通过电容器层间压强的增强提高电容器寿命

通过对圆柱体电容器内部压强进行计算分析,研究提升电容器寿命的方法。首先分析得出,电容器内部层间压强的提高可以减小自愈能量,降低脉冲放电过程中的电容量损失,因此可以提升电容器寿命。其次,提出了电容器内部层间压强的计算模型。试验结果表明:通过合理的热处理工艺,电容器寿命可提高30%;当外包膜层数从20层增加到100层时,电容器寿命提高38%。     

电压反峰对脉冲电容器寿命特性的影响

以金属化聚丙烯膜电容器电压反峰为研究对象,探讨了脉冲电流对电容器寿命的影响机制,通过对电容器在不同电压反峰系数下进行寿命测试,研究了电压反峰系数与电容器寿命之间的关系。研究结果表明:当电压反峰系数在10%~65%时,电容器寿命成指数下降。基于试验结果,通过统计分析的方法,拟合出寿命随电压反峰系数变化的经验公式,可以用来预测不同反峰系数下金属化聚丙烯膜电容器的寿命特性。     

Sweep direction and efficiency of the swept-frequency two pulse phase modulated scheme for heteronuclear dipolar-decoupling in solid-state NMR

We present here a bimodal Floquet theoretical and experimental investigation of the direction of sweep in the swept-frequency two pulse phase modulated (SW(f)-TPPM) scheme used for heteronuclear dipolar decoupling in solid-state NMR. The efficiency of the decoupling turns out to be independent of the sweep direction.     

Johnson‐Lindenstrauss lemma for circulant matrices**

We prove a variant of a Johnson‐Lindenstrauss lemma for matrices with circulant structure. This approach allows to minimize the randomness used, is easy to implement and provides good running times. The price to be paid is the higher dimension of the target space k = O(ε^?2 log³ n) instead of the classical bound k = O(ε^?2 log n). © 2011 Wiley Periodicals, Inc. Random Struct. Alg., 2011     

New insights from variable-temperature and variable-pressure studies into coupling and decoupling processes for ion transport in polymer electrolytes and glasses

A fresh analysis of literature data shows how the influences of temperature and pressure on ion transport and structural relaxation in glass-forming systems may be combined within the framework of ‘master plots’ based on the equation E_A = M · V_A, to reveal new insights into coupling and decoupling effects in a wide range of systems. E_A,σ and V_A,σ are, respectively, instantaneous activation energies and volumes for ionic conductivity and the parameter, M_σ, is a corresponding ‘process modulus’. For structural relaxations occurring at the glass transition, the appropriate modulus is given by M_s = T_g · dP/dT_g. We can now identify typical behaviour patterns for fragile liquids on the one hand, and typical inorganic glasses on the other. Thus, the parameters, M_σ and M_s, for fragile systems such as molten Ca(NO₃)₂:KNO₃ (CKN) or a typical polymer electrolyte such as a complex of LiCF₃SO₃ in PPG, are found to remain constant over a wide range of temperatures down to T_g, despite changes in the temperature (and pressure) dependences of the ionic conductivities, as indicated for example by a return to Arrhenius behaviour in the case of CKN, or by so-called Stickel plots and changes in the VTF parameters for the polymer electrolytes. If E* and V* are activation energies and volumes assigned to elementary steps, when again E* = M · V*, we can go further and identify the microscopic processes driving forward structural relaxation. In the case of inorganic glasses, where usually we find the decoupling index R_τ ≈ 10¹², we identify two distinct decoupling paradigms represented by strong and fragile systems respectively, where in both cases the activation volumes for ion transport are very similar to the corresponding ionic volumes. In the former case (typified by the strongly cross-linked silicate and aluminosilicate systems), the negative activation volumes for structural relaxation (negative values of dT_g/dP) are clearly indicative of a ‘water-like’ behaviour attributable to the collapse of the network under pressure. On the other hand, for the more fragile fast-ion conducting silver iodomolybdate glass, the experimental results show that M_s (at T_g) ≈ M_σ (in glass), implying some recoupling of structural relaxation to ion transport. Arguments based on the dynamic structure model lead us to predict that a similar close link should exist between M_s (at T_g) and M_σ in the relatively fragile lithium and sodium borate glasses, thus highlighting the need for more information concerning the effects of pressure on the glass transition temperatures of common inorganic glasses.     

Magnetization-induced double-layer capacitance enhancement in active carbon/Fe304 nanocomposites

The effects of magnetic fields on electrochemical processes have made a great impact on both theoretical and practical significances in im- proving capacitor performance. In this study, active carbon/Fe304-NPs nanocomposites （AC/Fe304-NPs） were synthesized using a facile hy- drothermal method and ultrasonic technique. Transmission electron micrographs （TEM） showed that Fe304 nanoparticles （Fe304-NPs） grew along the edge of AC. AC/Fe304-NPs nanocomposites were further used as an electrochemical electrode, and its electrochemical performance was tested under magnetization and non-magnetization conditions, respectively, in a three-electrode electrochemical device. Micro-magnetic field could improve the electric double-layer capacitance, reduce the charge transfer resistance, and enhance the discharge performance. The capacitance enhancement of magnetized electrode was increased by 33.1% at the current density of 1 A/g, and the energy density was improved to 15.97 Wh/kg, due to the addition of magnetic particles.     

脉冲功率应用中的金属化膜电容器寿命预测

分析了金属化膜电容器的寿命特性, 根据电容器寿命Weibull分布、可靠性分析和影响因素等建立了从元件到整机的电容器寿命预测模型。通过电容器元件和整机的寿命试验, 验证了该预测模型的适用性。研究结果表明：储能密度0.5 kJ/L下寿命达到6万次以上;储能密度0.95 kJ/L下寿命达到2万次以上;通过新技术措施改进, 2.0 kJ/L下寿命达到1000次以上。     

重复频率脉冲电容器寿命测试平台

通过建立脉冲重复频率寿命测试平台研究ms级脉冲放电领域的脉冲电容器的寿命特性。平台采用的输出功率为30 kW重复频率充电机, 其输出电压0~12 kV;脉冲放电回路采用晶闸管作为开关, 最高重复频率为100 Hz;负载采用电阻和电感调节回路波形。通过该测试平台可以研究各个阶段的经历时间和重复频率对电容器寿命特性的影响以及重复频率工况下脉冲电容器的失效机制。     

随机阈值冲击模型金属化膜脉冲电容器可靠性分析

基于惯性约束聚变强激光装置中金属化膜脉冲电容器自愈的失效机理,提出了更为适合的冲击模型金属化膜脉冲电容器可靠性评估方法。现有的加速退化试验中,失效阈值往往是事先确定的,但考虑可靠性产品个体差异及环境影响的不同,这是不合理的,针对此问题提出了基于随机阈值的冲击模型金属化膜脉冲电容器退化建模的方法。由于模型过于复杂,采用MCMC方法进行参数估计。最后通过对电容器的仿真实验,将随机阈值下的评估结果与固定阈值的情况相对比,说明了该模型和方法的合理性,并进一步分析了不同的阈值分布均值和方差对产品可靠性的影响。     

高场强下金属化膜电容器绝缘电阻特性

分析了高电场强度下金属化膜电容器绝缘电阻的特点,得出高电场强度下金属化膜电容器绝缘电阻是电容器介质膜泄漏和自愈过程的综合反映。针对高场强下的应用条件提出一种金属化膜电容器绝缘电阻的测试方法,并对高场强下电容器的绝缘电阻进行测试。测试结果表明：在300~400 V/m工作场强范围内金属化膜电容器绝缘电阻随工作场强增大会急剧减小,由于自愈的作用,绝缘电阻下降幅度随场强增加会逐渐变缓;在20~50 ℃工作温度范围内,金属化膜电容器绝缘电阻随温度升高会急剧减小,同样由于自愈的作用,绝缘电阻下降幅度随温度增高会逐渐变缓。分析得出,高温高场强下金属化膜电容器介质膜泄漏将会导致电容器电压出现明显下降,影响电容器和脉冲功率系统的储能效率。