Ceramic (multilayer) capacitor and ceramic composition for use in such capacitor

A ceramic composition on the basis of a doped BaTiO.sub.3, a ceramic multilayer having such ceramic composition and a monolithic capacitor having such a composition are provided according to the invention. The composition corresponds to the formula EQU (Ba.sub.1-a-b Ca.sub.a Dy.sub.b)(Ti.sub.1-c-d-e-f Zr.sub.c Mn.sub.d Nb.sub.e).sub.f O.sub.3+.delta. wherein: 0.00<a.ltoreq.0.20 PA1 0.006.ltoreq.b.ltoreq.0.016 PA1 0.00<c.ltoreq.0.25 PA1 0.3b+0.7e<d.ltoreq.0.014 PA1 0.001.ltoreq.e.ltoreq.0.005 PA1 1.000<f.ltoreq.1.007. Capacitors having this ceramic composition as a dielectric material show an increased life-time well as a good resistance against degradation of their electrical properties if used at high temperatures under dc conditions.

BACKGROUND OF THE INVENTION 
The invention relates to a ceramic composition on the basis of doped 
BaTiO.sub.3, which ceramic composition can suitably be used in capacitors. 
The invention also relates to a ceramic multilayer capacitor comprising a 
number of ceramic layers on the basis of a doped BaTiO.sub.3 as well as a 
number of electrode layers predominantly consisting of Ni, the ceramic 
layers and the electrode layers being alternately stacked to form a 
multilayer structure which is provided with electric connections at two 
side faces, said electric connections being connected to a number of the 
electrode layers. The invention further relates to a ceramic capacitor 
comprising two electrode layers predominantly consisting of Ni between 
which layers a dielectric ceramic layer on the basis of a doped 
BaTiO.sub.3 is situated. 
Ceramic compositions of the type mentioned in the opening paragraph for use 
in monolithic ceramic capacitors are known per se. They are described, 
inter alia, in United States Patent document U.S. Pat. No. 5,264,402. This 
patent discloses, more particularly, non-reducible dielectric compositions 
consisting essentially of a modified barium titanate system, said main 
composition consisting essentially of oxides of Ba, Ca, Ti, Zr and Nb in a 
specific ratio, to which certain additives A and B have been added. This 
barium titanate (BaTiO.sub.3) system has a so-called perovskite structure. 
In the case of the known material, a specific quantity of Ca ions is 
substituted at the Ba sites of the perovskite structure, and specific 
quantities of Zr ions and Nb ions are substituted at the Ti sites. 
The known ceramic compositions show interesting properties which make them 
suitable for use in so-called `base metal electrode` capacitors. Firstly, 
said compositions can be sintered in a reducing atmosphere at relatively 
low temperature. Therefore, instead of the expensive noble metal Pd the 
base metal Ni can be used for electrode layers. This relatively low 
sintering temperature is necessary to preclude that the Ni of the 
electrode layers melts during the sintering process. Secondly, said known 
compositions show a relatively high dielectric constant around 10000 and 
even higher, in combination with relatively low losses. 
Capacitors, both of the monolithic type and of the multilayer type, which 
comprise said known ceramic composition as a dielectric show a clear 
disadvantage. It has been found that, in practice, the indicated 
composition does not lead to optimum properties of the capacitor. It has 
been shown that especially the electrical resistance of the known ceramic 
composition decreases rapidly if such capacitor is used at a relatively 
high temperature under direct current (dc) conditions. 
SUMMARY OF THE INVENTION 
It is an object of the invention to obviate the above disadvantages. The 
invention more particularly aims at providing a ceramic composition which 
is suitable for use in multilayer capacitors or monolithic capacitors 
comprising electrode layers predominantly consisting of Ni, which 
composition, in addition to the relatively high dielectric constant and 
the relatively low losses, also shows a strong resistance against dc 
fields at high temperature. The inventions also aims at providing 
monolithic and multilayer capacitors comprising said ceramic composition 
as a dielectric. 
These and other objects of the invention are achieved by a ceramic 
composition on the basis of doped BaTiO.sub.3, characterized in that the 
composition corresponds to the formula 
EQU (Ba.sub.1-a-b Ca.sub.a Dy.sub.b)(Ti.sub.1-c-d-e-f Zr.sub.c Mn.sub.d 
Nb.sub.e).sub.f O.sub.3+.delta. 
wherein: 0.00&lt;a.ltoreq.0.20 
0.006.ltoreq.b.ltoreq.0.016 
0.00&lt;c.ltoreq.0.25 
0.3b+0.7e&lt;d.ltoreq.0.014 
0.001.ltoreq.e.ltoreq.0.005 
1.000&lt;f.ltoreq.1.007. 
The invention is based on the unexpected, experimentally gained insight 
that the incorporation of a small amount of Dy in the Ba sites of the 
doped BaTiO.sub.3 material according to the prior art significantly 
increases the resistance of the material against degradation of its 
electrical properties under dc field application at high temperatures. 
This kind of degradation is a severe problem which suffers many base metal 
electrode capacitors. It is believed that stabilisation of the charge of 
Mn in base metal electrode materials is essential to solve this problem. 
The inventors found that the presence of Dy plays an essential role in 
such stabilisation. 
The incorporation of various dopants in BaTiO.sub.3 is necessary to render 
the inventive material suitable for use as a dielectric material in 
ceramic base metal electrode capacitors, both of the monolithic type and 
of the multilayer type. In the present case, Ca and Dy ions are 
incorporated in the Ba sites and Zr, Mn and Nb ions are incorporated in 
the Ti sites of the BaTiO.sub.3 material. It is noted that the indicated 
quantities of dopants are calculated as parts of the overall quantity of 
available Ba and Ti sites in this material. 
The presence of Ca and Dy in the indicated quantities at the Ba sites of 
the ceramic material is regarded as an essential prerequisite to obtain 
properly functioning ceramic capacitors. As mentioned before, it has been 
found that maintenance of the electrical properties of the inventive 
material is strongly dependent on the presence of Dy. If the quantity of 
Dy is below 0.006 parts or above 0.016 parts, the electrical properties 
are negatively influenced for capacitors comprising such compositions, 
especially if they are used at high temperature under dc conditions. 
Especially the electrical resistance of the ceramic material becomes 
rather low. This effect causes numerous rejects in highly accelerated life 
span tests (HALT). The best results are achieved if the quantity of Dy at 
the Ba sites of the ceramic composition according to the present invention 
ranges between 0.008 and 0.014 parts. 
The presence of a certain amount of Ca at the Ba sites of the presently 
invented ceramic composition serves to widen the so-called dielectric peak 
of the ceramic material. However, a quantity above 0.20 parts of Ca in the 
ceramic material leads to a dielectric constant of said material which is 
too low. An optimum compromise between both undesirable effects is 
achieved if the quantity of Ca at the Ba sites of the ceramic material 
ranges between 0.05 and 0.15 parts. 
The presence of Zr, Mn and Nb in the indicated quantities at the Ti sites 
of the ceramic material is also regarded as an essential prerequisite to 
obtain a properly functioning ceramic capacitor material. The presence of 
Zr causes the maximum value of the dielectric constant (the Curie 
temperature) of BaTiO.sub.3 to shift to a lower temperature range. If no 
Zr is present or if the quantity of Zr is more than 0.25 parts, the Curie 
temperature is too high or too low, respectively, for practical use. In 
both cases this leads to a dielectric constant which appears to be too low 
at the operating temperature of the material. If the quantity of Zr ranges 
between 0.10 and 0.20 parts, and especially between 0.13 and 0.15 parts, 
the position of the Curie temperature is optimally chosen for most 
applications. 
Mn appears to play an essential role in the sintering behaviour of the 
ceramic material of the capacitor in accordance with the invention. Said 
sintering process takes place in a reducing atmosphere. During sintering, 
reduction of BaTiO.sub.3 may occur. This leads to a reduction of the 
resistance of the ceramic material formed in the sintering process. This 
is undesirable. In experiments it has been established that the presence 
of a specific quantity of Mn at Ti sites of the ceramic material can 
preclude this undesirable reduction of the ceramic material. It is 
believed that the ability of Mn to protect against reduction occurs, in 
particular, in the grains of the ceramic material. If the Mn content is 
below a minimal amount, no protection takes place. The inventors have 
found that this minimum amount is strongly related to the amounts of Dy 
and Nb present in the ceramic composition. Said minimum amount appears to 
equal 0.3b+0.7e. If, on the other hand, the quantity of Mn exceeds 0.014 
parts, the life span of the ceramic appears to be reduced considerably. 
The ceramic material of the ceramic capacitors in accordance with the 
invention should also contain a small quantity of Nb. The presence of this 
element has a positive effect on the life span of the ceramic material, 
and thus on the service time of the capacitors comprising such material. 
As mentioned before, said life span is determined by means of highly 
accelerated life tests (HALT). If the material contains less than 0.001 
parts of Nb, the life span-extending effect is insufficient. If the 
material contains more than 0.005 parts of Nb, the electric resistance of 
the material decreases. This is undesirable. Preferably, the quantity of 
Nb is chosen in the range between 0.002 and 0.004 parts. 
As there is an unequal number of Ti sites and Ba sites (f is unequal to 
zero), the number of O sites is not equal to 3. The deviation from 3 is 
indicated in the formula by .delta., which has a small value. 
The invention also relates to a ceramic multilayer capacitor comprising a 
number of ceramic layers on the basis of a doped BaTiO.sub.3 as well as a 
number of electrode layers consisting predominantly of Ni, the ceramic 
layers and the electrode layers being alternately stacked to form a 
multilayer structure which is provided with electric connections at two 
side faces, said electric connections being connected to a number of the 
electrode layers. This ceramic multilayer is characterized in that the 
composition of the main component of the doped BaTiO.sub.3 corresponds to 
the formula 
EQU (Ba.sub.1-a-b Ca.sub.a Dy.sub.b)(Ti.sub.1-c-d-e-f Zr.sub.c Mn.sub.d 
Nb.sub.e).sub.f O.sub.3+.delta. 
wherein: 0.00&lt;a.ltoreq.0.20 
0.006.ltoreq.b.ltoreq.0.016 
0.00&lt;c.ltoreq.0.25 
0.3b+0.7e&lt;d.ltoreq.0.014 
0.001.ltoreq.e.ltoreq.0.005 
1.000&lt;f.ltoreq.1.007. 
The invention further relates to a ceramic capacitor comprising two 
electrode layers consisting predominantly of Ni between which layers a 
dielectric ceramic layer on the basis of a doped BaTiO3 is situated. The 
monolithic capacitor is characterized in that the composition of the main 
component of the doped BaTiO3 corresponds to the formula 
EQU (Ba.sub.1-a-b Ca.sub.a Dy.sub.b)(Ti.sub.1-c-d-e-f Zr.sub.c Mn.sub.d 
Nb.sub.e).sub.f O.sub.3+.delta. 
wherein: 0.00&lt;a.ltoreq.0.20 
0.006.ltoreq.b.ltoreq.0.016 
0.00&lt;c.ltoreq.0.25 
0.3b+0.7e&lt;d.ltoreq.0.014 
0.001.ltoreq.e.ltoreq.0.005 
1.000&lt;f.ltoreq.1.007. 
Multilayer capacitors and monolithic capacitors comprising a dielectric 
ceramic layers as defined by the formula show a relatively high dielectric 
constant, relatively low losses as well as a relatively high life span in 
highly accelerated life tests.

It is noted that, for clarity, the parts shown in the Figures are not drawn 
to scale. 
DETAILED DESCRIPTION OF THE INVENTION 
These and other aspects of the invention will be apparent from and 
elucidated with reference to the embodiments described hereinafter. 
FIG. 1 shows a multilayer capacitor in accordance with the present 
invention. This capacitor comprises a number of ceramic layers 1 on the 
basis of a doped BaTiO.sub.3. The capacitor also comprises a number of 
electrode layers 2 which consist predominantly of Ni. The capacitor 
additionally has two electric connections 3 which, in this case, are 
provided on two oppositely located side faces of the capacitor. These 
connections contain a solderable material, for example copper. In 
practice, the electrode layers are provided on a ceramic foil by means of 
screen printing, whereafter a number of these screen-printed foils are 
stacked. As shown in the FIG. 1, the ceramic foils are stacked so that 
successive electrode layers are connected alternately to the one or the 
other electric connection. 
For clarity, only 6 electrode layers are shown in FIG. 1. In practice, 
ceramic multilayer capacitors comprise minimally ten and maximally several 
hundred electrode layers. Their thickness typically ranges from 
approximately 0.5 to 2.0 micrometers. The thickness of the ceramic foils 
typically ranges from 5 to 20 micrometers. In practice, the multilayer 
capacitors are provided with a protective layers 4 on the upper side and 
the lower side of the stacked, printed foils. This protective layer is 
usually composed of a number of unprinted ceramic foils which, during 
stacking of the printed foils, are incorporated in the stack. 
The ceramic multilayer capacitors in accordance with the invention are 
manufactured as follows. First, a powder mixture is prepared by mixing 
powders of oxides and/or carbonates of the desired metals in quantities 
corresponding to the intended composition. In practise, BaCO.sub.3, 
CaCO.sub.3, TiO.sub.2, ZrO.sub.2, MnCO.sub.3, Dy.sub.2 O.sub.3, and 
Nb.sub.2 O.sub.5 are used for this purpose. This powder mixture is 
suspended in 2-propanol to which a small quantity of a dispersing agent is 
added. The suspension is ground in a ball-mill with ZrO.sub.2 balls for 
about 20 hours, so that powder particles having an average size below 0.4 
micrometer are obtained. Subsequently, the powder is dried. 
Next, the dried powder is calcined in air for about 4 hours at 
approximately 1100.degree. C. This results in the desired, doped 
BaTiO.sub.3. This calcined powder is ball-milled again for several hours. 
The powder thus formed has an average particles size below 1.0 micrometer. 
A binder solution with a dispersant is added to this powder. Subsequently, 
green, ceramic foils having a thickness of, for example, 40 micrometers 
are drawn from this powder-binder mixture. Electrode layers are screen 
printed on these foils by means of techniques which are known per se. For 
this purpose, use is made of a screen-printing paste which contains metal 
particles which are predominantly composed of Ni. The metal content of 
such a paste consists for at least 90 wt. %, preferably at least 98 wt. %, 
of Ni. Sintered electrode layers of such pastes are considered to consist 
predominantly of Ni. The layer thickness of the non-sintered electrode 
layers is approximately 2 micrometers. 
Subsequently, printed foils having a desired size are stacked in such a 
manner that the electrode layers of the even layers and of the odd layers 
are slightly displaced relative to each other. The stacked foils are 
uniaxially subjected to a high pressure (approximately 300 bar) at an 
increased temperature (approximately 80.degree. C.) to form a multilayer 
structure. This structure is subsequently broken in one direction to form 
rods and in a second direction (at right angles to the first direction) to 
form separate multilayer-capacitor bodies. These bodies are sintered in a 
reducing (hydrogen/nitrogen) atmosphere at about 1300.degree. C. for about 
2 hours. 
Finally, two oppositely located surfaces of the multilayer capacitor bodies 
are provided with electric connections of copper by means of dip coating. 
These connections are galvanically reinforced and provided with a 
solderable NiSn alloy. The mechanical and electrical properties of the 
ceramic multilayer capacitors thus produced can be subsequently measured. 
FIG. 2 shows a monolithic capacitor according to the present invention. 
This capacitor comprises a monolithic disc 11 made of a ceramic 
composition the basis of a doped BaTiO.sub.3 as claimed. The capacitor 
also comprises two electrode layers 12 which consist predominantly of Ni. 
Said electrode layers 12 are applied on the two main surfaces of the disc 
11 by common techniques, preferably by means of screen printing. On said 
electrode layers, additional layers may be provided, f.i. layers 
comprising a solderable material, for example copper. Electrical leads may 
be soldered to said additional layers. 
The monolithic capacitors according to the present invention are 
manufactured as follows. First a calcined powder mixture is made as 
described before for multilayer capacitors. However, instead of making 
sheets, discs are pressed from this powder, which discs are subsequently 
sintered in a reducing (hydrogen/nitrogen) atmosphere at about 
1300.degree. C. for about 2 hours. After sintering, the main surfaces of 
the discs are ground and polished for electrical property measurements. 
Electrode layers consisting of predominantly Ni are finally applied on 
said main surfaces of the discs by means of screen printing. 
Below, a number of experiments are described, which demonstrate the 
advantageous aspects of the presently invented compositions as well as the 
advantages of their use in ceramic capacitors of the multilayer type and 
of the monolithic type. 
Table 1 depicts a number of dielectric compositions 1-5, which are used in 
ceramic multilayer capacitors. These capacitors comprise a stack of 18 
layers (thickness 15 micrometer) of ceramic material, the central 10 
layers of the stack being provided with Ni electrodes (thickness about 1 
micrometer). The capacitors were dimensioned according to the so-called 
1206 size. Table 2 depicts a number of electrical properties of the 
capacitors with the dielectric compositions as described in table 1. More 
particularly, the capacitance C (nF at 25.degree. C.), the tanD losses 
(percentage), the insulation resistance IR (M.Ohm) and the life time L 
(hours) under HALT-conditions (27 V/micrometer at 140.degree. C.) are 
shown. 
TABLE 1 
______________________________________ 
item a c f b d e 
______________________________________ 
1 0.04 0.14 1.003 
0.012 0.01 0.004 
2 0.13 0.14 1.001 
0.012 0.01 0.004 
3 0.13 0.14 1.003 
0.0 0.01 0.004 
4 0.13 0.14 1.003 
0.006 0.01 0.004 
5 0.13 0.14 1.003 
0.012 0.01 0.0 
______________________________________ 
TABLE 2 
______________________________________ 
C tanD I.R. L (hrs) 
______________________________________ 
163.6 1.66% 3.8*E5 &gt;100 
94 2.34% 1.40*E5 &gt;100 
62 5.84% 0.27*E5 0 
109 4.62% 0.45*E5 45 
96 3.33% 2.01*E5 28 
______________________________________ 
In these tables, example 3 clearly shows that the absence of Dy (b=0) in 
the doped BaTiO.sub.3 results in a short life time of the capacitor if 
used at high temperature under dc conditions. If Dy is present, the life 
time under these conditions increases dramatically. Example 4 shows that 
the use of a small amount of Dy (b=0.006) increases the life-time already 
to 45 hours in HALT tests. However, the best results are obtained by using 
higher amounts of Dy (examples 1 and 2). It is also observed that the 
insulation resistance IR of examples 3 and 4, having no Dy and low Dy 
content respectively, is much lower than of examples 1 and 2 having higher 
Dy content. Example 5 demonstrates the importance of the presence of Nb in 
the inventive material. The life-time in HALT tests appears to be 
drastically reduced if no Nb is present (e=0). 
Table 3 depicts a number of dielectric compositions 11-21, which are used 
in ceramic monolithic capacitors. Said capacitors comprises a disc having 
a thickness of 1.0 mm and a radius of 7.5 mm. Table 4 depicts a number of 
electrical properties of the capacitors described in table 3. More 
particularly, the dielectric constant K (at 25.degree. C.), the tanD 
losses (percentage), the insulation resistance IR (M.Ohm) and the density 
D of the dielectric composition are given. 
TABLE 3 
______________________________________ 
item a c f b d e 
______________________________________ 
11 0.04 0.14 1.003 
0.012 0.01 0.004 
12 0.09 0.14 1.003 
0.012 0.01 0.004 
13 0.13 0.14 1.000 
0.012 0.01 0.004 
14 0.13 0.14 1.008 
0.012 0.01 0.004 
15 0.13 0.14 1.003 
0 0.01 0.004 
16 0.13 0.14 1.003 
0.017 0.01 0.004 
17 0.13 0.14 1.003 
0.012 0.007 
0.004 
18 0.13 0.14 1.003 
0.012 0.015 
0.004 
19 0.13 0.14 1.003 
0.012 0.01 0.007 
20 0.13 0.14 1.001 
0.012 0.01 0.004 
21 0.13 0.14 1.005 
0.012 0.01 0.004 
______________________________________ 
TABLE 4 
______________________________________ 
K tanD I.R. D 
______________________________________ 
13000 0.2% 4.3*E5 5.90 
12050 0.3% 3.9*E5 5.86 
8975 1.98% 1.9*E3 5.56 
12350 1.84% 8.8*E2 5.81 
10190 0.93% 7.5*E3 5.72 
10766 0.97% 8.3*E2 5.78 
12500 2.35% 5.6*E1 5.72 
6500 0.15% 4.2*E5 5.82 
13560 2.81% 2.5*E2 5.80 
9850 0.35% 5.1*E5 5.79 
11022 0.62% 2.5*E5 5.82 
______________________________________ 
Example 15 and 16 demonstrate that a low insulation resistance is obtained 
if the Dy-content is lower respectively higher than the claimed range. 
HALT tests (data not shown) confirm that the life-time of these capacitors 
is relatively low. Example 18 clearly demonstrates that a low dielectric 
constant K is obtained if the Mn-content is larger than the claimed upper 
limit of 0.014. Example 17 proves that a too low Mn content (i.e. lower 
than 0.3b+0.7d) is accompanied with a low insulation resistance IR. 
As relates to the Ba/Ti-sites ratio, example 13 proves that a to low ratio 
(f=1.000) is correlated with a relatively small insulation resistance. The 
too high Ba/Ti-sites ratio of example 14 (f=1.008) on the other hand also 
results in a relatively small insulation resistance. 
As concerns the Nb content of the compositions, example 19 teaches that too 
much Nb (e=0.007) causes a relatively low insulation resistance. A low 
insulation resistance of capacitors generally is accompanied with a short 
life-time. 
Good examples of the present invention are depicted as compositions 1 and 2 
for the multilayer capacitors and examples 11, 12, 20 and 21 for the 
monolithic capacitors. 
From the above experiments, it can be concluded that the life span of 
bme-capacitors according to the prior art is strongly enhanced if a small 
but effective amount of Dy is incorporated in the dielectric composition.