Capacitive liquid metal-based humidity sensor, preparation method and application thereof

Abstract

The application discloses a capacitive liquid metal-based humidity sensor and a preparation method and application thereof. The application adopts liquid metal nanoparticles wrapped with oxides to form a sensing layer, and utilizes the liquid metal oxides to realize active sensing responding to humidity changes. The application adopts one-step laser direct writing technology to prepare the capacitive liquid metal-based humidity sensor, and the capacitive liquid metal-based humidity sensor has the advantages of simple operation, small energy consumption, energy saving and emission reduction, green environmental protection, and good response time and recovery characteristics. Due to the photo-thermal effect of ultraviolet laser, the liquid metal nanoparticles wrapped with oxides can be selectively sintered and converted from insulating wires into conductive wires, and the resistivity of the conductive wires is 0.19 Ω·cm, and the untreated area is an active sensing layer responding to humidity changes. Under 95% relative humidity, the humidity sensor exhibits highly stable performance, fast response and recovery time.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of flexible sensor preparation, and relates to a capacitive liquid metal-based humidity sensor and a preparation method and application thereof. BACKGROUND

[0002] In recent years, flexible humidity sensors have made great progress in the application aspects of human health detection, plant health management and non-contact human-machine interface. The flexible capacitive humidity sensor is concerned due to its reliable humidity sensing performance, low power consumption and simple structure design. Generally, the performance of the capacitive humidity sensor is closely related to the dielectric constant of the functional nanomaterial between the electrodes.

[0003] So far, various active materials have been studied as flexible capacitive humidity sensors, such as carbon materials, cellulose paper, metal oxides, metal sulfides and conductive polymers. They usually have high hydrophilicity, large exposed surface area and rich active sites for interaction with water molecules.

[0004] As a potential metal oxide, Ga2O3 has good chemical and thermal stability, and has been proved to be used as an active material for humidity sensors. Currently, the manufacturing techniques of Ga2O3-based humidity sensors mainly include chemical vapor deposition, vapor-liquid-solid growth mechanism, heat treatment and hydrothermal method, etc. For example, C.G. Hou et al. synthesized single-crystal Ga2O3 nanobelt by using chemical vapor deposition method with metal gallium and oxygen as raw materials. The preparation process includes: firstly, high-purity gallium particles are placed on an alumina substrate. Secondly, the whole is placed in a quartz tube and slowly heated to a growth temperature of 1000℃. Then, after keeping for a period of time, it is naturally cooled to room temperature. It is generally believed that metal gallium has good binding capacity with oxygen at high temperature, so the preparation of Ga2O3 usually needs to be carried out at high temperature. In addition, Y.M. Juan et al. synthesized Ga2O3 nanowires by heating gallium nitride / sapphire template using vapor-liquid-solid mechanism. The specific operation process includes: (1) immerse the gallium nitride / sapphire in a dilute hydrochloric acid-water solution to remove the natural oxide layer on the surface and purify it with argon gas. (2) Grow Ga2O3 nanowires at high temperature. The temperature in the furnace is increased to 500℃ at a rate of 30℃ / min, then O2 gas is introduced into the system, and the furnace temperature is increased to 1050℃. (3) After the growth of Ga2O3 nanowires, the furnace is naturally cooled to room temperature. As can be seen, the traditional processing method has high energy consumption, complex steps, tedious process and high required preparation temperature. Tsai et al. proposed a metal organic chemical vapor deposition growth method to form Ga2O3 nanowires as a sensitive material to respond to humidity changes, and studied the response characteristics of the humidity sensor under the condition of 42% to 92% relative humidity (RH). However, the response time and recovery time of the device to humidity need to be improved, which cannot meet the actual application requirements. In summary, the traditional preparation method usually has high annealing temperature, complex manufacturing process and diverse material system, which hinders its practical application.

[0005] Therefore, the present application proposes to use oxide-wrapped liquid metal nanoparticles to form a sensing layer, and to use the oxide to realize active sensing in response to humidity changes. In addition, the present application uses one-step laser direct writing technology to prepare a capacitive liquid metal-based humidity sensor. The preparation process is simple in operation, low in energy consumption, energy-saving and emission-reducing, green and environmentally friendly, and the humidity sensor has good response time and recovery characteristics. Due to the photo-thermal effect of ultraviolet laser, the oxide-wrapped liquid metal nanoparticles can be selectively sintered and converted from insulating wires to conductive wires, with a resistivity of 0.15-0.19 Ω·cm, while the untreated area is an active sensing layer responding to humidity changes. Under 95% RH, the humidity sensor shows highly stable performance, fast response and recovery time. With these superior properties, the humidity sensor of the present application can monitor the human respiratory frequency and the palm skin humidity under different physiological states, and is used for medical health monitoring. SUMMARY

[0006] The present application aims at the deficiencies of the prior art, and provides a preparation method of a capacitive liquid metal-based humidity sensor and a method for improving the conductivity of a flexible electronic device.

[0007] The technical scheme of the present application is as follows:

[0008] Step S1: at room temperature, the liquid metal is dispersed in an ethanol dispersion solution for ultrasonic treatment to achieve sufficient dispersion, and meanwhile the liquid metal is oxidized to obtain a liquid metal nanoparticle ethanol dispersion solution with a diameter of 100-300 nm; the liquid metal nanoparticles are particles with an outer layer of liquid metal wrapped by an oxide.

[0009] Preferably, the mass-volume ratio of the liquid metal to the ethanol dispersion solution is 150-250 mg: 10-15 ml.

[0010] Preferably, the ethanol dispersion solution is an ethanol solution with a volume content of 99%.

[0011] Preferably, the power of the ultrasonic treatment is 15-30 W, and the ultrasonic treatment time is 10-30 min.

[0012] Preferably, the liquid metal is a gallium-indium alloy, and the weight ratio of Ga to In in the gallium-indium alloy is 75:25.

[0013] Step S2: a polyimide film is used as a flexible substrate and placed on a heating table at 60-80°C; then the liquid metal nanoparticle dispersion solution is uniformly dropped onto the upper surface of the polyimide substrate, and after the ethanol in the liquid metal nanoparticle dispersion solution is volatilized, the liquid metal nanoparticles are left, and the liquid metal nanoparticle layer is formed after repeated drop coating.

[0014] Preferably, the thickness of the polyimide film is 100-125 µm.

[0015] Step S3: according to a specified pattern, the liquid metal nanoparticle layer of the polyimide film is processed by using a UV laser direct writing technology in a one-step method to obtain a capacitive liquid metal-based flexible humidity sensor; due to the photo-thermal effect of the UV laser, the oxide on the surface of the upper layer of the liquid metal nanoparticle layer is sintered, so that the upper layer of the liquid metal nanoparticles without the surface oxide forms a conductive network, so that even if the surface of the subsequent liquid metal nanoparticle layer is oxidized, it can also realize conduction; the area that is not sintered has an upper layer composed of liquid metal particles wrapped by an oxide, and the oxide is used to realize active sensing in response to humidity changes.

[0016] The ultraviolet laser processing parameters are: a scanning speed of 350mm / s-550mm / s, a laser wavelength of 355μm, and a laser flux of 2.8-9.4J / cm 2 .

[0017] Another object of the present application is to provide a capacitive liquid metal-based flexible humidity sensor.

[0018] Yet another object of the present application is to provide the application of the capacitive liquid metal-based flexible humidity sensor in wearable sensors, or monitoring the respiratory rate of the human body, or the palm skin humidity under different physiological states.

[0019] The beneficial effects of the present application are:

[0020] The present application proposes to use liquid metal nanoparticles with surface layer oxides removed as a conductive network, which is composed of liquid metal particles wrapped in oxides, and uses oxides to achieve active sensing in response to humidity changes, which can make the humidity sensor more miniaturized, significantly improve the detection response time (about 1.2s) and recovery time (about 1.6s), and further improve the performance of flexible electronic devices.

[0021] The present application proposes an ultraviolet laser one-step direct writing technology to realize the simple preparation of liquid metal in a humidity sensor, and the laser processing parameters (scanning speed, laser wavelength, laser flux) need to be controlled at appropriate parameters to enable the oxides on the surface of the upper layer of liquid metal nanoparticles of reasonable thickness to be sintered; the liquid metal nanoparticles wrapped in oxides can be selectively sintered and converted from insulating wires to conductive wires, with a resistivity of 0.15-0.19Ω·cm.

[0022] Compared with traditional humidity sensors, the present application uses ultraviolet laser one-step direct writing technology to prepare flexible humidity sensors, which simplifies the processing flow of flexible humidity sensors, improves the conductivity of flexible thin film wires, improves the stability, fast response and recovery time of flexible humidity sensors, and thus improves the performance of flexible electronic skin sensors. In addition, the minimum resolution of the laser wire preparation is about 48μm, which can realize fine and miniaturized processing. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 SEM images of the sintered and unsintered liquid metal nanoparticle regions of the ultraviolet laser, where the area above the black dashed line is the unsintered liquid metal nanoparticle, and the area below the black dashed line is the sintered liquid metal particle by ultraviolet laser.

[0024] Figure 2 Distribution histogram of the diameter size of the unsintered liquid metal nanoparticles.

[0025] Figure 3Relationship between resistivity and laser flux parameters in the conductive path of laser-induced liquid metal nanoparticles.

[0026] Figure 4 At a laser flux of 9.4 J / cm 2 The minimum resolution of the prepared conductive liquid metal path.

[0027] Figure 5 Image of a U-shaped flexible liquid metal wire.

[0028] Figure 6 A schematic diagram of the interdigitated electrodes of a flexible humidity sensor.

[0029] Figure 7 Long-term stability measurement of the flexible humidity sensor at 95% RH (50 cycles).

[0030] Figure 8 The study monitors the subject's nasal breathing rate, sensor reaction time, and recovery time in real time while the subject is at rest. Detailed Implementation

[0031] As mentioned above, in view of the shortcomings of the prior art, the inventors of this case, through long-term research and extensive practice, proposed the technical solution of this invention, the main basis of which includes at least the following: 1) This invention proposes to use liquid metal nanoparticles with the surface oxide removed as a conductive network, which is composed of liquid metal particles wrapped with oxides. The active sensing of humidity changes is achieved by using oxides, which can make the humidity sensor more miniaturized and significantly improve the detection response time (about 1.2s) and recovery time (about 1.6s), further improving the performance of flexible electronic devices. 2) This invention proposes a one-step ultraviolet laser direct writing technology to simplify the preparation of liquid metal in humidity sensors. The laser processing parameters (scanning speed, laser wavelength, laser flux) need to be controlled at appropriate parameters to achieve the sintering of oxides on the surface of the upper layer of liquid metal nanoparticles of a reasonable thickness. The liquid metal nanoparticles wrapped with oxides can be selectively sintered and transformed from insulating wires into conductive wires with a resistivity of 0.15~0.19Ω·cm.

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0034] A method for fabricating a capacitive liquid metal-based humidity sensor includes the following steps:

[0035] Step S1: at room temperature, liquid metal is dispersed in ethanol dispersion solution and is subjected to ultrasonic treatment to achieve sufficient dispersion, while the liquid metal is oxidized, to obtain liquid metal nanoparticle ethanol dispersion solution with a diameter of 100-300 nm; the liquid metal nanoparticles are particles with an outer layer of liquid metal wrapped by its oxide.

[0036] Preferably, the mass-volume ratio of the liquid metal to the ethanol dispersion solution is 150-250 mg: 10-15 ml.

[0037] Preferably, the ethanol dispersion solution is an ethanol solution with a volume content of 99%.

[0038] Preferably, the ultrasonic treatment is performed at a power of 15-30 W for 10-30 min.

[0039] Preferably, the liquid metal is a gallium-indium alloy, and the weight ratio of Ga to In in the gallium-indium alloy is 75:25.

[0040] Step S2: polyimide film is used as a flexible substrate and is placed on a heating table at 60-80°C; then the liquid metal nanoparticle dispersion solution is uniformly dropped onto the upper surface of the polyimide substrate, and after the ethanol in the liquid metal nanoparticle dispersion solution is volatilized, the liquid metal nanoparticles are left behind; the process of dropping and coating is repeated multiple times to form a liquid metal nanoparticle layer.

[0041] Preferably, the thickness of the polyimide film is 100-125 μm.

[0042] Step S3: according to a specified pattern, the liquid metal nanoparticle layer of the polyimide film is processed by using a direct writing technology of ultraviolet laser in one step to obtain a capacitive liquid metal-based flexible humidity sensor; due to the photo-thermal effect of the ultraviolet laser, the oxide on the surface of the upper layer of the liquid metal nanoparticle layer is sintered, causing the upper layer of the liquid metal nanoparticles without the surface oxide to form a conductive network, so that even if the surface of the subsequent liquid metal nanoparticle layer is oxidized, it can also achieve conductivity; the area that is not sintered has an upper layer of liquid metal particles wrapped by oxide, and the oxide is used to achieve active sensing in response to humidity changes.

[0043] The processing parameters of the ultraviolet laser are as follows: the scanning speed is 350-550 mm / s, the laser wavelength is 355 μm, and the laser flux is 2.8-9.4 J / cm 2 .

[0044] The application is described in detail below by way of examples, but it should not be understood as a limitation of the application.

[0045] Example 1: Preparation of Ga2O3 / liquid metal-based humidity sensor

[0046] S1 Preparation of liquid metal nanoparticle dispersion

[0047] Take 150 mg of liquid metal (gallium-indium alloy with a weight ratio of Ga to In of 75:25) and 10 ml of ethanol (99%) as the dispersion. At room temperature, the liquid metal ethanol dispersion is subjected to ultrasonic treatment with a total power of 15 W. In order to obtain fully dispersed liquid metal nanoparticles, the dispersion solution is subjected to ultrasonic treatment for 10 min. The size distribution histogram of the liquid metal nanoparticles after ultrasonic treatment is shown in Figure 2 .

[0048] S2 Preparation of liquid metal nanoparticle layer

[0049] A polyimide film with a thickness of 100 μm is selected as the flexible substrate, and the polyimide film is placed on a heating table at 60°C. The well-dispersed liquid metal ethanol dispersion is uniformly dropped onto the polyimide substrate using a disposable dropper. After the evaporation of the ethanol solution, the liquid metal nanoparticles are left behind. After repeated drop coating, a liquid metal nanoparticle layer is formed.

[0050] S3 Preparation of liquid metal conductive path by ultraviolet laser

[0051] A U-shaped pattern (see Figure 5 ) required for processing the electrode is drawn on the CorelDraw software matched with the ultraviolet laser. A U-shaped electrode with a total length of 45 mm and a width of 13 mm is designed. The polyimide film with the deposited liquid metal nanoparticle layer is processed using the ultraviolet laser. The parameters of the ultraviolet laser in the final processing process are as follows: the scanning speed is fixed at 350 mm / s, the laser wavelength is 355 μm, and the laser flux is 2.8 J / cm 2 .

[0052] As can be seen from the SEM images in Figure 1 , the area above the black dashed line is the unsintered liquid metal nanoparticles, and the right image in the first row is the SEM of the unsintered area with a magnification of 13k. The area below the black dashed line is the liquid metal particles after ultraviolet laser sintering. The second row is the SEM of the liquid metal particles after ultraviolet laser sintering with magnifications of 700x and 13k, respectively.

[0053] S4 Conductivity test

[0054] After the preparation of the liquid metal wire (i.e., the sintered area), a digital multimeter is used to test the conductivity of the wire. As shown in Figure 3 , the relationship between the resistivity of the liquid metal nanoparticle conductive path induced by the ultraviolet laser and the laser flux parameter.

[0055] Example 2: Flexible Ga2O3 / liquid metal-based humidity sensor

[0056] S1 Preparation of liquid metal nanoparticle dispersion

[0057] Take 250 mg of liquid metal (gallium-indium alloy with a weight ratio of Ga to In of 75:25), 15 ml of ethanol (99%) as the dispersion. At room temperature, the liquid metal ethanol dispersion is ultrasonicated with an ultrasonic machine with a total power of 30 W. In order to obtain fully dispersed liquid metal nanoparticles, the dispersion solution is ultrasonicated for 30 min.

[0058] S2 Preparation of liquid metal nanoparticle layer

[0059] Select a polyimide film with a thickness of 125 μm as the flexible substrate, and place the polyimide film on a heating table at 80°C. Use a disposable dropper to evenly drop the dispersed liquid metal ethanol dispersion onto the polyimide substrate. After the evaporation of the ethanol solution, the liquid metal nanoparticles are left behind. Repeat the drop coating multiple times to form a liquid metal nanoparticle layer.

[0060] S3 Preparation of liquid metal conductive path by ultraviolet laser

[0061] Draw the interdigital electrode pattern of the flexible humidity sensor on the CorelDraw software matched with the ultraviolet laser (see Figure 6 ), and design the interdigital electrode with a length of 20 mm and a width of 15 mm. Deposit the polyimide film with the liquid metal nanoparticle layer using the ultraviolet laser. The ultraviolet laser parameters in the final processing process are as follows: the scanning speed is fixed at 550 mm / s, the laser wavelength is 355 μm, and the laser flux is varied within the range of 9.4 J / cm 2 As shown in Figure 4 , when the laser flux is 9.4 J / cm 2 , the minimum resolution of the prepared conductive liquid metal path is about 48 μm.

[0062] S4 Conductive performance test

[0063] After the preparation of the liquid metal wire (i.e., the sintered area), test the conductive performance of the wire using a digital multimeter.

[0064] S5 Humidity test on wearable capacitive Ga2O3 / liquid metal-based flexible humidity sensor

[0065] In the temperature and humidity oven, periodically change the humidity from 30% RH to 95% RH, and test the capacitive cycle data of the Ga2O3 / liquid metal-based humidity sensor. The test results are as follows Figure 7As shown, the flexible humidity sensor has good long-term stability at 95% RH under 50 cycles. In addition, a subject wears a commercial mask with the humidity sensor attached, and performs a mouth breathing test in a resting state, and obtains the response and recovery time of the sensor (see Figure 8 ).

[0066] The foregoing is considered as illustrative only of the principles of the application. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the application to the exact construction and practice described. Accordingly, all such variations are intended to be included within the scope of the present application as defined in the following claims.

Claims

1. A capacitive liquid metal-based humidity sensor, wherein the resistivity of the sintered region using a one-step ultraviolet laser direct writing technique is 0.15-0.19 Ω·cm, and the unsintered region has a detection response time of 1.2 s and a recovery time of 1.6 s at 95% RH, characterized in that... The humidity sensor is prepared using the following method: Step S1: At room temperature, the liquid metal is dispersed in an ethanol dispersion and subjected to ultrasonic treatment to achieve full dispersion. Simultaneously, the liquid metal undergoes oxidation to obtain a dispersion of liquid metal nanoparticles with a diameter of 100 nm to 300 nm. The liquid metal nanoparticles are particles in which the outer layer of the liquid metal is coated with its oxide. The liquid metal is a gallium-indium alloy. The ethanol dispersion is a 99% ethanol solution by volume. Step S2: Use a polyimide film as a flexible substrate and place it on a heating stage at 60℃~80℃; then uniformly drop a liquid metal nanoparticle dispersion onto the surface of the polyimide substrate. After the ethanol in the liquid metal nanoparticle dispersion evaporates, liquid metal nanoparticles remain. Repeat the drop coating process multiple times to form a liquid metal nanoparticle layer. Step S3: According to the specified pattern, a liquid metal nanoparticle layer of polyimide film is processed in one step using ultraviolet laser direct writing technology to obtain a capacitive liquid metal-based flexible humidity sensor. Due to the photothermal effect of the ultraviolet laser, the oxide on the surface of the upper liquid metal nanoparticle layer is sintered, causing the liquid metal nanoparticles with the surface oxide removed to form a conductive network, transforming the insulating wires into conductive wires. This allows the liquid metal nanoparticle layer to remain conductive even if oxidation occurs on the surface. The unsintered areas are composed of liquid metal particles encapsulating oxides, which enable active sensing in response to humidity changes. The ultraviolet laser processing parameters are: scanning speed of 350 mm / s to 550 mm / s, laser wavelength of 355 μm, and laser flux of 2.8 to 9.4 J / cm².

2. The humidity sensor according to claim 1, characterized in that... In step S1, the mass-to-volume ratio of the liquid metal to the ethanol dispersion is (150 mg~250 mg): (10 ml~15 ml).

3. The humidity sensor according to claim 1, characterized in that... In step S1, the ultrasonic treatment power is 15 W to 30 W, and the ultrasonic treatment time is 10 min to 30 min.

4. The humidity sensor according to claim 1, characterized in that... In step S1, the weight ratio of Ga to In in the gallium-indium alloy is 75:

25.

5. The humidity sensor according to claim 1, characterized in that... In step S2, the thickness of the polyimide film is 100 μm to 125 μm.

6. The humidity sensor according to any one of claims 1-5 is used in wearable sensors, or in monitoring human respiratory rate, or in measuring the humidity of the palm skin under different physiological conditions.

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