Crystalline Phase Transformation Research Articles

Experimental study of delayed ettringite formation under geothermal high-temperature environment

The geothermal high-temperature environment will potentially damage engineering, which is mainly reflected in the expansion cracking caused by delayed ettringite formation (DEF). The solution method was used to simulate the temperature and humidity coupled liquid-phase environment of ettringite formation, investigate the temperature limit of ettringite generation in concrete pore solution, and analyze the effect of gypsum admixture on DEF under a geothermal environment. The results show that the high temperature has a promoting effect on ettringite generation from the early hydration of cement, and the high temperature causes the AFt generated early to be gradually converted into AFm as the curing age increases. 70–75 °C is the key temperature interval for the transformation of the AFt crystalline phase into the AFm crystalline phase, and it is recommended that the core temperature of concrete during hardening be controlled below 60 °C to avoid the potential risk of AFt decomposing into AFm in large quantities and DEF. The sulfate content of the Portland cement used in the high-temperature region should be strictly controlled, and the amount of gypsum used should not exceed 4%.

Residual stress associated with crystalline phase transformation of 3–6 mol% yttria-stabilized zirconia ceramics induced by mechanical surface treatments

Monolithic dental prostheses made of 3–6 mol% yttria-stabilized zirconia (3–6YSZ) have gained popularity owing to their improved material properties and semi-automated fabrication processes. In this study, we aimed to evaluate the influence of mechanical surface treatments, such as polishing, grinding, and sandblasting, on the residual stress of 3–6YSZ used for monolithic prostheses in association with crystalline phase transformation. Plate specimens were prepared from five dental zirconia blocks: Aadva Zirconia ST (3YSZ), Aadva Zirconia NT (6YSZ), Katana HT (4YSZ), Katana STML (5YSZ), and Katana UTML (6YSZ). The specimens were either polished using 1, 3, or 9 μm diamond suspensions, ground using 15, 35, or 55 μm diamond discs, or sandblasted at 0.2, 0.3, or 0.4 MPa. The residual stress, crystalline phase, and hardness were analyzed using the cosα method, X-ray diffraction (XRD), and Vickers hardness test, respectively. Additionally, we analyzed the residual stress on the surfaces of monolithic zirconia crowns (MZCs) made of 4YSZ, 5YSZ, and 6YSZ, which were processed using clinically relevant procedures, including manual grinding, followed by polishing using a dental electric motor on the external surface, and sandblasting on the internal surface. Residual stress analysis demonstrated that grinding and sandblasting, particularly the latter, resulted in the generation of compressive residual stress on the surfaces of the plate specimens. XRD revealed that the ground and sandblasted specimens contained a larger amount of the rhombohedral phase than that of the polished specimens, which may be a cause of the residual stress. Sandblasting significantly increased the Vickers hardness compared to polishing, which may possibly be due to the generation of compressive residual stress. In the case of MZCs, compressive residual stress was detected not only on the sandblasted surface, but also on the polished surface. The difference in the residual stress between the plate and crown specimens may be related to the force applied during the automated and manual grinding and polishing procedures. Further studies are required to elucidate the effects of the compressive residual stress on the clinical performance of MZCs.

Phase-change-driven tuning of linear and nonlinear optical properties of Te/As2Se3 bilayer thin films for optoelectronic applications

The present work reports the amorphous-crystalline phase transformation by interdiffusion of Te into the As2Se3 matrix with annealing at various temperatures (100, 150, and 200 °C). Te/As2Se3 bilayer thin films were prepared by electron beam evaporation under high vacuum conditions. The amorphous nature of As2Se3 thin film was confirmed from the X-ray diffraction study. The presence of small peaks in Te/As2Se3 thin film indicates the As4Te4 (monoclinic) and As2Se3 (monoclinic) phases, confirming the amorphous to crystalline phase transformation upon Te interdiffusion and subsequent annealing. The impact of Te top layer deposition on the vibrational modes were also observed from the Raman spectroscopy. The formation of Te agglomerates is confirmed from the morphological analysis which disappeared with further annealing. Furthermore, the water contact angle measurements have provided insight into surface interaction mechanisms of these materials. The linear and nonlinear optical properties were changed upon interdiffusion of Te as observed from UV–Visible spectroscopy. The increase in Eu indicated an increase in disorder hence the decrease in the bandgap. The 3rd order nonlinear susceptibility (χ3) and nonlinear refractive index (n2) increased with annealing. The tuning of linear and nonlinear optical properties by annealing can be used in various optoelectronic applications. Specifically, the phase transformation behaviour of Te/As2Se3 bilayer thin films enables their utilization in phase-change material-based optoelectronic devices.

Optical coherence tomography (OCT) for the analysis of zirconia crystalline phase transformation

This study aimed to validate the Optical coherence tomography (OCT) for the analysis of the transformed zone of two dental zirconia-based materials after hydrothermal ageing and correlate the values with biaxial flexural strength. Kinetics of tetragonal to monoclinic phase transformation (t→m) was calculated. X-ray diffraction (XRD) results showed a sigmoidal transformation rate over time due to the limited X-ray maximum penetration depth. Scanning electron microscope (SEM) and OCT showed a linear relationship between the thickness of the transformed layer and the ageing time. The apparent activation energy was 104.5 kJ/mol and 106.7 kJ/mol (SEM) and 106 kJ/mol and 99.4 kJ/mol (OCT) for the infrastructure and monolithic dental zirconia, respectively. Mechanical strength decreased after 150 h. of ageing at 150 °C for both materials showing a correlation with the depth of the transformed zone observed by OCT. Therefore, both monoclinic phase percentage and the depth of the transformed layer are critical concerning zirconia mechanical properties upon hydrothermal ageing. OCT is a non-destructive, fast, innovative, and accurate method for the analysis of zirconia's t→m phase transformation depth kinetics after hydrothermal ageing.

Amorphous-crystalline phase transformation induced changes in linear and non-linear optical properties of Te/As2Se3 bilayer thin films

The current work describes the amorphous state to crystalline state transition upon diffusion of the top Te layer into the bottom As2Se3 layer at 200 °C annealing temperature. The Te/As2Se3 bilayer thin films were prepared by electron beam (e-beam) evaporation technique. The XRD study confirms the amorphous to crystalline phase transformation. The UV–Visible spectroscopy data shows significant changes in both linear and non-linear optical properties due to the diffusion of Te into As2Se3 upon thermal annealing. The phase transformation behaviour of the Te/As2Se3 bilayer thin films can be utilized in various optoelectronic applications.

Meltable, Glass-Forming, Iron Zeolitic Imidazolate Frameworks

We describe the first meltable iron-based zeolitic imidazolate framework (ZIF), denoted MUV-24. This material, elusive from direct synthesis, is obtained from the thermal treatment of [Fe3(im)6(Him)2], which yields Fe(im)2 upon loss of the neutral imidazole molecules. Different crystalline phase transformations are observed upon further heating, until the material melts at 482 °C. Vitrification upon cooling of the liquid phase gives rise to the first Fe-metal-organic framework glass. X-ray total scattering experiments show that the tetrahedral environment of the crystalline solids is maintained in the glass, whereas nanoindentation measurements reveal an increase in Young's modulus, in agreement with stiffening upon vitrification.

Network structure, crystallization behavior, and microwave dielectric properties of ZnO-B2O3 glass-ceramics with ZrO2 additions

The effects of ZrO2 addition on the glass network structure, crystallization behavior and microwave dielectric properties of ZnO-B2O3 glass-ceramics were investigated. The introduction of ZrO2 decreases the connectivity of the superstructure in the borate glass network and thus affects the crystallization behavior of glass, leading to the transformation of the main crystalline phase from Zn4B6O13 to ZnB4O7 and Zn3B2O6. With the increase of ZrO2 content, the microstructure changed from the branch-like structure into the net-like structure. For the sintered glass-ceramics, the relative permittivity increased with the ZrO2 content, which relates to the decrease in the vibrational energy of the B-O bond. In addition, the glass-ceramic with 0.25 mol% ZrO2 exhibits the maximum Q×f value due to the low disorder of B3+ cations. The ZnO-B2O3 glass-ceramic with 0.25 mol% ZrO2 sintered at 660 °C for 5 h exhibits the optimum microwave dielectric properties with εr = 6.18, Q×f = 23,670 GHz, τf = −65 ppm/°C. The high Vickers hardness of 662 kgf/mm2 and excellent chemical compatibility with Ag electrode demonstrated that the glass-ceramic is a good candidate for ULTCC application.

Radio-frequency controlled crystalline phase transformation of MoS2 thin film fabricated by unique vapour-plasma mixing technique

Successful fabrication of molybdenum disulfide thin films on silicon substrates by a unique vapour-plasma mixing technique at a relatively low temperature of 300 °C is reported. This mixing technique involves simultaneous sputtering of a molybdenum target and thermal evaporation of sulphur flakes to form MoS2 molecules. Raman studies confirm the formation of MoS2 with characteristic E12g and A1g modes observed at ∼384 and ∼408 cm−1. The effects of radio frequency (RF) power on the physical properties of MoS2 film at a constant sulphur evaporation rate are investigated. The Raman and X-Ray photon spectroscopy reveal that a controlled crystalline phase transition between metallic tetragonal (1 T-MoS2) and semiconducting hexagonal (2H–MoS2) has been successfully achieved upon variation of the RF power. Furthermore in X-Ray diffraction analysis, Scherrer's, Williamson-Hall and Size-Strain Plot methods are used to extract relevant correlations between structural properties (viz. Crystallite sizes, strains) and the thin film growth controlling parameters.

Effect of Temperature Sintering on Grain Growth and Optical Properties of TiO2 Nanoparticles

Titanium dioxide (TiO2) nanoparticles were prepared by the sol–gel method and the structural, morphological, and optical properties were investigated at different sintering temperatures of 500, 600, 700, 800, and 900°C. A tetragonal structure of anatase, mixed (anatase–rutile), and rutile phases are observed in the X-ray diffraction (XRD) analysis. Pure anatase phase formation occurred at 500°C, whereas anatase-to-rutile phase transformation began at 600°C, and reaching complete conversion to rutile at 800°C. The average crystallite size increased from 23 to 34 nm for anatase and 38 to 62 nm for the rutile when sintering temperature increased from 500 to 900°C. The scanning electron microscope (SEM) result presented the increase of grain size (25–100 nm) with increasing the sintering temperature. The Fourier transform infrared (FTIR) spectra demonstrated the presence of TiO2 vibrational bonds in all samples. The optical bandgaps of the sintered TiO2 nanoparticles decreased with rising sintering temperature. Photoluminescence spectra exhibited three characteristic peaks centered at 380, 450, and 550 nm. The emission intensity increased with increasing the sintering temperature. The Mie analysis was also studied to calculate scattering cross-section, forward scattering, and asymmetry for different grain sizes. The results showed that by increasing the nanoparticle diameters, the peaks of all spectra are redshifted for larger grain sizes and cross-section peaks shift to high values. In this study, the sintering temperature is observed to have a strong influence on crystalline phase transformation, microstructure, and optical properties of TiO2 nanoparticles. The TiO2 sample sintered at 900°C shows the best result to be used as luminescent material due to the low optical bandgap energy.

The Influence of Polishing on the Mechanical Properties of Zirconia—A Systematic Review

Purpose: To systematically review studies that investigated the consequences of various polishing protocols on the mechanical properties of zirconia. The effects on the roughness and crystalline phase transformation were also evaluated. Materials and methods: The Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) was followed. The electronic searches were conducted via OVID MEDLINE (R) and Scopus for publications between 1996 and August 2022. The search strategy was limited to full texts in the English language and in vitro studies. The influences on flexural strength, hardness, fracture strength, fracture toughness, wear resistance, roughness and phase transformation were collected. Various methodologies to measure these properties were also outlined and compared. The risk of bias for included studies was evaluated according to a modified Consolidated Standards of Reporting Trials (CONSORT) checklist. Results: After removing duplicates, the systematic search identified a total of 419 studies. Nineteen studies satisfied the inclusion criteria and were selected for final analysis. Fifteen of the included studies observed the changes in surface roughness along with the mechanical properties and ten studies detected the tetragonal (T) to monoclinic (M) phase transformation. Eight studies also investigated the change in properties after polishing the ground surface. Testing parameters were not consistent among studies due to the varying methods. Conclusions: To a certain extent, polishing influences the strength, hardness, toughness and wear resistance. The damage in some mechanical properties, as well as the roughened surface, from grinding can be restored via an appropriate polishing treatment. The polishing process itself barely induces the transition from the tetragonal to monoclinic phase of zirconia, while this commonly occurs after grinding. If the subsequent polishing is adequate, the transformed monoclinic phase can be eliminated with the removal of the outermost surface layer. In dentistry, polishing is an imperative step to maintain the superior functions and service life of zirconia for patients.

Crystal strain engineering of AgBr/TiO2 for visible-light photocatalysis: Balancing light absorption and charge carrier kinetics

Addition of noble metals and/or their halides certainly improves the visible-light photocatalytic activity of TiO2, but whether this improvement is due to better photoabsorption capacity of the noble metal or enhanced photoelectrical properties of TiO2 itself, is still a subject of debate. Furthermore, the optimized load of noble metal halides at which best photocatalytic activity in TiO2 occurs hasn’t been explained in the literature consistently. Herein, we investigate the change in intrinsic lattice strain of TiO2 at different AgBr loadings which imparts selective optoelectronic properties turning AgBr/TiO2 system into a visible-light photocatalyst. XRD analysis reveals that beyond a maximum strain developed at an optimum AgBr loading level, the grain boundary interface can no longer withstand the increased strain at Ti—Ti, and Ti—O bonds and a chain of thermodynamically governed crystalline phase transformation within TiO2 begins. Faster dye degradation kinetics corroborate the hypothesis that the catalyst possessing maximum lattice strain shows superior photocatalytic properties. Photoelectrochemical studies was used to quantify the rate of charge transfer (kct) and rate of recombination (kr) of photogenerated electrons and holes. These studies revealed that although higher AgBr concentration may increase charge transfer through plasmon resonance but ultimately kr controls the total charge carrier flux that determines the photocatalytic efficicency of TiO2. Our work highlights methods in which the optimum loading of AgBr can be determined and presents fundamental insight into the role of lattice strain in balancing optical and electrical properties of TiO2 for constructing high-performance photocatalysts suitable for visible light application.

Impact of Annealing on ZrO2 Nanotubes for Photocatalytic Application

This work aims to study the structural, optical, and photocatalytic properties of ZrO2 nanotubes (NTs) that have been synthesized using the electrochemical anodization method. The structural and morphological characteristics of unannealed and annealed (400 °C, 500 °C, and 700 °C) ZrO2 NTs were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Details of the structural and morphological results are depicted to clarify the effect of annealing temperature on the NTs. Furthermore, the reflectivity and photoluminescence of ZrO2 NTs were found to depend on the annealing temperature. The resulting bandgap values were 3.1 eV for samples annealed at 400 °C and 3.4 eV for samples annealed at 550 and 700 °C. Thus, amorphous and annealed ZrO2 NTs were tested in terms of their photocatalytic degradation of Black Amido (BA) dye. Samples annealed at 400 °C exhibited 85.4% BA degradation within 270 min compared to 77.5% for samples annealed at 550 °C and 70.2% for samples annealed at 700 °C. The anodized ZrO2 NTs that were annealed at 400 °C showed the coexistence of tetragonal and monoclinic crystalline phases and exhibited the fastest photocatalytic performance against the BA dye. This photocatalytic behavior was correlated to the crystalline phase transformation and the structural defects seen in anodized ZrO2.

Migration of lattice oxygen during chemical looping dry reforming of methane with Ca2Fe2O5/Zr0.5Ce0.5O2 oxygen carrier

Ca2Fe2O5/Zr0.5Ce0.5O2 exhibits excellent reactivity in chemical looping dry reforming of methane. The migration of lattice oxygen is crucial to the cyclic stability of oxygen carrier, but the lattice oxygen migration in Ca2Fe2O5/Zr0.5Ce0.5O2 were not extensively explored. This work explored the lattice oxygen migration of Ca2Fe2O5/Zr0.5Ce0.5O2 during redox process. During the reduction process, the lattice oxygen reacted with the hydrogen radical, resulting in the decrease of lattice oxygen content and the increase of oxygen vacancy. As the lattice oxygen released, the spatial structure of Ca2Fe2O5 was destroyed and transferred into Fe8Ca8O20 with oxygen defects due to lattice distortion. The exothermic reaction between H2 and Ca2Fe2O5 promoted the formation of FeH with hexagonal close-packed structure. After reduction for 160 min, the unstable FeH completely converted into FeC with stable structure and the complete reduction was reached. As for the oxidation process, Fe and CaO reacted with the oxygen radical to form Ca2Fe2O5, leading to the complete recovery of lattice oxygen within 40 min. However, crystalline phase transformation could lead to the spatial structure instability and sintering after multiple redox cycles. Overall, the migration of oxygen during redox process is clarified in detail, providing a reference for improving the cyclic stability of Ca2Fe2O5/Zr0.5Ce0.5O2.

The effects of multiphase composition on structure and mechanical properties of ZrO2/Si–C–O ceramic fibers by sol-gel process

High temperature thermally stable fiber materials with flexibility are expected to be applicable to severe conditions. In this article, ZrO2/Si–C–O ceramic fibers have been synthesized by the sol-gel process combined with the electrospinning technique. We focus on the changes of composition, structure and morphology with the increasing heat treatment temperature. More importantly, the effects of ZrO2 crystalline phase transformation on mechanical property of ZrO2/Si–C–O ceramic fibers have been investigated. The results show that the increased temperature can lead to an increase of the fiber diameter and grain size, as well as the transformation of ZrO2 phase. It is also found that the ZrO2/Si–C–O ceramic fibers possess refined crystalline grain and improved mechanical properties at a heat treatment temperature of 800 °C, which is mainly due to the fact that the amorphous Si–C–O phase containing free carbon facilitates the refinement of the crystalline grain, and the ZrO2 in the tetragonal structure improves the flexibility of ceramic fibers. Furthermore, the obtained fibers exhibit excellent thermal stability up to 1300 °C, therefore they have broad application prospect in high temperature environment.

Performance of iron sand as an oxygen carrier at high reduction degrees and its potential use for chemical looping gasification

Iron sand as an industrial by-product has a reasonable iron content (35 wt% Fe) and low economical cost. The reactivity of iron sand as an oxygen carrier was examined in a bubbling fluidized bed reactor using both gaseous and solid fuels at 850–975 °C. Pre-reductions of iron sand were performed prior to fuel conversion to adapt the less-oxygen-requiring environment in chemical looping gasification (CLG). Based on the investigations using CO and CH4, iron sand has an oxygen transfer capacity of around 1 wt%, which is lower than that of ilmenite. The conversion of pine forest residue char to CO and H2 was higher when using iron sand compared to ilmenite. Depending on the mass conversion degree of iron sand, the activation energy of pine forest residue char conversion using iron sand was between 187 and 234 kJ/mol, which is slightly lower than that of ilmenite. Neither agglomeration nor defluidization of an iron sand bed occurred even at high reduction degrees. These suggests that iron sand can be utilized as an oxygen carrier in CLG. Furthermore, this study presents novel findings in the crystalline phase transformation of iron sand at various degrees of oxidation, altogether with relevant thermodynamic stable phases.

Synthesis of high temperature resistant ZrO2-SiO2 composite aerogels via supercritical fluid deposition

A series of ZrO2-SiO2 composite aerogels have been synthesized by adjusting the volume of tetraethoxysilane (TEOS) during supercritical fluid drying process. ZrO2 aerogels have been prepared via the sol-gel reaction using ZrOCl2ꞏ8H2O and the gel initiator malic acid as raw materials. Then the ZrO2-SiO2 composite aerogels have been obtained by supercritical fluid deposition (SCFD). The samples have been characterized applying XRD, FTIR, SEM, TEM, TGA and Nitrogen adsorption-desorption. The introduction of SiO2 can effectively restrain the growth of the nanoparticles and the transformation of crystalline phase, and reduce the linear shrinkage of the aerogels. It indicates the improvement of the thermostability of the ZrO2-SiO2 composite aerogels.

Crystalline phase transformation of Co-MOF derivatives on ordered mesoporous carbons for high-performance supercapacitor applications

Preparation and phase transformation of Co-based metal–organic frameworks derivatives.

Tuning magnetic and luminescent properties of iron oxide@C nanoparticles from hydrothermal synthesis: Influence of precursor reagents

Magneto-luminescent nanocomposites were obtained by coating coprecipitated iron oxide nanoparticles with a functionalized carbon layer by hydrothermal treatment, using different organic precursors (D-glucose and p-phenylenediamine). The nanomaterials were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), RAMAN, infrared (FTIR), UV-vis and photoluminescence (PL) spectroscopic analysis, vibrating sample magnetometry (VSM) and zetametry. Remarkable contrasting changes in the structural and magnetic properties of nanocomposites were identified after the carbon coating, which was directly related to the precursor used. Crystalline phase transformations were observed with a decrease in the saturation magnetization (-80%) using p-phenylenediamine and an increase (+22%) using glucose, relative to uncoated magnetic core. The distinct photoluminescent properties with multicolored emissions on long-wavelength are ascribed to the carbon shell and to the chemical moieties, such as amino, carbonyl, carboxylate, and hydroxyl groups on the surface. This makes it possible for different surface states to be responsible for emission, and these chemical groups are also responsible for a higher colloidal stability of nanocomposites in alkaline medium. The tunability of the properties of nanocomposites from the conditions of synthesis opens a wide range of applications for these nanomaterials, such as ion detection, magnetohyperthermia, and theranostics.

The effect of copper on phase transformation, microstructure and mechanical characterization of Ni50-xTi50Cux shape-memory alloy

This research is related to the investigation of the influence of copper additions on phase transformation, microstructure and mechanical characterization of equiatomic NiTi shape-memory alloys. A typical vacuum arc melting furnace was used in the preparation of Ni50-xTi50Cux (x = 2, 5, 10, and 15 at.%) shape-memory alloys. The crystalline phase formation and phase transformation temperature were studied using X-ray diffraction and a differential scanning calorimeter. The morphology and chemical composition of the annealed samples were investigated using a field emission scanning electron microscope and energy-dispersive X-ray spectrometer analysis. The Vickers microhardness test was utilized in the analysis of the homogeneity of the alloy. The experimental results obtained from X-ray diffraction confirmed the coexistence of austenite and martensite phases along with the presence of a Ti2Ni and minor rutile phases. The crystalline sizes of the alloys were in the range of 42 to 78nm. An energy-dispersive spectrometer validated the distinct phases and precipitates in the alloys. The size and dispersion of the secondary phase in the alloy showed a significant influence on the increased hardness from 407HV to 470HV. The following NiTiCu alloy compositions were employed for low-temperature applications since their transition temperatures were below 100°C.

In situ surface engineering enables high interface stability and rapid reaction kinetics for Ni-rich cathodes

Layered oxide cathodes with high Ni content promise high energy density and competitive cost for Li-ion batteries (LIBs). However, Ni-rich cathodes suffer from irreversible interface reconstruction and undesirable cracking with severe performance degradation upon long-term operation, especially at elevated temperatures. Herein, we demonstrate in situ surface engineering of Ni-rich cathodes to construct a dual ion/electron-conductive NiTiO3 coating layer and Ti gradient doping (NC90–Ti@NTO) in parallel. The dual-modification synergy helps to build a thin, robust cathode–electrolyte interface with rapid Li-ion transport and enhanced reaction kinetics, and effectively prevents unfavorable crystalline phase transformation during long-term cycling under harsh environments. The optimized NC90–Ti@NTO delivers a high reversible capacity of 221.0 mAh g−1 at 0.1C and 158.9 mAh g−1 at 10C. Impressively, it exhibits a capacity retention of 88.4% at 25 ​°C after 500 cycles and 90.7% at 55 ​°C after 300 cycles in a pouch-type full battery. This finding provides viable clues for stabilizing the lattice and interfacial chemistry of Ni-rich cathodes to achieve durable LIBs with high energy density.