CN1313842A

CN1313842A - Barium titanate dispersions

Info

Publication number: CN1313842A
Application number: CN99809978A
Authority: CN
Inventors: 戴维·V·米勒; 斯利德哈·维尼加拉; 唐纳德·J·克兰西
Original assignee: Cabot Corp
Current assignee: Cabot Corp
Priority date: 1998-06-23
Filing date: 1999-06-21
Publication date: 2001-09-19
Also published as: AU4581299A; BR9911406A; SI20526A; JP2002518290A; RU2001101934A; EP1109758A1; TW552250B; CA2335927A1; WO1999067189A1; IL140454A0; KR20010034928A

Abstract

The invention provides slurries, dispersions, or slips of barium titanate-based particles in a non-aqueous medium and methods of their production. The particles have a coating comprising a metal oxide, metal hydrous oxide, metal hydroxide or organic acid salt of a metal other than barium or titanium. At least 90 percent of the particles have a particle size less than 0.9 micrometer when the coated particles are dispersed by high shear mixing.

Description

Barium titanate dispersion

Background

The present invention relates to a barium titanate dispersion, and more particularly to a barium titanate dispersion in a nonaqueous medium.

Barium titanate-based materials are suitable for use in multilayer ceramic capacitors, commonly referred to as "MLCs" because of their high dielectric constant, which comprise alternating layers of dielectric material and layers of conductive material. Examples of MLC's are disclosed in US3612963 and US 4435738. Palladium, silver, palladium-silver alloys, copper and nickel are conventional conductive materials used in MLC. The dielectric layers of MLC are typically prepared using higher solids dispersions known in the art as slurries. Such slurries typically include a powdered barium titanate-based material and a polymeric cement in an aqueous or non-aqueous solvent. A thin film of powder stabilized with a cement is formed by casting or coating a slurry and dried to provide a green body of ceramic dielectric layer. The green body is coated with a conductive material in a pattern and subsequently laminated to form a laminate of alternating ceramic dielectric and conductive layers. The stack is cut into MLC-sized cubes and heated to burn off the organic materials such as cement and dispersant, followed by firing to sinter the barium titanate-based material particles into a capacitor structure having stacked, dense ceramic dielectric and conductive layers. The sintering temperature is typically 1000-. During sintering, the density of the ceramic dielectric layer is increasing as the particles melt and consolidate to form grains. Despite the use of grain growth inhibitors, the ceramic grain size in MLC dielectric layers is typically 3-5 times larger than the original grain size, for example. And all pores cannot be removed during firing. Typically, about 2-10% porosity still remains in the MLC dielectric layer. These pore or void defects in the dielectric layer may be larger in larger grain ceramics. Certain critical capacitor properties such as breakdown voltage and DC leakage are affected by dielectric layer thickness, grain size and void defects. For example, it is believed that: an effective dielectric layer needs only a few grains thick, such as at least 3-5 grains thick. Because defects in any one layer of an MLC can critically affect its performance, MLCs are produced with dielectric layers that are thick enough to effectively reduce the impact of ceramic defects formed by disordered large grains or pores, which can adversely affect the performance of the MLC.

Due to market demand for miniaturization of electronic device designs, the MLC industry needs ceramic raw materials that enable thinner dielectric layers without causing catastrophic consequences from large grain and pore sizes relative to the dielectric layer thickness.

Barium titanate powder produced by conventional methods such as calcination or hydrothermal methods has large particles and/or fine particles having a particle size of about 1 μm, and is strongly agglomerated. Such particles and agglomerates have been determined to be difficult to use to produce fine-grained, ultra-thin dielectric layers, such as MLC's having a thickness of less than 4-5 microns. Accordingly, it would represent a novel advance in the art to provide a barium titanate-based material and dispersion suitable for producing MLC's having relatively thin dielectric ceramic layers, e.g., less than 4 microns in thickness, and satisfactory or exceptional electrical properties, including DC leakage and breakdown voltage, without the need for additional ball milling.

Summary of The Invention

In one aspect, the present invention provides a slurry, dispersion or slip comprising particles of a barium titanate-based material dispersed in a non-aqueous medium. The particles include a coating comprised of a metal oxide, metal hydrous oxide, metal hydroxide or organic acid salt of a metal other than barium or titanium, wherein at least 90% of the particles have a particle size less than 0.9 micron when dispersed by high shear agitation.

The term "barium titanate-based" as used herein refers to barium titanate, barium titanate with another metal oxide coating, and barium titanate having the structural formula ABO₃In which a represents one or more divalent metals, such as barium, calcium, lead, strontium, magnesium and zinc, and B represents one or more tetravalent metals, such as titanium, tin, zirconium and hafnium.

In another aspect, the present invention provides a method of forming a slurry, dispersion or slip, the method comprising the steps of: barium titanate-based particles are dispersed in an anhydrous medium by high shear agitation until 90% of the particles have a particle size less than 0.9 micron. These particles have a coating layer composed of a metal oxide, a metal hydrous oxide, a metal hydroxide or an organic acid salt of a metal other than barium or titanium.

In yet another aspect, the present invention provides another method of forming a slurry, dispersion or slip comprising the step of forming a slurry of barium titanate-based particles in an aqueous medium by a hydrothermal process. The method further comprises the step of forming a coating layer composed of a metal oxide, a metal hydrous oxide, a metal hydroxide or an organic acid salt of a metal other than barium or titanium on the particles. The method further comprises the steps of replacing the aqueous medium with a non-aqueous medium and dispersing the barium titanate-based particles in the non-aqueous medium by high shear agitation.

Brief description of the drawings

FIGS. 1A,1B and 1C are particle size distribution curves of a preferred embodiment of the barium titanate particles of the present invention, wherein the three particles are separately high shear stirred for 45 minutes, additional stirred in a horizontal media mill for 30 minutes and additional stirred in a horizontal media mill for 45 minutes.

Fig. 2A and 2B are particle size distribution curves of another embodiment of the barium titanate particles of the present invention, wherein the two particles are high-shear stirred for 10 minutes and high-shear stirred for 30 minutes.

Other novel features and aspects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims.

Detailed Description

The present invention relates to a slurry, dispersion or slip of barium titanate particles dispersed in a non-aqueous medium. The particles include a coating on at least a portion of the surface of the particles. The coating comprises a metal oxide, hydrous oxide, hydroxide or salt of an organic acid of a metal other than barium or titanium or a mixture thereof. When the coated particles were dispersed by high shear stirring, 90% of the particles had a particle size of less than 0.9 microns.

The barium titanate-based particles can be dispersed in a non-aqueous medium to form a submicron-sized dispersion without the need for ball milling, which is advantageous for the production of MLCs having thinner dielectric layers with submicron grain sizes and higher breakdown voltages. The high shear agitation effectively reduces the size of the barium titanate-based particle agglomerates and depolymerizes or separates the agglomerates into smaller, coated particles without ball milling. The ball milling process involves impacting the particles with a hard milling medium such as rods or spheres of zirconia or the like. Since ball milling can break the particles so that their particle size is smaller than the primary particle size, thereby producing non-equiaxed particles having bare, uncoated surfaces, in preferred embodiments, the particles of the present invention are not ball milled and most of the surface of the particles is coated. In another aspect of the invention, the unground particles are equiaxed or spherical.

Barium titanate-based particles are used to provide monolithic capacitors comprising a ceramic body having a particle size of less than 0.3 microns. Preferred MLCs exhibit X7R and Y5V temperature coefficients of capacitance, and have a dielectric thickness of less than 5 microns and a dielectric strength of at least 50 watts/micron.

The primary particle size of the barium titanate-based particles can be determined by methods well known to those of ordinary skill in the art. Examples of such methods include the use of Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM). It should be appreciated that the barium titanate-based particles may include primary particles of varying sizes, and that the coated barium titanate-based particles have an average primary particle size of less than 0.6 micron. The primary particle size of the particles is preferably less than 0.5 microns, more preferably less than 0.4 microns; more preferably the particle major diameter is less than 0.3 microns, most preferably the particle major diameter is less than 0.2 microns.

The barium titanate-based particles may also be present in forms other than primary particles, such as agglomerates of primary particles and/or clusters of agglomerates of primary particles. SEM and TEM do not effectively distinguish between primary particles, agglomerates of primary particles, and particle size distributions between clusters of primary particle agglomerates. Analysis of the particle size distribution, for example by light scattering techniques, is a preferred method for determining the particle size of barium titanate-based particles, provided that the preparation of the analysis does not include a treatment to alter the distribution of the agglomerate or conglomerate particles, for example due to ultra-scattering techniquesSonication is depolymerization by high shear agitation or grinding. An automatic light scattering technique uses HORIBALA-910^TMA laser scattering particle size analyzer or similar device. Such analysis typically represents a volume fraction (in terms of frequency) of various particles consisting of size discontinuities, including primary particles, agglomerates, clusters, and the like, with the size of the discrete particles divided into ten groups, or tenths. Thus, as used herein, the term "particle size" refers to the size of each particle in the powder, and such particles may include primary particles, agglomerates of agglomerates, and mixtures thereof. In one embodiment of the present invention, at least 90% of the barium titanate-based particles coated with metal oxide in the barium titanate-based particle dispersion have a particle size of less than 0.8 micrometer, preferably less than 0.7 micrometer, and more preferably less than 0.6 micrometer. In a preferred embodiment of the invention, at least 90% of the particles in the barium titanate dispersion are less than 0.5 micron, preferably less than 0.4 micron, and more preferably less than 0.3 micron.

The characteristics of the particle size distribution include D₉₀,D₅₀,D₁₀,D₉₀Is the smallest particle size of the ten-fold-size largest particles, D₅₀Is an intermediate particle diameter, D₁₀Is the largest particle size of the ten equal fractions of thesmallest particles. D₉₀/D₁₀Is a simple characteristic for determining the width of the particle size distribution curve. In the different embodiments of the invention, the particle size distribution is narrow, D is preferred₉₀/D₁₀Ratio less than 4, more preferably D₉₀/D₁₀A ratio of less than 3, particularly preferably D₉₀/D₁₀The ratio is less than 2.5.

As used herein, dispersion refers to a two-phase system of solid particles suspended in a non-aqueous medium. In a preferred embodiment, the stability of the dispersion, or the resistance of the suspended solid particles to settling, is improved by the use of dispersants.

In addition to the meaning of metal oxide as clearly explained herein, the term "metal oxide" as used herein is also used to describe a coating of metal oxide, metal hydroxide, aqueous metal oxide or organic salt of a metal. Such organic acid salts may be converted to oxides or hydroxides, for example, by thermal decomposition that occurs during heating to burn off the ceramic binder and/or during sintering of the ceramic.

The term "high shear agitation" as used herein means agitation in a liquid while imparting sufficient energy to separate the coated particle agglomerates into smaller particles without impinging upon them with a solid medium such as a stick, cylinder or hard spherical medium such as zirconia balls. Hard media is used in certain high shear mixing devices where a smaller sized media is used to generate shear forces but not impact. While high shear mixing can be effectively performed with the various devices described below, it is difficult to accurately determine the force applied to separate agglomerates during high mixing.

As stated above, "barium titanate-based" refers to barium titanate, barium titanate with another metal oxide coating, and barium titanatehaving the structural formula ABO₃In which A represents one or more divalent metals, such as barium, calcium, lead, strontium, magnesium and zinc, and A represents one or more tetravalent metals, such as barium, tin, zirconium and hafnium. Preferred titanium typically useful in Y5A applicationsThe structural formula of the barium acid-based material is B_(1-X)A_XO·Ti_(1-Y)BYO₂Wherein X and Y may be 0 to 1, wherein A represents one or more divalent metals other than barium, such as lead, calcium or strontium, and B represents one or more tetravalent metals other than titanium, such as tin, zirconium and hafnium. When other metals are present as impurities, the X and Y values can be very small, for example less than 0.1. In other cases, other metal or metals may be introduced to provide a distinctly discernable compound such as calcium-barium titanate, strontium-barium titanate, barium titanate-barium zirconate, and the like. In other cases where X and Y are 1, barium or titanium may be substituted with some other metal of suitable valence to provide a compound such as lead titanate or barium zirconate. In other cases, the compound may have one or more partial substitutions of barium or titanium. Examples of such various partially substituted compositions are represented by the following structural formula:

Ba_{(1-X-X’-X”)}Pb_XCa_X’Sr_X”O·Ti_{(1-Y-Y’-Y”)}Sn_YZr_Y’Hf_Y”O₂

wherein X, X 'and Y, Y' are all ≧ 0, and (X + X '+ X ")<1, and (Y + Y' + Y")<1. In many cases, barium titanate-based materials exhibit a perovskite structure.

When hydrothermal barium titanate particles are conventionally dried to a powder, the particles form a more strongly agglomerated agglomerate of particles that are not readily and efficiently depolymerized with simple high shear agitation. Dispersions made from such dried, agglomerated barium titanate-based particles having submicron primary particle sizes require a relatively long period of impact milling to provide micron-sized particles, while for submicron particles, a longer period of more intensive milling is required. In contrast, slurries, dispersions, slurries containing agglomerated, metal oxide-coated particles having submicron primary particle sizes in a non-aqueous medium can be deagglomerated into submicron sized coated particles by moderate high shear agitation.

The barium titanate-based particles produced by the hydrothermal process can be used to prepare a non-aqueous slurry, dispersion, slurry of the barium titanate-based particles of the present invention, and these particles are maintained in an aqueous environment, for example, as an aqueous slurry, at least until after they are provided with a coating, and the aqueous medium is then replaced with a non-aqueous medium as described below.

Slurries of sub-micron barium titanate-based particles are prepared, for example, using the hydrothermal process disclosed in US4832939, US4829033 and US 4863833. In these hydrothermal processes, typically no more than 20% excess barium hydroxide solution is added to an aqueous titania slurry and typically heated to a temperature of about 100-. The particle size and particle size distribution can be controlled by controlling various process variables such as the temperature of the slurry and solution, the feed rate, the heating rate and cooling rate from the perovskite formation temperature. The choice of process variables to achieve a satisfactory granular product can be readily determined by the skilled person in the light of the general rules of crystallography. For example, larger particles may be prepared by relatively slow addition of barium hydroxide to a slurry maintained at a lower temperature, such as about 35℃, while smaller particles may be prepared by faster addition of barium hydroxide to a slurry maintained at a higher temperature, such as about 95 ℃. Good stirring is essential for the preparation of homogeneous particles.

The slurry is heat-treated to give a perovskite structure to the barium titanate particles. The particles are preferably washed to remove unreacted metal species such as barium ions. An effective rinse with ammoniated deionized water at pH 10 may be used to prevent barium from dissolving from the particles. The rinse water may be removed by filtration or draining of the supernatant from the precipitated particles. The number of flushes is determined by the desired purity in the aqueous phase to provide a slurry having a low ionic concentration and a conductivity of less than 5 millisiemens, preferably less than 1 millisiemens, and it has been found that 4 to 5 flushes are suitable for reducing the ionic content of the aqueous phase to a low ionic content characterized by a conductivity of no more than about 100 millisiemens.

The barium titanate particles may be held in the aqueous phase until after the coating process. As described above, the barium titanate-based particles may include a coating of at least one metal oxide, hydrous oxide, hydroxide, organic acid salt other than barium and titanate. Such coatings can be provided by adding an aqueous solution of a salt of the desired metal of the coating, such as a nitrate, borate, oxalate, etc., to a stirred barium titanate-based particulate slurry. A suitable pH may promote precipitation to form the coating. The salt solution may be added as a mixture of salts to form a uniform coating of a single layer, or separately and continuously to form individual oxide, aqueous oxide, hydroxide or organic acid salt layers. In the case of metals with higher solubility, such as cobalt and nickel, the oxide coating may be more difficult to apply and difficult to no longer dissolve. Itis therefore often preferred to apply an oxide coating of these more soluble metals as an outer coating over the more readily deposited metal oxide coating. The alkaline environment also minimizes the solubility of barium and facilitates the provision of barium-free or titanium-free coatings to the particles. Particle coatings for ceramic capacitor applications typically have a thickness of less than 10% of the particle diameter, often less than 20 nanometers in thickness, preferably no more than 5-10 nanometers in thickness.

Useful organic acid salt coatings include those metal organic acid salts that have relatively low solubility. Examples of such organic acid salt coatings are metal salts of oxalic acid, citric acid, tartaric acid and palmitic acid (e.g. niobium oxalate). It is believed that the organic acid salt will be converted to a metal oxide during the binder burnout process. The choice of metal should be based on the improvement in processability or performance of the MLC. The metal in the coating is typically selected from bismuth, lithium, magnesium, calcium, strontium, scandium, zirconium, hafnium, vanadium, niobium, tantalum, tungsten, manganese, cobalt, nickel, zinc, boron, silicon, antimony, tin, yttrium, lanthanum, lead, and lanthanides. Preferably, the barium titanate particles have a barium-free and titanium-free metal oxide coating. When it is desired that the ceramic capacitor has X7R dielectric properties, it is useful to provide barium titanate particles with a dopant, such as niobium oxide, titanium oxide, or neodymium oxide, which may be used in combination with nickel oxide or zirconium oxide. When it is desired to provide a ceramic capacitor that is sintered at low temperatures, such as about 1000 c to 1200 c, it is practical to provide barium titanate particles with a dopant that facilitates low temperature sintering as compared to sintering at 1300 c to 1600 c. Such low temperature sintering aids include bismuth oxide, zinc borate, zinc vanadate, lithium borate, and mixtures thereof. Oxides for dielectric modification and low temperature sintering may be added to barium titanate-based particles after rinsing and prior to forming a dispersible wet cake.

After coating the hydrothermal barium titanate-based particles, the slurry is rinsed and the water content of the slurry is reduced to provide a dense slurry, wet cake or powder. Also, the slurry, wet cake or flour may be treated with a dispersant to provide a dispersion. Further treatment with a binder and other additives to provide a slurry. The water is preferably removed by means such as calcination which avoids or at least minimizes the occurrence of strong agglomeration of the particles. Some metal oxide coatings may retain their hydrated metal oxide form unless they are sintered or dried, and these hydrated oxides may be dissolved if they are not maintained at a pH at which the solubility of the metal oxide is minimized. For example, if nickel oxide or cobalt oxide is not kept at a pH around 10, they may have some solubility. Thus, in order to retain the particles in a suitable coating, the pH of the aqueous composition is preferably maintained within the range of about 9-10. A metal oxide coated barium titanate-based particle slurry having a relatively low solids content, e.g., less than 30% of barium titanate particles, can be readily produced. Higher solids contents are typically preferred for the production of MLC. Thus, in the case of the direct use of the slurries according to the invention for the production of MLCs, it is practical to thicken the slurries, for example by removing the water, for example by filtration, to a solids content of at least 40% by weight, preferably at least 50% by weight, more preferably at least 55% by weight, and particularly preferably at least about 60 to 75% by weight. A dispersant and a binder may be added to the concentrated slurry to provide a slurry of barium titanate-based particles or a stable dispersionof barium titanate-based particles.

As described above, the aqueous phase may be replaced with a non-aqueous phase after the barium titanate-based particles are coated. The aqueous phase may be replaced by an organic liquid phase by solvent exchange or distillation. During the solvent exchange, a filtration device may be used. In a preferred embodiment, the filter device is a Funda comprising a flat pan^TMA filter lined with a ultrafiltration membrane mounted over the central water collector. The liquid stream was passed through Funda^TMThe filter and removed via a central water collector. Once the filtration operation is completed, the filter cake is removed from the pan by centrifugal force. The solids are then dropped through a valve into the next process step. Since the hydrothermal method produces barium titanate particles in submicron sizes, ultrafiltration membranes are used.

In operation, the solvent exchange process first involves pumping an aqueous slurry of barium titanate particles into a filtration apparatus. The water was removed through an ultrafiltration membrane. After removal of most of the water, an anhydrous but water-miscible organic solvent such as Methyl Ethyl Ketone (MEK) or toluene (described below) is added to the filtration apparatus. The water-miscible solvent is added to and removed from the filter material apparatus through the ultrafiltration membrane material until the water content of the barium titanate filter cake reaches the desired value. Once the water in the filter cake is removed using the water soluble solvent, a water insoluble solvent is then added. Sufficient water insoluble solvent is added and removed through the ultrafiltration material to dilute the concentration of water soluble solvent to the desired value. The barium titanate is then removed from the ultrafiltration material and allowed to fall into a stirred tank located below the filtration apparatus.

Once the barium titanate has entered the stirred tank located below the filtration device, the dispersant is added thereto and the contents of the vessel are stirred until a homogeneous slurry is obtained. If a lower moisture content is desired, a drier such as activated alumina may also be added. The dispersed slurry is then pumped to a grinding apparatus or a sealing apparatus.

Another method of conducting the solvent exchange is by distillation. The process involves a distillation vessel for the mixed materials, a condensing heat exchanger, and a phase separation tank. In operation, an aqueous barium titanate slurry is pumped to a distillation tank and the desired organic solvent is added. If the desired organic solvent is immiscible with water, a small amount of a water miscible solvent such as a high molecular weight alcohol may be added, and the ingredients may then be stirred into an emulsion and heated. When using a distillation process, it is preferred to use a high molecular weight solvent with a boiling point higher than that of water. Heating the mixture selectively removes water, thereby dispersing the remaining barium titanate in the organic medium. A phase separation tank located downstream of the condensing unit separates the water and any solvent that will also be removed during the distillation. The solvent is then pumped back into the distillation vessel and the water is pumped into the waste water vessel. The phase separation tank may be subjected to a vacuum to facilitate distillation at low temperatures. During the distillation, a dispersant may be added to prevent the slurry from solidifying in the distillation vessel. Once the desired water content is reached, no further solvent is recovered and the mixture is subsequently concentrated by condensation to achieve the desired solids content.

Similar to the ultrafiltration process described above, if a lower water content is desired, the mixture in the distillation tank can be passed into a bed containing a dry substance such as activated silica and activated alumina, noting that the thermodynamic efficiency of the distillation process can be improved by adding an ultrafiltration module upstream of the addition of the aqueous slurry of barium titanate into the distillation vessel. These ultrafiltration modules remove a substantial portion of the water from the aqueous slurry of barium titanate prior to entering the distillation process.

The non-aqueous slurry may also be concentrated, for example by filtration, to provide a wet solid cake comprising metal oxide coated barium titanate particles and liquid non-flowing solids. The non-aqueous wet cake can be in a solid state with 60% by weight solids mixed with non-aqueous solution. More preferably, the wet cake comprises at least 65 wt.% solids, and particularly preferably at least 70 wt.% solids. The wet cake may include no more than 85 wt.% solids. In a non-aqueous wet cake, the non-aqueous solvent may have a pH greater than 8 in order to prevent dissolution of the metals. The preferred pH range is between about 8 and about 12, more preferably between about 9 and about 11. The wet cake of barium titanate-based particles is a precursor to the colloidal dispersion. That is, the wet cake can be dispersed, for example, by blending a dispersant. If necessary, an additional liquid medium is required in order to convert the wet cake from a solid state to a fluid dispersion.

At least in the case of the wet cake containing no water, the particles in the wet cake remain weakly agglomerated for at least a longer period of time as long as the liquid content in the wet cake remains at least 15% by weight, more preferably at least 20% by weight, and particularly preferably at least 25% by weight.

The wet cake is preferably wrapped in a moisture-retaining film to prevent loss of moisture that can promote the particles to form agglomerates that do not readily disaggregate. The moisturizing film, such as a polyethylene bag or a fibrous tympanic membrane coated with polyethylene, can extend the useful lifetime by at least one or more days, preferably by at least 3 days, more preferably by at least 30 days, and most preferably by at least 90 days. This feature facilitates easy storage and transport of the wet cake in the present solution.

The wet cake can be easily converted to a fluid dispersion by introducing a dispersing agent into the wet cake without adding too much fluid. Although a fluid may be added to the wet cake, the amount of dispersant required to convert the solid wet cake to a fluid dispersion is very small, e.g., typically less than 2% by weight (based on the barium titanate-based material). In some cases, no additional fluid other than the dispersant fluid is required when converting the wet cake to a fluid dispersion. The recommended dispersant is a polyelectrolyte comprising an organic polymer having both anionic and cationic functional groups. Anionic functional polymers include carboxylic acid polymers such as polystyrene sulfonic acid and polyacrylic acid; cationic functional polymers include polyimines such as polyetherimines and polyethyleneimines. For many applications, polyacrylic acid is preferred. Although the polymeric acid groups may be protonated, it is preferred that the polymeric acid groups have counter cations that avoid lowering the pH of the dispersion to values that promote dissolution of barium and other metal species, such as metals that may be present in the doped coating. For capacitor applications, the preferred cation is ammonium. In some cases, it is feasible to use a doped metal as a counter cation to the polymeric acid dispersant. Regardless of the substance of the dispersant, one skilled in the art can readily determine the appropriate amount of dispersant by titration. When the amount of dispersant selected is that which provides the lowest viscosity to the dispersion, the concentration of dispersant in the dispersion can be reduced when the dispersion is used, for example by diluting the dispersion or reacting it with additives, resulting in an increase in viscosity to an undesirably high level. Thus, for many applications, it is desirable to use a dispersant in an "amount to minimize viscosity".

A preferred dispersant that has been found to be used and tested in colloidal dispersions for capacitor applications is aminated polyacrylic acid having a number average molecular weight of about 8000. For example, it has been found that 0.75 wt.% of such an aminated polyacrylic acid (as a 40 wt.% aqueous dispersion) can be used to convert the wet cake into a liquid dispersion. The dispersing agent can be introduced by conventional methods, for example by mechanical blending of the dispersing agent into the wet cake. When high shear agitation is used, the deagglomeration exposes new surface area of the particles, thereby consuming excess dispersant. Thus, it is convenient to add the dispersant incrementally during the high shear agitation.

Wet cake and slurry, dispersant, the difference of slurry is: wet cake is an immobile solid; slurry, dispersant, slurry is a flowing liquid; dry powders are flowable solids. The wet powder may or may not flow, depending on the amount of liquid present. When more liquid is removed, the wet powder will largely dry. However, it should be appreciated that: the dry powder need not be completely dehydrated. Spray drying, freeze drying, and low temperature vacuum assisted drying are preferred methods for providing dry powders of coated barium titanate-based particles that can be maintained in good dispersion simply by stirring them in a solution containing a dispersing agent, such as by high shear stirring. The coated barium titanate-based particulate dry powder can be surprisingly dispersed into a dispersion of submicron particles without the need for prolonged impact milling. Unlike known materials, it does not require hours of high energy milling to reduce the particle size to a desired level that enables the coated barium titanate-based particle dispersion or slurry to be used to produce capacitors having fine grains, thin dielectric layers and high breakdown voltages.

Another aspect of the present invention is to provide a method for dispersing submicron coated barium titanate-based particles in a solution that deagglomerates larger (greater than 1 micron) weakly agglomerated barium titanate-based particle dispersions coated with metal oxide until substantially all of the particles are less than 1 micron. In a preferred process of the invention, a relatively high solids dispersion comprising 30 to 70% by weight of particles can be depolymerised by using a dispersant and subjecting it to high shear agitation. The optimum time for high shear stirring can be readily determined by routine experimentation. High shear agitation can be effectively carried out in a centrifugal pump depolymerization mill, which is commercially available, for example, from Silverson Machine Inc (East Longmeadow, MA). Other equipment used to deagglomerate the dispersion includes known ultra-fine mills, colloid mills, and cavitation mills. The micronizer Mill has a milling chamber containing media and equipped with a high speed rotating disk at the central shaft, which is commercially available, for example, from Premier Mill (Reading, PA). Colloid mills having a grinding gap between the extended surface of the high speed rotor and the stationary stator are also commercially available, for example from Premier Mill. In a cavitation mill, such as that available from Arde Barinco inc. (Norwood, NJ), fluid is pumped through a series of rapidly opening and closing chambers that rapidly pressurize and depressurize the fluid therein, thereby imparting a high frequency shearing effect that they can deagglomerate the particles. It is desirable that the concentrated slurry, dispersion, wet cake, wet powder or dry powder perform equally well in order to provide a slurry for producing high performance capacitors, but this is dependent upon the unique capacitor production equipment and process.

A test to determine the effective amount of dispersant for weakly agglomerated and coated barium titanate-based particles includes using a high shear mixer, such as a Silverson Model L4R high shear test mixer equipped with a square-hole high shear screen to high shear mix a 500 gram sample of a dispersion comprising 70 weight percent coated particles in an alkaline non-aqueous solution at a temperature of 25 to 30 ℃ and a pH that does not dissolve the coating, and the dispersion further contains an effective amount of dispersant to de-polymerize the coated particles within an effective time. The effective amount of dispersant is sufficient to keep the separated agglomerates and clusters in a small particle size without re-agglomeration. The effective amount of dispersant will vary depending on factors such as particle size, coating and dispersant powder characteristics. Effective amounts and effective times of dispersants can be readily determined by those skilled in the art with minimal routine experimentation by observing the effect of various variable parameters, such as dispersant concentration and high shear mixing time, on reducing the particle size distribution range. The effective values of these variable parameters enable one to obtain particle size analysis data reflecting the true effect of high shear stirring on depolymerization. In many cases, an effective amount of an aminated polyacrylic acid (number average molecular weight of about 8000) dispersant has been found to be about 1% by weight of the total weight of the particles and dispersant. While the effective high shear stirring time is about 1 minute.

The coated bariumtitanate-based particles produced by the hydrothermal process are substantially spherical and are equiaxed in appearance. Such particles remain substantially spherical after size reduction with high shear agitation. Occasionally, substantially spherical particles may be paired. It is expected that there will be very little chance of pairs of particles appearing. The use of spherical particles provides a powder with a particularly high specific surface area, compared to milled particles that are non-spherical, with a BET specific surface area of at least about 4 m/g; preferably at least about 8 m/g; and more preferably at least 12 square meters per gram.

The submicron coated barium titanate particles can be suspended using a non-aqueous solvent and a number of different cements, dispersants and release agents to provide a ceramic slip slurry.

Barium titanate-based particles are dispersed in an organic solvent containing dissolved polymeric cement and optionally other dissolved materials such as plasticizers, mold release agents, dispersants, strippers, deodorants and wetting agents. Useful organic solvents have relatively low boiling points and include benzene, methyl ethyl ketone, acetone, xylene, methanol, ethanol, propanol, 1,1, 1-trichloroethane, tetrachloroethylene, amyl acetate, 2,2, 2-triethylpentanediol-1, 3-monoisobutyrate, toluene, dichloromethane, turpentine, ethanol, bromochloromethane, butanol, diacetone, methyl isobutyl ketone, cyclohexanone, methanol, n-propanol, isopropanol, n-butanol, n-octanol, benzyl alcohol, glycerol, ethylene glycol, benzaldehyde, propionic acid, n-octanoic acid, ethyl acetate, butyl butyrate, n-hexane, and the like, and mixtures thereof with water such as methanol/water mixtures. In addition, a few azeotropic organic solvent mixtures have lower boiling points and can be used as carriers. The solvent mixture may include, for example: 72% trichloroethylene/28% ethanol, 66% methyl ethyl ketone/34% ethanol, 70%methyl ethyl ketone/30% ethanol, 59% methyl ethyl ketone/41% ethanol, 50% methyl ethyl ketone/50% ethanol, 80% toluene/20% ethyl alcohol, 70% toluene/30% ethanol, 60% toluene/40% ethanol, 70% isopropanol/30% methyl ethyl ketone, 40% methyl ethyl ketone/60% ethanol, and mixtures thereof.

Dispersing agents (deagglomerator/wetting agents) which may be used in the submicron metal oxide coating barium titanate-based particle non-aqueous dispersion in addition to the above-disclosed materials include, for example, phosphate esters, ethylene glycol trioleate, ethoxylates, 2-amino-2-methyl-1-propanol, tall oil hydroxyethyl imidazoline, oleic acid hydroxyethyl imidazoline, fatty acids such as glyceryl trioleate, lanolin fatty acids, polyvinylbutyraldehyde, sodium bis (tridecyl) sulfosuccinate, sodium diisobutylsulfosuccinate, sodium dioctylsulfosuccinate, ethoxylated alkylguanamine, sodium dihexylsulfosuccinate, sodium diisobutylsulfosuccinate, phenylsuccinic acid, oil soluble sulfonates, alkyl ethers of polyethylene glycol, ethylene oxide adducts of oleic acid, sorbitan trioleate, ethylene oxide adducts of stearic acid amide, alkylaryl polyether alcohols, ethers of polyethylene glycols, ethyl benzene glycols, polyoxyethylene acetates, polyoxyethylene esters, and the like. Preferred dispersants for organic solvent dispersions and slurries include menhaden oil, corn oil, polyethyleneimine, and aminated polyacrylic acid.

In polymeric cement materials used in non-aqueous slurries, the cementing agent is, for example, polyvinyl butyral, polyvinyl acetate, polyvinyl alcohol, cellulosic polymers such as methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, methyl hydroxyethyl cellulose, cellulose acetate butyrate, nitrocellulose, polypropylene, polyethylene, silicon polymers such as poly (methyl siloxane), poly (methyl phenyl siloxane), polystyrene, butadiene/styrene copolymers, poly (ethylene pyrrolinone), polyamides, polyethers, poly (ethylene oxide-propylene oxide), polyacrylamides, and acrylic polymers such as sodium polyacrylate, poly (methyl acrylate), poly (methyl methacrylate), polyacrylates, and copolymers such as copolymers of ethyl methacrylate and methyl acrylate, polyvinyl alcohol, polyvinyl chloride, vinyl chloride acetate, poly (tetrafluoroethylene), poly (2-methylstyrene), and the like. The amount of polymeric cement used is typically about 5-20% by weight. A preferred commercially available polymer is ACRYLOID^TMB-7 acrylate Polymer (Rohm)&Hass Co.,Philadelphia,PA)。

The organic medium often contains a small amount of plasticizer in order to lower the glass transition temperature (Tg) of the cement polymer. The choice of plasticizer is determined primarily by the polymer that must be modified, and it may include phthalate esters (and phthalate blends) such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, butyl benzyl phthalate, alkyl phosphates, poly (alkyl glycols), polyethylene glycols, glycerol, poly (ethylene oxide), hydroxyethylated alkylphenols, dialkyl dithiophosphates, poly (isobutylene), butyl stearate, methyl rosinate, tris-hydroxy-phenyl phosphate, dipropyl ethylene dibenzoate, and the like.

In one embodiment, the organic solvent-based slurry of the present invention comprises 100 parts by weight of barium titanate-based particles and:

25 to 40 parts by weight of an organic solvent,

2-5 parts by weight of a dispersant,

5 to 20 parts by weight of a polymeric cement, and

0-15 parts by weight of plasticizer.

The application of various coatings, such as coupling agents, to the surface of the particles can facilitate dispersion of the barium titanate-based particles in various non-aqueous solvents. In particular, a variety of commercially available silane coupling agents provide a means for adapting the surface of the particles to the support. Coating barium titanate-based particles obtained by a hydrothermal process with a silane coupling agent is suitable not only for dry powders but also for particles held in solution. Although the coupling agent may be coated onto the surface of the dried or undried particles, it is preferred that the coupling agent is coated onto the dried particles to facilitate dispersion thereof in the carrier. Furthermore, dry powders provide an easy way to determine whether a complete coating has been obtained by attempting to disperse the treated powder in water. The uncoated or partially coated powder will be partially or completely dispersed in water, while the completely coated particles will float on the water surface, even with stirring.

The particles may be coated in one carrier by solvent exchange (as described above) or by distillation prior to transfer to another carrier. Although the silane coupling agent may be applied to the powder surface before or after the solvent exchange, it is preferred that the coating is applied after the solvent exchange process. The coupling agent is adhered to the surface of the particles to facilitate dispersion of the particles in the desired carrier, which also helps to passivate the surface of the particles.

General formula R of organosilanes_nSiX_(4-n)Two types of functional groups are indicated. The X group is involved in the reaction with the inorganic substrate. In a coupling agentThe bond between X and the silicon atom is replaced by a bond between the inorganic substrate and the silicon atom. X is a hydrolysable group, typically alkoxy, acyloxy, amino or chlorine. The most common alkoxy groups are methoxy, ethoxy. It may produce methanol and ethanol by-products during the coupling reaction. Chlorosilanes are used less often than alkoxysilanes because they generate hydrogen chloride during the coupling reaction. R is a non-hydrolyzable organic group possessing functionality that allows the coupling agent to bond to organic resins and polymers. Most widely used organosilanes have organic substituents. In most cases, the silane can be hydrolyzed prior to surface treatment. Immediately after hydrolysis, reactive silanol groups are formed, which can undergo condensation with other silanol groups, such as those on the surface of the siliceous filler, to form siloxane bonds. And also with other oxides such as aluminum, zirconium, tin, titanium, and nickel oxides. Form less stable bonds with oxides of boron, iron, and carbon. Alkali metal oxides and carbonates do not form stable bonds with Si-O-.

The water used for hydrolysis may be of several sources. Either added water, water present on the surface of the substrate, or water from the atmosphere. The reaction of these silanes with trimethoxy hydrolysable X groups proved to involve the following four steps:

1. initially, three X groups, which are readily bonded to silicon, are hydrolyzed,

2. then condensed to oligomers:

3. the H in the oligomer then bonds to OH groups in the matrix,

4. finally, during drying or curing, covalent bonds are formed with the matrix, with concomitant loss of water.

At the interface, there is typically only one bond formed between the silicon in the organosilane and the surface of the substrate. The two remaining silanol groups are present in bonded form withthe silicon of the other coupling agent or in free form.

The green tape is formed on the surface of the support using an organic solvent-based slurry by methods well known to the skilled artisan. See, for example, J.C. Williams, Ceramic manufacturing Process (Ceramic Fabric Processes), p.173-197, Treation Materials Science and Technology, Vol.9, Academic Press (1976), and US3717487 and US4640905, both of which are incorporated herein by reference.

In addition, there are a number of ways to convert the slurry into a film, a green laminate and a sintered ceramic. It is believed that minor modifications of the dispersions of the present invention can be applied to the above-described applications using a variety of different ceramic processes for making MLC dielectric layers, such as selecting the preferred dispersion medium and binder, diluting it to the desired flow viscosity, etc. The slurry can be formed into a film by spraying, forming a layer on a moving plate or mold (doctor blade) exiting the waterfall and other methods used in the MLC industry. When sufficient non-aqueous liquid is removed from the thin film, a solid "green" film may be provided which is coated on one or both sides in a predetermined pattern with a conductive material or conductive material precursor, such as an ink containing fine particles of palladium, silver, nickel or palladium-silver alloy. Such conductive inks may contain fine particles of metals and ceramics. The green film sheet is typically stacked, e.g., no more than 250 or more layers, and cut into MLC-sized cubes which, when sintered, burn off the polymer cement and dispersant and are sintered into a dense multilayer capacitor structure having a fine grain structure dielectric layer. The conductive metal coated on the terminals may connect the conductive inner layers forming the MLC in an alternating arrangement.

The unique particle size properties of thebarium titanate-based particles of the present invention make them promising for the production of novel MLC's with submicron-grained ultra-thin dielectric ceramic layers. Such dielectric materials contribute to a significant increase in bulk capacitance. Furthermore, MLCs are expected to have unexpectedly high breakdown voltages. The lack of larger, e.g., greater than 1 micron, particles allows for high-yield commercial production, e.g., greater than 98% yield of MLCs comprising multiple, e.g., more than 40, dielectric layers. The particles of the present invention are expected to be preferably used for producing MLC having a maximum particle diameter of 0.9 μm or less, more preferably less than 0.8. mu.m, and most preferably 0.7 μm or less in the dielectric ceramic layer. Other aspects of the invention provide X7R or Y5V capacitors comprising more than 20 dielectric layers of barium titanate-based materials sintered into a ceramic structure, wherein the dielectric layers are less than 5 microns thick, such as 2-4 microns thick. A higher number of dielectric layers, such as 250-500 layers, may be preferred depending on the MLC design. The thinner dielectric layer allows MLC with increased number of dielectric layers to be used for standard-sized MLC with fixed number of layers to meet the requirement of miniaturization of package size. The result is that the MLC capacitance of a standard package size can easily be increased by a factor of 5-10 or more.

To obtain a monolithic X7R MLC, the particles used to provide the dielectric layer are preferably coated with oxides of nickel, cobalt and manganese. The preferred metal oxide coating may contain bismuth oxide in view of the possibility of low-temperature firing, for example, sintering at 1200 ℃ or lower. To obtain ultra-thin dielectric layers with a thickness of less than 4 microns, the primary particle size (major diameter) of the particles is preferably less than 0.3 microns, most preferably less than 0.1-0.2 microns. The uniform fine grain size, e.g., less than 0.3 microns, in ultra-thin dielectric layers provides excellent dielectric strength in excess of 100 watts/micron and lower dissipation factor. These properties provide high reliability to high capacitance, high voltage ceramic capacitors. The ability to provide thinner dielectric layers enables the production of capacitors having a capacitance that exceeds 5-10 times that of standard size capacitors. Such an MLC preferably comprises a monolithic ceramic body, for example a barium titanate ceramic body doped with a metal oxide, two sets of internally digitised electrodes embedded in said ceramic body and extending respectively to two opposite ends of said ceramic body, and two electrically conductive ends connecting said two sets of electrodes at said two opposite ends. The MLC with X7R characteristic has a temperature coefficient of capacitance which does not vary by more than + -15% of the capacitance at 25 ℃ between-55 and 125 ℃. In a preferred embodiment of the invention, the ceramic grains in the x7r.mlc have a grain size of less than 0.3 μm and comprise from 93 to 98 wt% of the barium titanate-based ceramic and from 2 to 7 wt% of other metal oxides.

The following examples are not meant to limit the scope of the invention.

Example 1

To determine the efficiency of barium titanate-based particles dispersed in a non-aqueous solvent, dried, low-temperature calcined X7R particles obtained from a hydrothermal process were dispersed in an 80 toluene/20 ethanol mixed solvent with a phosphate ester dispersant.

The barium titanate wet cake, X7R, formed by the hydrothermal process, containing 72 wt.% solids and 28 wt.% water, was dried at 200 ℃ in a rotary drying apparatus with vacuum applied.

Twenty (20) pounds of dried hydrothermal X7R barium titanate particles were mixed with 6.7 pounds (3041.8 grams) of an 80 toluene/20 ethanol mixed solvent to form a slurry. By DISPERSATOR^TMThe high shear mixer (Premier) stirred the slurry for 45 minutes while 0.8 pound (363.2 g) of RHODAFAC RS-410 was added^TMPhosphate ester dispersant (Rhone-Poulenc). The final slurry was then tested for particle size distribution (sample 1) and the results are presented below and illustrated in figure 1A.

Then in PREMIER^TMThe slurry was stirred in a horizontal media Mill for 30 minutes (Premier Mill). The final slurry was then tested for particle size distribution (sample 2) and the results are presented below and illustrated in fig. 1B.

Then in PREMIER^TMThe slurry was stirred in the horizontal media mill for an additional 15 minutes (total 45 minutes). The final slurry was then tested for particle size distribution (sample 3) and the results are shown below and illustrated in figure 1C. The final loading was determined to be 78 wt%.

Test specimen	D₁₀	D₅₀	D₉₀	D₉₀/D₁₀
Test specimen	D₁₀	D₅₀	D₉₀	D₉₀/D₁₀	1.45 min. high shear Mixer	10.6	65.3	225.6	21.3

2.30 min. horizontal medium Texture mill	0.136	0.205	0.347	2.6
2.30 min. horizontal medium Texture mill	0.136	0.205	0.347	2.6	3.45 min horizontal medium Texture mill	0.133	0.192	0.356	2.7

The above experimental results show that the X7R powder obtained by the hydrothermal method after drying can be dispersed in an 80 toluene/20 ethanol mixed solvent using a phosphate ester dispersing agent. This also demonstrates that by choosing a suitable dispersant, it is possible to obtain (D) from X7R powder obtained by hydrothermal method₉₀/D₁₀) A slurry of particles in a non-aqueous solvent having a ratio of less than 3. It is believed that other solvents (such as those disclosed above) with suitable dispersants may also be used to obtain particles (D) therein₉₀/D₁₀) Slurries with ratios less than 3.

Although the results show that: the barium titanate wet cake X7R obtained from the hydrothermal process can be dried and redispersed in a non-aqueous solvent to form particles (D)₉₀/D₁₀) Slurries with ratios less than 3, but with high shear agitation do not effectively deagglomerate these dried particles which have undergone stronger agglomeration. The dispersion formed by drying the agglomerated barium titanate-based particles in this manner is intended to have a major particle size (major particle diameter) in the submicron range, and requires a relatively long period of media milling in order to provide the submicron-sized particles. Furthermore, it is believed that the heating process used to dry the wet cake can have a potential impact on the barium titanate-based particulate coating, particularly if used to dry the wet cakeThis is especially the case if the heating is carried out at elevated temperatures and/or for longer periods of time. Such potential negative effects include, for example, bonding of the hydrous oxide coating between particles, such that separation of the particles becomes difficult without peeling or flaking the coating from somebarium titanate-based particles.

Example 2

To determine the efficiency of dispersion of barium titanate-based particles in a non-aqueous solvent, cryo-calcined X7R particles obtained from a hydrothermal process were subjected to a solvent exchange process followed by dispersion with a phosphate dispersant. The low-temperature calcined X7R particles obtained in the hydrothermal process were initially present in water. The water was then replaced by a 80 toluene/20 ethanol mixed solvent.

A 1 kilogram (1kg) hydrothermal process formed a wet cake of X7R barium titanate containing 72 wt% solids and 28 wt% water, the wet cake was slurried with 1 kilogram (1kg) ethanol, and the slurry was placed in a Buchner funnel containing an ultrafiltration membrane as the filter medium. The ethanol is filtered through the wet cake formed and the cracks formed are mechanically eliminated. Once the first filtration was near completion, an additional 1kg (1kg) of pure ethanol was added and the cake was filtered through, and the procedure was repeated once more as the second filtration was near completion.

After filtration with ethanol was completed, 1kg (1kg) of toluene was added and filtered through the wet cake, which was then dried to a solids loading of 75 total%.

Subsequently in DISPERSATOR^TMThe final wet cake (859.3 g) was mixed with 26.81 g of RHODAFAC RS-410 in a high shear blender (Premier Mill)^TMMixing with phosphate dispersant (Rhone-Poulenc). The wet cake was allowed to stir by a high shear mixer for 10 minutes (sample 4) and 30 minutes (sample 5). The particle size distribution was then tested and the results are presented below and illustrated in fig. 2A and 2B.

Test specimen	D₁₀	D₅₀	D₉₀	D₉₀/D₁₀
Test specimen	D₁₀	D₅₀	D₉₀	D₉₀/D₁₀	4.10 min high shear Mixer	0.381	0.869	1.908	5.0
5.30 min high shear Mixer	0.351	0.764	1.805	5.1	4.10 min high shear Mixer	0.381	0.869	1.908	5.0

The above experimental results show that solvent exchange can be used in the process of substituting a non-aqueous solvent for an aqueous solvent, followed by addition of a suitable dispersant, if necessary, to obtain satisfactory (D)₉₀/D₁₀) The solvent exchange method, in contrast, allows the provision of a dispersion with a very narrow particle size distribution by high shear agitation alone (without the need for a horizontal media mill) in a shorter time than the time required to form a dispersion from a dry powder (sample 1, as shown in example 1). Furthermore, it is believed that (D) of less than 3 (D) can be achieved by selecting an appropriate dispersant, hydrothermal X7R powder in a non-aqueous solvent (via solvent exchange) followed by a few treatments in a horizontal media mill, depending on other factors such as the material size₉₀/D₁₀) And (4) the ratio. It is also believed that other solvents (such as those disclosed above) may also be used to form particles having a satisfactory D by selecting an appropriate dispersant₉₀/D₁₀) Specific slurry.

In addition to the foregoing, the solvent exchange process avoids the potential negative effects on the barium titanate-based particle coating when drying the wet cake, particularly at elevated temperatures and/or long drying times.

Example 3

To determine the effect of using a silane coupling agent to promote dispersion of barium titanate-based particles in a non-aqueous solvent, X7R powder obtained from a hydrothermal process was coated with methyltrimethoxysilane coupling agent.

The methyltrimethoxysilane provides a hydrophobic coating on the X7RBaTiO obtained by hydrothermal method₃The particle surface, the process of which is as follows:

1. 95 ml of ethanol were mixed with 5 ml of deionized water. With 0.1M HNO₃The solution was adjusted to pH 4. Methyltrimethoxysilane (5 g) was added to the acidified ethanol/water solution and stirred for 5 minutes to hydrolyze the three labile methoxy groups.

2. X7R BaTiO dilution with 250 ml ethanol₃Wet cake (70 wt% solids) and emulsified by stirring at 7000 rpm for 1 minute.

3. An ethanol/water solution containing a hydrolyzed silane coupling agent was added to a slurry containing X7R particles obtained by a hydrothermal method, and emulsified by stirring at 7000 rpm for 30 seconds.

4. The final slurry was air dried for 24 hours to remove excess carrier. The final material was then placed in a vacuum oven at 80 ℃ for 12 hours to completely dry the powder.

Dilution of the X7R wet cake with ethanol caused the dispersion to become very viscous due to emulsification. The final dry powder was tested using a sink or float test in water to determine if the hydrophobic coating was successfully adhered to the powder surface. The powder was lightly ground in a mortar with a pestle and then sprinkled on the water surface contained in the beaker. The particles started to float on the water surface, but some particles went into solution while stirring. Thus, when the X7R wet cake was initially diluted with ethanol, the surface only achieved partial coating. While partial coating may result in higher viscosity when the X7R wet cake is initially diluted with ethanol. Whereas the high viscosity equivalent particle coating broke when mixed in the immersion/flotation test.

Example 4

The methyltrimethoxysilane provides a hydrophobic coating, and the hydrophobic coating is located on the X7R BaTiO obtained by the hydrothermal method₃The particle surface, the process of which is as follows:

1. 95 ml of methanol was mixed with 5 ml of deionized water. With 0.1M HNO₃The solution was adjusted to pH 4. Methyltrimethoxysilane (5 g) was added to the acidified methanol/water solution and stirred for 5 minutes to hydrolyze the three labile methoxy groups.

2. X7R BaTiO dilution with 250 ml methanol₃Wet cake (70 wt% solids) and emulsified by stirring at 7000 rpm for 1 minute.

3. A methanol/water solution containing a hydrolyzed silane coupling agent was added to a slurry containing X7R particles obtained by a hydrothermal method, and emulsified by stirring at 7000 rpm for 30 seconds.

The final dry powder was tested using a submerged floating test in water to determine if the hydrophobic coating was successfully adhered to the powder surface. The powder was lightly ground in a mortar with a pestle and then sprinkled on the water in a beaker. The particles float on the water surface and remain on the water surface even while stirring. Thus, when the X7R wet cake was initially diluted with methanol, complete coating of the surface was achieved. Initial dilution of the X7R wet cake with methanol produced a slurry of lower viscosity and good dispersion, which ensured complete coating of the particles when a hydrolyzed silane coupling agent was used.

Those skilled in the art will readily understand that: all parameters listed herein are exemplary only, and the actual parameters will depend on the particular application of the methods and apparatus of the present invention. It is, therefore, to be understood that the foregoing is illustrative only and that, within the scope of the appended claims or equivalents thereto, the invention may be practiced otherwise than as specifically described.

Claims

1. A slurry, dispersion or slip comprising barium titanate-based particles dispersed in a non-aqueous medium, said particles having a coating comprised of a metal oxide, metal hydrous oxide, metal hydroxide, or organic acid salt of a metal other than barium or titanium, wherein at least 90 percent of said particles have a particle size less than 0.9 micrometer when dispersed by high shear agitation.

2. The slurry, dispersion or slip of claim 1, wherein said particles have a ten-equal part time D₉₀/D₁₀A particle size distribution with a ratio of less than 4.

3. The material according to claim 1Slurry, dispersion or slip, wherein said particles have ten equal parts of time D₉₀/D₁₀A particle size distribution with a ratio less than 3.

4. The slurry, dispersion or slip of claim 1, wherein said particles have a ten-equal part time D₉₀/D₁₀A particle size distribution with a ratio of less than 2.5.

5. The slurry, dispersion or slip of claim 1, wherein at least 90 percent of said particles have a particle size of less than 0.8 microns when said particles are dispersed by high shear agitation.

6. The slurry, dispersion or slip of claim 1, wherein at least 90 percent of said particles have a particle size of less than 0.7 microns when said particles are dispersed by high shear agitation.

7. The slurry, dispersion or slip of claim 1, wherein at least 90 percent of said particles have a particle size of less than 0.6 microns when said particles are dispersed by high shear agitation.

8. The slurry, dispersion or slip of claim 1, wherein at least 90 percent of said particles have a particle size of less than 0.5 microns when said particles are dispersed by high shear agitation.

9. The slurry, dispersion or slip of claim 1, wherein at least 90 percent of said particles have a particle size of less than 0.4 microns when said particles are dispersed by high shear agitation.

10. The slurry, dispersion or slip of claim 1, wherein at least 90 percent of said particles have a particle size of less than 0.3 microns when said particles are dispersed by high shear agitation.

11. The slurry, dispersion or slip of claim 1, comprising at least 50 weight percent of said particles.

12. The slurry, dispersion or slip of claim 1, comprising at least 60 weight percent of said particles.

13. The slurry, dispersion or slip of claim 1, comprising at least 75 weight percent of said particles.

14. The slurry, dispersion or slip of claim 1, further comprising a dispersant.

15. The slurry, dispersion or slip of claim 1, wherein said particles comprise a coating of a coupling agent on the surface of said particles.

16. The slurry, dispersion or slip of claim 15, wherein said coupling agent comprises an organosilane.

17. The slurry, dispersion or slip of claim 1, further comprising 3 to 20 weight percent of a binder composition comprising a dissolved or suspended film-forming polymer.

18. The slurry, dispersion or slip of claim 1, wherein substantially all of said particles are equiaxed or spherical.

19. The slurry, dispersion or slip of claim 1, wherein said particles are hydrothermally produced.

20. The slurry, dispersion or slip of claim 1, wherein said coating coats a substantial portion of the surface of the particles.

21. The slurry, dispersion or slip of claim 1, wherein said coating comprises at least one metal selected from the group consisting of: bismuth, lithium, magnesium, calcium, strontium, scandium, zirconium, hafnium, vanadium, niobium, tantalum, tungsten, manganese, cobalt, nickel, zinc, boron, silicon, antimony, tin, yttrium, lanthanum, lead, or a lanthanide.

22. The slurry, dispersion or slip of claim 1, wherein the non-aqueous medium comprises an organic solvent.

23. The slurry, dispersion or slip of claim 22, wherein the non-aqueous medium comprises a mixture of anorganic solvent and water.

24. The slurry, dispersion or slip of claim 22, wherein the organic solvent is selected from the group consisting of: benzene, methyl ethyl ketone, acetone, xylene, methanol, ethanol, propanol, 1,1, 1-trichloroethane, tetrachloroethylene, amyl acetate, 2,2, 4-triethylpentanediol-1, 3-monoisobutyrate, toluene, dichloromethane, turpentine, ethanol, bromochloromethane, butanol, diacetone, methyl isobutyl ketone, cyclohexanone, methanol, n-propanol, isopropanol, n-butanol, n-octanol, benzyl alcohol, glycerol, ethylene glycol, benzaldehyde, propionic acid, n-octanoic acid, ethyl acetate, butyl butyrate, n-hexane, and mixtures thereof.

25. The slurry, dispersion or slip of claim 24, wherein the organic solvent is ethanol.

26. The slurry, dispersion or slip of claim 1, wherein the non-aqueous medium comprises a mixture of more than one organic solvent.

27. The slurry, dispersion or slip of claim 26, wherein said mixture is selected from the group consisting of: 72% trichloroethylene/28% ethanol, 66% methyl ethyl ketone/34% ethanol, 70% methyl ethyl ketone/30% ethanol, 59% methyl ethyl ketone/41% ethanol, 50% methyl ethyl ketone/50% ethanol, 80% toluene/20% ethyl alcohol, 70% toluene/30% ethanol, 60% toluene/40% ethanol, 70% isopropanol/30% methyl ethyl ketone, 40% methyl ethyl ketone/60% ethanol, and mixtures thereof.

28. The slurry, dispersion or slip of claim 27, wherein the non-aqueous medium is 80% toluene/20% ethanol.

29. A method of forming a slurry, dispersion, or slip, comprising the steps of:

forming a barium titanate-based particulate slurry in an aqueous medium using a hydrothermal process;

forming a coating layer comprising a metal oxide, metal hydrous oxide, metal hydroxide, or organic acid salt of a metal other than barium or titanium on said particles;

replacing the aqueous medium with a non-aqueous medium; and

the particles are dispersed in a non-aqueous medium by high shear agitation.

30. The method of claim 29, wherein said particles are dispersed in a non-aqueous medium by high shear agitation until 90% of said particles have a particle size of less than 0.9 microns.

31. The method of claim 29, wherein the process of replacing the aqueous medium with a non-aqueous medium comprises a solvent exchange process.

32. The method of claim 31, wherein the solvent exchange process comprises:

filtering the barium titanate-based particulate slurry in an aqueous medium; and

adding the filtered particles to a non-aqueous medium.

33. The method of claim 29, wherein the process of replacing the aqueous medium with a non-aqueous medium comprises a distillation process.

34. The method of claim 33, wherein the distillation process comprises:

adding a non-aqueous medium to the barium titanate-based particle slurry in an aqueous medium; and

the aqueous medium is allowed to evaporate.

35. The method of claim 29, further comprising applying a coupling agent to the surface of said particles after the non-aqueous medium displaces the aqueous medium.

36. The method of claim 29 further comprising applying a coupling agent to the surface of said particles after said particles have formed a coating and before the non-aqueous medium displaces the aqueous medium.

37. A method according to claim 35 or 36, wherein the coupling agent comprises an organosilane.

38. A method of forming a slurry, dispersion or slip, comprising the steps of:

dispersing barium titanate-based particles in a non-aqueous medium by high shear agitation until 90% of said particles have a particle size of less than 0.9 micrometer; the particles have a coating comprising a metal oxide, metal hydrous oxide, metal hydroxide, or organic acid salt of a metal other than barium or titanium.