Process for preparing powders with superior homogeneity from aqueous solutions of metal nitrates

A process for preparing a powdered product with superior homogeneity. A precursor solution is prepared by adding an additive selected from glycerol, glyceryl nitrate, polyglycerols, glycols or polyglycols with a concentration of 0.1-2.0 percent by weight to an aqueous solution of metal nitrate. The precursor solution is atomized into droplets and thereafter heat treated to obtain a powdered product of particles.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for preparing powders, and in 
particular to a process for preparing spherical hollow powders with 
superior homogeneity. 
For better fabrication of electronic or structural ceramics, powder 
synthesis based on wet chemistry has gained wide attention these past few 
years. The high purity of starting materials, the molecular scale mixing 
of ingredients and the ability to control the composition precisely are 
all typical features of the solution process. As a consequence, better 
stoichiometric and microstructural control as well as extreme 
compositional homogeneity should be possible with the process. Various 
techniques using co-precipitation, sol-gel processing, freeze drying, 
controlled hydrolysis and high temperature pyrolysis, to name a few, were 
developed for the wet process. The aerosol technique, in which the 
well-mixed liquid precursor is nebulized into isoltated fine droplets and 
then reacted at a high temperature to form powders, has been evaluated to 
be beneficial in maintaining the homogeneity in the wet state and in 
eliminating the problems during calcination. 
However, the homogeneity of powders prepared by the aerosol technique is 
still not satisfactory. For example, in the preparation of MgAl.sub.2 
O.sub.4 powder by aerosol method, using nitrate solutions as precursors, 
partial separation of MgO on the surface of aerosolized powders was 
observed, and the chemical homogeneity could not be restored even 
subjected to a post heat treatment at temperature up to 1000.degree. C. 
Moreover, the shape of the particles produced by the aerosol technique are 
usually not perfectly spherical and the surfaces thereof are rough. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a process for 
preparing spherical hollow powders with superior homogeneity. 
In order to attain the object and according to the present invention, the 
process for preparing spherical powders with superior homogeneity includes 
(a) preparing an aqueous solution of metal nitrate; (b) adding 0.1-2.0 
percent by weight of an additive selected from the group consisting of 
glycerol, glyceryl nitrate, polyglycerols, glycols and polyglycols to the 
aqueous solution to form a precursor solution; (c) atomizing the precursor 
solution into droplets; (d) subjecting the droplets to pyrolysis with the 
introduction of gases at a temperature of above 400.degree. C. to obtain a 
powdered product of hollow particles; and (e) optionally, heat treating 
the powdered product to obtain a crystallized powdered product. 
one aspect of the present invention is that the produced powders are hollow 
and spherical with smooth surfaces and superior homogeneity. Therefore, no 
impurity phases and off-stoichiometry due to segregation are observed. 
Another aspect of the present invention is that only 0.1-2.0 percent by 
weight of additives are added to the aqueous solution of metal nitrate to 
form a precursor solution. 
A further aspect of the present invention is that more than one metal 
nitrate can be used as raw material to prepare multicomponent powder 
materials.

DETAILED DESCRIPTION OF THE INVENTION 
According to the present invention, more than one metal nitrate can be used 
as raw material to prepare the aqueous solution. For example, if NiO 
powders are to be produced, preferably a nickel nitrate aqueous solution 
in a molar concentration of 0.1 to 0.4 are prepared. If MgAl.sub.2 O.sub.4 
powders are to be produced, the aqueous solution can be prepared by 
dissolving magnesium nitrate and aluminum nitrate into water so that the 
resultant aqueous solution contains magnesium and aluminum ions at a molar 
ratio of 1:2. 
The additives suitable to be used in the present process include glycerol, 
glyceryl nitrate, polyglycerols, glycols and polyglycols. Among them, 
glycerol is preferred. The amount of the additives added is 0.1-2.0 
percent by weight based on the aqueous solution. 
The precursor solution can be atomized by a gas stomizer or by an 
ultrasonic atomizer. The method of atomization is not limited to the 
above-mentioned two, but an ultrasonic atlomizer exciting at an frequency 
of 1.7 MHz is particularly suitable for the purpose of the present 
invention. 
PREFERRED EMBODIMENT OF THE INVENTION 
Referring to FIG. 1, there is shown a schematic diagram of an apparatus 
usable to prepare powders by a process according to the present invention. 
The apparatus of FIG. 1 includes an ultrasonic atomizer 10, a reaction 
tube 20, and an oven 30. A precursor solution prepared for the production 
of powders is contained inside the ultrasonic atomizer 10 to be atomized 
into a flow of droplets. The atomized droplets are introduced into the 
reaction tube 20. Heat is subsequently applied to the reaction tube 20 for 
a duration long enough to obtain a powdered product. If necessary, the 
produced powder product is further heat treated in oven 30 to obtain a 
well crystallized powder product. 
EXAMPLE 1 
An aqueous solution having a Ni concentration of 0.29 mole/l was prepared 
by adding nickel nitrate to water. To the aqueous solution was then added 
0.5 percent by weight of glycerol to obtain a precursor solution. The 
precursor solution was then atomized into a flow of droplets by the 
ultrasonic atomizer 10 at an excitation frequency of 1.7 MHz. The droplets 
flow were then guided along with air into the reaction tube 20 and heated 
therein at a temperature of 600.degree. C. to obtain a mass of hollow 
N.sub.i O particles. 
A SEM picture at 15,000 magnification of the samples of the produced hollow 
NiO particles is shown in FIG. 2. As can be clearly seen from FIG. 2, the 
samples of hollow NiO particles are more smoothly formed and shapes of the 
same are closely spherical. In addition, an X-ray diffraction pattern of 
the samples of hollow NiO particles is shown in FIG. 4. 
The hollow NiO particles were further heat treated at a temperature of 
500.degree. C. under a reducing atmosphere to obtain a crystallized 
powdered product of hollow Ni particles. An X-ray diffraction pattern of 
the crystallized hollow Ni particles is shown in FIG. 6. 
COMATIVE EXAMPLE 1 
A sample of powdered NiO was prepared by the same procedures as in Example 
1, except no glycerol was added to the aqueous solution. A SEM picture at 
15,000 magnification of this sample of hollow NiO particles is shown in 
FIG. 3. As can be seen from FIG. 3, the size and morphology of the samples 
of hollow NiO particles are irregularly formed. An X-ray diffraction 
pattern of the sample of hollow Ni particles is shown in FIG. 5, and is 
substantially the same as that shown in FIG. 4. 
EXAMPLE 2 
Mg(NO.sub.3).sub.2 6H.sub.2 O and Al(NO.sub.3).sub.3 9H.sub.2 O at a molar 
ratio of Mg:Al=1:2 were dissolved in deionized water to form 0.125M/l 
aqueous solution. To this aqueous solution was added 0.5 percent by weight 
of glycerol to obtain a presursor solution. The precursor solution was 
then atomized into a flow of droplets by the ultrasonic atomizer 10 at an 
excitation frequency of 1.7 MHz. The atomized droplets were then guided 
along with air into the reaction tube 20 and heated therewithin at a 
temperature of 600.degree. C. to obtain a powdered product of MgAl.sub.2 
O.sub.4 particles. 
A SEM picture at 15,000 magnification of the sample of hollow MgAl.sub.2 
O.sub.4 particles is shown in FIG. 7, which shows that the powders are 
spherical and in which the broken one indicates the powders are hollow 
balls with even wall thickness. 
The samples of MgAl.sub.2 O.sub.4 powders were subjected to heat treatment 
respectively at temperatures of 500.degree. C. 600.degree. C., 800.degree. 
C., and 1000.degree. C. for one hour. The X-ray diffraction patterns of 
the heat treated samples are shown in FIG. 8. As can be seen from FIG. 8, 
the amorphous state of the powder was hardly changed by heating at 
500.degree. C., and the spinel structure started to appear after heat 
treating at 600.degree. C. As temperature rose, the spinel peaks become 
more clearly defined indicating growth of crystallities. The MgO phase was 
never detected. Accordingly, it is revealed from the XRD pattern of FIG. 8 
that heat treating the sample of MgAl.sub.2 O.sub.4 powders amorphous at a 
temperature above 800.degree. C. would obtain well crystallized single 
phase MgAl.sub.2 O.sub.4. 
The powders were also examined under a JEOL JEM2000FX transmission electron 
microscope(TEM), and the TEM picture is shown in FIG. 10. As shown in FIG. 
10, no surface layer exists indicating no segregation of ingredients from 
the bulk material. 
COMATIVE EXAMPLE 2 
Samples of powdered MgAl.sub.2 O.sub.4 were prepared by the same procedures 
as in Example 2, except no glycerol was added to the aqueous solution. 
These samples of hollow MgAl.sub.2 O.sub.4 particles were subjected to the 
same heat treatments as in Example 2, and the corresponding X-ray 
diffraction patterns are shown in FIG. 9. As can be seen from FIG. 9, MgO 
and spinel phase crystallized out appreciably when heat treated at 
500.degree. C. As temperature rose, both MgO and spinel had improved 
crystallinity. The relative intensity of MgO peaks decreased with 
increasing temperature but never disappeared even after treating at 
1000.degree. C. for 1 hr. Compared with the test results of FIGS. 8A-8C, 
it is revealed that adding glycerine to the aqueous solution of magnesium 
nitrate and aluminum nitrate can eliminate impurity phases. 
The TEM picture of the powders is shown in FIG. 11. It is evident from the 
picture that some surface layers exist indicating the segregation of 
ingredients from the bulk material. 
For the detailed inspection of the microstructural features of the 
synthesized powders, microanalysis was done with a TEM equipped with a 
electron energy loss spectrometer(EELS). The EELS spectrum obtained from 
the surface layer region around the particles is shown in FIGS. 12a and 
12b. This spectrum revealed that the chemical composition consists of Mg 
and O. The results confirmed that partial separation from the bulk 
material during heating. 
EXAMPLE 3 
An aqueous solution having a Y.sub.3 Al.sub.5 O.sub.12 concentration of 0.2 
mole/l was prepared by dissolving yttrium nitrate and aluminum nitrate at 
a molar ratio of Y:Al=3:5 in water. To this aqueous solution was added 0.5 
percent by weight of glycerol to obtain a precursor solution. The 
precursor solution was atomized into a flow of droplets by the ultrasonic 
atomizer 10 at an excitation frequency of 1.7 MHz. The atomized droplets 
were then guided along with air into the reaction tube 20 and heated 
therewithin at a temperature of 800.degree. C. to obtain a powdered 
product of Y.sub.3 Al.sub.5 O.sub.12 particles. 
The powdered product of Y.sub.3 Al.sub.5 O.sub.12 particles were subjected 
to heat treatment of temperatures of 775.degree. C., 800.degree. C., 
825.degree. C., 850.degree. C., 875.degree. C., and 900.degree. C. 
respectively for eight hours. The X-ray diffraction patterns of the heat 
treated samples are shown in FIG. 13. It can be seen from FIG. 13 that a 
substantially pure phase of Y.sub.3 Al.sub.5 O.sub.12 can be obtained when 
heat treated at a temperature above 875.degree. C. 
COMATIVE EXAMPLE 3 
Samples of powdered Y.sub.3 Al.sub.5 O.sub.12 were prepared by the same 
procedure as set forth in Example 3, except that no glycerol was added to 
the aqueous solution. The samples of powdered Y.sub.3 Al.sub.5 O.sub.12 
were respectively subjected to heat treatment at temperatures of 
600.degree. C., 800.degree. C., 825.degree. C., 850.degree. C., 
875.degree. C., 900.degree. C., 950.degree. C., and 1000.degree. C. 
respectively for 8 hours. The X-ray diffraction patterns of the heat 
treated samples are shown in FIG. 14. It is seen that impurity phases of 
YAlO.sub.3, Y.sub.2 Al.sub.4 O.sub.9, and Y.sub.2 O.sub.3 are present 
along with the desired phase of Y.sub.3 Al.sub.5 O.sub.12. 
The present invention has been described hitherto with examples. However, 
it is to be understood that the scope of the present invention need not be 
limited to these disclosed examples. On the contrary, it is intended to 
cover variations within the scope defined in the following appended 
claims. The scope of the claims should be accorded the broadest 
interpretation so as to encompass all such variations.