Crystalline Phase Transition Research Articles

Hexagonal Phase Formation and Crystalline Structural Transition in Long-Spaced Aliphatic Polyesters with Side Groups.

Side substitution is an effective method for the chemical modification and functionalization of linear polyesters. The presence of side groups can have a profound effect on the crystalline structure and phase transition of semicrystalline polyesters. Herein, we synthesized the long-spaced polyesters with -OH and -CH3 side groups and various methylene segment lengths and studied the effects of the side groups on the crystal polymorph and phase transition of substituted polyesters. The substituted polyesters grow in the thermally stable phase (form I) at a higher temperature. However, the polyesters crystallize in a metastable hexagonal phase (form II) with trans chain conformation at a lower temperature. The metastable form II transforms into the more stable form I during long-time annealing or upon heating; this phase transition is accompanied by chain tilting and crystal lamellar thickening. This study has elucidated the critical role of side groups in the polymorphic crystallization and phase transition of linear polyesters.

Alkali metal and alkaline earth metal perovskites for oxidative coupling of methane and oxidative dehydrogenation of ethane

In this study, alkali metal and alkaline earth metal perovskites have been synthesized using simple preparation methods for catalyzing the oxidative coupling of methane (OCM) and oxidative dehydrogenation of ethane (ODHE). The influence of their structural parameters (such as tolerance factor t, unit cell free volume (Vf), electronegativity of A- and B-site elements, average metal-oxygen energy, and crystalline phase transition) on basic sites and reaction performance is also systematically investigated. The results show that these two perovskites have similar active sites for OCM and ODHE reactions, with the moderate and strong basic sites favoring the OCM reaction and the moderate basic site facilitating the ODHE reaction. The surface basicity of these perovskites reflects the electron-donating ability of lattice oxygen, which in turn affects the production of reactive oxygen species (O2−, O22−, and O−). Moderate basic sites are abundant and closely related to oxygen vacancies, with high oxygen vacancy concentrations generating abundant reactive oxygen species. Meanwhile, Vf values of perovskites govern the CH4, C2H6, and O2 conversions, the electronegativity of elements affects the catalyst basicity, and the average metal-oxygen energy and crystalline phase transition alter the moderate basic sites. Overall, it concludes that perovskites should meet the following conditions to achieve excellent catalytic performance for OCM and ODHE: moderate Vf, low electronegativity of A- and B-site elements, low average metal oxygen energy, and a crystalline structure that changes with temperature.

Conducting 2D Nanosheets: Photo Driven Crystallinity in Self-Assembled Cu (II) Metalated Porphyrin-Bispyridylquinoxaline

Tuning the crystallinity in self-assembled nanostructures has a significant impact on the electronic properties. Although several approaches have been adopted for organic π-conjugated systems to generate the amorphous to crystalline structures, the bulky structure of extended macrocyclics is hitherto unknown. Herein, we have developed three symmetric freebase and metallo substituted porphyrin appended bispyridylquinoxalines (PFb-DPQ, PCu-DPQ and PZn-DPQ) in which copper metalated P-DPQ exhibits crystalline phase transitions under light stimuli. Notably, light irradiated P-DPQ derivatives undergo a protonation, demetalation, and anion binding mechanism, resulting in the formation of self-assembled open, closed 0D nanospheres together with 2D nanosheets via methanol vapor diffusion approach. Interestingly, light stimulation regulates the morphology from 0D nanosphere to 2D nanosheets in PCu-DPQ and H+[PCu-DPQ(Cl)]−, which promotes the amorphous to crystalline nature. Consequently, the bulk conductivity of 2D nanosheets exhibits ∼32 mS cm–1, and it is remarkably 102 times higher than 0D nanospheres. Thus, these results promote that the stimuli responsive metallo substituted macrocyclic systems is a significant initiative to manipulate the electronic properties in organic electronics.

Polypropylene Nanocomposites Attained by In Situ Polymerization Using SBA-15 Particles as Support for Metallocene Catalysts: Effect of Molecular Weight and Tacticity on Crystalline Details, Phase Transitions and Rheological Behavior.

In this study, nanocomposites based on polypropylene are synthesized by the in situ polymerization of propene in the presence of mesoporous SBA-15 silica, which acts as a carrier of the catalytic system (zirconocene as catalyst and methylaluminoxane as cocatalyst). The protocol for the immobilization and attainment of hybrid SBA-15 particles involves a pre-stage of contact between the catalyst with cocatalyst before their final functionalization. Two zirconocene catalysts are tested in order to attain materials with different microstructural characteristics, molar masses and regioregularities of chains. Some polypropylene chains are able to be accommodated within the silica mesostructure of these composites. Thus, an endothermic event of small intensity appears during heating calorimetric experiments at approximately 105 °C. The existence of these polypropylene crystals, confined within the nanometric channels of silica, is corroborated by SAXS measurements obtained via the change in the intensity and position of the first-order diffraction of SBA-15. The incorporation of silica also has a very significant effect on the rheological response of the resultant materials, leading to important variations in various magnitudes, such as the shear storage modulus, viscosity and δ angle, when a comparison is established with the corresponding neat iPP matrices. Rheological percolation is reached, thus demonstrating the role of SBA-15 particles as filler, in addition to the supporting role that they exert during the polymerizations.

Subtle lattice distortion-driven phase transitions in layered ACu4As2 (A = Eu, Sr)

The compounds composed of transition metal cations and pnictide anions provide a rich platform for studying novel physical phenomena. Here we report on the observation of a phase transition at ∼ 70 K and 145 K in layered compound EuCu4As2 and SrCu4As2, respectively. from both the transport and heat capacity. The thermal expansion measurements show that the variation of the lattice parameters (ΔLb /Lab ) around T P is much less than that for a typical crystalline phase transition. Our experimental results reveal that the transition in EuCu4As2 and SrCu4As2 should be driven by subtle structural-distortion.

Atomic-scale in situ observation of electron beam and heat induced crystallization of Ge nanoparticles and transformation of Ag@Ge core-shell nanocrystals.

Crystallization of amorphous materials by thermal annealing has been investigated for numerous applications in the fields of nanotechnology, such as thin-film transistors and thermoelectric devices. The phase transition and shape evolution of amorphous germanium (Ge) and Ag@Ge core-shell nanoparticles with average diameters of 10 and 12nm, respectively, were investigated by high-energy electron beam irradiation and in situ heating within a transmission electron microscope. The transition of a single Ge amorphous nanoparticle to the crystalline diamond cubic structure at the atomic scale was clearly demonstrated. Depending on the heating temperature, a hollow Ge structure can be maintained or transformed into a solid Ge nanocrystal through a diffusive process during the amorphous to crystalline phase transition. Selected area diffraction patterns were obtained to confirm the crystallization process. In addition, the thermal stability of Ag@Ge core-shell nanoparticles with an average core of 7.4 and a 2.1nm Ge shell was studied by applying the same beam conditions and temperatures. The results show that at a moderate temperature (e.g., 385°C), the amorphous Ge shell can completely crystallize while maintaining the well-defined core-shell structure, while at a high temperature (e.g., 545°C), the high thermal energy enables a freely diffusive process of both Ag and Ge atoms on the carbon support film and leads to transformation into a phase segregated Ag-Ge Janus nanoparticle with a clear interface between the Ag and Ge domains. This study provides a protocol as well as insight into the thermal stability and strain relief mechanism of complex nanostructures at the single nanoparticle level with atomic resolution.

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.

Reactivation of hydrated cement powder by thermal treatment for partial replacement of ordinary portland cement

Cement is the strength-forming component of concrete. It has been a major building material for more than a century. However, its production is accountable for a considerable percentage of global CO2 emissions and is very energy-intensive. The Ordinary Portland Cement (OPC) production is a thermal process at around 1450 °C. This study shows that the reactivation of Hydrated Cement Powder (HCP) can be successful at a much lower temperature. Therefore, the possibility of using HCP to replace parts of OPC in concrete reduces the energy consumption and the CO2 emissions associated with OPC production. HCP, which may ultimately stem from recycled concrete, needs treatment to produce new concrete of the required mechanical strength. Using reactivated HCP in concrete, an optimum strength is achieved by heating the HCP in the range of 400–800 °C. Among other factors, the type of cement used influences the optimum heating temperature and attainable strength. This paper shows that 600 °C is an optimum heating temperature using the OPC type CEM I 52.5R. The crystalline phase transitions resulting from the thermal treatment were analyzed by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetry (TG). The heat released during hydration was investigated, and scanning electron microscopy (SEM) displays the microstructure evolution. OPC can be partially replaced by thermally treated HCP in mortar, attaining similar mechanical strength values.

Preparation and characterization of foamed ceramics from coal bottom ash for construction insulation materials

Foamed ceramic is a novel lightweight wall material combining high strength, low density, and excellent thermal insulation. This study used coal bottom ash (CBA) to fabricate microporous foamed ceramics by the high-temperature foaming method. The CBA content and heating rate effects on the bulk density, compressive strength, pore structure, and phase components of foamed ceramics were systematically investigated. The results demonstrate that using CBA in foamed ceramics can effectively improve the green body foaming capacity. With the increased CBA content, the bulk density of foamed ceramics dropped, while the average pore size and shape factor increased, jointly reducing the compressive strength. Meanwhile, the quartz phase content decreased, and the mullite phase appeared gradually. The transition of the main crystalline phase from MgAl2O4 to Mg0.7Fe0.23Al1.97O4 with the increased CBA content gave rise to a change in the color of the foamed ceramic. The elevation in the heating rate can improve the foaming ability of the green body, contributing to increased porosity. However, the rapid heating rate of 10 °C/min results in an uneven pore size distribution in the foamed ceramics containing more than 70 wt % CBA. It increases the temperature difference in the sample, promoting a stratified distribution of pore size in samples with a CBA content of less than 30 wt %.

Proposal for Shape Memory Alloy Positioning Actuator using Electric Parameter Based Controller

This paper proposes the controller design for a shape memory alloy (SMA) positioning actuator system. The system utilizes spiral-formed SMA wire as the driving power source. SMA generates force via temperature-based crystalline phase transitions. The wire is deformed in the spiral form under low temperature and heated by applying direct current, it recovers to the original form and generates the recovering force, which rotates the attached load. This paper proposes using the electric resistance of the SMA, which is related to its crystalline phase status, for designing the controller of the actuator system. The feasibility of the proposed system and controller design are evaluated experimentally with a sample test device, manufactured using a 3D printer, and the device achieves the designed rotation.

Experimental study on calcium carbonate phase transition from amorphous to crystalline forms in a reverse emulsion

Calcium carbonate (CaCO3) nanoparticles for lubricant additives are typically manufactured in reverse emulsion. The amorphous form is sterically stabilised in oil to create the so-called, ‘overbased detergents’, for motor engine oils, whilst crystalline CaCO3 nanoparticles are produced for specific applications, like lubricating greases. Here, the phase transition of amorphous calcium carbonate to crystalline forms in a reverse water/oil emulsion was studied in a 23L jacketed batch reactor equipped with agitator, temperature and pressure control. During the process, several acids, such as acetic, alkylaryl and 12-hydroxystearic (HSA), were added. The impact of acid type and HSA quantity were studied with all other aspects constant. The amorphous to crystalline phase transition kinetics were measured, the crystal habits were observed and the solid forms were characterized. With no HSA, a more acidic environment resulted in faster transition kinetics from amorphous CaCO3 to calcite. However, the presence of HSA tended to slow down the transition kinetics. Indeed, calcite crystal growth was asymmetrically inhibited by HSA, leading to elongated rod-shaped crystals. Finally, a very high HSA concentration leads to the formation of vaterite crystals, vaterite being a less stable phase than calcite.

Preparation and properties of anorthite‐based ceramics by using metallurgical solid waste and fly ash

AbstractA large amount of metallurgical solid waste accumulation poses a serious threat to the environment. Study on synergistic reinforcement of synthetic process of metallurgical solid waste‐based ceramics with fly ash is of great significance in reducing environmental pollution and resource utilization. A metallurgical solid waste‐based ceramic used as building ceramic was developed with the erosion part of used MgO–C bricks and fly ash as main raw materials, and the amount of solid waste added to the prepared ceramics was at least 60 wt% and up to 90 wt%. The effects of fly ash content and sintering temperature on the crystalline phase transitions, morphologies, and the main physical and mechanical properties of ceramics were investigated by X‐ray diffraction, scanning electron microscopy, and mechanical testing. The results show that the obtained ceramics presented maximum bending strength and minimum water absorption, 80.14 MPa and 5.04%, respectively, when the raw material proportions were the erosion part of used refractories accounted for 60 wt%, fly ash 20 wt%, pyrophyllite 10 wt%, and quartz sands 10 wt%, and the process parameters were the sintering temperature 1150°C, sintering time 120 min, and molding pressure 15 MPa.

A Thermo-Responsive Polymer Micelle with a Liquid Crystalline Core.

An amphiphilic diblock copolymer (PChM-PNIPAM), composed of poly(cholesteryl 6-methacryloyloxy hexanoate) (PChM) and poly(N-isopropyl acrylamide) (PNIPAM) blocks, was prepared via reversible addition-fragmentation chain transfer radical polymerization. The PChM and PNIPAM blocks exhibited liquid crystalline behavior and a lower critical solution temperature (LCST), respectively. PChM-PNIPAM formed water-soluble polymer micelles in water below the LCST because of hydrophobic interactions of the PChM blocks. The PChM and PNIPAM blocks formed the core and hydrophilic shell of the micelles, respectively. With increasing temperature, the molecular motion of the pendant cholesteryl groups increased, and a liquid crystalline phase transition occurred from an amorphous state in the core. With further increases in temperature, the PNIPAM block in the shell exhibited the LCST and dehydrated. Hydrophobic interactions of the PNIPAM shells resulted in inter-micellar aggregation above the LCST.

Optical band-gap evolution and local structural change in Ge2Sb2Te5 phase change material

The amorphous to crystalline phase transition in Ge2Sb2Te5 (GST) phase change material is investigated using XRD and the systematic variation in the optical band-gap (Eg ) and structural disorder (B 1/2) employing UV-Vis-NIR spectroscopy. The amorphous phase is observed to have Eg value of 0.708 eV while crystalline phase (200 °C) shows 0.442 eV. Variation in B 1/2 slope of 13.4 % is noted around the crystallization temperature (150 °C), depicting structural disorder reduction and hence structural ordering in the material forming the cubic phase. The change in the optical band-gap and local structural disorder upon crystallization occurs due to alternation in the atomic bonding configurations, which is explored using XPS technique. The findings reveal Ge-Te (~1218.35 eV binding energy) and Sb-Te (~528.8 eV) bonds in the amorphous phases. However, their bond lengths increase (hence binding energy reduces) as the annealing temperature rises, demonstrating phase switching occurs upon reaching the crystallization temperature. Insight into the optical band-gap, local structural disorder, and atomic arrangement governs many vital features of phase change material, such as fast crystallization speed, better thermal stability, high endurance, and large resistance contrast, which provide the path for non-volatile memory and nanophotonic applications.

Phase Transitions in Three New Hybrid Crystals: (Me3NR)4[Ni(NCS)6] (R=Ethyl, Propyl, and Butyl)

AbstractUnderstanding the microscopic origin of phase transitions in hybrid crystals is of significant important but challenging for designing novel phase‐transition materials. Here, three new hybrid crystals, (Me3NR)4[Ni(NCS)6] (R=ethyl for 1, propyl for 2, and butyl for 3) were synthesized and comprehensively studied on their crystal structures, intermolecular interactions, and phase transitions. They possess a same anti‐XeF4 structure mode but exhibit different phase transitions arising from their subtly changed alkyl groups. Specifically, 1 undergoes four‐step P21/c‐P21/c‐P21/c‐Pbca‐Cmce crystalline transitions at 165, 203, 244, and 280 K, respectively; 2 undergoes three‐step P212121‐Pbca‐P21/n‐P 1c crystalline transitions at 167, 393, and 410 K, respectively, following by a solid‐liquid transition at 453 K; 3 undergoes an iso‐space‐group P21/n‐P21/n crystalline phase transition at 257 K and a solid‐liquid transition at 410 K. These instances well demonstrate the key roles of delicate and complicated intermolecular interactions on inducing nontrivial phase transitions in hybrid crystals.

Hollow hemispherical Si-doped anatase for efficient carbamazepine degradation via photocatalytic activation of peroxymonosulfate

Photocatalytic persulfate activation on semiconductor (such as TiO2) supported transition metal catalysts exhibits great potential in removing refractory organic pollutants, but it often encounters the issues of unsatisfactory activity and undesirable metal leaching. Herein, novel three-dimensional (3D) Si-doped TiO2 materials without transition metal doping are proposed for efficient photocatalytic activation of peroxymonosulfate (PMS). Unlike creating an inverse opal structure using polymer sphere templates in prior research, the SiO2 opal template enables the creation of hollow hemispherical structured Si-doped TiO2 (denoted as HHS-Si/TiO2) with hierarchical macro-mesopores. The SiO2 template also endows the HHS-Si/TiO2 materials excellent sintering-resistant properties and completely inhibits the crystalline phase transition from anatase to rutile at high temperatures, benefiting from the confinement effect and formation of Si-O-Ti bonds. The HHS-Si/TiO2 materials calcined at 400–900 °C show similar and excellent activities and stability in PMS activation under solar light irradiation to fast degrade carbamazepine (CBZ) and various other pollutants. The influences of PMS dosage, pH value and water matrix on CBZ degradation are illustrated. The HHS-Si/TiO2 materials are advantageous (especially at high calcination temperatures) over the commercial, the template-free synthesized and the inverse opal structured TiO2 materials for photocatalytic activation of PMS, because of their 3D hollow hemispherical structure, interconnected small anatase nanocrystals and Si-doping. PMS is mainly activated by the photo-generated electrons. HO• (dominant), SO4•− and holes are the reactive species responsible for CBZ degradation. This work provides enhanced understanding on the synthesis of 3D Si-doped TiO2 materials and it would provide useful information on developing efficient catalysts for photocatalytic PMS activation.

Characterizing crystalline phase transitions in zeolitization of coal fly ash using focused ion beam-scanning electron microscopy

Coal fly ash (CFA) accounts for a significant portion of coal ash, which is a by-product of thermal power plants. Herein, CFA collected in Korea was used to synthesize a zeolitic material via fusion and hydrothermal synthesis. The crystallographic characteristics of CFA, alkali-fused CFA, and synthesized zeolitic material from CFA (SZC) were investigated via cross-section analysis using focused ion beam-scanning electron microscopy (FIB-SEM) with energy-dispersive X-ray spectroscopy (EDS), which has not been attempted in previous studies. Cross-section milling and EDS line scan were performed using FIB-SEM to develop the existing analysis methods, focusing on the crystalline phase and chemical composition of the surface of synthetic materials. Following the alkali treatment, the amorphous CFA exhibited a biphase comprising both crystalline and amorphous phases. After the synthesis process, both the surface and the interior exhibited a uniform Na-A zeolite crystalline structure with sharp-edged cubes. Furthermore, the SZC comprised crystals of Na-A zeolite with a uniform structure and a Si/Al molar ratio of approximately 1.4.

Long chain ceramides raise the main phase transition of monounsaturated phospholipids to physiological temperature

Little is known about the molecular mechanisms of ceramide-mediated cellular signaling. We examined the effects of palmitoyl ceramide (C16-ceramide) and stearoyl ceramide (C18-ceramide) on the phase behavior of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) using differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SAXS, WAXS). As previously published, the presence of ceramides increased the lamellar gel-to-lamellar liquid crystalline (Lβ–Lα) phase transition temperature of POPC and POPE and decreased the Lα-to-inverted hexagonal (Lα–HII) phase transition temperature of POPE. Interestingly, despite an ~ 30° difference in the main phase transition temperatures of POPC and POPE, the Lβ–Lα phase transition temperatures were very close between POPC/C18-ceramide and POPE/C18-ceramide and were near physiological temperature. A comparison of the results of C16-ceramide in published and our own results with those of C18-ceramide indicates that increase of the carbon chain length of ceramide from 16 to 18 and/or the small difference of ceramide content in the membrane dramatically change the phase transition temperature of POPC and POPE to near physiological temperature. Our results support the idea that ceramide signaling is mediated by the alteration of lipid phase-dependent partitioning of signaling proteins.

Phase Transition of MoTe2 Controlled in van der Waals Heterostructure Nanoelectromechanical Systems.

This work reports experimental demonstrations of reversible crystalline phase transition in ultrathin molybdenum ditelluride (MoTe2 ) controlled by thermal and mechanical mechanisms on the van der Waals (vdW) nanoelectromechanical systems (NEMS) platform, with hexagonal boron nitride encapsulated MoTe2 structure residing on top of graphene layer. Benefiting from very efficient electrothermal heating and straining effects in the suspended vdW heterostructures, MoTe2 phase transition is triggered by rising temperature and strain level. Raman spectroscopy monitors the MoTe2 crystalline phase signatures in situ and clearly records reversible phase transitions between hexagonal 2H (semiconducting) and monoclinic 1T' (metallic) phases. Combined with Raman thermometry, precisely measured nanomechanical resonances of the vdW devices enable the determination and monitoring of the strain variations as temperature is being regulated by electrothermal control. These results not only deepen the understanding of MoTe2 phase transition, but also demonstrate a novel platform for engineering MoTe2 phase transition and multiphysical devices.

Characterization of TiO2 nanofibers with enhanced photocatalytic properties prepared by plasma assisted calcination

In this article, we present a novel approach to produce titania (TiO 2 ) nanofibers with enhanced properties using plasma pre-treatment. Nanofibers were produced from electro-spun polyvinylpyrrolidone (PVP) based nanofibers with titanium(IV) isopropoxide (TTIP) as a precursor. PVP/TTIP fibers were plasma treated using diffuse coplanar surface barrier discharge (DCSBD) followed by thermal calcination at 500 °C, 600 °C and 700 °C. The morphology of nanostructures was characterized using SEM, BET and XRD, the chemical composition was analyzed by XPS, ATR-FTIR, the photocatalytic activity was evaluated by degradation of methylene blue and band gap was determined from diffuse reflectance spectra (DRS) using Tauc plot. Initial plasma treatment caused decomposition of polymer matrix, which led to faster oxidation and crystalline phase transition at lower temperature during the following thermal processing/calcination. The results showed enhanced photocatalytic properties of plasma pre-treated fibers despite the fact they possess higher proportion of rutile and lower specific surface area. UV–vis DRS measurements exhibit lower band gap energy at 700 °C, but difference between plasma treated (PT) and non-treated (NT) samples at the same temperature was not observed.