Patent Application
20110297358
This disclosure generally relates to techniques for controlling thermal energy transfer through materials, and deals more particularly with a thermal barrier coating and method of making the coating.
Managing the flow of thermal energy through materials is important in a wide range of industries. Thermal energy flows through a material in the form of thermal quanta known as phonons. The flow of phonons through a material may be reduced by the use of insulation which presents a resistance to the phonon flow, resulting in a thermal differential or temperature gradient. However, the use of insulation to control thermal energy transfer may have limitations in some applications. For example, in the aerospace industry, the use of insulation to reduce thermal energy transfer may add undesired weight to a vehicle or may result in parts that are too large or thick to meet application requirements.
Phonon flow through a material may also be reduced by providing the material with a reflective surface which reflects heat from the material before it can be absorbed. However, reliance on highly reflective surfaces to control thermal energy transfer may also be problematic in some applications, because of the need to maintain the reflective surface polished and/or mirror-like, which can add to operating costs.
More recently, it has been discovered that layered, two dimensional arrays of small metal particles can interfere with the flow of phonons, thereby reducing the flow of heat through these layers in a direction substantially normal to the layers. However, the use of layered two dimensional arrays of small metal particles to reduce heat flow through phenomena known as phonon interference has been largely limited to laboratory experimentation. Known layered two dimensional arrays may be time consuming and costly to produce, and may result in a surface that may not be sufficiently durable for high performance applications, such as those found in the aerospace industry.
Accordingly, there is a need for a coating that may be used as a thermal barrier to manage the flow of heat through a material using phonon interference, and which is durable and robust. There is also a need for a method of making the coating and applying it to a substrate which is both relatively economical and may be used to cover relatively large areas of a substrate surface.
The disclosed embodiments provide a thermal barrier coating that blocks or reduces the flow of heat using a composite structure comprising nano-scale materials having high and low thermal conductivities arranged in thin films or as an array of small particles held in a matrix. In one embodiment, the composite structure comprises a quasi-regular 3-D array of metal nano-spheres having a relatively high thermal conductivity embedded in a glassy enamel matrix having a relatively low thermal conductivity. Heat waves traveling through the coating encounter the discontinuities in thermal conductivity and are partially reflected at the internal interfaces between the matrix material and the nano-particles. The summation of partial reflections from a multitude of interfaces at the discontinuities results in an aggregate, substantially reflective material. In some cases, these internal reflections can be caused to interfere with each other on a quantum level. This interference results in a highly directional scattering property that acts to strongly limit the forward flow of heat, giving the material a very low thermal conductivity. In effect, the disclosed coating is internally “shiny” in the thermal energy band. When properly sized, the coating may exhibit as little as 2% of the thermal conductivity of the same matrix and nano-particles materials mixed together in a bulk alloy. Since the disclosed thermal barrier coating does not depend on surface properties to manage heat flow, it may be embedded inside or between elements or substrates.
The disclosed thermal barrier coating is light weight and relatively economical to apply over large areas of a substrate. The coating may be tailored to reduce thermal energy transport over a relatively wide range of temperatures, yet is highly durable and is suitable for use in high performance applications, such as those in the aerospace industry. In some applications, use of the disclosed thermal barrier coating may reduce the need for relatively expensive materials such as titanium that may be designed to withstand higher temperatures. In other applications, the coating may also be tailored for lower temperature applications suitable for use on composites such as graphite/epoxy composite materials. The disclosed thermal barrier coating produces phonon interference from any direction of thermal energy flow, in contrast to prior techniques where layered 2-D arrays may reject only tuned phonon frequencies perpendicular to those layers.
According to one disclosed embodiment, a coating is provided which is adapted to be applied to a substrate for managing the flow of heat traveling through the substrate. The coating comprises an array of metal nano-particles held in a glassy matrix material. The array may be a substantially quasi-regular 3-D. The nano-particles may comprise a metal and may be spherical in shape. The spacing of the nano-particles in the array may be substantially constant, and the molecular mass of the nano-particles is substantially greater than that of the matrix material. In one embodiment, the ratio of the molecular mass of the nano-particles particle materials to the molecular mass of the matrix material is greater than approximately 10. In other words, the metal spheres are made of heavy material such as tungsten, and the matrix is made of light material such as silicon. Other material choices may be made without loss of generality.
According to another disclosed embodiment, a coating is provided which is adapted to be applied to a substrate for managing the flow of heat traveling through the substrate. The coating comprises an array of nano-particles held in a matrix where the nano-particles have a thermal conductivity substantially greater than that of the matrix. The matrix may a glassy compound comprising one of fused quartz, soda lime glass, boro-silicate glass and alumino-silica glass. The glass matrix may be in the form of a ceramic such as aluminum oxynitride. The size of the nano-particles is substantially similar to the wavelength of the phonons transporting heat through the coating at a preselected temperature.
According to a further embodiment, a thermal barrier coating is provided. The coating comprises at least two layers each including an array of metal nano-particles held in a glassy matrix material. The layers have characteristics respectively tailored to reduce thermal transport in at least two ranges of temperatures. The characteristics may include at least one of the spacing between the nano-particles, the ratio of the masses of the nano-particles to the glassy matrix material, and the ratio of the elastic constants of the nano-particles to the glassy matrix material. The thermal barrier coating may further comprise a third layer including an array of metal nano-particles held in a glassy matrix material wherein the third layer has characteristics tailored to reduce thermal transport in a third temperature range different than the first and second temperature ranges.
According to still another embodiment, a thermal barrier coating is provided for an aircraft part. The coating comprises a glassy matrix, and a plurality of metal nano-particles held in the matrix. The nano-particles are arranged in a 3-D array and are spaced apart at substantially constant distances substantially equal to the wavelength of phonons transporting thermal energy through the coating.
According to a further embodiment, a method is provided of making a thermal barrier coating. The method comprises applying a glassy compound to metal nano-particles, and fusing the glassy compound into a glass matrix holding the nano-particles. The coating may be formed by spraying a glassy powder or coating with a sol-gel silica compound on the nano-particles. The method further comprises assembling the nano-particles into a quasi-regular 3-D array.
According to still another disclosed embodiment, a method is provided of forming a thermal barrier coating on a substrate. The method comprises coating metal nano-particles with a glassy compound and self assembling the coated nano-particles into a quasi-regular 3-D array. The method also includes applying the assembled nano-particles to the substrate and fusing the glassy compound coatings into a substantially homogeneous matrix.
The disclosed embodiments satisfy the need for a low cost, durable, high performance thermal barrier coating useful in a wide range of applications for controlling the transport of thermal energy through a substrate.
Referring to
The disclosed thermal barrier coating 30 has a wide range of applications in various industries, such as, without limitation, the aerospace industry. For example,
Other typical examples of aerospace applications of the coating 30 include, but are not limited to components located near an APU (auxiliary power unit), hot hydraulic components, engine nacelles, titanium nozzle surfaces, turbine blades and the walls of a combustion chamber, to name only a few. The thermal barrier coating 30 may be used in other industries, such as to maintain cryogenic cables (not shown) within a double walled vacuum system (not shown). The application of the coating 30 may reduce the need to maintain the vacuum, providing much lower cost of fabrication and operation.
Referring to
More than one thermal barrier coating 30 may be used to control the transport of thermal energy through one or more substrates 34. For example, as shown in
Referring now to
The materials that are selected for the supporting matrix 66 should exhibit phonon mean free path lengths at least as long as the phonon wavelength λ in order to prevent phonon scattering from material dislocations. The ratio of the atomic masses between the material used for the matrix 66 and for the nano-particles 60 should be as high as practical, and the mechanical compliance of the material of the matrix 66 may be optimized in order to achieve a large phonon capture ratio for the coating 30. In one practical embodiment, the ratio of the atomic masses of the nano-particles 60 and the matrix 66 is at least approximately 10.
The coating 30 may have a thickness t that is sufficient to provide tolerance for dislocations or errors in the self-assembly process. The compounds useful as the matrix 66 may be selected such that the coefficient of thermal expansion of the coating 30 is substantially matched to that of the substrate 34 to which it applied. The performance of the coating 30 may depend on the ratio of atomic masses, ratio of thermal conductivities, elastic constants and the geometry of the composite structure. The greater the difference between the two atomic masses in the super lattice, the more the super lattice acts as a phonon mirror, reflecting heat as long as a significant elastic constant difference exists between the matrix 66 and the nano-particles 60.
As used herein, “glass”, “glassy”, “glass compound”, “glass compound” and “silica-based compound” refer to compounds containing silica. In one embodiment, the matrix may comprise a glassy compound such as, without limitation, a ceramic, a fused quartz, soda lime glass, boro-silicate glass, alumino-silica glass, to name only a few. The fused quartz may include amorphous SiO2, and the soda lime glass may include SiO2, Na2O, CaO, Al2O3 and MgO. The boro-silicate glass may include B2O3, Na2O, K2O and CaO. The alumino-silica glass may include SiO2, Na2O, CaO, Al2O3 and MgO. The boro-silicate glass may include SiO2, B2O3, Al2O3, Na2O, MgO and CaO. Fused quartz and fused silica are types of glass containing primarily silica in amorphous form. Soda-lime glass, also called soda-lime-silica glass, is the most prevalent type of glass and is commonly used for window glass and glass containers.
Well known and proven glass engineering techniques may be used to formulate a glassy enamel for use as the matrix 66. Suitable ceramics may comprise oxides, non-oxides and composites. Typical ceramics include aluminum oxinitride, silica glass fibers, and silica aerogel. These three types of ceramics may have an operating maximum safe working temperature of approximately 1150° F. to 1200° F. A matrix 66 comprising a silica-based compound of the type mention mentioned above may be in the form of glass frit. When melted through the application of heat, the glass frit fuses into a vitreous, low-cost, durable enamel coating that exhibits good adhesion properties and bonds readily with surface metal oxides. The enamel coating may also have desirable quantum properties, and may be useful to over approximately 1400° F.
The phonons 64 encountering the nano-particles 60 are partially reflected as shown by the arrows 70. If the spacing of the nano-particles 60 is close to the wavelength λ of the phonons 64, the reflections 70 may interfere with each other. These interferences sum to produce an aggregate thermal reflection which reduces the thermal energy flow through the coating 30 and lowers its effective thermal conductivity. The difference in thermal conductivities of the matrix 66 and the particles 60 which result in interfering reflections 70 of the phonons 64 may be dependent on the mass ratio, effective matrix spring constant, phonon dispersion and scattering in the matrix over short path lengths, and relative period ordering of the nano-particles 60. It should be noted here that the 3-D spatial arrangement of the nano-particles 60, including the distance D (
Attention is now directed to
At step 84, the solvent or gel is evaporated from the slurry on the substrate 34, causing the coated nano-particles 58 to self-assemble into a quasi-regular 3-D array. As an alternative to self-assembly of the coated nano-particles 58 into the 3-D array by the solution technique described above, self-assembly may be achieved by electrostatically assisted dry deposition. Finally, at step 86, the shells 62 around the nano-particles 60 are fused together to form a substantially homogeneous matrix by sintering the coated nano-particles 58. The sintering and fusion of the matrix material shells 62 may be may caused either by heating the coated nano-particles 58 in an oven to melt and fuse the glassy frit-like shells 62, or by using a laser to melt the shells 62 into an enamel surface that bonds to the substrate 34. The use of a laser to fuse the shells 62 allows lower processing temperatures which may permit the thermal barrier coating to be applied to composite structures (not shown), such as graphite/epoxy composites. The coating 30 may be applied to the substrate 34 by dipping, spraying or other suitable techniques.
Embodiments of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace, marine and automotive applications. Thus, referring now to
Each of the processes of method 90 may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on.
As shown in
Systems and methods embodied herein may be employed during any one or more of the stages of the production and service method 90. For example, parts, structures and components corresponding to production process 198 may be fabricated or manufactured in a manner similar to parts, structures and components produced while the aircraft 92 is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages 98 and 100, for example, by substantially expediting assembly of or reducing the cost of an aircraft 92. Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft 92 is in service, for example and without limitation, to maintenance and service 106.
Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.
