A humidity sensor based on external electric field polarized bismuth ferrite and its preparation method and application

Abstract

The present invention relates to a humidity sensor based on external electric field polarized bismuth ferrite, and its preparation method and application. The humidity sensor based on external electric field polarized bismuth ferrite comprises: a substrate, interdigital electrodes arranged on the surface of the substrate, and a polarized bismuth ferrite humidity-sensitive film attached to the surface of the interdigital electrodes; the polarized bismuth ferrite humidity-sensitive film comprises polarized bismuth ferrite powder; preferably, the chemical formula of the polarized bismuth ferrite powder is BiFeO 3‑a , where 0<a≤0.05.

Description

Technical Field

[0001] The invention relates to a humidity sensor based on external electric field polarized bismuth ferrite and a preparation method and application thereof, belonging to the technical field of humidity sensors. Background Art

[0002] With the development of smart IoT and artificial intelligence, humidity sensors for human-related humidity detection play an important role in smart wearable devices. As the core component of humidity sensors, the performance of humidity-sensitive materials determines the quality of humidity sensors. Currently, most humidity sensors used on the human body are based on two-dimensional materials, polymer materials, and their composite materials. However, these materials often involve complex preparation processes, and their sensors have low sensitivity, long response and recovery times, and a narrow range of applications, which are not conducive to humidity sensors monitoring human-related humidity. As a typical ferroelectric material, bismuth ferrite has broad application prospects in ferroelectric storage, piezoelectric transduction, dielectric energy storage, and other fields due to its simple preparation process, large spontaneous polarization, and high Curie temperature. Summary of the Invention

[0003] To address the low sensitivity and long response and recovery times of current humidity sensors used for human-related humidity detection, the present invention provides a humidity sensor based on external electric field-polarized bismuth ferrite material, as well as its preparation method and application. Specifically, the polarized bismuth ferrite humidity sensor is prepared using polarized bismuth ferrite powder.

[0004] On the one hand, the present invention provides a humidity sensor based on external electric field polarized bismuth ferrite, which comprises: a substrate, interdigital electrodes arranged on the surface of the substrate, and a polarized bismuth ferrite humidity-sensitive film attached to the surface of the interdigital electrodes; the polarized bismuth ferrite humidity-sensitive film comprises polarized bismuth ferrite powder; preferably, the chemical formula of the polarized bismuth ferrite powder is BiFeO 3-a , where 0<a≤0.05.

[0005] The inventors have discovered for the first time that for polarized bismuth ferrite ferroelectric materials, its unique ferroelectric polarization has a strong interaction with the adsorption of water molecules on its surface. In particular, the polarized bismuth ferrite has a residual polarization strength and there is a bound charge on the surface. Due to the dissociative adsorption of water molecules on the surface, the negatively charged OH - This charge compensates for surface oxygen vacancies. Under polarization, oxygen vacancies accumulate on the surface, increasing reactive sites. The inventors have creatively exploited this property to create a highly sensitive and fast-responding polarized bismuth ferrite humidity sensor. The humidity sensor fabricated by this invention exhibits high sensitivity, rapid response and recovery, a wide response range, and stable performance.

[0006] Preferably, the particle size of the polarized bismuth ferrite powder is 0.1 to 10 μm, preferably 0.1 to 5 μm, more preferably 0.2 to 2.5 μm, and most preferably 2.5 μm.

[0007] Preferably, the thickness of the polarized bismuth ferrite humidity-sensitive film is 20-80 μm.

[0008] Preferably, the interdigital electrodes are made of gold or silver.

[0009] Preferably, the material of the substrate is selected from one of alumina, polyimide and mica.

[0010] On the other hand, the present invention provides a method for preparing a bismuth ferrite humidity sensor based on external electric field polarization, comprising:

[0011] (1) mixing bismuth ferrite powder and a binder, pressing and sintering to obtain a bismuth ferrite ceramic sheet;

[0012] (2) after plating electrodes on the upper and lower surfaces of the obtained bismuth ferrite ceramic sheet, polarizing the sheet, then grinding off the electrodes, and then crushing and grinding the sheet to obtain polarized bismuth ferrite powder;

[0013] (3) The obtained polarized bismuth ferrite powder and the binder are fully mixed and then printed on a substrate with interdigital electrodes, and then annealed to obtain the humidity sensor based on external electric field polarized bismuth ferrite.

[0014] Preferably, in step (1), the bismuth ferrite powder has a cubic particle shape, a rhombohedral crystal phase (rhombohedral phase), belongs to the R3c space group, and has a particle size of 0.1 to 10 μm, preferably 0.2 to 2.5 μm, and more preferably 2.5 μm; the binder is selected from at least one of polyvinyl alcohol, carboxymethyl cellulose, polyacrylamide (PAM), and polyvinyl pyrrolidone (PVP); and the amount of the binder added is 5 to 30 wt% of the mass of the ferroelectric ceramic powder.

[0015] Preferably, in step (1), the sintering temperature is 750-850° C. and the sintering time is 1-2 hours;

[0016] Preferably, debonding is performed before sintering, and the debonding temperature is 600-650° C. and the time is 30-60 minutes;

[0017] Preferably, the heating rate of the sintering is 3-5°C / min.

[0018] Preferably, in step (2), the polarization treatment environment is silicone oil; the polarization treatment voltage is 12 to 15 kV / cm; and the polarization treatment time is 20 to 40 minutes.

[0019] Preferably, in step (3), the annealing temperature is 160-300° C. (e.g., 200-250° C.); the annealing time is 1-2 hours, as long as the binders such as terpineol, butyl carbitol acetate, and dibutyl phthalate are completely volatilized to avoid affecting the results of the humidity sensitivity test.

[0020] Preferably, in step (3), the binder is selected from at least one of terpineol, butyl carbitol acetate, and dibutyl phthalate; and the amount of the binder added is 10% to 30wt% of the mass of the polarized bismuth ferrite powder.

[0021] On the other hand, the present invention provides an application of a humidity sensor based on external electric field polarized bismuth ferrite in detecting human respiration.

[0022] The present invention measures humidity by changing the resistance of a polarized bismuth ferrite thin film. Specifically, the sensor measures the humidity of human exhaled air, enabling real-time monitoring of respiration. The polarized bismuth ferrite humidity sensor is placed 2 cm from the tip of the nose and measures the humidity difference between exhaled air and the surrounding environment.

[0023] Beneficial effects:

[0024] The bismuth ferrite particles of the present invention have a uniform surface morphology and good crystallization properties. The humidity-sensitive film of the humidity sensor of the present invention is composed of bismuth ferrite powder polarized by an external electric field. The ferroelectric polarization not only greatly improves the sensitivity of the sensor to humidity, but also shortens the response and recovery time. At the same time, the sensor improves repeatability and hysteresis, reducing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the polarized bismuth ferrite humidity sensor of the present invention, wherein: 1-polarized bismuth ferrite film; 2-interdigitated electrodes; 3-substrate;

[0026] Figure 2 is a scanning electron microscope image of the bismuth ferrite powder prepared in Example 1;

[0027] Figure 3 is an X-ray diffraction pattern of the polarized bismuth ferrite powder prepared in Example 1;

[0028] Figure 4 The resistance-time response curve and hysteresis test results of the polarized bismuth ferrite humidity sensor of Example 1 as the humidity changes;

[0029] Figure 5 Response time and recovery time curves of the polarized bismuth ferrite humidity sensor of Example 1;

[0030] Figure 6is a repeatability graph of the polarized bismuth ferrite humidity sensor of Example 1 at 20% RH and 80% RH;

[0031] Figure 7 This is the result of real-time monitoring of respiration by the polarized bismuth ferrite humidity sensor in Example 1;

[0032] Figure 8 The humidity sensor test results of polarized bismuth ferrite with two different particle sizes in Example 1 are as follows;

[0033] Figure 9 These are the test results of the humidity sensor of unpolarized and polarized bismuth ferrite at different sintering temperatures in Example 1. DETAILED DESCRIPTION

[0034] The present invention is further described below through the following embodiments. It should be understood that the following embodiments are only used to illustrate the present invention, rather than to limit the present invention.

[0035] In the present disclosure, the structure of the humidity sensor based on external electric field polarization of bismuth ferrite is as follows Figure 1 As shown, it includes: a substrate, on which interdigital electrodes are arranged; and a polarized bismuth ferrite humidity-sensitive film layer, which is attached to the interdigital electrodes.

[0036] In the present invention, an appropriate amount of bismuth ferrite powder is pressed into a ceramic body, sintered into a ceramic, and double-sided silver-plated electrodes are applied, and voltage is applied for polarization. The polarized bismuth ferrite ceramic is fully crushed and ground to obtain a polarized bismuth ferrite humidity-sensitive material. Finally, an appropriate amount of the above-mentioned polarized bismuth ferrite powder is fully mixed with a binder such as terpineol, printed on a substrate with interdigitated electrodes, and heated and aged to obtain a bismuth ferrite humidity sensor. The resulting humidity sensor has a resistance variation range of 4 orders of magnitude and a response time of less than 100ms. It has a wide response range and good performance stability and can be applied to real-time monitoring of human respiration.

[0037] The following is an exemplary description of a method for preparing a humidity sensor based on external electric field polarized bismuth ferrite.

[0038] Preparation of initial bismuth ferrite powder material (referred to as bismuth ferrite powder unless otherwise specified): In the present invention, bismuth ferrite powder is prepared using a hydrothermal method, a sol-gel method, a solid-phase reaction method, or the like, and only requires that the bismuth ferrite be in a rhombohedral crystal phase with an average size of 0.1 to 10 μm (preferably 0.2 to 2.5 μm, more preferably 2.5 μm).

[0039] As an example of preparing initial bismuth ferrite powder using a hydrothermal method, the process includes: adding ferric nitrate nonahydrate and bismuth nitrate pentahydrate to a dilute nitric acid solution and stirring continuously until completely dissolved to obtain a mixed solution. A potassium hydroxide solution of a certain concentration is then added to adjust the pH and react. The resulting reaction product is centrifuged and washed. The reaction product is then added to an aqueous solution containing potassium hydroxide and potassium nitrate, stirred evenly, and transferred to a hydrothermal reactor for heating. The initial bismuth ferrite powder is then obtained after centrifugation, washing, and drying.

[0040] In an optional embodiment, the concentrations of ferric nitrate nonahydrate and bismuth nitrate pentahydrate in the mixed solution may be 12 to 18 mmol, respectively. The volume of the dilute nitric acid solution is 100 mL, and the concentration is 1 to 3 mol / L. The concentration of the potassium hydroxide solution may be 10 to 12 mol / L, so that the pH of the mixed solution is 8 to 14. The concentration of potassium hydroxide in the aqueous solution containing potassium hydroxide and potassium nitrate is 10 to 12 mol / L, and the concentration of potassium nitrate is 2.2 to 2.8 mol / L. The hydrothermal time may be 20 to 24 hours, and the hydrothermal temperature may be 180 to 220°C.

[0041] Initial bismuth ferrite powder and a binder are mixed and pressed into a ceramic body. The binder is selected from at least one of polyvinyl alcohol, carboxymethyl cellulose, polyacrylamide (PAM), and polyvinyl pyrrolidone (PVP). The binder is added in an amount of 5 to 30 wt% of the ferroelectric ceramic powder.

[0042] The ceramic body is sintered to obtain a ceramic sheet. The sintering temperature is 750-850°C, the sintering time is 1-2 hours, and the heating rate is 3-5°C / min. Preferably, a debonding step is performed before sintering. The debonding step is performed at a temperature of 600-650°C, the sintering time is 30-60 minutes, and the heating rate is more preferably 3-5°C / min.

[0043] After silver electrodes are plated on both sides of the ceramic sheet, a voltage is applied in silicone oil for polarization. The polarization voltage is 12-15 kV / cm, and the duration is 20-40 minutes. Humidity-sensing performance is not significantly different when the polarization voltage is between 12 and 15 kV / cm. Due to the inherent properties of this material, the polarization voltage does not have a large adjustment range. Naturally, the voltage cannot be increased, as it will cause breakdown of the bismuth ferrite ceramic.

[0044] Grind off the silver electrode on the polarized ceramic sheet, and then crush (or break) and grind the polarized ceramic sheet to obtain polarized bismuth ferrite powder. It is necessary to ensure that the particle size of the obtained polarized bismuth ferrite powder can be 0.1 to 10 μm (preferably 0.2 to 2.5 μm, more preferably 2.5 μm). The change in the particle size of the polarized bismuth ferrite powder has a certain influence on the humidity sensitive performance, see Figure 8 The humidity sensing performance of particles with different sizes after polarization treatment is demonstrated.

[0045] Polarized bismuth ferrite powder is thoroughly mixed with a binder such as terpineol. The resulting slurry is printed onto a substrate with interdigitated electrodes and then annealed to produce a polarized bismuth ferrite humidity sensor. The annealing temperature is 200-250°C for 1-2 hours. The annealing temperature does not significantly vary within a suitable range; it only needs to ensure complete evaporation of the terpineol.

[0046] The polarized bismuth ferrite humidity sensor is placed in different humidity environments and the humidity is measured by the change in its resistance.

[0047] The following examples are further given to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention all fall within the scope of protection of the present invention. The specific process parameters and the like in the following examples are only examples within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description herein, and are not limited to the specific numerical values exemplified below.

[0048] Example 1

[0049] A humidity sensor based on external electric field polarized bismuth ferrite and its preparation method and application, the specific steps are as follows:

[0050] (1) adding 15 mmol of ferric nitrate nine hydrate and 15 mmol of bismuth nitrate pentahydrate to a 100 mL volume and 2 mol / L concentration of dilute nitric acid solution, stirring continuously until completely dissolved, adding 12 mol / L of potassium hydroxide to make the solution pH 14 and then reacting, centrifuging and washing the obtained reaction product; adding the reaction product to a solution containing 12 mol / L potassium hydroxide and 2.5 mol / L potassium nitrate and stirring evenly, transferring 120 mL of the solution to a hydrothermal autoclave for heating reaction at a temperature of 200° C. for 24 h, centrifuging, washing, and drying to obtain an initial cubic bismuth ferrite powder with a particle size of 2.5 μm;

[0051] (2) Add 3 drops of polyvinyl alcohol solution (concentration of 7wt% to 0.5g of original BiFeO3 powder. The amount of polyvinyl alcohol added is 10-30wt% of the original BiFeO3 powder), stir evenly and use a tablet press to press a ceramic body with a diameter of 1cm at 6MPa. Heat the ceramic body to 650℃ at a heating rate of 3℃ / min and keep it warm for 60min, then heat it to 850℃ at 5℃ / min and keep it warm for 2h to obtain BiFeO3 ceramic. After silver electrodes are plated on both sides, a voltage is applied in silicone oil for polarization. The polarization voltage is 13kV / cm and the polarization time is 30min. Then, the silver electrodes are ground off and the polarized ceramic pieces are crushed and ground to obtain polarized bismuth ferrite powder. The average particle size of the obtained polarized bismuth ferrite powder is 2.5μm.

[0052] (3) The polarized bismuth ferrite powder prepared in step (2) is thoroughly mixed with pinene alcohol to obtain a slurry. The obtained slurry is screen-printed on an alumina substrate containing gold interdigitated electrodes, and then annealed at 220° C. for 1 h to obtain a polarized bismuth ferrite humidity sensor with a film thickness of approximately 50 μm.

[0053] (4) The polarized bismuth ferrite humidity sensor prepared in step (3) is placed in different humidity environments, and the humidity is measured by the change in its resistance.

[0054] The scanning electron microscope image of the bismuth ferrite powder prepared in this Example 1 is shown in FIG. Figure 2 ,from Figure 2 It can be seen that the bismuth ferrite powder is in the form of uniform cubic particles with a size of 2.5 μm.

[0055] The X-ray diffraction pattern of the polarized bismuth ferrite powder prepared in Example 1 is shown in FIG. Figure 3 ,from Figure 3 It can be seen that the polarized bismuth ferrite powder has a rhombohedral phase structure and the diffraction peak is relatively sharp, indicating that the polarized bismuth ferrite has very good crystallinity.

[0056] The resistance-time response curve and hysteresis test results of the polarized bismuth ferrite humidity sensor as humidity changes in Example 1 are shown in FIG. Figure 4 ,from Figure 4 It can be seen that the resistance of the sensor decreases with the increase of humidity, and the hysteresis difference is small.

[0057] The response time and recovery time curve of the polarized bismuth ferrite humidity sensor in Example 1 is shown in FIG. Figure 5 ,from Figure 5 It can be seen that the resistance of the sensor is extremely sensitive to changes in humidity, with a response time of 84ms and a recovery time of 376ms.

[0058] The repeatability of the polarized bismuth ferrite humidity sensor in Example 1 at 30% RH and 80% RH is shown in FIG. Figure 6 ,from Figure 6It can be seen that the sensor exhibits good repeatability.

[0059] The results of real-time monitoring of human respiration by the polarized bismuth ferrite humidity sensor in Example 1 are shown in FIG. Figure 7 ,from Figure 7 It can be seen that the sensor has a good monitoring effect on slow breathing, normal breathing and fast breathing.

[0060] Example 2

[0061] A humidity sensor based on external electric field polarized bismuth ferrite and its preparation method and application, the specific steps are as follows:

[0062] (1) adding 12 mmol of ferric nitrate nine hydrate and 12 mmol of bismuth nitrate pentahydrate to a 100 mL volume and 1.5 mol / L dilute nitric acid solution, stirring continuously until completely dissolved, adding 10 mol / L of potassium hydroxide to adjust the solution pH to 8 and then reacting, centrifuging and washing the obtained reaction product; adding the reaction product to a solution containing 12 mol / L potassium hydroxide and 2.5 mol / L potassium nitrate and stirring evenly, transferring 120 mL of the solution to a hydrothermal autoclave for heating reaction at a temperature of 200° C. for 24 h, centrifuging, washing, and drying to obtain an initial cubic bismuth ferrite powder with a particle size of 0.2 μm;

[0063] (2) Add 3 drops of polyvinyl alcohol solution (concentration of 7wt% to 0.5g of original BiFeO3 powder. The amount of polyvinyl alcohol added is 10-30wt% of the original BiFeO3 powder) and stir evenly. Press the ceramic body with a diameter of 1cm at 6MPa using a tablet press. Heat the ceramic body to 650℃ at a heating rate of 3℃ / min and keep it warm for 60min, then heat it to 800℃ at a rate of 5℃ / min and keep it warm for 2h to obtain BiFeO3 ceramic. After silver electrodes are plated on both sides of the sintered ceramic, a voltage is applied in silicone oil for polarization. The polarization voltage is 12kV / cm and the polarization time is 30min. Then, the silver electrodes are ground off and the polarized ceramic pieces are crushed and ground to obtain polarized bismuth ferrite powder. The average particle size of the obtained polarized bismuth ferrite powder is 0.2μm.

[0064] (3) The polarized bismuth ferrite powder prepared in step (2) is thoroughly mixed with pine alcohol to obtain a slurry. The obtained slurry is screen-printed on a polyimide substrate containing gold interdigitated electrodes, and then annealed at 200° C. for 1 hour to obtain a polarized bismuth ferrite humidity sensor with a film thickness of approximately 40 μm.

[0065] (4) The polarized bismuth ferrite humidity sensor prepared in step (3) was placed in different humidity environments, and the humidity was measured by the change in its resistance. The resulting sensor's resistance was extremely sensitive to changes in humidity, with a response time of 1.1 s and a recovery time of 3.2 s.

[0066] Example 3

[0067] The preparation process of the humidity sensor in Example 3 was similar to that in Example 1, with the only difference being that in step (1), the ceramic body was heated to 650°C at a heating rate of 3°C / min and held for 60 minutes, followed by sintering at 750°C for 2 hours to obtain the BiFeO3 ceramic. The resulting humidity sensor had a response time of 0.7 seconds and a recovery time of 3.8 seconds.

[0068] Example 4

[0069] The preparation process of the humidity sensor in Example 4 was similar to that in Example 1, with the only difference being that in step (1), the ceramic body was heated to 650°C at a heating rate of 3°C / min and held for 60 minutes, followed by sintering at 800°C for 2 hours to obtain the BiFeO3 ceramic. The resulting humidity sensor had a response time of 0.3 seconds and a recovery time of 3.1 seconds.

[0070] Depend on Figure 8 It is known that BiFeO3 particle size has a great influence on gas sensing performance. The humidity sensitive film printed with small BiFeO3 particles is relatively dense. In contrast, the humidity sensitive film printed with large BiFeO3 particles is porous, which is conducive to the adsorption of more water molecules. This data is the data of Example 1 and Example 2. Figure 8 The response and recovery times shown are the values obtained by testing in a test chamber. Figure 5 The data of Example 1 is the fastest response time that can be achieved by human exhalation. Since the humidity of human exhaled air can reach 95% more quickly, the response times of the two test methods are different.

[0071] Figure 9 The humidity sensing performance of the unpolarized and polarized BiFeO3 sensors prepared in Example 4, Example 3 and Example 1 were obtained by sintering BiFeO3 ceramics at 750℃, 800℃ and 850℃, respectively. Figure 9 It can be seen that as the sintering temperature of the ceramic increases, the humidity sensing performance is significantly improved. The humidity sensing response of BiFeO3 powders prepared at all temperatures is significantly improved after polarization, and the response and recovery time are greatly reduced.

Claims

1. A humidity sensor based on external electric field polarized bismuth ferrite, characterized in that: Including in order: A substrate, an interdigital electrode arranged on the surface of the substrate, and a polarized bismuth ferrite humidity-sensitive film attached to the surface of the interdigital electrode; the polarized bismuth ferrite humidity-sensitive film comprises polarized bismuth ferrite powder; the chemical formula of the polarized bismuth ferrite powder is BiFeO 3-a , wherein 0<a≤0.05; the preparation method of the polarized bismuth ferrite powder comprises: (1) mixing bismuth ferrite powder and a binder, pressing and sintering to obtain a bismuth ferrite ceramic sheet; The sintering temperature is 750-850°C; (2) After electrodes are plated on the upper and lower surfaces of the obtained bismuth ferrite ceramic sheet, a polarization treatment is performed, and then the electrodes are ground off, and the sheet is crushed and ground to obtain polarized bismuth ferrite powder; the voltage of the polarization treatment is 12 to 15 kV / cm; and the time of the polarization treatment is 20 to 40 minutes.

2. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 1, characterized in that: The particle size of the polarized bismuth ferrite powder is 0.1-10 μm.

3. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 2, characterized in that: The particle size of the polarized bismuth ferrite powder is 0.1 to 5 μm.

4. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 2, characterized in that: The particle size of the polarized bismuth ferrite powder is 0.2 to 2.5 μm.

5. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 1, characterized in that: The thickness of the polarized bismuth ferrite humidity-sensitive film is 20 to 80 μm.

6. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 1, characterized in that: The material of the interdigital electrodes is gold or silver; the material of the substrate is selected from one of aluminum oxide, polyimide and mica.

7. A method for preparing a humidity sensor based on external electric field polarized bismuth ferrite according to any one of claims 1 to 6, characterized in that: include: The polarized bismuth ferrite powder and the binder are fully mixed and then printed on a substrate with interdigital electrodes, and then annealed to obtain the humidity sensor based on external electric field polarized bismuth ferrite.

8. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 1, characterized in that: In step (1), the particle shape of the bismuth ferrite powder is cubic particles, the crystal phase is rhombohedral system, and belongs to the R3c space group; the particle size of the bismuth ferrite powder is 0.1 to 10 μm; the binder is selected from at least one of polyvinyl alcohol, carboxymethyl cellulose, polyacrylamide (PAM), and polyvinyl pyrrolidone (PVP); and the amount of the binder added is 5 to 30 wt% of the mass of the ferroelectric ceramic powder.

9. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 8, characterized in that: The particle size of the bismuth ferrite powder is 0.1 to 5 μm.

10. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 8, characterized in that: The particle size of the bismuth ferrite powder is 0.2 to 2.5 μm.

11. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 1, characterized in that: In step (1), the sintering time is 1 to 2 hours.

12. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 11, characterized in that: Debonding is performed before sintering. The debonding temperature is 600-650° C. and the time is 30-60 minutes.

13. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 11, characterized in that: The heating rate of the sintering is 3-5°C / min.

14. The humidity sensor based on external electric field polarized bismuth ferrite according to claim 1, characterized in that: In step (2), the polarization treatment environment is silicone oil.

15. The preparation method according to claim 7, wherein The annealing treatment temperature is 160-300° C.; the annealing treatment time is 1-2 h.

16. Use of a humidity sensor based on external electric field polarized bismuth ferrite according to any one of claims 1 to 6 in detecting human respiration.

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