U.S. patent number 3,621,343 [Application Number 05/005,354] was granted by the patent office on 1971-11-16 for polar vapor sensing means.
This patent grant is currently assigned to NL Industries, Inc.. Invention is credited to Charles F. Pulvari, Stephen F. Urban.
United States Patent |
3,621,343 |
|
November 16, 1971 |
POLAR VAPOR SENSING MEANS
Abstract
The sensing means comprises a body of metal oxide dielectric
material having an active surface layer exhibiting surface
conductive characteristics wherein the resistivity of the surface
layer varies in the presence of a polar vapor and wherein the
resistivity of the surface layer varies between a first condition
in the absence of water vapor and a second condition in the
presence of water vapor by a factor on the order of 1: 10,000 or
more. At least two separate spaced electrically conductive
electrodes are electrically connected with portions of the surface
layer. Means is provided for impressing an AC voltage across said
electrodes, and a load impedance is connected in series therewith
from which an output signal can be derived.
Inventors: |
Charles F. Pulvari (Washington,
DC), Stephen F. Urban (Kenmore, NY) |
Assignee: |
NL Industries, Inc. (New York,
NY)
|
Family
ID: |
21715442 |
Appl.
No.: |
05/005,354 |
Filed: |
January 23, 1970 |
Current U.S.
Class: |
361/434 |
Current CPC
Class: |
G01N
27/121 (20130101) |
Current International
Class: |
G01N
27/12 (20060101); H01g 009/00 () |
Field of
Search: |
;317/230,231,238,258,262
;106/39 ;324/61 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: James D. Kallam
Attorney, Agent or Firm: Charles F. Kaegebehn Robert L.
Lehman Fred Floersheimer
Parent Case Text
CROSS REFERENCE TO THE RELATED APPLICATION
The present application is a continuation-in-part of copending U.S.
Pat. application Ser. No. 709,642, filed Mar. 1, 1968, now
abandoned.
Claims
1. Polar vapor sensing means comprising a body of polarizable metal
oxide dielectric material having an active surface layer including
means for varying the resistivity of the layer in response to
contact with a polar vapor whereby the resistivity of the surface
layer varies between a first condition in the absence of a polar
vapor and a second condition in the presence of a polar vapor, said
varying being at least on the order of 1:10,000, and at least two
separate and spaced electrically conductive
2. Apparatus as defined in claim 1 including means for impressing
an AC voltage across said electrodes, and a load impedance
connected with said
3. Apparatus as defined in claim 1, wherein said body is insoluble
in
4. Apparatus as defined in claim 2, wherein said body comprises a
ferroelectric crystal, and the means for impressing said AC voltage
provides a voltage which is smaller but near to the coercive
voltage of
5. Apparatus as defined in claim 1, wherein said body is selected
from the group consisting of ferroelectric crystal, ferrielectric
crystal,
6. Apparatus as defined in claim 1, wherein the surface portions of
said
7. Apparatus as defined in claim 1, wherein the thickness of said
surface layer is within the range of a fraction of a micron up to
about 20
8. Apparatus as defined in claim 1, wherein said surface layer
provides a
9. Apparatus as defined in claim 1, wherein said body comprises a
ferroelectric crystal selected from the group consisting of sodium
potassium niobate, sodium niobate vanadate, lithium niobate and
barium
10. Apparatus as defined in claim 1, wherein said body comprises a
ferroelectric crystal having additive means selected from the
group
11. Apparatus as defined in claim 1, wherein said body comprises a
ferroelectric crystal having additive means therein for enhancing
capacity variation of the crystal, said additive means being
selected from the
12. Apparatus as defined in claim 1, wherein said body comprises a
ferroelectric crystal having additive means therein for enhancing
resistivity variation of the crystal, the additive means being
selected from the group consisting of lanthanum, niobium, tantalum
and molybdenum.
13. Apparatus as defined in claim 1, wherein said body comprises a
ferroelectric crystal for sensing polar vapors of crude oils, said
crystal comprising bismuth titanate having additive means therein
selected from
14. Apparatus as defined in claim 1, wherein the free area of the
crystal is substantially greater than the area of the electrodes,
the outer
15. Apparatus as defined in claim 1, wherein at least one of said
electrodes includes a plurality of relatively narrow elongated
portions
16. Apparatus as defined in claim 1, wherein said electrodes are
each disposed on one side of the body, each of the electrodes
comprising a plurality of spaced elongated interconnected portions,
whereby the
17. Apparatus as defined in claim 1, wherein said body comprises a
single ferroelectric crystal plate, said electrodes being disposed
on opposite faces of said plate, each of said electrodes including
a plurality of elongated portions for increasing the peripheral
dimension relative to the area thereof, and protective means
disposed adjacent said crystal and
18. Apparatus as defined in claim 17, wherein said protective means
comprises a housing disposed in substantially surrounding
relationship to said crystal and said electrodes, said housing
having hole means formed therethrough for permitting polar vapors
to come into intimate contact with said sensing means.
Description
BACKGROUND TO THE INVENTION
The apparatus according to the present invention may be used in any
application wherein it is desired to detect the presence of certain
polar vapors. It may advantageously be employed for example to
detect the humidity of ambient air and may be connected in a
suitable electric circuit for indicating the amount of humidity or
for operating humidity control mechanisms. It may also be employed
in many other fields as for detecting the polar vapors of crude
oils during oil prospecting operations. Additionally, many other
applications of the invention will suggest themselves to one
skilled in the art. For example, the invention may be used in the
medical, biological, manufacturing and controlling fields as well
as in chemical processes and the like.
Prior art devices for measuring humidity have employed many
different arrangements. Typically, such devices have utilized
hairs, strings and the like for operating a suitable linkage.
Electrical circuits have also been employed wherein a resistor in
the circuit may be provided with an organic coating.
Such prior art structures for measuring humidity have not proved to
be satisfactory since the components thereof were subject to
breakage or other physical damage, and further due to the fact that
the results obtained therewith were not sufficiently reliable due
to the nature of the substances utilized as the sensing means
thereof.
The present invention provides a so-called solid state sensing
means in the form of a body of metal oxide dielectric material
including an active surface layer having surface conductive
characteristics and in a dry condition at room temperature having a
resistance on the order of 10.sup.10 -10.sup.12 ohms between its
electrodes. Certain additive means may be incorporated in the body
to enhance changes in the electrical surface properties of the
body. At least two separate spaced electrically conductive
electrodes are electrically connected with spaced portions of said
surface layer.
This type of sensing means does not comprise organic substances, is
very rugged and not as susceptable to physical damage as prior art
arrangements. Furthermore, the sensing means of the present
invention is very reliable, the sensing means when removed from a
polar vapor atmosphere returning to its original sensitivity
condition or dry resistance whereby the repeatability of the
apparatus is very good. It should be noted that due to the fact
that the operation of this sensing means is based on the active
surface properties of the metal oxide body its response is fast and
can be expressed in seconds rather than in minutes, which is a most
important feature when it is used for automatically controlling the
conditions of large rooms.
The term "polarizable metal oxide dielectric material" includes a
group of materials which may be termed Moxies, this expression
including ferroelectric crystals, ferrielectric crystals,
ferroelectric glass ceramics including ferroelectrics grown in a
glassy matrix, fused quartz and various other glassy compositions
such as silica glasses. All of these materials are capable of being
treated so as to provide a thin active surface layer having surface
conductive characteristics as hereinafter described. These
materials are also insoluble in water.
The term "ferroelectric crystal" as used herein is intended to
denote a crystal which has ferroelectric properties such as a Curie
point and wherein the polarization thereof can be reversed with an
electric field lower than the breakdown voltage of the crystal. A
typical example of such a crystal is also intended to include
ferrielectric crystals such as bismuth titanate which have
characteristics similar to ferroelectric crystals and additionally
include a threshold switching field when the voltage applied
thereto is reversed.
In the sensing means of the present invention, the capacity and
resistivity between the electrodes of the device vary in the
presence of polar vapors and this variation is essentially a
surface effect occuring due to the interaction of the dipoles
adsorbed on the surface rather than absorbed in the bulk of the
crystal. It has been found that a highly polished crystal surface
was more sensitive and gave better repeatable results than a
frosted surface which indicates that only adsorption of the polar
vapors and no absorption is required to obtain the sensing
effect.
Although the exact mechanism of the surface conductivity induced by
the polar vapors is not yet known, it is believed that the surface
conductivity is a result of the fact that the surface layer
includes atomic arrangements such as hydroxyl groups which are
active with respective to polar molecules so as to produce
rearrangement of the atoms of the polar molecules when in contact
therewith thereby producing charge carriers. The continuous free
carrier or charge production occuring on the active surface acts
like a donor for the semi-insulating surface layer to thereby
substantially decrease the dry resistance of the sensing means in
the presence of polar vapors.
The active surface layer exhibits a surface conductivity when
subjected to polar vapors. This surface conductivity is believed to
be a result of molecular interaction at the activated metal oxide
surface which rearranges the molecular structure of the polar
vapor, and as a result, active groups are created on the surface of
the metal oxide, For example, in the case of water vapors and a
silicon-based glass, the active groups formed are surface hydroxyl
groups spaced sufficiently far apart so that they do not interact
with one another, surface hydroxyl groups which are so close
together that they are hydrogen-bonded to one another, and
molecular water which is physically adsorbed on the surface of the
glass. In this case, the first two groups are of major interest
from the viewpoint of adsorption.
The nature of the active surface is controlled by the temperature
at which the metal oxide dielectric material is melted and the loss
of volatile components during such melting, the loss of volatiles
from the surface during the activating process and the impurities
and vapors introduced are adsorbed during the forming and cooling
processes.
Although each of the various processing phases have an effect on
the formation of active groups on the surface of the metal oxide
dielectric material, the thermal history of the activating process
is most significant. It should be noted that some impurities
enhance the formation of active surface groups while others inhibit
the formation of such active surface groups. Various types of metal
oxide dielectric materials may form different active surface groups
whereby the surface is responsive to various polar vapors. It has
been found that in case of water vapor adsorption which has been
extensively studied, the rehydration of silica or bismuth titanate
metal oxide surfaces depends on the previous thermal history. Up to
temperatures of about 400.degree. C., the dehydroxylation of the
surface is reversible, but at temperatures above 400.degree. C.,
the removal of the adjacent hydroxyl groups from the surface causes
the surface to become hydrophobic. It was found that a heat
treatment or activation step was effectively obtained when the
surface of the metal oxide dielectric material was subjected to a
temperature of about 500.degree. C. for an extended period of about
1 to 2 hours.
It has been found that due to the interaction of a polar vapor and
the active surface layer of a Moxi, the capacity of a capacitor
defined by the Moxi as a dielectric and its associated electrodes
changes depending upon how much polar vapor is deposited on the
free crystal surface.
In addition, the resistance between the two electrodes changes as a
consequence of the amount of deposited polar vapor. Although the
exact physics of this phenomenon is not entirely clear, it is
apparent that the factors responsible in obtaining a high
sensitivity sensing means is a thin active skin layer and the
interaction of the field of polar molecules with the active skin
layer. An external electric field is applied to the electrodes
through a load impedance from which an output signal can be
derived.
The sensing ability of the active surface layer may be due to three
factors, namely, the active surface properties of the sensing
means, the interaction of polar vapors with said active surface,
and some rather weak semiconductive properties of the surface layer
of the Moxi which has in a dry condition resistance comparable to
insulators and therefore is more correctly called a
semi-insulator.
The metal oxide dielectric materials used in the present invention
are essentially insulators with a resistance between electrodes
spaced about 12-20 mils apart on the order of 10.sup.10 -10.sup.12
ohms in a dry condition. If a thin surface layer is activated as
described hereinabove, the interaction of polar vapors and the
active surface layer causes a resistance to decrease to a range on
the order of 10.sup.4 -10.sup.5 ohms. This represents a change in
resistance of the surface layer between a first condition in the
absence of water vapor and a second condition in the presence of
water vapor by a factor on the order of 1:10.sup.6.
The bulk of the body of metal oxide dielectric material does not
change its resistance and is not effected by the polar vapors. The
active surface layer is so thin that it can be removed by
scratching the surface with an abrasive paper in which case the
sensitivity of the sensing means vanishes.
The term "semi-insulators" is used for insulating materials which
due to surface conditions as described above or impurities render
an insulating material such as ceramic, glass or Moxies to become
slightly conductive and normally the lower range of resistance on
the order of 10.sup.4 -10.sup.5 ohms is obtained when subjected to
polar vapors as contrasted to a resistance of 10.sup.10 -10.sup.11
ohms between electrodes when not subjected to polar vapors.
The utilization of such high resistivity changes was not possible
until the recent development of high impedance input integrated
operational amplifiers, and their small size made it feasible to
transform the high impedance changes of the semi-insulating surface
conductive layer of the metal oxide dielectric material into low
impedance and high current outputs.
Although it has been found that the surface layer of a number of
Moxies have been successfully converted to a metal oxide active
layer which responds to the presence of polar molecules such as
humidity in the form of water vapor and the like in that the
capacity and/or resistivity between the electrodes of the sensing
means changes in the presence of such vapors when disposed on said
active surface layer, it has been found that bismuth titanate
exerts a particularly strong effect, probably because this material
possesses a low dielectric constant. Bismuth titanate possesses a
dielectric constant of .epsilon.'.congruent.19 and a very high
Curie temperature such as approximately 675.degree. C. Furthermore,
the Ti atom lends itself very well to form very active surface
groups.
The existence of a threshold switching field in bismuth titanate
permits the maintenance of a relatively large driving voltage
without causing switching of the domains. This is one reason that
the response of bismuth titanate crystals may be superior to other
ferroelectrics. Other crystals possessing a threshold switching
field and exhibiting good response are sodium potassium niobate,
sodium niobate vanadate, lithium niobate and barium sodium
niobate.
It has been found that the presence of certain additive means in
the Moxi body and the active surface layer formed thereon in some
instances substantially enhances the changes in the electrical
surface conductive properties of the sensing means when subjected
to polar vapors. Certain additives have been found to be especially
effective when sensing particular polar vapors as hereinafter fully
described. Crystals grown with particular additive means as
hereinafter discussed exhibit extraordinarily strong changes in the
dielectric and conductivity properties when subjected to polar
vapors.
The presence of such additive means on the crystal surface
apparently acts like catalyzers act in chemical processes since it
has been clearly ascertained that the presence of particular
additive means renders the crystal responsive to particular
chemicals present in polar vapors. The capacity and resistance
changes of the sensing means may be measured in a suitable
electrical circuit as hereinafter described. Means is provided for
impressing an AC voltage across the electrodes and a load impedance
is connected in series with said means for impressing the AC
voltage and the active surface layer.
It has been found that the sensing means has maximum sensitivity or
response when a ferroelectric crystal is employed and when the AC
driving voltage applied to the electrodes of the sensing means is
smaller but close to the coercive voltage of the device kept on a
value which does not switch the domain configuration of the
crystal. Typical operation frequencies are in the range of 20-500
kilocycles, but higher frequencies also provide a good
response.
The sensing means must also be so constructed that the free crystal
area of that part of the crystal upon which electrodes are mounted
must be large compared to the electrode area. If the electrodes
cover substantially the entire crystal area, the response of the
sensing means is practically nil. It has been found that the free
crystal area must be at least as much as the C.sub.s /C ratio as
hereinafter defined, and generally, the free crystal area should be
at least ten times larger than the area of the electrode. Sample
devices which do not incorporate the proper ratio between free
crystal area and electrode area have not provided sufficient
measuring range. Additionally, it has been found that the electrode
should have a large contour or peripheral dimension for its area in
order to be most effective.
The objectives of the present invention are to provide a new and
novel polar vapor sensing means which can be adapted for measuring
or controlling humidity or for detecting certain polar vapors such
as hydrogen sulfite and the like which are present in crude oils
and wherein the apparatus could be employed for detecting the
presence of a crude oil source and would permit test holes to be
drilled to a less depth for ascertaining the presence of oil in a
particular drilling area. The apparatus of the presence invention
is quite simple and inexpensive in construction, and yet at the
same time, it is quite compact and versatile in application. The
sensing means is additionally applicable to many different fields
such as chemical process control.
FIG. 1 is a front view of a sensing means according to the present
invention illustrating schematically thereon the presence of domain
walls within the crystal having oppositely polarized areas at
either side thereof;
FIG. 2 is a top view of a modified form of the invention;
FIG. 3 is a top view of still another form of the invention;
FIG. 4 is a longitudinal section through still another form of the
invention;
FIG. 5 is a schematic wiring diagram illustrating an electrical
circuit including the sensing means of the present invention;
FIG. 6 is a schematic wiring diagram of still another electrical
circuit employing the present invention; and,
FIG. 7 is a schematic wiring diagram of yet another electrical
circuit incorporating the sensing means according to the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the illustrated embodiments, ferroelectric crystals are
described. It should, of course, be understood, that in each case
any suitable body of metal oxide dielectric material may be
employed as discussed hereinbefore, and that the ferroelectric
crystals as disclosed are merely for the purpose of
illustration.
Referring now to the drawings, a first form of the invention is
illustrated in FIG. 1 wherein a ferroelectric crystal is indicated
by reference numeral 10. This crystal comprises a single crystal
plate which may comprise any of the conventional or aforementioned
ferroelectric and ferrielectric crystals. In the preferred
embodiment, the crystal is formed of bismuth titanate having a
relatively high Curie temperature as aforedescribed. Electrode
means is provided in the form of a pair of electrically conductive
electrodes 12 and 14 which are disposed in contact and suitably
secured to the opposite faces of the crystal. These electrodes may
for example comprise a conductive oxide deposition formed in the
usual manner. In a typical example the crystal 10 may have a
thickness of approximately 2 mils with the length and width thereof
being approximately two-eighths of an inch and one-eighth of an
inch, while the electrodes may have a diameter of approximately
one-sixteenth of an inch, wherein the electrodes are deposited in a
substantially circular configuration. Suitable electrical leads 16
and 18 are secured to electrodes 12 and 14 respectively for
connecting the sensing means in an associated electrical
circuit.
The crystal is indicated as having a multiplicity of domain walls
schematically indicated by the lines 20, the surface of the crystal
including a plurality of oppositely polarized areas as indicated by
the positive and negative signs adjacent the surfaces thereof,
adjacent oppositely polarized areas being separated by one of the
domain walls. When the polar vapors are deposited on the surface of
the crystal, as pointed out previously, the capacity of the
capacitor defined by member 10, 12 and 14 changes in accordance
with the amount of polar vapor deposited on the crystal
surface.
It appears that the sensing device in effect constitutes a device
similar to a mass spectrograph responsive to polar vapors in
general. It should be noted, however, that the sensing means of the
present invention is very much simpler than any other device which
serves a similar function. It is clear that the change in capacity
and resistivity between the electrodes of the device is more a
surface effect occuring essentially on the free crystal area not
covered by an electrode rather than in the bulk of the crystal
since the time response of the sensing means to a particular polar
vapor occurs rather fast, in seconds or minutes, depending on the
hysteresis cycles. Bismuth titanate, among others, exhibited a very
large response to polar vapors and particularly to humidity if
hafnium, chromium, niobium and tantalum additives are included in
small amounts within the range of about 0.05 to 3 mole percent to
provide optimum results. The amount of additive should be within
the extreme ranges of approximately 0.001 to 10 mole percent.
An AC voltage is impressed through leads 16 and 18 on electrodes 12
and 14, and optimum operating conditions were obtained when the
additive means comprised tantalum, and optimum operation was also
obtained when the additives comprised hafnium, chromium or
niobium.
It has also been found that the firing temperature of electrodes 12
and 14 provides best results if carried out at approximately
400.degree. C. since in the case of water vapors the
dehydroxylation of the surface is reversible.
In the method of making the sensing means of the present invention,
the surface portions of the body of metal oxide dielectric material
to which the electrodes are to be connected are preferably
initially highly polished to provide a mirror polished, smooth
surface.
The electrodes are then suitably fixed to the surface of the body
as by firing the electrodes on as previously described, the
temperature of firing being lower than the heat treatment
temperature where the body surface is activated as hereinafter
described. The electrodes may also be connected to the body by a
vacuum deposition process or by a high temperature hydrolysis
process.
The active surface layer having surface conductive characteristics
may be formed on the Moxi by several different processes. This
active surface layer may firstly be formed by a heat treatment
process wherein the Moxi body is heated to a temperature of at
least about 500.degree. C. for at least about 1 hour.
The thin active surface layer may also be formed by irradiating the
surface by electrons or ions such as performed in a conventional
ion implantation process or a conventional deoxidation process as
will appear to one skilled in the art.
The Moxi body and the electrodes connected thereto are mounted in a
support socket and a protective means is then mounted in place
relative to the socket.
The relationship of the free crystal area to the electrode area
should be within the ranges as aforedescribed, and the free crystal
area in this case represents on the faces of the crystal on which
the electrodes are disposed the total area not covered by the
electrodes. The voltage impressed on the electrodes should be
within the ranges aforedescribed, and especially should be smaller
but near to the coercive voltage of the crystal if the material has
ferroelectric properties.
As mentioned previously, certain additive means are included in the
crystal to enable the desired results. Reference is now made to
table I which represents results obtained when various additives
are included in a bismuth titanate crystal.
The first column of the table labeled GROUP represents the
different elements which comprise the additive means in a bismuth
titanate crystal, these elements having been given their
conventional chemical symbol.
The next column represents capacitance and resistance ratios
obtained when the polar vapor comprises water. The next column
represents the results obtained when the polar vapor was linseed
oil, and the subsequent columns indicate the ratios obtained when
the polar vapors were oleic acid, dimethyl formamide and acetic
acid respectively.
An explanation is now in order as to what the ratios indicated in
these vertical columns represent. The first ratio defined is
C.sub.s /C. C.sub.s ##SPC1##represents the capacitance of the
capacitor illustrated in FIG. 1 when the polar vapors deposited
thereon were deposited from ambient air at 100 percent humidity. C
represents the capacitance of the capacitor illustrated in FIG. 1
when the ambient air from which the polar vapor is deposited is
substantially free of moisture. It will be noted that the results
vary considerably in accordance with the particular elements which
form the additive means in the crystal, and further dependent upon
what particular polar vapor is to be detected.
The second ratio indicated in this table is G.sub.s /G. G.sub.s
represents the resistivity of the capacitor shown in FIG. 1 when
the polar vapor is at 100 percent humidity and G represents the
resistivity when the polar vapor is substantially free of moisture.
In each case involving both ratios, it should be understood that an
AC voltage is impressed across the electrodes of the sensing means.
These ratios were obtained at room temperature and in air at room
temperature. The additives included in the crystal were within the
range of approximately 0.001 to 10 mole percent. From an inspection
of table I, it is apparent that polar vapors have a profound effect
on the capacitive and conductive properties of ferroelectric or
ferrielectric capacitors. Furthermore, it shows that certain
additives enhance the capacity variation response such as
strontium, lanthanum, zirconium and chromium. Some other additives
enhance the resistivity variation response such as lanthanum,
niobium, tantalum and molybdenum for example. In some cases, the
two effects are about equal. In some cases, the resistivity change
dominates the response. In other cases, the capacity change effect
dominates the response.
Table I illustrates how this effect can be utilized for measurement
and control of humidity, for example, and wherein certain bismuth
titanate crystals having additives such as zirconium, hafnium,
niobium and tantalum are especially well suited. In selecting the
additives, the strength of the response and also the time of
response must be considered, the time of response usually being
about 5 minutes, while for certain stronger responding additives
such as chromium, the response time was as high as 20 minutes,
which is relatively slow.
It also may be seen from table I that other polar vapors may give
excellent response with certain additives in bismuth titanate. For
example, manganese provides a good response in a bismuth titanate
crystal when it is desired to detect acetic acid, while the same
additive gives a rather small response for humidity.
In another example, the additive samarium, while showing a very low
effect for humidity, shows a very high resistivity variation for
dimethyl formamide.
It has also been found that the rate earths may be included as
additives in the ferroelectric crystals when it is desired to
detect hydrogen sulfite which is present in the polar vapor of
crude oils.
Referring now to FIG. 2, modification of the invention is
illustrated wherein a crystal 30 is provided, this crystal being
formed of any suitable ferroelectric or ferrielectric substance as
described hereinabove. The domain walls are again indicated
schematically by reference numerals 32, and the positive negative
signs indicate the oppositely polarized area separated by the
domain walls. The few domains illustrated schematically in this
figure are only examples of domains as viewed from the top, these
domains usually being distributed over the entire crystalline
structure in a sort of semirandom fashion.
Similar electrodes may be suitably secured to opposite sides of the
crystal, one of the electrodes 34 being visible in FIG. 2. This
electrode is indicated generally by reference numeral 34 and
includes a generally cylindrical central portion having a plurality
of elongated portions 38 radiating outwardly therefrom. A suitable
electric lead 40 is connected with the electrode, and it will be
understood that a similar electrode structure is provided on the
opposite side of the crystal.
This particular electrode configuration provides a large contour or
peripheral dimension for the area of the electrode, which has been
found to be more effective than a simple circular electrode as
discussed in connection with FIG. 1.
Various other electrode configurations may be employed in order to
increase the peripheral dimension for a given area of electrode. It
is again emphasized that the free crystal area should be large
relative to the electrode area and the ratios of the free crystal
area to the electrode area should be at least 10 times as great as
the electrode area, and in some cases, as many as 50 times or more
as great.
Referring now to FIG. 3 of the drawings, a crystal 50 similar to
the crystals previously described is provided, and in this form of
the invention, the two electrodes in contact with the crystal are
both disposed on one face of the crystal. The first electrode
includes a central portion 52 having a plurality of elongated
portions 54 extending radially outwardly therefrom. An electrical
lead 56 is connected with this electrode. The other electrode
includes a generally circular portion 58 having elongated portions
60 extending radially inwardly therefrom and in spaced relationship
to the elongated portions 54 of the other electrode. An electrical
lead 64 is connected with the second electrode.
It is apparent that this arrangement insures a maximum contour or
peripheral dimension for each of the spaced separate electrodes for
the area involved.
Here again, the free crystal area should be substantially greater
than the area of the electrodes on the same order as previously
described. In this instance, the free crystal area comprises that
portion of the surface of the crystal which is visible in FIG. 3
and is not covered by the electrodes. The free crystal area in this
modification would not include the opposite face of the crystal,
since in defining free crystal area, only that surface of the
crystal upon which the electrodes are disposed is included.
Referring now to FIG. 4 of the drawings, a single crystal plate 70
similar to those previously described is provided, an electrode 72
being secured to one surface thereof and defining a plurality of
elongated portions 74 extending therefrom. A similar electrode 76
is affixed to the opposite face of the crystal and is provided with
the same configuration to increase the contour or peripheral
dimension for the area of the electrode.
A header 80 is provided, and two lead connections 82 and 84 are
supported within a body of insulating material 86 carried by the
header. This insulating material may be similar to that employed in
transistor headers. Lead connections 82 and 84 are secured to
electrodes 72 and 76 respectively by conductive connecting portions
90 and 92 respectively such as silver paste or by welding and the
like. Here again, the relative sizes of the free crystal area and
the electrode area conform with the aforementioned
requirements.
The crystal of the sensing means is protected by a cap 96 which
slips over the header and has a large number of holes 98 formed
therethrough which permit a polar vapor to reach the surface of the
crystal. It is apparent that many other types of protective
housings may be provided for supporting and protecting the sensing
means of the present invention.
Referring now to FIG. 5, an electrical circuit is illustrated
incorporating the sensing means of the present invention. The
capacitor sensing means of any of the previously described forms of
the invention may be employed as the capacitor 100 which is one
member of a balanced Wheatstone bridge. The bridge balance can be
set by the variable capacitor 102 and the variable resistor 104 and
by proper choice of resistor 108. An CA voltage is impressed across
terminals 110 and 112 which causes the bridge is be driven across
terminals 110 and 118, the potential difference between terminals
114 and 116 is zero for the balanced bridge.
The terminals 114, 116 and 118 of the bridge are connected to a
comparator 120 which may comprise a differential amplifier. The
output of the amplifier is in turn connected with an electrical
indicating instrument 122 such as a milliammeter or the like.
When the sensing capacitor 100 is subjected to polar vapors, the
capacity or resistivity or both varies and upsets the balance of
the bridge. As a result, the output of comparator 120 supplies a
voltage or current to the indicating device and the amount of vapor
present in the environment can be read out. It is apparent that a
suitable recording instrument could also be employed in place of
the indicating means 122.
Referring now to FIG. 6 of the drawings, the sensing capacitor 130
may be of the construction illustrated in any of the forms shown in
FIGS. 1-4, for example, in accordance with the present invention.
This capacitor is also connected in a Wheatstone bridge arrangement
including a variable capacitor 132, a variable resistor 134 and a
resistor 136. The means for providing the AC drive in this form of
the apparatus comprises a conventional small transistor Colpit
oscillator 150 connected through a coupling capacitor 152 with the
Wheatstone bridge. The oscillator is provided with a suitable DC
current in the usual manner. The oscillator provides the required
alternating current and proper voltage which is set so that it does
not switch the ferroelectric crystal sensing element around the
hysteresis loop.
The bridge balance can be set by the variable capacitor 132 and the
variable resistor 134 and by proper choice of resistor 136.
The output of the Wheatstone bridge is connected with the input
terminals 154 and 156 of an operational amplifier 160. The
amplifier is a commercially available Amelco type integrated
circuitry operational amplifier, type 809C, and manufactured by
Amelco Semiconductor, Division of Teledyne, Inc., 1300 Terra Bella
Avenue, Mountain View, California. This is a well known and widely
used operational amplifier prepared on a very small silicon chip
encased in a TO5 can.
Between the output terminal 162 and the input terminal 154 of the
amplifier, a negative feedback is provided through resistor 164.
Between the terminals 166 and 168 of the amplifier, resistor 172
and capacitor 174 are connected to provide compensating components
to avoid oscillations of the amplifier. The resistor 176 connected
between input terminal 156 and ground also forms part of the
compensating network. A positive battery voltage is connected to
terminal 180, and the negative battery voltage is connected to
terminal 182.
When the operational amplifier is fed from a balanced bridge, no
output appears at terminal 162. However, if the sensing means or
capacitor 130 of the present invention is subjected to polar vapor,
the capacity or resistivity or both change according to the amount
of polar vapor present and the bridge is unbalanced. Accordingly,
the output of the operational amplifier will be proportion to the
amount of vapors deposited on the surface of the crystal of the
sensing means 130.
The output terminal 162 of the amplifier is connected through
resistors 190 and 192 to a silicon-controlled rectifier 194 which
has a load impedance 196 series with a positive voltage source. The
silicon-controlled rectifier is biased by a negative potential
source 198. As the potential on the output terminal 162 of the
amplifier increases, it is possible to set by varying the resistor
190 a certain level at which the silicon-controlled rectifier
becomes conductive and switches a suitable control device such as a
relay, motor and the like by placing the driving coil or resistance
of such a control device in place of the load impedance 196.
Accordingly, it is possible to set a level for initiating control
of a humidifier or dehumidifier so that suitable humidity control
apparatus may be energized and deenergized by the circuit. By
setting the bias level, the switching operation may be controlled
so as to produce a desired level of humidity. It is apparent that
instead of the silicon-controlled rectifier, a magnetic relay or
transistor switch and the like may also be employed.
The use of an operational amplifier in the circuit illustrated in
FIG. 6 increases the sensitivity of the arrangement considerably
and also provides a linear relationship between the ferroelectric
crystal sensing means output and the output of the operational
amplifier.
Referring now to FIG. 7 of the drawings, two electronic oscillators
200 and 202 are provided, these oscillators being disposed within a
sealed envelope indicated schematically by dotted line 204.
Oscillator 200 is used as a reference oscillator and includes a
stable nonvariable capacitor 208 which sets a stable frequency.
This could also be a crystal stabilized oscillator if desired.
The second oscillator 202 is tuned by ferroelectric sensing means
210 according to the present invention, this particular sensing
means being similar to that shown in FIG. 4 of the drawings.
Capacitor 210 will of course change capacity under the influence of
polar vapors.
The two oscillators are coupled through a mixer 212 so as to
produce a beat frequency which is proportional to the capacity
change caused by the amount of polar vapor deposited on the sensing
means 210. It is of course apparent that suitable means is provided
for providing access of the ambient air to the crystal of sensing
means 210 for detecting the presence of polar vapor.
The initial beat frequency of the circuit can be set by tuning the
oscillator 200 to the oscillator 202, and this permits a desirable
scale for the humidity range to be set. The output of the sensing
means as illustrated in FIG. 7 would be a digital output, since the
beat frequency can drive digital counters and display devices. As a
result, the humidity data may be transmitted through a telephone
line because attenuation or fading of signals does not affect the
frequency.
It is apparent from the foregoing that there is provided according
to the present invention new and novel polar vapor sensing means
which is responsive to changes of polar vapor content, such as
humidity and the like in the ambient environment, and that the
output of the circuits according to the present invention may be
employed for providing an indication of the vapors or for operating
suitable recording equipment or control devices for operating any
suitable desired apparatus.
It is apparent from the foregoing that this invention may be
embodied in several forms without departing from the spirit or
essential characteristics thereof. The present embodiment is
illustrative and not restrictive, and since the scope of the
invention is defined by the appended claims, all changes that fall
within the metes and bounds of the claims or that form their
functional as well as conjointly cooperative equivalents are
therefore intended to be embraced by those claims.
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