[Show abstract][Hide abstract] ABSTRACT: As one of magnetic refrigerants with giant magnetocaloric effect (GMCE),
MnFePGe-based compounds had drawn tremendous attention due to their many
advantages for practical applications. In this paper, correlations among
preparation conditions, magnetic and crystal structures, and magnetocaloric
effects (MCE) of the MnFePGe-based compounds are reviewed. Structure evolution
and phase transformation in the compounds as a function of temperature,
pressure, and magnetic field were reported. Influences of preparation
conditions to the chemical composition and microstructure homogeneity of the
compounds, which play key role to the MCE and thermal hysteresis of the
compounds, were introduced. Based upon these experimental results, a new method
to evaluate MCE of the compounds via DSC measurements was proposed. Moreover,
the origin of virgin effect of the MnFePGe-based compounds was discussed.
Chinese Physics B 11/2014; 24(1). · 1.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The single-phase n-type Mg2(Si0.4−xSbxSn0.6) (0≤x≤0.025) solid solutions were prepared by an induction melting and Spark Plasma Sintering method using buck of Magnesium, Silicon, Tin and Antimony. The unique multiple nanostructures of samples containing nanoscale precipitates and mesoscale grains are formed by the non-equilibrium preparing technique, resulting in remarkably decreasing of lattice thermal conductivities, particularly for samples with the nanoscale precipitates having the size of 20–30 nm. Meanwhile, the electrical properties were increased by Sb-doping. The thermoelectric performance of Sb doped samples with the nanostructures are remarkably improved, and the dimensionless figure of ZT for Mg2(Si0.38Sb0.02Sn0.6) sample shows highest value of 1.30 at 773 K, which is very much higher than that of the non-doped sample.
[Show abstract][Hide abstract] ABSTRACT: Single-phase polycrystalline solid solutions (La1−xSmx)B6 (x = 0, 0.2, 0.4, 0.8, 1) are fabricated by spark plasma sintering (SPS). This study demonstrates a systematic investigation of structure–property relationships in Sm-doped LaB6 ternary rare-earth hexaborides. The microstructure, crystallographic orientation, electrical resistivity, and thermionic emission performance of these compounds are investigated. Analysis of the results indicates that samarium (Sm) doping has a noticeable effect on the structure and performance of lanthanum hexaboride (LaB6). The analytical investigation of the electron backscatter diffraction confirms that (La0.6Sm0.4)B6 exhibits a clear (001) texture that results in a low work function. Work functions are determined by pulsed thermionic diode measurements at 1500–1873 K. The (La0.6Sm0.4)B6 possesses improved thermionic emission properties compared to LaB6. The current density of (La0.6Sm0.4)B6 is 42.4 A cm−2 at 1873 K, which is 17.5% larger than that of LaB6. The values of ΦR for (La0.6Sm0.4)B6 and LaB6 are 1.98 ± 0.03 and 1.67 ± 0.03 eV, respectively. Furthermore, the Sm substitution of lanthanum (La) effectively increases the electrical resistivity. These results reveal that Sm doping lead to significantly enhanced thermionic emission properties of LaB6. The compound (La0.6Sm0.4)B6 appears most promising as a future emitter material.
Physica Status Solidi (A) Applications and Materials 03/2014; 211(3). · 1.53 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 10/2013; 31(10):975–978. · 1.34 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 09/2013; 31(9):885–890. · 1.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 higher than that of the non-doped samples.
[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 08/2013; 31(8):778–783. · 1.34 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 07/2013; 31(7):674–678. · 1.34 Impact Factor
[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 02/2013; 23(1):18–22. · 1.14 Impact Factor
[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). · 2.19 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. · 1.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Polycrystalline Cr-doped higher manganese silicides (HMSs) Mn1−x
Si1.80 (x = 0 to 0.03) were prepared by the in situ spark plasma sintering method. The phase structure and microstructure of the bulk samples were investigated, and their thermoelectric (TE) properties were measured from 473 K to 873 K. x-Ray diffraction patterns show that almost all of the Cr-doped samples possess single-phase HMS structure. However, minor amounts of MnSi phase can be observed in scanning electron microscopy images. The electrical conductivity increases continuously with increasing Cr substitution, and is enhanced by up to 15% for bulk Mn0.97Cr0.03Si1.80 over the entire measurement range from 473 K to 873 K. Meanwhile, the Seebeck coefficient is slightly depressed, so that the power factor is increased by about 10%. Cr doping affects the thermal conductivity negatively, and the overall dimensionless TE figure of merit of all the samples decreases slightly due to the higher thermal conductivity.
Journal of Electronic Materials 12/2011; 41(6). · 1.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The magnetocaloric effect of Gd5Si2Ge2 alloys under heat treatment conditions are investigated in low magnetic fields. The magnetocaloric effect (MCE) is studied by measuring magnetic entropy change (ΔS
M) and adiabatic temperature change (ΔT
ad) in a magnetic field of 1·5 T using a vibrating sample magnetometer (VSM) and a home-made magnetocaloric effect measuring apparatus, respectively. The maximum ΔS
M of the alloys increases by 200% from 4·38 to 13·32 J kg−1 K−1, the maximum ·T
ad increases by 105% from 1·9 to 3·9 K when compared to the as-cast due to the homogeneous composition distribution and microstructure, while the magnetic ordering temperature is slightly reduced. These results indicate that the annealed Gd5Si2Ge2 compounds are promising as high-performance magnetic refrigerants working room temperature in relatively low magnetic fields.
Bulletin of Materials Science 07/2011; 34(4). · 0.87 Impact Factor
[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 06/2011; 29(6):580-584. · 1.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The magnetocaloric properties of the as-consolidated nanocrystalline and coarse-grained gadolinium metals were studied in
the present work. With the decrease of Gd grains from micrometer to nanometer range, magnetic entropy change drops surprising
from 10.07 to 4.47Jkg−1K−1 at a magnetic-field change of 5T, and their resultant magnetic entropy change uniformly peaks at 294, 290, and 288K, respectively,
corresponding to the magnetic transition temperature of the three samples. The Curie temperature T
C of the nanocrystalline Gd shifts by more than 6K below that of coarse-grained Gd sample. However, the values of magnetic
entropy change of the nanocrystalline metals exhibit a more constant tendency compared with the coarse-grained sample. The
Arrott plots indicate the second-order character of magnetic phase transition still in the nanocrystalline Gd metals. The
refrigerant capacity calculated is also used to evaluate material refrigeration capacity.
KeywordsMagnetocaloric effect–Magnetic entropy changes (∆S)–Nanocrystalline metals
[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 05/2011; 29(5):471-473. · 1.34 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: 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 12/2010; 28:382-384. · 1.34 Impact Factor