Mechanochemical fabrication of electroceramics

Details Mechanochemical fabrication of electroceramics Mechanochemical fabrication of electroceramics

2001-03-26

2003-09-30

Wood, Elizabeth D. (Department: 1755)

Compositions

Piezoelectric

Lead, zirconium, titanium or compound thereof containing

C252S069000

Reexamination Certificate

active

06627104

ABSTRACT:

This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/SG99/00062 which has an International filing date of Jun. 26, 1999, which designated the United States of America.
FIELD OF THE INVENTION
The invention relates to the fabrication of technologically demanding electroceramic materials, such as PbO-based relaxor ferroelectrics with the general formula of Pb(B′B″)O
3
, where B′ is low valence cations such as Mg
2+
, Fe
2+
, Zn
2+
, Ni
2+
, Co
2+
, Mn
2+
, Sc
3+
, Al
3+
and In
3+
, and B″ is high valence cations such as Nb
5+
, Ta
5+
and W
6+
, and piezoelectrics. The required electroceramic phases of perovskite structure are formed by mixing and reacting the constituent oxides in a mechanochemical reaction chamber. Intermediate compounds, such as pyrochlore phases, may or may not be involved in the mechanochemical synthesis. The loss of volatile components, such as lead (PbO), may be avoided as the reaction may be activated by mechanical energy or a combination of mechanical and thermal energies at room temperature or a temperature considerably lower than the calcination temperature in the conventional ceramic processing routes. Sintered electroceramics are fabricated by directly shaping and sintering the resulting electroceramic powders, while all the phase-forming and powder milling steps prior to sintering are preferably omitted. The mechanochemical processing technique of the invention has been used to produce: (a) electroceramic powders of nanometer to micrometer in size, without having to pre-react the starting oxides or nonoxides at an intermediate temperature which is almost always involved in synthesizing the ceramic powders via other fabrication routes (e.g. Columbite method); and (b) sintered electroceramics at much lower sintering temperatures than those required by all the currently used industrial processes.
BACKGROUND OF THE INVENTION
Electroceramics, such as lead oxide-based relaxor ferroelectrics (e.g. PMN, PMN-PT, and PMN-PZN-PT) and piezoelectrics (e.g. PZT and PLZT) are functional materials indispensable in almost all the electronic and microelectronic devices. They exhibit a perovskite structure and demonstrate unique dielectric, piezoelectric and ferroelectric properties, depending on the type and composition of phases retained in the sintered ceramic form. For example, PMN-based relaxor ferroelectrics are characterized by a very high dielectric constant (>15,000) and broad maxima, together with a high electrostriction coefficient. PZT exhibits some of the most desirable piezoelectric properties required in the applications such as transducers, mechanoelectrical sensors and actuators.
Each of these electroceramic materials can be fabricated via several processing routes, although the suitability of these techniques and the microstructure and properties of resulting electroceramic vary considerably from one to another. Using PMN-based relaxor ferroelectrics as example, there are at least four types of very different fabrication routes: (i) Conventional ceramic processing technique, which involves mixing of the starting oxide constituents and reacting them together by a calcination treatment at intermediate temperatures in the range of 700 to 1000° C., followed by milling of the pre-reacted ceramic phases prior to shape forming and sintering at a high enough temperature. See, for example, M. Lejeune and J. P. Boilot, Ceramics International, Vol. 8, 1982, pp.99-103; (ii) Columbite method which is a modification of the conventional mixed oxide method whereby the constituent MgO and Nb
2
,O
5
are first mixed and reacted together to form the columbite phase prior to mixing and reacting with PbO. See, for example. S. L. Swartz and T. R. Shrout, Materials Research Bulletin, Vol. 17 (1982) pp.1245-1250. Similarly, the perovskite PMN phase may also be synthesized by pre-reacting PbO and Nb
2
O
5
before MgO is mixed and reacted in. See, for example, J. P. Guha and H. U. Anderson. Journal of American Ceramic Society, Vol. 69, 1986, pp.c-287-288. Both of the two processes inevitably involves multiple calcination and milling steps, before a PMN ceramic can be made by consolidating the resulting electroceramic powder of perovskite structure at the sintering temperature; (iii) Wet chemistry routes, where a PMN precursor is first synthesized via one of the many wet chemistry-based routes such as co-precipitation, sol-gel routes, citrate, oxalate and partial oxalate methods. Perovskite ceramic phase is then formed by calcining the resulting chemical precursor at an intermediate temperature, prior to shape forming and sintering at a high enough temperature. See, for examples, F. Chaput, J. P. Boilot, M. Lejeune, R Papiernik and L. Hubert-Pfalzgraf, Journal of American Ceramic Society, Vol. 72, 1989, pp.1355-1357; Y. Yoshikawa and K. Uchino, Journal of American Ceramic Society, Vol. 79,1996, pp.2417-2421; A. Watanabe, H. Haneda, Y. Moriyoshi, S. Shirasaki, S. Kuramoto and H. Yamamura, Journal of Materials Science, Vol. 27, 1992, pp.1245-1249; J. C. Ho, K. S. Liu and I. N. Lin, Journal of Materials Science, Vol. 30, 1995, pp.3936-3943; and (iv) molten salt method, in which the required perovskite PMN phase is synthesized by reacting the constituent oxides in a molten medium of low melting point, such as KCI, NaCI and PbO. See, for examples, K. Katayana, M. Abe and T. Akiba, Ceramics International, Vol. 15, 1989, pp.289-295; H. M. Jang, S. H. Oh and J. H. Moon, Journal of American Ceramic Society, Vol. 75, 1992, pp.82-88. These four types of processing route for PMN are considerably different in many aspects, including the types of starting material required and the ability to deliver a sintering-reactive PMN powder of perovskite phase and therefore in the microstructure and electrical properties of sintered PN. Most of the wet chemistry-based processing routes use high purity inorganic or organometallic chemicals, such as nitrates, chlorides and alkoxides, as the starting materials. They are many times mole expensive than the industrial ceramic oxide powders and many of these chemicals are highly sensitive to moisture and therefore are difficult to handle in a large production scale for industrial applications. Furthermore, almost all these wet chemistry-based processing routes are very low in production yield and most of them have yet demonstrated any significant advantages than the conventional ceramic processing route.
In contrast, the conventional mixed oxide route involves mixing and calcining oxide powders at an intermediate temperature and subsequently milling the pre-reacted electroceramic phase, although inexpensive oxides are used as the starting materials. Its application is, however, limited by the inevitable occurrence of pyrochlore phases (e.g. Pb
3
Nb
4
O
13
, Pb
3
Nb
2
O
8
, and Pb
2
Nb
2
O
7
) in the pre-reacted PMN powders and it is impossible to eliminate them from the sintered electroceramic. This has effectively made the conventional mixed oxide route inapplicable in fabricating PbO-based relaxor ferroelectrics. The Columbite method, which has been dominating the fabrication of PMN and PMN-based relaxor ferroelectrics over the last 15 years since its discovery in 1982 by S. L. Swartz and T. R. Shrout, see Materials Research Bulletin, Vol. 17 (1982) pp.1245-1250, is able to deliver a PMN powder of predominant perovskite phase. However, it is associated with such fetal disadvantages as the multiple calcination and milling steps required, which cause contamination to the electroceramics and significantly degrade their electrical properties. As pointed out by A. J. Moulson and J. M. Herbert (Electroceramics, Chapman and Hall, London, 1990), contamination in the level of 0.1 to 2 wt % is common under normal milling conditions. This is not acceptable for most of the electroceramic materials. As a result of the poor powder characteristics, a sintering temperature in the range of 1200 to 1350° C. is needed for the Columbite-der

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