Recycling of additively printed rare-earth bonded magnets.

In this work, we describe an efficient and environmentally benign method of recycling of additive printed Nd-Fe-B polymer bonded magnets. Rapid pulverization of bonded magnets into composite powder containing Nd-Fe-B particles and polymer binder was achieved by milling at cryogenic temperatures. The recycled bonded magnets fabricated by warm compaction of ground cryomilled coarse composite powders and nylon particles showed improved magnetic properties and density. Remanent magnetization and saturation magnetization increased by 4% and 6.5% respectively, due to enhanced density while coercivity and energy product were retained from the original additive printed bonded magnets. This study presents a facile method that enables the direct reuse of end-of-life bonded magnets for remaking new bonded magnets. In addition to magnetic properties, mechanical properties comparable to commercial products have been achieved. This research advances efforts to ensure sustainability in critical materials by forming close loop supply chain.

Size control of highly ordered HfO2 nanotube arrays and a possible growth mechanism.

Highly ordered HfO2 nanotube arrays were prepared through an electrochemical anodization in the presence of NH4F and ethylene glycol. The voltage-dependent pore size, wall thickness and porosity were studied using scanning electron microscopy and a wall thickness to pore size ratio was proposed on the basis of the results to serve as a boundary condition additional to the 10% porosity rule introduced by the Gosele group. The average distributions of the tube sizes and wall thicknesses of the nanotubes prepared at 20 V were determined from the small-angle x-ray scattering data using a simple polydisperse core-shell cylinder model fit. Temperature-dependent x-ray diffraction measurements show that the as-grown amorphous nanotube arrays can be converted into crystalline nanotube arrays at a temperature above 500 degrees C. Transmission electron microscopy study of the dimple layer under the as-grown nanotube arrays reveals the presence of a layer of ordered HfO2 nanocrystals. Further microscopic investigation of the nanotube root region indicates that the nanotubes develop from bulbs produced during anodization. A possible gas bubble initiated growth mechanism based on these observations was proposed.

Germanium-catalyzed hierarchical Al(2)O(3) and SiO(2) nanowire bunch arrays.

Germanium (Ge), a Group IV semiconductor, was recently used as an effective catalyst to grow individual, single-crystalline ZnO nanowires through a vapor-liquid-solid (VLS) process [Pan et al., Angew. Chem., Int. Ed., 2005, 44, 274-278]. Here, we show that Ge can also act as an efficient catalyst for the large-scale growth of highly aligned, closely-packed polycrystalline Al(2)O(3) and amorphous SiO(2) nanowire bunch arrays. The Ge-catalyzed Al(2)O(3) and SiO(2) nanowire growth exhibits many interesting growth behaviors including (i) multiple nanowire growth catalyzed by one micrometer-size Ge particle, (ii) branching growth and (iii) batch-by-batch growth. These growth phenomena are distinct from the conventional Au-catalyzed nanowire growth but are analogous to the recently reported Ga-catalyzed SiO(2) nanowire growth. It is anticipated that many other oxide nanowires and nanowire assemblies can be synthesized through the Ge-catalyzed VLS process. The Ge-catalyzed Al(2)O(3) and SiO(2) nanowires emit strong visible light under ultraviolet light excitation.