Metal powders

This invention relates to metal powders. In particular, the invention provides a novel metal-refractory composite powder which will withstand high temperatures, and means for its production. The metals for which the invention will have special application are platinum, palladium, rhodium, ruthenium, iridium, osmium and gold and silver. According to the invention, a metal powder suitable for use at high temperatures comprises an intimate mixture, other than a mere physical admixture, of particles of platinum, palladium, rhodium, ruthenium, iridium, osmium, gold or silver, or an alloy containing one or more of said metals, and particles of a refractory material.

This invention relates to metal powders. In particular, the invention 
provides a novel metal-refractory composite powder which will withstand 
high temperatures, and means for its production. The metals for which the 
invention will have special application are platinum, palladium, rhodium, 
ruthenium, iridium, osmium and gold and silver. 
A disadvantage of using prior art metal powders at high temperatures, 
particularly temperatures approaching or exceeding the melting point of 
the metal, is that the metal, even if it does not actually melt, tends to 
sinter. Thus, the individual metal powder particles tend to fuse together 
and form agglomerates or aggregates with the result that the physical 
properties associated with the metal in powder form are destroyed. 
In the case of a metallic gold powder which is frequently incorporated in 
decorative compositions applied to, for example, ceramic ware, the 
decorated ware is fired at temperatures in the region of 860.degree. C. 
However, if firing of the decorated ware is carried out at higher 
temperatures such as those up to and exceeding the melting point of gold, 
the resultant gold decoration (usually in the form of a film) shows 
improved chemical and mechanical durability. Further for some time there 
has been a need for gold decorations which may be fired at temperatures 
equal to those used for fire resistant decorations, that is at 
temperatures at which the glaze composition is soft. Such temperatures 
generally lie between 1200.degree. C. and 1400.degree. C. 
In this specification, reference to high or higher temperatures refers to 
temperatures up to and exceeding by, for example 100.degree. C. to 
150.degree. C., the melting point of the metal concerned. In the case of 
gold, the said high temperature would be in the region of 1400.degree. C. 
It is an object of the present invention to provide a metal powder which, 
at temperatures approaching, equal to or exceeding the melting point of 
the metal, does not suffer from the foregoing disadvantage. 
According to one aspect of the invention, a metal powder suitable for use 
at high temperatures comprises an intimate mixture, other than a mere 
physical admixture, of particles of platinum, palladium, rhodium, 
ruthenium, iridium, osmium, gold or silver, or an alloy containing one or 
more of said metals, and particles of a refractory material. Such powders 
will hereinafter be referred to as "metal-refractory composite powders" or 
merely as "composite powders". 
The refractory material may be any material from the range of 
naturally-occurring and synthetic refractories. Examples of 
naturally-occurring refractories are clays, silica, alumina, titania, 
zirconia, bentonite, boehmite and/or mixtures of these and examples of 
synthetic refractories are silicon nitride, silicon carbide and/or 
mixtures of these. In addition, re-processed naturally occurring 
refractories may be used such as sol-gel alumina. 
The refractory materials which we prefer to use are those the particles of 
which, under the pH conditions obtaining during the preparation of the 
metal-refractory composite powders, are positively charged over at least 
part of their surface area. Of these, we particularly prefer to use 
sol-gel alumina, which has an overall positive surface charge, and china 
clay, the particles of which present a positive charge along their edges 
but not on their faces. We believe that the positively charged sites are 
attractive to the negatively charged metal or alloy powders, thus 
producing a strongly associated metal-refractory composite powder. It is a 
feature of composite powders according to the invention that the 
individual particles or crystallites of metal and of refractory material 
have a very fine particle size. The preferred particle size range of the 
metal component of the composite powders is from about 0.2-2.0 microns; 
for the refractory materials we prefer to use colloidal materials such as 
sol-gel alumina which have particle sizes in the region of 50-100 A. 
In the case of a gold-refractory composite powder according to the first 
aspect of the invention for use in decorating compositions, the preferred 
relative amounts of metal and refractory material are determined by the 
need to achieve the required thermal stability coupled with a satisfactory 
decorative effect in a fired film of the decorating composition. 
Increasing the relative amount of refractory material increases the 
thermal stability and also, up to a certain level, the decorative effect, 
due to the physical bulk of the refractory material inhibiting or 
preventing globule formation; above the said level, however, the bulk 
volume of the refractory material is greater than or at least equal to 
that of the gold and the reflection properties of the fired film suffer in 
consequence. We have found in practice that the preferred percentage by 
weight of refractory material in powders according to the invention is 
within the range 10-25%, a more preferred range being 15-20%. 
We already know that, in the manufacture of a metal powder for use at 
ordinary temperatures, by which is meant temperatures significantly below 
the melting point of the metal or alloy concerned, control over the 
particle size distribution can be exercised by a precipitation technique 
involving the stages of nucleation and controlled growth. We have now 
found, surprisingly, that the application of this technique to heat 
resistant substrates yields metal powders having remarkable 
high-temperature properties. 
According to a further aspect of the invention, therefore, a method of 
producing a metal-refractory compsite powder comprising an intimate 
mixture, other than a mere physical admixture, of particles of platinum, 
palladium, rhodium, ruthenium, iridium, osmium, gold or silver, or an 
alloy containing one or more of said metals, and particles of a refractory 
material, comprises the steps of nucleating and, thereafter, growing 
particles of said metal or alloy in association with particles of said 
refractory material. 
Broadly speaking, therefore, the method of the present invention includes 
the steps of nucleating the metal, for example gold, or alloy onto the 
surface of the refractory particles and then growing further metal or 
alloy in bulk onto the resulting nuclei. The resultant composition forms 
an intimate mixture, other than a mere physical admixture, of refractory 
and metal or alloy particles. It is generally necessary, in order to 
produce and to reproduce consistently a metal powder in which the 
particles are of the desired size, to control closely the steps of 
nucleation and growth. 
The method will now be described in greater detail by way of example. 
A first stage of the process is to activate the substrate. By "activate" we 
mean to ensure that the maximum surface are of the substrate becomes 
available for nucleation and subsequent growth. Methods of activation vary 
according to the nature of the substrate. For example, activation of a 
clay or a naturally-occurring mineral can be achieved by boiling it in 
water. Optionally, the water may also contain a solution of a strong 
reducing agent, such as hydrazine hydrate or sodium sulphite. An 
alternative activation procedure for a clay or a naturally-occurring 
mineral is to boil it in dilute mineral acid. On the other hand, an 
activation procedure for a synthetic refractory compound, if the inherent 
activity is too low, is to deposit active sites on the surface of the 
refractory. This may be carried out by using any or all of the methods of 
preparing ceramic and other heat resistant substrates for catalytic 
purposes, which methods are well known to those skilled in the art. 
A second stage of the process is to nucleate particles of metal or alloy 
onto the surface of the activated substrate particles and this may be 
achieved by adding a suspension of the substrate particles in the 
activating agent to an aqueous solution of a salt of the metal or metals 
or applying an organo compound of the metal or metals and subsequently 
decomposing the same. 
By way of example and with particular reference to gold powders, nucleation 
may be achieved by dispersing the refractory substrate particles in a 
solution of an organic sulphur-containing gold compound in an organic 
solvent, evaporating the solvent and thermally decomposing the gold 
compound. 
Nucleation is then initiated by reducing the metal, e.g. gold salt, with a 
strong reducing agent which may be that already optionally present in the 
activation solution. If no reducing agent is present in the activation 
solution, nucleation may be induced by adding a strong reducing agent to 
the suspension of substrate in the mixture of activating agent and metal 
salt solution. The chemical nature of the strong reducing agent added to 
initiate nucleation may be similar to that of the reducing agent added to 
the activation solution, that is to say, hydrazine hydrate or sodium 
sulphite, for example. Vigorous stirring is desirable at this stage to 
ensure adequate and uniform dispersion of the substrate particles in the 
metal salt solution. 
We prefer to add to the solution of metal salt, prior to the addition of 
the suspension of substrate particles in the activation solution, a 
colloidal protective agent. This agent controls nucleation and prevents 
agglomeration of the nucleated substrate particles. Examples of suitable 
colloidal protective agents are gum acacia, gum arabic, gelatin, egg 
albumin and dextrin, but in general the requirements of the colloidal 
protective agents are that they should have a high molecular weight and be 
capable of being adsorbed onto the surface of the nucleated substrate 
particles so that their agglomeration is physically prevented. 
A third stage of the process is to grow further metal in bulk on the nuclei 
already present on the substrate surface. This is achieved by adding to 
the second-stage suspension a weak reducing agent, such as hydrogen 
peroxide or hydroquinone, for example. We prefer to add the reducing agent 
in portions and any foam generated may readily be suppressed by a spray 
of, for example, isopropanol. After all the weak reducing agents has been 
added, the suspension is stirred for some hours to complete the growth 
stage of the process. The resulting metal powder is then filtered off, 
washed and dried. 
It is, of course, possible to provide a metal powder by combining the 
stages of nucleation and growth into a one-step process, using either a 
strong reducing agent alone or a weak reducing agent alone instead of the 
former followed by the latter. We have found, however, that the use of a 
strong reducing agent alone results in rapid nucleation and a fast growth 
rate; the two stages overlap and control of the process is poor. The use 
of a weak reducing agent alone improves the process control somewhat, but 
in these circumstances, the nucleation stage is somewhat slow and more 
difficult to control than compared with the case where a strong reducing 
agent is used. As the time factor is increased by slowing the rate of 
reduction, side effects occur, such as nucleation and growth on dust 
particles, the walls of vessels and stirrers and the like. 
The major factor controlling the performance and stability during firing of 
films or coatings incorporating metal-refractory composite powders 
according to the invention is the maximum particle size of the metal 
component of the composite. We have found that the metal maximum particle 
size is governed by the preparative reaction conditions of temperature, 
initial metal concentration and the degree of induced nucleation. These 
are discussed below in turn. 
(i) Temperature. We have found that increasing the temperature leads to an 
increase in the maximum metal particle size which in turn causes a 
decrease in stability. The optimum temperature is around 25.degree. C.; 
temperatures approaching 60.degree. C. result in poor stability. 
(ii) Initial metal concentration. It has become apparent that this is the 
least important of the three variable conditions, but nevertheless an 
increase in the initial metal concentration (towards 100 g/l for gold) 
leads to an increased particle size with attendant loss of stability. An 
initial concentration of approximately 20 g/l is preferred. 
(iii) Degree of induced nucleation. An increase in the degree of induced 
nucleation from a "low" figure of about 0.25 ml/g towards about 2.0 ml/g 
leads to a reduction in the maximum particle size and a consequential gain 
in stability. 
The preferred maximum particle size for gold is approximately 1 micron; 
increasing the maximum particle size leads to a corresponding decrease in 
stability such that a particle size of 20 microns gives poor stability.

The method will now be described by means of the following Examples, in 
which the preparation of a gold powder according to the invention is 
described. 
EXAMPLE 1 
75.0 g of gold as gold ammonium chloride was dissolved in 2.4 liters of 
distilled water in a 5 liter beaker. 40 ml of a 10% gum acacia solution 
was added and the mixture stirred to ensure complete dissolution of the 
gold salt. Meanwhile, 25 g of china clay was activated by boiling in 100 
ml of distilled water containing 10 drops (.ident.0.45 ml) of a 6% 
hydrazine hydrate aqueous solution. The china clay/hydrazine hydrate 
suspension was then added with vigorous stirring to the gold solution. 
Upon addition, the colour of the mixture changed from yellow to 
yellow-green. After stirring for 10 minutes, 400 ml of "40 volume" 
hydrogen peroxide was added, as a result of which the colour changed from 
green to brown and foam was formed from reaction gases generated. The foam 
was suppressed using the minimum quantity of isopropanol from a laboratory 
spray. After 10 minutes, a further 100 ml of "40 volume" hydrogen peroxide 
was added, which caused further foaming, and the final 100 ml of hydrogen 
peroxide was added after a further 10 minutes. The reaction mixture was 
then stirred for 5 hours to complete the reaction, after which the clear 
supernatant liquor was decanted off and the powder was filtered off, 
washed and dried until constant weight was achieved. 
EXAMPLE 2 
80 g of gold as gold ammonium chloride was dissolved in 2 liters of 
distilled water in a 5 liter beaker. 40 ml of a 10% aqueous gum acacia 
solution was added and the mixture stirred to ensure complete dissolution 
of the gold salt. Meanwhile, 20 g of bentonite was activated by boiling in 
200 ml of distilled water containing 0.45 ml of an aqueous 6% hydrazine 
hydrate solution. The bentonite/hydrazine hydrate mixture was then added 
with vigorous stirring to the gold solution. After stirring for 10 
minutes, a 6% hydrazine hydrate aqueous solution was added to the mixture 
until the reaction was complete. The foam formed from the reaction gases 
generated was suppressed using the minimum quantity of isopropanol spray. 
When the reaction was complete the gold/bentonite composite was allowed to 
settle, the clear supernatant liquor was decanted off and the powder was 
filtered, washed and dried to constant weight. 
EXAMPLE 3 
80 g of gold as gold ammonium chloride was dissolved in 2 liters of 
distilled water in a 5 liter beaker. 40 ml of a 10% aqueous gum acacia 
solutin was added and the mixture stirred to ensure complete dissolution 
of the gold salt. Meanwhile, 20 g of zirconium dioxide powder was 
dispersed in 100 ml of water containing 0.45 ml of 6% hydrazine hydrate 
aqueous solution. The zirconia/hydrazine hydrate suspension was boiled and 
added with vigorous stirring to the gold solution. After stirring for 10 
minutes, a solution of 200 g of sodium sulphite dissolved in 1 liter of 
water was added. The reaction was complete within 10 minutes. The 
resulting powder was allowed to settle and the supernatant liquid was 
decanted off. The powder was filtered, washed free from dissolved salts 
and was dried to constant weight. 
EXAMPLE 4 
90 g of platinum as sodium chloroplatinate was dissolved in 1500 ml water 
in a 5 liter beaker. 50 ml of a 10% aqueous gum acacia solution was added 
and the mixture was thoroughly stirred to allow the platinum salt to 
dissolve. Meanwhile, 10 g china clay was dispersed in 125 ml of a 1% 
hydrazine hydrate solution by boiling for 5 minutes. The clay dispersion 
was added to the platinum salt solution during vigorous stirring. After 
stirring for 10 minutes, sufficient aqueous 6% hydrazine solution was 
added to bring the reaction to completion. The resulting powder was 
allowed to settle, washed and dried to constant weight. 
We have found that metal-refractory composite powders according to the 
invention exist as intimate mixtures, as shown by examination of a 
selection of gold-alumina powders by the techniques of x-ray photoelectron 
spectroscopy and electron microscopy. 
In the photoelectron spectroscopy examination, the powders, each containing 
a different proportion of gold:alumina, were subjected to analysis of the 
gold:aluminium ratio at the surface. The specimens were prepared and were 
mounted on the plane faces of the specimen holders by coating a thin layer 
of conducting silver paste with the appropriate powder. The mounted 
specimens were introduced into the ultra high vacuum of the spectrometer 
and irradiated with monochromatic x-ray radiation. The photoelectrons 
emitted from the specimens were analysed for energy distribution and a 
spectrum of the distribution obtained. 
A wide scan (1000 eV) covering the general pattern of emitted energies was 
followed on each specimen by a narrower scan, containing the 2 p and 2 s 
peaks of aluminium and the 4f doublet of gold. Measurement of peak heights 
of these lines enabled an approximate atomic ratio of gold to aluminium 
lying within an electron mean free path of the solid/vacuum interface of 
the specimens to be determined. 
The surface ratios of gold to aluminium obtained by x-ray photoelectron 
spectroscopy were compared with the calculated theoretical gold:aluminium 
ratios for each powder. 
______________________________________ 
Gold:Aluminium ratios as calculated from 
experimental results 
Powder type A B C D E F 
______________________________________ 
Au:Al using Al 2 p line 
3.7 1.5 0.6 4.2 3.6 7.2 
Au:Al using Al 2 s line 
4.6 1.6 0.6 3.0 4.4 5.9 
______________________________________ 
Note: 
The variation in peak height ratios between 2p and 2s emissions from 
aluminium are in line with statements by Jorgenson & Berthou that 
quantitative measurements may only be accurate to within .+-. 20%. 
Comparison of Experimental and 
Theoretical Data 
Au:Al Au:Al 
Au:Al.sub.2 O.sub.3 
Experimental Theroetical 
Powder weight ratio 
Atomic Ratio (% Au) 
% Au 
______________________________________ 
C 70:30 37.5 41.5 
B 80:20 61 55 
A 90:10 80 73.4 
D 95:5 80 85.5 
E 90:10 79 73.4 
F 85:15 68 63.2 
______________________________________ 
Note: 
The theroetical Au:Al ratio was calculated by assuming the gold alumina 
relationship to be that of an homogeneous particulate mixture. 
Taking into account the .+-.20% accuracy obtainable by this technique, 
comparison of the experimental and theoretical data shows only slight 
variations. This supports the assumption that the composite exists as an 
intimate mixture of gold and alumina particles. Furthermore an absence of 
"peak masking" implies that the encapsulation of one species by the other 
is not occurring. 
The powders subjected to x-ray photoelectron spectroscopic analysis were 
also examined using transmission electron microscopy. Following ultrasonic 
dispersion in acetone, samples were mounted on copper grids and observed 
at magnifications between .times.20,000 and 100,000. 
A sample of a gold alumina composite that gave good results on porcelain 
was also examined by scanning electron microscope up to magnifications of 
12,500. 
A typical transmission micrograph can be seen in FIG. 1. The crystallite 
size of the alumina was below the maximum resolution of the instrument and 
appears in the micrograph as the grey diffuse areas, whilst the gold 
particles are evident as the sharper black areas. Observations confirm 
that the composites consist mainly of an intimate mixture of spherical 
gold particles and highly flocculated alumina. 
A typical scanning electron micrograph can be seen in FIG. 2. The gold 
appears as two distinct species; spherical particles with a size range of 
0.25-0.5 .mu.m, and crystalline hexagonal platelets of approximately 2-3 
.mu.m. The alumina particles are not as easily distinguishable as in the 
transmission micrographs. 
We have found that gold-refractory composite powders according to the 
invention are excellent for use as pigments in a "burnish gold" 
preparation for decorating pottery and porcelain and for firing at high 
temperatures. Firing schedules for decorations and in common use at 
present employ a peak temperature of about 800.degree. C., but modern 
furnaces are designed to operate at temperatures up to about 1050.degree. 
C. (for chinaware) and up to about 1400.degree. C. (for porcelain). At 
these temperatures, using burnish gold preparations containing standard 
gold powders, breakdown of the film occurs due to the gold sintering and 
forming into agglomerates. However, using a gold-refractory composite 
powder according to the invention, the resulting film has surprisingly 
high cohesive properties at temperatures as high as 1400.degree. C. and 
the spatial configuration of the gold powder particles in the film is 
maintained. The resulting films are capable of being burnished to a 
continuous decorative film with good adhesion and no wrinkling, and 
exhibit outstanding physical and chemical durability. 
Although the invention has been exemplified with reference to the 
preparation of a gold-refractory composite powder for use in a burnish 
gold preparation for pottery and porcelain dcoration, it is to be 
emphasized that the method of production may equally be applied to making 
metal powders of the platinum group metals and silver or alloys containing 
one of these metals, and uses are by no means limited to the decoration of 
pottery and porcelain. Metal-refractory composite powders according to the 
invention maintain their spatial configuration at high temperatures, in 
whatever application to which they are submitted. Powders according to the 
invention have the properties of the metals at ordinary temperatures, in 
that they can be formed into shapes, for example, and in addition the 
substrate particles provide a rigidity and dimensional stability at high 
temperatures that would cause an ordinary metal powder to break down. 
One advantage of a metal-refractory composite powder according to the 
invention is that economy of the metal is achieved, as the refractory 
material can be considered as an "extender" which does not "dilute" or 
weaken in any way the physical properties of the metal, which is the case 
with the usual extenders which are used in the form of an intimate mixture 
or dispersion in the untreated metal powder. 
Examples of some disadvantages of the platinum group metals and silver at 
high temperatures, which could be overcome by using the metal in the form 
of a metal-refractory composite powder according to the invention, are as 
follows 
Alloys of platinum and rhodium, near their softening point, lose their 
cohesive strength with the result that faults such as sagging develop. 
Therefore, according to the invention, thinner than normal sections of 
metal may be prepared by powder metallurgy. 
Adjacent spirals of resistance thermometer elements tend to fuse together, 
causing short circuits. 
During the metallising of heat resistant substrates by firing pads of 
conductor material, for example silver or silver-palladium, onto the 
substrate, adhesion of the metallising layer to the substrate may be good 
but the cohesive strength of the metallising layer is often low. 
Expansion coefficients of substrates and metallising layers of conductors 
are often significantly different, leading to early metallising breakdown 
at only moderate temperatures. 
Ordinary metal powders and formulations containing them have poor 
resistance to leaching by molten solders. 
If desired, the heat resistant substrate particles may have applied thereto 
two or more coatings of different metals or alloys. 
In addition to using metal powders in accordance with this invention for 
decorative purposes as already mentioned, the particles may be used as 
pelleted catalysts in, for example, oxidation and reduction reactions. 
Particular applications of such catalysts are: 
(1) the purification of waste or tail gases from industrial plants; and 
(2) the purification of automobile exhaust gases.