US20100227134A1

US20100227134A1 - Method for the prevention of nanoparticle agglomeration at high temperatures

Info

Publication number: US20100227134A1
Application number: US12/713,147
Authority: US
Inventors: Tushar K. Shah; Brandon K. Malet; Jordan T. Ledford; Harry C. Malecki
Original assignee: Lockheed Martin Corp
Current assignee: Applied Nanostructured Solutions LLC
Priority date: 2009-03-03
Filing date: 2010-02-25
Publication date: 2010-09-09
Also published as: EP2403714B1; US10138128B2; BRPI1007928A2; KR101722524B1; KR20110134396A; ES2467921T3; US20140154412A1; AU2010221614A1; WO2010101784A1; JP5757881B2; ZA201105400B; EP2403714A1; US20100224129A1; KR101727116B1; CN102341234B; CA2751732A1; CN102341234A; EP2789579A1; JP2012519643A; KR20140032017A

Abstract

A method includes: (a) conformally depositing a barrier coating, provided in liquid form, on at least one surface of a substrate; (b) embedding a plurality of nanoparticles in the barrier coating to a selected depth; and (c) fully curing the barrier coating after embedding the plurality of nanoparticles; the embedded plurality of nanoparticles are in continuous contact with the cured barrier coating. The order in which the barrier coating and nanoparticles are deposited on the substrate can be switched or they can be deposited simultaneously. An article includes a substrate having a cured barrier coating conformally disposed on at least one surface of the substrate and a plurality of nanoparticles embedded to a selected depth in the barrier coating creating an embedded portion of each of the plurality of nanoparticles. The embedded portion of each of the plurality of nanoparticles in continuous contact with the cured barrier coating.

Description

STATEMENT OF RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. §119(e) to provisional applications 61/157,096 filed Mar. 3, 2009, and 61/182,153 filed May 29, 2009 each of which is incorporated by reference herein in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

The present invention relates to nanoparticles, more specifically to nanoparticles used in high temperature processes.

BACKGROUND OF THE INVENTION

Many nanoparticles are used in applications that involve gas phase reactions which can expose the nanoparticles to elevated temperatures. It has been observed that with decreasing particle size, the melting point of the nanoparticle also decreases. The reduced melting point coupled with surface diffusion characteristics (driven by the large surface area to volume ratio) of nanoparticles on various substrates can cause sintering and particle agglomeration during high temperature processes. Sintering occurs when the surfaces of nanoparticles approach temperatures close to their melting point, resulting in an outer surface layer change to a liquid phase, while the nanoparticle core remains solid. The outer liquid surface of multiple particles may then mix together forming solid agglomerates upon cooling. The sintering and agglomeration of the nanoparticles result in reduced particle specific surface areas and consequently diminished effectiveness in various applications.
As an example, in the specific application of transition metal nanoparticles as catalysts for carbon nanotube (CNT) growth, nanoparticle sintering and agglomeration due to high temperature exposure of more than 450° C. can contribute to reduced ability to nucleate CNTs.
It has been demonstrated that a ceramic coating encapsulating individual nanoparticles can provide a thermal and physical barrier to prevent agglomeration. This encapsulation, while keeping the particles separated to prevent sintering and agglomeration, can also prevent the nanoparticles from achieving target temperatures for catalytic reactions and/or can present a physical barrier to active catalytic surfaces of the nanoparticle. Such discrete particles can also be mobile on a surface leading to a lack of consistent distribution and density.
In the silicon wafer industry, it has been indicated that nanoparticle mobility and subsequent sintering can be reduced by providing, for example, an alumina layer on the silicon surface. This alumina layer, however, channels the nanoparticles into a pre-determined template. Moreover, the channels can be of substantial depth and dimensions such that they impact the subsequent reaction chemistry of the nanoparticle. For example, when growing carbon nanotubes (CNTs), CNT morphology such as CNT diameter, growth direction, and branching can all be affected by the pre-constructed alumina template.
Methods that aid in the prevention of nanoparticle sintering and agglomeration during high temperature processes while maintaining catalytic activity, reducing particle mobility, and providing consistent targeted nanoparticle densities would be useful. In particular, providing a method for preventing sintering and agglomeration under CNT growth conditions, while reducing or preventing the effects of templating on CNT morphology is also desirable. The present invention satisfies these needs and provides related advantages as well.

SUMMARY OF THE INVENTION

In some aspects, embodiments disclosed herein relate to a method that includes a method that includes: (a) conformally depositing a barrier coating on at least one surface of a substrate; the barrier coating provided in liquid form; (b) embedding a plurality of nanoparticles in the barrier coating to a selected depth creating an embedded portion of each of the plurality of nanoparticles; and (c) fully curing the barrier coating after embedding the plurality of nanoparticles; the embedded portion of each of said plurality of nanoparticles being in continuous contact with the cured barrier coating.
In some aspects, embodiments disclosed herein relate to a method that includes: (a) depositing a plurality of nanoparticles on at least one surface of a substrate; (b) conformally depositing a barrier coating over the substrate and at least a portion of each of the plurality of nanoparticles, creating an embedded portion of each of the plurality of nanoparticles, the barrier coating provided in liquid form; and (c) fully curing the barrier coating; the plurality of nanoparticles are in surface contact with the substrate and the embedded portion of each of the plurality of nanoparticles is in continuous contact with the cured barrier coating.
In some aspects, embodiments disclosed herein relate to an article that includes a substrate having a cured barrier coating conformally disposed on at least one surface of the substrate, and a plurality of nanoparticles embedded to a selected depth in the barrier coating creating an embedded portion of each of the plurality of nanoparticles. The embedded portion of each of the plurality of nanoparticles in continuous contact with the cured barrier coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows nanoparticles embedded in a barrier coating disposed conformally over a substrate.

FIG. 2 shows nanoparticles embedded in a barrier coating and in surface contact with a substrate.

FIG. 3 shows exemplary growth of carbon nanotubes on the embedded nanoparticles shown in FIG. 1.

FIG. 4 shows a system for producing a high temperature ceramic fiber composite.

FIG. 5 shows a system for producing CNTs on carbon fiber.

DETAILED DESCRIPTION

The present disclosure is directed, in part, to methods that employ barrier coatings on a substrate to “lock” nanoparticles distributed on a substrate surface in place to substantially reduce nanoparticle sintering and agglomeration at high temperatures. The barrier coatings employed in the methods disclosed herein are in contact with the nanoparticles. In some embodiments, the barrier coating does not fully encapsulate the nanoparticles, allowing the nanoparticles to be exposed to desired reaction environments while preventing nanoparticle sintering and agglomeration. In some embodiments, the barrier coating does fully encapsulate the nanoparticles. In such applications, the function of the nanoparticle can be, for example, as a means of absorbing high energy radiation. The heat associated with such absorption can be sufficient to cause nanoparticle sintering in the absence of the barrier coating. The barrier coating and nanoparticles can be disposed on the substrate surface sequentially in any order or they can be applied to the substrate simultaneously.
The barrier coatings employed in methods disclosed herein can be provided as a sufficiently thin layer (equal to or less than the effective nanoparticle diameter) that the barrier coating itself does not influence the reactivity profile and/or course of the reactions catalyzed or seeded by the nanoparticles. For example, when using CNT growth catalysts embedded in nanochanneled template materials for aligned CNT growth, the template dictates the CNT dimensions, including width, and direction of CNT growth (Li et al. App. Phys. Lett. 75(3):367-369 (1999)).
In some embodiments, the barrier coating can completely embed the nanoparticles. In some embodiments, a barrier coating can embed the nanoparticles while also allowing a degree of diffusion through the barrier coating to allow access to the embedded nanoparticles. Methods of the invention embed nanoparticles in the barrier coating in a dense array without the restrictions of any kind of pre-formed template. This can provide a greater nanoparticle density, as well as a more uniform density of nanoparticles. These benefits are realized by providing the barrier coating in a liquid form which allows the barrier coating to conform to the nanoparticle dimensions. This is particularly beneficial in CNT synthesis applications because sintering is prevented and CNT morphology is controlled by the nanoparticle itself, rather than a pre-determined channel in which the CNT resides.
The barrier coatings employed in methods disclosed herein provide a means to prevent sintering and agglomeration of nanoparticles under high mobility conditions by preventing nanoparticle-to-nanoparticle interactions. The barrier coatings can also prevent nanoparticle-to-substrate interactions by means of physical separation and mechanical interlocking of the nanoparticles in the barrier coating, as exemplified in FIG. 1. For example, a metallic nanoparticle can form an alloy with a metal substrate. The barrier coating can prevent such alloy formation. Similarly, in the area of CNT growth, the barrier coating can prevent nanoparticle-to-substrate interactions between a transition metal catalyst and a carbon rich substrate. Such nanoparticle-to-substrate interaction can poison the transition metal nanoparticle catalyst by providing an excessive amount of carbon as feedstock under CNT growth conditions. More generally, the barrier coatings employed in methods disclosed herein facilitate the use of nanoparticles with substrates that would otherwise be incompatible in the absence of the barrier coating.
In some embodiments, the embedded nanoparticles can be in surface contact with the substrate as shown in FIG. 2 while still avoiding or reducing nanoparticle-to-substrate interactions. For example, the barrier coating can be used to minimize the contact area between the substrate and the nanoparticles. In some embodiments, even where there is still appreciable contact area between the nanoparticles and the substrate, a sufficiently thick barrier coating can provide a thermal barrier so that the nanoparticle-substrate contact interface is at a sufficiently low temperature to avoid any deleterious interactions. In some embodiments, when the nanoparticle is in contact with a substrate surface, a barrier coating thickness can be used that encapsulates the nanoparticle while still allowing diffusion of reactive materials through the barrier coating to allow nanoparticle catalyzed reactions to take place. For example, in the case of CVD CNT growth, carbon atoms from a CVD carbon feedstock can diffuse through an appropriate barrier coating material. In such embodiments, it can be desirable to have a barrier coating thickness that is approximately the same or just slightly more than the effective diameter of the nanoparticle catalysts.
An additional use of the barrier coating can be to protect sensitive substrates from high temperature and/or reactive environments used in connection with reactions of the embedded nanoparticles. For example, some carbon-based substrates may not be stable under high reaction temperatures or when exposed to a variety of reaction conditions, such as a strongly oxidative environment.
The present invention is also directed, in part, to articles that include a substrate having a barrier coating conformally disposed on at least one surface of the substrate with a plurality of nanoparticles embedded in the barrier coating. Such articles can be used in further reactions to modify the substrate and hence properties of the article. For example, CNTs can be grown on the surface of the substrate, when employing transition metal nanoparticles. Such CNTs can be useful in the manufacture of organized CNT arrays for use in surface enhanced Raman applications and microelectronic structures, in the preparation of reinforcing materials in composites and other composite applications such as EMI shielding, signature control, and lightning strike protection. Articles of the invention can also include barrier coated substrates with embedded nanoparticles in which the nanoparticles serve as catalysts for other reactions where high temperatures are employed, but in which the article remains unchanged. For example, articles can include immobilized catalyst nanoparticles for combustion reactions, as might be employed in a catalytic converter.
As used herein, the term “conformally depositing,” when used in reference to the application of a barrier coating to a substrate, refers to a process in which the barrier coating is deposited on, and in surface contact with a substrate, regardless of substrate geometry. Conformal deposition of a barrier coating on a substrate to which nanoparticles have already been deposited does not interfere with the exposure of at least a portion of the nanoparticle surface when desired. In such embodiments, the barrier coating can be formulated to fill the voids between nanoparticles without completely encapsulating the nanoparticles. This can be achieved by altering the concentration and/or viscosity of the liquid form of the barrier coating.
As used herein, the term “barrier coating” refers to any coating used to reduce or prevent undesirable nanoparticle-to-nanoparticle interactions such as sintering and agglomeration on a substrate surface. The term also includes coatings used to reduce or prevent undesirable nanoparticle-to-substrate interactions. “Barrier coatings” can be further selected for adherence to particular substrates and/or to protect a substrate from a reactive environment that is used in a reaction in which a nanoparticle is used as a catalyst, seed material, or reactant. Barrier coatings of the invention are thermal insulators that can be applied to a substrate in liquid form, such as gels, suspensions, dispersions, and the like. By providing the barrier coating in a liquid form, it can be subsequently partially or fully cured. The curing process generally involves the application of heat. Exemplary barrier coatings include, for example, spin-on glass or alumina.
As used herein, the term “agglomeration” refers to any process in which nanoparticles disposed on a substrate are fused together. Conditions for agglomeration can include heating to a melting point of the entire nanoparticle or a portion of the nanoparticle, such as its surface. In addition, agglomeration refers to conditions that accelerate surface diffusion of the nanoparticles on the substrate, which includes heating. With respect to the latter conditions, the term “agglomeration” can be used interchangeably with the term “sintering.”
As used herein, the term “nanoparticle” or NP (plural NPs), or grammatical equivalents thereof refers to particles sized between about 0.1 to about 100 nanometers in equivalent spherical diameter, although the NPs need not be spherical in shape. Such nanostructured materials encompass any geometry lacking a large aspect ratio with respect to all dimensions.
As used herein, the term “effective diameter” refers to the average nanoparticle diameter of approximately spherical nanoparticles.
As used herein, the term “embedding,” when used in reference to nanoparticles in barrier coatings, refers to the process of surrounding the nanoparticles with the liquid form of the barrier coating to any depth, including in surface contact with a substrate, and/or encapsulating the nanoparticle completely. “Embedding” the nanoparticles of the invention in the barrier coating and curing the barrier coating can mechanically lock the particles in place preventing their migration and subsequent agglomeration. “Embedding” the nanoparticles in the barrier coating can include placing the particles in the barrier coating to a depth that the nanoparticles are also in surface contact with the substrate on which the barrier coating is deposited, while still maintaining an exposed surface of the nanoparticle. Nanoparticles can also be “embedded” in the barrier coating by applying the barrier coating after placing nanoparticles on a substrate. Nanoparticles can also be embedded in the barrier coating by simultaneous application of the barrier coating and the nanoparticles.
As used herein, the term “carbon nanotube” or “CNT” refers to any of a number of cylindrically-shaped allotropes of carbon of the fullerene family including single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTS), multi-walled carbon nanotubes (MWNTs). CNTs can be capped by a fullerene-like structure or open-ended. CNTs include those that encapsulate other materials.
As used herein, the term “transition metal” refers to any element or alloy of elements in the d-block of the periodic table. The term “transition metal” also includes salt forms of the base transition metal element such as oxides, carbides, nitrides, acetates, and the like.
In some embodiments, the present invention provides a method that includes (a) conformally depositing a barrier coating on at least one surface of a substrate; the barrier coating is provided in liquid form; (b) embedding a plurality of nanoparticles in the barrier coating to a selected depth creating an embedded portion of each of the plurality of nanoparticles; and (c) fully curing the barrier coating after embedding the plurality of nanoparticles. The embedded portions of each of the plurality of nanoparticles are in continuous contact with the cured barrier coating. The barrier coating does not affect the arrangement of the plurality of nanoparticles embedded therein. Thus, the barrier coating does not behave as a template dictating the relative placement of the nanoparticles. The result of this process is a barrier-coated substrate with locked nanoparticles that can be used in a variety of contexts depending on the exact choice of nanoparticle and substrate employed, as further described below. In some embodiments, the step of conformally depositing the barrier coating and embedding the plurality of nanoparticles is simultaneous. Thus, the barrier coating material can also be applied to the substrate in situ with the nanoparticles via solutions that contain both the barrier coating and nanoparticle material (‘hybrid solutions’).
In some embodiments, the methods described herein control particle dispersion on a variety of shaped objects. This includes an efficient means of coating composite materials like fibers or fabrics and irregular shaped materials. Moreover, methods of the invention control and maintain a nanoparticle density on substrate surfaces, even when exposed to conditions that might cause NP diffusion and/or sintering.
In some embodiments, the present invention provides a method that includes (a) conformally depositing a barrier coating on at least one surface of a substrate and (b) embedding a plurality of nanoparticles in the barrier coating, wherein the thickness of the barrier coating is about the same or greater than the effective diameter of the plurality of nanoparticles. In such embodiments, the thickness of the barrier coating can be between about equal to the effective diameter of the plurality of nanoparticles up to about 5,000% greater than this effective diameter. Thus, the thickness of the barrier coating can be 0.01% greater than this diameter or 0.1%, or 0.5%, 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, 150%, 200%, 250%, 300%, 350%, 400%, 500%, 1,000%, 1,500%, 2,000%, and so on up to about 5,000% greater than the effective diameter of the plurality of nanoparticles, including an value in between and fractions thereof.
In some embodiments, the nanoparticles are prevented from agglomerating when subjected to heating, for example. In some embodiments, a barrier coating that encapsulates the plurality of nanoparticles can be useful in applications where reactant access to the NPs is not employed. For example, in electromagnetic interference (EMI) shielding applications, the barrier coatings can be transparent to electromagnetic radiation, but the NPs can effectively absorb the EM radiation. This absorption can cause the NPs to heat; thus, the barrier coating can prevent sintering in such instances. In some embodiments, the barrier coating can encapsulate the plurality of NPs without denying access to the particle when, for example, a porous barrier coating is employed. In such embodiments, although the particle is technically encapsulated, the porous nature of the barrier coating allows access to reactive surfaces of the NP.
In some embodiments, the plurality of nanoparticles can be embedded partially in the barrier coating providing a physical boundary between the nanoparticle and the substrate, as shown in FIG. 1. In other embodiments, the embedded nanoparticles can be in surface contact with substrate, as shown in FIG. 2. In still further embodiments, the embedded nanoparticles can be a mixture of a first portion separated from the substrate and a second portion and in surface contact with the substrate. In some applications it can be beneficial to avoid direct surface contact between the substrate and the nanoparticles. For example, with a metal substrate and a metal nanoparticle, partial embedding of the nanoparticle can help avoid formation of alloys when the nanoparticle is exposed to high temperatures. Similarly, in the case of CNT growth with transition metal nanoparticle catalysts, it can be useful to separate the catalyst from a carbon rich substrate that might react with the nanoparticle.
In some embodiments, the nanoparticles are completely encapsulated in the barrier coating, but an exposed surface is created through a number of subsequent processes. For example, when fully curing the barrier coating some materials can form fissures in the coating in the vicinity of nanoparticles which can provide an interface between the nanoparticles and a reactive environment. Other barrier coating materials can create the necessary access to the nanoparticles through the formation of a porous cured structure.
In some embodiments, fully encapsulated nanoparticles can be treated with a plasma to roughen the surface of the barrier coating and create exposed nanoparticle surfaces. Similarly, the barrier coating with encapsulated nanoparticles can be treated with a wet chemical etching agent for a period sufficient to expose a portion of the surface of the nanoparticles.
In still further embodiments, fully encapsulated nanoparticles can be treated under mechanical roughening conditions to expose a portion of the surface of the nanoparticles. This can be done through any physical abrasive method such as sand blasting, laser ablation, ball milling, plasma etching, and the like.
Regardless of the degree with which the nanoparticles are embedded in the barrier coating, the barrier coating can serve to mechanically lock the nanoparticles in place to prevent their agglomeration or sintering when subjected to heat. Without being bound by theory, this is accomplished by restricting the movement of the nanoparticles on the substrate surface reducing NP diffusion. Thus, the nanoparticle-to-nanoparticle interaction is substantially reduced or eliminated by the presence of the barrier coating.
The barrier coating can also provide a thermal barrier for use with low melting substrates. In this regard, the barrier coating can minimize or reduce to zero the surface area contact between the plurality of nanoparticles and the substrate to mitigate the effects of the exposure of the substrate to temperatures which the nanoparticles might be heated or, more generally, to avoid exposure of the substrate to the reaction environment to which the plurality of nanoparticles can be at least partially exposed.
In some embodiments the thickness of the barrier coating is generally chosen to be about equal to, less than, or slightly less than the effective diameter of the plurality of nanoparticles so that there remains an exposed nanoparticle surface for subsequent exposure to a reaction environment. In other embodiments, the thickness can also be more than the effective diameter of the nanoparticles by employing any number of techniques described above to create an exposed surface of the nanoparticles. In some embodiments, the thickness of the barrier coating is between about 0.1 nm and about 100 nm. In some embodiments, the thickness can be less than 10 nm, including 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, and any value in between. The exact choice of barrier coating thickness can be chosen to approximately match or be less than the effective diameter of the plurality of nanoparticles. In some embodiments, the embedded plurality of nanoparticles maintains an exposed surface even when the nanoparticles are in surface contact with the substrate. In some embodiments, the thickness of the barrier coating coats is such that it covers about half the nanoparticle surface area. In some embodiments, the thickness of the barrier coating covers about 10% of the nanoparticle surface area, while in other embodiments, the thickness of the barrier coating covers about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, and 100% of the surface area of the nanoparticles, including all values in between. In still other embodiments, the barrier coating covers the nanoparticle when applied but a portion of the nanoparticle is exposed upon further treatments or choice of porous barrier coating.
In some embodiments, the methods of the invention can include treating the substrate with a plasma prior to conformally depositing the barrier coating. Treating the substrate in a plasma process can serve a dual role of creating functional groups and roughening the substrate surface, thereby increasing its effective surface area, to improve the wetting properties of the substrate and thus improve the conformal deposition of the barrier coating. Substrate surface modification can be achieved using a plasma of any one or more of a variety of different gases, including, without limitation, argon, helium, oxygen, ammonia, hydrogen, and nitrogen.
In some embodiments, the step of depositing the barrier coating is accomplished by a technique selected from dip coating and spraying. Thus, the barrier coating can be solution based and applied via dip bath configuration, spray methods, or the like in some embodiments. The exact choice of method can be dictated by a number of factors, including, for example, the substrate geometry. For irregular shaped substrates, it can be useful to employ dip methods that avoid the use of directionally applied barrier coatings, such as in spray applications. For substrates in which a single side should be coated, such as a wafer substrate, it can be useful to apply the barrier coating with spray or related techniques (nebulizers, for example) to assure coating on only one side. Other factors to consider in applying the barrier coating can depend on the barrier coating material itself including, for example, the ability to form solutions or homogenous suspensions for dip or spray coating.
When applying the barrier coating via dip or spray methods, for example, the thickness of the barrier coating can be controlled by use of diluents. Diluents can include any solvent compatible with both the substrate and nanoparticle materials. For dip coating, in particular, the thickness of the barrier coating can be a function of concentration of the barrier coating material and the residence time in the dip bath. The residence time can also aid in providing uniformity of the coating. Uniformity can also be insured by employing multiple dip baths.
The barrier coating includes a material selected from a siloxane, a silane, an alumina, a silicon carbide ceramic, a metal, and mixtures thereof. In some embodiments, the choice of barrier coating can be chosen for its ability to adhere to the substrate. There are many types of barrier coating materials including, for example, those that are siloxane-based, silane-based, alumina-based, silicon carbide-based ceramics, and metallic based. Alumina based materials include, for example, alumoxane, alumina nanoparticles, and alumina coating solutions, including, for example, alumina-based coatings available from Zircar Ceramics, such as Alumina Rigidizer/Hardener Type AL-R/H. In some embodiments, glass coatings such as spin on glass, glass nanoparticles, or siloxane-based solutions, such as methyl siloxane in isopropyl alcohol, can be used as barrier coating materials. Metallic based barrier coatings useful in the invention include, for example, molybdenum, aluminum, silver, gold, and platinum. Silicon carbide based ceramics include, for example, SMP-10, RD-212a, Polyaramic RD-684a and Polyaramic RD-688a available from Starfire.
Barrier coatings can also act as multifunctional coatings tailored to specific applications. A specific type of barrier coating can be selected to both prevent sintering as well as promote adhesion to the substrate. For composite applications, a barrier coating can selected to prevent sintering as well as bond well to the composite matrix material. In still further embodiments, the barrier coating material can be selected for adhesion both to the substrate as well a composite matrix material. In yet further embodiments, more than one barrier coating can be employed. A first barrier coating can be selected for its ability to adhere to the substrate surface. A second barrier coating can be selected for its ability to adhere, for example, to a composite matrix material such as a resin, ceramic, metal, or the like.
In some embodiments, methods of the invention include partially curing the barrier coating prior to embedding said plurality of nanoparticles. Partial curing of the barrier coating can provide a “sticky” surface to embed the nanoparticles while preventing movement of the applied nanoparticles to minimize particle-to-particle interaction. Partial curing can also be caused by the method used to apply the nanoparticles to the barrier coating. In such a case, the partial curing step and embedding step are performed simultaneously. Partial curing temperatures are generally below the normal cure temperature, and can include temperature that are between about 50 to about 75% of the normal cure temperature and for residence times on the order of seconds.
In some embodiments, methods of the present invention further include heating the environment about the embedded plurality of nanoparticles, in the presence of a feedstock material, to a temperature promoting growth of a plurality of nanostructures from the feedstock material. In some embodiments, the embedded plurality of nanoparticles can catalyze the growth of the nanostructures. In some embodiments, the nanoparticles act as a seed for growth of the nanostructure, without behaving as a true catalyst. In still further embodiments, the nanoparticles catalyze a reaction which does not alter the substrate, barrier coating, or the nanoparticles. Thus, the nanoparticle can catalyze a gas phase reaction in which the products remain in the gas phase, for example. In some embodiments, the temperature of a given reaction is sufficient to cause agglomeration of the plurality of nanoparticles in the absence of the barrier coating. Thus, the barrier coating provides an effective means for preventing sintering.
In some embodiments, the nanoparticles include a transition metal. The catalyst transition metal nanoparticle can be any d-block transition metal as described above. In addition, the nanoparticles can include alloys and non-alloy mixtures of d-block metals in elemental form or in salt form, and mixtures thereof. Such salt forms include, without limitation, oxides, carbides, and nitrides. Non-limiting exemplary transition metal NPs include Ni, Fe, Co, Mo, Cu, Pt, Au, and Ag and salts thereof, such as acetates and chlorides, and mixtures thereof. In some embodiments, the transition metal is used as a CNT forming catalyst. Many of these transition metal catalysts are readily commercially available from a variety of suppliers, including, for example, Ferrotec Corporation (Bedford, N.H.).
In some embodiments, the feedstock material is a carbon source, which when used in conjunction with the aforementioned transition metals, allows for the synthesis of nanostructures such as carbon nanotubes (CNTs). These CNTs can be single-walled, double-walled, or other multi-walled CNTs. One skilled in the art will recognize the relationship between nanoparticle size and the type of CNTs that can be grown. For example, single-walled CNTs are normally accessible with nanoparticle catalysts less than about 1 nm. CNT growth conditions are typically between about 500 to about 1,000° C., a temperature at which sintering is observable and can impact successful CNT growth.
Many substrate types, such as carbon and stainless steel, are not normally amenable to CNT growth of high yields when only a catalyst nanoparticle is applied to the surface due to high levels of sintering. Barrier coatings are useful, however, for high-yield CNT growth, even on these challenging substrates.
On the surface of a substrate, a catalyst nanoparticle's ability to nucleate CNT growth can depend on the presence of sufficient barrier coating material at that location of the substrate surface to substantially reduce or prevent sintering. CNT growth can be performed when the catalyst nanoparticles are applied to the substrate prior to the barrier coating ('reverse order'). The benefit of a ‘reverse order’ process is that the barrier coating keeps the catalyst locked onto the substrate, and therefore allows for anchoring of the CNTs to the substrate surface. Without being bound by theory, when barrier coating is applied prior to catalyst coating the CNT nanoparticle catalyst tends to follow the leading edge of CNT synthesis, that is, tip-growth results. The ‘reverse order’ coatings can promote base-growth.
In some embodiments, the feedstock can be a carbon source mixed with other gases as might be found, for example, in a combustion process. In such embodiments, embedded transition metal nanoparticles, such as platinum, palladium, rhodium, cerium, manganese, iron, nickel, or copper can be used to modulate the oxidation of the carbon source. The favorable surface area to volume of a nanoparticle can improve the catalytic performance in such combustion processes. This type of reaction can find application, for example, in catalytic converters. It can also be useful in various industrial petroleum processes such as in refining and in downhole operations to catalyze the cracking of heavy hydrocarbons for enhanced oil recovery, thus maximizing formation productivity.
In some embodiments, other uses of transition metal nanoparticles include the manufacture of high density magnetic recording media that employ FePt nanoparticles. One skilled in the art will recognize that sintering of FePt nanoparticles is problematic when attempting to induce phase the change to obtain the useful face-centered tetragonal FePt structure. This phase change is generally conducted by heating at about 550° C. and is accompanied by sintering. The barrier coatings disclosed herein are useful in preventing this sintering.
In some embodiments, a transition metal nanoparticle can be used in desulfurization processes. For example, nickel and molybdenum catalysts have been used in the desulfurization of bitumen. In such processes, expensive supports such as uranium oxide have been employed to prevent sintering during recycle of the catalyst. Methods of the present invention employing a barrier coating can be employed to prevent such sintering, while avoiding the use of expensive support materials.
In some embodiments, a transition metal nanoparticle can be used in syngas production processes. It has been determined that sintering of CeO₂in Rh—CeO₂catalysts limits the use of this catalyst system. The barrier coating employed in methods disclosed herein can be used to prevent this sintering and enhance the biomass to syngas transformation, for example.
In some embodiments, the nanoparticles can include other metal containing materials such as ceramics, for example, oxides, carbides, borides, of zinc, titanium, aluminum, and the like. Other materials that do not contain transition metals such as clays, silica, silicates, aluminosilicates and the like can also be used.
Any of the aforementioned nanoparticles can range in size from between about 0.1 nm to about 100 nm. In some embodiments, the size of the nanoparticles can be in a range from between about 1 to about 75 nm, and between about 10 to 50 nm in other embodiments. In some embodiments, the size of the nanoparticles is in a range from between about 0.1 to about 1 nm. In other embodiments, the size of the nanoparticles is in a range from between about 2 to about 10 nm. In still further embodiments, the size of the nanoparticles is in a range from between about 10 to about 20 nm, from between about 20 to about 30 nm, from between about 30 to about 40 nm, from between about 40 to about 50 nm, from between about 50 to about 60 nm, from between about 60 to about 70 nm, from between about 70 to about 80 nm, from between about 80 to about 90 nm, and from between about 90 to about 100 nm, including all values in between. The choice of size can depend on the application. In catalytic processes, as described above, it can be desirable to utilize smaller particles to benefit from the larger surface area to volume. More generally, at the nanoparticle scale, one skilled in the art will recognize the quantized nature of the properties of the nanoparticles and that an appropriate size can be determined through theoretical considerations and calculations. For example, a particular particle size can be designed to absorb specific wavelengths of radiation.
The rate of sintering of a metallic nanoparticles can vary depending on the substrate on which it is disposed. However, by employing the barrier coatings in methods of the present invention, any substrate type can be used. For example, the substrate can include a metal, a ceramic, a silica wafer, a fiber, a graphite sheet, high temperature plastics, such as polyimides, PEEK, PEI and the like.
In some embodiments, the present invention provides a method that includes: (a) depositing a plurality of nanoparticles on at least one surface of a substrate; (b) conformally depositing a barrier coating over the substrate and at least a portion of each of the plurality of nanoparticles, creating an embedded portion of each of the plurality of nanoparticles; the barrier coating is provided in liquid form; and (c) fully curing the barrier coating. The plurality of nanoparticles are in surface contact with the substrate in such embodiments, and the embedded portion of each of the plurality of nanoparticles is in continuous contact with the cured barrier coating. This is described above as “reverse order” process and is shown graphically in FIG. 2. In this configuration, the barrier coating can also prevent the agglomeration of the plurality of nanoparticles when exposed to heat, or other processes that might cause sintering. As described above, the thickness of the barrier coating can be about the same or slightly less than the effective diameter of the plurality of nanoparticles allowing the plurality of nanoparticles to maintain an exposed portion of their surface. Alternatively the thickness of the barrier coating can be greater than effective diameter of the plurality of nanoparticles. In some embodiments, the methods described above for post barrier coating handling can be used when the barrier coating encapsulates the nanoparticles completely.
When employing the “reverse order” process, the substrate can be treated with a plasma prior to depositing the plurality of nanoparticles. This can provide the exposed substrate surface with good wetting characteristics as described above. Similarly, the step of depositing the barrier coating can be accomplished by a technique selected from dip coating and spraying as described above. Moreover, any of the above applications, conditions and general considerations apply equally to the “reverse order” methods of the invention.
The methods of the invention can be used to produce an article that includes a substrate having a barrier coating conformally disposed on at least one surface of the substrate and a plurality of nanoparticles embedded in the barrier coating. The barrier coating function can be to prevent the agglomeration of the plurality of nanoparticles when subjected to heat or other chemical and/or physical processes.
The thickness of the barrier coating in articles of the invention can be about the same or slightly less than the effective diameter of said plurality of nanoparticles allowing said plurality of nanoparticles to maintain an exposed portion of their surface when said nanoparticles are, optionally, in surface contact with the substrate. In particular embodiments, the embedded plurality of nanoparticles are in surface contact with the substrate. Articles of the invention can include a substrate that is a metal, ceramic, silica wafer, fiber, graphite sheet, and high temperature plastic, as describe above.
Any of the nanoparticle types and sizes described above can be used in connection with the articles of the invention. In some embodiments, articles of the invention include, composite materials having a matrix material and carbon nanotubes infused to a fiber. In combustion and related catalyst applications articles of the invention include a) catalytic converters, b) catalyst reaction beds used in refining, syngas production, desulfurization and the like, c) downhole tools used in oil recovery, and d) high density storage media.
It is understood that modifications which do not substantially affect the activity of the various embodiments of this invention are also included within the definition of the invention provided herein. Accordingly, the following examples are intended to illustrate but not limit the present invention.

Example I

This example shows how a barrier layer can be used in a ceramic fiber composite structure to prevent sintering of iron nanoparticles applied to the ceramic fiber surface for enhanced signature control characteristics.
FIG. 4 depicts system 400 for producing a high temperature ceramic fiber composite with enhanced signature control characteristics in accordance with the illustrative embodiment of the present invention. System 400 includes a ceramic fiber 402, barrier coating solution bath 404, nanoparticle solution bath 406, coating curing system 408, filament winding system 410, and a resin infusion system 412, interrelated as shown.
The ceramic fiber 402 used is a Silicon Carbide Sylramic™ fiber tow (1600 denier10 micron diameter) (COI Ceramics, Inc).
A barrier coating 404, consisting of the Starfire SMP-10, RD-212a solution is applied to the ceramic fiber 402 via a dip process. A diluted solution of 1 part SMP-10 and 10 parts isopropyl alcohol is used in the dip process to apply a 2-4 nm thick coating.
The nanoparticle solution 406 used is GTP 9700 (NanoChemonics), an iron oxide nanoparticle mixed in a toluene solution. The nanoparticle solution is used to apply a uniform distribution of iron oxide nanoparticles on the surface of the barrier coating 404. Solutions containing less than 10% iron oxide by weight is used to create nanoparticle coatings with 20-40 nm spaced nanoparticles.
The coating curing system 408 consists of a set of heaters used to cure the combine barrier and nanoparticle coating 409. The coated fiber is exposed to a temperature of 200 C for 2 hours along with a platinum-based catalyst to aid in the curing process.
The cured coating locks the nanoparticles into position, and the coated fiber is wound into a component using the filament winding system 410.
The filament wound component 411 is then infused with a bismaleimide matrix using the resin infusion system 412.
The final cured high temperature ceramic fiber composite structure 413 is able to withstand brief high temperature exposure as high as 600 C while maintaining signature control characteristics which are imparted due to the dispersed iron oxide nanoparticle coating. This nanoparticle coating will not sinter as a result of its interaction with the cured barrier coating.

Example II

This example shows how carbon nanotubes (CNTs) can be grown on the surface of a carbon fiber using a barrier coating to prevent sintering of the iron nanoparticle catalyst.
FIG. 5 depicts system 500 for producing CNTs on carbon fiber (34-700 12 k unsized carbon fiber tow with a tex value of 800—Grafil Inc., Sacramento, Calif.) in accordance with the illustrative embodiment of the present invention. System 500 includes a carbon fiber material payout and tensioner station 505, plasma treatment station 515, barrier coating application station 520, air dry station 525, catalyst application station 530, solvent flash-off station 535, CNT-growth station 540, and carbon fiber material uptake bobbin 550, interrelated as shown.
Payout and tension station 505 includes payout bobbin 506 and tensioner 507. The payout bobbin delivers an unsized carbon fiber material 560 to the process; the fiber is tensioned via tensioner 507. For this example, the carbon fiber is processed at a linespeed of 2 ft/min.
Unsized fiber 560 is delivered to plasma treatment station 515. For this example, atmospheric plasma treatment is utilized in a ‘downstream’ manner from a distance of 1 mm from the spread carbon fiber material. The gaseous feedstock is comprised of 100% helium.
Plasma enhanced fiber 565 is delivered to barrier coating station 520. In this illustrative example, a siloxane-based barrier coating solution is employed in a dip coating configuration. The solution is ‘Accuglass T-11 Spin-On Glass’ (Honeywell International Inc., Morristown, N.J.) diluted in isopropyl alcohol by a dilution rate of 40 to 1 by volume. The resulting barrier coating thickness on the carbon fiber material is approximately 40 nm. The barrier coating can be applied at room temperature in the ambient environment.
Barrier coated carbon fiber 590 is delivered to air dry station 525 for partial curing of the nanoscale barrier coating. The air dry station sends a stream of heated air across the entire carbon fiber spread. Temperatures employed can be in the range of 100° C. to about 500° C.
After air drying, barrier coated carbon fiber 590 is delivered to catalyst application station 530. In this example, an iron oxide-based CNT forming catalyst solution is employed in a dip coating configuration. The solution is ‘EFH-1’ (Ferrotec Corporation, Bedford, N.H.) diluted in hexane by a dilution rate of 200 to 1 by volume. A monolayer of catalyst coating is achieved on the carbon fiber material. ‘EFH-1’ prior to dilution has a nanoparticle concentration ranging from 3-15% by volume. The iron oxide nanoparticles are of composition Fe₂O₃and Fe₃O₄and are approximately 8 nm in diameter.
Catalyst-laden carbon fiber material 595 is delivered to solvent flash-off station 535. The solvent flash-off station sends a stream of air across the entire carbon fiber spread. In this example, room temperature air can be employed in order to flash-off all hexane left on the catalyst-laden carbon fiber material.
After solvent flash-off, catalyst-laden fiber 595 is finally advanced to CNT-growth station 540. In this example, a rectangular reactor with a 12 inch growth zone is used to employ CVD growth at atmospheric pressure. 98.0% of the total gas flow is inert gas (Nitrogen) and the other 2.0% is the carbon feedstock (acetylene). The growth zone is held at 750° C. For the rectangular reactor mentioned above, 750° C. is a relatively high growth temperature. The addition of the barrier coating prevents sintering of the catalyst nanoparticle at CNT growth temperatures, allowing for effective high density CNT growth on the surface of the carbon fiber.
CNT coated fiber 597 is wound about uptake fiber bobbin 550 for storage. CNT coated fiber 597 is loaded with CNTs approximately 50 μm in length and is then ready for use in composite materials.
It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other processes, materials, components, etc.
Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.

Claims

1. A method comprising:

(a) conformally depositing a barrier coating on at least one surface of a substrate; said barrier coating provided in liquid form;

(b) embedding a plurality of nanoparticles in said barrier coating to a selected depth creating an embedded portion of each of said plurality of nanoparticles; and

(c) fully curing said barrier coating after embedding said plurality of nanoparticles; said embedded portion of each of said plurality of nanoparticles being in continuous contact with said cured barrier coating.

2. The method of claim 1, wherein conformally depositing said barrier coating and embedding said plurality of nanoparticles is performed simultaneously.

3. The method of claim 1, wherein a thickness of said barrier coating is about the same or less than the effective diameter of said plurality of nanoparticles.

4. The method of claim 1, wherein a thickness of said barrier coating is in a range from between about the same as the effective diameter of said plurality of nanoparticles up to about 5,000% greater than the effective diameter of said plurality of nanoparticles.

5. The method of claim 1, wherein said substrate is treated with a plasma prior to conformally depositing said barrier coating.

6. The method of claim 1, wherein said embedded plurality of nanoparticles maintain an exposed surface when said plurality of nanoparticles are in surface contact with said substrate.

7. The method of claim 1, wherein the step of depositing said barrier coating is accomplished by a technique selected from dip coating and spraying.

8. The method of claim 1, wherein said barrier coating comprises a material selected from a siloxane, a silane, an alumina, a silicon carbide ceramic, and a metal; said barrier coating being chosen for its ability to adhere to said substrate.

9. The method of claim 1 further comprising partially curing said barrier coating prior to embedding said plurality of nanoparticles.

10. The method of claim 1, wherein said substrate is selected from the group consisting of a metal, a ceramic, a silica wafer, a fiber, a graphite sheet, and a high temperature plastic.

11. The method of claim 1 further comprising heating the environment about said embedded plurality of nanoparticles, in the presence of a feedstock material, to a temperature promoting growth of a plurality of nanostructures from said feedstock material, said embedded plurality of nanoparticles catalyzing said growth; wherein the temperature is sufficient to cause agglomeration of said plurality of nanoparticles in the absence of said barrier coating.

12. The method of claim 11, wherein said nanoparticles comprise a transition metal.

13. The method of claim 11, wherein said feedstock material is a carbon source.

14. The method of claim 11, wherein said nanostructure is a carbon nanotube.

15. The method of claim 1, wherein said nanoparticles comprise a clay.

16. The method of claim 1, wherein said nanoparticles comprise silica or alumina.

17. The method of claim 1, wherein said nanoparticles range in size from between about 0.5 nm to about 500 nm.

18. A method comprising:

(a) depositing a plurality of nanoparticles on at least one surface of a substrate;

(b) conformally depositing a barrier coating over said substrate and at least a portion of each of said plurality of nanoparticles, creating an embedded portion of each of said plurality of nanoparticles, said barrier coating provided in liquid form; and

(c) fully curing said barrier coating;

wherein said plurality of nanoparticles are in surface contact with said substrate and said embedded portion of each of said plurality of nanoparticles is in continuous contact with said cured barrier coating.

19. The method of claim 18, wherein a thickness of said barrier coating is about the same or less than the effective diameter of said plurality of nanoparticles.

20. The method of claim 18, wherein a thickness of said barrier coating is in a range from between about the same as the effective diameter of said plurality of nanoparticles up to about 100% greater than the effective diameter of said plurality of nanoparticles.

21. The method of claim 18, wherein said substrate is treated with a plasma prior to depositing said plurality of nanoparticles.

22. The method of claim 18, wherein the step of depositing said barrier coating is accomplished by a technique selected from dip coating and spraying.

23. The method of claim 18, wherein said barrier coating comprises a material selected from a siloxane, a silane, an alumina, a silicon carbide ceramic, and a metal; said barrier coating being chosen for its ability to adhere to said substrate.

24. The method of claim 18, wherein said substrate is a metal, a ceramic, a silica wafer, a fiber, a graphite sheet, and a high temperature plastics.

25. The method of claim 18 further comprising heating the environment about said embedded plurality of nanoparticles, in the presence of a feedstock material, to a temperature promoting catalyzed growth of a plurality of nanostructures from said feedstock material, said embedded plurality of nanoparticles catalyzing said growth; wherein the temperature is sufficient to cause agglomeration of said plurality of nanoparticles in the absence of said barrier coating.

26. The method of claim 25, wherein said plurality of nanoparticles comprise a transition metal.

27. The method of claim 25, wherein said feedstock material is a carbon source.

28. The method of claim 25, wherein said nanostructure is a carbon nanotube.

29. The method of claim 18, wherein said nanoparticles comprise a clay.

30. The method of claim 18, wherein said nanoparticles comprise silica or alumina.

31. The method of claim 18, wherein said nanoparticles range in size from between about 0.5 nm to about 500 nm.

32. An article comprising:

a substrate having a cured barrier coating conformally disposed on at least one surface of said substrate; and

a plurality of nanoparticles embedded to a selected depth in said barrier coating creating an embedded portion of each of said plurality of nanoparticles, said embedded portion of each of said plurality of nanoparticles in continuous contact with said cured barrier coating.

33. The article of claim 32, wherein said embedded plurality of nanoparticles are in surface contact with said substrate.

34. The article of claim 32, wherein said barrier coating comprises a material selected from a siloxane, a silane, an alumina, a silicon carbide ceramic, and a metal; said barrier coating being chosen for its ability to adhere to said substrate.

35. The article of claim 32, wherein said substrate is selected from the group consisting of a metal, a ceramic, a silica wafer, a fiber, a graphite sheet, and a high temperature plastic

36. The article of claim 32, wherein said plurality of nanoparticles comprise a transition metal.

37. The article of claim 32, wherein said plurality of nanoparticles comprise a clay.

38. The article of claim 32, wherein said plurality of nanoparticles comprise silica or alumina.

39. The article of claim 32, wherein said nanoparticles range in size from between about 0.5 nm to about 500 nm.

40. The article of claim 32, wherein a thickness of said barrier coating is about the same or less than the effective diameter of said plurality of nanoparticles.

41. The article of claim 32, wherein a thickness of said barrier coating is in a range from between about the same as the effective diameter of said plurality of nanoparticles up to about 5000% greater than the effective diameter of said plurality of nanoparticles.