CN114953806A

CN114953806A - Magnetic microfilaments and security media

Info

Publication number: CN114953806A
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: 2022-08-30
Anticipated expiration: 2042-06-06
Also published as: CN114953806B

Abstract

The invention provides a magnetic microfilament and a safety medium. The magnetic microfilament has a core-shell structure, the magnetic metal microfilament is a core, 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 more than 160m ^‑1 The bending curvature of (2). A security medium comprising said magnetic microwire. The magnetic microwire has a predetermined curvature, and has a larger detectable angle and detectability than the existing magnetic microwire.

Description

Magnetic microfilaments and security media

Technical Field

The present invention relates to the field of security material preparation technology, and more particularly, to a magnetic microwire and a security medium, which enables the security medium to detect the presence of the security medium through an electromagnetic wave theft prevention instrument or other electromagnetic detectors.

Background

Information security is becoming increasingly important as the pace of global informatization speeds. In order to prevent the outflow of printed matter such as paper with confidential information, various kinds of paper and components including magnetic media have been studied. The user can limit the confidential information to be printed only on the paper containing the magnetic medium, and the electromagnetic wave anti-theft instrument arranged at the entrance can detect the existence of the paper containing the magnetic medium, thereby preventing the leakage of the confidential information and ensuring the safety of the information. However, in the detection process of the electromagnetic wave anti-theft instrument, when the direction of the magnetic field 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 that can make the object to be detected have good isotropy.

Disclosure of Invention

In view of the deficiencies in the prior art, it is an object of the present invention to address one or more of the problems in the prior art as set forth above. For example, it is an object of the present invention to provide a magnetic microwire and a security medium, the magnetic microwire enabling the security medium to obtain better isotropy, enabling better detection of the presence of the security medium by means of an electromagnetic wave theft protection instrument or other electromagnetic detector.

One aspect of the present invention provides a magnetic micro-wire having a core-shell structure, a magnetic metal micro-wire as a core, and a glass layer as a coating layer, wherein the magnetic micro-wire may have a thickness of 50m in a free state ^-1 Above and not more than 160m ^-1 The bending curvature of (2).

Further, the magnetic micro-wires may have a 70m in a free state ^-1 Above and not more than 130m ^-1 The bending curvature of (2).

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

Further, the cross section of the magnetic metal microwire 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 micro-wires may be greater than or equal to 6 μm and less than or equal to 30 μm, and the average diameter of the magnetic metal micro-wires may be greater than or equal to 5 μm and less than or equal to 20 μm.

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

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

Another aspect of the present invention provides a security medium 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, label or security device.

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

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

The invention also provides an application of the magnetic microfilament in a magnetic sensitive element and a wave-absorbing material.

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

(1) the magnetic microwire has a predetermined curvature, and has a larger detectable angle and detectability than the existing magnetic microwire.

(2) The magnetic microfilament is 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 factor is reduced.

Drawings

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

FIG. 1 is a schematic structural view of a magnetic microfilament according to the present invention;

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

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

FIG. 4 is a physical diagram of the release paper with 8mm long magnetic microfilaments of example 1;

FIG. 5 is an optical micrograph of magnetic microwires of 8mm in length according to example 1;

FIG. 6 is a physical diagram of the magnetic microfilament of comparative example 1 with a length of 8mm scattered on the surface of release paper;

FIG. 7 is an optical microscopic image of magnetic microwires 8mm in length of comparative example 1;

fig. 8 is a schematic view of the magnetic micro-wires of example 2 distributed on a printing paper at equal intervals;

fig. 9 is a schematic view showing the rotation of the magnetic micro-wires of example 2 after the magnetic micro-wires are equally spaced on the printing paper;

fig. 10 is a schematic view of the magnetic micro-wires of comparative example 1 being equally spaced on a printing paper;

FIG. 11 is a graph comparing the variation of the harmonic signal voltage with the angle to the magnetic field of the magnetic microwires of example 2 and comparative example 1;

FIG. 12 is a graph of the harmonic voltage spectrum measured for the magnetic microfilament of sample number 2-1 in Table 2;

FIG. 13 is a graph of the harmonic voltage spectrum measured for the magnetic microfilament sample number 2-2 in Table 2;

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

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

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

Detailed Description

Hereinafter, a magnetic micro-wire 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 microwire has a core-shell structure comprising a magnetic metal microwire as a core or a magnetic metal wire as a core part, the surface of the core being coated with a glass layer as a skin. The magnetic microfilament has a thickness of 50m in free state ^-1 Above and not more than 160m ^-1 The bending curvature of (2). For example, the magnetic microwires have a length of greater than or equal to 50m in the free state ^-1 Greater than or equal to 55m ^-1 Greater than or equal toIs 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 157m or less ^-1 144m or less ^-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 combinations of the above ranges. Preferably, the magnetic microfilaments have a thickness of 70m in the free state ^-1 Above and not more than 130m ^-1 The curvature of (a); in the preferred range, the magnetic microfilament has good process realizability and large Barkhausen effect, and can ensure better isotropy of the safety medium. The bending curvature here can be measured, i.e. by taking the inverse of the radius of curvature of the magnetic microwires. The composition of the magnetic metal microwire may be Co, Fe, Si and/or B containing other elements known in the art that may be used to make the magnetic microwire, such as Mn, Ni or Cr, etc.

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

In some embodiments, the magnetic metal microwire cross-section and the glass layer cross-section are in an eccentric circle configuration. The circle center of the cross section of the magnetic metal microwire is not overlapped with the circle center of the cross section of the glass layer, and a nested structure is formed. The cross section of the magnetic metal microwire and the cross section of the glass layer are of an eccentric circle structure, the structure can be realized by the glass layer with nonuniform thickness in the radial direction of the magnetic metal microwire, and the formed eccentric circle structure can make the stress borne by the magnetic microwire nonuniform so as to spontaneously form a certain bending rate without external force.

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 can 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 ranges therein. The eccentric circle structure influences the bending curvature of the magnetic microfilament and the difficulty degree of a preparation process, if the eccentricity is less than 0.4 mu m, the bending curvature of the magnetic microfilament is smaller, and the magnetic microfilament is basically in a straight state; if the eccentricity is more than 5 μm, 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. As for the lower limit of the diameter of the magnetic micro-filament, when the diameter of the magnetic micro-filament is less than 6 μm, the micro-filament is too thin to be easily broken, and further, the requirement for the purity of the raw material is high and the production efficiency is also low, and thus 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 combinations thereof.

In some embodiments, the average diameter of the magnetic metal microwires can 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 micro-wires may be greater than or equal to 7 μm, greater than or equal to 11 μm, greater than or equal to 14 μm, greater than or equal to 17 μm, greater than or equal to 19 μm, less than or equal to 18 μm, less than or equal to 13 μm, less than or equal to 9 μm, less than or equal to 6 μm, or a combination of ranges above. Compared with the safety paper type safety medium, for the plastic shell of the U disk, the thickness is about 1mm, the magnetic micro-wires can be thicker properly, but the diameter of the magnetic metal micro-wires is not more than 20 μm, otherwise, the length of the magnetic micro-wires needs to be increased to maintain the large Barkhausen effect.

In some embodiments, the magnetic microfilaments may have a magnetostriction coefficient greater than or equal to 1 × 10 ^-8 And is less than or equal to 1 × 10 ^-6 In the range of the magnetostriction coefficient, the small coercive force and the large Barkhausen effect can be considered at the same time, so that the detection of an electromagnetic wave anti-theft instrument or other electromagnetic detectors is facilitated. For example, the magnetic micro-wires may have a magnetostriction coefficient of 2 × 10 or more ^-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 combinations of the above ranges.

In some embodiments, the average thickness of the glass layer may be 0.5 μm to 5 μm. If the average thickness of the glass layer is more than 5 mu m, the magnetic microfilament accounts for a smaller proportion, and the detectable performance of the microfilament per unit mass is reduced; if the average thickness of the glass layer is less than 0.5 μm, the process difficulty is large. For example, the average thickness of the glass layers can 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), 1(B), and 1(C) are schematic structural views of a magnetic microwire 10 as described herein. FIG. 1A shows a single chopped magnetic microfilament 10 having a radius of curvature R (in m) which is the inverse 1/R (in m) of the radius of curvature ^-1 ). It is easily understood that, when the curvature is larger, the radius of curvature is smaller; when the magnetic microwire is in a linear state, the curvature is infinitesimal. Fig. 1(B) is a partial enlarged view of a portion ii in fig. 1(a), and the magnetic microwire 10 has a magnetic metal microwire 12 as a core portion, which is a magnetic portion of the magnetic microwire. Of magnetic microfilaments 10The surface layer is an optically transparent glass layer, and the glass layer is coated along the radial direction of the magnetic metal microwire 12 in a non-uniform thickness, wherein the thicker area is 14, and the thinner area is 16. In 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 micro-filament is cooled from a high temperature, since the glass layer has a non-uniform thickness, the cooling shrinkage amount of the thick region and the thin region will be different, so that stress imbalance is generated, and the magnetic micro-filament is spontaneously bent to a certain curvature. The inner side 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 microwire 10, and it can be clearly seen that the cross-section of the magnetic metal microwire 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 microwire is 50m ^-1 Above and not more than 160m ^-1 It has a good detection rate and detection angle, for example, the detectable angle can reach 80 ° or more, and at the same time it is advantageous to form an isotropic security medium.

Fig. 2 shows the variation tendency of the harmonic signal voltage with the bending curvature of the magnetic micro-wire (the magnetic micro-wire of example 1) in which the diameter of the magnetic micro-wire is 22 μm, the diameter of the magnetic metal micro-wire is 16.5 μm, the frequency of the excitation signal is 1kHz, and the detection signal is 19 th harmonic. When the bending curvature is higher than 160m ^-1 It was found that the signal of the harmonic voltage is significantly reduced, and therefore the bending curvature is preferably not higher than 160m ^-1 。

Another aspect of the present invention provides a security medium, which in some embodiments comprises at least one magnetic microwire as described herein. For example, the security medium comprises hundreds, tens of thousands, millions, or billions of magnetic microwires.

In some embodiments, the security medium may be a value document or value device or some other valuable item that records or contains confidential information or the like, for example, security paper, security cardboard, security document, security strip, patch or label. The above security media such as security paper, security cardboard, security document, security strip, patch or label are used in many fields. It can be placed with a given product into the packaging of pre-sold goods or products. The security media may contain information relating to the goods or products. It can also be applied in clothing products with a specific production place, a specific design, securities, stock for the production of lottery tickets, in the production of different kinds of folders and other publications, or in the production of different kinds of labels, such as labels for cans, bottles or other packaging units containing drugs and medicines. When it is important to say that the information sheet or document comes from a specific company, the security sheet according to the present invention can be used as an information sheet to be transmitted from a different company to the general public or a specific target group. The above security medium may be applied in a wide range of packaging applications, e.g. may be converted into a box or any other kind of container for containing e.g. drugs, cigarettes, perfumes, chocolate, etc. The valuable device or the valuables can be a U disk plastic shell, a computer shell and the like. In certain embodiments, when the security medium is security paper, the security paper is prepared using magnetic microfilaments having a specific curvature as described herein, preferably using air flow (dry paper making) or water flow (wet paper making), and is less likely to form oriented structures due to imbalance of forces under the action of a fluid such as air flow or water flow, thus forming a more isotropic security paper.

In some embodiments, the magnetic microwires can have lengths greater than or equal to 3mm and less than 10 mm. When the length of the magnetic micro-wires exceeds 10mm, the magnetic micro-wires are easy to tangle in both paper making and injection molding. In addition, when the magnetic microfilament is used for preparing a safety medium, the length is less than 3mm, and the shearing is difficult; lengths above 10mm are not easily dispersed into the safety media or easily entangled. For example, the length of the magnetic microwires can 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 a combination of ranges therein.

In some embodiments, the magnetic microfilaments, when formed into a security paper-like security medium, preferably have a diameter close to the diameter of the pulp fibers, otherwise the magnetic microfilaments in the security paper tend to wrinkle around.

The eccentricity D of the magnetic microwire of the present invention has a rough correlation with the bending curvature. In general, the greater the eccentricity D, the greater the bending curvature. The applicant has found that there are other factors affecting the curvature of the bend, with eccentricity D being the main factor. When the eccentricity is equal to 0.4 μm, the bending curvature of the magnetic microwires is approximately 50m ^-1 (ii) a When the eccentricity is equal to 5 μm, the bending curvature of the magnetic microwires is approximately 160m ^-1 。

The magnetostriction coefficient of the magnetic microwires is a key factor determining the detectivity thereof. Contrary to some aspects 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. A small coercivity can be achieved by controlling the demagnetizing field and adjusting the internal stress. The magnetic microwire has a specific bending curvature, and the demagnetization field of the magnetic microwire is obviously different from that of a straight wire in the prior art. In addition, the glass layer of the invention is coated in a non-uniform thickness along the radial direction of the magnetic metal microwire, and the distribution of the internal stress is also obviously different from that of the straight wire in the prior art. When the magnetostriction coefficient of the magnetic micro-wires is less than 1 x 10 ^-8 In time, the large barkhausen effect is not obvious, and high higher harmonic voltage is not easy to obtain; when the magnetostriction coefficient of the magnetic micro-wires is more than 1 x 10 ^-6 When the coercive force is too large, the excitation signal is difficult to magnetize the magnetic microwire.

In a further aspect of the invention, there is provided the use of a magnetic microwire as described herein in a magnetosensitive element and in a wave-absorbing material.

For a better understanding of the present invention, the following further illustrates the contents of the present invention with reference to specific examples, but the contents of the present invention are not limited to the following examples.

Example 1

An improved Taylor-Uliotvsky method is adopted to prepare the magnetic microfilament. The preparation method comprises using a material with a diameter of 20mm, an inner diameter of 15mm, and a thermal expansion coefficient of 3.3 × 10 ^-6 K ^-1 Is highA borosilicate glass tube. The original glass tube has uniform wall thickness, and the wall thickness of one side is thinned from the bottom, the thinnest part of the wall thickness is about 1mm, and the other side is 2.5 mm. The thinning treatment is a mode that the glass layer is coated in a non-uniform thickness mode along the radial direction of the microwire, the thinning treatment can comprise the steps of etching away glass on one side by hydrofluoric acid, thinning by a mechanical grinding method or hot processing, and the embodiment can be thinned by the mechanical grinding method. The cobalt-based alloy is added into a glass tube, and the components of the cobalt-based alloy are 70.5 percent of Co, 4.5 percent of Fe, 11 percent of Si, 12 percent of B and 2 percent of Cr according to atomic percentage, and the spinning process is carried out, wherein the spinning speed is 150 m/min. The diameter of the prepared magnetic microwire is 22 μm, and the diameter of the magnetic metal microwire is 16.5 μm. The magnetic microfilament is easy to be prepared in batches under the process parameters, as shown in figure 3, the magnetic microfilament can be wound on a reel, and the length of a single microfilament can reach 100 km. As shown in FIG. 4, FIG. 4 shows that magnetic microfilaments with a length of 8mm are spread on the surface of release paper, and exhibit a spontaneous curved shape with a curvature of 90m ^-1 . As shown in FIG. 5, when the magnetic micro-wires shown in FIG. 4 were observed under an optical microscope, it was found that the glass layer was thick and thin, the thickness of the thick region glass layer was 4.1 μm, the thickness of the thin region glass layer was 1.4 μm, and the eccentricity was 1.35. mu.m.

Comparative example 1

An improved Taylor-Uliotvsky method is adopted to prepare the magnetic microfilament. The material has a diameter of 20mm, an inner diameter of 15mm, and a thermal expansion coefficient of 3.3 × 10 ^-6 K ^-1 The high borosilicate glass tube has uniform wall thickness. Adding a cobalt-based alloy, wherein the components of the cobalt-based alloy are 70.5% of Co, 4.5% of Fe, 11% of Si, 12% of B and 2% of Cr in atomic percentage, and carrying out a spinning process, wherein the spinning speed is 150 m/min. The diameter of the prepared magnetic microwire is 22 μm, and the diameter of the magnetic metal microwire is 16.5 μm. As shown in fig. 6, the magnetic microfilaments with a length of 8mm are spread on the surface of the release paper and take on a straight shape. As shown in FIG. 7, the magnetic microfilaments shown in FIG. 6 were observed under an optical microscope to find a glass layer with a thickness of 2.75 μm on both sides. This corresponds to the shape of the magnetic microwires of the prior art.

Example 2

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

The magnetic microfilaments were replaced with the magnetic microfilaments 20 of comparative example 1, also 10mm in length and 16 in number, and were distributed on a 60mm by 45mm piece of printing paper at equal intervals in accordance with fig. 8, as shown in fig. 10. The above test procedure was repeated.

The above test results are shown in fig. 11, where the curve labeled "invention" in the figure represents the harmonic voltage values corresponding to different included angles using the magnetic micro-filament prepared in example 1, and the curve labeled "prior art" in the figure represents the harmonic voltage values corresponding to different included angles using the magnetic micro-filament prepared in comparative example 1, it can be found that: the harmonic signal voltage of the magnetic microwires of example 1 and comparative example 1 both attenuated with increasing included angle of the magnetic microwires and the magnetic field, and when the angle was 90 °, i.e., the magnetic microwires were perpendicular to the magnetic field, no harmonic signal voltage was detected. However, the harmonic signal voltage of the magnetic micro-wire 10 of embodiment 1 of the present invention decays more slowly. 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 at most included angles. In addition, the detectable angle of the magnetic micro-wires 20 of comparative example 1 was 70 °, the detectable angle of the magnetic micro-wires 10 of example 1 of the present invention was 80 °, and the detectable angle of the magnetic micro-wires 10 of the present invention was larger. The magnetic microwire 10 of the present invention presents advantages because of its inherent bending curvature.

Example 3

An improved Taylor-Uliotvsky method is adopted to prepare the magnetic microfilament. Using a diameter of 18mm, insideDiameter of 12mm, thermal expansion coefficient of 3.3X 10 ^-6 K ^-1 The high borosilicate glass tube has uniform wall thickness. In order to obtain the magnetic microwires 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 other side is 3 mm. Adding a cobalt-based alloy into a glass tube, and carrying out a spinning process at a spinning speed of 100-300 m/min, wherein the cobalt-based alloy comprises 70.5 atomic% of Co, 4.5 atomic% of Fe, 11 atomic% of Si, 12 atomic% of B and 2 atomic% of Cr. The geometry of the magnetic microfilaments prepared is shown in table 1. In general, in the case where the diameters of the magnetic microwires and the magnetic metal microwires are the same, the larger the eccentricity is, the larger the bending curvature of the magnetic microwires is; the smaller the diameter of the magnetic microwire, the larger the bending curvature thereof under the same eccentricity.

TABLE 1 magnetic microwire parameters

Example 4

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

TABLE 2 magnetostriction coefficient and Large Barkhausen Effect test results

FIG. 12 is a graph showing that the sample number in Table 2 is 2-1 and the magnetostriction coefficient is-1X 10 ^-8 The fundamental frequency of the harmonic voltage spectrum measured by the magnetic microfilament of (1) is 1 kHz. It can be seen that the higher harmonic voltage is smaller and the attenuation is faster. FIG. 13 is a graph showing that the sample number in Table 2 is 2-2 and the magnetostriction coefficient is 3X 10 ^-7 The fundamental frequency of the harmonic voltage spectrum measured by the magnetic microfilament of (1 kHz). It can be seen that the higher harmonic voltage is larger and the attenuation is slower.

In view of the above, therefore, the magnetic micro-wires have a magnetostriction coefficient of 1 × 10 or more ^-8 And is less than or equal to 1 × 10 ^-6 Is preferred.

Example 5

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

The resulting security paper was cut to a size of a4, with the long edge of a4 security paper proceeding in the MD direction. The number of magnetic microfilaments on the a4 paper was counted. The detection rate of the A4 safety paper is tested by an electromagnetic wave anti-theft instrument. Cutting out a wafer with the diameter of 200mm by taking the geometric center of the A4 safety paper as a circle center, and testing signals in the MD direction and the TD direction at a fixed position of an electromagnetic wave anti-theft instrument to observe the distribution orientation condition of the magnetic microfilaments. The above results are reported in table 3.

TABLE 3 test results corresponding to different security papers

It can be seen from the test results that the magnetic metal micro-wires of the present invention, which have a diameter of 16.5 μm and an overall diameter of 22 μm, have a high detection rate when the length is greater than 6 mm. In addition, the MD direction and TD direction signal values of the security paper of the present invention are approximately the same, indicating that the magnetic micro-wires with a certain bending curvature of the present invention do not have significant orientation under the action of water flow, and a generally isotropic security paper can be obtained. Fig. 14 is a schematic representation of a security paper made with magnetic microfilaments 10 according to the invention (microfilament length 8mm), it being 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 generate a moment on the bent micro-wires, which tends to turn around in the water flow, and thus the orientation of the micro-wires tends to be random. Fig. 15 is an optical microscope image of the security paper 30 of the present invention (the security paper shown in fig. 14), in which the magnetic microfilaments 10 have a diameter comparable to that of the pulp fibers 18, and have good fusion properties, and no wrinkles are observed.

The security paper prepared using the magnetic microfilaments 20 of comparative example 1 had strong signal values in the MD direction and weak signal values in the TD direction, impairing the overall detection rate. As shown in fig. 16, most of the magnetic micro-wires 20 are oriented more uniformly in the MD direction.

In summary, according to the magnetic microwire and the safety medium provided by the invention, the magnetic microwire is designed into a shape capable of spontaneously bending into a certain curvature, so that the detectability of the magnetic microwire 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 in connection with exemplary embodiments, it will be apparent to those skilled in the art that various modifications and changes may be made to the exemplary embodiments of the present invention without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A magnetic microfilament is characterized in that the microfilament has a core-shell structure, a magnetic metal microfilament is taken as a core, a glass layer is taken as a coating layer, wherein,

the magnetic microfilament has a thickness of 50m in free state ^-1 Above and not more than 160m ^-1 The bending curvature of (2).

2. The magnetic microwire of claim 1, wherein the magnetic microwire has a free state of 70m ^-1 Above and not more than 130m ^-1 The bending curvature of (2).

3. The magnetic microwire of claim 1 or 2, wherein the glass layer has a non-uniform thickness in the radial direction of the magnetic metal microwire.

4. The magnetic microwire of claim 1 or 2, wherein the magnetic metal microwire has a cross-section in eccentric circle configuration with the cross-section of the glass layer.

5. The magnetic microwire of claim 4, 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.

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

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

8. The magnetic microfilament according to claim 1 or 2, wherein the glass layer has an average thickness of between 0.5 μm and 5 μm.

9. A security medium comprising at least one magnetic microwire according to any of claims 1 to 8.

10. The security medium of claim 9, wherein the security medium is a security paper, a security cardboard, a security document, a security strip, a patch, a label, or a security device.

11. A security medium according to claim 9 or 10, wherein the length of the magnetic microwires is greater than or equal to 3mm and less than 10 mm.

12. Use of a magnetic microwire according to any of claims 1 to 8 in a magnetosensitive element and in a wave-absorbing material.