LT6821B - Gas sensor with capacitive micromachined ultrasonic transducer structure and functional polymer layer - Google Patents

Abstract

The invention relates to a gravimetric gas sensor for the selective detection of inorganic "acidic" gases, such as sulfur dioxide and carbon dioxide, in a gas mixture, without limiting the possibility of use with other gases. The sensor consists of capacitive micromachined ultrasonic transducer (CMUT) structure and functional material. The prior-art patents are close to this invention in that they rely on the measurement of changes in the CMUT structure resonant frequency. However, different functional materials are required to selectively detect different gases. The present invention is based on the use of functional polymers having specific properties, such as mPEI (methylated polyethyleneimine), without limiting the possibility of using other polymers having suitable properties. The problem is solved: more accurate, faster and more efficient gas detection and concentration measurement. The invention utilizes cross-sensor selectivity for different gases, achieved by refraining from modifying the sensor with different materials, instead employing more sophisticated measurement of detector dynamic parameters. CMUT structure is modified by functional material. The interaction with the gas changes the physical properties of the functional material (mass, modulus of elasticity), resulting in changes in the CMUT membrane loading parameters that are measured. The invention can be applied by mining companies, a wide range of household consumers.

Description

TECHNICAL FIELD

The invention relates to sensors for materials, and more particularly to gravimetric gas sensors and meters having resonators with a modified surface modified with functional polymers that react with target gas molecules and alter the dynamic properties of the resonator.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (MEMS) are currently used as resonant, acoustic, gravimetric chemical or biochemical sensors. The Capacitive Micromachined Ultrasonic Transducer (CMUT) consists of cells that are capacitors with a single moving plate (membrane) separated from the structural base by a vacuum gap and insulating brackets. Disc, quadrangular or polygonal membranes are used. The main structure of the transducer is formed on a plate of silicon or other material used as the lower electrode; the upper electrode is mechanically aligned with the moving plate. When voltage is applied to the electrodes, the moving plate bends toward the base due to coulomb interaction. The vibration of the moving plate can be induced by an alternating electric field. CMUT elements may have one or more cells; the converter consists of a set of elements. When the CMUT element is loaded with additional mass, for example with a polymer coating, the dynamic parameters of the transducer change: the resonant frequency and the electromechanical impedance.

To date, a number of attempts have been made to use a micromembrane electrostatically activated CMUT structure for gas sensors.

CMUT converters can have a single element as well as one-dimensional and two-dimensional element arrays with the ability to measure simultaneously in multiple channels while maintaining small converter dimensions. A high potential for parallel measurements is obtained with a large number of measuring probes in a small area. Multidimensional and multi-channel CMUT sensors are mentioned in U.S. Pat. No. 9,366,651B2.

For selective gas detection, the surface of CMUT sensors is modified with polymers that specifically interact with the analyte gas by selectively adsorbing them to the modified surface, thus allowing the detection of physical and / or chemical changes in the modified surface. The ability to detect mass changes due to the adsorption of gas molecules is created by analyzing the changes in the resonant frequency. Modification of CMUT sensor membranes with functional materials and polymers is described in prior art patent documents WO2011026836A1, DE200910040052, US9366651B2, US7871569B2.

Typically, various materials and polymers are deposited on the surface of the CMUT sensor by drop-by-layer deposition. Also, the surface of the sensor is modified by a centrifugation method, which allows repetitive layers of thin polymer to be obtained on the surface of the sensor. CMT coating by spinning is described in US7074634B2, US20070180916A1, CN107151864B.

WO2019032938A1 describes a technology for the production of an optically transparent capacitive micromountable ultrasonic transducer (CMUT) and states in paragraph [0092] that it can be configured as a gravimetric sensor with a selective functional layer on a vibrating membrane, where the vibration frequency of the membrane oscillates the target molecules bind (or adsorb) on the functional layer. Such a sensor can be designed to operate in a gaseous or liquid medium and function as a gas sensor or biosensor. However, real experiments show, for example, the source Microchimica Acta. October 2014, Volume 181, Issue 13-14, pp. 1749-175, DOI: 10.1109 / ULTSYM.2014.0646 that under certain conditions the specific interaction between gas molecules and the functional coating changes the modulus of elasticity of the functional coating or reduces the damping coefficient, and as a result the resonant frequency may not only decrease but also increase. Also, this patent application WO2019032938A1 does not disclose the use of CMUT to accurately and selectively measure concentrations of certain gases.

Gas sensors currently on the market, such as electrochemical sensors, have drawbacks and often react to different gas mixtures without the possibility of distinguishing between different gases according to the received signal, which limits the advantage of the information received. The specificity allows the selective detection of different gases in their mixture.

U.S. Pat. No. 8,689,606B2 (WO2010109363A2 / CN102362178B) describes a gas sensor chip for transmitting and receiving ultrasound configured over a sufficiently wide frequency range and measuring the concentration of at least one of the gas components in a range of at least two responses. The frequency range can be achieved by resizing the cell membranes, changing the diagonal voltage, and / or changing the air pressure for the CMIIT or MEMS microphone set. A sensor chip can be used to detect CO ₂ gas. The chip is implemented as a stand-alone device that does not require separate sensors. The present invention uses an ultrasonic propagation time measurement that has a lower accuracy potential than selective gravimetric. Also, functional materials with their properties are not used in the present invention, so cross-selective gas detection is not possible.

Patent application WO2011026836A1 (DE102009040052A1) describes a carbon dioxide sensor having a gas-sensitive layer made of linear polymer chains having side chains having a primary amino group. The amino group reacts with CO ₂ to form the carbamate, and the physical properties of the substance change during this reaction. This change can be measured with a field effect transistor (FET), a Kelvin probe, or by measuring changes in capacitance or mass. Examples of materials are siloxanes such as polyaminopropylmethyldiethoxyl, carbon nitride and cysteamine. The sensitive material can be mixed with a hydrophobic material or polymerized with hydrophobic monomers.

U.S. Pat. No. 9,366,651B2 describes surface modifications of a sensor array that provide different molecular adsorption or binding functions to the sensors. The first sensor in the array includes a first resonant element having a first surface having a receptor material coated over the first base material. The second sensor includes a second resonant element having a second surface comprising a receptor material coated with a second base material that is different from the first base material. The first base material, the second base material, and the receptor material are selected such that the first resonant element having a combination of receptor and first base material has a different ability to adsorb or bind one or more masses. The second resonant element having a combination of the receptor material with the second parent material is analytical. The receptor substances are 3-aminopropyltrimethoxysiloxane (AMO) and propyltrimethoxysiloxane (PTMS).

U.S. Patent No. 7871569B2 describes systems and methods for detecting biomolecules in a sample using biosensors with resonators whose surfaces have the functionality to react with target biomolecules. In an embodiment, the device comprises a piezoelectric resonator, the functionalized surface of which is configured to react with the target molecules, thereby varying the mass and charge of the resonator, which changes the frequency response of the resonator accordingly. The resonator frequency response affected by the sample is compared to reference parameters such as frequency response before exposure to the sample, retained initial frequency response, or control resonator frequency response.

The closest prior art sources to the present invention are US9366651B2 and US7871569B2. However, they do not describe how to selectively and accurately measure the concentrations of certain gases in a gas mixture, such as carbon dioxide and sulfur dioxide. These prototype patents also use other types of resonators (such as quartz crystal microbalance (QCM) or other resonators based on piezoelectric crystal properties) that have a lower sensitivity potential than the CMUT structure. It is therefore an object of the present invention to find functional polymers and measurement methods that allow accurate measurement of certain gas concentrations by CMUT sensors with a single CMUT converter (or transducers of the same type connected in parallel).

SUMMARY OF THE INVENTION

The invention solves the problem of selective gas detection in a gas mixture by a single CMUT converter. The developed system allows to gravimetrically measure and analyze the amounts of different gases in the gas mixture. In one embodiment of the invention, the gas is carbon dioxide and sulfur dioxide. Also, the invention increases the amount of information available about interactions between gas molecules and functional materials. This improves the reliability of the measurement channel and reduces uncertainties, enriches the sensor signal information, detects ongoing specific interactions with the sensor cells. The invention uses imine group polymers for selective gas detection.

An embodiment of the experiment of the invention comprises a harmonized system for detecting and measuring the concentration of carbon dioxide and sulfur dioxide in a gas mixture comprising:

• Modified surface polymer modified methylated polyethyleneimine (mPEI), where the surface is polymer coated by spincoating;

• the electronics of the measuring electronics circuit, which consists of the power supply electronics, and the oscillator, the frequency of which is given by the CMUT structure;

• Measurement data digitization and digital processing software.

In an embodiment, the system consists of an array of CMUT sensors modified with a thin layer of polymer and connected to a biasing electrical circuit for optimal operating point. The CMUT frequency and electromechanical impedance amplitude characteristics are monitored using an oscillator circuit that is powered by a separate power supply connected to an AC voltage source. The amplified signal and its parameters, including the resonant frequency and the amplitude of the oscillator output signal, are digitized by an oscilloscope and transmitted to a computer via a digital interface in digital format for digital processing, storage and display.

The embodiment is used for the selective measurement of a mixture of carbon dioxide and sulfur dioxide with CMUT sensors, for the use of these sensors in the analysis of gas mixtures, and for the accurate determination of the concentrations of a mixture of carbon dioxide and sulfur dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying diagrams and drawings form an integral part of the description of the invention and are provided as a reference to a possible embodiment of the invention, but are not intended to limit the scope of the invention. The drawings and diagrams do not necessarily correspond to the scale of the details of the invention. Details that are not necessary to explain the operation of the invention and are not related are not provided.

Fig. Figure 1 shows a schematic of the single cell components of a CMUT sensor.

The following elements are numbered in the figure: 101 - CMUT cell structure; 102 - insulating layer; 103 - upper electrode contact area; 104 upper electrode; 105 - surface functional polymer layer; 106 - vacuum gap; 107 - membrane; 108 - lower electrode contact area; 109 - lower electrode.

Fig. Figure 2 shows a block diagram of a measuring system.

The following elements are marked with numbers in the figure: 201 - bias generator; 202 - CMUT sensor; 203 - surface modification; 204 - oscillator circuit; 205 - oscillator circuit power supply; 206 - oscillograph; 207 computer.

Fig. Figure 3 shows a structural diagram of the measurement system.

The following elements are numbered in the figure: 301 - CMUT sensor; 302 - PCB; 303 - surface functional polymer; 304 - upper electrode (gold); 305 CMUT cells; 306 - CMUT cell array; 307 - epoxy layer; 308 - contact area; 309 - lower electrode contact area; 310 - gold thread; 311 - PCB track for lower electrode; 312 - lower electrode / ground contact; 313 - PCB track for upper electrode; 314 - upper electrode contact.

Fig. Figure 4 shows the 400 - resonant frequency shift and amplitude of the oscillator output signal as a function of time: before the reaction, during CO ₂ and SO ₂ reaction and after the reaction.

The following elements are numbered in the figure: 401 - initial frequency; 402 change in frequency during reaction with CO ₂ ; 403 - frequency change during reaction with CO ₂ and SO ₂ ; 404 - frequency change after reaction with CO ₂ and SO ₂ ; 405 - initial amplitude; 406 CO ₂ reaction onset; 407- amplitude during reaction with CO ₂ and SO ₂ ; 408- onset of CO ₂ and SO ₂ reaction; 409- end of CO ₂ and SO ₂ reaction;

Fig. _{Figure 5 shows the characteristics of the resonant frequency F re} z of a CMUT membrane (107) coated with a mPEI polymer layer, depending on the target gases in the test gas mixture. 501 - concentrations of carbon dioxide CO ₂ , 502 - concentrations of sulfur dioxide SO ₂ at ambient temperature 23 ° C.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses a surface-modified CMUT sensor for the selective detection of gas concentrations in a mixture of two gases, such as carbon dioxide and sulfur dioxide, using additional electronic components and measurement channels.

The surface of the membranes of the CMUT sensor is modified with polymers of the imine group, which can in principle be suitable for the selective detection of gases other than carbon dioxide and sulfur dioxide in the gas mixture.

Sensor design. The CMUT sensor (301) is comprised of at least one element (306) comprising at least one cell (101,305) having a diameter of 42 μm, the number of cells per element (306) being 1600. Each cell comprises a membrane (106) which is insulated by holders ( 102) is separated from the cell structural base (109) to form a vacuum gap (106). The top layer (104) of the membrane is made of a material having high electrical conductivity and good adhesion to the functional material, such as gold, on which an active functional material layer (105, 305) can be technologically formed to increase the selectivity of the transducer in the gas mixture. Other layers of materials can be used that combine the electrical, mechanical, morphological, and other properties of the membrane and the functional layer as needed. The shape of the membrane may be disk-shaped, quadrangular or polygonal, or other shapes. At least one layer of the membrane is electrically conductive and is used as the upper electrode (104, 105, 304). An insulating layer (102), electrode separation (108) and a vacuum gap (106) are formed between the membrane (107) and the substrate to vibrate the membrane. The sensor cells (101,305) are formed on an alloyed silicon wafer (109) which is used as the lower electrode (308). A voltage is applied to the upper electrode (104, 304) and the lower electrode (109, 309). The membrane (107) bends towards the base due to the Coulomb interaction, and the vibration of the membrane is induced by changing the bending of the membrane by a changing electric field. Voltage is applied to the upper and lower electrodes through special connectors (312, 314) constructed on a printed circuit board (302) whose tracks (311, 313) are connected to the electrodes of the CMUT sensor contacts (309, 304) coated with epoxy coating. (307).

The CMUT sensor cells (101, 305) are connected in parallel to form an array (306) of sensor elements. The array of membranes (306) is coated with gold (304) connecting all sensor cells (305) in parallel.

When the CMUT array is loaded with additional mass, the dynamic parameters of the sensor change: resonant frequency and electromechanical impedance. In order for the CMUT sensor to act as a chemical sensor, its cell surface is modified by a polymer of the imine group of the active substance (105, 303), such as methylated polyethyleneimine (mPEI) (303), which interacts with the target gas molecules under study. The surface modification (203) is performed by coating a layer of imine group polymer (e.g., mPEI) (303) on the sensor membranes (104, 106, 305) by centrifugation. As the gas molecules interact with the polymer layer, the resonant frequency and electromechanical impedance of the dynamic characteristics of the sensor (301) change, which determines the amplitude of the oscillator output signal. These variable parameters are recorded on a computer (207) to which an oscilloscope (206) is connected, receiving a real-time signal from the resonant frequency and amplitude of the CMUT membrane from the oscillator circuit (204) powered by a separate power supply (205). In this circuit, the sensor (301) acts as an electromechanical resonator. The operating point optimization is performed by bias-generating electronics (201).

The computer stores in real time data about changes in the resonant frequency of the CMUT sensor (301) and the amplitude of the oscillator signal output as the sensor surface modification layer (303, 105) reacts with carbon dioxide and sulfur dioxide to change the concentrations of this gas mixture.

Experimental system. A harmonized system (200) comprising an array of CMUT transducers (202) with a surface modified polymer (in this experiment: methylated polyethyleneimine or mPEI) coated on a CMUT sensor by centrifugation and a measuring electronics circuit consisting of power supply electronics, signal amplification electronics, an oscilloscope use for data storage in conjunction with a MatLab interface capable of detecting the concentration of carbon dioxide and sulfur dioxide in a mixture of these gases. This system consists of:

• A CMUT sensor array (202) modified with a thin polymer layer (203) connected to a bias-generating electrical circuit (201) for optimal operating point;

• The CMUT frequency and signal amplitude characteristics are monitored using a Colpitts or other suitable type of oscillator circuit (204) that is powered by a separate power supply (205) connected to an AC voltage source.

• The amplified signal and signal parameters including resonant frequency and amplitude are transmitted by the oscilloscope (206) to a computer (207) via a serial USB connection and stored with the MatLab software using a harmonized code that communicates with the oscilloscope (206) and stores the received data on the computer (207). .

• A computer (207) in the MatLab software package processes and analyzes the collected signal data and determines the _{concentrations of the target gases (in the case of this experiment, carbon dioxide CO 2} and sulfur dioxide SO ₂ ) in the test mixture.

Measurement data processing and results. The results of the experiment are related to cross-selective gas detection and real-time measurement, as shown in Fig. 4, (400). Experiments were performed with dry _{mixtures of nitrogen N 2} , carbon dioxide, CO2, and sulfur dioxide SO2 to determine changes in the membrane resonance frequency of the polymer-mPEI-functional CMUT sensor and changes in the amplitude of the oscillator output signal. First, the test chamber was vented with a constant flow of dry nitrogen N2 to bring the CMUT sensors to equilibrium. The initial resonance frequency (401) and signal amplitude (405) were measured in real time and recorded continuously throughout the experiment. When CO2 gas was introduced into the chamber due to the weak interaction of mPEI and CO2 molecules, a resonance frequency shift Δί ₂ –Δίι was observed (402), with no obvious detectable changes in the resistance spectra (406). _{When SO 2} gas was introduced into the mixture, changes in the resonance frequency and signal amplitude due to the strong chemical _{interaction between mPEI and SO 2 molecules were observed at the time SO 2} gas was introduced and reacted with the mPEI layer, as shown in Figure 4. This phenomenon is related to the weak _{interaction of CO 2} and mPEI, which does not change the physical structure of the polymer, but the strong chemical interaction of the functional polymer (mPEI) with SO2 molecules changes the polymer film properties, as shown by the ₂ gas (409). These changes increase the oscillation coefficient and the membrane mass, which in turn results in a shift in the resonant frequency and a decrease in the amplitude of the oscillator output signal (404), which can be seen in Figure 4.

Importantly, in the present invention, the results shown in Figure 4 show that a _{mixture of two "acid" gases, SO 2} and CO ₂ , can be detected in each of the separate and rule out their effects.

Also, the experiments performed in the present invention with the functional coatings of the gravimetric sensor and the different gases show that the resonant frequency of the membrane (107) can change not only in the direction of decrease but also in the direction of increase. Due to the specific interaction between the gas molecules and the functional coating is not only on the membrane weight rn _in increased additional mass m _a, but may change the membrane with the functional coating having an elasticity modulus k _m additional amount _k m or decrease the damping ratio, and membrane resonance frequency F _res can and increase, according to the formula:

F -

This is observed by measuring not only the resonant frequency of the membrane (F _re z) but also the resonance quality (Q) or the amplitude of the oscillator output signal due to the membrane mass m _m and the damping factor b _ap i, depending on the measurement environment factors such as temperature, pressure and humidity.

<2 = 2π / ₀ ^ apt

Combinations of the dynamic changes of _{these two parameters F rez} and Q are possible using CMUT functional coatings and investigating different gases. Differentiation of dynamic changes is possible according to the positive change (+ AF _rez , + AQ), negative change (AFrez, -AQ) or no change (AF _re z = 0, AQ = 0) of any parameter - thus 8 combinations of non-zero changes are obtained, allowing identify different gaseous reagents and / or their states. In this way, changes in these parameters due to different gas interactions can be cross-selectively separated and rejected.

By recording the differences in the CMUT sensor signal at different gas concentrations and ambient conditions, reference characteristics of the target gas and the functional coating can also be established, which can then be used to detect and measure the target gas concentration in a gas mixture of unknown composition. An example of such a characteristic is shown in Figs. _{Figure 5 shows the resonant frequency F re} z of the CMUT membrane (107) coated with the mPEI polymer layer as a function of the concentration of carbon dioxide CO2 (501) and the concentration of sulfur dioxide SO ₂ (502) in the test gas mixture in the presence of to an ambient temperature of 23 ° C. These characteristics (501) and (502) show that the _{interactions of CO 2} and SO ₂ with the functional coating differ from each other, and from these interactions, the concentration of the target gas in the study can be estimated by processing and comparing the dynamic changes of the CMUT signal with the reference characteristics. in a mixture.

The described embodiment and experiment with SO ₂ and CO ₂ gases are an integral part of the present invention, but are not intended to limit the scope of the invention. Aspects of the invention, such as the use of imine group polymers in the membrane functional layer, the cross-selectivity of the CMUT sensor for different target gases, and their detection and concentration by analyzing the dynamic parameters of the CMUT sensor signal, can be used for other target gas detection and concentration measurements. Also, the described methods of digitally processing a CMUT sensor signal form an integral part of the present invention, but are not intended to limit the scope of the invention: a variety of other digital signal processing methods and algorithms can be used to increase the cross-selectivity of a single CMUT sensor with different coatings. gases and their concentrations in the mixture.

Claims (9)

DEFINITION OF THE INVENTION 1. Gravimetric gas sensor comprising - at least one capacitive micromountable ultrasonic transducer (CMUT) (101) with a membrane (107), a lower base electrode (109) and an upper membrane electrode (104), - a layer of functional polymer (105) covering the membrane (107) of the CMUT converter, - an electrical CMUT converter control circuit, an oscilloscope and a computer for storing and processing measurement data (200), characterized in that the functional polymer layer (105) covering the CMUT converter membrane (107) is an imine group polymer for accurate measurement of gas concentrations with a single CMUT converter (101) or several CMUT converters of the same type connected in parallel (101). A gas sensor according to claim 1, characterized in that the CMUT converter (101) has a matrix (301) consisting of a topology of more than one CMUT element, in which the electrodes of the CMUT elements (101) are connected in parallel. 3. The gas sensor of claim 1, wherein the membranes (107) of the CMUT sensor elements are coated with the functional polymer (105) by centrifugation. 4. The gas sensor of claim 1, wherein the cross-selectivity of the functional polymer (105) for at least two different target gases in the test gas mixture is used. 5. The gas sensor of claim 4, wherein the target gas is detected in the test gas mixture and its concentration is determined by analyzing the resonant frequency F _rez and the resonant quality Q of the dynamic parameters of the CMUT signal membrane (107). 6. The gas sensor of claim 1, wherein the functional polymer (105) is an imine group methylated polyethyleneimine (mPEI). A gas sensor according to claim 6, characterized in that it is used to measure the concentration of carbon dioxide in the gas mixture (402). A gas sensor according to claim 6, characterized in that a sulfur dioxide concentration is measured in the gas mixture (404). A gas sensor according to claim 6, characterized in that sulfur dioxide and carbon dioxide are used to measure their concentration in the gas mixture (403).