WO2002082475A1

WO2002082475A1 - Micro wires and process for their preparation

Info

Publication number: WO2002082475A1
Application number: PCT/GB2002/001587
Authority: WO
Inventors: V. S. Larin; A. V. Torcunov; L. V. Panina; D. J. Mapps; A. Zhukov; M. Vazquez; J. Gonzalez; A. Hernando; Steven M. Frazier; Simon Debues
Original assignee: Fourwinds Group Inc; Amotec S.R.L.; Tamag Iberica S.L.
Priority date: 2001-04-03
Filing date: 2002-04-03
Publication date: 2002-10-17
Also published as: GB0108373D0; GB2374084A

Abstract

A Co-Fe-Mn-B-Si alloy suitable for use in the manufacture of micro wires which exhibit bistable magnetic behaviour, the alloy comprising: Iron from 2 to 15 at . % Manganese from 3 to 7.5 at . % Boron from 12 to 16 at. % Silicon From 8 to 14 at . % Cobalt - balance Wherein the alloy exhibits an essentially rectangul ar magnetisation loop.

Description

Micro wires and a process for their preparation

The present invention relates to magnetic micro wires and a process for their preparation and, in particular, to micro wires which exhibit bi-stable magnetic switching behaviour.

Magnetic labels and markers may be used for identification and sorting, verification, authentication, tracking and security applications, and are typically composed of one or more micro wires.

As shown in Figure 1, a micro wire comprises a metallic core having and a coating formed from an insulating material, such as glass. The core has a diameter d_c, while the total diameter of the micro wire, i.e. core and the coating, is D_w. The metallic core typically has an amorphous and/or microcrystalline microstructure in order to achieve the desired magnetic properties, for example magnetic anisotropy and coercivity. The ratio of the metal core and glass coating thickness also affects the magnetic properties.

Upon exposure of the micro wire to an ac magnetic field (for example triangular or sinusoidal) whose field strength exceeds a predetermined threshold value, a voltage pulse is generated. For this signal to be readily detected and easily distinguished, it should ideally have a large amplitude and a short duration.

A bistable magnetic wire is characterised by an essentially rectangular magnetisation loop, as shown in Figures 2 and 3. If the external magnetic field H applied in the axial direction is sufficiently large, the wire may become uniformly magnetised along its axis, reaching a saturation magnetisation B_s. As the external field is decreased, the magnetisation decreases a little, having a value of B_r(remanence magnetisation) at H = 0. The ratio B_r/B_s is slightly smaller than 1. For an ideal loop (Figure 2), the ratio B_r/B_s = 1. Upon reversing the field direction, the magnetisation abruptly jumps reaching a negative saturation at some critical field H_c, which is referred to as the switching field. Such a magnetisation loop is important to obtain sharp voltage pulses, as shown in Figures 4 and 5. A pick up coil is a principle element of the external detector. The pick-up coil may be mounted tightly on the wire, or it may have a larger diameter (up to 0.5 m for remote sensing) so that the wire is inserted inside the detector, or the wire may be located at one side of the detector in the case of a flat plane reader. The pick-up coil detects a voltage U due to the rapid change in the magnetisation caused by the external magnetic field (ac triangular field, for example, as shown in Figure 4) . The voltage pulse may be characterised by its amplitude U_s and its duration Δt. For practical applications, it is often desirable for the voltage peak to be large and the pulse duration to be short. This requires not only that B_r/B_s is 1 or almost 1, but also small fluctuations in the switching field. The ability to alter the value of the switching field is also important.

There are essentially four properties of micro wires which characterize their switching behaviour and are relevant for their application in magnetic markers and/or labels.

A: The magnitude (amplitude) of the voltage pulse at remagnetisation (in arbitrary units) , which is determined as the ratio (A=U/V) of the voltage signal U in an electronic circuit used to the volume V of the metallic core. The parameter A may be represented as (KN/ (S_cS_ral) ) dl/dt, where K is the amplifying parameter of the measuring circuit used, N is the number of turns of the measuring coil S_c is its cross-section, S_m is the sample cross-section, 1 is the sample length. dl/dt is the rate of change in the current of the magnetising coil. A has arbitrary units .

B: The voltage pulse duration (μmsec) .

C: The switching field (A/m) , which corresponds to the coercivity of the micro wire.

D: The fluctuation of the switching field

(μsec) that corresponds to the fluctuations in the position of the voltage pulse peak.

In a first aspect the present invention provides a method of selecting an N-component alloy for a micro wire of the type capable of generating a well distinguished voltage pulse in a detector when the micro wire is exposed to an alternating magnetic field, wherein the N-component alloy is represented by Pι r ?2 r -r P_ ••• t P_N, where P_X,P₂, ..., Pi, ..., P_N are the elemental constituents of the alloy in atomic mass percent, the method comprising the step of determining the relative amount (s) of one or more of the elemental constituents by solving one or more of the equations:

In a second aspect, the present invention provides a method of predicting the switching behaviour of an alloy for use in the manufacture of a micro wire of the type capable of generating a voltage pulse in a detector when the micro wire is exposed to an alternating magnetic field, wherein the N-component alloy is represented as Pι,P₂, ..., Pi, ...,P_N, where Pι,P_2? ..., Pi, ...,P_N are the elemental constituents of the alloy in atomic mass percent, the method comprising the step of solving one or more of the equations:

α₀, α_l α₁₃ , β₀, β_x, β₁₃ , γ₀, γ_{l f} γ_1D , δ₀, δ_x and δ_1D are constants .

In both the first and second aspects of the present invention, for convenience, each parameter A, B, C and D may be characterised by a general matrix constructed as follows (example for N = 5 component alloy) :

For N-component alloy, the matrix will be written in a corresponding form of rank N+l.

In both the first and second aspects the alloy will generally be a 5 component alloy, i.e. N = 5.

Required magnetic alloys are designed by varying the concentrations of one or more of the elements within certain limits. Equations (l)-(4) above are then solved imposing specific conditions for Pi.

In both the first and second aspects, the method will generally involve providing at least one desired bi-stable magnetic switching property, i.e. A, B, C and/or D. The at least one switching property may then be expressed as a quadratic-polynomial function (as recited above) in respect of the element concentrations in the alloy, with imposed boundary conditions for the element concentrations. This is followed by the step of determining the at least one matrix for the selected boundary conditions, and then calculating the element concentrations for the at least one desired switching property.

In both the first and second aspects of the present invention, the alloy is preferably an Fe and Co-based alloy. For example (P_lf P₂, P₃, P₄, P₅) may represent (P_Fe, P_Co, P_Mn, P_B, P_si) , that is a pentary Fe-Co- Mn-B-Si alloy. Such an alloy may preferably consist essentially of iron, manganese, boron, silicon and cobalt, together with unavoidable impurities. However, it will be appreciated that this alloy may also include small amounts of other elements

In both the first and second aspects of the present invention, the multi-component alloy is required to satisfy a number of properties. The alloy is preferably an Fe and Co-based alloy in order to obtain stable magnetic properties with controlled parameters. As the content of Fe is increased, the magnetostrictive constant monotonically increases • going through zero at P_Fe = 4 to 4.5%. This allows a magnetic system with the coercivity varying within a wide field region to be designed. This behavior can be further refined using one or more additives selected from, for example, one or more of Mn, Cr and Re.

In order to produce a microwire having a metallic core in an amorphous or micro/ultra-crystalline state, elements responsible for amorphisation may also be included in the alloy. Suitable examples of such elements include one or more of Si, B and Ge . The total content of such elements preferably does not exceed a certain limit to avoid relaxation of properties with time. Along with this, certain requirements on composition may be imposed by the technological regime.

The preferred alloy is based on the pentary alloy system, Fe-Co-Mn-B-Si alloy, that is, in equations (l)-(4) N = 5 and ( P_{l r} P₂, P₃, P₄, P₅) may represent (P_Fe, P_Co, P_Mn, P_B, P_Sl) . Therefore, such an alloy may preferably consist essentially of iron, manganese, boron, silicon and cobalt, together with unavoidable impurities. However, it will be appreciated that this alloy may also include small amounts of other elements, such as, for example, one or more of Cr, Νi, Ge and Re .

In both the first and second aspects of the present invention, the values of the switching parameters are related to the alloy composition via a set of coefficients determined empirically. The parameters A, B, C and D are represented as quadratic- polynomial functions of the alloy element concentrations P_x, in atomic mass percent, where i may, for example, designates the elements Fe, Co, Mn, B, and Si.

The method according to both the first and second aspects may be embodied in a computer program product, which when read by a computer causes the said computer to solve the equations (1) to (4) to determine the characteristics matrix (5) -(8). The computer program product may be designed using FORTRAN and will typically be stored on a storage medium. In both the first and second aspects, the micro wire will typically exhibit bistable magnetic behaviour when exposed to an alternating magnetic field and, preferably, the micro wire exhibits a substantially rectangular magnetisation loop when exposed to an alternating magnetic field. In this case, the ratio B_r/B_s for the micro wire is preferably in the range of from 0.7 to 1, more preferably from 0.8 to 1, still more preferably from 0.9 to 1, where B_r is the residual magnetisation and B_s is the saturation magnetisation when the micro wire is exposed to an alternating magnetic field.

In the first aspect, the method will typically involve selecting values for one or more of A, B, C and D. For example, if a micro wire having an essentially rectangular hysteresis loop and exhibiting a sharp and large voltage pulse at remagnetisation is desired, then one may select a value of A such that it is maximal and a value of B that is minimum, with the values of the parameters C and D being arbitrary. The equations may then be solved to determine appropriate levels of (P_lf P₂, P₃, P₄, P₅) . An example of an alloy system that fulfills these requirements is given below:

14 . 2 < P_x = P_Fe < 15 . 0 54 . 4 < P₂ = P_Co ≤ 56 . 8 7 . 0 < P₃ = P_Mn ≤ 7 . 5

12 . 0 < P₄ = P_B < 12 . 5 10 . 0 < P₅ = P_si ≤ 10 . 6

A micro wire formed using such an alloy system may exhibit an essentially rectangular hysteresis loop with B_r/B_s being typically equal to or approximately equal to 1.

Similarly, if the amplitude of the voltage signal at remagnetisation is not of critical importance, then the search for a system possessing good switching characteristics can be realised by imposing the condition of a minimal voltage pulse duration (B) and a minimal fluctuation switching field (D) . In other words, one may select values of B and D that are minimum, with the values of the parameters A and C being arbitrary. An example of an alloy system that fulfills these requirements is given below:

70 . , 9 < p₂ = Pco < 73

10 . , 0 < p₅ = Psi < 10 . 6

A micro wire formed using such an alloy system may exhibit partial bi-stability with the ratio B_r/B_s being typically equal to or approximately equal to 0.9.

For certain applications, as for coding-decoding systems, alloy systems with small switching fields are desired. In this case, the requirement of a maximal amplitude A is also important. An example of an alloy system that fulfills these requirements is given below:

2 . 0 ≤ P_x = P_E < 2 . 2

66 . 1 < P, = P_r C_oo < 68 . 2

4 . 8 ≤ P, P_Mn ≤ 5 . 1

15 . 0 < P₄ = P_B < 16 . 0 10 . 0 ≤ P₅ = P_si ≤ 10 . 6

A micro wire formed using such an alloy system may exhibit partial bi-stability with the ratio B_r/B_s being typically equal to or approximately equal to 0.7.

For the application of micro wires in multi-bit tags, not only are good switching parameters required (maximal amplitude A of the switching signal and its short duration B) , but also a number of different switching fields are required. Additionally, the fluctuations of the field have to be minimised (minimal D) . To obtain the switching field values varying within a wide range, for example 0-300 A/m, two or more alloy systems will generally be required.

For systems where the coercivity C is smaller than approximately 100 A/m, the parameter B will typically be approximately 35 μS or less (for example falling in the range of from 10 to 35 μS) , preferably approximately 25 μS or less (for example falling in the range of from 12 to 20 μS) , more preferably approximately 17 μS or less. For higher coercivity systems C > 100 A/m) , the parameter B will typically be approximately 90 μS or less (for example falling in the range of from 30 to 90 μS) , preferably approximately 50 μS or less (for example falling in the range of from 25 to 50 μS) , more preferably approximately 30 μS or less.

The parameter C (i.e. the switching field) will typically fall in the range of from 3 to 330 A/M. For certain applications, C may range from 5 to 20 A/M, preferably from 5 to 15 A/M, more preferably from 8 to 14 A/M. For other applications, C may range from 20 to 40 A/M, preferably from 25 to 35 A/M, more preferably from 26 to 34 A/M. For other applications, C may range from 50 to 120 A/M, preferably from 60 to 110 A/M, more preferably from 70 to 100 A/M. For other applications, C may range from 100 to 220 A/M, preferably from 110 to 210 A/M, more preferably from 120 to 200 A/M. For other applications, C may range from 190 to 330 A/M, preferably from 200 to 310 A/M, more preferably from 210 to 300 A/M.

For systems where the coercivity C is smaller than approximately 100 A/m, the parameter D (i.e. fluctuation of the switching field) will typically be approximately 5 μS or less (for example falling in the range of from 1 to 5 μS) , preferably approximately 2 μS or less, more preferably approximately 1 μS or less. For higher coercivity systems, the parameter D will typically be approximately 25 μS or less, preferably approximately 15 μS or less, more preferably approximately 10 μS or less.

In both the first and second aspects of the present invention, the micro wire will typically comprise a metallic core formed of the desired alloy, together with an insulating or dielectric coating thereon, preferably a glass and/or a ceramic coating.

The metallic core of the micro wire will typically have a diameter in the range of from 1 to 30 μm. For certain applications, the diameter of the metallic core may range from 2 to 20 μm, preferably from 4 to 16 μm, more preferably from 5 to 15 μm. For other applications, the diameter of the metallic core may range from 1 to 7 μm, preferably from 2 to 6 μm, more preferably from 3 to 5 μm. For other applications, the diameter of the metallic core may range from 10 to 30 μm, preferably from 10 to 25 μm, more preferably from 15 to 20 μm.

The total diameter of the micro wire (i.e. the diameter of the metallic core and coating) will typically in the range of from 5 to 35 μm, preferably^* from 7 to 30 μm, more preferably from 10 to 25 μm.

The ratio of the metal core and glass coating thickness affects the magnetic properties and thus different ratios result in micro wires having different magnetic switching properties.

In both the firs't and second aspects of the present invention, the method may further comprise the step of manufacturing a micro wire having a metallic core formed of an alloy with the calculated element concentrations .

In a third aspect, the present invention provides a Co-Fe-Mn-B-Si alloy for use in the manufacture of micro wires which exhibit bistable magnetic behaviour, the alloy comprising:

Iron - from 2 to 15 at . %

Manganese - from 3 to 7.5 at.%

Boron - from 12 to 16 at.%

Silicon - from 8 to 14 at.% cobalt - balance

wherein the alloy exhibits an essentially rectangular magnetisation loop.

In a first embodiment of the third aspect of the present invention, the alloy preferably comprises: Iron from 14.2 to 15 at.%

Manganese from 7 to 7.5 at.%

Boron from 12 to 12.5 at.%

Silicon from 10 to 10.6 at.% cobalt from 54.4 to 56.8 at,

In a second embodiment of the third aspect of the present invention, the alloy preferably comprises:

Iron from 2 to 2.2 at.%

Manganese from 4.8 to 5.1 at.%

Boron from 15 to 16 at.%

Silicon from 10 to 10.6 at.% cobalt from 66.1 to 68.2 at.

In a third embodiment of the third aspect of the present invention, the alloy preferably comprises:

Iron from 4.2 to 4.5 at.%

Manganese from 3.1 to 3.5 at.%

Boron from 12 to 12.5 at.%

Silicon from 10 to 10.6 at.% cobalt from 68.9 to 70.1 at.

In a fourth embodiment of the third aspect of the present invention, the alloy preferably comprises:

Iron from 9 to 9.5 at.%

Manganese from 3.5 to 4 at.%

Boron from 13.3 to 13.7 at.%

Silicon from 10.5 to 11 at.% cobalt from 61.8 to 63.7 at.%

In a fourth aspect, the present invention provides a Co-Fe-Mn-B-Si alloy for use in the manufacture of micro wires which exhibit bistable magnetic behaviour, the alloy comprising:

Iron from 4.2 to 4.5 at.%

Manganese from 0.8 to 1.5 at.%

Boron from 12 to 12.5 at.%

Silicon from 10 to 10.6 at.% cobalt from 70.9 to 73 at.%

wherein the alloy exhibits an essentially rectangular magnetisation loop.

In a fifth aspect, the present invention provides a Co-Fe-Mn-B-Si alloy for use in the manufacture of micro wires which exhibit bistable magnetic behaviour, the alloy comprising:

Iron from 11 to 24 at.%

Manganese from 0.1 to 2.5 at.

Boron from 13 to 16 at.%

Silicon from 9 to 15 at.% cobalt balance

In a first embodiment of the fifth aspect, the alloy preferably comprises:

Iron from 12.8 to 13.5 at.%

Manganese from 0.1 to 0.4 at.%

Boron from 13 to 13.5 at.%

Silicon from 10 to 10.6 at.% cobalt from 62 to 64.1 at.%

In a sixth aspect, the present invention provides a Co-Fe-Mn-B-Si alloy for use in the manufacture of micro wires which exhibit bistable magnetic behaviour, the alloy comprising: Iron - from 24 to 30 at.%

Manganese - from 0.1 to 1.5 at.%

Boron - from 12 to 15 at.%

Silicon - from 9 to 14 at.% cobalt - balance

In a first embodiment of the sixth aspect, the alloy preferably comprises:

Iron - from 26.5 to 27.5 at.%

Manganese - from 0.5 to 0.8 at.%

Boron - from 14 to 14.5 at.%

Silicon - from 9.6 to 10.6 at.% cobalt - from 46.6 to 49.4 at.%

For all of the alloys according to the present invention, the ratio B_r/B_s lies in the range of from 0.7 to 1, preferably from 0.8 to 1, more preferably from 0.9 to 1.

For all of the alloys according to the present invention, the alloy may have an amorphous, crystalline, and/or ultra/micro-crystalline microstructure . Typically, the alloy will have an amorphous and/or ultra/micro-crystalline microstructure .

For all of the alloys according to the present invention, the alloy may consist essentially of iron, manganese, boron, silicon and cobalt, together with unavoidable impurities.

For all of the alloys according to the present invention, the values of the switching parameters A, B, C and D may be as herein described in relation to any of the embodiments of the first and/or second aspects .

The present invention also provides a micro wire which exhibit bistable magnetic behaviour, wherein the micro wire is formed from an alloy as herein defined. The micro wire will typically have coating on at least part of the outer surface thereof. The coating is preferably a dielectric or insulating coating. An example of a suitable coating is glass, although it is envisaged that the coating could also comprise or consist of a ceramic material.

The metallic core of the micro wire will typically have a diameter in the range of from 1 to 30 μm, while the total diameter of the micro wire (i.e. the metallic core and the coating) will typically fall in the range of from 5 to 35 μm. The micro wire is preferably of the type that is capable of generating a voltage pulse in a detector when the micro wire is exposed to an alternating magnetic field. Advantageously, the micro wire exhibits a voltage pulse at remagnetisation of from 5 to 35 μS, preferably from 5 to 30 μS, more preferably from 8 to 20 μS . The micro wire may be provided on its own or on a suitable carrier. Two or more micro wires may be also be provided either with or without a suitable carrier. The two or more micro wires may be formed of the same alloy as herein described or may, alternaively, be formed of diffrent alloys as herein described.

The present invention also provides a magnetic marker or label for identification, sorting, verification, authentication, recognition, tracking or security, said marker comprising an alloy as herein defined or one or more micro wires as herein defined. The magnetic marker or label may comprise at least two micro wires as herein defined, wherein the micro wires have different coercivities . Accordingly, each wire switches at a different field strength of the external field. As a consequence, each wire can be considered as a "bit" of encoded information, which can be read out in an external alternating electromagnetic field. A plurality of micro wires may advantageously be arranged in a label having closely spaced different coercivities, leading thus to a high coding density.

The present invention also provides an article having a magnetic marker or label attached thereto or associated therewith, said marker or label being as herein described. The article may be a container for a foodstuff for example, such as a metal or plastic can, a ring pull or top portion therefor, a glass or plastic bottle, or a plastic, paper or cardboard carton. Such articles are typically recycled after use and the provision of a magnetic marker or label assists in the identification, sorting and/or tracking of a particular type of article. Thus, the present invention also provides a method of identification, sorting and/or tracking of two or more types of article, wherein each type of article has a micro- wire, magnetic marker or label as herein described attached thereto or associated therewith, said micro- wire, magnetic marker or label being representative of said type of article. Accordingly, aluminium-based cans can readily be distingusihed from steel-based cans or plastic bottles for example. This facilitates economic recycling. The micro-wire, magnetic marker or label is advantageously provided in the ring-pull component of a can. Alternatively, the micro-wire, magnetic marker or label may be provided in the seal region of the top protion of the can. When provided with a plastic bottle, an advantage of the glass or ceramic coating in the micro wire is that the metal core remains substantially unannealed during the blow- moulding or thermo-forming process. The magnetic marker or label may comprise at least two micro wires as herein defined, wherein the micro wires have different coercivities. The alloy according the the third aspect of the present invention is preferably used for a micro-wire, magnetic marker or label for a metal container, for example metal can. The alloy according the the fifth aspect of the present invention is preferably used for a micro-wire, magnetic marker or label for a non-metallic container, for example a plastic bottle. The alloy according the the sixth aspect of the present invention may be used for a micro-wire, magnetic marker or label for a metal or non-metallic container.

In a seventh aspect of the present invention, there is provided a process for preparing a micro wire which exhibits bistable magnetic behaviour and which comprises a metallic core comprising Co, Fe, Mn, B and Si and a glass or ceramic coating, which process comprises:

(i) providing a glass or ceramic tube containing an alloy comprising Co, Fe, Mn, B and Si;

(ii) heating the alloy in the tube to a temperature sufficient to melt the alloy and to soften the tube;

(iii) superheating the alloy in the tube;

(iv) drawing a capillary tube from the softened tube; and (v) cooling the capillary tube with the alloy therein to form said micro wire.

Step (i) may preferably involve providing a glass or ceramic tube and delivering inside the tube an alloy comprising Co, Fe, Mn, B and Si in the form of a rod. A suitable glass composition is given below.

In step (ii) , heating of the alloy is preferably such so as to form a melting metal droplet covered by softened glass.

In step (iii) , superheating of the alloy in the tube is desirable for the following reasons. First, the coefficients of the viscosity which depend on temperature for metal and glass have to be in a certain correspondence to produce a wire of a given geometry. Second, to control the microstructure (amorphous or microcrystalline or mixed amorphous/crystalline) since the initial temperature affects the cooling rate and the metal structure in the liquid state. Third, to produce a wire with stable properties along its length, it is important to maintain a constant temperature of the melting metal before the drawing process starts, which gives a homogeneous structure in the liquid metal.

In step (iv) , a glass fibre will generally first be drawn and wound on a coil. Under certain drawing conditions, a glass capillary is then formed where molten metal penetrates, thus resulting in a micro wire, i.e. a metal core covered by glass or ceramic.

Step (v) typically involves cooling the micro wire on its way to a receiving coil to thereby obtain a metallic core in an amorphous or fine microcrystalline state.

The micro wires fabricated according to the process of the present invention may be optimized for application in magnetic markers or labels .

The micro wires are manufactured by use of a modified Taylor-Ulitowski process, based on direct casting from the melt.

In the process according to the present invention, an alloy droplet in a quantity of typically from 3 to 6 gm is delivered inside a glass tube held directly over suitable heating means, for example an inductor heater. The droplet is heated up to the melting point T_m of the alloy at which temperature the glass softens and forms an envelope for the metal. Then, the metal is superheated and kept at a temperature T_s = T_m + ΔT for a time t. From softened glass, a glass fibre is drawn and preferably wound on to a coil. Under certain drawing conditions, a glass capillary is formed where molten metal penetrates, thus forming a micro wire in which a metal core covered by glass or ceramic. The micro wire is rapidly cooled by any suitable cooling means, for example water, on its way to the receiving coil.

The melting temperature T_ra of the alloy will typically fall in the range of from 1230 to 1400°C, preferably from 1250 to 1380°C, more preferably from 1270 to 1360°C.

The alloy is typically superheated by from 10 to 300°C above its melting point.

The alloy is preferably superheated for a period t of from 30 seconds to 8 minutes, preferably from 1 to 5.5 minutes, more preferably from 2 to 5 minutes. The parameter t is preferably found empirically for a given alloy composition, magnetic properties and values of the metallic core diameter and the glass thickness. To find a suitable time t, the process of casting micro wires with a length of, for example, from 200 to 300 m is repeated several times with different times t. The magnetic properties of the obtained samples are then measured as a function of t. Using extrapolation methods, the exposure time for the required magnetic and geometrical parameters may then be determined.

Cooling of the capillary tube with the alloy therein is preferably carried out under conditions to yield a microcrystalline and/or amorphous microstructure.

Cooling of the capillary tube with the alloy therein is preferably carried out under conditions to yield a ultra/micro-crystalline and/or amorphous microstructure. The cooling rate chosen depends on geometry of micro wire (diameter d_c of the metal core and total wire diameter D_w) , as well as on the parameters of the technological process. To obtain an amorphous microstructure in the metal core, the cooling rateV preferably satisfies the following empirical equation: 2aθ (r_m +A) v = p_mr_m ²C_m + p_gC_g(2r_mA+A²)

where P_m, Pg , C_m and Cg are density and specific

heat for metal and glass, respectively, C is the parameter of thermoconductivity at the boundary microwire/cooling medium, u is the difference in temperature between the metallic core and cooling

medium, ^rm ⁼"_ml i_s the radius of the metal core, and

A={D_w—u_m)/2 is the thickness of the glass cover.

The characteristic cooling rate for Co-Fe micro wire having a diameter of from 5 - 10 μm with a glass cover thickness of from 2 - 5 μm, with casting rates of from 200 - 500 m/min, using as a cooling medium water is typically 10⁵ - 10⁶°/S.

In the process according to the present invention, the alloy may have any one of the compositions as herein defined.

The process may further include the step of providing one or more of the thus formed micro wires and attaching it or them to a suitable carrier.

The process may further include an initial step of selecting an alloy for a micro wire according to the method as herein defined.

The values of the switching parameters A, B, C and D depend, inter alia , on the alloy composition, the super heating temperature, the time the alloy is held at the super heating temperature, the drawing rate, the cooling rate, the diameter of the metallic core diameter d_c and the overall diameter D_w.

The present invention is described further with reference to the following examples and accompanying drawings (Figures 1 to 9) , which are provided by way of example .

Examples

The micro wires are preferably manufactured by use of a modified Taylo^Ulitowski process, based on direct casting from the melt, as shown in Figures 6 and 7. A glass tube and metallic alloy in the form of a rod are provided (separately) . The glass tube is placed vertically with its bottom end above a high frequency inductor. One end of the metallic rod is inserted inside the bottom end of the glass tube

(hence, located above the heating inductor) . This small portion (about 3-6 gm) is melted up to the melting point T_m, thereby softening the surrounded portion of glass and a droplet is formed covered completely by glass. Then, the metal is superheated and kept at a temperature T_s = T_m + ΔT for a time t. From the softened glass, a glass fibre is drawn and wound on to a coil. Under certain drawing conditions a glass capillary is formed where melting metal penetrates and thus a micro wire is formed, which has a metal core covered by glass or ceramic. The micro wire is rapidly cooled by suitable cooling means, for example a flow of water or oil, on its way to the receiving coil. The glass consumption is continuously compensated during the process by moving the glass tube in the area near the inductor. The amount of metal is restricted by the initial melting droplet.

The melting temperature T_m of the alloy will typically fall in the range of from 1230 to 1400°C, preferably from 1250 to 1380°C, more preferably from 1270 to 1360°C.

The alloy is typically superheated from 10 to 300°C above its melting point.

For an alloy (from Example 2)

70 . 9 < P₂ = P_Co < 73

0 . 8 < P₃ = P_Mn < 1 . 5 12 . 0 < P₄ = P_B < 12 . 5 10 . 0 < P₅ = P_Sl < 10 . 6

the following parameters of the technological process were set: frequency of the inductor 440 ± 10% voltage at the inductor 18 - 20 V rate of casting 250 - 300 m/min rate of glass tube delivery 3 - 5 mm/min cooling liquid water with temperature 18-25 °C distance from the bottom of the melting droplet to the cooling water flow is 10-15 mm pressure inside the glass tube 20 - 60 Pa

In this case, an amorphous structure of the metal core was obtained for micro wires with geometry: d_c/Δ_g < 1.75, Δ_g = (D_w-d_c)/2, for d_c < 18 μm. The initial alloy material was in the form of a thin rod having a diameter of approximately 3-4 mm, the glass tube was made of glass "Pirex" with an outer diameter of approximately 11-12 mm and a wall thickness of approximately 1-2 mm. The glass composition was (Si0₂) ₇₄.₅ (A1₂0₃) ₂ (B₂0₃) i_s (Na₂0) ₃ (K₂0) _λ.₅.

The obtained magnetic and geometrical parameters are given in Table 2.

The dependence of the magnetic parameters on the alloy compositions are demonstrated in Figures 8 and 9. A large number of similar plots have been used to model equations (1) -(4) and determine the characteristic matrixes (5) -(8). In the case of Figures 8 and 9, the range of concentration is chosen such that the deviation from a linear law is substantially not noticeable. This results in small coefficients corresponding to the quadratic terms. Provided are examples of the experimental plots for magnetic parameters vs . alloy compositions . These data are obtained for alloy constituent concentrations changing within certain limits.

Alloy system (a)

Here, an alloy system was considered, where the concentrations were set by the following boundary conditions :

3.0 ≤ P, ≤ 7.5

12.0 < P₄ = P_B < 16.0

P_si ≤ 14.0

In this case, the parametric matrixes were calculated to be

A

Example 1

This example relates to designing a micro wire having a rectangular hysteresis loop and giving a sharp and large voltage pulse at remagnetisation. In this case, the composition was chosen such that A is maximal and B is minimum. The parameters C and D are arbitrary. This problem is resolved with an alloy having the following composition (which corresponds to the limiting conditions) :

Pco = 55 . 6 + 1 . 2

P_* = P_S1 = 10 . 3 ± 0 . 3 The temperature of the melting alloy was fixed in the range of from 1270 to 1350°C. Varying the exposure time, micro wires having the desired magnetic properties and geometrical parameters were fabricated. The results of are provided in Table 1.

Table 1

This system allows the best known characteristics of the voltage pulse at remagnetisation to be obtained, i.e. maximal amplitude and very short duration. For example, amorphous wires obtained by in-rotating-water-spinning method (Unitika) with further treatment (cold drawn and tension annealed) exhibit a voltage pulse at remagnetisation having duration of more than 20 μsec (for a critical length which is about 2 cm) . The obtained micro wires have stable parameters if the exposure time (t) is up to 5.5 min. Example 2

If the amplitude of the voltage signal at remagnetisation is not critically important, then the search for a system possessing good switching characteristics can be realised by imposing the condition of a minimal duration and a minimal fluctuation field. In this case, the alloy composition was found to be:

Pi 4.35 ± 0.15 71.95 ± 1.05

P = P = 1.15 ± 0.35

P — P_B — 12.25 ± 0.25 , = P_a< = 10.3 ± 0.3

The temperature of the melting alloy was fixed in the range of from 1280 to 1360°C. Varying the exposure time, micro wires having the desired magnetic properties and geometrical parameters were fabricated. The results are provided in Table 2.

Table 2

Example 3

For certain applications, such as coding-decoding systems, materials with small switching fields are required. In this case, the requirement of a maximal amplitude A is also important. A suitable alloy was found to be :

P, = Pr Co = 67 . 15 ± 1 . 05

P, = P, Mn = 4 . 95 ± 0 . 15

Psi = 10 . 3 + 0 . 3

The temperature of the melting alloy was fixed in the range of from 1280 to 1360°C. In this case, the exposure time was varied to obtain very thin wires . The results are provided in Table 3.

Table 3

For applications where micro wires are to be used in multi-bit tags, there is a requirement for a maximal A, a minimal B, and also a wide range in respect of the switching field C. Additionally, the fluctuations have to be minimised (minimal D) . To obtain the switching field values varying within a wide range of from 0 to 300 A/m, two alloy systems were designed. The temperature of the melting alloy was fixed in the range of from 1280 to 1360°C. Fabricating wires of specified composition, with different values of the metallic core diameter and glass cover thickness, and varying the exposure time, allowed the desirable variation in magnetic-switching parameters to be obtained

Alloy system (b)

0.1 < P₃ = P_Mn ≤ 2.5 13.0 < P₄ = P_B < 16.0 9.0 ≤ P₅ = P_si < 15.0

z=l

In this case, the parametric matrixes

Λ Λ Λ Λ

A, B, C , D are calculated to be

B=()

Alloy system (c)

0.1 < P₃ = P_Mn < 1.5 12.0 < P₄ = P_R < 15.0

9.0 < P₅ = P_si < 14.0

In this case, the parametric matrixes

Λ Λ Λ Λ

A, B, C, D are calculated to be

Example 4

Certain variations in coercivity near 30 A/m (for example from 25 to 35 A/m) were obtained for alloy system (a) having the following composition:

P, = Pco = 69 . 8 ± 0 . 8

Psi = 10 . 3 + 0 . 3

as represented in Table 4 :

Table 4

Example 5

To increase the coercivity value to cover the range of from 70 to 100 A/m, the following alloy (from system (a) ) was used:

P, = P Co = 62.75 ± 0.95 = 3.75 ± 0.25

P₅ = Psi = 10.75 + 0.25

The geometrical and magnetic parameters are provided in Table 5:

Table 5

Example 6

Using alloy system (c) , micro wires with coercivity values in the range of from 100 to 200 A/m were fabricated. The alloy had the following composition:

Pco = 48 ± 1. 4

PB = 14.25 + 0.25

The parameters are listed in Table 6

Table 6

Example 7

Using alloy system (b) , micro wires with coercivity values in the range of from 200 to 300 A/m were fabricated. The alloy had the following composition:

P_λ = P Fe = 13 . 15 + 0 . 35

P. = PB = 13 . 25 ± 0 . 25

P₅ = Psi = 10 . 3 + 0 . 3

The parameters are listed in Table 7

Table 7

With reference to Figure 10, a number of wires approximately 3 cm long formed from alloys according to (a) , (b) and (c) as herein described were attached to a glass substrate (usual microscope glass) at a distance of approximately 2 mm from each other. The assemblies were placed in a middle of coaxial magnetising and detection coils of approximately 25 cm in diameter. The magnetising coils produced a sinusoidal magnetic field directed along the wires of up to 400 A/m amplitude and having a frequency of 100 Hz. The detected signal showed a number of non- overlapping voltage pulses. Each of these pulses can be regarded as a bit of information. Measurements were made for n = 1-9. Therefore, n-bit magnetic code for remote reading (at distance of about 10 cm) has been realised using microwires fabricated according to Examples 1 to 7.

Claims

CLAIMS :

1. A Co-Fe-Mn-B-Si alloy suitable for use in the manufacture of micro wires which exhibit bistable magnetic behaviour, the alloy comprising:

Iron - from 2 to 15 at.%

Manganese - from 3 to 7.5 at.%

Boron - from 12 to 16 at.% Silicon - from 8 to 14 at.% cobalt - balance

wherein the alloy exhibits an essentially rectangular magnetisation loop.

2. An alloy as claimed in claim 1, comprising:

Iron - from 14.2 to 15 at.%

Manganese - from 7 to 7.5 at.% Boron - from 12 to 12.5 at.%

Silicon - from 10 to 10.6 at.% cobalt - from 54.4 to 56.8 at.%^'

3. An alloy as claimed in claim 1, comprising:

Iron - from 2 to 2.2 at.%

Manganese - from 4.8 to 5.1 at.%

Boron - from 15 to 16 at.%

Silicon - from 10 to 10.6 at.% cobalt - from 66.1 to 68.2 at.%

4. An alloy as claimed in claim 1, comprising:

Iron - from 4.2 to 4.5 at.% Manganese - from 3.1 to 3.5 at.%

Boron - from 12 to 12.5 at.% Silicon - from 10 to 10.6 at.% cobalt - from 68.9 to 70.1 at.%

5. An alloy as claimed in claim 1, comprising:

Iron - from 9 to 9.5 at.%

Manganese - from 3.5 to 4 at.%

Boron - from 13.3 to 13.7 at.%

Silicon - from 10.5 to 11 at.% cobalt - from 61.8 to 63.7 at.%

6. A Co-Fe-Mn-B-Si alloy suitable for use in the manufacture of micro wires which exhibit bistable magnetic behaviour, the alloy comprising:

Iron - from 4.2 to 4.5 at.%

Manganese - from 0.8 to 1.5 at.%

Boron - from 12 to 12.5 at.%

Silicon - from 10 to 10.6 at.% cobalt - from 70.9 to 73 at.%

wherein the alloy exhibits an essentially rectangular magnetisation loop.

7. A Co-Fe-Mn-B-Si alloy suitable for use in the manufacture of micro wires which exhibit bistable magnetic behaviour, the alloy comprising:

Iron - from 11 to 24 at.% Manganese - from 0.1 to 2.5 at.%

Boron - from 13 to 16 at.%

Silicon - from 9 to 15 at.% cobalt - balance

8. An alloy as claimed in claim 7, comprising: Iron - from 12.8 to 13.5 at.%

Manganese - from 0.1 to 0.4 at.%

Boron - from 13 to 13.5 at.%

Silicon - from 10 to 10.6 at.% cobalt - from 62 to 64.1 at.%

9. A Co-Fe-Mn-B-Si alloy suitable for use in the manufacture of micro wires which exhibit bistable magnetic behaviour, the alloy comprising:

Iron - from 24 to 30 at.%

Manganese - from 0.1 to 1.5 at.%

Boron - from 12 to 15 at.%

Silicon - from 9 to 14 at.% cobalt - balance

10. An alloy as claimed in claim 9, comprising:

Iron - from 26.5 to 27.5 at.% Manganese - from 0.5 to 0.8 at.%

Boron - from 14 to 14.5 at.%

Silicon - from 9.6 to 10.6 at.% cobalt - from 46.6 to 49.4 at.%

11. An alloy as claimed in any one of claims 1 to 10, wherein the ratio B_r/B_s lies in the range of from 0.7 to 1, preferably from 0.8 to 1, more preferably from 0.9 to 1.

12. An alloy as claimed in any one of claims 1 to 11, wherein the alloy has an^' amorphous, crystalline, and/or ultra/microcrystalline microstructure, preferably a combination of an amorphous and ultracrystalline microstructure.

13. An alloy as claimed in any one of claims 1 to 12, consisting essentially of iron, manganese, boron, silicon and cobalt, together with unavoidable impurities .

14. A micro wire which exhibit bistable magnetic behaviour, wherein the micro wire is formed from an alloy as defined in any one of claims 1 to 13.

15. A micro wire as claimed in claim 14, which has coating on at least part of the outer surface, preferably a dielectric or insulating coating.

16. A micro wire as claimed in claim 15, wherein the coating is or comprises a glass and/or a ceramic.

17. A micro wire as claimed in any one of claims 14 to 16, wherein the metallic core of the micro wire has a diameter in the range of from 1 to 30 μ .^'

18. A micro wire as claimed in any one of claims 15 to 17, wherein the total diameter of the micro wire is in the range of from 5 to 35 μm.

19. A micro wire as claimed in any one of claims 14 to 18 of the type capable of generating a voltage pulse in a detector when the micro wire is exposed to an alternating magnetic field.

20. A micro wire as claimed in claim 19, which exhibits a voltage pulse at remagnetisation of from 5 to 35 μS, preferably 5 to 30 μS, more preferably 8 to 20 μS.

21. A micro wire as claimed in any one of claims 14 to 20 which is provided on a carrier.

22. A magnetic marker for identification, sorting, verification, authentication, recognition, recycling, tracking or security, said marker comprising an alloy as defined in any one of claims 1 to 13 or a micro wire as defined in any one of claims 14 to 21.

23. An article having a magnetic marker attached thereto or associated therewith, said marker being as defined in claim 22.

24. An article as claimed in claim 23 which is a container.

25. An article as claimed in claim 23 or claim 24 which is a metal or plastic can, a ring pull or top portion therefor, a glass or plastic bottle, or a plastic, paper or cardboard carton.

26. A process for preparing a micro wire which exhibits bistable magnetic behaviour and which comprises a metallic core comprising Co, Fe, Mn, B and Si and a glass or ceramic coating, which process comprises :

(iii) superheating the alloy in the tube;

27. A process as claimed in claim 26, wherein the melting temperature of the alloy is in the range of from 1230 to 1400°C, preferably from 1250 to 1380°C, more preferably from 1270 to 1360°C.

28. A process as claimed in claim 26 or claim 27, wherein the alloy is superheated by from 10 to 300°C above its melting point.

29. A process as claimed in any one of claims 26 to 28, wherein the alloy is superheating in the tube for a period of from 30 seconds to 8 minutes, preferably from 1 to 5.5 minutes, more preferably from 2 to 5 minutes .

30. A process as claimed in any one of claims 26 to 29, wherein cooling of the capillary tube with the alloy therein is carried out under conditions to yield a micro/ultra-crystalline and/or amorphous microstructure .

31. A process as claimed in any one of claims 26 to 30, wherein the alloy has a composition as defined in any one of claims 1 to 13.

32. A process as claimed in any one of claims 26 to 31, wherein the metallic core of the micro wire has a diameter in the range of from 1 to 30 μm.

33. A process as claimed in claim 32, wherein the total diameter of the micro wire is in the range of from 5 to 35 μm.

34. A method of selecting an alloy for a micro wire of the type capable of generating a voltage pulse in a detector when the micro wire is exposed to an alternating magnetic field, wherein the N-component alloy is represented by Ε^> _l, P₂r ■i ?_>- -ι P^ where

P₁,P₂, ..., Pi, ..., P_N are the elemental constituents of the alloy in atomic mass percent, the method comprising the step of determining the relative amount (s) of one or more of the elemental constituents by solving one or more of the equations:

where

A = is the ratio of a voltage pulse generated in a detector positioned adjacent the micro wire when the micro wire is exposed to an alternating magnetic field to the volume of the alloy in the micro wire,

B is the duration of said voltage pulse, C is the strength of the alternating magnetic field corresponding to the maximum of said voltage pulse,

D is the fluctuation in the position of the peak of said voltage pulse,

α₀, α_x, α₁₃, β₀, β_x, β_i3 γ₀, y_{ι r} γ₁₃ , δ₀, δ_x and δ₁ are constants.

35. A method of predicting the switching behaviour of an alloy for use in the manufacture of a micro wire of the type capable of generating a voltage pulse in a detector when the micro wire is exposed to an alternating magnetic field, wherein the alloy is represented by P_lfP₂, ..., P^ ..., P„, where Pχ,P₂, ..., P^ ..., P_N are the elemental constituents of the alloy in atomic mass percent, the method comprising the step of solving the following equations

N N N A = a₀ + ∑a_lP_l + ∑ ∑«_y^ _; d>

«=1 j≥i ;=1

where A = is the ratio of a voltage pulse generated in a detector positioned adjacent the micro wire when the micro wire is exposed to an alternating magnetic field to the volume of the alloy in the micro wire,

B is the duration of said voltage pulse,

C is the strength of the alternating magnetic field corresponding to the maximum of said voltage pulse,

D is the fluctuation in the position of the peak of said voltage pulse,

α₀, α_x, α₁₃, β₀, β_x, β₁₃, γ₀, γ_∑, γ₁₃, δ₀, δ_x and δ„ are constants.

36. A method as claimed in claim 34 or claim 35, wherein N=5 and (P_lf P₂, P₃, P„, P₅) = (P_Fe, P_Co, P_Mn, P_B, P_Sl) .

37. A method as claimed in any one of claims 34 to

36, wherein the alloy consists essentially of iron, manganese, boron, silicon and cobalt, together with unavoidable impurities.

38. A method as claimed in any one of claims 34 to

37, wherein the micro wire exhibits bistable magnetic behaviour when exposed to an alternating magnetic field.

39. A method as claimed in any one of claims 34 to

38, wherein the micro wire exhibits a substantially rectangular magnetisation loop when exposed to an alternating magnetic field.

40. A method as claimed in any one of claims 34 to 39, wherein the ratio B_r/B_s for the micro wire is in the range of from 0.7 to 1, preferably from 0.8 to 1, more preferably from 0.9 to 1, where B_r is the residual magnetisation and B_s is the saturation magnetisation when the micro wire is exposed to an alternating magnetic field.

41. A method as claimed in any one of claims 34 to 40, comprising the step of selecting values for one or more of A, B, C and D.

42. A method as claimed in any one of claims 34 to

41, wherein B is in the range of from 5 to 35 μS, preferably 5 to 30 μS, more preferably 8 to 20 μS .

43. A method as claimed in any one of claims 34 to

42, wherein C is in the range of from 3 to 330 A/M.

44. A method as claimed in claim 43, wherein C is in the range of from 5 to 20 A/M, preferably from 5 to 15 A/M, more preferably from 8 to 14 A/M.

45. A method as claimed in claim 43, wherein C is in the range of from 20 to 40 A/M, preferably from 25 to

35 A/M, more preferably from 26 to 34 A/M.

46. A method as claimed in claim 43, wherein C is in the range of from 50 to 120 A/M, preferably from 60 to 110 A/M, more preferably from 70 to 100 A/M.

47. A method as claimed in claim 43, wherein C is in the range of from 100 to 220 A/M, preferably from 110 to 210 A/M, more preferably from 120 to 200 A/M.

48. A method as claimed in claim 43, wherein C is in the range of from 190 to 330 A/M, preferably from 200 to 310 A/M, more preferably from 210 to 300 A/M.

49. A method as claimed in any one of claims 34 to 48, wherein D is in the range of from 0.3 to 100 μS.

50. A method as claimed in claim 49, wherein D is in the range of from 0.5 to 7 μS, more preferably from 1 to 5 μS.

51. A method as claimed in claim 49, wherein D is in the range of from 0.3 to 3 μS, more preferably from 0.5 to 2.5 μS.

52. A method as claimed in claim 49, wherein D is in the range of from 20 to 100 μS, more preferably from 30 to 90 μS.

53. A method as claimed in claim 49, wherein D is in the range of from 15 to 80 μS, more preferably from 25 to 70 μS.

54. A method as claimed in claim 49, wherein D is in the range of from 0.5 to 7 μS, more preferably from 1 to 5 μS.

55. A method as claimed in any one of claims 34 to 54, wherein the micro wire comprises a metallic core formed of said alloy and an insulating or dielectric coating thereon, preferably a glass ^'and/or a ceramic coating.

56. A method as claimed in claim 55, wherein the metallic core of the micro wire has a diameter in the range of from 1 to 30 μm.

57. A method as claimed in claim 56, wherein the metallic core of the micro wire has a diameter in the range of from 2 to 20 μm, preferably from 4 to 16 μm, more preferably from 5 to 15 μm.

58. A method as claimed in claim 56, wherein the metallic core of the micro wire has a diameter in the range of from 1 to 7 μm, preferably from 2 to 6 μm, more preferably from 3 to 5 μm.

59. A method as claimed in claim 56, wherein the metallic core of the micro wire has a diameter in the range of from 10 to 30 μm, preferably from 10 to 25 μm, more preferably from 15 to 20 μm.

60. A method as claimed in any one of claims 34 to

59, wherein the total diameter of the micro wire is in the range of from 5 to 35 μm, preferably from 7 to 30 μm, more preferably from 10 to 25 μm.

61. A method as claimed in any one of claims 34 to

60, wherein the alloy has a composition as defined in any one of claims 1 to 13.

62. A method as claimed in any one of claims 34 to 61, further comprising forming an alloy for a micro wire of the desired composition.

63. A method as claimed in any one of claims 34 to 62, further comprising preparing a micro wire which exhibits bistable magnetic behaviour and which comprises a metallic core comprising Co, Fe, Mn, B and Si and a glass or ceramic coating.

64. A method as claimed in claim 63, wherein the micro wire is prepared by a process as defined in any one of claims 26 to 33.

65. A process as claimed in any one of claims 26 to 33 further comprising the step of attaching the micro wire to a metal or plastic can, a ring pull or top portion therefor, a glass or plastic bottle, or a plastic, paper or cardboard carton.