Manufacture of electrical and magnetic graded and anisotropic materials for novel manipulations of microwaves

Spatial transformations (ST) provide a design framework to generate a required spatial distribution of electrical and magnetic properties of materials to effect manipulations of electromagnetic waves. To obtain the electromagnetic properties required by these designs, the most common materials approach has involved periodic arrays of metal-containing subwavelength elements. While aspects of ST theory have been confirmed using these structures, they are often disadvantaged by narrowband operation, high losses and difficulties in implementation. An all-dielectric approach involves weaker interactions with applied fields, but may offer more flexibility for practical implementation. This paper investigates manufacturing approaches to produce composite materials that may be conveniently arranged spatially, according to ST-based designs. A key aim is to highlight the limitations and possibilities of various manufacturing approaches, to constrain designs to those that may be achievable. The article focuses on polymer-based nano- and microcomposites in which interactions with microwaves are achieved by loading the polymers with high-permittivity and high-permeability particles, and manufacturing approaches based on spray deposition, extrusion, casting and additive manufacture

Spontaneous induction of the uniform lying helix alignment in bimesogenic liquid crystals for the flexoelectro-optic effect

Using in-plane electric fields, the electrical induction of the uniform lying helix (ULH) alignment in chiral nematic liquid crystals is reported. This process permits spontaneous induction of the ULH alignment to give an in-plane optic axis, without the need for complex processing. Flexoelectro-optic switching is subsequently obtained by holding the in-plane electrodes at a common voltage and addressing via a third, plane-parallel electrode on a second, or upper, substrate to give a field across the device in the viewing direction. For this device, in optimized bimesogenic materials, we demonstrate full intensity modulation and sub-millisecond response times at typical device temperatures. (C) 2012 American Institute of Physics. [doi:10.1063/1.3682305]11Nsciescopu

Increasing the flexoelastic ratio of liquid crystals using highly fluorinated ester-linked bimesogens

We present experimental results on the bulk flexoelectric coefficients e and effective elastic coefficients K of non-symmetric bimesogenic liquid crystals when the number of terminal and lateral fluoro substituents is increased. These coefficients are of importance because the flexoelastic ratio e/K governs the magnitude of flexoelectro-optic switching in chiral nematic liquid crystals. The study is carried out for two different types of linkage in the flexible spacer chain that connects the separate mesogenic units: these are either an ether or an ester unit. It is found that increasing the number of fluorine atoms on the mesogenic units typically leads to a small increase in e and a decrease in K, resulting in an enhancement of e/K. The most dramatic increase in e/K, however, is observed when the linking group is changed from ether to ester units, which can largely be attributed to an increase in e. Increasing the number of fluorine atoms does, however, increase the viscoelastic ratio and therefore leads to a concomitant increase in the response time. This is observed for both types of linkage, although the ester-linked compounds exhibit smaller viscoelastic ratios compared with their ether-linked counterparts. Highly fluorinated ester-linked compounds are also found to exhibit lower transition temperatures and dielectric anisotropies. As a result, these compounds are promising materials for use in electro-optic devices.11Nsciescopu

Hyper-precarious lives : Migrants, work and forced labour in the Global North

This paper unpacks the contested inter-connections between neoliberal work and welfare regimes, asylum and immigration controls, and the exploitation of migrant workers. The concept of precarity is explored as a way of understanding intensifying and insecure post-Fordist work in late capitalism. Migrants are centrally implicated in highly precarious work experiences at the bottom end of labour markets in Global North countries, including becoming trapped in forced labour. Building on existing research on the working experiences of migrants in the Global North, the main part of the article considers three questions. First, what is precarity and how does the concept relate to working lives? Second, how might we understand the causes of extreme forms of migrant labour exploitation in precarious lifeworlds? Third, how can we adequately theorize these particular experiences using the conceptual tools of forced labour, slavery, unfreedom and precarity? We use the concept of ‘hyper-precarity’ alongside notions of a ‘continuum of unfreedom’ as a way of furthering human geographical inquiry into the intersections between various terrains of social action and conceptual debate concerning migrants’ precarious working experiences