Title of Invention

A CERAMIC SUSPENSION AND A METHOD FOR PREPARING A CERAMIC SUSPENSION

Abstract

The present invention relates to a ceramic suspension and a method for preparing the same. The suspension has a quantitz of ceramic powder comprising at least one metal component, said powder uniformelz suspended in an aqueous fluid ranging up to 30 percent, bz volume, of total solids in said suspension and having an average particle size of less than 0.5 micrometer a quantity of passivating agent ranging from 0.5 percent to 5 percent bz weight of said ceramic powder and a quantitz of dispersing agent of equal to or greater than 1 percent, bz weight, of said ceramic powder, said passivating agent and said dispersing agent being present in amounts such such that said suspension has an apparent viscositz of less than 3 10-<sup>6</sup> Mpas (3000 cps).

Full Text

CERAMIC SLIP COMPOSITION AND METHOD FOR MAKING THE SAME FIELD OF THE INVENTION The present invention pertains to a • ceramic suspension , — — — ■ — and i method- for producing the same. In particular, the present invention pertains to the use of an optimal range of passivation and dispersing agents in an aqueous suspension of finely divided ceramic powder. BACKGROUND OF THE INVENTION The importance of being able to produce thinner dielectric layers is becoming increasingly recognized by the producers of multilayer capacitors (MLC"s) due to end user requirements of reduced size and cost. These capacitors are typically manufactured by co-firing, i.e., sintering a ceramic dielectric formulation and a conductive electrode material in an oxidizing atmosphere at a temperature in the range of about 1000=" to 1400°C. Dielectric layers have traditionally been produced preparing a suspension of ceramic powder in a liquid usually containing a dispersant, and then adding an organic resin matrix which functions to bind the -eramic particles within the suspension. A variety of nethods are "inown for applying the suspension and binder mixture (hereafter defined as slip) to a substrate to form very :hin layers of the suspended solids. Methods such as wet :oating, tape-caating (casting), or doctor-blading are readily known to those skilled in the art. The thin, iried layers generally termed as green layers, may then coated with conductive electrodes and stacked together similar layers to form a green body. The stack is :hen trimmed and co-fired to produce a structure :insisting of alternating layers of sintered electrode ;end dielectric which is terminations to form the finished capacitor. Suspensions used for dielectric compositions in the past have used both aqueous and organic liquids, but because of the environmental and safety concerns, the tendency has been to increase the use of aqueous suspensions for making the dielectric layers. Another trend in the capacitor industry has been to make the dielectric layers thinner to obtain more capacitance per unit volume. Therefore, the thickness of dielectric layers have been reduced from 25 microns to 10 microns. It is now desirable to reduce the thickness to 5 microns or less. These thinner layers necessitate the use of extremely srriall solid ceramic particles in the suspension to produce the required high density and fine grain size in the final fired layer. When ceramic powders are reduced to such small particle sizes, i.e., less than 0.5 microns, they tend to have a significant soluble portion that dissolves in an aqueous suspension thus causing chemical reactions with the dispersants and binders in solution. Smaller particles are also more difficult to handle making automated systems unduly complex and expensive. Barium titanate, the base material of choice for capacitor formulations due to its dielectric characteristics, forms a soluble cation. Since the binder contains dispersing agents, any reaction of the soluble cation or its companion hydroxyl ion with the chemical dispersants in the binder can cause agglomerates of binder and "salting out" or precipitation of the metal cation - dispersant complex. These complexes or agglomerates often create voids in the ceramic body during the binder burnout phase prior to sintering and can result in either elevated levels of electrical leakage or electrical shorting paths. Void formation is particularly unforgiving in layers having a thickness of less than 10 microns. r Another problem that occurs when making suspensions with ceramic powder of less than 0.5 microns in diameter is that both the interfacial area between the solids and the liquid carrier and the number of particles in a given volume are greatly increased. This results in a high physical chemical interaction between the solid particles in the liquid phase, and diminished processability, especially at commercially acceptable solids loading levels. Hence it can be expected that the benefit of finer particle sizes can be countered by the necessity of going to lower solids loadings in the suspensions or slips. Manufacturing processes which expose the suspension to high shear conditions such as those encountered in pumping or tape casting, result in excessive gelling and in the worst case, dilatant-like conditions which are characterized by unworkable suspensions with shear thickening characteristics and high viscosities. A variety of attempts have been made to prepare finely divided ceramic powders in aqueous suspensions and slips. For example, US Patent No. 3,495,008 discloses the ball milling of a ferroelectric material such as barium titanate in a 60% by weight solids loading level of milled material to water. The mixed suspension is rediluted to a desirable consistency for spray application. In US 3,551,197 a dielectric composition is prepared with between 40 to 90 weight percent of a ceramic powder in water. The ceramic powder is selected from a group including barium titanate, strontium titanate, calcium titanate, and lead titanate, and has a particle size of 0.5 to 3 micron. The suspended ceramic material is combined with a binder such as polymethylene glycol or diethylene glycol for example. In US 4,968,460, an aqueous emulsion of water soluble polymeric binder is combined with an aqueous suspension of ceramic material in a solids loading of at least 50 weight percent. The polymeric binder is used in a range of 0.5 to 35 weight percent and optionally with up to 5 weight percent of a selected dispersing agent. Tapes prepared from the slip composition had a thickness of between 30 microns and 2.540mm. Particle sizes in the range of 0.5 to 12 micron are disclosed. These references however, do not address the problems encountered in the preparation of aqueous suspensions or slips of ceramic powders having particles of less than 0.5 micron in diameter. A suspension of ceramic powder having a diameter of 0.5 micron or less which remains suspended in an aqueous carrier fluid for extended periods of time in a substantially unagglomerated state and which maintains an apparent viscosity of less than 3000 centipoise (cps) without solidifying when determined from high shear rates of between 50 to 100/sec, would be a desirable improvement in the art of ceramic suspensions, slips, and the processes for producing them. Yet another object of the present invention is to produce an aqueous suspension which has a ceramic powder loading of up to about 30% by volume and has an apparent viscosity of less than 3000 cps. Yet another objective of this invention is to passivate the surface of the particle making up the ceramic powder with respect to soluble anions and cations, with a very thin layer of a relatively insoluble passivating agent, such as barium oxalate, then apply a dispersing agent to create a stable suspension. Surface passivation is necessary to prevent interactions of the barium, or other ions, which can cause cross-linking with the dispersing agent(s). An additional objective of the invention is to enable the formation of stable slips at high solids loading of up to about 30% by volume of total solids (70 weight percent) or more of barium titanate, which have low enough viscosities to qive aood flow Dronpr-h-i o = necessary to make thin green layers; which contain the necessary binder to form a cohesive film; and which have a uniform distribution of ceramic particles. A further object of the present invention is to provide a slip which is non-dilatant in high shear applications such as pumping and spraying. SUMMAEY OF THE INVENTION Accordingly, the present invention provides a method for preparing a ceramic composition by suspending a quantity of ceramic powder, having an average particle size range of 0.5 micron or less in a range of up to 30% by volume of total solids in a carrier fluid composed of water and a quantity of passivating agent ranging from 0.5 to 5% by weight of ceramic powder. A quantity of either anionic or cationic dispersant is added to the suspended material in a range of at least 1% by weight based on the ceramic powder. The suspension and dispersing agent are then diluted with water and exhibit an apparent viscosity in the range of less than 3000 centipoise. In an alternate embodiment, a quantity of an organic binder is added to the suspension in a range of 12 weight percent or less and preferably 3 to 12 weight percent to form a slip composition. The slip may then be applied to a substrate or mold to form a green layer having pore sizes of less than 0.5 microns. The features and advantages of the present invention are meant to be illustrative rather than exhaustive. Further advantages and features of the present invention will become apparent while reviewing the detailed description of the invention. DETAILED DESCRIPTION OF THE INVENTION TO form a thin dense layer of a ceramic or dielectric powder, such as BaTiOj from an aqueous suspension of the ceramic powder, it is necessary to create a stable suspension which resists settling at high solids loading; which is processable, i.e. has an apparent viscosity level which is sufficiently low to allow casting or pouring of thin films; has a Bingham yield point which provides that the applied layer of suspension achieves a gel state but which does not become dilatant; and where desired, contains an organic binder that will hold the particles together during the subsequent operations in the process prior to the ultimate high temperature sintering operation. An additional characteristic of the present invention is to provide a very thin coating on the particles which will produce a zeta potential which weakens the inherent interparticle attraction yet which will be more hydrophobic in nature, and will aid with the organic binder to control the physical properties of the green layer. The inventors have discovered that for the preparation of suspensions and slips containing elevated ceramic powder loading levels in aqueous carrier fluids, there exists a critical concentration range for passivating agents which will result in reduced agglomeration and more uniform pore size distributions in the green layers and green bodies made therefrom. In one embodiment, ceramic compositions for tape fabrication were prepared by uniformly suspending ceramic powder up to about 30 volume percent and more particularly from 20 to 3 0 volume percent in an aqueous carrier fluid of deionized water to which the passivating agent had been previously added. The term "uniform" is defined to mean pore sizes of 0.5 microns or less formed in green layers which were prepared from suspensions or slips of the present invention. Prepared suspensions had consistencies of from fluid-like to paste-like depending on the loading level of ceramic powder. The dispersant was subsequently added to the mixture to obtain a uniform suspension. While a variety of ceramic powders will be known to those skilled in the art, the powders that will benefit most from the passivation-dispersion technology are those powders that display phase instability in the presence of water or those powders with relatively high solubilities or leachability of at least one of the metal ^ Moles per liter components of the ceramic powder between 10" / to 10 "" Kfoles per litei in a pH range of 4 to about 11. Furthermore, powders with an average particle size of 0.5 micron or less and preferably between 0.05 micron and 0.5 micron (and having at least one metal component in the solubility or leachability range noted above) will benefit most from the passivation-dispersion technology disclosed herein. There are a number of methods for measuring average particle size of ceramic powders that are widely known to those skilled in the art. In the present invention, average particle size was determined by BET gas adsorption surface area analysis. This method is particularly useful in calculating average particle size when the powder particles are substantially spherical as is the case of the powders used in the present invention. The term ceramic powders are further defined to include metallic oxides such as zinc oxide, bismuth oxide, or aluminum oxide; metallic sulfides, metallic borides, metallic nitrides, metallic carbides, metallic tellurides, metallic arsenides, metallic silicides, metallic selenides, and metallic halides; and including mixed materials such as metallic titanates, metallic tantalates, metallic zirconates, metallic silicates, metallic germanates, and metallic niobates. The metal component of the metallic oxides may include those metals of the periodic table of elements found in groups IIA through IIB, and also including the Lanthanum and Actinium series. Ceramic powders are further defined to include complex oxides having the general formula ABOj wherein A consists of one or more of the metallic species which are of a sicnilar ionic radius and ionic charge and are selected from Che group consisting of barium, calcium, magnesium, lead, strontium, and zinc. Group B consists of one or more metallic species selected from the group consisting of hafnium, tin, titanium, zirconium, and may further include mixtures or solid solutions thereof. Those skilled in the art will recognize the similarity of the cited B species with ticanate based on reported values for ionic radius and ionic charge. In an alternate embodiment, ceramic powders are further defined to include the above identified complex oxides having the general formula ABO3, and may also contain one or more dopants. As the quantity of dopants to be added is normally a small weight percent of the total solids, the addition of a dopant generally does not affect the physical characteristics of the slip or suspension. Those skilled in the art will therefore recognize that a variety of "dopants" may be used. The term "dopants" shall be defined to include an additive which is used to tailor the electrical performance of the ceramic powder in the finished capacitor. In the present invention, dopants may be defined Co include one or more metals selected from Che oxides of the group consisting of aluminum, antimony, bismuth, boron, calcium, cadmium, chromium, copper, cobalC, hafnium, iron, lanthanum, lead, manganese, molybdenum, neodymium, nickel, niobium, praseodymium, samarium, scandium, silicon, silver, tantalum, titanium, tin, tungsten, vanadium, yttrium, zinc, and zirconium. The passivating agent can be any acid or base which (i) burns out cleanly from Che suspension or slip at a temperature of about lOBCC or less, (ii) provides a uniform surface charge on Che ceramic particles as a function of the carrier fluid pH, (iii) has reasonably flat solubility over a pH range of 4 to about 11, (iv) forms relatively insoluble precipitaCes with at least 1 metallic species of the ceramic powder, (v) promotes adsorption of desired anionic or cationic dispersants, and (vi) after adsorption of the dispersant, has "improved settling characteristics", defined hereafter to mean a sol which remains opaque for a period of one week without appreciable precipation when compared to control samples prepared in an aqueous carrier, at the same solids loading but without a passivating agent. Analytical methods for determining the opacity of the sol are known to those skilled in the art and will not be elaborated upon herein. While a number of passivating agents will be known to those skilled in the art, particularly desirable agents may include compounds or mixtures of succinates, benzoates, formates, cupferons, and S-hydroxyquinoline. Although not wishing to be so limited, oxalic acid is the preferred passivating agent as discussed and evaluated in the Examples presented below. Preferably the oxalic acid is dissolved in deionized water at 1 to 3% by weight of ceramic powder. The inventors have also discovered that for a ceramic powder loading level of up to 30 volume percent, an optimum passivating agent loading level of 1 to 3 weight percent exists for a quantity of dispersing agent of greater than or equal to 1 weight percent. At lower concentrations of passivating agent, the ceramic powder begins to form agglomerates. At higher concentrations, the excess passivating agent in the carrier fluid forms precipitates with dilute metals and dispersants and apparent viscosity increases to unacceptably high levels, i.e. to greater than 3000 centipoise as determined at 50 to 100/s. The term "apparent viscosity" is known to those skilled in the art and shall be defined hereafter to mean a value determined by fitting a power function to a viscosity-shear rate curve which is prepared from actual viscosity measurements. The resulting function is used to calculate the viscosity at shear rates of 50 sec" and 100 sec"". A point-slope method is then used to Extrapolate to zero shear rate using the linear fit to the viscosity-shear rate curves to determine the apparent viscosity. The present invention contemplates that dispersant concentrations of greater than 5% by weight may be used depending on the passivating agent and binder selected for a given slip composition. While not wishing to be so limited, dispersing agents preferably include the characteristics of being polymers which are; (i) compatible with a ceramic powder particle passivated in the manner discussed above and which uniformly coat the particle; (ii) which have trains which stretch in a generally parallel manner across the particle surface rather than extending radially therefrom; (iii) which minimize crosslinking or "salting out" in the bulk suspension solution; and (vi) and which have a zeta potential in excess of +10 millivolts and preferably either in the range of +10 millivolts to about +40 millivolts or of -10 millivolts to about -40 millivolts for the loading levels discussed above. A zeta ^ cr -10 potential magnitude which is less than +10/millivolts results in a suspension where there is insufficient electrostatic repulsion to prevent particle agglomeration. Various anionic and cationic surfactants having molecular weight in the range from less than 1000 to greater than 30,000 are contemplated as dispersants. Included are sodium, potassium, or preferably ammonia salts of stearate, lauryl sulfate, alkyl polyphosphate, dodecyl benzene sulfonate, disopropylnaphthalene sulfonate, dioctylsulfosuccinate, ethoxylated and sulfated lauryl alcohol, and ethoxylated and sulfated alkyl phenol. Various cationic surfactants include polyethyleneimine, ethoxylated fatty amine and stearylbenzyldiraethylammonium chloride or nitrate. Alternate dispersants contemplated in the present invention include: polyethylene glycols, lecithin, polyvinyl pyrrolidone, polyoxyethylene, isoctylphenyl ether, polyoxyethylene nonylphenyl ether, amine salts of alkylaryl sulfonates, polyacrylate and related salts, polymethacrylate and related salts, and fish oil. Additional anionic and cationic dispersants having the characteristics described above may be found in the reference entitled McCutcheon"s. Volumes 1 and 2, McCutcheon Division, published by The Manufacturing Confectioner Publishing Co. In operation, up to 30 volume percent of a ceramic powder was suspended in an aqueous carrier fluid containing from 0,5 to 5 weight percent of passivating agent. A quantity of dispersing agent of greater than or equal to 1% by weight of the ceramic powder was then added to the passivating agent containing carrier fluid. An additional quantity of water was then introduced to achieve the stated weight percent loading level. A final suspension pH of between about 4 and about 11 and preferably between 7 and 10 was achieved. Suspensions prepared by this method had Bingham yield points of less than 230 dynes/cm^ and apparent viscosities of less than 3000 cps. The inventors have discovered that the step of adding passivating agent to the carrier fluid prior to the addition of the ceramic powder is critical to the performance of the suspension or slip as a green layer. While not wishing to be limited to any specific theory, it is considered that the dissolution reactions at the powder surface occur rapidly which requires the passivating agent to be available upon introduction of the particles into the carrier media. If the passivating agent is added after the powder is introduced, the precipitation very likely occurs in the bulk solutiop rather than at the particle surface. In an alternate embodiment, a quantity of dispersant may be added directly to the carrier fluid with the passivating agent prior to the addition of the ceramic powder. In a method for producing a slip of the ceramic powder, a quantity of organic binder of 12% or less was added to the suspension prepared according to the process discussed above and diluted further with vater. The criteria for selecting a binder are that the binder must uniformly disperse throughout the suspension and bind the passivated or dispersant coated ceramic particle while minimizing separation of phases. The uniform distribution of passivated particles facilitates particle coating by the binder thereby allowing less binder to be used. Although not wishing to be so limited, polyethylene glycol, commercially available under the trademark "Carbowax ^" was utilized. Carbowax is a trademark of Union Carbide of Danbury, Connecticut. In another embodiment, polyvinylpyrrolidone was used as a binder at 12% loading levels by weight of the ceramic material. In yet another embodiment a total of 13% of polyethyieneimine was used as a combined dispersant and binder. Slip materials prepared at the loading levels discussed above were then applied on a substrate by methods known in the art to form green layers having a thickness of 5 microns or less. By sandwiching conductive layers between multiple green layers, green bodies may be prepared. Those skilled in the art will be familiar with the techniques for preparing green bodies and therefore such techniques will not be further discussed. Accordin^y. the present inveation provides i cenmic suspension, comprising: i quastity of cenmic powder comprising at least one metal component, said powder ttniformly suspended in to aqueous fluid ringing up to 30 percent, by volume, of total solids in said suspension and having an average particle size of less than 0.5 micrometer; a quantity of passivating agent ranging from 0.3 percent to 5 percent by weight of said ceramic powder; and a quantity of dispersing agent of equal to or greater than 1 percent, by weight, of said ceramic powder, said passivating agent and said dispersing agent being present in amounts such that said su^eosion has an apparent viscosity of less than 3 X lO"^Mpa-s (3000 cps). Accordingly, the present invention also provides a me&od for preparii^ a ceramic suspension as described above, comprising: mixing up to 30 volume percent solids of ceramic powder comprising at least one metal component in an aqueous fluid, said powder having an average particle size of less than 0.5 micrometer, said fluid containiag a quanti^ of passivating agent ranging from 0.3 percent to 5 percent, by weigjit, of said ceramic powder; and mixing a quantity of a dispersing agent of equal to or greater than 1 percent, by weight, of said ceramic powder with said suspension, said passivating agent and said dispersing agent being present in amounts such that said suspension has an apparent viscosity of lera dian3 x lO"^Mpa^ (3000 cps). Referring to Figures lA and B, scanning electron photo micrographs made from 8uq>ension8 prepared in accordance witii Example 1, are illustrated at two different magnifications. Loading rates of 0.3 weight percent of oxalic acid and 1 weight percent polyetfayleneimine were used As ilhistrated, agglomerated areas are clearly visible. Considerable improvement in Che degree of agglomeration is seen in the micrographs of Figures 2 and 3, A and B, where green layers were prepared in accordance with the procedures of Examples 2 and 3. Loading rates of 0.5 weight percent oxalic acid at 5 weight percent of polyethyleneimine and 1 weight percent oxalic acid and 1 weight percent of polyethyleneimine, were used. Figures 4 A and B are micrographs of cast green layers prepared in accordance with Example 5 where 1 weight percent of passivating agent and 3 weight percent of polyethyleneimine were used. When the amount of polyethyleneimine was increased to 5 weight percent, green layers illustrated in Figures e A and B were obtained which are similar to Figure B. Where levels of oxalic acid from 2 to 3% were used and levels of polyethyleneimine from 1 to 5%, were used {Figures 7 to 9}, commercially acceptable green layers were obtained. However, the micrographs of Figure 1, illustrate that where oxalic acid loading levels of 0.5 weight percent were used at solids loading up to 30% by volume, and 1 weight percent of polyethyleneimine was used, excessive agglomeration and unworkable viscosity was obtained. The micrographs demonstrate the criticality of the lower range of passivating loading for aqueous suspensions. While not wishing to be bound by any particular theory, one explanation is that the passivating effect of the oxalic acid is insufficient at 0.5 weight percent to passivate the barium titanate at the indicated level of solids loading. Referring now to Figure 10, aqueous slips were prepared in accordance with the procedure described in Examples 16 and 17 where 3 weight percent of oxalic acid and 1 weight percent of polyethyleneimine were combined with 3 weight percent of polyethylene glycol binder. The micrographs illustrated in Figure lOA and B show that the surface of the green layer has a uniform pore size distribution of 0.5 micron or less. In Figure 11, an aqueous slip was prepared in accordance with the procedure described in Example 18 where 3 weight percent of oxalic acid, 1 weight percent of polyethyleneitnine was combined with 6 weight percent of polyethylene glycol binder. Commercially acceptable green layers were obtained having a pore sizes 0.5 micron or less. In Figure 12, an aqueous slip was prepared in accordance with the procedure described in Example 19 where 3 weight percent of oxalic acid, 1 weight percent of polyethyleneitnine was combined with 12 weight percent of polyvinyl pyrroilidone binder- Commercially acceptable green layers were obtained having a pore size of less Chan O.S micron. In Figure 13, an aqueous slip was prepared in accordance with the procedure described in Example 20 where 3 weight percent of oxalic acid and 2 weight percent of polyethyleneimine was combined with 11 weight percent of polyvinyl pyrroilidone binder. Tin apparent viscosity of 1050 cps was obtained. In Figure 14, an aqueous slip was prepared in accordance with the procedure described in Example 21 where 3 weight percent of oxalic acid and 1 weight percent of polyethylene- imine was added as a dispersant. An additional 12 weight percent of polyethyleneimine was added as the binder. An apparent viscosity of 779 cps was obtained. The green layer formed had a pore size of 0.5 micron or less. The present invention will become more readily understood by the following non-limiting Examples. Apparent viscosity and loading levels for Examples 1-21 are summarized in Table 1 hereunder. Percentages are in weight percent of ceramic powder unless otherwise designated. EXAMPLE 1 An aqueous solution of oxalic acid was prepared with 0.2810 g oxalic acid (as H,C204-2H20) and 23.2 g H3O, To the oxalic acid solution 56.B068 g of barium titanate, BT-10, produced by the assignee of the present invention, Cabot Corporation, having an average particle size of about 0.1 micron was slowly added with high shear mixing (normally 5000 rpra). About l.l433g of polyethyleneimine or "PEI", made by Kodak, 50% in water is added to the oxalic acid/barium titanate suspension at the same mixing shear rate as above. The final suspension contained 0.5% oxalic acid and 1% PEI. The suspension obtained had an unacceptably high apparent viscosity in excess of 3000 cps and was therefore unsuitable for the preparation of green, ceramic layers by wet lay-down techniques. Actual viscosity measurements were performed on the suspensions at 25"*C using a cone-plate viscometer at shear rates ranging from 0.6 to 120 sec"". If the measured viscosity of the suspensions was beyond the range permitted by the measuring head of the viscometer or if insufficient data points were obtained to observe a plateau regime for the viscosity-shear rate plot, the suspension viscosity was reported as "NA". The cone plate viscometer was a Digital Viscometer, either model # LVTDCP or model DV-III Rheometer made by Brookfield Engineering Laboratory, Inc. of Stoughton, Massachusetts. All suspensions for which rheological data could be obtained were pseudoplastic. Many of the suspensions also exhibited a Bingham yield stress, below which the slurry did not flow. This latter property is desirable for most tape or layer forming processes to ensure that the slurry does not flow off of the substrate after deposition. Suspensions which were undergoing actual viscosity measurements were evaluated also for Bingham yield by recording the shear stress at a point where the suspension or slurries tested exhibited no flow using a minimum shear rate equal to 0.6 sec"". Zeta Potential Test Procedure A quantity of suspension or slip prepared in accordance with the Examples was measured for zeta potential on a Brookhaven ZetaPlus, produced by Brookhaven Instruments Corporation, in Holtsville, New York. Preparation of Green Layers Green layers of the suspension or slip were prepared by the following method: About 2 - 3cc of the suspension or slip was added onto a glass side and manually dispersed by means of running a metal blade run over the material to obtain uniform thickness. In another method, a manually operated doctor blade was run over the deposited ^ suspension and slip materials. The suspension was then dried for 10 to 15 minutes at room temperature to form the green layer. EXAMPLES 2-15 were prepared in accordance with the procedure of EXAMPLE 1 employing the quantities of reactants as indicated in Table 1. Percent by weight of oxalic acid as passivating agent and polyethyleneimine as dispersant obtained in the final suspension, as well as apparent viscosity values are presented in Table 1. EXAMPLE 16 An aqueous solution of oxalic acid was prepared that contained 1.7068 g oxalic acid (as H2C204-2H20) to 20.0468 g HjO. To the oxalic acid solution 56:8370 g of Cabot B-10 barium titanate is slowly added with high shear mixing {nominally 5000 rpm). Polyethyleneimine (PEI made by Kodak, 50% in water) at 1.1349 g was added to the oxalic acid/barium titanate suspension under the same mixing shear as discussed above. Then 1.6778 g of binder (Carbowax PEG 1450F, Union Carbide) was added to the existing suspension. The final slip contained 3% oxalic acid, 1% PEI, and 3% binder as compared to the total weight percent of 69.82 (28.63 volume percent) for the barium titanate powder. The slip was shear thinning with an apparent viscosity of 428 cps. Green layers were prepared in accordance with the procedure of EXANPLE 1. EXAMPLE 17-21 were prepared in accordance with the procedure of EXAMPLE 16 employing the quantities of reactants as indicated in Table 1. Percent by weight of oxalic acid as passivating agent and dispersant obtained in the final suspension, as well as apparent viscosity values are presented in Table 1. The dispersant utilized in EXAMPLE 19 and EXAMPLE 20 was polyvinyl pyrroilidone, available from GAP Corporation of Wayne, New Jersey as PVP K-30. In EXAMPLE 21, polyethyleneimine at a concentration of 1 weight percent was used as the dispersant and 12 weight percent was used as a binder. Green layers were produced by the doctor blade procedure presented in Example 16. As is evident from Table 1, both apparent viscosity and Eingham Yield Point values are in acceptable ranges for commercial applications. WE CLAIM: 1. A ceramic suspension, comprising; a quantity of ceramic powder comprising at least one metal component, said powder uniformly suspended in an aqueous fluid ranging up to 30 percent, by volume, of total solids in said suspension and having an average particle size of less than 0.3 micrometer; a quantity of passivating agent ranging from 0.5 percent to 5 percent by weight of said ceramic powder; and a quantity of dispersing agent of equal to or greater than 1 percent, by weighty of said ceramic powder, said passivating agent and said dispersing agent being present in amounts such that said suspension has an apparent viscosity of less (3000 cps). 2. The ceramic suspension as claimed in claim 1, wherein said ceramic powder is a complex metal oxide having the formula of ABO3, wherein A is at least one metallic species selected from the group consisting of barium, calcium, magnesium, lead, sfrontium and zinc, and wherein B is at least one metallic species selected from the group consisting of hafriium. tin, titanium and zirconium, or mixtures or solid solutions thereof 3. The ceramic suspension as claimed in claim 2, wherein said complex metal oxide is barium litanate. 4. The ceramic suspension as claimed in claim 1, wherein said passivating agent is selected from the group consisting of succinates, benzoates, formates, cupferrons, 8-bydroxyquinoline, oxalic acid and mixtures Aereof. 3. The cerimic suspension is claimed in claim 1. wherein a quantity of between 17.72 percent and 30 percent, by volume, of cerimic powder is umformly suspended in Slid aqueous fluid, and said suspension has in apparent viscosity of less than 3 x IC Mpi"s (30 dynes/cm1 measured »t 25°C tod at ahetr rates ranging from 0.6 to 120 sec". 6. The ceramic suspension as claimed in claim 3, wherein said complex metal oxide includes one or more dopants selected from the group consisting of oxides of aluminum, antimooy, foismutfa, boron, calcium, cadmium, chromium, copper, cobalt, faiihium, iron, lanthanum, lead, mauginese, molybdenum, , nickel, niobium, praseodymium, samarium, scandium, silicon, silver, tantslum, titanium, tin, tungsten, vinidium, yttrium, zinc and zirconium. 7. The ceramic suspension as claimed in claim 1, wherein said ceramic powder has a zeta potential in the range of +10 to +40 millivolts or -10 to -40 millivolts. S. The ceramic suspension as claimed in claim 1. wherein said suspension his a Bingham yield point of less than 2300 N/m" (230 dyae/cm1. 9. The ceramic suspension is claimed in claim 1, wherein said metal component solubility in said fluid is between lO"1moles per liter to 10* moles per liter. 10. The ceramic sugpension is claimed in claim 1, wherein a quantity of an organic binder is present. U. A method for preparing I ceramic suspension as claimed in claim 1, comprising: mixing up to 30 volume percent solids of ceramic powder comprising it least one metal component in an aqueous fluid, said powder having in average particle size of less than 0.5 micrometer, said fluid containing a quantity of passivating agent ranging from 0.5 percent to 5 percent, by weight, of said ceramic powder; and mixing a quantity of a dispersing agent of equal to or greater than 1 percent, by weight, of said ceramic powder with said suspension, said passivating agent and said dispersing agent being present in amounts such that said suspension has an apparent viscosity of less dian 3 x 10"1 MPa s (3000 cps). 12. The method as claimed in claim 11, wherein said dispersing agent is mixed with the ceramic powder which is a complex metal oxide having the formula of ABO3, wherein A is at least one metallic species selected from the group consisting of barium, calcium, magnesium, lead, strontium, zinc, and wherein B is at least one metallic species selected from the group consisting of hafnium, tin, titanium and zirconium, or mixtures or solid solutions thereof, 13. The method as claimed in claim 12, wherein said dispersing agent is mixed with barium titanate. 14. The method as claimed in claim 11, wherein said ceramic powder is mixed with the aqueous fluid containing said passivating agent selected from the group consisting of succinates, benzoates, formates, cupferrons, 8-hydroxyquinoline, oxalic acid and mixtures thereof. 15. The method as claimed in claim 11, wherein an additional quantity of aqueous fluid is introduced into said ceramic suspension to achieve an apparent viscosity of less than 3 x 10"1 MPas (30 dynes/cm1) measured at 25°C and at shear rates ranging from 0.6 to 120 sec". 16. The method as claimed in claim 13, wherein the said dispersing agent is mixed with the complex metal oxide having one or more dopants selected from the group consisting of oxides of aluminum, antimony, bismuth, boron, calcium, cadmium, chromium, copper, cobalt, hafnium, iron, lanthanum, lead, manganese, molybdenum, neodymium, nickel, niobium, praseodymium, samarium, scandium, silicon, silver, tantalum, titanium, tin, tungsten, vanadium, yttrium, zinc and zirconium. 17. The method as claimed in claim 11, wherein said ceramic powder mixed with said aqueous fluid has a zeta potential in the range of +10 to +40 millivolts or -10 to ~ 40 milUvolts. 18. The method as claimed in claim 11, wherein an organic binder is added to said ceramic suspension to form a slip composition. 19. The method as claimed in claim 11, wherein said ceramic powder mixed with the said aqueous fluid comprises metal component having solubility in said fluid between 10"" moles per liter to 10"" moles per liter. 20. The ceramic suspension as claimed in any one of claims 1-10, wherein the ceramic powder is coated with an insoluble precipitate formed from the passivating agent and the metal component of the ceramic powder. 21. The method as claimed in any one of claims 11-19, wherein the ceramic powder is coated with an insoluble precipitate formed from the passivating agent and the metal component of the ceramic powder. 22. A ceramic suspension, substantially as herein described and exemplified with reference to the accompanying drawings. 23. A method for preparing a ceramic suspension, substantially as herein described and exemplified with reference to the accompanying drawings.

Documents:

1068-mas-1995 abstract.pdf

1068-mas-1995 claims-duplicate.pdf

1068-mas-1995 claims.pdf

1068-mas-1995 correspondence-others.pdf

1068-mas-1995 correspondence-po.pdf

1068-mas-1995 description (complete)-duplicate.pdf

1068-mas-1995 description (complete).pdf

1068-mas-1995 drawings-duplicate.pdf

1068-mas-1995 drawings.pdf

1068-mas-1995 form-1.pdf

1068-mas-1995 form-26.pdf

1068-mas-1995 form-4.pdf

1068-mas-1995 petition.pdf