RH) range of 7% to 120 kg/mm² at 0.4% RH, levelling off to 0.02% RH which was the lower limit of humidity in this experiment. At relative humidities higher than 0.4% weathering and stress corrosion were observed, while at relative humidities lower than 0.4%, weathering was not observed but the decrease of the strength with a dynamic loading duration was observed, although the behavior was somewhat different from the typical stress corrosion. The strength measured in vacuum was almost equal to that measured at relative humidities lower than 0.4%, although the data were more scattered. The strength of boric oxide glass was much lower than that of silica glass, even when weathering and stress corrosion were depressed to the extreme. This weakness of boric oxide glass may be attributed to its layer structure.     

New observations on scratch deformations of soda lime silica glass

In spite of the wealth of literature, the role of the scratching speed in affecting the material removal mechanism in soda lime silica (SLS) glass is yet to be comprehensively understood. Here we report the surface and sub‐surface deformation mechanisms of SLS glass scratched under three different normal loads of 5, 10 and 15 N at various speeds in the range of 100–1000 μm/s with a diamond indenter of ~ 200 μm tip radius. The results show that at any given applied normal load, the width, depth, wear volume of the scratch grooves and wear rate of the SLS glass decreased with an inverse power law dependence on the applied scratching speed. The surface damage also reduced with the increase in scratching speed. A new, simple model was developed to explain these observations. The significant contributions of the time of contact, the tensile stress behind the indenter and the shear stress active just underneath the indenter in governing the material removal mechanisms of the SLS glass were discussed.     

Viscosity, relaxation and elastic properties of photo-thermo-refractive glass

Viscometry, ultrasonic echography and mechanical spectroscopy were applied to explore the thermo-mechanical properties of a photo-thermo-refractive (PTR) glass. Newtonian shear viscosity and ultrasonic velocity were used to determine the Maxwell time of structural relaxation. Two fast relaxation modes are found in PTR glass below glass transition temperature, which are attributed to alkali and mixed cation mobility. Respective activation energies, E_γ = 72 ± 4 kJ mol⁻¹ and E_β = 117 ± 4 kJ mol⁻¹, are considerably lower than that of viscous flow at the glass transition E_α ≈ 465 kJ mol⁻¹ as quantified from mechanical spectroscopy. Decoupling ratios of E_γ/E_α = 0.155 and E_β/E_α = 0.255 are similar to those of sodium trisilicate glass but at considerably longer timescales. These observations imply that β- and γ-relaxations are delayed as local structural arrangements are more complex for the multi-component PTR glass.     

Enhancement in nanohardness of soda-lime-silica glass

For a thin commercial soda-lime-silica glass cover slip a significant increase in nanohardness (e.g. up to 74%) occurred with variations in the loading rate (10-20,000 μN·s^− 1) along with presence of serrations in load-depth plots and deformation band formation inside the nanoindentation cavity. These results are explained in terms of shear stress acting at positions of structural weakness close to the tip of the nanoindenter and the time scale of interaction between the nanoindenter and the local microstructure of the glass.     

Crystallization of silica and titanium oxide-silica corning glasses (codes 7940 and 7971)

The crystallization kinetics of silica glass (Corning 7940) and titanium oxide-silica glass (Corning 7971) was investigated. Special attention was paid to prevent the contamination of the surface of the samples during their heat treatment. It was found that the thickness of the crystallized layer depends linearly on the time of heat treatment for both types of the investigated glasses. The beginning of the crystallized-layer growth is preceded by the quite considerable induction period. The temperature dependences of the induction period and of the rate of crystal growth for silica glass coincide practically and are characterized by the activation energies E_τ ≈ E_u ≈ 160 kcal/mol. For titanium oxide-silica glass E_τ >E_u. The measured value of the activation energy of viscous flow differs considerably from the activation energy of the crystal growth rate. This may be evidence of the fact that the crystallization and shear viscosity depend on different kinetic units of silica glass.     

Glass formation through hydrolysis of Si(OC2H5)4 with NH4OH and HCl solution

Monolithic silica glass was obtained by heating the gels prepared by hydrolyzing Si(OC₂H₅)₄ with NH₄OH and HCl solution. The effect of the condition of hydrolysis of Si(OC₂H₅)₄ on glass formation was examined by measuring the bulk density, the infrared spectra and the thermal shrinkage of the gel on heating. The gel prepared by hydrolysis with NH₄OH solution consisted of numerous spherical particles, the bulk density being about 0.8. This gel abruptly shrank at about 1050°C, being converted to the pore free material similar to fused silica. The conversion of the gel to glass followed the sintering model in which the viscous flow controlled the sintering process. The viscosity and the activation energy for viscous flow were calculated on the basis of the Frenkel equation. On the other hand, the spherical particles were not observed in the gel prepared by hydrolysis with HCl solution. The bulk density of the gel was about 1.8. This gel was converted to glass at about 700°C, which was lower than the temperature of glass formation for the gel obtained by hydrolysis with NH₄OH solution.     

BaO–B2O3–SiO2–Al2O3 sealing glass for intermediate temperature solid oxide fuel cell

Glasses in the formulation close to BaSiO₃–BaB₂O₄ eutectic compound are developed for sealing of intermediate-temperature (500–650 °C) solid oxide fuel cell (IT-SOFC). Thermal and microstructural analyses of the glasses with 0–10 mol% Al₂O₃ are also conducted. Detailed crystallization kinetics and interfacial stability of the glass in contact with yttria-stabilized zirconia (YSZ) and samaria-doped ceria (SDC) are investigated and compared. The results show that the formulation, 47BaO–21B₂O₃–27SiO₂–5Al₂O₃ (G1A5), performs the best on glass forming ability (GFA) among all tested formulations, and shows matched thermal expansion and working temperature to CeO₂-based electrolytes of IT-SOFC. Two major crystalline phases that precipitate from G1A5 above 750 °C are platy hexacelsian and BaSiO₃ grains.     

Sintering process of glasses in the system Na2O-B2O3-SiO2

In this work, the sintering process of different glasses in the system Na₂O-B₂O₃-SiO₂ has been studied. The studied compositions are suitable for sealing the gas manifolds of molten carbonate fuel cells. Sealing glasses are usually applied on the surfaces to be sealed using powder glass mixed with an organic medium. The agglomerant elimination and the sintering of the glass powder take place during the thermal treatment. Three different particle sizes of glass powder and different sintering temperatures and times have been used to reveal the influence of the specific surface area and viscosity on sintering. The control of these parameters allows optimization of the sealing conditions of the glasses. Dense materials have been characterized as well as the sintering mechanism. Two processes take place during the thermal treatment: the sintering process and the quartz crystallisation. Both processes act in opposite directions on the glass densification. Crystallisation is the dominant process at long times and high temperatures. The viscous flow Scherer model has been adequately applied to the experimental data.     

Development and characterizations of BaO-CaO-Al2O3-SiO2 glass-ceramic sealants for intermediate temperature solid oxide fuel cell application

Several compositions based on BaO-CaO-Al₂O₃-SiO₂ (BCAS) glass system have been studied in this investigation to see their applicability as sealant for solid oxide fuel cell (SOFC). The glasses as well as the corresponding glass-ceramics have been systematically characterized by differential thermal analysis, dilatometry, X-ray diffractometry, electron microscopy and impedance analysis to examine their suitability as sealant. While the glass transition temperature (T_g) determined from DTA are within 600-665 °C, the coefficient of thermal expansion (CTE) can be tailored between 9.5 and 13.0 × 10⁻⁶ K⁻¹. These glasses are found to be well adhered with metallic interconnects, such as commercial ferritic steel (Crofer22APU), at an optimum sealing temperature of 850 °C. The shrinkage behavior of the developed glasses in their pellet form has also been investigated. The resistivities of the glass-ceramics, as obtained from impedance analysis, are found to be within 10⁴-10⁶ Ω cm at 800 °C. Under sandwiched condition between two metals, some of the developed compositions are found to maintain this high resistivity even after 100 h of operation. One of the glass compositions has shown a low leak-rate of the order of ∼10⁻⁷ Pa m² s⁻¹.     

Role of marangoni tension effects on the melt convection in directional solidification process for multi-crystalline silicon ingots

We carried out global simulations to investigate the marangoni tension effect on the thermal and flow fields in the silicon melt of the directional solidification process for multi-crystalline silicon ingots. The argon flow rate was varied to provide different solidification conditions and to change the relative values between the argon shear stress and the marangoni tension at the melt free surface. We found that the marangoni tension together with the shear stress mainly influences the upper layer melt convection while the thermal buoyancy force dominates the bulk flow of the melt. At low argon flow rates, the argon shear stress can be neglected and the marangoni tension alone enhances the melt convection intensity near the gas–melt–crucible triple junction point. The marangoni tension is so weak that it cannot modify the melt flow pattern in this case. For medium flow rate, the marangoni tension can significantly weaken the shear stress effect at the outer part of the melt free surface, leading to a distinctive flow pattern in the silicon melt. With further increase in argon flow rate, the shear stress sharply increases and dominates the upper layer melt flow, limiting the marangoni tension effect to the triple point. The numerical results are helpful for better understanding and controlling of the directional solidification process for high quality multi-crystalline silicon ingots.     

Precipitation of intermetallic phase in Zn‐Ti alloy single crystals

Pure and 0.14 at.%Ti alloyed Zn single crystals have been grown by the Bridgman technique. The alloyed crystal exhibits a sheet‐like precipitation structure parallel to the basal plane of the hexagonal solid solution Zn‐Ti matrix. The tetragonal precipitates are intermetallic compounds of Zn₁₆Ti composition. They are needle‐shaped with the needle axis peferentially aligned along the <a> axis. The approximate orientation relationship between precipitate and matrix is (0001)<11$\bar 2$ 0>_Matrix||(010)[001]_Zn16Ti. To study the influence of alloying on the mechanical properties, the critical resolved shear stress for basal slip has been measured in compression at different temperatures and strain rates. It is shown that the critical resolved shear stress can be very well explained by theories of solid solution and precipitation hardening. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relaxations in polystyrene below the glass transition studied by viscosity measurement

The shear viscosity of organic glass polystyrene has been determined under pure shear deformation mode from room temperature up to the glass transition temperature. A mechanical model of series connection of anelasticity and viscosity was used to determine the viscosity of the material. Relaxation time for the viscous flow was determined as a function of temperature. The relaxation was composed of two thermal-activation type relaxation processes: the high temperature relaxation (HTR) and the low temperature relaxation (LTR). In both relaxations the relaxation time was represented as τ=τ_o exp(E/k_BT), and the values of τ₀ and E were different in specimens treated differently – aged, loaded, and annealed. The observed τ₀ and E were not independent of each other but a compensation effect, a linear decrease of logτ₀ with increased E, was seen. The results were explained using the idea of cooperative relaxations of relaxing elements. HTR and LTR were considered to correspond to the structural and the slow relaxations, respectively, and the relaxing elements could respectively be a single atom or molecule and a segment in molecular chains.     

Dimensional changes of silica-, borosilicate- and germania-glasses and quartz under irradiation

Length changes of crystalline SiO₂, of two SiO₂ glasses (HERASIL1, SUPRASIL1), of two SiO₂–B₂O₃ glasses (DURAN, VYCOR) and of vitreous GeO₂ were measured during proton, electron and UV irradiations under various applied tensile stresses at temperatures up to 400 °C. No change was observed under UV irradiation, while proton and electron irradiation initially caused transient compaction and subsequent irradiation induced creep with linear stress dependence (viscous flow). Asymptotic compaction strain is reduced at elevated temperatures due to recovery processes, while creep rate is virtually independent of temperature. Compaction tends to decrease while creep rate increases with decreasing SiO₂ content of the glasses. A comparison of flow rates to electronic and nuclear stopping power gives no safe discrimination of the underlying mechanisms. Quartz shows continuous dilatation under proton irradiation, with a very low stress dependence, which allows only determination of an upper limit of viscous flow compliance.     

Stabilization of color conversion process of various colors in glass production

A new color conversion process has been proposed for minimizing the glass defect occurring during the color conversion caused by the glass flow turbulence in glass melting furnace. Glass flow turbulence is caused by the variation of bottom furnace temperature and also due to the change of high temperature thermal conductivity of glasses. Iron oxide was examined as an additive to control the high temperature conductivity of glasses. The effect of iron oxide on the high temperature thermal conductivity was examined. Based on these results, optimum content of iron oxide needed to keep the high temperature conductivity constant between the glasses of different colors was determined. Finally color coordinate of glasses which contained estimated optimum iron oxide content was also examined to check if it can be used as a panel of cathode ray tube.     

Redox behavior of platinum-group metals in nuclear glass

This study highlights the role of two platinum-group metals, ruthenium and palladium, in the redox equilibria of nuclear waste containment glass. Electrochemical measurements in simplified R7/T7 glass melts were used to develop a thermodynamic model of ruthenium redox equilibrium. The oxygen fugacity at equilibrium, corresponding to the coexistence of ruthenium oxide and ruthenium metal dispersed in the molten glass, was measured at different ratios at temperatures ranging from 1000 °C to 1200 °C. Experiments were carried out on glass with and without Pd, revealing the combined role of palladium and tellurium on redox equilibria in the glass. The formation of palladium-tellurium alloys in nuclear glass was observed to result in oxidation of the elements dissolved in the melt.