US 3365400 A
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Jan.23, 1968 c. F, PULVARI 3,365,400
` ELECTRICAL DEVICES EMEODYIN FERRIELECTRIC SUBSTANCES Filed March 14, 1960 IN1/EN TOR.
" @www E Palm@ United States Patent O 3,365,400 ELECTRECAL DEVICES EMBODYING FERRIELEC'ERIC SUBSTANCES Charles Ferenee Pulvari, 2014 rIaylor St. NE., Washington, D C. 20018 Filed Mar. 14, 1960, Ser. No. 14,585 Claims. (Cl. 252-623) This invention relates to new substances having ferroelectric properties and to electrical devices utilizing such substances.
When ordinary ferroelectrics are compared with magnetic materials it is observed that the most striking difference is the fact that unlike magnetic materials ferroelectrics do not have a threshold switching -field, because dipole-dipole interaction perpendicular to the direction of polarization is weak as compared to the spin-spin interaction in magnets in the same direction. ln single domains of ordinary ferroelectrics dipoles point in the same direction, as a result, the interaction between dipoles side by side cannot be strong. In antiferroelectric materials, opposite dipoles are side by side to each other, therefore interaction between their opposite dipoles is strong. The interaction of antidipoles in antiferroelectrics is so strong that no polarization reversal of practical usefulness is observed, and the net spontaneous polarization in these materials appears to be zero. These facts led to the present invention.
According to this invention, it was visualized that if a material with an antidipole structure can be produced similar to an antiferroelectric material, the dipoles of which however would be unequal, a new material should result with a strong dipole-dipole interaction comparable to the spin-spin interaction in magnetic materials and a threshold switching field should appear. This new type of Vmaterial was first discovered when two antiferroelectric materials were in a solid solution. In the following the name ferrielectrics, analogous to its ferrimagnetic counterpart, will be used to identify this new group of materials or substances. On a macroscopic scale below the Curie temperature the electrical behavior of ferrielectrics is essentially similar to that which is associated p with ferroelectrics. However, on a microscopic scale the sub-lattices do not contain equal and identical numbers of dipole moments as do the antiferroelectrics, and a net spontaneous polarization results. In general, materials exhibiting ferroelectric properties composed of sub-lattices with interlaced unbalanced anti-dipole structure, representing an incompletely compensated antiferroelectric material, will be referred to as ferrielectrics regardless whether they are composed of two different antiferroelectrics or they have said structure genuinely.
According to this invention two antiferroelectric materials are brought in a solid solution which form then a ferrielectric material, with an interlaced antidipole struc- .f
ture and incompletely compensated antiferroelectricity. The internal fields of such a structure combine so as to represent a threshold switching field. This special structure justifies the name ferrielectricity.
Because ferrielectrics differ from ferroelectrics in their microscopic structure they have a number of heretofore not observed interesting electrical characteristics.
The favorable switching properties of these new materials render them adaptable to a number of heretofore not feasible practical applications. Among these as examples are mentioned their use as a condenser dielectrics and as piezoelectric elements. For these purposes, ferrielectrics may be used in the form of single crystals or in the form of polycrystalline materials such as ceramics. Other applications are the uses of such condensers as improved transpolarizers, information storage elements and matrices, logical circuits, etc. Another application is 3,365,400 iyatented dan. 23, 1968 rice the use of these materials as improved piezoelectric elements for sensing various physical properties such as pressure, temperature, volume changes, in general, their use as transducers. Again, another application of these new materials is their combined use as a condenser dielectric and piezoelectric element, for example, in an electromechanical lter-network utilizing the unusual dielectric qualities of these new ferrielectrics.
Among the unusual electrical properties of ferrielectrics it was already mentioned that their switching characteristic exhibits a higher non-linearity and a more pronounced threshold switching field than common ferroelectrics. Their switching properties are similar to magnetic materals and switching transients appear only beyond a certain field strength. FIGURE l shows switching characteristics of two different electrical condensers, one of which comprises common ferroelectrics as a dielectric, while the other one comprises the new ferrielectrics as a dielectric. The applied switching field E is plotted on the abscissa, and the maximum values of the switching currents measured on a load resistance are plotted on the ordinate. The dashed line 10 -represents the characteristic switching behavior of heretofore known ferroelectrics, and the solid lines 11 represent the switching behavior of the newly invented ferrielectric dielectric and show a higher order nonlinearity than ordinary ferroelectrics. The switching behavior of ferrielectrics can not be represented by the simple exponential relationship:
found for heretofore known or ordinary ferroelectrics (I. Appl. Phys. 29, 1315-1321 (1958), instead an initial coercivity, i.e., a threshold switching eld E0 has to be considered below which switching practically does not occur. This threshold ield was heretofore not observed in ordinary ferroelectrics and as a consequence it limited their practical usefulness.
Since a number-of these ferrielectrics are composed of antiferroelectrics possessing a rather high Curie temperature (375 C.), a considerably improved domain stability was obtained in single crystals as compared with ordinary ferroelectrics Domains once aligned do not change into other type of domains as easily as they do in heretofore known ferroelectrics. Again, another advantage of ferrielectrics is that since it is a complex compound it lends itself for engineering various types of ferrielectrics with properties desired for a given application. Coercivity as well as polarization of these materials can be varied within a wide region which is very desirable for a number of important applications.
The first ferrielectric material discovered was the heretofore not reported mixed crystal composed of two antiferroelectric materials, namely, sodium niobate and sodium vanadate:
The Curie temperature may vary depending on the composition, values obtained for various compositions are as follows:
This material exhibited well-saturated hysteresis loops from room temperature up to 225 C. This limit may be further improved when more perfect crystals will be produced. Good quality single crystals could be grown within a wide 4range of compositions. The solid lines in FIG. 1 show the characteristic switching behavior of this new material. Polarization, coercivity switching behavior can be varied within a wide range depending on the cornposition.
Another mixed crystal composed of two antiferroelectric materials, namely, sodium niobate and silver vanadate is also ferrielectric.
Another example of this novel ferrielectric group is composed of the two antiferroelectric lead zirconate and lead hafnate:
4with a Curie temperature of about 210 C. Here again the hafnium atom tits very well into the zirconate structure similarly as vanadium does in the niobate structure. Wellsaturated hysteresis loops are again obtained from room temperature up to the Curie temperature.
Other combinations of antiferroelectric materials resultiug in a ferrielectric are:
A further modification of ferrielectric materials is obtained when small amounts of impurities are introduced in the crystal in order to cause changes in the various properties or behavior, such as, for example, to influence the conductivity of the crystal or the crystal growth habit, etc. One method by which crystals of the ferrielectric substances of this present invention have been prepared is by reacting: 2.1 624 gm. (0.02 M) of sodium carbonate, 0.3638 gm. (0.002 M) of vanadium pentoxide, and 4.7844
i gm. (0.018 M) of niobium pentoxide, as follows:
This mixture, with the addition of some acetone, was thoroughly ground in a tungsten carbide lined electric grinder and was then placed in a small platinum crucible and loaded, at room temperature, into the furnace. The temperature was then raised to l390 C. and a soaking time of two and one half hours was allowed. Cooling was done on a 5 degrees per hour down to 1300o C. After this, the cooling rate was increased to 30 degrees per hour down to 1200 C., after which a more rapid cooling ended the cycle.
The crystals were cubic-like, light brown with clear-cut shiny faces and had easy cleavage planes.
Thin plates which have been cleaved from larger cubes and have been prepared for investigation show large homogeneous domain areas, which could be observed in cross-polarized light under a microscope. It was for the rst time that in mixed crystals large uniform domain structure was obtainable. It is noted that this good domain structure was quite abundant and even from a small batch a large number of good samples could be produced.
. Crystals were also prepared by reacting at 1200o C. a mixture having the weight proportion of the desired composition. For illustration purposes, a sodium vana- Y date and 85% sodium niobate mixture is given. This calcined material was finely ground and packed in a platinum Crucible with sodium iiuoride ux in a ratio of i The furnace was heated again to ll00 C. and the mixture soaked for three hours, after which a 5 degrees per hour cooling rate was used until 1000 C. The flux was then poured off and cubic like crystals were obtained.
FIGURE 2 is a perspective view of an electrical device which is useful as a condenser or as a piezoelectric element and comprises the ferrielectric materials of this invention.
FIGURE 3 is a perspective view of a similar electrical device as shown in FIG. 2 with two condensers.
FIGURES 4 and 5 exemplify other configurations of condensers useful as electrical capacitors or as piezoelectric elements comprising the ferrielectric materials of this invention.
FIGURE 6 represents an example of a multicondenser device embodying the ferrielectric substances of this invention.
In the devices shown in FIGS. 2-6, the novel ferrielectric `body of this invention is shown in form of various shapes of slabs or circular discs (FIGS. 4 `and 5). The electrodes are adherent metal coatings l2 and 13 formed on the opposite sides of the bodies 14. Lead wires 15 and 16 are electrically connected to the electrodes for example by soldering. The devices shown in FIGS. 2-6 may serve, when properly utilized, as an electric condenser or as a piezoelectric device.
When used as an electric condenser, the device makes use of the high dielectric constant,` and the novel heretofore not available dielectric properties of these new ferrielectric materials.
In order to expose the dramatic differences between ordinary ferroelectrics and ferrielectrics, it is sufficient to refer to a few of the numerous novel electrical properties already mentioned. The unusual high nonlinearity of the switching characteristic with a threshold switching field, domain stability, the fact that polarization and coercivity can -be varied within wide limits depending on the composition of these materials are all contributing to produce improved capacitors for various purposes.
When the devices shown in FIGS. 2-6 are used as a piezoelectric element, it may be operated While subjected to a constant direct-current biasing field. j
When subjected to such a field, the bodies 14 exhibit piezoelectric properties in that it changes in physical size in response to changes of a potential applied across the body/ in a direction having a component parallel to the direction of the biasing field, and in that, when subjected to mechanical stress, it generates a potential, in the direction of the biasing field, which varies with variations in lthe applied stress. The effectiveness of the piezoelectric element increases as the superimposed direct-current field 1s increased.
The direct-current biasing field for piezoelectric use may be established by maintaining a direct-current Voltage across the electrodes 12 and 13 While the device is in use. A similar result can be achieved by subjecting the ferrielectric body to a high direct-current potential gradient for a substantial period of time prior to use. Upon removal of this direct-current potential, a residual polarization remains in the body which can be used as the source of the requisite direct-current field without the use of an externally applied direct-current potential.y Good permanent polarization can be induced in ferrielectrics because of their threshold field, i.e. high coercivity. The residual polarization may be obtained more effectively if the body is heated to a temperature above the Curie temperature and is then allowed to cool to room temperature under a high direct-current potential gradient.
The devices of FIGS. 2 to 6, when operated with an adequate direct-current bias either externally applied or vresulting from remnant polarization in the `bodies 14,
`.ay be used for any of the known piezoelectric purposes. Thus, it may be used in the usual manner as a frequency control device or as an electromechanical filter, the alternating-current voltage being applied across the leads and 16 and the external direct-current biasing voltage, if any, being applied across the same leads. A suitable externally applied direct-current biasing gradient may be between about 2,000 volts per centimeter and 40,000 volts per centimeter.
The device may also be used in the usual manner as an electromechanical transducer where it is desired to convert variations of electrical current or potential into corresponding mechanical variations, or vice versa, as in supersonic sound generators, microphones, telephone receivers, phonograph pick-ups, piezoelectric relays and similar devices. In such devices, the usual mechanical means are supplied for either transmitting mechanical energy to the body 14 as in microphones and phonograph pick-ups, or utilizing the mechanical energy generated in the body, as in supersonic generators, telephone receivers and relays.
As mentioned above, the body 14 of FIGS. 2 to 6 may be formed of the ferrielectric substance in the orm of a single crystal or crystal section or in the form of a coherent polycrystalline body, such as a ceramic body prepared by sintering together finely divided particles of the ferrielectric crystals.
The devices of the present invention have been described as made up essentially of electrodes; spaced by a coherent body of one or more ferrielectric crystals. These devices may be manufactured according to the techniques known in the art for the manufacture of analogous devices embodying other ferroelectric crystal bodies. The best results are obtained when the electrodes consist of an adherent conductive coating formed directly on the ferrielectric body, as by the application of a conventional silver paste, which is later red to produce an adherent durable solid conductive coating, or by applying a sprayed or evaporated metal layer.
It may obviously be desirable to form the devices of the present invention with more than two electrodes in some instances. When the ferrielectric body is in a polycrystalline substance, it may obviously be readily formed into various shapes other than those shown in the drawings. Thus, it may be formed in the shape of a tube having an internal metal coating and an external metal coating as electrodes or it may be formed as an annular ring having suitably disposed electrodes.
The invention has been described above in terms of its specic embodiment and, since modiiication and equivalents will be apparent to those skilled in the art, the description is intended to be illustrative of, and not a limitation upon, the scope of the invention.
1. A ferrielectric body having fer-roelectric properties, said body consisting essentially of nely divided crystal particles including at least two antiferroelectric substances, each of said substances having the formula A1303 wherein A is a member selected from the group consisting of sodium, silver, and lead, and B is a member selected from the group consisting of niobium, vanadium, zirconium, and hafnium.
2. A ferrielectric body having ferroelectric properties, said body consisting essentially of a plurality of mixed crystals composed of sodium niobate and sodium vanadate.
3. A ferrielectric body having ferroelectric properties, said body consisting essentially of a piuralilty of mixed crystals composed of sodium niobate `and silver vanadate.
4. A ferrielectric body having ferroelectric properties, said body consisting essentially of a plurality of mixed crystals composed of lead zirconate and lead hafnate.
5. A ferrielectric body having erroelectric properties, said body consisting essentially of a plurality of mixed crystals composed of ammonium dihydrogen phosphate and ammonium dihydrogen arsenate.
6. A ferrielectric body 'having ferroelectric properties, said body consisting essentially of a plurality of mixed crystals composed of ammonium dihydrogen phosphate and deuteroammonium dideuterium phosphate.
7. A ferrielectric body having ferroelectric properties, said body consisting essentially of a plurality of mixed crystals composed of ammoniumv dihydrogen arsenate and deuteroamrnonium dideuterium arsenate.
8. The method of making a ferrielectric body having erroele-ctric properties which comprises the steps of mixing together sodium carbonate, vanadium pentoxide, and niobium pentoxide, heating the mixture to a predetermined temperature, maintaining said temperature for a predetermined time, and then gradually cooling the heated material until it becomes crystalline.
9. The method of making a ferrielectric body having ferroelectric properties which comprises the steps of mixing together sodium Vanadate and sodium niobate, adding a metal uoride ux to said mixture, heating said mixture and iiux to a predetermined temperature, maintaining said temperature for a predetermined period of time, cooling the heated material gradually to a predetermined temperature, and then pouring olf said ux when said cooled material becomes crystalline.
10. An electric device comprising at least two conducting electrodes spaced by a ferrielectric body composed ot a plurality of mixed crystals consisting essentially of at least two antiferroelectric substances, each of said substances having the formula ABO3 wherein A is a member selected from the group consisting of sodium, silver, and lead, and B is a member selected from the group consisting of niobium, vanadium, zirconium, and hafnium, said ferrielectric body having ferroelectric properties.
References Cited UNITED STATES PATENTS 2,911,370 11/1959 Kulcsar 252-629 2,960,411 11/1960 Brajer et al.
2,976,246 3/ 1961 Egerton et al. 252-629 2,906,710 9/1959 Kulcsar 310-8 2,928,032 3/ 1960 Daniel 317-262.
OTHER REFERENCES Goldsmith et al., Ferroelectric Behavior of Thiourea, Journal of Chemical Physics, vol. 3l, November 1959, pp. 1175-1187; 1187 most pertinent.
Megaw, Ferroelectricity in Crystals, Mehuen & Co., Ltd., 1957, pp. 110, 111, and 121.
TOBIAS E. LEVOW, Primary Examiner.
SAMUEL yBlilRNSTEIN, JOHN L. BURNS,
MAURICE A. BRINDISI, Examiners.
J. D. KALLAM, I. S. RAPPAPORT,
R. D. EDMONDS, Assistant Examiners,