The effects of preparation method on the characteristics of alumina-zirconia powders

Two series of Al₂O₃-ZrO₂ powders with various contents of ZrO₂ were synthesized by sol-gel (chemical polymerization) and coprecipitation methods. The effect of ZrO₂ content and preparation method on the structure and texture of the resultant powders were determined. The samples were characterized by nitrogen sorption, thermal gravimetric analysis, X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. The sol-gel method yields more homogeneous powder and retards the aggregation of particles. It produces powders with a larger surface area, a smaller pore size, and a narrower pore size distribution than the coprecipitated one. The X-ray diffraction results show that the powders are amorphous at the calcination temperature below 700°C for both methods. Incorporation of zirconia into alumina greatly affects the surface properties of the powders upon heating, exerting a protective effect against sintering, and inhibiting both the crystallization of the -Al₂O₃ phase and the - to -phase transformation. Zirconia is dispersed in alumina matrix and does not form a new structure.     

Preparation and Characterization of Amorphous ZrO2-SiO2 Composite Powders Processed by Sol-Gel Chemistry

Composite ZrO₂-SiO₂ powders, with different ZrO₂ contents, including pure ZrO₂ powders, were prepared by precipitation in SiO₂ suspensions, of zirconia gels from solutions of zirconyl chloride at pH = 11. These products were investigated in connection with the phase changes in ZrO₂ caused by heat-treatments. ZrO₂-SiO₂ mixtures containing 0–100% mol ZrO₂, were studied by differential thermal analysis (DTA), X-ray powder diffraction (XRD), temperature programmed desorption combined with mass spectroscopy (TPD-MS), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), to obtain information on the morphological and structural features of the particles before and during the heat treatment up to 1200°C. Specific surface areas were determined using nitrogen adsorption by the BET method. The results offer an explanation about some of the factors which can be influencing on the stabilization of metastable-cubic/tetragonal (C/T) phase of ZrO₂ and the evolution of surface areas (vulcano profile) observed in the composites.     

Atomic-Scale Structure of Water-Based Zirconia Xerogels by X-Ray Diffraction

The structural evolution of zirconia thin films and gel powders has been evaluated by X-ray diffraction. Maxima (r ₁ and r ₂) of the experimental radial distribution function RDF and the bond angles were determined and correlated with TGA (thermogravimetric analysis), DTA (differential thermal analysis) and MS (mass spectrometry). The results indicate that the topological short-range structure (<5 Å) of amorphous zirconia thin films, independent of drying temperature, resembles that of crystalline tetragonal ZrO₂. In contrast, amorphous zirconia powder gels dried at temperatures below 120°C show atomic arrangements similar to that of tetragonal ZrO₂. The structure of these gels annealed at temperatures between 165–340°C resembles a distorted tetragonal ZrO₂, monoclinic-like structure. Zirconia powders and films contain crystalline tetragonal ZrO₂ at 400°C.     

Generation of WO3-ZrO2 catalysts from solid solutions of tungsten in zirconia

WO₃-ZrO₂ samples were obtained by precipitating zirconium oxynitrate in presence of WO₄ species in solution from ammonium metatungstate at pH=10.0. Samples were characterized by atomic absorption spectroscopy, thermal analysis, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and energy filtered-TEM. The ammonia retained in the dried sample produced a reductive atmosphere to generate W⁵⁺ ions coexisting with W⁶⁺ ions to produce a solid solution of tungsten in the zirconia lattice to stabilize the zirconia tetragonal phase when the sample was annealed at 560 °C. When the sample was annealed at 800 °C, the W atoms near crystallite surface were oxidized to W⁶⁺, producing patches of WO₃ on the zirconia crystallite. The HR-TEM analysis confirmed the existence of the solid solution when the sample was annealed at 560 °C, and two types of crystalline regions were identified: One with nearly spherical morphology, an average diameter of 8 nm and the atomic distribution of tetragonal zirconia. The second one had a non-spherical morphology with well-faceted faces and dimensions larger than 30 nm, and the atom distribution of tetragonal zirconia. When samples were annealed at 800 °C two different zirconia crystallites were formed: Those where only part of the dissolved tungsten atoms segregated to crystallite surface producing patches of nanocrystalline WO₃ on the crystallite surface of tetragonal zirconia stabilized with tungsten. The second type corresponded to monoclinic zirconia crystallites with patches of nanocrystalline WO₃ on their surface. The tungsten segregation gave rise to the WO₃-ZrO₂ catalysts.     

The thermodynamic viability of yttria-stabilized zirconia pH sensors for high temperature aqueous solutions

The thermodynamic viability of the yttria-stabilized zirconia sensor (YSZS) [H₂O, H⁺/ZrO₂(Y₂O₃)/HgO/Hg] for the measurement of pH in high temperature aqueous solutions is evaluated by measuring potentials for this electrode and a conventional hydrogen electrode (HE) against a common reference electrode in a variety of solutions [0.01m H₃PO₄, 1m Na₂SO₄, 0.01m B(OH)₃+0.01m KOH, and 0.01m KOH] at temperatures from 298.15K (25°C) to 573.115K (300°C). In order to compare theoretical and experimental potentials for the cell

Effect of Fluorine Doping on Semiconductor to Metal-Like Transition and Optical Properties of Cadmium Oxide Thin Films Deposited by Sol–Gel Process

Highly conducting fluorine doped n-type cadmium oxide thin films have been synthesized by sol–gel dip coating process on glass and Si substrates for various fluorine concentrations in the films. X-ray diffraction pattern confirmed the cubic CdO phase formation and SEM micrograph showed fine particles of CdO with size ~0.3 m. F concentration in the films was varied from 1.8% to 18.7% as determined from energy dispersive X-ray analysis (EDX). The resistivity of the CdO films decreased with increase of F doping and increase of temperature below 14.6% of F, as usual for semiconductors. Above this F concentration the resistivity increased with increase of temperature like metals. Hall measurement showed very high carrier concentrations in the films lying in the range of ~2.93 × 10²⁰ cm^–3 to 4.56 × 10²¹ cm^–3. UV-VIS-NIR spectrum of the films showed the optical bandgap energy increased with increase of F doping and corresponding carrier concentrations obtained from Burstein-Moss shift also support the Hall measurement results.     

The Effect of Precursor Chemistry on the Crystallisation and Densification of Sol-Gel Derived Mullite Gels and Powders

Stoichiometric and silica-rich mullite gels and powders were prepared using four different sol-gel methods. Thermal analysis, X-ray powder diffraction and dilatometry techniques were used to investigate the thermal decomposition, crystallisation and sintering of these mullite precursor gels. The method of preparation, by controlled hydrolysis of various mixtures of tetraethylorthosilicate, aluminium sec-butoxide and aluminium nitrate, affected the texture of the gels, producing single-phase or diphasic samples.The crystallisation sequence of the gels depended on the composition and method of preparation. Single phase mullite crystallised from homogeneous gels at 980°C, while diphasic gels initially formed of a mixture of -Al₂O₃ spinel and mullite, or simple -Al₂O₃ spinel, which subsequently transformed to mullite at 1260°C.Dilatometry and density measurement were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 500°C. Varying the heating rates from 2 to 10°C min^-1 had little effect on the densification to 1500°C. However, the densification rate was sensitive to the degree of crystallinity and the amount and type of phases present at the sintering temperature. The presence of -Al₂O₃ spinel in the structure initially promoted densification, but the sintering rate was reduced considerably after mullite crystallised. Diphasic materials, especially those with an excess amount of silica in the original gel, sintered to higher densities due to the presence of excess silica promoting densification by viscous phase sintering.     

Microstructure and properties of large ceramic parts made from a Sol-Salt yttried zirconia powder

Yttried zirconia powders (3% mole Y₂O₃: Y-TZP) were produced with a specific Sol/Salt process. The advantage of Sol/Salt process is that the powder reactivity (grain size) is determined by the synthesis of the initial Sol (hydrolysis conditions), and the shaping ability (ceramic manufacture) can be adjusted by control of surface state (calcination).Specific grades are derived from the Sol/Salt powder to produce high performance ceramic parts. After sintering, the microstructure is made of fine grains (<0.6 µm), without porosity and aggregates, with density values close to the theoretical one. The close packing of particles obtained by slip-casting leads to a high densification rate (relative density d>6.05), and high level of strength and toughness. Good properties are also obtained by pressing.     

Studies on lattice thermal expansion and XPS of ThO2NdO1.5 solid solutions

The lattice parameter changes with respect to temperature (T) have been measured by high temperature X-ray diffraction (HTXRD) technique for ThO₂NdO_1.5 solid solutions containing 23.8 and 42.5 mol% NdO_1.5 in the temperature range from 298 to 2000 K. The temperature versus lattice parameter data have been made use of in calculating the lattice thermal expansivity. The values of thermal expansion of the solid solutions were found to be increased with increase in neodymium oxide content and temperature. The mean linear thermal expansion coefficients in this temperature range for ThO₂NdO_1.5 solid solutions are 12.28 × 10⁻⁶ and 12.90 × 10⁻⁶ K⁻¹, respectively. The binding energies of Th 4f_7/2 and Nd 3d_5/2 energy levels of the solid solutions containing 13.1, 23.8, 31.9, 37.2 and 42.5 mol% NdO_1.5 and two-phase mixtures containing 47.6 and 51.8 mol% NdO_1.5 were experimentally determined by X-ray photoelectron spectroscopy (XPS).     

Investigation of the complex thermal behavior of fats

The thermal behavior of three ural fats (displaying very different composition), cocoa butter (CB)², lard, and a stearin obtained from anhydrous milk-fat (AMF) fractionation, were studied by both DSC and X-ray diffraction as a function of temperature (XRDT). To perform temperature explorations between –30C and +80C, at rates identical to those used for DSC and ranging from 0.1 K min^–1 to 10 K min^–1, a new set of X-ray sample-holders, temperature-controlled by Peltier effect, has been developed. It is shown that the three more stable polymorphic forms of CB were easily characterized by either X-ray diffraction or DSC, and existence of two -3L forms was confirmed. On the contrary, the more complex polymorphism of lard and AMF required combined examination by DSC and XRDT and the brightness of the synchrotron source for studies at the highest heating rates. Quantitative analysis of the long spacings of XRDT recordings is invaluable for interpretation of thermal events. For instance, it was found that the simultaneous formation of two polymorphic forms, of apparent long spacing of 34 and 42 å, at the onset of lard crystallization might explain the difficulty of its fractionation.Special thanks to Courtney P. Mudd (NIH, Bethesda) for his pertinent advice on the mounting and use of thermoelectric devices. The study of lard crystallization was initiated by Valerie Portalier and suggested by Jean-Luc Vendeuvre of CTSCCV (Maisons-Alfort). For the AMF part of this study, stearin was fractionated by ADRIA Normandie, while characterization of its thermal properties was performed as part of a research program funded by ARILAIT Recherches and the French Ministry of Research and Technology.     

Direct analysis of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powders by total reflection X-ray fluorescence spectrometry

A direct analysis procedure for the determination of trace impurities of Ca, V, Cr, Mn, Fe, Ni, Cu, Zn and Ga in Al₂O₃ ceramic powders by total reflection X-ray fluorescence spectrometry (TXRF) is described. The powders were analysed in the form of slurries containing 1–10 mg mL^–1 of powder. The use of the procedure in the case of powders with differing grain size and for different slurry concentrations was investigated. Three different quantification possibilities were compared, namely the use of Al as a matrix component, the use of Fe as a trace element contained in the sample or of Co added in concentrations of 200 g g^–1 as internal standard. The homogeneity of elemental distributions in sample layers deposited on the TXRF quartz carriers by evaporating 5 L of the 10 mg mL^–1 slurries was studied by scanning the 4- to 5-mm-diameter spots of two samples by synchrotron radiation TXRF at Hasylab. For powders with differing graininess but mainly finer than about a few 10 m, no systematic influence of the grain size on the accuracy of the determinations of Ca, V, Fe, Ni, Cu and Zn could be observed. The measurement precision, however, seemed to be limited by inhomogeneous distributions of the trace elements in the samples as testified by the synchrotron radiation TXRF scans. Detection limits of the developed TXRF procedure for Ca, V, Cr, Mn, Fe, Ni, Cu, Zn and Ga were found to be in the 0.3–7 g g^–1 range and were shown to increase slightly with the grain size of the samples. Quantification using Al (matrix) as internal standard led to systematically higher values out of the accuracy required, whereas the other two approaches in all cases led to reliable results.Dedicated to the memory of Wilhelm Fresenius     

Thermal Treatment of Sol-Gel Derived Nickel Oxide Xerogels

Very fine nickel hydroxide and oxide xerogel powders were prepared using a new sol-gel synthesis procedure in which nickel ethoxide was produced through the reaction of nickel chloride, as a precursor, with sodium ethoxide in dehydrated ethanol, followed by the hydrolysis of nickel ethoxide with ammonia and drying the resulting hydrogel under subcritical pressures to form the xerogel. The effects of thermal treatment on the surface area, pore volume, crystallinity and particle structure of the resulting xerogels were investigated and found to have significant effects on all of these properties. Overall, the xerogel remained amorphous as Ni(OH)₂ space up to 200°C, with little change in the surface area and pore volume. At 250°C, the Ni(OH)₂ began to decompose and form crystalline NiO with the uniformity of the crystals increasing with an increase in temperature. The surface area and pore volume decreased sharply when increasingthe temperature beyond 250°C; this was the temperature where maximums of about 270 m²/g and 0.33 cm³/g were exhibited by this composite amorphous Ni(OH)₂ and crystalline NiO xerogel powders. At the higher calcination temperatures, very uniform NiO crystals with average crystallite sizes of 1.7 nm and 14.5 nm were obtained at 400 and 600°C, respectively.     

Some significant advances in sol-gel processing of dense structural ceramics

This review deals with the solution-sol-gel processing of some structural ceramics such as alumina, zirconia, mullite and cordierite and brings out the most significant advances in the preparation of dense ceramics. In the Al₂O₃ system, seeding of gels with -Al₂O₃ and other isostructural seeds led not only to lower crystallization temperature but also enhanced densification with refined microstructure through solid-state epitaxy. This breakthrough of seeding has led to improved abrasive grains with large commercial market. Although highly dense, partially stabilized zirconia ceramics were prepared using monodisperse and spherical or nanophase zirconia sol-gel powders, no commercial applications seem to have been realized thus far. In the Al₂O₃–SiO₂ system, compositionally different sol-gel nanocomposites (diphasic gels) led to enhanced densification of mullite at lower temperatures because of the occurrence of densification and crystallization processes almost simultaneously. Mullite powders derived from the diphasic gel route are a breakthrough and are now commercially marketed by Chichibu Cement Company. Highly dense cordierite was prepared by using three sols, i.e., the compositionally different sol-gel nanocomposites in an analogous manner to that of mullite. Although this process appears to be highly cost-effective, especially for the fabrication of substrates, it has not yet been utilized for commercial applications. The use of sol-gel nanocomposites in the processing of various ceramics is expected to be fully exploited in the future.     

Kinetics of colloid formation during the preparation of sol-gel zirconia

The formation of zirconia colloids by hydrolysing zirconium n-propoxide in n-propanol has been investigated by simultaneous, multi-angle static and dynamic light scattering, and vibrational spectroscopy, as a function of reactant concentration, water-to-alkoxide mole ratio and temperature.The overall hydrolysis/condensation reaction followed pseudo 2nd-order kinetics at 303 K, with an induction period of <1 to 24 hours. The induction period could be substantially reduced by increasing the temperature to 348 K. For hydrolysis with 3.6 moles of water per mole of alkoxide, the apparent activation energy was 24 kJ mol^–1. Such a low activation energy implies that hydrolysis occurs readily over the temperature range investigated (303–348 K). During the induction period, processing with stoichiometric, or excess, water produced oxy-hydroxides, while hydrated oxides formed under water-deficient conditions.The hydrolysis reactions yielded zirconia colloids with equivalent spherical, z-averaged diameters of <200 nm. The colloids exhibited fractal dimensions of 3.0, with a low size-polydispersity, inferring the formation of dense, monodispersed spherical particles. SEM observations confirmed these results.     

Synthesis of nanocrystalline tetragonal zirconia by a polymeric organometallic method

A polymeric precursor method based on the Pechini process was successfully used to synthesize zirconia–12 mol% ceria ceramic powders. The influence of the main process variables (citric acid—ethylene glycol ratio, citric acid—total oxides ratio and calcination temperature) on phase formation and powder morphology (surface area and crystallite size) were investigated. The thermal decomposition behavior of the precursor is presented. X‐ray diffraction (XRD) patterns of powders revealed a crystalline tetragonal zirconia single‐phase, with crystallite diameter ranging from 6 to 15 nm. The BET surface areas were relatively high, reaching 95 m² g⁻¹. Nitrogen adsorption/desorption on the powders suggested that nonaggregated powders could be attained, depending on the synthesis conditions. Copyright © 1999 John Wiley & Sons, Ltd.     

Mesoporous TiO<Subscript>2</Subscript> thin films prepared from hydrothermally treated precursor powder sols

Two series of TiO₂ thin films were prepared based on soluble precursor powders: The first run originated directly from an alcohol-based coating solution whereas for the second batch the aqueous precursor powder sol had previously undergone a hydrothermal treatment. The respective microstructures were characterized by electron microscopy, the phase evolution was monitored by X-ray diffraction. Ellipsometric porosimetry (EP) was employed to reveal changes of porosity and pore size induced by thermal treatment of the films.

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Soluble TiO₂ precursor powders were hydrothermally treated to yield coating solutions. Films from these sols were compared with those directly obtained by dissolving the precursor powders. Results indicate that crystallization to anatase is induced under hydrothermal conditions and the resulting films mostly maintain their porosity throughout thermal treatment. In contrast to that coatings processed from as-dissolved precursor powders undergo more extensive densification

     

Colloidal stability of electrostatically stabilized sol particles. Part I: The role of hydration in coagulation and repeptization of ferric hydroxide sol

FeO(OH)·0.5 H₂O powders were prepared by drying portions of a FeO(OH)·0.5 H₂O sol at different relative vapor pressures, and the adsorption of water on the powders was determined. The magnitude of electrostatic potential barrier for the sol is of about 9 mJ·m^–2. The reduction in the immersion enthalpy and in the surface free-energy in a range of the relative vapor pressures of 0.0–0.9 is as high as 140 mJ·m^–2 and 130 mJ·m^–2, respectively.It follows from the foregoing that two potential barriers may form. The electrostatic barrier presumably regulates the rate of flocculation and the hydration barrier (at closer separations) regulates the rate of particle coalescence or sintering.Peptizability of the FeO(OH)·0.5 H₂O powders dried at relative vapor pressures between 0.4 and 0.9 was found to be fairly high, presumably because the adsorbed water prevented the formation of close contacts between the primer particles. Lowering the vapor pressure, however, resulted in a notable decrease in the peptizable amount, and also a considerable increase in the particle size of the peptized sol.     

Studies on recrystallised aluminium trihydroxide precipitates: The energetics of dissolution by sodium hydroxide solutions

Summary Aluminium hydroxide gels were recrystallised in high pH solution for 2–1000 hr to give a series of gibbsite powders of different surface areas, 1`11/2, P11/8, P11/48, P11/360 and P11/X; their average platelet lengths were 30, 60, 130, 260 and 450 nm. The dissolution equilibria of these powders in sodium hydroxide solutions of C=1-8 M were studied at 20° to 80°C.The equilibrium constants (for formation of diaquo-tetrahydroxoaluminate anion) increased with temperature and with increasing surface area; the K value for P11/2 was about four times that of P 11/X at 20 °C and about three times (that for P 11/X) at 80 °C. The dissolutions were endothermic; however, the value over this temperature range increased from –29.5 kJ mol<sup>–1</sup> (for P11/X) to –24.7 kJ (for P 11/2). This increase was related to the appreciable surface energies of the high surface-area materials.The (– G°) values increased with increasing temperature and in turn with increasing surface area; the (– G°) values for 1`11/2 dissolution at any temperature were about 4 kJ mol^–1 higher than those for P11/X dissolution.With 1 figure and 1 table     

Photoluminescent coordination polymer bulk glasses and laser-induced crystallization

We synthesized luminescent coordination polymer glasses composed of d¹⁰ metal cyanides and triphenylphosphine through melt-quenching and mechanical milling protocols. Synchrotron X-ray total scattering measurements and solid-state NMR revealed their one-dimensional chain structures and high structural dynamics. Thermodynamic and photoluminescence properties were tunable by the combination of heterometallic ions (Ag⁺, Au⁺, and Cu⁺) in the structures. The glasses are moldable and thermally stable, and over centimeter-sized glass monoliths were fabricated by the hot-press technique. They showed high transparency over 80% from the visible to near-infrared region and strong green emission at room temperature. Furthermore, the glass-to-crystal transformation was demonstrated by laser irradiation through the photothermal effect of the glasses.

Over centimeter-sized luminescent coordination polymer glasses were fabricated. They showed high transparency (over 80%) and strong green emission at room temperature. The glass-to-crystal transformation by laser irradiation was demonstrated.     

123Sb NQR and 19F NMR study of potassium,rubidium, and cesium heptafluorodiantimonates(iii)

The dynamics of crystal lattices of potassium, rubidium, and cesium heptafluorodiantimonates(iii) and specific features of internal rotations of the Sb₂F₇ fluoride groups in these compounds were studied using ¹²³Sb NQR in the temperature interval from 77 to 325 K and ¹⁹F NMR in the temperature interval from 240 to 470 K in combination with X-ray diffraction and thermogravimetric analyses. The distinctions in the dynamic behavior of the fluoride ions with changing the size (polarizability) of outer-sphere cations are discussed. The structural phase transition in CsSb₂F₇ was revealed at 425—430 K accompanied by the appearance of a high ion conductivity ( 1.3·10^–3 S cm^–1 at 450 K). A second type phase change can exist at 220—270 K.