Toshio Kagotani

M-type ferrite composite as a microwave absorber with wide bandwidth in the GHz range

The electromagnetic wave absorption properties of Ba M-type (BaFe/sub 12-x/(Ti/sub 0.5/Mn/sub 0.5/)/sub x/O/sub 19/) ferrite-epoxy resin composites were investigated. After measuring the complex permittivities of the samples, the regions of complex permeabilities in which the reflection loss (R.L.) becomes less than -20 dB (-20 dB Regions), were calculated. Altering the conditions such as sintering temperature, particle size and the ratio of ferrite powder in the composite samples, resulted in frequency dependencies of permeabilities that agree well with the calculated -20 dB regions, and which lead to an expansion in bandwidth (frequency range (/spl Delta/f) of R.L. <-20 dB). The BaFe/sub 9/(Ti/sub 0.5/Mn/sub 0.5/)/sub 3/O/sub 19/ ferrite-resin composite, which was produced by sintering at 1573 K for 20 h, crushing into powder (150-300 /spl mu/m) and mixing with epoxy resin at a ratio of 70 mass%, exhibited a wide bandwidth (/spl Delta/f=7.2 GHz) from 13.75 GHz to 20.95 GHz. /spl Delta/f values of 1.4-5.4 GHz in the frequency range 6.35-15.65 GHz, were obtained by changing the composition of the BaFe/sub 12-x/(Ti/sub 0.5/Mn/sub 0.5/)/sub x/O/sub 19//epoxy resin composite from x=3.5 to 4.5.

GHz microwave absorption of a fine α-Fe structure produced by the disproportionation of Sm2Fe17 in hydrogen

Abstract In this study, the possibility of using the disproportionation reaction of the Sm 2 Fe 17 compound for the production of powders with a fine α-Fe structure was investigated, for use as electromagnetic wave absorbers that can operate in the GHz frequency range. A fine α-Fe/SmH 2 or α-Fe/SmO structure with a sub-micrometer size is formed from the Sm 2 Fe 17 compound after disproportionation in hydrogen or air, respectively. In this way, magnetic powders with a fine structure of α-Fe were obtained. The powders disproportionated in hydrogen, were heated in air in order to oxidize the Sm hydride, to give a α-Fe/SmO two-phase microstructure, and thereby increase the resistivity of the powders. Toroidally shaped epoxy-resin composites were made from this powder, and the microwave absorption properties of these samples were measured. An undisproportionated sample did not show any electromagnetic wave absorption in this frequency range. However, the disproportionated samples (heated in hydrogen at 873 K for 1 h, milled for 30 min and oxidized at 473 K for 2 h in air) exhibited electromagnetic wave absorption (RL f m ) range 0.73–1.30 GHz, for absorber thicknesses ( d m ) ranging from 13.1 to 7.9 mm, respectively. The disproportionated samples in air (heated at 473 K for 2 h and milled for 30 min) also showed good electromagnetic wave absorption properties ( f m =1.21–2.50 GHz and d m =8.0–4.2 mm). It is concluded that the disproportionation reaction, in either hydrogen or in air, is a useful method for the production of powders with a fine α-Fe structure, which can be used for microwave absorbers that operate in the several GHz range.

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.

An improved HDDR treatment for the production of anisotropic Nd-Fe-B ternary powders

Abstract The magnetic properties of Nd12.2Fe81.8B6.0 alloys processed using a new HDDR (hydrogenation disproportionation desorption recombination) treatment were investigated. This newly proposed HDDR treatment is a combination of heat treatments at hydrogen pressures close to the recombination pressure of the Nd2Fe14B compound, in both the disproportionation and recombination stages. In other words, this new treatment is a combination of the l-HD (heating in a low H2 pressure during the hydrogenation disproportionation stage), and s-DR (heating in Ar or in a relatively high pressure of hydrogen, at the start of recombination) treatments. In this investigation, the influence of the s-DR conditions on the magnetic properties of anisotropic Nd–Fe–B HDDR-treated powders, were investigated. It was found that an s-DR treatment in which the hydrogen pressure was decreased in steps from 0.1 MPa to 0.5 kPa, enhanced the remanence (1.45 T) and anisotropy (Br/Js=0.94). The rate at which the hydrogen pressure decreases to 6 kPa during s-DR is also considered to be an important factor in obtaining both a high remanence and a high coercivity. In addition, the coercivity was found to increase after an s-DR treatment in which the temperature was decreased in a step by step manner.

Giant magnetoresistance of Cu3Al-Cu2MnAl melt-spun ribbons

Abstract The giant magnetoresistance of Cu–Mn–Al melt-spun ribbons with compositions along the Cu3Al–Cu2MnAl tie line, was investigated. X-ray diffraction and TEM observations revealed that the as-spun ribbons have either an L21 or a D03 type structure. The Cu–15Mn–25Al ribbon exhibited the highest MR ratio of 1.1% (measured at room temperature) among the as-spun ribbons. Two phase decomposition into the Cu3Al and Cu2MnAl phases, occurs after heat treatment within the miscibility gap, and the coherent two phase microstructure grows with increasing heat treatment temperature. This results in an increase in the magnetization and the MR ratio of the ribbons. The Cu–10Mn–25Al ribbon showed the highest MR ratio of 3.9% (measured at room temperature), after a heat treatment at 423 K for 6 h. From the phase diagram, the volume fraction of Cu2MnAl phase in this ribbon is 35.7%, which corresponds to the optimum volume fraction of ferromagnetic phase reported in other granular type GMR materials. The high MR ratio region is narrow and closely related to the two phase decomposition area of Cu–Mn–Al.

Improvement of coercivity of anisotropic Nd–Fe–B HDDR powders by Ga addition

The effect of Ga addition on the increase of coercivity of HDDR-treated Nd-Fe-B powders was investigated. The Ga addition suppresses grain growth and may decrease the region where magnetocrystalline anisotropy is reduced. In addition, a heat treatment in which the hydrogen pressure was decreased in steps during the recombination reaction resulted in good magnetic properties of B r = 1.44T, j H c = 0.97 MA m -1 and (BH) max = 308 kJ m -3 .

Magnetic properties of SrO/spl middot/nFe/sub 2/O/sub 3/ (0.5<n<6.0) and phase diagram in SrO-Fe/sub 2/O/sub 3/ system

Ferrite compound of the chemical composition SrO/spl middot/nFe/sub 2/O/sub 3/ (2.0<n<6) showed high magnetic properties comparable to that of SrO/spl middot/6Fe/sub 2/O/sub 3/ (M-ferrite). The saturation magnetisation Ms is 50/spl sim/75 emu/g in the range of n=6.0/spl sim/2.0. Coercive force iHc is 2.4 kOe near n=6.0/spl sim/6.5 and n=1.5. Furthermore, SrO/spl middot/nFe/sub 2/O/sub 3/ indicated hexagonal (M-type) orthorhombic and tetragonal phases in the range of n=6.5/spl sim/2.0, 2.0/spl sim/0.5 and less than 0.5, respectively. The fact means that a new magnetic phase is in existence near n=1.0/spl sim/2.0. Also, it is never recognized that the compound of chemical formula 3Sr0/spl middot/2Fe/sub 2/O/sub 3/, which is described in the current phase diagram and have been prevailed for more than two decades, is in existence.

Magnetic properties and microstructures of the (SmFe10V2) 1 − x-(Sm2Fe17)x cast alloys

Abstract The present study shows the magnetic properties of (SmFe 10 V 2 ) 1 − x -(Sm 2 Fe 17 ) x . cast alloys ( x = 0–0.3), The alloys heat-treated below 700 °C show low coercivity but the alloys heat-treated at 800–1000 °C exhibit a relatively high coercivity of around 3 kOe. In particular, the stoichiometric SmFe 10 V 2 alloy heat-treated at 900 °C for 20 h exhibits the highest coercivity of 3.7 kOe, which is the highest value among the reported values of ThMn 12 -type magnets in the cast state. This alloy consists of the SmFe 10 V 2 phase, with grain sizes of several micrometers, and a small amount of the α-(Fe,V) phase. In the alloys heat-treated above 1100 °C, the SmFe 10 V 2 phase disappears and the α-(Fe,V) phase exists as the major phase. It is considered that the annealing temperature at 1100 °C is higher than the peritectic temperature. The existence of the α- (Fe,V) phase as the major phase is the main reason for the decrease of coercivity.

Effect of crystal orientation relationship on anisotropic inducement in the HDDR treated Nd-Fe-B alloys

Summary form only given. Nd-Fe-B powders produced using HDDR process, are useful for the production of high performance anisotropic bonded magnets. Recently, the authors clarified the temperature dependence of the recombination pressure of the Nd/sub 2/Fe/sub 14/B compound, and established the necessary HDDR treatment conditions for the production of high performance anisotropic ternary alloy powders. The newly proposed HDDR treatment was a combination of heat treatments, at hydrogen pressures close to the recombination pressure of the Nd/sub 2/Fe/sub 14/B compound, in both the disproportionation and recombination stages. Namely, the combination of v-HD or l-HD (heating in vacuum or in a low H/sub 2/ pressure, respectively, during the Hydrogenation Disproportionation stage), and s-DR (heating in Ar or in hydrogen with hydrogen pressure close to the recombination pressure for 10 min, at the start of the Recombination) treatments. In this investigation, using anisotropic sintered bodies, the crystal orientation relationship between the parent Nd/sub 2/Fe/sub 14/B phase and the disproportionated mixture during the new HDDR treatment was investigated by using XRD and TEM, and its effect on the anisotropic inducement was discussed.