Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
1997-09-04
2001-07-31
Le, Hoa T. (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S404000, C501S137000, C501S138000, C501S139000
Reexamination Certificate
active
06268054
ABSTRACT:
BACKGROUND OF THE INVENTION
The high dielectric constant of barium titanate-based materials make them suitable materials for multilayer ceramic capacitors, commonly referred to as “MLC's”. MLC's comprise alternating layers of dielectric and electrical conductor materials. Examples of MLC's are disclosed in U.S. Pat. Nos. 3,612,963 and 4,435,738. Palladium, silver, palladium-silver alloys and nickel are common electrical conductor materials used in MLC's. The dielectric layers of an MLC are usually prepared from a high solids dispersion, known in the art as a “slip”. Such slips typically comprise powdered barium titanate-based material and a polymeric binder in an aqueous or non-aqueous solvent. Films of binder-stabilized powder made by casting or coating with a slip are dried to provide a “green” layer of ceramic dielectric. Green layers are coated with conductor materials in a pattern and are then stacked to provide a laminate of alternating layers of green ceramic dielectric and conductor. The stacks are diced into MLC-sized cubes which are heated to burn off organic materials such as binder and dispersant and then fired to sinter the particles of barium titanate-based material to form a capacitor structure with laminated, dense ceramic dielectric and conductor layers. Sintering temperatures are typically in the range of 1000 to 1500° C. During sintering increased ceramic dielectric density is achieved as a result of the fusion and consolidation of the particles to form grains. Even with the use of grain growth inhibitors, ceramic grain size in an MLC dielectric layer is typically larger, e.g. by a factor of 3 to 5, than the size of the original primary particles. Moreover, not all porosity is removed during the sintering process. Typically, 2 to 10% porosity remains in MLC dielectric layers. These pores, or hole defects, in the dielectric layer, tend to be larger in larger grain size ceramics. Certain critical capacitor properties such as break down voltage and DC leakage are influenced by dielectric thickness, grain size and pore defects. For instance, it is believed that effective dielectric layers need to be several, e.g. at least 3 to 5, grains thick. Because a defect in any one of the layers of an MLC can be fatal to its performance, MLC's are manufactured with a sufficient thickness of dielectric layer to effectively reduce the impact of ceramic defects which can be caused by random large grains or pores, adversely affect the properties of the MLC.
With the market demand for miniaturization in the design of electronic devices there is a need in the MLC industry for ceramic materials that will allow thinner dielectric layers without incurring the catastrophic effects of large grain and pore size relative to dielectric thickness.
Barium titanate powders produced by prior art processes, e.g. calcination or hydrothermal processes, have large particles and/or strongly-agglomerated fine particles of a size substantially larger than 1 &mgr;m and that such particles and agglomerates are not readily amenable to the production of MLC's with fine grained, ultrathin dielectric layers, e.g. less than 4-5 &mgr;m. Thus, it would represent an advance in the art to provide a barium titanate-based material that would be suitable for making MLC's with thinner dielectric ceramic layers, e.g. less than 4 &mgr;m, with acceptable or exceptional electrical properties including DC leakage and breakdown voltage without the need for extended milling.
SUMMARY OF THE INVENTION
This invention provides barium titanate-based particles having a coating comprising 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 the coated particles are dispersed by high shear mixing. As used herein the term “barium titanate-based” refers to barium titanate, barium titanate having another metal oxide coating and other oxides based on barium and titanate having the general structure ABO
3
, where 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. This invention also provides compositions comprising such barium titanate-based particles of this invention, e.g. in a variety of forms such as slurry, wet cake, powder, dispersion and slip.
Such particles are easily dispersible without the need for milling into submicron dispersions which are advantageous in the manufacture of MLC's with thin dielectric layers having submicron grain size and high breakdown voltage. High shear mixing is effective in reducing the size of agglomerates of particles of this invention and involves de-agglomeration or separation of agglomerates into smaller coated particles without milling, e.g. impacting the particles with hard, milling media such as rods, balls or zirconia particles, etc. Since milling can split particles into smaller than the primary particle size resulting in non-equiaxed particles with exposed, i.e. uncoated, surface, in a preferred embodiment the particles of this invention are unmilled, e.g. characterized by particles having a major portion of the surface covered by the coating. In another aspect of the invention unmilled particles are characterized as equiaxed or spherical.
Another aspect of this invention provides a method for making submicron, barium titanate-based particles with a metal oxide coating comprising:
(a) providing submicron, barium titanate-based particles in a liquid medium,
(b) adding to the liquid medium one or more soluble metal salts to provide submicron, particles with a coating comprising an oxide, hydrous oxide, hydroxide or organic acid salt of said metal.
Still another aspect of this invention provides a method of making a dispersion of submicron, barium titanate-based particles in a liquid medium, said method comprising de-agglomerating a dispersion of barium titanate-based particles in the liquid medium until the particle size distribution is less than 0.9 micrometer. Such de-agglomerating is preferably effected by high shear mixing.
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paten
Costantino Stephen A.
Hard Robert A.
Venigalla Sridhar
Cabot Corporation
Le Hoa T.
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