539 research outputs found
Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes
Monolayer graphene exhibits exceptional electronic and mechanical properties,
making it a very promising material for nanoelectromechanical (NEMS) devices.
Here, we conclusively demonstrate the piezoresistive effect in graphene in a
nano-electromechanical membrane configuration that provides direct electrical
readout of pressure to strain transduction. This makes it highly relevant for
an important class of nano-electromechanical system (NEMS) transducers. This
demonstration is consistent with our simulations and previously reported gauge
factors and simulation values. The membrane in our experiment acts as a strain
gauge independent of crystallographic orientation and allows for aggressive
size scalability. When compared with conventional pressure sensors, the sensors
have orders of magnitude higher sensitivity per unit area.Comment: 20 pages, 3 figure
Array of lenses with individually tunable focal-length based on transparent ion-implanted EAPs
We report on the fabrication and characterization of 2x2 arrays of mm-diameter PDMS lenses whose focal length can be electrically tuned. Dielectric elastomer actuators generally rely on carbon powder or carbon grease electrodes, which are not transparent, precluding the polymer actuator from also being a lens. However compliant electrodes fabricated by low-energy ion implantation are over 50% transparent in the visible, enabling the polymer lens to simultaneously be an actuator. We have developed a chip-scale process to microfabricate lens arrays, consisting of a molded socket bonded to a Pyrex chip supporting 4 membrane actuators. The actuators are interconnected via an incompressible fluid. The Pyrex chip has four through-holes, 1 to 3 mm in diameter, on which a 30 ÎŒm thick Polydimethysiloxane (PDMS) layer is bonded. The PDMS layer is implanted on both sides with 5 keV gold ions to define the transparent electrodes for EAP actuation. Applying a voltage to one of the lens/actuators leads to an area expansion and hence to a change in radius of curvature, varying the focal length. We report tuning the focal length from 4 mm to 8 mm at 1.7 kV, and present changes in optical transmission and membrane stiffness following gamma and proton irradiation
Optimal Percolation of Disordered Segregated Composites
We evaluate the percolation threshold values for a realistic model of
continuum segregated systems, where random spherical inclusions forbid the
percolating objects, modellized by hard-core spherical particles surrounded by
penetrable shells, to occupy large regions inside the composite. We find that
the percolation threshold is generally a non-monotonous function of
segregation, and that an optimal (i. e., minimum) critical concentration exists
well before maximum segregation is reached. We interpret this feature as
originating from a competition between reduced available volume effects and
enhanced concentrations needed to ensure percolation in the highly segregated
regime. The relevance with existing segregated materials is discussed.Comment: 5 pages, 4 figure
Temperature and moisture are minor drivers of regional-scale soil organic carbon dynamics
Storing large amounts of organic carbon, soils are a key but uncertain component of the global carbon cycle, and accordingly, of Earth System Models (ESMs). Soil organic carbon (SOC) dynamics are regulated by a complex interplay of drivers. Climate, generally represented by temperature and moisture, is regarded as one of the fundamental controls. Here, we use 54 forest sites in Switzerland, systematically selected to span near-independent gradients in temperature and moisture, to disentangle the effects of climate, soil properties, and landform on SOC dynamics. We estimated two SOC turnover times, based on bulk soil 14C measurements (Ï14C) and on a 6-month laboratory soil incubation (Ïi). In addition, upon incubation, we measured the 14C signature of the CO2 evolved and quantified the cumulated production of dissolved organic carbon (DOC). Our results demonstrate that Ïi and Ï14C capture the dynamics of contrasting fractions of the SOC continuum. The 14C-based Ï14C primarily reflects the dynamics of an older, stabilised pool, whereas the incubation-based Ïi mainly captures fresh readily available SOC. Mean site temperature did not raise as a critical driver of SOC dynamics, and site moisture was only significant for Ïi. However, soil pH emerged as a key control of both turnover times. The production of DOC was independent of Ïi and not driven by climate, but primarily by the content of clay and, secondarily by the slope of the site. At the regional scale, soil physicochemical properties and landform appear to override the effect of climate on SOC dynamics
Large-Stroke Dielectric Elastomer Actuators With Ion- Implanted Electrodes
In this paper, we present miniaturized polydimethyl- siloxane (PDMS)-based diaphragm dielectric elastomer actuators capable of out-of-plane displacement up to 25% of their diameter. This very large percentage displacement is made possible by the use of compliant electrodes fabricated by low-energy gold ion im- plantation. This technique forms nanometer-scale metallic clusters up to 50 nm below the PDMS surface, creating an electrode that can sustain up to 175% strain while remaining conductive yet having only a minimal impact on the elastomerâs mechanical properties. We present a vastly improved chip-scale process flow for fabricating suspended-membrane actuators with low- resistance contacts to implanted electrodes on both sides of the membrane. This process leads to a factor of two increase in breakdown voltage and to RC time constant shorter than mechanical time constants. For circular diaphragm actuator of 1.5â3-mm diameter, voltage- controlled static out-of-plane deflections of up to 25% of their diameter is observed, which is a factor of four higher than our previous published results. Dynamic characterization shows a mechanically limited behavior, with a resonance frequency near 1 kHz and a quality factor of 7.5 in air. Lifetime tests have shown no degradation after more than 4 million cycles at 1.5 kV. Conductive stretchable electrodes photolithographically defined on PDMS were demonstrated as a key step to further miniaturiza- tion, enabling large arrays of independent diaphragm actuators on a chip, for instance for tunable microlens arrays or arrays of micropumps and microvalves
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