Thermal Analysis of Reactions in Soda–Lime Silicate Glass Batches Containing Melting Accelerants: II, Multicomponent Systems

The glass melting reactions in a multicomponent system (sand–soda ash–calcite–dolomite–feldspar) were studied using data from DTA, TGA, and XRD interactively. The first-formed liquid phase occurred at 700°C from eutectic melting among CaCO₃, Na₂CO₃, and MgO. Further liquid phase formed at the CaCO₃–Na₂CO₃, eutectic at 785°C and a fusion reaction among SiO₂, CaO, and the molten phase at 812°C. Reactions between molten soda ash and silica grains to form a sodium disilicate coating also occurred in this temperature range. The effects of reaction accelerant additions (Na₂SO₄, NaNO₃, NaCI) on batch fusion were analyzed. Sodium chloride was found to be the most effective melting accelerant due to the formation of a NaCI–Na₂CO₃ eutectic liquid phase at ∼636°C, which effectively attacked the silica relic. CO₂ gas release terminated ∼80°C earlier with 1 wt% NaCI additions to the base glass.     

Phase Equilibria in the Na2O-Cs2O-SiO2 System

The Na₂O-Cs2O-SiO₂ system has been investigated by means of a new differential thermal analysis apparatus. Two compounds have been observed for the first time in the metasilicate and disilicate joins. The CsNaSi₂O₅ disilicate melts congruently at 1217 K and the peritectic fusion of the CsNaSiO₃ metasilicate occurs at 1120 K.     

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.     

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 .     

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.     

CO2 Retention in High-Alkali Borate Glasses

A study of the high-alkali region of glass formation in the system Na₂O +B₂O₃ reveals that retention of CO₂ from carbonate starting materials can become a serious preparative problem at the high-alkali extreme. Results presented for glasses prepared using both Na₂O and Na₂CO₃ show that residual CO₂ can lead to major differences in physical properties which in this work are represented by the viscosity-related glass transition temperature .     

Chemical Reaction in a Hot-Pressed Al2O3-Glass Composite

The effects of hot-pressing variables on a chemical reaction within a two-phase system were studied. The reaction between alumina and sodium disilicate glass to form nephelite (Na₂O·-Al₂O₃·2SiO₂) was investigated using X-ray diffraction, electron microprobe, and petrographic techniques. The nephelite nucleated at the surface of the alumina and grew into the glass matrix.     

Direct Formation and Photocatalytic Performance of Anatase (TiO2)/Silica (SiO2) Composite Nanoparticles

Anatase (TiO₂)/silica (SiO₂: 23.9–27.7 mol%) composite nanoparticles were directly synthesized from (i) the reaction of titanyl sulfate (TiOSO₄) and sodium metasilicate (Na₂SiO₃) under mild hydrothermal conditions, (ii) the acidic precursor solutions of TiOSO₄ and tetraethylorthosilicate (TEOS) by thermal hydrolysis, and (iii) the metal alkoxides, i.e., tetraisopropoxide (TTIP) and TEOS, by the sol–gel method. Their photocatalytic activities were evaluated by measurements of the relative concentration of methylene blue after UV irradiation. The as-prepared TiO₂/SiO₂ composite nanoparticles showed far more improved photocatalytic activity than the pure anatase-type TiO₂. The composite nanoparticles formed from (i) TiOSO₄ and Na₂SiO₃ as well as those from (ii) TiOSO₄ and TEOS showed fairly good photocatalytic activity, and it was better than that of those synthesized from (iii) the metal alkoxides, which was suggested to be due to the difference in crystallinity of the anatase.     

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.     

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.     

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.     

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.     

The System Sodium Fluoride—Alumina Investigated by Quenching Methods

Alumina was found to react with sodium fluoride on fusion to produce sodium aluminate and cryolite according to the reaction 6NaF + 2Al₂2c₃= 3NaA10₂+ Na₃A1F₆. An insoluble sodium aluminate phase was observed under the polarizing microscope in samples quenched from as high as 1400°C. The equilibrium crystallization temperature of sodium fluoride in the presence of solid sodium aluminate was found to be slightly depressed with added alumina. A maximum lowering of 6°C was found for a starting alumina content of 5.4%. Further alumina additions resulted in the secondary precipitation of β-Al₂O₃. The shallow depression of the sodium fluoride crystallization temperature and the observed limited alumina solubility are attributed to the formation of cryolite. The composition of the liquid in equilibrium with sodium aluminate and sodium fluoride or sodium aluminate and β-alumina is represented in terms of the pseudo-ternary system NaF-Al₂O₃-Na₃A1F₆.     

Electrochemical Studies in Glass: I, The System NiO-Na2Si2O5

A solid electrolyte electrochemical cell of the type Pt|Ni:NiO _{a =1}∥ZrO₂+7.5% CaO∥Ni:NiO _{a <1}+glass|Pt was used to measure the activities of NiO in sodium disilicate glass from 750° to 1100°C. The data indicate a solubility varying from 11 mol% (5.0 wt%) at 800° to 20 mol% (9.3 wt%) at 1100°C. From the variation in NiO activity, the activity of sodium disilicate in glass solution was estimated; from these combined data partial molar free energies and entropies of solution of NiO and Na₂Si₂O₅ and free energies and entropies of mixing were calculated. A partial phase diagram for the system NiO-Na₂Si₂O₅ proposed from solubility data indicates a eutectic at ∼12 mol% (5.3 wt%) NiO at 830°C.     

Densification and Microstructure of Sodium-Doped Colloidal Mullite

A series of mullite precursor gels to prepared by hetero-coagulation of boehmite and colloidal silica. Samples were doped with up to 1.5 wt% Na₂O. Na₂O additions as low as 0.16 wt% led to the formation of α-alumina and a stable glassy phase in addition to mullite. No evidence for the incorporation of sodium into the mullita structure was found by either XRD or ²⁷AlMAS NMR. SEM showed mullite doped with 0.16 wt% and greater Na₂O had an inter-granular glassy phase and large crystalline grains. It is thought that the majority of the sodium was present in the glassy phase. Sodium addition did not decrease the mullite crystallization temperature and did not lead to enhanced sintering.     

New Sodium Borate Glasses

The glass-forming regions for the sodium borate system were determined using a roller-quencher. The new regions are R=0 to 0.8 and R=1.2 to 4.9for samples prepared from sodium carbonate (where R=mol% Na₂O/mol% B₂O₃). Above R=2.0 retention of'C02 occurs. Quantitative measures of CO₂ retention as a function of R are reported. The results using sodium carbonate extend by a considerable amount the limits reported by Imaoka and are comparable to the recent data of Ota and Soga.     

Viscosity of Molten Na2O─B2O3 Slags

The viscosity of sodium borate slags at high Na₂O concentrations (37.3 to 49.4 mol%) and high temperatures (1000° to 1300°C) follows an Arrhenius-type relationship. This relationship was also observed for sodium borate slags (mass% Na₂O/mass% B₂O₃= 0.86) containing CaO and CaF₂ for the same temperature range. There has been a reduction in viscosity of the sodium borate slags (mass% Na₂O₃mass% B₂O = 0.53 to 0.86) with increase in Na₂O concentration. On adding CaO (10 to 50 mass%) to the sodium borate slag (mass% Na₂O/mass% B₂O₃= 0.86), the viscosity increased considerably, while an addition of CaF₂ (S to 15 mass%) to the slag (30.9 mass% Na₂O₃ 35.8 mass% B₂O₃, 33.3 mass% CaO) decreased the viscosity. The average activation energies of Na₂O─B₂O₃, Na₂O─B₂O₃─CaO₃ and Na₂O─B₂O₃─CaO─CaF₂ slag systems have been estimated as 14.6, 124.7, and 41.4 kJ/mol, respectively, for the given composition ranges and 1000° to 1300°C temperature range.     

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.     

Crystallization of Beta NaFeO2 from a Glass Along the Na2SiO3-Fe2O3 Join

Fine-particle beta sodium ferrite (β-NaFeO₂), rather than α-Fe₂O₃, may be responsible for superparamagnetic behavior in a glass of composition (in mole fractions) 0.37Na₂O-0.26Fe₂O₃-0.37SiO₂. The 700°C isothermal section of the phase diagram of the Na₂O-Fe₂O₃-SiO₂ system is given, showing a three-phase field bounded by Na₂SiO₃-NaFeO₂-Fe₂O₃; there is no evidence for the existence (at 700°C) of compounds of molar composition 6Na₂O-4Fe₂O₃-5SiO₂ or 2Na₂O-Fe₂O₃-SiO₂. The Moessbauer spectrum of β-NaFeO₂ has an internal magnetic field of 487 kOe at room temperature.     

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.