TECHNICAL FIELD [0001] The invention pertains to heat spreader constructions and methods of forming heat spreaders. BACKGROUND OF THE INVENTION [0002] Thermal management and heat dissipation in electronic and electro- optical devices is important for proper device performance. Thermal management components such as heat sinks and heat spreaders are utilized to decrease potential negative impacts of heat generating components in a wide range of devices by aiding in the transfer of heat to the ambient environment. [0003] Referring to Fig. 1 , such shows an exemplary heat dissipation configuration 10 having a heat generating device 100 and an associated heat spreader 200. Generally, heat spreaders are larger than the associated heat generating device, at least in the x and y directions (as shown). As depicted in Fig. 2, heat spreader 200 can have opposing surfaces 202 and 204 where surface 202 can be characterized as being a heat receiving surface and can be disposed in heat receiving relation relative to a surface 102 of heat generating device 100. [0004] In particular applications, opposing face 204 can be disposed interfacing an appropriate heat sink (not shown). An associated heat sink typically is configured to have an x-y dimensional area or "footprint" that is larger than that of the heat spreader. The relative x-y size configuration of the heat spreader (typically intermediate that of the heat generating device and the heat sink) can allow effective spreading of the generated heat from the small device to the heat sink, thereby increasing the efficiency of the thermal management system. [0005] Referring again to the exemplary assembled thermal management system shown in Fig. 1 , heat removal from device 100 can be most efficient where heat spreader 200 is able to remove heat efficiently both in a direction substantially parallel to the plane of the interfacing surface of heat generating device 100 (surface 102 of Fig. 2), and in a second orthogonal direction perpendicular to such interfacing surface. In other words, with reference to the depicted coordinate system, it can be desirable to efficiently conduct heat in at least one direction parallel to the direction of the x-y plane (e.g. in the x and/or y direction), and also in the z direction perpendicular to the x-y plane. The increased dimensional heat conduction can maximize heat spreading ability while minimizing the temperature differential between opposing heat spreader surfaces 202 and 204. [0006] Conventional heat spreaders often utilize materials such as copper which have isotropic thermal conductivity Kth of 360-400 Watts/meter-Kelvin (W/mK), with specific values depending upon purity of the material. However, thermal conductivities within this range are often insufficient for heat removal from particular devices. Materials which have isotropic thermal conductivities higher than copper include single crystalline silicon carbide having a thermal conductivity of approximately 500 VWmK (where the exact value is dependent upon crystallographic polymorph or microstructure) and single-crystalline diamond having a thermal conductivity of from about 1000 to about 3000 W/mK (where the specific value depends upon perfection and purity of the diamond). The high cost of these relatively high thermal conductivity materials can be cost prohibitive for their use in heat spreader and other thermal management applications. [0007] It is desirable to develop heat conducting materials and thermal management articles for utilization in heat spreader and/or heat sink applications where the heat conducting article has high thermal conductivity in at least two orthogonal directions. SUMMARY OF THE INVENTION [0008] In one aspect the invention encompasses a heat spreader construction comprising a plurality of panels, each of which includes woven carbon fibers and a matrix material. In one aspect the heat spreader construction has woven carbon fibers having axes of the carbon fibers aligned in two or more directions. [0009] In one aspect the invention encompasses a heat spreader comprising a plurality of panels wherein the panels include a high thermal conductivity matrix material and one or more of discontinuous graphitic carbon particulates, diamond particulates, discontinuous graphitic carbon whiskers, and discontinuous graphitic carbon fibers. [0010] In one aspect the invention encompasses a method of forming a heat spreader construction. The method includes providing a plurality of panels which comprise woven carbon fibers and joining the plurality of panels to form a block having a surface configured to interface a heat generating device. A first portion of the carbon fibers are substantially axially aligned parallel to the surface and a second portion of the fibers are aligned substantially orthogonally to the surface. BRIEF DESCRIPTION OF THE DRAWINGS [0011] Preferred embodiments of the invention are described below with reference to the following accompanying drawings. [0012] Fig. 1 is an isometric view of an exemplary thermal management system. [0013] Fig. 2 is an exploded isometric view of the thermal management system depicted in Fig. 1. [0014] Fig. 3 is an isometric view of a heat spreader in accordance with an aspect of the present invention. [0015] Fig. 4 is an isometric view of a single panel portion of the heat spreader shown in Fig. 3. [0016] Fig. 5 is a side view of the heat spreader panel shown in Fig. 4, depicting a particular weave pattern in accordance with the invention. [0017] Fig. 6 is a side view of a heat spreader panel having an alternative weave pattern relative to that shown in Fig. 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0018] One aspect of the invention is to develop methodology and heat spreader configurations to allow cost effective manufacture of heat spreaders capable of maintaining integrity and performance of electronic and electro-optical devices. In particular, heat spreader materials and configurations of the invention can allow efficient heat removal from heat generating devices in at least two orthogonal directions. In particular instances it can be preferable that efficient heat removal occurs in at least two orthogonal directions where a first direction is substantially parallel to an interfacing surface of a heat generating device, and in a second direction substantially perpendicular to the interfacing surface. The heat conducting articles of the invention can be particularly useful in applications where the heat spreader is to be utilized to transfer heat between a heat generating device and a heat sink. [0019] In general, heat conducting articles of the invention can be formed of a block of material such as depicted by the exemplary heat spreader 200 shown in Figs. 1 and 2. Block 200 can comprise a single piece or block of material as shown in Fig. 2, or can alternatively have a multi-plate or multi-panel configuration such as that shown in Fig. 3. For purposes of ease of the present description, where a three dimensional block comprises a plurality of panels, the panels can be referred to as "two-dimensional" although each panel does have a thickness in a third dimension. [0020] A single block heat spreader in accordance with the invention can comprise continuous carbon fibers woven in three dimensions. The continuous carbon fibers can be, for example, pitch-derived carbon fibers. The continuous fibers can be woven in more than two directions or dimensions to form a pre-form. In particular aspects, pre-forms can preferably be woven to comprise fibers in three mutually orthogonal directions (where fiber direction refers to the direction of the fiber axis). In particular applications, the total fiber content of the pre-form will consist of three fiber portions: a first portion being substantially axially aligned in a first direction, a second portion being substantially axially aligned in a second direction which is orthogonal to the first direction, and a third portion being substantially axially aligned in a third direction orthogonal to each of the first and second directions. The fiber fraction in any of the three directions can be tailored. [0021] Woven pre-forms in accordance with the invention are not limited to any particular size or shape and can be fabricated to have geometrical dimensions engineered to conform to final desired dimensions of articles such as heat spreaders for electronic or electro-optical devices and circuits (near net shape). Alternatively, a woven pre-form can be fabricated as a single piece which will be further processed to produce multiple units having similar or varying dimensions. [0022] Heat spreader 200 or other thermal management articles of the invention can be fabricated by multidimensional weaving of continuous carbon fibers so as to provide a sufficiently high fiber fraction in each direction for efficient removal of heat from interfacing surface 102 of heat generator 100. [0023] The woven pre-form can be densified with, for example, a carbon comprising matrix material. Densification can be accomplished by, for example, infiltration with liquid pitch resin or carbon matrix, or by chemical vapor deposition of carbon matrix material. Where liquid resin is utilized, thermal annealing can be conducted in a non-oxidizing ambient to remove non-carbon components from the matrix. Such processing can produce a matrix that consists essentially of, or consists of carbon. After densification, the resulting article can be heated in a non-oxidizing ambient to a temperature above about 20000C to effectively graphitize the article in order to increase thermal conductivity of the fiber and matrix combination. [0024] Although fabrication of the single piece material is described above as comprising graphitization after densification, it is to be understood that the invention contemplates graphitization of the fiber pre-form prior to densification. Graphitization can optionally be performed both prior to and after densification. Alternatively, the pre¬ form can be partially densified to increase rigidity, graphitized, and subsequently be more fully densified. [0025] The invention also contemplates embodiments where the woven carbon fiber pre-form (graphitized or non-graphitized) is infiltrated with a polymeric matrix such as, for example, epoxy or an inorganic non-carbon matrix (e.g. silicon, silicon carbide, boron nitride or a metal or metallic material). [0026] Utilizing the carbon pre-form/matrix composite material described above, physical properties such as thermal expansion coefficients in different directions can be controllably tailored. Sizing and/or shaping can be performed to produce a desired dimension utilizing, for example, machining. [0027] The single piece article 200 of the invention can be disposed either in direct contact with surface 102 of heat generating device 100 as depicted in Figs. 1 and 2, or can be utilized in combination with one or more interfacing materials (not shown). [0028] Upon completion of three dimensional weaving, densification, and thermal annealing described above, single piece thermal management articles of the invention can exhibit conductivities greater than about 400 W/mK in three mutually perpendicular directions. Additionally, the carbon-carbon fiber matrix composites described have high mechanical strength and toughness with strength increasing with increasing temperature. This is in direct contrast with strength of copper, diamond or silicon carbide materials which decreases with increasing temperature. The composite materials of the invention can also be advantageous since the composite materials are less brittle than materials such as diamond and silicon carbide. Additionally, the materials of the invention can be tailored to achieve a particular desired thermal expansion coefficients in particular directions in contrast to materials such as diamond and silicon carbide which have substantially fixed thermal expansion coefficients. [0029] In one aspect, the invention includes multi-part heat conducting articles such as the heat spreader depicted in Fig. 3. As shown in Fig. 3, heat spreader 200 can comprise a plurality of plates or panels 210. Exemplary configurations for panels 210 are discussed with reference to Figs. 4-6. [0030] Referring to Fig. 4, the depicted panel 210 can be described as a two- dimensionally woven carbon-fiber carbon matrix composite. Although the depicted panel is shown to have substantial thickness in the y-direction, it is to be understood that the depicted panel can represent a thin woven single-ply sheet of material as described herein. A plurality of panels 210 can be bonded together to form a three- dimensional structure such as that shown in Fig. 3. Panel 210 can comprise woven carbon fibers which are preferably woven such that the axes of the woven fibers lie substantially along the x and z directions. In particular applications, each panel can preferably have a total amount of fibers consisting of two portions, a first potion of the fibers being substantially axially aligned in a first direction, and a second portion being substantially axially aligned in a second direction orthogonal relative to the first direction. [0031] In application, when the article shown in Fig. 3 is utilized in a heat spreader application, it can be preferable that surface 202 interface (directly or indirectly) the surface of the heat generating device from which heat is to be removed. Accordingly, the axes of the woven fibers in each of panels 210 lie substantially in the x and z direction. This configuration can result in the highest thermal conductivity in these two directions with respect to the x-y surface plane of the heat generating electronic device and thereby provide enhanced thermal management relative to conventional articles or alternative configurations. [0032] In particular applications, each of the panels 210 such as shown in Fig. 4 can preferably comprise pitch-derived carbon fibers in a carbon matrix. [0033] The single-ply two-dimensional woven carbon fiber panels 210 are not limited to any particular weave and can comprise, for example, a plane weave such as that shown in Fig. 5, or alternatively a five-harness weave such as that shown in Fig. 6. Alternatively, the woven panel can have a non-uniform weave with one or more areas of the panel having an increased fiber concentration (not shown) relative to other areas. Such high-concentration areas can advantageously remove heat form areas of particularly high heat generation or "hot-spots" of a heat generating device. The non¬ uniform weave configuration can be more cost effective than a panel having uniform weave with the high concentration of fibers uniform throughout the panel. It is to be understood that the invention contemplates additional weave patterns in addition to those specifically depicted in the figures or discussed above. [0034] Since the x and z planes are parallel to corresponding fiber axial directions, thermal conductivity can be substantially higher in these directions within the panels since thermal conductivity is substantially higher axially along the pitch-derived carbon fiber relative to fiber radial directions (corresponding to the y direction which extends into the plane of the two-dimensional depiction in Figs. 5 and 6). [0035] Panels 210 can be fabricated individually or multiple panels can be produced from a single sheet of single-ply two-dimensional fabric. Such fabrics can be prepared, for example, using methodology described in European Patent No. EP 891530 B1 which is hereby incorporated by reference. Such fabrication can typically comprise weaving in two dimensions, densification and heat treatment analogous to those described above with respect to the three-dimensional woven materials. The single two-dimensional panels 210 in accordance with the invention can typically exhibit thermal conductivities in each of the two axial fiber directions (the x and z directions as depicted in the figures) as high as from about 450 W/mK to about 1 100 VWmK, depending upon particular materials and processes. [0036] Formation of the three-dimensional article depicted in Fig. 3 from individual 2-dimensional panels such as shown in Fig. 4, can be achieved by several alternative routes. For example, a plurality of substantially flat two-dimensional panels 210 (after densification and heat treatment) can be placed one on top of another to create a stack. Prior to being placed in the stack, each of the panels can optionally be dipped in or otherwise coated with a coating such as, for example, pitch carbon resin or other suitable carbon resin. Alternatively, coating material can be applied to fewer than all of the panels to be included in the stack, or can be provided over a portion of some or all of the panels. [0037] Maintaining a substantially rectangular shape can be achieved by, for example, placing panels 210 into a suitably shaped container during stack formation. Although rectangular stack shapes are depicted and described, the invention contemplates alternatively shaped panels. [0038] Application of a slight pressure onto the top panel of a stack (by, for example, placing a block of suitable material onto a top panel) can insure achieving at least a minimally sufficient contact between panels during stack construction. The entire stack assembly can then be annealed, for example, in a furnace in non-oxidizing ambient to effect carbonization of the liquid carbon resin or other coating material. Such annealing can typically be performed at a temperature of from about 8000C to about 1000°C for at least about 2 hours. [0039] The annealing event can produce a solid block which can be cut into appropriately sized articles such as high thermal conductivity heat sink structures. When properly oriented in heat receiving relation relative to a heat generating device, the article can be utilized as a high thermal conductivity heat spreader exhibiting highest thermal conductivity in the x and z directions as discussed above. The described manufacturing operation can be automated to reduce costs. [0040] In an alternative example of article formation, each of a plurality of individual 2-dimensional panels 210 can be only partially processed and can be cut to size in a "pre-preg" state, which refers to a state where a woven fiber construction is impregnated with a relatively small amount of resin and cured. The pre-preg can be formed or cut into an appropriate shape and size for the desired application. A stack can then be created as described above without additional carbon resin. The stack of panels can then be processed as a single unit to convert the carbon resin into carbon and thereby increase thermal conductivity and density (see the incorporated reference EP 891530). The processing as a unit can include steps of carbonization, rigidization, and densification of the stacked panels by, for example, chemical vapor infiltration of carbon and subsequent heat treatments. Chemical vapor infiltration can be performed, for example, in an isothermal, isobaric reactor known to those skilled in the art, or alternatively by accelerated or rapid infiltration processes such as those described in U.S. Patent No. 5,348,774, incorporated by reference herein. [0041] Upon completion of processing steps, a solid block can be obtained which can be cut into appropriately sized articles to be utilized in heat sink and/or heat spreader applications. Again, when properly oriented, the articles can exhibit very high thermal conductivity in the x and z directions as discussed above. [0042] In addition to the stacking described above, it is to be understood that 2- dimensional panels 210 can be offset relative to one another during stacking or two or more types of weave can be utilized in various patterning arrangements within a particular stack. Such can result in either a balanced or an unbalanced weave arraignment and enable engineering of thermal conductivities and other physical properties of the final heat spreader or heat sink article. For instance, a first type of panel 210 having a first weave pattern can be utilized in conjunction with a second type of panel having a second weave pattern. Stacking can comprise alternating between weave types in an every-other-panel fashion, or can utilize sub-stacking of a plurality of a first type of panel followed by addition of one or more of a second type of panel to the sub-stack. It is to be understood that the invention contemplates utilization of any number of stacking patterns, each of which can utilize two or more different panel weaves. [0043] An additional variation of stacking can involve staggering of placements of woven panels such that the offset is substantially equivalent to a width of one fiber bundle or one tow, to result in different fiber orientations in alternate panels (i.e. displacement in the y direction) relative to the heat generating device. [0044] Additional variations can include application of an electrically insulating or electrically conducting coating or paste material to one or more surfaces of a heat conducting article. For example, a coating or paste material can be applied over surface 202 (Fig. 3) which is to contact or interface the surface of a heat generating device. Such coatings can be utilized to facilitate thermal contact and/or cushion differences in thermal expansion coefficients between the thermal management device 200 and the heat generating device. Exemplary coatings can comprise, for example, one or more metal or metal alloy. [0045] In addition to the aspects described above, the invention contemplates utilization of three-dimensional woven materials discussed above as panels to be employed in stacks described with reference to the '2-dimensional' panels. Such can allow a substantially equal fiber fraction present in the x and z directions which have a smaller fiber fraction present in the y direction. The advantages of such constructions can include obviating a need to stack and bond a large number of two-dimensional patterns and can avoid incurring expense of a full three-dimensional (or higher dimensionality) woven construction. [0046] The invention additionally contemplates utilization of various two- dimensional panels in combination with one or more three-dimensional woven materials. Such can be especially useful for preparation of a desired size or shape of article which does not conform to a particular 3-dimensional and/or 2-dimensional panel or stack configuration. [0047] In alternative aspects of the invention, panels 210 can comprise a two- component material comprising discontinuous graphitic carbon and/or diamond particulates, whiskers or fibers (as opposed to continuous fibers) in a high thermal conductivity matrix. Examples of particulates which can be utilized include single- crystalline diamond particles, graphite flakes, graphite fibers (such as chopped P25 class fibers or vapor grown carbon fiber (VGCF)), or single-wall or multi-wall carbon nanotubes and agglomerates of such carbon nanotubes in a number of forms and morphologies (such as nanoropes). [0048] Exemplary matrix materials can include carbon, especially highly oriented rough-laminar graphitized carbon which can be deposited by, for example, chemical vapor deposition and/or infiltration. Alternatively, carbon can be derived from liquid mesophase pitch. Other appropriate matrix materials can comprise, for example, copper, silver, aluminum, silicon and alloys thereof. [0049] The described two-component panels of the invention can be fabricated individually or cut or machined from larger prepared heat conducting materials. Fabrication of individual panels can comprise, for example, molding from a slurry, chemical vapor infiltration of a dry pre-form, or liquid metal infiltration of a dry pre-form. In order to obtain high thermal conductivity in discontinuously reinforced materials, fractions of both particulate and matrix should be chosen such that there is substantial continuous heat flow within each component of the material. Preferably at least the component having the higher thermal conductivity (relative to the other component) is sufficiently abundant within the material to provide continuous heat flow. For example, in a carbon-nanotube/ aluminum composite where the intrinsic thermal conductivity of carbon nanotubes is several times (e.g., 15 times) higher than that of the aluminum matrix, a sufficient nanotube fraction should be present and is preferably distributed so as to satisfy the "percolation threshold" for thermal conductivity to allow the composite to exhibit relatively high thermal conductivity overall. Preferably, the percolation threshold is satisfied for each of the components of the composite material, although the actual fiber or component fractions can be above the minimum content for establishing percolation threshold. [0050] lnterfacial properties between particulates and matrix materials can affect aggregate thermal conductivity of panels. Accordingly, panel fabrication methods may be adjusted to optimize for each combination of phases to produce a desired interfacial property for optimum thermal transfer and mechanical strength. [0051] Where individual panel thicknesses are smaller than a length or width of a heat generating device, bonding of two or more panels may be utilized to achieve an appropriate size. Bonding of panels can be achieved utilizing, for example, gluing (utilizing for example, carbon resins such as pitch derived resin) and subsequent annealing. Gluing techniques can be especially useful for materials having carbon matrix. Additional joining techniques which can be utilized alone or in combination include soldering or brazing (especially for those materials having a metal matrix), welding, and solid state bonding (especially for materials having a silicon or metallic matrix). The various bonding techniques can be facilitated by first applying a surface coating utilizing, for example, electroless plating or electroplating, sputtering or physical vapor deposition. Such coating can improve adhesion of the bonded panels and increase efficient transfer of heat across panel interfaces. [0052] In particular instances, the thermal conductivity in the y in-plane direction within each panel 210 can be substantially lower (for example, 5-20 times lower) than thermal conductivity in either the x or the z direction. Further, thermal conductivity in the y direction across a plurality of bonded panels can in particular instances be equal to or lower than thermal conductivity within a single panel due to increased interracial thermal resistances of bonding between panels. [0053] It is also to be understood that the panel bonding methods described can be augmented by any of several mechanical attachment techniques to provide an enhanced bond. Mechanical attachment techniques can include, for example, surface roughening of areas to be bonded prior to bonding which can be achieved utilizing mechanical or chemical etching methods, machining of grooves or grooves and counter grooves of various shapes, machining of holes and providing pins, bolts, rivets, etc., or combinations of these methods. It is also to be understood that the invention additionally contemplates utilization of these mechanical attachment techniques in an absence of gluing or other non-mechanical bonding and specifically without any bonding process which would utilize an annealing temperature above about 3000C. [0054] Although the description above sets forth particular panel shapes and article shapes, it is to be understood that the invention and methods can be adapted to provide alternative shapes appropriate for a given thermal management application.