Piezoresistive Characteristics Research Articles

Piezoresistive characterization of multi-walled carbon nanotube-epoxy based flexible strain sensitive films by impedance spectroscopy

In this study, we investigate the piezoresistive properties of flexible, strain sensitive multi-walled carbon nanotubes (MWCNTs)/epoxy composites. The deformation over the sensor area was tested by digital image correlation (DIC) under quasi-static uniaxial tension. The piezoresistive characteristics of the films were investigated quantitatively by electrochemical impedance spectroscopy (EIS) over a wide range of frequencies from 40 Hz to 110 MHz. Scanning electron microscopy (SEM) images confirmed that MWCNTs/epoxy composites with different CNT concentrations have a good homogeneity and dispersion. Additionally, in order to tailor the piezoresistivity of the strain sensor, an RC equivalent circuit was derived based on the impedance responses and the corresponding parameters were extracted under tensile strain. Compared with traditional strain gauges, higher sensitivity is obtained in particular at the concentrations close to the percolation threshold (13.6 for 0.3 wt.%). Due to the tunneling effect, a non-linear piezoresistivity is observed at low concentrations. It was found that sensors with 1 wt.% shows the highest linearity with a correlation coefficient of 0.999. The standard deviation of the cyclic readings was found to be 0.05%, indicating a high repeatability.

Styrene‐butadiene‐styrene copolymer compatibilized interfacial modified multiwalled carbon nanotubes with mechanical and piezoresistive properties

ABSTRACTIn this study, styrene‐butadiene‐styrene tri‐block copolymer/multiwalled carbon nanotubes (SBS/MWNTs) were prepared by means of a solution blending method. To enhance the compatibility between SBS and MWNTs, the SBS grafted MWNTs (SBS‐g‐MWNTs) were used to replace MWNTs. The MWNTs were chemically hydroxylated by the dissolved KOH solution with ethanol as solvent and then reacted with 3‐Aminopropyltriethoxysilane (APTES) to functionalize them with amino groups (MWNT‐NH2). The SBS‐g‐MWNTs were finally obtained by the reaction of MWNT‐NH2 and maleic anhydride grafted SBS (MAH‐g‐SBS). The SBS‐g‐MWNTs were characterized by X‐ray photoelectron spectroscopy (XPS), Fourier transform‐infrared spectroscopy (FT‐IR), transmission electron microscopy (TEM), scanning electron microscope (SEM), and thermogravimetric analysis (TGA). The results showed that the SBS molecules were homogeneously bonded onto the surface of the MWNTs, leading to an improvement of the mechanical and electrical properties of SBS/SBS‐g‐MWNTs composites due to the excellent interfacial adhesion and dispersion of SBS‐g‐MWNTs in SBS. A series of continuous tests were carried out to explore the electrical‐mechanical properties of the SBS/SBS‐g‐MWNTs composites. We found out that, near the percolation threshold, the well‐dispersed SBS/SBS‐g‐MWNTs composites showed good piezoresistive characteristics and small mechanical destructions for the development of little deformation under vertical pressure. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42945.

Soft piezoresistive sensor model and characterization with varying design parameters

Recent robots target safety, reconfigurability and interactivity by addressing the “softness” of the hardware either by endowing additional degrees-of-freedom or through inherent compliancy. These robots require distributed sensing with flexibility and softness that would not interfere with the robot's agility. There have been various sensing solutions using soft conductive materials including conductive silicone, liquid metal-filled micro-channels, and conductive-ink based sensors. However, we still lack a comprehensive study on their potentials, drawbacks, and the different parameters that affect their response.We present our design, fabrication process and characterization results for conductive silicone polymer and carbon ink-based curvature sensors. These sensors are flexible, mechanically robust under large strains, scalable, and easy to fabricate in large numbers. We propose an equivalent mechanical system to model sensors’ response. This model is unique for its extensive characterization of these polymer based sensors. Based on the characterization results, we systematically categorize and compare the performance of conductive silicone and carbon ink-based sensors with different design parameters.

Strain gauge sensors comprised of carbon nanotube yarn: parametric numerical analysis of their piezoresistive response

Carbon nanotube (CNT) yarns are micron-size fibers that contain thousands of intertwined CNTs in their cross sections and exhibit piezoresistance characteristics that can be tapped for sensing purposes. Sensor yarns can be integrated into polymeric and composite materials to measure strain through resistance measurements without adding weight or altering the integrity of the host material. This paper includes the details of novel strain gauge sensor configurations comprised of CNT yarn, the numerical modeling of their piezoresistive response, and the parametric analysis schemes that determines the highest sensor sensitivity to mechanical loading. The effect of several sensor configuration parameters are discussed including the inclination and separation of the CNT yarns within the sensor, the mechanical properties of the CNT yarn, the direction and magnitude of the applied mechanical load, and the dimensions and shape of the sensor. The sensor configurations that yield the highest sensitivity are presented and discussed in terms of the mechanical and electrical properties of the CNT yarn. It is shown that strain gauge sensors consisting of CNT yarn are sensitive enough to measure strain, and could exhibit even higher gauge factors than those of metallic foil strain gauges.

Preparation of electroless copper coated glass fiber and piezoresistive properties of copper coated glass fiber reinforced plastics

Metallized glass fibers have the excellent electric conductivity and low price compared with carbon fibers. Metallized glass fibers were fabricated by coating copper on the surface using different processing technologies and the effects of processing technology on the properties of copper coated glass fibers were discussed. By hand laying up molding, metallized glass fiber reinforced plastic (MGFRP) with good dispersion and interfacial cohesion was prepared successfully. The piezoresistive properties of MGFRP were compared with those of carbon fiber reinforced plastic (CFPR). The results show that copper coated on glass fibers with volume resistivity of 10−5Ω·cm have the good electrical conductivity and cohesion between two phases through optimizing the pretreatment technology of glass fibers and the formula of chemical plating solution. MGFRP has the distinct and similar piezoresistive characteristics with CFRP and exhibits the better piezoresistive sensitivity to fracture.

Piezoresistive characterization of bottom-up, n-type silicon microwires undergoing bend deformation

The piezoresistance of silicon has been studied over the past few decades in order to characterize the material's unique electromechanical properties and investigate their wider applicability. While bulk and top-down (etched) micro- and nano-wires have been studied extensively, less work exists regarding bottom-up grown microwires. A facile method is presented for characterizing the piezoresistance of released, phosphorus-doped silicon microwires that have been grown, bottom-up, via a chemical vapour deposition, vapour-liquid-solid process. The method uses conductive tungsten probes to simultaneously make electrical measurements via direct ohmic contact and apply mechanical strain via bend deformation. These microwires display piezoresistive coefficients within an order of magnitude of those expected for bulk n-type silicon; however, they show an anomalous response at degenerate doping concentrations (∼1020 cm−3) when compared to lower doping concentrations (∼1017 cm−3), with a stronger piezoresistive coefficient exhibited for the more highly doped wires. This response is postulated to be due to the different growth mechanism of bottom-up microwires as compared to top-down.

Characterization of piezoresistive PEDOT:PSS pressure sensors with inter-digitated and cross-point electrode structures.

The piezoresistive characteristics of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) pressure sensors with inter-digitated (IDE) and cross-point electrode (CPE) structures have been investigated. A small variation of the resistance of the pressure sensors with IDE without bottom indium-tin-oxide (b-ITO) film and with CPE structures was observed owing to the single carrier-conducting pathway. For the IDE pressure sensors with b-ITO, the piezoresistive characteristics at low and high pressure were similar to those of the pressure sensors with IDE without b-ITO and with CPE structures, respectively, leading to increased piezoresistive pressure sensitivity as the PEDOT:PSS film thickness decreased. A maximum sensitivity of more than 42 kΩ/Pa was achieved. When the normal pressure was applied, the increased number of conducting points or the reduced distance between the PEDOT oligomers within the PEDOT:PSS film resulted in a decrease of the resistance. The piezoresistive pressure sensors with a single carrier-conducting pathway, i.e., IDE without b-ITO and CPE structures, exhibited a small relaxation time and a superior reversible operation, which can be advantageous for fast piezoresistive response applications.

Research on the Piezoresistive Properties of Conductive Silicone Rubber under Water Pressure

The conductive silicon rubber samples have been prepared in different filler ingredients, the platform is builded for testing the piezoresistive characteristics of conductive silicon rubber when the material is under water pressure and the test methods are designed, the body piezoresistive properties and surface piezoresistive properties under the water pressure within a certain range are studied respectively. The results show that, the sample of 6% acetylene black/2% nanoSiO2 blends exhibit the best piezoresistive performance when the samples are under water pressure, the piezoresistive properties of the material can be improved with the increase of the thickness of the materials (2mm-4mm), and the conductive silicon rubber used for the pressure-sensitive materials of piezoresistive gauge is feasible.

A novel temperature compensated piezoresistive pressure sensor

The main drawback of current piezoresistive pressure sensors is the drop of output voltage with increase in the operating temperature which severely reduces the measurement accuracy. This paper presents a novel passive technique for temperature compensation of silicon piezoresistive pressure sensors. The built-in compensation technique eliminates the need for expensive and time consuming calibration process required for each sensor inside a fabricated batch. In this technique, extra polysilicon resistors with negative Temperature Coefficient of Resistivity (TCR) are employed for compensation purpose. Through applying this technique, Temperature Coefficient of Sensitivity (TCS) of the conventional non-compensated sensor was reduced to zero. Analytically derived equations and verified Finite Element Model considering mechanical, piezoresistive and electrical characteristics of the sensor are adapted to analyze the behavior of the sensor. The implementation of the introduced technique is compatible with conventional MEMS devices fabrication process. The compensated sensor is advantageous for pressure measurement in harsh environments with temperature variations.

Design and modeling of sensor behavior for improving sensitivity and performance

This paper describes the design process of the MEMS-based pressure sensor and analysis of circuit characteristics and parameters such as mechanical and electrical variation. Using the industry standard electrical and mechanical tools for design and analysis, in short duration low cost mechatronics equipment with improved accuracy and performance can be achieved. In this paper, piezoresistive material characteristics and theoretical signal conditioning analyses are also investigated for improving its performance. A newly proposed design technology for MEMS-based pressure sensors make a small circuit containing the electromagnetic component, transformer and analyzed for the said purpose.

PYSICAL ASPECTS OF LOW-TEMPERATURE MECHANICAL SENSORS BASED ON SILICON MICROCRYSTALS

The complex studies of conductivity, magnetoresistance and piezoresistance of boron doped p-type silicon microcrystals with <111> orientation in the temperature range 4.2–300 K and at the magnetic field up to 14 T were carried out. The influence of uniaxial strain on the resistivity and magnetoresistance of Si microcrystals with different boron doping at low temperatures was studied. The giant non-classic piezoresistance at helium temperatures was revealed in p-Si microcrystals with boron concentration in the vicinity of metal-insulator transition (MIT) from the insulating side: gauge factor of such crystals at 4.2 K equals GF4.2 K= –5.7×105 at compressive strain. Non-classic piezoresistance is caused by hopping conductivity of these crystals at low temperatures; the activation energies E2 and E3 and their values under uniaxial strain were determined. Piezoresistive characteristics of Si(B) crystals, mounted on the invar spring elements, in the wide ranges of temperature 4.2–300 K and strain ε=0–±1.2×10-3 rel. un. were measured. The obtained characteristics gave the possibility to predict the characteristics of piezoresistive mechanical sensors based on these crystals at low temperatures. It was shown the possibility to develop high-sensitive mechanical sensors, operating at cryogenic temperatures, based on non-classic piezoresistance of Si(B) microcrystals. The examples of developed strain sensors, pressure sensor and sensor of level of cryogenic liquids are presented.

PIEZORESITIVE CHARACTERISTICS OF SILICON MICROCRYSTALS AT CRYOGENIC TEMPERATURES

To create strain sensors operating at cryogenic temperatures piezoresistive characteristics of boron doped p-type Si whiskers, mounted on spring elements fabricated of steel, were studied at fixed temperatures 4.2, 77 and 300 K. It was shown that the effect of thermal strain that appears due to the difference between thermal expansion coefficients of silicon and material of the spring element (steel), when the crystals are mounted on the beam, on the piezoresistive characteristics of silicon crystals is very strong, especially at cryogenic temperatures. This fact should be accounted to develop sensors for strain measurements of constructions, fabricated from different materials. P-type Si whiskers with ρ300К =0.010 Ohm×cm with boron concentration near metalinsulator transition (MIT) from the metallic side of MIT with gauge factor GF4.2K =–1020 at the compressive strain are most suitable to create strain sensors for cryogenic temperatures, particularly, for 4.2 K. To create strain sensors for the temperature of liquid nitrogen the most suitable are p-Si whiskers with ρ300К =0.025 Ohm×cm with gauge factor GF77K =263.

Optimized Technical Characteristics of Polysilicon Nanofilm

In order to take good advantage of polysilicon nanofilm, optimized technical characteristics of the polysilicon nanofilm are very necessary to investigated. In this paper, the polysilicon nanofilms were prepared under different technical parameters, including thickness and doping concentration, which are very important for preparation of the nanofilm. The experimental results of piezoresistive and temperature characteristics show that the optimized technical characteristics are followed, the thickness is about 90nm, and the doping concentration is about 4.1×?1019cm-3 or between 2.0×?1020cm-3 and 4.1×?1020cm-3 from different point of view. The investigations of optimized technical characteristics are very useful for application of the polysilicon nanofilm to piezoresistive sensor.

Ultra-sensitive strain sensors based on piezoresistive nanographene films

Graphene shows promise on strain sensor applications, but the piezoresistive sensitivity of perfect graphene is low due to its weak electrical conductivity response upon structural deformation. In this paper, we used nanographene films for ultra-sensitive strain sensors. The piezoresistive sensitivity of nanographene films with different thicknesses and conductivities was systematically investigated and a nearly inverse proportional correlation was found. A gauge factor over 300, the highest so far for graphene-based strain sensors, was achieved. A charge tunneling model was used to explain the piezoresistive characteristics of nanographene films, which indicates our results provide a different rout toward ultra-sensitive strain sensors.

Effects of the Substrate on Piezoresistive Properties of Silicon Carbide Thin Films

Silicon carbide (SiC), in bulk or film form, has been shown as a promising material to replace the silicon as sensing element in devices for harsh environments. This has motivated several studies on growth and characterization of SiC thin films on different substrates such as silicon, silicon dioxide (SiO2), aluminum nitride (AlN) and silicon nitride (Si3N4), among others. However, less attention has been given to the investigation on how substrates affect the piezoresistive properties of SiC thin films. In this work, we have investigated the effect of substrates on the piezoresistive properties of SiC thin films produced by magnetron sputtering. Three types of substrates were utilized: (100) monocrystalline silicon, AlN on silicon and thermally oxidized silicon. Test structures were fabricated using photolithography, metallization lift-off and RIE (reactive ion etching) processes in order to perform piezoresistive characterization of the SiC samples produced.

Design of Ultra Low Temperature Pressure and Temperature Sensor Structure

In order to measure the pressure in the ultra-low temperature condition, the structure of ultra-low temperature piezoresistive pressure sensor is designed. Polysilicon nanometer thin film is used as a varistor according to its temperature and piezoresistive characteristics. The effect of the dimensions of silicon elastic membrane for the sensor sensitivity and the strain dimensions of the elastic membrane are analyzed, then layout position of resistances is arranged. The package structure of pressure sensor is designed. Meanwhile, a low-temperature sensor is designed to compensate the temperature influence to the pressure sensor.

Piezoresistive characteristic of conductive rubber for flexible tactile sensor

In the research of 2D flexible tactile sensor matrix, pressure-sensitive conductive rubber was developed and tested in which carbon black was used as its conductive phase and silicon rubber as its matrix layer. Experiments were undertaken and the resultant data were used for its piezoresistive characteristics investigation for two kinds of electrode connection configurations, the surface directive connection and embedded connection. It is found that due to the rather strong nonlinearity of the piezoresistive characteristic curves obtained, a higher correlation relationship can be obtained by means of quadratic polynomial fitting. It also showed that the embedded electrode assembling has higher fitting accuracy while the surface directive connection has better mechanical sensitivity.

Simultaneous global and local strain sensing in SWCNT–epoxy composites by Raman and impedance spectroscopy

Polymer composites can be benefited in many ways through the addition of carbon nanotubes (CNT). For instance, CNT can build up a percolated network within the polymer matrix, which results in a composite material with electrical conductivity and piezoresistive characteristics. This has very important implications for the realization of self-stress sensing structural composites. Moreover, the remarkable optical and transport properties of CNT permit to obtain information about the stress state of the composite at different scales. In the present work, the local and global stress response of SWCNT–epoxy composites is characterised by simultaneous Raman spectroscopic and electrical measurements on nanocomposite specimens submitted to different levels of surface strain. Both the Raman G′-band resonance frequency and the electrical resistance of the composite are found to change monotonically with strain until an inflection point is reached at ∼1.5% strain. Increased sensitivity of the piezoresistive network and lower load transfer efficiency occur beyond this strain level, and are considered to be the result of CNT slippage from the polymer. The reversibility of the stress sensitivity of the composites is verified by performing cyclic loading tests. Hysteresis loop are found to develop earlier on the Raman curves as in the resistance curves, which indicates that even at low strain levels, permanent damage is induced in the vicinity of carbon nanotubes. The use of Raman spectroscopy in combination with electrical methods provides a further insight on the stress sensing capabilities of CNT and the factors which affect the sensitivity and reproducibility of this behaviour.

Fabrication and characteristics of an ultra-thin carbon nanotube/parylene membrane at large strain

Multi-walled carbon nanotubes were embedded in a parylene membrane by filtering dispersed carbon nanotube solution through a template of anodic aluminum oxide, following by physical vapor deposition of parylene. The fabricated membrane was 2μm thick and showed special piezoresistive characteristics even at large strain. Its variations of the electrical resistance and exerted force due to tensile strain were fully characterized for the strain up to 20%. Linear relation between the force and strain was obtained when the strain was smaller than 2%. Positive correlation between the resistance and force was observed for a long range of larger strains. This investigation can be applied for designing flexible electronics and sensors based on carbon nanotube/polymer composites.

Piezoresistive Microcantilevers From Ultrananocrystalline Diamond

This paper reports on the temperature-dependent electrical resistivity and piezoresistive characteristics of boron-doped ultrananocyrstalline diamond (UNCD) and the fabrication of piezoresistive microcantilevers using boron-doped and undoped UNCD. The devices consist of 1-μm-thick doped UNCD on either 1- or 2-μm-thick undoped UNCD. The electrical resistivity of doped UNCD is 0.1 Ω · cm at room temperature, which is five orders of magnitude smaller than the electrical resistivity of undoped UNCD. Over the temperature range of 25°C-200vC, the doped UNCD has a temperature coefficient of electrical resistance of (-1.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ) per °C. The doped UNCD exhibits a significant piezoresistive effect with a gauge factor of 7.53 ± 0.32 and a piezoresistive coefficient of 8.12 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> Pa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at room temperature. The piezoresistive properties of UNCD are constant over the temperature range of 25°C-200°C. Microcantilevers having a length of 300 μm have a deflection sensitivity of 0.186 mΩ/Ω per micrometer of cantilever end deflection. These measurements of electrical and piezoresistive properties of doped UNCD could aid the design of future diamond microsystems.