Ceramics based on aluminum titanate, process for their production and their use

Ceramics having principal phases of aluminum iron titanate and mullite corresponding to the overall chemical composition: PA0 50 to 62% by weight Al.sub.2 O.sub.3, PA0 36 to 49.5% by weight titanium oxide, expressed as TiO.sub.2, PA0 0.5 to 10% by weight SiO.sub.2, and PA0 0.2 to 15% by weight iron oxide, expressed as Fe.sub.2 O.sub.3, PA0 up to 1% by weight impurities, PA0 with the sum of Al.sub.2 O.sub.3, TiO.sub.2 and SiO.sub.2 being 100%, wherein the sum of the crystalline phases Al.sub.2 O.sub.3, TiO.sub.2, SiO.sub.2 (other than the principal phases) is less than 6% by weight, based on the sintered body, are useful where a ceramic having a high thermal-shock-resistance is required, such as exhaust port liner internal combustion engines, for example.

This invention relates to sintered ceramics based on principal phases of 
aluminum iron titanate and mullite for the following chemical composition 
50 to 62% by weight Al.sub.2 O.sub.3, 
36 to 49.5% by weight titanium oxide, expressed as TiO.sub.2, 
0.5 to 10% by weight SiO.sub.2, 
the sum of these three components being 100%, and in addition from 0.2 to 
15% by weight iron oxide, expressed as Fe.sub.2 O.sub.3, and up to 1% by 
weight impurities, to processes for the production of these sintered 
ceramics and to their use. 
BACKGROUND OF THE INVENTION 
There is a need in practice for thermal-shock-resistant materials which are 
suitable for use at temperatures of .gtoreq.1000.degree. C. and which, at 
the same time, have a strength level of .gtoreq.40 MPa which provides for 
engineering constructions, such as: in melt metallurgy, for example, for 
throughflow controllers, in machine construction, for example hot gas 
fans, in engine construction, for example thermal insulations of the 
exhaust gas port (port liners), in chemical engineering, for example as 
filters or catalyst supports. 
Although ceramics based on pure aluminum titanate, or tialite, show 
interesting properties, such as a low thermal expansion coefficient (TEC) 
and a low Young's modulus, they are of only limited technological value on 
account of their very poor strength and their tendency to decompose at 
temperatures in the range from about 900.degree. to about 1300.degree. C. 
At temperatures in this range, tialite decomposes into the starting oxides 
Al.sub.2 O.sub.3 and TiO.sub.2, accompanied by a marked increase in the 
TEC. 
Numerous proposals have been made for the production of ceramics based on 
aluminum titanate. Thus, U.S. Pat. No. 2,776,896 relates to a 
non-decomposing, thermal-shock-resistant aluminum titanate ceramic of 
which the improved properties are achieved by additions of iron, magnesium 
and silicon. 
According to U.S. Pat. No. 2,776,896, the addition of 1 to 2 mol-% Fe.sub.2 
TiO.sub.5 to Al.sub.2 TiO.sub.5 is sufficient to produce substantial 
resistance to decomposition and up to 50 mol-% Fe.sub.2 TiO.sub.5 may be 
added without the low TEC being significantly affected. For a ceramic of 
90 mol-% Al.sub.2 TiO.sub.5 and 10 mol-% Fe.sub.2 TiO.sub.5, a TEC 
(RT-400.degree. C.) of -2.35.times.10.sup.-6 l/K is measured after 
sintering. 
In addition to iron, silica may be added to the composition. If silicon 
dioxide is added, it should be added in the form of clay for practical 
reasons. The silica content of the mass should not exceed 10% by weight. 
The formula is preferably calculated in such a way that one additional mol 
TiO.sub.2 is added for 2 mol SiO.sub.2 (column 7, line 21). This means an 
excess of free TiO.sub.2 which results in inadequate strength. Thus, all 
the Examples disclosed show relatively high contents of free oxidic 
components. 
EP-B 133 021 describes an aluminum titanate/mullite ceramic consisting of 
60 to 75% by weight Al.sub.2 O.sub.3, 15 to 35% by weight TiO.sub.2 and 1 
to 16.5% by weight SiO.sub.2. In another embodiment, 0.5 to 5% by weight 
Fe.sub.2 O.sub.3 and/or 0.5 to 5% by weight rare earth oxides are also 
added. The quoted Examples described compositions of the sintered ceramic 
comprise mullite contents of 20 to 40% by weight, Al.sub.2 TiO.sub.5 
contents of 50 to 70% by weight and Al.sub.2 O.sub.3 contents of 10 to 12% 
by weight. In addition, oxides of iron, lanthanum and neodymium are added. 
Adequate strength values of &gt;40 MPa are only obtained at sintering 
temperatures of or above 1500.degree. C. and by addition of expensive rare 
earth oxides. 
EP-A 210 813 describes an aluminum titanate/mullite ceramic, in the 
production of which at least one of the two components aluminum titanate 
or mullite is presynthesized and then sintered at temperatures in the 
range from 1500.degree. to 1700.degree. C. The overall composition is 
53-74% by weight Al.sub.2 O.sub.3, 14-33% by weight TiO.sub.2, 6-20% by 
weight SiO.sub.2 and 1.2-5% by weight Fe.sub.2 O.sub.3. Due to the 
necessary presynthesis, this process is relatively expensive. In addition, 
the ceramics produced by this process show inadequate strength values. 
De-PS 2 741 434 describes an aluminum titanate ceramic which, in addition 
to 2 to 13% by weight SiO.sub.2, contains 0.5 to 10% by weight rare earth 
oxides and 1.5 to 20% by weight of SnO.sub.2. This ceramic shows 
inadequate strength values. 
According to DD-B 29 794, high thermal-shock-resistance is produced by a 
very low, preferably negative, linear thermal expansion coefficient. For 
the production of a highly refractory oxidic material showing high 
thermal-shock-resistance, this publication proposes compositions of 
MgO-Al.sub.2 O.sub.3 -TiO.sub.2 or of MgO-Al.sub.2 O.sub.3 -TiO.sub.2 
-SiO.sub.2, the TiO.sub.2 content being said to amount to between 15 and 
75% by weight, the Al.sub.2 O.sub.3 content to between 70 and 25% by 
weight and the contents of SiO.sub.2 and MgO up to 40 and 20%, 
respectively. The obtainable coefficient of linear thermal expansion is 
said to be &lt;4.times.10.sup.-6 l/K in the range from 10.degree. to 
700.degree. C., preferably being negative or differing only slightly from 
zero. The mixing ratios of Al.sub.2 O.sub.3 to TiO.sub.2 shown in the 
Examples encompass the broad range from 1:0.7 to 1:1.7, the latter range 
applying to a silicate-free composition containing 8% by weight MgO. 
Corresponding materials are of little value on account of their poor 
strength. 
The safe use of the ceramic in practice presupposes a number of properties 
which the material is required to show in a reproducible form. For many 
applications, as for example in melt metallurgy, thermal-shock-resistance 
is an important criterion. Not only a low TEC, but also a low Young's 
modulus and high strength are crucially important to 
thermal-shock-resistance. The shrink in behavior of the ceramic is another 
crucial property in cases where a metal jacket is to be cast around a 
ceramic tube insert, as for example the exhaust port liners in cylinder 
heads. The ceramic has to yield to the solidifying and shrinking metal 
sleeve without breaking. This requires a ceramic material showing high 
fracture strain in combination with small shrinkage backstresses, i.e. a 
low Young's modulus and high strength. In addition, the use of the 
ceramic, for example as a hot gas fan, requires excellent decomposition 
resistance above 900.degree. C. 
BRIEF DESCRIPTION OF THE INVENTION 
Ceramics according to the present invention have principal phases of 
aluminum iron titanate and mullite, and have an overall chemical 
composition: 
50 to 62% by weight Al.sub.2 O.sub.3, 
36 to 49.5% by weight titanium oxide, expressed as TiO.sub.2, 
0.5 to 10% by weight SiO.sub.2, and 
0.2 to 15% by weight iron oxide, expressed as Fe.sub.2 O.sub.3 
up to 1% by weight impurities, 
with the sum of Al.sub.2 O.sub.3, TiO.sub.2 and SiO.sub.2 being 100% 
wherein the sum of the crystalline phases Al.sub.2 O.sub.3, TiO.sub.2, 
SiO.sub.2 (other than the principal phases) is less than 6% by weight, 
based on the sintered body. 
DETAILED DESCRIPTION 
The object of the present invention is to provide materials which exhibit 
these properties. In addition, the material is intended to be obtainable 
by a simple, inexpensive process, for example by reaction sintering. 
The reaction sintering of the raw materials containing Al, Ti, Si, Fe, etc. 
has the advantage over the use of presynthesized products, such as for 
example Al.sub.2 TiO.sub.5 clinker, that it is possible to use raw 
materials which are commercially available relatively inexpensively and in 
large quantities with readily reproducible properties. For economic 
reasons, reaction sintering should be carried out at the lowest 
practicable temperatures, for example at temperatures below 1500.degree. 
C. Economic considerations also rule out expensive additives, such as rare 
earth oxides. 
Despite the wide variety of patent literature and other publications on 
materials based on aluminum titanate, it has so far not been possible to 
produce a satisfactory material showing this combination of properties by 
the inexpensive method of reaction sintering. 
The material according to the invention described in the following, 
produced by careful choice of the composition and accurately adapted 
processing, particularly during sintering, satisfies all the technical and 
economic requirements listed in the foregoing. 
The present invention relates to sintered ceramics based on principal 
phases of aluminum iron titanate and mullite for the following chemical 
overall composition: 
50 to 62% by weight Al.sub.2 O.sub.3 
36 to 49.5% by weight titanium oxide, expressed as TiO.sub.2, 
0.5 to 10% by weight SiO.sub.2, and preferably between 1 to 5.5%, 
the sum of these three components being 100%, and in addition from 0.2 to 
15% by weight iron oxide, expressed as Fe.sub.2 O.sub.3, and up to 1% by 
weight impurities, comprising at most 0.3% by weight MgO and at most 0.7% 
by weight of K.sub.2 O+P.sub.2 O.sub.5, characterized in that the sum of 
the crystalline phases Al.sub.2 O.sub.3, TiO.sub.2, SiO.sub.2 present in 
addition to the principal phases is less than 6% by weight, based on the 
sintered body. 
In its sintered state, the material according to the invention consists 
essentially of two crystalline phases, namely: an Al.sub.2 TiO.sub.5 
-Fe.sub.2 TiO.sub.5 solid solution and the mullite phase which is taken as 
3Al.sub.2 O.sub.3.2SiO.sub.2. 
Particularly preferred sintered ceramics are those in which the sum of the 
crystalline phases Al.sub.2 O.sub.3, TiO.sub.2 and SiO.sub.2 present in 
addition to the principal phases is less than 4% by weight, based on the 
sintered body. 
The presence of iron, for example in the form of finely dispersed Fe.sub.2 
O.sub.3, has a favorable, very complex effect on microstructure and on 
resistance to decomposition. Particularly preferred sintered ceramics 
according to the invention have a content of iron oxide, expressed as 
Fe.sub.2 O.sub.3, of from 0.4 to 5% by weight. 
The formation of aluminum titanate--from pure Al.sub.2 O.sub.3 and 
TiO.sub.2 in the case of reaction sintering--does not take place 
spontaneously at temperatures in the range from .about.1280.degree. to 
.about.1350.degree. C., but requires a nucleating agent to form grains of 
tialite. Iron, for example in the form of finely dispersed Fe.sub.2 
O.sub.3 with a mean particle size of less than 1 .mu.m, has proved to be 
an effective nucleating agent for tialite. 
The sintered ceramics according to the invention are distinguished by a 
combination of particularly good physical properties. Thus, their flexural 
strength at room temperature is in the range from 40 to 150 MPa, their 
Young's modulus is in the range from 5 to 20 GPa and their TEC (RT 
-1000.degree. C.) is in the range of .+-.2.times.10.sup.-6 l/K. Even after 
more than 100 hours at 1000.degree. C., they are still resistant to 
decomposition. 
Another advantage and characteristic feature of ceramics according to the 
invention is the fact that they can be sintered at relatively low 
temperatures with short holding times. These ceramics are sintered at 
temperatures in the range from 1250.degree. to 1500.degree. C. and 
preferably in the range from 1300.degree. to 1450.degree. C. with a 
holding time of from 0.5 to 100 h and preferably from 1 to 50 h. 
The present invention also relates to a process for the production of 
ceramics according to the invention which is characterized in that it 
comprises the steps of mixing, shaping and sintering suitable materials. 
To this end, a green body may be prepared in known manner from a slip. The 
shaping process may also be carried out by the compression molding of a 
powder mixture containing the usual temporary additives for the 
composition according to the invention. Other shaping techniques known to 
those skilled in the art may as well be employed. The use of a spray-dried 
granulate can be of advantage. Sintering is preferably carried out as 
reaction sintering under the sintering conditions mentioned above. 
Pre-reacted mixtures may of course also be used without any disadvantage. 
After sintering, the ceramic may be machined in known manner. 
The present invention also relates to the use of the sintered ceramics 
according to the invention. 
They are particularly suitable for the production of hollow, tubular 
metal/ceramic composites. They are also suitable for the lining or 
supports of combustion chambers, pistons and cylinder heads. The ceramics 
according to the invention may also be used as catalyst supports, 
optionally after doping with catalytically active substances. By virtue of 
their favorable thermal and mechanical properties, the ceramics according 
to the invention are also suitable as materials in melt metallurgy.