Temperature Dependence Research Articles

Trap states and carrier diffusion lengths in NiO/β-Ga2O3 heterojunctions

We report the electrical properties, deep trap spectra, and diffusion lengths of non-equilibrium carriers in Ni Schottky diodes and NiO/Ga2O3 heterojunctions (HJs) prepared on the same n−/n+ β-Ga2O3 epi structures. The heterojunctions decrease the reverse current of Ga2O3 high-power rectifiers. In HJs, in contrast to Schottky diodes, the capacitance and AC conductance show a prominent frequency and temperature dependence, suggesting the presence of two temperature activation processes with activation energies of 0.17 and 0.1 eV. The deep trap spectra of the Schottky diodes and HJs differ by the absence in the HJ of deep electron traps E2* with level near Ec − 0.7 eV considered to be an important center of non-radiative recombination. This correlates with the observed increase in the diffusion length of non-equilibrium charge carriers in the HJs to 370 nm compared to 240 nm in the Schottky diodes. The diffusion length of charge carriers in p-NiO was found to be quite short, 30 nm. Possible reasons for the observed differences and possible origin of the minority-trap-like feature commonly reported to be present in the deep level spectra of HJs and also observed in our experiments are discussed.

Improved power and temperature performance of half-disk diode microlasers.

The power and temperature characteristics of Ø200 µm half-disk microlasers with a half-ring metal contact and high-density InGaAs/GaAs quantum dots are studied. In a continuous wave (CW) mode, the maximal optical power at 20°C was 134 mW, and the maximal CW lasing temperature reached 113°C. In a pulsed regime the maximal optical power of 1.6 W, limited by catastrophic degradation, was achieved. By comparing the CW and pulsed current-voltage characteristics, the dependence of a microlaser temperature on CW pumping current was determined. At CW currents corresponding to the maximal wall-plug efficiency, the maximal optical power, and complete lasing quenching, the laser temperatures were 60, 99, and 149°C, respectively.

Temperature effects in narrow-linewidth optical cavity control with a surrogate quasi-second-harmonic field

Fabry–Perot cavities are widely used in precision interferometric applications. Various techniques have been developed to achieve the resonance condition via the direct interrogation of the cavity with the main laser field of interest. Some use cases, however, require a surrogate field for cavity control. In this study, we construct a bichromatic cavity to study the surrogate control approach, where the main and the surrogate fields are related by the second-harmonic generation with nonlinear optics. We experimentally verify the temperature dependence of the differential reflection phase of a dielectric coating design optimized for the surrogate control approach of the optical cavities of the light-shining-through-a-wall experiment Any Light Particle Search II and develop a comprehensive cavity model for quasi-second-harmonic resonances that considers also other important factors, such as the Gouy phase shift, for a detailed analysis of the surrogate control approach.

Study of the Effects of Remote Heavy Group Vibrations on the Temperature Dependence of Hydride Kinetic Isotope Effects of the NADH/NAD+ Model Reactions.

It has recently been observed that the temperature(T)-dependence of KIEs in H-tunneling reactions, characterized by isotopic activation energy difference (ΔEa = EaD - EaH), is correlated to the rigidity of the tunneling ready states (TRSs) in enzymes. A more rigid system with narrowly distributed H-donor-acceptor distances (DADs) at the TRSs gives rise to a weaker T-dependence of KIEs (i.e., a smaller ΔEa). Theoreticians have attempted to develop new H-tunneling models to explain this, but none has been universally accepted. In order to further understand the observations in enzymes and provide useful data to build new theoretical models, we have studied the electronic and solvent effects on ΔEa's for the hydride-tunneling reactions of NADH/NAD+ analogues. We found that a tighter charge-transfer (CT) complex system gives rises to a smaller ΔEa, consistent with the enzyme observations. In this paper, we use the remote heavy group (R) vibrational effects to mediate the system rigidity to study the rigidity-ΔEa relationship. The specific hypothesis is that slower vibrations of a heavier remote group would broaden the DAD distributions and increase the ΔEa value. Four NADH/NAD+ systems were studied in acetonitrile but most of such heavy group vibrations do not appear to significantly increase the ΔEa. The remote heavy group vibrations in these systems may have not affected the CT complexation rigidity to a degree that can significantly increase the DADs, and further, the ΔEa values.

A Self‐assembly of New Cobalt Coordinating Polymer Based on Fluorinated Secondary Building Unit and Pyrazine

The direct two‐step synthesis of new coordination polymer [Co2(pyz)(tfa)2(dmf)6(H2O)][Co3(μ3‐F)(tfa)6(Htfa)2(pyz)(dmf)]2·2dmf, (where pyz = pyrazine, Htfa = trifluoroacetic acid, dmf = dimethyl formamide) containing the fluoro‐centered secondary building block [Co3(μ3‐F)(tfa)6], is reported. The substance characterization using single‐crystal XRD, powder XRD, IR‐spectroscopy and thermogravimetry was performed. SCXRD data revealed the presence of cationic and anionic subsystems, both containing cobalt(II) ions and pyrazine bridging ligands. The magnetic proper‐ties of the compound were collected and discussed. Temperature dependence of magnetic susceptibility shows Curie – Weiss like behavior at all studied temperatures.

PARAMETERS OF THE PERTURBED STATE OF THE DENDRITE TIP IN DEEPLY SUPERCOOLED NICKEL AND COPPER MELTS

The object of study is the morphological stability of the tip of a free dendrite in a supercooled melt of a pure substance. Areas of deep supercooling are considered: for nickel DT > 166 K, for copper – DT > 180 K. A distinctive feature of the processes being studied is the presence of two propagation velocities of small disturbances (velocities of “sound”). The dependences of these rates on the supercooling of the melt were determined. The periodic and coordinate-aperiodic modes of perturbation of the growth line have been studied in detail. For these stable regimes, the possibility of observing the same speed of the disturbance wave in two processes, differing from each other in the size of the spatial inhomogeneity of the background in front of the wave and the characteristic wave attenuation times, was discovered. It is shown that aperiodic instability appears if, after the passage of the wave front, the width of the growth line inhomogeneity zone decreases. The resonant excitation mode of the dendrite tip demonstrates important differences between the properties of the nickel and copper melts. First of all, this relates to the temperature dependences of the resonant frequency and the speed of a standing wave formed in the vicinity of the vertex. Numerical calculations are presented and graphic information is presented illustrating the patterns of growth of nickel and copper dendrites.

Microstructure and Physico-Mechanical Properties of Biocompatible Titanium Alloy Ti-39Nb-7Zr after Rotary Forging

The evolution of microstructure, phase composition and physico-mechanical properties of the biocompatible Ti-39Nb-7Zr alloy (wt.%) after severe plastic deformation by rotary forging (RF) was studied using various methods including light optical microscopy, scanning and transmission electron microscopies, X-ray diffraction, microindentation, tensile testing and investigation of thermophysical properties during continuous heating. The hot-rolled Ti-39Nb-7Zr with initial single β-phase structure is subjected to multi-pass RF at 450 °C with an accumulated degree of true deformation of 1.2, resulting in the formation of a fibrous β-grain structure with imperfect 500 nm subgrains characterized by an increased dislocation density. Additionally, nano-sized α-precipitates formed in the body and along the β-grain boundaries. These structural changes resulted in an increase in microhardness from 215 HV to 280 HV and contact modulus of elasticity from 70 GPa to 76 GPa. The combination of strength and ductility of Ti-39Nb-7Zr after RF approaches that of the widely used Ti-6Al-4V ELI alloy in medicine, however, Ti-39Nb-7Zr does not contain elements with limited biocompatibility and has a modulus of elasticity 1.5 times lower than Ti-6Al-4V ELI. The temperature dependences of physical properties (elastic modulus, heat capacity, thermal diffusivity) of the Ti-39Nb-7Zr alloy after RF are considered and sufficient thermal stability of the alloy up to 450 °C is demonstrated.

Distinguishing Quasiparticle-Phonon Interactions by Ultrahigh-Resolution Lifetime Measurements.

We present a determination of quasiparticle-phonon interaction strengths at surfaces through measurements of phonon spectra with ultrahigh energy resolution. The lifetimes of low energy surface phonons on a pristine Ru(0001) surface were determined over a wide range of temperatures and an analysis of the temperature dependence enables us to attribute separate contributions from electron-phonon interactions, phonon-phonon interactions, and defect-phonon interactions. Strong electron-phonon interactions are evident at all temperatures and we show they dominate over phonon-phonon interactions below 400K.

Phase transition thermodynamics of organic semiconductors: N,N,N’,N’-tetraphenylbenzidine and 4,4′-bis(N-carbazolyl)-1,1′-biphenyl

In this work, we report a comprehensive study of the phase change thermodynamics of two compounds, N,N,N’,N’-tetraphenylbenzidine (TPB) and 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), used in the production of organic light-emitting diodes and photovoltaics. The vapor pressures and phase transition enthalpies of these compounds have been previously studied at the elevated temperatures which may not be relevant to the conditions during fabrication and use TPB- and CBP-based materials. In this work, a combination of experimental and computational methods was employed to establish the reliable temperature dependences of the sublimation, fusion, and vaporization enthalpies of TPB and CBP in a wide temperature range. The vapor pressures of both compounds over solid and liquid phases were measured using fast scanning calorimetry. The fusion enthalpies at the melting point were determined by DSC. The isobaric heat capacities of the compounds in crystalline and liquid phases, including the supercooled liquids, were measured using scanning calorimetry. For the first time, the heat capacities of the ideal gas TPB and CBP were computed, considering internal rotation effects. The solution enthalpies in benzene at 298.15 K were measured, providing an independent estimate of the fusion enthalpy temperature dependence. Its consistency with the data on the fusion enthalpies was confirmed using Hess’s and Kirchhoff’s laws of Thermochemistry. Using the obtained results, the available literature data were critically evaluated. The obtained thermodynamic parameters may facilitate optimization of vapor deposition conditions and description of the thermodynamic and kinetic stability of the glassy state of TPB and CBP.

Laboratory and Field Performance Evaluation of Cracking of Airfield Warm Mix Asphalt with Reclaimed Asphalt Pavement at the National Airport Pavement and Materials Research Center

The U.S. Federal Aviation Administration is exploring initiatives to decrease transportation greenhouse gas emissions by developing carbon reduction strategies, including using low-embodied carbon materials, such as reclaimed asphalt pavement (RAP), in airport pavement construction. However, verifying RAP’s viability for airfield pavements with laboratory and field performance measures is essential. This study compared the cracking susceptibility of a P-401 hot mix asphalt and warm mix asphalt (WMA)+RAP airfield mixes using laboratory performance tests and conducted a preliminary evaluation of pavement responses from accelerated pavement testing (APT) fatigue tests. The observations and outcomes presented in this paper found that adding 20% of recycled materials with WMA additives had a decreasing impact in mix performance but did not overly affect the asphalt mix properties, as cracking properties from the different test procedures were found with comparable outcomes. Additional observations from strain gauges and temperature probes in the National Airport Pavement and Materials Research Center test cycle 2 APT fatigue test sections showed that the maximum tensile strains in the WMA surface lane 5 south section were consistently higher than those in the WMA+RAP lane 6 south section, which may be because of the higher load level that it was exposed. Additionally, the hardened or aged RAP binder in the entire lane 6 south section may have increased its stiffness, resulting in lower strain levels than in the lane 5 south section. Notably, both sections showed an increase in tensile strains with an increase in asphalt concrete temperature, which confirmed the temperature dependency behavior of asphalt concrete.

Confined Water Dynamics in the Scaffolds of Polylactic Acid.

Resorbable polylactic acid (PLA) ultrathin fibers have been applied as scaffolds for tissue engineering applications due to their micro- and nanoporous structure that favor cell adhesion, besides inducing cell proliferation and upregulating gene expression related to tissue regeneration. Incorporation of multiwalled carbon nanotubes into PLA fibers has been reported to increase the mechanical properties of the scaffold, making them even more suitable for tissue engineering applications. Ideally, scaffolds should be degraded simultaneously with tissue growth. Hydration and swelling are factors related to scaffold degradation. Hydration would negatively impact the mechanical properties since PLA shows hydrolytic degradation. Water absorption critically affects the catalysis and allowance of the hydrolysis reactions. Moreover, either mass transport and chemical reactions are influenced by confined water, which is an unexplored subject for PLA micro- and nanoporous fibers. Here, we probe and investigate confined water onto highly porous PLA microfibers containing few amounts of incorporated carbon nanotubes by Fourier transform infrared (FTIR) spectroscopy. A hydrostatic pressure was applied to the fibers to enhance the intermolecular interactions between water molecules and C=O groups from polyester bonds, which were evaluated over the wavenumber between 1600 and 2000 cm-1. The analysis of temperature dependence of FTIR spectra indicated the presence of confined water which is characterized by a non-Arrhenius to Arrhenius crossover at T0 = 190 K for 1716 and 1817 cm-1 carbonyl bands of PLA. These bands are sensitive to a hydrogen bond network of confined water. The relevance of our finding relies on the challenge detecting confined water in hydrophobic cavities as in the PLA one. To the best of our knowledge, we present the first report referring the presence of confined water in a hydrophobic scaffold as PLA for tissue engineering. Our findings can provide new opportunities to understand the role of confined water in tissue engineering applications. For instance, we argue that PLA degradation may be affected the most by confined water. PLA degradation involves hydrolytic and enzymatic degradation reactions, which can both be sensitive to changes in water properties.

A Simplified Correlation for Burning Velocity of Gaseous Fuel-Air Mixtures

Using ideas of burning velocity dependence on the adiabatic flame temperature at any mixture ratio (equivalence ratio), and the behavior of pressure and initial temperature dependences, simple correlations have been obtained for hydrocarbons and alcohols, hydrogen air as well as syngas compositions. These exploit simple behavior of the burning velocities with adiabatic flame temperature with equivalence ratio, and for a combination of equivalence ratio and a specific compositional parameter and the pressure dependence is taken with the pressure exponent as −0.3 for hydrocarbon-air and 0 for hydrogen air compositions in a mean sense since the dependence varies slightly over the other parameters like equivalence ratio and initial temperature. Observed behavior also brings out limited universality in the behavior. In the case of syngas compositions, a parameter that depends on the mixture composition that treats even carbon dioxide as an inert seems to provide reasonable predictions for a wide range of compositions. One anomaly that comes up is that methyl alcohol appears distinctly more reactive (by 15%) than methane and deserves examination since the results at higher initial temperatures seem to follow the trend noted in the general correlation.

Body Size, Cerebral Blood Flow, Ambient Temperature, and Relative Brain Temperatures in Newborn Infants under Incubator Care.

Subtle changes in body temperature affect the outcomes of ill newborns. However, the temperature profile of neonatal brains remains largely unknown. In open-cot care, increased cerebral perfusion is correlated with higher superficial brain temperatures. This study investigated the dependence of brain temperature (relative to rectal temperature) on ambient temperature, body size, cerebral perfusion, and metabolism in infants receiving incubator care. Rectal, scalp, and brain temperatures, superior vena cava flow, and brain oxygenation were assessed using echocardiography, thermo-compensatory temperature monitoring, and near-infrared spectroscopy in 60 newborns. These infants had a mean postconceptional age of 36.9 (2.2) weeks and weighed 2348 (609) g at the time of evaluation. The ambient temperature was maintained at 30.0 (1.0) °C. A higher rectal temperature was associated with greater postconceptional age (p = 0.002), body weight (p < 0.001), and head circumference (p < 0.001). Relative scalp, superficial brain, and deep brain temperatures were associated with smaller head circumference (p < 0.001, p = 0.030, and p = 0.015, respectively) and superior vena cava flow (p = 0.002, p = 0.003, and p = 0.003, respectively). In infants receiving incubator care, larger head sizes and increased brain perfusion were associated with lower relative scalp and brain temperatures. When considered alongside previous reports, cerebral perfusion may contribute to maintaining stable cerebral tissue temperature against ambient temperature changes.

The coexistence and competition of charge-density wave and superconductivity in the electron-doped H- MX2 (M = Mo, W and X=S, Se)

In the group VI transition metal dichalcogenides (TMDs), an insulator family with two-dimensional structure, MoS2 has recently demonstrated to be involved in charge-density wave (CDW) and superconductivity (SC). Nevertheless, there is a lack of a cohesive theoretical framework and fragmentation in the existing theoretical models of the relationship between CDW and SC for this system. Base on first-principles calculations, the effects of electron doping on the electronic properties, electron-phonon interaction and SC are investigated comprehensively for monolayer H- MX2 (M = Mo, W and X = S, Se) here. We confirmed that two CDW phases, 23×23 and 2 × 2, are originated from Fermi surface nesting (FSN) and electron-phonon coupling (EPC), respectively. A complicated phase diagram of doping concentration dependencies of CDW and SC critical temperatures (TC) is created, by SC coexisting with 23×23 CDW phase while competing with 2 × 2 CDW phase. The physical mechanisms of such coexistence and competition are also explained here. Our results extend the theoretical investigation by providing a comprehensive and in-depth analysis of CDW and SC in the group VI TMDs, and open up an alternative path for the exploration and regulation of collective phases of two-dimensional materials.

Plant-Mediated Synthesis of Magnetite Nanoparticles with Matricaria chamomilla Aqueous Extract.

Magnetite nanoparticles (NPs) possess properties that make them suitable for a wide range of applications. In recent years, interest in the synthesis of magnetite NPs and their surface functionalization has increased significantly, especially regarding their application in biomedicine such as for controlled and targeted drug delivery. There are several conventional methods for preparing magnetite NPs, all of which mostly utilize Fe(iii) and Fe(ii) salt precursors. In this study, we present a microwave hydrothermal synthesis for the precipitation of magnetite NPs at temperatures of 200 °C for 20 min and 260 °C for 5 min, with only iron(iii) as a precursor utilizing chamomile flower extract as a stabilizing, capping, and reducing agent. Products were characterized using FTIR, PXRD, SEM, and magnetometry. Our analysis revealed significant differences in the properties of magnetite NPs prepared with this approach, and the conventional two-precursor hydrothermal microwave method (sample MagH). FTIR and PXRD analyses confirmed coated magnetite particles. The temperature and magnetic-field dependence of magnetization indicate their superparamagnetic behavior. Importantly, the results of our study show the noticeable cytotoxicity of coated magnetite NPs-toxic to carcinoma cells but harmless to healthy cells-further emphasizing the potential of these NPs for biomedical applications.

Anomalous thermal transport and high thermoelectric performance of Cu-based vanadate CuVO3

Thermoelectric (TE) conversion technology, capable of transforming heat into electricity, is critical for sustainable energy solutions. Many promising TE materials contain rare or toxic elements, so the development of cost-effective and eco-friendly high-performance TE materials is highly urgent. Herein, we explore the thermal transport and TE properties of transition metal vanadate CuVO3 by using first-principles calculation. On the basis of the unified theory of heat conduction, we uncover the hierarchical thermal transport feature in CuVO3, where wave-like tunneling makes a significant contribution to the lattice thermal conductivity (κl) and results in the anomalously weak temperature dependence of κl. This is primarily attributable to the complex phononic band structure caused by the heterogeneity of Cu–O and V–O bonds. Simultaneously, we report a high power factor of 5.45 mW·K−2·m−1 realized in hole-doped CuVO3, which arises from a high electrical conductivity and a large Seebeck coefficient enabled by the multiple valleys and large electronic density of states near the valence band edge. Impressively, the low κl and the high power factor make p-typed CuVO3 have ZT of up to 1.39, with the excellent average ZT above 1.0 from 300 to 600 K, which is superior to most reported Cu-based TE materials. Our findings suggest that the CuVO3 compound is a promising candidate for energy conversion applications in innovative TE devices.

Estimating the effects of temperature on transmission of the human malaria parasite, Plasmodium falciparum

Despite concern that climate change could increase the human risk to malaria in certain areas, the temperature dependency of malaria transmission is poorly characterized. Here, we use a mechanistic model fitted to experimental data to describe how Plasmodium falciparum infection of the African malaria vector, Anopheles gambiae, is modulated by temperature, including its influences on parasite establishment, conversion efficiency through parasite developmental stages, parasite development rate, and overall vector competence. We use these data, together with estimates of the survival of infected blood-fed mosquitoes, to explore the theoretical influence of temperature on transmission in four locations in Kenya, considering recent conditions and future climate change. Results provide insights into factors limiting transmission in cooler environments and indicate that increases in malaria transmission due to climate warming in areas like the Kenyan Highlands, might be less than previously predicted.

Optimized dielectric mirror coating designs for quasi-harmonic cavity resonance

High-finesse optical cavities have a wide range of applications, some of which are bichromatic. The successful operation of high-finesse bichromatic cavities can demand careful control on the temperature dependence of the wavelength-dependent reflection phase from the dielectric mirror coatings that constitute the optical cavity. We present dielectric coating designs that are optimized for minimal differential change in the reflection phase between a quasi-second-harmonic field and its fundamental field under temperature changes. These designs guarantee cavity resonance at a wavelength of interest via the control of its quasi-harmonic field. The proposed coating designs are additionally examined for their sensitivity to manufacturing errors in the coating layer thickness with promising results.

Source–drain contact impacts on electrical performances and low frequency noise of InZnO thin-film transistors down to 7 K

Source–drain contacts seriously affect low frequency noise (LFN) in amorphous IZO (a-IZO) thin-film transistors with bilayer ITO/Mo electrodes at cryogenic temperatures. In the range of 7–300 K, electrical and LFN performances of devices are measured, and the temperature dependence of channel resistances (Rch) and contact resistances (Rsd) is also analyzed. The carrier transport transition both occurs in a-IZO channels at 80 K and in ITO/Mo electrodes at 100 K, which causes the variation in Rsd and Rch. Below 80 K, the variable range hopping conduction dominates in the channels and contacts with Rsd/Rch ratio &amp;gt; 1. The LFN measured results indicate that the noise is contact-dominated. At higher temperatures above 120 K, the band conduction dominates in the channel, with Rsd/Rch ratio &amp;lt; 1, and the noise is channel-dominated. The main mechanism of noise transfers from the mobility fluctuation to the carrier number fluctuation. We consider that the contribution of contact noise to total LFN is determined by the Rsd/Rch ratio. This work provides comments for the selection of electrode materials in devices applied to cryogenic integration circuits.

Investigation of cryogenic current–voltage anomalies in SiGe HBTs: Role of base–emitter junction inhomogeneities

The deviations of cryogenic collector current–voltage characteristics of SiGe heterojunction bipolar transistors (HBTs) from ideal drift-diffusion theory have been a topic of investigation for many years. Recent work indicates that direct tunneling across the base contributes to the non-ideal current in highly scaled devices. However, cryogenic discrepancies have been observed even in older-generation devices for which direct tunneling is negligible, suggesting that another mechanism may also contribute. Although similar non-ideal current–voltage characteristics have been observed in Schottky junctions and were attributed to a spatially inhomogeneous junction potential, this explanation has not been considered for SiGe HBTs. Here, we experimentally investigate this hypothesis by characterizing the collector current ideality factor and built-in potential of a SiGe HBT vs temperature using a cryogenic probe station. The temperature dependence of the ideality factor and the relation between the built-in potential as measured by capacitance–voltage and current–voltage characteristics are in good qualitative agreement with the predictions of a theory of electrical transport across a spatially inhomogeneous junction. These observations suggest that inhomogeneities in the base–emitter junction potential may contribute to the cryogenic non-idealities. This work helps to identify the physical mechanisms limiting the cryogenic microwave noise performance of SiGe HBTs.