I.R. Harris

Structural analysis of hydrothermally synthesized Sr1−xSmxFe12O19 hexagonal ferrites

Abstract M-type hexagonal ferrites of composition Sr1−xSmxFe12O19 with x=0, 0.06, 0.11, 0.2 and 0.33 were produced by hydrothermal synthesis. The purity of the samples was investigated by X-ray diffraction and Mossbauer spectrometry. The analyses reveal for all the samples the presence of the hexagonal M-type phase as the main phase, but also the presence of Sm containing secondary phases of the (Sr,Sm)FeO3−δ type. The results of the Mossbauer analysis show that the hyperfine parameters of the M-type phase contribution do not vary with the Sm content, indicating that the Sm content in the M-type phase is very weak. This is in agreement with the fact that no Fe2+ is detected in the spectra. As both anisotropy field and saturation magnetization remain constant, the increase of the coercivity is thus attributed to microstructural changes, in relation with the presence of the Sm containing secondary phases.

Structural and magnetic properties of hydrothermally synthesised Sr1-xNdxFe12O19 hexagonal ferrites

Abstract M-type hexagonal ferrites of composition Sr 1− x Nd x Fe 12 O 19 with x =0, 0.06, 0.08, 0.11, and 0.20 were produced by hydrothermal synthesis. The phase composition of the samples was investigated by X-ray diffraction and Mossbauer spectrometry. The analyses reveal for all the samples the presence of the hexagonal M-type phase as the main phase. However, Nd containing secondary phases of the (Sr,Nd)FeO 3− δ type are also detected. Mossbauer spectrometry indicates that the Nd content in the M-type phase is very weak, as the hyperfine parameters of the M-type phase contribution do not vary with the Nd content. This is in agreement with the fact that both saturation magnetisation and remanence remain almost constant as the Nd content increases. The variations of the coercive field are thus attributed to microstructural changes, in relation with the presence of the Nd containing secondary phases.

Resistivity measurements on hydrogenation disproportionation desorption recombination phenomena in NdFeB alloys with Co, Ga and Zr additions

Abstract Resistivity measurements on the hydrogenation disproportionation desorption recombination (HDDR) phenomena in near stoichiometric NdFeB alloys were carried out in the temperature range of T=740–900 °C in order to investigate the effect of additional elements such as Co, Ga and Zr. These studies were combined with detailed temperature-pressure-analysis to relate the observed changes in resistivity with the solubility of hydrogen in the rare-earth metal. Higher relative resistivity changes can be observed when lower disproportionation temperatures are applied. This tendency depends on the amount of hydrogen absorbed in the disproportionated mixture. The addition of Co leads to a longer completion time of the disproportionation reaction. In particular, the alloy with both Co and Ga additions exhibits the longest completion time. These observations suggest that Co stabilises the 2-14-1 phase and Ga addition enhances this effect. These additional elements decrease the number of hydrogen atoms absorbed per formula unit and also increase the onset hydrogen pressure of the disproportionation and recombination reactions. On desorption, Co and Ga additions decrease the activation energy of the recombination reaction and lead to a higher reaction rate of these alloys compared to that of the ternary alloy, which is especially pronounced during the initial stages of recombination and which may be relevant for the development of anisotropic features in the recombined state.

Characterisation of high temperature oxidation of NdFeB magnets

Abstract The long term high temperature oxidation properties of a Nd 16.4 Fe 75.7 B 7.9 commercial sintered magnet were investigated in pure oxygen atmosphere up to 500°C and in air between 350 and 600°C. In pure oxygen atmosphere, three exothermic reactions occur, corresponding to the oxidation of the Nd-rich intergranular regions, the Nd 2 Fe 14 B matrix phase, and the α -Fe phase that forms during the oxidation of Nd 2 Fe 14 B. In air, the oxidation of Nd 2 Fe 14 B was investigated further. Instead of the oxidation proceeding along the grain boundaries, the Nd 2 Fe 14 B matrix dissociates to form an adherent grey surface layer which grows transgranularly into the magnet. This is probably due to a reaction occurring in the Nd-rich regions which prevents fast path oxygen diffusion along the grain boundaries. The main reaction is the dissociation of the Nd 2 Fe 14 B matrix into α -Fe nanocrystals which contain small precipitates of oxides of Nd. The products of this reaction form the adherent grey layer which grows transgranularly into the magnet. The activation energy and the diffusivity pre-exponential factor for this reaction were found to be 114 kJ mol- −1 and 0.7 mm 2 s −1 , respectively. After further oxidation of the dissociated grey layer, α -Fe is oxidised to form α Fe 2 O 3 and finally, from about 600°C, some of the α -Fe 2 O 3 reacts with the small precipitates of oxides of Nd to form FeNdO 3 .

Hydrides of the RNiIn (R=La, Ce, Nd) intermetallic compounds: crystallographic characterisation and thermal stability

LaNiInH2.0, CeNiInH1.8 and NdNiInH1.7 intermetallic hydrides were synthesised by the reaction of gaseous hydrogen with RNiIn compounds at 298 K and hydrogen pressures 1–100 bar and characterised by X-ray diffraction and thermal desorption studies. The hexagonal symmetry of the initial ZrNiAl-type structure is not changed on hydrogenation. Hydrogen insertion causes a pronounced anisotropic expansion of the unit cells along [001] (Δc/c=14.9–18.3%) and results in a volume increase of 8.9–9.3%. Possible interstitial sites for the accommodation of hydrogen atoms in the lattices of dihydrides RNiInH1.7–2.0 were proposed. A reversible formation of equiatomic RNiIn ternaries accompanies a complete hydrogen desorption from the dihydrides and takes place at temperatures near 800 K. Hydrogen evolution proceeds through two steps with peaks at 425–540 and 630–710 K and at temperatures 500–600 K leads to the formation of lower hydrides LaNiInH0.9, CeNiInH0.8 and NdNiInH0.85, which were structurally characterised as isotropically expanded ZrNiAl-type compounds. The melting points were determined for the LaNiIn (1057 K) and CeNiIn (1083 K) intermetallics. The NdNiIn compound exhibits high thermodynamic stability and does not disproportionate in hydrogen at PH2=1 bar up to 1023 K. RNiIn compounds formed with Y or the heavier rare earth metals (R=Sm, Gd, Tb, Dy, Ho, Er and Tm) do not form hydrides at hydrogenation pressures up to 100 bar, both at room temperature or on heating in hydrogen gas up to 1143 K.

Structural and Mössbauer analyses of ultrafine Sr1−xLaxFe12−xZnxO19 and Sr1−xLaxFe12−xCoxO19 hexagonal ferrites synthesized by chemical co-precipitation

Ultrafine M-type hexagonal ferrites of nominal composition Sr1−xLaxFe12−x ZnxO19 and Sr1−xLaxFe12−xCoxO19 with x = 0, 0.1, 0.2, 0.3 and 0.4 were produced by chemical co-precipitation. The phase make-up of the samples was investigated by x-ray diffraction. The analyses show that the hexagonal M-type phase is the main phase in all the samples. Secondary (La,Sr)FeO3, ZnFe2O4 and CoFe2O4 phases are also detected, indicating that the La, Zn and Co content in the M-type phase is lower than the nominal content. A complete Mossbauer analysis of the substitution effects in the M-type phase was made. The results show that the M-type phase contains La3+ in the same proportion as Zn2+ and La3+ in the same proportion as Co2+, in the La–Zn and La–Co samples, respectively. The evolution with x of the hyperfine parameters of the components used to fit the contribution of the M-type phase have been interpreted consistently in relation to the substitution effects by means of the comparison between the spectra of the La–Zn and La–Co substituted samples. The results show unambiguously that La3+ ions are located in the Sr2+ sites, Zn2+ ions are located in the 4f1 sites and Co2+ ions are located in both 4f2 and 2a sites.

Magnetostrictive properties of polymer bonded Terfenol-D

Abstract A resin bonding route has been used to produce rods of the highly magnetostrictive alloy Tb 0.27 Dy 0.73 Fe 1.93 . The influence of the resin load on the magnetic properties has been studied. Two phenomena could be observed during the measurement of the magnetomechanical factor. The first was a threshold value of applied magnetic field. The second was abnormal behaviour of the characteristic frequencies with applied bias field.

Densification of a Nd13Fe78NbCoB7-type sintered magnet by (Nd,Dy)-hydride additions using a powder blending technique

Abstract This work reports improvements in the density ( ρ ) and permanent magnetic properties of sintered magnets made from a Nd 13 Fe 78 NbCoB 7 -type alloy, as a result of additions of Nd-, Dy- or (Nd+Dy)-powder in the form of hydrides, using a powder blending technique. Under the conditions employed in this work, it proved impossible to achieve a full density in the sintered magnets, processed from this alloy in the cast state, using a standard HD-powder metallurgical route. Increasing the sintering temperature and the duration of the sintering did not result in any significant improvements in the ρ values and the highest possible density achieved in the sintered magnets was only 7.05 g cm −3 . In an attempt to improve these densities, the effects of powder blending Nd and Dy additions in the form of their respective hydrides on the magnetic properties and microstructural appearance of the sintered magnets have been studied. Using the same processing conditions, it was found that, by an addition of only 2 at% Nd or Dy, practically full densification could be achieved and magnets with densities >7.40 g cm −3 could be produced consistently. The total addition of (Nd+Dy), with varying proportions of Nd and Dy, was limited to 2 at% and at each addition, a full density in the sintered state was attained. Using 1 at% Nd and 1 at% Dy, an intrinsic coercivity (H ci ) of ∼780 kA m −1 with a corresponding energy density [(BH) max ] value of ∼324 kJ m −3 were obtained. These values were improved to ∼950 kA m −1 and ∼331 kJ m −3 , respectively, by applying a standard annealing treatment of 600°C for 1 h, followed by air-cooling to room temperature. These studies have confirmed that powder blending using Nd and Dy hydrides is a very effective means of promoting liquid phase sintering and optimising magnetic properties in Nd–Fe–B-type magnets.

Blending additions of cobalt to Nd16Fe76B8 milled powder to produce sintered magnets

Abstract A blending process involving the mixing of powders of NdFeB and pure cobalt has been developed. This process has been shown to be an effective and simple way of adding cobalt to the composition. This allows the composition and hence properties of the finished magnets to be adjusted subsequent to the casting and milling of the basic alloy. After standard sintering treatments, the cobalt substitutes into the matrix phase, causing a linear increase in the Curie temperature of 11°C per at% Co in the range 0–10 at% Co. As the amount of cobalt increases, so the remanence is improved, but there is a corresponding decrease in the coercivity. With increasing cobalt content, the proportion of grain boundary phase decreases and cobalt-containing phases are observed. The increase in remanence is attributed to the increased proportion of Nd 2 (Fe, Co) 14 B and the decrease in coercivity is attributed to the reduced magnetic isolation of the grains and to the introduction of ferromagnetic, cobalt containing grain boundary phases.

The improvement of the hydrogenation properties of nickel-metal hydride battery alloy by surface modification with platinum group metals (PGMs)

Abstract Because of their higher power densities, low memory effects and more environmentally friendly constituents, Nickel Metal Hydride (NiMH) batteries have become a strong competitor to NiCad batteries. The use of multicomponent alloys has been found to improve cell lifetimes, while further improvements to the cell performance are obtained by surface coating or ‘microencapsulation’ techniques with various metals or metal oxides. In the present work, a technique of coating hydride powders with one or more Platinum Group Metals (PGMs) has been developed, which enables the alloy to be charged/discharged rapidly with hydrogen with the additional advantage of being extremely resistant to deactivation. The hydrogenation kinetics of the uncoated and PGM doped alloys were assessed by gravimetric analysis, with periodic exposure to the atmosphere in an attempt to deactivate the powders. It was observed that, in the solid–gas studies, the Ru and Pd/Ru coated alloys were readily charged/discharged with hydrogen, even after long periods in air. Thus, the use of PGMs in this application represents a valuable advance both in the technology of NiMH batteries and of hydrogen storage.