Unusual Thermal Behavior Research Articles

Polyethylene oxide mixtures with thymol: Unusual thermal behavior and morphology due to a crystallizable hydrogen-bonded complex

Formation of cocrystals in polymer-containing systems attracts a lot of attention lately due to their promise in pharmaceutics, being a poorly studied phenomenon in polymer science. For the first time, we report on the cocrystallization in mixtures of polyethylene oxide and thymol and study in detail the structure and thermal behavior of the mixtures by differential scanning calorimetry, optical microscopy, wide angle X-ray scattering, attenuated total reflection Fourier-transformed infrared spectroscopy, and quantum chemical simulation. The results are summarized into the experimental phase diagram that describes well all the transformations of the systems during their heating and cooling. On the molecular level, we conclude the existence of noncovalent-bonded complexes composed of three monomeric units of polyethylene oxide and two molecules of thymol, which might become structural units of cocrystals upon cooling. This results in a growth of huge centimeter-sized spherulites in the stoichiometric mixture and to the coexistence of two types of spherulites in the mixtures of other compositions. The peculiarities of thymol release from the complexes into different media are also investigated.

Thermal expansion properties of three-dimensional relaxor ferroelectric single crystals

Large-size Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) and Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) single crystals have been successfully produced. Nevertheless, the susceptibility of these crystals to cracking substantially hampers their application. This study investigates the thermal expansion characteristics of PMN-PT and PIN-PMN-PT crystals oriented along the 〈100〉, 〈110〉, and 〈111〉 directions, revealing no significant anisotropy. The average thermal expansion coefficients for the PMN-PT and PIN-PMN-PT crystals range from 7.77 to 8.21 × 10−6 K−1 and from 6.59 to 6.79 × 10−6 K−1, respectively, within the temperature range of 20 to 600 °C. Interestingly, the PMN-PT and PIN-PMN-PT crystals demonstrate unusual thermal expansion behavior around the phase transition temperature. The magnitudes of their thermal expansion coefficients are comparable to those of certain commercial crystals but are lower than that of platinum. The main causes inferred for crystal cracking include the pressure exerted by the platinum crucible and the abnormal thermal expansion behavior around the phase transition temperature.

Unveiling the underlying mechanism of unusual thermal conductivity behavior in multicomponent high-entropy (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 ceramics

Previously, the research on thermal conductivity of ceramic thermal barrier coatings mainly focused on phonon and photon thermal conductivity (thermal radiation effect). However, electrical conductivity is remarkable in some systems. Hence, the contribution of phonon, photon and electronic heat conduction to thermal conductivity of high-entropy systems was evaluated in this study. The (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 (x = 0–0.5) high-entropy ceramics with single defective fluorite structure were successfully prepared via a solid reaction method. Below 600 °C, the thermal conductivities decrease with increasing temperature for x = 0.1–0.5 components, then reveal a drastic temperature dependent increase. Moreover, the composition dependent thermal conductivities are also unusual based on the conventional phonon thermal conduction mechanism. The increased electronic thermal conductivity, improved photon thermal conductivity (at high temperatures) and reduced phonon-grain boundary scattering should be responsible for the unusual thermal conductivity behavior. This can be verified by the significantly increased electrical conductivity, optical transmittance and grain size, as well as reduced emissivity for (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 high-entropy ceramics. The present study also broadens the way to investigate the thermal conductivity of ceramic thermal barrier coatings, and is helpful to design thermal barrier coatings with low thermal conductivity.

On the specific heat and mass loss of thermochemical transition

Poly(vinyl alcohol) PVA is a synthetic polymer used in a variety of applications and exhibits unusual thermal behavior. In this work, differential scanning calorimetry (DSC), isothermal and non-isothermal thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), optical microscopy, viscometry and kinetic modeling were used to study the thermal behavior of PVA for providing further insights about the thermochemical transition. It is shown that the value of specific heat of thermochemical transition is comparable to theoretical values derived from chemical bond dissociation energy values. Activation energy's contribution to the specific heat of thermochemical transition seems to be low. Contradictions among the conclusions from various analytical techniques, regarding the glass transition of PVA, are discussed and interpreted. Settlement of these contradictions leads to the conclusion that like PVA's melting point, its glass transition is also a thermochemical transition but with no detectable mass loss.

Optimization of Eu3+-to-Host Emission Ratio in Double-Perovskite Molybdenites for Highly Sensitive Temperature Sensors

Double-perovskite phosphors exhibit unusual thermal and luminescent behavior and could potentially be applied as efficient and sensitive luminescent temperature sensors. Four double perovskite hosts─Ba2MgMoO6 (BMM), Sr2MgMoO6 (SMM), Ba2ZnMoO6 (BZM), and Sr2ZnMoO6 (SZM)─are studied for the correlations between the symmetry, energetic structure, luminescence, and thermometric performance. The quantum yield of Eu3+ emission in those hosts is higher for the samples with high cubic symmetry and ranges up to over 20% for Ba2MgMoO6. The Eu3+ ions can be excited indirectly by the host absorption bands in the UV and charge transfer band (CTB) in the blue range. The symmetry of the host plays a crucial role in the thermal stability of the host emission, which together with varying activation energy of Eu3+ luminescence provides a basis for thermometric sensitivity optimization. The relative sensitivities (Sr) obtained by the Eu-doped molybdenite hosts are 3.2% at 75 °C for Sr2MgMoO6 and 9.2% at −196 °C for Ba2ZnMoO6. It is also demonstrated that the sensing performance is higher in hosts with a uniform quenching profile of host luminescence and steep quenching of Eu3+ luminescence.

Spin-liquid signatures in geometrically frustrated layered kagome compounds YBaCo4−xFexO7+δ

Experimental detection of spin dynamics of the quantum spin liquid phase, which occurs when the quantum fluctuations lead to fractionalized spin states, remains a challenge. In this paper, we illustrate spin-liquid signatures in the geometrically frustrated layered kagome system $\mathrm{YBa}{\mathrm{Co}}_{4\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{7+\ensuremath{\delta}}$ ($x=0$ and $x=0.6$) over a wide temperature range using micro-Raman spectroscopy. The thermal response of the gapped broad band over a high temperature range possibly marks a spin liquid phase in this system. We report the appearance of sharp spectral features in the high temperature regime with unusual thermal behavior for $\mathrm{YBa}{\mathrm{Co}}_{4\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{7+\ensuremath{\delta}}$ ($x=0.6$). Besides, atypical oscillation of a phonon mode frequency with temperature suggests an unconventional electron-lattice coupling in the doped compound via a strong correlation between their lattice vibration and magnetic spin dynamics.

Extruded PVDF composites with anatase and gamma-alumina: a study on properties and stability

ABSTRACT The importance of alumina and titania is exceptionally high for modern life since they appear in all materials science fields and perform essential tasks without which modern life would not be possible. Every day, the window of application of these oxides expands, including polymer science and technology. Alumina and titania as nanofillers are explored in the present work, aiming to expand the window of application of these oxides as a secondary phase in extruded PVDF composites. Unexpected findings lead to the belief that PVDF induces an unknown phase transition/oxidation in γ-alumina and anatase, which causes their composites to be dark and present unusual thermal behavior; Tmax decreased 75 and 61°C upon the incorporation of anatase and γ-alumina to PVDF matrix, respectively. All the titania and alumina composites possess lower mechanical properties and degradation temperatures than pure PVDF; however, the crystallinity degree is 3% higher for rutile, which could have positive application in membranes and pipes with selective permeability. The formation of the transformation mentioned above in γ-alumina and anatase decreased the viscosity of the polymer by 6 and 24% respectively, which is closely related to the unexpected thermal behavior of those composites and possess a new research subject about the causes of this transformation induced by polymer and its possibility for future applications.

Anomalous Temperature Dependence of the Lowest-Frequency Lattice Vibration in Crystalline γ-Aminobutyric Acid.

Crystalline γ-aminobutyric acid (GABA) exhibits unusual thermal behavior in a low-frequency lattice vibration that occurs at 37.2 cm-1 at 290 K but decreases dramatically by 34.0% when the sample is cooled to 78 K. Lattice vibrations in molecular crystals are indicators of intermolecular force characteristics, and the extraordinary temperature sensitivity of this vibration offers new insight into the local environment within the solid. Solid-state density functional theory simulations of the GABA crystal have found this anomalous frequency shift is based in unexpected differences in the strengths of the intermolecular hydrogen bonds that are cursorily the same. This was accomplished through mapping of the potential energy surfaces governing the terahertz-frequency motions of the GABA solid and use of the quasi-harmonic approximation to model the response of all the lattice vibrations to temperature-induced unit cell volume changes brought about through the anharmonic character of the intermolecular interactions. The analysis reveals that the vibration in question is rotational in nature and involves the significant distortion of a specific weak intermolecular N-H···O hydrogen bond in the crystal that results in its unique thermal response.

Melting of nucleobases. Getting the cutting edge of "Walden's Rule".

Walden's Rule is an empirical observation of an invariant fusion entropy during fusion of non-associated organic compounds. For the five nucleobases, adenine, thymine, cytosine, guanine, and uracil, surprisingly high fusion temperatures and enthalpies have been measured using a specially developed fast scanning calorimetry method that prevents decomposition. Even when nucleobases admittedly possess very high fusion temperatures, e.g. the value of 862 K measured for guanine really exceeds all expectations of the feasible dimension of the fusion temperature for such a relatively small and simple organic molecule. Hirshfeld surface analysis has been applied in order to find out an explanation for such extremely unusual thermal behavior of nucleobases. We rationalized the observed trends in terms of fusion entropy (Walden's constant = 56.5 J K-1 mol-1) as the entropic penalty of fusion not only for "non-associated", as proposed by Walden in 1908, but also for "ideal associated" systems like nucleobases.

Temperature-Induced Self-Assembly of the Group B Streptococcus (GBS) Fusion Antigen GBS-NN

Group B Streptococcus (GBS) is a leading cause of serious bacterial neonatal infections worldwide, which provides an unmet medical need for a globally effective vaccine. The recombinant GBS fusion antigen GBS-NN contains the N-terminal regions of the GBS Rib and Alpha C proteins. It shows promising immunogenicity eliciting protective immunity in mice and encouraging results in early human clinical trials. Understanding the physical stability of GBS-NN containing conformational B-cell epitopes is crucial to ensure optimal vaccine stability, efficacy, and safety. We initially discovered that GBS-NN is prone to form higher-order structures at elevated temperatures. We therefore investigated the self-assembly behavior of GBS-NN and characterized the higher-order conformational structures as a function of temperature. In the native state, GBS-NN exists as a monomer and has a secondary structure containing α-helix and β-sheet. Langmuir studies demonstrated that the native protein is highly surface-active and forms a monolayer film at the air-water interface because of its amphipathic properties. The conformational stability of GBS-NN was measured as a function of temperature. GBS-NN has an unusual thermal behavior with a phase transition of approximately 61 °C, which is not accompanied by any major changes in the secondary structure. However, the antigen showed irreversible self-assembly as a function of temperature into higher-order structures with a hydrodynamic diameter of approximately 100 nm. Cryo-transmission electron microscopy analyses demonstrated that these self-assemblies consist of vesicular, ring-like structures with a hollow aqueous interior. Therefore, GBS-NN is a physically stable monomeric protein but is prone to temperature-induced self-assembly above 61 °C.

Silicon sheds its harmonicity

Materials Science The widespread technological uses for silicon make understanding this element's physical properties very important. Kim et al. performed inelastic neutron scattering experiments on single crystals of silicon to measure the vibrational properties up to 1500 K. Silicon has some odd thermal properties at certain temperatures, and these experiments show the need to account for a number of factors to explain the unusual thermal expansion behavior. This in-depth look at silicon helps refine theoretical models and provides a better understanding of this technologically important material. Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1707745115 (2018).

A Case of Self‐Organization in Highly Emissive EuIII Ionic Liquids

Intrinsic photoluminescent ionic liquids based on europium(III) tetrakis(β‐diketonate) complexes with a tetraalkylphosphonium as the counterion were synthesized. Two are room temperature ionic liquids; [P6,6,6,14][Eu(fod)4] (1) {fod is 1,1,1,2,2,3,3‐heptafluoro‐7,7‐dimethyloctane‐4,6‐dionate} and [P6,6,6,14][Eu(fod)3(dbm)] (2) {dbm is dibenzoylmethanate}, while calorimetric measurements showed a melting point at 415 K for [P6,6,6,14][Eu(dbm)4] (3). The luminescence quantum yields determined for 1, 2, and 3 are 34, 59 %, and 64 %, respectively. The photoluminescence and thermochemical properties of the complexes were investigated in detail and the influence of the long chain tetraalkylphosphonium counterion, together with the aromatic diketone ligand, on the unusual thermal behavior of 2 is discussed.

Synthesis of a Sulfate Enclathrated Zeolite with Intermediate Framework Structure between Sodalite and Cancrinite

A sulfate enclathrated microporous zeolite of composition Na7.4[SiAlO4]6(SO4)0.7(H2O)4.8 with intermediate framework structure (INT) between the well known zeolites sodalite (SOD) and cancrinite (CAN) can be prepared by the autothermal synthesis, a method first demonstrated in reference. The product (abbreviated as “SO4‐INT”) was characterized by X‐ray powder diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X‐ray analysis (EDX‐analysis), Fourier transform infrared spectroscopy (FT‐IR) and thermogravimetry (TG). SO4‐INT was found to crystallize as spheres with 1–6 μm in diameter with a rough surface. Sometimes the spheres are agglutinated to each other by a few amount of amorphous material within the sample. The thermal behavior of the new phase was further studied. At elevated temperature (1000 °C) a transformation of SO4‐INT into a large amount of nosean was observed beside some parts of nepheline, formed by crystallization of the amorphous byproduct. The nosean phase was found to be stable up to 1250 °C, before the onset of a total destruction by melting during further heating. Finally a re‐crystallization of the melt during cooling yielded carnegieite as the final product (low form at room temperature). This unusual thermal behavior of SO4‐INT is discussed in terms of structural interactions of the sulfate guests inside the aluminosilicate host‐framework and in comparison with literature data of the intermediate phases CO3‐INT, as well as NO3‐INT.

Poly(hydroxybutyrate‐co‐hydroxyvalerate) and bovine serum albumin blend prepared by electrospinning

ABSTRACTElectrospinning is a method for the preparation of nanosized polymer fibers. Here, electrospinning is used to prepare a blend of a polyester, poly(hydroxybutyrate‐co‐hydroxyvalerate) (PHBV), and a globular protein, bovine serum albumin (BSA). The electrospun blend film is compared with a solution‐cast blend film and with single‐component electrospun films made of PHBV and BSA. In the electrospun blend films, BSA manifests itself as flat ribbons and a fine network formed from fibers less than 50 nm in diameter. The dissolution rate of BSA from the electrospun blended film is lower than from the solution‐cast one. The films are characterized using scanning electron microscopy, differential scanning calorimetry, and contact‐angle measurements. The obtained PHBV+BSA blend films have several emergent properties: a slow BSA dissolution rate, a fine BSA network, and unusual thermal behavior. Thus, the PHBV+BSA blend films introduce a new class of polymer–protein blends. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45090.

Monte Carlo - Metropolis Investigations of Shape and Matrix Effects in 2D and 3D Spin-Crossover Nanoparticles

The Ising like model, taking into account short-, long-range interaction as well as surface effects is used to investigate size and shape effects on the thermal behaviour of 2D and 3D spin crossover (SCO) nanoparticles embedded in a matrix. We analyze the role of the parametert, representing the ratio between the number of surface and volume molecules, on the unusual thermal hysteresis behaviour (appearance of the hysteresis and a re-entrance phase transition) at small scales.

Local Structural Distortion Induced Uniaxial Negative Thermal Expansion in Nanosized Semimetal Bismuth.

The corrugated layer structure bismuth has been successfully tailored into negative thermal expansion along c axis by size effect. Pair distribution function and extended X-ray absorption fine structure are combined to reveal the local structural distortion for nanosized bismuth. The comprehensive method to identify the local structure of nanomaterials can benefit the regulating and controlling of thermal expansion in nanodivices.

Cooperative Switching in Nanofibers of Azobenzene Oligomers.

Next-generation molecular devices and machines demand the integration of molecular switches into hierarchical assemblies to amplify the response of the system from the molecular level to the meso- or macro-scale. Here, we demonstrate that multi-azobenzene oligomers can assemble to form robust supramolecular nanofibers in which they can be switched repeatedly between the E- and Z-configuration. While in isolated oligomers the azobenzene units undergo reversible photoisomerization independently, in the nanofibers they are coupled via intermolecular interactions and switch cooperatively as evidenced by unusual thermal and kinetic behavior. We find that the photoisomerization rate from the Z-isomer to the E-isomer depends on the fraction of Z-azobenzene in the nanofibers, and is increased by more than a factor of 4 in Z-rich fibers when compared to E-rich fibers. This demonstrates the great potential of coupling individual photochromic units for increasing their quantum efficiency in the solid state with potential relevance for actuation and sensing.

Coexistence of Three Ferroic Orders in the Multiferroic Compound [(CH3 )4 N][Mn(N3 )3 ] with Perovskite-Like Structure.

The perovskite azido compound [(CH3 )4 N][Mn(N3 )3 ], which undergoes a first-order phase change at Tt =310 K with an associated magnetic bistability, was revisited in the search for additional ferroic orders. The driving force for such structural transition is multifold and involves a peculiar cooperative rotation of the [MnN6 ] octahedral as well as order/disorder and off-center shifts of the [(CH3 )4 N](+) cations and bridging azide ligands, which also bend and change their coordination mode. According to DFT calculations the latter two give rise to the appearance of electric dipoles in the low-temperature (LT) polymorph, the polarization of which nevertheless cancels out due to their antiparallel alignment in the crystal. The conversion of this antiferroelectric phase to the paraelectric phase could be responsible for the experimental dielectric anomaly detected at 310 K. Additionally, the structural change involves a ferroelastic phase transition, whereby the LT polymorph exhibits an unusual and anisotropic thermal behavior. Hence, [(CH3 )4 N][Mn(N3 )3 ] is a singular material in which three ferroic orders coexist even above room temperature.

Unusual thermal transport behavior in self-assembled fullerene nanorods

Ultralow thermal conductivity (less than 0.06 W m−1 K−1) is observed for self-assembled C60 nanorods at room temperature.

Thermal shock behavior of Cr2AlC in different quenching media

Many ternary ceramics exhibit an abnormal thermal shock behavior as quenched in water. It is not clear that the unique thermal shock behavior remains valid during quenching in other quenching media. This work reports on the investigation of the thermal shock behavior of Cr2AlC as a representative member of the ternary ceramics in various quenching media. The quenching tests revealed that the abnormal thermal shock behavior of Cr2AlC was observed during quenching in water, oil and molten salt. Crack healing is the main mechanism responsible for the unusual thermal shock behavior.