JP7126238B2 - Combustible gas analysis method - Google Patents

Description

The present invention relates to a gas sensor group and a combustible gas analysis method.

Various odor components exist in the living environment. Most of these odor components are very small amounts of organic molecules and are called volatile organic compounds (VOC). VOCs include compounds with various properties. Various low-concentration VOCs, equivalent to several tens to hundreds of ppb in total, exist indoors due to emissions from building materials, interior decoration, furniture, etc., and use of cosmetics, air fresheners, etc. by residents. For this reason, a certain concentration of water vapor and a low concentration of volatile impurities are usually present in a living environment in which a particular VOC is detected.

For example, in the medical field, it has been pointed out that VOCs contained in exhaled breath are related to bad breath, metabolism, and diseases, and that VOCs in exhaled breath are correlated with health conditions. In the field of foods, it is said that there is a correlation between VOCs, which correspond to odors emitted from foods, and freshness and quality. Therefore, monitoring specific VOCs is useful for health and food management. However, specific VOCs are detected indoors, and volatile impurities are present indoors. Therefore, it is necessary to detect specific VOCs without being affected by volatile impurities.

On the other hand, in order to detect combustible gases represented by VOC, a semiconductor gas sensor using a metal oxide n-type semiconductor has been conventionally used. Semiconductor sensors are inexpensive and have low running costs. A typical semiconductor-type gas sensor is a tin oxide sensor. Oxygen in the air is adsorbed on the surface of the tin oxide particles that make up the sensitive part, and carrier electrons are captured by the adsorbed oxygen. increases, increasing the electrical resistance of grain boundaries. Therefore, the combustible gas reaches the surface of the tin oxide particles and consumes the oxygen adsorbed on the surface, thereby reducing the potential barrier of the grain boundary and reducing the electrical resistance. Here, the tin oxide sensor is classified as a so-called grain boundary responsive semiconductor gas sensor.

However, it is difficult for the grain boundary responsive semiconductor gas sensor to exhibit gas selectivity due to its nature.

Under these circumstances, it is desired to develop a group of gas sensors capable of detecting specific combustible gases in a room containing volatile impurities.

Therefore, methods of performing statistical analysis such as principal component analysis of signals obtained from a gas sensor group having a plurality of gas sensors typified by semiconductor gas sensors have been studied.

Due to its nature, it is difficult to detect only a specific combustible gas with individual gas sensors. 3).

However, it is not possible to sufficiently separate and detect combustible gases in humidified air containing volatile impurities.

Patent Document 4 discloses a gas sensor group including a plurality of types of grain boundary responsive semiconductor gas sensors and one or more types of bulk responsive semiconductor gas sensors.

JP-A-10-170422 JP-A-2006-53059 Japanese Unexamined Patent Application Publication No. 2004-12193 JP 2017-150944 A

Generally, it is desired to miniaturize the gas sensor group. Here, in order to reduce the size of the gas sensor group, it is conceivable to dispose the plurality of gas sensors constituting the gas sensor group on the same substrate. preferable.

However, the bulk-response semiconductor gas sensor is generally driven at a higher temperature than the grain-boundary-response semiconductor gas sensor, making it difficult to reduce the size of the gas sensor group.

An object of one aspect of the present invention is to provide a gas sensor group that can be easily miniaturized and that can sufficiently separate and detect combustible gases in humidified air containing volatile impurities.

One aspect of the present invention has a plurality of types of n-type semiconductor semiconductor gas sensor elements and one or more types of p-type semiconductor type semiconductor gas sensor elements, and the n-type semiconductor type semiconductor gas sensor element includes: a gas sensor in which at least one of the p-type semiconductor type semiconductor gas sensor elements and at least one of the p-type semiconductor type semiconductor gas sensor elements are arranged on the same substrate; a heater pattern is formed on the lower surface of the substrate; The heater pattern has a heat generating portion that generates heat in a part thereof, and the semiconductor gas sensor element having the highest optimum temperature among the n-type semiconductor gas sensor element and the p-type semiconductor semiconductor gas sensor element. is arranged directly above the heat-generating portion of the substrate, and the semiconductor gas sensor element having a low optimum temperature is arranged away from the heat-generating portion of the substrate, and is used for detecting combustible gas. , wherein the plurality of n-type semiconductor semiconductor gas sensor elements and the one or more p-type semiconductor semiconductor gas sensor elements contain at least one combustible gas sensor element. a step of detecting a gas and obtaining a response value; , wherein the eigenvector of the n-type semiconductor type semiconductor gas sensor element obtained by principal component analysis of the response value indicates a direction parallel to the axis of the first principal component, and the p The eigenvector of the semiconducting gas sensor element of the semiconducting type indicates a direction parallel to the axis of the second principal component .

According to one aspect of the present invention, it is possible to provide a gas sensor group that can be easily miniaturized and that can sufficiently separate and detect combustible gases in humidified air containing volatile impurities.

FIG. 4 is a top view showing a gas sensor substrate used in Examples. It is a bottom view showing a substrate for a gas sensor used in Examples. It is a gas flow diagram of the measurement system used in the examples. FIG. 10 is a diagram showing principal component scores and eigenvectors of the first principal component and the second principal component in Example 1; FIG. 10 is a diagram showing principal component scores and eigenvectors of the first principal component and the second principal component in Comparative Example 1;

Next, the form for implementing this invention is demonstrated.

[Gas sensor group]
The gas sensor group of the present embodiment is used for detecting combustible gas, and has a plurality of types of n-type semiconductor type semiconductor gas sensor elements and one or more types of p-type semiconductor type semiconductor gas sensor elements. The gas sensor group of the present embodiment uses grain boundary responsive semiconductor gas sensor elements with different detection principles, thereby improving the accuracy of principal component analysis, which will be described later. In addition, in the gas sensor group of the present embodiment, the n-type semiconductor type semiconductor gas sensor element and the p-type semiconductor type semiconductor gas sensor element have a small difference in driving temperature, and can be arranged on the same substrate.

In the specification and claims of this application, combustible gas is defined as a substance that can exist as a gas and that can be detected by a semiconductor gas sensor element. That is, the combustible gas is a substance that can be oxidized on the surface of the metal oxide (particles) constituting the semiconductor type gas sensor element, which will be described later.

Combustible gases include organic compounds such as VOCs, and inorganic compounds such as hydrogen sulfide, hydrogen, and carbon monoxide.

VOCs include aliphatic hydrocarbons, aromatic hydrocarbons, esters, alcohols, aldehydes, ketones, terpenes, halogenated hydrocarbons and the like. Among them, nonanal, decane, acetone, acetoin, and other components contained in patient's breath are preferred. It has been reported that acetoin, nonanal, and decane are correlated with lung cancer patient's breath, and acetone with diabetes patient's breath. Examples reported are: acetoin (G. Songa, et al., Lung Cancer 67 (2010) 227-231), nonanal (P. Fuchs et al., Int. J. Cancer 126 (2010) 2663-2670. ), Decane (D. Poli et al., Respiratory Research 6 (2005) 71-80.), Acetone (C. Wang, et al., IEEE Sens. J. 10 (2010) 54-63.)
In the present embodiment, the semiconductor gas sensor element of the n-type semiconductor type has a sensing portion made of metal oxide particles of the n-type semiconductor. Oxygen in the air is adsorbed on the surface of the metal oxide particles to supplement carrier electrons, thereby reducing the concentration of free electrons at the grain boundaries of the metal oxide particles and increasing the electrical resistance. When the combustible gas reaches the surface of the metal oxide particles and oxidizes them, the adsorbed oxygen on the surfaces is consumed in the process, so that the concentration of free electrons at the grain boundaries increases and the electrical resistance decreases. The amount of decrease in electrical resistance depends on the type and concentration of the combustible gas that reaches it. Therefore, the surface of the metal oxide particles adsorbs not only oxygen but also water molecules. When the atmospheric humidity changes, the amount of water molecules adsorbed on the surface of the metal oxide particles changes, which affects the amount of oxygen molecules adsorbed on the surface of the metal oxide particles. As a result, the n-type semiconductor type semiconductor gas sensor element is susceptible to changes in humidity.

In the present embodiment, the p-type semiconductor type semiconductor gas sensor element has a sensing portion made of p-type semiconductor metal oxide particles. Oxygen in the air is adsorbed on the surface of the metal oxide particles to supplement carrier electrons, thereby increasing the hole concentration at the grain boundaries of the metal oxide particles and reducing the electrical resistance. When the combustible gas reaches the surface of the metal oxide particles and oxidizes them, the adsorbed oxygen on the surfaces is consumed in the process, so that the hole concentration at the grain boundaries decreases and the electrical resistance increases. The amount of decrease in electrical resistance depends on the type and concentration of the combustible gas that reaches it. The effect of the exchange of oxygen on the surface of the metal oxide particles on the electrical resistance is similar to that of the n-type semiconductor type semiconductor gas sensor element. Molecules are more likely to be adsorbed, so the p-type semiconductor type semiconductor gas sensor element is less susceptible to changes in humidity.

The p-type semiconducting metal oxide is preferably lanthanum iron oxide (LaFeO ₃ ). As a result, the difference between the drive temperature of the p-type semiconductor type semiconductor gas sensor element and the drive temperature of the n-type semiconductor type semiconductor gas sensor element can be particularly reduced.

It should be noted that the response characteristics to the combustible gas can be controlled by supporting the noble metal catalyst particles on the p-type semiconductor metal oxide particles.

Examples of noble metals include, but are not limited to, platinum, palladium, gold, silver, rhodium, ruthenium, and the like, and two or more of them may be used in combination. Among these, combined use of platinum, gold and palladium is preferred.

In the gas sensor group of the present embodiment, at least one of the n-type semiconductor type semiconductor gas sensor elements and at least one of the p-type semiconductor type semiconductor gas sensor elements are arranged on the same substrate. Includes gas sensor.

At this time, the gas sensor preferably has a heater capable of directly heating the substrate. As a result, the semiconductor gas sensor elements arranged on the same substrate can be driven.

[Method for analyzing combustible gas]
The combustible gas analysis method of the present embodiment is a method of analyzing combustible gas using the gas sensor group of the present embodiment. A p-type semiconductor type semiconductor gas sensor element detects one or more combustible gases and obtains a response value in response.

In the combustible gas analysis method of the present embodiment, principal component analysis is performed on data consisting of response values of a plurality of types of n-type semiconductor type semiconductor gas sensor elements and one or more types of p-type semiconductor type semiconductor gas sensor elements. It is preferable to further include a step. As a result, one or more combustible gases in the humidified air can be sufficiently separated and detected.

The combustible gas analysis method of the present embodiment can be applied to, for example, a health management system in the medical field and a food management system in the food field by detecting breath.

Examples of the present invention are described below, but the present invention is not limited to the following examples.

[Example 1]
In Example 1, six types of n-type semiconductor type semiconductor gas sensor elements (two types of SnO ₂ +M (SnO ₂ +M(1), SnO ₂ +M(2)), ZnO, ZnO+M, FeZrO, FeZrO+M), A total of eight types of gas sensor elements including two types of p-type semiconductor type semiconductor gas sensor elements (LaFeO ₃ and LaFeO ₃ +M) were used. Specifically, a compact gas sensor in which four different semiconductor gas sensor elements are mounted on two gas sensor substrates (see FIGS. 1 and 2) having four comb-shaped electrodes on the upper surface and a heater pattern on the lower surface. A gas sensor group consisting of two pieces was produced.

The eight types of gas sensor elements used are described below.

SnO ₂ +M(1) and SnO ₂ +M(2) are gas sensor elements having SnO ₂ as a sensitive part to which 1 mass % each of Pt, Pd and Au are added, respectively, manufactured by the National Institute of Advanced Industrial Science and Technology. (See Sens. Actuators B 187 (2013) 135-141). Here, Pt, Pd and Au are added to SnO ₂ to improve it so that it exhibits a certain sensitivity to various VOCs. SnO ₂ +M(1) and SnO ₂ +M(2) differ in the thickness of the sensing part by adjusting the manufacturing method. Although the thickness of the sensitive part of SnO ₂ +M(1) and SnO ₂ +M(2) cannot be directly measured, the thickness of SnO ₂ +M(1) and SnO ₂ +M(2) produced by the same manufacturing method were 4.6 μm and 2.8 μm, respectively. Therefore, SnO ₂ +M(1) and SnO ₂ +M(2) have different response characteristics. SnO ₂ +M(1) and SnO ₂ +M(2) were prepared according to the method described in the above literature.

ZnO and ZnO+M are a gas sensor element with ZnO as a sensitive part and a gas sensor element with ZnO as a sensitive part to which 1 mass % each of Pt, Pd and Au are added, respectively, which were produced by the National Institute of Advanced Industrial Science and Technology. Here, ZnO is obtained by adding an aqueous sodium hydroxide solution dropwise to an aqueous zinc nitrate solution to obtain a precipitate of zinc hydroxide, with reference to the method for producing cerium oxide particles (see Sensors 2015, 15, 9427-9437). It was prepared by uniformly mixing with carbon powder and firing. ZnO+M was prepared in the same manner as SnO ₂ +M.

FeZnO and FeZnO+M are gas sensor elements _each having a sensitive part made of a mixture of Fe ₂ O ₃ and ZnO _, manufactured by the National Institute of Advanced Industrial Science and Technology. and ZnO as a sensitive part. Here, FeZnO is prepared by dropping ammonia water into a mixed aqueous solution of iron nitrate and zirconyl nitrate to form a mixed hydroxide precipitate, referring to the method for producing cerium oxide particles (see Sensors 2015, 15, 9427-9437). After obtaining, carbon powder was uniformly mixed and fired to produce. FeZnO+M was prepared in the same manner as SnO ₂ +M.

LaFeO ₃ and LaFeO ₃ +M are respectively gas sensor elements having LaFeO ₃ as a sensing element and a gas sensor having LaFeO ₃ as a sensing element to which 1% by mass each of Pt, Pd and Au are added, both of which were manufactured by the National Institute of Advanced Industrial Science and Technology. element. Here, LaFeO ₃ is obtained by adding ammonia water dropwise to a mixed aqueous solution of iron nitrate and lanthanum nitrate, with reference to the method for producing cerium oxide particles (see Sensors 2015, 15, 9427-9437) to obtain a mixed hydroxide precipitate. After obtaining, carbon powder was uniformly mixed and baked. LaFeO ₃ +M was prepared in the same manner as SnO ₂ +M. LaFeO ₃ and LaFeO ₃ +M are classified as p-type semiconductor type semiconductor sensors because they exhibit a response in which electrical resistance increases in the presence of combustible gas.

The eight types of gas sensor elements described above were driven so that the maximum temperature of the substrate was about 300.degree. The heater pattern on the lower surface of the substrate shown in FIG. 2 is a thick film of platinum. In the drawing, the pattern on the left is thinner than the two thick wires on the right with a border of about 3 mm from the left. , the pattern on the left heats up because the current density increases. Note that the two thin wires on the right side are wires for controlling the voltage by the four-terminal method. Of the four comb-shaped electrodes on the upper surface of the substrate shown in FIG. 1, the two comb-shaped electrodes on the left are directly above the heat-generating locations, and the two comb-shaped electrodes on the right are located away from the heat-generating locations. . As a result, the driving temperature can be differentiated, and by arranging the gas sensor element with the higher optimum temperature on the left side and the gas sensor element with the lower optimum temperature on the right side, the gas sensor elements with different optimum temperatures can be arranged on the same substrate. be able to.

FIG. 3 shows a gas flow diagram of the measurement system used in this example.

The two gas sensors were placed in a sensor chamber 1 in which the gas atmosphere can be controlled and sealed.

Next, the gas that flows into the sensor chamber 1 will be specifically described.

First, the total flow rate of the nitrogen-oxygen mixed gas flowing into the sensor chamber 1 during measurement was always set to 500 mL/min. Next, the volume ratio of nitrogen and oxygen flowing into the sensor chamber 1 during measurement was always kept at 4:1. That is, the nitrogen flow rate is 400 mL/min and the oxygen flow rate is 100 mL/min. Of these, 200 mL/min of nitrogen was passed through the water bubbler 2 containing distilled water, and 100 mL/min of the total amount of oxygen was passed through the water bubbler 2 containing distilled water. Here, the water bubbler 2 was controlled so that the water temperature was always 20°C. That is, 200 mL/min of nitrogen and 100 mL/min of oxygen are saturated with water vapor and are mixed with 200 mL/min of dry nitrogen to reach the sensor chamber 1. The humidity of the oxygen mixed gas is equivalent to 60% RH at 20°C (hereinafter referred to as synthetic air).

The four target VOCs used in this example were acetoin, nonanal, decane, and acetone, and were adjusted so that the concentration when reaching the sensor chamber 1 was 1 ppm.

These target VOCs were produced by passing nitrogen gas through a gas from a nitrogen-based gas cylinder or a permeator (manufactured by Gastech) as a gas generator 3 that produces gas from a solvent. Specifically, acetone corresponds to the former, and acetoin, nonanal, and decane correspond to the latter. When flowing these target VOCs, the flow rate of dry nitrogen is reduced by the amount corresponding to the flow rate of the flowed gas, so that the total gas flow rate is 500 mL / min and the volume ratio of nitrogen and oxygen is always 4: 1. maintained.

The principal component analysis used the response value S of the gas sensor. The response value S of the semiconductor type sensor element of the n-type semiconductor type is expressed by the formula S=Ra/Rg
(Wherein, Ra is the electrical resistance of the gas sensor when synthetic air is flowed, and Rg is the electrical resistance of the gas sensor when synthetic air containing 1 ppm target VOC is flowed.)
defined by

The n-type semiconductor sensor element responds with a decrease in electrical resistance to the target VOC, whereas the p-type semiconductor sensor element exhibits an increase in electrical resistance to the target VOC. indicates the response to Therefore, the response value S of the semiconductor sensor element of the p-type semiconductor type is expressed by the formula S=Rg/Ra
defined by

Four types of target VOCs were each measured twice using eight types of semiconductor type gas sensor elements. A principal component analysis was performed using the 64 response values S obtained.

FIG. 4 shows principal component scores and eigenvectors of the first principal component and the second principal component. Each axis shows the contribution rate of each principal component.

From FIG. 4, it can be seen that the cumulative contribution rate up to the second principal component is 87.8%. This indicates that the value of 7.024 types (=8 types×0.878) out of the 8 types of semiconductor gas sensor elements is concentrated. As shown in FIG. 4, the principal component scores among acetoin, nonanal, decane, and acetone are separated, and it can be seen that they can be sufficiently separated and detected.

Note that when focusing on the eigenvector of each semiconductor gas sensor element indicated by the arrow in FIG. 4 (however, the arrows of ZnO and SnO ₂ +M(2) overlap), it can be classified as an n-type semiconductor type semiconductor gas sensor element. The six types of eigenvectors are generally parallel to the axis of the first principal component, or parallel to the direction slightly downward to the right. On the other hand, the two types of eigenvectors classified as semiconductor sensor elements of p-type semiconductor are substantially parallel to the axis of the second principal component in a direction substantially perpendicular to the eigenvector of the semiconductor gas sensor element of n-type semiconductor. showing direction. From this, it is considered that the resolution in the axial direction of the second principal component is generally provided by the p-type semiconductor type semiconductor sensor element, which contributes to the improvement of the separability of the principal component scores.

[Comparative Example 1]
In Comparative Example 1, six types of n-type semiconductor type semiconductor gas sensor elements (two types of SnO ₂ +M, ZnO, ZnO+M, FeZrO, and FeZrO+M) were used. Comparative Example 1 corresponds to Example 1 with the p-type semiconductor type semiconductor gas sensor element removed.

FIG. 5 shows principal component scores and eigenvectors of the first principal component and the second principal component. Each axis shows the contribution rate of each principal component.

It can be seen from FIG. 5 that the cumulative contribution rate up to the second principal component is 94.8%. This indicates that the values of 5.688 types (= 6 types x 0.948) of the 6 types of gas sensors are aggregated. In addition, it can be seen from FIG. 5 that the principal component score (▽) of acetone and the principal component score (□) of nonanal are close to each other as compared with Example 1 (see FIG. 4). Specifically, although the second principal components of the principal component scores of acetone and nonanal were divided into positive and negative in FIG. 4, they are both negative in FIG. 5, and the values are almost the same. This is thought to be due to the fact that the information of the p-type semiconductor type semiconductor gas sensor, which was mainly responsible for the resolution of the second principal component in the axial direction in Example 1, was lost.

REFERENCE SIGNS LIST 1 sensor chamber 2 water bubbler 3 gas generator 4 valve 5 flow meter with needle valve 6 three-way valve 7 crossover four-way valve

Claims (5)

having a plurality of types of n-type semiconductor type semiconductor gas sensor elements and at least one type of p-type semiconductor type semiconductor gas sensor elements,
a gas sensor in which at least one of the n-type semiconductor type semiconductor gas sensor elements and at least one of the p-type semiconductor type semiconductor gas sensor elements are arranged on the same substrate;
A heater pattern is formed on the lower surface of the substrate,
The heater pattern has a heat generating portion that generates heat in a part thereof,
Of the n-type semiconductor type semiconductor gas sensor element and the p-type semiconductor type semiconductor gas sensor element, the semiconductor type gas sensor element having a higher optimum temperature is arranged directly above the heat generating portion of the substrate, and is the semiconductor type gas sensor element having a low coefficient of
A method for analyzing combustible gas using a group of gas sensors used for detecting combustible gas,
a step of obtaining a response value in response to detection of one or more combustible gases by the plurality of n-type semiconductor semiconductor gas sensor elements and the one or more p-type semiconductor semiconductor gas sensor elements ;
a step of principal component analysis of the response values of the plurality of types of n-type semiconductor type semiconductor gas sensor elements and the one or more types of p-type semiconductor type semiconductor gas sensor elements ;
including
The eigenvector of the n-type semiconductor type semiconductor gas sensor element obtained by principal component analysis of the response value indicates a direction parallel to the axis of the first principal component, and the eigenvector of the p-type semiconductor type semiconductor gas sensor element is A combustible gas analysis method, wherein the eigenvector indicates a direction parallel to the axis of the second principal component . 2. The heater pattern according to claim 1, wherein the heater pattern has two thick wires arranged at a location away from the heat-generating portion, and a wire thinner than the thick wire, which constitutes the heat-generating portion. combustible gas analysis method . 3. The combustible gas analysis method according to claim 1, wherein the n-type semiconductor semiconductor gas sensor element contains n-type semiconductor metal oxide particles. 4. The combustible gas analysis method according to claim 1, wherein the p-type semiconductor type semiconductor gas sensor element contains p-type semiconductor metal oxide particles. 5. The method for analyzing combustible gas according to claim 4, wherein the p-type semiconductor metal oxide is lanthanum iron oxide.

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