rus
Issue #6/2021
E.M.Glebova, V.I.Molomin
Production of anisotropic magnetic powders of the neodymium – iron – boron system with high magnetic properties at an enlarged pilot plant
Production of anisotropic magnetic powders of the neodymium – iron – boron system with high magnetic properties at an enlarged pilot plant
DOI: 10.22184/1993-8578.2021.14.6.374.380
The study was carried out on highly coercive anisotropic powders of the Nd-Fe-B alloy system, which were prepared by HDDR process. The HDDR process has been proven to be possible under optimum conditions in a large volume of magnetic powder to be processed. Experimental batch of Nd-Fe-B powder was produced and the magnetic characteristics of the powder were determined.
The study was carried out on highly coercive anisotropic powders of the Nd-Fe-B alloy system, which were prepared by HDDR process. The HDDR process has been proven to be possible under optimum conditions in a large volume of magnetic powder to be processed. Experimental batch of Nd-Fe-B powder was produced and the magnetic characteristics of the powder were determined.
Теги: hddr-process hddr-процесс hydrogen decomposition magnetic powders magnetic properties nd-fe-b permanent magnets водородная обработка магнитные порошки постоянные магниты
PRODUCTION OF ANISOTROPIC MAGNETIC POWDERS OF THE NEODYMIUM – IRON – BORON SYSTEM WITH HIGH MAGNETIC PROPERTIES AT AN ENLARGED PILOT PLANT
The study was carried out on highly coercive anisotropic powders of the Nd-Fe-B alloy system, which were prepared by hydrogen decomposition desorption recombination (HDDR process). In order to reduce the temperature gradient created by the exothermic hydrogenation reaction and the endothermic dehydrogenation reaction in a large volume of powder, a thin layer of powder was loaded on plates which were placed one onto other inside the isothermal zone of the plant. The HDDR process has been proven to be possible under optimum conditions in a large volume of magnetic powder to be processed. Experimental batch of Nd-Fe-B powder was produced. The magnetic characteristics of the powder were determined: coercive force up to 840 kA/m, residual induction up to 1.2 Tesla and magnetic product up to 218 kJ/m3.
INTRODUCTION
Neodymium – iron – boron alloys are unique. They can be used to produce permanent magnets using powder metallurgy and rapid ingot hardening techniques. There are several ways to produce magnetic powders, one of which is the high-temperature hydrogen processing method (hereinafter HDDR-process). The hydrogen treatment method is the only way to produce a coarse-grained powder of Nd–Fe–B alloy which has high magnetic characteristics and anisotropy of magnetic properties. The powder structure consists of large crystallites with a particle size of 50–100 nm. This structure is not an equilibrium one, and if hydrogen processing conditions deviate from the optimum, it can easily change, resulting in reduced magnetic properties of the resulting powder. This is why maintaining the optimum process temperature and time regime is a prerequisite for achieving reproducible high magnetic properties of powder loaded into the plant.
It should be noted that the majority of the published papers examined this process performed under laboratory conditions, and the weight of the material was usually a few grams. For such quantities of material, the hardware design of production was not a problem. However, when the charge is increased to 1.0 kg or more, difficulties arise due to heat transfer problems. Due to the low thermal conductivity of the powder, removal of reaction heat during hydrogenation of the powder and the supply of heat during dehydrogenation become more difficult as the loading volume increases. This creates a temperature gradient in the volume of the powder to be treated and, consequently, leads to deviations from the optimum mode and reduction of the magnetic characteristics. Therefore, the magnetic characteristics of industrially produced powders are worse than the powders produced under laboratory conditions [1].
To reduce the influence of the volume factor on the hydrogen treatment process and to increase the magnetic properties of neodymium-iron-boron powders, various designs of reaction plants in which the thermal contact of the powder with the heat removal and supply surfaces of the plant casing were proposed (rotary furnaces, multiple-hearth furnaces (reactors), screw-type reactors, etc.).
Our specialists applied the following technical solution at the enlarged pilot plant for hydrogen treatment of alloy powder: in order to set up similar conditions for heat supply and removal in the loading volume, the processed powder is sifted as in a thin layer on plates, which are placed one on another in the isothermal zone of the plant. Such arrangement prevents undesirable temperature gradients in the powder layer and makes it possible to increase the single charge without affecting the magnetic properties of the powder.
This study deals with experimental testing of a plant of the chosen configuration for hydrogen treatment of magnetic powders with a single charge of 10 kg or more, as well as studying the process of obtaining magnetic powders with properties not worse than those of the powders obtained under laboratory conditions.
Initial materials and research methodology
The study was conducted on the alloys whose compositions are shown in Table 1.
The initial materials for alloy melting were: metallic neodymium, TU 48-4-205-70 [2], ferroboron FB-17, GOST 14848-69 [3], low-carbon electrotechnical steel 10895E, GOST 11036-75 [4], and technical metal gallium, Gl-1, GOST 12797-77 [5].
The ingots were melted in a UPPF-3M vacuum induction furnace in an inert atmosphere. After ingot melting, the oxygen content was monitored by chromatography. For this system it is 0.01–0.02%, and for nitrogen 0.06–0.007%.
The component composition of these alloys was determined by the emission-spectral method. Control was carried out with the help of the standard sample composition of magnetic alloy OSO-4-49, developed in SE "Spetsmagnet", Moscow. The measurement error of the measurements did not exceed 2%.
The phase composition of these alloys in the initial state was monitored by nuclear gamma resonance method by a Mössbauer spectrometer.
After melting the ingots were subjected to high-temperature hydrogen treatment (HDDR-treatment). The oxygen content in the obtained NdFeB alloy powders was monitored by chromatography.
Analytical VLA-200 scales was used for weighing powders. Then the powders were placed in weighing boats under draught, then the boat was moved into an open crucible furnace and heated to a temperature of 100–200 °C with an exposure time of 100–600 h. The humidity at room temperature (25–30 °C) was 50–70%.
The magnetic characteristics (coercive force, residual induction (or magnetisation) and energy product) were measured by a VSM LDJ 9600 vibromagnetometer with pre-pulse magnetisation.
Metallographic analysis of the powders before and after testing was carried out with a "Leiсa" optical microscope.
Process design hardware
Figure 1 shows a diagram of a stainless steel unit with a volume of 0.05 m3 (2). The unit includes a trap stand with a source of pure hydrogen which is filled with titanium hydride. Pumping is carried out by rotary forevacuum and diffusion pumps 2HVRD-5AM and NVDM-250, respectively.
The powder was loaded onto 10–16 plates (3) which were placed one on top of the other in the isothermal zone of the unit. The reaction stand was heated by a three-section resistance furnace, with a power output of 15 kW in each section. Temperature control was carried out by P133 analog regulators with thyristor amplifiers.
The powder layer thickness on all plates was the same and equaled 15–20 mm. The powder weight on each plate was ~ 1000 g. The total loading weight in this unit can reach up to 30 kg.
Hydrogen treatment of Nd–Fe–B alloy powders on an enlarged plant
When the temperature rises, Nd–Fe–B alloy powders begin to absorb gas impurities trapped into the plant through "leaks" and released from the plant walls, i.e. the powder works as a getter. Therefore, carrying out thorough cleaning of the internal surfaces of the plant during preparation and assembly is one of the main tasks before commencement of the hydrogen treatment process. The preparation stage quality can be checked by the contents of oxygen, nitrogen and carbon in the resulting powder.
The modes of hydrogen treatment were selected on the basis of the conclusions and recommendations contained in [6, 7].
Since the thermal response of the unit in the enlarged plant is greater than in the laboratory unit, the heating and cooling time of the large plant is longer. However, this difference does not lead to a deterioration of the magnetic properties of the obtained powder.
RESULTS AND DISCUSSION
Ten pilot hydrogen treatment processes of NdFeB alloys of the melted composition were carried out at the enlarged plant (Table 1).
Samples were taken from the centre and periphery of each plate over the entire loading height and their magnetic characteristics were determined (Table 2).
From the data shown in Table 2 it is clear that the chosen method of charging the powder on plates made it possible to obtain material with practically identical magnetic properties throughout the entire loading volume along the height of the unit and along the radius. The deviations of the values of the magnetic properties from the average value did not exceed the error of their determination with the aid of magnetometer.
Experimental HDDR-treatment processes of alloy powder No. 1 and No. 2 were carried out. The average values of magnetic properties of the obtained powders were determined along the axis of light magnetization (Table 3).
It is clear from table 3 that the hydrogen treatment processes carried out according to the optimal regime in the enlarged pilot plant ensure high reproducibility of the obtained: the magnetic properties of powders of the same composition obtained in different processes differ insignificantly. The analysis of powders from different processes showed that there was no contamination of powders with carbon, nitrogen and oxygen during hydrogenation and vacuum degassing: the nitrogen and carbon content was determined to be 0.03% or less, the oxygen content was up to 0.1–0.2 % wt.
The obtained powders have magnetic properties similar to those of Nd-Fe-B alloy powders reported in the literature for alloys of similar composition.
The last line in Table 3 shows the magnetic properties of the powder after the second hydrogen treatment. Repetition of the process resulted in a significant increase of the magnetic properties of the resulting powder. Such a technique can be useful for the correction of defects.
Thus, based on the analysis of the obtained results, it was shown that the chosen design of the reaction unit can be recommended for the equipment designed to produce several tens of kilograms of magnetic powder per cycle of operation.
CONCLUSIONS
In order to ensure optimum conditions for the process, in the entire volume of alloy powder loaded into the reaction unit, a scheme for charging the powder as a thin layer onto plates that are positioned one on top of the other in the isothermal zone of the reaction vacuum stand has been applied.
The design of the reaction unit tested in this study can be recommended for the equipment designed to produce several tens of kilograms of magnetic powder in a single operation and can be scaled up using the larger volume autoclaves.
The research carried out on the high-temperature hydrogen treatment of neodymium-iron-boron alloys in the enlarged pilot plant enabled to obtain good and reproducible results. ▪
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The study was carried out on highly coercive anisotropic powders of the Nd-Fe-B alloy system, which were prepared by hydrogen decomposition desorption recombination (HDDR process). In order to reduce the temperature gradient created by the exothermic hydrogenation reaction and the endothermic dehydrogenation reaction in a large volume of powder, a thin layer of powder was loaded on plates which were placed one onto other inside the isothermal zone of the plant. The HDDR process has been proven to be possible under optimum conditions in a large volume of magnetic powder to be processed. Experimental batch of Nd-Fe-B powder was produced. The magnetic characteristics of the powder were determined: coercive force up to 840 kA/m, residual induction up to 1.2 Tesla and magnetic product up to 218 kJ/m3.
INTRODUCTION
Neodymium – iron – boron alloys are unique. They can be used to produce permanent magnets using powder metallurgy and rapid ingot hardening techniques. There are several ways to produce magnetic powders, one of which is the high-temperature hydrogen processing method (hereinafter HDDR-process). The hydrogen treatment method is the only way to produce a coarse-grained powder of Nd–Fe–B alloy which has high magnetic characteristics and anisotropy of magnetic properties. The powder structure consists of large crystallites with a particle size of 50–100 nm. This structure is not an equilibrium one, and if hydrogen processing conditions deviate from the optimum, it can easily change, resulting in reduced magnetic properties of the resulting powder. This is why maintaining the optimum process temperature and time regime is a prerequisite for achieving reproducible high magnetic properties of powder loaded into the plant.
It should be noted that the majority of the published papers examined this process performed under laboratory conditions, and the weight of the material was usually a few grams. For such quantities of material, the hardware design of production was not a problem. However, when the charge is increased to 1.0 kg or more, difficulties arise due to heat transfer problems. Due to the low thermal conductivity of the powder, removal of reaction heat during hydrogenation of the powder and the supply of heat during dehydrogenation become more difficult as the loading volume increases. This creates a temperature gradient in the volume of the powder to be treated and, consequently, leads to deviations from the optimum mode and reduction of the magnetic characteristics. Therefore, the magnetic characteristics of industrially produced powders are worse than the powders produced under laboratory conditions [1].
To reduce the influence of the volume factor on the hydrogen treatment process and to increase the magnetic properties of neodymium-iron-boron powders, various designs of reaction plants in which the thermal contact of the powder with the heat removal and supply surfaces of the plant casing were proposed (rotary furnaces, multiple-hearth furnaces (reactors), screw-type reactors, etc.).
Our specialists applied the following technical solution at the enlarged pilot plant for hydrogen treatment of alloy powder: in order to set up similar conditions for heat supply and removal in the loading volume, the processed powder is sifted as in a thin layer on plates, which are placed one on another in the isothermal zone of the plant. Such arrangement prevents undesirable temperature gradients in the powder layer and makes it possible to increase the single charge without affecting the magnetic properties of the powder.
This study deals with experimental testing of a plant of the chosen configuration for hydrogen treatment of magnetic powders with a single charge of 10 kg or more, as well as studying the process of obtaining magnetic powders with properties not worse than those of the powders obtained under laboratory conditions.
Initial materials and research methodology
The study was conducted on the alloys whose compositions are shown in Table 1.
The initial materials for alloy melting were: metallic neodymium, TU 48-4-205-70 [2], ferroboron FB-17, GOST 14848-69 [3], low-carbon electrotechnical steel 10895E, GOST 11036-75 [4], and technical metal gallium, Gl-1, GOST 12797-77 [5].
The ingots were melted in a UPPF-3M vacuum induction furnace in an inert atmosphere. After ingot melting, the oxygen content was monitored by chromatography. For this system it is 0.01–0.02%, and for nitrogen 0.06–0.007%.
The component composition of these alloys was determined by the emission-spectral method. Control was carried out with the help of the standard sample composition of magnetic alloy OSO-4-49, developed in SE "Spetsmagnet", Moscow. The measurement error of the measurements did not exceed 2%.
The phase composition of these alloys in the initial state was monitored by nuclear gamma resonance method by a Mössbauer spectrometer.
After melting the ingots were subjected to high-temperature hydrogen treatment (HDDR-treatment). The oxygen content in the obtained NdFeB alloy powders was monitored by chromatography.
Analytical VLA-200 scales was used for weighing powders. Then the powders were placed in weighing boats under draught, then the boat was moved into an open crucible furnace and heated to a temperature of 100–200 °C with an exposure time of 100–600 h. The humidity at room temperature (25–30 °C) was 50–70%.
The magnetic characteristics (coercive force, residual induction (or magnetisation) and energy product) were measured by a VSM LDJ 9600 vibromagnetometer with pre-pulse magnetisation.
Metallographic analysis of the powders before and after testing was carried out with a "Leiсa" optical microscope.
Process design hardware
Figure 1 shows a diagram of a stainless steel unit with a volume of 0.05 m3 (2). The unit includes a trap stand with a source of pure hydrogen which is filled with titanium hydride. Pumping is carried out by rotary forevacuum and diffusion pumps 2HVRD-5AM and NVDM-250, respectively.
The powder was loaded onto 10–16 plates (3) which were placed one on top of the other in the isothermal zone of the unit. The reaction stand was heated by a three-section resistance furnace, with a power output of 15 kW in each section. Temperature control was carried out by P133 analog regulators with thyristor amplifiers.
The powder layer thickness on all plates was the same and equaled 15–20 mm. The powder weight on each plate was ~ 1000 g. The total loading weight in this unit can reach up to 30 kg.
Hydrogen treatment of Nd–Fe–B alloy powders on an enlarged plant
When the temperature rises, Nd–Fe–B alloy powders begin to absorb gas impurities trapped into the plant through "leaks" and released from the plant walls, i.e. the powder works as a getter. Therefore, carrying out thorough cleaning of the internal surfaces of the plant during preparation and assembly is one of the main tasks before commencement of the hydrogen treatment process. The preparation stage quality can be checked by the contents of oxygen, nitrogen and carbon in the resulting powder.
The modes of hydrogen treatment were selected on the basis of the conclusions and recommendations contained in [6, 7].
Since the thermal response of the unit in the enlarged plant is greater than in the laboratory unit, the heating and cooling time of the large plant is longer. However, this difference does not lead to a deterioration of the magnetic properties of the obtained powder.
RESULTS AND DISCUSSION
Ten pilot hydrogen treatment processes of NdFeB alloys of the melted composition were carried out at the enlarged plant (Table 1).
Samples were taken from the centre and periphery of each plate over the entire loading height and their magnetic characteristics were determined (Table 2).
From the data shown in Table 2 it is clear that the chosen method of charging the powder on plates made it possible to obtain material with practically identical magnetic properties throughout the entire loading volume along the height of the unit and along the radius. The deviations of the values of the magnetic properties from the average value did not exceed the error of their determination with the aid of magnetometer.
Experimental HDDR-treatment processes of alloy powder No. 1 and No. 2 were carried out. The average values of magnetic properties of the obtained powders were determined along the axis of light magnetization (Table 3).
It is clear from table 3 that the hydrogen treatment processes carried out according to the optimal regime in the enlarged pilot plant ensure high reproducibility of the obtained: the magnetic properties of powders of the same composition obtained in different processes differ insignificantly. The analysis of powders from different processes showed that there was no contamination of powders with carbon, nitrogen and oxygen during hydrogenation and vacuum degassing: the nitrogen and carbon content was determined to be 0.03% or less, the oxygen content was up to 0.1–0.2 % wt.
The obtained powders have magnetic properties similar to those of Nd-Fe-B alloy powders reported in the literature for alloys of similar composition.
The last line in Table 3 shows the magnetic properties of the powder after the second hydrogen treatment. Repetition of the process resulted in a significant increase of the magnetic properties of the resulting powder. Such a technique can be useful for the correction of defects.
Thus, based on the analysis of the obtained results, it was shown that the chosen design of the reaction unit can be recommended for the equipment designed to produce several tens of kilograms of magnetic powder per cycle of operation.
CONCLUSIONS
In order to ensure optimum conditions for the process, in the entire volume of alloy powder loaded into the reaction unit, a scheme for charging the powder as a thin layer onto plates that are positioned one on top of the other in the isothermal zone of the reaction vacuum stand has been applied.
The design of the reaction unit tested in this study can be recommended for the equipment designed to produce several tens of kilograms of magnetic powder in a single operation and can be scaled up using the larger volume autoclaves.
The research carried out on the high-temperature hydrogen treatment of neodymium-iron-boron alloys in the enlarged pilot plant enabled to obtain good and reproducible results. ▪
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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