Changing dielectric properties of an elastically deformed solid material is called dielectrostriction. This physical
response enables a concept of self-sensing in dielectric materials such as polymers and polymeric composites. In
addition, dielectrostriction response is governed by same material parameters as the electrostriction effect which is
suitable for self-actuation applications. Designed planar capacitor sensor is employed for monitoring dielectrostriction
effect without mechanical contact with a loaded specimen. Such sensor can also be arranged in a rosette to directly
obtain the principal values of the stress/strain and the principal directions. This study investigates dielectrostriction and
electrostriction effects in carbon nanotube (CNT) composites. Preliminary results show tenfold increase in
dielectrostriction response of nanocomposites having 2 vol. % of randomly distributed CNTs. Current study targets
CNT composites having microstructure modified using applied electric field for optimizing sensing and actuation
Two types of the electroactive response in polymeric suspensions can be considered - dielectrostriction and
piezoresistance. Dielectrostriction is a variation of dielectric properties of a material with deformation while
piezoresistance involves a change in conductivity with deformation. Both phenomena have similar microscopic
foundation - they arise from variation of local electric field due to the redistribution of polarized or conductive
inclusions. Both dielectrostriction and piezoresistance are determined by the pair distribution function of inclusions
and are sensitive to a material's microstructure, which renders them effective for material characterization. In this study,
dielectrostriction effect of silicone/aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) and piezoresistance effect in silicone/graphite suspensions
during oscillatory shear deformations are detected by a rosette of planar sensors with mutually perpendicular electrodes.
In both measurements, the electric responses are found to be scaled with the deformation-induced stresses. Moreover, the
variation of dielectrostriction response with suspensions having various particle size distributions indicates the high
sensitivity of dielectrostriction to material's microstructure. Dielectrostriction and piezoresistance constitute new
approaches to study the rheological properties of suspensions and compliment each other for revealing the microstructure
in various systems.
A variation of dielectric response with deformation, called dielectrostriction, provides a new approach for in-line monitoring properties and structure of materials. The dielectrostriction effect resembles a well-known birefringence phenomenon which has been widely used for NDE of transparent materials. While birefringence is described by the stress-optic rule, the stress-dielectric rule applies to dielectrostriction. However, dielectrostriction measurements can be applied to both transparent and opaque dielectric materials, require a much simpler measurement technique, are capable of measuring local stresses/strains and can be implemented for material processing and health monitoring of structures. Planar capacitor sensor setup is implemented to detect the dielectrostriction effect in both liquid and solid polymers. Dielectrostriction effect and the stress-dielectric relationship are studied for solid polycarbonate subjected to uniaxial tensile load. Similar results are obtained for liquid polymers in oscillatory shear flow.
This paper promotes the general paradigm that a composite’s internal structure can be micro-tailored to achieve a multifunctional physical response through the use of the Field Aided Micro Tailoring (FAiMTa) technique. The FAiMTa technique relies on curing a polymer composite while in its liquid state in the presence of an electric field. The particles within the composite align themselves in the direction of the electric field and create an orthotropic composite structure. This technology can lead to composite materials having a micro-tailored structure mimicking biological systems. As an initial step towards this goal, uniformly orthotropic composites, which are prepared by the FAiMTa technique, are mechanically characterized. Two epoxy based systems are considered: a composite having micro-sized graphite particles whereas the other has micro-sized aluminum particles. Mechanical tests show the change of material properties according to direction of the particle alignment within the composite. Optical microscopy also confirms the created orthotropic microstructure. The next step in development of FAiMTa technique is the reduction of stress concentration near a geometric discontinuity by properly orienting particulate structures within the composite. Our on-going efforts toward optimization of the composites are briefly outlined.
A sensing approach, based on resonance frequency shifts of an oscillating micro- or nano- cantilever, can potentially provide ultimate sensitivity for detection of a single molecule. However, implementation of this sensing technology on a micro-scale has intrinsic limitations: The quality, Q, of an oscillating microcantilever vibrating in air is approximately in the 30-100 range and this value dramatically drops in a liquid environment. Feedback control of the oscillations can improve the quality of the system but multiple challenges are encountered with the sensing and actuation. Traditional data acquisition approaches, which include optical, piezoresistance, piezoelectric and capacitance methods, have very limited application in signal transduction from micro- or nano- cantilever beams. In addition, electrostatic and thermal actuations are not appropriate for liquid environments. A novel approach, utilizing the self-sensing and self-actuation response of electroactive materials is proposed for control of cantilever beam vibration. As far as sensing is concerned, we exploit the fact that any dielectric material exhibits dielectrostriction effect; this is defined as variation of dielectric properties of the material with deformation. Similarly, on the actuation side electrostriction response can also be used. In this work, control challenges and approaches for such nonlinear systems with self-sensing and self-actuation capabilities will be discussed.
This paper introduces polymer composites with locally micro-tailored electric and thermal conductive properties. We concentrate on specially designed orthotropic composites that have modified thermal properties in one preferable direction. This preferable direction can vary from region to region in the composite part to fulfill design objectives. Required local micro-tailoring and optimization of structure for given thermal applications is achieved by exposing liquid polymer suspensions to an electric field and then curing the obtained structure. We present testing results for epoxy resin with various fillers including graphite, silica etc. Obtained orthotropic composites are tested for mechanical and thermal and electrical properties. Elastic modulus, thermal expansion, and thermal conduction are measured for various compositions, directions and degree of orthotropy. The potential of obtained materials for electronic, aerospace and automotive applications are briefly discussed.
A novel approach for NDE of polymeric materials utilizing the dielectrostriction effect is presented. Any dielectric material exhibits dielectrostriction effect which is defined as variation of dielectric properties of the material with deformation. This phenomenon resembles well known photoelastic effect which has been widely used for NDE of transparent materials. The major difference is that dielectrostriction measurements can be conducted using a lower frequency than optical range of electromagnetic field. Thus, the dielectrostriction measurements can be administered to any, even non-transparent, material for in-line monitoring of strains and stresses. In addition, no mechanical contact is required for dielectrostriction measurements. Therefore, there is no problem with interface and attachment of the sensing element to the monitored part. A potential of dielectrostriction phenomenon for NDE is not completely explored at this time and would be the subject of future extensive studies. We will show its feasibility for monitoring undesirable features such as fatigue, cracks and residual stresses in dielectric materials. In this paper, we will present theoretical background and experimental data for dielectrostriction study of polymeric parts manufactured under various processing conditions.
Contemporary applications could benefit from multifunctional materials having anisotropic optical, electrical, thermal or mechanical properties. These desirable features, but with locally-controlled distribution of directional response, would be even more attractive. Such materials are difficult to engineer by conventional methods. However, the field-aided technology presented herein is able to locally tailor electroactive composites. By applying an electric field to a polymer in its liquid state, we have developed ability to orient chain- or fiber-like inclusions or phases from what was originally an isotropic material. Such composites can be formed from liquid solutions, melts, or mixture of pre-polymer and cross-linking agent. Upon curing, a "created composite" results consisting of these "pseudo fibers" embedded in a matrix. One can also create orientated composites from embedded spheres, flakes or fiber-like shapes in a liquid plastic. Orientation of the externally applied electric field defines the orientation of field-aided self-assembled composites. The strength and exposure duration of the electric field control the degree of created anisotropy. Results of electromechanical testing of these modified materials relevant to sensing and actuation applications are presented. Material microstructure is analyzed using microscopy and X-ray diffraction. The performance of these novel materials having different composition and morphology is being investigated.
Deformation induced variations in dielectric properties of an elastic material is called electrostriction. This effect can be detected using capacitance measurements and employed for sensing shear and normal strains. Almost any solid dielectric can be used as a sensing medium, but electroactive polymeric composites present a unique advantage. A composite’s structure can be locally micro-tailored and optimized for a given sensing application by exposing liquid polymeric suspensions to an electric field and curing the obtained structure. A single plane sensor configuration for shear and normal loads is analyzed. Such a design has no moving electrodes, can be implemented using surface micro-machining and supplied with on-chip electronics. Advantages of electroactive polymeric sensors include: intrinsic sensitivity to shear strain, robustness and flexibility, high tolerance to overloads, a large selection of candidate material for specific applications, simplicity in the manufacture, and operation and potential for miniaturization.
Meeting the stringent error budget of 157-nm lithography for manufacturing devices in the sub-100 nm regime requires that all mask-related distortions be minimized, corrected, or eliminated. Sources include the pellicle system, which has been previously identified as a potential cause of image placement error. To characterize pellicle-induced distortions, finite element (FE) models have been developed to simulate system fabrication, including soft pellicles as well as prototype fused silica (hard) pellicles. The main sources of distortions are: (a) temperature variations, (b) initially distorted components, and (c) sag-induced refraction. Temperature variations are an issue if pellicle mounting and exposure take place at different temperatures. Sources of attachment-induced distortions include the initial frame curvature, initial reticle shape, attachment method (mounting tools-induced), frame and gasket materials, and the hard pellicle bow. These attachment-induced distortions were modeled using experimentally measured values of Young's modulus for adhesive gaskets. Refraction aberration is an issue with bowed hard pellicles which act as optical elements and induce image degradation. These effects were assessed and found to be significant. Results from the experiments and comprehensive FE simulations have characterized the relative importance of the principal sources of pellicle-induced photomask distortions for 157-nm lithography.