Note: Descriptions are shown in the official language in which they were submitted.
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METAL OXIDE VARISTOR WITH CONTROLLABLE BREAKDO~N
VOLTAG AND CAPACITANCE
Back~round of the Inventicn
This invention relates to metal oxide varistors and, in
part.icular, to lithium-doped zinc oxide based varistors with
controllable breakdown voltage and capacitance.
In general, a metal oxide varistor comprises a zinc
oxide (ZnO3 based ceramic semiconduct.or device with a highly
nonlinear current~voltage relationshi? ~hich may be represented
~y ~.he equation I = (V/C)a, where V is the voltage between two
points separated by the varistor material~ I is the current
flowing between the points, C is a constant, and ~ is 2 measure
of device nonlinearity. If ~ = 1, the device exhibits ohmic
propertiesO For values of ~ greater thar 1 ~typically 20~-50 or
more for ~nO based varistors~ the voltage-current character
istics are similar to those exhibited by back-to-bark connected
Zener diodes. Varistors, however, have much greater voltage,
curren~., and energy-handling capabilities. If the voltage
applied to t.he varistor is less than the varistor breakdown
voltaye, only a small leakage current will flow between the
electrodes and the device is essentially an insulaton having a
resistance of many megohms. However, if the applied voltage is
greater than the varist.or breakdown voltage, the varistof
resistance drops to low values permitting large currents to flow
through the varistor. Under varistor breakdown conditions,
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the curren~. throu~h the varistor varies greatly for small chdnges
in applied voltage so that the voltage across the varistor is
effectively limited to a narrow range of values. The voltage
limitiny or clamping action is enhanced dt higher values of ~.
Metal oxide varistors have been widely employed as surge
arresters for protecting electrical equipment from transients
on AC power lines created by lightning strikes or switching of
electrical apparatus. Such applications require the use of
varistors having breakdown voltages slightly greater than ~he
maximum input voltage of the system to be protected. Thus, for
exa~nple, d ty~ical system powered from l70 volts peak voltage
(120 volts ~ns) AC power Inains would require the use of a varistor
having a breakdo~n Yoltage somewhat greater than 170 volts.
Varistor device behaviorlnay be approximately modeled by
a variable resistor in parallel with a capacitor. The parasitic
capacitance modeled by the capacitor is an intrinsic property
associated with the particular varistor composition9 and is
generally undesirable as it may affect varistor performanc2 in
surge-protective or swi tching applications, fo r example. In
typical surge-arrest;r applications, the varistor is subjected ~o a
continuously applied voltage. Although the applied voltage is
lower than t.he varistor breakdown voltage, an undesirable current,
due prednminantly to the parasitic capacitance~ flows through the
varistor. In high ~requency circuits this current flow may be
large enouyh to affect. normal operation of the cireuitO
Another capacitance-related problem (described in greater
detail in U.S. Patent. 4,276,578, issued to L~M. Levinson, and
assigned ~.o the same assignee as the present invention) arises
in surge-arrest.er devices made up of stacked metal oxide
varistors. In such devices, eaeh varistor in the stack has
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in addition t.o the pdrasitic capacitance assooiated therewith,
a coupling cap~citance to ground~ As a result of the combined
effect of ~he parasitic an~ ground capacitance, particularly
ground capacitance, a larger current flows through the top
varistors (t.hose nearesf. the line3 in the stack since these
varistors also pass the capacitive ground currents which flow
through the lower varistors. The u~per varistors therefore are
required to dissipate greater power, resulting in higher
operating temperature, inferior stability, and ~oncomittantly
shorter useful li~e due ~.o premature failure. In conventional
systems, discrete, low dissipation capacitors are connected in
parallel with the varistors to achieve a more uniform voltage and
power distribution throughout the staoked varistors. Use of
capacitors with graded intrinsic capacitances, as described in the
aforementioned patent, is a more effective solution.
Varistor elements may also be used as switching elPments
forInultiplexing, for example, liquid crystal displaysO In
such applications, the parasitic capacitance is also a problem,
since it appears in series with the capacitance of the liquid
crystal material~ fonning a capacitive voltage divider. A
lower electric field t.han would otherwise be available is thus
used to maintain the liquid crystal material in its active
state. Additionally, if the varistor capacitance is too high
nonselected ele~ents in the liquid crystal array may be
inadvertently activated by pulses applied to the displayO
A more detailed description of multiplexing liquid crystal
displays usi ng Ya ristors appears in U.S. Patent 4~?23~6o3 issued
to D.E. Castleberry and in Canadia~ Serial No. 39.4,621 filed
~an 21/82 - by L.M. Levinson, both assigned to the same assignee
as the present invention.
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Fro~ll the ~oregoiny the importance and desirability of
reducing varistor capacitance is apparent. Aforementioned
U.S. Patent 4,276,578 discloses the inclusion of antimony oxide
(Sb203) in the varistor for the purpose of decreasing intrinsic
capacitance. The present invention provides varistors with high
breakdown volt.age and low capacitance by controlled diffusion of
lithium into conventional zinr oxide varistor material.
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In accordance with the present invention, a zinc oxid~ based
varistor exhibiting a high breakdown voltage and low capacitance is
fabricated by diffusing lithium into conventional metal oxide
varistor material at elevated temperatures. The diffusion of
lithium mus~. be carefully controlled, otherwise the varistor
becomes insulating for applied voltages even as high as ten or morP
times the normal breakdown voltageO Lithium may be diffused into
the varistor materidl by placing a so1ution containing LiN03 or
Li20 on the varistor surface. Solvents such as alconol or
acet~"e ~ay be air dried while aqueous solutions should be heated
in air to remove t.he water. Following the drying stept lithium
surface concentration should not exceed approximately 2 mg/cm2O
The varistor material is then heated at9 for instance~ 800C for
approximat.ely one hour. Temperatures between 500C and 1100C,
however may ~e employedO The penetration of lithiwn into the
varistor is determined by the time and temper~ture o~ the diffusion
step. GiYen sufficient time, lit.hium may be diffused completely
throuyh the varis~.or mat.eriat. For varistors in which lithium
diffusion is limited t.o d thin layer on one side of the varistor,
conventional surface electrodes may be employed.
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It is an obJec~. of the invention to provide a metal oxide
varistor exhibiting high breakdown voltage and low capacitance~
It is another object of the invention to provide a metal
oxide varistor exhibiting controllable breakdown vol~age and
capacitance characteristics.
It is still another object of the invention to provide
a zinc oxide varistor containing diffused lithium and which has
high breakdown vol~.a~e, low capacitance, and low leakage current.
Brie~ Descri~tion of the ~
The features of the invention believed to be novel are set
fort.h with particularity in tne appended claimsO The invention
itself~ however, both as to its organization and method of
operation, together with fur~her obJects and advantages thereof,
may best ~e understood by reference to the fallo~ing description
taken in conjunction with the acc~npanying drawings in which
/the single Figure depicts voltage-current characteristic curves
of a metal oxide varistor produced in accordance with the present
i nvention.
Det.ailed Deseription of the Invention
In the past, high resistance surface layers containing
lithium and potassium have been produced by diffusion of Li2C03
or Li~0 and K2C03 or K20 into zinc oxide varistor materials.
The lithium and potassium are diffused into the sides of the
varistor disk or rod~ for example, while the eleotrodes are
affixed to the flat end portions. In this manner, the non-
linearity fo the varistor is unaffeeted in the undoped varistor
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material portions, while the doped regions provide a high-
resistance. Since the doped layer has a high resistance9 it does
not appear to havP a nonohmic voltage characteristic, typical of
varistor behavior. In Fact, by virtue of its high resistance, the
doped layer could aid in avoiding voltage flashover between the
electrodes from occurring along the sides of the varistor disk
or rod.
In contrast, in accordance with ~he present invention,the
quantity of lithium dif-fused into the varistor material is
carefully controlled to preserve the nonohmic voltage character-
istics associat~d with the varistor material, If rel aki vely
large anlounts of lithium (described hereinaFter) are diffused,
the varistor material becomes insulating for applied voltages
even as high as ten or more times the nonnal breakdown voltage.
Such highly doped varistor materials do not exhibit varistor
breakdown conduction. If the applied voltage is increased
sufficiently, catastrophic conduction results. For smaller
amounts of lithium dopant, however, a varistor having a high
~, increased breakdown voltage, and lower capacitance than that
obtained with similar undoped varistor material is realized.
In order to practice the invention, lithium may be
diffused into any conventional zinc oxide varistor materialO
Such varistor materials may conveniently comprise any of the
standard cwnpositions employed in fabricating metal oxide
varistors by conventional methods. Typically9 such varistors
have zinc oxide (ZnO) as the primary constituent (typically,
90 mole ~ercent or more) and include smaller quantities of other metal
oxide additives, such as bismuth oxide (Bi~03), cobalt oxide ~Co203),
chromium oxide (Cr203) as well as other additives which may
include additional metal oxides. Examples of such addikives include
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mdnyanese oxide (MnO2), antilnony trioxide (Sb203), silicon
dioxide (SiO2), nickel oxide (NiO), magnesium oxide ~MgO),
aluminum nitrate (Al(N03)3 9(H20)), tin oxide (SnO2),
titanium oxide (TiO2), nickel fluoride (NiF2), barium carbonate
(BaC03), and boric acid (H3B03). The li5t of additives is
not intended to be exhaustive~ nor, generally are all of the above-
enumerated materials employed in a single varistor composition.
By way of example, and not limitation, a varistor material suitable
for practicing the invention may comprise 0.5 mole percent each
of Bi203, Co2039 MnO2, and SnO2, 0.1 mole percent each
cf H3~03 and ~aC03, 1 mole percent Sb203, the remainder
being ZnO~ The additive elernenrs may be added to ~he unfired
varistor mixture as any convenient salt of the additive element
since upon sintering these compounds decompose into oxides of the
element.
Lithium may be diffused into varistor material by placing
thereon a suitable paste or a solution of lithium nitrate (LiN03)
or lithium oxide (Li20). Solutions using alcohol (sueh as,
methanol) or acetone may be air dried. If an aqueous solution is
used, the varistor is initially heated at a low temperature such
as 100~C to evaporate the w~terO Resulting surface concentration
of LiN03 or Li20 on the varistor should not exceed approximately
Z mg/om2. The varistor material is then heated in air at
temperatures as high as 1100Co The usual time versus temperature
tradeoffs apply and the penetration of lithium into the varistor is
determined by the time and temperature of the diffusion step. For
a varistor heated for one hour at 600C, lithium penetration is in
~he order of a few mils9 while at 9Q0C it is on the order of. a few
millimeters~ If sufficient time is allQwed, the lithium can be
made to completely penetratQ the varistor.
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In applications where attaching electrodes to the opposite
sides of the varistor material is inconvenient7 irnpractical,
or where it is desired to control electrode separation, electrodes
may be attached adjacent to one another on the doped side of the
varistor material.
The Figure illustrates voltage-current characteristics of
lithium doped and undoped varistor material having the afore-
described ex~lnplary composition into which lithium has been
diffused by heating in air at 800C for one hour, and on which
surface electrodes were positiuned 1 mm apart. Varistor breakdown
vol~age is indicated on the ver~ical axis9 while corresponding
current values are shown on the horizortal axis. Curves A9 B, and
C depict varistor characterilstics oF a lithium-doped varistor
surf;ace correspondiny to depths of ~, 7.5, and 15 thousandths of
an inch, respectively. In obtaining the voltage-current character-
istics at various depths, to illustrate the dependence of breakdown
voltage and varistor capacitance on lithium dopant concentration,
successive varistor material layers were removed by lapping,
electrodes attached, and the varistor characteristics measured.
Curves A, B, and C represent progressively lower lithium
concentrations. Curve D depicts the charcteristics o~ an undoped
varistor surface. It will be observed that for curves A, B, and C,
capacitance values are 20 pf, 40 pf9 and 70 pf, respectively,
while breakdown voltages are ~40, 410, and 155 voltsg respectively.
For undoped varistor material the capacitance and breakdown
voltage are 1~0 pf and 11~ volts, resepctively. It is apparent,
there~ore, tha~ near the varistor surface (Curve A7 highest
lithium doping), the breakdown voltage is approximately eight
times larger and t.he cdpacitance dpproximately fiYe times smaller
than the undoped surface (Curve D)~
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It is apparent from the foreyoing that the present invention
provides a me~al oxide based varistor with a controllable breakdown
voltage and capacitance. More specifically, the invention provides
d zinc oxide varistor containing lithium and which has high
breakdown voltage, low capacitance, and low leakage current.
While certain preferred features of the invention have
been shown by way of illusl:ration, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all
such modifications and changes as fall within the true spirit of
the invention.
