Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2001-01-10
2003-12-02
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
Reexamination Certificate
active
06656590
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to dielectric materials and, more particularly, to coated barium titanate-based particles and process of producing the same.
Barium titanate-based materials, which include barium titanate (BaTiO
3
) and its solid solutions, may be used as dielectric materials in electronic devices such as multilayer ceramic capacitors (MLCCs). Typically, barium titanate-based materials are processed in particulate form and subsequently sintered to form the dielectric material. Pure barium titanate undergoes several phase transformations which causes it to have an unstable capacitance over the typical operating temperature range for MLCC applications (−55° C. to 125° C.). To achieve a higher degree of capacitance temperature stability required in certain MLCC applications, dopant compounds may be added to pure barium titanate. Dopants may also be added to barium titanate-based materials to improve other electrical properties or for processing purposes. Typically, the dopants are metallic compounds, often in the form of oxides.
In some cases, dopant compounds are added to a barium titanate-based particulate composition in the form of discrete particles. The dopant particles may be mixed with the barium titanate-based particles and, in some cases, further milled to yield the desired particle size. The particulate mixture may then be dispersed to form a ceramic slurry which may be further processed, for example, to form a dielectric material suitable for use in MLCC applications. In some cases, the inhomogeneity of particle size and non-uniform distribution of dopant particles in such particulate mixtures may limit the ability to fabricate reliable MLCCs with thin dielectric layers having a thickness of below about 5 microns.
Certain processes have been developed which may improve the distribution of dopants in barium titanate-based compositions. These techniques may be especially important when the barium titanate-base particles have submicron particle sizes. For example, processes have been developed to coat dopant compounds on the surface of barium titanate-based particles in an aqueous-based precipitation process. In some cases, the dopant compounds are coated on to the barium titanate-base particles as oxides or hydroxides from alkaline (pH>7) aqueous solutions. Such dopant metal oxides, for example, Y
2
O
3
, MnO
2
, and MgO, are therefore insoluble in water under these conditions. However certain dopant oxides are soluble in water at alkaline conditions, thereby limiting their ability to be coated onto particles from alkaline aqueous solutions. Furthermore, coated particles are oftentimes subjected to further processing in alkaline aqueous environments to form dielectric layers. Therefore, an alternative process may be needed to coat barium titanate-based particles with dopant metals that form oxides that are soluble in alkaline environments.
SUMMARY OF THE INVENTION
The invention provides coated barium titanate-based particles and a process to coat the particles. The coating includes a dopant metal compound that is insoluble in water under alkaline conditions. The dopant metal in the coating is selected from the group of metals which form oxides or hydroxides that are soluble in water under alkaline conditions. The group of metals includes tungsten, molybdenum, vanadium, and chromium. The process involves precipitating the insoluble compound from an aqueous medium as a coating on surfaces of barium titanate-based particles. The coated barium titanate-based particles may be further processed, for example, to form dielectric materials which may be used in many electronic applications such as in MLCCs applications.
In one aspect, the invention provides a barium titanate-based composition comprising barium titanate-based particles, wherein at least a portion of the barium titanate-based particles are at least partially coated with a coating comprising a dopant metal compound that is insoluble in water at alkaline conditions. The dopant metal is selected from the group consisting of tungsten, molybdenum, vanadium, and chromium.
In another aspect, the invention provides a method of coating barium titanate-based particles. The method includes the steps of providing an aqueous slurry of barium titanate-based particles and adding dopant metal ions to the aqueous slurry. The dopant metal is selected from the group of metals consisting of tungsten, molybdenum, vanadium, and chromium. The method further includes the step of reacting the dopant metal ions with an ionic species in the aqueous slurry to form a coating covering at least part of the surfaces of at least a portion of the barium titanate-based particles. The coating comprises a dopant metal compound that is insoluble in water at alkaline conditions.
Other advantages, aspects, and features of the invention will become apparent from the following detailed description.
DETAILED DESCRIPTION
The present invention is directed to coated barium titanate-based particles and a process to coat the particles. In the process, barium titanate-based particles are coated with at least one layer that includes a dopant metal compound which is insoluble in water under alkaline conditions (pH>7). The dopant metal is selected from the group of metals including tungsten, molybdenum vanadium, and chromium. The process utilizes barium titanate-based particles which may be dispersed in an aqueous medium to form a slurry. A solution containing the dopant element in ionic form is mixed with the aqueous slurry. The dopant element reacts with other species in the slurry to form an insoluble compound which deposits on surfaces of the barium titanate-based particles. The species in the slurry with which the dopant metal reacts may be added to the slurry or may be residual species from previous processing steps. The slurry of coated particles may be further processed, for example, to form dielectric layers in electronic components such as MLCCs.
As used herein, “barium titanate-based compositions” refer to barium titanate, solid solutions thereof, or other oxides based on barium and titanium 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. One type of barium titanate-based composition has the structure Ba
(1−x)
A
x
Ti
(1−y)
B
y
O
3
, where x and y can be in the range of 0 to 1, where A represents one or more divalent metal other than barium such as lead, calcium, strontium, magnesium and zinc and B represents one or more tetravalent metals other than titanium such as tin, zirconium and hafnium. Where the divalent or tetravalent metals are present as impurities, the value of x and y may be small, for example less than 0.1. In other cases, the divalent or tetravalent metals may be introduced at higher levels to provide a significantly identifiable compound such as barium-calcium titanate, barium-strontium titanate, barium titanate-zirconate, and the like. In still other cases, where x or y is 1.0, barium or titanium may be completely replaced by the alternative metal of appropriate valence to provide a compound such as lead titanate or barium zirconate. In other cases, the compound may have multiple partial substitutions of barium or titanium. An example of such a multiple partial substituted composition is represented by the structural formula Ba
(1−x−x′−x″)
Pb
x
Ca
x′
Sr
x″
O.Ti
(1−y−y′−y″)
Sn
y
Zr
y′
Hf
y″
O
2
, where x, x′, x″, y, y′, and y″ are each greater than or equal to 0. In many cases, the barium titanate-based material will have a perovskite crystal structure, though in other cases it may not.
The barium titanate-based particles may have a variety of different particle characteristics. The barium titanate-based particles typically has an average primary particle size of less than abou
Thrush Kathleen A.
Venigalla Sridhar
Cabot Corporation
Le H. Thi
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