Negative thermal expansion of laminates

Measurements have been carried out on the in-plane and through the thickness thermal expansion coefficients of glass polypropylene fibre composites of 50% volume fraction between room temperature and 120°C. Only in the temperature range 20° to 75°C are reproducible results obtained. It is confirmed that in-plane negative values may be obtained in specific directions. The physical reason for this and its connection with the necessary appearance of a large Poisson ratio is pointed out. The expansivity of the matrix material depends strongly on temperature. Very good agreement between the experimental values and those predicted from the properties of the two constituents is found provided that the value for the expansivity of the polyoropylene is that within the temperature range considered. It is shown that the polypropylene matrix does not provide a matrix which is stable enough in its properties to enable the system to yield consistent negative values of expansivity. An alternative system is proposed and a single experiment confirms that a negative value of the thermal expansivity of as large as –50 × 10^–6 K^–1 may be obtained.     

BEES: exploring Mars with bioinspired technologies

The bioinspired engineering of exploration systems (BEES) applies insects' biological sensory and flight control abilities to the design of real-time autonomous, visual-navigation and control systems for small unmanned flying vehicles. The bioinspired engineering of exploration systems focuses on using the variety of nature-tested mechanisms successfully implemented by biological organisms but not easily accomplished by conventional methods. We apply BEES technology to the development of bioinspired visual navigation sensors integrated on small flyers to enable autonomous flight. BEES technology extracts the salient principles from a variety of diverse organisms adept at flight, and applies them to machines that can fly on Mars.     

Filtres microondes en microruban suspendu

The authors present the design and the development of integrated microwave filters using suspended substrate microstrip, a technology that provides many advantages. Through quasi-static approximation the propagation structure has been treated using integral equations solved with the method of moments. Thanks to this tool of analysis and to an accurate synthesis method, a series of bandpass filters have been designed and manufactured. The results presented validate the accuracy of our design method.     

Connective Ethnography for the Exploration of e-Science

E-science comprises diverse sites, connected in complex and heterogeneous ways. While ethnography is well established as a way of exploring the detail of the knowledge production process, some strategic adaptations are prompted by this spatial complexity of e-science. This article describes a study that focused on the biological discipline of systematics, exploring the ways in which use of a variety of information and communication technologies has become a routine part of disciplinary practice. The ethnography combined observation and interviews within systematics institutions with mailing list participation, exploration of web landscapes, and analysis of expectations around information and communications technologies as portrayed in policy documents. Exploring connections among these different activities offers a means of understanding multiple dimensions of e-science as a focus of practice and policy. It is important when studying e-science to engage critically with claims about the transformative capacity of new technologies and to adopt methodologies that remain agnostic in the face of such claims: A connective approach to ethnography offers considerable promise in this regard.     

Hot compaction of woven nylon 6,6 multifilaments

We describe a study of the hot compaction of woven nylon 6,6 multifilaments produced by a patented procedure, developed at the University of Leeds, for creating novel single‐polymer composites. In this process, an assembly of oriented elements, often in the form of a woven cloth, is held under pressure and taken to a critical temperature so that a small fraction of the surface of each oriented element is melted, which on cooling recrystallizes to form the matrix of the single‐polymer composite. This process is therefore a way of producing novel high‐volume‐fraction polymer/polymer composites in which the two phases are chemically the same material. Nylon is an obvious candidate material for this process because oriented nylon multifilaments are available on a commercial scale. The aim of this study was first to establish the conditions of temperature and pressure for the successful hot compaction of oriented nylon 6,6 fibers and second to assess the mechanical properties of the manufactured hot‐compacted nylon sheets. A crucial aspect of this work, not previously examined in hot‐compaction studies of other oriented polymers, was the sensitivity of the properties to absorbed water, with a significant change in the properties measured immediately after hot‐compaction processing and 2 weeks later when 2% water had been absorbed by the compacted nylon sheets. As expected, the water uptake had a greater effect on those properties that depended on local chain interactions (e.g., the modulus and yield strength) and less effect on those properties that depended on the large‐scale properties of the molecular network (e.g., strength). The only negative aspect of the properties of the hot‐compacted nylon sheets was the elevated‐temperature performance of the wet sample, with the modulus falling to a very low value at a temperature of 80°C. However, apart from the elevated‐temperature performance, the majority of the measured properties of the hot‐compacted nylon sheets were comparable to those of hot‐compacted polypropylene and poly(ethylene terephthalate). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 991–997, 2006     

The hot compaction of polypropylene fibres

Investigation of the compaction of unidirectionally arranged high-tenacity polypropylene fibres is described. A combination of techniques, with the major emphasis being morphological studies, show that controllable selective surface melting is not achieved at a high enough proportion to give substantial fibre-to-fibre bonding, and hence good lateral strengths.     

Studies of the pH of the membrane surface in a laboratory chlor-alkali cell

The pH on the surface of an ion-exchange membrane was measured with a microprobe in a laboratory scale chlor-alkali cell to study the leakage of OH^– through the membrane. The solution pH in the vicinity of the membrane facing the anode was found to be considerably higher than that in the bulk solution because of penetration and leakage of OH^–. The pH varied with the membrane type. This explains why the carboxylate membrane is not protonated and can be used as a separator in chlor-alkali cells.Deceased.     

Assessment and Early Instruction of Preschool Children at Risk for Reading Disability.

Preschool children at familial risk for reading disability were assessed on cognitive and linguistic variables and compared with preschoolers without familial risk. Risk children displayed performance profiles resembling those of older children with reading disability. Each group received intensive instruction in phonemic awareness and structured book reading. Instructed risk children made somewhat smaller gains than the nonrisk and required more teaching sessions to reach criterion. Rhyme and phoneme awareness predicted instruction outcome levels, and vocabulary and verbal short-term memory predicted number of teaching sessions to criterion. In kindergarten, the nonrisk group outperformed the risk group on reading and spelling, although the risk group reached grade-appropriate levels. At-risk children can be helped by appropriate preschool instruction, but they require more sustained teaching than nonrisk preschoolers. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effect of the measurement frequency on the elastic anisotropy of fibre laminates

Static tests and ultrasonic measurements (2.25 MHz) have been carried out on a series of composite laminates of glass fibres in a polypropylene matrix. A range of angle ply laminates were prepared for this study, with laminate angles θ of ±0, 10, 20, 30 and 40^∘. The high frequency measurements were made using the ultrasonic immersion technique, which allows the determination of a complete set of the elastic constants of a material. The relationship between the ultrasonically determined elastic constants of the angle ply laminates was found to be in excellent agreement with theoretical predictions, as previously validated for carbon fibre/epoxy angle ply laminates. A comparison between the ultrasonic and statically measured values was made for two of the angle ply laminates (θ = 0 and 20^∘). It was found that the static values were lower than those measured at ultrasonic frequency, particularly those constants that were more matrix dominated (for example the transverse moduli of the laminates). Measurements on a pure polypropylene sample at both testing frequencies confirmed that the change in matrix properties with frequency was the cause of this difference. The change in properties with test frequency is likely to be much larger in this system than in other composite materials because the glass transition temperature of polypropylene is close to ambient temperature. Dynamic mechanical tests (1 Hz) were carried out on a sample of pure polypropylene to assess this effect. We also give an appropriate method of estimating the dependence of glass transition temperature on frequency. The results for polypropylene are compared with those for other commonly used polymer matrix materials: epoxy resin, nylon and polyetheretherketone (PEEK): DMTA measurements were also made on these samples. The effect of test frequency on matrix properties, for the glass/PP laminates, was further investigated by examining the relationship of the Poisson's ratios with laminate angle using a mixture of ultrasonic experiments and theoretical predictions. Previously we have shown that the degree of anisotropy between the reinforcing fibre and the matrix phase is paramount in determining whether the material will show a negative Poisson's ratio at a critical laminate angle. The ultrasonic measurements carried out in this study on the glass/PP laminates showed a minimum in one of the Poisson's ratio at a laminate angle of 32°, but the value did not become negative. However, theoretical predictions showed that for a static frequency measurement (1 Hz), where the matrix is softer and hence the anisotropy of each laminate ply is higher, the laminate will show a negative Poisson's ratio with a minimum at a laminate angle of around 28°.