TITANATE CERAMICS FOR ELECTROMECHANICAL PURPOSES Hans Jaffe The Brush Development Company, Cleveland, Ohio
A development of increasing scientific and technical importance is the electromechanical effect that is exhibited by barium titanate ceramic under the influence of a high electric polarizing field. Polarized polycrystalline barium titanate shows certain striking analogies to ferromagnetic materials, and it also exhibits properties usually associated with piezoelectric single crystals. The outstanding piezoelectric effects in barium titanate ceramic are the development of alternating electric charges when a n alternating stress (impinging sound waves) is applied parallel to the direction of polarization, and conversely,
expansion and contraction in the direction of an electric alternating current field superimposed parallel to the prepolarization. This latter effect is especially useful for the production of ultrasonic waves in liquids. The ceramic can be shaped into large plates, spherical radiators, and the like, which would be costly to build from piezoelectric crystals such as quartz. Introduction of titanate ceramic radiators into the ultrasonic field may revolutionize the application of ultrasonic radiation to chemical processes involving interaction between solids and liquida or solids and gases.
T
tions based on barium titanate were discovered around 1940 (7, IS, 16). Much pioneering development on these materials was done by E. Wainer and co-workers a t the Titanium Alloy Mfg. Company,
ITANIUM dioxide (rutile) has been known for some 30 years to show a dielectric constant of about 100, which is a t least e factor of 10 higher than that of typical insulators. About 1940 it was found that certain titanates, barium titanate in particular, had a dielectric constant ten or more times higher than rutile. Titanate ceramics have become an important raw material for the manufacture of capacitors, especially in certain types of high voltage condensers such as those used in television sets.
IOOO/K
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010-
~ a ~ i 0 3
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SrTiOQ
- 100- 50 -
Titanate Dielectrics
- -
Ceramic components based on titanium dioxide and alkaline earth titanates have found important applications in the electronic industry as capacitors, and, more recently, as converters of electrical into mechanical energy and vice versa. These applications are based on the unusually high values of dielectric constant in some of these titanates and certain electromechanical phenomena that are connected with the high dielectric constant. The high values of dielectric constant are, at least in part, a consequence of the high electron polarizability exhibited by compounds containing titanium atoms surrounded by oxygen atoms, in particular where the oxygens are located at the six corners of an octahedron. This high electron polarizability is also responsible for the high refractive index exhibited by titanium dioxide, especially in the form of rutile, and many titanates. The high refractive index is the reason for some of the advantages of titanium pigments in paints, enamels, etc. The dielectric constant of titanates useful for electric circuit components ranges all the way from 13 for magnesium titanate to several thousand for solid solutions of strontium titanate in barium titanate. For practical as well as theoretical reasons, dielectric constants should be compared on a reciprocal rather than a direct scale (Figure 1). Titanium dioxide in the form of rutile and several titanates of the perovskite type appear grouped together at the top of the graph. All these compounds have titanium atoms surrounded octahedrally by oxygens (Figure 2); magnesium titanate, which has a different arrangement, is closer to other common dielectrics. Ceramic capacitors containing rutile were introduced around 1930. The much higher values of dielectric constant of composi-
- 20-
50
MgTiO3
- 100-
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MgO
10-
- 8-
-
150-
Steatite
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6
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5-
-.
1-
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3-
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Mica -
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SJ:f,.r cryEtallIne
- 300- 350Polystyrene -400
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-2.5-
Figure 1. Dielectric Constants of Capacitor Materials Plotted on Reciprocal Scale 264
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but pertinent information remained restricted in this country until the end of the war. The dielectric properties of rutile, magnesium titanate, calcium titanate, strontium titanate, barium titanate, and barium-strontium titanate mixtures have been described in a comprehensive publication by von Hippel and coworkers a t the Laboratory for Insulation Research (14).
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weight % barium titanate and 27% strontium titanate, but the dielectric constant falls off sharply with either increase or decrease of temperature. Some improvement in temperature characteristics is obtained by replacing a fraction of the titania with stannic oxide or zirconia. Powdered titaniaand alkalineearthtitanates of aformand purity suitable for ceramic preparations are commercially available from the Titanium Alloy Manufacturing Division of The National Lead Company. The materials may be shaped by one of the usual ceramic procedures, such as pressing, extruding, or slip-casting, followed by firing a t temperatures in the vicinity of 2500 ’F. The biggest field of application for the various types of titanate ceramic capacitors is in television set manufacture, where their compactness is of great value (IS). Nevertheless, the total consumption of titanium oxide for dielectric uses hardly exceeds 1% of the demands of the pigment and allied industries.
Figure 2. Cubic Unit Cell of Perovskite-Type Compound BaTiOs above 120° C.
Since the war, titanate ceramic capacitors have found an increasingly important place in the electronic industry. Capacitors based on rutile combine a dielectric constant around 100 with a very low loss factor and moderate negative temperature coefficient of capacity. A dielectric constant of 37 with substantially zero temperature coefficient has been found for a composition of 83 mole yo titanium dioxide and 17 mole yo barium oxide (8). Compositions approximating the formula of barium titanate, BaTiOs, have a dielectric constant of more than 1000, but this high value is accompanied by a rather large dielectric loss (dissipation factor 0.01 to 0.02), and marked dependence of capacitance on temperature (Figure 3, top). A peak dielectric constant of 10,000 at room temperature is obtained in a combination of 73
2400 9000
1600
1900 800 9300
99.m
2100
0.94
0 .PO 0 .I
-80
-40
0
TEMPERATURE,
40 C.
80
120
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Figure 3. Dielectric Constant, Frequency Constant, and Piezoelectric Coupling Coefficient of Prepolarized Barium Titanate Bar in Lengthwise Resonant Vibration
A
6
Figure 4. Polarization us. Voltage Oscillograms for Various Peak Voltages at 60 Cycles A.
BaTiOs ceramic showing fairly low dielectric hysteresis. Extreme curve a t 30 kv./cm. peak Length of horizontal calibration line -20 t o +20 kv./cm. peak; vertical line, 5 to + 5 microooulombs/sq. om.
B . BaTiOs ceramic showing high hysteresis.
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Ferroeleotricity of Barium Titanate Barium titanate excels not only in the high level of its dielectric constant, but also in its characteristic temperature and voltage dependence of dielectric polarization, which make it the closest electric analog known so far to magnetic iron. The salient facts are: The dielectric constant shows a very sharp maximum of almost 10,000 near 120’ C., the “Curie point”; below this temperature, the dielectric constant is strongly dependent on voltage, first increasing and then decreasing with applied voltage gradient; and there is pronounced dielectric hysteresis (Figure 4). These dielectric properties are not markedly dependent on the frequency of the applied electric field between 100 cycles per second and several megacycles; this proves that the hysteresis phenomenon is not a frictional effect but is due to the existence of an essentially stable polarized state of the electric material, which is analogous to the magnetic condition of a permanent magnet. This does not imply that a piece of barium titanate which has been subjected to an electric polarizing field is the permanent source of an external electric field, and that it attracts or repels electrically charged particles. The absence of such external forces is due to the neutralization of the internal polarization by compensating electrit surface charges; to these there is no magnetic analog. When a barium titanate element is polarized for the first time by an appropriately high voltage, such as 20 kv. per cm., a high charging current flows onto the electrodes to provide external compensation of the induced internal polarization. Only a rather small discharge current will flow through a short circuit applied after polarization, but if subsequently the element is heated to above the Curie point, the remainder of the electric charge will flow off the electrodes. Crystal structure analysis shows that the hysteresis phenomena of barium titanate reside in crystalline domains which are of
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remain after the polarizing field is removed (1 1 ), In other words, a polycrystalline ceramic material, barium titanate, may be given a permanent piezoelectric response comparable to the response previously obtained only from a single crystal of intrinsic polar nature by treatment with an electric polarizing field applied for a short period of time. The piezoelectric response of prepolarized barium titanate is frequently found to drop by about 20% in an initial period after the application and removal of the polarizing field, but will then become practically constant. The curves of Figure 6 illustrate the electrostrictive and piezoelectric response of a barium titanate sample. The curve on the left shows essentially 24 26 96 96 94 electroatrictive response with a certain amount Figure 5. X-Ray Rack-Reflection Diagrams, Copper Radiation of hysteresis. The curve on the right shows a superposition of electrostrictive (parabolic) and Right. ,,(Ba, Sr)TiOz cubic symmetry Center. BaTiOs, tetragonal. 26 line is split 'up by deviation from cubic symmetry, and piezoelectric (linear) response t,o an applied shifted outward owing to larger lattice constant. Left. Back-reflection from stationary BaTiOa sample; number and intensity of individual alternating- electric field; the linear resDonR spots give indication of grain size. is due to a polarized condition of the material. Compositions of barium and strontium titanate having the peak dielectric constant-that is, their Curie pointpolar nature. This polarity disappears at the Curie point; benear room temperature are in general not so desirable for eleclow this point the crystal symmetry is tetragonal, above it is tromechanical purposes as plain barium titanate, because they cubic (Figure 5). require a maintained polarizing field in order to show linear elecThe internal state of polarization can be detected by optic or tromechanical response. Addition of lead titanate, on the other x-ray methods (4, IO) and, most important, by the electromehand, raises the Curie point and may be of advantage for some chanical behavior. applications (P). Electromechanical Action in Barium Titanate
It had been known for many years that all dielectric bodies are subject to electrostriction-that is, a mechanical deformation proportional to the square of an applied electric field. In ordinary dielectrics this effect is extremely small. However, in barium titanate the effect is of a very different order of magnitude than in previously investigated dielectric materials. A direct current polarizing field of 1000 volts per mm. produces a deformation of about 1 part in 3000. A smaller alternating field superimposed on the polarizing field will produce additional deformation which is essentially proportional to the alternating current signal. Furthermore, the polarized piece of barium titanate shows an electric response to an applied mechanical force. These actions of the polarized barium titanate ceramic are similar to the piezoelectric effect commonly associated only with single crystals of a polar type of symmetry.
A
B
Figure 6.
Electrostrictive Strain Perpendicular to Applied Field Calibration lines. Horizontal, - 10 t o + 10 kv./cm.
vertical, - 2 to + 2 X 10-5 relative change in length. Frequency 100 cycles/sec. BaTiOo ceramic, not prepolarized, quadratic response BaTiOs ceramic, prepolarized, considerable linear component in response
peak; A.
B.
The need for an electric polarizing field adds some complications. It is therefore of great significance that the proportional mechanical response to an applied alternating field and the proportional electric response t o an applied external force
Figure 7.
Electrode Disk of Titanate Ceramic
Arrows indicate directions of motion produced by electric field.
The best idea of the strength of a piezoelectric effect is given by the piezoelectric coupling coefficient. This is a dimensionless magnitude which in the ideal case would have the value 1 0 0 ~ o . That would mean that all elastic reaction to an applied external force would be of directly useful dielectric nature and, conversely, that all dielectric polarization in the material would appear as an elastic stress. It has been found that prepolarized barium titanate has a coupling coefficient up to 5070, which brings i t almost to the top of the list of available piezoelectric or electromechanical materials. The type of mechanical motion induced in barium titanate by an applied electric voltage is illustrated in Figure 7 . The principal action is an expansion parallel to the applied electric field (9, 6, 9). With this goes a lateral contraction in all directions a t right angles to the electric field. Resonant frequency for a bar of unit length, and coupling coefficient for the lateral effect as a function of temperature, are shown in Figure 3. The total sidewise contraction amounts to about 80% of the lengthwise expansion, leaving a net volume effect (6). If an electric alternating current signal is applied parallel to a direct current polarization, the element will pulse in and out in the direction of the applied field and correspondingly expand and contract sidewise. Applied alternating pressure is most effective if it acts parallel to the direc-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
February 1950
Figure 8. 1
267
Electroded Spherical Bowl of Titanate Ceramic Arrows indicate converging motion.
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tion of polarization; it produces an alternating voltage in the same direction.
Ceramio Transdnoers I n the field of electromechanical transducers two single-crystal piezoelectric materials are predominant. One is Rochelle salt, which shows a high piezoelectric effect but a somewhat limited thermal, mechanical, and chemical stability. The other is quartz, which has excellent thermal and mechanical properties but a rather low piezoelectric effect and is of limited availability. Ammonium dihydrogen phosphate (ADP) and ethylenediamine tartrate ( E D T ) crystals, more recently introduced, show useful intermediate properties without reaching either the sensitivity of Rochelle salt or the stability of quartz. Some of the electromechanical coefficients of these crystals are compared in Table I with data obtained on prepolarized barium titanate.
Table I.
Piezoelectric Data on Common Electromechanical Transducer Materials
Rochelle Salt (340C.) Quartz
Mode
d 10 -11 Meter per Volt
K
Volt-Meter per Newton
k
Lateral Parallel Lateral Lateral Parallel Lateral
165 2.3 2.3 24 190 78
200 4.5 4.5 15.3 1700 1700
0.093 0.058 0.058 0.177 0.0125 0.0052
0.54 0.10 0.10 02.8 0.46 0.19
P
N H ~ ~ ~ P(ADP) OI, Barium titanate ceramic prepolarbed d. Piezoelectric modulus, indicates mechanical strain per applied electrical field strength. R. Relative dielectric constant. 0. Volta e output coefficient, indicates piezoelectric field strength per applied mec%anical stress. k. Piezoelectric coupling coefficient. ~
Barium titanate shows good chemical stability. It can be exposed to very high temperatures and operated a t temperatures up to 100' C. It has a high level of piezoelectric activity. Most of all, because it is a polycrystalline material, it can be shaped in large and complicated forms which would be difficult if not imp*
high in barium titanate. Frequently it is desired to obtain an extended sound field of high-frequency ultrasonic waves which requires large plates of the electromechanically sensitive material. It may further be de-
Figure 9. Titanate Bowl under Oil, Driven a t Its Thickness Resonance, about 500 Kilocycles Converging ultrasonic waves a r e produced in liquid; a spout rises from focus.
Praotical Ultrasonic Generator Design and operation of barium titanate ceramic transducers may be illustrated by some data on a laboratory size ultrasonic generator. The transducer element in this generator is a spherical bowl of about 10-om. chord diameter and 6-cm. radius of curvature. The bowl is fabricated to a uniform thickness of between 2.5 and 10 mm. and equipped with fired-on silver electrodes. Operation is usualiy a t the fundamental thickness mode resonance, which is given by the relation:
Figure 10.
Electroded Tube of Titanate Ceramic
Arrows indicate directions of motion produced by electric field. Ultrasonic waves focused along axis of tube are obtained if it is filled.with a liquid and driven a t resonant frequency correspondlng t o wall thickness.
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Resonance frequency (in kilocycles/sec. ) X thickness (in mm.)
=
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ultrasonic energy to processes involving emulsification, dispersion, catalytic action of solids suspended in liquids, and direct acceleration of chemical reactions (1). Biological action of ultrasonic energy on suspended living matter has also been reported. Much remains t o be done in the laboratory to give us the understanding and control needed for successful use of ultrasonic energy on an industrial scale. Titanate ceramic transducers will provide the plant scale ultrasonic apparatus a t reasonable cost when needed.
2550
Because of the high piezoelectric coupling of barium titanate, tuning to resonance is not so critical as with quartz transducers. In view of the high dielectric constant, the electric energy is supplied at the comparatively low potential of about 50 volts. Power rating for continued operation is 1.2 watt per square centimeter, although the ceramic can stand a t least ten times as much without losing its polarization if kept from overheating. The transducer is surrounded by transformer oil of low viscosity and enclosed in a housing able to withstand up to 40 atmospheres’ internal pressure. The ultrasonic power emitted from the inner surface of the bowl is concentrated in a focal area of less than 0.5 sq. em. near the center of curvature of the bowl. A sound energy flow of 100 watts per square centimeter in the oil corresponds to a pressure amplitude of 15 atmospheres. The sound-transmitting oil is maintained under hydrostatic pressure to suppress cavitation (vapor-bubble formation) which would otherwise occur during the negative pressure phase of the sound wave, and would cause a high rate of sound absorption. The medium to be treated, if in liquid form, is contained in a glass cylinder whose rounded bottom is just below the sound wave focus. The material in the glass container is usually not under pressure, so that cavitation occurs freely in the substance to be treated. Cavitation is probably the phenomenon responsible for the.majority of reported chemical and physical effects of ultra3onics.
dcknowledgment Most of the piezoelectric data in this paper are based on measurements by G. X. Cotton. Practical transducer development is due to C. K. Gravley, G. D. Gotschall, and others a t The Brush Development Company.
Literature Cited Bergmann, L., “Ultrasohall,” 3rd German ed., Ann Arbor, Mich., Edwards Bros., 1944. Bunting, E. N., Shelton, G. R., and Creamer, A. S., J . Research Xatl. Bur. Standards, 38, 337 (1947).
Cherry, W. L., Jr., and Adler, R., Phys. Res., 72, 981 (1947). Danielson, G., Acta Crystallographica,2, 90 (1949). Donley, H. L., RCA Rev., 9, 218 (1948). Jaffe, H., Phys. Rev., 73, 1260 (1948); Electronics, 22 ( l ) , 128 (July 1948). Jonker, G. H., and Van Santen, J. H., Science, 109, 632 (1949). Koren, H. W., J . Acoust. Soc. Am., 21, 198 (1949). Mason, W. P., Phys. Idew., 74, 1134 (1948) ; Bell Lab. Record, 27,
Industrial Application of Ultrasonic Energy The laboratory size ultrasonic generator described is comparable to available quartz crystal generators in total ultrasonic power produced, but offers the features of efficient focusing and a pressurized coupling medium between transducer element and work load, permitting a concentration of power on the latter otherwise not obtainable. A pilot plant size transducer of 1-foot diameter and 1.5-kw. rating has been developed. The introduction of barium titanate electromechanical ceramic thus opens the field of ultrasonic energy to a wide range of practical uses. There have been many publications on application of
285 (1949).
Matthias, B., and von Hippel, A., Phys. Rev., 73, 1378 (1948). Itoberts, Shepard, Ibid., 7 1 , 8 9 0 (1947). Thurnauer, H., and Deaderick, J., I?.S. Patent 2,429,588 (filed Oct. 2, 1941). Van Arsdell, Elm. M f g . , 41, 104 (1948). Von Himel, Breckenridge, CHEW, - Chesley, and Tisza, IND.EKG. 38, 1697 (1946).
Wainer, E., U. S. Patent 2,467,169 (filed Nov. 12, 1942). R E C E I V ~September D 26, 1949.
COURTESY BUREAU OF MINE8
Titanium Powder and Blocks of Consolidated Metal Prepared from Powder by Pressing and Sintering
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e
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END O F SYMPOSIUM (The paper,
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Determination of Oxygen in Titanium,’’ by Dean I. Walter is in the February 1950 issue of Analytical Chemistry. Reprints of the Titanium Symposium may be purchased from the Reprint Department, American Chemical Society, 1155 Sixteenth St., Y. W., Washington 6, D. C.)
