Highly effective sensor for the vapors of volatile organic compounds

Abstract

Molybdenum oxide is doped with vanadium, nobidium, tantalum or titanium to form a stable M.sub.5O.sub.14 theta phase crystal structure as a vapor sensitive material for detecting vapors of volatile organic compounds. That material is used between electrodes connected to a measuring device to measure a change in an electrical quality in the presence of a vapor of a volatile organic compound. Concentration of the vapor is measured to low part per million ranges.

Claims

1. An apparatus comprising a sensor for detecting vapors of organic compounds, comprising plural electrodes, a single layer of vapor sensitive material in contact with each of the plural electrodes, the vapor sensitive material being molybdenum oxide doped with sufficient vanadium, niobium, tantalum or titanium to form a stable M.sub.5O.sub.14 theta phase crystal structure, where M is predominantly molybdenum and which exhibits a response in a form of an increase or a decrease in an electrical property of the vapor sensitive material in presence of a vapor of a volatile organic compound.

2. The apparatus of claim 1, wherein the sensor detects vapor of a liquid volatile organic compound.

3. The apparatus of claim 2, wherein the sensor detects ppm concentrations of isobutylene vapor in air.

4. A method comprising effecting determination of a presence of a vapor of a volatile organic compound which comprises providing a vapor sensor, providing plural electrodes, providing a single layer of vapor sensitive material and each of the plural electrodes in contact with the vapor sensitive material, wherein providing the vapor sensitive material comprises providing molybdenum oxide doped with sufficient vanadium, niobium, tantalum or titanium to form a stable M.sub.5O.sub.14 theta phase crystal structure, where M is predominantly molybdenum, wherein the vapor sensitive material exhibiting a response in the form of increasing or a decreasing in an electrical property of the vapor sensitive material in the presence of the vapor of the volatile organic compound by detecting the change in the electrical property of the vapor sensitive material, measuring the change in the electrical property and determining a concentration of the vapor of the volatile organic compound.

5. The method of claim 4, further comprising sensing the vapor and determining the concentration of the vapor of the volatile organic compound in presence of humidity.

6. The method of claim 4, wherein the vapor is vapor of liquid organic compound.

7. The method of claim 4, wherein the measuring further comprises measuring resistance of the vapor sensitive material, measuring capacitance of the vapor sensitive material or measuring impedance of the vapor sensitive material.

8. The method of claim 4, further comprising determining a concentration of the vapor of the organic compound in the presence of air and humidity.

9. The method of claim 4 comprising determining concentration of isobutylene.

10. The method of claim 4 comprising determining concentration of benzene in air.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1-4 show examples of sensor structures and electrical components shown in U.S. Pat. No. 6,173,602.

(2) FIG. 5 shows sensitivity of two tantalum-doped Mo.sub.5O.sub.14 sensors to repeated exposures to a sequence of concentrations of 20 ppm, 10 ppm and 5 ppm isobutylene in air.

(3) FIG. 6 shows relative humidity changes during the time periods shown in FIG. 5.

(4) FIG. 7 shows the varied ratio of isobutylene in the air at the time periods shown in FIG. 5.

DETAILED DESCRIPTION

(5) FIG. 1 shows a sensor 9 comprising a gas sensitive material 4 and, in contact with the gas sensitive material, gold electrodes 2 and 3. The gas sensitive material may be carried by a substrate, e.g. of alumina, not shown.

(6) Conductors 5 are provided to connect the electrodes 2 and 3 respectively to electrical measuring means 6 for measuring the resistance and/or capacitance, and/or impedance of the vapor sensitive material 4.

(7) In operations vapor of a volatile organic compound in air or in a gaseous mixture is contacted with the vapor sensitive material 4.

(8) The resistance and/or conductance, and/or impedance are measured by the electrical measuring means 6. Changes in the composition of the vapors which result in a change of resistance and/or conductance, and/or capacitance, and/or impedance are observed as changes in the resistance and/or conductance, and/or capacitance and/or impedance recorded by the measuring means 6. Sensor 9 may include a temperature sensor 17 for sensing temperature and a heater 15 for heating the sensor.

(9) FIG. 2 shows (in plan view) an insulating substrate 1, for example an alumina or ceramic tile, upon which is formed a first electrode 2, for example of gold, a vapor sensitive material layer 4 comprising a vapor sensitive material in accordance with the present invention and a second electrode of gold.

(10) A parallel plate sensor 9, as shown in FIG. 2, may be fabricated by applying the first electrode 2 of gold to the insulating substrate 1 by screen printing or sputtering and a vapor sensitive material layer 45 by deposition by screen printing or doctor blading from a suspension or a colloidal dispersion and firing at a temperature in the range 450-950° C. to promote adhesion and mechanical integrity. A second electrode 3 of gold is formed on the vapor sensitive material layer 4 by screen printing or sputtering.

(11) FIG. 3 shows a parallel plate sensor 9 of the type shown in FIG. 2 partially completed, inasmuch as the second electrode 3 has not been formed. FIG. 3 shows the insulating substrate 1, the first electrode 2, and the vapor sensitive material layer 4. The portion of the first electrode 2 covered by the vapor sensitive material layer 4 may preferably extend in area to substantially the same extent as the second electrode 3.

(12) The first electrode 2 and second electrode 3 are connected to an electrical measuring means for measuring the resistance and/or capacitance, and/or impedance of the vapor sensitive material layer 4, and the sensor is contacted with air or a gas or gaseous mixture which may contain a vapor of a volatile organic compound. The resistance and/or capacitance, and/or impedance is measured by the electrical measuring means, and changes in the concentration of the vapor are observed and measured as changes in the resistance, capacitance, and/or impedance recorded by the electrical measuring means.

(13) FIG. 4 shows an insulating substrate 1, an alumina ceramic tile, upon which are formed electrodes 2 and 3, both of gold, and a vapor sensitive material layer 4. Portions of the first electrode 2 and second electrode 3 covered by the vapor sensitive material layer 4 are interdigitated.

(14) Sensors composed of the vapour detecting materials are selective in that they display virtually no response to changes over a wide range of relative humidity, along with very little response to the presence of oxidizing gases.

(15) FIG. 5 shows sensitivity of two tantalum-doped Mo.sub.5O.sub.14 sensors to repeated exposures to a sequence of concentrations of 20 ppm, 10 ppm and 5 ppm isobutylene in air. The two sensors, sensor 51 and sensor 52, are both made from similar vapor sensitive material. The difference in output is due to the difference in thickness of the vapor sensitive material. The data clearly illustrate the very high sensitivity of the material. Sensitivity indicated on the y-axis or ordinate of FIG. 1 is effectively the resistance Ro of the sensing element in clean air divided by the sensing element's resistance Rg when it is exposed to the target vapor.

(16) FIG. 6 shows relative humidity changes during the time periods shown in FIG. 1.

(17) FIG. 7 shows the varied ratio of isobutylene in air at the time periods shown in FIG. 1. The isobutylene to air ratio was periodically reduced from 20 ppm, to 10 ppm, to 5 ppm and then to zero ppm. At about 750 minutes the isobutylene supply was terminated.

(18) While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention, which is defined in the following claims.