Abstract:
A hydrogen sensor ( 100 ) with instant response that uses one or more quartz tuning forks ( 101, 102 ) while no chemical reactions or other material modifications are involved. Sensor ( 100 ) can be used in any application to measure percent range of hydrogen concentrations.

Description:
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/719,548, filed Sep. 22, 2005. 

   TECHNICAL FIELD 
   This invention relates to gas sensors, and more particularly to hydrogen sensors. 
   BACKGROUND 
   There are many technologies for sensing hydrogen, some of them are commercialized. Most of the technologies can be split into two categories—chemical sensors and palladium-based sensors. They use properties of hydrogen to interact with materials, either via chemical reactions as in metal oxide sensors, or by dissolving in Pd and changing the physical properties of Pd—H system. The property of Pd to dissolve and store hydrogen is implemented in many approaches to designs of the sensors, including quartz microbalance sensors, which detect a change in resonance oscillation frequency as the palladium film changes its mass while it absorbs hydrogen. A related patent is U.S. Pat. No. 6,029,500. However, using palladium or similar hydrogen dissolving metals as active coatings results in poor stability of the devices as the metal surface oxidizes with time, and also in case of Pd, it can be poisoned by sulfur or delaminated due to the phase change occurring at high hydrogen concentrations. Thus, it is desired to have a sensor stable in time at various chemical and physical conditions. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  illustrates an embodiment of the present invention; 
       FIG. 2  illustrates a graph of sensitivity of a sensor configured in accordance with an embodiment of the present invention; 
       FIG. 3  illustrates another embodiment of the present invention; 
       FIG. 4  illustrates a graph of sensitivity of a sensor configured in accordance with an embodiment of the present invention; 
       FIG. 5  illustrates a circuit for outputting results of embodiments of the present invention; 
       FIG. 6  illustrates another embodiment of the present invention; and 
       FIG. 7  illustrates an exemplary application of embodiments of the present invention. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
   Hydrogen is known as the element with the smallest atomic mass. In a gas mixture in thermodynamic equilibrium, molecules have a mean energy of ˜3/2 kT, whether they are molecules of hydrogen, nitrogen, oxygen, etc. The momentum of a molecule is mν, where m is molecular mass, and ν is the mean molecular velocity equal to (8 kT/πm) 1/2 . So, the momentum of a gas molecule at a given temperature will depend on its mass as (m) 1/2 . The difference in momentum and size (effective diameter) of molecules leads to the difference in other macroscopic parameters of gases, such as viscosity and diffusion rate. 
   During oscillations in a gas environment, a vibrating object, such as tuning fork, tines impart momentum to gas molecules resulting in mechanical energy loss in the tines. This loss causes a change in the resonant oscillation frequency of the fork, and the frequency shift will depend on a momentum that the tines impart to gas molecules. This means that in a gas that contains light molecules, such as hydrogen, the losses due to interaction with the gas molecules will be less than in an environment without hydrogen. Hence, the frequency of oscillations will be higher in an environment having a presence of hydrogen. 
   In a tuning fork quartz oscillator, the fork tines symmetrically vibrate in an antiphase flexure mode, wherein the tines move in opposite directions against each other at any moment in time. The speed at which the tines oscillate can be estimated as follows. The amplitude of the tine deflection is approximately 60 nm/V. If the driving voltage on the tines is about 1V, then at a frequency of 32768 Hz, the tines will have a characteristic speed of ˜2 mm/sec. That is much less than the speed of gas molecules (hundreds of meters per second), and it is possible to consider a quasi-static case for this interaction. Therefore, it is mostly the macroscopic characteristics of gases that will affect tuning fork oscillation frequency. 
   The described tuning fork sensor is not selective to hydrogen, and other light gases like, helium, may interfere with the H2. To avoid interference, it is possible to use H2-permeable membranes, like Pd (palladium) membranes, to improve selectivity. 
   The frequency change in the tuning fork resonator is usually small, so a differential frequency detection method may be used for the detection of small frequency deviations. 
   Along with frequency, the quality factor Q and the electric impedance of the tuning fork resonator changes as the oscillation energy is dissipated in gas environment. 
   Referring to  FIG. 1 , embodiments of the present invention illustrate two ECS-327SMO type oscillators  101 ,  102  which may be used for gas detection. The tuning fork oscillator cans&#39; tops  103 ,  104  are sanded off for access of gas, and the oscillators&#39; outputs  110 ,  111  are connected to D and CLK inputs of a D flip-flop trigger  105 . The primary oscillator (OSC 1 )  101  is located in a suspected hydrogen-containing gas environment. The reference oscillator (OSC 2 )  102  is used to account for changes in gas composition (such as humidity changes) and for temperature compensation. The frequency of the primary oscillator  101  will increase with the hydrogen concentration. The frequency difference at the flip-flop output  106  is thus proportional to the hydrogen concentration. 
   The sensor  100  may be characterized using hydrogen gas mixed with air at volume concentrations of 0 to 16% at room temperature. The hydrogen-air mixture may be prepared using two 100 sccm mass flow controllers. The interval between the frequency beats may be measured using a Tektronix CDC250 counter. 
   The response of the sensor  100  to hydrogen is quite linear in all ranges of concentrations. As can be seen in  FIG. 2 , a 9% change in the differential frequency may be observed when 16% H 2  concentration is achieved in the chamber of oscillator  101 . 
     FIG. 3  illustrates an alternative embodiment where two oscillators  303 ,  304  can make a sensor device  300  and be used to detect hydrogen in a gas chamber  301 . Sensor  300  has the reference oscillator  304  sealed (the can&#39;s top is not sanded off) to protect from access to hydrogen. Along with the open oscillator  303  they can be placed into the measured gas environment next to each other. As noted above, an H 2 -permeable membrane  302  may be used. In this embodiment, cross-compensation can be done for the temperature, but not for humidity changes. Sensor  300  comprises circuitry  307  for converting the difference in frequencies to an output parameter  305  (e.g., a number or voltage level) proportional to the concentration of hydrogen C H2 , and a display  306  for presenting the output parameter  305 . 
   The concentration of hydrogen can then be calculated as follows:
 
 f   1   =f   10   +kC   H2 ,
 
 f   2   =f   10   +f   12 ,
 
Δ f=f   1   −f   2   =f   10   +kC   H2   −f   10   +f   12   =kC   H2   +f   12 ,
 
where f 10  is the frequency of oscillator OSC 1   303  without hydrogen, f 12  is the difference between the frequencies of OSC 1   303  and OSC 2   304  without hydrogen, k is the proportionality factor, and C H2  is the concentration of hydrogen. The last expression can be recalculated as follows:
 
 C   H2 =(Δ f−f   12 )/ k.  
 
   Test results for sensor  300  with sealed and open can oscillators are shown in  FIG. 4 . The sensor  300  was tested at room temperature, the flow rate of nitrogen was 200 sccm, and the flow rate of hydrogen changed from 0 to 10 sccm. The sensor  300  shows a near linear response to H 2  concentration changes. 
   Referring to  FIG. 5 , since a difference in frequencies of two oscillators can be as small as several Hz, it may be more convenient to measure time intervals between two frequency beating pulses. In this case, a separate high-frequency oscillator (not shown) can be used to fill the time intervals with pulses at a fixed frequency f 0 . For precision measurements, oven-controlled crystal oscillators (OCXO) can be used to generate such pulses. The time interval between the frequency beatings will be
 
 T= 1/Δ f= 1/( f   1   −f   2 )=1/( kC   H2   +f   12 ).
 
If the OCXO stabilized generator has a frequency of f 0 , then the number of pulses at the output N of pulse counter  501  will be
 
 N=f   0   T=f   0   /Δf=f   0 /( kC   H2   +f   12 ).
 
   In another embodiment, a device for measuring oscillation parameters of the tuning fork detects changes in Q factor and an impedance of the tuning fork as hydrogen will change the energy that is dissipated by the tuning fork tines. The energy dissipation in the tuning fork can be described as follows. 
   If a mechanical system such, as a tuning fork, has a mechanical resistance R M , the quality factor Q at a resonant frequency f 0  will be
 
Q˜f 0 /R M .
 
The mechanical resistance R M  is a function of the gas viscosity V, and thus R M  can be described as the following series:
 
 R   M   =R   M0 (1+ c   1   V+c   2   V   2 + . . . )
 
where c1, c2, . . . , are the proportionality coefficients. For media with low viscosity, such as a gas, this can be rewritten as
 
 R   M   =R   M0 (1+ c   1   V )
 
where R M0  is the mechanical resistance in vacuum and does not depend on a gas viscosity.
 
   Hydrogen has approximately two times lower viscosity than air (8.4×10 −6  Pa*s vs. 17.4×10 −6  Pa*s at 0° C.), and, hence, the mechanical resistance will decrease at higher relative hydrogen concentrations. Thus:
 
 R   M   =R   M0 ( a−bC   H2 ),
 
where a and b are functions of viscosities of hydrogen and a balanced gas (such as air), and C H2  is a relative concentration of hydrogen in the gas mixture. Then the concentration can be defined as
 
C H2 ˜(a−f 0 /QR M0 )/b
 
where Q can be measured experimentally. The quality factor can be easily found when the quartz tuning fork is a part of an electrical circuit, since, by definition,
 
 Q=f   0   /Δf.  
 
Measuring Δf can be done by conventional methods used in electronics (frequency sweeping around f 0 , measuring amplitude attenuation of oscillation pulses (damping factor), etc.). Since the electric impedance is also a function of the quality factor,
 
 |Z (ω)| 2 ˜(1 /Q   2 −1)+(ω/ω 0 ) 2 +(ω 0 /ω) 2 ,
 
it can be used for determination of hydrogen concentration as well.
 
   Referring to  FIG. 6 , another embodiment is based on measurements of the frequency and/or the Q factor of a periodic movement (vibration) of a nanowire or a nanotube  601 , such as a carbon nanotube, in flexure mode. The measurement system  603  of the sensor will include a means to detect and quantify such oscillations. An example of a system that is electrically coupled to a nanotube  601  is shown in  FIG. 6 . Nanotube  601  vibrates due to an applied external force (not shown), such as mechanical or electrostatic force. An electric probe  602  coupled to the nanotube  601  either by field electron emission to/from the nanotube  601 , or through the capacitance between the nanotube  601  and the probe  602 , or by any other coupling mechanism that is known to one skilled in the art. The probe  602  forms a part of the measuring electric circuit  603  that can measure deviations in coupling parameters (such as capacitance) and determine the frequency of these deviations. Details of such a measuring circuit are not described, but would be within the capability of one skilled in the art. 
   As described above, the vibration frequency of the nanotube  601  or nanowire will depend on the viscosity of a surrounding gas, which, in turn, will depend on the concentration of hydrogen in the gas. 
   The sensitivity range of the sensor is 0 to 100% H 2 , with a detectivity limit of at least 1%, as can be seen from the sensitivity graphs shown above in  FIGS. 2 and 4 . The lower flammability level of hydrogen is 4%, and lower explosive limit (LEL) is 17%. Thus, the described sensors may be used as a leak detector for many applications. For example, referring to  FIG. 7 , in a fuel cell powered car, the sensor can be installed near the fuel cell reactor, near the passenger seats, or in the exhaust system. 
   An LEL detector that uses the described sensor may be a portable handheld device, with a sensor incorporated in the device body, or placed at the end of an attachable sampling probe. The device may have indications of concentration on a display along with a sound alarm if the concentration of hydrogen reaches a certain critical level. Other applications include water electrolysers, hydrogen storage systems, industrial equipment, etc. 
   Improvements can be made to stabilize the sensor response against temperature, humidity, atmospheric pressure, quartz aging, and other conditions of operation. 
   A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.