Flexible stress and humidity sensor, and preparation method and application thereofTechnical Field
The invention belongs to the field of sensors, and particularly relates to a flexible stress and humidity sensor, and a preparation method and application thereof.
Background
Stresses play an important role in the fields of civil engineering, construction, mechanical engineering, mineral engineering, material science, and the like. For example, in the building construction process, such as the processes of steel structure installation, unloading, transformation, reinforcement, concrete pouring and the like, the stress change needs to be monitored, an alarm is given when a monitored value is close to a control value, the safety of construction is ensured, and the method can also be used for checking whether the construction process is reasonable or not. In the preparation and processing processes of the material, the existence of stress reduces the strength of the workpiece, so that the workpiece generates technological defects such as deformation, cracking and the like in the preparation and application processes. The mechanical properties of the material are reduced due to component fracture, fatigue failure, stress corrosion and the like. Therefore, the measurement of stress is of great significance.
The measurement of the stress is often accomplished indirectly through force sensors, which are devices that convert various mechanical signals into electrical signals. The following mechanical sensors are commonly used at present:
1. strain type mechanical sensor
The working principle of the resistance strain type sensor is that a resistance strain sensitive element is adhered or mounted on an elastic element with a certain shape by other methods. When the contact force acts on the elastic element, the elastic element deforms, and the resistance value of the resistance strain sensitive element changes accordingly. The change of the resistance value is changed into the change of voltage (or current) through a conversion circuit, a signal of the change of the resistance value is output to a measuring circuit, and the magnitude and the acting position of the contact force are obtained through simulation calculation according to the change of the voltage (or current). The temperature influence is large (temperature compensation is sometimes needed), the process is complex, and the manufacturing cost is high.
2. Inductance type pressure sensor
The principle of an inductive pressure sensor is to measure pressure by means of changes in inductance of an inductive coil. The air gap type and differential transformer type are common. The air gap type operation principle is that the measured pressure acts on the diaphragm to generate displacement, so that the magnetic circuit reluctance of the inductance coil is changed, the air gap between the diaphragm and the magnetic core is increased and decreased, and the inductance is decreased and increased, so that the inductance differential change is formed, and an alternating voltage corresponding to the measured pressure is output through an electric bridge consisting of electric poles. The differential transformer has the working principle that a measured pressure acts on a spring tube to generate displacement in direct proportion to the pressure, and simultaneously drives an iron core connected to the tail end of the spring tube to move, so that two symmetrical secondary windings connected in series in the opposite direction of the differential transformer lose balance, and a voltage in direct proportion to the measured pressure is output to obtain related mechanical information. However, the frequency response of the sensor is not high, the sensor is not suitable for rapid dynamic measurement, and the requirements on the stability of the frequency and the amplitude of an excitation power supply are high.
3. Capacitive mechanical sensor
The capacitance pressure sensor generally adopts a circular metal film or a metal-plated film as one electrode of a capacitor, when the film deforms under pressure, capacitance formed between the film and a fixed electrode changes, and an electric signal which forms a certain relation with voltage can be output through a measuring circuit. The capacitive pressure sensor belongs to a polar distance variation type capacitive sensor and can be divided into a single capacitive pressure sensor and a differential capacitive pressure sensor. In practice, the output impedance is high, the load capacity is poor, the instability phenomenon is easily caused by the influence of external interference, and the influence of parasitic capacitance is large.
4. Piezoelectric type mechanical sensor
The main principle of piezoelectric mechanical sensors is the piezoelectric effect. Certain materials develop an electrical charge on their surface when they are altered by pressure or tension in a certain direction. When the external force is removed, the surface of the glass sheet returns to the uncharged state again. The piezoelectric mechanical sensor converts the force to be measured into electric quantity by utilizing an electric element and other machines, and obtains relevant information of the force after analog calculation. The piezoelectric sensor can not be used in static measurement, charges subjected to external force can be stored when the loop has infinite input impedance, and therefore, the piezoelectric sensor can only be used for dynamic measurement.
5. Piezomagnetic type mechanical sensor
When the piezomagnetic element formed by laminating the silicon steel sheets is not stressed, the magnetic resistance of the iron core is consistent in all directions. The magnetic lines of force of the primary coil are symmetrically distributed, and are not interlinked with the secondary coil, so that induced electromotive force is not generated in the secondary coil. When the sensor is stressed, the magnetic permeability decreases in a direction parallel to the applied force, and the magnetic resistance increases. The permeability increases perpendicular to the direction of the applied force and the reluctance decreases. The magnetic force lines generated by the primary coil are redistributed, and at the moment, a part of the magnetic force lines are linked with the secondary winding to generate induced electromotive force. The higher the pressure, the more the magnetic lines of force of the linkage, and the greater the induced electromotive force. The induced electromotive force is calculated by simulation, and then grams represent the measured force.
In addition to stress, humidity has a large effect on devices and materials. The humidity sensor is a sensor which can sense the content of water vapor in gas and convert the water vapor into usable output signals, and is mainly applied to the aspects of mechanical engineering, sensors, gas and humidity sensors and the like. The humidity sensor is the simplest humidity sensor and mainly comprises a resistance type sensor and a capacitance type sensor.
The humidity sensitive resistor features that a layer of humidity sensitive material is coated on the substrate, and when water vapor in air is adsorbed onto the humidity sensitive film, the resistivity and resistance of the element are changed. The humidity-sensitive resistor has advantages of high sensitivity but poor linearity and interchangeability of products.
Humidity sensitive capacitors are typically made from polymer film capacitors. When the environmental humidity changes, the dielectric constant of the humidity sensitive capacitor changes, so that the capacitance of the humidity sensitive capacitor also changes, and the capacitance change quantity of the humidity sensitive capacitor is in direct proportion to the relative humidity. The precision of the humidity sensitive capacitor is generally lower than that of the humidity sensitive resistor.
In addition, the humidity sensor needs to select a low-humidity section sensor, a medium-humidity section sensor and a high-humidity section sensor according to needs to improve detection accuracy.
At present, most sensors cannot simultaneously realize stress (pressure and tension) and humidity sensing, and different types of sensors are required to be selected according to different use environments. If a plurality of sensors are arranged in a lattice mode, continuous and complete universe sensing information of the surface to be tested cannot be obtained; if a plurality of sensors are integrated into a single device, the related, preparation and integration processes are very complicated, and if continuous and complete global information to be sensed needs to be obtained, a large number of sensors need to be installed, so that the sensing device is rigid and the irregular surface cannot be monitored; these methods are complicated in installation and monitoring processes.
Disclosure of Invention
The invention provides a flexible stress and humidity sensor and a preparation method thereof, aiming at solving the defects of the prior art, and the flexible stress and humidity sensor can simultaneously realize the detection of stress and humidity, and has the advantages of simple method and high accuracy.
In order to solve the technical problems, the invention adopts the following technical scheme:
a flexible stress and humidity sensor comprising the following raw materials: elastic conductive colloid with chain structure and flexible high molecular polymer.
The elastic conductive material has a chain structure on the molecular structure, can be intertwined with molecular chains in a flexible high polymer to form a physical intertwined structure, can sensitively sense external stress and humidity change, and can sense humidity of 20 microliter liquid and deformation of 0.1 percent.
The principle of the invention is as follows: the corresponding current of the sensor is related to the cross-sectional area S and the length L of the sensor under a constant applied voltage. When the sensor is subjected to stress, S and L of the sensor change, resulting in a change in the response current I. According to the change of the response current, the magnitude of the stress borne by the sensor can be calculated by combining the compression modulus and the elastic modulus of the sensor. In addition, the motion condition of ions in the sensor directly influences the conductivity of the sensor, the water content in the sensor has obvious influence on the motion of the ions in the sensor, and the water content of the sensor is determined by the environmental humidity, so that the sensor can judge the water content according to the size of the response current and then judge the environmental humidity.
Preferably, the elastic conductive material is an elastic conductive colloid prepared according to Chinese patent application with the publication number of CN 104558699A and the invention name of "an elastic conductive colloid, a preparation method and application thereof".
Preferably, the flexible high polymer is polydimethylsiloxane, or polyolefin elastomer, or ethylene-vinyl acetate copolymer, or ethylene-propylene-diene monomer.
Preferably, the volume ratio of the elastic conductive material to the polydimethylsiloxane is 1: 3-4: 1, the volume ratio with the optimal effect is 3:1, the elongation at break of the sensor prepared according to the ratio is highest and can reach 600%, and the sensor is more suitable for application in complex use environments.
A method of making a flexible stress and humidity sensor comprising the steps of:
(1) mixing an elastic conductive material with a flexible high-molecular polymer;
(2) shaping the mixture;
(3) and curing the formed mixture to obtain the flexible stress and humidity sensor.
The flexible stress and humidity sensor is a flexible high molecular polymer modified by an elastic conductive material, the flexible high molecular polymers are mixed with each other before being cured, the mixture has injectability, the flexible high molecular polymer can be randomly shaped before being cured, and the stress and humidity can be simultaneously sensed after being cured.
Preferably, in the step (3), the curing temperature is 50-120 ℃, and the curing time is 1-12 h.
The invention also provides application of the flexible stress and humidity sensor, and the flexible stress and humidity sensor can be used for preparing a stress detection device or a humidity detection device.
The flexible stress and humidity sensor is applied to stress detection and/or humidity detection.
The invention has the following beneficial effects:
1. the sensor is a flexible sensor and can be used for surfaces with any curvature;
2. the sensor can simultaneously or independently sense stress (including pressure and tension) and humidity, and has high accuracy;
3. the sensor can obtain continuous and complete sensing information of the whole domain of the surface to be tested; the sensing range is wide: compressive sensing
The range is 0-80%, the range of stretching sensing is 0-600%, and the range of humidity sensing is 0-100%;
4. the preparation method is simple, the required time is short, and the cost is low.
Drawings
Fig. 1 is a graph of the compressive strength and the amount of deformation (pressure detection) of example 1.
Fig. 2 is a graph of the amount of deformation and the response current (pressure detection) of example 1.
FIG. 3 is a graph showing tensile strength and deformation amounts (tensile test) of example 1.
Fig. 4 is a graph of the amount of deformation and the response current of example 1 (pull force detection).
Fig. 5 is a graph of humidity and response current (humidity detection) of example 1.
Fig. 6 shows the response current of the present invention at different pressures.
FIG. 7 shows the response current of the present invention at different degrees of index finger bending (pull force).
Fig. 8 shows the response current of the present invention at different humidities.
Detailed Description
The present invention will be described in further detail with reference to specific examples.
Example 1
A flexible stress and humidity sensor comprising the following raw materials: the flexible conductive material is characterized by comprising an elastic conductive material with a chain structure and a flexible high polymer, wherein the flexible high polymer is polydimethylsiloxane.
The preparation method comprises the following steps: measuring the elastic conductive colloid and the polydimethylsiloxane according to the volume ratio of 3:1, mixing the elastic conductive colloid and the polydimethylsiloxane according to the volume ratio of 3:1 at room temperature before the polydimethylsiloxane is not cured, and curing for 12 hours at the temperature of 60 ℃ after molding to obtain the flexible stress and humidity sensor.
The elastic conductive colloid is prepared according to Chinese patent application with the publication number of CN 104558699A and the invention name of 'an elastic conductive colloid, a preparation method and application thereof', and the specific preparation method is as follows: dissolving potassium nitrate in deionized water according to the mass percent of 50% to obtain an electrolyte solution; adding the cassava starch into the electrolyte solution according to the mass ratio of 10%, and fully stirring at 50 ℃ until the starch is completely gelatinized to obtain viscous liquid; and standing the viscous liquid at 50 ℃ for 2h to obtain the elastic conductive colloid with the chain structure.
Example 2
A flexible stress and humidity sensor comprising the following raw materials: the flexible conductive material is an ethylene propylene diene monomer rubber.
The preparation method comprises the following steps:
(1) mixing elastic conductive colloid and ethylene propylene diene monomer according to the volume ratio of 1: 3;
(2) shaping the mixed mixture;
(3) after the mixture is formed, the mixture is heated to 50 ℃ and reacted for 6 hours, and the mixture is solidified, so that the flexible stress and humidity sensor is obtained.
The preparation method of the elastic conductive colloid comprises the following steps: dissolving aluminum chloride in deionized water according to the mass percent of 40% to obtain an electrolyte solution; adding the sticky rice starch into the electrolyte solution according to the mass ratio of 10%, and fully stirring at 70 ℃ until the starch is completely gelatinized to obtain viscous liquid; and standing the viscous liquid at 70 ℃ for 1h to obtain the elastic conductive colloid with the chain structure.
Example 3
A flexible stress and humidity sensor comprising the following raw materials: the elastic conductive material with a chain structure and the flexible high molecular polymer are polyolefin elastomers.
The preparation method comprises the following steps:
(1) mixing an elastic conductive colloid prepared by neodymium nitrate with a polyolefin elastomer according to a volume ratio of 4: 1;
(2) shaping the mixed mixture;
(3) after the mixture was shaped, it was heated to 120 ℃ and reacted for 1 hour to cure the mixture, resulting in a flexible stress and humidity sensor.
The preparation method of the elastic conductive colloid comprises the following steps: dissolving neodymium nitrate into deionized water according to the mass percent of 20% to obtain an electrolyte solution; adding corn starch into an electrolyte solution according to the mass ratio of 30%, and fully stirring at 60 ℃ until the starch is completely gelatinized to obtain a viscous liquid; and standing the viscous liquid at 60 ℃ for 3h to obtain the elastic conductive colloid with the chain structure.
Example 4
A flexible stress and humidity sensor comprising the following raw materials: the flexible high-molecular polymer is an ethylene-vinyl acetate copolymer.
The preparation method comprises the following steps:
(1) mixing an elastic conductive colloid prepared by sodium chloride with an ethylene-vinyl acetate copolymer in a volume ratio of 2: 1;
(2) shaping the mixed mixture;
(3) after the mixture is formed, the mixture is heated to 100 ℃ and reacts for 3 hours, so that the mixture is solidified, and the flexible stress and humidity sensor is obtained.
The preparation method of the elastic conductive colloid comprises the following steps: dissolving 25% by mass of sodium chloride in deionized water to obtain an electrolyte solution; adding potato starch into the electrolyte solution according to the mass ratio of 10%, and fully stirring at 80 ℃ until the starch is completely gelatinized to obtain viscous liquid; and standing the viscous liquid at 80 ℃ for 0.5h to obtain the elastic conductive colloid with the chain structure.
Example 5
A flexible stress and humidity sensor comprising the following raw materials: the flexible conductive material is characterized by comprising an elastic conductive material with a chain structure and a flexible high polymer, wherein the flexible high polymer is polydimethylsiloxane.
The preparation method comprises the following steps:
(1) mixing elastic conductive colloid prepared by calcium chloride with polydimethylsiloxane according to the volume ratio of 1: 2;
(2) shaping the mixed mixture;
(3) after the mixture is formed, the mixture is heated to 80 ℃ and reacted for 8 hours, and the mixture is solidified, so that the flexible stress and humidity sensor is obtained.
The preparation method of the elastic conductive colloid comprises the following steps: dissolving calcium chloride in deionized water according to the mass percent of 40% to obtain an electrolyte solution; adding sweet potato starch into an electrolyte solution according to the mass ratio of 10%, and fully stirring at 35 ℃ until the starch is completely gelatinized to obtain a viscous liquid; and standing the viscous liquid at 35 ℃ for 6h to obtain the elastic conductive colloid with the chain structure.
Example 6
The flexible stress and humidity sensor manufactured in example 1 was subjected to pressure detection, tension detection, and humidity detection.
The working principle is as follows:
according to the resistivity formula:
wherein,
it is possible to obtain:
where U is the voltage applied across the sensor, S is the sensor cross-sectional area, L is the sensor length, and ρ is the sensor resistivity. It can be seen that for a given applied voltage, the corresponding current of the sensor is related to S and L of the sensor. When the sensor is subjected to stress, S and L of the sensor change, resulting in a change in the response current I. According to the change of the response current, the magnitude of the stress borne by the sensor can be calculated by combining the compression modulus and the elastic modulus of the sensor.
In the invention, the motion condition of ions in the sensor directly influences the conductivity of the sensor, the water content in the sensor has obvious influence on the motion of the ions in the sensor, and the environmental humidity determines the water content of the sensor, so that the sensor can judge the water content according to the magnitude of the response current and then judge the environmental humidity.
And (3) pressure detection: fig. 1 and fig. 2 are respectively a relationship diagram of the compressive strength and the deformation amount of the sensor, and the response current and the deformation amount, and the deformation amount parameter of the sensor can be obtained through the detected response current, and the magnitude of the pressure applied to the sensor can be judged by referring to the compressive strength of the sensor through the obtained deformation amount parameter. For example, if the sensor is deformed by pressure and the response current is 4mA, the deformation amount of the sensor is known to be 50%, and the compression strength of the sensor at that time is known to be 2.3MPa according to 50% of the deformation amount, and the magnitude of the pressure applied to the sensor can be determined according to the size of the sensor.
And (3) tension detection: fig. 3 and 4 are relationship diagrams of the tensile strength and the deformation amount, and the response current and the deformation amount of the sensor of the present embodiment, respectively, the deformation amount parameter of the sensor can be obtained through the detected response current, and the magnitude of the tensile force applied to the sensor can be judged by referring to the tensile strength of the sensor through the obtained deformation amount parameter. For example, if the sensor is deformed by a tensile force and the response current is 120 μ a, the deformation amount of the sensor is known to be 130%, and if the deformation amount is 130%, the tensile strength applied to the sensor at that time is known to be 60kPa, and the magnitude of the tensile force applied to the sensor can be determined from the size of the sensor. The volume ratio of the elastic conductive colloid to the polydimethylsiloxane is 3:1, the elongation at break of the prepared sensor is the highest, as shown in figure 3, the elongation at break can reach 600%, and the sensor is more suitable for application in complex use environments.
And (3) humidity detection: fig. 5 shows response currents of the sensor of the present embodiment at different humidities, and the detected response currents can determine the humidity of the environment where the sensor is located. As can be seen from fig. 5, when the sensor response current is 18mA, the ambient humidity of the sensor is 50%.
Through multiple tests, the range of compressive sensing is 0-80%, the range of tensile sensing is 0-400%, and the range of humidity sensing is 0-100%. The invention can carry out humidity sensing on 20 microliter of liquid; the sensor can sense the deformation of 0.1% magnitude and has high precision.
Example 7
Changes in pressure, tension and humidity were detected using the sensor manufactured in example 1.
As shown in fig. 6, when subjected to different pressures, the strain was different (the strain amounts of 0% and 50% were changed alternately), and the response current was changed (the response current was-9 ma at 0% strain amount and-13 ma at 50% strain amount, and was changed alternately in accordance with the change in strain amount).
The sensor is fixed on the index finger to respectively detect the change of the sensor when the finger is stretched and bent, as shown in figure 7, when the finger is stretched, the response current is 42-43 microamperes, when the finger is bent, the sensor deforms, so that the response current changes and becomes 39-40 microamperes, and figure 7 shows the response current changes when the finger is circularly stretched-bent, therefore, the invention can be applied to monitoring the limb movement or other related applications.
The sensor is placed under different humidity conditions, response current values are collected, as shown in fig. 8, a cross line is a humidity change curve of manual control, a dotted line is a change curve of response current, the response current can be found to change along with the humidity change, and the humidity of the environment where the sensor is located is judged through the detected response current.
The experiment shows that the invention can directly detect the humidity, the pressure or the tension through the deformation quantity, has higher precision and wider detection range, and can also be applied to devices for detecting the humidity, the pressure or the tension.
The above description is only a preferred embodiment of the present invention, and the scope of the present invention is not limited thereto, and any simple changes or equivalent substitutions of the technical solutions that can be obviously obtained by those skilled in the art within the technical scope of the present invention are within the scope of the present invention.