CN110857894B - Flexible mechanical sensor capable of detecting stress direction based on ordered graphene and preparation method thereof - Google Patents
Flexible mechanical sensor capable of detecting stress direction based on ordered graphene and preparation method thereof Download PDFInfo
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- CN110857894B CN110857894B CN201810972816.9A CN201810972816A CN110857894B CN 110857894 B CN110857894 B CN 110857894B CN 201810972816 A CN201810972816 A CN 201810972816A CN 110857894 B CN110857894 B CN 110857894B
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- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/18—Measuring force or stress, in general using properties of piezo-resistive materials, i.e. materials of which the ohmic resistance varies according to changes in magnitude or direction of force applied to the material
Abstract
The invention provides a flexible mechanical sensor capable of detecting stress direction based on ordered graphene, which comprises: the sensor comprises a first ordered graphene sensing unit, a second ordered graphene sensing unit and a third ordered graphene sensing unit, wherein the first ordered graphene sensing unit comprises ordered graphene sheet layers, polydimethylsiloxane embedding the graphene sheet layers, and a lead and an electrode which are led out from the polydimethylsiloxane; and a second ordered graphene sensing unit comprising ordered graphene sheets, polydimethylsiloxane embedding the graphene sheets, and leads and electrodes leading out of the polydimethylsiloxane; and aligning and overlapping the direction parallel to the graphene arrangement of the first ordered graphene sensing unit and the direction vertical to the graphene arrangement of the second ordered graphene sensing unit, adhering a contact surface with polydimethylsiloxane, curing and packaging. The flexible mechanical sensor can measure the stress magnitude and the stress direction simultaneously.
Description
Background
The invention belongs to the technical field of flexible sensing, and particularly relates to a flexible mechanical sensor capable of detecting a stress direction based on ordered graphene and a preparation method thereof.
Technical Field
The sensor is one of important functional units of modern society measurement, measurement and control and intelligent automation systems, so that research and development of sensing technology have important scientific research and social values. In recent years, flexible sensing technologies such as flexible electronics, bionic skin and wearable electronics become one of hot spots of domestic and foreign researches, the traditional sensing technology is replaced by the flexible sensing technology, and the cognition of the forms and functions of traditional electronic devices, sensing devices, systems and the like is overturned. The flexible sensing technology senses mechanical signal triggering through the flexible functional unit to realize measurement of the flexible form system.
At present, two methods are mainly used for testing mechanical signals in a flexible form system: a force-electricity sensitive detection method and a force-light sensitive detection method. Mechanical signals are sensed through the force-sensitive functional unit, signal sensing is achieved by combining a test circuit to conduct signal processing, but the problems that wires are broken due to stretching, lead interconnection interfaces fall off, the functional unit is adhered to metal and the like exist.
The force-electricity sensitive detection is used as a common mode for testing mechanical signals, and the function is realized mainly through a piezoresistive sensor: the resistance value of the sensing material is regularly changed under the stress action to realize measurement. The advantages of low power consumption, wide stress range, easy signal reading, simple device structure and manufacturing process and the like are widely researched. Over the last several decades, researchers have developed a variety of materials to develop high performance piezoresistive sensors. Generally, piezoresistive sensing fillers are materials with better electrical conductivity, such as composite materials of Conductive Polymers (CPs), metal particles, Carbon Nanotubes (CNTs), and reduced graphene oxide (rGO) and elastic polymers (such as Polyurethane (PU) and Polydimethylsiloxane (PDMS)), and in addition, conductive fibers, metal nanowires and films can also be used as sensing materials of piezoresistive sensors. For example, Hanif et al, university of richia, malaysia, reported a flexible pressure sensor constructed of defective graphene sheets with typical piezoresistive effects. Compared with the complete graphene sheet, the defect-introduced graphene can keep good linear sensing characteristics within the pressure range of 0-50 kPa; in addition, the application of some conductive polymers with characteristics of biocompatibility, super hydrophobicity and the like in the piezoresistive pressure sensor is also researched. However, most of the elastomer polymer composite piezoresistive sensors based on the planar structure show poor sensing performance in a low-load state, and cannot accurately correspond to stress changes, so that the application of the sensors in flexible mechanical sensing is limited, because the design application scenes of most of the flexible mechanical sensors are positions with relatively small stress, such as the skin surface of a human body, joints and the like. Therefore, researchers have developed and designed new materials and structures to improve their low-load performance, such as sensing materials with 3D grid structures, porous structures and other microstructures. For example, korea seoul university Ko et al proposed a resistive electronic skin sensor using an array of interlocked micro-cylinders: and (3) attaching 2 CNT composite films with the micro-cylinder arrays to form an interlocking geometric structure. The interlocking micro-cylinder array pressure sensor realizes the measurement of weak stress signals by utilizing the change of the contact area and the tensile ratio of the nanowires on the micro-cylinder under the stress action. Compared with a planar composite film structure, the interlocking micro-cylinder device is 3-4 times faster in response time (about 18ms) and recovery time (about 10 ms). The fast response allows the interlocking micro-cylinder sensor to also recognize mechanical shock signal patterns of a certain frequency. Meanwhile, the excellent pressure sensing performance can realize the detection of pulse waves so as to realize the measurement of blood pressure, and is used for monitoring other human physiological signals in real time and the like. It can be seen that the "traditional" elastic polymer composite sensing material realizes high-sensitivity resistive pressure sensing only through the innovation of the microstructure of the material. A slight disadvantage is that such pressure sensors are still limited to measuring stress in a single direction.
In recent years, as a type of ordered graphene, graphene fiber materials have developed and enriched the research field of functional and structural materials constructed by graphene, so that the graphene fiber materials have macroscopic mechanical and electrical properties, and lay a foundation for the application of the graphene fiber materials in the fields of mechanical sensors, electrical devices and the like. The device has obvious effect in the fields of stretchable supercapacitors, portable solar cells, flexible sensors and the like. However, there are some problems in the application research of graphene fibers, such as insufficient mechanical properties, etc. This defect of the graphene fiber can be improved by optimizing the preparation process, or by using other materials. Meanwhile, although the process of manufacturing a mechanical sensor by using graphene fibers is continuously developed, single tension or pressure sensing is mainly used, and planar graphene fibers are easily broken or stacked due to strain, so that the advantage of the graphene material cannot be fully exerted on the performance, and the limitation of a single use function cannot be broken through. For example, flexible mechanical sensors based on graphene materials cannot simultaneously detect the direction and magnitude of stress. Therefore, the application of the graphene material in the flexible sensor is still a field in which new technology, new process and new method are urgently needed.
The existing flexible sensor has the following defects: (1) reliability needs to be improved. The existing flexible sensor has the problems that a metal material and a flexible substrate are easy to fall off, and a metal electrode is easy to break to cause sensor failure when large deformation occurs. In practical application, good contact between the flexible substrate and the metal material is required to ensure the normal service of the device. If the flexible sensor fails due to short service life of the flexible sensor or environmental influence under the condition of non-irreversible serious damage, the flexible sensor cannot be put into practical application. At present, most flexible sensors have serious problems, such as poor electrode contact, large influence of environmental disturbance on measurement results, short repeatable service life and the like. Although reliability is not the most important part in the research of the sensor, the lack of reliability is a fatal weakness of a flexible mechanical sensor which is designed to be applied to monitor smaller stress for a longer time, especially human physiological characteristic signals and the like. Not only can the measured signal value become meaningless, but also the service cost is greatly increased due to too short service life. (2) Low load sensing performance is poor. Most of the elastic polymer composite piezoresistive sensors using a planar structure show poor sensing performance in a low load state. For example, the novel material such as graphene has many excellent physicochemical properties, but is not fully excavated in a mechanical sensor, and the common application is limited to a simple structure of laying graphene. This structure responds poorly to small strains. Meanwhile, the mechanical sensor is usually designed in a tensile or compressive stress environment, the tiled graphene is easily broken or stacked by tensile and compressive stress, the structure is irreversibly damaged, and the reliability is reduced. (3) The sensing function is single. At present, a common pressure sensor is limited to measuring stress in a single direction, so that when a certain stress is measured in a measuring space, at least three sensors are needed to measure components of the stress in x, y and z directions, the complexity of measurement is improved, and the measurement efficiency and accuracy are reduced to some extent. While the stress in the application scenario faced by the flexible mechanical sensor usually has variable spatial directions, the design of the traditional mechanical sensor makes the accurate measurement in this case complicated and inefficient.
In summary, with the advancement of technology, most of the existing flexible mechanical sensors can meet the requirements of applications in terms of measurement range and sensitivity, but the measurement function is still relatively single, such as: only the point at which the stress change transient is measured and does not reflect an accurate stress value, or only the stress in a certain direction. In daily life and production applications, information about the direction and magnitude of the applied force often needs to be acquired at the same time. Therefore, the flexible mechanical sensor is still a field to be developed and improved, and more diversified sensor materials are needed, for example, orderly-arranged graphene materials such as graphene fibers are applied to the flexible mechanical sensor, so that the development of the flexible mechanical sensor with more complete and comprehensive functions is promoted. At present, there is no report of application of ordered graphene in a flexible mechanical sensor, and ordinary disordered graphene can only realize measurement of stress in a single direction, so that development of a flexible mechanical sensor capable of simultaneously measuring stress magnitude and stress direction is urgently needed.
Disclosure of Invention
An object of the present invention is to provide a flexible mechanical sensor capable of measuring the magnitude and direction of stress simultaneously.
Another object of the present invention is to provide a method for preparing the flexible mechanical sensor.
In order to achieve the above object, the present invention provides a flexible mechanical sensor capable of detecting a stress direction based on ordered graphene, including:
the sensor comprises a first ordered graphene sensing unit, a second ordered graphene sensing unit and a third ordered graphene sensing unit, wherein the first ordered graphene sensing unit comprises ordered graphene sheet layers, polydimethylsiloxane embedding the graphene sheet layers, and a lead and an electrode which are led out from the polydimethylsiloxane; and
a second ordered graphene sensing unit comprising ordered graphene sheets, polydimethylsiloxane embedding the graphene sheets, and leads and electrodes leading out of the polydimethylsiloxane;
and aligning and overlapping the direction parallel to the graphene arrangement of the first ordered graphene sensing unit and the direction vertical to the graphene arrangement of the second ordered graphene sensing unit, adhering a contact surface with polydimethylsiloxane, curing and packaging.
According to the flexible mechanical sensor, the lead and the electrode are preferably located at the edge of the graphene structure.
Preferably, the positions of the wires and electrodes are four positions in the parallel and perpendicular directions of the graphene sheets.
Preferably, the wire includes, but is not limited to, a silver wire or a copper wire.
Preferably, the material of the electrode includes, but is not limited to, silver paste and polyethylenedioxythiophene.
Although there are many types of existing flexible mechanical sensors, the variety of flexible mechanical sensors manufactured based on graphene is not abundant. Most of such flexible graphene mechanical sensors are formed by laying graphene sheets on a flexible material, and the electrical conductivity of the sensor is changed by utilizing gaps between the sheets under the action of stress (usually tensile stress) of graphene. The structure has the defects of being not beneficial to the recovery of the graphene after stretching and having single function. As an improvement to this sensor structure, the present inventors have developed flexible mechanical sensors based on vertically and orderly arranged graphene sheets. PDMS (polydimethylsiloxane) is selected as a carrier and an encapsulating material of graphene, has good elasticity and biocompatibility, and is a common carrier material of a flexible sensor. The graphene sheet layers of the vertical structure are embedded in the PDMS, and stretch and rebound along with the PDMS under the action of stress. Compared with the tiled graphene, the structure has better tolerance to repeated stretching, so that the service life of the manufactured sensor is greatly prolonged.
The ordered graphene lamellar array is realized by adding an inducer in the process of growing a few-layer graphene lamellar structure by chemical vapor deposition. The main principle of the invention is that an electric field is applied in the direction vertical to a substrate (a stainless steel sheet), and graphene sheets are orderly arranged on the steel sheet under the induction of the electric field force. In the ordered graphene obtained in the way, due to the two-dimensional structure of the graphene sheet layers, the contact between the sheet layers has large anisotropy, namely the contact overlapping area along the plane direction of the graphene sheet layers is large, the contact overlapping area perpendicular to the plane direction of the graphene sheet layers is small, and the contact area in the middle direction is also in the middle. This makes the conductivity of the whole graphene array anisotropic, and the conductivity-direction relationship can be quantified by the included angle between the parallel or perpendicular direction of the sheet plane. In the process that the graphene sheet layer array is stretched, the rules (strain rates) of resistance in different directions changing along with stress are different, and the strain rates and angles have quantitative relations, so that the stress direction is measured by the sensor according to the principle.
Compared with the graphene mechanical sensor manufactured by a spin coating or tiling method, the flexible mechanical sensor provided by the invention has the characteristics of diversified functions: the tensile stress in different directions can be measured on a plane, the direction of the stress can be judged at the same time, and even the pressure perpendicular to the plane can be measured on the basis (measurement of three-dimensional stress). When the tensile stress changes, the electric signals of the two loops on the plane are transmitted into the measuring equipment, compared with the standard quantity, the magnitude of the tensile stress applied on the plane is calculated, and meanwhile, the included angle between the stress direction and the specified positive direction is judged. Compared with the existing mechanical sensor, the two-dimensional flexible mechanical sensor realizes the simultaneous measurement of the stress magnitude and the stress direction.
The invention also provides a preparation method of the flexible mechanical sensor, which comprises the following steps:
(1) preparing a graphene sheet layer which vertically grows by utilizing a chemical vapor deposition technology, placing a metal inducer on a stainless steel substrate on which the graphene grows, inducing a component in a parallel direction from a vertical electric field on the surface of the substrate to prepare a graphene sheet layer which is vertically and orderly arranged, and transferring and embedding the graphene sheet layer into polydimethylsiloxane;
(2) inserting a lead into polydimethylsiloxane containing graphene, pouring an electrode at a position where the lead penetrates through the surface of the polydimethylsiloxane, leading out the lead, coating the polydimethylsiloxane on the surface where the electrode and the lead are led out, and curing to obtain a first ordered graphene sensing unit and a second ordered graphene sensing unit;
(3) aligning and overlapping the direction parallel to the graphene arrangement of the first ordered graphene sensing unit and the direction perpendicular to the graphene arrangement of the second ordered graphene sensing unit, adhering a contact surface with polydimethylsiloxane, curing and packaging.
Preferably, in the step (2), the position where the wire is inserted is located at the edge of the graphene structure.
Preferably, in the step (2), the positions where the lead and the pour electrode are inserted are four positions in the parallel direction and the perpendicular direction of the graphene sheet layer.
Preferably, in step (2), the conductive wire includes, but is not limited to, a silver wire or a copper wire.
Preferably, in step (2), the material for the perfusion electrode includes, but is not limited to, silver paste and polyethylenedioxythiophene.
In the preparation process of the flexible mechanical sensor based on the ordered graphene, the graphene sheet layer array needs to be integrally transferred from the stainless steel sheet to the flexible PDMS film. After the vertically ordered graphene is transferred to PDMS, the graphene structure is almost entirely embedded in the cured PDMS, exposing only few roots on the PDMS surface. And inserting a metal wire (copper wire or silver wire) into the PDMS containing the graphene, wherein the inserting position requires that the graphene is uniformly distributed and positioned at the edge of the graphene structure. And (3) filling silver glue at the position where the wire penetrates the surface of the PDMS to form an electrode, and leading out the wire. The selected locations for inserting the leads and the perfusion electrodes are typically four locations in the parallel and perpendicular directions of the ordered graphene lamellar structure. In order to fully ensure that the connection part is well contacted without looseness, after the manufacturing of the lead and the electrode is completed, a thin layer of PDMS is coated on the surface of PDMS, which is exposed out of the root of the graphene (namely the surface where the electrode and the lead are led out), and then the thin layer of PDMS is cured, so that the manufacturing of a single sensing unit is completed. The anisotropy of the conductivity of the transferred graphene structure is proved by measurement, and the conductivity parallel to the arrangement direction of the graphene sheet layers is obviously superior to that in the vertical direction.
And finally, aligning and overlapping the direction parallel to the arrangement of one unit graphene and the direction perpendicular to the arrangement of the other unit graphene by adopting two pieces of ordered graphene sensing units, adhering a contact surface with PDMS (polydimethylsiloxane) and curing, and packaging into a complete sensor. The purpose of using two sensing cells is to make it difficult to distinguish the actual angle of stress from the complementary angle of the angle when only one sensing cell is used, and to more clearly measure the actual angle of stress when two sensing cells perpendicular to the direction in which graphene is arranged are stacked. Different resistance values measured by 4 groups of loops of 8 electrodes are compared and analyzed, so that the direction and the magnitude of the stress can be calculated, and the magnitude of the stress can be calculated according to the elastic modulus of PDMS. Meanwhile, the structure can also measure pressure, and multi-dimensional mechanical sensing is realized.
In the above scheme, silver paste is used as an electrode at the connection between the lead and the sensor substrate material (PDMS). However, the adhesion between PDMS and silver paste is not good, so that there is a certain difficulty in device fabrication, and the reliability of the electrode is also affected. A high-molecular conductive material with better conductivity (such as PEDOT (polyethylene dioxythiophene)) can also be selected as an electrode material, so that the electrode is firmer; or the surface hydrophilicity and hydrophobicity of the PDMS is changed through ion etching, so that the PDMS can be fully soaked by the silver colloid, the contact quality between the silver colloid and the graphene is improved, and a better measurement effect is achieved.
Drawings
Fig. 1 is a schematic structural diagram of a first ordered graphene sensing unit of an ordered graphene-based flexible mechanical sensor capable of detecting stress direction according to the present invention.
Fig. 2 is a schematic structural diagram of a second ordered graphene sensing unit of the ordered graphene-based stress direction detectable flexible mechanical sensor according to the present invention.
Fig. 3 is a structural diagram of a flexible mechanical sensor capable of detecting stress direction based on ordered graphene according to the present invention after packaging.
Fig. 4 is a side view in elevation of a graphene arrangement of an ordered graphene sensor cell.
Fig. 5 is a vertical direction side view of graphene arrangement of an ordered graphene sensor cell.
Fig. 6 is a schematic diagram of chemical vapor deposition growth of ordered graphene.
FIG. 7 is a chart of example data for a joint force measurement.
Fig. 8 is a graph of example pulse measurement data.
Fig. 9 is a partial neck activity measurement data chart.
Detailed Description
The technical solution of the present invention is further described below with reference to the following specific examples and the accompanying drawings, but the present invention is not limited to the following examples.
It is to be understood that for the sake of brevity, no detailed description of known operations or procedures in the art is set forth below, and that such related operations and/or materials will be apparent to those skilled in the art unless so specified.
Example 1: preparation of ordered graphene flexible mechanical sensor
Vertical graphene is first grown using Chemical Vapor Deposition (CVD). One of microwave plasma enhanced chemical vapor deposition (MPCVD) or inductively coupled plasma enhanced chemical vapor deposition (ICPCVD) can be selected as the growth mode of the vertical graphene. In both growth modes, a stainless steel sheet 61 is used as a matrix material for graphene growth and is placed on a cathode sample table in a growth cavity in a deposition system.
MPCVD is to vacuumize the growth chamber to below 5Pa, then introduce 100sccm (standard milliliter/minute) of hydrogen to about 220 + -10 Pa, and then apply 500W of microwave to initiate glow. Then a-100V bias was applied to the cathode to enhance the plasma, with a gradual increase in chamber temperature to 460 ± 20 ℃. After 20 minutes, the chamber was purged with 7sccm of methane and the chamber temperature and pressure were changed to 490. + -. 20 ℃ and then stabilized while the bias applied to the cathode was increased to-200V. Keeping the growth conditions stable, finishing the growth after continuously growing for 30 minutes, and cooling the cavity to room temperature under the state of keeping vacuum.
ICPCVD is first to vacuum the growth chamber to 5X 10-4Pa or less, and heating the stainless steel substrate 61 to 900 + -50 ℃. Then 15sccm of hydrogen and 15sccm of argon were introduced into the chamber, the pressure was changed to 0.4 + -0.02 Pa, then 900W of RF source was applied to ignite, and a bias of-100V was applied to the cathode to enhance the plasma. After 15 minutes, introducing 60sccm methane and adjusting the hydrogen flow to 10sccm, changing the cavity gas pressure to 0.4 +/-0.02 Pa, then increasing the radio frequency power and the cathode bias voltage to 1000W and-200V respectively, continuing to grow for 20 minutes, stopping growing, and finally keeping the cavity in vacuum and cooling to room temperature.
The growth of the ordered vertical graphene 64 is realized by arranging the metal inductors 62 at the two ends of the stainless steel sheet 61 substrate to change the electric field distribution. In the invention, the metal inductor 62 is a rectangular parallelepiped block made of stainless steel or titanium, the length of the metal inductor is the same as that of the stainless steel substrate 61 grown by graphene, the width of the metal inductor is 0.5-2 cm, the height of the metal inductor is 1-3 cm, and the metal inductor is arranged at two ends of the stainless steel substrate 61 in a pair in parallel. In the growth process, a vertical electric field between the cathode and the anode in the growth cavity is influenced by the inducer, the surface of the stainless steel substrate is bent and is parallel to the surface of the substrate at the position close to the inducer. Under the influence of such an electric field 63, the vertical graphene is induced to grow into ordered graphene 64 having a specific orientation. The schematic diagram of the placement position of the metal inducer 62 and the induced growth of the ordered vertical graphene is shown in fig. 6.
The steel sheet loaded with the ordered graphene is placed into a culture dish, air bubbles are removed through centrifugation of prepared PDMS, and then the steel sheet is slowly guided into a vacant area of the culture dish according to the direction of a channel arranged by the graphene, so that the phenomenon that the graphene structure is damaged by too fast flowing of PDMS liquid is avoided. The petri dish was placed in an oven at 60 ℃ for 12 hours, and after removal of the petri dish, the cured PDMS was removed. At this time, the solid PDMS is embedded with vertical graphene and a steel sheet, and the PDMS is bent in a direction without the steel sheet, so that the steel sheet is gradually peeled off from the PDMS, and thus the vertically-ordered graphene is completely embedded in the PDMS, as shown in fig. 1, 2, 4, and 5.
The method comprises the steps of enabling one surface, containing graphene, of PDMS to face upwards, inserting copper wires or silver wires into an area at the edge of a graphene structure, injecting silver glue to enable the copper wires or the silver wires to be fully bonded with the graphene, manufacturing an electrode, and ensuring good electrical contact between the graphene and a lead. Each graphene sensing unit is provided with four electrodes, and a pair of each two electrodes are respectively positioned at the edge parallel to the graphene arrangement direction and the edge perpendicular to the graphene arrangement direction, as shown by electrodes 1, 2, 3 and 4 in fig. 1 and electrodes 1 ', 2', 3 'and 4' in fig. 2, wherein points 1, 2, 3 and 4 represent connection points of a copper wire and graphene; points 1 ', 2', 3 'and 4' are the same; f1、F2Respectively tensile stresses in two directions. The parallel direction of the graphene 11 arrangement in fig. 1 is parallel to the 2 and 4 connecting lines, and the parallel direction of the graphene 21 arrangement in fig. 2 is parallel to the 1 'and 3' connecting lines. In the process, the wires 13 and 23 are marked, and the direction parallel to the arrangement direction of the graphene and the direction perpendicular to the arrangement direction of the graphene are recorded, so that the comparison and verification of experimental data are facilitated.
And then, paving a layer of PDMS on the device to encapsulate the graphene and the electrode, and standing until the PDMS is completely cured. The structure of graphene in PDMS is shown in fig. 4 (parallel side view) and fig. 5 (vertical side view), where graphene 11, 21 is embedded in PDMS12, 22.
If observing the appearance of the ordered vertical graphene transferred to the PDMS, one surface of the steel sheet containing the ordered vertical graphene is placed downwards to face the bottom of the culture dish, then the PDMS is slowly injected, after the PDMS is completely solidified, the PDMS containing the graphene is gently taken down by using a pair of tweezers, and the ordered vertical graphene structure in the sample is observed by using a Transmission Electron Microscope (TEM) after thinning.
Preparing two samples with the same size according to the steps, overlapping one sample with the other sample by rotating one sample for 90 degrees along the arrangement direction of the graphene, adhering PDMS (polydimethylsiloxane) on a contact surface, and curing to obtain the ordered graphene flexible mechanical sensor, as shown in FIG. 3, wherein the flexible mechanical sensor comprises two overlapped ordered graphene sensing units 31, a PDMS packaging carrier 32 and 8 wires 33.
Example 2: measurement of ordered graphene flexible mechanical sensor corresponding force
The calibration and calibration of the sensing unit is first performed as follows.
1. The elastic extension range of the sensing element is measured. Along F in FIGS. 1 and 21Or F2The sensing unit is directionally stretched and the resistance values of the 13 (or 1 '3') loop and the 24 (or 2 '4') loop are simultaneously measured until the resistance R no longer exhibits a linear relationship with the change of the strain epsilon, at which time the strain epsilonlimitI.e. the elastic limit. The less elastic of the 13 (or 1 '3') loop and the 24 (or 2 '4') loop is taken as the elastic limit of the sensing unit.
2. Within elastic limits, the strain rate G at a fixed angle is measured. Defining the direction parallel to the ordered graphene alignment as 0 °, setting a series of fixed stretching angles, such as 5 °, 10 °, 15 °, 25 °, …, 175 °, 180 °, stretching the sensing element within the elastic limit, resulting in measured resistance values R of a series of 13 (or 1 '3') and 24 (or 2 '4') loops13And R24Strain epsilon in the same 13 (or 1 '3') direction and 24 (or 2 '4') direction13And ε24According to the formula G ═ Δ R/R0) Calculating the strain rate G of the sensing unit when the sensing unit is stretched in each direction13And G24. Wherein R is0Δ R is the resistance value measured after applying stress and R is the resistance value when no stress is applied0The difference of (a). Epsilon13And ε24The components of the total strain epsilon in the 13 (or 1 '3') and 24 (or 2 '4') directions are taken.
3. And obtaining a relation function of the strain rate G and the tensile stress angle theta. By fitting a series of strain rates G at fixed stretch angles13And G24Obtaining a relation function G according to the relation of the same tensile stress angle theta13F (θ) and G24＝g(θ)。
The unknown tensile stress can then be measured. Assuming that the strain caused by the unknown tensile stress is epsilon ', the angle is theta', and the resistance value measured by the sensing unit is R13' and R24', the following system of equations can be established:
G13’＝(ΔR13’/R13 0)/ε13’ (1)
G24’＝(ΔR24’/R24 0)/ε24’ (2)
G13’＝f(θ’) (3)
G24’＝g(θ’) (4)
tanθ’＝ε13’/ε24’ (5)
solving equations (1) - (5) can obtain the angle theta' and the strain component epsilon13’，ε24' and further calculates the strain ∈ ═ v (∈ √ s)13’2+ε24’2) The stress is σ ═ ε' E, where E is the elastic modulus of PDMS.
Note that since the ordered graphene structure has a certain symmetry, the stress angle θ' calculated by only one sensing unit is actually a solution of two nearly complementary angles. At this time, the results calculated by the other sensing unit are compared, and the group of angles with smaller difference is the actual stress angle.
Example 3
The electric signal is measured by an electric measuring device such as a voltammeter, a power meter and the like (a portable voltammeter can also be used for improving the practicability of the sensor), a 1 '3' electrode is connected as a loop, a 2 '4' electrode is connected as a loop, and two groups of data of a person are measured at the same time in a natural state, so that the current of the 1 '3' electrode loop is larger (or the resistance is smaller) and the current of the 2 '4' electrode loop is smaller (or the resistance is larger) under the same voltage.
Example 4
Simultaneously connecting 1 '3' electrode as a loop 1, 2 '4' electrode as a loop 2, fixing a tension point on two sides of 1 '3', and stretching the device by using fixed tension, wherein the current of the loop 1 and the current of the loop 2 are reduced simultaneously (or the resistance of the loop 1 and the resistance of the loop 2 are increased simultaneously), but the current of the loop 1 is obviously larger than that of the loop 2 (or the resistance of the loop 1 is obviously smaller than that of the loop 2); the same method fixes 2 '4' on both sides as tension points and measures the current or resistance, again with loop 1 having a significantly higher current than loop 2 (or loop 1 having a significantly lower resistance than loop 2).
Example 5
Simultaneously connecting 1 '3' electrode as a loop 1, 2 '4' electrode as a loop 2, fixing two sides of a tension point 1 '3' as force application points, and stretching the device by variable force to find that the current of the loop 1 and the current of the loop 2 are reduced simultaneously (or the resistance of the loop 1 and the resistance of the loop 2 are increased simultaneously), but the current of the loop 1 is obviously larger than that of the loop 2 (or the resistance of the loop 1 is obviously smaller than that of the loop 2); the same method fixes 2 '4' on both sides as tension points and measures the current or resistance, again with loop 1 having a significantly higher current than loop 2 (or loop 1 having a significantly lower resistance than loop 2).
Application example 1: joint stress measurement
The ordered graphene flexible mechanical sensor is attached to the back of the hand of a testee close to the mouth of the tiger, and the fist is gradually clenched by the palm from flat extension. The real-time resistance value change of the electrode 4 loop of the multi-channel source meter measuring sensor in the fist making process is utilized to calculate the size and the direction of stress, and the measurement of the fist making degree is realized. The measurement results are shown in fig. 7, and the resistance value change measured by 4 loops shows that the sensor is subjected to stretching with gradually increasing angle and strength as the palm is gradually gripped, and quantized tensile stress data is obtained through calculation.
Application example 2: pulse measurement
The ordered graphene flexible mechanical sensor is attached to the wrist of a subject, the initial resistance value of an electrode 4 loop of the sensor 8 and the real-time resistance value change in pulse measurement are measured by using a multi-channel source meter, and a curve of the resistance value changing along with time is drawn. By comparing the fluctuation of the four groups of resistance values of the two sensing units, the measurement error caused by the environmental influence (such as violent body movement of the subject and looseness of the sensor) is eliminated, so that an accurate pulse fluctuation curve is obtained, as shown in fig. 8.
Application example 3: respiration measurement
The ordered graphene flexible mechanical sensor is attached to the neck of a testee, the initial resistance value of an electrode 4 loop of the sensor 8 and the real-time resistance value change in respiration measurement are measured by using a multi-channel source meter, and a curve of the resistance value changing along with time is drawn. By comparing the fluctuation of four groups of resistance values of the two sensing units, the measurement error caused by environmental influence (such as rapid rotation of the neck of a subject and looseness of a sensor) is eliminated, and therefore an accurate breathing curve is obtained. Meanwhile, the neck motion states of the subjects, such as coughing and sneezing, can be detected more intensely, and more abundant physiological signal monitoring is provided, and the measurement data is shown in fig. 9.
Claims (8)
1. A flexible mechanical sensor capable of detecting stress direction based on ordered graphene is characterized by comprising:
the sensor comprises a first ordered graphene sensing unit, a second ordered graphene sensing unit and a third ordered graphene sensing unit, wherein the first ordered graphene sensing unit comprises ordered graphene sheet layers, polydimethylsiloxane embedding the graphene sheet layers, and a lead and an electrode which are led out from the polydimethylsiloxane; and
a second ordered graphene sensing unit comprising ordered graphene sheets, polydimethylsiloxane embedding the graphene sheets, and leads and electrodes leading out of the polydimethylsiloxane;
wherein the direction parallel to the graphene arrangement of the first ordered graphene sensing unit is aligned with the direction perpendicular to the graphene arrangement of the second ordered graphene sensing unit, and the first ordered graphene sensing unit and the second ordered graphene sensing unit are overlapped, and the contact surface is adhered by polydimethylsiloxane, cured and packaged;
the lead and the electrode are positioned at the edge of the graphene structure;
the positions of the wires and the electrodes are four positions in the parallel direction and the perpendicular direction of the graphene sheet layer.
2. The flexible mechanical sensor according to claim 1, wherein the conductive wire is a silver wire or a copper wire.
3. The flexible mechanical sensor according to claim 1, wherein the electrode is made of silver colloid or polyethylene dioxythiophene.
4. A method for manufacturing a flexible mechanical sensor according to any one of claims 1 to 3, characterized by comprising the following steps:
(1) preparing a graphene sheet layer which vertically grows by utilizing a chemical vapor deposition technology, placing a metal inducer on a stainless steel substrate on which the graphene grows, inducing a component in a parallel direction from a vertical electric field on the surface of the substrate to prepare a graphene sheet layer which is vertically and orderly arranged, and transferring and embedding the graphene sheet layer into polydimethylsiloxane;
(2) inserting a lead into polydimethylsiloxane containing graphene, pouring an electrode at a position where the lead penetrates through the surface of the polydimethylsiloxane, leading out the lead, coating the polydimethylsiloxane on the surface where the electrode and the lead are led out, and curing to obtain a first ordered graphene sensing unit and a second ordered graphene sensing unit;
(3) aligning and overlapping the direction parallel to the graphene arrangement of the first ordered graphene sensing unit and the direction perpendicular to the graphene arrangement of the second ordered graphene sensing unit, adhering a contact surface with polydimethylsiloxane, curing and packaging.
5. The method according to claim 4, wherein in the step (2), the position where the lead is inserted is located at the edge of the graphene structure.
6. The method according to claim 4, wherein in the step (2), the positions of inserting the lead and pouring the electrode are four positions in the parallel direction and the perpendicular direction of the graphene sheet layer.
7. The method according to claim 4, wherein in the step (2), the conductive wire is a silver wire or a copper wire.
8. The method according to claim 4, wherein in the step (2), the material for pouring the electrode is silver colloid or polyethylene dioxythiophene.
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| CN114010166B (en) * | 2021-11-17 | 2023-10-31 | 齐鲁工业大学 | Flexible sensor manufacturing method |
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