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  • Author or Editor: B. Chowdhury x
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Abstract  

Functional parameters contributing to the recognizable good performance of a highly crosslinked olefinic polymer based on cyclopentadiene have been evaluated by a limited range of stress decay and creep recovery from a fixed deformation over constant time periods. The experimental design for these involved a calculated stress (load) at the initial sample deformation (displacement) for determination of changes in stress in bending mode and flex creep under applied load at three constant temperature segments of 6 h each with 20 min recovery time between segments. The results have identified a behavior which is typical of interfacial orientational effect in a two-component system. Thermal conductivity, heat capacity and density at the same temperatures were measured for determination of variations in thermal diffusivity as an indication of the competing effects of densification and thermal expansion for such a system. Static charging and polarization experiments were performed for evaluation of other desired uses of this material, in moving parts and in direct or induced voltage applications. The latter allows measurement of dynamic charge transport through the resin matrix. Calculations indicate generation of charge carriers from weak secondary bonding typical of polymer interlaces. The polarization itself consists of dipole orientation characterized by relatively high activation energy. The intrinsic mechanical properties for this material had been determined by the manufacturer, but the performance parameters reported in this study have been determined for the first time for exploration of further uses for this material.

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Abstract  

Pyrolysis of normally insulating aromatic polyimide is known to impart electrical conductivity to the polymer due to the formation of carbonized regions in an insulating matrix with a concomitant change in the polymer’s structural arrangement. The wholly pyrolyzed polyimide is potentially useful for specific applications in certain types of semiconductor devices because of the polyimide’s insulator/conductor transition which creates a barrier type conduction. Pyrolysis, however, degrades the required mechanical integrity of the polyimide for construction of such devices. In order to evaluate the fundamental aspects of barrier conduction by high voltage electron transfer from metal contact that can still produce measurable current in thermally treated non-pyrolyzed polyimide, the nature of depolarization in Kapton was assessed by the thermally stimulated depolarization current (TSDC) technique. The results show that thermal treatment of polyimide without pyrolysis and therefore without loss of mechanical integrity offers a viable means of steady electron conduction for semiconductor operation.

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Abstract  

The mechanics of thermally-activated gaseous diffusion in polymers have long been studied for development of theoretical understanding of the interactive forces responsible for the non-linear nature of diffusion and the resultant enthalpic changes in the polymer. Methodologies and calculations have been developed in this work for treatment of experimental data for elimination of thickness-related anomalies in water vapor diffusion and for delineation of pressure effects. Linearized data for different polymer thicknesses and the attendant internal thermal effects have been generated by using calculated single molecule diffusion values. Equally linear data are obtained for different pressures by the use of a graphical method from which identical diffusion values are obtained, independent of material thickness and external pressure. True comparisons and classification of polymers as to their diffusivities are thus possible for development of barrier materials for food and drug packaging and for protective encapsulation of electronic devices. Results will be reported for three structurally different polymers.

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Abstract  

Measurement of voltage-induced thermal depolarization current and calculation of the rate of depolarization as well as the parameters of drift mobility and conductivity of charge carriers for melt-extruded neat unreinforced grade A950 VECTRA resin - a wholly aromatic copolyester - strongly suggest that an irreversible minor transition centered around 65C is the primary thermal process related to molecular realignment. Changes in capacitance values with temperature also show this to be the most active temperature region. A major depolarization peak at 100C having the characteristics of a Tg cannot be justified as due to glass transition but likely to result from molecular motions involving long range intermolecular order. The interpretation for both transitions can be supported by the mechanical response of this polymer. An important outcome of this work is the assertion that contrary to current thinking, it is the number of charge carriers and not viscosity alone that will have to be considered in future development of fast response liquid crystal displays along with the development of newer liquid crystal polymer structures.

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Abstract  

Physical deterioration is a natural consequence of moisture attack in finished concrete. Moisture is retained in open voids in prepared cement during mixing and hydration stages. Further atmospheric moisture uptake occurs following dehydration under use conditions in polymer-containing cement formulations, since polymers act as conduits for moisture ingress. The initial attribute of added strength from the polymer in improved cement formulations is thus nullified. Customary solution to the moisture problem has centered around improving moisture resistance by significant reduction of inherent macro defects. A more complete method of moisture blocking by the reaction of activated carbon with a vinyl polymer included in the cement formulation is described here. The moisture blocking action is due to the mitigation of open voids by the occlusion of a carbon-vinyl additive, essentially resulting in filled voids or no voids in the prepared cement, producing a durable finished concrete product. Supporting evidence for the above by thermal, polarization, X-ray and SEM are reported in this paper.

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Abstract  

A logical approach to electron transport studies for barrier conduction in layered structures was adopted by thermally stimulated discharge current (TSDC) measurement. The scope and applicability of this technique to the evaluation of the thermoelectric parameters of relaxation time, detrapping energy and depolarization rates are demonstrated here. These are characterized by the controlling factors of layer resistance and the resultant thermal and voltage gradients which apply to the drift of electrons arising from both dipolar and interfacial charges. The methodologies used in this study are suitable for parametric evaluation of structured electronic devices.

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Abstract  

A neutron induced prompt γ -ray spectrometry (NIPS) facility has been developed at the Nuclear Chemistry Research Division, of the Korea Atomic Energy Research Institute (KAERI) with the aim of analyzing the major components of various elements in aqueous samples. The facility is equipped with a 252Cf neutron source and a γ-γ coincidence setup with two n-type coaxial HPGe detectors based on NIM spectrometric modules in association with data acquisition and spectral analysis systems. The development of the system, its set-up and the calibration of detection efficiency up to 8 MeV using a set of radionuclides and the (n,γ) reactions of chlorine are described in the paper.

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