David Book

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.

Reversible Interpenetration in a Metal-Organic Framework Triggered by Ligand Removal and Addition

Interpenetration is known for the structures of many minerals and ice; most notably for ice, it exists in doubly interpenetrating (VI, VII, and VIII) and non-interpenetrating (Ih) forms with the latter being porous and having nearly half of the density of the former. In synthetic materials, specifically in metal–organic frameworks (MOFs), interpenetration is generally considered undesirable because it reduces porosity. However, on the contrary, many advantageous properties also arise when MOFs are interpenetrated, such as selective guest capture, stepwise gas adsorption, enhanced framework robustness, photoluminescence control, and guest-responsive porosity. Therefore, various strategies have been suggested to control interpenetration during synthesis. However, once these extended network materials are prepared as interpenetrating or non-interpenetrating structures, the degree of interpenetration generally remains unchanged, because numerous chemical bonds must be broken and subsequently reformed in a very concerted way during the process unlike some interlocked coordination compounds in solution (Figure 1a).

Hydrogen absorption/desorption and HDDR studies on Nd16Fe76B8 and Nd11.8Fe82.3B5.9

Abstract The disproportionation and recombination reactions in which Nd 2 Fe 14 B decomposes into αFe, Nd hydride and Fe 2 B in hydrogen and then reforms under vacuum, have been investigated in Nd 16 Fe 76 B 8 and Nd 11.8 Fe 82.3 B 5.9 (Nd 2 Fe 14 B) alloys by TPA, DTA and Faraday balance measurements. The hydrogen content of a range of NdFeB alloys was found to increase in value with increasing Nd content, and desorption measurements of Nd 16 Fe 76 B 8 hydride showed that hydrogen desorbs from the Nd-rich grain boundary at 250 °C from NdH ≈2.7 to NdH 2 and at 660 °C from Nd dihydride to Nd metal. Optical microscopy of partially disproportionated Nd 16 Fe 76 B 8 showed that the disproportionation of the Nd 2 Fe 14 B phase begins at the Nd-rich grain boundary, which suggests that the Nd-rich phases at the grain boundary act as hydrogen diffusion paths during the disproportionation reaction. This was supported by TPA measurements, which showed that the temperature range of the disproportionation reaction decreases with increasing proportion of Nd-rich phase. DTA measurements showed that the disproportionation temperature in Nd 16 Fe 76 B 8 decreases with increasing hydrogen pressure. It was found that, in disproportionated NdFeB, the hydrogen desorption kinetics for NdH ≈2.7 and the dihydride were different for Nd in the Nd-rich grain boundary and Nd in the disproportionated matrix.

Characterisation of porous hydrogen storage materials: carbons, zeolites, MOFs and PIMs

Porous materials adsorb H2 through physisorption, a process which typically has a rather low enthalpy of adsorption (e.g. ca. 4 to 7 kJ mol(-1) for MOFs), thus requiring cryogenic temperatures for hydrogen storage. In this paper, we consider some of the issues associated with the accurate characterisation of the hydrogen adsorption properties of microporous materials. We present comparative gravimetric hydrogen sorption data over a range of temperatures for different microporous materials including an activated carbon, a zeolite, two MOFs and a microporous organic polymer. Hydrogen adsorption isotherms were used to calculate the enthalpy of adsorption as a function of hydrogen uptake, and to monitor the temperature dependence of the uptake of hydrogen. Under the conditions investigated, it was found that the Tóth equation provided better fits to the absolute isotherms compared to the Sips (Langmuir-Freundlich) equation at low pressures, whereas it appeared to overestimate the maximum saturation capacity. The isosteric enthalpy of adsorption was calculated by either: fitting the Sips and Tóth equations to the adsorption isotherms and then applying the Clausius-Clapeyron equation; or by using a multiparameter Virial-type adsorption isotherm equation. It was found that the calculated enthalpy of adsorption depended strongly upon the method employed and the temperature and pressure range used. It is shown that a usable capacity can be calculated from the variable temperature isotherms for all materials by defining a working pressure range (e.g. 2 to 15 bar) over which the material will be used.

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.

MgH2 → Mg phase transformation driven by a high-energy electron beam: An in situ transmission electron microscopy study

The dynamics of a phase change have been studied using the electron beam in a transmission electron microscope to transform MgH2 into Mg. The study involved selected-area diffraction and electron-energy-loss spectroscopy (EELS). The orientation relation ( and ), obtained from the electron diffraction study, has been used to propose a model for the movements of magnesium atoms during the phase change. The in situ EELS results have been compared with the existing H-desorption model. The study aims to describe the sorption dynamics of hydrogen in MgH2, which is a base material for a number of promising hydrogen storage systems.

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 .

The HDDR behaviour of crystalline and amorphous rapidly quenched NdFeB

Abstract The disproportionation and recombination reactions in which Nd 2 Fe 14 B decomposes into αFe, Nd hydride and Fe 2 B in hydrogen and then reforms under vacuum, have been investigated in melt-spun ribbon with the compositions Nd 13.1 Fe 82.4 B 4.5 and Nd 18 Fe 76 B 6 . TPA and DTA investigations have shown that hydrogen absorption and desorption are more rapid in the amorphous Nd 13.1 Fe 82.4 B 4.5 ribbon cast at 30 m s −1 than in the fully crystalline ribbon of this composition, spun at 10 m s −1 . Disproportionation was found to occur in two stages in 30 m s −1 ribbon, the first stage in the range 400–500 °C associated with the amorphous phase, and the second stage in the range 500–700 °C due to the presence of crystalline Nd 2 Fe 14 B phase. By selecting ribbons with the smallest thicknesses, it was possible to investigate totally amorphous material on a Faraday balance, and these studies showed that, in this material, the disproportionation reaction was very rapid, and finished at 500 °C. This is in agreement with the observations on the two-phase material. In addition, DTA desorption measurements showed that recombination was more rapid and took place at a lower temperature in 30 m s −1 ribbon disproportionated at 500 °C compared with that disproportionated at 800 °C, suggesting a coarsening of the disproportionated microstructure at the higher temperature.