CA1070852A - Base metal electrode capacitor and method of making the same - Google Patents

Abstract

BASE METAL ELECTRODE CAPACITOR
AND METHOD OF MAKING THE SAME

Abstract of the Disclosure A monolithic ceramic capacitor with base metal el-ectrodes fired in an atmosphere of oxygen at low partial pressure in which the reaction between the electrodes and the ceramic prevents conversion of the ceramic into the semiconductive state. The base metal is a transition metal or a transition metal alloy, preferably nickel. The method is usable with any green ceramic without changing the firing temperature. The only change required is from the normal oxygen partial pressure in the kiln open to the atmosphere to an atmosphere of much lower oxygen partial pressure.

Description

Th~s invention is intended to improve monolit~ic titanate ceramic capacitor~ by firing the ceramic under conditions at which the base metal electrode reacts with the ceramic to an extent sufficient to neutralize unlocal-ized electrons which normally occur when a barium titanate ceramic i8 flred ln a reducing atmosphere and which lead to an n-type (electron) conductivlty in the dlelectric. The use of the electrode to protect the ceramic from r~duction to the semiconductive state is appllcable generally and ls not lim-- 10 ited to special ceramic formulations. To use this invention lt is not necessary to devise any special ceramlc formulations or to develop new firing temperatures.
In the drawing Fig. 1 is a plan view of one of the ceramic layers used in making a monolithic capacitor which -..?.~

., . , : ', ' '' - ' ' - ''''' .

,:
.

, ::: ' '' : - ., :: . :, ' : ' .. " ,, ~, : ' ha3 been coated wlth an electrode pattern of base metal paint; Fig. 2 i8 a cro~s Bectional view of a multllayer monollthic capacitor befor¢ firlng; Fig. 3 i8 a simllar view after flring, Flg. 4 18 a dlagrammatic enlarged ~ec-tlon through a portion of one Or the electrode layers after flrlng; Flg. 5 iB a diagram o~ re~istivity agalnst oxygen partlal pres~ure; Flg. 6 i8 a diagram of the change ln capacl~y wlth temperature.
In a preferred form of the above lnventlon illustrated in the drawings the capacltor is mad~ from a plurallty of layers l of green ceramlc dlelèctric which comprls~ mix-tures of barlum titanate wlth other oxides, tltanates, zlrconates, stannates, or precursors thereof. The layer l also contalns temporary binders and other ingredlents whlch ald in processlng. There 18 a large body of patent llterature describing these dlelect~lcs and the procedures for preparing the ~ame in ~heet or layer form. On the layer l i8 applled an electrode pattern 2 whlch extends to one end 3 and 1B marglned lnward from the Blde~ 4 and from the other end 5. The electrode pattern 18 applied as a paint ln which the pigment is one of the transition metals ~uch as nlckel or a tran~ltlon metal alloy. For application a~ a palnt the metal pigment is dispersed or suspended in a vehicle which is vaporized-or burned during the early stages of the ceramlc firing. The sectlon Or the pattern

2 oppo~lte bracket 6 18 the capaclty sectlon of the el-ectrode and the section opposlte bracket 7 i8 a terminal extension by whlch electrical connection i8 made to the capaclty sectlon. Ad~acent to end 5 of the sheet l 18 a shleld pattern 8 which i8 conveniently applled at the same time and with the same palnt as the pattern 2. The shield ....

~(;i76J85z pattern 8 i~ electrically in~ulated rrom the elec~rode pat;tern 2 by a ~pace 9. The layer~ 1 are stacked one on toE) of the other wlth alternate layer~ turned end ror end as ~hown in Fig. 2. At both the upper and lower end~ o~
the ~tack there is applled a sheet 1 of a dielectri¢ having an electrode pattern whlch i~ not turned end for end re-latlve to the immediately underlylng electrode but hae lts terminal extension 7 to the same end of the stack as the immediately underlying electro~e. me electrode pattern of th~ upper and lower ends i8 ldentlcal to that of Fig. 1, wlth the electrode pattern 2 and shield pattern 8 of the upper and lower ends dlrectly overlapping the electrode pattern 2 and shield 8 of the next underlying elec~rode.
The assembly of the green ceramic body i~ completéd by several plaln layers 10 of green ceramic. The stacked layers are then pres~ed together and rired or Binged lnto a monolith as Bhown ln Flg. 3.
In one example the ceramlc was a ~tandard commercial body known as K7000 having approximately 80~ BaT103, 10%
CaZrO3 and 1% other ingredients mixed with an organic binder and the paint was a metallic nickel pigment dlspersed ln an organic vehicle. The ~tack was baked at 530 F to burn of~ the organlc materlals present and the baked stack wa~
then ~ired at about 1370 C for 2-1/2 hours ln an atmosphere havlng a partlal pressure o~ oxygen b~tween 1.8 X 10 7 and 1.5 X 10 atmospheres. The alternate electrodes extending to opposite ends o~ the capacltors are Joined by a common condu¢tor 11, 12 o~ a simllar composition to the electrode~
either before or a~ter ~iring. Fig. 3 1~ representative o~ the fired monolithic oeramic capacitor8 wlth nickel el-eotrode~ rabricated by the de~cribed method with dielectrlc layer~ 1 and interleaved condu¢tive nickel ~lectrode~, ' ;., V~5~

having alternate electrodes connected at opposite end~ 2 by a nickel conductor ll, 12.
Durlng firing Or the monollthic ceramic capacitors with nlckel electrodes, an equllibrium is established be-tween the atmospheric oxygen, the oxyg0n in th~ ceramic dielectric, and the nickel electrode, all o~ whlch ~hare the available oxygen. There i8 insufflcient oxygen ln an at-mosphere containlng, ~or example, 6 X 10 7 atmo~pheres of oxygen at 1370 C to cause oxldation of nickel, 80 that the nickel electrode~ remain metallic and conductive ln the bulk.
However, at the interface between the nickel electrode and the oxygen bearlng dlelactric, oxidatlon o~ the nickel takes place by the sharing o~ oxygen wi~h the dielectric. The oxidized nlckel ~l.e. Nl ) then reacts with the neighbor-lng oxlde. At the ~ame time, the titanate based dlelectric 1~ reduced (releases oxygon to the riring atmosphore) be-cau~e of the low oxygen content of the flrlng atmosphere.
The reduction of a barlum titanate based dlelectrlc normally leads to an undesirably high conductivity ln the dielectric. Thi~ reactlon can be repre#ented by:
(l) Ba Ti+4 0 Ni~2 heat and low Ba Ti(l )e l o(2

3 x oxygen atmosphere -x x 3-x) For charge balance, the reduction reactlon requires the pro-ductlon o~ unlocalized electron~ which lead to an n-type (electron) conductlvlty in the dielectrlc.
However, while the oxygen ic being removed from the dielectric, nickel, as Nl 2, enters the dielectric and the total reaction 18:
t2) Ba 2 Ti 4 032 Nl+2 heat and low Ba 2 Titl x) Ni+X20(3x) x oxygen atmosphere No excess of unlocallzed electrons are requlr~d for charge - . . .. ..

balance, and n-type conductivity i~ not induced ln the di-electric.
Fig. 4 18 a schematic repre~entation of ~he localized oxidatlon Or the nlckel electrode with concomltant Ni 2 in-corporation lnto the dlelectrlc. The oxidatlon o~ the nl-cke]. electrode at it~ interface wlth the dlelectric is read-lly observed mlcroscopically and i~ shown schematlcally in Flg. 4 where 13 ~nd 14 represent nickel oxide skins on the core 15 of metallic nickel. Flg. 4 repre~ent~ qualitative-ly what happens in patterns 2 and 8. In the upper and lowerelectrode pattern~ the oxide skins are thicker becau~e the~e patterns are outermos~ and have a greater bulk Or ceramic from whlch oxygen can be obtalned. Thsse outer patterns ~hicld the el~ctrode patterns 2. Since these outer patterns are electrically inert, they can be ~acrlficed without af-reotlng the capacitor. The ~hiold patterns 8 prevent ex-ceoslve oxidatlon Or the tormlnal ~ectlon~ 7 of the pattern-8 2. Whlle oxidation of ths electrodes iR oesentlal to the reaction of equatlon 2, excessive o~ldatlon is destructive and 18 prevented by the ~hield pattern~ 8, and the overlap-plng upper and lower patterns.
It is nece~sary that there be enough oxygen to oxidlze - the skins of the electrode~ but not 80 much oxygen as to completely oxidlze the electrodes. Thl~ 1B illustrated ln Fig. 5 whlch show~ the results o~ flrlng K7000 ceramlc at a temperature o~ 1370 C and at oxygen partlal pres~ure of 1.8 X lO 7, 2.7 X lO 7~ 8.9 X lO 7, and 1.5 X lO atmos-pheres. At 1.8 X lO 7 atmospheres (deslgnated by the num-eral 16) ther~ 1B not enough oxygen in the atmosphere to produoe the requlred oxldation Or the ~kins of the elect-rode~ at the ni~kel-dielectric inter~ace. At thi~ pres~ure ': ' , ' .'.: . ;

1~7~
a greater amount of oxygen was w~thdrawn from the dielectrlc resulting in unacceptable lo~ resl~tivity of the dielectric.
At an oxygen partial pres~ure of 1.5 X 10-6 atmo~pheres (designated by the numeral 19) there wa~ too much oxygen re-sulting in exca~slve oxidization in the nickel electrodes resultlng ln 10~8 of conductivity of electrodes and 1088 of capacitance. At oxygen partial pressures 2.7 X 10 7 at-mo~pheres (deelgnated by the numeral 17) and 8.9 X }0 7 at-mosphere~ (designated by numeral 18), the oxidation Or the skin~ of the electrodes was sufflcient to protect the die-lectric from excessive 1088 of oxygen and was not great enough to destroy the conductiYity of the electrodes. The numeric values apply to the K7000 body and would be differ-ent for other bodies receivlng different firing temperatures.
However, the same kind of phenomenon 18 observed in all titanate ceramlc bodles.
Fig. 6 curve 20 shows the varlatlon ln percent of cap-acltance change with temperature for a K7000 monollthic cer-amlc capacitor with prec$ous metal electrodes fired in air and curve 21 i8 a slmilar curve for a like capacitor with nickel electrode~ rlred at the ~ame temperature in 5 X 10 7 atmospheres oxygen partial pres~ure, a preasure midway be-tween points 17 and 18 of the Fig. 5 curve. In addition to the change in electrical properties evldent from curves 20 and 21 there 18 also a change in the microstructural physlcal propertie~. The grain size of the dielectric curve 20 was 9 microns while the grain size of the dielectric of curve 21 was 2 microns. The incorporation of Ni+2 into the dielectric mo~diries both the electrical and the micro-structural physical properties of the dielectric.
Monolithlc ceramic capacitors were fabricated accord-ing to the pr~ceding description from an unaltered K7000 1~7~)85;2 ceramic dielectrlc formulation. Three groups of capacitors were rabricated: one group with platinum electrode~, another wlth a mlxture Or nickel and platlnum in the electrodes, and a final group with pure nlckel electrodes. These cap-acitors were fired slmultaneously at 1370 C for 2-1/2 hours in a 25:1 mixture Or CO2:CO. This mixture ylelds an oxygen partial pre~oure of 7.9 X 10~7atmo~phere~ a~ 1370 C. The room temperature products of resi~tance a~ter one minute of charging at 77 VDC/mll and capacitance (RC product) were ~0.05 sec, 586 sec, and 1272 ~ec, respectlvely. The~e data show that the increase in nlckel content in the electrode and coin¢identally in the dielectric leads to an increase in the resistivl~y of the dlelectric. These data are con-~istent with the prevlous technical explanatlon.
No appreciable shirt ln the Curie Temperature could be ascertained when the nlckel electroded monollthlc ceraml¢
capacitors were flred over the partlal prossure Or oxygen range from 2.7 X 10 7atmospheres to 1.5 X 10 atmospheres, whlle the capacltance remalned within \ 10% of the mean over the range 1.8 X 10 7 to 9 X 10 7 atmo~phere8 of oxygen.
These data indicate the method employed ln this patent i8 capable of the reproduciblllty required for a production method for fabricating monolithic ceramic capacitors with embedded ba~e metal electrodes.
Capacitors ~abricated according to the method herein described were life tested at 85 C by imposing 77 VDC/mil stress for extended periods of time. The results are sum-mllrized a~ follows:
(A) Initial capa¢itance and dissipation factor at 1 Khz and 1 volt, and insulation re81s-tance at 77 VDC/mil after one minute charging ,: , , . ... - .

~7~135Z

and ~C product all mea~ured at 25 C were 387 nF, 0.99%, 2.52 GJ~ , and 975 ~Q F ~ec).
(B) Af~er 100 hours - 294 nF, 1.10%S 2.22 G~ , and 874 ~ec, re~pectlvely at 25 C.
(C) Arter 200 hours - 362 nF, 1.00%, 3.33 GvQ , and 1205 sec. re~pectively at 25 C.
(D) Aft~r 5O0 hours - 373 nF, 1.0%, 1.98 a ~Q, and 739 ~ec, re~pectlvely at 25 C.
Th~se data show that the base metal electroded mono-llthlc ceramlc capacltors encompassed by thl6 patent pro-duce~ capacltors havlng uaeful propertles over the ll~e tlme expected for these capacltors ln normal operation.
The foregolng example~ are based on K7000 dlelectrlc wlth nlckel electrode~.
The lnventlon hao produced some potontlally userul properties omploylng other dlelectrlc composltlons and non-preclous metal electrode~.
A dlelectrlc compo~ltlon known as K2000 havlng the composltlon 92% BaT103, 3.5~ CaZrO3, 1.5% SrTiO3, and 3%
other lngredlents normally flred wlth pr~clou~ metal el-ectrodes was prepared as monolithi¢ capacltors wlth nickel electrodes and wlth preclous metal electrodes. The two types of capacitors were rlred slmultaneously at 2560 F ~or 2-l/4 hours wlth an atmosphere of 25 parts CO2 to 1 part CO (2 X 10 6 atmo~pheres of oxygen). Th0 slx nlckel el-~ctro~ed units had average value~ as ~ollow~: capaoltanco - 122 nF, dissipation ractOr ~ ~.5%, ia~u~atlon reslstance o . 94 a ~, RC product 3 115 sec. The precious metal el-ectroded unlts had re~i~tance8 of 2000~Q to 8000~ at 1.5 3 VDC, far too conductlve to meaoure any capacltance. Clear-ly, th~ ~abricatlon method hereln described yielded ~uper-lor results.

A dielectrlc composltion known aa NPO having a compo~-ltlon of 59% nd(C03)4, 26% T102, and 15% BaTiO3, whlch i8 normally employed with preciou~ metal elertrodes wa~ prepared both wlth a mlxture of platlnum and nlckel and wlth a pre-clow metal electrode. Firing was at 2350 F ror 2-1/2 hours ln a 25 parts C02 to l part CO mlxture ylelding an oxygen pressure Or 1 . 05 X 10 7 atmosphere~. Reslstance~ meaBured at 1.5 VDC wlth an ohmmeter were 270,000 for the platlnum-nlckel electrodes, and lo~Q for the preclous metal electrodes.
m e beneflclal e~fect of the nickel is ob~ervable in thls case .
A dielectrlc composltlon known as K1200 havlng the com-position 80% BaT103, ll~ Bi2Ti207, and 9% CaSnO3 wa~ prepared in two groups with nlckel ~lectrodes and platinum electrode~, re~pectlvely. Flrlng was as ~or NPO abovo. The nickel el-ectroded unltB had capacitance ~ 33 nF, dis~lpatlon ractor -6.8S, lnsulatlon reclstance 1.58 G~n and RC product - 52 ~ec. The platlnum electroded capacltors had no continuou~
internal electrodes apparently because the atmo~phere, al-ong with the bismuth ~rom the dielectric compositlon, loweredthe meltlng point o~ platinum 80 that meltlng took place.
A compositlon composed Or barium titanate, BaT103, with a 8mall addition of lanthanium oxide, La203 was prepared as thin discs. This composit~on, when ~ired at low oxygen partial pre3sure~, ha~ been used to glve highly conductive BaTiO3. Some of the discs were painted wlth nickel external electrodes before ~irlng, whlle others were fired wlthout electrode~. Firing was at 2500 F for 2-l/2 hours at 25:1 ratio of C02 to CO. The nickel elec~roded discs yielded a capacitance o~ 13 nF, a dlssipatlon factor of 18%, and an insulation resistance of .4 a~ at lOO VDC. The calculated _g_ ~C~7~85Z

dielectric con~tant was lO,000. The unelectroded di~c~ were painted wlth ~ilver palnt and yielded a reslstance of 408~1l measured at 1.5 VDC. The nickel electrodes greatly improv-ed the reslstance of the dielectric.
An electrode sy~tem containlng 90% nickel and 10% tin was employed ln monollthic capacltors with the K7000 dle-lectrlc, and flred at 1370 C for 2-1/2 hours at an oxygen pres~ure of 7.9 X 10-7 atmo~phere. The prop~rties were:
capacitance = 43 nF, RC product = 516 ~ec. In terms of the technlcal discussion, entry of Sn (oxldized t~n) into the dlelectrlc could not cause charge balsnce within the die-lectrlc wlthout excess electrons belng presen~. Hence the inBulatlon resl~tance 18 lowered. Also, ~ince the nickel ln thl~ ca~e 18 diluted by tin, the ni¢kel did not enter the dlelectrlc to the extent to whlch lt could if undlluted, and, ~here~ore, could not improve the in~ulatlon resistance to the extent which was accomplished with pure nickel el-ectrodes.
The oxldation re~lstance of several electrode metals or ¢ombination o~ metals have been studied. The electrode~
~tudied are a~ rollsws: nickel, nickel-copper, nickel-zinc, nickel-60dlum oxlde, nick~l-cobalt, cobalt-iron, nlckel-lron, nickel-tantalum, chromium, nlckel-chromium, cobalt, lron, nickel-tln, nickel-chromium oxide.
The rationale of selecting an ele¢trode metal ~ys~em can be mo~t conveniently illu~trated by consldering a ~pec-ific case. For the previouBly mentioned dielectrlc, NP0 the required ~iring temperature ln near 2300 F (1250 C).
Pure nickel will remaln unoxldlzed in the bulk at oxygen partial pr~Bcure of leBs than 2 X 10-7 atmosphere8 at thi8 temperature. However, thio oxygen partial pressure will : , ~()t35Z

cau~e conductivlty to be high in fired NP0 unless the nickel as Ni+2 enters the dlelectrlc in ~ufflcient quantity as pre-viously de~¢ribed. Also, nlckel metal will not melt at thls temperature. In order to produce les~ conductivity in the NPOJ lt would be desirable to fire at higher oxygen partial pre~sures. Gne possible alternatlve would be to select an electrode composed of copper whlch will remain unoxidlz~d up to partial pres~ures Or oxygen of 2.3 X 10 atmospheres at 2300 F (two order~ o~ magnitude higher). How~ver, copper melt~ at 1980 F and would be unsuitable on this account. A
solid solution of copper and nlckel in the ratio of 40% by weight nlckel and 60S by welght copper melts at about 232~ F
and once the solld oolutlon i8 rormed, thl~ ~olld ~olution ~hould yield oxldation re~lstance ouperlor to nickel but lnferlor to pure copper. This effect has already been pro-ven by slmultaneous ~lrlng of pure nickel on the top surface o~ a piece of dlelectrlc and the above mixture o~ metals on a piece of dielectric. In this case, the nickel oxidized completely, whereaa the solution had a resistance o~ about 0.1 ~Q at 1.5 VDC. Also, the color of the dielectric with the nickel-copper electrodes was a brown or orange a~ com-pared wlth the normal dark grey color caused by reduced oxygen firlng. The resl~tance at 100 volta was 2 G~as com-pared wlth a very hlgh conductivity (but not measured) for NP0 fired ~imultaneoucly but with precious metal electrode~.
The dif~erence in color and reslstance indicate~ that a beneficial modificatlon of the dielectric has occurred due to the presence of the copper-nickel electrodes. The uni-que feature~ of this method of fabricatlon are as rollowB:
(A) The electrode material must make a contribution by reaction to the properties of the dielectric.

7~3SZ

This 18 easily observed by ~iring ba~e metal electroded and precl~us metal el-ectroded capacitors simultaneously in the same low oxygen partial pre~sure at-mosphere. In the prevlous example~ and the Disclosure itself, thls 18 observed as ln-creased resistance of the ceramlc wlth base metal electrodes, modificatlon of grain aize of the ceramlc, and shif$ in the Curie temperature (temperature o~ maximum cap-acltance).
(B) The dielectric compo~itlon i8 not prepared es-peclally for reduced oxygen partial pressure ~lring. Improvements to the dielectric com-positlon come about because Or (A) above.
(C) Tho quantlty of oxygen ln the flrlng atmosphere 18 determlned by the base metal-ceramic combln-atiQn 80 that the base metal 18 not oxidized in the bulk but 18 oxldized at the dielectrlc base metal lnterface, and the dielectric 18 not r~duced any more than requlred ~or the ba3e metal reaction to take place.

,.

Claims (20)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of making a monolithic capacitor having a sintered unitary body of titanate ceramic and electrodes embedded in the body which comprises preparing a green ceramic body with embedded electrodes of metal M, where M is selected from the group consisting of transition metals and/or transi-tion metal alloys, and firing the green ceramic body at a temperature to mature the ceramic and in an atmosphere of oxygen partial pressure low enough so that the skins of the electrodes oxidize and react with the ceramic and prevent conversion of the ceramic to the semi conductive state and the cores of the electrodes remain in the metallic state.

2. The method of Claim 1 in which the ceramic comprises barium titanate.

3. The method of Claim 1 in which the electrodes have terminal extensions with alternate extensions extending to different outer surfaces of the body and a shield element of metal M is interleaved between alternate extensions and is spaced from the electrodes and extensions.

4. The method of Claim 1 in which a shield layer of metal M in the outer part of said green ceramic body overlaps the immediately underlying electrode.

5. The method of Claim 1 in which the reaction between the electrodes and the ceramic is in accordance with the equation

6. The method of Claim 1 in which the oxygen partial pressure is greater than 10-7 atmosphere.

7. The method of Claim 1 in which the metal M is nickel.

8. The method of Claim 1 in which the metal M is selected from the group consisting of nickel, nickel-copper, nickel-zinc, nickel, sodium-oxide, nickel-cobalt, cobalt-iron, nickel-iron, nickel-tantalum, chromium, nickel-chromium, cobalt, iron, nickel-tin, nickel-chromium oxide.

9. The method of Claim 1 in which terminations comprising metal M are applied to outer surfaces of the green ceramic body and make connections to said terminal extensions.

10. A monolithic capacitor having a sintered unitary body of titanate ceramic and electrodes embedded in the body, the electrodes being of metal M, where M is selected from the group consisting of transition metals and/or transition metal alloys, the skins of the electrodes having been oxidized and reacted with the ceramic to prevent conversion of the ceramic to the semi conductive state and the cores of the electrodes remaining in the metallic state.

11. The capacitor of Claim 10 in which the ceramic comprises barium titanate.

12. The capacitor of Claim 10 in which the electrodes have terminal extensions with alternate extensions extending to different outer surfaces of the body and a shield element of metal M is interleaved between alternate extensions and is spaced from the electrodes and extensions.

13. The capacitor of Claim 10 in which a shield layer of metal M in the outer part of said green ceramic body overlaps and shields the immediately underlying electrode.

14. The capacitor of Claim 10 in which the reaction between the electrodes and the ceramic is in accordance with the equation

15. The capacitor of Claim 10 in which the oxygen partial pressure is greater than 10-7 atmospheres.

16. The method of Claim 9 in which the metal M comprises nickel.

17. The capacitor of Claim 10 in which the metal M is selected from the group consisting of nickel, nickel-copper, nickel-zinc, nickel-sodium oxide, nickel-cobalt, cobalt-iron, nickel-iron, nickel-tantalum, chromium, nickel-chromium, cobalt, iron, nickel-tin, nickel-chromium oxide.

18. The capacitor of Claim 10 in which terminations comprising metal M applied to outer surfaces of the green ceramic body make connections to said terminal extensions.

19. The capacitor of Claim 18 in which the metal M
comprises nickel.

20. The capacitor of Claim 10 in which the metal M
comprises nickel.