Abstract:
A flexible thin metal film thermal sensing system is provided. A self-metallized polymeric film has a polymeric film region and a metal surface disposed thereon. A layer of electrically-conductive metal is deposited directly onto the self-metallized polymeric film&#39;s metal surface. Coupled to at least one of the metal surface and the layer of electrically-conductive metal is a device/system for measuring an electrical characteristic associated therewith as an indication of temperature.

Description:
ORIGIN OF THE INVENTION 
     The invention was made by an employee of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to thin metal films. More specifically, the invention is a flexible thin metal film thermal sensing system. 
     2. Description of the Related Art 
     Electrically-based thermal sensing is typically accomplished using a thermocouple or resistance temperature device (RTD). A thermocouple is a passive device made out of wire or a metal film deposited on an insulative substrate. Wire thermocouples are relatively inflexible thereby making them a poor choice when the sensor must be wrapped about or otherwise conformed to the shape of a structure. Metal film-based thermocouples are typically made by thermal evaporation or sputtering of a metal film onto a dielectric surface, e.g., a plastic. If this sensor is to be mounted on a structural surface for temperature measurement thereof, the dielectric surface must be (i) doped to make it thermally conductive, and (ii) pre-shaped to fit the portion of the structural surface on which it is to be mounted. 
     RTDs are active devices in that an electric current must be supplied thereto with a resulting electrical resistance being read therefrom. The electrical resistance is indicative of temperature. RTDs are typically made from metal foils that are thicker and less flexible than a metal film thermocouple. Accordingly, RTDs may not be suitable choices for temperature sensing applications requiring relatively small and flexible temperature sensors. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a flexible thin metal film thermal sensing system. 
     Another object of the present invention is to provide a flexible thin metal film thermal sensing system adaptable for operation as a passive thermocouple or an active RTD. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, a flexible thin metal film thermal sensing system uses a self-metallized polymeric film having a polymeric film region and a metal surface disposed on the polymeric film region. A layer of an electrically-conductive metal is deposited directly onto the metal surface of the self-metallized polymeric film. Operatively coupled to at least one of the metal surface and the layer of electrically-conductive metal is a device/system for measuring an electrical characteristic associated therewith as an indication of temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a flexible thin metal film thermal sensing system in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic view of a flexible thin metal film thermal sensing system in accordance with another embodiment of the present invention; and 
         FIG. 3  is plan view of an example of a patterned metal layer forming an electrical resistance element. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to  FIG. 1 , an embodiment of a flexible thin metal film thermal sensing system is shown and is referenced generally by numeral  100 . Thermal system  100  is a passive thermoelectric device in that it requires no applied excitation voltage or current. Thermal system  100  is based on a flexible thin metal film system  10  previously disclosed in U.S. patent application Ser. No. 11/279,009, filed Apr. 7, 2006, the contents of which are hereby incorporated by reference. Flexible thin metal film system  10  obtains its flexibility from a self-metallized polymeric film base  12  that, in general, has an underlying sheet  12 A of polymeric material with a surface layer  12 B that is a conductive metal. In general, the structure of self-metallized polymeric film  12  is created/developed in one or more processing stages. Conventional two-stage processing involves preparing/fabricating polymer sheet  12 A and then depositing surface layer  12 B onto sheet  12 A. However, absent a pre-treatment process, there will be adhesion problems between sheet  12 A and surface layer  12 B. 
     The adhesion between sheet  12 A and metal surface layer  12 B is greatly improved if self-metallized polymeric film  12  is created/developed by single-stage processing of, for example, a homogenous solution of a native metal precursor (as a positive valent metal complex) and a selected poly(amic acid) precursor of the final polymer. Single-stage thermal or light processing simultaneously causes the polymer to form while most of the metal atoms aggregate at the surface of the polymer in a very thin layer on the order of about 500-2000 Angstroms (Å) in thickness. Such single-stage processing is disclosed by R. E. Southward et al., in “Inverse CVD: A Novel Synthetic Approach to Metallized Polymeric Films,” Advanced Materials, 1999, 11, No. 12, pp 1043-1047, the contents of which are hereby incorporated by reference as if set forth in its entirety. 
     The resulting self-metallized polymeric film  12  is flexible and does not suffer from the aforementioned adhesion problems. As a result of such single-stage processing, underlying sheet  12 A retains some of the metal atoms (i.e., the same metal forming surface layer  12 B) so that underlying sheet  12 A possesses thermal conductivity while metal surface layer  12 B is electrically conductive. Accordingly, as will be readily apparent from the ensuing description, underlying sheet  12 A can be coupled to a structural surface (not shown) when the temperature thereof is to be measured. Further, since thin metal film system  10  is flexible, it can be easily conformed to the shape of the structural surface. 
     Flexible thin metal film system  10  further includes a layer  14  (or multiple layers) of electrically conductive metal directly deposited onto surface layer  12 B. Further, in at least one embodiment of the present invention, metal layer  14  is deposited directly onto surface layer  12 B without any adhesion pretreatment of layer  12 B. Additionally, in at least one embodiment, metal layer  14  can comprise multiple sub-layers, wherein the first sub-layer is directly deposited onto the surface layer  12 B, and each sub-layer may comprise the same or different electrically conductive metals. 
     In other words, surface layer  12 B serves as a strike layer for metal layer  14  that is deposited onto surface layer  12 B by one of a variety of electrodeposition methods to include electroplating. However, it is to be understood that layer  14  could also be deposited directly onto surface layer  12 B by means of a variety of electroless deposition/plating techniques without departing from the scope of the present invention. For a description of electroless plating techniques, see Chapter 17 of “Electroplating” by Frederick A. Lowenheim, McGraw-Hill Book Company, New York, 1978. Still other techniques for depositing metal layer  14  include, for example, immersion or displacement plating, chemical reduction deposition such as silvering, thermal evaporation, sputtering and chemical vapor deposition. Thin metal film systems fabricated in this fashion are typically on the order of 0.05 to 1 micron in thickness. By comparison, metal foil RTDs are considerably thicker, i.e., typically 4-50 microns in thickness. 
     As shown in  FIG. 1 , coupled to flexible thin metal film system  10  is a voltmeter  20 . More specifically, as shown, voltmeter  20  has its leads  22  coupled to metal surface layer  12 B and metal layer  14  where the choice of either as an anode or cathode is not a limitation of the present invention. A voltage difference measured by voltmeter  20  is indicative of temperature experienced by thin metal film system  10 . Thermal system  100  can be calibrated by measuring voltages (i.e., the Seebeck voltage) at known temperature intervals with the recorded voltages and known temperatures being used to determine the equation of a line with the slope thereof being the Seebeck Coefficient. 
     The present invention can also be adapted/configured to function as an active resistance temperature device (RTD) as will now be explained with the aid of  FIGS. 2 and 3 . In  FIG. 2 , a flexible thin metal film thermal sensing system  200  utilizes a flexible thin metal film system similar to that described above. The difference is that metal layer  14  is shaped, formed or etched to define a pattern  16  ( FIG. 3 ) that exhibits changes in electrical resistance in correspondence with changes in temperature. In operation of thermal sensing system  200 , electric current is supplied to metal layer  14  by a current source  30  while the electrical resistance of pattern  16  is measured by a resistance measuring device  32  electrically coupled across pattern  16 . 
     The advantages of the present invention are numerous. The flexible thin metal film thermal sensing systems of the present invention overcome the comparatively inflexible prior art thermocouples and RTDs. In addition, since the underlying polymeric sheet is thermally conductive, these thermal sensing systems are well suited to sense temperatures of structures to which they are coupled as they do not require doping with thermally conductive materials. 
     The present invention can be made using a variety of self-metallized polymeric films. Referring again to  FIGS. 1 and 2 , metal surface layer  12 B of self-metallized polymeric film  12  as well as metal layer  14  can be selected from the group of metals to include palladium, platinum, gold, silver, nickel, copper, tantalum, tin, lead, and mercury. Alloys of these metals could also be used. Furthermore, the metal for surface layer  12 B need not be the same as the metal used for metal layer (or multiple layers)  14 . 
     Althouqh the invention has been described relative to specific embodiments thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. For example, while the present invention has been described with respect to electroplating and electroless plating fabrication techniques and materials associated therewith, the present invention is not so limited. Other metals and associated fabrication techniques (e.g., thermal evaporation, sputtering, etc.) could also be used to construct thermal sensing systems in accordance with the present invention. These alternative fabrication methods could be used to deposit pure metals such as iron, copper, nickel, manganese, aluminum, silicon, platinum, rhodium and chromium, as well as alloys thereof such as constantan (55% copper, 45% nickel), alumel (95% nickel, 2% manganese, 2% aluminum, 1% silicon), chromel (90% nickel, 10% chromium) and nichrome (80% nickel, 20% chromium). Additionally,  FIG. 3  shows one example of a pattern  16  that exhibits changes in electrical resistance in correspondence with changes in temperature, other such patterns are within the scope of the present invention. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.