PHOTOACOUSTIC SPECTROSCOPE HAVING A VIBRATING STRUCTURE AS SOUND DETECTOR

Abstract

The invention relates, in a first aspect, to a photoacoustic spectroscope for analyzing gas, comprising an infrared emitter (3), which can be modulated, an analysis volume (1), which can be filled with gas, and a sound pressure detector. The sound pressure detector comprises a structure (5) capable of vibrating, an actuator and a measurement unit, wherein the actuator is configured to actively excite vibration of the structure (5) capable of vibrating and the measurement unit can measure the vibration properties of the structure (5) capable of vibrating, which measurement depends on the formation of the sound pressure waves.

In an additional aspect, the invention relates to a method for analyzing gas, comprising the provision of a photoacoustic spectroscope for analyzing gas, irradiating the gas with infrared radiation, modulated by a modulation frequency, to generate sound pressure waves, exciting the structure (5) capable of vibrating at an excitation frequency, measuring the vibration properties of the structure (5) capable of vibrating, which measurement depends on the sound pressure, and determining the sound pressure of the gas based on the measured vibration properties.

Claims

1. A photoacoustic spectroscope for analyzing gas, comprising a modulatable infrared emitter, an analysis volume that can be filled with gas, and a sound pressure detector, wherein the infrared emitter, the analysis volume, and the sound pressure detector are arranged such that the infrared radiation modulatably emittable from the infrared emitter can excite gas in the analysis volume to form sound pressure waves which can be measured using the sound pressure detector, wherein the sound pressure detector comprises a structure capable of vibrating, an actuator, and a measuring unit, wherein the actuator is configured to actively excite vibration of the structure capable of vibrating and the measuring unit is configured for measuring the vibration properties of the structure capable of vibrating, which measurement depends on the formation of the sound pressure waves, and wherein the modulation frequency of the infrared emitter is preferably between 1 Hz and 200 Hz, while the excitation frequency of the structure capable of vibrating is between 1 kHz and 200 kHz.

2. The photoacoustic spectroscope according to claim 1, wherein the spectroscope comprises a control unit which is configured to excite the structure capable of vibrating to vibrate using an excitation frequency and to control the modulatable infrared emitter in such a manner that it emits infrared radiation modulated with a modulation frequency, wherein the modulation frequency of the infrared emitter is smaller than the excitation frequency of the structure capable of vibrating by a factor of 2 or more.

3. The photoacoustic spectroscope according to claim 1, wherein the excitation frequency of the structure capable of vibrating corresponds to a resonance frequency of the structure capable of vibrating.

4. The photoacoustic spectroscope according to claim 1, wherein the spectroscope comprises an array of sound pressure detectors.

5. The photoacoustic spectroscope according to claim 1, wherein the actuator is a MEMS actuator.

6. The photoacoustic spectroscope according to claim 1, wherein the structure capable of vibrating comprises a bending beam, a valve, and/or a membrane.

7. The photoacoustic spectroscope according to claim 1, wherein the sound pressure detector comprises a piezoelectric beam which is preferably arranged as a cantilever in the analysis volume, wherein the piezoelectric bending beam preferably comprises two electrodes and a piezoelectric intermediate layer made of a material selected from the group containing lead-zirconate-titanate (PZT), aluminum nitride (AlN), or zinc oxide (ZnO).

8. The photoacoustic spectroscope according to claim 1, wherein the analysis volume comprises a sample chamber and a reference chamber, wherein the infrared emitter is arranged in such a manner that it irradiates the sample chamber and not the reference chamber, and wherein a connection channel is present between the sample chamber and reference chamber in which channel the structure capable of vibrating is located.

9. The photoacoustic spectroscope according to claim 1, wherein the measuring unit of the sound detector is an optical measuring unit, preferably comprising a photon emitter for generating a photon beam and a photodetector, wherein the photon emitter is aligned with the structure capable of vibrating in such a manner that the vibration properties of the structure capable of vibrating can be measured by means of the photodetector.

10. The photoacoustic spectroscope according to claim 1, wherein the measuring unit of the sound detector is an electrical measuring unit.

11. The photoacoustic spectroscope according to claim 1, wherein the modulatable infrared emitter comprises a heating element.

12. The photoacoustic spectroscope according to claim 11, wherein the heating element comprises a substrate onto which at least partially a heatable layer of a conductive material is applied, which substrate comprises contacts for a current and/or voltage source.

13. The photoacoustic spectroscope according to claim 12, wherein the substrate is selected from a group consisting of silicon, monocrystalline silicon, polysilicon, silicon dioxide, silicon carbide, silicon germanium, silicon nitride, nitride, germanium, carbon, gallium arsenide, gallium nitride, and indium phosphide and/or the conductive material for forming the heatable layer is selected from the group consisting of platinum, tungsten, (doped) tin oxide, monocrystalline silicon, polysilicon, molybdenum, titanium, tantalum, titanium-tungsten alloy, metal silicide, aluminum, graphite, and copper.

14. The photoacoustic spectroscope according to claim 1, wherein the control device is configured to regulate the temperature of the heating element in a range between 50° C. and 1000° C.

15. A method for analyzing gas, comprising; a. providing a photoacoustic spectroscope for analyzing gas according to claim 1, b. irradiating the gas with infrared radiation modulated at a modulation frequency to generate sound pressure waves, c. exciting the structure capable of vibrating with an excitation frequency, d. measuring the vibration properties of the structure capable of vibrating, which depends on the sound pressure, and e. determining the sound pressure of the gas based on the measured vibration properties.

16. The method for analyzing gas according to the claim 15, further comprising determining a time profile of the sound pressure waves generated by means of the modulated infrared radiation.

17. The photoacoustic spectroscope according to claim 2, wherein the modulation frequency of the infrared emitter is smaller than the excitation frequency of the structure capable of vibrating by a factor of 5 or more.

18. The photoacoustic spectroscope according to claim 5 wherein the MEMS actuator is selected from the group consisting of an electrostatic actuator, a piezoelectric actuator, an electromagnetic actuator, and a thermal actuator.

19. The photoacoustic spectroscope according to claim 10 wherein the electrical measuring unit is for measuring the vibration characteristics of the structure capable of vibrating by an impedance measurement and/or a capacitive measurement.

20. The photoacoustic spectroscope according to claim 14, wherein the control device is configured to regulate the temperature of the heating element in a range between 100° C. and 1000° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0153] FIG. 1 Analysis volume comprising a sample chamber and a reference chamber as well as a sound pressure detector with a structure capable of vibrating in the connecting channel between the two chambers.

[0154] FIG. 2 Temporal progression of the pressure of a sound pressure wave (top) as well as a vibration amplitude or intensity, once without the damping effect of a sound pressure wave (center) and once with the influence of the sound pressure wave (bottom).

[0155] FIG. 3 The influence of an ambient pressure on the vibration properties of the structure capable of vibrating, particularly on the resonance frequency.

DETAILED DESCRIPTION OF THE DRAWINGS

[0156] FIG. 1 shows the analysis volume 1 comprising a sample chamber 7 and a reference chamber 9 as well as a sound pressure detector with a structure capable of vibrating 5 in the connecting channel 11 between the two chambers. In this exemplary embodiment, both chambers contain the same gas to be analyzed, wherein only the sample chamber is exposed to modulated IR radiation 3. The structure capable of vibrating 5, in this case a bending beam or cantilever in the connecting channel 11 between the two chambers, is excited to an active vibration by the actuator (not shown), and the measuring unit (not shown) measures the vibration properties of the structure capable of vibrating, e.g. vibration amplitude, vibration frequency, phase of the vibration with respect to the excitation and/or resonance frequency of the structure capable of vibrating 5. These vibration properties depend on the formation of the sound pressure waves that the structure capable of vibrating 6 experiences. These can also contain sound pressure waves from undesired sound sources in addition to the sound pressure waves due to an absorption of the modulated IR radiation 3 of the gas to be analyzed in the sample chamber 7. Since the undesired sound pressure waves preferably reach both the reference chamber 9 and the sample chamber, they preferably act on both sides of the cantilever to the same extent, wherein the sound pressure waves of the PAS from the sample chamber 7 only act on the structure capable of vibrating 5 from the direction of this sample chamber 7 and therefore can be measured by it substantially without a measurement of the undesired sound pressure waves.

[0157] The top part of FIG. 2 shows an example of a time curve of a pressure to be measured of a sound pressure wave due to a PAS excitation of gas molecules. The center part of the figure shows the vibration properties of the structure capable of vibrating 5 excited by the actuator without the external influence due to the sound pressure wave. Particularly, the vibration amplitude or the vibration intensity is plotted over time, the periodicity of which, however, also allows conclusions to be drawn about the vibration frequency. The bottom part of the figure shows the resultant if the sound pressure waves of the top part of the figure interact with a vibration of the structure capable of vibrating 5 from the center part of the figure. The illustration shows schematically how the vibration properties of the structure capable of vibrating 5 are influenced by the sound pressure waves, which, by measuring the vibration properties, allows direct conclusions to be drawn about the formation of the sound pressure waves and thus about the composition of the gas in the sample volume according to FIG. 1. Particularly, influencing the vibration amplitude or vibration intensity of the structure capable of vibrating 5 can be seen, particularly by modulating the envelope of the vibration.

[0158] FIG. 3 shows the influence of an ambient pressure on the vibration properties of the structure capable of vibrating 5, particularly on the resonance frequency [1] of the structure capable of vibrating and the respective vibration amplitude. During this measurement, the pressure was varied statistically, and the resonance frequency and its vibration amplitude were measured at different static pressures. Particularly at pressures in the range of atmospheric pressure, a strong dependence of the resonance frequency on the ambient pressure of the structure capable of vibrating 5 is visible, due to a damping of the vibration through impacts of the structure 5 with the gas molecules of the environment. At a sufficiently high sampling rate, this measurement at static pressures can also be used for dynamic pressure variations due to sound pressure waves which are caused by the absorption of modulated IR radiation by the gas molecules. At low pressures from about 10 mbar, only a smaller influence of the ambient pressure on the vibration properties of the structure capable of vibrating 5 is shown. At such low pressures, the influence of the intrinsic damping of the structure 5 generally outweighs that of external damping due to the pressure of the surrounding gas. However, even at these low pressures, statements can preferably be made on the basis of a measurement of the vibration properties, if the resolution of the measurement of the frequency and the vibration amplitude or vibration intensity would be increased, for example.

UST OF REFERENCE NUMERALS

[0159] 1 analysis volume [0160] 3 modulated infrared radiation [0161] 5 structure capable of vibrating [0162] 7 sample chamber [0163] 9 reference chamber [0164] 11 connection channel

REFERENCES

[0165] [1] Abdallah Ababneh, A. N. Al-Omari, A. M. K. Dagamseh, H. C. Qiu, D. Feili, V. Ruiz-Díez, T. Manzaneque, J. Hemando, J. L. Sánchez-Rojas, A. Bittner. U. Schmid; H. Seidel: Electrical characterization of micromachined AlN resonators at various back pressures, Microsyst Technol 20: 663-670, 2014. [0166] [2] G Pfusterschmied, M. Kucera, E. Wistrela, T. Manzaneque, V. Ruiz-Díez, J. L. Sánchez-Rojas, A. Bittner and U. Schmid et al: Temperature dependent performance of piezoelectric MEMS resonators for viscosity and density determination of liquids, J. Micromech. Microeng. 25 105014, 2015.