CA1158932A - Deposition of tantalum or niobium on ceramic particles using a nucleating agent - Google Patents

Abstract

E.L. Bush - E.J. Workman 21-1 COATING POWDERED MATERIAL of the disclosure A process for coating a particulate insulating material, e.g. alumina, with a uniform layer of a valve metal. The particles are exposed to a gaseous nitrogen compound, or to nitrogen gas together with hydrogen and a volatile halide of the valve metal. It is thought that nitrogen provides nucleation of the surface for subsequent metal deposition. The coated powder may be used in the fabrication of electrolytic capacitor anodes.

Description

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COATING POWDERED ~TERIAL
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This invention relates to processes for coating partlculate materials, and in particular to a process for the chemlcal vapour deposition of a metal surface layer on a powder.
In the manu:Eacture of metal coated powders, such as are employed in the construction of electrolytic capacitor anodes, it is necessary to provide a uniform metal coating on a r01atively finely divided insulating material. Typ-ically the finely divided material has a particle size of the order of 13 mi-crons, but coated particles of the order of 3 microns would be preferable.
Coating of such finely divided material involves difficulties that were previ-ously thought to be insuperable.
In order to manufacture a capacitor anode from a metal coated powder, the material is compressed so that the metal flows away from the points of con-tact between the coated particles and the whole mass cold welds into a solid porous body. It is highly advantageous to limit the thickness of the metal coatin-g so that the compaction process becomes self limiting, there being in-sufficient metal to fill the voids between the particles. The technique is more fully described in Canadian Patent No. 1,056,922. To achieve such a compaction process with relatively fine particles it is essential that the coating metal be of a corresponding "thinness".
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It has been demonstrated that, for example, alumina particles with an average diameter of 13 microns can be provided, albeit with difficulty, with a tantalum coating via a chemical vapour deposltion process. In such a process it is possible to achleve a tantulum coating on 90% of the part;cles when the average tantalum content is greater than 40% by weight. Deta;led investigation of the material during the deposition process has shown that the metal growth develops from island nuclei on the surface of the substrate particles.
Some of the particles, even at an advanced stage of the deposition process, remain completely devoid of metal presumably due to the absence of a nucleation centre from which metal growth can occur. Thus to ensure an adequate metal coating ;t is therefore necessary to deposit excess metal. This problem is of course exacerbated as the particle size becomes smaller and the number of particles lacking a nucleation centre increases.
It has previously been considered that, for the satisfactory manufacture of tantalum capacitor anodes from coated powders of 200 to 230 mesh, a minimum tantalum coating thickness of 2.5 microns is required. Theoretical calculations however, indicate that, to achieve anodized layers of 50 volts a minimum metal coating thic~ness of only 0.05 microns will suffice. This apparent discrepancy is t~ought to arise from the fact that previous workers in this field have achieved island gro~th only. ~hus, it is possible 3 to obtain a metal 'coated'- powder which, when pressed and sintered, cannot be anodized as there ls no inter-connection of the individual metal islands.
In the manufacture of tantalum electrolytic capacitors it is ad~antageous ko manufacture the capacitor anodes from tantalum coated powder rather than from tantalum metal powder. In order for tantalum coated powders to be economic however, it is necessary to utilise substrate powd-ers of a particle size comparable to that of commercially available tantalum pow-ders. Such tantalum metal powders have particle sizes within the range 1 to 5 microns and typically provide specific capacitance values oE 30,000 to 60,000 microcoulombs/cc and 4,000 to 12,000 microcoulombs/g. To achieve similar propor-tions with tantalum coated powders it is essential to employ a powder substrate with a particle diameter of 3 microns or less. The tantalum coating should also be limited to an average thickness of about 0.3 micron which is equivalent to 75 weight ~ of the tantalum metal in the coated product. It would in fact be pref-erable to limit the tantalum content to 50 weight ~, which represents a coatingthickness of only 0.1 micron.
According to one aspect of the invention there is provided a process for depositing a substantially uniform layer of tantalum or niobium on a particu-late insulating ceramic substrate comprising particles of 30 microns or less in diameter by chemical vapor deposition of said valve metal, the process co.mprising:
nucleating said substrate by exposing said substrate at a predetermined elevated temperature to a gaseous mixture of tantalum or niobium halide and a nucleating agent, wherein said nucleating agent is selected from the group con-sisting of ammonia, an ammonium halide, a hydrazine-hydrohalide, a hydroxylamine-hydrohalide, nitrogen, a boron halide, a sulphur halide, a phosphorous halide, asilicon halide, a boron hydride, a sulphur hydride, a phosphorous hydride or a silicon hydride and the substrate is exposed to said gaseous mixture for a prede-termined time to provide nucleation of said substrate with a tantalum- or niobium-containing material via an irreversible reaction; and depositing tantalum or niobium on said nucleated substrate by then expo-sing at a predetermined elevated temperature said nucleated substrate to a gase-ous mixture of tantalum or niobium halide and hydrogen for a predetermined period ,~ -3-~3~5~Z

of time sufficient to deposit a layer of tantalum or niobium. Alumina is a pre-ferred ceramic substrate.
According to another aspect oE the invention there is provided a metal coated powder made by the above process.
According to a further aspect of the invention there is provided an electrolytic capacitor anode including a compacted body of a metal coated powder made by the above process.
The predetermined temperature in the nucleating step is preferably bet-ween 600C and 1400C.
In order to provide a tantalum, or other valve metal, coating of not greater than 0.3 microns thickness on a 3 micron diameter powder substrate it is essential to employ a deposition process which provides uniform growth rather than "fill-inl' of island growth. We have found that uniform growth of a metal film on an insulating ceramic surface can proceed only after a complete nucleat-ion of the surface has occurred. This necessitates a high degree of supersatura-tion of the depositing metal species.
In many metal deposition processes the chemical reaction is thermodyna-mically reversible, the forward and reverse reactions being stimulated by control of temperature and the partial pressures of -the various gaseous species. Thus, for example, the deposition of such valve metals as tantalum and niobium may pro-ceéd via the following typical reactions:-1000C + excess H Ta + 4HCL

1000C + excess H Ta + 5HCL

1000C + excess H Nb + 4HCL

~S~32 NbC15 ~ 21H2 1000 C ~ excess H2 Nb = 5HCL

Even at 1000C and a 20-fold excess of hydrogen, the conversion of tantalum chloride to tantalum metal is only 96% efficient. The result is that, ln the absence of extensive nu¢leation, once tantalum growth has been initiated on parts of the surface there is a tendency for these regions to grow at the expense of the remainder of the surface and that the remainder o~ the surface remains uncovered.
We have found that it is possible to modlfy the chemical transport reaction to provide an irreversible deposition step to give a meta~-compatible deposit such that metal deposition then proceeds uniformly over the nucleated surface. Such a modified reaction process is postulated as follows:-Ta ~ 4HCQ 5O C ~ TaC~4 + 2H2 lO00C
reversible 1 ~ 1000C
reversible ~ nucleating gas RX
Intermediate TaC14 adduct irre~ersible ~ 1000C
Ta ~ TaX + HCl ~e have made the unexpected dlscovery that this reaction sequence may in ~act be achieved with the aid of certain nitrogen containing reactive gases, e.g. ammonia. When ammonia is employed as the nucleating gas, then the tantalum deposits over the complete surface of all the particles within the reaction region. The tantalum film thus deposited may contain up to 10 weight % nltrogen, possibly in the form of one or more nitrides. Further deposition o~ tantalum by the normal thermodynamically reversible process results in the build up of substantially uniform layers of tantalum on al~ particles. The electrical and physical properties of the final coated particulate -6~ 3~
product are substantially unaffected by the thln inter~acial underlayer of nltrided tantalum.
All attempts to achieve uniform surface coverage o~ fine alumina powder with tantalum without initial treatment with a nucleating gas have proved unsuccess~ul unless uneconomically large quantities of tantalum are deposited. Ammonia is relatively cheap and readily obtainable and is thus an attractive choice as the nucleating gas. However, other nitrogen containing gas may of course be employed, and we have achieved successful nucleatlon with, e.g. ammonium chloride, hydrazine-hydrochlorlde, hydroxylamine hydrochloride or their other halides and, under certain conditions, e~en nîtrogen gas. We have also found that nucleation can be achieved not only with nitrogen compounds but also other materlals which react in an irreversible way to form a surface layer compatible with a subsequent depositing species. Thus, volatile borides, sulphides, phosphides and silicides may all be employed as nucleating agents. Nitrogen compounds are to be preferred however as they are relatively cheap and, in general, easler to handle.
The techni~ue is appllcable to various types of processes for the production of uniformly metal coated particles.
In the process uncoated particles are mixed with coated or partially coated particles and are then treated with a mixture of a hydrogen halide and a nucleating gas so as ~o transport metal from the coated particles to the uncoated partlcles and provide a substantially uniform surface coverage of all the particles. In an alternative process non-uniformly coated particles are treated with a hydro~en halide and a nucleating gas to 'spread' the metal into a uniform coating.

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In a modification of these two processes, uncoated insulating particles are mixed with metal particles e.g. in a fluidiæed bed. The mix is treated with a hydrogen halide together with a nucleating gas so as to transport the metal to the surface of each insulating particle.
In a further process, uncoated particles are treated with a mixture of a metal halide vapour and a nucleating gas to cause decomposition of the halide and substantially uniform deposition of the metal.
For use as a capacitor anode material the substrate powder should be between l and 30 microns in diameter. The process described herein has been found to produce uniform metal coatings ~nd such powders met the required metal thickness of less than l micron.
In a typical deposition process, a mixture of finely divided alumina and tantalum powders or partly coated alumina powder is dispersed in a fluidised bed in a flow of hydrogen at a temperature of 900C to l,lO0C. Nucleation of the alumina is then effected by exposing the powder to an atmosphere of hydrogen containing e.g. lO volume % ammonia and lO volume ~ hydrogen chloride for a peri-od of 5 minutes. During the nucleation period metallic tantalum reacts with the hydrogen chloride to form a volatile tantalum chloride whereby tantalum is trans-ported to the alumina particles and is deposited on the alumina surface via an irreversible reaction to yield a nitrided tantalum surface. Further deposition of tantalum can then take place via the normal reversible reaction process. The deposited tantalum contains up to lO weight % nitrogen.
The material thus formed may then be pressed into electrolytic capaci-tor anodes e.g. by the process described in Canadian Patent No. 1,056,922.

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In such a process the powder is pressed such that the relatively soft valve metal flows away from the points of contact of the relatively hard alumina par-ticles, the whole mass cold welding into a porous body. The metal coating is of sufficient "thinness" that the metal cannot completely fill the voids between thc particles.
The following example illustrates the invention:-Example A series of mixtures of alumina, tantalum and optionally silica pow-ders were prepared and exposed to mixtures of hydrogen, hydrogen chloride and ammonia so as to coat the alumina particles with tantalum metal. The various powder treatments are summarized in the following table:-~.,, ':
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Some of the coated particles were then employed to manufacture capacitor anodes. The capacitance yield was measured in each case and the results summarized in Table 2.
The results demonstrate the feasibility of the use of the process described herein for the manufacture of metal coated powders suitable for capacitor anode constructions.

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Claims (11)

E.L. Bush-E.J. Workman 21-1 (Revision) THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for depositing a substantially uniform layer of tantalum or niobium on a particulate insulating ceramic substrate comprising particles of 30 microns or less in diameter by chemical vapor deposition of said valve metal, the process comprising:
nucleating said substrate by exposing said substrate at a predetermined elevated temperature to a gaseous mixture of tantalum or niobium halide and a nucleating agent, wherein said nucleating agent is selected from the group consisting of ammonia, an ammonium halide, a hydrazine-hydrohalide, a hydroxylamine-hydrohalide, nitrogen, a boron halide, a sulphur halide, a phosphorous halide, a silicon halide, a boron hydride, a sulphur hydride, a phosphorous hydride or a silicon hydride and the substrate is exposed to said gaseous mixture for a predetermined time to provide nucleation of said substrate with a tantalum- or niobium-containing material via an irreversible reaction;
and depositing tantalum or niobium on said nucleated substrate by then exposing at a predetermined elevated temperature said nucleated substrate to a gaseous mixture of tantalum or niobium halide and hydrogen for a predetermined period of time sufficient to deposit a layer of tantalum or niobium.

2. The process of claim 1 wherein said nucleating step provides substantially complete nucleation of said substrate with a substantially uniform layer of said tantalum- or niobium-containing material.

E.L. Bush-E.J. Workman 21-1 (Revision)

3. The process of claim 2 wherein said nucleating step predetermined temperature is between 600°C and 1400°C.

4. The process of claim 3 wherein the thickness of said layer of tantalum or niobium does not exceed an average of 0.3 microns.

5. The process of claim 4 wherein said depositing step is carried out by a reversible reaction.

6. The process of claim 5 wherein both of said predetermined temperatures are within the range of 900°C to 1100°C.

7. The process of claim 1 wherein gaseous tantalum halide is used in said nucleating step, the deposited metal of said depositing step comprises tantalum and said substrate is alumina.

8. The process of claim 7 wherein said nucleating agent comprises ammonia.

9. The process of claim 8 wherein said tantalum halide is tantalum chloride and said gaseous mixtures are provided by dispersing a mixture of alumina and tantalum powders on a fluidized bed in a hydrogen atmosphere at a temperature of 900°C-1100°C, and exposing the powders to a mixture of hydrogen, hydrogen chloride and ammonia so as to nucleate the surface of each alumina particle and transport tantalum metal thereto.

10. A metal coated powder made by the process of claim 1.

11. An electrolytic capacitor anode including a compacted body of the metal coated powder of claim 10.