Heat-induced Phase Transition Research Articles

High-Resolution 17O Solid-State NMR as a Unique Probe for Investigating Oxalate Binding Modes in Materials: The Case Study of Calcium Oxalate Biominerals.

Oxalate ligands are found in many classes of materials, including energy storage materials and biominerals. Determining their local environments at the atomic scale is thus paramount to establishing the structure and properties of numerous phases. Here, we show that high-resolution 17O solid-state NMR is a valuable asset for investigating the structure of crystalline oxalate systems. First, an efficient 17O-enrichment procedure of oxalate ligands is demonstrated using mechanochemistry. Then, 17O-enriched oxalates were used for the synthesis of the biologically relevant calcium oxalate monohydrate (COM) phase, enabling the analysis of its structure and heat-induced phase transitions by high-resolution 17O NMR. Studies of the low-temperature COM form (LT-COM), using magnetic fields from 9.4 to 35.2 T, as well as 13C-17O MQ/D-RINEPT and 17O{1H} MQ/REDOR experiments, enabled the 8 inequivalent oxygen sites of the oxalates to be resolved, and tentatively assigned. The structural changes upon heat treatment of COM were also followed by high-resolution 17O NMR, providing new insight into the structures of the high-temperature form (HT-COM) and anhydrous calcium oxalate α-phase (α-COA), including the presence of structural disorder in the latter case. Overall, this work highlights the ease associated with 17O-enrichment of oxalate oxygens, and how it enables high-resolution solid-state NMR, for "NMR crystallography" investigations.

Interfacial second harmonic generation switching with 2D monolayer/VO2 heterostructure

To establish a facile nonlinear optical switching mechanism activated by thermal field, that is compatible with on-chip integration, we develop a physical model to quantify the interfacial second harmonic generation (SHG) of 2D monolayer/3D phase-changing material heterostructure. Our results show that heat-induced phase transition of VO2 can realize temperature-reversible interfacial SHG switching, with an ON-OFF ratio of about 3 and a low temperature threshold of about 60 °C. Experimental results can be well addressed by quantitative calculations based on the physical model. This work constitutes substantial insights into interfacial SHG switching, which will benefit design and construction of on-chip nonlinear optical devices based on 2D monolayers.

Multivalent manganese-based composite materials for sodium energy storage in ether electrolyte

The structural collapse of manganese-based materials during cycling greatly restricts its development in Sodium-ion batteries (SIBs). Hence, two-phase structures with different manganese valence states are designed based on an alternating voltage approach to mitigate volume expansion via two-phase coordination. Here distinct manganese valence states of precursor are driven via atmosphere. And in the following heat-induced phase transition, the elemental valence state is the decisive key for the structural and compositional evolution. In a batch of synthetic materials, MnO/MnS with bivalent states of manganese performs best as expected. Furthermore, NaPF6 in DEGDME is taken to replace the traditional carbonate electrolyte which is prone to produce unstable SEI film and leading to polarization. A series of characterization methods confirm that the energy barrier of charge transfer at the interface between electrolyte and electrode is the main factor determining the electrochemical characteristics of the interface and thus the energy storage performance. Ultimately, carbon dots (CDS) with rich functional groups are introduced in an effort to further promote the conductivity of MnO/MnS, and the in-situ generated C-S-Mn bond enhances electrochemical kinetic behaviors. This work provides an approach for designing manganese-based materials and the selection of electrolytes in sodium energy storage.

A Simpler Fabrication for Thermal Energy Storage Wood

Using thermal energy storage wood with phase change materials (PCM) as a building material can save thermal energy during heat-induced phase transition, and can reduce the energy consumption of indoor heating. In our work, three thermal energy storage poplars (TESPs: TESP-1, TESP-2 and TESP-3) were prepared by directly infiltrating three PCMs (fatty alcohol/acid materials: lauryl alcohol, decanoic acid and myristic acid myristyl ester), respectively, into the longitudinal-cutting plantation poplar woods and by directly encapsulating the PCMs in the poplar-based materials with SiO2 films. The phase-changing temperature ranges of TESP-1, TESP-2 and TESP-3 were at 19–30 °C, 26–39 °C and 33–54 °C, respectively. The phase-changing temperature peaks were at ~24 °C, ~31 °C and ~42 °C, respectively. After the same heat treatment on TESPs and original poplar (OP), the average temperature of TESPs was higher than that of OP after 35 min, thus proving that TESPs can save more thermal energy than OP. The radial bending strengths of TESP-1, TESP-2 and TESP-3 had increased by 50.85%, 70.16% and 70.31%, respectively, as compared to with that of OP. Additionally, the radial bending elastic modules of TESP-1, TESP-2 and TESP-3 had increased by 47.14%, 67.38% and 74.57%, respectively, as compared to OP. The tangential section hardness of the TESPs also had also increased by 67.09%, 71.80% and 80.77%, respectively. These improved mechanical properties of TESPs are almost close to that of ash wood (ash wood is a common building material), therefore, this proves that our TESPs can be used as thermal energy-saving building materials.

Effect of Sr doping and temperature on the optical properties of BaTiO3

Ceramics with low solar absorptivity and high infrared emissivity play an important role in radiative cooling applications. BaTiO3, for instance, is one promising radiative cooling ceramic, although it has been mainly explored only in electronics. In this work, the solar-reflective and infrared-radiative properties of Sr-doped BaTiO3 are studied systematically. Ba1-xSrxTiO3 (x = 0, 0.2, 0.25, 0.3, 0.35) ceramics synthesized by sol-gel method were investigated in terms of their microstructure, phase transition, and optical properties. Increasing infrared emissivity and solar absorptivity were observed with increasing Sr content. The optimized Ba1-xSrxTiO3 achieves solar absorptivity of 0.17 and infrared emissivity of 0.78. The lowest absorptivity-emissivity ratio of 0.24 was observed for Ba0.8Sr0.2TiO3. Meanwhile, the solar absorptivity and infrared emissivity exhibited temperature stabilization, even affront heat-induced phase transition. Finally, the coupling relation between band gap, Ti–O vibration, and optical properties of Ba1-xSrxTiO3 ceramics are clarified, which is promising to improve their radiative cooling performance.

Accounting for heat release in small volumes of matter on the example of the growth of ZnO micro-rods: search for a modeling technique

Using examples of an exothermic chemical reaction and self-heating of the region of a conducting filament of a memristor, heat-induced phase transitions, disadvantages of applying the classical Fourier approach on the nanoscale, and advantages of the molecular mechanics method at modeling the temperature factor are discussed. The correction for Arrhenius relationship, taking into account that the temperature becomes a random variable is proposed. Based on the introduced concepts (elementary act of heat release, distance and region of thermal impact) method for taking into account the thermal factor, is proposed.The correction is based on splitting the entire pool of particles into several, each of which corresponds to a fixed temperature value taken from a certain range. Although continuous and discrete correction options are given both, but the discrete option is more preferable. This is due to the fact that the methodology focuses on the application of methods of molecular mechanics, and, intentionally, in the most primitive version. The role of amorphization is noted as an example of the structural restructuring of matter in nano-volumes. It is indicated that the phonon spectra themselves, which determine heat transfer, depend on temperature. The technique is consistent with the ideology of multiscale modeling. The integral temperature increase is calculated outside the region of thermal exposure, where nonequilibrium effects are significant, by solving the standard equation of thermal conductivity.

The thermochromic characteristics of Zn-doped VO2 that were prepared by the hydrothermal and post-annealing process and their polyurethane composite films

In this paper, Zn-doped VO2 nanoparticles have been successfully fabricated by a two-step hydrothermal-annealing process, and the thermally induced visible light transmittance enhancement of Zn-doped VO2 has been studied for the first time. It is found that Zn-doped VO2 not only exhibits excellent solar modulation ability (ΔTsol = 15.27%) but also can reduce the phase transition temperature and increase the visible light transmittance after the heat-induced phase transition (ΔTlum=+5.78%). Moreover, with the increase of Zn doping concentration, the phase transition temperature (Tc) and phase transition hysteresis (ΔT) both decrease. It is shown that the Zn-doped VO2-PU films not only have good solar light modulation ability and properties of improving visible light transmission after phase transition, but also have good durability. The research result is of great significance for improving the visible light transmittance after phase transition and realizing the practical application of VO2 in the field of smart windows.

Exploring the transition of polydopamine-shelled perfluorohexane emulsion droplets into microbubbles using small- and ultra-small-angle neutron scattering.

Perfluorocarbon emulsion droplets are interesting colloidal systems with applications, ranging from diagnostics and theranostics to drug delivery, due to their controllable phase transition into microbubbles via heat application or acoustic droplet vapourisation. This work highlights the application of small- and ultra-small-angle neutron scattering (SANS and USANS, respectively), in combination with contrast variation techniques, in observing the in situ phase transition of polydopamine-stabilised perfluorohexane (PDA/PFH) emulsion droplets into microbubbles during heating. Results show peak USANS intensities at temperatures around 90 °C, which indicates that the phase transition of PDA/PFH emulsion droplets occurs at significantly higher temperatures than the bulk boiling point of pure liquid PFH (56 °C). Analysis and model fitting of the SANS and USANS data allowed us to estimate droplet sizes and interfacial properties at different temperatures (20 °C, 90 °C, and 20 °C after cooling), giving valuable insights about the transformation of these polydisperse emulsion droplet systems.

Entropy Rules: Molecular Dynamics Simulations of Model Oligomers for Thermoresponsive Polymers.

We attempted to attain atomic-scale insights into the mechanism of the heat-induced phase transition of two thermoresponsive polymers containing amide groups, poly(N-isopropylacrylamide) (PNIPAM) and poly(2-isopropyl-2-oxazoline) (PIPOZ), and we succeeded in reproducing the existence of lower critical solution temperature (LCST). The simulation data are in accord with experimental findings. We found out that the entropy has an important contribution to the thermodynamics of the phase separation transition. Moreover, after decomposing further the entropy change to contributions from the solutes and from the solvent, it appeared out that the entropy of the solvent has the decisive share for the lowering of the free energy of the system when increasing the temperature above the LCST. Our conclusion is that the thermoresponsive behavior is driven by the entropy of the solvent. The water molecules structured around the functional groups of the polymer that are exposed to contact with the solvent in the extended conformation lower the enthalpy of the system, but at certain temperature the extended conformation of the polymer collapses as a result of dominating entropy gain from “released” water molecules. We stress also on the importance of using more than one reference molecule in the simulation box at the setup of the simulation.

Modeling of a Vertical Hybrid Plasmonic Switch With VO2 Fin Bragg Grating

A 3-D vertical hybrid plasmonic switch having vanadium oxide (VO2) Bragg grating element inside a silicon waveguide has been modeled and simulated in this letter. Narrow bandwidth (30 nm) switching has been achieved maintaining 11.14-dB extinction ratio (ER) at 1.55 $\mu \text{m}$ , effectively due to tailoring VO2 refractive index via heat-induced phase transition. Furthermore, switching response has been assessed by tailoring the number of VO2 fins within the silicon waveguide, oxide spacer dimension, width and height of the silicon waveguide as well, which have impact on the switch behaviour. From numerical simulations, we have found that for 933.4-nm device length with three VO2 fins ~ 11.14-dB ER and 1.57 figure of merit at 1.55- $\mu \text{m}$ wavelength can be obtained.

Effect of laser pulse propagation on ultrafast magnetization dynamics in a birefringent medium

Light propagation effects can strongly influence the excitation and the detection of laser-induced magnetization dynamics. We investigated experimentally and analytically the effects of crystallographic linear birefringence on the excitation and detection of ultrafast magnetization dynamics in the rare-earth orthoferrites (Sm0.5Pr0.5)FeO3 and (Sm0.55Tb0.45)FeO3, which possess weak and strong linear birefringence, respectively. Our finding is that the effect of linear birefringence on the result of a magneto-optical pump-probe experiment strongly depends on the mechanism of excitation. When magnetization dynamics, probed by means of the Faraday effect, is excited via a rapid, heat-induced phase transition, the measured rotation of the probe pulse polarization is strongly suppressed due to the birefringence. This contrasts with the situation for magnetization dynamics induced by the ultrafast inverse Faraday effect, where the corresponding probe polarization rotation values were larger in the orthoferrite with strong linear birefringence. We show that this striking difference results from an interplay between the polarization transformations experienced by pump and probe pulses in the birefringent medium.

Effect of chain architecture on the phase transition of star and cyclic poly(N-isopropylacrylamide) in water

The heat-induced phase transition of aqueous solutions of Poly(N-isopropylacrylamide) (PNIPAM) in water is examined for a four-arm PNIPAM star (s-PNIPAM), a cyclic PNIPAM (c-PNIPAM), and their linear counterparts (l-PNIPAM) in the case of polymers (1.0 g L−1) of 12,700 g mol−1 < Mn < 14,700 g mol−1. Investigations by turbidity, high-sensitivity differential scanning calorimetry (HS-DSC), and light scattering (LS) indicate that the polymer architecture has a strong effect on the cloud point (Tc: decrease for s-PNIPAM; increase for c-PNIPAM), the phase transition enthalpy change (ΔH decrease for s-PNIPAM and c-PNIPAM), and the hydrodynamic radius of the aggregates formed above Tc (RH: c-PNIPAM < s-PNIPAM < l-PNIPAM). The properties of s-PNIPAM are compared with those of previously reported PNIPAM star polymers (3 to 52 arms). The overall observations are described in terms of the arm molecular weight and the local chain density in the vicinity of the core of the star, by analogy with the model developed for PNIPAM brushes on nanoparticles or planar surfaces. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2059–2068.

Phase transitions of ε-HNIW in compound systems

The heat-induced phase transitions of ε-HNIW, both neat and coated with various additives used in plastic bonded explosives, were investigated using powder X-ray diffraction and differential scanning calorimetry. It was found that ε-HNIW, after being held at 70°C for 60h, remained in the ε-phase. Applying other conditions, various phase transition parameters were determined, including Tc (the critical phase transition temperature), T50 (the temperature at which 50% of the phase transition is complete) and T180 (the percentage of γ-HNIW present in samples heated to 180°C). According to the above three parameters, additives were divided into three categories: those that delay phase transition, those that raise the critical temperature and the transition rate, and those that promote the phase transition. Based on the above data, a phase transition mechanism is proposed.

Remarkable distinctions in the heat-induced phase transition processes of two poly(2-isopropyl-2-oxazoline)-based mixed aqueous solutions.

Detailed phase transition processes of poly(2-isopropyl-2-oxazoline) (PIPOZ)/poly(N-isopropylacrylamide) (PNIPAM) and PIPOZ/poly(N-vinylcaprolactam) (PVCL) mixtures in aqueous solution were investigated by DSC, temperature-variable (1)H-NMR, optical microscopy and FT-IR spectroscopy measurements accompanied with two-dimensional correlation spectroscopy (2Dcos) and perturbation correlation moving window (PCMW) analytical methods. Through the comparison of these two systems, it is revealed that PVCL chains can interact with PIPOZ chains directly through the polymer-water-polymer cross-linking hydrogen bonds (C[double bond, length as m-dash]OD-O-DO[double bond, length as m-dash]C), which induce their transition process as one. However, in the PIPOZ/PNIPAM mixture, the phase transition of the given component (PNIPAM or PIPOZ) is indirectly affected by the presence of the second component, because the strong hydrogen bonds C[double bond, length as m-dash]OD-N in PNIPAM components forbid the direct connection with PIPOZ, which induces two phase transition processes separately with no liquid-liquid phase separation (LLPS). Additionally, the formation of polymer-water-polymer hydrogen bonds (C[double bond, length as m-dash]OD-O-DO[double bond, length as m-dash]C) is highlighted as the key process in macroscopic LLPS.

How Lewis acidity of the cationic framework affects KNaNbOF(5) polymorphism.

The valence matching principle is used to explain the loss of inversion symmetry in the noncentrosymmetric (NCS) polymorph of KNaNbOF5 in comparison to its centrosymmetric (CS) polymorph. The [NbOF5](2-) anion has five contacts to both potassium and sodium in the NCS polymorph, whereas in the CS polymorph there are only four contacts to potassium and six contacts to sodium. The lower average Lewis acidity of the cationic framework in the NCS polymorph relative to the CS polymorph reflects the loss of inversion symmetry. This lower average Lewis acidity is achieved during hydrothermal synthesis with a potassium-rich solution when the K:Na ratio in the reaction is greater than 1:1, as the Lewis acidity of potassium is lower than that of sodium. The contrasting coordination environments are manifested in secondary distortions that weaken the primary Nb═O interaction and lengthen the Nb═O bond in the NCS polymorph. An unusual heat-induced phase transition from the CS to the NCS polymorph was studied with in situ powder X-ray diffraction. The transition to the NCS polymorph upon cooling occurs through an intermediate phase(s).

Heating-Induced Phase Transition of Bupropion Hydrobromide Polymorphs

Crystal phase transition and isothermal crystallization kinetics of bupropion hydrobromide is studied by thermal analysis, X-ray powder diffractometry, and scanning electron microscopy. As well known, bupropion hydrobromide has two stable polymorphic forms, form I and form II, during the production. Here it is found that form II will convert into form I with thermal treatment. Avrami exponent n is evaluated to be around 2.0 in the range 170°C-190°C, which indicates that the phase transition is a process of one-dimensional nucleation growth. This viewpoint is also confirmed by microscope morphology study. In addition, phase-transition active energy is calculated to be 239.4 kJ/mol, which means that a high energy barrier needs to be overcome to start up the phase transition.

LCST behavior of copolymers of N-isopropylacrylamide and N-isopropylmethacrylamide in water

The lower critical solution temperature (LCST) behavior of copolymers of N-isopropylacrylamide (NiPA) and N-isopropylmethacrylamide (NiPMA) in water was studied as a function of the copolymer composition, using a combination of turbidity measurements and differential scanning calorimetry (DSC). The copolymers were prepared by free radical polymerization using N,N-dimethylformamide as a solvent and α,α′-azobis(isobutyronitrile) as an initiator. The copolymer composition was determined by elemental analysis. It was found that the temperature (T c) at which the copolymer undergoes a phase transition, i.e., LCST, increases linearly with increasing the mole fraction (f m) of NiPMA in the copolymer, within the T c range from 32 °C (at f m = 0; NiPA homopolymer) to 42 °C (at f m = 1; NiPMA homopolymer). Also found from heating DSC thermograms were the linear dependencies of the enthalpy (ΔH) and entropy (ΔS) changes at T c upon f m. However, the ΔH (5.5 kJ/unit-mol) at f m = 1 was slightly smaller than that (5.7 kJ/unit-mol) of poly(N-n-propylacrylamide) but considerably smaller than that (7.8 kJ/unit-mol) of poly(N-n-propylmethacrylamide). The same trend was observed in the f m dependence of ΔS. These results were discussed in terms of the structural effects of the NiPMA monomer unit on the heat-induced phase transition in water of poly(NiPA-co-NiPMA)s. It was suggested that a strong interaction of water with the amide group in the NiPMA would raise the transition temperature, but a local dehydration which occurs around the isopropyl side chain would not lead to large changes in the enthalpy and entropy at T c.

Size‐Controlled Fabrication of Polydiacetylene‐Embedded Microfibers on a Microfluidic Chip

A microfluidic technique was employed to fabricate polydiacetylene (PDA)-embedded hydrogel microfibers. By taking advantage of calcium ion-induced insoluble hydrogel formation, supramolecularly assembled diacetylene (DA)-surfactant complexes were successfully immobilized in the calcium alginate fibers. Thus, instantaneous microfiber formation was observed when the core flow of DA supramolecules-containing alginate solution met the sheath flow of calcium ions. UV irradiation of the resulting fibers afforded blue colored PDAs, and the formation of a conjugated polymer was confirmed by heat-induced phase transition and by Raman spectroscopy. By adjusting the core and sheath flow rates, PDA-embedded hydrogel fibers of various sizes were obtained.

Heat induced nanoforms of zinc oxide quantum dots and their characterization

In our studies we observed heat induced phase transitions of Zinc oxide quantum dots at 60, 200, 360 and 400°C, where all the transitions were irreversible except the transition at 60°C which wasa reversible one. The phase transition at 60°C indicated a heat induced conformational change which was supported here by studying polarizing micrographs of ZnO quantum dots thin film. The X-ray diffraction studies of the sample fired at different temperatures as indicated by the thermal analysis were performed in order to understand the changes occurred due to transitions. The study also indicated a new and simple approach to develop ZnO nanorods by just thermal decomposition of the ZnO quantum dots firing in furnace at 200°C with 2h soaking. In order to have a proper insight of the structural changes we performed scanning electron microscopy. Optical characterization was done by UV-Vis and fluorescence spectrophotometer.

Thermomagnetic power generation performance of first-order phase transition material Mn1.2Fe0.8P0.4Si0.6

In this paper, we report on the magnetism and the thermomagnetic power generation performance of a first-order phase transition material Mn1.2Fe0.8P0.4Si0.6,which can be used for thermomagnetic power generation that turns heat directly into electricity. The compound is synthesized by using the high-energy ball milling and solid state reaction method. Magnetic measurements show that the compound undergoes a ferromagnetic-to-paramagnetic first-order phase transition at 337 K, accompanied by a giant magnetization change. According to this feature of the material, we design a demonstration device for thermomagnetic power generation, and study the electric current generated by heat induced phase transition. The current increases with the increase of the heat-flow temperature and the mass of material. This study shows that the Mn1.2Fe0.8P0.4Si0.6 compound possesses the high performance of thermomagnetic power generation.