Thermal–moisture dynamics and thermal stability of active layer in response to wet/dry conditions in the central region of the Qinghai–Tibet Plateau,China

The amount of rainfall varies unevenly in different regions of the Qinghai-Tibet Plateau, with some regions becoming wetter and others drier. Precipitation has an important impact on the process of surface energy balance and the energy-water transfer within soils. To clarify the thermal-moisture dynamics and thermal stability of the active layer in permafrost regions under wet/dry conditions, the verified water-vapour-heat coupling model was used. Changes in the surface energy balance, energy-water transfer within the soil, and thickness of the active layer were quantitatively analyzed. The results demonstrate that rainfall changes significantly affect the Bowen ratio, which in turn affects surface energy exchange. Under wet/dry conditions, there is a positive correlation between rainfall and liquid water flux under the hydraulic gradient;water vapour migration is the main form under the temperature gradient, which indicates that the influence of water vapour migration on thermalmoisture dynamics of the active layer cannot be neglected. Concurrently, regardless of wet or dry conditions,disturbance of the heat transport by conduction caused by rainfall is stronger than that of convection by liquid water. In addition, when rainfall decreases by 1.5 times(212 mm) and increases by 1.5 times(477 mm), the thickness of the active layer increases by 0.12 m and decreases by 0.21 m, respectively. The results show that dry conditions are not conducive to the preservation of frozen soil;however, wet conditions are conducive to the preservation of frozen soil, although there is a threshold value. When this threshold value is exceeded, rainfall is unfavourable for the development of frozen soil.

A Study of the Structural Properties and Thermal Stability of Chitosanases EAG1 by Molecular Dynamics Simulations

Chitosanases EAG1, a classical glycoside hydrolase from Bacillus ehimensis, is relatively unstable with higher temperature. This shortcoming seriously restricts its industrial application. Therefore, it is crucial to clarify the theoretical basis of thermo stability and to produce enzymes with high activity and stability. Using the structural modeling and molecular dynamical simulation, residues Leu84, Gly113, Asp116, Ala207 and Leu286 were believed to be the key residues for structural stability. Then the predicted residue Leu84 was mutated to ALA. It was shown that the L84A mutation can improve the thermal stability of chitosanases EAG1. Together with previous studies, mutations of G113C, D116C, A207C and L286C forms two sulfur bonds can change the thermal stability of EAG1. The results suggest that the thermal stability of EAG1 could be engineered by site-directed mutagenesis on the conserved residues. This protocol could be employed for improving thermal stability of other chitosanases EAG1.

Thermal Stability and Hot Carrier Dynamics of Gold Nanoparticles of Different Shapes

Anisotropic shapes of gold nanoparticles are prepared using a modified seed method in the presence of silver ions or clusters in order to study the thermal stability and the dynamics of the hot carriers induced by femtosecond laser pulses. Although gold nanospheres are stable towards thermal treatment, the decomposition of the gold nanorods into spherical nanoparticle aggregates upon thermal treatment is mechanistically different from the case of nanoprisms. Great enhancement of thermal stability is achieved by modifying the surface of the nanoparticles by adding specific amounts of polyvinyl pyrrolidone (PVP) after preparation of gold particles of different shapes capped with cetyltrimethylammonium bromide (CTAB). The surface plasmon resonance spectra of the gold nanostructures are used to monitor their morphological changes. In regards to the hot carrier dynamics, it is found that the phonon-phonon (ph-ph) coupling is much slower in dots than in rods and prisms while electron-phonon (e-ph) coupling is almost the same in these particles.

Thermal instability and dynamic response analysis of a tensioned carbon nanotube under moving uniformly distributed external pressure

Single-walled carbon nanotubes(SWCNTs)are receiving immense research attention due to their tremendous thermal,electrical,structural and mechanical properties.In this paper,an exact solution of the dynamic response of SWCNT with a moving uniformly distributed load is presented.The SWCNT is modelled via the theories of Bernoulli-Euler-thermal elasticity mechanics and solved using Integral transforms.The developed closed-form solution in the present work is compared with existing results and excellent agreements are established.The parametric studies show that as the magnitude of the pressure distribution at the surface increases,the deflection associated with the single walled nanotube increases at any mode whilst a corresponding increase in temperature and foundation parameter have an attenuating effect on deflection.Moreover,an increase in the Winkler parameter,as well as a decrease in the SWCNT mass increases its frequency of vibration.Furthermore,an increase in the speed of the external agent decreases the total external pressure as a result of the removal of dead loads.The present work is envisaged to improve the application of SWCNT as nanodevices for structural,electrical and mechanical systems.

Onset and Stability of Thermally-Driven Fluid Convection in a Vertical Rock Crack and Their Implication for Hydrothermal Ore-Forming Processes

Dynamic equations controlling the thermally-driven fluid convection in a single rock crack are established in the paper . The critical criteria for the onset of convection in such a configuration are found by linear dynamic analysis of the equations and the stability of the convection discussed by nonlinear analysis method . The research demonstrates that the critical Rayleigh number has a magnitude 103. While the Rayleigh number R of real ore-forming fluids exceeds this value , the convection happens , and as R becomes larger , the fluid convection pattern develops from nonequilibrium steady states to double-periodically produced limit cycles and eventually to chaos (turbulences ).The implication of these dynamic analyses for the ore-forming processes of late-magma tic hydrothermal deposits is also discussed in the paper .

Structure,stability,and motion of dislocations in double-wall carbon nanotubes

In this paper,a novel double-wall carbon nanotube（DWCNT） with both edge and screw dislocations is studied by using the molecular dynamics（MD） method.The differences between two adjacent tubule indexes of armchair and zigzag nanotubes are determined to be 5 and 9,respectively,by taking into account the symmetry,integrality,and thermal stability of the composite structures.It is found that melting first occurs near the dislocations,and the melting temperatures of the dislocated armchair and zigzag DWCNTs are around 2600 K-2700 K.At the premelting temperatures,the shrink of the dislocation loop,which is comprised of edge and screw dislocations,implies that the composite dislocation in DWCNTs has self-healing ability.The dislocated DWCNTs first fracture at the edge dislocations,which induces the entire break in axial tensile test.The dislocated DWCNTs have a smaller fracture strength compared to the perfect DWCNTs.Our results not only match with the dislocation glide of carbon nanotubes（CNTs） in experiments,but also can free from the electron beam radiation under experimental conditions observed by the high resolution transmission electron microscope（HRTEM）,which is deemed to cause the motion of dislocation loop.

Thermal stability of silicon nanowires:atomistic simulation study

Using the Stillinger Weber （SW） potential model,we investigate the thermal stability of pristine silicon nanowires based on classical molecular dynamics （MD） simulations.We explore the structural evolutions and the Lindemann indices of silicon nanowires at different temperatures in order to unveil atomic-level melting behaviour of silicon nanowires.The simulation results show that silicon nanowires with surface reconstructions have higher thermal stability than those without surface reconstructions,and that silicon nanowires with perpendicular dimmer rows on the two （100） surfaces have somewhat higher thermal stability than nanowires with parallel dimmer rows on the two （100） surfaces.Futher-more,the melting temperature of silicon nanowires increases as their diameter increases and reaches a saturation value close to the melting temperature of bulk silicon. The value of the Lindemann index for melting silicon nanowires is 0.037.

Design of Balanced Reactor Welded with Water-Cooled Aluminum Busbars

A new process of welding aluminum water-cooled busbars is proposed, It can not only reduce the weight and cost, but also improve the dynamic and thermal stability. Furthermore~ both finite element method analysis and a prototype test testify the advantages of the design which is not limited by load current and provides a new approach for water-cooled reactors.

Structural Origins for Enhanced Thermal Stability and Glass‑Forming Ability of Co–B Metallic Glasses with Y and Nb Addition

The effects of Y and Nb addition on thermal stability,glass-forming ability(GFA),and magnetic softness of Co75B25 metallic glass(MG)were comprehensively investigated.The experimental results indicated that the thermal stability,GFA,and magnetic softness of the studied MGs increase in the order Co_(75)B_(25)<Co_(73)Nb_(2)B_(25)<Co_(71.5)Y_(3.5)B_(25)<Co_(69.5)Y_(3.5)Nb_(2)B_(25).The structural origins of the improved properties were revealed by ab initio molecular dynamics(AIMD)simulations and density functional theory(DFT)calculations.Results showed that the B-centered prism units are the primary structure-forming units of the four MGs,connect through vertex-,edge-,and face-shared(VS,ES,and FS)atoms,and Co-centered units tend to connect with Co/B-centered units via the intercross-shared(IS)atoms.The addition of Y and Nb not only plays the role of connecting atoms but also enhances both bond strengths and the fractions of icosahedral-like units in increasing order Co_(75)B_(25)<Co_(73)Nb_(2)B_(25)<Co_(71.5)Y_(3.5)B_(25)<Co_(69.5)Y_(3.5)Nb_(2)B_(25),which is conducive to the enhancement of the structural stability,atomic packing density,and viscosity,thereby improving thermal stability and GFA.In addition,the improvement of structural stability and homogeneity leads to enhanced magnetic softness.

典型海上热发射动力学效应及影响因素分析

考虑海上发射环境特殊性,分析了发射船承受的典型海浪以及风环境,针对陆基发射时运载火箭直接坐落在发射台上的裸箭热发射方式,分析了该支撑形式在海上动态环境下的射前稳定性,获得海上裸箭热发射方式的承受晃动极限。为解决陆基热发射支撑方式不满足三级海况发射稳定性的问题,提出运载火箭的框架式扶稳措施,通过框架内的柔性支撑保证运载火箭的射前稳定性。构建由火箭、等效甲板、导向框架、导轨、柔性支撑、发射台等组成的发射系统动力学仿真模型,开展复杂海况条件下的发射动力学分析,获得了框架式热发射方式在典型海况条件下,不同因素对火箭出框过程飞行姿态、安全间隙等影响规律,可为运载火箭框架式热发射的海上动基座发射动力学安全性评估提供技术支撑,并用于框架式热发射方式的海上动基座环境适应性的综合评价。

考虑功率变化的过负荷输电线路热稳安全裕度计算方法

随着电网互联复杂程度的提高,热稳定性已成为限制电网输电能力的主要因素。特别是在输电线路过负荷情况下,现有方法忽略了线路功率变化引起的动态热特性,使得线路热稳安全裕度计算存在误差,可能导致保护过早动作而切除线路,甚至威胁整个电力系统的安全稳定运行。为此,本文分析了影响输电线路温升过程的因素,建立了机组运行方式、功率调节与过负荷线路功率之间的灵敏度模型,并结合线路动态热平衡方程,提出了一种考虑功率变化的过负荷线路热稳安全裕度计算方法。算例表明,该方法充分考虑了过负荷线路功率变化对热稳定性的影响,在确保输电线路安全运行的同时,能最大限度地延迟保护的动作,从而减小停电范围和连锁故障。

Quasi-two-dimensional strong liquid-like dynamics of surface atoms in metallic glasses

The fast dynamic properties of the surface of metallic glasses(MGs) play a critical role in determining their potential applications. However, due to the significant difference in thermal history between atomic simulation models and laboratory-made samples, the atomic-scale behaviors of the fast surface dynamics of MGs in experiments remain uncertain. Herein, we prepared model MG films with notable variations in thermal stability using a recently developed efficient annealing protocol, and investigated their atomic-scale dynamics systematically. We found that the dynamics of surface atoms remain invariant, whereas the difference in dynamical heterogeneity between surface and interior regions increases with the improvement of thermal stability. This can be associated with the more pronounced correlation between atomic activation energy spectra and depth from the surface in samples with higher thermal stability. In addition, dynamic anisotropy appears for surface atoms, and their transverse dynamics are faster than normal components, which can also be interpreted by activation energy spectra. Our results reveal the presence of strong liquid-like atomic dynamics confined to the surface of laboratory-made MGs, illuminating the underlying mechanisms for surface engineering design, such as cold joining by ultrasonic vibrations and superlattice growth.

磷系阻燃改性聚酯的流变性能

为探究2-羧乙基苯基次磷酸(CEPPA)作为阻燃剂改性合成的阻燃聚酯的可纺性,分别用DSC、TG、毛细管流变仪、旋转流变仪对阻燃改性聚酯进行热稳定性和流变性能测试。结果表明:阻燃聚酯的熔融温度、初始降解温度略微降低,残碳率提高,说明阻燃聚酯的加工温度可降低且成碳效果变好。阻燃聚酯是典型的非牛顿流体,其剪切黏度随着剪切速率的增加而降低,随温度的升高而降低,相比普通聚酯,改性后的阻燃聚酯黏度更低,流动性更好;阻燃改性聚酯的非牛顿指数n均小于1,且随着剪切速率的增加而减小;其黏流活化能随着剪切速率的增加而降低,反映了阻燃聚酯随着剪切速率的增加而对温度的敏感性变小。该研究结果可为阻燃聚酯纤维的制备工艺和参数调整提供参考。

SBS改性沥青热储存稳定性探讨及其等级划分

针对SBS改性沥青储存稳定性问题日益突出的现状,该文对改性沥青离析现象的本质进行了探讨,提出了离析指标等级划分建议,并对改性沥青储存稳定性快速检测新方法进行了总结。综合分析表明,改性沥青储存稳定性的优劣与体系的相容性有关,而改性沥青的离析现象是由热力学与动力学因素共同决定的;为实现对SBS改性沥青储存稳定性的精准评价,建议将离析指标划分为Ⅰ~Ⅴ五个等级;随着分析手段的不断进步,快速化、精准化、标准化已成为改性沥青储存稳定性快速检测技术的发展方向。

单体中仲胺类化合物含量对长碳链尼龙1012性能的影响

以癸二胺和十二碳二元酸为原料制备尼龙1012盐,然后将双六甲基三胺作为含仲胺结构的化合物引入到尼龙1012盐聚合反应中制备尼龙1012,考察双六甲基三胺含量对尼龙1012性能的影响。结果表明:随着双六甲基三胺含量的增加,尼龙1012的相对分子质量逐渐增大,相对分子质量分布明显变宽,双六甲基三胺质量分数从0增加至2.0%,聚合物分散性指数从2.48上升到4.29;当双六甲基三胺质量分数大于1.0%时,尼龙1012的动态流变性能表现出弹性大于黏性的行为;随着双六甲基三胺含量的增加,尼龙1012的熔融温度和结晶温度均呈现逐渐降低趋势,而热分解温度逐渐上升;双六甲基三胺质量分数从0增加至1.0%,尼龙1012的拉伸强度逐渐降低,缺口冲击强度逐渐增大;双六甲基三胺的仲胺结构参与了聚合反应,形成了支化、交联结构,交联结构的增加有利于提高尼龙1012的拉伸强度。

Molecular dynamics simulation of thermal stability of nanocrystalline vanadium

The microstructure and thermal stability of nanocrystalline vanadium with an average grain size ranging from 2.86 to 7.50 nm are calculated by means of the analytic embedded-atom method and molecular dynamics. The grain boundary and nanocrystal- line grain atoms are differentiated by the common neighbor analysis method. The results indicate that the fraction of grain boundary increases with the grain size decreasing, and the mean energy of atoms is higher than that of coarse crystals. The thermal-stable tem- peratures of nanocrystalline vanadium are determined from the evolution of atomic energy, fraction of grain boundary and radial distribution function. It is shown that the stable tem- perature decreases obviously with the grain size decreasing. In addition the reasons which cause the grain growth of nanocrystalline vanadium are discussed.

DYNAMIC STABILITY ANALYSIS OF EMBEDDED MULTI-WALLED CARBON NANOTUBES IN THERMAL ENVIRONMENT

In the present paper, the dynamic stability of multi-walled carbon nanotubes （MW- CNTs） embedded in an elastic medium is investigated including thermal environment effects. To this end, a nonlocal Timoshenko beam model is developed which captures small scale effects. Dynamic governing equations of the carbon nanotubes are formulated based on the Timoshenko beam theory including the effects of axial compressive force. Then a parametric study is conducted to investigate the influences of static load factor, temperature change, nonlocal parameter, slenderness ratio and spring constant of the elastic medium on the dynamic stability characteristics of MWCNTs with simply-supported end supports.

Polymer Fiber Rigid Network with High Glass Transition Temperature Reinforces Stability of Organic Photovoltaics

Organic photovoltaics(OPVs)need to overcome limitations such as insufficient thermal stability to be commercialized.The reported approaches to improve stability either rely on the development of new materials or on tailoring the donor/acceptor morphology,however,exhibiting limited applicability.Therefore,it is timely to develop an easy method to enhance thermal stability without having to develop new donor/acceptor materials or donor–acceptor compatibilizers,or by introducing another third component.Herein,a unique approach is presented,based on constructing a polymer fiber rigid network with a high glass transition temperature(T_(g))to impede the movement of acceptor and donor molecules,to immobilize the active layer morphology,and thereby to improve thermal stability.A high-T_(g) one-dimensional aramid nanofiber(ANF)is utilized for network construction.Inverted OPVs with ANF network yield superior thermal stability compared to the ANF-free counterpart.The ANF network-incorporated active layer demonstrates significantly more stable morphology than the ANF-free counterpart,thereby leaving fundamental processes such as charge separation,transport,and collection,determining the device efficiency,largely unaltered.This strategy is also successfully applied to other photovoltaic systems.The strategy of incorporating a polymer fiber rigid network with high T_(g) offers a distinct perspective addressing the challenge of thermal instability with simplicity and universality.

The Application of Thermomechanical Dynamics (TMD) to Thermoelectric Energy Generation by Employing a Low Temperature Stirling Engine

A thermoelectric generation Stirling engine (TEG-Stirling engine) is discussed by employing a low temperature Stirling engine and the dissipative equation of motion derived from the method of thermomechanical dynamics (TMD). The results and mechanism of axial flux electromagnetic induction (AF-EMI) are applied to a low temperature Stirling engine, resulting in a TEG-Stirling engine. The method of TMD produced thermodynamically consistent and time-dependent physical quantities for the first time, such as internal energy ℰ(t), thermodynamic work Wth(t), the total entropy (heat dissipation) Qd(t)and measure or temperature of a nonequilibrium state T˜(t). The TMD analysis produced a lightweight mechanical system of TEG-Stirling engine which derives electric power from waste heat of temperature (40˚CT100˚C) by a thermoelectric conversion method. An optimal low rotational speed about 30θ′(t)/(2π)60(rpm) is found, applicable to devices for sustainable, clean energy technologies. The stability of a thermal state and angular rotations of TEG-Stirling engine are specifically shown by employing properties of nonequilibrium temperature T˜(t), which is also applied to study optimal fuel-injection and combustion timings of heat engines.

功能化石墨烯改性聚乙烯热力学特性的分子动力学模拟

聚乙烯绝缘材料在我国高压电缆中有着广泛的应用.为了提高其耐热稳定性和力学性能,利用石墨烯对聚乙烯进行掺杂改性,并基于分子动力学模拟的研究方法分别建立了低密度聚乙烯(LDPE)、石墨烯和3种官能团接枝石墨烯掺杂聚乙烯的复合模型.研究表明,相比石墨烯直接掺杂聚乙烯,羧基(-COOH)、氨基(-NH)和羟基(-OH)接枝石墨烯能够更加有效地提高聚乙烯的玻璃化温度(分别提高了16 K、7 K、5 K),减弱聚乙烯分子链的移动和降低聚乙烯的热膨胀系数、均方位移(MSD),从而使得聚乙烯复合体系的热学性能得到了有效增强.此外发现石墨烯的掺杂能够提高复合模型的力学模量,其中官能团接枝石墨烯改进效果更明显,室温下弹性模量和剪切模量的提升幅度由不接枝的33.98%、36.18%,提升到了44%和42.89%(羧基功能化体系).研究结果可为聚乙烯绝缘材料的热老化抑制和力学性能的改善提供有益的参考.