EP2226828A1

EP2226828A1 - Cold cathode and a method for the production thereof

Info

Publication number: EP2226828A1
Application number: EP07873394A
Authority: EP
Inventors: Radik Rafikovich Mulyukov; Yulai Mukhametovich Yumaguzin; Linar Raisovich Zubairov; Rinat Khamzovich Khisamov
Original assignee: Institute For Metals Superplasticity Problems Of Russian Academy Of Sciences (IMSP RAS)
Current assignee: Institute For Metals Superplasticity Problems Of Russian Academy Of Sciences (IMSP RAS)
Priority date: 2007-12-28
Filing date: 2007-12-28
Publication date: 2010-09-08
Also published as: WO2009084976A1

Abstract

The invention relates to the electronic engineering and, more specifically, to the fields thereof where the physical phenomenon of electron, or ion-electron, or cold emission is used, and concerns a cold cathode and a method for fabricating the same. Provided is a method for fabricating a cold cathode, according to which a cathode workpiece of a metal or alloy is subjected to severe plastic deformation to transform an initial structure of the workpiece into a fragmented structure comprising fragments of nanometric size or into a mixed structure comprising grains and fragments of nanometric size, characterized in that, after the severe plastic deformation, the cathode workpiece is subjected to a low-temperature annealing for transforming at least part of said fragments into grains of nanometric size at a temperature not lower than the operating temperature T_oper of the cathode. The invention is an improvement of the method for fabricating a cold cathode using severe plastic deformation and allows to further enhance efficiency of the cathode via a reduction in the electronic work function value.

Description

Field of the invention

The invention relates to the electronic engineering and, more specifically, to the fields thereof where the physical phenomenon of electron, or ion-electron, or cold emission is used, and concerns a cold cathode and a method for fabricating the same.

Background of the invention

Cold cathodes operating on the principle of secondary electron, or ion-electron, or cold emission are used widely. They are utilized in such devices as vacuum devices and gas-discharge devices, including optical quantum generators, helium-neon lasers, detection devices, ion sources. The latter are used, in particular, as important components of widespread devices such as plasmatrons, mass spectrometers. Operation accuracy and efficiency of the aforementioned devices are determined by cold cathode characteristics. With advances in microelectronics and nanoelectronics, requirements for cold cathode characteristics are becoming more stringent.
In most of the aforementioned devices, cathodes made of aluminum, beryllium, magnesium, iron, nickel, tantalum, molybdenum and other metals or alloys based thereon are used as the cold cathodes [1]. The choice of a specific cathode material is governed by the requirements for the cathode service life and efficiency, in the latter case with regard to the most important emission characteristic of a metal or alloy, i.e. an electronic work function. Other things being equal, a reduction in the electronic work function value allows, e.g. in an ion source, a greater ionic current to be obtained at a lower value of applied voltage. Furthermore, a reduction in the work function value, e.g. in gas-discharge devices, allows a gas pressure to be decreased, thereby reducing the load on a vacuum system of the device, which eventually results in improving reliability of the device.
Values of the electronic work function for various materials, including metals, can be found in the reference literature [2].
A cold cathode of a gas-discharge device is known in [3], said cathode being made of substantially pure aluminum which is doped with silicon in the amount of 0.5-1.65 wt.%.
The addition of silicon to aluminum promotes formation of homogeneous microstructure and reduction in the grain size, that in turn promotes improved resistance of the oxide layer and prevents cathode sputtering.
In contrast to the [3], a cold cathode is known in [4] a material of which further comprises iron in the amount of 0.1-2.0 wt.%. The addition of iron to an aluminum-silicon alloy reduces lattice parameters due to the formation of a homogeneous substitution solid solution in aluminum. Thereby, resistance of the cathode surface to ion bombardment is improved and sputtering is further reduced.
A cold cathode of a gas-discharge device is known in [5], where nickel with addition of aluminosilicate of cesium or rubidium in the amount of 0.5-25 wt.% is used as the cathode material, which allows stable operation of the cathode under intensive ion bombardment conditions and increases the emission current, including due to a reduced work function.
A method for fabricating the cathode involves the use of hot plastic deformation, in particular, hot forging, die forming, rolling, for producing a cathode workpiece. The cathodes can be also fabricated using powder metallurgy techniques, in particular, by compacting a mixture of powders into a desired cathode configuration and then sintering the compacted material [5].
A drawback of the techniques associated with changing the alloy composition by addition of special additives [3, 4, 5] is that it is impossible to fabricate a cathode having predetermined properties with a high degree of accuracy, which is caused by the fact that it is difficult to keep the same alloy composition in different melts. This is the reason for indication of rather wide ranges of the additive amounts in the known solutions. Such cathodes cannot be interchangeable when used in the aforementioned state-of-the-art devices. In other words, methods for fabricating cold cathodes that involve changing the alloy composition by addition of special additives have limited capabilities for improving the cathode efficiency. Furthermore, changing the composition of any commercial alloy is extremely uneconomic.
In connection with the latter assertion, commercial aluminum alloys like a D16 alloy containing, in wt.%: Al is the base; Cu - 4.3; Mg - 1.5; Mn - 0.6; Fe ≤ 0.5; Si ≤ 0.5, and a AD1 alloy (commercially pure Al, 99.3%) are known to be used as a cold cathode material [6].
To improve efficiency of a cold cathode, a method for fabricating the same [6] involves using cold plastic deformation for strengthening a near-surface layer of the cathode. The plastic deformation is carried out by rolling a working surface of the cathode by a distributing ball head. The strengthening via plastic deformation results in improving durability of the cathode. This technique is however reported to be unacceptable for processing beryllium due to its high hardness and brittleness. That is, capabilities of the cold plastic deformation carried out in this fashion are also restricted.
A cathode (cathode target) and a method for fabricating the same are known in [7]. The method comprises the steps of providing a cathode workpiece of a metal or alloy, subjecting the cathode workpiece to cold deformation in two stages into a desired cathode configuration. The first stage comprises cold rolling a card-shaped workpiece (card blank) with rolling directions varied, which is necessary to avoid formation of a pronounced texture. In the second cold deformation stage, preferably by forming, a cylinder or cup shape is imparted to the semi-finished cathode.
Between the cold deformation stages, the workpiece can be subjected to thermal treatment, such as high-temperature tempering or annealing, to relieve internal stresses.
As the result of the two-stage deformation, the cathode has a homogeneous texture and a uniform fine-grained structure with a grain size of 15 µm and greater.
Owing to the uniform fine-grained structure, a homogeneity of sputtering the metal from the target surface is achieved. But the known solution [7] is silent how the cathode structure, in particular grain size, can influence the electronic work function and the cathode strength.
Disadvantages of this method may be referred to that the rolling operation is rather power-intensive and labor-consuming, including the necessity for varying the rolling direction. In addition, the cold rolling is unsuitable for processing rather brittle metals like beryllium [6].
Therefore, the known methods for fabricating a cold cathode using plastic deformation do not involve techniques that could remarkably improve the cold cathode efficiency owing to a reduction in the work function, that is, the manufacturing capabilities thereof are also restricted as in the case of changing the alloy composition.
An effect of plastic deformation on the electronic work function in various stress-strain state conditions is known from [8]. A procedure was proposed for calculating a variation of the work function, based on a model of its relationship with electronegativity of metals and taking into account the formation of nanometric surface defects. However, the capabilities of using the model and the calculation procedure based thereon in fabrication of a cathode exhibiting a reduced work function value are also restricted. This is primarily caused by the fact that the procedure, being rather complicated and cumbersome, takes into account only the effect of surface defects on the work function variation, which can be cancelled out in further processing of the cathode or during its service.
It is known from [8] that a reduction in the work function correlates with an increase in the degree of plastic strain.
It is known that a maximum possible degree of strain without fracture can be accomplished by methods of severe plastic deformation (SPD) [9, 10].
Most commonly known SPD methods include equal-channel angular pressing (ECAP) and torsion under quasi-hydrostatic pressure on a Bridgemen anvil-type setup [9, 10]. By using the above methods, a degree of strain e ≥ 4 can be attained, where e is the true logarithmic degree of strain.
In light of the aforementioned, a closest prior art (prototype) to the present invention is a cold cathode and a method for fabricating the same as disclosed in [11]. In this method, an experimental cathode sample was fabricated from tungsten of 99.99% purity by an SPD deformation method, namely, by torsion under quasi-hydrostatic pressure on a Bridgemen anvil-type setup to a true logarithmic degree of strain e ∼ 7. Thereby, a structure with a nanometric grain size of about 100 nm was obtained in the sample. An electronic work function was determined by measuring the contact potential difference, more precisely the difference between the work function of the cathode sample subjected to the SPD and having the nanometric grain size and the work function of a cathode sample in a normal coarse-grained state. It was found that the formation of a structure with nanometric size of grains leads to a reduction in the work function of the metal. The reduction of the work function for tungsten with the grain size of about 100 nm was ∼0.8 eV. Complex studies of the sample were conducted, including transmission electron microscopy and also numerical simulation of the experiment.
It was revealed that the observed specific behavior of the nanocrystalline material was primarily caused not by the fine grain size, but by a special, "non-equilibrium" state of grain boundaries defined by internal stresses, as demonstrated by the diffused contrast at the grain boundaries observed under a transmission electron microscope. Substantial (up to 1-3%) distortions of the lattice by internal stresses were observed near the boundaries in a region of about 10 nm wide.
Thus, this allows a hypothesis to be put forward that the formation of a structure with nanometric size of grains gives rise to the occurrence of tubes of current with a reduced work function. Such a tube of current includes the grain boundaries per se and their vicinities of about 10 nm wide.
In addition to the reduced work function, the SPD leads to a remarkable improvement in the cold cathode strength, which may result in an increase of its durability [6].
Furthermore, as compared to the other known methods for fabricating a cold cathode [6], the ability appears to use cold deformation for strengthening a cold cathode of brittle materials. The latter is explained by the fact that the SPD is generally carried out under the conditions of all-around compression, as with the ECAP, or under the conditions close to all-around compression, as with the torsion under quasi-hydrostatic pressure at a Bridgemen anvil-type setup.
However, further experiments have shown that, with increasing the degree of strain, in particular, of a sample of the same tungsten, the SPD failed to give the expected reduction in the work function value as known from [8]. In particular, with the SPD deformation method for a sample made of a commercial aluminum alloy AMG6 (6.3% Mg; 0.6% Mn; Cu<0.1%; Zn<0.2%; Fe<0.4; Si<0.4%, the balance Al), the result expected according to [11] was unobtainable at all.
Therefore, there is a problem associated with revealing factors that influence the variation in the electronic work function in cathode samples subjected to SPD and with the need of finding techniques allowing to maximally use advantages of SPD in fabrication of cathodes made of various metals and alloys conventionally used for these purposes.

Disclosure of the invention

The object of the invention is to improve a method for fabricating a cold cathode using severe plastic deformation, which is capable of further improving the cathode efficiency through a reduction of the electronic work function, and also to expand manufacturing capabilities of the method for fabricating cold cathodes from various metals and alloys.
The object is attained in the case a method for fabricating a cold cathode, according to which a cathode workpiece of a metal or alloy is subjected to severe plastic deformation to transform an initial structure of the workpiece into a fragmented structure comprising fragments of nanometric size or into a mixed structure comprising grains and fragments of nanometric size, is characterized in that after the severe plastic deformation the cathode workpiece is subjected to a low-temperature annealing for transforming at least part of said fragments into grains of nanometric size at a temperature not lower than the operating temperature T_oper of the cathode.
In the present invention, by the term "severe plastic deformation" or SPD (also referred to as "intensive plastic deformation" in the Russian literature) is understood the plastic deformation of a metallic material up to highest degrees of plastic strain (up to some thousands percent) at relatively low homologous temperatures (typically below 0.3 of the melting temperature), which leads to subdivision of grains in the initial structure of the material.
Preferably, the severe plastic deformation of the cathode workpiece is carried out at a degree e ≥ 4, where e is the true logarithmic degree of strain. It should be noted that, in accordance with the invention essence, the lower limit of strain degree e is determined by the attainment of nanometric size of elements of the cathode workpiece structure (i.e. grains and fragments) and by the provision for a desired effect of the work function reduction. The upper limit of strain degree e may be substantially any one, and is selected so that to maximize the refinement of grains and fragments and to minimize manufacturing costs. Therefore, a preferred range of the strain degree e is from 4 to 20, more preferably from 5 to 10, and most preferably from 6 to 8. With that, the initial structure of the cathode workpiece before the severe plastic deformation may be substantially any one, such as a grained structure (e.g. a coarse-grained structure with grains of a size much higher than 10 µm, or a fine-grained structure with grains of a size slightly higher than 1 µm), or even a partially fragmented structure possibly obtained at preceding deformation steps if they are desirable.
Preferably, the severe plastic deformation of the cathode workpiece is carried out by torsion under quasi-hydrostatic pressure on a Bridgemen anvil-type setup, which often allows the required degree of strain to be attained in a single pass and, hence, allows the manufacturing costs to be reduced.
Preferably, the low-temperature annealing of the cathode workpiece is carried out at a temperature selected in the temperature range of from T_oper to 0.4×T_melt, where T_melt is the melting temperature of said metal or alloy. As the cold cathode may warm up during its service, generally from room temperature to an operating temperature in the order of 50 to 100°C, the lower annealing temperature limit should be not lower than the cathode operating temperature T_oper to avoid thermal instability of the cathode structure during its service. The upper annealing temperature limit is restricted by the grain size in the workpiece structure not going beyond the nanometric range (i.e. does not exceed 1 µm) and is determined by maximizing the work function reduction effect as a result of the annealing. At an annealing temperature above 0.4×T_melt, an inadmissible grain growth takes place in the workpiece structure, which cancels, at least partly, the work function reduction effect as a result of the severe plastic deformation. More preferably, the annealing temperature is selected in the range of from T_oper to 0.3×T_melt, and most preferably in the range of from 0.1×T_melt to 0.25×T_melt.
Preferably, the low-temperature annealing of the cathode workpiece is carried out for a time period of from 1 minute to 100 hours, and more preferably from 0.5 hour to 2 hours. It should be noted here that the annealing time limits in accordance with the invention may be substantially any ones, and are selected so that to maximize the work function reduction effect and to minimize manufacturing costs, provided the grain size in the workpiece structure does not fall outside the nanometric range (i.e. does not exceed 1 µm).
It should be also noted here that the annealing time and temperature in accordance with the invention are preferably selected with regards to each other. Typically, the higher the annealing temperature, the shorter the annealing time, and vice versa.
Preferably, the at least part of said fragments transformed into grains during the low-temperature annealing is from 10 to 100%, more preferably from 20 to 100%, and even more preferably from 30 to 100%, of their number in the fragmented or mixed structure of the workpiece after the severe plastic deformation. It should be noted here that generally the greater the number of fragments transformed into grains by annealing, the higher is the work function reduction effect. For this reason, it is most preferable that the cathode workpiece after the low-temperature annealing has a predominantly grained structure, i.e. comprises at least 50% grains, and in the best case it has a substantially grained structure, i.e. comprises at least 90% grains.
Preferably, a machining or plastic working of the cathode workpiece is carried out after the severe plastic deformation and the low-temperature annealing, wherein the deformation temperature in the plastic working of the cathode workpiece is regulated so that to preserve the nanometric size of grains. Moreover, any further cathode processing, if necessary, is carried out so that to preserve the nanometric size of grains.
In the device aspect, the object of the invention is attained in the case a cold cathode of a metal or alloy having a structure comprising grains of nanometric size and obtained by severe plastic deformation, is characterized in that the cathode has a predominantly grained structure with the grains of nanometric size, said structure being obtained by the method according to any of embodiments of the invention.

Explanation of the Invention Matter

In severe plastic deformation (SPD), the structure of a metal or alloy undergoes considerable modifications. A subdivision of initial grains occurs to form fragments finer than the initial grains, which fragments represent disoriented regions separated by unformed boundaries of dislocational type. In recrystallization process, the fragments transform into grains of about the same size as the fragments, with grain-type boundaries being formed between them. The formation of grains and their respective boundaries proceeds more intensively in pure metals, than in alloys, due to hindered boundary migration in alloys. A subdivision of the newly formed grains occurs further in the course of SPD. The above processes proceed simultaneously and continuously while SPD is being carried out. That is why, when the SPD is interrupted upon the desired size of grains or fragments has been reached, both grains of nanometric size and fragments of nanometric size are inevitably present in the metal or alloy, i.e. the metal or alloy has a mixed structure or a fragmented structure after the SPD. Meanwhile, alloys generally have a fragmented structure for the reason mentioned above. The former pattern was revealed, in particular, in additional studies of the structure of a sample made of tungsten, and the latter pattern was revealed in studies of the structure of a sample made of the AMG6 aluminum alloy.
Electronic microscopy studies of the samples subjected to SPD show that increasing the degree of strain above a certain value depending on physical properties of a metal or alloy does not lead to the transformation of a fragmented structure into a grained structure in the SPD process. So the situation is possible where both a metal and an alloy will have a predominantly fragmented structure after the SPD (see Fig. 10 below illustrating the process under discussion for a cathode sample of the AMG6 alloy).
Boundaries of fragments have a dislocational nature, i.e. they are crystallographically unformed, and remain such during and after the deformation. The unformed fragment boundaries cannot provide a further reduction in the electronic work function. Furthermore, the work function in cathode samples with a completely fragmented structure after the SPD may not differ from that in cathode samples having a coarse-grained (above 1 µm) structure obtained by conventional methods.
A transformation of the mixed structure into a predominantly grained or even completely grained structure preferably takes place at the low-temperature annealing. The annealing temperature sufficient only for transformation of fragments into grains does not result in increasing the sizes of structure elements. The nanometric size of grains remains approximately equal to the size of fragments. As the absolute equality of grain and fragment sizes in such a physical process as the transformation of a metal or alloy structure is substantially infeasible, the sizes of grains may be several nanometers greater, preferably in the range of ∼20-30 nm, than the fragment sizes. Boundaries of the formed nanometric-size grains give an additionally contribution to the electronic work function reduction with respect to the contribution which gives SPD alone, in case of a less fragmented structure, e.g. as in the method for fabricating a cathode of tungsten. With a predominantly fragmented structure after SPD, only annealing can provide the work function reduction effect described in [11].
After the preferred low-temperature annealing, the presence of a small number of fragments is still possible, but the fragments no longer affect the general pattern of occurrence of tubes of current and the work function reduction.
A low-temperature annealing temperature for most of metals and alloys traditionally used for cold cathode fabrication is recommended to be selected from about 0.1×T_melt to about 0.2×T_melt. It has been experimentally found that such an annealing temperature is well suited to both substantially pure metals such as nickel, tungsten, titanium, molybdenum, and alloys such as AMG6, E125, etc. Annealing at a lower temperature frequently fails to lead to sufficient modifications in the strained sample structure. Furthermore, a cold cathode may warm up during its service, typically from room temperature to an operating temperature in the order of 50-100°C, so the annealing temperature should not be below the cathode operating temperature to avoid thermal instability of the cathode structure during service.
Therefore, the method in accordance with the present invention, as compared to the closest prior art method, allows severe plastic deformation capabilities to be more completely used for reducing the electronic work function value. In combination with the low-temperature annealing, the SPD can be used to improve efficiency of a cathode made of substantially any metal or alloy conventionally used for this purpose. At the same time, the use of SPD alone cannot result in improvement of efficiency of a cathode made of a metal or alloy prone to intense fragmentation under SPD.
Owing to the use of SPD and low-temperature annealing, a cold cathode and a method for fabricating the same in accordance with the invention are more cost-effective as compared to the closest prior art because the cathode is fabricated from commercial alloys without incorporating additional additives to reduce the electronic work function.
In addition, a method and a cold cathode in accordance with the invention preserve all other advantages of the closest prior art.
More particularly, a method in accordance with the invention, like the closest prior art method, allows to improve the cold cathode strength. However, in contrast to conventional coarse-grained cathodes, the strength improvement is to a somewhat less amount than in the closest prior art due to a partial removal of SPD-induced work hardening during the low-temperature annealing.
Like the closest prior art, the invention provides the possibility of using cold deformation for strengthening a cold cathode made of brittle materials.
Recommended is a most economical and power and labor saving embodiment of the inventive method, which includes processing a disk-shaped cathode workpiece by torsion under quasi-hydrostatic pressure on a Bridgemen anvil-type setup. The setup has a simple design, though a required degree of strain can be attained in a single pass.
A setup for ECAP-pressing has a more complex design and, thus, several passes are performed to reach a required degree of strain, but an intermediate cathode workpiece can be produced in the form of a rod of a predetermined diameter which can be separated into semi-finished products of a predetermined thickness by cutting techniques.
The SPD can be also used to consolidate particles or clusters of atoms obtained by techniques conventional in the field of powder metallurgy. A thus preformed block can be used as the cold cathode workpiece. However, the cost of such a cold cathode will be higher, so this embodiment of the method seems to be the least economically sound.
SPD methods have certain restrictions associated with small dimensions of workpieces to be processed and with the necessity to make the workpieces into specified shapes: a rod for the ECAP or a disc for the torsion under quasi-hydrostatic pressure.
This fact is not however a constraint to the method for fabricating a cold cathode as an object having small dimensions suitable for SPD methods and a tendency to further decrease of dimensions. Moreover, the possibility arises of industrial use, in combination with the low-temperature annealing, of an SPD method such as, for example, torsion under quasi-hydrostatic pressure, which has been traditionally considered appropriate for processing samples of metals and alloys in laboratory conditions only.
It should be also noted that in the operation of a cathode heated to implement the thermionic emission phenomenon, a considerable grain growth is taking place, and the role of grain boundaries as a factor contributing to the work function reduction will be lost against this background. For this reason, the method feature reflecting its designation, in particular, a method for fabricating precisely a cold cathode, becomes essential.
As the obtained nanometric grain size and the state of boundaries are retained in a finished article, i.e. a cold cathode, these are the features of the cathode per se as a device.
In light of the foregoing, it can be concluded that the objects of the invention can be solved only with the associated use of the entire combination of essential features of the invention.

Brief description of the drawings

The present invention will be illustrated by the appended drawings, wherein:

Fig. 1 shows a schematic diagram of how a cathode is fabricated;
Fig. 2 shows a photograph of an experimental sample of a cold cathode;
Fig. 3 shows a photograph of an external view of a diode cell;
Fig. 4 shows a photograph of gas discharge in the diode cell;
Fig. 5 shows current-voltage characteristics of the diode cell having a cold cathode of nickel;
Fig. 6 shows current-voltage characteristics of the diode cell having a cold cathode of E125 alloy;
Fig. 7 shows current-voltage characteristics of the diode cell having a cold cathode of AMG6 alloy;
Fig. 8 shows current-voltage characteristics of the diode cell having a cold cathode of tungsten;
Fig. 9 shows a structure of an AMG6 alloy sample before SPD;
Fig. 10 shows a structure of an AMG6 alloy sample after SPD;
Fig. 11 shows a structure of an AMG6 alloy sample after SPD and annealing at 75°C;
Fig. 12 shows a structure of an AMG6 alloy sample after SPD and annealing at 150°C;
Fig. 13 shows a structure of an AMG6 alloy sample after SPD and annealing at 250°C;
Fig. 14 shows a structure of an AMG6 alloy sample after SPD and annealing at 300°C;
Fig. 15 shows the relationship between microhardness of a cold cathode sample and grain size.

Description of preferred embodiments of the invention

It is intended that the scope of the invention be limited not with detailed description of exemplary specific embodiments described below, but rather by the claims appended hereto.
In order to fabricate a cold cathode in accordance with a method of the present invention, it is possible to use, for example, as in [1], aluminum, beryllium, iron, nickel, tungsten, tantalum, molybdenum, titanium, niobium, zirconium and other metals or alloys based thereon, i.e. materials conventionally used for fabrication of a cold cathode.
To illustrate the method for fabricating a cold cathode, the following materials were selected as a cold cathode material: commercially pure nickel (Ni 99.98 wt.%), commercially pure tungsten (99.99 wt.%), a commercial aluminum alloy (AMG6 having a nominal composition, in wt.%: Mg 6.3%; Mn 0.6%; Cu<0.1%; Zn<0.2%; Fe<0.4%; Si<0.4%, the balance Al) and a commercial zirconium alloy (E125 having a nominal composition, in wt.%: Nb 2.5%, the balance Zr). The presented examples do not exclude the possibility of using all other metals and alloys which a cold cathode can be fabricated from.
As the method steps are identical, a single example will be described for fabricating cold cathode samples using the above materials.
In all cases, a cathode workpiece was deformed by an SPD method, in particular, by torsion under quasi-hydrostatic pressure on a Bridgemen anvil-type setup (Fig. 1). In Fig. 1, a cathode workpiece is denoted by the reference number 1 and blocks of an anvil are denoted by the reference number 2.
The presented example does not exclude the possibility of using other SPD methods in fabrication of a cold cathode, including new methods if they will be developed after creating the present invention.
For performing SPD, discs 10 mm in diameter and 0.2 mm in thickness were cut out from a commercial hot-pressed rod of AMG6 alloy, as well as from ingots of nickel and tungsten and of E125 alloy.
The disc workpieces were put between two blocks of the anvil and were compressed under pressure P ∼ 7 GPa at room temperature.
The structure obtained by the severe plastic deformation had the following elements: in the form of predominantly fragments of nanometric size in the cathode workpieces of E125 and AMG6 alloys, and in the form of fragments and predominantly grains of nanometric size in cathode workpieces of tungsten and nickel. Deformation of all the workpieces was performed up to the true logarithmic degree of strain e ∼ 7.
The degree of strain was determined by the formula: $e = l n (Θ \cdot r / 1),$

where Θ is the angle of rotation, in radians; r and 1 are the workpiece radius and thickness, respectively [10].
An extent of fragmentation of the workpiece structure and sizes of the structure elements were determined by means of a JEM-2000EX transmission electron microscope (TEM).
An average size of the fragments and grains in the cathode workpieces of nickel and tungsten was about 100 nm. The extent of fragmentation of the nickel workpiece was greater than that of the tungsten workpiece.
An average size of the fragments in the cathode workpieces of AMG6 alloy was about 100 nm.
An average size of the grains and fragments in the cathode workpiece of E125 alloy was about 90 nm.
The extent of fragmentation of the structure in the latter two workpieces was considerable.
After the SPD, the cathode workpieces were subjected to a low-temperature annealing. The annealing was carried out at the following temperatures:

for the AMG6 alloy workpiece at 150°C;
for the E125 alloy workpiece at 300°C;
for the nickel workpiece at 250°C;
for the tungsten workpiece at 500°C.

The annealing time was about 1 hour in this case.
After the low-temperature annealing, a predominantly grained structure was obtained in all the workpieces. The average grain size and the number of grains in % (measured by statistical analysis of the number of grains and fragments in several fields of view in TEM), respectively, were:

in the AMG6 alloy workpiece - about 130 nm, more than about 60%;
in the E125 alloy workpiece - about 110 nm, more than about 60%;
in the nickel workpiece - about 120 nm, more than about 70%;
in the tungsten workpiece - about 120 nm, more than about 70%.

To prevent oxidation processes, the low-temperature annealing was carried out in vacuum in an electric furnace.
After the SPD and the low-temperature annealing, all the four cathode workpieces were subjected to an additional deformation to obtain a slightly concave-convex shape. The workpieces were deformed at room temperature.
The finished cold cathode samples were disc-shaped, 10 mm in diameter and ∼0.1 mm in thickness. An appearance of one experimental sample of the finished cold cathode is shown in Fig. 2. The cathode can be used as a component of an ion source.
Then, the electronic work function was measured for the cathodes fabricated by the method described in the above example.
The work function was measured by a contact potential difference method [12]. A relative work function variation Δϕ was determined directly from the shift of delay lines relative to each other for the cathode samples fabricated by the invented method and the closest prior art method.
To provide reliable results, surfaces of all samples were cleaned by ionic etching in an atmosphere of inert gases before the work function measurements directly in the measurement system, with the samples being hold free from contact with air.
An error of the measurements was ∼2%.
For illustration, the results have been summarized in Table below:

No. Material of sample Δϕ = ϕ₁ - ϕ₂, eV

1 AMG6 alloy 0.3

2 Nickel 0.4

3 E125 alloy 0.3

4 Tungsten 0.2

where ϕ₁ and ϕ₂ are the electronic work functions of the cold cathode in accordance with the invention and in accordance with the closest prior art, respectively.
Thus, these measurements have demonstrated that the samples after SPD and low-temperature annealing have a lower work function value than the samples just after SPD alone.
The reduction of the work function results in improving the cold cathode efficiency, e.g. when the cathode is operated in an ion source.
The ion source is an electric vacuum device intended to produce a directional flow of ions, whose action is based on the use of various kinds of electric discharges in a gas (inert gases, hydrogen) or metal vapors. A simplest ion source is a diode cell (Figs. 3, 4) comprising a bulb filled with an inert gas; and properties of the ion source are determined by the electron flow/gas medium interaction and by the electric field between electrodes (an anode and a cathode).
A characteristic of the operation efficiency of an ion source with a cold cathode is the ability to produce a higher operating current of ion beam at a lower operating voltage, as well as to provide ion beam of homogeneous composition and ionic current of constant density. At the same conditions, the cathode operation efficiency is dependent on cathode material properties, in particular, on the electronic work function. The lower the electronic work function is, the greater is the secondary ion-electron emission ratio and, hence, the ion beam intensity.
Studies of voltage-current (U-I) characteristics of a self-sustained discharge in a diode cell comprising the cold cathodes fabricated by the method in accordance with the invention and by the closest prior art method revealed that the cathode efficiency was improved. By way of example, the current of self-sustained discharge at a 1 kV voltage in a cell with the cold cathode fabricated by the invented method was higher than the current of self-sustained discharge at the same voltage in a cell with the cold cathode fabricated by the closest prior art method by:

55% for nickel, see Fig. 5;
35% for E125 alloy, see Fig. 6;
40% for AMG6 alloy, see Fig. 7;
25% for tungsten, see Fig. 8.

The voltage-current characteristics of the cold cathodes fabricated by the method in accordance with the invention and by the closest prior art method are shown in Figs. 5,6,7,8.
In Figs. 5, 6, 7, 8, curve 1 is a current-voltage characteristic of a diode cell with the cold cathode fabricated by the method in accordance with the invention from nickel, E125 and AMG6 alloys and tungsten, respectively; curve 2 is a current-voltage characteristic of a diode cell with the cold cathode fabricated by the closest prior art method from the same materials.
As has noted above, studies of the microstructures of the samples were conducted under a JEM-2000EX transmission electron microscope. Samples of all four materials (nickel, tungsten, E125 and AMG6 alloys) were studied. In all cases, reliable results relating to an evolution of the structure in the samples were obtained and confirmed that the object of the invention was attainable. For brevity, a data (graphic materials) will be further presented only in respect of AMG6 alloy.
Figs. 9, 10, 11, 12, 13, 14 show the structure of the AMG6 alloy under study before SPD, after SPD and after SPD and annealing at a temperature T_ann = 75°C, 150°C, 250°C or 300°C, respectively.
Before SPD, the AMG alloy had a typical partially recrystallized structure of a hot-pressed intermediate product. Coarse inclusions of primary phases were present in the matrix. Chemical analysis revealed that the composition of light-color particles corresponds to an Al₁₀Mg₂Mn phase, while the composition of dark particles corresponds to a Mg₂Si phase.
After SPD, the matrix structure became fragmented in construction and was comprised substantially of fragment elements with the average size of ∼100 nm. Nonuniform contrast in the fragment body was an evidence of the presence of severe distortions in the crystal lattice and of high internal stresses, while the absence of diffused contrast was an evidence of dislocational nature of boundaries and their unformed state.
Annealing at a temperature of 75°C for about 1 hour did not result in appreciable modifications of the AMG6 alloy fragmented structure.
In the process of annealing at a temperature of 150°C, the fragmented structure transformed into a predominantly grained structure comprising ∼60% of grains. The process was accompanied by a slight coarsening of grains (up to ∼130 nm). A further temperature increase to 250°C activated the normal grain growth that has formed an equilibrium and equiaxial structure with the average grain size of ∼540 nm. A further increase of the annealing temperature was accompanied by a substantial coarsening of the grains. As a result, the grain size increased to 2 µm after annealing at 300°C. Therefore, in the present experiment conditions, the temperature of low-temperature annealing in accordance with the invention for AMG6 alloy was found to be from 75°C to 250°C.
The obtained data also allowed a microhardness of the sample to be determined in various structural states. The microhardness was evaluated in accordance with the Russian State Standard GOST 9450-76.
Fig. 15 shows the relationship between microhardness and annealing temperature and, consequently, grain size for AMG6 alloy. According to the obtained relationship, a microhardness of the cold cathode sample subjected to SPD and low-temperature annealing is 2.5 times higher than that of the sample in coarse-grained state. This results in 2.5 times higher strength of the cathode and, as has been already mentioned for some alloys, extends its service life. Moreover, the microhardness of the sample after SPD and low-temperature annealing is insignificantly lower than that after SPD only due to a partial removal of work hardening (cold work).
A drastic reduction in the microhardness seen in Fig. 15 at higher temperatures (about 200-300°C) is caused by the above-described structural transformations and is associated with removal of work hardening effects.

References cited:

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Claims

A method for fabricating a cold cathode, according to which a cathode workpiece of a metal or alloy is subjected to severe plastic deformation to transform an initial structure of the workpiece into a fragmented structure comprising fragments of nanometric size or into a mixed structure comprising grains and fragments of nanometric size, characterized in that, after the severe plastic deformation, the cathode workpiece is subjected to a low-temperature annealing for transforming at least part of said fragments into grains of nanometric size at a temperature not lower than the operating temperature T_oper of the cathode.
The method according to claim 1, characterized in that the severe plastic deformation of the cathode workpiece is carried out at a degree e ≥ 4, where e is the true logarithmic degree of strain.
The method according to claim 1, characterized in that the severe plastic deformation of the cathode workpiece is carried out by torsion under quasi-hydrostatic pressure on a Bridgemen anvil-type setup.
The method according to claim 1, characterized in that the low-temperature annealing of the cathode workpiece is carried out at a temperature selected in the temperature range of from T_oper to 0.4×T_melt, where T_melt is the melting temperature of said metal or alloy.
The method according to claim 1, characterized in that the low-temperature annealing of the cathode workpiece is carried out for a time period of from 1 minute to 100 hours, and preferably from 0.5 hour to 2 hours.
The method according to claim 1, characterized in that the at least part of said fragments transformed into grains during the low-temperature annealing is from 10 to 100%, and preferably more than 50%, of their number in the fragmented or mixed structure of the workpiece after the severe plastic deformation.
The method according to claim 1, characterized in that a machining or plastic working of the cathode workpiece is carried out after the severe plastic deformation and the low-temperature annealing, wherein a deformation temperature during the plastic working of the cathode workpiece is regulated so that to preserve the nanometric size of grains.
A cold cathode of a metal or alloy, having a structure comprising grains of nanometric size and obtained by severe plastic deformation, characterized in that the cathode has a predominantly grained structure with the grains of nanometric size, said structure being obtained by the method according to any one of claims 1 to 7.