US3621343A

US3621343A - Polar vapor sensing means

Info

Publication number: US3621343A
Application number: US5354A
Authority: US
Inventors: Charles F Pulvari; Stephen F Urban
Original assignee: Individual
Current assignee: NL Industries Inc
Priority date: 1970-01-23
Filing date: 1970-01-23
Publication date: 1971-11-16
Anticipated expiration: 1988-11-16

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.

Description

United States Patent [72] Inventors Charles F. Pulvari Washington, D.C.; Stephen F. Urban, Kenmore, N.Y. [21] Appl. No. 5,354 [22] Filed Jan. 23, 1970 [45] Patented Nov. 16, 1971 [73] Assignee NL Industries, Inc.

New York, N.Y. by said Urban Continuation-impart of application Ser. No. 709,642, Mar. 1, 1968, now abandoned.

[54] POLAR VAPOR SENSING MEANS a 18 Claims, 7 Drawing Figs. [52] US. 317/231, 317/262, 324/61 [51] Int. Cl 01g 9/00 [50] Field 01 Search 317/230, 231, 238, 258, 262; 106/39; 324/61 [56] Reierences Cited UNITED STATES PATENTS 2,924,814 2/1960 Simpson 317/262 Primary Examiner-lames D. Kallam Attorneys-Charles F, Kaegebehn, Robert L. Lehman and Fred Floersheimer 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.

POLAR VAPOR SENSING MEANS CROSS REFERENCE TO THE RELATED APPLICATION The present application is a continuation-in-part of copending US. Pat. application Ser. No. 709,642, filed Mar. 1, 1968, now abandoned.

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 willsuggest 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 resister 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.

SUMMARY OF THE INVENTION 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 IO -l 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 retuming 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 effeet 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 C., the dehydroxylation of the surface is reversible, but at temperatures above 400 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 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 l-l0 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 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.

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 l0l0 ohms is obtained when subjected to polar vapors as contrasted to a resistance of l0-l0 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 semiinsulating 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 e' l9 and a very high Curie temperature such as approximately 675 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 ,/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 pennit 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.

BRIEF DESCRIPTION OF THE DRAWINGS 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 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 40 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.

7 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 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 C. for at least about i 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 formarnide 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,/C. C,

TABLE 1 Dimothyl formarnldo H O ratios Linseed oil ratios Oleic acid ratios ratios Acetic acid ratios Group C. C./O GJG C./C G./G 0.]0 (L/U C./C (14G C./C 0.](1

Sr 23. 5 3 1 1 1 1 14 7. (l 36 5. ti Ba 1.3 8.2 1 1 1 1 1.3 1.0 2.3 16.0

1 1 1 1.4 2.4 2.8 iii 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,/G. G, 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 elecroom 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 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 posi tive 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 slsst l ad i sslsq tht s ss bs 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 afiixed 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 fonns 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 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. 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 Ee usu al manner. The oscillator provides the required alt ernating current and proper voltage which is set so that it does not switch the ferroelectric crystal sensing element around the 3 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 tenninals 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 T05 can.

Between the output terminal 162 and the input terminal 154 of the amplifier, a negative feedback is provided through resistor 164.

Betweenthe 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 tenninal 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 siliconcontrolled 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 siliconcontrolled 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 mag netic 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 Z QQ an d 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 thesensingriieans iiilffiis ofcotirs e 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 ac cording 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 equipmentor 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 bythe 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.

What is claimed is:

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 witira polar vapor wherebythe 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 1210,000, and at least two separate and spaced electrically conductive electrodes electrically connected with said layer.

2. Apparatus as defined in claim 1 including means for impressing an AC voltage across said electrodes, and a load impedance connected with said electrodes.

3. Apparatus as defined in claim 1, wherein said body is insoluble in water.

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 the crystal.

5. Apparatus as defined in claim 1, wherein said body is selected from the group consisting of ferroelectric crystal, ferrielectric crystal, ferroelectric glass ceramics, quartz and glassy compositions.

6. Apparatus as defined in claim I, wherein the surface portions of said body to which said electrodes are connected are highly polished.

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 microns.

8. Apparatus as defined in claim 1, wherein said surface layer provides a continuous surface conductive path between said spaced electrodes.

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 sodium niobate.

10. Apparatus as defined in claim 1, wherein said body comprises a ferroelectric crystal having additive means selected from the group consisting of hafnium, zirconium, niobium and tantalum.

11. Apparatus as defined in claim I, 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 group consisting of sEonTiTIrhT lanthanum, zirconium and chromium.

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 the group consisting of the rare earths.

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 periphery of the electrodes being large relative to the area thereof.

15. Apparatus as defined in claim 1, wherein at least one of said electrodes includes a plurality of relatively narrow elongated portions defining a large contour for the area thereof.

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 electrodes have a large peripheral dimension for the area thereof.

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 increas-

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 electrodes electrically connected with said layer.

3. Apparatus as defined in claim 1, wherein said body is insoluble in water.

6. Apparatus as defined in claim 1, wherein the surface portions of said body to which said electrodes are connected are highly polished.

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 group consisting of strontium, lanthanum, zirconium and chromium.

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 electrodes to prevent damage thereto.

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.