Patent Application
20190338401
The present invention relates to a MnAl alloy manufacturing method and, more particularly, to a MnAl alloy manufacturing method using a molten salt electrolysis method.
As a manufacturing method for a MnAl-based alloy, melting methods such as an arc melting method and a high-frequency induction melting method are known, and molten metal obtained by melting is cooled/solidified using an atomizing method or a roll quenching method, whereby a MnAl-based alloy is obtained. For example, Patent Document 1 reports a MnAl-based ferromagnetic material having, as a main phase, a τ-phase having a tetragonal crystal structure of L10 type as a main component. Further, Patent Document 2 proposes a Mn—Al—C-based magnetic material and discloses that an alloy obtained by the melting method becomes a MnAl-based ferromagnetic material having a τ-phase as a main phase.
In MnAl-based alloy manufacturing methods using such a melting method, the stoichiometric ratio of the τ-phase is Mn/Al=1:1 originally; however, in order to transform an ϵ-phase (hcp) to the τ-phase as completely as possible by heat treatment as post-processing, many ϵ-phases need to be contained at the time of rapid quenching as pre-processing. Thus, it has been important to set the alloy composition at the time of melting to Mn/Al=55:45, which results in a disadvantage that the τ-phase containing excess Mn is generated.
On the other hand, in the Mn—Al—C-based magnetic material, addition of carbon makes it possible to directly obtain the τ-phase without passing through the ϵ-phase as an intermediate generated phase and without application of heat treatment (with only melting and rapid quenching). However, the addition of carbon disadvantageously generates a slight amount of a different phase (Mn3AlC).
A molten salt electrolysis method is also known as the MnAl-based alloy manufacturing method. Non-Patent Document 1 discloses that a MnAl alloy is deposited on the surface of an electrode by electrolyzing molten salt having an Al compound as a main component and a Mn compound as an additive component. Non-Patent Document 2 reports that a MnAl-based magnetic material having, as a main phase, the τ-phase exhibiting ferromagnetism is deposited by adjusting a Mn compound to be added to molten salt having an Al compound as a main component and applying electrolysis under predetermined conditions.
[Patent Document] 1 JP S34-30435 B
[Patent Document] 2 JP S37-57224 B
J. Uchida et al., Tetsu-to-hagane Vol. 77 (1991) No. 7, p.931
G. R. Stafford et al., J. Alloy Compd. 200 (1993) 107-113
In a MnAl alloy manufacturing method using the conventional molten salt electrolysis method, it is possible to obtain a MnAl-based alloy whose Mn content is lower than 55% by adjusting the amount of a Mn compound to be added to molten salt having an Al compound as a main component and thus to obtain a MnAl-based magnetic material having a single-phase τ structure and having Mn/Al ratio close to the stoichiometric ratio. However, the concentration of the Mn compound in the molten salt gradually decreases as electrolysis progresses, causing a variation in the composition of a MnAl alloy to be deposited. Thus, stable production conditions cannot be maintained.
The present invention has been made in view of the above situation, and the object thereof is to reduce a variation in the composition of a MnAl alloy to be deposited in the MnAl-based alloy manufacturing method using the molten salt electrolysis method so as to obtain high magnetic characteristics.
The present inventors have intensively studied to solve the above problem and attain the object and, as a result, they found that by making the concentration of the Mn compound in the molten salt stable, a variation in the composition of the MnAl alloy to be deposited decreases, which in turn makes it possible to obtain high magnetic characteristics. The present invention has been made based on such a technical finding, and a MnAl alloy manufacturing method according to the present invention is a method that electrolyzes molten salt containing a Mn compound and an Al compound to deposit a MnAl alloy and is characterized by additionally charging the Mn compound into the molten salt during electrolysis.
According to the present invention, the concentration of the Mn compound is maintained by additionally charging the Mn compound, so that a variation in the composition of the MnAl alloy to be deposited decreases, making it possible to maintain stable production conditions. The concentration of the Mn compound in the molten salt is preferably maintained at 0.2 mass % or more by additional charging of the Mn compound. This allows the MnAl alloy having high magnetic characteristics to be produced stably. The molten salt may further contain an alkali metal halide and, additionally, a rare earth halide or an alkaline earth halide. The temperature of the molten salt during electrolysis is preferably 150° C. or more and 600° C. or less. The electricity amount per electrode area 1 cm2 is preferably 30 mAh or more and 120 mAh or less. Various magnetic characteristics can be imparted to the MnAl alloy according to the temperature of the molten salt during electrolysis. Specifically, when the temperature of the molten salt during electrolysis is set to 150° C. or more and less than 400° C., ferromagnetism can be imparted to the MnAl alloy. Further, when the temperature of the molten salt during electrolysis is set to 400° C. or more and less than 600° C., metamagnetism can be imparted to the MnAl alloy. Further, when the temperature of the molten salt during electrolysis is set to 600° C. or more and 700° C. or less, ferromagnetism can be imparted to the MnAl alloy, and residual magnetization can be increased as compared with the MnAl alloy produced at less than 600° C.
In the present invention, heat treatment may be applied to the MnAl alloy deposited by electrolysis. When heat treatment is applied to the deposited MnAl alloy, various magnetic characteristics can be imparted to the MnAl alloy according to heat treatment conditions. Specifically, when heat treatment temperature is set to 400° C. or more and less than 600° C., metamagnetism can be imparted to the MnAl alloy. Further, when heat treatment temperature is set to 600° C. or more and 700° C. or less, residual magnetization can be increased when compared with the MnAl alloy before application of heat treatment. The heat treatment is preferably performed in an inert gas atmosphere or in a vacuum atmosphere.
In the present invention, a powdery MnAl alloy may be deposited by performing electrolysis with an electricity amount of 50 mAh or more per 1 mass % of concentration of the Mn compound in the molten salt and per electrode area 1 cm2. This allows high productivity to be obtained and allows a desired product shape to be obtained by compression-molding the powdery MnAl alloy.
As described above, according to the present invention, it is possible to reduce a variation in the composition of the MnAl alloy to be deposited and thus to obtain high magnetic characteristics in a MnAl-based alloy manufacturing method using the molten salt electrolysis method.
Hereinafter, preferred embodiments of the present invention will be described. The present invention is not limited to the embodiments and examples described below. Further, the constituent elements shown in the following embodiments and examples can be appropriately combined or selected for use.
The electrodeposition apparatus illustrated in
The molten salt 3 contains at least a Mn compound and an Al compound. As the Mn compound, MnCl2 can be used. As the Al compound, AlCl3, AlF3, AlBr3, or AlNa3F6 can be used. The Al compound may be composed of AlCl3 alone, and a part of AlCl3 may be substituted with AlF3, AlBr3, or AlNa3F6.
The molten salt 3 may contain another halide in addition to the above-described Mn compound and Al compound. As another halide, an alkali metal halide such as NaCl, LiCl, or KCl is preferably selected, and a rare earth halide such as LaCl3, DyCl3, MgCl2, or CaCl2, and an alkaline earth halide may be added to the alkali metal halide.
The above Mn compound, Al compound, and another halide are charged in the alumina crucible 2 and heated and melted by the electric furnace 4, whereby the molten salt 3 can be obtained. The molten salt 3 is preferably stirred sufficiently by the stirrer 8 immediately after the melting so as to make the composition distribution of the molten salt 3 homogeneous.
The electrolysis of the molten salt 3 is performed by making current flow between the cathode 5 and the anode 6 through the constant current power supply device 7. This allows the MnAl alloy to be deposited on the cathode 5. The heating temperature of the molten salt 3 during the electrolysis is preferably 150° C. or more and 600° C. or less depending on the composition of the molten salt 3 and the intended characteristics of MnAl alloy. The electricity amount is preferably 30 mAh or more and 120 mAh or less per electrode area of 1 cm2 depending on the composition of the molten salt 3 and the intended characteristics of MnAl alloy to be obtained. During the electrolysis, the sealed vessel 1 is preferably filled with inert gas such as N2.
Further, the electricity amount of the current made to flow between the cathode 5 and the anode 6 is set to 50 mAh or more per 1 mass % of concentration of the Mn compound in the molten salt 3 and per electrode area 1 cm2, whereby a powdery MnAl alloy can be deposited on the cathode 5. That is, the higher the concentration of the Mn compound in the molten salt 3, the more rapidly the deposition is accelerated, and the more the electricity amount per unit electrode area, the more rapidly the deposition is accelerated, and the MnAl alloy to be deposited easily becomes powdery when the above value range (50 mAh or more) is satisfied. When the MnAl alloy deposited on the cathode is powdery, the deposition of the MnAl alloy is not stopped even when electrolysis is performed for a long time, and hence, the productivity of the MnAl alloy can be enhanced. Further, it is possible to obtain a desired product shape by compression molding the obtained powdery MnAl alloy.
The initial concentration of the Mn compound in the molten salt 3 is preferably 0.02 mass % or more and, more preferably, 0.2 mass % or more and 3 mass % or less. Further, in the present embodiment, the Mn compound is preferably additionally charged during electrolysis so as to maintain the concentration of the Mn compound in the molten salt 3. More specifically, powdery Mn compound or Mn compound in the form of pellets (obtained by molding powder) may additionally be charged in the molten salt 3 continuously or periodically. When the Mn compound is additionally charged during electrolysis of the molten salt 3, reduction in the concentration of the Mn compound associated with the progress of the electrolysis is suppressed, whereby the concentration of the Mn compound in the molten salt 3 can be maintained at a predetermined value or more. This makes it possible to suppress a variation in the composition of the MnAl alloy to be deposited.
The MnAl alloy deposited by electrolysis is then subjected to heat treatment, whereby predetermined magnetic characteristics can be imparted to the MnAl alloy. Specifically, by setting heat treatment temperature to 400° C. or more and less than 600° C. and setting heat treatment time to about 0.5 hours, metamagnetism can be imparted to the MnAl alloy. Further, by setting heat treatment temperature to 600° C. or more and 700° C. or less and setting heat treatment time to about 0.5 hours, residual magnetization of the MnAl alloy can be increased. The heat treatment is preferably performed in an inert gas atmosphere or in a vacuum atmosphere. When heat treatment is performed for a longer period of time under the condition where the heat treatment temperature is 400° C. or more and less than 600° C., sharp metamagnetism can be obtained. When heat treatment time exceeds a predetermined time length under the condition where the heat treatment temperature is 600° C. or more, the MnAl alloy becomes nonmagnetic.
When the heating temperature of the molten salt 3 during electrolysis is 400° C. or more and 700° C. or less, heat treatment is effectively applied during electrolysis, so that it is possible to impart various magnetic characteristics to the MnAl alloy according to the heating temperature during electrolysis without applying heat treatment to an electrodeposit. Specifically, when the heating temperature of the molten salt 3 during electrolysis is set to 400° C. or more and less than 600° C., metamagnetism can be imparted to the MnAl alloy and, when the heating temperature of the molten salt 3 during electrolysis is set to 600° C. or more and 700° C. or less, residual magnetization of the MnAl alloy can be increased when compared with that before heat treatment.
The metamagnetism refers to a property in which magnetism undergoes first-order phase transition from paramagnetism (PM) or antiferromagnetism (AFM) to ferromagnetism (FM) by a magnetic field. The first-order phase transition by a magnetic field refers to the occurrence of discontinuity in a change in magnetization under a magnetic field. The metamagnetic material is classified into a PM-FM transition type metamagnetic material which undergoes transition from paramagnetism to ferromagnetism by a magnetic field and an AFM-FM transition type metamagnetic material which undergoes transition from antiferromagnetism to ferromagnetism by a magnetic field. In the PM-FM transition type metamagnetic material, the first-order phase transition occurs only in the vicinity of the Curie temperature; on the other hand, in the AFM-FM transition type metamagnetic material, the first-order phase transition occurs at a temperature equal to or less than the Neel temperature where antiferromagnetism disappears. The MnAl alloy according to the present embodiment is the AFM-FM transition type metamagnetic material, so that it exhibits metamagnetism over a wide temperature range.
As described above, in the MnAl alloy manufacturing method according to the present embodiment, by additionally charging the Mn compound during electrolysis, the concentration of the Mn compound in the molten salt can be maintained, thus making it possible to suppress a variation in the composition of the MnAl alloy to be deposited. Further, by applying heat treatment to the deposited MnAl alloy, predetermined magnetic characteristics can be imparted to the MnAl alloy. Further, by adjusting the concentration of the Mn compound in the molten salt 3 and the electricity amount per unit electrode area, the MnAl alloy to be deposited on the cathode 5 can be made into powder.
It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.
First, an electrodeposition apparatus having the structure illustrated in
Then, 50 mol % anhydrous AlCl3 which is an Al compound, 50 mol % NaCl which is another halide, and 0.1 mass % MnCl2 dehydrated in advance as the Mn compound are weighed and charged into the alumina crucible 2 such that the total weight thereof was 1200 g. Thus, the weight of MnCl2 was 1.2 g. The dehydration was performed by heating MnCl2 hydrate at about 400° C. for four hours or longer in an inert gas atmosphere such as N2.
The alumina crucible 2 into which the materials had been charged was moved inside the sealed vessel 1, and the materials were heated to 350° C. by the electric furnace 4, whereby the molten salt 3 was obtained. Then, rotary vanes of the stirrer 8 were sunk into the molten salt 3, and stirring was performed at a rotation speed of 400 rpm for 0.5 hours. Thereafter, a constant current of 60 mA/cm2 (2.4 A) per unit electrode area was conducted between the cathode 5 and the anode 6 for 0.5 hours, and the current conduction and heating were stopped. Then, the electrode was removed before cooling and solidification of the molten salt 3, and the cathode 5 was subjected to ultrasonic washing using acetone. A film-like electrodeposit was deposited on the surface of the cathode 5. The film-like electrodeposit was collected by dissolving and removing Cu constituting the cathode 5 by concentrated nitric acid. The collected electrodeposit was pulverized with a mortal to obtain a powdery sample of Comparative Example 1.
Electrolysis conditions, form of electrodeposit, concentration unevenness, and magnetic characteristics of Comparative Example 1 are shown in
Samples of Comparative Examples 2 to 5 were produced in the same manner as Comparative Example 1 except that the concentration of MnCl2 as the Mn compound was changed. Further, samples of Comparative Examples 6 to 15 were produced in the same manner as Comparative Examples 1 to 5 except that the conduction time was set to 1 hour or four hours. As shown in
As described above, in the Comparative Examples 1 to 15, a film-like ferromagnetic electrodeposit is obtained. Although the residual magnetization of the electrodeposit tends to be larger as the initial concentration of the Mn compound is higher, the obtained residual magnetization is comparatively small. This is probably because Mn in the molten salt is consumed as the electrolysis progresses to cause a reduction in Mn ratio in the generated electrodeposit. As a result, a ferromagnetic τ-phase with a low Mn ratio and a non-magnetic γ2-phase or γbrass-phase with a low Mn ratio are generated, which may cause the reduction in the residual magnetization. On the other hand, when the initial concentration of the Mn compound is as excessively high as 3 mass %, the residual magnetization is slightly reduced in Comparative Examples 5 and 10 in which the conduction time is short. This is probably because when the initial concentration of the Mn compound is too high, the Mn compound is saturated with respect to the molten salt to be dispersed in the molten salt as a solid, which may lower the current density to, e.g., less than 30 mA/cm2 to cause inhibition of electrochemical reaction.
Samples of Examples 1 to 5 were produced in the same manner as Comparative Examples 6 to 10, respectively, except that MnCl2 as the Mn compound was added during electrolysis.
The addition of MnCl2 was performed as follows. First, MnCl2 hydrate was dehydrated in advance at about 400° C. for four hours or longer in an inert gas atmosphere such as N2, and obtained anhydrous MnCl2 was pulverized with a mortal in an inert gas atmosphere. The obtained powder was molded into columnar pellets each having a diameter of 5 mm to produce pellets of anhydrous MnCl2. The thus obtained pellets were added to the molten salt 3 during electrolysis. The addition was carried out every 10 minutes, and the charging amount per one time was 0.38 g in Examples 1 to 5.
As shown in
The powdery electrodeposit was in part left on the cathode 5, but the rest is deposited on the bottom portion of the alumina crucible 2. Therefore, the powdery electrodeposit sunk into the molten salt 3 was filtered and collected. At the same time, the molten salt was subjected decantation, and the mixture of the powdery electrodeposit left on the bottom portion and the molten salt was cooled and solidified, followed by washing using acetone and filtering/collection. The powdery electrodeposits obtained by both the above collection methods were mixed with a powdery sample obtained by pulverizing the film-like electrodeposit to obtain the samples to be evaluated.
As described above, in Examples 1 to 3, the file-like ferromagnetic electrodeposit is obtained, while in Examples 4 and 5, both the film-like ferromagnetic electrodeposit and powdery electrodeposit are obtained. In Examples 1 to 5, the electricity amounts per 1 mass % of concentration of the Mn compound in the molten salt and per electrode area 1 cm2, are 6 mAh, 12 mAh, 30 mAh, 60 mAh, and 180 mAh, respectively.
The residual magnetization of the electrodeposit increases under the condition where the initial concentration of the Mn compound is in the range of 0.1 mass % to 1 mass % and slightly decreases under the condition where it is 3 mass %. That is, when the initial concentration of the Mn compound is as excessively low as 0.1 mass %, the Mn ratio in the generated electrodeposit is low even though an operation for maintaining the initial concentration is performed, and a ferromagnetic τ-phase with a low Mn ratio and a non-magnetic γ2-phase or γbrass-phase with a low Mn ratio are easily generated, with the result that the film-like electrodeposit is easily formed. When the initial concentration of the Mn compound is as adequate as 0.2 mass % or more, the τ-phase is generated, and the operation for maintaining the initial concentration of the Mn compound is performed, so that the τ-phase that has not been formed in a film shape is obtained as the powdery electrodeposit. On the other hand, when the initial concentration of the Mn compound is as high as 3 mass %, the additional charge amount of the Mn compound exceeds Mn amount to be consumed by electrolysis to cause the Mn compound to be saturated with respect to the molten salt, so that the Mn compound is dispersed as solid matter in the molten salt, which may cause inhibition of electrochemical reaction.
Samples of Examples 6 to 10 were produced in the same manner as Comparative Examples 11 to 15, respectively, except that MnCl2 as the Mn compound was added during electrolysis. The pellets to be used and charging conditions were the same as those in Examples 1 to 5, respectively.
As shown in
As described above, in Examples 6 to 10, both the film-like ferromagnetic electrodeposit and powdery electrodeposit are obtained. The residual magnetization of the obtained ferromagnetic electrodeposit increases under the condition where the initial concentration of the Mn compound is in the range of 0.1 mass % to 1 mass % and slightly decreases under the condition where it is 3 mass %. That is, when the initial concentration of the Mn compound is as excessively low as 0.1 mass %, the Mn ratio in the generated electrodeposit is low, and a ferromagnetic τ-phase with a low Mn ratio and a non-magnetic γ2-phase or γbrass-phase with a low Mn ratio are easily generated. When the initial concentration of the Mn compound is as high as 3 mass %, the additional charge amount of the Mn compound exceeds Mn amount to be consumed by electrolysis to cause the Mn compound to be saturated with respect to the molten salt, so that the Mn compound is dispersed as solid matter in the molten salt, which may cause inhibition of electrochemical reaction.
Samples of Examples 11 to 16 were produced in the same manner as Examples 4 and 9 except that the conduction time was set to 0.5 hours and that the obtained electrodeposit was subjected to heat treatment. The electrodeposits in all the samples has a film shape. In the heat treatment, the powdery sample was heated to 300° C. to 700° C. for one hour in an N2 gas flow, and this state was maintained for 0.5 hours.
As illustrated in
Samples of Examples 17 to 19 were produced in the same manner as Examples 4 and 9 except that the conduction times were set to 0.5 hours, two hours, and three hours, respectively. As shown in
Thus, when the conduction time is short, the electrodeposit has a film shape, while when the conduction time is long, the electrodeposit becomes powdery. That is, in a case where electrolysis is performed with an electricity amount of 60 mAh/cm2 per 1 mass % of concentration of the Mn compound, the thickness of the film-like electrodeposit increases to about 10 μm to 20 μm as the conduction time becomes longer, so that flatness is lost, when compared with the electrode surface in the initial state, causing dendrite growth to start at the projecting part of the ruggedness, allowing generation of powdery electrodeposit. Further, also in all the samples of Examples 17 to 19, the evaluation on the concertation unevenness is “O”.
The sample of Example 17 was produced under the same conditions as for Examples 11 to 16 except that heat treatment was omitted. As shown in
Samples of Comparative Examples 16 to 21 were produced in the same manner as Examples 11 to 16, respectively, except that addition of MnCl2 was not performed during electrolysis.
As shown in
Samples of Examples 20 to 25 were produced in the same manner as Example 9 except that obtained electrodeposits were subjected to heat treatment for 0.5 hours. The electrodeposits obtained in all the samples of Examples 20 to 25 have film and powdery shapes.
As illustrated in
Samples of Examples 26 to 30 were produced in the same manner as Examples 17, 4, 18, 19, and 9, respectively, except that the electricity amount per unit electrode area was set to 30 mAh/cm2.
As shown in
Samples of Examples 31 to 35 were produced in the same manner as Examples 26 to 30, respectively, except that the electricity amount per unit electrode area was set to 120 mAh/cm2 and that the conduction times were set to 0.2 hours, 0.4 hours, 0.5 hours, one hour, and two hours, respectively.
As shown in
Samples of Examples 36 to 48 were produced in the same manner as Example 9 except that the type and ratio of the Al compound and the type and ratio of the halide were changed. The type and ratio of the Al compound and those of the halide are shown in
As shown in
Samples of Examples 49 to 56 were produced in the same manner as Example 9 except that the temperatures of the molten salt during electrolysis were set to 150° C., 200° C., 300° C., 400° C., 450° C., 500° C., 550° C., and 600° C., respectively. In Examples 55 and 56, the ratio between AlCl3 and NaCl was set to 49:51 considering the temperature of the molten salt.
As shown in
Samples of Examples 57 to 60 were produced in the same manner as Example 9 except that the current densities during electrolysis were set to 15 mA/cm2, 30 mA/cm2, 120 mA/cm2, and 150 mA/cm2.
As shown in
Samples of Examples 61 to 73 were produced in the same manner as Example 23 except that the type and ratio of the Al compound and the type and ratio of the halide were changed. The type and ratio of the Al compound and those of the halide are shown in
As shown in
Samples of Examples 74 to 81 were produced in the same manner as Example 23 except that the temperatures of the molten salt during electrolysis were set to 150° C., 200° C., 300° C., 400° C., 450° C., 500° C., 550° C., and 600° C., respectively. In Examples 80 and 81, the ratio between AlCl3 and NaCl was set to 49:51 considering the temperature of the molten salt.
As shown in
Samples of Examples 82 to 85 were produced in the same manner as Example 23 except that the current densities during electrolysis were set to 15 mA/cm2, 30 mA/cm2, 120 mA/cm2, and 150 mA/cm2.
As shown in
1: Sealed Vessel
2: Alumina crucible
3: Molten salt
4: Electric furnace
5: Cathode
6: Anode
7: Constant current power supply device
8: Stirrer
9: Gas passage
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-000365 | Jan 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/046986 | 12/27/2017 | WO | 00 |
