Frederick E. Pinkerton

Pr‐Fe and Nd‐Fe‐based materials: A new class of high‐performance permanent magnets (invited)

We report the properties of a new class of high‐performance permanent magnets prepared from Nd‐Fe‐B and Pr‐Fe‐B alloys. Magnetic hardening is achieved by rapid solidification. Energy products of these isotropic materials can exceed 14 MGOe with intrinsic coercivities of ∼15 kOe. X‐ray and microstructural analyses indicate that the alloys exhibiting optimum characteristics are comprised of roughly spherical crystallites, strongly suggesting that the coercivity mechanism is of the single‐domain particle type. The crystallites are composed of an equilibrium R‐Fe‐B intermetallic phase having tetragonal symmetry, and the stability of this phase with respect to other rare earths and other metalloids has been investigated.

Thermodynamic and magnetic properties of metastable FexCu100−x solid solutions formed by mechanical alloying

Metastable solid solutions of Fe and Cu, which are immiscible in equilibrium, have been formed using high-energy ball milling of elemental powder mixtures. Single-phase face-centered-cubic (fcc) solid solution was obtained for 0<x≤60, and body-entered-cubic (bcc) solid solution for 75≤x<100. The transition from fcc to bcc occurred near x=70, where a mixture of fcc and bcc phases was obtained. The enthalpy of transformation to equilibrium was measured using differential scanning calorimetry. The average atomic volume of the phases exhibits a positive deviation from Vegard’s law, in qualitative agreement with the large positive enthalpy of mixing in this system. The magnetic moments and Curie temperatures for the metastable solid solutions have been determined and compared with those reported for Fe-Cu alloys formed by vapor deposition. Calculations of the formation enthalpy (ΔH) and free energy (ΔG) have been performed based on calphad data, with corrections based on our magnetization measurements. The cal...

Cerium: An Unlikely Replacement of Dysprosium in High Performance Nd–Fe–B Permanent Magnets

Replacement of Dy and substitution of Nd in NdFeB-based permanent magnets by Ce, the most abundant and lowest cost rare earth element, is important because Dy and Nd are costly and critical rare earth elements. The Ce, Co co-doped alloys have excellent high-temperature magnetic properties with an intrinsic coercivity being the highest known for T ≥ 453 K.

Magnetization process in rapidly solidified neodymium‐iron‐boron permanent magnet materials

Initial magnetization and demagnetization data are reported for three forms of rapidly solidified Nd‐Fe‐B permanent magnet materials: melt‐spun ribbons, hot‐pressed magnets, and die upset magnets. In all three materials, the results are consistent with domain‐wall pinning at Nd2Fe14B grain boundaries as the coercivity mechanism. Thermally demagnetized ribbons are comprised of single domain Nd2Fe14B grains, and both initial magnetization and demagnetization occur by depinning and wall motion. Domain walls move easily in die upset magnets until they become strongly pinned at grain edges. Complete magnetization requires an applied field greater than the coercive field. Hot‐pressed magnets show a mixture of ribbon and die upset behavior.

The kinetic enhancement of hydrogen cycling in NaAlH4 by melt infusion into nanoporous carbon aerogel

Enhanced kinetic performance and reversibility have been achieved with uncatalyzed NaAlH4 by incorporation into nanoporous carbon aerogel. Aerogel with a pore size distribution peaked at 13 nm and a pore volume of 0.8 cm(3) g(-1) was filled with NaAlH4 to 94% capacity by melt infusion at 189 degrees C under 183 bar H(2) gas overpressure. Dehydrogenation to NaH + Al with reasonable kinetics was accomplished at 150 degrees C, well below the NaAlH4 melting temperature (183 degrees C), compared to hydrogen release above 230 degrees C for bulk uncatalyzed NaAlH4. Uncatalyzed bulk samples did not rehydrogenate under laboratory conditions, whereas NaAlH4 in a carbon aerogel host was readily rehydrogenated at approximately 160 degrees C and 100 bar H(2) to approximately 85% of its initial capacity. Ball-milled NaAlH4 catalyzed with 4 mol% TiCl3 showed somewhat better kinetics compared to the infused aerogel; nevertheless, the large kinetic enhancement obtained by incorporation into carbon aerogel, even in the absence of a catalyst, demonstrates the substantial benefit of confining the NaAlH4 to nanoscale dimensions.

Magnetic hardening of Ce2Fe14B

We report an effort to optimize the room-temperature permanent magnet properties of Ce-Fe-B materials rapidly solidified by melt spinning. Starting alloy compositions in the ternary phase diagram were selected systematically. Ribbons were melt spun from them at a quench wheel velocity of 35 m/s, corresponding to a solidification rate high enough to yield mostly amorphous or nanocrystalline material. Heat treatment above 450 °C crystallizes Ce2Fe14B, as x-ray diffraction clearly indicates, with the concomitant development of hard magnetic properties. The anneal temperature yielding optimum remanence Br, intrinsic coercivity Hci, and energy product (BH)max was determined in each case. For the ingot composition Ce17Fe78B6, we obtain Br = 4.9 kG, Hci = 6.2 kOe, and energy product (BH)max = 4.1 MGOe in ribbons comprised principally of Ce2Fe14B. This composition differs substantially from the optimum Nd13Fe82B5 stoichiometry for melt-spun magnets based on Nd2Fe14B and can be understood from a comparison of the ...

Deformation-induced nanocrystallization in an Al-based amorphous alloy at a subambient temperature

Abstract The compressive region of amorphous Al 90 Fe 5 Gd 5 , bent at −40 °C, was investigated by transmission electron microscopy. A high density of nanocrystals is observed within shear bands. Severe plastic deformation and precipitation of nanocrystallites are observed at the fracture surface. It is argued that deformation-assisted atomic transport leads to nanocrystallization.

Effect of strain rate on the formation of nanocrystallites in an Al-based amorphous alloy during nanoindentation

The effect of deformation by nanoindentation on nanocrystallization in amorphous Al90Fe5Gd5 was investigated by transmission electron microscopy. Massive precipitation of nanocrystallites is observed within the indents. Under the quasistatic condition used, a temperature rise due to adiabatic heating is likely negligible, confirming that plastic deformation can induce crystallization without a heating effect. The nucleation of nanocrystallites is significantly affected by the strain rate.

Sodium magnesium amidoborane: the first mixed-metal amidoborane

The first example of a mixed-metal amidoborane Na(2)Mg(NH(2)BH(3))(4) has been successfully synthesized. It forms an ordered arrangement in cation coordinations, i.e., Mg(2+) bonds solely to N(-) and Na(+) coordinates only with BH(3). Compared to ammonia borane and monometallic amidoboranes, Na(2)Mg(NH(2)BH(3))(4) can release 8.4 wt% pure hydrogen with significantly less toxic gases.

Integrated magnetic field sensor

A magnetic field sensor, such as a magnetoresistor, includes a strip of a layer of a high electron mobility semiconductor whose electrical characteristics vary when a magnetic field is applied thereto on the surface of a body (substrate) of an insulating layer. Conductive contacts are on the strip at the ends thereof and conductive shorting bars are on and spaced along the strip to divide the strip into active regions. The body is mounted on a permanent magnet assembly which includes a magnet and a layer of a ferromagnetic material with the ferromagnetic material extending over the strip. The ferromagnetic layer is in close proximity to only the strip and, more preferably, to only the active regions of the strip so as to confine the magnetic field to the strip.