Abstract:
An embodiment of the invention relates to providing an electrical component that provides an electrical functionality, the component comprising: a fiber felt comprising a tangle of fibers and characterized by a fill factor; and at least two layers of material formed on the fibers that contribute to providing the electrical functionality.

Description:
RELATED APPLICATIONS 
     The present application is a US National Phase of PCT Application No. PCT/IB2012/051646, filed on Apr. 4, 2012, which claims benefit under 35 U.S.C. §119(e) of U.S. Provisional Application 61/471,576 filed on Apr. 4, 2011 the disclosure of which is incorporated herein by reference. 
     RELATED APPLICATIONS 
     The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application 61/471,576 filed on Apr. 4, 2011, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD 
     Embodiments of the invention relate to capacitors and batteries comprising layers of material formed on fiber felts 
     BACKGROUND 
     Capacitors are passive, two terminal electrical devices for storing energy in electric fields and are commonly found in electrical and electronic circuits used in a wide variety of applications. They typically comprise a dielectric sandwiched between two generally parallel conductors and store energy in an electric field generated in the dielectric by negative charge accumulated on one of their conductors and positive charge accumulated on the other of their conductors. By way of example, capacitors are used to reduce ripple in voltage provided by power supplies, to multiply voltage in charge pumps, to isolate signal circuits from direct current (DC) signals and voltages, and to filter out noise in signal processing circuits. Capacitors function as snubbers in switching circuits, pulse power sources in weapons systems, and energy storage components in solid state circuits. 
     Electrical features of a capacitor are characterized by capacitance “C”, which is a coefficient that relates an amount of energy stored in an electric field in the dielectric of the capacitor to voltage between the capacitor conductors. If “E” represents an amount of energy stored in the capacitor for a given voltage difference “V” between the conductors the stored energy is given by an expression E=(½)CV 2 . An amount of charge “Q” that resides on the conductors is related to V by an expression Q=CV. 
     Capacitance C is determined by geometric features of the capacitor and permittivity of the dielectric. Assuming that the conductors are parallel planar conductors separated by a distance “d” and have area “A”, and that the relative permittivity of the dielectric is “∈ r ”, C may be given by an expression C=∈ o ∈ r A/d, where ∈ o  is the permittivity of free space. In the MKS (meter, kilogram, second) system of units, ∈ o =10 −9 /(36π), d is in meters (m), area A is in square meters (m 2 ) and C is given in Farads (F). A useful figure of merit of a capacitor is its specific capacitance, herein represented by “C*”, which is the capacitor&#39;s capacitance per unit volume of the capacitor. The specific capacitance provides an indication of how much space a capacitor occupies in a circuit for an amount of capacitance it provides. 
     As electronic components and circuits that comprise the components decrease in size and their three dimensional volumes shrink it is generally advantageous that capacitors also shrink and provide increasing specific capacitance. However, as capacitors shrink and their specific capacitances increase, for a given desired range of operating voltages, magnitudes of electric fields generated between their conductors, in their dielectrics, and between their terminals, increase. As a result, it becomes increasingly difficult to configure high specific capacitance capacitors to support the large fields they generate for operating voltage ranges and breakdown voltages required by many applications. 
     A product of an operating voltage of a capacitor and its specific capacity, may be used to provide a figure of merit for a capacitor that is responsive to both the specific capacitance and operating voltage of the capacitor. The product, hereinafter referred to as a “specific operating charge” Q*, is equal to a charge a capacitor stores at its operating voltage per unit volume of the capacitor. For convenience of presentation, specific operating charge is given below in units of Volts×μF/mm 3  (microfarad/cubic millimeter), rather than coulombs per m 3  (cubic meter). 
     Electrolytic capacitors, which comprise a metal terminal typically formed from Aluminum or Tantalum, on which a thin dielectric layer of an oxide of the metal is grown by an electrolytic process, are often used for applications that require high specific capacitance. By way of example, KEMET Electronic Corp markets a 1000 μF (microfarad) Tantalum electrolytic capacitor under the catalog number “T530X108M003AS” for surface mount applications. The KEMET capacitor operates at 2 volts, has dimensions 7.3 mm×4.3 mm×4.3 mm and therefore a specific capacitance equal to about 7.4 μF/mm 3 , and a specific operating charge equal to about 14.8 V μF/mm 3 . Whereas electrolytic capacitors provide relatively large capacitances their oxide-metal interfaces are rectifying. As a result they are generally polar capacitors having a positive terminal and a negative terminal and they must be connected to a circuit so that voltage on the positive terminal is always greater than or equal to a voltage on the negative terminal. 
     Applications that require very high capacitances and high specific capacitances may use electrochemical capacitors, also referred to as, super-capacitors, ultra-capacitors, or electric double-layer capacitors. The electrochemical capacitors comprise electrodes in contact with a liquid or solid electrolyte. As the name “electric double-layer capacitor” implies, these capacitors store energy in electric fields generated between two layers of opposite charge that are separated by very small distances. The charge double-layers may be separated by distances between about 0.3 nm (nanometers) to about 0.5 nm and are formed at the interfaces between the capacitors electrodes and the electrolyte. 
     As a result of the very small separation distances of the charge layers in their double-layers, electrochemical capacitors may have very large capacitances, and very large specific capacitances. An electrochemical capacitor may have capacitance of hundreds of Farads and a specific capacitance of a few milliFarads per cubic millimeter (mF/mm 3 ). However, their operating voltages are limited by breakdown voltages of their electrolytes, which are typically between 1 and 3 volts and they exhibit relatively large effective series resistance (ESR), which limits their operational frequency bandwidths. 
     Batteries have structures similar to that of capacitors and generally comprise two terminals coupled respectively to an anode electrode and a cathode electrode that sandwich between them an electrolyte. However, whereas in capacitors materials in the electrodes do not chemically react with material in the electrolyte, in batteries materials in the anode and cathode undergo oxidation and reduction reactions to store electrical energy and deliver stored electric energy to a load. 
     SUMMARY 
     An aspect of the invention relates to providing a capacitor, hereinafter referred to as a “fiber felt capacitor”, characterized by a relatively high specific capacitance that comprises a layer of a dielectric material sandwiched between first and second conductors formed by atomic layer deposition (ALD) on a fiber felt “scaffolding”. 
     A fiber felt refers to a material comprising randomly oriented, entangled, synthetic or natural fibers. The fibers may by way of example, comprise conductive and/or non-conductive fibers and may be formed from any of a variety of materials, including plastic, glass, ceramic, and/or metal. Any of various methods, such as by way of example, turning, drawing, cutting, chopping, and solgel and/or aerogel processes may be used to produce the fiber felt. A fiber felt may be configured to have different surface areas per unit volume by adjusting a fraction, hereinafter referred to as a fill factor (FF), of a unit volume of the fiber felt that is occupied by its fibers. For a given fiber cross section, the surface area per unit volume of the fiber felt is proportional to the fill factor. 
     For use as scaffolding for a fiber felt capacitor in accordance with an embodiment of the invention, a fiber felt is configured having a relatively large surface area per unit volume. In an embodiment of the invention, the first conductor of the fiber felt capacitor is formed as a layer directly on the surfaces of the felt fibers in the scaffolding or on at least one substrate layer formed on the felt fibers. In embodiments of the invention for which the felt fibers are formed from a conductive material, optionally, the fibers may function as the first conductor. 
     Each of the conductors and the dielectric layer of the fiber felt capacitor that are formed on the fibers of the scaffolding have surface areas at least as large as that of the fibers on which they are formed. As a result, per unit volume, the fiber felt capacitor in accordance with an embodiment of the invention has a relatively large value for the surface area “A” that appears in the expression for capacitance discussed above. In addition, forming the dielectric layer by ALD provides a relatively high integrity uniform dielectric layer that may have a relatively small value for thickness, “d”, of the dielectric layer that appears in the expression for the capacitance. The large surface area, A, per unit volume, and small thickness, d, result in the fiber felt capacitor having a relatively large specific capacitance. The large surface area provides the fiber felt capacitor with a relatively large specific operating charge. 
     In an embodiment of the invention, the fiber felt capacitor comprises a first terminal that is electrically connected to the first conductor prior to forming the dielectric layer. Optionally, the first terminal comprises a planar conducting plate brazed to the first conductor. In an embodiment of the invention, the fiber felt capacitor comprises a second electrical terminal, which is optionally brazed to the second conductor to electrically connect the second terminal to the second conducting layer. Optionally, the second terminal comprises a planar conducting plate parallel to the plate of the first terminal. 
     In an embodiment of the invention, a spacer formed from an insulating material separates the first and second terminals and surrounds the fiber felt scaffolding and the layers formed thereon. The insulating spacer may be formed prior to forming the dielectric layer or prior to forming the second conductor. 
     Hereinafter, a fiber felt scaffolding comprised in a fiber felt capacitor or other component in accordance with an embodiment of the invention may also be referred to simply as a fiber felt. Unless otherwise specified, a given layer of material said to be formed “on” fibers comprised in a fiber felt in accordance with an embodiment of the invention may be formed directly on the fibers or on a layer of material intermediate the given layer of material and the fibers. 
     An aspect of some embodiments of the invention relate to providing an electrochemical fiber felt capacitor comprising layers of material formed using ALD processes on a fiber felt. The layers include a solid electrolyte sandwiched between an anode layer and a cathode layer that store energy in electric fields generated by electric double layers at the interfaces between the electrolyte and the anode and cathode when a voltage difference is applied between the anode and cathode. 
     An aspect of some embodiments of the invention relate to providing a fiber felt battery in which layers of material are formed using ALD processes on a fiber felt that include a solid electrolyte layer sandwiched between an anode layer and a cathode layer. The layers comprise active materials that undergo oxidation reactions and reduction reactions at the anode and cathode respectively to support current that the battery provides to a load. 
     There is therefore provided in accordance with an embodiment of the invention, an electrical component that provides an electrical functionality, the component comprising: a fiber felt comprising a tangle of fibers and characterized by a fill factor; and at least two layers of material formed on the fibers that contribute to providing the electrical functionality. Optionally, the layers are formed by atomic layer deposition (ALD). 
     Additionally or alternatively, the fibers have a radius less than or equal to about 1 micron. Optionally, the fibers have a radius less than or equal to about 500 nm (nanometers). Optionally, herein the fibers have a radius less than or equal to about 100 nm. 
     In an embodiment of the invention, the fill factor is greater than or equal to about 0.4. Optionally, the fill factor is greater than or equal to about 0.5. Optionally, the fill factor is greater than or equal to about 0.6. 
     In an embodiment of the invention, the at least two layers comprise a first layer formed from an electrically conducting material. Optionally, the at least two layers comprise a second layer that underlies the first layer and is formed from a dielectric material. Optionally the electrical component comprises a conductor underlying the second layer. Optionally, the conductor comprises a third layer underlying the second layer that is formed on the fibers from a conducting material. Alternatively or additionally, the fibers are formed from a metal and the conductor comprises the fibers. 
     In an embodiment of the invention the electrical component comprises a first terminal electrically connected to the first layer and a second terminal electrically connected to the conductor underlying the second layer. 
     In an embodiment of the invention the fill factor and layers formed on the fibers are configured to have a specific operating charge of the fiber felt that is greater than or equal to about 20 VμF/mm 3 , wherein the specific operating charge is equal to an amount of charge stored per mm 3  of the fiber felt volume for a voltage between the first and second terminals that is about 0.6 times a voltage at which the dielectric layer between the first and second layers breaks down. Optionally, the specific operating charge is greater than or equal to about 120 μVμF/mm 3 . Optionally, the specific operating charge is greater than or equal to about 600 VμF/mm 3 . 
     In an embodiment of the invention, the at least two layers comprise a second layer that underlies the first layer and is formed from a solid electrolyte. Optionally, the electrical component comprises a third layer underlying the second layer and wherein the second layer is contiguous and interfaces with the first layer and the second layer and when a voltage difference is applied between the first layer and the third layer, a charge double-layer is formed at the interfaces. 
     In an embodiment of the invention, the at least two layers comprise a battery anode layer contiguous with a solid electrolyte layer and in addition, a battery cathode layer contiguous with the solid electrode layer and wherein materials in the electrolyte react with materials in the anode and cathode to undergo oxidation and reduction reactions. Optionally, the battery cathode layer has thickness greater than or equal to about 100 nm. Alternatively or additionally, the battery anode layer has thickness greater than or equal to about 100 nm. In an embodiment of the invention, the solid electrolyte layer has thickness between about 10 nm and about 50 nm. 
     In the discussion, unless otherwise stated, adjectives such as “substantially” and “about” modifying a condition or relationship characteristic of a feature or features of an embodiment of the invention, are understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       Non-limiting examples of embodiments of the invention are described below with reference to figures attached hereto that are listed following this paragraph. Identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. 
         FIG. 1A  schematically shows a fiber felt to be prepared for use as a scaffolding in providing a fiber felt capacitor in accordance with an embodiment of the invention; 
         FIGS. 1B and 1C  schematically show the fiber felt shown in  FIG. 1A  after preliminary processing to prepare the fiber felt for producing a fiber felt capacitor in accordance with an embodiment of the invention; 
         FIG. 1D  schematically shows a greatly enlarged view of entangled fibers in the fiber felt following preliminary processing, in accordance with an embodiment of the invention; 
         FIG. 1E  schematically shows the entangled fibers shown in  FIG. 1D  after a substrate layer is formed on the fibers and a first conductor of the fiber felt capacitor is formed as a layer on the substrate layer, in accordance with an embodiment of the invention; 
         FIG. 1F  schematically shows a first terminal of the fiber felt capacitor brazed to the entangled fibers shown in  FIG. 1E , in accordance with an embodiment of the invention; 
         FIG. 1G  schematically shows a cross section of the fiber felt scaffolding and the first terminal after brazing of the first terminal to the first conducting layer, in accordance with an embodiment of the invention; 
         FIG. 1H  schematically shows the entangled fibers shown in  FIG. 1F  after a dielectric layer of the fiber felt capacitor is formed on the first conducting layer shown in  FIGS. 1E and 1F , in accordance with an embodiment of the invention; 
         FIG. 1I  schematically shows a cross section of the fiber felt scaffolding and the first terminal after formation of an insulating spacer on the first terminal, in accordance with an embodiment of the invention; 
         FIG. 1J  schematically shows the entangled fibers shown in  FIG. 1H  after a second conductor of the fiber felt capacitor is formed as a layer on the dielectric layer shown in  FIG. 1H , in accordance with an embodiment of the invention; 
         FIG. 1K  schematically shows a cross section of the fiber felt scaffolding after the second conductor is formed and a second terminal brazed to the second conductor to complete the fiber felt capacitor, in accordance with an embodiment of the invention; 
         FIG. 2A  shows a scanning electron microscope (SEM) image of a fiber felt comprising sintered 1 μm (micrometer) diameter stainless steel fibers for use as a scaffolding in a fiber felt capacitor, electrochemical capacitor, or a battery, in accordance with an embodiment of the invention; 
         FIG. 2B  shows a SEM image of the fiber felt scaffolding shown in  FIG. 2A  after the fibers have been coated by atomic layer deposition with layers for a capacitor, in accordance with an embodiment of the invention; 
         FIG. 3A  schematically shows an enlarged view of the tangled fibers shown in  FIG. 1D  on which a cathode layer, an electrolyte layer, and an anode layer of an electrochemical fiber felt capacitor are formed, in accordance with an embodiment of the invention; and 
         FIG. 3B  schematically shows an enlarged view of the tangled fibers shown in  FIG. 1D  on which layers of a fiber felt battery are formed, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description an example of a process by which a fiber felt capacitor is produced in accordance with an embodiment of the invention is schematically illustrated in  FIGS. 1A-1K  and discussed with reference to the figures. A numerical example of a fiber felt capacitor produced in accordance with a process similar to that described with reference to  FIGS. 1A-1K  and details of features of the process are discussed following the discussion of  FIGS. 1A-1K . SEM images of an actual fiber felt scaffolding before and after having formed on its fibers ALD layers to provide a fiber felt capacitor in accordance with an embodiment of the invention are shown in  FIGS. 2A and 2B  respectively. A discussion and examples of fiber felt electrochemical capacitors and fiber felt batteries in accordance with embodiments of the invention schematically shown in  FIGS. 3A and 3B  respectively follow the discussion of the production and specification of a fiber felt capacitor illustrated in  FIGS. 1A-1K . Fiber felt 
       FIG. 1A  schematically shows a fiber felt  20  for use in producing a fiber felt capacitor  100 , schematically shown in  FIG. 1K , in accordance with an embodiment of the invention. Fiber felt  20  comprises a tangle of fibers  22  that are fused or bonded to each other at points at which fibers in the tangle touch. Fibers  22  may be any of various synthetic and/or natural fibers, and may be formed from any of a variety of materials, including by way of example, plastics, polymers, glasses, ceramics, and/or metals. The fibers may be bonded at contact points by sintering, or bonding with a suitable bonding agent. Optionally, fibers  22  have an average diameter greater than or equal to about 100 nm (nanometers). Optionally, the fibers have an average diameter equal to or greater than about 500 nm. In an embodiment, fibers  22  have an average diameter equal to or greater than about 1 μm. 
     Optionally, fiber felt  20  is a commercially available fiber felt that may be used for filtering a fluid. A fluid filtration fiber felt often has a relatively small fill factor to moderate a degree to which it may interfere with flow through the fiber felt of a fluid that the fiber felt is used to filter. The fill factor may be smaller than about 0.15, and is typically between about 0.10 and about 0.15. 
     In accordance with an embodiment of the invention, fiber felt  20  is trimmed and compressed to configure the fiber felt to a desired shape and increase its fill factor so that it has a surface area per unit volume sufficiently large to provide a fiber felt capacitor having a desired specific capacitance.  FIG. 1B  schematically shows a cross section of fiber felt  20  after the fiber felt has been trimmed to a desired size. Optionally, fiber felt  20  is trimmed to a shape of a circular disk.  FIG. 1C  schematically shows a perspective view of fiber felt  20  shown in  FIG. 1B  after it has been shaped to a circular disk and compressed to increase its surface area per unit volume. Optionally, fiber felt  20  is compressed to a degree so that it has a fill factor greater than or equal to about 0.40. Optionally, the fiber felt is compressed to a fill factor greater than or equal to about 0.50. In an embodiment of the invention fiber felt  20  is compressed to a fill factor greater than or equal to about 0.60. 
       FIG. 1D  schematically shows a greatly enlarged view of portions of entangled fibers  22  in fiber felt  20  after the fiber felt has undergone compression, as indicated by  FIG. 1C  referenced to  FIG. 1B . Fibers  22  are fused or bonded together at a common contact region  24  and are shown severed to exhibit their cross sections. 
     Following trimming compression and fusing or bonding, fibers  22  are optionally coated with a layer of material that provides an advantageous substrate on which to form a first conductor of fiber felt capacitor  100  ( FIG. 1K ).  FIG. 1E  schematically shows entangled fibers  22  shown in  FIG. 1D  following coating with a substrate layer  30  and formation of a conducting layer  31  on the substrate layer. Conducting layer  31  may function as a first conductor of the fiber felt capacitor and may be formed from any of various conducting materials such as by way of example, Tungsten Nitride (WN), Titanium Nitride (TiN), Tantalum Nitride (TaN), Copper (Cu), Platinum (Pt), Tungsten (W) and/or Silver (Ag). Optionally, layers  30  and  31  are formed by ALD. In embodiments of the invention for which fibers  22  are formed form a conducting material having suitable conductivity and resistance to oxidation, the fibers may function as the first conductor of fiber felt capacitor  100 . 
     In an embodiment of the invention, following formation of conducting layer  31 , a conducting terminal  41 , schematically shown in cross section in  FIG. 1F , is mechanically and electrically connected to conducting layer  31 , optionally by brazing. During brazing a filler metal  50  is heated to wet and electrically and mechanically bond regions of conducting layer  31  and regions of terminal  41  that are in contact. In  FIG. 1F  a region  131  of conducting layer  31  located on a fiber  22  that is assumed to be in close proximity to terminal  41  is schematically shown wetted by filler metal  50 .  FIG. 1G  schematically shows a “zoomed-out” cross section view of fiber felt  20  and terminal  41  after brazing. Optionally, as shown in  FIG. 1G  and figures that follow, terminal  41  extends beyond fiber felt  20  to form a shelf region  42  surrounding a “bottom” edge  26  of the fiber felt. 
     Following brazing, a dielectric layer  32  schematically shown in  FIG. 1H  is formed, optionally in an ALD process on conducting layers  31  formed on fibers  22  and on shelf region  42  of terminal  41 . By way of example dielectric layer  32  may comprise Al 2 O 3 , ZrO 2 , HfO 2 , SiO 2 , SrTiO 3 , BaTiO 3  and/or Ta 2 O 5 . Optionally, subsequent to formation of dielectric layer  32 , an insulating spacer  60 , schematically shown in a cross section view of fiber felt  20  and terminal  41  in  FIG. 1I  is formed on shelf  42 . Insulating spacer  60  has a top surface  61  and side surfaces  62  and may completely surround fiber felt  20  and have a height substantially equal to thickness of the fiber felt. Any of various insulating materials that are not damaged by processes in the production of fiber felt capacitor  100 , such as a glass or plastic may be used to form insulating spacer  60 . A second conducting layer  33 , schematically shown in  FIG. 1J , which functions as a second conductor of fiber felt capacitor  100  is formed, optionally by ALD, on dielectric layer  32  and on exposed surfaces of insulating spacer  60 , such as top surface  61  and side surface  62  of the insulating spacer shown in  FIG. 1I . Optionally, second conducting layer  33  comprises a conducting material from which first conducting layer  31  may be formed. 
     In an embodiment of the invention, a second terminal  43  schematically shown in a cross section view in  FIG. 1K  is brazed to second conducting layer  33  ( FIG. 1J ) to mechanically and electrically connect the second terminal to the second conducting layer and complete production of fiber felt capacitor  100 . 
     By way of a particular numerical example, a fiber felt capacitor in accordance with an embodiment of the invention similar to fiber felt capacitor  100  ( FIG. 1K ) may be formed from a fiber felt  20  ( FIG. 1A ) having an initial fill factor about 0.20 and comprising fibers  22  having an average diameter equal to about 1 μm. Optionally, the fibers are formed from stainless steel  316 . In an embodiment of the invention, the fiber felt is trimmed ( FIG. 1B ) and compressed ( FIG. 1C ) to form a disk shaped fiber felt scaffolding having a diameter of about 10 mm, thickness equal to about 1.0 mm, and fill factor equal to about 0.50. The compressed disk may be sintered in a vacuum furnace at a temperature of about 1020° C. (degrees Celsius) for about ten minutes to cause stainless steel fibers  22  in the disk to fuse in regions where they are in contact.  FIG. 2A  shows a scanning electron microscope (SEM) image of a region of a fiber felt scaffolding after sintering formed in accordance with an embodiment of the invention and comprising 1 μm diameter stainless steel  316  fibers. 
     In an embodiment of the invention, conducting layer  31 , dielectric layer  32 , and second conducting layer  33  are formed by ALD from Platinum (Pt), Al 2 O 3  (alumina, or Aluminum oxide) and Pt respectively. Optionally, alumina dielectric layer  32  and conducting layers  31  and  33  have thickness equal to about 10 nm. Substrate layer  30  on which conducting layer  31  is formed is optionally produced from Al 2 O 3  deposited on stainless steel fibers  22  by an ALD process. Al 2 O 3  accelerates deposition of Pt in ALD processes.  FIG. 2B  shows a SEM image of a fiber felt scaffolding similar to that shown in  FIG. 2A  after forming layers  30 - 33  noted above using ALD, in accordance with an embodiment of the invention. 
     To minimize strain in and between various components of fiber felt capacitor  100  generated by changes in ambient and/or operating temperatures of the fiber felt capacitor it is advantageous that components of the fiber felt capacitor have coefficients of thermal expansion that are substantially the same and/or are relatively small. It is therefore generally advantageous that first and second terminals  41  and  43  ( FIG. 1K ) be formed from a same material from which fibers  22  are formed. In an embodiment of the invention similar to that discussed above for which fibers  22  are formed from stainless steel  316 , which has a coefficient of thermal expansion equal to about 16.5×10 −6  mm° C., optionally terminals  41  and  43  are also formed from stainless steel  316 . Optionally, terminal  41  and  43  are formed from copper, which has a coefficient of thermal expansion equal to about 17×10 −6  mm/° C. 
     Brazing of terminals  41  or  43  to conducting layers  31  and  33  respectively may be performed with a filler metal that melts at a temperature below a temperature at which materials already integrated in the production of capacitor  100  at the time of brazing may be damaged. The filler metal should also be such that it itself is not damaged by a process in the production of the capacitor subsequent to brazing with the filler metal. 
     For example, silver melts at a temperature below the melting temperatures of stainless steel, alumina, and platinum and above temperatures used in ALD processes used to produce fiber felt capacitor  100 . Silver also melts at a temperature above that which capacitor  100  may be subjected to in installing, for example by soldering, the fiber felt capacitor in a circuit. Silver is therefore a suitable candidate of use as a filler metal  50  to braze terminal  41 . As for second terminal  43 , a filler metal used to braze terminal  43  advantageously satisfies all the constraints that a filler used to braze terminal  41  satisfies and in addition should have a melting temperature lower than that of the filler metal used to braze terminal  41 . Zinc satisfies the constraints satisfied by silver and has a melting temperature less than that of silver. Zinc is therefore a suitable candidate as a filler metal for brazing second terminal  43 . It is noted that whereas in the above discussion pure metals were cited as filler metals, a filler metal is generally a composite of more than one metal tailored to melt at a desired temperature. 
     Insulating spacer  60  may be formed from any of various insulating materials that are not damaged at processing temperatures at which dielectric layer  32  and conducting layer  33  are formed or terminal  43  is brazed to conduction layer  33 . Advantageously, the insulating materials have a coefficient of thermal expansion similar to that of stainless steel  316 . ALD formation of the dielectric and conducting layers may be performed at temperatures between about 250° C. and about 300° C. Brazing of terminal  43  to conducting layer  33  with a zinc filler metal may be performed at a temperature of about 420° C. Various glasses and polymers are available that satisfy the temperature and coefficient of thermal expansion constraints noted above and may be used to provide insulating spacer  60 . 
     Let “R” and “τ” represent the radius and thickness respectively of a disk shaped fiber felt scaffolding comprised in a fiber felt capacitor similar to fiber felt capacitor  100  in accordance with an embodiment of the invention, and let “FF” represent the fill factor of the fiber felt scaffolding  20 . If the fibers in the fiber felt scaffolding have radius φ then a total length “L” of fiber  22  in the scaffolding may be estimated by an expression L=FF(R 2 τ/φ 2 ). A total area “A” of the surfaces of fibers  22  in the fiber felt scaffolding may then be estimated by an expression A=2 πφFF(R 2 τ/φ 2 )=2 πFF(R 2 τ/φ). If dielectric layer  32  has a thickness “d”, then a total capacitance C of the fiber felt capacitor may be estimated by an expression C=∈ o ∈ r 2 πFF(R 2 τ/φd). If an operating voltage of fiber felt capacitor  100  is represented by V o  volts then a specific operating charge, Q* of capacitor  100  may be expressed by Q*=∈ o ∈ r 2FF(1/φd)V o . Optionally, an operating voltage is equal to about 0.6 of a voltage at which dielectric layer  32  breaks down. 
     Evaluating the expressions for C and Q* for the above numerical example of fiber felt capacitor  100 , (for which, as noted above FF=0.5, φ=0.5×10 −6  m, d=10×10 −9  m and ∈ r =7 for Al 2 O 3 ) C is about equal to 969 μF. The 10 nm thick Al 2 O 3  dielectric layer  32  has a breakdown voltage equal to about 8 volts and therefore fiber felt capacitor  100  may have an operating voltage V o  equal to about 5 volts, and a corresponding specific operating charge Q* equal to about 62 VμF/mm 3 . 
     It is noted that in general operating voltage V o  is proportional to thickness d of dielectric layer  32 . As a result, specific operating charge Q* is substantially independent of d. However Q* is inversely proportional to radius φ of fibers  22  and fiber felt capacitor  100  may be configured to have different values of Q* by forming layers  30 - 33  on fibers  22  having different radii. A fiber felt capacitor in accordance with an embodiment of the invention similar to fiber felt capacitor  100  may have a Q* greater than or less than 62 VμF/mm 3  by forming layers  30 - 33  on fibers  22  having radii less than or greater than 0.5×10 −6  m respectively. For example, forming layers  30 - 33  on fibers  22  having radii about equal to 0.25×10 −6  m provides a fiber felt capacitor similar to fiber felt capacitor  100  having Q* equal to about 124 VμF/mm 3 . Forming layers  30 - 33  on fibers  22  having radii equal to about 50×10 −9  m provides a fiber felt capacitor similar to fiber felt capacitor  100  having Q* equal to about 620 VμF/mm 3 . 
     Whereas in the figures and the above discussion a fiber felt capacitor comprising a dielectric layer sandwiched between conducting layers is shown and described, practice of embodiments of the invention is not limited to dielectric type capacitors. For example, methods and components used to provide capacitor  100  may be used to provide an electrochemical fiber felt capacitor or a fiber felt battery. 
     In an embodiment of the invention, a fiber felt electrochemical capacitor is produced by forming on fibers  22  of fiber felt  20  ( FIG. 1C ) a solid electrolyte layer sandwiched between a cathode layer and an anode layer, optionally by an ALD process.  FIG. 3A  schematically shows an enlarged view of tangled fibers  22  shown in  FIG. 1D  on which a cathode layer  101 , a solid electrolyte layer  102 , and an anode layer  103  comprised in an electrochemical capacitor (not shown), in accordance with an embodiment of the invention are formed on fibers  22 . A terminal  41  shown in  FIG. 3A  provides an electrical connection to cathode layer  101  and is brazed to the cathode layer before electrolyte layer  102  is formed using an appropriate filler metal  50 . A terminal (not shown) is similarly brazed to anode layer  103  to provide electrical connection to the anode layer. An insulating spacer (not shown) optionally similar to insulating spacer  60  ( FIGS. 1I and 1K ) electrically isolates the terminals one from the other. 
     Optionally, cathode layer  101  is formed on fibers  22  ( FIGS. 1A-1D ) from a material that may comprise at least one of: LiFePO 4 , LiCoO 2 , LiMn 2 O 4 , LiNiO 2 , LiMPO 4 , where “M” represents a metal which may be Fe, Co, Mn, or Ti; and LiFe 0.95 V 0.05 PO 4  and A 2 FePO 4 , where “A” represents Na, or Li. Solid electrolyte layer  102  is formed on cathode layer  101  and may comprise at least one of: Lithium Phosphorous Oxynitride (Lipon), Lithium Lanthanum Titanate (LLT), Beta-alumina complexed with a mobile ion such as Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+ or Ba2+, non-stoichiometric Sodium Aluminate, Yttria-stabilized zirconia (YSZ) and (Li,La) x Ti y O z . Anode layer  103  formed on the solid electrolyte layer may be formed from at least one of: Li 4 Ti 5 O 12 , Ge(Li 4.4 Ge), Si(Li 4.4 Ge), Lithium-Titanate or Lithium Vanadium Oxide. It is noted that whereas in  FIG. 3A  cathode layer  101  is formed on fibers  22 , the order of layers  101 ,  102  and  103  may be reversed, with anode layer  103  formed on fibers  22  instead of cathode layer  101 . 
     Whereas  FIG. 3A  shows three layers formed on fibers  22  an electrochemical capacitor, in accordance with an embodiment of the invention is not limited to three layers. For example, an electrochemical capacitor may comprise a five layer structure in which the three layer structure schematically shown in  FIG. 3A  is sandwiched between conducting layers. 
     A fiber felt battery may be formed on fiber felt scaffolding  20  in accordance with an embodiment of the invention by forming an optionally five layer structure on fibers  22 . The five layer structure may comprise first and second conductors that sandwich between them a battery anode, a solid electrolyte, and a battery cathode. If fibers  22  are sufficiently conducting, the fibers may function as the first conductor, otherwise a conducting layer of a suitable metal may be formed on fibers  22  to function as the first conductor of the battery. In an embodiment, a fiber felt battery may have a battery anode and a battery cathode that are sufficiently conducting, so that they function as first and second conductors respectively. The fiber felt battery may then comprise a three layer structure. 
       FIG. 3B  schematically shows an enlarged view of tangled fibers  22  shown in  FIG. 1D  on which a five layer structure  200  comprised in a fiber felt battery (not shown) in accordance with an embodiment of the invention, is formed. Five layer structure  200  optionally comprises a first conducting layer  201 , a battery anode layer  202 , a solid electrolyte layer  203 , a battery cathode layer  204 , and a second conducting layer  205 . Optionally layers  201 - 205  are formed by ALD. A terminal  41  shown in  FIG. 3B  provides an electrical connection to first conducting layer  201  and is brazed to the conducting layer optionally before battery anode layer  202  is formed using an appropriate filler metal  50 . A second terminal (not shown) is similarly brazed to second conducting layer  205  to provide electrical connection to the conducting layer. An insulating spacer optionally similar to insulating spacer  60  ( FIGS. 1I and 1K ) electrically isolates the first and second terminals one from the other. 
     Optionally anode layer  202  of the fiber felt battery is formed on fibers  22  and comprises at least one of Li 4 Ti 5 O 12 , Ge(Li 4.4 Ge), Si(Li 4.4 Ge), Lithium-Titanate or Lithium Vanadium Oxide. Solid electrolyte layer  203  formed on the anode layer may comprise at least one of: Lithium Phosphorous Oxynitride (Lipon), Lithium Lanthanum Titanate (LLT), Beta-alumina complexed with a mobile ions such as Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+, or Ba2+, nonstoichiometric Sodium Aluminate, Yttria stabilized zirconia (YSZ) and (Li,La)xTiyOz. Cathode layer  204  formed on the electrolyte layer may comprise at least one of: LiFePO 4 , LiCoO 2 , LiMn 2 O 4 , LiNiO 2 , LiMPO 4 , where M stands for a metal such as Fe, Co, Mn, Ti, and LiFe 0.95 V 0.05 PO 4 . Conducting layers  201  and  205  are formed from any suitable conducting material and may comprise a metal, such as by way of example Ag, Cu, Al, W, Zn, Ni, Fe, and/or Pt. Whereas in  FIG. 3B  anode layer  202  is formed on conducting layer  201 , the order of layers  202 ,  203  and  204  may be reversed, with cathode layer  202  formed on conductor  201  instead of cathode layer  202 . 
     In an embodiment of the invention, fiber felt  20  ( FIG. 1C ) has a fill factor equal to about 0.5, and fibers  22  on which layer structure  200  is formed have a diameter equal to about 1 μm. Assuming fiber felt  20  is disk shaped, as shown in  FIG. 1C , and has a diameter equal to about 10 mm and thickness equal to about 1 mm, fibers  22  in the fiber felt have a total surface area equal to about 1,500 cm 2 . The battery comprising layer structure  200  has an ion transport cross section substantially equal to the total surface area of fibers  22 , and is therefore also equal to about 1,500 cm 2 . Optionally, cathode and anode layers  202  and  204  have thickness equal to about 100 nm and electrolyte layer  203  has a thickness between about 10 nm and about 50 nm. An ion transport distance of the battery may therefore be estimated as being equal to between about 100 nm and 200 nm. 
     A conventional button battery may have an ion transport cross section of about 1 cm 2  and an ion transport distance equal to about 0.5 mm. Therefore a battery in accordance with an embodiment of the invention having dimension similar to those given in the preceding paragraph may be expected to have an internal ion transport resistance at least, 1/10 6  that of conventional batteries. 
     In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of components, elements or parts of the subject or subjects of the verb. 
     Descriptions of embodiments of the invention in the present application are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described, and embodiments of the invention comprising different combinations of features noted in the described embodiments, will occur to persons of the art. The scope of the invention is limited only by the claims.