Patent GB-1440968-A (GB)
Title: Laminated magnetically soft material and method of making same
IPC: H01F1/18
Filed: 1973-06-28
Abstract: 1440968 Making transformer and like cores C N WHETSTONE 28 June 1973 30862/73 Heading B3A [Also in Division H1] A magnetically soft laminate, e.g. for transformer cores, magnetic shields, electromagnets and recording heads comprises plural layers of magnetically soft material and at least one continuous layer of an inorganic material having a coefficient of thermal expansion substantially identical with that of the magnetically soft material and a resistivity of > 10<SP>-4</SP> ohm  cmÀ  located between adjacent layers thereof and heat bonded thereto. A laminated billet (Figs. 1, 2) is fabricated from layers 10 of soft magnetic material separated by an insulative compounding material 12 which reacts on heating to form an electrically insulating intermetallic compound with the magnetic material. Plural such layers are sandwiched into an etch resistant casing 16 which is then canned with a mechanically similar filler, capped, and evacuated; after which the can assembly is heated and extruded through a laminar flow die so that layers 10, 12 are coreduced and diffusion bonded. Thereafter the can and filler 18, 20 are etched away leaving the extruded laminate (Figs. 4, 5, not shown). This is mechanically shaped into an ultimate form, e.g. toroidal cores and heat treated to anneal the magnetic material and impart high permeability and low coercive force, and to form insulant intermetallic compounds between the insulative compounding material and the magnetic laminations. The compounding material layers may comprise refractory metals, e.g. Nb, Ta, Zr, Ti, Hf or Va. Mg, Al, Zr, or Cd may also be used. Additional layers may be interposed (Fig. 3, not shown) between the layers 12, 10 to form further intermetallic compounds without modifying the magnetic properties of layers 10, which may, e.g. be of Ni-Fe-Mo alloy. Layers 12 may be of Zr and the sandwich may be surrounded with an etch proof layer 16 of T<SP>i</SP> and placed in a low carbon steel can 18 with a low carbon steel or Cu-Ni alloy filler 20. The can and filler may be removed after extrusion by ferric chloride etch, and square section toroidal cores machined from the stock with subsequent cleansing in HF + PNO and annealing, with subsequent furnace cooling during which the resistive intermetallic compounds are formed. Layers 12 may alternatively be of T<SP>i</SP> between adjacent layers of oxygen free Cu, and Cd may replace T<SP>i</SP>, while Ni may replace Cu. The intermediate metals are diffusable into the magnetic material. A shielded sandwich 145 (Fig. 6) may comprise layers of copper 46 interposed between adjacent layers 48 of T<SP>i</SP> and soft magnetic material 10 fabricated as described; the T<SP>i</SP> forming insulating intermetallic compounds with the copper and the magnetic material, so that both electrostatic and magnetic shielding is provided. The extrusion billet may comprise magnetic sandwiches 14 (Fig. 2) separated by a magnetic/ electrostatic shielding sandwich 14 (Fig. 6) in a composite structure which after extrusion is suitable for the cores of multitrack recording heads. The compounding substances may be deposited by electroplating or electrodeposition; and As or Sb may be deposited on Al foil used as a laminate to separate the magnetic laminates, and after compression and annealing heat treatment they combine to form semiconductor or insulant layers between the magnetic layers and diffusion bonding of the As and Sb layer thereto. In modifications the resistive material per se may be deposited directly on the soft magnetic layers and heated for bonding without interreaction, and may be Se or SiO 2 . Se may be deposited on Pb, Sn, or Hf and CdS, PbS, CdO, ZnS, ZnO, NiO 2 , GeS, SnS may be deposited by plating or vapour deposition. Alternatively the magnetic layers may be coated with glass and heatbonded; the glass being selected so as to have a similar coefficient of thermal expansion to the magnetic material; with subsequent annealing. Layers of fretted magnetic material sheets (Fig. 8) photoetched to define magnetic elements 64, e.g. tape head sections may alternate with sheets of powdered glass formed into tapes with a volatile organic binder; the frets being located by holes 66 engaging pins 68 of platen 70. The assembly is heated in an oxidizing atmosphere to bond the magnetic layers together and compressed by weight 72; after which it is annealed in an evacuated furnace and compressed to a thickness defined by gauge blocks 76. Thereafter the laminate is furnace cooled to below its Curie point, cut into required shapes, ground, and polished. The magnetic material combined with glass may be of Fe-Ni alloy. In further modifications the resistive layers may be of Hf or Mo powder mixed with Se powder or Al powder mixed with Sb or Ag powder and a volatile binder; to form a tape interleaved with magnetic laminae and heated to volatilize the binder and to chemically form a resistive or semi-conductor layer. Mo or Hf may alternatively be reacted with S, and H 2 , O 2  or N 2  may be thermally diffused into the insulant layers to form hydrides, oxides or nitrides of metals. Specifications 1,440,969 and 1,440,970.