Phase Transformation Rate Research Articles

Effects of dehydration temperature on water vapor adsorption and dissolution behavior of carbamazepine

Anhydrous carbamazepine was prepared by heating carbamazepine dihydrate at 60, 80, 100, 120, and 140 °C and used to investigate the effects of dehydration temperature on water vapor adsorption and dissolution behavior. The hydration rate of anhydrous carbamazepine at 75, 83, and 95% relative humidity and 25 °C decreased with increasing heating temperature. From the dissolution study by the rotating disk method, the calculated solubility of anhydrous carbamazepine was about 2.5 times higher than that of the dihydrate. The rate of phase transformation from the anhydrous form into the dihydrate during the dissolution process decreased with an increase in sample preparation temperature. These phenomena were further studied by thermal analysis, specific surface area measurement, density measurement, small-angle X-ray scattering, and wide-angle powder X-ray diffraction. As the heating temperature was raised, the specific surface area was reduced and the density was increased; furthermore, the average of the solid part calculated by the Debye method with small-angle X-ray scattering increased. The anhydrous carbamazepine prepared at lower heating temperatures was found to have a more porous structure and was seen by wide-angle powder X-ray diffraction to comprise both anhydrous forms I and II.

Invasion percolation model of co-interpenetrating ceramic-metal composites

An invasion percolation model for simulating the reactive metal infiltration of porous ceramics to form a co-interpenetrating composite is presented. By combining the pore-level dynamics of percolation models with kinetic Monte Carlo phase transformation methods, two thermodynamic driving forces for infiltration are simulated: the applied pressure and the chemical reaction at the ceramic/metal interface. In doing so the model describes both the capillary fingering effects that dominate at high pressures and the evolution of reaction byproduct phases that may cause pore space closure at high temperature. At very high temperature and/or low pressure, a `core-shell' morphology forms. Fingerwidth and pore cluster probabilities are computed to serve as a means for quantifying microstructures. It is demonstrated that the adjustable parameters favourable for the formation of the most highly interconnected composites are high phase transformation rate, low to intermediate invasion pressure, low initial transformation contact angle (low temperature), low kinetic growth rate constant, and low initial matrix porosity. The optimal combinations relating to expected mechanical strength of the composite are presented in the form of Weibull survival probability plots and process maps.

Flow characteristics of metal with phase transformation and prediction of its microstructure

We propose a flow-stress characteristic of SUS430F steel that takes in account the effects of temperature, strain rate and their deformation history. In the framework of this characteristic, the history effects of strain rate and temperature are estimated through the plastic strain energy stored. The formulated characteristic (σ) may be shown as a function of temperature (θ), strain rate ((έ), and the stored energy W or the reference stress (σst). The energy is stored through plastic deformation and released during annealing process. The energy is also referred by the yield flow stress (σst), which is measured under the reference condition. The discussion on the characteristic is extended to the material at high temperature (1073–1473 K) with the σ + gamma phase of a given phase ratio. The equilibrium ratio of (α)- or (γ)-phase at a given temperature can be estimated on the basis of an equilibrium phase diagram. In order to introduce the flow-stress characteristic with phase transformation using the proposed formulation, we also analyze the phase transformation rate from (α) to α + γ with temperature elevation, and from (α) to >γ + α in the cooling process on the basis of the “time–temperature–transformation” diagram that includes the quenching process as well. The flow stress characteristic and phase ratio are estimated simultaneously for a hot forging process.

Dopants for synthesis of stable bimodally porous titania

Abstract Bimodally porous titania powders doped with alumina, zirconia, and silica were made by wet precipitation from organometallic precursors (for Al/Ti=0.05-0.4, and Zr/Ti=Si/Ti=0.1). Doping retards not only the anatase-to-rutile phase transformation, but also the crystallite growth of titania. So it was used to control the powder phase composition and pore structure at high temperatures. The extent of the retarding effect on pore structure and phase transformation increased with increasing alumina concentration. The effectiveness of these dopants follows the order of: zirconia>silica>alumina. The dopants also reduce the loss of surface area of the calcined powders by decreasing the sintering and phase transformation rates. All powders exhibited bimodal pore size distributions (PSD) with fine intra-particle pores (1–4 nm) and larger inter-particle pores (10–120 nm). However, the intra-particle pores of the pure titania disappeared at 600°C, while the bimodal PSD of doped titania was maintained up to 750°C.

The electrochemistry of platinum phthalocyanine microcrystals. IV. Temperature dependence of the electrochemical behaviour in non-aqueous solution

The effect of temperature on the electrochemical behaviour of platinum phthalocyanine microcrystals was investigated by cyclic voltammetry and electrochemical impedance spectroscopy in acetonitrile containing 0.1 mol dm −3 of the tetrabutylammonium salt of either ClO 4 −, BF 4 − or PF 6 −. Varying the temperature leads to a shift in the peak potential and a change in the magnitude of the peak current, accompanied by the formation of new peaks and a change in peak shape. The temperature dependence of the electrochemical behaviour is controlled by the identity of the doping anions. The shift in peak potentials is explained using an electrochemically stimulated conformational relaxation model. The changes in peak current depend on the kinetics of the redox processes. For lightly doped film, the peak current is decreased on increasing the temperature. For the highly doped films, a nucleation–growth-like phase transformation is the rate-determining process during oxidation. The phase transformation rate is controlled by both ionic charge transport in the film and the nucleation–growth kinetics. The nucleation–growth rate decreases with increasing temperature, whereas the ionic transport rate increases with temperature. A kinetics model of crystallisation developed by Keith and Padden is used to describe the effect of temperature on the rate of phase transformation.

Microstructural Evolution in Liquid‐Phase‐Sintered SiC: Part II, Effects of Planar Defects and Seeds in the Starting Powder

The effects of planar‐defect density in a β‐SiC starting powder and the addition of α‐SiC seeds to that powder on microstructural evolution in liquid‐phase‐sintered (LPS) SiC have been studied separately. Planar‐defect density is altered by appropriate heat treatment of an as‐received β‐SiC starting powder. It was found that a decrease in the planar‐defect density in the powder retards the β→α phase transformation rate. It is proposed that, because nucleation of α‐SiC occurs on the planar defects present in the β‐SiC starting powders, the nucleation rate and the attendant rate of transformation decrease with a reduction in planar‐defect density. Consequently, this reduces the frequency of formation of elongated β/α composite grains, resulting in lower average aspect ratios, as the initial untransformed β‐SiC grains coarsen in an equiaxed manner. In contrast, addition of external α‐SiC seeds has no effect on the β→α phase transformation rate, although a significant reduction in the average aspect ratio occurs. It is proposed that preferential equiaxed coarsening of the α‐SiC seeds over elongated coarsening of β/α composite grains occurs, resulting in a reduction of overall coarsening anisotropy.

Studies into the phase transformation of Bi-2223 precursor powders using X-ray diffraction and SQUID susceptibility measurements

An optimized processing route for formation of the Bi-2223 phase, from a highly reactive precursor powder, is of obvious importance in achieving a high J/sub c/ in powder-in-tube (PIT) tapes, Here, we report the dependence of the SQUID susceptibility signal on the phase content and heat-treatment of the Bi-2223 precursor powders. Magnetic susceptibilities as measured by SQUID reveal differences in the powder quality not seen by x-ray powder diffraction. In particular, Bi-2223 detection is enhanced using SQUID. The rate of phase transformation to the 2223 phase in various precursor powders exhibiting significantly different SQUID signals will be discussed.

Effects of Dehydration Temperatures on Moisture Absorption and Dissolution Behavior of Theophylline.

Anhydrous theophylline was prepared by heating theophylline monohydrate at temperatures between 60 degrees C and 140 degrees C. The effects of dehydration temperatures on the moisture absorption and dissolution behavior of anhydrous theophylline were investigated in this study. The hydration rate of anhydrous theophylline at 95% relative humidity and 25 degrees C decreased with increasing dehydration temperatures. From the fitting analysis of solid-state reaction models, the hydration reaction was found to be governed by the phase boundary reaction model for samples prepared at lower dehydration temperatures (<100 degrees C) but the reaction obeyed the growth of nuclei reaction model when samples were dehydrated at higher temperatures. The dissolution rates of various anhydrous theophylline samples were measured by the rotating disk method. The calculated solubility of anhydrous theophylline prepared by heating was about 2.5 times higher than that of theophylline monohydrate. The phase transformation rate from the anhydrous form to the monohydrate during dissolution tests decreased with higher dehydration temperatures. It was found that the anhydrous theophylline prepared at different dehydration temperatures transformed to the monohydrate by way of different growth of hydrate nuclei mechanism.

Pressure-Induced Pyrochlore-Perovskite Phase Transformation in PLZST Ceramics

The influence of applied pressure on the pyrochlore-perovskite phase transformation in the PLZST system was studied. A stoichiometric and homogeneous ferroelectric PLZST pyrochlore powder was prepared by the coprecipitation and freeze-drying method. In the absence of applied pressure, the phase transformation from pyrochlore to perovskite occurred at temperatures above 550 °C, with 100% perovskite phase being obtained at 750 °C. Because the molar volume of the pyrochlore is larger than that of perovskite phase by 8.5%, applied pressure is expected to accelerate the pyrochlore-perovskite phase transformation. When the pyrochlore powder was uniaxially cold-pressed, the temperature for initiation of the phase transformation was lowered, and the rate of transformation was increased. This effect is interpreted as an increased nucleation frequency due to the defects created at particle contacts. For evaluation of pressure effects during annealing at elevated temperature, the powder was hot-pressed at various temperatures and pressures in air. Hot-pressing was found to greatly enhance the phase transformation rate. This is attributed to the additional P · ▵ Vmolar driving force for the phase transformation provided by applied pressure.

An Investigation of Solvent-Mediated Polymorphic Transformation of Progesterone Using in Situ Raman Spectroscopy

Many analytical techniques, such as differential scanning calorimetry (DSC), X-ray diffraction (XRD), infrared spectroscopy (IR) and Raman spectroscopy can be used to differentiate between crystalline polymorphs of the same chemical entity. While all of these techniques are routinely applied to off-line analysis of materials, Raman spectroscopy has the advantage over these other techniques in that Raman technology currently exists for in situ monitoring of the solid-phase behavior within a mixed suspension of liquid and solid. In this work, we present our results from an in situ Raman study, demonstrating the solvent-mediated polymorphic phase transformation of progesterone. In situ Raman analysis has shown that the appearance of Form I progesterone is always preceded by the formation of Form II progesterone. Phase transformation rates were found to increase monotonically as the temperature increases, which indicates that the polymorphic system is monotropic. Form I was found to be thermodynamically more ...

Structure, optical absorption and electronic states of Zn + ion implanted and subsequently annealed sol–gel anatase TiO 2 films

Zn + or both Zn + and O + ions were implanted in porous anatase TiO 2 films prepared by sol–gel method and subsequently annealed in N 2 or O 2 atmosphere. The results were compared with that obtained after Ar + ion implantation and subsequent annealing. The samples were evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron diffraction (ED) and X-ray diffraction (XRD). Optical absorption and electronic states were characterized by photothermal deflection spectrometry (PDS) and electron energy loss spectrometry (EELS). The porous film was densified by Zn + ion implantation up to the ion penetration depth. After the subsequent annealing at 800°C the phase transformation from anatase to rutile accompanied with grain growth up to the film thickness was observed. In addition, the phase transformation was not induced by the annealing up to 800°C with or without preceding Ar + ion implantation. Thus, the implanted impurity Zn assisted the phase transformation. Annealing in O 2 tends to reduce the rate of phase transformation and create ZnTiO 3. Optical absorption above the photon energy of 2.9 eV was increased remarkably by the Zn + or Zn + and O + ion implantation and subsequent annealing. EELS spectra of the Zn + implanted and annealed sample is consistent with the results of PDS. The change in the optical absorption above 2.9 eV is due to the phase transformation.

The equilibrium phase boundary between hexagonal and cubic boron nitride

Abstract The equilibrium phase boundary between hexagonal and cubic boron nitride was determined over the pressure range 3–6 GPa and the temperature range 1200–2200°C. Absolute pressure and temperature were estimated based on the minimum P–T point of diamond formation in the conventional metal catalyst system using the Kennedy–Kennedy equilibrium line between graphite and diamond. Above 3.8 GPa, we determined the phase boundary by detecting the formation of cubic BN in the catalyst-containing system and by the reverse transformation from cubic BN to hexagonal BN. Below 3.8 GPa, we determined the boundary by careful observation of the transformation behavior of cubic BN powder. The rate of phase transformation from cubic BN to hexagonal BN was evaluated from the ratio of X-ray diffraction peak intensities of both phases. We found that the amount of cubic BN phase clearly changed at the phase boundary. Two sets of the results could be plotted with a uniquely determined boundary line expressed by the equation P(GPa)=T(°C)/465+0.79, which is located about 300°C higher at 5 GPa than the line determined by Bundy and Wentorf in 1963.

Definitive effects of chloride ions on the formation of spherical hematite particles in a forced hydrolysis reaction

The effects of chloride ions on the morphology and structure of hematite particles, produced from a forced hydrolysis reaction of acidic Fe(III) solution, were investigated. Spherical hematite particles were precipitated under all conditions employed which changed the molar fraction of coordinative Cl− to non-coordinative NO3− ions to Fe(III) ions in the solutions. The size of the particles was increased with increase in the molar fraction of FeCl3, FeCl3/[FeCl3+Fe(NO3)3] (XFeCl3) although no remarkable change was observed in varying that of HCl, HCl/(HCl+HNO3) (XHCl). TEM and XRD measurements suggested that the hematite particles formed at XFeCl3⩽0.2 exhibited a single-crystal nature, although those produced at XFeCl30.4 are polycrystalline with an enlarged c edge length in a unit cell possessing OH− ions in the lattice. The rate of phase transformation from β-FeOOH to hematite decreased with increase in XFeCl3 and was closely related to the crystal lattice distortion. The results obtained were explained by an aggregation mechanism taking into account the difference in the coordination properties of Cl− and NO3− ions to Fe(III) ions. The hydrolysis reaction of Fe(III) was slowed in the presence of Cl− by its strong coordination to produce electrically neutral, extremely low molecular weight polynuclear (PN) species, [FeO2/3(OH)4/3(H2O)xCl1/3]p. Therefore, this system gave a large spherical particle through the aggregation of the neutral small PNs with less mutual electrostatic repulsion force. In contrast, NO3− ions exhibited a fast hydrolysis reaction owing to their non-coordinative nature and produced positively charged, high molecular weight PNs, [FeO2/3(OH)4/3(H2O)x]p(p/3)+, resulting in less aggregated small hematite particles owing to their repulsion force. This aggregation mechanism was further confirmed by gas adsorption, TEM and addition of NaCl.

Kinetics of hydrogen induced diffusion phase transformation in intermetallic compound TbFe 2

The effect of temperature on the rate of isothermal diffusion phase transformation in the intermetallic compound TbFe 2 in hydrogen atmosphere has been investigated by means of magnetic and powder X-ray diffractometry methods. It was shown, that in the temperature range of 450–580°C the rate of decomposition was increased with increasing temperature. The final products of decomposition were TbH 2 and α-Fe.

Microstructure in silicon nitride containing β-phase seeding: Part I

A proper powder preparation technique was used to develop elongated β–Si3N4 particles as seeds from raw materials. The phase transformation and development of microstructure in Si3N4 ceramics containing β–Si3N4 seeds were investigated. The specimens of seeded silicon nitride had higher phase transformation rates than the specimens without seed. A core/shell structure was observed by transmission electron microscopy in seeded specimens owing to the difference in aluminum concentration in Si3N4 grains. The misfit between the core and the shell was accommodated by interfacial dislocation that has a rotation character. The growth mode was epitaxial, although there was some compositional difference between the core and the shell. A relatively larger grain size and wider grain size distribution in seeded Si3N4 specimens was due to the amount of β-phase seeds that could act as the nuclei, and the final modification of the microstructure was due to the coalescence process. The crack wake process characterized the mechanism of toughening.

Impedance of metal hydride electrodes. Rate of phase transformation limited by nucleation and growth mechanisms

A mathematical model for the electrochemical impedance spectroscopy of a metal hydride electrode was developed. Ac impedance data of phase transformation were derived by considering a nucleation and growth mechanism based on the theory developed by Johnson–Mehl–Avramy. Different mechanisms such as grain edge and grain boundary preferential nucleation sites are discussed. Global Nyquist plots of the metal hydride electrode are obtained by adding a surface charge transfer reaction and a double-layer capacitance to the model.

New kinetic model for the nanocrystalline anatase-to-rutile transformation revealing rate dependence on number of particles

Existing kinetic models are unable to describe published experimental data for the anatase-to-rutile phase transformation in nanocrystalline samples. A new kinetic model is proposed that combines nucleation at certain contact areas between two anatase particles and formation and of rutile nuclei. Kinetic equations, incorporating mass-balance considerations, derived for this interface nucleation and constant growth model fit the experimental data of Gribb and Banfield (1997) fairly well. Results confirm that the transformation is second order with respect to the number of particles of anatase. Over shorter reaction times, the net transformation rate is determined by the rate of nucleation, which is initiated from rutile-like structural elements in the contact area. The activation energy of 165.6+ or -1.1 kJ/mol for rutile nucleation within nanocrystalline anatase particles is much lower than values previously measured for rutile nucleation in coarse anatase samples (>330 kJ/mol). Nuclei proceeds at a constant rate with a very small activation barrier. Over longer reaction times, the net transformation rate is determined both by nucleation and nuclei growth. Results quantitatively explain the origin of the size dependence of phase transformation rates in this system.

High-pressure/low-temperature metamorphism and the dynamics of an accretionary wedge

Summary The preservation and exhumation of high-pressure rocks is an important observation in understanding the geodynamics of orogenic processes. A numerical tool is developed to estimate quantitatively the effect of the complex interplay between the mechanical and thermodynamical behaviour, and to assess under which conditions the preservation of metastable denser phases is possible. A finite difference numerical method is used to solve the continuity, Navier–Stokes and thermal equations for a Newtonian compressible fluid medium. In the model we take into account a typical forcing induced by a subduction process in a collisional environment according to a corner flow model. We follow the evolution of different phases in the crust including a pressure–temperature-dependent phase transition in the numerical code. Although eclogite is formed at depth when the phase diagram is only prescribed from thermodynamics, it cannot reach the surface. The kinetic effects of thermally activated diffusion and of the nucleation processes are taken into account in the modelling of the phase transition. Our simplified model does not explicitly take into account the presence of water. It assumes that the rate of phase transformation can be computed from a knowledge of pressure, temperature and phase content. The parameters of the kinetic equations are empirically chosen to reproduce qualitatively the typical pressure–temperature–time paths recorded in the Alpine belt. To obtain significant concentrations of high-pressure phases at the surface, different activation energies for the prograde and retrograde reactions are needed. This difference may be related to changes in the water content of the crust between its burial and its exhumation.

Mechanical Behavior of an Ni-Ti Shape Memory Alloy Under Axial-Torsional Proportional and Nonproportional Loading

Several biaxial proportional and nonproportional loading experiments are reported for thin-wall tubes of a pseudoelastic Ni-Ti shape memory alloy (SMA). In addition to the mechanical behavior, temperature was measured during the experiments. It is shown that the phase transformation exhibits asymmetrical behavior in the case of tension-compression cycling. The transformation strain rate is determined for selected histories by numerical differentiation of data. Under nonproportional loading, the rate of phase transformation does not follow a generalized J2-J3 criteria based on results of micromechanical simulations for proportional loading. The role of simultaneous forward and reverse transformations on the nonproportional transformation response is examined using a simple micromechanical model, and the direction of the inelastic strain rate is adequately predicted. Load- and strain-controlled experiments at different strain rates, with and without hold times, are reported and coupled thermomechanical effects are studied.

Differences in the Tetragonal to Monoclinic Phase Transformation Rate in Hot Water of 3mol% Y2O3-ZrO2 Ceramics under Different Surface Conditions.

Hot-isostatic-pressed (HIP'd) tetragonal zirconia ceramic containing 3mol% Y2O3 was soaked in hot water at 95-100°C to evaluate the transformation rate from the tetragonal (T) to the monoclinic (M) phase. The transformation rate differed considerably between the polished surface and the surface cut by a diamond wheel. After about 1000h of soaking in hot water, the volume of the M-phase was 50% in the cut surface and less than 20% in the polished surface. Tetragonal ZrO2 with polished and cut surfaces were annealed at 1000°C for 2h in air. When the annealed specimens were soaked in hot water, the transformation rate in the cut surface was the same as that before annealing; on the other hand, the M-phase volume in the polished surface increased to 70% after 1000h of soaking. Surface roughness, a rhombohedral phase generated by cutting or machining and orientation of the T-phase crystal due to machining, are thought to be causes of the transformation rate change. However, not one of them on its own could explain all the phenomena without contradiction.