Multicaloric effect in a piezoelectric layer

Changes in the temperatures and entropy of a piezoelectric layer under the electric field and stresses applied to its boundaries (a multicaloric effect) have been investigated. It has been shown that these changes are composed of three terms, which describe the electrocaloric, elastocaloric, and piezoelectrocaloric effects, respectively. If the influence of the strain gradient on the polarization is taken into account, one more term associated with the flexoelectrocaloric effect arises in the entropy change. The influence of the multicaloric effect on losses and motion of domain walls in ferroelectrics has been discussed.     

Multicaloric effect in a solid: New aspects

The multicaloric effect that is the result of interaction between various caloric effects has been studied theoretically. The effects attributable to the pairwise interactions of fields (piezomagnetocaloric, piezoelectrocaloric, and magnetoelectrocaloric effects) have been added to the previously known electrocaloric, magnetocaloric, and elastocaloric effects that exist when the electric, magnetic, and elastic fields change. These new effects are shown to be determined by the temperature dependence of the piezomagnetic (magnetostrictive), piezoelectric (electrostrictive), and magnetoelectric coefficients. According to the estimates obtained, the change in entropy in an isothermal process under the magnetoelectrocaloric effect for Cr₂O₃ is 2–5 mJ kg^?1 K^?1. The caloric effects caused by the influence of the gradient in one of the fields on other fields are shown to contribute to the multicaloric effect. One of these gradient effects, the flexocaloric one, which consists in a change in temperature and/or entropy when a strain gradient is applied or removed, has been studied in detail as an example. It follows from the derived formulas that the greatest values of this effect should be expected for materials with strong temperature dependences of the flexocaloric coefficient, permittivity, or permeability. The change in temperature calculated from experimental data for a PMN ferroelectric is estimated as 2–6 mK at a strain gradient of 1 m^?1. The interaction between fields of a different nature is known to lead to the synergetic effect, and the multicaloric effect can reach values that are commonly called giant ones, expanding considerably the possible domains of its application.     

Evaluation of the Thermodynamic Efficiency of Solid-State Coolers and Generators Based on the Multicaloric Effect

Technical Physics - The efficiency of using the multicaloric effect (μCE) in solid-state cooling systems is investigated and compared with single caloric effects. The proposed approach is...     

Temperature-electric field hysteresis loop of multicaloric effects in PbZr0.8Ti0.2O3 thin films

The Multicaloric effect in the PbZr_0.8Ti_0.2O₃ thin films is investigated with the application of sine wave electric field, dc electric field and stress using a phase field method combined with the thermodynamic analysis. The simulation results show that the adiabatic temperature change-electric field curve presents a shape of butterfly in the presence of the sine wave electric field. In order to detect the effect of the sine wave electric field, the multicaloric effect and the domain structures under the direct electric field and the sine wave electric field are compared. It is found that the domain switching behaviors are quite different under the different applied electric fields. And the negative multicaloric effect in the PbZr_0.8Ti_0.2O₃ thin film is attribute to the domain switching under the external field.     

Electrical Conductivity of Noble Gases at High Pressures

We present experimental and theoretical results for the electrical conductivity of noble gases (He, Ne, Ar, Kr, Xe) up to high pressures where a transition from nonmetallic to metallic‐like conductivities occurs. In addition, we show the behavior of the thermal conductivity and thermopower for xenon as an example. The experiments were performed using explosively driven shock waves. Different geometries allow to probe various parameter regions up to several megabars. Besides single‐shock experiments along the principal Hugoniot curve, also multiple‐shock experiments were performed which follow almost an isentrope. The theoretical calculations were performed within a partially ionized plasma model. The composition is determined by solving a system of mass action laws. The transport coefficients are calculated within linear response theory taking into account the relevant scattering mechanisms of electrons at different ion species, atoms, and other electrons. The general trends of the experimental results can be explained within this theoretical approach. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Negative thermal expansion: Mechanisms and materials

Negative thermal expansion (NTE) of materials is an intriguing phenomenon challenging the concept of traditional lattice dynamics and of importance for a variety of applications. Progresses in this field develop markedly and update continuously our knowledge on the NTE behavior of materials. In this article, we review the most recent understandings on the underlying mechanisms (anharmonic phonon vibration, magnetovolume effect, ferroelectrorestriction and charge transfer) of thermal shrinkage and the development of NTE materials under each mechanism from both the theoretical and experimental aspects. Besides the low frequency optical phonons which are usually accepted as the origins of NTE in framework structures, NTE driven by acoustic phonons and the interplay between anisotropic elasticity and phonons are stressed. Based on the data documented, some problems affecting applications of NTE materials are discussed and strategies for discovering and design novel framework structured NET materials are also presented.     

One-dimensional fluctuating nanodomains in the charge-transfer molecular system TTF-CA and their first-order crystallization

We report on the direct observation by x-ray diffuse scattering measurements of thermally induced one-dimensional nanoscale ordered fluctuations in the mixed-stack charge-transfer molecular system tetrathiafulvalene-p chloranil (TTF-CA), the prototype for the neutral-ionic phase transition. The unusual physical properties of this compound are considered to be driven by such one-dimensional excitations. The results are discussed in relation to previous experimental and theoretical experiments both at thermal equilibrium and under light irradiation.     

相场再结晶储能释放模型与显微组织演变的模拟研究

在相场再结晶模型中提出了形式为 f(η_i^k,η_j^k) = E_s (η_i^de )² (1-(η_j^re)²)的冷变形储能项,并应用该模型模拟了 AZ31镁合金的再结晶过程,模拟结果和实验观测结果符合很好.研究表明, 引入储能释放模型可以实现再结晶形核物理过程的模拟; 模拟结果可以把合金在冷变形后退火的过程按照机理分为再结晶和热晶粒 长大两个阶段,模拟得出的理论再结晶时间是实验再结晶时间的2/3. 考察了冷变形应变大小对形变金属的亚晶粒尺寸和储能的影响机理和 试验结果,并将考察结果代入到改进后的再结晶模拟模型, 成功地再现了一个经典实验结果:随预先应变量的增加, 存在临界应变量对应的一个再结晶晶粒尺寸峰值.同时还给出了 这一经典实验结果的理论解释.     

Nonlinear phenomena in quantum thermoelectrics and heat

We review recent developments in nonlinear quantum transport through nanostructures and mesoscopic systems driven by thermal gradients or in combination with voltage biases. Low-dimensional conductors are excellent platforms for analyzing both the thermoelectric and heat dynamics beyond the linear response because, due to their small size, a small temperature difference applied across regions gives rise to large thermal biases. We offer a theoretical discussion based on the scattering approach to highlight the differences between the linear and the nonlinear regimes of transport. We discuss recent experiments on quantum dots and molecular junctions subjected to strong temperature differences. Theoretical predictions concerning the Kondo effect and heat rectification proposals are briefly examined. An important issue is the calculation of thermoelectric efficiencies including nonlinearities. Cross Seebeck effects and nonlinear spin filtering arise in superconductors and topological insulators, while mixed noises between charge and heat currents are also considered. Finally, we provide an outlook on the possible future directions of the field.     

低杂波电流驱动的数值模拟

通过求解二维FokkerPlanck方程,得到了HT7托卡马克低杂波驱动电流随时间的演化关系及其空间分布,理论值与实验结果基本符合,为HT7托卡马克实验提供了理论依据.通过解电流扩散方程,考虑了快电子的径向扩散效应对驱动电流的影响 关键词： 低杂波电流驱动 数值模拟 径向扩散     

低功率激光热透镜效应的模拟及研究

与高功率激光相比,低功率激光更安全,其热透镜效应对激光功率变化更敏感,更适合用于实验教学.本文利用有限元法模拟低功率激光的热透镜效应,模拟介质温度场和等效热透镜焦距,定量探究低功率激光热透镜焦距与激光功率以及介质热吸收率、厚度的关系.模拟结果与实验数据符合良好,为探究低功率激光热透镜效应实验提供理论依据.     

Understanding the role of fire retardants on the discontinuous ignition of wildland fuels

This work reports on a theoretical and experimental study on the role of fire retardant treatments on the discontinuous ignition of wildland fuels. The effect of the concentration of fire retardant in the solution applied to the vegetation is as expected to increase the ignition delay time. We found that the fire retardant modifies the fuel bed effective thermophysical properties, delaying the thermal response of the specimen when subjected to an incident heat flux. Nevertheless, the critical heat flux remains unaltered within the experimental error. We followed a proven approach based on the thermal ignition theory and testing which however has not been previously employed to study fire retardants on wildland fuels. To carry this out, we performed experiments on the I-FIT apparatus, which yields repeatable results and controlled boundary conditions. The theoretical model shows a good agreement with the experimental results, delivering simple expressions for pencil-and-paper calculations of the ignition delay time and analytical tools to evaluate effective fuel properties. These results will help CONAF and other forest services around the world to gain insight on the optimal concentrations and delivery methods for these types of products during wildfire response.     

Response of plates with unconstrained layer damping treatment to random acoustic excitation,part II: Response evaluation

Theoretical and experimental investigations on the response of a plate with unconstrained layer damping treatment to random acoustic excitation have been carried out. The theoretical response evaluation consisted of determining the power spectral density of the acceleration response of the layered plate by the use of generalized harmonic analysis under a specific random acoustic excitation, with use being made of modal frequencies and associated loss factors estimated as described in Part I. A study was made on the contribution of cross coupling terms of the acceleration response for the two boundary conditions investigated: namely, all edges simply supported and all edges clamped. In the experimental investigation, plates with different damping layer thicknesses were subjected to high intensity random acoustic excitation generated by an exponential horn driven by an electropneumatic transducer. The acceleration responses were recorded and later analyzed to yield the power spectral densities. Experimental and theoretical results are compared.     

用于光生毫米波的双频微片激光器热致频差特性研究

研究了激光二极管端面抽运的双纵模双频微片激光器的热效应对其输出频谱的影响. 通过对端面抽运Nd:YVO₄微片的热传导方程求解得出晶体内部的温度场分布; 并进一步对微片晶体热致折射率变化进行分析, 由此计算出双频微片激光器的热致频谱变化. 实验结果表明: 当抽运功率较小时得到稳定的双纵模双频激光输出; 当抽运功率增大时, 晶体热效应使双频激光频谱双峰之间的频差减小, 各纵模波长频谱宽度加宽, 实验结果与理论分析相符合. 关键词： 光载无线通信 双频微片激光器 热效应 频率差     

The solution of the kinetic equation for phonon heat conductivity by the method of momenta and the influence of isotopic disorder on phonon heat conductivity of germanium and silicon crystals at T=300 K

The problem of solving the kinetic equation for the heat conductivities of dielectric substances and semiconductors by the method of momenta is discussed. Microscopic models are used to estimate the effect of isotopic disorder on the thermal resistance of germanium crystals in the multimomentum approximation. The contributions of acoustic and optical phonons are taken into account. Excess thermal resistance ΔW of samples with a natural isotopic composition compared with highly enriched samples is calculated. For germanium, the theoretical and experimental Δ W values are in close agreement with each other. For silicon, the theoretical ΔWvalue is much smaller than its experimental excess thermal resistance.     

WALL CONDUCTION EFFECT ON THE ONSET OF THERMAL INSTABILITY IN HORIZONTAL RECTANGULAR CHANNELS

The effect of peripheral thermal conduction in the bottom wall on the onset of thermal instability in the thermal entrance region of horizontal, rectangular, bottom-uniform-heated channels was studied by experiment and theoretical simulation. The predicted results of the onset of thermal instability for the cases of uniform peripheral wall heat flux are in agreement with the published experimental data, but the predicted results for the cases of uniform peripheral wall temperature exceed the previous data by about an order of magnitude. The present experimental data reconfirm the present predictions. The effect of increasing the peripheral wall conduction is shown to stabilize the flow.     

纳米流体液滴内的光驱流动实验及其解析解

在光透过性的流体介质中添加具有高光响应特性的纳米颗粒,可以形成光驱动纳米流体,实现对光能的高效利用.本文针对光驱纳米流体流动行为开展实验观察和理论分析研究,这是实现光驱纳米流动精确调控的理论基础.首先利用粒子图像测速技术对液滴中直径为300 nm的Fe3O4颗粒在不同光源照射下受Marangoni效应诱导的运动进行了实验观测,研究光能向动能的高效转化机制.实验结果表明,当颗粒浓度大于临界数密度时,可诱导出垂向具有对称结构的涡,在液滴底部颗粒由四周向中心运动,顶部则由中心向四周运动,光源频率和颗粒数密度是这一过程的主导因素.随后,针对光强高斯分布的紫外光驱动下大颗粒数密度、特征流速约mm/s的光驱纳米流体,通过Stokes方程和表面张力梯度边界条件实现了其流场分布的解析求解,理论获得的流场分布解析解与实验测量结果保持一致,证实定量理论分析的有效性.最后,讨论了引入表面张力与在液滴底部引入表面压力及体相中集中引入光辐射力的不同驱动模式之间的相关性.这一研究成果为光微流控系统中流动行为的精确调控及光能的高效转化等提供了理论支持.     

On the work mechanism of MIM point contact diodes

We have performed systematic measurements on a W-Ni MIM point contact diode at different IR, FIR, and IF frequencies in order to investigate the mechanisms responsible for the different response times observed in the diode operation.Our results led to the experimental confirmation of a second effect of thermal nature, besides tunnel effect, responsible for the diode operation. We measure the cutoff frequency of this second effect, which results in good agreement with our theoretical interpretation.     

强脉冲电子束辐照材料表面形貌演化的模拟

在回顾和总结强脉冲电子束表面改性实验的基础上, 利用有限元数值计算方法对强脉冲电子束辐照铝和304不锈钢产生的温度场进行模拟, 给出了靶的近表面区域流体状态存在的特征尺度和特征时间, 并对不同材料特性下熔坑的产生原因进行了讨论. 采用两相流模型, 通过水平集方法和有限元方法结合的计算流体力学模拟了熔坑和表面突起形貌在表面处于熔融状态下的运动特征, 通过和实验数据相对比, 验证了对于高黏度, 高表面张力的高熔点金属, 表面处于流体状态下的张力驱动效应是熔坑等表面形貌演化的重要原因.