Patent Application
20060012282
1. Field of the Invention
This invention relates to a dielectric device using dielectric properties and electrical-mechanical conversion properties.
2. Description of the Related Art
In the related art, a type of dielectric device using ferroelectric ceramics is known. The dielectric device using this ferroelectric ceramics can usually be obtained by forming a laminated structure, by patterning an electrode on a ferroelectric ceramic thin plate formed by the screen printing method or the green sheet method, or joining to another metal or a ceramic thin plate.
Here, the screen printing method is a method of obtaining a ceramic thin plate used as the base of a dielectric device by forming a film on a predetermined substrate from a slurry containing a ceramic powder dispersed in an organic binder by screen printing, and sintering this film at a high temperature of 900° C. or higher. The green sheet method is a method of obtaining a ceramic thin plate by forming a thick film of predetermined thickness from the aforesaid slurry, drying to obtain a green sheet, and sintering at a high temperature in the same way as above after hole-punching. In recent years, this kind of dielectric device has been used as an electron emitter, for example in field emission displays (FEDs) or electron beam sources in electron microscopes.
The electron emitter is operated in a predetermined vacuum level, and when a predetermined electric field is applied to the electron emission portions (hereafter, emitters), electrons are emitted from the emitters. When this electron emitter is used in an FED, plural electron emitters are arranged in two dimensions, and plural fluorescent bodies are respectively disposed at a predetermined interval from the electron emitters so as to correspond to the plural electron emitters. By selectively driving one of these plural electron emitters arranged in two dimensions at any desired position, electrons can be emitted from an electron emitter in any desired position, fluorescence can be emitted from a phosphor in any desired position due to collision between the emitted electrons and the phosphor, and a desired display can thus be obtained.
Some early examples of this electron emitter are given for example in the following Patent References 1 to 5. These electron emitters do not use the aforesaid dielectric device, but instead have an emitter which includes minuscule conductive electrodes with sharp tips. When a predetermined drive voltage is applied between a reference electrode opposite to this emitter and the emitter, electrons are emitted from the tips of the emitter. Therefore, to form this minuscule conductive electrode, microfabrication by etching, forming or the like is required. Also, to emit a predetermined electron amount into a predetermined vacuum level from the tips of the aforesaid conductive electrodes, a sufficiently high voltage is required as the drive voltage, and an expensive drive element which can supply a high voltage, such as an IC for driving this electron emitter, is required.
As stated above, the problem of the electron emitter using a conductive electrode as an emitter has been that the production costs of not only the electron emitter itself but also the device to which the electron emitter is applied increase. In recent years, therefore, an electron emitter using the aforesaid dielectric device, i.e., an electron emitter wherein the emitter is comprised of a dielectric, has been designed, as disclosed for example in the following Patent References 6 and 7. General knowledge on the electron emission in the case of using a dielectric as the emitter is disclosed in the following Non-Patent References 1 to 3.
The electron emitter disclosed in Patent References 6 and 7 is configured so as to: cover a part of the upper surface of an emitter including a dielectric with a cathode electrode; and dispose an anode electrode at a position on or below the lower surface of the emitter or a position apart from the cathode electrode at a prescribed interval on or above the upper surface of the emitter. That is, the electron emitter is configured so that the exposed surface portion, where neither a cathode electrode nor an anode electrode is formed, of the emitter exists on the upper surface side of the emitter in the vicinity of the outer edge of the cathode electrode Then as the first step, voltage is applied between the cathode electrode and the anode electrode so that the cathode electrode has a higher potential, and the electric field formed by the applied voltage makes the emitter (the exposed portion in particular) get into a prescribed polarized state. Next as the second step, voltage is applied between the cathode electrode and the anode electrode so that the cathode electrode has a lower potential. At this time, primary electrons are emitted from the outer edge of the cathode electrode, the polarization of the emitter is reversed, the primary electrons collide with the exposed portion of the emitter where the polarization has been reversed, and thereby secondary electrons are emitted from the emitter (the exposed portion in particular). The secondary electrons fly toward a prescribed direction caused by a prescribed electric field applied from outside and thereby electrons are emitted from the electron emitter.
In the dielectric device of the related art, as described above, a ceramic thin plate simply formed by the screen printing method or the green sheet method is used, so the following problems occurred.
First, the ceramic thin plate obtained by only the screen printing method or green sheet method is obtained by simply drying and sintering a slurry in which a ceramic powder is dispersed, so many types of ceramic particles having a broad particle size distribution from fine particles to coarse particles are present in the ceramic thin plate. In a dielectric device (in particular, an electronic device) such as the aforesaid electron emitter or surface acoustic wave (SAW) device wherein the physical properties in the vicinity of the surface of the ceramic thin plate have a large effect on performance, it is difficult to obtain a desired surface state and obtain better characteristics. Hence, in the related art, no consideration is given to the matching of the application of the dielectric device and its required characteristics to a suitable structure of the ceramic thin plate.
In the screen printing method or green sheet method, an organic binder from a slurry film or green sheet is decomposed/vaporized, and ceramic crystals are grown at a high temperature to cause adjacent ceramic particles to fuse together. By this process, the ceramic thin plate which forms the base of these dielectric devices can be obtained, but voids between adjacent ceramic particles cannot be completely eliminated, and it is difficult to obtain a high filling density of the ceramic material in the ceramic thin plate. Specifically, taking the case where there are no voids as the theoretical density, the density achieved is of the order of 70 to 80% of the theoretical density. Therefore, if a dielectric device is used as an electronic device employing dielectric properties such as the aforesaid electron emitter, a high electric field could not be applied.
Since sintering is performed at a high temperature of 900° C. or higher, the ceramic layer is damaged by differences of thermal expansion coefficient with the substrate underneath the ceramic layer, a material with low heat resistance such as glass cannot be used as the substrate, and in particular, it is difficult to increase screen size when applied to an FED.
Further, in the electron emitter disclosed in Patent References 6, 7 using the aforesaid related art dielectric device (hereafter, referred to simply as “related art electron emitter”), when electrons are emitted from a cathode electrode toward an emitter, electrons are emitted at the portion, on the surface of the cathode electrode, where lines of electric force concentrate and electric field intensity increases (here, the fact that lines of electric force concentrate on the surface of an electrode which is a conductor and thereby the electric field intensity at the portion where lines of electric force concentrate increases as stated above is hereunder referred to simply as “electric field concentration” and the portion where the electric field concentration occurs is hereunder referred to simply as “electric field concentrated portion”).
Here, an example of a conventional electron emitter is schematically shown in
It is therefore an object of the present invention, which is conceived in view of the problems inherent in the aforesaid related art, to provide a dielectric device of higher performance. It is a further object of the present invention to provide a dielectric device wherein the electron emission amount of an electron emitter can be increased by applying the invention to the electron emitter.
In order to achieve the aforesaid objects, the dielectric layer forming the dielectric device according to the present invention includes plural first dielectric particles and plural second dielectric particles having a smaller particle size than that of the first the electric particles, and the second dielectric particles are disposed in spaces surrounded by the adjacent plural first dielectric particles. In this arrangement, the second dielectric particles are interposed in the gaps between the plural first dielectric particles in the dielectric layer, so the filling density of the dielectric material in the dielectric layer can be increased. Therefore, the physical properties of the dielectric material constituting the dielectric layer can be utilized with less diminished in any way, and a higher performance dielectric device can be provided. In particular, the surface state (surface roughness, density) of the dielectric layer can be easily controlled by the filling degree of the second dielectric particles in the upper surface of the dielectric layer. Herein, if the first dielectric particles and second dielectric particles are of an identical material, a more uniform dielectric layer can be obtained.
The first dielectric particles are preferably coarse particles having an average particle size of micron order (at least 1 μm or more), and the particle size distribution may be fairly wide. The second dielectric particles are preferably fine particles whose average particle size is of submicron order with a narrow particle size distribution. In this context, the average particle size is a computed value obtained by image analysis of the cross-section. For example, each particle can be approximated by a circle having an identical area to that of the particle, the diameter of this circle is taken as the particle size, and the particle size distribution is computed in terms of area.
In this invention, in addition to the aforesaid construction, the first electric particles and/or second dielectric particles forming the dielectric layer are preferably formed by an aerosol deposition method from powder particles of a dielectric. In the aerosol deposition method, the powder is dispersed in a gas by vibration or the like to form a smoke-like state (aerosol), this aerosol is transported to a film-forming chamber under a predetermined reduced pressure, and is then sprayed onto a predetermined substrate via a nozzle to form a dense, crystallized solid film. According to this aerosol deposition method, a dielectric layer with few voids (e.g., about 90 to 95% of the theoretical density) and high filling density is obtained, so the physical properties required of the dielectric layer can be stably and definitively obtained.
In the present invention, the dielectric layer may be formed also by impregnating an aggregation of the first dielectric particles in the form of a layer with a dispersion of the second dielectric particles or a solution of a precursor, and evaporating a dispersion medium or solvent from the dispersion or solution (hereafter, this method will be referred to as “sol impregnation method”). In this way, a dielectric layer having few voids and high filling density identical to those obtained by the aerosol deposition, may be obtained by a very simple process. In this case, the first dielectric particles may form a layer by the screen printing or green sheet method of the prior art, or by the aerosol deposition method. In other words, the first dielectric particles have a so-called “powder origin” due to the screen printing, green sheet or aerosol deposition method, whereas the second dielectric particles have a “sol origin” due to the sol impregnation method. By packing the spaces between the particles of powder origin with particles of sol origin, the filling density of the dielectric material in the emitter can be increased.
Here, when the dielectric layer is prepared using the sol impregnation method, the aggregation of the first dielectric particles preferably contains coarse particles having a particle size of 1 μm or more, and fine particles having a particle size of 1 μm or less. In this case, the second dielectric particles have a smaller particle size than the aforesaid fine particles. In this arrangement, the filling density of the dielectric material in the emitter can be further increased.
It is a feature of the invention that, in addition to the aforesaid arrangement, the dielectric layer contains a metal which is mixed therewith. Specifically, metal fine particles and/or a metal film may be interposed between adjacent dielectric particles in the dielectric layer. Alternatively, the aforesaid metal objects such as the metal particulates may be interposed between the dielectric particles and a substrate at the interface between the dielectric layer and a predetermined substrate supporting the dielectric layer.
By filling the voids in the dielectric layer by a metal in this way, the dielectric constant of the dielectric layer can be increased. Also, by making the metal function as a binder using the ductility of the metal, the film of the dielectric layer can be formed satisfactorily.
In this case, the metal is preferably implanted so that it is dispersed (dispersed is discontinuously) throughout the dielectric layer. Here, the term “discontinuously” represents the state wherein gaps exist among metallic fine particles and/or metal films adjacent to each other along the thickness direction so as not to yield electric conductivity over the thickness direction of the dielectric layer (namely from the upper surface to the lower surface of the dielectric layer).
As a result, in addition to a higher dielectric constant and improved film-forming properties, satisfactory properties (dielectric properties such as electrostatic/electron emission properties and electrical-mechanical conversion properties such as piezoelectric/electrostriction properties) in the dielectric layer are obtained.
The present invention moreover includes an emitter having the aforesaid dielectric layer, and first and second electrodes formed on or above the emitter to which a drive voltage is applied to discharge electrons from the emitter's upper surface.
According to this arrangement, the emitter is a dielectric layer including plural first dielectric particles, and plural second dielectric particles having a smaller particle size than that of the first dielectric particles, the second dielectric particles being disposed in the spaces surrounding the adjacent, plural first dielectric particles. When a predetermined drive voltage is applied between the first electrode and second electrode, a predetermined electric field is applied to the emitter having the aforesaid structure, and electrons are emitted from the upper surface of the emitter. If the emitter is an emitter having the aforesaid structure, due to the relatively coarse first particles, the basic electron emission properties required of the emitter can be obtained, while at the same time due to the second dielectric particles, the filling density of the emitter as a whole can be increased. Further, if the emitter is an emitter having the aforesaid structure, and the upper surface of the dielectric layer is the upper surface of the emitter of the electron emitter, the physical properties (e.g., surface roughness, relative dielectric constant and D-E hysteresis properties) obtained are such that more electrons can be emitted when a predetermined electric field is applied.
Herein, the dielectric layer is preferably subjected to heat treatment at a lower temperature than the sintering temperature of the ceramics. Due to this heat treatment, the relative dielectric constant of the dielectric layer formed by the aerosol deposition method and/or the sol impregnation method can be increased, so a high-performance electronic device to which a higher electric field can be applied, can be provided. Further, when the first electrode or second electrode forming process includes a step for heating to the aforesaid temperature, the electrode-forming step may conveniently be combined with the heat treatment step to increase the dielectric constant. The aforesaid temperature is more preferably in the region of 500° C. Due to this, the aforesaid dielectric device can be formed even on an economical glass substrate which does not have such a high heat resistance temperature, so when it is applied to an electron emitter of an FED, large screens can be manufactured at low cost.
In the present invention, the first electrode is provided on the upper surface of the emitter, and air gaps are provided between a surface opposite to the upper surface of the emitter and the upper surface of the emitter in an edge of the first electrode. By adopting this configuration, the aforementioned triple junction is formed at a portion different from the tip of the edge of the first electrode (including the outer edge of the first electrode and, in the case where the first electrode has an opening as described later, the inner edge at the opening) and hence the electric field concentrated portion is formed also at the tip of the edge of the first electrode in addition to the aforementioned triple junction. Thereby the number of the electric field concentrated portions serving as electron emission sites increases more than ever and the amount of the emitted electrons at the electron emitter can be increased.
Here, it is preferable to dispose the second electrode to the lower surface side, namely on or above (below) the lower surface opposite with the upper surface of the emitter. Thereby the direction of the electric field applied to the emitter is identical to the thickness direction of the dielectric layer composing the emitter and only the first electrode is formed and disposed on or above the upper surface of the emitter. As a result, the area occupied by the electron emitter on a plan view reduces in comparison with the configuration wherein both the first and second electrodes are formed and disposed on the same surface of the emitter, and thus a higher degree of the integration of electron emitters becomes possible. In the case where electron emitters using the dielectric devices are applied to an FED in particular, the higher resolution of a display can easily be attained.
Further in this case, since an air gap (gap) is formed between the surface of the edge portion of the first electrode opposite to the upper surface of the emitter and the upper surface of the emitter, the electrostatic capacity of the virtual condenser formed between the surface of the edge portion of the first electrode opposite to the upper surface of the emitter and the upper surface of the emitter becomes smaller than that in the case. where such a gap is not formed. As a result, most part of the drive voltage is substantially applied to the gap portion, and thus the field intensity at the edge of the first electrode increases and the amount of emitted electrons also increases in comparison with a conventional electron emitter not having such a gap as stated above.
Furthermore, it is preferable that the first electrode is provided with plural openings that expose the upper surface of the emitter toward the exterior of the dielectric device. Thereby triple junctions and the tips at the edge of the first electrode serving as electric field concentrated portions are formed also at the inner edges of the plural openings in addition to the outer edge of the first electrode. As a result, the number of the electric field concentrated portions serving as electron emission sites further increases and thus the amount of emitted electrons at the electron emitter can also be increased. In addition, since the electric field concentrated portions serving as electron emission sites are formed at the outer edge and the plural openings located inside the outer edge in the first electrode, uniform electron emission having less deviation and dispersion in the region occupied by the first electrode can be obtained.
Here, it is particularly preferable that the electron emitter, to which a dielectric device is applied, according to the present invention functions as follows: as the first step, electrons are emitted (supplied) from the first electrode to the emitter by applying such a drive voltage that the potential of the first electrode is lower than that of the emitter, namely electrons are accumulated on the emitter (the emitter is electrified); and as the second step, the electrons already accumulated on the emitter are emitted by applying such a drive voltage that the potential of the first electrode is higher than that of the emitter. This sort of functions can be carried out in the following manner for example.
As the drive voltage applied between the first and second electrodes, for example the voltage which is applied as pulse voltage or alternating voltage relative to a prescribed reference potential (for example, 0 V) is used.
Firstly, at the first step, a drive voltage is applied between the first and second electrodes so that the potential of the first electrode is lower than that of the reference potential and the potential of the second electrode is higher than that of the reference potential. Then, by the electric field caused by the drive voltage, the polarization of the emitter is directed toward such a direction that positive electric charge appears on the upper surface of the emitter, the electric field concentration occurs at the aforementioned electric field concentrated portions, and electrons are supplied from the first electrode to the emitter. Thereby electrons are attracted toward the positive electric charge appearing on the upper surface of the emitter and thereby accumulate at the portion, on the upper surface of the emitter, corresponding to an opening of the first electrode. That is, the portion, on the upper surface of the emitter, corresponding to the opening at the first electrode is electrified. At this time, the first electrode serves as the supply source of the electrons.
Next, at the second step, the drive voltage changes drastically, and the drive voltage is applied between the first and second electrodes so that the potential of the first electrode is higher than that of the reference potential and the potential of the second electrode is lower than that of the reference potential. Then, by the electric field caused by the drive voltage, the polarization direction of the emitter is reversed and negative electric charge appears on the upper surface of the emitter. Thereby, at the first step, the electrons attached to the portion, on the upper surface of the emitter, corresponding to the opening of the first electrode undergo electrostatic repulsive force caused by the polarization reversal and thereby fly from the upper surface of the emitter, and the flown electrons are emitted outside through the opening.
According to such functions, the amount of electrostatic charge at the emitter can be controlled relatively easily at the first stage and hence controllability that is stable and allows a large amount of electrons to be emitted can be secured. In particular, the configuration formed by disposing the first electrode having an opening on the upper surface of the emitter and the second electrode on the lower surface of the emitter is the most suitable as the configuration of an electron emitter for the functions.
Further, an opening, of the first electrode, secluded from the upper surface of the emitter can function as a gate electrode or a focusing electron lens for the electrons emitted from the upper surface of the emitter and hence the linearity of the emitted electrons can also be improved.
Furthermore, a feature of the present invention is that the edge of the first electrode has such a shape as to concentrate the lines of electric force. Such a shape of an edge as to concentrate the lines of electric force can be implemented, for example: either by disposing a portion having an acute-angled shape on the inner wall surface of the edge in sectional side view; or by attaching to the inner wall surface protrusions or conductive fine particles the size of which is equal to or smaller than the thickness of the first electrode. Otherwise, it can also be implemented by forming the inner wall surface of the edge into a hyperbolic shape (particularly a hyperbolic shape configured so that both the upper end and lower end portions of the edge portion in a sectional side view have acute angles). Otherwise, such a shape of the first electrode as to concentrate the lines of electric force at an edge can also be implemented by various means in addition to the above measures. Thereby the degree of the electric field concentration at the tips of the edges of the first electrode (the outer edge of the first electrode and the inner edge of an opening) rises and the amount of the electrons emitted from the tips to the emitter can be increased.
In addition, a feature of the present invention is that the first electrode is the aggregation of conductive particles having a shape extending in the longitudinal direction in a sectional side view and the conductive particles are arranged so that the longitudinal direction is parallel with the upper surface of the emitter. Thereby it is possible to easily realize such a shape as stated above having an air gap between the face of the edge portion of the first electrode, opposite to the upper surface of the emitter and the upper surface of the emitter (hereunder referred to simply as “overhanging shape” occasionally).
Here, as the conductive particles, composing the first electrode, having a shape extending in the longitudinal direction in a sectional side view, various shapes of particles can be adopted, for example: particles of a scale-like shape, a disc-like shape, a coiled spring-like shape or a hollow cylindrical shape; or particles of, in a sectional side view, a rod-like shape, an acicular shape, a hemispherical shape, an elliptical shape or a semi-elliptical shape. Then the conductive particles are arranged in plurality on the upper surface of the emitter so that the longitudinal direction thereof is along the upper surface of the emitter. In this case, the longitudinal direction is not necessarily parallel precisely with the upper surface of the emitter and it is generally acceptable if the conductive particles are arranged on the upper surface of the emitter in the state of “lying” to the extent of forming the gap or overhanging shape causing such functions as described earlier. For example, it is preferable that an angle between the longitudinal direction of the conductive particles and the upper surface of the emitter in a sectional side view is about 30 degrees or less.
Further, in the case where the first electrode has an opening, it is preferable that the opening is formed by the outer edges of plural conductive particles. That is, merely by arranging the conductive particles in plurality on the upper surface of the emitter by coating or another means, a space surrounded by the outer edges of the adjacent conductive particles in a plan view is formed and thus the opening having the aforementioned overhanging shape can easily be formed.
Furthermore, it is preferable that the first electrode is formed by arranging the primary particles of the conductive particles and/or the secondary particles formed by assembling the primary particles in plurality along the upper surface of the emitter and the length of the primary or secondary particles in the longitudinal direction in a sectional side view is larger than the average grain size of the first dielectric particles at the upper surface of the emitter. That is, the dielectric layer composing the emitter is generally a polycrystalline material and concavities are likely to form at the crystal grain boundaries or the junctions of the powder particles. Hence, as long as the concavities are utilized, merely by arranging the primary or secondary particles of the conductive particles in plurality on the upper surface of the emitter, the aforementioned overhanging shape can easily be formed.
Still further, it is preferable that the first electrode is comprised of graphite. Here, the graphite powder is conductive particles of a shape having relatively sharp edges, such as a scale-like shape, a flake-like shape or the like. In other words, it has a shape extending in the longitudinal direction in a sectional side view. Hence, by composing the first electrode of the graphite powder, it is possible to easily form gaps (air gaps) between the emitter and the edges of the first electrode, overhanging shape at the edges of the first electrode, and a shape allowing the lines of electric force to concentrate at the edges of the first electrode, as stated above.
Still further, it is preferable that the first electrode further contains conductive fine particles. In addition to that, it is preferable that the conductive fine particles are deposited also on the upper surface of the emitter. Thereby, since the fine particles exist on the upper surface of the first electrode like protrusions, the fine particles can serve as electric field concentrated portions by the effect of the protrusions and hence the sites of electron emission can further be increased.
In addition, it is more preferable that the fine particles are deposited also on the upper surface of the emitter corresponding to the edges of the first electrode. Thereby fine float electrodes including the fine particles are formed on the emitter composed of a dielectric. The float electrodes are suitable for abundantly accumulating electrons emitted from the first electrode to the emitter and can further increase the amount of the emitted electrons at the electron emitter. Consequently, by forming the float electrodes with the fine particles, it becomes possible to form the float electrodes on the upper surface of the emitter through a simple process, for example, a process of coating the upper surface with the fine particles together with the material composing the first electrode when the first electrode is formed on the upper surface of the emitter.
Furthermore, it is preferable that the fine particles are comprised of silver. Thereby it becomes possible to produce the first electrode containing conductive fine particles easily at a low cost. In particular, when graphite is used as the first electrode and a heating process in an atmosphere containing an oxygen gas is included in the process of forming the first electrode, the graphite around fine silver particles is oxidized and eroded during the heating process. Thereby an edge of the first electrode is likely to have a sharp tip or an opening is likely to be formed by the perforation in the interior of the electrode. As a consequence, the electric field concentrated portions further increase and a more preferable electrode shape can be obtained.
Here, the first electrode stated above can be formed on the emitter by the following method.
That is, the first electrode is formed by: preparing paste produced by dispersing in a dispersing medium conductive particles having a shape extending in the longitudinal direction in a sectional side view; forming a film including the paste on the upper surface of the emitter; and heating the film at a temperature lower than the sintering temperature of ceramics (a preferable temperature is around 500° C.).
Thereby (by properly adjusting the viscosity and; compounding ratio of the paste), it becomes possible to put conductive particles in the state of “lying” as stated earlier by the effects of the self-weight, the surface energy and others of the conductive particles after the aforementioned film forming and before the heating of the film is finished and to easily produce a preferable electron emitter having gaps (air gaps) between the emitter and the openings of the first electrode and overhanging shapes at the openings of the first electrode. Further, it is possible to combine the process of forming the first electrode and the process of heat-treating the emitter as stated above.
Further, in the above production method, it is preferable to disperse conductive fine particles in the dispersing medium when paste is prepared. Thereby it becomes possible to easily produce an electron emitter having a larger number of electric field concentrated portions and an increased amount of emitted electrons as stated above.
As described above, according to the dielectric device of the present invention, a dielectric device having higher performance can be provided. Further, by applying this dielectric device to an electron emitter, the electron emission amount of the electron emitter can be increased.
Preferred embodiments of a dielectric device according to the present invention are hereunder explained referring to drawings. The present embodiments show examples wherein a dielectric device according to the present invention is applied to an electron emission device used as an electron beam source in various devices, using electron beams, such as a display including an FED, an electron beam irradiation device, a light source, an alternative of an LED, electronic parts manufacturing apparatus, and an electronic circuit part.
Firstly, an electron emitter as a dielectric device according to the first embodiment is explained on the basis of FIGS. 1 to 8.
The emitter 12 includes a polycrystalline body of a dielectric material, including plural first particles 12e and plural second particles 12f. The first particles 12e are relatively coarse particles having an average particle size of 1 μm or more, and the second dielectric particles 12f are of an identical material to the first dielectric particles 12e, but they are fine particles of submicron order with a sharp particle size distribution. The second dielectric particles 12f are filled in spaces surrounding the plural first dielectric particles 12e.
Herein, the upper surface of the upper layer 12c forms the upper surface 12a of the emitter 12 which has the role of emitting electrons when a predetermined electric field is applied. If the particle size is too small, crystal grain boundary and other defects increase and the relative dielectric constant falls, so the applied field strength decreases which is undesirable. Hence, the amount of coarse particles having a particle size of micron order is preferably as large as possible. Therefore, the particle size distribution of the dielectric particles (including the plural first particles 12e and plural second particles 12f) constituting the emitter 12 preferably contains 30% or more particles having a particle size of 1 μm or more, more preferably contains 50% or more particles having a particle size of 1 μm or more, and still more preferably contains 50% or more particles having a particle size of 2 μm or more. Here, the above particle sizes and proportions are the values calculated by subjecting a section to image analysis and, for example, the particle size of each particle is determined by regarding the diameter of a circle which has the same area as the relevant particle as the particle size and the proportion is determined as the value obtained by calculating the distribution of the particle sizes through the area standard.
The thickness of the emitter 12 is determined in relation to a drive voltage so that the polarization of the emitter 12 is reversed and field intensity is applied up to the extent of not causing the dielectric breakdown when a drive voltage is applied between the upper electrode 14 and the lower electrode 16. For example, assuming that the dielectric breakdown voltage of the emitter 12 is 10 kV/mm or more, when an applied drive voltage is 100 V, the necessary thickness of the emitter 12 is 10 μm or more theoretically, but it is preferable to set the thickness of the emitter 12 at about 20 μm in consideration of the allowance enough to avoid the dielectric breakdown.
The upper electrode 14 is formed so that the thickness thereof is in the range from 0.1 to 20 μm and has plural openings 20 through which the upper surface 12a of the emitter 12 is exposed to the exterior. As shown in
As it is obvious from
The lower electrode 16 is composed of a metal thin film and a desirable thickness thereof is 20 μm or less, more desirably 5 μm or less. Then, a pulse generator 18 to apply drive voltage Va between the upper electrode 14 and the lower electrode 16 is connected to the upper electrode 14 and the lower electrode 16.
The electron emitter 10A is actuated in a prescribed vacuum atmosphere and, for example, a desirable vacuum level in the atmosphere (in particular the space above the upper surface 12a of the emitter 12 in
As described above, the emitter 12 is a polycrystalline body including the plural dielectric particles 12e and second dielectric particles 12f, so on the upper surface 12a of the emitter 12, microscopic unevenness are formed in the join parts between the adjacent dielectric particles 12e or crystal grain boundaries, and as shown in
Further, as shown in
Here, in the present embodiment, the maximum angle θ between the upper surface 12a (the face in the vicinity of the top of the convex portion of a jog) of the emitter 12 and the lower face 26a of the overhang 26 in the upper electrode 14 is set so as to satisfy the expression 1°≦θ≦60°. Also, the maximum distance d in the vertical direction between the upper surface 12a of the emitter 12 and the lower face 26a of the overhang 26 in the upper electrode 14 is set so as to satisfy the expression 0 μm<d≦10 μm.
Then a triple junction (a triple point where an upper electrode 14, an emitter 12, and vacuum have contact with each other) 26c is formed at a portion where the upper surface of the emitter 12, the upper electrode 14 and the medium surrounding the electron emitter 10A (for example vacuum) contact. Then the triple junction 26c is the place (electric field concentrated portion) where the lines of electric force concentrate (electric force concentration) when drive voltage Va is applied between the upper electrode 14 and the lower electrode 16. Here, “the place where the lines of electric force concentrate” means the place where the lines of electric force originated from the lower electrode 16 at equal intervals concentrate when the lines of electric force are drawn assuming that the upper electrode 14, the emitter 12 and the lower electrode 16 are the flat plates extending infinitely in a sectional side view. The state of the accumulation of the lines of electric force (electric force concentration) can easily be simulated by the numerical analysis using the finite element method.
Further, in the present embodiment, the opening 20 is formed so that the inner edge 26b of the opening 20 serves as an electric field concentrated portion. More specifically, the overhang 26 of the opening 20 is formed so as to sharply protrude at an acute angle toward the inner edge 26b which is the tip of the overhang 26 (so as to reduce the thickness gradually) in a sectional side view. The upper electrode 14 having an opening 20 of such a shape can be formed by a simple method, namely, as stated above, by disposing the conductive particles 15 having a shape extending in the longitudinal direction in a sectional side view in the state of “lying” so that the longitudinal direction of the conductive particles 15 is along the upper surface 12a of the emitter 12 in a sectional side view. Note that, the electric field concentrated portion at the inner edge 26b of the opening 20 and the triple junction 26c as stated above are also formed at positions corresponding to the outer edges 21 at the outer periphery of the upper electrode 14 (refer to
Here, each of the openings 20 is formed so that it has the aperture 20a the diameter of which, the diameter being represented by the diameter of a virtual circle having the same area as the aperture 20a in a plan view, is in the range from 0.1 μm to 20 μm in average. The reasons are as follows.
As shown in
Next, the principle of electron emission in an electron emitter 10A is explained on the basis of FIGS. 4 to 6. In the present embodiment, as shown in
The principle is further explained on the assumption that, in the initial state, the emitter 12 is polarized in one direction and for example the negative pole of a dipole is in the state of being directed to the upper surface 12a of the emitter 12 (refer to
Firstly, in the initial state wherein the reference voltage is applied, as shown in
Thereafter, when the negative voltage V2 is applied, the polarization is reversed (refer to
Thereafter, when the negative voltage V2 returns to the reference voltage again as shown in
Here, also at the outer edge of the outer periphery, of the upper electrode 14, where no opening 20 is formed, electrons are discharged in the same way as described above.
Further, in the present embodiment, as shown in
In actual operation, the condenser C1 caused by the emitter 12 is not serially connected to the integrated condenser C2 as it is and the components of the serially connected condensers vary in accordance with the number of the formed openings 20 of the upper electrode 14, the overall formed area and others.
Here, as shown in
Since the portion connected in series and the remaining portion are connected to each other in parallel, the overall capacitance is 27.5 pF. The capacitance corresponds to 78% of the capacitance 35.4 pF of the condenser C1 caused by the emitter 12. It means that the overall capacitance is smaller than the capacitance of the condenser C1 caused by the emitter 12.
In this way, with regard to the integrated capacitance of the condensers Ca caused by the plural gaps 28, the capacitance of the condensers Ca caused by the gaps 28 is relatively small and thus most part of the applied voltage Va is applied to the gaps 28 and a high output of electron emission can be secured at the gaps 28 on account of the voltage divided from the condenser C1 caused by the emitter 12.
Further, since the integrated condenser C2 is serially connected to the condenser C1 caused by the emitter 12, the overall capacitance is smaller than the capacitance of the condenser C1 caused by the emitter 12. As a result, preferable effects such as a high electron emission output and a low overall electric power consumption can be obtained.
Next, referring to
The aerosol deposition equipment 60 is provided with a film forming chamber 70 and a first aerosol feeder 80. The film forming chamber 70 is provided with: a vacuum chamber 71 the interior of which is maintained to a prescribed vacuum level; an X-Y-Z-θ stage 72 that holds a glass substrate 11 in the vacuum chamber 71 and allows the glass substrate 11 to move in an arbitrary direction; a first nozzle 73 fixed in the vacuum chamber 71 in order to spray aerosol on the glass substrate 11 held on the X-Y-Z-θ stage 72; and a vacuum pump 76 to maintain the interior of the vacuum chamber 71 to a prescribed vacuum level.
The vacuum level in the interior of the vacuum chamber 71 is set at around 50 to 1,000 Pa with the vacuum pump 76. The first nozzle 73 has an opening in the shape of a slit 10 mm×0.4 mm in size and ejects through the opening aerosol supplied from the first aerosol feeder 80 toward the glass substrate 11 in the vacuum chamber 71 having the above vacuum level. The glass substrate 11 is transferred in an arbitrary direction by the X-Y-Z-θ stage 72 and thereby, while moving relative to the first nozzle 73, aerosol is sprayed.
The first aerosol feeder 80 is provided with: a first aerosolizing chamber 82 to store material powder 81; a compressed gas supply source 83 to store a carrier gas used for mixing it with the material powder 81 and generating aerosol in the first aerosolizing chamber 82; a compressed gas feed pipe 84 to feed the carrier gas from the compressed gas supply source 83 to the first aerosolizing chamber 82; a vibration agitator 85 to impose vibration on the first aerosolizing chamber 82 in order to mix the material powder 81 with the carrier gas and aerosolize them in the first aerosolizing chamber 82; an aerosol feed pipe 86 to feed aerosol from the first aerosolizing chamber 82 to the first nozzle 73; and a control valve 87 to control the amount of aerosol ejected from the first nozzle 73 to the glass substrate 11 by adjusting the flow rate of the aerosol in the aerosol feed pipe 86.
The material powder 81 is a mixture of a relatively coarse dielectric powder having an average particle size of 1.5 to 5 μm (nominal value by the maker; measured by laser diffractometry or with a Coulter Multisizer (a registered trademark)), and fine dielectric particles having an average particle size of 1 μm (nominal value by the maker; the same as above).
This material powder 81 is aerosolized by undergoing vibration from the vibration agitator 85 and thereby being mixed with a carrier gas violently in the first aerosolizing chamber 82. Since the aerosol behaves like a fluid, in the state of opening the control valve 87, the aerosol flows toward the vacuum chamber 71 due to the pressure difference between the first aerosolizing chamber 82 and the vacuum chamber 71 and is ejected toward the glass substrate 11 at a high speed through the first nozzle 73. Here, as the carrier gas stored in the compressed gas supply source 83, besides compressed air, an inert gas such as a nitrogen gas or a noble gas including a helium gas, an argon gas or the like can be used. Then by opening the control valve 87 and ejecting the aerosol of the material powder 81 toward the glass substrate 11, the emitter 12 is formed on the glass substrate 11 (more precisely on the lower electrode 16).
The aerosol deposition equipment 160 is provided with a film forming chamber 70, a first aerosol feeder 80, and a second aerosol feeder 90. The film forming chamber 70 is provided with, besides the aforementioned first nozzle 73, a second nozzle 74 to eject aerosol supplied from the second aerosol feeder toward the glass substrate 11 in the film forming chamber 70. The first aerosol feeder 80 is the same as the case of
The material powder 181 is a dielectric in the form of relatively coarse powder 1.5 to 5 μm in average particle size (nominal value by a maker; the same as above) and the material powder 191 is a dielectric in the form of fine powder 1 μm or smaller in average particle size (nominal value by a maker; the same as above). Then by opening one or both of the control valve 87 and control valve 97 as required, the emitter 12 filled with the second dielectric particles 12f packed in the spaces surrounding the first dielectric particles 12e, is obtained on the glass substrate 11 (more precisely, on the lower electrode 16).
Note that, though situations differ in accordance with the ejection conditions of aerosol, mechanical properties of the material powder 81, 181 and 191, and the like, since those particles of the material powder 81, 181 and 191 are ejected toward the glass substrate 11 and undergo impulsive force when they collide with the glass substrate 11 and the like, the shapes of the particles of the material powder 81, 181 and 191 are generally different from the shapes of the first dielectric particles 12e and the second dielectric particles 12f composing the emitter 12 formed on the glass substrate 11.
Further, the emitter 12 can be formed also by the sol impregnation method. In the case of using the sol impregnation method, firstly, a dielectric thin plate of the first dielectric particles 12e is first prepared. This dielectric thin plate is formed by pasting dielectric powder with a disperse medium such as an organic binder, forming the paste into prescribed shape and thickness by screen printing or the like, and thereafter decomposing and transpiring the disperse medium at the temperature in the range from 400° C. to 500° C. The dielectric powder used to form this dielectric thin plate is a relatively coarse dielectric powder of average particle size 1.5 to 5 μm (nominal value by the maker; the same as above), or a mixture of this dielectric powder with a fine dielectric powder of average particle size 1 μm (nominal value by the maker; the same as above) or less. Then fluid dispersion (hereunder referred to as “sol solution”) of the second dielectric particles 12f is dripped on the dielectric thin plate and the sol solution is impregnated into the dielectric thin plate. Thereafter, after excessive sol solution is removed with a spin coater, the dielectric layer is dried at the temperature in the range from 100° C. to 200° C. and then subjected to heat treatment at the temperature in the range from 400° C. to 500° C. The impregnation of sol solution, drying and heat treatment are generally repeated several times. Then finally, by applying heat treatment at a temperature of 700° C. or lower, the emitter 12 is formed. Here, in the above processes, as the dielectric layer before the sol solution is dripped, a green sheet or even a film formed by the aerosol deposition method can also be used.
In this way, the emitter 12 formed by the aerosol deposition method or the sol impregnation method is formed as a very dense dielectric thin film having a high filling density, and adequate dielectric properties are obtained without sintering at 900° C. or higher. Therefore, film-forming can be performed at the low temperature of 700° C., 600° C. or lower. The upper surface 12a of the emitter 12 can also be formed in any desired surface state (surface roughness) depending on the film-forming conditions of the aerosol deposition method or the sol impregnation method.
As the dielectric material composing the emitter 12, a dielectric of a comparatively high relative dielectric constant, for example 1,000 or more, can preferably be adopted. As such a dielectric, adopted can be, besides barium titanate: a substance such as lead zirconate, lead magnesium niobate, lead nickel niobate, lead zinc niobate, lead manganese niobate, lead magnesium tantalate, lead nickel tantalate, lead antimony stannate, lead titanate, lead magnesium tungstate, or lead cobalt niobate; ceramics containing arbitrary combination thereof; a substance mainly containing a chemical compound composed of those substances by 50 wt. % or more; or a substance produced by further adding to the above ceramics oxides of lanthanum, calcium, strontium, molybdenum, tungsten, barium, niobium, zinc, nickel, manganese, or the like, the combination thereof, or other chemical compounds in an appropriate manner.
For example, in the case of binary system nPMN-mPT (n and m are mole fractions) including lead magnesium niobate (PMN) and lead titanate (PT), by increasing the mole fraction of PMN, the Curie point can be lowered and thus the relative dielectric constant at room temperature can be increased. In particular, the relative dielectric constant is preferably 3,000 or more in the case of n=0.85 to 1.0 and m=1.0−n. For example, a relative dielectric constant of 15,000 can be obtained at room temperature in the case of n=0.91 and m=0.09, and even 20,000 at room temperature in the case of n=0.95 and m=0.05.
Next, in the case of ternary system including lead magnesium niobate (PMN), lead titanate (PT) and lead zirconate (PZ), in addition to the increase of the mole fraction of PMN, to obtain a composition close to the morphotropic phase boundary (MPB) between tetragonal system and pseudo-cubic system or between tetragonal system and rhombohedral-system contributes preferably to the increase of a relative dielectric constant. For example, particularly preferably, the relative dielectric constant is 5,500 in the case of PMN:PT:PZ=0.375:0.375:0.25 and the relative dielectric constant is 4,500 in the case of PMN:PT:PZ=0.5:0.375:0.125.
Further, it is preferable to improve a relative dielectric constant by further mixing metal such as platinum with such a dielectric in the range allowing the insulativity to be secured. In this case, it is preferable, for example, to mix platinum by about 20% in weight (about 17% in volume) with a dielectric.
More specifically, for example, metal fine particles (for example about 0.01 to 1 μm) having a particle size larger than that of the dielectric particles 12f and smaller than that of the dielectric particles 12e, are mixed with the material powder 81. Alternatively, the metal fine particles may be mixed with the material powder 181 and/or 191. Due to this, the structure shown in
That is, the particle size of the dielectric particles (hereafter referred to as “coarse particles”) contained in the material powder 81 or 181 constituting the dielectric particles 12e is much larger than that of the particle size of the dielectric particles (hereafter referred to as “fine particles”) contained in the material powder 81 or 191 constituting the dielectric particles 12f. Hence the kinetic energy when the coarse particles are sprayed from the nozzle 73 is much larger than the kinetic energy of the fine particles sprayed from the nozzle 73 or 74. Therefore, in some sizes and material quality of the coarse particles, there are cases where almost all of the coarse particles are flicked on the glass substrate 11 (on the lower electrode 16) or the layer (hereafter, “film-forming layer”) of the dielectric particles 12e, 12f formed on the glass substrate 11. In those cases, it is very difficult to form the film with these coarse particles, and there is a possibility that the coarse particles will damage the surface of the glass substrate 11/surface of the lower electrode 16, or the film-forming layer. To cope with the problem, by mixing the aforesaid metal fine particles with the material powder 81, and using the ductility of the metal (causing the metal to function as a bond), the film-forming layer can be formed satisfactorily on the glass substrate 11.
Here, as metallic fine particles, besides the aforementioned platinum, used can be fine particles of noble metal such as gold, silver or the like, base metal such as nickel, copper, iron or the like, or alloy such as silver-palladium, platinum-rhodium, brass or the like. In the case of alloy, the capability of forming a layer improves by properly adjusting the composition and thus regulating ductility, and therefore it is preferable to use alloy fine particles as the metal fine particles.
Further, a preferable mix rate is 0.01 to 20%, yet preferably 0.05 to 10% in volume.
By taking above measures, the metal portions 12k function as a binder between adjacent dielectric particles 12e and 12f in the emitter 12′ and also between the emitter 12′ and the lower electrode 16, and thereby the capability of forming the upper layer 12c and the lower layer 12d improves.
Further, by filling the gaps between adjacent dielectric particles 12e and 12f with the metal portions 12k, the dielectric constant of the emitter 12′ increases and it becomes possible to realize good dielectric performances at the emitter 12′.
Further, in order not to electrically connect the upper electrode 14 (refer to
Further, a piezoelectric and electrostrictive layer, an antiferroelectric layer, or the like can be used as the emitter 12 as stated above and, when a piezoelectric and electrostrictive layer is used as the emitter 12, as such a piezoelectric and electrostrictive layer, adopted can be, for example: a substance such as lead zirconate, lead magnesium niobate, lead nickel niobate, lead zinc niobate, lead manganese niobate, lead magnesium tantalate, lead nickel tantalate, lead antimony stannate, lead titanate, barium titanate, lead magnesium tungstate, or lead cobalt niobate; or ceramics containing the arbitrary combination thereof.
It goes without saying that a substance containing those chemical components by 50% or more in weight as the main component can also be used. Among the above ceramics, ceramics containing lead zirconate are most frequently used as a constituent material of a piezoelectric and electrostrictive layer composing the emitter 12.
Further, in the case of composing a piezoelectric and electrostrictive layer of ceramics, used may also be a ceramics produced by further adding to it: oxides of lanthanum, calcium, strontium, molybdenum, tungsten, barium, niobium, zinc, nickel, manganese, or the like; combination of some of them; or other chemical compounds in an appropriate manner. Furthermore, ceramics produced by adding SiO2, CeO2, Pb5Ge3O11, or combination of some of them to the above ceramics may also be used. More specifically, a material produced by adding SiO2 by 0.2 wt. %, CeO2 by 0.1 wt. %, or Pb5Ge3O11 by 1 to 2 wt. % to a PT-PZ-PMN system piezoelectric material may preferably be used. For example, it is preferable to use ceramics containing a component including lead magnesium niobate, lead zirconate, and lead titanate as the main component and further containing lanthanum and strontium. A piezoelectric and electrostrictive layer may be either dense or porous and, if porous, a preferable porosity is 40% or less.
In the case of using an antiferroelectric layer as the emitter 12, as the antiferroelectric layer, desirable is: a substance containing lead zirconate as the main component; a substance containing a component including lead zirconate and lead stannate as the main component; a substance produced by further adding lanthanum oxide to lead zirconate; or a substance produced by adding lead zirconate and lead niobate to a component including lead zirconate and lead stannate. Further, the antiferroelectric layer may be porous and, if porous, a preferable porosity is 30% or less.
Further, in the case of using strontium tantalate bismuthate (SrBi2Ta2O9) as the emitter 12, polarization reversal fatigue is preferably small. Such a material having small polarization reversal fatigue is a laminar ferrodielectric chemical compound and expressed by the general expression (BiO2)2+(Am−1BmO3m+1)2−. Here, the ions of the metal A are Ca2+, Sr2+, Ba2+, Pb2+, Bi3+, La3+, etc. and the ions of the metal B are Ti4+, Ta5+, Nb5+, etc. Further, it is also possible to render semiconducting capability by further adding an additive to piezoelectric ceramics of a barium titanate system, a lead zirconate system, or a PZT system. In this case, it is possible to allow an uneven electric field distribution to form in an emitter 12 and thus to concentrate electric fields in the vicinity of the interface with the upper electrode 14 that contributes to electron emission.
Furthermore, it is possible to lower the sintering temperature by mixing for example a glass component such as lead borosilicate glass or another low melting chemical compound (for example bismuth oxide or the like) with piezoelectric, electrostrictive and antiferroelectric ceramics.
Yet further, in the case of composing an emitter 12 of piezoelectric, electrostrictive and antiferroelectric ceramics, the shape thereof may be any of a solid sheet, a laminated sheet, or a shape formed by laminating or bonding one of them on a support substrate.
In addition, by using a non-lead type material as the emitter 12 and thus raising the melting point or the transpiration temperature of the emitter 12, the emitter 12 becomes unlikely to be damaged by the collision of electrons or ions.
As the conductive particles 15 composing the upper electrode 14 of the present embodiment, scale-like powder such as scale-like graphite powder or metallic powder, or acicular or rod-like powder such as carbon nanotube is preferably used. As a method for forming the upper electrode 14, used is the method including the processes of: dispersing the scale-like powder (while using a disperser if necessary) into an organic solvent (a binder) such as ethyl cellulose and thus forming paste; applying the paste by spin coating, screen printing, dipping, spraying, or the like and thus forming a thick film of the paste on the upper surface 12a of the emitter 12; and then subjecting the thick paste film to heat treatment. In the case of forming the thick film, when the viscosity of the paste is adjusted to about 100,000 to 200,000 cps so as to be suitable for the thick film forming, a preferable film thickness after printing is considered to be about 1 to 25 μm, more preferably about 3 to 15 μm. If the film thickness is excessive, the size of an opening 20 is too small and inversely, if the film thickness is too thin, the electrical conductivity in an upper electrode 14 cannot be secured. Then after the thick film in the above thickness range is formed, by applying heat treatment, the formed film to be the upper electrode 14 on the emitter 12 is transformed into only the electrode material along with the decomposition of the binder and simultaneously plural openings 20 are formed. As a result, without specific patterning treatment such as masking treatment being applied, plural openings 20 and overhangs 26 are formed in the upper electrode 14 as shown in
When the upper electrode 14 is formed, as stated above, it is preferable to add also conductive fine particles 19. As such conductive fine particles 19, besides metallic fine particles, carbonaceous fine particles such as spheroidized graphite powder, carbon black, or the like can be used. When conductive fine particles 19 are added, in addition to metallic fine powder classified into a prescribed particle size range, a substance that can be finally transformed into conductive fine particles by sintering can also be used, for example resinate or the like may be used.
When a carbonaceous material is used as the conductive particles 15 (particularly in an oxygen atmosphere such as the atmospheric air or the like), the temperature at the heat treatment of the upper electrode 14 is preferably 500° C. or lower and further, when conductive fine particles 19 are added, it is necessary to select a temperature at which the conductive fine particles 19 do not aggregation or cause grain growth in excess of a prescribed particle size.
Meanwhile, as the lower electrode 16, a material having electrical conductivity such as metal is used and the lower electrode 16 is composed of platinum, molybdenum, tungsten, or the like. Further, the lower electrode 16 is: composed of a conductor showing resistance to a high-temperature oxidizing atmosphere, such as pure metal, alloy, mixture of insulative ceramics and pure metal, mixture of insulative ceramics and alloy or the like; or preferably composed of high-melting noble metal such as platinum, iridium, palladium, rhodium, molybdenum or the like, a substance mainly composed of alloy such as silver-palladium, silver-platinum, platinum-palladium or the like, or a cermet material composed of platinum and ceramics. More preferably, the lower electrode 16 is composed of platinum only or a material mainly composed of platinum type alloy. Furthermore, as the lower electrode 16, a material of a carbon or graphite type may also be used. Here, the proportion of the ceramic material added to the electrode material is preferably 5 to 30% in volume. It goes without saying that the same material as used for the aforementioned upper electrode may also be used. Then when the lower electrode 16 is made of the aforementioned metal or carbonaceous material, the aforementioned thick film forming method is preferably used.
As described in detail above, in the electron emitter 10A according to this embodiment, the emitter 12 includes the relatively coarse first dielectric particles 12e of micron order, and the fine second dielectric particles 12f of submicron order, and the second dielectric particles 12f are disposed so that they fill the spaces surrounding the plural first dielectric particles 12e. Therefore, by suitably adjusting the filling degree of the second dielectric particles 12f in the spaces surrounding the first dielectric particles 12e, the physical/electrical properties required of the emitter 12 can be set appropriately without difficulty.
Also, microscopic unevenness are formed by the joins and crystal grain boundaries of the adjacent plural dielectric particles 12e, 12f in the upper surface 12a of the emitter 12. If these microscopic unevenness are used, an overhanging shape such as that of the aforesaid overhang 26 can easily be formed. In particular, merely by arranging the conductive particles 15 having a shape extending in the longitudinal direction in a sectional side view on the upper surface 12a of the emitter 12, the aforementioned overhangs 26 are formed easily and abundantly. Also, due to the microscopic unevenness in the upper surface 12a of the emitter 12, the surface area of the upper surface 12a which has the electron emission function in the emitter 12 is enlarged, and the electron emission amount can be increased. Also, the second dielectric particles 12f are of an identical material to that of the first dielectric particles 12e, so there is no difference of surface energy between the two in the joins, the first dielectric particles 12e and second dielectric particles 12f are highly compatible in the contact parts, and the filling density of the dielectric material in the emitter 12 can easily be increased.
If the emitter including the dielectric is formed into a film and filled by the aerosol deposition method and/or the sol impregnation method, a high dielectric thin layer of high dielectric constant with few defects and a high filling ratio can be obtained by a relatively low temperature process. Therefore, the electron emitter 10A can be formed onto a relatively economical substrate with a relatively low heat resistance such as a glass substrate, so a large size screen for an FED can be obtained at low cost. The physical properties of the emitter 12 can be adjusted as desired by the film-forming conditions of the aerosol deposition method or the sol impregnation method. Further, by performing heat treatment at 500° C. or less when the upper electrode 14 is formed, the dielectric constant of the emitter 12 increases and the electron emission performance improves.
Further, in an electron emitter 10A according to the present embodiment, by forming a overhang 26 at the upper electrode 14, a triple junction 26c is formed at a site other than the inner edge 26b of the upper electrode. Further, an opening 20 has such a shape as to allow the inner edge 26b of the opening 20 to serve as an electric field concentrated portion. Hence it is possible to further increase the number of the electric field concentrated portions than in the case where the overhangs 26 are not formed. In particular, since a overhang 26 in the present embodiment is formed so that it protrudes at an acute angle toward the inner edge 26b that is the tip of the overhang 26, the degree of electric field concentration is higher than the case where the shape of the inner edge 26b is a right angle or an obtuse angle and it becomes possible to increase the amount of the electrons accumulated on the upper surface 12a of the emitter 12.
Furthermore, in the upper surface 12a of the emitter 12, the maximum angle θ between the upper surface 12a (the face in the vicinity of the top of the convex portion of a jog) of the emitter 12 and the lower face 26a of a overhang 26 in the upper electrode 14 is set so as to satisfy the expression 1°≦θ≦60° and the maximum distance d in the vertical direction between the upper surface 12a of the emitter 12 and the lower face 26a of the overhang 26 in the upper electrode 14 is set so as to satisfy the expression 0 μm<d≦10 μm, and by the configuration it becomes possible to increase the degree of the electric field concentration at a gap 28 and thus to increase the amount of the electrons accumulated on the upper surface 12a of the emitter 12.
Further, by forming the aforementioned overhang 26, a gap 28 is formed between the lower face 26a, opposite to the emitter 12, of the overhang 26 at the opening 20 in the upper electrode 14 and the emitter 12, most part of a drive voltage Va is substantially applied to the gap 28 by the influence of the electrostatic capacity of the virtual condenser at the gap 28, and thus the electric field at the opening 20 is intensified. Thereby it becomes possible to decrease the absolute value of the drive voltage Va required for obtaining the same field intensity at the opening 20.
Further, since the overhang 26 of the upper electrode 14 functions as a focus electron lens or a gate electrode (control electrode), the linearity of the emitted electrons can be improved. This is advantageous to the reduction of cross talk in the case of arraying many electron emitters 10A and using those, for example, as the electron sources of a display.
In addition, as stated above, the openings 20 are formed numerously in a region of the upper electrode 14 in a plan view, and the electric field concentrated portions at the inner edges 26b of the openings 20 and the triple junctions 26c are also formed numerously in a region of the upper electrode 14 in a plan view (they are formed numerously also at the positions corresponding to the outer edge 21 at the outer periphery (refer to
Next, a display 100 using electron emitters 10A according to the present embodiment is explained on the basis of
In the display 100, as shown in
The reason why such a range is selected is that, if the vacuum level is low, there arise the following risks; (1) since the gas molecules are abundant in the space, plasma tends to be generated and, if plasma is generated in too large quantity, a large number of positive ions collide with the upper electrode 14, resulting in the increase of damages, and (2) the emitted electrons undesirably collide with gas molecules before they reach the collector electrode 132 and the excitation of the phosphor 134 by the electrons fully accelerated by the collector voltage Vc is insufficiently secured.
On the other hand, if the vacuum level is high, though electrons are likely to be emitted from the points where electric fields concentrate, the problem is that the sizes of the supports of the structure and the sealing system for vacuum increase and thus that is disadvantageous for downsizing.
Next, an electron emitter 10B according to the second embodiment is explained referring to
Further, an electron emitter 10C according to the third embodiment is explained referring to
Here, it is preferable that the conductive fine particles 19 are exposed on the top surface of the upper electrode 14, in particular at the overhangs 26 as shown in
Further, when a carbonaceous material (for example graphite) is used as the conductive particles 15 and silver is used as the conductive fine particles 19 and then heating treatment is applied at the time of the forming of the upper electrode 14, during the heat treatment, since the graphite or the like in the vicinity of the silver fine particles is eroded by oxidation, the outer edge of the upper electrode 14 is likely to take the shape having a sharp edge or to have openings caused by perforation inside the electrode. Thereby the electric field concentrated portions further increase and it becomes possible to obtain a more suitable electrode configuration.
An electron emitter 10D according to a fourth embodiment will now be described referring to
The surface layer 12g includes plural dielectric particles 12g1. The dielectric particles 12g1 include a dielectric material prepared beforehand by sintering or the like. For example, the dielectric particles 12g1 are of a piezoelectric/electrostrictive ceramics thin plate material which is sintered so that it has predetermined dielectric properties (dielectric properties such as for example piezoelectric/electrostrictive properties or electron emission properties). Specifically, the dielectric particles 12g1 are obtained by for example breaking and/or cutting the previously sintered piezoelectric/electrostrictive ceramics thin plate to a predetermined size.
The size of this dielectric element 12g can be suitably selected according to the surface roughness and flatness of the upper surface 12a′ of the dielectric layer due to the dielectric particles 12e, 12f. In order to obtain satisfactory adhesion properties between the upper surface 12a′ and surface layer 12g and satisfactory dielectric properties (as few voids as possible) in the emitter 12″ even if the surface roughness and flatness of the upper surface 12a′ is not so good, the thickness is preferably approximately in the range from 10 to 20 μm, and the particle size (dimensions in the longitudinal direction i.e., the direction perpendicular to the thickness when viewed in lateral cross-section of the emitter 12″) is preferably in the range from 20 to 200 μm. The particle size of the dielectric particle 12g is a value obtained by analyzing the scanning electron microscope (SEM) image in an upper plan view and computation, e.g., the value obtained by approximating the particle to a circle having an equivalent area to that of the particle, taking the diameter of this circle as the particle size and computing the particle size distribution in terms of the area.
This surface layer 12g is joined to and formed in one piece with the upper surface 12a′, for example by sintering or the like. Specifically, films of dielectric layers of the dielectric particles 12e, 12f are formed on the glass substrate 11, plural dielectric particles 12g1 are laid over the upper surface 12a′ of these dielectric layer films, heat treatment is performed to form a one-piece construction, and the emitter 12″ is thereby obtained.
Herein, if the dielectric layers of the dielectric particles 12e, 12f are formed by for example the above-described aerosol deposition method, an additive including glass or metal fine particles is mixed with the material powder and sprayed on the glass substrate 11, the additive functions as a binder, so the emitter 12″ can be formed at a low heat treatment temperature (if metal fine particles are mixed in, as shown in
In the electron emitter 10D according to the embodiment having the aforesaid construction, predetermined dielectric properties can definitively be implemented on the upper surface 12a′ of the emitter 12″, so very good electron emission properties can be obtained. Specifically, the method of forming the dielectric layers of the dielectric particles 12e, 12f has a considerable electrical/mechanical influence on the dielectric layers, so the dielectric properties in the vicinity of the upper surface 12a′ of the dielectric layers may be less than those expected. Hence, in this embodiment, the dielectric layers of the dielectric particles 12e, 12f function as an insulating film of high dielectric constant. By forming the upper surface 12a of the emitter 12″ by the surface of the surface layer 12g having predetermined dielectric properties which is fixed to the insulating film, an electron emitter having predetermined properties (dielectric element) can easily be manufactured.
When the upper surface 12a′0 of the dielectric surface due to the dielectric particles 12e, 12f is formed very smooth and flat, the surface layer 12g may be formed from one dielectric thin plate instead of an aggregation of the plural dielectric particles 12g1.
Hereinabove, a typical embodiment of the present invention has been described, but it will be understood that the invention is not to be construed as being limited in any way thereby, and various modifications may be made without departing from the scope and spirit thereof.
For example, in the above embodiments, the upper electrode 14 and the lower electrode 16 are formed on the different surfaces (the upper surface and the lower surface) of the emitter 12, but they may be formed on an identical surface (the upper surface).
The emitter 12′ containing metal (
Further, with regard to the shape of an opening 20 serving as an electric field concentrated portion at the inner edge, in addition to the shapes explained in the above embodiments, various shapes may be adopted. For example, as shown in
Further, when an opening 20 having such a shape as to serve as an electric field concentrated portion at the inner edge thereof is adopted as stated above, the upper surface 12a of the emitter 12 formed by the aerosol deposition method and/or the sol impregnation method may be smooth like a mirror surface. Thereby, by selecting the shape of the opening 20, it becomes possible to secure a high packing density and a high dielectric constant of the emitter 12 while increasing the number of the electric field concentrated portions, and hence to increase the field intensity at the electric field concentrated portions and also increase the amount of the emitted electrons more than ever.
In the sol impregnation method of the aforesaid embodiment, the dielectric fine particles are dispersed in an organic solvent, but a precursor solution of dielectric fine particles which precipitates the dielectric fine particles due to heating or the action of a reaction initiator or the like, may also be used.
Furthermore, in the production method employed in the above embodiments, the opening 20 of the upper electrode 14 can be formed merely by forming a thick film by controlling the viscosity and compounding ratio of paste and the film thickness even though specific masking or the like is not applied, but, as shown in
| Number | Date | Country | |
|---|---|---|---|
| 60588194 | Jul 2004 | US | |
| 60646676 | Jan 2005 | US |
