SPECIFICATIONProcess for controlling density of fine particlesBACKGROUND OF THE INVENTIONField of the InventionThe present invention relates to a process for controlling a density of fine particles, employed for transportation or blowing of fine particles and adaptable to film forming, formation of composite material, doping etc. with fine particles, or a field of fine particle formation.
In the present specification, the fine particles include atoms, molecules, ultra-fine particles and general fine particles. The ultra-fine particles means those generally smaller than 0.5 ,um, obtained for example by evaporation in gas, plasma evaporation, chemical vapor reaction, colloidal precipitation in a liquid or pyrolysis of liquid spray. The general fine particles mean fine particles obtained by ordinary methods such as mechanical crushing, crystallization or precipitation. A beam means a flow with a substantially constant cross section along the flow direction independently of the geometry of said cross section. Further, a density of fine particles means the one in the flow. In other words, it is defined as the number of fine particles per unit volume in case of general fine particles or a mass of fine particles per unit volume in other cases.
Description of the Related ArtIn general, fine particles are dispersed and suspended in a carrier gas and are transported by the flow of said carrier gas.
Conventionally, the control of flow of fine particles in the transportation thereof has merely been achieved by defining the entire flow of the fine particles flowing together with the carrier gas by means of a pipe or a casing utilizing the pressure difference between the upstream and downstream sides. Consequently the flow of fine particles is inevitably dispersed over the entire pipe or casing defining the flow path, though there is certain distribution in the flow.
In case of blowing the fine particles to a substrate, they are generally ejected with carrier gas from a nozzle. The nozzle employed in such fine particle blowing is a straight or convergent nozzle, and the cross section of the flow of fine particles immediately after the ejection is constricted according to the area of the nozzle outlet. However the flow is at the same time diffused at the nozzle outlet, so that said constriction is only temporary and the flow velocity does not exceed acoustic velocity. However, in case of transporting the fine particles while dispersing them in a whole pipe or casing defining the flow path, for example, contact of the fine particles with the walls of the pipe or casing results in a broad density-distribution of the fine particles in the flow.Therefore, for example, in contact of the fine particles with a gas to be reacted on the way of transportation thereof, the contacting condition varies because of a nonuniform density of the fine particles. Thus, a stable and uniform contacting condition can not be afforded.
On the other hand, the conventional straight or convergent nozzle generates a diffused flow in which the fine particles show a large districution in density. Therefore, in case of blowing fine particles onto a substrate, it is difficult to achieve uniform blowing and to control the area in which such uniform blowing is obtained.
The United States Patent No. 4,200,264 discloses an apparatus for producing metallic Mg orCa by carbon reduction method.
In said apparatus, a reduction reaction is caused by heating an oxide of Mg or Ca with carbon in a reaction chamber, and the resulting gaseous mixture is introduced into a divergent nozzle to cause adiabatic expansion for cooling thereby obtaining fine particles of Mg or Ca.
The divergent nozzle employed in said patent is limited to that functioning under an underexpansion condition.
The use of such nozzle under such condition certainly allows to create an supersonic velocity in the passing gas, but the passing gas flow is diffused at the nozzle orifice so that a flow with a substantially constant cross section cannot be obtained.
Consequently there results a low trapping yield in case of trapping the resulting fine particles for example on a substrate.
SUMMARY OF THE INVENTIONThe present invention is to overcome the above-explained problems.
More specifically, an object of the present invention is to provide novel process for controlling a density of fine particles.
The above-mentioned object can be achieved by the present invention as will be explained in the following.
According to one aspect of the present invention, there is provided a process for controlling a density of fine particles, comprising: providing a convergent-divergent nozzle in a flow path of said fine particles; and allowing said fine particles to pass through said nozzle.
According to another aspect of the present invention, there is provided process for controlling a density of fine particles, comprising: providing a convergent-divergent nozzle in a flow path of said fine particles; and suitably selecting a pressure ratio P/Po of a pressure P in a downstream side to a pressure Po in a upstream side, and a ratio A/A* of an aperture cross-sectional area A of an output to that A* of a throat of said nozzle.
BRIEF DESCRIPTION OF THE DRAWINGSFigure 1 is a schematic view showing the basic principle of the present invention;Figure 2 is a schematic view of a film-forming process with ultra-fine particles embodying the present invention;Figures 3A to 3C are views showing embodiments of gas exciting means;Figures 4A to 4C are views showing shapes of the convergent-divergent nozzle;Figure 5 is a schematic view of a skimmer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSThe present invention is illustrated below referring to the Figures.
Fig. 1 is a schematic view showing the basic principle of the present invention, that is, of a process for controlling a density of the fine particles, comprising: providing a convergentdivergent nozzle in a flow path of said fine particles; and allowing said particles to pass through said nozzle. The present invention based on the principle can cause a flow of fine particles to become uniform and constitute a beam.
A convergent-divergent nozzle 1 employed in the present invention has an aperture cross section which, as shown in Fig. 1, is gradually reduced from an inlet 1 a to an intermediate throat 2 and is then gradually enlarged toward an outlet 1 b. For the convenience of explanation, in Fig. 1, the inlet and outlet of the convergent-divergent nozzle 1 are respectively connected to a closed upstream chamber 3 and a closed downstream chamber 4. However, the inlet and outlet of the convergent-divergent nozzle 1 of the present invention may be connected to closed or open systems as long as the fine particles are caused to pass together with carrier gas by a pressure difference therebetween.
An optimum expansion condition in the present invention means that the pressure P, at the nozzle outlet 1 b is equal to the pressure P in the downstream chamber 4, whereby the flow from the nozzle has a beam characteristic.
Under an under expansion condition P1 > P, the ejected flow rapidly diverges toward the outside, starting from the outlet of the nozzle, so that a uniform flow cannot be obtained. On the other hand, under an overexpansion condition P1 < P, the flow is liable to cause peeling-off in the nozzle, thus becoming unstable and also tends to generate a shock wave and being not suitable for the present purpose.
For obtaining an optimum expansion flow, pressure sensors are provided at or around the outlet of,the nozzle and the downstream chamber respectively, and P at the upstream portion and P at the downstream portion is controlled so that the pressures detected by the sensors may be approximately equal to each other.
In the present invention, a pressure difference between the upstream chamber 3 and the downstream chamber 4 is generated, as shown in Fig. 1, by supplying the upstream chamber 3 with carrier gas in which the fine particles are dispersed in a suspension state, and evacuating the downstream chamber 4 with a vacuum pump 5, whereby the supplied carrier gas containing the fine particles flows from the upstream chamber 3 to the downstream chamber 4 through the convergent-divergent nozzle 1.
The convergent-divergent nozzle 1 performs functions not only of ejecting the fine particles together with carrier gas according to the pressure difference between the upstream and downstream sides, but also of a diffuser, that is, rendering a density of the ejected fine particles uniform. Such uniform density of fine particles can be utilized for blowing the fine particles uniformly onto a substrate 6.
The convergent-divergent nozzle 1 is capable of accelerating the flow of fine particles ejected with the carrier gas, by suitable selection of a pressure ratio P/P of the pressure P in the downstream chamber 4 to the pressure P0 in the upstream chamber, and a ratio A/A of the aperture cross-sectional area A of the outlet 1 b to that A of the throad 2. If said ratio P/PO of the pressures in the upstream and downstream chambers 3, 4 is larger than a critical ratio of pressure, the flow velocity at the outlet of the nozzle 1 becomes subsonic or less, and the fine particles and the carrier gas are ejected at a reduced velocity. On the other hand, if said pressure ratio P/PO is the critical ratio of pressure or less, said flow velocity at the outlet becomes supersonic, so that the fine particles and the carrier gas are ejected at an supersonic velocity.
In such ejection with a pressure ratio lower than the critical ratio of the pressure, the carrier gas and the fine particles form a uniform diffused flow, and the fine particles are uniformly dispersed over a relatively large area. Thus, the density of the fine particles can be rendered to be relatively low.
On the other hand, in case of the aforementioned ultrasonic emission, the carrier gas and the fine particles linearly proceed as a beam, substantially maintaining the cross section immediately after the emission from the nozzle. In this manner the flow of the fine particles is also formed as a beam and a high density flow, which is transported at an ultrasonic speed with minimum diffusion in the downstream chamber 4, in a state spatially independent from the walls thereof.
If the flow of the fine particles is assumed to a compressive one-dimensional flow with adiabatic expansion, the Mach number M that can be reached by the flow is determined by the pressure P0 of the upstream chamber and the pressure P of the downstream chamber, according to a following formula:
wherein u is the velocity of the fine particle flow, a is the local acoustic velocity at this point, and y is the ratio of specific heats of said fluid; and M exceeds 1 when the ratio P/PO is the critical ratio of pressure or less.
The acoustic velocity a can be determined by a formula:
wherein T is the local temperature and R is gas constant. Also there stands a following relation among the aperture cross sections A, A of the outlet 1 b and throat 2, and the Mach number
It is therefore possible to regulate the velocity of the flow of fine particles ejected from the nozzle 1, by selecting the aperture ratio A/A* according to the Mach number M determined by the equation (1) from the pressure ratio P/PO of the upstream and downstream chambers, or by regulating said ratio P/PO according to the value of M determined by the equation (2) from the aperture ratio A/A*.The velocity u of the flow of fine particles can be determined by a following equation (3):
wherein To represents the temperature of the upstream chamber 3.
Fig. 3 schematically show an embodiment in which the present invention is applied to a process for film formation with ultra-fine particles, wherein illustrated are a convergent-divergent nozzle 1; an upstream chamber 3; a first downstream chamber 4a; and a second downstream chamber 4b.
The upstream chamber 3 and the first downstream chamber 4a are constructed as an integral unit, and, to said first downstream chamber 4a, there are detachably connected a skimmer 7, a gate valve 8 and the second downstream chamber 4b in similar unit structures, through flanges of a common diameter, which will hereinafter be referred to as common flanges. The upstream chamber 3, first downstream chamber 4a and second downstream chamber 4b are maintained at successively higher degrees of vacuum by a vacuum system to be explained later.
To a side of the upstream chamber 3 there is connected, by a common flange, a gas exciting means 9 which generates active ultra-fine particles by plasma and sends said particles to the confronting convergent-divergent nozzle 1, together with carrier gas such as hydrogen, helium, argon or nitrogen. The upstream chamber 3 may be provided with an anti-adhesion treatment on the inner walls thereof, in order to prevent the adhesion of thus generated ultra-fine particles onto said inner walls. Due to the pressure difference between the upstream chamber 3 and the first downstream chamber 4a caused by the higher degree of vacuum in the latter, the generated ultra-fine particles flow, together with said carrier gas, through the nozzle 1 to the first downstream chamber 4a.
As shown in Fig. 3(A), the gas exciting device 9 has a rod-shaped first electrode 9a housed in a tubular second electrode 9b, wherein the carrier gas and raw material gas are supplied in said second electrode 9b and an electric discharge is induced between said electrodes 9a, 9b. The gas exciting device 9 may also be constructed, as shown in Fig. 3(B), with a porous first electrode 9a for supplying the carrier gas and raw material gas therethrough to the space between the first and second electrodes, or, as shown in Fig. 3(C), with a tube composed of semi-circular electrodes 9a, 9b separated by insulators 9c into which the carrier gas and raw material gas are supplied.
The convergent-divergent nozzle 1 is mounted, by a common flange, on a lateral end of the first downstream chamber 4a directed toward the upstream chamber 3 so as to protrude in the upstream chamber 3, with the inlet 1 a opened in said upstream chamber 3 and the exit 1 b opened in said first downstream chamber 4a. Said nozzle 1 may also be mounted so as to protrude in the first downstream chamber 4a. The protruding direction of the nozzle 1 is determined according to the size, quantity and nature of the ultra-fine particles to be transported.
As explained before, the cross section of the convergent-divergent nozzle 1 is gradually reduced from the inlet 1 a to the throat 2, and is then gradually expanded to the outlet 1 b, and the differential coefficient of the streamline of the channel changes continuously and reaches zero at the throat 2, thereby minimizing the formation of grow boundary layers in the nozzle 1. In the present invention, the curve of flow path in the nozzle 1 means the curve of the internal wall on a cross-section along the direction of flow. In this manner it is rendered possible to select the effective cross section of the flow in the nozzle 1 close to the designed value and to fully exploit the performance of the nozzle 1.As shown in a magnified view in Fig. 4A, the internal periphery in the vicinity of the outlet 1 b is preferably substantially parallel to the central axis, or, has a differential coefficient equal to zero, in order to facilitate the formation of a parallel flow, since the direction of flow of the ejected carrier gas and fine particles is affected, to a certain extent, by the direction of the internal periphery in the vicinity of the outlet 1 b. However, if the angle a of the internal wall from the throat 2 to the outlet 1 b with respect to the central axis is selected smaller than 7 , preferably 5 or less as shown in Fig. 4B, it is possible to prevent the peeling-off phenomenon and to maintain a substantially uniform state in the ejected carrier gas and ultra-fine particles. Consequently, in such a case, the above-mentioned parallel internal peripheral wall can be dispensed with, and the manufacture of the nozzle 1 can be facilitated by the elimination of said parallel wall portion. Also a slit-shaped ejection of the carrier gas and ultra-fine particles can be obtained by employing a rectangular nozzle 1 as shown in Fig. 4C.
The above-mentioned peeling-off phenomenon means a formation of an enlarged boundary layer between the internal wall of the nozzle 1 and the passing fluid, caused for example by a projection on said internal wall, giving rise to an uneven flow, and such phenomenon tends to occur more frequently in the flow of a higher velocity. In order to prevent such peeling-off phenomenon, the aforementioned angle a is preferably selected smaller when the internal wall of the nozzle 1 is finished less precisely. The internal wall of the nozzle 1 should be finished with a precision indicated by three, preferably four, inverter triangle marks as defined in the JIS B 0601.Since the peeling-off phenomenon in the divergent portion of the nozzle 1 significantly affects the flow of carrier gas and ultra-fine particles thereafter, the emphasis on the surface finishing should be given to said divergent portion, in order to facilitate the fabrication of the nozzle 1. Also for preventing said peeling-off phenomenon, it is necessary to form the throat portion 2 with a smooth curve and to avoid the presence of an infinitely large differential coefficient in the change rate of the cross-sectional area.
Examples of the material of the convergent-divergent nozzle 1 include metals such as iron and stainless steel, plastics such as acrylic resin, polyvinyl chloride, polyethylene, polystyrene andpolypropylene, ceramic materials, quartz, glass etc. Said material can be selected in consideration of absence of reaction with the ultra-fine particles to be generated, ease of mechanical working, gas emission in the vacuum system. Also the internal wall of the nozzle 1 may be plated or coated with a material that prevents adhesion of or reaction with the ultra-fine particles. An example of such material is polyfluoroethylene coating.
The length of the convergent-divergent nozzle 1 can be arbitrarily decided, in consideration, for example, of the length of the apparatus. The thermal energy is converted into kinetic energy while the carrier gas and fine particles pass the nozzle 1. Particularly in case of a subsonic ejection, the thermal energy is significantly reduced to reach a supercooled state. Thus, if the carrier gas contains condensable components, it is also possible to form the ultra-fine particles by condensing said components by such super-cooling. Such method allows to obtain homogeneous ultra-fine particles, due to the formation of homogeneous nucleation. Also in such case, the convergent-divergent nozzle 1 should preferably be longer for achieving sufficient condensation. On the other hand, such condensation increases the thermal energy and reduces the kinetic energy. Consequently, in order to maintain high-speed ejection, the nozzle 1 should preferably be shorter.
By passing the flow of ultra-fine particles through the aforementioned convergent-divergent nozzle 1, with an appropriate selection of the pressure ratio P/PO of the upstream chamber 3 and the downstream chamber 4 and of an aperture area ratio A/A* of the throat 2 and the outlet 1 b, said flow is formed as a relatively high density of a beam or a relatively low density of a diffused flow, flowing at a highspeed from the first downstream chamber 4a to the second downstream chamber 4b.
The skimmer 7 is a variable aperture which can be externally regulated to stepwise vary the area of the aperture between the first downstream chamber 4a and the second downstream chamber 4b, in order to maintain a higher degree of vacuum in the second downstream chamber 4b than in the first 4a. More specifically, said skimmer is composed, as shown in Fig. 5, of two adjusting plates 11, 11' which are respectively provided with notches 10, 10' and which are slidably positioned in such a manner that said notches 10, 10' mutually oppose. Said adjusting plates 11, 11' can be moved externally, and the notches 10, 10' cooperate each other to define an aperture which allows the beam to pass and still is capable of maintaining a sufficient degree of vacuum in the second downstream chamber.Also the shape of the notches 10, 10' of the skimmer 7 and of the adjusting plates 11, 11' is not limited to the foregoing but may be semicircular V-shape shown in Fig. 7a or otherwise.
The gate valve 8 is provided with a dam-shaped valve member 13 opened or closed by a handle 12, and is fully opened when the beam flows. By closing said gate valve 8, it is rendered possible to exchange the unit of the second downstream chamber 4b while maintaining the upstream chamber 3 and the first downstream chamber 4a in vacuum state. In case the ultrafine particles are easily oxidizable metal particles, it is rendered possible to replace the unit without danger of rapid oxidation by employing a ball valve or the like as said gate valve 8 and replacing the second downstream chamber 4b together with said ball valve.
In the second downstream chamber 4b, there is provided a substrate 6 for capturing the transported ultra-fine particles as a film. Said substrate is mounted on a substrate holder 16 at an end of a sliding shaft 15 which is mounted in the second downstream chamber 4b through a common flange and is moved by a cylinder 14. In front of the substrate 6 there is provided a shutter 17 for intercepting the beam when required. Also the substrate holder 16 is capable of heating or cooling the substrate 6 to an optimum condition for capturing the ultrafine particles.
On the top and bottom walls of the upstream chamber 3 and the second downstream chamber 4b, glass windows 18 are mounted by common flanges as illustrated for enabling observation of the interior. Though not illustrated, similar glass windows are mounted by common flanges on the front and rear walls of the upstream chamber 3, first downstream chamber 4a and second downstream chamber 4b. These glass windows, when removed, may be utilized for mounting various measuring instruments or a load lock chamber through the common flanges.
In the following there will be explained a vacuum system to be employed in the present embodiment.
The upstream chamber 3 is connected to a main valve 20a through a pressure regulating valve 19. The first downstream chamber 4a is directly connected to the main valve 20a, which is in turn connected to a vacuum pump 5a. The second downstream chamber 4b is connected to a main valve 20b which is connected to a vacuum valve 5b. Rough pumps 21a, 21b are respectively connected to the upstream side of the main valves 20a, 20b through preliminary vacuum valves 22a, 22b, and are also connected to the vacuum pumps 5a, Sc through auxiliary valves 23a, 23b. Said rough pumps 21a, 21b are used for preliminary evacuation of the upstream chamber 3, first downstream chamber 4a and second downstream chamber 4b. Leak/purge valves 24a-24h are provided for the chambers 3, 4a, 4b and pumps 5a, 5b, 21a, 21b.
At first the preliminary vacuum valves 22a, 22b and the pressure regulating valve 19 are opened to effect preliminary evacuation of the upstream chamber 3 and first and second downstream chambers 4a, 4b by means of the rough pumps 21a, 21b. Then the preliminary vacuum valves 22a, 22b are closed and the auxiliary valves 23a, 23b and the main valves 20a, 20b are opened to sufficiently evacuate the upstream chamber 3 and the first and second downstream chambers 4a, 4b by the vacuum pumps 5a, 5b. In this state the opening of the pressure regulating valve 19 is controlled to achieve a higher degree of vacuum in the first downstream chamber 4a than in the upstream chamber 3, then the carrier gas and the raw material gas are supplied and the skimmer 7 is regulated to achieve a still higher degree of vacuum in the second downstream chamber 4b than in the first downstream chamber 4a.Said regulation can also be achieved by the main valve 20b. Further control is made in such a manner that each of the chambers 3, 4a, 4b is maintained at a constant degree of vacuum throughout the generation of ultra-fine particles and the film formation by beam ejection. Said control can be achieved either manually or automatically by detecting the pressures in the chambers 3, 4a, 4b and accordingly driving the pressure regulating valve 19, main valves 20a, 20b and skimmer 7.
The upstream chamber 3 and the first downstream chamber 4a may be provided with separate vacuum pumps for the above-mentioned vacuum control. However, if a single vacuum pump Sa is employed, as explained above, for evacuation in the direction of beam flow to control the degrees of vacuum in the upstream chamber 3 and the first downstream chamber 4a, the pressure difference therebetween can be maintained content even when the vacuum pump 5a has certain pulsation. It is therefore made easier to maintain a constant flow state, which is easily affected by a change in the pressure difference.
The suction by the vacuum pumps 5a, 5b is preferably from upside, particularly in the first and second downstream chambers 4a, 4b, since such suction from upside will prevent, to a certain extent, the descent of beam by gravity.
The above-explained apparatus of the present embodiment can also be subjected to following modification.
Firstly, the convergent-divergent nozzle 1 may be inclined vertically or horizontally, or may be so constructed as to perform a scanning motion over a certain range to form a film over a layer area. Such inclination or scanning motion is advantageous when combined with the rectangular nozzle shown in Fig. 4C.
It is also possible to form the nozzle 1 with an insulator such as quartz and to supply a microwave thereto, thereby generating active ultra-fine particles therein, or to form the nozzle with a translucent material and to irradiate the flow with light of various wavelength such as ultraviolet light, infrared light or laser light. Also there may be provided plural nozzles 1 to generate plural flows simultaneously. Particularly, the connection of plural nozzles 1 with independent upstream chambers 3 enables to simultaneously generate flows of different fine particles, thereby realizing lamination or mixed capture of different fine particles, or even generation of new fine particles through collisions of crossing flows.
The substrate 6 may be rendered vertically or horizontally movable, or rotatably supported, in order to receive the flow over a wide area. Also the substrate may be unwound and advanced from a roll to receive the flow, thereby subjecting a web-shaped substrate to the treatment with fine particles. Furthermore the treatment with the fine particles may be applied to a rotating drum-shaped substrate 6.
The above-explained embodiment consists of the upstream chamber 3, first downstream chamber 4a and second downstream chamber 4b, but it is also possible to eliminate the second downstream chamber 4a and second downstream chamber 4b, or to connect additional downstream chamber or chambers to the second downstream chamber. The first downstream chamber 4a may be operated under an open system if the upstream chamber 3 is pressurized, or, the upstream chamber 3 may be operated under an open system if the first downstream chamber 4a is reduced in pressure. It is also possible to pressurize the upstream chamber 3 as in an autoclave and to depressurize the first and ensuing downstream chambers.
In the foregoing explanation the active ultra-fine particles are generated in the upstream chamber 3, but they can also be generated elsewhere and supplied to said chamber together with the carrier gas. It is furthermore possible to provide a valve for opening and closing the constricting-spreading valve 1 and to intermittently open and close said valve thereby temporarily storing the fine particles in the upstream chamber 3. The energy supply in the downstream side, including the throat 2, of the nozzle 1 may be synchronized with the opening and closing of said valve to significantly reduce the load of the vacuum system and to obtain a pulsating flow of fine particles, while achieving effective utilization of the raw material gas. For a given evacuating condition, a high degree of vacuum can be more easily attained in the downstream side, by such intermittent opening and closing.In such case, there may be provided a chamber for temporarily storing the fine particles, between the upstream chamber 3 and constricting-spreading nozzle 1.
It is furthermore possible to employ plural nozzles 1 in series and to regulate the pressure ratio between the upstream side and downstream side of each nozzle, in order to maintain a constant beam speed, and to employ spherical chamber to prevent the formation of dead spaces.
According to the present invention, fine particles can be transported as a uniformly dispersed and relatively diffused flow or as a high density of a beam while controlling the density. In particular, a supersonic transportation of fine particles can be achieved in a spatially independent state, and thus the density can surely be controlled. It is also rendered possible to securely transport the active fine particles to the capturing position in a desired density, and to exactly control the area of capture by the control of the capturing surface. It is also expected to obtain a new field of reaction, realized by the presence of a beam in the form of an ultra-high speed beam flow, and by the conversion of thermal energy into kinetic energy at the beam formation, to maintain the fine particle in energetically frozen state. Furthermore, utilizing the above-mentioned energetically frozen state, the process for controlling the density of the present invention is capable of defining a microscopic state of the molecules in the fluid to handle a transition from a state to another. More specifically, there is opened a possibility of a novel gaseous chemicial reaction in which the molecule is defined to the energy level thereof and is given an energy corresponding to said energy level. There is provided a new field of energy transfer, which can be easily utilized for obtaining intermolecular compounds formed with relatively weak intermolecular forces such as hydrogen bond or van der Waals force.