Temperature-induced Structural Evolution Research Articles

Effect of Micellar Morphology on the Temperature-Induced Structural Evolution of ABC Polypeptoid Triblock Terpolymers into Two-Compartment Hydrogel Network.

We investigated the temperature-dependent structural evolution of thermoreversible triblock terpolypeptoid hydrogels, namely poly(N-allyl glycine)-b-poly(N-methyl glycine)-b-poly(N-decyl glycine) (AMD), using small-angle neutron scattering (SANS) with contrast matching in conjunction with X-ray scattering and cryogenic transmission electron microscopy (cryo-TEM) techniques. At room temperature, A100M101D10 triblock terpolypeptoids self-assemble into core-corona-type spherical micelles in aqueous solution. Upon heating above the critical gelation temperature (T gel), SANS analysis revealed the formation of a two-compartment hydrogel network comprising distinct micellar cores composed of dehydrated A blocks and hydrophobic D blocks. At T ≳ T gel, the temperature-dependent dehydration of A block further leads to the gradual rearrangement of both A and D domains, forming well-ordered micellar network at higher temperatures. For AMD polymers with either longer D block or shorter A block, such as A101M111D21 and A43M92D9, elongated nonspherical micelles with a crystalline D core were observed at T < T gel. Although these enlarged crystalline micelles still undergo a sharp sol-to-gel transition upon heating, the higher aggregation number of chains results in the immediate association of the micelles into ordered aggregates at the initial stage, followed by a disruption of the spatial ordering as the temperature further increases. On the other hand, fiber-like structures were also observed for AMD with longer A block, such as A153M127D10, due to the crystallization of A domains. This also influences the assembly pathway of the two-compartment network. Our findings emphasize the critical impact of initial micellar morphology on the structural evolution of AMD hydrogels during the sol-to-gel transition, providing valuable insights for the rational design of thermoresponsive hydrogels with tunable network structures at the nanometer scale.

Temperature-induced structure evolution in CoxBy liquids studied by ab initio molecular dynamics simulation

The relationship between the local structures and the thermodynamic properties of metallic liquids has been a fundamental issue, which significantly impacts the comprehensive performances of metallic materials. The temperature-induced structure evolution in CoxBy alloy liquids, involving CoB, Co2B, Co3B, and Co23B6, was investigated using ab initio molecular dynamics simulations. The results show that the clusters of icosahedral-like polyhedrons are found predominantly in all alloys at low temperature, while the heating induces the decrease in ideal icosahedral and the increase in defected icosahedral. At high temperature, the polyhedron clusters decrease significantly and the ideal icosahedra transforms into smaller short-range polyhedrons, signifying the temperature-induced chemical structure change from long-range to short-range ordering. The aggregation of B atoms at high temperature is confirmed by atomic configuration, charge density, and B-centered Voronoi polyhedrons, and the α-Co single phase is observed below 1600 K. This study is helpful for understanding the local structure variability in eutectic Co-B alloys and guiding solidification path in theory.

High-precision nuclear magnetic resonance probe suitable for in situ studies of high-temperature metallic melts

High-temperature nuclear magnetic resonance (NMR) has proven to be very useful for detecting the temperature-induced structural evolution and dynamics in melts. However, the sensitivity and precision of high-temperature NMR probes are limited. Here we report a sensitive and stable high-temperature NMR probe based on laser-heating, suitable for in situ studies of metallic melts, which can work stably at the temperature of up to 2000 K. In our design, a well-designed optical path and the use of a water-cooled copper radio-frequency (RF) coil significantly optimize the signal-to-noise ratio (S/NR) at high temperatures. Additionally, a precise temperature controlling system with an error of less than ± 1 K has been designed. After temperature calibration, the temperature measurement error is controlled within ± 2 K. As a performance testing, 27Al NMR spectra are measured in Zr-based metallic glass-forming liquid in situ. Results show that the S/NR reaches 45 within 90 s even when the sample’s temperature is up to 1500 K and that the isothermal signal drift is better than 0.001 ppm per hour. This high-temperature NMR probe can be used to clarify some highly debated issues about metallic liquids, such as glass transition and liquid–liquid transition.

Temperature-induced shear-thinning in catalyst inks

Temperature is a key parameter for ionomer adsorption on carbon supports in catalyst inks for fuel cells, and thus it affects the properties of the ink. This study aimed to investigate the temperature-induced structural evolution and viscosity transition in a catalyst ink. The effect of temperature on the nanostructures of the catalyst ink fabricated from surface-modified carbon supports without platinum nanoparticles was investigated using contrast-variation small-angle neutron scattering (CV-SANS) analysis, through which the concentration and thickness of the adsorbed ionomer layer on the surface of carbon were evaluated. The CV-SANS analysis revealed that the thickness of the shell ionomer layer at 70 °C was twice that at 25 °C, while at elevated temperatures the catalyst ink exhibited shear-thinning behavior. It was surmised that plausible factors in the shear-thinning behavior were a reduction in electrostatic repulsion and the appearance of bridging attraction. Our findings indicate that temperature is a key parameter in the fabrication of catalyst inks, which may be overlooked during the formulation of catalyst inks and catalyst layers.

Temperature-induced structural evolution in liquid Ag-Ga alloys

Temperature-dependent atomic structural evolutions of liquid ${\mathrm{Ag}}_{60}{\mathrm{Ga}}_{40}$ and ${\mathrm{Ag}}_{70}{\mathrm{Ga}}_{30}$ alloys have been studied by in situ high-energy x-ray-diffraction (HEXRD) experiments combined with ab initio molecular-dynamics simulations. The experimental data show a reversible structural crossover at about $1050\ensuremath{\sim}1100 (\ifmmode\pm\else\textpm\fi{}50)\phantom{\rule{4pt}{0ex}}\mathrm{K}$ in both liquid ${\mathrm{Ag}}_{60}{\mathrm{Ga}}_{40}$ and ${\mathrm{Ag}}_{70}{\mathrm{Ga}}_{30}$ alloys. Obvious changes of the electrical resistivity, absolute thermoelectric power, and atomic diffusivity around the similar temperature range for both Ag-Ga liquids strongly support the HEXRD results. The origin of the liquid-to-liquid crossover in both Ag-Ga liquids was suggested to link with the rearrangements of Ag and Ga atoms, i.e., Ag and Ga atoms prefer to associate with themselves in the higher temperature range above 1100 K, consistent with the accelerated increase of the strong covalently bonded Ga--Ga dimers in both Ag-Ga liquids. In addition, more studies from the energy aspect are still desirable to understand the rearrangements of Ag and Ga atoms in the higher temperature range in both Ag-Ga liquids.

In situ high-temperature nuclear magnetic resonance characterization of structural evolution in pure gallium melt

The structure of liquid gallium (Ga) is a long-standing issue in the studies of metallic liquids. In this paper we investigate pure Ga melt in the temperature range of 307--1356 K by high-temperature $^{69}\mathrm{Ga}$ and $^{71}\mathrm{Ga}$ NMR. A structural crossover at around 1000 K is revealed through the analyses of NMR observables including the Knight shift and the spin-lattice relaxation time, which may result from the temperature-induced structural evolution of the covalently bonded clusters in pure Ga melt. However, structure and diffusivity anomalies around 450 and 730 K at ambient pressure, which may be induced by the liquid-liquid critical point, are not observed.

Temperature-induced structure evolution of Ag nanoparticles

The molecular dynamics method with a modified tight-binding (TB-SMA) potential has been used to study the thermal stability of the initial fcc phase in perfect silver clusters to 2.0 nm in diameter, including the “magical” icosahedral size of N = 55 and 147 atoms. It has been shown that the temperature factor can cause a transition from the initial fcc phase to other structural modifications in small Ag clusters (up to 200 atoms). Thus, the fcc structure of clusters with N > 200 atoms is thermally stable to the melting point. For silver nanoparticles of smaller size, the situation is much more complicated, and numerous cases of thermally induced changes in the cluster structure have been observed, often occurring in different scenarios. Thus, to use such small silver clusters, it is necessary to study in detail the problems of the thermal stability of the cluster structure, apparently taking into account the influence of various “magic” numbers.

Temperature-induced structural evolution in liquid Sn85Zn15 eutectic alloy

Temperature-dependent atomistic structural evolution in liquid Sn85Zn15 eutectic alloy has been investigated by using in situ high energy X-ray diffraction and ab initio molecular dynamics simulations. We reveal a structural crossover at around 900 K in liquid Sn85Zn15 eutectic alloy, below and above which the liquids exhibit different changes in first peak position shifts on S(q) and G(r) curves, the nearest coordination number and activation energies for self-diffusion. This structural crossover is likely ascribed to the spatial connectivity variation of local clusters. Our finding will trigger more studies on the liquid-to-liquid structural transition in metallic liquids.

Study of temperature-induced structural evolution in (Na0.5Bi0.5)TiO3-(K0.5Bi0.5)TiO3-(K0.5Na0.5)NbO3 lead-free ceramics

In this work, the temperature-induced structural evolution in 0.79(Na0.5Bi0.5)TiO3-0.2(K0.5Bi0.5)TiO3-0.01(K0.5Na0.5)NbO3 (NKBNT) lead-free ceramics was investigated by Raman microscopic spectroscopy combined with electrical macroscopic measurements. The NKBNT ceramics possess the local structure with the coexisted rhombohedral R3c and tetragonal P4bm polar-nano-regions (PNRs). The R3c and P4bm PNRs coexist in a wide temperature range, then the local structure transforms to the P4bm PNRs around the temperature of dielectric maximum (Tm) evidenced by the doublet splitting of Ti-O modes (peak B) and oxygen octahedral vibrational modes (peak C). The discontinuous changes of wavenumber and line-width of peak B2 and peak C3 as well as the dielectric local maxima around the rhombohedral-tetragonal phase transition temperature (TRT) are considered to result from the thermal evolution of R3c and P4bm PNRs. The macroscopic changes of non-polar phase with electric field and temperature were investigated by the temperature-dependent polarization-electric field (P-E) loops, current-electric field (I-E) loops and bipolar strain (S-E) curves. The electric-field level necessary to form the long-range ferroelectric order from non-polar phase associated with the stability of the induced ferroelectric phase depends strongly on the temperature.

Temperature-induced structural evolution of Sm nanoparticles on Al2O3 thin film: An in-situ investigation using SRPES, XPS and STM

The structural evolution of Sm nanoclusters on ultrathin film of Al2O3 epitaxially grown on Ni3Al(111) substrate at elevated temperatures was investigated in-situ using synchrotron radiation photoemission spectroscopy (SRPES), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). The vapor-deposited metallic Sm onto the Al2O3 thin film at 300K is partially oxidized, leading to the appearance of both Sm2+ and Sm3+ states at low coverages, due to the charge transfer from Sm to oxide film. The complete oxidation of the Sm, i.e., all Sm2+ converted to Sm3+, occurs when the sample is annealed to 500K. Further annealing results in the diffusion of Sm into the Al2O3 lattice. At ∼900K, the formation of a SmAlO3 complex is observed. However, this complex starts to decompose and desorb from the surface at temperature higher than 1200K. Interestingly, it is found that Sm can promote the oxidation of Ni3Al substrate and thicken the alumina film when Sm is deposited at room temperature onto the Al2O3/Ni3Al(111) substrate followed by annealing in oxygen environment at ∼800K.

A new orthorhombic boron phase B51.5–52 obtained by dehydrogenation of “α-tetragonal boron”

Abstract

Influence of molecular weight on polymorphs and temperature-induced structure evolution of regioregular poly(3-dodecylthiophene)

Formation of form I and form II polymorphs and their phase transitions were investigated for poly(3-dodecylthiophene) (P3DDT) films with molecular weights of 5300, 14 800, and 59 000 respectively, using temperature-dependent FTIR spectroscopy in conjugation with X-ray diffraction and differential scanning calorimetry. For the P3DDTs with the higher molecular weights (14 800 and 59 000), the form I polymorph was obtained by fast drying a solution at ambience or crystallization from the melt, whereas for the low molecular weight one (5300), the form II structure was produced, the amount of which can be increased by slowing down the process. Temperature-induced phase transitions of the three P3DDTs were explored. The results show that form II is thermodynamically more stable at room temperature, but its formation is kinetically hindered, and possible reasons are discussed.

Stress-versus temperature-induced structural evolution in metallic glasses

Structure evolution induced by shear deformation was investigated via molecular dynamic simulation on CuZr metallic glass system and compared with that induced by temperature. Voronoi tessellation analysis found that the local structures evolve to a liquid-like state as shear stress increases, similar to the temperature-induced structure evolution. However, shear stress induces little change to the radial distribution functions (RDFs) compared to temperature, indicating that the global glassy state still sustains. This is in contrast to the opinion that shear deformation may be similar to temperature increase and induce a metallic glass system to experience glass-liquid transition. The analysis of anisotropic part of RDFs reveals that shear deformation can induce significant anisotropic structural evolution, while pure increase of temperature cannot. Our findings demonstrate that shear deformation may induce different structural evolution in metallic glasses compared to temperature.

Thermal expansion and high temperature structure evolution of zoisite by single-crystal X-ray and neutron diffraction

The thermo-elastic behaviour and the temperature-induced structure evolution of a natural Fe-free zoisite have been investigated by in situ single-crystal X-ray and neutron diffraction. Neither discontinuities in volume expansion nor changes in symmetry have been observed up to 1,023 K. Zoisite shows a negative thermal expansion along [100] at T > 700 K, while a continuous positive expansion occurs on the (100) plane. Two different regimes in the anisotropic thermal behaviour of zoisite can be distinguished (i.e. at T < 700 K and T > 700 K), corresponding to an increase in the volumetric thermal expansion at T > 700 K. The structure evolution with temperature has been described by a series of X-ray and neutron refinements at different temperatures. In particular, the M(3) polyhedra show a significant octahedral flattening and expansion in the equatorial plane with T. All [SiO4] tetrahedra show a regularization with increasing T. The neutron refinements show no change in the configuration of the hydrogen bonding at least up to 873 K. The effects of the T-induced main deformation mechanisms on the anisotropic elastic behaviour of zoisite are discussed. A comparison with the thermal behaviour of epidote has been carried out.

Composition and temperature-induced structure evolution in Bi0.5Na0.5TiO3-based solid solutions

In this study, the temperature and composition dependence of the dielectric, ferroelectric properties, and polarization current characteristics of the (0.935 − x)Bi0.5Na0.5TiO3–0.065BaTiO3–xSrTiO3 (BNBSTx) with x ranging from 0.02 to 0.22 were systematically investigated. At elevated temperature the BNBSTx solid solution with x 0.18, which may be related with the formation of certain sublattices in the complex solid solution.

Composition and temperature-induced structural evolution in La, Sm, and Dy substituted BiFeO3 epitaxial thin films at morphotropic phase boundaries

Detailed structural investigations on the substitution-induced structural phase transition from the rhombohedral phase to an orthorhombic phase in (Bi,RE)FeO3 epitaxial thin films (RE = La, Sm, and Dy) grown on (100) SrTiO3 substrates are presented. X ray diffraction reveals that the unit cell dimensions of the orthorhombic phase are strongly dependent on the type of the RE dopant. For RE = La3+ ion, which has an ionic size comparable to the Bi3+ ion, the unit cell is found to be a0 × a0 × 2a0, where a0 is the pseudo-cubic lattice parameter. This is in contrast with the √2a0 × √2a0 × 2a0 unit cell for the case of smaller ionic radius RE (= Sm and Dy) elements. While clear double-hysteresis loops in the polarization versus electric-field curves due to field-induced transitions are observed for smaller ionic radius RE (= Sm and Dy), no signature of the double hysteresis loops is observed for the RE = La case across the structural boundary. We have also performed systematic tracking of the structural phases as functions of the RE composition and temperature, based on which we propose a phase diagram. This work reveals that the ionic size of the RE element plays a critical role in the evolution of the structural and functional properties of RE-substituted BiFeO3 thin film materials systems.