AU4801890A - Process for producing highly crystalline and homogeneous sub-micron doped and undoped piezoelectric ceramic powders with controlled stoichiometry and particle size - Google Patents

Description

PROCESS FOR PRODUCING HIGHLY CRYSTALLINE AND

HOMOGENEOUS SUB-MICRON DOPED AND UNDOPED PIEZOELECTRIC CERAMIC POWDERS WITH CONTROLLED STOICHIOMETRY AND PARTICLE SIZE

FIELD OF THE INVENTION

The present invention relates to a process for making powders, particularly crystalline perovskite/lead zirconate titanate stannate powders. The powders may comprise single crystal, solid solution particles of lead zirconium titanate and may also contain other secondary ingredients, such as dopants and solid

solution substitutions, that alter electrical

properties. The powders have utility in commercial electrical element applications as, for example, dielectric ceramics, piezoelectric materials,

electrostrictive ceramics, transparent electrooptic ceramics, and piezoelectric composites.

BACKGROUND OF THE INVENTION

Perovskite compounds have the general formula ABO₃ and a close-packed structure with corner-shared octahedra, where the A cation is large and has 12-fold

coordination, the B cation is of intermediate size and has 6-fold coordination, and the O anion has 8-fold coordination. Elements which can fill the A position include Ba²⁺, Sr²⁺, Ca²⁺, Pb²⁺, K⁺, and rare earth elements. Elements which can fill the B site include Ti^{4 +}, Zr^{4 +}, Sn⁴⁺ , Nb⁵⁺ , and Ta⁵⁺ . Examples of materials with perovskite structure include "ideal" cubic SrTiO₃, tetragonal BaTiO₃, rhombohedral LaA10₃, and orthorhombic GdFeO₃. Ceramics, i.e., powders that have been sintered into a solid mass, particularly perovskite ceramics, have numerous commercial applications, mainly because of their useful electrical properties. These applications include: dielectric ceramics for capacitors;

piezoelectric materials for transducers and sensors;

electrostrictive ceramics for micropositioners and actuator devices; transparent electrooptic ceramics for information storage and optical signal processing; and 0-3 composites for hydrophone and medical applications. A good discussion of perovskite materials is given in the June, 1988 issue of Scientific American,

PEROVSKITES, page 74 to 81, in an articie by Robert M. Hazen.

The perovskite structure as typified by SrTiO₃ has a cubic structure. This structure consists of a regular array of oxygen ions at the corners, small tetravalent titanium ions in the center, and large, divalent

strontium ions located at the face centers. In

ferroelectric perovskite compounds, the crystal

structure is distorted at low temperatures, such that it exhibits tetragonal, orthorhombic or rhombohedral symmetry. At higher temperatures, the structure

transforms to cubic; this transition temperature from the distorted phase to the cubic phase is called the Curie point. Ferroelectric behavior is caused by distortions in the crystal lattice due to shifts in the position of the central B cation (e.g., the Zr or Ti ion in Pb(Zr,Ti)O₃) ; this results in a displacement of the centers of the positive and negative charge of the ions within the structure and thus a net (or "spontaneous") polarization of the structure. The electrical properties of perovskite ceramics are significantly influenced by ferroelectricity, giving rise to useful dielectric, piezoelectric, electrostrictive, and electrooptic properties. The electrical properties of perovskite ceramics can be controlled by the wide range of

compositional modifications that are possible, allowing the electrical properties to be tailored to those required for a specific application. The electrical properties of perovskite ceramics are also affected by manufacturing and processing conditions, as more fully described below.

The requirements of a powder for the numerous electrical applications of perovskite ceramics depend on the specific material and its application. However, in most applications, the "ideal" powder is considered to have a fine particle size, narrow or no particle size

distribution, chemical homogeneity, controlled

stoichiometry, equiaxed particle shape, and to be agglomerate free. After a powder has been prepared, several processing steps are required to form the powder into a shape and to densify it into a finished

functional electrical ceramic product. A powder is first formed or compacted into a partially dense shape called a green body. The exact shape depends on the electrical product's intended function and application, e.g., an electromechanical sensor or transducer. Once the powder is formed into a green body (e.g., by dry pressing or tape casting), the compact must be densified by hot pressing, sintering, or the like. Sintering involves heating the green body to high temperature and allowing densification to occur by diffusional

processes. The sintering conditions, e.g., time, temperature, pressure, and atmosphere, are dictated by the nature of the starting powder, the powder

compaction, and the desired microstructure (e.g., grain size, microstructural uniformity and distribution of secondary phases) of the electrical ceramic product.

Some characteristics of the microstructure which can significantly affect the electrical properties of the ceramic product include grain size, grain size

distribution, amount and location of porosity, pore size and distribution, and controlled distribution of

secondary phases. Sintering is a key aspect of the manufacturing process of ceramic products and must be controllable to ensure that the production of high quality ceramic materials is reproducible. However, reproducibility of the sintering process and the ceramic product is highly dependent on the reproducibility of the powder production.

The piezoelectric effect is a third order crystal property that relates a microscopic strain (or

displacement) of a material in response to an applied electric field. The piezoelectric effect is useful for most transducer and sensor applications. Very strong piezoelectric effects can be induced in ferroelectric perovskite ceramics by application of an electric field, which polarizes (or "poles") the ceramic by partially aligning the directions of spontaneous polarization within each grain of the ceramic, resulting in a net remanent polarization and piezoelectric activity.

Most commercial piezoelectric ceramic applications are based on perovskite Pb(Zr,Ti,Sn)O₃ solid solutions between PbZrO₃, PbTiO₃, and PbSnO₃. The term PZT is used herein to describe those compositions belonging to the binary system lead zirconate-lead titanate, which may in part be in solid solution with lead stannate, and which may comprise secondary ingredients as additives or substitutions.

Compositional modifications can be made to PZT to tailor the piezoelectric properties for specific applications. For instance, piezoelectric properties are highly sensitive to the Zr/Ti ratio, particularly in relation to the morphotropic phase boundary. Also, the

piezoelectric properties are significantly affected by solid solution substitutions of Ba or Sr (for Pb) and Sn (for Zr/Ti) and by dopant additions such as Fe, Ni, La, Nb, Sb, and others.

Commercial manufacturing of PZT ceramic products has typically experienced high rejection rates which can be related to poor reproducibility of batch production of PZT powder and related manufacturing and processing of ceramics. Significant commercial benefits can be made by increased powder process control, shorter production cycles, and reduced environmental restrictions and controls. Lower sintering temperatures of PZT powder compacts would .significantly reduce the problem of PbO volatility, increase sintering reproducibility, and simplify the processing of ceramic products. Electrical property enhancements of the ceramic product would also be expected with more homogeneous solid solutions, more uniform dopant distributions, and reduced impurity levels. In addition to the production of sintered piezoelectric ceramic products, improved piezoelectric composites can be fabricated with the piezoelectric powders.

Composites with 0-3 connectivity can be manufactured using this powder by loading a three-dimensionally connected polymer matrix with the powder. Property enhancements would be expected due to the highly crystalline nature of the powder, as well as the

properties outlined above.

Relaxor ferroelectrics are a relatively new class of PbO-based complex perovskites, with the. general formula

Pb(B1, B2)O₃, where the B1 cation can be one of several low valent cations (e.g., Mg ²⁺, Zn²⁺, Ni²+, Fe³⁺, etc.), and the B2 cation is of higher valence (e.g., Nb 5+,

Ta ⁵⁺, W⁵⁺, etc.). These materials have promise for dielectric (e.g., capacitor), piezoelectric, and

electrostrictive actuator (e.g., micropositioner) applications, depending on their composition.

Compositions of interest for dielectric applications are based on PbMg_⅓ Nb_⅔ O₃ (PMN) with solid solution additions of PbTiO₃ and/or PbZn _⅓ Nb_⅔ O₃ (PZN). PMN- based ceramic elements have higher dielectric constants than the BaTiO₃-based dielectrics, and thus have the potential, for improved volumetric efficiency. In addition, these PbO-based ceramic elements sinter at low temperatures, e.g., < 1000º C, so that when used in multilayer capacitor applications, the use of less expensive electrode materials will be possible. Electrostriction is a phenomenon that occurs in all materials, and relates strain to an applied electric field. It differs from piezoelectricity in that the electrostrictive strain is proportional to the square of the electric field, whereas piezoelectric strain is directly proportional to the electric field. In most materials, electrostrictive strain is extremely small and thus cannot be used in transducer applications.

However, the electrostrictive strains generated in some relaxor ferroelectrics are comparable with piezoelectric strains in PZT ceramic elements . Electrostrictive materials can be used in devices where more precise motion control is required. Most commercial and

research electrostrictive devices are based on a PMN composition in solid solution with PbTiO₃. The processing of relaxor ferroelectric ceramics by conventional milling and calcination techniques is difficult, and this has limited their applications potential. For example, it is extremely difficult to produce PbMg_⅓ Nb_⅔ O₃ by conventional mixed oxides processing due to the formation of a stable Pb-niobate pyrochlore phase during calcination. Repeated

calcination at high temperature, e.g., > 1000 ° C is required to form the PMN powder. Another complication of conventional mixed oxides processing arises from the required high calcination temperature; the volatility of PbO alters the stoichiometry and prevents complete reaction. A two-step formation sequence in which the columbite MgNb₂O₆ is first formed and then reacted with

PbO to form PMN has been developed. However, the requirement of a two step calcination process

complicates powder manufacturing and limits the ultimate process control. Advanced powder preparation techniques (such as coprecipitation) have not been successful in the preparation of phase-pure PMN powders.

Perovskite ceramic products based on lead lanthanum zirconate titanate, (Pb,La) (Zr,Ti) O₃ or PLZT, are useful because they can be prepared in transparent forms having good electrooptic properties. The electrooptic effect relates to a change in refractive index in response to an applied electric field. Thus PLZT electrooptic ceramic products can be used in several optical

applications, including shutters, modulators, displays, color filters, image storage devices, and linear gate arrays for optical data processing. The key to achieving transparency in PLZT ceramic products is to produce a pore-free ceramic with uniform microstructure. Starting with a PLZT powder (which can be prepared by several methods), transparent PLZT ceramics are typically produced by hot pressing or liquid-phase sintering. Hot pressing involves the application of pressure at high temperature. The pressure enhances the densification, and pore-free PLZT ceramics can be prepared. With the liquid phase

sintering technique, an excess of PbO is added to the PLZT powder prior to sintering. The PbO melts during sintering, forming a liquid phase which facilitates densification into a pore-free ceramic. As sintering takes place, the excess PbO evaporates from the ceramic; the sintering operation is then carried out until no excess PbO remains.

The crucial step in both of the above PLZT fabrication techniques is powder preparation which ensures the optical quality of the final transparent PLZT ceramic products. The optical quality of PLZT ceramics produced from conventionally prepared powders is limited.

Improvements in optical quality of PLZT ceramics have been demonstrated using PLZT powders prepared by various chemical coprecipitation techniques. However, the powder produced from these methods suffers from agglomeration, chemical inhomogeneity, and lack of reproducibility.

Another method of producing anhydrous crystalline products, including perovskite compounds, utilizes a hydrothermal treatment step. Recently, the emphasis of research on this method has been on dielectric barium titanate compounds and piezoelectric lead zirconate titanate (PZT) compounds. These investigations have all shown that sub-micron crystalline, products can be formed. It was reported by K. Abe et al., U.S. Patent 4,643,984, that perovskite compounds with the general formula ABO₃ could be produced using a three step procedure. The first step involved subjecting a mixture of A and B hydroxides to hydrothermal reaction in an aqueous media. Next, an insolubilizing agent, such as carbon dioxide, was added to the reacted mixture so as to precipitate unreacted A element materials to adjust the A to B stoichiometry. This step was necessary due to the soluble nature of the A elements, including Pb, Sr, Ca, Ba, and Mg, under the conditions of the hydrothermal treatment. The mixture formed after the second step contains both a B-rich crystalline oxide phase formed during the hydrothermal reaction and an A-rich non- crystalline, non-oxide phase.

Alternatively, the product slurry of the hydrothermal reaction was first filtered and washed, and then added to an aqueous medium containing the supplemental A elements. The product stoichiometry could then be adjusted by adding an insolubilizing agent. The final step was to filter and wash the product with the

corrected A to B elemental ratio. This process was demonstrated for the preparation of compounds containing the A elements listed above and the B elements Ti, Zr, Hf, and Sn.

Although the process described by Abe et al. was shown to result in the formation of compounds with the desired stoichiometries, several problems are expected from the method of production. The primary problem is the method chosen to control the A to B elemental ratio. The second step in the process described above not only adds impurities which can be detrimental to the sintering step , it also introduces inhomogeneities to the product. The washing steps are expected to remove some of unreacted A elements. This problem is most severe for compounds containing lead and strontium on the A-site.

Several investigators have reported a similar process for producing perovskite compounds in which the salts, or in some cases hydroxides or carbonates, of many of the A and B constitutents are combined in an aqueous mixture. The mixture is adjusted to a basic pH through the addition of an alkaline material or ammonia. This mixture is then reacted under hydrothermal conditions to produce the crystalline perovskite compounds. The product slurry is cooled, filtered and washed with water to remove impurities remaining from the salts and the pH adjusting compounds. Examples of processes which employ these general steps have been reported by Fuji Titan Kogyo Co., Japanese patent number JP61031345; Yonezawa et al., U.S. Patent 3,963,630; M. and D. Watson et al., Proceedings of the First International Conference on Ceramic Powder Processing Science, Orlando, 1987.

The Japanese patent reported that barium and strontium titanate could be produced by this method with high yield and complete incorporation of the A site elements, strontium and barium, presumably at a reaction pH of much greater than seven. However, complete

incorporation of strontium and barium is possible only as long as the reaction is operated under alkaline conditions and in the absence. of chlorides. In

addition, this approach does not allow complete

incorporation of other elements, such as lead and antimony, because of the presence of anionic impurities (chlorides or nitrates) and due to the alkaline pH condition. Another problem is the introduction of unsuitable quantities of sodium impurities into the reaction product which arises from the high

concentration of sodium hydroxide employed in the reaction.

In another investigation, by Yonezawa et al., complex lead zirconate titanate compounds were produced. In this process, an acidic aqueous solution of the positive elements consisting of Pb, Ti, Zr, Mn, Sb, Nb, and Ta was prepared with predetermined mole ratios. The solution was neutralized by use of NaOH, KOH or NH₄OH to a neutral or slightly basic pH. The mixture was then directly reacted in an autoclave at temperatures between 150° and 300° C. The resultant product was cooled, and the precipitate was filtered from the solution and washed to remove impurities. The process was reported to have a high yield; although the product was analyzed and found to contain concentrations of unreacted Pb, Ti, Zr, Mn, and Sb ions of 30, 40, 800, 400, and 1500 parts per million in a 700 ml filtrate respectively. For electrical applications, these solution losses are highly significant and can adversely affect electrical properties due to loss of control over product

stoichiometry. The solution losses are a direct result of the anionic impurities left in the reacting solution. Other problems associated with the solution losses include disposal of hazardous effluents or increased plant complexity necessary to provide for recovery of these elements. Because the cationic impurities were not removed before the hydrothermal reaction, it is expected that the products would also contain excessive amounts of sodium or potassium. These impurities are also detrimental to the sintering properties of the powder and the electrical properties of the sintered ceramics. Finally, because the ionic impurities were not removed in the above examples, exotic materials of construction are required for the hydrothermal reactor to prevent corrosion of manufacturing equipment

associated with high temperature aqueous solutions containing chloride, nitrate, and ammonium ions. This increases equipment costs and increases the possibility of product contamination. In the work by Watson et al., the formation conditions for lead titanate were determined under hydrothermal conditions. Only analyses of the lead titanate products were reported. Filtrate solutions were not analyzed with respect to lead and titanium ion concentrations, and therefore no conclusion can be made about the yield of the process. However, high levels of lead are expected to remain in the solution phase in the presence of high concentrations of either chloride or hydroxide ions in the hydrothermal reaction. This results in a product with a poorly defined A to B stoichiometry. The products formed by Watson et al. were particles of non- uniform shape and size. These types of particles are typical if chloride ions are not removed from the precipitated solution prior to hydrothermal reaction.

Another hydrothermal process for production of PZT compounds was described by K. Beal in a presentation at the American Ceramic Society Conference in Boston in August, 1986. In this process, the zirconium and titanium were dissolved and neutralized. The resultant mixed hydroxide precipitate was then filtered and washed extensively to remove all traces of ionic impurities. The hydroxide gel was then mixed with lead oxide and reacted in an aqueous slurry by a hydrothermal reaction. It was determined that at a temperature of 300° C the reaction to the desired perovskite crystalline powder would not occur unless significant quantities of

mineralizers were added. These mineralizers included the fluorides and hydroxides of potassium, sodium and lithium. These mineralizers were shown to introduce significant concentrations of impurities to the resultant PZT products. The impurities from the

mineralizers are detrimental to the sintering and electrical properties of the ceramic products. Also, the problem of corrosion of the reaction vessel in which the hydrothermal reaction occurs is expected to be severe in the presence of such mineralizers.

Kutty et al. have described the preparation of several perovskite materials including PZT (Mat. Res. Bull., Vol. 19, pp. 1479-1488, 1984), strontium titanate (Mat. Res. Bull., Vol. 22, pp. 641-650, 1987), and Ba(Ti,Zr)O₃ (Mat. Res. Bull., Vol. 22, pp. 99-108, 1986). In this work, hydroxide gels were prepared by neutralization of an acidic salt solution of the B elements. The gels were washed to remove ionic impurities and were mixed with oxides or hydroxides of the A elements. The slurries were then reacted under hydrothermal conditions to form the sub-micron powders of the desired compounds. The concentrations of the unreaσted A and B elements were not determined in this work, so it is impossible to discern whether complete reaction took place. In fact, excess amounts of A element were added, and the products were then leached with acid to remove water insoluble by-products and to adjust the product A to B

stoichiometry.

Other investigators have employed organic precursors as a feed material for a hydrothermal synthesis process. These materials add excessive cost to the process and also introduce carbon based impurities which are

detrimental to the sintering properties of perovskite compounds. Examples of such processes include those reported by K. Abe et al., U.S. Patent 4,643,984. SUMMARY OF INVENTION It is an object of the present invention to provide a single crystal, solid solution, chemically homogenous powder of a modified lead zirconate titanate perovskite compound of predetermined average particle size and composition having useful electrical properties and the general formula:

Pb(Zr_xTi_ySn_z)O₃

wherein x, y, and z have any value less than 1 and x + y + z = 1.00; and having a sintering temperature less than about 1100ºC. The powder has a primary crystallite size of less than about 0.4 microns and a secondary particle size of less than about 2 microns. This powder also has a distribution of average particle size ranging from about 0.4 to about 1.2 microns. The average particle size may be controlled by adjusting the pH at which the powder is made.

Various elements may individually or in combination substitute for at least a portion of the lead component in the perovskite compound. These elements include Ba, Sr, Ca, Mg, and La.

Various other elements may individually or in

combination substitute for at least a portion of the zirconium or titanium components in the perovskite compound. These elements include Nb, Ta, Hf, Y, Sr, Zn, Mn, Co, and W.

The powder may also contain assorted dopant elements which may include Bi, Ce, Nd, Sm, Mg, Fe, Sb, Ni, Cr, V, Dy, Pr, Th, Li, In, and combinations thereof.

It is another object of the present invention to provide an improved method of producing improved modified crystalline lead zirconate titanate stannate powders comprising the steps of: (a) preparing a first acidic solution containing zirconium and titanium ions and preferably, in partial replacement of at least a portion of the zirconium and titanium, at least one ion selected from the group consisting of Sb, La, Fe, Nb, Ta, Cr, Mn, Ni, Sn, Bi, Li, Ce, Zn, Nd, Sm, Mg, Fe, V, Dy, Pr, Th, Li, In, Co, Hf, and Y;

(b) preparing a second basic solution containing a sufficient concentration of at least one of the

hydroxides of sodium, ammonium, and potassium to provide a predetermined pH when mixed with the first solution;

(c) adding the first acidic solution to the second to precipitate a substantially pure mixture of zirconium hydroxide and titanium hydroxide;

(d) washing the precipitate to remove hydroxide and salt impurities that solubilize lead or other

constituent elements of the powder;

(e) preparing an aqueous slurry of the washed

precipitate and adding a lead compound and preferably, in partial replacement of at least a portion of the lead component, at least one ion selected from the group consisting of Sr, Ba, Ca, Mg, Mn, Co, Zn, and Ni;

(f) hydrothermally treating the slurry at a temperature and pressure for a time sufficient to form the powder; and

(g) drying the powder.

A preferred composition of the powder provided by the present invention has the general formula: Pb (Zr_1-xTi_x) O₃ wherein x has a value between 0.44 0.55; and having 0 to 13 mole% total dopants and solid solution substitutions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing all possible basic

compositions of the first two perovskite compounds of the present invention.

FIG. 2 is a diagram showing the basic compositions of the third perovskite compound of the present invention. FIG. 3A and B represent ICP analyses of hydrothermal solutions which respectively show the unreacted

concentrations of the elements Pb, Sr, and Sb.

FIG. 4 is a diagram showing the relationship between coprecipitation pH and the average agglomerate particle size of PZT powders.

FIG. 5A and B respectively show scanning electron micrographs (magnified 25,000x) of powders produced in Examples 1 and 3 and which depict the effect of reaction pH on particle morphology of PZT powders.

FIG. 6 represents the X-ray diffraction patterns of PZT powders produced hydrothermally according to Examples 1, 2, and 3.

FIG. 7A and B respectively show scanning electron micrographs (magnified 10,000x) of fracture specimens of ceramics produced by sintering PZT powders according to Examples 2 and 1. FIG. 8 represents the X-ray diffraction patterns of samples of hydrothermally produced powders according to Examples 4-7.

FIG. 9 represents the X-ray diffraction patterns of samples of hydrothermally produced powders according to Examples 10, 11, and 14.

DETAILED DESCRIPTION OF THE INVENTION

The process of the invention is for making PZT powders, particularly perovskite, ABO₃, powders, which consist of single crystal, solid solution particles, preferably containing secondary ingredients and having a primary crystallite size less than about 0.4 microns and a secondary particle size of less than about 2 microns. The term powder is used herein to describe unsintered crystalline lead zirconate titanate stannate, which may comprise secondary ingredients as additives or

substitutions.

The constituents of the compositions of the present invention may be categorized as "principal" and

"secondary", the former whole composition and the latter making up a minor fraction. The principal ingredients are lead titanate and either or both lead zirconate and lead stannate. The secondary ingredients comprise: (1) alkaline and alkaline earth and lanthanum additions substituted for an equivalent quantity of lead in the principal ingredients and (2) certain optional additions and substitutions which are described hereinbelow. For ease of description, the principal ingredients will be considered as basic compositions to which the secondary ingredients are added as direct additions or as

substitutions. The basic compositions fall into three categories: (1) those belonging to the binary system lead zirconate-lead titanate; (2) those belonging to the binary system lead stannate-lead titanate; and (3) those belonging to the ternary system lead zirconate-lead stannate-lead

titanate. The designations binary and ternary are used in conjunction with the basic compositions and in disregard of the secondary ingredients. Furthermore, as is known by those skilled in the art, hafnium occurs as an impurity in varying amounts in zirconium. For the purposes of the claims of the present invention, hafnium may be regarded as the substantial equivalent of zirconium and the presence of hafnium either as an impurity or as a substituent for zirconium is acceptable. However, because the high relative cost of hafnium, as compared to zirconium, renders its use uneconomical in the commercial

manufacture of the compositions under discussion, the present description will disregard the possible presence of hafnium. It will also be appreciated that various rare earth elements, because of their scarcity and relatively high cost, would not be economically

competitive with other elements though fully operative from the technical standpoint.

All possible basic compositions coming within all three of the systems defined above are represented by the triangular diagram constituting Figure 1 of the

drawings. All compositions represented by the diagram as a whole, however, are not ferroelectric, and many are electromechanically active only to a very slight degree. The basic compositions utilized in the present invention are those exhibiting piezoelectric response of

appreciable magnitude and which are commercially suited for dielectric, piezoelectric, electrooptic, composite, and electric wave filter or other applications requiring the same combination of properties.

The general empirical formula for these basic

compositions may be expressed by the general formula Pb(Zr_xTi_YSn_Z)O ₃ wherein the subscripts x, y, and z represent the mole fraction of the respective component symbols with which each is associated and x + y + z = 1.00. Preferred numerical values of the subscripts x, y, and z are from 0 to 0.90, 0.10 to 0.60, and 0 to 0.65, respectively.

A wide variety of elements may serve as secondary constituents of the lead zirconate titanate stannate perovskite compound. For example, the elements Ba, Sr, and Ca may individually or in combination substitute for a portion of the lead in the compound. Ba, Sr, and Ca, when used as secondary constituents, preferably comprisefrom 0.01 to 0.30 mole percent of the perovskite

compound.

Other elements, when used as secondary constituents, may partially substitute for the lead, zirconium, titanium, or tin in the perovskite compound. These elements include Nb, Ta, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sb. These elements preferably

comprise from about 0.01 to 5..0 weight percent of the perovskite compound on an oxide basis. Additional elements which can be used as secondary ingredients include Cr, U, Fe, Ni, Co, Zn, Mg, La, K, Sc, In, Tn, W, Bi, V, and Mn. These elements have varying ranges for their preferred mole percent

incorporation in the lead zirconate stannate perovskite compound. Cr and U are preferably incorporated in the perovskite compound on a mole basis corresponding to a range from about 0.1 to 1.5 weight percent. Fe, Ni, Co, and Zn preferably comprise from about 0.01 to about 1.0 weight percent of the perovskite compound on an oxide basis. La preferably comprises from about 0.1 to 28 mole percent of the perovskite compound and Mg

preferably comprises from about 0.01 to 1.0 mole percent of the perovskite compound. K, Sc, and In preferably comprise an amount sufficient to increase the ability of the ceramic to withstand depoling, decrease the

dielectric losses of the ceramic, and increase the mechanical Q to decrease the mechanical losses of the ceramic. Th comprises from about 0.1 to about 8.0 mole percent of the perovskite compound. W and Bi preferably comprise from about 0.1 to about 5.0 weight percent of the perovskite compound on an oxide basis. Finally, V and Mn preferably comprise from about 0.05 to about 0.8 weight percent of the perovskite compound on an oxide basis. Furthermore, the basic compositions utilized in the present invention also include those exhibiting a mechanically induced material phase transition from a ferroelectric state to an antiferroelectric state with an accompanying release of electrical energy. Such a transition may be induced with respect to materials located close to the ferroelectric-antiferroelectric phase boundary (at a given temperature) by the

application of hydrostatic pressure or a directional stress. The mechanical input required to effect a phase transition has been found to be dependent on the

distance of the composition from the phase boundary on a temperature-composition phase diagram and will be minimum in the case of the borderline compositions.

Accordingly, compositions are preferable which are in close compositional proximity to the phase boundary. Numerous known perovskite ceramic compositions possess the above-mentioned characteristics and are suitable for the practice of this invention. Particularly suitable is lead zirconate modified by the partial substitution of titanate and stannate for zirconate as expressed by the basic compositional formula Pb(Zr_1-t-vSn_tTi_v)O₃ wherein the subscripts t and v represent the mole fraction of the respective component symbols with which each is associated and preferably have the numerical values: t + v < 0.60 and v = 0.03 to 0.20.

It is known that electrical properties of lead zirconate titanate solid solutions can be improved by the

substitution for one of the principal metallic elements of an element of higher or lower valency in amounts of 0.1 to 3.0 atom percent.

It is also known that barium, calcium, and/or strontium may be substituted for lead in equal atomic percents.

In this case the limits on t and v in the above equation are modified. More calcium and strontium favors the antiferroelectric state and barium favors the

ferroelectric state.

In addition to the above, the basic compositions

utilized in the present invention also include those exhibiting favorable switching characteristics both with respect to speed and the number of times satisfactory switching may be obtained with little to no degradation. These basic compositions are particularly suited and easily adapted for use in ferroelectric memories. The basic compositions which are preferable utilize a three component solid solution of oxides forming a

ferroelectric ceramic at a region of the triangular coordinate phase diagram near the point where the phase areas come together with proportions of the constituent having a boundary between the tetragonal ferroelectric and tetragonal antiferroelectric phases very close to the antiferroelectric region.

The basic composition provides a ferroelectric ceramic having a fairly substantial piezoelectric coupling factor with extremely minimal lattice distortion. The basic composition may be modified in such a manner as to raise the electrical volume resistivity, to lower the coercive field and to improve poling properties and squareness of the slow hysteresis loop. Particularly suitable is lead zirconate modified by the partial substitution of titanate and stannate for zirconate as expressed by the basic compositional formula

[Pb(Zr_xTi_ySn_z)O₃]_1-v(LaFeO₃)_v wherein the subscripts v, x, y, and z represent the mole fraction of the

respective component symbols with which each is

associated. These subscripts preferably have the following numerical values: v = 0.05 to 0.15; x = 0.50 to 0.75; z = 0.00 to 0.15; and x + y = 0.85 to 1.00.

The proportions of lead titanate, lead zirconate, and lanthanum ferrite in the preferred compositions are chosen as to be represented by a point lying within the area 17 on Figure 2 which is close to the intersection point of the boundaries between phases, F_R in which the composition is ferroelectric with a rhombohedral

structure, F_T, ferroelectric with a tetragonal

structure, and A_T antiferroelectric with a tetragonal structure. The area 17 is within the ferroelectric tetragonal region F_T but lies very close to the

antiferroelectric region A_T . It will be understood that in this type of ternary system there is also a

paraelectric region C, in which the crystal structure is cubic. However, the area 17 is chosen to be closer to the boundary between the ferroelectric rhombic and the ferroelectric tetragonal regions, F_R and F_T, respectively. The exact locations of the boundary lines between the regions F_T, F_T, and A_T depend upon the perovskite compounds and the substitutions employed. Consequently, the area is best defined by its proximity to these boundary lines and the common point, rather than by percentages of each constituent.

In order to obtain adequate resistivity with measurable quasi-static hysteresis loops, relatively low coercive field with a good slow squareness of the hysteresis loop, and good poling properties, it is known that a donor additive or substitution may be employed, such that one of the principal metallic elements of an element of higher valency in amounts of about 0.01 to 5.0 atom percent is the donor. These improved

properties occur with the substitution of small five- valent elements such as Nb, Sb, and Ta in the B-site position and/or the substitution of large three-valent elements such as La, Bi, and W in the A-site position. In the case of La, it is present both as a substituent for a position of the lead and also in the lanthanum ferrite constituent of the solid solution.

It is also known that barium, calcium, and/or strontium may be substituted for lead in equal atomic percents.

The preferred proportions of each element substituent to the total of lead is between 0.1 and 15 atom percent, preferably between 3 and 10 percent. In the perovskite lattice, as hereinabove described, the metals in the B position are tetravalent. A mixed oxide may also be employed in which metals of different valences are employed in the B-site position in such proportions as to give an average valance of four. For example, the composition may be represented by the formula [Pb_.99D_.02B_.98O₃]_0.9(LaFeO₃)_0.1 where the B position metals are (X_1/3Y_2/3)⁴⁺ and where X is a

bivalent metal such as zinc or magnesium and Y is a pentavalent metal, such as niobium, which gives an average valence of four, and D is a donor metal such as antimony, niobium, or tantalum, which is pentavalent and may be substituted in amounts of 0.01 to 5 atom percent. Donor elements such as La, Bi, and/or W may be

substituted in the A position in amounts of 0.01 to 5 atom percent. Isovalent elements such as barium, calcium, and/or strontium may be substituted in the A position in amounts of 0.1 to 15 atom percent.

These powders may be sintered at 100° to 400°C below conventional sintering temperatures to ceramics

exhibiting uniform microstructure and homogeneous chemical composition. The powders are expected to result in electrical properties superior to those of traditional electric ceramics. The major steps in the process are:

(1) Acidic salts including chlorides,

oxychlorides, and/or nitrates of titanium and/or zirconium are codissolved in an acid/water solution to form an acidic solution with dopant oxide salts,

including chlorides and/or nitrates of any of all of the following: niobium, tantalum, uranium, iron, antimony, lanthaum, bismuth, nickel, manganese, promethium, europium, gadolinium,

terbium, holmium, erbium, thulium, ytterbium, lutetium, scandium, potassium, neodymium, samarium, cobalt, tungsten, tin, hafnium, yttrium, cerium, chromium, vanadium, disprosium, praseodymium, thorium, lithium, and indium.

(2) The codissolved acidic salt solution is added slowly to a vigorously agitated basic solution containing a predetermined concentration of at least one of the hydroxides of sodium, ammonium, and potassium until a pH value between about 4 and 12 is reached. The precise pH value is dependent on the constituents dissolved in step (1) and on the desired particle size. For lead containing compounds, for example, the pH must be below about 7.5 in order to obtain nearly complete reaction of lead oxide in step

(5). For strontium and barium containing compounds, the pH must be greater than about 6.5 to achieve nearly complete incorporation of these components. The pH value of the mixed hydroxide slurry also affects, to a lesser extent, the incorporation levels of antimony and tin. Manganese, chromium, nickel, and zinc are expected to be likewise affected. This method of coprecipitation results in the formation of a mixed hydroxide

precipitate containing all of the above mentioned metal hydroxides in a

homogeneous mixture. The pH can be adjusted, within the range allowable to achieve complete reaction, in order to control the particle size of the reacted product. Increased pH levels favor increased particle size.

(3) The mixed metal hydroxide is separated from the resultant salt solution and is washed until the chloride, nitrate, and free hydroxide content of the hydroxide gel are reduced to a sufficiently low level to favor complete incorporation of all constituents into the product phase in step (5), e.g. specific conductivity of wash filtrates less than about 20 mmho.

(4) The washed hydroxide gel is then

redispersed in water and is vigorously mixed until a homogenous slurry with a pH of between about 10.0 and 13.0 is formed with the remaining perovskite components including oxides and/or hydroxides of the following: lead, barium, strontium, calcium, magnesium, manganese, cobalt, zinc, nickel, and yttrium in the proper ratio to produce a mixture with a

stoichiometry equal to the desired ABO₃ stoichiometry of the target powder.

(5) The homogeneous slurry is hydrothermally treated. It is introduced into a

pressure reactor, which can be either a stirred autoclave or a plug flow vessel, and is heated to a temperature between about 100° and 350° C, depending on the composition of the target powder, under the vapor pressure of the solution or in the presence of an oxidizing gas for less than about 30 minutes. It is cooled below about 100° C, and is removed from the pressure vessel. (6) The crystalline solids formed in the reaction step (5) are separated from the liquid phase which is essentially free of soluble oxide components and is washed to remove any remaining ionic impurities.

The solids have essentially the same metal ion stoichiometry as the feed material for the process. This process differs significantly from the previous state-of-the-art process technology. Specifically, the process involves the adjustment of pH in step (2) to control both the extent of incorporation of elemental constituents and the particle size of products formed in step (5). The effects of the procedures employed in steps (1), (3), and (4) are essential to take advantage of this process. Removing impurities in step (3) enables substantially complete stoichiometric reaction of all of the A and B constituents in the same

proportions as the feed material. In the prior art, the powders produced did not contain all of the added constituents due to incomplete reaction. This invention also involves adjustment of pH in both steps (2) and (4) to control the particle size of the hydrothermally produced powder. Particle size can be controlled in the range of 0.04 to 1.2 microns, depending on pH and composition of the powder.

Powder products formed by this method have been

demonstrated to be reactive toward sintering. For example, PZT powders containing strontium, iron and lanthanum have been produced which were sintered to greater than 98 percent theoretical density at 900° C. This is a reduction in the sintering temperature of approximately 400° C. A demonstration on the use of the process for reduction of sintering temperature of barium titanate-based dielectric compositions has also been shown.

Advantages obtained by this improved process include reduced hydrothermal reaction temperature and time and improved control over product stoichiometry and particle size. Advantages of the improved product powders are uniform, controlled, fine particle size, improved sintering reactivity, improved chemical homogeneity, reduced impurity contamination, and controlled

agglomeration- These advantages are expected to result in the manufacture of ceramics with improved structural and electrical properties by processes which are less polluting, more reliable, and therefore more economical.

Evidence has been collected in experimental

investigations that expands the current understanding of the hydrothermal process with respect to the control of the A to B elemental ratio. In previous patents, e.g., that by K. Abe et al., it was considered inevitable that a portion or all of the A elements would remain as a soluble species in the hydrothermal reaction mixture. Therefore, measures had to be taken after the reaction to correct the product stoichiometry. The present invention avoids this problem by process conditions that provide nearly complete incorporation of all elements of a complex perovskite compounds.

The expanded understanding of the invention reactions was derived from a detailed study of the neutralization or coprecipitation step employed in similar hydrothermal processes (step (2) from above) . It was determined that the conditions under which the precipitation is carried out has a significant impact on the stoichiometry of the final product. Specifically, it has been shown that the primary B constituents, zirconium and titanium, can be codissolved and neutralized to hydroxides over a wide range of pH. This pH range has been demonstrated to cover 6.0 to 11.7 and is expected to extend to at least 4 and 12 based on known solubility relationships.

Likewise, many of the dopant elements: Nb, Ta, Mn, Zn, Sb, La, Fe, U, and Cr, can be coprecipitated over all or a portion of this pH range. Since the coprecipitated mixture is washed to remove ionic impurities after the coprecipitation step, it was not expected that the coprecipitation pH would have any effect on the

incorporation of these elements into the complex

perovskite compound in the hydrothermal reaction. This, however, is not the case. Other elements, such as strontium, barium, lead,

magnesium, and calcium are not added in the

coprecipitation step because of the relatively narrow range of pH under which they can be precipitated. Several experiments have been carried out for

hydrothermal production of perovskite compounds

containing the elements: Ti, Zr, La, Fe, Sb, Sn, Ca, Sr, Pb and Ba. Ti, Zr, La, Fe, Sb, and Sn were added in the first process step as chlorides or oxychlorides. Sr and Ba were added in step (4) as crystalline hydroxides and Ca and Pb were added as CaO and PbO, also in step (4). The chemicals used were assayed for oxide content by various chemical methods. The acidic salt solution was first prepared and was then slowly added to a solution of sodium or potassium hydroxide, which contained the proper concentration of these bases to reach a desired pH endpoint. The experiments were carried out over the range of pH from about 6.0 to 11.7. The coprecipitation was carried out in a blender or mixer which provides sufficient agitation to ensure a uniform solution composition throughout. In these experiments, the blending speed was varied from about 3000 to about 6000 rpm. As an alternative, the sodium hydroxide or

potassium hydroxide solution can be added to the acidic salt solution to cause the coprecipitation. This has been found, however, to result in products which do not sinter as readily. This is presumably due to the creation of inhomogeneities in the precursor hydroxide gel. As the basic solution is added, the least soluble hydroxide will precipitate first, followed by the next least soluble and so on. The result is a mixture of several separate hydroxide components in the precursor gel. On the other hand, if the acidic solution, containing all of the coprecipitation components, is added to the basic solution, a uniform mixture of hydroxides is formed. This is caused by the

instantaneous precipitation of the mixture as it comes in contact with the basic solution. After the desired pH endpoint was reached, the hydroxide was separated from the solution by filtration or

centrifugation. The filtrate or centrate was analyzed by inductively coupled plasma (ICP) analysis to

determine the extent of precipitation for each of the elements. In the pH range of 6.0 to 11.7, it was verified that zirconium and titanium are essentially completely precipitated. Iron, antimony, and lanthanum were also precipitated under the conditions studied. Only tin was found to remain partially soluble at a pH of 11.80. Table 1 summarizes the results of the coprecipitation experiments.

N/A - Not applicable

After completing steps (1) and (2), the coprecipitated mixture was washed with water to remove ionic

impurities. This was accomplished by blending the gel with a volume of distilled water equal to two-thirds the volume from which it was formed. Blending is continued for 10 to 20 minutes, and the gel is again separated from the wash liquid. This procedure can be repeated to remove additional salts. It has been shown that three wash cycles were sufficient to remove essentially all the leachable salts.

The gel was then redispersed in water and the remaining components added to the mixture. These components can include oxides or hydroxides of barium, strontium, lead, magnesium, and calcium. In general, these compounds are mildly basic and will increase the pH of the mixture to between 10 and 13, depending on the concentration. The mixture is then introduced to an autoclave or other pressure containing vessel. The slurry is heated to the reaction temperature, which can be as low as 100ºC or as high as 350ºC, depending on the composition of the ceramic element being formed. After reaching the reaction temperature, a short hold period may be

utilized; although, it has been demonstrated that this may not be necessary. For example, fully developed PZT compositions have been produced at 300°C without any hold period. The need for a holding period can be readily determined by those skilled in the art. The slurry is then cooled to below 100ºC, and the product powder is separated from the liquid phase by

centrifugation, filtration, or settling. The filtrates or centrates were analyzed by ICP to determine concentrations of unreacted elements.

Complete incorporation of all elements is essential to the control of the product stoichiometry. It was determined that the concentration of Unreacted

components was strongly related to the pH at which the precursor hydroxide gel was precipitated in step (2). Figure 3 shows the relationships between unreacted element concentrations and coprecipitation pH. Lead and antimony have increased solubility levels when the coprecipitations were carried out at elevated pH values. On the other hand, the solubilities of barium and strontium increase with decreasing pH. Incorporation levels of zirconium, titanium, lanthanum, iron, and calcium were not affected by coprecipitation pH in the ranges investigated.

Figure 4 shows the relationship between coprecipitation pH and the agglomerate size of PZT powders. The

agglomerate size was determined by ultracentrifugation in a Horiba particle analyzer. The crystallite size was also affected as can be seen in the scanning electron micrographs shown in Figure 5. The average crystallite size was roughly doubled when coprecipitation pH was increased from 7.16 to 10.94. These results have dramatically altered our view on the use of the hydrothermal process for preparation of ceramic powders with electrical properties. The primary concern in preparation of these materials has been control of stoichiometry. Because of the investigation into the incorporation of elements as a function of processing conditions, it is now possible to specify conditions for each processing step depending on the target composition. For example, to produce a compound (Ba,Sr)TiO₃, one must perform the coprecipitation step within a pH range of roughly 7 to 12. The particle size of the products can be controlled by selecting a pH within this range. This allows one to have some

additional control over the formation of the ceramic microstructure in the sintering step. To produce a compound (Ba,Sr,Pb)TiO₃, the coprecipitation pH must be close to 7, in order to preserve control over product stoichiometry. Addition of zirconium, lanthanum, or iron to either of these compounds does not change the conditions under which the process may be carried out. Likewise, all other elements which can be incorporated into the perovskite structure should have similar solubility relationships. By determining these

relationships, one can define sets of conditions for production of any perovskite material, using the six step process of the invention.

Ranges of pH in step (2) have been determined which result in complete or near complete incorporation of A and B site cations into the perovskite structure in step (5). For example, it has been shown that in the

compound (Pb,Sr) (Zr,Ti,Sb) O₃ , each of the elemental constituents has a pH range in which it is 100 percent incorporated. Values of 4 and 12 have been selected as the boundaries of the acceptable pH range since many of the elements of interest have appreciable solubility beyond these bounds. Therefore, pH bounds can be set for each element in the compound within this range. The approximate boundaries, that have been determined are shown in Table 2.

ELEMENTAL INCORPORATION VERSUS GEL PREPARATION pH (DASHED LINES ARE EXPERIMENTAL; ASTERICKS ARE PREDICTED)

Lead is fully incorporated when gels are prepared at pH levels below approximately 6.5. Strontium has a pH range of 7 to 12 and antimony has a pH range of 6.5 to 12. According to this data, the pH condition which will result in optimum incorporation of all elements is between 6.5 to 7.0. Inside of this range, minor

concentrations of strontium and lead will remain in the hydrothermal solution phase. However, these levels are minimized because of the method employed to ensure complete precipitation. For example, to incorporate greater than 99.8 percent of all elements, the

acceptable pH range could be increased to between 6.5 and greater than 8. In this pH range, slight

adjustments can be made in the feedstock stoichiometry to account for solution losses. It has been determined that similar incorporation relationships can be determined for other elements employed in the perovskite structure. These

relationships are thought to be dependent on the solubility of perovskite compounds in the hydrothermal solutions from which they are formed. Hydroxide gel pH affects the solubility relationships, probably due to the influence of free hydroxide and other anionic species which remain in the gel after washing. For example, relationships have been determined for barium, strontium, lead, calcium, tin, titanium, zirconium, antimony, lanthanum, and iron. These are summarized in Table 3. By matching the pH range of perovskite elements, a high yield product of well defined

stoichiometry can be produced for any potential

com ound.

pH RANGES FOR NEARLY COMPLETE (LESS THAN 50

PPM SOLUBILITY) INCORPORATION INTO PEROVSKITE COMPOUNDS (DASHED LINES ARE EXPERIMENTAL;

ASTERISKS ARE PREDICTED) Likewise, it is expected that relationships could be determined for all other elements incorporated into the perovskite structure by an ordinary person skilled in the art. To do this, synthesis experiments need to be performed employing stoichiometrically balanced

feedstocks of the compounds of interest. For example, barium titanate formulations could be produced with other dopant elements, including zinc, magnesium, bismuth, cobalt, tungsten, manganese, niobium, tantalum, samarium, and neodymium as minor substitution elements. The hydroxide gel should be precipitated and washed in accordance with the invention as described above.

Several pH values should be selected within the range of 4 to 12 to perform the precipitations. Synthesis experiments can then be carried out as a function of hydroxide gel pH. The relationships can then be

determined by analyzing product filtrates for all of the elemental constituents.

The invention will now be illustrated in the following examples:

Example 1

A PZT composition was produced with the formula

(Pb_0.94Sr_0.06)-(Zr_0.52Ti_0.48)O₃ and containing minor concentrations of iron and lanthanum as dopants

incorporated in the ABO₃ perovskite structure. About 150 grams of zirconium oxychloride solution containing about 20.45 percent ZrO₂ by weight was mixed with 2.80 grams of dilute iron(III) chloride solution, about 6.69 grams of dilute lanthanum chloride solution, and about

200 grams of distilled water. The solution was mixed with about 78.57 grams of a titanium oxychloride

solution containing about 22.534 percent TiO₂ by weight.

In a 4-liter capacity blender, a caustic solution was prepared by dissolving about 112.0 grams of a 50 percent by weight NaOH solution in 1500 grams of distilled water. While blending the caustic solution at a rate of

approximately 3000 rpm, the mixed solution containing Ti, Zr, La, and Fe chlorides was slowly poured into it. The hydroxide slurry which was formed was allowed to blend for an additional 15 minutes. The slurry was then filtered through a Buchner funnel, and 1300 ml of clear filtrate having a pH of 7.16 was collected. The

precipitate was redispersed in the blender with 1000 ml of distilled water and was blended for 20 minutes. It was then recovered by filtration and washed two

additional times in the same manner.

The precipitate was again redispersed in 1000 ml of water, and 7.29 grams Sr(OH)₂.8H₂O and 98.94 grams of PbO, including an excess amount added to aid in

sintering, were blended in. The slurry was diluted to a total volume of 1.5 liters and the pH was measured to be 11.76. The slurry was poured into a 3.785-liter capacity stainless steel autoclave, heated to 300° C over 150 minutes at a stirring rate of about 350 rpm and held at temperature for about 30 minutes. It was then cooled to 90° C and removed from the autoclave.

The slurry was immediately filtered, and 1250 ml of clear filtrate was collected. The filtrate was analyzed by ICP and was determined to contain 200 ppm Pb and 2.5 ppm Sr. From these measurements, incorporation levels of Pb and Sr were calculated to be 99.72 and 99.87 percent, respectively. All other elements were

essentially 100 percent incorporated. The solids were washed in the same manner as the hydroxide gel and then dried in a vacuum oven at 60° C for several hours. The solids were pale yellow in color and were analyzed by XRD for crystalline phase. The XRD pattern is shown in Figure 6. The product contains only perovskite

structure materials. The average particle size of the powder was determined to be about 0.60 microns and the crystallite size was estimated to be about 0.1 to 0.2 microns based on the SEM micrograph shown in Figure 5.

The powder was compacted into a small disc using an isostatic press which was then cold isostatically pressed to a green density of about 55 percent. The specimen was sintered at about 950° C for 2 hours to a final density of about 7.59 g/m3. The microstructure was uniform and had a grain size of about 0.5 to 1.0 microns. An electron microscopic photograph of a frature specimen of the sintered ceramic product is shown in Figure 7.

Examples 2-9

Several experiments were carried out using the same general formulation as in Example 1. The conditions employed in each experiment are summarized in Table 2. Process variables including reaction time and hydroxide gel preparation pH were varied.

In Examples 2 and 3, hydroxide gels were prepared in the same manner as in Example 1; however, the pH endpoints for gel preparation were about 11.80 and 10.94, compared to 7.16 for Example 1. The average particle sizes of materials produced under these conditions increased from about 0.60 to over 1 micron. Primary crystallite sizes were increased from about 0.1-0.2 microns to about 0.2- 0.5 microns, by SEM analysis. Electron microscope photographs of powders produced in Examples 1 and 3 are shown in Figure 5. The powders were compacted, as described above and were sintered at about 950° C for 2 hours into dense ceramics. The microstructure of ceramics produced from the Examples 1 and 2 powders are compared in Figure 7. The grain size of the two

specimens was directly proportional to the particle size of the hydrothermally produced powders. This

demonstrates the use of the process for controlling the microstructures of the ceramics. The powders produced in Examples 4 through 7 differed only in reaction time. The perovskite solid solution was formed immediately upon heat up to about 300° C.

Additional reaction time up to one hour had little effect on the reaction products. XRD patterns of the powders formed in Examples 4 through 7 are compared in Figure 8.

In Examples 4 through 9, high amounts of excess lead oxide were added to the process. These are responsible for the relatively high solution losses in these

examples. The remainder of the examples show more clearly the relationship between hydroxide gel pH and solution lead loss. In order to produce larger

particles of PZT, for example, to create a large grained microstructure, the hydroxide gel pH should be near the upper end of the range 4 to 12. Minor concentrations of excess PbO must be added to the process in order to compensate for lead losses under these conditions. Lead losses were calculated to be a maximum of about 1.53 percent for Example 2, in which the hydroxide gel was produced at a pH of about 11.80.

Example 10

A PZT composition was produced with the formula

(pb_0.88Sr_0.12)-(Zr₀.56Ti_0.44)O₃ and containing minor concentrations of antimony as a dopant incorporated in the ABO₃ perovskite structure. An amount of about 160 grams of zirconium oxychloride solution containing about 20 percent ZrO₂ by weight was mixed with 9.76 grams of a dilute antimony chloride/hydrochloric acid solution, 200 grams of distilled water, and about 75.0 grams of a titanium oxychloride solution containing 22.534 percent TiO₂ by weight. In a 4-liter capacity blender, a caustic solution was prepared by dissolving 115.00 grams of a 50 percent by weight NaOH solution in 1500 grams of distilled water.

While blending the caustic solution at a rate of

approximately 3000 rpm, the mixed solution containing Ti, Zr, and Sb chlorides was slowly poured into it. The hydroxide slurry which was formed was allowed to blend for an additional 15 minutes. The slurry was then filtered through a Buchner funnel, and 1650 ml of clear filtrate having a pH of 7.88 was collected. The

precipitate was redispersed in the blender with 1000 ml of distilled water and was blended for about 20 minutes.

It was then recovered by filtration and washed two additional times in the same manner.

The precipitate was again redispersed in 1000 ml of water, and about 14.69 grams Sr(OH)₂.8H2O and about 94.32 grams of PbO, including an excess amount added to aid in sintering, were blended in. The slurry was diluted to a total volume of 1.5 liters and the pH was measured to be about 11.78.

The slurry was poured into a 3.785-liter capacity stainless steel autoclave, heated to about 300° C over 150 minutes at a stirring rate of about 350 rpm and held at that temperature for about 30 minutes. It was then cooled at about 90° C and removed from the autoclave.

The slurry was immediately filtered, and 1220 ml of clear filtrate was collected. The filtrate was analyzed by ICP and was determined to contain about 135 ppm Pb and 0.35 ppm Sr. From these measurements, incorporation levels of Pb and Sr were calculated to be 99.81 and 99.99 percent, respectively. All other elements were essentially 100 percent incorporated. The solids were washed in the same manner as the hydroxide gel and were then dried in a vacuum oven at 60° C for several hours. The solids were white in color and were analyzed by XRD for crystalline phase. The XRD pattern is shown in Figure 9. The product contains only perovskite

structure materials. The average particle size of the powder was determined to be about 0.80 microns.

The powder was compacted in a small disc using an isostatic press which was then cold isostatically pressed to a green density of about 55 percent. The specimen was sintered at about 950° C for 2 hours to a final density of about 7.45 g/cm 3. The microstructure was uniform and had a grain size of about 0.5 to 1.0 microns.

Examples 11-14

Several experiments were carried out using the same general formulation as in Example 10. The conditions employed in each experiment are summarized in Table 3. Hydroxide gel preparation pH was varied for these experiments.

Perovskite compounds were formed under all conditions. As in the previous examples, as the hydroxide gel pH increased, the particle size increased, the degree of lead incorporation decreased, and the degree of

strontium incorporation increased. It was also shown that antimony incorporation levels decreased as

hydroxide gel pH was increased. At the highest gel pH studied, the level of incorporation of lead was reduced to 99.23 percent, and at the lowest pH, it was 99.99 percent. Levels of strontium incorporation ranged from 96.16 to 99.95 percent. Particle size increased, from 0.47 to 1.07 microns over the hydroxide gel pH studied. The ceramic formed from the powder produced in Example 11 showed increased grain size from that of Example 10, when sintered under identical conditions.

Example 15

The PZT composition from Example 1 was mixed with a 20 percent by weight solution of polyvinyl alcohol (PVA) so that the PVA content was 3.20 percent by weight. The mixture was then dried at 80°C, ground with mortar and pestle, and sieved through a 40 mesh screen. Pellets of between 1.1 and 1.3 grams were uniaxially pressed in a 0.55 inch diameter steel die at a pressure of 10,000 psi, and then isostatically pressed at 55,000 psi. The binder was burned out by heating at 100° C per hour to 600° C and holding for four hours. The pellets produced in this matter had average green densities of

4.63 g/cm³.

The pellets were sintered on an alumina plate separated by a thin layer of zircon sand. About 4 grams of PbZrO₃ powder were used to provide a PbO atmosphere during sintering. A 100 cc alumina dish was used as a cover.

Sintering of the pellets was done by heating to a

temperature between 900° and 1000°C with a heating rate between 50° and 200° C per hour and holding for a time between 1 or 2 hours. The sample weights were recorded before and after sintering and the weight loss was

calculated. The densities were measured using the

Archimedes method with isopropyl alcohol as the solvent. Sintering data is presented in Table 4 below:

As is apparent from the data in Table 6, the

densification of the pellets of the Example 15

composition was completed at temperatures as low as 900º C. This is a reduction of sintering temperature of

approximately 400°C. A second method for sintering was performed on the Example 16 composition in which PbZrO₃ powder was used as a PbO source. The pellets were

placed on Pt foil on an alumina plate covered with a 100 cc alumina dish, and sintered at temperatures of 850° and 900° C for times of 1 to 4 hours, with a heating

rate to 50° C per hour. Weight loss and sintering data are presented in Table 7 below:

As is apparent from the data in Table 7, it is possible to sinter pellets of the Example 15 composition without a PbO atmosphere source at temperatures as low as 850°C.

Conventionally prepared PZT powder requires careful control of the PbO atmosphere by a source in order to density the ceramic product without significant lead volutilization.

Although the present invention is described herein with some specificity, persons of skill in the art will recognize modifications and variations that are within the spirit of the invention as described. It is intended that such modifications and variations also be encompassed by the

following claims.

Claims (27)

We claim :

1. A single crystal, solid solution, chemically homogenous powder comprising a lead zirconate titanate

perovskite compound having useful electrical properties and the general formula:

Pb(Zr1-_xTi_x)O₃

wherein x has a value between 0.44 and 0.56; and having 0 to 13 mole% total dopants and solid solution substitutions.

2. The powder according to claim 1 wherein said powder has a sintering temperature less than about 1100°C.

3. The powder according to claim 1 wherein said powder has an average particle size ranging from about 0.4 to about 1.2 microns.

4. The powder according to claim 1, wherein a portion of the lead in said perovskite compound is replaced by at least one element selected from the group consisting of Ba, Sr, and Ca.

5. The powder according to claim 1 wherein a portion of the lead, zirconium, or titanium in said perovskite compound is replaced by at least one element selected from the group consisting of Nb, Ta, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ce, Y, Sb, Cr, U, Fe, Ni, Co, Zn, K, Sc, In, Th, W, Bi, V, and Mn.

6. The powder according to claim 1 wherein said powder has a primary crystallite size distribution ranging from about 0.20 microns to about 0.60 microns, and a secondary particle size distribution ranging from about 0.4 microns to about 2.0 microns.

7. A piezoelectric transducer comprising a sintered mass of the powder of claim 1.

8. A piezoelectric wave filter device comprising a sintered mass of the powder of claim 1.

9. The powder according to claim 1 wherein a portion of the lead in said perovskite compound is replaced by an element selected from the group consisting of La and Mg.

10. The powder according to claim 1 wherein said element is La and comprises about 0.1 to about 10 mole percent of said perovskite compound.

11. An electrooptic device comprising a sintered mass of the powder of claim 10.

12. A process for making a crystalline lead zirconate titanate powder having the general formula: Pb(Zr₁-_xTi_x)O₃ wherein x has a value between 0.44 and 0.56; and having 0 to 13 mole% total dopants and solid solution substitutions, comprising the steps of:

(a) preparing a first acidic solution containing zirconium and titanium ions;

(b) preparing a second basic solution containing a sufficient concentration of hydroxide to provide a predetermined pH when mixed with the first solution;

(c) adding the first acidic solution to the second basic solution to precipitate a substantially pure mixture of zirconium hydroxide and titanium hydroxide;

(d) washing the precipitate to remove hydroxide and salt impurities that solubilize lead or other constituent elements of the powder; (e) preparing an aqueous slurry of the washed precipitate and adding a lead compound;

(f) hydrothermally treating the slurry at an elevated temperature and pressure for a time sufficient to form the powder; and

(g) drying the powder.

13. The process according to claim 12, wherein said hydroxide of step (b) is selected from the group consisting of sodium hydroxide, ammonium hydroxide, and potassium hydroxide.

14. The process according to claim 12 further

comprising the step of controlling the pH to select the average particle size of said powder.

15. The process according to claim 12 wherein said first acidic solution is added to said second basic solution at a predetermined pH ranging from about 4 to about 12.

16. The process according to claim 12 wherein said lead compound is added to said aqueous slurry of the washed precipitate at a predetermined pH ranging from about 10 to about 13.

17. The process according to claim 12 wherein said precipitate is washed with distilled water.

18. The process according to claim 12 further

comprising adding to said aqueous slurry of the washed precipitate at least one ion selected from the group consisting of Pb, Sr, Ba, Ca, Mg, Mn , Co, Zn, Ni, and Y.

19. The process of claim 12 further comprising the step of including in said first acidic solution at least one ion selected from the group consisting of Sb, La, Fe, Nb, Ta, Cr, Mn, Ni, Sn, Bi, Li, Ce, Zn, Nd, Sm, Mg, Fe, V, Dy, Pr, Th, In, Co, Hf, Y, Pm, Ev, Gd, Tb, Ho, Er, Tm, Yb, Lu, Sc, and K.

20. The process according to claim 19 wherein said powder has a primary crystallite size of less than about 0.4 microns and a secondary particle size of less than about 2 microns.

21. The process according to claim 19 wherein said powder has a primary crystallite size ranging from about 0.20 microns to about 0.60 microns, and a secondary particle size ranging from about 0.4 microns to about 2.0 microns.

22. The process according to claim 19 wherein the ion is La and comprises about 0.1 to about 10 mole percent of said powder product.

23. A process for making a crystalline lead zirconate titanate powder having the general formula:

Pb(Zr_1-xTi_x)O₃ wherein x has a value between 0.44 and 0.56; and having 0 to 13 mole% total dopants and solid solution substitutions, comprising the steps of:

(a) codissolving acidic salts, selected from the group consisting of chlorides, oxychlorides, and nitrates of titanium and zirconium, and dopant oxide salts, selected from the group consisting of chlorides and nitrates of at least one of Sb, La, Fe, Nb, Ta, Cr, Mn , Ni, Sn, Bi, Li, Ce, Zn, Nd, Sm, Mg, Fe, V, Dy, Pr, Tn, In, Co, Hf, Y, Pm, Eu, Gd, Tb, Ho, Er, Tm, Yb, Lu, Sc, and K, in water solution to form an acidic solution;

(b) slowly adding the acidic solution to an agitated basic solution containing a sufficient concentration of hydroxide to provide predetermined pH upon said addition and to precipitate a substantially pure zirconium and titanium hydroxide gel; (c) separating the precipitate from the resultant salt solution and washing the precipitate with distilled water until the chloride, nitrate, and free hydroxide content of the hydroxide gel are reduced to a sufficiently low level to favor complete incorporation of all constituents into the powder;

(d) redispersing the washed hydroxide gel in water and mixing until a homogeneous slurry with a pH of between about 10.0 and 13.0 is formed with the remaining perovskite components including oxides and hydroxides of Pb, Sr, Ba,

Ca, Mg, Mn, Co, Zn, Ni, and Y in the proper ratio to produce a mixture with a stoichiometry equal to the desired ABO₃ stoichiometry of the powder;

(e) hydrothermally treating the slurry by intro- ducing the slurry into a pressure reactor and heating the reactor to a temperature between about 100° and 350°C, depending on the composition of the powder;

(f) cooling to below about 100°C and removing from the pressure vessel, separating the crystalline solids formed in the reaction in step (e) from the liquid phase and washing it with distilled water to remove any remaining ionic impurities.

24. The process according to claim 23 further

comprising the step of controlling the pH to select the average particle size of said powder.

25. The process according to claim 23 wherein said acidic solution is added to said basic solution at a predetermined pH ranging from about 4 to about 12.

26. The process according to claim 23 wherin said remaining perovskite components are added to said aqueous slurry of the washed precipitate at a predetermined pH ranging from about 10 to about 13.

27. A process for making a crystalline lead zirconate titanate powder having the formula (Pb _0.94Sr_0.06)- (Zr_0.52Ti_0.48)O₃ and containing minor concentrations of iron and lanthanum as dopants incorporated in the ABO₃ perovskite structure comprising the steps of;

(a) preparing an acidic solution containing zirconium and titanium ions by mixing about 150 grams of zirconium oxychloride solution containing about 20.45 % ZrO₂ by weight of 2.80 grams of dilute iron (III) chloride solution, about 6.69 grams of dilute lanthanum chloride solution about 200 grams of distilled water, and about 78.57 grams of a titanium oxycholoride solution containing about 22.534% TiO₂ by weight;

(b) preparing a caustic solution containing hydroxide ions by dissolving about 112.0 grams of a 50% by weight NaOH solution in 1,500 grams of distilled water;

(c) while blending said caustic solution at a rate of about 3000 rpm, slowly pouring the mixed acidic solution containing the Ti, Zr, La and Fe chlorides into said caustic solution and allowing the hydroxide slurry precipitate to blend for an additional 15 minutes;

(d) washing the precipitate three times in succession to remove hydroxide and salt impurities by

redispersing the precipitate in a blender with 1,000 ml of distilled water and blending it for 20 minutes;

(e) redispersing the precipitate in 1,000 ml of water, blending the precipitate with 7.29 grams of

Sr(OH)₂.8H2O and 98.94 orams of PbO to form a slurry, and diluting the slurry to a total volume of 1.5 liters with a pH of 11.76;

(f) hydrothermally treating the slurry at an elevated temperature and pressure for a time sufficient to form the powder by pouring the slurry into a 3.785-liter capacity stainless steel autoclave, heating to 300°C over 150 minutes at a stirring rate of about 350 rpm, holding the slurry at that temperature for about 30 minutes, and cooling the slurry to 90°C and removing the slurry from the autoclave;

(g) preparing a dry, pale-yellow powder from the slurry by immediately filtering the slurry, washing the solids in the slurry in the same manner as the hydroxide gel in step (d), and then drying the slurry in a vacuum oven at 60°C for several hours.