Enhancing Effect of Iron Chlorides on the Anatase-Rutile Transition in Titanium Dioxide

The effect of solid Fe₂O₃ and gaseous iron chlorides on the anatase-rutile phase transition was investigated in the temperature range of 750°-950°C via X-ray diffractometry and scanning electron microscopy. In both cases, iron diffusion in the anatase lattice decreased the transition temperature and increased the anatase-rutile transformation rate, in comparison with that in TiO₂ fired in air. The enhancement effect of iron on the anatase-rutile transition is understood on the basis of oxygen-vacancy formation, which favors rutile nucleation. Solid-state iron diffusion between the contact points of TiO₂ and Fe₂O₃ particles and vapor mass transport through gaseous chlorides were the primary mechanisms of iron mass transport to the TiO₂ surface in the presence of both Fe₂O₃ and gaseous iron chlorides, respectively. The transformation rate at a given reaction temperature increased in the following order of reaction conditions: pure TiO₂ in air, TiO₂ in the presence of Fe₂O₃ in air, TiO₂ in the presence of Fe₂O₃ in chlorine, and TiO₂ in the presence of gaseous iron chlorides.     

Effect of B2O3 and hBN Crystallinity on cBN Synthesis

Four kinds of BN powders—amorphous BN with B₂O₃, partially crystallized BN without B₂O₃, well-crystallized hBN with B₂O₃, and well-crystallized hBN without B₂O₃—were prepared to determine the effect of B₂O₃ on the crystallization of amorphous BN and the effect of BN crystallinity on the formation of cBN under high pressure (4–5 GPa) and at high temperature (1350–1450°C). The amorphous BN with B₂O₃ easily crystallized and transformed to cBN in the presence of A1N catalyst, while the partially crystallized BN without B₂O₃ did not. The well-crystallized hBN transformed very slowly to cBN without B₂O₃, in contrast to fast transformation with B₂O₃. It is thus found that the transformation from hBN to cBN in the presence of AIN catalyst is determined by the degree of BN crystallinity as well as the presence of B₂O₃. Cubic BN can be synthesized only from crystallized hBN under the experimental conditions used. The formation of cBN from amorphous BN is possible through its prior crystallization, which can occur in the presence of B₂O₃.     

Anatase-Type TiO2 and ZrO2-Doped TiO2 Directly Formed from Titanium(III) Sulfate Solution by Thermal Hydrolysis: Effect of the Presence of Ammonium Peroxodisulfate on their Formation and Properties

Anatase-type TiO₂ powder containing sulfur with absorption in the visible region was directly formed as particles with crystallite in the range 15–88 nm by thermal hydrolysis of titanium(III) sulfate (Ti₂(SO₄)₃) solution at 100°–240°C. Because of the presence of ammonium peroxodisulfate ((NH₄)₂S₂O₈), the yield of anatase-type TiO₂ from Ti₂(SO₄)₃ solution was accelerated, and anatase with fine crystallite was formed. Anatase-type TiO₂ doped with ZrO₂ up to 9.8 mol% was directly precipitated as nanometer-sized particles from the acidic precursor solutions of Ti₂(SO₄)₃ and zirconium sulfate in the presence and the absence of (NH₄)₂S₂O₈ by simultaneous hydrolysis under hydrothermal conditions at 200°C. By doping ZrO₂ into TiO₂ and with increasing ZrO₂ content, the crystallite size of anatase was decreased, and the anatase-to-rutile phase transformation was retarded as much as 200°C. The anatase-type structure of ZrO₂-doped TiO₂ was maintained after heating at 1000°C for 1 h. The favorable effect of doping ZrO₂ to anatase-type TiO₂ on the photocatalytic activity was observed.     

Effect of the Presence of Ammonium Peroxodisulfate on the Direct Precipitation of Ceria and Ceria–Zirconia Solid Solutions from Acidic Aqueous Solutions

Direct precipitation of nanometer-sized particles of ceria–zirconia (CeO₂–ZrO₂) solid solutions with cubic and tetragonal structures was successfully attained from acidic aqueous solutions of cerium(III) nitrate (Ce(NO₃)₃) and zirconium oxychloride (ZrOCl₂) through the addition of ammonium peroxodisulfate ((NH₄)₂S₂O₈), because of promotion of the hydrolysis via the oxidation of Ce³⁺ ions, together with the simultaneous hydrolysis of ZrOCl₂ under hydrothermal conditions. Ultrafine CeO₂ particles also could be formed from relatively concentrated aqueous solutions of the same trivalent cerium salt in the presence of (NH₄)₂S₂O₈ via hydrolysis. The crystallite size and lattice strain of as-precipitated solid solutions varied, depending on the composition within the CeO₂–ZrO₂ system. Creation of a solid solution of ZrO₂ into a fluorite-type CeO₂ lattice clearly introduced lattice strain, as a consequence of the decreasing crystallite size. Both the direct precipitation process and the effectiveness of the presence of (NH₄)₂S₂O₈ for the synthesis of CeO₂–ZrO₂ solid solutions were discussed.     

Glass-Ceramic Formation in the ZnO-P2O5 System and the Effect of Silica as a Nucleating Agent

Crystallization of a series of ZnO-P₂O₅ based glasses was investigated. ZnO-P₂O₅-CaO glasses could be converted most readily to glass-ceramics and crystallization of these led to formation of alpha-Zn₂P₂O₇, alpha-CaZn₂(PO₄)₂, and ß-CaZn₂(PO₄)₂ phases. A further phase has been tentatively identified as monoclinic (Zn,Ca)₂P₂O₇. The most promising glass-ceramic composition Z15 (59.4ZnO·33P₂O₅·6.6CaO·1SiO₂) crystallized to alpha-Zn₂P₂O₇ and ß-CaZn₂(PO₄)₂, the latter phase being stabilized by the presence of SiO₂ which also encouraged volume nucleation giving a fine-scale (submicrometer) microstructure.     

Strength and Microstructure in Lithium Disilicate Glass-Ceramics

The effect of heat treatment on the microstructure of Li₂O-Al₂O₃·SiO₂ glass-ceramics which contain crystals of either Li₂SiO₃, Li₂Si₂O₅, or both was investigated quantitatively. Strength determinations for abraded rods were correlated with heat treatment on the basis of both size and distribution of crystals and the type and amount of crystal phases present. The presence of Li₂Si₂O₅ crystals enhanced the strength, whereas the presence of Li₂SiO₃ crystals did not change the strength of the abraded parent glass. The interrelation between strength and microstructure is discussed.     

Synthesis and Irreversibility of Oxygen Weight Loss in 80 K YBa2Cu4O8 Superconductor

Synthesis of YBa₂Cu₄O₈ superconductor usually requires oxygen pressure greater than 20 MPa and very long heating schedules at high temperatures. Here we report synthesis of YBa₂Cu₄O₈ superconductor by the standard solid-state reaction between the precursors BaCu₂O₃ and Y₂O₃ in the presence of alkali nitrate. NaNO₃, under ambient O₂ pressure. The resulting sample showed zero resistance at 78 K. The presence of 124 was confirmed by high-resolution X-ray diffraction studies. Irreversibility of oxygen weight loss was studied by thermogravimetric analysis and differential thermal analysis.     

Characterization of Interfacial Reactions Between β-Al2O3 and Y2O3-Partially-Stabilized ZrO2

The interfacial reaction between Y₂O₃-partially-stabilized ZrO₂ and α-Al₂O₃ was studied. It was noted that α-Al₂O₃ forms inside the periphery of the β-Al₂O₃ grains; its formation suggests the loss of Na₂O from the p-Al₂O₃, either by evaporation or by dissolution in the ZrO₂ matrix. The presence of Na₂ZrO₃ is suspected.     

Stability of YBa2Cu3O7-x in Molten Chloride Salts

The decomposition products of YBa₂Cu₃O_7-x depend on the composition of the molten chloride salt for exposure at 1173 K in air. The presence of dichloride salts such as CuCl₂, CaCl₂, or MgCl₂ promote formation of CuO, Cu₂Y₂O₅, and loss of barium to the chloride salt as BaCl₂. Salts based on BaCl₂ or containing LiCl result in YBa₂Cu₃O_7-x decomposition products of Y₂BaCuO₅, CuO, and BaCl₂. High barium activity in the salt supports formation of the Y₂BaCuO₅ phase and reaction of CO₂ with the salt producing BaCO₃. Decomposition is most sluggish in binary NaCl-KCl salts where minimal amounts of reaction or decomposition products are observed.     

Fluoride Model Systems: IV, The Systems LiF-BeF2 and PbF2-BeF2

Additional information on phase equilibria in the subsolidus region of the system LiF-BeF₂ was obtained and a study was made of the system PbF₂–BeF₂. In the system LiF-BeF₂ the compound LiBeF₃ occurs; it decomposes below the solidus at 300° C. to yield Li₂BeF₄ and the quartz form of BeF₂. An additional subsolidus compound having the probable composition LiBe₂F₅ forms below 275°C. No evidence was found for the existence of the alleged compound LiBe₂F₅. In the system PbF₂-BeF₂ two compounds occur: 3PbF₂.BeF₂ and PbF₂-BeF₂; the former melts congruently at 482°± 5°C. and the latter at 585°± 5°C. Extensive solid solution exists between PbF₂.BeF₂ and BeF₂. The cristobalite form of BeF₂ crystallizes from glasses of high BeF₂ content at temperatures of 450°C. or lower but is converted to the quartz form by heating at higher temperatures or in the presence of liquid.     

Dopants in Flame Synthesis of Titania

The effect of dopants on the characteristics of titania particles made by oxidation of TiCl₄ in a laminar diffusion flame reactor is presented. Introduction of dopant SiCl₄ inhibits the transformation of anatase to rutile, due to the formation of interstitian solid solution of SiO₂ and TiO₂. Silica decreases the sintering rate of titania and decreases the primary particle size, and, as a result, the specific surface area increases. Intruduction of SnCl₄ enhances the transformation of anatase to rutile, due to the similar crystalline structure of SnO₂ and rutile titania. However, the presence of SnO₂ and rutile titania. However, the presence of SnO₂ does not affect the primary particle size or the specific surface area of titania particles. Introduction of AICI₃ enhances the transformation of anatase to rutile, due to the formation of excess oxygen vacancies as Al₂O₃ and TiO₂ form a substitutional solid solution.     

Hydration Study of the System Ca3Al2O6–CaSO4· 2H2O–Ca(OH)2–H2O with and without Citric Acid

Hydration occurring in the system Ca₃Al₂O₆–CaSO₄· 2H₂O–Ca(OH)₂–H₂O has been studied at different temperatures and it was found that the reactions are diffusion controlled. The kinetic data obeyed Jander's equation and the rate of reaction increased with increasing temperature. X-ray diffraction studies and calorimetric measurements show that when gypsum is consumed, ettringite is converted into monosulfate. The rate of this conversion also increased with the increasing temperature and decreased in the presence of citric acid. Spectroscopic studies showed that there was some interaction between citric acid and the cement and that the product of hydration is of colloidal nature. Zeta potential measurements show that retardation of Ca₃Al₂O₆ hydration in the presence of gypsum and Ca(OH)₂ is not due to SO²⁻₄ adsorption. Electrical conductivity and thermoelectric potential measurements of solid Ca₃Al₂O₆ show that Ca₃Al₂O₆ is an n -type semiconductor and contains defects. The retardation of Ca₃Al₂O₆ may be due to poisoning of reaction sites by gypsum and Ca(OH)_2.     

Formation of Cubic ZrO2 with Transition Metals of Groups V and VI and Their Oxides

Cubic ZrO₂ can be formed at 1500°C from tetragonal ZrO₂ and transition metals of groups V and VI or their monoxides only in the presence of TiO₂. The 1500° isotherm of the pseudo-ternary system ZrO₂-TiO₂-VO was investigated and the extent of the single-phase cubic ZrO₂ region was determined. In the system Ta-Ta₂O₅ the lower oxides cannot be formed by reacting Ta and Ta₂O₅ between 1450° and 1815°C.     

Crystallization Behavior of Al2O3 in the Presence or SiO2

The independent crystallization sequence of an Al₂O₃ component is modified in the presence of SiO₂ and vice versa. Mixed SiO₂-Al₂O₃, gel (28 wt% SiO₂ and 72 wt% Al₂O₃) forms neither cristobalite nor γ-Al₂O₃ and corundum at 1000°C but forms Si-Al spinel; an amorphous aluminosilicate phase invariably also forms after the gel is heated. However, the composition of this amorphous aluminosilicate phase is not as yet known.     

Vaporization and Corrosion of Refractories in the Presence of Pressurized Pulverized Coal Combustion Slag

The corrosion and vaporization behavior of eight commercial refractories containing Cr₂O₃ were investigated under simulated pressurized pulverized coal combustion (PPCC) conditions at 1723 K in the presence of liquid slag. The corrosion resistance of the refractory materials decreased with increased content of free Cr₂O₃, because bursting (reaction of Fe₃O₄ with Cr₂O₃) occurred. Refractories containing MgCr₂O₄ with dissolved Al₂O₃ showed the highest corrosion resistance. Thermodynamic calculations showed that CrO₃ and CrO₂(OH)₂ were the most volatile species in air and in PPCC flue gas, whereas additions of A₂O (A = Na and K, minor slag component) increased the chromium vaporization significantly because of A₂CrO₄ formation. Knudsen effusion mass spectrometry measurements showed that the chromium vaporization of refractories was directly correlated to the Cr₂O₃ content of the material. In contrast, refractories containing MgCr₂O₄ showed a significant decrease in chromium vaporization.     

Stability of Si3N4 and Liquid Phase(s) During Sintering

Advanced sintering techniques for consolidation of Si₃N₄ powders in the presence of an oxygen-rich liquid phase(s) require high temperatures and usually high nitrogen pressures. A stability diagram is constructed for Si₃N₄ as a function of the partial pressures of nitrogen (P_N2) and silicon (P_Si). High P_N2 (20 to 100 atm) increases the stability of Si₃N₄ and the oxygen-rich liquid phase by reducing the P_Si and P_Si0, respectively. The region of high sinterability is outlined for submicrometer Si₃N₄ powders containing 7 wt% BeSiN₂ and 7 wt% SiO₂ as densification aids .     

Hot-Pressing of Si3N4 with Y2O3 and Li2O as Additives

The rates of densification and phase transformation undergone by α-Si₃N₄ during hot-pressing in the presence of Y₂O₃, Y₂O₃−2SiO₂, and Li₂0−2Si0₂ as additives were studied. Although these systems behave less simply than MgO-doped Si₃N₄, the data can be interpreted during the early stages of hot-pressing as resulting from a solution-diffusion-reprecipitation mechanism, where the diffusion step is rate controlling and where the reprecipitation step invariably results in the formation of the β-Si₃N₄ phase.     

Mullite–β-Spodumene Composites from Aluminosilicates

Sintering studies were conducted using kaolin, metakaolin, zeolite 4A, and various synthetic mixtures of Al₂O₃ and SiO₂ in the presence of Li₂CO₃ and LiCl as fluxing agents. Various compositions of the above were prepared, and conventional sintering studies were conducted at temperatures of 900°–1450°C with soaking periods of 1–3 h. Kaolin, metakaolin, and amorphized kaolin in the presence of Li₂CO₃ showed nucleation centers of β-spodumene as pink specks, whereas synthetic mixtures of Al₂O₃ and SiO₂ failed to behave in the same manner. To determine whether the pink specks formed were color centers or F centers, the samples were subjected to UV, IR, and X-ray irradiation; however, the samples showed no tenebrescence properties. External addition of iron as an impurity in a nonlayered system also resulted in pink speck formation. This observation indicated that impurities present in the natural kaolin were the cause of this phenomenon. Moreover, the LiCl-based samples did not result in pink specks, even though the kaolinitic samples contained iron as an impurity. Therefore, although β-spodumene was formed in aluminosilicates in the presence of Li₂CO₃ and LiCl, the pink variety of β-spodumene (kunzite) formation occurred only in the presence of lithium-rich aluminosilicates and in the presence of iron as an impurity. The phase identification and microstructure were explained based on XRD, DTA, and SEM studies.     

Hydration in the System Ca2SiO4–Ca3(PO4)2 at 90°C

Compositions along the Ca₂SiO₄–Ca₃(PO₄)₂ join were hydrated at 90°C. Mixtures containing 15, 38, 50, 80, and 100 mol% Ca₃(PO₄)₂ were fired at 1500°C, forming nagelschmidtite + a ¹-CaSiO₄, A -phase and silicocarnotite and a -Ca₃(PO₄)₂, respectively. Hydration of these produces hydroxylapatite regardless of composition. Calcium silicate hydrate gel is produced when Ca₂SiO₄≠ 0 and portlandite when Ca₂SiO₄ is >50%. Relative hydration reactivities are a -Ca₃(PO₄)₂ > nagelschmidtite > α ¹-Ca₂SiO₄ > A -phase > silicocarnotite. Hydration in the presence of silica or lime influences the amount of portlandite produced. Hydration in NaOH solution produces 14-A tobermorite rather than calcium silicate hydrate gel.     

Suppression of Rhombohedral-Phase Appearance and Low-Temperature Sintering of Scandia-Doped Cubic-Zirconia

Inhibition of cubic-rhombohedral phase transformation and low-temperature sintering at 1000°C were achieved for 10-mol%-Sc₂O₃-doped cubic-ZrO₂ by the presence of 1 mol% Bi₂O₃. The powders of 1-mol%-Bi₂O₃–10-mol%-Sc₂O₃-doped ZrO₂ were prepared using a hydrolysis and homogeneous precipitation technique. No trace of rhombohedral-ZrO₂ phase could be detected, even after sintering at 1000°–1400°C. The average grain size of the ZrO₂ sintered at 1200°C was >2 μm because of grain growth in the presence of Bi³⁺. Cubic, stabilized Bi-Sc-doped ZrO₂ sintered at 1200°C had sufficient conductivity at 1000°C (0.33 S/cm) to be used as an electrolyte for a solid-oxide fuel cell (SOFC) and at 800°C (0.12 S/cm) for an intermediate-temperature SOFC.