Compositions – Piezoelectric
Patent
1996-12-30
1999-06-22
Kunemund, Robert
Compositions
Piezoelectric
117 4, 117 5, 117 6, 117945, 501134, C30B 2922
Patent
active
059140682
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to Bi-layer oxide ferroelectrics and more particularly, to Bi-layer oxide ferroelectrics which are particularly useful as capacitors, ultrasonic oscillators and the like and also to a method for making such Bi-layer oxide ferroelectrics in high accuracy.
BACKGROUND ART
Known oxide ferroelectrics which are employed as capacitors and transducers include, for example, Pb(Zr,Ti)O.sub.3 having the Perovskite structure and Bi-layer Perovskite compounds.
The term "Perovskite structure" used herein is intended to mean a crystal structure represented by the general formula, ABO.sub.3, wherein the BO.sub.6 oxygen octahedra with consists of six-coordinated octahedra of oxygen ions (O) about a given cation (B) undergo mutual corner sharings to form a three-dimensional fundamental structure in such a way that the resultant spaces are occupied with another type of cation (A). The above-indicated Pb(Zr,Ti)O.sub.3 represents a compound having the Perovskite structure wherein Pb corresponds to the A ion and the (Zr, Ti) corresponds to the B ion. In the above formula, the parentheses ( ) mean that at least one element selected from the elements included in the parentheses should be principally contained. It is a matter of course, however, that if a mixing ratio is defined, the elements being mixed are contained in such a compositional ratio.
Moreover, the term "layer Perovskite structure" is intended to mean a crystal structure wherein pseudo-Perovskite structures and intermediate layers (i.e. a layer intercalated between adjacent pseudo-Perovskite structures) are alternately located, as having such a structure as of pseudo-Perovskite structure/intermediate layer/pseudo-Perovskite structure/intermediate layer/ . . . , at a given period in one direction. The term "pseudo-Perovskite structure" means a structure which fundamentally consists of a Perovskite structure but whose elemental ratios are not exactly represented by ABO.sub.3 because the pseudo-Perovskite structure is invariably contiguous to the intermediate layers.
Further, the term "Bi-layer Perovskite structure" means such an intermediate layer as mentioned above wherein the cation in the intermediate layer consists of Bi. For instance, known compounds having a typical layer Perovskite structure include (Bi.sub.2 O.sub.2)(A.sub.n-1 B.sub.n O.sub.3n+1). In this instance, the intermediate layer consists of (Bi.sub.2 O.sub.2) and the pseudo-Perovskite structure consists of (A.sub.n-1 B.sub.n O.sub.3n+1). The suffix, n, means that corner-shared BO.sub.6 oxygen octahedral are stacked n times within the pseudo-Perovskite structure along the direction wherein the pseudo-Perovskite structures and intermediate layers are alternatively located.
FIG. 1(b) is a schematic projection view of a crystal structure, as viewed from the side of the layer, of a Bi-layer Perovskite compound of the formula wherein n=3, (Bi.sub.2 O.sub.2)(A.sub.2 B.sub.3 O.sub.10). In the figure, rectangles 11 each having a "cross" symbol therein, respectively, represent a BO.sub.6 octahedron formed by oxygen ions octahedrally coordinated about the B ion. These BO.sub.6 octahedra are three-dimensionally shared at the corners thereof to form a pseudo-Perovskite structure 18. The space between the adjacent BO.sub.6 oxygen octahedra 11 is occupied with an A ion 12. A region surrounded by a dotted line 15 indicates a Bi.sub.2 O.sub.2 intermediate layer isolating adjacent pseudo-Perovskite structures 18 from each other. This intermediate layer is constituted of Bi ions 13 and O ions 14.
A pure Perovskite structure has elemental ratios of A:B:O of 1:1:3. With the pseudo-Perovskite structure, the elemental ratios are n-1:n:3n+1, disenabling one to express the elemental composition as ABO.sub.3. As having set out hereinbefore, this is because the pseudo-Perovskite structures and the intermediate layers adjoin each other at a given period. As will be apparent from FIG. 1(b), the sites to be occupied by the A ions 12 are occupied by the Bi ions 13.
The compounds of the afor
REFERENCES:
patent: 5519566 (1996-05-01), Perino et al.
patent: 5683614 (1997-11-01), Boyle
"Structural Basis of Ferroelectricity in the Bismuth Titanate Family", R.E. Newnham, R.W. Wolfe and J.F. Dorrian, Materials Research Laboratory, pp. 1029-1039, vol. 6, No. 10, Jul. 28, 1971.
Ikegama et al, "Piezoelectricity in Ceramics of Ferroelectrics Bismuth Compound with Layer Structure" Jpn. Jr. of App. Phy. vol. 13, No. 10 Oct., 1975.
Hiratani Masahiko
Imagawa Kazushige
Kushida Keiko
Takagi Kazumasa
Hitachi , Ltd.
Kunemund Robert
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