Preparation method of transmission electron microscope sample of sintered neodymium iron boron block material

Abstract

The invention discloses a preparation method of a transmission electron microscope sample of a sintered neodymium-iron-boron block material, which comprises the following steps of block sample cutting, mechanical grinding, sample punching, ring sticking, pre-thinning and final thinning, so that the transmission electron microscope sample to be observed is obtained. In addition, the method is simple, feasible, economical and practical, and the sample preparation success rate can be remarkably increased.

Description

Technical Field

[0001] This invention belongs to the field of transmission electron microscopy sample preparation technology, and relates to a method for preparing neodymium iron boron transmission electron microscopy samples, specifically, a method for preparing transmission electron microscopy samples of sintered neodymium iron boron bulk materials. Background Technology

[0002] Neodymium iron boron (NdFeB) rare-earth permanent magnets possess properties such as high remanence, high coercivity, and high energy product. Furthermore, this material is free of Co, has a lower cost than Sm-Co permanent magnets, and is more competitive in the market. It is widely used in high-tech and environmental fields such as computer hard drives, nuclear magnetic resonance imaging, electric vehicles, wind power generation, magnetic machinery, and magnetic levitation, making it an important basic functional material.

[0003] Since "structure determines properties, and properties determine applications," the analysis of a material's microstructure is crucial. Transmission electron microscopy (TEM) is one of the most powerful tools for studying the microstructure of materials and has been applied to the study of neodymium iron boron (NdFeB). Obtaining high-quality TEM samples is a prerequisite for reflecting the true microstructure of the material and obtaining high-quality TEM images.

[0004] Currently, there are three common methods for preparing bulk metal materials into transmission electron microscopy (TEM) samples: electrolytic double-jet thinning, ion thinning, and focused ion beam (FIB) cutting.

[0005] Sintered NdFeB bulk materials are characterized by low strength and brittleness. The NdFeB-rich phase is distributed at the grain boundaries of the matrix phase, exhibiting higher chemical reactivity and lower hardness compared to the matrix phase. Based on these characteristics of sintered NdFeB bulk materials, any single one of the three methods mentioned above has its own drawbacks, making it difficult to obtain satisfactory NdFeB transmission electron microscopy (TEM) samples with large thin areas at a low cost and in a short time. If only electrolytic double-jet thinning is used, the significant electrochemical phase differences between the phases can lead to preferential corrosion of the NdFeB-rich phase, resulting in preferential dissolution of the grain boundaries and an excessively thick matrix phase, causing either no thin area or an unevenly distributed thin area. Currently, there is no suitable electrolytic double-jet process to solve this problem. For ion thinning, the thinner the sample before thinning, the better the thinning effect. However, due to the fragility of NdFeB materials, it is difficult to mechanically grind them to a sufficiently thin thickness before ion thinning. For example, even if the sample is ground to below 30 micrometers, there is still a high possibility of breakage due to stress during subsequent thinning, transfer, and electron microscope mounting, rendering the entire process unusable. If ion thinning begins when the sample is already relatively thick, the thinning time needs to be increased. However, prolonged thinning will result in a small thin area and uneven thickness, leading to an unsatisfactory thinning effect. In addition, due to the overall thickness of the sample, excessive magnetism may cause severe optical path deflection in the transmission electron microscope, which is detrimental to observation. If only FIB is used to prepare transmission electron microscope samples, sample pretreatment only requires polishing one surface of the sample, and precise micro-area sampling can be achieved. The resulting transmission electron microscope sample thickness is relatively uniform, down to tens of nanometers. However, the equipment and consumable costs are higher, and the observable area is very small. For example, although the length and width are only a few micrometers, the sample preparation process is cumbersome, time-consuming, and inefficient.

[0006] Therefore, it is urgent to develop an efficient, high-quality, economical and practical transmission electron microscopy sample preparation method suitable for sintered NdFeB bulk materials. Summary of the Invention

[0007] To address the aforementioned technical problems in existing methods, the inventors, through repeated and meticulous research, discovered a method for preparing transmission electron microscopy (TEM) samples of sintered NdFeB bulk materials. This method is economical and practical, capable of efficiently preparing high-quality NdFeB TEM samples with a large-area thin region.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for preparing a transmission electron microscopy (TEM) sample of sintered NdFeB bulk material, comprising the following steps:

[0010] 1) Cutting of bulk samples: The sintered NdFeB bulk samples are cut to obtain thin sheets with a thickness of 0.5-1 mm;

[0011] 2) Mechanical grinding: The thin sheet obtained in step 1) is mechanically ground to a thickness of 30-120μm to ensure that both sides are smooth and free of obvious scratches.

[0012] 3) Punching: Punch the thin sheet ground in step 2) into a round sheet with a size close to Φ3mm using a punching tool;

[0013] 4) Attaching the ring: Attach the Φ3mm disc obtained from the punching in step 3) to the transmission electron microscope support ring with an outer diameter of 3mm, and cover the hole in the center of the ring.

[0014] 5) Pre-thinning - Electrolytic double spray: Place the disc after the ring is attached in step 4) into the electrolytic double spray thinning instrument, use a suitable electrolyte solution, and control the temperature, voltage and current of the electrolyte solution to perform double spray thinning. Spray until the pre-thinning termination time, remove the sprayed sample disc and rinse it in 3 cups of ethanol about 10 times by immersion and lifting. Finally, use filter paper to absorb the ethanol on the surface.

[0015] 6) Final thinning – ion thinning: Place the sample pre-thinned in step 5) in the sample stage of the ion thinner, with the angle between the ion gun and the sample being ±2° to ±8° and the ion beam energy being 0.1keV to 6keV. After thinning to the point where the sample exits through the center hole, appropriately reduce the electron gun angle and energy for fine finishing and expand the thin area, thereby obtaining the transmission electron microscope sample.

[0016] Preferably, the sample cutting method in step 1) is wire cutting, precision cutting machine or slow saw cutting, etc.

[0017] More preferably, the cutting method described in step 1) is slow-speed sawing. Since common NdFeB block samples are not large in size and their hardness is not high, slow-speed sawing can meet the cutting accuracy requirements, is economical and practical, and is easy to operate.

[0018] Preferably, the mechanical grinding process in step 2) involves grinding the thin sheet with sandpaper. This can be done manually or using a metallographic sample preparation and polishing machine. Taking manual grinding as an example, before mechanical grinding, one side of the thin sheet can be coated with rosin or 502 glue onto a small, thick glass slide that is easy to hold. Then, one side of the thin sheet is ground with sandpaper, starting with 500# and decreasing the grit size sequentially. Each time, it is not necessary to grind away a large thickness; just remove the scratches from the previous step. The last sandpaper used should be greater than or equal to 2000#. Afterward, the thin sheet is removed from the glass slide, and the smoothed side is attached to the glass slide. The grinding operation of the first side is repeated, with the grit size of the sandpaper decreasing sequentially. At the same time, attention should be paid to the grinding thickness. Grind until the thickness of the thin sheet is 30-120μm, and the surface is smooth without obvious scratches. Finally, the thin sheet is removed from the glass slide.

[0019] More preferably, the neodymium iron boron sample is ground to 50-80 μm, which can ensure a certain strength and prevent the magnetic field from being too strong and affecting electron microscopy observation.

[0020] More preferably, the sample is bonded to a glass slide using rosin or 502 glue for grinding. If rosin is used, the sample can be softened by heating it in an electric furnace after grinding, and the slide can be carefully removed with tweezers. If 502 glue is used, the ground sample and glass slide need to be soaked in acetone for several hours, then the slide can be carefully removed with tweezers, rinsed with ethanol, and dried with filter paper.

[0021] Preferably, in step 3), a label paper should be placed underneath the sample during the punching process, and the sample should be punched at a uniform speed with appropriate force. Because neodymium iron boron is very brittle after being ground to the target thickness, it is difficult to punch out a Φ3mm round piece with a smooth edge using a sampler, and it is easy to crack in the middle, causing sample preparation failure. Therefore, to prevent the above situation from occurring, this invention proposes to first use a blade to press vertically downwards to divide the thin sheet into square pieces slightly larger than 3mm*3mm. Then, ordinary label paper is cut to approximately the same size. The sample piece and label paper are simultaneously held with tweezers, aligned with the hole of the sampler, and the sampler lever is gently and uniformly pressed down until the thin sheet is completely cut. The punched round piece is then removed, and any debris that may exist on the edges is cleaned off, resulting in a sample round piece with a diameter slightly less than 3mm. This method avoids the possibility of leaving sample protrusions exceeding 3mm in diameter when repairing the edge of a disc with a blade. This would prevent the sample from falling completely to the bottom of the sample slot during loading, affecting the screw tightening effect and even cracking the sample, thus ruining all previous efforts.

[0022] Preferably, the transmission electron microscope (TEM) support ring in step 4) is a molybdenum ring with an insulating layer on its surface, hereinafter referred to as an insulating molybdenum ring, with an inner diameter of 0.5–2.0 mm. One of the inventive aspects of this invention is the use of a ring-attached design before electrolytic double-spraying and ion thinning. This is because the sintered NdFeB structure is not dense, resulting in brittle, easily broken sheets under external force. During electrolytic double-spraying, ion thinning, and subsequent TEM sample loading, tweezers are required for handling the sample; if the sample breaks during this process, all previous work is wasted. Therefore, this invention increases the edge strength of the sheet and disperses pressure through ring attachment, thus better protecting the sample. Furthermore, by using a molybdenum ring with an insulating layer, the current distribution during subsequent double-spraying is improved, enhancing the double-spraying effect. Additionally, if a molybdenum ring without an insulating layer is used as the support ring, the electrolyte may cause molybdenum ring corrosion, sample graying on the molybdenum ring side, and preferential thinning of the area near the molybdenum ring during the double-spraying process. This results in a still thick sample center area, increasing the subsequent ion thinning time.

[0023] Preferably, in step 5), when placing the sample to be double-sprayed into the fixture, the side of the bonded insulating molybdenum ring is positioned opposite the platinum electrode. This is to ensure that one side of the sample is in direct contact with the platinum electrode, while the other side is in contact with the electrolyte, thus creating a pathway during double spraying.

[0024] Preferably, the electrolyte solution in step 5) is a 3% to 10% perchloric acid methanol or ethanol solution, or a 5% to 40% nitric acid methanol or ethanol solution.

[0025] More preferably, the electrolyte solution in step 5) is a perchloric acid methanol solution with a volume fraction of 3% to 10%, and even more preferably a perchloric acid methanol solution with a volume fraction of 5% to 7%.

[0026] Preferably, the temperature of the electrolytic solution in step 5) is -30 to -40°C, the voltage is 10 to 20V, and the current is 40 to 100mA.

[0027] Preferably, the termination time of the electrolytic double spraying in step 5) can be achieved in two ways: one is to stop immediately when the light is slightly transmitted through an optical fiber, and the other is to determine an exit time by double spraying a sample of the same thickness until it is obviously transmitted. Then, under the same double spraying conditions, the exit time is reduced by 10 to 20 seconds as the thinning termination time.

[0028] Preferably, in step 6), the angle between the ion gun and the sample during the ion thinning and pore-forming stage is ±5° to ±8°, the energy is 4 to 6 keV, and the time is about 10 min to 1 h; during the finishing stage, the angle between the ion gun and the sample is ±2° to ±4°, the energy is 1 to 4 keV, and the time is about 10 min to 1 h, until the diameter of a single pore reaches 100 to 300 micrometers.

[0029] In addition, samples thinned in this way can be observed using transmission electron microscopy.

[0030] The beneficial effects of this invention are:

[0031] According to the preparation method of the present invention, an insulating molybdenum ring is bonded to one side of a ground NdFeB sample, followed by pre-thinning of the sample using electrolytic double-jet spraying, and finally final thinning using ion thinning. This yields a NdFeB transmission electron microscopy (TEM) sample that is easy to handle, not easily broken, and has a large thin area. This method of bonding the insulating molybdenum ring not only enhances the mechanical strength of the sample but also improves the current distribution during double-jet spraying, resulting in a pre-thinned sample with bright and smooth surfaces on both sides. Furthermore, the electrolytic double-jet pre-thinning method shortens the total time required for ion thinning and also helps to increase the area of ​​the electron beam-transmittable region. This can be explained in detail from the following aspects:

[0032] 1. Using this method to process bulk NdFeB materials, satisfactory NdFeB transmission electron microscopy samples with relatively large thin areas can be obtained.

[0033] 2. This preparation method does not require the sample to be ground to a very thin thickness, therefore the preparation method is easy to operate.

[0034] 3. By attaching a ring to a Φ3mm circular piece, the sample strength is improved, enhancing the operability of subsequent sample preparation steps.

[0035] 4. This preparation method greatly shortens the usage time of ion thinning equipment, making it a fast and low-cost method.

[0036] Therefore, it can be said that this invention is simple, easy to implement, economical and practical, and can significantly improve the success rate of sample preparation. Attached Figure Description

[0037] Figure 1 The figure shows a transmission electron microscope (TEM) image of the neodymium iron boron (NdFeB) sample prepared in Example 1 of the present invention. In the figure, (a) is the bright field TEM image of the sample, (b) is the high-resolution electron micrograph of the edge region of the sample, and the inset is the corresponding selected area electron diffraction (SID) spectrum.

[0038] Figure 2 The image shows a transmission electron microscope (TEM) image of the neodymium iron boron (NdFeB) sample prepared in Comparative Example 1 of this invention. Detailed Implementation

[0039] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments and accompanying drawings, further clarifies the invention. Those skilled in the art should understand that the specific description below is illustrative rather than restrictive and should not be construed as limiting the scope of protection of the present invention.

[0040] The neodymium iron boron bulk sample of this invention is a common commercially available product.

[0041] Example 1

[0042] Preparation method of transmission electron microscopy samples of sintered NdFeB bulk materials

[0043] The specific steps are as follows:

[0044] 1) Cutting of block samples: The commercially available sintered NdFeB block samples were cut into thin slices with a thickness of 0.7 mm using a slow saw;

[0045] 2) Mechanical grinding: Attach one side of the cut sheet to a thick glass slide using 502 glue, and grind the other side smooth with sandpaper ranging from coarse to fine. Then soak the sheet in acetone to separate it from the glass slide. Attach the ground side back to the glass slide using 502 glue and grind the other unground side. Monitor the sample thickness with a micrometer until it reaches 65 μm. Finally, grind with 5000# sandpaper until both sides are smooth and free of obvious scratches.

[0046] 3) Punching: The ground sheet is punched into a round disc with a size close to Φ3mm using a punching tool;

[0047] 4) Attaching the ring: Attach the Φ3mm circular piece from step 3) to the transmission electron microscope special support ring with an outer diameter of 3mm, and cover the hole in the center of the ring. The support ring is an insulating molybdenum ring.

[0048] 5) Pre-thinning – Electrolytic double-jet: Place the disc with the adhered ring in the electrolytic double-jet thinning apparatus. Use a 6% perchloric acid-methanol solution as the electrolyte. Control the temperature of the electrolyte solution to -35℃ by adding liquid nitrogen for cooling. Begin double-jet thinning, adjusting the voltage to 14V and the current to approximately 50mA. After about 2 minutes, when the sample shows slight translucency when observed through the optical fiber during double-jet thinning, immediately turn off the double-jet apparatus. Remove the pre-thinned disc and rinse it approximately 10 times in 3 cups of ethanol using an immersion and lifting method. Finally, blot the surface of ethanol with filter paper.

[0049] 6) Final thinning – Ion thinning: Place the pre-thinned disc in the sample stage of the ion thinner, adjust the angle between the ion gun and the sample to ±6°, and set the ion beam energy to 4keV. After about 20 minutes, thin to the center of the sample through the hole, and then appropriately lower the electron gun angle to ±3° and the energy to 3keV for fine finishing. After about 20 minutes, stop thinning when the diameter of the center hole reaches about 100μm, and obtain the transmission electron microscope sample to be observed.

[0050] The obtained transmission electron microscopy samples were observed, and the results are as follows:

[0051] Figure 1 The figures show the transmission electron microscopy (TEM) observation results of the sintered NdFeB sample in Example 1. Figure (a) is the bright-field TEM image of the sample. As can be seen from Figure (a), the edge of the sample is brighter, indicating that the electron beam can penetrate the sample well. In addition, the brightness also indicates that the sample obtained by this invention is thinner and the observable area is large enough. Figure (b) is a high-resolution electron micrograph of the edge region of the sample. The inset above it is the corresponding selected area electron diffraction (SED) spectrum. As can be seen from this figure, the high-resolution image is clearer, indicating that the sample is thin enough to meet the high-resolution requirements. There are no obvious amorphous rings in the SED spectrum, indicating that the sample surface is clean and uncontaminated.

[0052] Comparative Example 1

[0053] Steps 1) to 4) are the same as in the example. In step 5), the sample is thinned by electrolytic double spraying until a clear hole is formed. After that, no ion thinning is performed to obtain the transmission electron microscope sample to be observed.

[0054] The obtained transmission electron microscopy samples were observed, and the results are as follows: Figure 2 The transmission electron microscopy (TEM) observation results of the sintered NdFeB sample in Comparative Example 1 show that there are many sieve pores in the bright field TEM image. The pores are neat and pure black, indicating that the sample is too thick and the electron beam cannot penetrate it, i.e., there is not enough thin area.

[0055] In summary, the preparation method of the present invention is economical and practical, and can efficiently prepare high-quality NdFeB transmission electron microscopy samples with a large thin area, which provides strong assistance for the transmission electron microscopy observation of NdFeB samples.

[0056] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for preparing transmission electron microscopy (TEM) samples of sintered NdFeB bulk materials, characterized in that, Includes the following steps: 1) Cutting of bulk samples: The sintered NdFeB bulk samples are cut to obtain thin sheets with a thickness of 0.5-1 mm; 2) Mechanical grinding: The thin slices obtained in step 1) are mechanically ground to a thickness of 30-120μm to ensure that both sides are smooth and free of obvious scratches; 3) Punching: Punch the thin sheet ground in step 2) into a Φ3mm round sheet using a punching tool; 4) Attaching the ring: Attach the circular piece with a size close to Φ3mm obtained from the punching in step 3) to a transmission electron microscope support ring with an outer diameter of 3mm, and cover the hole in the center of the ring; 5) Pre-thinning - Electrolytic double spray: Place the disc after the ring is attached in step 4) into the electrolytic double spray thinning instrument, use a suitable electrolyte solution, and control the temperature, voltage and current of the electrolyte solution to perform double spray thinning. Spray until the pre-thinning termination time, take off the sprayed sample disc and rinse it in 3 cups of ethanol by immersion and lifting about 10 times. Finally, use filter paper to absorb the ethanol on the surface. 6) Final thinning – ion thinning: Place the sample pre-thinned in step 5) in the sample stage of the ion thinner, with the angle between the ion gun and the sample being ±2° to ±8° and the ion beam energy being 0.1keV to 6keV. After thinning to the point where the sample exits through the center hole, appropriately reduce the electron gun angle and energy for fine finishing and expand the thin area, thereby obtaining the transmission electron microscope sample.

2. The method for preparing transmission electron microscopy samples of sintered NdFeB bulk materials according to claim 1, characterized in that: In step 1), the cutting method can be wire cutting, precision cutting machine or slow saw cutting.

3. The method for preparing transmission electron microscopy samples of sintered NdFeB bulk materials according to claim 1, characterized in that: The mechanical grinding process described in step 2) involves manually grinding the thin sheet with sandpaper or grinding it with a metallographic sample preparation and polishing machine.

4. The method for preparing transmission electron microscopy samples of sintered NdFeB bulk materials according to claim 1, characterized in that: In step 3), a label should be placed under the sample and the sample should be fired at a uniform speed with appropriate force.

5. The method for preparing transmission electron microscopy samples of sintered NdFeB bulk materials according to claim 1, characterized in that: In step 4), the transmission electron microscope support ring is a molybdenum ring with an insulating layer on its surface, and the inner diameter of the molybdenum ring is 0.5 to 2.0 mm.

6. The method for preparing transmission electron microscopy samples of sintered NdFeB bulk materials according to claim 1, characterized in that: In step 5), when placing the double-sprayed sample into the fixture, the side with the bonded insulating molybdenum ring should be placed on the opposite side of the platinum electrode.

7. The method for preparing transmission electron microscopy samples of sintered NdFeB bulk material according to claim 1, characterized in that: In step 5), the electrolyte solution is a perchloric acid methanol or ethanol solution with a volume fraction of 3% to 10% or a nitric acid methanol or ethanol solution with a volume fraction of 5% to 40%.

8. The method for preparing transmission electron microscopy samples of sintered NdFeB bulk material according to claim 1, characterized in that: In step 5), the temperature of the electrolytic solution is -30 to -40°C, the voltage is 10 to 20V, and the current is 40 to 100mA.

9. The method for preparing transmission electron microscopy samples of sintered NdFeB bulk materials according to claim 1, characterized in that: In step 6), during the ion thinning and pore-forming stage, the angle between the ion gun and the sample is ±5° to ±8°, the energy is preferably 4 to 6 keV, and the time is about 10 min to 1 h; during the finishing stage, the angle between the ion gun and the sample is ±2° to ±4°, the energy is preferably 1 to 4 keV, and the time is about 10 min to 1 h.

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