Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2003-05-27
2004-12-28
Brunsman, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C501S135000, C501S136000, C501S137000, C501S138000, C501S152000, C252S06290R, C252S06290R
Reexamination Certificate
active
06835684
ABSTRACT:
TECHNICAL FIELD
The invention relates to a ceramic material, especially an electrically conductive ceramic material, as well as its production and use.
STATE OF THE ART
Electrically conductive ceramics are typically used as connecting elements between components. These ceramics ensure on the one hand an electrical contact between two components with which these connecting elements have as a consequence of a joining process an intimate material to material contact. On the other hand electrical conductive paths can be formed from such ceramics and which are located in or on a component and either as a consequence of an applied electric current can induce a physical-chemical change in the component (actuator function) or can transform a change of a physical-chemical characteristic of the component into an electrically measurable magnitude (sensor function).
From the literature a series of manufacturing processes are known which give rise to being sintered at low temperatures. Very often synthesis methods have involved sol-gel intermediate products or nanophasal powders to lower the sintering temperatures of ceramics. Such processes are characterized by the fact that they require very expensive (frequently metal-organic) starting materials and auxiliary materials and produce only very small quantities, see for example U.S. Pat. No. 4,636,378.
Another method of obtaining low sintering ceramics is known from EP 0 280 033 B1. In this case, precipitation reactions are carried out with, for example, hydroxides or oxalates. These have the drawbacks that the salts which are frequently formed, tend to separate out in complex compositions on the one hand or because of different solubility products of the salts precipitate only incompletely, and can thus give rise to deviations from stoichiometry of the products. A further disadvantage is frequently the need for organic solvents or purification agents which increase the cost of the fabrication process.
Furthermore, from U.S. Pat. No. 3,330,697, a fabrication process for niobates, zirconates and titanates of lead and alkaline earth metals is known. In this case, into a solution of polyhydroxy alcohol and citric acid compounds of titanium, zirconium and niobium are mixed with lead or alkaline earth salts. By heating the solution, the organic components are removed. This method of production is known in the literature as the Pechini method. This method is however not suitable for ceramics sintering at low temperatures as described in the patent. The polyhydroxy alcohols which are there used have the disadvantage that the basic solution with the cations and the citric acids, upon heating, is transformed into a viscous resin. In addition, the higher proportions of organic auxiliary materials can give rise to a spontaneous combustion of the viscous resin with further temperature elevation in the examples given. Investigations of this fabrication process as well as of other fabrication processes which are based on uncontrollable ignition of the intermediate product have shown a significant limitation of the sinterability of the ceramic powder which is obtained. A heating like that which results from the combustion of the resin is thus to be strictly avoided in the fabrication of being sintered at low temperature.
OBJECTS AND SOLUTION
The object of the invention is to obtain a ceramic material which, by comparison with the state of the art, has improved sinterability and reduced sintering temperature. In addition the ceramic material should have good electrical properties. Further it is an object of the invention to provide a fabrication process for such a ceramic material.
DESCRIPTION OF THE INVENTION
With the method according to the invention of claim
1
, at least two different metal nitrates or metal carbonates are dissolved in an aqueous solution together with a metal complex former, concentrated, and at low temperature converted to a solid.
The metal can be at least one metal from the first group A′=(Y, Sc, Ce, La, Pr, Nd, Sm, Eu, Gd) and at least one metal from a second group B=(Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, W, Sn, Sb, Pb, Bi).
As the metal complex former, any complex former which is soluble in water is suitable which can complex with the above-mentioned metals and bring them as metal ions into solution. The metal complex formers which are to be counted in this group are especially the lactic acids, the citric acids, the citric acid esters or also the tartaric acid complexers. Furthermore, suitable are also other polycarboxylic, polyhydroxycarboxylic acids or polyaminocarboxylic acids like, for example, EDTA, (ethylenediamenetetraacetic acid).
The solution of metal complex former and metal salts is heated in such a way that the metal complex former decomposes. This can be achieved by for example splitting off gaseous carbon monoxide or carbon dioxide (CO, CO
2
) or by splitting off also gaseous nitrogen oxides (NO
x
). By means of the heating, water is simultaneously evaporated from the aqueous solution so that the initially dissolved or complex metal ions form a solid.
In an advantageous embodiment of the method, at least on metal of the group A″=(Mg, Ca, Sr, Ba) is dissolved as a nitrate or as a carbonate in the aqueous solution.
In an advantageous embodiment of the method, the metal compounds are used in a predetermined ratio. To that effect, the stoichiometry of the metals in a ceramic of the following compositions should be met: ABO
3
, A
2
BO
4
or A
2
B
2
O
7
where A stands for the elements of the mentioned groups A′ and A″ and B for the mentioned elements of group B.
It has been found to be advantageous further when, with the method according to the invention, different metals of group B are used in different proportions. In this case, especially the following compositions have been found to be especially desirable:
Shorthand
Designation
Composition
in FIG. 2:
a)
La
0.8
Ca
0.2
Cr
0.1
Co
0.6
Cu
0.3
O
3
LCC-A
b)
La
1.6
Ca
0.4
Cr
0.1
Co
0.6
Cu
0.3
O
4
c)
La
1.6
Ca
0.2
Cr
0.1
Co
0.3
Cu
0.6
O
3
LCC-B
d)
La
0.8
Ca
0.2
Cr
0.1
Co
0.5
Bi
0.1
Cu
0.3
O
3
LCC-C
e)
La
0.6
Y
0.2
Ca
0.2
Mn
0.2
Fe
0.3
Co
0.3
, Cu
0.2
O
3
f)
La
0.4
Y
0.4
Ca
0.2
(Mn
0.8
Co
0.1
, Cu
0.1
)
0.9
O
3
LCC-D
g)
La
0.75
Bi
0.05
Ca
0.2
(Mn
0.3
Co
0.4
, Cu
0.3
)O
3
h)
La(Mn
0.4
Co
0.4
Cu
0.2
)
0.95
O
3
LCC-E
i)
La(Mn
0.45
Co
0.35
Cu
0.2
)O
3
LCC-F
j)
LaMn
0.35
Co
0.45
Cu
0.2
)O
3
k)
LaFe
0.6
Ni
0.4
O
3
l)
La
0.8
Ca
0.2
(Pb, Zr, Ti)
0.2
Co
0.5
Cu
0.3
O
3
m)
La
0.95
Ca
0.05
(Pb, Zr, Ti)
0.2
Co
0.5
Cu
0.3
O
3
n)
La(Pb, Zr, Ti)
0.1
Mn
0.3
Co
0.45
Cu
0.15
O
3
o)
Y
0.5
Ca
0.5
Mn
0.4
Co
0.4
Cu
0.2
O
3
p)
Y
0.5
Ba
0.5
Mn
0.3
Co
0.4
Ti
0.15
Cu
0.15
O
3
The compositions indicated under a) to k) are advantageously suitable for use in fuel cells while the compositions described under l) to p) are advantageous for use in piezoceramics.
The heating of the aqueous solution is carried out especially slightly initially until the major part of the water is evaporated. Then the temperature is increased further, especially to up to 700° C. At these temperatures the materials according to the invention are transformed advantageously into a Perovskite or a multiphase ceramic which has as a major component a Perovskite.
The ceramic material according to the invention of claim
8
is produced by the method of the invention and has the following composition:
A′
1-x-y
A″
x
B′
1-a-b
B″
a
B′″
b
O
3
with
A′=(Y, Sc, Ce, La, Pr, Nd, Sm, Eu, Gd)
A″=(Mg, Ca, Sr, Ba)
B′=(Mn, Fe, Co)
B″=(Ti, V, Cr, Ni, Zn, Pb, Sb, W, Zr)
B′″=(Cu, Bi)
x=0-0.6 y=0-0.2
a=0-1 b=0-0.8
The material is electrically conductive and usually has a significantly improved sinterability by comparison with conventional ceramics. By an improved sinterability is to be understood a lower temperature range for the sintering which can be in the range from 800° C. to significantly below 1000° C. In addition, such materials have a smaller grain size dist
Basu Rajendra Nath
Jungen Wolfgand
Meschke Frank
Tietz Frank
Brunsman David
Dubno Herbert
Forschungszentrum Julich GmbH
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