Crystalline Phase Transformation Research Articles

Effect of phase transition on room temperature ferromagnetism in cerium doped ZnS nanorods

The effects of Ce doping on the structure, optical, oxidation, thermal and magnetic properties of ZnS:Ce nanorods synthesized by a chemical co-precipitation method were reported. The crystalline phase transformation from cubic to hexagonal structure was observed upon doping ZnS with Ce. Magnetic measurements showed the existence of room temperature ferromagnetism in Ce-doped ZnS nanorods. X-ray photoelectron spectroscopic (XPS) measurements provided evidence for Zn—S bonds and oxidation state of Ce in the near-surface region. Raman spectrum provided evidence for the presence of defects as well as hexagonal structure of 5 wt.% Ce doped ZnS nanorods. Ce substitution induced shape evolution was studied by using TEM. DRS spectra further validated the incorporation of Ce3+ ions. The present study reveals that Ce doped ZnS nanorods may find applications in spintronic devices.

In situ investigations on the amorphous to crystalline phase transformation of precursors for methanol synthesis catalysts

Two in situ methods are presented to study phase transformations in the synthesis of Cu/Zn hydroxycarbonates as catalyst precursors for industrial methanol production. In contrast to the well-established application of in situ FTIR probes in the detection of liquid phase concentrations, we evaluated spectra arising from the structural changes in the solid phase during the transformation of amorphous georgeite to nanocrystalline zincian malachite. Further, a novel microreaction system is employed to gain statistical insight on the scattering of the reaction time as a function of various process parameters. The obtained transformation kinetics during aging are described by a model based on the theory of solvent-mediated phase transformations showing excellent agreement with experimental results. The quantitative impact of temperature, concentration and zinc content on dissolution, nucleation and growth of the respective phases is discussed.

Melt electrowriting of electroactive poly(vinylidene difluoride) fibers

AbstractPoly(vinylidene difluoride) (PVDF) has piezoelectric properties suitable for numerous applications such as flexible electronics, sensing and biomedical materials. In this study, individual fibers with diameters ranging from 17 to 55 µm were processed using melt electrowriting (MEW). Electroactive PVDF fibers can be fabricated via MEW, while the polymer can remain molten for up to 10 h without noticeable changes in the resulting fiber diameter. MEW processing parameters for PVDF were investigated, including applied voltage, pressure and temperature. A rapid fiber characterization methodology for MEW that automatically determines the fiber diameters from camera images taken of microscope slides was developed and validated. The outputs from this approach followed previous MEW processing trends already identified with different polymers, although overestimation of fiber diameters <25 µm was observed. The transformation of the PVDF crystalline phase to the electroactive β phase was confirmed using piezo‐force microscopy and revealed that the PVDF fibers possess piezoelectric responses showing d33 ≈ 19 pm V–1. © 2018 Society of Chemical Industry

Photo-induced cubic-to-hexagonal polytype transition in silicon nanowires

Crystalline phase transformation in silicon nanowires from cubic diamond to hexagonal diamond under strong laser excitation, caused by inhomogeneous heating-induced mechanical stresses.

Polycrystalline to preferred-(100) single crystal texture phase transformation of yttrium iron garnet nanoparticles.

Nanocrystalline Ce-substituted yttrium iron garnet (YIG) powders of different compositions, Y3−xCexFe5O12 (0 ≤ x ≤ 2.0), were synthesized by a combination of sol–gel auto-combustion and solid-state synthesis techniques. The as-obtained powder samples were sintered at 1150 °C for 10 h. The garnet structure formation is confirmed by the X-ray diffraction pattern, which shows that the calculated lattice parameter increased for x = 1.0 and shows a decreasing trend for x ≥ 1.0 with the addition of cerium ions. The lattice parameter increased from 12.38 Å to 12.41 Å for x ≤ 1.0 whereas it decreased from 12.412 Å to 12.405 Å with the cerium composition for x > 1.0. The average particle size determined by high resolution transmission electron microscopy is in the range of 50 to 90 nm and found to increase with the substitution of cerium ions in YIG. The room temperature magnetic parameters such as saturation magnetization, coercivity and remanence magnetization are greatly affected by the substitution of cerium ions. The values of saturation magnetization decrease from 25.5 to 15 emu g−1 whereas coercivity increases from 1 to 28 Oe with the substitution of cerium ions. The pure YIG sample shows polycrystalline nature that changed towards a single-crystal structure leading to a preferred-(100) orientation with the Ce substitution. The change from a ring to a spotty pattern observed in SAED confirmed the crystalline phase transformation and is well supported by HRTEM and magnetic measurements. The behavior of magnetic and electrical properties is well supported by the poly- and single-crystalline nature of YIG and Ce-YIG, respectively. The crystal structure transformation in YIG brought about by Ce substitution could unveil enormous opportunities in the preparation of single-crystal materials from their polycrystalline counterparts.

The Stability of Polymers in Liquid Li-S Battery

Polymers have been widely employed as the binders, separators, separator modifiers and gel polymer electrolytes in lithium-sulfur batteries. Considering the special electrolyte system and discharge/charge mechanism, to verify whether the common polymer is applicable for Li-S battery is extremely necessary but often overlooked. In this paper the stability of some frequently-used polymersin the ether electrolyte(1 M LiTFSI + 0.2 MLiNO3/dioxolane (DOL) and 1,2-dimethoxyethane(DME) (1:1, v/v)) on the cycling and storage performances of Li-S batteryis examined by testing the solubility of the PEO, PVDF and PVDF-HFP particles/membranes in ether electrolyte and detecting the structure and composition changes of PVDF and PVDF-HFP membranes after cycling and storage as the separators of Li-S batteries. The results show that PEO particle and membrane both can dissolve in DOL and DME(1:1). PVDF and PVDF-HFP particles also dissolve in DOL and DME(1:1) but the membranes remain relatively stable, whereas PVDF-HFP react with Li2Sx in liquid Li-S cellto form sulfone, which induce the crystalline phase transformation (from α-phase to γ-phase) and cystallinity decrease. PVDF can also react with Li2Sxin liquid Li-S cell, but the action is less intensive than PVDF-HFP.

Phase transformation and photoluminescence of undoped and Eu3+-doped zinc stannate (Zn2SnO4) nanocrystals synthesized by hydrothermal method

In this work we report a hydrothermal approach for synthesis of zinc stannate (Zn2SnO4–ZTO) nanocrystals. Structural properties and morphology of the samples were investigated in detail. In particular, our research focused on the effect of hydrothermal duration on the phase composition of ZTO nanocrystals. By combining X-ray diffraction analysis, scanning electron microscopy and selected area energy-dispersive X-ray spectroscopy, a crystalline phase transformation during the hydrothermal process was enlightened. The ZTO nanocrystals were doped with europium ions. The room-temperature emission spectra of the undoped ZTO and Eu3+-doped ZTO nanocrystals were recorded. The emission spectra of the undoped ZTO nanocrystals showed two broad bands related to the lattice defects, while the spectra of Eu3+-doped ZTO nanocrystals exhibited the narrow emission peaks, which were assigned to the radiative intra-configurational f–f transitions of Eu3+ ions.

Thermal annealing induced evolution of AgIn5Se8 phase from Ag/In2Se3 bilayer thin film

In the present report, we have investigated the AgIn5Se8 phase formation upon annealing the Ag/In2Se3 bilayer film prepared by the thermal evaporation technique. The optical, morphology as well as structural parameters was tuned with different annealing temperature. The prominent crystalline phase developed at 250 °C annealing where as there is no signal of this phase in the as-prepared film (amorphous). The analysis using XRD study showed the amorphous to crystalline phase transformation with annealing. The indirect as well as the direct band gap is found to be increased with annealing (up to 150 °C) and then decreased due to the growing up new phase as explained by phase transformation and decrease in density of localized states. The formation of dangling bonds at the surface near the crystallite sites during crystallization process reduces the band gap. The Tauc parameter change shows the degree of chemical disorderness in the films. The transmittivity, absorption coefficient, oscillator energy, refractive index, dispersion energy and extinction coefficient are found to be changed accordingly with annealing condition. The micro structural study done by Scanning Electron Microscopy shows the formation of crystallites upon annealing. The Raman study of these films supports the formation of new phase.

Solvent-free hydrothermal synthesis of gamma-aluminum oxide nanoparticles with selective adsorption of Congo red

Aluminum hydroxide and oxide have been widely used for decontamination due to their environmentally friendly nature and cost effectiveness. Aluminum (hydro) oxides are the main phases of aluminum-derived environment materials. Herein, the solvent-free hydrothermal synthesis of gamma-aluminum oxide (γ-Al2O3) nanoparticles and phase transformation of AlOOH into γ-Al2O3 are reported. Hydrothermal treatment of NH3·H2O-induced aluminum precipitate resulted in the formation of AlOOH, which was an intermediate product of γ-Al2O3. AlOOH was transformed into highly crystalline 20-nm γ-Al2O3 particles through calcination at 500 °C due to dehydration. The transformation was confirmed through X-ray diffraction (XRD) and thermogravimetric (TG) analyses. The resulting γ-Al2O3 had superior adsorption ability for the anionic Congo red (CR) dye than for the cationic methylene blue (MB) and malachite green (MG) dyes. The selective adsorption ability of CR instead of MB was attributed to the electrostatic attraction and hydrogen bonds between the amino group and azo double bond of CR, and between the amino group and hydroxyl group in γ-Al2O3. Thus, this study investigated crystalline phase transformation into γ-Al2O3 and selective adsorption capacity of CR, which provides important information regarding the synthesis of crystalline γ-Al2O3 adsorbent, with selective adsorption ability for decontamination applications.

Crystalline to Crystalline Phase Transformations in Six Two-Dimensional Dynamic Metal–Organic Frameworks: Syntheses, Characterizations, and Sorption Studies

Six dynamic metal–organic frameworks, namely, {[Cd(1,4-bib)(glut)]·(4H2O)}n (1), {[Zn(1,4-bib)(glut)]·(4H2O)}n (2), {[Co(1,4-bib)(3,5-pydc)]·(2H2O)}n (3), {[Mn(1,4-bib)(3,5-pydc)]·(2H2O)}n (4), {[Z...

Effect of dispersant content and drying method on ZrO2@Al2O3 multiphase ceramic powders

ZrO2@Al2O3 composite ceramic powders were prepared by solution combustion method with aluminum nitrate (Al (NO3)3) and 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y- TZP) as the main raw materials, ammonium polyacrylate (PAA-NH4) as a dispersant, urea (CO (NH2)2) as a reducing agent. The effects of PAA-NH4 concentration and drying method on the microstructure and morphology of the ZrO2@Al2O3 powders were investigated. The results showed that when the concentration of PAA-NH4 was 1.5 wt%, and the molar ratio of Al (NO3)3 to CO (NH2)2 was 1:2, the ZrO2@Al2O3 powders with uniform grain size and high crystallinity could be synthesized by solution combustion drying method. Additionally, the abnormal growth of 3Y- TZP grain in ZrO2@Al2O3 was suppressed and the crystalline phase transformation trend (t-ZrO2 to m-ZrO2) was obviously decreased after sintering at 1600 °C.

The effect of metal ions doping on the electrochemical performance of molybdenum trioxide

Transition metal elements (Ni, Cu, Mn, Co and Cr) have been doped into the host structure of molybdenum trioxide via in situ hydrothermal method. The characterizations indicate that Ni and Cu doping has no influence on the structure of the host MoO3 while Co and Cr doping not only cause the crystalline phase transformation but also leads to the morphology change of the doped MoO3. These crystalline and morphology alteration have great impact on the electrochemical performance of the doped samples. The electrochemical tests demonstrate that the specific capacitance and the rate capability are enhanced after Ni, Cu and Mn doping while decline after Co and Cr doping, which may be caused by the structure and morphology change of MoO3 as well as the formation of SEI (solid electrolyte interface) layer after Cr doping. The results manifest that metal ions doping has great impact on the electrochemical property of host materials.

Colloidal Gradated Alloyed (Cu)ZnInS/ZnS Core/Shell Nanocrystals with Tunable Optical Properties for Live Cell Optical Imaging

AbstractIn this work, Cd‐free, oleophilic (Cu)ZnInS(core)−ZnS(shell) core/shell quantum dots (QDs) with varying Cu content in the core and shell thickness were synthesized by heating‐up method (core) followed by hot‐injection route (shell). Effect of Cu ion doping (in core) and shell growth on the structural and optical properties of (Cu)ZnInS−ZnS core/shell QDs is reported. Incorporation of Cu ions causes the crystalline phase transformation of zinc blende ZnInS ternary alloy to wurtzite Cu−Zn−In−S quaternary alloy. The (Cu)ZnInS alloyed core QDs are encapsulated with a wider bandgap ZnS shell with gradient interface. The photoluminescence of (Cu)ZnInS core QDs exhibits wide tunable multicolor emissions from green (535 nm) to deep red (683 nm) with large Stokes shift. After the growth of ZnS layer, the PL peak position and absorption edge undergo a hypsochromic shift with enhanced photoluminescence quantum yield up to 47.31%, which is due to the partial cation exchange of Cu+ and In3+ with Zn2+, yielding a graded core/shell structure with smooth potential interface. The radiative excited state lifetime is lengthened from 22.91 ns to 291.11 ns. The mercaptoundecanoic acid (MUA) functionalized QDs have been successfully used for in vitro fluorescence cellular imaging of HEK‐293 and HeLa cells. These QDs are cell permeable and suitable for visible light induced multicolor fluorescence‐based cellular imaging.

Phase change studies in Se85In15−xZnx chalcogenide thin films

This research work describes the phase change studies in Se85In15−xZnx thin films at various annealing temperatures. Glassy samples of Se85In15−xZnx were synthesized by the melt quenching method and thin films of thickness 400 nm were prepared by the vacuum evaporation technique on a glass/Si wafer substrate. The glass transition temperature (Tg) and the on-set crystallization temperature (Tc) of the prepared alloys were evaluated by non-isothermal differential scanning calorimetry studies. Thin films were annealed at three temperatures 330 K, 340 K, and 350 K (which are in between Tg and Tc of the synthesized samples) in a vacuum furnace for 2 h. High resolution X-ray diffraction studies demonstrate that the as-prepared films are amorphous in nature whereas the annealed films are of crystalline/polycrystalline in nature. Field emission scanning electron microscopy studies of thin films (as-deposited and crystallized) confirm the phase transformation in Se85In15−xZnx thin films. Optical band gaps were calculated from the Tauc's extrapolation procedure and were found to be enhanced with the Zn concentration and decrease with the increasing annealing temperature. Various optical parameters were evaluated for as-prepared and annealed Se85In15−xZnx thin films. The changes in optical parameters with annealing temperature were described on the basis of structural relaxation as well as changes in defect states and density of localized states during amorphous to crystalline phase transformation in Se85In15−xZnx thin films.

Effects of laser polishing on surface microstructure and corrosion resistance of additive manufactured CoCr alloys

Laser polishing of 3D printed metal components has drawn great interest in view of its potential applications in the dental implant industries. In this study, corrosion resistance, surface composition and crystalline structure of CoCr alloys were investigated. The corrosion resistance, micromorphology, composition, phase transformations and crystalline structures of samples were characterized using an electrochemical analyzer, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and transmission electron microscope (TEM), respectively. The results indicate that high laser powers and low object distances within a certain range can facilitate the formation of complex oxide films, which exhibits high corrosion resistance. Further, object distances have a significant influence on cooling rates during the solidification of the melt pool in laser polishing, and fast cooling generates vast amounts of vacancies and defects, which result in the crystalline phase transformation from γ to ε. Consequently, the formed oxides play an important role in corrosion resistance on the outer layer, and inner layer with γ phase also helps keep the CoCr alloys in a stable structure with high resistant to corrosion. The two process parameters in laser polishing, laser power and object distances, are demonstrated as being important for controlling the surface microstructures and corrosion resistance of the additive manufactured CoCr alloy components.

Mechanical properties of amorphous Ge2Sb2Te5 thin layers

The working principle of a Phase Change Memory (PCM) cell exploits the repeated reversible transition between a crystalline and an amorphous phase of chalcogenide alloys typically Ge2Sb2Te5, that are characterized respectively by a high (SET) and a low (RESET) conductive state. The change in density between the two phases (6%) induces a very high compressive stress to the active amorphous region by the surrounding crystalline materials. Moreover, the physical iterative transformation between crystalline and amorphous phase transformation introduces a swelling and deswelling effect. This is one of the key failure mechanisms that are limiting the reliability of the final integration of the PCM system. Knowledge of the mechanical properties of the amorphous phase is then an important factor. Amorphous structure, i. e. its short-range order, depends on the adopted formation procedure. In this paper we analyze the mechanical characteristics of sputtered amorphous Ge2Sb2Te5 thin layers and the modification introduced by ion irradiation, a procedure adopted to simulate the amorphous state produced by melt quenching. Measurements of Young's Modulus and Hardness were performed using Ultra High-Nano Indentation on plane samples. The values of both quantities increase of about 10–20% in the 30 keV Ge+ irradiated samples. This trend is due to the reduction of homopolar wrong bonds (GeGe and TeTe) present in the as deposited film. Thermal spikes associated to the impinging ion cause a local atomic rearrangement that results in a structure similar to that of the crystalline phase. The investigation was extended to cantilevers of length in the range 10–200 μm, with a layer of 100 nm Ge2Sb2Te5 deposited on 280 nm thick SiN. The cantilever modal analysis and the out of plane deflection measurements were correlated using a Finite Element modeling, that makes use of the mechanical values measured by Ultra high Nano Indentation. After deposition the amorphous Ge2Sb2Te5 layer is subject to a compressive mechanical shrinkage, this internal stress is released after ion implantation.

An investigation into the crystalline morphology transitions in poly-L-lactic acid (PLLA) under uniaxial deformation in the quasi-solid-state regime

The mechanical behaviour, crystalline and macromorphology structure development during uniaxial deformation and annealing of poly-l-lactic acid (PLLA), with varying strain rate and draw temperatures (Td) above Tg, have been investigated using small- and wide-angle X-ray scattering (SAXS/WAXS), microscopy, thermal and mechanical techniques. The mechanical behaviour of PLLA, was strongly dependent on Td where embrittlement and eventual failure were observed as Td was increased, during uniaxial drawing of the amorphous polymer. This was mirrored in the bulk surface morphology where crazing, microvoiding and cavitation occurred with increasing Td. SAXS/WAXS data showed that strain-induced crystallization occurs on drawing, but crystallite orientation decreased with increasing Td, due to chain relaxation at temperatures ≥30 °C above Tg. However, no long-range oriented lamellar macromorphology was observed post-draw directly and only developed in the samples that were step annealed at temperatures above Td. Also, the disordered α′ crystal form was observed post-draw at Td between 60 and 80 °C, whereas Td ≥ 90 °C, resulted in the ordered α crystal form directly. However, on annealing at temperatures of ≥110 °C, the α′-α crystal transition ensued and in all samples, an oriented lamellar macromorphology developed. Therefore, Td and post-draw annealing, have a significant influence on the mechanical properties, crystallinity and crystalline phase transformation in PLLA, which in-turn, affects the polymers medical and industrial applications.

Evolving affinity between Coulombic reversibility and hysteretic phase transformations in nano-structured silicon-based lithium-ion batteries

Nano-structured silicon is an attractive alternative anode material to conventional graphite in lithium-ion batteries. However, the anode designs with higher silicon concentrations remain to be commercialized despite recent remarkable progress. One of the most critical issues is the fundamental understanding of the lithium–silicon Coulombic efficiency. Particularly, this is the key to resolve subtle yet accumulatively significant alterations of Coulombic efficiency by various paths of lithium–silicon processes over cycles. Here, we provide quantitative and qualitative insight into how the irreversible behaviors are altered by the processes under amorphous volume changes and hysteretic amorphous–crystalline phase transformations. Repeated latter transformations over cycles, typically featured as a degradation factor, can govern the reversibility behaviors, improving the irreversibility and eventually minimizing cumulative irreversible lithium consumption. This is clearly different from repeated amorphous volume changes with different lithiation depths. The mechanism behind the correlations is elucidated by electrochemical and structural probing.

Room temperature ionic liquid assisted synthesis of highly stable amorphous Se nanoparticles: A rapid and facile methodology

Herein, Selenium (Se) nanoparticles are synthesized in the host matrix of a neat imidazolium based room temperature ionic liquid (RTIL) by a rapid and facile approach at mild conditions. From the microscopic imaging studies, the size of the nanoparticles was determined to be 45 ± 5 nm with globular morphology. XRD and TEM studies revealed the amorphous phase of the Se nanoparticles. This was further evident from the Raman mapping of the nanoparticles. Interestingly, the well-known amorphous to crystalline phase transformation of Se nanoparticles was found to be much slower (>2 months) in RTIL compared to few minutes-to-days as reported in earlier studies. The underlying mechanism for the formation of the nanoparticles and their phase stability has been explained taking into account of the structural and the fluidic aspects of the RTIL.

Influence of Temperature on the Mechanical Properties and Reactive Behavior of Al-PTFE under Quasi-Static Compression.

Al-PTFE (aluminum-polytetrafluoroethylene) is a typical kind of Reactive Material (RM), which has a variety of potential applications in weapon systems. In this paper, quasi-static compression experiments were carried out for a pressed and sintered mixture of Al and PTFE powders using a microcomputer-controlled electronic universal testing machine. The results show that both the mechanical property and reactive behavior of Al-PTFE are strongly temperature-dependent. The material undergoes a brittle-ductile transition associated with a temperature-induced crystalline phase transformation of the PTFE matrix. At low temperatures (−18, 0, and 16 °C), samples of Al-PTFE failed with shear crack and no reaction was observed. As the temperature increased (22, 35, and 80 °C), Al-PTFE exhibited a high toughness and violent reaction occurred in all of the tested samples. Scanning electron microscope observations showed different fracture mechanisms of the PTFE matrix and the increase in toughness was due to the formation of PTFE fibrils which could dissipate energy and bridge crack plane during plastic deformation.