RU2550697C1 - Sensor based on surface acoustic waves to measure concentration of carbon dioxide - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: sensor comprises a tight body, inside of which there is a piezoelectric acoustic line, on the working surface of which there is a transceiving interdigital transducer (IDT), loaded to an antenna, which is arranged outside the tight body, a support reflecting IDT and a reflecting IDT loaded to impedance located outside the tight body, the value of which is sensitive to the measured value, and an acoustic absorber applied at the ends of the acoustic line. The impedance is made in the form of a lattice of parallel connected nanorods of zinc oxide.

EFFECT: provision of maximum sensitivity of a sensor to concentration of carbon monoxide due to usage of zinc oxide rods as impedance depending on carbon monoxide concentration, with resistance close to resistance of radiation of a reflecting IDT.

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Description

The invention relates to piezoelectric sensors designed for remote monitoring of various physical and chemical quantities.

Sensors based on surface acoustic waves (SAWs) are known, containing a housing inside which a piezoelectric sound duct is located, on the working surface of which there are two interdigital transducers (IDT) and an acoustic absorber deposited on the ends of the sound duct [1, 2] (Dias JF Hewlett-Packard J. - 1981 / - V.32, N 12. - P.21-37. [1], Kostromin AC, Rozanov I.A., Chernykh E.V., Kuvakhara Hiroyuki, Tomilova L.G., Zefirov N. .C. Sensor on surface acoustic waves for the detection of carbon dioxide. Patent of the Russian Federation RU 2132584 C1, IPC 6 H01L 41/18 from 1999.06.27 [2]). In one of the sensors, the sound duct with IDT on the working surface is a delay line, which is included in the feedback circuit of the amplifier and is an electric oscillation generator, the frequency of which depends on the temperature or the magnitude of the sound duct deformation [1]. The signal from the sensor using a transmitting antenna connected to the generator is transmitted to the receiving device, which performs remote monitoring. The device of another sensor [2] is similar, only between the IDT there is a film that can selectively absorb various substances. In this case, the sensor can monitor the appearance of various substances. In this case, the housing cannot be made airtight, which reduces the reliability of the sensor, since various aggressive substances can destroy the metal film of which the IDT is made. Since the sensor includes an amplifier, the sensor needs a power source that must be changed periodically and which may fail (discharge) at an unexpected time, which reduces the reliability of the sensor. In addition, the presence of semiconductor elements in the amplifier can lead to its failure in the presence of ionizing radiation, which also reduces the reliability of the sensor.

These defects can be eliminated by a sensor containing a sealed enclosure, inside which a piezoelectric sound duct is located, on the working surface of which there is a bi-directional transceiver interdigital transducer (IDT) loaded on an antenna that is located outside the sealed enclosure, a reference reflective unidirectional IDT with internal reflectors and reflective unidirectional IDT with internal reflectors, loaded on the impedance located outside the sealed enclosure, on which it is sensitive to the measured value, and akustopoglotitel supported on the ends of the acoustic line. The impedance value may be sensitive to temperature, pressure, humidity, ionizing radiation, electromagnetic radiation, the presence of various substances [3] (Bagdasaryan AS, Bagdasaryan SA, Karapetyan G.Ya., Dneprovsky VG Physical sensor values on surface acoustic waves.Patent of the Russian Federation 2387051 C1, IPC G01D 5/12 of 04/20/2010), taken as a prototype.

In this device, the reflection coefficient depends on the magnitude of the impedance, the magnitude of which depends on the measured physical quantity. Since the housing is sealed, the IDT and the substrate are isolated from the environment, which increases the reliability of the sensor. The absence of semiconductor elements in the sensor makes this sensor insensitive to ionizing radiation. The lack of a power source allows you to place this sensor in hard-to-reach places only once. The sensor is polled using a reader that sends a polling electromagnetic pulse, which is received by the sensor antenna and converted into surface acoustic waves (SAWs), which, reflected from the reflective IDTs, are received by the IDT transceiver and converted again into the electromagnetic signal received by the reader receiver. In this case, the delay and amplitude of the reflected pulses is different due to the fact that the reflectance of the surfactant from the reflective IDT loaded on the impedance differs from the reflectance of the surfactant from the unloaded reflective IDT, and they are located at different distances from the transceiver IDT. A comparison of the amplitudes of the reflected pulses makes it possible to determine the measured value from the known dependence of the reflection coefficient on the magnitude of the impedance. To determine the concentration of carbon monoxide, it is necessary to use an impedance sensitive to its concentration. However, the existing impedances based on thin semiconductor films are quite high-resistance and significantly weaken the influence of the change in impedance on the reflection coefficient of the surfactant. In addition, the resistance of these films is extremely sensitive to contamination.

The problem to which the invention is directed, is to increase the measurement accuracy by creating an impedance sensitive to the concentration of carbon monoxide, in which even its slight change causes a noticeable change in the reflection coefficient of surfactants from IDT.

The technical result that the invention provides provides for maximum sensitivity of the sensor to the concentration of carbon monoxide due to the use of zinc oxide nanorods, the resistance of which is close to the radiation resistance of reflective IDT, as an impedance depending on the concentration of carbon monoxide.

This is achieved by the fact that in the sensor, which contains a sealed enclosure, inside which there is a piezoelectric sound duct, on the working surface of which there is a transceiver interdigital transducer (IDT) loaded on an antenna that is located outside the sealed enclosure, a reference reflective IDT and a reflective IDT loaded the impedance located outside the sealed enclosure, the value of which is sensitive to the measured value, and the acoustic absorber applied to the ends of the sound duct, the impedance is made in the form of a lattice of parallel-connected zinc oxide nanorods.

Figure 1 shows the topological structure of a device for surfactants in accordance with the invention.

The sensor contains a piezoelectric sound duct 1, on the working surface of which there is a transceiver IDT 2, a reflective unidirectional IDT 3, which is loaded on an impedance 4 of connected zinc oxide nanorods, another reflective unidirectional IDT 5. Acoustic absorbers 6 are located at the ends of the sound duct, and IDT 2 is loaded to the transceiver antenna 7. The sound pipe along with the IDT is placed in a sealed enclosure 8.

Figure 2 shows a schematic representation of a sensitive element of the SAW sensor CO (impedance). The upper electrode 9 is applied directly to the lattice of zinc oxide nanorods 10, which are grown on a buffer sublayer of zinc oxide 11 to improve the vertical orientation of ZnO nanorods 10. This film sublayer of zinc oxide 11 is sprayed onto a sapphire substrate 12. The lower gold electrode 13 is sprayed onto the sublayer of zinc oxide 11 which protrudes to the right from under the nanorods 10.

The sensor operates as follows. When an interrogating electromagnetic pulse is applied to the transceiving antenna 7, it is converted by means of IDT 2 into SAW pulses that are reflected from IDT 3 and IDT 5. Reflected SAW pulses from IDT 3, 5 are converted back by IDT 2 to electromagnetic pulses that are emitted by antenna 7 Since a unidirectional IDT 5 is not loaded, the surfactants incident on it will almost completely be reflected from it, since a unidirectional IDT in idle mode (in the absence of load) should reflect all surfactants incident on it back. In the mode of full coordination of IDT 3 with a load in the form of bound nanorods of zinc oxide, surfactants reflected from active electrodes and surfactants reflected from internal reflectors are in antiphase. In this case, IDT 3 will not reflect SAW. But it’s very difficult to achieve full coordination, and this can be done only on one frequency. Therefore, the interrogating pulse will be reflected from such IDT much less than from unloaded IDT 5, and the reflection coefficient will depend on the degree of matching IDT 3 with load (impedance) 4, i.e. on the magnitude of this load Z. This magnitude, in turn, will depend on the measured concentration of carbon monoxide. Therefore, the reflection coefficient from the sensor will depend on the measured concentration. Since the reading pulse reflected from the unloaded reflecting IDT 5 will have a constant amplitude, comparing this amplitude with the amplitude of the reading pulse reflected from IDT 3, one can judge the measured physical quantity. The ratio of these amplitudes will not depend on the relative position of the sensor reader antennas, but will depend only on the ratio of the reflection coefficients from IDT 3 and 5, which depends on the magnitude of the impedance loaded on IDT 3, and, consequently, on the measured physical quantity.

The ZnO lattice of nanorods 10 on a sapphire substrate 12 (see Fig. 2) was sprayed by us using pulsed laser spraying at high argon pressure [4] (M. Lorenz, M .; Kaidashev, EM; Rahm, A .; Nobis, Th .; Lenzner, J .; Wagner, JG; Spemann, D .; Hochmuth, H .; Grundmann, M. Appl. Phys. Lett. 2005, 86, 143113). Spraying was carried out in an evacuated quartz cell with an external resistive heater. Laser radiation from a KrF laser (λ = 248 nm, E = 300 mJ) was focused on the surface of a rotating ceramic ZnO target. The power density on the target surface was 2 J / cm ² . The target-substrate distance was 5-35 mm. The laser pulse repetition rate was 3-10 Hz. The synthesis of micro- and nanocrystals was carried out for 12000-24000 laser pulses. The substrate temperature ranged from 850-950 ° C. The argon flow was 50 sccm at a pressure of 75-300 mbar. Single-crystal substrates of c- and a-sapphire 10 × 10 × 0.5 mm in size with a buffer layer of zinc oxide 11 were parallel to the laser plume. The buffer film sublayer of zinc oxide 11 was sprayed to improve the vertical orientation of ZnO nanorods and to connect the lower gold electrode 13. The ZnO film was sprayed for 10,000 laser pulses at an oxygen pressure of 2 × 10 ^-2 mbar, target-substrate distance ~ 70 mm and a temperature of 670 ° C. As a catalyst, we used a gold film with a thickness of 1-2 nm.

To create the upper electrode 9 to the lattice of zinc oxide nanorods, gold was sputtered at an angle of 45 ° in high vacuum through a mask with a round hole. Due to the dense arrangement, the nanorods shield each other, which prevents dusting of the base of the structure. Thus, a gold contact is created at the top of the nanorods and combines them together.

Execution example. The sensor is made on sound duct 1 from a YX-section of lithium niobate with dimensions of 8 × 1.4 × 0.5 mm. IDTs 3 and 5 are made with internal reflectors with a period of two SAW lengths at the center frequency f ₀ = 870 MHz and a length of 33 SAW lengths at the center frequency, which provides a unidirectional mode of 15 dB. IDT 2 is made bidirectional with the width of the electrodes equal to a quarter of the length of the surfactant at the center frequency, and has a length equal to 20 lengths of the surfactant. IDT 5 is located at a distance of 2.15 mm from IDT 2, and IDT 3 is at a distance of 4 mm, which provides a delay between reflected pulses of 1 μs, and reflected from IDT 5 and reflected from IDT 3 pulses - 0.074 μs with a reading pulse length 0.037 μs, i.e. these pulses do not overlap. The IDT apertures 2, 3, 5 were chosen equal to 80 SAW lengths at the central frequency, which allows neglecting diffraction losses at given distances. The radiation resistance of reflective IDTs is close to 50 Ohms. The insertion loss when applying a signal to IDT 2 and removing it from IDT 3 was 10 dB. The sound pipe along with the IDT is placed in a sealed enclosure. Antenna 7 in the form of a half-wave vibrator and impedance 4 are located outside the housing.

The sensitive element of the surfactant of the CO sensor (impedance) consists of parallel connected zinc oxide nanorods 11. By increasing the area of the upper electrode 9, it is possible to reduce the total resistance of the associated lattice of zinc oxide nanorods. The magnitude of the impedance Z, depends on the concentration of CO. When CO adsorption, the resistance of the sensitive element 4 (impedance) varies in the range of 50 to 400 Ohms.

Information sources

1. Dias J.F. Hewlett-Packard J. - 1981 / - V.32, N 12. - P.21-37.

2. Kostromin A.S., Rozanov I.A., Chernykh E.V., Kuwahara Hiroyuki, Tomilova L.G., Zefirov N.S. Surface acoustic wave detector for detecting carbon dioxide. RF patent RU 2132584 C1, IPC 6 H01L 41/18 from 1999.06.27.

3. Bagdasaryan A.S., Bagdasaryan S.A., Karapetyan G.Ya., Dneprovsky V.G. The sensor of a physical quantity on surface acoustic waves. RF patent 2387051 C1, IPC G01D 5/12 of 04/20/2010.

4. M. Lorenz, M .; Kaidashev, E.M .; Rahm, A .; Nobis, Th .; Lenzner, J .; Wagner, J.G .; Spemann, D .; Hochmuth, H .; Grundmann, M. Appl. Phys. Lett. 2005, 86, 143113.

Claims (1)

A sensor on surface acoustic waves for measuring carbon monoxide concentration, containing a sealed enclosure, inside which there is a piezoelectric sound duct, on the working surface of which there is a transceiver interdigital transducer (IDT) loaded on an antenna that is located outside the sealed enclosure, a reference reflective IDT and a reflective IDT loaded on an impedance located outside the sealed enclosure, the magnitude of which is sensitive to the measured value, and acoustic absorption Tel supported on ends acoustic conductor, characterized in that the impedance is made as a lattice of parallel-connected zinc oxide nanorods.

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