Type I dielectric composition based on neodymium titanate

A type I dielectric composition based on neodymium titanate is formed of a mixture comprising 58% to 78% by weight of neodymium titanate (Nd.sub.2 O.sub.3 -3TiO.sub.2), 10 to 21% by weight of lead titanate (PbTiO.sub.3), 5% to 14% by weight of barium titanate (BaTiO.sub.3), 4% to 15% by weight of barium zirconate (BaZrO.sub.3) and 0.2% to 1.2% by weight of yttrium oxide (Y.sub.2 O.sub.3).

BACKGROUND OF THE INVENTION 
The present invention pertains to type I dielectric compositions that frit 
at high temperatures, especially type I dielectric compositions based on 
neodymium titanate. 
Dielectric compositions having useful properties for making capacitors have 
been classified according to various criteria such as their temperature 
coefficient (defined as the product of the relative variation of 
capacitance and the converse of the temperature variation 
##EQU1## 
and their dielectric constant. Thus type I dielectric compositions are 
those that have a low dielectric constant, ranging notably between 10 and 
90. 
Prior art type I dielectric compositions notably exhibit a dielectric 
constant temperature coefficient which is practically zero within very 
wide range of temperatures. They also display very low dielectrical losses 
(generally smaller than 6.10.sup.-4) at high frequences of greater than 1 
MHz and over a very wide range of temperature between -55 degrees C. and 
+125 degrees C., when materials such as lead titanate and neodymium 
titanate. This type of composition is generally used to make either 
disk-shaped or multi-layered ceramic capacitors. Now, multi-layered 
ceramic capacitors are generally made by casting or shaping layers of 
dielectric, depositing conductive metallic electrodes on the insulating 
layers, stacking the resultant elements to form a multi-layered capacitor 
and fritting the material at a high temperature to densify it and form a 
solid structure. 
Multi-layered ceramic capacitors have various applications and the 
specialist knows that the presence of bismuth in a type I dielectric 
ceramic composition does not enable capacitors of this type to be used at 
high frequencies. It is therefore indispensable to eliminate the bismuth 
from any type I dielectric composition. Bismuth, however, has the 
advantage of reducing the fritting temperatures of a ceramic. 
An object of the present invention, therefore, is to remove these 
disadvantages by proposing a new type I dielectric composition which frits 
at high temperatures, this composition being chosen from the following 
diagram: 
EQU Nd.sub.2 O.sub.3.3TiO.sub.2 -PbTiO.sub.3 -BaTiO.sub.3 -BaZrO.sub.3 
while at the same time preserving the performances of capacitors that use 
type I dielectric compositions. For a frequency of 1 MHz, these 
performances are the following: 
Dielectric constant ranging from 75 to 85, 
Dielectric loss factors of 4 to 8.times.10.sup.-4 which are stable at high 
frequency, 
A temperature coefficient .alpha. varying from 0 to .+-.30 ppm 
corresponding to the NPO class. 
SUMMARY OF THE INVENTION 
An object of the present invention is therefore a type I dielectric 
composition based on neodymium titanate, formed of a mixture comprising 
58% to 70% by weight of neodymium titanate, 10% to 21% by weight of lead 
titanate, 5% to 14% by weight of barium titanate, 4% to 15% by weight of 
barium zirconate and 0.2% to 1.2% by weight of yttrium oxide. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
These dielectric ceramic compositions are stable under temperature 
(variations of 0 to .+-.30 ppm) and result in high dielectric constants 
associated with low dielectric loss factors. The fritting temperatures of 
compositions of this type range between 1280.degree. C. and 1300.degree. 
C., and this fact enables the use of palladium-silver alloy electrodes 
(containing 70% by weight of palladium to 30% by weight of silver) when 
making multi-layered ceramic capacitors. 
The ceramic compositions are prepared according to the method, well-known 
to the specialist, of weighing and mixing raw materials in order to obtain 
a sufficiently homogeneous mixture, providing for complete reaction of the 
elements during fritting.

The present invention will be better understood from the following examples 
which do not exhaust the possibilities of the invention. 
EXAMPLE I 
We shall give a detailed description of the preparation of a special 
composition, and it is understood that the compositions described 
subsequently would be made in an identical way. This first composition 
will be designated A. The raw materials used are commonly employed in 
manufacturing and have not been chosen specially for application to a type 
I dielectric composition. 
The following materials are mixed in polyethylene containers: 200 grams of 
zircon balls, 100 cm3 of de-ionized water and a quantity, ranging between 
50 and 60 g., of a powder with the following composition (the percentages 
are with reference to the total weight of the powder): 
Neodymium titanate Nd.sub.2 O.sub.3 -3TiO.sub.2 : 64.08% 
Lead titanate PbTiO.sub.3 : 11.65% 
Barium titanate BaTiO.sub.3 : 12.62% 
Barium zirconate BaZrO.sub.3 : 10.68% 
Yttrium oxide Y.sub.2 O.sub.3 : 0.97% 
All the constituent elements are mixed for two hours. The suspension 
obtained is dried after incorporation of a binder, and the powder is 
sifted. Disks 1 mm thick with a diameter of 8.4 mm. are pressed. The 
fritting is done in a furnace wih an oxidizing atmosphere at temperatures 
ranging from 1280.degree. to 1300.degree. C. After silver electrodes have 
been deposited on both sides, the capacitors are tested electrically. 
Table I gives the following values: the density d of the ceramic, the 
dielectric constant .epsilon., measured at 20.degree. C. and at 1 MHz, the 
dielectric loss factor tg .delta. measured at 20.degree. C. and 1 MHz, the 
temperature coefficient .alpha. and the loss factor for frequencies of 1 
kHz and 100 Hz. 
It can be seen that the dielectric according to the invention as well as 
the capacitors made from this dielectric possess good qualities and 
correspond, in particular, to an NPO class. 
EXAMPLE II 
Six mixtures, the proportions by weight of which are given in the table II, 
are prepared under the same conditions as above. 
Table III shows the electrical characteristics B to G. These 
characteristics are measured under the same conditions as above. 
The variations of the coefficient .alpha. shown in table III are mainly due 
to the variation in the neodymium titanate content. By acting on the 
neodymium titanate content, the temperature coefficient .alpha. can be 
adjusted zero. The compositions E, F and G are especially useful at this 
coefficient. 
EXAMPLE III 
Three mixtures, the proportions by weight of which are given in table IV, 
are prepared under the same conditions as above. The main characteristic 
of this table is the variation in the barium zirconate content. 
Table V shows the electrical characteristics of the compositions H, I and 
J. These characteristics are measured under the same conditions as above. 
The variations of the temperature coefficient .alpha. shown in the table V 
are mainly due to the variation of the barium zirconate content. 
EXAMPLE IV 
Three mixtures, the proportions by weight of which are given in the table 
IV, are prepared under the same conditions as above. The main 
characteristic of this table is the variation in the lead titanate and 
barium titanate content. 
Table VII shows the electrical characteristics of the compositions K, L and 
M. These characteristics are measured under the same conditions as above. 
According to the various examples cited and on the basis of the variations 
in electrical characteristics depending on variations in the proportions 
of the constituent elements, we can choose compositions comprising the 
following by weight: 
58% to 70% of neodymium titanate, 
10% to 21% of lead titanate, 
5% to 14% of barium titanate, 
4% to 15% of barium zirconate, 
0.2% to 1.2% of yttrium oxides. 
A composition with a barium zirconate content of about 10.7% by weight (see 
composition A) is especially useful. An yttrium oxide content of about 
0.97% by weight would seem to be the best choice. 
TABLE I 
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d 5,49 
.epsilon. 77,6 
tg .delta. at 1 MHz 4 .times. 10.sup.-4 
.alpha. -4 
tg .delta. at 1 kHz 12 .times. 10.sup.-4 
at 100 Hz 10 .times. 10.sup.-4 
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TABLE II 
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Composition 
B C D E F G 
__________________________________________________________________________ 
PbTiO.sub.3 
17,977 
17,20 
16,495 
15,84 
15,236 
14,679 
BaTiO.sub.3 
10,112 
9,677 
9,278 
8,91 8,57 8,257 
BaZrO.sub.3 
12,359 
11,828 
11,34 
10,891 
10,48 
10,092 
Nd.sub.2 O.sub.3 --3TiO.sub.2 
58,427 
60,215 
61,85 
63,366 
64,762 
60,055 
Y.sub.2 O.sub.3 
1,125 
1,08 1,037 
0,993 
0,952 
0,917 
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TABLE III 
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Composition 
B C D E F G 
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.epsilon. 77,6 74,5 76 76,3 75 74,3 
tg .delta. at 1 MHz 
5 5 4 4 6 5 
(10.sup.-4) 
.alpha. -54 -37 -25 -4 +4 +8 
tg .delta. at 1 kHz 
9 10 9 9 10 9 
at 100 Hz 0 10 0 0 10 0 
(10.sup.-4) 
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TABLE IV 
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Composition H I J 
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PbTiO.sub.3 11,881 12,12 12,373 
BaZrO.sub.3 14,851 13,13 11,34 
BaTiO.sub.3 12,871 12,13 13,402 
Y.sub.2 O.sub.3 
0,991 1,014 1,03 
Nd.sub.2 O.sub.3 --3TiO.sub.2 
59,406 60,606 61,855 
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TABLE V 
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Composition H I J 
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.epsilon. 74 73 77 
tg .delta. at 1 MHz 
5 5 4 
(10.sup.-4) 
.alpha. -95 -77 -53 
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TABLE VI 
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Composition K L M 
______________________________________ 
PbTiO.sub.3 12,368 16,5 20,62 
BaTiO.sub.3 13,404 9,282 5,152 
BaZrO.sub.3 11,335 11,335 11,335 
Nd.sub.2 O.sub.3 --3TiO.sub.2 
61,86 61,85 61,86 
Y.sub.2 O.sub.3 
1,033 1,033 1,033 
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TABLE VII 
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Composition K L M 
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.epsilon. 75 75 75 
tg .delta. at 1 MHz 
6 4 13 
(10.sup.-4) 
.alpha. -57 -28 +4 
tg .delta. at 1 kHz 
11 11 11 
at 100 Hz 10 10 10 
(10.sup.-4) 
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