Publications

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Multilayer coextrusion is applied to produce a tape containing layers of alternating electrical properties to demonstrate the potential for using coextrusion to manufacture capacitors. To obtain the desired properties, we develop two filled polymer systems, one for conductive layers and one for dielectric layers. We describe numerical models used to help determine the material and processing parameters that impact processing and layer stability. These models help quantify the critical ratios of densities and viscosities of the two layers to maintain stable layers, as well as the effect of increasing the flow rate of one of the two materials. The conducting polymer is based on polystyrene filled with a blend of low-melting-point eutectic metal and nickel particulate filler, as described by Mrozek et al. (2010). The appropriate concentrations of fillers are determined by balancing measured conductivity with processability in a twin screw extruder. Based on results of the numerical models and estimates of the viscosity of emulsions and suspensions, a dielectric layer composed of polystyrene filled with barium titanate is formulated. Despite the fact that the density of the dielectric filler is less than the metallic filler of the conductive phase, as well as rheological measurements that later showed that the dielectric formulation is not an ideal match to the viscosity of the conductive material, the two materials can be successfully coextruded if the flow rates of the two materials are not identical. A measurable capacitance of the layered structure is obtained.

This report summarizes the results generated in FY13 for cable insulation in support of the Department of Energy's Light Water Reactor Sustainability (LWRS) Program, in collaboration with the US-Argentine Binational Energy Working Group (BEWG). A silicone (SiR) cable, which was stored in benign conditions for %7E30 years, was obtained from Comision Nacional de Energia Atomica (CNEA) in Argentina with the approval of NA-SA (Nucleoelectrica Argentina Sociedad Anonima). Physical property testing was performed on the as-received cable. This cable was artificially aged to assess behavior with additional analysis. SNL observed appreciable tensile elongation values for all cable insulations received, indicative of good mechanical performance. Of particular note, the work presented here provides correlations between measured tensile elongation and other physical properties that may be potentially leveraged as a form of condition monitoring (CM) for actual service cables. It is recognized at this point that the polymer aging community is still lacking the number and types of field returned materials that are desired, but Sandia National Laboratories (SNL) -- along with the help of others -- is continuing to work towards that goal. This work is an initial study that should be complimented with location-mapping of environmental conditions of Argentinean plant conditions (dose and temperature) as well as retrieval, analysis, and comparison with in- service cables.

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The purpose of this project is to develop multi-layered co-extrusion (MLCE) capabilities at Sandia National Laboratories to produce multifunctional polymeric structures. Multi-layered structures containing layers of alternating electrical, mechanical, optical, or structural properties can be applied to a variety of potential applications including energy storage, optics, sensors, mechanical, and barrier applications relevant to the internal and external community. To obtain the desired properties, fillers must be added to the polymer materials that are much smaller than the end layer thickness. We developed two filled polymer systems, one for conductive layers and one for dielectric layers and demonstrated the potential for using MLCE to manufacture capacitors. We also developed numerical models to help determine the material and processing parameters that impact processing and layer stability.

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Aluminum oxide (ALOX) filled epoxy is the dielectric encapsulant in shock driven high-voltage power supplies. ALOX encapsulants display a high dielectric strength under purely electrical stress, but minimal information is available on the combined effects of high voltage and mechanical shock. We report breakdown results from applying electrical stress in the form of a unipolar high-voltage pulse of the order of 10-{micro}s duration, and our findings may establish a basis for understanding the results from proposed combined-stress experiments. A test specimen geometry giving approximately uniform fields is used to compare three ALOX encapsulant formulations, which include the new-baseline 459 epoxy resin encapsulant and a variant in which the Alcoa T-64 alumina filler is replaced with Sumitomo AA-10 alumina. None of these encapsulants show a sensitivity to ionizing radiation. We also report results from specimens with sharp-edged electrodes that cause strong, localized field enhancement as might be present near electrically-discharged mechanical fractures in an encapsulant. Under these conditions the 459-epoxy ALOX encapsulant displays approximately 40% lower dielectric strength than the older Z-cured Epon 828 formulation. An investigation of several processing variables did not reveal an explanation for this reduced performance. The 459-epoxy encapsulant appears to suffer electrical breakdown if the peak field anywhere reaches a critical level. The stress-strain characteristics of Z-cured ALOX encapsulant are measured under high triaxial pressure and we find that this stress causes permanent deformation and a network of microscopic fractures. Recommendations are made for future experimental work.