Electrical conduction at low fields of composites sparsely filledwith aluminum particles

The dc and ac electrical conduction properties have been investigated in four composites containing hydroxyl-terminated polybutadiene (HTPB) as binder and different proportions of aluminum particles as filler in a temperature range of -60 to +70°C and a frequency range of 40 Hz to 100 kHz. A change in conduction mechanism at a temperature of 4±3°C was displayed explicitly in de measurements. The transition was shown to be due to crystallization of the HTPB upon cooling. dc conduction is explained in terms of ionic hopping in HTPB. Distinct polarization mechanisms, below and above the transition temperature, have been identified as interfacial polarisation and electrode blocking effect, respectively. The change of conduction behaviors at the crystallization temperature, 4±3°C, was not revealed explicitly in ac measurements. However, using the free volume concept to analyze the frequency shift factor a_T, a discontinuity in the fractional free volume curve was displayed. The temperature of the break point, 6.7°C, corresponds to the crystallization temperature of HTPB. The study showed that the crystallization of a polymer is accompanied by an abrupt reduction in free volume, and this structural change affects dc as well as ac conduction behaviors     

Blood longitudinal (T 1) and transverse (T 2) relaxation time constants at 11.7 Tesla

Object

The goal of the study was to determine blood T ₁ and T ₂ values as functions of oxygen saturation (Y), temperature (Temp) and hematocrit (Hct) at an ultrahigh MR field (11.7?T) and explore their impacts on physiological measurements, including cerebral blood flow (CBF), blood volume (CBV) and oxygenation determination.

Materials and methods

T ₁ and T ₂ were simultaneously measured. Temperature was adjusted from 25 to 40°C to determine Temp dependence; Hct of 0.17?C0.51 to evaluate Hct dependence at 25 and 37°C; and Y of 40?C100% to evaluate Y dependence at 25 and 37°C. Comparisons were made with published data obtained at different magnetic field strengths (B ₀).

Results

T ₁ was positively correlated with Temp, independent of Y, and negatively correlated with Hct. T ₂ was negatively correlated with Temp and Hct, but positively correlated with Y, in a non-linear fashion. T ₁ increased linearly with B ₀, whereas T ₂ decreased exponentially with B₀.

Conclusion

This study reported blood T ₁ and T ₂ measurements at 11.7?T for the first time. These blood relaxation data could have implications in numerous functional and physiological MRI studies at 11.7?T.     

Several effects of electric fields on liquid extraction

The effects of electric fields on dispersed liquid-phase flow behavior and mass transfer efficiency have been investigated for several liquid-liquid systems using single/multiple-nozzle extraction columns. Drop size, drop surface charge density, and drop terminal velocity were correlated with the applied field strength by typical models. Drop surface area and velocity were increased significantly by the electric fields. The end effects were separated from the overall mass transfer process by varying the plate gap. The mass transfer coefficients for drop freefall region were calculated. The overall extraction efficiencies increased 51% for heptane-furfural and 24% for the toluene-water-methanol system     

对极低频场健康风险的观点与分析

针对电磁场环境健康风险,WHO组织全球60多个国家、8个国际权威组织,开展了为期10年的协同研究,其结论性文件《环境健康准则,极低频场．（EHC 238）》已于2007年正式发布。国际上一些身为利益相关者的学者,在其研究结果因研究质量欠佳或未能被WHO采纳的情况下,为了抵制WHOW“国际电磁场计划”的科学评估结论及WHO促进全球标准协同化的努力,匆匆推出了由14个学者署名,但又“不对整个结论负责”的名为“Bioinitiative Working Group Report”的报告,对WHO及其组织的全球性“国际电磁场计划”研究成果进行武断的攻击,在国际上严重影响了公众的风险感受,也遭致多个国家权威部门的责疑。文章摘引国家电网公司电力设施电磁影响实验室对该报告的研究与分析,提出中国应认同并采纳WHO对电磁场环境健康风险的科学评估结论;建议决策者充分重视若干利益相关者的行为,并遵照WHO的建议,适时组织开展利益相关各方围绕极低频场健康风险的风险沟通及对话,以消除公众的误解和焦虑。     

Changes in proton transverse relaxation times of rat myocardium that has suffered a previous oxidative insult

An oxidative insult can induce severe damage, as in the phenomenon of myocardial ischemia and reperfusion. However, there are situations in which the damage is not so obvious (e.g., silent ischemia or aging), and the negative effects will be seen only in time. Our aim was to reveal these small changes in the myofilaments by using the nuclear magnetic resonance (NMR) technique. We used Wistar rat hearts in a constant-pressure Langendorff system, perfused with oxygenated Krebs-Henseleit buffer at 37°C. After 15 minutes of stabilization, the hearts were perfused with buffer supplemented with H₂O₂ at 50, 75, or 100 μmol/L for 15 or 30 minutes. Fifteen-minute and 45-minute perfusion controls and unperfused hearts were also collected. Heart rate (HR) and left ventricular developed pressure (LVDP) were determined with the help of a latex balloon, inserted in the left ventricle and connected with a pressure transducer. Proton transverse relaxation times (T ₂) were determined at the end of the experiment.T ₂ values were measured again in the same tissue fragments after they had been glycerinated and incubated in relaxation and rigor media. The functional parameters (HR, LVDP, coronary flow) were not significantly changed in control and 50 μmol/L H₂O₂ groups but were increased in the 75 μmol/L H₂O₂ group and significantly decreased in the 100 μmol/L H₂O₂ group.T ₂ is significantly decreased in rigor media starting with 50 μmol/L H₂O₂ administrated for 30 minutes and does not correlate with dose and duration of the oxidative insult.T ₂ in rigor is shorter than in relaxation media within the groups, and this difference is increased in the treated hearts.     

Biological effects of narrow band pulsed electric fields

This paper describes the process of narrow band pulsed electric fields (NPEFs) and its effect on mammalian cells. The NPEF consists of a pulse modulated sinusoidal wave (PMSW), which allows delivery of well-defined electric fields in terms of frequency, field strength and deposition energy to the biological systems. 100 mus long sinusoidal electric fields with a frequency of 0.02, 2 or 50 MHz and field strengths of up to 2 kV/cm are applied to CHO cells with variation in the DNA density in the cells investigated by means of Acridine Orange assay. The experiments indicate that 50 MHz fields cause DNA degradation without cell membrane defects, while 0.02 MHz fields lead to an increase in membrane permeability which is similar to the effect known as electroporation. The intermediate frequency of 2 MHz influences both the membrane and DNA. It is demonstrated that the MHz range narrowband electric fields with the amplitude level of 1 kV/cm cause intracellular effects in mammalian cells.     

间断式横缝设计及其对大坝地震响应的影响

介绍强震区高碾压混凝土重力坝横缝的创新性设计及其对大坝地震响应的影响.为提高大坝的整体稳定性,对坝体每一碾压浇筑层,横缝的切缝深度仅切割2/3浇筑层厚度,并在切缝中填充无纺布,使横缝具有弱连接诱导缝的性质,达到既能满足温控要求,又可增强大坝的整体性.开发了反映横缝法向开合非线性和径向滑移动接触模型,采用非线性指数型动接触本构模型模拟了横缝的张开、闭合的非线性力学行为,运用摩尔库伦定律模拟了横缝的切向非线性特性.对在建的金安桥碾压混凝土重力坝3条横缝、10条横缝的情形分别进行了地震响应分析.结果表明,间断式横缝有效地提高了大坝的整体性,大坝横河向位移响应明显减小;金安桥大坝靠左右岸坝段的横缝的张开度明显大于中间坝段横缝的开度值,相应的张开一闭合频率也远高于中间坝段的横缝.所提设计方案已用于实际工程,实践表明具有较好的效果,可推广用于类似碾压混凝土大坝的施工设计.     

脉冲电流电磁场对生物膜的非热效应分析

通过简单的生物体细胞模型,研究脉冲电磁场对生物体所产生的生物效应的机制问题。证明了高电压脉冲电流放电环境中,低强度的脉冲电磁场可引起膜内外离子的输运过程,而当脉冲电磁场足够强时,则引起细胞膜的击穿。因此高电压脉冲电流放电环境中脉冲电流电磁场分量以及电磁防护的研究是非常有必要的。     

Brain T2 relaxation times correlate with regional cerebral blood volume

We previously reported cerebellar and putaminal transverse relaxation time (T2) differences in children with ADHD and in adults with childhood trauma. As brain T2 can be altered by deoxyhemoglobin concentration ([dHb]) and because [dHb] is proportional to regional cerebral blood volume (rCBV), at steady state we attributed those differences to rCBV changes. Studies in other species have established a correlation between T2 and rCBV; however this has yet to be demonstrated in human brain. Echo planar imaging (EPI) T2 relaxometry and dynamic susceptibility-contrast (DSC) MRI were used to measure T2 and rCBV in 11 healthy adults. Significant T2-rCBV correlations were observed in both cerebellar vermis and putamen (r=0.759,p=0.007;r=0.782,p=0.004, respectively). These correlations predict 9±3% and 10±3% rCBV changes, respectively, for each 1-msec change in T2. Consequently, brain T2 measurements may be useful for estimating steady-state rCBV.     

Automated keratometry at low cost

Describes an automatic optical system that is attached to a slit lamp in order to provide automatic keratometry at low cost. The system consists of projecting a light ring as a target onto the patient's cornea as well as analysis of the deformation of the target in order to obtain the radius of curvature and the axis of the associated astigmatism. The reflected image of the target is displayed on a PC monitor, while dedicated software performs the analysis of the image, which provides the corneal keratometry     

外施横磁下真空断路器弧后金属蒸汽击穿过程仿真研究

从微观的角度分析外施横向磁场下直流真空断路器弧后金属蒸汽击穿过程。首先,建立了可以描述直流真空断路器弧后金属蒸汽击穿过程的粒子模拟-蒙特卡洛碰撞仿真模型，确定了弧后金属蒸汽击穿判据；其次对比了不同横向磁场强度下弧后金属蒸汽击穿的发展过程，分析外施横磁强度对弧后金属蒸汽击穿的作用；最后，分析了外施横向磁场强度对弧后金属蒸汽击穿时刻的影响。研究结果表明：场致发射提供了金属蒸汽击穿的初始电子，是诱发金属蒸汽击穿的“种子”，电子崩的出现意味着金属蒸汽击穿的发生；横向磁场会降低真空间隙内的电子数和铜离子数，这是因为间隙内的电子和铜离子会在横磁的作用下横向扩散；横向磁场越大，金属蒸汽击穿速度越慢，当外施磁场由0 mT增加至2 000 mT时，击穿时刻由1.913×10^-7 s增加至2.72×10^-7 s。本文的研究结果有助于理解外施横磁下直流真空开断的弧后金属蒸汽击穿过程，可为低压直流真空断路器的开发提供理论依据。     

Information theoretic evaluation of Lorentzian,Gaussian, Voigt,and symmetric alpha-stable models of reversible transverse relaxation in cervical cancer in vivo at 3 T

Magnetic Resonance Materials in Physics, Biology and Medicine - To better characterize cervical cancer at 3 T. MRI transverse relaxation patterns hold valuable biophysical information...     

Flow electrification of transformer oil effects of mixed fields

Static electrification in transformer oil is investigated experimentally using a laboratory synthetic closed cycle, where the oil is pumped in a coaxial electrode arrangement. The electrode system is electrically energized radially with dc and/or ac, and the electrostatic charging tendency (ECT) of the oil is quantified by measuring the streaming current. The results indicate that for the same voltage ratio (K=Vˆ/(Vˆ+V)), increasing the ac or the dc voltage component leads to higher ECT of oil, although the ECT under only ac field is much lower than that under dc field. The unenergized streaming current is inherently affected by the electrode material and configuration; where electrodes having a lower work function give higher positive ECT of oil at high temperature. The energized streaming current increases with oil temperature, oil velocity and electric field; where negative dc voltage application to the outer electrode gives both higher conduction and streaming currents. While the conduction current decreases with oil velocity, increasing the frequency of mixed ac voltage has no significant effect on streaming current. Moreover, the effect of combined radial electric (ac or dc) and axial magnetic (ac or dc) fields on ECT of oil is investigated, and the results reveal that the ECT is enhanced by the magnetic field while the radial conduction current decreases. Derived formulas for the streaming current for unenergized and ac energized cases are also presented together with expressions of the volume and surface charge densities for the coaxial electrode system     

A comparative study of strain relaxation effects on the performanceof InGaAs quantum-well-based heterojunction phototransistors

The performance of a GaAs based heterojunction phototransistors (HPTs) using an n-p-i-n configuration, where the absorption is provided by InGaAs quantum wells (QWs) have been studied. Structures with differing numbers of QW were investigated. This allowed the tradeoff between the benefits of increased light absorption and the drawbacks of increased lattice relaxation, caused by the mis-match between InGaAs and the GaAs substrate, to be examined. All the HPT's investigated showed responsivities (A/W) far larger than unity, as well as large wavelength tolerance, for example 44 A/W±15% from 950-970 nm, for 10 μW incident optical. Electrical common-emitter current gains, of up to 3000 were measured for our HPT's and then confirmed by subsequent HBT measurements. Small relaxation levels (<10%) had no significant detrimental effects, allowing a large improvement in HPT performance. More heavily relaxed HPTs showed a degradation in both the inherent photodetector and transistor action, though this was not catastrophic in nature. A simple simulation of the results is carried out, suggesting that the dislocations adversely effect the carrier transport across the collector region, and also reduce the minority carrier diffusion length in the base     

Biological effects of power frequency electric and magnetic fields

The Institute of Electrical and Electronics Engineers (IEEE) has made an analysis of six recent major scientific committee reports dealing with the biological effects of exposure to electric and magnetic fields associated with electric power distribution and utilization. The conclusions drawn from these committee reports are presented in this summary     

Ultrathin gate-oxide breakdown-reversibility at low voltage

Solid-insulator breakdown always leads to a permanent conduction path that is irreversible. This is a built-in assumption in all gate-oxide breakdown reliability measurement and lifetime projection. This assumption is not valid when the gate-oxide thickness is less than 2 nm and the operation voltage is 1 V or less. The authors examine the impact of reversible breakdown using breakdown data from 12 000 devices stressed by plasma charging damage. The data support the notion that when the surge current is limited at breakdown, the breakdown event may not leave any mark such as a permanent conduction path. The implication is that the commonly used accelerated-stress test, such as time dependent dielectric breakdown (TDDB), may be underestimating the actual gate-oxide lifetime by as much as a million folds.     

Resistive states in strontium titanate thin films: Bias effects and mechanisms at high and low temperature

A study on charge transport properties of thin film Fe-doped SrTiO₃ epitaxially grown on Nb-doped SrTiO₃ is reported. Electric measurements between 350 °C and 750 °C show a transition from predominant ionic to electronic conduction and lower conductivity of the thin films compared to the bulk of polycrystalline samples. Defect chemical changes at elevated temperature were investigated by applying a bias voltage. A model is described which successfully predicts additional features such as inductive loops or extra semicircles measureable by impedance spectroscopy as well as the complicated time dependence of electric DC-measurements. With this model it is also possible to calculate the negligibly small ionic conductivity next to the dominating electronic conductivity in the high temperature regime. The ionic conductivity is referenced by oxygen isotope depth profiling. Changes of resistive states in Fe-doped SrTiO₃ thin films at high temperature and moderate fields are compared to room temperature resistive switching phenomena at high electric fields. A conductive filament based switching process is observed at room temperature, and the capability for forming such filaments and their electric properties is further analysed using microelectrodes.