Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2000-08-31
2001-10-30
Brunsman, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C501S136000
Reexamination Certificate
active
06309995
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to multilayer ceramic capacitors meeting the COG standard and having a ceramic dielectric that is based upon a magnesium zinc titanate combined with a glass containing sintering flux, which combined materials are capable of being sintered to maturity at well below 1100° C.
BACKGROUND OF THE INVENTION
Temperature stable ceramic capacitors meeting the EIA standard, COG, (a.k.a. NPO), must exhibit a change in capacitance of within +/−30 ppm over the temperature range of from −55° C. to 125° C. Such capacitors also must have a quality factor, Q, that is greater than 1,000 measured at 1 megahertz, which is equivalent to the dissipation factor (DF) being no greater than 0.01. Ceramic precursor powders having been fired under conditions to achieve a mature ceramic at 95 percent or more of the theoretically possible density are most likely to provide such high quality dielectric ceramics. The sintering of high-firing ceramic precursor materials such as magnesium zinc titanate is typically achieved at a temperature of 1100° C. It is well known to add a small quantity of glass containing flux as a sintering aid to the high-firing ceramic powders to reduce the sintering temperature necessary for yielding a mature dielectric ceramic. Examples of COG ceramic compositions sinterable at about 1100° C. are found in patents U.S. Pat. No. 4,882,650 issued Nov. 11, 1989 and U.S. Pat. No. 4,533,974 issued Aug. 6, 1985.
When making a multi-layer capacitor (MLC), a metal sheet electrodes are interspersed between successive layers of the green precursor ceramic powder which includes the flux if any. Thus the buried metal electrodes in the green (unfired) ceramic must be subjected to a temperature high enough to sinter the ceramic to maturity. The most commonly used formulation for the metal electrode is by weight 70% Ag and 30% Pd. This formulation has a melting temperature of 1150° C. and is typically used for the electrodes in MLC capacitors that are to be heated to temperatures no higher than 1140° C. to avoid the risk that the metal would melt and run out. When using sintering furnaces that cannot be relied upon to hold the temperature to within that 10° C. difference, an even lower furnace temperature setting must be used, and a further 10° C. safety factor is typically used in MLC manufacturing.
The addition of larger amounts of the sintering flux will enable a lower MLC sintering temperature hut at the cost of a reduced dielectric constant (K) and other degraded performance measures such as Q. Known start formulations of ceramic precursor plus flux, that can he sintered to a high density at lower temperatures than about 1100° C. are relatively rare, and the industrial use of such a start powder mixture is even more rare because the formulations and the sintering conditions become more critical leading to lower yields.
The cost of palladium is an order of magnitude greater than the cost of silver and palladium is typically the greatest cost factor in the manufacture of a MLC capacitor. One alternative to the use of palladium in a buried electrode is the use of a base metal such as nickel and or copper. However with base metal electrodes, sintering must be accomplished at less than the melting points of the base metal, which for nickel is 1453° C. but for copper is 1083° C. And sintering must be effected in a rare-oxygen atmosphere which greatly complicates the process. Control of a low oxygen pressure atmosphere itself leads to increased costs, and the choices of ceramic composition that do not become semiconducting due to loss of oxygen at sintering are severely limiting with respect to any particular dielectric ceramic performance that may be obtained.
The flux used in many air-fired MLC capacitors contains oxides of bismuth cadmium and lead which are especially effective for lowering the melting point of the flux. For a given amount of flux, this advantageously leads to a further reduction of the sintering temperature. However, these oxides tend to reduce the Q of MLC capacitors and bismuth can react with silver palladium electrodes leading to an even poorer quality factor. These volatile heavy metal oxides also contaminate the sintering ovens leading to irregular sintering results. Perhaps most importantly they represent a hazard to the environment and especially a health hazard to personnel involved in making both the ceramic powder and the MLC capacitors.
It is therefore an object of this invention to provide a ceramic powder for use in making multilayer ceramic capacitors meeting the COG standard, which start powder is capable of being sintered to maturity in an air atmosphere at a temperature of 1000°+/−50° C. so that the MLC may contain electrodes of a more silver rich and less costly composition e.g. 85% Ag/15% Pd.
It is a further object of this invention to provide such a ceramic powder that includes a high firing part comprised of a magnesium zinc titanate and a low firing flux part having essentially none of the hazardous heavy metal oxides of lead, bismuth and cadmium.
SUMMARY OF THE INVENTION
A dielectric ceramic powder mixture consists essentially of agglomerates and each agglomerate includes a homogenous group of two kinds of powder particles having been mildly calcined and superficially co-reacted to bind the group of powder particles together to form the agglomerate. One kind is a high-firing ceramic-precursor powder that is a magnesium zinc titanate which makes up from 87 to 98 weight percent of the dielectric ceramic powder mixture. The other kind is a powdered barium lithium boro-silicate sintering flux which makes up from 2 to 13 weight percent of the dielectric ceramic powder mixture.
The magnesium zinc titanate powder is preferably the fully reacted compound Mg
2/3
Zn
1/3
TiO
3
, wherein there may be substituted for up to 20 mole percent of the Mg an equal molar amount of an alkaline earth metal, and alternatively in the case of only substituted barium up to 60 mole percent may be so substituted.
A particularly effective range of compositions for the barium lithium boro-silicate flux is from 10 to 55 weight percent Li
4
SiO
4
, from 3 to 40 weight percent BaO.B
2
O
3
and 10 to 76 weight percent 3BaO.B
2
O
3
. Another preferred range of flux compositions is from 22 to 26 weight percent Li
4
SiO
4
, from 20.5 to 23.5 weight percent BaO.B
2
O
3
and from 50 to 56 percent 3BaO.B
2
O
3
.
The above-described dielectric ceramic powder mixture is made by combining from 87 to 98 weight percent of a magnesium zinc titanate and from 2 to 13 weight percent of a powdered barium lithium boro-silicate sintering flux; mixing the combined powders to form a homogenous powder mixture; and mildly calcining the homogeneous mixture at from 600° C. to 750° C. to obtain a powder comprised of agglomerates of the homogeneous powder mixture wherein each of the agglomerates has essentially the same composition of magnesium zinc titanate and barium lithium boro-silicate as in the combined powders. The mildly calcined agglomerates may then be comminuted to render the dielectric ceramic powder mixture a free flowing powder having an average agglomerate size of about 1.2 micrometers.
A method for making a multilayer ceramic capacitor entails making a slurry by milling in an organic vehicle the dielectric ceramic powder mixture described above, forming layers of the slurry and drying the layers. A first patterned film of palladium silver alloy is deposited on one of the dried layers. Then a stack is formed by stacking at least a second green ceramic layer over the first patterned alloy film. A second patterned alloy film is deposited over the second green ceramic layer, and at least a third green ceramic layer is stacked on top of the second patterned film. The stack is then sintered at a temperature within the range of 950° C. to 1120° C. and after cooling a silver paste is applied to the ends of the body to which an edge of the first and second alloy patterns may extend. after heat curing the silver paste, termination
Maher Galeb H.
Maher Samir G.
Brunsman David
MRA Laboratories, Inc.
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