GB2243917A - Gas sensing device - Google Patents

Abstract

A device for detecting the presence of trace levels of organic nitro-compounds such as nitrobenzene or ethylene glycol dinitrate comprises a sensor element 14 comprising a film of halo-aluminium phthalocyanine or halo-aluminium naphthalocyanine and at least one further sensor element 15 which is not sensitive to organic nitro-compounds. Electronic circuitry 18 compares the conductivity of the two sensor elements, a differential indicating the presence of organic nitro-compounds. The device may comprise two air tight cylinders 10, 11, the inner cylinder 10 being rotatable through 180 DEG . Vacuum pump 17 draws air in over preconcentrators 12, 13. Cylinder 10 is then rotated through 180<0> bringing the preconcentrators 12, 13 into close proximity with the sensor elements 14, 15. Sensor element 14 is sensitive to organic nitro-compounds and interfering gases whereas sensor element 15 is sensitive to interfering gases only. <IMAGE>

Description

GAS SENSING DEVICE The invention relates to a sensor device for the detection of trace levels of vapours of organic nitro-compounds.

There is a requirement for highly specific monitors which can identify the presence of vapours of organic nitro-compounds in the air environment.

One such compound is nitrobenzene (NB), a toxic substance widely used in industry which needs to be monitored for health and safety reasons to protect personnel working in the vicinity of industrial plant. Another compound of interest is ethylene glycol dinitrate (EGDN), a component commonly used in commercial explosives and which has a relatively high vapour pressure. Detection of EGDN in the atmosphere can therefore be used as an indication of the presence of explosive material in the vicinity of the detector.

Demand for the development of a portable vapour detection device for detecting organic nitro-compounds is by no means new and a large number of potential techniques have been investigated. These include: nuclear detection techniques; electron captive detection; gas chromatography; ion mobility spectrometry; plasma chromatography, bioluminescence; and solid state microelectronic chemical sensors. These all suffer from various problems such as susceptibility to interference, lack of sensitivity, lack of robustness, short lifetime and high cost. Chemical sensors have shown the most promise, in particular sensors which utilise changes in electrical conductivity of a semiconducting material.

A semiconductor suitable for use in a gas sensing microelectronic device must meet certain criteria. It must have measurable conductivity and be heat resistant to temperatures high enough to allow the chemisorption reaction of interest to proceed quickly and reversibly and also to prevent water vapour condensing on the surface of the film. Thus it must be resistant to relatively high temperatures and yet be capable of producing coherent films by sublimation of deposition from solution. The material should also be highly pure and resistant chemically to degradation by atmospheric gases. It is also desirable that the material used in the construction of a sensing device can be synthesised easily and cheaply.

Most importantly, the conductivity of the semiconductor must change reproducibly and reversibly with the concentration of the gas in ambient air.

The conductivity of an organic semiconductor can change by many orders of magnitude when a gas adsorbs on its surface, thus making such materials ideal for detecting very low concentrations of gases in the environment.

For example, 1 ppb of NO2 can be detected by measuring semiconductivity changes in metal derivatives of phthalocyanine. The chemisorption of gas molecules on the surface of a semiconductor may lead to transfer of an electron or electrons from one to the other depending upon the electronegativity of the gas and the electronic structure of the solid.

Either surface conductivity or surface and bulk conductivities can be changed.

Although the presence of certain gases has long been known to affect the electrical conductivity of some solid organic materials, the interest in the past has been mainly in transition metal oxides, some of which now form the basis of commercially available sensors.

The use of organo-metallic semiconductors such as metallo-phthalocyanines for the detection of reactive gases such as NO2 and C12 has been known for a few years. Many materials have been investigated for the detection of these, and other, strong electron acceptor gases.

However, detection of gases which are not so reactive is also required, such as, for example, vapours emitted by explosives materials.

Furthermore, many explosives materials have very low vapour pressures and thus the lower detection limit of a sensing device will need to be down to ppb levels.

The invention provides a sensor for the detection of trace levels of vapours of organic nitro-compounds comprising at least two sensor elements and means to measure changes in the conductivity of the sensor elements wherein a first sensor element comprises a film of a halo-aluminium phthalocyanine or a halo-aluminium naphthalocyanine and one or more further sensor elements is adapted to detect levels of interfering gases but is not sensitive to organic nitro-compound vapours.

Preferably the sensor device is suitable for the detection of trace levels of vapours, such as NB, emitted by some industrial processes and for the detection of vapours, such as EGDN, emitted by explosive materials.

Preferably the first sensor element comprises a film of one of chloro-aluminium phthalocyanine (AlpcCl), fluoro-aluminium phthalocyanine (AlpcF), chloro-aluminium naphthalocyanine (AlncCl) or fluoro-aluminium naphthalocyanine (AlncF).

Preferably the first sensor element comprises a film of one of these compounds deposited on a substrate with long electrode length, short electrode separation, good thermal conductivity and high electrical resistance. Advantageously the substrate is aluminium oxide as this enables the temperature of the substrate to be maintained and varied over a wide range and films can easily be deposited on it.

Nitrogen oxides (NOx) are the main interfering gases present in the air.

Advantageously at least one of the further sensor elements is sensitive to NOx gases such that the effect of those gases on the detection of the target vapour can be eliminated or significantly reduced. The or each further sensor element may advantageously be a film or material known to be sensitive to particular NOx gases but not to the target vapour which may be NB or EGDN. Possible materials are the metal derivatives of phthalocyanine or the metal, especially copper,derivative of tetraazadibenzo (14) - annulene (M-TADA, especially Cu-TADA).

The concentrations of the target vapours in air are likely to be very low, in the ppb range, and thus the sensor device is required to be extremely sensitive. The sensitivity can be increased by including a preconcentrator for each sensor element in the sensor device. The preconcentrator provided for the first sensor element is preferably a conventionally-prepared metallic surface of a type specifically for explosives vapours. Enrichment factors in the range 100 to 1000 are generally obtainable.

The sensitivity of the sensor device may advantageously be further increased by providing a secondary means to detect the presence of organic nitro-compounds. This may involve the inclusion of means to heat the preconcentrator associated with one of the further sensor elements sufficiently to cause thermal cracking of any adsorbed explosives vapours, the nitrogen oxides resulting from the fragmentation then being detected in addition to the background levels of nitrogen oxides. Background levels alone are detected by another sensor element and the result subtracted from the first to enable a value for the levels of nitrogen oxides obtained by the cracking of the target vapours to be deduced. This provides a secondary detection means to the direct detection offered by the first sensor element.

The invention further provides an explosives sensor device comprising at least two chambers and pumping means to enable air to be drawn into and extracted from each chamber, wherein each chamber has a dividing means which in one position separates first and second parts of the chamber in a gas tight manner and in a second position enables the first and second parts of the chamber to communicate, and wherein the first part of each chamber houses a vapour concentrator and the second part of each chamber houses a sensor element, a first sensor element comprising a film of a halo-aluminium phthalocyanine or a halo-aluminium naphthalocyanine and at least one further sensor element being adapted to detect levels of interfering gases but not being sensitive to organic nitro-compound vapours, and there is included means to drive adsorbed vapour off the vapour concentrators for detection by the sensor elements, and means to provide an output signal related to the concentrations of detected gases from each sensor element.

By subtracting the output from the or each further sensor element from the output from the first sensor element a sensitive detection of organic nitro-compounds is provided.

At least one of the further sensor elements is advantageously sensitive to NOx gases and may conveniently be of a compound such as a metal-TADA derivative, for example Cu-TADA.

In further variations more chambers may be provided with sensor elements for detecting specific explosives vapours or interfering species.

The sensitivity of the electrical response of organic films to a given vapour is dependent upon chemical structure while the rate and reproducibility of the response are influenced by the crystallographic features of the film which determines the strength and distribution of various active adsorption sites.

The preparation of the first sensor element preferably comprises vacuum ( < 10'4 mm Hg) sublimation of the organic film material onto a substrate such that the rate of deposition of the film is controlled to be less than about 1 A per second. This gives a smooth and uniform film of a better quality than faster sublimation rates. This leads to improved response times, and improved reproducibility of results. Alternatively the sensor may be prepared by Organic Molecular Beam Epitaxy, which produces very high quality films but is very expensive.

The further step of heat treating the film, for example by baking for about 12 hours at about 1800C in air and then heat treating under vacuum ( 10~2mum Hg) at about 240"C for about 12 hours further, enhances the crystallisation of the film and hence leads to increased active site populations.

Preferably the method also includes the further step of exposing the film to saturated vapours of target materials to encourage the creation of strongly adsorbed vapour molecules to obtain maximum film sensitisation.

Hereinafter, the phthalocyanine ring (C32H16N8) will be denoted by "pc" and the naphthalocyanine ring (C48H24N8) will be denoted by "nc".

The invention will now be described, by way of example only, with reference to the drawings, of which: Figure 1 shows the structure of phthalocyanine (a) and the cofacially stacked polymeric derivative fluoro-aluminium phthalocyanine, (AlPcF) (b); Figure 2 shows graphs of the rate and magnitude of response of a fresh AlpcCl film1 at a substrate temperature of 60"C, to saturated vapours of nitrobenzene after (A) and before (B) completing the film sensitization process; Figure 3 shows a graph of the rate of reversal of the electrical response of a fresh, initiated AlpcCl film at 150 C substrate temperature after pumping the target vapour away;; Figure 4 shows graphs of the response kinetics of AlpcF film at 600C to saturated vapour of nitrobenzene at 53 C, overall (A) and initially (B); Figure 5 shows a graph of the electrical response of an AlpcCl film on exposure to saturated vapours of EGDN at 80do; Figure 6 shows a graph of the rate of reversal of conductivity of an AlpcCl film after exposure to EGDN at a substrate temperature of 240"C; Figure 7 shows a graph of the electrical response of an (AlpcF)n film on exposure to vapours of EGDN; Figure 8 shows a graph of the rate of reversal of an (AlpcF)n film current, after exposure to EGDN, at a substrate temperature of 200"C;; Figure 9 shows graphs of the rise in current of an AlncCl film at 80"C on exposure to static vapours of nitrobenzene (60"C, approx 14.8mg/l) for the initial (B) and long exposure (A); Figure 10 shows graphs of the kinetics of response (A) and the initial rate of response (B) for a baked AlncCl film to saturated vapours of EGDN (60"C) at a substrate temperature of 80"C; Figure 11 shows a graph of the rate of conductivity reversal of an EGDN doped AlncCl film at a substrate temperature of 2400C; Figure 12 shows a schematic drawing of an explosives sensor device according to the invention;; Figure 13 shows a typical response of initial (A) and general (B) rate of current rise achieved using an explosives sensor device of the type shown in Figure 12 for the detection of EGDN using AlncCl film (80"C) and a platignum preconcentrator triggered at 1860C when fully loaded.

The structure of the basic compound phthalocyanine (pc-H2) is shown in Fig.l(a). It consists of four isoindole units linked by four nitrogen atoms. The structure of the fluoro-aluminium derivative (AlpcF) is shown in Fig.l(b). This is a polymer with (Al-F)n backbones. The backbones are surrounded by a sheath of phthalocyanine rings stacked face to face. In contrast AlpcCl is a square pyramidal non-polymeric phthalocyanine complex. Both derivatives exhibit a very low solubility and volatility.

The naphthalocyanine derivatives are of similar structures to their phthalocyanine counterparts with AlncF being polymeric and AlncCl being monomeric.

For electrical measurements the solid films of the organic semiconductors were made on a commercially available alumina substrate (Platfilm, Rosemount Engineering, Bognor Regis, UK) using an Edwards Model E306A coater. The film currents are measured by passing current from a variable DC power supply unit through the semiconducting film and into an amplifier.

Parameters such as rate of coating, film thickness and substrate temperature, which control the conditions for sublimation of the film onto the substrate, must be controlled carefully to achieve consistent results.

The thermal conditions during sublimation markedly influence such properties as crystalline structure and perfection and the tensile stresses which might result if any differential thermal contraction of the substrate and film takes place.

A very slow rate of depositon ( < 1 A/s) gives a smooth and uniform film, with consequently better film quality.

Following depositon the films are heat treated to enhance the crystallisation, which leads to increased active site populations, giving improved electrical responses.

The heat treated films were then exposed to saturated vapours of target materials at moderate temperatures for several hours to encourage the creation of traces and strongly adsorbed vapour molecules to obtain full film sensitisation.

In the description of the following Figures the current-voltage characteristics of each individual film with applied d.c.field, and d.c.

conductivity as a function of film temperature and time are described.

The film currents in all the measurements will correspond to an applied voltage of 12 volts unless otherwise stated.

Figure 2 shows the rate and response of a film of AlpcCl to saturated vapours of nitrobenzene (NB). As shown in Figure 2(b) the rate and magnitude of response before full initiation to the vapour is very discouraging. However, as shown in Figure 2(a) the rate and magnitude of response are significantly improved after initiation. The film current on exposure to NB vapour (6.05mg/l) at equilibrium state is about 105 times higher than the current of the undoped film under vacuum (about 2xlO" A). The semiconducting film exhibits a maximum electrical response if the alumina base is maintained at about 60"C. Higher substrate temperatures tend to prevent the charge transfer interaction between the film and the target vapour from taking place and a rise in film temperature above 150 0C is adequate to drive the adsorbed vapour molecules off the surface of the AlpcCl film, leading to a sharp drop in the film current. The current eventually approaches the undoped film level if heating is continued for a sufficient length of time. The ease with which the conductivity effect can be reversed is shown by the plot in Figure 3.The removal of the target vapour followed by a rise in the film temperature to 1500C leads to a sharp drop in the film current from an equilibrium value of about 3.5x10-6A to a level corresponding to the current of an undoped AlpcCl film of about 2xlO-tlA. A subsequent reduction in the film operating temperature in the presence of NB gives rise to a gradual increase in the semiconductivity of the film. This effect can be reproduced successively and has no visible effect on the lifetime of the AlpcCl film.

Referring now to Figure 4, it can be seen that exposure of a fresh thick film of AlpcF, after heat treatment at 15O0C for 20 hours, to vapour of NB (6.05mg/l) under static conditions produces a very rapid response. The rate and magnitude of the initial response are shown in Figure 4(B) whereas Figure 4(A) illustrates the effect over a longer period of time.

The graphs show that the magnitude of response of a fully sensitized AlpcF film to NB vapour is increased by about five orders of magnitude (105 to 106) when compared to the current of an undoped film under vacuum (about 2xlO-llA) An increase in the temperature of the alumina base up to 2000C leads to some reduction in the semiconductivity but the inability to reverse the electrical response fully in this range contrasts with the behaviour of AlpcCl, indicating that the chemisorption interaction between the NB molecules in the vapour phase and the active sites on the AlpcF film is much stronger and may not be broken off by moderate heat. Speedy and complete reversal of the electrical response is only possible if the film temperature is raised to above 2500C.At this temperature the conductivity initially jumps to 10-5A and then drops suddenly to give a film current of 5x10-7A in a matter of 2 minutes. The film current drops to a value corresponding to the undoped film conductance in less than 20 minutes. Reducing the temperature of the substrate to the optimum temperature (60 CC) in the presence of NB vapour results in a rapid rise of the film current again. This process can be demonstrated repeatedly with no visible effect on the quality of electrical response of the sensitized film to nitrobenzene vapour.

Figure 5 shows the rate and magnitude of electrical response of a fresh heat-treated AlpcCl film on exposure to saturated vapours (60"C) derived from an EGDN sample. The charge transfer interaction increases the film current from a vacuum level of lxlO-l A to over lx10-7A thus increasing the conductivity by at least three orders of magnitude.

The rate of conductivity reversal of the AlpcCl film becomes noticeable only when the substrate temperature is increased to about 200"C, though a temperature of about 2400C is needed to give rapid desorption of the adsorbed vapours, as shown in Figure 6.

The behaviour of AlpcCl films on exposure to EGDN at other temperatures has been examined. An increase in the substrate temperature in the presence of the target vapour leads to a corresponding increase in the film current up to 160 cm. Above this temperature the current no longer rises, due to desorption, suggesting that the optimum charge-transfer interaction occurs below a film temperature of 160 cm.

Figure 7 shows the rate and magnitude of electrical response of a fresh heat treated (AlpcF)n film at 800C on exposure to saturated vapours (600C, 1.7 mbar) of EGDN. The film current is increased from a vacuum conductivity of about 1x10-10 amps to over 5x10-7 amps.

The film current in the presence of vapours increased as the substrate temperature was raised to about 1600C. A complete reversal of the conductivity is achieved if the film is maintained for a sufficient length of time at a temperature above 200"C.

The naphthalocyanine macrocycles exhibit a number of advantages when compared with their phthalocyanine counterparts. Results of the response of AlncCl to NB and EGDN are shown in Figures 9 and 10. The applied voltage to the films was 9 volts.

At moderate temperatures the dark conductivity of fresh, heat-treated AlncCl in vacuum is about lxlO-t A. A rise in the film temperature of 240 CC increases the current to about 2x10-8A.

The kinetics of response of AlncCl film on exposure to saturated vapours of NB (at 60 ) at a substrate temperature of 80"C are shown in Figure 9.

The current shows a two orders of magnitude increase over that of an undoped film in vacuum, though the initial rate of conductivity rise is slow.

Exposure of a film of AlncCl to saturated vapours of EGDN (60cm, 1.7mbar) at a substrate temperature of 80"C induces the electrical response shown in Figure 10. An increase in the film current by 3 orders of magnitude, from an undoped value of lxlO-t A to about 7x10-8A is observed. This suggests the chemisorption of the EGDN molecules on the film surface and the subsequent generation of charge carriers. An increase in the film temperature up to 160 CC induced a corresponding increase in the film current.

The rate of reversal of conductivity of a fully EGDN-doped AlncCl film under vacuum at 2400C is shown in Figure 11. This appears to be the minimum temperature necessary for a complete desorption of EGDN molecules in an acceptable timescale.

Figure 12 shows an explosives sensor device which enables a reading of the presence of target vapours and possible interfering species to be obtained. The main body consists of two air-tight PTFE cylinders 10,11, with the internal cylinder 10 being moveable by 180C during operation, either manually or electrically. Heating wires (not shown) can be wound into the external surfaces of the cylinders 10, 11 to prevent condensation of the explosives vapours and possible build up of a high background concentration.

Two small platinum coil preconcentrators 12, 13 are mounted within the internal cylinder 10 and two sensor heads 14, 15 are mounted within the external cylinder 11. A three way solenoid valve 16 and a vacuum pump 17 control air flow through the device and a printed circuit board 18 provides the necessary electronic circuitry to drive the device and produce output signals to an operator.

Operation of the device is simple: ambient air is drawn over the two platinum coils 12, 13 by the pump 17 for a number of seconds to collect sufficient quantities of explosive vapours for the detection stage. The internal cylinder 10 is then turned by 1800 to position the two preconcentrators 12, 13 parallel to and within a few millimetres of their corresponding sensor heads 14, 15. One of the sensor heads1 14, is of a material sensitive to organic nitro-compound vapours such as NB or EGDN, for example AlpcF1 and the other sensor head, 15, is of a material sensitive to nitrogen oxides but not to organic nitro-compounds, for example Cu-TADA.

The turning movement of the cylinder 10 automatically interrupts the flow of ambient air through the pump 17. A change in the switch mode of the solenoid valve 16 creates a partial vacuum in each of the two small housings 19, 20 containing the sensor elements 14, 15 and the preconcentrators 12, 13 before the pumping action is interrupted. At the final step, the preconcentrators 12, 13 are heated by passing a current through the coils to drive off adsorbed vapours for detection by the sensors 14, 15 by inducing changes in their respective conductivities.

Sensor head 14 detects the presence of explosives vapours and sensor head 15 detects the presence of interfering species such as NO2. A comparison between the readings is made by the electronic circuitry 18 and if the presence of explosives vapours is confirmed a danger signal is given.

The reversal of the conductivity increases by raising of the temperatures of the sensor films 14, 15, followed by pumping off of the evolved vapours terminates the operation, leaving the device ready for the next measurement.

Figure 13 shows the results obtained using a device of the type shown in Figure 12, having a sensor head 14 of AlncCl, for the detection of EGDN vapours. The platinum preconcentrator 12 adsorbs EGDN vapour and when it is fully loaded the cylinder 10 is rotated and the preconcentrator 12 then heated to a triggering temperature of 186"C to drive off the adsorbed vapours for detection by the AlncCl film of sensor head 14. The initial flat region of the plot represents the film background current before triggering the coil at point "T". A noticeable rise in the film current was observed after triggering, with a good signal-to-noise ratio.

A device of the type shown in Figure 12 is not limited to two cells but may have further cells for example containing: a pyrolytic sensor, such as GapcCl or AlncCl for detecting decomposition products of explosives; AlpcF sensor for dual detection of NB and EGDN vapours;and AlpcCl or AlncCl sensor for detection of EGDN vapours.

Claims (11)

CLAIMS:

1. A sensor device for the detection of trace levels of vapours of organic nitro-compounds comprising at least two sensor elements and means to measure changes in the conductivity of the sensor elements wherein a first sensor element comprises a film of a halo-aluminium phthalocyanine or a halo-aluminium naphthalocyanine and one or more further sensor elements is adapted to detect levels of interfering gases but is not sensitive to organic nitro-compound vapours.

2. A sensor device according to claim 1 wherein the first sensor element comprises a film of one of chloro-aluminium phthalocyanine, fluoro-aluminium phthalocyanine, chloro-aluminium naphthalocyanine or fluoro-aluminium naphthalocyanine.

3. A sensor device according to claim 1 or claim 2 wherein the first sensor element film is deposited on an alumina substrate.

4. A sensor device according to any preceding claim wherein at least one of the further sensor elements is sensitive to NOx gases.

5. A sensor device according to any preceding claim wherein a preconcentrator is provided for each sensor element.

6. A sensor device according to claim 5 wherein at least one preconcentrator has means for heating it sufficiently to cause thermal cracking of any adsorbed explosives vapours.

7. An explosives sensor device comprising at least two chambers and pumping means to enable air to be drawn into and extracted from each chamber1 wherein each chamber has a dividing means which in one position separates first and second parts of the chamber in a gas tight manner and in a second position enables the first and second parts of the chamber to communicate, and wherein the first part of each chamber houses a vapour concentrator and the second part of each chamber houses a sensor element, a first sensor element comprising a film of a halo-aluminium phthalocyanine or a halo-aluminium naphthalocyanine and at least one further sensor element being adapted to detect levels of interfering gases but not being sensitive to organic nitro-compound vapours, and there is included means to drive adsorbed vapour off a first vapour concentrator for detection by the first sensor element, and means to provide an output signal related to the concentrations of detected gases from each sensor element.

8. An explosives sensor device according to claim 7 wherein the output signal from the or each further sensor element is subtracted from the output signal from the first sensor element.

9. An explosives sensor device according to claim 7 or claim 8 wherein at least one of the further sensor elements is sensitive to NOx gases.

10. An explosives sensor device according to claim 9 wherein at least one of the further sensor elements is a metal-TADA derivative, such as Cu-TADA.

11. An explosives sensor device substantially as hereinbefore described with reference to Figure 12 of the accompanying drawings.