CN110988005B

CN110988005B - Strong permanent magnet device for magnetizing material under vacuum system

Info

Publication number: CN110988005B
Application number: CN201911301311.0A
Authority: CN
Inventors: 孙喆; 王鹏栋; 刘毅; 李昱良; 张国斌
Original assignee: University of Science and Technology of China USTC
Current assignee: University of Science and Technology of China USTC
Priority date: 2019-12-17
Filing date: 2019-12-17
Publication date: 2021-10-22
Anticipated expiration: 2039-12-17
Also published as: CN110988005A

Abstract

The invention discloses a strong permanent magnet device for magnetizing materials in a vacuum system, which comprises: the permanent magnet structure, vacuum cavity, sample frame, wherein sample frame and permanent magnet are installed inside vacuum cavity, are responsible for respectively cooling and adding the magnetic field for the sample. The device does not need to use large current (>100A) and a heat insulation device to maintain a magnetic field, so that additional interference on a measurement system caused by extra current is avoided, the use cost is low, and the performance is stable. The magnetic field generated by the device exceeds 1T and is far higher than the magnetic field intensity of the traditional magnet, the magnetic field is uniformly distributed in a magnetization area, and the repeatability of an experiment is improved. Compared with the linear Halbach array magnet which is widely applied, the annular magnet has smaller volume and reasonable distribution of the strongest points of the magnetic field, and is suitable for being applied to the ultrahigh vacuum environment.

Description

Strong permanent magnet device for magnetizing material under vacuum system

Technical Field

The invention relates to the fields of vacuum technology, angle-resolved photoelectron spectroscopy characterization and permanent magnet application, in particular to a strong permanent magnet device for magnetizing materials in a vacuum system.

Background

Magnetic materials have been the focus of research due to their complex physical properties and broad application prospects. Technical means for measuring the magnetism of materials in normal pressure environment are mature at present, and measuring instruments are also commercial products (property measuring system PPMS) (see Lashley J C, Hundley M F, Migliori A, et al. Critical evaluation of property measurement on quantity Design property measurement system [ J ]. Cryogenics,2003,43(6): 369-378.). However, the technical difficulty of performing a regulation measurement on the magnetism of a material in a vacuum system, especially in an ultrahigh vacuum environment, is high. This is determined by the complexity and sensitivity of the vacuum system. At present, the means for applying a high-strength magnetic field to a magnetic material in an ultrahigh vacuum environment only adopts two schemes of a permanent magnet or an electromagnet. However, the magnetic field strength of a simple permanent magnet is difficult to reach one tesla (1T), and the magnetic field strength is limited on the surface of the magnet, so that the requirement of extreme tests is difficult to meet. The use of an electromagnet to achieve a strong magnetic field above tesla requires a very large copper coil, even a superconducting coil, which requires an additional design of a large vacuum cavity, which hinders miniaturization of equipment; and the electromagnet needs to be provided with a high-power supply and a magnet cooling device, so that the equipment cost and the experiment cost are further increased.

Halbach Array (Halbach Array) is a new type of arrangement of permanent magnets. The permanent magnets of different magnetization directions are arranged in a sequence such that the magnetic field is significantly enhanced on one side of the array and significantly reduced on the other side.

Halbach arrays (see reference https:// www.kjmagnetics.com/blog. aspp. Halbach-arrays) were first proposed in 1979 by professor Klaus Halbach of Lorentbergli national laboratory, USA, and were mainly applied to high-energy physical fields such as particle accelerators, free electron laser devices, synchrotron radiation devices, and the like. Due to its excellent magnetic field distribution characteristics, the application range is gradually expanded to industrial fields such as nuclear magnetic resonance, magnetic levitation, permanent magnet special motors, and the like.

The special magnet arrangement mode can connect the magnetic fields of the magnets into a whole and concentrate the magnetic fields in a narrow range, so that the strength is improved in a multiplied way. The peak magnetic strength of the strongest permanent magnet is about 0.6T at present. And the use of a halbach magnet array of ideal design can break through this value.

In view of research value of magnetic materials and blank of the field of vacuum magnetization, the invention provides a strong permanent magnet device which can be widely used in vacuum equipment and has no interference and energy consumption, and has important significance in the field of scientific research.

Disclosure of Invention

The invention aims to provide a strong permanent magnet device for magnetizing materials in a vacuum system, which can apply a strong magnetic field higher than 1T to the materials in ultrahigh vacuum equipment such as angle-resolved photoelectron spectroscopy (ARPES), and the like, and simultaneously ensures that the magnet does not interfere with an electronic energy analyzer of the equipment, and the residual magnetic field of a sample cannot be damaged by temperature in the transmission process.

The technical scheme adopted by the invention is as follows: a strong permanent magnet apparatus for magnetizing a material under a vacuum system, comprising: permanent magnet structure, vacuum chamber and sample frame, wherein:

the permanent magnet structure consists of eight or twelve N52 neodymium magnets or samarium cobalt magnets with magnetic poles distributed according to Halbach, and the number and the material of the magnets are selected according to actual needs; the magnet is fixed through a specially designed shell and is connected to the vacuum cavity through a screw rod, and a constant magnetic field exceeding 1T can be applied to the magnetic material;

the vacuum cavity is mainly formed by a CF63 vacuum pipeline with the radial length of 15-20cm, three CF35 flange ports are symmetrically and orthogonally distributed on the cavity, and another CF35 flange port is deviated from an orthogonal axis by 20 degrees and is used as an observation window. One side of the vacuum cavity is provided with a CF63 or larger type (large CF flange) flange for connecting various vacuum test instruments, and the other large CF flange is used for connecting a sample rack;

the sample holder consists of a large CF flange, a transfusion pipe and a copper cold head. Wherein, the cold head is internally provided with a liquid circulation passage and is externally carved with a groove for placing a sample support. The circulation passage is connected with the infusion tube. The infusion tube is fixed on the flange in a welding way. The sample holder is connected with the vacuum cavity through the flange and can pass through the center of the permanent magnet without hindrance to cool the magnetic material and reach the position with the strongest magnetic field.

Furthermore, magnesium carbonate is coated at the tail end of the sample support to serve as a heat insulation coating, so that the sample support can be effectively prevented from contacting a normal-temperature component to be heated in the process of being transferred to other cavities.

Further, the size of the permanent magnet array has been designed optimally based on magnetic field software simulation, where the inner ring diameter of the array is 30mm and the outer ring diameter is 60mm, which can be scaled up by a fixed ratio of 1: 2. The fixed cover plate of the magnet is made of a 316 stainless steel perforated plate with the thickness of 1mm, and the base is made of 316 stainless steel. The gap between the fixed base steel pipe and the cover plate is not more than 0.2 mm. The gap between the brass ferrule and the magnet is not more than 0.2 mm. The materials and the clearance fit degree ensure that the magnetic field shape of the magnet is not interfered and the intensity is not leaked.

Furthermore, the sample holder is made of an iron-cobalt-nickel alloy material and has no residual magnetism. The table surface is provided with four equidistant screw holes which can fix a film sample; a rectangular metal area (not covered with the coating) with the size of 1mm multiplied by 10mm is reserved in the center of the magnesium carbonate coating on the two sides of the table top and is used for being in contact with the electric brush to ensure that the sample holder is conductive with other testing equipment; the height of the table top is 1mm, and the table top is compatible with sample holders of various types of vacuum equipment.

The invention has the advantages and positive effects that:

compared with an electromagnet, the device does not need to use a large current (>100A) and a heat insulation device to maintain a magnetic field, so that additional interference on a measurement system caused by extra current is avoided, the use cost is low, and the performance is stable. The magnetic field generated by the device exceeds 1T and is far higher than the magnetic field intensity of the traditional magnet, the magnetic field is uniformly distributed in a magnetization area, and the repeatability of an experiment is improved. Compared with the linear Halbach array magnet which is widely applied, the annular magnet has smaller volume and reasonable distribution of the strongest points of the magnetic field, and is suitable for being applied to the ultrahigh vacuum environment.

Drawings

FIG. 1 is a schematic view of a linear Halbach array;

fig. 2 is a schematic view of the orientation of the magnetic poles of a ring magnet used in the present invention, in which fig. 2(a) is an 8-step halbach array ring, and fig. 2(b) is a 12-step halbach array ring.

FIG. 3 is a schematic view of a magnet clamp, wherein FIG. 3(a) is a top view and a front view of a base, FIG. 3(b) is a top view and a front view of a cover plate, and FIG. 3(c) is a top view and a front view of a ferrule;

FIG. 4 is a top view of the vacuum chamber;

FIG. 5 is a design drawing of a sample holder, in which FIG. 5(a) shows an overall structure of the sample holder, and FIG. 5(b) shows a structure of a liquid circulation path inside a cold head and a sample tank;

FIG. 6 is a sample holder layout;

FIG. 7 is a schematic view of a magnet clamp assembly, wherein FIG. 7(a) shows a magnet upper cover plate structure and FIG. 7(b) shows a magnet fixing base structure;

FIG. 8 is a schematic view of the assembled permanent magnet;

FIG. 9 is a schematic view of the robot;

FIG. 10 is a top view of the structure of the completed device;

fig. 11 is a side view of the structure of the completed device.

Detailed Description

The invention is further described with reference to the following figures and detailed description.

1. Permanent magnet array design

As shown in fig. 2, the present invention adopts a structure of a permanent magnet designed by a circular halbach array. 8 or 12 magnets with different magnetization directions are used and arranged in a ring, the magnetic pole directions of which are shown in FIG. 2 (the arrows point to the south pole). The materials of the magnets are selected in two ways, the N52 nd-Fe-B magnet can obtain the strongest permanent magnetic field, but the working temperature cannot exceed 80 ℃; samarium cobalt magnet also can obtain the magnetic field intensity that exceeds 1T, and operating temperature can reach 180 degrees centigrade moreover, can be according to whether equipment needs high temperature to toast and select. The magnetic field intensity of the centers of the 8 th-order Halbach array ring and the 12 th-order Halbach array ring selected by the invention is uniformly distributed, and the test material does not need to be accurately positioned at the circle center.

2. Assembly of permanent magnet arrays

Conventional approaches fail to arrange the magnets close together in the manner shown above due to the strong repulsion between the magnets. As shown in fig. 3, the present invention contemplates a set of assembly fixtures. Fig. 3(a) and 3(b) are schematic views of the base and clip of the magnet, which may be secured in place by the ferrule of fig. 3 (c). Wherein the base and the clamping piece are made of non-magnetic 316 stainless steel. All weak portions are subjected to heat treatment of induction heating and quenching so as to have strength enough to resist the repulsive force of the magnet. The material of the ferrule is brass, and because the component is the main part bearing the stress of the magnet, the brass with good elasticity and mechanical strength is used. The whole set of clamp is made of magnetic-free materials, so that interference on magnetic field distribution of the Halbach array is prevented. The magnet assembly effect is shown in the attached drawings.

3. Design of vacuum chamber

In order to enable the magnet to be used in a vacuum device such as an angle-resolved photoelectron spectrometer, the present invention uses the following vacuum chamber design, as shown in fig. 4. The flange openings of the first flange 1, the second flange 2 and the third flange 3 are standard CF35 flanges used for mounting vacuum components such as a vacuum gauge, an angle valve, a manipulator and the like, and the three flanges are distributed orthogonally. The flange opening of the fourth flange 4 is a CF35 flange for installing a glass observation window, and the angle of the flange opening is 80 degrees with the first flange 1. The angular tilt here is in the region of 20 degrees in order to facilitate observation of the sample as it is transported into the magnet. The fifth flange 5 and the sixth flange 6 are upper and lower flanges of the chamber, and both use the CF standard, and the present invention does not limit a unique model, but uses CF63, 100, 150, etc. according to the interface of the docking device.

4. Design of sample holder

Because many magnetic materials have a low curie transition temperature, they can only be magnetized well below the transition temperature, into a ferromagnetic state. The invention designs a set of sample rack capable of keeping low temperature, which is used for cooling a test material. As shown in fig. 5, includes: the cold head part 51 for clamping the sample holder is made of brass, has a heat conductivity coefficient of 108.9/(m.k), and also has good wear resistance. The flange 52 of the butt joint vacuum cavity and the infusion tube 53 for conveying liquid nitrogen are made of 304 stainless steel, are communicated with the flange through welding and have a bearing effect. An observation window 54 for observing whether the sample reaches the magnetization position. In the fine structure of the cold head, a cooling liquid circulation loop 55 inside the cold head is filled with flowing cooling liquid to ensure that the cold head maintains low temperature. The groove 56 for placing the sample holder can place the sample holder transferred from other cavity body.

5. Design of sample holder

The invention only lists the sample holder design suitable for an ARPES spectrometer because the sample holder designs of various vacuum instruments are different, wherein the heat insulation design is the core concept of the invention, and the shape of the holder can be modified according to specific conditions.

Fig. 6 shows a sample holder for use with a sample holder according to the present invention. The innovation point of the invention is that the surface of the metal support is coated with a magnesium carbonate heat insulation layer, and metal parts with the width of 1mm are exposed at a first position 61 and a second position 62 in fig. 6. The core idea of this design is to use magnesium carbonate with a thermal conductivity of only 0.0692 w/(m.k) as a thermal barrier layer to prevent heating during sample transfer. Because other vacuum cavities and manipulators in a room temperature state are strong heat sources relative to a sample support cooled by liquid nitrogen, white magnesium carbonate with low heat conductivity coefficient can effectively prevent contact heat transfer and infrared radiation of surrounding heat sources, and a good heat insulation effect is achieved.

FIG. 7 is a schematic view of the magnetic clamp parts in the apparatus, the material is 316 stainless steel, and the weak parts of the cover plate are all quenched.

Fig. 8 is an effect view of the present device after the permanent magnet is assembled, in which the bottom is the base of fig. 6 and the upper portion covers the cover plate of fig. 6. The assembly process is that the hoop is sleeved on the base, then the magnets are oppositely arranged, each pair of magnets is pressed by the cover plate after being arranged, and the magnets are fixed by screws.

Fig. 9 is a structural diagram of a robot in the present apparatus. The invention adopts a commercialized magnetic rod type manipulator to correct the position of the sample in the magnet, and the specific structure is not repeated.

Fig. 10 and 11 are the effect diagrams after the device of the invention is assembled. The permanent magnet array 101 is fixed on the vacuum cavity 102 through a connecting rod 108, and the observation window flange 103 and the pumping port flange 104 are used for connecting a vacuum pump. The vacuum in the vacuum chamber is read by a current vacuum gauge 105. The cold head of the sample holder 106 passes right through the hole in the center of the magnet to the point where the magnetic field is strongest. A magnetic bar robot 107 for some corrective action of the transfer of the sample holders from the other vacuum chambers to the sample holders.

1. The installation step:

the device is used for example in angle-resolved photoelectron spectroscopy. The device needs to be installed at a position far away from the test cavity, and the model of the butting flange is selected from CF 63. After the installation finishes, need through taking out a mouthful flange joint vacuum pump package, take out the vacuum of cavity to below 10^ 8mbar after steps such as leak hunting, if need high temperature toast, then the magnet selects samarium cobalt material. The infusion tube of the device is connected to a liquid nitrogen or liquid helium dewar, cooling liquid is introduced into the sample holder 106, and after more than ten minutes, the temperature of the cold head is stabilized and the cold head can be magnetized.

2. Preparation of samples

The sample needs to be adhered to a special sample support coated with magnesium carbonate, and metal parts reserved on the left side and the right side of the sample support are in contact with a testing device of a tester, so that the conductivity is ensured. The sample holder is finally transferred into the magnetization cavity through the multi-stage vacuum cavity.

3. Process of magnetization

And (3) selecting cooling liquid according to the Curie temperature of the material, placing the sample holder at the groove of the cold head after the temperature is stable, wherein the sample holder is the strongest point of the magnetic field of the permanent magnet, keeping for more than 10 minutes, and magnetizing. The magnetic material can be analyzed and tested by rapidly transferring the sample into a test cavity of an angle-resolved photoelectron spectrometer.

Claims

1. A strong permanent magnet device for magnetizing materials under a vacuum system is characterized in that: the method comprises the following steps: permanent magnet structure, vacuum chamber and sample frame, wherein:

the permanent magnet structure consists of eight or twelve N52 neodymium magnets or samarium cobalt magnets with magnetic poles distributed according to Halbach orientation; the magnet is fixed through the shell and connected to the vacuum cavity through the screw rod, and a constant magnetic field exceeding 1T is applied to the magnetic material;

the vacuum cavity is characterized in that a CF63 vacuum pipeline with the radial length of 15-20cm is used as a main body, three CF35 flange ports are symmetrically and orthogonally distributed on the cavity, and another CF35 flange port deviates from an orthogonal axis by 20 degrees and is used as an observation window; one side of the vacuum cavity is provided with a large CF flange of CF63 or larger type for connecting various vacuum test instruments, and the other side is provided with a large CF flange for connecting a sample rack;

the sample rack consists of a large CF flange, a liquid conveying pipe and a copper cold head, wherein a liquid circulation passage is arranged inside the cold head, a groove for placing a sample holder is carved outside the cold head, the liquid conveying pipe is connected with the liquid conveying pipe, the liquid conveying pipe is fixed on the flange in a welding mode, the sample rack is connected with the vacuum cavity through the flange, passes through the center of the permanent magnet without obstruction, cools the magnetic material and reaches the position with the strongest magnetic field;

magnesium carbonate is coated at the tail end of the sample holder to serve as a heat insulation coating, so that the sample holder is effectively prevented from contacting a normal-temperature component to be heated in the process of being transferred to other cavities;

the size of the permanent magnet array is designed to be optimal according to magnetic field software simulation, wherein the diameter of an inner ring of the array is 30mm, the diameter of an outer ring of the array is 60mm, the material of a fixed cover plate of the magnet is a 316 stainless steel plate with holes with the thickness of 1mm, the base is also made of 316 stainless steel, the gap between the fixed base steel pipe and the cover plate is not more than 0.2mm, the gap between a hoop made of brass and the magnet is not more than 0.2mm, and the magnetic field shape of the magnet is not interfered and the leakage strength is not ensured by the materials and the clearance fit degree.

2. A strong permanent magnet apparatus for magnetizing a material under a vacuum system according to claim 1, wherein: the sample holder is made of iron-cobalt-nickel alloy, has no residual magnetism, and the table top of the sample holder is provided with four equidistant screw holes for fixing a film sample; a rectangular metal area which is 1mm multiplied by 10mm and is not covered with the coating is reserved in the center of the magnesium carbonate coating on the two sides of the table top and is used for being in contact with the electric brush, and the sample holder is ensured to be conductive with other testing equipment; the height of the table top is 1mm, and the table top is compatible with sample holders of various types of vacuum equipment.