ELECTRO-DEPOSITION PROCESS, ELECTRO-DEPOSITION BATH, AND METHOD FOR PREPARING RARE EARTH PERMANENT MAGNETIC MATERIAL THROUGH ELECTRO-DEPOSITION

Abstract

The present invention discloses an electro-deposition process, an electro-deposition bath, and a method for preparing a rare earth permanent magnetic material through electro-deposition. The electro-deposition process is used for depositing a heavy rare earth element on the surface of a sintered R.sup.2-T-B type master alloy, and comprises Step 1: providing an electro-deposition bath, comprising a main salt containing the heavy rare earth element, an induction salt for inducing the heavy rare earth element to deposit, and an organic ionic liquid as the solvent, wherein the main salt is a tetrafluoroborate of the heavy rare earth element; and Step 2: electroplating the sintered R.sup.2-T-B type master alloy in the electro-deposition bath at a temperature of 0 to 200° C. The present invention has the following beneficial effects: deposition of the heavy rare earth element on the surface of the sintered R.sup.2-T-B type master alloy is rapid, so that the electro-deposition process time can be saved, and the production efficiency is improved. In addition, a higher plating thickness of up to 10 to 40 μm can be achieved.

Claims

1. An electro-deposition process for depositing a heavy rare earth element on the surface of a sintered R.sup.2-T-B type master alloy, comprising: Step 1: providing an electro-deposition bath, comprising a main salt containing the heavy rare earth element, an induction salt for inducing the heavy rare earth element to deposit, and an organic ionic liquid as the solvent, wherein the main salt is a tetrafluoroborate of the heavy rare earth element; and Step 2: electroplating the sintered R.sup.2-T-B type master alloy in the electro-deposition bath at a temperature of 0 to 200° C.

2. The electro-deposition process according to claim 1, wherein the heavy rare earth element is selected from at least one of Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and preferably selected from at least one of Dy, Tb, and Ho.

3. The electro-deposition process according to claim 1, wherein the induction salt is Fe(BF.sub.4).sub.2 and/or Co(BF.sub.4).sub.2.

4. The electro-deposition process according to claim 1, wherein when the induction salt is Fe(BF.sub.4).sub.2 and Co(BF.sub.4).sub.2, the molar concentration of the main salt in the electro-deposition bath is 0.1 to 2 mol/L; the molar concentration of Fe(BF.sub.4).sub.2 is 0.1 to 2 mol/L; and the molar concentration of Co(BF.sub.4).sub.2 is 0.1 to 1 mol/L.

5. The electro-deposition process according to claim 4, wherein the molar concentration ratio of Fe(BF.sub.4).sub.2 to Co(BF.sub.4).sub.2 in the electro-deposition bath is 1 to 2.5:1.

6. The electro-deposition process according to claim 1, wherein the organic ionic liquid is selected from at least one of a tetrafluoroborate, a bis[(trifluoromethyl)sulfonyl]imide salt, and a bis(fluorosulfonyl)imide salt; preferably, the tetrafluoroborate is selected from N-methoxyethyl-N-methyldiethylammonium tetrafluoroborate or N-methylethylpyrrolidinium tetrafluoroborate; the bis[(trifluoromethyl)sulfonyl]imide salt is selected from 1-ethyl-3methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, N-methoxyethyl-N-methyldiethylammonium bis[(trifluoromethyl)sulfonyl]imide, trimethylpropylammonium bis[(trifluoromethyl)sulfonyl]imide, trimethylbutylammonium bis[(trifluoromethyl)sulfonyl]imide, N-methylbutylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide, N-methyl,propylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide, N-methylethylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide, N-methylmethoxyethylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide, N-methylpropylpiperidinium bis[(trifluoromethyl)sulfonyl]imide, N-methylbutylpiperidinium bis[(trifluoromethyl)sulfonyl]imide, and 1,2-dimethyl-3-propylimidazolium bis[(trifluoromethyl)sulfonyl]imide; and the bis(fluorosulfonyl)imide salt is selected from 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, N-methylpropylpyrrolidinium bis(fluorosulfonyl)imide, and N-methylpropylpiperidinium bis(fluorosulfonyl)imide.

7. The electro-deposition process according to claim 1, wherein the electro-deposition bath further comprises a conducting salt.

8. The electro-deposition process according to claim 7, wherein the conducting salt is selected from at least one of LiClO.sub.4, LiCl, LiBF.sub.4, KCl, and NaCl.

9. The electro-deposition process according to claim 1, wherein, in the process, the cathode is the sintered R.sup.2-T-B type master alloy; and the anode may be one of graphite, platinum, silver, and gold, preferably, in the sintered R.sup.2-T-B type master alloy, wherein R.sup.2 is at least one of the rare earth elements, preferably at least one of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and more preferably Nd or Pr; and is present in an amount of 17 to 38 wt % based on the weight of the master alloy; T comprises iron (Fe), which is present in an amount of 55 to 81 wt % based on the weight of the master alloy; and at least one element, which is present in an amount of 0 to 6 wt % based on the weight of the master alloy, selected from Al, Cu, Zn, In, Si, P, S, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag, Cd, Sn, Sb, Hf, Ta, and W; B is the elemental boron, which is present in an amount of 0.5 to 1.5 wt % based on the weight of the master alloy; and impurity elements.

10. The electro-deposition process according to claim 1, wherein the electroplating is conducted at a constant voltage of 0.5 to 2 V and preferably 0.8 to 1.6 V, preferably at a temperature ranging from 0 to 100° C. and more preferably from 30 to 40° C., and for a period of time of 20 to 500 min and preferably 50 to 300 min.

11. The electro-deposition process according to claim 1, wherein after Step 2 is completed, the mean thickness of the heavy rare earth element plating on the surface of the sintered R.sup.2-T-B type master alloy is 10-40 m.

12. An electro-deposition bath for depositing a heavy rare earth element on the surface of a sintered R.sup.2-T-B type master alloy, comprising a main salt containing the heavy rare earth element, an induction salt for inducing the heavy rare earth element to deposit, and an organic ionic liquid as the solvent, wherein the main salt is a tetrafluoroborate of the heavy rare earth element.

13. The electro-deposition bath according to claim 12, wherein the heavy rare earth element is selected from at least one of Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and preferably selected from at least one of Dy, Tb, and Ho; the induction salt is Fe(BF.sub.4).sub.2 and/or Co(BF.sub.4).sub.2; the organic ionic liquid is selected from at least one of a tetrafluoroborate, a bis[(trifluoromethyl)sulfonyl]imide salt, and a bis(fluorosulfonyl)imide salt; preferably, the tetrafluoroborate is selected from N-methoxyethyl-N-methyldiethylammonium tetrafluoroborate or N-methylethylpyrrolidinium tetrafluoroborate; the bis[(trifluoromethyl)sulfonyl]imide salt is selected from 1-ethyl-3methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, N-methoxyethyl-N-methyldiethylammonium bis[(trifluoromethyl)sulfonyl]imide, trimethylpropylammonium bis[(trifluoromethyl)sulfonyl]imide, trimethylbutylammonium bis[(trifluoromethyl)sulfonyl]imide, N-methylbutylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide, N-methyl,propylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide, N-methylethylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide, N-methylmethoxyethylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide, N-methylpropylpiperidinium bis[(trifluoromethyl)sulfonyl]imide, N-methylbutylpiperidinium bis[(trifluoromethyl)sulfonyl]imide, and 1,2-dimethyl-3-propylimidazolium bis[(trifluoromethyl)sulfonyl]imide; and the bis(fluorosulfonyl)imide salt is selected from 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, N-methylpropylpyrrolidinium bis(fluorosulfonyl)imide, and N-methylpropylpiperidinium bis(fluorosulfonyl)imide; more preferably, the main salt and the induction salt in the electro-deposition bath are formulated in such a manner that the molar concentration of Tb(BF.sub.4).sub.3 is 0.1 to 2 mol/L, the molar concentration of Fe(BF.sub.4).sub.2 is 0 to 2 mol/L, and the molar concentration of Co(BF.sub.4).sub.2 is 0 to 1 mol/L; and more preferably, the molar concentration ratio of Fe(BF.sub.4).sub.2 to Co(BF.sub.4).sub.2 in the electro-deposition bath is 2:1.

14. The electro-deposition bath according to claim 13, further comprising a conducting salt; and preferably the conducting salt is selected from at least one of LiClO.sub.4, LiCl, LiBF.sub.4, KCl, and NaCl.

15. A method for preparing a sintered R.sup.1R.sup.2-T-B type permanent magnetic material, comprising: Step 1: providing a sintered R.sup.2-T-B type master alloy; Step 2: depositing a heavy rare earth element R.sup.1 on the surface of the R.sup.2-T-B type master alloy according to the electro-deposition process as set forth in claim 1; and Step 3: performing thermal treatment on the master alloy having the heavy rare earth element R.sup.1 plated on the surface thereof, to obtain the R.sup.1R.sup.2-T-B type permanent magnetic material; preferably, the thermal treatment comprises first-stage high-temperature thermal treatment at 820 to 920° C. under vacuum or under an Ar atmosphere for 1 to 24 hours; and heating and maintaining at a low temperature of 480 to 540° C. for 1 to 10 hours.

Description

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

[0049] FIG. 1 is an SEM image of a specimen according to an embodiment of the present invention at 100× magnification;

[0050] FIG. 2 is an SEM image of a specimen according to an embodiment of the present invention at 300× magnification; and

[0051] FIG. 3 is an SEM image of a specimen according to an embodiment of the present invention at 500× magnification.

DETAILED DESCRIPTION

[0052] Hereinafter, the embodiments of the present invention will be described in detail with reference to examples, in which where no specific conditions are defined, the conventional conditions or the conditions recommended by the manufacturer are followed; and the used reagents or instruments with no manufacturer indicated are all conventional products commercially available.

[0053] The main salts used in the following examples are obtained by reacting terbium oxide, metallic iron and cobalt carbonate with HBF.sub.4, respectively.

[0054] The specific preparation processes are as follows:

[0055] The chemical reaction formula for producing Fe(BF.sub.4).sub.2 is: Fe+2HBF.sub.4═Fe(BF.sub.4).sub.2+H.sub.2↑.

[0056] In the experiment, Fe(BF.sub.4).sub.2 is produced through a displacement reaction.

[0057] Excessive HBF.sub.4 is added into reductive iron powder, then the mixture is heated until the reductive iron powder disappears, and until most of H.sub.2O and HBF.sub.4 are distilled off. After reaction, the system is cooled to room temperature, and heated in a vacuum oven at 100° C. for 15 hours, to obtain Fe(BF.sub.4).sub.2. The Fe(BF.sub.4).sub.2 prepared in the experiment is easily oxidized, and thus it should be stored in an inert gas atmosphere. After preparation, the Fe(BF.sub.4).sub.2 should be used as soon as possible before it is oxidized into Fe(BF.sub.4).sub.3 to prevent the failure of the experiment.

[0058] The chemical reaction formula for producing Co(BF.sub.4).sub.2 is: CoCO.sub.3+2HBF.sub.4═Co(BF.sub.4).sub.2+H.sub.2O+CO.sub.2↑.

[0059] In the experiment, Co(BF.sub.4).sub.2 is produced through a metathetical reaction. Excessive HBF.sub.4 is added into CoCO.sub.3, the mixture is heated until the CoCO.sub.3 disappears, and until most of H.sub.2O and HBF.sub.4 are distilled off. After reaction, the system is cooled to room temperature, and heated in a vacuum oven at 100° C. for 15 hours, to obtain Co(BF.sub.4).sub.2.

[0060] The chemical reaction formula for producing Tb(BF.sub.4).sub.3 is: Tb.sub.2O.sub.3+3HBF.sub.4=2Tb(BF.sub.4).sub.3+3H.sub.2O.

[0061] In the experiment, Tb(BF.sub.4).sub.3 is produced through a metathetical reaction. Excessive HBF.sub.4 is added into Tb.sub.2O.sub.3. After reaction, the system is cooled to room temperature, and heated in a vacuum oven at 100° C. for 15 hours, to obtain Tb(BF.sub.4).sub.3.

[0062] The following experimental procedures need to be carried out in a glove box. All the experimental processes need to be completed in a stringent environment free of oxygen and water vapor, and the ionic liquid used should be dried for over 2 hours with activated 4A molecular sieve.

Example 1

[0063] The cathode material used in this example was an R.sub.2FeMB (neodymium iron boron) magnetic material of D7×3 mm, and the anode was a platinum sheet of 10×10×1 mm. The electro-deposition bath contained a main salt comprising a heavy rare earth element, an induction salt for inducing the heavy rare earth element to deposit, and an organic ionic liquid as the solvent. The main salt was a tetrafluoroborate of the heavy rare earth element. In the electro-deposition bath, the molar concentrations of Tb(BF.sub.4).sub.3, Fe(BF.sub.4).sub.2, and Co(BF.sub.4).sub.2 were 1 mol/L, 1.2 mol/L, and 0.6 mol/L respectively, and the ionic liquid was 1-butyl-3-methylimidazolium tetrafluoroborate ([EMIM] BF.sub.4). The electroplating was conducted at a temperature of 50° C. and a constant voltage of 1.9 V for 300 min, to obtain a Fe—Co—Tb plating, as shown in FIG. 1. Its surface was analyzed by EDS. The result is shown in Table 1.1. The thermal treatment process was performed by maintaining at 900° C. for 150 min and then cooling, heating at 480° C. and maintaining at 480° C. for 150 min, followed by cooling. A non-electroplated blank sheet (a blank sheet with no heavy rare earth added in the experiment) was treated with the same thermal treatment process. The comparison results of the properties of the two magnets are shown in Table 1.2.

TABLE-US-00001 TABLE 1.1 Energy spectrum analysis results Percent Atomic Element by weight percent C K 4.91 15.73 F K 10.11 26.25 Mg K 0.69 0.81 Cl K 0.18 0.15 Fe K 41.69 33.44 Co K 20.04 18.41 Nd L 5.87 1.17 Tb L 16.51 4.04

[0064] The energy spectrum analysis results show that, the more the content of the heavy rare earth (for example, Tb and so on) is, the better the improvement of the coercive force is.

TABLE-US-00002 TABLE 1.2 Magnetic property analysis of magnetic materials Magnetic Hcj (BH).sub.max Hk property (kA/m) (kJ/m.sup.3) Br (T) (kA/m) Blank sheet 1275 357.3 1.355 1234 Magnet 1355 353.6 1.351 1324 according to the present invention

Example 2

[0065] The cathode material used in this example was an R.sub.2FeMB (neodymium iron boron) magnetic material of D7×3 mm, and the anode was a platinum sheet of 10 10×1 mm. The electro-deposition bath contained a main salt comprising a heavy rare earth element, an induction salt for inducing the heavy rare earth element to deposit, and an organic ionic liquid as the solvent. The main salt was a tetrafluoroborate of the heavy rare earth element. In the electro-deposition bath, the molar concentrations of Tb(BF.sub.4).sub.3, Fe(BF.sub.4).sub.2, and Co(BF.sub.4).sub.2 were 0.5 mol/L, 1 mol/L, and 0.5 mol/L respectively, and the ionic liquid was N-methylethylpyrrolidinium tetrafluoroborate. The electroplating was conducted at a temperature of 0° C. and a constant voltage of 0.5 V for 500 min, to obtain a Fe—Co—Tb plating. The thermal treatment process was performed by maintaining at 820° C. for 24 hours and then cooling, heating at 540° C. and maintaining at 540° C. for 1 hour, followed by cooling. An R.sub.1R.sub.2FeMB magnetic material was obtained by forming a network-like granular crystalline plating of about 10-30 μm in thickness on the surface of R.sub.2FeMB through the electro-deposition process described in this example. A non-electroplated blank sheet (a blank sheet with no heavy rare earth added in the experiment) was treated with the same thermal treatment process. The comparison results of the properties of the two magnets are shown in Table 2.

TABLE-US-00003 TABLE 2 Magnetic property analysis of magnetic materials Magnetic Hcj (BH)max Hk property (kA/m) (kJ/m.sup.3) Br (T) (kA/m) Blank sheet 1291 356.4 1.352 1259 Magnet 1435 351.6 1.348 1396 according to the present invention

Example 3

[0066] The cathode material used in this example was an R.sub.2FeMB (neodymium iron boron) magnetic material of D7×3 mm, and the anode was a platinum sheet of 10×10×1 mm. The electro-deposition bath contained a main salt comprising a heavy rare earth element, an induction salt for inducing the heavy rare earth element to deposit, and an organic ionic liquid as the solvent. The main salt was a tetrafluoroborate of the heavy rare earth element. In the electro-deposition bath, the molar concentrations of Tb(BF.sub.4).sub.3, Fe(BF.sub.4).sub.2, and Co(BF.sub.4).sub.2 were 0.2 mol/L, 0.5 mol/L, and 0.1 mol/L respectively, and the ionic liquid was 1-ethyl-3methylimidazolium bis[(trifluoromethyl)sulfonyl]imide. The electroplating was conducted at a temperature of 200° C. and a constant voltage of 2 V for 350 min, to obtain a Fe—Co—Tb plating. The thermal treatment process was performed by maintaining at 920° C. for 1 hour and then cooling, heating at 480° C. and maintaining at 480° C. for 10 hours, followed by cooling. An R.sub.1R.sub.2FeMB magnetic material was obtained by forming a network-like granular crystalline plating of about 10-30 m in thickness on the surface of R.sub.2FeMB through the electro-deposition process described in this example. A non-electroplated blank sheet (a blank sheet with no heavy rare earth added in the experiment) was treated with the same thermal treatment process. The comparison results of the properties of the two magnets are shown in Table 3.

TABLE-US-00004 TABLE 3 Magnetic property analysis of magnetic materials Magnetic Hcj (BH)max Hk property (kA/m) (kJ/m.sup.3) Br (T) (kA/m) Blank sheet 1370 353.8 1.352 1331 Magnet 1515 350.4 1.349 1460 according to the present invention

Example 4

[0067] The cathode material used in this example was an R.sub.2FeMB (neodymium iron boron) magnetic material of D7×3 mm, and the anode was a platinum sheet of 10 10×1 mm. The electro-deposition bath contained a main salt comprising a heavy rare earth element, an induction salt for inducing the heavy rare earth element to deposit, and an organic ionic liquid as the solvent. The main salt was a tetrafluoroborate of the heavy rare earth element. In the electro-deposition bath, the molar concentrations of Tb(BF.sub.4).sub.3, Co(BF.sub.4).sub.2, and Fe(BF.sub.4).sub.2 were 0.5 mol/L, 0.3 mol/L, and 0.8 mol/L respectively, and the ionic liquid was trimethylbutylammonium bis[(trifluoromethyl)sulfonyl]imide. The electroplating was conducted at a temperature of 80° C. and a constant voltage of 0.8 V for 200 min, to obtain a Fe—Co—Tb plating. The thermal treatment process was performed by maintaining at 900° C. for 5 hours and then cooling, heating at 500° C. and maintaining at 500° C. for 6 hours, followed by cooling. An R.sub.1R.sub.2FeMB magnetic material was obtained by forming a network-like granular crystalline plating of about 10-30 μm in thickness on the surface of R.sub.2FeMB through the electro-deposition process described in this example. A non-electroplated blank sheet (a blank sheet with no heavy rare earth added in the experiment) was treated with the same thermal treatment process. The comparison results of the properties of the two magnets are shown in Table 4.

TABLE-US-00005 TABLE 4 Magnetic property analysis of magnetic materials Magnetic Hcj (BH)max Hk property (kA/m) (kJ/m.sup.3) Br (T) (kA/m) Blank sheet 1285 354.7 1.359 1250 Magnet 1435 351.1 1.351 1379 according to the present invention

Example 5

[0068] The cathode material used in this example was an R.sub.2FeMB (neodymium iron boron) magnetic material of D7×3 mm, and the anode was a platinum sheet of 10×10×1 mm. The electro-deposition bath contained a main salt comprising a heavy rare earth element, an induction salt for inducing the heavy rare earth element to deposit, and an organic ionic liquid as the solvent. The main salt was a tetrafluoroborate of the heavy rare earth element. In the electro-deposition bath, the molar concentrations of Tb(BF.sub.4).sub.3, Co(BF.sub.4).sub.2, and Fe(BF.sub.4).sub.2 were 1 mol/L, 1 mol/L, and 1.2 mol/L respectively, and the ionic liquid was 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide. The electroplating was conducted at a temperature of 120° C. and a constant voltage of 1.6 V for 500 min, to obtain a Fe—Co—Tb plating. The thermal treatment process was performed by maintaining at 890° C. for 20 hours and then cooling, heating at 490° C. and maintaining at 490° C. for 8 hours, followed by cooling. An R.sub.1R.sub.2FeMB magnetic material was obtained by forming a network-like granular crystalline plating of about 10-30 μm in thickness on the surface of R.sub.2FeMB through the electro-deposition process described in this example. A non-electroplated blank sheet (a blank sheet with no heavy rare earth added in the experiment) was treated with the same thermal treatment process. The comparison results of the properties of the two magnets are shown in Table 5.

TABLE-US-00006 TABLE 5 Magnetic property analysis of magnetic materials Magnetic Hcj (BH)max Hk property (kA/m) (kJ/m.sup.3) Br (T) (kA/m) Blank sheet 1272 357.6 1.352 1196 Magnet 1435 350.1 1.347 1365 according to the present invention

Example 6

[0069] The cathode material used in this example was an R.sub.2FeMB (neodymium iron boron) magnetic material of D7×3 mm, and the anode was a platinum sheet of 10×10×1 mm. The electro-deposition bath contained a main salt comprising a heavy rare earth element, an induction salt for inducing the heavy rare earth element to deposit, an organic ionic liquid as the solvent, and a conducting salt. The main salt was a tetrafluoroborate of the heavy rare earth element. In the electro-deposition bath, the molar concentrations of Tb(BF.sub.4).sub.3, Fe(BF.sub.4).sub.2, and Co(BF.sub.4).sub.2 were 1 mol/L, 2 mol/L, and 1 mol/L respectively. The ionic liquid was N-methylethylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide, and the conducting salt NaCl had a concentration of 0.5 mol/L. The electroplating was conducted at a temperature of 150° C. and a constant voltage of 1.5 V for 300 min, to obtain a Fe—Co—Tb plating. The thermal treatment process was performed by maintaining at 900° C. for 3 hours and then cooling, heating at 480° C. and maintaining at 480° C. for 2 hours, followed by cooling. An R.sub.1R.sub.2FeMB magnetic material was obtained by forming a network-like granular crystalline plating of about 10-30 m in thickness on the surface of R.sub.2FeMB through the electro-deposition process described in this example. A non-electroplated blank sheet (a blank sheet with no heavy rare earth added in the experiment) was treated with the same thermal treatment process. The comparison results of the properties of the two magnets are shown in Table 6.

TABLE-US-00007 TABLE 6 Magnetic property analysis of magnetic materials Magnetic Hcj (BH)max Hk property (kA/m) (kJ/m.sup.3) Br (T) (kA/m) Blank sheet 1410 344.8 1.341 1334 Magnet 1595 339.4 1.335 1516 according to the present invention

[0070] In the above examples, the experimental results show that the coercive force Hcj of the magnets prepared through the electro-deposition process of the present invention have been improved, while there is little influence on the remanent magnetization Br.

[0071] In addition, it should be noted that with the same temperature and the same organic solvent, the solubility of the tetrafluoroborate of a heavy rare earth element (for example, Tb(BF.sub.4).sub.3) is about ten times of the solubility of other kinds of heavy rare earth salt (for example, TbCl.sub.3). The solubility of Tb(BF.sub.4).sub.3 is generally about 1 mol/L, and the solubility of TbCl.sub.3 is about 0.1 mol/L. With the same period of time (for example, the electro-deposition time is 60 min), a plating with a thickness of about 10 m can be formed in a system having Tb(BF.sub.4).sub.3 as the main salt, while a plating with a thickness of only about 1 m is formed in a system having TbCl.sub.3 as the main salt. Even though the former is an alloy, and the content of the heavy rare earth is about 15-20%, the rate in the former case is still 1 time faster than that in the latter case. Furthermore, considering the high solubility, the supplementation time cycle of the main salt during production can be extended, which desirably meets the practical requirement in massive production.

[0072] The above embodiments are merely exemplary embodiments of the present invention and are not intended to limit the protection scope of the invention, which is defined by the claims. Various modifications or equivalent substitutions may be made to the present invention by a person skilled in the art within the spirit and protection scope of the present invention, and such modifications or equivalent substitutions are also deemed to fall within the protection scope of the present invention.