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Shi Y.,University of Akron | Yoonessi M.,Ohio Aerospace Institute | Weiss R.A.,University of Akron
Macromolecules | Year: 2013

High switching temperature shape memory polymers (SMPs) were developed from metal salts of sulfonated PEEK (M-SPEEK) ionomer and composites of the M-SPEEK ionomers and a fatty acid salt. The neat M-SPEEK ionomers exhibited reasonable shape memory behavior, but the composite SMPs showed very promising shape memory behavior. The composites were prepared from 70 wt % M-SPEEK (M = sodium or zinc) and 30 wt % sodium oleate (NaOl). Ionic nanodomains formed by the interactions of ionic groups provided a permanent physically cross-linked network, and strong dipolar interactions between the ionomer and a dispersed phase of crystalline NaOl provided the temporary network. A temporary shape was achieved and fixed by deforming the material above the melting temperature (Tm) of NaOl and then cooling under stress to below Tm. The permanent shape was recovered by reheating the material above Tm without applying stress. Shape fixing efficiencies of 96% were achieved, and shape recovery reached 100%. Triple shape memory behavior was also achieved for M-SPEEK/NaOl compounds using the glass transition of the ionomer and the melting point of the NaOl as two separate switching temperatures. © 2013 American Chemical Society. Source

Yoonessi M.,NASA | Yoonessi M.,Ohio Aerospace Institute | Gaier J.R.,NASA
ACS Nano | Year: 2010

Graphene nanosheet-bisphenol A polycarbonate nanocomposites (0.027-2.2 vol %) prepared by both emulsion mixing and solution blending methods, followed by compression molding at 287 °C, exhibited dc electrical percolation threshold of ∼0.14 and ∼0.38 vol %, respectively. The conductivities of 2.2 vol % graphene nanocomposites were 0.512 and 0.226 S/cm for emulsion and solution mixing. The 1.1 and 2.2 vol % graphene nanocomposites exhibited frequency-independent behavior. Inherent conductivity, extremely high aspect ratio, and nanostructure directed assembly of the graphene using PC nanospheres are the main factors for excellent electrical properties of the nanocomposites. Dynamic tensile moduli of nanocomposites increased with increasing graphene in the nanocomposite. The glass transition temperatures were decreased with increasing graphene for the emulsion series. High-resolution electron microscopy (HR-TEM) and small-angle neutron scattering (SANS) showed isolated graphene with no connectivity path for insulating nanocomposites and connected nanoparticles for the conductive nanocomposites. A stacked disk model was used to obtain the average particle radius, average number of graphene layers per stack, and stack spacing by simulation of the experimental SANS data. Morphology studies indicated the presence of well-dispersed graphene and small graphene stacking with infusion of polycarbonate within the stacks. © 2010 American Chemical Society. Source

Ferguson D.C.,Air Force Research Lab | Vayner B.V.,Ohio Aerospace Institute
IEEE Transactions on Plasma Science | Year: 2013

Electrostatic discharges on the spacecraft solar array surfaces have been studied for over 40 years. Multiple tests in many laboratories have been performed to reveal the mechanisms of current pulse formation, to determine plasma expansion speed and dimensions of neutralized surface area, and to define the parameters of the electric circuits to be able to adequately simulate space discharges in plasma chambers. Initially, a thorough analysis of all the available experimental data is performed to achieve progress in the creation of a satisfactory theoretical description of pulse wave forms and surface neutralization processes. It is shown that there are two main reasons for the significant variations observed in pulse wave forms, durations, and amplitudes; differences in test arrangements and pure statistical variations caused by the random nature of arc inception and development. Then, the theoretical model of flashover current pulse formation due to coverglass charge neutralization at the plasma perimeter is carefully considered and confronted with existing experimental data. It is found that this theory contains some appropriate elements but cannot be applied for quantitative explanation of pulse wave forms. The comparison of theoretical results and experimental data clearly shows that there is no (and there cannot be) such thing as a standard pulse. To estimate the possibility of detrimental effects from primary arcs, these events must be studied and analyzed statistically. © 2013 IEEE. Source

Leib S.J.,Ohio Aerospace Institute
AIAA Journal | Year: 2013

An acoustic analogy approach is used to predict the turbulent mixing noise emitted by cold, subsonic, rectangular jets of aspect ratios 2, 4, and 8. Mean flow interaction effects are accounted for in terms of a Green's function under the locally parallel mean flow approximation. The problem for the Green's function is reduced to a set of coupled ordinary differential equations by modeling the mean flow as having level surfaces in crossflow planes coinciding with one coordinate in a cylindrical elliptical coordinate system and using a conformal mapping method. The Green's function is combined with a previously developed hybrid source model to complete the prediction method. Source model coefficients are chosen based on the results of low-speed flow surveys in the nozzles considered. Results are compared with acoustic data, and it is found that the method is generally capable of predicting the measured azimuthal directivity characteristics of the sound field. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. Source

Ohio Aerospace Institute and US Government | Date: 2010-05-07

Ceramic oxide aerogels having improved flexibility are disclosed. Preferred embodiments exhibit high modulus and other strength properties despite their improved flexibility. The gels may be polymer cross-linked via organic polymer chains to further improve strength properties, without substantially detracting from the improved flexibility. Methods of making such aerogels are also disclosed.

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