B. Gebel

Highly coercive SmCo5 magnets prepared by a modified hydrogenation-disproportionation-desorption-recombination process

The hydrogenation-disproportionation-desorption-recombination (HDDR) process was applied to SmCo5 using extreme conditions, namely high hydrogen pressures and reactive milling under hydrogen. Investigations on the hydrogen absorption behavior of SmCo5 by differential scanning calorimetry under hydrogen pressures between 1 and 7 MPa showed absorption events due to an interstitial absorption at about 100 °C and a disproportionation reaction at about 600 °C. X-ray diffraction showed the disproportionation of SmCo5 into Sm hydride and fcc-Co. A favorable effect of high hydrogen pressures on the disproportionation reaction was observed which can be explained by a decrease of the free enthalpy of the samarium hydride for increasing hydrogen pressures. Reactively milled SmCo5 showed also the products of the disproportionation reaction. The recombination to the original SmCo5 phase on hydrogen desorption in a subsequent heat treatment in vacuum was successful for both methods. However, Sm2O3, Sm2Co17, and Sm2Co7 ...

Permanent magnets prepared from Sm10.5Fe88.5Zr1.0Ny without homogenization

Abstract In as-cast Sm 2 Fe 17 the high amount of α-Fe caused by a peritectic reaction can be considerably reduced by a small addition of about 1 at% Zr. X-ray diffraction showed that as-cast Sm 10.5 Fe 88.5 Zr 1.0 mainly consists of a phase with the Th 2 Zn 17 -type structure and SmFe 3 . Non-homogenized Sm 10.5 Fe 88.5 Zr 1.0 was milled and (i) annealed in vacuum or (ii) treated with a hydrogenation-disproportionation-desorption-recombination (HDDR) process. The annealed and subsequently nitrogenated powder is magnetically anisotropic and has a coercivity μ O J H C up to 2.0 T and an energy product ( BH ) max up to 136 kJ/m 3 . HDDR-treated and nitrogenated powder is isotropic and exhibits values of μ O J H C = 3.1 T and ( BH ) max = 103 kJ/m 3 . Consequently, Sm 10.5 Fe 88.5 Zr 1.0 N y ( y ≈ 16) permanent magnets with very good properties can be prepared without the time-consuming homogenization procedure.

Metastable borides and the inducement of texture in Pr/sub 2/Fe/sub 14/B-type magnets produced by HDDR

A modified hydrogenation-disproportionation-desorption-recombination (HDDR) process has been applied to Pr/sub 2/(Fe,Co,Zr)/sub 14/B-type alloys. XRD and Rietveld analysis show that a Fe/sub 3/B phase observed in a Pr/sub 13.7/Fe/sub 80.3/B/sub 6/ alloy solid-disproportionated at 875/spl deg/C is transformed to Fe/sub 2/B and bcc Fe on further hydrogen annealing and that the formation of Fe/sub 3/B is partly suppressed when the alloy is processed at 800/spl deg/C. No residual 2-14-1-type phases are observed after disproportionation of Pr/sub 13.7/Fe/sub 63.5/Co/sub 16.7/Zr/sub 0.1/B/sub 6/ alloy but here a new intermediate boride phase, Pr(Fe,Co)/sub 12/B/sub 6/ (rhombohedral R~3m), is found. Pr(Fe,Co)/sub 12/B/sub 6/ transforms to (Fe,Co)/sub 2/B and /spl alpha/-(Fe,Co) on further hydrogen annealing or conventional disproportionation. For both alloys it is found that processing at higher temperature leads to a better texture on recombination. The role of the Co and Zr additives is not primarily the stabilization of the 2-14-1 matrix phase against hydrogen disproportionation but more importantly the stabilization of intermediate boride phases.

Thermostability of Sm2(FeGa)17Cy prepared by gas-solid reaction (GSR)

The gas-solid-reaction (GSR) was used to introduce interstitial carbon atoms into Sm2Fe17−xGax compounds with x=0, 0.5, 1, and 2. For this process, powders made from homogenized ingots were annealed at 500 °C under methane for different times. The thermostability increases for small amounts of Ga and the investigation shows that Sm2Fe16.5Ga0.5Cy is stable up to 750 °C. In the case of Sm2Fe15Ga2Cy carburized for 6 h (y=2.0) and 18 h (y=2.2), the x-ray diffraction patterns show the Th2Zn17-type structure only. After annealing at 800 °C for 20 min the 6 h carburized sample shows a small amount of α-Fe and other phases and there is a large Fe content after annealing at 850 °C. For an 18 h carburized sample, less Fe and no other phases have be seen after annealing at 800 °C, i.e., the material is nearly single phase. The result that longer carburization times stabilize the Th2Zn17-type structure could also be manifested by Kerr microscopy. A comparison with mechanically alloyed Sm2Fe15Ga2C2 powders prepared wi...

Permanent magnet properties of Sm3(Fe0.93Ti0.07)29Xy (X = C or N)

Abstract Magnets from Sm 3 (Fe 0.93 Ti 0.07 ) 29 material were prepared from powder, which was milled, carburized or nitrided, then field compacted in a die press and resin-bonded. Both types of magnet exhibit coercivities of about 0.4 T. The coercivity can be increased up to 0.9 T by additional milling after nitrogenation. Zn-bonding of these powders seems to be less effective for enhancing the coercivity than for Sm 2 Fe 17 interstitials.

Nd2(Fe,Co,M)14B-type magnet powders produced by the HDDR process

Abstract The relationship between the various HDDR processing parameters and the magnetic properties of Nd 2 (Fe,Co,M) 14 B-type materials with partial replacement of Fe by Zr, V, Cr, Ga and Al or Al–Cr have been studied. Aligned anisotropic powders show values of ( BH ) max ∼ 180 kJ m −3 for Nd 16 Fe 66.4− x Co 11.6 M x B 6 (M x =Zr 0.5 , Cr 0.25 and Al 1 Cr 2 ) materials.

Thermodynamics of the (Sm2Fe17−xGax+H2) system

Abstract Sm 2 Fe 17− x Ga x samples (with x =0, 0.5, 1, and 2) were disproportionated into SmH 2 and α-(Fe,Ga) by heating the samples in 4 MPa hydrogen up to 650°C. It was possible to disproportionate even the more stable compounds with a higher Ga content under these conditions. The recombination into the original 2:17 phase was carried out by annealing in low hydrogen pressures of 2.5, 12 and 60 kPa. The values of the hydride formation enthalpy, Δ H , and of the change of entropy, Δ S , for the Sm 2 Fe 17− x Ga x compounds were obtained by using the Van’t Hoff relation. It was found that the negative Δ H increases monotonically from 141 to 229 kJ/mol H 2 and the negative Δ S increases monotonically from 112 to 258 J/Kmol H 2 for x =0 to x =2, respectively. These results show that the higher stability for the compounds with increasing Ga content is mainly attributed to the increasing negative change of entropy −Δ S and not, as expected before, to a decrease of −Δ H .

Influence of M=Al, Ga and Si on microstructure and HDDR-processing of Sm2(Fe, M)17 and magnetic properties of their nitrides and carbides

Abstract Microstructural investigations by means of scanning electron microscopy and X-ray diffraction showed that the formation of high amounts of α-Fe in as-cast Sm 2 (Fe,M) 17 alloys can be suppressed by using the substitutions M=Al or Si together with excess Sm. Ga, on the other hand, does not induce this beneficial effect. The hydrogen absorption and desorption behaviour was investigated by temperature–pressure–analysis and hydrogen differential thermal analysis. Decreasing amounts of hydrogen were absorbed in Sm 2 (Fe,M) 17 for increasing contents of all substitutions M=Ga, Al and Si due to their stabilising effect on the 2:17 phase with regard to its disproportionation. Compared with the homogenised alloys, the as-cast materials show a weaker interstitial hydrogen absorption and a stronger disproportionation reaction at lower temperatures due to the higher content of Sm-rich phases in the as-cast alloys. A second cycle of the hydrogenation–disproportionation–desorption–recombination (HDDR) process leads to a faster disproportionation reaction at lower temperatures due to the grain refinement during the first cycle. Magnetic properties of as-cast and homogenised materials were investigated after HDDR treatment and nitrogenation or carburisation of pre-milled powders. Nitrogenated and carburised samples showed coercivities up to 3.0 T and 2.3 T, respectively. There were only slight differences of the magnetic properties of materials prepared from homogenised and from as-cast samples.

Effect of small Zr additions on the microstructure of Sm/sub 2/Fe/sub 17/

Microstructure and phase composition of Sm/sub 10.5+/spl delta//Fe/sub 89.5-/spl delta/-/spl infin//Zr/sub /spl infin// samples with /spl delta/=0, 1.5 and x=0, 1, 2 were investigated. The Zr addition reduces the amount of Sm-rich phase and free /spl alpha/-Fe in as-cast materials, whereas the Sm addition (only investigated for Zr-containing alloys) reduces the amount of free /spl alpha/-Fe on cost of a higher amount of Sm-rich phase. The Zr is not uniformly distributed in the as-cast materials, forming Zr-rich (Sm, Zr)Pes phase (10-20 at% Zr) besides 1:3 and 2:17 phases with lower Zr content (/spl les/1 at% Zr). However, after homogenization, these Zr- rich regions disappeared and the Zr was then uniformly distributed in the 1:3 phase regions. The hard magnetic properties of all samples were studied after milling and nitrogenation. The highest energy product (BH)/sub max/ of 146 kJm/sup -3/ was obtained for the homogenized Zr-free sample. However, the properties of the as-cast materials are improved by the Zr-addition, with the optimum composition of Sm/sub 12/Fe/sub s/spl tau//Zr/sub 1/N/sub y/ showing a (BH)/sub max/ of 125 kJm/sup -3/.

Study of hydrogenation of Sm2Fe17-yGay by means of X-ray diffraction

Abstract The hydrogenation process of Sm 2 Fe 17− y Ga y ( y =0–2) was studied. X-ray investigations show a decreasing hydrogen solubility in the intermetallic alloy with increasing Ga-content from 4.0±0.3 atoms per formula unit for Sm 2 Fe 17 to 2.85±0.05 for Sm 2 Fe 15 Ga 2 . The larger Ga atoms reduce the size of the interstitial sites and thereby the maximum hydrogen concentration is decreased. The behaviour of the lattice parameters a and c with increasing Ga content points to a changed hydrogen distribution on the interstitial sites, becoming more statistical. In situ observations by means of high temperature X-ray diffraction show that the hydrogen absorption process is diffusion controlled. The hydrogen absorption starts at an annealing temperature of 120–140°C in all cases. The solubility of hydrogen decreases with increasing temperature. The hydrogen is completely desorbed above 350°C in all cases. The absorption/desorption process is reversible between room temperature and 400°C. Annealing at temperatures above 400°C leads to the decomposition of the Sm 2 Fe 17 phase, indicated by emerging of α-Fe. The formation of SmH x is established at 600°C. The decomposition temperature increases with increasing Ga-content. Up to 750°C, only Sm 2 Fe 17 is completely decomposed.