Zoubeida Ounaies

Dispersion of single wall carbon nanotubes by in situ polymerization under sonication

Single wall nanotube reinforced polyimide nanocomposites were synthesized by in situ polymerization of monomers of interest in the presence of sonication. This process enabled uniform dispersion of single wall carbon nanotube (SWNT) bundles in the polymer matrix. The resultant SWNT-polyimide nanocomposite films were electrically conductive (antistatic) and optically transparent with significant conductivity enhancement (10 orders of magnitude) at a very low loading (0.1 vol%). Mechanical properties as well as thermal stability were also improved with the incorporation of the SWNT.

Electrical properties of single wall carbon nanotube reinforced polyimide composites

Electrical properties of single wall carbon nanotube (SWNT) reinforced polyimide composites were investigated as a function of SWNT concentration. AC and DC conductivities were measured, and the frequency behavior of the specific admittance was investigated. The experimental conductivity was found to obey a percolation-like power law with a relatively low percolation threshold. The current-voltage measurement results exhibited a non-ohmic behavior, indicating a quantum tunneling conduction mechanism. Analytical modeling and numerical simulation using high aspect ratio, rigid, spherocylinders as models for the SWNT were carried out to aid in understanding these results. The predictions were in good agreement with the experimental results. Published by Elsevier Ltd.

A Free Energy Model for Hysteresis in Ferroelectric Materials

This paper provides a theory for quantifying the hysteresis and constitutive nonlinearities inherent to piezoceramic compounds through a combination of free energy analysis and stochastic homogenization techniques. In the first step of the model development, Helmholtz and Gibbs free energy relations are constructed at the lattice or domain level to quantify the relation between the field and polarization in homogeneous, single crystal compounds which exhibit uniform effective fields. The effects of material nonhomogeneities, polycrystallinity, and variable effective fields are subsequently incorporated through the assumption that certain physical parameters, including the local coercive and effective fields, are randomly distributed and hence manifestations of stochastic density functions associated with the material. Stochastic homogenization in this manner provides low-order macroscopic models with effective parameters that can be correlated with physical properties of the data. This facilitates the identification of parameters for model construction, model updating to accommodate changing operating conditions, and control design utilizing model-based inverse compensators. Attributes of the model, including the guaranteed closure of biased minor loops in quasistatic drive regimes, are illustrated through examples.

A domain wall model for hysteresis in piezoelectric materials

This paper addresses the modeling of hysteresis and nonlinear constitutive relations in piezoelectric materials at moderate to high drive levels. Hysteresis and nonlinearities are due to the domain structure inherent to the materials and both aspects must be addressed to attain the full potential of the materials as sensors and actuators in high performance applications. The model employed here is based on previously developed theory for hysteresis in general ferroelectric materials. This theory is based on the quantification of the reversible and irreversible motion of domain walls pinned at inclusions in the material. This yields an ordinary differential equation (ODE) model having five parameters. The relationship of the parameters to physical attributes of the materials is detailed and algorithms for determining estimates of the parameters using measured values of the coercive field, differential susceptibility and saturation properties of the materials are detailed. The accuracy of the model and its capability for the prediction of measured polarization at various drive levels is illustrated through a comparison with experimental data from PZT5A, PZT5H and PZT4 compounds. Finally, the ODE model formulation is amenable to inversion which facilitates the construction of an inverse compensator for linear control design.

Determination of the orientation distribution function in aligned single wall nanotube polymer nanocomposites by polarized Raman spectroscopy

This work focuses on the derivation of the orientation distribution function (ODF) for a uniaxial-axially symmetric system using polarized Raman spectroscopy. A numerical methodology is proposed to determine the ODF that is formulated in terms of Legendre polynomials and the principle of maximum information entropy. The ultimate goal is to quantify the alignment of single wall nanotubes (SWNTs) in a polymer matrix using the experimental information from the Raman intensity. Some of the mathematical and numerical steps in the determination of ODF, not clarified in the current literature, are shown in this work. The proposed numerical methodology to obtain the ODF is illustrated using an experimental case. Electric field–aligned SWNT-urethane dimethacrylate/1,6-hexanediol dimethacrylate nanocomposites are investigated at different processing conditions to bring forward factors that may enhance the alignment of SWNT inclusions in the polymer. ODF results indicate that the higher electric field frequencies produce a good alignment of the SWNT inclusions; a result also corroborated by optical microscopy imaging and electrical conductivity measurements.

Harvesting Energy Using a Thin Unimorph Prestressed Bender: Geometrical Effects

Mathematical models and circuitry necessary for optimal energy conversion have been developed for piezoelectric devices because of their ability to convert mechanical energy to electrical energy. The piezoelectric device that is the focus of this study is a curved, thin unimorph prestressed bender. This device consists of layers of piezoelectric material, polyimide, and metal bonded at high temperatures. Effects of its layer composition and geometry on energy harvesting and actuation are investigated. Through this investigation, a method for developing empirical relationships is established and it is demonstrated that an actuator can be engineered so that the same energy output could be obtained with different materials by adjusting relevant parameters.

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.

Preparation, characterization, and modeling of carbon nanofiber/epoxy nanocomposites

There is a lack of systematic investigations on both mechanical and electrical properties of carbon nanofiber (CNF)-reinforced epoxy matrix nanocomposites. In this paper, an in-depth study of both static and dynamic mechanical behaviors and electrical properties of CNF/epoxy nanocomposites with various contents of CNFs is provided. A modified Halpin-Tsai equation is used to evaluate the Youngs modulus and storage modulus of the nanocomposites. The values of Youngs modulus predicted using this method account for the effect of the CNF agglomeration and fit well with those obtained experimentally. The results show that the highest tensile strength is found in the epoxy nanocomposite with a 1.0 wt% CNFs. The alternate-current (AC) electrical properties of the CNF/epoxy nanocomposites exhibit a typical insulator-conductor transition. The conductivity increases by four orders of magnitude with the addition of 0.1 wt% (0.058 vol%) CNFs and by ten orders of magnitude for nanocomposites with CNF volume fractions higher than 1.0 wt% (0.578 vol%). The percolation threshold (i.e., the critical CNF volume fraction) is found to be at 0.057 vol%.

Dielectric and piezoelectric properties of microwave sintered PZT

In this paper, the dielectric and piezoelectric properties of sol–gel derived and microwave sintered PZT are presented. It has been observed that the microwave sintering results in a material with a higher dielectric constant (≈1600) than that sintered by conventional methods (≈1480). It also offers a low dielectric loss tangent and improved d33. Pellets sintered by the microwave method at 1100 ◦ C are found to have high hardness (≈1460 MPa) on the Vickers hardness scale with a theoretical density of ≈94% in comparison with that sintered by the conventional method (i.e. ≈980 MPa and ≈84%). From a hysteresis study, the remanent polarization (Pr) and coercive field (Ec) of the microwave sintered PZT are observed to be 340 mC m −2 and 10 kV cm −1 , respectively. The value of Ec in the microwave sintered sample is low and this property is desirable for device fabrication of PZT-based smart structures.

Low-Field and High-Field Characterization of THUNDER Actuators

THUNDER (THin Unimorph DrivER) actuators are pre-stressed piezoelectric devices developed at NASA 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 form 1 mil to 20 mils. The various devices were evaluated to determine low-field and high-field displacement as 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 degree(s)C to 200 degree(s)C while the displacement was being measured. Second, the samples were isothermally ages at 0 degree(s)C, 50 degree(s)C, 100 degree(s)C, and 150 degree(s)C in air, and the isothermal decay of the displacement was measured at room temperature as a function of time.