Jiuxing Zhang

Enhanced thermoelectric performance of Mg2Si0.4Sn0.6 solid solutions by in nanostructures and minute Bi-doping

n-type Mg2(Si0.4Sn0.6)Bix (0 ≤ x ≤ 0.04) solid solutions with minute amounts of Bi were prepared by induction melting, melt spinning (MS), and spark plasma sintering (SPS) method, namely the non-equilibrium technique MS-SPS, using bulks of Mg, Si, Sn, Bi as raw materials; the phase components, microstructures as well as the thermoelectric properties were systematically investigated. The multiple localized nanostructures within the matrix containing nanoscale precipitates and mesoscale grains were formed, resulting in remarkably decreasing of lattice thermal conductivities, particularly for samples with the nanoscale precipitates having the size of 10–20 nm. Meanwhile, the electrical resistivity was reduced and the Seebeck coefficient was increased by Bi-doping, causing improved electrical performance for the Mg2(Si0.4Sn0.6)Bix (0 ≤ x ≤ 0.04) compounds. The dimensionless figure of merit ZT was significantly improved and the maximum value reaches 1.20 at 573 K for the Mg2(Si0.4Sn0.6)Bi0.03 sample, greatly h...

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.

A study of secondary electron emission properties of the molybdenum cathode doped with RE2O3

The secondary electron emission properties of rare-earth-doped molybdenum emitter made by aqueous solution-solid doping method, its microstructure and surface behavior are discussed in this paper. The experimental results show that adding rare-earth oxides La 2 O 3 ,Y 2 O 3 and Gd 2 O 3 into molybdenum raises the maximum secondary electron yield δ m from 1.25 of clean Mo to 2.0-3.24 of doped molybdenum when heated to 1100-1600 °C in vacuum. With increasing percentage content of doped rare-earth oxide δ m is enhanced in some extent. δ m remains nearly constant during 1000 h of test period when cathodes are operated at 1100 °C and under primary electron bombardment with bombarding energy of 1000 eV and bombarding power of 300 W/cm 2 . This demonstrates such cathodes have good anti-bombing insensitivity. A work function of about 2.7-2.8 eV is obtained from Richardson lines. SEM observations and EDS analysis show that distribution of rare-earth oxides in the cathode body is inhomogeneous and tends to aggregate at molybdenum grain boundaries. By AES analysis it is found, that atomic concentrations of La and Y on the surface are about 2.5 times higher than in the bulk for a sintered body and further increase after activation. We believe the rare-earth atoms segregated at the grain boundaries diffuse along the grain boundary during the process of cathode activation. Therefore, the surface of the cathode is covered by a certain amount of rare-earth oxide. As a result, the secondary electron yield is improved.

Crystal structure and magnetic properties of ultrafine nanocrystalline SmCo3 compound

The single-phase ultrafine nanocrystalline SmCo(3) compound with a high coercivity of 33 kOe and a Curie temperature of 925 K was prepared using a simple and efficient method, which took advantages of the concurrent processes of nanocrystallization and densification during spark plasma sintering. The crystal structure of the nanocrystalline SmCo(3) compound was constructed. As compared with the conventional microcrystalline SmCo(3) compound, a large axial ratio c/a = 4.920 and an expansion of the unit cell volume of 2.97% were obtained in the lattice structure of the nanocrystalline SmCo(3). The relationship between the magnetic properties and the nanocrystalline structure was analyzed. A specific magnetic transition from the weak ferromagnetic to the strong ferromagnetic state was discovered in the nanocrystalline SmCo(3) compound, which was considered to be related to the large anisotropic strain in the crystal lattice.

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.

Mössbauer spectroscopy study on the magnetic transition in Mn1.1Fe0.9P0.8Ge0.2

The magnetic transition has been studied in Mn1.1Fe0.9P0.8Ge0.2 by magnetic measurements and F57e Mossbauer spectroscopy. The alloy crystallizes in the hexagonal Fe2P-type structure with lattice constants of a=6.0476(4) A and c=3.4766(7) A. Both bulk magnetization measurements and Mossbauer spectroscopy show that the as-prepared sample has a significantly lower transition temperature on first cooling (TC1≈200 K) than after it has undergone thermal cycling to 20 K (TC20 K=240 K). The behavior of the material stabilizes after the first cooling/heating cycle and no further changes are observed in TC. Working with a stabilized sample, we find that the temperature dependence of the hyperfine field, Bhf(T), is more rapid than that predicted by a simple mean field Brillouin function, and in addition, Bhf(T) shows a thermal hysteresis of 10 K upon cooling versus heating. These results show that the magnetic transition at TC is definitely first order and suggest that there is an additional irreversible magnetostru...

Improvement of coercivity and corrosion resistance of Nd-Fe-B sintered magnets by doping aluminium nano-particles

Nd-Fe-B permanent magnets with a small amount of Al nano-particles doping were prepared by conventional sintered method. Effect of Al content on magnetic property, corrosion resistance and oxidation properties of the magnets were studied. Investigation showed that the coercivity rose gradually, while the remanence decreased simultaneously with increase of Al doping amount. Further investigation revealed that most Al element diffused into the main phase and some Al element diffused into the Nd-rich phase. The autoclave test results showed that the corrosion rate of the magnets decreased with Al content increasing. After oxidation, the maximum energy product losses of the magnets with 0.0 wt.% and 0.2 wt.% Al nano-particles doping were 6.13% and 3.99%, respectively. Therefore, Al nano-particles doping was a promising way to enhance the coercivity and corrosion resistance of sintered Nd-Fe-B magnet.

Effects of lanthanum substitution on microstructures and intrinsic magnetic properties of Nd-Fe-B alloy

Abstract (Nd 1– x La x ) 30 Fe 69 B ( x =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) alloys were prepared by inducting melting, and the effect of substitution of La for Nd on their microstructure and intrinsic magnetic properties were investigated. With the increase of La content, Curie temperature ( T c ) decreased from 582.4 to 557.4 K, saturation magnetization ( M s ) decreased from 1.59 to 1.37 T, and anisotropy field ( H A ) decreased from 5394 to 3911 kA/m. However, the reductions of the intrinsic magnetic properties were relatively gentle as La content increased, which meant that the intrinsic magnetic properties still had the potential to fabricate permanent magnets. Moreover, further microstructure observations showed that La tended to diffuse into the Nd-rich grain boundary phase instead of main phase during the substitute process. Such aggregation behavior was beneficial to fabricating La containing magnet with high M s .

Crystal structure and magnetic properties of SmCo6.6Nb0.4 nanoflakes prepared by surfactant-assisted ball milling

Abstract SmCo 6.6 Nb 0.4 nanoflakes with TbCu 7 structure were successfully prepared by surfactant-assisted high energy ball milling (SA-HEBM) with heptane and oleic acid as milling medium. The microstructure, crystal structure and magnetic properties were studied by scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometer, respectively. The effects of ball milling time on the c -axis crystallographic alignment and coercivity of the nanoflakes were systematically investigated. The research showed that the nanoflakes had an average thickness of 100 nm, an average diameter of 1 μm, with an aspect ratio as high as 100. As the ball milling time increased from 2 to 8 h, the reflection peaks intensity ratio I (002) / I (101) , which indicated the degree of c -axis crystal texture of the SmCo 6.6 Nb 0.4 phase, increased first, reached a peak at 4 h, and then decreased. Meanwhile, the coercivity of the nanoflakes also increased first, reached a peak at 13.86 kOe for 4 h, and then decreased.

Bulk Nanocrystalline

We prepared bulk nanocrystalline SmCo6.6Nb0.4 sintered magnet material by spark plasma sintering technique. X-ray diffraction patterns show that the magnet exhibits a stable TbCu7 structure. Transmission electron microscopy indicates that the microstructure of the magnet is composed of SmCo6.6Nb0.4 single-phase grains with an average grain size of 30 nm. Magnetic measurement shows that under a 7 T magnetic field, the coercivity of the magnet reaches as high as 2.8 T; the saturation magnetization and the remanence are 69.6 and 51.4 emu/g, respectively. The magnet exhibits good thermal stability with the coercivity of 0.48 T at 773 K, and the coercivity temperature coefficient beta of -0.169%/K.