Abstract:
Disclosed are bimetallic strips that incorporate magnetostrictive materials to enhance and provide sensing, actuating and energy harvesting functions. The bimetallic strips include a positive magnetostrictive Fe-based alloy layer and a flexible layer. The flexible layer may be a negative magnetostrictive layer or a permanent magnet layer. One or more permanent magnet materials may also be used in the arrangement. The bimetallic strips are inexpensive and easily manufactured, and have characteristics that enhance sensing and actuator applications, and enables energy harvesting.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/882,259, filed Dec. 28, 2006, which is incorporated herein by reference. 
     
    
     STATEMENT OF GOVERNMENT INTEREST 
       [0002]    The following description was made in the performance of official duties by employees of the Department of the Navy, and, thus the claimed invention may be manufactured, used, licensed by or for the United States Government for governmental purposes without the payment of any royalties thereon. 
     
    
     TECHNICAL FIELD 
       [0003]    The following description relates generally to bimetallic strips, more particularly, bimetallic strips that incorporate magnetostrictive materials to enhance and provide sensing, actuating and energy harvesting functions. 
       BACKGROUND 
       [0004]    Bimetallic strips have been extensively used for numerous applications in the past. Such applications include electrical/mechanical actuation and temperature sensing. Typically, bimetallic strips consist of two strips of different metals that have different expansion and heating characteristics. Consequently, when heated, the strips expand at different rates. Typically, the strips are adjacently attached along their length. Because of the different expansion and heating characteristics, one strip bends over the other when heated, so that the bimetallic strip bends in a predictable manner. When cooled, the bimetallic strip bends in the opposite direction. 
         [0005]    Because of the above-recited properties, bimetallic strips are used as sensors and actuators. As a sensor, a bimetallic strip can be used to detect changes in temperature, heat, or other environmental conditions. As an actuator, a bimetallic strip may for example, push a switch as it moves, thereby changing the state of a system. In some applications, a bimetallic strip may be provided in a linear form, and in others in curved or coiled form. Generally, bimetallic strips usually comprise materials such as steel and copper. However, it is desirable to have bimetallic strips that are easy to manufacture and are functional in a greater number of working environments. 
       SUMMARY 
       [0006]    In one aspect, the invention is a bimetallic strip. In this aspect, the bimetallic strip includes a first substantially flat layer. According to the invention, the first substantially flat layer includes a positive magnetostrictive material. The bimetallic strip also has a second substantially flat layer attached to the first substantially flat layer forming a dual layered strip. The second substantially flat layer has a flexible material. 
         [0007]    In another aspect, the invention is a method of energy harvesting. The method includes the providing of a bimetallic strip having a first end and a second end. The bimetallic strip further includes a first substantially flat layer having a positive magnetostrictive material. According to the method, the magnetostrictive material is an Fe-based alloy having, Fe 100-x Al x  wherein x is from about 5 to about 25, or Fe 100-y Ga y  wherein y is from about 5 to about 35. The magnetostrictive material may also be formed of a combination of the Fe 100-x Al x  and the Fe 100-y Ga y . The providing of the bimetallic strip further includes providing a second substantially flat layer attached to the first substantially flat layer forming a dual layered strip. According to the method, the second substantially flat layer has a permanent magnet or a negative magnetostrictive material. The method further includes the attaching of the first end of the bimetallic strip to a first surface, and the attaching the second end of the bimetallic strip to an inertial mass or to a second surface. In this aspect, the method also includes the initiating of a changing magnetic arrangement and accompanying changing magnetic flux in the bimetallic strip by vibrating the first surface to produce a mechanical stress in the bimetallic strip. The method also includes the inducing of an AC voltage from the changing magnetic flux in a coil. This is done by wrapping the coil around the outer surface of the bimetallic strip. The associated induced current is stored or converted into a desired output form. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Other features will be apparent from the description, the drawings, and the claims. 
           [0009]      FIG. 1A  is a schematic illustration of a bimetallic strip according to an embodiment of the invention; 
           [0010]      FIG. 1B  is a schematic illustration of a bimetallic strip according to an embodiment of the invention; 
           [0011]      FIG. 2A  is a schematic illustration of a bimetallic strip according to an embodiment of the invention; 
           [0012]      FIG. 2B  is a schematic illustration of a bimetallic strip according to an embodiment of the invention; 
           [0013]      FIG. 2C  is a schematic illustration of a bimetallic strip according to an embodiment of the invention; 
           [0014]      FIG. 3A  is method of harvesting energy according to an embodiment of the invention; 
           [0015]      FIG. 3B  is a schematic illustration of an energy harvesting arrangement according to an embodiment of the invention; and 
           [0016]      FIG. 3C  is a schematic illustration of an energy harvesting arrangement according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1A  is a schematic illustration of a bimetallic strip  100  according to an embodiment of the invention. As shown, the bimetallic strip  100  is a two-layer structure having a first substantially flat layer  110  attached to a second substantially flat layer  120 . In this embodiment, the first layer  110  is a metallic strip having positive magnetostrictive properties. The first layer  110  may be an alloy of aluminum (Al) and iron (Fe), preferably Fe 100-x Al x , wherein x is from about 5 to about 25. The first layer  110  may also be an alloy of gallium (Ga), preferably Fe 100-y Ga y , wherein y is from about 5 to about 35. Alternatively, the first layer  110  may be a combination of Fe 100-x Al x , wherein x is from about 5 to about 25, and Fe 100-y Ga y , wherein y is from about 5 to about 35. 
         [0018]    To maximize the performance of the bimetallic strips in operations such as energy harvesting, actuating, and sensing, alloys having substantial amounts of the above outlined Fe-based Ga and Al alloys may also be used. As such, each of the above outlined Fe-based alloy layers, may optionally include one or more elements as small additions, such as carbon (C), manganese (Mn), sulfur (S), beryllium (Be), or Tin (Sn). Thus, for example, layer  110  may be a combination of Fe 100-x Al x  wherein x is from about 5 to about 25, carbon, and tin. In another example, layer  110  may be a combination of Fe 100-y Ga y  wherein y is from about 5 to about 35, and carbon. In yet another example, layer  110  may be a combination of Fe 100-x Al x , wherein x is from about 5 to about 25, and Fe 100-y Ga y , wherein y is from about 5 to about 35, carbon, and tin. It should be noted that the above outlined Fe-based alloys for layer  110  are merely examples, and other combinations may be used. 
         [0019]    The above outlined alloys have desirable properties such as strength, durability, and the ability to be welded. Additionally, the Fe—Al and Fe—Ga based alloys have high magnetostrictive levels that are only weakly dependent on temperature. The manufacture of the above mentioned Fe-based alloys can be readily accomplished because the alloys can be trained by stress annealing and/or magnetic field annealing, and can be inexpensively prepared. 
         [0020]    In this embodiment, the second substantially flat layer  120  is a flexible layer. In this embodiment, layer  120  is an alloy having negative magnetostrictive properties. The second layer may be nickel (Ni) or an appropriate Ni alloy, or any other alloy having negative magnetostrictive properties. Ni possesses a magnetostriction of about 50 ppm (negative). Consequently, when the second substantially flat layer  120  is Ni, the magnetostriction of the first Fe-based layer  110  can be readily magnetiostrictively matched to that of Ni by adjusting the amounts of Al or Ga. The bimetallic strip  100  composed of the two magnetostrictively active layers  110  and  120 , of the materials outlined above would be inexpensive. The Fe alloy as well as the Ni alloy may be prepared from bar stock, rolled stock, or by melt spinning techniques. The first and second layers  110  and  120  may be joined by means of welding, brazing, soldering, or any other means of adhesion. Although  FIG. 1A  shows the bimetallic strip  100  being substantially rectangular, the strip may have any shape associated with bimetallic strips in general. It should be noted that the size and the dimensions of the bimetallic strip and its components may vary depending on the application. 
         [0021]      FIG. 1B  is a schematic illustration of a bimetallic strip  150  according to an embodiment of the invention. In this embodiment, the bimetallic strip generally does not require a magnetic bias field to operate properly. As shown in  FIG. 1B , the bimetallic strip  150  is a two-layer structure having a first substantially flat layer  110  attached to a second substantially flat layer  160 . In this embodiment, the first layer  110  is a metallic strip having positive magnetostrictive properties, as described with respect to the embodiment of  FIG. 1A . Thus, as outlined above, the first layer  110  may be Fe 100-x Al x , wherein x is from about 5 to about 25, or Fe 100-y Ga y , wherein y is from about 5 to about 35, or a combination thereof. With respect to the Fe 100-y Ga y , an alloy with a very large magnetostriction of about 400 ppm may be employed. As outlined above, each of the above outlined Fe-based alloy layers, may optionally include one or more additional elements, such as for example, carbon (C), manganese (Mn), sulfur (S), beryllium (Be), or Tin (Sn). 
         [0022]    The second layer  160  shown in  FIG. 1B  is a flexible material. In this embodiment, layer  160  is a permanent magnet material. The permanent magnet material is not required to be particularly magnetically strong because fields are only required to be less than about 300 Oe. The permanent magnet material of layer  160  provides the proper bias field for the magnetostrictive Fe-based alloy. The permanent magnet may be a long thin Alnico magnet material or alternatively a thin coat of common ferrite material in a rubber-like matrix. As with the embodiment of  FIG. 1A , the first and second layers  110  and  160  may be joined by means of welding, brazing, soldering, or any other means of adhesion. Although  FIG. 1B  shows the bimetallic strip  100  being substantially rectangular, the strip may have any shape associated with bimetallic strips in general. Additionally, the size and the dimensions of the strip and its components may vary depending on the application. 
         [0023]      FIGS. 2A ,  2 B, and  2 C show bimetallic strips according to other embodiments of the invention.  FIGS. 2A ,  2 B, and  2 C show bimetallic strips  200 ,  250 , and  275  respectively, each having three or more layers. Each bimetallic strip is composed of the two strips (elements) of  FIG. 1A , and one or more permanent magnetic strips.  FIG. 2A  shows a three layer bimetallic strip  200  in which a permanent magnet material layer  140  is added to the bimetallic strip arrangement ( 110 ,  120 ) of  FIG. 1A . Although,  FIG. 2A  shows the permanent magnet material layer attached to the Fe-based alloy  110 , the permanent magnet material may alternatively attached to layer  120 , thereby sandwiching the negative magnetostrictive layer  120  between layers  110  and  140 .  FIG. 2B  shows a four layer bimetallic strip  250  in which permanent magnet material layers  140  sandwich the bimetallic strip arrangement ( 110 ,  120 ) of  FIG. 1A .  FIG. 2C  shows a three layer bimetallic strip  275  in which a permanent magnet material layer is positioned between the Fe-based alloy layer  110 , and the negative magnetostrictive layer  120 . In each bimetallic strip ( 200 ,  250 ,  275 ), the alloys can be trained by stress and or magnetic field annealing to obtain the best magnetic domain configuration to maximize energy transfer. Additionally, as stated above, although  FIGS. 2A ,  2 B, and  2 C show the bimetallic strips being substantially rectangular, the strip may have any shape associated with bimetallic strips in general. Additionally, the size and the dimensions of the strips and their components may vary depending on the application. 
         [0024]      FIGS. 3A ,  3 B, and  3 C illustrate a method  300  of harvesting energy according to an embodiment of the invention.  FIG. 3A  shows a flowchart of the method  300 , and  FIGS. 3B and 3C  schematically show the arrangement of elements of the energy harvesting method. Step  310  is the providing of a bimetallic strip  301  having a first end  302  and a second end  303 . According to the method  300 , the bimetallic strip  301  may have a structure according to any of the embodiments as outlined with respect to  FIGS. 1A ,  1 B,  2 A,  2 B, and  2 C. For example, if bimetallic strip  301  has a structure as outlined with respect to  FIG. 1A , then the strip has a first substantially flat layer that may be Fe 100-x Al x  wherein x is from about 5 to about 25, or Fe 100-y Ga y  wherein y is from about 5 to about 35, or a combination thereof. As outlined above, each of the above outlined Fe-based alloy layers may optionally include one or more additional elements, such as carbon (C), manganese (Mn), sulfur (S), beryllium (Be), or Tin (Sn). The strip would also have a second substantially flat layer that has negative magnetostrictive properties. The second layer may be nickel (Ni) or an appropriate Ni alloy, or any other alloy having negative magnetostrictive properties. 
         [0025]    Step  320  is the attaching of the first end  302  of the bimetallic strip  301  to a first surface  365 . The first surface is a surface that is positioned within a vibration rich environment. For example, the first surface  365  may exist in an aircraft or automotive environment. The first surface  365  may also be associated with a common household appliance, such as a refrigerator, a washing machine, microwave oven. The surface may also, for example, be associated with industrial equipment, buildings, or bridges. 
         [0026]    Step  330  is the attaching of the second end  303  of the bimetallic strip  301  to a second surface  366 . The second surface may also be a surface located within a vibration rich environment. At step  330 , the second end  303  may also be attached to an inertial mass  367 . Any known bonding or means of adhesive may be used to attach the strips to the respective surfaces and/or mass. 
         [0027]    Step  340  is the initiating of a changing magnetic arrangement and accompanying changing magnetic flux in the bimetallic strip  301 . This is accomplished by vibrating the first surface to produce a mechanical stress in the bimetallic strip.  FIGS. 3B and 3C  show the first surface vibrating in direction  380 . However, vibration may take place in other directions. In  FIG. 3B , the second surface may be stationary or may optionally vibrate as shown by arrow  381 . The vibrating of the surface may be initiated by mechanical means within the vibration rich environment. For example, if the first surface is in an aircraft or automobile, the vibrating of the surface may result from starting an engine. If for example, the surface is on a bridge, the vibration may result from natural means such as winds and/or water currents, or by other means such as the transportation of vehicles across the bridge. 
         [0028]    Because of the manner in which the bimetallic strip  301  is secured as shown in  FIGS. 3B and 3C , the strip undergoes mechanical stress due to the vibration. Because the bimetallic strip comprises magnetostrictive materials, the resulting mechanical stress rearranges the magnetic makeup of the strip, as well as the associated magnetic flux. 
         [0029]    Step  350  is the inducing of an alternating current from the changing magnetic flux. As shown in  FIGS. 3B and 3C , a coil  375  a wrapped around the bimetallic strip  301 . An alternating current is induced in the coil as a result of the changing magnetic flux of the strip  301 , which is caused by the mechanical stresses and resulting alterations in the bimetallic strip. Although the coil  375  is shown in one orientation, the coil may be positioned in other orientations to maximize the induction of the current. At step  360 , the induced current is stored via an electrical storage element. Alternatively, the induced current may be converted to another desired output using the appropriate circuitry. The energy harvesting method  300  is maximized by matching the bimetallic strip components and dimensions to the vibration amplitude and stress capability of the energy source. 
         [0030]    A number of exemplary implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, suitable results may be achieved if the steps of described techniques are performed in a different order and/or if components in a described component, system, architecture, or devices are combined in a different manner and/or replaced or supplemented by other components. For example, the Fe-based alloy for use in the bimetallic strips, may not necessarily be limited to Fe—Al or Fe—Ga, but other elements may be used for ease of alloy preparation, modification of device temperature dependence, and other auxiliary effects. Accordingly, other implementations are within the scope of the following claims.