Abstract:
The present invention relates to an inductive heating apparatus, particularly using a liquid deposition precursor, for densifying porous articles. The inductive heating apparatus using induction coils that are made from machined or milled flat metal plates, such as copper plate, instead of bending a member (especially a hollow member) into a coil shape. The induction coils according to the present invention can be more precisely and more consistently fabricated than conventional bending techniques.

Description:
PRIOR APPLICATIONS 
       [0001]    This application claims the benefit of priority from U.S. Provisional Application No. 60/821,596 filed on Aug. 7, 2007, the entire content of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to densifying porous articles with a desirably high rate of production, particularly, but not necessarily only, with respect to the field of friction braking articles, such as aircraft brakes. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the field of friction materials, it is generally known to use porous material substrates to manufacture friction members, such as friction brake disks. 
         [0004]    The manufacture of such friction members generally begins with the construction of a porous preform. For example, in many friction brake applications, annular preforms are used. The annular preforms can be constructed using several different known methods. For example, carbon fiber fabric plies can be needled together and annular preforms can be cut from the stacked material. Oxidized polyacrylonitride (“PAN”) fibers or pitch-based fibers are common examples of starting fibers used in this type of application. Subsequently, these fibers may be carbonized in a high temperature treatment step prior densification. 
         [0005]    Thereafter, the porous preforms are densified. A generally known method of densification uses a liquid precursor of a material that deposited within the porous preform to densify the preform. This kind of densification is sometimes referred to in the art as “film boiling.” 
         [0006]    In general, film boiling densification involves immersing a porous preform in a liquid hydrocarbon so that the liquid substantially completely infiltrates the pores and interstices of the preform. Thereafter, the immersed preform is inductively heated to a temperature above the decomposition temperature of liquid hydrocarbon (typically 1000° C. or more, typically up to about 1400° C.). More particularly, the liquid hydrocarbon adjacent to the inductively heated preform structure dissociates into various gas phase species within the preform porosity. Further thermal decomposition of the gas phase species results in the formation of pyrolitic carbon on interior surfaces in the open regions of the porous material. 
         [0007]    The use of liquid precursors for densification is discussed in, for example, U.S. Pat. Nos. 4,472,454, 5,389,152, 5,397,595, 5,733,611, 5,547,717, 5,981,002, and 6,726,962. Each and every one of these documents is incorporated herein by reference in its entirety. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention will be even more clearly understandable with reference to the drawings appended hereto, in which: 
           [0009]      FIG. 1  is a cutaway schematic side view of a liquid densification apparatus; 
           [0010]      FIG. 2  is a cutaway schematic top view of a liquid densification apparatus generally corresponding to that seen in  FIG. 1 ; 
           [0011]      FIG. 3  is a schematic cross sectional view of a porous article mounted relative to conventional induction coils for inductively heating the article; 
           [0012]      FIG. 4  is a schematic cross sectional view of a porous article mounted relative to induction coils according to the present invention, for inductively heating the article; 
           [0013]      FIG. 5  is a close-up cross sectional view of a part of an induction coil as illustrated in  FIG. 4 ; 
           [0014]      FIG. 6  is an enlarged partial view of the arrangement illustrated in  FIG. 4 ; 
           [0015]      FIG. 7  is a schematic side view illustrating an arrangement for mounting a porous article in the apparatus of the present invention; 
           [0016]      FIG. 8  is a schematic perspective view of an exemplary apparatus for liquid densification of multiple porous articles; 
           [0017]      FIG. 9  is a schematic cross sectional view of an exemplary apparatus for liquid densification of multiple porous articles; and 
           [0018]      FIGS. 10-12  are X-ray images of a densified porous preform for illustrating an effect that the structure of the inductive heating coils has on densification. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    A typical densification facility according to the present invention includes one or more reactor chambers generally sized and constructed to hold one or more preform members therein, with corresponding induction heating coils, while substantially immersed in the liquid precursor of choice. For example, the reactor chambers could be suitably manufactured from aluminum. Other suitable materials are non-magnetic and include, without limitation, glass, stainless steel, ceramic or combinations thereof. Dedicated inner buses and coils are provided depending on the size of the preforms being densified. Typical examples of preform diameters range from about 10″ to about 25″ and the thickness ranges from 15 mm to 50 mm. Particularly common examples include 15″ and 20″ diameter preforms. 
         [0020]    The one or more reaction chambers are each provided with induction heating coils corresponding to each preform that is to be processed. The induction coils and the preforms being processed are preferably mounted in position relative to each other using chemically non-reactive and thermally stable supports made from, for example, an electrically non-conductive glass composite material that can withstand a liquid hydrocarbon environment (such as that known in the art as “G-10”). 
         [0021]    The induction heating coils are preferably water-cooled pancake spiral coils, and are commonly made from copper metal. For this reason, it is desirable to provide a heat exchanger dedicated to the water-cooling system for the induction coils, as will be discussed in further detail below. In use, a preform (or worn brake) being densified is heated by direct coupling from the induction coil. 
         [0022]      FIGS. 1 and 2  are side and top schematic view of a coil/reactor chamber  100  generally in accordance with the present invention. Loading and unloading of one or more preforms  160  require, for example, a locking mechanism is provided for the selectably openable top lid  150 . 
         [0023]    Each reactor chamber  100  (as many as are provided) is connected with a common precursor supply line  180  and a common exhaust line  140  to the condenser and thermal oxidizer. Each reactor chamber  100  can be filled, drained, and monitored from the computer control system. The exhaust vapor from the densification process is condensed and fed back to the reactors, whereas the residual effluent gas is preferably then taken to a conventional thermal oxidizer (not shown) and burned. 
         [0024]    For the preform to reach its full density, it is desirable that the center of the preform initially be heated above the pyrolysis temperature of the precursor liquid. As densification proceeds, the densification moves radially outwards from the center portions of the preform because of the heating profile created by induction heating. In one example, the frequency of the power supply is set (in view of the material characteristics of the preform) to provide a skin depth roughly near the center of the preform. As the center of the preform densifies, the frequency of the power supply increases in response to changes in the electrical characteristics of the preform caused by densification. 
         [0025]    Certain features of the presently contemplated invention are now discussed hereinbelow, and are distinguished where appropriate from conventional aspects of film boiling densification. 
         [0026]    In one contemplated example, the induction coils in a respective reaction chamber are associated with a corresponding power supply. An example of a commercially-available power supply suitable for this application is a Model No. LSP 14-400/30 Lepel induction-type power supply. 
         [0027]    The power from the power supply is transferred to the induction coils by custom-built (depending on the physical arrangements of a given densification facility) copper bus bars with external water cooling networks. 
         [0028]    The Lepel power supply, for example, has remote PID loop control capability and can be monitored and controlled from a computer control terminal. Power density, voltage control, current control, frequency control, and/or temperature control of the densification process by known methods is also within the scope of the contemplated arrangement. 
         [0029]    An appropriate liquid precursor is supplied to the reaction chamber(s) from a precursor storage tank. The system includes a conventional fluid transfer system for managing the transport of new (and used) precursor liquid (not shown). The fuel transfer system may include one or more remote storage tanks for storing a new liquid precursor supply as well as possibly temporarily storing used precursor liquid. A relatively smaller local storage tank for new precursor liquid can also be provided. 
         [0030]    One example of a precursor liquid used according to the present system is cyclohexane (C 6 H 12 ). However, depending on the selection of the precursor liquid, the corresponding deposited material could be any of, strictly by way of example and without limitation, carbon, silicon carbide, silicon nitride, carbon-silicon carbide, or carbon-silicon nitride. To deposit carbon within the preform, for example, a hydrocarbon such as cyclohexane, n-hexane, benzene, toluene, or combinations thereof can be used. Methyltrichlorosilane, dimethyldichlorosilane, and methydichiorosilane, tris-n-methyl amino silane, or other organosilane mixtures can be used for depositing silicon carbide and silicon nitride. 
         [0031]    Moreover, the precursor liquid can be chosen to co-deposit materials. For example, a mixture of silicon carbide and silicon nitride can be deposited using tris-n-methyl amino or other silane compound. Also, co-deposited materials such as carbon-silicon carbide and carbon-silicon nitride can be produced with an appropriately selected mixture of precursors. Depending upon the desired matrix, silicon carbide or silicon nitride interface coating can be produced during an early stage of the densification or co-deposited throughout the matrix densification. 
         [0032]    Preferably, sufficient liquid precursor is provided to substantially immerse the preform being densified, as well as the induction heating coils associated therewith. As mentioned before, the film boiling process creates gaseous species that eventually cause the formation of pyrolitic carbon on interior surfaces of the preform porosity. Precursor vapor is preferably captured to the extent possible and condensed for possible recycling in the process. Effluent gas that is still left over is preferably conveyed to a thermal oxidizer of a known configuration to burn off residual hydrocarbons in the effluent gas. 
         [0033]    The structure of the induction coils  130  used in film boiling densification processes is very important for obtaining densified preforms having suitable mechanical characteristics, including friction characteristics. 
         [0034]    Using conventional pancake coils constructed from hollow copper tubing (square or round), as illustrated in  FIG. 3  for example, can result in undesirable “spiral” densification (sometimes referred to as a “mirror effect”) corresponding to the constructions of spiral induction coil. It has been determined that conventional copper tubing is relatively difficult to form (i.e., bend) into a pancake coil, particularly with respect to maintaining a consistent gap between adjacent turns of the coil. In turn, the desire to maintain a consistent (and relatively small) gap between coil turns causes manufacturing and structural issues with respect to suitably bending the copper tubing to provide the tight turns necessary. 
         [0035]    A particular difference between the convention inductive coil of  FIG. 3  and that of  FIG. 4  is the fact that the prior induction heating coil  200  shown in  FIG. 3  is made from square or round hollow copper tubing  230  bent into the desired coil form. The copper tubing functions both to transmit the power for inductive heating (through the copper metal) and convey cooling water through the inlet port  250  and the outlet port  260 . The coil  200  of  FIG. 3  has, for example, a nominal ¾″ gap with an inner diameter (ID) coverage of 18″ and an outer diameter (OD) coverage of 8.5″. For the purpose of illustration,  FIG. 3  shows a carbonized preform  240  placed inside of an alumina holder  220  and supported with a G-10 holder  270 . 
         [0036]    In contrast,  FIG. 4  shows an example of an induction heating coil  300  according to the present invention is fabricated from, for example, ¼″ solid copper plate that is milled or otherwise machined into the desired coil form  330 , instead of being turned or bent into shape as the coil in  FIG. 3  is. Preferably, a high precision machining operation is used to form the coil from the copper plate, such as, for example, CNC milling and the like. A hollow metal (copper, for example) water cooling tube  335  is brazed on an exterior (i.e., opposite the side facing the preform in use) side of the machined copper coil  330 . The braze composition may be any commercially available composition suitable for brazing copper as long as it can withstand the expected temperature environment. 
         [0037]    Because the currently contemplated induction coil uses a machined flat copper plate, it can be produced with greater precision (particularly with respect to gaps between turns of the coil) than bending copper tubing. In addition, the machined copper plate does not undergo stress and strain deformation like bent copper tubing. 
         [0038]    A gap between adjacent turns of the coil according to the invention is substantially consistent, and is between about 4 mm to about 8 mm. 
         [0039]    In addition, it has been observed that in some cases, the conventional copper tube becomes distended and at least susceptible to rupture (in a manner not unlike a vascular aneurysm) (if rupture does not in fact occur). It is tentatively believed that the conventional copper tube induction coil can become too hot for the cooling water to adequately dissipate the heat. It is speculated that this can cause vaporization of the cooling water within the conventional copper tubing with an unacceptable pressure increase therein. This pressure is believed to result in the bulging tubing deformation and an elevated risk of tube rupture. 
         [0040]    The possibility that the copper tubing could rupture during a densification process cannot be neglected, bearing in mind that the coil is immersed in a liquid hydrocarbon precursor during densification. If cooling water were to escape the coil during densification (for example, because of a tubing failure), the water would flash to steam, which would provide an oxygen source. Because the liquid hydrocarbon is typically already above its self-combustion temperature during densification, the introduction of an oxygen source would present a grave risk of explosion. 
         [0041]    Also, a prior induction coil made from copper tubing (such as that illustrated in  FIG. 3 ) suffers from a spiral densification characteristic and weak densification at the radially inner and outer peripheries, as can be seen below in  FIGS. 10 and 11 . 
         [0042]    “Spiral densification” is an art-recognized term which generally refers to non-homogenous densification, particularly characterized by generally circumferential “streaks”—regions in which the densification is non-trivially different from adjacent regions. This problem is significantly related to imperfections in coil design, especially with respect to the gap distance between adjacent turns of the coil. It will be appreciated that if the coil structure is not carefully monitored, gaps between coil turns may occur that lead to insufficient heating of the adjacent preform. This results in turn in poor densification. In addition, pancake coil distance is usually in a fixed position which does not allow for any adjustment for the depth penetration. 
         [0043]    The preforms illustrated in  FIGS. 10 and 11  were formed with a conventional copper tubing induction coil in a film boiling densification process. In both images, large, circumferentially extending regions of density variation are evident. In particular, a region of very low density (corresponding to the white region at approximately the “10 o&#39;clock” position of both preforms is noticeable. In addition, as mentioned above, poor densification can be seen, for example, at the radially outward periphery of the preforms. See, for example, the white region along approximately the bottom halves of each X-ray scan. 
         [0044]      FIG. 12  is a similar type of X-ray scan which illustrates the densification of a preform densified using an induction coil made from a machined copper plate and having a cooling water tube brazed thereto, as described above with reference to  FIG. 4 . The important feature to note between  FIGS. 10 and 11  relative to  FIG. 12  is the absence of distinct or prominent circumferentially extending blotches and the like in  FIG. 12  (unlike in  FIGS. 10 and 11 ), which indicate large variations in densification levels, and more particularly, regions of undesirably low densification. 
         [0045]      FIG. 5  hereinbelow is a cross-sectional view of one segment of an induction coil according to the present invention. A cooling water tube  400  brazed to an exterior (with respect to a preform being densified) side of the machined copper coil  402  used as the induction heating coil. A mounting fixture, such as a bolt shaft  401  may be attached to an exterior of the cooling water tube  400  for mounting the coil to a support within the reaction chamber. 
         [0046]    As discussed above, the reaction chamber according to the present invention includes at least one induction heating coil as described herein, for inductively heating a preform mounted adjacent thereto. In a typical arrangement, pairs of coils are provided and a preform being treating is mounted between them. 
         [0047]    It is known in the art that preforms are generally soft and pliable before being densified. Therefore, it is desirable to hold a preform relative to the inductive heating coil in such a manner so as to reduce bending and other forms of warping or deformation before the preform is densified. 
         [0048]    In one arrangement, support members may be used to hold the preform in place. See, for example,  FIGS. 6 and 7 . In the above-illustrated example, the preform is held in place by one or more supports  530  mounted, for example, on the same supports on which the induction coils are mounted. The supports may be, for example, non-reactive quartz glass or alumina cylinders having an appropriately sized and aligned notch formed therein. As indicated by the bidirectional radially extending arrows in  FIGS. 6 and 7 , the radial (with respect to the preform) position of some or all of the supports can be adjusted to, for example, accommodate insertion of the preform relative to the heating coils, or to accommodate preforms of different diameters. 
         [0049]    The distance between coils and the preform can be adjusted by screwing in or out of the bolt shaft  520  (corresponding to the bolt shaft  401  seen in  FIG. 5 ) in order to provide effective heating during initial densification. Control of this spacing between the induction coils and the preform can help control current concentrations in the preform during inductive heating, so as to avoid “mirror effect” or “spiral” densification on the one hand, and to increase power yield by not spacing the coil too far away from the preform. 
         [0050]      FIG. 8  hereinbelow is a perspective view of a reaction chamber  700  according to the present invention into which three preforms (and associated induction heating coils) are loaded. This illustration is strictly by way of example to illustrate the densification of multiple porous parts, and other loading arrangements with other numbers of preforms are possible. 
         [0051]    Overall, the multiple-part densification shows another way of producing carbon disks in at least a three-part densification configuration. In this arrangement, the liquid precursor is supplied to the reaction chamber  700  through ports  710 . The power from the power supply is transferred to the induction coils  730  by appropriately arranged (depending on the physical arrangements of a given densification facility) copper bus bars  760  with external water cooling networks. 
         [0052]    As previously mentioned, the porous parts are secured and positioned in the center of the pancake coils  730  during densification. The Lepel power supply, for example, has remote PID loop control capability and can be monitored and controlled from a computer control terminal. Power density, voltage control, current control, and/or temperature control of the densification process by known methods is also within the scope of the contemplated arrangement. 
         [0053]    The liquid precursor is supplied to the reaction chamber(s)  700  from the local precursor storage tank. Preferably, sufficient liquid precursor is provided to substantially immerse the preform(s) being densified, as well as the induction heating coils associated therewith. The film boiling process creates gaseous species that in part causes the formation of pyrolitic carbon on interior surfaces of the preform porosity. Precursor vapor is released thru an exhaust line  720  and vented to an external condenser (not shown) for possible recycling in the process. Effluent gas that is still left over is preferably conveyed to a thermal oxidizer (not shown). 
         [0054]    In this example of a coil/reactor chamber  700 , loading and unloading of the porous preforms require, for example, bolting and unbolting a front reactor cover panel  750  and corresponding external bus connection  760 . Each reactor chamber  700  (as many as are provided) is connected with a common precursor supply line and a common exhaust line  720  to the condenser and thermal oxidizer. Each reactor chamber can be filled, drained, and monitored from the computer control system. The exhaust vapor from the densification process is condensed and fed back to the reactors, whereas the residual effluent gas is then taken to the thermal oxidizer and burned. 
         [0055]      FIG. 9  illustrates another arrangement for the liquid densification of multiple porous parts. This illustration shows the densification of multiple porous parts  870  inside of a sealed reaction chamber  800 . A load stack consisting of multiple porous parts  870  and graphite plates  860  therebetween are pre-assembled. The load stack is placed on a ceramic or electrically insulated stand  890  and the furnace lid  805  is removed during the loading and unloading operation. The power from the power supply is transferred to the multi-zone coil  810 ,  820 , and  830  through power port  880 . Individual PID zone control is made possible through power density, voltage control, current control, frequency control, and/or temperature control for the densification process. 
         [0056]    The heating of the porous parts  870  is accomplished by using, for example, a multi-zone induction coil  810 ,  820 ,  830  which is directly coupled to the graphite plates  860 . The annular graphite plates  860  have similar ID and OD dimensions as the annular porous parts and the thickness of the annular graphite plate ranges between 5 mm to 25 mm. 
         [0057]    In this case the densification fronts start from the heated preform surfaces which are in contact with the graphite plates  860 . Liquid precursor is supplied to the reactor chamber  800  and the liquid can be filled and drained through port  895 . A liquid level indicator  840  indicates the level of precursor fluid during the fill, drain, and densification. Sufficient liquid precursor is provided to substantially immerse the load stack inside of the reactor chamber cavity  806 . A continuous feed of liquid precursor is also possible during the densification process. 
         [0058]    As mentioned previously, the densification process creates gaseous species and the vapor is released from port  850  and directed to the condenser for possible recycling purpose. Effluent gas that is not captured is conveyed to the thermal oxidizer to burn off the residual hydrocarbons. 
         [0059]    It is thought that it might be useful to mount a preform with respect to corresponding coils such that the preform could be rotated about its center during densification, under influence of the heating coils. For example, a mounting assembly could be provided at the center of a given preform that could be appropriately connected to a driving device located outside of the respective reaction chamber. The rate of rotation of the preform might be from about one to about five revolutions per minute, as currently contemplated. 
         [0060]    It can be useful according to the contemplated system to provide the coils and/or the buses with a non-stick electrically insulating coating, such as epoxy or polytetrafluoroethylene to prevent electrical load faults or shorting caused by inadvertent contact between the preform and the coil and/or bus. 
         [0061]    While the present invention has been described with respect to what are believed to be the most practical embodiments thereof, it is particularly noted that this is by way of example only, and appropriate modifications and variations thereof are possible within the scope of the claims appended hereto.