Granular metal-carbon nanocomposites as piezoresistive sensor films-Part 2: Modeling longitudinal and transverse strain sensitivity

Granular and columnar nickel-carbon composites may exhibit large strain sensitivity, which makes them an interesting sensor material. Based on experimental results and morphological characterization of the material, we develop a model of the electron transport in the film and use it to explain its piezoresistive effect. First we describe a model for the electron transport from particle to particle. The model is then applied in Monte Carlo simulations of the resistance and strain properties of the disordered films that give a first explanation of film properties. The simulations give insights into the origin of the transverse sensitivity and show the influence of various parameters such as particle separation and geometric disorder. An important influence towards larger strain sensitivity is local strain enhancement due to different elastic moduli of metal particles and carbon matrix

Improving the electrical and structural stability of highly piezoresistive nickel–carbon sensor thin films

The family of sputter deposited granular metal-based carbon-containing sensor films is known for their high sensitivity transforming force-dependent strain into electrical resistance change. Among them nickel–carbon thin films possess a gauge factor of up to 30, compared to only 2 for traditional sensor films of metal alloys. This high sensitivity is based on disordered interparticle tunneling through barriers of graphite-like carbon walls between metal–carbon particles of columnar shape. Force and pressure sensors would benefit a lot from the elevated piezoresistivity. A disadvantage, however, is a disturbing temporal creep and drift of the resistance under load and temperature. This contribution shows how to stabilize such sensor films. A significant stabilization is achieved by partially replacing nickel with chromium, albeit at the expense of sensitivity. The more chromium used in these NixCr1−x-C layers, the higher the optimum annealing temperature can be selected and the better the electrical stabilization. A good compromise while maintaining sensitivities well above the standard of 2 is identified for films with x=0.5 to 0.9, stabilized by optimized temperature treatments. The stabilizing effect of chromium is revealed by transmission electron microscopy with elemental analysis. The post-annealing drives segregation processes in the layer material. While the interior of the layer is depleted of chromium and carbon, boundary layers are formed. Chromium is enriched near the surface boundary, oxidized in air and forms chromium-rich oxide sub-layers, which are chemically very stable and protect against further reactions and corrosion. As a result, creep and drift errors are greatly reduced, so that the optimized sensor coatings are now suitable for widespread use

Granular metal-carbon nanocomposites as piezoresistive sensor films - Part 1: Experimental results and morphology

We have produced granular films based on carbon and different transition metals by means of plasma deposition processes. Some of the films possess an increased strain sensitivity compared to metallic films. They respond to strain almost linearly with gauge factors of up to 30 if strained longitudinally, while in the transverse direction about half of the effect is still measured. In addition, the film's thermal coefficient of resistance is adjustable by the metal concentration. The influence of metal concentration was investigated for the elements Ni, Pd, Fe, Pt, W, and Cr, while the elements Co, Au, Ag, Al, Ti, and Cu were studied briefly. Only Ni and Pd have a pronounced strain sensitivity at 55- €±- €5- €at.- €% (atomic percent) of metal, among which Ni–C is far more stable. Two phases are identified by transmission electron microscopy and X-ray diffraction: metal-containing nanocolumns densely packed in a surrounding carbon phase. We differentiate three groups of metals, due to their respective affinity to carbon. It turns out that only nickel has the capability to bond and form a stable and closed encapsulation of GLC around each nanoparticle. In this structure, the electron transport is in part accomplished by tunneling processes across the basal planes of the graphitic encapsulation. Hence, we hold these tunneling processes responsible for the increased gauge factors of Ni–C composites. The other elements are unable to form graphitic encapsulations and thus do not exhibit elevated gauge factors

Finite element modeling and validation of a soft array of spatially coupled dielectric elastomer transducers

Dielectric elastomer (DE) transducers are suitable candidates for the development of compliant mechatronic devices, such as wearable smart skins and soft robots. If many independently-controllable DEs are closely arranged in an array-like configuration, sharing a common elastomer membrane, novel types of cooperative and soft actuator/sensor systems can be obtained. The common elastic substrate, however, introduces strong electro-mechanical coupling effects among neighboring DEs, which highly influence the overall membrane system actuation and sensing characteristics. To effectively design soft cooperative systems based on DEs, these effects need to be systematically understood and modeled first. As a first step towards the development of soft cooperative DE systems, in this paper we present a finite element simulation approach for a 1-by-3 silicone array of DE units. After defining the system constitutive equations and the numerical assumptions, an extensive experimental campaign is conducted to calibrate and validate the model. The simulation results accurately predict the changes in force (actuation behavior) and capacitance (sensing behavior) of the different elements of the array, when their neighbors are subjected to different electro-mechanical loads. Quantitatively, the model reproduces the force and capacitance responses with an average fit higher than 93% and 92%, respectively. Finally, the validated model is used to perform parameter studies, aimed at highlighting how the array performance depends on a relevant set of design parameters, i.e. DE-DE spacing, DE-outer structure spacing, membrane pre-stretch, array scale, and electrode shape. The obtained results will provide important guidelines for the future design of cooperative actuator/sensor systems based on DE transducers

Fully Polymeric Domes as High-Stroke Biasing System for Soft Dielectric Elastomer Actuators

The availability of compliant actuators is essential for the development of soft robotic systems. Dielectric elastomers (DEs) represent a class of smart actuators which has gained a significant popularity in soft robotics, due to their unique mix of large deformation (>100%), lightweight, fast response, and low cost. A DE consists of a thin elastomer membrane coated with flexible electrodes on both sides. When a high voltage is applied to the electrodes, the membrane undergoes a controllable mechanical deformation. In order to produce a significant actuation stroke, a DE membrane must be coupled with a mechanical biasing system. Commonly used spring-like bias elements, however, are generally made of rigid materials such as steel, and thus they do not meet the compliance requirements of soft robotic applications. To overcome this issue, in this paper we propose a novel type of compliant mechanism as biasing elements for DE actuators, namely a threedimensional polymeric dome. When properly designed, such types of mechanisms exhibit a region of negative stiffness in their force-displacement behavior. This feature, in combination with the intrinsic softness of the polymeric material, ensures large actuation strokes as well as compliance compatibility with soft robots. After presenting the novel biasing concept, the overall soft actuator design, manufacturing, and assembly are discussed. Finally, experimental characterization is conducted, and the suitability for soft robotic applications is assessed

Dielectric Elastomer Cooperative Microactuator Systems : DECMAS

This paper presents results of the first phase of “Dielectric Elastomer Cooperative Microactuator Systems” (DECMAS), a project within the German Research Foundation Priority Program 2206, “Cooperative Multistable Multistage Microactuator Systems” (KOMMMA). The goal is the development of a soft cooperative microactuator system combining high flexibility with largestroke/high-frequency actuation and self-sensing capabilities. The softness is due to a completely polymer-based approach using dielectric elastomer membrane structures and a specific silicone bias system designed to achieve large strokes. The approach thus avoids fluidic or pneumatic components, enabling, e.g., future smart textile applications with cooperative sensing, haptics, and even acoustic features. The paper introduces design concepts and a first soft, single-actuator demonstrator along with experimental characterization, before expanding it to a 3 × 1 system. This system is used to experimentally study coupling effects, supported by finite element and lumped parameter simulations, which represent the basis for future cooperative control methods. Finally, the paper also introduces a new methodology to fabricate metal-based electrodes of sub-micrometer thickness with high membrane-straining capability and extremely low resistance. These electrodes will enable further miniaturization towards future microscale applications

Electrode Impact on the Electrical Breakdown of Dielectric Elastomer Thin Films

Dielectric Elastomer Actuators (DEAs) enable the realization of energy-efficient and compact actuator systems. DEAs operate at the kilovolt range with typically microampere-level currents and hence minimize thermal losses in comparison to low voltage/high current actuators such as shape memory alloys or solenoids. The main limiting factor for reaching high energy density in high voltage applications is dielectric breakdown. In previous investigations on silicone-based thin films, we reported that not only do environmental conditions and film parameters such as pre-stretch play an important role but that electrode composition also has a significant impact on the breakdown behavior. In this paper, we present a comprehensive study of electrical breakdown on thin silicone films coated with electrodes manufactured by five different methods: screen printing, inkjet printing, pad printing, gold sputtering, and nickel sputtering. For each method, breakdown was studied under environmental conditions ranging from 1 ◦C to 80 ◦C and 10% to 90% relative humidity. The effect of different manufacturing methods was analyzed as was the influence of parameters such as solvents, silicone content, and the particle processing method. The breakdown field increases with increasing temperature and decreases with increasing humidity for all electrode types. The stiffer metal electrodes have a higher breakdown field than the carbon-based electrodes, for which particle size also plays a large role

Fully Polymeric Domes as High-Stroke Biasing System for Soft Dielectric Elastomer Actuators

The availability of compliant actuators is essential for the development of soft robotic systems. Dielectric elastomers (DEs) represent a class of smart actuators which has gained a significant popularity in soft robotics, due to their unique mix of large deformation (>100%), lightweight, fast response, and low cost. A DE consists of a thin elastomer membrane coated with flexible electrodes on both sides. When a high voltage is applied to the electrodes, the membrane undergoes a controllable mechanical deformation. In order to produce a significant actuation stroke, a DE membrane must be coupled with a mechanical biasing system. Commonly used spring-like bias elements, however, are generally made of rigid materials such as steel, and thus they do not meet the compliance requirements of soft robotic applications. To overcome this issue, in this paper we propose a novel type of compliant mechanism as biasing elements for DE actuators, namely a three-dimensional polymeric dome. When properly designed, such types of mechanisms exhibit a region of negative stiffness in their force-displacement behavior. This feature, in combination with the intrinsic softness of the polymeric material, ensures large actuation strokes as well as compliance compatibility with soft robots. After presenting the novel biasing concept, the overall soft actuator design, manufacturing, and assembly are discussed. Finally, experimental characterization is conducted, and the suitability for soft robotic applications is assessed