BACKGROUND

Field of the Disclosure

The present application relates to methods of fabricating field emission cathode devices and, more particularly, to methods of forming field emission cathodes incorporating a carbon nanotube matrix material modified to improve adhesion between the material and a substrate and to improve field emission characteristics of the cathode and field emission cathode devices implementing such cathodes.

Description of Related Art

A field emission cathode device generally includes a cathode substrate (usually comprised of a metal or other conducting material such as alloy, conductive glass, metalized ceramics, doped silicon), a layer of a field emission material (e.g., nanotubes, nanowires, graphene) disposed on the substrate, and, if necessary, an additional layer of an adhesion material disposed between the substrate and the field emission material. Some typical applications of a field emission cathode device include, for example, electronics operable in a vacuum environment, field emission displays, and X-ray tubes.

Carbon nanotubes may be used in the fabrication of cold field emission cathodes. However, the matrix materials comprising the carbon nanotubes used to produce such cathodes have less than desirable characteristics, particularly regarding adhesion strength, conductivity, cleanliness, and defects of the carbon nanotubes. For example, such field emission cathodes are typically hard to totally clean, so as to be free of any loose particles, thereby resulting in poor adhesion strength, not only between the carbon nanotubes and the matrix materials, but also between the matrix materials and the substrates. Additionally, after a standard deposition and activation process, the cathode is still capable of releasing loose carbon nanotubes and other small impurities and/or matrix particles over time, which may severely contaminate the vacuum environment, causing vacuum electrical arcing and electrode shortage.

Thus, there is a need for a process for improving the adhesion of the carbon nanotubes within a matrix material and between the matrix materials and the surface of substrates, which may improve the uniformity of an electric field at a cathode surface, reduce the effects of ion bombardment and oxidation, increase conductivity of cathodes, improve the work function of the carbon nanotubes, and improve the life time of the cathodes.

SUMMARY OF THE DISCLOSURE

The above and other needs are met by aspects of the present disclosure which includes, without limitation, the following example embodiments and, in one particular aspect, a method of forming a field emission cathode device, where the method includes forming a field emission material by introducing a plurality of carbon nanotubes to a matrix material, depositing a layer of the field emission material on to at least a portion of a substrate to form the cathode, exposing the cathode to an activation process, and depositing a layer of a thin metal film on to the cathode.

Another example aspect provides another method of forming a field emission cathode, where the method includes depositing a layer of a field emission material on to at least a portion of a substrate to form the cathode, where the field emission material comprises a plurality of carbon nanotubes in a matrix material, exposing the cathode to an activation process, and depositing a layer of a thin metal film on to the cathode.

Yet another example aspect provides for a field emission cathode device, where the cathode is fabricated in accordance with any one of the proceeding aspects to obtain a cathode device having improved uniformity of an electric field at a cathode surface, reduced impact from ion bombardment and oxidation, increased conductivity, improved work function of the carbon nanotubes, and improved cathode life time.

The present disclosure thus includes, without limitation, the following example embodiments:

Example Embodiment 1: A method of forming an electron field emission cathode, comprising forming a field emission material by introducing a plurality of carbon nanotubes to a matrix material; depositing a layer of the field emission material on to at least a portion of a substrate to form the electron field emission cathode; exposing the cathode to an activation process; and depositing a layer of a thin metal film on to the cathode.

Example Embodiment 2: The method of any preceding example embodiment, or combinations thereof, wherein depositing the layer of the field emission material comprises depositing the layer of the field emission material on to the at least a portion of the substrate by a printing process or an electrophoretic deposition.

Example Embodiment 3: The method of any preceding example embodiment, or combinations thereof, wherein depositing the layer of the field emission material comprises depositing the layer of the field emission material on to the at least a portion of the substrate by the printing process comprising an inkjet printing process or a screen printing process.

Example Embodiment 4: The method of any preceding example embodiment, or combinations thereof, wherein depositing the layer of the thin metal film comprises depositing the layer of the thin metal film on to the cathode via a physical vapor deposition process.

Example Embodiment 5: The method of any preceding example embodiment, or combinations thereof, wherein depositing the layer of the thin metal film comprises depositing the layer of the thin metal film on to the cathode via the physical vapor deposition process selected from the group consisting of e- beam evaporation, ion assisted deposition, thermal evaporation, pulse laser deposition, magnetron sputtering, or ion beam sputtering.

Example Embodiment 6: The method of any preceding example embodiment, or combinations thereof, wherein depositing the layer of the thin metal film comprises depositing the layer of the thin metal film comprised of a pure metal or a metal alloy on to the cathode.

Example Embodiment 7: The method of any preceding example embodiment, or combinations thereof, wherein exposing the cathode to an activation process comprises exposing the cathode to the activation process, the activation process removing a portion of the matrix material from a surface of the layer of the field emission material and orienting the carbon nanotubes upwardly via taping, chemical etching, electrochemical etching, or particle blasting.

Example Embodiment 8: The method of any preceding example embodiment, or combinations thereof, wherein depositing the layer of the field emission material comprises depositing the layer of the field emission material on to the at least a portion of the substrate comprising a metal, a glass, or a ceramic.

Example Embodiment 9: The method of any preceding example embodiment, or combinations thereof, comprising repeating the step of depositing the layer of a thin metal film on to the cathode to provide a plurality of thin film metal layers on the cathode.

Example Embodiment 10: A method of forming an electron field emission cathode, comprising depositing a layer of a field emission material on to at least a portion of a substrate to form the electron field emission cathode, wherein the field emission material comprises a plurality of carbon nanotubes in a matrix material; exposing the cathode to an activation process; and depositing a layer of a thin metal film on to the cathode.

Example Embodiment 11: The method of any preceding example embodiment, or combinations thereof, wherein depositing a layer of a field emission material comprises depositing the layer of the field emission material on to the at least a portion of the substrate by a printing process or an electrophoretic deposition.

Example Embodiment 12: The method of any preceding example embodiment, or combinations thereof, wherein depositing a layer of a field emission material comprises depositing the layer of the field emission material on to the at least a portion of the substrate by the printing process comprising ink-jet printing or screen printing.

Example Embodiment 13: The method of any preceding example embodiment, or combinations thereof, wherein depositing the layer of the thin metal film comprises depositing the layer of the thin metal film on to the cathode via a physical vapor deposition process.

Example Embodiment 14: The method of any preceding example embodiment, or combinations thereof, wherein depositing the layer of the thin metal film comprises depositing the layer of the thin metal film on to the cathode via the physical vapor deposition process selected from the group consisting of e- beam evaporation, ion assisted deposition, thermal evaporation, pulse laser deposition, magnetron sputtering, or ion beam sputtering.

Example Embodiment 15: The method of any preceding example embodiment, or combinations thereof, wherein depositing the layer of the thin metal film comprises depositing the layer of the thin metal film comprising a pure metal or a metal alloy on to the cathode.

Example Embodiment 16: The method of any preceding example embodiment, or combinations thereof, wherein exposing the cathode to the activation process comprises exposing the cathode to the activation process, the activation process removing a portion of the matrix material from a surface of the layer of field emission material and orienting the carbon nanotubes upwardly via taping, via chemical etching, electrochemical etching, or particle blasting,

Example Embodiment 17: The method of any preceding example embodiment, or combinations thereof, wherein depositing the layer of the field emission material comprises depositing the layer of the field emission material on to the at least a portion of the substrate comprising a metal, a glass, or a ceramic.

Example Embodiment 18: A field emission cathode device comprising a cathode fabricated in accordance with the method of any preceding example embodiment, or combinations thereof.

These and other features, aspects, and advantages of the present disclosure will be apparent from a reading of the following detailed description together with the accompanying drawings, which are briefly described below. The present disclosure includes any combination of two, three, four, or more features or elements set forth in this disclosure, regardless of whether such features or elements are expressly combined or otherwise recited in a specific embodiment description herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosure, in any of its aspects and embodiments, should be viewed as intended, namely to be combinable, unless the context of the disclosure clearly dictates otherwise.

It will be appreciated that the summary herein is provided merely for purposes of summarizing some example aspects so as to provide a basic understanding of the disclosure. As such, it will be appreciated that the above described example aspects are merely examples and should not be construed to narrow the scope or spirit of the disclosure in any way. It will be appreciated that the scope of the disclosure encompasses many potential aspects, some of which will be further described below, in addition to those herein summarized. Further, other aspects and advantages of such aspects disclosed herein will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the described aspects.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 schematically illustrates an example of a field emission cathode and the nature of the field emission material deposition layer engaged with the cathode substrate, according to one or more aspects of the present disclosure;

FIG. 2 schematically illustrates a field emission cathode having a substrate and a carbon nanotube matrix layer after activation, according to one or more aspects of the present disclosure;

FIG. 3 schematically illustrates the field emission cathode according to an aspect of FIG. 2 after modification, according to one or more aspects of the present disclosure; and

FIG. 4 illustrates one example of a method of forming a field emission cathode, according to one or more aspects of the present disclosure. DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all aspects of the disclosure are shown. Indeed, the disclosure may be embodied in many different forms and should not be construed as limited to the aspects set forth herein; rather, these aspects are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

FIG. 1 illustrates one example of a field emission cathode 100 that includes a substrate 102 and a layer of a field emission material 104 disposed on the substrate 102, and, if necessary, an additional layer of an adhesion material (not shown) disposed between the substrate 102 and the field emission material 104. The substrate 102 may be made of an electrically conductive material, such as a metallic material, such as a solid metal or alloy (e.g., stainless steel, doped silicon), conductive glass (e.g., Indium Tin Oxide (ITO) coated glass or other fused glass having a conductive coating on the surface); or a conductive ceramic (e.g., a metalized ceramic, such as aluminum oxide, beryllium oxide, and aluminum nitride). The field emission material 104 comprises a plurality of carbon nanotubes disposed within a matrix material, as is generally known in the art. The layer of field emission material 104 is formed via deposition of the field emission material on to the substrate 102 by, for example, spray coating, dip coating, inkjet printing, screen printing, or electrophoresis.

FIG. 2 illustrates a field emission cathode 200 having a carbon nanotube/matrix composite film 204 deposited on to a substrate 202, similar to those described hereinabove. The cathode of FIG. 2 has been exposed to an activation process, where a portion of the matrix material has been removed and the carbon nanotubes 206 are generally oriented upwardly from the film layer 204. The field emission material 204 may be applied by, for example, a printing process or electrophoretic deposition.

FIG. 3 depicts the cathode 200 of FIG. 2 after it has been exposed to a modification process, such as the application of a coating 208 of a metal film on a top surface of the cathode 200. By depositing a layer of a metal fdm on the surface of a cathode as shown in FIG. 3, the adhesion between the carbon nanotubes and the matrix material can be significantly improved, at least in part, because the roots of the carbon nanotubes 208 will now be embedded in the modification layer or coating 208. Additionally, as shown in FIG. 3, the layer of field emission material is further secured to the substrate 202 by the encapsulation of the layer 204 by the coating 208.

Moreover, the modification process (300 in FIG. 4) can significantly increase the conductivity of layers of the field emission material 204 and improve the uniformity of the electric field at the cathode surface. The modification process can also prevent loose particles (see FIG. 1) from releasing out of the cathode surface during a vacuum device operation, which may greatly decrease the chance of arcing and damage to the vacuum device, thereby resulting in a device with a longer life time. In some cases, the modification process may modify or correct certain defects within the carbon nanotubes within the matrix material, improving the work function of the carbon nanotubes and the field emission characteristics of the cathode. Furthermore, the modification process can be repeated to form multiple layers 208 of thin metal films on the cathode surface. In some cases, the multiple layers may have the same composition or each layer may have a different composition to suit a particular application. Additionally, certain metals or metal alloys may be deposited on the surfaces of the carbon nanotubes 206 to modify or correct certain defects therein. The modified carbon nanotubes may produce a larger field emission current and have a longer life time.

FIG. 4 illustrates a method 300 of using carbon nanotubes in the fabrication of field emission cathodes. In one aspect of the method, a substrate, such as those described hereinabove, is provided to a deposition process (step 310). The substrate may be provided to the appropriate equipment via, for example, a robotic material handling system or manually by a user. The substrate is configured to receive a layer of a field emission material thereon.

The field emission material is formed by mixing a plurality of carbon nanotubes into a matrix material as known in the art (step 320). During the deposition step (step 330), the field emission material is deposited on to the substrate. The field emission material may be deposited on to the substrate via a printing process, such as, for example, inkjet printing or screen printing, or by an electrophoretic deposition process.

After the field emission material has been deposited on to the substrate, the cathode is activated. In some cases, the field emission material layer or cathode may be exposed to an additional process prior to activation, such as, for example, a curing or drying process. The activation process (step 340) is performed to remove a portion of the matrix material from a surface of the layer of the field emission material to better expose/align the carbon nanotubes. In some cases, the activation process includes taping, chemical etching, electrochemical etching, or particle blasting.

The essentially completed cathode is then exposed to a modification process (step 350) that includes depositing one or more thin metal fdms on to a surface of the layer of the field emission material and/or a surface of the cathode. The process/step 350 may be carried out via a physical vapor deposition process, such as, for example, e-beam evaporation, ion assisted deposition, thermal evaporation, pulse laser deposition, magnetron sputtering, or ion beam sputtering. In some cases, step 350 is repeated any number of times to suit a particular application. For example, multiple layers of different metals may be applied to the surface(s). The thin metal film(s) may include a pure metal or a metal alloy, for example, a metal or alloy with a melting point higher than 600 °C.

Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these disclosed embodiments pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that embodiments of the invention are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the invention. Moreover, although the foregoing descriptions and the associated drawings describe example embodiments in the context of certain example combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative embodiments without departing from the scope of the disclosure. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated within the scope of the disclosure. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. It should be understood that although the terms first, second, etc. may be used herein to describe various steps or calculations, these steps or calculations should not be limited by these terms. These terms are only used to distinguish one operation or calculation from another. For example, a first calculation may be termed a second calculation, and, similarly, a second step may be termed a first step, without departing from the scope of this disclosure. As used herein, the term “and/or” and the “/” symbol includes any and all combinations of one or more of the associated listed items.

As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes”, and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Therefore, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.