EXPANDABLE SINTERED NEODYMIUM-IRON-BORON MAGNET, PREPARATION METHOD THEREFOR AND APPLICATION THEREOF

Abstract

An expandable sintered neodymium-iron-boron magnet, a preparation method, and an application are provided. The magnet has a sintered neodymium-iron-boron magnet and an expandable coating coated on the surface of the sintered neodymium-iron-boron magnet. The sintered neodymium-iron-boron magnet coated with the expandable coating is used to replace a conventional assembly method of an epoxy resin adhesive coating magnet and potting resin glue, so that the magnet coated with the expandable coating may be inserted into a magnetic steel groove. The irreversible expansion of the coating itself is used to fix the magnet in the magnetic steel groove. Meanwhile, the use of the expandable coating shortens the assembly time of motors and improves the assembly accuracy of the motors.

Claims

1. An expandable sintered neodymium-iron-boron magnet, comprising a sintered neodymium-iron-boron magnet and an expandable coating that coats the surface of the sintered neodymium-iron-boron magnet, wherein preferably, the expandable coating has a thickness of 50-300 ?m, preferably 80-150 ?m; preferably, the expandable coating softens at 60-100? C.; preferably, in a pressure-free state, the expandable coating has an expansion rate of 200%-400%, preferably 300%-400%; preferably, the expandable coating is honeycomb-shaped after expansion; preferably, the expandable coating has a morphology substantially as shown in FIG. 1 after expansion.

2. The expandable sintered neodymium-iron-boron magnet according to claim 1, wherein the expandable coating comprises at least a water-soluble resin and a foaming agent, preferably, the water-soluble resin is selected from at least one of a water-soluble acrylic resin, a water-based epoxy resin, and a water-based polyurethane resin; preferably, the solid content of the water-soluble resin in the expandable coating is 30%-50%.

3. The expandable sintered neodymium-iron-boron magnet according to claim 1, wherein the foaming agent is thermoplastic expandable microspheres; preferably, the thermoplastic expandable microspheres have a diameter of 5-30 ?m, preferably 5-20 ?m; preferably, after expansion of the expandable coating, the area of the expandable microspheres accounts for 60%-90% of the cross-sectional area of the expandable coating; preferably, the thermoplastic expandable microspheres have an average diameter of 10-15 ?m; preferably, the thermoplastic expandable microspheres have an expansion temperature of 110-210? C.; preferably the thermoplastic expandable microspheres have a maximum heat-resistant temperature of 145-235? C.

4. The expandable sintered neodymium-iron-boron magnet according to claim 1, wherein the expandable coating is prepared by coating with an expandable coating material comprising at least components of a water-soluble resin and a foaming agent, preferably, the water-soluble resin in the expandable coating material has a weight percentage of 45%-65%, e.g., 50%-60%; preferably, the foaming agent in the expandable coating material has a weight percentage of 10%-30%, e.g., 15%-25%.

5. The expandable sintered neodymium-iron-boron magnet according to claim 4, wherein the expandable coating material further optionally comprises hectorite, preferably, the hectorite has a weight percentage of 0.1%-0.5%, e.g., 0.2%-0.4%; preferably, the expandable coating material further optionally comprises diethylene glycol butyl ether; preferably, the diethylene glycol butyl ether has a weight percentage of 0.5%-3%, e.g., 0.8%-2.5%; preferably, the expandable coating material further optionally comprises propylene glycol; preferably, the propylene glycol has a weight percentage of 1%-3%, e.g., 1.5%-2.5%; preferably, the expandable coating material further optionally comprises an acrylic thickener; preferably, the acrylic thickener has a weight percentage of 0.2%-0.8%, e.g., 0.3%-0.6%; preferably, the expandable coating material optionally further comprises a dispersant; preferably, the dispersant has a weight percentage of 0.1%-0.5%, e.g., 0.2%-0.4%; preferably, the dispersant is ethylene glycol, sodium oleate, carboxylate, or the like; preferably, the expandable coating further optionally comprises a leveling agent; preferably, the leveling agent has a weight percentage of 0.1%-0.5%, e.g., 0.2%-0.4%; preferably, the leveling agent is silicone oil, organosiloxane, or the like; preferably, the expandable coating material further comprises water; preferably, the sum of the weight percentages of the components in the expandable coating material is 100%.

6. The expandable sintered neodymium-iron-boron magnet according to claim 1, wherein the expandable coating is prepared by coating with an expandable coating material comprising the following components in percentage by weight: 45%-65% of water-soluble resin, 10%-30% of foaming agent, 0.1%-0.5% of hectorite, 0.5%-3% of diethylene glycol butyl ether, 1%-3% of propylene glycol, 0.2%-0.8% of acrylic acid thickener, 0.1%-0.5% of dispersant, and 0.1%-0.5% of leveling agent, preferably, the sintered neodymium-iron-boron magnet consists of a main phase Nd.sub.2Fe.sub.14B, a Nd-rich phase, and a B-rich phase.

7. A preparation method for the expandable sintered magnet according to claim 1, wherein the preparation method comprises coating with the expandable coating material comprising the components described above the surface of the sintered neodymium-iron-boron magnet, and performing pre-curing treatment to prepare the expandable sintered magnet; preferably, the sintered neodymium-iron-boron magnet further comprises a step of performing surface pretreatment before coating the expandable coating material; preferably, the surface pretreatment comprises processes of chemical ultrasonic degreasing, acid washing, and water washing of the surface of the sintered neodymium-iron-boron magnet; preferably, the degreasing liquid used in the degreasing process is a composite solution of a base and a surfactant; preferably, the base is sodium hydroxide or sodium carbonate at a concentration of 10-20 g/L; preferably, the surfactant is sodium dodecyl sulfonate or sodium dodecyl sulfate at a concentration of 2-6 g/L; preferably, the degreasing liquid has a temperature of 30-70? C., and the degreasing is performed for 1-20 min; preferably, the acid used for acid washing can be an aqueous nitric acid or citric acid solution; preferably, the acid used for acid washing is at a concentration of 5-30 wt %, and the acid washing is performed for 5-30 s; preferably, the coating method includes, but is not limited to, spray coating, printing, dipping, applying, and the like; preferably, the coating method is spray coating; preferably, the coating has a thickness of 50-300 ?m, preferably 80-150 ?m, illustratively 50 ?m, 80 ?m, 110 ?m, 150 ?m, 180 ?m, 200 ?m, 250 ?m, or 300 ?m.

8. Use of the expandable sintered magnet according to claim 1 in a motor rotor.

9. A motor rotor workpiece, comprising the expandable sintered magnet according to claim 1.

10. An assembly method for the motor rotor workpiece according to claim 9, comprising assembling the expandable sintered magnet into a magnetic steel groove, and performing heating expansion treatment to prepare the motor rotor workpiece, wherein: preferably, the heating expansion treatment is performed by a two-stage heating method, wherein the first expansion stage has an expansion stage of 110-160? C., and the first expansion stage has a heating rate of 5-15? C./min; preferably, the second expansion stage of the heating expansion treatment has an expansion temperature of 180-210? C., and the second expansion stage has a heating rate of 30-60? C./min.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] FIG. 1 is an electron micrograph (at a magnification of 300) of the expandable coating prepared in Example 1 after expansion.

[0077] FIG. 2 is an electron micrograph (at a magnification of 250) of the expandable coating (sample 5) prepared in Example 5 after expansion.

[0078] FIG. 3 is an electron micrograph (at a magnification of 250) of the expandable coating (sample 6) prepared in Example 6 after expansion.

[0079] FIG. 4 is an electron micrograph (at a magnification of 500) of the expandable coating (sample 10) prepared in Example 10 after expansion.

DETAILED DESCRIPTION

[0080] The embodiments of the present disclosure will be further illustrated in detail with reference to the following specific examples. It should be understood that the following examples are merely exemplary illustrations and explanations of the present disclosure, and should not be construed as limiting the protection scope of the present disclosure. All techniques implemented based on the content of the present disclosure described above are included within the protection scope of the present disclosure.

[0081] Unless otherwise stated, the starting materials and reagents used in the following examples are all commercially available products, or can be prepared using known methods.

Example 1

[0082] In this example, a sintered neodymium-iron-boron magnet (not magnetized) with the specification of 35.5 mm?16.5 mm?5.5 mm was used, and a magnetic steel groove with the assembled motor rotor had a size of 36 mm?17 mm?6 mm.

[0083] Magnet surface pretreatment: the sintered neodymium-iron-boron magnet was degreased at 60? C. for 2 min by using a composite degreasing liquid with sodium hydroxide at a concentration of 15 g/L and sodium dodecyl sulfonate at a concentration of 3 g/L, and then subjected to acid washing for 15 s by using 25 wt % of an aqueous citric acid solution, and finally placed in deionized water for ultrasonic cleaning for 2 min.

[0084] Expandable coating preparation: the expandable coating material was formulated according to the following components (in percentage by weight): 55% of water-soluble acrylic resin, 30% of water, 10% of foaming agent, 0.2% of hectorite, 1.5% of diethylene glycol butyl ether, 2% of propylene glycol, 0.5% of acrylic acid thickener, 0.4% of ethylene glycol, and 0.4% of polydimethylsiloxane, wherein the foaming agent was selected from thermoplastic expandable microspheres with an average particle size of 13 ?m.

[0085] Coating treatment: the expandable coating material described above was coated on the surface of the magnet by compressed air spray coating, wherein the spray coating speed was 120 mm/s, the coating thickness was 110 ?m, the distance between a nozzle and a workpiece was 15 cm, the angle between a spray gun and the workpiece was 25?, and the argon pressure of the spray gun was 0.6 MPa.

[0086] Pre-curing treatment: the surface of the magnet coated with the expandable coating material was heated to 50? C., and subjected to a pre-curing treatment to obtain a coating with a thickness of 110 ?m.

[0087] Coating expansion: the magnet coated with the expandable coating was placed into a high-temperature oven under a non-pressure state, and subjected to two-stage heating, that is, the magnet was first heated to 120? C. and maintained at this temperature for 5 min, and then quickly heated to 170? C. and maintained at this temperature for 3 min. The first heating stage was at a heating rate of 5? C./min, and the second heating stage was at a heating stage of 50? C./min. With the increase of the temperature, the gas pressure within the shell of the expandable microspheres increases. The thermoplastic shell softens, and the expansion volume of the microspheres increases significantly. At this time, the resin within the expandable coating softens, and the thickness of the expandable coating increases with the increase of the volume of the expandable microspheres. As shown in FIG. 1, it can be seen that the microspheres expand uniformly, and the walls of the microspheres and the resin are cross-linked with each other to form a stable supporting structure. The expandable coating consists of a water-soluble acrylic resin and expandable microspheres. The thickness of the expandable coating was increased from 110 ?m to 394 ?m after the pre-curing treatment, with an expansion rate of 358%. After expansion, the expandable coating is honeycomb-shaped, and the cross-sectional area of the expandable microspheres accounts for 82% of that of the expandable coating. (In the present disclosure, the proportion of the cross-sectional area of the expandable microspheres to the cross-sectional area of the expandable coating in a cross section is calculated as follows: firstly, taking a scanning electron microscope photo of the cross-section of the expandable coating, then identifying gaps in the cross section through the image, calculating the sum of the areas of the gaps, and taking the areas of the gaps as the proportion of the cross section of the expandable microspheres to the cross section of the expandable coating).

[0088] The magnet coated with the expandable coating was assembled into a magnetic steel groove of a motor rotor, placed into a high-temperature oven, and subjected to two-stage heating, that is, the magnet was first heated to 120? C. and maintained at this temperature for 5 min, and then quickly heated to 170? C. and maintained at this temperature for 3 min. The first heating stage was at a heating rate of 5? C./min, and the second heating stage was at a heating stage of 50? C./min. (When the magnet is installed in the magnetic steel groove, the expandable coating extrudes the inner wall of the magnetic steel groove after being heated to expand, and the gap between the magnetic steel groove and the magnet is filled, and meanwhile, the resin in the coating is cross-linked with the expandable microspheres to form a honeycomb-shaped coating structure. Due to the limitation of the inner wall of the magnetic steel groove, the coating cannot expand to the maximum in the expansion process, so that the expanded honeycomb-shaped structure is compressed and wrinkled.) After the heating, the motor rotor was cooled under natural conditions. The volume of the expandable microspheres increased due to the change of ambient temperature, and the change was irreversible. Due to the increase in the volume of the coating, the gap between the magnet and the inner wall of the magnetic steel groove was filled, so that the magnet was tightly fixed in the magnetic steel. At this time, the magnet product obtained was named sample 1.

[0089] In this state, the normal-temperature bonding thrust force and the high-temperature bonding thrust force of the magnet were determined. The bonding thrust force at room temperature (25? C.) was 1200 N/cm.sup.2, and the bonding thrust force at high temperature (170? C.) was 530 N/cm.sup.2.

Examples 2-5

[0090] The surface pretreatment method, the expandable coating material, and the coating process were the same as those in Example 1. A magnet with an expandable coating with a thickness of 110 ?m was inserted into a magnetic steel groove, and different first expansion temperature, second expansion temperature, heating rate, and expansion time were used to obtain the optimal assembly process conditions. The results are shown in Table 1 below.

TABLE-US-00001 TABLE 1 Sample Sample Sample Sample Sample Name 1 2 3 4 5 First Expansion 120 110 160 110 160 expansion temperature/? C. stage Heating rate ? C./min 5 5 15 40 5 Holding time min 5 5 5 5 5 Second Expansion 170 180 210 180 210 expansion temperature/? C. stage Heating rate ? C./min 30 40 60 60 5 Holding time min 3 3 3 3 3 Parameter Normal-temperature 1200 1182 1192 1002 1006 thrust force (N/cm.sup.2) High-temperature 530 540 535 410 375 thrust force (N/cm.sup.2) Rupture W.sub.1 mg/l 5.66 5.81 4.15 3.8 6.41 proportion of W.sub.2 mg/l 1.24 1.08 2.74 3.08 0.58 microspheres q 82% 84% 60% 55% 92% *Wherein: W.sub.1 and W.sub.2 represent the content of isooctane, and q represents the rupture proportion of microspheres.

[0091] As can be seen from the results in Table 1, the normal-temperature thrust force and the high-temperature thrust force are correlated with the expansion temperature and the heating rate, and the optimal assembly process conditions can be obtained by optimization.

[0092] Meanwhile, the rupture proportion of the microspheres was deduced by determining the release amount of isooctane, and it could be found that when the ruptured microspheres were cross-linked with the resin coating in the case of the rupture proportion in the range of 60%-85%, the obtained coating has a stable structure and can stably support the gap between the magnet and the magnetic steel groove.

[0093] FIG. 1 shows the state of the coating after the expansion of sample 1, from which it can be seen that: the microspheres expand uniformly to cross-link with the walls of the microspheres and the resin, thereby forming a stable support structure.

[0094] FIG. 2 shows the state of the coating after the expansion of sample 5, from which it can be seen that there are many ruptured microspheres, and the microspheres are fused and adhered to each other to form relatively large voids, so that the coating has lower bonding force in a high-temperature state.

Example 6

[0095] In this example, a sintered neodymium-iron-boron magnet (not magnetized) with the specification of 35.5 mm?16.5 mm?5.5 mm was used, and a magnetic steel groove with the assembled motor rotor had a size of 36 mm?17 mm?6 mm.

[0096] The surface pretreatment method same as that in Example 1 was used. The sintered neodymium-iron-boron magnet was degreased at 60? C. for 2 min by using a composite degreasing liquid with sodium hydroxide at a concentration of 15 g/L and sodium dodecyl sulfonate at a concentration of 3 g/L, and then subjected to acid washing for 15 s by using 25 wt % of an aqueous citric acid solution, and finally placed in deionized water for ultrasonic cleaning for 2 min.

[0097] The expandable powder coating material produced by AKZO-Nobel company was selected. The coating consists of 50% of epoxy resin powder, 20% of curing agent, 10% of elastomer resin, and 20% of thermoplastic expandable microspheres. The expandable coating material described above was coated on the surface of the magnet by compressed air spray coating, wherein the spray coating speed was 60 mm/s, the coating thickness was 110 ?m, the distance between a nozzle and a workpiece was cm, the angle between a spray gun and the workpiece was 25?, and the argon pressure of the spray gun was 0.6 MPa.

[0098] The magnet coated with the expandable coating was assembled into a magnetic steel groove of a motor rotor, placed into a high-temperature oven, and heated at 190? C. for 20 min. The volume of the thermoplastic expandable microspheres was increased due to temperature change, resulting in the expansion of the overall coating. At the same time, the epoxy resin was cured, making the coating stable and non-retractable. The cross-sectional structure of the expandable coating was observed using an electron microscope. As shown in FIG. 3, the thickness of the expandable coating was increased from 110 ?m to 180 ?m after the pre-curing treatment, with the expansion rate reaching 163%. After expansion, the edges of the thermoplastic expandable microspheres can be clearly seen. The expandable microspheres, under heating conditions, increased in microsphere volume solely due to the vaporization of the liquid alkane within the microspheres. Whether the microspheres in the coating are ruptured is speculated by determining the released amount of isooctane in the expandable microspheres. When the coating is heated at 190? C. for 20 min, the released amount w1 of isooctane is determined to be 1.52 mg/L. When the coating is heated to 240? C. and maintained at this temperature for 3 h, the expandable microspheres in the coating are completely ruptured, and the released amount w2 of isooctane is 5.34 mg/L. According to the formula, the rupture proportion of the expandable microspheres in the actual assembly process can be calculated as q=w1/(w1+w2)=1.52/(1.52+5.34)=22%.

[0099] As can be seen from FIG. 3, the cross-sectional area of the expandable microspheres accounts for 35% of that of the expandable coating.

[0100] In this state, the normal-temperature bonding thrust force and the high-temperature bonding thrust force of the magnet were determined. The normal-temperature (25? C.) bonding thrust force was 920 N/cm.sup.2, and the high-temperature (170? C.) bonding thrust force was 310 N/cm.sup.2.

Examples 7-14

[0101] In this example, a sintered neodymium-iron-boron magnet (not magnetized) with the specification of 35.5 mm?16.5 mm?5.5 mm was used, and a magnetic steel groove with the assembled motor rotor had a size of 36 mm?17 mm?6 mm.

[0102] The surface pretreatment process and expandable coating material same as those in Example 1 were used. The expandable coating described above was coated on the surface of the magnet by compressed air spray coating, wherein the thicknesses for the spray coating were 80 ?m, 90 ?m, 100 ?m, 110 ?m, 120 ?m, 130 ?m, 140 ?m, and 150 ?m, respectively.

[0103] The magnet coated with the expandable coating was assembled into a magnetic steel groove of a motor rotor, placed into a high-temperature oven, and subjected to two-stage heating, that is, the magnet was first heated to 120? C. and maintained at this temperature for 5 min, and then quickly heated to 170? C. and maintained at this temperature for 3 min. The first heating stage was at a heating rate of 5? C./min, and the second heating stage was at a heating stage of 50? C./min. The normal-temperature thrust force and the high-temperature thrust force of the magnet in the operating state were determined. The results are shown in Table 2 below:

[00001]$\mathrm{Expansion}\mathrm{rate}=H_2/H_0$$\mathrm{Compression}\mathrm{rate}=\left(H_1-H_0\right)/\left(H_2-H_0\right)$

[0104] wherein H.sub.0 is the coating thickness, H.sub.1 is the thickness of the expandable coating after expansion in the magnetic steel groove, and H.sub.2 is the thickness of the expandable coating after expansion in the natural state.

TABLE-US-00002 TABLE 2 Free Expansion Coating expansion thickness H1 Normal- High- thickness thickness Expansion in magnetic Compression temperature temperature No. H0 H2 rate steel groove rate thrust force thrust force Sample 7 80 280 350% 250 85% 1100 380 Sample 8 90 318 353% 250 70% 1120 385 Sample 9 100 355 355% 250 59% 1275 410 Sample 10 110 394 358% 250 49% 1310 445 Sample 11 120 434 362% 250 41% 1290 480 Sample 12 130 478 368% 250 34% 1250 495 Sample 13 140 525 375% 250 29% 1105 415 Sample 14 150 585 390% 250 23% 1003 400

[0105] FIG. 4 shows the coating state of sample 10 in the magnetic steel groove. As can be seen from FIG. 4, in the expansion process of the coating, the expandable coating is compressed and wrinkled due to the limitation of the inner wall of the magnetic steel groove.

[0106] When the expandable coating is seriously compressed, the contact surface of the expandable coating and the contact surface of the inner wall of the magnetic steel groove generate relatively strong stress. The stress on the unit area of the coating is increased, and the internal defects of the coating are exponentially increased, so that the cohesive strength of an adhesive layer is reduced. Therefore, by controlling the compression rate to be above 35%, the magnet with better normal-temperature thrust force and high-temperature thrust force can be prepared.

[0107] The greater the compression ratio is, the better the economic effect is obtained. The inventors find through a large number of experiments that: when the compression rate is greater than 65%, the coating is more sensitive to the shrinkage stress and thermal stress caused by temperature change, which will result in the loss of cohesive strength of the magnet, thereby reducing normal-temperature thrust force and high-temperature thrust force of the magnet.

[0108] The above examples illustrate the embodiments of the present disclosure. However, the present disclosure is not limited to the embodiments described above. Any modification, equivalent, improvement, and the like made without departing from the spirit and principle of the present disclosure shall fall within the protection scope of the present disclosure.