Abstract:
A smoke detector replaces the americium source of alpha particles with a field emission device using carbon nanotubes as the field emitters, or some other field emitter, in order to provide an ionization of the air potentially caring smoke particles through the smoke detector.

Description:
This application for patent claims priority to U.S. Provisional Patent Applications Ser. No. 60/891,927 filed Feb. 27, 2007, Ser. No. 60/941,858 filed Jun. 4, 2007, and Ser. No. 60/844,761 filed Sep. 15, 2006 which are hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     Current smoke detector technology is based on one of two general approaches. Photoelectric-based detectors are based on sensing light intensity that is scattered from smoke particles. Light from a source (LED) is scattered and sensed by a photosensor. When the sensor detects a certain level of light intensity, an alarm is triggered. Ionization-type smoke detectors are based on a radioactive material that ionizes some of the molecules in the surrounding gas environment. The current of the ions is measured. If smoke is present, then smoke particles neutralize the ions and the ion current is decreased, triggering an alarm. 
     Referring to  FIG. 1 , ionization sensor smoke alarms  100  contain a small amount of radioactive material, americium  101 , embedded in a gold foil matrix within an ionization chamber  103 . The matrix is made by rolling gold and americium oxide ingots together to form a foil approximately one micrometer thick. This thin gold-americium foil is then sandwiched between a thicker (˜0.25 millimeter) silver backing and a 2 micron thick palladium laminate. This is thick enough to completely retain the radioactive material, but thin enough to allow the alpha particles  102  to pass. 
     The ionization chamber  103  is basically two metal plates  104  a small distance apart. One of the plates  104  carries a positive charge, the other a negative charge. Between the two plates  104 , air molecules received through the screen  105 , made lip mostly of oxygen and nitrogen atoms, are ionized when electrons are kicked out of some molecules and picked up by other molecules as a result of collisions with alpha particles  102  from the radioactive material  101 . The result is oxygen and nitrogen molecules that are both positively and negatively charged, such as NO + , O 2   − , OH − , HCO 3   + , and many other similar ions. 
       FIGS. 1 and 2  illustrate how ionization technology works. Referring to  FIG. 1 , the positive atoms flow toward the negative plate, as the electrons or negative ions flow toward the positive plate. The movement of the charges registers as a small but steady flow of current. Referring to  FIG. 2 , when smoke particles  106  enters the ionization chamber  103 , the current is disrupted as the smoke particles  106  attach to the charged ions and restore them to a neutral electrical state. The large smoke particles  106  can shield the electric charge due to their size, or neutralize the charge through a chemical reaction. The net result is that fewer charged ions make it to the electrode. This reduces the flow of current between the two plates  104  in the ionization chamber  103 . When the electric current drops below a certain threshold, an alarm is triggered. 
     There are problems with the radioactive material that is currently used as an ionizer. 
     1. The radioactive material is a small amount and does not pose a health hazard to the homeowner as long as the material is not tampered with. It is possible that someone could tamper with the americium-based smoke detector and inadvertently inhale or ingest the americium. This can be a serious health hazard. 
     2. Although the amount of americium in each smoke detector is small (about 1 microCurie), the accumulated amount of material can add up. The typical user disposes of the smoke detector by throwing it in the household trash. It is possible that this material can then find its way into recycled material that could then find its way back into a home in a form that is not so innocuous as the original smoke detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  illustrate a prior art smoke detector; 
         FIG. 3A  demonstrates an effect of an electric field at a point source; 
         FIG. 3B  illustrates a field emission device; 
         FIG. 4A  illustrates a smoke detector; 
         FIG. 4B  illustrates another embodiment of a smoke detector; 
         FIG. 5  illustrates another embodiment of a smoke detector; 
         FIG. 6A  illustrates another embodiment of a smoke detector; 
         FIG. 6B  illustrates another embodiment of a smoke detector; 
         FIG. 7  illustrates another embodiment of a smoke detector; 
         FIG. 8  illustrates sensor electronics for a smoke detector; 
         FIG. 9  illustrates electronic circuitry for a smoke detector; 
         FIG. 10  illustrates an analog comparator; 
         FIG. 11  illustrates a digital latch; and 
         FIG. 12  illustrates an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention replace the radioactive element of the standard ionization-type smoke detector with a field emission or field ionization ion source that is non-radioactive and uses no radioactive materials. The field emission ion source will operate at atmospheric pressures and will operate over a wide temperature range. What is described in detail below are embodiments that use carbon nanotubes as the field ionizer material, but there are many other materials that could be used for this application: 
     1. Functionalized or coated carbon nanotubes may be used to improve durability and lifetime and also reduce operating voltage. One example of this would be alkali-metal coated or alkali-salt coated carbon nanotubes. 
     2. Nanotubes or nanowires of other materials, such as Si, ZnO, GaAs, etc. These nanowires may also be functionalized or coated. 
     3. Metal or semiconducting microtips may be used, such as W or Mo (metals) or Si or Ge (semiconductors). It may be possible that a Spindt microtip configuration may be used with an emitter structure and a gate electrode. 
     An electric field on the order of several megavolts/cm (˜several 100 V/μm) is sufficient to produce electron emission from materials. One way to achieve these fields practically is to use conducting or semiconducting structures, or materials that have very high aspect ratios (they are tall and thin), and place them in an electric field. Because the high aspect ratios will concentrate the electric fields at the ends or tips of the structure, electron field emission can be achieved with applied electric fields as low as 1-10 V/μm since the electric field at the tips of these high aspect features can be as high as 100-1000 V/μm. 
       FIG. 3A  illustrates how a high aspect ratio (h/r) conductor concentrates the applied electric field (F 0 ) so that the field at the tip of the conductor (F) is magnified.  FIG. 3B  illustrates a diode-type (anode sued cathode electrodes only) field emission display structure using carbon nanotube emitters. See, Tonegawa et al., “Development of Large Size CNT-FED,” IDW/AD &#39;05, Takamatsu, Japan, p. 1659. 
     Initially, metal or Si microtip structures were designed and built to be used for field emission applications. See, C. A. Spindt and L. N. Heynick, U.S. Pat. No. 3,665,241, May 23, 1972. The first field ionization experiments were performed by Müller. See, E. W. Müller, Phys. Rev. Vol. 102, p. 618 (1956). There are two cases or methods to use field emitter structures as gas ionization sources: 
     Case 1) Electrically bias the structures negatively such that electrons are pulled from the field emitters into the gas environment (producing ions by electron-impact or electron capture); or 
     Case 2) Electrically bias the structures positively (the reverse of above) such that electrons are pulled from the gas molecules into the tips of the structures, thus producing positive ions. 
     In both cases, it is well documented (See, Robert Gomer, Field Emission and Field Ionization, pub. by the Am. Inst. of Physics, 1993, pp. 1-31 and 64-102) that the phenomenon that controls the behavior is quantum mechanical tunneling of electrons from the conduction band of the metal into the vacuum or gas environment as a result of high local electric fields (Case 1), or the reverse, electrons tunneling from the gas molecules into the metal (Case 2) from similar applied electric fields but polarized in the opposite direction. 
     There are issues that are considered for implementing embodiments of the present invention: 
     Gas adsorption and changing work function of the tip emitters: Since these emitters will operate in air, gas can form physical and chemical bonds to the surface, changing work function and aspect ratio and degrading emission properties. Carbon nanotubes are relatively inert compared to most metals (i.e., an oxide layer is not formed on the surface). They are flexible yet strong, (Young&#39;s Mod. of SWNT=1 TPa, max tensile strength=30 GPa. See, M.-F. Yu et al., Phys. Rev. Lett. 84, 5552 (2000)) and have high thermal conductivity. See, Savas Berber et al., “Unusually High Thermal Conductivity of Carbon Nanotubes”, PRL, V84, p. 4613, (2000). Based on these properties, the carbon nanotube is a good choice, as it is expected to be the most stable. 
     Ion erosion of the emitter: Water or oxygen ions may attach to the carbon nanotube material, converting it to CO or CO 2 . This may limit the life of the carbon emitters. It is found that this is true in high vacuum conditions. See, L. H. Thuesen, R. L. Fink, et al., J. Vac. Sci. Technol. B 18(2), p. 968, March/April 2000. For embodiments herein, the electrons are emitted into air at atmospheric pressure at low energy; thus, the electrons do not gain significant energy before impacting a molecule. Therefore, ions are created by electron capture (i.e., they are negative ions) and are repelled from the CNT electrode. The only concern may be positive ions. Positive ions can be created if the electron energy striking the molecule is high. Embodiments herein may adjust both the gap between the electrodes and the bias of the electrodes to change the electron impact energy and tune it for optimal performance. Furthermore, the impact of ions on the CNT emitters may be limited because ion energy will be imparted to other molecules as a result of high collision rates at atmospheric pressure. 
     There are examples of using carbon emitters as gas ionization sources in the literature. Dong et al. and Choi et al. used CNT emitters in ionization vacuum gauges. See, C. Dong et al., APL., 84, p. 5443, 2004, and In-Mook Choi, et al., APL., 87, p. 173104, 2005. They operated their devices in partial vacuum, different from the proposed approach, and with much higher electron impact energy than proposed herein. Riley et al. used multiwall carbon nanotubes to ionize He. See, D. J. Riley et al., “Helium Detection via Field Ionization from Carbon Nanotubes,” NanoLetters, 3, p. 1455 (2003). They were successful in ionizing He atoms at low pressures (4×10 −5  mbar). 
     Peterson et al. measured the performance of both carbon nanotubes and polycrystalline diamond as a gas ionizer at atmospheric pressure and in the Case 1 mode, very similar to what is disclosed here. See, M. S. Peterson, W. Zhang, et al., Plasma Source Sc. and Technol., Vol. 14, pp. 654-660. (2005). First, using the highly graphitic polycrystalline diamond material, they were able to generate a current between 5 pA and 10 μA with voltages of 20 V and 340 V respectively, using a gap of 10 μm. They were able to maintain the current in one case over 40 hours in continuous DC mode. This demonstrates that oxygen ions did not significantly degrade the performance of the carbon-based electron source operating in air. 
       FIG. 4  illustrates a first embodiment, which may be easiest to make and may be the lowest cost to manufacture of all the designs. It comprises two conductor plates or metal coated glass panels  404 ,  405 . One conductor  404  is coated with CNTs  403 . Air  407  is allowed to flow in between the plates  404 ,  405 . Either DC or AC voltage may be applied between the electrodes. It is also possible to bias die electrodes with an AC voltage having a DC offset. In DC mode or with a DC offset, ions created will drift toward an electrode; the direction of the drift will depend on the charge of the ion. An alternative (not shown) is that both sides may be coated with a CNT film to take advantage of the AC swing that may be needed to neutralize ion drift. The substrates are shown as glass that is metallized. It in fact may also be a metal foil or other conducting sheet of material. Gas may be forced through the gap in one direction or there may be no forced flow at all and left open to the prevailing air currents in the room, entering the gap from any direction. Sensor electronics  406  are coupled to the electrodes  404 ,  405  to detect changes in the current and set off an alarm if a threshold of smoke particles are detected. 
       FIG. 4B  illustrates a second embodiment that is similar except the sensor electrodes (electrode rings  424  and  425 ) are separate and independent of the gas ionization electrodes ( 404  and  405 ) although they may be formed or deposited onto the same substrate  426 . The substrate material in this case is insulating, such as ceramic materials or glass. In this embodiment, ions are created in a similar manner as in  FIG. 4 , but here the ion current is measured by collecting the negative ions oil the positive electrode ( 425 ) and by collecting the positive ions oil the negative electrode ( 424 ). These electrodes can be various shapes and sizes but they are in close proximity to the ion source so that they measure the ions created in the gap between electrodes  404  and  405 . As described before, the parameters of the ion source electrodes can be adjusted to create positive ions, negative ions or both. Sensor electronics  406  are coupled to the electrodes  424  and  425  to detect changes in the current and set off an alarm if a threshold of smoke particles is detected. 
       FIG. 5  illustrates a third embodiment that is similar except it has a smaller area and thus the capacitance will be lower. The + and − lines may be easily patterned using standard photoresist-patterned metal lines  504 ,  505  on glass substrates, or by screen printing. Design factors, such as the gap between electrode plates and the spacing between the electrodes on the same plate, may be varied. The drive parameters, such as the bias level, frequency and duty factor, may be easily varied to achieve optimal conditions. The electrodes  504 ,  505  may be operated in DC or AC mode, or in an AC mode with a DC offset. In this design, an insulating substrate may be required to maintain the potentials on the separate electrodes. Gas  507  may be forced through the gap in one direction or there may be no forced flow at all and left open to the prevailing air currents in the room, entering the gap from any direction. A CNT coating  503  is deposited on the electrodes  504 ,  505  in a desired manner. 
       FIG. 6A  illustrates two metal grids  601 ,  602  that are parallel to each other; one or both may be coated with CNTs  603 . If the other grid is coated, then the coating would face the coating on the other grid. The gas flow  607  is through the grids  601 ,  602 . Once the grid material is chosen, the only mechanical parameter that may be changed is the gap between the grids  601 ,  602 . As with extraction grids used for vacuum microelectronics, such as displays and x-ray tubes, a good rule of thumb is that the grid dimension (size of grid openings) is about the same as the gap between the grids. The metal grids  601 ,  602  or electrodes may be operated in DC or AC mode, or in an AC mode with a DC offset. The drive parameters, such as DC voltage bias level, AC or DC potential and frequency of AC signal may also be optimized. This design allows for easy flow of gas through the grid. 
       FIG. 6B  illustrates an embodiment similar to the embodiment illustrated in  FIG. 6A  except there is only one “grid” that comprises CNT coated wires  610 ,  611  that are biased relative to each other. 
     Referring to  FIG. 7 , several grids may be used, spaced apart from each other similar to the 2-grid design shown in  FIG. 6A . The grids may be biased opposite each other (+ then − then + and so on).  FIG. 7  shows the CNT coating on selected surfaces, but the CNT coating may be on both sides of the grid. The bias between the grids may be DC, or it may be AC or could be AC with a DC offset. 
     The embodiments described herein detect smoke in the same way that the prior art detects smoke, but from monitoring the change in current through one or more of the grids or electrodes. For example, if electrons are emitted into the gas on a negative electrode, this would create negative ions that would be collected at the positive grid or electrode. However, if there is smoke present, then the negative ions may react with the smoke particles (typically carbon particles or hydrocarbon aerosols) and neutralize or mask the negative charge to create neutral particles; thus no current would arrive at the positive electrode. The current at each electrode may be monitored, and if a current decreases below a set value, an alarm may be triggered. 
       FIG. 8  illustrates a block diagram of sensor electronics  406  that may be utilized within each of the embodiments illustrated in  FIGS. 4 ,  5 ,  6 A,  6 B.  7  and  11 .  FIG. 9  illustrates a detailed circuit diagram of the sensor electronics of  FIG. 8 .  FIG. 10  illustrates a detailed circuit diagram of the analog comparator  802  within the sensor electronics  406 .  FIG. 1  illustrates a detailed circuit diagram of the digital latch  803  of the sensor electronics  406 . As can be seen from  FIGS. 8-11 , any of the embodiments of the smoke detector described above may be coupled to the sensor electronics  406  at the sensor inputs  801 . An alarm  804  or some other type of external output  805  may be the result for output from the sensor electronics  406  when a threshold current level is detected at senor input  801 , as predetermined by a threshold detection set point  806 . A further description of the sensor electronics  406 , including the component parts of the analog comparator  802  and digital latch  803  are not further described herein for reasons of brevity. The sensor electronics  406  are to be designed to receive an input from a smoke detector using CNT coatings, as described herein, and provide an appropriate output in order to make the smoke detector practically effective. The design of such sensor electronics is not pertinent to an understanding of the present invention. Other sensor electronics may be substituted in order to arrive at the same result. 
       FIG. 12  illuminates a combination smoke detector that combines the CNT-based ionization smoke detector (see  FIGS. 4. 5 ,  6 A,  6 B, and  7 ) with one or more other sensors or smoke detector technologies. Some fires give off gases as incipient indicators of a fire. The smoke from the fire may be different depending on what is burning. Some fires give off heavy black smoke; other fires may give of a gray smoke or very little at all. The combination of tie different sensors will help sense a fire faster while at the same time lower the chances of a false alarm. The combination sensor will also have the lo possibility of determining what kind of fire is present and relay this information to 911 or a subscription security service. 
     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.