Kapton Research Articles

Multiple Cracking of Metallized Polyimide Kapton Films

Mechanical behaviour of metallized polyimide Kapton films was studied. The adhesion of metallized films to Kapton films is approximately hundred-folds higher than that of coatings made by usual sputter deposition of a metal on a polymer substrate. Multiple cracking of a metallized film was observed. Cracks<br />of two different scale levels are observed. The cracks of the first type (microcracks) are short with 1-2 μm length. The cracks of the second type are much longer and have length of several hundreds microns. The cracks of this type appear at higher strains as compared with the shorter ones. In thinner 7-μm-thick samples<br />long cracks were not observed.

A study of the electromagnetic characteristics of no-insulation GdBCO racetrack coils under an external magnetic ripple field

Here we report the effect of an external magnetic ripple field on the electromagnetic characteristics of GdBCO racetrack coils being operated with a constant DC current. Two types of GdBCO racetrack coils, one wound without turn-to-turn insulation (NI) and the other wound with Kapton tape (INS), were examined under external ripple fields generated by a permanent magnet mounted on a rotor, which was driven by a separate AC motor. The voltage fluctuations and magnetic field variations were measured with respect to the external ripple field intensity (BERF), rotating speed, and the operating condition. When the INS and NI coils were exposed to an external ripple field (herein, Iop = 80 A, BERF = 2 mT, and 5 rpm), a voltage fluctuation occurred because a time-varying magnetic field interacted with an electric circuit creating an electromotive force. The peak-to-peak voltage (Vpp = 0.29 mV) of the NI coil was ∼1.86 times lower than that (0.54 mV) of the INS coil, because the voltage response of the NI coil lagged behind dB/dt due to the existence of turn-to-turn contact. Furthermore, the Vpp of the INS coil increased with increasing BERF and rotating speed, while those of the NI coil were barely affected due to the delay of electromagnetic induction. In excessive current and ripple field conditions (Iop = 1.125 Ic, BERF = 8 mT, and 50 rpm) the INS coil eventually quenched while the NI coil did not, implying that the electromagnetic stability of the NI coil in excessive time-varying field conditions was superior to that of the INS coil.

Analytical and experimental investigation of electrical characteristics of a metallic insulation GdBCO coil.

This paper presents results, experimental and analytical, of the electrical characteristics of GdBCO single-pancake coils co-wound with a brass tape as metallic insulation (MI coil). The GdBCO pancakes were subjected to sudden discharge, charge-discharge, and over-current tests. The sudden discharge and charge-discharge test results of the MI coil demonstrated that MI coils can be charged and discharged significantly faster than non-insulated coils that are wound only with GdBCO tape. In over-current tests at 150 A (1.25I(c)), the MI coil exhibited better electrical behavior, i.e., self-protecting features, than its counterpart co-wound with Kapton tape, an insulator. Moreover, the experimental and analytical results are in agreement, validating the use of a concise equivalent parallel-RL circuit model for the MI coil to characterize its electrical behavior. Overall, the MI winding technique is highly promising to help build compact, mechanically robust, and self-protecting magnets composed of REBCO pancake coils. With no organic material in the winding, MI REBCO pancakes will be immune to neutron radiation damage, making the MI winding technique a viable option for fusion reactors, such as for toroidal field, poroidal field magnets, and central solenoid.

A vibrating reed apparatus to measure the natural frequency of multilayered thin films

An apparatus for measuring the natural frequency of sub-micrometric layered films in cantilever beam configuration is presented. The instrument comprises a specially designed test rig with a sample holder, an electronic excitation source, a vibration sensor and an automated software for the excitation and data recollection. The beam is excited by means of an air pulse and the oscillation amplitude of its free end is measured through a laser diode-photosensor arrangement. The instrument provides a very low uncertainty (∼1 mHz, for frequencies of the order of tens Hz) for repeated sequential tests and the major source of uncertainty (∼0.2 Hz, corresponding to a coefficient of variation of 0.18%) arises from the difficulty of placing the sample in an exactly identical location upon clamping. This high sensitivity renders the capability of measuring very small frequency shifts upon deposition of sub-micrometric films over thicker substrates. In order to assess the reliability of the apparatus, cantilever beams of 125 μm thick neat Kapton (substrate) and thin layered films of Au/Kapton and Al/Au/Kapton of 200–250 nm film thickness were fabricated and their natural frequency and damping factor were measured. Calculations of the natural frequency of such beams by finite element analysis further support the accuracy of the experimental measurements.

Kapton-derived carbon as efficient terahertz absorbers

The focus of this study is the preparation of carbon-based materials that satisfy the requirements of developing quasi-optic controllable absorbers for terahertz technology. These materials have been obtained from a commercially available organic polymer, Kapton® HN polyimide, through a pyrolysis process conducted in an inert atmosphere at different temperatures. The pyrolysis of Kapton film up to 1200 °C left a black residue principally composed of graphitic-carbon with a yield of about 55%. The D.C. conductivity of materials measured at 295 K increases with increasing pyrolysis temperature. The starting organic polymer is turned from an insulator to conductor material when heat-treated at over 700 °C. Moreover, in the frequency range 220–500 GHz, the pyrolyzed materials show distinct optical properties, which have variant degrees of terahertz absorption. Kapton® HN polyimide that is thermally converted into carbon-based materials may be used for the realization of calibrated terahertz absorbers, particularly for thermal transducers.

Experimental analysis on AC loss and fault current test of HTS coils co-wound with various inserted materials

A significant number of recent studies have explored the impact of many kinds of winding insulation conditions on high-temperature superconducting (HTS) coils for the application of electric devices. HTS coil co-wound with turn-to-turn inserted materials could be an appropriate alternative to the no-insulation coil because of the poor charge–discharge delay and unexpected quench behavior of the no-insulation coil. In addition, the co-wound HTS coil has good thermal stability and mechanical integrity, making it useful for superconducting applications such as superconducting magnetic energy storage (SMES) and superconducting fault current limiter (SFCL). However, studies of the co-wound HTS coil have explored dc electrical characteristics, but not ac electrical characteristics. Research into ac electrical characteristics is essential because of the operation sequence of the SMES and fault occurrence for the SFCL. In this paper, the ac electrical characteristics of HTS coils co-wound with various inserted materials were experimentally analyzed. Tested coils were co-wound with either Kapton, stainless steel, or copper tape at every turn of the winding, and one no-insulation coil served as a reference. AC loss, fault current, and recovery time were measured at 77 K, self-field.

Experimental investigation on the starting vortex induced by symmetrical dielectric barrier discharge plasma actuator

Flow control using plasma actuator is a promising research field of aeronautical applications. Due to its low energy consumption, rapid response and simple construction, this actuator has been investigated in various aerodynamics problems, such as boundary layer flow control, drag reduction, lift enhancement, noise reduction, and flow separation control. In order to understand the controlling mechanism of plasma actuator, many researchers have been carried out some experiments on the plasma actuator characterization in quiescent air and obtained the evolution process of starting vortex induced by plasma actuator. But the plasma actuator always works under flow condition. Therefore, understanding the interaction process between the starting vortex and incoming flow is a key to promote this technology development. In this paper, the starting vortex induced by symmetrical Dielectric Barrier Discharge (DBD) plasma actuator in quiescent air or under flow condition was investigated using Particle Image Velocimetry (PIV). Compared with the asymmetrical DBD plasma actuator, the symmetrical plasma actuator adopted the whole metal plate model as the insulated electrode. Three layers of kapton film as dielectric material covered the testing model and the thickness of each layer was 0.05 mm. The copper foil which was 2 mm in width and 0.05 mm in thickness was mounted on the trailing edge of the plate and oriented along the spanwise direction to induce a wall jet in the streamwise direction. The input AC voltage was 8 kV p-p and the frequency of the power source was 3 kHz. The wind speed was 1 m/s. The results suggested that the symmetrical actuator produced one pair of counter-rotating starting vortexes on each side of upper electrode and the trajectory of the starting vortex core was shown to scale with t0.7 in quiescent air. Compared to the evolution law of starting vortex in still air, the development evolution and life time of starting vortex under flow condition was different due to the interaction influence between incoming flow and starting vortex. The breakdown time of downstream starting vortex was earlier and the location of the starting vortex core scaled with t0.45 under flow condition. Conversely, the life time of upstream starting vortex which was in the opposite direction of incoming flow was delayed. The incoming flow enhanced the upstream starting vortex's capability of promoting mixing and entraining high-momentum fluid into boundary layer, therefore the boundary layer became more energetic and capable of withstanding adverse pressure gradient. The jet effect and mixing function could be achieved by the symmetrical plasma actuator. These investigations laid the groundwork for flow control using DBD plasma actuator at high wind speed or high Reynolds number.

Chloride catalytic effect on hydrogen desorption in NaAlH4

Hydrogen is considered as an environmental friendly energy carrier because it generates no pollution. Chlorides are promising catalyst for the hydrogen desorption in sodium alanate. All reactions and operations were performed under argon atmosphere in glove box. Crystalline NaAlH4 and catalyst TiCl3, Nb2O5 PbCl2 and CeCl3 were mixed in a ratio of (1, 2) wt%. Due to the air sensitivity of the samples they were covered with a kapton tape layer to minimize any contamination for characterization purpose. Structural characterizations and phase compositions of the samples were studied by XRD. The hydrogen storage capacity is measured by seivert type apparatus. Amount of desorbed hydrogen was found to be 3.6 wt % to 5.5 wt% at 250 °C with different type of catalyst. In the present work effect of doping of TiCl3, Nb2O5 PbCl2 and CeCl3 catalysis in NaAlH4 are investigated to understand the hydrogen desorption kinetics. It is concluded that enhanced storage capacity of NaAlH4 can be achieved due to various catalysts with reduction in temperature 285 °C–250 °C for desorption.

Soluble and transparent polyimides with high Tg from a new diamine containing tert‐butyl and fluorene units

ABSTRACTA novel diamine monomer 1, 4,4'‐(9H‐fluorene‐9,9‐diyl)‐bis(2‐tert‐butylaniline), was synthesized from 9‐fluorenone and 2‐tert‐butylaniline by the condensation reaction. Then it was polymerized with several commercial aromatic dianhydrides, respectively, to produce polyimides (PIs) by the one‐pot method. The number‐averaged molecular weights of the resulting PIs are in the range of (4.54–8.82) × 104 with polydispersity indices from 2.51 to 4.33 by gel permeation chromatography measurement. They are soluble in many organic solvents and can form transparent and tough films by solution‐casting. The cut‐off wavelengths of UV–vis absorption for the PI films are below 360 nm, which are much lower than that of Kapton film. The light transparency of them is above 90% in the visible light range from 400 to 760 nm. They also display relatively low dielectric constants (from 2.79 to 3.00), low water absorption rates (<1%), and high tensile strength (> 50 MPa). Their excellent solubility and transparency can be attributed to the incorporation of tert‐butyl groups and fluorene units into the rigid backbones of PIs. Simultaneously, they still maintain the high thermal stability (the 5% weight loss temperature in the range from 526 to 539 °C in nitrogen) and the high glass transition temperatures (Tg > 340 °C). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 976–984

Elasto-Aerodynamics-Driven Triboelectric Nanogenerator for Scavenging Air-Flow Energy.

Efficient scavenging the kinetic energy from air-flow represents a promising approach for obtaining clean, sustainable electricity. Here, we report an elasto-aerodynamics-driven triboelectric nanogenerator (TENG) based on contact electrification. The reported TENG consists of a Kapton film with two Cu electrodes at each side, fixed on two ends in an acrylic fluid channel. The relationship between the TENG output power density and its fluid channel dimensions is systematically studied. TENG with a fluid channel size of 125 × 10 × 1.6 mm(3) delivers the maximum output power density of about 9 kW/m(3) under a loading resistance of 2.3 MΩ. Aero-elastic flutter effect explains the air-flow induced vibration of Kapton film well. The output power scales nearly linearly with parallel wiring of multiple TENGs. Connecting 10 TENGs in parallel gives an output power of 25 mW, which allows direct powering of a globe light. The TENG is also utilized to scavenge human breath induced air-flow energy to sustainably power a human body temperature sensor.

Correlative Ultrastructural Analysis of Functionally Modulated Synapses Using Automatic Tape-Collecting Ultramicrotome - SEM Array Tomography

To understand synaptic plasticity, correlative microscopic approaches using functional imaging by light microscopy combined with electron microscopy (EM) are necessary. Ultrastructure of synapses can be visualized only by EM, but re-finding the same synapses that were manipulated and imaged by light microscope in the EM is a very laborious and time-consuming task when performing the conventional serial ultrathin-sectioning and observation method with transmission EM. Recently, new imaging applications to obtain volume data sets from brain tissues were introduced using an automatic tapecollecting microtome (ATUM) [1] or a focused ion beam [2] with scanning electron microscopy (SEM). We used an ATUMtome (RMC/Boeckeler) and automatic array tomography software, Atlas AT5 (Zeiss) and established a workflow to investigate ultrastructural changes of synapses. We first performed glutamate uncaging by 2-photon laser microscope to induce/modify spines, and obtained the whole cell image with a confocal microscope. Then the tissues are prepared with conventional pre-embedding immuno-EM using nanogold labeling for EM visualization as explained below. 2-photon glutamate uncaging and confocal laser scanning microscopy (CLSM) imaging: We uncaged MNI-glutamate with 4 mW (720 nm, 2 photon), 2 Hz X 40 stimulation on smooth dendrites of pyramidal neurons expressing EGFP in mouse hippocampal organotypic slice cultures. After the new spine formation visually confirmed, tissue slices were fixed in a mixture of 4% paraformaldehyde and 0.5-1% glutaraldehyde. CLSM Z-stack images were captured with a lower magnification to overview the whole tissue slice containing GFP fluorescent cells. Once the target neuron was identified, it was imaged to visualize its dendritic arbor and to define the position of the newly developed spines. Immuno-staining and resin embedding: Slices were cryo-protected with sucrose, permeabilized by liquid nitrogen, blocked and immunostained with anti-GFP antibody followed by nanogold conjugated secondary antibody. Slices were then silver enhanced, post-fixed with OsO4, en-block stained with uranyl acetate, dehydrated with a series of ethanol and acetone, and embedded in Durcupan resin. ATUM and SEM imaging with Atlas AT5/ Merlin VP Compact (Zeiss): The area containing the target cell was trimmed based on the CLSM image to a size of 1 x 1.5 mm. The sample was sectioned at 60-70 nm thickness with an ATUMtome and collected on Kapton tape automatically, then the kapton tape was placed on silicon wafers. The overview image of the Si wafer was captured using a digital camera, the image was loaded into the Atlas AT5 and aligned to the physical location of the wafer in the SEM using mesh grids as fiducials on the wafer. Low-resolution SE2 images for all sections were first captured with a 2 µm/pixel xy-resolution protocol (4 mm 2 imaging area) and overlaid on the overview image. An approximate area of the target cell was demarcated on the serial sections, and montage images were captured with a 30 nm pixel resolution protocol using BSE. After finding the target cell, a rough 3D reconstruction was performed to see the orientation of the target dendrite and spine. The high-resolution images of the target spine were captured with a 4-7 nm pixel resolution protocol using an InlensDuo detector in the BSE mode.

Pressure-dependent electrolytic conduction of track-etched single conical nanopore

In this paper, a systematic investigation of the influence of pressure on electrolytic conduction of conical nanopores in track-etched polyimide (Kapton) films is described. The effect of pressure is shown to be dependent on the pore orifice size and the rectification ratio of nanopore. For larger Kapton nanopores, the disruption of cation and anion equilibrium distributions within the nanopore caused by pressure resulted in the decrease of the rectification, but the pressure effect on the I–V also exhibited some differences for larger nanopores with different diameter. For smaller nanopores, pressure had a negligible influence on rectification.

A Study on Normal Zone Propagation Behavior of Partially Insulated GdBCO Coil

This paper reports the quench initiation and propagation characteristics of two GdBCO single-pancake coils, one wound with Kapton tape every two turns and the other completely wound with Kapton tape, to investigate the effect of a partial insulation (PI) winding technique. The thermal/electrical stabilities of the PI coil were improved by turn-to-turn contacts between the partially uninsulated turns, leading to an increased minimum quench energy. Furthermore, transverse normal zone propagation velocities of the PI coil increased because local heat was well transferred through uninsulated turns during quench events.

Experimental and Analytical Studies on Electromagnetic Behaviors of the GdBCO Racetrack Coils in a Time-Varying Magnetic Field

The electromagnetic responses of two GdBCO racetrack coils, one wound without turn-to-turn insulation (NI) and the other wound with Kapton tape (INS), were examined in a time-varying magnetic field generated by a permanent magnet mounted on a rotor, which was driven by a separate motor. The currents induced by the electromotive forces were measured with respect to the rotating speed, number of turns, and the distance (d) between the permanent magnet and the coil. In the case of the NI coil, delay of the induced current was observed in a time-varying magnetic field (5 rpm, d = 10 mm) due to anisotropy of the current path, suggesting that delay of the induced current will affect the electromagnetic induction by dB/dt. Consequently, the maximum induced current (0.35 A) of the NI coil was 8.2 times lower than that of the INS coil (2.88 A). Moreover, the induced currents in the INS coil increased with increasing rotating speed and number of turns, as well as with decreasing d, while those of the NI coil were barely affected due to the anisotropic current path. The experimental results showed good agreement with the simulated ones, which validates the proposed approaches involving an NI equivalent circuit model.

A Flexible, Stretchable and Shape‐Adaptive Approach for Versatile Energy Conversion and Self‐Powered Biomedical Monitoring

A flexible triboelectric nanogenerator (FTENG) based on wavy-structured Kapton film and a serpentine electrode on stretchable substrates is presented. The as-fabricated FTENG is capable of harvesting ambient mechanical energy via both compressive and stretching modes. Moreover, the FTENG can be a bendable power source to work on curved surfaces; it can also be adaptively attached onto human skin for monitoring gentle body motions.

Cost Effective Fabrication and Characterization of in-Membrane Micro-Fuel Cell

Micro fuel cells (μFCs) have been attracting much attention as a leading candidate for prospective portable power sources and battery replacements [1-3]. Their ability to create an efficient and clean source of energy combined with the ease of portability makes μFCs available to meet the needs of various portable electronic applications in the future [3, 4]. To this purpose, making more efficient devices with cost effective processes and materials is crucial. Many efforts on fuel cell miniaturization are focused on silicon-based techniques as silicon is the most common substrate in MEMS technology [5, 6]. However, combining silicon devices with polymeric fuel cells at mm or sub-mm scale presents many challenges, none of which have been solved in a completely satisfactory manner [7]. Distinct from previous attempts at fabricating in-plane μFCs by other studies, in this work we present a novel IMμFC (In-Membrane Micro Fuel Cell) design where the micro-flow channels for fuel and oxidant input and the MEA are fabricated using a Nafion membrane as the starting substrate. The fabrication procedure of IMμFCs has been illustrated in Fig. 1. In preparation for the microfabrication process, 100 μm thick SU-8 photoresist was spin coated, baked and developed to produce a micro stamp. The developed SU-8 on Si wafer was hard baked at 200°C for an hour to enhance the rigidity of the stamps. Fabricated stamps were cut and pressed onto Nafion 1110 membrane at 140°C and 300 psi by a NX2000- Nanoimprinter (Nanonex Corp., NJ, USA). Catalyst loading of conventional Pt/C dispersion on a selected area of microchannels was made by screen printing. Silver wire connections were secured in the designed electrical channels by Kapton tape and carbon black paste. In the sealing step, a laser patterned acrylic sheet was pressed onto the Nafion substrate by the nanoimprinter machine. The fabrication process was completed by mounting the gas inlet tubes on the acrylic sheet side of IMμFCs. The micro device performance was characterized by I-V polarization curves and impedance spectroscopy under dry and humidified conditions. Comparing to other studies on μFCs, the presented fabrication approach consists of cost effective processes with a great potential for scale-up production. This new architecture may lead to the deployment of an ultra-low weight, high energy density power source for flat-board-oriented electronic structures. Refrences: 1. Sundarrajan, S., Allakhverdiev, S. I., & Ramakrishna, S. (2012). Progress and perspectives in micro direct methanol fuel cell. International Journal of Hydrogen Energy, 37(10), 8765-8786. 2. Taylor, A. D., Lucas, B. D., Guo, L. J., & Thompson, L. T. (2007). Nanoimprinted electrodes for micro-fuel cell applications. Journal of Power Sources, 171(1), 218-223. 3. Kamarudin, S. K., Daud, W. R. W., Ho, S. L., & Hasran, U. A. (2007). Overview on the challenges and developments of micro-direct methanol fuel cells (DMFC).Journal of Power Sources, 163(2), 743-754. 4. Cao, J., Xu, J., Chen, Z., Wang, W., Huang, Q., & Zou, Z. (2013). A silicon‐based micro direct methanol fuel cell stack with a serial flow path design.International Journal of Energy Research, 37(4), 370-376. 5. Shah, K., Shin, W. C., & Besser, R. S. (2004). A PDMS micro proton exchange membrane fuel cell by conventional and non-conventional microfabrication techniques. Sensors and Actuators B: Chemical, 97(2), 157-167. 6. Lu, G. Q., Wang, C. Y., Yen, T. J., & Zhang, X. (2004). Development and characterization of a silicon-based micro direct methanol fuel cell.Electrochimica Acta, 49(5), 821-828. 7. Omosebi, A., & Besser, R. S. (2013). Fabrication and performance evaluation of an in membrane micro-fuel cell. Journal of Power Sources, 242, 672-676. Figure1. (a) Top-view illustration of μFC design imprinted on Nafion and (b) schematic fabrication steps of the μFC production Figure 1

Determination of charge-trapping sites in saturated and aromatic polymers by quantum chemical calculation

We studied the trapping of positive and negative charges in the chemical structures of polymers under a high electric field using a space charge measurement system. Positive charges accumulated in low-density polyethylene (LDPE), whereas positive and negative charges accumulated in polyimide (Kapton) and also in ethylene tetrafluoroethylene (ETFE) subjected to electron beam irradiation. To determine the charge-trapping sites in the chemical structures, a quantum chemical calculation was carried out using Density Function Theory (DFT) with Gaussian 09. The relationship between the energy band and the isosurface of orbital electrons at various energy levels was obtained. A three-dimensional (3D) electrostatic potential distribution map was obtained for positively and negatively charged polymers to determine the relationship between a trapping site and the charge accumulation center in the 3D potential distribution map. Positive and negative charges in Kapton and ETFE films are trapped in trapping sites in chemical structures and the positive charges in an LDPE film are trapped in physical defects.

Optothermal profile of an ablation catheter with integrated microcoil for MR-thermometry during Nd:YAG laser interstitial thermal therapies of the liver—an in-vitro experimental and theoretical study.

Flexible microcoils integrated with ablation catheters can improve the temperature accuracy during local MR-thermometry in Nd:YAG laser interstitial thermal therapies. Here, the authors are concerned with obtaining a preliminary confirmation of the clinical utility of the modified catheter. They investigate whether the thin-film substrate and copper tracks of the printed coil inductor affect the symmetry of the thermal profile, and hence of the lesion produced. Transmission spectroscopy in the near infrared was performed to test for the attenuation at 1064 nm through the 25 μm thick Kapton substrate of the microcoil. The radial transmission profile of an infrared high-power, light emitting diode with >80% normalized power at 1064 nm was measured through a cross section of the modified applicator to assess the impact of the copper inductor on the optical profile. The measurements were performed in air, as well as with the applicator surrounded by two types of scattering media; crystals of NaCl and a layer of liver-mimicking gel phantom. A numerical model based on Huygens-Fresnel principle and finite element simulations, using a commercially available package (COMSOL Multiphysics), were employed to compare with the optical measurements. The impact of the modified optical profile on the thermal symmetry was assessed by examining the high resolution microcoil derived thermal maps from a Nd:YAG laser ablation performed on a liver-mimicking gel phantom. Less than 30% attenuation through the Kapton film was verified. Shadowing behind the copper tracks was observed in air and the measured radial irradiation correlated well with the diffraction pattern calculated numerically using the Huygens-Fresnel principle. Both optical experiments and simulations, demonstrate that shadowing is mitigated by the scattering properties of a turbid medium. The microcoil derived thermal maps at the end of a Nd:YAG laser ablation performed on a gel phantom in a 3 T scanner confirm that the modified irradiation pattern does not disrupt the thermal symmetry, even though, unlike tissue, the gel is minimally scattering. The results from this initial assessment indicate that microcoils can be safely integrated with ablation catheters and ensure that the complete necrosis of the liver tumor can still be achieved.

Layer-by-layer polyelectrolyte films for contact electric energy harvesting

We report how self-assembly of polyelectrolyte thin films alters the contact electrification of polyimide polymer films used in contact based triboelectric energy harvesting systems. Polyimide films of the same size do produce a very small current when brought into contact. However, by covering one of the polyimide films with a polyelectrolyte thin film terminated by positively charged poly(allylamine hydrochloride) (PAH), the current is reversed and a much larger current and voltage are generated upon contact with the other polyimide film. A similar increase in contact current is not seen for polyelectrolyte thin films terminated by the negatively charged poly(sodium 4-styrenesulfonate). The PAH-terminated Kapton films are used to create an energy harvesting system providing a voltage of about 60 V and a current of 10 μA. At an average power of 11 μW for a load resistance of 100 MΩ, the energy harvester is able to power several light emitting diodes. Further studies on the contact electrification of the polyelectrolyte demonstrate that nanostructuring of the polymer surface using reactive ion etching does not give rise to polarity reversal. This is explained as hidden pockets of charge not accessible to PAH molecules, but which become accessible when the polymer is put under stress. Although the current originating for a PAH-terminated multilayer film does initially have the opposite sign to that of bare polyimide, it is found that the polarity will switch after subjecting it to a periodical mechanical force. Characteristic changes in current signatures associated with the switch are found, and are interpreted as mechanical interpenetration of the charged layers.

A Structured Approach to Analyze the Influence of Channel Dimensions on Heat Extraction via Superfluid Helium

For the luminosity upgrade of the LHC at CERN, the final focusing quadrupole magnets will be exposed to an increased energy deposition in their coil windings. To have a higher heat transfer rate between cable and superfluid helium bath, the cable insulation has been subject of many studies. Improved cable insulation designs, making use of several layers of Kapton tape wound around Rutherford type cables, allow helium to penetrate via micro-channels, which are left open between adjacent Kapton tape sections. To better understand the thermal behaviour at operating temperatures below and some degrees above the helium lambda transition, besides experimental work, several numerical models have been developed to study the influence of the cable insulation. Here we present a new numerical model based on an earlier presented FEM model, which makes use of coupling variables such that the user can easily and quickly change the parameters of interest, giving the possibility to analyse new ideas effectively.