Patent Publication Number: US-8525633-B2

Title: Fusible substrate

Description:
This application claims priority to, and the benefit of, U.S. Provisional Application 61/046,653, filed Apr. 21, 2008, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates, generally, to circuit protection devices. More particularly, it relates to fusible substrates that fracture upon reaching a predetermined temperature to provide overcurrent protection. 
     Existing fuses have several issues regarding both failing when they should not fail and not failing when they should fail. Severe surges such as lightning strikes should cause the fuse to fail; however, the fuse needs to withstand smaller surges such as those that occur upon initial current flow through the circuit. Brief, severe surges are not the only condition that should cause fuse failure. A phenomenon known as a sneak current can also overload a circuit resulting in fuse failure. Sneak currents occur by an incident such as a power line falling on top of a telephone line, which induces a low level increase in current that exceeds the capacity of the circuit. Present fuse technology allows for complete fuse failure within 30 seconds under a sneak current. Although this time appears to be short, circuit damage can still occur within these 30 seconds. 
     A phenomenon known as arcing can also be problematic in that it allows the fuse to carry current after the onset of melting. The fuse element begins to melt at its hottest spot, typically in the middle of the fuse. Metal vapor remains in the air gap between the melted ends. The metal vapor continues to conduct the current across the gap which is fed by the voltage in the circuit. The arc generates a plasma of ionized gases which then takes over the current. The ionized arc creates more heat, pressure, and current in the gap. 
     SUMMARY 
     In an embodiment, a fuse element includes a substrate disposed between first and second terminals. The substrate includes an electrically insulative material. A conductive film is disposed on a first surface of the substrate and in electrical contact with the first terminal and second terminals. In an embodiment, the substrate includes a ceramic material. In an embodiment, the film includes a metal selected from the group consisting of copper, gold, and mixtures thereof. In an embodiment, the coefficient of thermal expansion of the substrate is lower than a coefficient of thermal expansion of the coating. 
     In an embodiment, the substrate has a cylindrical shape. In an embodiment, the conductive film is disposed on an outer surface of the substrate. In another embodiment, the substrate has a rectangular cross section and four outer surfaces extending between the terminals. In an embodiment, the conductive film is disposed on one of the outer surfaces of the substrate. 
     In an embodiment, a fuse element includes a substrate disposed between first and second terminals. The substrate includes a conductive polymer material. In an embodiment, the conductive polymer material includes metal particles dispersed in a polymer matrix. In another embodiment, the conductive polymer material includes a doped polymer material. 
     In an embodiment, a fuse element includes a substrate disposed between first and second terminals. The substrate is composed of a material with a melting point between 300° C. and 800° C. A layer including a conductive material is disposed over the substrate. In an embodiment, the substrate is composed of a wax. In an embodiment, the substrate is capable of withstanding a temperature of 260° C. for at least 2 minutes without melting. 
     In an embodiment, a fuse element includes a conductive material disposed between the first terminal and the second terminal. A substrate is disposed between the conductive material and one of the first terminal and the second terminal. The substrate is composed of a material with a melting point between 300° C. and 800° C. In an embodiment, the substrate includes a first substrate, further including a second substrate disposed between the conductive material and the other of the first terminal and the second terminal. In an embodiment, the substrate is capable of withstanding a temperature of 260° C. for at least 2 minutes without melting. 
     Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  is an isometric view of an embodiment of a fuse element. 
         FIG. 1B  is a cross-section view of the fuse element of  FIG. 1A . 
         FIG. 2  is an isometric view of another embodiment of a fuse element. 
         FIG. 3A  is an isometric view of another embodiment of a fuse element. 
         FIG. 3B  is a cross-section view of the fuse element of  FIG. 3A . 
         FIG. 4  is an isometric view of another embodiment of a fuse element. 
         FIG. 5  is an isometric view of another embodiment of a fuse element. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides a fuse element that fractures rather than melts, which reduces failure time and provides overcurrent protection. 
     The present disclosure provides a fuse that breaks a current quickly when operating parameters are exceeded without the potential for arcing. The fuse is particularly useful for telecommunications circuit boards. Specifically, the present disclosure provides fuse elements including an insulating substrate with a conductive coating. Unlike existing fuses, which generally rely on a melting mechanism for failure, the fuse elements disclosed herein fracture rather than melt. By eliminating the need for melting in the fuse element, the chance for arcing is reduced. By breaking a conductive material apart from an insulating substrate as an alternative to melting, a large gap between the contacts is created, raising the arcing voltage. The fuse elements disclosed herein capitalize on a mismatch in the coefficients of thermal expansion between the substrate and conductive layer. 
       FIGS. 1A and 1B  illustrate a fuse element  10  including a conductive coating  14  on a substrate  12 . The substrate  12  is preferably constructed from a ceramic with a low coefficient of thermal expansion. The substrate  12  may be alumina or quartz. The conductive coating  14  may be applied to the substrate  12  using a deposition process or by painting a conductive slurry onto the substrate  12 . The coating  14  may also be applied by deposition processing or sputter coating. A mismatch of thermal expansion coefficients between the substrate  12  and the coating  14  results in a large induced stress that causes the coating  14  to break apart from the substrate  12  at a critical current or temperature. The fuse element  10  may also include an intermediate layer (not shown) between the conductive coating  14  and the substrate  12 . The intermediate layer may be a sol-gel material. Upon heating, the sol-gel layer undergoes a phase transformation resulting in a large volume change, thus enhancing the fracturing of the fuse element  10 . 
     The induced stress may be caused by the conductive coating  14  undergoing electrical resistance heating and expanding at a different rate than the substrate  12 , increasing the strain at the coating/substrate interface  19 . The stress at the interface  19  is large enough at a certain critical temperature to cause the conductive coating  14  to break off from the substrate  12  in a brittle manner, stopping the current through the device  10  without much potential for arcing. 
     The geometry of fuse element  10  includes a flat ceramic substrate  12  with a conductive coating  14  applied to only one surface  11 . The other four surfaces  13 ,  15 ,  17  are left uncoated. Another embodiment of the fuse element includes a cylindrical ceramic rod with a 360-degree conductive coating. It is believed that heat transfer from the planar design may be more efficient than a cylindrical design as there is a free, non-conducting surface. Also, a more uniform deposition of the conductive coating may be achieved in a planar geometry. 
       FIG. 2  illustrates an embodiment of a polymer based fuse element  20 . The fuse element  20  includes of a fuse body  26  and terminals  22 ,  24 . The fuse body  26  is composed of a material such as a conductive polymer, a conductive polymer containing dispersed metal particles, or a non-conductive polymer containing dispersed metal particles. Metal particles in a polymer matrix can raise the electrical conductivity of the system. The principle of the design relies on the fuse undergoing electrical resistance heating and melting at a critical current. The fuse element  20  is formed to the desired length and diameter using an extruder. Metal particles may be mixed with the polymer during extrusion if necessary. The failure method for this fuse element would produce a quick and predictable failure at the melting temperature. 
       FIGS. 3A and 3B  illustrates a fuse element  40  including terminals (not shown) disposed at either end  46 ,  48 . The fuse element  40  includes a cylindrical substrate  42  with a conductive metal thin film coating  44 . The substrate  42  melts at a fixed temperature, preferably between about 300° C. and 800° C. The substrate  42  may be composed of wax or a similar material. The wax core  42  melts upon heating, causing the conductive coating  44  to disperse, eliminating conduction between the terminals. The wax core  42  may be produced through the use of molds. Molten wax is poured into a mold of the desired shape and allowed to cure. The conductive thin film coating  44  is then applied through deposition of copper or gold. The failure method produces a predictable failure at the melting temperature of the wax core  42 . The wax is preferably capable of withstanding 260° C. for 2 minutes. 
       FIG. 4  illustrates a fuse element  60  including a conductive material  66  disposed between terminals  62 ,  64 . A least one substrate  68  is disposed between the conductive material  66  and one of the terminals  62 ,  64 . The substrate  68  is composed of a conductive material with a set melting point between 300° C. and 800° C. A second substrate  70  may be disposed between the conductive material  66  and the terminal  64 . The conductive material of substrate  68  melts upon the heating of the fuse element  60 , thus causing the conductive material  66  (such as a copper wire) suspended between the terminals  62 ,  64  to fall from connection with the terminals  62 ,  64 , eliminating current flow throughout the circuit. 
     Processing fuse element  60  is similar to that of the previously described extruded polymer design or the wax core design. The conductive substrates  68 ,  70  may be produced through the use of molds or extrusion. The substrates  68 ,  70  may be melted, poured into a mold of the desired shape, and allowed to cure if a wax-like material was chosen. If a conductive polymer is used, extrusion may be used to create cylinders of desired length and diameter. The conductive material  66  and terminals  62 ,  64  are inserted into the pre-molded or extruded material. The melting of the substrates  68 ,  70  produces a quick and accurate failure point for the fuse element  40 . 
     As shown in  FIG. 5 , fuse element  80  is a variation of the fuse element  10  discussed above. Element  80  includes a substrate with restrained ends and using a ceramic with a high coefficient of thermal expansion. Constraining the ends of the substrate  12  with elements  82 ,  84  reduces the amount of freedom that the ceramic has to expand, resulting in large internal stresses as the temperature of the ceramic rises. At a critical stress, the ceramic substrate  12  fails catastrophically, resulting in an immediate break of the fuse element  10 . 
     The fuse elements disclosed herein are preferably smaller than 10×1×1 mm, are able to withstand a temperature of 260° C. for 2 minutes, can conduct a current of 0.5 Ampere DC indefinitely, will fail under severe surge currents, and will fail under low level currents of 2.2 Ampere rms AC within ten seconds. 
     EXAMPLES 
     Experimental Procedure 
     Two experimental fuse elements were fabricated. Both fuse elements consisted of a 0.79 mm diameter, 30 mm long alumina rod painted with a Hobby Colorobbia Bright Gold slurry that, upon firing, became 22 karat gold. Paint uniformity was checked by visual inspection. The slurry was fired in a kiln at pyrometric cone  018  (about 695° C.). 
     After firing, both fuse elements were tested in a test apparatus. The fuse elements were connected to a circuit by inserting each element in series with the other components. The electrical current was increased from zero Amperes in increments of 0.1 A with a minute long hold at each current. Once a current of 0.5 A was reached, a five minute hold was performed. After holding at 0.5 A, current was once again increased in 0.05 A to 0.1 A increments with one minute holds until fuse failure. 
     Test Results 
     Two experimental fuse elements were fabricated by the same method, as discussed above in the experimental section. The coating thickness was approximately 10 μm. Both of these elements were tested in a test apparatus configured to subject the fuse element to a controlled current and voltage. The gold-coated alumina rod in Test 1 was placed in the circuit in series to test the conducting capabilities of the basic design idea of a thin film of gold on a ceramic substrate. The fuse element survived for one minute at 0.15 A, 0.2 A, 0.3 A, and 0.4 A at 30 V DC. The fuse element conducted an operating current of 0.5 A for five minutes. The current abruptly stopped when increased to 0.75 A, with the fuse showing no signs of melting or fracture. 
     A second gold-coated alumina road was used in Test 2 with the same experimental set-up. The fuse element survived for one minute at 0.15 A, 0.2 A, 0.3 A, 0.4 A and survived for five minutes at 0.5 A. The current was increased by a smaller increment in Test 2 after reaching 0.5 A. The fuse element survived for one minute at 0.6 A, 0.7 A, and 0.75 A. Within 20 seconds at 0.8 A, the color of the center of the fuse became bright orange due to an increase in temperature. The fuse element survived when held at 0.8 A for a total of five minutes. The current was increased to 0.825 A at which point the fuse element stopped conducting after 1 min 35 sec. To the naked eye, the fired coating on the failed fuse element used in Test 1 appeared to be similar in color and roughness across the length of the rod. No failure location could be identified in Test 1. 
     The fuse element in Test 2 was examined both by optical and scanning electron microscopy. The failure location was clearly visible as a gray ring around the circumference of the element. The gold layer appeared to have melted and due to surface tension, separated at the center and receded to expose the alumina substrate. 
     After analysis of the fuse elements, theories were developed regarding the failure mechanism. It is theorized that gold may diffuse rapidly into alumina. The glowing orange color of the fuse indicated the temperature was somewhere in the range of 800-1100° C. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.