CN101198851A - Polymer Strain Sensor - Google Patents

Abstract

A strain sensor consisting of a non conducting polymer incorporating conductive nanoparticles below the percolation threshold and preferably less than 10 % v/v of the polymer. The polymer is a polyimide and the conducting nanoparticle is carbon black having an average particle size of 30-40 nm and an aggregate size of 100-200 nm. The sensor can sense strain in extension, compression and torsion.

Description

Polymer Strain Sensor

technical field

The present invention relates to strain sensors, in particular micro-strain sensors, which are easy to manufacture and are used for continuous monitoring of structures under strained conditions.

Background technique

Polymer strain gauges have been proposed.

US Patent 5,989,700 discloses the preparation of pressure sensitive inks that can be used in the manufacture of pressure sensors such as strain gauges, where the resistance is indicative of the applied pressure. The ink has an elastic polymer component, and semiconductor nanoparticles are uniformly dispersed in the polymer binder.

US Patent 5,817,944 discloses a strain sensor for concrete structures containing electrically conductive fibers.

US Patent 6079277 discloses a strain or stress sensor consisting of a polymer composite material and a carbon filament matrix.

US Patent 6276214 discloses strain sensors using conductive particle-polymer complexes. Carbon black is dispersed in ethylene-vinyl acetate copolymer to form a conductive polymer matrix.

All of these polymer sensors are fabricated by preparing conductive particles, which are then mixed into polymers by solution or melt methods, and then formed into films. The assembly is then glued to an insulating support and embedded in the mechanical structure to be monitored. Electrical leads need to be connected to the sensor. Polymer strain gauges that rely on a change in the resistance of a conductive film are generally unsatisfactory and suffer from poor service life due to hysteresis. Metallic strain gauges are generally preferred.

The object of the present invention is to develop polymeric strain sensors with improved performance characteristics and low hysteresis.

Contents of the invention

To this end, the present invention provides a composite polymer strain sensor consisting of a non-conductive polymer mixed with conductive nanoparticles below the percolation threshold, preferably less than 10% of the volume of the polymer.

The conductive particle loading is relatively low compared to state-of-the-art polymer strain sensors (typically 30% v/v), which means that compared to the metal-like properties exhibited by state-of-the-art sensors, the The complex is semiconducting.

The polymer is generally polyimide material, and the conductive particles are carbon in different forms, including graphite, carbon black and glassy carbon, which have an average particle size of 30-70nm and aggregate particle size of 100-200nm. This nanocomposite strain sensor element, along with conductive traces, can be printed or adhered directly on the substrate under test by various casting, printing, or conventional attachment techniques, allowing the element to be connected to external circuitry.

The conductive particle loading is relatively low compared to state-of-the-art polymer strain sensors (typically 30% v/v), which means that compared to the metal-like properties exhibited by state-of-the-art sensors, the The composite material described above is semiconducting. The proposed composition is significantly lower than the percolation threshold compared with prior art composite sensors that rely on physical contact between conductive particles providing a percolation network and are subject to micromechanical hysteresis displacement role. The conductivity measurements of prior art polymer sensors are reduced due to the breakdown of permeate conduction paths in the composite. The low loading minimizes the degradation of the micromechanical properties of the polymer composite due to high volume loading.

These composites exhibit enhanced electrical conductivity through an electron hopping mechanism. The conductivity properties (temperature dependence/deformation dependence/voltage dependence, etc.) of such systems depend on the carbon particle size, the concentration of carbon nanoparticles, and the particle distance. When the concentration of carbon nanoparticles increased from 1% v/v to 8% v/v, the electrical conductivity of the composite structure gradually changed from 10 ⁻⁷ to 10 ⁻² S/cm. Thus, these composite membranes are semiconducting in their temperature behavior, which are not used for strain sensing, but are used as extremely low hysteresis strain sensor membranes due to the nature of their impermeable electron transport mechanism. The deformation-dependent change in electrical properties of carbon-polyimide nanocomposite membranes, which critically depends on the intergranular gaps that occur during deformation, is exploited in these membranes to obtain strain sensors for use as these membranes.

Unlike state-of-the-art polymer strain sensors (where the conductivity at zero strain depends on the presence of a percolation network of conductive particles), the conductivity of these carbon-polymer nanocomposite films critically depends on Electron hopping between nanoparticles in a polymer matrix separated by spacing. The semiconducting properties of these nanocomposite films at zero strain also provide a compensation mechanism for the temperature dependence of their resistance.

This allows the strain sensor element (SSE) of the present invention to respond to:

a) extensional (i.e. stretching) deformation, which results in an increase in the electrical resistance of the film through intergranular spacing expansion under tensile strain, and

b) Compressive deformation, the decrease in SSE membrane resistance caused by the decrease of interparticle spacing under compressive load, which is different from the state-of-the-art polymer-based strain sensors, which are insensitive to compressive load due to the presence of percolation network, and

c) Torsional deformation, by virtue of its response to both tensile and compressive deformation.

This SSE can be easily fabricated and used in any shape and size, including thin or thick film or any solid shape, depending on the specific application and sensitivity requirements.

The unique properties of these SSEs enable quantitative monitoring such as tensile and compressive deformation and force, torsional deformation and force, vibration, shock and sinusoidal deformation.

A suitable polymer is polyimide which is commonly used in microelectronic devices. Polyimide has excellent micromechanical, chemical and electrical properties over a wide temperature range of -270°C to 260°C.

A preferred conductive nanoparticle is carbon black with an average particle size of 30-70 nm and an aggregate particle size of 100-200 nm. A more preferred carbon content is about 1% v/v.

Description of drawings

Figure 1 illustrates the preparation steps used in one embodiment of the invention;

Figure 2 illustrates the conductivity as a function of carbon content at 20 °C;

Figure 3 illustrates the temperature dependent change in electrical resistance between a free standing membrane and a lined membrane;

Figure 4 illustrates the electromagnetic hysteresis caused by thermal cycling;

Figure 5 illustrates the general micromechanical behavior of the sensor of the invention compared to an unfilled polymer;

Figure 6 illustrates the general electromechanical behavior of the sensor of the present invention;

Figure 7 illustrates the strain resistance change and gauge factor of the sensor of the present invention;

Figure 8 is a schematic diagram of a carbon fiber composite rowing Oar, showing that the position of the SSE is placed along the axis of the oar;

Figure 9 is a graph of the resistance ratio of the strain sensor element versus time during periodic deformation of the paddle;

Figure 10 is a plot of the resistance of a strain sensor element as a function of applied load;

Figure 11 is a diagram of the resistance change of the strain sensor element SG1 in a cyclic load experiment at two different temperatures;

Figure 12 is a plot of resistance change versus time for specified strain sensor elements during cyclic loading;

Figure 13 is a graph of the relative change in electrical resistance of the SSE when subjected to tensile and compressive deformation;

Figure 14 is the relative change in resistance of all strain sensor elements placed along the paddle axis under tensile deformation and compressive deformation, and the deformation is produced by applying a force of 200N;

Fig. 15 is a graph corresponding to time of resistance change when periodic torsional deformation is applied on the paddle shaft in a clockwise or counterclockwise direction;

Figure 16 is a schematic diagram giving details of the positioning of the carbon fiber composite tube for torsional deformation measurements using an Instron testing machine;

Figure 17 shows the variation of a) the torque applied to the tube, b) the variation of torsional deformation angle (degrees) and c) the electrical resistance of the SSE as a function of time when a periodic torsional deformation is imposed on the carbon fiber composite tube.

Detailed description of the invention

As shown in Figure 1, the nanocomposite film is prepared by mixing carbon black into the precursor of polyimide (i.e. polyamic acid of benzophenone tetracarboxylic dianhydride), and using n-methyl 2- Film formation of 4,4'-aminodiphenyl ether (BPDA-ODA) in pyrrolidone (NMP) solvent. The cast film is in the range of 50-100 microns. The carbon black has an average particle diameter of 30-70 nm and an aggregate particle size of 100-200 nm. The carbon loading was kept below 10% v/v so that the conductivity was in the range of 10 ⁻⁶ to 10 ⁻² Scm ⁻¹ and in the semiconducting range, as shown in FIG. 2 .

Figure 3 shows the resistance-temperature diagram of a nanocomposite film with a carbon content of 5% v/v cast on a silicon substrate. Resistance decreases with increasing temperature, which is typical for semiconductors. The figure also shows reduced resistive hysteresis behavior upon thermal cycling.

Figure 4 shows the temperature-dependent resistance change in freestanding and lined carbon-polyimide nanocomposite films. The difference in the resistance change of the two films shows the effect of the matrix on the electrical behavior of the polymer nanocomposite film.

An advantage of the present invention is that the magnetic hysteresis is very low compared to polymer membranes where the particle loading is in the percolation range, as shown in FIG. 3 . Due to the relatively low loading, the micromechanical properties of the composite are similar to pure polyimide, as shown in Fig. 5. Resistance vs. static strain of the sensor of the present invention is shown in FIGS. 6 and 7 . The free-standing strain sensor membrane had a gauge factor of 8 in tension mode (Figure 6), and a gauge factor of 12 in flexure mode for the strain sensor membrane fixed on a silicon substrate. Gauge factors as high as 25 can be obtained when strain sensors are used on different substrates.

A gauge factor of 25 may be obtained with certain substrates. Conventional metal strain gauges have gauge factors typically <5.

An illustration of the application of these unique properties of this SSE material is its application in monitoring the micromechanical behavior of carbon fiber composite paddles.

Below are examples obtained by placing these strain sensor elements on paddles, demonstrating their potential application.

Figure 8 shows a schematic diagram of the left hand paddle (LO). The distance to the blade is measured from the point of attachment of the shaft to the blade. The position is measured according to the paddle. Table 1 gives the exact geometric location of the SSE on the paddle in the experiments.

Table 1: Detailed locations of SSEs on paddles and their respective resistance values at room temperature

Strain gauge Distance to paddle (mm) position on paddle Angle with the paddle axis Resistance (KΩ) Right hand paddle (RO) SG1 300 Front 0° 87.7 SG2 500 Front 45° 93.6 SG3 600 Front 0° 83.3 SG4 900 Front 0° 84.2 SG5 800 the bottom 0° 80.7

Experimental arrangement

The SSE used in this example consisted of strips 5 mm long, 1 mm wide, and about 0.06 mm thick. The resistance of the SSE was measured using a computer-controlled data acquisition system with a multimeter, and the rowing motion was simulated with a universal testing machine (INSTRON) by which the paddle was fixed by clamping it horizontally with the blade face down. to the handle portion of the paddle and pull the end of the paddle shaft up. The handle-to-sleeve portion of the paddle was fixed to a concrete bench to ensure that this portion of the paddle did not move or deform during the experiment. The end of the propeller shaft (ie where the propeller shaft meets the blade) is attached to the INSTRON by a specially designed fixture. For a force of 300 N, the resulting vertical displacement of the blade here is about 130 mm. The paddles were subjected to periodic deformation at a speed of 1000 mm per minute (approximately 112 load cycles in 1450 seconds in a continuous experiment).

Figure 9 shows the change in resistance over time during the last 10 cycles: SSEs placed at different locations experienced different amounts of strain, which was reflected in their respective changes in resistance ratio. Strain gauge SG3 (located at 600 mm from the center of the blade) and strain gauge SG4 (located at 900 mm from the center of the blade) produced an approximate load-induced strain response, indicating that the deformation characteristics of the paddle at these two locations are similar. These two SSEs also show a maximum response, indicating that the shaft deflection is greatest at these locations. Strain gauge SG1 (located at 300mm) shows a lower strain (two-thirds) compared to SG3 and SG4, showing a lower deformation of the propeller shaft at this location, and SG2 (located at 500mm) shows the smallest stress. The strain gauge SG5 located at 800 mm along the axis (top position) exhibited compressive behavior when the paddle was subjected to a tensile load of 300 Newtons.

The experiments described above demonstrate the ability of these SSEs in quantitatively monitoring paddle deformation, which allowed us to determine the maximum and minimum strain locations on the paddle. This experiment also demonstrates the ability of the inventive strain sensor element to respond to compressive deformation, as shown by the behavior of strain sensor element SG5, which is placed along the axis of the propeller shaft, but at 90° to the position of the other strain sensor elements.

Figure 10 shows a plot of resistance versus applied load. The resistance went from 83,000 ohms with no load to 83,700 ohms with a 300 Newton load. A linear change in resistance with applied load is achieved. This behavior is the same for all strain sensor elements placed along the axis. This resistance response is highly repeatable across all strain sensor elements under cyclic loading when the temperature of the strain sensor is held constant.

Due to its semiconducting properties, the resistance under no-load conditions varies with temperature. However, the rate of change of the resistance of the strain sensor element with temperature remains constant. For example, Figure 11 shows the resistance of strain sensor element SG1 as a function of applied load at two different temperatures. The effect of ambient temperature is to shift the resistance-applied load curve along the Y-axis. However, the load factor (slope) of the resistor remains the same.

Demonstration of the compressive deformation characteristics sensed by the strain sensor element of the present invention.

In Figure 8, the strain sensor element SG5 placed along the paddle axis but at 90° to the other SSEs shows a decrease in resistance with increasing applied load. This is caused by the SG5 laterally compressing the assembly along the paddle axis.

Using INSTRON, a load is applied to the paddle shaft from the opposite direction such that all strain sensor elements that were previously deformed in tension are now compressed under this load configuration.

Figure 12 shows the change in resistance over time for a given strain sensor element when a cyclic load is applied to it. During the tensile deformation of the strain sensor element, the maximum load applied to the paddle was maintained at 300N, and during the deformation test, the maximum load applied to the paddle shaft in the opposite direction was maintained at 200N.

Figure 12 shows the continuous change in resistance of the strain sensor element under periodic forward and reverse loads. In both directions, the observed deformation is proportional to the load.

This can be seen more clearly in Figure 13, where the above data are plotted as a graph of the relative change in resistance with both applied tensile and compressive loads.

Figure 14 shows the relative change in electrical resistance of different strain gauges placed on a paddle shaft along its axis, subjected to tensile and compressive changes induced by a 200 Newton load. The small variation in the values in each strain gauge may be due to the small variation in the placement of the SSE membrane along the paddle axis in the experiment.

Due to the unique ability of the strain sensor element to electrically respond to tensile and compressive changes, by placing the SSE strip on a shaft at a specific geometric location, it can be used to measure the torsional deformation of a material under test.

In experiments illustrating the behavior of these carbon-polymer nanocomposite films, the SSE in the form of thin ribbons was placed with its length at 45° to the axis. The paddle shaft is then subjected to torsional deformation in clockwise and counterclockwise directions. With this arrangement, the SSE is stressed in tension when the torsional force is applied in one direction and in compression when the torsional force is reversed. Thus, the electrical response from the SSE is a positive change in resistance when a twisting force is applied in one direction, and a negative change when the direction is reversed. The relative change also varies with the amount of torsional deformation.

As can be seen in Figure 15, torque is exerted on the strain sensor element SG2 by twisting the paddle clockwise and counterclockwise. SG2 is under compressive stress in one direction and tensile stress in the opposite direction. The change in resistance value depends on the degree of torque and therefore the degree of rotation experienced, and the sign of the change depends on the direction of the applied torque.

The carbon fiber shaft used for the above-mentioned torsional deformation measurement is a hollow tube, and its diameter gradually decreases from the handle to the blade, so the quantitative measurement of the torsional deformation is a complicated task. Separate experiments were performed using the INSTRON test machine to quantitatively demonstrate the performance of the SSE. A schematic diagram of the experimental setup is shown in FIG. 16 .

A hollow tube 11 made of homogeneously porous carbon fiber composite material is used. The device consists of a tube 11, one end of which is held by a holder 14 on a fixed base 12, and the other end supported by a bearing 15 is subjected to torsion. The dimensions of the tube are: length 1500 mm, inner diameter 44.7 mm, outer diameter 46.2 mm. The length direction of the strip-shaped SSE17 is 45° to the pipe axis and placed 100mm away from the pipe support point. The tube 11 is then torsionally deformed by applying a torque of 150 Nm in the clockwise direction and 120 Nm in the counterclockwise direction by using the moving arm 16 (lever) and the INSTRON testing machine. The torque is applied at 1160 mm from the support point and 1060 mm from the sensor location. In order to minimize the effect of the bending of the paddle caused by the applied torque, said torque is applied between two fixed ball bearings at a distance of 360 mm. In this configuration, the SSE 17 experiences a net effective tensile stress when the torsional force is applied in a clockwise direction, and a net effective compressive stress when the torsional force is applied in a counterclockwise direction. Therefore, the resistance change of the SSE17 is positive when the twisting force is applied in the clockwise direction and negative when the twisting force is applied in the counterclockwise direction. This relative change also varies with the amount of twisting force applied.

When a periodic torsional deformation is applied, the change in a) the torque applied to the tube, b) the change in the angle of torsional deformation (degrees), and c) the change in electrical resistance of the SSE over time are illustrated in FIG. 17 .

The change in resistance value depends on the degree of torque and therefore the degree of rotation experienced, the sign of the change depending on the direction of the applied torque.

As can be seen from the above, the present invention provides a strain gauge that can be used to measure both large and small strains. The polymer film can be easily cut and bonded over most surface types and shapes.

Those skilled in the art will appreciate that the present invention may be practiced otherwise than in the described embodiments without departing from the core teachings of the invention.

Claims (10)

1. composition polymer strain transducer, it is made up of non-conductive polymer mixed conductive nano-particles, and described conductive nano-particles is lower than percolation threshold, and preferred less than 10% of described polymer volume.

2. each strain transducer in the aforementioned claim, wherein said polymkeric substance is a polyimide.

3. each strain transducer in the aforementioned claim, wherein said conductive nano-particles are the carbon blacks with aggregate size of the mean grain size of 30-70nm and 100-200nm.

4. each strain transducer in the aforementioned claim, wherein conductivity is 10 ^-6To 10 ^-2Scm ^-1In the scope.

5. each strain transducer in the aforementioned claim, wherein deposit conductive traces on described composition polymer strain transducer can be connected described device with external circuit.

6. the method for preparing polymeric strain sensor, it may further comprise the steps: the conductive nano-particles of capacity is dispersed in the polymer solution, this polymer film of curtain coating comes film forming subsequently, exists in the amount of conductive nano-particles described in this film with the percolation threshold that is lower than described polymkeric substance.

7. prepare the method for polymeric strain sensor in the claim 6, wherein said polymkeric substance is a polyimide, and described conductive nano-particles is the carbon black with aggregate size of the mean grain size of 30-70nm and 100-200nm.

8. the method for preparing polymeric strain sensor in the claim 6 or 7, wherein said conductive nano-particles exists with 10% amount less than described polymer volume.

9. the method for preparing polymeric strain sensor in the claim 6 or 7, wherein said conductive nano-particles is can provide described polymer composites with 10 ^-6To 10 ^-2Scm ^-1The amount of the conductivity in the scope exists.

10. the strain sensor element of being made by each polymer composites among the claim 1-5, it can detect the strain in stretching, compression and the distortion.

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