FR2490245A1 - Superconductor prodn. by chemical vapour phase deposition on fibre - by exposing niobium or tantalum to nucleating gas and nucleating agent to give uniform film - Google Patents

Abstract

Superconductor prodn. involves deposition of a uniform Ta and/or Nb (cpd) film on a fibrous support by chemical vapour phase reaction. The fibres with Ta/Nb are exposed to N2 gas or a gaseous N cpd. and a H halide, so that the H halide reacts with Ta/Nb to form a gaseous cpd. which then reacts with the N2 or N cpd., so that a uniform film is formed on the surface of the fibres by an irreversible reaction. The uniform nucleation over the entire surface, required for the prodn of a uniform coating is obtd by using this irreversible process.

Description

 The present invention relates to methods for coating particulate material, especially for the chemical vapor deposition of a layer of retal on the surface of a powdery substrate.

 In the manufacture of metal-coated powders such as those used for electrolytic capacitor anode making, it is necessary to provide a uniform metal coating on a relatively thinly divided insulating material. This relatively finely divided material generally consists of particles having a size of the order of 13 nierons; but coated particles of the order of 3 microns would be preferable. However, the coating of particles of such finesse between difficulties that were considered insurmountable until now.

 To make a capacitor anode from a coated powder, the material is compressed so that the metal spreads from the points of contact between coated particles, so that the breakage in its enserble is cold-welded. a massive porous body. It is very advantageous to limit the thickness of the metal coating so that the compaction process becomes self-limiting, the amount of natal available is not sufficient to fill all the voids between the particles. This technique is described in more detail in British Patent Publication No. 1,506,667. In order to obtain such a compaction process with relatively fine particles, it is essential that the coating material has an equivalent "fineness".

 It is known, for example, that alumina particles having an average diameter of 13 microns can receive a tantalum-bound rave by a chemical vapor deposition process, even though this is at the cost of serious difficulties. With such a process, it is possible to rave about 90% of tantalum when the average proportion of tantalum exceeds 40% by weight. Detailed studies of the material during the deposition process have shown that the retinal debate develops from small isolated nuclei on the surface of the particles of the substrate. Some of the particles, even at an advanced stage of the deposition process, remain completely free from natal probably due to the absence of a nucleation center from which the retal could grow. To obtain a suitable metal coating, it is therefore necessary to deposit excess metal. This problem is naturally amplified when the particle size becomes smaller, thus when the number of particles having no nucleation center increases.

 It has previously been considered that a tantalum coating with a minimum thickness of 2.5 microns was necessary to allow satisfactory production of tantalum capacitor anodes from 200-230 particle size coated powders. However, theoretical calculations indicate that a thickness of the metal coating of only 0.05 microns would suffice to obtain anodized layers of 50 volts.

This difference is believed to be due to the fact that so far only a nucleation growth coating method has been used. But it is possible to obtain a metal-coated powder which can not be anodized when it is compressed and agglomerated because there is no bond between the individual nucleation centers.

 For the production of tantalum electrolytic capacitors, it is advantageous to make the capacitor anodes from tantalum-coated powder rather than from tantalum powder. However, in order for tantalum-coated powders to be economically reusable, it is necessary to use powder substrates whose particle size is comparable to that of commercial tantalum powders. The particle size of such tantalum powders is of the order of 1 to 5 microns and these powders generally offer specific capacitance values of 30,000 to 60,000 microcoulombs per cm3 and 4,000 to 12,000 microcoulombs per gram. to obtain comparable values with tantalum-coated powders, it is imperative to use a powdery substrate whose particles have a diameter of 3 microns or less. The tantalum coating should also be limited to an average thickness of about 0.3 micron, which corresponds to a proportion of tantalum in the coated product of 75% by weight. It would in fact be preferable to limit the tantalum content to 50% by weight, which corresponds to a coating thickness of only 0.1 microns.

 One aspect of the invention relates to a method for chemical vapor deposition of a substantially uniform coating of anodizable metal on an insulating powdery substrate, which method comprises inserting a nucleating agent during the deposition process, this nucleating agent being such that it causes the deposition of an initial surface coating of anodisable metal via an irreversible reaction.

 In one exemplary application, this method allows the deposition of a substantially uniform layer of tantalum or niobium on a pulverulent alumina substrate, which method comprises exposing the alumina particles (at least some of which are entirely or partially coated with tantalum or niobium) to nitrogen gas or to a gaseous nitrogen compound comprising a hydrogen halide, which hydrogen halide reacts with the tantalum or niobium coating to form a gaseous compound reactive with nitrogen or with the nitrogen compound to thereby deposit a uniform surface layer on these particles through an irreversible reaction.

 Another aspect of the invention relates to a method for producing a powder comprising exposing a mixture of anodisable metal powder and particularly hard powdery insulating material to a mixture of nitrogenous vapors and hydrogen halide. at a temperature of 700 to 14000C for a time sufficient to allow the transfer of the anodizable metal to the insulating material.

 To obtain a thickness of the tantalum (or any other anodizable metal) coating not exceeding 0.3 micron on a powdered substrate with a particle diameter of 3 microns, it is imperative to use a deposition process that allows for growth. uniformity of the coating rather than "filling" spaces between nucleation centers. The uniform growth of a metal film on an insulating ceramic surface can only occur after complete nucleation of the surface. This requires a high degree of supersaturation of the components of the deposition metal.

The chemical reaction of many metal deposition processes is thermodynamically reversible, with direct and inverse reactions being stimulated by temperature regulation and partial pressures of the various gaseous compounds. For example, the deposition of metals such as tantalum and niobium can occur through the following standard reactions
TaC14 + 2 H2 10000C + H2 in excess Ta + 4 HC1
5000C
TaC1S ± 2h H2 10000C + H2 in excess Ta + 5 HC1
5000C
NbC14 + 2 H2 10000C + H2 in excess Nb + 4 HC1
5000C

<tb> NbC15 <SEP> + <SEP> 2i <SEP> H2 <SEP> 10000C <SEP> + <SEP> H2 <SEP> in <SEP> excess <SEP> Nb <SEP> + <SEP> 5 <SEP > HCl
<tb><SEP> 400 "C
<Tb>
The conversion of tantalum chloride to tantalum metal is only 96% efficient, even at 10000C and under a 20-fold excess of hydrogen. Therefore, in the absence of extended nucleation, once the growth of tantalum has been initiated on some parts of the surface, these parts will tend to increase in growth to the detriment of the rest of the surface. remains uncoated.

However, it is possible to modify the chemical transfer reaction to achieve an irreversible deposit deposition step consistent with the metal so that deposition of the metal subsequently occurs uniformly over the nucleated surface. Such a modified reaction process is postulated as follows ::

<tb> Ta <SEP> + <SEP> 4 <SEP> HC1 <SEP> 500 Q <SEP> TaC14 <SEP> + <SEP> 2 <SEP> H2
<tb><SEP> NO
<tb><SEP> 1000 <SEP> C <SEP> -I
<tb><SEP> reversible <SEP> | <SEP> t1000 C
<tb><SEP> reversible <SEP> + <SEP> gas <SEP> nucleating <SEP> RX
<tb><SEP> Adding <SEP> Intermediate <SEP> of <SEP> TAC14
<tb><SEP> irreversible <SEP> + <SEP> 1000C
<tb><SEP> Ta <SEP> + <SEP> TaX <SEP> + <SEP> HC1
<Tb>
This reaction sequence can in fact be obtained using nitrogen containing reactive gases such as ammonia. When ammonia is used as a nucleating gas, tantalum is deposited on the entire surface of all the particles in the reaction zone. The tantalum film thus deposited can contain up to 10% of nitrogen in the reaction zone. weight, probably in the form of one or more nitrides. Further deposition of tantalum by the thermodynamically reversible normal process results in the formation of substantially uniform layers of tantalum on all particles. The electrical and physical properties of the final coated powdery product are virtually unaffected by the thin tantalum nitride interfacial sublayer.

 All attempts to obtain a uniform surface coating of fine powders of alumina with tantalum without initial treatment with a nucleating gas have been unsuccessful unless very high amounts of alumina are deposited and therefore not economically viable.

Ammonia is relatively cheap and readily available; this product therefore represents a good nucleating gas. However, it is possible to use other nitrogen gases and good nucleation is obtained with, for example, ammonium chloride, hydrazine hydrochloride, hydroxylamine hydrochloride or other halides thereof and even with nitrogen gas under certain conditions. Nucleation can be achieved not only with nitrogen compounds but also with other materials which react irreversibly to form a surface layer compatible with the compounds to be deposited together.
It is therefore possible to use volatile silicides, phosphides, sulphides and borides as nucleating agents. However, nitrogen compounds are preferable because they are relatively inexpensive and generally easier to handle.

 This technique is applicable to different types of uniform metal coating particle production processes.

 In one method, the uncoated particles are mixed with particles coated in whole or in part and are then treated with a mixture of hydrogen halide and nucleating gas to transfer the metal from the coated particles to the particles. uncoated and to obtain a substantially uniform surface coating of all the particles. In a variation of this process, non-uniformly coated particles are treated with hydrogen halide and a nucleating gas to spread the metal to obtain a uniform coating.

 In a modification of these two methods, the uncoated insulating particles are mixed with metal particles, for example in a fluidized bed. This mixture is treated with a hydrogen halide mixed with a nucleating gas to transfer the metal to the surface of each of the insulating particles.

 In another method, the uncoated particles are treated with a mixture of metal halide vapors and nucleating gas so as to decompose the halide and result in substantially uniform metal deposition.

 For use as a capacitor anode, the particle diameter of the insulating substrate should be between 1 and 30 microns. The process described here produces uniform metal coatings and such powders provide the required native thickness which is less than 1 micron.

 In a typical method of deposition, a mixture of finely divided powders of alumina and tantalum or partially coated alumina powder is dispersed in a fluidized bed in a hydrogen stream at a temperature of 900 to 1100 C. Nucleation of the alumina is then carried out by exposing the powder to a hydrogen atmosphere containing, for example, 10% ammonia by volume and 10% hydrogen chloride by volume for 5 minutes. During the nucleation period, the tantalum metal reacts with hydrogen chloride to form a volatile tantalum chloride, which allows tantalum to be transferred to the alumina particles and deposited on the surface of these particles via a irreversible reaction, to obtain a surface of tantalum nitride.The subsequent deposition of tantalum can then take place via the normal reversible reaction process. The deposited tantalum contains up to 10% by weight of nitrogen.

 The material thus formed can then be compressed to form anodes of electrolytic capacitors, for example by means of the method described in the patent published under No. 1,507,667.

In this process, the powder is compressed in such a way that the relatively soft anodizable metal flows from the points of contact between the relatively hard alumina particles, thus the whole mass is welded into a porous body. The metal coating is of sufficient fineness that the metal can not completely fill all the voids between the particles.

The following example illustrates the invention
A series of mixtures of alumina, tantalum and optional silica powders were prepared and exposed to mixtures of hydrogen, hydrogen chloride and ammonia to coat the tantalum alumina particles. metal. The various treatments of the powder are summarized in Table 1.

TABLE 1
Gas flow, l / min Duration of Charge Time
Ref. H2 HCl report treatment deposition Bed type Weight Description
NH3 / HCl (in minutes) of tantalum (in grams) (in hours)
MP 10 30 3 - - 13 Fluid 4000 (dynamic) 3 Al2O3
MP 17 40 3 - - 16 Fluid 4000 3 Al2O3 / 150 sand 3/1 by weight
MP 21 35 3 - - 19 Fluid 4000 3 Al2O3 / 150 sand 3/1 by weight
MP 7 25 4 - - 10 Fluid 5000 3 Al2O3 / 120 sand 3/1 by weight
MP 2 60 6 - - 10 Fluid 4000 13 Al2O3
Nt 36 1,2 - 0,4 / 0,2 1 x 5 - Static 30 MP 7 - 3 AlD2O3 coated
LR 118 8 0.5 0.5 / 0.5 2 x 10 8 Fluid 250 3 Al2O3 / 120 Al2O3 4/1 by weight
LR 119 8 0.4 0.4 / 0.2 3 x 10 12 Fludie 215 3 Al2O3 / 120 Al2O3 3/1 by weight 3
MP 25 35 4-5 3/2 4 x 10 10- Fluid 4600 3 Al2O3 / 120 Al2O3 7/1 8
NT 55 (Before 1,5 - 0,6 / 0,3 5 x 10 - Static 35 3 Al2O3 Each treatment after new nucl adding tantalum
NT 54)
NT 55
After 1 - 0.4 / 0.2 1 x 5 - Static 35 nucl.

 Some of the coated particles were then used for the production of capacitor anodes. Capacitance was measured in each case and these results are summarized in Table 2.

 These results demonstrate the feasibility of the process described herein for the manufacture of metal-coated powders suitable for producing capacitor anodes.

It is obvious that the above description has been given by way of non-limiting example and that other variants can be envisaged without departing from the scope of the invention. TABLE 2
Ref. Resistivity, Voltage Weight Accuracy, ohm / cm tnntnle, anodizing in microcoulombs (C) Remarks in% C / g C / cm3
MP 10,700 k 51.3, - - - Depletion Ta on alumina 3 m anns proèédé of nucléntion. Difflelle powder at
MP 16 1,3 M 51,2 - - - compaeter and can not be used for the refurbishment of capacitors
MP 21 490 76.5 - -
MP 7 2.3 52.7 7000 38000
MP 2 0.4 38 8300 Alumis 13 m; deposit is ta without stalling
MP 36 52.7 12 14300 58000 MP 7 subject to registration only, without further deposition of Ta
LR 118 0.1 64.1 24 14200 57700 Alumina 3 m, deposition of Ta, subjected to nucleation process
LR 119 0.4 53.1 24 20400 71700
MP 25 0.1 58 24 19600 80300
NT 55 # 10 M 40 - - - Physical mixture of 40% by weight of Ta before and 60% by weight of alumina, subjected to the nucleation process only
NT 55 0.3 40 12 10000 30000 after nucl.

Claims (15)

 A method for deposition of a metal layer on a powdery substrate in which the deposition is chemical vapor phase and provides a substantially uniform layer of anodizable metal on an insulating powdery substrate, which process is characterized by the fact that it comprises the introduction of a nucleating agent during the deposition process, this nucleating agent being such as to cause the deposition of an initial surface coating of the anodizable metal via an irreversible reaction.  A process for depositing a metal layer on a powdery substrate in which a substantially uniform layer of an initial tantalum or niobium surface coating is obtained on a pulverulent alumina substrate, which process is characterized by the it comprises exposing the alumina substrate - partially coated with nitrogen or a nitrogen gas mixture comprising a hydrogen halide, where this hydrogen halide reacts with tantalum or niobium surfaces partially coated to drill a gaseous compound reacting with the nitrogen or nitrogen compound in question to deposit a uniform metal layer via an irreversible reaction.  A method for depositing a metal layer on a powdery substrate in which a substantially uniform layer of tantalum or niobium surface metal coating is obtained on a pulverulent alumina substrate, which process is characterized by the fact that comprises exposing the alumina substrate to tantalum or niobium halide, with or without additional hydrogen halide in combination with nitrogen or a nitrogen gas mixture, wherein the nitrogen or nitrogen mixture reacts with the halide of tantalum or niobium in question to deposit a uniform layer of a tantalum or niobium compound via an irreversible reaction.  4. Process for depositing a metal layer on a powdery substrate, characterized in that it comprises exposing a mixture of anodizable metal powder and relatively hard powdery material to a mixture of nitrogen and halide vapors. hydrogen at a temperature between 600 and 14000C for a time sufficient to allow the transfer of the anodizable metal to the surface of the insulating material.  5. Process for the deposition of a metal layer on a powdery substrate, according to claim 4, characterized in that the reaction temperature is between 900 and 1100 C.  6. Process for the deposition of a metal layer on a powdery substrate according to any one of claims i to 5, characterized in that the deposited metal is an alloy of the anodizable metal.  7. Process for the deposition of a metal layer on a powdery substrate according to any one of Claims 2 to 6, characterized in that the nucleating agent is ammonia, an ammonia halide, a hydrohalide hydrohalide or hydroxylamine hydrohalide.  8. Process for the deposition of a metal layer on a powdery substrate according to any one of claims 1 to 6, characterized in that the nucleation gas is nitrogen.  9. Process for the deposition of a metal layer on a powdery substrate according to any one of claims 1 to 6, characterized in that the nucleation gas is a halide or a hydride of boron, sulfur, phosphorus or silica.  10. Process for the deposition of a metal layer on a powdery substrate according to any one of claims 1 to 9, characterized in that the particle diameter of the substrate is 1 to 30 microns.  11. Process for the deposition of a metal layer on a powdery substrate according to any one of claims 1 to 10, characterized in that the thickness of the deposited metal is less than 1 micron.  12. Process for the deposition of a metal layer on a powdery substrate in which a substantially uniform coating of tantalum is obtained on alumina powder, this process being characterized in that it comprises the dispersion of a mixture powders of alumina and tantalum in a fluidized bed in a hydrogen atmosphere at a temperature between 900 and 11000C and the exposure of the powder to a mixture of hydrogen, hydrogen chloride and ammonia to achieve nucleation of the surface of each of the alumina particles and transfer the tantalum there.  13. Process for the deposition of a metal layer on a powdery substrate according to any one of Claims 1 9 12, characterized in that the metal-coated powder serves to constitute a compacted element which will produce an electrolytic capacitor anode. .  14. Process for the deposition of a metal layer on a powdery substrate according to claim 12, characterized in that the thickness of said coating is less than 0.3 micron and that at least a portion of the anodizable coating metal. is in the form of a nitride.  15. Process for the deposition of a metal layer on a powdery substrate according to claim 14, characterized in that the nitride represents up to 10% by weight of said coating.