Abstract:
This invention relates to polymer derived ceramics (PDC&#39;s) and more particularly, to methods and product made by using polymeric derived ceramic precursors to synthesize dense, crack-free bulk ceramics in a technique using sacrificial molds, coating processes, replication processes, assembly processes and finishing processes; where gas release paths are created and maintained during these processes to release gases generated during pyrolysis of the ceramic precursor. 
     It is a primary objective of the present invention to provide a well defined method to create PDC voxels which are interconnected as a bulk (high density) material. Such a material is effectively a lattice with face centered cubic or hexagonal close pack geometry. 
     A second objective of the present invention is to provide a method for bulk, high density material to be combined with fully dense material in a hybrid material. 
     A third objective of the present invention is to provide a method for different types of bulk and fully dense materials, and other items (optionally), to be combined in to a composite material.

Description:
[0001]    The present application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 61/190,755 filed on Sep. 2, 2008 and is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to polymer derived ceramics (PDC&#39;s) and more particularly, to methods and product made by using polymeric derived ceramic precursors to synthesize dense, crack-free bulk ceramics in a technique using sacrificial molds, coating processes, replication processes, assembly processes and finishing processes; where gas release paths are created and maintained during these processes to release gases generated during pyrolysis of the ceramic precursor. 
       BACKGROUND AND PRIOR ART 
       [0003]    Recently it has been demonstrated that covalent bonded ceramics, such as silicon carbides (SiC, SiOC), silicone nitrides, (Si3N4, SiCN), aluminum nitride (AlN) and hafnium carbide (HfC) can be synthesized by thermal decomposition of suitable polymeric precursors, and are referred to as polymer derived ceramics. Two of the inventors have previously made application for a patent on related work entitled ‘Method For Synthesizing Bulk Ceramic And Structures From Polymeric Ceramic Precursors’ (U.S. Patent Application 61/029,651). That patent application refers to a method for creating ‘bulk ceramic’ . . . which is a ceramic which has a high density. 
         [0004]    When PDCs are polymerized from their liquid state, they shrink slightly, which can introduce stresses between them and objects incorporated in them, or onto which they are applied. When the polymerized PDCs are pyrolyzed, they undergo significant shrinkage and can deform, separate and fracture, if not defined and supported properly. 
         [0005]    Other inventors, such as Chen et al, (U.S. Pat. No. 4,837,230) have worked with ceramic matrix composites, where a ceramic matrix material is applied to ceramic reinforcement fabric. If a PDC matrix material is applied to a ceramic reinforcement fabric, it can deform, separate and fracture. The chief purpose of Chen&#39;s invention is to coat the reinforcement fabric and utilize the properties of the ceramic matrix material as an adhesive and bonding agent which provides some structure. The composite created is not high density. Flaws introduced into the composite are distributed throughout its volume. In order to create a more dense and defect free article, repetitions of coating, polymerizing and pyrolyzing are required. These sequential processes compound stresses and defects. 
         [0006]    Other inventors, such as Petrak et al (U.S. Pat. No. 5,707,471) have worked with fiber reinforced ceramic matrix composites where the fibers are coated and pyrolyzed under refractory conditions. The use of these pre-coated fibers to enhance bonding in a greater object is the subject of Petrak&#39;s work. Again, the composite created is not high density, and in order to create a more dense and defect free article, repetitions of coating, polymerizing and pyrolyzing are required. 
         [0007]    Other inventors, such as Sapieszko et al (U.S. Pat. No. 6,521,246) create a matrix which they apply to a porous sponge. The sponge element is coated with the ceramic matrix, and the sponge itself is burned out leaving the ceramic matrix . . . which is further pyrolyzed into a ceramic object. Sapieszko acknowledges that the result can be a powder which is morphologically uniform and of small dimension. Sapieszko achieves highly and uniformly porous bodies with this approach when they don&#39;t crumble into a powder. In his approach, porosity of at least 75% is preferred. 
         [0008]    Other inventors, such as Ostertag et al (U.S. Pat. No. 5,632,834) utilize a ceramic matrix material to bond and adhere sandwich structures. PDCs can be incorporated into the ceramic matrix, which can also include fillers. 
         [0009]    In general, the prior art includes means of developing low density bulk ceramic from PDCs. Other prior art refers to means of creating moderate density composite structures using repeated infiltration cycles and process steps, using more infiltration process steps to increase overall density where desired and when possible. 
         [0010]    In summary, polymer derived ceramic precursor (PDC) is a fluid which can be made (polymerized) into a plastic which can be transformed (pyrolyzed) into a ceramic. 
         [0011]    Scaffold has been used to provide a surface for coating with PDCs, or to provide randomized gas channels for their use by functioning as gross mechanical passageways. 
         [0012]    PDC pre-ceramic and ceramic voxels, volumetric elements, are those volumes occupied by PDC during and after polymerization and pyrolysis. 
         [0013]    A PDC green body is formed of a collection of PDC voxels substantially interconnected with one another. In some of the aforementioned art, the majority of the body is not ceramic matrix material, and the PDC is just a binder. A PDC green body formed by a scaffold which constrains the PDC into an interconnected network of voxels would have a high volume of PDC and the majority of the body would become ceramic matrix material. 
         [0014]    The ‘Critical Dimension’ (maximum dimension of an irregular voxel) or ‘Critical Diameter’ (maximum dimension of a spherically regular voxel) is notable, as for each PDC there is a ‘Critical Dimension’ up to which a ‘solid’ voxel can be formed without catastrophically failing during pyrolysis. 
         [0015]    A ‘bulk’ ‘white, black or gray’ precursor, scaffold and resultant polymer green body and bulk ceramic of one constituent, can be created using a scaffold, scaffold design or lattice design to render a high density pre-ceramic and ceramic material from one of the ‘white, black or gray’ constituents. 
         [0016]    A ‘hybrid’ white, black or gray precursor, scaffold and resultant polymer green body and fully dense and bulk ceramic of one constituent, is formed from a combination of bulk (high density), as well as fully dense material in combination in a part. 
         [0017]    A ‘composite’ ‘white and/or black and/or gray’ precursor, scaffold and resultant polymer green body and ceramic of two or more of the ‘white, or black or gray’ constituents . . . thus having multiple constituent bulk and fully dense ceramic, and non-ceramic components; having multiple densities (eg. high density bulk; and fully dense=hybrid characteristics) as well as multiple constituent materials. 
       SUMMARY OF INVENTION 
       [0018]    It is a primary objective of the present invention to provide a well defined method to create PDC voxels which are interconnected as a bulk (high density) material. Such a material is effectively a lattice with face centered cubic or hexagonal close pack geometry. 
         [0019]    A second objective of the present invention is to provide a method for bulk, high density material to be combined with fully dense material in a hybrid material. 
         [0020]    A third objective of the present invention is to provide a method for different types of bulk and fully dense materials, and other items (optionally), to be combined in to a composite material. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0021]      FIG. 1 . Hexagonal Close Pack or Face Centered Cubic Geometric Lattices 
           [0022]      FIG. 2   a . Sacrificial Spherical Voxels in a Lattice Geometry 
           [0023]      FIG. 2   b . Sacrificial Spherical Voxels in a Lattice Geometry with a Sacrificial Interstitial Material 
           [0024]      FIG. 2   c . The Sacrificial Interstitial Material saturated with PDC 
           [0025]      FIG. 2   d . The remaining PDC with the interstitial material removed 
           [0026]      FIG. 3 . Hybrid Combination of Alternate PDCs and Scaffolds 
           [0027]      FIG. 4   a . Hybrid Combination of Bulk Object, Coated with Several Fully Dense Surface Areas 
           [0028]      FIG. 4   b . Hybrid Combination of Bulk Object and Connected Fully Dense Feature 
           [0029]      FIG. 4   c . Hybrid Combination of Bulk Object with One Fully Dense Surface 
           [0030]      FIG. 5   a . Hybrid Combination of Bulk Material Framing a Fully Dense Element 
           [0031]      FIG. 5   b . Hybrid Combination of Bulk Element and Cantilevered Fully Dense Element 
           [0032]      FIG. 6   a . Hybrid Construction of Bulk element Coated with Two Different Fully Dense Elements 
           [0033]      FIG. 6   b . Hybrid Construction of Bulk element Coated with Two Different Fully Dense Areas 
           [0034]      FIG. 7   a . Composite Hybrid Construction of Bulk element assembled to another Bulk element 
           [0035]      FIG. 7   b . Composite Hybrid Construction of Bulk element assembled to another Bulk element where each bulk element has integrated fully dense element 
           [0036]      FIG. 8 . Composite Hybrid Construction of Bulk element assembled to Two pass thru conductors 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]    Polymer derived ceramic (PDC) precursors can be polymerized into solid shapes. Typically as these shapes are pyrolyzed into ceramic, they do not survive as a large solid element. The polymeric material (pre-ceramic) consists of molecules which dismiss some of their atoms during transformation from polymer into ceramic. This dismissal occurs as an off-gassing process. This off-gassing process can be violent to the extent that the ceramic solid formed, fractures. 
         [0038]    For any particular PDC precursor, being transformed from a pre-ceramic polymer material (through pyrolysis occurring through a temperature range and through a pressure range), there will exist a critical gas diffusion length which cannot be exceeded without causing mechanical or structural failure. This is one reason that many PDCs are used to create pure ceramic powders with small dimensions. 
         [0039]    So for any particular PDC precursor, there is a critical diameter or critical dimension of ceramic volume (voxel) which can be reliably and repeatably formed. 
         [0040]    When a large number of these pre-ceramic polymer voxels are created and organized in a lattice like pattern, they can form a face centered cubic (FCC) or hexagonal close pack (HCP) structure which is highly dense and which exhibits homogeneous mechanical properties. The PDC can be formed as substantially spherical units which are organized into a lattice like structure with interstitial spaces. A lattice like structure can be created in these interstitial spaces into which fluid PDC is introduced and then polymerized After the PDC is polymerized, the scaffold into which it is introduced can be selectively removed leaving the interconnected PDC voxels. 
         [0041]    The result is a homogeneous bulk solid ceramic having isotropic properties that can be formed from PDC. The theoretical density of FCC and HCP structures is on the order of 75%. Because the PDC voxels intersect where they touch, the bulk pre-ceramic and then ceramic solids thus formed have a density which typically exceeds 80%. 
         [0042]    One means of creating the bulk ceramic is to assemble a large number of spheres of polymerized PDC. These spheres can have a uniform distribution of size, which does not exceed the critical diameter for them to remain intact upon pyrolysis ( FIG. 1   a .). Another means of creating the bulk ceramic is to form a lattice of sacrificial spheres ( FIG. 2   a ,  201 ) and then fill the interstitial network with a second sacrificial material ( 210 ). The sacrificial spheres are removed leaving the interstitial network ( FIG. 2   b ,  210 ) The interstitial network is then filled with PDC ( 220 ), contained in a predictable and desirable geometry ( FIG. 2   c ,  220 ). And after the PDC is polymerized, the second sacrificial material ( FIG. 2   c ,  210 ) is selectively removed leaving the interstitial passages which provide means for gas to escape during pyrolysis ( FIG. 2   d .). 
       Bulk Objects 
       [0043]    Because the homogeneous bulk solids described herein have interstitial networks existing in them which preserve a critical voxel diameter (or maximum solid geometry allowing non-catastrophic gas diffusion during pyrolysis) they are not ‘fully dense’. Bulk materials are very dense, but not fully dense. As has been discussed, very dense means greater than 75% density. 
         [0044]    Bulk ceramic articles can be created from different PDCs with different critical dimensions of voxels, starting with solid voxels not exceeding a critical dimension, or being derived from a scaffold structure of predetermined geometry into which the PDC is introduced, rendering an interconnected network of channels of PDC material which can be polymerized and then pyrolyzed non-destructively. 
         [0045]    Referring to ( FIG. 3 ) a pre-ceramic white body with first polymer derived ceramic from a first spongy material and a first PDC precursor ( 301 ) is created in tandem with a pre-ceramic black body from a second polymer derived ceramic from a second spongy material and a second PDC precursor ( 330 ). 
         [0046]    The pre-ceramic white body can share the same scaffold with the pre-ceramic black body. The pre-ceramic white body ( 301 ) shares an adjoining surface ( 350 ) with the pre-ceramic black body ( 330 ). 
         [0047]    Where both the white body and black body have independent scaffolds, with independent critical variables for their respective PDCs, there is still an adjoining surface ( 350 ) which satisfies the critical requirements of the more sensitive PDC constituent. The combination of the white body and black body is a ‘hybrid’. 
         [0048]    The spongy scaffold material is created and then utilized to form and support a network of solid polymerized PDC (pre-ceramic) ( FIG. 1   b .). The spongy material is then removed, leaving the solid network which is interlaced with an interstitial network where the spongy material previously was ( FIG. 1   c .). 
         [0049]    The polymerized PDC material can be formed in to other shapes and assembled into a unitary assembly of polymerized PDC (pre-ceramic). 
         [0050]    In its simplest form the bulk material is a pre-ceramic gray body which can be transformed into a polymer derived ceramic which started from a spongy material and a PDC precursor, where the spongy material constrains the PDC which is poured into it in a manner consistent with the critical dimensions required to preserve its integrity upon transformation into ceramic. The matrix of PDC voxels which conform to the required critical dimensional criteria may also be achieved by assembling spherical voxels directly from polymerized gray PDC. 
       Hybrid Objects 
       [0051]    The bulk homogeneous pre-ceramic and ceramic material made from PDC will have an interstitial network of passages between interconnected voxels, where the gases dismissed during pyrolysis can escape without initiating structural failure. 
         [0052]    Consider a cube of material where one surface of the cube has a fully dense coating ( 460 ) of PDC upon it ( FIG. 4   c .). The coating thickness does not exceed the critical thickness for the PDC, the outward facing surface can out-gas to the atmosphere directly and the coating&#39;s ‘contact surface’ can breath through its ‘inward face’ in touch with the interstitial network. Thus a ‘hybrid’ component can be formed which has contiguous volumes which are bulk pre-ceramic, and fully dense pre-ceramic. These hybrid pre-ceramic green bodies may then be formed to be unitary parts with both bulk ceramic and fully dense ceramic volumes. These bulk and fully dense regions co-exist on a pre-ceramic and then ceramic component ( FIGS. 4   a ,  4   b ,  4   c ). 
         [0053]    Consider two cubes of material where one surface of each cube may capture a fully dense film of PDC between it and the other. The film thickness does not exceed the critical thickness for the more sensitive PDC, the film can out-gas through the respective interstitial network in the cubes on either side of the constrained film ( FIG. 3 ) ( 350 ). Thus a ‘hybrid’ component can be formed which has some volumes which are bulk pre-ceramic ( 301 ,  330 ), and some volumes that are fully dense pre-ceramic ( 310 ,  350 ,  340 ). These pre-ceramic volumes may then be formed to be both bulk ceramic and fully dense ceramic. These bulk and fully dense regions co-exist in a pre-ceramic and then ceramic component. 
         [0054]    Consider a cube of material where one surface of the cube has a fully dense coating ( 420 ) of PDC upon it ( FIG. 4   a .). The coating thickness does not exceed the critical thickness for the PDC, the outward facing surface can outgas to the atmosphere directly and the ‘contact surface’ can breath through the ‘inner facing’ interstitial network. Other surfaces ( 430 ) of the cube can also be coated with a fully dense film of PDC which does not exceed the critical thickness, as discussed. However, these other surfaces ( 430 ) which are coated must include sufficient ‘openings’ so as to allow the overall solid volume to breath and out-gas through the remaining interstitial channels ( FIG. 4   a .). Thus a ‘hybrid’ component can be formed which has volumes which are bulk pre-ceramic, and fully dense pre-ceramic. These pre-ceramic parts may then be formed to be both bulk ceramic and fully dense ceramic. These bulk and fully dense regions co-exist on a pre-ceramic and then ceramic component. 
         [0055]    A cube or other shape of material can be formed from one or more components of material made from ‘bulk’ PDC with ‘fully dense films’ formed where component parts share a surface at an adjoining location ( FIG. 3 ,  350 ) and with complete ‘fully dense films’ on some outer surfaces ( 310 ,  340 ) and interrupted ‘fully dense films’ on other outer surfaces ( FIG. 4   a ,  430 ) . . . where the unitary pre-ceramic component may be transformed into a ceramic component. 
         [0056]    ( FIG. 5   a ) shows a front view of a bulk pre-ceramic frame ( 220 ) with a fully dense ( 510 ) window in it. ( FIG. 5   b ) shows a right side cross section of the same hybrid assembly with the fully dense window ( 510 ), which has a thickness not exceeding the critical dimension for the PDC, supported in bulk material ( 220 ). The whole assembly will shrink at the same rate when transformed into ceramic, and the object will be preserved. 
         [0057]    ( FIG. 5   c ) shows a front view of a bulk pre-ceramic block ( 220 ) onto which a fully dense pre-ceramic feature is attached ( 530 ). ( FIG. 5   d ) is a right side view of this assembly showing the line of attachment at the surface of the bulk part. The whole assembly will shrink at the same rate when transformed into ceramic, and the object will be preserved. This is similarly shown in  FIG. 4   b , with bulk material ( 220 ) connected to Fully dense material ( 450 ). 
         [0058]    ( FIG. 6   a ) shows a cross section of a bulk part ( 220 ) which has an uninterrupted coating ( 610 ) on a primary surface, which may be of the same PDC as used for the bulk part, and which has an interrupted coating ( 640 ) over other places in the part, where the PDC utilized may be a different PDC than was used for the bulk object. The whole assembly will shrink at substantially the same rate when transformed into ceramic, and the coating will be preserved when conforming to the constraints of critical dimensions, and the object will be preserved. 
         [0059]    ( FIG. 6   b ) shows a cross section of a bulk part ( 220 ) which has an uninterrupted coating ( 660 ) a primary surface, which may be of the same PDC as used for the bulk part, and which has an interrupted coating ( 660 ) over other places in the part, where the PDC utilized for coating is the same PDC than was used for the bulk object. The whole assembly will shrink at the same rate when transformed into ceramic, and the object will be preserved. 
       Composite Objects 
       [0060]    As discussed previously, a cube or other shape can be formed from one or more volumes of material made from ‘bulk’ PDC with ‘fully dense films’ formed where component parts share a surface at an adjoining location and with complete ‘fully dense films’ on some outer surfaces and interrupted ‘fully dense films’ on other outer surfaces . . . where the unitary pre-ceramic component may be transformed into a hybrid ceramic component. 
         [0061]    Introduced onto this shape can be an additional feature of PDC which does not exceed the critical thickness ( FIGS. 5   a,b ). Such features can take the form of a wall, a rib, a fin or some other geometry which is attached to the bulk geometry or a coating on the bulk geometry, where the hybrid geometry has sufficient exposed interstitial passages to allow gas diffusion to occur without a catastrophic effect to the component as it is being transformed from PDC pre-ceramic to ceramic. 
         [0062]    Such a feature can take the form of a wall which is attached to the bulk geometry like a window in a bulk ceramic frame ( FIGS. 5   a,b ), where the hybrid geometry has sufficient exposed interstitial passages to allow gas diffusion to occur without a catastrophic effect to the component will it is being transformed from PDC pre-ceramic to ceramic. 
         [0063]    Such a feature can take the form of a wall which is attached to the bulk geometry like a rib connected at a coated surface ( FIGS. 5   c,d ), where the hybrid geometry has sufficient exposed interstitial passages to allow gas diffusion to occur without a catastrophic effect to the component will it is being transformed from PDC pre-ceramic to ceramic. 
         [0064]    Such a feature can take the form of a wall which is attached to the bulk geometry like a fm connected at an uncoated surface ( FIG. 4   b .), where the hybrid geometry has sufficient exposed interstitial passages to allow gas diffusion to occur without a catastrophic effect to the component while it is being transformed from PDC pre-ceramic to ceramic. 
         [0065]    ( FIGS. 7   a  and  7   b ) show hybrid objects with many complementary features. In  FIG. 7   a , item  222  is a bulk ceramic object with a pin which passes through a hole in object  221 . There is an interface of PDC film which does not exceed the critical thickness, which occupies the adjoining surfaces at  222  and  221 , and which transforms to ceramic at the same time as  222  and  221  pass to ceramic from pre-ceramic. 
         [0066]    In  FIG. 7   b , a bulk ceramic item ( 242 ) shares an adjoining surface ( 250 ) with bulk ceramic object  232 . Bulk ceramic object  232  has a fully dense ceramic wall that was formed at its base ( 280 ). Bulk ceramic object  242  has window like ribs ( 262 ) that were attached at three edges to the bulk ceramic frame formed by the upper bulk ceramic object ( 242 ). 
       Composite Hybrid 
       [0067]    Consider multiple features and elements of bulk and fully dense pre-ceramic which are created from different scaffolds and PDCs. These may be assembled into unique structures which are wholly pre-ceramic and scaffold, or wholly ceramic. Several bulk and/or fully dense elements can be combined from pre-ceramic, ceramic and non-ceramic components. 
         [0068]    In addition, objects can be introduced into the pre-ceramic object during its construction, which are pre-ceramic, ceramic or non-ceramic or combinations of these ( FIG. 8 ). Consideration may be given for these constituent parts to adapt or conform to the overall structure as it transforms from pre-ceramic to ceramic. An example would be a twisted strand conductor of stainless steel with a polymeric insulating jacket ( 320  and  340 ). This ‘wire’ form may be supported in a PDC hybrid structure ( 810 ) to create a composite structure. When the pre-ceramic structure which contains the jacketed wire is exposed to the heat of pyrolysis, the polymeric jacket supporting the stranded conductors can burn or sublimate in much the same way as the spongy scaffold burns or sublimates in the bulk solid, preserving the interstitial network of breathing passages. The strands of stainless steel wire are constrained when the structure surrounding them shrinks onto them and supports them again. 
         [0069]    There is a bulk material ( 806 ) which is coated on an outer face with fully dense material ( 804 ) forming an object ( 810 ), which traps jacketed wire ( 320 ) and jacketed wire ( 340 ). 
         [0070]    Another example of a composite structure would be a glass disk mounted in a frame of pre-ceramic polymer, which is encased in ceramic during the pyrolysis process. The glass can deform elastically as the pre-ceramic material transforms into ceramic material.