Phase Transformation Rate Research Articles

Evolution of Pore and Phase Structure of Sol-Gel Derived Lanthana Doped Titania at High Temperatures

Sol-gel derived porous titania was doped with lanthana at different dopant concentrations by a solution-sol mixing method to improve its phase and pore structure stability at high temperature. Very different effects of doping lanthana on the phase and pore structures of the sol-gel derived titania are found between the samples with low lanthana loadings (<0.27 g/100 m 2 ) and those with high lanthana loadings (≥0.27 g/100 m 2 ). The critical lanthana loading (0.27 g/100 m 2 ) corresponds to the monolayer coverage of La 2 O 2 on the grain surface of titania. At the low lanthana loadings (<0.27 g/100 m 2 ), doped lanthana covers the surface of titania particles. This decreases the sintering rate and retards the anatase to rutile phase transformation. Doping lanthana also reduces the loss of surface area (or change of the pore structure) of heat-treated samples as a result of decreasing sintering and phase transformation rates. At the high lanthana loadings, lanthanum titanates are formed because of reactions between lanthana and titania which are facilitated at high temperature and lanthana concentration

Effect of Peptization on Densification and Phase‐Transformation Behavior of Sol–Gel‐Derived Nanostructured Titania

Porosity reduction, packing, pore‐size distribution, and anatase to rutile phase transformation behavior of nanostructured titania ceramics prepared from both peptized and unpeptized sols were studied and compared using XRD, DSC, and nitrogen‐gas physisorption techniques. Precursor gels prepared from the peptized sol had a green density of about 70% after drying at 40°C, whereas the samples prepared from the unpeptized sol had a green density of only 50%. Samples prepared from the peptized sol showed higher sintering and phase transformation rates compared to the unpeptized sols.

Raman characterization of alumina supported MoVFe catalysts: Influence of calcination temperature

Alumina supported V 2O 5, MoO 3, Fe 2O 3, Fe 2O 3V 2O 5, and MoO 3V 2O 5 catalysts were investigated by Raman spectroscopy as a function of calcination temperature under ambient conditions where the surfaces are hydrated. The single metal oxide systems (V 2O 5, MoO 3, or Fe 2O 3) and the mixed metal oxide systems (Fe 2O 3V 2O 5 and MoO 3V 2O 5) are present as surface metal oxide species on the alumina support at intermediate calcination temperatures. For the single metal oxide species, the surface vanadium oxide species consists of metavanadate species ([VO 3] x− n ), the surface molybdenum oxide species consists of heptamolybdate (Mo 7O 6− 24) and octamolybdate (Mo 8O 4− 26) species, and the surface iron oxide species is primarily present as a surface spinel containing Fe 3+ ions in the surface region of the alumina support. Addition of iron oxide and molybdenum oxide to the V 2O 5/Al 2O 3 transforms the surface metavanadate species to surface pyrovanadate species (V 2O 2− 7) and surface decavanadate species (V 10O 6− 28), respectively. The thermal stability of the Supported metal oxide on the alumina support depends on the specific nature of the surface metal oxide species. The supported iron oxide on alumina has a high thermal stability, similar to the unpromoted alumina, which is probably related to Fe 3+ absorption into the surface region of the alumina lattice. In contrast, the supported molybdenum oxide on alumina system is not stable above 800°C due to significant loss in the alumina surface area and volatilization of molybdenum oxide. The surface vanadium oxide species on alumina is stable up to a calcination temperature of 800°C and transforms to crystalline V 2O 5 at higher calcination temperatures due to significant loss in surface area of the alumina support. The mixed metal oxide species (Fe 2O 3V 2O 5 and MoO 3V 2O 5) on alumina could not stabilize the surface vanadium oxide on alumina above 800°, and accelerated the rate of surface area loss and phase transformations. No FeVO 4, AlVO 4, or VMoO compounds were formed in these mixed metal oxide species on alumina at elevated temperatures.

Microstructural changes on the reduction of imperial smelting furnace sinters

The reduction reactions of Imperial Smelting Furnace (ISF) sinter microstructure were investigated in simulated zinc blast furnace conditions. Initial and partially reduced samples were examined using optical, electron-probe microanalysis, scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), and X-ray diffraction (XRD) to characterize the struc-tural and compositional changes occurring during the reduction reaction. The reaction mecha-nisms and reduction sequences for the various oxide phases within the sinter structure during reduction of ISF sinters under the system studied are discussed. The reduction of sinters resulted in the structural modification of zincite, franklinite, slag phases, and the formation of new oxide and metallic phases. The rate and sequence of these complex phase transformations were found to be dependent upon reduction time, temperature, and the reacting gas composition.

Modelling the CO-induced surface phase transition on Pt{100}: implications for kinetic oscillations

A recent molecular beam study of the CO-induced hex →(1 × 1) surface phase transition on Pt{100} showed that the rate of phase transformation has a high reaction order (ca. 4.1) with respect to the local CO coverage on the rotated hexagonal (hex-R) phase. This result is used to formulate a new kinetic model for the CO + Pt{100} system. It is used to investigate features of CO temperature-programmed desorption spectra, and to predict the temperature and pressure dependence of the CO and NO coverages on the (1 × 1) phase during the hex →(1 × 1) surface phase transition (SPT) and under equilibrium conditions. At temperatures of around 450 K, where kinetic oscillations in the CO + NO reaction on Pt{100} are accompanied by oscillations in the relative area of the (1 × 1) phase, the adsorbate coverages on the (1 × 1) phase are too low to inhibit NO dissociation. An alternative mechanism for these oscillations is presented, involving the hex ↔(1 × 1) SPT and not the rate limitation of NO dissociation as the driving force. Our new model is used to develop a more detailed understanding of this new mechanism.

Factors influencing the κ-Al 2O 3 → α-Al 2O 3 phase transformation during CVD growth

Chemical vapour deposition (CVD) of Al 2O 3 layers on silica substrates by the AlCl 3/CO 2/H 2 process was studied at temperatures from 950 °C to 1080 °C. κ-Al 2O 3 was grown initially in the whole temperature range. The growth direction of the κ-Al 2O 3 layers changed during the CVD process and eventually a phase transformation to α-Al 2O 3 occured. The phase transformation rate increased with increasing process temperature. Additionally, the influence of different background contamination sources on the phase transformation was studied. X-ray diffraction analysis and morphological examination of the Al 2O 3 layers showed that the AlCl 3/CO 2/H 2 process is extremely sensitive to impurities. The κ/α phase content as well as the morphology and microstructure of the aluminium oxide layers were strongly influenced by the presence of impurities and the air leak rate of the CVD system.

Thermally stabilized transitional alumina prepared by fume pyrolysis of boehmite sols

Alumina prepared by fume pyrolysis of sols consisting of fibrillar boehmite, 100 nm in length and 10 nm in diameter, was subjected to the thermal resistant tests. Even after calcination at 1473 K for 30 h, the alumina still consisted of fibrils and possessed a surface area as high as 50 m 2/g. This was ascribed to the suppression of the rate of phase transformation to α-alumina, caused by the fact that the alumina particles are of crude structures assembled by fibrils.

X-ray diffraction study of anisotropy by the formation and decomposition of nickel hydride Part II: Decomposition kinetics

The role of the microstructure on the decomposition of β-NiH formed as a result of the H cathodic charging of nickel was studied with the help of X-ray diffraction methods. This decomposition follows Kolmogorov-Avrami-Johnson-Mehl kinetics. The microstructural anisotropy induces anisotropy in the rate of phase transformation which is governed by the chemical reaction β-NiH→Ni+H, i.e. it proceeds without diffusion limitations.

CBr 4 vapor growth morphologies near the polymorphic transition point II. Crystals with large-angle grain boundaries

The polymorphic phase transition of CBr 4 crystals with large-angle grain boundaries or twin boundaries (extended macro-defects), was investigated during growth from the vapor by high resolution microscopy and image processing. We have found that on heating the monoclinic to cubic transition nucleates on the surface at the defects at temperatures distinctively below that of the bulk phase transition T tr. The phase transformation rate increase rapidly as the temperature approaches T tr. At a given supersaturation, the transformation rate is large as compared to the growth rates observed below and above the bulk transition temperature. Experimental evidence is given that these high growth rates result predominantly from surface diffusion and only to a small extent from vapor condensation. Thus the new phase grows mostly at the expense of the old phase, leading to surface depressions around the new growth locations. This rapid growth mode shows a new form of morphological instability.

Controlled Gelation and Sintering of Monolithic Gels Prepared from γ‐Alumina Fume Powder

The gelation, phase transformation, and densification of a colloidal monolithic gel made from γ‐Al2O3 fume powder are investigated. Among the six gelation agents that we use, formamide and urea are quick in causing gelation and easy to burn off. The densification rate of this gel decreases rapidly after the γ‐to‐α phase transformation. TiO2 is an effective sintering aid to overcome this bottleneck of densification because (1) it enhances the phase transformation rate so that the sintering of α‐alumina occurs at a lower temperature, and (2) it promotes sintering rates at the initial and intermediate stages after phase transformation. On the other hand, MgO has an inappreciable effect on gel sintering. The effect of MgO at the final sintering stage is obstructed by this densification barrier after transformation. The titania‐doped gel monoliths can be sintered to high density and fine microstructure at 1400°C.

High pressure polymorphs in ceramics and minerals GeO 2 and ZrO 2

Two examples of recent studies in high pressure polymorphs of inorganic substances were reported on GeO 2 and ZrO 2. The first one was the measurement of rate of phase transformation from α-quartz to rutile type of GeO 2. The phase change proceeded in two steps: induction period and structural change. From the analysis of measurement it was possible to calculate the activation energy for the phase transformation. The second was the investigation of ZrO 2. Its high pressure phases were difficult to quench, but thermodynamic data were obtained which enabled the calculation of slopes of phase boundaries.

Phase and structural transitions on heating or monoaxial deformation of rigid- and semirigid-chain polymers

The dependences of the completeness of phase transformations and structural features of fibres from rigid-chain polymers on temperature, and on the time and rate of mechanical actions has been discussed. It has been found out, to create an equilibrium fibre structure, the rate of mechanical actions should be comparable with the rate of phase and relaxation transformations.

The effect of certain processing variables on the form ii to form i phase transformation in polybutene‐1

AbstractDensity and wide angle x‐ray scattering techniques were used to study the form II to form I phase transformation in polybutene‐1. The influence of deformation by cold rolling, orientation produced by melt processing (film blowing), and certain additives blended with the homopolymer on the phase transformation behavior were examined. A new technique based on a combination of wide angle X‐ray diffraction and density measurements was devised to determine the relative fractions of form I, form II, and amorphous phases present. Small reductions in thickness by cold rolling cause a rapid partial transformation to form I followed by further transformation at enhanced rates. The enhancement of crystal transformation by cold rolling is primarily the result of the stresses or strains applied rather than the orientation produced. The introduction of molecular orientation by melt processing also enhances the rate of the form II to form I phase transformation, but it is rather less effective than small amounts of cold rolling which themselves produce comparatively little change in molecular orientation. The mechanical blending of polypropylene with polybutene‐1 was found to accelerate the form II to form I phase transformation, while another additive (1‐naphtylacetamide) known to be a good nucleating agent for crystallization of polybutene‐1 did not increase the rate of phase transformation when added at concentration levels needed to nucleate crystallization from the melt.

Effect of titanium on the rate of phase transformations in Fe-Cr-Co alloys

1. As the titanium content of the alloy is increased, the range in which γ phase exists becomes narrower, the initial temperature of the high-temperature α→γ+α transformation drops, and the time required for the transformation increases. 2. The temperature range in which the effect of a magnetic field is observe during TMT of Fe-Cr-Co alloys containing 12% Co and alloyed with Ti is 500–660°C.

A chlorine NMR study on the phase transformation of chloral hydrate

The rate of phase transformation between phase I and phase II (high and room temperature phase, respectively) of chloral hydrate was studied by measuring the change of 13Cl-NQR line intensity during the transformation. Decomposition of the material was found to have strong influence on the rate of the transformation II → I. The rate of the reverse transformation depends remarkably on the sample history. The transformation point of the enantiotropic transformation, T II→I, is found in the range 303 ⩽T II→I/K⩽308.

Measurement of solid-solid transformation rates by positron annihilation

Positron lifetime measurements have been used to obtain the rates of solid-solid phase transformations at 21°C in certain binary mixtures of n-eicosane and n-docosane.

Polymorphic phase transformation rates in shock-loaded potassium chloride

Single crystals of potassium chloride (KCl) were impacted on quartz stress gauges at velocities ranging from 0.0363 to 0.0755 cm/μsec, producing plane shocks with stress amplitudes between 17 and 31 kbar. Stresses greater than 21 kbar are sufficient to induce a polymorphic phase transformation in KCl. For impact stresses above the phase transition point, the theory of continuum mechanics is used to relate measured stress history at the impact interface to transformation rate between the phases. Experimental results show that, when shocked above the measured dynamic transition pressure of 20.8±0.5 kbar, KCl rapidly transforms to a metastable nonequilibrium mixture of phases. Phase transformation to these initial metastable states proceeds at a rate greater than 500 μsec−1 for shock propagation along the 〈100〉 and 〈111〉 directions, based on the magnitude of initial transformation and the minimum response time of the measuring system. The degree of partial transformation is observed to depend only on impact velocity and is independent of crystallographic orientation of the KCl. The observed metastable states are consistent with the postulate that the heat of formation of the nonequilibrium high-pressure phase is characterized by a specific entropy 3.9 × 10−6 cm2 μsec−2°K−1 higher than the equilibrium high-pressure phase. Subsequent phase transformation proceeds at a rate that depends on crystallographic orientation of the KCl. When the shock propagates along a 〈111〉 direction, it is observed that transformation proceeds from the metastable state to completion at a rate of 25±5 μsec−1, with final states in agreement with thermodynamic equilibrium calculations. When the shock propagates along a 〈100〉 direction, transformation proceeds at a rate of less than 5 μsec−1. For the 〈100〉 orientation, data are inadequate to allow determination of whether transformation was completed during the experiment, because of uncertainty in late-time quartz gauge response.

Experimental investigation of the rate of phase and chemical transformations in a high-velocity high-temperature stream

A procedure for the experimental-theoretical investigation of the evaporation and combustion of a liquid injected into a supersonic high-temperature stream is proposed.

Singularities of heat treatment of the cladded VT15 alloy

1. The optimum quenching temperature of cladded sheets of the VT15 alloy 3 mm thick is 800°C. 2. The cooling rate during quenching must be no lower than 3 deg/min. 3. Since the cladding has a certain passivating effect, the rate of phase transformations in the base metal is slowed down. 4. The areas of the metal close to the cladding layer are enriched in molybdenum and consist of undecomposed β-phase.