Effects of Nb substitution for Zr on the phases,microstructure and magnetic properties of Co80Zr18−xNbxB2 melt-spun ribbons

The phases, microstructure, and magnetic properties of Co₈₀Zr_18−xNb_xB₂ (x=1, 2, 3, and 4) melt-spun ribbons were investigated. The small substitution of Nb for Zr in the Co–Zr–B melt-spun ribbons resulted in the improvement of magnetic properties, especially the coercivity. The main effect of added Nb on the coercovity of Co–Zr–Nb–B melt-spun ribbons, originated from modification of the grain size of Co₁₁Zr₂ phase. The coercivity of the Co–Zr–Nb–B melt-spun ribbons depends on the annealing temperature. The optimal magnetic properties of H_c=5.1 kOe, and (BH)_max=3.4 MGOe were obtained in the Co₈₀Zr₁₅Nb₃B₂ melt-spun ribbons annealed at 600 °C for 3 min.     

Double magnetic entropy change peaks and high refrigerant capacity in Gd1-xHoxNi compounds in the melt-spun form

Gd_1-xHo_xNi melt-spun ribbons were fabricated by a single-roller melt spinning method. All the compounds crystallize in an orthorhombic CrB-type structure. The Curie temperature (T_C) was tuned between 46 and 99 K by varying the concentration of Gd and Ho. A spin reorientation (SRO) transition is observed around 13 K. Different from T_C, the SRO transition temperature is almost invariable for all compounds. Two peaks of magnetic entropy change (ΔS_M) were found. One at the higher temperature range was originated from the paramagnet-ferromagnet phase transition and the other at the lower temperature range was caused by the SRO transition. The maximum of ΔS_M around T_C is almost same. The other maximum of ΔS_M around SRO transition, however, had significantly positive relationship with x. It reached a maximum about 8.2 J kg⁻¹ K⁻¹ for x = 0.8. Thus double large ΔS_M peaks were obtained in Gd_1-xHo_xNi melt-spun ribbons with the high Ho concentration. And the refrigerant capacity power reached a maximum of 622 J kg⁻¹ for x = 0.6. Gd_1-xHo_xNi ribbons could be good candidate for magnetic refrigerant working in the low temperature especially near the liquid nitrogen temperature range.     

FePd melt-spun ribbons and nanowires: Fabrication and magneto-structural properties

Fe-30%Pd alloys in ribbon and nanowire geometry have been prepared. Ribbon samples were produced by the melt-spinning technique in the Ar environment. FePd nanowires, having about 35 nm in diameter, 105 nm inter-nanowires distance and around 4 μm in length, were synthesized into nanoporous anodic alumina membranes as templates. Energy dispersive X-ray microanalysis of ribbons shows an average atomic composition of Fe (73.2%) and Pd (26.8%). The X-ray diffraction at RT analysis was performed on both surfaces, free and wheel side, of the melt-spun ribbon. It shows the coexistence of two phases: fct and bct FePd, but with differences between both surfaces consisting the existence of Fe oxides (Fe₂O₃ and Fe₃O₄) and a textured 200 reflection in the free side. Heating and cooling thermomagnetic curves exhibit a reversible behaviour from RT to 720 K, but around 770 K a clear irreversible transformation takes place.     

Hard magnetic properties of melt-spun Co82Zr18−xTix alloys

The phases and magnetic properties of Co-Zr-Ti melt-spun ribbons were studied by XRD analysis and magnetic measurements. The optimal magnetic properties of Ms=59.0 emu/g, Mr=4.0 kG, Hc=2.9 kOe, and (BH)_max=3.0 MGOe were obtained in Co₈₂Zr₁₄Ti₄ ribbons produced at a wheel speed of 30 m/s. In this work, we found that Ti was one of the few large atomic radius elements, which could improve hard magnetic properties of Co-Zr alloy.     

Near room temperature magnetocaloric properties of melt-spun Gd100−xBx (x=0, 5, 10, 15, and 20 at%) alloys

The magnetocaloric properties of melt-spun Gd-B alloys were examined with the aim to explore their potential application as magnetic refrigerants near room temperature. A series of Gd_100−xB_x (x=0, 5, 10, 15, and 20 at%) alloys were prepared by melt spinning. With the decrease in Gd/B ratio, Curie temperature (T_C) remains constant at ∼293 K, and saturation magnetization, at 275 K, decreases from ∼100 to ∼78 emu/g. Negligible magnetic hysteresis was observed in these alloys. The peak value of magnetic entropy change, (−ΔS_M)_max, decreased from ∼9.9 J/kg K (0-5 T) and ∼5.5 J/kg K (0-2 T) for melt-spun Gd to ∼7.7 J/kg K (0-5 T) and ∼4.0 J/kg K (0-2 T), respectively for melt-spun Gd₈₅B₁₅ and Gd₈₀B₂₀ alloys. Similarly, the refrigeration capacity (q) decreased monotonously from ∼430 J/kg (0-5 T) for melt-spun Gd to ∼330 J/kg (0-5 T) for melt-spun Gd₈₀B₂₀ alloy. The near room temperature magnetocaloric properties of melt-spun Gd_100−xB_x (0≤x≤20) alloys were found to be comparable to few first-order transition based magnetic refrigerants.     

Structure and magnetic properties of melt-spun Tb0.27Dy0.73Fex ribbons

The structure and magnetic properties of the melt-spun ribbons of Tb_0.27Dy_0.73Fe_x alloy are investigated as a function of various wheel speeds during melt-quenching using a single-roll technique. It is found that Tb_0.27Dy_0.73Fe_x alloy is difficult to be fabricated as amorphous state by using the melt-quenching method. X-ray diffractions show that all these ribbons for x=1.7−2.0 are the MgCu₂-type phase at the wheel speed of 45 m s⁻¹. For Tb_0.27Dy_0.73Fe_x alloy, the high wheel speed is beneficial to eliminate the RFe₃ phase and form the perfect MgCu₂-type phase. Compared with the bulk of Tb_0.27Dy_0.73Fe_1.95, these ribbons exhibit higher intrinsic coercivity value and their saturation magnetizations increase as well. The magnetostriction of Tb_0.27Dy_0.73Fe_1.95 composite with 4% epoxy resin is 640×10⁻⁶ at 900 kA m⁻¹.     

Phase composition, microstructures and magnetic properties of melt-spun Nd1.2Fe10.5Mo1.5 ribbons and their nitrides

The effect of wheel speed on the phase compositions and microstructures of melt-spun Nd_1.2Fe_10.5Mo_1.5 ribbon was investigated. It is found that the Nd(Fe,Mo)₁₂ phase can be obtained at the wheel speed of 10 m/s, and TbCu₇-type Nd(Fe,Mo)₇ phase is formed with the wheel speed higher than 10 m/s. The amorphous phase is achieved at 65 m/s. The average grain size of phases decreases linearly with increasing wheel speed. The Nd(Fe,Mo)₁₂N_1.0 nitride obtained from annealed ribbons quenched at 65 m/s shows a coercivity much higher than that from the ribbons quenched at 10 m/s, which is due to the smaller grain size in the former ribbons.     

Melt-spun magnetically anisotropic SmCo5 ribbons with high permanent performance

We have succeeded in preparing magnetically anisotropic SmCo₅ ribbons with high permanent performance by single-roller melt spinning at low wheel velocity. The anisotropy is associated with a crystallographic texture formed during melt-spinning process, with the c-axis parallel to the longitudinal direction of the ribbons. The formation of the crystallographic texture is attributed to a directional solidification process resulting from a thermal gradient. A remanence of 9.1 kG, remanence ratio of 0.9, intrinsic coercivity of 16.2 kOe and energy product of 18.2 MGOe at room temperature are obtained in the melt-spun and subsequently annealed SmCo₅ ribbons prepared at 5 m/s.     

Enhanced coercivity of melt-spun Sm(Co,Fe,Cu,Zr)z ribbons annealed by improved process

The effects of conventional thermal annealing (CTA), rapid thermal annealing (RTA) and rapid recurrent thermal annealing (RRTA) on coercivity of the melt-spun Sm(Co_0.6Fe_0.27Cu_0.1Zr_0.03)_7.5 ribbons were systematically studied. The results show that the annealing parameters greatly affect the coercivity of the ribbons. The optimum coercivity is 9.8, 8.9 and 10.2 kOe for the CTA-treated, RTA-treated and RRTA-treated ribbons, respectively, indicating that the coercivity is not enhanced only by elevating the heating rate. Nevertheless, the coercivity increases to 15.1 kOe for the RRTA-treated ribbons when the cooling rate decreases to 1 °C/min.     

Facile synthesis of nanostructured n-type SiGe alloys with enhanced thermoelectric performance using rapid solidification employing melt spinning followed by spark plasma sintering

SiGe alloy is widely used thermoelectric materials for high temperature thermoelectric generator applications. However, its high thermoelectric performance has been thus far realized only in alloys synthesized employing mechanical alloying techniques, which are time-consuming and employ several materials processing steps. In the current study, for the first time, we report an enhanced thermoelectric figure-of-merit (ZT) ∼ 1.1 at 900 °C in n-type Si₈₀Ge₂₀ nano-alloys, synthesized using a facile and up-scalable methodology consisting of rapid solidification at high optimized cooling rate ∼ 3.4 × 10⁷ K/s, employing melt spinning followed by spark plasma sintering of the resulting nano-crystalline melt-spun ribbons. This enhancement in ZT > 20% over its bulk counterpart, owes its origin to the nano-crystalline microstructure formed at high cooling rates, which results in crystallite size ∼7 nm leading to high density of grain boundaries, which scatter heat-carrying phonons. This abundant scattering resulted in a very low thermal conductivity ∼2.1 Wm⁻¹K⁻¹, which corresponds to ∼50% reduction over its bulk counterpart and is amongst the lowest reported thus far in n-type SiGe alloys. The synthesized samples were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy, based on which the enhancement in their thermoelectric performance has been discussed.