方波脉冲下纳米氧化铝掺杂对聚酰亚胺表面放电特性影响

聚酰亚胺薄膜(polyimide,PI)以其卓越的介电性能广泛应用于变频电机匝间绝缘,纳米掺杂能改善PI膜的绝缘性能。为研究方波条件下纳米Al_2O_3对PI膜表面放电特性的影响,文中将粒径为60nm的Al_2O_3纳米粒子作为无机填料添加到PI基体中,制作了掺杂量为1%,2%,5%,7%,10%的PI薄膜。测量了PI/Al_2O_3薄膜耐电晕性能和局部放电次数随温度的变化,用扫描电镜(scanning electron microscope,SEM)观察了放电前后PI/Al_2O_3薄膜微观形貌。利用傅里叶红外光谱(Fourier transform infrared spectroscopy,FTIR)测试了复合薄膜老化前后的化学键情况。研究结果表明:纳米Al_2O_3在PI基体中弥散分布,PI/Al_2O_3复合薄膜的红外吸收峰与纯PI膜基本一致,但吸收峰深度有所加强。老化4h后PI/Al_2O_3分子链上的C-O-C(醚键)以及C-N-C(酰亚胺环)消失,其余化学键吸收峰有所减弱;Al_2O_3纳米粒子的掺入提高了复合薄膜的耐电晕性能并减少了局部放电次数。并且温度的上升会导致局部放电次数减少。表面放电导致的化学键断裂是复合薄膜降解的主要原因,纳米粒子所引入的两相界面以及其优良的导热性能提高了复合薄膜的绝缘性能。     

聚酰亚胺/二维纳米银复合薄膜的介电性能研究

采用原位聚合法制备聚酰亚胺(PI)基二维纳米银复合薄膜。利用扫描电镜(SEM)表征纳米银片的原始尺寸及复合薄膜的断面形貌,采用傅里叶红外光谱(FTIR)、X射线衍射(XRD)分析薄膜的相结构,并测试复合薄膜的介电常数、电导率和介质损耗因数。结果表明:当二维纳米银体积分数为0.3%时,复合薄膜内银片尺寸改变明显,随着体积分数的升高,银片尺寸变化不大;复合薄膜XRD图谱的特征峰与纯聚酰亚胺薄膜和银的特征峰吻合;纳米银片在PI中具有良好的分散性,与PI高分子链形成无机-有机复合结构。纳米银片的加入明显提升复合薄膜的介电常数,银片尺寸对复合薄膜介电性能影响较大;复合薄膜的电导率与介质损耗因数接近纯PI薄膜。     

聚酰亚胺薄膜在电晕放电下击穿特点分析

为探讨电晕放电引起聚酰亚胺(PI)薄膜绝缘老化的原因,利用扫描电镜(SEM)分别研究了纯PI薄膜和PI/Al_2O_3复合薄膜在电晕放电下老化击穿的特点,并通过重复电晕老化击穿实验统计了薄膜击穿位置出现的规律。结果表明:电晕老化击穿后纯PI薄膜和PI/Al_2O_3复合薄膜均形成了直径约为20μm的圆形击穿孔,统计击穿孔的位置发现大多数击穿位置集中在特定的圆心区和边缘区,只有少数击穿发生在电晕破坏环上,说明电晕放电对薄膜的破坏并不是引起薄膜最终老化击穿的主要原因,电晕放电产生大量的热量在薄膜上积累引发的热击穿导致薄膜最终发生了热老化击穿。     

聚酰亚胺/氧化铝纳米复合材料的热刺激电流研究

采用热刺激电流（TSC）对自制的4种不同纳米Al2O3含量的聚酰亚胺（PI）薄膜进行测试,发现复合材料的热刺激电流峰值随着纳米Al2O3填充量的增加而降低,当纳米Al2O3的填充量大于10%后,热刺激电流峰的峰值随着温度升高持续一段时间后才出现下降趋势。对4种薄膜的TSC高温峰分别进行计算,得出薄膜中陷阱电荷的数量。并结合纳米复合材料的多核模型和陷阱理论,得出该现象的出现是由于PI/Al2O3纳米复合薄膜中界面结构影响了载流子输运过程造成的。     

聚酰亚胺薄膜的热刺激电流谱

用热刺激电流法(Thermally Stimulated Currents,简称TSC)对聚酰亚胺薄膜的分子结构及其运动机理进行了研究。在-140℃—250℃的温度范围內,其热刺激电流谱出现二个峰,-70℃附近的γ峰,80℃—170℃范围内的β峰。而在190℃以后,热刺激电流随温度升高而一直上升。γ峰与试样所吸收的水有关。β峰大致上能反映薄膜的亚胺化程度。而高温时的电流,是空间电荷运动的结果。因为空间电荷与结构的不均匀性有关,因此这一电流可能在一定程度上反映了薄膜的结晶情况。     

基于双极性载流子输运模型的聚酰亚胺薄膜空间电荷数值模拟

聚酰亚胺（PI）薄膜因其优异的电气性能广泛应用在电力设备中,但空间电荷会在PI薄膜内部积聚导致其电气绝缘性能劣化。为了获得绝缘性能优异的PI薄膜,需要进一步研究薄膜内部空间电荷动态积累和迁移过程,优化其内部结构。本研究基于双极性载流子输运模型,分析了PI薄膜在直流电压下空间电荷的分布,探讨了电场强度与温度对PI薄膜内部...     

方波脉冲下不同纳米添加物对聚酰亚胺薄膜电气性能影响

为了研究不同纳米添加物对聚酰亚胺(polyimide,PI)电气性能的影响,采用原位聚合法制备了纯PI薄膜、PI质量分数10%的PI/SiO_2和PI/Al_2O_3纳米复合薄膜,测试其电导率(表面、体积电导率)、介电频谱、方波脉冲下的局部放电以及耐电晕性能,并用SEM观察击穿点周围的表面形貌。结果表明:PI/SiO_2膜的电导率大于PI/Al_2O_3膜,其中表面电导率是PI/Al_2O_3膜的6倍;PI/Al_2O_3膜、PI/SiO_2膜、PI膜的介电常数依次降低;PI/SiO_2膜和纯PI膜的介电损耗角正切值(tanδ)随频率的增加先减小后增大,PI/Al_2O_3膜的tanδ值在6 k Hz后最大;由于空间电荷弛豫,PI/Al_2O_3膜的tanδ值在0.02Hz左右出现了一个峰值;另外,因为电荷扩散能力不同,PI/SiO_2膜、PI/Al_2O_3膜以及PI膜的局部放电起始电压和耐电晕时间依次减小,而局部放电的平均幅值则依次增大;电晕放电使得3种薄膜表面都形成了很多微孔、裂纹,纳米复合薄膜表面出现块状物。研究结果表明:复合薄膜中界面体积分数和纳米粒子极性,是造成PI/SiO_2薄膜和PI/Al_2O_3薄膜电气性能差异的主要原因。     

溶胶–凝胶法在聚酰亚胺表面沉积氧化铝薄膜及性能

采用溶胶凝胶法利用表面处理和离子交换技术,在纯聚酰亚胺(polyimide,PI)薄膜表面提拉制备了氧化铝(Al2O3)薄膜。利用扫描电子显微镜(scanning electron microscopy,SEM),原子力显微镜(atomic force microscopy,AFM),X射线衍射仪(X-ray diffractions,XRD)对薄膜进行了表征,并对表面覆盖Al2O3薄膜的PI薄膜(复合膜)的热稳定性(thermogravimentric analyses,TGA)、击穿和耐电晕性能进行分析。结果显示Al2O3薄膜在PI基体表面呈连续分布、表观平整且致密。断面显示Al2O3薄膜和PI基体有非常清晰、明显的三层结构,Al2O3薄膜的厚度约为1?m;Al2O3薄膜主要由?-AlOOH,??-AlOOH和?-AlOOH三种一水合氧化铝构成;热稳定性显示在初始升温阶段复合膜的热失重比纯膜快,后期趋向于持平;击穿测试结果表明由于复合膜中含有极性基团而比纯膜的击穿场强有所下降;耐电晕测试结果表明,复合膜的耐电晕时间由纯膜的4.4 min提高到了104.6 min,耐电晕寿命获得了极大的提升,比纯膜提高了23.8倍。     

AgNWs@SiO2/PI复合薄膜制备及性能研究

为了提高聚酰亚胺薄膜热导率的同时维持绝缘性,采用SiO2对AgNWs进行表面绝缘包覆获得AgNWs@SiO2核壳结构,首先通过静电纺丝技术将AgNWs@SiO2分散在聚酰胺酸(PAA)电纺纤维内部,规划导热路径,同时改善AgNWs@SiO2在PI基体中的分散性,再用含AgNWs@SiO2的PAA胶液浸渍PAA电纺膜,热亚胺化后得到E-AgNWs@SiO2/PI复合薄膜.研究其填料改性和含量对复合薄膜导热性能和绝缘性能的影响.结果表明:当填料质量分数为25％时,E-AgNWs/PI和E-AgNWs@SiO2/PI复合薄膜的热导率分别为2.92 W/(m·K)和2.80 W/(m·K),分别是纯PI薄膜的14.6倍和14倍.E-AgNWs@SiO2/PI复合薄膜的介电常数降低至5以下,并且介质损耗因数维持在0.015以下,体积电阻率提升至1.79×1013Ω·m.     

纳米氧化铝杂化聚酰亚胺薄膜的制备与性能研究

由铝的纳米氧化物溶胶制得纳米氧化铝杂化的聚酰亚胺(PI)薄膜。对PI薄膜的耐电晕性进行了测试,结果表明氧化铝质量百分数为23%的PI薄膜在棒板电极系统、气隙间距0.1 mm、50 Hz、90 MV/m条件下耐电晕时间达120 h以上,比纯PI薄膜提高30倍以上;使用介电谱仪测试其介电常数和介质损耗;采用FTIR、SEM分别表征了杂化PI薄膜与纯PI薄膜的化学结构和表面形貌。     

耐电晕PI/无机纳米氧化物复合薄膜设计及性能

采用热液法制备了纳米无机氧化物分散液,其中水解所需的水由醇缩水成醚反应提供.所得的MTES改性的纳米氧化铝与聚酰亚胺复合制成杂化聚酰亚胺复合薄膜(PI/Al2O3 - SiO2),另外还制备了未改性的纳米氧化铝杂化聚酰亚胺复合薄膜(PI/Al2O3),在试样厚度均为25 μm的情况下,采用双极性脉冲方波电压、峰-峰值2500V、频率20 kHz、占空比50％、测试温度155℃的条件下,分别测试上述两种薄膜以及Kapton 100 CR薄膜的耐电晕时间,结果表明,PI/Al2O3 - SiO2薄膜的耐电晕寿命最长,是Kapton 100 CR薄膜的6倍以上,是PI/Al2O3薄膜的12倍以上.由SEM的测试结果分析表明,PI/Al2O3 - SiO2薄膜中的无机纳米复合结构可以更有效地保护PI基体,从而提高材料的耐电晕性.     

Alternating Current Conduction and Impedance Spectroscopy Analysis on Pulsed Excimer Laser Ablated (Pb,La)TiO 3 Thin Films

Lanthanum doped lead titanate (PLT) thin films were identified as the most potential candidates for the pyroelectric and memory applications. PLT thin films were deposited on Pt coated Si by excimer laser ablation technique. The polarization behavior of PLT thin films has been studied over a temperature range of 300 K to 550 K. A universal power law relation was brought into picture to explain the frequency dependence of ac conductivity. At higher frequency region ac conductivity of PLT thin films become temperature independent. The temperature dependence of ac conductivity and the relaxation time is analyzed in detail. The activation energy obtained from the ac conductivity was attributed to the shallow trap controlled space charge conduction in the bulk of the sample. The impedance analysis for PLT thin films were also performed to get insight of the microscopic parameters, like grain, grain boundary, and film-electrode interface etc. The imaginary component of impedance Z" exhibited different peak maxima at different temperatures. Different types of mechanisms were analyzed in detail to explain the dielectric relaxation behavior in the PLT thin films.     

绝缘纸无定形区玻璃转化的分子动力学模拟

绝缘纸无定形区的玻璃转化温度是其热稳定性的重要标度之一。为研究变压器绝缘纸的玻璃转化过程的微观机理,继而挖掘其中的热老化信息,利用分子动力学对绝缘纸纯纤维素、纤维素-水两个模型的玻璃转化过程进行了模拟。用比体积-温度曲线法确定了纯纤维素和纤维素-水模型的玻璃化转变温度分别为448、418K。模拟结果表明,玻璃转化过程中,绝缘纸纤维素链的运动和机械特性均发生突变,400～500K之间自由体积的突变给纤维素链运动提供了更多空间,导致其发生玻璃化转变;水分子浸入无定型区可以破坏纤维素链间的氢键,显著地降低玻璃转化温度,影响绝缘纸热稳定性。     

纳米硅/铝氧化物杂化聚酰亚胺薄膜电性能研究

为了提高聚酰亚胺薄膜的电学性能,将纳米硅/铝氧化物掺杂到聚酰亚胺(PI)基体中,通过固定掺杂总量,调整纳米硅/铝氧化物的相对质量百分含量,制备出一系列的无机纳米杂化聚酰亚胺薄膜。采用SEM(扫描电镜)表征了薄膜的表面形貌,对薄膜电气强度和耐电晕寿命进行了测试。结果表明:无机纳米杂化聚酰亚胺薄膜与纯的聚酰亚胺薄膜相比其电性能大幅度提高。     

Pulsed Laser Deposited Ba(Mg1/3Ta2/3)O3 Microwave Dielectric Thin Films

Ba(Mg_1/3Ta_2/3)O₃ (BMT) microwave dielectric thin films were successfully synthesized by a modified pulsed laser deposition (PLD) process, which includes low temperature (200°C) deposition and high temperature (>500°C) annealing. Crystalline structured BMT thin films were obtained when the PLD-deposited films were post-annealed at a temperature higher than 500°C in oxygen atmosphere. The characteristics of BMT thin film, including crystallinity, grain size, film roughness, and dielectric properties were improved with annealing temperature, achieving dielectric constant K = 23.5 and dissipation factor tan δ = 0.015 (at 1 MHz) for the 800°C-annealed films.     

Increase in breakdown strength of PE film by additives ofazocompounds

PE films with various additives were prepared by the solution-grown method. The thickness of the films ranged from 5 to 25 μm. Six different azocompounds were used as additives. The results show that BDS 0 for the film with additives was higher than that for the film without additives in the temperature range from -35 to 30°C. The increasing ratio of BDS depended on the composition of the additives used. Our results indicate that the breakdown of the film is induced by an electron avalanche. The conduction current through the film in the high electric field region was reduced by using the additives. The dependence of the conduction current on additives corresponds to that of BDS; a higher BDS is realized for films with additives by which the conductive current is controlled at the lower value. It was considered that the reduction of current by the additive is due to either the trapping effect or the excitation effect of the additive. The trap level for the additive and the excitation energy depend upon the type of radical connected on the benzene ring of the additive: electron-accepting or electron-donating type. The results suggested that high BDS and small current are obtained when the azocompound on which the electron-accepting radical is connected is used as the additive     

Structure and Properties of Preferential (110) Orientation BiFeO3 Thin Films by Sol-Gel Method

Abstract

Multiferroic materials, coexisting of ferroelectric, ferromagnetic and ferroelastic properties, possess potential applications in functional devices. BiFeO₃ (BFO) is a unique room temperature multiferroic material with high ferroelectric Curie temperature and Neel temperature. The BFO thin films were prepared on Si (111) substrate by sol-gel method in this paper. XRD analyses show that the thin films exhibit pure phase and preferred (100) orientation when annealing temperature is 500?°C. Field emission scanning electron microscopy shows that the crystallization degree of the films is getting better with the increase of annealing temperature. The thickness of the sample is about 400?nm. The hysteresis loop of BFO films annealing at 500?°C show 1.93?µC/cm² remnant polarizations. However, the hysteresis loop is not perfect, which may be caused by a large leakage current. The magnetic hysteresis loop of BFO films is tested as well, indicating that the BFO film is antiferromagnetic and the residual magnetization (Mr) and coercive field (Hc) of the BFO films were 0.054?emu/g and 1026.4?Oe, respectively.     

电子束辐照对聚酰亚胺电气性能的影响

研究了高能电子束辐照对聚酰亚胺薄膜(PI)试样电气性能的影响。实验发现高能电子束辐照后PI的介电常数减小,电导率增大,电损耗增大。利用原位聚合法把70纳米Al2O3粒子引入到PI薄膜中,对材料进行改性,实验发现改性后PI薄膜抗高能电子束辐射能力得到加强。     

Annealing temperature effect on ferroelectric and magnetic properties in Mn-added polycrystalline BiFeO3 films

We fabricated 5 at.% Mn-added polycrystalline BiFeO₃ films and investigated the annealing temperature effect on structural, ferroelectric and magnetic properties. In the x-ray diffraction patterns, only the diffraction peaks due to the BiFeO₃ structure were observed and no secondary phase could be observed at annealing temperatures between 773 and 923 K. Adding Mn suppressed the leakage current density in the high electric field region when compared to pure BiFeO₃ films. The conduction mechanism of the Mn-added BiFeO₃ films was dominated by Ohmic conduction. Remanent polarization of the Mn-added polycrystalline BiFeO₃ films for an applied electric field of approximately 1.5 mV/cm was 63 μC/cm² for the specimen annealed at 773 K and 46 μC/cm² for the specimen annealed at 923 K, although the remanent polarization still exhibited a tendency to increase with an increase in the electric field. Spontaneous magnetization was obtained at high annealing specimens. This study revealed that the annealing temperature strongly affected the ferroelectric and magnetic properties in Mn-added polycrystalline BiFeO₃ films. In addition, by optimizing the annealing temperature, we realized multiferroics coexistent with spontaneous magnetization and spontaneous polarization at room temperature in the Mn-added polycrystalline BiFeO₃ film.