Wenwu Xu

Abnormal crystal structure stability of nanocrystalline Sm2Co17 permanent magnet

Abnormal crystal structure stability is discovered in the single-phase nanocrystalline Sm2Co17 permanent magnet. Three kinds of crystal structures, namely the rhombohedral Th2Zn17-type (2:17 R), the hexagonal TbCu7-type (1:7 H), and the hexagonal Th2Ni17-type (2:17 H), are claimed to exist at room temperature in the Sm2Co17 alloy system. The strong dependence of the magnetic properties on the structure characteristics in the single-phase Sm2Co17 alloy is interpreted in view of the atom space occupancy and the exchange coupling between substructures especially in the nanocrystalline alloy.Abnormal crystal structure stability is discovered in the single-phase nanocrystalline Sm2Co17 permanent magnet. Three kinds of crystal structures, namely the rhombohedral Th2Zn17-type (2:17 R), the hexagonal TbCu7-type (1:7 H), and the hexagonal Th2Ni17-type (2:17 H), are claimed to exist at room temperature in the Sm2Co17 alloy system. The strong dependence of the magnetic properties on the structure characteristics in the single-phase Sm2Co17 alloy is interpreted in view of the atom space occupancy and the exchange coupling between substructures especially in the nanocrystalline alloy.

Nanoscale thermodynamic study on phase transformation in the nanocrystalline Sm2Co17 alloy

The characteristics of phase transformation in nanocrystalline alloys were studied both theoretically and experimentally from the viewpoint of thermodynamics. With a developed thermodynamic model, the dependence of phase stability and phase transformation tendency on the temperature and the nanograin size were calculated for the nanocrystalline Sm(2)Co(17) alloy. It is thermodynamically predicted that the critical grain size for the phase transformation between hexagonal and rhombohedral nanocrystalline Sm(2)Co(17) phases increases with increasing temperature. When the grain size is reduced to below 30 nm, the hexagonal Sm(2)Co(17) phase can stay stable at room temperature, which is a stable phase only at temperatures above 1520 K in the conventional polycrystalline alloys. A series of experiments were performed to investigate the correlation between the phase constitution and the grain structure in the nanocrystalline Sm(2)Co(17) alloy with different grain size levels. The experimental results agree well with the thermodynamic predictions of the grain-size dependence of the room-temperature phase stability. It is proposed that at a given temperature the thermodynamic properties, as well as the phase stability and phase transformation behavior of the nanocrystalline alloys, are modulated by the variation of nanograin size, i.e. the grain size effects on the structure and energy state of the nanograin boundaries.

Thermodynamic study on metastable phase: From polycrystalline to nanocrystalline system

First an approach based on the Debye model is developed to quantify the thermodynamic parameters of metastable phase in the conventional coarse-grained polycrystalline systems. Subsequently, by combining the experimental measurements on heat capacity with the nanothermodynamic calculations for nanocrystalline alloys, a method is established to determine the fundamental thermodynamic functions of the metastable phases in both polycrystalline and nanocrystalline alloy systems. Taking the typical metastable-phase SmCo7 alloy as an example, the thermodynamic properties of polycrystalline and nanocrystalline systems are studied, and good agreement between model calculations and experimental results is achieved.First an approach based on the Debye model is developed to quantify the thermodynamic parameters of metastable phase in the conventional coarse-grained polycrystalline systems. Subsequently, by combining the experimental measurements on heat capacity with the nanothermodynamic calculations for nanocrystalline alloys, a method is established to determine the fundamental thermodynamic functions of the metastable phases in both polycrystalline and nanocrystalline alloy systems. Taking the typical metastable-phase SmCo7 alloy as an example, the thermodynamic properties of polycrystalline and nanocrystalline systems are studied, and good agreement between model calculations and experimental results is achieved.

Thermodynamic study on phase stability in nanocrystalline Sm–Co alloy system

To study the phase stability and the phase transformation behavior in the nanocrystalline (NC) stoichiometric alloys, a thermodynamic model has been developed in the present paper. Using the NC Sm–Co alloy as an example, the thermodynamic properties of various phases in the alloy system were evaluated systematically. In particular, the grain-size-dependence of the Gibbs free energy of each alloy phase at different temperatures was provided. Based on the model calculations, the stabilities of different phases in the NC Sm–Co system were analyzed. As distinctly different from the phase stability in the conventional polycrystalline alloys, the Gibbs free energies of some NC phases become positive at the room temperature when the nanograin size is reduced to below a certain critical value, which implies that these phases cannot stably exist at the room temperature. In order to verify the thermodynamic model, the stoichiometric Sm–Co alloy was prepared, and the grain structure and the phase constitution of the alloy were characterized by combining x-ray diffraction and transmission electron microscopy analyses. The experimental findings have confirmed the thermodynamic model predictions for the NC alloy system.

A nanocrystalline Sm–Co compound for high-temperature permanent magnets

The inherently high magnetic anisotropy and nanoscale grain size in a Sm5Co19 compound result in an intrinsic coercivity far higher than those of known Sm-Co compounds prior to orientation treatment. The combination of ultrahigh intrinsic coercivity, high Curie temperature and low coercivity temperature coefficient of nanocrystalline Sm5Co19 as a single phase material shows it to be a very promising compound to develop outstanding high-temperature permanent magnets.

MULTIPHASE EQUILIBRIUM, PHASE STABILITY AND PHASE TRANSFORMATION IN NANOCRYSTALLINE ALLOY SYSTEMS

A nanoscale thermodynamic approach is developed to quantify the multiphase equilibrium in nanocrystalline alloy systems. The nanoscale multiphase equilibrium calculations indicate that the activities and mole numbers of phases in alloy systems change diversely as a function of the nanograin size at constant temperature, which results in distinctly different characteristics of phase stability and phase transformation behavior in nanocrystalline alloys with respect to coarse-grained polycrystalline counterparts. Take the nanocrystalline Sm–Co alloy system as an example, the diverse phase stabilities and phase transformations are demonstrated. To verify the thermodynamic predictions, a series of experiments on preparation and characterization of nanocrystalline SmCo3, Sm2Co7 and SmCo2 alloys have been performed. The experimental results are in agreement with the model calculations, which validate the present thermodynamic approach for nanoscale multiphase equilibrium.

Crystal structure and magnetic performance of nanocrystalline SmCo9.8 alloy

Starting with the ingot with a nominal stoichiometric composition of SmCo9.8, the nanocrystalline bulk with ultrafine nanograin structure was prepared by a route combing ball milling and spark plasma sintering. The phase formation, microstructure, crystal lattice characteristics, and magnetic performance of the nanocrystalline SmCo9.8 alloy were investigated. The TbCu7-type solid solution of the SmCo9.8 composition was obtained, and the crystal structure of the nanocrystalline 1:9.8 H was constructed. The magnetic features were disclosed through nanostructuring to obtain the single metastable 1:9.8 H. It was found that the nanocrystalline SmCo9.8 alloy has high saturation magnetization and Curie temperature. The study presents a new understanding of the nanoscale-stabilized TbCu7-type solid solution of the SmCo9.8 composition and may promote the development of SmCo9.8-type alloys as candidates for the high-temperature permanent magnets, based on the coercivity improvement.

Nanoscale phase stability in Li ion battery anode materials

A thermodynamic model was developed in particular for nanocrystalline partially ionic solids, which represent a group of Li ion battery anode materials. The lithium compounds were used as examples to demonstrate the model applications in studies of phase stability and phase transformation behavior in the nanoscale anode system. The peritectic and eutectic transformations were described systematically concerning the reaction temperatures and liquid concentrations at various equilibria, in which the grain size effects on the equilibrium, stability and transformation of Li-containing phases were quantified. To verify the model predictions, a series of experiments were performed using the nanocrystalline Li–Si system as sample materials. The experimental finding confirmed the model calculations, based on which the correlation of phase stability, temperature, grain size and critical grain size was proposed.