U.S. patent number 3,671,913 [Application Number 05/020,620] was granted by the patent office on 1972-06-20 for aging-proof humidity sensing element and method for the production thereof.
This patent grant is currently assigned to Kabushiki Kaisha Saginomiya Seisakusho, Tokyo, JP. Invention is credited to Ryoichiro Yajima, Shigeo Mamiya.
United States Patent |
3,671,913 |
|
June 20, 1972 |
AGING-PROOF HUMIDITY SENSING ELEMENT AND METHOD FOR THE PRODUCTION
THEREOF
Abstract
A humidity sensing element comprises an insulating base plate
provided with gold electrodes and a film or layer metal oxide
particles or a mixture of metal oxide particles and carbon
particles. The micro-gaps between said particles are filled up with
a hydrophilic high polymer, thereby minimizing the deterioration by
aging. The humidity sensing element can be produced by a method
which comprises forming a film made of metal oxide particles or a
mixture of metal oxide particles and carbon particles on an
insulating base plate provided with gold electrodes, immediately
whereupon said film is impregnated with a solution of a hydrophilic
high polymer in a water-miscible volatile solvent and subsequently
volatilizing off said solvent. The thus produced humidity sensing
element shows a substantially constant electric resistance at a
given humidity even with the passage of a long period of time,
thereby enabling a hydrometer capable of exactly indicating
humidity with high durability to be provided.
Inventors: |
Shigeo Mamiya (Tokyo, JP),
Ryoichiro Yajima (Tokyo, JP) |
Assignee: |
Kabushiki Kaisha Saginomiya
Seisakusho, Tokyo, JP (N/A)
|
Family
ID: |
12026808 |
Appl.
No.: |
05/020,620 |
Filed: |
March 18, 1970 |
Foreign Application Priority Data
|
|
|
|
|
Mar 19, 1969 [JP] |
|
|
44/20430 |
|
Current U.S.
Class: |
338/35; 73/73;
73/335.05; 73/29.05 |
Current CPC
Class: |
G01N
27/121 (20130101) |
Current International
Class: |
G01N
27/12 (20060101); H01c 013/00 () |
Field of
Search: |
;338/34,35
;73/29,73,336.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Samuel Feinberg
Assistant Examiner: R. Kinberg
Attorney, Agent or Firm: Woodhams, Blanchard and Flynn
Claims
1. A humidity sensing element, comprising: an insulating base plate
having gold electrodes thereon; a thin layer of particles on said
electrodes, said layer being of substantially uniform thickness and
consisting essentially of particles selected from the group
consisting of (1) .gamma.-ferric oxide particles and (2) a mixture
of .gamma.-ferric oxide particles and up to 40 per cent by weight
of carbon particles based on the weight of said .gamma.-ferric
oxide particles, said particles being of a size in the range of
about 10.sup.-.sup.5 to 10.sup.-.sup.7 cm and there being
micro-gaps between the particles throughout the depth of said
layer, and a hydrophilic high polymer filling the micro-gaps
between the particles throughout the entire depth of said layer for
improving the resistance to
2. A humidity sensing element as claimed in claim 1 in which said
layer has
3. A humidity sensing element as claimed in claim 1, in which the
weight
4. A humidity sensing element as claimed in claim 1, in which said
hydrophilic high polymer is disposed substantially entirely in
the
5. A humidity sensing element as claimed in claim 1, wherein said
hydrophilic high polymer is selected from the group consisting
of
6. A humidity sensing element as claimed in claim 1, wherein said
gold electrodes comprises a pair of comb-shaped symmetrical gold
electrodes.
7. A method for preparing a humidity sensing element, which
comprises the steps of: applying a suspension of particles in water
onto an insulating base plate having gold electrodes thereon to
form a thin layer of particles thereon, said particles being
selected from the group consisting of (1) .gamma.-ferric oxide
particles and (2) a mixture of .gamma.-ferric oxide particles and
up to 40 per cent by weight of carbon particles based on the weight
of .gamma.-ferric oxide particles, said particles being of a size
in the range of about 10.sup.-.sup.5 to 10.sup.-.sup.7 cm,
immediately thereafter and while said layer still contains said
water, applying onto said layer a solution containing about 1 to 5
per cent by weight of a hydrophilic high polymer dissolved in a
water-miscible volatile solvent so that the hydrophilic high
polymer enters the micro-gaps between the particles, and then
volatilizing the water and solvent to obtain a humidity sensing
element in
8. A method as claimed in claim 7, in which said high polymer is
selected from the group consisting of polyvinyl acetate and
cellulose acetate and said solvent is selected from the group
consisting of acetone and ethyl
9. A method as claimed in claim 8, in which said solution is
applied to
10. A method as claimed in claim 8, in which said solution is
applied by
11. A method as claimed in claim 8, in which said suspension
contains in the range of about 1 to 4 per cent by weight of said
particles and in which said plate is at a temperature in the range
of about 40.degree. to 60.degree. C., when the suspension is
applied thereto and said plate is at room temperature when said
solution is applied thereto.
Description
This invention relates to a humidity sensing element and a method
for producing the same, and more particularly to an aging-proof
humidity sensing element and a method for the production
thereof.
Heretofore, there has been known an electric resistance type
hygrometer in which there is used an insulating plate provided with
gold electrodes and a metal oxide film formed thereon, so that the
electric resistance between said gold electrodes changes in
response to a change of humidity (Japanese Patent No. 473306).
Furthermore, the present inventors have disclosed in Japanese
Patent Application No. 83043/1967 that an improved humidity sensing
element, which comprises a steatite plate provided with gold
electrodes and a graphite-containing .gamma.-ferric oxide film
formed thereon, has the advantage that it does not deteriorate on
exposure to air under severe conditions of high temperature and
high humidity, it requires no temperature compensation and it has
an improved linear relationship between humidity and electric
resistance.
The conventional humidity sensing element comprising a metal oxide
film has, however, the severe disadvantage that it does not possess
resistance to repeated great changes in humidity or aging
resistance. Illustratively stated, when the humidity sensing
element which comprises a porcelain, for example steatite base
plate provided with gold electrodes and a film made of metal oxide
particles or a mixture of metal oxide particles and carbon
particles is exposed to an atmosphere of low humidity for a long
time, its electric resistance, observed at a given humidity,
increases more and more and, thus it will be unable to be used as a
humidity sensing element. As a result of the present inventors'
investigation on the aging of the conventional humidity sensing
element, to wit, the change in the electric property (relationship
between relative humidity and electric resistance) of the element
on exposure to the air, it has been found that the difficulty
stated above is due to a number of hairline cracks which are formed
on the surface of the oxide film. These hairline cracks
occasionally have a width of 1.5 .times. 10.sup.-.sup.3 mm and
appear in a network form. Such hairline cracks causes the electric
resistance to greatly change, thereby destroying the function of
the humidity sensing element.
The unfavorable aging of the conventional humidity sensing element
will be illustrated in Table 1 which is set forth later. As an
example, when a titanium oxide film is employed as the metal oxide
film, the humidity sensing element which shows, at the beginning,
an electric resistance of 2.2 .times. 10.sup. 5 ohms at a relative
humidity of 50 percent will change in quality after exposure to air
(temperature: 15.degree. to 25.degree. C., humidity (R.H.): 30 to
50 percent for 3 weeks and show an electric resistance of 6.2
.times. 10.sup. 5 ohms at the same relative humidity as at the
beginning. Such change of electric resistance will not cease even
after an exposure for 5 weeks. When .gamma. -ferric oxide is used,
a similar change of electric resistance is observed. In this
connection, it is noted that as the rate of such change of electric
resistance of the humidity resistance element decreases with the
passage of time of exposure, the element is occasionally employed
in a hygrometer at the time when the rate of change of electric
resistance has been minimized. However, because of the lack of
reproducibility of the rate of change of the electric resistance
with aging, it is difficult to obtain a humidity sensing element
having a predictable property (relationship between relative
humidity and electric resistance) only by regulating the period of
time of exposure to the air. For this reason, the rate of
production of acceptable products is less than 10 percent. Thus, it
is disadvantageous in view of economic consideration.
As a result of intensive study by the present inventors, it has
been found that a humidity sensing element capable of resisting
repeated great changes of humidity can be obtained by filling up
the gaps formed between the oxide particles in the oxide film with
a hydrophilic high polymer. The present invention has been made on
the basis of such novel finding.
Therefore, it is an object of the present invention to provide a
humidity sensing element which is aging-proof and especially is
capable of resisting repeated great changes of humidity.
It is another object of the present invention to provide a humidity
sensing element of the character described which is capable of
sensing humidity with certainty as well as with accuracy.
It is a further object of the present invention to provide a
humidity sensing element of the kind described which can be
mass-produced and sold at reasonable cost.
It is a still further object of the present invention to provide a
method for producing a humidity sensing element of the kind
described which is simple and is excellent in the rate of
production of acceptable products.
The foregoing and other objects, features and advantages will be
apparent in the following detailed description taken in connection
with the accompanying drawing in which:
FIG. 1 is a partly cut-away plan view of a humidity sensing element
according to the present invention;
FIG. 2 is an enlarged sectional view taken along the line II -- II,
showing the condition prior to application of the treatment
according to the present invention;
FIG. 3 is a view similar to FIG. 2, showing the condition following
the application of the treatment according to the present
invention;
FIG. 4 is a view similar to FIG. 2, in which a humidity sensing
film, for comparison purposes, is formed by spraying a suspension
containing metal oxide particles, graphite particles and a
hydrophilic high polymer; and
FIG. 5 is a diagram showing the characteristics of the respective
humidity sensing elements having the structures of FIGS. 2, 3 and
4.
According to the present invention, there is provided a humidity
sensing element which comprises an insulating base plate provided
with gold electrodes and metal oxide particles or a mixture of
metal oxide particles and carbon particles, and forming a film on
the base plate. The micro-gaps between said particles are filled up
with a hydrophilic high polymer, thereby enabling the deterioration
by aging to be minimized. Such a humidity sensing element can be
produced by a method which comprises forming a film made of metal
oxide particles or a mixture of metal oxide particles and carbon
particles on an insulating base plate provided with gold
electrodes, immediately whereupon said film is impregnated with a
solution of a hydrophilic high polymer in a water-miscible volatile
solvent and subsequently volatilizing off said solvent. In this
connection, it should be noted that the application of said
hydrophilic high polymer to the film must be carried out
immediately after the film is formed. Accordingly, said application
of the hydrophilic high polymer is conducted at the time when the
moisture still remains in the film. For this reason, the volatile
solvent to be used for the preparation of a solution of the
hydrophilic high polymer should be selected from the solvents of
the kinds which are easily miscible with water. The hydrophilic
high polymers which can be used in the present invention embrace
various high molecular weight compounds; for example, polyvinyl
acetate or cellulose acetate may be employed with advantage. As the
solvent for these high polymers, acetone or ethyl acetate may
suitably be employed. In applying a solution of the high polymer,
when the concentration of the solution is too high, too thick a
film of the high polymer is formed on the surface of metal oxide,
leading to a lowering of the sensibility of metal oxide to
humidity. Therefore, the concentration of the solution is of
importance and is suitably in the range of 1 to 5 percent by
weight, preferably 2 to 4 percent. As to a method for impregnating
the metal oxide film with the solution of high polymer, dipping or
spraying using a spray gun is preferably employed. In this
connection, it is to be noted the purpose of application of the
solution of high polymer is not to simply coat the metal oxide film
nor to simply adhere the metal oxide to the base plate, but rather
is to fill up the micro-gaps between the component particles of the
metal oxide film. Accordingly, a method in which the suspension
prepared by dispersing the metal oxide particles in the solution of
high polymer is sprayed onto the insulating base plate for form a
humidity sensing film or a method in which the concentrated
solution of high polymer is sprayed onto the metal oxide film to
form a simply strong film, can not achieve the desired purpose. On
the other hand, for example when the solution of high polymer is
sprayed onto the film using a spray gun, when the position of the
spray gun is too high, the volatile solvent, e.g., acetone,
volatilizes and dissipates before reaching the metal oxide film,
hence only the high polymer is sprayed onto the metal oxide film
while the surface is in a dry condition. As a result, the purpose
of filling up the micro-gaps between the component particles of the
oxide film is not achieved and the desired product can not be
obtained. Accordingly, it is necessary to hold the surface of the
metal oxide film always wet with the solution.
Referring now to FIGS. 1 to 3, the metal oxide film is formed by,
for example, applying, using a spray gun, a suspension prepared by
colloidally dispersing 1 to 4 percent by weight of metal oxide
particles 4 (diameter: 10.sup.-.sup.5 to 10.sup.-.sup.7 cm)
containing carbon particles e.g. graphite particles 3 of 0 to 40
percent, preferably 10 to 20 percent, by weight based on the weight
of the metal oxide particles in water onto a porcelain base plate
e.g. steatite plate 2 provided with a pair of comb-shaped
symmetrical gold electrodes 1, 1. As the metal oxide, .gamma. -
ferric oxide, titanium oxide, tri-iron tetroxide, silicon oxide,
aluminum oxide, chromium oxide or the like may suitably be
employed. Yet, .gamma. - ferric oxide is most preferably employed
because of its lower electric resistance as well as its high
resistance to deterioration by oxidation. Thus, the metal oxide
particles and graphite particles are fixed onto the base plate
thereby to form a film having an electric conductivity. The metal
oxide film containing graphite particles may preferably be 5 to 10
.times. 10.sup.-.sup.3 mm thick. In order to obtain such thickness,
it is preferable to make a film by spraying a suspension of a
regulated concentration (1 to 4 percent by weight) two or more
times. During the spraying operation, the base plate is preferably
maintained at a temperature above room temperature, preferably
40.degree. to 60.degree. C. Subsequently, the temperature of the
base plate having the metal oxide film thereon is lowered to room
temperature, whereupon a 1 to 5 percent by weight solution of a
hydrophilic high polymer in a water-miscible volatile solvent is
applied onto the metal oxide film by spraying or dipping thereby to
fill up the micro-gaps 5 between the component particles of the
metal oxide film. The amount of high polymer employed is suitably
in the ratio of 1 : 2 to the weight of metal oxide film. The thus
obtained humidity sensing element is placed in a room with a
ventilator and then is allowed to stand at room temperature for 3
weeks. As a result, the volatile solvent is completely volatilized
and dissipated to obtain the desired product in which the
micro-gaps between the component particles of the film are
completely filled up with the high polymer 6.
When the treatment according to the present invention is not
applied, the mixture of metal oxide particles 4 and graphite
particles 3 forms a film on the steatite base plate 2 leaving
micro-gaps 6' therebetween as depicted in FIG. 2. On the other
hand, when the suspension prepared by dispersing a mixture of metal
oxide particles 4 and graphite particles in a solution of the high
polymer in the solvent is sprayed onto the base plate 2 to form a
film, the high polymer is caused to be mixedly present between the
metal oxide particles 4 and the graphite particles 3 as depicted in
FIG. 4.
As is clearly seen in FIG. 5, a comparison of the characteristics
of these three kinds of humidity sensing elements is as follows:
The humidity sensing element according to the present invention A
(FIG. 3) shows almost no change in the humidity indication at a
given humidity with the passage of days. By contrast, the humidity
sensing element not subjected to the treatment of this invention B
(FIG. 2) and the humidity sensing element produced by spraying the
suspension prepared by dispersing the mixture of metal oxide
particles 4 and graphite particles in a solution of the high
polymer in the solvent C (FIG. 4) both show great changes in the
humidity indication at a given humidity with the passage of
days.
Furthermore, it is to be noted that the remarkable effect of the
present invention is a noticeable improvement in the rate of
production of desired products. According to the actual example,
the products capable of standing use for more than one year can be
obtained at a rate of production of desired products of 85 percent
or more according to this invention, while the conventional
humidity sensing element having a film composed only .gamma.-
ferric oxide particles containing graphite particles capable of
standing use for a half year can be obtained at a rate of
production of desired products of 10 percent or less.
The present invention is illustrated by the following Examples.
EXAMPLE 1
An aqueous suspension containing 3.2 percent by weight of a mixture
of .gamma.-ferric oxide particles and graphite particles (15
percent by weight based on the weight of .gamma.-ferric oxide
particles) was sprayed three times onto a steatite base plate
provided with a pair of comb-shaped symmetrical gold electrodes to
form a fixed film having a thickness of 7 .times. 10.sup.-.sup.3
mm. During the spraying operation, the temperature of the base
plate was maintained at 50.degree. C. Subsequently, the temperature
of the base plate having the metal oxide film thereon is returned
to room temperature, whereupon a 2 percent by weight acetone
solution of polyvinyl acetate was sprayed, using a spray gun, onto
the metal oxide film three times in such manner that each-spraying
operation was conducted for 3 seconds, followed by drying and such
spraying operation was repeated three times. The thus obtained
product was placed in a room with a ventilator at room temperature
for 3 weeks thereby to completely volatilize and dissipate acetone,
and the desired product was obtained.
EXAMPLE 2
The same operation as in Example 1 was repeated with exception that
titanium oxide was employed in place of .gamma.-ferric oxide. Thus,
a desired product was obtained.
EXAMPLE 3
The same operation as in Example 1 was repeated with exception that
cellulose acetate was employed in place of polyvinyl acetate. Thus,
a desired product was obtained.
The aging test (Condition: temperature 0.degree. to 20.degree. C.,
Relative humidity (R.H.) 20 to 60 percent was conducted on the
respective products obtained in Examples 1, 2 and 3. The test
results are summarized in Table 1, in comparison with the test
results of the convention humidity sensing elements not subjected
to the treatment of this invention. Table 1
Aging test (Electric resistance in R.H. 50 percent) ##SPC1##
As described, according to the present invention, an aging-proof
humidity sensing element can be obtained which is capable of giving
a substantially constant humidity indication at a given humidity
even with the passage of time.
* * * * *