SURFACE ACOUSTIC WAVE OSCILLATOR GAS DETECTOR

Abstract: An environment detector or analyzer (10) is formed using a pair of surface acoustic wave oscillators (11), (12), each having a surface acoustic wave propagation channel (17, 23). A thin film (22) selectively sorptive of a gas or other substance to be detected is formed on one of the channels. A difference detector (35, 36, 38) is operatively coupled to the outputs of the two surface acoustic wave oscillators for generating a signal proportional to the difference in surface acoustic wave propagation velocities in the first and second channels in response to selective sorption of gas or other substance by the thin film. The difference detector responds to the difference in oscillation frequencies of the two SAW oscillators as a result of the change in surface wave velocity in the channel containing the sorptive thin film.

Description: SURFACE ACOUSTIC WAVE OSCILLATOR GAS DETECTOR Technical Field This invention relates to gas detectors and analyzers and more generally to detectors and analyzers of environ- mental-parameters which affect the properties of selectively sorptive and responsive thin films. In particular the invention is concerned with application of multi-channel surface acoustic wave (SAW), delay line oscillators, reson¬ ators and similar SAW devices as differential detectors and analyzers. Background Art The recent development of surface acoustic wave CSAW oscillators has made available a small, rugged and inexpen¬ sive oscillator of great stability and sensitivity. Such an oscillator generally comprises an . amplifier such as a microwave amplifier with a SAW delay line coupled in a feedback loop between the output and input. The SAW delay line is generally provided by spaced apart interdigital transducers formed on a substrate, for example of quartz or LiNbO-, by photolithography or similar processes. Rayleigh waves are launched and detected by the interdigital trans¬ ducers and travel along the surface of the piezoelectric substrate material. SAW delay line oscillators offer great advantages over conventional quartz and piezoelectric crystal bulk oscillators, because of their substantially greater stability and sensitivity. Further characteristics of SAW delay lines and oscilla¬ tors can be found in the following articles: Meirion Lewis, "The Surface Acoustic Wave Oscillator", Royal Radar Estab- lishment, Malvern, Worcs, United Kingdom; Richard M. White, "Surface Acoustic Waves'', Proceedings of IEEE, Vol. 58, Ho. 8 (August 19.70.1; and Roger H. Tancrell, "Analytical Design of Surface Wave Bandpass Filters", IEEE Transactions on Sonics and Ultrasonics, Vol.. SU-=-21, No. 1 (January 1974) . A review of the prior art reveals that bulk oscillators and other conventional oscillators have been applied as differential detectors where the frequency difference be¬ tween the oscillators is indicative of the presence, for example, of a gas. Furthermore, SAW oscillators have been SUBSTITUTE SHEET applied as differential strain gauges. However, the com¬ bination of multi-channel SAW oscillators with the proper¬ ties of selectively sorptive thin films in accordance with this patent application for highly stable and selective differential detection of gases and similar environmental parameters has not been accomplished prior to the present invention, to the inventors' knowledge. Prior Art Statement The Kiewit U.S. Patent No. 4,107,626 assigned to Gould, Inc. describes the use of a surface acoustic wave device as a force sensor. Two SAW delay lines are coupled as oscilla¬ tors in respective feedback circuits through respective amp¬ lifiers providing loop gain greater than one. The oscilla¬ tors are arranged so that the SAW delay line portion of. one is along the compression surface of a beam while the SAW delay line portion of the other is along the tension part of the same beam. Upon deflection of the beam by an unknown force, compression of one surface portion and tension on the other respectively speeds and slows the SAW propagation rate thereby changing the resonant frequencies of the two oscill¬ ating circuits. The frequency difference is measured by a mixer and counter for readout in digital form. The reading is proportional to the beam deflection and the force causing the deflection- The Schwarz U.S. Patent 3,888,115 assigned to Texas Instruments describes another strain sensor using SAW de¬ vices. A first SAW oscillator is applied to a first sub¬ strate forming a first leg which is to be stressed and mea¬ sured. The second SAW oscillator is applied to a second substrate forming an unstressed reference leg. The frequency difference between the signals derived from the stressed leg and reference leg is utilized as a measure of the strain applied to the first substrate. Yet another SAW oscillator force-sensing device is described in the Dias et al U.S. Patent 3,878,477 assigned to Hewlett-Packard. Dual SAW oscillators are applied to a single substrate which inversely change their respective frequencies in response to a force applied to the substrate. A mixer circuit provides a difference frequency signal as a SUBSTITUTE SHEET OMPI function of the applied force. None of the foregoing patents teaches or suggests the use of selectively sorbant thin films, nor do they teach application of SAW devices for gas detection. U.S. Patent No. 4,265,124 assigned to Rockwell International describes the use of a single SAW oscillator for remote sensing of physical variables. The variables suggested by Lim et al include pressure, temperature, and atmospheric loading. Gas detection is not suggested nor is the use of a frequency differential between two SAW oscillators, nor the use of selective thin films. In addition to these SAW device patents, applicants are aware of three gas analysis patents in which detection is based upon the occurrence of a frequency difference between a conventional sample oscillator whose resonant frequency is modified by the presence of a specified sub¬ stance, and a conventional reference oscillator which main¬ tains a relatively constant frequency signal. In the most pertinent patent, U.S. Patent 3,374,659 to Sanford et al__ assigned to Phillips Petroleum Company, wrcr© ■ s-eKEEiat- a? circuits are controlled by bulk piezoelectric crystals,-".one of which is coated with a material preferentially sorptive for one component of the gas being analyzed in the chroma- tographic column. The sample oscillator signal is changed in frequency by the amount of the component preferentially absorbed, and is passed through a mixer with the reference signal. The resulting difference signal is appropriately converted for driving a recorder and for separating the sample peak from peaks of other constituents not preferen- tially absorbed. This patent describes the comparative use of relatively unstable and unresponsive bulk oscillations from two crystal oscillators rather than the use of SAW waves or Rayleigh waves in selective thin films. The Testerman et al U.S. Patent 3,144,762 also assigned to Phillips Petroleum describes a frequency difference anal¬ yzer in which two mechanical/pneumatic oscillators are provided at the output of a gas chroma ographic analyzer. The two oscillators use the principle of oscillation of a jet stream by obstruction of the stream near the orifice. S UBSTITUTE SHEET / ^O OMPI The pulsation at the intersection of a feedback stream with the jet stream is a function of the concentration of a sampled constituent of the gas stream. The oscillations of a sample wave generator oscillator and reference wave generator oscillator and mixed or heterodyned to provide a difference signal to drive a digital or analog indicator device. Another Testerman et al U.S. Patent No. 3,273,377 of Phillips Petroleum describes the use of two fluid oscilla- tors in a chromatographic analyzer. In the sample fluidic oscillator, the gas stream to be detected is directed through an orifice and divided into two branch streams. Feedback streams alternately direct the stream back and forth between the two branches at a frequency which is a function of the molecular weight of the fluid stream. A reference oscillator is of course also provided. The two oscillations are detected by microphone pickups to provide the two electrical signals for mixing, and the difference signal is indicative of the content of the sample fluid from the column. United States Patent 4,182,986 of Parker discloses the concept of using two coupled oscillators locked or synchron¬ ized in frequency as a detector for testing metal parts. Defects or variations in a tested metal part are used to vary the impedance in the resonant circuit of one of the oscillators. This variation in impedance produces a phase shift between the frequency locked signals of the two coupled oscillators. The detected phase shift provides a measure of the defect or other tested property of the solid metal. This device uses transistor oscillators which are frequency locked or synchronized by coupling and exchange of energy between solid material that may introduce an impedance var¬ iation in a resonant circuit. Furthermore, the phase shift is used to provide a phase difference signal rather than a frequency difference signal. SAW devices are not utilized, nor are selective thin films. It is apparent from these patents that further patent protection is available for the SAW gas analyzer and detec¬ tor of the present invention, comprising a structure with SUBSTITUTE SHEET two SAW oscillators, one of which contains a thin film selectively sorptive of a substance to be detected by its effect on the frequency of oscillation of the particular SAW channel containing the thin film. Objects of the Invention It is an object of the present invention to provide a highly selective and stable gas detector using the combined advantages of surface acoustic wave oscillators and select¬ ively responsive and sorbant thin films. Another object of the invention is to utilize the prin¬ ciple that thin films formed on a SAW delay line channel will change the velocity of propagation of the surface waves in proportion to the sorption of gases or other environ¬ mental substances to which the thin film is selectively responsive. A feature and advantage of the invention is that a very high sensitivity is achieved by the- synergistic interaction of the surface wave propagating along and con¬ fined to the SAW delay line channel/thin film interface. A further object of the invention is to provide gas detectors applicable for detecting and measuring a wide range of gases, vapors and other substances of contemporary interest and concern while maintaining the high selectivity. The invention offers the advantage of a wide range of appli¬ cability according to the selective characteristics of the thin film incorporated into the detector. ' Other objects features and advantages of the present invention will become apparent in the following specifica¬ tion and accompanying drawings. Disclosure of the Invention In order to accomplish these results the present in¬ vention provides a first SAW oscillator having a first SAW propagation channel, a second SAW oscillator having a second SAW channel, and a thin film selectively sorptive of a gas or other substance to be detected formed on one of the first or second channels. A difference detector is opera¬ tively coupled to the outputs of thw two SAW oscillators for generating a signal proportional to the difference in SAW propagation velocities in the first and second channels in response to selective sorption of gas or other substance SUBSTITUTE SHEET ^RE OMPI by the thin film. In the preferred form of the invention, the difference detector responds to the difference in oscillation frequencies of the two SAW oscillators as a result.of the change in surface wave velocity in the channel containing the sorptive thin film. In preparing the SAW gas detector, two substantially identical surface acoustic wave delay lines or channels are fabricated side-by-side on a single piezo electric substrate. A liquid or solid film selectively absorptive or adsorptive to a particular toxic or non-toxic gas is placed on the sub¬ strate between the pair of interdigital transducers of one of the delay lines. Each delay line is connected in the feedback loop of a respective RF amplifier.to produce microwave oscillations in each loop of nearly equal frequency. When a gas or other substance to be detected is sorbed by the thin film, the surface acoustic wave properties, and in particular the velocity of propagation is retarded or other¬ wise changed resulting in a shift in the oscillation fre¬ quency of the SAW delay line or channel containing the film. The outputs of the two oscillators are coupled together through a mixer and the difference, frequency is selected by a low pass filter. A signal is thereby obtained whose fre¬ quency is proportional to the quantity of sorbed gas. This signal may be converted to operate a recording device. A small inexpensive, reliable and accurate gas detector is thereby provided. The SAW delay lines or dual channel or dual path delay line may be formed on a piezo electric substrate of, for example, quartz or lithium niobate (LiNbO_) . For the thin film, a variety of organic or metallic materials may be used according to the gas or substance to be detected. For example, an organic triethonalamine film may be used for the detection of S0_. In one example according to the invention using such a film on either quartz or lithium niobate oscil- lators, sensitivity in detection of SO- to less than 70- parts per billion has been achieved. The triethonalamine film may also be used for the detection of NO-,. • For detection of or anophosphorus compounds such as pesticides and nerve gases, films of CuCl_, FeCl-., NiCl_, or SUBSTITUTE SHEET -gϋs.E CdCl ? may be used. Triphenylamine films and films of ter¬ tiary amines may be used for detecting and measuring hydro¬ gen chloride (CHI) while ascorbic acid films or films of ascorbic acid with silver nitrate may be used for detecting ammonia (NH ) . For the detection of aromatic hydrocarbons, films of trans-chlorocarbonyl-bis (triphenyl-phosphine) are applicable, and alternatively, irridium films may be used. In many instances, metallic films are appropriate and provide films of greater stability than the organic compound films. For example, in the detection of hydrogen sulfide (H_S) high vacuum deposited films of either silver or copper may be used as well as lead acetate or acetone extracts re¬ sulting from the burning of organochlorine compounds. Gold film may be used for the detection of mercury, mercury com- pounds, and vapors. Thin films of other compounds, elements and materials may also be used which are found to be select¬ ively sorptive of the gas or substance sought to be monitored by the SAW gas detector. In the operation of the SAW gas detector, according to the invention, the selective film of desired sorptive characteristics is formed on one of the SAW delay lines or channels and the device is calibrated for substantial coin¬ cidence of resonant frequencies for the two oscillators. The SAW gas oscillator is then placed in the test chamber or environment to be monitored for exposure of the thin film. After an exposure time factor.of, for example, one to three minutes or longer, a measurement is made of the frequency difference signal which shifts or increases in proportion to sorption of the targeted gas or substance and this is in turn a function of the concentration of the gas or substance in the monitored environment. The SAW gas detector thin film system is reversible and after an exposure and measurement, the device is removed to the room air or outside air for desorption of the sorbed monitored gas or gases from the film. The SAW gas detector is therefore reversible and reusable and is prepared for subsequent reuse by desorption in an uncontaminated atmosphere. SUBSTITUTE SHEET Brief Description of the Drawings .Fig. 1 is a schematic block diagram of the dual channel surface acoustic wave oscillator gas detector according to the present invention. Fig. 2 is a graph of the difference frequency drift of the dual channel SAW oscillator with thin film formed on one of the SAW delay line channels indicating the stability of the film and hence stability of the oscillation frequency with time, prior to exposure. Fig. 3 is a graph of the frequency shift of the thin film SAW oscillator channel from time of exposure of the thin film to target gas or gases in a test cell, for five different test runs, all shown in solid lines. Fig. 4 is a graph comparing the sensitivity of response of the SAW gas detector according to the present invention indicated by the solid line with the response of a bulk wave conventional piezoelectric oscillator detector indicated by the dotted line. Data points for the SAW gas detector are shown by a circle along the solid line. Best Mode for Carrying Out the Invention In the SAW oscillator gas detector 10 illustrated in Fig. 1, dual SAW oscillators 11 and 12 are provided. SAW socillator 11 includes a first SAW delay line 13 formed by spaced apart interdigital transducers 14 and 15 deposited on the piezoelectric substrate 16. The piezoelectric sub¬ strate may be formed e.g. from quartz or lithium niobate. A surface wave path or channel 17 couples the interdigital transducers 14 and 15 and in the case of the first SAW oscillator 11 does not include a thin film along the path or channel. The second SAW oscillator 12 includes a second SAW delay line 18 composed of interdigital transducers 20 and 21 depo¬ sited on the common substrate 16. In the case of the second SAW delay line 18 a thin film 22 is deposited along the first surface wave path or channel 23, which film is select¬ ively absorptive or adsorptive of the gas sought to be moni¬ tored or detected by the cell. The first SAW delay line 13 is coupled in feedback loop through amplifier 25 to form the first SAW oscillator 11. SUBSTITUTE SHEET Amplifier 25 is adjusted to afford a gain of one, the oscillating condition for this feedback circuit. Similarly, the second SAW delay line 18 is coupled in a feedback loop through amplifier 26 to form the second SAW oscillator 12. The amplifier 26 is also adjusted to provide a gain of one to establish the oscillating condition in the second feed¬ back circuit. The difference frequency of the two oscil¬ lators is used as a baseline. The surface acoustic wave portions and components of SAW oscillator gas detector 10 may be housed, for example, in a gas tight test cell 30 with sufficient capacity to admit a representative sample of the air or atmosphere to be moni¬ tored by the gas detector. In the instant example, the gas type test cell may have a volume, for example, of 144 ml. To provide a representative example of the parameters and dimensions of the device, the SAW delay lines 13 and 18 may have dimensions to provide a surface wave path or channel having a length of approximately one centimeter. The coincident frequency of oscillation of the SAW oscilla- tors 11 and 12 may be, for example, in the range of 100- 150 megahertz (mHz) . The frequency shift of the resonant frequency of SAW oscillator 12 relative to SAW oscillator. 11 as a result of selective sorption of gas by thin film 22 in delay line channel 23 may be, for example, in the order of 100 kiloherz (kHz) up to, for example, 2 mega¬ hertz (mHz) . Thus, the frequency difference between the two oscillators upon detection of the gas is no more than a few percent of the original resonant frequency of the oscillators. The delay in the propagation of surface acoustic waves across channel 23 covered with thin film 22 as a result of absorption or adsorption of a targeted gas and change in the propagation velocity causes the shift in the resonant frequency of the SAW oscillator 12 relative to oscillator 11. Output signals from SAW oscillators 11 and 12 are de¬ rived respectively through directional couplers 32 and 33. These output signals are coupled to the input of mixer 35 where the signals of shifted frequency are mixed. The sum and difference signals from the output of mixer 35 are fed SUBSTITUTE SHEET ^SHREΛ to low pass filter 36 which passes the difference frequency signal to signal converter 38 which in turn converts the frequency signal to a voltage signal proportional to the frequency for driving the reocorder 40. A dual channel SAW oscillator gas detector of the type illustrated in Fig. 1 was fabricated with delay lines formed on a lithium niobate substrate. A triethonalamine film was coated on the channel 23 of delay line 18 for detecting the presence of sulphur dioxide (S0_) in a test cell. The stability of the frequencies of the two oscillators in the absence of the targeted gas is demonstrated by the graph of Fig. 2. The graph of Fig. 2 plots the frequency difference between oscillators 11 and 12 over an extended period of time. The dramatic frequency shift of the resonant frequency of oscillator 12 relative to oscillator 11 for five differ¬ ent tests is shown in the graph of Fig. 3. Exposure to the targeted gas in specified concentration was limited to only a few minutes. Finally, the sensitivity of the SAW oscillator gas de¬ tector in terms of the frequency shift ΔF relative to different concentrations of S0 2 is illustrated in the graph of Fig. 4. By way of contrast the sensitivity of a conven¬ tional bulk wave oscillator gas detector is also shown by the dotted line 42. The solid line 44 represents the test data for the surface acoustic wave oscillator gas detector of the present invention. While the invention has been described with reference to the application of dual channel SAW oscillators, dual port SAW resonators may also be used. Furthermore, the SAW oscillator gas detector of the present invention may be used in combination with a gas chromatograph column to achieve even greater selectivity. SUBSTITUTE SHEET OMPI

Claims: Claims 1. A SAW oscillator gas detector for detecting selected gases comprising: first SAW oscillator means having a first SAW propaga- tion channel; second SAW oscillator means having a second SAW propa¬ gation channel; thin film means selectively sorptive of a gas or gases to be detected, said thin film means formed on one of said first or second SAW propagation channels; and difference detector means operatively coupled to the output of said first and second SAW oscillators for generating a signal proportional to the difference in SAW propagation velocity in the first and second channels in response to selective sorption of gas in the thin film. 2. The SAW oscillator gas detector of Claim 1 wherein said thin film comprises a film of triethanolamine. 3. The SAW oscillator gas detector of Claim 1 wherein said thin film means comprises a metallic film. 4. The SAW oscillator gas detector of Claim 1 wherein said thin film means comprises an organic film. 5. The SAW oscillator gas detector of Claim 1 wherein said difference detector means comprises means for detecting the difference in frequency of the signals from said first and second SAW oscillator means. 6. The SAW oscillator gas detector as set forth in Claim 1 wherein said first and second SAW oscillators comprise a two port SAW resonator. 7. A SAW oscillator gas detector comprising: a first SAW oscillator comprising a first SAW delay line coupled in the feedback loop of a first amplifier, said delay line having a first SAW propagation channel; a second SAW oscillator comprising a second SAW delay line coupled in the feedback loop of a second amplifier, said second delay line having a second SAW propagation channel; a thin film selectively sorptive of a gas or gases to be detected and formed on one of said first or second SAW propagation channels; SUBSTITUTE SHEET * URE_ and signal frequency difference detector means operatively coupled to the output of the first and second SAW oscillators for generating a gas detecting output signal-proportional to the difference in frequency between the first and second oscillators as a result of selective sorption of gas in the thin film. 8. The SAW oscillator gas detector of Claim 7 further comprising gas chromatographic column means having an output coupled to the channels of said SAW delay line. 9. The SAW oscillator gas detector of Claim 7 wherein said thin film comprises an organic film. 10. The SAW oscillator gas detector of Claim 7 wherein said thin film comprises a metallic film. SUBSTITUTE SHEET

Inventors: LEE DONALD L, VETELINO JOHN F, LEE, DONALD, L., VETELINO, JOHN, F.
Applicants: UNIV MAINE, BOARD OF TRUSTEES OF THE UNIVERSITY OF MAINE
Application year: 1982
Main classification: G01N31
IPC codes: G01N29