U.S. patent application number 10/416910 was filed with the patent office on 2004-03-18 for metal and alloy powders and powder fabrication.
Invention is credited to Chen, George Zheng, Fray, Derek John.
Application Number | 20040052672 10/416910 |
Document ID | / |
Family ID | 9903259 |
Filed Date | 2004-03-18 |
United States Patent
Application |
20040052672 |
Kind Code |
A1 |
Fray, Derek John ; et
al. |
March 18, 2004 |
Metal and alloy powders and powder fabrication
Abstract
A precursor powder comprising a metal compound is formed into a
sample for electro-deoxidation, for example by slip-casting. The
sample is then immersed in a melt comprising a molten salt and a
cathodic potential applied to remove non-metal species from the
precursor powder by electro-deoxidation and dissolution in the
melt. This typically forms a metallic sample which can be
fragmented to form a metallic powder. In a second aspect of the
invention a powdered feed material is formed into a shaped
precursor and more extensive electro-deoxidation carried out so as
to form a near-net shaped product.
Inventors: |
Fray, Derek John;
(Cambridge, GB) ; Chen, George Zheng; (Cambridge,
GB) |
Correspondence
Address: |
David R Saliwanchik
Saliwanchik Lloyd & Saliwanchik
A Professional Association
2421 41st Street N W Suite A-1
Gainesville
FL
32606-6669
US
|
Family ID: |
9903259 |
Appl. No.: |
10/416910 |
Filed: |
October 2, 2003 |
PCT Filed: |
November 15, 2001 |
PCT NO: |
PCT/GB01/05031 |
Current U.S.
Class: |
419/30 |
Current CPC
Class: |
C22B 34/129 20130101;
C25C 5/00 20130101; C22B 5/02 20130101; C22B 4/00 20130101 |
Class at
Publication: |
419/030 |
International
Class: |
B22F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 15, 2000 |
GB |
0027929.9 |
Claims
1. A method for producing a metallic powder, comprising the steps
of: treating by electro-deoxidation a precursor powder comprising a
compound (M.sup.1X) between a metal (M.sup.1) and an non-metal
species (X), the precursor powder forming a cathode contacting a
melt comprising a fused salt (M.sup.2Y), under conditions such that
the non-metal species dissolves in the melt; and processing a
product of the electro-deoxidation as required to form the metallic
powder.
2. A method according to claim 1, in which the electro-deoxidation
is Carried out under conditions whereby a cathodic potential less
than a potential for the deposition of a cation (M.sup.2) from the
melt is applied to the cathode.
3. A method according to claim 1, in which the melt comprises a
mixture of salts, including two or more cations (M.sup.2), and the
electro-deoxidation is carried out under conditions whereby a
cathodic potential less than a potential for the deposition of any
cation (M.sup.2) from the melt is applied to the cathode.
4. A method according to claim 1, 2 or 3, in which the precursor
powder is a conductor and is used as the cathode.
5. A method according to claim 1, 2 or 3, in which the precursor
powder is an insulator and is used in contact with a conductor to
form the cathode.
6. A method according to any preceding claim, in which
electro-deoxidation is carried out at a temperature of
700-1000.degree. C.
7. A method according to any preceding claim, in which the
precursor powder comprises particles between 0.05 and 20 .mu.m in
size.
8. A method according to any of claims 1 to 6, in which the
precursor powder comprises particles between 0.25 and 2 .mu.m in
size.
9. A method according to any preceding claim, in which the metallic
powder comprises particles between 1 and 30 .mu.m in size.
10. A method according to any preceding claim, in which the fused
salt comprises as a cation species (M.sup.2) Ca, Ba, Li, Cs and/or
Sr.
11. A method according to any preceding claim, in which the fused
salt comprises as an anion (Y) Cl or F.
12. A method according to any preceding claim, in which the
non-metal species comprises O, S, C or N.
13. A method according to any preceding claim, in which the metal
(M.sup.1) comprises Ti, Si, Ge, Zr, Hf, Sm, U, Al, Mg, Nd, Mo, Cr
or Nb, V, Ta, Mb, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd,
Pt, Cu, Ag, Au, Be, Sr, Ga, In, Tl, lanthanides or actinides, or an
alloy thereof.
14. A method according to any preceding claim, in which the
precursor powder is formed into a sample for electro-deoxidation by
powder processing techniques, for example slip-casting or
compaction, and the product of the electro-deoxidation is ground or
fragmented to form the metallic powder.
15. A method according to claim 14, in which the formation of the
precursor powder comprises sintering.
16. A method according to any preceding claim, in which the
precursor powder comprises a mixture or solid solution of one or
more metal compounds, and optionally one or more metals or
alloys.
17. A metallic powder produced according to a method as defined in
any preceding claim.
18. An apparatus for carrying out the method defined in any
preceding claim.
19. A method for forming a near net-shaped product, comprising the
steps of: forming a shaped precursor from a powdered feed material
comprising a compound (M.sup.1X) between a metal (M.sup.1) and a
non-metal species (X); treating the precursor by
electro-deoxidation, the precursor forming a cathode contacting a
melt comprising a fused salt (M.sup.2Y) under conditions such that
the non-metal species dissolves in the melt, the
electro-deoxidation being carried out for a sufficiently long time
and/or at a sufficiently high temperature to form interconnections
between metallic powder particles produced by the
electro-deoxidation and to produce the near net-shaped product
strong enough for further processing.
20. A method according to claim 19, in which the
electro-deoxidation is carried out under conditions whereby a
cathodic potential less than a potential for the deposition of a
cation (M.sup.2) from the melt is applied to the cathode.
21. A method according to claim 19, in which the melt comprises a
mixture of salts, including two or more cations (M.sup.2), and the
electro-deoxidation is carried out under conditions whereby a
cathodic potential less than a potential for the deposition of any
cation (M.sup.2) from the melt is applied to the cathode.
22. A method according to claim 19, 20 or 21, in which the
precursor is a conductor and is used as the cathode.
23. A method according to claim 19, 20 or 21, in which the
precursor powder is an insulator and is used in contact with a
conductor to form the cathode.
24. A method according to any of claims 19 to 23, in which
electro-deoxidation is carried out at a temperature of
700-1000.degree. C.
25. A method according to any of claims 19 to 24, in which the
powdered feed material comprises particles between 0.05 and 20
.mu.m in size.
26. A method according to any of claims 19 to 24, in which the
powdered feed material comprises particles between 0.25 and 2 .mu.m
in size.
27. A method according to any of claims 19 to 26, in which the
fused salt comprises as a cation species (M.sup.2) Ca, Ba, Li, Cs
and/or Sr.
28. A method according to any of claims 19 to 27, in which the
fused salt comprises as an anion (Y) Cl or F.
29. A method according to any of claims 19 to 28, in which the
non-metal species comprises O, S, C or N.
30. A method according to any of claims 19 to 29, in which the
metal (M.sup.1) comprises Ti, Si, Ge, Zr, Hf, Sm, U, Al, Mg, Nd,
Mo, Cr, Nb, V, Ta, Mb, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni,
Pd, Pt, Cu, Ag, Au, Be, Sr, Ga, In, Tl, lanthanides or actinides,
or an alloy thereof.
31. A method according to any of claims 19 to 30, in which the
powdered feed material is formed into a sample for
electro-deoxidation by slip-casting or compaction.
32. A method according to any of claims 19 to 31, in which the
formation of the precursor comprises sintering.
33. A method according to any of claims 19 to 32, in which the
powdered feed material comprises a mixture or solid solution of one
or more metal compounds, and optionally one or more metals or
alloys.
34. A method according to any of claims 19 to 33, in which the near
net-shaped product is subsequently treated by sintering and/or
machining.
35. A near net-shaped product formed by a method as defined in any
of claims 19 to 34.
36. An apparatus for producing a near net-shaped product as defined
in claim 35.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a method and an apparatus for
preparing metallic powders of well-defined particle sizes and
composition, and to metallic powders so produced. In a further
aspect the invention relates to powder fabrication and the
production of near net-shaped products.
BACKGROUND TO THE INVENTION
[0002] Metallic powders have many applications and these
include:
[0003] (a) As feed materials for powder metallurgical techniques,
which offer the possibility of making near net-shaped products
rather than having to machine a component from a large billet. In
some cases 90% of the material is removed during the machining
process, and has to be recycled. A method for making near-net
shaped products may advantageously reduce this wastage.
[0004] (b) Alloys, the use of metal powders in alloy preparation
results in rapid dissolution and minimal segregation within an
alloy.
[0005] (c) For their aesthetic properties; metal powders are used
often in metallic paints.
[0006] (d) As fuels in rockets.
[0007] (e) As fine powders for-mixing as alloy constituents, for
example-in making many high intensity magnetic phases.
[0008] There are a variety of conventional ways of making metallic
particles. These include crushing and grinding, which are
particularly energy-intensive processes as metals inherently resist
deformation, and for reactive metals the grinding process needs to
take place under inert conditions to avoid oxidation. Metal powders
can also be obtained by the reduction of metal compounds such as
oxides by hydrogen but this is generally restricted to oxides that
are less stable than water vapour. To reduce the oxides of very
reactive metals would require reactants such as calcium and the
powders are then likely to be contaminated with calcium oxide. The
injection of molten metal onto a spinning disc results in fine
particles of liquid being centrifugally expelled from the disc as
droplets that subsequently solidify. Liquid metals can be atomised
by impinging a high velocity gas into a stream of molten metal.
Metal powders can be produced by shock-cooling metallic vapours.
For some metals with substantial solubility of hydrogen, it is
possible to form brittle hydride phases which can subsequently be
crushed or decrepitated into fine particles. By heating at elevated
temperatures, the hydrides simply decompose to form metallic
particles. Lastly, electrochemical deposition of metal from a
compound of the metal dissolved in an aqueous or fused salt
electrolyte can result in a dendritic deposit that can easily be
crushed to a fine powder. Overall, these methods can give fine
powders but frequently the powders are highly oxidised and
contaminated with oxide products, and there is generally a
substantial range of particle sizes. This is a particular problem
when a metallic powder of a given particle size is required, which
typically necessitates sieving of the product and rejection of a
sizeable fraction. These problems are exacerbated when alloy
powders are required, especially for those of the most reactive
metals.
[0009] Metal oxide powders are much easier to obtain by grinding,
as oxides are typically highly brittle and crush readily. Being
oxides, they do not suffer from oxidation during this process. Very
fine oxide powders can also be produced by precipitation from an
aqueous or fused salt solution. Alternatively, by reacting a
volatile compound with oxygen, it can be possible to form a fine
oxide powder. For example the reaction of titanium tetrachloride
with oxygen results in a very fine oxide powder. Frequently, these
particles are of a uniform size, but the problem remains of
producing fine metal powders.
SUMMARY OF THE INVENTION
[0010] In a first aspect, the invention provides a method and an
apparatus for producing a metallic powder, and a metallic powder,
as defined in the appended independent claims. Preferred or
advantageous features of the invention are set out in dependent
subclaims.
[0011] This aspect of the invention is based on the surprising
finding that powdered metal compounds such as metal oxide powders
may be treated electrochemically to yield metal powders with a
uniform structure and size. Thus, a method for producing a metallic
powder may advantageously be provided, in which a precursor powder
comprising a compound (M.sup.1X) between a metal (M.sup.1) and a
non-metal species (X) is treated by electro-deoxidation. In this
process, the precursor powder forms a cathode contacting a melt
comprising a fused salt (M.sup.2Y), under conditions such that the
non-metal species dissolves in the melt. This may advantageously
form a porous metallic sample which may be processed as required to
form the metallic powder.
[0012] Surprisingly, the metal powders produced according to
embodiments of the invention have been found to have a uniform
microscopic structure, both in terms of the particle size of the
metal powder and the microstructure of individual particles. In
addition, it has been found that particles of similar shapes may be
produced. For example, the powders may form a cube structure. The
small, consistent particle sizes, and the metal purity, produced by
this method may be particularly advantageous as the production of
metal powders by prior art methods has failed to produce high
yields of such materials; in prior art methods, sieving is
generally required to produce consistent particle sizes and entails
very significant wastage.
[0013] The term electro-deoxidation is used herein to describe the
process of removing the non-metal species (X) from a compound in
the solid state by contacting the compound with the melt and
applying a cathodic voltage to it such that the non-metal species,
or anionic species, dissolves. In electrochemistry, the term
oxidation implies a change in oxidation state and not necessarily a
reaction with oxygen. It should not, however, be inferred that
electro-deoxidation always involves a change in the oxidation
states of both (or all) of the components of the compound; this is
believed to depend on the nature of the compound, such as whether
it is primarily ionic or covalent. In addition, it should not be
inferred that electro-deoxidation can only be applied to an oxide;
any compound may be processed in this way. Other terms to describe
the electro-deoxidation process in particular instances may be
electro-decomposition, electro-reduction or solid-state
electrolysis.
[0014] In a preferred embodiment, the cathodic voltage applied to
the metal compound is less than the voltage for deposition of
cations from the fused salt at the cathode surface. This may
advantageously reduce contamination of the intermetallic compound
involving the cations. It is believed that this may be achieved
under the conditions of an embodiment in which the decomposition
potential of the salt, or electrolyte, is not exceeded during
electro-deoxidation, or electro-reduction, or under the conditions
of an embodiment providing a method for producing a metallic powder
by treating a powder of a metal compound (M.sup.1X) by electrolysis
in a fused sal, M.sup.2Y or a mixture of salts, under conditions
whereby reaction, or ionisation, of X rather than M.sup.2
deposition occurs at an electrode surface, and X dissolves in the
electrolyte M.sup.2Y.
[0015] Further details of the electro-deoxidation process are set
out in International patent application number PCT/GB99/01781,
which is incorporated herein by reference in its entirety.
[0016] In the method of invention, it is preferable that the metal
produced has a higher melting point than that of the melt, or
salt.
[0017] Further, other metal compounds, such as metal oxides, may be
present and the electrolysis product may be an alloy powder.
[0018] The method of the invention may advantageously give a
product which is of very uniform particle size and free of oxygen
or other contaminants.
[0019] In accordance with a preferred embodiment of the present
invention, it has been found that electrochemical reduction of
metal oxide powders, by cathodically ionising the oxygen away from
the oxide, results in agglomerates of pure metal powder, the
particle size of which depends upon the conditions of pre-forming
and sintering of the metal oxide powders and the time and
temperature of electro-deoxidation, or electrolysis. Other
electrolysis parameters such as voltage, current and salt
composition may also be varied to control the metal powder
morphology. Control of these parameters may advantageously be
applicable to precursor powders other than oxides.
[0020] The metal compound or oxide should show at least some
electronic conductivity or be used in contact with a conductor.
[0021] Metal alloy powders may advantageously be formed by
electro-deoxidation of precursor powders comprising a mixture or
solid solution of two or more metal compounds or one or more metals
or alloys with one or more metal compounds.
[0022] In a second aspect, the invention may advantageously provide
a method for forming a near net-shaped product. In this method, a
shaped precursor is formed from a powdered feed material comprising
a compound (M.sup.1X) between a metal (M.sup.1) and a non-metal
species (X). The precursor is then treated by electro-deoxidation,
the precursor forming a cathode contacting a melt comprising a
fused salt (M.sup.2Y) under conditions such that the non-metal
species dissolves in the melt. The electro-deoxidation is carried
out for a sufficiently long time and/or at a sufficiently high
temperature to form interconnections between the metallic powder
particles produced by the electro-deoxidation, in order to produce
the near net-shaped product strong enough for further
processing.
[0023] The advantages of the powder production aspect of the
invention described above may also be applicable to this aspect of
the invention. For example, carrying out electro-deoxidation at a
cathodic potential less than the potential for cation deposition
from the melt may advantageously reduce contamination of the near
net-shaped product, and using a feed material comprising a mixture
or solid solution of two or more metals may advantageously produce
a near net-shaped product of a desired alloy. The skilled person
would readily appreciate that other advantages described above may
also be applicable to near net-shaped product formation.
[0024] Specific Embodiments and Best Mode of the Invention
[0025] Embodiments of the invention will now be described by way of
example, with reference to the drawings, in which;
[0026] FIG. 1 illustrates an apparatus for the electro-deoxidation
of a metal oxide powder according to a first embodiment of the
invention;
[0027] FIG. 2 illustrates an apparatus according to a second
embodiment of the invention;
[0028] FIG. 3 is a photomicrograph of a titanium oxide powder, as
used as the starting material in Examples 1 and 2;
[0029] FIG. 4 is a photomicrograph of a titanium powder produced
from the oxide of FIG. 3 in Example 1;
[0030] FIG. 5 is a photomicrograph of a titanium powder produced
from the oxide of FIG. 3 in Example 2;
[0031] FIG. 6 is a photomicrograph of a chromium powder as produced
in Example 3;
[0032] FIG. 7 is a photomicrograph of an AlNi.sub.3 powder as
produced in Example 5;
[0033] FIG. 8 is an XRD (X-ray diffraction) spectrum for the powder
of FIG. 7, overlaid on a spectrum for a reference sample of
AlNi.sub.3;
[0034] FIG. 9 is a photomicrograph of a niobium oxide powder, as
used as the starting material in Example 6;
[0035] FIG. 10 is a photomicrograph of a niobium powder as produced
in Example 6 from the oxide powder of FIG. 9;
[0036] FIG. 11 is a schematic diagram of an apparatus for
electro-deoxidation as used in Example 6; and
[0037] FIG. 12 is a plot of an XRD analysis of a niobium powder
produced as in Example 6.
[0038] FIGS. 1 and 2 show pellets 2 of metal oxide in contact with
a cathode conductor. Each pellet is prepared by powder processing
techniques, such as pressing or slip-casting a submicron or
micron-sized powder (FIG. 3) such as titanium dioxide. The pellet
may then be fired to give it structural strength before being made
the cathode in a cell in which a crucible 6 contains a fused salt
8. In the embodiment, the cell contains chloride salts, being
either. CaCl.sub.2 or BaCl.sub.2 or their eutectic mixture with
each other or with another chloride salt such as NaCl.
[0039] In the embodiment of FIG. 1, the pellets are annular and are
threaded onto a cathodic conductor in the form of a Kanthal wire 4.
The crucible is an inert crucible of graphite or alumina. In the
embodiment of FIG. 2 the crucible 12 is made of a conducting
material such as titanium or graphite. The pellets sink in the melt
and contact the crucible, to which the cathodic voltage is applied.
The crucible itself thus acts as a current collector.
[0040] In both embodiments the electrochemical process is the same,
as follows. On the application of current, the oxygen ionises,
dissolves in the salt and diffuses to a graphite anode 10 where it
is discharged. The oxygen is thereby removed from the oxide,
leaving the metal behind. The metal product is a very fine powder
of very uniform size, as shown in FIG. 4. It should be noted that
the metal powder produced has a much larger grain size than the
initial grain size of the oxide powder. By varying the temperature,
the time of electro-deoxidation (reduction), the voltage, the
current and/or the salt, it is possible to change and control the
size and morphology of the metallic powder.
[0041] The embodiment described above produces titanium metal
powder but it is possible to make alloy powders by the same route
simply by mixing the oxide powders together, and preferably firing
or sintering them to strengthen the pellet. The pellet may also be
fired so as to form a solid solution of the oxides. It is
preferable that the oxide powders are not greater than microns in
particle size and are finer than the metal powder to be
produced.
[0042] The electrolyte should consist of salts which are more
stable than the equivalent salts of the metal which is being
produced and, preferably, the salt should be as stable as possible
to remove the oxygen to as low as concentration as possible. The
choice of salt includes the chloride or other halide salts of
alkali and/or alkaline earth metals, particularly barium, calcium,
cesium, lithium, strontium and yttrium.
[0043] To obtain a salt with a lower melting point than that given
by a pure salt and/or to modify the interactions between the
cathode and the electrolyte, a mixture of salts can be used,
preferably the eutectic composition.
[0044] At the end of reduction, the reduced compact is withdrawn
from the molten salt. Some of the salt is contained within the
withdrawn pellet, however, and stops the powder oxidising. The salt
can simply be removed by washing in water or an organic solvent
such as ethanol. Generally, the pellets are very brittle and can
easily be crushed to reveal the metal powder.
[0045] The following Examples illustrate the invention.
EXAMPLE 1
[0046] Three pellets, 5 mm in diameter and 1 mm in thickness
prepared by pressing moisturised 0.25 .mu.m titanium dioxide powder
(FIG. 3) followed by drying and sintering at 950.degree. C. in air
for 2 hours, were placed in a titanium crucible filled with molten
calcium chloride at 950.degree. C. The cell arrangement is as shown
in FIG. 2. A potential of 3 V was applied between a graphite anode
and the titanium crucible. After 10 hours, the electro-deoxidation
was terminated, the salt allowed to solidify and then dissolved in
water to reveal a black/metallic pellet which was then removed from
the crucible and dried. Examination under the scanning electron
microscope showed that the particulate structure of the pellet had
been transformed from 0.25 .mu.m particles of titanium dioxide to
12 .mu.m particles of titanium (FIG. 4). The titanium particle size
was advantageously very uniform, being about 12 .mu.m+/-3 .mu.m.
Within experimental errors, no oxygen was detected by energy
dispersive X-ray analysis.
[0047] It should be mentioned that in other experiments it was
observed that increasing the time of electrolysis would increase
the size of the particles and, at the same time, interconnections
between individual particles became significantly stronger. This
could eventually lead to the production of strong metallic pellets
which could not be crushed to powders and which are therefore a
form of near net-shaped product. In addition, such strong pellets
may be used directly as a feed-stock for various fabrication
techniques, such as sintering. The microstructure in these strong
pellets is believed to be similar to that in the conventional Kroll
titanium sponge. The formation of the titanium pellets depended
also on the nature of the molten salts and other experimental
conditions such as the pre-forming conditions and sintering of the
pellet.
EXAMPLE 2
[0048] The TiO.sub.2 powder as in Example 1 was mixed with water to
form a slurry which was then slip cast into small pellets, dried
and sintered at 950.degree. C. in air for 2 hours. The sintered
pellets were about 8 mm in diameter and 2 mm in thickness. A hole,
1.5 mm in diameter, was drilled in each of the sintered TiO.sub.2
pellets. Two of them were threaded onto a Kanthal wire, 1.5 mm in
diameter, and then inserted into a molten eutectic mixture of
calcium chloride and barium chloride at 950.degree. C. An alumina
crucible was used to accommodate the salts and the cell arrangement
is as shown in FIG. 1. A potential of 3.1 V was applied between a
graphite anode and the Kanthal wire. After about 20 hours, the
temperature was lowered to 700.degree. C., the pellets on the
Kanthal wire were removed from the crucible, cooled in air and then
washed in water to reveal grey/metallic pellets. Examination under
the scanning electron microscope showed that the particulate
structure of each pellet had been transformed from 0.25/m particles
of titanium dioxide to two types of titanium particles, about 3
.mu.m and about 20 .mu.m respectively (see FIG. 5).
[0049] As shown in Example 1 above, it is possible to produce
titanium powder of a more consistent particle size than this by
appropriate control of process parameters but it should be noted
that the particle size range produced in Example 2 may
advantageously be substantially more uniform than that produced by
prior art methods.
EXAMPLE 3
[0050] A 1 .mu.m powder of chromic oxide was mixed with water to
form a slurry which was slip cast into small samples, or pellets,
about 8-10 mm in diameter and 3-5 mm in thickness, followed by
drying and sintering at 950.degree. C. in air for 2 hours. After
sintering, no significant change was observed on the colour (green)
and size of the samples, but the mechanical strength was enhanced
significantly. Three of the sintered samples were placed in a
graphite crucible filled with molten calcium chloride at
990.degree. C. as shown in FIG. 1. Better results have been
obtained by adding NaCl to the melt to reduce dissolution of
chromic oxide in the melt. A potential of 2.7 V was applied between
a graphite anode and the graphite crucible. After 15 hours, the
electrolysis was terminated, the salt allowed to solidify and then
dissolved in water to reveal the grey/metallic pellets. Examination
under a scanning electron microscope (FIG. 6) revealed aggregates
of crystallites in two sizes in the reduced samples: the larger
crystallites were 20-50 .mu.m in size while the smaller ones were
5-8 .mu.m. Energy dispersive X-ray analysis confirmed both types of
crystallites were pure chromium metal.
[0051] The particle size range produced in this Example may be
reduced through process parameter control but is significantly
narrower than the chromium particle size range produced by prior
art methods, typically by mechanical grinding.
EXAMPLE 4
[0052] Powders of titanium dioxide (0.25 .mu.m particle size),
alumina (0.25 .mu.m) and vanadium oxide (1-2 .mu.m) were mixed in a
ratio such that the ratio of the metal elements was the same as a
desired alloy, being in this example the Ti--6Al-4V alloy. The
mixture was then made into a slurry with water and slip cast into
pellets, followed by drying and sintering at 950.degree. C. for 2
hours in air. After sintering, the colour of the pellets changed
from light green to dark brown. The size of the sintered pellets
was about 8 mm in diameter and 6 mm in thickness. After drilling a
hole of 1.5 mm in diameter, one of the sintered pellets was
threaded onto a Kanthal wire, and then inserted into a molten
eutectic mixture of barium chloride and calcium chloride at
950.degree. C. An alumina crucible was used to accommodate the
molten salts and the cell arrangement is shown in FIG. 1. A
potential of 3.1 V was applied between a graphite anode and the
Kanthal wire. After 20 hours, the temperature of the salt was
allowed to cool to 700.degree. C. and then the electro-deoxidation
terminated. The pellets on the Kanthal wire were removed from the
crucible, cooled in air and then washed/leached in water to reveal
the grey/metallic pellets. Examination under the scanning electron
microscope showed that the particulate structure of the pellet was
similar to that shown in FIG. 3 for titanium. EDX analysis revealed
no oxygen in the pellets and confirmed the presence of titanium,
aluminium and vanadium in individual particles in the desired
ratio, within experimental error.
EXAMPLE 5
[0053] Powders of Al.sub.3O.sub.3 and NiO were mixed in a molar
ratio of 1:6, pressed into small cylindrical pellets (10 mm
diameter, 5-10 mm height), and sintered in air at 980-1000.degree.
C. for about 2 hours. After sintering, the grey-green colour of the
pellets became only slightly paler. Holes of 1.7 mm diameter were
drilled into the sintered pellets. Four of the sintered pellets,
weighing about 4 grams, were threaded onto a Kanthal wire (1.0 mm
diameter) to form an assembled cathode. Electro-deoxidation was
carried out between the assembled cathode and a graphite anode in
argon-protected molten CaCl.sub.2 at 950.degree. C. and 3.1 V for
about 18 hours, as shown in FIG. 1. The pellets were removed from
the molten salt upon reduction, cooled first in argon and then in
air to room temperature. Water was used to wash the reduced pellets
which were then dried in air, showing a grey metallic colour. The
surfaces and cross sections of the reduced pellets consisted of
nodular particles of 2-20 microns in size (see FIG. 7) and which
contained Al and Ni in the atomic ratio of 1:3. No oxygen was
detected. The pellets were then manually ground into powder in an
agate mortar. XRD (X-ray diffraction) was applied to the powder and
the spectrum showed an almost identical pattern to a standard
AlNi.sub.3 sample (see FIG. 8).
EXAMPLE 6
[0054] Nb.sub.2O.sub.5 powders used in experiments were 99.97 wt %
and 99.99 wt % pure, with mean particle sizes of 4.03 .mu.m and
12.71 .mu.m, respectively. The powders were pressed into porous
compacts that were then strengthened by sintering. The sintered
pellets were attached onto a cathodic current collector to form an
assembled oxide cathode. The CaCl.sub.22H.sub.2O and NaCl employed
for the melt were analytical reagents. All the chemicals were
supplied by Aldrich Chemical. The CaCl.sub.22H.sub.2O was
dehydrated in air at 373 K for 1 hour, heated up slowly to 573 K
and then was held at 573 K for 12 hours. The dehydrated CaCl.sub.2
and dried NaCl were mixed thoroughly and the mixture was then dried
at 473 K before use. High-density graphite rods of 10 mm in
diameter and 100 mm long were purchased from Graphite Technologies
and were used as the anodes. A Kanthal.RTM. wire, 1.5 mm in
diameter, was employed as the cathodic current collector.
[0055] The electrolytic cell for electrolysis is schematically
shown in FIG. 11. Two Farnell LS30-10 Autoranging Power Supplies
were used for conducting the electrolysis under constant voltages.
A first wire 50 for connecting the pellets of Nb.sub.2O.sub.5 60
was led to the negative end of one power supply. The stainless
steel crucible 56 for holding the molten electrolyte 58 was
connected by a second wire 62 to the negative end of the other
power supply. Two positive ends of the two power supplies were both
connected to the graphite rod anode 52. All electrical connections
from the individual electrodes to the power supplies were made by
Kanthal.RTM. wires 50, 62. The electrolyte temperature was measured
using a type K thermocouple in an alumina sheath 54. The cell was
placed in a vertical Inconel.RTM. reactor tube closed at one
end.
[0056] The electrolytic cell was flushed with high purity argon
while it was heated to the required temperatures. When the cell
reached its electrolysis temperature, the graphite rod anode was
dipped into the molten electrolyte and the pre-electrolysis was
performed at U.sub.2=2.8-3.0 V and 1173 K until no anode bubbles
could be visually observed, usually for a period of 12 hours. After
completing the pre-electrolysis, the oxide cathode was immersed in
the melt. The electrolysis was carried out under constant voltages
(U.sub.1 and U.sub.2) applied respectively to the cell as shown in
FIG. 11. The applied voltage (U.sub.1), along with the resulting
currents, were displayed and logged by a PC with Serial RS232 plus
ADAMS 4017-8 Channel Analogue-to-Digital Convertor during the
course of electrolysis.
[0057] The samples as-reduced were removed quickly from the melt
under a flow of the argon at 873 K and quenched and washed in cold
water, followed by acid leaching, water rinse, and acetone wash.
The resulting porous pellets were made into powders by grinding
manually. The obtained niobium metal powders were then cleaned with
acetone again and dried under vacuum at room temperature.
[0058] Morphology of the sintered or reduced pellets was observed
using a Jeol JSM-5800LV scanning electron microscope (SEM) with an
energy dispersive X-ray analysis (EDXA) attachment. Concentrations
of impurities were determined by EDXA. Various phases present in
the prepared powders were examined by powder X-ray diffractometry
(XRD) using a Philips diffractometer PW1710 with Cu K.varies..sub.1
radiation. Contents of oxygen were also determined by weighing the
prepared niobium metal powders before and after re-oxidation in
air, where a complete re-oxidation of the metal powders to the
Nb.sub.2O.sub.5 was confirmed by XRD analysis. A level of chlorine
in the off-gasses was monitored using a Drager QuadGard Chlorine
Detector.
[0059] The final product remaining at the cathode after
electrolysis was found to be metallic niobium, in the form of
porous pellets. FIGS. 9 and 10 show the typical microstructures of
cross sections of Nb.sub.2O.sub.5 pellets before (for 4.03 .mu.m
particle size Nb.sub.2O.sub.5) and after electro-deoxidation at
U.sub.1=3.1 V and 1123 K for 24 hours. It was interesting to
observe that after the reduction the form of the as-reduced product
was essentially a powder compact which had loosely sintered
together and also the particle sizes were enlarged to some extent.
The prepared niobium metal powder contained 2311 mass ppm
oxygen.
[0060] A typical measured XRD (X-ray diffraction) pattern is shown
in FIG. 12 for the niobium metal powders reduced at 1173 K for 48
hours, from which one can see that the powder is pure niobium, free
of any oxide phases.
[0061] Overall, the present experimental results provide evidence
that porous pellets of Nb.sub.2O.sub.5 can be easily deoxidised to
the metallic niobium. The niobium metal powders prepared are
obviously acceptable for subsequent purification treatments, such
as high vacuum sintering at high temperatures. Our experiments
indicated that various ranges of particle sizes of the niobium
metal powders could be readily prepared by a proper control of
experimental conditions and by changing the particle sizes of
Nb.sub.2O.sub.5 powders.
* * * * *