Use of a finely divided, refractory, oxidic micropowder for preparing ceramic masses and moldings

The present invention pertains to the use of a finely divided magnesium oxide micropowder for preparing ceramic masses and moldings.

SPECIFICATION 
The present invention pertains to the use of a finely divided magnesium 
oxide micropowder for preparing ceramic masses and moldings. 
A magnesium oxide in the form of a fine powder and its use for preparing 
high-density ceramics have been known from [Austrian Patent] AT 392 464 B. 
The finely divided magnesium oxide has a particle size of &lt;15 .mu.m and a 
specific surface of &lt;20 m.sup.2 /g (determined according to the BET method 
from the nitrogen adsorption isotherm). It is further characterized by a 
primary particle shape factor between 1 and 1.5 as well as by a coating 
consisting of a hydrophobizing substance. The said particle shape factor 
describes essentially spherical particles, whose degree of dispersity is 
increased by the said addition of a hydrophobizing substance. 
Even though (undesired) agglomerates are prevented from forming by the 
hydrophobization, recrystallization with grain growth and consequently 
growth of the finely divided MgO particles, which leads to an irregular 
structural constitution, will ultimately occur during firing. A finely 
divided magnesium oxide powder, which is obtained from an aqueous 
suspension by spray drying, has also been known from practice; a coating 
agent, which is to precipitate possibly as a monomolecular layer on the 
surface of the oxide particles, is added to the suspension. The coating 
agent consists of, e.g., a carboxylic acid. However, due to processing in 
an aqueous suspension, hydration of the magnesium oxide into magnesium 
hydroxide cannot be prevented, even though the degree of hydration can be 
limited to values less than 10 wt. % by the use of the coating agent and 
the subsequent spray drying. However, hydration leads to a change 
(increase) in the shape of the particles, which is undesirable. It can be 
stated as a rule of thumb that a degree of hydration of 1.0 wt. % leads to 
a linear increase by 1.0% in the size of the corresponding particle. 
In addition, the prior-art finely divided magnesium oxide powders can be 
processed only together with binders in a coarse ceramic matrix.

The basic task of the present invention is to provide a possibility for 
preparing refractory, ceramic masses and moldings which lead to sufficient 
green bond and high density after firing even without expensive processing 
of the starting components. 
The present invention is based on the finding that this can be achieved by 
using a finely divided, refractory, oxidic micropowder in a coarse 
ceramic, refractory matrix, wherein the micropowder occurs very 
extensively in the form of single-fraction particles. 
It was found that purely physical binding can be achieved between the 
particles by using such single-fraction particles, and this binding leads 
to a basic strength that is sufficient even in the green state and is 
sometimes higher than that according to the state of the art. In addition, 
recrystallization with grain growth, during which smaller particles 
coagulate with larger particles during sintering, as a result of which 
irregular sintering characteristics will develop, is advantageously 
prevented from occurring by the use of single-fraction particles. 
In contrast, the use of the finely divided oxidic micropowder with very 
extensively uniform particle diameter leads to physical adhesion of the 
particles to one another already in the green state, and this adhesion 
continues in a uniform sintering during firing. The finely divided 
component advantageously fills the wedges between the coarser particles in 
the form of a very dense spherical packing, thus making it possible to 
achieve a markedly reduced porosity in the finished (fired) product. 
It can be calculated statistically that a residual porosity of 8 to 10 vol. 
% is obtained in the finished (fired) product in the case of, e.g., an 
initial porosity of 20 vol. % in the coarse-grain matrix material (with a 
maximum particle size of 3 mm and a minimum particle size above the fine 
grain fraction) as well as by addition of 15 wt. % of the finely divided 
single-fraction oxide powder with a particle size slightly less than 1.0 
.mu.m. 
Under these premises, the present invention pertains, in its most general 
embodiment, to the use of a finely divided, refractory, oxidic micropowder 
of very extensively uniform particle size, which is obtained after 
dispersion in a non-aqueous dispersing agent, in a coarse ceramic, 
refractory matrix material for preparing ceramic masses and moldings of 
high green bond and high density after firing. 
The micropowder should be used in a particle size less than 10 .mu.m, and 
particle sizes less than 1 .mu.m have a particularly favorable effect 
according to the present invention. 
The terms "single-fraction particles" and "very extensively uniform 
particle size" are not meant to imply that exactly single-fraction 
particles are used, because these would be able to be prepared only at 
increased expense if at all, even though they would be particularly 
preferred; thus, according to an embodiment variant, the use of a finely 
divided micropowder is proposed, in which 90 wt. % of the particles have a 
maximum deviation of .+-.10% from the mean particle diameter. 
The amount of micropowder used may vary in an application-specific manner, 
but it should be fundamentally between 5 wt. % and 18 wt. % in relation to 
the total mass, and a percentage between 10 wt. % and 15 wt. % might be 
considered for most fields of application. 
Conventional screen characteristics, which are in the particle size range 
of &lt;5 mm and &gt; than the micropowder, can be used for the coarse ceramic 
matrix material. 
As was initially explained, the essential advantage of the described use is 
that the combination of coarse ceramic matrix material and dispersed, 
finely divided micropowder leads to the possibility of operating without 
binder, because the finely divided single-fraction component acts quasi as 
an "in situ binder." 
To optimize the dispersing action of the finely divided component, the 
corresponding weight percentage is first prepared in a non-aqueous 
dispersing medium (as a result of which hydration is prevented in the case 
of hydration-sensitive oxides such as MgO), wherein the dispersing medium, 
which is inert with respect to the solids, may consist of, e.g., 
naphthene-basic oils or fatty alcohols, and, e.g., a modified alkyd resin 
(polyester) may be used as the dispersing agent. 
The refractory, oxidic micropowder may consist of various oxides, e.g., 
MgO, Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, and/or TiO.sub.2. The oxidic 
micropowder may also consist of only one of these oxides; however, it is 
also possible to use mixtures of these oxides as the micropowder, 
especially when spinel formation is desirable. It is advantageous to use, 
e.g., an MgO/Cr.sub.2 O.sub.3 dispersion in this case. 
The use according to the present invention achieves its most advantageous 
properties when a dispersion with a very high solids content of the oxidic 
micropowder is used. These are understood to be dispersions which have a 
solids content exceeding 85 wt. % in relation to the total dispersion. The 
percentage of the dispersing agent correspondingly amounts to up to 15 wt. 
%. 
It was surprisingly observed that such highly concentrated dispersions can 
be prepared in high-energy mixers by using a suitable dispersing agent 
(e.g., polyester), in which case the solids content can be set even at 
values exceeding 90 wt. %. 
The coarse ceramic matrix material is selected as a function of the desired 
properties of the material. 
Thus, it is possible, for purely magnesitic products, to prepare a 
magnesitic, coarse ceramic matrix material with an MgO micropowder. 
However, the micropowder may also consist of a spinel-forming MgO/Cr.sub.2 
O.sub.3 or MgO/Al.sub.2 O.sub.3 dispersion in this case. 
When a coarse ceramic matrix material based on Al.sub.2 O.sub.3 is used 
(e.g., alumina, tabular alumina, corundum) is used, it is advantageous to 
use an Al.sub.2 O.sub.3 micropowder, but it may again be replaced 
completely or partially by TiO.sub.2 or another refractory, oxidic 
micropowder. 
Even when oxides that are not sensitive to hydration, e.g., Al.sub.2 
O.sub.3, are used, this should preferably be prepared in a non-aqueous 
dispersing medium in order to maintain the water content in the mass 
prepared as low as possible. The preparation method with a non-aqueous 
dispersing medium offers the advantage that the above-mentioned, unusually 
high solids concentrations in the dispersion can be reached more easily, 
as a consequence of which not only the green bond of the ceramic mass 
(after mixing with the coarse ceramic matrix material), but also the 
density after firing can thus be substantially improved. 
Thus, densities of 3.25 g/cm.sup.3 can be reached after firing by using an 
MgO micropowder in purely magnesitic bricks. 
However, it is also possible to separate the refractory, oxidic micropowder 
dispersion from the dispersing medium used for preparation by, e.g., spray 
drying or freeze drying of the dispersion, prior to mixing with the coarse 
ceramic matrix material. The dispersing material is thus removed, e.g., by 
suction filtration, so that a very fine, pure and surface-modified 
micropowder is prepared. This micropowder can then also be used in aqueous 
systems. 
Experiments have shown that moldings with a green product cold compression 
strength of 50 to 60 N/mm.sup.2 can be prepared from a mass that contains 
85 wt. % coarse ceramic MgO matrix material of the &gt;1.0 .mu.m and &lt;3 mm 
particle fraction as well as 15 wt. % of a previously dispersed, finely 
divided MgO micropowder with a particle size of 0.9 .mu.m; the 
above-mentioned cold compression strength is several times higher than the 
cold compression strength values known from the state of the art. 
At the same time, densities above 3.15 g/cm.sup.3 are reached (these values 
apply to both the green products and the fired bodies). 
Another essential advantage is the fact that the moldings prepared from the 
mass described here are not subject to any appreciable shrinkage during 
firing.