Thermal Analysis of Reactions in Soda–Lime Silicate Glass Batches Containing Melting Accelerants: I, One- and Two-Component Systems

To identify each glass melting reaction in a multicomponent system, one-component and two-component reaction processes were studied using DTA, TGA, and XRD. Two-component mixtures were prepared by choosing pairs in the same ratio as in a commercial container glass batch composition (sand-soda ash-calcite-dolomite-feldspar). The presence of silica in the silica-calcite system decreased the termination temperature of the decomposition of calcite, but did not alter the onset of decomposition. Similar behavior was found in the dolomite-silica system. A double carbonate (Na₂Ca(CO₃)₂) formed via solid-state reaction in the calcite-soda ash system, and metasilicate/disilicate phases were detected during the fusion process in the soda ash-silica system. The effects of reaction accelerant additions in the soda ash-silica system were investigated using 1 wt% additions of sodium sulfate, sodium nitrate, and sodium chloride. Sodium chloride was the most effective melting accelerant, lowering the termination temperature of CO₂ release by ∼80°C compared with the soda ash-silica system with no additives. NaCl additions caused complete reaction and/or fusion of Na₂CO₃ prior to its melting temperature. Sodium sulfate additions acted to suppress metasilicate/ disilicate formation by coating quartz grains and shifted consequent CO₂ release to higher temperature.     

Low-Temperature Synthesis of CaZrO3 Powder from Molten Salts

Calcium zirconate (CaZrO₃) powder was synthesized using calcium chloride (CaCl₂), sodium carbonate (Na₂CO₃), and zirconia (ZrO₂) powders. On heating, CaCl₂ reacted with Na₂CO₃ to form NaCl and CaCO₃. NaCl–Na₂CO₃ molten salts provided a liquid reaction medium for the formation of CaZrO₃ from in situ -formed CaCO₃ (or CaO) and ZrO₂. CaZrO₃ started to form at about 700°C, increasing in amount with increasing temperature and reaction time, with a concomitant decrease in CaCO₃ (or CaO) and ZrO₂ contents. After washing with hot-distilled water, the samples heated for 5 h at 1050°C were single-phase CaZrO₃ with 0.5–1.0 μm grain size.     

Phase-Pure Mullite from Kaolinite

Sintering of kaolinite in the presence of certain carbonate mineralizers, viz., CaCO₃, Na₂CO₃, and K₂CO₃, has been conducted at 950°–1350°C. The influence of these mineralizers at the above temperatures is evaluated using XRD and SEM techniques. A comparative study of phase formation of the above compositions shows that the sodium- and calcium-fluxed samples give rise to multiphase systems, while K₂CO₃-incorporated samples give phase-pure mullite. The observation that kaolinite in the presence of K₂CO₃ can act as a precursor material for phase-pure mullite is of great industrial significance.     

Refractory Selection for High-Temperature Black Liquor Gasification

A methodology has been established for the selection of materials for refractory linings in black liquor gasifiers that are to be employed by the pulp and paper industry. As a first step, a thermodynamic software package was used to determine that black liquor smelt was composed of a liquid solution of primarily Na₂CO₃ and Na₂S under the operating conditions of the gasifier (950°C, 1 atm). Next, the software was used to predict the interaction of the black liquor smelt with various ceramics such as aluminosilicates, single component oxides, and binary oxides that are candidates for the application. Finally, experiments were performed to verify or disprove the predictions. Using sessile drop testing, contact angles were determined for molten Na₂CO₃ on candidate refractory compounds. All of the candidate ceramics were wet by molten Na₂CO₃. Among the candidates, MgAl₂O₄ was found to have the highest contact angle (∼13°). Post-mortem analysis was performed on sessile drop specimens using X-ray diffraction analysis and scanning electron microscopy with energy dispersive spectroscopy to determine if molten Na₂CO₃ had penetrated into or reacted with the ceramics. From the candidates, MgO and CeO₂ were found to have the best resistance to attack, while MgAl₂O₄ was also found to be a promising candidate.     

Effect of Calcination Conditions and Excess Alkali Carbonate on the Phase Formation and Particle Morphology of Na0.5K0.5NbO3 Powders

Sodium-potassium niobate [Na_0.5K_0.5NbO₃] powders were prepared following the conventional mixed oxide method. An orthorhombic XRD pattern, consistent with single-phase Na_0.5K_0.5NbO₃, was obtained after calcination at 900°C for 6 h. Introducing 5 mol% excess Na₂CO₃ and K₂CO₃ into the starting mixture allowed milder calcination conditions to be used, for example 800°C for 2 h. Primary particles in 5 mol% excess samples were cuboid, with maximum sizes of ∼2.5 μm. Equiaxed 0.3–0.4-μm particles were formed for non-excess powders, and also for powders prepared with 1 and 3 mol% excess alkali carbonates. The results suggest liquid formation during calcination of the excess 5-mol% starting powders.     

Kinetics and Mechanism of Corrosion of SiC by Molten Salts

Corrosion of sintered α-SiC under thin films of Na₂CO₃/CO₂, Na₂SO₄/O₂, and Na₂SO₄/SO₃ was investigated at 1000°C. Chemical analysis was used to follow silicate and silica evolution as a function of time. This information couples with morphology observations leads to a detailed corrosion mechanism. In all cases the corrosion reactions occur primarily in the first few hours. In Na₂CO₃/CO₂ case, rapid oxidation and dissolution lead to a thick layer of silicate melt in ∼0.25 h. After this, silica forms a protective layer on the carbide. In the Na₂SO₄/O₂ case, a similar mechanism occurs. In the Na₂SO₄/SO₃ case, a porous nonprotective layer of SiO₂ grows directly on the carbidem and a silicate melt forms above this. In addition due to reaction of silicate with SO₃ and SO₂+O₂. The reaction slows when the lower silica layer becomes nonporous.     

Melting Reactions of Soda-Lime-Silicate Glasses Containing Sodium Sulfate

Various Na₂SO₄-Na₂CO₃-CaCO₃-SiO₂ combinations were studied by differential thermal analysis to elucidute the role of Na₂SO₄ in soda-lime-silica glassmelting reactions. It was found that Na₂SO₄ encourages the formation of wollastonite at 850° to 900°C. The solid-state reaction of Na₂Ca(CO₃)₂ occurs very readily at temperatures in the vicinity of 400°C. The Na₂Ca(CO₃)₂ must therefore be considered a major constituent in glass batches containing both soda ash and limestone .     

Molten-Salt Corrosion of Silicon Nitride: I, Sodium Carbonate

Corrosion of Si₃N₄ under thin films of Na₂CO₃ was investigated at 1000°C. Pure Si₃N₄ and Si₃N₄ with various additives were examined. Thermogravimetric analysis and morphology observations lead to the following detailed reaction mechanism: (I) decomposition of Na₂CO₃ and formation of Na₂SiO₃, (II) rapid oxidation, and (III) formation of a protective silica layer below the silicate and a slowing of the reaction. For Si₃N₄ with Y₂O₃ additions, preferential attack of the grain-boundary phase occurred. The corrosion of pure Si and SiC was also studied for comparison to Si₃N₄. The corrosion mechanism generally applies to all three materials. Silicon reacted substantially faster than Si₃N₄ and SiC.     

The Role of Li2CO3 and PbO in the Low-Temperature Sintering of Sr, K, Nb (SKN)-Doped PZT

It is demonstrated that the use of ∼0.9 mol% Li₂CO₃ (LC) as a sintering aid for Sr, K, Nb modified Pb(Zr_{1− x} ,Ti _x )O₃ (PZT) ceramics is effective only in the presence of excess PbO (∼2 mol%). It is shown that LC and PbO react to form the compound, Li₂PbO₃ (LPO) which has a melting temperature of ∼836°C. Using dilatometry, we were able to correlate shrinkage during heating of a green ceramic to the melting of the LPO. Consequently, complete densification and sizeable grain growth are achieved by solution-precipitation of the ceramic through the liquid phase. Importantly, sintering is not particularly effective with such small additions of either LC or PbO alone. In confirmation of this model, the LPO compound was presynthesized and used as the only sintering aid in the same PZT composition. The densification behavior of this mixture compared well with the case of separate additions.     

Studies in Germanium Oxide Systems: II, Phase Equilibria in the System Na2O—GeO2

Phase equilibrium relations in the system Na₂O-GeO₂ have been determined using standard quenching techniques supplemented by differential thermal analysis. Two congruently melting compounds, Na₂O·GeO₂ and 2Na₂O·9GeO₂, exist; the melting points are 1103°± 15°C and 1073°± 3°C, respectively. The eutectic temperature between GeO₂ and 2Na₂O·9GeO₂ is 950°±f 10°C at 94.5 wt GeO₂. The eutectic temperature between 2Na₂O · 9GeO₂ and Na₂O·GeO₂ is 790° f 10°C at about 75 wt% GeO₂. Both the refractive index and the density of glasses in the system Na₂O-GeO₂ exhibit maximum values at about 16 to 18 mole % Na₂O. The Ge-O-Ge absorption band at 890 cm⁻¹ shifts toward lower wave numbers with the addition of Na₂O.     

Microstructure in Hydrated Silicate Glasses

A microstructure ranging from 5 to 20 nm in size was detected by small-angle X-ray scattering in Na₂O·3SiO₂ and 4EaO·28Na₂O·68SiO₂ glasses hydrated at ∼100° and 80°C, respectively. This observation suggests the presence of silica gel-solvent (water) phase separation.     

Lithium Carbonate/Sodium Sulfate Eutectic—An Additive for Improving Glass Production

A phase diagram for the system lithium carbonate/sodium sulfate was developed by DTA measurements in a CO₂ atmosphere. The lithium carbonateisodium sulfate eutectic composition from that system improved glassmelting, speeded fining, and reduced the viscosity of a standard fiber glass when used as an additive. Improved melting rates were postulated to result from reaction of lithia with glass-forming components at a relatively low temperature. Lithia is formed by decomposition of Li₂CO₃ or Li₂CO₃/Na₂SO₄ eutectic in air at or near their melting points.     

Sodium Carbonate Flux Effects on the Luminescence Characteristics of (Y0.5Gd0.5)2O3:Eu Phosphor Particles Prepared by Spray Pyrolysis

Na₂CO₃ flux was introduced in the preparation of phosphor particles by spray pyrolysis to improve the photoluminescence (PL) characteristics of (Y_0.5Gd_0.5)₂O₃:Eu phosphor particles. The phosphor particles directly prepared by spray pyrolysis at 1300°C from solutions with 20 wt% Na₂CO₃ flux had the highest PL intensity, which corresponded to 130% of that of particles prepared from solution without flux. On the other hand, the maximum PL intensity of the annealed particles, which were as-prepared at 900°C and posttreated at 1200°C for 3 h, was obtained from a solution with 5 wt% Na₂CO₃ flux. The maximum PL intensity of particles directly prepared by spray pyrolysis without posttreatment was 86% of that of posttreated phosphor particles. Na₂CO₃ flux was also important in control of morphology of (Y_0.5Gd_0.5)₂O₃:Eu phosphor particles.     

Mechanism of Strength Degradation for Hot Corrosion of α-SiC

Sintered α-SiC was corroded by thin films of Na₂SO₄ and Na₂CO₃ molten salts at 1000°C. This hot corrosion attack reduced room-temperature strengths by as much as 50%. Strength degradation was proportional to the degree and uniformity of corrosion pitting attack as controlled by the chemistry of the molten salt. Extensive fractography identified corrosion pits as the most prevalent source of failure. The fracture strength was correlated with pit depth and could be roughly estimated from a simplified fracture mechanics treatment.     

Effects of MoO3, on Phase Separation of Na2O-B2O3-SiO2 Glasses

Immiscibility temperatures of Na₂O-B₂O₃-SiO glasses, with andwithout 1 mol% MoO₃, additions, were determined and the effect of MoO₃ additions on the 65O°C immiscibility isotherms was established. In addition, immiscibility temperature and phase-separation morphology of an Na₂O-B₂O₃-SiO₂ glass with progressive additions of MoO₃, were investigated. It was found that the addition of small amounts of MoO₃ extends the immiscibility boundary of the system and raises the immiscibility temperature by ∼l8°C for each mol % MoO₃, addition. Analysis of phase-separation morphology suggests that the MoO₃, additions do not significantly alter the tie lines of phase separation in the system, although such additions cause a lowering of the viscosities and the glass-transition temperatures of these glasses.     

Hot Corrosion of Sintered α-Sic at 1000°C

The hot corrosion of sintered α-Sic by thin films of Na₂SO₄ and Na₂CO₃ was studied at 1000°C in controlled gas atmospheres. Under all conditions corrosion led to 10 to 20 times the amount of SiO₂ formed in pure oxidation after a 48-h exposure. In addition, small amounts of sodium silicate formed. Melts of Na₂SO₄/SO₃ caused uniform pitting of the Sic substrate; Na₂CO₃/CO₂ melts caused localized pitting and grain-boundary attack. In all cases the protective SiO₂ layer dissolved to form silicate, leading to corrosion. In the sulfate case, free carbon in the Sic promotes this process. In all cases the presence of liquid films is responsible for rapid transport rates and the subsequent rapid reaction.     

Raman Study of Grain Size Effects in the Melting Behavior of 15Na2CO3-10BaCO3-75SiO2 Batches

By means of Raman spectroscopy the melting behavior of 15Na₂CO₃−10BaCO₃−75SiO₂ batches with different grain sizes of raw materials was investigated both qualitatively and quantitatively. The results show that the reaction rate at low temperatures ( T ≤800° to 900°C) increases when finer grains of all raw materials are used; upon pelletizing the fine batch the reaction rate increases even further. At high temperatures ( T > 900°C) the grain size of SiO₂ is the main determining factor, the melting rate being increased when fine SiO₂ grains are used.     

Thermodynamic Characterization of the Eutectic Phase Mixture NaNbO3/Na3NbO4. I: Literature Survey

The literature about the thermodynamic properties of NaNbO₃ and Na₃NbO₄ has mainly been governed by estimations. The only exceptions are two current calorimetric investigations on the standard enthalpy of formation of NaNbO₃ and, in addition, an old and inappropriately evaluated study on the carbon dioxide equilibrium gas pressure over the phase mixture NaNbO₃/Na₃NbO₄/Na₂CO₃. Upon reevaluating the latter results, first experimentally proven data on the difference of the Gibbs-free energies have been obtained (715°–822°C):      

Further Studies on Thermal Evolution of Glass-Forming Gels

Glasses of composition Na₂O·2SiO₂ containing iron oxide are obtained by the gel technique. The results show that the iron atoms accelerate the formation of the glass, whereas when they are present in low percentage (∼2%), the temperature necessary to obtain the glass phase is higher. During heat treatment at 250°C, two silicates are formed: Na₂SiO₃ and Na₄SiO₄. When the iron content is low, they change at 650°C into Na₂Si₂O₅, but when the iron content is higher (∼6%), this silicate does not form and the system becomes vitreous.     

Effects of Particle Size on the Fusion of Soda–Lime–Silicate Glass Containing NaCl

The coupled effects of particle size and 1 wt% NaCl additions on the sequence of melting reactions in a multicomponent system (sand–soda ash–calcite–dolomite–feldspar) were studied using data from DTA, DTGA, and XRD interactively. Glass batches varied in average particle size from 250 μm to finer than 45 μm. Milestone events in the fusion process of the coarse particle base glass were elucidated. The termination temperature of the last significant reaction associated with CO₂ release was 35°C lower in the fine particle size batch than with the coarsest batch. Liquid-phase formation at ∼523°C in the batch with 1 wt% NaCl occurred to an increasingly sizable extent with decreasing particle size. This contrasts with a similar effect at ∼630°C for a comparable batch without NaCl via eutectic melting between soda ash and dolomite. Sodium chloride additions significantly enhanced dissolution of CaO relic.