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Ounaies, Z.

Publications and source records attributed to Ounaies, Z..

11 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Discriminating Uranium Isotopes Using the Time-Emission Profiles of Long-Lived Delayed Neutrons

In nuclear nonproliferation and safeguards, detecting and accurately characterizing special nuclear material remains one of the greatest challenges. Uranium enrichment determination is typically achieved by measuring the ratio of characteristic γ -ray emissions from 235 U and 238 U. Fission also produces β -delayed neutrons, which have been used in the past to determine uranium enrichment from the time dependence of the long-lived delayed-neutron emission rate. Such measurements typically use moderated 3 He-tube detectors. We demonstrate an alternative measurement technique that employs a fast neutron active-interrogation probe and a scintillation detector to measure the enrichment of uranium using both the buildup and decay of β -delayed-neutron emission. Instead of 3 He tubes, a capture-based heterogeneous composite detector consisting of scintillating Li glass and polyvinyl toluene is constructed and used, offering a prospect to scale delayed-neutron measurements to larger detector sizes. Since the technique relies on the existing tabulated nuclear data, no calibration standards are required. It is shown that the buildup of delayed-neutron emission can be used to distinguish between uranium samples and infer the uranium enrichment level, with accuracy that rivals the method that employs the time-dependent decay of delayed-neutron emission.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS2002

Polymer-Single Wall Carbon Nanotube Composites for Potential Spacecraft Applications

Polymer-single wall carbon nanotube (SWNT) composite films were prepared and characterized as part of an effort to develop polymeric materials with improved combinations of properties for potential use on future spacecraft. Next generation spacecraft will require ultra-lightweight materials that possess specific and unique combinations of properties such as radiation and atomic oxygen resistance, low solar absorptivity, high thermal emissitivity, electrical conductivity, tear resistance, ability to be folded and seamed, and good mechanical properties. The objective of this work is to incorporate sufficient electrical conductivity into space durable polyimides to mitigate static charge build-up. The challenge is to obtain this level of conductivity (10(exp -8) S/cm) without degrading other properties of importance, particularly optical transparency. Several different approaches were attempted to fully disperse the SWNTs into the polymer matrix. These included high shear mixing, sonication, and synthesizing the polymers in the presence of pre-dispersed SWNTs. Acceptable levels of conductivity were obtained at loading levels less than one tenth weight percent SWNT without significantly sacrificing optical properties. Characterization of the nanocomposite films and the effect of SWNT concentration and dispersion on the conductivity, solar absorptivity, thermal emissivity, mechanical and thermal properties were discussed. Fibers and non-woven porous mats of SWNT reinforced polymer nanocomposite were produced using electrospinning.

Park, C.↗
NASA NTRS2001

Piezoelectric Polymers

The purpose of this review is to detail the current theoretical understanding of the origin of piezoelectric and ferroelectric phenomena in polymers; to present the state-of-the-art in piezoelectric polymers and emerging material systems that exhibit promising properties; and to discuss key characterization methods, fundamental modeling approaches, and applications of piezoelectric polymers. Piezoelectric polymers have been known to exist for more than forty years, but in recent years they have gained notoriety as a valuable class of smart materials.

Harrison, J. S.↗
NASA NTRS2001

Low-Field and High-Field Characterization of THUNDER Actuators

THUNDER (THin UNimorph DrivER) actuators are pre-stressed piezoelectric devices developed at NASA Langley Research Center (LaRC) that exhibit enhanced strain capabilities. As a result, they are of interest in a variety of aerospace applications. Characterization of their performance as a function of electric field, temperature and frequency is needed in order to optimize their operation. Towards that end, a number of THUNDER devices were obtained from FACE International Co. with a stainless steel substrate varying in thickness from 1 mil to 20 mils. The various devices were evaluated to determine low-field and high-field displacement its well as the polarization hysteresis loops. The thermal stability of these drivers was evaluated by two different methods. First, the samples were thermally cycled under electric field by systematically increasing the maximum temperature from 25 C to 200 C while the displacement was being measured. Second, the samples were isothermally aged at 0 C, 50 C, 100 C. and 150 C in air, and the isothermal decay of the displacement was measured at room temperature as a function of time.

Ounaies, Z.↗
NASA NTRS2000

Polarization Stability of Amorphous Piezoelectric Polyimides

Amorphous polyimides containing polar functional groups have been synthesized and investigated for potential use as high temperature piezoelectric sensors. The thermal stability of the piezoelectric effect of one polyimide was evaluated as a function of various curing and poling conditions under dynamic and static thermal stimuli. First, the polymer samples were thermally cycled under strain by systematically increasing the maximum temperature from 50 C to 200 C while the piezoelectric strain coefficient was being measured. Second, the samples were isothermally aged at an elevated temperature in air, and the isothermal decay of the remanent polarization was measured at room temperature as a function of time. Both conventional and corona poling methods were evaluated. This material exhibited good thermal stability of the piezoelectric properties up to 100 C.

Park, C.↗
NASA NTRS1999

Design Requirements for Amorphous Piezoelectric Polymers

An overview of the piezoelectric activity in amorphous piezoelectric polymers is presented. The criteria required to render a polymer piezoelectric are discussed. Although piezoelectricity is a coupling between mechanical and electrical properties, most research has concentrated on the electrical properties of potentially piezoelectric polymers. In this work, we present comparative mechanical data as a function of temperature and offer a summary of polarization and electromechanical properties for each of the polymers considered.

Ounaies, Z.↗
NASA NTRS1999

Electrical Properties and Power Considerations of a Piezoelectric Actuator

This paper assesses the electrical characteristics of piezoelectric wafers for use in aeronautical applications such as active noise control in aircraft. Determination of capacitive behavior and power consumption is necessary to optimize the system configuration and to design efficient driving electronics. Empirical relations are developed from experimental data to predict the capacitance and loss tangent of a PZT5A ceramic as nonlinear functions of both applied peak voltage and driving frequency. Power consumed by the PZT is the rate of energy required to excite the piezoelectric system along with power dissipated due to dielectric loss and mechanical and structural damping. Overall power consumption is thus quantified as a function of peak applied voltage and driving frequency. It was demonstrated that by incorporating the variation of capacitance and power loss with voltage and frequency, satisfactory estimates of power requirements can be obtained. These relations allow general guidelines in selection and application of piezoelectric actuators and driving electronics for active control applications.

Jordan, T.↗
NASA NTRS1999

Ferroelectric and Piezoelectric Properties of Blends of Poly(Vinylidene-Trifluoroethylene) and Graft Elastomer

A piezoelectric polymeric blend system has been developed. The system contains two components: ferroelectric poly(vinylidene-trifluoroethylene) and graft elastomer. The remanent polarization, Pr, and the piezoelectric strain coefficient, d31, of the blends have been studied as a function of relative composition of the two components, temperature and frequency. Both blended copolymer and graft unit in the elastomer contribute to the total crystallinity of the blend-system, and hence to the remanent polarization and piezoelectricity. The piezoelectric strain coefficient, d31, of the blend systems shows dependence on both the remanent polarization and the mechanical stiffness, which in turn are determined by the fraction of the two components in the blends. This mechanism makes it possible for the piezoelectric strain response of the blend to be tailored by adjusting the relative composition. Although Pr of the copolymer is higher than that of the blends, the blend films containing 75 wt.% copolymer exhibit a higher d31 at room temperature, possibly due to their lower modulus. The blend films containing 50 wt.% copolymer exhibit a constant value of d31, from room temperature to 70 C.

Su, J.↗
NASA NTRS1997

Dielectric Properties of Piezoelectric Polyimides

Molecular modeling and dielectric measurements are being used to identify mechanisms governing piezoelectric behavior in polyimides such as dipole orientation during poling, as well as degree of piezoelectricity achievable. Molecular modeling on polyimides containing pendant, polar nitrile (CN) groups has been completed to determine their remanent polarization. Experimental investigation of their dielectric properties evaluated as a function of temperature and frequency has substantiated numerical predictions. With this information in hand, we are then able to suggest changes in the molecular structures, which will then improve upon the piezoelectric response.

Ounaies, Z.↗