Fluctuation phenomena in nanoscale nickel films near the melting point

Voltage fluctuations in thin nanoscale nickel films which arise during current application through a film sample upon slow heating are studied. It is shown that positive voltage fluctuations (surges) arise due to an increase in the resistance of local regions of the film under study, caused by its local thinnings and discontinuities which result from the film melting onset. The temperature of the melting onset of nanoscale nickel films on oxidized silicon was experimentally determined as 740, 815, and 875 K for films 5, 20, and 40 nm thick, respectively.     

氟掺杂对非晶硅薄膜特性的影响

采用电容结构研究了氟(F,Flourine)掺杂非晶薄膜漏电、击穿以及温度特性。结果表明,氟掺杂钝化了非晶膜中的缺陷和悬挂键,使非晶薄膜漏电降低两个数量级,同时使击穿电压升高。掺杂非晶薄膜的漏电与温度呈指数增长,增长系数为0.0375 K^-1,击穿电压随温度线性减小,系数为0.012 V·K^-1。     

Embossed Hollow Hemisphere‐Based Piezoelectric Nanogenerator and Highly Responsive Pressure Sensor

Harvesting energy using piezoelectric materials such as ZnO, at nanoscale due to geometrical effects, are highly desirable for powering portable electronics, biomedical, and healthcare applications. Although one‐dimensional nanostructures such as nanowires have been the most widely studied for these applications, there exist a limited number of piezomaterials that can be easily manufactured into nanowires, thus, developing effective and reliable means of preparing nanostructures from a wide variety of piezomaterials is essential for the advancement of self‐powered devices. In this study, ZnO embossed hollow hemispheres thin film for highly responsive pressure sensors and nanogenerators are reported. The asymmetric hemispheres, formed by an oblique angle deposition, cause an unsymmetrical piezoelectric field direction by external force, resulting in the control of the current direction and level at about 7 mA cm^‐2 at normal force of 30 N. The nanogenerators repeatedly generate the voltage output of ≈0.2 V, irrespective of the degree of symmetry. It is also demonstrated that when one piece of hemisphere layer is stacked over another to form a layer‐by‐layer matched architecture, the output voltage in nanogenerators increases up to 2 times.     

有机绝缘层材料聚(4-乙烯基苯酚)喷墨打印工艺研究

通过压电喷墨打印方式在氧化铟锡(ITO)玻璃上直接图案化打印聚(4-乙烯基苯酚)(PVP),研究了不同浓度PVP的电学特性,从电容、漏电流、击穿场强几个方面进行了分析。结果表明,采用打印方式得到的PVP绝缘层在0～40V的外加电压下,漏电流密度在10-11～10-8 A/cm2范围内,为打印方式制备高性能交联PVP(CL-PVP)介电层提供了必要的参考。     

BaTiO_3压电薄膜的制备及其性能研究

利用旋涂技术,在聚二甲基硅氧烷(PDMS)混合液中掺入不同质量分数(30%,50%,70%)的钛酸钡(BaTiO_3)纳米颗粒,并将混合物均匀涂在洁净的硅片表面进行旋涂处理;加热固化制得压电薄膜,并对压电薄膜进行极化处理,分别运用扫描电子显微镜(SEM),X线衍射(XRD)仪分析薄膜表面和BaTiO_3粉末。实验结果表明,薄膜内部BaTiO_3分布相对均匀,且其中BaTiO_3纳米颗粒为四方相。设计振动能量采集测试系统测试分析薄膜的输出开路电压和供电能力,分别用单悬臂梁振动和激振器敲击的形式对压电薄膜的输出特性进行研究。压电薄膜的输出电压峰-峰值与BaTiO_3的质量分数具有高度的一致性,在w(BaTiO_3)=70%时,输出电压最高,对应的峰-峰值为3.50V。     

Piezoelectric Sensors Operating at Very High Temperatures and in Extreme Environments Made of Flexible Ultrawide-Bandgap Single-Crystalline AlN Thin Films

Extreme environments are often faced in energy, transportation, aerospace, and defense applications and pose a technical challenge in sensing. Piezoelectric sensor based on single-crystalline AlN transducers is developed to address this challenge. The pressure sensor shows high sensitivities of 0.4–0.5 mV per psi up to 900 °C and output voltages from 73.3 to 143.2 mV for input gas pressure range of 50 to 200 psi at 800 °C. The sensitivity and output voltage also exhibit the dependence on temperature due to two origins. A decrease in elastic modulus (Young's modulus) of the diaphragm slightly enhances the sensitivity and the generation of free carriers degrades the voltage output beyond 800 °C, which also matches with theoretical estimation. The performance characteristics of the sensor are also compared with polycrystalline AlN and single-crystalline GaN thin films to investigate the importance of single crystallinity on the piezoelectric effect and bandgap energy-related free carrier generation in piezoelectric devices for high-temperature operation. The operation of the sensor at 900 °C is amongst the highest for pressure sensors and the inherent properties of AlN including chemical and thermal stability and radiation resistance indicate this approach offers a new solution for sensing in extreme environments.     

SILC as a measure of trap generation and predictor ofTBD in ultrathin oxides

The theoretical basis of stress-induced leakage current (SILC) as a measure of bulk trap density within thin oxide films is explored. Contrary to popular belief, this measure is neither absolute, nor do most papers in the literature sufficiently specify the measurement conditions to make their comparison meaningful. We also explore the relationship between SILC generation rate and the time-to-breakdown, and show that only a very specific definition of SILC generation can capture the voltage dependence of the time-to-breakdown     

压电-电磁复合式俘能器的设计与实验研究

针对现有压电、电磁俘能器不能同时输出大电压和大电流,设计了一种压电-电磁复合式俘能器。根据设计的复合式俘能器结构进行了理论建模,推导出了电压、电流、振幅和输出功率的表达式,并利用Ansys和Ansoft仿真软件对复合式俘能器的输出特性进行了仿真分析。最后通过实验对比分析了压电、电磁与压电-电磁复合式俘能器的输出特性,分析得到在0.6 g(g=10m/s2)加速度作用下,压电-电磁复合式俘能器的最优输出功率比电磁、压电俘能器分别提高了118%、38%,同时3dB带宽可增大67%、25%。     

变截面悬臂梁压电俘能器动力学特性分析

提出一种变截面悬臂梁压电俘能器结构,通过有限元仿真分析其振动特性和输出电压,有利于提高发电性能。该俘能器结构固定端为等截面梁,自由端为变截面梁,压电层粘贴在悬臂梁根部等截面梁表面,改变悬臂梁自由端与固定端的宽度比,得到多种不同形式的变截面悬臂梁。对比分析了三角形梁、矩形梁和具有不同宽度比梯形梁的固有频率、应力和应变分布及简谐激励输出电压响应。结果表明,三角形梁固有频率较大,输出电压最大,同时分析了不同变截面段长度对输出电压的影响。该文还分析了具有相同一阶频率、不同宽度比俘能器的输出电压,表明三角形结构单位体积压电层输出电压最大。对比分析了基体层上根部粘贴压电片和全部粘贴压电片的输出电压特性。结果表明,前者输出电压较大,发电性能更好。     

基于非完美声学黑洞的压电能量收集系统

针对高频压电振动能量回收效率低的缺点,提出了一种基于非完美声学黑洞的压电阵列式能量收集系统。首先建立了非完美声学黑洞薄板结构模型,并分析了在时域和频域条件下能量聚集特性,非完美声学黑洞结构能量集聚于中心平台的多个位置。然后建立了阵列式压电俘能系统实验模型,结构中心平台上能量聚集点的输出电压存在较大的相位差,通过引入整流电路消除此相位差,对比分析了整流前后串、并联电路的输出电压,结果表明,整流后系统输出电压能力得到了较大提高。     

悬臂梁双晶压电片不同连接方式发电性能

为了更加有效提高双压电振动能量采集器的发电效率,以双晶压电片为研究对象,设计了一种可收集电动机机械振动能量的双压电振动能量采集器。探究了电动机不同转速下,双晶压电片不同连接方式的内阻和发电特性;分析了电动机在不同转速下,双晶压电片不同连接方式时双压电振动能量采集器输出电压和输出功率的特性。理论计算与实验结果进行对比,选择一种最优的双晶压电片连接方式。     

On low-frequency electric power generation with PZT ceramics

Piezoelectric materials have long been used as sensors and actuators, however their use as electrical generators is less established. A piezoelectric power generator has great potential for some remote applications such as in vivo sensors, embedded MEMS devices, and distributed networking. Such materials are capable of converting mechanical energy into electrical energy, but developing piezoelectric generators is challenging because of their poor source characteristics (high voltage, low current, high impedance) and relatively low power output. In the past these challenges have limited the development and application of piezoelectric generators, but the recent advent of extremely low power electrical and mechanical devices (e.g., MEMS) make such generators attractive. This paper presents a theoretical analysis of piezoelectric power generation that is verified with simulation and experimental results. Several important considerations in designing such generators are explored, including parameter identification, load matching, form factors, efficiency, longevity, energy conversion and energy storage. Finally, an application of this analysis is presented where electrical energy is generated inside a prototype Total Knee Replacement (TKR) implant.     

Mobile Ferroelastic Domain Walls in Nanocrystalline PZT Films: the Direct Piezoelectric Effect

Ferroelastic (90°) domain wall motion occurs readily in bulk samples of displacive ferroelectrics such as Pb(Zr,Ti)O₃ (PZT), dictating critical piezoelectric, dielectric, and polarization switching properties. Many prior studies have used converse piezoelectric measurements to probe the dynamics of ferroelastic domains in thin films; however, such experiments are strongly influenced by the mechanical clamping effect of the substrate, which inhibits electric field‐induced 90° domain wall motion. Nevertheless, these observations raise a tantalizing question: Does the application of mechanical stress, rather than electric field, result in an entirely different response in thin films? Here we report biaxial stress‐driven crystallographic reorientation of (100)/(001) textured, 70 nm thick Pb(Zr_0.25Ti_0.75)O₃ films via 90° domain wall motion, measured in situ by both x‐ray diffraction and piezoforce microscopy. Visual evidence of nanoscale mechanisms that underlie the direct piezoelectric effect is shown. Mobile 90° domain walls effect complete orientation switching in the grains in which they operate, without apparent wall pinning, indicating that bulk‐like ferroelastic behavior can extend to nanocrystalline films in the absence of substrate clamping.     

Performance Enhancement of Flexible Piezoelectric Nanogenerator via Doping and Rational 3D Structure Design For Self‐Powered Mechanosensational System

With the rapid development of the Internet of things (IoT), flexible piezoelectric nanogenerators (PENG) have attracted extensive attention for harvesting environmental mechanical energy to power electronics and nanosystems. Herein, porous piezoelectric fillers with samarium/titanium‐doped BiFeO₃ (BFO) are prepared by a freeze‐drying method, and then silicone rubber is filled into the microvoids of the piezoelectric ceramics, forming a unique structure based on silicone rubber matrix with uniformly distributed piezoelectric ceramic. When subjected to external force stimulation, compared with conventional piezocomposite films found on undoped BFO without a porous structure, the PENG possesses higher stress transfer ability and thus boosts output performance. The notable enhancement in the stress transfer ability and piezoelectric potential is proven by COMSOL simulations. The PENG can exhibit a maximum open‐circuit voltage (V_oc) of 16 V and short‐circuit current (I_sc) of 2.8 µA, which is 5.3 and 5.6 times higher than those of conventional piezocomposite films, respectively. The PENG can be used as a triggering signal to control the operation of fire extinguishers and household appliances. This work not only expands the application scope of lead‐free piezoelectric ceramic for energy harvesting, but also provides a novel solution for self‐powered mechanosensation and shows great potential application in IoT.     

用于微传感器中PZT压电薄膜的制备和图形化

采用溶胶-凝胶法在Si/Si3N4/Poly-Si/Ti/Pt基片上制备PZT压电薄膜, 为了选择更适合微电子机械系统(MEMS)器件的压电薄膜,采用一般热处理和快速热处理对锆钛酸铅(PZT)压电薄膜进行干燥和结晶.首先,采用V(H2O):V(HCL):V(HF)＝280 mL:120 mL:4drops(4滴HF溶液)配比的腐蚀液在室温下对未结晶的PZT压电薄膜进行了湿法腐蚀微细加工;然后,对图形化好的压电薄膜进行再结晶的热处理,实验结果表明这种方法可用于压电薄膜微器件的制备.     

Characterization of the vortex dynamics in Abrikosov superconductive flux flow transistors

In order to develop superconducting thin films or multilayer structures suitable for Superconducting Flux Flow Transistors (SFFTs), we propose a model to describe the current–voltage characteristics measured on microbridges in the flux creep regime and we show how the effective superconducting thickness of the samples, as well as the pinning potential range, the maximum velocity, the pinning energy, and the depinning current of the vortices can be determined. This model is applied to YBCO microbridges in order to compare samples and to develop films which may easily be driven into the flux flow regime.     

Mobile Domain Walls as a Bridge between Nanoscale Conductivity and Macroscopic Electromechanical Response

The interfaces in complex oxides present unique properties exploitable in nanoscale devices. Recent studies on ferroelectric BiFeO₃, BaTiO₃, and Pb(Zr,Ti)O₃ have revealed an unusually high electric conductivity of the domain walls (DWs), adding another degree of freedom for controlling the local properties of these materials. While most of the investigations are focused on thin films for nanoscale applications, many practical devices, including piezoelectric sensors, actuators, and transducers, rely on the macroscopic properties of bulk polycrystalline materials where the average effect of local properties should be small. It is shown that in polycrystalline BiFeO₃ the local domain‐wall conductivity interferes with the dynamics of the DWs within the grains, resulting in an unexpectedly large effect on the macroscopic piezoelectric response. The results thus bridge the local conductivity and the macroscopic piezoelectricity via domain‐wall dynamics, revealing that the domain‐wall conductivity must be considered when interpreting and controlling macroscopic electromechanical properties.     

Bi2VO5.5铁电薄膜的制备及电学性质研究

利用溶胶凝胶法在n-Si(100)衬底上成功制备了钒酸铋(Bi2VO5.5)铁电薄膜.利用X射线衍射和原子力显微镜对薄膜的微结构进行了分析,结果表明,Bi2VO5.5薄膜与n-Si衬底有着良好的晶格匹配并表现出高度的c轴择优取向,晶粒大小均匀.对薄膜电学性质的研究表明,Bi2VO5.5薄膜具有良好的C-V特性,在±4V偏压下,存储窗大于0.4V.当外加偏压为3.2V时,漏电流密度为5×10-8Acm-2.1kHz下介电常数和介电损耗分别为95和0.22.这些结果说明,Bi2VO5.5在铁电存储器方面具有较大的应用前景.     

基于涡致振动的压电能量收集阵列的仿真与实验

为了解决部分微电子设备供电需求大,而单一的压电能量收集结构无法满足的问题,该文对基于涡致振动的压电能量收集阵列进行流-固-电耦合仿真,并与风洞实验数据进行对比.首先对前置阻流体的俘能结构进行测试,验证结构的可行性,然后对串列、并列、错列、长方阵型的压电俘能结构进行研究.仿真与实验结果表明,压电能量收集阵列随风速的增大呈...     

Direct Observation of Conducting Nanofilaments in Graphene‐Oxide‐Resistive Switching Memory

Determining the presence of conducting filaments in resistive random access memory with nanoscale thin films is vital to unraveling resistive switching mechanisms. Bistable resistive switching within graphene‐oxide (GO)‐based resistive memory devices, recently developed by many research groups, has been generally explained by the formation and rupture of conducting filaments induced by the diffusion of metal or oxygen ions. Using a low‐voltage spherical aberration‐corrected transmission electron microscopy (TEM), we directly observe metallic nanofilaments formed at the amorphous top interface layer with the application of external voltages in an Al/GO/Al memory system. Atomic‐resolution TEM images acquired at an acceleration voltage of 80 kV clearly show that the conducting nanofilaments are composed of nanosized aluminum crystalline within the amorphous top interface layer after applying a negative bias (ON state). Simultaneously, we observe the change in the crystallinity of GO films by the back‐diffusion of oxygen ions. The oxygen‐deficient regions are clearly confirmed by energy‐filtered TEM oxygen elemental mapping. This work could provide strong evidence to confirm the resistive switching mechanism previously suggested by our group.