Low Temperature Region Research Articles

Carrier transport across PtSe2/n-type GaN heterojunction

We fabricated PtSe2/n-type GaN heterojunction diode by transferring a large-scale grown two-dimensional (2D) PtSe2 layered film onto a 3D GaN substrate. The PtSe2 film transferred onto GaN was ∼5.84-nm thick and had 11 layers with interlayer spacing of 0.52 nm. An exploration of the temperature-dependence of the current–voltage (I–V) characteristics of the PtSe2/n-type GaN heterojunction in the range 125–400 K revealed a strong dependency of the ideality factor and barrier height on measurement temperature. This indicates a presence of low- and high-barrier-height patches resembling barrier inhomogeneities along the interface. A barrier inhomogeneity analysis with a consideration for the Gaussian barrier heights distribution demonstrated the existence of a double Gaussian barrier distribution having a mean barrier height of 0.97 and 1.41 eV in low- and high-temperature regimes, respectively. An examination of the temperature-dependency of reverse leakage current indicated the dominance of variable-range hopping conduction in lower temperature region (<225 K) and Poole–Frenkel emission mechanism in higher temperature region (>250 K). The interface state density Nss of the PtSe2/n-type GaN heterojunction determined from the capacitance method was lesser than that attained from the forward I–V characteristics that can be accredited to the inhomogeneous spreading of the Nss.

Simultaneous control of mechanical strength and hierarchical structure in freeze‐casted porous alumina by two‐step sintering

AbstractTo increase mechanical strength of porous ceramics, here, an effective two‐step sintering (TSS) technique capable of producing highly porous alumina with enhanced mechanical strength is suggested. Based on the sintering theories, here, a significantly lower activation energy for densification at low temperature region allowed a beneficial temperature range for the TSS to be deduced. With a specific TSS regime (T1 = 1550°C and T2 = 1400°C), significantly higher compressive strength levels (8.00–15.24 MPa) were measured with an apparent porosity of 56.49% compared to conventional sintering (1.03–1.86 MPa) with similar apparent porosity of 57.84%. Specifically, another TSS regime (T1 = 1550°C and T2 = 1380°C) left submicron‐sized open pores within the lamellar walls, providing a hierarchical porous structure with enhanced mechanical strength. An evaluation of the mechanical stability by a finite element analysis indicated outstanding compressive strength even with small pores in the lamella walls.

Pyrolysis characteristics and kinetics of Prunus avium L. leaves using thermogravimetric analysis

Differences in the pyrolysis characteristics of leaves of sweet cherry tree (Prunus avium L.) under rain-shelter cultivation (RS) or under open-field cultivation (CK) were analysed using thermogravimetry (TG), derivative thermogravimetry (DTG), and differential scanning calorimetry (DSC) at three heating rates of 10, 20, and 30 °C·min-1. There were two obvious mass loss peaks at 280 °C and 330 °C, which were manifested by the slow pyrolysis of hemicellulose in the low temperature region and the rapid pyrolysis of cellulose in the high temperature region, respectively. The curve in the pyrolysis range after 440 °C was stable, and the mass change corresponded to the pyrolysis of a small amount of macromolecular organic extracts and inorganic salts. When the temperature reached 600 °C, approximately 69% and 73% of the CK and RS leaves were thermally destroyed, respectively. The Coats-Redfern method was used for kinetic calculations to obtain an activation energy of 29.8 to 36.1 kJ·mol-1 in the first-order pyrolysis kinetics stage. The second-order pyrolysis kinetics stage can fit the pyrolysis process well. A significant difference was observed in the pyrolysis characteristics or the kinetics between CK and RS, which were related to the heating rate and the hemicellulose content, cellulose content, and lignin ratio in each sample.

Statistical structural inference from edge weights using a mixture of gamma distributions

Abstract The inference of reliable and meaningful connectivity information from weights representing the affinity between nodes in a graph is an outstanding problem in network science. Usually, this is achieved by simply thresholding the edge weights to distinguish true links from false ones and to obtain a sparse set of connections. Tools developed in statistical mechanics have provided particularly effective ways to locate the optimal threshold so as to preserve the statistical properties of the network structure. Thermodynamic analogies together with statistical mechanical ensembles have been proven to be useful in analysing edge-weighted networks. To extend this work, in this article, we use a statistical mechanical model to describe the probability distribution for edge weights. This models the distribution of edge weights using a mixture of Gamma distributions. Using a two-component Gamma mixture model with components describing the edge and non-edge weight distributions, we use the Expectation–Maximization algorithm to estimate the corresponding Gamma distribution parameters and mixing proportions. This gives the optimal threshold to convert weighted networks to sets of binary-valued connections. Numerical analysis shows that it provides a new way to describe the edge weight probability. Furthermore, using a physical analogy in which the weights are the energies of molecules in a solid, the probability density function for nodes is identical to the degree distribution resulting from a uniform weight on edges. This provides an alternative way to study the degree distribution with the nodal probability function in unweighted networks. We observe a phase transition in the low-temperature region, corresponding to a structural transition caused by applying the threshold. Experimental results on real-world weighted and unweighted networks reveal an improved performance for inferring binary edge connections from edge weights.

Computer simulation of tritium release behavior of Li4SiO4 ceramic breeder with an improved packed bed model

In order to have a deep understanding of the tritium migration and release mechanisms in ceramic breeders, computer simulation of the tritium release behavior of Li4SiO4 ceramic breeder was performed by using an improved tritium release model. The influences of various factors, such as surface adsorbed water, water vapor in the purge gas, hydrogen in the purge gas, grain size and tritium production amount, on the tritium release process were systematically investigated. The simulation results have shown that: 1) The surface adsorbed water and water vapor in the purge gas can remarkably facilitate tritium release as tritiated water (HTO) in low temperature region (typically <450 °C); 2) Adding hydrogen to the purge gas is effective to promote tritium release as tritium gas (HT), but the effectiveness is sensitive to surface adsorbed water and water vapor in the purge gas; 3) Large grain size and tritium production amount can relatively weaken the effects of surface adsorbed water and water vapor in the purge gas. In general, the simulation results were consistent with the observations in tritium release experiments, which were helpful to further elucidate the complex behavior of tritium release from Li4SiO4.

An experimental and numerical study on the blade tip conjugate heat transfer performance

In the gas turbine, the blade tip is under extreme heat load and thereby it is necessary to adopt cooling method for the tip region. In this work, a blade tip conjugate heat transfer model was conducted in a linear cascade. The tip overall cooling effectiveness (φ) and adiabatic cooling effectiveness (η) were measured by the pressure sensitive paint (PSP) and the Infrared thermography (IR), respectively. Impacts of coolant blowing ratios (M = 1.0 ∼ 2.5), tip gap size (τ = 1.98%, 3.97%C, chord length), and film holes locations on the tip φ distributions were investigated. Then, the numerical simulations were performed to obtain the tip solid temperature distributions, and to reveal the conjugate heat transfer characteristics. Within the scope of this work, both the tip φ distributions and the tip solid temperature distributions indicate that the convection in film holes is a major factor of the tip conjugate heat transfer process. The tip area-averaged φ for the CL_H case are increased evidently by 13.1%∼38.9% with the increased coolant M, whereas the increase of η is not obvious. The tip gap size has less effect on the tip φ distributions than the coolant M, especially at a low M. The higher φ and the lower tip solid temperature region are detected near the film holes for any film hole location. The area-averaged φ values of the cases with film holes along the chamber line are 10.6% and 3.0% higher than those of the cases with film holes along the pressure-side or suction-side rim, respectively. This work revealed the major factor in the tip conjugate heat transfer process and may serve as a guide for tip cooling design.

Phase transitions and magnetoelectricity in Fe implanted TlInS2 and TlGaSe2 ferroelectric crystals

The dielectric constant measurements of Fe implanted TlInS2 and TlGaSe2 crystals without and under application of constant magnetic field have been performed. It has been observed that the implantation of the crystals with Fe ions caused the formation of ferromagnetic layer in the irradiated region, which leads to considerable shifts of the successive incommensurate and commensurate phase transitions to high temperatures in the heating regime. Remarkable shifts of the phase transition temperatures back to the low temperature region on heating of the samples were observed on applying the magnetic field perpendicularly to the implanted surface. None changes of the phase transition temperature points on measurements of the samples have been observed in cooling regime. The obtained results are interpreted as magnetodielectric effects that resulted from magnetoelectric coupling between ferromagnetic and ferroelectric constituents of the composite structure.

Impedance Spectroscopy and Study of Electrical Properties of Pb(Zr&lt;sub&gt;0.52&lt;/sub&gt;Ti&lt;sub&gt;0.48&lt;/sub&gt;)&lt;sub&gt;1-x&lt;/sub&gt;Fe&lt;sub&gt;x&lt;/sub&gt;O&lt;sub&gt;3-x&lt;/sub&gt; (0≤x≤0.20) Ceramics

Iron substituted Lead zirconate titanate nanoparticles Pb (Zr0.52Ti0.48) 1-x FexO3-x (PZTFx) (for (x=0.00, 0.025, 0.05, 0.075, 0.10, 0.15 and 0.20) was prepared using the sol-gel route in the morphotropic phase boundary (MPB) region. The X-ray diffraction data revealed the formation of both rhombohedral and tetragonal structures. The microstructural properties of the compounds were examined through the scanning electron microscopy (SEM) technique. The impedance spectroscopy and conductivity spectroscopy were carried out over a wide range of temperatures (RT–400°C) and frequencies (100 Hz–2 MHz) to investigate the grain and grain boundary effect on the electrical properties of PZTFx. The complex impedance analysis data have been presented in the Nyquist plot which is used to identify the corresponding equivalent circuit and fundamental circuit parameters. Cole– Cole plots indicate Debye-type dielectric relaxation and the grain boundaries resistance is dominant at room temperature. The Nyquist plot showed the negative temperature coefficient of resistance (NTCR) character of PZTFx ceramics. The dielectric properties of (PZTFx) ceramics as a function of temperature are studied and displayed a resonance phenomenon for all samples. Temperature-dependent conductivity behavior indicated an Arrhenius type of thermally activated process in the low-temperature region. Activation energy has been calculated from the temperature-dependent DC electrical conductivity measurements for all the samples.

Structural and magnetic properties of nanoparticles of La3Ni2NbO9 double perovskite

The nanoparticles of La3Ni2NbO9 (LNNO) have been produced via the glycine-nitrate combustion technique followed by a process of calcination with an average crystallite size of ∼ 25 nm. The powder X-ray diffraction (XRD) pattern was analyzed for structure analysis by Rietveld structure refinement. Study on the XRD pattern reveals that LNNO crystallizes in the single phase of monoclinic crystal structure with P21/n space group. Analysis of temperature-dependent magnetization reveals that nanoparticles of LNNO undergo paramagnetic to antiferromagnetic phase transition at Neel temperature TN ∼ 43 K and antiferromagnetic to spin glass transition at Tg ∼ 31.45 K. Analysis of magnetic ac-susceptibility reveals that the LNNO nanoparticles follow Vogel-Fulcher and dynamic scaling power laws in the low-temperature region.

Phase boundary mapping for high thermoelectric performance β-Zn4Sb3 in Zn–In–Sb ternary system

Zn4Sb3 with complex structure shows excellent thermoelectric performance in the low temperature region. Herein, three batches of In doped Zn4Sb3 were synthesized and the influence of In dopant and Zn content on the thermoelectric properties of Zn4Sb3 was systematically investigated by using a phase boundary method. The isothermal ternary phase diagram of Zn–In–Sb is explored at 623 K. The solubility limit of In in the single phase region with β-Zn4Sb3 structure is strongly related to the ratio between Zn + In and Sb. In atoms preferentially occupy C interstitial of 48f site in the crystallographic Wyckoff position and Zn deficiency increases the concentration of Zn vacancies on this site, enhancing the solubility limit of In in the system. The modification of structure via doping with In hinders the movement and diffusion of Zn interstitial in Zn4Sb3, accompanying with the disappearance of the low temperature phase transitions from α′ to α phase and from α to β phase. The regulation of Zn and In contents in single phase region with β-Zn4Sb3 structure effectively optimizes the thermoelectric performance. A maximum ZT value of 1.43 is attained at 673 K for (Zn0·96In0.04)4.15Sb3 compound, which demonstrates a 47% improvement in comparison with that of Zn4·15Sb3 compound. This work provides a new avenue to further improve thermoelectric properties of Zn4Sb3 compounds.

Inflatable aerodynamic decelerator for CubeSat reentry and recovery: IAD geometrical effects on the flowfield structure

In this investigation, Direct Simulation Monte Carlo computations were carried out to assess the influence of Inflatable Aerodynamic Decelerator (IAD) geometrical configurations on the flowfield during the atmospheric reentry. The non-reacting hypersonic rarefied flow over three IAD configurations coupled to a CubeSat was simulated considering a 0∘ angle of attack at an altitude of 105 km. According to the results, it was observed the formation of a strong and diffuse shock wave for all geometries considered in this investigation. However, a lower inflatable aeroshell angle is associated with a thinner shock wave and a maximum shock wave temperature closer to the shield's surface. These differences decrease in the flow expansion over the IAD shoulder. In the rear of the inflatable shields, a low-temperature and low-velocity region is observed, indicating that the IADs geometries successfully mitigate the harsh reentry conditions experienced by the payload. Finally, it was noticed that the wake region is larger for aerodynamic elongated shapes, in contrast to blunt geometries with the expense of slightly higher gas temperature closer to the front surface of the shield. No recirculation zone was observed in any of the simulated IAD configurations considered in this investigation.

Numerical simulations on effect of cooling hole diameter on the outlet temperature distribution for a gas turbine combustor

The outlet temperature distribution characteristics of a gas turbine combustor are critical to the durability and lifetime of the turbine. Regarding to an advanced high-temperature-rise combustor, the air flow rate for swirling increases while the volume of air used to cool the wall and control the outlet temperature field decreases, causing severe challenges in effectively regulating the outlet temperature field, consequently. In the present work, with a focus on regulating the outlet temperature field under conditions of low cooling and mixed gas volume, numerical simulations of a single-sector centrally-staged combustor are carried out. The effusion cooling structure of multi-inclined cooling holes with 30° inlet angle are used on the liner wall. The impact of liner wall cooling holes with different diameters on the outlet temperature distribution characteristics is analyzed, while the variation in relative areas and distribution shapes of low-temperature regions near the outlet wall and hot streaks are obtained. The results suggest that under a constant inlet mass flow rate, both pattern factor (PF) and radial pattern factor (RPF) of the combustor increase with the increment of cooling hole diameter. Compared to the 0.5 mm hole diameter, when the diameter of each single cooling hole is doubled to 1.0 mm, the PF and RPF increase respectively by 1.14 times and 1.83 times, meanwhile, the combustion efficiency decreases from 99.97% to 97.74%, and the pressure drop reduces from 4.3% to 2.4%. Increasing the diameter of cooling holes enhances the cooling film thickness on the liner wall, resulting in the expansion of low-temperature regions near the outlet wall. Furthermore, in comparison with the smaller diameter, the larger diameter forces the combustion reaction zone to shift toward the outlet, leading to the expansion of hot streaks and a deterioration in the uniformity of the outlet temperature field. The combustor corresponding to the design scheme of multi-inclined cooling holes with 0.5 mm diameter has the most uniform outlet temperature field and the highest combustion efficiency, showing the optimum performance.

Global isomorphism approach: Attractive Yukawa fluid, 2D case

In this paper we apply fluid-lattice gas global isomorphism approach to liquid-vapor equilibrium of Yukawa attractive fluid in the two dimensions. The construction of tangent to the binodal of the fluid in the low-temperature region is performed based on the Zeno-element. The dependence of Zeno-element and Boyle parameters on interaction parameters is also studied. It is shown that the asymptotic behavior of the Zeno-element parameters can serve as a marker for liquid phase instability in this case. We also provide the relation between critical temperatures of bulk (3D) and monolayer (2D) fluid

Catalytic Conversion of Oil in Model and Natural Reservoir Rocks

The catalytic activity of metal oxides in the processes of low- and high-temperature oxidation (LTO and HTO, respectively) of oil was studied on model systems consisting of oil-saturated quartz sand with additives of Al2O3, Cr2O3 and MgO using thermal analysis methods. The used additives were shown to shift the LTO and HTO processes to the low-temperature region. The catalytic activity of a natural reservoir carbonate rock without and with water was studied. This study established that at room temperature in dry carbonate rock, the oil components undergo dealkylation and polycondensation of aromatic fragments for a week. In the presence of water, the polycondensation processes are suppressed, and the cracking of resin and asphaltene occurs. The cracking reactions lead to a decrease in the content of heteroatoms in resins and asphaltenes.

Differential diffusion effects in the near field of non-premixed NH3/H2/N2-air jet flames at elevated pressure

Differential diffusion (diff-diff) effects are analyzed in both non-premixed laminar and turbulent NH3-H2-N2 jet flames at 5 bar using 1D quantitative Raman scattering measurements. The two target flames feature fuel compositions simulating a 14% and a 28% NH3 cracking ratio. A diff-diff parameter is introduced for ammonia combustion, analogous to the definition used for hydrocarbon flames. Measurements in laminar flames, compared to Chemkin simulation featuring multi-component transport show that the instrument is capable of measuring the mean diff-diff parameter, but that single-shot measurements are compromised by the large sensitivity of the parameter to errors in N2 and NH3 mass fractions. The analysis of the diff-diff parameter in turbulent flames shows that both flames have strong differential diffusion in the near field up to an axial distance of 10 diameters (Z/D = 10), then the turbulent mixing becomes dominant further downstream. The flame with a 28% NH3 cracking ratio exhibits a higher diff-diff parameter especially in the low-temperature region (<1000 K) due to the higher H2 content. The competition between differential diffusion and turbulent mixing in the near field is also reflected in the NH3, H2, and NH3/H2 profiles, falling in between prediction for Le = 1 and multi-component transport. Interestingly the NH3/H2 profiles in the far field converge to a profile matching that of Le = 1, but with an increased NH3/H2 ratio, showing the persistence of the differential diffusion effects in the region dominated by turbulent mixing.

Phase Transition Field Effect Transistor Observed in an α-(BEDT-TTF)2I3 Single Crystal

The metal–insulator transition induced by the gate electric field in the charge order phase of the α-(BEDT-TTF)2I3 single-crystal field-effect transistor (FET) structure was clearly observed near the phase transition temperature. An abrupt increase in the electrical conductance induced by the applied gate electric field was evident, which corresponds to the partial dissolution of the charge order phase triggered by the gate electric field. The estimated nominal dissolved charge order region (i.e., the gate-induced metallic region) was overestimated in 130–150 K, suggesting additional effects such as Joule heating. On the other hand, in the lower temperature region below 120 K, the corresponding dissolved charge order was several monolayers of BEDT-TTF, suggesting that it is possible to dissolve the charge order phase within the bistable temperature region.

Distress Characteristics in Embankment-Bridge Transition Section of the Qinghai-Tibet Railway in Permafrost Regions

The Qinghai-Tibet Railway has been operating safely for 16 years in the permafrost zone and the railroad subgrade is generally stable by adopting the cooling roadbed techniques. However, settlement caused by the degradation of subgrade permafrost in the embankment-bridge transition sections (EBTS) is one of the most representative and severe distresses. A field survey on 440 bridges (including 880 EBTSs) was carried out employing terrestrial laser scanning and ground-penetrating radar for comprehensively assessing all EBTSs in the permafrost zone. The results show that the types of distresses of EBTSs were differential settlement, upheaval mounds of the protection-cone slopes, subsidence of the protection-cone slopes, surface cracks of the protection cones and longitudinal and transverse dislocation of the wing walls. The occurrence rates of these distresses were 78.93, 3.47, 11.56, 3.36, 21.18 and 4.56%, respectively. The most serious problem was differential settlement, and the average differential settlement amount (ADSA) was 15.3 cm. Furthermore, the relationships between differential settlement and 11 influencing factors were examined. The results indicate that ADSA is greater on the northern side of a bridge than on the southern side and on the sunny slope than on the shady slope. It is also greater in the high-temperature permafrost region than in the low-temperature permafrost region and in the high-ice content area than in the low-ice content area. The EBTSs are more influenced by ice content than by ground temperature. The ADSA increases when the embankment height increases, the particle size of subgrade soil decreases and the surface vegetation cover decreases.

Exploring the framework of small pore zeolites for passive NOx adsorption

The use of a passive NOx adsorber (PNA) for NOx storage during the cold-start period is crucial for eliminating NOx emissions from the diesel exhaust. Palladium (Pd)-loaded zeolites have garnered significant attention owing to their high potential as PNA. In this study, small-pore zeolites with different frameworks were synthesized, and their structural characteristics and passive NOx adsorption abilities were investigated. Five zeolites, with CHA, AEI, AFX (low- and high- Si/Al ratio), and LEV framework structures, were synthesized by the hydrothermal conversion of faujasite-type (FAU) zeolite. Pd (1 wt%) was impregnated in combination with hydrothermal treatment. NOx adsorption and desorption behaviors were studied based on breakthrough NO adsorption and subsequent temperature-programmed desorption (TPD) experiments. Pd-loaded CHA, AEI, and LEV zeolites and high-silica AFX zeolites adsorbed approximately 93.88, 107.93, 97.57 and 106.53 μmol-NO g−1, respectively. On the other hand, the incorporation of Pd into AFX zeolite with a low Si/Al ratio led to significant hydrothermal damage and resulted in lower levels of adsorption and desorption capabilities. This demonstrates that hydrothermally stable small-pore zeolites on Pd-incorporation have great potential for passive NOx adsorption. The zeolite with different framework exhibited the different temperature range of NOx release. CHA, AEI, and high-silica AFX zeolites desorbed NOx in the low-temperature region (T < 250 °C), whereas the LEV zeolite showed a relatively high NOx desorption amount in the middle-temperature region (250 °C < T < 350 °C).

Molecular simulations of the vapour-liquid coexistence curve of square-well dimer fluids

In this work, we evaluate the liquid-vapour coexistence diagram and the critical point for two different ranges, and of the square-well dimer fluid, using two different simulation methods: (1) In the critical point vicinity, we use a new algorithm, based on transition rates, that can obtain the chemical potential as a function of the density at a given temperature and (2) Molecular Dynamics simulations using the direct coexistence technique for temperatures far below the critical region. The transition rate method has been proposed by Sastre and was used for the evaluation of the critical temperature of square-well monomers with high accuracy. The simulations in the low-temperature region were carried out using molecular dynamics simulations with the direct coexistence method and a continuous version of the square-well potential proposed recently by Zerón et al.

Structural and dielectric properties of GdFeAsO ceramic material

The iron-based ceramic GdFeAsO has been prepared by the solid-state reaction method. To identify the ceramic and its multiple applications we have performed structural, micro structural and electrical characterizations of the sample. The crystalline size and structure are calculated at room temperature using the X-ray Diffraction (XRD) technique. Tetragonal structure has been ensured from XRD. The lattice strain of the crystal is obtained using W.H. Plot, which ensured the structural stability of the crystal. The field emission scanning electron microscopy (FESEM) technique is used to study the surface morphology of the sample. Polycrystalline grains having spherical and cylindrical sizes of ranges 1-3 micrometer are obtained. The observed prominent peaks at 107.1cm−1,150.4cm−1,216.6cm−1,296.6cm−1,344.6cm−1 and 440.37cm−1 in the Raman spectra of the sample, are for the cubic phase of the Gd2O3 in the GdFeAsO compound material. The peak at 623.0cm−1 corresponds to α-Fe2O3 in the spectra reveals the presence of impurity oxide phases. The broad peaks in the Differential Scanning Calorimetry (DSC) graph at about (290°C) and (600°C) reveals the initiation of slow crystallization of the sample within the given temperature range. The dielectric properties such as the dielectric constant (εr) and loss tangent (tanδ) of the sample have been studied at different temperatures and frequencies with LCR meter. The high value of dielectric constants is seen to exist in the materials. There are two dielectric anomalies that are observed at lower and higher temperature region. The magnetic measurement has been performed by using Vibrating Sample Magnetometer (VSM) at room temperature and showed the non linearity in the magnetization versus field curve.