Measuring and testing – Gas analysis – By vibration
Patent
1997-12-05
2000-07-04
Patel, Harshad
Measuring and testing
Gas analysis
By vibration
73579, G01N 2100
Patent
active
060821787
DESCRIPTION:
BRIEF SUMMARY
Photoacoustic techniques are based on a heat effect which is known as the photothermal effect. These techniques utilize the principle that absorbed radiation energy, in particular from infra-red (IR) radiation results in pressure variations in a given volume of gas, the pressure variations being proportional to the absorbed amount of energy. These pressure variations can then be detected by means of a sensitive pressure sensor.
This invention is directed to a photoacoustic gas detector comprising a chamber for receiving the gas or gas mixture concerned, a path for pulsed or modulated IR radiation into, through, and out of the chamber, and a pressure sensor adapted to measure pressure changes in the chamber caused by applied IR radiation.
Examples of known uses of such photoacoustic techniques are found, inter alia, in patent publication US-H651 and in an article by C. F. Dewey Jr, R. D. Kamm, and C. E. Hackett: Acoustic amplifier for detection of atmospheric pollutants, Appl. Phys. Lett., Vol. 23, No. 11, December 1973.
The present invention is aimed at substantial improvements in a photoacoustic gas detector as referred to above, by taking as a starting point a design based on semiconductor materials, in particular silicon or quartz elements as being for example commonly employed within the semiconductor technology. What is novel and specific in the gas detector according to the invention consists thus, in the first place in that the chamber for the gas or gas mixture is formed by joining together at least two semiconductor elements, in particular silicon or quartz elements manufactured by planar technology, whereby at least a first element from a major surface has an etched recess which constitutes a substantial portion of the volume of the chamber.
In order to provide a measuring signal corresponding to the membrane oscillations as a result of the pressure changes in the chamber, various principles can be contemplated, for example a capacitive measurement principle. Such principles and techniques are well known in connection with silicon pressure sensors and microphones or the like, and will not be discussed further here.
The design and manufacture of photoacoustic gas sensors on the basis of silicon or quartz micromechanics involve a number of substantial advantages. Such gas sensors are cheap in production, and with advantage can have one or more membranes formed as an integrated structure in one or more of the elements being joined together, so as to form the gas chamber. In most embodiments according to the invention planar technology will be utilized in an optimal way when each of the elements has a substantially plate-like shape.
Silicon and quartz are materials being well suited for the manufacture of photoacoustic gas sensors, among other things because they are transparent to IR radiation. Quartz has somewhat varying transmission properties for IR radiation depending upon the wavelength. It may also be possible to employ other semiconductor materials, for example semiconductor materials of the III-V semiconductor type, in gas detectors according to the present invention.
Another advantage has to do with the small dimensions being possible with such a gas detector, so that the chamber volume for the gas can be made very small. Accordingly the sensitivity of the detector is improved. Moreover the small chamber dimensions also mean that it will be easy to keep the temperature under control and the same applies to the pressure of the gas in the chamber. Not less important in some Uses, is this factor when the gases to be handled and measured are toxic or for other reasons should or can be present only in small amounts for the measurement of interest here.
The planar technology mentioned, which constitutes a substantial prerequisite for the invention, in general makes possible the formation or application of various layers or coatings in the gas detector structure in view of different effects and purposes, as will appear also from the following description.
In addition to utilization for gas measure
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Bernstein Ralph
Ferber Alain
hlckers Per A.
Leiv Eiriksson Nyfotek AS
Patel Harshad
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