Hybrid materials refer to artificial combinations of constituents that differ in structure and/or composition. The combination results in properties that none of the individual constituents can provide. The structure and composition within each constituent, the degree of bonding between the constituents, the relative positions of the units of the constituents in the hybrid material, and the shape and size of each unit of the constituent in the hybrid material are all important in governing the properties of the hybrid material. An example of a hybrid material is one that consists of two constituents that are bonded together within each unit of the hybrid material, with each constituent comprising a plurality of units, such that each unit is in the form of a sheet. In an example of the relative positions of the units of the constituents, the sheets of the two constituents are regularly alternating.
Hybrid materials are particularly attractive when the size of each unit of a constituent is small (e.g., less than 10 μm, preferably less than 1 μm). This is because the small constituent unit size and the consequent large area of the interface between adjoining constituent units enhance the extent of interaction between the adjoining constituents. For example, such interaction may cause the structure of one constituent to affect that of an adjoining constituent, as in the case of an epitaxial thin film on a crystalline substrate, with the structure of the thin film affected by the structure of the substrate, such that the thin film is crystallographically in registry with the substrate. This interaction provides an avenue for controlling or tailoring the structure of a material. Furthermore, the small constituent size allows the hybrid material unit size to be small while the hybrid material remains representative of the structure of the hybrid. The ability for the hybrid material to be small in unit size widens the scope of applications for the hybrid material. An example of an application that benefits from a small hybrid material unit size is the use of the hybrid material as a filler in a composite material.
A challenge in the development of hybrid materials relates to the imperfect bonding between the constituents of the hybrid material. The imperfect bonding results in weakness at the interface between the adjoining constituents. The smaller is the constituent unit size, the larger is the amount of the interface area per unit volume, and the more severe is the problem associated with the weak interface.
A composite material is an artificial combination of components that are different in structure and/or composition, such that the composite material involves solid ingredients that are bound by a binder, which forms the matrix of the composite material. There are various types of solid ingredients, including continuous fibers, short fibers, tubes, particles, platelets, sheets, etc. A filler is a solid ingredient that is discontinuous and small in size, most commonly in the form of particles.
Because the desired properties of a composite material tend to be enhanced by an increase in the volume fraction of a solid ingredient, the volume fraction of the matrix should be kept low. For example, the elastic modulus of a composite with fiber reinforcement increases with increasing reinforcement volume fraction. Thus, the binder or matrix needs to be effective for binding the solid ingredients together even when it is at a low volume fraction. This requirement on the binder or matrix poses a challenge for the development of composite materials.
Another challenge in the development of composite materials relates to the imperfect binding of the solid ingredients of the composite material. The imperfect binding results in weakness at the interface between the units of a solid ingredient and the matrix of the composite. The smaller is the size of each unit of the solid ingredient, the larger tends to be the amount of the interface area per unit volume, and the more severe tends to be the problem associated with the weak interface.
High-temperature materials are materials that can resist high temperatures, such as temperatures above 1000° C. Preferably they can resist high temperatures in the presence of oxygen, which is in air. They are needed for numerous applications, such as missiles, reentry space vehicles, aircraft brakes, furnace components, power plant components and high-temperature industrial process components. For missiles, reentry space vehicles and aircraft brakes, the materials also need to be low in density, i.e., lightweight, for the purpose of fuel saving.
Ceramics are inorganic compounds such as oxides, carbides, nitrides, sulfides, silicates, oxynitrides, oxycarbides, etc. Because they are compounds (having already undergone reactions that result in these compounds), they tend to be thermally more stable than elemental carbon, metals and polymers. Therefore, high-temperature materials are commonly in the form of ceramics, which tend to be able to resist high temperatures even in the presence of oxygen.
Another class of high-temperature materials is elemental carbon in the graphite family, i.e., elemental carbon (not necessarily pure carbon) that substantially exhibits sp2 hybridization in each carbon atom. Graphite (such as natural graphite flakes) is crystalline, with a crystal structure that consists of graphite layers that are stacked in the AB sequence, with a combination of covalent bonding and metallic bonding within each layer and weak secondary bonding (van der Waals force) between the layers. In contrast to graphite, elemental carbon in the graphite family may or may not be crystalline. In case that the elemental carbon in the graphite family is not crystalline, the carbon still consists of layers that exhibit a combination of covalent bonding and metallic bonding within each layer, but the layers are limited in area and are not well ordered. Because the crystalline form is thermodynamically stable, whereas the noncrystalline form is only metastable, noncrystalline carbon can be converted to graphite by heating at a sufficiently high temperature that is known as the heat-treatment temperature. The higher is the heat-treatment temperature, the greater is the degree of crystallinity. This crystallization process is known as graphitization. Carbon fibers, carbon nanofibers, carbon nanotubes, graphene, graphite flakes, graphite nanoplatelets, intercalated graphite, exfoliated graphite, carbon black and activated carbon are examples of elemental carbon in the graphite family. In particular, carbon fibers are commonly noncrystalline, although carbon fibers that exhibit a degree of crystallinity exist. On the other hand, diamond is elemental carbon that is not in the graphite family. Both diamond and diamond-like carbon exhibit sp3 hybridization of the carbon atoms. Diamond is not a high-temperature material, because it changes to elemental carbon in the graphite family at high temperatures. Graphite is the thermodynamically stable form of elemental carbon at room temperature and pressure.
Although elemental carbon in the graphite family is a high-temperature material, it suffers from the tendency to be oxidized at high temperatures in the presence of oxygen, such that the carbon forms carbonaceous gases, such as carbon dioxide. As a consequence of the oxidation, the elemental carbon experiences mass loss, which is undesirable.
Due to the superior oxidation resistance of ceramics compared to carbons, the oxidation resistance of a carbon material is commonly enhanced by coating the carbon material with a ceramic material (such as silicon carbide). However, the coating tends to suffer from the tendency to be detached, due to the high-temperature use and the difference in thermal expansion coefficient between the coating material and the carbon material. Furthermore, the coating process adds to the cost of material production. In addition, due to the low thermal conductivity and low electrical conductivity of typical ceramic materials compared to elemental carbon materials, the ceramic coating is undesirable for applications that require heat dissipation, lightning protection, electromagnetic interference shielding or electrical conduction.
For the purpose of improving the oxidation resistance, elemental carbon can be coated with silicon carbide (U.S. Pat. No. 5,380,556, U.S. Pat. No. 5,225,283, U.S. Pat. No. 6,668,984, U.S. Pat. No. 4,668,579, U.S. Pat. No. 5,955,197, U.S. Pat. No. 6,668,984), which is more oxidation resistant than carbon. Other coating materials are boron carbide (U.S. Pat. No. 4,668,579, U.S. Pat. No. 7,160,618), boron nitride (U.S. Pat. No. 7,160,618), silicon oxide (U.S. Pat. No. 4,894,286), boron oxide (U.S. Pat. No. 4,894,286), titanium oxide (U.S. Pat. No. 4,894,286), alkali or alkaline earth metal silicates (U.S. Pat. No. 7,160,618), alkali metal hydroxide (U.S. Pat. No. 7,160,618), glass (U.S. Pat. No. 5,955,197) and glass-ceramic (U.S. Pat. No. 5,955,197, U.S. Pat. No. 5,427,823).
Both ceramic materials and elemental carbon materials are typically brittle compared to metals. In order to reduce the brittleness of these materials, fiber reinforcement is commonly added, thus resulting in fiber-reinforced ceramic materials (i.e., fibrous ceramic-matrix composite materials) and fiber-reinforced carbon materials (i.e., fibrous carbon-matrix composite materials). The fiber reinforcement also serves to enhance the strength and stiffness (modulus of elasticity). The fibers tend to be more effective when they are continuous rather than being discontinuous. Due to the high-temperature use of the ceramic-matrix and carbon-matrix composites, the fibers and the matrix are preferably essentially the same in composition, so that they are essentially equal in the thermal expansion coefficient. Hence, the fibers in a ceramic-matrix composite are preferably ceramic fibers and the fibers in a carbon-matrix composite are preferably carbon fibers. A ceramic-fiber ceramic-matrix composite (e.g., a silicon-carbide-fiber silicon-carbide-matrix composite) is also known as a ceramic-ceramic composite. A carbon-fiber carbon-matrix composite is also known as a carbon-carbon composite.
Due to the extremely high melting temperatures of ceramics and carbons, the fabrication of shaped articles made of ceramics or carbons does not involve melting. A method of fabrication involves sintering (U.S. Pat. No. 5,294,387), which is heating without melting, such that the heating provides sufficient thermal energy for the atoms to move in the solid state. For example, in case that the ceramic or carbon is in the form of particles, the atomic movement enables connection resembling a neck to be formed between the particles, thereby resulting in bonding. However, the temperature required for sintering is still very high. Furthermore, sintering is expensive and restrictive in the size of the resulting article, due to the high temperatures and pressures involved. However, sintering allows a wide choice of ceramic compositions, e.g., mullite (U.S. Pat. No. 5,294,387, U.S. Pat. No. 5,376,598), alumina (U.S. Pat. No. 6,887,569), and zirconia (U.S. Pat. No. 6,887,569).
Yet another method of fabrication involves the use of a precursor material, which upon heating, undergoes thermochemical decomposition, thermal transformation and/or chemical reaction, thereby forming the ceramic or carbon material desired. The temperature required for this process is typically lower than that required for sintering. A ceramic precursor is a material that, upon heating, forms a ceramic material. A carbon precursor is a material that, upon heating, forms an elemental carbon material. The heating process is known as pyrolysis in case that the precursor is an organic material and the process involves thermochemical decomposition. In the case of the formation of elemental carbon from a carbon precursor, the process is also known as carbonization. The mass of the ceramic or elemental carbon that results from a unit mass of the precursor through the pyrolysis process is known as the yield of the precursor. In case of a carbon precursor, the yield is known as the carbon yield. A high yield is preferred, since the porosity in the resulting carbon increases with decreasing carbon yield of the carbon precursor. The precursors are commonly in a liquid form (e.g., a resin). The ceramic precursor has a composition that includes the atoms of the elements that constitute the resulting ceramic. For example, a polymer that contains silicon and carbon atoms (e.g., polycarbosilane) can serve as a precursor for silicon carbide; a polymer that contains carbon atoms (e.g., pitch) can serve as a precursor for elemental carbon. The liquid form of the precursor is attractive in that it allows convenient shaping of the final product and also allows conformability with the surface topography of the solid ingredients present. In order to avoid oxidation, the heating is commonly performed in the essential absence of oxygen. This is particularly important for the heating of a carbon precursor to form carbon. Examples of inert atmospheres are nitrogen gas, argon gas and vacuum.
The method of fabrication involving precursors is particularly common in the fabrication of ceramic-matrix and carbon-matrix composites. The precursor serves as the binder for the solid ingredients (such as fibers) and forms the matrix of the resulting composite material during the heating associated with the fabrication of the composite material. Hence, the precursor is often known as the matrix precursor. In the fabrication of a ceramic-ceramic composite, ceramic fibers and a ceramic precursor are used together, such that the ceramic precursor binds the ceramic fibers together and forms the ceramic matrix of the resulting composite. In the fabrication of a carbon-carbon composite, carbon fibers and a carbon precursor are used together, such that the carbon precursor binds the carbon fibers together and forms the carbon matrix of the resulting composite.
A shortcoming of the precursor method is the porosity in the resulting ceramic or carbon material. The porosity is a consequence of the fact that the volume of the resulting ceramic or carbon material is smaller than the volume of the precursor material, due to the fact that the yield of the precursor is less than 1 (due to the partial loss of the precursor material as gases during the fabrication). Pores are detrimental to the mechanical properties and numerous other properties of the resulting ceramic or carbon material. Therefore, the pyrolysis process is commonly followed by densification, which is a process aimed at decreasing the porosity. The densification process commonly involves impregnation of the pores with the precursor and subsequent pyrolysis of the newly impregnated precursor. As porosity still occurs after the second round of pyrolysis, multiple cycles (e.g., more than six cycles) of impregnation and pyrolysis are commonly necessary in order to attain an adequate degree of densification. Another method of densification involves chemical vapor infiltration (abbreviated CVI), which is a process that involves the infiltration of a precursor gas into the pores, followed by heating to form the ceramic or elemental carbon. CVI is commonly used as a final step of densification, since the pores become smaller as the material becomes more densified and a precursor gas can penetrate the small pores more easily that a precursor liquid. The densification process is tedious and costly and contributes significantly to the high cost of ceramic-matrix and carbon-matrix composites.
A method of facilitating the densification process involves the addition of a filler in the form of a ceramic powder that can resist high temperatures, such as aluminum oxide, boron carbide, silicon carbide, titanium carbide, molybdenum silicide, titanium silicide and silicon germanide (U.S. Pat. No. 7,211,208, U.S. Pat. No. 7,575,799, U.S. Pat. No. 6,261,692). However, this method does not provide enough positive effects on the mechanical properties.
Ceramic-matrix composites tend to be superior in the ability to resist high temperatures than carbon-matrix composites. However, they tend to be more difficult to fabricate than carbon-matrix composites, due to the greater complexity of the chemical/thermal process associated with the conversion of a ceramic precursor to a ceramic compared with the process associated with the conversion of a carbon precursor to elemental carbon.
Due to the superior thermal stability of ceramics compared to carbon, ceramic-carbon hybrids have been used, mostly commonly as the matrix of a composite material. For example, a ceramic precursor (such as a polycarbosilane, which is a precursor for silicon carbide) and a carbon precursor (such as pitch) are mixed in the liquid state and converted by heating to a multi-phase material that comprises the ceramic that is formed from the ceramic precursor and the elemental carbon that is formed from the carbon precursor (U.S. Pat. No. 5,254,397, U.S. Pat. No. 6,221,475). The phases in the multi-phase material are randomly and homogeneously distributed and are intermixed in a fine scale, since they are formed from ceramic and carbon precursors that are randomly and homogeneously distributed and are finely mixed. In other words, there is no control of the configuration of the spatial distribution of the phases in the multi-phase material. This means that there is no control of the shape of the units of each phase in the multi-phase material. Furthermore, this method limits the choice of the ceramic phase in the multi-phase material to those that can be formed by processing ceramic precursors that are in a liquid form. In particular, ceramic alloys such as mullite (which exhibits an attractively high melting temperature) cannot be obtained by using this method.
In a related but different method, a ceramic-carbon hybrid is formed by the pyrolysis of a suitable copolymer (U.S. Pat. No. 5,552,505). However, this method also suffers from the difficulty of forming specific ceramic alloys and the difficulty of controlling the configuration of the hybrid.
A ceramic-carbon hybrid can also be in the form of a carbon-carbon composite with a silicon carbide particles dispersed in the matrix (U.S. Pat. No. 4,863,773). This form of ceramic-carbon hybrid is limited to the configuration in which the ceramic is in particle form, such that the particles are dispersed in carbon and are bound by the carbon.
A ceramic-carbon hybrid can also be in the form of a ceramic-matrix composite with carbon fibers as a reinforcement embedded in the composite. An example is a carbon fiber composite with silicon carbide as the ceramic matrix (US 2010/0331166, U.S. Pat. No. 5,635,300). Another example is a carbon fiber composite with silicon oxycarbide as the ceramic matrix (U.S. Pat. No. 5,587,345). This form of ceramic-carbon hybrid is limited to the configuration in which the ceramic component is the matrix (binder) and the carbon component is the carbon fiber reinforcement.
A ceramic-carbon hybrid can also be in the form of a ceramic-matrix composite with a carbon-carbon composite embedded in the composite (U.S. Pat. No. 7,297,368). This form of ceramic-carbon hybrid is limited to the configuration in which the ceramic component is the matrix (binder) and the carbon component is the carbon-carbon composite.
A ceramic-carbon hybrid can also be in the form of a composite with a combination of ceramic fibers and carbon fibers, such as a yarn comprising the ceramic fibers and carbon fibers (U.S. Pat. No. 6,051,313). In the final product, this hybrid is used as the reinforcement in a composite material that consists of the hybrid and a certain matrix material that binds the components of the hybrid together. This form of ceramic-carbon hybrid is limited to the configuration in which the two types of fibers are bound together by a certain matrix material, such that the fibers are substantially not in direct contact. In the absence of the matrix material, the hybrid does not exhibit adequate mechanical properties, due to the air gaps between the adjacent fibers.
Yet another form of ceramic-carbon hybrid is a calcium silicate hydrate (a material like the cement in concrete) with embedded carbon (e.g., carbon in the form of graphite particles) can be formed by adding the carbon to the hydrate before the hydrate sets, followed by the setting and curing of the hydrate (US 2012/0308813). The resulting hybrid is a composite with the calcium silicate hydrate as the matrix and with the carbon as the filler. This method suffers from the fact that the ceramic in the hybrid is limited to silicate hydrates, which, partly due to their porosity (as in cement), tend to be inferior to conventional ceramics (such as alumina, silicon carbide, silicon dioxide and mullite) in the mechanical properties.
In order to avoid degradation of the reinforcement during the process of forming a ceramic-matrix or carbon-matrix composite, the reinforcement should be able to withstand the temperatures of the process (typically in the range from 1,000° C. to 2,500° C.). Therefore, the choice of reinforcement is limited to reinforcement materials that can withstand high temperatures. This limitation reduces the flexibility of composite material design. Furthermore, due to the relatively high cost of reinforcements that can withstand high temperatures, this limitation adds cost to the composite material.
Wear and friction properties are important for brake applications. For the purpose of improving these properties of carbon-carbon composites, ceramic powder such as silica and silicon carbide can be introduced to the composite during composite fabrication (US 2008/0090064). A related method involves the formation of a silicon carbide phase in the composite during composite fabrication (U.S. Pat. No. 6,221,475). A still related method involves the introduction of a carbide-forming metal during composite fabrication (U.S. Pat. No. 6,514,562, US 2010/0292069). However, this method does not provide adequate positive effects on the mechanical properties and does not facilitate the densification process.
For the purpose of improving the impact resistance of a carbon-carbon composite, carbon nanotubes can be introduced to the matrix of the composite (U.S. Pat. No. 7,407,901). Carbon nanotubes are advantageous due to their small diameter. The diameter is typically much smaller than the size of ceramic particles. Due to the small diameter and filamentous shape of carbon nanotubes, the area of the interface between the carbon nanotube and the carbon matrix per unit volume of the composite is substantial. Slippage at this interface provides a mechanism of consuming the mechanical energy. Thus, the substantial interfacial area results in improved impact resistance. However, the filamentous shape of a carbon nanotube is not optimum for providing a large interfacial area. For example, a planar shape would provide a larger interfacial area per unit volume than a fiber shape. In addition, carbon nanotube is very expensive compared to ceramic particles. Furthermore, this method does not facilitate the densification process.
For the purpose of improving the oxidation resistance of carbon-carbon composites, boron compounds can be introduced during composite fabrication (U.S. Pat. No. 4,937,101). However, this method does not provide adequate positive effects on the mechanical properties, and does not facilitate the densification process.
Clay is a layered silicate material, with the silicate layers exhibiting long-range crystal structural order. Clay is inexpensive, since it is a natural and abundant mineral.
Organoclay (also known as nanoclay) is layered silicate having organic molecules between the silicate layers. Interactions involving the organic molecules and the matrix polymer enable organoclay to be particularly suitable for use as a filler (i.e., a nanofiller) in polymer-matrix composites. The effectiveness of organoclay as a reinforcement for increasing the toughness stems from the platelet shape of the silicate, the small thickness of the platelets, and the consequent large area of the interface between the platelets and the polymer matrix per unit volume of the composite material.
Organoclay is used in polymer-matrix composites for the purpose of increasing the stiffness (US 2006/0276579, U.S. Pat. No. 7,928,156, US 2007/0032585), strength (US 2007/0032585), toughness (US 2006/0276579), heat resistance (U.S. Pat. No. 7,928,156), flame-retardant ability (U.S. Pat. No. 6,610,770) and color stability (US 2007/0197711). Organoclay is also used in combination with basalt fiber in insulating polymer-matrix composites (US 2010/0205929). In addition, organoclay is used in aqueous dispersions for enhancing the thermal shock resistance (US 2005/0059765). The polymer-matrix composites can withstand temperatures up to about 350° C. (more commonly up to 200° C.); the aqueous dispersions can withstand temperatures up to about 80° C. This means that these are not high-temperature materials. The ability to withstand high temperatures is needed for numerous applications.
The present invention is directed to overcoming these and other deficiencies in the art.