Gibbs Free Energy of Formation of Zircon from Measurement of Solubility in H2O

We exploited the large difference in the solubility of SiO₂ and ZrO₂ in H₂O to constrain precisely the Gibbs energy of formation of zircon (ZrSiO₄). Solubility in H₂O was determined at 800°C, 1.2 GPa, by weight loss of synthetic zircon crystals. The experiments yielded fine-grained monoclinic ZrO₂ as an incongruent solution product uniformly coating zircon crystals. Experiments on the ZrO₂-coated zircon crystals were also carried out with an initially slightly SiO₂-oversaturated fluid, causing weight gain by zircon regrowth. The mean SiO₂ concentration for forward and reverse experiments was 0.069±0.010 mol/kg H₂O (2σ). When combined with precise activity–composition measurements for aqueous SiO₂, the data constrain the Gibbs free energy of zircon from its oxides at 298 K, 10⁵ Pa, to be −19.30±1.16 kJ/mol (2σ). This determination is comparable in precision to the best measurements obtainable by more conventional methods, which suggests that determination of the thermochemical properties of other important ceramic materials may also be amenable to this method.     

Standard Enthalpy of Formation of Lanthanum Zirconate

The enthalpy of formation of lanthanum zirconate, La₂Zr₂O₇, which is shown to have a distorted pyrochlore structure, was determined. High-temperature solution calorimetry in molten lead borate was used to determine the heat of formation from the constituent oxides as −135.8 ± 6.4 kJ/mol at 974 K. Using additional data on the enthalpies of formation of the oxides and heat contents, this value was converted to a standard enthalpy of formation from the elements: Δ_fH°_298.15=−4130.4 ± 6.8 kJ/mol.     

Calorimetric Study of Nickel Molybdate: Heat Capacity, Enthalpy, and Gibbs Energy of Formation

The thermodynamic properties of the α and β polymorphs of NiMoO₄ were directly investigated by calorimetry. The standard entropies of formation, Δ_f S ° _T , of α and β were determined from measuring the molar heat capacity, C _p,m, from near absolute zero (2 K) to high temperature (1380 K) by a relaxation method and differential scanning calorimetry. The standard enthalpies of formation, Δ_f H ° _T , of α and β were determined by combining C _p,m with the standard enthalpy of formation, Δ_f H °₂₉₈, at 298 K obtained from drop solution calorimetry in molten sodium molybdate at 973 K. The standard Gibbs energies of formation, Δ_f G ° _T , of α and β were determined from their Δ _f S ° _T and Δ _f H ° _T values. The Δ _f G ° _T values indicate that the polymorphic transformation from α to β occurs at 1000 K, consistent with the observed phase transformation at 1000 K.     

Communications of the American Ceramic Society Estimation of Thermochemical Data for Calcium Silicate Hydrate (C-S-H)

Calcium silicate hydrate (C-S-H) can be viewed as a solid solution, 0.833Ca(OH)₂.SiO₂.0.917H₂O-xCa(OH)₂, at equilibrium at 30°C. On this basis, the change in Gibbsfree energy (ΔG_r) in the solid-solution reaction was calculated from solubility duta for C-S-H in water. The change in ΔG_r with real ratio decreased notably for the higher calcium contents (CaO/Si0₂1.7; ×0.867). Thermochemical values for C-S-H (CaO/SiO₂=1.7) were estimated to be ΔH°=-2890 kJ/mol, ΔG°=-2630 kJ/mol, and S°=200 J1/mol.K at 298 K .     

Assessment of the Thermodynamic Values for PuO1.5 and High-Temperature Determination of the Values for PuC1.5

Thermodynamic values for PUO_1.5 were assessed using an improved method for estimating fef ° 1.5 and new data for S°₂₉₈ 1.5. Based on the assessment, a value of ΔH°₂₉₈, 1.5=–828 kJ/mol is recommended. Measurements of (CO) pressure over the nominal equilibrium 1.5+ 1.5+ C were performed between 1348 and 1923 K, yielding pressures between 0.644 and 11600 Pa. Second- and third-law analyses were used to obtain a value for ΔH°₂₉₈ 1.5=–93.3°3.3 kJ/mol.     

Direct Calorimetric Measurement of Enthalpies of Phase Transitions at 2000°–2400°C in Yttria and Zirconia

High-temperature differential thermal analysis provided data on phase transitions in zirconia and yttria. The tetragonal form of ZrO₂ transforms to the cubic fluorite structure at 2311°±15°C with an enthalpy of 3.4±3.1 kJ/mol. Cubic C-type Y₂O₃ transforms, probably to the fluorite structure, at 2308°±15°C with Δ H =47.7±3.0 kJ/mol. This high-temperature polymorph melts at 2382°±15°C with an enthalpy of fusion of 35.6±3.0 kJ/mol.     

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.     

Thermodynamic Properties of RuO2

The free energy change for the reaction RuO₂( s )+4Cu( s ) = 2Cu₂O( s )+Ru( s ) was determined from 600° to 1000°C from emf measurements on a solid oxide galvanic cell using a stabilized ZrO₂ electrolyte. The cell was designed to minimize the reduction of RuO₂ by the gas phase. The results were used to develop an equation for the standard molar free energy of formation of RuO₂:
The standard molar enthalpy and entropy of formation of RuO₂ at 298°K were calculated to be −72,430 ±200 cal/mol and –40.44±0.2 eu, respectively, using the available heat capacity data. The absolute entropy of RuO₂ at 298°K was calculated to be 15.46±0.2 eu.     

Effect of Bi2O3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Bi₂O₃ was added to a nominal composition of Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi₂O₃ content was <8.0 mol%, a porous microstructure with Bi₄(SiO₄)₃ and SiO₂ second phases was developed in the specimen sintered at 885°C. However, when the Bi₂O₃ content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛ_r=7.6, and τ_f=−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi₂O₃ sintered at 885°C for 2 h.     

Zircon Synthesis via Sintering of Milled SiO2 and ZrO2

The formation of zircon (ZrSiO₄) via sintering of milled SiO₂ and ZrO₂ powders was studied, and the effects of slurry vs dry milling, sintering time, and particle size on zircon yield were examined. It was found that very high zircon yields could be obtained via slurry milling, cold pressing, and sintering of the oxide precursors. The controlling factor in determining zircon yield was found to be the particle size of the SiO₂ and ZrO₂ powders. Zircon yield as a function of sintering time was examined, and found to be similar to previous studies in which sol-gel precursors seeded with zircon were used. SEM studies reveal a homogeneous product with particle sizes on the order of 1–5 µm. It was found that complete reaction to zircon can be achieved from a once-through milling, pressing, and sintering process of SiO₂-ZrO₂ powders.     

Preparation of High-Purity ZrSiO4 Powder Using Sol–Gel Processing and Mechanical Properties of the Sintered Body

Effects of the concentration of ZrOCl₂, calcination temperature, heating rate, and the size of secondary particles after hydrolysis on the preparation of high-purity ZrSiO₄ fine powders from ZrOCl₂.8H ₂ O (0.2 M to 1.7 M ) and equimolar colloidal SiO₂ using sol–gel processing have been studied. Mechanical properties of the sintered ZrSiO₄ from the high-purity ZrSiO₄ powders have been also investigated. Single-phase ZrSiO₄ fine powders were synthesized at 1300°C by forming ZrSiO₄ precursors having a Zr–O–Si bond, which was found in all the hydrolysis solutions, and by controlling a secondary particle size after hydrolysis. The conversion rate of ZrSiO₄ precursor gels to ZrSiO₄ powders from concentrations other than 0.4 M ZrOCl₂.8H₂O increased when the heating rate was high, whereupon the crystallization of unreacted ZrO₂ and SiO₂ was depressed and the propagation and increase of ZrSiO₄ nuclei in the gels were accelerated. The density of the ZrSiO₄ sintered bodies, manufactured by firing the ZrSiO₄ compacts at 1600° to 1700°C, was more than 95% of the theoretical density, and the grain size ranged around 2 to 4 μm. The mechanical strength was 320 MPa (room temperature to 1400°C), and the thermal shock resistance was superior to that of mullite and alumina, with fairly high stability at higher temperatures.     

Thermodynamic Regularities in Perovskite and K2NiF4 Compounds

It has been found that the enthalpy of formation of perovskite compounds, Δ_fH° (ABO₃, B = transition metais), from binary oxides can be well characterized in terms the tolerance factor, t≡(r_A+ r_o)√2 (r_B+ r_o), where r_A and r_B are the radii of A-site ions with 12-coordination and B-site ions with 6-coordination, respectively, and Δ_fH°=−168 + 270(1 − t) kJ·mol⁻¹ for A^IB^VO₃, Δ_fH°=−125 + 1000(1 − t) kJ·mol⁻¹ for A^IIB^IVO₃, and Δ_fH°=− 90 + 720(1 − t) kJ·mol⁻¹ for A^IIIB^IIIO₃. Although the thermodynamic data of K₂NiF₄ compounds are not extensive, a similar regularity can be found when use is made of the radii of A-site ions with 9-coordination for the K₂NiF₄ compounds. These correlations will be quite useful in predicting.     

Formation Mechanism and Synthesis of Apatite-Type Structure Ba2+xLa8−x(SiO4)6O2−δ

The formation process of Ba₂La₈(SiO₄)₆O₂ was clarified using thermogravimetry–differential thermal analysis (TG-DTA) and a high-temperature powder X-ray diffraction (HT-XRD) method. Phase changes identified from the HT-XRD data surprisingly corresponded to the weight loss and/or endothermic peaks observed in the TG-DTA curves. Raw material with the composition Ba₂La₈(SiO₄)₆O₂ was completely reacted at 1400°C and produced only an apatite-type compound without a secondary phase. Moreover, the synthesis of Ba_{2+ x} La_{8− x} (SiO₄)₆O_2−δ crystals with x = 0–2 was attempted using a solid-state reaction.     

Volatility of PuO2 in Nonreducing Atmospheres

The vapor pressure of plutonium dioxide (PuO₂) was investigated in the range 1450° to 1775°C in air, argon, and oxygen atmospheres by a transpiration technique. There were strong indications that PuO₂ can vaporize congruently or as a suboxide species, depending on the atmosphere. The δH°₂₉₈ for vaporization in 1 atm of oxygen is approximately 154,000 cal per mole. The estimated standard free energy of formation (δG°_f) of gaseous PuO₂ is −121,000 + 10.7 T from 1227° to 1827°C.     

Fluoride Model Systems: II, The Binary Systems CaF2–BeF2, MgF2–BeF2, and LiF–MgF2

Data obtained by quenching, thermal, and high-temperature X-ray techniques are presented for the three binary systems CaF₂–BeF₂, MgF₂–BeF₂, and LiF–MgF₂. The systems CaF₂–BeF₂ and MgF₂–BeF₂ are presented as weakened models of the systems ZrO₂–SiO₂ and TiO₂–SiO₂, respectively. The compound CaBeF₄ is a model of ZrSiO₄ (zircon). New data obtained for the system LiF–MgF₂ explain many discrepancies among the results of previous authors. Solid solution is almost complete between LiF and MgF₂ at elevated temperatures, but a small gap occurs at the eutectic (735°C.) with extensive exsolution at lower temperatures.     

The Thermodynamic Properties of Silicon Oxynitride

A critical assessment of thermal and equilibrium data for silicon oxynitride (Si₂N₂O) is presented. Selected values for the heat of formation of Si₂N₂O from the elements and the absolute entropy of Si₂N₂O at 298.15 K are ΔH⁰_f,298=−947.7 ± 5.4 kJ/mol and S⁰₂₉₈= 45.35 ± 0.4 J/mol·K, respectively. A table of thermodynamic functions for Si₂N₂Ofrom 298 to 2500 K, which has been calculated from the analysis of the literature data, is also presented.     

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.     

Transformation Kinetics of Ba2Ti9O20 with ZrO2 Additive

Pure Ba₂Ti₉O₂₀ (BT29) was synthesized by a solid-state reaction in one step with various amounts of ZrO₂ powder additive. The transformation kinetics of BT29 were investigated by quantitative X-ray diffractometry (XRD). The results show that stoichiometric powder mixtures transform to the BT29 phase by nucleation and growth mechanism between 1200° and 1300°C with 1.0 mol% ZrO_2. The activation energy of the transformation was found to be 620±60 kJ/mol, but decreases to 515±30 kJ/mol when doped with 1.0 mol% ZrO₂. The addition of ZrO₂ possibly changes the phase transformation mechanism of BT29 from diffusion controlled to interface controlled.     

Knudsen Cell Studies of the Vaporization of Samarium Dicarbide

The vapor pressure of SmC₂ in equilibrium with graphite was measured by the Mnudsen effusion technique. Rates of weight loss from the cells were measured with an automatic recording balance. The apparent pressures varied with orifice size, and equilibrium pressures were calculated by extrapolation to zero orifice area. This work was combined with other studies to obtain log₁₀ P(atm) = - 13.869 × 10³/T + 3.752 (1300°-2050°K) for the Sm vapor pressure above SmC₂-C. Estimates of S°₂₉₈ and c_p were made for SmC₂, and δH°₂₉₈ was calculated to be 72.0 ± 2 kcal/mol for the reaction SmC₂(s) = Sm(g) + 2C(s). This value combined with δH°_{v, 298}= 48.6 kcal/mol for Sm gives a δ°_f298 for SmC₂ of 23.4 ± 2 kcal/mol.     

Phase Diagram of Wollastonite-Zirconia

A reexamination of the system CaO·SiO₂–ZrO₂ has been conducted in order to use this system to obtain ZrO₂-toughened wollastonite materials. The results have shown that CaO·SiO₂–ZrO₂ is a pseudobinary system with an invariant peritectic point at 1467°± 2°C, which is in disagreement with previously reported results.