Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2001-08-24
2003-09-16
Brunsman, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C252S06290R
Reexamination Certificate
active
06620752
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to lead magnesium niobate perovskite compounds. More particularly, the invention relates to lead magnesium niobate-lead titanate solid solution compounds and products thereof. The present invention also relates to textured lead magnesium niobate-lead titanate solid solution compounds.
BACKGROUND OF THE INVENTION
Ceramic compounds which have a perovskite crystal structure have numerous commercial applications. These applications include: dielectric materials for capacitors; piezoelectric materials for transducers and sensors; electrostrictive materials for micropositioners and actuator devices; and transparent electrooptic materials for information storage and optical signal processing.
The perovskite structure, as typified by BaTiO
3
above 135° C., is cubic. This structure is a regular array of oxygen ions at the face centers, small tetravalent titanium ions in the center, and big, divalent barium ions located at the corners. The perovskite structure in ferroelectric compounds is distorted at low temperatures and exhibits tetragonal, orthorhombic, or rhombohedral symmetry. At higher temperatures, the structure transforms to cubic. The transition temperature at which the distorted phase transforms to the cubic phase is called the Curie point. The ferroelectric behavior is caused by distortions in the crystal lattice caused by shifts in the position of the central cation.
A relatively new class of ferroelectric materials is PbO-based complex perovskite corresponding to the formula Pb(B
1
,B
2
)O
3
. The B
1
cation can be one of several low valence cations such as Mg
2+
, Zn
2+
, Ni
2+
, and Fe
3+
, and the B
2
cation can be one of several higher valence cations such as Nb
5+
, Ta
5+
, and W
5+
. These ferroelectrics have promise for dielectrics such as ceramic capacitors, piezoelectrics, and electrostrictive actuators (e.g., micropositioner) applications, depending on composition.
Ceramic processing of ferroelectrics of lead magnesium niobate (“PMN”) by conventional milling and calcination techniques is difficult. For example, it is extremely difficult to produce PbMg
1/3
/Nb
2/3
O
3
by conventional mixed oxides processing due to formation of a stable Pb-niobate pyrochlore phase during calcination. Repeated calcination at high temperature (1000° C.) is required to form PMN powder. Moreover, at these high temperatures, the volatility of PbO alters stoichiometry and prevents complete reaction. As a result, excess PbO is required.
Several processing steps are required to form a PMN powder into a shape and to densify it into a functional electrical ceramic element. The powder first is formed into a green body such as by dry pressing. The green body then is densified by sintering. Sintering is a key aspect of the manufacturing process and must be controlled to produce uniform, dense ceramic products. The uniformity and density of the products produced, however, are highly dependent on the ceramic powder employed.
Lead-based relaxor ferroelectric-PbTiO
3
solid solutions of the perovskite crystal structure which have the general formula Pb(B
1
B
2
)O
3
—PbTiO
3
, (“PMN-PT”)where B
1
can be any of Zn, Mg, Sc, Ni, Yb, Fe, Co, Cu, and Cd and B
2
is any of Nb, Ta, Ti, Zr, Hf, and W have excellent dielectric and electromechanical properties. Compounds of this formula which are slightly on the rhombohedral side of the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases have excellent dielectric and electromechanical properties. For example, the compound 0.67PMN-0.33PT (67PMN-33PT) which is slightly on the rhombohedral side of the MPB has a longitudinal piezoelectric coefficient (d
33
) as high as 640-700 pC/N. Also it is known that <001> oriented cuts of single crystal 65PMN-35PT have piezoelectric coefficients (d
33
) >1500 pC/N and longitudinal electromechanical coupling coefficients (k
33
) >0.9. See Park et al.,
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control,
vol. 44, pp. 1140, 1997.
These properties of PMN-PT type ceramics have spawned renewed interest in growing PMN-PT type ceramics because of their potential for improving the performance of transducers and actuators.
Various methods have been used to grow lead-based ferroelectric single crystals. Typically, these methods employ the high-temperature flux process and the Bridgeman method. These methods, however, have not been satisfactory. For example, the high-temperature flux process has suffered the disadvantage of difficulty of control over the crystallographic growth direction of the single crystals, as well as control of the size of the single crystals produced. In addition, the Bridgman method suffers the disadvantage of poor control over the chemical uniformity of the crystals produced. In addition to these disadvantages, each of these methods requires excess PbO to enhance crystal growth. Excess PbO, however, can limit the properties attainable in the single crystals as well as cause processing difficulties. As a result, control over the chemical uniformity of the PMN-PT crystals is expensive and difficult.
A need therefore exists for a method of manufacture of PMN-PT type textured ceramics which overcome these disadvantages. A need also exists for ceramic powders useful in manufacture of uniform, dense ferroelectrics such as those based on PMN, particularly those based on solid solutions of PMN and lead titanate (“PT”).
SUMMARY OF THE INVENTION
The present invention relates to PMN compounds, powders and products thereof, especially to PMN-PT compounds, powders and products which have the perovskite structure.
The PMN-PT compounds are characterized by the formula (1−x)Pb(Mg
1/3
Nb
2/3
)O
3
−xPbTiO
3
where x may vary from about 0.0 to about 0.95, preferably about 0.0 to about 0.40. More preferably, x is about 0.35. The formula (1−x)Pb(Mg
1/3
Nb
2/3
)O
3
−xPbTiO
3
also can be expressed as (1−x)PMN−xPT where x may vary from about 0.0 to about 0.95, preferably about 0.0 to about 0.40. More preferably x is about 0.35.
In a first aspect, the invention relates to preparing lead magnesium niobate-lead titanate compounds of the formula (1−x)Pb(Mg
1/3
Nb
2/3
)O
3
−xPbTiO
3
where x=0.0-0.95. The method entails mixing a blend including a source of lead oxide with magnesium niobate and fumed titanium oxide to form a mixture. Examples of useful sources of lead oxide include lead acetates-lead hydroxides such as Pb(CH
3
COO)
2
Pb(OH)
2
, lead acetates such as Pb(CH
3
COO)
4
, lead carbonate-hydroxides such as(PbCO
3
)
2
Pb(OH)
2
, and lead carbonates such as PbCO
3
. The mixture is milled to produce a blend of a particle size less than about 3 &mgr;m. Preferably, milling is performed by ball milling in distilled water. The blend is heat treated to produce a dried precursor powder. The dried precursor powder is sintered at about 900° C. to about 1300° C. to produce a lead magnesium niobate-lead titanate compound.
In a further aspect, the blend may include an oxide of any of Zr, Ta, La, Fe, Mn, Ni, Zn, and W and mixtures thereof. The blend also may include a binder such as polyvinyl alcohol, polyethylene glycol, methylcellulose, carboxymethylcellulose, ethylcellulose, hydroxpropylcellulose, polyethylene oxide base high polymers, acrylic base high polymers, maleic anhydride base high polymers, starch, gelatine, polyoxyethylene alkyl ether, polyvinyl butyrol and waxes.
In another aspect, the invention relates to a process for manufacture of 0.65PMN-0.35PT ceramics such as 0.65PMN-0.35PT single crystals. The process entails mixing (PbCO
3
)
2
Pb(OH)
2
of a particle size of less than about 6 &mgr;m, MgNb
2
O
6
having a specific is surface area of more than about 5 m
2
/g and fumed TiO
2
having a specific surface area of more than about 30 m
2
/g to form a mixture. The(PbCO
3
)
2
Pb(OH)
2
, fumed TiO
2
, and MgNb
2
O
6
are present in amounts sufficient to produce a ratio of (PbCO
3
)
2
Pb(OH)
2
:MgNb
2
O
6
:fumed TiO
2
of abou
Kwon Songtae
Messing Gary L.
Sabolsky Edward M.
Brunsman David
Law Offices of John A Parrish
The Penn State Research Foundation
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