JP5374293B2 - Infrared gas detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an infrared gas detector made easy to manufacture by a constitution such that infrared rays of an unnecessary wavelength region are filtered out by a filter and suppressing the temperature rise of the filter. <P>SOLUTION: The infrared gas detector is equipped with an infrared light source 1 for radiating infrared rays and an infrared sensor 2 for detecting the infrared rays radiated from the infrared light source 1 and passed through a gas detection pipe 3. The infrared sensor 2 is equipped with light detection elements 27x and 27y composed of pyroelectric elements, the transmission filter 25a and 25b arranged between the gas detection pipe 3 and respective light detection elements 21a and 21b and selectively transmitting infrared rays of a specific wavelength band and the filter 26 arranged between the gas detection pipe 3 and the transmission filters 25a and 25b and absorbing infrared rays of a wavelength region excepting the specific wavelength band transmitted through the transmission filters 25a and 25b. The light detection elements 21a and 21b have sensitivity with respect to the infrared rays of the specific wavelength band transmitted through the transmission filters 25a and 25b and the infrared light source 10 is driven so as to intermittently radiate infrared rays. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an infrared gas detector that projects infrared light into a monitoring space into which a detection target gas is introduced, and detects a change in infrared light due to the presence of the detection target gas.

  Conventionally, there has been provided an infrared gas detector for projecting infrared radiation emitted from an infrared light source into a monitoring space for introducing a detection target gas and detecting the detection target gas based on the amount of infrared absorption and reflection. Yes.

  This type of infrared gas detector emits infrared light emitted from an infrared light source into a monitoring space where the detection target gas (methane, carbon monoxide, carbon dioxide, etc.) is introduced, and receives light according to the amount of received infrared light. The type and concentration of the detection target gas is detected by detecting the infrared absorption wavelength and the amount of light absorption by the detection target gas using a light receiving element that outputs a signal.

  The light receiving element receives infrared rays through a transmission filter that selectively passes only infrared rays having a wavelength absorbed by the detection target gas among infrared rays emitted from the infrared light source. Therefore, the type of the detection target gas is specified by the wavelength that selectively passes through the transmission filter, and the concentration of the detection target gas is measured according to the amount of received infrared light (the output signal level of the infrared sensor).

  In this type of infrared gas detector, in order to detect the amount of light absorption in the detection target gas, the detection target gas is irradiated with infrared rays intermittently a plurality of times in one analysis. Therefore, in order to shorten the time for detecting the detection target gas, it is necessary to shorten the time interval for intermittently using an infrared light source capable of high-speed modulation. Moreover, since infrared rays are irradiated a plurality of times, it is necessary to increase the amount of infrared rays emitted relative to the input energy in order to reduce the energy (electric power) required for infrared rays emission. Furthermore, in order to improve the detection accuracy, it is also necessary to make the amount of infrared radiation emitted from the infrared light source constant.

  The infrared light source has a structure in which a filament formed in a coil shape with a material such as tungsten or platinum is housed in a light-transmitting airtight container, or the surface of a coiled core material made of tungsten or platinum is made of ceramic such as alumina. There is known a structure in which a filament whose infrared radiation efficiency is enhanced by coating is housed in a light-transmitting airtight container.

  However, since this type of infrared light source cannot be used in a state where the filament is exposed to the outside air, it is impossible to avoid an increase in size due to an airtight container like an incandescent bulb, and it is difficult to reduce the size of the apparatus. . In addition, since the heat capacity including the filament and the airtight container is increased, the rise time required for the infrared rays to rise to the required intensity after turning on the power, and the fall time required for the infrared rays to decrease to the specified intensity after the power is turned off are long. Steep characteristics cannot be obtained at the rise and fall.

  That is, with this type of infrared light source, the frequency at which the infrared light is interrupted can only be set to about 0.1 to 10 Hz. In addition, since the rise and fall of the infrared radiation intensity are not steep, detection errors due to waveform rounding tend to occur.

  On the other hand, as an infrared light source that can be miniaturized, an infrared light source formed using MEMS (Micro Electro Mechanical Systems) technology has been proposed (see, for example, Patent Document 1). The infrared light source described in Patent Document 1 has a rectangular frame-shaped support substrate formed by etching a silicon substrate, and polysilicon doped on one surface of the support substrate via a thermal insulating layer of silicon nitride An infrared radiation layer of the film is formed, and an incandescent filament that is a metal layer is embedded in an insulating layer of silicon nitride that covers the infrared radiation layer.

  This infrared light source has an electrode connected to an incandescent filament, and has an indirectly heated configuration in which the incandescent filament is heated by energizing between the electrodes, and the infrared radiation layer is heated by the incandescent filament.

  This infrared light source forms an opening in the support substrate on the back side of the infrared radiation layer, and surrounds the incandescent filament that contributes to infrared radiation and the infrared radiation layer with highly heat-insulating air. The heat capacity difference between the part contributing to the heat and the surroundings is large, and it is possible to respond relatively quickly to the interruption of the current flowing through the incandescent filament. That is, in the infrared light source including the above-described airtight container, the response is about 0.1 to 10 Hz, but when the configuration described in Patent Document 1 is adopted, the response up to 200 Hz is possible. This is described in Patent Document 1.

JP-A-9-184757

  By the way, in order to increase the detection accuracy of the detection target gas in the infrared gas detector, it is necessary to accurately detect the amount of infrared absorption or reflection in the detection target gas, so that it is absorbed or reflected by the detection target gas. It is necessary to increase the amount of light received by the light receiving element for infrared rays in the wavelength range.

  As described above, this object can be achieved to some extent by providing a transmission filter. However, the emission wavelength of the infrared light source having the above-described configuration has a wide wavelength range including a wavelength range (mid-infrared to far-infrared range) where absorption by a detection target gas such as methane, carbon monoxide, and carbon dioxide occurs. Therefore, the wavelength selectivity is insufficient only by using the transmission filter, and there is a problem that the detection accuracy as the infrared gas detector cannot be sufficiently increased.

  In other words, the infrared wavelength range required to detect the detection target gas is much narrower than the infrared wavelength range emitted by the infrared light source, so that not only the transmission filter but also the infrared in the unnecessary wavelength range is removed. It is desirable to provide a removal filter.

  As a removal filter that removes infrared rays in an unnecessary wavelength range, a multilayer filter configured to remove infrared rays in an unnecessary wavelength range by using a multilayer filter in which a plurality of layers having different refractive indexes are laminated, an infrared ray is removed. Absorption filters made of absorbing materials are known.

  The multilayer filter needs to control the film thickness and refractive index of each layer, so there is a problem that it takes time to manufacture, and the absorption filter is easier to manufacture than the multilayer filter, but absorbs infrared rays. Along with this, there is a problem that when the temperature rises, infrared rays are emitted from the removal filter, or the wavelength shift of the transmission characteristics of the removal filter occurs.

  The present invention has been made in view of the above reasons, and its purpose is to facilitate manufacture by using a removal filter that removes infrared rays in an unnecessary wavelength region, and to increase the temperature of the removal filter. An object of the present invention is to provide an infrared type gas detector that prevents infrared rays from being radiated from the removal filter and suppresses wavelength shift of the removal filter.

The present invention is, Yes in order to achieve the object described above, an infrared light source emitting an infrared ray, an infrared sensor for detecting infrared rays having passed through the monitored space to the detection target gas is emitted is introduced from the infrared light source and, wherein an infrared gas detector that detects the detection target gas in the monitored space by using the output of the infrared sensor, the infrared sensor includes a light receiving element for converting the infrared into an electrical signal, from said monitored space the specific wavelength band that passes through the transmission filter to the arranged infrared incident path of the light receiving element selectively transmits infrared of a specific wavelength band, the transmission filter is disposed between said monitoring space the transmission filter and a removal filter for removing infrared broadband by absorbing infrared rays in the wavelength range excluding the specific wave the light receiving element is configured to transmit the transmission filter Has sensitivity to the infrared band, the infrared light source is intermittently driven so as to emit infrared radiation, the infrared rays heat generated by the energized a thin film formed on one surface of the substrate An infrared radiation layer to be radiated; a holding layer formed on one surface of the substrate so as to support the infrared radiation layer by laminating the infrared radiation layer on one surface; and the other surface of the holding layer on the substrate. A gas layer in contact, the gas layer having a frequency of an applied voltage of f [Hz], a thermal conductivity of the gas layer of αg [W / mK], and a volumetric heat capacity of the gas layer of Cg [J / m ³ K], Lg '= (2αg / ωCg ) 1/2, when the omega = 2 [pi] f, the thickness Lg is, 0.05Lg'<is set in the range of Lg <3LG ', the infrared radiation layer Suppresses a decrease in temperature of the holding layer during energization of the red When the external radiation layer is not energized, heat transfer from the holding layer to the substrate is promoted .

The elimination filter, the multilayer filter formed by laminating a plurality of layers having different refractive index, said at least one layer of the multilayer film filter absorbs far-infrared wavelength longer than the specific wavelength band that passes through the transmission filter An infrared absorbing layer is desirable.

Further, the material of the infrared absorbing layer, _Al 2 _{O 3,} may be selected from _Ta 2 _{O 3} Prefecture. Further, the removing filter has a filter substrate made of a plate-like Si, it is desirable to laminate the multilayer film filter on the filter substrate.

The light receiving element is preferably a thermal element having sensitivity to all wavelengths of infrared rays emitted from the infrared light source.

  According to the configuration of the present invention, by adopting a configuration that removes infrared rays in a wide band by absorbing infrared rays as a removal filter, most of infrared rays having wavelengths irrelevant to detection of the detection target gas are removed and detected. Sensitivity to the target gas can be increased. In addition, if a removal filter that removes unnecessary infrared rays by absorbing infrared rays is used, infrared rays may be emitted from the removal filter due to temperature rise due to absorption of infrared rays. Absorbing unnecessary infrared light because it radiates suppresses the temperature rise of the removal filter, and it is also possible to suppress wavelength shift caused by temperature change of the removal filter caused by infrared radiation from the infrared light source. As a result, it is possible to detect the detection target gas with high accuracy by effectively using the function of removing unnecessary infrared rays by the removal filter. Moreover, since the infrared light is intermittently emitted from the infrared light source, the input power can be reduced as compared with the infrared light source that continuously emits the infrared light.

In particular, and using an infrared light source and a holding layer and the gas layer and the infrared radiation layer, when turning on the power to the infrared radiation layer gas layer increases the amount of infrared radiation for the input power of suppressing the temperature drop of the retaining layer On the other hand, if the power supply to the infrared radiation layer is stopped, the gas layer promotes heat transfer and lowers the temperature of the holding layer, so that the infrared radiation can be stopped in a short time. In other words, the infrared radiation layer can be radiated and stopped with high responsiveness to the turning on and off of the power to the infrared radiation layer, and the infrared light can be radiated efficiently with respect to the inputted power. Compared with the case where an incandescent lamp is used or a structure in which a filament is provided in a dielectric film is used, power consumption can be reduced.

  When a multilayer filter including an infrared absorption layer is used as the removal filter, it is possible to adjust the wavelength to be removed using not only absorption but also reflection. In particular, the wavelength range to be removed is determined by selecting the material of the infrared absorption layer. However, since the wavelength range can be adjusted according to the refractive index and film thickness of each layer, the absorption and reflection are used complementarily. This makes it possible to widen the range of the wavelength range to be removed. In other words, while using an infrared light source that emits broadband infrared light, the infrared light with a wavelength unnecessary for detection of the detection target gas is removed as much as possible, and the infrared light with a wavelength unnecessary for detection of the detection target gas is incident on the light receiving element. As a result, the sensitivity to the detection target gas is increased.

Moreover, if the material of the infrared absorption layer is selected from Al ₂ O ₃ and Ta ₂ O ₃ , the far-infrared absorptance can be increased as compared with the case of using SiO _x or SiN _x for the infrared absorption layer. . In particular, when Al ₂ O ₃ is used as an infrared absorption layer and a multilayer filter is laminated on a plate-like Si filter substrate, the change in ambient temperature is due to the close hardness of Si and Al ₂ O _3. Even if expansion and contraction occurs due to the above, there is no possibility that the multilayer filter is peeled off from the filter substrate, and stability and reliability can be improved.

  Furthermore, if a thermal infrared detector that has sensitivity to all wavelengths of infrared rays emitted from an infrared light source is used as a light receiving element, it can be used for various types of detection target gas by simply changing the design of the transmission filter and removal filter. This makes it possible to share parts for infrared gas detectors of various specifications with different detection target gases, and as a result, can be provided at low cost.

It is a schematic block diagram which shows the whole structure of embodiment. It is sectional drawing of the transmission filter used for the same as the above. It is a characteristic view of the transmission filter and removal filter used for the same as the above. It is sectional drawing which shows an example of the radiation element used for the same as the above. It is operation | movement explanatory drawing of the radiation element used for the same as the above.

  In the present embodiment, an infrared gas detector used for applications such as a gas leak alarm will be described. As shown in FIG. 1, this type of infrared gas detector has an infrared light source 1 that emits infrared rays by an input of an electrical signal and an infrared sensor 2 that detects infrared rays. The detection target gas is introduced into the path to the sensor 2. A gas detection tube 3 is disposed at a site where the detection target gas is introduced.

  The gas detection tube 3 is formed with a conduit 31 for guiding infrared rays from the infrared light source 1 to the infrared sensor 2, and the inner peripheral surface of the conduit 31 reflects infrared rays. That is, a reflection film that reflects infrared rays is formed on the inner peripheral surface of the pipe 31 of the gas detection tube 3, or the gas detection tube 3 is formed of a material that reflects infrared rays. The reflective film is formed by depositing a metal thin film such as Au on the entire inner peripheral surface of the duct 31 by a thin film forming method such as sputtering. As described above, the infrared rays are reflected on the inner peripheral surface of the pipe line 31, so that a part of the infrared rays emitted from the infrared light source 1 is reflected on the inner peripheral face of the pipe line 31 as indicated by a broken line in FIG. 1. Is repeated until the infrared sensor 2 is guided.

  The gas detection pipe 3 is provided with a large number of communication holes 32 through which the internal space of the pipe line 31 communicates with the external space. Therefore, the detection target gas existing in the external space is introduced into the internal space of the conduit 31 through the communication hole 32. When the detection target gas is introduced into the gas detection tube 3, a part of the infrared ray emitted from the infrared light source 1 is absorbed or reflected by the detection target gas, and thus the received light intensity of the infrared sensor 2 changes. By detecting this change, it becomes possible to detect the presence and concentration of the gas to be detected. That is, the detection target gas is detected using the internal space of the pipe 31 provided in the gas detection pipe 3 as a monitoring space.

  The pipe 31 of the gas detection pipe 3 can be formed in a straight tube shape, but is preferably formed in a meandering shape as shown in the figure. Here, since the inner peripheral surface of the duct 31 reflects infrared rays, the infrared rays can be transmitted from the infrared light source 1 to the infrared sensor 2 even if the duct 31 is formed in a meandering shape.

  As described above, when the pipe line 31 is formed in a meandering shape, the infrared path from the infrared light source 1 to the infrared sensor 2 is extended, and the detection target gas introduced into the pipe line 31 is expanded. Since the distance through which the infrared ray passes becomes longer, it becomes easier to detect the influence of the detection target gas on the infrared ray. Moreover, since the distance through which the infrared ray passes through the detection target gas becomes longer even when the infrared rays are repeatedly reflected on the inner peripheral surface of the pipe 31, the sensitivity to the detection target gas can be improved by providing the meandering gas detection tube 3. Can be increased.

  The infrared light source 1 can intermittently emit infrared rays by intermittently applying a voltage from the drive circuit 4, has a short rise time from the start of energization, and a fall time from the end of energization. Is configured to be shorter. A specific configuration of the infrared light source 1 will be described later.

  The voltage waveform applied from the drive circuit 4 to the infrared light source 1 is a single pulse wave or a burst wave composed of a plurality of (about 5 to 10) pulses. Moreover, the time interval which applies a voltage to the infrared light source 1 is set to 10-60 s, for example. The time for applying a voltage to the infrared light source 1 (pulse width per pulse) depends on the response speed of the infrared sensor 2, but is set to 100 μs to 10 ms, for example. When burst waves are used, the duration is about 100 ms, for example. And

  In the illustrated example, the infrared light source 1 includes a radiating element 11 housed in a metal package 10 (can). A window hole 12 is formed in the package 10 in front of the radiating element 11, and a projection lens 13 is attached to the window hole 12. The light projection lens 13 is made of Si and is formed by a semiconductor process. In addition, two lead pins 14 for connecting the radiating element 11 to the drive circuit 4 are projected from the package 10. The light projection lens 13 is formed with an infrared antireflection film on both sides so as to suppress reflection in a specific wavelength band necessary for gas detection. A condensing mirror may be provided in place of the light projecting lens 13.

  On the other hand, in the infrared sensor 2, two light receiving elements 21a and 21b made of pyroelectric elements are housed in a metal package 20 (can), and in the package 20, in front of the two light receiving elements 21a and 21b. One common window hole 22 is formed. A filter for selecting the wavelength of infrared rays incident on the light receiving elements 21 a and 21 b from the internal space of the pipe 31 provided in the gas detection pipe 3 is attached to the window hole 22. Furthermore, lead pins 24 for connecting the light receiving elements 21 a and 21 b to the detection circuit 5 are provided on the package 20 in a protruding manner.

  The light receiving elements 21a and 21b can be either a thermal type or a quantum type. By using a thermal type element such as a pyroelectric element, the light receiving elements 21a and 21b are easy to handle and have high sensitivity. It can be offered at a low price.

  The filters are respectively arranged on the infrared incident paths from the monitoring space, which is the internal space of the pipe line 31, to the light receiving elements 21a and 21b, and each of the two transmission filters 25a and 25a selectively transmits infrared light in a specific wavelength band. 25b and a removal filter 26 that absorbs infrared light in a wavelength range excluding infrared light in a specific wavelength band that passes through the transmission filters 25a and 25b.

  Specifically, two transmission filters 25a and 25b are formed on one surface of the light receiving elements 21a and 21b on the filter substrate 23 made of Si, and a removal filter 26 is formed on the other surface of the filter substrate 23. The In other words, the transmission filters 25a and 25b are respectively arranged on the infrared incident paths from the pipe line 31 serving as the monitoring space to the light receiving elements 21a and 21b, and the removal filter 26 is connected to the pipe line 31 serving as the monitoring space and the transmission filter. It is arranged between 25a and 25b. Therefore, after the wavelength band transmitted through the conduit 31 is narrowed by the removal filter 26, only the specific wavelength band is selectively transmitted by the transmission filters 25a and 25b to reach the light receiving elements 21a and 21b.

  Each of the transmission filters 25a and 25b has a narrow-band transmission characteristic in a wavelength range of mid-infrared to far-infrared emitted by the radiating element 11. Transmission characteristics are designed so that one transmission filter 25a transmits a specific wavelength band where absorption by the detection target gas occurs, and the other transmission filter 25b transmits a specific wavelength band where absorption by the detection target gas does not occur.

  For example, the transmission filter 25a is configured to transmit a specific wavelength band centered on 4.3 μm when the detection target gas is carbon dioxide, and 4.7 μm when the detection target gas is carbon monoxide. When the detection target gas is methane, the value is set to 3.3 μm. Further, the transmission filter 25b is configured to transmit, for example, a specific wavelength band centered on 3.9 μm so that absorption by these detection target gases does not occur.

  As shown in FIG. 2, the narrow band transmission filters 25a and 25b are composed of a plurality of thin films 27x and 27y having a periodic structure with different refractive indexes and equal optical thicknesses, and a periodic structure (for example, two types of refraction). There is a multilayer filter having a structure in which a wavelength selection layer 28 having a different optical film thickness is sandwiched in the middle of the periodic structure, which is different from the thin films 27x and 27y. Each layer of the thin films 27x and 27y having the periodic structure is configured to have an optical film thickness of a quarter wavelength.

On the other hand, the removal filter 26 has an infrared absorption layer made of a material that absorbs infrared rays, and Al ₂ O ₃ or Ta ₂ O ₃ is used as the material of the infrared absorption layer. Although a configuration including only an infrared absorption layer may be employed as the removal filter 26, it is desirable to use a multilayer filter having a plurality of layers having different refractive indexes, similarly to the transmission filters 25a and 25b. For example, in the case where two types of refractive index layers are provided alternately, the low refractive index layer is an infrared absorption layer. When the infrared absorption layer is formed of the above material, Ge may be used as the material for the high refractive index layer.

  By selecting the above materials as the infrared absorption layer of the removal filter 26, the infrared rays longer than the specific wavelength band are spread over a wide band without absorbing the infrared of the specific wavelength band that passes through the transmission filters 25a and 25b. Can be absorbed. In addition, by using a multilayer filter as the removal filter 26, it is possible to prevent infrared rays in unnecessary wavelength ranges from entering the light receiving elements 21a and 21b by using not only absorption of infrared rays but also reflection.

  FIG. 3 shows the relationship between the characteristics of the transmission filters 25a and 25b (characteristic B is the transmission filter 25a and characteristic B is the transmission filter 25b) and the removal filter 26 (characteristic C). As can be seen from FIG. 3, in the removal filter 26, an unnecessary wavelength region (far infrared wavelength region) on the long wavelength side is removed by being absorbed without being transmitted, and the short wavelength side transmitted through the removal filter 26. In the specific wavelength band (mid-infrared or far-infrared wavelength band), the infrared of a specific wavelength band passes through the narrow band transmission filters 25a and 25b.

  The filter substrate 23 has a function of supporting the transmission filters 25 a and 25 b and the removal filter 26, and also has a function of radiating heat from the removal filter 26 by being thermally coupled to the package 20. As a material of the filter substrate 23, Ge or ZnS can be used in addition to Si described above. For the filter substrate 23, a material that transmits infrared rays in a specific wavelength band selected by the transmission filters 25a and 25b is selected.

  In order to detect the presence / absence and concentration of the gas in the gas detection tube 3 using the outputs of the two light receiving elements 21a and 21b, the difference or ratio between the output values of both the light receiving elements 21a and 21b is obtained. Now, it is assumed that the outputs of the light receiving elements 21a and 21b are Va and Vb, respectively, in the absence of the detection target gas, and that only the output of the light receiving element 21a is decreased by ΔV in the presence of the detection target gas. In the above-described example, the light receiving element 21a in which the transmission filter 24a is disposed in the front has a reduced light receiving intensity due to the presence of carbon dioxide, and the light receiving element 21b in which the transmission filter 24b is disposed in the front receives light even if carbon dioxide is present. Since the intensity does not decrease, a setting that only the output of the light receiving element 21a decreases is appropriate. Here, the change in the output difference between the light receiving elements 21a and 21b is (Va−Vb) → (Va−ΔV−Vb), and the change in the ratio is (Va / Vb) → {(Va−ΔV) / Vb. }.

  When the difference between the outputs of the light receiving elements 21a and 21b is obtained, if a pyroelectric element is used as the light receiving elements 21a and 21b, the polarity is used, and both the light receiving elements 21a and 21b are connected in reverse series to each other. Output differences can be used. Regardless of the type of the light receiving elements 21a and 21b, the difference between the outputs of the light receiving elements 21a and 21b can be obtained by inputting the output to the differential amplifier.

  Regardless of whether the difference between the outputs of the light receiving elements 21a and 21b or the ratio is used, simply receiving infrared light corresponding to a single pulse wave from the infrared light source 1 may be misidentified as noise such as external light. Therefore, it is desirable to receive infrared light corresponding to the burst wave from the infrared light source 1. If infrared light corresponding to a burst wave is received, the average value of differences or ratios obtained by a plurality of light receptions can be obtained, thereby reducing the influence of noise and improving S / N.

  In this embodiment, since pyroelectric elements are used as the light receiving elements 21a and 21b, the light receiving elements 21a and 21b are connected in reverse series to output a difference. The outputs of the light receiving elements 21a and 21b are given to the detection circuit 5. In the detection circuit 5, for example, carbon dioxide having a predetermined concentration or more exists as a target gas in the gas detection tube 3 due to the difference between the outputs of the light receiving elements 21a and 21b. Is detected. The detection circuit 5 may be provided either inside the package 20 or outside.

  In the present embodiment, the detection circuit 5 determines whether or not the concentration of the detection target gas in the gas detection tube 3 is equal to or higher than a specified value. Is output. The alarm signal is output to an alarm device that gives an alarm audibly or visually.

  The detection circuit 5 that enables this operation includes, for example, a current-voltage conversion circuit having an integration function that averages the outputs of the light receiving elements 21a and 21b corresponding to burst waves, and the output value of the current-voltage conversion circuit as a threshold value. It is possible to employ a configuration including a comparator for comparing with the output circuit and an output circuit for outputting an alarm signal according to the comparison result. The detection circuit 5 can adopt a configuration other than this configuration, and if it generates an output corresponding to the concentration of the detection target gas instead of an alarm signal, it is not a comparator but a current-voltage conversion circuit. May be provided with a function of converting the output value into the concentration of the detection target gas.

  By the way, the radiating element 11 used in this embodiment needs to radiate infrared rays in response to a single pulse from the driving circuit 4 or a burst wave composed of a plurality of pulses, and a response speed of about 10 μs to 10 ms. Is required. An example of the radiation element 11 that satisfies this requirement is shown in FIG. The configuration of the radiating element 11 illustrated in FIG. 4 is an example, and is not intended to be limited to this configuration.

  The radiating element 11 shown in the figure is a thin film holding layer 42 formed on one surface of a substrate 41, and a thin film laminated on the opposite surface side of the substrate 41 with respect to the holding layer 42. And an infrared radiation layer 43 that radiates infrared rays. Further, a gas layer 44 having a small thickness surrounded by a part of the substrate 41 and the holding layer 42 is formed on one surface of the substrate 41 where the holding layer 42 is provided. The gas layer 44 is formed in a shape in which a holding layer 42 is covered on an opening surface of a shallow recess formed by leaving a peripheral portion of one surface of the substrate 41. In the illustrated example, a cavity without any gas layer 44 is provided, but a support in the form of a column may be provided between the bottom surface of the recess and the holding layer 42 in order to support the holding layer 42.

  The substrate 41 is a semiconductor substrate (for example, a single crystal silicon substrate) and is formed in a rectangular parallelepiped shape. The holding layer 42 is formed by making the substrate 41 porous by anodizing a region where the peripheral portion of one surface of the substrate 41 is left. The conditions for anodic oxidation (electrolyte composition, current density, processing time, etc.) are appropriately set according to the conductivity type and conductivity of the substrate 41.

  Anodization is performed in an aqueous hydrogen fluoride solution, and is formed as a porous semiconductor layer (for example, a porous silicon layer) having a porosity of approximately 70%. The conductivity type of the substrate 41 may be either p-type or n-type. However, the p-type silicon substrate has a porosity higher than that of the n-type silicon substrate when anodized. Since it tends to be large, it is desirable to use a p-type silicon substrate for the substrate 41.

  The holding layer 42 made porous by anodization has the characteristics that the heat capacity and thermal conductivity are small, the heat resistance is high, and the surface is smooth. Further, in order to reduce the thermal conductivity of the holding layer 42, oxidation or nitridation may be performed on a part or all of the holding layer 42. If the oxidation or nitridation is performed, the electrical insulation is increased.

  On the other hand, a semiconductor oxide film formed by thermal oxidation may be used for the holding layer 42 instead of making it porous by anodic oxidation. Further, instead of forming the semiconductor oxide film that becomes the holding layer 42 by thermal oxidation, it is also possible to form the holding layer 42 of a material containing oxide by a CVD method. If the holding layer 42 is formed by thermal oxidation or CVD, the manufacturing process becomes simpler than the case where the holding layer 42 is formed by making it porous, so that mass productivity can be improved. In the case where the holding layer 42 is formed by the CVD method, it is possible to use an oxide with high thermal insulation such as alumina or a material containing this kind of oxide. The holding layer 42 can also be formed of a porous body of this kind of material.

  The infrared radiation layer 43 is formed of a material selected from TaN and TiN. Since these materials are excellent in heat resistance and oxidation resistance, the infrared radiation layer 43 can be used in an air atmosphere. These materials have a physical property that the sheet resistance becomes a desired value in a thickness dimension (several tens of nm) suitable for ensuring the responsiveness while satisfying the durability as the infrared radiation layer 43.

  Since nitrogen gas is introduced when forming the infrared radiation layer 43 using these materials, the sheet resistance can be controlled by the partial pressure of the introduced nitrogen gas. For example, the infrared radiation layer 43 can be formed at a predetermined position by reactive sputtering of TaN. Then, by controlling the partial pressure of nitrogen gas, the sheet resistance can be formed to a desired value at a predetermined heat generation temperature. However, materials other than TaN and TiN can be used as the material for forming the infrared radiation layer 43, and other metal nitrides or metal carbides may be used.

  A pair of electrodes 45 formed of a highly conductive metal material is provided on the surface of the infrared radiation layer 43. As the metal material used for the electrode 45, iridium which does not easily react with the material of the infrared radiation layer 43 and has excellent stability at high temperatures is suitable. Further, when the temperature increase of the infrared radiation layer 43 is small, a material such as aluminum can be used for the electrode 45. The material of the electrode 45 is not limited to these metal materials, and other conductive materials can be used.

  As described above, the radiating element 11 is housed in the package 10 to form the infrared light source 1, and each electrode 45 is connected to each lead pin 14 via the bonding wire 15.

  When the radiating element 11 having the above-described configuration is energized between the electrodes 45, the infrared radiation layer 43 is heated by Joule heat, and infrared radiation is emitted from the infrared radiation layer 43. Further, when the energization is stopped, the infrared radiation from the infrared radiation layer 43 is stopped.

  If the energization time is relatively short when the infrared radiation layer 43 is energized, heat transfer by heat conduction and convection is not performed in the gas layer 44, and the temperature drop of the holding layer 42 is suppressed. As a result, the infrared radiation layer 43 Is maintained at a high temperature to promote infrared radiation.

  On the other hand, if there is a temperature difference between the substrate 41 and the holding layer 42 when the infrared radiation layer 43 is not energized, heat transfer from the holding layer 42 to the substrate 41 and heat transfer by convection due to the gas present in the gas layer 44. The heat radiation of the holding layer 42 is promoted, and as a result, the infrared radiation layer 43 is rapidly cooled to stop the infrared radiation immediately.

  That is, as shown in FIG. 5A, when a pulse voltage is applied to the infrared radiation layer 43 (between a pair of electrodes 45 provided on the infrared radiation layer 43), the infrared radiation layer 43 becomes as shown in FIG. Infrared rays are radiated with good responsiveness from the rise of the voltage, and the emission of infrared rays is stopped in a relatively short time after the fall of the voltage.

The peak wavelength λ [μm] of infrared rays radiated from the infrared radiation layer 43 satisfies the Vienna displacement law, and the absolute temperature T [K] of the infrared radiation layer 43 has the following relationship.
λ = 2898 / T (1)
Therefore, by changing the temperature of the infrared radiation layer 43, the peak wavelength of infrared radiation emitted from the infrared radiation layer 43 can be changed. In order to adjust the temperature of the infrared radiation layer 43, the amplitude or waveform of the voltage applied to the electrode 45 is adjusted to change the Joule heat generated per unit time.

  For example, by applying a sinusoidal voltage of about 100 V between both electrodes 45, it is possible to design to emit infrared light having a peak wavelength of 3 to 4 [μm]. If the voltage is adjusted, It is also possible to emit infrared rays having a peak wavelength of 4 [μm] or more.

By the way, assuming that a sinusoidal voltage is applied to the electrode 45 of the infrared light source configured as described above, the thermal conductivity of the holding layer 42 is αp [W / mK], and the volumetric heat capacity of the holding layer 42 (specific heat capacity and density). ) Is Cp [J / m ³ K], and the frequency at which the infrared radiation layer 43 can respond (twice the frequency of the applied voltage) is f [Hz], the thermal diffusion length μ of the holding layer 42 is It is expressed by the following formula.
μ = (2αp / ωCp) ^1/2 (2)
However, ω = 2πf.

  When the heat which changes alternatingly from the infrared radiation layer 43 is given from the infrared radiation layer 43, the infrared rays radiated from the infrared radiation layer 43 toward the retention layer 42 are the boundary between the retention layer 42 and the gas layer 44. It is necessary to set the thickness dimension Lp so as to be passed to the gas layer 44 on the surface. That is, it is desirable to set the thickness dimension Lp of the holding layer 42 to a value that is at least smaller than the thermal diffusion length μ (Lp <μ).

The holding layer 42 is made of porous silicon, and the frequency f at which the infrared radiation layer 43 can respond, the volumetric heat capacity Cp of the holding layer 42, and the thermal conductivity αp of the holding layer 42 are f = 10 [kHz] and αp, respectively. = 1.1 [W / mK], Cp = 1.05 × 10 ⁶ [J / m ³ K] and substituting into the equation (2), μ = 5.8 × 10 ⁻⁶ [m]. Therefore, it is necessary to make the thickness dimension Lp of the holding layer 42 smaller than 5.8 [μm].

  In order to further increase the infrared radiation efficiency, it is desirable to establish a resonance condition in the holding layer 42. That is, the amount of infrared rays radiated to the rear of the infrared radiation layer 4 is reduced by reflecting the infrared rays from the infrared radiation layer 43 toward the retention layer 42 at the boundary surface between the retention layer 42 and the gas layer 44, and as a result. The intensity of infrared rays emitted from the infrared emission layer 43 can be increased. In order to enable this operation, the thickness dimension of the holding layer 42 is set so as to satisfy the resonance condition of the infrared ray of the target wavelength.

In order to satisfy the resonance condition in the holding layer 42 for the infrared ray of the target wavelength, the optical path length of the holding layer 42 for the infrared ray must be an odd multiple of a quarter wavelength of the infrared ray. Now, assuming that the thickness dimension of the holding layer 42 is Lq [m] and the refractive index of the holding layer 42 is n, the optical path length is n · Lq. Therefore, the wavelength of the target infrared ray in vacuum is λ [m]. If so, the resonance condition is expressed by the following equation.
n · Lq = (2m−1) λ / 4 (3)
Here, m is a positive integer.

  As described above, when the holding layer 42 is made of porous silicon, the refractive index of the holding layer 42 is, for example, n = 1.35, so that the target infrared wavelength is 4 [μm], and m = 1. Is set, the thickness dimension Lq of the holding layer 42 is 0.74 [μm]. That is, since Lp = 0.74 [μm] <5.8 [μm], the expression (2) can also be satisfied.

  That is, the holding layer 42 does not hinder the temperature rise of the infrared radiation layer 43. In addition, since the holding layer 42 is formed using porous silicon, the volume heat capacity of the holding layer 42 is reduced as compared with the case where the holding layer 42 is formed using a dense material. The volumetric heat capacity as a whole with the layer 42 can be reduced.

  Thus, since the volumetric heat capacity of the holding layer 42 is reduced and the temperature rise of the infrared radiation layer 43 is not hindered, the temperature rise efficiency of the infrared radiation layer 43 is increased, and a response to a change in applied voltage is made at high speed. Is possible.

  Further, the surface of the holding layer 42 opposite to the surface in contact with the infrared radiation layer 43 is in contact with the gas layer 44, and the gas layer 44 has a lower thermal conductivity than the holding layer 42. The thermal resistance of the heat conduction path through the layer 42 is increased, and heat dissipation to the periphery of the infrared radiation layer 43 is suppressed.

Incidentally, it is desirable to set the thickness dimension Lg of the gas layer 44 under the following conditions. Now, the applied voltage to the infrared radiation layer 43 is sinusoidal, the frequency of the applied voltage is f [Hz], the thermal conductivity of the gas layer 44 is αg [W / mK], and the volumetric heat capacity of the gas layer 44 is Cg [J / M ³ K]. At this time, the thickness dimension Lg of the gas layer 44 is set to a range represented by the following formula.
0.05Lg '<Lg <3Lg' (4)
However, Lg ′ = (2αg / ωCg) ^1/2 and ω = 2πf.

For example, the applied voltage to the infrared radiation layer 43 is a sine wave having a frequency f = 10 [kHz], and the thermal conductivity αg of the gas layer 44 and the volumetric heat capacity Cg of the gas layer 44 are respectively αg = 0.0254 [W / MK], Cg = 1.21 × 10 ³ [J / m ³ K], from (4), 1.3 [μm] <Lg <77.5 [μm]. By setting the thickness dimension Lg to 25 [μm], for example, the expression (4) can be satisfied. Desirably, the thickness is set so that the temperature amplitude ratio is maximized within this range.

  If the temperature of the substrate 41 is constant, the gas layer 44 has either a heat insulating function or a heat radiating function depending on the temperature and the thickness dimension Lg of the holding layer 42. By appropriately adjusting Lg in the condition range of the equation (4), the gas layer 44 is provided with heat insulation during the period in which the voltage applied to the infrared radiation layer 43 increases, and the voltage applied to the infrared radiation layer 43 is reduced. During the descending period, the gas layer 44 can be provided with heat dissipation.

  That is, the timing of using the heat insulation and heat dissipation of the gas layer 44 can be substantially matched with the timing of increase / decrease of the voltage applied to the infrared radiation layer 43, and the voltage applied to the infrared radiation layer 43 is high frequency. Even in the case of modulation by the above, it is possible to change the temperature of the infrared radiation layer 43 so as to be substantially synchronized with the frequency of the voltage. That is, it is possible to improve the responsiveness compared to the case where the gas layer 44 is not provided.

  In the configuration example described above, a direct heating configuration in which the infrared radiation layer 43 is energized to generate heat and emit infrared rays by energizing the infrared radiation layer 43, but between the holding layer 42 and the infrared radiation layer 43 is exemplified. A heating layer is provided separately from the infrared radiation layer 43, or a heating layer is provided on the opposite side of the holding layer 42 with the infrared radiation layer 43 interposed therebetween. The infrared radiation layer 43 is heated to emit infrared rays by energizing the heating layer. It is also possible to use an indirectly heated configuration.

  In the latter configuration, the infrared radiation layer 43 can also be used as the holding layer 42. In the latter configuration, the heating layer is formed so that the resonance condition is established with respect to the infrared ray of the target wavelength so that the infrared ray radiated from the infrared radiation layer 43 is transmitted through the heating layer. It is necessary to increase the radiation efficiency. In addition, you may employ | adopt the structure which provided the reflection layer (not shown) which reflects infrared rays instead of providing the gas layer 44. FIG. In short, the radiating element 11 used in the infrared light source 1 may be of any configuration as long as the infrared intensity changes following a pulse having an ON period of about 10 μs to 10 ms.

DESCRIPTION OF SYMBOLS 1 Infrared light source 2 Infrared sensor 3 Gas detection tube 21a, 21b Light receiving element 23 Filter substrate 25a, 25b Transmission filter 26 Removal filter 27x, 27y Thin film 28 Wavelength selection layer 30 Gas detection tube 41 Substrate 42 Holding layer 43 Infrared radiation layer 44 Gas layer

Claims (6)

An infrared light source that emits infrared light;
And a infrared sensor for detecting infrared rays having passed through the monitored space to the detection target gas is emitted is introduced from the infrared light source,
A infrared gas detector for detecting the detection target gas in the monitored space by using the output of the infrared sensor,
The infrared sensor is
A light receiving element that converts infrared light into an electrical signal;
A transmission filter for the arranged from the monitoring space to incident path of the infrared to the light receiving element selectively transmits infrared of a specific wavelength band,
And a removal filter for removing infrared broadband by absorbing infrared rays in a wavelength range other than the specific wavelength band that passes through the transmission filter is disposed between the monitoring space and the transmission filter,
The light receiving element has a sensitivity to infrared of the specific wavelength band that passes through the transmission filter,
The infrared light source is
It is driven to emit infrared rays intermittently ,
An infrared radiation layer that emits infrared light by heat generated by energization, which is a thin film formed on one surface of the substrate;
A holding layer formed on one surface of the substrate to support the infrared radiation layer by laminating the infrared radiation layer on one surface;
A gas layer in contact with the other surface of the holding layer in the substrate,
The gas layer is
The frequency of the applied voltage is f [Hz], the thermal conductivity of the gas layer is αg [W / mK], the volumetric heat capacity of the gas layer is Cg [J / m ³ K], Lg ′ = (2αg / ωCg) ^{1 / 2} , ω = 2πf, the thickness dimension Lg is set in the range of 0.05Lg ′ <Lg <3Lg ′,
When energizing the infrared radiation layer, suppress the temperature drop of the holding layer,
An infrared gas detector, wherein heat transfer from the holding layer to the substrate is promoted when the infrared radiation layer is not energized . The removal filter is:
A multilayer filter in which a plurality of layers having different refractive indexes are laminated,
2. The infrared gas detection layer according to claim 1, wherein at least one of the multilayer filters is an infrared absorption layer that absorbs far infrared rays having a longer wavelength than the specific wavelength band that transmits the transmission filter. vessel. The infrared gas detector according to claim 2, wherein the material of the infrared absorption layer is Al ₂ O ₃ . The infrared gas detector according to claim 2, wherein a material of the infrared absorption layer is Ta ₂ O ₃ . The removal filter has a filter substrate made of plate-like Si,
The infrared gas detector according to claim 2 , wherein the multilayer filter is laminated on the filter substrate . The infrared gas detector according to claim 1 , wherein the light receiving element is a thermal element having sensitivity to all wavelengths of infrared rays emitted from the infrared light source .

Images