CA1040847A - Piezoelectric ceramic compositions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention relates to a piezoelectric ceramic composition of a solid solution of a material, Pb (Sn1/3Nb2/3)xTiyZrzO3, wherein 0.01?x?0.25, 0?y?0.75, 0?z?0.875 and x+y+z=1, which further contains a quantity of cobalt equivalent to from 0.1 to 3 weight percent of cobalt oxide (CoO). This ceramic has a high mechanical quality factor, high stability of resonant frequency over wide temperature range and high stability of capacitance ratio over a wide time range.

Description

1~4~B47 This invention relates to piezoelectric ceramic compositions and articles fabricated therefrom. More parti-cularly, this invention pertains to novel ferroelectric ceramics which are polycrystalline aggregates Pf certain constituents. -These piezoelectric compositions are sintered to ceramics by ordinary ceramic techniques and the thus made ceramics are polarized by applying a D.C. voltage between the electrodes to impart thereto electromechanical transducing properties ,. ,~ - .
`r similar to the well known piezoelectric effect. The invention -also encompasses the calcined product of raw ingredients and the articles of manufacture such as electromechanical transducers fabricated from the sintered ceramic.
The ceramic bodies materialized by this invention exist basically in the solid solution comprising the systems Pb(Sn /3Nb )o3-PbTiO3-PbZrO3~ pb(snl/3Nb2/3) 3 3 Pb(Snl/3Nb2/3)03-PbZrO3, modified with CoO additive from 0.1 to 3 weight percent, respectively.
The use of piezoelectric materials in various transducer applications in the production, measurement and _1- ~' ~
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', . ' ,' ' ' ' 1~4~47 sensing of sound, shock, vibration, pressure, etc. has increased greatly in recent years. Both crystal and ceramic types of transducers have been widely used. But, because of their potentially lower cost and facility in the fabrication of ceramics with various shapes and sizes and their greater durability at high temperatures and/or humidity than crystalline substances such as Rochelle salt, piezoelectric ceramic materiaLs have become important recently in various transducer applications.
The required piezoelectric characteristics of ceramics apparently vary with species of applications. For example, electromechanical transducers e.g. for phonograph pick-ups and microphones require piezoelectric ceramics characterized by substantially high electromechanical coupling coefficient 7 and dielectric constant. On the other hand, it is desired in ceramic filter application of piezoelectric ceramics that the materials exhibit a high value of mechanical quality factor.
Furthermore, ceramic materials require a high stability with temperature and time in resonant frequency and in other electri-cal properties.
As more promising ceramics for these requirements, lead titanate-iead zirconate is in wide use up to now. However, -~
it is difficult to get a very small temperature coefficient `~
of resonant frequency in the lead titanate-lead zirconate ceramics. Moreover, the dielectric and piezoelectric properties of the lead titanate-lead zirconate ceramics change greatly with the firing technique, which relates to evaporation of PbO.
It is, therefore, the fundamental object of this invention to provide novel and improved piezoelectric ceramic materials which overcome at least one of the problems outlined above.
A more specific object of the invention is to provide improved polycrystalline ceramics characterized by high

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mechanical quality factor along with high stability over wide temperature range in resonant frequency and high stability over wide time range in capacitance ratio.
Another object of the invention is the provision of novel piezoelectric ceramic compositions, certain properties of which can be adjusted to suit various applications.
. A further object of the invention is the provision of an improved electromechanical transducer utilizing, as the active elements, an electrostatically polarized body of the novel ceramic compositions.
These objects of the invention and the manner of their attainment will be readily apparent from the following description and from the accompanying drawing, in which Fig. 1 (single figure) is a cross-sectional view of an electromechanical transducer embodying this invention.
Before proceeding with a detailed description of the piezoelectric materials contemplated by the invention, their ; piezoelectric application in electromechanical transducers will be described with reference to Figure 1 wherein reference character 7 designates, as a whole, an electromechanical transducer having, as its active element, a preferably disc shaped body 1 of a piezoelectric ceramic material according to : this invention.
The body 1 is electrostatically polarized in a manner hereinafter set forth, and is provided with a pair of electrodes 2 and 3 applied in a sùitable manner on two opposed surfaces thereof. Wire leads 5 and 6 are attached conductively to the electrodes 2 and 3, respectively, by means of solder 4. When the ceramic is subjected to shock, vibration or other mechanical stress, tXeelectrical output generated can be taken from the wire lead 5 and 6. Conversely, as with other piezoelectric transducers, application of electrical voltage to the electrodes ... ~ .

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2 and 3 will result in mechanical deformation of the ceramic body. It is to be understood that the term electromechanical -transducer as used herein is taken in its broadest sense and ` includes piezoelectric ceramic filters, frequency control devices, and the like, and that the invention may also be used and adapted to various other applications requiring mat-erials having dielectric, piezoelectric and/or electrostrictive properties.
` According to this invention, the ceramic body 1, Fig. 1, is formed from a novel piezoelectric composition which is a polycrystalline ceramic composed of Pb(Snl/3Nb2~3)03-PbTiO3-PbZrO3' Pb(Snl/3Nb2/3)o3-PbTiO3 or Pb(Snl/3Nb2/3)o3 PbZ 3;
modified with a CoO additive, respectively.
This invention is based on the discovery that within particular ranges of these systems, the specimens modified with a CoO additive exhibit relatively high mechanical quality ~-factor along with high stability with temperature in resonant ~ ~
frequency and high stability with time in capacitance ratio. ~ -This invention has various advantages in the 20 manufacturing process in application for ceramic transducers.
It has been known that the evaporation of PbO during firing is a problem when sintering of lead compounds such as lead titanate zirconate. The inventive composition, however, has a smaller amount of evaporated PbO than usual lead titanate zirconate does. The invented system can be fired without any ... 5, ~ .
particular control PbO atmosphere. A well sintered body of the composition according to this invention is obtained by ~iring in a ceramic crucible with a ceramic cover made of an A1203 ceramic. A high sintered density is desirable for humidity resistance and high piezoelectric response when the sintered body is employed in a resonator etc.

Some compositions falling within the system - -, 1~14'~)847 ':
Pb(Snl/3Nb2/3)03-PbTiO3-PbZrO3 do not exhibit high piezo-` electricity, and many are electromechanically active only to a slight degree.
This invention is concerned with piezoelectric ceramic compositions comprising a solid solution of a material represented by the formula: ;
Pb(snl/2Nb2/3)xTiyzrzo3 !~ wherein the ranges for x, y and z are O.Ol x<0.25, O y_0.75 and O_z_0.875, and wherein x+y+z=l, and further containing a quantity of cobalt equivalent to from Q.l to 3 weight percent of cobalt oxide (CoO) and electromechanical trans-ducers comprising said compositions.
The compositions described herein may be prepared in accordance with various well-known ceramic procedures.
One preferred method which will be more fully described hereinafter contemplates the use of PbO or Pb304, SnO or SnO2, Nb205, Tio2~ ZrO2 and CoO, Co203 or Co304. The following method, however, should not be construed as limitative.

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l'~g~ 7 The starting materials, viz., lead oxide (PbO), stannic oxide (SnO2~, niobia (Nb2O5), titania (TiO2), zirconia (ZrO2) and cobalt oxide (CoO), all of relatively pure grade (e.g., C.P. grade) are intimately mixed in a rubber-lined ball mill with distilled water. In milling the mixture, care should be taken to avoid (or the pro-portions of ingredients should be varied to compensate for) contamination by wear of the milling ball or stones.
Following the wet milling, the mixture is dried.
Thereafter, the mixture is pre-reacted by calcination at a temperature of around 850C for 2 hours.
After calcination, the reacted material is al-lowed to cool and is then wet-milled to a small particle size.
CoO additive may be added to the reacted material after calcin-ation of raw materials which do not include CoO, and the reacted material with CoO additive may be milled to a small particle size. Once again, care should be taken to avoid (or the "" -.

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.: -104~ 7 proportions of ingredients should be varied to compensate for) contamination by wear of the milling balls or stones.
Depending on preference and the shapes desired, the material may be formed into a mix or slip suitable for pressing, slip casting, or extruding, as the case may be, in accordance with conventional ceramic procedures. The samples for which data are given hereinbelow were prepared by mixing 100 grams of the ;~ milled pre-sinte~ed mixture with 5 cc of distilled water.

The mixture was then pressed into discs of 20 mm diameter and 2 mm thickness at a pressure ~f 700 kg/cm . The pressed discs are fired at 1180-1290C for about 45 minutes. According to this invention, there is no need to fire the composition in an atmosphere of PbO and no special care is required for the temperature gradient in the furnace, as compared with the prior art. Thus, according to this invention, uniform and excellent piezoelectric ceramic products can be easily obtained simply -by covering the samples with an alumina crucible during firing.
The sintered ceramics are polished on both surfaces to the thickness of 1 mm. The polished disc surfaces may then be coated with silver paint and fired to form silver electrodes.

Finally, the discs are polarized while immersed in a bath of silicone oil at 100C. A voltage gradient of D.C. 4 KV/mm is obtained for one hour, and the discs are field-cooled to room temperature in thirty minutes.
The piezoelectric and dielectric properties of the polarized specimen were measured at 20C in a relative humidity of 50 %. Examples of specific ceramic compositions according to this invention and various pertinent electromechanical and `

dielectric properties thereof are given in Table 1.
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1~4f~847 Table 1.
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. . _ .. _~ .
-- -- -- Compositions- 24 hours Example ~le % of basic o~sitions ~CoO -poling P-T fr~TC
No. I additi~e% % %
PbSnv3Nb2/3O3 P~Ti03 PbZrO3 by ~eight P QM
1 25.0 75.0 _ O.S 39.2 5101.320.23 2 25,0 37.5 37.5 O.S 4.7 ~800.980.28

3 12.5 A3.5 44.0 0.1 2.7 1700,590.08 12,5 43.5 44.0 1.0 3.0 3900.530.17 12,5 q3,5 44.0 3.0 4,5 2300.380.25 10` 6 ` 12,5 _ 87.5 O.S 40.9 620 1.71 0.28 7 B.O ~6.0 48.0 0.1 2.8 190 1.32 0.09 ~ 6.0 ~6.0 ?~8.~ 1.0 3.1 530 0.91 0.15 9 3.0 47.0 50.0 0.2` 3.3 230 0.89 0.22 3.0 47.0 50.0 0.5 3.4 500 1.37 0.17 ~1 1.n _ _ ~7.0 52.0 -- 0.2 - 3.4 250 0.70 ~

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p is a capacitance ratio - resonant frequenc [P 2(antiresonant frequency-resonant frequency]
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QM is mechanical quality factor. P-T is the change in capacitance ratio(p) between 24 hours after poling and 1000 hours after ` 20 poling. fr-TC is the change in resonant frequency (fr) within the range -40C to 80C. -~
The compositions of this invention modified with -a-- CoO additive exhibit a relatively high mechanical quality ,' factor (QM) as compared with that of basic compositions, that is the basic compositions without CoO exhibit QM of about 100.
! From Table 1 it is evident that the piezoelectric ceramics of this invention exhibit a high stability in resonant frequency with temperature over wide temperature range, and high stability in capacitance ratio with time after poling.
The composition of Example No. 3 shows a very high resonant frequency stability with temperature. These properties are , :, ,, : ., ' . ', ., : . , :. . :

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important to the use of piezoelectric ceramic compositions in ceramic filter, frequency control device etc..
Ceramic compositions containing CoO additive less than 0.1 weight ~ exhibit relatively low mechanical quality factor. And with compositions containing CoO additive more than 3 weight ~, mechanical quality factor is relatively low.
For this reason, they are excluded from the scope of the present invention.
In addition to the superior properties shown above, the compositions according to this invention yield ccramics of good physical quality and which polarize well. It will be understood from the foregoing that the ternary solid solution Pb(Snl/3Nb2/3)O3-PbTiO3-PbZrO3 modified with a CoO additive form an excellent piezoelectric ceramic body.
It will be evident that the starting materials to be used in this invention are not limit~d to those used in the above examples. Those oxides, hydroxides and carbonates may be used instead of any starting material of above examples, which are easily decomposed to form required compositions at elevated temperature.

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Claims (5)

The embodiments of the invention in which exclusive property or privilege is claimed are defined as follows:

1. A piezoelectric ceramic composition comprising a solid solution of a material represented by the formula:
Pb(Sn1/3Nb2/3)xTiyZrzO3 wherein the ranges for x, y and z are 0.01?x?0.25, 0?y?0.75 0?z?0.875, and wherein x+y+z=1, and further containing a quantity of cobalt equivalent to from 0.1 to 3 weight percent of cobalt oxide (CoO).

2. An electromechanical transducer element comprising a ceramic composition as claimed in claim 1.

3. A piezoelectric ceramic composition consisting essentially of a material represented by the formula:
Pb(Sn1/3Nb2/3)0.125Ti0.435Zr0.440O3, and further containing a quantity of cobalt to 0.1 weight percent of cobalt oxide (CoO).

4. An electromechanical transducer element comprising a piezoelectric ceramic composition as claimed in claim 3.

5. A method for the preparation of the ceramic compos-ition of claim 1, comprising: intimately wet-mixing PbO, SnO2, Nb2O5,TiO2 ZrO2 and CoO; drying said mixture; pre-reacting said mixture by calcining at a temperature of around 850°C
for 2 hours; cooling said calcined mixture; milling said calcined material to a smaller particle size, shaping said particulate mixture and firing said shaped mixture.