Influence of size effect and boundary conditions on temperature overshooting in nanoscale thermal conduction

The temperature overshooting phenomenon in one-dimensional nanoscale heat conduction in thin films is studied for various boundary conditions.The results show that when ballistic heat transport strongly affects the heat transport process,temperature overshooting is more likely to occur.A sudden increase of temperature on only one surface of a thin film cannot trigger temperature overshooting,while symmetric boundary temperature perturbations lead to the largest temperature overshooting.Twodimensional heat conduction is also studied in a nanoscale area.The analytic results show that Fourier’s law may severely underestimate the temperatures in nanofilms as well as in nanoareas when temperature overshooting occurs.     

地物温度波及其应用

根据热传导方程和热传导的付里叶定律,推导出地物温度波和热流密度波的表达式,提出一些修正因素,并讨论温度波在热红外遥感,气象学和建筑学中的应用。     

Non-Fourier Heat Conduction Effects During High-Energy Beam Metalworking

Non-Fourier heat conduction induced by ultrafast heating of metals with a high-energy density beam was analyzed. The non-Fourier effects during high heat flux heating were illustrated by comparing the transient temperature response to different heat flux and material relaxation times. Based on the hyperbolic heat conduction equation for the non-Fourier heat conduction law, the equation was solved using a hybridmethod co mbining an analytical solution and numerical inversion of the Laplace transforms for a semiinfinite body with the heat flux boundary. Analysis of the temperature response and distribution led to a criterion for the applicability of the non-Fourier heat conduction law. The results show that at a relatively large heat flux, such as greater than 108 W/cm2, the heat-affected zone in the metal material experiences a strong thermal shock as the non-Fourier effects cause a large step increase in the surface temperature. The results provide a method for analyzing transient heat conduction problems using a high-energy density beam, such as electron beam deep penetration welding.     

Molecular dynamics study on thermal conductivity of nanoscale thin films

A simple and effective model of heat conduction across thin films is set up and molecular dynamics simulations are implemented to explore the thermal conductivity of nanoscale thin dielectric films in the direction perpendicular to the film plane. Solid argon is selected as the model system due to its reliable experimental data and potential function. Size effects of the thermal conductivity across thin films are found by computer simulations: in a film thickness range of 2–10 nm, the conductivity values are remarkably lower than the corresponding bulk experimental data and increase as the thickness increases. The consistency between the approximate solution of the phonon Boltzmann transport equation and the simulation results ascribes the thermal conductivity size effect to the phonon scattering at film boundaries.     

Thermal conductivity of nanoscale thin nickel films

The inhomogeneous non-equilibrium molecular dynamics (NEMD) scheme is applied to model phonon heat conduction in thin nickel films. The electronic contribution to the thermal conductivity of the film is deduced from the electrical conductivity through the use of the Wiedemann-Franz law. At the average temperature of T=300 K, which is lower than the Debye temperature ΘD=450 K, the results show that in a film thickness range of about 1?11 nm, the calculated cross-plane thermal conductivity decreases almost linearly with the decreasing film thickness, exhibiting a remarkable reduction compared with the bulk value. The electrical and thermal conductivities are anisotropic in thin nickel films for the thickness under about 10 nm. The phonon mean free path is estimated and the size effect on the thermal conductivity is attributed to the reduction of the phonon mean free path according to the kinetic theory.     

Non-Fourier heat conduction study for steady states in metallic nanofilms

As a fundamental theory of heat transfer, Fourier’s law is valid for most traditional conditions. Research interest in non-Fourier heat conditions is mainly focused on heat wave phenomena in non-steady states. Recently, the thermomass theory posited that, for steady states, non-Fourier heat conduction behavior could also be observed under ultra-high heat flux conditions at low ambient temperatures. Significantly, this is due to thermomass inertia. We report on heat conduction in metallic nanofilms from large currents at low temperatures; heat fluxes of more than 1×1010 W m 2 were used. The measured average temperature of the nanofilm is larger than that based on Fourier’s law, with temperature differences increasing as heat flux increased and ambient temperature decreased. Experimental results for different film samples at different ambient temperatures reveal that non-Fourier behavior exists in metallic nanofilms in agreement with predictions from thermomass theory.     

氮气氛围下热处理的Na（0.5）Bi（0.5）TiO3薄膜的低温介电性能研究

采用溶胶凝胶法在LiNaO3（LNO）衬底上制备Na0.5Bi0.5TiO3（NBT）薄膜.部分NBT薄膜在氮气氛围下进行了热处理,记为NBT-N2.在100～400 K的温度范围内,测量了NBT-N2的介电性能.NBT-N2薄膜的介电频谱中出现了损耗峰,存在弛豫现象.利用Arrhenius公式拟合弛豫所对应的激活能,发现在NBT-N2薄膜中存在2个热激活过程：一个对应氧空位电离引入的极化子的跳跃跃迁;另一个对应导电电子与偏离氧八面体中心的Ti离子之间的耦合.此外,采用复阻抗谱分析了NBT-N2薄膜中的介电弛豫过程,在测试温度范围内,随着温度的升高,该弛豫过程逐渐趋向于理想的德拜模型.     

带第二声速的变系数热弹性力学方程组间断解的性态

主要研究一维空间变量的带第二声速的变系数线性及半线性热弹性力学方程Cauchy问题间断解的渐近性态.当松弛系数趋于零时,得到弹性体温度的间断将消失,弹性波和热流量的间断将以弹性波的速度传播;这些间断随时间增长呈指数衰减,且衰减率与非线性项的增长率、热传导系数、弹性波速度的变化率有关.     

基于辛方法的二维非稳态热传导问题研究

基于二维热传导理论,通过引入对偶变量,推导了非稳态热传导温度场问题的辛对偶方程组。采用分离变量法和本征展开方法,建立起一种本征值和本征解的直接求解方法,得到了适用于任意跨厚比的平面非稳态问题的解析解。由于在求解过程中不需要事先人为地选取试函数,而是从基本方程出发,直接利用数学方法求出问题的解,使得问题的求解更加合理化。探讨不同跨厚比、不同时间步长情况下温度和热流密度的分布规律,并与已有解进行比较。结果表明,辛方法是一类可行的研究非稳态热传导的方法。考虑到非零本征值本征解具有局部性特点,进一步讨论不同跨厚比、不同时间情况下温度和热流密度分布的端部效应问题。为非稳态问题的理论及实际应用研究提供了新的途径。     

脉冲热流作用下球体内的非傅立叶热传导

分析了球形物体表面遭受一随时间变化脉冲热流时的双曲型非傅立叶热传导问题．给出了此类超急速传热情形下的热传导方程、边界条件及初始条件的无量纲形式,采用Ｌａｐｌａｃｅ变换技术,求得了任意时刻球体内部温度分布的解析解．作为算例,计算了方波脉冲这类随时间变化热流作用下球体内的温度变化,结果表明,该类超急速热传导问题与常规的傅立叶热传导相比具有明显不同的特征     

燕尾形轴向槽道热管的蒸发冷凝传热特性

建立了燕尾形轴向槽道热管蒸发和冷凝薄液膜传热特性理论模型,并对模型进行了数值求解.对蒸发薄液膜区液膜厚度、接触面温度和热流密度分布进行了分析,给出了汽液接触面蒸发/冷凝传热系数沿轴向的变化.研究表明:在蒸发薄液膜区域,薄液膜厚度沿槽壁方向呈线性增加;汽液接触面的温度在起点几乎和壁面温度相同,随着薄液膜厚度的增加而迅速降低;在薄液膜的起始段,热流密度快速达到最大值,随即迅速减小.蒸发段的蒸发传热系数大于冷凝段的冷凝传热系数,蒸发/冷凝传热系数在整个绝热段并不都为零.同时,通过实验验证了模型的正确性.     

金属薄膜在短脉冲激光加热下的温度响应

利用非经典的双相延迟DPL(Dual曲ase lag)热传导方程,采用稳定的有限差分方法求解了金属薄膜在超短脉冲激光辐射作用下的温度响应.探讨了薄膜内温度响应随温度延迟相与热流延迟相的不同而产生的变化趋势,讨论了在微时间和微空间尺度条件下金属薄膜随不同的激光加热以及薄膜厚度而引起的热响应特征.     

膜状冷凝初期过程的分子动力学模拟研究

采用分子动力学模拟方法,对氩蒸气在铂金属表面发生的膜状冷凝过程进行了研究.为保证冷凝过程在相对较长时间范围内持续稳定进行,提出了一种改进的气态分子补充方法.通过逐时对系统内局部温度及密度进行统计,获得了不同时刻的参数分布.结果显示:在模拟时间范围内,液膜厚度近似线性增加,壁面附近液相分子受固壁势能作用而呈现出密度振荡的"液体层状化"分布;液膜内产生温度梯度,固液界面处温度跳跃现象明显.考察了气体温度以及壁面润湿性变化产生的影响,结果表明:随着气体温度的升高,温度梯度以及温度跳跃均增大;液相密度略有下降,液体内层状区域的密度振荡范围略有减小,气液界面厚度增加;质量流率以及液膜厚度增长速率也都增大,反映出更大的气固温差加快了冷凝过程的进行,这一点与宏观规律一致.随着润湿性增强,液膜厚度增长加快,液体层状区内的密度振荡范围增加,液膜内温度梯度增大,温度跳跃大幅减小,冷凝过程得到显著强化.显然,近壁面区内的热传导对整个冷凝过程进行的速度具有重要影响.     

生物质材料在秒量级脉冲激光加热下的导热特性研究

通过系统的实验,对常温下生物质材料在秒量级脉冲加热下的导热特性进行了研究,并且针对本实验的条件分别进行了抛物线型和双曲线型两种导热模型的数值模拟,结果发现,傅里叶模型的计算结果和实验结果非常吻合,而双曲线模型的计算结果和实验结果相差很大,这说明在本实验的条件下,在所用的生物质材料中没有发现非傅里叶现象,同时也说明某些文献所述各种生物质材料的热松弛时间为秒量级的结论还有待商榷。     

微桥量热计测量铝薄膜热容

用表面硅微加工工艺研制了一种用于薄膜热容测量的新型微桥量热计，热性能测试与分析表明，微桥量热计温度响应快，温度均匀性较好，采用脉冲量热法，通过测量量热计的热功耗、瞬态温度以及稳态热功耗，可计算量热计和样品薄膜的热容．在真空中300～420K测量了40～1150nm厚的Al薄膜热容，并测量了样品的贡量，获得了样品的比热容，1150nm厚的Al薄膜比热容与Al体材料比热容的文献值吻合较好．随着厚度的减小，Al薄膜的比热容增强，这种现象在高温时更为显著．     

BiFeO_3薄膜的so-lgel制备及其铁电性能研究

采用sol-gel方法在Pt(111)/Ti/SiO2/Si(100)衬底上制备出了(100)择优取向的BiFeO3薄膜.XRD研究表明,600～650℃退火的薄膜结晶较好.AFM形貌显示,650℃退火的薄膜中等轴状晶粒大小均匀(直径100～150nm),薄膜较为致密.电学性能测量结果表明,650℃退火、厚度为840nm的薄膜的2Pr值为2.8mC/cm2;在50kV/cm外加电场下,漏电流为2.7×10-5 A/cm2.电流-电压特性显示,在欧姆区之上,薄膜的主要导电机制为波尔-弗兰克尔发射导电.     

超快脉冲激光烧蚀过程中的靶材温度分布

讨论了超快脉冲激光烧蚀靶材时的烧蚀特性及激光频率对温度分布的影响．从包含热源项的导热方程出发，详细研究了在纳秒级脉冲激光作用下单次脉冲和多次脉冲时靶材表面温度的变化规律．结果表明，用超快脉冲激光制膜可以有效地减小烧蚀期间靶材温度的大幅度波动．从而为克服传统PLD制膜技术上的制膜厚度的不均匀及大颗粒粒子的产生从理论上提供了依据。     

A1掺杂ZnO薄膜的结构研究

采用直流反应磁控溅射方法,在玻璃衬底上制备了Al掺杂ZnO薄膜.系统研究了不同氩氧体积比、不同退火温度和氛围、不同Al掺杂浓度对薄膜结构的影响.结果显示,Al掺杂ZnO薄膜为多晶薄膜,呈六角纤锌矿结构,制备条件和后期处理以及掺杂浓度对薄膜的微结构有着很大的影响.在氩氧体积比6:1条件下制备的薄膜,经真空500℃退火后具...     

CVD金刚石膜的压阻及其温度特性

在Fuchs-Sondheimer(F-S)薄膜理论的基础上，通过求解弛豫近似下的Bohzmann方程，求出了P-型金刚石膜的电导率。在价带分裂模型下得到了压阻的计算公式，计算了微应力作用下金刚石膜的压阻，压阻和应力的变化(△ε)有比较好的线性关系，张应力情况下压阻大于零，压应力情况下压阻小于零。从理论上给出了压阻和温度的关系，计算了压阻随温度的变化，压阻随着温度的上升而单调减小。     

时域间断Galerkin有限元法在激光热加工过程中应用

针对半无限体和薄膜结构受多种激光热源作用下的非傅里叶热传导过程,采用时域间断Galerkin有限元法进行数值仿真.其主要特点是在时域内对温度及其时间导数分别进行三次Hermite插值和线性插值.对于半无限激光热源热传导问题,其计算结果与解析解吻合良好.算例表明,时域间断Galerkin有限元法在高频激光脉冲问题中,没有...