Patent Grant
12334296
The present invention is directed to the field of high current field emission cathodes and more particularly to high current field emission cathodes based on nanomaterials.
Nanomaterials, especially carbon nanotubes (CNTs), are known to have excellent electron field emission properties due to their unique structure, outstanding mechanical strength, excellent electric and thermal conductivity, superb thermal stability, and good chemical stability. CNTs have very low electron emission threshold fields (1˜2V/μm) and can emit electrons at very high current densities. Electron emitting currents as high as 0.2 mA have been drawn from a single CNT in the laboratory.
Due to their superb field emission properties, tremendous efforts have been made in the past few decades to develop novel vacuum electronic devices based on CNT field emitters. For example, CNT-based Multi-Beam X-Ray (MBX) tubes enable stationary X-ray scanners for medical and security applications, which provide significant advantages over the conventional rotating gantry X-ray systems on the market today. These advantages include higher energy efficiency, higher imaging resolution, faster scanning, smaller system footprints, improved safety, reduced system acquisition and maintenance costs, etc. Traveling wave tubes (TWTs) are key components for high frequency and high-power communications, such as for radar and electronic warfare. TWTs in active service today use thermal cathodes as electron sources. The heat generated by a thermionic cathode is difficult to dissipate, significantly limiting the efficiency, current density, and lifetime of the TWT. A TWT without a cathode requiring heating would overcome all of technical difficulties mentioned above. Payload weight may be lowered because high current, high voltage transformers would not be needed. Such a TWT could also operate instantaneously. The beam current would be modulated directly at the cathode and focusing electron beams would become much easier.
Despite decades of research and development, the above-mentioned field emission devices are still at a prototyping stage. Their commercial success is mostly hindered by technical challenges in producing high current CNT cathodes. Although extremely high current densities (>1000 A/cm2) can be achieved from a single CNT in the laboratory, high current densities cannot be readily obtained in the macroscopic scale at the cathode level because of cathode non-uniformities and the CNT emitter screening effect. Cathode non-uniformity can be caused by CNT material non-uniformity, composite mixture non-uniformity, cathode deposition non-uniformity, and cathode post processing (sintering and activation) non-uniformity, etc. These non-uniformities cause the electron emission to concentrate on a limited number of emission hot spots, significantly reducing the overall current density of the cathode when the total emission current is normalized by the cathode emission area. The emitter screening effect reduces the electric field enhancement of CNT emitters, and also significantly reduces the emission current from CNT cathodes. Presently, state-of-art CNT cathodes can operate stably at a current density up to 100 mA/cm2. This is not enough for key high-end field emission MBX tube applications nor most microwave applications. One example is full body medical CT scanners, where 0.5 up to 1 A/cm2 is required. Many TWT applications also require over 1 A/cm2.
Thus, there is a need for high current field emission cathodes, and methods to make the same, to address the above mentioned challenges, and others, associated with conventional fabrication techniques.
The present invention describes processes and methods for producing high current field emission cathodes including nanomaterials. It provides a path forward for the successful development of numerous novel vacuum electronic devices, including field emission-based flat panel displays, MBX, and microwave tubes, etc.
The present disclosure provides a process for producing high current field emission cathodes based on nanomaterials with a specific multi-layer structure. The multi-layer structure improves emission uniformity and emission stability of the nanomaterials-based field emission cathodes. The present disclosure also discloses processes for patterning field emission cathodes based on nanomaterials to alleviate the electric field screening effect on nano-emitters.
According to one embodiment, a method comprises forming a uniform composite mixture of a nonconductive matrix material and a conductive filler material, depositing the composite mixture uniformly on a cathode substrate, and subjecting the deposition to a number of heating cycles to form an adherent, electrically resistive layer on the cathode substrate; forming a uniform composite mixture of adhesion promoting materials, depositing the composite mixture uniformly on top of the electrically resistive layer, and subjecting the deposition to a number of heating cycles to form an adherent, adhesion promoting layer on the cathode substrate; and forming a uniform composite mixture including electron emitting nanomaterials, depositing the composite mixture uniformly on top of the adhesion promoting layer on the cathode substrate, and subjecting the deposition to a number of heating cycles to form an electrically resistive layer backed (with an electrically resistive layer as an under layer), adherent, nanomaterial-based electron emissive layer on the cathode substrate.
The electrically resistive layer adds a ballistic resistor to each individual nano-emitter, which suppresses emission hot spots and improves overall emission uniformity. It also limits the amount of current that passes through each individual nano-emitter, which protects the individual nano-emitters from over-current damage and improves the lifetime of the overall cathode. The adhesive promotion layer improves the adhesion of nano-emitters to the electrically resistive layer, which also serves to improve the stability and lifetime of the nanomaterials-based cathode.
According to yet another embodiment, the present disclosure also provides a method for producing high current field emission cathodes based on nanomaterials with a patterned structure. This method comprises forming a uniform composite mixture of a nonconductive matrix material and a conductive filler material, depositing the composite mixture uniformly on a cathode substrate to form a patterned deposition, where the patterned deposition includes islands of deposited composite mixture with well-defined gaps between the islands, subjecting the deposition to a number of heating cycles to form an adherent, and patterned electrically resistive island layer on the cathode substrate; forming a uniform composite mixture of adhesion promoting materials, depositing the composite mixture uniformly only on top of the islands on the cathode substrates, subjecting the deposition to a number of heating cycles to form adherent islands of adhesion promoting layer on the cathode substrate; and forming a uniform composite mixture of electron emitting nanomaterials, depositing the composite mixture uniformly only on top of the islands on the cathode substrate, and subjecting the deposition to a number of heating cycles to form electrically resistive layer-backed, and adherent islands with a nanomaterials-based electron emissive layer on the cathode substrate. Each set of heating cycles, in various embodiments, comprises three successively hotter firings.
This patterned cathode design can effectively reduce the electrical field screening effect between nano-emitters and serves to improve the overall emission current density of the nanomaterials-based field emission cathodes.
Exemplary processes that are consistent with the principles of the present disclosure, and corresponding structures and devices, are described as follows. In general, methods of the present disclosure can include a combination of some or all of the following steps, (1) forming a uniform composite mixture of nonconductive matrix materials and conductive filler materials, (2) depositing the composite mixture uniformly on a cathode substrate, (3) heating the composite deposition to form an adherent, electrically resistive layer on the cathode substrate, (4) forming a uniform composite mixture of adhesion promoting materials, (5) depositing the composite mixture uniformly on top of the electrically resistive layer, (6) heating the composite deposition to form an adherent, adhesion promoting layer on the cathode substrate, (7) forming a uniform composite mixture of electron emitting nanomaterials, (8) depositing the composite mixture uniformly on top of the adhesion promoting layer, and (9) heating the composite deposition to form an adherent, electrically resistive layer backed, nanomaterial-based field emissive layer on the cathode substrate. In some embodiments, rather than forming a continuous deposition, a patterned deposition is formed with individual electron emissive islands and well-defined gaps between them is preferred in the above deposition process to achieve a high emission current from the produced field emission cathodes.
The method 105 optionally begins with a step of depositing the electrically resistive layer 115 on the cathode substrate 110. Depositing the electrically resistive layer 115 begins by forming a uniform composite mixture of one or more nonconductive matrix materials and one or more conductive filler materials from pre-formed raw material or raw material mixture. The nonconductive matrix materials can include, for instance, glass powders (e.g., sealing glass powders) containing various oxides, such as silicon oxide (SiO2), aluminum oxide (Al2O3), calcium oxide (CaO), barium oxide (BaO), boron oxide (B2O3), zirconium oxide (ZrO2), magnesium oxide (MgO), sodium oxide (Na2O), potassium oxide (K2O), iron oxide (Fe2O3), lead oxide (PbO), nickel oxide (NiO), tin oxide (SnO2), etc. The nonconductive matrix materials can also be a mixture of glass powders containing various fluorides, such as zirconium fluoride (ZrF4), aluminum fluoride (AlF2), and beryllium fluoride (BeF2), etc. The nonconductive matrix materials can also include glass powders containing various chlorides such as calcium chloride (CaCl2)), in some embodiments. The nonconductive matrix materials can also be any mixture of oxides, fluorides, and chlorides. Preferably, the nonconductive matrix materials are characterized by high melting temperatures (>400° C.) and low vapor pressures (<1×10−6 torr at 600° C.).
The conductive filler materials can be various conductive materials, including CNTs, conductive nanowires (copper (Cu) nanowires, silver (Ag) nanowires, cerium hexaboride (CeB6) nanowires, and lanthanum hexaboride (LaB6) nanowires, etc.), conductive carbon black, metal powders such as silver (Ag), copper (Cu), iron (Fe), Cobalt (Co), Nickel (Ni), molybdenum (Mo), and tungsten (W), and oxide powders including, for example, indium oxide (In2O3), zinc oxide (ZnO), tin oxide (SnO2), gallium oxide (Ga2O3), cadmium oxide (CdO), and indium tin oxide (ITO)), or any mixture of these. Preferably, the conductive filler materials are also characterized by high melting temperatures (>800° C.) and low vapor pressures (<1×10−6 torr at 600° C.). The conductive filler materials in the electrically resistive layer 115 can either comprise nanomaterials or not.
The cathode substrate 110 should be high vacuum compatible material with a high melting temperature and a low vapor pressure. It can comprise a metal, such as Cu, 304 stainless steel, Mo, etc., and alloys thereof. Substrates 110 can also be made from metalized nonconductive materials, including Ag, Cu, Ni, or Mo metalized silicon (Si), silicon nitride, quartz, sodium lime glass, or alumina (Al2O3, such as with 90% or better purity).
The composite mixture of adhesion promoting materials can comprise, in some embodiments, a large concentration of a glass powder and a small amount of nanomaterials, where the glass powder comprises a mix of compounds. The glass powder can either be the same glass powder of nonconductive matrix materials used to form the electrically resistive layer 115, or can be a glass powder with a different composition. The glass powder used here is selected from the same oxides, fluorides, chlorides, and their mixtures, as listed previously.
The composite mixture of electron emitting nanomaterials consists of a large concentration of electron emissive nanomaterials and a small amount of a glass powder. The glass powder here can either be the same as either glass powder used in the prior layers or can be of a different composition, and is selected from the same oxides, fluorides, and chlorides listed previously.
The nanomaterials used herein optionally can comprise single-walled carbon nanotubes (SWNTs) with diameters between 1 and 2 nm, small diameter multiwalled carbon nanotubes (SDMWNTs) with diameters between 2 to 5 nm.
The above-mentioned CNT raw materials can be fabricated according to a number of different techniques known in the art, including laser ablation (LA) techniques, chemical vapor deposition (CVD) techniques, and arc discharge (AD) techniques. After fabrication, the raw CNTs are subjected to a purification process. A number of techniques for purifying raw CNTs can be used to produce high purity CNTs, including an acid wash such as with hydrochloride acid (HCl), sulfuric acid (H2SO4), or nitric acid (HNO3), or a hydrogen peroxide (H2O2) reflux, or an air oxidation followed by an acid wash.
Besides pristine CNTs, the nanomaterials used above can also comprise doped CNTs including nitrogen (N), boron (B), or sulfur(S), for example, as a dopant. N doped CNTs have a reduced work function, which can contribute to an enhanced emission current. Doped CNTs can be produced by in situ doping, e.g., introducing doping agents when the CNTs are synthesized. Alternatively, doped CNTs can be produced by a substitutional doping reaction between dopants and pristine CNTs.
The nanomaterials used above can also comprise materials other than CNTs including nanotubes that are not principally made of carbon, and nanowires. CeB6 and LaB6 nanowires are two good examples due to their exceptionally low work functions (2.5 eV and 2.7 eV respectively). LA, CVD, and AD techniques can also be used to produce CeB6 and LaB6 nanowires. The as-grown, raw nanowires can also be purified by an acid wash with the acids noted above, followed by sonication, and a filtration process. For field emission applications, the diameter of the CeB6 and LaB6 nanowires should be less than about 20 nm, and preferably, less than about 10 nm. For these applications, the lengths of CeB6 and LaB6 nanowires should be greater about 1 μm, and preferably from about 2 to 5 μm.
Compared with pristine CNTs, doped CNTs and other nanowires often have more structural defects and less structural integrity which can degrade the field emission properties of these materials. In some embodiments a high temperature annealing process is used to remove structural defects and improve structural integrity. According to one embodiment, the purified nanowires or doped CNTs are annealed in a controlled environment, for instance, in an inert gas environment such as argon (Ar), nitrogen (N2), mixtures thereof, or a high vacuum (e.g., 2×10−4 torr or better), at high temperature for an extended period of time to improve their structural integrity. The annealing temperature of a nanomaterial, in some embodiments, should be between about 50% to 90%, and preferably about 80% of the melting temperature of that material. For example, about 2000° C. is an appropriate annealing temperature for CeB6 nanowires and about 1800° C. is an appropriate annealing temperature for LaB6 nanowires. Since the melting temperature for carbon materials is extremely high (>3000° C.), doped CNTs can be annealed at temperature of at least about 2000° C., and preferably about 3000° C. The annealing time should be as long as possible, and at least 3 hours, to achieve the optimal results.
To form uniform composite mixtures in the steps above, glass powders, metal particles, and other conducting oxide particles with small particle sizes should be used. Ideally, materials with submicron particle sizes should be used. Various mechanical milling processes can be used to process solid materials to reduce their particle sizes, including high shear milling techniques such as planetary, rotary ball milling, 3 roll milling, and impact ball milling techniques.
To form a uniform composite mixture in the steps above, mixture components (e.g., glass powders, metal particles, conducting oxide particles, CNTs, doped CNTs, CeB6 nanowires, and LaB6 nanowires, etc. must be mixed thoroughly. Again, high shear milling techniques (such as planetary, rotary ball milling, and 3-roll milling) can be used to achieve this goal. According to one embodiment, sub micron-sized glass powders are mixed with CNTs and an organic solvent and processed by a planetary, rotary ball mill 300 at high power (such as up to 500 RPM for at least 30 minutes) to achieve a highly uniform mixture with well dispersed CNTs. In another embodiment, sub micron-sized glass powders mixed with CNTs and an organic solvent are processed by a 3-roll mill 350 at a high shearing force of at least 5 N/mm and preferably 10 N/mm or more, to achieve a highly uniform mixture with well dispersed CNTs. CNTs and nanowires tend to aggregate together due to van der Waals force and are very difficult to disperse. High shear force is often required to break down CNT or nanowire agglomerations and disperse them. In some embodiments, repeated processing at a high shear force is used to process CNTs or nanowire mixtures to achieve uniform mixtures with well dispersed CNTs, and/or nanowires.
The deposition of composite mixtures onto the cathode substrates as described above can be performed using different deposition techniques, including electrophoretic deposition (EPD), screen printing (SP), stencil printing, spray coating, doctor blade coating, and slot die coating, etc.
The second step is a deposition process, e.g., electrophoresis. In such a method, as shown in
The SP process, shown in
The heating process following each of the composite depositions described above can include three firing cycles, in some embodiments. The first firing cycle is in air at a temperature from 80° C. to about 150° C. for up to about 30 minutes. The first firing cycle removes the organic solvent trapped in the composite deposition. The second firing cycle can be performed in an inert gas (e.g., Ar, N2, or mixtures thereof) at a temperature up to about 500° C. for 30 minutes. This step will burn out any polymeric binder or dispersant in the composite deposition. The last firing cycle is in a high vacuum (1×10−5 torr or better) with a temperature up to about 1500° C. for up to 3 hours. The third firing cycle serves to activate the glass powders in the composite deposition and produce a fully solidified, and adherent composite layer on the cathode substrate 110. In various embodiments, rather than performing three firings after each deposition it is also possible to perform all three firings after the final deposition, or to perform the first two firings after the first two depositions and then all three firings after the third deposition. Other combinations can also be used.
The high current field emission cathodes 100 disclosed herein have a unique multi-layer structure. The electrically resistive layer 115 contains a mixture of a nonconductive material and a conductive material. The weight ratio of the nonconductive material to the conductive material is preferably in the range from about 1000:1 to about 1:1. More preferably, this ratio should be in the range from about 100:1 to about 5:1. The thickness of the electrically resistive layer 115 should be less than about 30 μm and preferably less than about 10 μm. The adhesion promoting layer 120 contains a large amount of a nonconductive material and a small amount of nanomaterials. The weight ratio of the nonconductive material to the nanomaterial in adhesion promoting layer 120 is preferably in the range from about 100:1 to about 2:1. More preferably, this ratio should be in the range from about 10:1 to about 5:1. The thickness of the adhesion promoting layer 120 should be less than about 10 μm and preferably less than about 5 μm. The electron emissive layer 125 contains a large amount of nanomaterials and a small amount of nonconductive material. The weight ratio of the nonconductive material to the nanomaterial is preferably in the range from about 1:1000 to about 1:1, and more preferably in the range from about 1:200 to about 1:20. The thickness of the electron emissive layer 125 should be less than about 10 μm and preferably less than about 5 μm.
The electrically resistive layer 115 adds a ballistic resistor to each individual nano-emitter, which suppresses emission hot spots and improves the overall emission uniformity. It also limits the current that passes through each individual nano-emitter, which protects the individual nano-emitters from over-current damage and improves the lifetime of the overall cathode 100. This mechanism can be illustrated by the schematic in
Patterning the high current field emission cathode 100 is a further way to improve the emission current thereof. The emission current density of a CNT field emission cathode is reduced when the emission area of the cathode increases, as shown in the graph of
The patterned field emission cathodes can be produced using different approaches. According to one embodiment, shown in
As noted, patterned field emission cathodes 100 can be made on a single cathode substrate 110.
After production, nanomaterials-based field emission cathodes should be activated before use to achieve optimal performance. Taping the cathode emission surface using an adhesive tape is one effective activation approach, as illustrated by
Removing (peeling off) the adhesive tape 1010 at a constant force and uniform speed is also important to achieving optimal emission performance. Before peeling off the adhesive tape 1010, the field emission cathode 1000 should be secured on a base 1030. There should not be any movement of the cathode 1000 during the entire tape peeling process. Preferably, the adhesive tape 1010 should be peeled off from the cathode 1000 along a straight line on its surface. It should initiate from one edge of the cathode, proceed through the entire cathode surface, and complete when the cathode 1000 is completely separated from the adhesive tape 1010. The peeling force should be slightly larger than the peeling strength of the adhesive tape 1010 and the peeling force should be maintained uniformly along the peeling line. Additionally, the adhesive tape 1010 should be peeled off at an angle from the surface of the cathode 1010. The angle can be between 15° to 90°, preferably around 45° to achieve optimal activation results.
Subjecting the taped nanomaterial cathode 1000 to high voltage pulses for an extended period of time (up to 100 hours in some embodiments) can also improve the field emission performance of the cathode 1000. The voltage used for this purpose will depend on the distance between the nano-emitters and the electron extraction electrode. Generally speaking, an electric field of <10V/μm is preferable, which corresponds to <3 kV for a 300 μm distance between the nano-emitters and the electron extraction electrode. The high voltage pulse is preferably a square wave, in some embodiments. The single pulse width should be <250 ms, and preferably between 100 us and 10 ms. The rest period between consecutive pulses should be at least 10 times longer than the voltage pulse width, preferably between about 100 times and about 1000 times the voltage pulse width. For example, the cathode 1000 can be run at a constant current density of 100 mA/cm2 at 1 ms pulse at 10 Hz for 20 hours to activate the cathode 1000.
The cathode 1000 can also be activated by current or voltage sweeping. In specific embodiments, the emission current of the cathode 1000 can be increased from 0 to 100 mA/cm2 and then back down from 100 mA/cm2 to 0 to complete one sweeping cycle. Sweeping cycles can be repeated for 100 to 500 times to achieve activation, in various embodiments. The taping process serves to fracture the nanomaterial electron emission layer 125 and expose loose ends of nanotubes or nanowires. The repeated electric field exposure can serve to align the nanotubes or nanowires along the electric field direction to improve field enhancement on their emitting ends.
To form a practical field emission cathode, a mesh gate electrode is placed above the field emission cathode to apply an electric field on the nano-emitters to extract electrons from the nano-emitters. The electric field on the nano-emitters is not only closely related to the voltage difference between the mesh gate and the cathode but also depends on the distance between the mesh gate and nano-emitters. The higher the voltage difference, and the smaller the separation, the higher the electric field on the nano-emitters and the higher the emission current that can be drawn from the nano-emitters. To reduce the cost of driving electronics, a small separation distance is often desired. The practical emitter to mesh gate gap distance is <500 μm and preferably <300 μm.
The embodiments discussed herein are illustrative of the present invention. As these embodiments of the present invention are described with reference to illustrations, various modifications or adaptations of the methods and or specific structures described may become apparent to those skilled in the art. All such modifications, adaptations, or variations that rely upon the teachings of the present invention, and through which these teachings have advanced the art, are considered to be within the spirit and scope of the present invention. Hence, these descriptions and drawings should not be considered in a limiting sense, as it is understood that the present invention is in no way limited to only the embodiments illustrated.
This application claims the benefit of U.S. Provisional Application No. 63/517,512 filed on Aug. 3, 2023, which is incorporated herein by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 20050153618 | Kuo | Jul 2005 | A1 |
| 20120007490 | Liu | Jan 2012 | A1 |
| 20120177887 | Inoue | Jul 2012 | A1 |
| 20160013007 | Du | Jan 2016 | A1 |
| Number | Date | Country |
|---|---|---|
| 102208317 | Oct 2011 | CN |
| 2005302607 | Oct 2005 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 63517512 | Aug 2023 | US |
