A method for manufacturing a sandwich-structured capacitive humidity sensor

Abstract

This invention discloses a method for fabricating a sandwich-structured capacitive humidity sensor, comprising the following steps: S1, preparing a humidity-sensitive thin film layer: adding multi-walled carbon nanotube (MWCNT) dispersion and zinc oxide (ZnO) dispersion to a hydrophilic polyacrylamide (PAM) dispersion and mixing, followed by drying to obtain a humidity-sensitive thin film layer; S2, preparing upper and lower electrode layers: using foamed nickel as the electrode layer material, cutting the foamed nickel to obtain sheet-like upper and lower electrode layers; S3, encapsulation: laying the humidity-sensitive thin film layer flat between the upper and lower electrode layers, then encapsulating it with polyimide tape, leaving at least one exposed area in contact with air, to obtain a sandwich-structured capacitive humidity sensor. The humidity sensor prepared by this invention has good repeatability, high long-term stability, low hysteresis, and is less affected by temperature, exhibiting higher sensitivity in high humidity environments.

Description

Technical Field

[0001] This invention relates to the field of humidity sensor technology, specifically to a method for fabricating a sandwich-structured capacitive humidity sensor. Background Technology

[0002] Humidity is a physical quantity that describes the degree of dryness or wetness of air. Humidity monitoring plays a crucial role in food storage, electronics manufacturing, agricultural planting, and textile production to ensure that production operations are conducted in an environment with appropriate humidity. With further research into humidity detection technology, traditional hair hygrometers and wet-bulb hygrometers, based on fixed principles, have been gradually phased out in many applications due to their inability to meet electronic requirements. Humidity sensors manufactured using semiconductor technology have become mainstream due to their low cost and high performance. Most humidity sensors are detection devices that convert environmental humidity information into electrical signals based on a humidity-sensitive element. The principle is that when the moisture-sensitive medium on the element comes into contact with water molecules in the air, adsorption or desorption occurs, causing a change in its moisture-sensing characteristic. This change is usually a non-electrical quantity, so a signal processing circuit is needed to convert this non-electrical quantity into an electrical quantity. Currently, humidity sensors on the market are mainly divided into resistive and capacitive types.

[0003] Resistive humidity sensors are easy to manufacture, thus dominating the low-end humidity sensor market due to their low cost. Their main problems are:

[0004] (1) The humidity sensing range is small, and the humidity sensing material used in resistive humidity sensors is usually not sensitive to low humidity.

[0005] (2) Long response time. Most of the current market products have a response time of more than 60 seconds, which is due to their moisture-sensing mechanism;

[0006] (3) The measurement accuracy is low, the product consistency is poor and it is easily affected by dust, etc., resulting in poor repeatability;

[0007] Resistive humidity sensors rely on charge transfer between the sensing layer and the contact electrode, which is susceptible to flicker noise and incomplete charge recovery. In contrast, capacitive humidity sensors primarily detect changes in the dielectric properties of the sensing layer and do not involve any charge transfer, resulting in faster response times, higher linearity of the output characteristic curve, and lower power consumption.

[0008] The sensing medium of capacitive humidity sensors is mainly composed of polymers and porous oxides. Among them, polyimide (PI) has become the primary choice for the sensing medium of capacitive humidity sensors due to its advantages such as high temperature resistance, compatibility with CMOS processes, and suitability for mass production. Polyimide thin-film capacitive humidity sensors have high linearity and fast response speed, but they also have the following shortcomings:

[0009] (1) The sensitivity is low. With the trend of sensor miniaturization, the capacitance change corresponding to a unit humidity change is getting smaller and smaller. The lower the sensitivity, the higher the requirements for the capacitance signal detection circuit.

[0010] (2) Water molecules tend to cluster in high humidity, causing moisture stagnation, which will lead to a decrease in the detection accuracy of the sensor;

[0011] (3) Capacitive humidity sensors have poor temperature independence, and changes in ambient temperature can easily cause the detection capacitance value to drift.

[0012] In recent years, with the rapid development of nanomaterials technology, carbon nanotubes (CNTs), silver nanowires, two-dimensional graphene, nano-metal oxides, and various organic / inorganic composite materials have been used to prepare capacitive humidity sensors. Compared with humidity sensors made of traditional moisture-sensitive materials, humidity sensors based on nanomaterials have advantages in sensitivity and response time. Nanomaterials have the characteristics of high variable area-to-volume ratio and high specificity. Changes in environmental humidity will cause changes in the ionic and electronic states of the surface or cross-section of nanomaterials, and the changes are fast and highly sensitive. Therefore, humidity sensors made of nanomaterials can utilize the interface effect of nanomaterials to improve the thermal stability, chemical stability, sensitivity, and selectivity of the sensor. However, the use of nanomaterials to manufacture capacitive humidity sensors also faces many problems, such as poor product consistency, weak long-term stability, and high cost of some raw materials. Summary of the Invention

[0013] The technical problem to be solved by the present invention is to overcome the existing defects and provide a method for preparing a sandwich structure capacitive humidity sensor, which has good repeatability, high stability in long-term use, low humidity hysteresis, little influence from temperature, and higher sensitivity in high humidity environments, and can effectively solve the problems in the background art.

[0014] To achieve the above objectives, the present invention provides the following technical solution: a method for fabricating a sandwich-structured capacitive humidity sensor, comprising the following steps:

[0015] S1. Preparation of humidity-sensitive thin film layer:

[0016] Add 0.02 mol acrylamide (AM), 0.015 g N,N'-methylenebisacrylamide (MBA), and 0.015 g potassium persulfate crosslinking agent (KPS) to 10 ml of deionized water and mix to prepare a hydrophilic polyacrylamide (PAM) dispersion;

[0017] Add 0.1 ml of multi-walled carbon nanotube (MWCNT) dispersion and 0.1 ml of zinc oxide (ZnO) dispersion to the prepared polyacrylamide dispersion, and stir on a magnetic stirrer for 20 min to obtain PAM / MWCNT / ZnO composite dispersion.

[0018] The prepared PAM / MWCNT / ZnO composite dispersion was poured into a 5×5cm mold plate with a depth of 3mm, placed in a vacuum drying oven for vacuum heating and drying, and then the dried film was taken out and cut into a 3×3cm, 3mm thick moisture-sensitive film layer.

[0019] S2. Preparation of upper and lower electrode layers:

[0020] Using foamed nickel as the electrode layer material, the foamed nickel was cut to make an upper electrode layer and a lower electrode layer with a thickness of 2 mm and a diameter of 3×3 cm.

[0021] S3, Packaging:

[0022] A humidity-sensitive thin film layer is laid flat to cover the lower electrode layer, and then an upper electrode layer is laid flat to cover the humidity-sensitive thin film layer. Corresponding wires are connected to both the upper and lower electrode layers. The upper electrode layer, humidity-sensitive thin film layer, and lower electrode layer are then wrapped and encapsulated with polyimide tape, leaving at least one exposed area in contact with air, to obtain a sandwich structure capacitive humidity sensor.

[0023] Preferably, in step S1, the concentration of the multi-walled carbon nanotube dispersion is 8 mg / ml, and the concentration of the ZnO dispersion is 8 mg / ml.

[0024] Preferably, in step S1, the vacuum drying oven is first evacuated for 20 minutes, the temperature of the vacuum drying oven is set to 70°C, and drying is carried out for 60 minutes.

[0025] Preferably, step S1 further includes detecting the prepared humidity-sensitive film layer and performing Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analysis on the humidity-sensitive film layer.

[0026] Compared with the prior art, the beneficial effects of this invention are as follows: The preparation of the humidity-sensitive thin film layer utilizes acrylamide (AM) to initiate cross-linking under the action of initiator N,N'-methylenebisacrylamide (MBA) and potassium persulfate cross-linking agent KPS, forming a PAM hydrogel cross-linked network structure, which greatly increases the water absorption rate. After adding multi-walled carbon nanotubes, they are distributed throughout the film surface. The layered structure of the multi-walled carbon nanotubes increases the porosity of the film surface, further improving the water absorption rate. The addition of zinc oxide, with its layered structure and large specific surface area, increases the adsorption sites for water molecules, making it more conducive to water molecule adsorption. The prepared humidity sensor exhibits higher sensitivity in high humidity environments; it has low humidity hysteresis characteristics, resulting in excellent humidity detection performance; the response time and recovery time are 70s and 140s, respectively; and it possesses excellent long-term stability and repeatability. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the humidity sensor structure of the present invention;

[0028] Figure 2 This is an FTIR image of the humidity-sensitive thin film layer of the present invention;

[0029] Figure 3 This is an EDS image of the humidity-sensitive thin film layer of the present invention;

[0030] Figure 4 This is a graph showing the moisture absorption and desiccation curves of the humidity sensor of the present invention.

[0031] Figure 5 This is a humidity hysteresis curve diagram of the humidity sensor of the present invention;

[0032] Figure 6 This is a graph showing the capacitance change of the humidity sensor of the present invention at different temperatures;

[0033] Figure 7 This is a graph showing the response / recovery time of the humidity sensor of the present invention.

[0034] Figure 8 This is a repeatability analysis curve of the humidity sensor of the present invention;

[0035] Figure 9 This is a graph showing the stability changes of the humidity sensor of the present invention. Implementation

[0036] The technical solution of the present invention will now be described with reference to the accompanying drawings of the embodiments of the present invention. In the description, it should be understood that the terms "upper," "lower," "front," "rear," "left," and "right," etc., indicating directions or positional relationships, are only used to correspond to the accompanying drawings of the present invention for the purpose of facilitating the description of the present invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation.

[0037] Please see Figure 1-9 This invention provides a technical solution: a method for preparing a sandwich-structured capacitive humidity sensor, comprising the following steps:

[0038] S1. Preparation of humidity-sensitive thin film layer:

[0039] Add 0.02 mol acrylamide (AM), 0.015 g N,N'-methylenebisacrylamide (MBA), and 0.015 g potassium persulfate crosslinking agent (KPS) to 10 ml of deionized water and mix to prepare a hydrophilic polyacrylamide (PAM) dispersion;

[0040] Add 0.1 ml of multi-walled carbon nanotube (MWCNT) dispersion and 0.1 ml of zinc oxide (ZnO) dispersion to the prepared polyacrylamide dispersion, and stir on a magnetic stirrer for 20 min to obtain PAM / MWCNT / ZnO composite dispersion.

[0041] The prepared PAM / MWCNT / ZnO composite dispersion was poured into a 5×5cm mold plate with a depth of 3mm, placed in a vacuum drying oven for vacuum heating and drying, and then the dried film was taken out and cut into a 3×3cm, 3mm thick moisture-sensitive film layer.

[0042] S2. Preparation of upper and lower electrode layers:

[0043] Using foamed nickel as the electrode layer material, the foamed nickel was cut to make an upper electrode layer and a lower electrode layer with a thickness of 2 mm and a size of 3×3 cm.

[0044] S3, Packaging:

[0045] A humidity-sensitive thin film layer is laid flat to cover the lower electrode layer, and then an upper electrode layer is laid flat to cover the humidity-sensitive thin film layer. The upper electrode layer and the lower electrode layer are connected with wires. Then, the upper electrode layer, the humidity-sensitive thin film layer and the lower electrode layer are wrapped and encapsulated with polyimide tape, leaving at least one exposed area in contact with air, to obtain a sandwich structure capacitive humidity sensor.

[0046] A humidity-sensitive membrane (PMZ) was prepared using multi-walled carbon nanotubes (MWCNTs), zinc oxide (ZnO), and hydrophilic polyacrylamide (PAM). This PMZ exhibits high water absorption and increased porosity on its surface, creating more adsorption sites for water molecules and thus enhancing water adsorption. Based on the PMZ, a sandwich-structured capacitive humidity sensor was fabricated using nickel foam as electrodes. This sensor demonstrates higher sensitivity, lower humidity hysteresis, excellent humidity detection performance, and superior long-term stability and repeatability.

[0047] like Figure 1As shown, the specific structure of the sandwich structure capacitive humidity sensor includes an upper electrode, a humidity-sensitive thin film layer, and a lower electrode layer, as well as corresponding wires connecting the upper electrode layer and the lower electrode layer. It should be noted that during the encapsulation process, the polyimide tape does not completely cover the upper electrode layer, the humidity-sensitive thin film layer, and the lower electrode layer. An exposed area that comes into contact with air needs to be reserved to allow moisture in the air to enter the humidity-sensitive thin film layer. Since both the upper electrode layer and the lower electrode layer are made of foamed nickel, which is also breathable, the exposed area can be set on the upper part of the upper electrode layer, the lower part of the lower electrode layer, or the side, which can ensure that air enters the humidity-sensitive thin film layer.

[0048] Furthermore, in step S1, the concentration of the multi-walled carbon nanotube dispersion is 8 mg / ml, and the concentration of the ZnO dispersion is 8 mg / ml; in step S1, the vacuum drying oven is first evacuated for 20 min, the temperature of the vacuum drying oven is set to 70℃, and drying is carried out for 60 min.

[0049] It also includes testing the prepared humidity-sensitive film layer, and performing Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analysis on the humidity-sensitive film layer;

[0050] like Figure 2 and Figure 3 As shown, FTIR analysis reveals that in the PAM infrared spectrum, the characteristic absorption peak of associated -NH2 is at 3344, the characteristic absorption peak of carbonyl is at 1662, attributed to amide I, C=O stretching vibration, and the characteristic absorption peak of amide II, N-H bending vibration, is at 1613. The characteristic peaks of the PAM / MWCNT / ZnO film show little change, indicating successful preparation of the humidity-sensitive film. EDS / morphology analysis shows that the morphology of the humidity-sensitive film surface magnified 1000 times in the left image reveals surface wrinkling, increasing the contact area with water molecules in the air. EDS surface scanning analysis, using an electron beam to scan the film surface, shows that the Zn element on the humidity-sensitive film surface appears white on the screen. This indicates that ZnO is uniformly distributed in the film and its content is low, consistent with the added ZnO content, further proving the successful preparation of the humidity-sensitive film.

[0051] like Figure 4As shown, the prepared humidity sensor was tested. Before each measurement, the humidity sensor was placed in an atmospheric humidity environment for 30 minutes to ensure stability. It can be seen that during the moisture absorption and dehumidification process, the response of the humidity sensor is linearly related to the relative humidity. The sensitivity is 1.33pF / %RH in the detection range of 20-90%RH, the capacitance is almost unchanged in the range of 20-30%RH, the sensitivity is 0.63pF / %RH in the range of 30-70%RH, and the sensitivity is 3.4pF / %RH in the range of 70-90%RH. It can be concluded that the humidity sensor has higher sensitivity in high humidity environments.

[0052] like Figure 5 As shown, the maximum hysteresis of the prepared humidity sensor is 2.26%RH, which indicates that the humidity sensor has low humidity-sensitive hysteresis characteristics and excellent humidity detection performance.

[0053] like Figure 6 As shown, to study the effect of temperature on the prepared humidity sensor, the humidity response at 70% RH was tested at temperatures of 25°C, 30°C, 35°C, 40°C, 45°C, and 50°C to investigate the effect of temperature on the humidity sensor. It can be concluded that the capacitance of the humidity sensor exhibits a linear change as the temperature increases, which means that the humidity sensor has excellent linearity and negligible temperature dependence, indicating that it is suitable for working in harsh environments in high-temperature regions.

[0054] like Figure 7 As shown, the response time and recovery time refer to the time when the humidity changes from 30%RH to 90%RH and from 90%RH to 30%RH, respectively. It can be seen that the response time and recovery time of this humidity sensor are 70 seconds and 140 seconds, respectively.

[0055] like Figure 8 As shown, the prepared humidity sensor was subjected to six alternating cyclic tests in an environment with humidity ranging from 20% to 90%. The results showed that the humidity sensor has good repeatability, with a maximum repeatability deviation of 2.5% across the entire relative humidity range.

[0056] like Figure 9 As shown, the prepared humidity sensor was placed at relative humidity of 30%, 50%, 70%, and 90% for 120 hours. It can be seen that the test capacitance remained stable. Under continuous testing for 120 hours, the maximum error rate of the humidity sensor's stability was 3%RH, which proves the excellent long-term stability of the humidity sensor.

[0057] The parts of this invention not described in detail are prior art. It will be apparent to those skilled in the art that this invention is not limited to the details of the above exemplary embodiments, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the above embodiments should be regarded as exemplary and non-limiting in all respects. The scope of this invention is defined by the appended claims rather than the foregoing description. Therefore, it is intended to include all changes that fall within the meaning and scope of the equivalents of the claims within this invention, and no reference numerals in the claims should be regarded as limiting the content of the claims.

Claims

1. A method for fabricating a sandwich-structured capacitive humidity sensor, characterized in that: Includes the following steps: S1. Preparation of humidity-sensitive thin film layer: Add 0.02 mol acrylamide (AM), 0.015 g N,N'-methylenebisacrylamide (MBA), and 0.015 g potassium persulfate crosslinking agent (KPS) to 10 ml of deionized water and mix to prepare a hydrophilic polyacrylamide (PAM) dispersion; Add 0.1 ml of multi-walled carbon nanotube (MWCNT) dispersion and 0.1 ml of zinc oxide (ZnO) dispersion to the prepared polyacrylamide dispersion, and stir on a magnetic stirrer for 20 min to obtain PAM / MWCNT / ZnO composite dispersion. The prepared PAM / MWCNT / ZnO composite dispersion was poured into a 5×5cm mold plate with a depth of 3mm, placed in a vacuum drying oven for vacuum heating and drying, and then the dried film was taken out and cut into a 3×3cm, 3mm thick moisture-sensitive film layer. S2. Preparation of upper and lower electrode layers: Using foamed nickel as the electrode layer material, the foamed nickel was cut to make an upper electrode layer and a lower electrode layer with a thickness of 2 mm and a diameter of 3×3 cm. S3, Packaging: A humidity-sensitive thin film layer is laid flat to cover the lower electrode layer, and then an upper electrode layer is laid flat to cover the humidity-sensitive thin film layer. Corresponding wires are connected to both the upper and lower electrode layers. The upper electrode layer, humidity-sensitive thin film layer, and lower electrode layer are then wrapped and encapsulated with polyimide tape, leaving at least one exposed area in contact with air, to obtain a sandwich structure capacitive humidity sensor.

2. The method for fabricating a sandwich-structure capacitive humidity sensor according to claim 1, characterized in that: In step S1, the concentration of the multi-walled carbon nanotube dispersion is 8 mg / ml, and the concentration of the ZnO dispersion is 8 mg / ml.

3. The method for fabricating a sandwich-structure capacitive humidity sensor according to claim 1, characterized in that: In step S1, the vacuum drying oven is first evacuated for 20 minutes, the temperature of the vacuum drying oven is set to 70°C, and drying is carried out for 60 minutes.

4. The method for fabricating a sandwich-structure capacitive humidity sensor according to claim 1, characterized in that: Step S1 also includes detecting the prepared humidity-sensitive film layer and performing Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analysis on the humidity-sensitive film layer.

Images