JPS6015142B2 - How to coat powder with valve metal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to coating powders with valve metal, further coating such coated powders with valve metal coatings, and manufacturing capacitors from the coated powders so formed. .

It is known, for example from GB 1,030,004, to form capacitors by a process which involves coating a powder with valve metal by a chemical vapor deposition reaction method. When coating with tantalum, this is typically done by reaction of tantalum halide with hydrogen. In the case of coating with aluminum, this can be done by decomposing the aluminum tri-isobutyl into aluminum and isobutylene. Typically, the powder to be coated is supported in a fluidized bed. A key problem with aluminum vapor deposition methods is that the starting materials are organic and therefore the products are highly susceptible to carbon contamination.

In the case of tantalum deposition, one of the problems is the problem of contamination from highly reactive halide vapors reacting with the materials forming the coating device. The applicant has found that the above problem can be avoided by coating the powder with valve metal obtained by evaporation.

One advantage of this is that evaporation/rather than heterogeneous chemical reactions
The condensation-dependent deposition reaction allows the use of lower substrate temperatures for the powder, which in turn allows the use of a wider range of substrate materials, and allows the use of a wider range of substrate materials at the interface between the valve metal coating and the non-valve metal material of the substrate. This means that the problem of mechanical stress can be reduced. Another advantage is that evaporation of the valve metal, such as aluminum, can be easily performed to provide a growth morphology that provides a high specific surface area. The advantage of physical vapor deposition over chemical vapor deposition concerns the problem of nucleation. At the surface of the substrate, it is essential that a high degree of supersaturation exists in order for the species to be deposited to form substantially complete coverage of the substrate from the beginning of the deposition process. Moderate supersaturation is difficult to achieve for reversible reactions. For chemical vapor deposition reactions, high substrate temperatures are generally required for the reaction to proceed at a reasonable rate, thus preventing supersaturation and
Physical vapor deposition, on the other hand, generally does not require high temperatures.
Two methods of coating powders with aluminum obtained by evaporation are described in US Pat. No. 0,396,25 (M.P.
Drake-S. Y. This is noteworthy because it is described in Hu teaching es7-4). In both of these methods, the powder is caused to fall from a hopper into a coating area where the powder particles come into contact with aluminum vapor and after leaving this area the particles are collected in a suitable receiver. To obtain a thicker aluminum coating, the bottled powder can be returned to the dosing hopper and the coating cycle can be repeated multiple times. However, it has been found that this method becomes more difficult to perform satisfactorily when the coating layer becomes thicker. A preferred method of coating the powder to be described first involves generating aluminum vapor using an equipment arrangement substantially the same as that used in the No. 20,000 process described in the aforementioned patent application.

In this case, an aluminum wire 10 is fed to one side of the heater 1, which is relatively resistant to attack by molten aluminum. A preferred heater has the form of an electrically conductive refractory slab 11, which can consist of a compressed mixture of, for example, shelmetal nitride, titanium dishelfride and a small amount of aluminum nitride. In the "green" state, this material can be easily machined. The slab is provided with a pair of fortresses 12 which create a higher temperature area near the ends, which wets the surface of the slab and makes it cooler, limiting the movement of aluminum that is susceptible to migration into the section. The slab is
It is held between a pair of water-cooled steel electrodes 13 terminating in copper jaws. Wrap graphite paper (not shown) around one end of the slab before inserting it into the jaws to account for different expansion effects and to provide a low electrical impedance, high thermal impedance connection between the slab and the electrode jaws. . In one example, it is mounted vertically and has a room temperature resistance of 500-550 ohms and about 16
A slab was used that required approximately 120 amps to heat to 0,000.

Aluminum wire, typically 0.7 m in diameter, is passed from a reel (not shown) through tube 14 to hot slab 11.
from which aluminum vapor is released.

The rotating drum 15 is arranged such that the majority of the emitted vapor is directed inward, where it condenses on the charged powder particles.
The drum is held with its axis of rotation tilted between horizontal and vertical, thereby displacing the powder as the drum rotates. Typically it rotates at about 3 pm, but speed selection varies widely depending on size and conditions. For a drum with a diameter of 7 skins and a depth of 5 arcs, a typical charge powder particle is 100 g of 29 micron aluminum powder leached with a dilute solution of hydrochloric acid, thoroughly washed with demineralized water, and thoroughly dried. Consisting of For capacitor manufacturing, electrically insulating powders are generally preferred so that if the valve metal coating is penetrated by the electrolyte, exposure of the underlying substrate material will not adversely affect the electrical properties. If desired, a larger drum process can be used and finer particle size powders can also be used. Thus, a drum of diameter 15 mm and depth of 15 mm and a 30 mm charge will produce a satisfactory coating. The drum is provided with inwardly directed radial vanes to modify the rolling action of the powder, and its mouth is provided with an inwardly projecting lip. Also, in addition to the 29 micron coating, 13 micron powder was continuously coated. To perform the coating process, the drum, wire and intermetallic slab assembly is mounted in a vacuum chamber (not shown) into which a pump is pumped.
It is drawn to a pressure of less than 1 pitor, typically about 2 x 10-5 torr.

As the deposition progresses, the coated powder particles have an increasing tendency to stick together and form agglomerates that cannot be broken up by the rolling action of drum rotation. However, the transfer action was found to destroy the agglomerates under the remote presence of oxygen. Therefore, it is expedient to continue the deposition until agglomerates accumulate. At this stage, power is cut to the intermetallic slab and the supply of aluminum wire is stopped, while pure oxygen or air is supplied to the drum, which is still rotating. When oxygen is used, the vacuum system pressure is typically 10-
It should not be increased above 4 Torr, whereas when air is used, higher pressures, typically from 1 Torr to atmospheric pressure, are found to be suitable and are applied. The reason for maintaining a low pressure at this stage is to reduce the pumping time required to restore the desired level of vacuum before restarting deposition once the agglomerates have been sufficiently broken up. Breaking up this agglomerate may take approximately 15 minutes. Time is pressure dependent and therefore there is a trade-off between pump run time and agglomerate break time.
e-off) exists. The deposition process and the introduction of oxygen or air to break up the mulberry agglomerates are typically repeated 10 to 15 times or more to form a deposited layer about 2 to 3 microns thick. At this stage, when destroying 29 micron alumina, the proportion of aluminum is 25-3% by weight in depth. As an alternative to this intermittent introduction of oxygen or air to aid in the destruction of agglomerates, agglomerate formation can be inhibited by supplying oxygen directly to the drum at a continuous, controlled rate during the entire deposition process. It can be eradicated. If the coated powder is to be used in the production of condenser anodes,
The actual amount of aluminum required for the powder will of course depend on the forming voltage requirements for the anode.

Generally, if lower formation voltages are to be used, less aluminum is expected to be used. All that is required is that there be enough aluminum to leave a continuous metal film after the anodization is complete. However, Applicants have observed that the growth of aluminum that occurs with this process appears to be rough on a microscopic scale, and Applicants believe that this is due to the increase in specific surface with increasing aluminum loading. I think there is. capacitance yield
FIG. 2 shows a typical example of In Figure 2, 29
20V, 50V and 1 using micron alumina powder
The capacitor field obtained with a formation voltage of 00y is shown. Note that the capacitance shield is expressed as a unit volume, that is, the value of microfarads per 3 volts, and is used to compare the performance of electrolytic capacitors. It was removed and compacted using a carefully defatted die.

A 0.05 g powder sample was compressed in a die with a cross section of 6 ribs x 12 willows applying a force of approximately 1350 newtons. The anode contact was provided by an aluminum wire contained in the die in a known manner used in the manufacture of compressed tantalum capacitor anodes. A compacted anode can be formed by anodization using a 3% solution of ammonium tartrate at room temperature in a conventional manner. Alternatively, a compressed anode can be formed using an 8% solution of shelf acid at an elevated temperature of 85q0. When formed with a shelf acid, using an alkali to buffer the solution to 5.5 states,
This results in the problem of white precipitate formation in the pongee hole, which is why unbuffered solutions are preferred. FIG. 3 shows a typical construction of a capacitor using a wet electrolyte, in which a number of anodized compact coated powder slabs 30 with parallel connected conductors 31 and a plurality of anodized compact coated powder slabs 30 with parallel connected conductors 33 are shown in FIG. Interposed between a set of non-anodized compact coated powder slabs 32.

Slab 32 is identical to slab 30 except that it is not anodized, but may be subjected to passivation treatments known for etch/oil cathodes. To achieve the total capacitance, the number of slabs 30 is
The number of slabs 32 is one less than the number of slabs 32 so that each slab 30 is sandwiched between a pair of slabs 32. The slabs are separated from one thick roll of manila paper 34 wrapped around the components of the assembly. This construction allows the slab to be held in close proximity to the paper 34, which acts as a reservoir for the working electrolyte. This can be, for example, a standard organic electrolyte composition based on butyrolactone and n-methyl-2-pyrrolidone. 6 each
4 parallel connected "willow x 12 posts x 0.7 ribs"
A typical capacitor constructed in this manner from a cathode slab 32 had an anode formed with 200 volts of 3% ammonium tartrate to provide an operating voltage of 160 volts.

After assembly, the unit had an anode formed by 200 volts. After assembly, the unit has a maximum operating voltage of 200 volts and 2000 volts at 25°C.
An organic working electrolyte with a measured conductivity of micro-Siemens was immersed in a vacuum chamber. Vacuum impregnation was performed using a vacuum dessicator and water pump. No pre-firing was performed. After this treatment, place the unit in a 4-cup glass container,
It was closed with a sealed plastic lid and the leads 31 and 33 were removed from the lid. The device was then post-formed at 180 volts for 2 hours and again exposed to vacuum to remove gases generated during post-forming. Once completed, the capacitor has a capacitance of 12.0 microfarads, as determined by measurements taken at 160 volts and a frequency of 120 Hz at the bridge.
It was found to have a negative current of 8.5% and a leakage current of 35 microamps. The device was then subjected to a 14-hour life test at 160 volts, after which the measured parameters were a capacitance of 11.8 microfarads, a formal delta of 8.5%, and a leakage current of 14 microamps. The coated powder can also be used to form the anode of a solid electrolytic capacitor, where the anode is usually in the form of a single block and is formed to provide the block with an anodized layer. Standard manganese filling method (MA) used in the manufacture of solid electrolytic tantalum capacitors
The pores are filled with a solid electrolyte, such as manganese dioxide, using a process.

Another method of coating the powder is described with reference to FIG.

The equipment used in this method is designed for an evaporator system in which the valve metal vapor is evaporated upward from the surface of the melt.
Four drums containing the powder to be coated, typically alumina, are rotated around a horizontally held wire.
The rotational speed is such that even at the highest point the powder is carried along with the drum by centrifugal force. An electron beam evaporator 41 of 270 mm or other suitable type arranged to form a molten pool of valve metal, typically aluminum or tantalum, is stationary within the rotating drum. Vapors evaporated from this melt pool condense on the powder as it advances to the top. To ensure that the same surface of the powder particles is not exposed to steam with each revolution of the drum, the powder is transported through a spoiler (not shown) near the bottom of the drum (a plate used to break up and disrupt the smooth flow). or through a series of plates in each cycle. Similar to the apparatus of Figure 1, this apparatus is contained in a vacuum vessel (not shown).During the coating process,
The pressure in this vessel is typically maintained at a pressure of about 2 x 10-5 Torr. Aluminum has an evaporation source of 1250~
By maintaining the temperature at 1300°C, it can be evaporated at an appropriate rate. At a temperature of 12°C, the vapor pressure of aluminum is 1 Torr.

A temperature of 3500° C. is required to obtain a corresponding vapor pressure for vaporizing tantalum. Obviously, at this higher temperature, the heat loss due to radiation is very large. 12
At 84 qo, the radiant energy is 33.5 watts/m, while at 3500 qo, it is 1.15 kW/m. However, on the other hand, the latent heat of vaporization of tantalum is less than that of aluminum, so approximately 8.3 kilowatts are required to provide the latent heat energy required to vaporize aluminum at a rate of 1 g/s. However, about 5 kilowatts is sufficient to provide the latent heat energy necessary to vaporize the tantalum at the same rate. It therefore follows that a 100 kilowatt electron beam evaporator can readily vaporize tantalum at a reasonable rate, despite the fact that it involves very high temperatures. In fact, because the latent heat is lower and the melting point is higher, tantalum can be deposited at a faster rate than aluminum without post-diffusion problems due to heating of the substrate by the heat of condensation. Furthermore, tantalum has a much higher melting point than aluminum, so
It is possible to use higher substrate temperatures without reducing the supersaturation level to the point where nucleation problems become significant.

The production of tantalum capacitors from this tantalum-coated powder is carried out by methods commonly used in the production of all-tantalum compacted powder capacitors, including compaction or sintering or both compaction and casting, followed by molding and manganese filling.

In a modified version of the device of FIG. 4, the lips 42 on either end of the drum, which serve to prevent powder scattering, are omitted, and the powder is gradually fed in an axial direction from one end to the pond end. A spoiler (not shown) is arranged in the .

In this way, uncoated powder can be fed semi-continuously to one end of the drum so that it emerges completely coated from the pool end. For aluminum evaporation, an RF induction heating vessel can be used instead of an electron beam evaporator. In the apparatus of FIG.
Shown with F coil 51. The container has an aluminum charge 5 which provides a source of aluminum vapor which is reflected by the container lid 53 and is caused to emerge substantially horizontally from the container.
Contains 2. To this end, the lid can be arranged to be heated by coupling to the RF region of the evaporation vessel 50 as well as by radiation from a molten metal source. A rotating drum 54, whose axis is held vertically, surrounds a stationary container 50.

At the back of the drum is a fixed lid 55 from which hangs a stationary conical chute 56 having the general shape of a Belleville washer. The powder 57a on the upper surface of the chute 56 moves down the slope under gravity toward the center hole, and freely falls from there into the curtain 57b around the container 50. As it passes through the gap between the container and the lid, the aluminum is coated. The powder then falls to the bottom of the drum 53, from where it is forced up the outwardly tapered side of the drum by centrifugal action. Near the top of the drum, the outward taper changes to an inward taper, where powder collects (57).
℃). The outer rim of stationary chute 56 is immersed in this powder accumulation. The rim slows the rotation of the powder near it, so the powder just above the rim moves down its slope and is replenished with more powder from the bottom of the drum.
The base powder to be coated is essentially alumina, and the coating process is carried out in a vacuum container (not shown). In the above description of examples of the invention, the only material exemplified for depositing the coating was alumina powder.

However, it goes without saying that many other materials can be used in its place. In providing aluminum coated powders for capacitor manufacturing, the desired properties of the underlying substrate powder are such that the agglomerates are more easily rendered porous in the processing required to form the agglomerates from the coated powder. The ductility of the powder must be lower than that of aluminum. If the valve metal coating does not completely cover the powder or is destroyed in some step, the substrate material will be exposed to the electrolyte. Therefore, the base powder must be compatible with the electrolyte in the sense that the electrolyte must not significantly deteriorate the electrical properties of the capacitor. For this reason, it is generally preferred to use dielectric materials in the powder. Alumina was mentioned in all examples.

This is because the use of alumina as an industrial abrasive means that alumina is readily available in the appropriate form of a particular particle size. Alumina is inert and relatively inexpensive in this application. However, deposition can be carried out on a wide variety of powder substrate materials by considering compatibility in terms of melting point, vapor pressure, and ductility. The choice of substrate properties will also depend on compatibility with product end use, cost and availability considerations.
Glasses and resins will prove suitable for many applications for the deposition of alumina. In the case of aluminum electrolytic capacitor production, an important reason for choosing coated powder over the use of pure aluminum powder relates to the processing required to produce porous aggregates of compacted powder. This is difficult to achieve with pure all-aluminum powder. This is because its ductility is very high, and therefore, when compressed, the pongee holes tend to disappear from the structure. In the case of tantalum electrolytic capacitor manufacturing, the reason for choosing this over pure all-tantalum powder is primarily a cost issue. Therefore, the amount of tantalum used in powder coating is kept to a minimum that meets performance requirements. When applying a tantalum coating directly to a dielectric powder substrate, the calculation of the required coating thickness depends not only on the amount of metal converted to oxide by the chemical conversion process, but also on the amount of converted metal remaining in the substrate to form the electrode structure. The required amount must also be taken into account. This unconverted metal need not be tantalum, but can be a less expensive valve metal such as aluminum. Thus, the powder can be coated first with aluminum and then with tantalum. Tantalum is provided in just enough amount to satisfy the formation requirements. The breakdown of the tantalum dioxide production anode coating will be filled with material produced by anodization of the underlying aluminum. The produced capacitor anode will have more anodized tantalum than anodized aluminum, and therefore its properties will be closer to those of an all-tantalum coated powder capacitor than an all-aluminum coated powder capacitor. This type of method of depositing a high melting point metal onto a relatively lower melting point metal is generally possible by physical vapor deposition, but can be difficult with chemical vapor deposition due to conflicting requirements regarding substrate temperature. Or it will be understood that it is impossible.

The literature describes many alloys that exhibit valve metal properties. Such valve metals can be alloys containing one or more constituent elements that are not themselves valve metals. Although the only valve metals illustrated were elemental metals, the invention is also applicable to coatings with valve metals that are alloys. It is well known to deposit alloy systems by evaporation using common or separate sources.

[Brief explanation of the drawing]

1 is a schematic diagram of an apparatus for coating powder with aluminum; FIG. 2 is a graph showing a typical capacitance shield of a capacitor made from powder coated with the apparatus of FIG. 1; FIG. The figure shows a typical capacitor structure diagram,
4 and 5 are schematic illustrations of two different sets of apparatus for coating powders. 10... Aluminum wire, 11... Heater, 13.
...Electrode, 14...Tube, 15...Drum, 30...Anodized slab, 31...Conductor, 32...Non-anodized slab, 33...Conductor, 34...Paper, 40...・Drum, 41... Evaporator, 50... Container, 51... RF
Coil, 52...aluminum, 53...lid, 54...drum, 56...chute, 57a...powder, 57b...curtain. O.G. 4. F'G. 7. F'6.3. ○6.2. F'G. 5.

Claims (1)

[Claims] 1. A method for producing valve metal-coated powder particles for an anode or a cathode of an electrolytic capacitor, comprising: (a) rotating powder particles made of a dielectric substance in a rotating drum under vacuum; (b) contacting the rotating powder particles contained in said drum with valve metal vapor obtained by vacuum evaporation of at least one valve metal source; and (c) bringing said powder particles into contact with air or oxygen gas. substantially preventing the formation of agglomerates by intermittent or continuous contact with
The method characterized in that. 2. The method of item 1 above, wherein the powder particles are less ductile than the coating valve metal material. 3. A method according to paragraph 1 or 2 above, wherein the coating step is carried out while introducing air or oxygen into the powder at a controlled rate to substantially prevent the formation of agglomerates. 4. A method according to paragraphs 1 or 2 above, wherein the coating step is stopped while the agglomerates are broken up by continuously rotating the drum while introducing air or oxygen into the coating powder. 5. The drum is rotated about a horizontal axis at a speed that allows the powder to form a coating while retaining the powder against the drum wall by centrifugal action as it passes over the evaporation source; The method according to item 1 or 2 above. 6
The drum is rotated about a horizontal axis at a speed that allows the powder to form a coating while centrifugal action holds the powder against the drum wall as it passes over the evaporation source. 3. A method according to paragraph 1 or 2 above, wherein the method is advanced axially along the drum. 7. The drum is rotated about a vertical axis and the drum cooperates with a fixed plate to circulate the powder within the drum, raising the sides of the drum, lowering the fixed plate and causing a curtain around the evaporation source. 3. A method according to paragraph 1 or 2 above, wherein a tapered sidewall is provided to cause the fall.