Note: Descriptions are shown in the official language in which they were submitted.
~2;~736
GAS DETECTING ELEMENTS AND
PROCESS FOR PRODUCING Lowe SAME
BACKGROUND OF THE INVENTION
This invention relates to cJas detecting elements
and to processes for producing the same and more
particularly to a gas detecting element capable of
detecting a reducing gas in air with high sensitivity
and selectivity and a process for producing the same.
Heretofore, known elements for detecting a reduce
in gas in air are those gas detecting elements where-
in a sistered body of a metal oxide semiconductor
exhibiting N-type semiconductor characteristics such
as Snow, Zoo, or Foe is used. In such elements,
when these metal oxide semiconductors come into con
tact with a reducing gas, their electric conductivity
is increased. A gas is detected by measuring the
change of electric resistance value.
In recent years, compact and multi functional
elements have been sought. Studies have been carried
out on the use of thin film-type elements in place of
the sistered body-type gas detecting elements describe
Ed above. Such a thin film-type element has a thin
film structure obtained by depositing a metal oxide
semiconductor having gas sensitivity as described
above by a thin film-forming process such as sputter-
in, vapor deposition, or CUD.
'I
I
In both sistered body-type yes detecting elements
and thin ~ilm-type gas detecting elements, the use of
a metal oxide semiconductor alone generally provides
a gas detecting element having low sensitivity and
insufficient selectivity. For this reason, ordinarily
noble metals such as platinum (Pi) and palladium (Pod)
are being used as catalysts with the aim of increasing
the sensitivity of the element. That is, Pi or Pod is
directly added to the metal oxide semiconductor.
Alternatively, a catalyst layer supporting Pi or Pod is
formed on the metal oxide semiconductor.
When such measures are carried out, the sensitivity
is improved as compared with that in the case wherein
no catalyst is used. However, the gas detecting element
still does not exhibit sufficient sensitivity to detect
a reducing gas having a low concentration. Further,
when various reducing gases are present, highly sense-
live and selective detection of a particular reducing
gas is extremely difficult because erroneous function-
in of the element is induced by the influence of other reducing gases. Particularly in the case of gases which
adversely affect a human body even at a low concentra-
lion, such as COY it has been extremely difficult to
eliminate erroneous functioning due to the other reduce
in gases in order to detect them. Furthermore, when the gas detecting element is to be used in a general
home, the elimination of erroneous functioning of the
36
gas detecting element due to miscellaneous gases,
particularly alcohol vapor is an important problem.
While an element having a stunk oxide thin
film (Japanese Patent Laid Open Publn. Noah/
198~) and an element having an indium oxide thin
film (Japanese Patent Laid-Open Publn. Noah/
1984) are known as gas detecting elements heretofore
proposed, even greater improvement of sensitivity as
well as selectivity is desired.
SUMMARY OF THE INVENTION
It is therefore an object of the present invent
lion to provide a thin film-type gas detecting element
by which a reducing gas having a low concentration can
be detected with high sensitivity, particularly a
detecting element by which carbon monoxide (CO) can
be selectively detected with high sensitivity at a
low temperature range (from room temperature to about
120C), and methane (SHEA) and propane (C3H8) can be
selectively detected with high sensitivity at a high
-0 temperature range (from 350 to 450C).
It is another object of the present invention to
provide a process for producing a thin film-type gas
detecting element.
According to one aspect of the present invention,
there is provided a gas detecting element comprising
an insulating substrate provided with a pair of
electrodes, an Insane thin film so provided on the
surface of said insulating substrate as to cover said
electrodes, and a catalyst layer laminated on said
thin elm and comprising a carrier and a-t least one
of platinum group metals supported on said carrier.
According to another aspect of the present invent
lion, there is provided a process for producing gas
detecting element which comprises a first step of
forming an Insane thin film on the surface of an
insulating substrate provided with a pair of electrodes
by applying as a coating a thin film-forming starting-
material solution obtained by dissolving an organic
compound containing In and an organic compound contain-
in Sun in a solvent, onto the surface of said insulate
in substrate so as to cover said electrodes, drying
said applied solution, and thereafter firing resulting
structure to pyrolyze said organic compounds; and (2)
a second step of laminating a catalyst layer on said
thin film by applying onto said thin film a slurry
containing a catalyst powder obtained from an aqueous
solution containing one or more compounds of platinum
group metal wherein a carrier powder is immersed there-
in, drying said slurry, and thereafter firing the result-
in structure.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims
particularly pointing out and distinctly claiming the
subject matter which is regarded as the present
12;~ I
invention, it is believed that the invention will be
better understood from the following detailed descrip-
lion taken in connection with the accompanying
drawings, in which:
FIG. 1 is a cross-sectional view of a gas detect-
in element according to the present invention;
FIG. 2 is a perspective view of a gas detecting
device in which the element shown in FIG. 1 is used;
and
I FIG. 3 is a graph indicating the variation of
sensitivity with compositional ratio of In and Sun.
DETAILED DESCRIPTION OF THE INVENTION
-
FIGS. 1 and 2 illustrate an example of an element
according to the present invention. FIG. 1 is a
cross-sectional view of a cylindrical element, and
FIG. 2 is a perspective view showing one example of
the state of use of the element.
Referring to FIG. 1, the outer cylindrical sun-
face of a cylindrical substrate 1 made of an insulate
in material such as alumina or Malta is provided
with a pair of electrodes 2. The substrate 1 and the
electrodes 2 are covered with an Insane thin layer 3.
The thin layer 3 is covered with a catalyst layer 4.
Thus an element of the present invention is formed
The film thickness of the Insane thin film is
preferably in the range of from 1,000 A to 1 micrometer.
If the film thickness is more than 1 micrometer, its
~;~Z~'73~
sensitivity to reducing gases is reduced. If -the film
thickness is less than 1,000 A, sufficient sensitivity
cannot be obtained. The thickness of the catalyst
layer 4 is preferably in the range of from 10 to 50
micrometers. If the thickness of the catalyst layer
is outside this range, catalytic effects such as those
on sensitivity and selectivity are reduced.
As shown in FIG. 2, the thus constituted clement
of the present invention is mounted and retained on
pins 6 vertically provided on an insulating plate 5 in
such a state that the element does not come into con-
tact with other parts. Lead wires 7 are provided for
electrodes. A heater 8 is provided centrally through
the element in order to adjust the surface temperature
(working temperature) of the element.
A process for producing an Insane thin film will
be described.
The Insane thin film according to the present
invention is formed by pyrolyzing an organic compound
containing In and an organic compound containing Sun.
Specific amounts of In-containing organic compounds
such as In metal soap (e.g., indium octylate), In-
containing resin salts, alkoxides and Run wherein R is
an alkyd or aureole group, and specific amounts of Sun-
containing compounds such as Sun metal soap (e.g., tinoctylate), Sn~con~aining resin salts, alkoxides and
Run wherein R is an alkyd or aureole group art dissolved
9L'~2~'73~
in suitable solvents such as Tulane, Bunsen and n-
bottle alcohol -to prepare thin film forming starting-
material solutions having desired concentrations.
A solution of this character is then applied to
the outer cylindrical surface of the insulatincJ sub-
stroke 1 having a pair of electrodes 2. It is allow-
Ed to stand for a specific period of time ordinarily
from 30 minutes to one hour) in air. Thereafter, the
coated substrate is heated to a suitable temperature
(ordinarily about 120C) to evaporate the solvent
used and then fired for from 30 minutes to one hour in
air at a temperature of from 400C to 700C. Thus, the
organic compound containing In and the organic come
pound containing Sun are pyrolyzed to form an Insane
thin film.
The number of repetitions of this application-
firing step varies depending upon the concentration
of the starting material solution used and thus cannot
be generally specified. when the application-firing
step (a firs-t step) is repeated from once to four times,
a thin film having a specific film thickness is formed.
It is believed that when the content of Sun in the thin
film it less than about 10~ of In in atomic ratio percent
((Sn/In)xlOO), Sun acts as an impurity of indium oxide,
whereas when the content of Sun is 10% or more, polyp
crystals of indium oxide and tin oxide are formed.
In a second step, a catalyst layer 4 is then
I
laminated on the thus formed thin film 3 by the follow-
in process.
The catalyst layer 4 according to the present
invention comprises a catalyst wherein at least one of
platinum group metals, e.g. any one of palladium (Pod),
platinum (Pi) and rhodium (Rho) or any one of palladium-
platinum (Pd-Pt), palladium-rhodium (Pd-Rh) and platinum-
rhodium (Try) is supported on a carrier, e.g.,
aluminum oxide (Allah).
lo This catalyst is prepared as follows:
First, chlorides such as H2PtCl6 OWE, PdC12 and
RhCl3 OWE or ammonium salts such as (MH~)2PtCl6,
(NH4)2PdC16, and (NH4)3RhC16 are used to prepare an
aqueous solution containing Pod, Pi or oh at a specific
temperature. When Pod, Pi or Rho alone is supported on
Allah, a specified amount of Allah is immersed in the
respective solutions. When Pd-Pt, Pd-Rh or Try is
supported on AYE, aqueous solutions containing Pod,
Pi or Rho are mixed at a specific ratio to prepare a
mixture, and a specific amount of OWE is immersed in
the mixture.
oh components are thoroughly stirred and mixed.
The mixture is then dried for, for example, l to 2
hours under reduced pressure and heated and dried at a
temperature of about 100C. The dried material is
ground into a powder, for example, in a mortar. The
powder is placed in a quartz crucible and fired at a
'73~
temperature of from 400 to 800C. As a result, a
catalyst wherein a predetermined amount of Pi, Pi, Rho
Pd-Pt, Pd-Rh or Try is supported on OWE is obtain-
Ed
The amount of Pod, Pi or Rho supported on AYE is
as follows: when each component is used, it is prefer-
able that Pod, Pi or Rho be in the range of from 0.05%
to 2Ø0% by weight based on the weight of AYE. If
the amount is outside this range the amount of Pod,
Pi or Rho will not contribute to the improvement of the
sensitivity of the element. The amount of Pd-Pt, Pod--
Rho or Try supported on OWE is as else in the
cases of Pd-Pt and Pd-Rh, it is preferable that Pod be
present in an amount of from 0.05% to 20.0% by weight
based on the weight of AYE, and Pi or Rho be present
in an atomic ratio of Pi or Rho to Pod (Pt/Pd or Rh/Pd)
of from 0.05 to 1.0; and in the case of Try, it is
preferable that Pi be present in an amount of from
0.05% to 20.0% by weight based on the weight of AYE
and Rho be present in an atomic ratio of Rho to Pi
Rapt of from 0.05 to 1Ø
The thus prepared catalyst is then slurries by
using, for example, an aqueous solution containing
aluminum hydroxychloride or the like as a binder. This
slurry is applied onto the thin film and dried to obtain
a prescribed thickness. Thereafter, the whole assembly
is fired at a temperature of from 300 to 400C to form
I
a catalyst layer according to the present invention.
The following non-limiting examples are set
forth to more fully illustrate the present invention.
EXAMPLE 1
Indium octylate and tin octylate were used as
starting materials for an Insane thin film. These
materials were so dissolved in Tulane that the con-
tent of metal atoms became 50% in atomic ratio percent
((Sn/In)xlO0), whereby a starting-material solution was obtained.
This solution was then applied onto the outer
cylindrical surface of a cylinder of an insulating
substrate 1 previously provided with a pair of elect-
nodes 2 as shown in FIG. 1. The substrate thus coated
was allowed to stand for one hour in air and thereafter
heated to a temperature of 120C to evaporate off the
Tulane.
The resulting structure was then fired for one
hour at a temperature of 500C in air. This apply-
cation-firing step was repeated three times to form
a thin film having a thickness of about 3,000 A.
Thereafter, a catalyst layer was formed on this
thin film. First, PdC12 was dissolved in water to
form an aqueous solution containing 1.0~ by weight of
Pd. AYE fine powder having a surface area of about
100 m go was i~nersed in the aqueous solution, and the
mixture was thoroughly stirred. The mixture was dried
for 1.5 hours under reduced pressure to remove
-10~
~Z~'7~
moisture and then evaporated to dryness. The dried
material was then ground in a mortar and -the result-
in powder was placed in a quartz crucible and fired
at a temperature of 400C.
This catalyst powder was placed in an aluminum
hydroxychloride azaleas solution (1% AYE) with water
to form a slurry. This slurry was applied onto the
Insane thin film and dried. The structure thus obtain-
Ed was fired at a temperature of 400C. Thus, a Pod-
AYE catalyst layer having a thickness of 20 micro-
meters and containing 1.0% by weight of Pod in supported
state was formed.
In a similar manner, elements were prepared where-
in Tao or Roy catalysts as well as Pd-Pt-
AYE, Pd-Rh-A12O3 and Try catalysts were used.
In catalysts wherein any one of Pal, Pi and Rho was
supported on AYE, the amount of Pod, Pi or Rho was 1.0%
by weight based on the weight of AYE. In the cases
of Pd-Pt and Pd-Rh, the amount of Pod supported was 1.0%
by weight based on the weight of AYE, and the atomic
ratio of Pi or Rho to Pod was 0.5. In the case of Try,
the amount of Pi was 1.0% by weight based on the weight
of AYE, and the atomic ratio of Rho to Pi was 0.5.
A device as shown in FIG. 2 was assembled by using
each of thus prepared gas detecting elements wherein
the type of each catalyst layer was different. the
sensitivity to CO, Ho, SHEA, and C3H8 having concentrations
~22~'~3~
of 200 Pam and KIWI having a concentration of 1,000
Pam was measured as Roarers using each device.
The measurement was carried out at working tempera-
lures of 100C and ~00C. As used herein, Roger is
the resistance value exhibited by elements in air con-
twinning no measuring gas, and Russ is the resistance
value exhibited by elements in air containing the
above gases having respective concentrations. Accord-
tingly the larger the ratio Reargues
lo is the sensitivity.
The results obtained are shown in the following
Table.
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. .
73~
o o o o o o o ___ owe
o o o o o o o . . . . . .
mu O O O I , 1 co co
owe, . r-i
Q o o o o o o o o o o o o
Q . . . . . . . . . . . .
Jo Jo Jo or I ED ED
I o __ _ Jo N I 1~1
K I Q o o o o Jo o o o o o o o
m Jo Jo Jo 00
ox
Jo ___ _ _ __ _
rJ
I: R o ;` In us I o o o o o o
Al m O Jo I or
En I _ _ __
O O O O O O Us I
ox O O O O O O Jo Jo Jo
__ _ __
Jo
ox ox ox ox ox ox
o
I I r-l 1-l
o o o o o o ICKY
rl
I
O I! I; K lo I K I;
I Al)
I I) Ed Ed Ed Ed Jo
O I K Jo I 11 Pi Pi K Pi Pi Pi
U U
_ __ _ _ _ _
I U
Ed o
o o
I O o o
S l I it!
o a
En
-13
I I
As can be seen from the Table, in all catalysts,
the sensitivity of CO (200 Pam) is higher than that
of KIWI (1,000 Pam) at a working temperature of 100
C. It is apparent that the gas detecting elements
exhibit extremely high sensitivity to CO gas. On the
other hand, it is apparent that the sensitivity of
C3H8 and SHEA (200 Pam) is higher than that of KIWI
(1,000 Pam) at a working temperature of 400C.
EXAMPLE 2
Elements were produced as in Example 1 except
that the contents of In and Sun in the Insane thin film
were varied. The sensitivity to the percent content of
Sun to In was measured under the same conditions as
described in Example 1.
The results obtained by using an element having
a Pd-A12O3 catalyst as a catalyst layer to measure -the
sensitivity to SHEA (200 Pam) gas at a working tempera-
lure of 400C are shown in FIG. 3 as one example. As
can be seen from FIG. 3, when tune atomic ratio percent con-
tent ((Sn/In)xlO0) is Nero, i.e., the thin film is free
of Sun and contains only Inn (In-O system), the sense-
tivity is low. The sensitivity is increased as the
percent content of Sun is gradually increased. However
the sensitivity is gradually decreased from about 50~D
When the percent content is 100r i.e., the thin film
is free of In and contains only Snow (Snow system),
the sensitivity is approximately the same as that of the
-14-
73Ç;
thin film having a percent content of zero.
As can be seen Eros the foregoing, the Insane
thin film provides higher sensitivity as compared with
the thin film formed from :[n~03 (In-O system) only or
Snow (Snow system) only. Particularly, when the atomic
ratio percent of Sun to In is about 50%, the gas
detecting element exhibits excellent sensitivity.
Such a tendency was similarly observed even when the
type of the catalyst layer and gases to be measured
were varied.
As can be also seen from the results of Examples
set forth above, the gas detecting element of the
present invention exhibits high sensitivity to reduce
in gases of low concentration and shows excellent
sensitivity to CO gas at a Ion temperature range ox
from room temperature to about 120C and high sense-
tivity to SHEA and C3H8 at a high temperature range of
from 350 to 450C. Thus, the gas detecting element
of the present invention exhibits excellent silicate-
viny. Accordingly, by varying the working tempera-
lure, erroneous functioning due to miscellaneous
gases such as KIWI can be eliminated to achieve
highly sensitive detection of various reducing gases.
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