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Патент USA US3023177

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Feb. 27, 1962
T. s. DUNNE
3,023,167
ROOM TEMPERATURE FERROELECTRIC MATERIALS
Filed July 8, 1960
FIG.2
3
/Il/
12
6
mHJm8>E6Sm.
4
2
0
20
140
100
TEMPERATURE (°C)
15
_
m5
180
TEM PERATURE (°C)
INVENTOR
THOMAS C. DUNNE
ATTORNEY
_
atent 5.;
w
3,023,167
CC
Patented Feb. 27, 1962
1
2
Thomas G. Dunne, Poughkeepsie, N.Y., assignor to In
The general process of the preparation of the mixed
ferrolectric crystals of the present invention may be illus
trated with reference to the following speci?c examples.
EXAMPLE I
ternational Business Machines Corporation, New
York, N.Y., a corporation of New York
Accordingly, LiF (12.07 grams), BaFz (87.68 grams)
3,023,167
ROOM TEMPERATURE FERROELECTRIC
'
MATERIALS
and AlF3-3H2O (0.34 gram) were heated at elevated tem
peratures, preferably at about 1300“ C., for about 4 hours
in air in a 250 cc. platinum crucible and cooled slowly,
‘This invention relates to ferroelectric materials and 10 suitably at the rate of about 40° C. per hour, to below
more particularly to a new class of ferroelectrics exhibit
about 800° C., the furnace thereafter shut off and the
ing a spontaneous polarization at room temperature.
mass cooled to room temperature. The resulting product
As is known in the art, the so-called ferroelectric char
consisted of a large polycrystalline mass interspersed with
acteristic manifests itself as a resultant or spontaneous
a smaller amount of hexagonal platelets of about 1 to 4
polarization, in the material following the application 15 mm.2 in area and about 0.1 mm. thick. The platelets
and removal of a force applied thereto. As is also well
then were collected by mechanical separation from the
known in the art, only a limited number of materials
polycrystalline mass. The mixed crystal platelets ana
Filed July 8, 1960, Ser. No. 41,527
5 Claims. (Cl. 252—63.2)
are known to exhibit this property.
lyzed to be the composition BaLiMZAlMBFOMOHB.
The present invention adds a new class of ferro-electric
EXAMPLE II
materials to those previously available to the art. The 20
new class comprises mixed crystals of compounds having
the general formula:
Following the procedure described in Example I, LiF
(12.07 grams), BaF2 (87.68 grams) and AlF3-3H2O
M1+2M2+3X4-2
(0.38 gram) yielded a mixed ferroelectric crystal which
analyzed to be BaLiMAlmFMOw.
designated as group I and
25
M1+2M2+1X4-1
designated as group II, wherein the M and X elements are
related by a similar ionic crystalline radii as will be set
forth hereinafter.
EXAMPLE III
As above, LiF (12.07 grams), BaF2 (87.68 grams) and
AlF3-3H2O (8.5 grams) gave a mixed crystal which ana
lyzed BaLlwsAllAFLgoag.
The table shows the variation of some electrical prop
erties as the composition of the mixed crystal is changed
combination of group I and group II, the materials of
in the formula BaLi2xAl2_2xF4XO4_4x, where P is the spon
the present invention include compounds within the class:
taneous polarization at 25° C., Ec is the coercive ?eld at
25° C., Tc is the Curie temperature and ETC/e25, C. is the
35 ratio of the dielectric constant at the Curie temperature
A preferred subclass of compounds of the present in
to that at 25 ° C.
vention are those having the general formula:
Table
30
Stated in terms of a composite formula representing a
where “x" preferably ranges from 0.15-0.30.
40
12
ere/625°C. ’I‘.,° C.
Pine/cm.z E.,kv./cm.
As is recognized in the art, ferroelectric materials have
found use in a number of practical devices, including di
1. 27
148
0. O8
7. O
l. 51
127
0. l0
8. 5
electric ampli?ers, non-linear capacitors and memory ele
1. 68
140
0. 14
20. 1
ments in computer circuitry. In use in the latter applica
tion, it is highly desirable that the material exhibit a rela
The crystal structure of the mixed ferroelectric crystals
tively low coercive force, a high spontaneous polarization, 45
of the present invention possesses a hexagonal unit cell.
and a more nearly rectangular hysteresis loop charac
teristic. Another particularly sought after feature for
For example, the compound BaLiOIGAlMFLZOOM has the
unit cell dimensions a: 10.44 A. and c=8.77 A. Fer
ferroelectric materials used as memory elements is that
they be capable of operation at or near room tempera 60 roelectric crystals isomorphous with the base system,
ture, i.e., that they exhibit a spontaneous polarization in
this temperature range.
Accordingly, it is a general object of the present inven
BaLi2XAl2_2XF4xO4_4X, are produced by replacing one or
more of the ions in the crystal lattice by another of the
same valence and of nearly the same size. For example,
tion to provide a new class of useful ferroelectric mate
Ba+2 is suitably replaced by the ions, U02“, Cu(NH3)4+2
rials exhibiting a spontaneous polarization at or near 65 or Ni(NH3)6+2. Similarly, Al+3 may be replaced by the
trivalent cations Ga+3, Cr!‘3 and M“.
In a similar manner, ferroelectric crystals are produced
The foregoing and other objects, features and advan
by substituting Na+ and K+ into the preferred crystal
tages of the invention will be apparent from the follow
structure for Li*'. Cl-1 and Br“1 also substitute for
ing more particular description of preferred embodiments
of the invention, as illustrated in the accompanying draw 60 fluoride in the crystal lattice.
room temperature.
ings.
In order to aid those skilled in the art to use the novel
In the drawings:
FIGURE 1 shows a typical ferroelectric hysteresis loop
ferroelectric materials of the present invention the fol
lowing typical physical properties and device performance
will now be presented with reference to FIGURES 1-3.
cordance with the present invention, having the formula: 65 As shown in FIGURE 1, for a typical material of the
characteristic of the ferroelectric mixed crystals in ac
BaLio.sA11.4F1.2O2.a
present invention, BaLio,6Al1_4F1,2O2_8, it is seen that they
exhibit a nearly rectangular hysteresis loop which makes
FIGURE 2 is a graphic representation of spontaneous
them especially desirable for use as memory elements in
polarization and coercive ?elds plotted as a function of
computer circuitry. In FIGURE 2 there is shown curves
temperature for the same composition.
70 of the relatively slight variation of the spontaneous po
FIGURE 3 is a graph of dielectric constant versus tem
larization and coercive ?eld versus temperature over the
perature for the same typical mixed crystal composition.
range from room temperature to about 140° C. The di
3,023,167
.
.
4
3
_
having the formula BaLi0_6Al1_4F1_2O2_8 which has the
electric constant as a function of temperature is shown in
hexagonal unit cell dimensions a: 10.44 A. and c=8.77 A.
FIGURE 3, the Curie temperature being about 140° C.
4. A method of making a room temperature ferroelec
accompanied by a slight increase in the dielectric con
tric crystal which has the formula BaLi-ZXFHQP;x where
stant. The switching interval from one state of polariza
in “x” ranges from 0.15-0.30 comprising the steps of
tion to another is approximately 5 microseconds for 300
heating 12.07 grams of LiF, 87.68 grams BaFg and be
volt pulses applied to a crystal of 0.1 millimeter in thick
tween 0.34 and 8.5 grams A1F3-3H20 at about 1300° C.
ness, which indicates that high speed switching devices
for about 4 hours in air and cooling at the rate of about
may be constructed using the materials of the present
40° C. per hour to below about 800° C. and thereafter
invention.
While the invention has been particularly shown and 10 cooling the mass to room temperature and separating the
hexagonal crystals from the mass.
described with reference to preferred embodiments there
5. The method according to claim 4 wherein x=0.15
of, it will be understood by those skilled in the art that
and the amount of AlF3-3H2O is 0.34 gram.
various changes in form and details may be made therein
without departing from the spirit and scope of the inven
15
tion.
References Cited in the ?le of this patent
What is claimed is:
UNITED STATES PATENTS
1. A ferroelectric material having the formula
wherein “x” ranges from 0.15-0.30.
20
2. A room temperature ferroelectric having the for
1111113 BaLiD.32A11.68F0£403.36
3. A single crystal, room temperature ferroelectric
.
2,899,321
2,934,443
2,945,744
2,952,514
Mockrin _____________ __ Aug. 11,
Shell et al. __________ __ Apr. 26,
Knox ________________ __ July 19,
Smith ______________ __ Sept. 13,
1959
1960
1960
1960
2,954,300
Triebwasser __________ __ Sept. 27, 1960
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