The System K2SO4-Cs2SO4

The phase diagram for the system K₂SO₄-Cs₂SO₄ was determined by using DTA for melting relations and DTA and high-temperature X-ray diffractometry for subsolidus relations. At the solidus the system shows complete solid solubility, with a minimum at 940°C and 50 mol% Cs₂SO₄. Orthorhombic K₂SO₄ and Cs₂SO₄, the stable low-temperature forms, show mutual solid solubility and form a eutectoid at 50 mol% Cs₂SO₄ and 430°C, the lowest temperature of stability of the high-temperature hexagonal solid-solution phase. Isothermal plots of the a and c dimensions of this hexagonal phase vs composition show large positive deviations from linearity for c. These deviations are interpreted on the basis of the crystal structure of KNaSO₄ with a similar unit cell.     

Kinetics and Mechanism of Hot Corrosion of γ-Y2Si2O7 in Thin-Film Na2SO4 Molten Salt

γ-Y₂Si₂O₇ is a promising candidate material both for high-temperature structural applications and as an environmental/thermal barrier coating material due to its unique properties such as high melting point, machinability, thermal stability, low linear thermal expansion coefficient (3.9 × 10⁻⁶/K, 200°–1300°C), and low thermal conductivity (<3.0 W/m·K above 300°C). The hot corrosion behavior of γ-Y₂Si₂O₇ in thin-film molten Na₂SO₄ at 850°–1000°C for 20 h in flowing air was investigated using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS). γ-Y₂Si₂O₇ exhibited good resistance against Na₂SO₄ molten salt. The kinetic curves were well fitted by a paralinear equation: the linear part was caused by the evaporation of Na₂SO₄ and the parabolic part came from gas products evolved from the hotcorrosion reaction. A thin silica film formed under the corrosion scale was the key factor for retarding the hot corrosion. The apparent activation energy for the corrosion of γ-Y₂Si₂O₇ in Na₂SO₄ molten salt with flowing air was evaluated to be 255 kJ/mol.     

In Situ Laser Raman Determination of Electrochemical Reactions of Y2O3-Stabilized ZrO2 and Molten Na2SO4

When used as components of advanced energy conversion systems, oxide ion conductors and molten electrolytes may react or decompose according to various mechanisms. Understanding these mechanisms is important in improving system performance. In situ laser Raman spectroscopy was used successfully to study the reactions between an oxide ion conductor and a molten salt. Changes in the Raman spectrum of molten Na₂SO₄ were observed near Y₂O₃-stabilized ZrO₂ electrodes when they were polarized anodically in the molten salt. These changes were interpreted to indicate the formation of yttrium sulfate complexes.     

Vaporization Kinetics of Na2SO4 from 900° to 1200°C

Continuous weight-change measurements were used to determine the vaporization kinetics of Na₂SO₄ from 900° to 1200°C in 150 torr O₂. Simple evaporation of the sulfate was indicated. An activation energy for vaporization of 71 kcal/mol was calculated. Vaporization results obtained for Na₂SO₄ on oxide substrates indicated an Na₂SO₄-Cr₂O₃ reaction; however, no Na₂SO₄-ZrO₂ reaction was observed.     

The Activation Effect of K2SO4 on the Hydration of Gypsum Anhydrite, CaSO4(II)

The effect of K₂SO₄ activator on the hydration of chemical anhydrite obtained from burned FGD-gypsum has been studied by different experimental techniques. Results obtained show that the degree of hydration increases when the K₂SO₄ concentrations increase from 0.5 to 3.3 wt%. Their heat evolution rate and maximum value also increase with the increase of K₂SO₄ concentration. The highest values were obtained when the hydration degree was about 50%. Also a correlation between the hydration degree and the total heat evolved was obtained. The X-ray and SEM/EDX studies have shown that K₂SO₄ is adsorbed at the surface of CaSO₄ even within 5 min of hydration and a syngenite—a double salt K₂SO₄·CaSO₄·H₂O is formed (even in the presence of 1.0 wt% K₂SO₄). Also, important changes in the morphology of the dihydrate crystal are detected. Finally, in the presence of 1.0 wt% K₂SO₄ (water/anhydrite ( W / A ) ratio=0.33), it was found that the resonance frequency, modulus of elasticity, compressive strength, and tensile strength increase with the degree of hydration whereas the total porosity decreases.     

Na2SO4-lnduced Corrosion of Si3N4 Coated with Chemically Vapor Deposited Ta2O5

Hot isostatically pressed Si₃N₄ was coated with chemically vapor-deposited Ta₂O₅, and subjected to oxidative and corrosive environments to determine the feasibility of using a Ta₂O₅ coating for protecting Si₃N₄ from hot corrosion. The coated structure was relatively stable at 1000^deg;C in pure O₂. However, the Ta₂O₅-Si₃N₄ system became unstable in an environment containing Na₂SO₄ and O₂ at 1000^deg;C because (1) Ta₂O₅ and Na₂SO₄ reacted rapidly to form NaTaO₃ and (2) subsequently NaTaO₃ interacted destructively with the underlying Si₃N₄ substrate to form a molten phase.     

Size Control of α-Al2O3 Platelets Synthesized in Molten Na2SO4 Flux

α-Al₂O₃ platelet powders were synthesized in molten Na₂SO₄ flux. The size of α-Al₂O₃ platelets was significantly reduced when partially decomposed rather than pure Al₂(SO₄)₃ was used as the source of Al₂O₃; a further reduction in the platelet size was realized through additional seeding with nanosized α-Al₂O₃ seeds. The addition of microsized α-Al₂O₃ platelet seeds significantly influenced the platelet morphology of the final powder, as well. The platelet size of the final powder was in direct proportion to the size of the platelet seeds, and was in reverse proportion to the cube root of the platelet seed content.     

Investigation of the Phase Diagram for the Pseudobinary System Li2SO4–La2(SO4)3 and Its Ionic Conductivity

The phase diagram of the pseudobinary system Li₂SO₄–La₂(SO₄)₃ has been investigated by means of X-ray diffraction and differential thermal analysis. LiLa(SO₄)₂ is formed by a peritectic reaction in this system; the peritectic temperature is 653±3°C. The eutectic reaction of Li₂SO₄ and LiLa(SO₄)₂ occurs at 553±3°C; the composition at the eutectic point is 17 mol% La₂(SO₄)₃. LiLa(SO₄)₂ is monoclinic with a=1.375 nm, b=0.6744 nm, c=0.7068 nm, and β=105.4°. The ionic conductivity of LiLa(SO₄)₂ has been studied from room temperature to 350°C and is found to be relatively low at room temperature or at lower temperatures. Its activation energy is 0.66 eV. Thus it is not suitable as a fast ion conductor.     

Characterization of Si3N4 Coated with Chemically-Vapor-Deposited Mullite after Na2SO4-lnduced Corrosion

Si₃N₄ substrates coated with chemically-vapor-deposited, crystalline mullite (3Al₂O₃.2SiO₂) were subjected to a corrosive environment containing Na₂SO₄ and O₂ at 1000°C for 100 h. The composition and microstructure of the as-deposited and corroded specimens were examined and compared. The coating appeared to be effective in preserving and therefore protecting the surface microstructure of the underlying Si₃N₃ substrates. However, a small degree of Na penetration through mullite grain boundaries was observed to a coating depth of ∼1 μm.     

Decomposition of Ni2SiO4 in an Oxygen Potential Gradient

Polycrystalline Ni₂SiO₄ was exposed to a gradient in oxygen potential at 1336°C to cause kinetic decomposition into its component oxides, NiO and SO2. At the higher-oxygen-potential side NiO formed, and at the lower-oxygen-potential side SiO₂ formed. This spatial distribution of SiO₂ and NiO is consistent with diffusion data for silicate olivines which indicate that Ni diffuses much faster than either Si or O.     

Electron Microscopy of Annealed (Ni,Zn,Co)Fe2O4

( Ni,Zn)Fe₂O₄ samples containing small amounts of Co were characterized in a transmission electron microscope to ascertain the micro structural changes accompanying low-temperature oxidation of the samples. Although no new features resulting from oxidation were observed, prominent surjace reduction occurred in the thin foil specimens. Formation and growth of Ni particles on the ferrite surface are explained using the heats of formation of the oxides.²     

Activated Sintering of ThO2 and ThO2-Y2O3 with NiO

The effect of additives on the sintering of ThO₂ and ThO₂-Y₂O₃ compacts and loose powders was studied by isothermal shrinkage measurements and by scanning electron micrography. Small amounts of the oxides of Ni, Zn, Co, and Cu reduced the sintering temperature. The behavior of NiO at a concentration of 0.8 wt% (2.5 mol%) was studied in detail and found to yield high-density bodies at temperatures below 1500°C. The presence of Y₂O₃ as a separate phase increases the rate of sintering of ThO₂, but smaller amounts of NiO are much more potent. The major portion of the densification occurs very rapidly and is followed by a much slower sintering process typical of volume diffusion. The fast early shrinkage may be caused by the capillary forces of a liquid, but since no evidence of melting was found, a solid-state mechanism may be responsible.     

Kinetics of NiCr2O4 Formation and Diffusion of Cr3+ Ions in NiO

The reaction kinetics for NiCr₂O₄ formation and the diffusion of Cr³⁺ ions into single-crystal NiO were studied between 1300° and 1600°C in air. The experimental activation energy for NiCr₂O₄ formation was about 83 kcal/mol. After incubation, NiCr₂O₄ formed by a diffusion-controlled process. The origin of pores at the NiO/NiCr₂O₄ interface is discussed. The concentration profiles of Cr³⁺ in NiO were linear because the interdiffusion coefficient was directly proportional to the mol fraction Cr³⁺. Theoretical considerations indicate that the interdiffusion coefficient equals 3/2 the self-diffusion coefficient of Cr³⁺, which is rate-determining. The interdiffusion coefficient at 1 mol% Cr₂O₃ can be expressed as =4×10⁻³ exp (−55,000/RT) cm² s⁻¹.     

Solid Solubility and Polymorphism in the System Sr2SiO4-Sr2GeO4-Ba2GeO4-Ba2SiO4

The reciprocal salt pair Sr₂SiO₄-Sr₂GeO₄-Ba₂GeO₄-Ba₂SiO₄ was investigated using X-ray powder diffraction and DTA. Unlimited solubility in the low-K₂SO₄ structure type (α') occurs throughout the system above 85°C. The nonlinear changes of some lattice constants of the solid solutions are discussed. A stable monoclinic low-temperature (β) form of Sr₂SiO₄ was found which converts reversibly to the high-temperature α'-modification at 85°C. The enthalpy of the β-α' transition is 51 cal/mol. In the reciprocal salt pair the β-form solid solutions occur in a very narrow region below 85°C.     

Effect of Physical Nature of Powders and Firing Atmosphere on ZnAI2O4 Formation

The rate of ZnA1₂O₄ formation for binary powder mixtures of ZnO and α-Al₂O₃ (dense coarse particles and weak agglomerates of fine powder) fired in air or O₂ atmospheres was measured and the microstructures of those systems were observed by scanning electron microscopy. With dispersed dense particles of α-Al₂O₃, the Al₂O₃ surfaces were covered with ZnO and the spinel grew into the particles maintaining essentially a constant reaction interface area. Calculations based on geometric measurements and use of Jander's equation gave a similar high activation energy, 354 kJ/mol, which corresponds to the activation energy of volume diffusion of Zn²⁺ in ZnAl₂O₄. An oxygen atmosphere had no effect. With a matrix of fine α-Al₂O₃ powder and dispersed granules of ZnO, a higher reaction rate occurred because of an increase in reaction interface area due to penetration of the powder compact matrix by ZnO vapor, which was enhanced by an O₂ atmosphere. The reaction layer grew into the alumina matrix adjoining the ZnO granules with a parabolic rate law. Apparent activation energies below ∼200 kJ/mol were calculated.     

TiO2 Produced by Vapor-Phase Oxygenolysis of TiCI4

The formation of TiO₂ powders by oxygenolysis of TiCI₄ was studied with emphasis on the effects of reaction conditions on the particle size of the products. The particle size of TiO₂(anatase) decreased with increasing reaction temperature or O₂concentration and with decreasing TiCI₄ concentration. The results are compared with those for the oxygenolysis of AlBr₃and SiCI₄. It was found that the reactivity of metal halides with O₂ is closely related to the ease of dissociation of the first halogen atom.     

Products of Reaction Between PbO and Nb2O5 in Molten KCl or NaCl

Lead oxide and niobium oxide were heated in molten KCl or NaCl to examine the participation of chlorides in the reaction between the oxides. Alkali ions partially replace Pb ions in PbNb₂O₆, and Cl ions form PbCl₂. The substitution of alkali ions for Pb stabilizes the tetragonal form ofPbNb₂O₆. The reactivity of NaCl with Nb₂O₅ was much greater than that of KCl; the extended substitution resulted in the formation of an NaNbO₃ phase with incorporated Pb .     

Stability of CaNiSi2O6 ("Niopside") and Activity–Composition Relations of CaMgSi2O6– CaNiSi2O6 Solid Solutions at 1350°C

A complete solid-solution series exists between diopside (CaMgSi₂O₆) and its nickel analogue, "niopside"(CaNiSi₂O₆). Activity–composition relations within this solid solution, and the stability of the end member CaNiSi₂O₆, have been determined by equilibrating CaNiSi₂O₆ with SiO₂, CaSiO₃, and metallic Ni in atmospheres of known oxygen pressures. Within limits of accuracy of the experiments, the solution is ideal at 1350°C. From the experimental data obtained in the present investigation, the standard free energy (Δ G °) of formation of CaNiSi₂O₆ according to the equation CaO + NiO + 2SiO₂= CaNiSi₂O₆ is calculated to be Δ G °=−165862 + 42.40 T J. Experiments in the system CaO–NiO–SiO₂ have shown that the nickel analogue of the phase pseudo-enstatite (MgSiO₃) is unstable with respect to SiO₂ and nickel olivine (Ni₂SiO₄), and the nickel analogues of the phases akermanite (Ca₂MgSi₂O₇) and monticellite (CaMgSiO₄) are unstable relative to the phase assemblage pseudo-wollastonite (CaSiO₃) plus NiO. In the system CaO–MgO–NiO–SiO₂, however, substitution of Ni for Mg in these phases was observed. The percentage substitution of Ni for Mg in the phases is given in parentheses: diopside (100%), olivine (100%), enstatite (18%), akermanite (20%), and monticellite (57%).     

Formation of ZnAl2O4 in the Presence of Cl2

The formation of ZnAl₂O₄ spinel in diffusion couples of Al₂O₃ and ZnO was investigated between 1000° and 1390°C in air and in air containing 4.8 vol% Cl₂ by X-ray diffraction, electron probe microanalysis, and scanning electron microscopy. The rate of formation of a spinel layer obeyed a parabolic rate law and was accelerated remarkably by the presence of Cl₂. The interdiffusion coefficient, , and the activation energy, E, were calculated to be 10⁻⁸ to 10⁻⁹ cm²/s and 123 kcal/mol (514 kJ/mol) in air and 10⁻⁷ cm²/s and 31 kcal/mol (130 kJ/mol) in air containing 4.8 vol% Cl₂, respectively.     

Static-Fatigue Life of Ce-TZP and Si3N4 in a Corrosive Environment

Stress rupture testing was performed in four-point flexure at 1000°C to determine the effects of Na₂SO₄-induced corrosion on the static-fatigue life of a Ce-TZP and a MgO-doped Si₃N₄. The results showed that the static-fatigue life of the Ce-TZP was unaffected by this corrosive environment. However, the static-fatigue life of a MgO-doped Si₃N₄ was reduced by the introduction of Na₂SO₄.