Temperature Dependence Of Structural Properties Research Articles

Temperature-dependent crystal structure and fluorescence properties of a tetranuclear copper(I) cluster from in situ [2 + 3] cycloaddition synthesis

Hydrothermal reaction of CuSO4·7H2O and NaN3 with 2-cyanothiophene in aqueous ethanol yielded a novel tetranuclear copper(I) polymer, {[Cu4(tptz)4]·0.5(C2H6O)}n (1) (Htptz = 5-(thiophen-2-yl)-1H-tetrazole). The tptz ligand in the complex was generated through an in situ [2 + 3] cycloaddition reaction involving the cyano groups of the precursor 2-cyanothiophene. In a rare example, three different geometries of Cu(I) cations, namely the linear, triangle, and tetrahedron geometries of Cu(I) centers, are created and linked by Htptz ligands into a 1-D ladder structure. Additionally, 1 exhibits strong blue fluorescence (with λ = 467 nm) at room temperature in the solid state.

Low-Temperature Associated Interface Influence on the Black Phosphorus Nanoflakes.

The temperature-dependent structure properties of few-layer black phosphorus (BP) nanoflakes supported on the Si substrate and suspended on the holes are investigated using the Raman spectroscopy in the range of 77-293 K. The temperature coefficients of three main Raman modes for supported BP are larger than those of suspended BP because of the interface effects, mainly the tensile strain and the different thermal expansion coefficients (TEC) between the substrate and BP nanoflakes. The out-of-plane vibration is more apt to be affected by the initial strain, whereas the influence on the in-plane vibrations is negligible. During the varied temperature circulation, the temperature coefficients in the heating process are much larger than those in the cooling process because of the interface influences. Thus, the interfacial interaction between BP and the substrate plays an important role in the thermal-mechanical properties of BP. These findings pave a way for the further study of thermal-mechanical related applications of flexible BP nanodevices.

Temperature-dependent structural properties of water molecules confined in TiO2 nanoslits: Insights from molecular dynamics simulations

The confinement of titanium dioxide (TiO2) significantly affects the nanoconfined water molecules behaviors, manifested structurally as fluid layering and dynamically as slowing down of fluid mobility near the confining surface. In this work, we carried out molecular dynamics simulations to investigate temperature-dependent structural characteristics of water molecules confined in rutile (110) nanoslits. Specifically, we studied the microstructure of two layers of water molecules near TiO2 surface under temperatures ranging from 27 to 800 °C. The simulation results showed that the mean residence time of the first layer of water molecules decreased with temperature. A dramatic decreasing rate occurred when the temperature went beyond 300 °C. Detailed microstructural investigation of confined water molecules showed that with the increase of temperature, the orientation of some water molecules changed. The possible reason is that water molecules obtained more kinetic energy due to the higher interfacial temperature, so as to increase the probability of forming hydrogen bonds between water molecules in layer I. Moreover, dimers of water molecules, which exhibit higher mobility than water monomers near TiO2 surfaces, began to form at about 300 °C. The formation of hydrogen bonds within the first layer of water molecules is largely responsible for the reduction of mean residence time. The results of this work provide perceptive guidelines for the application of TiO2 at high temperatures, such as TiO2-supported catalysts.

Temperature-dependent magnetic and local structural properties of diamagnetic α-HgI2

We examined the temperature-dependent magnetic and structural properties of α-HgI2 in the temperature range of 300–400 K using temperature-dependent DC-magnetization, x-ray diffraction (XRD), and x-ray absorption fine structure (XAFS) measurements. DC-magnetization measurements of α-HgI2 single crystals show a sudden reduction of diamagnetic susceptibility for T > 350 K. XRD measurements with Rietveld refinements reveal a small but substantial amount of decrease in disorder in both Hg and I sites above 350 K, with neither a structural transition nor vacancies in the temperature range. XAFS demonstrates a shift of the I K edge for T > 350 K but no changes at the Hg L3 edge. A stronger bonding of Hg-I pairs above 350 K likely affects the electrons in the outermost shells of iodine atoms and enhances the intrinsic paramagnetic properties, thereby producing a reduction in diamagnetism.

Three-Dimensional Thermomechanical Simulation and Experimental Validation on Failure of Dissimilar Material Welds

Dissimilar material weld joints, consisting of low-alloy steel and 304LN austenitic stainless steel (SS), have critical application in boiling water reactors in the nuclear industry. It was predicted that phase transformation adjacent to the fusion boundary and stress distribution across the transition joint play a key role in the structural degeneration of these welds. Quantitatively, to evaluate their contribution, two different joints were considered. One was fabricated with buttering material 309L SS (M/S Mishra Dhatu Nigam Limited, Hyderabad, India), and the other was produced with buttering material IN182 (M/S Mishra Dhatu Nigam Limited, Hyderabad, India). Base materials remained the same for both. Thermomechanical simulation on dissimilar material welds was performed using finite-element modeling to predict the thermal effect and stress prone area. Temperature-dependent thermal and structural properties were considered for simulation. Simulation results were compared with microstructural characteristics, and data were obtained from the in-situ tensile test. Simulation results exhibited that stress was at maximum in the buttering material and made the zone weaker with respect to adjacent areas. During the validation of results, it was observed that failure occurred through buttering material and endorsed the inference. The variation in mechanical properties of the two welds was explained considering the effect of thermal state and stress distribution.

Structural and optical properties of Ge60Te40: experimental and theoretical verification

The temperature-dependent structural and optical properties of Ge60Te40 during transformation from amorphous to crystalline phases are studied by spectroscopic ellipsometry, grazing incidence x-ray diffraction (GIXRD), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) as well as ab initio molecular dynamics (MD) simulations. Our findings, derived from GIXRD results and dielectric functions, indicate that the crystallization process of Ge60Te40 occurs at ~260 °C and the crystalline phase adopts the rhombohedral GeTe phase along with the cubic Ge phase. Our calculations further indicate that the Ge atoms also prefer to clump together in amorphous Ge60Te40. Such a segregation of Ge leads to a high-temperature phase-change in Ge60Te40 samples. According to the XPS results, the local atomic environment between amorphous and crystalline states gets a little bit of change. And the result of the atomic cluster alignment suggests that both the amorphous and crystalline phases have the rhombohedral local structure.

Synthesis and temperature-dependent local structural and electrical properties of VO2 films

We examine the local structural and electrical properties of VO2 films showing metal-to-insulator transition (MIT) by using in-situ x-ray absorption fine structure (XAFS) measurements at the V K edge and resistance measurements in the temperature range 20–120 °C. VO2 films are synthesized on Al2O3 (0001) substrates by DC magnetron sputtering deposition. X-ray diffraction measurements show that the films have b-oriented monoclinic-phase crystals at room temperature. XAFS measurements reveal a local structural transition in the films from the monoclinic (M1) to the rutile (R) phase at ∼70 °C during their heating; further, temperature-dependent resistance (R–T) measurements showed a sharp MIT in the films at ∼75 °C. Extended XAFS (EXAFS) measurements reveal non-rigid changes of V–O and V–V bond lengths from the M1 to the R phase via the M2 phase. In-situ EXAFS and R–T measurements show that the synthesized VO2 films act as Mott insulators and that their electrical property change is not proportional to their structural property change at their MIT temperature.

Structure evolution and large strain response in BNT–BT lead-free piezoceramics modified with Bi(Ni0.5Ti0.5)O3

The composition and temperature-dependent structure, phase stability, strain behavior, dielectric, ferroelectric (FE), piezoelectric, and pyroelectric properties of lead-free Bi(Ni0.5Ti0.5)O3(BNiT)–modified 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 (BNT–BT–xBNiT, 0 ≤ x ≤ 0.06) have been systematically investigated. The correlation between ferroelectric-to-relaxor transition temperature TF-R and depolarization temperature Td was unraveled based on temperature-dependent the stability of electrically induced long-range ferroelectric order and piezoelectricity. The phase diagram was established and the relationship between the phase structure and the electrical properties of the BNT–BT–xBNiT system has been clarified. As the BNiT content increases, the destabilization of the FE order resulting in a degradation of the remanent polarization, coercive field, Td and TF-R, is accompanied by a large electric-field-induced strain of 0.35% at 60 kV/cm with normalized strain Smax/Emax = 590 pm/V at around critical composition of x = 0.035. The Raman-spectra analysis, macroscopic properties, thermal depolarization results, and temperature-dependent relationships of both polarization and strain manifested that the observed large strain for the investigated system is contributed by a reversible field-induced relaxor to FE phase transformation.

Temperature dependent structural properties and bending rigidity of pristine and defective hexagonal boron nitride

Structural and thermodynamical properties of monolayer pristine and defective boron nitride sheets (h-BN) have been investigated in a wide temperature range by carrying out atomistic simulations using a tuned Tersoff-type inter-atomic empirical potential. The temperature dependence of lattice parameter, radial distribution function, specific heat at constant volume, linear thermal expansion coefficient and the height correlation function of the thermally excited ripples on pristine as well as defective h-BN sheet have been investigated. Specific heat shows considerable increase beyond the Dulong–Petit limit at high temperatures, which is interpreted as a signature of strong anharmonicity present in h-BN. Analysis of the height fluctuations, , shows that the bending rigidity and variance of height fluctuations are strongly temperature dependent and this is explained using the continuum theory of membranes. A detailed study of the height–height correlation function shows deviation from the prediction of harmonic theory of membranes as a consequence of the strong anharmonicity in h-BN. It is also seen that the variance of the height fluctuations increases with defect concentration.

Magnetic order and phase transitions in Fe50Pt50–x Rh x

Polarized and unpolarized neutron diffraction techniques have been applied to study the temperature-dependent magnetic and structural properties of four 200 nm-thick Fe50Pt50−x Rh x films with x = 5, x = 10, x = 17.5 and x = 25. Similar to the bulk system, an antiferromagnetic to ferromagnetic transition can be found in the films with decreasing Rh concentration. The application of structure factor calculations enables one to determine the microscopic magnetic configuration of the different films as a function of temperature and Rh concentration. The developed models indicate a magnetic transition from a dominant antiferromagnetic order in the out-of-plane direction to a dominant ferromagnetic order in the in-plane direction with decreasing Rh concentration. The different magnetic configurations can theoretically be described by a phenomenological model which includes a two-ion and a one-ion interaction Hamiltonian term with different temperature dependencies of the anisotropy constants.

Temperature-Dependent Local Structural Properties of Redox Pt Nanoparticles on TiO2 and ZrO2 Supports

This study examined the local structural properties of Pt nanoparticles on SiO2, TiO2–SiO2, and ZrO2–SiO2 supports to better understand the impact of oxide-support type on the performance of Pt-based catalysts. In situ X-ray absorption fine structure (XAFS) measurements were taken for the Pt L3-edge in a temperature range from 300 to 700 K in He, H2, and O2 gas environments. The XAFS measurements demonstrated that Pt atoms were highly dispersed on TiO2–SiO2 and ZrO2–SiO2 forming pancake-shaped nanoparticles, whereas Pt atoms formed larger particles of hemispherical shapes on SiO2 supports. Contrary to the SiO2 case, the coordination numbers for Pt, Ti, and Zr around Pt atoms on the TiO2–SiO2 and ZrO2–SiO2 supports were nearly constant from 300 to 700 K under the different gas environments. These results are consistent with the improvements in thermal stability of Pt nanoparticles achieved by incorporating TiO2 or ZrO2 on the surface of SiO2 supports. XAFS analysis further indicated that the enhanced dispersion and stability of Pt were a consequence of the strong metal support interaction via Pt–Ti and Pt–Zr bonds. From XANES measurements, oxygen coordination numbers of a Pt atom on different supports of (a) SiO2, (b) TiO2-SiO2, and (c) ZrO2-SiO2 were determined. As temperature increases, oxygen coordination numbers gradually decreased in a He gas, whereas it is dramatically decreases in an H2 gas.

Reliability Studies on High-Temperature Operation of Mixed As/Sb Staggered Gap Tunnel FET Material and Devices

The reliability of structural and electrical properties of mixed As/Sb staggered gap tunnel field-effect transistors (TFETs) for high-temperature operation was comprehensively investigated from 25 °C to 150 °C. Temperature-dependent X-ray measurements showed identical strain relaxation of the active region, indicating that no additional dislocations were introduced at 150 °C. Symmetric 2-D surface crosshatch patterns before and after annealing suggested no significant structural properties change during high-temperature operation. No extra interdiffusion of species at the source/channel heterointerface was observed at 150 °C, confirmed by secondary ion mass spectrometry measurement. The leakage current of the fabricated reverse-biased p+-i-n+ diode exponentially increased with increasing temperature due to Shockley-Read-Hall generation-recombination mechanism. The on-state drain current of the TFET device showed weak temperature dependence, and it decreased with increasing temperature from 25 °C to 100 °C due to the variation of Fermi distribution and the increase in channel resistance but increased from 100 °C to 150 °C due to the reduction of both band-gap energy and the effective tunneling barrier height. The subthreshold slope has a strong positive temperature-dependent property particularly at higher temperature due to trap-assisted tunneling process. These experimental results demonstrated stable structural properties and distinguished device characteristics of the mixed As/Sb staggered gap TFETs at higher operating temperature. The temperature-dependent structural and device properties of the mixed As/Sb staggered gap TFET highlight the importance of the reliability on high-temperature operation of TFETs for future low-power digital logic applications.

Heat-Treated Fe<sub>3</sub>O<sub>4</sub> - Activated Carbon Nanocomposite for High Performance Electrochemical Capacitor

The impact of heat treatment temperature on the electrochemical performance of Fe3O4-activated carbon nanocomposite electrodes was investigated using constant current charge-discharge and Electrochemical Impedance Spectroscopy (EIS). An improved capacitive behaviour was observed due to the effect of enhanced ionic and electronic conductivities of the 4 wt% Fe3O4/AC by thermally heating at 200 °C for 6 hours. It was found that the internal resistance of 4 wt% Fe3O4/AC composite electrode calcined at 200 °C for 6 hours is the smallest (2.97 Ω) in comparison to those untreated (4.36 Ω) composite electrodes. The ion mobility inside the porous composite electrodes is favourable at 200 °C, accompanying with the enhanced electronic conductivity of oxide electrode as a result of improved crystallinity. The EIS results and analysis not only have significant impact on the fundamental understanding of the temperature-dependent structural and electrochemical properties of electrode but also provide the insights on the diffusion mechanism of the nanocomposite in neutral Na2SO3 electrolyte.

Temperature-dependent mechanical properties of alpha-/beta-Nb5Si3 phases from first-principles calculations

The temperature-dependent structural properties and anisotropic thermal expansion coefficients of α-/β-Nb5Si3 phases have been determined by minimizing the non-equilibrium Gibbs free energy as functions of crystallographic deformations. The results indicate that the crystal anisotropy of α-Nb5Si3 phase is much more temperature dependence than that of β-Nb5Si3 phase. The total/partial density of states of α-/β-Nb5Si3 phases are discussed in detail to analyze their electronic hybridizations. It is demonstrated that the bonding of the two phases is mainly contributed from the hybridization between Nb-4d and Si-3p electronic states. The temperature-dependent mechanical properties of α-/β-Nb5Si3 phases are further investigated via the quasi-harmonic approximation method in coupling with continuum elasticity theory. The calculated single-crystalline and polycrystalline elasticity shows that both phases are mechanically stable and exhibit the intrinsic brittleness. The results also suggest that α-Nb5Si3 phase possesses a superior ability of compression resistance but an inferior ability of high-temperature resistance of mechanical properties than those of β-Nb5Si3 phase. The bonding features of α-/β-Nb5Si3 phases are discussed by means of charge density difference analysis in order to explain the difference of the temperature-dependent mechanical properties between the two phases.

AB INITIOMOLECULAR DYNAMICS SIMULATIONS ON LOCAL STRUCTURE AND ELECTRONIC PROPERTIES IN LIQUIDSbFROM 913 K TO 1193 K

Ab initio molecular dynamics simulations on liquid Sb have been carried out at five different temperatures from 913 K to 1193 K. We have investigated the temperature dependence of structure properties including structural factor S(Q), pair correlation function g(r), bond-angle distribution function g3(θ), cluster properties and bond order parameter Q4and Q6. A shoulder was reproduced in the high wave number side of the first peak in the S(Q) implying that the residual structure units of crystalline Sb remain in liquid Sb . There is a noticeable bend at around 1023 K in the temperature dependence of the first-peak height of S(Q), the cluster properties and bond order parameter Q4, respectively, indicating that an abnormal structural change may occur at 973–1023 K.

Structural and transport properties of Sn–Mg alloys

In this work, the structural and transport properties of Mg-doped Sn-based alloys have been investigated. The temperature-dependent transport and structural properties of Sn–Mg alloys were investigated for five different samples (Pure Sn, Sn-1.0 wt% Mg, Sn-2.0 wt% Mg, Sn-6.0 wt% Mg and Pure Mg). Scanning electron microscopy (SEM), X-ray diffraction and energy dispersive X-ray analysis measurements were carried out in order to clarify the structural properties of the samples. It was found that the samples had tetragonal crystal symmetry, except for pure Mg which had hexagonal crystal symmetry. We also found that the cell parameters changed slightly with the addition of Mg element. The SEM micrographs of the samples showed that they had smooth surfaces with a clear grain boundary. The electrical and thermal conductivity of the samples were measured by four-point probe and the radial heat flow method, respectively. The electrical resistivity of the samples increased almost linearly with the increasing temperature. The thermal conductivity values ranged between 0.60 and 1.00 W/Km, while they decreased slightly with temperature and increased with Mg composition. The thermal conductivity values of the alloys were in between the values of pure Sn and Mg. The thermal conductivity results of the alloys were compared with other available results, and a good agreement was seen between the results. In addition, the temperature coefficients of electrical resistivity and thermal conductivity were determined; these were independent of the composition of the alloying elements.

Effect of distribution of stickers along backbone on temperature-dependent structural properties in associative polymer solutions

Effect of distribution of stickers along the backbone on structural properties in associating polymer solutions is studied using self-consistent field lattice model. Only two inhomogeneous morphologies, i.e., microfluctuation homogenous (MFH) and micelle morphologies, are observed. If the system is cooled, the solvent content within the aggregates decreases. When the spacing of stickers along the backbone is increased the temperature-dependent range of aggregation in MFH morphology and half-width of specific heat peak for homogenous solutions-MFH transition increase, and the symmetry of the peak decreases. However, with increasing spacing of stickers, the above three corresponding quantities related to micelles behave differently. It is demonstrated that the broad nature of the observed transitions can be ascribed to the structural changes which accompany the replacement of solvents in aggregates by polymer, which is consistent with the experimental conclusion. It is found that different effect of spacing of stickers on the two transitions can be interpreted in terms of intrachain and interchain associations.

Modulating Supramolecular Assemblies and Mechanical Properties of Engineered Protein Materials by Fluorinated Amino Acids

Here we describe the biosynthesis and characterization of fluorinated protein block polymers comprised of the two self-assembling domains (SADs): elastin (E) and the coiled-coil region of cartilage oligomeric matrix proteins (C). Fluorination is achieved by residue-specific incorporation of p-fluorophenylalanine (pFF) to create pFF-EC, pFF-CE, and pFF-ECE. Global fluorination results in downstream effects on the temperature-dependent secondary structure, supramolecular assembly, and bulk mechanical properties. The impact of fluorination on material properties also differs depending on the orientation of the block configurations as well as the number of domains in the fusion. These studies suggest that integration of fluorinated amino acids within protein materials can be employed to tune the material properties, especially mechanical integrity.

Thermal Cycling Effects on the Thermoelectric Properties of n-Type In,Ce-Based Skutterudite Compounds

n-Type In-filled CoSb3 is a known skutterudite compound that has shown promising thermoelectric (TE) properties resulting in high dimensionless figure of merit values at elevated temperatures. Use in various waste heat recovery applications will require survival and operation after exposure to harsh thermal cycling environments. This research focused on uncovering the thermal cycling effects on TE properties of n-type In0.2Co4Sb12 and In0.2Ce0.15Co4Sb12 skutterudite compositions as well as quantifying their temperature-dependent structural properties (elastic modulus, shear modulus, and Poisson’s ratio). It was observed that the Seebeck coefficient and resistivity increased only slightly in the double-filled In,Ce skutterudite materials upon thermal cycling. In the In-filled skutterudites the Seebeck coefficient remained approximately the same on thermal cycling, while the electrical resistivity increased significantly after thermal cycling. Results also show that the thermal conductivity marginally decreases in the case of In-filled skutterudites, whereas the reduction is more pronounced in In,Ce-based skutterudite compounds. The possible reason for this kind of reduction can be attributed to grain pinning effects due to formation of nanoinclusions. High-temperature structural property measurements (i.e., Young’s modulus and shear modulus) are also reported. The results show that these structural properties decrease slowly as temperature increases and that the compounds are structurally stable after numerous (up to 200) thermal cycles.

Temperature dependence of structure and density for D2O confined in MCM-41-S

Using neutron diffraction, we have tracked the temperature dependence of structural properties for heavy water confined in the nanoporous silica matrix MCM-41-S. By observing the correlation peak corresponding to the pore–pore distance, which is determined by the scattering contrast between the silica and the water, we monitored the density of the confined water. Concurrently, we studied the prominent first diffraction peak of D2O at ≈1.8 Å−1, which furnishes information on the microscopic arrangement of the water molecules. The data show the presence of a density maximum at ≈275 K (±10 K), a property similar to bulk water, and the occurrence of a density minimum at ≈180 K (±10 K). The prominent diffraction peak of D2O is found to shift and sharpen over a wide T range from 200 to 270 K, reflecting structural changes that are strongly correlated with the changes in density. We also observe the continuous formation of external ice, arising from water expelled from the pores while expansion takes place within the pores. An efficient method for monitoring the density of the confined D2O using a triple-axis spectrometer is demonstrated.