Patent Application
20060121196
The present invention relates to composite materials and especially to chemical vapor composites and to methods of making them.
Composites are a class of materials that mix two or more distinct phases generally with the objective of achieving a mixture with improved properties such as improved mechanical or thermal properties. Composite technology has been used in a number of applications such as the production of structural components. For example, metal matrix composites (typically metal particles mixed with a ceramic base) can have desired performance features relating to high-temperature stability, chemical inertness, hardness and toughness. Composite design can also provide other desired properties relating to magnetic, electrical and optical features. It is often important to be able to control the microstructure (grain size and grain distribution). Composites can be produced utilizing high temperature treatment of liquid or solid phase mixtures, but with these processes control of grain size is difficult. In the case of ceramic and other high temperature composites, sintering agents are typically used to promote reactions of the separate components at reasonable temperatures. However, these agents act as impurities that may degrade performance of the resulting composite.
The direct application of solid materials to various substrates by chemical vapor deposition (CVD) is well known. For example, methyltrichlorosilane (CH3SiCl3) gas decomposes on contact with hot surfaces to SiC (a solid which plates out on the hot surfaces) and gaseous HCl, which is drawn off.
U.S. Pat. Nos. 5,154,862 and 5,348,765 assigned to Applicants employer describe processes by which a composite article may be formed in a single step process from the coupling of a chemical vapor deposited matrix with a fine particle second phase embedded within the matrix. Such articles are formed at high deposition rates and may obviate the above-described prior art disadvantages. These prior art processes, known as chemical vapor composite (CVC) processes, utilize particles with sizes in the range of about 1 nm to 60 microns or larger with the particle mass comprising about 5 percent of the composite mass or greater, typically about 1 to 10 percent. With these prior art CVC processes deposition rates were much higher than CVD deposition rates but the densities of the resulting products were substantially reduced as compared to similar products produced with CVD processes. Prior art CVC processes utilize relatively small reactors having work zones smaller than one cubic meter. With the limited work zone volume and fact that composite runs generally require at least a few days to complete, the result is high costs of the composite products. In addition, prior art CVC processes have not provided techniques for good control of either composite density or grain size.
A surface of the substrate often has the opposite shape of a desired shape of a product to be produced so that when the deposited material is removed for the substrate the deposited material has approximately desired shape with minimal machining or polishing required. This is important because these CVD products can be extremely hard to machine and polish. In this specification and the claims we will refer to the shape of a surface of a substrate that is the mating, complementary, or inverse surface of the desired product surface as a “replicative surface”.
There are known advantages of using silicon carbide structures for large mirrors. Large mirrors typically must be very dimensionally stable and should be relatively light weight. As the mirrors become larger maintaining dimensions becomes more challenging.
What is needed is a CVC method for efficiently producing light weight rigid ceramic composites without the need for substantial machining and polishing.
The invention is a method for forming, within a reactor having a work zone of at least one cubic meter, composite articles, particularly ceramic composite articles, for high temperature applications. The invention provides composite articles formed from the deposition as a solid matrix on hot surfaces of a chemical vapor having entrained solid particles. A composite material is produced comprising the chemical vapor deposition matrix with the solid particles dispersed within the matrix. By carefully controlling the reactor gas flows and pressure within a large work zone, as well as the number of solid particles per flow rate of reactor gas, Applicants are able to efficiently produce composites with substantially improved quality as compared with CVD produced articles and as compared with articles produced with prior art CVC processes. Preferred embodiments include processes for applying special coatings to prepared substrates in order to produce net shape or near net shape CVC products.
The reactant gases referred to above must be heated to a temperature high enough to cause decomposition of the gas. A preferred technique is to fabricate an underlying material, a substrate, into a desired shape, such as a coil, wire or a more complex configuration such as a vane, turbo rotor, rocker arm, or other engine component. The shaped substrate is then maintained at the required elevated temperature, thereby providing the thermal activation necessary for the decomposition of the chemical precursor gas. The exact temperature range is dependent upon the ultimate CVD matrix composition selected.
A gaseous mixture containing the precursor gas, a carrier gas, and particles of the second phase material is then injected onto and over the heated substrate. The present invention can be utilized with a large number of precursor gasses to produce a variety of matrix materials. In this application 33 separate composite processes have been specifically identified. The particles of solid phase materials can be any of a large number of materials and shapes. Materials such as SiC, Si3N4, and ZrO2 are examples of materials. Preferred shapes include random shaped particles of various mesh sizes, fibers, wiskers, nanoparticles and nanotubes.
A preferred composite material made by according to the present invention is silicon carbide composite materials. For example, a stream of methyltrichlorosilane and hydrogen is injected into the CVD chamber accompanied by a simultaneous flow of silicon carbide particles of 40-14,000 mesh. The gas mixture with the entrained particles is introduced into the reactor at a relatively low temperature. The CH3SiCl3 breaks down into solid SiC and gaseous HCl when the CH3SiCl3 gas contacts very hot surfaces in the reactor. The SiC along with some of the entrained particles deposits on the hot surfaces in the reactor, in particular graphite substrates having the general shape of desired articles. Gaseous HCl and hydrogen are pumped out of the reactor and disposed of. When desired thicknesses of the SiC-particle composite have been deposited, the reactor is cooled and the substrate with the coating of SiC-particle matrix is removed from the reactor. The substrate may then be removed leaving the SiC-particle composite article having qualities substantially superior to SiC deposited utilizing conventional CVD processes. The coated article thus produced contains a shaped underlying substrate fused to a CVD produced silicon carbide matrix having a uniform and random distribution of silicon carbide particles embedded therein.
Preferably, the reactor should have a work zone of at least one cubic meter for efficient production of a large number of small composite articles or the production of a smaller number of large items. A vertically oriented reactor is described with a cylindrical work zone 64 inches high and a diameter of 64 inches providing a work zone volume of 3.37 cubic meters and permitting production of large products or simultaneous production of a large number of small products. Large horizontally oriented reactors are also described specifically designed for the production of tubular shaped ceramic composites.
In preferred embodiments a special coating is placed on the substrate so that after the composite material (such as a silicon carbide composite) is deposited and the substrate with the deposited material is cooled down the deposited material is easily removed having a shape matching the substrate with precision so that polishing is minimized or rendered unnecessary. In preferred embodiments the substrate is silicon carbide and the coating is a layer of silicon dioxide-carbon and the deposited material is a silicon carbide composite material.
Reactor Shell
A reactor shell is comprised of a 304L stainless steel cylinder 104, a rounded stainless steel top cover 106 and a rounded stainless steel bottom cover 108. The cylinder and both top and bottom covers utilize a double wall design. A 10 psig pressure relief device is provided on the chamber. Six power ports 118 are provided to accommodate electric power feed through assemblies for the heating elements 122. Twelve additional ports (not shown) are provided for the installation of instrumentation and control components. A water-cooled exhaust port is also provided on the chamber. The reactor shell is equipped with a cooling water jacket providing cooling water flow in the spaces between the two walls of the shell. The outside wall temperature of the reactor is maintained at about 25-35 degrees C. when internal work zone temperatures are at about 1400 degrees C. Thermal insulation consists of 2 inches of carbon felt on the side of the hot zone, and 3 inches of insulation on the top and bottom of the hot zone. The carbon felt is mounted on the inner surface of a stainless steel support cage assembly 107. Cooling water manifolds incorporating shut off capability on both the supply and return side are mounted to the chamber support frame. Flow sensors with adjustable minimum level settings are provided for each cooling circuit. Interlocks are provided for connection to the power supply and alarms.
Heat and Pressure
In preferred embodiments graphite heating elements 122 in reactor 102 heat the internal components of the reactor and the substrate material to temperatures of about 1200-1500 degrees C. prior to the injection of the feed gas—particle mix. Heating elements 122 are a three-phase resistance configuration for a balanced electrical loading. A modular design is utilized for easy part replacement during maintenance cycles to minimize downtime. A total of six water-cooled power feed through assemblies 118 are connected to the six graphite heating elements. A VRT type, low voltage, three phase power supply 160 as shown in
Work Zone Enclosure
The chamber provides a 64 inch internal diameter, 64 inches high work zone 124 providing a volumetric work zone of about 3.37 cubic meters. The work zone is surrounded by a graphite enclosure 105 consisting of a bottom cover 105B, top cover 105A, and a graphite tube 105C assembly to keep the heating elements and thermal insulation clean to minimize maintenance. A uniquely designed exhaust region is included to minimize both un-reacted process gases and pyrophoric reactant byproduct downstream. The exhaust region is a subsidiary graphite compartment below the main chamber, separated by a graphite plate with between 6-12 exhaust holes. This compartment directs the exhaust gases to the exhaust plumbing along hot graphite surfaces which help to completely react any un-reacted pre-cursor gases or partially reacted subsidiary byproducts. The work zone enclosure and the bottom portion of the insulation can be lowered together with the bottom cover to allow easy access as shown in
Reactor Frame
A steel frame 103, as shown in
Vapor Delivery System
A vapor delivery system consists of seven methyltrichlorosilane vaporizers 180 (with a total capacity of over 100 lbs/hr) and a gas flow distribution/measurement system, with safety interlocks and shut-off devices. Connections are provided for tie-ins to a liquid MTS source 156A, bulk hydrogen source 156B, bulk argon source (not shown), and utilities. Porter/Bronkhorst Mass flow controllers are included to provide accurate measurement and flow-control for consistent product quality. Seven injectors and interconnect piping are also included. Components of the vapor delivery system are enclosed in a ventilated hood (not shown). The pumping system is designed for extremely corrosive applications and is connected to a vacuum chamber 162 (as shown in
Instrumentation
Field instruments include 3 type C thermocouples for furnace temperature control, 7 type K thermocouples for vaporizer control, 14 mass flow controllers, 7 scales for vaporizers, 7 MTS mass flow controllers, 2 pressure transducers, 16 water flow switches and 4 local pressure gauges in the vaporizer cabinet. A PC based (LabView) control system is integrated into the system. The flow of CH3SiCl3 gas into reactor is monitored very accurately by measuring the flow rate of liquid CH3SiCl3 in the vaporizers.
Substrates
Silicon carbide composite parts are typically produced in reactor 102 by depositing the composites on graphite substrates having the general shape of the desired article to be produced. For example, as shown in
Any material may be selected as the underlying substrates so long as it does not decompose at the required CVD temperature nor become subject to chemical reaction with the reactants or products of the process. It should be noted in this regard that the desired decomposition of CH3SiCl3 occurs at a temperatures greater than about 1300 degrees C., producing highly corrosive hydrochloric acid which can easily etch a plethora of common substrate materials. However, since the process of the invention is not solely directed at the decomposition of CH3SiCl3 into silicon carbide, but instead can be used with any matrix which can be produced through chemical vapor deposition, there will be a plurality of embodiments in which less corrosive gases will be produced at less elevated temperatures. In such embodiments, a broad range of materials may be incorporated as the underlying substrate without resulting in decomposition or corrosion during application of the disclosed process.
Special Coatings for Substrates for Near Net Shape
In preferred embodiments special coatings are applied to the substrates to avoid the deposited CVC sticking too solidly to the substrate. Candidate materials are metal oxides, boron nitride and carbon rich silicon carbide coatings. These coatings and the process is described in the attachment to this specification. The CVC process is capable of producing near net shape materials by replicating the surface of the mandrel very precisely. Through the proper selection and preparation of the mandrel material and surface, Applicants can replicate mirrors directly from the mandrel, completely eliminating conventional polishing of the resulting CVC SiC mirror, or at least greatly reducing the extent of the polishing. This is the Holy Grail for high-grade optics and provides important commercial advantages in both cost and quality in the production of mirrors. The following is a preferred process for obtaining near net shape silicon carbide products:
As described above, a preferred reactant gas employed in the formation of composite articles according to the invention is a mixture of methyltrichlorosilane (donor gas) and hydrogen (carrier gas), and a preferred particle material is silicon carbide. The mixture of reactant gas and entrained particles is made by introducing the particles and a carrier gas such as hydrogen from a powder feeder 157 into a stream of reactant gas carried by the line 121. The reactant gas and particles typically are supplied to the reactor 120 at or slightly (about 10 to 20 degrees C.) above room temperature. A continuous flow of particles from the feeder 157 is typically utilized to ensure a uniform build-up both of the CVD matrix produced from thermal activation of the reactant gas and of the particles which are co-deposited with the matrix. The particles may include long or short particles, or both, with selection dependent on the desired application of the composite article. Silicon carbide particles of 325-600 mesh size (dimensions of about 2 mils) have been found to be especially suitable in forming composite tubes.
In alternative embodiments precursor gases other than methyltrichlorosilane may be used to produce the SiC composite article of interest, provided a carbon containing precursor gas (e.g. hydrocarbons such as methane, propane, butane, etc.) and a silicon containing precursor gas (e.g., SiH4, SiCl4, SiHxCl4-x, etc.) are included. Reaction temperatures in these cases may range between about 800 to 1350 degrees C. For matrixes other than SiC as discussed in more detail below, the precursor gasses used are preferably those typically used in normal CVD processes to produce the matrix material.
CVD produced material with solid particles suspended therein has been successfully deposited onto flat, square, rectangular, cylindrical, and spherical substrates. These composite layers of CVD matrix and particles uniformly and randomly disposed within the matrix provide a hard, impact and corrosion-resistant covering for otherwise soft materials which are readily susceptible to chemical attack. Hence, relatively common materials such as tungsten, molybdenum and carbon can be manufactured into a final desired embodiment and then subjected to coating with silicon carbide composite utilizing one of the above disclosed methods. The result is a relatively inexpensive produce with an extremely hard, chemically resistant product.
The present invention is not limited to a specific CVC produced material, such as CVC silicon carbide, but could additionally include other carbides (HfC, TaC, WC, B4C, etc.), nitrides (Si3N4, BN, HfN, AlN, etc.), oxides (SiO2, Al2O3, HfO2, Ta2O5, TiO2, BaTiO3, SrTiO3), silicides (WSi2, TiSi2, etc.), and metals (Cu, Al, W, Fe, etc.). Thus the scope of the matrix material which can be produced by the present invention is limited only by the capability of the chemical vapor deposition process to produce the desired chemical composition. However, the present invention provides for the addition of particles as described above that are deposited along with the vapor deposited material. Examples of matrix materials that can be produced utilizing the principals of the present invention are listed in the Table I below which includes preferred precursor gasses as well as preferred solid particulate materials.
The incorporation of particles can lead to porosity in the deposit due to incomplete formation of the CVD matrix around the particles. Applicants have discovered that this porosity depends on the feed rate of particulate compared to the CVD matrix growth rate. The porosity of the CVC deposit can thus be controlled by adjusting the feed rate of the particulate from a fully dense deposit to a deposit with as much as 40% porosity, as desired by the specific application. Other deposition parameters also play a role by affecting the CVD matrix growth, including pressure, gas flows and substrate temperatures.
It is an important advantage of the invention that this co-deposition occurs at a high rate—e.g., 10-20 mils/hour as contrasted with about 2-5 mils/hour in a conventional process depositing silicon carbide by CVD only. Conventional CVD requires the use of low growth rates to minimize internal stress levels. The distinct grain structure afforded by the additional of particles results in a low stress deposit enabling much higher reactant feed rates than is achievable by conventional CVD.
Particles with at least one dimension in the range of a few nanometers to a few tens of nanometers (called nanoparticles) may be substituted for the 30 micron particles referred to in the above descriptions. The nanoparticles may be carbon nanotubes, or nanotubes formed from silicon carbide or other metal carbides. Use of these nanoparticles in place of the much larger particles permit a very large increase in the number of particles for the same particle percentage in the resulting composite. Since the composite grain size is determined by the number of particles per composite volume, the larger number of particles mean smaller grain size. Applicants have determined that smaller grain size results in increased fracture toughness. Therefore, these ceramic nanocomposites have greater toughness than composites formed using larger particles or fibers. In addition, the use of nanoparticles can result in unique electrical and optical properties, for example, due to the phenomenon of quantum confinement. The deposition method is applicable to any ceramic material currently obtainable via a CVD process. Carbon nanotubes are known for their extremely high tensile strength, and therefore these nanotubes should engender high strength properties for the CVC phase, where the matrix may be silicon carbide, silicon nitride, or any other phase that can be derived via chemical vapor deposition.
Another preferred variation is one in which particles are generated within the CVD reactor itself, which are then incorporated within the CVD material. In doing so, the same stress relief as the CVC process is accomplished without the need for additional particles to the gas stream. The advantages achieved are higher purity and simplification of the reactor design, while maintaining high density, good mechanical properties, and high growth rates. Methyltrichlorosilane is preferably used as the reactant precursor for the growth of silicon carbide via CVD. MTS vapor is injected into a high temperature furnace at about 1300-1400° C. using a carrier gas of hydrogen. The SiC is deposited on a graphite perform, while simultaneously, SiC particulates are generated above the part. The furnace and preform are designed in the former process to lengthen the residence time of the chemical in the high temperature reaction zone. This serves to increase the probability of SiC particles nucleating from the gas phase. Through control over the pressure, temperature, and feed rates of MTS and H2, the degree of particle formation can be controlled. Optimization of these parameters yields the desired amount of stress relief, while maintaining fully dense, low porosity material.
The technique can also be applied to other materials, including other carbides, nitrides, oxides, suicides and metals. There are a number of applications, which can benefit from the high purity, low porosity, low stress, and high mechanical strength of the ceramic materials deposited via this technique. Examples of these applications include optics, high purity chemical processes, and components for extreme high temperature environments.
The chemical vapor composites method involves the addition of solid particulates (normally polycrystalline silicon carbide particles) to a chemical vapor deposition reaction stream. Molecular ratios can be varied using special process variations of the basic CVC process. In preferred embodiments particles other than polycrystalline silicon carbide can be added to the feed gas stream. These alternative added particles could include various forms of silicon carbide other than polycrystalline silicon carbide; single crystal silicon particles could be used, or mixtures of silicon carbide particles and silicon particles could be used. Also, the matrix material could be altered by using variations in the feed gas. For example, softer optical surfaces may be produced for mirrors that are more amenable to polishing. Thus, for the mirror substrate shown in
Applicants have developed techniques for producing multiple planar type SiC products during a single production run. Applicants multi-product technique is shown in
The techniques and reactors described above can be modified slightly to produce metal boride composites, metal carbide composites and metal nitride composites, which are suitable, for example, for ultra high temperature applications. As in the case of the silicon carbide composites, solid particles are entrained in a feed gas stream and the particles are deposited on a substrate along with a matrix material that is vapor deposited from the feed gas. The proposed method is able to maintain the high purity required for ultra-high temperature applications, while achieving a low internal stress in the composites. Table I lists several of these composites along with preferred chemical routes and preferred particle and fiber materials.
Boride Family CVC
Preferred embodiments of the invention involves the production of metal boride ceramics via the general process:
MCl4(g)+2BCl3(g)+5H2(g)→MB2(s)+10HCl(g)
where M=Hf, Zr, Ta, or Ti, BCl3 is boron trichloride, and H2 is hydrogen gas. The metal chloride is introduced into the reaction stream by either direct sublimation of the solid, or via in process production of MCl4 vapor from solid metal and a chlorine containing gas species. To the reaction mixture is added solid micron or nanometer scale particles, whose chemical composition is identical to the metal boride species being formed, or entirely different. This embodiment allows for the production of high purity residual stress free ultra high temperature metal boride ceramic materials.
Carbide Family CVC
Preferred embodiments of the invention involves the production of metal carbide ceramics via the general process:
MCl4(g)+CH3Cl(g)+H2(g)→MC(s)+5HCl(g)
where M=Hf, Zr, to Ta, CH3Cl is chloromethane, and H2 is hydrogen gas. The metal chloride is introduced into the reaction stream by either direct sublimation of the solid, or via in process production of MCl4 vapor from solid metal and a chlorine containing gas species. To the reaction mixture is added solid micron or nanometer scale particles, whose chemical composition is identical to the metal boride species being formed, or entirely different. These embodiments allow for the production of high purity residual stress free ultra high temperature ceramic materials of the carbide family.
Nitride Family
Preferred embodiments of the invention involves the addition of solid particulates to a chemical vapor deposition reaction stream. This invention involves the production of metal nitride ceramics via the general process:
2MCl4(g)+N2(g)+4H2(g)→2MN(s)+8HCl(g)
where M=Hf, Zr, to Ta, and N2 and H2 are nitrogen and hydrogen gas, respectively. The metal chloride is introduced into the reaction stream by either direct sublimation of the solid, or via in process production of MCl4 vapor from solid metal and a chlorine containing gas species. To the reaction mixture is added solid micron or nanometer scale particles, whose chemical composition is identical to the metal boride species being formed, or entirely different. These embodiments provide for the production of high purity residual stress free ultra high temperature ceramic materials of the Nitride family.
Net and near-net CVC deposition require effective mass transport of reactants into (and reaction products away from) the topography of the substrate. In certain substrate geometries, the growth of the deposited material results in a loss of mass transport efficiency to certain locations of the substrate. To minimize this result in some cases Applicants utilize variable reaction pressure to optimize process efficiencies and mass transport rates. In the early periods of the deposition, high reactor pressures may be employed because the complex substrate structure is considered “open” and facilitates efficient reactant and product mass transport. As the growth of the deposited material proceeds and significant constriction of reactant (product) flow to (from) certain locations in the structure occurs, the reaction pressure is systematically reduced to increase mass transport rates.
The advantage of this technique lies in the ability to optimize reactant flow rates with regard to mass transport and process efficiency. If high reactant pressures are employed throughout the deposition, certain locations within the complex structure will exhibit deposits that are thinner than desired. However, if low pressures are employed throughout the deposition, including the early periods when the complex structure is “open”, the process efficiency will be reduced due to the enhanced linear velocity of the reactant gases, with consequent losses of reactant to the exhaust system.
The chemical vapor composite process and a reactive melt infiltration process can be used in conjunction to produce ceramic products having special shapes such as straight multi-section tubes, angled tubes or “elbows”, and tube sections in the form of a “tee”. Separate ceramic parts can be produced using the chemical vapor composite process. The finished ceramic sections will be ground (such as with either an internal or an external taper) so the individual components will fit tightly together to form the required shapes. The individual components are then bonded using a reactive melt infiltration process. Techniques for joining ceramic section via reactive melt are described in detail in various NASA publications available on the Internet.
Composites may be produced comprising thin films of material consisting of two or more distinct phases, using physical transport of nanometer-scale particles along with a physical vapor deposition stream(s). Composite thin film materials, i.e., a film containing a mixture of two or more chemically distinct phases, can exhibit a wide variety of interesting properties, such as giant magneto-resistance, enhanced magnetic coercivities, and quantum well behavior. These properties arise from the interaction between the different phases, and depend strongly on the grain structure of the film, i.e., grain size, grain boundaries, and arrangement. The common method to form these composite films is to co-deposit material from separate sources by physical vapor deposition (PVD), followed by an anneal to achieve the desired grain structure. However, the annealing step gives limited control over the grain structure and can lead to undesired interdiffusion between the separate phases. The new technique is the formation of composite films by physical transport of nanometer scale particles to a substrate, coincident with a conventional chemical vapor stream. The added particles thus become embedded in the CVD matrix. The key advantage of this method is the ability to precisely control the grain size in each film, with minimal interdiffusion between the phases, since the requirement for high temperature anneal is removed. Various different films can be provided by changing to size and/or number of particles and/or changing the gas chemical or physical properties.
In this embodiment, a deliberate sequence of particle types is added to a chemical vapor deposition stream. The materials constituting the different particles are selected for their coefficients of thermal expansion (CTE). The added particle materials may have CTE values higher or lower than that of the matrix phase that is produced by the chemical reaction. The effective CTE of the particle-matrix composite will be a function of the CTE values of the matrix and particle materials. By controlling the volume fraction and type of particle material added to a given layer or local region of the deposited material, the magnitude and distribution of residual stresses in the deposited object can be controlled.
An example application would be the CVC deposition of silicon carbide, wherein the initial particle additives would be low CTE silicon nitride (Si3N4). After a selected period of SiC/Si3N4 composite growth, the particle additive is changed to high CTE zirconia (ZrO2). After a selected period of SiC/ZrO2 growth, the particle additive is changed back to Si3N4. Upon cooling, the differential CTE properties of the three composite layers in the deposit result in compressive surface stresses and tensile internal stresses. The effect is analogous to the condition accomplished in tempered glass, where rapid cooling of the surface layers of a molten sheet, followed by slow cooling of the interior results in compressive surface forces and a remarkable enhancement of fracture toughness. The example above assumes the final use temperature is lower than the deposition temperature. The CVC designed stress concept can also be employed to engender compressive surface stresses when the application temperature is higher than the deposit temperature.
Composite ferroelectric material may be produced using selected secondary phase particles with a reactive chemical vapor deposition stream. Ferroelectrics are a class of insulating materials, which can exhibit a spontaneous polarization whose direction can be changed via an applied electric field. The phenomenon is tied to the placement and symmetry of ions in a crystalline lattice, which can be altered by straining the material. A common method of producing ferroelectric materials is metal-organic chemical vapor deposition, which reacts a metal-organic complex at high temperature and under controlled conditions of pressure and gas composition to achieve the desired ferroelectric state. A ferroelectric with altered material properties can be produced by adding a second phase particulate stream to the metal-organic vapor stream. The strain state of the ferroelectric material can be changed by adding a particulate with a different coefficient of thermal expansion (CTE) than the ferroelectric. Upon cool down from the high deposition temperature, the particulate can introduce a tensile or compressive stress on the material, depending on the difference in CTE's between the particle and the ferroelectric. Anticipated benefits could include reduced dielectric loss materials, enhanced dielectric constant, and increased dielectric tunability.
This embodiment involves the addition of a particulate stream that includes high aspect ratio fibers or whiskers. The chemical composition of these fibers or whiskers is such that they can be removed from the deposit structure via chemical etching or combustion. The matrix material produced by the chemical vapor deposition process is typically refractory metals or ceramics. Removal of the fiber/whisker components result in a structure of controlled porosity and pore size. The resulting structure can serve as a particle filter device for high temperature, highly corrosive environment applications.
In cases where a vapor deposition process is used to deposit a ceramic matrix on a substrate the present invention can be utilized to minimize stresses due to differences in thermal expansion between the substrate and the matrix material. In this case the particle material size and composition can be chosen for adjusting and grading the effective coefficient of thermal expansion of the deposited phase in order to improve bonding of the deposited phase with the substrate phase.
As stated above silicon carbide is a superior material for lightweight mirror applications because of its high stiffness-density ratio, high thermal conductivity and low coefficient of thermal expansion. SiC is particularly favorable for space optics applications because of its resistance to plasma and radiation damage. While SiC is highly reflective in certain limited regions of the infrared spectrum, achieving high reflectivity in other spectral domains requires the addition of a highly reflective coating, for example a metal or dielectric stack structure, either of which may comprise a non-SiC material. Applicants propose to provide a synthesis of a highly reflective Bragg stack via electrochemical etching. The SiC material may be single crystal or polycrystalline and derived via chemical vapor deposition or chemical vapor composites methods. The SiC material may be either n-doped or p-doped to a level sufficient to allow electrochemical etching. The etching would be achieved using an ethanolic hydrofluoric acid solution under either potentiostatic or galvanostatic conditions. Repeated alternating exposures to high and low current densities (or anodic potentials) result in layers of alternating porosity and therefore alternating layers of varying index of refraction. By controlling the anodization current density and etching time for each layer, it is possible to prepare a multiple layer structure, where each layer fulfils a λ/4 condition necessary for high reflectivity over a selected wavelength range. The advantage of this technique lies in the dielectric Bragg stack being comprised of SiC material, rather than another substance with lower radiation and/or plasma tolerance.
Preferred embodiments of the present invention can be used to produce toughened ceramics. Fibers and/or whiskers can be added to the reactant gas mixture and injected into a chemical vapor deposition reactor. The fibers and/or the whiskers will be co-deposited to form a ceramic composite. The interweaving fibers serve as the medium to increase the strength of the composite. The added fibers will stop the progression of cracks.
The basic CVC process produces grains of varying sizes. Applicants have discovered that grain sizes can be increased by adding an annealing step to the CVC process. For example after producing CVC material at the normal deposition temperature of about 1400 degrees C., Applicants increased the temperature in the reactor to 1700 degrees for two hours. Subsequent analysis indicated a significant growth in grain size and an approximately 20 percent increase in thermal conductivity, from about 200 Watts/mK to about 240 Watts/mK.
Applicant's CVC SiC can be made translucent through lowering the pressure to about 10 torr. This reduces the grain size to the point where the material transmits light. This material is potential useful for optical applications, such as conformal optics, missile nose cone, ballistic windows for aircraft and vehicles, and high temperature windows among many other applications. Applicants can produce large transparent surfaces, especially with the 3.37 cubic meter reactor shown in
Preferred embodiments of the present invention involves the addition of nanometer sized solid particles to a CVD reaction stream, where the solid particle material and the material deposited through the CVD reaction represent components of a potential homogenous composite. The CVC deposition process results in a composite which is heterogeneous at the molecular scale, but homogenous at the nanometer scale. Because of the high surface—volume ratio of the additive nano-particles, the effective fusion temperature of these particles is lower than that of micron sized particles of the same material. Subsequent heat treatment leads to true homogeneous mixing of the two components. A key advantage of this process is that the composite material can be fabricated at a lower temperature than conventional processes, hence achieving a savings in energy and cost.
Chemical vapor deposition of structural materials requires a precise control over reactant feed rates. When a reactant is a gas at ambient temperatures, a standard gas flow controller can be used. When a reactant is liquid at ambient temperatures and pressures, a liquid vaporizer unit is typically employed, and the control over reactant feed rate is accomplished via control of liquid flow into the vaporizer, and a feedback system through which liquid flow in and vapor flow out maintain an approximately constant vaporizer mass.
If a reactant in a chemical vapor deposition scheme is solid under ambient conditions, reactant feed rate is difficult to control. In preferred embodiments of the present invention, the rate of sublimation is determined by heat and/or carrier gas flow rate into the sublimator unit. The rate of sublimation is monitored by a mass compensator system, namely a device that delivers a powder or a low vapor pressure liquid to a receptacle on the top of the sublimator unit. A scale monitors the mass of the sublimator and the added liquid or powder. A control loop delivers mass data to the heater and/or carrier gas controls. As more solid sublimes and leaves the unit, more compensating powder or liquid is added to maintain a constant mass. The rate at which the compensating powder or liquid is delivered to the receptacle is, under conditions of zero sublimator unit mass change, equivalent to the rate at which the sublimed material is being delivered to the reactor.
It is understood that the preceding description is given merely by way of illustration and not in limitation of the invention and that various modifications may be made thereto without departing from the spirit of the invention as claimed. For example, variations in the toughness and structure of composite articles formed by the method may be achieved by varying process parameters such as reactant gas stream flow and temperature, and the size, shape, and materials of the particles or fibers used as a second phase material. High temperature CVD techniques as well as plasma enhanced CVD (PECVD) techniques can be utilized along with the addition of particles using the techniques described above.
The scope of the invention is indicated by the appended claims, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.
This application claims the benefit of provisional parent application Ser. No. 60/644,916, filed Jan. 18, 2005 and is a continuation in part application of Ser. No. 11/006044, filed Dec. 7, 2004.
| Number | Date | Country | |
|---|---|---|---|
| 60644916 | Jan 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11006044 | Dec 2004 | US |
| Child | 11335041 | Jan 2006 | US |
