DE3422281A1 - Process for manufacturing mouldings from magnetic metal alloys, and mouldings thus produced - Google Patents

Abstract

Ferromagnetic glassy metal powder having a particle size in the range from approximately 2 mu m to approximately 2 mm is compacted by thermomechanical processes. The resulting compacted mouldings can be heat-treated in order to improve their magnetic properties. The compacted bodies show excellent ferromagnetic properties, such as low coercive force, high permeability and low alternating-current core loss.

Description

■ _4- "" "342228t

Process for the production of Formun

genes made of magnetic metal alloys and molded products made in this way

'■ The invention relates to magnetic objects made of magnetic cores, There are pole pieces and the like, and a method for producing the same from metal glass powder.

Amorphous metal alloys and articles made therefrom are described by Chen and PoIk in US Pat. No. 3,856,513. This patent describes certain new metal alloy compositions which can be obtained in the amorphous state and which are better than previously known crystalline alloys based on the same metals. These compositions are easily quenched into amorphous state and have desirable physical properties. It was too already described that powder of such amorphous metals with particle sizes in the range of about 10 μπι to 250 μπι obtained can be by atomizing the molten alloy to form droplets thereof and the Quenching droplets in a liquid such as water, chilled saline, or liquid nitrogen.

The manufacture of magnetic objects by densifying permalloy and other crystalline alloy powders is known. New applications that require improved magnetic properties have made efforts necessary to develop alloys and to develop compacting or compaction processes, which at the same time increase the strength and the magnetic response of magnetic objects.

The present invention provides amorphous metal alloy powders that are particularly useful for densification or compaction or pressing into bodies with excellent strength and magnetic response are suitable. Also supplies the invention a process for the production of magnetic objects, in which a compression or compaction

vitreous metal powder using mechanical pressure and / or a binder at elevated temperature takes place below the crystallization temperature of the material. This invention also teaches the optimal sizes and preferred alloy compositions of the powders, suitable binder materials and heat treatments for Post processing.

Articles made by the method of the invention have low core loss and high permeability. Typically such have compacted or cortipacted magnetic glassy metal alloys have an initial relative magnetic permeability of at least about 70 at a frequency of 5 kHz and an induction of 0.1 Tesla. The term "relative permeability" used here shall be the ratio of the magnetic induction generated by a certain field in a medium to the mean magnetic induction generated by the same field in vacuum.

The high permeability magnetic compact bodies of the present invention are generally made of vitreous or amorphous metal alloys in powder form. The general process for making metal glass powders of alloys involves a rapid quenching of molten alloys and a subsequent step Pulverization stage. A vitreous alloy can be made by using the methods disclosed in U.S. Patent No. 3,856,553 method described is followed. The resulting sheets, ribbons, strips, and wires are useful precursors for that materials described here. These amorphous materials are pulverized as follows: An amorphous one Material in the form of a sheet, ribbon, wire or flakes is above its embrittlement temperature, but heated below its crystallization temperature and mechanically crushed. The resulting flakes and Powders of different sizes are sieved or otherwise classified to give flakes and powders of desired sizes

342228 *

to collect. Alternative methods to glassy metal alloy powder to . are described in U.S. Patent 4,290,808.

Starting with the powder, densification or compaction of the powder is the initial stage in the manufacture of a Body. Powder suitable for compaction or pressing can be fine powder (with a particle size in the range from about 2 to 100 μΐη), coarse powder (with particle sizes between 100 μΐη and 1000 μπι) and flakes (with particle sizes between 1000 μπι and about 2 mm) include. These Flakes and powders are hereinafter referred to simply as powder or powder particles. Compaction or compaction can be obtained by pressing glassy metal alloy powders or with a binder binds.

In the event that low permeabilities are desired, a particle diameter of about 5 to 10 μm is used. For high permeabilities, larger particle diameters of about 180 μm or more are used. A combination of relatively high permeability (e.g. on the order of about 200 to 5 kHz and at 0.1 Tesla induction) and excellent mechanical hardness (e.g. on the order of about 400 kg / mm ² ) is achieved by using particles with a mesh size (US Standard Sieve Series) of about 80. Flake cores use larger particles with parallel planes. In this case, the properties come closer to those of lamellar cores.

Powders can be evacuated for compaction or contacting Cans included and in desired shapes at temperatures below the glass transition temperature of the alloy rolled or hot isostatically pressed. In addition, powders can be below their glass transition temperature in a vacuum, in air or in other protective gas atmospheres in any desired form according to conventional methods hot pressed. The powders are preferably

_ * 7 -

pressed at a pressure of at least 7 MPa at a temperature between 85% and 95% of the glass transition temperature. Poetry Pressed amorphous bricks close to the theoretical maximum can be made with high pressures and temperatures with short pressing times can be achieved. The effects of time, temperature and pressure on the density of the compacted body are in Table I shown. When the temperature to the glass transition temperature is increased and the pressure is increased, the relative 'density increases. The remaining time can be in seconds can be minimized by having high pressure and a temperature used near the glass transition temperature.

Table I.

Relationship between pressing time, temperature,
Pressure and density for vitreous alloy Fe _7R B, _Si "-powder * of +80 meshes,
to an annular body with a
Outside diameter of 4.15 cm and one
Deformed inner diameter of 2.25 cm

Pressing time Pressing temperature pressure Density** (min) ( ⁰ C) (MPa) 78.7 10 400 520 80.4 20th 400 520 70.9 30th 400 210 77.2 30th 400 310 80.2 30th 400 420 85.2 30th 400 520 90.6 30th 425 52 0

* Powder size larger than 80 mesh
** 100% corresponds to 7.2 g / cm ³

The powders can be mixed with a suitable organic binder such as paraffin, etc., and then mixed cold pressed into suitable shapes. As an insulator and

'"" _B _ 34222Si

As the binder, resins such as phenol-formaldehyde resins such as Bakelite (trade name of Union Carbide Corporation) are used. Other suitable binders include synthetic resins, drying oils, residues from the distillation of oils or fats, solutions of vegetable gums or resins and oxidized oil and wax compounds. Certain oxides such as SiO ₂ , MgO and B 3 O ₃ can be mixed with powders, and the mixtures can be compacted under pressure at elevated temperatures below the glass transition temperature or crystallization temperature of the powders. Certain acids such as borate (boric acid) can be mixed with powders, and the mixtures can be compacted or densified under pressure at elevated temperatures below the glass transition or crystallization temperature of the powders. In this case the acids decompose during the compaction to form certain oxides, such as B "0" in the case of borate, and water, which evaporates during the compaction. The amount of binder can be up to 30% by weight and is preferably less than 10% by weight and more preferably between 0.1 and 4% by weight for cores with high permeability. Such a shaped alloy can have a density of at least 60% of the theoretical maximum. The pressed article can be cured at a relatively low temperature below the glass transition temperature to provide greater strength and then ground to final dimensions. The preferred product of this process comprises moldings useful as magnetic parts.

The hardening process can be carried out with the simultaneous application of a Magnetic field are carried out. Preferably the curing process is carried out in the absence of oxygen. The methods are adapted to the optimal heat treatment cycles in such a way that a desired magnetic and structural product made of vitreous metal alloy.

After compacting or compacting, the end product becomes

ground to final dimensions. This method is suitable for producing large machine tools with a simple geometry. In addition, the finished product may optionally vary depending on the particular alloy used in the case Application was used to be annealed. The solid body has a density of not less than about 60% and preferably 95% of the alloy in the state as it was in the molding process comes from.

A metal glass is an alloy melt product that has been cooled to a rigid state without crystallization. Such metal glasses generally have at least some of the following properties: high hardness and scratch resistance, great smoothness of a glass-like surface, dimensional and dimensional stability, mechanical rigidity, strength, ductility, high electrical resistance compared with corresponding metals and alloys thereof, excellent magnetic softness and a diffuse X-ray diffraction pattern.

As used herein in the conventional sense, the term "alloy" means a solid mixture of two ors more metals (Condensed Chemical Dictionary, 9th Edition, Van Norstrand Reinhold Co., New York, 1977). These alloys also contain at least one non-metallic element mixed in. The terms "glassy metal alloy", When used herein, "metal glass" and "amorphous metal alloy" are all equivalent to one another.

Alloys suitable for the process according to the present invention are, for example, those of the composition MM ¹ , Z, in which M is at least one of the elements Fe, Ni and Co, M ^{1 is} at least one of the elements Cr, Mo, W, V, Nb, Ta, Ti, Zr and Hf, Z is at least one of the elements B, Si, C and P, "a", "b" and "c" are the atomic percent, where a + b + c = 100 and a in the range of about 65 to 88, b in the range from about 0 to 7 and c in the range from about 12 to 28.

Preferred ferromagnetic alloys according to the present invention are based on iron, cobalt and / or nickel. The iron-based alloys have the general composition Fe _40- Og (Co and / or Ni) _0-40 (Cr, Mo, W, V, Nb, Ta, Ti, -Zr and / or Hf) _Q __ (B, Si , C and / or ^p ) i ₂ -28 ' ^where ^ ^e subscripts denote the atomic percentages. The cobalt-based alloys have the general composition Co _40-88 (Fe and / or Ni) _{0 - 40} (Cr, Mo, W, V, Nb, Ta, Ti, Zr and / or Hf) q_7 (B, Si, C and / or ^p ) i2_28 ' ^where i ^d i ^e subscripts denote the atomic percentages. The nickel-based alloys have the general composition Ni _40-68 (Co and / or Fe) _20-40 (Cr, Mo, W, V, Nb, Ta, Ti, Zr and / or Hf) ___ (B, Si, C and / or ^p) ι 9-28 ^'w ° kei the subscripts represent the atomic percentages.
15th

Preferred alloys have atomic percentages of less than 5 atomic percent carbon, 25 atomic percent boron, 20 atomic percent silicon and 10 atomic percent phosphorus.

During compaction, each powder shows different Degree of external stress, which in turn changes the magnetic properties of the powder when it is magnetostrictive is. So it is desirable to use powders with low saturation magnetostriction to compact. This is particularly applicable to compacted materials for use with high frequencies. A saturation magnetostriction λ is related to a change in length Δα, / ä, which in a magnetic material occurs when it goes from the demagnetized to the saturated ferromagnetic state transforms. The magnetostriction value, a dimensionless quantity, is often given in micro-deformation units (i.e., a unit of micro-deformation is a change in length of 1 ppm). The quantity λ can either be positive or be negative and depends on the material, whose soft magnetic properties are strongly influenced by this size will. The preferred absolute value of λ for the powders

_ c S-

is below about 10 χ 10 (or 10 ppm), which is the case with the most of the vitreous alloy powders based on cobalt and

"™ _" "■" '342228t

Nickel-based and, in the case of the vitreous iron-based alloy powders, with about 40 atomic percent iron and less than about 2 atom% of at least one element from the group Cr, Mo, W, V, Nb, Ta, Ti, Zr and / or Hf and between about 40 and 88 atom% iron with more than 2 atom% at least one of the elements from the group Cr, Mo, W, V, Nb, Ta, Ti, Zr and / or Hf can be realized. The most preferred value for λ for the powders is close to zero or zero, whatever in the case of the vitreous alloy powders based on cobalt with a ratio of the iron content to the cobalt content in the Range between about 0.03 to about 0.13 can be achieved. If magnetostriction is not an important factor, As with most low frequency applications (i.e. in the 50/60 Hz range), these considerations may be unnecessary be.

Amorphous metal powders can be compacted to make parts suitable for a variety of applications, such as electromagnetic cores, pole pieces and the same. The glassy metal alloys have high permeability. You can use a lot less nickel than conventional ones contain pressed alloy bodies of comparable permeability. The resulting cores can be used as transformer cores and used in other AC applications.

The following examples are provided to aid in a more complete understanding of the invention. The special methods, conditions, Materials, proportions and parameters that are given to explain the principles and implementation are exemplary only and should not be construed as limiting the spirit of the invention.

example 1
35

An amorphous metal powder with particle sizes less than a few millimeters from an alloy with the composition Fe B Si was produced. These powders were

* 342228a

sifts to classify them into different sizes. The sieved powders became a ring shape with an inner diameter of 3.2 and an outer diameter of 4.2 cm and compacted with a height of about 0.7 cm. The compaction temperature and the compaction pressure were about 350 ° C and 345 MPa, respectively. The effects of the size of the powder on the Magnetic properties of ring bodies in the as-molded state and in the annealed state are summarized in Table II. This table illustrates that larger powders tend to give better magnetic properties and that the powder sizes between 180 μΐη and 1.4 mm most are preferred in order to obtain the best overall magnetic properties. For example, have annealed ring bodies from powders with sizes in the range of about 80 μm and 1.4 mm Permeabilities at 5 and 50 kHz of more than 200 and 70, respectively.

Table II

_{Particle size and magnetic properties of ring bodies which were compacted from a powder with the composition Fe 7ft} B, -, Sip at 350 ° C. for 5 hours with a pressure of about 345 MPa. The inner diameter, outer diameter and height of the ring bodies are 3.2, 4.2 and 0.7 cm, respectively. 25th

30th 180 μΐη CD (At the) CD Tesla) permeability 5 kHz at 0 , 1 Tesla Particle ^ 1.4 mm Coercive AT Remanence AT at AT - at 50 kHz size 180 μπ \ force (10 ~ ⁴ 'AF AF AT AF ^ 500 μπι AF ' 87 AF 670 235 35 <_ 180 μm 180 55 75 1 75 μΐη 115 50 325 700 200 38 pounds around 140 40 56 77 89 200 355 160 118 170 95 92 30th 60 107 96 120 620 70 200 24 30th 136 84 70 30th 50 117 450

AF: like shaped

AN: annealed at 400 ° C. for 2 hours with a direct current field of 1600 A / m applied along the circumference of the ring body
5

Example 2

It should be noted that the permeability μ (±) of the ring bodies in Table II decreases with increasing frequency f.

This is a consequence of the eddy current loss resulting from the electrical conductivity between the powder particles, which is considerable since the powder particles are not electrically isolated. Eddy current loss in a metallic ferromagnet generally increases with frequency, material size and its electrical conductivity. For example, reducing the particle size and the electrical conductivity between the powder particles would reduce the eddy current loss. The effects of 2% by weight SiO ,,, with Fe, _O B, Si _n powders of different parts

Z / o Xj y

Small sizes mixed to increase the electrical resistance between the particles (the inverse of the conductivity) for the magnetic properties of compacted or densified cores are summarized in Table III. For the core made of powders larger than 180 μm, 7.5% by weight of borate was also used in order to see the effects of different insulation. Borate is a boric acid that decomposes to form boron oxide, which is an excellent binder and electrical insulation between the powder particles. As the results of Table III show, SiO ₂ or boron oxide-induced insulation is indeed effective in improving magnetic properties at high frequency. However, the results that larger powder sizes lead to magnetically better properties are surprising. For example, the permeability μ at 50 kHz and 0.1 T induction for the core, which was made from isolated powder particles with a size of more than 180 μπι, about 160 in comparison with a μ (50 kHz, 0.1 T) = 42 for those who, from isolated powder

particles with a size of less than 38 μπι produced became. It is therefore preferred that the size of the isolated powder is also greater than about 180 μm in order to have good magnetic properties To obtain properties in compacted cores according to the invention.

Table III

Effects of electrical insulation between the powder particles on the mag

Netic properties of the ring bodies that were manufactured under conditions which are identical to those of Table II

are.
15th

Permeability at 0.1 Tesla powder size Type of insulation f = 5 kHz f = 50 kHz

130 160

130 75

65 42

A number of cores were made from powders of various sizes containing either 2% by weight SiO or 7.5% by weight borate was isolated. The areas of magnetic properties that can be found in the annealed cores Powders with a size. of less than 75 μπι and from those obtained with a size between 180 μm and 1.4 mm are listed in Table IV. The overall characteristics of the Cores made from larger powders are better than those made from small

j> 180 μm 7, 5 wt. -% borate 360 20 _> 180 μm 2 Weight -% SiO ₂ 340 180 μm ^ 500 μπα 2 Weight -% SiO ₂ 210 12 5 μm ^ 180 μπι 2 Weight -% SiO ₂ ¹ 130 25 63 µπα * 12 5 μm 2 Weight -% SiO ₂ 110 £ 38 μm 2 Weight -% SiO ₂ 72 Example 3

ren powders. It should be noted that a core loss (L) is so as low as 10 W / kg and a permeability (μ) as high as 1000 at 5 kHz and 0.1 T in the iron-based powder cores can be obtained with a saturation induction of about 1.3 Tesla. These values are to be compared with μ (5 kHz, 0.1 T) of about 1000 and L of about 5 W / kg of a commercially available Ni-Zn ferrite with a saturation induction of about about 0.5 Tesla.

Table IV

Ranges of magnetic properties available for cores _{compacted from Fe 78} B. Sig powder at 350 ° C for 5 hours with a pressure of about 345 "MPa. The powder particles were either 2% by weight SiO ₂ or 7.5% by weight borate isolated The cores were annealed for 2 hours at 400 ° C. with a direct current field of about 1600 A / m, which was applied along the circumference of annular bodies with the dimensions given in the heading of Table II.

"Direct current

Coercive force properties at 5 kHz and 0.1 T powder size H (A / m) permeability core loss

<_ 75 μm 30 - 150 90 - 500 20 - 100

180 μm-1.4 mm 40-120 100-1000 20-40 30th

Example 4

The performance of a ferromagnet should very much be seen by the magnitude of its magnetostriction (λ), a magnetomechanism Effect, to be influenced. The quantity λ carries additional magnetic anisotropy energy through the internal stresses a. When magnetic powders are compacted, each powder is put under different degrees of stress.

3422231.

If the powder is magnetostrictive, this external tension in turn increases and changes the coercive force remanent magnetization and thus influences the AC properties the kernels. Table V shows the relationship between the magnetostriction value of the powder material and the AC properties of cores compacted from the powders of similar sizes. This table clearly shows that powder cores of lower magnetostriction have better magnetic properties. Low Thus, magnetostriction of less than about 10 10 is preferred, and the most preferred value of λ is close to zero for the best overall magnetic properties of the compacted cores of the present invention to obtain.

Table V

Effects of magnetostriction of powder materials on the properties of compacted ones Cores made from powders with particle sizes between 180 μm and 1.4 mm. The compacting conditions are in under the heading of Table II.

Magneto-permeability Composition of the striktion (at 5 kHz core loss Powder material

^Fe 81.5 ^B 13.5 ^Si 3.5 ^C 2 Fe ₇₈ B ₁₃ Si ₁

^{30 Fe} 40 ^Ni 38 ^M ° 4 ^B 18 ^C ° 72.2 ^Fe 5.8 ^Mo 2 ^B 15 ^Si 5

Example 5

To the magnetic properties of the compacted kernels according to the invention from low magnetostrictive powders listed in Table V, were further improved 4% by weight SiO and 4% by weight MgO added to the

λ (10 ~ ⁶ ) and 0.1 Tesla) (W / kg) 30th 300 76 30th 220 70 9 650 22nd ^ O 800 19th

342228t

To electrically isolate powder particles. The results are shown in Table VI. If the values in the Tables V and VI are compared, it can be clearly seen

'that electrically isolated low magnetostrictive powder cores have the best overall magnetic properties. The permeability (μ) at at 5 kHz and 0.1 Tesla from 800 to is comparable to or better than that (μ ^ 1000) for

Ni-Zn ferrites.

Table Vl

Magnetic properties of cores made of isolated powders of Fe.-Ni-oMo.B _1o (λ = 9 χ 10) and

4U JO 4 Xb

^Co 72 2 ^Fe 5 8 ^Μο 2 ^Β 15 ^{δ: ί} 5 ^ ^{λ % 0} ^ ^{with never (ari} 9 ^er magnetostriction and with particle sizes between 180 μm and 1.4 mm. The conditions of core compaction

were the same as those in the heading of Table II. The cores were left at 350 ° C for 2 hours annealed with a field of 1600 A / m applied along the circumference of the ring bodies.

Powder composition

insulation

DC coercive force (A / m)

²⁵ ° ^{C 72.2 Fe} 5.8

4 wt% SiO, 4 wt% MgO

7.9 7.7

^Fe 40 ^Ni 38

4 wt% SiO, 4 wt% MgO

14.2 16.7

- 18 - Table V continued

Core loss at

DC permeability at 0.1 T remanence 0.1 T 5 kHz 50 kHz

Fp
, 2 * ^e 5.8
15 ^Si 5 (10 ⁴ T) 5 kHz 50 kHz 5 (W / kg) Co ₇₂
Mon ₂ B 210 1600 650 210 , 9 240 100 ^Ni 38
18th 930 540 3 , 3 9 ^Fe 4 0
Mon ₄ B 130 830 460 300 , 5 440 140 1300 560 5 , 9

Claims (1)

Dr. Dieter Weber Klaus Seiffert Patent attorneys DIpL-Chem. Dr DieterWeber-DipL-Phys. Klaus Seiffert Poetfach 6145-6200 Wiesbaden German Patent Office Zweibrückenstr. 8000 Munich D-6200 Wiesbaden 1 Gustav-Frey tag-Straße 25 Telephone ΟβΙ 21/372720 + 372580 Telegram address : Wlllpwtent Telex: 4-186247
TWckopitiw Gr. IH OGl 21/3721 H PoBtsoheck: Frankfurt / Main 6763-602 Bank: Dresdner Bank AG, Wiesbaden, account number 27680700 (BLZi> 1080060) 81-2143 Date June 13, 1984 We / Wh Allied Corporation, Columbia Road and Park Avenue, Morristown, New Jersey 07960, USA Process for the production of moldings from magnetic metal alloys and molded products made in this way Priority: Serial No. 505 619 dated June 20, 1983 in USA Claims ^! _t 1 j A process for producing moldings from a magne-20 ^ - ^ tables metal alloy, characterized in that a ferromagnetic glassy metal powder having a particle size in the range of about 2 μΐη up to about 2 mm in a solidified body is compacted and presses. 25 2. The method according to claim 1, characterized in that the powder with a binder consisting of an oxide consists of the group SiO ₂ , MgO and BO ₃ , mixed before the compacting stage, the amount of binder in this mixture being in the range from about 1 to 20% by weight. 3. The method according to claim 2, characterized in that one uses a powder with a composition which essentially of the formula [Fe, Ni, Co] ₀₀ ^ ₁ - [Cr, Mo, W, V, Nb, - Öo-DD Ta ₇ Ti, Zr, Hf] ₇ [B, Si, C, P] _12-28 . 4. Compacted body of ferromagnetic vitreous metal made of a vitreous alloy of the composition which essentially of the formula Co. "Q ₀ Fe ₀ , _Ni _A ^ _n - 4U - OO (J - .LU U "jU M _n -Z, y_yn corresponds to where M is at least one of the elements Cr, Mo, W, V, Nb, To, Ti, Zr and / or Hf and Z is at least one of the elements B, Si, C and / or P. . 5. Annealed compacted body made of a ferromagnetic vitreous metal, produced from powders with particle sizes in the range between 180 μτα and 1.4 mm, the vitreous metal body having the composition Fe ₇₈ B, oSig and a relative initial magnetic permeability of at least about 70 at 5 kHz and 0.1 Tesla induction. 6. Annealed compacted body made of ferromagnetic vitreous metal, produced from powders with particle sizes between about 180 μm and 1.4 mm, the vitreous metal body having a composition Fe ₇₃ B ₁₃ Si ₉ and a relative initial magnetic permeability of at least about 100 at 5 kHz and Has 0.1 Tesla induction. 7. Ferromagnetic compacted body made of vitreous metal, produced from powders with particle sizes between about 180 μm and 1.4 mm, this vitreous metal body having the composition Fe _4n Ni ₃₀ Mo ₄ B ₁ Q, a relative initial magnetic permeability of about 650 and a core loss of about 22 W / kg at 5 kHz and 0.1 Tes- -3-Ia has induction. 342228t 8. Annealed compacted body made of ferromagnetic vitreous metal, produced from powders with particle sizes between about 180 μm and 1.4 mm, this vitreous metal body having the composition Fe ₄₀ Ni ₃₈ Mo ₄ -B, g, a relative initial magnetic permeability of about 1300 and has a core loss of about 6 W / kg at 5 kHz and 0.1 Tesla induction. 9. Ferromagnetic compacted body made of vitreous metal, produced from powders with particle sizes between about 180 μm and 1.4 mm, the vitreous metal body having the composition Co ~ ₇ ~ o ^Fe c oMOjB.rSir, a relative initial magnetic permeability of about 800 and has a core loss of about 19 W / kg at 5 kHz and 0.1 Tesla induction. 10. Annealed compacted body made of ferromagnetic vitreous metal, made from powders with particle sizes between about 180 μm and 1.4 mm, the vitreous metal body having the composition Co _{72 2} ^Fe sa ~ Mo ₂ B-Ji-Si ₁ -, a relative magnetic one Has initial permeability of about 900 and a core loss of about 3 W / kg at 5 kHz and 0.1 Tesla induction.