JPH0634543A - Infrared gas sensor - Google Patents

Abstract

PURPOSE:To downsize a sample cell of an infrared gas sensor and reduce its weight and also to enable execution of a highly-sensitive analysis. CONSTITUTION:A groove to serve as a gas flow passage is formed in the surface of each of two single-crystal silicon plates by etching. This groove is disposed in the shape of U or a semiellipse, the two silicon plates are stuck together and thereby a sample cell 11 is obtained. In the sample cell 11, gas flow passages 12A and 12B are formed by the grooves. Infrared rays are applied to the gas flow passages 12A and 12B and a detector 16 executes analysis of a gas component on the basis of absorption thereof. The sample cell 11 is made of silicon and, therefore, it is easy to make the sensor small in size and light in weight.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a small and lightweight infrared gas sensor used for, for example, a gas analyzer.

[0002]

_2. Description of the Related Art Gas molecules composed of different atoms such as CO and CO ₂ have their own vibrations. When such a molecule is irradiated with infrared rays by continuously changing the wavelength, infrared rays having the same frequency as the natural vibration of the molecule are absorbed,
A spectrum corresponding to the structure of the molecule is obtained. The method of analyzing the molecular structure from this spectrum is called infrared absorption spectroscopy. Quantitative and qualitative analysis of gas molecules using this infrared absorption spectrum method is performed using an infrared gas sensor. For example, an infrared gas sensor is used for quantitative and qualitative analysis of gas molecules composed of different atoms such as CO, CO ₂ , CH ₄ , SO ₂ , or NOx.

In this infrared gas sensor, infrared rays emitted from two light sources become intermittent light due to a rotating sector, one of which reaches a detector through an interference filter cell and a sample cell, and the other of which is an interference filter cell and a comparison cell. To reach the detector. At this time, a quantitative and qualitative analysis of the gas is performed by measuring the difference in infrared absorption of the gas passing through the sample cell and the comparison cell.

That is, the detector is divided into two chambers, a comparison side and a sample side, and a condenser film is provided between them. Further, since the measurement component or the gas having the same infrared absorption band as that component is enclosed in the detector, only the infrared ray having a wavelength peculiar to the measurement target component is absorbed.

The comparison cell is filled with an inert gas such as N ₂ gas. For this reason, this comparative cell does not absorb the irradiation infrared rays. On the other hand, when the sample cell contains the component to be measured, infrared absorption occurs due to this component. As a result, the absorption of infrared rays by the detector is smaller on the sample side than on the comparison side. The difference in heat energy at this time becomes a pressure difference between the two chambers, and the capacitor film is displaced.
This change in capacitance is detected, processed as a signal, and extracted as an output signal.

The material of the sample cell and the comparison cell of the infrared type gas sensor having such a principle is usually stainless steel, iron or the like in consideration of sensitivity, robustness and corrosiveness due to the sample gas.

[0007]

However, when stainless steel or the like is used for the sample cell and the comparison cell as described above, welding technology or the like is required. Further, in order to obtain high sensitivity, it is necessary to increase the volume of the cell, and the cell itself becomes large, for example, 250 mm × 70 mm × 70 mm. As a result, there are drawbacks such as a long sample gas flow path and a long infrared optical path, which results in a long analysis response time. The cell weight is, for example, 0.5 to
Since the weight is as heavy as about 2.0 kg, the analytical operation is not simple and easy.

The present invention aims to provide an infrared gas sensor which has a small size and a light weight, and which has good analysis sensitivity and can perform gas analysis easily.
Its purpose is.

[0009]

This and other objects are achieved by the present invention described below. That is, in the present invention, the sample cell that defines the gas flow path of the sample gas in the infrared gas sensor is made of silicon. By using silicon for the sample cell in this way, it becomes possible to reduce the size and weight of the infrared gas sensor.

[0010]

In the present invention, silicon is used as the material of the sample cell. For example, a U-shaped or semi-elliptical gas flow path is formed by etching or the like on the surface of a silicon chip (silicon wafer) having a thickness of about 500 μm, and two silicon chips having the same shape are bonded together.
A smaller and lighter sample cell can be obtained. By using silicon as described above, fine processing can be easily performed by using a photolithography process, and the miniaturization and weight reduction can be easily achieved.

[0011]

EXAMPLES Examples of the present invention will be described in detail below. FIG. 1 is a cross-sectional view showing an infrared gas sensor according to an embodiment of the present invention. FIG. 2 is a perspective view showing the sample cell.

As shown in these figures, the infrared gas sensor has a sample cell 11 having a substantially rectangular parallelepiped shape. The sample cell 11 is formed by bonding two single crystal silicon plates of about 50 mm × 30 mm × 1.0 mm. In this case, a pair of semi-elliptical grooves is formed on each of the mirror-finished bonding surfaces of the two silicon plates. Then, the two silicon plates are attached so that these grooves are aligned with each other, and the sample cell 11 is obtained. To bond the silicon plates to each other, the SiO ₂ layers generated by natural oxidation may be anodically bonded to their interfaces. By this anodic bonding, oxygen in the SiO ₂ layer diffuses and disappears inside the silicon plate, so that the crystal uniformity of the silicon cell 11 is sufficiently maintained. Also, the groove cross-sectional shape can be any shape as long as it does not hinder the passage of infrared rays,
A rectangle or a circle is particularly preferable.

One of these parallel grooves is used as the sample gas flow path 12A and the other is used as the comparison gas flow path 12B. That is, the gas flow path 12
One of the two openings of A serves as a sample gas inlet 12AA and is connected to the gas supply means, and the other has a sample gas outlet 1
3A and communicates with the gas discharge means. Similarly, the gas flow path 12B also has a comparative gas inlet 12BB and a comparative gas outlet 13B. An etching method may be used to form the grooves to be the gas flow paths 12A and 12B. By using the etching method, a surface having high surface accuracy and smoothness is formed on the inner surfaces of the gas flow paths 12A and 12B, and diffused reflection of irradiated infrared light can be prevented. The width and depth of this groove are 1 to 5 mm.
And about 50 to 200 μm. If the size of the groove is more than this range, infrared light is transmitted, and if it is less than this range, the measurement volume of gas is reduced and the measurement sensitivity is reduced.

Further, in the infrared gas sensor of the present invention, infrared light is emitted along the longitudinal direction of the gas flow paths 12A and 12B. That is, as shown in FIG.
Light sources 14A and 14B are disposed on one side of the sample cell 11 so as to face the gas channels 12A and 12B, and an optical chopper 15 (rotary selector) is provided between the light sources 14A and 14B and the sample cell 11. It is installed. Since the silicon of the material of the sample cell 11 is transparent to infrared light, the irradiated infrared light passes through the silicon wall and passes through the gas channels 12A, 1A.
The detector 16 is reached via 2B. Therefore, the infrared light passage window conventionally required for the stainless sample cell can be omitted in the silicon sample cell 11 of the present invention.

The detector 16 is disposed so as to face the other side surface of the sample cell 11, and for example, the sample side chamber 17A and the comparison side chamber 17B are provided for each of the gas flow paths 12A and 12B.
And 2 chambers separated by a capacitor film 18.
These chambers 17A and 17B are filled with a measurement component or a gas having the same infrared absorption band as that component. Further, a motor-driven optical chopper 19 and a filter 20 are interposed between the sample cell 11 and the detector 16. Therefore, the sample gas is flown into the gas flow path 12A, and the comparative gas (inert gas without infrared absorption) is flowed into the gas flow path 12B, and the light source 1 is supplied to these gases.
Irradiate infrared rays from 4A and 14B. Then, due to absorption of infrared rays in the sample gas, both chambers 17A in the detector 16,
A pressure difference occurs between 17B. This is detected as a capacitance change of the capacitor film 18. Then, the sample gas can be analyzed by taking out this capacitance change as an output signal and performing a predetermined signal processing. This measurement can also be performed while continuously supplying the sample gas. The gas passage 12B on the comparison side may be filled with an inert gas that does not absorb infrared rays.

Although there is a concern that sensitivity will decrease due to miniaturization, Au, Ag, and P are formed on the inner walls of the gas passages 12A and 12B.
Such a phenomenon can be prevented by coating a highly reflective film using t, Al or an alloy of these metals.

The infrared gas sensor of the present invention thus manufactured is used for monitoring exhaust gas from a soot and smoke generating facility such as a large-scale boiler, monitoring automobile exhaust gas, monitoring working environment, monitoring and controlling atmosphere in firing furnace, and energy saving for power generation boiler. It can be used for applications such as performance quality control of burning appliances, and monitoring of fruit and vegetable storage.

The sample cell 11 is formed as follows. First, a groove for a gas flow path having a width of 2.5 mm and a depth of 150 μm is etched into a semi-elliptic shape in single crystal silicon, and a reaction preventive film having a thickness of 0.15 μm is deposited on the inner surface of the groove. After that, Au is vapor-deposited to a thickness of 1500 Å as a reflective film on the surface of the reaction preventive film.
Then, the two silicon plates are bonded together by the anodic bonding method. Then, a cell of 50 mm × 30 mm is cut out. Here, the thickness of the infrared light irradiation surface of the cell and the gas flow path surface is 1 mm. The weight of the silicon cell thus manufactured is 3.5 g.

Then, for example, this silicon cell was loaded into a non-dispersion type infrared gas sensor VIA-510 (manufactured by Horiba Ltd.), CO ₂ gas was selected as a gas to be measured, and infrared absorption analysis was carried out at room temperature. At this time, CO ₂
The gas flow rate was 0.2 l / min. The detection sensitivity at that time was 1 ppm, and it was confirmed that infrared absorption analysis can be performed with high sensitivity. Table 1 shows an example of the infrared type gas sensor of the present invention and the conventional type, and the specifications thereof are compared.

[0020]

[Table 1]

[0021]

The infrared gas sensor of the present invention can be made compact and lightweight and can perform gas analysis with high sensitivity.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an infrared gas sensor according to an embodiment of the present invention.

FIG. 2 is a perspective view of a silicon cell used in an infrared gas sensor according to an embodiment of the present invention.

[Explanation of symbols]

 11 sample cell 12A, 12B gas flow path 14A, 14B light source 16 detector

Claims (1)

[Claims] 1. An infrared gas sensor for measuring a specific component in a sample gas, wherein the sample cell provided with a gas flow path for the sample gas is made of silicon.