Domains and domain walls in multiferroics

Multiferroics are gathering solid-state matter in which several types of orders are simultaneously allowed, as ferroelectricity, ferromagnetism (or antiferromagnetism), ferroelasticity, or ferrotoroidicity. Among all, the ferroelectric/ferromagnetic couple is the most intensively studied because of potential applications in novel low-power magnetoelectric devices. Switching of one order thanks to the other necessarily proceeds via the nucleation and growth of coupled domains. This review is an introduction to the basics of ferroelectric/ferromagnetic domain formation and to the recent microscopy techniques devoted to domains imaging, providing new insights into the archetypal multiferroic domain morphologies. Some relevant examples are also given to illustrate some of the unexpected properties of domain walls, as well as the way these domain walls can be manipulated altogether thanks to various types of magnetoelectric coupling.     

Mechanisms and origin of multiferroicity

Motivated by the potential applications of their intrinsic cross-coupling properties, the interest in multiferroic materials has constantly increased recently, leading to significant experimental and theoretical advances. From the theoretical point of view, recent progresses have allowed one to identify different mechanisms responsible for the appearance of ferroelectric polarization coexisting—and coupled—with magnetic properties. This chapter aims at reviewing the fundamental mechanisms devised so far, mainly in transition-metal oxides, which lie at the origin of multiferroicity.     

Analytical Methods for the Determination of 90Sr and 239,240Pu in Environmental Samples

Artificial long-lived radionuclides such as ⁹⁰Sr and ^239,240Pu have been long released into the environment by human nuclear activities, which have a profound impact on the ecological environment. It is of great significance to monitor the concentration of these radionuclides for environmental safety. This paper summarizes and critically discusses the separation and measurement methods for ultra-trace determination of ⁹⁰Sr, ²³⁹Pu, and ²⁴⁰Pu in the environment. After selecting the measurement method, it is necessary to consider the decontamination of the interference from matrix elements and the key elements, and this involves the choice of the separation method. Measurement methods include both radiometric methods and non-radiometric methods. Radiometric methods, including alpha spectroscopy, liquid scintillation spectrometry, etc., are commonly used methods for measuring ^239+240Pu and ⁹⁰Sr. Mass spectrometry, as the representative of non-radiometric measurement methods, has been regarded as the most promising analytical method due to its high absolute sensitivity, low detection limit, and relatively short sample-analysis time. Through the comparison of various measurement methods, the future development trend of radionuclide measurement is prospected in this review. The fully automatic and rapid analysis method is a highlight. The new mass spectrometer with ultra-high sensitivity shows strong analytical capabilities for extremely low concentrations of ⁹⁰Sr, ²³⁹Pu, and ²⁴⁰Pu, and it is expected to develop determination methods with higher sensitivity and lower detection limit.     

两个相邻目标对平面波、高斯波束的光散射

基于等效原理和互易性定理研究了两个靠近目标对平面波、高斯波束的光散射问题,给出了这一复合光散射模型的二阶散射结果。通常一阶散射结果容易求解,但由于耦合效应的复杂性,很难给出二阶散射结果的解析形式。为了解决这一问题,应用互易性定理给出了求解任意相邻介质目标二阶散射场的公式,同时借助等效原理将求解散射场公式中的体积分简化为面积分的形式,从而降低了求解难度。求解了两相邻球形粒子的复合散射场,并将求解结果与应用时域积分方程法求得的结果进行了比较。同时,还讨论了束腰半径、目标位置对散射截面及偏振度的影响。     

高效耗能黏弹性阻尼框架结构减震性能研究

提出了高效耗能黏弹性阻尼器,研究了该阻尼器对框架结构的减震机理。首先给出了高效耗能黏弹性阻尼器的结构构造,推导了小震、大震与巨震作用下的阻尼放大系数计算公式,由此分析了该放大系数的变化规律。此外,还推导了设置高效耗能黏弹性阻尼框架结构的等效阻尼比。分析表明,框架结构设置高效耗能黏弹性阻尼器在阻尼等效前后顶部位移反应时程曲线相差不超过10%。     

当量比对圆盘结构下旋转爆震波传播的影响

为研究反应物当量比对旋转爆震波传播过程的影响,在圆盘形旋转爆震发动机上进行H₂/air的旋转爆震实验研究,并统计分析了当量比对爆震波传播模态及参数的影响规律.实验结果表明,固定质量流率,同一种传播模态下,随着当量比的增大,爆震波的压力峰值及传播速度增大,且旋转爆震波的传播过程更加稳定.不同质量流率条件下,当量比对传播模态的影响规律不同.空气质量流率小于100 g/s时,旋转爆震波皆以单波模态传播.空气质量流率大于150 g/s时,随着当量比的增大,旋转爆震波的传播模态由单波模态向双波模态转变,再转变为不对称双波模态,最后又回到单波模态.并且在不对称双波模态中发现了低频振荡现象,振荡频率约为300 Hz.质量流率继续增大,燃烧室中发现了同向三波传播模态.随着质量流率的增加,双波模态的当量比下限降低,不对称双波模态的当量比上限增大,而双波与不对称双波模态的分界线受质量流率的影响较小.      

N2与CO2对合成气爆炸特性影响的实验研究

为了研究惰性气体(氮气及二氧化碳)对合成气爆炸特性的影响,利用20L球形爆炸仪器,开展不同体积分数氮气与二氧化碳作用下不同当量比合成气的爆炸实验,从爆炸峰值压力、爆炸压力到达峰值时间、爆炸指数方面分析惰性气体对合成气爆炸特性的影响。研究结果表明:惰性气体体积分数的增加会降低合成气的爆炸压力和爆炸指数,推迟爆炸压力到达峰值的时间;在相同体积分数下,CO2比N2能更有效地降低合成气的爆炸峰值压力和爆炸指数,减小爆炸反应的剧烈程度,CO2在抑制合成气爆炸方面比N2的效果明显。     

Smooth equivalence of differential equations and linear automorphisms

For systems of ordinary differential equations admitting linear automorphisms, we consider the problems of smooth equivalence and linearization preserving these automorphisms. Translated fromMatematicheskie Zametki, Vol. 66, No. 4, pp. 567–578, October, 1999.     

细观等效理论预测再生混凝土宏观力学参数北大核心CSCD

预测分析再生混凝土各组分对再生混凝土宏观力学参数的影响是开展再生混凝土基本力学性能的一种方式.为了分析再生混凝土各组分对再生混凝土宏观力学参数的影响,根据再生混凝土的细观结构组成,建立了细观等效模型,利用扭转变形、细观夹杂理论、弹性等效思想和M-T模型方法,推导了由原生骨料、老界面层、老水泥砂浆、新界面层和新水泥砂浆等组成的再生混凝土的宏观力学参数预测模型.预测结果表明,随着再生骨料的取代率增加,水泥砂浆的含量不断增加,再生混凝土孔隙率也随之增大,导致再生混凝土的Poisson比随之增大,弹性模量、剪切模量和体积模量不断降低.模型的预测结果较好地反映了再生混凝土宏观力学参数随再生骨料取代率的增加不断变化的这一趋势,也为再生混凝土宏观力学参数的预测提供了一条简单实用的新方法,有利于再生混凝土基本力学性能的研究分析.     

平行柱体对平面波/高斯波束电磁散射

基于等效原理和互易性定理，研究了N个相互平行二维柱体对平面波/高斯波束的电磁散射特性，给出了求解N阶散射场公式.一阶散射可通过求解单个柱体的散射场得到，但对于高阶散射场而言，由于耦合散射的复杂性，很难给出精确的解析解.为了解决这一问题，借助等效原理和互易性定理给出了求解N阶散射场的面积分公式.只要给出柱体的i-1阶散射场及相关目标表面上的等效电流和(或)等效磁流，就可应用此公式求解i阶散射场.应用该近似方法计算了相互平行非均匀等离子体涂层导体圆柱的单/双站散射宽度，讨论了束腰半径、等离子体涂层厚度、电子密度、碰撞频率及雷达频率等对散射结果的影响.