US3418241A - Process for making aluminum-containing ferrites - Google Patents

Description

United States Patent 3,418,241 PROCES FOR MAKING ALUMINUM- CONTAINING FERRITES Joseph H. Weis, Syracuse, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Filed Mar. 29, 1965, Ser. No. 443,695 12 Claims. (Cl. 252-6258) ABSTRACT OF THE DISCLOSURE In aluminum-containing magnetic ferrites, the aluminum is more readily introduuced in the form of boehmite when making the ferrite.

This invention relates to magnetic ferrites. More particularly, it relates to magnetic ferrites of the MgMnAlFe and YAlFe or YGdAlFe types which are possessed of improved magnetic properties and to methods of preparing such ferrites.

Magnesium manganese iron ferrites and YGdAlFe ferrites have been found to be useful in microwave devices at the X band (8,000 to 10,000 megacycles) frequencies because of their low insertion losses, the saturation magnetization of these ferrites ranging in the order of from 2,000 to 2,400 gauss. It has been found that when aluminum is included in ferrites of the above types to replace a part of the iron, it is possible to reduce the saturation magnetization to as low as 300 gauss, thereby making such ferrites suitable for use at C band (6,000 to 8,000 megacycles), at S band (3,000 to 6,000 megacycles), and lower than 3,000 megacycles, depending upon the amount of aluminum present in the composition, the saturation magnetization being about inversely proportional to the aluminum content.

In the preparation of magnesium manganese ferrites, manganese carbonate and magnesium carbonate are generally used becasue as carbonates they are readily available, they are quite reactive and decompose at less than about 1000 C. Iron oxide (Fe O is also used. Its low cost and relatively low melting point of 1565 C. facilitates preparation of the ferrite. The resultant solid state material is MgMnFeO When aluminum is introduced into the batch to lower the saturation magnetization for use at lower than X band microwave frequencies, processing problems are encountered because aluminum is generally introduced into the batch as A1 0 its most commonly available form. The reactivity of A1 0 is low because of its refractory nature and high melting point of 2050 C. In an attempt to overcome the low reactivity of the A1 0 hydrated oxides and hydroxides of aluminum have been used as reactants. However, after the water content is removed, the refractory oxide with its relatively low reactivity tends to remain as such. The difficulty to introducing aluminum into magnetic ferrites has been recognized in the patent and other literature. It has been found that even with multiple calcinings to enhance the solid state reaction of the alumina, undesirable amounts of the A1 0 may remain as a distinct second phase ingredient unreacted with the other components. This relative inertness of the A1 0 prevents or seriously limits the formation of magnesia manganese ferro aluminate, YAlFe and YGdAlFe, the sought for products.

From the above it will be quite apparent that there is a definite need for magnetic ferrites of the above types containing aluminum in which the aluminum completely enters into the reaction and appears as an integrated component of the resultant ferrite, and it is a principal object of this invention to provide such magnetic ferrites and methods of making such ferrites.

3,418,241 Patented Dec. 24, 1968 "ice Those features of the invention which are believed to be novel are set forth with particularity in the claims appended hereto. Further objects and advantages of the invention will, however, be appreciated from a consideration of the following description.

It has been found that aluminum can be efliciently introduced into the structure of ferrites, for example, magnesium manganese iron ferrites and YAlFe and YGdAlFe ferrites as an integral constituent by using as the source of aluminum colloidal anisodiametric boehmite which is capable of being readily incorporated into the ferrite structure. The colloidal anisodiametric boehmite (AlOOH), hereinafter referred to as colloidal boehmite, is described in the literature, including Patent 2,915,475, issued Dec. 1, 1959. Colloidal boehmite is an alumina monohydrate having the boehmite crystal lattice in the form of particles of colloidal dimensions which are anisodiametric or which do not have equal diameters or axes. The form of the particles is rod-like and preferably fibrous. The colloidal particles have an average length from about 10 to 1500 millimicrons at the extremes and have axial ratios of at least 3:1. In the preferred case, the particles have lengths of 25 to 1,000 millimicrons, the preferred fibrils being in the shape of well-formed little fibers or fibrils having at least one dimension in the colloidal range or from 1 to millimicrons, with the fibril diameters being substantially uniform. The colloidal nature of the colloidal boehmite is indicated by its high specific surface area of 274 square meters per gram, its loose bulk density of 26 pounds per cubic foot and its absolute density of 2.28 grams per cc. Such colloidal boehmite is sold under the name Baymal by Du Pont. Since boehmite (AlOOH) contains 70.0 Weight percent of alumina after sintering, the amount of A1 0 called for in any particular ferrite is replaced by about 1.4 times the amount of colloidal boehmite.

For example, it has been found that superior magnesium manganese iron aluminum ferrites are produced by reacting together from about one to 15 mole percent colloidal boehmite calculated as A1 0 from about 45 to 55.0 mole perecnt magnesium carbonate, from about one to 10.0 mole percent manganese carbonate and from about 30 to 54.0 mole percent of Fe O When the oxides are used, the proportions are about one to 15.0 mole percent colloidal boehmite calculated as A1 0 21.5 to 26.3 mole percent MgO, 0.6 to 6.6 mole percent Mn O and 30 to 54.0 mole percent Fe O The metal contents are about 0.53 to 7.9 mole percent aluminum, 12.9 to 15.7 mole percent magnesium, 0.48 to 4.8 mole percent manganese and 21.0 to 37.7 mole percent iron. Improved ferrites of the YAlFe and YGdAlFe types are also provided as described below. In lieu of the carbonates, the correspondi g oxides can be used.

As opposed to the rigorous and expensive treatment in the form of multiple calcinings and relatively high temperatures required even for the incomplete and generally unsatisfactory incorporation of A1 0 into such ferrite compositions, when colloidal boehmite is used as taught herein, the aluminum is more readily incorporated into the ferrite structure. The coilloidal boehmite is mixed with the rest of the ferrite ingredients as by dry mixing with a V- shaped twin cone or ribbon or spiral blade or other type of dry blender until such time as the individual powders are indistinguishable when the product is rubbed on a smooth glass plate. The ferrite ingredients are readily mixed in the dry state according to the present invention, and a single calcining step after wet milling is generally sufiicient to promote complete solid state reaction of the colloidal boehmite with the other materials. Sintering times and temperatures to form desired shapes are typical for the magnesium manganese iron and other systems and can range from two hours to thirty-six hours of soak time at maximum temperature, the usual sintering temperature ranging fiom about 1250 C. to 1450 C.

Generally speaking, the raw powdered materials including the colloidal boehmite are mixed dry in a blender as described above and calcined at about 700 C. to 900 C., and preferably at about 800 C., for from about 1 to 8 hours to convert the magnesium carbonate and manganese carbonate constituents to oxides if the carbonates of these constituents are used as starting materials. If the corresponding oxides are used instead of carbonates, the dry blending and first calcining steps can be omitted. The dried, mixed powders derived from the carbonate process or the oxide starting materials are wet milled in ball mills, on rolls, or in attritors or micronizers for from about 4 to 16 hours to produce intimately mixed powders whose grain size is less than about 10 microns and preferably about two microns.This slurry is dried and the powder calcined as in ceramic saggers or a rotary tube kiln for from about 1 to 12 hours at from about 950 C. to about 1150 C., and preferably at about 1050 C. for the MgMnAlFe ferrite and about 950 C. to 1250 C. when rare earths are added. The calcined powder is then wet milled normally for from about 4 to 24 hours with any desired binder such as polyvinyl alcohol or other resinous material, natural or synthetic Waxes, oleates, stearates and the like and dried and pressed or molded into the desired final shape. Other binders for such materials are well known to those skilled in the art. Finally, the pressed, bonded parts are sintered at 1250 C. to 1450 C. after slowly burning ofI the organic content at their vaporization temperatures. Such sintering can be done in air or oxygen. When a carbon dioxide atmosphere is used up to the end of the high temperature soak period, annealing in oxygen is required beyond this heating period for maximum fired density.

When copper in the form of copper oxide or carbonate is introduced into the batch, the times and temperatures of treatment can be reduced because of the fiuxing effect of such copper compounds at high temperatures.

Rare earths such as yttrium, gadolinium and the like can also be introduced into batches in the ratio of 3 moles rare earth to each 5 moles of iron oxide to impart desirable characteristics. Such desirable characteristics include high powder capabilities and low losses at lower frequency ranges such as S band. The aluminum replaces the iron oxide in amounts up to 1.25 moles A1 0 equivalent. These rare earth ferrite crystallize in the garnet system whereas MgMnAl ferrites crystallize in the cubic system. Lesser or greater additions of rare earths result in poorer properties.

Microscopic examination of the completed sintered ferrites reveals the complete absence of aluminum as a separate phase which indicates its complete solid state reaction with the other ingredients. This as contrasted with the use of alumina which, unless subjected to intense fine milling or comminution and vigorous sintering remains as distinct I grains in the ferrite matix because of its great hardness and extreme refractory nature. When colloidal boehmite is employed, the resultant ferrite crystals are distinct with visible grain boundaries. The crystals range from about 1 to 40 microns in size depending upon the degree of pulverization and heat treatment. Maximum pulverization and minimum heat treatment result in smaller grain dimensions. Low colloidal boehmite contents in the order of 1 mole percent, calculated as A1 0 produce ferrites having Curie points over 300 C. and high saturation magnetization of about 2100 gauss. When higher colloidal boehmite contents of the order of 14 mole percent calculated as A1 0 are used, the Curie point is lowered to about 100 C. and the saturation magnetization is reduced to approximately 850 gauss, permitting the use of the present materials in various microwave bands.

The following examples will illustrate the practice of the present invention, it being realized that they are to be taken as exemplary only of the general teaching of the present concept and its use of colloidal boehmite as a reactant ingredient for magnetic ferrite materials.

Example 1 There were dry blended in a mixer 49.6 mole percent MgCO 39.4 mole percent Fe O 1.00 mole percent colloidal boehmite calculated as Al O and 10 mole percent MnCO the dry, mixed powder being calcined at a temperature of 800 C. for four hours to convert the carbonate to oxides. The calcined material was then wet milled in a ball mill using water for four hours, the slurry then being heated to the dry state. The dried powder was then calcined in a kiln for four hours at 1150 C. Next, the calcined material was wet milled for 16 hours with 2 percent by Weight of polyvinyl alcohol. After such milling the material was dried into granular free flowing grains and pressed into shape. The organic binder was burned oif at a temperature of 200 C. and the pressed parts sintered at a maximum temperature of 1350 C. for 36 hours, such sintering taking place in oxygen. The saturation magnetization of the material was 2000 gauss. The aluminum was completely reacted.

Example 2 Example 1 was repeated using as the starting materials 49.6 mole percent MgCl 39.2 percent Fe O 6.7 mole percent colloidal boehmite, calculated as A1 0 and 4.5 mole percent MnCO .The final ferrite material had a saturation magnetization of 1600 gauss and the aluminum had entered fully into the solid state reaction.

Example 3 Example 1 was repeated using as the starting materials mole percent MgCO 33.7.mole percent Fe O 13.3 mole percent colloidal boehmite calculated as A1 0 and 3.0 mole percent MnCO The resultant ferrite material had a saturation magnetization of 800 gauss.

As pointed out above, rare earth materials such as yttrium, gadolinium and the like may be included in ferrite compositions to impart high electrical power properties with low saturation magnetization for use at lower frequencies, such as S band. The following examples are illustrative of this aspect of the invention.

Example 4 Example 1 was repeated using as the starting materials 37.5 mole percent Y O 13.75 mole percent colloidal boehmite calculated as A1 0 and 48.75 mole percent Fe O except that the first calcining was omitted and the final heat reaction temperature ranged from about 1400 C. to 1480 C., such heat reaction being carried out in an oxygen atmosphere for maximum density. The saturation magnetization of this garnet material was 500 gauss, making it suitable for use at S band.

Example 5 be used. Also, as pointed out above, the time and tem-' perature of treatment can be reduced by introducing copper compounds into the starting batch. Generallly speaking, the magnetic ferrites prepared according to the present invention have a fired bulk density of about 4.23 as compared with a like density of 4.07 when'aluminum oxide of 2.5 micron size is used, for 800 gauss magnesium ferrite.

There are provided, then, by the present invention magnetic ferrites which are particularly characterized by the thorough incorporation and reaction of the aluminum constituent. The materials prepared according to the present invention are characterized by improved magnetic qualities such as lower losses and relatively higher permeabilities. The hysteresis loop of the materials has a higher remanence promoted by the relatively low calcining temperature which makes the materials particularly useful in phase shifters. Their grain size is small and uniform. From the present teaching, it is made apparent to those skilled in the art how improved aluminum containing ferrites can be prepared.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The process of producing aluminum-containing magnetic ferrites which comprises introducing the aluminum in the form of colloidal boehmite.

2. The process of producing magnesium manganese aluminum iron ferrite which comprises heat reacting together magnesium, manganese and iron-containing ingredients together with colloidal boehmite.

3. The process of producing yttrium aluminum iron ferrites which comprises heat reacting together yttrium and iron-containing ingredients along with colloidal boehmite.

4. The process of producing yttrium gadolinium aluminum iron ferrites which comprises heat reacting together yttrium, gadolinium and iron-containing ingredients along with colloidal boehmite.

5. The process of producing a magnetic ferrite of the magnesium manganese aluminum iron types which comprises heat reacting together from about one mole percent to about 15 mole percent colloidal boehmite calculated as A1 0 from about 45 mole percent to 55 mole percent magnesium carbonate, from about one mole percent to mole percent manganese carbonate, and from about 30 mole percent to 54 mole percent of F6203.

6. The process of producing a magnetic ferrite of the magnesium manganese aluminum iron type which comprises heat reacting together about 133 mole percent colloidal boehmite calculated as A1 0 about 50.0 mole percent magnesium carbonate, about 3.0 mole ercent manganese carbonate, and about 33.7 mole percent Fe O 7. The process of producing a magnetic ferrite of the magnesium manganese aluminum iron type which comprises heat reacting together about one mole percent colloidal boehmite calculated as A1 0 about 49.6 mole percent magnesium carbonate, about 10 mole percent manganese carbonate, and about 39.4 mole percent Fe O 8. The process of producing a magnetic ferrite of the magnesium manganese aluminum iron type which comprises heat reacting together about 6.7 mole percent colloidal boehmite calculated as A1 0 about 49.6 mole percent magnesium carbonate, about 4.5 mole percent manganese carbonate, and about 39.2 mole percent Fe O 9. The process of producing a magnetic ferrite of the yttrium gadolinium aluminum iron type which comprises heat reacting together 24.7 mole percent Y O 12.6 mole percent Gd O 7.7 mole percent colloidal boehmite calculated as A1 0 and 55.0 mole percent Fe O 10. The process of producing a magnetic ferrite of the yttrium aluminum iron type which comprises heat reacting together about 37.5 mole percent Y O 13.75 mole percent colloidal boehmite calculated as A1 0 and 48.75 mole percent Fe O 11. The process of producing magnetic ferrites which comprises heat reacting together ingredients containing about 0.53 to 7.9 mole percent aluminum, 12.9 to 15.7 mole percent magnesium, 0.48 to 4.8 mole percent manganese, and 21.0 to 37.7 mole percent iron, the aluminum being introduced as colloidal boehmite.

12. The process of producing magnetic ferrites which comprises heat reacting together about 1 to 15 mole percent colloidal boehmite calculated as A1 0 21.5 to 26.3 mole percent MgO, 0.6 to 6.6 mole percent Mn O and 30 to 54 mole percent Fe O References Cited UNITED STATES PATENTS 2,915,475 12/1959 Bugosh 23-141 2,981,903 4/1961 Van Uitert 25262.5 3,006,856 10/1961 Calhoun et al. 252-625 3,108,888 10/1963 Bugosh 23141 3,132,105 5/1964 Harrison et al. 252-62.5

TOBIAS E. LEVOW, Primaiy Examiner.

ROBERT D. EDMONDS, Assistant Examiner.