Process for preparing a ferromagnetic material

A process for preparing a ferrite in which a green body containing at least about 50 weight percent of iron compound is first sintered and then cooled. During the cooling cycle, from about 1,000 to about 700 degrees centigrade, the body is contacted with a reduced oxygen atmosphere containing less than 10 volume percent of oxygen. From 700 to 200 degrees centigrade, the reduced oxygen atmosphere contains less than 1 part per thousand of oxygen. Thereafter, the sintered body is treated to remove at least some sharp edges and to change its permeability; and it is then contacted with an etchant.

FIELD OF THE INVENTION 
A process for preparing a sintered ferromagnetic body. 
BACKGROUND OF THE INVENTION 
Processes for making shaped and sintered ferrite bodies with good physical, 
mechanical, magnetic, and electrical properties are known. However, with 
most of these prior art processes, reproducibility is poor; and the 
properties of the ferrite bodies produced vary substantially from lot to 
lot (and within each lot) even when substantially identical processing 
parameters are used. 
It is an object of this invention to provide a process for making shaped 
and sintered ferrite bodies with good physical, mechanical, magnetic, and 
electrical properties which will produce a substantially repeatable 
resulting product for a given set of conditions. 
It is another object of this invention to produce a process for making 
shaped and sintered ferrite bodies which has a substantially higher yield, 
yield rate, and throughput than prior art processes and which 
substantially reduces the need to rework articles. 
It is yet another object of this invention to provide a process of making 
shaped and sintered ferrite bodies in which the final geometric and 
magnetic configurations of the sintered ferrite bodies may be modified by 
tumbling the bodies. 
It is yet another object of this invention to provide a process for making 
shaped and sintered ferrite bodies in which the surfaces of such bodies 
may be altered by means other than tumbling. 
It is yet another object of this invention to provide a process for 
effecting the aforementioned goals with magnetostrictive materials other 
than ferrites. 
SUMMARY OF THE INVENTION 
In accordance with this invention, there is provided a process in which a 
specified ferrite green body is sintered and cooled while being subjected 
to a reduced oxygen environment, and then surfaces of the sintered body 
are then modified until the magnetic permeability of the body has changed 
by at least about five percent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process of this invention is suitable for producing improved ferrite 
bodies. 
As is known to those skilled in the art, a ferrite is a ferromagnetic 
compound containing iron in an oxygen lattice. See, for example, U.S. Pat. 
No. 3,576,672 of Harris et al., the entire disclosure of which is hereby 
incorporated by reference into this specification. 
As used in this specification, the term ferromagnetic refers to a permanent 
magnet composed of mixtures of ceramic and magnetic powder which have been 
pressed together and sintered. The term ferromagnetic material refers to 
any material displaying ferromagnetism; that is, having an abnormally high 
magnetic permeability, a definite saturation point, and appreciable 
hysteresis. See, for example, the aforementioned Van Aulock book. 
In one embodiment, the ferrite is a garnet. Iron garnet has the formula 
M.sub.3 Fe.sub.5 O.sub.12 ; see, e.g., pages 65-256 of Wilhelm H. Von 
Aulock's "Handbook of Microwave Ferrite Materials" (Academic Press, New 
York, 1965). Garnet ferrites are also described, e.g., in U.S. Pat. No. 
4,721,547, the disclosure of which is hereby incorporated by reference 
into this specification. 
In another embodiment, the ferrite is a spinel ferrite. Spinel ferrites 
usually have the formula MFe.sub.2 O.sub.4, wherein M is a divalent metal 
ion and Fe is a trivalent iron ion. M is typically selected from the group 
consisting of nickel, zinc, magnesium, manganese, and like. These spinel 
ferrites are well known and are described, for example, in U.S. Pat. Nos. 
5,001,014, 5,000,909, 4,966,625, 4,960,582, 4,957,812, 4,880,599, 
4,862,117, 4,855,205, 4,680,130, 4,490,268, 3,822,210, 3,635,898, 
3,542,685, 3,421,933, and the like. The disclosure of each of these 
patents is hereby incorporated by reference into this specification. 
Reference may also be had to pages 269-406 of the Aulock book for a 
discussion of spinel ferrites. 
In yet another embodiment, the ferrite is a lithium ferrite. Lithium 
ferrites are often described by the formula (Li.sub.0.5 Fe.sub.0.5).sup.2+ 
(Fe.sub.2).sup.3+ O.sub.4. Some illustrative lithium ferrites are 
described on pages 407-434 of the aforementioned Van Aulock book and in 
U.S. Pat. Nos. 4,277,356, 4,238,342, 4,177,438, 4,155,963, 4,093,781, 
4,067,922, 3,998,757, 3,767,581, 3,640,867, and the like. The disclosure 
of each of these patents is hereby incorporated by reference into this 
specification. 
In yet another embodiment, the preferred ferrite is a hexagonal ferrite. 
These ferrites are well known and are disclosed on pages 451-518 of the 
Van Aulock book and also in U.S. Pat. Nos. 4,816,292, 4,189,521, 
5,061,586, 5,055,322, 5,051,201, 5,047,290, 5,036,629, 5,034,243, 
5,032,931, and the like. The disclosure of each of these patents is hereby 
incorporated by reference into this specification. 
FIG. 1 is a flow diagram of one preferred process of the instant invention. 
Referring to FIG. 1, it will be seen that, in the first step of the 
process, to mixer 10 are charged the ingredients necessary to form the 
ferromagnetic body. The process will be described with reference to the 
formation of preferred ferrite materials, it being understood that a 
comparable process may be used to form other ferromagnetic materials (such 
as, e.g., alloys, metals, and the like). 
Referring to FIG. 1, an iron source is preferably charged via line 12. This 
iron source is preferably an iron oxide, such as ferrous oxide, ferric 
oxide, ferrous ferric oxide, mixtures thereof, and the like. 
Alternatively, or additionally, the iron source may be elemental iron, or 
an iron compound which is converted to iron oxide upon sintering. Thus, in 
addition to or instead of iron oxide, one may use iron carbonate, iron 
carbonyl, iron chloride, ferric hydroxide, ferric nitrate, ferric citrate, 
and the like. It will be apparent to those skilled in the art that, when 
an iron compound other iron oxide is used, a sufficient amount of material 
will be charged to the mixer 10 so that it will be converted to the 
desired concentration of iron oxide. 
In one preferred embodiment, the iron source is ferric oxide which, as 
those skilled in the art are aware, is Fe.sub.2 O.sub.3. This ferric oxide 
is often identified as "CAS: 1309-37-1"; it is a dense, material with a 
density of from about 5.12 to about 5.24 grams per cubic centimeter. 
It is most preferred to use an alpha-ferric oxide such as, e.g., the alpha 
ferric oxide described in U.S. Pat. No. 4,414,196. The entire disclosure 
of this patent is hereby incorporated by reference into this 
specification. 
As is known to those skilled in the art, alpha ferric oxide is commercially 
available and may be obtained, e.g., from the Chemrite Ltd. company of 
Kyoto, Japan as product number CSS-410E. 
It is preferred that the iron source charged via line 12 have a particle 
size distribution so that substantially all of its particles are smaller 
than 1,000 microns and, more preferably, smaller than 10 microns. In an 
even more preferred embodiment, it is preferred that substantially all of 
the particles of the iron source be smaller than about 2 microns. 
It is preferred to charge a sufficient amount of the iron source to mixer 
10 so that the mixture formed in mixer 10 is comprised of at least about 
50 weight percent (by dry weight of all materials) of the iron source (as 
iron oxide). In a more preferred embodiment, a sufficient amount of the 
iron source is charged so that at least about 65 weight percent of the 
mixture in mixer 10 comprises the iron source. 
As is known to those skilled in the art, the ferrite material may consist 
essentially of magnetite (Fe.sub.3 O.sub.4), in which case no other metal 
or metal oxide material is charged to mixer 10 in an amount exceeding 
about 1 weight percent. However, as is known to those skilled in the art, 
the iron or iron oxide(s) material may be substituted, in whole or part, 
by one or more other metal or metal compound materials. 
Thus, in one preferred embodiment, from about 0.1 to about 50 percent of a 
source of zinc (as zinc oxide, by total weight of dry material in mixer 
10) is charged via line 14. In this embodiment, it is preferred to charge 
from about 10 to about 20 weight percent (by dry weight) of a source of 
zinc. 
The source of zinc may be elemental zinc. It is preferred, however, that 
the source of zinc be a zinc compound such as zinc oxide, or a zinc 
compound which is converted to zinc oxide upon sintering. Thus, in 
addition to or instead of zinc oxide, one may use zinc carbonate, zinc 
chloride, zinc nitrate, zinc chlorate, zinc hydroxide, and the like. It 
will be apparent to those skilled in the art that, when the zinc source is 
either elemental zinc or a zinc compound other than zinc oxide, a 
sufficient amount of such material(s) is used so that, after sintering, it 
is converted into the desired concentration of zinc oxide. 
The preferred zinc compound is zinc oxide, and it is preferred to use zinc 
oxide with a particle size such that substantially all of its particles 
are smaller than about 10 microns and, more preferably, are smaller than 
about 5 microns. 
Zinc oxide is readily commercially available and may be obtained, e.g., 
from the Zochem Incorporated of Brampton, Ontario as product ZOCO-104. 
Two commonly used ferrites are comprised of nickel, Zinc, and iron, and 
manganese, zinc, and iron. Thus, in addition to the zinc, one may also 
charge a source of nickel oxide to mixer 10 via line 16. 
When a source of nickel oxide is added to mixer 10, it is preferred to 
charge from about 0.1 to about 50 weight percent (as nickel oxide, by dry 
weight of materials in mixer 10). However, it is preferred to use from 
about 10 to about 20 weight percent of the nickel (as nickel oxide). As 
before, one may use nickel oxide or a material which will be converted to 
nickel oxide such as, e.g., nickel, nickel carbonate, nickel carbonyl, 
nickel sesquioxide, nickel nitrate, nickel hydroxide, nickel chloride, and 
the like. 
It is preferred to use nickel oxide, and it is preferred that the source of 
nickel oxide have a particle size distribution such that substantially all 
of its particles are smaller than about 10 microns. 
Alternatively, or additionally, a source of manganese oxide may be charged 
via line 18 to mixer 10. In general, a sufficient amount of manganese 
oxide (or a source of such oxide, such as elemental manganese, or a 
non-oxide manganese compound) is charged so that the manganese oxide 
concentration is from about 0.1 to about 50 weight percent and, 
preferably, from about 10 to about 20 percent of manganese oxide. 
It will be apparent to those skilled in the art that, subject to the 
limitation that at least 50 weight percent of the ferrite should be iron 
oxide, the zinc and/or the nickel and/or the manganese may be present in 
trace amounts (less than about 1.0 weight percent), in minor amount 
amounts (from about 5 to about 30 weight percent), or in major amounts 
(from about 40 to about 50 weight percent). Additionally, other oxide 
materials (or sources of oxide materials) may also be charged to mixer 10. 
Thus, one may charge via line 20 oxides (or precursors of oxides) one or 
more of the oxides (or compounds) of lithium, fluorine, chlorine, and the 
like sodium, magnesium, aluminum, silicon. 
By way of illustration and not limitation, one may charge to mixer 10 
ingredients sufficient to form one or more of the ferrites disclosed in 
U.S. Pat. No. 5,359,479 (polycrystalline ferrite), U.S. Pat. No. 5,358,660 
(hexagonal ferrite), U.S. Pat. No. 5,354,610 (barium ferrite), U.S. Pat. 
No. 5,354,521 (strontium ferrite), U.S. Pat. No. 5,350,559 (ferrite 
steel), U.S. Pat. No. 5,334,955 (soft ferrite), U.S. Pat. No. 5,332,645 
(barium ferrite), U.S. Pat. No. 5,330,594 (ferrite--pearlite structure), 
U.S. Pat. No. 5,323,282 (polycrystalline manganese--zinc ferrite), U.S. 
Pat. No. 5,323,160 (nickel-zinc ferrite), U.S. Pat. No. 5,310,431 
(cobalt-nickel-molybdenum-titanium ferrite), U.S. Pat. No. 5,304,318 
(lithium--titanium--tin--germanium ferrite), U.S. Pat. No. 5,290,652 (zinc 
oxide ferrite), U.S. Pat. No. 5,271,907 (ZnFe.sub.2 O.sub.4 ferrite), U.S. 
Pat. No. 5,268,249 (ferrite containing divalent transition metal), U.S. 
Pat. No. 5,254,836 (silicon--manganese--chromium--niobium--copper 
ferrite), U.S. Pat. No. 5,243,911 (calcium oxide ferrite), U.S. Pat. No. 
5,238,508 (chromium--nickel stainless steel composition), U.S. Pat. No. 
5,223,049 (silicon--carbon--manganese--boron--titanium--aluminum ferrite), 
U.S. Pat. No. 5,217,545 (composition with ferrite number of from 1 to 15), 
U.S. Pat. No. 5,217,544 (martensite ferrite two-phased structure), U.S. 
Pat. No. 5,206,620 (ferrite and borosilicate glass), U.S. Pat. No. 
5,201,583 (ferrite with iron, chromium, nickel, and one or more of carbon, 
silicon, manganese, phosphorous, sulfur, molybdenum, and copper), U.S. 
Pat. No. 5,198,138 (iron oxide, nickel oxide, manganese oxide, 
nickel-manganese oxide, zinc oxide), U.S. Pat. No. 5,190,842 (lead 
ferrite), U.S. Pat. No. 5,183,709 (cobalt ferrite), and the like. The 
disclosure of each of these United States patents is hereby incorporated 
by reference into this specification. 
As will be apparent to those skilled in the art, the materials charged to 
mixer 10 will depend upon the composition of the ferrite desired. As is 
known to those skilled in the art, iron in the body-centered cubic form 
commonly occurs in steels, cast iron, and pig at about 910 degrees 
centigrade. Alpha and beta iron are the common varieties of ferrite, but 
the term "ferrite" is also applied to delta iron. 
The ferrimagnetic oxides which are commonly referred to as ferrites usually 
comprise a compound, a multiple oxide, of ferric oxide with another oxide, 
as sodium ferrite (NaFeO.sub.2), but more commonly a multiple oxide 
crystal. 
Referring again to FIG. 1, one may optionally charge liquid and/or binder 
via line 22 to mixer 10. In one preferred embodiment, liquid is charged 
via line 22. 
Substantially any liquid may be used which will not react with the solid 
ingredients in mixer 10 and, after sintering, not leave any harmful 
residues. Thus, by way of illustration, one may use an organic solvent 
such benzene, acetone, ketones, alcohols containing from about 1 to about 
6 carbon atoms, aliphatic hydrocarbons, aromatic hydrocarbons, and the 
like. 
It is preferred, however, that the liquid used be water, and that from 
about 10 to about 40 weight percent of water (by total weight of water and 
solid material) be used. In one preferred embodiment, from about 25 to 
about 35 weight percent of water is used. 
Additionally, or alternatively, one may charge from about 0.1 to about 5 
weight percent of binder (by total weight of material in mixer 10) via 
line 22. One may use any of the binders commonly used in ceramic 
processing. Thus, referring to James S. Reed's "Introduction to the 
Principles of Ceramic Processing" (John Wiley & Sons, New York, 1988), one 
may use one or more of the binders described on pages 152 et seq. of this 
book. Thus, e.g., one may use polyvinyl alcohol, polyethylene glycol, 
polymethyl metharcrylate, gum arabic, methyl cellulose, hydroxyethyl 
cellulose, lignosulfonates, and the like. Other suitable binders will be 
readily apparent to those skilled in the art. 
One may also charge to mixer 10 from about 0.1 to about 1 weight percent of 
an antifoaming agent (see page 179 of the Reed book) such as, e.g., tall 
oil, sodium alkyl sufate, propylene glycol ether, 2-octadecanoic acid, and 
the like). Additionally, or alternatively, one may also add to mixer 10 
from about 0.1 to about 1.0 weight percent of a deflocculant (see pages 
132 et seq. of the Reed book) such as, e.g., sodium polyacrylate, sodium 
pyrophosphate, sodium silicate, sodium borate, etc. A discussion of the 
identity and function of many common deffloculants is presented in U.S. 
Pat. No. 4,282,006, the entire disclosure of which is hereby incorporated 
by reference into this specification. 
The mixture in mixer 10 is then mixed until a substantially homogeneous 
mixture is produced. Referring to FIG. 1, samples from mixer 10 may be 
periodically removed via line 24 to laboratory 26, wherein the degree of 
homogeneity of the mixture may be evaluated by X-ray diffractometry 
analysis. X-ray diffractometer devices are well known to those skilled in 
the art and are described, e.g., in U.S. Pat. Nos. 5,359,640, 5,283,095, 
5,107,530, 5,084,910, 5,008,909, and the like. The disclosure of each of 
these United States patents is hereby incorporated by reference into this 
specification. 
When the mixture in mixer 10 has the desired degree of homogeneity, it then 
may be passed either to dryer 28 (via line 30), or calciner 32 (via line 
34). 
When the mixture in mixer 10 contains more than about 30 weight percent of 
liquid, it is preferred to dry it in dryer 28 until it contains less than 
about 20 weight percent of liquid and, preferably, less than about 1 
weight percent of liquid. Thereafter, the dried material may be passed via 
line 36 to calciner 32. 
It is preferred that, when the material is within dryer 28, it is subjected 
to a temperature of from about 50 to about 200 degrees centigrade and, 
more preferably, from about 100 to about 140 degrees centigrade. 
In one preferred embodiment, dryer 28 is a spray dryer. Thus, e.g., one may 
use a spray drier or a fluidized bed dryer manufactured by Niro Inc. of 
9165 Rumsey Road, Columbia, Md. 
Samples from dryer 28 may be periodically removed to laboratory 26 via line 
78 wherein analysis may be conducted to determine moisture content, angle 
of repose (which is a function of the granule size of spray-dried 
particles and, also, of moisture content), and particle size. It desired 
that, when the material from dryer 28 is to passed directly to former 42, 
the dry material have a moisture content of from about 0.05 to about 5.0 
percent (and, more preferably, from about 0.2 to about 1.0 weight 
percent), an angle of repose of less than about 32 degrees, and a particle 
size distribution such that at least about 80 weight percent of its 
particles are smaller than about 80 microns and substantially all of its 
particles are smaller than about 600 microns. However, when the material 
from dryer 28 is to be passed via line 36 to calciner 32, then material 
should have a moisture content of less than about 30 weight percent. 
Means of measuring the angle of repose of particulate materials are well 
known to those skilled in the art and are described, e.g., in U.S. Pat. 
Nos. 5,341,963, 5,324,097, 5,305,912, 5,305,535, 5,175,934, 5,130,106, 
5,129,164, and the like. The disclosure of each of these United States 
patents is hereby incorporated by reference into this specification. 
Means of measuring particle size distribution are well known and are 
described, e.g., in the aforementioned U.S. Pat. No. 4,282,006. 
When the dried particles in drier 28 have the desired moisture content, 
angle of repose, and particle size distribution, they may be passed via 
line 40 to former 42. Alternatively, or additionally, material from mixer 
10 may be passed via line 44 to former 42 when the material from said 
mixer contains less than about 20 weight percent of moisture. 
Referring again to FIG. 1, and in one of the preferred embodiments 
described therein, it is preferred that, when the material in dryer 28 has 
the desired particle size distribution, moisture content, and angle of 
repose, it be passed via line 36 to calciner 32. 
When the material is in calciner 32, it is preferably subjected to a 
temperature of from about 500 to about 1,600 degrees centigrade and, more 
preferably, from about 800 to about 1,150 degrees centigrade until a 
specified amount of such material has been converted to the spinel crystal 
phase. Samples of the material from calciner 32 may be periodically 
removed via line 42 to laboratory 26 and analyzed therein to determine the 
extent to which such material has been converted to the spinel crystal 
phase. 
As used in this specification, the term "spinel crystal phase" refers to 
the crystal lattice structure characteristic of spinel materials; see, 
e.g., the crystal structure illustrated on page 138 of J Smit et al.'s 
"Ferrites" (John Wiley & Sons, New York, 1959). As is known to those 
skilled in the art, one may determine whether the spinel crystal phase 
structure exists in a material, and to what it exists, by standard X-ray 
diffraction techniques. Thus, e.g., one may use the procedures described 
in, e.g., U.S. Pat. Nos. 5,354,637 and 5,217,37, the entire disclosures of 
which are hereby incorporated by reference into this specification. 
Referring again to FIG. 1, it is preferred to calcine the material in 
calciner 32 at a temperature of from about 500 to about 1,400 degrees 
centigrade and, more preferably, 900 to about 1,000 degrees centigrade 
until from about 20 to about 80 weight percent of the material in calciner 
32 (and, more preferably, from about 30 to about 70 weight percent of the 
material in calciner 32) has the characteristic spinel crystal structure. 
It is even more preferred to conduct such calcining operation until from 
about 40 to about 60 weight percent of the material in calciner 32 has the 
characteristic spinel crystal structure. 
After the material in calciner 32 has the desired structure, it is 
preferably passed via line 46 to grinder 48. Samples from grinder 48 may 
be periodically removed via line 38 to determine particle size 
distribution and the stoichiometry of the reagents in grinder 48. If, for 
example, there is an insufficient amount of iron oxide in the material in 
the material in grinder 48, it may be added via line 50. Alternatively, or 
additionally, one may add additional zinc oxide, manganese oxide, nickel 
oxide, and/or any other material which was deficient. Furthermore, one may 
add other reagents such as sources of silicon and calcium, lubricant(s), 
binder, deflocculant, plasticizer, and/or a pre-ground ferrite material 
with a specified particle size distribution necessary to adjust the 
particle size distribution of the mixture. 
Thus, e.g., instead of adding the deflocculant and/or the binder to mixer 
10 (as discussed hereinabove), one may add it at this stage. Thus, e.g., 
one may add water (if wet grinding is desired) to a concentration 
specified before or, alternatively, may dry grind. 
Referring again to FIG. 1, after the material in grinder 48 has some but 
not necessarily all of the desired properties, it may be passed via line 
52 to holding tank 54, where a sufficient amount of such material may be 
accumulated. In one average, the material in tank 54 may be continuously 
mixed, and its properties may be modified by the addition of further lots 
from grinder 48 and/or of reagents to tank 54. 
Referring again to FIG. 1, and in the preferred embodiment illustrated, the 
material from grinder 54 may be passed via line 56 to drier 58. 
Alternatively, or additionally, material from tank 54 may be passed to 
drier 58 by a line (not shown). Alternatively, material from grinder 48 
may be passed directly via line 49 to former 42. 
The material in drier 58 may be subjected to substantially the same 
conditions as the material in drier 28 prior to the time the former 
material is passed via line 60 to former 42. 
Regardless of which path used by the material to reach former 42, it is 
preferred that the material to be formed have certain properties. In the 
first place, such material preferably contains from about 0.1 to about 5.0 
weight percent of binder, but it need not contain any binder. In one 
embodiment, the binder used is a partially hydrolyzed polyvinyl alcohol 
sold by the Air Products and Chemicals Inc. of 7201 Hamilton Blvd., 
Allentown, Pa. as AIRVOL-205. 
In the second place, it is preferred that the material to be formed have a 
particle size distribution such that all its particles are smaller than 
about 1,000 microns and, more preferably, smaller than about 600 microns. 
In an even more preferred, substantially all of the particles of the 
material to be formed are smaller than about 400 microns. 
The material may be formed by any of the conventional shape forming ceramic 
processes. Thus, e.g., referring to the aforementioned Reed book, one may 
use die pressing (see pages 329 et seq.), hot or cold isotstatic 
compaction (see pages 349 et seq.), roll pressing (see page 351), 
extrusion (see pages 360-368), injection molding (see pages 373 et seq.), 
slip casting in a permeable mold (see pages 381 et seq.), tape casting 
(see pages 395 et seq.), forging, and the like. 
In one preferred embodiment, tool and die dry pressing is used as the 
forming technique. This technique is described, e.g., in U.S. Pat. Nos. 
3,717,693, 3,671,618, 3,523,344, and the like. The entire disclosure of 
each of these patents is hereby incorporated by reference into this 
specification. 
It is preferred that the green body formed in the forming operation have a 
density of from about 2 to about 7 grams per cubic centimeter. In one 
embodiment, the density of the green is from about 2.5 to about 3.5 grams 
per cubic centimeter. 
It is also preferred that the green body have mechanical integrity and be 
substantially free of physical defects such as cracks, pores, laminations, 
and the like. 
The green body formed in former 42 may be processed in one of several 
different methods. It may be passed via line 62 to rounding/etching 
operation 64 (see FIG. 2). It may be passed via line 66 to oxygen 
treatment operation 68. It may be passed via line 70 to surface alteration 
operation 72. 
Referring to FIG. 2, the formed green body (not shown) from the former 42 
(not shown in FIG. 2, but see FIG. 1) may be fed via line 100 to sintering 
chamber 102. 
As will be apparent to those skilled in the art, one may sinter with 
microwave energy and/or heat and/or induction heat. 
In one preferred embodiment, heat is used to sinter the green body. In this 
embodiment, the temperature of the green body is raised from ambient to 
about 100 degrees centigrade. 
When the green body is comprised of organic binder, it is preferred to 
raise its temperature from about 100 to about 275 degrees centigrade at a 
rate of from about 1 to about 30 degrees per hour. However, when the green 
body does not contain organic binder, the rate of temperature increase may 
be as high as about 400 degrees per hour. The atmosphere used during this 
portion of the cycle is preferably oxygen-containing gas (such as, oxygen, 
mixtures thereof, and the like). 
Thereafter, the temperature of the green body is raised from about 275 
degrees centigrade to a temperature between about 1,100 to about 1,500 
degrees centigrade at a rate of from about 1 to about 500 degrees per hour 
and, more preferably, at a rate of from about 100 to about 300 degrees per 
hour. It is preferred to use an oxygen-containing atmosphere during this 
portion of the cyle. 
In one embodiment, when the temperature of the green body is about 1,250 
degrees centigrade, it is then slowly raised to a temperature of from 
about 1,300 to about 1,350 degrees centigrade at rate less than about 25 
degrees per hour; although the green body may be maintained at this 
temperature for from about 4 to about 8 hours, but it need not be so 
soaked. 
In this embodiment, when the green body is not so soaked, thereafter, the 
temperature of the green body is raised to a sintering temperature of from 
about 1100.degree.-1500.degree. C., and preferably about 1,350 to about 
1,500 degrees centigrade and maintained at this temperature until the 
desired density, microstructural, electrical properties, and magnetic 
properties are obtained and, preferably, until the green body has a 
density of at least about 50 percent of its theoretical density (and, more 
preferably, at least about 90 percent of its theoretical density). Samples 
may be periodically withdrawn from chamber 102 via line 104 and evaluated 
in laboratory 106. 
After the green body has been sintered and obtained the desired density, it 
is then preferably subjected to a specified cooling cycle. The sintered 
body is cooled from the peak sintering temperature to ambient at rate of 
from 1 to about 500 degrees per hour. During this cooling cycle, the 
sintered body is contacted with an atmosphere whose composition changes. 
In one embodiment, when the sintered body is between a temperature of from 
about 1,000 degrees centigrade and 700 degrees centigrade, the sintered 
body is subjected to an atmosphere containing no more than about 10 volume 
percent and, more preferably, less than about 1 volume percent of oxygen. 
From about 700 degrees centigrade to about 200 degrees centigrade, the 
sintered body, while cooling, is subjected to an atmosphere containing 
less than about 1,000 parts per million of oxygen and, more preferably, 
less than about 100 parts per million of oxygen. 
The cooled and sintered body is then passed via line 104 to laboratory 106, 
wherein the magnetic properties of the of the body may be tested. Thus, 
e.g., one may test the permeability, the hysteresis magnetization, the 
loss factor, the volume resistivity, the amplitude permeability, the phase 
identification, the porosity, the morphology, and the like. These and 
other parameters, and means for evaluating them, are described in B. D. 
Cullity's "Introduction to Magnetic Materials" (Addison-Wesley Publishing 
Company, Reading, Mass., 1972). Reference also may be had to Richard M. 
Bozorth's "Ferromagnetism" (IEEE Press, Piscataway, N.J., 1993); page 822 
of this reference discloses, e.g., means for evaluating the permeability 
of a ferrite sample. 
The cooled and sintered body from sintering chamber 102 is then passed via 
line 108 to a mechanical finishing device 110 in which sharp edges of the 
body is removed by vibration and mechanical contact with other similar 
bodies. 
One means for effecting such mechanical finishing is illustrated in FIG. 
2A. Referring to FIG. 2A, it will be seen that edge rounding device 112 is 
preferably mounted on a table 114 and is comprised of a shaft 116 
connected by belt 118 and 120 to rotating cylindrical vessels 122 and 124; 
shaft 116 rotates in the direction of arrow 126, thereby causing tub 124 
to rotate in the direction of arrow 128, and tub 122 to rotate in the 
direction of arrow 130. 
The ferrites bodies (not shown) are disposed within either or both of tubs 
122 and 124. Additionally, tubs 122 and 124 may also contain water and or 
organic or inorganic solvent, grinding media, and the like. In one 
embodiment, ferrite parts with a geometry different than that of the parts 
being treated are used as the grinding media. 
In one embodiment, shaft 116 rotates at a rate of from about 10 to about 
100 revolutions per minute. In an even more preferred embodiment, the 
rotation rate is from about 60 to about 120 r.p.m. 
Samples may be periodically removed from mechanical finisher 110 (see FIG. 
2) via line 132 to laboratory 106 in order to evaluate the extent to which 
the edges of the ferrite bodies have been rounded and the extent to which 
the permeability of the ferrite has been changed. 
It will be apparent to those skilled in the art that the ability to edge 
round the ferrite bodies in the instant process is a substantial 
advantage. Referring to FIG. 4A, which depicts a ferrite body 150 made by 
a prior art process, it will be seen that such ferrite body 150 has sharp 
edges 152 and 154, 156, and 158, and a chip 160. The sharp edges pose a 
danger to wire (not shown) customarily wound around the ferrite body to 
make a magnetic device comprising the body. The chip 160 produces an area 
with undesirable magnetic properties. 
By comparison, FIG. 4B produces a body 170 with rounded edges 172, 174, 
176, and 178 and no dents or chips. 
Referring again to FIG. 2, the mechanical finishing process is preferably 
conducted until the permeability of the ferrite body is changed (plus or 
minus) from 1 to about 80 percent and, more preferably, from about 5 to 
about 60 percent. 
As will be apparent to those skilled in the art, other means of may be used 
to change the permeability of the ferrite body and remove sharp edges. 
Thus, by way of illustration and not limitation, one may shot peening, jet 
milling, ultrasonic vibration, sanding, machining, and the like. Any 
method which changes both the appearance and the permeability of the 
ferrite body may be used. 
The treated body from mechanical finisher 110 may then be (but need not be) 
passed via line 190 to etcher 192, wherein it is contacted with an 
etchant. 
One may use any of the etchants know to be effective with ferrites. Thus, 
e.g., one may use hydrochloric acid, phosphoric acid, nitric acid, 
sulfuric acid, aqua regia, hydrofluoric acid, alkali metal hydroxides, and 
the like. See, e.g., U.S. Pat. Nos. 4,169,026, 4,875,970, 4,214,960, 
4,781,852, 5,228,185, 5,250,150, 5,356,514, and the like. The disclosure 
of each of these United States patents is hereby incorporated by reference 
into this specification. 
Samples from ether 192 may be periodically evaluated in laboratory 106 by 
passing them to such laboratory via line 194. In one embodiment, 
laboratory 106 is an on line with the samples in etcher 192 and 
continually monitors their properties. 
The object of the etching operation is to move the permeability of the 
sample in the direction opposite to which it was moved in the mechanical 
finishing operation. In general, it is preferred to regain from about 2 to 
about 95 percent of change in permeability produced by the mechanical 
finishing step. 
By way of illustration, one ferrite body has an initial permeability of 
11,000. After mechanical finishing, its permeability was 5,500. After 
etching, its permeability rose to 10,700. 
By way of further illustration, another ferrite body has an initial 
permeability of 9,000 which rose to 12,000 after mechanical finishing and 
decreased to 10,000 after etching. 
The production of a surface modification of the ferrite body followed by a 
partial controlled undoing of such surface modification allows one to 
adjust the desired permeability property and cross it both ways, in order 
to optimize these properties. It will be apparent that, if the mechanical 
finisher produces the exact desired property or properties, there may be 
no need for the subsequent etching. However, as will also be apparent to 
those skilled in the art, because several properties need to be optimized, 
it is often advantageous to use both processes. 
One substantial advantage of the process, however, is that there is no need 
to worry about applying too much edge rounding treatment inasmuch as the 
effect of this treatment upon the properties can be at least in part 
counteracted by the etching. 
In one preferred embodiment, the enchant used is hydrochloric acid solution 
which, preferably, has a concentration of from about 10 to about 100 
percent. In this embodiment, the bath is at a temperature of from about 
ambient to about 100 degrees centigrade. In general, in this embodiment, 
the part is contacted with the hydrochloric acid for from about 10 seconds 
to about 50 hours and, more preferably, form about 5 to about 50 minutes. 
The etched ferrite is then passed via line 196 to washer 198, wherein the 
enchant is washed (preferably with water) to remove it from the surface of 
the ferrite. Thereafter, the washed ferrite is passed via line 200 to 
dryer 202. In one embodiment, the ferrite body is air dried under ambient 
conditions while being contacted with flowing air. 
Referring again to FIG. 1, instead of passing the green body via line 62 to 
rounding/etching step 64, one may pass the green via line 66 to oxygen 
treatment step 68. 
Oxygen step 68 is similar to rounding etching step 64 but differs from it 
that (1) it does not provide edge rounding, (2) during a portion of the 
cooling cycle, between from about 500 to about 200 degrees centigrade, the 
sample is contacted with oxygen containing gas (oxygen and/or air) during 
the cooling. In this process, the body is subjected to the steps sintering 
102 (see FIG. 2), but with the modification described above. 
Alternatively, one may subject the formed body to the surface alternation 
process 72 described in FIG. 1. In this process, the green body is 
calcined rather than sintered, and, thereafter, is edge-rounded and 
subsequently sintered. The process may be further enhanced by additional 
edge rounding and/or sintering steps, with or without the presence of 
oxygen-containing gas. 
FIG. 3 is a graph of processed ferrite bodies produced by various 
processes. In each process, however, the ferrite used was a maganese/zinc 
ferrite with an initial permeability of 10,000 which had been formed and 
sintered in substantial accordance with the procedure of FIG. 2. The 
graphs display the permeability of the body at various temperatures and 
under various conditions. 
Referring to FIG. 3, graph 500 shows the permeability of the body produced 
after sintering step 102 (see FIG. 2). By comparison, graph 502 shows the 
peremeability of the same body after mechanical finishing step 110 had 
occurred for ten minutes (see FIG. 2). However, after the body was etched 
(see step 102) at either 1 minute (see curve 504), or three minutes (see 
curve 506), or five minutes (see curve 508), the peremeability of the body 
was partially recovered. Curve 510 illustrates how the local maximum at a 
temperature of -20 to 40 degrees centigrade shifted as the processing 
conditions changed. 
It is to be understood that the aforementioned description is illustrative 
only and that changes can be made in the apparatus, in the ingredients and 
their proportions, and in the sequence of combinations and process steps, 
as well as in other aspects of the invention discussed herein, without 
departing from the scope of the invention as defined in the following 
claims.