Models and Experiments for Sodium Evaporation From Sodium-Containing Silicate Melts

Universal simulation models based on (a) validated mass transfer relations and (b) thermodynamic modeling procedures for glass melts are developed to predict the evaporation rates of volatile species from a large range of glass melt compositions. Depending on the glass composition, temperature of the surface of the melt, local composition of the atmosphere, exposure time of a melt layer to the combustion atmosphere, and local gas velocities above the glass melt surface, the evaporation rates of volatile species can be estimated. Laboratory-scale transpiration evaporation experiments have been used to study evaporation kinetics, to derive mass transfer relations, as well as to validate the sodium evaporation modeling results for sodium-silicate melts as well as for soda-lime-silicate melts. In these investigations, the molten sodium-silicate and soda-lime-silicate melts are exposed to atmospheres of flowing gases with controlled composition and gas flow rates. In a humid atmosphere for example, sodium mainly evaporates as NaOH. From the measured NaOH evaporation rates and the mass transfer relations, the NaOH vapor pressures in equilibrium with the molten glass at prevalent temperature and furnace atmosphere composition were derived. The NaOH vapor pressures are assumed to be in equilibrium with the glass melt composition at the surface of the melt. During the evaporation test, the Na₂O surface composition will change due to depletion. This leads to equilibrium vapor pressures decreasing with time, reflecting the changing chemical activity at the glass melt surface. The results of evaporation tests for sodium-disilicate and soda-lime-silicate glass melts are shown. Chemical activities derived from these measurements are compared with the results of thermodynamic modeling, using a method based on a glass melt from ideal mixtures of associate (stoichiometric) species of structural compounds with known thermodynamic properties.     

Dissolution Rates of Silicate Glasses in Water at pH 7

Dissolution rates of different glasses in buffered solutions of constant pH of 7 were measured by weight change, profilometry, and ion implantation with Rutherford backscattering. Rates with different techniques agreed within experimental error. Natural obsidian glass dissolved most slowly, at a rate comparable with those of quartz and crystalline aluminosilicate minerals. Commercial soda–lime glass containing alumina dissolved slowly, at about the same rate as vitreous silica; soda–lime silicate glasses both commercial and laboratory without alumina dissolved much more rapidly. Pyrex borosilicate glass dissolved at a rate intermediate between those of soda–lime silicate glasses with and without alumina; at room temperature Pyrex borosilicate glass dissolved about 100 times faster than a commercial soda–lime glass containing alumina. We suggest that surface structure is the main factor determining the relative dissolution rates of silicate glasses. Glasses with transformed surface layers caused by hydration dissolve most rapidly; phase separation and openness of the glass structure are also important factors.     

Sulfate Decomposition and Sodium Oxide Activity in Soda–Lime–Silica Glass Melts

Reaction equilibrium constants for the sulfate decomposition process, which releases oxygen and sulfur oxide gas in soda–lime–silica glass melts, have been determined. The chemical solubility of SO₂, probably in the form of sulfite ions in soda–lime–silica melts, has also been determined. The chemical solubility value of SO₂, dissolving as sulfite, ranges between 0.02 and 0.06 wt% SO ₃ ²⁻ at 1 bar SO₂ pressure in the temperature range of 1600–1800 K. Results of square-wave-voltammetry studies and measurements of the temperature-dependent sulfur retention after the fining process of commercial float glass melts and a model soda–lime–silica melt, with 74 wt% SiO₂, 16 wt% Na₂O, and 10 wt% CaO, are presented. The measured sulfur retention data and the results of the square-wave-voltammetry studies are used to determine the equilibrium constant of the sulfate decomposition reaction in the temperature range of 1600–1800 K. The thermodynamic relations and properties found for sulfate decomposition are used to derive activities of sodium oxide in soda–lime–silica melts. Literature values for sodium oxide activities in these glass melts are rare. In this study, these activities have been determined by a method, based on the measurement of sulfate decomposition equilibrium constants and the residual sulfate concentrations in glass melts, equilibrated with almost pure sodium sulfate galls.     

Radiation Characteristics of Glass Containing Gas Bubbles

In many materials processing and manufacturing situations such as steel, aluminum, ceramics, and glass, gas bubbles can form in liquid and solid phases. The presence of such bubbles affects the thermophysical properties and radiation characteristics of the two-phase system and hence the transport phenomena. This paper presents a general formulation of the radiation characteristics of semitransparent media containing large gas bubbles (bubble radius is much larger than the wavelength of radiation). Sample calculations for the spectral absorption and extinction coefficients and single scattering albedo of soda–lime silicate glass containing bubbles are discussed. Particular attention is paid to the effect of the volumetric void fraction and the bubble size distribution. Results clearly show that the presence of bubbles strongly affects the radiation characteristics of the semitransparent media containing entrapped gas bubbles, particularly if bubbles, void fractions, and the spectral absorption coefficient of the continuous phase are small.     

强制气流作用下溶液蒸发过程的传质规律

针对强制气流作用下溶液的蒸发过程,在分析气液相间力学特性的基础上,根据Levich涡流衰减理论和边界层理论,将气流流动状态与相间传质结合,研究了湍流气流横掠液面过程中气液相间的传质变化规律,分析了雷诺数、气流流道结构及普朗特混合长度对传质的影响,得到了气液相间的湍流质扩散系数的变化规律,湍流气流横掠液面条件下的对流传质准则数Sh=0.221Sc1/3Rex1/2. 结果表明,湍流扩散系数、传质系数与气流的流动状态密切相关,气流流道结构对气液相间的传质有重要影响.     

Vaporization of elemental mercury from molten lead at low concentrations

Experiments were conducted to measure the rate of vaporization of elemental mercury from molten lead to provide a basis for estimating radiological source terms for the APT (Accelerator Production of Tritium project) lead blanket. These data also have application to other accelerator targets in which mercury may be created by proton spallation in lead. Molten pools of lead with from 0.01% to 0.10% mercury were prepared under inert conditions. Experiments were conducted which varied in duration from several hours to as long as a month to measure the mercury vaporization from the lead pools. The melt pools and gas atmospheres were controlled at 340 degrees C during the tests, above the melting temperature of lead. Parameters which were varied in the tests included the mercury concentrations, gas flow rates over the melt, circulation in the melts, gas atmosphere compositions and the addition of aluminum to the melts. The vaporization of mercury was found to scale roughly linearly with the concentration of mercury in the pool. Variations in the gas flow rates were not found to have any effect on the mass transfer, however circulation of the melt by a submerged stirrer did enhance the mercury vaporization rate. The rate of mercury vaporization under a high-purity argon atmosphere was found to exceed that for an air atmosphere by as much as a factor of from ten to 20; the causal factor in this variation was the formation of an oxide layer over the melt pool with the air atmosphere which retarded mass transfer across the melt-atmosphere interface. Aluminum was introduced into the melt to investigate its effect upon the mercury vaporization rate. No effect was observed for a case under a high-purity argon atmosphere, which suggests that there are no chemical effects of the aluminum on the vaporization kinetics. With an air atmosphere, the presence of aluminum in the melt reduced the mercury vaporization by a factor of six in comparison to the identical test but without aluminum, suggesting that aluminum in the lead/ mercury melt retards the vaporization of mercury by creating a surface oxide layer in addition to the lead-oxide layer or by changing the character of the lead-oxide layer, thereby increasing the mass transfer resistance.     

VAPORIZATION OF ELEMENTAL MERCURY FROM MOLTEN LEAD AT LOW CONCENTRATIONS

Experiments were conducted to measure the rate of vaporization of elemental mercury from molten lead to provide a basis for estimating radiological source terms for the APT (Accelerator Production of Tritium project) lead blanket. These data also have application to other accelerator targets in which mercury may be created by proton spallation in lead. Molten pools of lead with from 0.01% to 0.10% mercury were prepared under inert conditions. Experiments were conducted which varied in duration from several hours to as long as a month to measure the mercury vaporization from the lead pools. The melt pools and gas atmospheres were controlled at 340 degrees C during the tests, above the melting temperature of lead. Parameters which were varied in the tests included the mercury concentrations, gas flow rates over the melt, circulation in the melts, gas atmosphere compositions and the addition of aluminum to the melts. The vaporization of mercury was found to scale roughly linearly with the concentration of mercury in the pool. Variations in the gas flow rates were not found to have any effect on the mass transfer, however circulation of the melt by a submerged stirrer did enhance the mercury vaporization rate. The rate of mercury vaporization under a high-purity argon atmosphere was found to exceed that for an air atmosphere by as much as a factor of from ten to 20; the causal factor in this variation was the formation of an oxide layer over the melt pool with the air atmosphere which retarded mass transfer across the melt-atmosphere interface. Aluminum was introduced into the melt to investigate its effect upon the mercury vaporization rate. No effect was observed for a case under a high-purity argon atmosphere, which suggests that there are no chemical effects of the aluminum on the vaporization kinetics. With an air atmosphere, the presence of aluminum in the melt reduced the mercury vaporization by a factor of six in comparison to the identical test but without aluminum, suggesting that aluminum in the lead/ mercury melt retards the vaporization of mercury by creating a surface oxide layer in addition to the lead-oxide layer or by changing the character of the lead-oxide layer, thereby increasing the mass transfer resistance.     

Mechanism of Mechanical Strength Increase of Soda–Lime Glass by Aging

Two models have been proposed to explain the mechanical strength increase of abraded or indented soda–lime glasses upon aging, namely, crack tip blunting and the release of residual tensile stress near the crack tip. To clarify the mechanism, the time dependence of the strengthening of an abraded soda–lime glass was investigated. Effects of aging media, such as moist air, distilled water, 1 N HCI and 1 N NaOH solutions, as well as the abrasion flaw depth, were determined. The strength increase rate in water of abraded soda–lime glass was compared with those of borosilicate and high-silica glasses. The effect of stressing during aging was also investigated. It was found that the rate of strength increase was faster with decreasing abrasion flaw depth and with decreasing chemical durability. For a given flaw depth, an acidic solution produced the fastest strengthening. The strengthening rate was found to accelerate because of the "coaxing'effect of stressing during aging. From these observations, it was concluded that the strengthening rates relate to the diffusion process and chemical reactions, especially the alkali–hydrogen (or hydronium) ion-exchange reaction, near the crack tip. The role of the residual tensile stress appears to be similar to that of the applied tensile stress, helping the diffusion process near the crack tip. The observed strength increase of soda–lime glass by aging was thus attributed to the effective blunting of the crack tip geometry by the glass–water reaction.     

Indentation Creep of Hydrated Soda–Lime Silicate Glass Determined by Nanoindentation

Time-dependent deformation behavior was investigated for soda–lime silicate glass with various water contents, using a nanoindentation technique. The complete indentation curve, loading and unloading part, is analyzed. It is shown that this deformation behavior may be represented in terms of a simple mechanical model analogous to a viscoelastic system. Values for Young's modulus were derived, a retardation spectrum was deduced, and apparent viscosity values were calculated. Structural rearrangements of the glass appear to be responsible for the observed changes of the viscoelastic properties. Water in the glass reduces Young's modulus and yield stress and thus promotes viscous flow.     

Recycle of Waste Glass into "Glass–Ceramic Stoneware"

The reuse of soda–lime–silica scrap or waste glasses as additives for traditional ceramics has been investigated extensively in the literature. Although interesting, this solution does not generally allow large quantities of glass to be recycled. This study reports a novel high glass recycle approach that replaces, in the formulation of porcelain stoneware, the feldspar flux with finely powdered glass derived from the melting of different waste products, e.g. lime from fume abatement systems, feldspar mining residues, and scrap soda–lime glass. At an optimized glass/clay ratio, the "glass–ceramic stoneware" samples sinter at 1000°C. The "glass–ceramic stoneware" has a bending strength approaching 90 MPa and a fracture toughness exceeding 2.0 MPa·m^0.5, similar to those of conventional porcelain stoneware, which requires sintering at higher temperatures. The high strength and fracture toughness are attributed to the interaction between the glass and clay residues upon sintering, which allows the development of several different crystalline phases.     

Fundamentals of melt rheology,heat generation,and heat transfer as applied to polymer processing

The flow of polymer melts is normally laminar, but pseudoplastic in character. That is, flow rate increases in proportion to a power higher then 1, usually between 1.5 and 4, of the applied pressure difference. Viscosities are extremely high—more then a mollion times that of water—so considerable heat is generated in the flowing melt by viscous dissipation of flow energy. This mode of heat generation is put to good practial use in the screw extruder, a device that converts plastic pellets into hot melt for shaping into pipe, sheets, coatings and molded products. Equations are given for computing flow rates and viscous dissipation rates. Heating (except by viscous working) and cooling of polymers are slow processes because all these materials are poor heat conducts nad their extreme viscosities make convection impracticable. The princliples of heat transfer are reviewed, and the solutions are given for some transient-conduction problems frequenlty encountered in processing. Equations are given for judging the operation of extruders.     

In Situ Cube-Corner Indentation of Soda–Lime Glass and Fused Silica

Indentation fracture with a cube-corner diamond pyramid on soda–lime silicate glass and fused silica is investigated during the entire indentation cycle in both silicone oil and ambient-air environments. Radial cracks form immediately on loading in all cases. The two-component, elastic-contact + elastic-plastic mismatch (residual) stress field model that has been used successfully to describe radial crack evolution at Vickers indentations fails to describe the fracture response with the cube-corner. The amplitudes of both elastic-contact and residual stress-intensity factors as deduced from these cube-corner experiments are up to a factor of 10 greater than have been previously observed.     

Phase Equilibria in Subsystems of the Quaternary Reciprocal System Na2O-SiO2-Al2O3-NaF-SiF4-AlF3

The phase relations in the pseudobinary system sodium fluoride-mullite have been investigated as a model system for fluoride attack on fire clay refractories in aluminum electrolysis cells. The phase composition below the solidus temperature 857°C changes from sodium fluoride, cryolite, nepheline, and ß-alumina to cryolite, albite, silica, and α-alumina with increasing oxide content. Albite was not observed to crystallize and an increasing amount of glass was observed with increasing mullite content. The phase diagram of the ternary system sodium fluoride-cryolite-nepheline was also measured in order to describe the composition of the most stable melt along the sodium fluoride-mullite composition join. The present findings are discussed in relation to the deterioration mechanism of fire clay refractories during fluoride attack. The formation of viscous albite-based melts is suggested to increase the resistance toward fluoride attack due to the reduced diffusion rate of fluorides.     

Blistering phenomenon of molten glass in contact with zirconia-based refractories

The contact between a glass melt and some refractories in glass industry is able to induce a blistering phenomenon, which is detrimental for obtaining high quality glasses. The objective of this study is to define the key parameters of the blistering process and to propose a mechanism for blistering of soda–lime–silica glass melts in contact with fused-cast zirconia-based refractories. The roles of oxygen semipermeability through the zirconia skeleton and of temperature are emphasized.     

Interrelation of Diffusion and Thermodynamic Characteristics in Silicate Glass and Glass-Forming Melts

A correlation was established between the character of the concentration dependence of the self-diffusion coefficients and the deviation from ideality for mixed alkali silicate glass and melts and a melt containing alkaline and alkaline-earth cations.     

Enhancement of mechanical properties and chemical durability of Soda-lime silicate glasses treated by DC gas discharges

We report for the first time a study on non-contact thermal poling of soda lime silicate glasses using DC gas discharge. In this work, the formation of a glow discharge is evidenced during the thermal poling treatment (longer than 30 minutes). The hardness and the chemical durability of glasses poled under different conditions (contact or non-contact) and atmospheres (nitrogen or air) are measured and compared to that of un-poled reference glass. The results reveal enhanced mechanical and chemical properties for samples poled under nitrogen as compare to air poled or soda lime silicate glass samples. A structural and chemical analysis of surface of the glass using IR-reflectance measurement and ToF-SIMS is also presented. The formation of a “silica-like” layer on the surface of nitrogen poled glasses is observed, which is likely associated with the enhancement of surface properties. On the other hand, the introduction of protons beneath the surface of glasses poled under air leads to the formation of a hydrated alkaline earth silica layer. Based on the observations a mechanism behind the sustainability of the plasma under DC conditions is proposed.     

Das Scaling‐Down‐Problem bei der Zweistoff‐Gaszerstäubung von Metallschmelzen

The production capacities of plants for metal pulverisation are frequently rated according to the minimum throughput of melt flowing from the distributor crucible to pulverisation. Especially in the case of high‐melting point metals and their alloys deficiencies occur in the thermal balance of the crucible in the exit section as a result of gas expansion at the pulverisation gas jets, which act as heat sinks. Minimum throughputs and associated limiting values of the convective heat transfer of the melt are stipulated in order to prevent "freezing" (solidification of the melt) in the crucible. The pertinent situation is illustrated for copper and steel melts and technical possibilities for compensating for heat deficiencies by inductive heating of the distributor exit are presented. In addition, the demand for minimum throughputs can be abandoned, and there result possibilities of scaling‐down and energy conservation as well as improvement of powder discharge.     

Shear viscosity of nylon 6 melts reinforced with microfibrous calcium silicate hydrate

Steady shear viscosity of nylon 6 melts reinforced with xonotlite, microfibrous calcium silicate hydrate (6CaO · 6SiO₂ · H₂O), is investigated. The highly filled nylon 6 melt tends to exhibit a yield value, resulting in remarkable viscosity increase particularly at low shear rates. Addition of the xonotlite significantly increases activation energy of viscous flow of the nylon 6 melt, leading the viscosity to be strongly temperature dependent. Comparisons with the melts filled with glass fibers and wollastonite are made. Flow-induced orientation becomes more important at low volume fractions. Shortening of the xonotlite during shear flow measurement can also be observed.     

Surface Crystal Formation During Acid Corrosion of Phosphate-Doped Lead Silicate Glass

The effect of small P₂O₅ additions on the acid corrosion behavior of lead silicate glass was studied at 22°C. Crystals of penta-lead phosphosilicate, a phase heretofore synthesized only by high-temperature melt techniques, formed on the surface of the phosphate-doped glass during the corrosion process. No crystals were observed to form on the pure lead silicate glass. Crystallization appears to result from the controlled release of phosphorus and lead at the glass surface and/or from the reactive catalytic properties of a silica gel surface film. Apparent lead diffusion rates into the acid solution for the phosphate-doped lead silicate glass are much less (by a factor of 11,3) than for the pure lead silicate glass. Application of these findings is suggested in two areas: formulation of low cation-release glass and glaze compositions and the synthesis of high-temperature phases in aqueous solution at ambient temperatures.     

PHOSPHATES IN CERAMIC WARE: II,ROLE OF PHOSPHORUS IN BONE CHINA*

Based on the behavior of phosphates in silicate melts, a possible picture of the constitution of bone-china bodies is presented. The characteristic features are the formation of fluorapatite and hydroxyapatite as the chief constituents of the rigid framework and of a glass high in lime having a higher refractive index than the glassy matrix of normal porcelain bodies. The translucency, the whiteness, and certain characteristic defects of bone china may be explained on this basis.