Process for the preparation of a suspension containing sphere-shaped oxide particles

A process is described for the preparation of a suspension containing sph-shaped oxide particles, in which process there is emulsified in an organic liquid an aqueous phase containing at least one element which can be precipitated as an oxide (hydrate) in dissolved form or in the form of a sol. In this process there is dissolved in this organic liquid before, during or after formation of the emulsion at least one compound which acts as a phase transfer catalyst, and which can replace the anions or cations present in the emulsified water droplets by hydroxide ions or protons and which can thereby cause precipitation of the oxide in the droplets.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for the preparation of a 
suspension which contains sphere-shaped oxide particles, and in particular 
to a process in which the sphere-shaped oxide particles are prepared in a 
water-in-oil emulsion with the aid of a compound which acts as a phase 
transfer catalyst. The invention furthermore relates to the use of the 
suspensions obtained in such a process for the preparation of (ceramic) 
oxide powders and/or (ceramic) shaped articles. 
A plurality of processes for the preparation of powders from emulsions is 
described in the literature. An example of this is the emulsification of 
metal alkoxides in liquids which are immiscible therewith (for example 
paraffin oil or propylene carbonate), followed by hydrolysis. The main 
disadvantage of this process is that the required alkoxides are relatively 
expensive compounds. In another existing process, salt solutions or 
stabilized sols are emulsified in hydrocarbons, and then, for example, 
ammonia is passed in to precipitate hydroxides, and the resulting mixture 
is added dropwise to boiling hydrocarbons so that the water evaporates, or 
the mixture is freeze-dried. The dry powders which can be obtained by this 
process contain the anions in the form of metal salts or ammonium salts. 
Thermal decomposition, which then takes place, brings about disintegration 
of the particles or the formation of open-pore, sponge-like structures. 
High green densities and good sinter properties can only be achieved by 
these processes in individual cases, if at all. 
In yet another existing process the extraction of acids or anions of acid 
salts from emulsified aqueous phases is achieved by adding long-chain 
amines to the organic phase. However, a complete exchange requires a large 
excess of amines or step-wise extraction. Such a process is not capable of 
splitting neutral salts (for example alkali metal compounds or alkaline 
earth metal compounds). 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a process 
for the preparation of a suspension which contains sphere-shaped oxide 
particles, in which process there is emulsified in an organic liquid an 
aqueous phase containing at least one element which can be precipitated as 
an oxide (hydrate), in dissolved form or in the form of a sol, and which 
is intended to have, in particular, the following advantages: 
Extraction of anions (cations) is also possible from neutral salt solutions 
and leads to the formation and precipitation of oxides (oxide hydrates) 
within the emulsion droplets. 
Inexpensive, inorganic starting compounds can be employed. 
The process leads to suspensions which can be further processed to give 
compact, spherical powders having a homogeneous chemical composition and 
good compaction and sinter properties. 
The organic phase as well as the compound which causes precipitation of the 
oxide (hydrate) can readily be recycled or regenerated. 
These advantages and others which will be illustrated below are obtained 
according to the invention bY a process for the preparation of a 
suspension which contains sphere-shaped oxide particles, in which process 
there is emulsified in an organic liquid an aqueous phase containing at 
least one element which can be precipitated as an oxide (hydrate), in 
dissolved form or in the form of a sol, and in which process there is 
dissolved in this organic liquid before, during or after the formation of 
the emulsion at least one compound which acts as a phase transfer catalyst 
and which can replace the anions or cations present in the emulsified 
water droplets by hydroxide ions or protons and which can thereby cause 
precipitation of the oxide (hydrate) in the droplets. 
DETAILED DESCRIPTION OF THE INVENTION 
In what follows, the process according to the invention will be illustrated 
in greater detail. 
The aqueous phase to be emulsified in the organic liquid contains at least 
one element which is in dissolved form or in the form of a sol and which 
can be precipitated as an oxide (hydrate) by raising or lowering the pH. 
Elements which are preferred according to the invention are those which are 
suitable for the preparation of glass or ceramics. Examples which may be 
mentioned in this context are metals in general, in particular alkali 
metals (for example Li, Na, K), alkaline earth metals (for example Mg, Ca, 
Sr and Ba), other main group metals, such as, for example, Al, Sn, Pb or 
Bi, sub-group metals, such as, for example, Ti, Zr, V, Mn, Nb, Ta, Cr, Mo, 
W, Fe, Co, Ni, Cu or Zn, and the lanthanides, for example Ce and Y. 
Non-metals, such as, for example, Si, B and P are also suitable according 
to the invention. 
Preferred examples for oxides to be precipitated are, inter alia, 
ZrO.sub.2, TiO.sub.2, lead zirconate titanate (PZT), BaTiO.sub.3, Al.sub.2 
O.sub.3, YBa.sub.2 Cu.sub.3 O.sub.7, ZrSiO.sub.4, Al.sub.6 Si.sub.2 
O.sub.1 3 and Mg.sub.2 Al.sub.4 Si.sub.5 O.sub.1 8. 
The above elements are preferably present in the emulsified aqueous phase 
in as high a concentration as possible. The higher the concentration, the 
greater the guarantee that sphere-shaped particles are formed. 
High concentrations can be obtained, for example, using aqueous salt 
solutions, or aqueous sols which have been peptized under acid conditions, 
and their combinations. 
If cations are to be precipitated as an oxide (hydrate), preferred salt 
solutions are those which have anions which readily undergo thermal 
decomposition, in particular nitrates, sulfates and anions of organic 
acids, such as acetates and formates. The nitrate ion .is particularly 
preferred since it has a high affinity to anion-exchanger resins, which is 
advantageous if an anion-exchanger resin is employed for regenerating the 
compound acting as the phase transfer catalyst. 
Acids which dissociate into the abovementioned anions are preferred in 
those cases when the sol is to be peptized under acid conditions. Peptized 
sols are to be preferred to the aqueous salt solutions in particular in 
those cases where the compound acting as the phase transfer catalyst is 
regenerated using ion exchangers, since the salt solutions contain a large 
number of anions to be exchanged and therefore cause more rapid exhaustion 
of the exchanger medium. 
Aqueous salt solutions and sols can be prepared using a variety of 
processes known to those skilled in the art. In the simplest case, the 
salt in question can simply be dissolved in water. Examples of other 
possibilities are dialysis of salt solutions, hydrolysis of alcoholates 
(for example in the case of zirconium), peptization of freshly 
precipitated hydroxides (for example in the case of aluminum), mixing of 
the sol with a salt solution or a peptized hydroxide etc. Specific 
examples for these processes will be mentioned further below. 
To form a water-in-oil emulsion, the aqueous phase is emulsified in an 
organic liquid. The emulsification process is expediently carried out in 
the presence of an emulsifier, preferably of a non-ionic emulsifier. 
Suitable organic liquids are generally all organic solvents which are 
inert under the reaction conditions and which are virtually 
water-immiscible. Examples of these liquids are (optionally halogenated, 
in particular fluorinated or chlorinated) aliphatic and aromatic 
hydrocarbons (for example hexane, heptane, decane, petroleum ether 
fractions, kerosene, mineral oil, benzene, toluene, xylene), higher 
alcohols (for example alcohols having 6 to 8 carbon atoms), ethers (such 
as, for example, tetrahydrofuran, dioxane, and ethers derived from 
polyethylene glycol and polypropylene glycol), esters and ketones. 
Preferred organic liquids according to the invention are petroleum ether 
fractions (boiling range for example 50 to 70.degree. C. or 100.degree. to 
140.degree. C.), hexane, heptane, toluene, the xylenes, and halogenated 
hydrocarbons, such as, for example, chloroform and chlorobenzene. In any 
case, the boiling point of the solvent should be below its decomposition 
point. Since the organic solvent will later be removed, preferably 
quantitatively, it should not have too high a boiling point, in particular 
a boiling point considerably above 180.degree. C. On the other hand, nor 
should the boiling point be too low, i.e., not considerably below 
50.degree. C., since this may cause difficulties when water may have to be 
removed at a later point in time. 
The best solvent for a given system depends on many factors, for example on 
the nature of the emulsifier employed, the compound which acts as the 
phase transfer catalyst and the type of the oxide (hydrate) to be 
precipitated, and it can be determined bY simple standard experiments. 
Emulsifiers which can be used are non-ionic, but also cationic and anionic 
emulsifiers Non-ionic emulsifiers are preferred since they permit 
processes to be carried out in a wide pH range and, on the other hand, do 
not introduce inorganic impurities into the system. In general, the 
emulsifier should be capable of forming a water-in-oil emulsion 
Emulsifiers of this type usually have a HLB value of less than 7, but 
water-in-oil emulsions can also be obtained in individual cases when 
emulsifiers having HLB values of 11 or more are used. This is because the 
type of emulsion depends not only on the emulsifier employed but also on 
the liquid used as the organic phase, the volumetric ratio of the phases 
etc. A survey of the interrelations in the formation of emulsions and the 
influences of various factors on the properties of oxide particles formed 
in such emulsions can be found in M. Akinc and K. Richardson, "Preparation 
of ceramic powders from emulsions", Mat. Res. Soc. Symp. Proc., Vol. 73, 
1986, Materials Research Society. Reference is made to the comments made 
therein. 
Specific examples of non-ionic emulsifiers which are particularly suitable 
according to the invention are sorbitan esters of lauric, palmitic, 
stearic and oleic acid (for example those commercially available under the 
trade mark Span.RTM.), polyoxyethylene derivatives of these esters (for 
example those commercially available under the trade mark Tween.RTM.) and 
alkyl(phenol) polyglycol ethers and fatty acid polyglycol esters (for 
example those commercially available under the trade mark Emulsogen.RTM.). 
Other examples which can be mentioned are emulsifiers of the Arlacel.RTM., 
Pluronic.RTM., Emulan.RTM., Malophen.RTM. and Malovet.RTM. type. Of 
course, the aforementioned emulsifiers only represent a small selection 
from amongst the w/o emulsifiers which can be employed according to the 
invention. 
Examples of anionic w/o emulsifiers are salts of highermolecular fatty 
acids containing divalent and trivalent cations, for example calcium, 
magnesium, aluminum and lithium. Examples of cationic emulsifiers are 
fatty amines and quaternary ammonium compounds. Finally, it is also 
possible to employ ampholytic emulsifiers, such as, for example, 
long-chain substituted amino acids and betains. 
It is perferable to employ the emulsifier in an amount of 0.1 to 25%, 
preferably 1 to 20% by weight, relative to the organic phase. However, in 
individual cases, concentrations which are lower or higher than the 
aforementioned can also be advantageous. 
According to the invention, the proportion of the aqueous phase in the 
emulsion is preferably 5 to 50, in particular 15 to 45, and particularly 
preferably 20 to 40% by volume. While there is no minimal value in 
practice for the volumetric proportion of the aqueous phase, it is 
generally very difficult to obtain a relatively stable w/o emulsion when 
the volumetric proportion of the aqueous phase is 50% or more. 
To prepare the emulsion, it is possible to use techniques known to those 
skilled in the art. For example, the emulsifier can first be dissolved in 
the organic liquid, and the aqueous phase is then added all at once or in 
portions, for example with stirring. The emulsion is preferably prepared 
at room temperature or at slightly increased temperatures (for example 
40.degree. to 50.degree. C.). Apparatus which can be used are, for 
example, ultrasonic equipment or rotor/stator systems having high shear 
rates. Specific emulsifiers and suitable apparatus (for example 
high-pressure homogenizer) make it possible to prepare emulsions of 
droplet size distributions down to the submicron range and a clear-cut 
upper limit. The size of the particles which form in the further progress 
of the process is determined by the droplet size of the emulsion and the 
solids content of the aqueous solution. The larger the emulsified water 
droplets, the larger later on the oxide particles in the suspension In 
general, droplet sizes of more than 20 .mu.m lead to the (in general 
undesired) formation of hollow spheres. If the suspensions prepared 
according to the invention are to be processed to form ceramic materials, 
the droplet size is preferablY 5 .mu.m or less, which corresponds to a 
final size (after calcination) of 1 to 2 .mu.m. The preferred lower limit 
for the droplet size is a function of the desired size of the oxide 
particles. 
Another procedure for the formation of the w/o emulsion involves converting 
an o/w emulsion into a w/o emulsion by adding an organic liquid. 
One of the most important aspects of the process according to the invention 
is the addition to the organic phase of a compound which acts as a phase 
transfer catalyst and which is capable of replacing the anions (or 
cations) present in the emulsified water droplets by hydroxide ions (or 
protons), thus causing the oxide (hydrate) to precipitate in the droplets. 
According to a particularly preferred embodiment of the present invention, 
neutral or basic (metal) oxides are precipitated in the droplets, so that 
the compound which acts as the phase transfer catalyst is bound to replace 
the anions present in the water droplets (for example nitrate, sulfate, 
formate, acetate, etc.) by hydroxide ions. In what follows, the process 
according to the invention will be illustrated with reference to this 
preferred embodiment. Acid oxides are precipitated analogously. 
Examples of the compound which acts as the phase transfer catalyst are a 
quaternary ammonium compound, a phosphonium compound or another onium 
compound. Crown ethers and kryptans, inter alia, are also suitable 
according to the invention. 
Ammonium compounds are preferred because they are readily accessible and 
because of their price. Examples which are particularly preferred are the 
quaternary ammonium salts, in particular tetraalkylammonium chlorides, 
tetraalkylammonium bromides and tetraalkylammonium iodides. In these 
tetraalkylammonium salts, the alkyl radicals, which can be identical or 
different, preferably have 1 to 20 carbon atoms. The total carbon number 
should be high enough that the salts are virtually insoluble in water, and 
is therefore preferably higher than 15, in particular higher than 20. 
Representatives of tetraalkylammonium salts which have proven particularly 
suitable are those which are provided with two to three long radicals (for 
example having 8 to 20 carbon atoms) and one or two short alkyl groups 
(for example one or two methyl or ethyl groups). An example of such a 
compound is didodecyldimethylammonium bromide. Another preferred class of 
tetraalkylammonium salts are those which are provided with one long 
hydrocarbon radical (for example having 16 or more carbon atoms) and three 
short alkyl groups (for example methyl or ethyl groups). An example of 
such a compound is octadecyltrimethylammonium bromide. In general, it can 
be said that tetraalkylammonium salts having radicals which are different 
from one another are usually more advantageous than those having four 
identical radicals. The alkyl radicals can also be substituted, for 
example by a phenyl group (for example benzylammonium salts). When 
selecting the tetraalkylammonium salt, it must also be taken into 
consideration that salts which have long alkyl chains can act as 
surfactants. Accordingly, simple standard experiments must be carried out 
to determine the most suitable phase transfer catalyst for each individual 
case. 
The compound which acts as the phase transfer catalyst can be dissolved in 
the organic liquid, but also in the finished emulsion. This can be carried 
out at room temperature or at a slightly increased temperature, preferably 
with stirring. If it is possible to dissolve the compound which acts as 
the phase transfer catalyst in the organic phase before the emulsification 
process, this has the advantage of simplifying the recycling of the 
organic phase. 
To facilitate the dissolution of the phase transfer catalyst in the organic 
liquid or in the organic phase of the emulsion, it is preferred to 
dissolve it in the smallest possible volume of a good solvent, in 
particular an alcohol having 4 to 8 carbon atoms, preferably hexanol, 
heptanol or octanol. The solvent for the phase transfer catalyst should be 
virtually insoluble in water so that a clear-cut phase separation in the 
emulsion is guaranteed. 
The compound which acts as the phase transfer catalyst, for example the 
tetraalkylammonium salt, can already be employed in the form of a 
hydroxide and also in stoichiometric amounts relative to the anions to be 
exchanged which are present in the aqueous phase. However, it is preferred 
to use the compound which acts as the phase transfer catalyst in 
substoichiometric amounts, in particular 1 to 20%, preferably 5 to 10%, 
relative to the ions (anions) to be exchanged which are present in the 
aqueous phase. In this case, of course, the phase transfer catalyst should 
be regenerated batchwise or continuously. According to a particularly 
preferred embodiment of the present invention, anion exchange and 
regeneration of the phase transfer catalyst occur simultaneously. If the 
phase transfer catalyst is present in the hydroxide form from the very 
beginning, this has the additional advantage that the first exchange 
occurs spontaneously, and hydroxides are immediately formed at the surface 
of the water droplets, which stabilizes the emulsion considerably so that 
this procedure is particularly recommended in the case of emulsions whose 
stability is low. 
Regenerating agents which are preferably employed for the compound which 
acts as the phase transfer catalyst are ion-exchanger resins. If the phase 
transfer catalyst is to exchange anions against hydroxide ions, an anionic 
ion-exchanger resin is used, while in the case of a compound which must 
exchange cations against protons, the regenerating agent is a cationic 
ion-exchanger resin. 
Suitable ion-exchanger resins according to the invention are those which 
are customary and commercially available. They come under trade marks such 
as Permutit, Lewatit, Amberlite, Amberlyst, Dowex, Wofatit etc. Inorganic 
ion exchangers, such as, for example, zeolites, montmorillonites, 
attapulgites, bentonites and aluminum silicates, can also be used. 
The compound which acts as the phase transfer catalyst can be regenerated 
in various ways. For example, the emulsion together with the phase 
transfer catalyst can be passed through an ion-exchanger column, the 
residence time of the emulsion on the column being chosen such that 
exchange and regeneration of the phase transfer catalyst are complete. For 
this purpose, the emulsion can also be passed over the column several 
times However, as already mentioned above, it is also possible to employ 
the phase transfer catalyst in stoichiometric amounts, to remove it after 
the exchange has taken place, to regenerate it and to re-use it. 
When phase transfer catalyst and regenerating agent are selected, it is 
self-evident to bear in mind that, for example, only an anion-exchanger 
resin which is more basic than the hydroxide form of the 
tetraalkylammonium salt can regenerate the latter. Likewise, the phase 
transfer catalyst in the process according to the invention should be more 
basic in its hydroxide form than the oxide (hydrate) which is to be 
precipitated. An analogous train of thought is applicable in the case of 
an exchange of cations, in which cationic ion exchangers can be employed 
as the regenerating agent and, for example, salts of long-chain organic 
acids (for example fattY acids and the analogous sulfonic acids) as the 
phase transfer catalyst 
If the emulsion is passed over a column or regenerated over an 
ion-exchanger membrane, it is easy to check if exchange or regeneration 
have already been brought to completion by using indicator strips (pH 
indicator strips or ion-selective indicator strips) 
When the desired oxide (hydrate) is precipitated (completely), the emulsion 
which was employed at the beginning is present in the form of a suspension 
of sphere-shaped oxide particles It is preferable to separate the water 
from this suspension in order to compact the oxide particles and to 
stabilize them. To this end, the water is subjected to azeotropic 
distillation (preferably under atmospheric pressure) Further small amounts 
of hydroxides dissolved in the aqueous phase precipitate upon removal of 
water Azeotropic mixtures containing 20 to 40% of water are achieved, for 
example, by adding a small amount (for example 5% by volume of the 
emulsion) of (n-)butanol and/or (n-)pentanol, i.e., alcohols which are 
still H.sub.2 O-miscible to a certain extent, to water/petroleum spirit 
mixtures. The water can be separated from the distillate by means of a 
Dean-Stark water trap. If appropriate, the organic phase can be recycled. 
The resulting oxide particles can now be isolated for example by filtration 
or centrifugation, rinsing and drying, or redispersing and spray-drying. 
Under suitable conditions this gives agglomerate-free powders of 
sphere-like, relatively densely compacted particles. These powders can be 
further processed in a customary manner, for example by calcining them, 
and, if appropriate, subsequently converting them into shaped articles by 
compressing and sintering. However, the powder suspension obtained can 
also be processed directly or after water has been removed, for example by 
feeding it to a filter press and processing it there to shaped articles. 
The oxide (hydrate) particles obtainable according to the invention from 
which the water has been removed in a suitable manner usually show a very 
low annealing loss on calcining (about 10 to 20%). Compared with 
mixed-oxide processes, the crystalline phases are formed at considerably 
lower temperatures. The individual particles in this process are compacted 
without the formation of hard agglomerates by beginning sintering 
mechanisms. These powders can subsequently be subjected to a processing 
step without grinding processes and hence without impurities due to 
abrasion. 
Compressing experiments using a variety of powders without the addition of 
compressing auxiliaries resulted in compressed particles of 55 to 65% of 
the theoretical density. It was then possible to sinter these particles 
without pressure to a density nearly as high as the theoretical density. 
In summary, the process according to the invention shows the following 
considerable advantages, inter alia: 
Extraction of (an)ions is also possible from neutral salt solutions and 
leads to the formation and precipitation of hydroxides within the emulsion 
droplets. 
Lower concentrations of the compound which acts as the phase transfer 
catalyst suffice to achieve complete exchange. These compounds can be 
regenerated continuously by an exchanger medium. 
Media which are possible as exchangers are ion-exchanger resins, but also, 
for example, aqueous or solid bases (for example NaOH pellets) which can 
be kept separate from the emulsion by diaphragms or ion-exchanger 
membranes. 
The exchanger media are either very cost-effective or can be regenerated in 
a simple manner; the organic solvents can be recycled. 
Inexpensive, inorganic starting compounds can be employed. 
The process according to the invention yields compact, spherical powders 
which have a homogeneous chemical composition and good compaction and 
sinter properties. 
The process can be applied, preferably to the exchange of anions, but in 
the same way also to the exchange of cations, for the preparation of 
powders from sols which are stabilized under basic conditions (for example 
SiO.sub.2 -powders of waterglass or silica sols). 
Accordingly, the process according to the invention is particularly 
suitable for the preparation of powders of sphere-shaped particles, of 
various compositions and for various intended uses: 
Structural or functional ceramics having a homogeneous chemical composition 
(also dopes), good compaction properties and low phase formation and 
sinter temperatures; 
Glass powders and glasses of compositions which are not accessible via 
melts, for example because the melting temperature is too high and/or the 
tendency to recrystallize is too high; 
Composites between ceramics, glasses, metals and organic components. 
The following examples illustrate the process according to the invention 
without restricting it thereto.

EXAMPLE 1 
Preparation of the aqueous solutions and/or sols 
(a) ZrO.sub.2 --Y.sub.2 O.sub.3 
109.2 g of Zr(OC.sub.3 H.sub.7). (75%), corresponding to 0.25 mole, are 
dissolved in 75 ml of ethanol and the solution is treated with 22 ml of 
conc. HNO.sub.3 (65% strength) and 25 ml of H.sub.2 O. The mixture is 
evaporated twice on the rotary evaporator at 60.degree. C. and again taken 
up in distilled water, and this results in a clear, aqueous sol. 5.76 g of 
Y(NO.sub.3).sub.3 .times.6H.sub.2 O are dissolved in 20 ml of H.sub.2 O, 
and the solution is added to the ZrO.sub.2 sol. The total volume of the 
sol is then adjusted to 100 ml. 
(b) ZrO.sub.2 -Al.sub.2 O.sub.3 
A procedure as described under a) is initially followed to prepare the 
ZrO.sub.2 sol. A completely peptizable aluminum oxide hydroxide (Disperal, 
made by Condea) is then added to this sol. In doing this, any desired 
mixing ratio between the two components can be chosen. For example, 9.1 g 
of AlOOH are added to the above-described 0.25 molar ZrO.sub.2 batch in 
order to prepare 20% of Al.sub.2 O.sub.3 and 80% of ZrO.sub.2. 
(c) ZrSiO.sub.4 
A procedure as described under a) is initially followed. 15 g of 
highly-disperse SiO.sub.2 (Aerosil, made by Degussa) are then added to the 
ZrO.sub.2 sol. 
(d) Pb(Zr,Ti)O.sub.3 
23.0 g of Zr(OC.sub.3 H.sub.7).sub.4 (74.1%) and 11.75 g of 
Ti(OC,H.sub.5).sub.4 (93.2%) are dissolved in 45 ml of ethanol, and the 
solution is treated with 7 ml of HNO.sub.3 (65% strength) and 10 ml of 
H.sub.2 O. The mixture is evaporated twice on a rotary evaporator at 
60.degree. C. and again taken up in distilled water, and this results in a 
clear, aqueous sol. 33.12 g of Pb(NO.sub.3).sub.2 are dissolved in 85 ml 
of H.sub.2 O, the solution is mixed with the sol, and the total volume is 
adjusted to 150 ml. The stoichiometric composition of this mixture 
corresponds to the reaction product Pb(Zr.sub.0.52Ti.sub.0.48)O.sub.3. 
(e) BaTiO.sub.3 
12.24 g of Ti(OC.sub.2 H.sub.5).sub.4 (93.2% pure) are dissolved in 15 ml 
of ethanol, and the solution is treated with 3.5 ml of HNO.sub.3 (65% 
strength) and 5 ml of H.sub.2 O. The mixture is evaporated twice on a 
rotary evaporator at 60.degree. C. and again taken up in distilled water, 
and this gives a slightly cloudy, aqueous sol. 13.07 g of 
Ba(NO.sub.3).sub.2 are dissolved in 125 ml of H.sub.2 O, the solution is 
mixed with the sol, and the total volume is adjusted to 150 ml. 
(f) YBa.sub.2 Cu.sub.3 O.sub.7 --x 
3.83 g of Y(NO.sub.3).sub.3 .times.6H.sub.2 O, 5.23g of Ba(NO.sub.3).sub.2 
and 7.25 g of Cu(NO.sub.3).sub.2 .times.3H.sub.2 O are dissolved under hot 
conditions in 120 ml of H.sub.2 O, and the complete solution is adjusted 
to 150 ml after cooling. 
EXAMPLE 2 
Preparation of the emulsions 
The processing of the aqueous solutions is identical for all the substance 
systems described under Example 1. 
Per 100 ml of aqueous phase to be emulsified 1 g of emulsifier 
(Emulsogen.RTM. OG, made by Hoechst) is dissolved in 250 ml of petroleum 
spirit (boiling range 50.degree. to 70.degree. C.), and the aqueous and 
organic phases are combined. The emulsion is then prepared using an 
Ultraturax (made by IKA) in a flow cell. 
EXAMPLE 3 
Ion exchange 
Before the ion exchange is started, 1 mole per cent of 
didodecyldimethylammonium bromide, relative to the amount of anions to be 
exchanged, is dissolved in n-octanol at a concentration of 1 g/5 ml, and 
the solution is added to the emulsion. The exchange is then carried out 
using a column packed with Dowex 1.times.8 (made by Dow Chemical), a 
strongly basic ion exchanger. The amount of ion exchanger is calculated 
from the ion-exchange capacity, the amount of anions to be exchanged and a 
sufficient safety reserve i.e., for example, 500 g of Dowex are employed 
for 250 mmoles of anions to be exchanged. The ion exchange is checked 
using pH indicator strips and nitrate indicator strips. 
EXAMPLE 4 
Drying 
Before the water is removed by azeotropic distillation, ten per cent by 
volume of n-butanol, relative to the total amount of water, are added to 
the emulsion; this causes the water concentration in the azeotropic 
mixture to rise from about 5% to 20%. After the distillation, the 
particles which have formed are filtered off, redispersed in petroleum 
spirit, refiltered and dried. 
EXAMPLE 5 
Calcining 
Calcination of the powders in a temperature range of up to 650.degree. C. 
leads to decomposition of traces of nonexchanged nitrates, to evaporation 
or pyrolysis and to burning out of adsorbed organic constituents (for 
example emulsifiers) and to formation of the oxides from the hydroxides 
(oxide hydrates). 
Depending on the substance system, even higher temperatures are in some 
cases then required for compacting the individual particles and for the 
formation of the desired phases. 
The powders are processed using conventional ceramic technologies.