多场耦合下铁电薄膜非线性行为的畴变模型

铁电薄膜在外加力场, 电场和温度场的作用下表现出明显的非线性, 为了更好的描述这种现象, 本文提出了一个热-电-力耦合场铁电薄膜下的畴变模型。该模型基于细观力学模型, 认为电畴自由能的改变提供电畴翻转的动力, 且180°电畴翻转由两步90°翻转构成。在本构关系中加入了铁电薄膜制备过程中产生的残余应变项, 以区别于块体铁电材料, 通过该模型计算出了铁电薄膜在不同外场下的响应, 结果与实验和其他模型的结果较为符合。     

GaN基半导体上BiFeO_3薄膜的生长与性能研究

采用脉冲激光沉积法在(0001)取向的GaN以及AlGaN/GaN调制掺杂结构上制备了(111)取向的BiFeO3(BFO)薄膜。首先在导电氧化物SrRuO3和TiO2缓冲层包覆的GaN上制备了BFO薄膜,分析了在GaN上生长的BFO薄膜的面外取向、外延关系、表面形貌以及电学性能等性质。然后,在AlGaN/GaN调制掺杂结构上采用TiO2缓冲层生长了BFO薄膜,并采用光刻工艺分别在AlGaN表面制备Ti/Al/Ti/Au欧姆电极和BFO表面制备Ni/Au肖特基电极以形成二极管结构。C-V测试表明,由于BFO铁电薄膜极化的作用,BFO/TiO2/AlGaN/GaN结构具有1 V左右的逆时针窗口。     

氧化钒薄膜的制备技术及特性研究

采用直流反应磁控溅射法,通过精确控制反应溅射电压优化了氧化钒薄膜的制备工艺.对制备的氧化钒薄膜,利用四探针测试仪检测了薄膜的方阻和方阻温度系数,用X射线光电子能谱(XPS)仪和原子力显微镜(AFM)对薄膜的钒氧原子比和薄膜的微观形貌分别进行了分析和表征.实验结果表明,利用精确控制反应溅射电压法生长出的氧化钒薄膜的性能得到了进一步的提高.     

循环冲击荷载下红砂岩细观损伤演化及能耗特性研究

为研究循环冲击荷载下红砂岩的细观损伤演化规律和能耗特性,利用带有围压装置的分离式霍普金森压杆(SHPB)装置和核磁共振仪,对红砂岩试件进行不同冲击气压(0.6、0.7、0.8、0.9 MPa)和围压(1、2 MPa)条件下的循环冲击试验,对冲击后岩石孔隙度、T₂谱曲线、核磁共振成像及能耗规律进行分析,同时提出一种新的损伤度计算方法。研究结果表明：总体上看,红砂岩孔隙度随循环冲击次数的增加呈指数增长趋势;前几次冲击孔隙度变化速率较缓,当冲击损伤累积到一定范围,岩石损伤加剧,表现为孔隙度增长幅度迅速增加;随着循环冲击次数的增加,T₂谱曲线起止点向两端移动,新孔隙不断萌生,不同尺度孔隙相互转化,孔隙尺寸和数量明显增长,中型孔隙对孔隙度增长起重要作用;MRI成像显示,随着冲击次数的增加,中心区域明亮斑点由小而分散向大而聚集转变,伴随有明亮条纹出现;提出一种损伤度定义新方法,明确了未损伤和完全损伤2种极端状态;红砂岩累积损伤度与累积比能量吸收值呈指数增长关系,能时密度随冲击次数的增加呈现出先增大,后减小,再增大的趋势,此外,能时密度与损伤度变化值呈正线...     

碲基复合薄膜在纳米尺度下的电学特性研究

分别采用AFM和原子力/扫描探针显微镜(AFM/SPM)在纳米尺度下对碲基复合(Te/TeO2-SiO2)薄膜的表面电势、电容梯度等电学特性进行测量。测试结果表明控制电压为-0.8V,反应时间为200s条件下制备的碲基复合薄膜的表面电势差达到700mV,相对介电常数小于以硅为主要成分的衬底相对介电常数。利用光谱分析,碲基复合薄膜的禁带宽度约为3.14eV。     

变截面压电悬臂梁能量采集器在高斯白噪声激励下的稳态统计特性研究

环境振动是一种非周期、随机的宽频激励,研究振动能量采集器在环境振动下的能量采集特性具有重要意义.本文采用改进随机平均法,求解了变截面压电梁在高斯白噪声激励下的等效振幅、位移、速度的稳态概率密度函数,位移与速度的联合概率密度函数以及稳态均方输出电压,随后研究了变截面压电梁的能量采集效能.结果表明:负载电阻一定时,截面系数β>0时的变截面压电梁相比β=0时的等截面压电梁有更好的稳态均方输出电压;截面系数β>0时,随着电阻电容乘积的倒数的增加,变截面压电梁的均方电压均呈现逐渐减小的趋势,当电阻电容乘积的倒数值一定时,β值越大,均方电压也越高;随着噪声强度的增加,变截面压电梁的均方电压均呈现逐渐增大的趋势,当噪声强度一定时,β值越大,均方电压也越高.本文研究结果可为变截面压电悬臂梁能量采集系统的设计及应用提供理论依据.     

SHPB劈裂试验下橡胶水泥砂浆的动态力学、能量特性及破坏机理试验研究

基于静载巴西劈裂原理,开展了橡胶水泥砂浆的分离式霍普金森压杆(SHPB)劈裂试验,对其动态力学、能量特性及破坏机理进行了研究.根据损伤力学,从强度和能量的角度分析了橡胶掺量、养护湿度和冲击荷载对水泥砂浆动态劈裂损伤的影响,并探讨了三种损伤因素下的六种损伤路径.结合圆盘试件的破坏模式,建立了冲击劈裂简化平面理想受力模型,分析了破裂方式-Ⅰ和破裂方式-Ⅱ两种截然不同的动态劈裂方式,并初步探讨了破裂方式-Ⅱ下圆盘试样的破坏机理.结果表明,掺入橡胶颗粒和降低养护湿度均降低了水泥砂浆的动态劈裂拉伸强度;普通水泥砂浆和橡胶水泥砂浆有着相同的应力率和应变率演化趋势;掺入橡胶颗粒和降低养护湿度均在一定程度上阻碍了能量在水泥砂浆圆盘试件中的传递;不同的单一损伤因素和复合损伤因素对水泥砂浆圆盘试样造成的动态劈裂损伤不同;在较大的冲击荷载下,圆盘试样会因"三角形压碎区"、"劈裂拉伸区"和"弯曲断裂区"的形成而被破坏.最后,从细观的角度分析讨论了界面过渡区(ITZ)对SHPB劈裂下橡胶水泥砂浆强度和抗冲击性能的影响.     

动载下含预制裂纹砂岩的力学特性及破裂过程研究EI北大核心CSCD

深入研究缺陷岩石的动力特性对保障深部煤矿巷道等地下硐室在掘进爆破、老顶来压等强扰动作用下的长效稳定具有重要意义。该研究基于天然裂纹长度的幂律分布假设,利用颗粒流软件建立了分离式霍普金森压杆试验系统以及含不同数量分布裂纹的砂岩试件模型,研究了在冲击荷载作用下预制分布裂纹数量对砂岩试件强度、破坏等力学特性的影响。结果显示:(1)随预制裂纹数量(number of prefabricated cracks, NPC)的增加,砂岩试件强度近似呈线性减小趋势,而弹性模量降低规律更接近三次方程,并且随着NPC的增加,试件强度、弹性模量的离散性逐渐增大,试件峰后应力的降低过程也更为复杂;(2)随着NPC的增加,试件的AE事件曲线由单峰值、高极值向多峰值、低极值转变;(3)在冲击荷载作用下,试件中新生裂纹沿应力波传播方向渐进分布,由散斑形式逐渐连通扩展为片层状,并且随着NPC的增加,试件的破坏模式由四周破坏形式转变为自左至右的渐弱破坏形式;(4)试件主要沿应力波传播方向发生破裂,并且NPC越少,倾角沿应力波传播方向的新生裂纹占比越大。     

钛酸铅陶瓷的介电和极化翻转特性研究

以水热纳米粉为前驱粉料,采用传统烧结工艺在950℃制备了致密性高、具有亚微米尺寸晶粒的钛酸铅(PbTiO3)陶瓷,并对其介电温谱和压电响应特性进行了系统的研究。结果表明,PbTiO3陶瓷在低温下的相对介电常数与测试频率无关,在室温以上,随着频率的增加,介电峰值向低温方向移动;压电力显微镜研究结果显示PbTiO3陶瓷中具有明显的微区极化翻转,表明具有较好的铁电性能。     

Manifestation of Local Magnetic Domain Reversal by Spin-Polarized Carrier Injection in (Ga,Mn)As Thin Films

Recently, we have reported that the spin-polarized holes generated by the irradiation with circularly polarized light can change the magnetization orientation of III–V ferromagnetic semiconductor (Ga,Mn)As via p–d exchange interaction. In this paper, we report that a small portion of change does not return to the initial state after the light is turned off. This residual component, named as the memorization effect, exhibits ferromagnetic characteristics. This fact strongly suggests that small magnetic domains having the perpendicular magnetic axis are rotated by photogenerated carrier spins.     

氧化镍溶胶-凝胶薄膜阻变特性研究

用溶胶-凝胶法在1TO基片上旋涂制备了NiO薄膜,通过对ITO/NiO薄膜/GaIn器件进行伏安特性测试,研究了溶胶浓度、退火、层数以及Cu掺杂等对其电学特性的影响.结果表明:所制备NiO薄膜具有良好可重复双极电阻开关特性.其中,2％Cu掺杂0.2 mol/L溶胶、双层、400℃退火1h制备的薄膜,阈值电压较低,约0.8 V;而开关比受以上因素影响不明显,约3× 102.分析发现薄膜高阻态的荷电输运符合空间电荷限制导电机制,而低阻态为欧姆特性,阻变开关机理为阈值电场及焦耳热导致的氧空位细丝的形成与断裂.     

Control of Domain Structures in Multiferroic Thin Films through Defect Engineering

Domain walls (DWs) have become an essential component in nanodevices based on ferroic thin films. The domain configuration and DW stability, however, are strongly dependent on the boundary conditions of thin films, which make it difficult to create complex ordered patterns of DWs. Here, it is shown that novel domain structures, that are otherwise unfavorable under the natural boundary conditions, can be realized by utilizing engineered nanosized structural defects as building blocks for reconfiguring DW patterns. It is directly observed that an array of charged defects, which are located within a monolayer thickness, can be intentionally introduced by slightly changing substrate temperature during the growth of multiferroic BiFeO₃ thin films. These defects are strongly coupled to the domain structures in the pretemperature‐change portion of the BiFeO₃ film and can effectively change the configuration of newly grown domains due to the interaction between the polarization and the defects. Thus, two types of domain patterns are integrated into a single film without breaking the DW periodicity. The potential use of these defects for building complex patterns of conductive DWs is also demonstrated.     

Better,Faster, and Less Biased Machine Learning: Electromechanical Switching in Ferroelectric Thin Films

Machine-learning techniques are more and more often applied to the analysis of complex behaviors in materials research. Frequently used to identify fundamental behaviors within large and multidimensional datasets, these techniques are strictly based on mathematical models. Thus, without inherent physical or chemical meaning or constraints, they are prone to biased interpretation. The interpretability of machine-learning results in materials science, specifically materials’ functionalities, can be vastly improved through physical insights and careful data handling. The use of techniques such as dimensional stacking can provide the much needed physical and chemical constraints, while proper understanding of the assumptions imposed by model parameters can help avoid overinterpretation. These concepts are illustrated by application to recently reported ferroelectric switching experiments in PbZr_0.2Ti_0.8O₃ thin films. Through systematic analysis and introduction of physical constraints, it is argued that the behaviors present are not necessarily due to exotic mechanisms previously suggested, but rather well described by classical ferroelectric switching superimposed by non-ferroelectric phenomena, such as electrochemical deformation, electrostatic interactions, and/or charge injection.     

Electrical Tunability of Domain Wall Conductivity in LiNbO3 Thin Films

Domain wall nanoelectronics is a rapidly evolving field, which explores the diverse electronic properties of the ferroelectric domain walls for application in low‐dimensional electronic systems. One of the most prominent features of the ferroelectric domain walls is their electrical conductivity. Here, using a combination of scanning probe and scanning transmission electron microscopy, the mechanism of the tunable conducting behavior of the domain walls in the sub‐micrometer thick films of the technologically important ferroelectric LiNbO₃ is explored. It is found that the electric bias generates stable domains with strongly inclined domain boundaries with the inclination angle reaching 20° with respect to the polar axis. The head‐to‐head domain boundaries exhibit high conductance, which can be modulated by application of the sub‐coercive voltage. Electron microscopy visualization of the electrically written domains and piezoresponse force microscopy imaging of the very same domains reveals that the gradual and reversible transition between the conducting and insulating states of the domain walls results from the electrically induced wall bending near the sample surface. The observed modulation of the wall conductance is corroborated by the phase‐field modeling. The results open a possibility for exploiting the conducting domain walls as the electrically controllable functional elements in the multilevel logic nanoelectronics devices.     

Three‐State Ferroelastic Switching and Large Electromechanical Responses in PbTiO3 Thin Films

Leveraging competition between energetically degenerate states to achieve large field‐driven responses is a hallmark of functional materials, but routes to such competition are limited. Here, a new route to such effects involving domain‐structure competition is demonstrated, which arises from strain‐induced spontaneous partitioning of PbTiO₃ thin films into nearly energetically degenerate, hierarchical domain architectures of coexisting c/a and a₁ /a₂ domain structures. Using band‐excitation piezoresponse force microscopy, this study manipulates and acoustically detects a facile interconversion of different ferroelastic variants via a two‐step, three‐state ferroelastic switching process (out‐of‐plane polarized c ⁺ → in‐plane polarized a → out‐of‐plane polarized c ^? state), which is concomitant with large nonvolatile electromechanical strains (≈1.25%) and tunability of the local piezoresponse and elastic modulus (>23%). It is further demonstrated that deterministic, nonvolatile writing/erasure of large‐area patterns of this electromechanical response is possible, thus showing a new pathway to improved function and properties.