Dipole

Summary

In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system is a pair of charges of equal magnitude but opposite sign separated by some typically small distance. (A permanent electric dipole is called an electret.) *A magnetic dipole is the closed circulation of an electric current system. A simple example is a single loop of wire with constant current through it. A bar magnet is an example of a magnet with a permanent magnetic dipole moment. Dipoles, whether electric or magnetic, can be characterized by their dipole moment, a vector quantity. For the simple electric dipole, the electric dipole moment points from the negative charge towards the positive charge, and has a magnitude equal to the strength of each charge times the separation between the charges. (To be precise: for the definition of the di

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Many technical advances of modern society sprout from the creation of specially customized polymer materials exhibiting unique combinations of properties uncharted so far. Amid this plurality of highly versatile materials, the class of electroactive polymers (EAPs) bears the special ability to undergo large shape changes when exposed to electrical stimulation. Such materials offer new perspectives in automation, robotics and medicine. Especially the dielectric elastomer actuators (DEA) technology is considered as highly promising among EAPs due to the simple working principle and actuation characteristics similar to mammalian muscles. Despite great research efforts and remarkable progress in regard to materials and devices, the commercial breakthrough of the DEA technology is still outstanding. A major obstacle for their application is the high driving voltage needed to obtain a reasonable actuation. The development of tailored elastomers that overcome this deficiency is therefore a main research focus in academia and industry. The exceptional elastic and dielectric characteristics of silicones nominate them as an ideal vantage point to start this endeavor. Their low permittivity inherently decreases the actuation performance and should therefore be optimized with all other properties remaining unaffected. In the present work, dipoles were covalently attached to a siloxane backbone in order to increase the permittivity thereof and achieve a material with an enhanced actuation performance at low driving voltages for DEAs. The introduction of the polar groups to the polymer side chains was carried out via thiol-ene chemistry, which enabled a quantitative functionalization of the polymer backbone. Three different polar groups, nitrile, thioether and sulfones, were investigated and the corresponding functionalized polysiloxanes were prepared and characterized. Thorough dielectric, mechanical and electromechanical investigations were conducted on polysiloxane based elastomers with different content of nitrile groups. In addition, first results were obtained with thioethers and sulfones which are also promising candidates to enhance the permittivity of polysiloxanes. Most of the synthesized polysiloxanes show glass transition temperatures well below room temperature, an important prerequisite to ensure elastic properties. Polysiloxanes that carry a butylthioether moiety at every repeat unit doubled the permittivity of the elastomer. With the same amount of the nitrile or sulfone dipoles, a six respectively eight times higher permittivity as compared to regular siloxane is achieved. However, with enhanced dipole content also an increase of the electrical conductivity of the polymer was observed. A simple one-step process to prepare cross-linked polar silicone films was developed using UV light triggered thiol-ene addition. A strong correlation between the dipole content in the material and the resulting actuation strain was found. Finally, a polar silicone elastomer was prepared that showed up to 20.5% lateral strain under a field as low as 10.8 V/um. Compared to commercial silicone this corresponds to an effective voltage reduction by about 70%. Processing such materials in films the thickness of which is below 10 um would result in an actuator that can be driven below 100 V.

EPFL2017

An early separation scheme for the LHC luminosity upgrade

Guido Sterbini

In this thesis we evaluate the potential of the Early Separation Scheme for the Luminosity Upgrade of the Large Hadron Collider (LHC). The main goal of the Early Separation Scheme is to reduce the crossing angle between the proton beams at the collision point in order to increase the luminosity performance of the machine and to alleviate, at the same time, the detrimental effects due to the electromagnetic interaction between the beams. The Early Separation Scheme consists of four dipoles for each of the two high luminosity Interaction Points of the LHC, corresponding to the ATLAS and CMS detectors. Two dipoles out of the four, the so-called D0 dipoles, have to be integrated in the experimental cavern. We show that, working in synergy with an increased beam current and with a stronger final focusing system, the Early Separation Scheme can provide an integrated luminosity of 3000 fb-1 over a period of 6.5 – 7 years with a leveled luminosity of 5.5 1034 cm-2 s-1. These figures are possible thanks to the luminosity leveling by using the crossing angle. We study the impact of the Early Separation Scheme from the beam dynamics point of view by evaluating its linear and non-linear effects. We show, using an analytical approach, that all induced linear effects are negligible. The non-linear effects, namely the electromagnetic interaction between the beams, are considered by means of numerical simulations and dedicated experiments. From the simulation and the experimental results, we have indications about the minimum beam crossing angle that is still compatible with the beam dynamics constraints. From these indications and by considering the different issues related to the integration of the scheme in the detectors, we propose to position the D0 dipole at 14 m from the Interaction Point. The integrated magnetic field required and the space constraints in the detectors imply the use of a superconducting D0 dipole: we optimize its aperture to reduce the power deposited on the magnet's coil by the collision debris. Due to the high power density impinging on the coil and to the 9 T magnetic field required, we propose a magnet cross-section using a Nb3Sn superconducting cable at the temperature of 4.2 K.

EPFL2010

Supramolecular architectures of molecularly thin yet robust free-standing layers

Mina Moradi, Patrick Shahgaldian, Henning Paul-Julius Stahlberg

Stable, single-nanometer thin, and free-standing two-dimensional layers with controlled molecular architectures are desired for several applications ranging from (opto-)electronic devices to nanoparticle and single-biomolecule characterization. It is, however, challenging to construct these stable single molecular layers via self-assembly, as the cohesion of those systems is ensured only by in-plane bonds. We herein demonstrate that relatively weak noncovalent bonds of limited directionality such as dipole-dipole (-CN center dot center dot center dot NC-) interactions act in a synergistic fashion to stabilize crystalline monomolecular layers of tetrafunctional calixarenes. The monolayers produced, demonstrated to be freestanding, display a well-defined atomic structure on the single-nanometer scale and are robust under a wide range of conditions including photon and electron radiation. This work opens up new avenues for the fabrication of robust, single-component, and free-standing layers via bottom-up self-assembly.

American Association for the Advancement of Science (AAAS)2019