Title of Invention

METHOD FOR SELECTIVELY DETECTING OF ONE OR MORE OF THE HIGHLY TOXIC CHEMICAL HAZARDOUS GASES,

Abstract

The invention relates to a detector for detecting highly toxic gaseous or vaporized hazardous substances or warfare agents. In order to be able to detect such substances quickly and easily at ambient temperature, a substrate comprising a hydrogen (H)-terminated or hydrogenated surface is used as a warfare agent detector. The substrate is equipped with a measuring instrument for measuring shifts of surface charges on the hydrogen (H)-terminated or hydrogenated surface. The substrate is made of a semiconductor material or a non-conductive material that has surface conductivity.

Full Text

FORM 2 THE PATENT ACT 1970 {39 of 1970) & The Patents Rules, 2003 COMPLETE SPECIFICATION (See Section 10, and rule 13 TITLE OF INVENTION DETECTOR FOR THE DETECTION OF CHEMICAL WARFARE AGENTS, PRODUCTION METHOD, AND USE OF & SUBSTRATE AS A WARFARE AGENT DETECTOR APPLICANT(S) a) Name b) Nationality c) Address EADS DEUTSCHLAND GMBH GERMAN Company WILLY-MESSERSCHMITT-STRASSE, 85521 OTTOBRUNN GERMANY PREAMBLE TO THE DESCRIPTION The following specification particularly describes the invention and the manner in which it is to be performed : - The invention relates to a warfare agent detector for detection of chemical warfare agents in a gas, to a production process and to a particular use of a substrate. Chemical warfare agents are substances which give rise to acute and chronic health risks. They are highly toxic to humans and animals and extremely environmentally damaging. Examples include phosphine (PH3), arsine (AsFfe), diborane (B2H6). These also form, for example, a group of highly toxic dopants which are also used routinely in the semiconductor industry. A further group relates to chemical warfare agents which were used as weapons of mass destruction in many countries at the command of the particular decision makers in past wars. Representatives of this group are mustard gases, organophosphorus substances, phosgene, VX, sarin, tabun, soman. Present-day endangerment by such substances is conceivable as a consequence of accidents with legacy materials or as a result of terrorist attacks. There is therefore a great need to be able to rapidly detect the presence of such highly dangerous substances in various environments, especially in the air. For detection of abovementioned gases and vapors, there are several sensor systems on the market. Some of these are expensive and highly complex analytical instruments which can be operated only by trained personnel and whose results can be interpreted reliably only by such personnel. Examples are gas chromatographs, mass spectrometers and ion mobility spectrometers. Sensors available inexpensively on the market, for example thick layer metal oxide sensors, can detect such hazardous gases in low concentrations. However, a serious disadvantage of such inexpensive sensors is the fact that they likewise react to a large number of disruptive gases which may likewise be present in the air in relatively high and highly variable concentrations. One reason for the inadequate selectivity is the high operating temperature of approx. 400°C, at which most molecules to be detected are burnt at the sensor surface and are only detected electrically as a result. For more exact identification, it is therefore necessary to operate a plurality of sensors with different cross-sensitivities in parallel in a sensor array (so-called electronic noses). "2" ,11 MAY 2009/ However, a clear detection of chemical warfare agents with the aid of electronic noses has to date not yet been possible with the necessary reliability in most cases, for the reasons mentioned above. It is therefore an object of the invention to enable rapid and uncomplicated detection of chemical warfare agents in the air at room temperature, without reacting to the disruptive gases which likewise occur in various industrial environments and/or in the outdoor environment. This object is achieved by the particular use of a substrate known per se according to claim 1, the warfare agent detector according to claim 14, and the production process according to claim 20. Advantageous embodiments of the invention are the subject matter of the dependent claims. According to the invention, a substrate with a hydrogen (H)-terminated surface is used as a warfare agent detector for detection of chemical warfare agents in a gas. The substrate is manufactured from a nonconductor material with an available surface conductivity or from a semiconductor material. The inventive use of the substrate makes it possible to detect chemical warfare agents in gases and especially in the air, without there being any disruption to the warfare agent detection by other substances present. This means that masking of the measured signal caused by the highly toxic or deadly hazardous materials or warfare agents by signals caused by troublesome background substances is prevented. Such disruptive gases are, for example, CO, 03, natural gas (especially CH4) or else alcoholic vapors. Advantageously, the density of the hydrogen coverage on the surface is reduced by partial oxidation. This achieves an enhancement in sensitivity. -3- According to the invention, the substrate is suitable especially for detection of highly toxic hazardous materials or warfare agents which, in chemical terms, belong to the groups of the III-H, V-H or VI-H compounds, and for detection of molecules which have such reactive groups as molecule constituents. The invention is based on the consideration that most of these substances have electron orbitals which are doubly occupied and are not involved in any covalent bond within the molecule to be detected. The hydrogenated or hydrogen-terminated surface of the substrate offers a means of docking via hydrogen bonds. The measured results described below demonstrate that electrical charges can be transferred between the warfare agent compounds and the sensor substrate with sufficiently high probability via such physisorption bonds. Such charge transfers lead to a change in the electrical conductivity at the substrate surface and hence to a detectable sensor signal. Displacements of surface charges are preferably measured at the hydrogen (H)-terminated surface by means of a measuring device. More particularly, a layer of hydrogen atoms applied to the substrate constitutes the hydrogen (H)-terminated surface. The layer of hydrogen atoms is, for example, a monoatomic layer. The substrate may be manufactured, for example, from diamond, hydrogenated amorphous silicon (a-Si:H), silicon carbide, a group III nitride or a metal oxide. Specific examples from the latter two groups are GaN or tin oxide, or else zinc oxide. The electrical resistance is preferably measured on the hydrogen (H)-terminated surface by means of a measuring device. The measurement for detection of the gases or vapors specified is effected preferably at a temperature below 100°C, especially at room temperature. -4- The sensitive hydrogen (H)-terminated substrate surface is preferably cleaned by purging with a fluid which does not contain the substances to be detected, especially air. The cleaning is preferably effected with addition of an oxidizing fluid, especially ozone. According to the invention, the substrate is used especially for detection of one or more of the following gases: mustard gas, sulfur mustard gas, organophosphorus warfare agents, nitrogen oxides, B2H6, PH3, ASH3 or other gases containing group III elements, especially A1(CH3)3, Ga(CH3)3, In(CH3)3. The inventive warfare agent detector is suitable for detection of chemical warfare agents in a gas and comprises a substrate which has a hydrogen (H)-terminated substrate surface for exposure to the gas, and a measuring device for measuring displacements of surface charges at the hydrogen (H)-terminated substrate surface, said substrate being manufactured from a nonconductor material with surface conductivity or from a semiconductor material. To form the hydrogen (H)-terminated substrate surface, a layer of hydrogen atoms is preferably applied to the substrate. Owing to the hydrogenated surface, the inventive warfare agent detector is suitable especially for detection of highly toxic hazardous material or warfare agent compounds of at least one group III, group V or group VI elements. A readout unit may be present, for example, in the form of a transistor. The readout unit and the sensitive layer, for example a-Si: H or H-terminated diamond, may be manufactured from different materials, i.e. the sensitive layer is first applied to the readout unit. According to the invention, accordingly, for example, a semiconductor surface is terminated with a layer of atomic hydrogen. Such a hydrogenated semiconductor -5" 1.1 MAY2009 surface constitutes the sensitive layer onto which the warfare agents to be detected dock. The H termination enables, for example, addition of a group V-H molecule to be detected, and the subsequent formation of an electrically charged complex of this molecule with the surface-bonded hydrogen at room temperature. Disruptive gases react at the surface only at relatively high temperatures and thus do not constitute any risk of distortion. The invention thus enables selective identification of chemical warfare agents in the form of group III-H, group V-H and group VI-H gases and vapors at room temperature. The hazardous material and warfare agent detector has a lower power consumption than existing competing products, since no heating power is required for the sensitive layer. Moreover, an uncomplicated construction and a low power consumption arise, since only a single sensor is sufficient and no further sensors are required to filter out disruptive gases. More particularly, the inventive sensor is notable in that it has no cross-sensitivity to environmental influences and disruptive gases in industrial manufacture. After one measurement, the sensor surface can be cleaned by a high concentration of ozone. The effect used in accordance with the invention, that of a change in the surface conductivity of hydrogenated semiconductor surfaces by various group V-H compounds, has already been known for a long time in the literature, at least in the case of hydrogenated amorphous silicon. This material is being used to an enhanced degree in photovoltaics. -6- 11 MAY 2009 For instance, the article "Effect of Adsorbates and Insulating Layers on the Conductance of Plasma Deposited a-Si:H*" by M. Tanielian et al. in Journal of Non-Crystalline Solids 35 & 36 (1980) 575-580 states that various adsorbed gases such as H2O and particularly NH3 have a strong effect on the conductivity and the photoconductivity of high-resistance layers of amorphous hydrogenated silicon. It is stated that NH3 acts as a surface donor for hydrogenated amorphous silicon and thus changes the surface conductivity. In contrast, selenium acts as an acceptor and likewise changes the surface conductivity. It is further stated that changes in surface charge caused by these substances can be reversed by heating under reduced pressure. Frank). Kampas describes, in "Chemical Reactions in Plasma Deposition", chapter 8 from "Semiconductors and Semimetals", Vol. 21A, Academic Press, Inc., 1984, which chemical processes proceed in the growth of hydrogenated amorphous silicon layers and how a naturally hydrogenated surface forms owing to the specific growth process. G. Muller et al. (Journal of Non-Crystalline Solids 59 & 60 (1983), p. 469-472) present, in the article "Hydrogen Incorporation, Doping and Thickness Dependent Conductivity in Glow Discharge Deposited a-Si: H Films", hydrogen depth profiles through a-Si: H layers of various thickness and show explicitly that a high surface density of hydrogen atoms, which is consistent with the forecasts of Kampas and Griffith, is present. R. Sung Gi et al. describe, in the article "Hall Effect Measurements of Surface Conductive Layer of Undoped Diamond Films in NO2 and NH3 Atmospheres", Jpn. J. Appl. Phys. Vol. 38 (1999), p. 3492-3496, NO2- and NHa-induced changes in the electrical surface conductivity in diamond samples with hydrogenated surfaces. The invention thus utilizes the effect of the displacement of surface charges through the H-terminated substrate or semiconductor surface, for the first time, for controlled detection of gaseous or vaporous chemical warfare agents. .7. 11 MAY2009 The working examples of the invention are illustrated in detail hereinafter with reference to the figures appended here. These figures show: Fig. 1 a graph which illustrates the resistance of a hydrogenated diamond at room temperature under different atmospheres; Fig. 2 a graph which shows the NH3 response at different temperatures; Fig. 3 a graph which shows how the response and decay rate can be enhanced by illumination with ultraviolet light; Fig. 4 a graph which illustrates the cleaning of a layer sensitive to various gases by means of ozone; Fig. 5 a graph which shows the sensitivity of the hydrogenated surface to NO2; Fig. 6 a graph which illustrates the enhancement of sensitivity achieved through "dilution" of the hydrogenated surface by partial oxidation. As a preparatory study for production of a warfare agent detector in the form of a gas sensor, an H-terminated surface was produced on a diamond substrate. The monoatomic layer present on the diamond substrate was studied for its sensitivity to various gases at several working temperatures. The layer has been found to be selective to ammonia and NO2 at room temperature. It is also possible for the response or sensitivity to be increased by partial replacement of the H termination by O termination. A gas sensor thus produced can be used as a threshold sensor for manufacture in the chemical industry and in the semiconductor industry. -8- 11 MAY2009 Further possible fields of use are air quality analysis, for example in the transport of foods, in agricultural farms, in biogas plants, in silos, or in the use of coolants. The studies shows that hydrogen (H)-terminated (hydrogenated) semiconductor surfaces are usable for detection of gaseous group III-H, group V-H and group VI-H compounds and other organic gases with these components as bonding partners. These compounds are present in most chemical warfare agents. Accordingly, the solution presented here relates to the use of hydrogen (H)-terminated (hydrogenated) semiconductor surfaces as a sensitive layer for detection of chemical warfare agents, in particular the very toxic, gaseous group III-H, group V-H and group VI-H compounds or other organic or inorganic gases with similar functional groups therein. As the basis for the creation of a warfare agent detector for the highly toxic gaseous group III-H, group V-H and group VI-H compounds, ammonia (NH3), which is relatively nontoxic in comparison, has been studied in detail as a representative of diborane (B2H6), phosphine (Ph3) and arsine (AsH3). Suitable possible substrates to which a hydrogenated sensitive layer can be applied are particularly semiconductor materials, for example diamond, silicon carbide, all group III nitrides (e.g. GaN) and metal oxides (e.g. tin oxide)> or else silicon. A hydrogenated surface is a surface terminated by hydrogen atoms. This termination can be applied in various processes, for example in a hydrogen plasma or in a "hot-wire" hydrogenation. A hydrogenated layer is a monoatomic layer consisting of hydrogen atoms bonded covalently to the lattice constituents of the substrate. The surface is thus coated with a layer of hydrogen atoms. One of several processes suitable for hydrogen termination is hydrogen glow discharge. Further useful processes include silanization and chemical functionalization of surfaces with organic molecules. '9' 11 MAY 2009 These surface hydrogen atoms, even at relatively low temperatures (e.g. room temperature - RT), promote adsorption, in this case of NH3 molecules, and form an NH3-H complex with them. There is a displacement of the surface charge, which can be read out electrically with the aid of suitable measurement technology. For this purpose, a measuring device is used to measure the surface resistance. Measuring devices for measuring ohmic resistance are well known and need not be explained any further here. Other gases which occur in the environment on a daily basis (e.g. ethanol, hydrocarbons, CO, hydrogen) require higher energies to add onto a hydrogenated surface and to react chemically. Thus, the reducing gases detected at relatively low temperatures, here lower than 100°C, are only NH3 and chemically related substances such as PH3 and AsH3. The detection of NH3 by means of a hydrogenated diamond was studied in more detail. The measured results are explained in detail hereinafter with reference to the figures appended. Fig. 1 shows the resistance behavior of a hydrogenated diamond under different gas atmospheres at room temperature RT. The background atmosphere used was synthetic air SA. In this experiment, gas pulses of different gases in unusually high concentration were introduced at time intervals. One gas pulse was then followed in each case by a purge pulse with synthetic air (SA), which was formed from 20% oxygen and 80% nitrogen. As is evident from the graph of fig. 1, first 5000 ppm (parts per million) of ethene, then 50 ppm of ethanol, then 500 ppm of carbon monoxide, then 1% hydrogen and finally 5000 ppm of ammonia were introduced. In the course of this, the surface resistance was measured permanently. It was clear that there was a change in the resistance only when ammonia was introduced. Fig. 2 shows the repetition of the experiment according to fig. 1, except at successively higher working temperatures of the diamond. Again, only when NH3 was added was an increase in the component resistance observed. When the time period within which NH3 was supplied is considered in more detail, a displacement "10" 11 MAY2009, in the sensor baseline toward a lower base resistance is observed in synthetic air, caused by the higher temperature. With increasing working temperature, however, significantly shorter reaction times for the adsorption and desorption of the NH3 are observed. Fig. 3 shows the accelerating effect of ultraviolet (UV) light on the response and decay behavior of the diamond sensor. UV illumination is therefore an important alternative to sensor operation at elevated temperatures. Fig. 4 shows how the desorption of HN3 is accelerated by addition of ozone. It can thus be seen very nicely in this figure how the component resistance at the start of the experiment increases as a result of addition of NH3. After the gas pulse supplied, a desorption of the ammonia gas from the diamond surface sets in. The resistance of the component decreases slowly. When a very high concentration of ozone is then supplied, the desorption process is accelerated. The resistance of the diamond is reduced drastically during the addition and remains at the lower level for a prolonged period even after the supply of gas. Cleaning of the sensor surface is thus possible with high concentrations of ozone. Sensitivity to oxidizing gases: When the sensitivity of a hydrogenated surface to oxidizing gases, for example ozone and NO2, is considered, it can be stated - as illustrated by fig, 4 - that a cleaning action at the surface sets in when ozone gas is supplied. At extreme ozone concentrations, which typically far exceed environmental concentrations, the hydrogen termination of the surface can be damaged, or replaced partially by oxygen coverage. This latter case is considered further specifically in fig. 6. Fig. 5 shows the influence of NO2. As can be discerned therefrom, NO2 is the only gas which causes reduction of the component resistance at room temperature. -11- 11 MAY 2009 Accordingly, this component can also be used for detection of NO2, and also of further nitrogen oxides, (NO, N20). Fig. 6 shows a repetition of the experiment shown in fig. 5. In this experiment, the gas supply sequence of fig. 5 was repeated four times in order to test the reproducibility of the results. This experiment was first carried out with a completely hydrogenated diamond and then repeated twice, and each repetition was preceded by exposure of the diamond to an extremely aggressive oxygen atmosphere. In these treatments, the number of H-terminated surface bonds is reduced each time. It can be seen that the "dilution" of the H termination and the associated increase in the base resistance cause a significant increase in the gas sensitivity. No occurrence of new gas sensitivities which have not been described to date were observed after these treatments. Applications for detection of other group III-H compounds or group V-H compounds: In further experiments which were performed similarly to the experiments explained above/ it was possible to demonstrate that it is possible to use a hydrogenated semiconductor surface, especially the above-discussed sensitive surface, likewise to detect diborane (B2H6) as group Ill-hydrogen compounds, and also phosphine (PH3) and arsine (ASH3) as well as NH3 as further group V-hydrogen compounds. The studies conducted show that the substrate with the hydrogenated surface safely and reliably detects the chemical warfare agents specified. -12- 11 MAY 2009 WE CLAIM: 1. The use of a substrate with a hydrogen (H)-terminated surface as a warfare agent detector for selective detection of highly toxic chemical hazardous materials and warfare agents in a gas, the substrate being manufactured from a nonconductor material with available surface conductivity or from a semiconductor material. 2. The use as claimed in claim 1, characterized in that the density of the hydrogen coverage on the surface has been reduced by partial oxidation. 3. The use as claimed in claim 1 or 2, characterized in that displacements of surface charges are measured by means of a measuring device at the hydrogen (H)-terminated surface. 4. The use as claimed in one of the preceding claims, characterized in that a layer of hydrogen atoms applied to the substrate constitutes the hydrogen (H)-terminated surface. 5. The use as claimed in claim 4, characterized in that the layer of hydrogen atoms is a monoatomic layer. 6. The use as claimed in one of the preceding claims, characterized in that the substrate is manufactured from diamond, amorphous silicon, silicon carbide, a group III nitride or a metal oxide. 7. The use as claimed in claim 6, characterized in that the substrate comprises GaN or tin oxide or zinc oxide. -13- 11MAY2009 8. The use as claimed in one of the preceding claims, characterized in that the electrical resistance at the hydrogen (H)-terminated surface is measured by means of a measuring device. 9. The use as claimed in one of the preceding claims, characterized in that the measurement for detection of the gases or vapors specified is effected at a temperature below 100°C, especially below approx. 55°C. 10. The use as claimed in claim 9, characterized in that the measurement is effected at room temperature. 11. The use as claimed in one of the preceding claims, characterized in that the sensitive hydrogen (H)-terminated substrate surface is cleaned by purging with a fluid which does not contain the substances to be detected, especially air. 12. The use as claimed in claim 11, characterized in that the cleaning is effected with addition of an oxidizing fluid, especially ozone, and/or with addition of ultraviolet light. 13. The use as claimed in one of the preceding claims for detection of one or more of the following gases: mustard gas, sulfur mustard gas, organophosphorus warfare agents, nitrogen oxides, B2H6, PH3, ASH3 or organic warfare agent gases with group III elements, especially A1(CH3)3, Ga(CH3)3, In(CH3)3. 14. A warfare agent detector for detection of highly toxic chemical hazardous materials or warfare agents in a gas, comprising a substrate which has a hydrogen (H)-terminated substrate surface for exposure to the gas, and a measuring device for measuring displacements of surface charges at the I1MAY2009' hydrogen (H)-terminated substrate surface, the substrate being manufactured from a nonconductor material with surface conductivity or from a semiconductor material. 15. The warfare agent detector as claimed in claim 14, characterized in that a layer of hydrogen atoms is applied to the substrate to form the hydrogen (H)-terminated substrate surface. 16. The warfare agent detector as claimed in claim 15, characterized in that the layer of hydrogen atoms is a monoatomic layer. 17. The warfare agent detector as claimed in one of claims 14 to 16, characterized in that the substrate comprises diamond, silicon carbide, a group III nitride or a metal oxide. 18. The warfare agent detector as claimed in claim 17, characterized in that the substrate comprises GaN or tin oxide. 19. The warfare agent detector as claimed in one of claims 14 to 18, characterized in that the measuring device for measuring the electrical resistance is configured on the hydrogen (H)-terminated substrate surface. 20. A production process for producing a warfare agent detector as claimed in one of claims 14 to 19, wherein a substrate is surface hydrogenated with the aid of a hydrogen plasma and connected to a measuring device for readout of a displacement of surface charge. -15- 11 MAY2009 21. The process as claimed in claim 20, characterized in that the substrate used is a semiconductor substrate, especially diamond, silicon carbide, a group III nitride, especially GaN, or a metal oxide, especially tin oxide. 22. The process as claimed in claim 20, characterized in that the substrate used is a nonconductor with available surface conductivity. HIRAL CHANDRAKANT JOSHI AGENT FOR EADS DEUTSCHLAND GMBH -16- HI! MAY 2009 Dated this 11th day of May, 2009

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