CA1038153A

CA1038153A - Pyrochlore-related oxides containing copper and/or silver and resistor compositions thereof

Info

Publication number: CA1038153A
Application number: CA190,868A
Authority: CA
Inventors: Robert J. Bouchard; Donald B. Rogers
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1973-01-26
Filing date: 1974-01-24
Publication date: 1978-09-12
Also published as: GB1447414A; US3896055A; DE2403667C3; FR2215403A1; DE2403667A1; NL158010B; DE2403667B2; IT1007117B; JPS5444400B2; FR2215403B1; JPS49111198A; NL7401057A

Abstract

Abstract of the Disclosure Novel polynary oxides of pyrochlore-related crystal structure having Cu and/or Ag ions in the A-site. Powder com-positions comprising such novel oxides and dielectric material plus optional components, useful for forming thick-film printed resistors on dielectric substrates; also resistors formed by firing such powder compositions.

Description

The novel compounds of this lnvention, and powder compc-sltlons containlng the same9 are lmprovements over those dlsclDsed - ~nd clalmed ln Bouchard U.S. Patent 3,583,931, whlch teaches the bene~lts of bismuth-snd ruthenlum and/or lrldlum ln polynary o~ides havlng pyrochlore-related crystal structure, for electri-cal reslstor appllcations Hoffman U.S. Patent 3,553,109 teaches reslstor composltions comprising such polynary oxldes (and re-lated polynary oxldes) plus inorganic blnder and rlnely dlvlded noble metals. Reslstor compositlons according to these teachlngs have en~oyed considerable com~erclal success because of the ex-cellent control they o~er ln provlding a range of reslstors wlth reproduclble values Or reslstlvlty, little afrected by tempera-ture or humidlty ln use, and readlly prlnted and flred on dielec-trlc supports.
A primary means of establishing the resistivity of a f~red resistor according to the teaching of Hoffman is to adjust the relative proportions of polynary oxide, noble metal, and inorganic binder in the composition. In general, the unusual properties of the resistor are adjusted toward hi~her resistivities by increasing the proporSion of binder and adjusted to~tard lower rcsistivities by increasing the proportion of finely divided noble metal. However~ increas-~n~ly large proportions of noble metal to polynary oxide also produce an increase in TCR (temperature coefficient of re-sistance) and obviate ~.qny of the advantages which have led to the gradual replacement of noble metal/glass compositions (such as the nalladi~/silver/~lass compositions of D'Andrea U. S. Patent 2,92~,5~0) by the more sophisticated polynary oxide containing compositions.
Chemical substitution in the polynary oxide itself has been investigated as a method for adjusting electrical

- 2 -" ~ ' 103~g153 properties. Thus the Bouchard and Hoffman patents cited above con-template, e.g., the substitution of yttrium, thalllum~ lndium, cadmium, lead or the rare earth metals of atomlc number 57-71 inclusive for some of the bismuth ln B12Ru207 sind Bi2Irz07; and platlnum, titanium, tin, chromium, rhodium, rhenlum, zirconium, antimony or germanium for some of the ruthenlum or irldlum. There is a need, however, for compositlons capable of produclng resls-tlvlties that are 6ubstantially lower than those obtalned with B12(Ru,Ir)207, whlle malntalnlng the de6irably flat temperature response. Such low resistlvities are often below 10 ohm8/square, and are preferably ln the range 1-5 ohms/square.
Summary of the Invention A portlon of the blsmuth ln B12(Ru,Ir)207 and lts modlfled oxldes can be replaced by the lon~ Ag and Cu, wlth retention of the pyrochlore-related crystal structure. Ag is unlvalent and Cu 1~ presumably unlvalent, although lt is possible that some dlvalent Cu may be present. Cu and/or Ag produces a level of electrlcal conductlvlty that extends the utility of previously known compositlons.
The polynary oxides of the invention are electrically conductive oxides of pyrochlore-related crystal structure having the formula MxM2-xM27-z wherein:
(1) M is at least one of Ag or Cu;
(2) M' is Bi or a mixture of at least one half Bi plus up to one half of onc or more cations from among (a) bivalent Cd or Pb and (b) trivalent Y, Tl, In and rare earth ~ metals of atomic number 57-71~ inclusive;

(3) M" is at least one of (a) Ru, (b) Ir, and (c) a mixture of at least three-fourths of at least one of Ru and Ir and up to one-fourth of at least one of Pt, Ti and Rh;

(4) x is in the range 0.10 to 0,60; and

(5) z is in the range 0.10 to 1.0, and is ~-equivalent to the sum of monovalent cations M and half of divalent cations in the polynary oxide.
Preferred polynary oxides are those wherein X is in ~--the range 0.10 to 0.5, and include Ago 5Bil 5Ru206 5 gO-5 0~75Bio~75RU206~s~ Ago.5Gdo 5BiRU26 5 and CuO 5Bil.sRU206.5.
Also a part of this invention are improved powder compositions useful for producing on dielectric substrates thick-film (printed) resistors of low resistivity. Powder compositions of polynary oxides plus dielectric material are known, with optional constituents such as noble metal powders (platinum, gold, etc.); and blnary oxldes (Co304, etc.) as disclosed in Hoffman, U. S. Patent 3,553,109; CdO as disclosed in Schubert U. S. Patent 3,560,410; and inert liquid vehicle. The improved powder compositions of this invention are those wherein the polynary oxide is a copper and/or silver-containing polynary oxide of this invention as described above.
Also of this invention are electrical elements, such as resistors comprising a dielectric substrate on which - : . ~ '-- , ~ , 103~1S3 such powder compositions have been deposited (as by known screen or stencil printing techniques) and then fired (sintered) to produce an electrically continuous unit.
Detailed Description of the Invention In its simplest embodiment this invention consists of the pyrochlore-type oxides of formula MXBi2 x(Ru,Ir)207_z whereln improved conductivity results from the replacement Or part of the Bi lons by ions of Ag or Cu. The univalent ions Ag, Li and Na have heretorore been known in certaln electrically insulating pyrochlore-type structures. Cu has not been known to participate in compositions of pyrochlore-type structures.
In the general pyrochlore formula A2+3B2 7 re-placement of a trivalent cation by a univalent one requires that stoichiometry be maintained by an equivalent oxygen de-ficiency (z = x). In the more complex situation where both univalent and bivalent cations are substituted, stoichiometry will require a formula Ax Ay A2 x yB2 7 z where z = x + y/2-It will be appreciated furthermore that small departures from exact stoichiometry may frequently be expected when a small number of ionic vacancies may exist with equivalent charge compensation by neighboring ions of variable valence.
Thus the essentlal character of thls invention typified by the slmple formula MxBi2_x(Ru,Ir)207_z also embraces the further substitution for Bi and (Ru,Ir) disclosed in Bouchard U.S. Patent 3,583,931 cited above, as well as the much smaller variation6 that result from well recognized crystal de-fects. A substantlally greater degree of substitution for bis-muth or a substantially greater oxygen deficiency z than given in the preceding formula or the claims are not conducive to obtaining a single-phase pyrochlore structure which makes :' po~sible the hi&h conductivity of these oxide compositions.
On-the other hand it will be appreciated that very minor amounts of the univalent metals Ag and Cu mi~ht be sub-stituted into the known pyrochlore compositions without pro-ducing a substantial effect on the propertiesO
As indicated above, while it is thought that the copper ions in the polynary oxides of the present inventicn are univalent9 this is not limiting. In fact9 it is possible that some of the copper may be divalent. Likewise, while heretofore it has not been possible to substitute more Ag or Cu in the pyrochlore than the amount claimed, it is recognized that under different conditions greater amount of Ag or Cu might be possible.
~: The polynary oxides of this invention are prepared by heating together the requisite oxides or the readily oxidizable metals or salts which provide a source of the particular elements. Reaction should be carried out under oxidizin conditions at a temperature ranging from about 6000C to about 1200C. Direct firing in air at ordinary pres-sure is usually most convenient, although an atmosphere ofoxygen or super-atmospheric pressures may be advantageous if oxidizable metals in finely divided form are used as a source of the requisite elements. As a source of the uni-valent cation essential to this invention finely-divided silver or copper may be used~ but repeated grinding;and firing in an oxygen-rich environment should then be used to insure complete oxidation. The preferred source Or silver is AgN03 which is easily converted to the oxide under firing conditions.
Cu20 is preferred as the source of univalent copper. Thorough grinding together of the reacting components assists in pro-moting complete react~on which is usually obtained in times

- 6 -between an hour or ~es; ~e.g., 15 min.~ and a day. Sllica or porcelaln vessel~ may be used, but Pt ~essels are pre-ferred at high temperature to avold any contamlnatlon. The completion of reactlon ls convenlently Judged by obtalnlng a slngle phase X-ray dlffractlon pattern corresponding to thc pyrochlorç structure. Electrical conducti~ity may be determ~ned on presced compact~ of the powdered oxide or, more functlonally, on composites Or the oxide product wlth low meltlng glasses ln the proportion desired to form electrlcal resistor elements.
The reslstor compostlons of the present invention are characterlzed ln that some or all of the polynary oxide ln polynary oxlde/dlelectric powder compositions 18 the Ag or Cu polynary oxlde o~ the present lnventlon. The novelty hereln resldes ln the use of these novel polynary oxldes.
Optional addltlves may be added to the powder composltlons, such as those dlsclosed in Schubert U.S. Patent 3,560,410;
Hof~man U.S. Patent 3~553,109; Popo~ich U.S. Patent 3,630,969; and Bouchard U.S. Patent 3,681,262.
Normally the powder composltions contain 5-90% polynary oxide and 10-95% dlelectrlc materlal~ the relatlve proportlons aelected dependlng upcn electrlcal propertles deslred in the r nal reslstor. m e presence (and amount) of optlonal addltlve~
are determlned by slmllar conslderatlons. Generally, up to lO~
or optlonal binary oxlde may be present (CdO~V205,Cr203~Mh203, Fe304,Co304,NiO, and CuO~ and up to 69% noble metal powder. When the powder composltlon ~8 to be dlsper8ed ln an lnert ll-quld vehlcle, the type and amount of vehlcle is a matter of selectlon by one skilled ln the art, the amount of vehicle generally being 10-90% of the resultlng dlsperslon.

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m e dielectric material may be any inorganic material which serves to bind the polynary oxide(s) and additives, where present, to the substrate. The inorganic binder can be any of the glass frits employed in resistor compositions for this general type. Such frits are generally prepared by melting a glass batch composed of the desired metal oxides, or compound~ which will produce the glass during melting, and pouring the melt into water. The coarse frit is then milled to a powder of the desired fineness. Larsen and Short U.S.
Patent 2,822,279, and Hoffman U.S. Patent 3,207,706, each describe glass frit compositions which can be employed either alone or in combination wlth glass wetting agents such as bismuth oxide. Typical frlt compositions usable as binders in the composition~ of this lnvention include borosilicate glasses such as lead borosilicate, cadmium borosilicate and similar borosilicates. Also, mixtures of various inorganic binders may be used.
Noble metals comprise the free metallic component of the resistor compositions of this lnvention. These include gold, silver, platinum and palladium.
m e compositions are used to produce thick film re-sistors as disclosed in the Hoffman, Schubert, Bouchard and Popowich patents cited above; printing may be by conventional screen or stencll techniques wlth optional inert liquid vehicle, as therein described; firing techniques are similarly described therein.
Generally, application of the resistor composition in paint or paste form to the sub6trate may be ef~ected in any desired manner. It will generally be desired, however, to effect the application in precise pattern form, whlch can be readily done in applying well-known screen stencil techniques or methods. The resulting print or pattern will ' - ~0381S3 then be fired in the usual manner at a temperature from about 650-950C in an air atmosphere employing the usual flring lehr.
The components of the powder compo~ltion are flnely divided 80 that they may be screen printed; generally, the average particle size is less than 20 microns.
In the examples and elsewhere ln the specification and ciaims, all parts, percentages and ratios are by weight, unless otherwise stated. X-ray measurements were made using a NORELCO* dlfrractometer.
Resistances were determined with a Non-Linear Systems Series X-l Ohmmeter.
Flred resistor film thicknesses were measured using a Brush Instrument~ Div. (Clevite Corp.) Surfanalyzer. m e thlckness, nominally one mil, was normally less than one mil; sheet re~istivity (ohm/square/mil) was determined by multiplying the resistance of the 100 x 200 mil resistor pald by the actual thlckness, and dividing by two. ~-~xample 1 Attempt to prepare "AgBlRu206~. Surflcient reactant6 are fired to produce AgBiRU206, but a hetergeneous product was obtained. 0.2666 g ~inely divided Ag, 0.5757 g Bi203, and O.6577 g Ru02 were ground together in an automatic mortar for 30 mlnutes, pressed into a pellet, and fired in a porcelain cru- -clble open to alr ~or 16 hours at ô50C. The hard, black pellet whlch resulted was shown by X-ray to contaln a phase having a cublc pyrochlore-type st~ructure along wlth some remalnlng Ru02 and Ag. A simllar mixture heated in porcelain at 950C for 24 hours ln air produced a blue-black product having a more crystal-line pyrochlore X-ray pattern wlth a cubic cell constant, aO, ~ denotes trade mark _ g _ .

about 10.24A, signi~lcantly smaller than that known for Bi2Ru207 (10.30A). Thus, AgBiRu206 was not formed under these condltlons, although lt is pos~ible that such polynary oxides where "x" i8 1.0 can be formed under more stringent and/or dl~erent reactlon condltlons.
Exam~le 2 Ag005Bil~5RU26~5~ 0.1~1~ g- AgN03, 0.74~4 g-Bi203, and 0~ 569~ g. Ru02 were ground together in an auto-matic mortar for 30 minutes, pressed into a pellet, and fired to 950C for 16 hrs. in an open Pt crucible in air. The black product had a single phase x-ray pattern corresponding to the pyrochlore structure; the cell constant9 aO, was 10.27.~ .
Exam~le 3 0.5Bil.5Ru206.5- 0-0~23 g. CU209 0-~045 g-~i203, and 0.6127 g. Ru02 were ground together in an automatic mortar for 30 minutes, pressed into a pellet, and fired to 950C for 16 hrs. in an open Pt crucible in air. The black product had a single-phase pyrochlore pattern, with a cell constant, aO, of 10.21A.
ExamDle l~
A~o . scdo . 75Bio . 75~u2o6 . 5 - 0- 2566 g. A~M03, 0.410~ g .
Gd203, 0.52~0 g. Bi203, and 0.~043 g. Ru02 were ground to-~ether in an automatic mortar for 30 minutes, and fired to 1100C in air in an open Pt crucible. The black product had j an x-ray pattern corresponding to the pyrochlore structure ¦ (cell constant, aO, of 10.26A) along with a small amount I of impurity.
¦ Rxam~le 5 Ago 5Gdo 5BiRU206 5. 0.2517 g. AgN03, 0.26~6 g.
Gd203, 0.6905 Bi203, and 0.7~ g. Ru02 were ground together .

lQ3~3 for 30 minutes in an agate mortar and pestle. The ground mixture was fired to 1000 for 16 hours in an open Pt cruclble in air.
The black product had a pyrochlore-related X-ray pattern (cell constant, aO, 10.25A) plus a trace of unreacted Ru02.
Exam~le 6 The polynary oxides of the present lnventlon are useful as components of screen-printable resistor compo61tions, as shown herein. Resistor compositionæ were prepared uæing the polynary oxldes of Examples 2, 3 and 4 (see Table I). The polynary oxides (and optional free metal powder), total conductlve phase 66 part~
were mixed wlth 14 parts powdered glass frit and 20 partæ of an organlc vehicle composed of 90~ ethylcelluloæe and 10% terplneol and screen-printed onto prefired al~ na substrates; the re-sultant structure waæ drled at 100C. for 10 minutes, fired slowly to 850C. for 10 mlnuteæ and then ælowly brought back to room temperature. The complete cycle took one hour. The re-sultant reslstor pad was 100 x 200 mlls and about 1 mil thick.
The glass frit consiRted of (wt.%) 25.7% PbO, 20.1% B203, 19.7%
S102, 7.9% A1203, 24.1% ZnO, 2.2~ ZrO2, and 0.3% Na20. For com-paratlve purposes the pyrochlore Bi2Ru207 descrlbed ln U.S.3,583,931 was also tested, wlth and wlthout free metal powder.

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tt - 12 -1()38153 The data reported in Table I are the a~erage of quadrupli-cate sampl~sO The best conductivity (lowest resisti~ity) was obtained using as conductor compositions the products of this invention9 (b), (c) 5 and ~f) in Table I. The con-ducti~ity is very ~uch greater than for Bi2Ru207 (com-position (a) of Table I), andS surprisingly, greater even than ~Ihen an equivalent amount o~ either metallic element, both excellent electrical conductors, is directly added to the composition as in compositions (d) and (e) of Table I.
The change in resistance (~ R), after standing for 42 hours ur.cer no electrical load at ambient temperature and ~ humidity can be quite small~ as seen in Table I.
j E~ample 7 The tests in Example 6 were duplicated9 except t~at the glass was (wt. %) 43.5,~0 Pb304, 4.3~ A1203, 9.g~o -~
CaO, 409~ B203, and 37. ~$ sio2. 60 parts conducti~e powder were used, 17 parts glass were used, and 23 parts ~ehicle were used (see T-~le II) .

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m ~ m eC m c~ s 1031~1S3 The results of Example 7 are similar to those of Example 6, except that the resistivity is lower with the glass of Example 7. It should be emphasized that these low resistivities are obtained without the addition of any noble precious metal powder, a result heretofore unobtainable.
It is also obvious that there are a wide number of glass com-positions, some of which may give even lower resistivities.
The polynary oxides of the present invention may be used as resistors, electrodes, etc. An advantage of the compounds of this invention is that a higher percentage of glass can be tolerated in compositions thereof to obtain the same resistivity as conductor compositions known in the art.
This is an advantage because in general, higher glass contents give smoother, more stable resistors.

.:. , .

Claims

THE INVENTION CLAIMED IS:

1. Electrically conductive polynary oxides of pyrochlore-related crystal structure having the formula MxM'2-xM"2O7-z wherein:
(1) M is at least one of Ag or Cu;
(2) M' is Bi or a mixture of at least one half Bi plus up to one half of one or more cations from among (a) bivalent Cd or Pb and (b) trivalent Y, Tl, In and rare earth metals of atomic number 57-71, inclusive;
(3) M" is at least one of (a) Ru, (b) Ir, and (c) a mixture of at least three-fourths of at least one of Ru and Ir and up to one-fourth of at least one of Pt, Ti and Rh;
(4) x is in the range 0.10 to 0.60 and (5) z is in the range 0.10 to 1.0, and is equiva-lent to the sum of monovalent cations M and half of divalent cations in the polynary oxide.

2. Polynary oxides of Claim 1 wherein X is in the range 0.10 to 0.5.

3. Polynary oxides of Claim 2 wherein M is Ag.

4. Polynary oxides of Claim 3 having the approxi-mate formula Ag0.5Bi1.5RU2O6.5.

5. Polynary oxides of Claim 3 having the approximate formula Ag0.5Gd0.75Bi0.75Ru2O6.5.

6. Polynary oxides of Claim 3 having the approximate formula Ag0.5Gd0.5BiRu2O6.5.

7. Polynary oxides of Claim 2 wherein M is Cu.

8. Polynary oxides of Claim 7 having the approximate formula Cu0.5Bi1.5Ru2O6.5.

9. In a powder composition comprising polynary oxides of a pyrochlore-related crystal structure and dielectric ma-terial, useful for producing resistors on dielectric substrates, improved compositions useful for producing low resistivity resistors wherein the polynary oxide is that of Claim 1.

10. In a powder composition comprising polynary oxides of a pyrochlore-related crystal structure and dielectric ma-terial, useful for producing resistors on dielectric substrates, improved compositions useful for producing low resistivity resistors wherein the polynary oxide is that of Claim 2.

11. In a powder composition comprising polynary oxides of a pyrochlore-related crystal structure and dielectric material, useful for producing resistors on dielectric sub-strates, improved compositions useful for producing low resis-tivity resistors wherein the polynary oxide is that of Claim 3.

12. In a powder composition comprising polynary oxides of a pyrochlore-related crystal structure and dielectric material, useful for producing resistors on dielectric sub-strates, improved compositions useful for producing low resistivity resistors wherein the polynary oxide is that of Claim 7.

13. An electrical element comprising a dielectric substrate having fired thereon the composition of Claim 9.

14. An electrical element comprising a dielectric substrate having fired thereon the composition of Claim 10.

15. An electrical element comprising a dielectric substrate having fired thereon the composition of Claim 11.

16. An electrical element comprising a dielectric substrate having fired thereon the composition of Claim 12.