LiCl-based flexible humidity sensor and preparation method thereof

Abstract

The invention discloses a LiCl-based flexible humidity sensor and a preparation method thereof, and belongs to the technical field of flexible sensors, and the preparation method comprises the following steps: mixing anhydrous lithium chloride and a polyimide solution according to a certain proportion, and stirring at normal temperature to form a mixed solution; uniformly coating a polyimide flexible substrate with the mixed solution, and drying the polyimide flexible substrate in a drying oven; and performing laser engraving on the surface of the dried film to form a laser-induced graphene interdigital electrode, thereby completing the manufacturing of the flexible humidity sensor. The flexible humidity sensing system adopts a novel nano composite material and a porous microstructure design, and can be tightly attached to the surfaces of objects in different shapes. The system is based on a capacitance change principle, can sensitively detect environment humidity change, and has good repeatability and stability. By optimizing the substrate material ratio and the electrode structure, accurate capture of weak signals such as respiratory airflow and skin surface humidity is realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of flexible sensors, and particularly relates to a flexible humidity sensor based on LiCl and a preparation method thereof. BACKGROUND

[0002] As an important representative of new generation sensing technology, flexible humidity sensors are showing great application potential in medical monitoring, environmental perception and other fields. Compared with traditional rigid sensors, flexible sensors can better adapt to complex surfaces and dynamic environments due to their bendable and stretchable characteristics, providing new possibilities for the development of wearable devices. This breakthrough is mainly due to the innovation of material science and the improvement of manufacturing processes, especially the development and application of new functional materials.

[0003] Among many substrate materials, lithium chloride stands out due to its unique physical and chemical properties. This inorganic salt material has excellent moisture absorption performance and ion conductivity characteristics. When the environmental humidity changes, its conductivity will change significantly and reversibly. Compared with other materials, lithium chloride has high sensitivity and can maintain stable performance in a wide humidity range.

[0004] The emergence of laser-induced graphene (LIG) technology provides an ideal solution for the substrate material of flexible sensors. This innovative technology uses laser to directly induce the formation of porous graphene structure on the surface of polymers such as polyimide, not only simplifying the complex preparation process of traditional graphene, but also realizing the direct patterning of conductive network. The unique three-dimensional porous structure of LIG greatly increases the active surface area, providing more attachment sites for humidity-sensitive materials, while its excellent conductivity ensures efficient signal transmission. This integrated preparation method lays the foundation for the mass production of flexible sensors.

[0005] Although flexible humidity sensors have made significant progress, there are still several aspects that need to be improved. In terms of materials, how to further improve the long-term stability and environmental adaptability of the sensor is a key challenge. In terms of manufacturing process, traditional flexible humidity sensors usually prepare humidity-sensitive materials and flexible substrates in steps: first, an independent humidity-sensitive film is prepared, and then it is assembled by gluing or sputtering metal electrodes. This "film-electrode secondary bonding" has two defects: first, there is a lack of chemical bonding between humidity-sensitive salts (such as LiCl) and flexible polymer substrates, and the salt layer is prone to local peeling when bending, resulting in humidity response drift; second, the metal electrode requires multiple patterning processes, which is costly and difficult to achieve micron-level interdigital structure on curved surfaces. In addition, the integration and intelligent development of sensors, as well as the collaborative design with energy harvesting, wireless transmission and other modules, are important directions for future research.

[0006] It is foreseeable that with the continuous progress of material science and micro-nano manufacturing technology, flexible humidity sensors will play an important role in more fields. From intelligent medical care to environmental monitoring, from industrial control to consumer electronics, this new type of sensor is opening a new chapter of human-computer interaction and environmental perception. Future research should not only focus on the improvement of performance indicators, but also pay attention to the needs of practical application scenarios, and promote the technology from the laboratory to industrialization, creating greater value for social development. SUMMARY

[0007] The present application designs a flexible humidity sensor based on LiCl. The present application directly incorporates LiCl into polyimide, once dried into a film, and then uses CO2 laser to induce the formation of graphene interdigital electrode on the same film, so that the humidity sensitive layer, flexible substrate and conductive network form a continuous whole, which not only eliminates the risk of interface peeling, but also simplifies the process, thereby solving the stability and scaling problems of traditional devices.

[0008] The sensor not only has a simple manufacturing process, but also has a wide detection range of humidity, short response / recovery time, excellent repeatability and good anti-interference performance. This makes the humidity sensor have wide application prospects in humidity detection, respiratory monitoring and non-contact human-computer interaction fields.

[0009] To achieve the above object, the technical scheme of the present application is as follows: a flexible humidity sensor based on LiCl, comprising a flexible substrate and a laser-engraved graphene interdigital electrode on the substrate, wherein the flexible substrate is composed of anhydrous lithium chloride LiCl and polyimide PI.

[0010] Preferably, the flexible substrate is a thin film formed by mixing and drying anhydrous lithium chloride and polyimide solution. The humidity-sensitive material is uniformly mixed into the PI solution, so that the substrate has both moisture absorption characteristics and bendability, avoiding the traditional "humidity-sensitive layer-substrate" interface peeling.

[0011] Preferably, the graphene electrode is a laser-induced graphene LIG interdigital electrode engraved on the film by laser direct writing technology. The surface of the PI film is printed by laser to generate porous graphene, thereby forming a conductive grid to transmit electrical signals.

[0012] Preferably, a preparation method of a flexible humidity sensor based on LiCl, the humidity-sensitive layer, the flexible substrate and the electrode are formed synchronously, reducing the cost and thickness. The method comprises the following steps: Step one, preparing a substrate solution, comprising: uniformly mixing anhydrous lithium chloride and polyimide solution according to a certain mass ratio to obtain a LiCl / PI composite solution; Step two, preparing a flexible substrate film, comprising: fixing the flexible PI film on a substrate, uniformly coating the mixed solution, and placing it in an oven to dry; Step three, preparation of LIG interdigital electrode, including: introducing the designed electrode pattern into the operation software, setting the carving parameters of the laser direct writing system, laser-induced carving of the LIG interdigital electrode, leading out at both ends of the electrode with copper tape, coating conductive silver paste and drying, completing the production of the flexible humidity sensor.

[0013] Preferably, step one includes: preparing a substrate solution, first, uniformly mixing anhydrous lithium chloride and a polyimide solution according to a mass ratio of 0.02:1, then placing it in a magnetic stirrer for stirring at room temperature for 2 hours to make the lithium chloride fully dissolved, obtaining a LiCl / PI composite solution, which not only ensures the hygroscopicity of the substrate, but also avoids salt aggregation, and takes into account sensitivity and flexibility; Preferably, step two includes: preparing a flexible substrate film, first, fixing the PI film with a thickness of 75 μm on the substrate to avoid cracks caused by shrinkage after drying. Then evenly coat the composite solution on the surface of the PI film, and then place it in an oven for drying at a temperature of 100℃ for 30 minutes, obtaining a flexible substrate film, which creates a substrate condition for laser printing; Preferably, step three includes: preparation of LIG interdigital electrode, under atmospheric conditions, using a CO2 laser direct writing system to laser induce graphene electrode on the substrate film at a carving speed of 110 mm / s and a laser power of 11 W. The single piece processing time is less than 1 minute, and the qualified rate is greater than 95%, which significantly reduces the manufacturing cost and improves the potential of batch production of flexible sensors that can be rolled and cut.

[0014] Preferably, the interdigital width of the electrode and the gap spacing between adjacent interdigital are both 400 μm, which is compatible with the minimum line width of low-cost CO2 laser, avoids processing defects caused by too narrow spacing, and provides stable process conditions for large-scale laser direct writing. The capacitance interdigital spacing and area are optimally compromised, and high sensitivity and linear output capacity are obtained within 1 cm² area. The logarithm of the interdigital electrode finger pair is 6, which maximizes the effective capacitance area and does not introduce too high series resistance, and can maintain the approximate linear relationship between capacitance change and humidity within the range of 11%-97%RH.

[0015] Preferably, step three includes: installing copper wires, using conductive silver paste to fix the copper tape at both ends of the electrode, and then placing it in an oven for baking at 100℃ for 30 minutes to ensure that the copper wires are firmly installed and establish a low-resistance lead-out interface.

[0016] Finally, the sensor is cut from the PI film and connected to the test system for detection.

[0017] Compared with the prior art, the beneficial effects of the present application are: 1) In terms of structural design, the laser-induced graphene LIG interdigital electrode structure adopts a miniaturization scheme, with an overall area of about 1 cm². Combined with a flexible substrate and a biocompatible LiCl / PI composite humidity-sensitive layer, it can perfectly fit various complex curved surfaces.

[0018] 2) In terms of performance, the humidity sensor exhibits high sensitivity. After mixing anhydrous LiCl and polyimide at a mass ratio of 0.02:1, coating, and drying at 100°C for 30 minutes, LiCl microcrystals are precipitated in the PI network, forming a continuous moisture absorption condition. When the humidity changes, the absorption / desorption of water by LiCl leads to a significant change in the dielectric constant, directly amplifying the capacitance signal. Meanwhile, a CO2 laser (110 mm / s, 11 W) is used to induce the generation of a three-dimensional porous graphene interdigital electrode (400 μm wide / gap, 6 pairs) on the same LiCl / PI film. The high specific surface area and microporous structure provide a large number of LiCl attachment sites, while reducing resistance and enhancing the response amplitude of the electric field to humidity changes. In the 11%-97% humidity range, the capacitance sensitivity is 1359.76 pF / %RH, and the signal output linearity is excellent. Experimental verification shows that this technology not only can track real-time respiratory frequency changes, but also can identify hand proximity actions, providing innovative solutions for intelligent medical care, smart wearable devices, and other fields. Its stable performance and sensitive response characteristics show good prospects for industrial application. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The overall structure of the LiCl-based flexible humidity sensor is shown; Figure 2 The scanning electron microscope (SEM) image of the LiCl / LIG composite material is shown; Figure 3 The Raman spectrum of the LiCl / LIG composite material is shown; Figure 4 The XPS spectrum of the LiCl / LIG composite material is shown; Figure 5 The response curve of the humidity sensor based on LiCl / LIG composite material in the 11-97% RH range is shown; Figure 6 The capacitance response curve of the LiCl-based flexible humidity sensor in a dynamic humidity environment is shown; Figure 7 The response time and recovery time of the sensor under specific humidity conditions are shown; Figure 8 The capacitance response curve of the sensor to human respiration is shown; Figure 9The capacitive response curves of the sensor at a distance of 2 cm and 0.5 cm from the finger are shown. DETAILED DESCRIPTION

[0020] The present application is further illustrated with reference to the accompanying drawings and detailed description, and it should be understood that the following detailed description is only used to illustrate the present application and is not used to limit the scope of the present application.

[0021] Embodiment: Reference Figure 1 The overall structure of the LiCl-based flexible humidity sensor is shown. The figure clearly depicts the design layout of the sensor, providing a basic framework for subsequent analysis. The LiCl-based flexible humidity sensor includes: a flexible substrate, a graphene interdigital electrode; the flexible substrate is LiCl / PI.

[0022] Example 1: Preparation of LiCl / PI substrate solution: 1) Mix anhydrous lithium chloride and polyimide solution uniformly according to a mass ratio of 0.02:1; 2) Put the mixture into a magnetic stirrer and stir at room temperature for 2 hours to make the lithium chloride fully dissolved, and the preparation of the LiCl / PI composite solution is completed.

[0023] Example 2: Preparation of LiCl / PI flexible substrate film: 1) Clean the glass substrate, then take a PI film with a thickness of 75 μm and stick it flat on the glass substrate with an appropriate size using adhesive tape; 2) Uniformly scrape the LiCl / PI composite solution on the PI film with a 120 μm film applicator, then put it into an oven and dry it at a temperature of 100°C for 30 minutes to make the solution solidify, and the preparation of the LiCl / PI flexible substrate film is completed.

[0024] Example 3: Preparation of LIG interdigital electrode: Import the designed electrode pattern into the operation software, and set the engraving speed of the laser direct writing system to 110 mm / s and the engraving power to 11 W, then perform laser-induced graphene electrode to obtain the graphene interdigital electrode; Paste copper tape at both ends of the electrode, smear conductive silver paste at the connection, and put it into an oven and bake it at 100°C for 30 minutes to ensure that the copper wire is installed firmly, and the preparation of the LiCl-based flexible humidity sensor is completed.

[0025] Example 5: Characterization of LiCl-based flexible humidity sensor: Figure 2 The microstructure characteristics of the present humidity sensor are further revealed. As can be seen from the scanning electron microscope (SEM) image, the surface of the sensor presents a large number of wrinkles and pore-like structures, which make the sensor have good water absorption, providing favorable conditions for detection in a humid environment.

[0026] 2) Figure 3 The Raman spectrum of the LiCl / LIG composite material is shown. These peaks are formed due to the structural changes of polyimide during the preparation process. −1 The D band corresponds to the vibration of carbon material defects and has a weak intensity; 1583.97 cm −1 The G band is the vibration of the ordered structure of carbon materials, with high intensity; 2699.87 cm −1 The 2D band is sensitive to the number of carbon material layers and has higher strength.

[0027] 3) Figure 4 The XPS spectra of the LiCl / LIG composite material are shown. In the O 1s spectrum, C=O has a peak at 532eV, COC / H2O has a peak at 533eV, O=C-OH has a peak at 534eV, and C-OH has a peak at 531eV. These are functional groups produced by oxidation of carbon materials and oxides in the hydrolysis products of LiCl hygroscopicity. In the C 1s spectrum, CC / C=C has a peak at 284eV, indicating that graphene structure has been formed after laser treatment. CO has corresponding peaks at 284.4eV and C=O at 285.3eV, indicating that the carbon material reacts with oxygen to form oxygen-containing functional groups during the laser treatment. In the Li 1s spectrum, LiCl has a peak at 55.62 eV, which is the residue of the raw material. LiOH has peaks at 54.2eV and Li2CO3 has peaks at 56.27eV. These are the hydrolysis of LiCl to form LiOH after absorbing moisture, and further react with CO2 to form Li2CO3. The 2p spectrum shows that C-Cl has a peak at 200eV, indicating that Cl - Combined with carbon surface defects to form covalent bonds, Cl - The peak at 198.39 eV is due to the residual or adsorption of raw material LiCl.

[0028] Example 6: Humidity performance test of LiCl-based flexible humidity sensor: 1) The sensor's humidity sensitivity was examined using different saturated salt solutions to create variations in humidity. Supersaturated salt solutions (25°C) of LiCl, CH₃COOK, MgCl₂, K₂CO₃, NaBr, CuCl₂, NaCl, KCl, and K₂SO₄ were used to generate relative humidity (RH) values ​​of 11%, 23%, 33%, 43%, 58%, 68%, 75%, 85%, and 97%, respectively.

[0029] 2) The test method is: Place the humidity sensor above the salt solution, connect the sensor using an LCR digital bridge, and record the capacitance response curve of the sensor at different humidity levels.

[0030] 3)Figure 5 The focus is on the performance of humidity sensors, showing the response curves of humidity sensors based on LiCl doping at different ratios within the 11-97% RH range. This graph intuitively reflects the sensor's response characteristics under different humidity conditions, providing an important basis for evaluating its performance. Figure 5 The capacitance response curves of humidity tests for humidity sensors with LiCl mass ratios of 1.4%, 1.6%, 2%, and 2.5% are shown in the figure. The test results show that the capacitance gradually increases as the humidity increases from 11% to 97%. The test curve for the humidity sensor with a LiCl mass ratio of 1.4% has good linearity.

[0031] 4) Figure 6 Through two subgraphs ( Figure 6 a and Figure 6 b) Detailed display of the capacitance response curve of the LiCl-based flexible humidity sensor under dynamic humidity environment. Figure 6 a depicts the response change from low humidity environment to high humidity environment, and Figure 6 Figure b shows the response change from high to low humidity. These graphs provide a deeper understanding of the sensor's dynamic performance under varying humidity conditions. The test results show that Figure a shows an increase in sensor capacitance from low to high relative humidity, while Figure b shows a decrease in capacitance from high to low relative humidity, exhibiting good stability.

[0032] 5) Figure 7 The figure further shows the response time and recovery time of the sensor under specific humidity conditions. This figure takes the capacitance response of a sensor prepared in a preferred embodiment at 23% RH and 58% RH as an example to intuitively demonstrate the fast response and recovery capabilities of the sensor. Figure 7 , followed by a response time and recovery time of 70 and 100 s in the RH range of 11-97%, respectively, demonstrating that the LiCl-based flexible humidity sensor has a good response speed.

[0033] Example 7: Humidity sensor detection of human breathing and finger distance test: Figure 8 and Figure 9 The applications of sensors in human respiration detection and finger non-contact humidity detection are discussed respectively. Figure 8 The capacitance response curve of the sensor to human breathing is shown. Figure 9 The capacitive response curves of the sensor at 2 cm and 0.5 cm from the finger are shown. These graphs not only demonstrate the sensor's performance in various application scenarios, but also provide strong support for its potential applications.

[0034] 1) Keep the sensor body mouth a certain distance, and connect with LCR digital bridge to record the capacitance response curve, human exhalation and inspiration and use the sensor to monitor the breathing signal. Refer to Figure 8 When the human exhales, the environmental humidity increases, and the sensor capacitance increases; when the human inhales, the environmental humidity decreases, and the sensor capacitance decreases.

[0035] 2) Keep the sensor finger a certain distance, and connect with LCR digital bridge to record the capacitance response curve, refer to Figure 9 When the finger is 0.5 cm away from the humidity sensor, the environmental humidity increases, and the sensor capacitance increases; when the finger is 2 cm away from the humidity sensor, the environmental humidity decreases, and the sensor capacitance decreases.

[0036] It should be noted that the above content only illustrates the technical idea of the present application, and cannot limit the protection scope of the present application. For ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, which fall within the scope of the claims of the present application.

Claims

1. A flexible humidity sensor based on LiCl, characterized in that: The invention comprises a flexible substrate material and graphene interdigitated electrodes laser-engraved on the substrate, wherein the flexible substrate is a film dried from a mixed solution of lithium chloride and polyimide.

2. The LiCl-based flexible humidity sensor according to claim 1, characterized in that: The graphene electrode is a laser-induced graphene LIG interdigitated electrode engraved on the film by laser direct writing technology.

3. A method for preparing the LiCl-based flexible humidity sensor according to claims 1 to 2, characterized in that: The following steps are involved: Step 1, preparing a substrate solution, comprising: The anhydrous lithium chloride and the polyimide solution are uniformly mixed in a certain mass ratio to obtain a LiCl / PI composite solution; Step 2: preparing a flexible substrate film, including: Fix the flexible PI film on the substrate, evenly apply the mixed solution, and dry it in an oven; Step 3: preparing LIG interdigitated electrodes, including: Import the designed electrode pattern into the operating software, set the engraving parameters of the laser direct writing system, and use laser-induced engraving of LIG interdigitated electrodes. Use copper tape to lead out at both ends of the electrode, apply conductive silver paste, and dry it to complete the production of the flexible humidity sensor.

4. The method for preparing a LiCl-based flexible humidity sensor according to claim 3, wherein: Step one includes: Prepare substrate solution: First, anhydrous lithium chloride and polyimide solution are mixed according to mass ratio; Then, the mixture was placed in a magnetic stirrer and stirred thoroughly at room temperature to form a mixed solution.

5. The method for preparing a LiCl-based flexible humidity sensor according to claim 3, wherein: Step 2 includes: Preparation of flexible substrate films: First, the PI film is fixed on the substrate; Subsequently, the mixed solution was evenly coated on the PI film; Then, it is placed in an oven for drying to form a flexible substrate film.

6. The method for preparing a LiCl-based flexible humidity sensor according to claim 3, wherein: Step three includes: First, the substrate film is placed in a laser engraving machine, and a CO2 laser direct writing system is used under atmospheric conditions to laser-induce the formation of graphene electrodes on the film. Subsequently, the graphene electrodes were led out with copper tape and fixed at both ends of the electrodes with conductive silver paste; Next, place it in an oven to bake it to ensure the copper tape is firmly installed; Finally, the sensor is cut from the film and connected to the test system for testing.

7. The method for preparing a LiCl-based flexible humidity sensor according to claim 6, wherein: The interdigital width of the electrodes and the gap spacing between adjacent interdigital fingers are both 400 μm, and the number of interdigital electrode finger pairs is 6.

Images