Solid material comminution or disintegration – Processes – With application of fluid or lubricant material
Reexamination Certificate
2000-10-16
2002-07-09
Rosenbaum, Mark (Department: 3725)
Solid material comminution or disintegration
Processes
With application of fluid or lubricant material
C241S021000
Reexamination Certificate
active
06415996
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to milling of ceramic powders and an improved process for the same.
BACKGROUND OF THE INVENTION
Ceramic powders are important in a variety of different fields. Examples include manganese zinc ferrites, aluminum nitride, zinc oxide, silicon dioxide, barium titanate, and iron oxide.
Manganese zinc ferrites (Mn,Zn,Fe)Fe
2
O
4
or MZF) are important ceramic materials for the manufacture of ferromagnetic devices including inductors and transformers. Conventionally, commercial MZF are prepared through calcination of mixtures of the single component precursor metal carbonates or oxides followed by milling to the desired particle size range. The milling operation usually involves suspension of the calcined, aggregated material in water to promote a higher milling efficiency than that encountered with dry milling. Aluminum nitride (AIN
3
) is commonly used in circuit substrates. Zinc oxide is used as a varistor material, and, of course, uses for silicon dioxide ceramics are ubiquitous. Barium titanate is often used in multi-layer capacitors and in piezoelectric devices such as transducers and ultrasonic vibrators.
Each of these ceramics presents its own characteristic problems when attempts are made to deposit those at a fine micron particle size level for their common ceramic uses as above-mentioned. For example, manganese zinc iron ferrite is often made by mixing a precursor mix of magnesium carbonate, zinc carbonate, iron oxide, which is then calcined at about 1000° C. The chemicals “homogenize” during the calcination process at the high temperature, but this results in primary particles which have sintered. Aluminum nitrides, as commercially prepared, often result in a polycrystalline aggregate which is made by chemical vapor deposition, but to be useful, the aggregate needs to be broken up, i.e., milled. Zinc oxide ceramic typically comes from calcining a mixture of zinc hydroxide and zinc carbonate or the single precursors, and the resultant product, too, needs milling to be useful. Silicon dioxide, as commercially provided, comes from chemical vapor deposition, but while it is very fine, it often aggregates in the process, and therefore also needs milling. Barium titanate is prepared by calcining either BaCO
3
and TiO
2
or a precursor such as (BaTiO)(C
2
O
4
)
2
·4H
2
O, which results in polycrystalline aggregates. All are in need of further treatment to make satisfactory submicron particles for their various ceramic material uses.
As an example, 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 alternating multilayers of the ceramic dielectric formulation and a conductive electrode material in a controlled atmosphere at a temperature in the range of about 1000° to 1400° C.
Dielectric layers have traditionally been produced by preparing a suspension of ceramic powder in a liquid vehicle, usually containing a dispersant, and then adding an organic resin matrix which functions to bind the ceramic particles after tape formation and drying. A variety of methods are known for applying the suspension and binder mixture (hereafter defined as slip) to a substrate to form very thin layers of the suspended solids. Methods such as wet coating, tape-casting (casting), or doctor-blading are readily known to those skilled in the art. The thin, dried layers generally termed as green layers, may then be coated with conductive electrodes and stacked together with similar layers to form a green body. The stack is then trimmed and co-fired to produce a structure consisting of alternating layers of sintered electrode and dielectric which is finally leaded with end 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 of late has been to increase the use of aqueous suspensions, which are therefore preferred.
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 has been reduced, e.g., from 25 microns to 10 microns. It is now desirable to reduce the thickness even less to, for example, 5 microns or less. These thinner layers necessitate the use of extremely small 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 which may create process problems.
Then too, ever 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 barium cation in aqueous conditions. The organic additive typically used in the processing contains chemical groups that can react with the soluble cation or its companion hydroxyl ion. Agglomerates of polymer and phase separation or “salting out” or precipitation of the metal cation organic complex can take place. 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 and/or a deletion effect on the strength of the ceramic. Void formation is particularly unforgiving in layers having a thickness of less than 10 microns and must be eliminated.
Another problem that occurs when making aqueous suspensions with ceramic powders 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, often the benefit of finer particle sizes needs to 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, unworkable suspensions with shear thickening characteristics and high viscosities. There is a continuing need to solve these problems.
A variety of attempts have been made to prepare finely divided ceramic powders in aqueous suspensions and slips. For example, U.S. Pat. No. 3,496,008 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 U.S. Pat. No. 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 U.S. Pat. No. 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 2.540 mm. Partic
Adair James H.
Mandanas Michael M.
Messing Gary L.
Pecora Tarah M.
Shrout Thomas R.
McKee Voorhees & Sease, P.L.C.
Rosenbaum Mark
The Penn State Research Foundation
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