CN118501182A - ZnO/TiO-based2Preparation method of chipless RFID ammonia gas detection sensor of nanocomposite, passive ammonia gas detection sensor of chipless RFID ammonia gas detection sensor, and indoor ammonia gas detection system and method - Google Patents

Abstract

The invention discloses a preparation method of a chipless RFID ammonia gas detection sensor based on ZnO/TiO ₂ nanocomposite, which belongs to the technical field of radio frequency sensors, and comprises the steps of preparing a microstrip patch antenna, loading ZnO/TiO ₂ nanocomposite on the microstrip patch antenna, and enabling the ZnO/TiO ₂ nanocomposite to have larger specific surface area, so that the sensitivity and response speed of the sensor can be improved during ammonia gas detection; the invention also provides a sensor prepared by the preparation method, wherein the microstrip patch in the sensor is provided with a rectangular groove, and the connection part of the microstrip patch and the feeder unit is provided with a metal short circuit, so that the sensitivity of the sensor is improved, and the response speed of the sensor is accelerated; on the other hand, the invention also discloses a system and a method for constructing indoor ammonia gas detection by the sensor, which are used for detecting the ammonia gas, and the sensor has better robustness in detecting the ammonia gas by improving the sensitivity and the response speed of the sensor.

Description

Preparation method of chipless RFID ammonia detection sensor based on ZnO/TiO2 nanocomposite material, passive ammonia detection sensor and indoor ammonia detection system, method

Technical Field

The present invention belongs to the technical field of radio frequency sensors, and specifically, relates to a method for preparing a chipless RFID ammonia detection sensor and a passive ammonia detection sensor prepared by the preparation method. The sensor is loaded with a ZnO/ _TiO2 nanocomposite material, and the robustness of ammonia detection is improved by loading the ZnO/ _TiO2 nanocomposite material. The present invention also relates to an indoor ammonia detection system and method constructed by the above-mentioned sensor.

Background Art

Ammonia is a common gas in agricultural planting and protein decomposition of agricultural products, and is used as a key detection indicator in the process of microbial and food protein decomposition. In agricultural greenhouse planting or other cultivation processes, excessive ammonia concentration will cause ammonia damage, affecting crop yield and quality. In the field of agricultural breeding, livestock and poultry are in an environment with excessive ammonia concentration for a long time, which will cause chronic poisoning, weakened physique, reduced resistance, decreased reproductive ability, and increased morbidity and mortality. Therefore, in the agricultural field, the detection of ammonia concentration in a closed environment is particularly important. In recent years, ammonia has been detected by active detection methods such as semiconductor, electrochemical, and optical. However, on the one hand, active detection methods increase energy consumption, and the heat accumulation generated by active work may affect the performance of the detection system. On the other hand, active detection requires wired connection to the detection system, and most closed environments cannot meet the needs of wired connection, so it causes certain inconvenience to the detection of ammonia.

As traditional agriculture gradually transforms to smart agriculture, and with the development of science and technology, the coexistence and integration of device sensing and wireless communication capabilities, radio frequency identification technology is widely used due to its lightweight, low-cost, markable, non-line-of-sight reading and other characteristics. Among them, chipless radio frequency identification (CRFID) tags have an ultra-long life, eliminate the use of integrated circuits, and are suitable for independent non-contact object monitoring. Therefore, chipless radio frequency identification tags have become an important medium for identification sensor information fusion. Radio frequency identification tags are essentially passive and non-traditional sensors. Passive sensors are prepared by loading different sensitive materials with conductivity, dielectric constant or magnetic permeability on the radio frequency identification tag. The passive sensor affects the power, frequency and phase of the radiated electromagnetic waves due to the different sensitive materials loaded with conductivity, dielectric constant or magnetic permeability, and can realize radio frequency cross-domain perception. The main structure of the passive sensor is composed of a planar resonator and a sensitive material. The planar resonator can be designed as a microstrip patch antenna with a patch structure of different shapes, and the microstrip patch antenna loaded with sensitive materials can be widely used in light, temperature, humidity, and gas detection. Therefore, the use of microstrip patch antennas loaded with sensitive materials to prepare passive sensors and apply them to closed environment detection effectively solves the drawbacks of active ammonia detection.

There are many studies in the prior art on preparing passive sensors for ammonia detection by load-sensitive materials. For example, the Chinese invention patent CN104730108A applied for by Nano New Energy (Tangshan) Co., Ltd. discloses an ammonia sensor based on zinc oxide and an ammonia detection device. The ammonia sensor uses ZnO as a sensitive material to prepare a sensor for detecting ammonia. Specifically, the sensor is provided with a first friction layer and a second friction layer, and the ZnO layer in the sensor is arranged on the second side surface of the first friction layer, and a friction interface is formed between the second side surface of the first friction layer and the ZnO layer arranged thereon and the second side surface of the second friction layer. When the sensor detects ammonia, due to the oxygen vacancies on the surface of ZnO, oxygen is adsorbed on the oxygen vacancies, thereby absorbing electrons on the conduction band of ZnO to become ions ( ^O2- , ^O- ), when ammonia in the environment is adsorbed on the ZnO surface, it replaces and reacts with the oxygen ions originally adsorbed on the ZnO to release electrons, so that the charge on the zinc oxide surface changes, resulting in a change in resistance, and then the purpose of ammonia concentration detection is achieved through the sensor. In order to improve the sensitivity of ammonia detection, when the above-mentioned sensor actually detects ammonia, the second friction layer rubs against the ZnO layer, so that the ZnO surface carries more electrons in addition to oxygen vacancies, increases the charge density, speeds up the oxygen absorption rate and oxygen absorption amount, thereby increasing the reaction rate of ammonia and oxygen, so as to improve the sensitivity of the ammonia sensor. However, when the ammonia concentration is actually detected by a passive sensor, it is difficult to improve the sensitivity of the sensor detection by friction. For this reason, it is very necessary to study a method for preparing a passive ammonia detection sensor with high sensitivity and fast response for preparing a passive ammonia detection sensor.

Summary of the invention

The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

In order to solve the above-mentioned problems of how to improve the sensitivity of the ammonia detection sensor and how to respond quickly, the present invention adopts the following technical solution.

On one hand, the present invention provides a method for preparing a chipless RFID ammonia detection sensor based on ZnO/ _TiO2 nanocomposite material, comprising preparing a microstrip patch antenna and loading the ZnO/ _TiO2 nanocomposite material on the microstrip patch antenna, and the specific steps are as follows:

Prepare a microstrip patch antenna: use an FR4 plate as a substrate, attach a ground patch to the lower surface of the substrate, attach a microstrip patch to the upper surface, the microstrip patch is connected to a feeder unit, the feeder unit is attached to the substrate and connected to the ground patch through a connector;

ZnO/ _TiO2 nanocomposite is coated on the surface of the microstrip patch.

Preferably, in the above-mentioned preparation method, the preparation method of the ZnO/ _TiO2 nanocomposite material is as follows:

After the zinc acetate is dissolved, it is dispersed by ultrasonication and then magnetically stirred, and ammonia water, sodium borohydride and polyvinyl pyrrolidone are added to obtain a suspension, the suspension is placed in a high-pressure reactor, reacted, cooled to room temperature, centrifuged to obtain a white precipitate, and the white precipitate is washed and dried to obtain ZnO powder;

The titanium tetrachloride is dissolved, dispersed by ultrasonic and then magnetically stirred, and the prepared ZnO powder is added, dispersed by ultrasonic and then magnetically stirred, and then placed in a high-pressure reactor for reaction, cooled to room temperature and centrifuged to obtain a white precipitate, which is washed and dried to obtain a ZnO/ _TiO2 nanocomposite material.

Preferably, the above-mentioned preparation method further includes a step of optimizing the parameters of the sensor, wherein the parameters include the length and width of the microstrip patch. The specific steps are as follows:

The width of the microstrip patch is a constant, and the length is a variable. The return loss is obtained by performing a frequency sweep analysis with a step amplitude of 1 mm, and the length corresponding to the minimum return loss is taken as the length of the microstrip patch.

The length of the microstrip patch is a constant, and the width is a variable. The return loss is obtained by performing a frequency sweep analysis with a step amplitude of 1 mm, and the width corresponding to the minimum return loss is taken as the width of the microstrip patch.

Preferably, in the above-mentioned preparation method, a rectangular gap is further provided on the microstrip patch, the length and width of the rectangular gap are respectively parallel to the diagonal of the microstrip patch and the center of the rectangular gap coincides with the center of the microstrip patch. The preparation method further includes a step of optimizing the size of the rectangular gap, the size of the rectangular gap includes the length and width of the rectangular gap, and the specific steps are as follows:

The width of the rectangular gap is a constant, and the length is a variable. The return loss is obtained by performing a frequency sweep analysis with a step amplitude of 1 mm, and the length corresponding to the minimum return loss is taken as the length of the rectangular gap.

The length of the rectangular gap is a constant, and the width is a variable. The return loss is obtained by performing a frequency sweep analysis with a step amplitude of 1 mm, and the width corresponding to the minimum return loss is taken as the width of the rectangular gap.

Preferably, in the above-mentioned preparation method, the connection between the microstrip patch and the feed unit is loaded with a metal short circuit, and the metal short circuit causes the current to be concentrated around the connection, which is used to change the radiation direction and gain of the microstrip patch antenna and change the current and magnetic field distribution, thereby changing the radiation characteristics of the microstrip patch antenna and reducing the cross-polarization of the microstrip patch antenna.

Preferably, in the above preparation method, a parasitic unit is arranged on one side of the microstrip patch on the substrate, a gap exists between the parasitic unit and the microstrip patch, and the current generated by the coupling between the parasitic unit and the microstrip patch radiates outward to improve the gain of the microstrip patch antenna.

Another aspect of the present invention also provides a passive ammonia detection sensor prepared by the above preparation method.

The present invention also provides an indoor ammonia detection system constructed by the above-mentioned passive ammonia detection sensor. The detection system also includes a reader and a vector network analyzer. The reader includes a transmitting antenna and a receiving antenna. The transmitting antenna and the receiving antenna are coaxially connected to the vector network analyzer. The reader transmitting antenna transmits a signal. When the passive ammonia detection sensor obtains a signal and is activated, it transmits the information and data it carries and detects to the reader. The reader receives the information of the microstrip patch antenna through the receiving antenna and transmits it to the reader. After demodulation and decoding, the reader transmits the received signal to the vector network analyzer through the coaxial cable, and the observation results are analyzed through the vector network.

On the other hand, the present invention also provides a method for detecting ammonia through the above-mentioned detection system. After the ZnO/ _TiO2 nanocomposite material in the passive ammonia detection sensor senses ammonia, the return loss of the microstrip patch antenna changes, and the receiving antenna transmits the signal to the reader. After processing, the reader feeds back the signal to the vector network analyzer, and the change in ammonia concentration is analyzed by the signal change of the vector network analyzer.

Preferably, in the above detection method, oxygen molecules in the air obtain electrons to form oxygen ions after contact with the ZnO/ _TiO2 nanocomposite material, and ammonia molecules in the ammonia gas react with the oxygen ions to release electrons and return to the surface of the ZnO/ _TiO2 nanocomposite material to change the resistance of the passive ammonia sensor and thus change the return loss for detecting the ammonia concentration.

Compared with the prior art, the present invention has the following beneficial effects:

(1) The preparation method of the passive ammonia detection sensor of the present invention is to load a ZnO/ _TiO2 nanocomposite material on the surface of a microstrip patch antenna for preparing the passive ammonia detection sensor, wherein the ZnO/ _TiO2 nanocomposite material has a large number of holes, good conductivity, and is sensitive to ammonia. When oxygen molecules in the air combine with the ZnO/ _TiO2 nanocomposite material to form oxygen ions, ammonia molecules in the ammonia react with the oxygen ions to release electrons back to the conduction band of the ZnO/ _TiO2 nanocomposite material, thereby changing the return loss of the microstrip patch antenna for detecting ammonia concentration. Since the ZnO/ _TiO2 nanocomposite material has a large specific surface area, the sensor prepared by this method has high sensitivity and fast response speed when detecting ammonia concentration.

(2) The sensor in the present invention also has a rectangular slot in the microstrip patch antenna. The rectangular slot enables the antenna to be circularly polarized, thereby improving the ability of the microstrip patch antenna to receive signals. In addition, the microstrip patch antenna in the present invention is loaded with a metal short circuit. The metal short circuit causes the current to concentrate around the short circuit point, thereby changing the radiation direction and gain of the microstrip patch antenna. In addition, the short circuit will change the current and magnetic field distribution, thereby changing the radiation characteristics of the microstrip patch antenna and reducing the cross-polarization of the microstrip patch antenna. The microstrip patch antenna in the present invention is also provided with a parasitic unit. The current generated by the coupling between the parasitic unit and the microstrip patch radiates outward, thereby improving the antenna gain while also suppressing the propagation of surface waves. Therefore, the passive sensor in the present invention has high sensitivity, fast response and good robustness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of a microstrip patch antenna in the present invention.

FIG. 2 is a schematic diagram of the split structure of FIG. 1 .

FIG. 3 is a front view of FIG. 1 .

FIG. 4 is a physical picture of the microstrip patch antenna in the present invention.

FIG. 5 is an EDS image of the ZnO/TiO ₂ nanocomposite material of the present invention.

FIG6 is a SEM image of the ZnO/ _TiO2 nanocomposite material of the present invention;

Wherein: Figure 6 (a) is a SEM image of ZnO/TiO ₂ nanocomposite at 2 μm;

Figure 6(b) is the SEM image of ZnO/TiO ₂ nanocomposite at 200 nm;

Figure 6(c) is the SEM image of the local ZnO/TiO ₂ nanocomposite at 200 nm;

Figure 6(d) is the SEM image of ZnO/TiO ₂ nanocomposite at 100 nm.

Figure 7 is a schematic diagram of the sensing mechanism of ZnO/ _TiO2 nanocomposite materials.

FIG8 is a schematic diagram showing the dimensions of the microstrip patch antenna of the present invention.

9 is a graph of return loss S11 according to the length and width adjustment of the microstrip patch when optimizing the microstrip patch antenna in the present invention;

Wherein: FIG9(a) is a graph showing the return loss S11 as the length L of the microstrip patch changes;

FIG9( b ) is a graph showing the return loss S11 as the width W of the microstrip patch changes.

FIG10 is a frequency sweep analysis diagram of the microstrip patch antenna in this embodiment;

Among them: Figure 10 (a) is an analysis diagram of the axis ratio changing with the length of the rectangular gap;

Figure 10(b) is an analysis diagram of the axis ratio changing with the width of the rectangular gap;

Figure 10(c) is an axial ratio diagram of the microstrip patch antenna;

Figure 10(d) is a diagram showing the circularly polarized current flow of the microstrip patch antenna.

FIG11 is a diagram showing the simulated distribution of electric and magnetic fields, an impedance simulation analysis diagram, and a radiation pattern of the microstrip patch antenna of the present invention;

Wherein: Figure 11(a) is a simulated electric field distribution diagram of the microstrip patch antenna;

Figure 11(b) is a simulated distribution diagram of the magnetic field of the microstrip patch antenna;

Figure 11(c) is a simulated distribution diagram of the normalized impedance of the microstrip patch antenna;

Figure 11(d) shows the three-dimensional radiation pattern of the microstrip patch antenna.

FIG12 is a radiation comparison diagram and a radiation gain diagram of a microstrip patch antenna loaded with a metal short circuit.

Wherein: Figure 12(a) is the polarization radiation diagram of the microstrip patch antenna without metal short circuit;

Figure 12(b) is the polarization radiation pattern of the microstrip patch antenna loaded with a metal short circuit;

Figure 12(c) is the polarization radiation gain diagram of the microstrip patch antenna.

FIG13 is an experimental diagram of an ammonia detection system in the present invention;

FIG14 is a comparison diagram of the test of the passive ammonia detection sensor in the present invention;

Wherein: FIG. 14 (a) is a result diagram of the S11 value of the sensor loaded with ZnO/TiO ₂ nanocomposite material under 0-100 ppm ammonia test;

FIG14( b ) is a graph showing the S11 value of the sensor loaded with ZnO material under ammonia test at 0-100 ppm;

Figure 14(c) is the fitting diagram of the sensor loaded with ZnO/ _TiO2 nanocomposite material under ammonia test at 0-100 ppm;

Figure 14(d) is a fitting diagram of the ammonia test of the sensor loaded with ZnO material at 0-100 ppm.

FIG15 is a comparison chart of the warning response time of the sensor loaded with ZnO/ _TiO2 nanocomposite material and ZnO material for ammonia detection.

FIG. 16 is a graph showing the reproducibility results of the repeatability test of the passive ammonia detection sensor in the present invention.

The corresponding relationship between the illustrations and component names in the figure is as follows:

100, substrate; 101, microstrip patch; 102, ground patch; 103, parasitic unit;

101a. Feeder unit.

DETAILED DESCRIPTION

In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not refer to the same embodiment, nor is it a separate or selective embodiment that is mutually exclusive with other embodiments. The present invention provides the following embodiments.

As shown in Figures 1 to 3, they are schematic diagrams of the structure of the passive ammonia detection sensor in this embodiment. The ammonia detection sensor in this embodiment is a passive ammonia detection sensor prepared by combining chipless RFID technology with ZnO/ _TiO2 nanocomposite materials, which solves the drawbacks of active detection in the prior art. The passive ammonia detection sensor in this embodiment, i.e., the chipless RFID ammonia detection sensor, is composed of a microstrip patch antenna loaded with a nanocomposite material sensitive to ammonia. This embodiment provides a method for preparing the above-mentioned sensor, specifically, first preparing a microstrip patch antenna, and then loading the nanocomposite material sensitive to ammonia on the microstrip patch antenna. The steps are as follows: a ground patch 102 made of all copper is attached to a dielectric constant FR4 The lower surface of the substrate 100 is provided with a microstrip patch 101, and the upper surface of the substrate 100 is also adhered with a microstrip patch 101. In this embodiment, a feeder unit 101a is also provided on the substrate 100. The material of the feeder unit 101a is also full copper. The feeder unit 101a is connected to the microstrip patch 101, and the feeder unit 101a is connected to the ground patch 102 through an SMA connector. As shown in Figure 4, the microstrip patch 101 in this embodiment is loaded with a nano-composite material sensitive to ammonia and together with the feeder unit 101a constitutes a passive ammonia detection sensor.

When the passive ammonia detection sensor in this embodiment is in use, oxygen molecules in the air ^form _O2- by adsorbing the conductive electrons on the surface of the ZnO/ _TiO2 nanocomposite material. When ammonia exists in the detection environment, ammonia reacts ^with _O2- and releases the captured electrons back to the conduction band in the nanocomposite material, thereby reducing the resistance of the passive ammonia detection sensor. When the resistance changes, the magnetic field generated by the inductance of the passive sensor will also change due to the change in the circuit, and the inductive reactance will change synchronously, thereby causing the impedance of the passive ammonia detection sensor to change. The impedance calculation formula of the passive ammonia detection sensor in this embodiment is as follows:

Z _in = R _in + jX _in

In the above formula: Z _in is the impedance of the sensor; R _in is the resistance component; Xin _is the reactance component, where the reactance component is composed of inductive reactance and capacitive reactance.

From the above formula, we can know that the inductive reactance in the microstrip patch antenna changes after ammonia reacts with oxygen ions and releases electrons back to the surface of the nanocomposite material, thereby changing the impedance of the antenna, causing the power of the microstrip patch antenna to change, and then causing the amplitude value of the return loss at the center frequency of the microstrip patch antenna to change. The purpose of ammonia concentration detection can be achieved by analyzing the change in the return loss amplitude value.

In this embodiment, the microstrip patch antenna is loaded with a nanocomposite material that is sensitive to ammonia gas, and the material is a ZnO/ _TiO2 nanocomposite material. In this embodiment, the ZnO/ _TiO2 nanocomposite material is prepared by a hydrothermal method, and the specific steps are as follows.

First, ZnO powder was prepared by dissolving 0.6585 g zinc acetate (Zn(CH ₃ COO) ₂ ) in 30 ml deionized water (H ₂ O) and then magnetically stirring for 5 minutes. Then, 1.5 mL ammonia water (NH ₃ ·H ₂ O), 1.8158 g sodium borohydride (NaBH ₄ ) and 0.2 g polyvinylpyrrolidone (PVP) were added. The obtained suspension was transferred to a stainless steel autoclave lined with polytetrafluoroethylene; the autoclave was reacted at a constant temperature of 120°C for 3 hours, and then cooled to room temperature after the reaction. The obtained product was centrifuged at a speed of 7000 rpm for 10 minutes to obtain a white precipitate. Then, it was washed with deionized water and anhydrous ethanol for 3 times, and finally placed in a vacuum oven at 80°C for vacuum drying overnight to obtain ZnO powder.

Secondly, 0.1650g of titanium tetrachloride (TiCl ₄ ) was dissolved in 30ml of deionized water (H ₂ O), and then the mixture was dispersed by ultrasonic and then magnetically stirred for 5 minutes. Then 0.2g of the prepared zinc oxide (ZnO) powder was added, and then the mixture was dispersed by ultrasonic and then magnetically stirred for 15 minutes. After the solution was dispersed by ultrasonic until it was evenly dispersed, it was poured into a high-pressure reactor and heated to 150°C for constant temperature reaction for 16 hours. After the reaction was completed, it was cooled to room temperature, and the obtained product was centrifuged at a speed of 7000rpm for 10 minutes to obtain a white precipitate. Then it was washed with deionized water and anhydrous ethanol for 3 times, and finally placed in a vacuum oven for vacuum drying at 70°C for 3 hours to prepare ZnO/TiO ₂ powder.

In this embodiment, the ZnO/ _TiO2 nanocomposite material is atomized into fine particles by an ultrasonic sprayer, and then the spray shape is precisely controlled by a carrier gas and evenly coated on the surface of the microstrip patch 101 to form a coating to prepare a passive ammonia detection sensor.

In this embodiment, the prepared nanocomposite material was analyzed by energy dispersive spectrometry (EDS), and the results are shown in FIG5 . It is not difficult to see from FIG5 that the peaks of Ti, Zn and O in the prepared nanocomposite material can be clearly observed. Only Ti, Zn and O elements were detected in the material, and no other impurity elements were detected. The O element had the highest content, and the Zn and Ti elements had higher contents in the mixture. Therefore, the two materials were successfully composited, and the prepared material was a ZnO/ _TiO2 nanocomposite material.

In addition, in this embodiment, the morphology of the ZnO/ _TiO2 nanocomposite material is characterized and measured by scanning electron microscopy (SEM), and the results are shown in Figure 6. As shown in Figure 6 (a), the ZnO in the prepared nanocomposite material is in a nanorod state and has a clear structure. In addition, it can be seen from Figure 6 (a) and Figure 6 (b) that the _TiO2 in the ZnO/ _TiO2 nanocomposite material is wrapped in good contact with the ZnO surface, indicating that the ZnO material and the _TiO2 material in the ZnO/ _TiO2 nanocomposite material prepared in this embodiment are successfully combined. In addition, as shown in Figure 6 (d), _TiO2 wrapped in the ZnO surface greatly increases the specific surface area of the nanocomposite material, which is conducive to the absorption and diffusion of ammonia molecules, and improves the ability of ammonia molecules to capture electrons, thereby improving the sensitivity of the passive case sensor in this embodiment to ammonia detection and accelerating the response speed.

As shown in Figure 7, it is a schematic diagram of the sensing mechanism of the ZnO/ _{TiO2 nanocomposite material in this embodiment. In this embodiment, the ZnO/TiO2 nanocomposite} material prepared by ZnO material and _TiO2 material has a higher specific surface area than pure _TiO2 material and pure ZnO material, which is conducive to the absorption and diffusion of ammonia molecules, and the conductivity of the ZnO/ _TiO2 nanocomposite material is greater than that of _TiO2 nanoparticles. By increasing the conductivity of the ZnO/ _TiO2 nanocomposite material, the charge transfer between _TiO2 and ZnO is achieved, thereby improving the response ability of the sensor. The addition of _TiO2 helps to increase the specific surface area and pore volume of the ZnO/ _TiO2 nanocomposite material, thereby improving the sensitivity of the passive ammonia detection sensor in this embodiment. Due to the large specific surface area, there are more active sites, which can adsorb more ammonia molecules; in addition, the presence of _TiO2 is also conducive to optimizing the energy band structure of the composite material, which helps to efficiently separate electrons and holes; in the absence of ammonia, the n-type ZnO semiconductor material is exposed to the air, and oxygen molecules are adsorbed to the surface of ZnO, acting as electron acceptors, and capturing free electrons to form oxygen ions, which leads to a decrease in the electron concentration on the surface of the material, a widening of the depletion layer, and a decrease in conductivity, thereby increasing the resistance of the material; when ammonia is present, ammonia molecules react with oxygen ions adsorbed on the surface of the material, leaving electrons on the surface of the nanocomposite material, and this process increases the electron concentration on the surface of the nanocomposite material, thereby reducing the resistance of the passive ammonia detection sensor. The specific reaction process is as follows.

In this embodiment, the above-mentioned ZnO/ _TiO2 nanocomposite material is coated on the surface of the microstrip patch 101. Therefore, when ammonia reacts with oxygen ions, electrons remain on the surface of the nanocomposite material, and the impedance of the microstrip patch antenna changes accordingly. These changes will be reflected in the amplitude of the antenna resonant frequency point to achieve the purpose of detecting the ammonia concentration.

Furthermore, in order to improve the sensitivity of the passive ammonia detection sensor and increase the response speed of the sensor, the preparation method of this embodiment also includes optimizing the parameters of the above-mentioned passive ammonia detection sensor to improve the performance of the sensor.

In this embodiment, the performance of the passive ammonia detection sensor is adjusted by adjusting the length and width of the microstrip patch 101. As shown in Figure 8, the longitudinal dimension of the substrate 100 is defined as the length SL; the lateral dimension of the substrate 100 is the width SW; the longitudinal dimension of the microstrip patch 101 is the length L, and the lateral dimension of the microstrip patch 101 is the width W; in this embodiment, the feeder unit 101a is divided into a connecting portion and an extension portion, and the longitudinal dimension of the extension portion is defined as the length L1, and the lateral dimension of the extension portion is defined as the width W1; the longitudinal dimension of the connecting portion is defined as the length L2, and the lateral dimension of the connection is defined as the width W2; the longitudinal dimension of the parasitic unit 103 is defined as the length L3, and the lateral dimension of the parasitic unit is defined as the width W3.

In this embodiment, the length L and width W of the microstrip patch 101 are used as variables to adjust the size of the microstrip patch 101 to perform frequency sweep analysis to obtain the return loss S11, and the result is shown in FIG9 .

As shown in FIG9(a), the width W of the microstrip patch 101 remains unchanged, and the length L of the microstrip patch 101 is swept with a step amplitude of 1 mm between 28.21 mm and 31.21 mm. The results show that when the length L of the microstrip patch 101 increases, the return loss S11 moves downward, and the resonant frequency moves to the left; as shown in FIG9(b), the length L of the microstrip patch 101 remains unchanged, and the width W of the microstrip patch 101 is swept with a step amplitude of 1 mm between 28.21 mm and 31.21 mm. The results show that when the width W of the microstrip patch 101 increases, the return loss S11 moves downward, and the resonant frequency remains unchanged. And it can be seen from FIG9(a) and FIG9(b) that when the length L of the microstrip patch 101 is 30.21 mm and the width W is 30.21 mm, the return loss S11 is the smallest.

In addition, since the electric field direction of the electromagnetic wave of the circularly polarized antenna will rotate in a circular trajectory over time, it has a better ability to receive signals. In the prior art, circular polarization is achieved by cutting corners and opening slits in the microstrip patch antenna, while the microstrip patch antenna in this embodiment is circularly polarized by opening a rectangular slit on the microstrip patch 101. In this embodiment, the length and width of the rectangular slit are respectively parallel to the diagonal of the microstrip patch 101, and the rectangular slit coincides with the center of the microstrip patch 101. The effect is shown in Figures 1 to 3. In this embodiment, the ability of the microstrip patch antenna to receive signals is improved by circular polarization and a nano-composite material sensitive to ammonia is loaded to prepare a passive ammonia detection sensor to detect ammonia.

Specifically, the bandwidth with an axial ratio of no more than 3 dB is defined as the circular polarization bandwidth of the antenna. The specific axial ratio formula is as follows:

Among them, AR is the axial ratio of the circularly polarized antenna; OA is the long axis of the electromagnetic wave projection, OB is the short axis of the electromagnetic wave projection, and the antenna with an axial ratio AR less than 3db has a good circular polarization effect.

In order to further improve the circular polarization effect of the microstrip patch antenna, in this embodiment, the length and width of the rectangular slot are adjusted to make the surface wave produce a 90-degree phase difference to achieve circular polarization, and the result is shown in FIG. 10 .

In this embodiment, the length and width of the rectangular slot are used as variables, specifically: the width of the rectangular slot is kept constant, the length of the rectangular slot is between 6mm-10mm, and the frequency sweep analysis is performed with a step amplitude of 1mm, and the result is shown in Figure 10(a); the length of the rectangular slot is kept constant, the width of the rectangular slot is between 6mm-10mm, and the frequency sweep analysis is performed with a step amplitude of 1mm, and the result is shown in Figure 10(b); As can be seen from Figure 10(a), as the length of the rectangular slot increases, the axial ratio of the microstrip patch antenna first decreases and then increases. The length of the rectangular slot is 8m m is optimal. As shown in Figure 10(b), as the width of the rectangular gap increases, the axial ratio of the microstrip patch first decreases and then increases, and it is optimal when the width of the rectangular gap is 2mm; therefore, the gap length of the microstrip patch antenna in this embodiment is 8mm and the width is 2mm; and as shown in Figure 10(c), the circular polarization effect of the microstrip patch antenna in this embodiment is best when the rectangular gap length is 8mm and the width is 2mm, and the corresponding axial ratio AR at the 2.25GHz resonance point is 2.77db, and as shown in Figure 10(d), the current of the microstrip patch antenna in this embodiment is right-handed.

In this embodiment, the performance of the microstrip patch antenna is improved by optimizing the length and width of the microstrip patch 101 and the rectangular gap, so that the microstrip patch antenna has good resonance characteristics, and the sensitivity and response speed of the microstrip patch antenna are improved. And as shown in Figures 11(a) and 11(b), the electric field strength and magnetic field strength of the microstrip patch antenna are concentrated at the edge of the microstrip patch 101, and radiate outward through the gap between the microstrip patch 101 and the ground patch 102; in addition, as shown in Figure 11(c), the normalized impedance value of the microstrip patch antenna is close to 1.0, and its impedance characteristics are good. As shown in Figure 11(d), the microstrip patch antenna in this embodiment has a uniform and rounded three-dimensional radiation pattern, and the antenna has good directivity.

Since the cross-polarization of the microstrip patch antenna will also greatly affect the signal receiving ability of the receiving antenna, excessive cross-polarization will reduce the signal quality and increase interference, thereby affecting the performance and reliability of the communication system; in addition, a high cross-polarization ratio may cause the orthogonality of the signal obtained from the antenna to weaken and the correlation between the two signals to increase, which is very unfavorable for information transmission. In this embodiment, in order to reduce the cross-polarization of the antenna, as shown in Figures 1 to 3, a metal short circuit is connected to the connection between the microstrip patch 101 and the feed unit 101a. The metal short circuit will cause the current to concentrate around the short circuit point, thereby changing the radiation direction and gain of the microstrip patch antenna. In addition, the short circuit will change the current and magnetic field distribution, thereby changing the radiation characteristics of the microstrip patch antenna and reducing the cross-polarization of the h plane (magnetic plane, parallel to the direction of the magnetic field) in the microstrip patch antenna. The steady-state value of the short-circuit current is the periodic component of the short-circuit current, so the periodic component of the short-circuit current is called the short-circuit capacity of the system. The larger the short-circuit capacity, the greater the load capacity of the system. Therefore, the microstrip patch antenna in this embodiment is provided with a metal short circuit at the connection between the feed unit 101a and the microstrip patch 101, so as to improve the carrying capacity of the microstrip patch antenna and also suppress the interference of the cross-polarization of the microstrip patch antenna on the co-polarization.

As shown in Figure 12, it is not difficult to see from Figure 12(a) and Figure 12(b) that the cross-polarization of the microstrip patch antenna is changed after loading the metal short circuit, and it can be seen from Figure 12(c) that the amplitude of the radiation pattern of the microstrip patch antenna is reduced from -11db to -14db after loading the metal short circuit, which means that the microstrip patch antenna after loading the metal short circuit effectively reduces the cross-polarization to improve the performance of the antenna.

In addition, in this embodiment, it can be seen from Figure 11 that the electric field strength and magnetic field strength of the microstrip patch antenna are concentrated at the edge of the microstrip patch 101, and radiate outward through the gap between the microstrip patch 101 and the ground patch 102 around it; for this reason, as a further optimization method of the microstrip patch antenna, in this embodiment, as shown in Figures 1 to 3, the passive ammonia detection sensor in this embodiment also includes a parasitic unit 103, the parasitic unit 103 is a full copper patch and is located on one side of the microstrip patch 101, and there is a gap between the parasitic unit 103 and the microstrip patch 101. It is worth noting that in this embodiment, the microstrip patch 101 and the feeder unit 101a together constitute a main radiating unit. When the microstrip patch antenna in this embodiment introduces the parasitic unit 103, the main radiating unit is excited after the electromagnetic wave energy is fed in, and the current generated by the coupling between the parasitic unit 103 and the main radiating unit radiates outward, thereby improving the antenna gain while also suppressing the propagation of surface waves.

On the other hand, the present embodiment provides an indoor ammonia detection system constructed by the above-mentioned passive ammonia detection sensor, the indoor ammonia detection system includes the above-mentioned optimized passive ammonia detection sensor, a reader and a vector network analyzer, the reader includes a transmitting antenna and a receiving antenna, the passive ammonia detection sensor in the present embodiment is placed in a closed space, the reader transmitting antenna transmits a signal within the identification range, when the passive ammonia detection sensor in the identification range obtains energy and is activated, the information and data carried and detected by itself are transmitted to the reader through the antenna, the reader receiving antenna receives the information of the microstrip patch antenna, sends it to the reader through the radio frequency interface, and after demodulation and decoding processing, the reader realizes the perception and detection of ammonia by receiving and decoding the signal data reflected by the sensor, and the reader transmits the received signal to the vector network analyzer through the coaxial line, and the detection result can be observed intuitively. In the process of ammonia perception and detection, the sensitive material will cause the relative dielectric constant of the antenna to change, the real part change of the relative dielectric constant corresponds to the resonant frequency shift, and the imaginary part change corresponds to the amplitude change of the resonant frequency, and then the signal change transmitted to the reader is changed to realize passive detection.

In order to illustrate that the indoor ammonia detection system in this embodiment can be well applied to detect ammonia in a closed agricultural environment, this embodiment constructs a model of the indoor ammonia detection system in a laboratory, as shown in FIG13 . In order to verify that the ZnO/TiO ₂ nanocomposite material can well sense ammonia, this embodiment also constructs an ammonia sensor by loading ZnO material on a microstrip patch antenna to detect ammonia as a comparative example. The process is as follows:

The passive ammonia detection sensor loaded with ZnO/ _TiO2 nanocomposite materials and the sensor loaded with ZnO nanomaterials were placed in the corresponding gas boxes, connected to the vector network analyzer through a coaxial line, and different concentrations of ammonia were introduced for detection after the data was stable. Specifically, 25ppm of ammonia was introduced into the gas box every 5 minutes and the change curve of S11 value was recorded. The result is shown in Figure 14 (b). The S11 value of the ammonia sensor loaded with ZnO material showed a downward trend as the ammonia concentration increased. Since the small specific surface area of ZnO is not conducive to the absorption and diffusion of ammonia, the response effect and sensitivity of the sensor are poor, and the S11 value changes by about 1.2db; as shown in Figure 14 (d), the fitting curve of the ZnO material ammonia sensor in the ammonia environment of 0-100ppm concentration, the fitting degree ^R2 = 0.90537, and the sensitivity of the sensor is 0.01db/ppm. As shown in Figure 14 (a), the passive ammonia detection sensor loaded with ZnO/ _TiO2 nanocomposite material has a decreasing S11 value as the ammonia concentration increases. However, compared with ZnO, ZnO/ _TiO2 nanocomposite material has a higher specific surface area, which is conducive to the absorption and diffusion of ammonia molecules. In addition, it has a certain selectivity for ammonia, which reduces the interference of other gases such as _CO2 in the ammonia detection of the sensor in the actual agricultural scene, and finally makes the response effect and sensitivity of the sensor better, and the S11 value changes by about 4.08db. As shown in Figure 14 (c), the fitting curve of the passive ammonia detection sensor loaded with ZnO/ _TiO2 nanocomposite material in the ammonia environment with a concentration of 0-100ppm, the fitting degree ^R2 = 0.97268, and the fitting degree is greatly improved while the sensitivity of the sensor is also improved to 0.04db/ppm. In summary, the passive ammonia detection sensor loaded with ZnO/ _TiO2 nanocomposite material has a better effect on ammonia detection.

In addition, the response time of the above two sensors to ammonia is also studied in this embodiment, and the results are shown in Figure 15. The passive ammonia detection sensor loaded with ZnO/ _TiO2 nanocomposite material reaches a stable resonance change in the ammonia environment after about 60 seconds, so the response time is 60 seconds, while the response time of the ammonia sensor of pure ZnO material is delayed to about 85 seconds. In comparison, the passive ammonia detection sensor of ZnO/ _TiO2 nanocomposite material has a better response effect and the effect is obvious. In addition, this embodiment repeats the test 3 times for the passive ammonia detection sensor loaded with ZnO/ _TiO2 nanocomposite material, and the results are shown in Figure 16. The results show that the reproducibility of the above passive ammonia detection sensor is good.

In summary, the passive ammonia detection sensor constructed by loading ZnO/ _TiO2 nanocomposite materials in this embodiment has high sensitivity, fast response and good reproducibility. The indoor ammonia detection system built by the passive ammonia detection sensor can be used in the agricultural field to detect ammonia concentration, which improves the robustness of the passive ammonia detection sensor in ammonia detection and has broad application prospects.

The above content is a further detailed description of the present invention in combination with specific implementation methods. It cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as belonging to the scope of protection determined by the claims submitted for the present invention.

Claims (10)

1. A method for preparing a chipless RFID ammonia detection sensor based on ZnO/ _TiO2 nanocomposite material, characterized in that it includes preparing a microstrip patch antenna and loading the ZnO/ _TiO2 nanocomposite material on the microstrip patch antenna, and the specific steps are as follows: Preparation of a microstrip patch antenna: using an FR4 plate as a substrate (100), a ground patch (102) being bonded to the lower surface of the substrate (100), a microstrip patch (101) being bonded to the upper surface, the microstrip patch (101) being connected to a feeder unit (101a), the feeder unit (101a) being bonded to the substrate (100) and connected to the ground patch (102) via a joint; The ZnO/ _TiO2 nanocomposite material is coated on the surface of the microstrip patch (101). 2. The method for preparing a chipless RFID ammonia detection sensor based on a ZnO/ _TiO2 nanocomposite material according to claim 1, wherein the method for preparing the ZnO/ _TiO2 nanocomposite material is as follows: After the zinc acetate is dissolved, it is dispersed by ultrasonication and then magnetically stirred, and ammonia water, sodium borohydride and polyvinyl pyrrolidone are added to obtain a suspension, the suspension is placed in a high-pressure reactor, reacted, cooled to room temperature, centrifuged to obtain a white precipitate, and the white precipitate is washed and dried to obtain ZnO powder; The titanium tetrachloride is dissolved, dispersed by ultrasonic and then magnetically stirred, and the prepared ZnO powder is added, dispersed by ultrasonic and then magnetically stirred, and then placed in a high-pressure reactor for reaction, cooled to room temperature and centrifuged to obtain a white precipitate, which is washed and dried to obtain a ZnO/ _TiO2 nanocomposite material. 3. The method for preparing a chipless RFID ammonia detection sensor based on ZnO/ _TiO2 nanocomposite material according to claim 2, characterized in that it also includes a step of optimizing the parameters of the sensor, the parameters including the length and width of the microstrip patch (101), and the specific steps are as follows: The width of the microstrip patch (101) is a fixed value, and the length is a variable. A frequency sweep analysis is performed with a step amplitude of 1 mm to obtain a return loss, and the length corresponding to the minimum return loss is used as the length of the microstrip patch (101); The length of the microstrip patch (101) is a fixed value, and the width is a variable. A frequency sweep analysis is performed with a step amplitude of 1 mm to obtain a return loss, and the width corresponding to the minimum return loss is used as the width of the microstrip patch (101). 4. The method for preparing a chipless RFID ammonia detection sensor based on ZnO/ _TiO2 nanocomposite material according to claim 3 is characterized in that a rectangular gap is also provided on the microstrip patch (101), the length and width of the rectangular gap are respectively parallel to the diagonal of the microstrip patch (101) and the center of the rectangular gap coincides with the center of the microstrip patch (101), the method further comprises the step of optimizing the size of the rectangular gap, the size of the rectangular gap includes the length and width of the rectangular gap, and the specific steps are as follows: The width of the rectangular gap is a constant, and the length is a variable. The return loss is obtained by performing a frequency sweep analysis with a step amplitude of 1 mm, and the length corresponding to the minimum return loss is taken as the length of the rectangular gap. The length of the rectangular gap is a constant, and the width is a variable. The return loss is obtained by performing a frequency sweep analysis with a step amplitude of 1 mm, and the width corresponding to the minimum return loss is taken as the width of the rectangular gap. 5. The method for preparing a chipless RFID ammonia detection sensor based on ZnO/ _TiO2 nanocomposite material according to claim 4 is characterized in that a metal short circuit is loaded at the connection between the microstrip patch (101) and the feeder unit (101a), and the metal short circuit causes the current to be concentrated around the connection, which is used to change the radiation direction and gain of the microstrip patch antenna and change the current and magnetic field distribution, thereby changing the radiation characteristics of the microstrip patch antenna and reducing the cross polarization of the microstrip patch antenna. 6. The method for preparing a chipless RFID ammonia detection sensor based on ZnO/ _TiO2 nanocomposite material according to claim 5 is characterized in that a parasitic unit (103) is arranged on one side of the microstrip patch (101) on the substrate (100), and there is a gap between the parasitic unit (103) and the microstrip patch (101), and the current generated by the coupling between the parasitic unit (103) and the microstrip patch (101) is radiated outward to improve the gain of the microstrip patch antenna. 7. A passive ammonia detection sensor prepared by the method for preparing a chipless RFID ammonia detection sensor based on ZnO/ _TiO2 nanocomposite material according to any one of claims 1 to 6. 8. An indoor ammonia detection system constructed by the passive ammonia detection sensor as described in claim 7, characterized in that it also includes a reader and a vector network analyzer, the reader includes a transmitting antenna and a receiving antenna, the transmitting antenna and the receiving antenna are coaxially connected to the vector network analyzer, the reader transmitting antenna transmits a signal, and when the passive ammonia detection sensor obtains a signal and is activated, it transmits the information and data it carries and detects to the reader, the reader receives the information of the microstrip patch antenna through the receiving antenna and transmits it to the reader, the reader transmits the received signal to the vector network analyzer through the coaxial cable after demodulation and decoding, and the observation result is analyzed by the vector network. 9. The detection method of the ammonia detection system according to claim 8 is characterized in that the return loss of the microstrip patch antenna changes after the ZnO/ _TiO2 nanocomposite material in the passive ammonia detection sensor senses ammonia, and the receiving antenna transmits the signal to the reader, which feeds back the signal to the vector network analyzer after processing, and analyzes the change in ammonia concentration through the signal change of the vector network analyzer. 10. The detection method according to claim 9 is characterized in that oxygen molecules in the air obtain electrons to form oxygen ions after contacting the ZnO/ _TiO2 nanocomposite material, and ammonia molecules in the ammonia gas react with the oxygen ions to release electrons and return to the surface of the ZnO/ _TiO2 nanocomposite material to change the resistance of the passive ammonia sensor and thus change the return loss for detecting the ammonia concentration.