U.S. patent application number 09/774472 was filed with the patent office on 2002-02-28 for gas sensors and the manufacturing method thereof.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Hattori, Akiyoshi, Inoue, Yoshikatsu, Yoshiike, Nobuyuki.
Application Number | 20020023480 09/774472 |
Document ID | / |
Family ID | 18549311 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020023480 |
Kind Code |
A1 |
Hattori, Akiyoshi ; et
al. |
February 28, 2002 |
Gas sensors and the manufacturing method thereof
Abstract
A gas sensor characterized in that the gas sensor has at least:
an insulating substrate; a pair of thin film electrodes which are
spaced apart at a given interval and provided on the insulating
substrate; a thin film gas sensitive layer which is provided on
both the substrate and the thin film electrodes, the gas sensitive
layer containing a given material as main ingredient; and a pair of
thick film electrodes which is correspondingly positioned over the
pair of thin film electrodes and provided on the thin film gas
sensitive layer, wherein the thin film electrodes and the thick
film electrodes are formed so as to sandwich portions of the thin
film gas sensitive layer between the two types of electrodes.
Inventors: |
Hattori, Akiyoshi;
(Yahata-shi, JP) ; Yoshiike, Nobuyuki; (Ikoma-shi,
JP) ; Inoue, Yoshikatsu; (Kusatsu-shi, JP) |
Correspondence
Address: |
RATNER AND PRESTIA
Suite 301
One Westlakes, Berwyn
P.O. Box 980
Valley Forge
PA
19482-0980
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
|
Family ID: |
18549311 |
Appl. No.: |
09/774472 |
Filed: |
July 18, 2001 |
Current U.S.
Class: |
73/31.05 ;
73/23.2 |
Current CPC
Class: |
G01N 33/02 20130101;
G01N 27/12 20130101; G01N 33/0047 20130101 |
Class at
Publication: |
73/31.05 ;
73/23.2 |
International
Class: |
G01N 009/00; G01N
007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2000 |
JP |
2000-023,104 |
Claims
What is claimed is:
1. A gas sensor characterized in that said gas sensor comprises at
least: an insulating substrate; a pair of thin film electrodes
which are spaced apart at a given interval and provided on said
insulating substrate; a thin film gas sensitive layer which is
provided on both said substrate and said thin film electrodes, said
gas sensitive layer containing a given material as main ingredient;
and a pair of thick film electrodes which is correspondingly
positioned over said pair of thin film electrodes and provided on
said thin film gas sensitive layer, wherein said thin film
electrodes and said thick film electrodes are formed so as to
sandwich portions of said thin film gas sensitive layer between
said two types of electrodes.
2. A gas sensor characterized in that said gas sensor comprises at
least: an insulating substrate; a pair of thin film electrodes
which are spaced apart at a given interval and provided on said
insulating substrate; a thin film gas sensitive layer which is
provided on both on said substrate and said thin film electrodes,
said sensitive layer containing a given material as main
ingredient; and a pair of thick film electrodes which is
correspondingly positioned over said pair of thin film electrodes
and provided in contact with said thin film electrodes.
3. The gas sensor according to claim 1 or 2 characterized in that
the interval between said pair of thick film electrodes is longer
than the interval between said pair of thin film electrodes.
4. The gas sensor according to claim 3 characterized in that said
given material comprises tin oxide as main ingredient and at least
one additional element selected from the group consisting of
palladium, iron, nickel, manganese, cobalt and zinc.
5. A method of manufacturing a gas sensor characterized in that a
process is provided wherein a thin film gas sensitive layer is
formed by coating an organic solution containing a metal tin salt,
an organic compound capable of coordinating to at least a tin and
an activator, and then by firing said solution.
6. The method of manufacturing a gas sensor according to claim 5
characterized in that said metal tin salt is at least one compound
selected from the group consisting of stannous chloride, tin
acetylacetonate complex and tin 2-ethylhexanoate.
7. The method of manufacturing a gas sensor according to claim 5 or
6 characterized in that said organic compound capable of
coordinating to a tin is at least one compound selected from the
group consisting of .beta.-diketones, etheralcohols, polyhydric
alcohols, and condensation products of the polyhydric alcohols.
8. A method of manufacturing a gas sensor characterized in that a
process is provided wherein a thin film gas sensitive layer is
formed by coating a paste comprising an organic solution containing
at least a metal tin soap, an activator and a viscosity controller,
and then by firing said paste.
9. The method of manufacturing a gas sensor according to claim 8
characterized in that said metal tin soap is tin 2-ethylhexanoate
and/or tin naphthenate.
10. The method of manufacturing a gas sensor according to claim 8
or 9 characterized in that said viscosity controller is polyvinyl
pyrrolidinone and/or ethyl cellulose.
11. The method of manufacturing a gas sensor according to claims 5
to 10 characterized in that said activator has at least one element
selected from the group consisting of palladium, iron, nickel,
manganese, cobalt and zinc, said activator being at least one
compound selected from the group consisting of metal chlorides,
metal acetylacetonate complexes and metal soaps.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to gas sensors and the
manufacturing method thereof to determine freshness and/or
putridity for vegetables or fruits by providing high sensitivity
and high selectivity to low levels of gases, such as ethylene,
ethanol, mercaptans, and amines, released from vegetables or
fruits.
[0003] 2. Related Art of the Invention
[0004] The freshness of foods or drinks is determined subjectively
through vision, taste and sense of throat, thereby making the
determination more or less vague. However, technical efforts are
actively being made to develop semiconductor gas sensors for
determining freshness. As shown in FIGS. 6 to 9, a semiconductor
gas sensor is generally composed of an insulating substrate 1, a
pair of electrodes 12, and a gas sensitive layer 13. Each of the
semiconductor gas sensors in FIGS. 6 and 7 is configured by placing
the pair of electrodes 12 on the insulating substrate 1 and forming
the gas sensitive layer 13 on the insulating substrate 1 and also
on the pair of electrodes 12. Each of the semiconductor gas sensors
in FIGS. 8 and 9 is configured by forming the gas sensitive layer
13 on the insulating substrate 1 and placing the pair of electrodes
12 on the gas sensitive layer 13. In FIGS. 6 to 9, the pair of
electrodes 12 indicates thick film electrodes.
[0005] A semiconductor gas sensor has recently been developed to
sense trimethylamine, a malodorous component emitted from raw fish,
for determining freshness for the fish. Oxide semiconductors based
on titanium dioxide are commonly used as a sensitive material for
the purpose described above, wherein addition of metal catalyst
components to the titanium dioxide improves the sensitivity of the
sensor. In this case, the sensitivity of the sensor depends on
action and dispersed state of the catalyst and the thickness of the
sensitive film, and the type of catalyst components and the amount
of their addition play an important role in improving catalytic
action described above. In addition to use of titanium dioxide as a
sensitive material as mentioned before, indium oxide supplemented
with magnesium is under study as sensitive material of a gas sensor
for trimethylamine, wherein atomic control by addition of 5 mol% of
magnesium oxide to indium oxide reduces electron density, thereby
increasing resistance of the sensor in air to make it more
sensitive. However, the study of sensitivity of this type of gas
sensor for trimethylamine is still at an early stage to apply, and
what is worse, too power-consuming to make its mass production
feasible.
[0006] For vegetables, which emit sulfides gas (mercaptans) unlike
trimethylamine coming out from raw fish, a sensor with an
excellent-sensitivity to determine freshness for vegetables has
already been developed. Japanese Patent No. 2875174 describes a
method of manufacturing a sensor to determine freshness for
vegetables, comprising the steps of adding a given amount of
palladium powders to tin oxide powders, mixing them, and then
crushing them; calcining the crushed powder mix of tin oxide and
palladium at a given temperature for a given time, and then mixing
it with an organic material to make paste; coating the paste onto
the electrode surface on the substrate to form a sensitive film;
and drying the coating and then sintering it at a given temperature
for a given time, thereafter connecting a lead wire to the
electrode surface.
[0007] It has become clarified that a trace amount of ethylene,
ethanol or aldehydes is emitted even from fresh vegetable or fruit,
while mercaptans are emitted as vegetable or fruit begins to rot
and amines, such as ammonia, are emitted as fruit begins to rot. It
has also become clarified that sensing a gas, such as ethylene,
ethanol or aldehydes is effective for determining freshness for
vegetable or fruit, while sensing mercaptans or amines, such as
ammonia, is suitable for determining putridity for vegetable or
fruit. However, for the conventional type of sensors to determine
freshness for vegetables described above, the sensitive film of
which is formed by mixing tin oxide powders with a given amount of
palladium powders, crushing and calcining them, and then mixing
them with an organic material, resulting in production of paste,
which is subsequently coated onto the electrode surface on the
substrate, dried and sintered, it is difficult to detect one ppm
level of ethylene, ethanol or aldehydes, which is necessary to
determine freshness for vegetables, and also is difficult to detect
one ppm level of mercaptans or amines which is necessary to
determine the putridity for vegetables and fruits.
[0008] To describe it more specifically, in conventional examples
illustrated in FIGS. 6 and 7, the gas sensitive layer 13 jammed
between the thick electrodes 12, 12 is obviously formed of a
correspondingly thick film due to the thickness of the electrodes
in order to achieve a good electric joint. As a result, the problem
that the sensor has a low sensitivity arises.
[0009] Further, in conventional examples illustrated in FIGS. 8 and
9, since the gas sensitive layer 13 is formed first on the
substrate 1, followed by formation of the electrodes 12 over the
gas sensitive layer 13, another problem arises that formation of
the electrodes 12 may cause contamination of the gas sensitive
layer 13 with some impurity.
SUMMARY OF THE INVENTION
[0010] The present invention is to eliminate the problems described
above, and to provide gas sensors highly sensitive to such gas as
ethylene, ethanol, aldehydes, mercaptans or amines, and the method
of manufacturing them with a good reproducibility.
[0011] The 1st invention of the present invention (corresponding to
claim 1) is a gas sensor characterized in that said gas sensor
comprises at least:
[0012] an insulating substrate;
[0013] a pair of thin film electrodes which are spaced apart at a
given interval and provided on said insulating substrate;
[0014] a thin film gas sensitive layer which is provided on both
said substrate and said thin film electrodes, said gas sensitive
layer containing a given material as main ingredient; and
[0015] a pair of thick film electrodes which is correspondingly
positioned over said pair of thin film electrodes and provided on
said thin film gas sensitive layer,
[0016] wherein said thin film electrodes and said thick film
electrodes are formed so as to sandwich portions of said thin film
gas sensitive layer between said two types of electrodes.
[0017] The 2nd invention of the present invention (corresponding to
claim 2) is a gas sensor characterized in that said gas sensor
comprises at least:
[0018] an insulating substrate;
[0019] a pair of thin film electrodes which are spaced apart at a
given interval and provided on said insulating substrate;
[0020] a thin film gas sensitive layer which is provided on both on
said substrate and said thin film electrodes, said sensitive layer
containing a given material as main ingredient; and
[0021] a pair of thick film electrodes which is correspondingly
positioned over said pair of thin film electrodes and provided in
contact with said thin film electrodes.
[0022] As mentioned before, the gas sensor according to the present
invention has a thinner film gas sensitive layer between the thin
film electrodes and thus a better electric joint between the thin
film gas sensitive layer and the thin film electrodes, compared
with the conventional thick film type of gas sensor. This results
in higher sensitivity, better stability and longer life of the gas
sensor.
[0023] Problems associated with the conventional methods that is,
adverse effects of the electrodes on the gas sensitive layer 3 in
the manufacturing process, such as contamination, cannot arise in
the method according to the present invention. It is because the
thin film electrodes 2 have been formed before the thin film gas
sensitive layer 3 is formed, raising no problems with the thin film
electrodes 2. Although the thick film electrodes 4 are formed after
the thin film gas sensitive layer 3, a portion of the gas sensitive
layer 3 which is directly related to gas detection, namely, the
portion P of the gas sensitive layer 3 placed between a pair of
thin film electrodes 2 is considerably distant from the thick film
electrodes 4, so that it is hardly affected adversely by formation
of the thick film electrodes 4.
[0024] Even when a pair of thick film electrodes, corresponding to
the pair of thin film electrodes, is provided on the thin film gas
sensitive layer, as in examples shown in FIGS. 1 and 2, the thin
film electrodes and thick film electrodes are configured so as to
sandwich the thin film gas sensitive layer. Since electric current
flows toward thinner electrodes, there exists virtually direct
electric connections between the thin film electrodes and the thick
film electrodes, indicating a good electric joint between the thin
film electrodes and the thick film electrodes.
[0025] The method of manufacturing a gas sensor according to the
present invention is characterized by providing a process of
coating and then firing an organic solution containing a metal tin
salt, an organic compound capable of coordinating to at least a
tin, and an activator in order to form a thin film gas sensitive
layer. Another method of manufacturing a gas sensor according to
the present invention is characterized by providing a process of
coating and then firing a paste consisting of a metal tin soap, an
activator, and an organic solution containing a viscosity
controller in order to form a thin film gas sensitive layer.
[0026] In the invention, an organic solution containing a metal tin
salt, an organic compound capable of coordinating to at least a
tin, and an activator is employed in order to form a thin film gas
sensitive layer. Generally, a metal tin salt tends to be
hygroscopic and/or hydrolyzable, and so it makes it difficult to
produce gas sensitive layers of the same thickness or composition
reproducibly. Therefore, an organic compound capable of
coordinating to the tin is added to form coordination compounds by
partial substitution, thereby achieving stabilization of the metal
tin salt. In addition, a paste consisting of a metal tin soap, at
least an activator, and an organic solution containing a viscosity
controller can also be employed. A metal tin soap forms a micell
with some organic solvents, resulting in increased viscosity of the
solution so that it may be pasty. A viscosity controller helps to
adjust the solution viscosity after it is dissolved in the organic
solvent. An activator described above is one of metal salts other
than tin salts. Addition of such a metal salt elevates the
sensitivity and/or selectivity of the gas sensitive layer to a gas
to be detected. A composition for producing a gas sensitive layer
comprising an organic solution which contains a metal tin salt, a
metal palladium salt, an organic compound capable of coordinating
to a tin, and an activator, or a paste which consists of a metal
tin soap, at least an activator, and an organic solution containing
a viscosity controller is coated and then fired, resulting in
reproducible production of gas sensitive layers where the activator
is dispersed uniformly. As a result, more highly sensitive gas
sensors, capable of detecting down to 1 ppm of different gases
(ethylene, ethanol, aldehydes, mercaptans, and amines), can be
manufactured, compared to the gas sensor to determine freshness for
vegetables, produced in the process comprising the steps of adding
a given amount of palladium powders to tin oxide powders, mixing
them, and then crushing them; calcining the crushed powder mix of
tin oxide and palladium at a given temperature for a given time,
and then mixing it with an organic material to make paste; coating
the paste onto the electrode surface on the substrate to form a
sensitive film; and drying the coating and then sintering it.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] [FIG. 1]
[0028] FIG. 1 is a schematic cross-sectional view, showing one
example of gas sensors according to the invention.
[0029] [FIG. 2]
[0030] FIG. 2 is a schematic cross-sectional view, showing another
example of gas sensors according to the invention.
[0031] [FIG. 3]
[0032] FIG. 3 is a schematic cross-sectional view, showing a
further example of gas sensors according to the invention.
[0033] [FIG. 4]
[0034] FIG. 4 is a graph showing a relation of the added amount of
palladium to the sensitivity of the sensor in the example according
to the invention.
[0035] [FIG. 5]
[0036] FIG. 5 is a graph showing effects of different metal
additives on the sensitivity of the sensor in the example according
to the invention.
[0037] [FIG. 6]
[0038] FIG. 6 is a schematic cross-sectional view of a conventional
semiconductor gas sensor.
[0039] [FIG. 7]
[0040] FIG. 7 is a schematic cross-sectional view of a conventional
semiconductor gas sensor.
[0041] [FIG. 8]
[0042] FIG. 8 is a schematic cross-sectional view of a conventional
semiconductor gas sensor.
[0043] [FIG. 9]
[0044] FIG. 9 is a schematic cross-sectional view of a conventional
semiconductor gas sensor.
DESCRIPTION OF SYMBOLS
[0045] 1 Substrate
[0046] 2 Thin film electrodes
[0047] 3 Thin film gas sensitive layer
[0048] 4 Thick film electrodes
[0049] 12 Electrodes
[0050] 13 Gas sensitive layer
PREFERRED EMBODIMENTS OF THE INVENTION
[0051] The embodiments of the invention will be described now.
[0052] FIGS. 1 and 2 are schematic cross-sectional views of typical
gas sensors according to the invention, respectively. In FIGS. 1
and 2, reference numeral 1 denotes an insulating substrate, such as
alumina, mullite or the like, reference numeral 2 denotes a thin
film electrodes made of a metal, such gold, silver or platinum,
reference numeral 3 denotes a thin film gas sensitive layer
consisting of metal oxides with a major component being tin oxide,
and reference numeral 4 denotes a thick film electrodes made of a
metal, such as gold, silver or platinum.
[0053] As the substrate 1, any material with an insulating surface
with heating function may be used. It is not limited with respect
to material or configuration. However, the substrate preferably a
surface roughness between 0.01-1 .mu.m in depth.
[0054] The thin film electrodes 2 and the thick film electrodes 4
serve to apply a voltage to the gas sensitive layer 3 to measure
its resistance. They are not limited for material, configuration,
pattern or manufacturing process. However, the thin film electrodes
2 are preferably 0.1-1 .mu.m thick, while the thick film electrodes
4 are preferably 3-20 .mu.m thick. As illustrated in FIG. 2, the
thin film electrodes 2 may be in direct and partial contact with
the thick film electrodes 4.
[0055] The thin film gas sensitive layer 3 is formed as below.
[0056] A composition for forming a gas sensitive layer is formed on
the substrate, and then it is fired at a temperature of several
hundred .degree. C. or more to form a thin film gas sensitive
layer. The composition for forming a gas sensitive layer may be
coated on the substrate by one of various methods, such as screen
printing, roll coating, dip coating and spin coating, and
preferably by dip coating or spin coating. The firing temperature
is established in a range above the decomposition temperature of a
composition for forming a gas sensitive layer and below the
deformation temperature of the substrate, and preferably in the
range of 400-800.degree. C. A composition for forming a gas
sensitive layer is prepared as below.
[0057] First, a metal tin salt is mixed with an organic compound
which is capable of coordinating to a tin. A metal tin salt usable
herein should have ligands to be replaced by an organic compound
which is capable of coordinating to a tin. Examples are stannous
chloride, tin acetylacetonate complex and tin 2-ethyl
hexanoate.
[0058] Preferably, a metal palladium salt is readily decomposed by
heat. Examples are palladium chloride and palladium acetylacetonate
complex.
[0059] An organic compound which is capable of coordinating to a
tin is necessary to stabilize a metal tin salt and to dissolve it
in an organic solvent, through partial coordination to the tin.
Examples include .beta.-diketones, such as acetylacetone,
etheralcohols, such as methoxyethanol, polyhydric alcohols, such as
ethylene glycol, and condensation products of the polyhydric
alcohols, such as diethylene glycol.
[0060] Subsequently, an organic solvent and an activator are added
to the solution mentioned above, and the resulting organic solution
undergoes heat treatment. Herein, an activator is a metal salt
additive used to elevate sensitivity and gas selectivity for the
gas sensitive layer. Examples include alkaline earth metal salts,
such as metal magnesium salts, metal calcium salts, metal strontium
salts or metal barium salts; transition metal salts, such as metal
titanium salts, metal zirconium salts, metal vanadium salts, metal
chromium salts, metal manganese salts, metal iron salts, metal
cobalt salts, metal nickel salts or metal copper salts or; metal
zinc salts; metal lead salts; metal cadmium salts; metal antimony
salts; metal bismuth salts; and metal palladium salts, and
preferably metal manganese salts, metal iron salts, metal cobalt
salts, metal nickel salts, metal zinc salts and metal palladium
salts. An activator compound should be relatively stable at room
temperature by itself, but decomposed readily by heat treatment,
whether it may be inorganic or organic. For example, inorganic
salts include nitrates, sulfates and chlorides, whereas organic
salts include carboxylates, dicarboxylates and acetylacetonate
complexes. The organic solvent described above may be any solvent
that can dissolve organic and inorganic compounds used according to
the invention. Examples are alcohols, such as ethanol and
isopropanol ketones, such as acetone and diethyl ketone,
tetrahydrofurane and the like. Further, if the activator mentioned
above is hardly soluble at room temperature, the metal tin salt,
the metal palladium salt, the organic compound capable of
coordination to a tin and the organic solution containing the
activator may be mixed and heated at the reflux temperature of the
organic solution or just below the temperature.
[0061] The present invention will be described in more detail by
the examples below.
[EXAMPLE 1]
[0062] A paste of the organic metal compound of gold was coated
onto an alumina substrate 0.4 mm thick using screen printing and
then dried, and subsequently it was fired at 800.degree. C. to form
thin film electrodes 0.3 .mu.m thick as the first layer.
[0063] In a 11 Erlenmeyer flask, to 8 g of stannous chloride
(Chemical formula 1) 16 g of methoxyethanol was added and the
mixture was mixed and made to be solved at a room temperature. To
the solution palladium chloride (Chemical formula 2) in an amount
such that the value of the expression 1 is 5 mol% and 130 g of
aceton, and the mixture was agitated and mixed to obtain the
desired composition for forming a gas sensitive layer.
SnCl.sub.2.2H.sub.2O [Chemical formula 1]
Pd/(Sn+Pd).times.100 [Expression 1]
PdCl.sub.2.2H.sub.2O [Chemical formula 2]
[0064] The composition for forming a gas sensitive layer was
applied on the alumina substrate 0.4 mm thick by dip coating, and
then fired at 600.degree. C. for 1 h to form the thin film gas
sensitive layer which consisted of metal oxides containing tin
oxide as main ingredient and had a thickness of 120 nm.
[0065] Gold paste for thick film printing was applied on the thin
film gas sensitive layer 3 by screen printing and dried, and then
fired at 600.degree. C. to form thick film electrodes 8 .mu.m thick
as the second layer.
[0066] The sensor element thus produced was submitted for
measurement to determine characteristics of response to ethylene
gas. The sensor element was fixed in the quartz tube, heated at
400.degree. C. by a heater, and then exposed to a flow of either
air or air containing 1 ppm of ethylene alternately to measure a
change in the resistance of the sensor element. Provided that the
resistance of the sensor element when it is in a flow of air is
denoted by RA, and its resistance 10 minutes after air is replaced
by the ethylene-containing air is denoted by RG, RG/RA refers to
the sensitivity of the sensor. The sensitivity thus obtained was
found 0.70.
[0067] FIG. 3 is a schematic cross-sectional view, showing a
further example of gas sensors according to the invention. In FIG.
3, reference numeral 1 denotes an insulating substrate, such as
alumina, mullite or the like, reference numeral 2 denotes thin film
electrodes made of a metal, such as gold, silver or platinum,
reference numeral 3 denotes a thin film gas sensitive layer
consisting of metal oxides with a major component being tin oxide,
and reference numeral 4 denotes thick film electrodes made of a
metal, such as gold, silver or platinum. The gas sensor here is
different from the one in FIGS. 1 and 2 in that the thick film
electrodes 4 are not formed on the thin film gas sensitive layer
3.
[0068] As the substrate 1, any material with an insulating surface
and with heating function maybe used. It is not limited with
respect to material or configuration. However, the substrate has
preferably a surface roughness between 0.01-1 .mu.m in depth.
[0069] The thin film electrodes 2 and the thick film electrodes 4
mainly serve to apply a voltage to the gas sensitive layer 3 to
measure its resistance. They are not limited with respect to
material, configuration, pattern or manufacturing process. However,
the thin film electrodes 2 are preferably 0.1-1 .mu.m thick, while
the thick film electrodes 4 are preferably 3-20 .mu.m thick.
[0070] The thin film gas sensitive layer 3 is formed as below.
[0071] A composition for forming a gas sensitive layer is formed on
the substrate, and then it is fired at a temperature of several
hundred .degree. C. or more to form the thin film gas sensitive
layer. The composition for forming a gas sensitive layer may be
coated on the substrate by one of various methods, such as screen
printing, roll coating, dip coating and spin coating, and
preferably by screen printing. The firing temperature is
established in a range above the decomposition temperature of the
composition for forming a gas sensitive layer and below the
deformation temperature of the substrate, and preferably in the
range of 400-800.degree. C. The composition for forming a gas
sensitive layer is prepared as below.
[0072] First, an organic solvent is added to an activator to make a
solution.
[0073] Herein, an activator is a metal salt additive used to
elevate sensitivity and gas selectivity for the gas sensitive
layer. Examples include alkaline earth metal salts, such as metal
magnesium salts, metal calcium salts, metal strontium salts or
metal barium salts; transition metal salts, such as metal titanium
salts, metal zirconium salts, metal vanadium salts, metal chromium
salts, metal manganese salts, metal iron salts, metal cobalt salts,
metal nickel salts or metal copper salts; metal zinc salts; metal
lead salts; metal cadmium salts; metal antimony salts; metal
bismuth salts; and metal palladium salts. An activator compound
should be relatively stable at room temperature by itself, but
decomposed readily by heat treatment, whether it may be inorganic
or organic. For example, inorganic salts include nitrates, sulfates
and chlorides, whereas organic salts include carboxylates,
dicarboxylates and acetylacetonate complexes. The organic solvent
described above should be able to dissolve both a metal tin soap
and a viscosity controller, and is exemplified by etheralcohols,
such as methoxyethanol and butylcarbitol, .beta.-diketones, such as
acetylacetone, esters, such as butylcarbitol acetate, and terpenoid
solvents, such as .alpha.-terpineol.
[0074] Secondly, a viscosity controller is added to the organic
solution and they are mixed.
[0075] The viscosity controller is any polymer to increase the
viscosity of the organic solution, namely, with a thickening effect
and is exemplified by polyvinyl pyrrolidinone and ethyl
cellulose.
[0076] Finally, metal tin soap is added to the organic solution
described above and they are mixed. Meanwhile, the viscosity
controller may be added to the organic solution after the viscosity
controller is mixed to the organic solution melted by heat.
[0077] Examples of the metal tin soap include tin 2-ethyl hexanoate
and tin naphthenate.
[0078] The invention will be described below by more detailed
examples, but it is not limited by the examples.
[EXAMPLE 2]
[0079] The paste of an organometallic gold compound was coated onto
an alumina substrate 0.4 mm thick by screen printing and then
dried, and subsequently fired at 800.degree. C. to form thin film
electrodes 2 of 0.3 .mu.m thickness as the first layer.
[0080] Gold paste for thick film printing was applied on the thin
film electrodes 2 of the first layer by screen printing and dried,
and then fired at 800.degree. C. to form thick film electrodes 6
.mu.m thick as the second layer.
[0081] In a 100 ml beaker, palladium chloride (having the chemical
formula 1, SnCl.sub.2.2H.sub.2O) as an activator was weighed so
that the value of the expression Pd/(Sn+Pd).times.100 may be 0-5
mol%, and then 4 g of butylcarbitol and 2 g of butylcarbitol
acetate were added. They were mixed for some time. Then, 2 g of
polyvinyl pyrrolidinone as viscosity controller was added and 6 g
of tin 2-ethyl hexanoate (having the formula 3)
(Sn(OOCCH(CH.sub.2CH.sub.3) (CH.sub.2)3CH.sub.3).sub.2) was also
added. The whole matter was stirred and mixed to obtain the desired
composition for forming a gas sensitive layer.
Sn(OOCCH(CH.sub.2CH.sub.3) (CH.sub.2)3CH.sub.3).sub.2 [Chemical
formula 3]
[0082] The composition for forming the gas sensitive layer was
applied on the alumina substrate 0.4 mm thick by screen printing
and then fired at 700.degree. C. for 1 h to form the thin film gas
sensitive layer, which was 200 nm thick and consisted of metal
oxides containing tin oxide as main ingredient.
[0083] Then, the sensor's sensitivity was measured for 1 ppm
ethylene similarly as in Example 1. The temperature for measurement
was 340.degree. C. The results are shown in FIG. 4. In addition,
the sensor's sensitivities for 1 ppm each of ethanol, acetaldehyde,
methylmercaptan and ammonia were found to be 0.20, 0.30, 0.60 and
0.70, respectively.
[EXAMPLE 3]
[0084] In a 100 ml beaker, metal 2-ethyl hexanoate (M.dbd.Mn, Fe,
Ni, Co, Zn) as an activator was weighed so that the value of the
expression 2 could be 1 mol%, and then a solution of 1 g of ethyl
cellulose in 12 g of butylcarbitol were added. They were mixed for
sometime. Finally, 6 g of tin 2-ethyl hexanoate
Sn(OOCCH(CH.sub.2CH.sub.3) (CH.sub.2)3CH.sub.3).sub- .2 was added.
The whole matter was stirred and mixed to obtain a desired
composition for forming a gas sensitive layer.
[0085] The composition for forming the gas sensitive layer was
applied on the alumina substrate 0.4 mm thick by screen printing
and then fired at 700.degree. C. for 1 h to form the thin film gas
sensitive layer, which was 240 nm thick and consisted of metal
oxides containing tin oxide as main ingredient.
[0086] Then, the sensor's sensitivity was measured for 1 ppm each
of dimethyl sulfide and ammonia similarly as in Example 1. The
temperature for measurement was 340.degree. C. The results are
shown in FIG. 5.
[0087] The present invention provides gas sensors having high
sensitivities to a low level of detectable gases (ethylene,
ethanol, aldehydes, mercaptans and amines) released from vegetables
and suitable for sensing freshness or putridity for vegetables.
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