Abstract:
An ultra-fast response, high sensitivity structure for optical detection of low concentrations of hydrogen gas, comprising: a substrate; a water-doped WO 3  layer coated on the substrate; and a palladium layer coated on the water-doped WO 3  layer.

Description:
[0001]    This application claims priority from U.S. Provisional Application Serial No. 60/202,155, filed May 5, 2000. 
     
    
     CONTRACTUAL ORIGIN OF THE INVENTION  
       [0002] The United States Government has rights in this invention under Contract No. DE-AC36-99GO10337 between the United States Department of Energy and the National Renewable Energy Laboratory, a division of the Midwest Research Institute. 
     
    
     
       TECHNICAL FIELD  
         [0003]    The invention relates to water doped tungsten oxide (WO 3 —nH 2 O) thin films for use in ultra-fast, high sensitivity gasochromic hydrogen sensors. The in-situ water-doped tungsten oxide is prepared during thermal evaporation of WO 3  on a glass substrate used as the chemochromic layer combined with a catalyst such as palladium, and utilized as an optical sensor to detect hydrogen.  
         BACKGROUND ART  
         [0004]    Hydrogen is a plentiful, clean, non-polluting fuel. Hydrogen is currently used in many industries, and the US demand for hydrogen is approximately 140 billion cubic feet per year and growing. However, hydrogen is explosive at 4% in air. Therefore, it is critical to measure, monitor and control hydrogen wherever it is used.  
           [0005]    In the gasochromic art where sensors and measurement instrumentation for hydrogen gases detect and/or measure hydrogen, typically there is required a portable sensing device, a kit (where hydrogen gas detection and/or measurement is required in existing equipment), and sensor heads installed at points where hydrogen leaks are possible, or where monitoring is necessary (i.e., in internal combustion engines which operate using hydrogen as a fuel).  
           [0006]    The problems associated with current H 2  gasochromic devices is that these devices are not of adequate durability in that they degrade quickly with cycling and time, are too moisture sensitive, and react too slowly in response to the presence of H 2  to produce an optical absorption change with a lengthy real time constant in the vicinity of 30 seconds.  
         DISCLOSURE OF INVENTION  
         [0007]    At present, H 2  detection is widely accomplished through the use of Pd/WO 3  hydrogen detecting gasochromic devices. However, several problems or drawbacks are associated with the use of Pd/WO 3  hydrogen detecting gasochromic devices. These problems are: 1) they are of inadequate durability; 2) they respond slowly to the presence of H 2 ; and 3) there is a conflicting cathodic-anodic optical response that results in a weakened color change.  
           [0008]    The item 2) slow response of the Pd/WO 3  hydrogen detecting gasochromic device in the presence of a H 2  leak is due to the hydrogen reaction in H x WO 3  which produces a slow optical absorption change within a lengthy room temperataure time constant of about 30 seconds.  
           [0009]    The slow response time of the Pd/WO 3  hydrogen detecting gasochromic device is exacerbated after storage of the sensor.  
         DISCLOSURE OF INVENTION  
         [0010]    One object of the present invention is to provide a water doped WO 3 /Pd thin film sensor structure for use in detecting the presence of hydrogen.  
           [0011]    Another object of the present invention is to provide a water doped WO 3 /Pd thin film structure for use in ultra-fast, high sensitivity hydrogen sensors.  
           [0012]    A further object of the present invention is to provide water doped WO 3 /Pd thin film sensor structures that provide faster response time to the detection of hydrogen than undoped WO 3 /Pd.  
           [0013]    In general, the invention is accomplished by providing a glass/W—WO 3 /Pd sensor thin film structure, wherein, during deposition of the WO 3  layer upon glass by thermal evaporation, the WO 3  is doped with water by in-situ addition.  
           [0014]    The in-situ addition results in improved hydrogen diffusion, and thus, improved response time. The in-situ addition also results in improved stability of the WO 3 /Pd. 
       
    
    
     BRIEF DESCRIPTION OF DRAWING  
       [0015]    [0015]FIG. 1 is a graph showing percent relative transmission versus time for a water-doped WO 3 /Pd sensor in a 1% hydrogen environment. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0016]    Due to the fact that Pd/WO 3  hydrogen sensors are encumbered by a slow response time, a need exists in the interest of safety to provide H 2  sensors of improved optical response time upon detecting hydrogen, which is explosive at 4% in air.  
         [0017]    The Pd/WO 3  sensor is of slow response time because hydrogen reaction in H x WO 3  produces optical absorption change with a room temperature time constant of about 30 seconds, as the rates of both hydrogen diffusion in the tungsten oxide layer and the charge transfer process at the palladium/tungsten oxide interface is slow.  
         [0018]    The improved water-doped WO 3 /Pd sensor of the invention is obtained by preferably depositing WO 3  on a glass substrate in a manner such that, during the deposition by thermal evaporation, the WO 3  is doped with water by in-situ addition.  
         [0019]    During the in-situ addition of water to affect doping of the WO 3  layer, the water content in the film may be controlled by using different water vapor pressures during thermal evaporation.  
         [0020]    The in-situ addition of water may also be accomplished by the use of other deposition processes such as plasma enhanced chemical vapor deposition (PECVD) and sputtering.  
         [0021]    The slow response time of the Pd/WO 3  sensor upon detection of hydrogen, and the even slower response time of the Pd/WO 3  sensor upon detection of hydrogen after storage is significantly improved as a result of the invention process of providing in-situ, water-doped WO 3 , during thermal evaporation deposition of the WO 3  layer onto a substrate.  
         [0022]    The effect of water doping of the WO 3  layer is shown in TABLE I.  
                                             TABLE I                                   Doped   Undoped                                        Full coloration time of fresh   1-2s   ˜30s           sensors           Full coloration time after   20s   &gt;600s, unable to reach           storage in ambient air for a       previous coloration           week                      
 
         [0023]    As can be seen from TABLE I, the undoped WO 3  containing a sensor device requires approximately 30 seconds to achieve full coloration when the sensor is fresh or new. By contrast, the water-doped WO 3  containing a sensor device requires only 1 to 2 seconds for response time upon detection of hydrogen, and within this 1 to 2 seconds, full coloration of the sensor is attained.  
         [0024]    In the case of storage of the undoped and doped sensors, after 1 week of storage of the sensor, full coloration of the undoped sensor could not be reached even after a period greater than 600 seconds. By contrast, the full coloration or reaction time after detection of hydrogen when the water-doped WO 3  containing sensor was stored for a week occurred in 20 seconds after detection of hydrogen.  
         [0025]    The in-situ water doping of the tungsten oxide layer during deposition by thermal evaporation resulted in faster proton diffusion and resulted in an improved interface charge transfer rate at the interface between the water-doped WO 3 /Pd layer.  
         [0026]    Reference is now made to FIG. 1, which is a graph showing the percent transmission versus time for a water-doped WO 3 /Pd sensor in a 1% hydrogen environment.  
         [0027]    It is clear that the water-doped WO 3  containing sensor provides an extremely fast response upon detection of hydrogen and that the sensor is highly responsive to the presence of low-concentrations of H 2 .  
         [0028]    However, since the WO 3 /Pd sensor becomes saturated at around the 2% level of hydrogen, the water-doped WO 3 /Pd sensor may be used in combination with a vanadium oxide sensor to detect a full hydrogen concentration range above the 2% level.  
         [0029]    The ultra-fast response to low concentrations of hydrogen gas as occasioned by the use of water-doped WO 3 /Pd sensors is a result of faster proton diffusion and improved interface charge transfer rates. These technical improvements clearly provide a better margin of safety in environments where rapid detection of hydrogen leaks are decisive harbingers of eminent explosion.