[show abstract][hide abstract] ABSTRACT: The inherently high magnetic anisotropy and nanoscale grain size in a SmCo 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 SmCo as a single phase material shows it to be a very promising compound to develop outstanding high-temperature permanent magnets.
[show abstract][hide abstract] ABSTRACT: The electrical resistivity of the as-consolidated and coarse-grained bulk gadolinium (Gd) metals was studied in the temperature range of 3–315 K. The experimental results showed that with decrease in the grain size of Gd grains from micrometer to nanometer range, the room temperature electrical resistivity increased from 209.7 to 333.0 μΩ cm, while the electrical resistivity at the low temperature of 3 K was found to increase surprisingly from 16.5 to 126.3 μΩ cm. The room temperature coefficient resistivity (TCR) values were obtained as 39.2×10–3, 5.51×10–3 and 33.7×10–3 K−1. The ratios of room temperature to residual resistivity [RRR=ρ(300 K)/ρ(3 K)] are 2.64, 11.0, respectively, for the as-consolidated samples at 280 °C and 700 °C with respect to that of the coarse-grained sample. All results indicate the remarkable influence of the nanostructure on the electrical resistivity of Gd due to the finite size effect and large fraction of grain boundaries.
Progress in Natural Science: Materials International. 02/2013; 23(1):18–22.
[show abstract][hide abstract] ABSTRACT: The compound Mn1.1Fe0.9P0.76Ge0.24 has been studied using neutron powder diffraction (NPD), differential scanning calorimeter (DSC), and magnetic measurements, in order to clarify the nature of the magnetic and structural transition and measure the associated entropy change (ΔS). The strongly first order transition occurs from a paramagnetic (PM) to a ferromagnetic (FM) phase and can be induced either by temperature or by an applied magnetic field. Our investigations indicate that the two processes exhibit identical evolutions regarding the crystal and magnetic structures, indicating they should have the same entropy change. We, therefore, conclude that the ΔSDSC obtained by the DSC method (where the transition is temperature induced) is valid also for the magnetically induced transition, thus avoiding uncertainties connected with the magnetic measurements. We have obtained the ΔSDSC = 33.8 J/kg · K for this sample upon cooling, which would increase to 42.7 J/kg · K for a impurity-free and completely homogeneous sample. For comparison, the magnetic entropy changes (ΔSM) induced by magnetic field and calculated using the Maxwell relation yields a ΔSM = 46.5J/kg · K, 38% higher than ΔSDSC. These entropy results are compared and discussed.
Journal of Applied Physics 01/2013; 113(4). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The rare earth Pr doped Ca1−xPrxMnO3 (x=0, 0.06, 0.08, 0.1, 0.12, and 0.14) compound bulk samples were prepared to study the effect of Pr doping on thermoelectric transport properties of CaMnO3 compound system. The doped samples exhibited single phase composition within the experimental doping range, with condensed bulk microstructure and small porosities. The electrical resistivity was remarkably reduced for doped samples, on account of the enhanced carrier concentration; the absolute value of Seebeck coefficient was deteriorated mainly due to enhanced electron carrier concentration. The electrical performances of the doped samples reflected by resistivity and Seebeck coefficient fluctuations were optimistically tuned, with an optimized power factor value of 0.342 mW/(m·K2) at 873 K for x=0.08 sample, which was very much higher comparing with that of the un-doped sample. The lattice thermal conduction was really confined, leading to distinctly repressed total thermal conductivity. The thermoelectric performance was noticeably improved by Pr doping and the dimensionless figure of merit ZT for the Ca0.92Pr0.08MnO3 compound was favorably optimized with the maximum value 0.16 at 873 K.
Journal of Rare Earths 01/2013; 31(9):885–890. · 1.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: Structure and magnetic properties were studied for bulk nanocrystalline Nd-Fe-B permanent magnets that were prepared at 650 °C for 3 min under 300 MPa using the SPS-3.20-MK-V sintering machine and the hot pressed magnets were then submitted to hot deformation with height reduction of 50%, 60%, 70%, 80%, and 85%. Effects of height reduction (HR) and deformation temperature on the structure and magnetic properties of the magnets were investigated. The crystal structure was evaluated by means of X-ray diffraction (XRD) and the microstructure was observed by transmission electron microscopy (TEM). The magnetic properties of the magnets were investigated by vibrating sample magnetometer (VSM). As the height reduction increased, the remanence (Br) of the magnets increased first, peaks at 1.3 T with HR=60%, then decreased again, and the coercivity (Hci) of the magnets decreased monotonically. On the other hand, as the deformation temperature increased, the Br of the magnets increased first, peaks at 1.36 T with HR=60%, then decreased again, and the Hci of the magnets decreased monotonically. Under optimal conditions, the hot deformed magnet possessed excellent magnetic properties as Br=1.36 T, Hci=1143 kA/m, and (BH)max=370 kJ/m3, suggesting the good potential of the magnets in practical applications.
Journal of Rare Earths 01/2013; 31(7):674–678. · 1.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Ba-, La- and Ag-doped polycrystalline Ca2.9M0.1Co4O9 (M=Ca, Ba, La, Ag) thermoelectric bulk samples were prepared via citrate acid sol-gel synthesis method followed by spark plasma sintering technique. The bulk samples were characterized and analyzed with regard to their phase compositions, grain orientations as well as microstructures. The high temperature thermoelectric transport properties of the bulk samples were studied in detail. All bulk samples were found to be single-phased with modified body texture. The electrical resistivity was modulated as a result of carrier concentration modification, however the carrier transport process was not influenced; the Seebeck coefficient was deteriorated simultaneously. The total thermal conductivity was remarkably reduced, on account of the decreasing of phonon thermal conductivity. The thermoelectric properties of the Ba-, La-, and Ag-doped bulk samples were optimized, and the Ba-doped Ca2.9Ba0.1Co4O9 system was found to have the highest dimensionless figure of merit ZT 0.20 at 973 K, which was remarkably higher than that of the un-doped sample.
Journal of Rare Earths 01/2013; 31(8):778–783. · 1.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
Journal of Rare Earths 01/2013; 31(1):65–68. · 1.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: SmCo6.6Nb0.4 nanoflakes with TbCu7 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 SmCo6.6Nb0.4 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.
Journal of Rare Earths 01/2013; 31(10):975–978. · 1.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: Bulk nanocrystalline Erbium metals were prepared via Spark Plasma Sintering (SPS) and subsequent annealing process. The nanocrystalline Er metals have the same hexagonal close packed structure as that of coarse-grained sample. Decrease in grain size results in remarkable changes in the three magnetic ordering temperatures of the nanocrystalline Er metal. At 5 K, the magnetization drops by 10.9%, while the coercivity increases by 4 times for nanocrystalline Er compared with those of coarse-grained sample. These results indicate the remarkable influence of the nanostructure on the magnetism of Er due to finite size effect.
[show abstract][hide abstract] ABSTRACT: The full densification polycrystalline cerium hexaboride (CeB6) cathode material was prepared by using the spark plasma sintering (SPS) method in an oxygen free system. The starting precursor nanopowders with an average grain size of 50 nm were prepared by high-energy ball milling. The ball-milled nanopowder was fully densified at 1550 °C under 50 MPa, which was about 350 °C lower than the conventional hot-pressing method and it was also lower than that of coarse powder under the same sintering condition. The mechanical properties of nanopowder sintered samples were significantly better than that of coarse powder, e.g., the flexural strength and Vickers hardness were 211% and 51% higher than that of coarse powder, respectively. The electron backscattered diffraction (EBSD) result showed that the (100) fiber texture could be fabricated by the ball-milled nanopowder sintered at 1550 °C and the thermionic emission current density was measured to be 16.04 A/cm2 at a cathode temperature of 1873 K.
Journal of Rare Earths 01/2011; 29(6):580-584. · 1.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: Crystallographic alignment and magnetic anisotropy were studied for NdxFe94-xB6 (x=8, 9, 10, 11) ribbons prepared via melt-spinning. Effect of Nd content and wheel speed on the crystal structure and magnetic properties of the ribbons was investigated. Both the free and wheel side of the ribbons could obtain strong c-axis crystal texture of Nd2Fe14B phase perpendicular to the ribbons surface at low wheel speed, but the texture weakened gradually with the increase of the wheel speed. Increase of Nd content led to better formation of crystal texture in the ribbons, indicating that the α-Fe phase might undermine the formation of crystal texture. Magnetic measurement results showed that the magnetic anisotropy of the ribbons exhibited corresponding behavior with the invariance of the c-axis crystal texture of Nd2Fe14B phase in the ribbons, and the coercivity of the ribbons rose with the increase of both Nd content and wheel speed during melt-spun process.
Journal of Rare Earths - J RARE EARTH. 01/2011; 29(5):471-473.
[show abstract][hide abstract] ABSTRACT: In this paper, we report on the structure and magnetic properties of bulk nanostructured Mn100-xBix (x = 40, 45, and 52) permanent magnets prepared by spark plasma sintering (SPS) technique. Effect of Mn/Bi ratio on the MnBi low temperature phase (LTP) formation and magnetic properties of the magnets was investigated. Increase of bismuth amount in the synthesized magnets leads to better formation of LTP, resulting in the improvement of both magnetization (at 2 Tesla) and ramanence, but reducing the coercivity of the magnets. Ms increases from 27.87 emu/g for Mn60Bi40 to 45.31 emu/g for Mn48Bi52, while the coercivity decreases from 10.5 kOe to 7.87 kOe at room temperature. For the Mn48Bi52 magnet, TEM observation shows that its microstructure is composed of fine and uniform grains with an average size of 140 nm. The density of the magnet is 8.7g/cm3, which is over 93% of its theoretical density. Further magnetic measurement at 423 K shows that the Mn60Bi40 magnet possesses a high coercivity of 19 kOe, indicating a strong positive temperature coefficient of coercivity of the bulk nanostructured MnBi permanent magnets.
[show abstract][hide abstract] ABSTRACT: The paper is focused on understanding the densification characteristics of WC–Co composite powders by the spark plasma sintering (SPS) technique, especially the essential mechanisms for the distinct differences of the effects of WC particle size on the densification behavior between the SPS and the conventional sintering technologies. For the particular combination and contacting state between WC and Co after ball milling in which Co forms thin films coating the WC particles, a model that quantitatively describes the densification process of SPS WC–Co powders with different WC particle sizes has been developed. The calculated results show that both the actual temperature in the Co film and the melting temperature of the Co film increase with the increase of the WC particle size. As a result, the formation and growth of the sintering necks due to the rapid melting and solidification of the Co films turn to be weakly influenced by the WC particle size, and hence the SPS densification is almost independent of the WC particle size. The model calculations are consistent with the experimental findings that in the SPS processes the temperatures corresponding to the start of the densification and the peak of the displacement rate, respectively, are nearly the same for the WC–Co powders with different WC particle sizes.
Journal of the American Ceramic Society 06/2010; 93(10):3153 - 3158. · 2.11 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: Ternary Tb-Fe-B ribbons were prepared via melt-spun technique under different wheel speeds of 5-25 m/s. Effect of wheel speed on the crystal structure and microstructure of the ribbons was investigated. All the ribbons quenched under different wheel speeds crystallized in single Tb2Fe14B phase with tetragon structure. Different crystallographic alignment evolutions were observed in the free side surface and wheel side surface of the ribbons. On the free-side surface, an in-plane c-axis crystal texture of Tb2Fe14B phase was found in the ribbons quenched at 5 m/s. However, with the increase in the wheel speed, the direction of the c-axis texture turns to perpendicular to the ribbon surface. On the wheel-side surface, a strong c-axis texture perpendicular to the ribbon surface was observed in the ribbons quenched at 5 m/s, and then weakened gradually with the increase in the wheel speed. Further investigation showed that the competition of the two types of temperature gradients during the quench process was responsible for the crystallographic alignment evolution in the ribbons.
Journal of Rare Earths - J RARE EARTH. 01/2010; 28:382-384.
[show abstract][hide abstract] ABSTRACT: Nanostructured polycrystalline LaB6 ceramics were prepared by the reactive spark plasma sintering method, using boron nanopowders and LaH2 powders with a particle size of about 30nm synthesized by hydrogen dc arc plasma. The reaction mechanism of sintering, crystal structure, microstructure, grain orientations and properties of the materials were investigated using differential scanning calorimetry, X-ray diffraction, Neutron powder diffraction, Raman spectroscopy, transmission electron microscopy and electron backscattered diffraction. It is shown that nanostructured dense LaB6 with a fibrous texture can be fabricated by SPS at a pressure of 80MPa and temperature of 1300°C for 5min. Compared with the coarse polycrystalline LaB6 prepared by traditional methods, the nanostructured LaB6 bulk possesses both higher mechanical and higher thermionic emission properties. The Vickers hardness was 22.3GPa, the flexural strength was 271.2MPa and the maximum emission current density was 56.81Acm−2 at a cathode temperature of 1600°C.
[show abstract][hide abstract] ABSTRACT: A novel reaction in-situ synthesis technique was developed for preparing pure bulk polycrystalline Gd dihydride materials by combination of the hydrogen plasma metal reaction (HPMR) and spark plasma sintering (SPS). The nanoparticles of Gd hydrides were firstly prepared by HPMR. SPS process was then introduced to synthesize the bulk GdH2 materials from the nanoparticles. The pure consolidated bulk GdH2 materials were achieved after sintering at temperature 750–900°C with a pressure of 50MPa in a vacuum atmosphere. The GdH2 phase with face centered cubic (FCC) and GdH3 phase with hexagonal condensed packed (HCP) exist in the nanoparticles, and the consolidated bulk, however, is a single phase GdH2 with FCC. Subsequent annealing process can make the bulk GdH2 material transform back to pure HCP Gd. Microstructure analysis reveals that the consolidated bulk material exhibits a good microstructure. These results indicate that the HPMR–SPS process is an effective method for the synthesizing bulk GdH2 materials.
[show abstract][hide abstract] ABSTRACT: The single-phase Sm2Co17 alloys with amorphous and nanocrystalline structures were prepared separately by a simplified route combining high-energy ball milling and spark plasma sintering. The microstructure evolution, as well as the phase transformation, were investigated for the prepared Sm2Co17 alloys. It was found that the nanocrystalline Sm2Co17 alloy has a stable hexagonal crystal structure at the room temperature, which shows different phase stability from the conventional polycrystalline Sm2Co17 alloys. In contrast with the coarse-grained Sm2Co17 ingot that shows very weak permanent magnetic properties, the amorphous and nanocrystalline Sm2Co17 alloys exhibit apparently improved magnetic properties. The first-principles calculations show that a much higher spin polarization is achieved in the nanocrystalline Sm2Co17, as compared with the coarse-grained polycrystalline counterpart. Due to the intense spin polarization at the Fermi level, the exchange interaction between Sm(4f) and Co(3d) electrons is enhanced, which results in the strong magnetism in the nanocrystalline Sm2Co17 alloy.
[show abstract][hide abstract] ABSTRACT: A novel route for the preparation of the single-phased Sm2Co17 nanocrystalline bulk with ultrafine grain sizes was proposed. It was found that the nanocrystalline Sm20Co17 has a hexagonal crystal structure at the room temperature, which shows a different thermal stability from the conventional polycrystalline alloy. The intrinsic coercivity of the nanocrystalline Sm2Co17 with a hexagonal crystal structure was greatly increased as compared with the single-phased polycrystalline alloy with a rhombohedral structure. The microhardness and the elastic modulus of the nanocrystalline Sm2Co17 bulk were increased as high as 1.8 and 2.6 times, respectively, when compared with the polycrystalline parent alloy.
Journal of Nanoscience and Nanotechnology 09/2009; 9(9):5141-4. · 1.15 Impact Factor