Y.C. Chuang

Characterization of ultrafine γ-Fe(C), α-Fe(C) and Fe3C particles synthesized by arc-discharge in methane

Ultrafine γ-Fe(C), α-Fe(C) and Fe3C particles were prepared by arc-discharge synthesis in a methane atmosphere. The phases, morphology, structure and surface layer of the particles were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) techniques and X-ray photoelectron spectroscopy (XPS). It was found that the mean particle size ranged from 9.8 to 12.8 nm. The surface of particles mostly consisted of a carbon layer and a little oxide. Phase transformation from γ-Fe(C) to α-Fe(C) was studied by annealing in vacuum and by differential thermal analysis and thermogravimetry (DTA–TGA) measurement. The abundance of γ-Fe(C) was determined by a magnetization measurement to be approximately 30%. Phase transformation occurred between 300 and 500 °C in a flowing argon atmosphere. The Fe3C particles oxidized to α-Fe2O3 and carbon dioxide at 610 °C or so.

The preparation and characterization of ultrafine Fe-Ni particles

Ultrafine Fe, Fe-Ni, and Ni particles were prepared by using the hydrogen plasma-metal reaction method in a mixture of H-2 and Ar of 0.1 MPa. The particles were characterized by x-ray diffraction, transmission electron spectroscopy, energy disperse spectroscopy, chemical analysis, and Mossbauer spectroscopy. In contrast with bulk Fe-Ni alloys, the distinguishing features in corresponding ultrafine particles are that two phases with fcc and bcc structures coexist in a wide composition range. Ultrafine Fe-Ni particles have higher resistance to oxidation than Fe and Ni particles. The mechanism of forming particles was analyzed by means of structural and magnetic measurements. It was found that quenching is a dominant mechanism for forming paramagnetic particles. Hyperfine interactions were studied by Mossbauer spectroscopy in comparison with those in bulk Fe-Ni alloys.

METASTABLE SM-FE-N MAGNETS PREPARED BY MECHANICAL ALLOYING

Abstract SmxFe100-x alloys with x = 11, 12, 12.5, 13 and 14 were synthesized by mechanical alloying (MA). It was found that with increasing annealing temperature from 600 to 900°C, the Curie temperature would decrease from 220 to 136°C which is still higher than that of as-melted and annealed Sm10.5Fe89.5. X-ray diffraction study indicated that the former tends to be of the TbCu7 structure and the latter is of the Th2Zn17 type. After nitrogenation, the Curie temperatures of two of these alloys were raised to about 480°C. High coercivities have been achieved in the nitrided samples. The best results obtained for the MA samples are iHc = 44.0 kOe, Br= 8.0 kG and (BH)max = 14.3 MGOe. High coercivities might originate from the SmFe7Nδ phase in the MA samples. Proper annealing is necessary to complete the solid state reaction for forming metastable phase SmFe7. It has been found that the optimum annealing temperature is 750°C. Too high annealing temperature leads to excessive loss of samarium. Transformation of SmFe7 to Sm2Fe17 occurs at 1000°C, resulting in the decrease of the coercivity.

First-order magnetic transition in (Nd, Pr)2Fe14B

Abstract The magnetization of magnetically-aligned (Nd1−xPrx)2Fe14B samples with x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 at 4.2 K has been measured in fields up to 35 T. The first-order magnetization process (FOMP) which has been observed earlier to occur in the magnetization of Nd2Fe14B, measured with the field applied perpendicular to the c axis, becomes more pronounced with increasing praseodymium content, whereas the transition field appears to decrease. A phenomenological analysis is presented in which the misalignment of the grains in the samples has been taken into account. The anisotropy constants up to sixth order obtained in this analysis allow for the calculation of the free energy as a function of the angle between the magnetization vector and the applied fields from which the first-order magnetic transition can be understood.

CHARACTERIZATION OF FE-NI(C) NANOCAPSULES SYNTHESIZED BY ARC DISCHARGE IN METHANE

Ultrafine Fe-Ni(C) particles of various compositions were prepared by are discharge synthesis in a methane atmosphere. The particles were characterized by x-ray diffraction, transmission electron microscopy, energy disperse spectroscopy, chemical analysis, x-ray photoelectron spectroscopy, Mossbauer spectroscopy, and magnetization measurement. The carbon atoms solubilizing at interstitial sites in gamma-(Fe,Ni, C) solution particles have the effects of forming austenite structure and changing microstructures as well as magnetic properties. A carbon layer covers the surface of Fe-Ni(C) particles to form the nanocapsules and protect them from oxidization, The mechanism of forming Fe-Ni(C) nanocapsules in the methane atmosphere was analyzed.

Rotation alignment for measuring easy-plane magnetic anisotropy

A new method for preparing aligned specimens of polycrystalline materials with easy-plane magnetization has been developed in which a powder sample mixed with epoxy is rotated in an applied field. X-ray diffraction and magnetization measurements show that a significant improvement has been achieved in aligning the specimens with this method. For a Y2Co14B specimen prepared by this method, a peak corresponding to the anisotropy field was observed in the second derivative of the room-temperature magnetization curve at a magnetic field of 2.8 T. Computer simulation of the magnetization process shows that there is a peak in the second derivative curve for a well-aligned easy-plane anisotropy sample.

Investigation of the Ce-Fe binary system

The CeFe binary system was investigated and an FeCe binary phase diagram was proposed. This system consists of 1. (i) two peritectic reactions, γ-Fe + L → Ce2Fe17 and Ce2Fe17 + L → CeFe2, occurring isothermally at 1063°C and 925°C respectively; 2. (ii) a eutectic reaction, L → CeFe2 + Ce, occurring isothermally at 592°C with eutectic containing 83.3 at.% Ce (92.6 wt.% Ce); 3. (iii) a peritectoid reaction, γ-Fe + Ce2Fe17 → α-Fe(Ce), occurring isothermally at 922 °C. The solid solubility of cerium in iron in the temperature range 850–900 °C was found to be less than 0.04 at.% (0.1 wt.%). The Curie temperature of α-Fe(Ce) was slightly lowered with increasing cerium content in solid solution.

Structure and magnetic properties of R2Fe14B1-xCx compounds (R=Y,Nd)

Abstract Magnetization curves parallel and perpendicular to aligned direction were measured in an extraction magnetometer (magnetic field up to 80 kOe) on R 2 Fe 14 B 0.5 C 0.5 compounds (R = Y,Nd) from 1.5 to 300 K. Modified Sucksmith-Thompson method was used to determine anisotropy constants K 1 and K 2 . It was found that the mean Fe moment, Curie temperature and spin reorientation temperature of the Nd-containing compound were decreased upon substitution of carbon for boron, however, the anisotropy field H a was increased with such substitution. X-ray diffraction analysis showed that values of (1 - α( c / a ) 2 ) of the compound in which boron was partially substituted by carbon was larger than that of the unsubstituted one. It is thought that the crystal field acting on the Fe sublattice is increased with carbon substitution. The decrease of T sr of Nd 2 Fe 14 B 0.5 C 0.5 compared with that of Nd 2 Fe 14 B is probably due to the increase of the Fe sublattice anisotropy. The value of H a determined by the SPD method differs significantly from that derived by 2( K 1 + 2 K 2 )/ M s . It seems that the anisotropy constants K 1 , K 2 determined by the Sucksmith-Thompson method does not completely represent the anisotropy behaviour of this type of compound. The nominal R 2 Fe 14 C samples are a mixture of two phases (R 2 Fe 17 C x + α − Fe) and the Nd 2 Fe 14 B type compound was not found in this case. The fact that R 2 Fe 17 C x has a higher Curie temperature compared to that of R 2 Fe 17 is worth noticing.

Nd - Fe - (C, B) permanent magnets made by mechanical alloying and subsequent annealing

The structure, phase transformation and magnetic properties of mechanically alloyed (MA) alloys Nd16Fe76-mCm (7 less than or equal to m less than or equal to 11) and Nd16Fe84-mCm-yBy (0 less than or equal to y less than or equal to m, m = 7, 8, 9) have been studied systematically. The Nd2Fe14C compound can be formed within a wide window for the range of composition and annealing temperature. More carbon than the stoichiometric content for Nd2Fe14C is necessary to stabilize the tetragonal structure. Substitution of boron for carbon can accelerate the phase transformation from Nd2Fe17Cx to Nd2Fe14(C, B) and increase the magnetic properties drastically. There is an fee structure Nd-rich phase in the mechanically alloyed Nd-FeC alloy. The lattice constant of the Nd-rich phase decreases as the boron content increases. Nd2Fe14(C, B) compounds formed in the alloys with same boron content and different carbon contents have different Curie temperatures. The Curie temperatures also vary with the variation of annealing temperature. The best magnetic properties achieved in Nd16Fe76B5C3 alloy are H-i(c) = 1480 kA m(-1), M(r) = 0.71 T and(BH)(max) = 91.5 kJ m(-3).

Structure, phase transformation and magnetic properties of Nd-Fe-C alloys made by mechanical alloying and subsequent annealing

Abstract Structure, phase transformation and magnetic properties of mechanically alloyed (MA) Nd y Fe 100−1.5 y C 0.5 y , Nd 16 Fe 84− m C m and Nd n Fe 92− n C 8 , alloys depend sensitively on the compositions and the annealing temperatures. Nd 2 Fe 14 C can be formed by a solid state reaction of α-Fe with Nd 2 C 3 as well as by a reaction of Nd 2 Fe 17 C x with Nd–C compounds. The thermal stability of Nd 2 Fe 14 C increases with the increase of carbon content, reaches a maximum and then decreases. Nd 2 Fe 14 C is stable in a triangular shaped area in a status diagram with axes of annealing temperature and composition. To make Nd 2 Fe 14 C the main phase, the Nd:C ratio must be kept within a certain range, because too little carbon leads to Nd 2 Fe 17 C x as the main phase, and too much carbon increases the α-Fe content, inhibiting the formation of Nd 2 Fe 14 C.