Development of elastomeric composite materials for the realization of piezoresistive sensors

Piezoresistive materials, able to sense geometrical deformation through variations of the electrical resistance, attracted an increasing interest in the scientific and industrial comparts during the last forty years, which increased significantly with the advent of nanostructured carbon-based conductive materials. These light, highly conductive and easy-to-obtain fillers have broadened the spectrum of materials that had been used up to that time, opening up the possibility of greater development of multifunctional materials. In particular, the carbonaceous fillers, homogeneously dispersed within a polymer matrix, immediately represented a valid alternative to the metals used in the field of piezoresistive systems. In the context of polymer nanocomposites and piezoresistive materials, a significant challenge for the scientific community is represented the by the achievement of an effective percolation pathway, which allows the passage of an electric current at the lowest percentage of filler (percolation threshold), and provides a direct correlation of the external forces in with the electrical resistance variations. Generally, the piezoresistive materials based on the exploitation of the polymer-based composites are designed by homogeneously dispersing the carbonaceous filler in the polymeric matrix. However, it is well known that a simple approach to reduce the content of filler and realize a conductive composite can be obtained by exploiting the concept of segregation of filler in the polymeric matrix. When the filler is not randomly dispersed, but segregated to build up a three-dimensional network, the electrical conductivity can be obtained with a significantly lower content of the carbonaceous filler. Amongst the several techniques for the realization of piezoresistive systems, in the last decade, the Additive Manufacturing (3D printing) technologies have aroused the greatest interest. The 3D printing processes lead to a considerable reduction in costs and times as compared with the traditional technologies of processing of polymers. Furthermore, as regards prototyping, they allow an almost total freedom to create even complex shapes and geometries in an automated and effective way. In particular, Selective Laser Sintering (SLS) is one of the most interesting technology, able to build up easily the segregated filler network, starting from polymeric powder adequately prepared. It is focused on the sintering of polymeric particles by a laser in the classic layer-by-layer mode. Many polymers can be used, from elastomeric to thermosetting, as well as conductive fillers. In this PhD research project, it was investigated the possibility of obtaining piezoresistive materials printed with 3D SLS using thermoplastic polyurethane (TPU) as a polymer matrix and graphene nanoparticles (GE) and multiwalled carbon nanotubes (MWCNTs) as conductive filler. The main objective of the doctoral research was to investigate the potential of SLS to create porous conductive materials with segregated distribution of the conductive filler, by evaluating the effect of different geometries and porosities (from 20% to 80%) and different shape of the conductive filler (i.e. 1D filler and 2D filler). Again, the aim was to evaluate, based on the complete characterization of the materials, what is the effect of the technology used, finding a possible correlation with the printed geometries. Thus, in the first part of the project, porous systems were printed using TPU modified with 1wt% of GE and starting from Diamond (D), Gyroid (G) and Schwarz (S) geometries for the building up of systems with regular porosity. The resulting three-dimensional porous structures show an effective conductive network due to the segregation of the graphene nanoplatelets previously assembled on the TPU powder surface in between the sintered elastomeric particles. The results confirm that GE nanoplatelets improve the thermal stability of the TPU matrix, while also increasing its glass transition temperature. Furthermore, porous structures made from S geometry show higher elastic modulus values in comparison with D and G based structures. After cyclic compression tests, all porous structures show robust negative piezoresistive behavior, regardless of their porosity and geometry, with exceptional sensitivity to deformation. Gauge Factor (GF) values of 12.4 at 8% deformation are obtained for S structures with 40 and 60% porosity, while GF values up to 60 are obtained for deformations lower than 5%. The thermal conductivity of TPU/GE structures significantly decreases with increasing porosity, while the effect of the structure architecture is less relevant. The second part of the project focused on the characterization of 3D printed TPU products with MWCNTs and a mixture of the two fillers, again at 1wt% but with a proportion of 70/30 wt/wt MWCNTs/GE with geometries D and G, in order to investigate a possible synergistic effect of the two conductive fillers. The results showed that the porous structures based on TPU with 1wt% MWCNTs/GE exhibit excellent electrical conductivity and mechanical strength. In particular, all the porous structures show a robust negative piezoresistive behavior, as demonstrated by the GF values that reach values of about -13 at 8% deformation. Moreover, the G20 porous structures (20% porosity) show microwave absorption coefficients ranging from 0.70 to 0.91 in the 12-18 GHz region and close 1 in the THz (300 GHz - 1 THz) frequency region. The results show that the simultaneous presence of MWCNT and GE brings a significant improvement in the specific functional properties of porous structures, which are proposed as potential piezoresistive actuators with relevant electromagnetic interference (EMI) shielding properties

Design and fabrication of a multipurpose compliant nanopositioning architecture

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 227-241).This research focused on generating the knowledge required to design and fabricate a high-speed application flexible, low average cost multipurpose compliant nanopositioner architecture with high performance integrated sensing. Customized nanopositioner designs can be created in ~~1 week, for 30x increase in sensing dynamic range over comparable state-of-the-art compliant nanopositioners. These improvements will remove one of the main hurdles to practical non-IC nanomanufacturing, which could enable advances in a range of fields including personalized medication, computing and data storage, and energy generation/storage through the manufacture of metamaterials. Advances were made in two avenues: flexibility and affordability. The fundamental advance in flexibility is the use of a new approach to modeling the nanopositioner and sensors as combined mechanical/electronic systems. This enabled the discovery of the operational regimes and design rules needed to maximize performance, making it possible to rapidly redesign nanopositioner architecture for varying functional requirements such as range, resolution and force. The fundamental advance to increase affordability is the invention of Non-Lithographically-Based Microfabrication (NLBM), a hybrid macro-/micro-fabrication process chain that can produce MEMS with integrated sensing in a flexible manner, at small volumes and with low per-device costs. This will allow for low-cost customizable nanopositioning architectures with integrated position sensing to be created for a range of micro-/nano- manufacturing and metrology applications. A Hexflex 6DOF nanopositioner with titanium flexures and integrated siliconpiezoresistive sensing was fabricated using NLBM. This device was designed with a metal mechanical structure in order to improve its robustness for general handling and operation. Single crystalline silicon piezoresistors were patterned from bulk silicon wafers and transferred to the mechanical structure via thin-film patterning and transfer. This work demonstrates that it is now feasible to design and create a customized positioner for each nanomanufacturing/metrology application. The Hexflex architecture can be significantly varied to adjust range, resolution, force scale, stiffness, and DOF all as needed. The NLBM process was shown to enable alignment of device components on the scale of 10's of microns. 150μm piezoresistor arm widths were demonstrated, with suggestions made for how to reach the expected lower bound of 25[mu]m. Flexures of 150[mu]m and 600[mu]m were demonstrated on 4 the mechanical structure, with a lower bound of ~~50[mu]m expected for the process. Electrical traces of 800[mu]m width were used to ensure low resistance, with a lower bound of ~~100[mu]m expected for the process. The integrated piezoresistive sensing was designed to have a gage factor of about 125, but was reduced to about 70 due to lower substrate temperatures during soldering, as predicted by design theory. The sensors were measured to have a full noise dynamic range of about 59dB over a 10kHz sensor bandwidth, limited by the Schottky barrier noise. Several simple methods are suggested for boosting the performance to ~~135dB over a 10kHz sensor bandwidth, about a <1Å resolution over the 200[mu]m range of the case study device. This sensor performance is generally in excess of presently available kHz-bandwidth analog-to-digital converters.by Robert M. Panas.Ph.D

Piezoelectric and Magnetoelastic Strain in the Transduction and Frequency Control of Nanomechanical Resonators

Stress and strain play a central role in semiconductors, and are strongly manifested at the nanometer-scale regime. Piezoelectricity and magnetostriction produce internal strains that are anisotropic and addressable via a remote electric or magnetic field. These properties could greatly benefit the nascent field of nanoelectromechanical systems (NEMS), which promises to impact a variety of sensor and actuator applications. The piezoelectric semiconductor GaAs is used as a platform for probing novel implementations of resonant nanomechanical actuation and frequency control. GaAs/AlGaAs heterostructures can be grown epitaxially, are easily amenable to suspended nanostructure fabrication, have a modest piezoelectric coefficient roughly twice that of quartz, and if appropriately doped with manganese, can form dilute magnetic compounds. In ordinary piezoelectric transducers there is a clear distinction between the metal electrodes and piezoelectric insulator. But this distinction is blurred in semiconductors. An integrated piezoelectric actuation mechanism is demonstrated in a series of suspended anisotype GaAs junctions, notably pin diodes. A dc bias was found to alter the resonance amplitude and frequency in such devices. The results are in good agreement with a model of strain based actuation encompassing the diode’s voltage-dependent carrier depletion width and impedance. A bandstructure engineering approach is employed to control the actuation efficiency by appropriately designing the doping level and thickness of the GaAs structure. Actuation and frequency are also sensitively dependent on the device’s crystallographic orientation. This combined tuning behavior represents a novel type of depletion-mediated electromechanical coupling in piezoelectric semiconductor nanostructures. All devices are actuated piezoelectrically, whereas three techniques are demonstrated for sensing: optical interferometry, piezoresistance and piezoelectricity. Finally, a nanoelectromechanical GaMnAs resonator is used to obtain the first measurement of magnetostriction in a dilute magnetic semiconductor. Resonance frequency shifts induced by field-dependent magnetoelastic stress are used to simultaneously map the magnetostriction and magnetic anisotropy constants over a wide range of temperatures. Owing to the central role of carriers in controlling ferromagnetic interactions in this material, the results appear to provide insight into a unique form of magnetoelastic behavior mediated by holes