Process for making multilayer capacitors

Multilayer capacitors of good electrical properties can be made by firing, in air at low temperatures, certain modified lead titanate dielectric compositions. The dielectric compositions have the formula EQU (Sr.sub.x Pb.sub.1-x TiO.sub.3).sub.a (PbMg.sub.0.5 W.sub.0.5 O.sub.3).sub.b wherein PA1 x is 0-0.10, PA1 a is 0.35-0.5, PA1 b is 0.5-0.65, and PA1 a plus b equals 1.

BACKGROUND OF THE INVENTION 
This invention relates to electrical capacitors, and more particularly to 
monolithic capacitors made by lamination and firing of electroded 
dielectric layers. 
Multilayer monolithic capacitors comprise a multiple number of dielectric 
layers, at least some of which bear metallizations (electrodes) in desired 
patterns. Such capacitors are made from green (unfired) tape of ceramic 
particles held together with an organic binder, by cutting pieces of tape 
from a sheet of tape, metallizing some of the tape pieces, stacking and 
laminating the pieces of tape, and firing the resultant laminate to drive 
off organic binders and any solvents and form a sintered (coherent) body, 
which is termed monolithic. Rodrieguez et al. U.S. Pat. No. 3,456,313 
discloses a process for making them. FIG. 1 of Fabricius U.S. Pat. No. 
3,223,905 shows a multilayer capacitor, which may be of alternating 
palladium and barium titanate layers. 
Metallizations useful in producing conductors for multilayer capacitors 
normally comprise finely divided metal particles, applied to dielectric 
substrates in the form of a dispersion of such particles in an inert 
liquid vehicle. 
Monolithic multilayer capacitors are typically manufactured by co-firing 
barium titanate formulations and conductive electrode materials in 
oxidizing atmospheres at temperatures of 1200.degree.-1400.degree. C. This 
process yields durable, well-sintered capacitors with high dielectric 
constant, e.g., greater than 1000. However, firing under these conditions 
requires an electrode material with high melting point, good oxidation 
resistance at elevated temperatures, sinterability at the maturing 
temperature of the dielectric, and minimal tendency to interact with the 
dielectric at the sintering temperature. These requirements normally limit 
the choice of electrode materials to the noble metals platinum and 
palladium, or to alloys of platinum, palladium, and gold. 
Significant savings in electrode costs could be realized if dielectirc 
materials could be modified to (1) yield good dielectric properties (high 
dielectric constant and low dissipation factor) after firing in reducing 
atmospheres, so that base metals could be used as electrodes, and/or (2) 
sinter at temperatures of 950.degree. C. or lower so that silver, which is 
significantly less costly than the other noble metals but has a lower 
melting point (962.degree. C.), could be used in electrode formation. 
Attempts have been made to modify barium titanate ceramics so that they may 
be fired in reducing (e.g., hydrogen) or inert (e.g., argon, nitrogen) 
atmospheres. The use of this approach has been somewhat limited in that 
the electrical properties, e.g., dielectric constant, dissipation factor, 
temperature coefficient of capacitance, etc., are compromised as compared 
with those of conventional air-fired compositions. In addition, 
maintaining an inert or reducing atmosphere involves an additional 
production cost as compared to firing in air. Exemplary of this approach 
is Buehler U.S. Pat. No. 3,757,177, disclosing capacitors of base metal 
electrodes (e.g., Ni, Co, Fe) and modified barium titanate (MnO.sub.2, 
Fe.sub.2 O.sub.3, CeO.sub.2, CaZrO.sub.3) fired in an inert atmosphere at 
about 1300.degree. C. (col. 3, lines 33-34). Even with these high firing 
temperatures and the expense of firing in an inert atmosphere, the highest 
dielectric constant reported there is 1800 (col. 3, line 67). 
Several attempts have been made to reduce the maturing temperature of 
dielectrics by mixing high temperature ferroelectric phases (titanates, 
zirconates, etc.) with glasses which mature at relatively low 
temperatures. Examples of this approach are given in Maher U.S. Pat. No. 
3,619,220; Burn U.S. Pat. No. 3,638,084; Maher U.S. Pat. No. 3,682,766; 
and Maher U.S. Pat. No. 3,811,937. The drawback of this technique is that 
the dilution effect of the glass often causes the dielectric constant of 
the mixture to be relatively low, in the 25-200 range. 
Another technique for lowering the sintering temperature of titanate-based 
dielectrics is by the use of "sintering aids." Additions of bismuth oxide 
or bentonite to barium titanate lowers the maturing temperature to about 
1200.degree. C. (Nelson et al. U.S. Pat. No. 2,908,579). Maturing 
temperatures of 1200.degree.-1290.degree. C. may be attained by addition 
of phosphates to titanates as described in Thurnauer et al. U.S. Pat. No. 
2,626,220. However, in each of these cases, the decrease in maturing 
temperature is not sufficient to permit the use of co-fired silver 
electrodes and dielectric properties are often degraded. 
There exists a need for a composition which can produce a high dielectric 
constant (e.g., 1000 or above) and low dissipation factor (e.g., less than 
5%, preferably less than 3%) and sinters in air at low temperatures (e.g., 
less than 1000.degree. C. or less). This would permit co-firing with 
silver or palladium/silver electrodes and hence would greatly reduce the 
cost of high dielectric constant multilayer capacitors. 
N. N. Krainik et al. (Soviet Physics-Solid State, 2, 63-65, 1960), report 
solid solutions between, inter alia, PbTiO.sub.3, and PbMg.sub.0.5 
W.sub.0.5 O.sub.3. Apparently a wide range of compositions, with 0-80% 
PbTiO.sub.3, was investigated (see FIG. 2). Firing was carried out in an 
atmosphere of PbO vapor, which precludes practical commerical 
applicability. No suggestion was made as to the manufacture of multilayer 
capacitors. In a second article from the same laboratory, G. A. Smolenskii 
et al. (Soviet Physics-Solid State 3, 714, 1961) report investigating 
certain solid solutions, including those of Krainik et al. Firing was 
similarly done in PbO. Phase transitions are discussed. In what is 
apparently a third article in this series, A. I. Zaslavskii et al. (Soviet 
Physics-Crystallography 7, 577, 1963), X-ray structural studies are 
reported. 
Brixner U.S. Pat. No. 3,472,777 discloses the manufacture of ferroelectric 
ceramic discs by a two step firing process. Each firing step is taught to 
occur in the range 800.degree.-1200.degree. C., in air. In the sole 
example firing was at 1050.degree. C. Brixner discloses various dielectric 
compositions such as PbMg.sub.1/3 Ti.sub.1/3 W.sub.1/3 O.sub.3, 
Pb.sub.0.8-0.9 Sr.sub.0.1-0.2 Mg.sub.1/3 Ti.sub.1/3 W.sub.1/3 O.sub.3 and 
Y-containing compositions. 
Incorporated by reference herein is Sheard U.S. Pat. No. 3,872,360, issued 
Mar. 18, 1975, relevant to the preparation of monolithic multilayer 
capacitors. 
SUMMARY OF THE INVENTION 
This invention is a monolithic capacitor fired in air at 1050.degree. C. or 
less, having a dielectric constant of at least 1000 and a dissipation 
factor of less than 5%, comprising a plurality of superimposed alternating 
layers of a dielectric composition and metal electrodes bonded together 
into a unitary body, the dielectric composition having the formula 
EQU (Sr.sub.x Pb.sub.1-x TiO.sub.3).sub.a (PbMg.sub.0.5 W.sub.0.5 
O.sub.3).sub.b 
wherein 
x is 0-0.10, 
a is 0.35-0.5, 
b is 0.5-0.65, and 
a plus b equals 1. 
Preferred capacitors are those where, in the dielectric composition, a is 
0.35-0.45 and b is 0.55-0.65. 
In one preferred embodiment, there is no strontium in the dielectric 
composition, that is x is 0. In other preferred embodiments, strontium is 
present such that in the dielectric composition x is 0.01-0.08. Preferred 
capacitors are those having silver electrodes or Pd/Ag electrodes. 
Preferably, for reasons of economy, the Pd/Ag electrodes comprise no more 
than 20% Pd, based on the total weight of Pd and Ag. 
Another embodiment of the invention is a method of making such monolithic 
capacitors comprising a plurality of superimposed alternating layers of a 
dielectric composition and metal electrodes bonded together into a unitary 
body, the method comprising the steps of 
a. calcining in air, at a peak temperature in the range 
750.degree.-900.degree. C., for at least 5 minutes, preferably for at 
least 15 minutes, and usually for 0.5-8 hours, a mixture of oxides or 
precursors thereof in such relative proportions to produce the desired 
dielectric composition described above, then comminuting the resultant 
calcined product to the desired fineness (usually substantially all the 
particles are 20 microns or less in largest dimension), 
b. preparing an unsintered flexible ceramic dielectric tape of the calcined 
product of step (a) in an inert liquid vehicle therefor, 
c. electroding two or more such tapes in the desired pattern with a 
dispersion of metal powder in an inert vehicle therefor, 
d. laminating a multiple number of such tapes as desired, the top layer 
being an unelectroded tape, and 
e. sintering the resultant laminate in air for at least 0.25 hour, 
preferably at least 0.5 hour, at a temperature in the range 
900.degree.-1050.degree. C. to form a unitary monolithic multilayer 
capacitor having a K of at least 1000 and a dissipation factor of no more 
than 5%. 
Where the metal powder of step (c) is silver powder, the sintering 
temperature of step (e) is preferably in the range 900.degree.-950.degree. 
C. Where the metal powder of step (c) is Pd/Ag, the sintering temperature 
of step (e) is normally in the range 900.degree.-1050.degree. C. A 
preferred method is that wherein the calcining step (a) is conducted for 
0.25-8 hours and the sintering of step (e) is conducted at 
900.degree.-1050.degree. C. for 0.5-4 hours.

DETAILED DESCRIPTION 
The essential feature of the present invention is the use of the dielectric 
compositions described herein in the manufacture of multilayer monolithic 
capacitors. The use of this composition permits the formation of 
capacitors of excellent characteristics, although fired at low 
temperatures in air. 
The dielectric of the present invention has the final composition set forth 
above. That final compositon may also be expressed as follows: 
EQU Sr.sub.0--0.10 Pb.sub.0.90-1.0 Ti.sub.0.35-0.50 Mg.sub.0.25-0.325 
W.sub.0.25-0.325 O.sub.3, 
the total of (Sr and Pb) being 1.0 and the total of (Ti and Mg and W) being 
1.0. It is well known that in dielectric materials of the perovskite 
structure the amount of oxygen may vary from the stoichiometric amount. 
These dielectric compositions may be prepared from the oxides of 
strontium, lead, titanium, magnesium, and tungsten, or from precursors 
thereof such as carbonates, hydroxides, nitrates, etc. Lead, magnesium, 
and strontium are conveniently supplied as carbonates, whereas titanium 
and tungsten are conveniently supplied as oxides. Lead oxide (PbO) also is 
a convenient source of lead and strontium nitrate is a convenient source 
of strontium. Mixtures of oxides and precursors thereof may, of course, be 
employed. 
The oxides or precursors are mixed together using conventional techniques 
(e.g., ball milling mortar and pestle, etc.) and then calcined in air (or 
an oxygen atmosphere) at a temperature not in excess of about 900.degree. 
C. Calcining normally occurs at a temperature in the range 
750.degree.-900.degree. C. for at least 5 minutes, preferably at least 15 
minutes, and usually for 0.5-8 hours. The preferred duration of calcining 
will be dependent upon the particular starting materials employed, e.g., 
lead oxide versus lead carbonate, etc.; the proportions of starting 
materials; the calcining temperatures; etc. As is well known, with lower 
temperatures longer duration of calcining will usually be employed. 
Calcining for more than 8 hours does not appear to cause any significant 
property improvements in the capacitors of this invention. Therefore, 
calcining may be conducted for longer than 8 hours yet still be within the 
purview of this invention. 
Following the calcining step, the calcined product may then be milled to 
the desired fineness. Normally, the calcined product is reduced in size so 
that substantially all the particles are 20 microns or less in largest die 
dimension. Usually the dielectric powders employed in preferred 
embodiments will have surface areas in the range 0.2-5 m..sup.2 /g. 
The calcined product is then dispersed in an inert liquid vehicle and cast 
using conventional techniques as a tape on a flat surface. The vehicle may 
be any of those conventionally used to form tapes, normally comprising 
polymeric components and organic liquids, such as that disclosed in U.S. 
Pat. No. 3,757,177, that is, an organic binder of acryloid plastic 
dispersed in ethylene dichloride, the binder often comprising about 45% of 
the total weight of the sheet. Individual dielectric pieces are punched 
out of the tape. 
The green (unsintered) dielectric tape is then electroded with a dispersion 
of a metal in a temporary vehicle therefor, the electroding being in the 
desired configuration. Such vehicles may be any of those commonly used in 
the art, including those disclosed in Sheard U.S. Pat. No. 3,872,360, 
which is incorporated by reference herein. In the present invention the 
preferred metal powder is silver or palladium/silver, there normally being 
no more than 20% palladium based upon the total weight of palladium and 
silver. 
After printing the electrode material on the green ceramic, the resulting 
electroded pieces are either then dry or wet stacked to the appropriate 
number of layers, pressed (up to 5000 psig with or without heat), 
optionally diced and then fired. 
A typical firing cycle for multilayer capacitors comprises two phases. The 
first, called bisquing normally reaches a peak temperature not in excess 
of 500.degree. C. The duration of this phase depends to some extent on the 
number of layers in the laminate. The purpose is the non-catastropic 
removal of vehicle (solvent and binder) both in the electrodes and in the 
green dielectric sheets. After this is accomplished, a rapid (several 
hours or less) heat up to the desired peak or soaking temperature normally 
occurs, for the purpose of maturing or sintering the ceramic dielectric. 
In the present invention a principal advantage is the ability of the 
dielectric to be sintered in air at temperatures below 1050.degree. C. 
versus 1400.degree. C. currently used. The actual sintering temperature 
employed depends upon the specific composition employed, the electrode 
composition employed, and the properties desired of the final capacitor. 
Sintering is conducted at a temperature in the range 
900.degree.-1050.degree. C., for a period not less than 0.25 hour 
(preferably at least 0.5 hour), nor more than 4 hours. 
In the following examples and elsewhere in the specification and claims, 
all parts, percentages, and ratios are by weight unless otherwise stated. 
EXAMPLES 1-6 
Three precalcined dielectric compositions were prepared as follows from the 
materials indicated in Table I. 
TABLE I 
__________________________________________________________________________ 
Components (wt. in g.) 
Dielectric Composition 
PbCO.sub.3 
MgCO.sub.3 
TiO.sub.2 
WO.sub.3 
Fired Composition 
__________________________________________________________________________ 
A 278.609 
31.8393 
21.1591 
78.5688 
PbMg.sub.0.325 Ti.sub.0.350 W.sub.0.3 
25 O.sub.3 
B 280.667 
29.7398 
23.5708 
73.0606 
PbMg.sub.0.300 Ti.sub.0.400 W.sub.0.3 
00 O.sub.3 
C 282.751 
27.4639 
38.0476 
67.4696 
PbMg.sub.0.275 Ti.sub.0.450 W.sub.0.2 
75 O.sub.3 
__________________________________________________________________________ 
The indicated starting materials for dielectric (reagent grade) were mixed 
together in a ball mill with 350 cc. water for about 1 hour (total solids 
weight about 400 g. of starting material). The milled samples were then 
calcined in air in mullite at 875.degree. C. for 2 hours (raised from room 
temperature to 875.degree. C. over 3 hours and held), and then crushed to 
minus 48 mesh and finally milled in a ball mill as before for 0.5 hour. 
The final particle size was such that substantially all the particles were 
less than 20 microns in largest dimension. 
Flexible tapes were prepared by mixing 100 g. dielectric powder with 125 g. 
of a vehicle to form a slurry. The tape vehicle was acryloid-based and 
comprised 40.3% acryloid B7 (Rohm and Haas), 2.8% santicizer 160 (Central 
Solvent Co.), 0.2% of a rosin solution (10% rosin in isopropyl alcohol), 
0.3% of a glycerine solution (10% glycerine in isopropyl alcohol) and 
56.4% trichloroethylene. The slurry was doctor-bladed on a flat plate 
using conventional techniques. The tape was dried at room temperature 
overnight to form green flexible tape about 1.5 mils (38 microns) thick. 
Then the tape was cut into 0.5 inch (1.3 cm.) diameter discs, and 
electroded as desired with the electrode composition indicated below. 
Electrodes were printed through a 325-mesh screen (U.S. scale), the 
resultant dried print being about 0.6 mil (15 microns) thick. 
Electroded discs were notched to provide for subsequent electrical contact. 
Two electroded discs were laminated with a third disc of tape by pressing 
at 5000 psig. (7.2 Kg./cm..sup.2) for a minute at room temperature, to 
provide two buried electrodes. Five such samples were prepared for each 
example. 
The laminated, unfired samples were placed in an unheated box furnace (air 
atmosphere). The temperature was raised to 500.degree. C. over 16 hours, 
then to the peak temperature (900.degree. C., 950.degree. C., or 
1000.degree. C., as indicated below), over 1 hour. The temperature was 
held at peak for 1 hour, then slowly cooled to room temperature over 2-3 
hours. 
Capacitance and dissipation factor (DF) were determined as follows. The 
fired multilayer capacitors were mounted in the jaws of an automatic RLC 
Bridge (General Radio Model No. 1683) where both capacitance and DF were 
automatically read. Knowing the capacitance, dimensions of electrode and 
thickness of the fired central dielectric layer, effective dielectric 
constant (K) was determined from: 
##EQU1## 
where E.sub.o is 8.82 .times. 10.sup.-12 Farads/m. 
EXAMPLES 1-3 
In Examples 1-3, the dielectric used was A, B, and C, respectively. Peak 
temperature was 950.degree. C. The electrode material was a silver 
composition of 60 parts silver and 40 parts of a vehicle of 70% rosin 
solution (10% Hercules Staybelite 470 in 90% of a mixture of kerosine, 
naphtha and terpineol); 15% damar varnish solution (30% varnish in 70% 
"Solvesco 150" aromatic solvent); 4% dibutylphthalate; 11% naphtha; and 1% 
soya lecithin. Results are indicated in Table II. 
TABLE II 
__________________________________________________________________________ 
EXAMPLES 1-3 
Dielectric 
Electrode 
Thickness 
Example 
Dielectric 
Cap. (pf.) 
D.F. (%) 
Area (cm..sup.2) 
mils 
microns 
__________________________________________________________________________ 
1 A 8950 1.3 0.1008 1.3 3350 
2 B 9450 3.1 0.1008 1.6 4350 
3 C 9650 4.2 0.1008 1.6 4440 
__________________________________________________________________________ 
EXAMPLES 4-6 
In these examples dielectric B was used, with a Pd/Ag conductor 
composition. Peak firing temperature was varied, as indicated in Table 
III. The Pd/Ag ratio was 83.3% Ag/16.7% Pd, there being 60 parts metal and 
40 parts of the same vehicle as Examples 1-3. 
EXAMPLE 7 
A strontium-containing dielectric of the following composition was 
prepared: 
EQU Pb.sub.0.96 Sr.sub.0.04 Mg.sub.0.27 Ti.sub.0.46 W.sub.0.27 O.sub.3. 
the starting materials, 9.025 g. Sr(NO.sub.3).sub.2, 273.457 g. PbCO.sub.3, 
27.044 g. MgCO.sub.3 (basic), 39.182 g. TiO.sub.2, 66.734 g. WO.sub.3, 
were mixed together in a ball mill with 350 cc. water for about an hour. 
The milled sample was then calcined in air at 600.degree. C. for 5 hours 
and 875.degree. C. for 2 hours, and then ground in a mortar and pestle to 
-48 mesh. Finally the sample was milled in a ball mill with 300 cc. water 
for 1 hour to -200 mesh. 
Dielectric tapes were made as in Example 1 using 44.6 parts dielectric 
powder and 53.4 parts of a vehicle (22.3 parts acryloid BT, 1.6 parts 
Santicizer 160, 0.1 part of the rosin solution of Example 1, 0.2 parts of 
the glycerine solution of Example 1, and 31.2 parts 
TABLE III 
__________________________________________________________________________ 
EXAMPLES 4-6 
Dielectric 
Firing Electrode 
Thickness 
Example 
Temp. (.degree. C.) 
Cap. (pf.) 
D.F. (%) 
Area (cm..sup.2) 
mils 
microns 
K 
__________________________________________________________________________ 
4 900 2390 3.5 0.1008 1.6 1100 
5 950 8470 2.9 0.1008 1.6 3900 
6 1000 11163 0.85 0.1008 1.6 5140 
__________________________________________________________________________ 
trichloroethylene). The dried green tape was 2.0 mils (50 microns) thick. 
Capacitors were prepared as in Example 2 (silver electrodes, 950.degree. 
C. peak temperature for 1 hour). Capacitance was 6.74 .times. 10.sup.-9 
Farads, D.F. was 3.4%, fired thickness was 4.32 .times. 10.sup.-5 meters, 
area was 1 .times. 10.sup.-5 meters, and K was 3300.