Influence of smooth temperature variation on hotspot ignition

Autoignition in thermally stratified reactive mixtures originates in localised hotspots. The ignition behaviour is often characterised using linear temperature gradients and more recently constant temperature plateaus combined with temperature gradients. Acoustic timescale characterisation of plateau regions has been successfully used to characterise the type of mechanical disturbance that will be created from a plateau core ignition. This work combines linear temperature gradients with superelliptic cores in order to more accurately account for a local temperature maximum of finite size and the smooth temperature variation contained inside realistic hotspot centres. A one-step Arrhenius reaction is used to model a H₂–air reactive mixture. Using the superelliptic approach a range of behaviours for temperature distributions are investigated by varying the temperature profile between the gradient only and plateau and gradient bounding cases. Each superelliptic case is compared to a respective plateau and gradient case where simple acoustic timescale characterisation may be performed. It is shown that hot spots equivalent with excitation-to-acoustic timescale ratios sufficiently greater than unity exhibit behaviour very similar to a simple plateau-gradient model. However, for larger hot spots with timescale ratios sufficiently less than unity the reaction behaviour is highly dependent on the smooth temperature profile contained within the core region.     

The sputtering of high Tc A15 Nb3Si and V3Ge

We report some initial results on the preparation of A15 Nb₃Si and V₃Ge using a getter sputtering technique. Under sufficiently clean conditions we observe an increase in the superconducting transition temperature. DC onsets in excess of 14 and 11 K have been observed for Nb₃Si and V₃Ge respectively. In each case a positive identification of the A15 phase has been made.     

Negative differential conductance in nano-scale normal metal/superconductor/normal metal junctions featuring Fe1 + y Te1 − x Se x

Iron-based superconductors have been the subject of intensive study due to their high transition temperature and intriguing physical mechanisms. We describe a unique experimental approach to fabricate nano-scale normal metal/superconductor/normal metal junctions involving microcrystals of Fe₁ ?₊?_y Te₁ ?_??_x Se _x , for which we have observed a distinct phenomenon of negative differential conductance (NDC) dips along with multiple plateau features in differential conductance spectra. The evolution of the NDC dips and the plateau features is further explored as a function of both temperature and magnetic field, and their physical origin is discussed.     

The resistance jump in the pure type-I superconductors tin,indium and lead

It is shown that for tin, indium and lead samples of sufficiently high purity, sufficiently large diameter (2–3mm), and sufficiently close to the critical temperature (ΔT<0.15K), there exists a common value close to $\text{2}\text{3}$(0.64?R(i_c)/R_n ? 0.69) of the resistance jump at the transition from the superconducting to the intermediate state.     

Interface traps contribution to capacitance of Al2O3/(GaN)AlGaN/GaN heterostructures at low frequencies

Influence of interface traps at Al₂O₃/(GaN)/AlGaN interface on low and high frequency capacitance of Al₂O₃/(GaN)/AlGaN/GaN heterostructure capacitor was studied. New features were observed in the capacitance curves. Obtained experimental results were modeled and simulated and accordance with the experiment has been obtained. For lower frequencies a new capacitance peak in the depletion and increase of the capacitance in a plateau region were measured. The capacitance peak in the depletion region was successfully explained by a capacitance response of the interface traps with U-shape density distribution. On the other hand the increase of the capacitance plateau was modeled by the homogeneous interface trap distribution. We assume that the traps located near the band edges having the highest density are able to respond to the low frequency measuring.     

Influence of absorbed hydrogen on the magnetic behavior of RCo3 (R = Gd,Dy and Ho) compounds

The pressure-composition isotherm for the RCo₃-H₂ system (R = heavy rare earth) shows two plateau pressures. For GdCo₃ the first plateau extends to the approximate composition GdCo₃H_2.2 (designated as β-hydride) and the second to the composition GdCo₃H_4.6 (γ-hydride). Magnetization measurements have been carried out on GdCo₃, GdCo₃H₂, GdCo₃H_4.6, DyCo₃, DyCo₃H_4.3, HoCo₃ and HoCo₃H_4.2 in the temperature range 4.2 K to 300 K. The results show that hydrogen absorption produces a reduction both in the magnetic moment per cobalt and the Curie temperature; the reduction is slight in going from GdCo₃ to GdCo₃H_2.2, but is drastic in going to GdCo₃H_4.6. In contrast, in isostructural GdFe₃ compound, the magnetic moment per iron increases on hydrogen absorption. Measurements on DyCo₃H_4.3 and HoCo₃H_4.2 suggest the possibility that the rare earth moments are not magnetically ordered even at 4.2 K.     

Suppression of ferroelectricity and quantum fluctuations in Ru-doped TbMnO<Subscript>3</Subscript>

We investigate the effects of Ru-doping in polycrystalline TbMn_1−x Ru _x O₃ (x≤0.10) on the multiferroicity. It is observed that the Ru substitution gradually melts away the dielectric anomaly at the ferroelectric transition point and the ferroelectricity by suppressing the polarization, accompanied with a surprising low-temperature dielectric plateau. While it is reasonable to observe the significant suppression of ferroelectricity, owing to the fact that the Ru-doping disrupts the Mn spiral spin ordering and reduces the Mn–Mn spin angle, quantum fluctuations associated with the Ru substitution, responsible for the low-temperature dielectric plateau, seems to be significant.     

Effect of Ni/Mn ratio on the performance of LiNi x Mn2 − x O4 cathode material for lithium-ion battery

Lithium nickel manganate is recognized as a type of promising cathode material for lithium-ion battery, due to its advantages such as high voltage, high power density, and relative lower cost. In this paper, a series of LiNi _x Mn_2???x O₄ cathode materials with various molar ratio of Ni/Mn have been prepared with a co-precipitation method, followed by a solid state reaction, and the effect of the molar ratio of Ni/Mn on the structure and properties of materials are intensively investigated by means of X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), scanning electron microscopy (SEM), and performance measurements, etc. It is revealed that all the samples with x from 0 to 0.5 have well-defined spinel structure and fit well to Fd-3 m space group. With the increase of the molar ratio of Ni/Mn, the diffraction peaks shift to higher angle slightly and the lattice parameter decreases gradually by the XRD results. Furthermore, it is found that the capacity at the 4.0 V plateau decreases while the capacity at 4.7 V plateau increases with the increase of the ratio of Ni/Mn, and the total discharge capacity shows growth trend with the increase of Ni content. It is important that all the samples with various molar ratio of Ni/Mn exhibit good cyclic stability. Based on the experimental results, we suggest that the Ni may incorporate into the lattice of LiMn₂O₄ substituting of Mn. The plateau at 4.7 V is related to the Ni ions and the plateau at 4.0 V is related to the Mn ions in the materials.     

Experiences sur l'anisotropie de la resistance de surface des supraconducteurs pres de Hc3

In this paper we present some experimental results in the anisotropy of the surface resistance near H_c3. The materials used in this work have values of κ between 1.2 and 4.6. The results seem to indicate that the actual theories are incomplete due to the fact that the influence of the κ parameter is not sufficiently taken into account.     

Effect of adding Ar gas on the pulse height distribution of BF<Subscript>3</Subscript>-filled neutron detectors

Boron trifluoride (BF₃) proportional counters are used as detectors for thermal neutrons. They are characterized by high neutron sensitivity and good gamma discriminating properties. Most practical BF₃ counters are filled with pure boron trifluoride gas enriched up to 96% ¹⁰B. But BF₃ is not an ideal proportional counter gas. Worsening of plateau characteristics is observed with increasing radius due to impurities in gas. To overcome this problem, counters are filled with BF₃ with an admixture of a more suitable gas such as argon. The dilution of BF₃ with argon causes a decrease in detection efficiency, but the pulse height spectrum shows sharper peaks and more stable plateau characteristics than counters filled with pure BF₃. The present investigations are undertaken to study the pulse height distribution and other important factors in BF₃+Ar filled signal counters for neutron beam applications. Tests are performed with detectors with cylindrical geometry filled with BF₃ gas enriched in ¹⁰B to 90%, and high purity Ar in different proportions. By analysing pulse height spectra, a value of 6.1 ± 0.2 has been obtained for the branching ratio of the ¹⁰B(n,α) reaction.      

啁啾激光与半周期脉冲形成的组合场驱动原子产生单个阿秒脉冲

提出了由波长为800 nm、脉冲宽度为5 fs的啁啾激光与半周期脉冲形成组合场,并利用这种组合场驱动一维模型氦原子获得单个阿秒脉冲. 通过数值求解一维氦原子的含时薛定谔方程,发现氦原子在组合场驱动下高次谐波谱的截止位置可以扩展到I_p+21.6U_p. 对第二平台区域不同范围内高次谐波的叠加都能得到单个阿秒脉冲,最短可达37 as,特别是对平台区域的前端进行叠加不仅能够得到较短的单个阿秒脉冲,而且与截止位置附近高次谐波构造的阿秒脉冲相比,强度提高了3个数量级. 关键词： 啁啾激光场 半周期脉冲 高次谐波 阿秒脉冲     

Disorder effect on the traffic flow behavior

The effects of some disorders, on the traffic flow behavior, are studied numerically. Especially, the effect of mixture of vehicles of different velocities and/or lengths, the effects of different drivers reactions, the position and the extraction rate of off-ramp in the free way. Using a generalized optimal velocity model, for a mixture of fast and slow vehicles, we have investigated the effect of delay times τ _f and τ _s on the fundamental diagram. It is Found that the small delay times have almost no effect, while, for sufficiently large delay time τ _s , the current profile displays qualitatively five different forms, depending on τ _f , τ _s and the fractions f _f and f _s of the fast and slow cars, respectively. The velocity (current) exhibits first-order transitions at low and/or high densities, from freely moving phase to the congested state, and from congested state to a jamming one, respectively. The minimal current appears in intermediate values of τ _s . Furthermore there exist, a critical value of τ _f above which the meta-stability and hysteresis appear. The effects of disorder due to drivers behaviors have been introduced through a random delay time τ allowing the car to reach its optimal velocity traffic flow models with open boundaries. In the absence of the variation of the delay time Δτ, it is found that the transition from unstable to meta-stable and from meta-stable to stable state occur under the effect of the injecting and the extracting rate probabilities α and β respectively. Moreover, the perturbation of the traffic flow behavior due to the off-ramp has been studied using numerical simulations in the one dimensional cellular automaton traffic flow model with open boundaries. When the off-ramp is located between two critical positions i _c1 and i _c2 the current remains constant (plateau) for β_0c1 < β₀ < β_0c2, and the density undergoes two successive first order transitions: from high density to plateau current phase and from average density to the low one. In the case of two off-ramps, these transitions occur only when the distance between ramps, is smaller than a critical value.     

Electrochemical reaction of 5 V cathode LiNi0.4Mn1.6O4

LiNi_0.4Mn_1.6O₄ was prepared under air and oxygen atmospheres using fine Mn₃O₄ particle and large MnO₂ particle at various temperatures. The sample prepared from Mn₃O₄ at 750 °C under air or oxygen atmosphere exhibited an ideal electrochemical behavior, which was based on three redox couples of Mn³⁺/Mn⁴⁺, Ni²⁺/Ni³⁺, and Ni³⁺/Ni⁴⁺. On the other hand, the sample prepared from MnO₂ had a larger capacity at 4 V plateau and smaller one at 5 V plateau. However, when using oxygen atmosphere, the sample exhibited more ideal behavior, which is similar to the sample prepared from Mn₃O₄. This means that some defects exist in LiNi_0.4Mn_1.6O₄, depending on preparation conditions. In order to confirm this point, the chemical composition and the valence state of Mn of the prepared sample was analyzed. From these results, it can be said that electrochemical reactions of LiNi_0.4Mn_1.6O₄ are well explained based on three redox couples of Mn³⁺/Mn⁴⁺, Ni²⁺/Ni³⁺, and Ni³⁺/Ni⁴⁺ by considering a presence of oxygen defects.     

Breakdown of an intermediate plateau in the magnetization process of anisotropic spin-1 Heisenberg dimer: Theory vs. experiment

The magnetization process of the spin-1 Heisenberg dimer model with the uniaxial or biaxial single-ion anisotropy is particularly investigated in connection with recent experimental high-field measurements performed on the single-crystal sample of the homodinuclear nickel(II) compound [Ni₂(Medpt)₂(μ-ox)(H₂O)₂](ClO₄)₂·2H₂O (Medpt=methyl-bis(3-aminopropyl)amine). The results obtained from the exact numerical diagonalization indicate a striking magnetization process with a marked spatial dependence on the applied magnetic field for arbitrary but finite single-ion anisotropy. It is demonstrated that the field range, which corresponds to an intermediate magnetization plateau emerging at a half of the saturation magnetization, basically depends on a single-ion anisotropy strength as well as a spatial orientation of the applied field. The breakdown of the intermediate magnetization plateau is discussed at length in relation to the single-ion anisotropy strength.     

Spin species conversion of rotational tunneling molecules studied by neutron transmission

The total proton spin of rotational tunneling molecules like CH₃, NH₃ or NH ₄ ⁺ groups has a strong influence on the total incoherent scattering cross section of long wavelength neutrons. At sufficiently low temperatures, a change in neutron scattering cross section after a rapid change in lattice temperature is the unequivocal signature of spin species conversion. This effect is used to study the time and temperature dependence of spin conversion. In a simple transmission experiment, we have screened 30 candidate substances of which 13 did not convert on a timescale of days. The other 17 converted: 10 contain paramagnetic ions and their spin temperature follows quasi instantaneously the lattice temperature. Seven substances convert slowly on a time scale of hours. Experimental results are presented and the advantages and limitations of the method are discussed.     

119Sn Mössbauer study of nickel–tin anodes for rechargeable lithium-ion batteries

¹¹⁹Sn Mössbauer spectrometry has been carried out on Ni–Sn alloys (Ni₃Sn LT, Ni₃Sn₂ LT and Ni₃Sn₄) and combined with ab initio calculations. Lithium insertion/extraction mechanisms of the most interesting compound (Ni₃Sn₄) have been studied from ¹¹⁹Sn Mössbauer measurements. The first discharge shows a plateau close to 0.0 V, which can be attributed to the formation of the Li₇Sn₂ alloy.     

Theoretical study of aging effects on LaNi<Subscript>5</Subscript> tritium storage

The electronic structure and energy of La₂Ni₁₀H₁₂ and La₂Ni₁₀H₁₁He (He is at tetrahedral, octahedral or twelve-face polyhedral interstitials) double cells have been calculated using the density functional theory. Their equilibrium structure, energy bands, electronic density of states (DOS) and X-ray diffraction are presented and discussed. The results indicate the helium-3 produced due to the decay of tritium is most possibly sited at twelve-face polyhedral and octahedral interstices and changes the thermodynamic properties of LaNi₅ tritide system. The changes due to aging such as the reduction in the isotherm plateau pressure, increase of the isotherm plateau slope, and appearance of deeply trapped hydrogen are caused not only by the lattice expansion, but also by modification of the electronic structure due to the presence of He.     

Theoretical study of aging effects on LaNi5 tritium storage

The electronic structure and energy of La₂Ni₁₀H₁₂ and La₂Ni₁₀H₁₁He (He is at tetrahedral, octahedral or twelve-face polyhedral interstitials) double cells have been calculated using the density functional theory. Their equilibrium structure, energy bands, electronic density of states (DOS) and X-ray diffraction are presented and discussed. The results indicate the helium-3 produced due to the decay of tritium is most possibly sited at twelve-face polyhedral and octahedral interstices and changes the thermodynamic properties of LaNi₅ tritide system. The changes due to aging such as the reduction in the isotherm plateau pressure, increase of the isotherm plateau slope, and appearance of deeply trapped hydrogen are caused not only by the lattice expansion, but also by modification of the electronic structure due to the presence of He.     

Hydrothermal synthesis and properties of manganese-doped LiFePO4

A series of carbon-coated LiFe_1???x Mn _x PO₄ compounds are prepared by a hydrothermal method at 170 °C for 12 h. The structure and morphology of the prepared composites are characterized to examine the effects of Mn²⁺ substitution. All LiFe_1???x Mn _x PO₄ compositions are found to have an ordered olivine-type structure with homogeneous Fe²⁺ and Mn²⁺ distributions. The substitution leads to grain refinement from ~500 to ~150 nm, as well as to increased initial capacity and improved electronic conductivity. The amount of carbon coating varies with increased doping amount. The discharge curves of the LiFe_1???x Mn _x PO₄/C materials reveal a high discharge plateau corresponding to Fe^2+/3+ and no obvious plateau assigned to Mn^2+/3+, although a slight contribution of manganese is detected. However, the electrochemical performance, including the discharge capacity and cyclic performance, deteriorates with increased Mn content in the composite.