JP2005083950A - Voc sensor and voc detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a VOC sensor for detecting density of VOC (Volatile Organic Compound) with improved sensitivity. <P>SOLUTION: The VOC sensor has a sensitive element section Rs, having a heater 42 driven by a pulse with a specified period including an energization OFF period and an energization ON period, and a catalyst layer 43 made of a mesoporous material that is provided thermally in contact with the heater 42, thus obtaining sensor output according to VOC density, based on the resistance of the heater 42 that becomes a peak value immediately after starting the energization ON period. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a VOC (Volatile Organic Compound) sensor and a VOC detection device.

  In recent years, buildings such as newly built houses have many structures having high airtightness and high heat insulation. In buildings with such a structure, new building materials and interior materials mainly made of synthetic resin or the like instead of natural building materials are mainly used. However, it is known that a large amount of VOC is generated from these new building materials and interior materials. The so-called “sick house syndrome”, in which indoor air is polluted by these VOCs and various symptoms such as irritation to eyes, nose and throat, headache, dizziness, etc., is a problem.

  As for VOC, more than 100 kinds of substances including formaldehyde, toluene and the like have been confirmed. Formaldehyde is contained in, for example, plywood, particle board, flooring material, adhesive, fungicide and the like. Toluene is contained in, for example, oil-based paints and organic solvent-based adhesives.

  In the above-mentioned building, these various types of VOC components are mixed and exist in the room. Therefore, not only whether the individual VOC gas concentration affects the human body, but the total amount of VOC in the entire room affects the human body. Whether or not to give will also be a problem. The total amount of VOC is called total volatile organic compound (TVOC).

For example, the World Health Organization (WHO) has established a limit value of 0.08 ppm for indoor formaldehyde, and the Ministry of Health, Labor and Welfare in Japan has also included a reference value that complies with the WHO limit value. “Reduction Guidelines” was announced in 2002. In this guideline, it is mentioned that concentration measurement is one of the measures that operators should take.

  However, since VOC and TVOC are gases that have a serious effect on the human body even if their concentrations are extremely small, for example, the gas concentrations of VOC and TVOC affect the human body in a new building after completion. Although a sensor for measuring whether or not the temperature is high is eagerly desired, a sensor that can detect the gas concentrations of VOC and TVOC in a short time with high sensitivity has not yet been proposed.

  Accordingly, in view of the above-described conventional problems, an object of the present invention is to provide a VOC sensor and a VOC detection device that can detect extremely small amounts of VOC and TVOC with high sensitivity.

  The VOC sensor according to the first aspect of the present invention includes a heater driven by a pulse having a predetermined cycle including an energization off period and an energization on period, and a catalyst layer made of a mesoporous material provided in thermal contact with the heater. And a sensor output corresponding to the VOC concentration is obtained based on the resistance value of the heater that becomes a peak value immediately after the start of the energization on period.

  The VOC sensor according to a second aspect of the invention is characterized in that, in addition to the feature of the first aspect, the mesoporous material is mesoporous silica.

  The VOC sensor according to the invention of claim 3 is provided in thermal contact with the first heater driven by a pulse having a predetermined cycle including an energization off period and an energization on period, and the first heater. A sensitive element having a catalyst layer made of mesoporous silica supporting a platinum group such as palladium (Pd), the second heater driven by the pulse, and the second heater are provided in thermal contact with each other. And a compensation element portion having a layer made of mesoporous silica, and a sensor output corresponding to the VOC concentration is obtained based on the resistance values of the first and second heaters during the energization-on period.

  According to a fourth aspect of the present invention, there is provided a VOC detection device according to any one of the first to third aspects, and a sensor that drives the VOC sensor with a drive pulse including a predetermined energization off period and an on period. Drive control means, integral value computing means for computing an integral value over a predetermined time of the sensor output of the VOC sensor during the energization ON period, and integral value data over the predetermined time corresponding to the VOC are stored in advance. A storage means, a comparison means for comparing the integral value calculated by the integral value calculation means with the integral value data, and an output for detecting the VOC based on the comparison result by the comparison means and outputting the result Means.

  According to the first aspect of the present invention, a large sensor output can be obtained when detecting VOC or TVOC, and for example, it is possible to detect a very low concentration (ppb level) VOC or TVOC concentration causing the onset of sick house syndrome. A VOC sensor can be obtained.

  According to the second aspect of the invention, by using a catalyst carrier called mesoporous silica, the ability to adsorb VOC is increased, and high sensitivity can be achieved.

  According to the third aspect of the present invention, a large sensor output compensated for temperature can be obtained when detecting VOC or TVOC.

  According to the fourth aspect of the present invention, it is possible to detect an extremely low concentration (ppb level) VOC or TVOC concentration, for example, by monitoring the VOC or TVOC concentration in a building room to avoid the onset of sick house syndrome. Yes, it has a great social contribution.

  The best mode for carrying out the present invention will be described below with reference to the drawings. 1A, 1B, and 1C are a plan view, a rear view, and a cross-sectional view taken along line AA of the VOC sensor according to the present invention, respectively.

As shown in FIGS. 1A and 1B, a VOC sensor according to the present invention has a predetermined thickness (for example, about 600 nm) on a silicon (Si) wafer 41 having a predetermined thickness (for example, about 400 μm). An insulating thin film of a silicon oxide (SiO ₂ ) film 48c, a silicon nitride (SiN) film 48b having a predetermined thickness (for example, about 250 nm), and a hafnium oxide (HfO ₂ ) film 48a having a predetermined thickness (for example, about 30 nm). A multilayer insulating film is formed sequentially.

On this multilayer insulating film, a platinum (Pt) heater 42 as a first heater having a predetermined thickness (for example, about 250 nm) is formed as the sensitive element portion Rs, and the platinum (Pt) heater 42 A catalyst layer comprising a platinum group, for example, a mesoporous material carrying palladium (Pd), for example, mesoporous silica (m-SiO ₂ ), ie, a Pd / m-SiO ₂ catalyst layer 43 having a predetermined thickness, is in thermal contact. (For example, about 1 to 40 μm).

The mesoporous material is a porous material having regularly arranged mesopores (1 to 50 nm in diameter) having a uniform diameter. For example, mesoporous silica (m-SiO ₂ ), which is one of mesoporous materials, is an aggregate of columnar structures having a hollow with an inner diameter of about 10 mm or more, and the specific surface area reaches 1500 m ² / g at the maximum.

  This mesoporous silica was first proposed as a production method in International Publication WO 91/11390 of Mobile Oil. In the production method, n-dodecyltrimethylammonium chloride and sulfuric acid dissolved in an aqueous solution of sodium hydroxide in silica sol were alternately added little by little, then the pH was adjusted to 13.5 to gel, and this was stirred at room temperature for 2 hours. After that, it is left in an autoclave at 140 ° C. for 48 hours to carry out hydrothermal synthesis, and the product is filtered, washed with water, dried, and then fired at 550 ° C. for several hours to remove residual organic matter. it can.

Further, on the multilayer insulating film, as the compensation element portion Rr, a platinum (Pt) heater 44 as a second heater having a predetermined thickness (for example, about 250 nm), and the platinum (Pt) heater 44 are thermally connected. In contact therewith, a layer made of mesoporous silica (m-SiO ₂ ), that is, an SiO ₂ layer 45 is also formed.

  Further, as shown in FIG. 1C, the silicon wafer 41 is anisotropically etched to form the recesses 46 and 47, thereby forming the thin film diaphragms Ds and Dr by the above-described insulating thin films.

  Next, a method for manufacturing the above VOC sensor will be described.

First, a silicon (Si) wafer 41 having a thickness of 400 μm is prepared, and the surface of the silicon (Si) wafer 41 is oxidized to form a silicon oxide (SiO ₂ ) film 48c having a thickness of 600 nm.

Next, using a sputtering apparatus (not shown), a silicon nitride (SiN) film 48b having a thickness of 250 nm is formed on the silicon oxide (SiO ₂ ) film 48c, and then on the silicon nitride (SiN) film 48b. Then, a hafnium oxide (HfO ₂ ) film 48a having a thickness of 30 nm is formed to form a multilayer insulating film.

Next, a pair of platinum (Pt) heaters 42 and 44 having a thickness of 250 nm are formed on the multilayer insulating film. Next, on the platinum (Pt) heater 42, mesoporous silica particles (Pd / m-SiO ₂ ) carrying palladium (Pd) are applied by screen printing, and a Pd / m-SiO ₂ catalyst having a thickness of 1 to 40 μm. Layer 43 is formed. On the platinum (Pt) heater 44, mesoporous silica particles (m-SiO ₂ ) not supporting palladium (Pd) are applied by screen printing to form an m-SiO ₂ layer 45 having a thickness of 1 to 40 μm.

The VOC sensor 40 manufactured as described above functions as an adsorption combustion type gas sensor as will be described later, and the specific surface area of the catalyst carrier is 1500 m ² / g, which is 10 times that of the catalyst carrier used in the conventional sensor. is there. Therefore, the amount of VOC gas adsorption increases, the sensor output generated by the combustion of the adsorption gas also greatly increases, and the sensitivity increases.

  Next, a gas detection apparatus using the VOC sensor 40 of the present invention will be described with reference to FIGS. 2 is a block diagram showing a configuration example of a gas detection device using the VOC sensor 40 of the present invention, FIG. 3 is a timing chart showing a waveform example of a drive pulse used in the gas detection device of FIG. 2, and FIG. FIG. 6 is a graph showing sensor output characteristics when toluene is detected by the VOC sensor 40 during a power-on period. FIG.

  The gas detection device 1 includes a controller 10, a drive power supply 20, an output unit 30, and a VOC sensor 40 including a detection bridge circuit. The controller 10 is connected to the drive power supply 20, the output means 30, and the VOC gas sensor 40.

  In the gas detection device 1, a desired gas type is detected and output based on the sensor output of the VOC sensor 40 when a drive pulse having a drive waveform as shown in FIG. The principle of this detection utilizes the fact that the sensor output of the VOC sensor 40 exhibits a unique peak waveform corresponding to the type of VOC from the start point of the energization on period.

  Specifically, as shown in FIG. 4, the sensor output of a VOC such as toluene has a peak most noticeably immediately after the start of the energization on period (200 milliseconds), so that this phenomenon is used effectively here. I have to. That is, as shown in FIG. 3, the sensor output is acquired from the VOC sensor 40 in the energization on period of 200 milliseconds after the energization off period of 30 seconds in the drive pulse. It is known that the energization off time is about 10 to 60 seconds in the ppm level measurement, and the energization on time is about 200 milliseconds.

  In the example of FIG. 4, sensor output characteristics when 100 ppm concentration of toluene is detected as VOC are exemplified, but in other VOCs such as ethanol, xylene, diethyl carbonate, dimethyl carbonate, acetic acid and formaldehyde, Similarly, it is known to exhibit a peak waveform.

  The controller 10 includes a correction unit 11, a sensor drive control unit 12, a time measurement unit 13, an integral value calculation unit 14, a storage unit 15, and a comparison unit 16. The controller 10 can be implemented by a microcomputer having a calculation unit, a storage unit, and the like.

  The correction unit 11 supplies the detection output from the bridge circuit with a predetermined correction to the integral value calculation unit 14 as a sensor output. That is, the detection output from the bridge circuit is measured in advance as an initial detection output in an environment where no VOC exists, and this initial detection output is subtracted from the detection output obtained from the bridge circuit in an environment where the VOC actually exists. Is output to the integrated value calculation means 14 as a sensor output. An amplifying means (not shown) may be inserted between the correcting means 11 and the bridge circuit.

  The sensor drive control unit 12 supplies the drive power source 20 to the VOC sensor 40 and the clock signal supplied from the time measuring unit 13 until the gas detection device 1 is powered on until an instruction to stop driving by a predetermined trigger is received. The energization control is performed with reference to continuously generate a drive pulse having a drive waveform in which one cycle is composed of an energization off period of 30 seconds and an energization on period of 200 milliseconds as shown in FIG. In the drive pulse shown in FIG. 3, the level of the drive pulse corresponding to the energization off period is not necessarily zero. That is, a DC bias may be applied to the drive pulse even during the energization off period so that a desired sensor output can be easily obtained by the integral value calculation means 14.

  The integral value calculation means 14 calculates the integral value over a predetermined time of the sensor output of the VOC sensor 40 during the energization ON period while referring to the clock signal supplied from the time measuring means 13. Specifically, the integral value calculation means 14 samples the sensor output from the correction means 11 and integrates them over a predetermined time in the energization ON period including the timing corresponding to the peak value of the gas to be detected.

  The storage means 15 stores in advance the integrated value data over the predetermined time corresponding to the gas to be detected. Specifically, since the integral value specific to the gas to be detected can be acquired in advance by experiments or the like, this is stored as integral value data. In the comparison unit 16, the integration value detected by the integration value calculation unit 14 is compared with the integration value data stored in the storage unit 15, and the comparison result is output to the output unit 30.

  The output means 30 detects the gas to be detected based on the comparison result by the comparison means 16 and outputs the result. As the output of this result, for example, a method using a visible signal such as a light emitting diode or a method using an audible signal such as a buzzer is assumed. Alternatively, a method using communication means such as a telephone line may be used. The output unit 30 includes a device that generates these visible or audible signals, a driver circuit thereof, a communication device, and the like. As the drive power source 20 connected to the controller 10, a known battery or a commercial power source can be used.

The VOC sensor 40 is intermittently driven by the driving pulse as shown in FIG. As shown in FIG. 1, the VOC sensor 40 includes a sensitive element portion Rs and a compensation element portion Rr. The sensitive element portion Rs includes a platinum (Pt) heater 42 and a Pd / m-SiO ₂ catalyst layer 43, and the compensating element portion Rr includes a platinum (Pt) heater 44 and an m-SiO ₂ layer 45. The sensitive element portion Rs and the compensation element portion Rr constitute a bridge circuit together with the fixed resistors R1 and R2 and the variable resistor Rv. In this bridge circuit, the drive pulse from the sensor drive control means of the controller 10 is at a predetermined interval at the connection point of the sensitive element unit Rs and the fixed resistor R1 and the connection point of the compensation element unit Rr and the fixed resistor R2. It is supplied intermittently. Further, the connection point of the sensitive element part Rs and the compensating element part Rr and the movable terminal of the variable resistor Rv are connected to the correction means 11 of the controller 10, whereby the voltage value as the sensor output of the VOC 40 is supplied to the correction means 11. Supplied.

  When using this VOC sensor 40, first, before starting the detection operation, the variable resistor Rv is adjusted so that the sensor output from the correction means 11 becomes zero. In this adjusted state, when a VOC such as toluene touches the sensitive element portion Rs, it is oxidized on the surface of the sensitive element portion Rs by the catalytic action to generate reaction heat. Due to this reaction heat, the resistance value of the platinum (Pt) heater 42 rises, and this rise in resistance value breaks the balance of the bridge circuit, and the sensor output is supplied to the controller 10. In this case, the platinum (Pt) heater 44 cancels out the variation in the resistance value of the platinum (Pt) heater 42 due to the variation in the ambient temperature, and the variation component of the resistance value of the platinum (Pt) heater 42 due to the reaction heat of the VOC. Compensate the temperature so that only

  In the gas detection device 1 having such a configuration, the drive pulses as shown in FIG. 3 are continuously output under the control of the sensor drive control means 12. With this drive pulse, the platinum (Pt) heaters 42 and 44 are heated to a temperature suitable for VOC detection (about 400 degrees) almost instantaneously at the start of the energization on period (200 milliseconds).

During this heating, since the Pd / m-SiO ₂ catalyst layer 43 uses a mesoporous material called mesoporous silica (m-SiO ₂ ) as a support material, it has a high specific surface area and the driving temperature of the VOC sensor 40 ( High thermal stability at about 400-450 ° C). That is, mesoporous silica (m-SiO ₂ ) does not collapse the mesopores even when the temperature rises to about 600 ° C., and can sufficiently withstand repeated use and long-term use of the VOC sensor 40.

The VOC gas molecules adsorbed onto the catalyst carrier during the energization off period (30 seconds) are combusted simultaneously with the start of the energization on period (200 milliseconds). At this time, the Pd / m-SiO ₂ catalyst layer 43 has good function of conducting heat to the platinum (Pt) heater 42 from the adsorption-combustion reaction of the VOC gas in addition to the function as an adsorbent or catalyst carrier. It also functions as a conductor.

  Due to the conduction of the combustion heat (reaction heat) of the gas molecules, a resistance change of the platinum (Pt) heater 42 is caused. This is detected as a potential difference in the bridge circuit. As described above, when the gas to be detected is detected by the VOC sensor 40, the sensor output is given to the integrated value calculation means 14 via the correction means 11. In the integral value calculation means 14, an integral value is calculated from the given sensor output, and the calculated integral value is given to the comparison means 16 and compared with the integral value data stored in the storage means 15. Then, the comparison result is output to the output means 30, and alarm output or the like is performed by a predetermined output method.

  As described above, VOC is detected by utilizing the integrated value of the timing period including the peak value, so that a larger sensor output can be obtained and the value necessary for detection is stable compared to the conventional contact gas sensor. As a result, the detection accuracy is greatly improved.

  Further, the larger the specific surface area of the mesoporous material that is the carrier material of the VOC sensor 40, the higher the ability to adsorb VOC, and the output (sensitivity) increases in proportion to the increase in the specific surface area.

FIG. 5 is a graph showing the relationship of integral sensitivity when detecting 100 ppm of toluene with respect to the specific surface area of the mesoporous catalyst support of the VOC sensor according to the present invention. Here, the integral sensitivity is an integral value obtained by integrating the area of the waveform in FIG. From this graph, it can be seen that the integral sensitivity (integral value) increases in proportion to the increase in the specific surface area of the mesoporous catalyst support. Moreover, according to the request | requirement resulting from the kind etc. of VOC, a highly sensitive VOC sensor is producible using the mesoporous catalyst support | carrier which has an arbitrary specific surface area of 500-1500 m < ² > / g.

  FIG. 6 is a graph showing the toluene concentration dependency of the VOC sensor of the present invention. Characteristic A indicates the integral sensitivity (au) with respect to the toluene concentration of the VOC sensor of the present invention, and characteristic B indicates the detection limit. FIG. 6 shows that the VOC sensor of the present invention has a limit value of 0.04 ppm (40 ppb), the detection limit is considerably lower than that of the conventional adsorption combustion type gas sensor, and the sensitivity is high.

  By the way, since there are differences in symptoms and onset VOC concentrations depending on the person, it is difficult to determine that VOC concentration detection up to this range is necessary as a measure against sick house syndrome. However, for example, the Ministry of Health, Labor and Welfare has published a guideline value for indoor air quality, which can be used as a reference value. The table below shows guide values for indoor concentrations for each volatile organic compound.

  Although the concentration value shown as a guideline varies depending on each VOC, it can be measured whether it is darker or lighter than this value. Actually, since the room air is in a state where various VOCs are mixed, the number of TVOCs of number 14 is measured instead of measuring individual VOCs. This TVOC shows how much each VOC in the room exists in terms of toluene concentration. A VOC sensor that accurately captures whether this guideline value is above or below 0.1 ppm is required.

  Therefore, in the gas detection apparatus of FIG. 2, the integrated value data over the predetermined time corresponding to the TVOC guideline value of 0.1 ppm is stored in the storage means 15 in advance. Then, the compared means 16 compares the calculated integrated value from the integrated value calculating means 14 with the above-described integrated value data stored in the storage means 15. The comparison result is output to the output means 30.

  For example, if the gas detector 1 is used in a room of a newly built house and the integrated value calculated from the integrated value calculating means 14 exceeds the above-described integrated value data, an alarm is output, and the indoor TVOC It is informed to the prospective resident that the value exceeds the guideline value.

  Thereafter, the TVOC is measured by the gas detection device 1 every certain number of days, and when it is confirmed that the calculated integrated value from the integrated value calculating means 14 is lower than the above-described integrated value data. Judging that there is no adverse effect of TVOC, the prospective residence person is allowed to live in the new house.

  As described above, when the VOC gas sensor according to the present invention is used, it has a resolution capable of detecting an extremely low concentration (ppb level) VOC or TVOC concentration that cannot be detected by a conventional gas sensor. VOC or TVOC concentration can be monitored to avoid the onset of sick house syndrome, and the social contribution is great.

  As described above, the embodiment of the present invention has been described. However, the present invention is not limited to this, and various modifications and applications are possible.

  For example, in the gas detection device described above, an example in which a desired VOC is detected using an integral value has been shown. However, instead of the integral value, a desired VOC is detected using a peak value of a sensor output. It is also possible to configure. In this case, instead of the integral value calculation means 14, a peak value acquisition means is provided, and peak value data of the energization on period corresponding to the gas to be detected is stored in the storage means 15 in advance. Specifically, since the peak value unique to the gas to be detected can be obtained in advance by experiments or the like, this is stored as peak value data. Further, the comparison unit 16 compares the peak value detected by the peak value acquisition unit with the peak value data stored in the storage unit 15 and outputs the result to the output unit 30.

  In the above description, toluene is used as an example of VOC. However, the present invention is not limited to detection of toluene alone, and can be similarly detected with a plurality of types of VOCs. For example, VOCs containing ethanol, xylene, diethyl carbonate, dimethyl carbonate, acetic acid and formaldehyde can also be detected. Accordingly, the energization on / off interval, the energization time, and the like can be appropriately changed.

  Further, as a support material, mesoporous materials having a high specific surface area other than mesoporous silica (for example, mesoporous alumina (a high specific surface area alumina manufactured by artificially controlling the pore structure using an organic template or the like), mesoporous silica alumina A similar effect can be obtained by using (aluminum element introduced into skeletal silica), and the combustion catalyst is not Pd but can be obtained by adding other platinum group elements.

  In addition, the VOC sensor of the present invention can be applied to detection of gases other than VOC as long as it is a combustible gas adsorbed on a carrier.

(A), (B), and (C) are the AA line sectional views in the top view, back view, and top view of the VOC sensor by the present invention, respectively. It is a block diagram which shows the structural example of the gas detection apparatus using the VOC sensor 40 of this invention. It is a timing chart which shows the waveform example of the drive pulse used with the gas detection apparatus of FIG. It is a graph which shows the sensor output characteristic at the time of toluene detection by the VOC sensor in an energization ON period. It is a graph which shows the relationship of the 100 ppm toluene sensitivity with respect to the specific surface area of the mesoporous catalyst support | carrier of the VOC sensor of this invention. It is a graph which shows the toluene concentration dependence of the VOC sensor of this invention.

Explanation of symbols

40 VOC sensor 41 Silicon wafer 42 Platinum (Pt) heater 43 Pd / m-SiO ₂ catalyst layer 44 Platinum (Pt) heater 45 m-SiO ₂ layer 46 Recess 47 Recess 48a Hafnium oxide (HfO ₂ ) film 48b Silicon nitride (SiN) ) Film 48c Silicon oxide (SiO ₂ ) film Ds Thin film diaphragm Dr Thin film diaphragm

Claims (4)

A sensitive element unit having a heater driven by a pulse of a predetermined cycle including an energization off period and an energization on period, and a catalyst layer made of a mesoporous material provided in thermal contact with the heater;
A sensor output corresponding to a VOC concentration is obtained based on a resistance value of the heater that becomes a peak value immediately after the start of the energization on period. The VOC sensor according to claim 1, wherein the mesoporous material is mesoporous silica. A first heater driven by a pulse having a predetermined period including an energization off period and an energization on period, and a platinum group such as palladium (Pd) provided in thermal contact with the first heater are supported. A sensitive element having a catalyst layer made of mesoporous silica;
A second heater driven by the pulse, and a compensation element portion having a layer made of mesoporous silica provided in thermal contact with the second heater,
A sensor output corresponding to a VOC concentration is obtained based on resistance values of the first and second heaters during the energization on period. The VOC sensor according to any one of claims 1 to 3,
Sensor drive control means for driving the VOC sensor with a drive pulse including an energization off period and an on period of a predetermined period;
Integral value calculating means for calculating an integral value over a predetermined time of the sensor output of the VOC sensor during the energization on period;
Storage means for previously storing integral value data over the predetermined time corresponding to the VOC;
A comparison means for comparing the integral value calculated by the integral value calculation means with the integral value data;
VOC detection apparatus comprising: output means for detecting the VOC based on a comparison result by the comparison means and outputting the result.

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