Modeling of the Phase Transformation Induced by Ball Milling in Anatase TiO2

A high-pressure and high-temperature phase of TiO₂ : TiO₂ II is formed transiently during room-temperature high-energy ball milling of anatase TiO₂ : TiO₂ anatase TiO₂ II TiO₂ rutile. Rutile is the only phase present after prolonged ball milling. The present paper focuses on the influences of physical and chemical processing conditions on the transformation kinetics. The effects of two milling parameters on the kinetics of phase transformation of anatase TiO₂ were investigated: the nature of milling tools and the powder-to-ball weight ratio R. Granulometric characterizations and TEM observations have demonstrated that the transformation of TiO₂ anatase into TiO₂ II occurs without fracturing of particles and that TiO₂ II nanograins form at the surface of anatase particles. The parameter R affects only the transformation rate. For a given R, the transformation rate is the largest with alumina grinding tools, intermediate with zirconia tools, and the smallest with steel tools. The parameters involved in current models of the mechanical alloying process do not suffice to explain the differences in transformation rates observed here. A parameter, which takes into account the influence of the mechanical properties of grinding materials, is considered.     

The crystalline phase stability of titania particles prepared at room temperature by the sol-gel method

Titania particles having anatase, brookite and rutile phase were prepared at various H⁺/TTIP (Titaniumtetraisopropoxide) mole ratios and room temperature by the sol-gel method. The crystalline phases according to the variation of the post heat treatment temperature were observed. The crystalline phase and the phase transformation, morphology, and crystallite size were identified by using XRD, TG/DTA, Raman spectroscopy and TEM. The brookite phase of titania particles prepared at the H⁺/TTIP mole ratio of 0.02 and room temperature was not transformed into anatase or rutile even with the heat treatment at 750°C, and also the anatase phase was stable at the temperature as high as 850°C. However, the titania particles prepared at the H⁺/TTIP mole ratio of 0.67, which contained the mixed phases of anatase, brookite, and rutile at room temperature, showed only rutile phase at temperature of 750°C. It was thus shown that the initial crystalline phase of the primary particles prepared at room temperature had an important effect on the phase transformation behavior upon post heating. Phase transformation from brookite to anatase and subsequently to rutile occurred with heating.     

TEM observation of growth and phase transformation in nanometer-sized titanium oxide powder

The growth and phase transformation in the nanocrystalline titania powders were studied using TEM and X-ray diffraction. Titania particles consisting of anatase phase grow through coalescence. After a nucleus of rutile is formed, it grows at the expense of the nearest anatase particles. A nucleus of rutile is formed in two ways. The nucleation of rutile occurs at the amorphous interface between the small anatase particles. In a large anatase particle, the martensitic transformation is dominant. It is estimated that these two kinds of phase transformation mechanisms are controlled by the intrinsic and extrinsic defects. A rutile particle has the low-energy surfaces with a lot of terraces. The growth direction of a rutile particle is related to the orientation of twin. A rutile particle grows along a \( \langle 1\overline{2} 1\rangle \) direction on the {101}-twin plane.     

Zero Sound Attenuation Near the Quantum Limit in Normal Liquid 3He Close to the Superfluid Transition

The zero sound attenuation of normal liquid ³He has been studied over a range of temperatures from slightly above the superfluid transition temperature, T _c, to approximately 10mK at the constant pressures of 1 and 5bar. Using longitudinal LiNbO₃ transducers, operating both on and off resonance, the experiment was performed at 15 discrete frequencies located in several broadband frequency windows, including 16–25, 60–70, and 105–111MHz. The results are compared to Landau's prediction for the attenuation of zero sound in the quantum limit, (k _B Tk _B T _F), where ₀(P,T, )= (P) T ²{1+(/2k _B T)²}. Calibration of the received zero sound signals was performed by measuring the temperature dependence of the first sound attenuation from 30 to 800mK at those same frequencies and pressures. The data are compared to previous results.     

Pair and Quasiparticle States of YBa2Cu3O7?x Films Deduced from the Surface Impedance and a Comparison with Nb3Sn

Surface impedance data at 19 and 87 GHz of high-quality epitaxial YBCO films on different substrates are compared with data for Nb₃Sn films on sapphire in terms of pair and quasiparticle (qp) transport. Surface resistance R _s and penetration depth of YBCO are strongly affected by temperature dependent qp scattering, which is depressed in films with enhanced lattice strain. All films showed a comparable residual resistance R _res(19 GHz)90 constituting a qp reservoir which is likely to be caused by the electronic configuration and by impurities. Subtracting R _res from R _s (T) revealed activated behavior with a reduced energy gap ₀/k _B T _c0.9 for a film on sapphire, but power-law behavior for the other films. The penetration depth did not reveal power-law dependences at T0.5 ·T _c, but was consistent with a reduced energy gap of 0.45 for a film on MgO. The increase of (T) at T0.5 · T _c was related to qp scattering, which also caused an extremal conductivity ₁(T). A shoulder in (T) at T=(0.6–0.7) · T _c confirmed evidence for the existence of two superconducting bands. The magnetic-field induced recovery of (B) of various YBCO films hinted for an important role of magnetic scattering. The results are in contradiction to a d-wave symmetry of the order parameter, at least for the chain band.     

Damped response of an axially excited rotating liquid bridge in zero-gravity

Summary The response of a solidly rotating finite liquid bridge due to axial excitation exhibits for frictionless liquid at the resonances singularities. For the experimenter in a spacelabmission the actual resonance amplitude is of quite some importance. For this reason damping, that has to be measured in ground tests, has been introduced into the results of the response.Notation a radius of the liquid bridge - h length of the liquid bridge - I ₀,I ₁ modified Besselfunctions - J ₀,J ₁ Besselfunctions - r, ,z polar coordinates - t time - excitation amplitude - elliptic case - hyperbolic case - abbreviation - damping factor of liquid - (z, t) free surface displacement - =² – ² surface tension - surface tension - liquid density - ₀ rotational speed of liquid bridge - forcing frequency of axial excitation - natural frequency of liquid bridge With 2 Figures     

Preparation and characterization of sodium titanate nanowires from brookite nanocrystallites

Sodium titanate nanowires from brookite (or anatase) nanocrystallites were synthesized by a hydrothermal treating in 10 M NaOH aqueous solution at 180 °C. Their structures were characterized by XRD, TEM, HRTEM, XPS, and Raman spectra. The results indicated that though the structures of sodium titanate nanowires derived from brookite and antase were similar, there existed differences between them. The decrease of (2 0 0) plane spacing resulted from the increase of Na amount intercalated into TiO₂. The species of short TiO bonds in sodium titanate from brookite was more than that in sodium titanate from anatase. UV–vis absorption spectra showed a strong absorption not only in the ultraviolet range but also in the visible-light range.     

Investigation of the dominant point defects in tetragonal YBa2Cu3Ox at elevated temperatures

The manner in which oxygen is incorporated into YBa₂Cu₃O _x (YBCO) at 800°C for values ofx close to 6 is shown to be in the form of neutral oxygen interstitials, O _i ^x . The experimental data on which this conclusion is based are obtained from measurements of oxygen partial pressure,P(O₂), as a function of compositionx and temperatureT (5.99x 6.35, 825T1120 K). The data are obtained by a solid-state electrochemical method. Other conclusions of this study include: (a) O _i ^x are noninteracting forx 6. (b) The stoichiometric composition of YBCO isx 6.0. (c) The reaction enthalpy of oxidation is 179 kJ/mol O₂. (d) The Fermi level changes by –0.2 eV asx increases from 6.05 to 6.35.     

Effect of temperature on interfacial shear strengths of SiC-glass interfaces

The in situ temperature dependencies of both the debonding, _d, and frictional, _f, shear stresses of a C-coated 140 m SiC monofilament (Textron SCS-6 SiC fibre) were measured using the single fibre pullout-test. Two matrices, a borosilicate (7740 Corning Glass) and a soda-lime (Thomas Scientific) with different thermal expansion coefficients, were tested. At lower temperatures both _d and _f were found to decrease linearly with increasing temperature as a result of the relaxation of the residual stresses developed during processing, which were compressive in both cases. The stress free debonding shear stress for the borosilicate matrix was found to be 3.5 ± 1 M Pa and the friction coefficient between that matrix and the fibres was calculated to be 0.18. Fibre oxidation are believed to be responsible for enhanced bonding between the fibres and the borosilicate matrix at higher temperatures which results in an increase in both _d and _f. The large thermal expansion mismatch between the soda-lime matrix and the SiC fibres resulted in radial cracking of the former during processing. A technique is described where the whole temperature dependence of the interfacial shear stresses can be measured by a single specimen.     

Selective synthesis of brookite,anatase and rutile nanoparticles: thermolysis of TiCl<Subscript>4</Subscript> in aqueous nitric acid

The three polymorphs of titania (anatase, rutile and brookite) have been obtained as nanoparticles selectively and with well definite morphologies (platelets of brookite, rods of rutile) by thermohydrolysis of TiCl₄ in concentrated aqueous nitric acid. The selectivity of the synthesis depends strongly on the acidity of the medium. The presence of concentrated nitrate ions seems to be the determining factor for the formation of brookite and its stabilization against recrystallization.     

Synthesis of nanocrystalline TiO<Subscript>2</Subscript> by tartarate gel method

A gel was formed when a mixture of TiOCl₂ and tartaric acid was heated on a water bath. Ultrafine powders of TiO₂ in the anatase phase were formed, when the gel was decomposed at 623 K and the mole ratio of tartaric acid to titanium was 2. The anatase phase was converted into rutile phase on annealing at higher temperatures, > 773 K. When initial ratio of titanium to tartaric acid was < 2, the decomposition of gel leads to the formation of mixed phases of rutile and anatase. However, pure rutile phase was not formed by the decomposition of gel for any ratio of tartaric acid and titanium. These powders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and surface area measurements. The average particle size obtained for anatase phase was 3 nm whereas it was 30 nm for rutile phase. Raman scattering experiments were also performed to confirm both anatase and rutile phases.     

Estimation of reaction conditions for synthesis of nanosized brookite-type titanium dioxide from aqueous TiOCl<Subscript>2</Subscript> solution

Ti O₂ nanoparticles with a mixture of brookite and rutile phases were prepared from aqueous TiOCl₂ solution at 80–150°C and pure rutile phase at 200°C. The volume fraction of brookite was gradually increased with increase of HCl concentration in the range of about 4.43 M to 6.28 M. The maximum volume fraction of brookite in the as-prepared TiO₂ particles was obtained when oxidation of Ti⁴⁺ to TiO₂ was completed but it was gradually decreased with increase of reaction time. The reaction time for complete oxidation of Ti^{4 +} to TiO₂ was about 15 h at 80°C, about 5 h at 100°C, about 2 h at 120°C, and about 1 h at 150°C, respectively, showing that the kinetics of oxidation is very dependent on the reaction temperature. Brookite phase was not transformed directly to rutile phase but to anatase phase by heat-treatment at about 750°C, which finally converted to rutile phase at 1100°C.     

Crystalline transformation of Ti-B-O,Al-Ti-B-O,Si-Ti-B-O and Al-Si-Ti-B-O by thermal treatments

The thermal transformations of Ti-B-O, Al-Ti-B-O, Si-Ti-B-O and Al-Si-Ti-B-O have been investigated using the methods of thermal analysis and X-ray powder diffraction. The materials are a crystalline series of TiO₂ with partial replacement of titanium by the elements, aluminium, boron and silicon. The anatase form of the materials was transformed to the rutile form at 520680 °C for Ti-B-O, 880950 °C for Al-Ti-B-O, 10801280 °C for Si-Ti-B-O and 11801330 °C for Al-Si-Ti-B-O. The rate constant for the anatase-rutile transformation of Ti-B-O was 6.908×10^–3 min^–1 under isothermal conditions at 680 °C. Analysis of the kinetic data obtained by differential thermal analysis (DTA) gave the activation energy for transformation of anatase into rutile as 663.7 Kcal mol^–1 for Al-Ti-B-O. The lattice parameters for the compounds studied at various temperatures were calculated by least-squares fitting of the X-ray powder diffraction data.     

Effect of thermal treatment on the structure and texture of titania

Anatase and rutile pigments, from two sources (B and T) were thoroughly purified from sulphate and chloride contaminants, thermally treated in the temperature range 150 to 550 ° C, and investigated using thermogravimetric (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), nitrogen and organic vapour adsorption techniques. TG analysis reveals two main dehydration steps, the first results from physically adsorbed water and the second from structural and ligand water. The number of ligand water molecules released through a unit surface area (nm²) is in the range 4.50 to 5.15, being evolved in the temperature range 250 to 300 ° C. Two dehydroxylation endotherms appear for the anatase samples in the temperature range 350 to 420 ° C which seem to arise from the presence of two types of hydroxyls. No transformation from the anatase to rutile sructure occurred in the temperature range investigated. Estimation of crystallite sizes showed a marked increase at temperatures >250 ° C for anatase (B), being greatest for the (101) plane, and >400 ° C for rutile (B), where the three planes (101), (110) and (111) increased distinctly. Maximum anisotropy was observed for the anatase heated at 550 ° C. Nitrogen adsorption data revealed a marked decrease in the specific area and total pore volume by thermal treatment 400 ° C for anatase and 250 ° C for rutile whereby it retains a nearly stable value with an average range of 2.62 nm. The anatase (B) samples are composed of a mixture of both meso- and micropores whereas for rutile (B) microporosity appeared only for the sample heated at 150 ° C, becoming predominantly mesoporous at higher temperatures. Most heated samples exhibited two group sizes in the mesopore range resulting from their existence in the form of particles constituted from a collection of small particulates. The rutile (B) products are generally characterized by possessing a wider pore system than those from anatase. The anatase (T) samples are predominantly microporous at 150 ° C and become mesoporous at 550 ° C — the reverse is true for rutile (T). Cyclohexane and benzene adsorption measure only a fraction of the nitrogen area. Specific interaction (H-bonding) is believed to exist in some cases between the cyclohexane molecules and titania surface hydroxyls, as well as some enhanced adsorption which is believed to occur with benzene adsorbate.     

Electron diffraction and HREM studies of the new phase and superstructures in Ca4Al6SO16

The fine structure of unhydrated cement clinker Ca₄Al₆SO₁₆ (C₄A₃S) and its four superstructures have been studied by means of electron diffraction and high resolution electron microscopy (HREM). The match between experimental images and simulated ones verifies the result of X-ray structural analysis: C₄A₃S having a cubic lattice with space group l_43m,Z = 2. The frequent apperance of forbidden reflections such as 1/2, –1/2, 0 along the 110^* directions of C₄A₃S imply the presence of superstructures which may be due to an orderly inverse occupation of Ca²⁺ ions. Various structure models based upon the observed structural images have been proposed for these four superstructures. A new cubic phase witha = 1.5 nm is also found by selected area electron diffraction (SAED).     

Thermodynamic Properties and Decomposition of Lithium Hexafluoroarsenate, LiAsF6

The heat capacity of lithium hexafluoroarsenate is determined in the temperature range 50–750 K by adiabatic and differential scanning calorimetry techniques. The thermodynamic properties of LiAsF₆ under standard conditions are evaluated: C _p ⁰(298.15 K) = 162.5 ± 0.3 J/(K mol), S ⁰(298.15 K) = 173.4 ± 0.4 J/(K mol), ⁰(298.15 K) = 81.69 ± 0.20 J/(K mol), and H ⁰(298.15 K) – H ⁰(0) = 27340 ± 60 J/mol. The C _p(T) curve is found to contain a lambda-type anomaly with a peak at 535.0 ± 0.5 K, which is due to the structural transformation from the low-temperature, rhombohedral phase to the high-temperature, cubic phase. The enthalpy and entropy of this transformation are 5.29 ± 0.27 kJ/mol and 10.30 ± 0.53 J/(K mol), respectively. The thermal decomposition of LiAsF₆ is studied. It is found that LiAsF₆ decomposes in the range 715–820 K. The heat of decomposition, determined in the range 765–820 K using a sealed crucible and equal to the internal energy change U _r(T), is 31.64 ± 0.08 kJ/mol.     

Heat treatment on TiO2 nanoparticles prepared by vapor-phase hydrolysis

TiO₂ nanoparticles are prepared by vapor-phase hydrolysis of TiCl₄ below 550°C and are characterized by TEM, XRD and XPS before and after heat treatment at various temperatures. It shows that anatase/amorphous particles start to transform to rutile at certain temperature. Such transformation temperature tends to decrease with the decrease of particle size. When the rutile phase starts to be formed, the particles grow rapidly, while they grow slowly before the appearance of rutile titania. The Ti2p_3/2 binding energies tend to increase as particle sizes decrease, especially for particles below 50 nm in size.     

Orientation relations between carbon nanotubes grown by chemical vapour deposition and residual iron-containing catalysts

Orientation relationships between the growth direction of carbon nanotubes and encapsulated residual iron-containing particles have been determined using transmission electron microscopy. The nanotubes that are prepared by Fe-catalysed chemical vapour deposition on sol–gel Fe(NO₃)₃-tetraethyl orthosilicate substrates are the helical multiwall type. Nanoscale particles of both the low-temperature α-Fe (ferrite) and high-temperature γ-Fe (austenite) were found in the cavity of the carbon nanotubes with , and parallel to the tube growth direction, respectively. Cementite Fe₃C, the most abundant Fe-containing phase in present samples was also found to be entrapped in nanotubes with or parallel to the tube axis. The metastable retention of γ-Fe particles at room temperature is ascribed to the strain energy induced at the particle-nanotube interface due to volume expansion upon the γ- → α-Fe phase transformation. The decomposition of initially high aspect-ratio, rod-shape particles into a string of ovulation, while encapsulated in carbon nanotubes is accounted for by the Rayleigh instability. Ovulation leading to reduced particle size has also contributed to increase the surface energy term that counterbalances the total free energy change of phase transformation from γ- to α-Fe and further aids to the metastable retention of γ-Fe.     

Raman scattering characterization of gel-derived titania glass

The preparation by the sol-gel method of bulk vitreous TiO₂ is reported and its transformation both by controlled thermal treatment and by laser-induced heating in the anatase and rutile crystalline forms is studied by Raman spectroscopy. The results are compared with those existing for amorphous TiO₂ thin films. The dependence of the Raman frequencies and linewidths on the incident laser power is presented and the Raman spectrum of the brookite (a third naturally occurring form of TiO₂) is also reported as a comparison.     

EDX analysis and microstructural properties of the Yba2Cu3O7−δ-Ba2HoSbO6 superconducting composites

We have fabricated and characterized Yba ₂ Cu ₃ O _7– -Ba ₂ HoSbO ₆ a realized high T _c superconductor - ceramic insulator composite system in which particles of the superconductor and ceramic insulator could coexist with well defined separated phases left intact by stringent processing conditions. All the composites exhibit superconductivity at 92K. EDX analysis show that element% and atomic% of the composites are in a good agreement with those of their respective components. SEM studies show homogenous surface morphology and particle size distribution. There is no detectable interface interaction between component grains and Ba ₂ HoSbO ₆ grains are distinguishably distributed in the Yba ₂ Cu ₃ O _7– matrix.