CN114953806B

CN114953806B - Magnetic microfilaments and security media

Info

Publication number: CN114953806B
Application number: CN202210635733.7A
Authority: CN
Inventors: 杨福桂; 刘运锋; 刘崇梁; 刘开煌
Original assignee: Ji'an Fabo Amorphous Technology Co ltd
Current assignee: Ji'an Fabo Amorphous Technology Co ltd
Priority date: 2022-06-06
Filing date: 2022-06-06
Publication date: 2023-05-12
Anticipated expiration: 2042-06-06
Also published as: CN114953806A

Abstract

The invention provides a magnetic microfilament and a safety medium. Magnetic microfilamentThe magnetic metal microfilament is in a core-shell structure, the core is a magnetic metal microfilament, and the glass layer is a coating layer, wherein the magnetic microfilament has a thickness of 50m when in a free state ^‑1 Above and not greater than 160m ^‑1 Is used for bending the material. A security medium comprising said magnetic microfilaments. The magnetic microfilament of the invention has a predetermined curvature and has a larger detectable angle and detectability than the existing magnetic microfilaments.

Description

Magnetic microfilaments and security media

Technical Field

The present invention relates to the field of security material preparation, and more particularly, to a magnetic microfilament and a security medium, which enable the security medium to detect the presence of the security medium through an electromagnetic wave anti-theft instrument or other electromagnetic detector.

Background

Information security becomes increasingly important as global informatization steps increase. In order to prevent the outflow of printed matter such as paper with confidential information, various papers and components containing magnetic media have been studied. The user can limit that confidential information can only be printed on the paper containing the magnetic medium, and the electromagnetic wave burglar alarm arranged at the inlet and the outlet can detect the existence of the paper containing the magnetic medium, so that the leakage of the confidential information is prevented, and the safety of the information is ensured. However, in the detection process of the electromagnetic wave anti-theft instrument, when the magnetic field direction is perpendicular to the easy magnetization direction of the magnetic medium, the detection rate of the object to be detected with poor isotropy is not high.

Therefore, it is necessary to develop a magnetic medium capable of making the object to be detected have a good isotropy.

Disclosure of Invention

An object of the present invention is to solve one or more of the problems occurring in the prior art, in view of the disadvantages of the prior art. For example, it is an object of the present invention to provide a magnetic microwire and a security medium, wherein the magnetic microwire enables the security medium to be more isotropic and the presence of the security medium to be better detected by an electromagnetic wave anti-theft device or other electromagnetic detector.

One aspect of the invention provides a magnetic microfilament having a core-shell structure, a core of magnetic metal microfilaments, a glass layer as a cladding,wherein the magnetic microfilament can have a length of 50m when in a free state ^-1 Above and not greater than 160m ^-1 Is used for bending the material.

Further, the magnetic microfilament can have a length of 70m when in the free state ^-1 Above and not greater than 130m ^-1 Is used for bending the material.

Further, the glass layer may have a non-uniform thickness in the radial direction of the magnetic metal microfilaments.

Further, the cross section of the magnetic metal microfilament and the cross section of the glass layer can be in an eccentric circle structure.

Further, the eccentricity of the eccentric circular structure may be greater than or equal to 0.4 μm and less than or equal to 5 μm.

Further, the average diameter of the magnetic microfilaments may be 6 μm or more and 30 μm or less, and the average diameter of the magnetic metal microfilaments may be 5 μm or more and 20 μm or less.

Further, the magnetostriction coefficient of the magnetic microfilament may be greater than or equal to 1×10 ^-8 And less than or equal to 1X 10 ^-6 。

Further, the average thickness of the glass layer may be 0.5 μm to 5 μm.

In another aspect of the invention, a security medium is provided comprising at least one magnetic microfilament as described above.

Further, the security medium may be security paper, security cardboard, security document, security tape, security strip, patch, tag or security device.

Further, the length of the magnetic microfilament may be greater than or equal to 3mm and less than 10mm.

Further, the diameter of the magnetic microfilaments is similar to the diameter of pulp fibers constituting the security paper.

In a further aspect, the invention provides an application of the magnetic microfilament in a magnetic sensor and a wave absorbing material.

Compared with the prior art, the invention has the beneficial effects that at least one of the following steps is included:

(1) The magnetic microfilament of the invention has a predetermined curvature and has a larger detectable angle and detectability than the existing magnetic microfilaments.

(2) The magnetic microfilaments are adopted to produce the safety medium, so that the safety medium can obtain better isotropy, the existence of the safety medium can be better detected through an electromagnetic wave anti-theft instrument or other electromagnetic detectors, and the omission ratio is reduced.

Drawings

The foregoing and other objects and features of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram of the structure of a magnetic microwire of the present invention;

FIG. 2 is a graph showing the trend of harmonic signal voltage of the magnetic microfilament according to the present invention along with the bending curvature;

FIG. 3 is a schematic view of the magnetic microfilament of example 1 being wound on a reel;

FIG. 4 is a physical image of the magnetic microfilaments of example 1 with a length of 8mm dispersed on the surface of the release paper;

FIG. 5 is an optical microscopy image of a magnetic microwire of 8mm length of example 1;

FIG. 6 is a physical image of the magnetic microfilaments of comparative example 1 with a length of 8mm dispersed on the surface of the release paper;

FIG. 7 is an optical microscopy image of a magnetic microwire of length 8mm of comparative example 1;

FIG. 8 is a schematic diagram of the magnetic microfilaments of example 2 equally spaced on a printing paper;

FIG. 9 is a schematic rotation diagram of the magnetic microfilaments of example 2 after they are equally spaced on a printing paper;

FIG. 10 is a schematic illustration of the equidistant distribution of the magnetic microfilaments of comparative example 1 on a printing paper;

FIG. 11 is a graph showing the harmonic signal voltage of the magnetic microwires of example 2 and comparative example 1 as a function of the angle with the magnetic field;

FIG. 12 is a plot of the harmonic voltage spectra measured for the magnetic microwires of sample No. 2-1 of Table 2;

FIG. 13 is a plot of harmonic voltage spectra measured on magnetic microwires with sample numbers 2-2 in Table 2;

FIG. 14 is a schematic view of security paper prepared from 8mm length magnetic microfilaments of example 1;

FIG. 15 is an optical micrograph of security paper prepared from 8mm length magnetic microfilaments of example 1;

FIG. 16 is a schematic view of security paper prepared from magnetic microfilaments of length 8mm of comparative example 1.

Detailed Description

Hereinafter, a magnetic microfilament and a security medium according to the present invention will be described in detail with reference to the accompanying drawings and exemplary embodiments.

One aspect of the present invention provides a magnetic microwire. In some embodiments, the magnetic microfilaments have a core-shell structure comprising a core of magnetic metal microfilaments or a core of magnetic metal filaments, the surface of the core being coated with a glass layer as a skin. The magnetic microfilament has a free state of 50m ^-1 Above and not greater than 160m ^-1 Is used for bending the material. For example, the magnetic microfilament has a free state of greater than or equal to 50m ^-1 Greater than or equal to 55m ^-1 Greater than or equal to 62m ^-1 Greater than or equal to 73m ^-1 87m or more ^-1 Greater than or equal to 97m ^-1 Greater than or equal to 105m ^-1 Greater than or equal to 116m ^-1 Greater than or equal to 121m ^-1 Greater than or equal to 137m ^-1 Greater than or equal to 142m ^-1 Less than or equal to 157m ^-1 Less than or equal to 144m ^-1 Less than or equal to 131m ^-1 Less than or equal to 111m ^-1 Less than or equal to 95m ^-1 Less than or equal to 84m ^-1 Or a combination of the above ranges. Preferably, the magnetic microfilament has a free state of 70m ^-1 Above and not greater than 130m ^-1 Is a curved curvature of (a); in the preferred range, the magnetic microfilaments have good process realizability and large Barkhausen effect, and can ensure good isotropy of the safety medium. The bending curvature may be measured, i.e. the bending curvature may be obtained by taking the inverse of the radius of curvature of the magnetic microfilament. The composition of the magnetic metal microfilaments may be a composition comprising Co, fe, si and/or B and other elements known in the art to be useful in the preparation of magnetic microfilamentsFor example, mn, ni, cr, or the like.

In some embodiments, the glass layer has a non-uniform thickness in the radial direction of the magnetic metal microfilaments, i.e., the glass layer is coated with a non-uniform thickness in the radial direction of the magnetic metal microfilaments.

In some embodiments, the magnetic metal microfilament cross-section and the glass layer cross-section are in an eccentric circular configuration. The circle centers of the cross sections of the magnetic metal microfilaments are not overlapped with the circle centers of the cross sections of the glass layers, and mutually form a nested structure. The cross section of the magnetic metal microfilament and the cross section of the glass layer are in an eccentric circle structure, the eccentric circle structure can be formed by the fact that the glass layer has uneven thickness in the radial direction of the magnetic metal microfilament, so that stress borne by the magnetic microfilament is uneven, a certain bending rate is spontaneously formed, external force is not needed, and compared with the existing curve configuration for realizing the magnetic microfilament through the external force, the efficiency is higher, and the distribution of the magnetic microfilament is more uniform.

In some embodiments, the eccentricity of the eccentric circular structure may be greater than or equal to 0.4 μm and less than or equal to 5 μm. For example, the eccentricity of the eccentric circular structure may be greater than or equal to 0.6 μm, greater than or equal to 0.9 μm, greater than or equal to 1.1 μm, greater than or equal to 1.5 μm, greater than or equal to 1.9 μm, greater than or equal to 2.1 μm, greater than or equal to 2.6 μm, greater than or equal to 3.1 μm, greater than or equal to 3.7 μm, greater than or equal to 4.2 μm, less than or equal to 4.8 μm, less than or equal to 4.3 μm, less than or equal to 3.5 μm, less than or equal to 2.9 μm, less than or equal to 1.7 μm, less than or equal to 0.5 μm, or a combination of the above ranges. The eccentric circle structure affects the bending curvature and the difficulty of the preparation process of the magnetic microfilaments, and if the eccentricity is smaller than 0.4 mu m, the bending curvature of the magnetic microfilaments is smaller, and the magnetic microfilaments are basically in a straight state; if the eccentricity is larger than 5 mu m, the preparation process is not easy to realize, and the preparation efficiency and the cost are affected.

In some embodiments, the average diameter of the magnetic microfilaments may be greater than or equal to 6 μm and less than or equal to 30 μm. For the lower limit of the diameter of the magnetic microfilament, when the diameter of the magnetic microfilament is smaller than 6. Mu.m, the microfilament is too thin to be broken easily, and furthermore, the purity of the raw material is required to be high and the production efficiency is also low, so that it is not preferable. For example, the average diameter of the magnetic microfilaments may be greater than or equal to 8 μm, greater than or equal to 11 μm, greater than or equal to 17 μm, greater than or equal to 22 μm, greater than or equal to 29 μm, less than or equal to 27 μm, less than or equal to 20 μm, less than or equal to 15 μm, less than or equal to 12 μm, or a combination of the above ranges.

In some embodiments, the average diameter of the magnetic metal microfilaments may be greater than or equal to 5 μm and less than or equal to 20 μm. For example, the average diameter of the magnetic metal microfilaments may be 7 μm or more, 11 μm or more, 14 μm or more, 17 μm or more, 19 μm or more, 18 μm or less, 13 μm or less, 9 μm or less, 6 μm or more. Compared with the security paper type security medium, the thickness of the plastic shell of the USB flash disk is about 1mm, the magnetic microfilaments can be thicker, but the diameter of the magnetic metal microfilaments is not more than 20 mu m, otherwise, the length of the magnetic microfilaments needs to be increased to maintain the large Barkhausen effect.

In some embodiments, the magnetostriction coefficient of the magnetic microfilament may be greater than or equal to 1X 10 ^-8 And less than or equal to 1X 10 ^-6 In the magnetostriction coefficient range, the smaller coercive force and the large Barkhausen effect can be combined, so that the detection of an electromagnetic wave burglar alarm or other electromagnetic detectors is facilitated. For example, the magnetostriction coefficient of the magnetic microfilament may be greater than or equal to 2X 10 ^-7 Greater than or equal to 3X 10 ^-7 Greater than or equal to 4X 10 ^-7 Greater than or equal to 5X 10 ^-7 Greater than or equal to 7X 10 ^-7 Greater than or equal to 9X 10 ^-7 Less than or equal to 8X 10 ^-7 Less than or equal to 6X 10 ^-7 Or a combination of the above ranges.

In some embodiments, the glass layer may have an average thickness of 0.5 μm to 5 μm. If the average thickness of the glass layer is more than 5 mu m, the magnetic microfilaments occupy smaller area, and the detectability of the microfilaments in unit mass is reduced; if the average thickness of the glass layer is less than 0.5 mu m, the process difficulty is high. For example, the average thickness of the glass layer may be greater than or equal to 0.8 μm, greater than or equal to 1.1 μm, greater than or equal to 1.9 μm, greater than or equal to 2.2 μm, greater than or equal to 3.1 μm, greater than or equal to 3.9 μm, greater than or equal to 4.2 μm, greater than or equal to 4.7 μm, less than or equal to 4.8 μm, less than or equal to 4.1 μm, less than or equal to 3.5 μm, less than or equal to 2.7 μm, less than or equal to 1.3 μm, less than or equal to 0.7 μm, or a combination of the foregoing ranges.

In some embodiments, as shown in fig. 1, fig. 1 (a), fig. 1 (B), and fig. 1 (C) are schematic structural diagrams of a magnetic microwire 10 as described herein. FIG. 1 (A) shows a single chopped magnetic microfilament 10 having a radius of curvature R (in m) which is the inverse 1/R (in m) ^-1 ). It is readily understood that the radius of curvature is smaller as the curvature is larger; when the magnetic microfilament is in a straight state, the curvature is infinitely small. Fig. 1 (B) is a partial enlarged view of a portion ii in fig. 1 (a), in which the core of the magnetic microwire 10 is a magnetic metal microwire 12 and is a portion of the magnetic microwire having magnetism. The surface of the magnetic microfilament 10 is an optically transparent glass layer which is coated with a non-uniform thickness along the radial direction of the magnetic metal microfilament 12, wherein the thicker region is 14 and the thinner region is 16. During the preparation process, for example, the Taylor-Ulitovsky method or the modified Taylor-Ulitovsky method known in the art or other methods, when the magnetic microfilaments are cooled from high temperature, the thick region and the thin region will have different cooling shrinkage due to the non-uniform thickness of the glass layer, thus generating stress imbalance, and the magnetic microfilaments will spontaneously bend to a certain curvature. The inside of the bend may be a region where the glass layer is thin or a region where the glass layer is thick. Fig. 1 (C) is a schematic cross-sectional view of the magnetic microfilament 10, and it can be clearly seen that the cross-section of the magnetic metal microfilament 12 and the cross-section of the glass layer form an eccentric circle structure, and the eccentricity is marked as D. When the bending curvature of the magnetic microfilament is 50m ^-1 Above and not greater than 160m ^-1 When the method is used, the method has better detection rate and detection angle, for example, the detectable angle can reach 80 degrees or more, and the isotropic security medium is formed.

FIG. 2 shows a magnetic microfilament (example 1Magnetic microfilaments) with a diameter of 22 μm and a diameter of 16.5 μm, the frequency of the excitation signal being 1kHz, the detection signal being 19 th harmonic. When the bending curvature is higher than 160m ^-1 It is found that the signal of the harmonic voltage drops significantly, so that the bending curvature is preferably not higher than 160m ^-1 。

Another aspect of the invention provides a security medium, in some embodiments, comprising at least one magnetic microfilament as described herein. For example, the secure media contains hundreds, tens of thousands, millions, or billions of magnetic filaments.

In some embodiments, the security medium may be a value document or value device or some other value item, such as security paper, security cardboard, security document, security belt, security strip, patch or tag, that records or contains confidential information or the like. The above-mentioned security papers, security documents, security strips, patches or labels, etc. have many fields of use. It can be placed in a package of pre-sold goods or products along with a given product. The security medium may contain information related to the goods or products. But also in clothing products with specific origin, in securities, in raw materials for the production of lottery tickets, in the production of folders and other publications of different kinds, or in the production of labels of different kinds, such as cans, bottles or other packaging units containing pharmaceuticals and medicines. When it is important to say that the information sheet or document comes from a specific company, the security paper according to the present invention can be used as an information sheet delivered from a different company to the general public or a specific target group. The above security media can be used in a wide variety of packaging applications, for example, can be converted into a box or any other kind of container for holding, for example, pharmaceuticals, cigarettes, perfumes, chocolate and the like. The valuable device or valuables can be a USB flash disk plastic housing, a computer housing, etc. In certain embodiments, when the security medium is security paper, the security paper is prepared using the magnetic microfilaments described herein with specific bending curvatures, preferably using air flow (dry papermaking) or water flow (wet papermaking), which is less likely to form an oriented structure due to imbalance in forces under the action of fluids such as air flow or water flow, thus enabling the formation of a better isotropic security paper.

In some embodiments, the length of the magnetic microwire may be greater than or equal to 3mm and less than 10mm. For the length of the magnetic microfilaments, if exceeding 10mm, the magnetic microfilaments are liable to become entangled, whether in paper making or injection molding. In addition, when the magnetic microfilament is used for preparing a safety medium, the length is lower than 3mm, and the shearing is difficult; the length is more than 10mm, so that the fiber is not easy to disperse in a safety medium or easy to tangle. For example, the length of the magnetic microfilaments may be greater than or equal to 4.5mm, greater than or equal to 5.2mm, greater than or equal to 6mm, greater than or equal to 7.5mm, greater than or equal to 8.9mm, less than or equal to 9.4mm, less than or equal to 7.4mm, less than or equal to 4.8mm, less than or equal to 3.2mm, or combinations of the above ranges.

In some embodiments, when the magnetic microfilaments are formed into a security medium for security papers, the diameter is preferably close to the diameter of the pulp fibers, otherwise the security paper is susceptible to wrinkling around the magnetic microfilaments.

The eccentricity D of the magnetic microfilament of the present invention has a general correlation with the bending curvature. In general, the larger the eccentricity D, the larger the bending curvature. The applicant has found that there are other factors affecting the curvature of the bend, the eccentricity D being the main affecting factor. When the eccentricity is equal to 0.4 μm, the bending curvature of the magnetic microfilament is about 50m ^-1 The method comprises the steps of carrying out a first treatment on the surface of the When the eccentricity is equal to 5 μm, the bending curvature of the magnetic microfilament is about 160m ^-1 。

The magnetostriction coefficient of the magnetic microwire is a key factor in determining its detection rate. Unlike some views of the prior art, the applicant has found that magnetic microfilaments have a small and positive magnetostriction coefficient, enabling a more stable large barkhausen effect to be obtained. The small coercive force can be achieved by controlling the demagnetizing field and adjusting the internal stress. The magnetic microfilament of the invention has a specific bending curvature, and the demagnetizing field is obviously different from that of the straight filaments in the prior art. In addition, the glass layer of the invention is along the radial direction of the magnetic metal microfilamentsThe coating is carried out in a non-uniform thickness, and the distribution of stress in the coating is obviously different from that of the prior art straight wire. When the magnetostriction coefficient of the magnetic microfilament is less than 1×10 ^-8 When the method is used, the large Barkhausen effect is not obvious, and high higher harmonic voltage is not easy to obtain; when the magnetostriction coefficient of the magnetic microfilament is more than 1 multiplied by 10 ^-6 When the coercivity is too large, it is difficult for the excitation signal to magnetize the magnetic microfilaments.

In yet another aspect of the invention, there is provided the use of a magnetic microwire as described herein in a magneto-sensitive element as well as a wave absorbing material.

For a better understanding of the present invention, the content of the present invention is further elucidated below in conjunction with the specific examples, but the content of the present invention is not limited to the following examples only.

Example 1

The improved Taylor-Ulitovsky method is adopted to prepare the magnetic microfilaments. The preparation method comprises using a material with a diameter of 20mm and an inner diameter of 15mm and a thermal expansion coefficient of 3.3X10 ^-6 K ^-1 Is a high borosilicate glass tube. The original glass tube has uniform wall thickness, the wall thickness of one side is thinned from the bottom, the wall thickness is about 1mm at the thinnest part, and the wall thickness of the other side is 2.5 mm. The thinning treatment is a mode capable of realizing non-uniform thickness cladding of the glass layer along the radial direction of the microfilaments, and can comprise etching away one side glass by hydrofluoric acid, thinning by a mechanical grinding method or thinning by a thermal processing method, wherein the thinning can be performed by a mechanical grinding method. The cobalt-based alloy is added into a glass tube, and the spinning process is carried out according to the components of 70.5% of Co,4.5% of Fe,11% of Si,12% of B and 2% of Cr by atomic percentage, wherein the spinning speed is 150m/min. The diameter of the prepared magnetic microfilament is 22 μm, and the diameter of the magnetic metal microfilament is 16.5 μm. The magnetic microfilaments can be easily prepared in batches under the process parameters, and as shown in fig. 3, the magnetic microfilaments can be wound on a reel, and the length of a single microfilament can be up to 100km. As shown in FIG. 4, FIG. 4 shows that magnetic microfilaments having a length of 8mm are dispersed on the surface of a release paper to take on a spontaneous curved shape with a curvature of 90m ^-1 . As shown in fig. 5, the magnetic microwire shown in fig. 4 is subjected to an optical microscopeAs a result of the observation, it was found that the glass layer was thick and thin on one side, the thickness of the glass layer was 4.1. Mu.m in the thick region, the thickness of the glass layer was 1.4. Mu.m in the thin region, and the eccentricity was 1.35. Mu.m.

Comparative example 1

The improved Taylor-Ulitovsky method is adopted to prepare the magnetic microfilaments. The thermal expansion coefficient is 3.3X10 when the diameter is 20mm and the inner diameter is 15mm ^-6 K ^-1 The high borosilicate glass tube has uniform wall thickness. Adding cobalt-based alloy, and spinning at a spinning speed of 150m/min, wherein the cobalt-based alloy comprises 70.5% of Co,4.5% of Fe,11% of Si,12% of B and 2% of Cr by atomic percentage. The diameter of the prepared magnetic microfilament is 22 μm, and the diameter of the magnetic metal microfilament is 16.5 μm. As shown in fig. 6, the magnetic microfilaments having a length of 8mm are dispersed on the surface of the release paper to take a straight shape. As shown in FIG. 7, the magnetic microfilaments shown in FIG. 6 were observed under an optical microscope to find that the thickness of both sides of the glass layer was 2.75. Mu.m. This is consistent in shape with the prior art magnetic microwires.

Example 2

The magnetic microfilaments 10 prepared in example 1 were cut into 10mm lengths, 16 were taken out and distributed on a sheet of 60mm 45mm printing paper at equal intervals as shown in fig. 8, wherein the tangent line of the midpoint of the magnetic microfilaments 10 was parallel to the long side of the printing paper and fixed with office glue. The harmonic voltages of the magnetic microwires were tested in a uniform alternating magnetic field. The magnetic field maximum was 1Oe and the frequency was 1kHz. As shown in fig. 9, the printing paper is rotated with the geometric center of the printing paper so that the included angle θ of the magnetic microfilaments and the magnetic field is 0 °, 10 °, 20 °, 30 °, 40 °, 50 °, 60 °, 70 °, 80 °, 90 ° in order, and 7 th harmonic voltage values of the respective angles are recorded.

The magnetic microwires were replaced with the magnetic microwires 20 of comparative example 1, which were also 10mm in length and 16 in number, and were equally spaced apart on a 60mm 45mm piece of printing paper as shown in fig. 10, consistent with fig. 8. Repeating the above test steps.

The above test results are shown in fig. 11, where the curve labeled "invention" represents the harmonic voltage values corresponding to the magnetic microfilaments prepared in example 1 at different angles, and the curve labeled "prior art" represents the harmonic voltage values corresponding to the magnetic microfilaments prepared in comparative example 1 at different angles, and it can be found that: the harmonic signal voltages of the magnetic microwires of example 1 and comparative example 1 were each attenuated as the angle between the magnetic microwire and the magnetic field increased, and were not detected when the angle was 90 °, i.e., the magnetic microwire was perpendicular to the magnetic field. However, the harmonic signal voltage of the magnetic microwire 10 of embodiment 1 of the present invention decays more slowly. At most angles, the harmonic signal voltage of the magnetic microwire 10 of example 1 of the present invention is higher than that of comparative example 1 of the prior art. In addition, the detectable angle of the magnetic microwire 20 of comparative example 1 was 70 °, the detectable angle of the magnetic microwire 10 of inventive example 1 was 80 °, and the detectable angle of the magnetic microwire 10 of the present invention was larger. The magnetic microfilament 10 according to the invention presents the advantage that it has a certain bending curvature itself.

Example 3

The improved Taylor-Ulitovsky method is adopted to prepare the magnetic microfilaments. The thermal expansion coefficient is 3.3X10 by using a diameter of 18mm and an inner diameter of 12mm ^-6 K ^-1 The wall thickness of the original glass tube is uniform. In order to obtain the magnetic microfilaments with different bending curvatures, the wall thickness of one side is thinned from the bottom of the glass tube, the thinnest part of the wall thickness is 0.5-2.6 mm, and the thickness of the other side is 3mm unchanged. The cobalt-based alloy is added into a glass tube, and the spinning process is carried out according to the components of 70.5% of Co,4.5% of Fe,11% of Si,12% of B and 2% of Cr by atomic percentage, wherein the spinning speed is 100-300 m/min. The geometry of the prepared magnetic microfilaments is shown in table 1. Generally, in the case where the diameter of the magnetic microfilament is the same as that of the magnetic metal microfilament, the larger the eccentricity is, the larger the bending curvature of the magnetic microfilament is; the smaller the diameter of the magnetic microfilament, the greater the bending curvature thereof at the same eccentricity.

TABLE 1 magnetic microwire parameters

Example 4

The improved Taylor-Ulitovsky method is adopted to prepare the magnetic microfilaments. The thermal expansion coefficient is 3.3X10 by using a diameter of 18mm and an inner diameter of 12mm ^-6 K ^-1 The wall thickness of the original glass tube is uniform. To obtain a bending curvature of about 100m ^-1 The thickness of one side of the magnetic microfilament is thinned from the bottom of the glass tube, the thinnest part of the wall thickness is 1.1mm, and the thickness of the other side is 3 mm. The different alloy compositions in table 2 were prepared into magnetic microfilaments (the alloy compositions in table 2 are in atomic percent), the diameter of the magnetic microfilaments was 22 μm, the diameter of the magnetic metal microfilaments was 16.5 μm, and the eccentricity was 1.75 μm. The hysteresis loop of the magnetic microfilaments was measured to observe the coercivity and the large Barkhausen effect with a test frequency of 1kHz, a sample length of 10mm and a magnetic field amplitude of 100A/m. The magnetostriction coefficient is measured by a small angle rotation method (SAMR). The test results are shown in Table 2. As can be seen from table 2, a magnetic microfilament with a small and negative magnetostriction coefficient is advantageous for obtaining a smaller coercivity, but the large barkhausen effect is weaker. A magnetic microfilament with a small but positive magnetostriction coefficient is advantageous for obtaining a strong large barkhausen effect. The coercive force can be adjusted by adjusting internal stress, aspect ratio of the sample, and bending curvature. In addition, the magnetostriction coefficient is of the order of 10 with stress (internal stress or test stress) ^-10 Correlation in MPa.

TABLE 2 magnetostriction coefficient and Large Barkhausen Effect test results

FIG. 12 is a graph showing that the sample No. 2-1 in Table 2 has a magnetostriction coefficient of-1X 10 ^-8 The harmonic voltage spectrum measured by the magnetic microfilament of (2) has a fundamental frequency of 1kHz. It can be seen that the higher harmonic voltages are smaller and decay faster. FIG. 13 is a graph showing that the sample No. 2-2 and the magnetostriction coefficient is 3X 10 in Table 2 ^-7 Harmonic voltage frequency measured by magnetic microfilamentsSpectrum, fundamental frequency 1kHz. It can be seen that the higher harmonic voltages are larger and the attenuation is slower.

In view of the above, the magnetostriction coefficient of the magnetic microfilament is thus 1×10 or more ^-8 And less than or equal to 1X 10 ^-6 Is preferable.

Example 5

The magnetic microfilaments of example 1 were cut into lengths of 3mm, 6mm, 8mm, 10mm and 12mm, respectively. The magnetic microfilaments of comparative example 1 were cut into 8mm lengths. The chopped magnetic microfilaments are respectively dispersed into paper pulp to manufacture safety paper on wet papermaking equipment, the weight ratio of the magnetic microfilaments to the paper pulp (dry weight) is about 1:400, and the technological parameters such as water flow speed and the like are kept identical.

The resulting security paper is cut to A4 size, wherein the length of the A4 security paper is taken in the MD direction. The number of magnetic microfilaments on A4 paper was counted. And (5) testing the detection rate of the A4 security paper by using an electromagnetic wave anti-theft instrument. Cutting out a wafer with the diameter of 200mm by taking the geometric center of the A4 security paper as the circle center, and testing signals in the MD direction and the TD direction at the fixed position of the electromagnetic wave anti-theft instrument so as to observe the distribution orientation condition of the magnetic microfilaments. The above results are recorded in table 3.

TABLE 3 detection results for different security papers

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From the test results, it can be seen that the magnetic metal microfilament of the present invention has a diameter of 16.5. Mu.m, and a total diameter of 22. Mu.m, and has a high detection rate when the length is greater than 6 mm. In addition, the signal values in the MD direction and the TD direction of the security paper of the present invention are substantially the same, which means that the magnetic microfilaments having a certain bending curvature of the present invention have no significant orientation under the action of water flow, and a substantially isotropic security paper can be obtained. Fig. 14 is a diagram of a security paper made of the magnetic microfilaments 10 (microfilaments length 8 mm) according to the present invention, and it can be seen that the magnetic microfilaments near the surface of the security paper are not oriented in the MD direction. This is because the water flow tends to create a moment on the bent microfilaments, which tends to turn over and advance in the water flow, and thus the orientation of the microfilaments tends to be randomly distributed. Fig. 15 is an optical microscope image of a security paper 30 (security paper shown in fig. 14) of the present invention, in which the diameter of the magnetic microfilaments 10 is equivalent to the diameter of the pulp fibers 18, and the fusion of the two is good, and no wrinkles are generated.

The security paper prepared by the magnetic microfilaments 20 of comparative example 1 has strong signal values in the MD direction and weak signal values in the TD direction, and weakens the overall detection rate. As shown in fig. 16, the majority of the magnetic microwires 20 exhibit a more uniform orientation in the MD direction.

In summary, the magnetic microfilaments and the safety medium provided by the invention have the advantages that the magnetic microfilaments are designed to be capable of spontaneously bending into a shape with a certain curvature, so that the detectability of the magnetic microfilaments is improved, a better uniform distribution effect can be achieved in the safety medium, and the better detectability of the safety medium is realized.

Although the present invention has been described above by way of the combination of the exemplary embodiments, it should be apparent to those skilled in the art that various modifications and changes can be made to the exemplary embodiments of the present invention without departing from the spirit and scope defined in the appended claims.

Claims

1. A magnetic microfilament is characterized by having a core-shell structure, wherein a magnetic metal microfilament is used as a core and a glass layer is used as a coating layer,

the magnetic microfilament has a free state of 50m ^-1 Above and not greater than 160m ^-1 Is a curved curvature of (a); the glass layer has a non-uniform thickness in the radial direction of the magnetic metal microfilaments.

2. The magnetic microfilament of claim 1 having a free state of 70m ^-1 Above and not greater than 130m ^-1 Is used for bending the material.

3. The magnetic microfilament of claim 1 or 2 wherein the cross section of the magnetic metal microfilament is of an eccentric circular configuration with the cross section of the glass layer.

4. The magnetic microfilament of claim 3 wherein the eccentricity of the eccentric circular structure is greater than or equal to 0.4 μm and less than or equal to 5 μm.

5. The magnetic microfilament of claim 1 or 2 wherein the average diameter of the magnetic microfilament is greater than or equal to 6 μm and less than or equal to 30 μm and the average diameter of the magnetic metal microfilament is greater than or equal to 5 μm and less than or equal to 20 μm.

6. The magnetic microfilament according to claim 1 or 2, wherein the magnetostriction coefficient of the magnetic microfilament is greater than or equal to 1 x 10 ^-8 And less than or equal to 1X 10 ^-6 。

7. The magnetic microfilament of claim 1 or 2 wherein the glass layer has an average thickness of 0.5 μm to 5 μm.

8. A security medium comprising at least one magnetic microfilament as claimed in any of claims 1 to 7.

9. The security media of claim 8, wherein the security media is a security document or a security device.

10. The security media of claim 8 or 9, wherein the length of the magnetic microwires is greater than or equal to 3mm and less than 10mm.

11. Use of a magnetic microfilament according to any of claims 1 to 7 in a magneto-sensitive element and a wave absorbing material.