Crystalline Phase Transformation Research Articles

Crystallization and oriented attachment of monohydrocalcite and its crystalline phase transformation

Crystallization and oriented attachment of monohydrocalcite (MHC) for the self-organization of dumbbell-like superstructures in the absence of any additives, as well as the crystalline phase transformation of MHC to anhydrous calcium carbonate (CaCO3), have been systematically described in this paper. In Mg2+-containing artificial seawater at 4 °C, the dissolution of initially formed amorphous calcium carbonate (abbreviated as ACC) cannot be detected prior to the crystallization of MHC, and the detected inter-particle hydrogen bonds probably induce the oriented attachment of nanocrystalline MHC building blocks with a protective Mg-rich layer. As for the crystalline phase transformation of MHC to an anhydrous CaCO3 (i.e., vaterite, aragonite or calcite), two novel aspects have been highlighted: (i) the unexpected detection of unstable vaterite for the calcination of solid-state MHC, (ii) the unique product of cube-shaped or dumbbell-like superstructures of tiny calcite rhombohedrons from Mg2+-free artificial seawater at 30 °C. In particular, the effect of Mg2+-chelators (organic acetylacetone or inorganic sodium hydroxide) on the abnormal solution-mediated transformation of MHC to calcite may be potentially applied for biomimetic synthesis purposes.

Non-isothermal crystallization in Ga–Se–Ag chalcogenide glass by differential scanning calorimetry

The differential scanning calorimetry (DSC) technique has been used to study the kinetics of amorphous to crystalline phase transformation in Ga15Se75Ag10 glass, prepared by rapid quenching method. The glass transition temperature and the crystallization temperature are found to be dependent on the heating rates. The dependence of the characteristic temperatures, glass transition temperature (Tg) and the crystallization onset temperature (Tc) on the heating rates are utilized to determine the activation energies for the glass transition (ΔEg) and crystallization (ΔEc). The transformation mechanism has been examined using the local Avrami exponents, which indicate the volume nucleation with two dimensional growths is responsible for the transformation process. The calculated values of ΔEc and ΔEg are found to be 134.63 and 152.51kJ/mol respectively.

Real-Time Visualization of Convective Transportation of Solid Materials at Nanoscale

Convective transportation of materials in the solid state occurring in a prototype solid bilayer system of Al and Si with negligible mutual solubility has been directly imaged in real time at nanoscale using a valence energy-filtered transmission electron microscope. Such solid-state convection is driven by the stress gradient developing in the bilayer system due to the amorphous to crystalline phase transformation of the Si sublayer. The process is characterized by compression experienced in the Si phase crystallizing within the Al sublayer, as well as by the development of mushroom-shaped "plumes" of Al nanocrystals in the Si sublayer as a result of compressive stress relaxation and discrete, new nucleation of crystalline Al. The real-time, atomistic observation and the thus-obtained fundamental understanding of solid-state convection enable highly sophisticated applications of such a complex process in advanced fabrication and processing of nanomaterials and solid-state devices.

Preparation of hollow TiO2 nanoparticles through TiO2 deposition on polystyrene latex particles and characterizations of their structure and photocatalytic activity

In a mixed solvent of water and ethanol, polystyrene/titanium dioxide (PSt/TiO2) composite particles of core-shell structure were prepared by hydrolysis of tetrabutyl titanate in the presence of cationic PSt particles or anionic PSt particles surface-treated using γ-aminopropyl triethoxysilane. Hollow TiO2 particles were obtained through calcination of the PSt/TiO2 core-shell particles to burn off the PSt core or through dissolution of the core by tetrahydrofuran (THF). An alternative process constituted of preheating the PSt/TiO2 particles at 200°C to allow partial crystallization followed by calcination or PSt dissolution by THF. The outcome TiO2 particles thus prepared were examined by TEM, and hollow TiO2 particles were observed. The crystalline phase structure and phase transformation were characterized, which revealed that preheating before the removal of the PSt core was useful to achieve the desired hollow TiO2 particles, and the calcination process was beneficial to the formation of anatase and rutile structures. The tests of TiO2 particles as catalyst in the photodegradation of Rhodamine B demonstrated that a much higher catalytic activity was observed with the TiO2 hollow particles prepared through calcination combined with preheating.

Polarity-dependent reversible resistance switching in as-deposited AgInSbTe phase change film

Abstract The electrical properties of a nanoscale prototype capacitor-like cell structure using an amorphous Ag8In14Sb55Te23 (AIST) as the active material were investigated. A polarity-dependent reversible switching effect which is different from the amorphous–crystalline phase transformation has been observed. The reversible resistance switching between a high resistance state (HRS) and low resistance state (LRS) was induced by bias amplitude and polarity. The electrical resistance ratio of HRS/LRS was ∼10:1. The AIST phase change material is shown to be very promising for multi-state data storage applications.

A study on electric properties for pulse laser annealing of ITO film after wet etching

The electric properties of ITO thin film after UV or IR laser annealing and wet etching was analyzed via grazing incidence in-plane X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectra and residual stress measurement. The laser annealing process readily induced microcracks or quasi-microcracks on the ITO thin film due to the residual tension stress of crystalline phase transformation between irradiated and non-irradiated areas, and these defects then became the preferred sites for a higher etching rate, resulting in discontinuities in the ITO thin film after the wet etching process. The discontinuities in the residual ITO thin film obstruct carrier transmission and further result in electric failure.

Synthesis and characterization of Fe3+ doped TiO2 nanoparticles and films and their performance for photocurrent response under UV illumination

Undoped TiO2 and Fe3+ doped (0.1, 0.3, 0.6 and 1wt.%) TiO2 nanoparticles have been synthesized by the acid-catalyzed sol–gel method. Iron cations are introduced in the initial solution, before gelification, what promotes their lattice localization. The Fe3+ doped TiO2 films have been fabricated using a dip-coating technique. The effect of iron content on the crystalline structure, phase transformation and grain growth were determined by X-ray diffraction (XRD), Raman spectroscopy, UV–visible diffused reflectance spectroscopy (DRS) and Electron paramagnetic resonance (EPR) spectroscopy. It has demonstrated that all catalysts are composed of mixed-phase crystals of anatase and brookite with anatase as dominant phase. The crystallinity of the brookite and anatase phases decreased with increasing the iron content. The analysis of EPR result further confirms that Fe3+ ion are successfully doped in the TiO2 lattice by substituting Ti4+. It was demonstrated that Fe3+ ion in the TiO2 films plays a role as the intermediate for the efficient separation of photogenerated hole–electron pairs and increases the photocurrent response of the film under UV light irradiation. The maximum photocurrent is obtained on the Fe3+ doped TiO2 film with 0.1% Fe, which is 1.46 times that achieved on undoped TiO2 film.

Amorphous to crystalline transformation in Ta2O5 studied by Raman spectroscopy

Many of the interesting properties that make Ta2O5 a strategic material for current and future applications in chemistry, microelectronics and optics, depend on its structural characteristics. In this work, we use Raman spectroscopy to probe structural modifications in amorphous Ta2O5 coatings submitted to thermal annealing. On the basis of previous knowledge on the crystalline material, we perform Raman spectrum simulations in disordered and partially ordered Ta2O5 from a phonon density of states. Calculated spectra are in good agreement with complex experimental spectra. Our original approach allows assignment of the vibrational features of the amorphous material, and quantitative interpretation of observed structural modifications in terms of ordering scales. In addition, it provides numerical indicators to analyse amorphous to crystalline phase transformation. Copyright © 2012 John Wiley & Sons, Ltd.

Evolution of composition, microstructure and optical properties of yttrium oxide thin films with substrate temperature

A series of yttrium oxide films has been fabricated on Si (100) wafers by different substrate temperatures based on reactive magnetron sputtering technology. And the evolution of composition, microstructure and optical properties of Y2O3 films has been investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and spectroscopic ellipsometry (SE). It is shown that stoichiometric Y2O3 with much strong YO bond and cubic nanophase with larger size notably enhance the optical properties, especially refractive index. At low substrate temperature, the composition of Y2O3 films has nonstoichiometry feature accompanying with much physisorbed oxygen (Oδ) compared with these at high temperature. Increasing substrate temperature promotes the formation of YO bonds, approaching the stoichiometry with lower Oδ content. The film commences crystallizing at room temperature, and transformation of crystalline phase from monoclinic to cubic occurs, along with preferred (111) orientation and large grain as temperature increases. The results facilitate clear relationship between the characterizations of Y2O3 film and growth parameters, also, it can be realized that adjustment of optical constants of Y2O3 films in a large range having the aid of substrate temperature to obtain desirable composition and structure for optical applications.

Preparation and characterization of mixed matrix membranes based on PVDF and three inorganic fillers (fumed nonporous silica, zeolite 4A and mesoporous MCM-41) for gas separation

Three types of inorganic fillers – fumed nonporous nano-silica, synthesized mesoporous MCM-41 and microporous zeolite 4A, were incorporated into the PVDF matrix to prepare mixed matrix membranes. The objective of this study was to investigate the effects of pore structure and particle size of the inorganic fillers on the gas transport properties of the mixed matrix membranes. The membranes were characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), contact angle tests and scanning electron microscopy (SEM). The mechanical properties of the membranes were determined using a tensile stress–strain machine. With the incorporation of the inorganic fillers, the thermal stability of the composite membranes was reduced with lower degradation temperature while the melting temperature and crystallizing temperature seemed to be unaffected. The DSC results showed that the crystallinity of the composite membranes had increased when compared with the pure PVDF membrane. In addition, the incorporation of MCM-41 had induced PVDF crystalline phase transformation from α to β phase. This phenomenon was not observed in PVDF/SiO2 and PVDF/4A composite membranes as indicated by the XRD results. In order to fully study the interface properties between the inorganic fillers and polymer chains, the density values of the membranes were experimentally determined and compared with the theoretical values. The experimentally determined density values of the composite membranes were lower than the theoretical values and the void volume fractions were calculated accordingly. The single gas (He, CO2, O2 and N2) permeabilities of the resulting membranes were carried out. The gas permeabilities of the three composite membranes exhibited similar behaviors, especially at lower inorganic filler loadings, although the inorganic fillers had different pore structures and particle sizes. The highest permeabilities for CO2 and O2 were obtained by PVDF/4A 32% composite membrane, which were 3.26 and 0.41Barrer, respectively, and the highest permeabilities for He and N2 were obtained by PVDF/MCM-41 32% composite membrane, which were 10.2 and 0.14Barrer, respectively. These permeabilities are much higher than those of pure PVDF membrane. The highest selectivities of 120.7, 33.1 and 4.6 for He/N2, CO2/N2, and O2/N2, were obtained by the PVDF/SiO2 4%, PVDF/SiO2 32% and PVDF/SiO2 24%, respectively, which are also higher than those of pure PVDF membrane.

Synthesis and Characterization of Fullerene Nanowhiskers by Liquid-Liquid Interfacial Precipitation: Influence of C60 Solubility

Fullerene nanowhiskers (FNWs) composed of C60 fullerene molecules were prepared using the liquid–liquid interfacial precipitation (LLIP) method in the carbon-disulfide (CS2) and isopropyl alcohol (IPA) system. The electron microscopic images reveal the formation of non-tubular FNWs. The X-ray diffraction (XRD) pattern studies indicate the presence of fcc crystalline structure and unusual triclinic structure in the FNWs. The selected area electron diffraction pattern (SAED) analysis demonstrates the existence of triclinic and electron beam assisted fcc to tetragonal crystalline phase transformation. The formation of triclinic structure might be validated due to the partial polymerization of FNWs at C60 saturated CS2-IPA interface. The high solubility of C60 in CS2 solvent system results in partial polymerization of FNWs. The polymerization of fullerene molecules in the FNWs has been further confirmed using Raman spectroscopy.

Observation of high pressure o-GeTe phase at ambient pressure in Si-Te-Ge glasses

Te-rich Si15Te85-xGex (1 ≤ x ≤ 11) glasses are found to exhibit an anomalous phase separations with germanium composition. The structural transformation of o-GeTe crystalline phase from o-GeTe with a = 11.76 Å, b = 16.59 Å, c = 17.44 Å, to high pressure o-GeTe with a new reduced lattice parameters a = 10.95 Å, b = 4.03 Å, c = 4.45 Å, is observed at Tc3 in the composition range 6 ≤ x ≤ 11. Raman studies support the possible existence of high pressure o-GeTe phase which is observed in X-ray diffraction experiments.

Solvated Crystals Based on [6,6]-Phenyl-C61-butyric Acid Methyl Ester (PCBM) with the Hexagonal Structure and Their Phase Transformation

This work focuses on the structural exploration of micro-sized crystals based on a well-known methanofullerene, [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). We have succeeded in producing PCBM crystals with the hexagonal symmetry through the liquid-liquid interfacial precipitation (LLIP) method. We found that smaller but more regular PCBM crystals tend to form in the oversaturated PCBM solutions with solvents of lower solubility for PCBM, such as tetrahydrofuran (THF) and 1,4-dioxane. The structure of the produced crystals also shows a dependence on the solvents, which can be attributed to the incorporation of different solvent molecules into PCBM crystals. Under thermal annealing, for the first time, we have observed a crystalline to crystalline phase transformation of these hexagonal PCBM crystals. Along with the phase transformation, the morphology of the crystals has also been transformed from the hexagon to long needles. In addition, the needlelike crystals arrange themselves with a relative angle of 60° to each other, which implies an intrinsic structural correlation between needlelike and hexagonal crystals.

Effect of Drawing Temperature on Structure and Properties ofα'-Phase of Poly(lactic acid)

Poly(lactic acid) (PLA) films consisting of α'-forms were prepared and uniaxially drawn. The effects of the drawing temperatures above the glass transition temperature on molecular weight and molecular weight distribution, chain conformation, degree of crystallinity and crystalline phase transformation were investigated by a combination of vibrational spectroscopy (infrared and Raman), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The general appearance 776 化 学 学 报 Vol. 70, 2012 of stress-strain behaviour of drawn PLA, and in particular its yield stress and Young’s modulus, is strongly affected by the drawing temperatures. The values decrease with increasing the stretching temperatures. The GPC results showed that the molecular weight decreases and its distribution becomes wider after drawing at various drawing temperatures. It was established that the α'-forms phase of PLA films does not transform into either an α or β crystals upon uniaxial drawing with various temperatures at a fixed draw ratio of 4. However, compared with undrawn α'-forms PLA films, the crystallinity and the degree of orientation of PLA products are greatly influenced by the drawing temperatures. The crystallinity is significantly increased upon deformation. The crystallinity and orientation of PLA products increase with increasing the draw temperature when the stretching temperature is <100 °C, however, the degree of crystallinity and deformation will decrease when the temperature is ≥100 °C. While the overall changes in physical properties can be attributed to changes in the degree of crystallinity as a function of drawing temperatures, polarized Raman spectra and XRD clearly illustrated changes and the differences in the amorphous and crystalline orientation as a function of processing conditions.

Preparation, structure and catalytic properties of magnetically separable Cu–Fe catalysts for glycerol hydrogenolysis

The Cu–Fe catalysts with stoichiometric proportion (Cu/Fe molar ratio was 0.5) were prepared by an epoxide assisted route. The structural properties of Cu–Fe catalysts were determined by X-ray diffraction (XRD), and Mossbauer spectroscopy measurements. These results indicated that a crystalline phase transformation from c-CuFe2O4 to t-CuFe2O4 occurred when elevating the calcination temperature from 500 to 600 °C. The M–H plots exhibited that all Cu–Fe catalysts had ferromagnetic nature and the saturation magnetization values monotonously increased with increasing calcination temperature irrespective of the phases composition. The significant superparamagnetic behavior was observed in the results of magnetic and Mossbauer spectroscopy measurements. The H2 temperature-programmed reduction (H2-TPR) was also conducted for examining the reducibility of Cu–Fe catalysts. The catalytic performance of Cu–Fe catalysts was examined for the hydrogenolysis reaction of glycerol. It is found that the formation of spinel CuFe2O4 greatly enhances the hydrogenolysis activity. The highest glycerol conversion (47%) was obtained over CuFe-500 catalyst, while the selectivity of 1,2-propanediol was maintained at about 92% for all catalysts.

Phase transformation of ZrO2 nanoparticles produced from zircon

This article focuses on the phase transformation of zirconia (ZrO2) nanoparticles produced from zircon using a bottom-up approach. The influence of mechanical milling and thermal annealing on crystalline phase transformation of ZrO2 nanoparticles was explored. It was found that the iron oxide, as an inherent impurity present in ZrO2 nanoparticles, produced from zircon stabilises the cubic phase after calcination at 600°C. The stabilised cubic phase of ZrO2 nanoparticles was disappeared and transformed into partial tetragonal and monoclinic phases after mechanical milling. The phase transformation occurred on account of the crystal defect induced by high-energy mechanical milling. The destabilisation of cubic phase into monoclinic phase was observed after the thermal annealing of ZrO2 nanoparticles at 1000°C. The phase transitions observed are correlated to the exclusion of iron oxide from the zirconia crystal structure.

Tetragonal and hexagonal polymorphs of BaTi1−xFexO3−δ multiferroics using x-ray and Raman analyses

The effect of Fe doping on the crystalline phase transformation and on the local environment around Fe dopant ions is investigated for BaTi1−xFexO3−δ (0.0 ≤ x ≤ 0.5) polycrystalline samples, using x-ray diffraction, x-ray absorption spectroscopy, and Raman scattering spectroscopy. Our experimental results show that the tetragonal-to-hexagonal transformation is gradually taken place when increasing the Fe content in the range 0.02 ≤ x ≤ 0.12. Although both hexagonal and tetragonal polymorphs coexist in this doping range, Fe ions preferably substitute for Ti sites in the hexagonal lattice and exist in both Fe3+ and Fe4+ forms. Our work is of paramount importance to provide a direct evidence to the preferable substitution of transition metal ions for Ti ions.

Characterization of Plasma Sprayed Yittria Stabilized Zirconia (8 wt %) Hydroxyapatite Coatings

Splitting problems at HA-coated implants are generally due to biological reasons. Bond-coatings were used to prevent the splitting problem of zirconia ceramics; this method can be widely seen in industrial applications. Two main groups were used; the first group consisted of spraying a bond layer of titania onto commercially pure titanium. This followed by a spray of HA with 5, 10 and 15 % zirconia (8 % yttria doped) as main layer onto the first bond-coating. For the second group, the samples were coated without bond-coating. Firstly, X-ray diffraction patterns of the starting powders were taken. Then x-ray diffraction patterns of the plasma sprayed samples were taken. In literature, it was seen that 20 % zirconia was sufficient for the transformation into a monoclinic structure for the bond-coated samples. For this study it was found that 10 % zirconia was sufficient to transform to the same structure of the desired crystalline phase transformation. The coating kept its crystal structure and relatively small amount of amorphous transformation was detected. A similar structure was produced using less zirconia. It was thought that the use of titanium-oxide bond-coating layer would play an important role as a third variable in the results. To further investigate these phenomena, more detailed researches must be conducted with using titanium-oxide yittria stabilized zirconia (8 wt %) hydroxyapatite bond-coatings with HA main coatings.

Electrochemical activity and durability of Pt–Sn alloys on carbon-based electrodes prepared by microwave-assisted synthesis

The microwave-assisted polyol (MP) reduction was used as the deposition technique for preparing binary Pt–Sn alloys with a mean size of 5 nm on carbon nanotubes (CNTs). A large number of CNTs with an average diameter of 40–50 nm were attached to carbon paper substrate using a catalytic chemical vapor deposition. The fast MP synthesis enabled the formation of Pt–Sn nanoalloys within 6 min. X-ray diffraction and X-ray photoelectron spectroscopy were applied to analyze the crystalline alloy structure. Postcalcination at 600 °C showed a positive effect in improving the activity and the long-term durability (i.e., 1000 cycles) of Pt–Sn catalysts toward the H 2SO 4 electrolyte, as demonstrated by cyclic voltammetry test and AC impedance spectroscopy. An equivalent circuit also was proposed to analyze the equivalent serial resistance of the electrocatalysts after the potential cycling. This improvement can be attributed to the reasons that the calcination process is prone to the following: (i) the reinforcement of the adherence between the catalyst particle and the carbon support, (ii) the transformation of crystalline phase on Pt–Sn alloy, and (iii) the decrease of Pt (II) content and chemisorbed oxygen species. The Pt–Sn crystallographic phase on the CNT-based support shows an enhanced activity and more corrosion resistance in acidic electrolyte.

Influence of heat treatment on thermally-reduced graphene oxide/TiO2 composites for photocatalytic applications

Thermally-reduced graphene oxide/TiO2 composites (TRGO/Ti) were prepared by the thermal reduction of graphene oxide/TiO2 composite that was obtained from a simple, environmentally friendly, one-step colloidal blending method. The changes in structural and textural properties as well as their corresponding photocatalytic activities were investigated as a function of calcination temperature. The presence of stacked TRGO sheets significantly retarded both the aggregation and the crystalline phase transformation of TiO2 as increasing the temperature from 200 to 600 °C. TRGO/Ti composites exhibited higher photocatalytic activity for the degradation of methylene blue in comparison with pure TiO2 due to the increase in specific surface area and the formation of π-π conjugations between dye compounds and aromatic regions of TRGO. However, increasing the calcination temperature resulted in the lower photoactivity and slower kinetics, which can be ascribed to the decrease in surface area, the reduction of oxygen vacancies, and the loss of functional groups at the edges or on the basal planes of the TRGO sheets.