Patent Document

RELATED APPLICATIONS 
   This application is related to commonly-assigned, co-pending application: U.S. patent application Ser. No. 11/877,593, entitled “COLD-CATHODE-BASED ION SOURCE ELEMENT”, filed Oct. 23, 2007. The disclosure of the above-identified application is incorporated herein by reference. 
   BACKGROUND 
   1. Field of the Invention 
   The invention relates to vacuum gauges and, particularly, to an ionization vacuum gauge employed in situations in which the vacuum system is sensitive to temperature and/or light and/or in high vacuum systems. 
   2. Discussion of Related Art 
   Ionization vacuum gauges have been used for several years. The conventional ionization vacuum gauge includes a hot filament, an anode electrode surrounding the hot filament, and an ion collector surrounding the anode electrode. The anode electrode and the ion collector are coaxial relative to the hot filament. In operation, electrons emit from the hot filament, travel toward and through the anode electrode and eventually are collected by the anode electrode. In their travel, electrons collide with the molecules and atoms of gas and produce ions, and eventually ions are collected by the ion collector. The pressure of the vacuum system can be calculated by the formula P=(1/k) (I ion /I electron ), wherein k is a constant with the unit of 1/torr and is characteristic of a particular gauge geometry and electrical parameters, I ion  is a current of the ion collector, and I electron  is a current of the anode electrode. 
   However, the conventional ionization vacuum gauge requires several watts of electrical power supply to the hot filament, the hot filament dissipates heat and light in the vacuum system, and consequently the conventional ionization vacuum gauge is unsuitable for use in a vacuum system sensitive to heat and/or light. Furthermore, the high temperature of the hot filament can cause evaporation, and thus the conventional ionization vacuum gauge tends to be unsuitable for high vacuum systems. 
   What is needed, therefore, is an ionization vacuum gauge that is suitable for use vacuum systems that are sensitive to temperature and/or light and/or that requires extremely high vacuum levels. 
   SUMMARY 
   In one embodiment, an ionization vacuum gauge includes a linear cathode, an anode, and an ion collector. The linear cathode, the anode, and the ion collector are coaxial and arranged in the order from center to outer. The linear cathode includes a linear base and a field emission film coating on the linear base, and the field emission film includes several carbon nanotubes. 
   Compared with the conventional ionization vacuum gauge, the cathode of the present ionization vacuum gauge includes the carbon nanotubes as the emission source. The electrical power supply to the present ionization vacuum gauge is able to be lower, and electrons are emitted from the carbon nanotubes of the cathode without dissipating heat and light and without promoting evaporation. Thus, the present ionization vacuum gauge is suitable for use in a vacuum system sensitive to heat and/or light and can be widely used to measure pressure in ultra-high and extremely high vacuum systems. 
   Other advantages and novel features of the present ionization vacuum gauge will become more apparent from the following detailed description of preferred embodiments, when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the present ionization vacuum gauge can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present ionization vacuum gauge. 
       FIG. 1  is a schematic, axial cross-section view showing an embodiment of the present ionization vacuum gauge; 
       FIG. 2  is a schematic, cross-sectional view of the present ionization vacuum gauge of  FIG. 1 ; and 
       FIG. 3  is a pressure graph displaying a ratio of ion current to electron current, as per an embodiment of the present ionization vacuum gauge. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one preferred embodiment of the present ionization vacuum gauge, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Reference will now be made to the drawings to describe in detail embodiments of the present ionization vacuum gauge. 
     FIGS. 1 and 2  are schematic axial and radial cross-sectional views, respectively, showing the present ionization vacuum gauge  100 . The ionization vacuum gauge  100  includes a linear cathode  102 , an anode  104 , and an ion collector  106 . The linear cathode  102 , the anode  104 , and the ion collector  106  are coaxial, and the arrangement thereof is in the order from center to outer. The linear cathode  102 , the anode  104 , and the ion collector  106  are spaced from one another, in such a manner so as to not be in direct electrical contact with one another. A radial space between the linear cathode  102  and the anode  104  (referred as r) is, beneficially, from about 1 millimeter (mm) to about 8 mm and, rather usefully, about 1˜2 mm. A radial space between the linear cathode  102  and the ion collector  106  (referred to as R) is from about 10 mm to about 15 mm, advantageously, about 12 mm. 
   The ionization vacuum gauge  100  further includes an enclosure  120  and three leads  122 . The enclosure  120  is part of a vacuum system in which the linear cathode  102 , the anode  104  and the ion collector  106  are disposed. The enclosure  120  is fluidly connected to a chamber (not shown) whose pressure is measured. The first ends of the leads  122  are electrically connected to the linear cathode  102 , the anode  104 , and the ion collector  106 , respectively, and the second ends of the leads  122  are extended out of the enclosure  120 . It is to be understood that the vacuum system incorporates one or more known evacuation mechanisms (not shown), as needed to achieve the desired level of vacuum. 
   The anode  104  and the ion collector  106  are made of an oxidation-resistant, conducting metal, such as aluminum (Al), copper (Cu), tungsten (W), or alloy thereof. The anode  104  has an apertured structure, such as a metallic ring, a metal-enclosed aperture, or a metallic net. The ion collector  106  has an apertured and/or plane structure, such as a metallic ring, a metal-enclosed aperture, a metallic net, or a metallic sheet. 
   The linear cathode  102  includes a linear base  108  and a field emission film  110  coated thereon. The linear base  108  is an electric conductive thread, such as an oxidation-resistant metal thread made, e.g., of nickel (Ni), tungsten, or copper. A diameter of the linear base  108  is about from 0.2 mm to 2 mm, advantageously, about 0.3 mm. The field emission film  110  is, usefully, composed, initially, of carbon nanotubes (CNTs), low-melting-point glass powders, conductive particles, and a an organic carrier/binder. The mass percents of the foregoing ingredients are respectively: about 5%˜15% of CNTs, about 10%˜20% of conductive particles, about 5% of low-melting-point glass powders, and about 60%˜80% of an organic carrier/binder, this latter component being evaporated and/or burned off in a drying step, leaving the other three ingredients in the final film composition. CNTs can be obtained by a conventional method such as chemical vapor deposition, arc discharging, or laser ablation. Rather suitably, CNTs are obtained by chemical vapor depositon. A length of CNTs is, advantageously, about from 5 microns (μm) to 15 μm, because CNTs less than 5 μm tend to be weak electron emitters, and CNTs more than 15 μm are often easily broken. 
   The an organic carrier/binder is composed of terpineol, acting as a solvent; dibutyl phthalate, acting as a plasticizer; and ethyl cellulose, acting as a stabilizer. The low-melting-point glass melts at an approximate temperature from 400° C. to 500° C. The function of the low-melting-point glass is to attach CNTs firmly to the linear base  108 , in order to avoid CNTs becoming dislodged/unbonded from the linear base  108 . The conductive particles can, usefully, be silver and/or indium tin oxide (ITO). The conductive particles help ensure, to at least a certain degree, that the CNTs are electrically connected to the linear base  108 . 
   A process for forming such an the linear cathode  102  is illustrated as per the following steps: 
   Step 1, providing and uniformly mixing carbon nanotubes (CNTs), low-melting-point glass powders, conductive particles, and an organic carrier/binder in a certain ratio to form a composite slurry; 
   Step 2, coating the composite slurry on the outer surface of the linear base  108 ; and 
   Step 3, drying and sintering the composite slurry to form the field emission film  110  on the linear base  108 . 
   In step 2, the composite slurry is beneficially provided onto the linear base  108  by a silk-screen printing process. In step 3, drying the composite slurry is performed to remove (e.g., evaporate and/or burn off) the an organic carrier/binder, and sintering the composite slurry is to melt the low-melting-point glass powders for firmly attaching the CNTs to the linear base  108 . After step 3, the field emission film  110  can, opportunely, further be scrubbed with rubber to expose end portions of CNTs, thus enhancing the electron emission capability thereof. 
   In another alternative, the field emission film  110  can be made essentially of CNTs. In this alternative, CNTs are deposited on the linear base  108  by a conventional method, i.e., CNTs are formed directly on the linear base  108 . 
   In operation of the ionization vacuum gauge  100 , an electric voltage is applied between the linear cathode  102  and the anode  104  to cause electron emission. After emitting, electrons are drawn and accelerated toward the anode  104  by the electric potential, then tending to pass through the anode  104  because of the inertia of the electrons and because of the apertured structure thereof. The ion collector  106  is supplied with a negative electric potential and thus decelerates the electrons. Therefore, before arriving at the ion collector  106 , electrons are drawn back to the anode  104 , and an electric current (I electron ) is formed. In the travel, electrons collide with gas molecules, ionize some of gas molecules, and produce ions. Typically, the ions are in the form of positive ions and are collected by the ion collector  106 , and, thus, an ion current (I ion ) is formed. A ratio of I ion  to I electron  is proportional to a pressure in the ionization vacuum gauge  100 , within a certain pressure range, covering the primary range of interest for most vacuum devices. Therefore, the pressure in the ionization vacuum gauge  100  and, by extension, the vacuum device (not shown), to which it is fluidly attached, can be measured according to the above relation. 
   Referring to  FIG. 3 , the ionization vacuum gauge  100 , according to the present invention, 25 volts of electric potential is supplied to the ion collector, 750 volts of electric potential is supplied to the anode, and ground potential is supplied to the linear cathode. The ratio of I ion  to I electron  is perfectly proportional to the pressure in the range from 10 −7  Torr to 10 −3  Torr, as can be seen in  FIG. 3 . 
   The present ionization vacuum gauge, employing a cathode having CNTs thereon, can be used instead of many kinds of the ionization vacuum gauges that use a hot-filament cathode, without dissipating heat and/or light. Further, the ionization vacuum gauge can be widely used in many fields (e.g., cases where a high degree of vacuum is necessary) in which use of the hot-filament cathode is not appropriate. 
   Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.

Technology Category: g