FACTORS AFFECTING THE DEGREE OF HOMOGENEITY OF GLASS*

The rate of homogenizing of soda-lime-silica glass under conditions of minimum convection mixing and in the absence of refractory materials was measured by the centrifuge density spread technique developed by Turnbull and Ghering. Measurements on melts prepared in pure platinum containers at 2550°F. showed that the density spread decreases rapidly during the first 16 hours of melting following which further decrease occurs at a much slower rate. The portion of glass responsible for the major part of the density spread was a light fraction, which tended to form or accumulate at the surface of the melt.     

EFFECT OF TEMPERATURE ON HOMOGENIZING RATE OF SODA-LIME-SILICA GLASS*

Density-spread determinations were made on a series of melts of a soda-lime-silica glass prepared at temperatures ranging from 1232° to 1454°C. for a four-hour period in the absence of furnace refractory and under conditions of minimum convection mixing. A sharp drop in density spread in the range 1232° to 1288°C., a minimum at about 1325°C., and a slowly increasing density spread at temperatures above 1325°C. were observed. The portion of glass responsible for the major part of the density spread was found to be concentrated in the top half of the glass at both 1232° and 1454°C. At the higher temperature, the improvement in the homogeneity of the bottom layer was more marked than that observed in the top layer as compared with the results obtained on the lower-temperature melt. The density spread of the glass in the bottom layer in the case of the high-temperature melt was in the range of that observed for commercial soda-lime-silica glass of good quality. A possible explanation for the observed increase in density spread at higher temperatures (above 1325°) is offered on the basis of more rapid initial segregation during melting at higher temperature as inferred from the density-spread data and from chemical analyses of the glass from the top and bottom portions of melts made at 1232° and 1454°C. Loss in weight of the glass due to volatilization was determined at 1232° and at 1454°C. and was found to amount to less than 0.01% for a four-hour melting period at both temperatures. Moderate mixing, achieved by repeated cracking and remelting of the glass and by melting in a rotating tilted crucible, had a marked effect in lowering the density spread. The data afforded by these experiments lend further emphasis to the view that convection currents in commercial tank operation are highly significant in improving the homogeneity of a soda-lime-silica glass.     

X-ray imaging of a high-temperature furnace applied to glass melting

The dynamics of soda-lime-silica glass grain melting is investigated experimentally using a nonintrusive technique. A cylindrical alumina crucible is filled with glass cullet and placed into a furnace illuminated by an X-ray source. This glass granular bed is gradually heated up to 1100°C, leading to its melting and the generation of a size-distributed population of bubbles rising in the molten glass. An image processing algorithm of X-ray images of the cullet bed during melting allows the characterization of bubbles size distribution in the crucible as well as their velocity. The introduction of tin dioxide μ-particles in the glass matrix before melting enhances the texture of the images and makes possible the determination of the bubble-induced molten glass velocity field by an optical flow technique. The bubble size distribution can be fitted by a log-normal law, suggesting that it is closely related to the initial size distribution in the cullet bed. The liquid motion induced by the bubbles in Stokes' regime is strongly affected by the flow confinement and the determination of bubble rising velocity along its trajectory unveils the existence of local tiny temperature fluctuations in the crucible. Overall, the measuring techniques developed in this work seem to be very promising for the improvement of models and optimization of industrial glass furnaces.     

EFFECT OF IRON OXIDE ON MELTING OF GLASS*

The addition of small amounts of iron oxide (0.1 to 0.2%) to soda-lime-silica glass batches exerts a profound influence in increasing the output of glassmelting tanks as well as in favoring the production of higher quality glass. The color produced by this addition, moreover, is not objectionable for many uses of the ware. The accelerated melting rate probably is the result of a chemical effect of iron oxide in the batch and a physical property possessed by such glass to absorb radiation from the flames more efficiently. Attempts to measure these effects were made by rioting the melting rate of glasses which contain varying additions of iron oxide and by determining the temperature gradient that exists in glass when it is melted in a miniature tank. The presence of iron oxide aids melting and fining in crucible melts at 1200°C., but no effect was observed at higher temperatures. The measurement of temperature gradients in a pot holding 45 lb. of glass and heated by flames passing over the glass surface showed that the temperature gradient increases with iron oxide content.     

SUBSTITUTION OF DOMESTIC MINERALS FOR INDIA KYANITE: VI,REFRACTORY PROPERTIES OF GEORGIA MASSIVE KYANITE*

The massive kyanite of Georgia is similar in structure to India kyanite, but it contains quartz with only occasional small amounts of corundum; sericite between the kyanite crystals is common. Excellent coarse grog (67% through 6- on 35-mesh) can be produced from this kyanite. Maximum expansion of the rock during calcining occurs at 1400°C. with slight shrinkage thereafter. Brick were made of the kyanite grog with 3% and 10%, respectively, of EPK (Florida) clay; both had excellent resistance to load at elevated temperatures and met the reheat specifications of the Navy Department and the A.S.T.M. In the panel spalling test, Georgia kyanite brick showed approximately 20% loss, whereas India kyanite brick of the same grain size and clay content showed only 0.3% loss. Intensive prospecting is necessary and the unusual mining and cleaning operations with present known deposits make large-scale commercial operation questionable.     

Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC

The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C.     

Processing of vacuum heat-treated MgO-densified silicon nitride crucible for molten cast iron handling

Silicon nitride with 3% MgO powder mix is uniaxially and cold isostatically pressed to form a green Si₃N₄ crucible. Liquid phase sintering was applied to the green Si₃N₄ crucible at 1600 °C for 30 min under the nitrogen atmosphere. Intergranular Mg–Si–O–N glass remained between the silicon nitride grains which reacted with the molten metal during melting. This grain boundary glass was removed by vacuum heat treatment at 1575 °C for 5 h. The vacuum heat treated crucible was used to melt cast iron to examine reactions between the molten metal and Si₃N₄ ceramic crucible. EDX spectra across the Si₃N₄–cast iron interface and XRD for silicon nitride sample after cast-iron melting side surface analysis were carried out. Optical microscopy and SEM image analysis were made to examine the interaction between Si₃N₄ crucible and cast iron melt. Surprisingly, no reaction was observed between the vacuum heat treated crucible and melted cast iron.     

Diamond film synthesis at low temperature

Sulfur-assisted hot-filament chemical vapor deposition was successfully employed to synthesize diamond films at relatively low substrate temperatures, sub 500 °C, on Mo and glass substrates. This result is ascribed to sulfur-containing species that create an additional carbon transport channel to the substrate, which explicitly requires a large temperature gradient to effectively operate. The grain size and roughness were found to decrease when the Mo substrate temperature was decreased, while the quality factor only showed a slight decrease. Moreover, the diamond quality, grain size, and roughness were found to increase on glass substrates in comparison to those on Mo substrates.     

Foaming in Glass Melts Produced by Sodium Sulfate Decomposition under Isothermal Conditions

The factors controlling the behavior of foam produced in molten glass by sulfate decomposition are sulfate oversaturation (determined by the initial sulfate concentration, fraction of sulfate dissolved, and melting temperature), bubble nucleation (occurring predominantly on the residual silica grains), and film stability. The connection between the foam generation and variation in melting temperature and silica grain size has been experimentally investigated by observing the melting process in a transparent silica crucible. A generation/release model for the time variation of gas-phase volume retained in molten glass has been developed. The analysis of experimental data using this approach suggests that two forms of sulfate produce foam during melting: (1) sulfate dissolved in melt and (2) sulfate from sulfate-rich melt surrounding sand grains.     

Chaotic mixing in electromagnetically controlled thermal convection of glass melt

A numerical investigation has been carried out to study the mixing behavior of electromagnetically controlled thermal convection of glass melt in a cylindrical crucible. Thermal convection caused by the internal heating of the glass melt is controlled by an external magnetic field applied along the axis of the crucible. Unlike in thermal convection without and with steady external magnetic fields, Lagrangian particle motion exhibits chaotic behavior in an oscillating magnetic field. The present study shows that the asymmetric thermal field caused by the gravitational body force is rotated in the clockwise and anticlockwise directions alternatively by the Lorentz force imposed by the oscillating magnetic field. As the magnetic field varies sinusoidally with time, the flow field undergoes periodic reorientation causing repeated stretching and folding of the material lines resulting in better mixing in the glass melt. The degree of mixing increases with the period of oscillation till it reaches a maximum and subsequently decreases with further increase in the period. A decline in the mixing performance is observed with increase in the magnetic field strength for a given period of oscillation. This is on account of the fact that the Lorentz forces try to nullify the asymmetry created by the gravitational body forces. The computational results presented here will be useful for developing better glass homogenization systems.     

METHOD FOR MEASURING FINING TIME OF GLASS*

A method is described for measuring the fining time of glass, and a formula is given for using the data obtained to estimate the tonnage-temperature schedules of continuous tank furnaces. Measurement of fining time at two and preferably three temperatures, in the range 1400° to 1500°C., is required to establish the fining characteristics of a glass. Data from fining-time measurements of four glasses show that fining is largely independent of the viscosity and probably also of the surface tension of glass. The use of fining-time measurements in accounting for differences in the melting efficiencies of tank furnaces and the value of systematic studies, involving fining agents and variations in composition, are suggested.     

提高配合料质量的技术措施

通过我厂在提高配合料质量方面取得的一些成效,系统地阐述了提高配合料质量,在原料组成、原料粒度、原料称量、原料混合等方面采取地切实可行的技术措施及实际控制指标.     

Rapid Grain Growth in 3Y‐TZP Nanoceramics by Pressure‐Assisted and Pressure‐Less SPS

Pressure‐less spark plasma sintering (SPS) is a new approach during which rapid densification of ceramic nanopowder green bodies is accompanied by rapid grain growth. Although the origin of this phenomenon has not yet been fully understood significant, difference in grain growth between pressure‐less and pressure‐assisted SPS was expected. In this work 3Y‐TZP nanopowder with average particle size of 12 nm was consolidated using two‐step approach: (1) at an intermediate temperature (600°C to 1000°C) SPS warm pressing followed by (2) high temperature (1400°C to 1600°C) pressure‐less SPS. The standard one step pressure‐assisted SPS experiments were quoted as references. Rapid grain growth was observed during both pressure‐less and standard SPS. The samples prepared by both approaches at the same sintering temperature (1400°C–1600°C) achieved identical grain size and grain size distribution, if large pores were eliminated in early stage by SPS warm pressing. The electric current, electromagnetic field, and mechanical pressure is proven to have a negligible direct influence on grain growth in 3Y‐TZP ceramics at temperatures above 1000°C under standard SPS conditions.     

TESTS OF FIRE-BRICK MADE FROM GANISTER,FLINT CLAY,AND PLASTIC CLAY MIXTURES,WITH SPECIAL REFERENCE TO SPALLING1

Influence of the alumina-silica ratio on properties of fire-brick.—Five experimental batches of fire-brick were made by mixing various proportions of ganister, flint clay and plastic clay in such a way as to vary the silica content from 53 to 77 per cent and the alumina content from 43 to 20 per cent. (1) The fusion points were found only slightly lower than those of corresponding pure silica-alumina mixtures. (2) Load tests at high temperature showed that the behavior under compression does not depend on chemical composition so much as on other factors such as the temperature of burning. (3) The resistance to spalling , as tested by alternate heating and dipping in cold water, was found to decrease as the temperature of burning was increased from 1300° to 1400 °C. The higher silica bricks were relatively more resistant at the lower temperature but not so at 1400 °C. Therefore the substitution of ganister for flint clay increases the resistance to spalling at moderate operating temperatures but is of no advantage at 1400° or above.     

A comparative study of physico-mechanical,bioactivity and hemolysis properties of pseudo-wollastonite and wollastonite glass-ceramic synthesized from solid wastes

This study aims to synthesize pseudo-wollastonite and wollastonite base glass-ceramic from solid wastes, and a comparative study of their physico-mechanical, bioactivity, and hemolysis properties is conducted. The stoichiometric amount of chicken eggshells derived CaO (~99%), and rice husk ash (RHA) derived nano SiO₂ (~99%) is used as ingredients for the formulation of wollastonite. The pseudo-wollastonite (α-W) is synthesized by sintering the ingredients at 1200 °C for 4 h, and wollastonite glass is made through the quenching method after melting of the ingredients at 1400 °C. The wollastonite glass-ceramic (GC) is prepared by control heat-treatment of wollastonite glass at 1200 °C for 1 h. Numerous characterizations like X-ray diffraction (XRD), scanning electron microscopy (SEM), apparent porosity and bending strength of α-W and GC are comparatively investigated. The GC is composed of the collaborative pseudo-wollastonite nucleation (~35%) and amorphous phases. α-W and GC show the apparent porosity about 4.23 % and 2.77 %, bending strength about 108 MPa and 139 MPa, and an average grain size about 2.30 μm and 0.60 μm, respectively. The obtained results exhibit that the waste-derived both α-W and GC have excellent bioactivity and good hemolysis index (<2%). Moreover, GC shows slightly more hydroxyapatite (HA) layer formation ability than α-W in the simulated body fluid (SBF) solution. This comparative study of α-W and GC is recommended that the GC is offering more promising characteristics for biomedical applications.     

VARIATIONS IN PYROMETRIC CONE EQUIVALENTS OF SILICA CEMENTS AND FIRE CLAYS1

In order to determine what variations might occur due to the method of preparing the sample for the pyrometric cone equivalent test a series of 100 silica cements and five fire clays were tested using the cement “as received,” ground to pass a 65-mesh Tyler screen, and after calcining a t 1400°C grinding to pass a 65-mesh Tyler screen. The pyrometric cone equivalent values were obtained by two operators, the cones being made by one operator. It was found that testing of silica cements “as received” gave closer checks than ground samples. The grinding of the sample lowers the pyrometric cone equivalent 1^1/2to 2 cones in general.     

Determination of Temperature‐Dependent Heat Conductivity and Thermal Diffusivity of Waste Glass Melter Feed

The cold cap is a layer of reacting glass batch floating on the surface of melt in an all‐electric continuous glass melter. The heat needed for the conversion of the melter feed to molten glass must be transferred to and through the cold cap. Since the heat flux into the cold cap influences the rate of melting, the heat conductivity is a key property of the reacting feed. We designed an experimental setup consisting of a large cylindrical crucible with an assembly of thermocouples (TC) that monitors the evolution of the temperature field while the crucible is heated at a constant rate. Then we used two methods to calculate the heat conductivity and thermal diffusivity of the reacting feed: the approximation of the temperature field by polynomial functions and the finite‐volume method (FVM) coupled with least‐squares analysis. Up to 680°C, the heat conductivity of the reacting melter feed was represented by a linear function of temperature.     

Influence of synthesis conditions on optical losses in aluminoborophosphate laser glass

The influence of the duration of melting, the purity of the reagents used for synthesis, and the material of crucibles on the optical losses in laser neodymium high-strength aluminoborophosphate glass has been investigated. The obtained results have demonstrated that the main reason of emerging of optical losses at the lasing wavelength in the glass under study is the interaction of the crucible material with the aluminoborophosphate melt which is aggressive at a synthesis temperature of 1350°C.     

CALCULATION OF SURFACE TENSIONS OF GLASSES*

Factors have been derived for the calculation of the surface tensions of glasses from their compositions at temperatures of 1200° and 1400°C. The nine oxides evaluated are those commonly found in significant amounts in glass compositions. The surface tension of a glass may be calculated with a fair degree of assurance from its composition to within about 4 dynes per cm.     

Investigation of the high-temperature behaviour of coal ash in reducing and oxidizing atmospheres

The high-temperature behaviour of ashes from a suite of coals exhibiting a wide range of mineralogies has been investigated. Phase analysis of ash samples quenched from various temperatures under either a reducing (60% CO/40% CO₂) or an oxidizing (air) atmosphere was performed by Mössbauer spectroscopy, scanning electron microscopy (SEM)/automatic image analysis (AIA), and X-ray diffraction (XRD). It was found that significant partial melting of the ashes occurred at temperatures as low as 200–400 °C below the initial deformation temperature (IDT) defined by the ASTM ash cone fusion test. Melting was greatly accelerated under reducing conditions, for which the percentage of melted ash increased rapidly between 900 and 1100 °C, saturating at temperatures above ≈ 1200 °C. The observation of such phases as wustite (FeO), fayalite (Fe₂SiO₄), hercynite (FeAl₂O₄), and ferrous glass in samples quenched from 900 to 1200 °C indicates that ash melting in a reducing atmosphere is usually controlled by the iron-rich corner of the FeO-Al₂O₃-SiO₂ phase diagram. Ashes rich in CaS are an exception to this rule, for large quantities of iron sulphide are formed and the melting behaviour is controlled in part by the FeO-FeS phase diagram. Under oxidizing conditions, potassium appears to be the most important low-temperature fluxing element, as the percentage of glass in samples quenched from temperatures below 1100 to 1200 °C was proportional to the amount of the potassium-bearing mineral illite contained in the coal. Above 1200 °C in air, calcium and, to a lesser extent, iron became effective as fluxing elements; melting accelerated between 1200 and 1400 °C, and was near completion between 1400 and 1500 °C for most ashes. To retard ash melting, it is generally concluded that aluminium is the most desirable constituent of ash, whereas the most undesirable constituents are iron, calcium, and potassium.