A.J. Williams

A zinc coating method for Nd–Fe–B magnets

Abstract A newly developed Low Pressure Pack Sublimation (LPPS) process has been used to coat fully dense Nd–Fe–B sintered magnets with zinc. The process was based on a form of sherardizing where the component to be coated is placed in a rotating chamber at 390°C in a mixture of sand and zinc dust. However, during the LPPS process, a moderate vacuum is applied and the chamber is not rotated. LPPS produced an adherent surface coating, which, when placed in a corrosive environment, displayed superior performance in terms of weight loss and reduction in magnetic properties compared to those exhibited by the commercially electroplated and uncoated samples. The severe corrosion conditions were imposed using an autoclave (100°C, 1bar pressure and saturated humidity). The composition and characteristics of the surface layers were examined using optical analysis, XRD and Scanning Electron Microscopy. Under the conditions employed in these studies, the LPPS process produced a small reduction in remanence and coercivity (≈10% and ≈5% respectively) by changing the surface conditions of the magnets. The reduction in properties was found to be related to coating thickness and temperature effects. By the use of LPPS a cheap and effective barrier to corrosion has been produced.

Mass spectrometer hydrogen desorption studies on some hydrided NdFeB-type alloys

Abstract The desorption of hydrogen from the alloys Nd 11.8 Fe 82.4 B 5.8 (formula unit Nd 2 Fe 14 B), Nd 15.5 Fe 77.5 B 7 , Nd 12.5 Dy 3 Fe 74.5 V 3 B 7 and Nd 12.5 Dy 3 Fe 69.5 Co 5 V 3 B 7 (the numbers are atomic percentages of the elements) and from 95% pure neodymium has been investigated by mass spectroscopy. These studies indicated that the desorption of hydrogen from the multiphase alloys is a three-stage process: (1) desorption from the matrix Nd 2 Fe 14 B, (2) partial desorption from the neodymium-rich material and (3) complete desorption from neodymium-rich material. The desorption behaviour of the cobaltcontaining alloy indicated a change in the character of the grain boundary phases in the alloy.

The effect of density on the corrosion of NdFeB magnets

Abstract The corrosion and degradation of Nd–Fe–B magnets by hydrogen from moisture in the environment has been termed the Negative Harris Effect. In order to provide more information on this phenomenon, different density Nd–Fe–B magnets have been subjected to vacuum desorption, XRD and electrochemical corrosion examination. During the normal vacuum sintering procedure used in producing the magnets, the residual hydrogen coming from the hydrogen decrepitation process is desorbed completely. However, the Nd–Fe–B magnets can absorb hydrogen readily from moisture in the environment. In these studies it has been found that the higher the density of the magnets the less is the hydrogen absorbed. The combined effects of the lattice expansion due to the formation of Nd 2 Fe 14 B–H solutions and NdH 2+ x phases as a result of hydrogen absorption and the volume expansion on formation of Nd(OH) 3 , lead to the eventual disintegration of the magnets. Thus hydrogen plays a vital role in the corrosion of Nd–Fe–B type magnets when they are exposed to a humid environment.

The oxidation of SmCo magnets

SmCo alloys form the basis of excellent permanent magnets with potential service applications to 500°C. It is suspected, however, that their oxidation behavior may limit their usefulness but this is a relatively unstudied subject. Two grades of Sm2(Co,Fe,Cu,Zr)17-based alloys were aged in air at temperatures between 300 and 600°C and the oxidation reaction characterized by post-test scanning electron microscopy examination. Both alloys formed a thin external oxide scale composed of Cu and mixed Fe∕Co oxides and an extensive internal oxidation layer consisting of Sm-rich oxide within a CoFe matrix. The depth of the internal oxidation zone was typically an order of magnitude thicker than the surface oxides and increased parabolically with time. This internal oxidation is an important degradation phenomenon because the transformation to CoFe causes a loss of magnetic properties proportional to the volume consumed. The morphologies of the oxidation zones, nature, and mechanisms of various precipitated phases ...

Hydrogen Decrepitation and Recycling of NdFeB-type Sintered Magnets

Abstract With the rapid growth in the use of NdFeB-type magnets and with the growing environmental need to conserve both energy and raw materials, the recycling of these magnets is becoming an ever important issue. In this paper it is demonstrated that hydrogen could play a vital role in this process. Fully dense, sintered NdFeB-type magnets have been subjected to the hydrogen decrepitation (HD) process. The resultant powder has been subsequently processed in one of two ways in order to produce permanent magnets. Firstly, the powder was subjected to a vacuum degassing treatment over a range of temperatures up to 1000°C in order to produce powder that would be suitable for the production of anisotropic bonded or hot pressed magnets. Secondly, the HD-powder has been used to produce fully dense sintered magnets; in which case optimisation of the milling time, sintering temperature and time was carried out. The optimum degassing temperature for coercive powder was found to be 700°C, giving powder with a remanence ( B r ) of ∼1350mT (±10 mT) and an intrinsic coercivity ( H cj ) of ∼750kAm −1 (±10 kAm −1 ). The best sintered magnet was produced by very lightly milling the powder (30 min, roller ball mill), aligning, pressing and vacuum sintering at 1080°C for 1 hour. The magnetic properties of this magnet were: ( BH ) max = 290 kJm −3 (±5 kJm −3 ), Sr = 1240mT (±5 mT) and H cj = 830 kAm −1 (±5 kAm −1 ); representing decreases of 15%, 10%, and 20% respectively, from the properties of the initial magnet.

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.

Mass spectrometer studies of hydrogen desorption from hydrided NdFeB powder

The desorption of hydrogen from NdFeB powder and unmilled material produced using hydrogen decrepitation as a premilling technique was investigated using a mass spectrometer to monitor the hydrogen partial pressure in the system as the sample was heated under vacuum from room temperature to approximately 1000 degrees C. A comparison was made between powder exposed to the atmosphere for 10 min and powder exposed for one month. There was found to be a reduction in the amount of hydrogen desorbed from the Nd-rich grain boundary phases, and there was a change in the shape of the desorption peak from the Nd/sub 2/Fe/sub 14/B phase as the level of oxidation increased. Studies were also carried out on alloy hydrided at room temperature in situ, and these showed a variability in the amount of hydrogen desorbed from the Nd-rich grain boundary phases. >

High anisotropy in Pr-Fe-Co-B-Zr HDDR powders

Abstract The magnetic behaviour of HDDR powders prepared from a Pr13.7Fe63.5Co16.7B6Zr0.1 alloy has been investigated. It has been shown that it is possible to produce highly anisotropic HDDR material from this alloy in the homogenised state (1373 K for 20 h). Remanent magnetisation in the easy direction of the aligned powder was 126±2 J/Tkg whereas, in the hard direction, it was 30±0.6 J/Tkg. HDDR powder based on the corresponding Nd-based alloy (subjected to the same heat treatments) and commercially available, HDDR powder, have both been used as a comparison. The degree of alignment for the HDDR powder based on the Pr alloy was higher (76%) than that of the commercial HDDR powder (69%).

The corrosion behaviour of uncoated bonded NdFeB magnets in humid environments

Abstract The corrosion behaviour of uncoated bonded NdFeB magnets prepared using different binders has been assessed in a humid environment (100°C, saturated water vapour) by means of mass change measurements and magnetic characterisation. Epoxy resin, zinc and polytetrafluoroethylene (PTFE) have been used to bond melt spun ribbon, MQP-D, as supplied by General Motors. It has been found that the PTFE bonded samples which had been heat treated prior to testing exhibit superior corrosion resistance to the other samples, as characterised by a smaller mass change and greater retention of magnetic properties.

The effects of long term annealing at 1000°C for 24 h on the microstructure and magnetic properties of Pr-Fe-B/Nd-Fe-B magnets based on Nd16Fe76B8 and Pr16Fe76B8

Abstract Magnets produced via a hydrogen decrepitation/roller-milling route have been subjected to a post sintering heat treatment of 1000°C for 24 h. Alloys of nominal composition Pr 16 Fe 76 B 8 and Nd 16 Fe 76 B 8 have been studied in terms of both microstructure and magnetic properties to determine the influence of this 24-h annealing treatment. The effect of annealing the Pr 16 Fe 76 B 8 magnets at 1000°C for up to 24 h resulted in a general increase in the overall magnetic properties, especially in the intrinsic coercivity. In contrast to these observations, the same heat treatment was found to be detrimental to all the magnetic properties of the Nd 16 Fe 76 B 8 magnets.