Chisato Mishima

Development of a Co-free NdFeB anisotropic bonded magnet produced from the d-HDDR processed powder

The NdFeB anisotropic bonded magnet produced by the HDDR method, which was developed in 1990, has not found general popularity due to its high cost and low serviceable temperature of 353 K. The authors have developed a new production method that overcomes these barriers to decrease the cost and increase the serviceable temperature. The use of the dynamic HDDR (d-HDDR) process eliminates the need for Co, thus greatly reducing the production cost of NdFeB anisotropic bonded magnets with a (BH) max of 159 kJ/m/sup 3/. The anisotropy of the NdFeB magnetic powder is induced by the control of the reaction rate of the NdFeB alloy and hydrogen, by controlling the hydrogen pressure in the HDDR process. The serviceable temperature is increased by the addition of Dy after the application of the d-HDDR method. An increase in intrinsic coercivity, iHc, from 1.12 MA/m to 1.56 MA/m, corresponds to a proportional increase in the serviceable temperature to 413 K.

Memory of texture during HDDR processing of NdFeB

In this paper, we describes the mechanism of texture generation in the recombined powder. Here Nd/sub 12.5/Fe/sub bal/B/sub 6.3/ and Nd/sub 12.5/Fe/sub bal/Ga/sub 0.3/Nb/sub 0.3/B/sub 6.3/ alloys have been HDDR processed and hydrogen partial pressures and dwell times during disproportionation and recombination have been varied systematically. The effect of these processing parameters on the microstructural and magnetic properties is described. Intermediate processing stages were characterized by XRD employing Rietveld analysis, field emission gun scanning electron microscopy, transmission electron microscopy using EDX and selected area diffraction. The degree of texture in aligned recombined powders has been characterized by magnetic measurements using a VSM and qualitatively by Kerr microscopy. Crystallographic relationships between parent, disproportionated and recombined phases are described.

Development of Nd-Fe-B anisotropic bonded magnet with 27 MGOe

The maximum energy product of bonded magnets is advancing every year. The highest maximum energy product of any bonded magnet achieved is 25 MGOe (200 kJ/m/sup 3/) in the Nd-Fe-B system by d-HDDR treatment. There is a great demand for even higher energy bonded magnets with the desire for smaller, more efficient electric motors. Bonded magnets made from d-HDDR treated anisotropic magnet powder have low squareness due to low squareness of the powder. The authors developed a method to increase squareness of d-HDDR powder, and succeeded in developing the worlds highest energy bonded magnet with 26.6 MGOe (213 kJ/m/sup 3/). This was achieved through Dy-diffusion treatment followed by d-HDDR treatment, as well as an increase in the density of the bonded magnet. This magnet has little aging loss after being held at 393 K for 878 h.

Texture inducement during HDDR processing of NdFeB

The microstructural changes during the hydrogenation disproportionation desorption recombination process have been characterized step by step. The magnetic properties and the degree of texture have been determined when varying the hydrogen pressure and a strong correlation was found: low hydrogen pressure during disproportionation yields an excellent degree of texture. Detailed electron microscopy revealed complex microstructures and, most importantly, we found crystallographically correlated Fe/sub 2/B grains within the disproportionated material and we propose the latter phase as the anisotropy-mediating phase throughout the-different stages. This mechanism is highly stable provided the appropriate hydrogen pressure is employed.

Development of anisotropic bonded magnet applied to 150/spl deg/C use

An anisotropic bonded magnet (MAGFINE25), is about to be applied to dc motors for automobile use such as power seat motors and window motors. These dc motors have achieved a 50% reduction in weight and in volume, with the heat resistance of 120/spl deg/C. However, the heat resistance of 150/spl deg/C is required for motors near the engine, such as fan motors and wiper motors. We improved the heat resistance of the bonded magnet by the addition of Dy element to magnet powder, and succeeded in developing a bonded magnet with the heat resistance of 150/spl deg/C.

Development of dy-free NdFeB anisotropic bonded magnet (New MAGFINE)

The authors have successfully developed a low-cost anisotropic NdFeB magnet powder with high coercivity value of 18kOe without any Dy additives by diffusion processing of Nd-Cu-Al element to anisotropic magnet powder produced by the d-HDDR method. This enhancement mechanism is to isolate the magnetic couple between grain of Nd2Fe14B phase by creation of non-magnetic grain boundaries of Nd and Cu. By surface coating on this anisotropic magnet powder, the long-term aging loss behavior under air at 150deg.C has been improved by 6% after 1,000hr compared to current MAGFINE material without powder surface coating. By using this new anisotropic bonded magnet (New-MAGFINE), small automotive motors with excellent thermal stability and low cost characteristics have become possible.

Development of Compound for Anisotropic Bonded Nd Magnets Using d-HDDR Magnet Powder

Bonded anisotropic Nd magnets with high-energy product (BH)max of 20MGOe and freedom of shape design are effective in realizing motor size and weight reductions. The anisotropic powders were produced by d-HDDR treatment, grain boundary diffusion treatment with NdCuAl, and powder surface coating treatment. However, during the compression molding process, the molding pressure can induce the magnet powder grains to crack, which leads to degradation in magnetic performance, in particular the squareness ratio. A compound made of d-HDDR magnet powder coated with resin, fine SmFeN powder, and lubricant can produce molded blocks with a high density of 6.0 g/cm3 at a low molding pressure of 1 t/cm2. In addition, by enabling molding at a low pressure, the loss of squareness of the magnet powder was reduced to 0.5%. From the above, through kneading and the addition of fine powder and lubricant, the surface treatment of d-HDDR magnet powder achieved high-density magnets at a low molding pressure and a compound capable of controlling the reduction in loss of squareness.

Development of anisotropic bonded magnet with heat resistance

The Nd-Fe-B anisotropic magnet powder was prepared through d-HDDR (hydrogenation, disproportionation, desorption, recombination) treatment. The powder mixed with fine Dy-Fe alloy hydride is heat treated in vacuum (Dy diffusion treatment). The magnet powder properties are measured by vibrating sample magnetometer (VSM) after alignment. Particle size is measured by laser diffraction method. The magnet properties of anisotropic bonded magnets are measured using BH tracer. Flux of the radially aligned ring-shape bonded magnet is measured by flux meter.

Anisotropy Mechanism in HDDR Processed NdFeB

The Hydrogenation Disproportionation Desorption Recombination (HDDR) process can yield anisotropic and highly coercive NdFeB-type powders with energy densities in excess of (BH)max=340 kJ/m3. The elucidation of the very unusual phenomenon of texture inducement is of great scientific and technological interest. Here, Nd12.5Feba1B6.3 and Nd12.5Feba1Ga0.3Nb0.3B6.3 alloys have been HDDR processed and hydrogen partial pressures and dwell times during disproportionation and recombination have been varied. The effect of these processing parameters on the microstructural and magnetic properties is detailed. Intermediate processing stages were characterized by XRD (Rietveld analysis), field emission gun scanning electron microscope (FEGSEM), transmission electron microscopy (TEM) using EDX, selected area diffraction and convergent beam electron diffraction modes. The degree of texture in aligned recombined powders has been characterized by magnetic measurements using a VSM and qualitatively by Kerr microscopy. The paper describes the mechanism of texture generation in the recombined powder. Crystallographic relationships between parent, disproportionated and recombined phases are described. It is suggested that it is the ironboride phase which acts as the anisotropy-mediating phase throughout the different stages of the process. The nucleation and growth of this phase is controlled by the hydrogen partial pressure during exothermic disproportionation, strongly effecting the final degree of texture. This d-HDDR (dynamic-) process allows to maximize texture without the Co addition. It is proposed that Nb is useful for the stabilization of the boride phase whereas Ga is beneficial during initial recombination.

Crystallographic alignment in the recombination stage in d-HDDR process of Nd-Fe-B-Ga-Nb powders

Nd-Fe-B-Ga-Nb magnetic powder was subjected to the dynamic hydrogen disproportionation desorption recombination treatment. For samples disproportionated at both 30 and 100 kPa of hydrogen pressure, the changes in the microstructure and grain orientation during recombination process were investigated. It was observed that even during the recombination process, the orientation relationship was maintained between α-Fe and NdH2+x grains formed after the disproportionation treatment at 30 kPa of hydrogen pressure, [110]α-Fe // [110]NdH2+x, (-110)α-Fe // (-220)NdH2+x. Additionally, the alignment of recombined Nd2Fe14BHy grains became clear after 30 min of DR treatment showing following orientation relationship: (001)Nd2Fe14BHy // (110)α-Fe and (110)NdH2+x. In contrast, such a relationship was not observed in the sample disproportionated at 100 kPa of hydrogen pressure. This difference in the degree of alignment was also confirmed by measuring the magnetic property of the respective samples.