Abstract:
A woven preform for a ceramic composite has a plurality of layers and structural members. The plurality of layer are of woven yarns of fibrous material. The structural members extend between the layers. The layers and members define interlayer spaces. One or more of the layers may have a plurality of openings extending therethrough. Low density ceramic insulation made be deposited in the interlayer spaces via a slurry that enters the preform, or the preform after it has been made a part of a composite, through the openings. The carrier of the slurry exits the preform, leaving the randomly packed fibers in the interlayer spaces. The structural members may be walls that, along with the layers, define channels. The channels may be used to direct fluid through so as the composite functions as an insulator. The channels may be directed in the warp direction for achieving increased benefits.

Description:
This application is a continuation-in-part of application Ser. No. 08/736,559 filed Oct. 24, 1996 and entitled Integrally Woven Ceramic Composites, the contents of which are expressly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to ceramic composites and, in particular, to integrally woven ceramic composite structures used in insulation. 
     2. Description of the Related Art 
     In certain high temperature operating environments, such as exterior surfaces of space reentry vehicles and combustion chambers and nozzles in jet engines, rocket engines, and power generators, for example, thermal barriers are necessary to protect supporting structures and equipment. A combustion chamber liner, for example, must be mounted on a strong surrounding structure, typically metal, which must be kept relatively cool and protected from heat, both radiant and conductive. Ceramic materials have utility as thermal barriers because of their high temperature stability. Moreover, since thermal barrier components typically comprise large panels or shell structures that are difficult to fabricate from monolithic ceramics, fiber reinforced ceramic composites are preferred. 
     In extremely high heat flux environments, such as in rocket engines, the thermal barrier material must also be actively cooled by some mechanism, such as an internally circulating fluid, because the operating temperatures exceed the capabilities of the exposed ceramic materials. All non-ablative rocket nozzles are currently designed in this manner, using high conductivity metals with internal channels for flow of coolant (usually high pressure fuel). The high conductivity is needed to maintain the temperature of the hot surface below the melting point of the metal. If ceramic composites could be used instead of than metals for such structures, large improvements in engine performance would result from: (I) reduced weight; and (ii) reduced heat flux absorbed at the hot surface because of the higher temperature capability of the ceramic. However conventional ceramic composite fabrication methods cannot produce structures capable of satisfying the combined requirements of high pressure containment and high heat flux management. 
     State-of-the-art ceramic composites are built up to the required thickness using stacked layers of fiber fabrics that are subsequently infiltrated with a ceramic matrix. Unfortunately, such layered composites are not suited to the formation of the structures needed for actively cooled panels because of their susceptibility to delamination of the layers, leading to catastrophic failure. 
     A preferred approach for forming such panels would be to begin with an integrally woven 3-dimensional fiber preform of the desired shape, with reinforcing fibers in walls and face sheets surrounding internal cavities aligned everywhere predominantly parallel to the stresses expected in use, and to infiltrate the preform with the desired ceramic matrix. Several methods are known for forming integrally woven structures consisting of face sheets connected by walls aligned along the weft direction during weaving. These walls form internal channels which could be used for coolant flow. However, such weft channel structures have several shortcomings for actively cooled structures: (1) the packing density of fiber yarns aligned around the circumference of weft channels (as needed for pressure containment) is inherently limited by the weaving process, so that thicker walls are required to achieve pressure containment, which defeats satisfaction of the heat flux requirements for high performance rocket nozzle and other applications; (2) low packing densities of fibers around channels makes it difficult to achieve hermetic containment of pressurized cooling fluids; (3) the channel lengths in weft channel structures cannot exceed the width of the loom, imposing severe restrictions to structural designs and increasing the difficulty of the weaving process; and (4) weft channel structures are not easily modified to incorporate connecting structures such as manifolds as part of the woven structure at the ends of the channels or elsewhere. 
     In some systems, passive thermal insulation systems as opposed to active for reentry vehicles is used. One such system includes space vehicle tiles. The passive thermal insulation systems typically comprise very low density ceramic materials bonded to the metal skin of the vehicle. Because of their low density, such materials are very fragile and susceptible to damage from contact with other objects. It would be desirable to provide such low density materials with an outer protective coating of dense tough ceramic composite material that is not susceptible to debonding, or to sandwich it between front and back faces of tough ceramic composite. 
     State-of-the-art passive thermal protection panels are built up to the required thickness by bonding a low density core of thermally insulating ceramic or other material to a protective skin or thin relatively dense composite sheet consisting of a fibers infiltrated with a ceramic matrix. Unfortunately, such sandwich structures are not durable as thermal barrier panels because of their susceptibility to delamination of the protective skin, leading to catastrophic failure. 
     A preferred approach for forming such panels would be to begin with an integrally woven 3-dimensional fiber preform of the desired shape, consisting of face sheets connected by walls or struts, the woven reinforcing fibers in the face sheets and walls or struts being infiltrated with a ceramic matrix, and the spaces between the face sheets and walls or struts being infiltrated with a low density insulation material. Several methods are known for forming integrally woven structures consisting of face sheets connected by walls aligned along the weft direction during weaving. These walls form internal channels which could be used for insertion of passive insulation. However, existing channel structures have the severe shortcoming that access to the space between the front and back skins of the structure for inserting passive insulation materials is limited to the ends of the channels—this is not suitable for the processing methods needed for certain preferred insulation materials. 
     SUMMARY OF THE INVENTION 
     The present invention comprises an integrally woven 3-dimensional ceramic composite structure with internal channels aligned in the warp weaving direction. The composite includes a multilayer fabric woven from yarns of fibers such as carbon, silicon carbide, silicon nitride, aluminum oxide, mullite, glass, yttrium aluminum garnet (YAG), polyethylene, and other fibrous materials. At least upper and lower layers (or skins) of the composite comprise woven warp and weft yams. The layers may form planes or curved surfaces or tubular structures that can be woven tightly for internal fluid pressure containment. The layers are joined or connected by integrally woven warp and weft yarns forming walls or rows of connecting columns so as to form interior channels in conjunction with the skins. 
     Weaving processes and designs are chosen in such a way that much higher packing densities of fibers are achieved around the perimeter of each channel to improve the ability of the channels to contain pressure without undue increase in the thickness of either the skins or the walls or columns that form the channel structure. 
     The woven yarns of the composite material are infiltrated or impregnated with a curing agent that may be in the form of fibers, particulates, powders, vapors, or liquids. The curing agent comprises a material, such as a curable polymer in uncured form or a ceramic precursor, for example, that can be cured by exposure to heat or light (such as infrared or ultraviolet radiation), for example, to form a rigid matrix for the infiltrated fiber yarns. A polymer agent optionally may include ceramic particles so that treatment at higher temperatures will sinter the ceramic particles into a ceramic matrix around the woven yarns and eliminate the polymer or convert it into a ceramic. Ceramic matrix material can also be added after either curing or initial heat treatment by chemical vapor infiltration (CVI) or infiltration of a liquid precursor followed by heat treatment. The resulting structure typically includes two or more layers (skins) connected by walls or struts, in which each of the skins and the walls or struts comprise ceramic reinforcing fibers in a ceramic matrix. The cavities of the open lattice structure can be used for circulation of active cooling fluids (liquids or gases), for example. 
     A principal object of the invention is a structural ceramic composite that includes utility as a high temperature thermal barrier material. A feature of the invention is a multilayer integrally woven ceramic composite structure with internal channels aligned in the warp weaving direction that can include cooling fluids and can be effectively bonded to a supporting structure. An advantage of the invention is a high packing density of reinforcing fibers aligned in the circumferential direction around the channels to allow containment of high pressure fluid and operation in high heat flux environment. Another advantage is that high packing densities of reinforcing fibers reduce gaps and promote hermetic containment of pressurized cooling fluids. Another advantage of the invention is that there is no limit to the length of channels that can be woven conveniently. Another advantage is that highly curved connecting parts or manifolds can be incorporated in the weave at the ends of the channels or elsewhere. 
     The above list of advantages may be achieved using either active or passive ceramic composite insulation, or a combination of both. Active ceramic composite insulation includes systems in that a fluid is directed through channels in the insulation. Passive ceramic composite insulation includes systems in that channels in the insulation are filled with randomly packed low density ceramic fibers. 
     In an aspect of the invention, a woven preform for a ceramic composite comprises a plurality of layers and structural members. The plurality of layers are of woven yarns of fibrous material. The structural members extend between the layers. The layers and the structural members define interlayer spaces. In a further aspect of the invention, a plurality of openings extend through at least one of the layers. 
     In a further aspect of the invention, low density ceramic insulation is disposed in the interlayer spaces. In a still further aspect of the invention, the low density ceramic insulation comprises fibers having a length shorter than an average width of the openings. 
     In an aspect of the invention, the plurality of layers of woven yarns comprise an upper layer, a lower layer, and one or more central layers disposed between the upper and lower layers. In an aspect of the invention, at least a portion of the plurality of openings extend through the upper layer. In a further aspect of the invention, at least a portion of the plurality of openings extend through at least one of the central layers. 
     In an aspect of the invention, the low density ceramic insulation is disposed in the interlayer spaces, the insulation comprising of fibers having a length shorter than an average width of the openings. In a further aspect of the invention, the opening average width is approximately 2 mm or greater. In an aspect of the invention, the low density ceramic insulation comprises Al 2 O 3  fibers or SiO 2  fibers that are randomly distributed in a three dimensional arrangement. 
     In an aspect of the invention, the interlayer spaces are channels. In a further aspect of the invention, wherein the channels extend in a warp direction. In an aspect of the invention, the low density ceramic insulation is disposed in the channels. 
     In an aspect of the invention, a ceramic composite comprises the woven preform and a matrix. 
     In an additional aspect, a woven preform for a ceramic composite comprises at least two layers and walls. The two layers are of woven yarns of fibrous material are of woven yarns of fibrous material. The walls extend between the layers. Further, the layers and the walls define channels that extend in a warp direction. In a further aspect of the invention, low density ceramic insulation is disposed in the channels and a plurality of openings extending through one of the layers. 
     In an additional aspect of the invention, a process for fabricating ceramic composite insulation comprising the steps of providing a woven perform and infiltrating the woven preform. The woven preform comprises a plurality of layers of woven yarns of fibrous material and structural members extending between the layers. The layers and the structural members define interlayer spaces. A plurality of openings extend through at least one the layers. The woven preform is infiltrated with a slurry of a carrier and low density ceramic insulation through the plurality of openings and into the interlayer spaces. At least a portion of the low density ceramic insulation is retained in the interlayer spaces. In an aspect of the invention, a ceramic composite insulation made by the above mentioned process. In a further aspect of the invention, the low density ceramic insulation comprises fibers having a length shorter than an average width of the openings. In a still further aspect of the invention, a ceramic composite that has the short fibers is made according to the above mention process. 
     In an additional aspect of the invention, a process of insulating a structure comprising the step of joining a ceramic composite to the structure, wherein the ceramic composite comprises a woven preform comprising a plurality of layers of woven yarns of fibrous material and structural members extending between the layers. The layers and the structural members define interlayer spaces. A plurality of openings extend through at least one of the layers. In a further aspect of the invention, fluid is directed through adjacent interlayer spaces. The plurality of layers comprises an upper layer, a lower layer, and at least one central layer disposed between the upper and lower layers. The adjacent interlayer spaces are disposed on opposing sides of a central layer with at least one of the openings extending between the adjacent interlayer spaces, such that the fluid flows between the adjacent channels. 
     Other aspects, objects, and benefits of the claimed invention are described herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic cross section of a woven preform of the present invention illustrating an upper layer, a lower layer, and integrally woven walls connecting the layers to form channels in warp weaving direction; 
     FIG. 2 is a schematic cross sections showing greater detail of arrangements of fiber yarns in the woven material illustrated in FIG. 1, in which walls and skins are formed predominantly of weft fiber yarns and high packing densities of circumferential fibers are achieved around channels; 
     FIG. 3 is a schematic cross section showing greater detail of an arrangement of fiber yarns in the woven material illustrated in FIG. 1, in which walls are formed of warp fiber yarns and gaps in the skins are avoided by arranging wall and skin yarns in the same vertical plane; 
     FIG. 4 is a schematic showing the incorporation of a third, central layer connected to both upper and lower skins by integrally woven walls or columns; 
     FIG. 5 shows a woven preform with walls of significant height according to an embodiment of the invention; 
     FIG. 6 shows a woven preform with walls of effectively zero height according to an embodiment of the invention; 
     FIG. 7 shows a schematic of the incorporation of attachments or manifolds in the woven structure at the terminus of the warp channels, the shown attachment being curved, according to an embodiment of the invention; 
     FIG. 8 shows a cross section of a woven material of the present invention with an upper skin, a lower skin, and an open lattice weave connecting the skins according to an embodiment of the invention; 
     FIG. 9 shows a schematic showing greater detail of the holes or open areas woven into one of the skins of the embodiment of the invention shown in FIG. 8; 
     FIG. 10 shows a schematic of a preform with open areas woven into a central layer according to an embodiment of the invention; and 
     FIG. 11 shows a schematic of a preform with a showing a lower skin and integrally woven loops according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, woven preform  10  is part of an embodiment of the present invention that comprises a multilayer, integrally woven, ceramic composite structure. The preform  10  comprises layers  12  and  14  of fabric woven from yarns comprising fibers of materials such as silicon carbide, silicon nitride, aluminum oxide, mullite, carbon, glass, yttrium aluminum garnet (YAG), polyethylene, and other fibrous materials. The woven yarns are infiltrated or impregnated with a curing agent, such as a curable polymer or a ceramic precursor, for example, that can be cured to form a rigid composite structure (not shown). 
     FIG. 1 illustrates schematic cross sections of portions of a woven preform of the present invention, with FIGS. 2 and 3 showing enlarged sections of the woven preform  10  of FIG.  1 . The woven preform  10  has top and bottom “skins” (not shown) including an upper layer  12  and a lower layer  14 . Layers  12  and  14  comprise woven fabrics formed of warp  16  and weft  18  yarns that run in warp direction  17  and weft direction  19 , respectively. Woven layers  12  and  14  are joined or connected by walls  20 , consisting of integrally woven warp and/or weft yarns that result in the three-dimensional woven preform  10  having channels  22 . 
     The structure and geometry of wall or column forming yarns may comprise various forms in embodiments of the invention, such as supporting struts or woven walls between layers  12  and  14 , that provide desired mechanical and thermal characteristics for the final composite structure. 
     Referring now to FIG. 4, a woven preform  24  has a central layer  26  of woven fabric. The central layer  26  may separate the preform  24  into two independent sets  28  and  30  of channels  22 . Embodiments of the invention include structures that may be modified in various ways by utilizing the separate interlayer channels  22 . For example, the channels  22  in set  28  between the upper layer  12  and the central layer  26  may be filled with a low density ceramic insulation material for additional heat insulation. As a further example, the channels  22  of the set  30  between the central layer  26  and the lower layer  14  may be used for circulation of a cooling liquid or gas. Other embodiments of the invention may have a plurality of central layers  26 . 
     The components of the woven preform  10 , comprising the layers  12 ,  14 ,  26  and the walls  20 , may be infiltrated or impregnated with a curing agent that may be in the form of fibers, particulates, powders, vapors, or liquids. The curing agent may comprise a material, such as a curable polymer in uncured form or a ceramic precursor, for example, that can be cured to form a rigid structure. Curing can be accomplished by exposure to heat or radiation, for example, to form a rigid matrix (not shown) reinforced by the embedded fibers of the warp and weft yarns  16  and  18 . A polymer agent may include optional ceramic particles so that treatment at higher temperatures sinters the ceramic particles into a ceramic matrix around the woven yarns  16  and  18 , thus eliminating the polymer agent or converting it into a ceramic material. Ceramic matrix material can also be added after either initial heat treatment or curing. Such ceramic material can be introduced by chemical vapor infiltration (CVI), or infiltration of a liquid precursor, followed by heat treatment, for example. 
     The two layer woven preform  10  shown in FIG. 1 or the multilayer woven preform shown in FIG. 4 forms an excellent thermal barrier for very high temperatures at the exposed upper layer  12  with cool temperatures at the protected lower layer  14  that can be joined or connected to a supporting structure, such as the exterior of a space vehicle (not shown). 
     In other embodiments, continuous tubes (not shown) can be inserted in the cavities of the open lattice weave, either during or after weaving, for containing a circulating cooling fluid. 
     Integrally woven composite structures of the present invention have advantages in the field of light-weight, high-stiffness, structural or thermal barrier components, particularly for use in high temperature and high thermal gradient environments. Additional advantages include: 
     (a) elimination of delamination as a potential failure mode because the skins and connecting struts contain integrally woven reinforcing fibers; 
     (b) efficient specific flexural stiffness provided by the open lattice structure; 
     (c) use of skins much thinner than would be required with conventional ceramic materials or composites because of the high flexural rigidity of the integral structure; 
     (d) high heat transfer with an actively cooled structure; 
     (e) ease of forming the matrix within the thin skins by liquid or vapor infiltration; 
     (f) accommodation of thermal strains by the flexibility of the thin through-thickness integral struts; and 
     (g) ease of attaching the relatively cool back skin to a supporting structure. 
     In addition to thermal barrier applications, integrally woven ceramic composites may be utilized as structural components, such as conformal pressure vessels, for example. 
     EXAMPLE 1 
     Active 
     From analysis of the thermal and mechanical loads on typical rocket engine nozzles, we calculate that a ceramic composite nozzle (not shown) would require a skin of thickness less than about 1 mm between the hot gas and the coolant, channels  22  of about 5 mm diameter for the flow of coolant, and a high volume fraction of the walls and skin consisting of fibers following load-bearing paths circumferentially around the channels. Larger skin thicknesses would lead to outer surface temperatures beyond the capability of the ceramic, while larger channel dimensions or small fiber volume fractions would lead to pressure-induced stresses in the skin exceeding the strength of the ceramic composite. 
     An embodiment of the invention comprises a specific class of weave structures that satisfies the above requirements consist of channels  22  in the warp weaving direction  17 . A feature of the warp channel structure (not shown) is that it possible to align a large volume fraction of fibers in the load bearing circumferential direction. The warp channel structure comprises has a larger volume fraction than is possible than with weft channel structures known in the prior art. 
     So-called “warp channel weaves” are formed by passing weft yarns  16  in the circumferential direction around the channels in the patterns shown in FIGS. 5 and 6. Each weft yarn  16  follows a path that alternates between the top and bottom skins or layers of adjacent channels, with the paths of alternate sets of weft yarns  16  being out of phase. High packing densities of the circumferential weft yarns  16  may be achieved by beating up the weft yarns and because the weft yarns in each skin can be woven in multiple layers on top of each other. 
     Note that for clarity in showing the arrangement of load bearing weft yarns  16 , the warp yarns  18  on the section normal to the channels are not shown in FIG. 2 however the warp yarns are shown in FIG. 2 on a section of the top skin  12  and on a wall  20  of the channel on the left. 
     The warp channel woven structures, preform  10  and  24  for example, differ fundamentally in possible packing densities from woven structures with channels in the weft direction, which are known in the weaving literature. In the prior art weft channel structures, the circumferential yarns are the warp weaving yams. The packing density of warp weaving yarns is limited by the necessary separation of the warp yarns by the heddles and comb, and by the need for adjacent warp weaving yarns to pass beside each other when they are raised and lowered to form a shed during the weaving process. Since adjacent yarns pass through opposite skins of each channel, the maximum density of circumferential warp yarns in each skin of a weft channel is limited to one yam for each distance of two yam diameters along the axis of the channel. In contrast, the warp channel structures do not suffer from this limitation. For example, in the case where each skin of the structure consists of yarns woven in the 2-layer angle interlock weave (as in FIG.  2 ), each skin of the warp channel configuration has two circumferential yarns for each distance of one yarn diameter—a factor of 4 higher packing density than for a corresponding weft channel structure. Multiple-layer angle interlock weaves with larger numbers of layers give even higher packing densities for the warp channel structures, but the same low packing density for weft channel structures. 
     The warp channel structure  50  of FIG. 5 may be woven in a flattened form as a multilayered fabric using a Jacquard loom. The “flattened form” of the structure may be obtained by collapsing it in shear. When woven, the flattened form can be erected by reversing the shear into the desired three-dimensional structure. These structures can be woven with several weave patterns within the face sheets, including plain weave and multi-layer angle interlock weave. 
     A special case of this type of weave is one with zero height of the wall that separates the channels  22 : a preform structure  60  results as shown in FIG.  6 . The preform structure  60  is a set of joined tubes  62 , the optimum shape for pressure containment. In this case the weaving process is greatly simplified and can be achieved using a harness loom, because each weft yarn can pass in one direction across the entire width of the woven structure, in contrast to the case for channels with finite walls, in which each weft yarn must change direction as it passes around each channel of the flattened structure. Details of the weave structure for joined tubes with 2-layer angle interlock face sheets are shown in FIG.  6 . 
     EXAMPLE 2 
     Active 
     Referring now to FIG. 3, warp channel structures can also be formed by so-called “distance weaves” that consist of two woven sheets  12  and  14  connected by warp yarns  21  that pass alternately from one sheet to the other. Such structures can be woven with several weave patterns within the face sheets, including plain weave and multilayer angle interlock. An example with plain weave is shown in FIG.  3 . In contrast to existing distance weave structures, the yarn pattern in FIG. 3 is modified so that warp yarns  16  forming a skin and warp yarns that form the wall  20  are gathered within the same gap in the beat-up comb, so that they lie over and under one another rather than next to one another where they contact in the fabric. This feature avoids the formation of gaps in the skins, which raises the packing density of fibers forming the channels and promotes hermetic containment of pressurized coolant in the channels. 
     EXAMPLE 3 
     Active 
     An actively cooled ceramic composite panel, similar to the embodiment of the invention shown in FIG. 2, comprises upper and lower skins  12  and  14  interwoven to form channels  22  that can be used for circulating cooling fluid. The upper and lower skins comprise tightly woven cloths of ceramic fibers (carbon, for example) with a matrix of SiC. Examples of weave patterns within the skins include angle interlock, plain weave, and satin weave. 
     Before infiltrating the fiber structure with the SiC matrix, it is also preferable to coat all of the fiber surfaces with a thin layer (approximately 0.2 μm) of pyrocarbon to provide good mechanical properties in the final ceramic composite. This is easily accomplished by chemical vapor infiltration. The fiber preform is held in the desired shape by carbon or refractory metal mandrels during the CVI processing. 
     The matrix material is introduced by infiltrating the fiber preform with a polymer precursor for SiC, or with a slurry consisting of crystalline SiC powder suspended in the polymer precursor. The matrix material consisting of combined SiC powder and polymer precursor can also be introduced by separate infiltrations of a slurry of crystalline SiC powder (in another fluid) and the polymer precursor. Various types of polycarbosilanes are known to be suitable precursors. The penetration of the matrix into the preform may be assisted by using vacuum infiltration. After infiltration, the part is heated to a temperature in the range 100 to 400° C. to cure the polymer. It is then heated to a temperature of approximately 1000° C. to pyrolyze the polymer and leave a matrix of SiC with some porosity. The infiltration and pyrolysis cycle is repeated up to about ten times, with each cycle reducing the fraction of residual porosity in the matrix. 
     EXAMPLE 4 
     Active 
     Alternative materials combinations can be processed using the same procedures as in Example 3. These include SiC for the fibers and Si 3 N 4  for the matrix. The woven yarns may be infiltrated by a precursor polymer that contains elements, which upon heating, decompose to form a desired ceramic matrix material. Examples of such precursor polymers include polyacrylnitrite (PAN) to produce a carbon matrix; polycarbosilane polymer to produce a SiC matrix; polysilazanes to form a Si 3 N 4  matrix; and polysilane to form a SiO 2  matrix. The precursor polymers may also contain other additives that react with the polymer or its decomposition products to provide elements of the final ceramic matrix. An example of this type of precursor is. polysilane polymer with a suspension of particulate Al metal to form a mullite matrix. 
     The foregoing precursor solutions are described as examples, not limitations, of the various precursors that can be used to impregnate the woven structures of the present invention. Precursor solutions are also known in the art for many other ceramic materials, including oxides such as Al 2 O 3 , ZrO 2 , SiO 2 , mullite, and yttrium aluminate, for example. These processing methods are known in the art. 
     EXAMPLE 5 
     Active 
     The woven yarns may be heated and infiltrated by a gas, or a combination of gases, that react or decompose upon contact with the heated yarns to form the desired ceramic matrix surrounding the yarns (chemical vapor infiltration, CVI). Examples include CH 4  gas to form a deposit of carbon on fibers heated above 1000° C.; and methyltrichlorosilane to form a deposit of SiC on fibers heated above 1100° C. 
     The woven yarns may be heated and infiltrated by a liquid, or a combination of liquid and solid, that react or decompose upon contact to form the desired ceramic matrix surrounding the yarns (melt infiltration). Examples include the reaction of carbon and liquid silicon to form SiC. These processing methods are known in the art. 
     EXAMPLE 6 
     Active 
     Referring now to FIG. 7, an actively cooled thermal barrier, represented by preform  10 , must be joined onto supply lines (not shown) for the ingress and exit of coolant from the coolant containment channels  22  in the barrier. The joining problem is greatly simplified by arranging that joints (not shown) are located away from the heat flux acting on the barrier. In rocket nozzle ramp applications, for example, one appealing solution is to extend the woven structure  12  at the end  70  of the channels  22  around an arc  72  so that the channels  22  curve back under the heated part of the thermal barrier, as shown by curved terminus  74 . Here the supply lines are protected from the heat flux and so joints to feed lines will remain relatively cool. 
     An advantage of warp channel integrally woven structures is that they permit such a curved terminus  74  to be woven in a straightforward manner. As shown in the detail of FIG. 7, curvature can be created by inserting additional weft yarns  18   a  on the top skin or layer  12 , so that when the yarns are beaten up that skin will be longer than the bottom skin or layer  14 . The added yarns  18   a  can be passed through the lower skin  14  so that they protrude on the cool side of the structure (where they do not impede heat flux into the coolant), rather than being woven into the lower skin. In that way, the lower skin  14  can be beaten up to a shorter length than the upper skin. 
     Referring now to FIGS. 8 and 9, a woven preform  110  for a ceramic passive thermal insulation structure (not shown) according to an embodiment of the invention comprises “skins” or an upper layer  112  and a lower layer  114  with walls  120  extending therebetween to form channels  122 . In other embodiments of the invention, columns, struts, and other structural members may extend between the layers. The structural members and the layers define interlayer spaces, which may be less delineated than the channels  122 . 
     In an embodiment of the invention, the preform  110  comprises materials described above and the ceramic passive thermal insulation structure is formed through infiltration of the preform as described below. Other embodiments of the invention may have one or more central layers, as is described above in connection with FIG.  4 . 
     Referring specifically to FIG. 9, the upper layer  112  and a lower layer  114  of the preform  110  comprise woven fabrics formed of warp  116  and weft  118  yarns that run in warp direction  117  and weft direction  119  respectively. Note that the yarns  121  that make up walls  120  and connect skins  112  and  114  may form individual struts or columns and that various arrangement of rows of columns may form channels that run in either the warp direction or the weft direction. Embodiments of the invention for a passive thermal insulation structure may have channels that run in any direction. 
     Woven layers  112  and  114  are joined or connected by integrally woven, but relatively sparse and loosely woven, warp  121  yarns . The warp yarns  121  form the walls  120  of the preform  110 . Embodiments of the invention may have the structure and geometry of the walls  120  comprise an open lattice weave of various forms, such as supporting struts between layers  112  and  114 , that provide desired mechanical and thermal characteristics for the final composite structure. 
     The woven preform  110  has a top layer  112  that is an open weave that results in openings  124  therethrough. The open weave of the top layer is used in various ways. In an embodiment of the invention, the channels  122  are filled with a low density ceramic insulation  125  material for additional insulation. The insulation  125  is disposed in the channels  122  through the openings  124  in the top layer  112 . A slurry (not shown) with the insulation is infiltrated through the openings  124  in the top layer  112  and the carrier of the insulation flows out the preform  110  through the bottom layer  114  or the sides (not shown). Therefore, the openings  124  are large enough for the insulation to pass through, but the relatively tight weave of the bottom layer  114  only permits the carrier to pass through. 
     Referring now to FIG. 10, in another embodiment of the invention, a preform  130  comprises a central layer  126  with openings  124  therethrough. The preform  130  may be used to make a ceramic composite (not shown) that enables fluid circulation between the channels  122 . Such a composite forms an excellent thermal barrier for very high temperatures at the exposed upper layer  112  with cool temperatures at the protected lower layer  114  that can be joined or connected to a supporting structure (not shown). 
     In another embodiment of the invention, the fibers and matrix of the upper and lower layers  112  and  114  and walls  120  may comprise different materials suited for different temperatures: the upper layer  112  may consist of ceramic fibers and matrix for exposure to high temperatures, while the lower layer  114  may consist of graphite fibers and epoxy matrix for low temperature structural efficiency. 
     EXAMPLE 1 
     Passive 
     The ceramic integrally woven material of the present invention may include a matrix formed by infiltration of the woven yarns by a precursor that produces a ceramic from the monazite and xenotime family, described in U.S. Pat. No. 5,514,474. The woven yarns can be impregnated with a solution, slurry, or solgel that converts to a monazite or xenotime when heated. As specific examples, a woven fiber preform comprising Al 2 O 3  fibers and another comprising mullite-silica fibers were impregnated with aqueous slurries containing particles of alumina (0.3 mm diameter α-Al 2 O 3 , as provided by the Sumitomo Corporation) and solution precursors for LaPO 4 , one containing lanthanum nitrate and methylphosphonic acid and the other containing lanthanum nitrate and phosphorous acid. These solutions contained lanthanum and phosphorus in the ratio of 1:1. Concentrations in the range of 1 to 2 moles per liter of the solution precursor were found to be suitable, with the lower concentrations preferred for thinner coatings of La-monazite and the higher concentrations preferred for thicker coatings. Phytic acid has also been used successfully as a precursor with lanthanum nitrate. The precursor slurries were able to wet and infiltrate between individual fibers of the woven material. After heating at about 1100° C. for an hour, the solution precursors converted to LaPO 4  (La-monazite) resulting in a two-phase ceramic matrix consisting of LaPO 4  grains and Al 2 O 3  grains and fine-scale porosity 30. 
     EXAMPLE 2 
     Passive 
     In an embodiment of the invention, the preform  110  may comprise a lower skin  114 , integral struts  120 , an upper skin  112 , and a low density ceramic insulation material  125 . The lower skin  114  may comprise a tightly woven cloth of Al 2 O 3  fibers (in a double layer angle interlock weave pattern, for example) with a matrix of Al 2 O 3 /LaPO 4 . The upper skin  112  may comprise the same materials, but with a weave pattern that contains periodic gaps of at least 2 mm width that do not contain either fibers or matrix. The connecting struts comprise individual fiber tows (yams), integrally woven with both skins, and having the same Al 2 O 3 /LaPO 4  matrix. 
     The matrix may be formed by infiltration with a slurry comprising Al 2 O 3  powder in an aqueous solution precursor for LaPO 4 , as described in example 1-passive. 
     The low density ceramic insulation material may comprise short ceramic fibers (Al 2 O 3  fibers of 100 μm length and 3 μm diameter, for example) distributed in a random three dimensional arrangement. Some rigidity and strength can be imparted to the network of insulating fibers by bonding pairs of fibers where they touch. The insulating fiber network is introduced into the ceramic composite lattice structure or framework by forming an aqueous slurry of dispersed fibers, placing the composite structure into a pressure filtration die with the fiber slurry, and applying pressure to remove excess water and concentrate the fiber network within the composite structure. The presence of openings in the upper skin with average widths being larger than the lengths of the dispersed fibers permits transportation and concentration of the dispersed fibers into the channels within the composite structure. The insulating composite panel, comprising the framework and consolidated fibers, is removed from the pressure filtration die and dried to remove the remaining water. Bonding at the fiber intersections is achieved using a solution or polymer precursor that is converted to ceramic during a subsequent heat treatment. The precursor is included in the slurry used to transport the insulating fibers into the composite framework and is deposited at the fiber intersections during the drying step described above. Alternatively, the precursor can be introduced by a separate infiltration step after the drying step. Bonding of the intersecting fibers can also be achieved by heating the consolidated composite to a temperature sufficiently high to cause solid state sintering of touching fibers or, in the case of fibers that contain a glass phase, melting and bonding of the glass phase where the fibers are in contact. 
     EXAMPLE 3 
     Passive 
     Referring now to FIG. 11, a woven preform  210  comprises a lower skin or bottom layer  214  with integral struts or loops  221  protruding therefrom in an upward direction  222 , and a low density ceramic insulation material  225 . The lower skin  214  may comprise a tightly woven cloth of Al 2 O 3  fibers (in a double layer angle interlock weave pattern, for example) with a matrix of Al 2 O 3 /LaPO 4 . The connecting struts  221  may comprise individual fiber tows (yarns ), integrally woven with the lower skin  214  and having the same Al 2 O 3 /LaPO 4  matrix. 
     A ceramic composite matrix (not shown) may be formed from the bottom skin  214  and the struts  221  from Al 2 O 3  and LaPO 4 by the same method described in Example 2-passive. 
     The low density ceramic insulation  225  comprises the same materials as in example 2-passive and is introduced into the space between and around the struts  221  in the same manner as well. The consolidated network of insulating fibers that forms the low density ceramic insulation  225  may form a layer of thickness  230  equal to the height of the struts  221  or a layer of thickness greater than the height of the struts, as determined by adjusting the volume of slurry used during the filtration step. In either case, the struts  221  serve to anchor the ceramic composite skin  214  to the body of the low density insulating material  225 . 
     EXAMPLE 4 
     Passive 
     The preforms  110 ,  130 , and  210  may be infiltrated by a precursor polymer that contains elements, which upon heating, decompose to form a desired ceramic matrix material. Examples of such precursor polymers include polyacrylnitrite (PAN) to produce a carbon matrix; polycarbosilane polymer to produce a SiC matrix; polysilazanes to form a Si 3 N 4  matrix; and polysilane to form a SiO 2  matrix. The precursor polymers may also contain other additives that react with the polymer or its decomposition products to provide elements of the final ceramic matrix. An example of this type of precursor is polysilane polymer with a suspension of particulate Al metal to form a mullite matrix. 
     The foregoing precursor solutions are described as examples, not limitations, of the various precursors that can be used to impregnate the woven structures of the present invention. Precursor solutions are also known in the art for many other ceramic materials, including oxides such as Al 2 O 3 , ZrO 2 , SiO 2 , mullite, and yttrium aluminate, for example. 
     EXAMPLE 5 
     Passive 
     The preforms  110 ,  130 , and  210  may be heated and infiltrated by a gas, or a combination of gases, that react or decompose upon contact with the heated yarns to form the desired ceramic matrix surrounding the yams. Examples include CH 4  gas to form a deposit of carbon on fibers heated above 1000° C.; and methyltrichlorosilane to form a deposit of SiC on fibers heated above 1100° C. 
     EXAMPLE 6 
     Passive 
     Two skins may be formed as two layers of plain, or satin, or angle interlock, or other weave comprising warp and weft yarns woven in a loom at approximately the desired (final) skin separation. Open spaces may be left intentionally in one or both skins by omitting selected warp or weft yarns or by incorporating temporary spacers in the skin as it is being woven. The two skins may be connected by warp or weft yarns (called pile yams) woven alternately into the fabric of one and then the other skin in a repeating pattern. With this method, friction and stiffness of the pile yarns is sufficient to maintain the desired separation of the skins. The pile yarns may be made to pass approximately at right angles from one skin to the other or at some other angle by selection of the particular yarns in the two skins around which they are to be woven. This weaving method is known in the art, but not with deliberately incorporated holes or open spaces for the purpose of introducing insulating material into the interior. 
     Although the present invention has been described with respect to specific embodiments thereof, various changes and modifications can be carried out by those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.