Low Temperature Side Research Articles

Microstructure and Martensitic Transformation Behavior in Thermal Cycled Equiatomic CuZr Shape Memory Alloy

Equiatomic CuZr alloy undergoes a martensitic transformation from the B2 parent phase to martensitic phases (P21/m and Cm) below 150 °C. We clarified the effect of the thermal cycling on the morphology and crystallography of martensite in equiatomic CuZr alloy using a transmission electron microscopy. The 10th cycled specimens consisted of different multiple structures at the maximum temperature of differential scanning calorimetry (DSC) measurement −400 °C and 500 °C, respectively. At the maximum temperature 400 °C of DSC measurement, it is composed of the fine plate-like variants, and a lamellar eutectoid structure consisting of Cu10Zr7 and CuZr2 phases on the martensitic variant. Concerning the maximum temperature of 500 °C of DSC measurement, it is observed the martensitic structure and the lamellar structure in which the martensitic phase was completely eutectoid transformed. The formation of this lamellar eutectoid structure, due to thermal cycling leads to the shift of forward and reverse transformation peaks to low and high temperature side. In addition, new forward and reverse transformation peaks indicating a new transformation appeared by thermal cycling, and the peaks remained around −20 °C. This new martensitic transformation behavior is also discussed.

Initial-stage reaction of methane examined by optical measurements of weak flames in a micro flow reactor with a controlled temperature profile

To examine methane oxidation at intermediate temperatures (ca., 900–1200 K), chemiluminescence observation and laser-induced fluorescence (LIF) measurements for CH2O and OH were conducted for methane weak flames in a micro flow reactor with a controlled temperature profile (MFR) at atmospheric and elevated pressures. Locations of CH2O–LIF, chemiluminescence, and OH-LIF in MFR were arranged from the low temperature side at 1.0 and 5.0 bar. Spatial separation of methane oxidation was successfully demonstrated. One-dimensional computations with five detailed kinetic mechanisms were conducted. Computational profiles of CH2O molar concentration, heat release rate (HRR), and OH molar concentration normalized by their own peak values were compared with experimentally obtained intensity profiles of the CH2O–LIF, chemiluminescence, and OH-LIF. Computational results obtained with AramcoMech 1.3 showed better agreements with experimentally obtained results among the mechanisms employed. However, the flame position computed with AramcoMech 1.3 showed a slightly higher temperature region than the experimental flame position, indicating underprediction of methane reactivity. Sensitivity analysis identified a set of dominant reactions for weak flame positions. Rate constants of the identified reactions were modified within uncertainty to reproduce experimentally obtained weak flame positions. The modified mechanism also well predicted ignition delay times and flame speeds, and significant improvement of prediction was identified particularly for ignition delay times of lowest temperature and pressure investigated. Reaction path analysis highlighted the importance of intermediate-temperature oxidation chemistry for methane such as CH3→CH3O2→CH3O→CH2O reactions at higher pressures. Two-stage oxidation of methane was observed by chemiluminescence observation and numerical simulations at higher pressures (6.0–10.0 bar).

Non-Tacky Fluorinated and Elastomeric STEM Networks.

Soft, elastomeric, non-tacky polymer networks are synthesized by reversible deactivation radical polymerization (RDRP). First, the pristine, structurally tailored and engineered macromolecular (STEM) networks are synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization and incorporated an atom transfer radical polymerization (ATRP) inimer into the network. Subsequently, poly(n-butyl acrylate) (PBA) and/or poly(octafluoropentyl acrylate) (POFPA) side chains are grafted from the network by photo-induced ATRP. These low glass transition temperature side chains produced soft materials (E= 104-178 kPa). However, only the POFPA-containing networks are also non-tacky. The fluorine content and material properties are investigated by dynamic mechanical analysis, elemental analysis, spectroscopy, and contact angle measurements.

Low-Temperature Side Contact to Carbon Nanotube Transistors: Resistance Distributions Down to 10 nm Contact Length

Carbon nanotube field-effect transistors (CNFETs) promise to improve the energy efficiency, speed, and transistor density of very large scale integration circuits owing to the intrinsic thin channel body and excellent charge transport properties of carbon nanotubes. Low-temperature fabrication (e.g., <400 °C) is a key enabler for the monolithic three-dimensional (3D) integration of CNFET digital logic into a device technology platform that overcomes memory bandwidth bottlenecks for data-abundant applications such as big-data analytics and machine learning. However, high contact resistance for short CNFET contacts has been a major roadblock to establishing CNFETs as a viable technology because the contact resistance, in series with the channel resistance, reduces the on-state current of CNFETs. Additionally, the variation in contact resistance remains unstudied for short contacts and will further degrade the energy efficiency and speed of CNFET circuits. In this work, we investigate by experiments the contact resistance and statistical variation of room-temperature fabricated CNFET contacts down to 10 nm contact lengths. These CNFET contacts are ∼15 nm shorter than the state-of-the-art Si CMOS "7 nm node" contact length, allowing for multiple generations of future scaling of the transistor-contacted gate pitch. For the 10 nm contacts, we report contact resistance values down to 6.5 kΩ per source/drain contact for a single carbon nanotube (CNT) with a median contact resistance of 18.2 kΩ. The 10 nm contacts reduce the CNFET current by as little as 13% at VDS = 0.7 V compared with the best reported 200 nm contacts to date, corroborated by results in this work. Our analysis of RC from 232 single-CNT CNFETs between the long-contact (e.g., 200 nm) and short-contact (e.g., 10 nm) regimes quantifies the resistance variation and projects the impact on CNFET current variability versus the number of CNT in the transistor. The resistance distribution reveals contact-length-dependent RC variations become significant below 20 nm contact length. However, a larger source of CNFET resistance variation is apparent at all contact lengths used in this work. To further investigate the origins of this contact-length-independent resistance variation, we analyze the variation of RC in arrays of identical CNFETs along a single CNT of constant diameter and observe the random occurrence of high RC, even on correlated CNFETs.

Human-Oriented Design of an Indoor Thermal Environment

In this study, an initial survey of clothing insulation and changes in the metabolic rate of individuals in office spaces was performed to establish the distribution of room temperatures at which individuals perceived a neutral thermal sensation. Subsequently, the indoor thermal environment in four offices was surveyed during the summer with different air-conditioning systems to determine the thermal environment stability in each case. The results revealed that for the required temperature, there was a noticeable difference between the average and most frequent values. Moreover, it was determined that the required temperature distribution is not normal, but rather, it is skewed to the low-temperature side. In addition, the radiant air-conditioning system was found to generate a narrow distribution of the equivalent temperature and hence, facilitated a more uniform thermal environment compared to a convective (multi-unit) air-conditioning system. Therefore, in buildings with convective air-conditioning systems, even if the planar average thermal environment is categorized as comfortable, it may be possible that workers who are sensitive to the cold or heat will complain of discomfort more frequently than those in buildings with radiant air-conditioning systems because the probability of workers sitting in cold- or hot-spot areas is higher in the former case.

Space charge characteristics in 160 kV DC XLPE cable under temperature gradient

A temperature gradient (TG) will be formed across a high voltage cable insulation resulting from Joule heat in the conductor. Therefore, it is important to measure space charge profiles in a full-scale cable under TG. In this study, an improved pulsed electro-acoustic system used for coaxial cables was established by injecting nanosecond pulses into the outer semi-conductive layer. An induced current heating and a water circulation system was used to regulate the temperatures of a cable conductor and the the outer semi-conductive layer. By this method, a TG between the cable conductor and the outer semi-conductor layer was controlled and set at various temperatures and the TG was formed across the radial insulation so as to avoid the influence of heat from the cable conductor. Space charge profiles in a 160 kV DC coaxial XLPE cable was measured under 10 kV/mm stress with polarity reversal. According to CIGRE TB496, the load cycle operation of the cable under 15 kV/mm with temperature difference of 40°C was performed for 48 and 72 h, including cable conductor cooling and heating. The results show that there is little space charge injection in the bulk at room temperature, but some remnant hetero-charges accumulates near the outer semi-conductive layer during voltage reversal. Under TG, hetero-charges appear near the outer semi-conductive layer on the low temperature side and hetero-charges increased with increasing TG. In addition, the hetero-charges increase during cable conductor heating and decrease during cooling.

Effect of combined addition of Cu51Zr14 inoculant and Ti element on the microstructure and damping behavior of a Cu-Al-Ni shape memory alloy

Grains of a Cu-Al-Ni shape memory alloy (SMA) were significantly refined by using the combined refining effect of Cu51Zr14 inoculant and Ti element. With the decrease of grain size, the damping capacity remarkably increased, and the Martensitic transformation (MT) temperatures shifted to low temperature side. Correlated mechanisms have been discussed.

Enhanced energy storage properties of BaTiO3-Bi0.5Na0.5TiO3 lead-free ceramics modified by SrY0.5Nb0.5O3

In this work, a novel lead-free relaxor-ferroelectric ceramics of (1-x)(0.65BaTiO3-0.35Bi0.5Na0.5TiO3)-xSrY0.5Nb0.5O3 ((1-x)BTBNT-xSYN x = 0.02–0.12), was designed and prepared by the conventional solid state reaction method. The results show that all of the sintered samples have a perovskite structure. The Curie Temperature (Tc) moves to the low temperature side which causes the high dielectric constant at room temperature, and the ferroelectric hysteresis loop becomes slimmer and slimmer with increasing the content of SYN. Meanwhile, the optimal energy storage properties can be obtained in 0.92BTBNT-0.08SYN ceramic with an impressive discharged energy density (Wrec) of 1.36 J/cm3 under 152 kV/cm, and the charge-discharge efficiency reaches 74.3%. Therefore, all these results indicate that (1-x)BTBNT-xSYN lead-free ceramics can be applied for pulsed power capacitors.

Thermal behaviour of two anti-inflammatory drugs (celecoxib and rofecoxib) and slow relaxation dynamics in their amorphous solid state. Comparison between the dynamic fragility obtained by dielectric spectroscopy and by thermostimulated currents

ABSTRACTThis study of the thermal behaviour of two glass formers showed that rofecoxib has a good ability to vitrify while celecoxib displays a moderate glass forming ability. On the other hand, celecoxib forms an instable glass while rofecoxib displays moderate glass stability. The α-relaxation of the two amorphous drugs was studied by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). The values of the dynamic fragility obtained by the two techniques are in good mutual agreement and suggest that both drugs are moderately fragile glass formers. These values are however considerably lower than those obtained by dielectric relaxation spectroscopy (DRS) and published in the literature. In the present work we will compare the values of the dynamic fragility obtained by TSDC and DRS for a set of ~30 glass-forming liquids, in order to evaluate their relative suitability for the determination of the m-index. The observed differences are discussed and interpreted. Finally, secondary mobility modes were detected by TSDC in both celecoxib and rofecoxib. The kinetic parameters are identical in both cases and similar to those found by DRS for the slow-b relaxation of celecoxib. These secondary relaxations, that are precursors of the main relaxation on its lower temperature side, are believed to be Johari-Goldstein relaxations.

Driven spin wave modes in XY ferromagnet: non-equilibrium phase transition

ABSTRACTThe dynamical responses of XY ferromagnet driven by linearly polarised propagating and standing magnetic field wave have been studied by Monte Carlo simulation in three dimensions. In the case of propagating magnetic field wave (with specified amplitude, frequency and the wavelength), the low temperature dynamical mode is a propagating spin wave and the system becomes structureless (or random) in the high temperature. A dynamical symmetry breaking phase transition is observed at a finite (non-zero) temperature. This symmetry breaking is confirmed by studying the statistical distribution of the angle of the spin vector. The dynamic non-equilibrium transition temperature was found to decrease as the amplitude of the propagating magnetic field wave increased. A comprehensive phase boundary is drawn in the plane formed by temperature and amplitude of propagating field wave. The phase boundary was observed to shrink (in the low temperature side) for longer wavelength of the propagating magnetic wave. In the case of standing magnetic field wave, the low temperature excitation is a standing spin wave which becomes structureless (or random) in the high temperature. Here also, like the case of propagating magnetic wave, a dynamical symmetry breaking non-equilibrium phase transition was observed. A comprehensive phase boundary was drawn. Unlike the case of propagating magnetic wave, the phase boundary does not show any systematic variation with the wavelength of the standing magnetic field wave. In the limit of vanishingly small amplitude of the field, the phase boundaries approach the recent Monte Carlo estimate of equilibrium transition temperature.

Thermoluminescence behaviour of GdAlO3:Yb3+ under gamma exposure

Thermoluminescence behaviour of Yb3+ doped GdAlO3 was studied for solution combustion synthesised phosphor. The TL glow curve was recorded under gamma radiation. The Tl glow curve was recorded for various heating rate for constant 0.25 kGy gamma radiation to optimize the heating rate. The optimized heating rate was obtained at 4 °C s−1. The effect of gamma dose on TL behaviour of the phosphor was studied by recording the TL for different gamma radiation dose from 0.25 to 1.25 kGy. For lower gamma dose the TL glow peak consists kinked peak with Tm at 397 and 432 K. As the gamma dose increases the Tm at lower temperature side vanished and the TL glow curve consists only one peak of higher temperature i.e. 432 K. To explain the variation in TL behaviour with variable gamma dose the TL glow curve for different gamma dose was analysed by using computerized glow curve deconvolution method. The kinetic parameters were calculated for all the deconvoluted peaks by using Chen’s peak shape method. Activation energy for higher temperature peak is more than lower temperature peak due to deeper and shallow traps respectively.

Enhanced persistent red luminescence in Mn2+-doped (Mg,Zn)GeO3 by electron trap and conduction band engineering

The effect of Zn substitution on the persistent luminescence properties of MgGeO3:Mn2+-Ln3+ (Ln = Eu and Yb) red phosphors was investigated. The intensity of the persistent luminescence of the Eu3+ co-doped phosphors increased with increasing Zn content, whereas that of the Yb3+ co-doped samples decreased. For both series of lanthanide co-doped samples, the thermoluminescence (TL) glow peak shifted to the lower temperature side with increasing Zn content. These persistent luminescence properties were well explained in terms of lowering of the bottom of the conduction band relative to the ground state of the divalent lanthanide ions. Especially, in Eu3+ co-doped system, TL peak shifted from 520 K to 318 K by 50% Zn substitution. The persistent radiance of the (Mg0.5 Zn0.5)GeO3: Mn2+-Eu3+ sample at 1 h after ceasing UV light was 46 times stronger than that of MgGeO3:Mn2+-Eu3+, and 11 times stronger than that of ZnGa2O4: Cr3+ standard deep red persistent phosphor.

Probing critical behavior of 2D Ising ferromagnet with diluted bonds using Wang-Landau algorithm

Randomness is an important subject in the study of phase transition as defect and impurity may present in any real material. The pre-existing ordered phase of a pure system can be affected or even ruined by the presence of randomness. Here we study ferromagnetic Ising model on a square lattice with a presence of randomness in the form of bond dilution. The pure system of this model is known to experience second order phase transition, separating between the high temperature paramagnetic and low-temperature ferromagnetic phase. We used Wang-Landau algorithm of Monte Carlo method to obtain the density of states from which we extract the ensemble average of energy and the specific heat. We observed the signature of phase transition indicated by the diverging peak of the specific heat as system sizes increase. These peaks shift to the lower temperature side as the dilution increases. The lower temperature ordered phase preserves up to certain concentration of dilution and is totally ruined when the bonds no longer percolates.

Effect of NiO substitution on the structural and dielectric behaviour of NaNbO3

The structural and dielectric properties of NiO substituted NaNbO3 ceramics are reported. The orthorhombic (Pmna) crystal structure of NaNbO3 transforms to a lower symmetry monoclinic phase (Pbma) after the dilute dispersion of NiO. X-ray photoelectron spectroscopy reveals pentavalent “Nb,” monovalent “Na,” and divalent “Ni” states along with the signatures of non-local screening effects. The antiferroelectric to paraelectric transition (TAFE) accompanied by a structural change from the orthorhombic to the tetragonal phase shifts by 55 °C toward the low-temperature side, whereas the morphotropic phase boundary (TO-M) moves toward a higher temperature by 28 °C for nominal substitutions (x≤0.10). The generalized Lyddane-Sachs-Teller expression (ε0−S′ε∞)=(ωlωt)2 and thermodynamic free energy models are employed to explain the anomalous behaviour of the temperature dependence of relative dielectric permittivity (εr(T)) across TAFE and TO-M. The frequency dependence of ac-conductivity σac(ω) follows the Jonscher power law (σac = σ(0) + Aωs), suggesting the dominance of the phonon-assisted hopping mechanism, whereas the frequency independent term (σ(0)) was explained by Funke's Jump-Relaxation Model.

Deformation and Stresses generated on a Bolted Flange Joint Assembly and Grayloc Clamp Connector at Elevated Temperatures

Finite element analyses were performed to investigate the leakage and structural integrity of the bolted flange joint assembly (BFJA) and Grayloc clamp connector (GCC), when each assembly was subjected to bolt preload, internal pressure and thermal loadings. The thermal loadings were divided into spatially-uniform and spatially-nonuniform temperatures. The internal pressure for both assemblies was 27 MPa. The BFJA was found to have satisfactory leakage and structural performance for the thermal loadings expected on the low-temperature (260 °C) side of the Carleton Supercritical Water (CSCW) loop. The GCC was found to have satisfactory leakage and structural performance for the thermal loadings expected on the high-temperature (600 °C) side of the CSCW loop. The leakage integrity of the GCC was found to fail for temperature gradients as high as 100 °C. The use of the GCC on the exit of the test section employed on the CSCW loop was not recommended.

Inert atmosphere processing of hydroxyapatite in the presence of lithium iron phosphate

The present study describes sintering behaviour of hydroxyapatite (HAp) upon addition of lithium iron phosphate (LFP) (1–10 wt.%) system in inert (Ar) atmosphere. The interaction between materials and melting of LFP influenced early and intermediate stages of HAp sintering, shifting the densification curves towards low-temperature side. Analysis of densification process indicated significant differences upon LFP addition. The reaction mechanism that assumes the initial interaction between phosphates from LFP and calcium from HAp was proposed, generating calcium vacancies and contributing to HAp densification. Cross-sections of sintered samples showed changes in microstructural properties, with uniform atomic distribution and presence of Li2Fe3O4 spherical inclusions (200 nm) located at grain boundaries of calcium phosphate matrix. The Rietveld refinement analysis indicated changes in structural and microstructural parameters like crystallite size, anisotropy and microstructural strain of HAp upon LFP addition. Mechanical characterisation indicated improvements in fracture behaviour upon LFP addition.

Improved mechanical properties of bitumen modified with acetylated cellulose fibers

Abstract Temperature sensitivity of bitumen modified with acetylated microfibrillated cellulose (MFCac), both in the low and high temperature ranges, is significantly decreased. The complex shear modulus for an MFCac content of 2 wt% was 10 times higher than for bitumen alone. In addition the phase angle at high temperatures indicates increased elastic behavior that results in favorable properties with respect to rutting. On the low temperature side (−25 °C), the deformation to failure in fracture toughness tests was around 100 times higher with MFCac modified bitumen than for the non-modified one whereas the failure forces were similar.

Structural, dielectric, electromechanical, piezoelectric, elastic and ferroelectric properties of lanthanum and sodium co-substituted barium titanate ceramics

In this paper, the polycrystalline ceramics Ba(1-x) (La, Na)xTiO30≤x≤0.08 were synthesized by microwave assisted heating of the starting materials. Room temperature X-ray diffraction along with Rietveld refinement analysis confirmed the formation of single-phase polycrystalline compound with the tetragonal crystal structure. A dense microstructure with several grains having different size distribution throughout the sintered pellet has been observed. The incorporation of La3+ and Na+ ions in BaTiO3 enhanced the room temperature relative permittivity. The studies of the temperature dependence of dielectric permittivity indicate ferroelectric-paraelectric phase transition with a clear shift in the Curie temperature (Tc) of BaTiO3 towards lower temperature side on co-substitution. The co-substituted sample with x = 0.08 exhibited superior dielectric constant (ɛ' = 4226) with low dielectric loss (tanδ = 0.0113) which is highly essential for the fabrication of ceramic capacitors. Resonance and antiresonance characteristics of impedance and phase for poled Ba(1-x)(La,Na)xTiO3 samples have been investigated in the frequency range of 300 kHz-1 MHz. Based on IRE standards for piezoelectric ceramics the piezoelectric response has been discussed. The room temperature P-E loop is investigated at an applied electric field of 80 kV/cm at 33 Hz frequency using Precision Premier II.

Structural and dielectric relaxor properties of Ba1-xMgxTiO3 ceramics prepared through a hydrothermal route

ABSTRACTPolycrystalline samples of lead-free Ba1-xMgxTiO3 (where x = 0.05, 0.10, 0.15) have been fabricated by a hydrothermal method. Single-phase perovskite-type X-ray diffraction patterns were observed and samples have a phase with a tetragonal structure at room temperature for all compositions. The SEM image of the powder shows the agglomeration with increment of Mg concentrations. Small grains with average grain sizes of ∼20 ± 5 nm are uniformly distributed over the mesocrystals. TEM specimens show well-dispersed spherical barium magnesium titanate nanoparticles and diameter of these particles is around 270–300 nm. A dielectric study of Ba1-xMgxTiO3 ceramics as a function of temperature suggested that with increasing substitution concentration the dielectric constant decreased and the Curie temperature shifted towards the lower temperature side. The dielectric diffusivity (γ) was found to be maximum (1.86) for substitution of x = 0.10 in Ba1-xMgxTiO3 compounds. The activation energy (Ea) was found to increases with increasing substitution concentration in BMT compounds.

Effect of sintering temperature on structural, magnetic and electrical transport properties of La0.67Ca0.33MnO3 ceramics prepared by Plasma Activated Sintering

La0.67Ca0.33MnO3 (LCMO) ceramics were prepared by Plasma Activated Sintering (PAS). The effect of sintering temperature on structural, magnetic and electrical transport properties of the LCMO ceramics were investigated. XRD and SEM characterizations of the LCMO ceramics indicated that all samples exhibited single orthorhombic phase. Also, through PAS dense LCMO ceramics were obtained. Certain interesting phenomena were observed from the PAS sintering temperature effect on the magnetic and transport behaviors of LCMO ceramics, which were significantly different from traditional sintering methods The magnetization decreased and the resistivity increased as the sintering temperature increased, whereas the TC and TIM shifted towards the lower temperature side. The conduction mechanism of all samples was electron-magnon scattering process in the ferromagnetic low-temperature region, while the paramagnetic high-temperature region mechanism was explained with the variable range hopping (VRH) model. All these behaviors were explained through the oxygen vacancy effect in combination with the grain size effect.