Gauge Factor Values Research Articles

High temperature measurement set-up for the electro-mechanical characterization of robust thin film systems

Due to economic and environmental requirements there is a strong need both to increase the efficiency and to monitor the actual status of gas turbines, rocket engines and deep drilling systems. For these applications, micromachined pressure sensors based on a robust substrate material (e.g. sapphire) as well as strain gauges made of platinum for long-term stable operation are regarded as most promising to withstand harsh environments such as high temperature levels, aggressive media and/or high pressure loads. For pre-evaluation purposes, a novel, custom-built measurement set-up is presented allowing the determination of electro-mechanical thin film properties up to 850 °C. Key components of the measurement set-up are the one-sided clamped beam made of Al2O3 ceramics which is deflected by a quartz rod and a high precision encoder-controlled dc motor to drive the quartz rod. The specific arrangement of the infrared halogen heaters in combination with the gold coated quartz half shells ensures a high degree of temperature homogeneity along the beam axis. When exposed to tensile as well as compressive load conditions, the corresponding gauge factor values of 1 µm thick platinum thin films show a good comparison at room temperature and in the temperature range from 600 up to 850 °C where the effects originating from grain boundaries or from the film surfaces are negligible. Between 150 and 600 °C, however, a strong deviation in the gauge factor determination depending on the mechanical load condition is observed, which is attributed to the gliding of adjacent grains.

Polypropylene-Carbon Nanofiber Composites as Strain-Gauge Sensor

Polymeric materials have been replacing other materials in various applications, from structural to electronic components. In particular, since the discovery of conducting polymers and the beginning of the manufacture of conducting composites with carbon fillers, their use in electronics has been growing. A group of electronic components with large potential for industrial applications such as structural monitoring, biomedical, or robotics are sensors based on the piezoresistive effect, fabricated from conductive polymers and/or composites. The aim of this article is to characterize the piezoresistive effect of conductive polymer composites based on polypropylene filled with carbon nanofibers, and to demonstrate a way of fabricating strain gauges from these materials, using industrial techniques. With this purpose, some films are prepared by shear extrusion, which allows the composites to be produced industrially in a standard nonexpensive process. Then, the dependence of the electrical response both on the preparation conditions and on the mechanical solicitations is measured. The obtained gauge factor values, up to 2.5, and piezoresistive coefficients up to 0.0019 mm2/N, prove the viability of these materials for fabricating strain-gauges, where their main advantages are the lower price and the ability to deal with much higher deformations, when compared to metal or semiconductor strain-gauges.

Mechanical, electrical and electro-mechanical properties of thermoplastic elastomer styrene–butadiene–styrene/multiwall carbon nanotubes composites

Composites of styrene–butadiene–styrene (SBS) block copolymer with multiwall carbon nanotubes were processed by solution casting to investigate the influence of filler content, the different ratios of styrene/butadiene in the copolymer and the architecture of the SBS matrix on the electrical, mechanical and electro-mechanical properties of the composites. It was found that filler content and elastomer matrix architecture influence the percolation threshold and consequently the overall composite electrical conductivity. The mechanical properties are mainly affected by the styrene and filler content. Hopping between nearest fillers is proposed as the main mechanism for the composite conduction. The variation of the electrical resistivity is linear with the deformation. This fact, together with the gauge factor values in the range of 2–18, results in appropriate composites to be used as (large) deformation sensors.

Study of the piezoresistivity of doped nanocrystalline silicon thin films

The piezoresistive response of n- and p-type hydrogenated nanocrystalline silicon thin films, deposited by hot-wire (HW) and plasma-enhanced chemical vapor deposition (PECVD) on thermally oxidized silicon wafers, has been studied using four-point bending tests. The piezoresistive gauge factor (GF) was measured on patterned thin-film micro-resistors rotated by an angle θ with respect to the principal strain axis. Both longitudinal (GFL) and transverse (GFT) GFs, corresponding to θ = 0° and 90°, respectively, are negative for n-type and positive for p-type films. For other values of θ (30°, 45°, 120°, and 135°) GFs have the same signal as GFL and GFT and their value is proportional to the normal strain associated with planes rotated by θ relative to the principal strain axis. It is concluded that the films are isotropic in the growth plane since the GF values follow a Mohr’s circle with the principal axes coinciding with those of the strain tensor. The strongest p-type pirezoresistive response (GFL = 41.0, GFT = 2.84) was found in a film deposited by PECVD at a substrate temperature of 250 °C and working pressure of 0.250 Torr, with dark conductivity 1.6 Ω−1cm−1. The strongest n-type response (GFL =− 28.1, GFT =− 5.60) was found in a film deposited by PECVD at 150 °C and working pressure of 3 Torr, with dark conductivity 9.7 Ω−1cm−1. A model for the piezoresistivity of nc-Si is proposed, based on a mean-field approximation for the conductivity of an ensemble of randomly oriented crystallites and neglecting grain boundary effects. The model is able to reproduce the measured GFL values for both n- and p-type films. It fails, however, to explain the transversal GFT data. Both experimental and theoretical data show that nanocrystalline silicon can have an isotropic piezoresistive effect of the order of 40% of the maximum response of crystalline silicon.

Investigation on Strain Films in the Thin Film Resistance Strain Gauge

Strain films in the thin film resistance strain gauge are prepared by magnetron sputtering method. Some results concerning the electromechanical and structural properties of nichrome (Ni80Cr20 wt.%) thin films are presented. As compared to the well-known Ni-Cu (constantan) alloy film, which are widely used for manufacturing pressure and force sensors, nichrome (Ni80Cr20 wt.%) thin films exhibit gauge factor values of the same order of magnitude, but they are much more corrosion resistant and adherent to the substrate. The influences of composition and post-deposition annealing on the electrical resistance, temperature coefficient of resistance (TCR) and gauge factor of nichrome (Ni80Cr20 wt.%) thin films are discussed.

The piezoresistive effect in diamond-like carbon films

Diamond-like carbon (DLC) film was deposited using plasma-assisted chemical vapour deposition (PACVD) at −350 V and −800 V. DLC strain gauges were integrated in bulk micromachined silicon. Optical bandgaps were found to be 1.2 eV and 1.03 eV at −350 V and −800 V, respectively. Films deposited at −350 V have a higher hydrogen percentage, hardness, sp3 content, resistivity and gauge factor compared to films deposited at −800 V. Piezoresistive gauge factors were measured under longitudinal and transversal strain configurations and in vertical and lateral current injection directions. It was found that the gauge factor was independent of the current injection direction and strain configurations. A model to explain the origin of the piezoresistive effect in DLC films along with parameters which can further enhance the gauge factor value of the films is discussed, which is confirmed experimentally.

Longitudinal and transversal piezoresistive effect in hydrogenated amorphous carbon films

We report a large piezoresistive effect of the plasma assisted chemical vapor deposited hydrogenated amorphous carbon (a-C:H) films deposited at bias voltages of − 350 and − 800 V. The gauge factors were measured in transversal, longitudinal strain configurations and lateral, vertical current flow directions with respect to the layer surface. The gauge factor measurements were performed in the temperature range of 23–60 °C. A model is proposed to explain the origin of piezoresistive effect in the a-C:H films. From the experimental results we suggest the parameters which can further enhance the gauge factor values in the films.

Strain Sensitivity of AlGaN/GaN HEMT Structures for Sensing Applications

Sensing elements based on AlGaN/GaN HEMT and Schottky diode structures have been investigated in relation with the strain sensitivity of their characteristics. Piezoresistance of the Al 0.3 Ga 0.7 N/GaN HEMT-channel as well as changes in the current-voltage characteristics of the Schottky diodes have been observed with gauge factor (GF) values ranging between 19 and 350 for the selected biasing conditions. While a stable response to strain was measured, the observed temperature dependence of the channel resistance demonstrates the need for a systematic characterisation of the sensor properties to allow compensation of the observed temperature effects.

The electrical resistance response of continuous carbon fibre composite laminates to mechanical strain

Abstract Measurements have been made of the effect of mechanical strain on potential distributions and resistance of unidirectional and multidirectional carbon fibre epoxy laminates. The effects of current flow direction and technique for current introduction on piezo-resistance have been studied. It was found that uniform current introduction at sample edges produced by sputtered Au–Cr contacts across the entire cross-section produced consistently low values of gauge factor of 1.75 for current flow parallel to the fibres and 2.7 for transverse current flow. Non-uniform current introduction, produced variously by local point introduction of current, or use of viscous adhesives producing intermittent contact, resulted in a wide range of apparent gauge factors ranging from 20.6 to −89. These anomalous values may be explained by a model in which the high anisotropy of resistance in unidirectional CFRP maintains initial non-uniform current throughout the sample. Under mechanical strain points of fibre contact will change, altering the distribution of current carrying fibres and leading to local changes in current. Thus changes in potential difference between two points produced by mechanical strain will not be exclusively caused by changes in local resistance. The presence of transverse plies in multidirectional laminates ensures that in plane non-uniform current distributions are largely eliminated, and the effect on piezo-resistance of non-uniform current introduction is minimised.

CuS thin films on flexible substrates

A report is presented on the electrical and structural properties of Cu7S4 films deposited on polyimide substrate at temperatures close to room temperature using the water solutions of complex-salt compounds. The X-ray diffraction data identified Cu7S4 crystalline monoclinic structure. The resistance of the film increased by 350% under tensile strain and by 5% under compressive strain. Maximal value of the gauge factor was found to be G≃103.

Piezoresistive sensors on plastic substrates using doped microcrystalline silicon

The piezoresistive behavior of n-type and p-type microcrystalline silicon films deposited on polyethylene terephthalate plastic substrates by hot-wire, and radio-frequency, plasma-enhanced chemical vapor deposition, at a substrate temperature of 100/spl deg/C, is studied. The crystallite size was 10 nm for hot-wire films and 6.5 nm for radio-frequency films and the crystalline fraction varied between 50 to 80%. A four-point bending jig allowed the application of positive and negative strains in the films. Repeated measurements of the relative changes in the resistance of the samples during the strained condition showed reversible behavior, with p-type microcrystal line films having positive gauge factor in the range from 25 to 30 and n-type /spl mu/c-Si:H films having negative values of gauge factor from -40 to -10. The induced strain in the films varied in the interval between 0 and /spl plusmn/0.3%. The films were used in the as-deposited size (50 mm /spl times/ 10 mm) as sensors, utilizing their piezoresistive properties to map the contour of an acrylic model with the shape of an Archimedes' spiral. Micron-sized devices were patterned and used to map the shape of the same model.

IrO 2 -based thick-film resistors

IrO 2 -based thick-film resistors were prepared and their microstructure/electrical property relationship was studied as a function of the firing temperature Tf and composition. The resistor microstructure exhibits a notable change, from a strongly segregated structure for Tf=750 °C, to a quasihomogeneous one when the firing temperature increases to Tf=850 °C. Likewise, a substantial change is observed in the sheet resistance (Rs) dependence on the volume fraction (v) of the conductive phase. For resistors prepared at Tf=750 °C the percolation power law Rs≈(v−vc)−t (where vc and t are the critical volume fraction and exponent, respectively) is observed, while samples fired at Tf⩾850 °C follow the relation ln Rs≈a(v−v0) where a and v0 are proper constants. In addition, the slope of plot a is affected by the firing temperature Tf as well as by the measuring temperature at T<50 K. Very high temperature coefficients of resistance are measured at cryogenic temperatures. The longitudinal gauge factor values are in the range from 3 to 12.4 according to the resistor composition and firing temperature, with the highest values in samples fired at 950 °C with sheet resistance in the range from 104 to 107 Ω/□.

Analysis of piezoresistive properties of CVD-diamond films on silicon

The piezoresistive properties of CVD-diamond are still very much in discussion since not only the materials energy band structure properties have to be considered but also the grain boundaries and internal stress distribution. Here, the experimental piezoresistive properties of CVD-diamond-on-silicon layers for free standing structures have been investigated comprehensively. The longitudinal gauge factor k l has been extracted using freestanding diamond cantilevers on silicon. The piezoresistors have been grown selectively onto the surface of diamond cantilevers near the mechanical suspension and doped with boron (acceptor). The electrical contacts are based on the tunneling mechanism with a silicon-based multilayer metalization leading to a linear IV-characteristic. Gauge factor values, k l, have been extracted on various structures with different doping concentrations and diamond film quality (highly oriented and textured, textured, randomly oriented), depending on temperature (room temperature, −350°C) and intrinsic stress. Highly oriented and textured films with grain sizes between 3 and 10 μm have been used to realize ‘single grain’ resistor structures enabling the investigation of grain boundaries in the electrical current path of the piezoresistor. Raman measurements have been performed to measure the intrinsic stress in the diamond grains. Gauge factors, k l of between 4 and 28 have been extracted. Largest k l values were observed on piezoresistors on highly oriented and textured diamond (HOD) films. Results of this work have been used in piezoresistive sensor applications.

Piezoresistive Sensors on Plastic Substrates Using Doped Microcrystalline Silicon

ABSTRACTThe piezoresistive behavior of optimized n-type and p-type microcrystalline silicon films deposited on polyethylene terephthalate plastic substrate by hot-wire and radio-frequency plasma-enhanced chemical vapor deposition, at a substrate temperature of 100 °C, is studied. A 4-point bending jig allowed the application of positive and negative strains in the films. Repeated measurements of the relative changes in the resistance of the samples during the strained condition showed reversible behavior, with p-type microcrystalline films having positive gauge factor in the range from 25 to 30 and n-type [.proportional]c-Si:H films having negative values of gauge factor from -40 to -10. The induced strain in the films was in the range between 0 and ±0.3%. A sensor utilizing the piezoresistive property of doped [.proportional]c-Si:H was used to map a contour with the shape of an Archimedes' spiral.

Strain sensitivity and temperature behavior of invar alloy films

The electrical resistance–strain characteristics of Invar films has been studied for their possible application as strain gauges. The Invar films were prepared by sputtering technique and the necessary details of film preparation are given. Temperature coefficient of resistance (TCR) of the films has been measured by systematic annealing of films in vacuum. It has been found that TCR of film is of the order of 10 −3/°C. The gauge factor which is an index of the strain sensitivity, was measured for as-deposited and annealed films. Films exhibited linear electrical resistance versus strain characteristics. The gauge factor for relatively thinner films was found to be as high as 5.5 and for films with thickness greater than 500 Å, it was lower. For films annealed by heating up to temperature less than 300°C, the gauge factor value did not change. However, films annealed at temperature above 300°C, showed reduction in gauge factor.

Ni–Ag thin films as strain-sensitive materials for piezoresistive sensors

Some results concerning the electrical, electromechanical and structural properties of Ni x –Ag 1− x (for x values between 0.35 and 0.50) thin films in view of their utilization for manufacturing pressure and force sensors are presented. As compared to the well known Ni–Cu (constantan) alloys, which are widely used for these type of sensors, Ni x –Ag 1− x thin films exhibit gauge factor values of the same order of magnitude, but they are much more corrosion resistant and adherent to the substrate. The influence of composition and post-deposition annealing on the electrical resistance, temperature coefficient of resistance and gauge factor of Ni x –Ag 1− x thin films is discussed.

Piezoresistivity of boron doped CVD diamond films

The piezoresistivity of chemical vapor-deposited (CVD) and boron doped polycrystalline diamond films has been measured for a wide range of resisitivities (from 0.1 Ω-cm up to 10 6 Ω-cm) at room ambient. Two branches of piezoresistivity of such films are identified—one for highly doped films (resistivities from 0.1 Ω-cm to 10 2 Ω-cm) and another one for slightly doped films (resistivities from 10 3 to 10 6 Ω-cm), the second one being reported for the first time. For both branches of piezoresistivities, the gauge factor increases when the resistivity increases, changing gradually from 7–9 up to 70–75 for the first branch (low resistivities) and from less than 0.1 up to 30–35 for the second branch (high resistivities). The two observed branches of piezoresistivity are attributed to the conduction mechanisms of impurity band conduction (branch 1) and valence band conduction (branch 2). For the first branch gauge factor values of approx. 70–75 are reached at room temperature, which are superior to that of polycrystalline silicon (30) and similar to those achieved for crystallised silicon (120) films. In addition to the instant piezoresistive response of the diamond films, slow creeping piezoresistive effects also have been observed with a time constant of several seconds. Such slow contributions to piezoresistive performance of diamond films may be attributed to the action of intergrain boundaries and in some cases significantly enhances the overall response of the devices.

5591635 Methods and apparatuses for rapid composting with closed air loop circulation for positive control: Young Richard N; Irwin Thomas J Atlanta, GA, United States assigned to DBS Manufacturing Inc

The piezoresistivity of chemical vapor-deposited (CVD) and boron doped polycrystalline diamond films has been measured for a wide range of resisitivities (from 0.1 Ω-cm up to 106 Ω-cm) at room ambient. Two branches of piezoresistivity of such films are identified—one for highly doped films (resistivities from 0.1 Ω-cm to 102 Ω-cm) and another one for slightly doped films (resistivities from 103 to 106 Ω-cm), the second one being reported for the first time. For both branches of piezoresistivities, the gauge factor increases when the resistivity increases, changing gradually from 7–9 up to 70–75 for the first branch (low resistivities) and from less than 0.1 up to 30–35 for the second branch (high resistivities). The two observed branches of piezoresistivity are attributed to the conduction mechanisms of impurity band conduction (branch 1) and valence band conduction (branch 2). For the first branch gauge factor values of approx. 70–75 are reached at room temperature, which are superior to that of polycrystalline silicon (30) and similar to those achieved for crystallised silicon (120) films. In addition to the instant piezoresistive response of the diamond films, slow creeping piezoresistive effects also have been observed with a time constant of several seconds. Such slow contributions to piezoresistive performance of diamond films may be attributed to the action of intergrain boundaries and in some cases significantly enhances the overall response of the devices.

New high gauge-factor thick-film transducer based on a capacitor configuration

Abstract A highly sensitive thick-film sensor developed by using a novel thick-film resistor composition, coupled with design and layout techniques, is reported. The thick film paste is based on the combination of oxides of bismuth, indium and ruthenium. By varying the layout and composition of the thick-film sensors, high gauge-factor values usually attributed to silicon semiconductor strain gauges have been achieved. Various strain gauge parameters have been analysed including linearity, hysteresis and temperature effects. Gauge factor values between 75 to 85 in both resistive and capacitive gauge applications have been observed.