Crystalline Phase Transformation Research Articles

Modeling the Stress-Induced Transformation Behavior of Shape Memory Alloys under Multiaxial Loading Conditions

A large number of criteria to model the onset of plasticity for ductile metals have been proposed by researchers in the last century. Strangely, very few researchers have tried to model the stress-induced crystalline phase transformation of Shape Memory Alloys (SMAs) according to yield criteria. This paper focuses on the question: is a yield criterion originally proposed for describing the plastic behavior of metals suitable to model the “pseudoelastic” behavior of SMAs? To answer this question, two yield criteria originally proposed by the present author are used to predict the initial surface of transformation onset of two different SMAs: Cu-Al-Be and Ni-Ti alloy. The predicted initial transformation onset surfaces of the two SMAs are compared with experimental results and existing theories reported in the literature and some significant conclusions and recommendations are given.

Systematic Structural and Optical Characterization of TiO2 Nanofibres Synthesised by Electrospinning

TiO2 nanofibres were synthesised by means of the electrospinning technique, which were annealed at high temperatures to achieve the crystalline phase transformation. The chemical stoichiometry of electrospun TiO2 nanofibres was estimated by EDS, finding that at low annealing temperatures excess of oxygen was detected and at high temperatures excess of titanium that originates oxygen vacancies. TEM images show clearly the formation of TiO2 nanofibres that exhibit a homogeneous and continuous aspect without the presence of crystalline defects, whose surface morphology depends strongly on the annealing temperature. The crystalline phase transformation was studied by Raman spectroscopy, which revealed that annealed TiO2 nanofibres showed a crystalline phase transformation from pure anatase to, first a mix of anatase-rutile, then pure rutile as the annealing temperature increased, which was corroborated by X-ray diffraction and high-resolution TEM microscopy. The average grain size, inside the nanofibres, increased with the crystalline phase transformation from 10 to 24 nm for anatase-TiO2 and from 30 to 47 nm for rutile-TiO2, estimated by using the Scherrer-Debye equation. The band gap energy (Eg), obtained from optical absorption spectra, decreases monotonically, where a local minimum is observed at 700 °C, which is ranged in 3.75  Eg  2.42 eV, caused by the anatase → rutile crystalline phase transformation. The photoluminescence shows that radiative bands present a gradual red-shift as the annealing temperature increases due to the continuous change of Eg.

Systematic Structural and Optical Characterization of TiO2 Nanofibers Synthesised by Electrospinning

TiO2 nanofibers were synthesised by means of the electrospinning technique, which were annealed at high temperatures to achieve the crystalline phase transformation. The chemical stoichiometry of electrospun TiO2 nanofibers was estimated by EDS, finding that at low annealing temperatures excess of oxygen was detected and at high temperatures excess of titanium that originates oxygen vacancies. TEM images show clearly the formation of TiO2 nanofibers (NF’s) that exhibit a homogeneous and continuous aspect without the presence of crystalline defects, whose surface morphology depends strongly on the annealing temperature. The crystalline phase transformation was studied by Raman spectroscopy, which revealed that annealed TiO2 NF’s showed a crystalline phase transformation from pure anatase to, first a mix of anatase-rutile, then pure rutile as the annealing temperature increased, which was corroborated by X-ray diffraction and high-resolution TEM diffraction. The average grain size, inside the NF´s, increased with the crystalline phase transformation from 10 to 24 nm for anatase-TiO2 and from 30 to 47 nm for rutile-TiO2, estimated by using the Scherrer-Debye equation. The band gap energy (Eg), obtained from optical absorption spectra, decreases monotonically, where a local minimum is observed at 700 °C ranged in 3.75 ≤ Eg ≤ 2.42 eV, caused by the anatase → rutile crystalline phase transformation. The photoluminescence shows that radiative bands show a gradual red-shift as the temperature increases due to the continuous change of Eg.

In situ inorganic conductive network formation in high-voltage single-crystal Ni-rich cathodes

High nickel content in LiNixCoyMnzO2 (NCM, x ≥ 0.8, x + y + z = 1) layered cathode material allows high specific energy density in lithium-ion batteries (LIBs). However, Ni-rich NCM cathodes suffer from performance degradation, mechanical and structural instability upon prolonged cell cycling. Although the use of single-crystal Ni-rich NCM can mitigate these drawbacks, the ion-diffusion in large single-crystal particles hamper its rate capability. Herein, we report a strategy to construct an in situ Li1.4Y0.4Ti1.6(PO4)3 (LYTP) ion/electron conductive network which interconnects single-crystal LiNi0.88Co0.09Mn0.03O2 (SC-NCM88) particles. The LYTP network facilitates the lithium-ion transport between SC-NCM88 particles, mitigates mechanical instability and prevents detrimental crystalline phase transformation. When used in combination with a Li metal anode, the LYTP-containing SC-NCM88-based cathode enables a coin cell capacity of 130 mAh g−1 after 500 cycles at 5 C rate in the 2.75-4.4 V range at 25 °C. Tests in Li-ion pouch cell configuration (i.e., graphite used as negative electrode active material) demonstrate capacity retention of 85% after 1000 cycles at 0.5 C in the 2.75-4.4 V range at 25 °C for the LYTP-containing SC-NCM88-based positive electrode.

Effect of electrochemical pretreatment on the control of scaling and fouling caused by circulating cooling water on heat exchanger and side-stream reverse osmosis membrane

An environment-friendly electrochemical pretreatment process was used to purify the circulating cooling water (Including salts extraction and organics removal), and the effect of electrochemical pretreatment on scale inhibition and reverse osmosis (RO) was explored. Results showed that the electrochemical pretreatment exhibits a good softening ability for different types of water with hardness from 100 to 500 mg/L (Calculated by CaCO3). Considering the treatment efficiency and energy consumption, 5 V and 7 V were suitable for softening treatment of low hardness water and high hardness water respectively (with 300 mg/L hardness as the limit). After electrochemical pretreatment, CaCO3 scale on heater surface can be effectively inhibited and the maximum scale inhibition rate can reach 73.9% at 7 V. Meanwhile, the CaCO3 scale is loose after electrochemical pretreatment, accompanied by the transformation of crystalline phase from calcite to aragonite. The long-term RO membrane flux experiment shows that the pure inorganic salt solution has the least contribution to RO membrane fouling, followed by the pure organic solution. Notably, when inorganic salt ions coexist with organic matters in water, a complex structure of gel network can be formed, which causes serious pollution to RO membrane and sharp decrease of the specific flux (J/J0). Interestingly, RO membrane fouling can be effectively controlled after electrochemical pretreatment even for actual circulating water. This work provides a simple strategy for the development and application of electrochemical technology in softening and RO membrane fouling control.

Nonthermal Transport of Energy Driven by Photoexcited Carriers in Switchable Solid States of GeTe

Phase change alloys have seen widespread use from rewritable optical discs to the present day interest in their use in emerging neuromorphic computing architectures. In spite of this enormous commercial interest, the physics of carriers in these materials is still not fully understood. Here, we describe the time and space dependence of the coupling between photoexcited carriers and the lattice in both the amorphous and crystalline states of one phase change material, GeTe. We study this using a time-resolved optical technique called picosecond acoustic method to investigate the \textit{in situ} thermally assisted amorphous to crystalline phase transformation in GeTe. Our work reveals a clear evolution of the electron-phonon coupling during the phase transformation as the spectra of photoexcited acoustic phonons in the amorphous ($a$-GeTe) and crystalline ($\alpha$-GeTe) phases are different. In particular and surprisingly, our analysis of the photoinduced acoustic pulse duration in crystalline GeTe suggests that a part of the energy deposited during the photoexcitation process takes place over a distance that clearly exceeds that defined by the pump light skin depth. In the opposite, the lattice photoexcitation process remains localized within that skin depth in the amorphous state. We then demonstrate that this is due to supersonic diffusion of photoexcited electron-hole plasma in the crystalline state. Consequently these findings prove the existence of a non-thermal transport of energy which is much faster than lattice heat diffusion.

Annealing Impact on the Evolution of Crystalline Phases and Emission in Si-Rich HfO2:Nd Films Obtained By Magnetron Sputtering

Hafnium oxide films doped with Si and Nd atoms have been produced by radio-frequency magnetron sputtering in pure argon plasma followed by an annealing in nitrogen atmosphere for 1-60 minutes at the temperature 1000 °C. The effect of post-deposition thermal annealing on the transformation of crystalline phases and photoluminescence spectra was investigated by the non-destructive methods (scanning electronic microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman scattering and photoluminescence(PL)). The XRD analysis of these samples demonstrate the phase separation process which depends on the time range and the annealing temperature. In the studed samples, the formation of the tetragonal HfO2, tetragonal SiO2 and cubic Si phases was observed by XRD and confirmed by the Raman study. Thermal annealing influents also on PL spectra where the main bands were detected at 380, 450, 550, 780 and 880 nm. The emission and excitation mechanisms for the Nd radiative defects, and the impact of phase transformation in studied films, are analyzed and discussed.

Structure and Emission Evolution in Si-Rich HfO2:Pr Films Versus Annealing Temperature

Pr-doped Si-rich-HfO2 films with specific emitting and structural properties have been prepared by radio-frequency magnetron sputtering using standard pure argon plasma approach. The effect of post-deposition thermal annealing on the transformation of crystalline phases, their compositions and photoluminescence spectra was investigated by means of the Energy Dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), scanning electronic microscopy (SEM), Raman scattering and photoluminescence (PL) methods. Thermal annealing was carried out at 800 °C, 900 °C, 1000 °C and 1100 °C within time of 30 min in nitrogen atmosphere. The SEM images demonstrate the grains with the size about 40 nm. It was observed that the influence of temperature at annealing causes the phase transformation and separation processes. In studied samples, the formation of the tetragonal HfO2 and SiO2 phases, were detected at XRD study after the annealing at 1000 and 1100 °C. PL spectra of these films are complex and demonstrate several PL bands centered at 442, 479, 523, 607, 637 and 716 nm. Their contribution depends on the annealing temperature and governs the shape of total PL spectra. The impact of crystalline phase separation on the peculiarities of PL spectra and XRD are analyzed and discussed.

Synthesis method comparison of compositionally complex rare earth‐based Ruddlesden–Popper n = 1 T′‐type cuprates

AbstractThe multicomponent approach has been successfully expanded to the Ruddlesden–Popper structure with the synthesis of two different high‐entropy cuprate compositions: (La0.2Nd0.2Gd0.2Tb0.2Dy0.2)2CuO4 and (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4. The effect of synthesis method is explored using both solid‐state reaction and polymeric steric entrapment (PSE) methods. It is found that PSE leads to more randomly distributed cation species, providing an advantageous method of synthesis for the growing field of high entropy oxides. In situ high‐temperature x‐ray diffraction tracks the amorphous to crystalline phase transformation in (La0.2Nd0.2Gd0.2Tb0.2Dy0.2)2CuO4 powder, synthesized using the PSE method. Using the High‐Temperature XRD data, a method for gaining information on the kinetic behavior is also applied. Magnetometry of both compositions indicates ferrimagnetic behavior at low temperatures.

Model fitted and model free approaches for crystallization kinetics in Se85Te15-xBix glasses

In this research work, we have described the model-fitted and model free approaches for the study of crystallization kinetics in Se85Te15-xBix chalcogenide glasses. Se85Te15-xBix bulk alloys were synthesized by melt quenching technique. High Resolution X- Ray diffraction (HRXRD) was used to confirm the amorphous nature of prepared alloys. Non-isothermal Differential Scanning Calorimetry (DSC) measurements were done at heating rates of 5, 10, 15, 20 and 25 K/min for crystallization kinetics studies in Se85Te15-xBix glasses. The various characteristic temperatures, such as glass transition (Tg), on-set crystallization (Tc) temperature, peak crystallization temperature (Tp) and melting temperatures (Tm) have been obtained from various DSC thermograms. The activation energies of glass transition (ΔEt) were calculated by using Kissinger and Moynihan approaches and found to be minimum for Se85Te12Bi3 chalcogenide glass which indicates that this alloy has maximum probability to jump into a less configurational energy state and has larger stability. The model-free approaches; Kissinger–Akahira–Sunose (KAS), Flynn-Wall-Ozawa (FWO), Tang and Straink (TS) reveal that the activation energy of crystallization varies with crystallization degree and temperature both. This variation shows that amorphous to crystalline phase transformation in Se85Te15-xBix chalcogenide glasses is a complex process with various nucleation and growth mechanisms.

Thermal behavior of different cocoa powder varieties and their physicochemical, phytochemical and microbiological characteristics

Four cocoa powder varieties processed in different European countries (Germany, Poland, Romania and Bulgaria) were subjected to physicochemical, phytochemical and microbiological analysis. The cocoa powders were extensively characterized by recording their pH and titratable acidity, respectively, the polyphenols and also the methylxantine derivatives content (theobromine and caffeine). The cocoa powders pH ranged between 5.37 and 8.23, while the titratable acidity was 3.2–4.3 miliequivalent (100 g)−1 of cocoa powder. Their total polyphenols content ranged between 0.986 ÷ 2.003 g GAE/(100 g)−1. The methylxanthine derivatives (theobromine and caffeine) were analyzed by the HPLC method and ranges of 0.992–1.174% for theobromine and 0.096–0.369% for caffeine were obtained. Thermal analysis (TG–DTA) combined with mass spectrometry (MS) elucidated the decomposition processes and the volatile substances (CO, CO2, H2O, NO, theobromine, caffeine). The thermal analysis revealed transformations in the cocoa powders composition: drying and water loss; decomposition of pectic polysaccharides; lipids, amino acids and proteins, crystalline phase transformations and carbonizations. The microbiological analysis tested the degree of preservation of the cocoa powders across time, specifically immediately after unwrapping and after 14 days.

Highly ordered Nb2O5 nanochannel film with rich oxygen vacancies for electrocatalytic N2 reduction: Inactivation and regeneration of electrode

We report the fabrication of highly ordered Nb2O5 nanochannel film (Nb2O5-NCF) onto niobium foil by an anodization method. After thermal treatment, the obtained Nb2O5-NCF with rich oxygen vacancies exhibits electrochemical N2 reduction reaction (NRR) activity with an NH3 yield rate of 2.52 × 10−10 mol cm-2 s-1 and a faradaic efficiency of 9.81% at −0.4 V (vs. RHE) in 0.1 mol/L Na2SO4 electrolyte (pH 3.2). During electrocatalytic NRR, the Nb2O5-NCF takes place electrochromism (EC), along with a crystalline phase transformation from pseudo hexagonal phase to hexagonal phase owing to H+ insertion. This results in the reduced NRR activity due to the decrease of oxygen vacancies of hexagonal phase Nb2O5, which can be readily regenerated by low-temperature thermal treatment or applying an anodic potential, showing superior recycling reproducibility.

A pyridinium salt with crystalline phase transformation under water vapor and reversible mechanochromic luminescent properties

The pyridinium salt of CPBBr undergoes crystalline phase transformation under water vapor, accompanied by emission turn-on from deep blue to green. The reversible mechanochromism luminescence indicates the formation of a charge transfer excimer.

Direct preparation of magnesium borate (Mg2B2O5) ceramic from boron-bearing slag: Super-gravity separation and microwave dielectric properties

In this study, a novel method was developed for direct preparation of magnesium borate (Mg2B2O5) ceramic from boron-bearing slag. The amorphous boron was efficiently enriched into Mg2B2O5 via crystalline phase transformation in the condition of B/S (B2O3/SiO2) = 1.20 and wt.% (MgO) = 36.00. High-purity Mg2B2O5 crystals were fully recovered from molten slag by super-gravity separation at G ≥ 500 and t ≥ 240 s. The Mg2B2O5 ceramic was directly prepared at various temperature by using the recovered Mg2B2O5, and the Mg2B2O5 ceramic displayed a favourable microwave dielectric properties including the dielectric constant (εr) of 6.05, the quality factor (Q × f) of 38,147 GHz and the temperature coefficient of resonant frequency (TCF) of -20.98 ppm/K. Compared with synthetic methods that use high-purity oxides, this study provides a green and economical method for direct utilisation of industrial slag to prepare microwave dielectric materials.

Understanding the Amorphous Lithiation Pathway of the Type I Ba8Ge43 Clathrate with Synchrotron X-ray Characterization

Tetrel (Tt = Si, Ge, and Sn) clathrates have highly tunable host–guest structures and have been investigated as novel electrode materials for Li-ion batteries. However, there is little understanding of how the clathrate structure affects the lithiation processes and phase evolution. Herein, the electrochemical lithiation pathway of type I clathrate Ba8Ge43 is investigated with synchrotron X-ray diffraction (XRD) and pair distribution function (PDF) analyses and compared to the lithiation of germanium with a diamond cubic structure (α-Ge). The results confirm previous laboratory XRD studies showing that Ba8Ge43 goes through a solely amorphous phase transformation, which contrasts with the crystalline phase transformations that take place during lithiation of micrometer-sized α-Ge particles. The local structure of framework-substituted clathrate Ba8Al16Ge30 after lithiation is found to proceed through an amorphous phase transformation similar to that in Ba8Ge43. In situ PDF and XRD during heating show that the amorphous phases derived from lithiation of Ba8Ge43 are structurally related to various Li–Ge phases and crystallize at low temperatures (350–420 K). We conclude that the Ba atoms inside the clathrate structure act to break up the long-range ordering of Li–Ge clusters and kinetically prevent the nucleation and growth of bulk crystalline phases. The amorphous phase evolution of the clathrate structure during lithiation results in electrochemical properties distinct from those in α-Ge, such as a single-phase reaction mechanism and lower voltage, suggesting possible advantages of clathrates over elemental phases for use as anodes in Li-ion batteries.

Enhanced thermoelectric properties of electrodeposited Bi2Te3 thin films using TiN diffusion barrier layer on a stainless-steel substrate and thermal annealing

Electrodeposition is an efficient wet-chemical approach to reduce the manufacturing cost of thin films. However, the thin films fabricated using this approach show properties inferior to those of the thin films fabricated using dry processes. In this study, a titanium nitride (TiN) diffusion barrier layer was formed on a stainless-steel (SS) substrate to improve the thermoelectric properties of electrodeposited Bi2Te3 thin films. The electrodeposition of the Bi2Te3 films was carried out in the potentiostatic mode using a standard three-electrode cell, followed by thermal annealing at 300–500°C. The X-ray diffraction and electron probe X-ray microanalysis results revealed that impurity diffusion from the SS substrate did not occur in the annealed films deposited on the TiN-coated SS substrate. However, the Bi2Te3-to-Bi4Te5 crystalline phase transformation occurred at higher annealing temperatures because of the preferential evaporation of Te atoms from the film surface, leading to the deterioration of the thermoelectric properties of the films. As a result, the 300°C-annealed film formed on the TiN-coated SS substrate showed the highest power factor of 4.4 μW/(cm·K2), which is approximately four times higher than that of the corresponding film formed on the bare SS substrate. Therefore, the TiN coating and optimum annealing temperature improved the thermoelectric properties of the electrodeposited Bi2Te3 thin films.

Nanoporous electrodes of phase-dealloyed Fe83.3-xCoxSi4B8P4Cu0.7 (x = 4, 10 and 20 at.%) precursors with superior Redox performances and high stabilities

Co substitutions for Fe83.3Si4B8P4Cu0.7 precursors are effective to stabilize the nanoporous structures during dealloying in H2SO4 solutions and improve Redox performances of nanoporous electrodes in alkaline conditions. Heterogeneous nanocrystalline structures of Co-substituted Fe83.3-xCoxSi4B8P4Cu0.7 (x = 0, 4, 10 and 20 at.%) alloys consist of two phases: α-Fe(Co) phases and continuously distributed residual amorphous phases. The nanopores on as-dealloyed Fe-Co-Si-B-P-Cu alloys are smaller in size with higher Co concentrations. This is ascribed to the finer α-Fe(Co) phases distributed in the precursor alloys after nanocrystallization. Nanoporous architectures after dealloying inherit the microstructural characteristics of Fe-Co-Si-B-P-Cu precursor alloys. The short-term and long-term CV curves of nanoporous Redox electrodes in 0.5 M KOH solution demonstrate that Redox reactions have been enhanced by partial substitution of Fe by Co and the best Redox performance can be obtained on as-dealloyed Fe79.3Co4Si4B8P4Cu0.7 alloys because of the highest Redox peak current density of 90 mA cm−2 after 150 CV cycles. However, the stabilities of nanoporous Fe79.3Co4Si4B8P4Cu0.7 electrodes have a predictable decline due to the overgrowth of Fe3O4 octahedra and destruction of the ligaments during long-term Redox cycling. On the other hand, the better Redox stabilities has achieved on as-dealloyed Fe63.3Co20Si4B8P4Cu0.7 alloys with stable oxidation peak current densities of 45–47 mA cm−2 from 950 to 3000 CV cycles, about 2.3 times the nanoporous Co-free Fe83.3Si4B8P4Cu0.7 electrodes. The stabilities are considered to result from the relatively small Fe3O4 octahedra, the stabilization of the amorphous Co3O4 outmost layers and the transformation of amorphous and crystalline Co3O4 phases during long-term Redox cycling. The Co substitution of Fe83.3Si4B8P4Cu0.7 precursor alloys can achieve the high Redox performances and good stabilities of nanoporous electrodes.

Formation of New Crystalline Phases upon High-Temperature Annealing of Iron Borate FeBO3 in Different Gas Media

Iron borate FeBO3 crystals are annealed in neutral, oxidizing, and reducing gas media on a specially designed setup. The effect of the chemical medium and annealing modes on the surface morphology and phase composition of the samples is established. It is shown that annealing in a neutral or oxidizing medium leads to the recrystallization of FeBO3 into the hematite α-Fe2O3 phase, whereas annealing in a reducing medium results in the formation of the pyroborate Fe2B2O5 phase and the metal α-Fe iron phase. The difference in the nature of structural transformations is associated with the change in the valence state of iron ions upon annealing in the reducing atmosphere. This result is of interest as a method for the transformation of crystalline phases.

Controlling oxygen vacancies of WOx suboxides by ZnWO4 nanophase hybridization

Substoichiometric tungsten oxide (WOx, 2 < x < 3) nanomaterials are important nanoscale semiconductor materials with broad applications in optoelectronic devices; however a controllable modification of their physical properties remains challenging task. Here, we report an alternative green synthesis of hybrid nanostructures composed by WOx and ZnWO4 phases in ethanol via a solvothermal process using ZnO and WCl6 as the precursors. The results indicate that the formation of ZnWO4 leads to the structural conversion of WOx from nanorods to nanoparticles due to the effect of ZnWO4 on the side growth of WOx nanostructures. Moreover, the surface morphology tailored by ZnWO4 and crystalline phase transformation induce different photoluminescence (PL) emission from pure WOx nanostructures and WOx/ZnWO4 hybrid nanostructures due to the increased number of oxygen vacancies.

Influence of phase transformation on structure–property relationship in quaternary In10Sb10Ag10Se70 chalcogenide films

This work reports the effect of thermal annealing (353–413 K) on structure and physical properties of thermally evaporated In10Ag10Sb10Se70 chalcogenide films. The samples were characterized by X-ray diffraction, transmission electron microscopy, TEM, Raman spectroscopy, optical absorption and photoconductivity measurements. Thermal annealing favours amorphous to polycrystalline structure transition with the growth of Se, AgInSe2 and AgSbSe2 phases. A significant change in position/shape of Raman bands upon annealing has been observed. The indirect optical gap decreases from 1.44 to 0.68 eV while tailing parameter increases with annealing. Photosensitivity decreases from 29.3 to 14.2 with annealing temperature. The decay of photocurrents is well fitted to stretched exponential function; the value of decay time constant increases while local minimum value of dispersion parameter at 353 K has been observed. These results are explained on the basis of amorphous crystalline phase transformation in chalcogenides and are important for developing new materials for emerging technologies.