H. Nakamura

Effects of HDDR treatment conditions on magnetic properties of Nd‐Fe‐B anisotropic powders

It is reported that Nd‐Fe‐B magnetic powders prepared by utilizing the HDDR (hydrogenation, disproportionation, desorption, and recombination) phenomena exhibit high coercivity, and the addition of Co, Ga, and Zr induces magnetic anisotropy in these powders. HDDR phenomena are caused by the heat treatment in hydrogen (H treatment) and subsequently in vacuum (V treatment). Present works describe the effects of V‐treatment conditions on magnetic properties of Nd12.6Febal.CoxB6.0 (x=0–17.4) alloys. The powders V‐treated at lower temperature show lower remanence and no noticeable magnetic anisotropy. V treatment at higher temperature enhances remanence and increases the differences of remanence between the powders aligned with and without magnetic field. Higher values are obtained in Co added alloys. This result suggests that a selective grain growth of Nd2Fe14B grains during V treatment plays an important role for the inducement of magnetic anisotropy in HDDR‐treated powders. The temperature for complete rec...

Effect of the disproportionation and recombination stages of the HDDR process on the inducement of anisotropy in Nd–Fe–B magnets

Abstract The magnetic properties of HDDR (hydrogenation disproportionation desorption recombination) treated Nd 12.2 Fe 81.8 B 6.0 alloys were investigated. Two different patterns were used for the disproportionation stage: (i) the alloys were heated to a certain processing temperature between 850–950°C in 0.1 MPa of hydrogen (conventional hydrogen treatment: c - HD treatment), and (ii) the alloys were heated under vacuum and hydrogen was only admitted when the processing temperature had been reached ( ν - HD treatment). The alloys were then held at the processing temperature for 1 or 2 h under hydrogen in order to cause complete disproportionation. Either an argon heat treatment, or a hydrogen heat treatment at various constant pressures below 0.1 MPa, was then used to control the hydrogen pressure during the recombination stage ( s - DR treatment), followed by the usual heat treatment in vacuum (conventional heat treatment in vacuum: c - DR treatment) to cause complete recombination. It was found that the magnetic properties of the ν -HD powders were more sensitive to the s -DR treatment time than those of the c -HD powders, which is thought to be related to differences in microstructure observed in the disproportionated state. The best magnetic properties were obtained for a ν -HD powder s -DR treated at 950°C for 20 min: B r =1.4 T, j H c =385 kAm −1 , B r / J s =0.92. It can be said that the inducement of anisotropy is influenced by the hydrogenation and desorption stages of the HDDR process, and that the combination of ν -HD and s -DR treatment can be an effective method of inducing anisotropy in Nd–Fe–B powders.

High-coercivity Sm2Fe17Nx powders produced by HDDR and nitriding processes

Abstract This paper describes the occurrence of HDDR (Hydrogenation Disproportionation Desorption Recombination or Hydrogen Decomposition Desorption Recombination) process in the processing of Sm2Fe17Nx powders with high coercivity. The hydrogen absorption and desorption behaviors of the Sm2Fe17 compound were studied. The Sm2Fe17 phase disproportionates into α-Fe and samarium hydrides after the heat treatment in hydrogen at around 600–1100 °C. The removal of hydrogen by following a heat treatment in vacuum so that desorption and recombination occur, results in the reforming of the Sm2Fe17 phase. This Sm2Fe17 phase has a cellular-like microstructure of about 50–500 nm in diameter. The nitrided Sm2Fe17 powders produced by the HDDR process at 775 °C have high coercivity of 8.7 kOe.

Microstructural evolutions during HDDR phenomena in Sm2Fe17Nx compounds

Abstract Microstructural evolutions during HDDR phenomena in Sm 2 Fe 17 particles were observed by transmission electron microscopy. Sm 2 Fe 17 compounds heat-treated over 510°C in a H 2 atmosphere disproportionate into SmH 2 and α-Fe phases with crystal relationship of α-Fe {110} // SmH 2 {111}. Recombination reactions were initiated by desorbing hydrogen from SmH 2 phases, and α-Fe phases diffuse into SmH 2 particles to form Sm 2 + δ Fe 17 phases by recombination. Prolonged aging in vacuum forms Sm 2 Fe 17 fine grains of 100–300 nm in diameter.

Development of an implantable artificial anal sphincter by the use of the shape memory alloy.

In this study, we developed and assessed an artificial anal sphincter driven by an shape memory alloy actuator (AS-SMA). The performance characteristics of the device were analyzed with a measurement system. Assessment showed that the AS-SMA could generate a pressure of 55 mm Hg at an atmospheric temperature of 36°C, and displacement of the SMA actuator was 7.5 mm when the temperature of the SMA plate was 55°C. To evaluate opening and closing, we studied a piglet colostomy model, in which the AS-SMA was implanted around the colostomy in the extraperitoneal space. Flow control tests using living porcine intestine revealed that the AS-SMA could maintain fecal continence against an intestinal pressure of 75 mm Hg. The high pressure zone corresponding to the location of the device was demonstrated in a manometric examination. For 6 days after surgery, we activated the AS-SMA twice a day and observed the bowel movements. The animal experiment indicated that the AS-SMA is able to control the bowel movements of patients with fecal incontinence if several problems, such as burning of tissue around the device and compression injury of the intestine, are resolved.

Low frequency (1/f) noise in YBa2Cu3O7−δ grain boundary junction dc superconducting quantum interference devices

We have studied low frequency (1/f) noise of YBa2Cu3O7−δ dc superconducting quantum interference devices (SQUIDs) on SrTiO3 bicrystal substrates. 1/f flux noise, either measured at different temperatures for optimized bias current or measured at 77 K for different bias currents, is almost constant. These facts imply that 1/f noise mainly comes from fluctuations of the critical current of the Josephson junction that form the SQUID. Also, we explain the critical current fluctuations in the junction by an equilibrium temperature fluctuation model.

Microstructural Changes with N Content in Sm-Fe-N Alloys

The microstructures of Sm<inf>2</inf>Fe<inf>17</inf>N<inf>x</inf> (0≪x≪6) powders were observed by transmission electron microscopy (TEM). Sm<inf>2</inf>Fe<inf>17</inf>N<inf>23</inf> powders nitrided in N<inf>2</inf> gas have high dislocation densities due to lattice expansion. Sm<inf>2</inf>Fe<inf>17</inf>N<inf>x</inf> powders nitrided in a mixture of NH<inf>3</inf> and H<inf>2</inf> gases (3≪x≪6) have cell-like structures. The dislocation density increases and the cell diameter decreases with increasing N content, and Sm<inf>2</inf>Fe<inf>17</inf>N<inf>5.9</inf> powders consist of 30 to 50 nm in diameter Sm<inf>2</inf>Fe<inf>17</inf>N<inf>x</inf> microcrystalline grains surrounded by an amorphous phase. Sm<inf>2</inf>Fe<inf>17</inf> powders nitrided above 600°C dissociate into SmN and ¿-Fe phases with characteristic sizes of 10 to 20 nm.

The effect of additional elements on the magnetic properties of hot-rolled NdFeB alloys

Abstract The magnetic properties of hot-rolled Nd16Febal.B6M1.5 (M = Cu, Ga and Al) and Nd16Fe76B5.5Ga1.5Al1 alloys were investigated, in order to study the role of additive elements in improving the magnetic properties in the NdFeB system. It is found that the original grain size of Cu, Ga or GaAl added alloys is much finer than that of the ternary and Al added alloys. But the grain size is almost identical for all the alloys after hot-rolling at 1000°C with 90% reduction in thickness. The coercivity of hot-rolled alloys with Cu, Ga or GaAl addition is not improved as was expected, because Nd-rich liquid phase in these alloys is very easily squeezed out during high-reduction-ratio rolling. Less quantity and nonuniform distribution of Nd-rich phase between distributed grains are believed to be the main reasons to depress the effect on the grain boundary smoothing. This effect is not the same as those observed in the PrFeB system. The highest magnetic properties achieved in this study are Br = 10 kG, iHc = 8.2 kOe, (BH)max = 18.5 MGOe for the Nd16Fe76.5B6Al 1.5 alloy.

Phase changes and magnetic properties of Sm2Fe17Nx alloys heat-treated in hydrogen

This paper describes the use of hydrogen in the processing of Sm 2 Fe 17 N x powders with good coercivity. The hydrogen absorption and desorption behavior of the compound Sm 2 Fe 17 was studied. Green compacts of Sm 2 Fe 17 powders were heat-treated in hydrogen at 600 to 1000°C, and the latter decomposed into ?-Fe and samarium hydrides. Samples underwent dehydrogenation by subsequent heat treatment in vacuum, resulting in the reformation of the Sm 2 Fe 17 phase.

Magnetic Properties of Sm 2 Fe 17 (C,N) x Melt-Spun Ribbons

The effects of nitrogenation on the magnetic properties of Sm<inf>2</inf>(Fe<inf>0.9</inf>M<inf>0.1</inf>)<inf>17</inf>C<inf>1.6</inf> (M=Ga,Cr) melt-spun ribbons were studied. The melt-spun amorphous ribbons are crystallized at temperatures above 550°C, forming a phase with the TbCu<inf>7</inf> type structure, and on heating to 680°C and above underwent a further transition to a phase with a Th<inf>2</inf>Zn<inf>17</inf> type structure. Nitrogenation at 550°C for four hours enhances the magnetic properties of annealed ribbons. The Sm<inf>2</inf>(Fe<inf>0.9</inf>Cr<inf>0.1</inf>)<inf>17</inf>C<inf>1.6</inf> ribbon annealed at 650°C, which consists of TbCu<inf>7</inf> structure, exhibits a high post-nitrogenation (BH)<inf>max</inf> of 11.0 MGOe. Sm<inf>2</inf>(Fe<inf>0.9</inf>Cr<inf>0.1</inf>)<inf>17</inf>C<inf>1.6</inf> ribbon annealed at 725°C, which consists of the Th<inf>2</inf>Zn<inf>17</inf> structure, shows a high coercivity of 19.0 kOe with a (BH)<inf>max</inf> of 7.4 MGOe after nitrogenation.