Crystallization Kinetics of a Complex Lithium Silicate Glass-Ceramic

Volume crystallization of this glass is nucleated by Li₃PO₄. On heating from room temperature, Li₂SiO₃ appears around 650°C and then converts to Li₂Si₂O₅ around 850°C by reaction with SiO₂ from the melt. Preheating the glass at 1000°C forms larger Li₃PO₄ nuclei that promote additional crystallization of cristobalite in the 650° to 850°C range. Crystallization activation energies calculated from scan-rate dependence of DTA peaks are 270 kJ/mol for Li₂SiO₃ and 360 to 570 kJ/mol for Li₂Si₂O₅.     

Effect of Silicon Carbide Additions on the Crystallization Behavior of a Magnesia-Lithia-Alumina-Silica Glass

Glass in the MgO-Li₂O-A1₂O₃-SiO₂ system was observed to crystallize readily at temperatures from 700° to 900°C. The primary crystalline phase evolved was Li₂Si₂O₅, and the secondary phase evolved was Li₂SiO₃. The glass was amorphous after heating in air at 1050°C for 30 min. The addition of 0.5 wt% SiC powder resulted in the crystallization of Li₂SiO₃ during heating in air at 1050°C for 30 min. It was suggested that the difference in crystallization behavior with Sic addition was due to dissolution of Sic into the oxide glass.     

Sol–Gel Route to Nanocrystalline Lithium Metasilicate Particles

Crystalline lithium metasilicate (Li₂SiO₃) nanoparticles have been synthesized using a sol–gel process with tetraethylorthosilicate and lithium ethoxide as precursors. The particle size examined by using transmission electron microscopy and BET-specific surface area techniques is in the range 5–50 nm, depending on the temperature at which the material is calcined. The crystalline Li₂SiO₃ forms at ambient temperature (∼40°C), and it remains in this phase after calcination at temperatures up to 850°C. The BET-specific surface area is ∼110 m²/g for material calcined at temperatures below 500°C, decreasing to ∼29 and ∼0.7 m²/g following calcination at 700° and 850°C, respectively. Solid-state ²⁹Si NMR spectroscopy shows the presence of only Q ² structural units in the material. The lithium metasilicate is further characterized using differential scanning calorimetry and thermogravimetric analysis, and Fourier transform infrared spectroscopy.     

Mechanical Properties of Polycrystalline Lithium Orthosilicate

Room-temperature mechanical properties and high-temperature creep deformation of lithium orthosilicate (Li₄SiO₄) were studied. Elastic constants, flexural strength, and fracture toughness were determined for specimens with densities between 68% and 98% of theoretical. Critical quenching temperature and thermal-shock resistance parameters for 90% dense specimens were also measured. High-temperature creep deformation was investigated by a constant-strain-rate test in an argon atmosphere at temperatures between 750° and 1025°C and strain rates ranging from about 10⁻⁶ to 10⁻³ s⁻¹. At 950°C and above, the stress exponent, n , was determined to be 3.6, with a creep activation energy of 715 kJ/mol. Selected results obtained for Li₄SiO₄ are compared with results obtained for other Li-containing ceramics that are under consideration as candidates for fusion reactor breeder blankets.     

Sintering and Crystallization of a Glass Powder in the Li2O–ZrO2–SiO2 System

Sintering and crystallization of a 23.12 mol% Li₂O, 11.10 mol% ZrO₂, 65.78 mol% SiO₂ glass powder was investigated. By means of thermal shrinkage measurements, sintering was found to start at about 650°C and completed in a very short temperature interval (Δ T similar/congruent 100°C) in less than 30 min. Crystallization took place just after completion of sintering and was almost complete at about 900°C in 20 min. Secondary porosity prevailed over the primary porosity during the crystallization stage. The glass powder compacts first crystallized into lithium metasilicate (Li₂SiO₃), which transformed into lithium disilicate (Li₂Si₂O₅), zircon (ZrSiO₄), and tridymite (SiO₂) after the crystallization process was essentially complete. The microstructure was characterized by fine crystals uniformly distributed and arbitrarily oriented throughout the residual glass phase.     

Activity-Composition Relations in Orthosilicate and Metasilicate Solid Solutions in the System MnO-CoO-SiO2

Activity-composition relations in Mn-Co orthosilicate and metasilicate solid solutions were determined at 1200° to 1250°C by equilibrating selected phase assemblages with metallic cobalt and a gas phase of known oxygen potentials at a total pressure of 1 atm. The orthosilicate solid solution shows a slight negative departure from ideality, whereas the metasilicate solid solution is ideal. The stability data derived for the Mn₂SiO₄ and MnSiO₃ end-members are in good agreement with those previously determined from the system MnO-"FeO"-SiO₂. The free-energy change for the reaction 2CoSiO₃= Co₂-SiO₄+ SiO₂ was determined to be -3.8 kcal at 1250°C.     

Phase Equilibria in the System Li2O-BeO-SiO2

The equilibrium phase diagram for the system Li₂O-BeO-SiO₂ contains only one ternary compound, Li₂BeSi0₄. Liquidus relations for compositions containing 33 mol% SiO₂ were determined; 10 liquidus invariant points were located and 7 subsolidus compatibility triangles. The most refractory compositions lie on the join BeO-Li₂BeSiO₄, with a solidus temperature of 1320°C. Metastable phases observed were a high-quartz phase, Li_2x(Si₁-_xBe_x)O₂, x 0.33; phase X which is probably a metastable orthosilicate between Li₂BeSiO₄ and Be₂SiO₄; and phase Y which lies on the join Li₂BeSiO₄-SiO₂. The crystal chemistry and glass network-forming properties of BeO are discussed.     

Ionic Conductivity of Li4SiO4, Li4GeO4, and Their Solid Solutions

Conductivity was measured for Li₄SiO₄ and its solid solutions with Li₄GeO₄ over a wide frequency range to separate clearly the effects of electrode polarization, conductance relaxations, etc., and to obtain true "dc" conductivities. The conductivities of all the electrolytes are markedly temperature-dependent, ranging from 10⁻⁸ to 10⁻¹⁰Ω⁻¹ cm⁻¹ at 100°C to 10⁻² to 10¹⁰Ω⁻¹ cm⁻¹ at 700°C. For solid solutions with the Li₄GeO₄ structure, conductivities fit the Arrhenius equation over a wide temperature range, but at higher temperatures a change in activation energy occurs, corresponding to a first-order phase transition. In contrast, solid solutions with the Li₄SiO₄ structure show changes in activation energy which do not correspond to phase transitions, but which appear to indicate changes in the conduction mechanism.     

Thermodynamic Properties of Alkali Silicates: Heat Capacity of Li2SiO3 and Lithium-Bearing Melts

The heat capacities of liquid Li₂SiO₃ and Li₂Si₃O₇ have been determined through drop calorimetry measurements to complement available data on lithium, sodium, and potassium silicate melts. The composition and temperature dependences of the heat capacity depends specifically on the nature of the alkali element, indicating that the temperature-induced structural changes that take place in the melts are specific. The properties of crystalline Li₂SiO₃ have also been determined above 298 K. The enthalpy of fusion at 1474 K is 71.3 ± 0.6 kJ/mol, in agreement with previous measurements. In contrast to other silicate compounds, and especially the isostructural crystal Na₂SiO₃, Li₂SiO₃ shows almost no premelting effects.     

The Join Ca2SiO4-CaMgSiO4

New data obtained on the join Ca₂SiO₄-CaMgSiO₄ established a limit of crystalline solubility of Mg in α-Ca₂SiO₄ corresponding to the composition Ca_1.90Mg_0.10SiO₄ at 1575°C. The α-α'Ca₂SiO₄ inversion temperature is lowered from 1447° to 1400°C by Mg substitution in the lattice. α'-Ca₂SiO₄ takes Mg into its lattice up to the composition Ca_1.94Mg_0.06SiO₄ at 1400°C and to Ca_1.96Mg_0.04SiO₄ at 900°C. A new phase (T) reported previously by Gutt, with the approximate composition Ca_1.70Mg_0.30SiO₄, was stable between 979° and 1381°C, and should be stable at liquidus temperatures in multicomponent systems involving CaO–MgO–SiO₂.     

Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF₄ were measured for the reactions 2SiO₂( c )+2BeF₂( d )⇋SiF₄( g )+Be₂SiO₄( c ) (log P _siF4(mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be₂SiO₄( c ) +2BeF₂( d )⇋SiF₄( g ) +4BeO( c )(log P _siF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF₂ was present in solution with LiF as molten Li₂BeF₄. The solubility of SiF₄ was low (∼0.04 mol kg^-1 atm^-1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H_f and Δ G_f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO₂ and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF₄ generated showed an increasing positive deviation from the expression given for the equilibrium involving Be₂SiO₄ and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO₂.     

Phase Equilibria in the System Li2O-CaO-SiO2

Phase relations in the system Li₂O-CaO-SiO₂ were studied by the quenching method. Four stable ternary compounds were found (Li₂Ca₃Si₆O_l6, Li₂Ca₄Si₄O₁₃, Li₂Ca₂Si₂O₇, and Li₂CaSiO₄) as well as phase Y , which is probably a metastable orthosilicate fairly close to Ca₂SiO₄ in composition. X-ray powder data are given for the new phases. Eleven subsolidus compatibility triangles and thirteen liquidus invariant points were located. Melting relations were determined for that part of the system bounded by Li₂SiO₃, Li₂CaSiO₄, Ca₂SiO₄, and SiO₂. The join Li₂SiO₃-CaSiO₃ is binary.     

Sol–Gel Preparation and Characterization of Lithium Disilicate Glass–Ceramic

A sol–gel process is described for preparation of crystalline lithium disilicate (Li₂Si₂O₅) from tetraethylorthosilicate and lithium ethoxide. The glass network structure and crystallinity resulting from heat treatment at temperatures from 150° to 900°C were investigated by nuclear magnetic resonance, X-ray diffraction, and differential scanning calorimetry/thermogravimetric analysis. Q³ structural units (SiO₄ tetrahedra with three bridging oxygen atoms) formed in the amorphous gel at a low temperature (≤150°C) persist to elevated temperature (≤500°C) and directly transform to crystalline Li₂Si₂O₅ at about 550°C. The heating schedule slightly affects the crystalline phase transformation.     

Crystallization of a Glass-Ceramic by Epitaxial Growth

The crystallization mechanism in a modified Li₂O-Al₂O₃-SiO₂glass containing P₂O₅ nucleating agent was investigated by transmission electron microscopy, X-ray diffraction, and differential thermal analysis. During an initial 1000°C treatment P₂0₅ and Li₂O react to precipitate Li₃PO₄ crystallites. At lower temperatures cristobalite, lithium metasilicate, and lithium disilicate crystallize by epitaxial growth on those Li₃PO₄crystallites. Crystallographic orientation relations for epitaxy were determined by electron diffraction, and lattice misfits were found to be in the range –5.3 to +3.8%. These results provide the first direct proof that glass-ceramics can crystallize by epitaxial growth on heterogeneous nuclei formed from intentionally added nucleating agents.     

Synthesis of Li4SiO4 by a Modified Combustion Method

We report for the first time the synthesis of Li₄SiO₄ by the modified combustion method, a rapid chemical process that takes 5 min for completion. This method uses nonoxidizer compounds instead of nitrate mixtures, which are not always commercially available.
The effects of the following parameters on the production of Li₄SiO₄ were studied: (1) different lithium hydroxide:silicic acid:urea (LiOH:H₂SiO₃:CH₄N₂O) molar ratios; (2) the presence of air flow in the furnace chamber; and (3) the furnace heating temperature. It was found that LiOH:H₂SiO₃:CH₄N₂O molar ratios 6:1:3 heated at 1100°C in the presence of additional air in the muffle chamber formed the best precursors to produce Li₄SiO₄.     

Preparation of Rapidly Quenched Glasses in Pseudobinary Systems Composed of Lithium Ortho-Oxosalts

Twin-roller quenching was attempted for 15 pseudobinary systems combining two chosen from the lithium ortho-oxosalts, i.e. Li₃BO₃, Li₄SiO₄, Li₄GeO₄, Li₃PO₄, Li₂SO₄, and Li₂WO₄. Glassy flakes were obtained in almost all systems. Glass formation was easier in systems containing large amounts of Li₃BO₃ or Li₂WO₄ compared to those containing Li₄GeO₄ or Li₃PO₄. Glass transition, crystallization, and liquidus temperatures were determined. Infrared spectra revealed that the glasses consisted of Li⁺ ions and discrete ortho-oxoanions only .     

Polymorphism of Calcium Orthosilicate

Therecentobservation of orthorhombic α'-Ca₂Si0₄ (bredigite) at all temperatures between about 850° and 1450°C. leads to a rational interpretation of the polymorphism of this substance, which is very satisfactory from the crystal-chemical point of view. The so-called β phase, of as yet unknown, complex structure, exists only meta-stably, and not above but below about 675°C, where γ is the stable phase. The monotropic β phase forms on cooling from α'near 675°C. as the result of the inhibition of the α'→γ inversion, at 850°C. The inhibition is caused by the need for a considerable atomic rearrangement and a 12% volume increase which accompanies the change of the coordinations CaO₂ and CaOlo, in α', to CaO₆, in γ. Among the solid phase equilibria with, Mg₂SiO₄ and Ca₃(PO₄)₂, unlimited solid solubility between γ-Ca₂SiO₄ and Mg₂SiO₄ is predicted, whereas the solubility of Mg₂SiO₄ is a limited one in α'and still more so in a-Ca₂SiO₄, as a result of the substitution, for calcium, of the smaller magnesium.     

Nucleation and Crystallization of a Lead Halide Phosphate Glass by Differential Thermal Analysis

The nucleation and crystallization mechanisms of a lead halide phosphate glass [40P₂O₅·30PbBr₂·30PbF₂ (mol%)] were investigated by differential thermal analysis (DTA) and X-ray diffraction analysis. There were two crystalline phases in the crystallized samples: the major phase was PbP₂O₄, and the minor phase was PbP₂O₆. The average activation energy for crystallization, E , for two different particle sizes of this glass was determined to be 119 ± 4 kJ/mol by the Kissinger method and 124 ± 4 kJ/mol by the Augis–Bennett method. The Avrami constants were determined to be 1.6 and 2.5 for particle sizes of 203 and 1040 μm, respectively, by the Ozawa equation, and 1.7 and 2.4 for particle sizes of 203 and 1040 μm, respectively, by the Augis–Bennett equation. The decrease in the crystallization peak height in the DTA curve with increasing particle size suggested that the particles crystallize primarily by surface crystallization. A nucleation-rate type curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T _p, or the height of the crystallization peak, (δ T )_p, as a function of nucleation temperature, T _n. The temperature where nucleation can occur for this glass ranges from 360°–450°C and the maximum nucleation rate is at 420°± 10°C.     

Synthesis and Microwave Dielectric Properties of MgSiO3 Ceramics

MgSiO₃ ceramics were synthesized and their microwave dielectric properties were investigated. The Mg₂SiO₄ phase was formed at temperatures lower than 1200°C, while the orthorhombic MgSiO₃ phase started to form by the reaction of SiO₂ and Mg₂SiO₄ in the specimen fired at 1200°C. The structure of the MgSiO₃ ceramics was transformed from orthorhombic to monoclinic when the sintering temperature exceeded 1400°C. A dense microstructure was developed for the specimens sintered at above 1350°C. The excellent microwave dielectric properties of ɛ_r=6.7, Q × f =121 200 GHz, and τ_f=−17 ppm/°C were obtained from the MgSiO₃ ceramics sintered at 1380°C for 13 h.     

Determining the Nucleation Rate Curve for Lithium Disilicate Glass by Differential Thermal Analysis

The crystallization of lithium disilicate (Li₂O·2SiO₂) glass nucleated at various temperatures was studied by differential thermal analysis (DTA). A plot of the DTA crystallization peak height versus nucleation temperature closely resembles the classical nucleation rate curve for lithium disilicate glass whose maximum is at 453°C. The glass becomes saturated with internal nuclei when heated at 453°C for 10 h. The DTA technique is a rapid, alternative method for determining the temperature for maximum nucleation. The activation energy for crystallization, E , and the heat of crystallization, H , are independent of the concentration of nuclei and are 249±10 and 67±3 kJ/mol, respectively. The Avrami exponent, n , depends strongly on the concentration of nuclei in the glass.