Dissolution and Reaction of Yttria-Stabilized Zirconia Single Crystals in Hydrothermal Solutions

Dissolution and reaction of yttria-stabilized zirconia (YSZ) single crystals were investigated in various solutions at 600° to 780°C under 100 MPa. YSZ crystals were not corroded in pure H₂O and neutral solutions such a LiF, LiCl, NaNO₃, KCl, KBr; K₂SO₄, and Na₂SO₄ even under severe conditions at 600°C, 100 MPa. They were, however, dissolved and reprecipitated in basic solutions such as NaF, K₂CO₃, KOH, NaOH, and LiOH with partial decompositon (destabilization in the last three solutions. YSZ crystals were completely decomposed into m -ZrO₂ in acidic solutions of Li₂SO₄, H₂SO₄, and HCl, whereas they reacted with the solution and formed other compounds in KF, NH₄F, and H₃PO₄ solutions.     

Preparation and Proton Conductivity of Surfactant-Templated Mesoporous Silica Gels Impregnated with Protonic Acids

Surfactant-templated mesoporous silica gel that was impregnated with various protonic acids has been prepared, and we propose this material as a new type of proton conductor. The conductivity of the mesoporous silica gel that was impregnated with 5.0 M H₂SO₄ was ∼1 × 10⁻⁴ S/cm at room temperature in a dry nitrogen-gas atmosphere after drying in vacuo at room temperature; this value was four orders of magnitude greater than that of silica gel that was prepared via the conventional sol–gel method and impregnated with H₂SO₄. The amount of impregnated H₂SO₄ in the mesoporous silica gel was ∼2.5 times larger than that of impregnated H₂SO₄ in the conventional silica gel. The H₂O molecules in the mesoporous silica gel that was impregnated with H₂SO₄ remained in the pores of the gel, even after drying in vacuo at room temperature.     

Reactivity of Alumina toward Phosphoric Acid

Reaction between alumina and 33.3 wt% orthophosphoric acid was investigated by monitoring the heat liberated under isothermal conditions at temperatures from 25° to 90°C. In a separate set of experiments, the H₃PO₄ concentration was varied from 0 to 50 wt%, at 25°C. Reactivities of five aluminas (three anhydrous and two hydrated) differing in particle size, surface area, and crystallinity were studied. Relationships between the properties of the aluminas and their reactivities toward phosphoric acid were established. The aluminas with the highest surface areas and the lowest degrees of crystallinity react more rapidly and produce overall more heat. Increasing the temperature and phosphoric acid concentration were also shown to increase heat evolution. However, increasing the H₃PO₄ concentration beyond 33.3 wt% (molar Al/P ratio = 1.0) for the anhydrous aluminas, and beyond 40 wt% for boehmite, does not result in a significant increase in the amount of heat evolved. Gibbsite continues to release greater amounts of heat when reacting with increasing concentrations of H₃PO₄ (up to 50 wt%). The anhydrous aluminas generally react faster than do the hydrates. Within the range of H₃PO₄ concentrations from 0 to 33.3 wt% the hydrates and the most reactive anhydrous alumina exhibit approximately the same degree of reactivity on a per mole of Al₂O₃ basis.     

Morphology of Zirconia Synthesized Hydrothermally from Zirconium Oxychloride

Monoclinic zirconia has been synthesized hydrothermally from zirconium oxychloride in the range 20–100 MPa, 250–650°C, for run duration from 20 to 100 h and in the presence of NaOH, Na₂CO₃, Na₂SO₄, NH₄F, HNO₃, and H₂SO₄ additives. Isometric, platelike, and elongated crystal morphologies were obtained depending on the additive. Both spherulitic and isolated textures were encountered with H₂SO₄. Growth of isometric crystals follows a general dissolution–precipitation mechanism. For H₂SO₄, two growth steps were identified: an early spherulitic step followed by an isolated crystals step resulting from Ostwald ripening of preexisting spherulites. The formation of spherulites is consistent with the specific properties of the sulfuric hydrothermal medium.     

Low-Temperature Formation of Aluminum Orthophosphate

Factors influencing the low-temperature formation of AIPO₄ and its precursor phases, AIPO₄· x H₂O (1 x 2), were investigated. AIPO₄ formed by reaction between 33.3 wt% H₃PO₄ solution and alumina. Five aluminas (three anhydrous and two hydrated) were utilized. Each differed in particle size, surface area, and crystallinity. The reaction temperatures investigated were 113°, 123°, and 133°C. The high-surface-area aluminas were sufficiently reactive in the phosphoric acid solution at these temperatures to produce crystalline reaction products. However, only hydrated forms of AIPO₄, AIPO₄· x H₂O (1 x 2), crystallized directly out of solution. x generally decreased as the curing temperature was increased. Upon dehydration of these hydrated reaction products, anhydrous AIPO₄ was formed, primarily in the berlinite and/or cristobalite modifications. Both the temperature of reaction and the alumina used influence the hydrates that form. In turn, the hydrates which form, the macroscopic assemblages into which they may crystallize, and the morphologies of the crystallites all affect the polymorphic form and the crystallinity of the anhydrous AIPO₄ phase ultimately produced on dehydration. Phase-pure and highly crystalline AIPO₄-cristobalite (the high-temperature modification) was formed by the dehydration of AIPO₄·H₂O at a temperature as low as 113°C.     

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.     

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.     

Early Stages of Alumina Sol-Gel Formation in Acidic Media: An 27Al Nuclear Magnetic Resonance Spectroscopy Investigation

The effects of processing conditions on the formation of alumina sols from the hydrolysis of alkoxide precursors have been investigated using ²⁷Al nuclear magnetic resonance (NMR) spectroscopy. In the range of acid ratios between ∼0.3 to 1.0 mol of HNO₃ per mole of alumina sec-butoxide, the NMR data show that sols processed at 20°C, low temperature (LT), differ greatly from those processsed at 90° to 95°C, high temperature (HT). In the LT sols, the aluminum is predominantly in the form of the polyoxycation species, Al₁₃O₄(OH)₂₄(H₂O)_xⁿ⁺ (where n is probably 7), rather than a folded defect boehmite structure as suggested by Pierre and Uhlmann. Furthermore, heating the LT sols at 90°C does not transform them to the HT form directly, but rather results in the growth of new tetrahedral and octahedral alumina species.     

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 .     

Preparation of Zirconia Whiskers from Zirconium Hydroxide in Sulfuric Acid Solutions under Hydrothermal Conditions at 200°C

Mixtures of Zr(OH)₄ and ZrO₂ particles, ∼10 nm in size, were hydrothermally treated in 0.25–1.5 mol/L H₂SO₄ solutions at a temperature of 200°C. After 3 h, very short ZrO₂ fibers, 10–30 nm in length, were obtained, with no other zirconium compounds observed. The particles grew with treatment time and resulted in whisker particles. In a higher concentration (3 mol/L) H₂SO₄ solution, ZrO₂ whiskers were not obtained, and clear solutions resulted with the starting ZrO₂ particles remaining. It was concluded that Zr(OH)₄ was useful as a starting material and that nanosized ZrO₂ particles served as seed crystals for whisker formation.     

Hydrothermal Treatment of Si3N4 for the Improvement of Oxidation Resistance at 1400°C

The surface of Si₃N₄ ceramics was hydrothermally treated with HCl or H₂SO₄ using an autoclave. The thickness of the oxide layers formed on the Si₃N₄ samples decreased to one-fourth after oxidation at 1400°C by the treatment. The oxide layer of the treated samples was dense, and flaw formation in and beneath the layer did not occur at 1400°C. The avoidance of low melting Y-silicates by leaching Y₂O₃ is the reason for the improved oxidation resistance of the hydrothermally treated Si₃N₄, despite an increase in surface porosity through a 70 μm layer.     

Synthesis of Titanate Derivatives Using Ion-Exchange Reaction

Two types of titanate derivatives, layered hydrous titanium dioxide (H₂Ti₄O₉· n H₂O) and potassium octatitanate (K₂Ti₈O₁₇) with a tunnellike structure, were synthesized using an ion-exchange reaction. Fibrous potassium tetratitanate (K₂Ti₄O₉· n H₂O) was prepared by calcination of a mixture of K₂CO₃ and TiO₂ with a molar ratio of 2.8 at 1050°C for 3 h, followed by boiling-water treatment of the calcined products for 10 h. The material then was transformed to layered H₂Ti₄O₉· n H₂O through an exchange of K⁺ ions with H⁺ ions using HCl. K₂Ti₈O₁₇ was formed by a thermal treatment of KHTi₄O₉· n H₂O. Pure KHTi₄O₉· n H₂O phase was effectively produced by a treatment of K₂Ti₄O₉ with 0.005 M HCl solution for 30 min. Thermal treatment at 250°–500°C for 3 h resulted in formation of only K₂Ti₈O₁₇.     

Low-Temperature Synthesis of ZrC Powder by Cyclic Reaction of Mg in ZrO2–Mg–CH4

We investigated the conditions for low-temperature synthesis of ZrC fine powder from ZrO₂–Mg–CH₄. The synthesis utilizes a thermite-type reaction, with Mg as the reducing agent, and a reaction between Mg and CH₄ gas as a carbon source. The Mg/ZrO₂ molar ratio as well as the heating rate were varied. Because C can be continuously fed into the reaction group by the cyclic reaction of Mg through the formation and decomposition of Mg₂C₃ (2Mg + 3CH₄→ Mg₂C₃+ 6H₂→ 2Mg + 3C), a molar ratio of 2.2 for Mg/ZrO₂ was sufficient for the synthesis of single-phase ZrC. ZrC powders were synthesized under the following conditions: Mg/ZrO₂ molar ratio = 2.2, heating rate = 20°C/min, and temperature maintained at 750°C for 30 min. The amount of reaction heat produced in the reduction reaction of ZrO₂ by Mg depended on the Mg/ZrO₂ molar ratio, specifically, the amount of ZrO₂ contained. Moreover, the cyclic reaction of Mg-Mg₂C₃–Mg was influenced by the amount of reaction heat described above and by the heating rate. The ZrC fine powder showed little aggregation and high dispersibility.     

Corrosion Kinetics and Strength Degradation of Sintered δ-Silicon Carbide in Potassium Sulfate Melts

Pressureless sintered α-SiC ceramics containing carbon and boron as sintering aids and hot-pressed SiC containing aluminum nitride as a sintering aid were corroded in K₂SO₄ melts at 1080° to 1150°C. SiC ceramics were oxidized and dissolved into K₂SO₄ melts. Since the corrosion of SiC ceramics in K₂SO₄ melts proceeded autocatalytically, a reaction product such as K₂S_1.44 was suspected to promote the corrosion reaction. The corrosion resistance of SiC containing AlN in K₂SO₄ melt was superior to that of SiC containing boron and carbon. Apparent activation energies for the corrosion of SiC ceramics were 309 to 331 kJ mol^-1. The fracture strength of the specimens corroded by K₂SO₄ melt degraded to 40% to 70% of the original values up to a 20% weight loss and then was almost constant up to 45% weight loss.     

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.     

Molten Salt Synthesis of Calcium Hydroxyapatite Whiskers

Calcium hydroxyapatite (HA) whiskers and crystals were produced by the route of molten salt synthesis. The effects on whisker morphology of chosen flux, flux-to-HA ratio, synthesis temperature, and reaction time were investigated. The thermal stabilities of the produced whiskers were tested at 1300°C in an air atmosphere. A tentative X-ray diffraction pattern was proposed for the HA whiskers. Molten salt synthesis with a K₂SO₄ flux was found to be a simple and sturdy technique for manufacturing short (≤60 μm) HA whiskers in the temperature range from 1080° to ∼1200°C. The alternative use of fluxes such as KCl, KBr, CaCl₂, or Na₂SO₄, rather than K₂SO₄, over the temperature range 850°–1000°C resulted in the formation of large (∼25 μm) single crystals of HA.     

Electroceramics from Source Materials via Molecular Intermediates: BaTiO3 from TiO2 via (Ti(catecholate)3)2-

Rutile or anatase may be depolymerized and complexed by sequential treatment with (i) H₂SO₄/(NH₄)₂SO₄, (ii) H₂O, and (iii) catechol/NH₄OH to produce the intermediate (NH₄)₂(Ti(catecholate)₃) · 2H₂O. Treatment with Ba(OH)₂· 8H₂O leads to an acid-base reaction generating Ba(Ti(catecholate)₃) · 3H₂O, in which the Ba:Ti ratio is held at 1:1 at the molecular level. Calcination produces BaTiO₃ powder.     

Molten-Salt Corrosion of Silicon Nitride: II, Sodium Sulfate

The corrosion mechanism of Si₃N₄+ Na₂SO₄/O₂ at 1000°C was investigated. Corrosion of both pure and additive-containing Si₃N₄ was studied. The reaction of Si₃N₄+ Na₂SO₄ consists of an initial period of slow weight loss due to Na₂SO₄ vaporization and oxidation-dissolution. This initial period was the same for all forms of Si₃N₄. A second region consisted of further oxidation or near termination of reaction, depending on the additive in the Si₃N₄. A 5- to 10-μm yttrium depletion zone was found after corrosion for the Si₃N₄ with yttria additives. For comparison, Si and SiC were corroded under similar conditions. These materials corroded substantially faster than all forms of Si₃N₄.     

Ion Exchange in Alkali Layers of Potassium β -Ferrite ((1 + x)K2O · 11Fe2O3) Single Crystals

The potassium ions in potassium β-ferrite ((1 + x)K₂O ·11Fe₂O₃) crystals were exchanged with Na⁺, Rb⁺, Cs⁺, Ag⁺, NH₄⁺, and H₃O⁺ in molten nitrates or in concentrated H₂SO₄. On the other hand, spinel and hexagonal ferrites were formed by soaking the crystals in the melt of divalent salts. The crystals of K⁺, Rb⁺, and Cs⁺β-ferrites decomposed to form α-Fe₂O₃ at high temperatures of 800° to 1100°C. In addition, H₃O⁺, NH₄⁺, and Ag⁺β-ferrites decomposed to form α-Fe₂O₃ at relatively low temperatures of 350° to 650°C, in accordance with the stabilities of the inserted ions. The electrical properties of some β-ferrites were measured.     

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.