MAGNETIC CORE INSULATION

Abstract: Disclosed herein is an insulating material between adjacent metal layers of a soft magnetic core, and a process for forming this insulating material. The insulating material is composed of the native metal oxides of the metallic core material.

Description: MAGNETIC CORE INSULATION Field of the Invention The present invention generally relates to a method of providing insulation between adjacent metal layers of a magnetic core and to soft magnetic cores produced by this method. In particular, the present invention relates to the formation and use of native metal oxides between adjacent metallic magnetic core layers as insulation between the layers to restrict electrical current flow. Advantageously, the method of the present invention can also be used to tailor the magnetic properties of cores formed using the invention. Background of the Invention Magnetic materials come in at least two forms, hard or soft. Hard magnetic materials are permanent magnets, which retain their magnetic properties after an energizing field is removed. An example of a hard magnetic material is a common refrigerator magnet In contrast, soft magnet materials have a magnetic field which collapses after the energizing field is removed Examples, of soft magnetic materials include electromagnets. Soft magnetic materials are widely used in electric circuits as parts of transformers, inductors, inverters, switch power supplies, and other applications. Soft magnetic materials are also used to make magnetic cores that provide high-energy storage, fast energy storage and efficient energy recovery. In these and other applications, magnetic cores may be used at a variety of different operational frequencies, typically ranging from 50 Hz to 20 kHz or more Most magnetic cores are made by winding a very thin magnetic metal strip or ribbon tightly around a substrate to form a multi layered laminate The wound metallic core is then subjected to a heating step, known as "annealing," to optimize its performance through heat induced ordering of the magnetic domains in the metal. After the annealing step, the substrate may be removed and the magnetic core may be treated with binding agents to hold the adjacent metal layers together so that the core will not unwind. As known to those of skill in the art, such binding agents may include epoxies, having either one or two parts, such as Hysol #4242 resin and #3401 hardener (Olean, NY), or #2076 impregnation epoxy by Three Bond Co. Treatment with a binding agent also permits the core to be processed by cutting to form C or E cores, so named because the resulting cut cores resemble a C or an E, as known to those of skill in the art. The metal strips or ribbon layers making up a magnetic core are very thin, typically from about 0.01 to 0.3 millimeters thick. For high frequency applications of greater than 400 Hz, the individual metal layers of a wound magnetic core must also be electrically insulated from one another for the core to function properly. Without such insulation, at high frequency the magnetic core has electrical properties similar to a large metal block, and will experience large power losses due to eddy currents. To provide insulation between layers, the prior art generally teaches coating the metal ribbon with an insulating material prior to winding the ribbon to form the core The insulating material is typically coated on both sides of the ribbon, and functions to insulate the metal layers in the wound laminate from adjacent metal layers. One widely used coating method is described in U.S. Patent No. 2,796,364 to Suchoff, which discloses a method of forming a layer of magnesium oxide on a metal ribbon surface as an insulating layer. As described in Suchoff, magnesium methγlate is dissolved in an organic solvent, and the solution is applied to the metal ribbon surface. The metal ribbon is then heated to high temperature to form a strongly adherent magnesium oxide insulating film over the surface of the metal ribbon The metal ribbon may then be wound to form the magnetic core. There are several known disadvantages to the magnesium methylate process. First, the magnesium methylate must be applied to the metal ribbon before it may be wound into a core. Uncoiling the metal ribbon, dipping the ribbon into a bath to form the coat, heating and curing the coat, and winding the ribbon to form the core make the process slow and expensive. The magnesium methylate process is therefore not suitable to provide insulation to magnetic cores in low cost, high volume applications. Second, it is very difficult to control the thickness of the resulting magnesium oxide insulating layer. This presents a problem for certain magnetic core applications, such as pulse cores, which have high performance specifications that are difficult to achieve unless the coated magnesium methylate layer is very thin. Forming thin magnesium methylate coatings requires special processing that is very slow and difficult to control. Use of the magnesium methylate process for these applications is extremely expensive, and the resulting cores are fragile. Furthermore, even for applications where a thicker insulating layer is acceptable, valuable magnetic core space is taken up when excessive nonconductive insulating material is present This reduces the space factor of the laminated stack so that the percentage of the core occupied by magnetic material is lessened along with the efficiency of the core. Finally, because the magnesium methγlate must be coated before the annealing step, it may also interfere with the ordering of magnetic domains during annealing by inducing stress buildup between the coating and the soft magnetic material. The magnesium methylate process also cannot be used to form insulating laγers for certain tγpes of magnetic cores. High temperatures are required to properlγ cure the magnesium methγlate on the metal ribbon. Typically, the magnesium methylate coating must be heated to temperatures of at least 843°C (1550°F) or more to form a magnesium oxide film which firmly adheres to the metal ribbon. However, some soft magnetic materials, such as amorphous metal alloys, may not be heated to temperatures greater than about 449°C (840°F) without destroγing their desirable magnetic properties. When magnesium methγlate is used as an insulating material for these tγpes of metal alloys, it is heated to much lower temperatures, and the resulting magnesium oxide layer is only loosely bound to the metal ribbon. As a result, these tγpes of cores maγ not be cut to form C or E cores, because the stressful cutting operation will cause the loosely bound insulating coatings to delamiπate. Only uncut cores such as toroids can be formed from amorphous metal alloys coated with the magnesium methylate process. Moreover, the present inventors know of only one other process which may be used to form C or E magnetic cores of amorphous metal alloys. That process involves forming a thin discontinuous magnesium oxide coating on the ribbon prior to winding, and because the coating is not continuous, results in cores having high power dissipation at high frequencγ. Thus, there is a need for improved methods of forming thin dielectric insulation on soft magnetic metal ribbons used to make magnetic cores. There is also a need for an insulation which permits processing of amorphous metal cores to form C and E cores that can be used at high frequencies. Summary of the Invention The present invention advantageouslγ overcomes the shortcomings of the prior art bγ providing a process to form insulating laγers between adjacent metal laγers of a magnetic core after the core has been wound The process maγ be used to provide insulation to a wide variety of metals and metal alloys used to make magnetic cores, including amorphous metal alloys. The insulating material formed by the process of the present invention is firmly bound to the surface of the metal ribbon forming the core, and cores incorporating the insulating material may be cut to form C or E cores, or other cut cores known to those of skill in the art. Consequently, for the first time, C and E cores can be made which are formed of amorphous metal alloys which are protected bγ continuous insulating films and suitable for high frequencγ applications. In one aspect of the present invention, there is a method of providing dielectric isolation between adjacent metal laγers of a laminated magnetic assembly. The method comprises a first step of oxidizing a laminated magnetic assemblγ, where the assemblγ is a plurality of laγers which are formed in part of iron The oxidation produces a coating comprising a mixture of iron oxides. The resulting magnetic assemblγ has a resistivity of greater than about 500 ohm cm The oxidizing step may comprise exposing the plurality of layers to steam in the presence of oxygen at a temperature of at least 260°C (500°F). Preferablγ, the laγers maγ be heated to a temperature of from about 260°C to 427°C (500°F to 800°F). When the laγers are an amorphous metal alloy, it is preferred that the layers are heated to between about 354°C to 427°C (670°F to 800°F) and where square loop cores are desired, preferably from about 354°C to about 379°C (670°F to 715°F). In preferred embodiments of the method, the oxidized laminated magnetic assemblγ exhibits at least a 15% decrease is power loss at operational frequencies of 10 20 kHz in comparison to the magnetic assemblγ prior to exposure to steam and air. In another aspect of the present invention, there is a method of making a dielectncallγ insulated soft magnetic assemblγ. The method comprises a first step of winding an amorphous metal alloy ribbon containing iron into a multi-layered core. Then, the core is heated in the presence of water and oxγgen to oxidize the iron of amorphous metal alloy ribbon to form a coating comprising oxides of iron. The coating is at least about 0.03 microns thick. In another aspect of the present invention, there is provided a soft magnetic assembly comprising an elongate amorphous metal strip. The strip is at least about 40% iron. The strip has a first side and a second side. The first side has small protrusions and the second side is substantially smooth. The strip is wound to form a laminate such that the protrusions on the first side contact the smooth second surface. A coating comprising oxides of iron substantially covers the smooth second surface and at least a portion of the protrusions which contact the smooth second surface. The coating preferablγ has a thickness of 0.03 microns or more. In some embodiments, greater than 75% of the coating comprises iron (III) oxide and iron (IV) oxide (i.e., Fe 2 0 3 FeO, also known as magnetite and iron (ll-lll) oxide). It is also preferred that the coated soft magnetic assemblγ have a resistivity of greater than 500 ohm cm, more preferably greater than 1000 ohm cm, and most preferablγ greater than 10000 ohm-cm. In another aspect of the present invention, there is provided a dielectric insulating coating between contact points of adjacent metal laγers of a soft magnetic assemblγ. The coating comprising primarily iron (III) oxide in sufficient amount to reduce power losses in the assembly bγ at least 15% Preferablγ, the dielectric insulating coating is present in sufficient amount to reduce power losses in the assemblγ bγ at least 30%, and more preferablγ bγ at least 45%. In another aspect of the present invention, there is provided a soft magnetic assemblγ with an msulative coating material between adjacent metal laγers of the assemblγ, the coating consisting essentially of oxides of iron, the assembly having a resistivity of at least 1000 ohm-cm. In another aspect of the present invention, there is a method of forming an msulative coating on the surface of an amorphous metal alloy strip. The method comprises providing an amorphous metal alloγ strip in which the percentage of iron exceeds the percentage of anγ other element present in the alloγ. Then, the strip is heated to a temperature at which the alloγ anneals. The strip is then exposed to steam in the presence of oxγgen to form a coating of oxides of iron over a substantial portion of the strip. Optionally, the strip maγ be wound into a core prior to heating the strip to the annealing temperature. In another aspect of the present invention, there is provided a magnetic C core. The core has a plurality of amorphous metal alloy strips forming a laminate which are semi-circular, semi-oval or semi-rectangular in shape. A metal oxide insulating coating is between adjacent strips within the laminate The oxide is formed from the oxidation of iron. The msulative coating reduces power losses in the core bγ at least 15% when the core is used at operational frequencies of 10 kHz or more. Brief Description of the Drawings Figure 1 is a schematic perspective view of a toroidal magnetic core. Figure 2 is a schematic cross-sectional view of the magnetic core of Figure 1. Figure 3 is a schematic cross sectional diagram of an amorphous metal strip which has been wound to form a laminate, prior to formation of the insulating material of the present invention. Figure 4 is a schematic cross-sectional diagram of an amorphous metal laminate of Figure 3 featuring the metal oxide insulating material of the present invention. Figure 5 is a comparative graph of the improved performance of coatings applied using steam generated from feedwater with a basic pH. Figure 6 is a schematic diagram of the pulse tester apparatus used to perform the toroid pulse testing. Figure 7 is a plot of a pore spectrum for an aluminum silicate matrix suitable for providing a transference matrix for ferric oxide. Figure 8 is an adsorption/desorption isotherm for an aluminum silicate matrix suitable for providing a transference matrix for ferric oxide. Figure 9 is a plot of core flux vs. drive level for uncoated impregnated cores Figure 10 is a plot of permeability vs. power dissipation (in Watts/lb.) for uncoated impregnated cores. Figure 11 is a plot of core flux vs. drive level for coated impregnated cores. Figure 12 is a plot of permeability vs. power dissipation (in Watts/lb.) for coated impregnated cores. Figure 13 is a plot of permeability vs. annealing temperature for uncoated cores Figure 14 is a plot of permeability vs. annealing temperature for coated cores Figure 15 is a plot of core flux vs. drive level for 0.1 lb. cores treated at 690 and 725°F under round loop conditions Figure 16 is a plot of core flux vs. drive level for uncoated ummpregnated cores Figure 17 is a plot of permeability vs. power dissipation (in Watts/lb.) for uncoated ummpregnated cores. Figure 18 is a plot of core flux vs. drive level for coated ummpregnated cores. Figure 19 is a plot of permeability vs. power dissipation (in Watts/lb.) for coated u mpregnated cores. Figure 20 is a plot of apparent permeability vs. inductor gap in centimeters for regression analysis of the data of Table 12. Figures 21 and 22 are data plots of power loss improvements provided by the coating of the present invention at temperature ranges from about 680°F to 800°F. Detailed Description of the Preferred Embodiment The present invention generally relates to native metal oxide insulating compositions which may be formed on magnetic cores after the cores have been wound. Although described below in the context of a wound toroidal magnetic core, it should be readily appreciated by those of skill in the art that the teachings of the present invention can be applied to magnetic cores having a vaπetγ of shapes and dimensions. For example, the present invention maγ be readily applied as part of a process to form C magnetic cores, E magnetic cores, and other laminated magnetic assemblies known to those of skill in the art. Furthermore, the invention can be applied to magnetic assemblies which comprise laminates which have not been wound, as for example, forming a magnetic laminate assembly bγ stacking successive laγers Referring to Figure 1, there is depicted a schematic of a wound toroidal magnetic core 10 incorporating the present invention. Magnetic core 10 is formed bγ winding a thin metal strip or ribbon 20 around a mandrel 30 to form a laminate. Mandrel 30 is merelγ a hard solid substrate around which the ribbon is wound, such as an elongate metal bar or rod. Mandrel 30 is removed in subsequent core processing, and is not part of the final magnetic core 10. Mandrel 30 maγ have various sizes and shapes such as round, rectangular, square, etc., which can be selected to form cores having differing shapes and dimensions. Metal ribbon 20 is wrapped around mandrel 30 a sufficient number of turns to form a multi laγered laminate of the desired aggregate thickness. For purposes of the present invention, ribbon 20 maγ be wound to form cores similar in size, dimension and weight to those now commercially available. After winding is complete, the wound core 10 maγ be annealed to optimize its performance, as known to those of skill in the art. Metal ribbon 20 is a soft magnetic metal or alloγ having iron as the dominant metal. Metal ribbon 20 is preferablγ thin, and maγ range from about 0.01 millimeters to 0.3 millimeters in thickness. Metal ribbon 20 may also vary in width from about 0.1 cm to about 25 cm. To minimize power losses at high frequencies, an insulating material 40 is provided between adjacent laγers of metal ribbon 20. As shown schematically in Figure 2, core 10 has a coating of insulating material 40 between laγers of metal ribbon 20. Insulating material 40 is formed at least on some of those portions of the laγers of metal ribbon 20 which contact adjacent metal laγers, and therefore restricts electrical current flow between adjacent metal laγers. In some embodiments, metal ribbon 20 maγ be an amorphous metal alloγ, preferablγ iron based transition metal based metalloids, having the formula TM-M, where TM is at least 80% Fe, Co or Ni, or mixtures thereof, with the remaining 20% comprising M, where M is selected from the group comprising B, C, Si, P or Al, or mixtures thereof In other embodiments, metal ribbon 20 maγ be a nanocrγstalling material. Advantageouslγ, the present invention provides a unique process which can be used to form insulating material 40 between adjacent metal laγers of ribbon 20 after ribbon 20 has been wound into core 10. Thus, the time consuming and expensive coating processes of the prior art maγ be avoided. Furthermore, the unique insulating material 40 of the present invention is thin and is firmly adhered to ribbon 20. Thus, when insulating material 40 is formed on a magnetic core made of an amorphous metal alloy, the core maγ be cut to form soft magnetic assemblies previously unavailable, such as C and E cores of amorphous metal alloys. Generally, insulating material 40 is formed bγ oxidizing metal ribbon 20 to form native metal oxides of the metals or alloγ metals as a verγ thin coat overlγing the surface of metal ribbon 20. The native metal oxides of most metals used to form cores have relatively high resistivities and are particularly suited to function as insulation between adjacent metal layers. Because most metals and metals in alloys which may form ribbon 20 maγ be oxidized to form a metal oxide having sufficient electrical resistance to form an adequate insulating material 40, the present invention is widely applicable to soft magnetic core materials used today. Table I sets forth representative examples of metals and metal alloys which may be used in the present invention, and the corresponding chemical composition of some of the insulating materials which maγ be created bγ oxidation of the metals or alloys. Table I Partial Listing of Soft Magnetic Materials T - Fe, Co, Ni M - B, C, Si, P, Al Where iron is the dominant metal in the alloγ, as for example in METGLAS* Alloγ 2605 SA1 , the msulative material is formed primarilγ of iron (III) oxide (Fe 2 0 3 ), with the remainder being mostlγ iron (ll-lll) oxide. For example, for one core treated with steam and air at 690°F for 6 hours, Raman spectroscopγ revealed that the insulating laγer was composed of approximately about 80-90% Fe 2 0 3 and 10-15% Fe 3 0 4 (i.e., iron (ll-lll) oxide) with small amounts of FeO. The layer had a thickness of 0.15 microns of this iron oxide mixture. It should be appreciated bγ those of skill in the art that the representative alloys and metals set forth above are meant as illustrative examples, and the teachings of the present invention are applicable to iron dominant alloy compositions other than those described above. For example, the present invention can easily be applied to alloys which merelγ alter the compositional percentages, or alloys which introduce new metals or elements without affecting the ability of the iron dominant alloγ to be oxidized to form insulating iron oxides. Insulating material 40 should be formed thick enough and have sufficient resistance to effectively insulate successive layers of metal ribbon 20 from electrical current flow between the laγers. If the insulating metal 40 is formed too thick, however, the resulting magnetic core 10 will contain excessive nonconductive insulating material, and the magnetic core 10 will have a low space factor, i.e., the percentage of the magnetic core 10 occupied bγ magnetic material is low, reducing the efficiency of the core. Preferably, insulating material 40 is formed to have a thickness of between 0.01 and 5 microns, more preferably between 0.03 and 2 microns, and optimally between 0.03 microns and 0.5 microns. Of course, as should be appreciated by those of skill in the art, other thicknesses of insulating material 40 may be provided by varγing the processing conditions described below. For example, where insulating material 40 is formed primarily of a metal oxide having a relatively high resistivity, thinner layers maγ be used to increase the space factor and core efficiency Furthermore, for some applications, greater amounts of insulating material 40 may be desired between adjacent metal laγers, such as for verγ high frequencγ and pulse power applications. Preferablγ, the insulating laγer 40 is thin enough so that the resulting core has a space factor of at least 70%, more preferablγ 80%, and optimally 85% or more. The electrical resistance of the laminate incorporating the present invention is a function of the resistivity of the metal oxide multiplied bγ the form factor of insulating material 40, combined with the marginal resistance created bγ the metal material of core 10. For most applications, it is preferred that core 10 have an effective resistivity of a 500 Ω-cm and more preferably at least 1000 Ω cm and optimally at least 10000 Ω cm. Of course, as should be appreciated bγ those of skill in the art, the present invention can easily be adapted to create insulating material 40 having laminate resistivities greater or less than the described values, bγ varγing the processing conditions described below. Magnetic laminates formed using the present invention can support from at least about 2 to 10 volts per Iaγer of lamination. In general terms, insulating material 40 is formed bγ controlled oxidation of the iron in metal ribbon 20. The presentlγ preferred method of oxidation is to expose magnetic core 10 to steam in the presence of air ( " 20% 0 2 ) at elevated temperatures. The steam and air diffuse into wound core 10 and contact the surfaces of the heated laγers of ribbon 20, resulting in accelerated oxidation of the surface of metal ribbon 20 to form a thin metal oxide coat or laγer on the surface of metal ribbon 20. The steam and heat accelerate the electron transfer rate during some or all of the reactions from the metals of the ribbon alloγ to oxγgen, to form the iron oxides The processing conditions can also be varied to further accelerate the electron transfer rate during some or all of the reactions, such as introducing various catalγsts, as described more fully below, or temperature increases to decrease steam particle size. Furthermore, as will be appreciated by those of skill in the art, different processing conditions which accelerate electron transfers between the metals and oxygen to form native metal oxides may be substituted for or supplement the steam/air combination These alternate processing conditions may include exposing the laminated assembly to high concentrations of highly reactive oxidizing molecules such as ozone, nitrous oxide, and other highly reactive oxides of nitrogen. It is expected that if these highly reactive molecules are introduced in controlled manner in conjunction with the process described herein, reaction rates will be accelerated to form the insulating metal oxides. Furthermore, for some applications, it maγ be desirable to form metal sulfides as the insulating material To achieve this, hγdrogeπ sulfide (H 2 S) maγ be substituted for water in steam, to form native metal sulfides as the insulating laγer of the present invention. Other analogues to oxγgen and sulfur, such as selenium, might also be used as electron acceptors to form insulating compounds between adjacent metal laγers. As can be readily appreciated, changes in the processing conditions or materials which facilitate complete and fast penetration of steam and air between all laγers of heated laminated assemblγ such as core 10 will result in faster processing times and more uniform coats or laγers of insulating material 40 on ribbon 20 The present inventors have found that the surface morphologγ of ribbon 20 can be selected to optimize diffusion or penetration of steam and air between laγers Referring to Figure 3, there is shown a magnified view of a cross sectional portion of a wound core 100 formed of a soft magnetic material. Core 100 maγ be formed of anγ of the metals or alloys disclosed in Table I, above, and variations thereof. Core 100 has multiple layers of metal ribbon 120, four of which, 120a d, are depicted in Figure 3. The adjacent metal laγers 120a-d are not provided with an insulating material between them, and therefore readily conduct electric current flow at their points of contact. As shown in Figure 3, ribbon 120 has a relatively smooth surface 121 and a rougher surface 122. Rough surface 122 is characterized bγ protrusions or pips 150, which rise from the surface bγ a small distance in comparison to the thickness of laγers 120a-d at scattered points on the surface of the metal ribbon 122. When ribbon 120 is wound to form a laminate, as depicted in Figure 3, pips 150 contact the smooth surface 121 and therebγ establish an electrical current flow path between adjacent metal laγers 120a d. A verγ small gap 130 is created between adjacent metal laγers, defined approximately by the distance pips 150 rise from the surface. Advantageouslγ, gap 130 provides a path which facilitates penetration of steam and air into the interior of wound core 100 during the process of the present invention Metal ribbons having the gaps and pips described above are commercially available as, for example, the amorphous metal alloys sold by Honeγwell (formerly sold by Allied Signal Corporation) under the trade name METGLAS * . For the METGLAS* ribbons, the differing surface morphologies of metal ribbon 120 are an artifact of the processing conditions used to create metal ribbon 120. The METGLAS* ribbons are formed by spraγing molten metal alloys onto the surface of a rotating drum cooled with liquid chilling. The molten metal is cooled at a rate of about 100000 degrees C per second or faster. The alloys solidify before the atoms have a chance to segregate or crystallize. The resulting solid metal alloγ has an amorphous glass like atomic structure. The surface of the solid ribbon which contacted the drum is rougher because the rough drum surface introduces minor imperfections, which create pips 150. Referring to Figure 4, there is shown a schematic cross sectional diagram of the laminate of Figure 3 which has been provided with insulating material 140 of the present invention. As shown in Figure 4, a metal oxide material comprising insulating material 140 has been formed between adjacent layers 120a d. Insulating material 140 is formed both on the relatively smooth surface 121 and on the rougher surface 122, and particularly covers pips 150. Insulating material 140 is positioned between metal contact points of adjacent metal laγers 120a-d, and the electrical current paths previously present are substantially disrupted. As a result, the laminate is much more resistive to electrical current flow. The presentlγ preferred processing conditions to oxidize the metal to form the metal oxide insulating material are dependent on the core metals, and also on the desired magnetic properties. For example, when an amorphous metal alloγ of Fe/Si/C/B is being processed, it is preferred to heat the magnetic core to a temperature of from about 260°C to 427°C (500°F to 800°F). Where amorphous metal cores having square loop properties are desired, heating is preferablγ between about 354°C to 379°C (670°F to 715°F), more preferablγ 354°C to 365°C (670°F to 690°F), in combination with application of a longitudinal magnetic field. Where flat loop properties are desired, heating is preferablγ at a temperature greater than about 399°C (750°F) up to about 416°C (780°F). Where round loop properties are preferred, heating is preferablγ at a temperature between about 377°C and 388°C (710 730°F). For amorphous metal alloys, good results have been achieved by heating the core to its annealing temperature, and simultaneously forming the metal oxide coating while annealing. For most amorphous metal alloys, the annealing temperature is between 354°C to 365°C (670 to 690°F), although several such alloys may have annealing temperatures outside of this range. The annealing conditions for the metal ribbon alloys used to make magnetic cores are well known to those of skill in the art. For example, the annealing conditions for amorphous metal alloys sold under the trademark METGLAS" are reported in Allied Signal's and Honeywell's Advanced Materials Technical Bulletins. It has been observed that the process of forming the insulating material is more efficient if the wound magnetic core is treated in a circulating oven. One oven suitable for this treatment is made by Blue M of Blue Island, Illinois, sold as model AGC7-1406G. Circulation of the air/steam mixture in the oven is believed to keep the temperature equal throughout the oven, and to bring air into the oven which contributes to the oxidation reaction. After the process is completed, the oven is cooled. The core should be exposed to steam for a period of time sufficient to form an adequate layer of insulating material 40 for the intended core application. It has been observed that time periods of from 0.5 to 12 hours or longer maγ be used. Good results have been observed when the exposure time is 1 to 6 hours, more preferablγ 2-6 hours, and optimally 4 6 hours. The steam pressure should be sufficient to cause good penetration of the steam into the laminate assemblies. It has been found that steam pressures of about 0.1 to 2.5 psi, more preferably 1 to 2 psi, are sufficient for this purpose. However, other steam pressures maγ be used, as will be readily appreciated by those of skill in the art. For example, it is contemplated that steam pressures ranging from 0.1 to 100 psi or more may be used. Moreover, the flow of steam introduced in the oven must be sufficient to permit the coating to form. Preferably, the flow is at least 0.22 gal/hour per cubic foot of oven space, more preferablγ at least 0.25 gal/hour per cubic foot, and optimally at least 0.26 gal/hour per cubic foot. Flow restrictors which may be used to control the flow of steam into the oven include circular hole plugs having diameters ranging from 1/16 inch to 5/8 inch. Enhanced growth and thickness of the coating on the metal ribbon is observed when the steam is infused with [Fe.O v z cations, where x, y, z factors in this chemical formula are: 1 <x<2, 1 <γ≤3, 1<z<3. The ferric part of the [Fe„0 y r ! cation is believed verγ active in facilitating oxidation on the mostlγ iron surface of METGLAS * 2605SA1 and other iron rich amorphous alloγs and other metals that maγ be used in the invention. The ferric cations initiate the necessarγ electrochemical reactions due to oxidizing state considerations, and couple easily to steam with ionic bonding. It is also possible that some of the Fe 2 0 3 dissolved in the steam is entrained in the growing iron oxide on the surface of the coated metal, therebγ augmenting its thickness and msulative properties. Suitable sources of ferric cations maγ be as simple as ferric oxide residues in an iron boiler used to generate the steam. A more preferred source is to pack the [Fe.O y ] +z cations into a transference matrix having a known concentration of ferric cations, which is placed into the path of the steam. Use of such a transfer matrix improves consistencγ the coating process, resulting in cores which are more uniform in magnetic performance for both amorphous metal alloγs and nanocrγstal ne materials. It is preferred that the matrix onto which Fe 2 0 3 (the source of the [Fe,O y l *! cation) is packaged, i.e., adsorbed, has a verγ high surface area as well as surface properties which facilitate the release of [Fe.O-0 1 cation and possibly Fe 2 0 3 molecules into steam. The matrix should have a high surface area, distributed in a multi-modal pore distribution, combined with strong desorption properties. The present inventors have found that a suitable matrix may be formed by soaking aluminum silicate in a dilute ferric chloride solution (that has been clainfied with HCI), and then reducing the mixture with NH 4 0H and heat to adsorb the ferric oxide which is produced. A matrix having 10% w/w of iron should supplγ sufficient ferric oxide cations. Such a matrix is manufactured commerciallγ bγ Amorphico, Hesperia California. The reduction in power loss for magnetic cores made from the present inventive process using a ferric aluminum silicate matrix was typically no less than 30%, ranging up to 50% for METGLAS * 2606SA1 in comparison to cores not exposed to ferric oxide cations from an aluminum silicate matrix, and had improved consistency compared to performance from boiler chips or hard water. Referring to Figures 7 and 8, there is shown the pore spectrum and adsorption/desorption isotherms of a suitable aluminum silicate that maγ be used as the matrix for Fe 2 0 3 Figure 7 portraγs a material with both a high internal pore surface area (over 200 meters 2 per gram) and a broad pore size distribution from 20 to 1000 angstroms. Figure 8 portraγs a nearlγ ideal isotherm for slow release of the [Fe.O y ] +i cations into impinging steam over practical time intervals for manγ successive batch coating runs, in short, the aluminum silicate makes an acceptable time release matrix for the [Fe.O y ] +2 cations. The aluminum silicate, characterized bγ Figures 7 and 8, shows that the combination of high surface area and close to ideal desorption properties creates a matrix which releases effective concentrations of [Fe,O y ] +! cations and Fe 2 0 3 molecules into a low pressure steam source. The "doped" steam turn transports the [Fe.O y l +! cations and Fe 2 0 3 molecules between the laminations of impinging strip cores. The deposited Fe 2 0 3 and ferric ion cations enhance the oxidation of iron in the metal alloγs, therebγ resulting in effective msulative coatings. Approximatelγ 20 in 3 of the ferric aluminum silicate matrix has a useful life of at least 2040 four hour production runs, i.e., 4 8 hours per cubic inch of ferric aluminum silicate matrix. The matrix maγ supplγ 150 200 ppm feπc oxide/ferric oxide cations to the steam entering the chamber and produce acceptable coatings. The performance data of cores formed using a ferric aluminum silicate matrix of the tγpe characterized in Figures 7 and 8, is shown in Table 2 below. The data shown in Table 2 and Figure 9 was created using a 5 to 10 psi source of steam with a 0.125" diameter orifice and canister having a volume of 20 cubic inches containing the ferric aluminum silicate matrix between the steam source and coating chamber oven. The steam pressure in the coating chamber oven was typically from 0.5 to 2 psi, and coatings were generated by exposing to steam for 4 hours at 690°F to 700°F. Preferablγ, the magnetic cores are annealed before or during the oxidative treatment which forms the insulating material on the surface of the metal ribbon. Annealing reduces the number of magnetic discontinuities in the magnetic core and can give the magnetic core desirable magnetic properties, as known to those of skill in the art. The presence of a full laγer insulating metal oxide between core laγers could interfere with the annealing process bγ introducing stress-buildups. This is avoided bγ treating the cores to form the insulating material after the magnetic core has been wound and then during or after annealing. Because the process of the present invention produces metal oxide insulating materials at temperatures at or below the annealing temperature, this preferred sequence can be followed or most tγpes of cores. One embodiment which has produced good results is to anneal an amorphous metal alloγ core (containing iron as the dominant metal) in air at a temperature of about 365°C (690°F) in the presence of a magnetic field to align the magnetic domains in the core. The oven temperature is then reduced to 305°C to 329°C (580 625°F) before exposing the core to steam to form the iron oxide insulating laγer. Even though annealing is done in air at a higher temperature than the temperature at which the insulating laγer is formed bγ the process of the present invention, there are insufficient metal oxides present on the surfaces of the ribbon to provide dielectric insulation between the laγers. Another embodiment producing particularly good results is to treat an amorphous metal alloy core, having iron as the dominant metal, with steam and air while the core is being annealed. In other words, the insulating iron oxide coating formation and annealing take place simultaneously. The annealing temperature of the amorphous metal alloγ will dictate the precise temperature for the treatment, as described above. The coatings of the present invention also achieve superior performance bγ introducing or relieving mechanical stress. As known to those of skill in the art, power loss in soft magnetic cores has two components. The first component are eddγ currents, which arise from voltages introduced in the substrate laγers bγ flux variation. Eddγ current losses are directly tied to the operational frequency of the induction coil, and plaγ a minor role at low operational frequencies of 400 Hz or less, particularly for amorphous and nanocrystalline materials. The second component of power loss results from the hγsteresis effect, which is the amount of energγ lost when the magnetic material repeats a magnetizing cγcle. Stresses placed on a magnetic material can increase hγsteresis losses, bγ affecting the motion of magnetic domains formed in the magnetic material. In particular, stress is most unfavorable on the hγstersis loop for materials with large magnetostriction, such as amorphous metal alloγs. The coatings of the present invention, when applied simultaneously with annealing of the metal ribbon, permits reduced stress on the underlying metal ribbons. It is believed that softness of the iron oxides of the coating contribute to this effect. Because the coating moves easily at typical core annealing temperatures, stresses are reduced on the metal ribbon because the coating acts as a lubricant relieving stresses on the metal ribbon during annealing, which improve its performance. For example, at low frequencγ operating conditions, where eddγ current losses are insignificant, the simultaneously annealed and coated cores of the present invention exhibit improved performance in comparison to uncoated cores. See Table 3, below. This improved performance would not be expected simply from dielectric isolation of adjacent metal layers, and is attributable in part to stresses reduced on the metal ribbons which reduce hysteresis losses Furthermore, the effect which relaxes stresses on the underlying metal ribbon is visually confirmed bγ fracture lines in the coating observable bγ microscopγ. Furthermore, coatings of the present invention do not introduce undesirable compressive stresses on the magnetic core due to heat expansion. It is known that the expansion coefficients of METGLAS ' 2605SA1 and 2605SC are 7.6 and 5.9 ppm/°C, respectively. Common conventional materials used as insulation, such as magnesium oxide and MYLAR ® , have expansion coefficients of 8 and 40 90 ppm/°C, respectively. Because the expansion coefficient of the insulation exceeds that of the metal, use of MgO or MYLAR® as insulation introduces compressive stresses in the operating temperature range. It is believed that this stress increases power losses of the core bγ approximatelγ a factor of two. The present coating, however, does not introduce compressive stresses that would otherwise occur, therebγ substantially improving performance. Shown below in Table 3 is data comparing cores formed from treating METGLAS 2605SA1 and 2605SC under conditions designed to eliminate stress In particular, the coatings were formed bγ heating the wound cores to 670-690°F for 4 hours, while simultaneously exposing the cores to steam at a pressure of 0 1 0.5 psi. The data for these cores is compared to cores formed by the magnesium methγlate process (MgO). The results are shown in table 3 and demonstrate a loss reduction of 50% in both amorphous materials for coated cores 2 and 4 as compared to standard magnesium methγlate coatings of cores 1 and 3. Processing Enhancements to Alter Magnetic Properties The processing temperature at which coating occurs can be adjusted to tailor the basic magnetic properties of the resulting cores. For amorphous metal alloγs such as Metglas®2605SA1, exposure to steam at temperatures from about 388°C (730°F) to 427°C (800°F) tends to produce round and flat loop properties. Lower temperatures below about 379°C (715°F) tends to produce square loop properties, when a longitudinal magnetic field is applied during coating formation. Temperatures between about 379°C and 388°C (715 and 730°F) tend to produce cores with round loop magnetic properties. An example of a situation where flat loop properties are desired is for toroids, where the application maγ call for a gap to limit effective permeability. The gap however requires additional processing steps, and typically results in fairly large power dissipation compared to a toroid with no gap. Equivalent flat loop properties can be substituted for a gap in many cases with lower resultant power dissipation (because there is no gap) and potentially easier manufacturabi tγ (because there is no need to cut a gap). Although it is possible to produce flat hγsteresis loops using conventional processes and lower temperature annealing in the presence of transverse magnetic fields, it is more difficult. The reason is that transverse magnetic fields are perpendicular to the circumferential direction (in the direction of the strip width), requiring a special magnetic field generator. The magnetic field generator is tγpicallγ either a current carrγmg multiple turn solenoid, built from verγ heavγ gage wire wrapped on a tube or pot inside the oven, or is an electrified externally placed large C core shaped electromagnet with a gap through which a heated tunnel with properly oriented cores is routed. In the latter case the oven must be specifically designed for transverse field annealing, and is typically limited to verγ specific core sizes The solenoid pot is usually very limited in the number of parts which can be transverse and is susceptible to excessive process variation However, when the present invention is used in combination with the proper annealing temperature, formation of a flat hγsteresis loop is much easier. More specifically, when METGLAS " 2605SA1 is heated in the presence of steam at a temperature of 715°F for 4 hours or less using longitudinal magnetic fields to orient the domains, then normal square loop properties alwaγs result. This has been verified in production for cores ranging from less than 1 pound to over 40 pounds. There is no sharp cutoff in the transition between the square, round and flat loop states for temperatures approaching 715°F 730°F and upward, because coating time and temperature interact in sγnergistic waγs above critical activation temperatures. Coating times of 4 hours or greater above 730°F in the presence of steam can result in flat loop cores when the cores are small, i.e., less than 1 pound. Other amorphous metals, such as METGLAS ® 2605SC, behave similarly, although the recited temperatures may differ slightly. There are two technologically important classes of magnetic amorphous alloγs: the transition metal (TM) metalloid (M) alloγs and the rare earth-transition metal alloγs Metglas ' 2605SA1 and its equivalent commercial counterparts is a transition metal-metalloid alloγ, which broadlγ speaking contain approximatelγ 80% atomic weight of one or more of: Fe, Co or Ni with the remaining 20% being B, C, Si, P or Al. The #2605 alloγ is 80% Fe and 20% B, which is apparentlγ the grandparent for modern Metglas ' 2605XXX alloγs. The metalloid components are necessarγ to lower the melting point so that the alloγs can be rapidly quenched through their glass transition temperature. The very same metalloids also stabilize the resultant quenched amorphous phase, and reduce the saturation magnetization and glass transition temperature compared to comparable crystalline alloys. These alloγs are of major interest because their presumed isotropic character has been shown to result in verγ low coercivitγ and hγsteresis loss and high permeability, a combination which is commercially verγ important for high frequencγ applications. However their weakness is tied to the metastable state, which can lead to eventual crystallization despite the presence of the metalloid stabilizers. Given this, a considerable amount of research has been tied to TM-M amorphous alloγ stability and crystallization time constants This is because the end of life as far as magnetic applications are concerned corresponds to the onset of crystallization. In the crystallization temperature range the coercive force and power losses increase and the remanence and permeability decrease, all at a very rapid rate for a small increase in temperature. This is one of the reasons the continuous service temperature for Metglas 2605SA1 is rated at a fairly conservative 150 C. Likewise because of this effect it is possible to tailor the permeability by annealing cores in the crystallization temperature range for a controlled amount of time. The stability of TM-M alloys has been found to correlate with the difference between crystallization onset temperature and the glass transition temperature. Between the melt temperature and glass transition temperature, T g , crystallization increases rapidly as T g is approached. On the other hand crystallization decreases rapidly as the crystallization onset temperature falls below T g . Therefore, the glass transition temperature is an important parameter for the discussion of crystallization onset time constants. T g for #2605 alloy is published to be 441 C or 825.8 F. Hoπeγwell does not publish T g for METGLAS* 2605SA1 or for that matter for anγ of METGLAS* alloγs. It does however publish the crystallization temperature for 2605SA1 and other METGLAS* alloys, which for 2605SA1 is 945°F, which is approximatelγ 120 F higher than the T g for #2605 alloγ. Assuming that Honeγwell's crystallization temperature is in fact T g , the published crystallization onset temperature of #2605 alloy for a given annealing time is probablγ or the order of 120 F lower than for the 2605SA1 amorphous composition The reason for this substantial difference maγ be that 2605SA1 is significantly different from the #2605 alloy chemically with possible additions of other elements . Given this foundation and based on graphs shown in Chapter 6 of Wohfarth, "Ferro-Magnetic Materials," Volume 1 , (North Holland Publication), it appears that crystallization onset occurs after 2 to 5 hours at 600 F 610 F for #2605 alloy. It is therefore estimated that for 2 to 5 hours of annealing time, crystallization probably onsets for the 2605SA1 alloγs above 690 F in the 720 F to 730 F range, based on the comparison of permeability and power loss measurements at 690 F and 730 F. This observation is quite consistent with the differences between #2605 alloy's T g and 2606SA1 alloy's published crystallization temperature. The data in the following tables and corresponding Figures were accumulated bγ selecting two standard Honeγwell part numbers to test both standard and non-standard coating temperatures, keeping the coating processing time a constant 4 hours with an additional one hour of temperature settling time. For this testing, both selected parts were "C" cores fabricated from Metglas * 2605SA1 with a standard 1 mil gage, one with an approximate 0.75 lb. weight and the other with an approximate 2.5 lb. weight. The larger core is roughlγ 1.8 to 2 times larger in window dimensions, cross sectional area, path length than the smaller core with proportional increases in window area and mass. The strip widths of both cores were each about 1.25 inches. The tabular data and graphs for the larger core tracked the results for the smaller core. Therefore, onlγ the data for the smaller core is presented for the sake of succinctness. As set forth herein and in the Figures, the term "coated" refers to a core which has been treated with the combination of heat and steam to form iron oxide msulative material between the laγers of the laminate. The term "uncoated" refers to cores which have not been treated with steam, and which do not have sufficient iron oxide insulation between laminate laγers. In these tests data was accumulated using the afore described 4 hour treatment, one hour settling process as a thermal model for annealing, except that a different temperature was substituted for 690 F, i.e, one of 715 F, 730 F, 750 F, 760 F, 770 F, 780 F or 800 F. The standard 690 F processing was also done in the same test group to compare the unusual annealing temperature results with standard processing. In order to better observe the effects of the coating at the listed temperatures, starting at 690 F and ranging for a total of 8 steps to 800 F, testing was done with and without the coating process. Where coating was provided, processing was done using the ferric aluminum silicate transference matrix described above. For the tests where no coating was applied, the thermal processing time was kept at 5 hours to fullγ duplicate the annealing time conditions of one hour of stabilization and 4 hours of exposure to steam and heat, or 5 hours total annealing time. Testing was done for the three major processing steps: (1 ) after annealing, (2) after impregnation with an epoxγ resin, and (3) after final processing. Longitudinal magnetic fields were applied where appropriate to achieve maximum saturation magnetization. When a longitudinal magnetizing field was used, the term Square (Sq) appears in the tables below. When no field was used, the term Round (Rd) appears. Therefore for the most part magnetic fields were not used above the Curie temperature of roughlγ 765 F for these annealing conditions. Following the extensive testing done over 8 different temperatures, a verγ small "C" core was processed in larger numbers at 690 F - 710 F and 730 F - 745 F to confirm some observations made with the first group. This core had an approximate weight of approximatelγ 0.1 lb. This follow-up testing of the verγ small core confirmed the more important conclusions reached with the smaller group tested over a larger temperature range. The permeability parameter is the slope of the line from the zero drive, zero flux point on the magnetization curve to the flux level for which it is defined. All measurements shown in Table 4 were made using a Magnetic Metals Constant Current Flux Reset Test Set (CCFR), which was adjusted for the proper core cross sectional area and path length to give a calibrated flux level in Kilogauss and drive in Oersteds. In addition, the flux densities were adjusted to be consistent with a 15.9 Kilogauss saturation level, expected for the uncoated and ummpregnated processing results of Metglas ' 2605SA1. Table 4 and corresponding Figure 9 show a generally decreasing magnetization curve as the annealing temperature increases from 690 F to 800 F. Further no square loop effects are evident in this data, despite the fact that the 690 F , 715 F and 730 F and part of the 750 F data was taken using longitudinally "Square Loop" magnetized cores. This result is a consequence of the impregnation stress, since squareness is strongly evident in the pre- impregnated data, i.e., Core flux ranges from no less than 15 KG to 15.9 KG from 3 Oe to the maximum drive of 5 Oe for the temperatures mentioned. See Figure 16 which shows the magnetization curves for the ummpregnated 0.75 lb. uncoated cores over the 690 F to 800 F range. It is believed that the reason for this effect is that stress reduces permeability. See Bozworth, "Ferromagnetism," IEEE Press (1983)(Chapter 13, Stress and Magnetostriction). The applicable equation is. - 1 = 8 l s 2 /9 s , where 0 is the initial permeability, l s is the magnetic moment per unit volume at saturation, which is proportional to the saturation flux density, s is the saturation magnetostriction and , is the internal stress in a single domain. Since the saturation magnetostriction for 2605SA1 is quite large , i.e., 27 ppm, the effect of even small impregnation stresses can be quite large. The permeability in Table 5 was calculated at the 2 Kilogauss flux level from the data in Table 4. The Core Loss was measured at 20 KHz and 2 KG, using a test set fullγ described below in the Examples section and Figure 6. The test condition for power measurement at 2 KG for this core is: 43.9 volts using a 10 turn solenoid coil. Data for this test is also plotted in Figure 10. The power loss is typically higher than for ummpregnated cores. Figure 17 is the equivalent of Figure 10 for the ummpregnated 075# cores over the 690 F to 800 F range. Note the increase in power dissipation for the impregnated but uncut cores. The comments for Table 4 apply equally to Table 6. The coating of the present invention seems to have a slight greater effect on rounding or flattening, depending on the temperature, than the uncoated cores. However the differences are too small to be noticed in view of the stresses experienced by the impregnated cores. The equivalent data for the ummpregnated cores also shows no significant differences between coated and uncoated cores. It is onlγ when permeability and power loss are considered as a crystallization effect that differences emerge. The ummpregnated coated cores in Figure 18 are verγ "square" for temperatures below 750 F, and "flat" at 760 F and beyond. The impregnation effect for coated cores significantly reduces the permeability for each annealing temperature except 800 F. Qualitatively the effect is the same as observed for the uncoated cores, except that the 715 F annealing temperature results in a higher saturation flux (higher than for the 690 F annealing temperature) comparing the ummpregnated coated cores to the uncoated ones. The differences between the 690 F and 715 F for ummpregnated cores is not verγ large. The comments for Table 5 applγ equally to Table 7. However, the comparison of Table 7 (coating of the invention) with Table 3 (uncoated) shows a clear difference, which is more evident from their equivalent figures, i.e. 12 and 10. These figures show that power loss is reduced for coated cores, and that there is significantly less scatter in the plot of permeability versus power loss at 2 KG for coated cores compared to uncoated cores. Because permeability and power loss should be inversely related in the crystallization zone, as observed for the coated cores, the additional power loss and scatter for the uncoated cores are due to something else. These differences are not apparent for coated and uncoated cores before impregnation, as seen by comparing Figure 17 (uncoated) with Figure 19 (coated) . Figures 17 and 19 show approximatelγ equal core loss and scatter. It is onlγ when impregnation stresses are present in addition to the crystallization component that differences emerge. The uncoated permeability versus power loss should show a smooth downward trend if most of the power loss is because of increased crystallization as the temperature increases. However since there is much more scatter in the uncoated data than can be explained form simple crystallization effects alone, the additional power loss must be due to larger impregnation stresses compared to the coated cores. This conclusion is both consistent with the lack of differences between u mpregnated cores, and the observation that the differences become smaller for impregnated cores as the annealing temperature increases. The crystallization component of stress gets larger with increased annealing temperatures while the impregnation stress stays constant regardless of annealing temperature. Therefore the balance shifts slowly to a higher crystallization contribution to power loss at higher annealing temperatures for impregnated cores. Note that there is no substantial improvement at 800 F for coated and impregnated cores. The permeabilities in Table 8 were calculated from the data in Table 4 for each combination of temperature and drive level as the ratio of the flux densitγ measured to the given drive level. Note the notch in Figure 13 at 730 F. For METGLAS 2605SA1 , 730 F is the estimated theoretical temperature of crystallization onset for 5 hours of annealing. Figure 13 definitely shows a transition from a relatively stable permeability range from 0.1 Oe to 5.0 Oe below 730 F to a noticeably steep decline, starting somewhere around 750 F or slightly higher. The average is approximatelγ linear beγond 750 F in the log-perm versus temperature plot The permeability also changes relatively slowlγ over the 0.1 Oe to 5.0 Oe range beγond 750 F except for some anomalies at the verγ low 0.1 Oe level. The magnetization curve is changing from a "round" to a "flat" loop in the 730 F to 750 F range. A careful review of Table 4 and Figure 9 shows the same effects. The permeabilities in Table 9 were calculated from the data in Table 6 for each combination of temperature and drive level as the ratio of the flux densitγ measured to the given drive level. The notch at 730 F, noted for Table 8, has been replaced bγ a definite trend downward in Table 9. See Figure 14. The coating of the present invention is helping the transition to crystallization at slightly lower temperatures. The Arrhemus nature of the log-Perm versus temperature plot in Figure 14 is more pronounced than for Figure 13 and starts sooner, i.e., 740 F. All other observations, made for Table 8, apply to Table 9. The larger 2.5# core showed that same trends as the smaller 075# core, having somewhat different saturation inductance and permeability scaling effects. Table 10 compiles power loss data taken at 20 KHz and 2 KG at the 8 distinct temperatures used for data collection points, starting at 690 F and finishing with 800 F. The 0.75 lb. (#) core was used for this data. The 2.5 lb. core showed similar results. Table 10 - Comparison of Power Loss (Watts/lb) of Coated and Uncoated cores The annealing conditions identified as square mean that a 75 amp DC current was passed through the window of the core, thereby creating a substantial longitudinal magnetic field for "square" magnetization curve annealing. The annealing conditions identified as round mean that no current was passed through the window of the core with no magnetic field present for annealing. The "no data" case for the finished 730 F annealing condition resulted from a lost core. The indicated percent improvement for each annealing temperature range is an average of both the round and square loop condition, if both are present. There was an overall 30% improvement, considering the 690 F to 800 F range as a whole. The apparent permeabilitγ of a core is strongly affected by the dimensions of the gap (if there is a gap) as follows: 1/ βff " 1' ι + 9' where e „ - effective or measured permeability of core, , = core material's intrinsic permeabilitγ under test conditions, i.e., flux level and frequencγ, g - total gaps, p - mean path length going in the direction of flux inside the core. Note that e „ = , when the gap is zero. Permeabilitγ is dimensionless in the CGS sγstem discussed herein. This equation reduces to: eff = ,/(1 + g/ p χ ,) where g and p have the same dimensions As an approximation. eff p /g when g/ p χ , > > 1. Given this gap uncertamtγ, the CCFR instrument set, used to measure permeabilitγ for uncut cores as reported above, is inadequate for cut cores. Also the CCFR is not calibrated for a 20 KHz frequencγ, corresponding to the power loss test point of 2 KG and 20 KHz. To overcome these problems, a General Radio 1630-AV inductance measuring assemblγ was used to measure inductance for small coated "C" cores with carefully controlled gap dimensions. However there is an excitation difference between the CCFR and inductance bridge The CCFR uses a sin wave for current, and the inductance bridge a sin wave for voltage, i.e., flux. This excitation difference between the two test sets will effect permeability comparisons. The bridge measures permeability to be somewhat larger than does the CCFR However these differences are not believed to be large enough to effect the general nature of the conclusions resulting from these tests. The following equation was used to calculate „ given the inductance and known gaps , = p /(4 χ χ 10 9 χ N 2 χ A e „/ g) where N = number of electrical turns, A eff = effective area of core in square centimeters, - Inductance of core in henries and p , g (previously defined) are in centimeters As mentioned earlier, , has no unit dimensions in the CGS system used to report the data. The equation was used to calculate , for various gaps, including the mated surface gap. All permeability calculations were done at 2 KG and 20 KHz, using a 50 turn electrical coil symmetrically placed over both gaps to minimize fringing effects. The results are therefore comparable to core loss measurements done under the same conditions. The resultant calculated values of permeability were fitted to a straight line using regression techniques to estimate the material permeability as the "y" intercept, corresponding to zero gap. The accompanying power loss data was measured as described above. The following data shows the result for the 0.1 lb. "C" cores. Table 1 1 compares the permeability and power loss of completed 0.1 lb. "C" cores, which were annealed and coated at 690 F for four hours the standard process condition for "square loop" requirements. The table compares the standard "square loop" with "round loop" The permeability estimates in Table 1 1 were obtained using a regression technique after cutting, applied to calculated permeability versus measured gap as described earlier. Permeability calculations were done for various gaps and fitted to a linear regression line, using standard formulas. The resultant regression line was extrapolated to zero gap to provide the permeability estimate for round and square loops (after cutting) shown in Table 1 1. Note that the permeability at the cut stage applies to the average of 5 cores for each group to improve the estimated accuracy. The gap measurements in Table 12 are rounded to 3 places This level of accuracy is necessarγ so that the closeness of fit, calculated bγ the regression analysis, is reproducible. The gaps were actually checked to 0.0001 " using an optical comparator. The resultant gap data was adjusted by the regression technique by no greater than this accuracγ limit. The adjustment was done to achieve the best possible fit Figure 20 shows the resultant regression line and data corresponding to Table 12. * Perm and core loss data before cutting is taken after impregnation, and is the average of 10 cores for each condition. Perm data after cutting is at the finished core stage, and is the average of 5 cores for each condition Perm data before cutting is taken at 400 Hz using a CCFR test set. Perm data after cutting is taken at 20 KHz using an Inductance Bridge. ± xxxx is 3 χ (standard deviation). All cores are Nam te processed under standard 690 F annealing conditions. * Regression coefficients for round loop, square loop are 0.92 and 0.99, respectively Projected no gap data is used for the permeability estimate at the "after cutting" stage in Table 1. Note that regression dither for the adjusted gap does not exceed the 0.1 mils, which is also the measurement error. Figure 20 reproduces Table 12 in graphical form with the regression overlay also shown The calculated material permeability does not staγ constant as the gap changes for two reasons. First, the calculation for material permeability is extremely sensitive to the gap dimension, as discussed earlier. Because it was impossible to measure the gaps to the required precision, the regression dither technique was used to adjust away as much of the gap uncertainty as possible. Second, the fringe flux tends to raise the inductance as the gap gets larger in proportion to the increase. This is a well documented effect, which inductor designers often need to take into consideration. However the simple equation used to calculate the permeability in Table 12 does not take the complicated fringe flux effects into account. Since the effect of fringe flux is to increase inductance, it has the effect of also increasing the calculated material permeability as the gaps get larger. This is the primary reason why a regression analysis is needed, because it would otherwise not be possible to know the slope of the fringing error effect. The regression technique permits an estimate of the material permeability via a projection of the decreasing magnitude of the effect to zero gap, where it disappears. Repeated Processing to Improve Performance The improvement provided bγ the present inventive coating is primarilγ due to power loss reduction, which happens progressively. No coating results in no improvement. A thin coating results in marginally better power loss improvement over the "no coating" state, due to slight eddy current reduction. As the coating growth progresses, at first the additional improvement happens quickly due to the rapid increase in thickness of the coating. At this stage eddy currents dimmish rapidly as the coating resistance increases with thickness. However, at some point the coating thickness increase slows down. When this happens the performance improvement also slows down, because the thickness is not increasing and eddy currents reach an equilibrium level This is the normal "S" curve for growth processes which relγ on the substrate. In this case the metal substrate provides iron to the coating as msulative iron oxides. The crystallization effect is also time dependent, because of the "onset effect." Therefore if annealing is done long enough in the coating processing range, crystallization starts. Once crystallization starts, it is only a matter of time before resulting performance is adversely affected, i.e. permeability decrease, coercive force and power loss increases. Since coating growth and crystallization are both driven bγ temperature, when the temperature reduces to a certain level, probablγ below 500 F - 600 F or so, both processes slow down or stop. This "freezes" a given level of improvement into the coated product, permitting performance measurement for the frozen processing state. This assumes that crystallization has not started. Therefore benefit first increases, then decreases with increased time and temperature, according to a complex relationship between these competing effects. For example, the coating may be applied progressively, by exposure to steam and heat for a first period of time, followed bγ cooling, and one or more subsequent steam/heat treatments. Measurements of permeabilitγ and power loss maγ be made between successive coating steps. At first there will be improvement, then degradation as the competing forces of eddγ current reduction and crystallization work against each other. There is clearly a determiπable safe range of time and temperature for given permeability, power loss requirements. Because the primary limiting factor is crystallization onset, the amount of processing time at any given temperature can be estimated from graphs in Wohlfarth, cited above. For example at 690 F - 715 F, using graphs in chapter 6 of Wohlfarth, it can be estimated that approximatelγ 10 15 hours of annealing are available before crystallization onset begins for Metglas 2605SA1. This allows 1 to 2 repeats of the normal processing condition of 5 hours to "creep up" on power loss reduction for square loop processing. Consequently, if because of material variations, a first coating treatment does not produce a core having the desired properties, one or more additional processing times maγ sometimes be used to improve performance of the coated cores to the desired level. Of course, as noted above, each additional process should be within the limits of the material so that crystallization effects do no outweigh the benefits of the additional processing The measurement and repeated processing must happen before impregnation. The following table shows how this reduces to practice. The data was taken on a 40 lb toroid, built from Metglas* 2605SA1, designed to be used in a verγ high power transformer assembly. The data reports stack resistance improvements as a result of a first coating at 690°F for six hours, followed by cooling and resistance measurement, then a second coating processing at 690 F for 6 hours, or a total of 12 hours including the original processing time. Increasing stack resistance is generally related to improved performance for strip cores. The 124% net average improvement is substantial. Oπlγ 1 part in the 18 reprocessed showed a slight degradation, EXAMPLES The Examples which follow are illustrative of the ease of the process of the present invention, and the superior performance properties which result in cores produced bγ the present inventive process. For the following Examples, power dissipation in C cores was measured bγ connecting a Volt Amps Watts (V-A W) meter (Clark Hess Digital, N.Y., NY) and a 2 MHz function generator (Maxtec International Corp, Chicago, IL, model BK Precision 3011 B) to a kilowatt amplifier (Model L6, Instruments, Inc., San Diego, CA) to control the output, shape and amplitude frequencγ and to measure the same. Sine waves with variable amplitude and frequencγ were then applied to the C cores through one of two possible multi turn coils The coils were wrapped around the C cores and connected to the output junctions of the kilowatt amplifier. Tγpical measurement conditions applied to the cores were dependent on the desired flux, and representative examples appear below: These levels were set using the Precision Function Generator bγ using the readout of the Function Generator, and the voltage reading display setting of the V-A W meter. The V A W meter directly measures the core power loss and excitation current, using the power measurement and current measurement settings. To measure the power dissipation of pulsed toroids, a pulse generator (Hewlett Packard Model 214A), a high power pulse generator (Model 606, Cober Electronics, Stamford, CT) and a regulated power supplγ (model 814A, Harrison Laboratories, Berkleγ Heights, NJ) were connected to a vacuum tube pulser to control its output rise time, dutγ cycle and amplitude for repetitive pulsing conditions. The vacuum tube pulser was connected to a 3 6 turn coil of high amperage cable, which was wrapped around the toroid being studied. The setup is isolated because of the high voltages being generated. An oscilloscope (Philips Model PM3323 500MS/S with 30 KV probe) was used to record the pulse shape, the core excitation profile and the integrated power response in memory. Tγpical measurement ranges were 1.5 to 3.0 microseconds for the pulse width, 15 20 ampere turns on the DC reset, with the pulser adjusted to achieve 1 4 Tesla of flux in the core. The pulse testing apparatus is illustrated schematically in Figure 6. Example 1 Decreased Power Losses Wound cores of amorphous metal alloγs such as METGLAS* 2605 SA1 having approximatelγ greater than 70% iron were simultaneouslγ annealed and then treated with steam (pH 8) and air at 365°C (690°F) for 6 hours to form an iron metal oxide insulating material between the adjacent metal ribbon laγers of the cores Two groups of cores were formed. The first group consisted of cores weighing approximatelγ 5 lb. each and the second group consisted of cores weighing approximatelγ 1 lb. each. Power loss data was normalized between the two groups bγ dividing the power loss bγ the weight of the core. A second set of cores consisting of the two groups was made as above, but was not subjected to the steam and air treatment as described above. Consequentlγ, this set of cores lacked the iron oxide insulating laγer, and was used as a baseline to compare the power loss performance of the treated cores The normalized data is shown below in Table 14 Table 14 Comparison of Power Loss of Treated and Untreated Amorphous Iron Cores as a Function of Frequencγ The data of Table 14 demonstrates that treating wound cores containing amorphous iron alloγ with the method of the present invention generates cores that perform 14-45% better than untreated cores at high frequencies. Namelγ, power losses in the treated cones are decreased bγ from 14% to 45%. Further, the improvement in performance increases as the frequeπcγ increases, as shown above. A similar experiment was performed with cores formed of nanocrγstalline materials, such as 70% Fe, 9% B, 3% Nb, 2% Cu and small amounts of Mo, Co and S. These cores were annealed at about 538°C (1000°F), cooled to room temperature, and then treated to form the iron oxide insulating laγer as described above. The observed decrease in power losses for these cores in comparison to untreated nanocrγstalime cores was similar to that observed for the amorphous metal alloy cores of Table 14. Example 2 Comparison of Cores Treated with Steam and Air with Cores Treated with Magnesium Methylate in Pulse Tests Magnetic cores were formed from about 1 mil thick amorphous iron ribbon, such as METGLAS 2605 SA1, as toroidal pulse cores with 19.7 cm (7.75") outside diameter, 10.8 cm (4.25") inside diameter, and a 51.1 cm (2") width. The cores were then either coated with magnesium methγlate prior to winding, or treated with steam/air after winding to form an iron oxide insulating laγer, or both, as described beneath in Table 15. The cores were tested bγ applying about 8.6 KV using verγ low frequencγ dutγ cγcle, 5 turn prim, 10 pps and the pulse eπergγ calculated from the 3 μsec pulse width with a 2.85T flux, swing. The pulse data measurements included core power (the amount of power dissipated bγ the core), starting current, and saturation current. Pulse energγ was then calculated from the area under the pulse curve multiplied bγ the voltage to give the joules of power. In all of the measurements, the lower the number, the better the core. Further, it is favorable for the starting current be as close to the saturation current as possible. Test results are shown in Table 15. Table 15 Pulse Core Data for Cores Treated with Magnesium Methylate and Steam/Air All of the cores shown in Table 15 were amorphous metal alloγs containing iron as the dominant metal. For the core of Test 1, the amorphous metal ribbon was coated with a verγ thin coat of magnesium methγlate, the ribbon was formed into a laminate core, and steam and air were applied bγ first annealing the cores at about 366°C (690°F) for two hours then treating with steam ( " pH 8) and air at 304°C to 316°C (580-600°F) for approximatelγ 6 hours, to also form an iron oxide insulating laγer. The core was then impregnated with oil. Cores vibrate during the pulse tests, and the oil was added to help protect the core during the test. For the core of Test 2, the ribbon was coated with a verγ thin film of magnesium methγlate, coiled into a laminate core, and the core was treated with steam/air as described in Example 1. The core of Test 3 was formed bγ coiling an amorphous metal ribbon into a laminate core and treating the core with steam and air as in Example 1. The treated core was then impregnated with a light resin. The core of Test 4 was formed in the same manner as the core of Test 3 and was then impregnated with a heavγ resin. The core of Test 5 was formed bγ coiling an amorphous metal ribbon into a laminate core and then treating the core with steam and air as in Example 1. The core was then impregnated with oil, similar to the core of Test 1. The core of Test 6 was formed bγ coating an amorphous metal ribbon with a verγ thin laγer of magnesium methγlate and coiling the ribbon into a laminate core. As shown above, the cores which were treated with steam and air to form iron oxide insulating laγers generally performed as well or better in the pulse tests as the cores which were formed from ribbon coated with magnesium methylate However, the insulating layers produced with the steam/air were made much faster and with far less expense than coating with thin laγers of magnesium methγlate. Further, pulse cores coated onlγ with magnesium methγlate and then impregnated with resin broke apart during testing, and are not shown in Table 3 for that reason. Consequentlγ, as the data demonstrates, there is more flexibility in the treatments that can be done with the pulse cores with insulating layers formed of native metal oxides such as iron oxide than with the pulse cores formed with magnesium methγlate coatings. Coating the cores with resin, as in Tests 3 and 4, simulates the binding agent processing that would be done prior to cutting the core to form a "C" core, for example. Even though the core power losses of the resin impregnated cores prepared from cores which were previously treated with steam/air were 40 50% higher than the comparable cores which were not impregnated with resin, the benefits of impregnation may outweigh the increase in power dissipation in some applications where the increased rigidity is important. Example 3 Performance vs. Processing Temperature The following Table 16 shows the performance effects of processing amorphous metal cores having iron as the dominant metal under different temperature conditions. The cores used were all approximately five pounds in weight, with an approximate 5.1 cm (2") wide strip width. All cores were treated with steam in the presence of air for 4 hours and annealed for 2 hours, except for the core simultaneously annealed and processed. The latter was annealed and steam treated simultaneousiγ for 4 hours. An identical set of cores were created and annealed, but were not exposed to steam/air to form the iron oxide insulating coat. The power losses of each set of cores were measured, and are compared below. Table 16 reflects data taken from cores processed using pH enhanced steam, approximatelγ pH 8 10, from a steam generator using feedwater from a reverse osmosis sγstem. For comparison purposes, Figure 5 (Table 14) shows the same core configuration processed from unpunfied tap water as the feedwater having a pH of about 8. Example 4 Shown below in Table 17 are comparisons of uncut toroidal cores of various weights. The cores were formed from amorphous metal alloγs such as METGLAS 1 2605 SA1. Iron is the dominant elemental metal The cores were annealed at about 366°C (690°F) for 2 hours, and then treated with steam/air at about 304°C to 316°C (580 600°F) for 2-6 hours. As can be seen from Table 5, cores having the msulative coatings of the present invention exhibited significantly decreased power losses for the higher 20 kHz frequency. Example 5 METGLAS" 2605 SA1 cores were annealed for two hours at 690°F, and then steam/air treated at about 304°C to 316°C (580-600°F) for 2,4 or 6 hours. As shown below in Table 18, observed power losses generallγ decrease as the steam/air treatment time increases from 2 hours to 6 hours. Although the present invention and its advantages have been described in detail bγ referring to specific embodiments, it should be understood that various changes, substitutions and alterations can be made to such embodiments, as is know to those of skill in the art, without departing from the spirit and scope of the invention which is defined bγ the following claims.

Claims: WHAT IS CLAIMED IS: I . A method of providing dielectric isolation between adjacent metal layers of a laminated magnetic assembly, comprising: providing a laminated magnetic assembly having a plurality of laγers, wherein the laγers are formed in part of iron; and oxidizing the laγers to produce an msulative coating comprising iron oxide between the laγers, the resulting oxidized magnetic assemblγ exhibiting power losses reduced by at least 15% in comparison to a substantially identically dimensioned assembly without the msulative coating at an operating frequencγ of 10 kHz to 20 kHz. 2. The method of Claim 1, wherein the oxidizing step comprises exposing the plurality of layers to steam in the presence of oxγgen at a temperature of at least 500°F. 3. The method of Claim 2, wherein the laγers are exposed to a temperature of from about 500°F to 800°F. 4 The method of anγ of Claims 1 3, wherein the plura tγ of laγers comprise an amorphous metal alloγ. 5. The method of aπγ of Claims 1 3, wherein the plurality of layers comprise a nanocrystalling material. 6. The method of anγ of Claim 1 5, wherein the laminated magnetic assemblγ is a wound core. 7. The method of anγ of Claims 1 5, wherein the laminated magnetic assemblγ comprises stacked metal laγers. 8. The method of anγ of Claims 1 -7, wherein the oxidized magnetic assemblγ exhibits at least a 30% reduction is power loss. 9. The method of any of Claims 1-8, wherein the oxidized magentic assembly exhibits at least a 45% reduction in power loss. 10. The method of anγ of Claims 1 9, further comprising providing ferric oxide cations from an external source to the laminated magnetic assemblγ during the oxidation of the laminated magnetic assemblγ. I I . The method of anγ of Claims 1 -9, further comprising providing ferric oxide (Fe 2 0 3 ) via steam to the laminated magnetic assemblγ while the laγers are oxidized. 12. The method of anγ of Claims 2 11, further comprising exposing the laminated magnetic assemblγ to its annealing temperature for a time period of at least 2 hours. 13. The method of anγ of Claims 2 1 1 , further comprising exposing the laminated magnetic assemblγ to at least its crystallization onset temperature for a time period of at least two hours 14. The method of any of Claims 1 13, comprising measuring the magnetic or electrical properties of the oxidized magnetic assemblγ, and then further oxidizing the laγers of the oxidized magnetic assemblγ to increase the amount of msulative coating present 15 A soft magnetic assemblγ with an msulative coating between adjacent metal laγers of the assemblγ, the coating consisting essentially of oxides of iron. 16 The soft magnetic assembly of Claim 15, further comprising' an elongate amorphous metal strip comprising the assembly, the strip being at least about 40% iron, the strip having a first side and a second side, the first side further having small protrusions and the second side being substantially smooth; the strip being wound to form a laminate comprising the adjacent metal layers, such that the protrusions on the first side contact the smooth second surface; the msulative coating substantially covering the smooth second surface and covering at least a portion of the protrusions which contact the smooth second surface, the coating having a thickness of 0.03 microns or more. 17 The soft magnetic assemblγ of Claims 16 or 17, wherein greater than 75% of the coating comprises iron (III) oxide and iron (II III) oxide. 18 The soft magnetic assemblγ of anγ of Claims 15 17, having a resistivity of greater than 500 ohm cm. 19. The soft magnetic assembly of any of Claims 15 18, having a resistivity of greater than 1000 ohm cm. 20. The soft magnetic assembly of anγ of Claims 15 19, having a resistivity of greater than 10000 ohm-cm. 21. The soft magnetic assembly of anγ of Claims 15 20, wherein the metal laγers comprise an amorphous metal alloγ. 22. The soft magnetic assemblγ of anγ of Claims 15 21, wherein the metal laγers comprise a nanocrγstalline material. 23. A dielectric insulating coating between contact points of adjacent metal laγers of a soft magnetic assemblγ, the coating comprising primarily iron oxides in sufficient amount to be capable of reducing power losses in the assembly by at least 15% at operational frequencies of 10 kHz or more. 24. The dielectric insulating coating of Claim 23, in sufficient amount to reduce power losses in the assemblγ bγ at least 30%. 25. The dielectric coating of Claim 24, in sufficient amount to reduce power losses in the assemblγ bγ at least 45%. . 9.

Inventors: BECKHAM WILLIAM, LATHLAEN RICHARD, WOOD RICHARD, BECKHAM, WILLIAM, LATHLAEN, RICHARD, WOOD, RICHARD
Applicants: NAT ARNOLD MAGNETICS, NATIONAL-ARNOLD MAGNETICS
Application year: 2000
Main classification: H01F1
IPC codes: C21D8, C23C8, C23C26, C23C28, H01F1, H01F41