U.S. patent application number 12/105683 was filed with the patent office on 2009-10-22 for non-lead resistor composition.
Invention is credited to Kenneth Warren Hang, Paul Douglas VerNooy, Alfred T. Walker.
Application Number | 20090261306 12/105683 |
Document ID | / |
Family ID | 40791418 |
Filed Date | 2009-10-22 |
United States Patent
Application |
20090261306 |
Kind Code |
A1 |
VerNooy; Paul Douglas ; et
al. |
October 22, 2009 |
NON-LEAD RESISTOR COMPOSITION
Abstract
A non-lead composition for use as a thick-film resistor paste in
electronic applications. The composition comprises particles of
Li.sub.2RuO.sub.3 of diameter between 0.5 and 5 microns and a
lead-free frit. The particles have had the lithium at or near
primarily the surface of the particle at least partially exchanged
for atoms of other metals.
Inventors: |
VerNooy; Paul Douglas;
(Hockessin, DE) ; Walker; Alfred T.; (Oxford,
PA) ; Hang; Kenneth Warren; (Hillsborough,
NC) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
40791418 |
Appl. No.: |
12/105683 |
Filed: |
April 18, 2008 |
Current U.S.
Class: |
252/518.1 |
Current CPC
Class: |
C04B 41/86 20130101;
C01G 55/00 20130101; C04B 41/009 20130101; C01P 2002/54 20130101;
H01B 1/22 20130101; C04B 2111/00844 20130101; C04B 41/52 20130101;
C01P 2004/61 20130101; H01C 17/06533 20130101; H01C 7/003 20130101;
C04B 41/5022 20130101; C01G 55/001 20130101; C01P 2004/62 20130101;
C04B 41/90 20130101; C03C 8/14 20130101; C01P 2002/72 20130101;
C04B 41/009 20130101; C04B 35/10 20130101; C04B 41/5022 20130101;
C04B 41/4539 20130101; C04B 41/4572 20130101; C04B 41/5027
20130101; C04B 41/52 20130101; C04B 41/4572 20130101; C04B 41/5116
20130101; C04B 41/52 20130101; C04B 41/4539 20130101; C04B 41/4572
20130101; C04B 41/5027 20130101 |
Class at
Publication: |
252/518.1 |
International
Class: |
H01C 1/00 20060101
H01C001/00; H01B 1/02 20060101 H01B001/02 |
Claims
1. A composition comprising particles of Li.sub.2RuO.sub.3 in which
Li atoms have been exchanged for Al, Ga, K, Ca, Mn, H, Na, Cr, Co,
Ni, V, Cu, Zn, or Ti atoms, or a combination thereof.
2. A composition according to claim 1 in which at least 50 mol % of
the Li atoms have been exchanged.
3. A composition according to claim 1 in which at least 75 mol % of
the Li atoms have been exchanged.
4. A composition according to claim 1 wherein a Li.sub.2RuO.sub.3
particle comprises first and second layers, and more Li atoms are
exchanged in the first layer than in the second layer.
5. A composition according to claim 4 wherein more than 80% of the
Li atoms in the first layer are exchanged.
6. A composition according to claim 4 wherein less than 20% of the
Li atoms in the second layer are exchanged.
7. A composition according to claim 4 wherein the first and second
layers are adjacent.
8. A composition according to claim 1 wherein an atom exchanged for
Li has a 2+ valence or a 3+ valence.
9. A composition according to claim 1 wherein an atom exchanged for
Li comprises one or more members of the group consisting of Al, Cu,
Mg, Zn, Ga, and Mn.
10. A composition according to claim 1 that is described by formula
as follows: M.sup.+1.sub.xM.sup.+2.sub.zLi.sub.2-x-2y-3zRuO.sub.3
where (x+2y+3z).ltoreq.1.5, and where M is selected from one or
more members of the group consisting of Al, Ga, K, Ca, Mn, Na, H,
Cr, Co, Ni, V, Cu, Zn, and Ti.
11. A composition according to claim 1 further comprising one or
both of an alkali metal, zinc alumino-borosilicate frit, and an
alkaline-earth metal, zinc alumino-borosilicate frit.
12. A resistor comprising a composition according to claim 11.
13. A resistor according to claim 12 that has a sheet resistance of
about 100 kilohms per square to about 10 megohms per square, and/or
a TCR of .+-.100 ppm/.degree. C.
14. An electronic device comprising a resistor according to claim
12.
15. A composition according to claim 10 further comprising one or
both of an alkali metal, zinc alumino-borosilicate frit, and an
alkaline-earth metal, zinc alumino-borosilicate frit.
16. A resistor comprising a composition according to claim 15.
17. A resistor according to claim 16 that has a sheet resistance of
about 100 kilohms per square to about 10 megohms per square, and/or
a TCR of .+-.100 ppm/.degree. C.
18. An electronic device comprising a resistor according to claim
16.
19-20. (canceled)
21. A composition comprising particles of Li.sub.2RuO.sub.3 in
which Li atoms have been exchanged for Al, Ga, K, Ca, Mn, Fe, H,
Na, Cr, Co, Ni, V, Cu, Zn, or Ti atoms, or a combination thereof,
wherein a Li.sub.2RuO.sub.3 particle comprises first and second
layers, and more Li atoms are exchanged in the first layer than in
the second layer.
22. A composition according to claim 21 in which at least 50 mol %
of the Li atoms have been exchanged.
23. A composition according to claim 21 wherein more than 80% of
the Li atoms in the first layer are exchanged.
24. A composition according to claim 21 wherein less than 20% of
the Li atoms in the second layer are exchanged.
25. A composition according to claim 21 wherein the first and
second layers are adjacent.
26. A composition according to claim 21 further comprising one or
both of an alkali metal, zinc alumino-borosilicate frit, and an
alkaline-earth metal, zinc alumino-borosilicate frit.
27. A resistor comprising a composition according to claim 26.
28. An electronic device comprising a resistor according to claim
27.
Description
TECHNICAL FIELD
[0001] This invention relates to a composition for use in the
production of resistors for electronic applications. The
composition is prepared from non-lead materials that include
lithium and ruthenium, and may be prepared in the form of a
thick-film paste.
BACKGROUND
[0002] Existing conductive intermediates (such as ruthenium
dioxide, silver/palladium solid solutions, and bismuth ruthenate)
combined with non-lead frits can form the low-resistance end of an
essentially lead-free resistor system (10 to 1000 ohms), while
existing conductives (such as ruthenium dioxide, bismuth ruthenate
and strontium ruthenate) with non-lead frits could be used to make
a 10 kilohm member. Ceramic resistor systems commonly include
individual decade members which range between 10 ohms/square and 1
megohm/square. Resistors in these series must be insensitive enough
to variations in thermal process conditions to be used on high
speed manufacturing lines. Currently, most commercial resistor
systems in the 100 kilohm to 1 megohm range utilize lead-containing
frits and/or lead-containing conductive phases, such as
formulations containing either lead ruthenate, or RuO.sub.2 and
high-lead frits.
[0003] Fukaya and Matsuo (1997, 97 ISHM Symposia Proceedings, pp.
65-71) describe a RuO.sub.2/sodium alkaline-earth
alumino-borosilicate frit resistor system that can be fired on
alumina substrates or an LTCC system as described therein. The
resistance of the system extends generally from 10 ohms to 500
kilohms. .+-.100 ppm/.degree. C. TCRs are reported from 100 ohms to
500 kilohms.
[0004] Hormadaly (2002, 02 IMAPS Symposia Proceedings, pp. 543-547)
describes resistors composed of M.sub.2-xCu.sub.xRuO.sub.7-.beta.,
where x is 0.2 to 0.4, .beta. is 0 to 1, and M is a rare earth
element. An example of a 6.15 megohm thick-film resistor is
given.
[0005] Atsushi et al (2002, JP 2002-101903) describe a resistor
composed of RuO.sub.2 and a bismuth-bearing frit with or without
bismuth ruthenate.
[0006] JP 2003-197405 describes RuO.sub.2 and several ruthenates
(such as CaRuO.sub.3) combined with frits composed of many alkali
and alkaline-earth borosilicates, and many transition metal
drivers.
[0007] There nevertheless remains a need to find a non-lead
conductive-oxide/frit combination that could provide resistor
compositions in the 100 kilohm to 10 megohm range, and preferably
with .+-.100 ppm/.degree. C. TCRs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows X-ray powder diffraction patterns for
Li.sub.2RuO.sub.3 with varying amounts of Li replaced with Al.
SUMMARY
[0009] In one embodiment, this invention provides a composition
that includes particles of Li.sub.2RuO.sub.3 in which Li atoms have
been exchanged for Al, Ga, K, Ca, Mn, Fe, H, Na, Cr, Co, Ni, V, Cu,
Zn, or Ti atoms, or a combination thereof.
[0010] In another embodiment, this invention provides a composition
described by formula as follows:
M.sup.+1.sub.xM.sup.+2.sub.yM.sup.+3.sub.zLi.sub.2-x-2y-3zRuO.sub.3
where (x+2y+3z).ltoreq.1.5, and where M is selected from one or
more members of the group consisting of Al, Ga, K, Ca, Mn, Fe, Na,
H, Cr, Co, Ni, V, Cu, Zn, and Ti.
[0011] In a further embodiment, the above described compositions
may be admixed with one or both of an alkali metal, zinc
alumino-borosilicate frit, and an alkaline-earth metal, zinc
alumino-borosilicate frit. The resulting composition may be
fabricated as a resistor that has desirable sheet resistance and
TCR properties, and the resistor so obtained may be used in an
electronic device.
[0012] In yet another embodiment, this invention provides a method
of preparing a Li.sub.2RuO.sub.3 composition by (a) providing
Li.sub.2RuO.sub.3 particles having an average particle diameter
between about 0.5 and about 5 microns; and (b) contacting the
Li.sub.2RuO.sub.3 particles with a solution comprising ions
prepared from one or more of the elements selected from the group
consisting of Al, Ga, K, Ca, Mn, Fe, Na, H, Cr, Co, Ni, V, Cu, Zn,
and Ti.
[0013] This invention provides a composition including particles of
Li.sub.2RuO.sub.3 particles wherein the Li atoms at or near the
particle surface have been replaced with atoms of other elements.
Resistors comprising this material can be made which show high
resistance and .+-.100 ppm/.degree. C. TCRs without the use of
toxic elements such as lead or cadmium.
[0014] The discovery that the conductive compositions disclosed
herein are suitable to shift the electrical resistance to a greater
extent than RuO.sub.2 at the same conductive volume in a fired
ceramic resistor formulation (employing, for example, glass,
conductive and medium) makes possible a six- or seven-decade value
resistor system in a lead-free chemistry.
DETAILED DESCRIPTION
[0015] This invention provides a chemically-modified lithium
ruthenate conductive oxide. This material, when combined with an
alkali- and/or alkaline-earth alumino-borosilicate frit, provides a
composition that may be fabricated as an unencapsulated resistor
having a desirably high-ohm sheet resistance, such as a sheet
resistance in the range of about 100 kilohms per square to about 1
megohm per square, preferably in the range of about 100 kilohms per
square to about 5 megohms per square, and more preferably in the
range of about 100 kilohms per square to about 10 megohms per
square. These resistors may also have TCR (thermal coefficient of
resistance) values of .+-.100 ppm/.degree. C.
[0016] The structure of Li.sub.2RuO.sub.3, as discussed in James
and Goodenough, Journal of Solid State Chemistry 74, pp. 287-294,
1988, is composed in general of two adjacent, alternating layers,
one layer containing only Li ions and the other containing both Ru
and Li ions (ignoring the oxygen atoms). The Li-only layer is
believed to contain about 75 mole % of the lithium in the
structure, and these lithium ions may be readily removed via ion
exchange. Although the lithium ions are mobile in the Li layer of
Li.sub.2RuO.sub.3, cations which have higher valence than Li (such
as Mg.sup.-2 or Al.sup.+3) are less mobile because of their higher
charge and concomitant stronger bonding. Thus, while the invention
is not limited to any particular theory of operation, it is
believed that the exchanging ion, such as magnesium, first
displaces lithium ions at or near the surface of the particle, and
in the layer that is Li-only, and remains in essentially that
position. The more magnesium ions that are available to exchange
with the lithium ions, however, the deeper into the particle the
magnesium ions will travel until all the exchangeable lithium has
been removed or the magnesium ions in solution are exhausted. When
Li ions in the Li-only layer are replaced by an amount of
exchanging ions that is not significantly greater than the amount
of Li ions in that layer, this tends to produce from the Li layer a
surface shell of the exchanging ion, and produce an internal core
of remaining lithium ions.
[0017] Because the lithium ions are being substituted for ions of
different charges and/or charge densities, the layer spacing in the
ion-exchanged portion of the Li.sub.2RuO.sub.3 crystal will change.
Cations with a higher charge, such as magnesium and aluminum, will
tend to decrease that layer spacing. The location of the (002) line
in the X-ray powder diffraction pattern of a composition in which
Li is replaced by Al reflects this shrinkage by shifting to higher
2.theta., as shown in FIG. 1. In contrast, cations with lower
charge densities, such as Na, will shift the (002) line to lower
2.theta.. Note in FIG. 1 how the largest (100%) peak in the
starting material [the (002) peak at .about.18.degree. 2.theta.]
shrinks with the extent of the ion exchange, and the new peak
(.about.19.degree. 2.theta.) grows in. Once all the lithium in the
lithium-only layer is removed (at about 75 mole % Li ion exchange),
the original 100% peak is gone.
[0018] To effect the exchange of Li ions in Li.sub.2RuO.sub.3,
particles of Li.sub.2RuO.sub.3 are preferably milled to a diameter
in the range of between about 0.5 and about 5 microns, which is a
size range that is generally suitable for later screen printing to
form a resistor. Any wet or dry milling technique can be used to
effect size reduction of the Li.sub.2RuO.sub.3 particles, such as
vibratory milling, ball milling, hammer milling, bead milling, rod
milling, jet milling, or disk milling. The milling step can be
performed sequentially prior to, or simultaneously while, the ion
exchange step is being performed. The milling and ion exchange
steps can be performed in separate vessels, or in the same
vessel.
[0019] In a particularly preferred embodiment, however, in order to
preserve what is essentially a core-shell arrangement between a
formerly Li-only layer that has been exchanged, the adjacent Li/Ru
layer, and the adjacent, next alternating Li-only layer, the
milling of the particles should be complete, or substantially
complete, before the ion-exchange step. If the ion-exchange step
takes place before the milling step, the lithium-containing cores
of the particles will be broken open and exposed to direct contact
with the frit, and a resistor made from the resulting composition
will very likely not have the desirable properties that
characterize resistors made from the compositions hereof.
[0020] In an alternative embodiment, this invention provides a
composition described generally by formula as follows:
M.sup.+1.sub.xM.sup.+2.sub.yM.sup.+3.sub.zLi.sub.2-x-2y-3zRuO.sub.3where
(x+2y+3z).ltoreq.1.5, and where M is selected from one or more
members of the group consisting of Al, Ga, K, Ca, Mn, Fe, Na, H,
Cr, Co, Ni, V, Cu, Zn, and Ti.
[0021] The formulae shown above describes each and all of the
separate, individual compounds that can be formed in that formula
by (1) selection from within the prescribed range for one of the
variable ions or numerical coefficients while all of the other
variable ions or numerical coefficients are held constant, and (2)
performing in turn the same selection from within the prescribed
range for each of the other variable ions or numerical coefficients
with the others being held constant. In addition to a selection
made within the prescribed range for any of the variable ions or
numerical coefficients of only one of the members of the group
described by the range, a plurality of compounds may be described
by selecting more than one but less than all of the members of the
whole group of ions or numerical coefficients. When the selection
made within the prescribed range for any of the variable ions or
numerical coefficients is a subgroup containing (i) only one of the
members of the whole group described by the range, or (ii) more
than one but less than all of the members of the whole group, the
selected member(s) are selected by omitting those member(s) of the
whole group that are not selected to form the subgroup. The
compound, or plurality of compounds, may in such event be
characterized by a definition of one or more of the variable ions
or numerical coefficients that refers to the whole group of the
prescribed range for that variable but where the member(s) omitted
to form the subgroup are absent from the whole group.
[0022] During the ion-exchange step, the particles are agitated, by
stirring or milling or other suitable means, in a solution
containing ions of Al, Ga, K, Ca, Mn, Fe, Na, H, Cr, Co, Ni, V, Cu,
Zn, Ti, or mixtures thereof. The ions are obtained by dissolving a
soluble salt of the desired element in a suitable solvent,
preferably water or a mixture of water and a water-miscible
solvent, such as an organic liquid such as methanol. Upon exposure
to the salt solution, lithium atoms within the Li.sub.2RuO.sub.3
particles are replaced with cations from the solution. Suitable
salts may be purchased commercially from suppliers such as Alfa
Aesar (Ward Hill, Mass.), City Chemical (West Haven, Conn.), Fisher
Scientific (Fairlawn, N.J.), Sigma-Aldrich (St. Louis, Mo.) or
Stanford Materials (Aliso Viejo, Calif.). Suitable salts are
nitrates, acetates, chlorides, fluorides, nitrites, sulfates,
carbonates, or others which have solubility in the solvent used.
The amount of lithium that is removed from the starting material
can be controlled in terms of the amount of metal ions provided to
be available for ion exchange, up to and including about 75 mole %
removal of the lithium. Typical removal is in the range of about 25
mole % to about 60 mole % of the amount of Li ions in the starting
material. For example, half a mole of divalent ions will displace a
mole of Li ions, and a third of a mole of trivalent ions will
displace a mole of Li ions. If water is used as the solvent,
protons from the water can also displace some lithium in the
lithium ruthenate. Thus, the lithium removed can be slightly more
than what would be stoichiometrically expected from the quantity of
metal salts used in the ion-exchange process.
[0023] The ion exchange process can be run for a period of time
(typically less than 24 hours) that has been determined, on
average, to be required to obtain ion exchange in the
Li.sub.2RuO.sub.3 composition to a desired extent; or the progress
of the exchange process can be monitored by analysis of the
increasing concentration of lithium in the solution. Such analysis
can be performed, for example, by induction coupled plasma-optical
emission spectroscopy. Alternatively, the depletion of the
ion-exchanging cation, such as magnesium, can be monitored to
signal the end of the process, such as the case where, for example,
the metal salt was the limiting reagent. The sample would then be
washed to remove any remaining salts. Washing can be done by any
convenient means, either in batch or continuous modes, including
centrifugation, decantation, re-suspension, filtration, or
combinations thereof. The washed particles are then dried, and, if
desired, deagglomerated by sieving.
[0024] To make a resistor, dried particles of an exchanged
Li.sub.2RuO.sub.3 composition may be mixed with one or more glass
materials known as frits. Frits suitable for preparation of a
composition from which a resistor may be fabricated herein include
one or both of an alkali metal, zinc alumino-borosilicate frit, and
an alkaline-earth metal, zinc alumino-borosilicate frit, including
without limitation the compositions described in the following
Table 1, but this list should be taken as representative only and
not exhaustive. Frits such as those described in Table 1 may be
acquired commercially from a variety of suppliers such as those
named above.
TABLE-US-00001 TABLE 1 Glass Compositions (all values are in weight
%). No. SiO2 Al2O3 ZnO CuO BaO MgO Na2O Li2O P2O5 B2O3 K2O TiO2 SrO
1 55.14 2.16 28.95 1.72 10.03 2 2 51.42 6.4 28.61 1.7 9.91 1.96 3
35.65 8.75 36.84 1.86 5.98 2.52 4.84 0.23 3.32 4 64 8.5 14.5 1 12 5
61 8.5 14.5 3 1 12 6 68 14.5 8.5 1 8 7 55.98 2.1 28.18 3.94 7.82
1.99 8 61.14 4.56 7.49 2.52 6.8 2.3 15.18 9 60.07 3.67 7.36 6.68
6.03 1.28 14.92
[0025] Frits suitable for use in this invention typically have an
average particle size in the range of about 0.5 to about 1.5 .mu.m,
and preferably in the range of about 0.8 to about 1.2 .mu.m. These
frits are suitable for firing to prepare a resistor at a
temperature in the range of about 800 to about 900.degree. C., and
more typically in the range of about 825 to about 875.degree. C.
Frits suitable for use herein may be produced by conventional
glass-making techniques. Glasses were prepared, for example, in 500
to 1000 gram quantities from metal oxide and carbonate raw
materials. Typically, the ingredients are weighed and mixed in the
desired proportions, and then heated in a bottom-loading furnace to
form a melt in platinum alloy crucibles. Heating is conducted to a
peak temperature (frequently about 1400 to about 1600.degree. C.)
and for a time such that the melt becomes entirely liquid and
homogeneous. The molten glass is quenched between counter-rotating
stainless steel rollers to form a 10 to 15 mil (0.25 to 0.38 mm)
thick platelet of glass. The resulting glass platelets are then
milled (typically in water and then dried) to form a powder with a
d50 (50% volume distribution) between 0.8 and 1.5 microns when
measured with an instrument such as a Microtrac X100 Laser Particle
Size Analyzer (Montgomeryville, Pa.).
[0026] The mixture of exchanged Li.sub.2RuO.sub.3 particles and
frit can be fabricated into a resistor by making a thick-film
paste. Typically, the paste contains conductive particles, glass
powder, and optional additives dispersed in an organic medium to
produce a screen-printable paste. The resistance of individual
resistor pastes can be varied by adjusting the content of the
conducting phase in the resistor compositions, and by varying the
weight ratio of the frits and conductive phases present in the
composition. The content of the conductive phase of a composition
may be adjusted in the same manner as is known for conventional
conductive compositions wherein, for example, the content of Ag/Pd
solid solution powders is adjusted for resistors having a sheet
resistance of less than about 10 ohms/sq., and the content of
RuO.sub.2 is adjusted for resistors having a sheet resistance equal
to or greater than about 10 ohms/sq. Using an exchanged lithium
ruthenate composition as the conductive phase and glass
compositions from Table 1, sheet resistances between 100
kilohms/sq. and 1 megohm/sq. can be achieved with loadings of the
conductive phase in the range of between about 15 and about 20
weight percent of the paste composition. The content of the
conductive phase and the frits together typically constitutes about
70 weight % of the paste composition.
[0027] The inorganic components are typically mixed with an organic
medium by mechanical mixing to form the type of viscous
compositions known as a paste, and an important property of the
paste is that it has a consistency and rheology suitable for screen
printing. A wide variety of inert viscous materials can be used as
the organic medium. The organic medium must be one in which the
inorganic components are dispersible with stability. The
rheological properties of the medium must be such that they lend
properties to the composition useful for printing, including: a
stable dispersion of solids, a viscosity and thixotropy that is
screen printable, ability to wet the substrate and the paste
solids, short drying rate, and stability during firing. The organic
medium used in a thick-film composition formed from a Li.sub.2RuO3
composition hereof is preferably a non-aqueous inert liquid. Use
can be made of any of various organic media, which may or may not
contain thickeners, stabilizers, and/or other conventional
additives. The organic medium is typically a solution of polymer(s)
in solvent(s). Additionally, a small amount of additives, such as
surfactants, may be a part of the organic medium. The most
frequently used polymer for this purpose is ethyl cellulose. Other
examples of suitable polymers include ethyl hydroxyethyl cellulose,
wood rosin, mixtures of ethyl cellulose and phenolic resins,
polymethacrylates of lower alcohols, and monobutyl ether of
ethylene glycol monoacetate. The most widely used solvents found in
thick-film compositions are ester alcohols and terpenes such as
alpha- or beta-terpineol or mixtures thereof with other solvents
such as kerosene, dibutylphthalate, butyl carbitol, butyl carbitol
acetate, hexylene glycol, and high-boiling alcohols and alcohol
esters. In addition, volatile liquids for promoting rapid hardening
after application on the substrate can be included in the medium.
Suitable surfactants for lithium ruthenate-based resistors include
soya lecithin and alkali phosphates. Various combinations of these
and other solvents are formulated to obtain the viscosity and
volatility requirements desired.
[0028] The polymer present in the organic medium is typically at a
content in the range of about 8 weight % to about 11 weight % of
the total composition. A thick-film resistor composition of this
invention may be adjusted to a predetermined, screen-printable
viscosity with the organic medium. The ratio of organic medium in
the thick-film composition to the inorganic components in the
dispersion may be adjusted in terms of the ability of the printing
equipment to handle a particular viscosity, thixotropy, and
volatility as influenced by the kind of organic medium used.
Usually, the dispersion will contain about 70 to about 95 weight %
of inorganic components and about 5 to about 30 weight % of organic
medium in order to obtain good wetting.
[0029] The powders are wetted by the organic medium by mechanical
mixing. Small samples can be hand mixed on a glass surface with a
spatula, but impeller stirrers are typically used for larger
volumes of paste. Final mixing and dispersion of powder particles
is accomplished by the use, for example, of a three-roll mill such
as the Ross (Hauppauge, N.Y.) three-roll mill [floor model with 4
inch (10.16 cm) diameter.times.8 inch (20.32 cm) long rolls]. A
final paste viscosity between about 150 and about 300 Pa-sec. [as
measured, for example, at 10 rpm and 25.degree. C. with a
Brookfield HBF viscometer (Middleboro, Mass.) with #6 spindle] is
suitable for screen printing. Screen printing may be accomplished,
for example, by use of an automatic screen printer (such as those
from Engineering Technical Products, Sommerville, N.J.). Either 200
or 325 mesh stainless steel screens may be used to achieve resistor
dried thickness of 18 microns (on resistors with 0.8 mm length and
width). The resistors may be printed on 1 inch (2.54 cm) squares of
96% alumina substrates. Substrates such as those that are 25 mils
(0.635 mm) in thickness, as are produced by CoorsTek (Golden,
Colo.), may be used for printing. The resistors may be printed on a
pattern of Ag thick-film terminations which have been previously
fired to 850.degree. C. DuPont 5426 terminations that have been
fired using the recommended 30 minute firing profile with 10
minutes at the peak firing temperature (DuPont MicroCircuit
Materials, Wilmington, Del.) are suitable. Resistors may also be
fired at 850.degree. C. using a 30 minute profile with 10 minutes
at the peak temperature. A furnace such as a Lindberg Model 800
(Riverside, Mich.) 10-zone belt furnace with 233.5 inch (593.1 cm)
belt length may be used for all firings.
[0030] The pastes may additionally contain one or more components
selected from CuO, P.sub.2O.sub.5, and TiO.sub.2.
[0031] In various alternative embodiments, this invention provides
a composition comprising particles of Li.sub.2RuO.sub.3 in which Li
atoms have been exchanged for Al, Ga, K, Ca, Mn, Fe, H, Na, Cr, Co,
Ni, V, Cu, Zn, or Ti atoms, or a combination thereof. In the
composition, at least 50 mol % of the Li atoms may be exchanged, or
at least 75 mol % of the Li atoms may be exchanged.
[0032] In the composition, a Li.sub.2RuO.sub.3 particle may
comprise first and second layers, and more Li atoms may be
exchanged in the first layer than in the second layer. For example,
essentially all (e.g., more than 80%, more than 90%, more than 95%
or more than 99%) of the Li atoms in the first layer may be
exchanged, and/or essentially none (e.g. less than 20%, less than
10%, less than 5%, or less than 1%) of the Li atoms in the second
layer may be exchanged. The first and second layers may be
adjacent. The second layer may also be adjacent on its other side
to a third layer in which more Li atoms may be exchanged than in
the second layer, or in which essentially all of the Li atoms may
be exchanged.
[0033] In the composition, an atom exchanged for Li may have a 2+
valence, or may have a 3+ valence. An atom exchanged for Li may
comprise one or more members of the group consisting of Al, Cu, Mg,
Zn, Fe, Ga and Mn. The composition may contain less than 100 ppm,
less than 50 ppm, or less than 10 ppm lead.
[0034] A resistor may be prepared from any of the compositions
according to this invention, and an electronic device such as a
circuit board may be prepared using such a resistor.
[0035] Also provided by this invention is a method of preparing a
Li.sub.2RuO.sub.3 composition by (a) providing Li.sub.2RuO.sub.3
particles having an average particle diameter between about 0.5 and
about 5 microns; and (b) contacting the Li.sub.2RuO.sub.3 particles
with a solution comprising ions prepared from one or more of the
elements selected from the group consisting of Al, Ga, K, Ca, Mn,
Fe, Na, H, Cr, Co, Ni, V, Cu, Zn, and Ti.
[0036] In such a method, the Li.sub.2RuO.sub.3 particles may be
milled to an average particle diameter between about 0.5 and about
5 microns prior to being contacted with the solution of ions, or
the Li.sub.2RuO.sub.3 particles may be milled to an average
particle diameter between about 0.5 and about 5 microns while being
contacted with the solution of ions. Also in this method, the
Li.sub.2RuO.sub.3 particles may be contacted with the solution of
ions in the presence of a solvent comprising either water, or a
mixture of water and a water-miscible organic liquid. And the
method may also include the steps of (c) washing the resulting
particles free of soluble salts; (d) drying the washed particles;
and (e) deagglomerating the dried particles, such as by
sieving.
[0037] Particle size herein may be determined according to methods
as disclosed in U.S. 2007/0102427, which is by this reference
incorporated in its entirety as a part hereof for all purposes.
EXAMPLES
[0038] The advantageous attributes and effects of the compositions
and methods hereof may be seen in a series of examples (Examples
1.about.13), as described below. The embodiments of these
compositions and methods on which the examples are based are
representative only, and the selection of those embodiments to
illustrate the invention does not indicate that materials,
conditions, components, reactants, ingredients, techniques or
protocols not described in these examples are not suitable for
practicing these compositions and methods, or that subject matter
not described in these examples is excluded from the scope of the
appended claims and equivalents thereof. The significance of the
examples is better understood by comparing the results obtained
therefrom with the results obtained from a trial run that is
designed to serve as a controlled experiment (Control A) and
provide a basis for such comparison since direct mixing of
components was used in Control A rather than ion exchange.
[0039] In the examples, RuO.sub.2 was obtained from Colonial Metals
(Elkton, Md.). The defoamer (Surfynol.RTM. DF-58) was obtained from
Air Products (Allentown, Pa.). All other chemicals were obtained
from Sigma-Aldrich (St. Louis, Mo.). The milling jars and media
were obtained from Paul O. Abbe (Bensenville, Ill.). The media were
3/8'' cylinders or 2 mm spheres of yttria-stabilized zirconia.
Example 1
Synthesis of Li.sub.2RuO.sub.3
[0040] Li.sub.2CO.sub.3 and RuO.sub.2 were dried at 100.degree. C.
overnight before use. 54.42 g Li.sub.2CO.sub.3 and 97.99 g
RuO.sub.2 were put into a 1 liter rubber-lined milling jar half
full of 3/8'' media (1700 g). The jar was rolled at 80 rpm for 24
hours. The contents were sieved to remove the media, and the powder
was placed into shallow alumina trays. The trays were heated at
1000.degree. C. for 12 hours in air. This synthesis yielded
approximately 120 g of Li.sub.2RuO.sub.3. Powder X-ray diffraction
confirmed the presence of Li.sub.2RuO.sub.3 with no impurity
phases.
Example 2
Al Ion Exchange
[0041] 100 g of Li.sub.2RuO.sub.3 prepared as in Example 1 was put
into a 1 liter rubber-lined milling jar with 1700 g of 3/8'' media.
Sufficient water was added to just cover the media and powder.
Defoamer (.about.1.5 g) was also added to prevent foaming. The jar
was rolled at 80 rpm for 48 hours. Then the jar was opened and
76.81 g of Al(NO.sub.3).sub.3.9H.sub.2O was added. Additional water
was also added to fill the jar about three-quarters full (to
minimize additional milling of the particles). Rolling was
continued another 24 hours. The contents of the mill were screened
to separate the slurry from the media, and the jar and the media
were washed to recover the sample. The slurry and the washings were
combined and centrifuged. The supernate (#1) was decanted; it
weighed 1133 g. Additional water was added to the solids, and the
solids were redispersed. The slurry was centrifuged again. The
supernate (#2) was decanted; it weighed 1301 g. Methanol was added
again, and the solids were redispersed. The slurry was centrifuged
again, and the supernate was decanted (#3); it weighed 1040 g. The
solids were dried at 70.degree. C. under vacuum and sieved to -325
mesh.
[0042] The three supernates were analyzed for Li, Al, and Ru by
ICP-OEP (induction coupled plasma-optical emission spectroscopy)
using a Perkin Elmer Optima 5300 V (Waltham, Mass.). The lithium
concentration decreased from 2985 ppm in #1 to 245 ppm in #2 to 115
ppm in #3. The aluminum concentration was 8 ppm in #1, 3 ppm in #2,
and undetected (<1 ppm) in #3. Ruthenium was undetected (<1
ppm) in all three solutions. By using the lithium concentrations in
the three supernates and their weights, it is possible to calculate
the total weight, and thus weight percent, of lithium removed. In
this case, 45% of the lithium originally present was removed and
replaced by aluminum. These data demonstrate that the aluminum
displaces about half of the lithium in the sample, cannot be washed
out, and remains firmly bound within the structure. The resulting
compound, Al.sub.0.3Li.sub.1.1RuO.sub.3, was analyzed with X-ray
powder diffraction (see the middle pattern of FIG. 1). The pattern
showed that what used to be the 100% peak of Li.sub.2RuO.sub.3
(.about.18.2.degree. 2.theta.) is now lower in intensity, and a new
peak has grown in at .about.19.degree. 2.theta..
Example 3
Cu Ion Exchange
[0043] The synthesis in Example 2 was repeated, except 62.49 g of
copper acetate dihydrate was substituted for the aluminum nitrate.
ICP-OES analysis was similar to Example 2, in that Li decreased
with each wash, and only trace amounts of Cu were detected in any
of the supernates. By using the lithium concentrations in the three
supernates and their weights, it is possible to calculate the total
weight, and thus weight percent, of lithium removed. In this case,
52.9% of the lithium originally present was removed and replaced by
copper. The resulting compound, Cu.sub.0.5LiRuO.sub.3, was analyzed
with X-ray powder diffraction. The pattern showed that what used to
be the 100% peak of Li.sub.2RuO.sub.3 (.about.18.2.degree.
2.theta.) is now lower in intensity, and a new peak has grown in at
.about.19.degree. 2.theta..
Example 4
Mg Ion Exchange
[0044] The synthesis in Example 2 was repeated, except 65.66 g of
magnesium acetate tetrahydrate was substituted for the aluminum
nitrate. ICP-OES analysis of the supernates was similar to Example
2, in that Li decreased with each wash, and only trace amounts of
Mg were detected in the supernates. By using the lithium
concentrations in the three supernates and their weights, it is
possible to calculate the total weight, and thus weight percent, of
lithium removed. In this case, 49.66% of the lithium originally
present was removed and replaced by magnesium. The resulting
compound, Mg.sub.0.5LiRuO.sub.3, was analyzed with X-ray powder
diffraction. The pattern showed that what used to be the 100% peak
of Li.sub.2RuO.sub.3 (.about.18.2.degree. 2.theta.) is now lower in
intensity, and a new peak has grown in at .about.19.degree.
2.theta..
Example 5
Zn Ion Exchange
[0045] The synthesis in Example 2 was repeated, except 68.64 g of
zinc acetate dihydrate was substituted for the aluminum nitrate.
ICP-OES analysis of the supernates was similar to Example 2, in
that Li decreased with each wash, and only trace amounts of Zn were
detected in the supernates. By using the lithium concentrations in
the three supernates and their weights, it is possible to calculate
the total weight, and thus weight percent, of lithium removed. In
this case, 39.9% of the lithium originally present was removed and
replaced by zinc. The resulting compound,
Zn.sub.0.4Li.sub.1.2RuO.sub.3, was analyzed with X-ray powder
diffraction. The pattern showed that what used to be the 100% peak
of Li.sub.2RuO.sub.3 (.about.18.2.degree. 2.theta.) is now lower in
intensity, and a new peak has grown in at .about.19.degree.
2.theta..
Example 6
H Ion Exchange
[0046] 81.47 g Li.sub.2RuO.sub.3, prepared as in Example 1, were
put into a 1 liter rubber-lined milling jar with 1700 g of 3/8''
media. A solution of 63.12 g 99.99% acetic acid and water,
sufficient to just cover the media and powder, was added, and the
jar was rolled 93 hours. The powder was isolated and washed as in
Example 2. ICP-OES analysis of the supernates was similar to
Example 2, in that Li decreased with each washing. By using the
lithium concentrations in the three supernates and their weights,
it is possible to calculate the total weight, and thus weight
percent, of lithium removed. In this case, 69.77% of the lithium
originally present was removed and replaced by protons. The
resulting compound, H.sub.1.40Li.sub.0.6RuO.sub.3, was analyzed
with X-ray powder diffraction. The pattern showed that what used to
be the 100% peak of Li.sub.2RuO.sub.3 (.about.18.2.degree.
2.theta.) is now much lower in intensity (as nearly all of the Li
in the Li-only layer has been removed), and a new peak has grown in
at .about.19.degree. 2.theta..
Example 7
Pre-Milling in Methanol Followed by Cu Ion Exchange
[0047] 100 g of Li.sub.2RuO.sub.3 prepared as in Example 1 was put
into a 1 liter nylon milling jar with 1700 g of 3/8'' media.
Sufficient methanol was added to just cover the media and powder.
The jar was rolled for 6 days at 80 rpm. Then 30.69 g copper
acetate monohydrate and 100 g water were added and the rolling
continued for 24 additional hours. The pattern showed that what
used to be the 100% peak of Li.sub.2RuO.sub.3 (.about.18.2.degree.
2.theta.) is now lower in intensity, and a new peak has grown in at
.about.19.degree. 2.theta.. ICP-OES analysis of the supernates was
similar to Example 2, in that Li decreased with each washing. No
copper was detected in any of the washes (<1 ppm). By using the
lithium concentrations in the three supernates and their weights,
it is possible to calculate the total weight, and thus weight
percent, of lithium removed. In this case, 37.03% of the lithium
originally present was removed and replaced by copper and
protons.
Example 8
Cu and Al Ion Exchange
[0048] 100 g of Li.sub.2RuO.sub.3 prepared as in Example 1 was put
into a 1 liter rubber-lined milling jar with 1700 g of 3/8'' media.
Sufficient water was added to just cover the media and powder.
Defoamer (.about.1.5 g) was also added to prevent foaming. The jar
was rolled at 80 rpm for 48 hours. Then the jar was opened and
61.67 g of Al(NO.sub.3).sub.3.9H.sub.2O and 14.27 g copper nitrate
hemipentahydrate were added. Additional water was also added to
fill the jar about three-quarters full (to minimize additional
milling of the particles). Rolling was continued another 24 hours.
The contents of the mill were screened to separate the slurry from
the media, and the jar and the media were washed to recover the
sample. The slurry and the washings were combined and centrifuged.
The supernate (#1) was decanted. Methanol was added, and the solids
were redispersed. The slurry was centrifuged again. The supernate
(#2) was decanted. Additional methanol was added, and the solids
were redispersed. The slurry was centrifuged again, and the
supernate was decanted (#3). The solids were dried at 70.degree. C.
under vacuum and sieved to -325 mesh. ICP-OES of supernate #1 found
5 ppm Al and no Cu, showing that the ion-exchange process had
proceeded until exhaustion of the added cations, resulting in a
final composition of Al.sub.0.267Cu.sub.0.1LiRuO.sub.3.
Example 9
Rate of Al Ion Exchange
[0049] This example demonstrates how rapidly the aluminum is
exchanged with the lithium in the structure. 100 g of
Li.sub.2RuO.sub.3, prepared as in Example 1, was placed into a 1
liter nylon milling jar with 1700 g of 3/8'' media. Sufficient
2-heptanone was added to just cover the media and powder. The jar
was rolled 96 h at 80 rpm. The sample was isolated with methanol.
The slurry was centrifuged and the supernate was decanted. The
solids were dried at 70.degree. C. under vacuum. 90.82 g of this
powder was placed into a 1 liter rubber-lined milling jar with 1700
g of 3/8'' media. 70.15 g Al(NO.sub.3).sub.3.9H.sub.2O were
dissolved in 100 g water and added to the jar. Additional water was
added to just cover the media and the powder. 1.46 g defoamer was
also added. The jar was rolled at 80 rpm and samples were taken at
intervals for ICP-OES analysis. At 1 hour, the Al concentration was
3990 ppm. At 2 hours, the Al concentration was 2306 ppm. At 4
hours, the Al concentration was 40 ppm. At 6 hours, the Al
concentration was less than 1 ppm.
Example 10
Fe Ion Exchange
[0050] 12.38 g Fe(NO.sub.3).sub.3.9H.sub.2O was dissolved in about
20 g of water. 10 g of Li.sub.2RuO.sub.3 was placed into a 125 ml
plastic bottle with 250 g of 2 mm media. The iron solution was
added with sufficient additional water to cover the media and
powder. The bottle was placed inside a larger bottle so that it
would tumble end-over-end as the larger bottle rolled. The sample
was rolled for 70 h. The solids were isolated as in Example 2.
X-ray diffraction of the resulting powder confirmed that the Fe
ions had replaced a fraction of the Li ions, in that the 100% peak
of Li.sub.2RuO.sub.3 was now smaller and a new peak had grown in at
19.degree. 2.theta..
Example 11
Ga Ion Exchange
[0051] 7.90 g gallium nitrate hydrate was dissolved in about 20 g
of water. 10 g of Li.sub.2RuO.sub.3 was placed into a 125 ml
plastic bottle with 250 g of 2 mm media. The gallium solution was
added with sufficient additional water to cover the media and
powder. The bottle was placed inside a larger bottle so that it
would tumble end-over-end as the larger bottle rolled. The sample
was rolled for 70 h. The solids were isolated as in Example 2.
X-ray diffraction of the resulting powder confirmed that the Ga
ions had replaced a fraction of the Li ions, in that the 100% peak
of Li.sub.2RuO.sub.3 was now smaller and a new peak had grown in at
19.degree. 2.theta..
Example 12
Mn Ion Exchange
[0052] 10 g of Li.sub.2RuO.sub.3 was placed into a 125 ml plastic
bottle with 250 g of 2 mm media. 18.05 g of a 9.32% Mn(II) nitrate
solution was added with sufficient additional water to cover the
media and powder. The bottle was placed inside a larger bottle so
that it would tumble end-over-end as the larger bottle rolled. The
sample was rolled for 70 h. The solids were isolated as in Example
2. X-ray diffraction on the resulting powder confirmed that the Mn
ions had replaced a fraction of the Li ions, in that the 100% peak
of Li.sub.2RuO.sub.3 was now smaller and a new peak had grown in at
19.degree. 2.theta..
Example 13
Resistor Formulation
[0053] Aluminum-exchanged Li.sub.2RuO.sub.3 (Example 2) was mixed
on a three-roll mill with one or several glass frits. The frit or
frit combination is lead free. The frit composition is in the range
of 50 to 63 weight % SiO.sub.2, 0 to 10% Al.sub.2O.sub.3, 0 to 10%
B.sub.2O.sub.3, 10 to 30% ZnO, 0 to 3% CuO, 3 to 8% BaO, 5 to 10%
Na.sub.2O, 7 to 17% SrO, 0 to 3% K.sub.2O, and 0 to 4%
P.sub.2O.sub.5. The solid powders were mixed with an organic medium
in accordance with the method described above. 70 weight % powder
and 30 weight % organics were used. The organics consisted of a
mixture of Aqualon T200 ethylcellulose (Hercules, Wilmington,
Del.), terpineol, and soya lecithin. Paste viscosity was between
220 and 260 Pa.-sec.
[0054] The resulting paste was printed on an alumina substrate in a
rectangular pattern, 0.8.times.0.8 mm length and width, and 18
microns dry thickness. Pre-fired 5426 Ag termination were used. The
parts were fired in a Lindberg 10-zone belt furnace with peak
firing temperature of 850.degree. C. (10 minute duration at peak
firing temperature). Fired thickness ranged between 10 to 14
microns. Electrical data for pastes with 14 and 16 weight %
conductive are given below.
[0055] Resistances are measured at -155, 25, and 125.degree. C.
using a two-point probe method. A Keithley 2000 multimeter and
Keithley 224 programmable current source (Cleveland, Ohio) are used
to carry out the measurements. An S & A Engineering 4220AQ
thermal test chamber (Scottsdale, Ariz.) is used to achieve the
three measurement temperatures. Sheet resistance data is reported
as ohms/square at 25.degree. C. Cold temperature coefficient of
resistance ("CTCR") is defined as [(R.sub.-55.degree.
C.-R.sub.25.degree. C.)/(R.sub.25.degree.
C..times..DELTA.T)].times.1,000,000. Hot temperature coefficient of
resistance ("HTCR") is defined as [(R.sub.125.degree.
C.-R.sub.25.degree. C.)/(R.sub.25.degree.
C..times..DELTA.T)].times.1,000,000. The units of both HTCR and
CTCR are ppm/.degree. C.
TABLE-US-00002 Sample A Sample B (14% conductive) (16% conductive)
Sheet resistance (ohms/sq.) 4679655 116117 HTCR (ppm/.degree. C.)
-239 15 CTCR (ppm/.degree. C.) -354 -45 HTCR - CTCR 115 60
[0056] This data indicate that a 100 kilohm/sq. resistor prepared
from a composition that includes a Li.sub.2RuO.sub.3 composition as
described herein would have an H/CTCR of +10/-40 ppm/.degree. C.,
which is well within the usual .+-.100 ppm specification limit for
thick-film resistor compositions.
Control A: Direct Synthesis of Al.sub.0.333LiRuO.sub.3
[0057] Al.sub.2O.sub.3, Li.sub.2CO.sub.3, and RuO.sub.2 were dried
at 100.degree. C. overnight before use. 8.497 g Al.sub.2O.sub.3,
18.473 g Li.sub.2CO.sub.3, and 66.535 g RuO.sub.2 were put into a 1
liter rubber-lined milling jar half full of 3/8'' media (1700 g).
The jar was rolled at 80 rpm for 24 hours. The contents were sieved
to remove the media, and the powder was placed into shallow alumina
trays. The trays were heated at 1000.degree. C. for 12 hours in
air. Powder X-ray diffraction showed the presence of AlLiO.sub.2,
RuO.sub.2, and Li.sub.2RuO.sub.3. The characteristic line at
.about.19.degree. 2.theta., which would indicate Al-doping of
Li.sub.2RuO.sub.3, was not present.
* * * * *