616 research outputs found
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The effect of water absorption on the dielectric properties of epoxy nanocomposites
In this research, the influence of water absorption on the dielectric properties of epoxy resin and epoxy micro-composites and nano-composites filled with silica has been studied. Nanocomposites were found to absorb significantly more water than unfilled epoxy. However, the microcomposite absorbed less water than unfilled epoxy: corresponding to reduced proportion of the epoxy in this composite. The glass transition temperatures of all the samples were measured by both differential scanning calorimetry and dielectric spectroscopy. The Tg decreased as the water absorption increased and, in all cases, corresponded to a drop of approximately 20K as the humidity was increased from 0% to 100%. This implied that for all the samples, the amount of water in the resin component of the composites was almost identical. It was concluded that the extra water found in the nanocomposites was located around the surface of the nanoparticles. This was confirmed by measuring the water uptake, and the swelling and density change, as a function of humidity as water was absorbed. The water shell model, originally proposed by Lewis and developed by Tanaka, has been further developed to explain low frequency dielectric spectroscopy results in which percolation of charge carriers through overlapping water shells was shown to occur. This has been discussed in terms of a percolation model. At 100% relative humidity, water is believed to surround the nanoparticles to a depth of approximately 5 monolayers. A second layer of water is proposed that is dispersed by sufficiently concentrated to be conductive; this may extend for approximately 25 nm. If all the water had existed in a single layer surrounding a nanoparticle, this layer would have been approximately 3 to 4 nm thick at 100%. This "characteristic thickness" of water surrounding a given size of nanoparticle appeared to be independent of the concentration of nanoparticles but approximately proportional to water uptake. Filler particles that have surfaces that are functionalized to be hydrophobic considerably reduce the amount of water absorbed in nanocomposites under the same conditions of humidity. Comments are made on the possible effect on electrical aging
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A "water shell" model for the dielectric properties of hydrated silica-filled epoxy nano-composites
The electrical properties of epoxy resin have been studied as a function of hydration. The epoxy was studied in an un-filled state, filled with 40 µm SiO2 particles, and filled with 50 nm SiO2 particles. The relative humidity was controlled by saturated salt solutions at ambient temperatures from 298-353 K. Measurements were made using dielectric spectroscopy over the frequency range 10-3-105 Hz. The hydration isotherm (i.e. the mass uptake of water) was established by measuring the mass as a function of relative humidity (RH). It was found that the nanocomposites absorb up to 60% more water than the unfilled and micro-filled epoxies. Dielectric spectroscopy shows different conduction and quasi-DC behaviours at very low frequencies (<10-2 Hz) with activation energies dependent on the hydration and temperature. These observations have led to the development of a “water shell” model to explain this phenomenon
Open-Source Workflows for Reproducible Molecular Simulation
We apply molecular simulation to predict the equilibrium structure of organic molecular aggregates and how these structures determine material properties, with a focus on software engineering practices for ensuring correctness. Because simulations are implemented in software, there is potential for authentic scientific reproducibility in such work: An entire experimental apparatus (codebase) can be given to another investigator who should be able to use the same processes to find the same answers. Yet in practice, there are many barriers which stand in the way of reproducible molecular simulations that we address through automation, generalization, and software packaging. Collaboration on and application of the Molecular Simulation and Design Framework (MoSDeF) features prominently.
We present structural investigations of organic molecule aggregates and the development of infrastructure and workflows that help manage, initialize, and analyze molecular simulation results through the following scientific applications (1) A screening study wherein we validate self-assembled poly-3-hexylthiophene (P3HT) morphologies show the same state dependency as in prior work, and (2) A multi-university collaborative reproducibility study where we examine modeling choices that give rise to differences between simulation engines. In aggregate, we reinforce the need for pipelines and practices emphasizing transferability, reproducibility, useability, and extensibility in molecular simulation
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Influence of absorbed water on the dielectric properties and glass-transition temperature of silica-filled epoxy nanocomposites
Work on dielectric spectroscopy of epoxy resin filled with nano-SiO2 at different relative humidities and temperatures is reported. Above the glass-transition temperature (Tg), dc-like imperfect charge transport (QDC or LFD) dominates the low frequency dielectric spectrum. Another mid-frequency relaxation process was found in the non-dried composites. Water also induces glass-transition temperature decreases, which can be measured both by dielectric spectroscopy and DSC. Both theory and experiment demonstrated that a higher water content could exist in nanocomposites than unfilled epoxy suggesting a bigger free volume when nanostructured. In our system, the hydrophilic surface of silica is likely to cause water to surround and lead to delamination of the epoxy from SiO2. This is a potential mechanical and dielectric weakness in the nanocomposites, which may lead to an ageing phenomenon. Hydrophobic surface group may reduce the water adsorption in nanocomposites
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Towards an understanding of nanometric dielectrics
Dielectric studies are described aimed at providing an understanding of the charge storage and transport of an epoxy resin containing TiO2 nanoparticles. Comparative results for conventionally filled composites are given, and the results discussed in terms of the underlying physics. It is shown that nanometric fillers mitigate the interfacial polarization characteristic of conventional materials with a reduction in the internal field accumulations
Relations of literature, science & philosophy in university education
RELATIONS OF LITERATURE, SCIENCE & PHILOSOHPY:
Thought distinguished from feeling;- series of minds founded on
this distinction. - The general and the particular distinguished,
and a second series of minds suggested;- a corresponding series cf
expressions of mentality;- diagram. Summary to this point. Of
the scientific method,- ancient philosophy. The development of
the inductive method,- Dr» Reid quoted;-the growth of science;-
oscillation;- the likeness of the new to the old philosophy
recapitulation.*THE FOREGOING ANALYSIS AS BEARING ON UNIVERSITIES:
A. Nature & Function of University Education:
Meaning of university Education defined,- Professor Butcher
quoted. The discussion introduced by a list of materials.B(1) Literature:
Why music and fine art are not essential in university education;-
literature completely representative of the artistic spirit, but
improperly used.- Ancient literature, modern literature.- Common
arguments in favour of literary training stated and refuted, The
real reason why literature must be studied;- the need for changed
method in the study of literature.* Summary and conclusions regarding literature.B.(2) Science & Philosophy
Limited nature of most arts courses;- gaps in arts courses;-
enough subjects included, but why always the same? The real
object of scientific training;- The best plan of education in
science & philosophy;- Omission of natural science from most arts
courses;- this omission not obviated by Scottish "honours"
courses,- comparison of these with those of Cambridge. Advantages of free choice of subjects. Need for philosophy and practical suggestions. The argument confirmed by reference to history. Summary,APPLICATION OF THE FOREGOING TO EDINBURGH UNIVERSITY:
The teaching organisation in Edinburgh. The use that is made of
this organisation, and the use that might be
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The decay of space charge in a glassy epoxy resin following voltage removal
The PEA technique is used to measure the distribution of space charge in an epoxy resin after polarisation for one week at an applied field of 7.14kV/mm over a range of temperatures. The decay of the space charge is followed for times up to 114 hours after removal of the voltage and analysed in terms of a number of alternative decay mechanisms. It is shown that the rate-determining stage of the decay mechanism is that of a thermally activated process that has been associated with charge de-trapping. At times greater than 102s the de-trapping process behaves as though the space charge field does not exist and the retention time of the space charge depends only upon the depth of the deepest occupied traps and the temperature
Structures of pyruvate kinases display evolutionarily divergent allosteric strategies
The transition between the inactive T-state (apoenzyme) and active R-state (effector bound enzyme) of Trypanosoma cruzi pyruvate kinase (PYK) is accompanied by a symmetrical 8° rigid body rocking motion of the A- and C-domain cores in each of the four subunits, coupled with the formation of additional salt bridges across two of the four subunit interfaces. These salt bridges provide increased tetramer stability correlated with an enhanced specificity constant (k(cat)/S(0.5)). A detailed kinetic and structural comparison between the potential drug target PYKs from the pathogenic protists T. cruzi, T. brucei and Leishmania mexicana shows that their allosteric mechanism is conserved. By contrast, a structural comparison of trypanosomatid PYKs with the evolutionarily divergent PYKs of humans and of bacteria shows that they have adopted different allosteric strategies. The underlying principle in each case is to maximize (k(cat)/S(0.5)) by stabilizing and rigidifying the tetramer in an active R-state conformation. However, bacterial and mammalian PYKs have evolved alternative ways of locking the tetramers together. In contrast to the divergent allosteric mechanisms, the PYK active sites are highly conserved across species. Selective disruption of the varied allosteric mechanisms may therefore provide a useful approach for the design of species-specific inhibitors
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Internal charge behaviour of nanocomposites
The incorporation of 23 nm titanium dioxide nanoparticles into an epoxy matrix to form a nanocomposite structure is described. It is shown that the use of nanometric particles results in a substantial change in the behaviour of the composite, which can be traced to the mitigation of internal charge when a comparison is made with conventional TiO2 fillers. A variety of diagnostic techniques (including dielectric spectroscopy, electroluminescence, thermally stimulated current, photoluminescence) have been used to augment pulsed electro-acoustic space charge measurement to provide a basis for understanding the underlying physics of the phenomenon. It would appear that, when the size of the inclusions becomes small enough, they act co-operatively with the host structure and cease to exhibit interfacial properties leading to Maxwell-Wagner polarization. It is postulated that the particles are surrounded by high charge concentrations in the Gouy-Chapman-Stern layer. Since nanoparticles have very high specific areas, these regions allow limited charge percolation through nano-filled dielectrics. The practical consequences of this have also been explored in terms of the electric strength exhibited. It would appear that there was a window in which real advantages accrue from the nano-formulated material. An optimum loading of about 10% (by weight) is indicated
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