Background of the Invention
1. Field of the Invention
[0001] The invention is concerned with electrical devices which depend for their operation
on ionic motion. Such motion may be macroscopic involving movement of ions as between
affixed electrodes or may be localized. Device uses include capacitors, electrolytic
cells, and bolometers.
2. Description of the Prior Art
[0002] An emerging field of interest involves rigid electrical devices which depend for
their function on some degree of ionic motion. Motion may be macroscopic with ions
moving between electrodes a field of interest here involves solid electrolytic primary
or secondary cells; motion may be extremely local (or macroscopic where blocking electrodes
are used) with devices functioning on the basis of attendant dielectric constants.
In the latter case, dielectric constant may be strongly dependent upon frequency,
as well as temperature or magnitude of applied electric field so that such devices
may be utilized, as well, for critical measurement of such parameters.
[0003] Effort to date has largely, but not exclusively, concerned crystalline materials,
for example, sodium beta alumina and related compositions for ionic conductivity (see
Journal of Applied Electrochemistry, Vol. 1, p . 153 (1971)). Attention on high dielectric
constant capacitive devices has been directed toward crystalline ferroelectric materials,
such as, substituted barium titanates in which ionic motion is localized within single
crystalline unit cells. (See "Multilayer Ceramic Capacitors--Materials and Manufacture"
by Z. F. Capozzi, pub. Sell Rex Co., Nutley, N.J. (1975)).
[0004] As in so many areas, the limitations inherent in the use of crystalline materials
has posed problems--i.e., anisotropy, as well as anomalous effects at crystallite
interfaces or, alternatively, practical difficulty in obtaining large sections of
near-perfect single crystal material. Where macroscopic ionic motion is desired, crystalline
materials pose a special problem in that permitted motion is due to an unusual combination
of properties which are highly structure and direction dependent. As a consequehce,
significant ionic conductivity in crystalline material is a rare phenomenon.
[0005] As in other areas of device investigation, workers have recognized that many of the
shortcomings associated with crystalline materials might be avoided in amorphous materials.
A fairly extensive survey of glassy compositions which have been considered for ionic
motion properties is contained in Journal of Non4Crystalline Solids, Vol. 21 (1976)
p. 343. One of the more promising material classes is based on Li
4Si0
4 and includes both non-stoichiometric variations, as well as compositions modified
by additions of titanium. See Vol. 3 Journal of Applied Electrochemistry, p. 327 (1973).
To date, realized ionic conductivity in amorphous materials have been at least two
orders of magnitude below that observed in the best crystalline materials as measured
near room temperature. (Titanium modified Li
4Si0
4, while attaining values of 10
-3 to 10
-4 ohm
-1cm
-1 at 300 degrees C is typically at a level of only about 10
47 ohm
-1cm
-1 at room temperature which compares with reported values for sodium beta alumina at
room temperature of the order of 10
42 ohm
-1cm
-1 (see Journal of Chemical Physics, Vol. 54 (1971) p. 414) or for lithium beta alumina
at room temperature of the order of 10
44 ohm
-1cm
-1 see Journal of Materials Science, Vol. 12 (1977) p. 15.)
[0006] There does not appear to be an extensive amount of work directed to limited motion
ionic phenomena, for example, in capacitors or other devices depending upon high or
variable dielectric constant, except in the particular case of ferroelectric materials.
[0007] From the device standpoint, high capacitance per unit area has been achieved by procedures
directed toward fabrication of extremely thin dielectric layers rather than by increasing
the degree of ionic motion to produce materials which, themselves, have high dielectric
constants. A good example of this approach is the anodized tantalum capacitor which
has a dielectric constant of about 30 and which, in thin layers typically yields capacitance
values as high as 0.1µ F per cm
2. Substituted barium titanate polycrystalline samples evidencing dielectric constants
as high as 5,000 are discussed in "Multilayer Ceramic Capacitor--Materials and Manufacture",
supra. Sample thicknesses as small as 1 mil (2.54 x20
-3 centimeters) result in capacitances as high as 0.2uF per cm
2. Neither of these prior art structures is substantially improved by increasing temperature.
Summary of the Invention
[0008] A series of glass compositions are found to manifest a degree of ionic motion which
leads to their use in devices using either ionically blocking or conducting electrodes
(e.g., capacitors or electrolytic cells). Significant device characteristics are found
in such glasses in which compositions are related to crystalline materials in which
cations are coordinated within an oxygen octahedron. Specific compositions considered
exemplary are the alkali metal niobates and tantalates44specifically, those of lithium,
potassium, and sodium. Retention and sometimes enhancement of device properties may
result from deviation from crystalline stoichiometry--a phenomenon sometimes observed
in crystalline materials. Since the ascribed mechanism is enhanced by voids in cation
positions, departures from stoichiometry are largely in the direction of cation-lean
compositions, although increase in cation content is also permitted.
[0009] Materials of the invention are amorphous in the traditional sense--i.e., no ordering
for distances greater than about 100 Angstrom units. As in other amorphous materials,
departure from stoichiometry are more easily accommodated than in crystalline counterparts.
Compositional ranges are considered to extend from compositions which are 50 atom
percent cation deficient to those which are 20 atom percent cation rich. Nominal stoichiometric
compositions based on crystalline materials are LiNb0
3, LiTao
3, RNbO
3, KTao
3, NaNbO
3, and NaTa0
3. Composition departures particularly by substitution of ions of valance values differing
from that of nominal site occupant may be tolerated or even enhance properties. Mixtures
of such compositions are permitted.
[0010] In general, glass compositions of the invention do not require stabilization by network
forming oxides, such as silica, but rather owe their existence to drastic heat treatment
(quenching) during formation. Nevertheless, glass forming additives are sometimes
utilized to expedite glass formation or even to stabilize the glass phase.
[0011] As expected, introduction of glass formers dilutes the properties upon which the
invention is based so that such modification is largely with a view to fabrication
expediency. Maximum network forming additive is desirably below about 10 percent by
weight. Other variations--intentional as discussed or unintentional--should not alter
the basic structure and to this end are generally limited to a maximum of 10 ion percent
for any ion occupancy or a maximum of 10 percent by weight of total composition.
Brief Description of the Drawing
[0012]
FIG. 1 is a perspective view of a device illustrative of that category of the invention
in which properties are dependent upon local ionic motion (macroscopic or microscopic
but always within the glass material);
FIG. 2 is a perspective view of a device illustrative of that category of inventive
devices in which device function depends upon macroscopic ionic motion through glass4electrode interfaces, i.e., solid electrolytic primary or secondary electric power
supplies; and
FIG. 3, on coordinates of logarithm of ionic conductivity (log a) on the ordinate,
and reciprocal temperature in degrees Kelvin on the abscissa, is a plot showing the
temperature dependence of ionic conductivity in a glassy material herein--a property
of significance in devices exemplified by bolometers, as well as other devices in
which varying values of conductivity/capacitance are useful for measurement purposes
or to compensate for other temperature-dependent parameters.
Detailed Description
1. The Composition and Preparation
[0013] Materials of the invention have two characteristics in common: (a) all materials
of the invention are amorphous in the sense that ordering, while detectable by state
of the art electron microscopy does not exceed about 100 Angstrom units (the approximate
resolution limit for conventional X-ray diffraction); and (b) all compositions, broadly
defined as lithium, potassium, or sodium niobates or tantalates or mixtures thereof,
are of nominal compositions which, as stoichiometric and unmodified in the crystalline
state, may result in octahedral coordination with an alkali metal cation within an
oxygen octahedron. Since materials of the invention are amorphous with attendant insensitivity
of structure, deviation from stoichiometry may be at least as great as to represent
a 50 percent deficiency of alkali metal cation; or, alternatively, a 20 percent excess
of such cation. While greater excess is possible, expected loss in conductivity or,
more generally, in ionic motion results. As in previous studies, cation deficiencies
tend to increase cation mobility with the upper limit on such deficiency being defined
by reduced total motion due to the now noticeable decrease in available mobile ions.
[0014] Device properties are attributed to nominal compositions which are invariably of
the form XZ'0
3, where X is at least one alkali ion selected from the group consisting of Li, K,
Na, and Z is at least one ion selected from the group consisting of Nb and Ta. As
noted, a cation deviation from stoichiometry of from -50 atom percent to +20 atom
percent is permitted, so resulting in the generalized formula X
0.5-1.2Z
1.1-0.96O
3, where X and Z are as above defined.
[0015] While properties of consequence are due to the nominal compositions noted, modification
is permitted, or even desired, for some purposes. As noted, glass formers (network
formers) may expedite or stabilize glass phase. Examples are P
20
5, B
20
3, Si0
2, Ge0
2, generally in amounts up to about 10 weight percent of total composition. Up to 20
ion percent--preferably up to 10 ion percent--of certain ions--may replace the alkali
metal, as well as Nb or Ta. The maxima, expressed in terms of percent for each cation
in the AB0
3 composition applies to Mg
2+ and/or Ca2+ considered to substitute for Li, K and/or Na, as well as to M
04+, Ti4+, Zr
4+ and/or W
4+ considered to substitute for Nb and/or Ta. Such substitution may induce vacancies
and so increase conductivity. Total compositional modification in other than addition
of glass former (or other effective diluent) and disregarding simple departures from
stoichiometry should not exceed about 10 weight percent, again based on total composition
(including unintentional inclusions), since further modification may adversely affect
the amorphous "structure" responsible for large values of ionic conductivity. While
exemplary compositions are produced by simple quenching, a desire to produce certain
configurations, perhaps thin films, may give rise to the desire to incorporate minor
amounts of glass forming ingredients. Since such ingredients only dilute the essential
device characteristics, addition is kept at a minimum. For most purposes, 10 weight
percent addition is a realistic compromise to accomplish the desired objective while
minimizing effect on device characteristics. So, addition of up to 10 weight percent
of a silicate may both expedite formation and stabilize compositions. Silicon- containing
glass formers may be added simply as silica or as silicates, again, desirably of any
of the alkali metal ions Li, K, Na. Other glass formers--e.g., Ge0
2, P
20
51 B203--may expedite formation and stabilization of the glass phase but may be non-tpreferred
by reason of device property deterioration and difficulty of introduction in that
order.
[0016] Experimental results reported herein are sometimes based on roller=quenched specimens.
For general description of this procedure, see Reviews of Scientific Instruments Vol.
41, (1970) p. 1237. Depending upon device design, it may be appropriate to utilize
alternative techniques, such as splat cooling, sputtering on a cold substrate, as
well as other procedures which may result in the desired amorphous state.
[0017] Device design considerations are interrelated with processing. Much of the study
reported in this disclosure relates to measurements made on discrete devices. Devices
of this nature are appropriately fabricated from samples made by roller quenching,
splat cooling, etc. It has been indicated that an aspect of the invention considered
of particular promise involves the extremely high dielectric constants attendant upon
the same ionic motion responsible for high conductivity values. These very high values,
which, in typical compositions tested at 1 kHz, range from 10
5 at temperatures of the order of 300 degrees C but still at a level as high as 150
at room temperature, present an alternative to the low dielectric constant thin film
approach exemplified by the familiar tantalum oxide capacitor. While there is little
hope that thicknesses of materials of the invention will get down to the range realizable
through anodization, it is quite likely that films of the order of fractions of a
micron or less producible by condensation techniques, may yield higher capacitance
values/unit area than are available from prior art anodized structures. Such condensation
techniques may take the form of evaporation, as well as sputtering--either reactive
or non«reactive. Applicable techniques are described in Handbook of Thin Film Technology,
edited by L. I. Maissel and Reinhard Glang, McGraw Hill, 1970. Sputtering techniques
which depend on choice of source, i.e., vapor phase reactants, if any, as well as
bias control effected through adjustment of such parameters as applied potential,
use of floating electrodes, shaping electric fields, etc., are developed to a degree
of sophistication as to'enable the worker to realize desired layer characteristics.
Device electrodes blocking or conducting may be applied in a manner familiar to workers
in the field of integrated circuits.
2. The Figures
[0018] FIG. 1 is illustrative of a category of devices in accordance with the invention
in which ionic motion is local--i.e., restricted to movement within the glassy material.
Devices of this category may serve a variety of uses. The high capacitance values,
characteristic of glass phase materials herein suggest construction of capacitors
possibly by a technique compatible with silicon integrated circuit or other integrated
or hybrid circuit fabrication. As would be expected, since dielectric characteristics
are due to ionic motion--a temperature dependent phenomenon--capacitance and, in fact,
all device characteristics of the invention, are also characterized by temperature
dependence. This dependence may be tolerable in categories of circuits some of which
may even be provided with close temperature control for other reasons. Alternatively,
temperature dependence of dielectric constant may be used to advantage, for example,
serving as dielectric bolometer for measuring temperature (or for indirectly measuring
any other condition which has the effect of altering temperature). Since typical circuitry
depends upon elements themselves characterized by temperature dependence, inclusion
of a device of the invention may serve to compensate such effect.
[0019] The device of FIG. 1 depicts the body 1 of an amorphous material in accordance with
the invention. Electrical connection is via leads 4 and 5 contacting electrodes 2
and 3, respectively. For the type of device contemplated, electrodes 2 and 3 are "blocking"
in that they do not show appreciable ionic conductivity. Suitable materials are electronic
conductors, such as, gold or aluminum, both of which lend themselves to ready fabrication.
[0020] FIG. 2 is illustrative of that class of devices in which at least some of the alkali
ions traverse the glass-electrode interface. While alkali ion-deficient compositions
are of general interest in all devices of the invention, it is in devices of this
category in which such compositions are of particular interest. The device depicted
which may be regarded, for example, as a primary or secondary solid electrolytic cell
consists of amorphous body 20 of a composition herein, intimately contacted by a first
alkali metal-containing electrode 21 and a second electrode 22, possibly of a transition
metal chalcogenide. Examples of such chalcogenides presently under study are
FeS
x, NbSe, TiS
2' VS
2, and NbS
2. It is the essence of structures of this type that electrodes, rather than blocking,
are ionic conductors. While alternatives are possible, it would be expected that electrode
21 and amorphous body (electrolyte) 20 would contain at least some alkali metal ions
in common. Much work reported in the literature depends upon use of the high mobility
of lithium; and it is expected that exemplary structures of the type depicted in FIG.
2 would make use of lithium- containing electrolyte and electrode material. The structure
of FIG. 2 is completed by electronic electrodes and leads 23, 24 and 25, 26.
[0021] FIG. 3 illustrates the temperature-dependent characteristics of typical compositions
of the invention. The particular coordinates chosen, logarithm of conductivity (in
terms of the symbol a which may, for example, be in units of ohm
-1cm
-1) on the ordinate, and reciprocal temperature (degrees Kelvin) on the abscissa conveniently
result in a straight line plot which may reliably be extrapolated beyond the data
for all temperatures in the amorphous phase. For the particular composition represented,
the room temperature conductivity is approximately 10
-5ohm
-1cm
-1. This value compares favorably with rigid ionic conductors, in general. While the
slope of the plotted line is generally characteristic of ionic conductors, the absolute
values of conductivity vary.
[0022] It has been indicated that devices of the invention all depend upon ionic motion--sometimes
macroscopic, sometimes quite localized. It follows that the data presented on FIG.
3, although directed to motion across glass-electrode interface (electrode-to-electrode
conductivity) is equally applicable to devices which do not depend upon ionic conductivity
in the conventional sense. Such devices, which may be included as capacitors, may
evidence ionic motion only on a localized scale or may depend upon blocking electrodes
to result upon charge accumulation where conductivity is, otherwise, macroscopic.
FIG. 3, which is a measure of ionic flow, is properly considered for its broader implication--ionic
movement, generally. In a very real sense, total charge accumulation-i.e., capacitance--is
sufficiently related to conductivity--net ionic movement responsive to biasing--to
permit use of the same data. An additional use of devices of the invention also dependent
upon localized movement--i.e., on charge accumulation-- is dependent upon the pyroelectric
effect either in biased material or in unbiased material which has previously been
polarized. Charges so produced are temperature dependent primarily due to the temperature
dependence of ionic motion. It has been noted that other device uses may also depend
upon temperature dependence of ionic motion. Such devices generally use blocking electrodes
(electrodes with large resistance to ionic conduction).
3. Examples
[0023] Material used in the following examples was prepared by roller quenching. Sintered
material of the appropriate composition was powdered in a mortar and pestle to produce
particles that would pass through a 120 mesh (125 micrometers) screen. Approximately
5 grams of powdered material was placed in an iridium crucible provided with a 10
mil (25.4 x 10
-3 centimeters) aperture, in the bottom surface. The crucible was covered with an apertured
platinum lid which was then evacuated through the aperture to maintain a small vacuum
of approximately 2 inches (5.1 centimeters) of water. Crucible and contents were then
heated with a radio frequency heater, heated sufficiently to melt contents. To facilitate
further processing, heating was actually carried out at a temperature somewhat in
excess of melting (100 degrees C - 300 degrees C excess). The purpose of the vacuum
is to prevent leakage of material during heating.
[0024] With the material still at temperature, the vacuum was replaced by a pressure of
about 10 psi (68,948 newtons per square meter) resulting in an exiting stream of molten
material which was directed between rotating 2 inch (5.1 centimeters) diameter chrome-plated
steel rollers (300 rpm). Conditions during roller quenching were such as to result
in exiting flakes. Flakes were typically 3 mm by 5 mm by 10 micrometers thick.
[0025] Flakes were inspected by X-ray diffraction, as well as differential thermal analysis,
to result in a finding that there was no long-range ordering over dimensions as great
as 100 Angstrom units and to indicate that the material was metastable (DTA exhibited
exotherm). Following, electrodes were affixed to the flake specimens or portions thereof--electrodes
were either blocking or ionically conducting, depending upon the nature of the experiment
to be conducted. Details are set forth in the examples which follow. In each instance,
a composition, as well as melt temperature actually utilized in its preparation, is
listed.
[0026] The following examples serve as a basis for comparison of the compositions reported,
since (a) as noted, preparation was, in all cases, similar and (b) insofar as feasible,
test conditions were maintained constant. With respect to the latter, all specimens
were biased at 1 volt with evaporated gold being used in all instances in which blocking
electrodes were utilized. Electrode area was, in each instance, 1 mm square with separation
between electrodes equal to the 10 micrometers thickness resulting from the constant
roller spacing utilized in quenching. In all but one instance, measurements were conducted
at 1 kilohertz--the exception being Example 7 in which capacitance/frequency dependence
was measured.
[0027] Example 9 is included as exemplary of a structure utilizing non-blocking electrodes.
It will be noted that oeasured ionic conductivity is that expected from measurements
conducted in the preceding examples.
![](https://data.epo.org/publication-server/image?imagePath=1979/08/DOC/EPNWA2/EP78100638NWA2/imgb0001)
Example 7
[0028] The specimen of Example 2--LiTaO
3-- was measured at frequencies of 120H, 400H, and 1 kH to reveal dielectric constant
dependence on this parameter. The 100 degree C ;dielectric constant was 7,000, 1,600,
and 800, respectively. Conductivity remained constant at a value of approximately
1.5 x 10
-6ohm
-1cm
-1 over this frequency range.
Example 8
[0029] In this Example, the dielectric constant of the specimen of Example 1--LiNbO
3--was measured at varying temperature to determine thermal response. Dielectric constant
was found to vary at the fractional rate of 6 percent/degrees C over the entire temperature
range from room temperature to 200 degrees C. For this configuration, absorption of
radiant energy results in a one degree temperature change per 5 microjoules absorption
of radiant energy.
Example 9
[0030] A specimen of the composition and dimensions of that of Example 1--LiNbO
3-- was provided with non-blocking electrodes of LiCl and the d.c. conductivity was
measured. Measured values were approximately the same as the 1 kilohertz values set
forth for Example 1.
1. An electrical device comprising a portion of material and spaced electrodes intimately
contacting the portion, the said material being amorphous within a region defining
a continuous path intermediate the said electrodes, the amorphous state being characterized
by absence of long-range ordering over a distance of at least 100 Angstrom units as
indicated by X-ray diffraction, CHARACTERIZED IN THAT the said material comprises
a composition which may be represented by the stoichiometry X0.5-1.2Z1.1-0.96O3 in which X is at least one element selected from Li, Na and K; Z is at least one element selected
from Nb and Ta; and O is oxygen.
2. Device according to claim 1, CHARACTERIZED IN THAT the said composition comprises
at least 80 percent of the said material, the said material containing up to 10 weight
percent of at least one glass former selected from P2O5, B203, SiO2, and GeO2.
3. Device according to claim 1 or.2, CHARACTERIZED IN THAT X contains up to 20 ion
percent of at least one ion selected from Mg2+ and Ca2+ and Z contains up to 20 ion percent of at least one ion selected from Mo4+, Ti4+, Zr4+, and W4+.
4. Device according to claim 1, 2 or 3, CHARACTERIZED IN THAT X contains up to 10
ion percent of at least one ion selected from Mg2+ and Ca2+ and Z contains up to 10 ion percent of at least one ion selected from Mo4+, Ti4+, Zr4+, and W4+.
5. Device according to claim 1, 2, 3 or 4, CHARACTERIZED IN THAT X consists essentially
of Li and Z consists essentially of Nb.
6. Device according to any one of the preceding claims, CHARACTERIZED IN THAT the
said electrodes are blocking - i. e., are essentially non-conducting for X ions.
7. Device according to claim 6, CHARACTERIZED IN THAT the said electrodes are metallic.
8. Device according to claim 6 or 7, CHARACTERIZED IN THAT a surface is provided which
is absorbing for radiation to be detected.
9. Device according to claim 8, CHARACTERIZED IN THAT the said radiation is in the
infrared wavelength range.
10. Device according to any one of the preceding claims 1-5, CHARACTERIZED IN THAT
the said electrodes are non-blocking with respect to X ions.
11. Device according to claim 10, CHARACTERIZED IN THAT the electrodes are of differing
electrochemical potential, a first such electrode acting as a source of X ions and
a second electrode acting as a sink for X ions, whereby an electric potential results
between the said first and second electrodes.
12. Device according to claim 11, CHARACTERIZED IN THAT the said first electrode comprises
lithium.
13. Device according to claim 12, CHARACTERIZED IN THAT the second electrode comprises
a chalcogenide selected from NbSe2, VS2, TiS2 and FeSx, where x equals a value of from 1 to 3.