US2724171A - Activation of ferroelectrics - Google Patents

Abstract

776,568. Ferroelectric materials. AMERICAN LAVA CORPORATION. Dec. 9, 1954 [Dec. 11, 1953], No. 35714/54. Class 37. [Also in Group XL (b)] A method of activating a ferroelectric ceramic comprises (i) applying an activating current to the ceramic with the ceramic heated to not more than 10‹ C. above its transition temperature and (ii) continuing to supply the activating current whilst allowing the ceramic to cool. The term " activated " means treatment to produce increased dielectric constant. The term " transition temperature " means the temperature at which the first crystal transformation above room temperature is effected. The current is D.C. and is preferably that which produces a change of at least 15 per cent and more preferably the maximum change in dielectric constant. The current may be maintained. substantially constant while the ceramic cools to at least 65‹ C. below its transition temperature. The maximum change in dielectric constant for a particular ceramic is found by using sample pieces of the material and subjecting each piece to activation until that current is found which produces the largest. change in dielectric constant. Detailed embodiment.-A ceramic cylinder 10, shown in Fig. 1, has silver electrode coatings 11, 12 and leads 13,14. The ceramic material comprises 96 per cent barium titanate and 4 per cent lead titanate. A number of such cylinders are immersed in chlorinated diphenyl and slowly heated to 10‹ C. above the transition temperature (120‹ C). A D.C. is then applied to leads 13, 14 and increased gradually until it reaches 14 micro-amperes per square inch of electrode surface. After a short time the temperature is lowered slowly and the current maintained constant by increasing the applied voltage as the cylinders cool to about 65‹ C. below the transition temperature. After complete cooling the diphenyl is removed by dipping the cylinders in acetone. The activating current value of 14 micro-amperes is that which produces the greatest change in dielectric constant for the given ceramic. This value is found by treating groups of cylinders using different current values for each group and measuring the capacity values and compiling results. Fig. 1 has application as a condenser or piezo-electric device. Ceramic materials.-The invention may be performed using the following ferro-electric ceramics : titanates of alkali metals and alkaline earth metals; tungstates, bismuth oxides, strontium cerate, boron phosphate, arsenic sesquioxide, lithium ferrite, lithium sulphate and antimony iodide; also titanate compositions comprising selectively as additives or minor proportions the stannates and zirconates of barium, lead and calcium and the titanates of calcium, strontium and lead.

Description

Nov. 22, 1955 J. D. WALLACE ACTIVATION OF FERROELECTRICS Filed Dec. 11, 1955 1 5 20 CURRENT (MICROAMPERES) INVENTOR JOHN D. WALLACE Fig. 3

.F. M A TTOR NE Y5 United States Patent- Ofiiice 2,724,171 Patented Nov. 22, 1955 2.724.171 ACTIVATION F FERRGELECTRIQS John D, Wallace, Qreland, Pa.

Application December 11, 1953, Serial No. 397,801

4 C a ms! 2 -35) (Granted under Title 35, U, S. Code (1952), See. 266) The invention described herein may be manufactured and used by or'for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a new method for activating ferroelectrics to impart to them electromechanical properties and to the product produced by the method,

Broadly, the process of the invention comprises sub: jecting ferroelectric materials to an external, electric field to produce materials of increased dielectric constant and having enhanced electromechanical properties. This procedure is referred to herein as activating the ferroelectric.

Previous methods directed to activating ferroeiectrics are based on the premise that the effect produce is due to a crystal realignment which may be effected by application of a voltage stress. For example, one prior art method comprises activating titanate ceramics by the application of high voltages at room temperature; and another method comprises the activation of barium titanate ceramic by the application of a voltage at the transition point of the material, both methods apparent- 1y based on the theory that a crystal realignment is effected by the voltage stress produced. These and other conventional methods are indicative of the fact that current criterion has apparently never been considered a factor in activating ferroclectrics.

Former methods for activating ferroelectrics are subject to the disadvantages that they do not produce a uniform'ly activated product or a completely activated product, due, mainly to the fact that without a consideration of current criteria there is no basis for control of the degree of activation of the material. Incomplete or non-uniform activation results in a relatively small increase in dielectric constant in the material as well as inferior electromechanical properties. Former methods lack reproducibility and also leave certain improvements to be desired in the product, such as higher coupling coefficient and improved hysteresis properties.

it is therefore an object of this invention to provide an improved method for activating ferroelectrics.

It is another object of the invention to provide a method for activating ferroelectric materials which is reproducible and by which complete activation may be obtained.

Itis a further object of this invention to provide an activated ferroelectric product for use as an electromechanical element having enhanced electromechanical properties, an increased dielectric constant, a high coupling coefiicient and improved hysteresis characteristics It has'been found that the above and other objects are accomplished in the preferred modification of the invention by the application of an optimum activation current to ferroelectric material at about its transition temperature, and maintaining this current substantially constan as the material cools, preferablyto a point at least 65 below the transition temperature.

In the accompanying drawings which are incorporated as a part of this application as an aid in illustrating the application of the invention to titanate ceramic,

Fig. 1 is an isometric View of a cylindrical transducer of barium titanate ceramic; v

Fig. 2 is a graph illustrating the percent increase in dl". electric constant in barium titanate ceramic elements re: sulting from activation by the present process; I

Fig. 3 shows a sampling of hysteresis loops made from data obtained on tranducers activated by the method of the invention, and I 4 Fig. 4 shows a sampling of hysteresis loops made from data obtained on tranducers activated by a former volt: age method.

The invention is illustrated in the specification and drawings by its application to barium titanate base ceramic, however, it is equally applicable to any ferro; electric ceramic, and particularly to titanate ceramics. Examples of other suitable materials are, titanates of other metals such as the alkali metals and other alkaline earth metals, niobates, and tantalates of alkali and alira: line earth metals and mixtures of all of the above. Addi: tional specific materials to which the invention is appli: cable are, tungstenatcs, bismuth oxides, strontium cerate, boron phosphate, arsenic sesquioxide, lithium ferrite, lithium sulfate and antimony iodide. The ceramics to which the invention applies may comprise of a single ferroelectric or mixtures of ferroelectrics, to, which ls m y b dde a mi r perc tag o v o s te orelectric additives for tailoring specific properties. Ex; amples of these additives are calcium titanate, calcium zirconate, strontium titanate, lead titanate, barium stan: mate and combinations thereof. 7

For h pu p of h pplicati n he tran iti n can perature is the temperature at which. the first crsytai transformation above room temperature is effected. In the case of barium titanate, for example, it is the tempera: ture at which the crystal structure changes from tetrag na t ic s u e n the spe ific i nd, c aims the transition temperature of the ceramic is to be inter: mat d to e h rans ti emperatu of the f r eelectric crystals contained in the ceramic. To ensure complete transformation, the material being activated is usua y eated to ab t 0 C- c' e t e trans t n e n: peraturc. This temperature is conventionally determ ed for y material by Peakin the d e t i c nst nt at the material. s

The op m a t o current ha b en ound is be that amount of current which effects the greatest change in dielectric constant between unactivated and activated material, and is a function of the cross sectional area .oi the material taken normal to the direction of polarization. It has been discovered that the percentage change in dielectric constant of a ferroelectric material upon activation is proportional to the degree of activation pro: duced in the material. This relationship between'the change in dielectric constant and the degree of activae tion of a material is an'important factor in the invention.

Activating current is necessarily expressed in terms of current per square inch ofelectrode surface for the particular material. The optimum activation current is con veniently obtained by maximizing the change in dielectric constant through activation using sample pieces of the material which have been electroded. That is, the material is subjected to the present process using various amounts of current until that amount of current is found which produces the largest change in dielectric constant.

It is believed that the reason for the effectiveness of the instant method is as follows: It is well established that the desired anisotrophy of barium titanate is QCCQITI? plished by the permanent displacement of the central titanium atom in the crystal lattice in the direction of oxygen atoms. According to the present invention, sufficient energy is provided throughout activation to hold the potential energy of the titanium atom at a constant value corresponding to a position near one of the oxygen atoms in the lattice. This requires furnishing enough energy to move the atom to the new position plus additional energy at a rate which equals its loss in thermal energy. This energy total can be calculated. Accordingly, the present process is based on the supplying of energy in the form of direct current to the element being activated. The amount of current, of course, depends on the size of the material being activated and the degree of activation required. In the preferred embodiment the current is maintained constant during the process by raising the voltage as the resistance increases with decrease in temperature of the material.

As an aid in illustrating the invention reference is made to Fig. 1. The ceramic cylinder was made by slip casting a composition of barium titanate, water and defiocculant and firing the formed casting to vitrification. However, various conventional processes may be used for making the ferroelectric ceramic elements. The electroded surfaces 11 and 12 are of glass base liquid silver and were applied by spraying on the silver and firing to approximately 1500 F. Leads 13 and 14 were then soldered on opposite electroded surfaces as shown using a 2% silver solder.

Activation of cylinders to produce transducers was accomplished as follows: a number of cylinders of barium titanate base ceramic were immersed in chlorinated biphenyl, a high resistance liquid, and slowly heated to a temperature about 10 C. above the transition temperature, this temperature being about 120 C. for barium titanate. Using an RA-38 rectifier power supply furnishing a maximum voltage and current, the current was applied through the leads and increased gradually until it reached 14 microamperes per square inch of electrode surface, the optimum amount of current for complete activation. This figure was obtained as explained above by maximizing the change in dielectric constant using samples of the material. After a short period, the temperature was lowered slowly and the current maintained constant by increasing the voltage as the pieces cooled to a temperature about 65 C. below the transition temperature. The current may be maintained substantially constant until the piece has cooled to room temperature, however, the above temperature is ordinarily an adequate minimum. After cooling, the biphenyl was removed from the pieces by dipping in acetone.

In operation as a transducer, acoustic waves or other mechanical stimuli striking the cylinder 10 result in the generation of a voltage which is taken off on leads such as those shown at 13 and 14. The device shown in Fig. 1 may also serve as a condenser when properly utilized.

The following tabulation of data obtained during activation of a batch of eight barium titanate cylindrical transducers was selected from a larger amount of similar data to illustrate the operation of the invention. The total current required for the group is shown in the table, a total current of 1.3 milliamperes giving 14 microamperes per square inch of total electrode surface. Activation was conducted over a period of about 90 minutes.

Table I Total Current (milliamperes) Temperature (degrees F.)

The elements used in obtaining the above data showed an average change in dielectric constant of about 19% as a result of the treatment. Activated material gave a coupling coefiicient of 25 percent.

Groups of barium titanate ceramic elements were activated by the process of this invention using difierent current values for each group, their average change in dielectric constant noted and the results tabulated in Table II below. The graph of Fig. 2 was made from the results shown in the table. The elements were cylindrical in shape and the outside surfaces were completely electroded with the exception of the end portions. The ceramic pieces had a composition of 96% barium titanate and 4% lead titanate. They had a height of 1 /2 inches, an average outside diameter of 1% inches and an average thickness of inch. The dielectric constant was ascertained as follows: A test bridge was used for obtaining the capacity values. The dielectric constant (k) was calculated using the following formula. For tubes:

height in inches. Groups 1, 2, 4 and 5 contained eight elements each and group 3 contained 36 elements.

Table 11 Percent Current change in Group (microdielectric amperes) constant (Average) The final dielectric constant is somewhat dependent upon the purity of the material, however, the percent change in dielectric constant does not vary appreciably with materials meeting reasonable purity standards.

The results in the above table show that the most effective current for titanate ceramics is a current of 14 microamperes per square inch of electrode surface. Use of this current in the process was found to produce complete activation as evidenced by the graph. However, other current values are shown to be effective, and particularly the range from 7.5 to 20 microamperes. The optimum activation current of 14 microamperes per square inch of electrode surface applies to barium titanate ceramics containing PbTiOa, CaTiOs, and the stannates and zirconates of barium, lead and calcium as additives. The invention, including the range of current values, is applicable to all ceramics having the perovskite type crystal structure. As the results indicate, the invention is not limited in its application to the optimum activation current alone, but in its broadest aspect includes hte application of quanta of current, i. e., activating currents to ferroelectrics under the stated conditions to effect activa tion through an intra-crystal rearrangement, as contradistinguished from prior methods which were based on the application of voltage directed to effecting a change in crystal orientation through stress.

Neither is the invention in its broadest sense restricted to maintaining the initial current constant during cooling, but it includes varying the current as the material cools, the requirement being that a substantial quantum of current, in the order of several microamperes, be supplied throughout the cooling period.

For purposes of comparison, elements of barium titanate ceramic similar to those used to obtain the results shown above were subjected to a standard voltage activation process, i. e., they were subjected to various voltages at the transition temperature and the voltage Table 111 Percent change in dielectric constant Voltage A comparison of the results of the standard voltage method and the current method of this invention shows the effectiveness of the present method, particularly when the optimum activation current is used. Further, the results constitute some evidence that activation is an energy phenomenon accompanied by a change in intra-crystal structure rather than a stress phenomenon accompanied by a realignment of crystals.

The method was found to be highly reproducible, a factor resulting from the use of current rather than voltage criterion as a basis for control of the degree of activation of the material.

For a further comparison of the effectiveness of the two methods, reference is made to Fig. 3 showing hysteresis loops produced by pieces activated by the method of this invention and to Fig. 4 showing hysteresis loops produced by pieces activated by the conventional voltage" method. The hysteresis loops were observed using an oscilloscope with a standard circuit. The observations recorded in Figs. 3 and 4 were made on the same type of elements for which results are shown in Tables I and II. The elements used for the loops of Fig. 3 were subjected to a drive of 4,000 and 5,000 volts respectively and those of Fig. 4 were subjected to a drive of 2,000 and 2,500 volts respectively. The area under the loops shown in Fig. 3 is quite small as compared to that under the loops of Fig. 4 even though the loops of Fig. 3 were produced with twice the drive of those of Fig. 4, indicating a significant difierence in power loss to hysteresis in the pieces activated by the two methods.

The product of the invention as illustrated in Fig. 1, when properly utilized, may serve in various applications other than transducers, such as, frequency control devices, electromechanical filters, supersonic sound genera tors, microphones, telephone receivers, phonograph pickups, piezoelectric relays and similar devices. evident from the properties set forth above.

The method of the invention provides a product highly suitable for electromechanical applications as well as for applications based on a change in dielectric constant of the material. The method ensures a high degree of activation as well as favorable hysteresis properties in the product, and is reproducible.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. it is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. The method of activating a ferroelectric ceramic which comprises applying electrodes to opposite sides of the ceramic, applying to the ceramic at approximately its transition temperature direct current within a range of about 7 to about 20 microamperes per square inch of electrode surface, and maintaining the supply of current substantially constant until the ceramic cools.

2. The method of activating a ferroelectric ceramic which comprises applying electrodes to opposite sides of the ceramic, applying to the ceramic at its transition temperature about 14 microamperes of direct current per square inch of electrode surface, and maintaining the supply of current substantially constant as the ceramic cools.

3. The method of making an electromechanical element which comprises subjecting a ferroelectric ceramic to an activating current at approximately the transition temperature of the ceramic and maintaining the activating current substantially constant until the ceramic cools.

4. The method of making an electromechanical element which comprises subjecting a ferroelectric ceramic to an activating current at approximately the transition temperature of the ceramic and maintaining the activating current substantially constant until the ferroclectric ceramic cools to about 65 degrees centigrade below its transition temperature.

This is Gray Nov. 1, 1949 Cherry Jan. 16, 1951

Claims (1)

1. THE METHOD OF ACTIVATING A FERROELECTRIC CERAMIC WHICH COMPRISES APPLYING ELECTRODES TO OPPOSITE SIDES OF THE CERAMIC, APPLYING TO THE CERAMIC AT APPROXIMATELY ITS TRANSITION TEMPERATURE DIRECT CURRENT WITHIN A RANGE OF ABOUT 7 TO ABOUT 20 MICROAMPERES PER SQUARE INCH OF ELECTRODE SURFACE, AND MAINTAINING THE SUPPLY OF CURRENT SUBSTANTIALLY CONSTANT UNTIL THE CERAMIC COOLS.

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