US2354331A - High-frequency ferroinductor - Google Patents

Description

July 25, 1944. w. J. POLYDOROFF FREQUENCY FERRO-INDUCTOR Filed May 5, 1941 F is. 1

Patented July 25, 1944 SW55: PATENT 7 Claims.

The invention relates to high frequency fixed or variable inductors of the type employing compressed powdered magnetic cores, said inductors being particularly useful at the higher audio and radio frequencies, and possessing high permeability with low losses.

The practice of constructing present day coils and cores for the above frequencies is well established and consists generally of individually insulating particles of small dimensions, according to the frequency, and compressing said particles with suitablebinders into solid bodies in the form of magnetic cores for the coils. The cores of such construction are enerally uniform in their magnetic and electrical characteristics regardless of the direction in which they are placed in the coil, as distinguished from the magnetic structures and coils used in lower frequencies where laminated magnetic sheets are bundled together in a definite relationship advantageous to their performance in the coils.

One object of this invention is to produce ferromagnetic inductors with cores of high efiective permeability and of low loss characteristics. Another object of this invention is to reduce the sizes and the amount of materials employed in the production of the inductors.

Still another object of this invention is to arrange the proportions so as to improve the operating characteristics of the inductors.

Another object of the invention is to provide a magnetic core for high frequency inductors formed of material comprising compressed flat metallic particles arranged with their flat surfaces forming substantially parallel planes and which core material will have a relatively high permeability and low loss characteristics in a direction parallel to the stratified planes and will have a much lower permeability in a direction normal to said stratified planes.

The invention will be better understood if reference is made to the accompanying drawing in which Fig. 1 represents a diagram of permeabilities obtainable by various powders and methods employed in construction of compressed magnetic cores. Fig. 2 shows a torroidol coil of the shape in which said measurements are made. Fig. 3 shows a core of thepresent invention. Figs. 4 and 4a show a variable inductor in accordance with the invention. Figs. 5 and 5a show a modification of the invention shown in Fig. 4, and Fig. 6 shows another form of .variable inductor.

The present invention is directed to the employment of metallic magnetic powders such as ner.

pure iron, iron nickel alloy, or other metallic alloys of high permeability.

Referring now to Fig. 1, the values of apparent permeability are plotted against the density of the ultimate core material. The above values were obtained as follows:

Three difierent mixtures of metallic powders, already insulated and containing a small amount of binder were pressed into the rings at various pressures, and after completion of such rings their density was calculated in the usual man- The toroidal rings were then wound with a single layer winding I5, Figure 2, and the inductance of each inductor measured. The in ductance of a toroidal air core can be very accurately calculated. The ratio between the measured inductance and the calculated air core inductance represents the effective permeability, which after correction for the difference in cross section between the coil and the coreis converted into apparent permeability. It is evident from the above curves which are substantially a straight line, that the permeability is proportionate to the density of the core material. Curve a represents the permeability obtainable with the iron powder produced by decomposition of carbonyl. It is known that the particles of this iron are of spherical form and because of the voids between the spheres the maximum density obtainable does not exceed 5.0 with a corresponding permeability of 12. The curve b represents the results obtainable with iron powder reduced from iron oxides by hydrogen, the particles being of the order of 1-30 microns. The maximum permeability obtainable is of the order of 22 at the density of 6.0. The curve 0 represents the permeability obtainable with electrolytic 'iron having substantially the same dimensions as the iron reduced by hydrogen, with the exception that the iron particles from the process of electrolytic deposition come in the form of thin flakes of minimum dimension of l3 microns and maximum dimensions (across the flat surface) of the order of 30 microns. The mold employed for the pressing of said cores was provided with a'tubular cavity into which the mixtures were loaded and the plunger compressed the mixtures from the top under pressures up to 50 tons per square inch. It has been ascertained that the particles of electrolytic iron, when loaded in the mold arrange themselves in a substantially horizontal plane, and after compression the cores examined under the microscope showed a structure mainly composed of numerous layers [6, Figure 2 of flat particles lying in a plane transverse to the principal axis of the toroid.

It has also been observed that due to the particular particle distribution relatively small pressures were required to obtain high densities. The permeabilities, as shownin Fig. l by curve are considerably higher than those obtainable with the hydrogen reduced iron, having substantially the same dimensions and at the samendensities. This unexpected increase of permeability was found to be due to the particular alignment of particles and was further verified by the powdered material shown on the curve d which represents the same hydrogen reduced iron powder as per curve b, the powder being placed for a considerable time in a ball mill as the result of which operation the particles were visibly fiattened. Curve d indicates much higher permeabilities than corresponding curve 1), although it does not reach the values obtainable on curve 0. This phenomenon of increase of permeability was further explored by pressing the cores in the form of rectangular bars, such as shown in Fig. 3. In forming this bar indicated by numeral l1 pressure was applied in a direction transversely to the longitudinal axis of the bar. A small rectangular piece I8 of square cross-section was cut from the bar, as shown by the dotted lines of Fig. 3, and this piece was measured with a search coil in two directions, (1) in the direction in which the particle alignment indicated by numeral 20 was parallel to the axis of the coil, and (2), in the direction in which the particle surfaces were perpendicular to said axis. In both cases, the same amount of magnetic materials were present in the field of the coil. When the coil was placed into an oscillatory circuit at 1,000 k. c. considerable differences in permeability and in the Q(wL/R) of the coil were observed. A marked increase of permeability and Q exists if the core piece is placed so as to have its stratified particles aligned in a direction parallel to the axis of the coil. It was also found that the normal insulating coatings deposited on the particles withstood high pressures very well between their flat surfaces, having high resistivity in this direction, and very much lower resistivity in the direction of alignment.

The invention, therefore, consists in preparing fiat metallic particles of extreme thinness and of reasonably large lengths across their surface, insulating said particles by the usual method, adding necessary binders in very small quantities, and then loading the mixture into the molds. It is desirable to mechanically vibrate the molds to facilitate the alignment of the flat particles so as to secure a stratified formation of the particles in the final core. The fiat particles are caused to align in parallel planes and which planes will be parallel to the magnetic axis of the coil to be associated with the core. When pressure is applied to the stratified core material the same is compressed to its required high density and since this pressure is in a transverse direction, that is, normal to said magnetic axis, the pressure further aids the Stratification. Suitable coils may then be placed on the completed cores so that the axis of the core material and the magnetic axis of the coils coincide.

Compared with high frequency ferro-inductors made according to present practices, the inductors produced in accordance with this invention possessed a much higher value of inductance per turn, and highly efiicient characteristics because of increased permeability of the core material,

One application of the invention is in toroidal. coils for high frequency loading coils and the like. in which, because of extreme thinness of the particles and high permeability, high inductance to resistance ratio is obtainable.

The rectangular bar core as shown on Fig. 3 is particularly suitable for fixed inductors of large value and of the open type or. if closed types are desired, the cores may be pressed into the usual L, E, or U shapes to complete the magnetic circuit with regard to the coil field.

A further increase in permeability is possible if the fiat shaped particles are insulated with a heat resisting binder such as water glass, kaolin and other suitable chemicals. The cores then are pressed in accordance with the inntion without any binder. A minute wetting of the mixture will help to retain the shape after extraction of the body from the mold. The body is then placed in a vessel containing a neutral or reducing atmosphere or in vacuum. Electrodes may be fixed to the ends of the core so that a current may be sent through the body in the direction of alignment of particles, which direction already oii'ers lesser resistance. The current may then be increased and further regulated until the body reaches its sintering temperature and maintained at that temperature for a certain length of time until the sintering is completed. The sintering may also be obtained by placing the vessel, containing the body into the field of an induction furnace in the direction of particle surfaces parallel to the field. Likewise, at a certain elevated temperature the sintering will occur, substantially in the desired direction, 1'. e., in the direction of an elongated core such as shown in Figure 3, thev electrodes are placed in contact with the respectlve ends of the bar I1 and the flow of current in a longitudinal direction is regulated so as to produce the desired high temperature. As a result the flat metallic particles in the various stratified planes are fused to each other at their edges. The cores when removed will exhibit a strength comparable with the solid metals, yet their magnetic properties are considerably increased in the di- 'rection of the magnetic axis, which, according to the invention, is parallel to the stratified planes in which the flat particles are arranged. However, the core losses, due to insulation, in a direction transverse to said magnetic axis, remain relatively low. Therefore it will be appreciated that cores produced accordingto the invention will have a relatively high apparent permeability in a direction parallel to the stratified planes and a lower permeability in a direction normal thereto.

In all cases, when loading the flattened particles into a mold it is preferable to subject the mold to mechanical vibrations or to a magnetic field in order to insure better initial alignment.

Another'application of the invention considerably simplifies the construction of variable inductors in which inductance variations are obtained'through the movement of magnetic cores in and out of the coil. As an example of such construction, a semi-toroidal coil I is shown in Fig. 4,

wound on an insulated tubing 2- and having its outside diameter of approximately 1%", the main diameter of the toroid being approximately 1%"..

, ther increase the inductance variations the core may be composed of additional core pieces 4 also in the form of a semi-ring which is stationary and closes the magnetic circuit when the moveable central part is moved inside of the coil as shown in Fig. 4a.

.Another construction to which the present invention is applicable is in a solenoidal coil with a cylindrical core moveable in and out. In order to. obtain the necessary inductance variations of the order of 10 to 1 very long solenoids are now being employed having their length to diameter ratio of the order of 6 with a correspondingly larger ratio in the case of cylindrical cores. The "consideration of the weight of the iron and of the working stroke of the core led to such constructions in which coils of extremely small diameter were employed, such coils having inherent poor electrical characteristics. Due to the considerable increase in the material permeability effected by the practice of this invention, it is now possible to produce cylindrical cores whose length to the diameter ratio is of the order of 4 or less and to employ solenoidal coils whose length to diameter ratio is of the order of three, so that the Q of the coil can be considerably improved. Such a coil and core are shown in Fig 5. The coil 5 is wound on a thin walled tube 6 of I. D. with a single layer or multi-layer winding of suitable Litz wire I, the lengths of the coil being less than 1%". Such a coil when employed for variable inductance with the core 8 withdrawn at a frequency of 1700 k. c. exhibits Q of the orderlof 150, as compared with the coils of the above mentioned designs whose Q does not exceed 70.

The cylindrical core suitable for the coil such as shown in Figure 5, has an outside diameter slightly under and a total length of 1 /2". This cylindrical core, in accordance with the invention, is made of thin fiat particles having a maximum dimension across their fiat surface of 30 microns and a minimum thickness of 1 micron, the particles being aligned in the mold so as to 3 A variable inductor in accordance with the invention may take the form shown in Fig. 6. A coil 10 is wound on a bobbin II and the core I! is rotatable inside the bobbin around the shaft I3. The core is cut out of the bar H, as already described so that at two positions at 90 the permeability is materially different. The rotation of the core by an external means produces a variable inductance suitable for periodic inductance changes at high frequency, the period depending upon the speed of the rotation of the core piece, which may additionally beof round disk shape.

What I claim is:

1. A method of making a compressed magnetic core of the character described, which consists in applying insulation to flat, metallic particles, loading said insulated particles into a mold and causing said flat particles to form in stratifled planes, applying pressure to the core material in a direction normal to said stratifled planes to compress the material to the required high density, removing the core, and then subjecting it to heat to sinter the particles in the various stratifled planes, whereby the flat particles in the various strata are fused to each other at their edges.

2. A method of making a compressed magnetic core as defined by claim 1, wherein the heat for sintering the flat particles is generated by causing an electric current to flow through the core in a direction parallel to said stratifled planes.

3'. A high frequency ferro-magnetic inductor including a solenoidal coil and a cylindrical core, s" id core comprising compressed metallic particies disposed with their'flat surfaces forming a stratify the particles forming them into parallel planes, and which planes will also be parallel to the magnetic axis of the coil. The pressure applied to compress the core material is in a direc tion transverse to the said magnetic axis. Such a core when moved into the coil will increase the inductance of the same approximately ten times,

which corresponds to the frequency range for the, I

coil, namely, 540-1700 k. c. It is evident that if a smaller frequency ratio is required the coil and core length may be reduced, or for improved Q both coil and core may be increased in diameter while retaining the same axial dimensions of the variable inductor. The above coil and the core when used with a'fixed capacitor across the coil form a circuit which may be tuned in a highly efficient, manner. If the coil is constructed with 20 strands of #44 wire the above mentioned Q of 150 will result at 1700 k. c. and similar frequencies and the Q may drop down to 120 at intermediatefrequencies within the frequency range when being moved in the coil. The advantage of a short stroke thus obtainable with high efiiciency will be appreciated in the construction of the mechanical movement for such device. It is possible to further decrease the stroke or the proportions of the coil and core by adding to the cylindrical core at one end of the core a small rin as shown in Fig, 5a by 9. This ring may be made in accordance with the invention, i. e., with the particle alignment as shown in the toroidal core, such ring extending the frequency variation of the device without increasing its stroke when the core is completely moved into the coil.

p urality of stratifled planes with insulation between the strata and having said particles in the various stratified planes fused to each other at. t eir edges, and said core havingassociation with the coil so that the magnetic axis of the coil is in a direction parallel to the stratifled formations of said particles.

4. A term-magnetic inductor including a toroid coil and an anular magnetic core, said core comprising compressed fiat metallic particles of magnetic nature having a thickness of approximately several microns and having maximum dimensions across their flat surfaces of approximately 10 to 30 microns, the particles of said core being arranged with their flat surfaces forming a plurality of stratifled planes with insulation between the various strata and having said particles in the various strata sintered to each other at their edges, whereby the core has a higher permeability in a direction parallel to the stratified formation of said fiat metallic particles than in a transverse direction.

5. A high frequency ferro-magnetic inductor including a coil and a magnetic core, said core being composed of comminuted material compressed to the desired high density, said material comprising particles of magnetic nature arranged in a manner forming a plurality of stratified planes, said Stratification resulting from the application of pressure perpendicularly to said planes, insulation between said planes, and saidv metallic. particles in the various planes being sinteredto each other at their edges, whereby the core has a higher permeability in a direction parallel to the stratified formations than in a trans verse direction. and said core having association with the coil so that the magnetic axis of the coil coincides withlthe axis of higher permeability of said core. 4

6. Amagnetic core composed of comminuted said material comprising particles of magnetic 1o nature arranged in a manner forming-a plurality of stratified planes, said stratification resulting from the application of pressure in a direction normal to said planes, insulation between said planes, and said particles in the various strat ed planes being sintered to each other at their edg s, whereby the core has a high permeability in a direction parallel to the stratified planes.

WLADIMIR J. POLYDOROFI".

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