Narrow Results

The two level tunnelling model is applied to analyze recent experimental data on low temperature thermal conductivity of Cu_xSn_1-x metallic glasses. An extra scattering mechanism due to Rayleigh is introduced to describe three characteristic regions of conductivity. Model parameters are determined using least-squares fitting procedure. Results derived from the parameters allow one to predict several interesting low temperature properties of metallic glasses in particular the characteristic plateau of conductivity. Theoretical results are also compared and contrasted with those of other metallic and non-metallic systems, and both common and uncommon features that exist between them are discussed.

Materials and mechanical characteristics of the low temperature PECVD silicon nitrides have been investigated using various analytical and testing techniques. TEM and SEM examinations reveal that there is no distinct microstructural difference existing between the films deposited under different conditions. However, their mechanical properties determined by nanoindentation indicate otherwise. The variations in mechanical properties with deposition conditions are found to be strongly correlated to the change in silicon-to-nitrogen ratio in the film.

The tensile deformation of polycrystalline aluminum (99.999%) was studied between 18 and 300 K. The stress-relaxation at constant strain was determined at strain intervals of about 0.5% with total strain exceeding about 3%. Stress-relaxation curves were logarithmic except at large "t" where they flatten. The relaxation rate "s" was determined by using equation s=d(Δσ)/d ln t, where Δσ(t)=σ₀-σ(t) is the amount of stress relaxed at any instant of time "t" from the initial stress level σ₀ at which relaxation was allowed to start. The slope ds/dσ₀ was observed to attain maxima at about 30 K and minima at about 60 K. The undulation in the temperature dependence of stress relaxation rate in the range 18–60 K is an outcome of changes in the substructures of dislocations which have developed during the deformation process. These changes then require stresses higher than that applied in the basic equations which describe the kinetics of the mode of deformation. The average intrinsic height of potential barrier U₀, estimated by means of the single barrier model of stress relaxation, was 1.2±0.3 eV. These values appeared to be compatible with the dislocation-dislocation intersection, controlling the rate processes in polycrystalline aluminum.

Stress relaxation rate in unirradiated and electron-beam-irradiated polycrystalline titanium (99.994%) was studied in the temperature range 300–100 K. Titanium specimens were irradiated with 12 MeV electrons to a dose of 0.01 dpa for 12 min. at 300 K. Tensile tests of the specimens were performed using a Universal Testing Machine in the given temperature range. To measure the relaxation of stress with time, the crosshead of the machine was arrested at different fixed loads. Stress relaxation rate s for a given stress level σ₀ was found to be temperature dependent, i.e., it decreased with decreasing temperature both in unirradiated and irradiated specimens. However, the decrease was more pronounced in irradiated specimens than that of unirradiated ones. The observed decrease in s values with decrease in temperature is ascribed to the retarding effect of unrelaxed dislocations pinned at defects, especially at the twin boundaries in the course of deformation, which became more conspicuous in irradiated specimens due to the interaction of glide dislocations with radiation-induced defects, in addition to mechanical twins. The activation energy for the movement of dislocations, calculated using the single-barrier model of stress relaxation, was found to be higher in irradiated specimens than that of unirradiated ones at all test temperatures.

Based on the analyzing of the thermodynamic performance of the heat transfer process in the low temperature heat exchangers, the exergy efficiency of the heat transfer process is defined and a general expression for the exergy efficiency is derived, which can be used to discuss the effect of heat transfer units number and heat capacity ratio of fluids on the exergy efficiency of the low temperature heat exchanger. The variation of the exergy efficiency for several kinds of flow patterns in the low heat exchangers is compared and the calculating method of the optimal values of heat capacity ratio for the maximum exergy efficiency is given.

The exergoeconomic analysis is carried out on enhanced heat transfer surfaces at low temperature. A new criterion for evaluating the performance of enhanced heat transfer surfaces at low temperature is proposed. It can be applied to various augmentation techniques and generalizes the performance evaluation criteria obtained by means of the first and second law analysis. The validity of the new performance evaluation criterion is illustrated by the analysis of heat transfer characteristics at low temperature and assessment of the heat transfer cost of two types of enhanced heat transfer surfaces.

The mechanical property of a new bulk metallic glass Zr_50.7Cu₂₈Ni₉Al_12.3, which owns 5% room-temperature plasticity, is investigated at low temperatures 193K and 123K. It is indicated that the yield strength and the plasticity significantly increase by 12.8% and 50% respectively as testing temperature is lowered from 298K (at room temperature) to 123K (in liquid nitrogen). The dense and multiple shear bands which contribute to the improved ductility are observed on the side surface of these samples fractured at low-temperature. Detailed study of the stress-strain curves for the samples deformed at low temperature show that more serrations and smaller stress drop form due to generation of more much shear bands.

We describe a result coming from an experiment based on an Al-Mg alloy (~5% Mg) suspended bar hit by an electron beam and operated above and below the temperature of transition from superconducting to normal state of the material. The amplitude of the bar first longitudinal mode of oscillation, excited by the beam interacting with the bulk, and the energy deposited by the beam in the bar are the quantities measured by the experiment. These quantities, inserted in the equations describing the mechanism of the mode excitation and complemented by an independent measurement of the specific heat, allow us to determine the linear expansion coefficient α of the material. We obtain α = [(10.9±0.4)T+(1.3±0.1)T³]×10^-10K^-1 for the normal state of conduction in the temperature interval 0.9 < T < 2 K and α = [(-2.45±0.60)+(-10.68±1.24)T +(0.13±0.01)T³]×10^-9K^-1 for the superconducting state in the interval 0.3 < T< 0.8 K.

The paper deals with low-temperature photoluminescence and deformation luminescence (mechanoluminescence) of a composite material based on fine disperse powder of phosphor SrAl₂O₄:(Eu²⁺, Dy³⁺) and photopolymerizing resin that is transparent in the visible region. New information about the energy levels of impurities and defects was obtained. It has been found that at low temperatures (T = 15 ÷ 200 K) the photoluminescence spectrum of SrAl₂O₄:(Eu²⁺, Dy³⁺) presents two partially overlapping wide bands with the maxima at λ_1max = 446 nm and λ_2max = 517 nm. The strong interaction of energy states of the bands results in temperature quenching of the short-wave band of luminescence (λ_1max = 446 nm) and its complete attenuation at T ≥ 200 K. The results were used to describe the mechanism of persistent luminescence and mechanoluminescence of SrAl₂O₄:(Eu²⁺, Dy³⁺).

We have investigated the cohesive and thermodynamic properties of (KBr)_1-x(KCN)_x using an extended three-body force shell model (ETSM), which has been found to be adequately suitable for the description of orientationally disordered mixed cyanide-halide crystals. The specific heat of (KBr)_1-x(KCN)_x for compositions (x=0.53, 0.65, 0.73, 0.84 and 0.93) at temperature 10 K≤T≤150 K have been computed using the ETSM for the first time. The paper also reports the calculated results on Debye temperature (Θ_D), cohesive energy (Φ), compressibility (β), molecular force constant (f) and Restrahlen frequency (ν₀) of (KBr)_1-x(KCN)_x. The results on specific heat and some other thermal properties are in good agreement with their available experimental data.

To study the geometrical scale dependency of thin film solid oxide fuel cells (SOFCs), we fabricated three thin films SOFCs which have the same cross-sectional structure but different electrode areas of 1 mm², 4 mm² and 9 mm². Since the activation and ohmic losses of SOFCs depend on their active region, we examined the variations of the power density of the cells with a Pt/YSZ/Pt structure and simulated the power density variations using the COMSOL software package.

The temperature range of $T$ = 77–770 K in the system alloys: Holl coefficient $(R)$, thermo-emf $(S)$, electric conductivity $(\sigma )$, measured $d$-density of components and analyzed. It has been established that samarium additive atoms contain donor-type properties and the effectiveness increases with the temperature increase: up to 40% proportional to $T\le 440$ K in $p$-type specimens, whereas in $n$-type samples this increase is higher and covers the contents of pH varying from $p$ to $n$. An electrical conductivity of compounds increased due to the carrier activation with further increase of temperature. The activation energy of carriers at low temperatures ($T\le 400$ K) is $\mathrm{\Delta }{E}_g\approx 0.37$ eV for $x=0.05$ mol.% and $x=0.10$ mol.% compounds at $T$ = 77–320 K and for $x=1.00$ mol.% and $x=3$ mol.% compounds are $\mathrm{\Delta }{E}_g\approx 0.39$ eV. $\sigma (T)\approx$ const at $T=300$–400 K for $x=0.05$ mol.% and $x=0.10$ mol.% compounds, and passing with minimum increases at $T$ = 400–500 K.

${\mathrm{\text{BaFe}}}_{11.1}{\mathrm{\text{Sc}}}_{0.9}{\mathrm{\text{O}}}_{19}$ hexaferrite compound was synthesized, the crystal structure was studied by the X-ray diffraction method. Powder samples were prepared from the mark “analytical grade” oxides ${\mathrm{\text{Fe}}}_2{\mathrm{\text{O}}}_3$, ${\mathrm{\text{Sc}}}_2{\mathrm{\text{O}}}_3$ and carbonate BaCO₃. It was found that under normal conditions and at room temperatures, the crystal structure of this compound has hexagonal symmetry with the P6₃mmc space group. The lattice parameters correspond to the values: $a=5.8842\mathrm{\text{Å}}$ and $c=23.1774\mathrm{\text{Å}}$. Atomic dynamics of ${\mathrm{\text{BaFe}}}_{11.1}{\mathrm{\text{Sc}}}_{0.9}{\mathrm{\text{O}}}_{19}$ hexaferrite in the range of low temperatures of $-190^{\circ }\mathrm{\text{C}}\le T\le 20^{\circ }\mathrm{\text{C}}$ and high temperatures of $20^{\circ }\mathrm{\text{C}}\le T\le 490^{\circ }\mathrm{\text{C}}$ were studied. As a result of the analysis of the spectra obtained by the Raman spectroscopy method, it was determined that the structural phase transition does not occur in this compound over a wide temperature range $-190^{\circ }\mathrm{\text{C}}\le T\le 490^{\circ }\mathrm{\text{C}}$. At the room temperature, the obtained Raman modes are observed at both low and high temperatures.

Quaternary SiCAlN nanoparticle films were produced by reactive rf magnetron co-sputtering technique with a chemically pure SiC and an Al target under low temperature. The crystalline structure, surface morphology, and element content of the films were studied in terms of sputtering parameters. The element content and chemical states of SiCAlN films were measured by means of energy dispersive X-ray fluorescence (EDX) and X-ray photoelectronic spectroscopy (XPS). Surface morphology of SiCAlN films was investigated by FE-SEM.

The relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ compositions are of interest owing to their excellent dielectric, electromechanical, electro-optical, and other properties. In this paper, the 0.80Pb(Mg_1/3Nb_2/3)O₃–0.20PbTiO₃ (PMN–PT 80/20) films with pure perovskite structure were synthesized by a single step at 150°C. The corresponding oxides were used as starting materials, namely PbO, MgO, Nb₂O₅, and TiO₂. By using oxides as precursors, we simplified the hydrothermal process, allowing the process to be more economical. The influences of the Ti metal substrate on the PMN–PT 80/20 films are investigated. By surveying the variations of films, it was suggested that the substrate reacted with the ions in the solution. The films were smooth and homogeneous. There were no cracks and abnormity crystals on the surface of the films. The thickness was about 20 μm. The frequencies dependence of the dielectric properties of PMN–PT 80/20 films was stable even at a high frequency range over 1 MHz.

The authors provide insight into the problem of selective flotation of fluorite from quartz–fluorite ores. The flotation of fluorite mineral from ores by using acidized water glass as depressant has been studied for substitution of the commonly used depressant. It was found that the acidized water glass could achieve higher selectivity of fluorite minerals against gangue minerals in the fluorite flotation. The experimental results showed that YF collector has high selectivity in the flotation of fluoride for low-grade fluorite ores. The use of the YF collector also made it unnecessary to heat the pulp to a high temperature and achieved an acceptable reduction in the flotation temperature to $13^{\circ }$C. The YF collector should be used in neutral pH to separate fluorite from gangue and it could also improve fluorite recovery and flotation rate.

Nanocrystalline silicon embedded in silicon oxide (nc-Si/SiO₂) films have been in situ grown at a low substrate temperature of 300°C by reactive magnetron co-sputtering of Si and SiO₂ targets in a mixed Ar/H₂ discharge. The influences of H₂ flow rate (F_H) on the microstructural properties of the deposited nc-Si/SiO₂ films were investigated. The results of XRD and the deposition rate of nc-Si/SiO₂ films show that the introduction of H₂ contributes to the growth of nc-Si grains in silicon oxide matrix. With further increasing F_H, the average size of nc-Si grains increases and the deposition rate of nc-Si/SiO₂ films decreases gradually. Fourier transform infrared spectra analyses reveal that introduction of hydrogen contributes to the phase separation of nc-Si and SiO_x in the deposited films. Moreover, the Si–O_4-nSi_n(n = 0, 1) concentration of the deposited nc-Si/SiO₂ films reduces with the increase of F_H, while that of Si–O_4-nSi_n(n = 2, 3) concentration increases. These results can be explained by that active hydrogen atoms increase the probability of reducing oxygen from precursor in the plasma and prompting oxygen desorption from the growing surface. This low-temperature procedure for preparing nc-Si/SiO₂ films opens up the possibility of fabricating the silicon-based thin-film solar cells onto low-cost glass substrates using nc-Si/SiO₂ films.

A simple aqueous solution-based block-polymer-assisted method was developed for the synthesis of Mg(OH)₂ nanotubes using inexpensive starting materials (MgSO₄ and NaOH) with significant advance of bringing down the average outer diameter to the regime below 10 nm. The synthesized nanotubes are found to possess a relatively high specific surface area up to 64 m²/g, which may suggest their potential value for use in the catalysis field. The Barrett–Joyner–Halenda pore-size distribution plot reveals a bimodal distribution centred at 3.4 and 17.3 nm, which may be from the tunnel cavity and the aggregated pores of Mg(OH)₂ nanotubes, respectively. This method can be widely used to fabricate other inorganic 1D nanostructured materials and easily realized in industrial-scale synthesis.

A new method for preparation of a carbon nanotube (CNT)–zinc oxide hybrid on a glass substrate at low temperature is introduced and the stability improvement of field emission of CNTs is reported. The emission current stability and substrate material are the two main commercial parameters for field emission applications of CNTs. The two alternative current (AC) and direct current (DC) plasma enhancement chemical vapor deposition (PECVD) is used to achieve low-temperature growth of vertically aligned CNTs and the hydrothermal processing is used for production of zinc oxide nanowires to improve the emission current stability and properties. The nanoscale junction between a semiconductor and conductive material are critical for electronic applications. The present study examined the field emission properties of CNTs near the crystalline structure of the zinc oxide hybrids and showed the improvement in the field enhancement factor and emission current stability of CNT–ZnO hybrid than the CNT emitters.

Mn-FeO_x/carbon nanotubes (CNTs) catalysts were firstly prepared via simple incipient wetness method and used for low-temperature selective catalytic reduction (SCR) of NO with NH₃. The structure and surface properties of the catalysts were characterized by N₂ sorption, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction by hydrogen (H₂-TPR). It was found that Mn-FeO_x/CNTs catalyst exhibited excellent low-temperature SCR activity and SO₂ resistance. XRD patterns revealed that metal oxides catalysts were possessed of amorphous structure. FESEM and TEM images showed that metal oxides catalysts were successfully supported on CNTs. The XPS results indicated that the obtained catalyst presented high Mn⁴⁺/Mn³⁺ and O_S/(O_S + O_L) ratios. The H₂-TPR profiles showed that Mn-FeO_x/CNTs catalyst possessed better low-temperature reducibility. Besides, the obtained catalyst exhibited better SO₂ resistance.