US3265787A - Method of processing ceramic type ferroelectric materials - Google Patents

Description

United States Patent 3 265,787 METHOD OF PROC ESSING CERAMIC TYPE FERROELECTRIC MATERIALS Ronald J. Brandmayr, Neptune, Arthur E. Brown, Red Bank, and Sam Di Vita and Robert J. Fischer, West Long Branch, N.J., assignors to the United States of America as represented by the Secretary of the Army No Drawing. Filed Feb. 6, 1963, Ser. No. 256,771

6 Claims. (Cl. 264-56) The invention described herein may be manufactured and use by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates in general, to a method of processing ceramic type ferroelectric materials, and in parti'cular, to a method of processing barium titanate.

Barium titanate and other ceramic type ferroelectric materials such as cadmium niobate (Cd Nb O3) lead titanate (PbTiO lead zirconate (PbZrO or mixtures of these materials are important in that their dielectric constant is about one hundred times higher than that of conventional dielectric materials provided their compositional and structural purity are high. The value of crystalline barium titanate bodies, for instance, as used in computer elements, ferroelectric amplifiers, memory devices, capacitors, I.F. transformers, etc., depends on the composition, crystalline structure, and grain size of the barium titanate or ferroelectric material. This purity will, in turn, depend on the purity of the raw materials used in manufacturing the barium titanate compounds and in the particular methods of manufacture which determine the particle size, homogeneity, and the structural purity of the crystals. US. Patent No. 2,990,602 issued July 4, 1961, to R. J. Brandmayr et a1. discloses the hot pressing of ceramic type ferroelectric materials in such a manner as to produce non-reduced pellets of extremely fine and homogeneous grain size and excellent dielectric properties. According to the patent, a prepressed pellet of ferroelectric material is completely imbedded in a casing of pressed zirconia powder and then hot pressed. A disadvantage of the method described in the above patent is that the high temperature required for hot pressing causes grain growth in the ceramic type ferroelectric material. A further disadvantage is that the resulting ceramic body exhibits too great a change in dielectric constant over a broad temperature range.

An object of this invention is to process ceramic type ferroelectric materials, particularly barium titanate, so

that a dense body having a very small average grain size is obtained with greatly restricted grain growth. Another object is to prepare a pure ceramic type ferroelectric body having no observable Curie point and an extremely small change in dielectric constant with temperature over a broad temperature range. A further object is to reduce the temperature at which ceramic type ferroelectric materials can be hot pressed.

It has now been found that the foregoing objects can be attained by a method of processing involving placing the ceramic type ferroelectric material in a chamber in a metallic tube, swaging the metallic tube down over the ferroelectric material to precompact the material to a relatively high density, and then maturing the ferroelectric material by firing or hot pressing.

Swaging is a special application of forging. It is basically a series of very rapid light blows, delivered within a machine consisting of a self-contained frame, rotary spindle, dies, hammers, cage assembly, and suitable motor drive. The spindle is the driven member, and houses the forming die and the two hardened hammers. This assembly rotates inside of the roll cage assembly, which contains hardened steel rollers. The head ring, which is pressed into the frame, forms an outer raceway BaTi(C O )2.4H O at 600 C. for 6 hours.

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for the rollers. As the hammers pass under two opposing rollers, there is no clearance between the die segments. As the hammers pass out from under the rollers, centrifugal force causes the hammers, and die segments, to be thrown outward, separating the die segments. This allows the work to be inserted further into the die, ready for the next forging stroke. Each forging stroke then results in a flow of material and a reduction in the crosssection of the piece being forged.

We have found that the swaging process can be used to precompact ceramic type'ferroelectric materials to a relatively high density. After hot pressing the swaged ferroelectric materials, dense bodies are obtained having a very small average grain size with greatly restricted grain growth, no observable Curie point, and an extremely small change in dielectric constant with temperature over a broad temperature range. Moreover, the temperature at which the particular ceramic type ferroelectric material is normally hot pressed can be reduced.

Some preferred embodiments of the invention are illustrated in the following examples.

Example I Barium titanate powder is first obtained by calcining The average particle size of the barium titanate powder obtained is about 0.1 micron after calcining. The barium titanate powder is then placed in a tube of 16 gauge, 1 /2 inches in diameter, 0.050 inch wall thickness, SAE #1020 steel tubing one end of which has been sealed with a cast aluminum plug. The barium titanate powder is then tamped down and the open end of the tube sealed with another aluminum plug such that the powder is tightly packed between the two aluminum plugs. 300 grams of barium titanate powder are used. The aluminum plugs are held in place by punching indentations into the steel tubes. The steel tube containing the barium titanate powder is then processed through a conventional swaging machine into which a 1.592 inch diameter die had been inserted. The result was a reduction in diarn- The swaged barium titanate is then removed from the steel tube by machining off the steel on a lathe. The barium titanate obtained is in the form of a tightly compacted rod. The bulk density of the swaged barium titanate is found to be 4.6 or about 76 percent of the X-ray density for barium titanate. Subsequent kiln firing of this material revealed that the material could be sintered to vitrification at 2250 F. to 2300 F. to 200 F. lower than the usual required firing temperature for barium titanate). Even greater bulk densities than above indicated can be obtained by evacuating the steel or brass tube containing the barium titanate powder before sealing and swaging. The term vitrification as used herein refers to a thermally maturing process involving the progressive reduction in porosity of a ceramic composition as a result of heat treatment or the process involved.

Example 2 A brass tube composed of 66.5 percent copper, 33.0 percent zinc, and 0.5 percent lead is de-aired and then lined by spraying a tetrafluoroethylene coating on the inside wall of the tube and baking at 700 F. Barium titanate powder obtained as in Example 1 is then placed in the brass tube and the brass tube then inserted inside an SAE No. 1020 steel tube telescope fashion; the steel tube then being swaged down over the brass tube until a tight seal exists between the two tubes. The combination of steel on brass with the barium titanate powder inside the brass is then swaged through the remaining dies down to 0.625 inch. The steel jacket is removed from the brass jacket by machining. The brass is dissolved from the barium titanate compact by immersing the swaged barium titanate encased in the brass jacket in a solution of 10 percent mercurous nitrate (HgNo and 1 percent nitric acid (HNO The brass having been work hardened by the swaging operation reacts with the mercury solution causing it to crack open enabling removal of the sample. Further ease in removing the compacted BaTio sample from the metal can be achieved if the sample is heat treated in vacuum at temperatures on the order of 600 to 1000 F. after treatment with the mercury solution. The sample is then hot pressed at 1900 F. at 10,000 pounds per square inch similar to the procedure described in US. Patent No. 2,990,602, except that the die case jacket is composed of Hastalloy X instead of stainless steel and the plunger rods are composed of Haynes metal ceramic LT-Z instead of zirconium oxide (ZrO The Hastalloy X is a high temperature nickel based alloy and LT-2 is a tungstenaluminum oxide (Al O )-molybdenum (Mo) cermet which enables hot pressing under higher temperatures and pressures than possible using Zr plungers and stainless steel die case jackets. Hastalloy X and LT-2 metal ceramic are marketed by the Haynes Stellite Division of Union Carbide Corporationf It is notted that the temperature of 1900 F. at which this material is vitrified by hot pressing is 100150 F. lower than that normally required. In the above menthod, as an alternative to removing the brass jacket from the barium titanate compact by; solvent means, one can slice discs from the compact while still encased in the brass tube and then hot press these discs at 1900 F. at 10,000 pounds per square inch similar to the procedure described in US. Patent 2,990,602. In such a case, the brass tube is then removed from the hot pressed barium titanate sample and the properties evaluated.

It is determined by means of electron microscopy that the barium titanate bodies prepared according to the procedure described in Example 2 are characterized by an average grain size of about 0.4 micron. This is a smaller average grain size than previously observed in similar material which is hot pressed at temperatures 100 to 150 F. higher than the bodies prepared in the manner described herein. It is further observed that the grains in the barium titanate bodies prepared as above fall within a narrow size range and there is evidence that very little grain growth has occurred. There is evidence that grain growth-occurs to a greater extent than above when barium titanate is hot pressed at temperatures in the range of 2000 F. to 2100 F. A further property of the barium titanate material prepared according to the above procedure is a very high dielectric constant and no observable Curie point. The dielectric constant undergoes a maximum change of +4 percent to -15 percent in the temperature range 24 C. to C. Barium titanate hot pressed according to the procedure outlined in U.S. Patent 2,990,602 usually undergoes a change of dielectric constant of +40 to -15 percent in the temperature range of 24 C. to 140 C.

In carrying out the invention, it is desired to precompact the material by swaging down to a point approaching the X-ray density of the material. The possibility of reaching this density will be limited by the strength of the metal tubing in which the material is encased.

The foregoing description is to be considered merely as illustrative of the invention and not in limitation thereof.

What is claimed is: I

1. The method of processing ceramic type ferroelectric materials comprising confining a powder of the ferroelectric material in a metallic tube, swaging the metallic tube down over the ferroelectric material to precompact the material to a relatively high density, removing the swaged ferroelectric material from the tube, and then thermally maturing the ferroelectric material.

2. The method according to claim 1 wherein the ceramic type ferroelectric material processed is barium titanate.

3. The method according to claim 2 wherein the ceramic type ferroelectric material is thermally matured by kiln firing at 2250 F. to 2300 F.

4. The method according to claim 2 wherein the ceramic type ferroelectric material is thermally matured by hot pressing at 1900 F. at 10,000 pounds per square inch.

5. The method of processing barium titanate powder comprising confining the powder in a steel tube, swaging the steel tube down until the diameter of the steel tube is reduced to 0.625 inch, removing the swaged barium titanate from the steel tube, and kiln firing the barium titanate at 2250 F. to 2300" F.

6. The method of processing barium titanate powder comprising confining the powder in a brass tube that has been de-aired and then inner lined with a tetrafluoroethylene coating, inserting the brass tube in a steel tube telescope fashion, swaging the steel tube down over the brass tube until a tight seal exists between the two tubes,

" further swaging down the steel tube until the di'lmeter of the steel tube is reduced to 0.625 inch, removin the barium titanate from the brass tube, and hot pressing the barium titanate at 1900 F. at 10,000 pounds per square inch.

References Cited by the Examiner UNITED STATES PATENTS 930,723 8/1909 Bolton 264l04 936,403 l0/1909 Bolton 264-104 2,990,602 4/1961 Brandmayr 264-332 FOREIGN PATENTS 613,273 1/1961 Canada.

ROBERT F. WHITE, Primary Examiner. R. B. MOFFITT, Assistant Examiner.

Claims (1)

1. THE METHOD OF PROCESSING CERAMIC TYPE FERROELECTRIC MATERIALS COMPRISING CONFINING A POWDER OF THE FERROELECTRIC MATERIAL IN A METALLIC TUBE,SWAGING THE METALLIC TUBE DOWN OVER THE FERROELECTRIC MATERIAL TO PROCOMPACT THE MATERIAL TO A RELATIVELY HIGH DENSITY, REMOVING THE SWAGED FERROELECTRIC MATERIAL FROM THE TUBE, AND THEN THERMALLY MATURING THE FERROELECTRIC MATERIAL.