Synthesis of Sm2Co17 alloy nanoparticles by mechanochemical processing

Sm₂Co₁₇ alloy nanoparticles of 10–250 nm in size were prepared by mechanochemical processing involving the co-reduction of Sm₂O₃ and CoO with Ca. The crystal structure of the nano-sized Sm₂Co₁₇ particles was mainly of the ordered Th₂Zn₁₇-type. When embedded in the CaO matrix the Sm₂Co₁₇ nanoparticles exhibited a high coercivity of 14.2 kOe. The CaO by-product could be removed by a carefully controlled washing process without significant oxidation of the ultrafine alloy particles. After washing, the cold-pressed powder exhibited a coercivity value of 11.8 kOe and a maximum magnetization of 92.0 emu/g under an applied field of 50 kOe.     

Effect of hot deformation on texture and magnetic properties of Sm-Co and Pr-Co alloys

Nanocrystalline PrCo₅, SmCo₅ and Sm₂(Co,Fe,Mn)₁₇ alloys were subjected to a high-degree plastic deformation at 950 °C with the height reduction ranging from 70% to 95%. With increasing degree of deformation, the PrCo₅ and SmCo₅ magnets showed improvement of the deformation-induced [0 0 1] texture. The PrCo₅ alloys, known to develop a superior texture at the lower degrees of deformation, showed only modest improvement and their magnetic performance was undermined by a low coercivity. The SmCo₅ alloys had their texture markedly enhanced and, after height reduction by 94.5%, they exhibited a remanence of 8.6 kG, maximum energy product of 18 MGOe and an intrinsic coercivity of 22.8 kOe. No induced texture was found in the alloys based on the Sm₂Co₁₇ structure. The microstructures of the hot-deformed alloys were studied with a transmission electron microscopy, and possible mechanisms of the texture development in the RCo₅ alloys (R=Pr, Sm) are briefly discussed.     

Magnetic orderings in La- and Y-substituted PrCo2Si2 compounds

The results of heat capacity (C), electrical resistivity (ρ) and magnetoresistance (Δρ/ρ) measurements on the compounds Pr_1−xLa_xCo₂Si₂ (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) and Pr_1−yY_yCo₂Si₂ (y=0.2, 1.0) are reported. The Pr sub-lattice dilution studies on the PrCo₂Si₂ compound indicate that in spite of the anomalously high antiferromagnetic ordering temperature (31 K), the RKKY interaction is responsible for magnetic ordering in this compound. The paramagnetic to antiferromagnetic transition temperature in Pr_0.8Y_0.2Co₂Si₂ is lower than that expected due to the positive chemical pressure effect. In contrast to the three magnetic transitions in PrCo₂Si₂ at 31, 17 and 9 K, there are only two magnetic transitions in Pr_0.8La_0.2Co₂Si₂ and Pr_0.8Y_0.2Co₂Si₂ above 3 K. The Δρ/ρ of PrCo₂Si₂ at 3 K is large (>20%) and positive for fields around 40 kOe. The metamagnetic transition observed in the Δρ/ρ data of Pr_1−xLa_xCo₂Si₂ and Pr_0.8Y_0.2Co₂Si₂ compounds has a systematic variation with composition.     

The influence of zirconium addition and process parameters on the magnetic properties of Pr-Fe-B sintered magnets

Sintered magnets based on the compositions Pr₁₆Fe₇₆B₈ and Pr₁₆Fe_75.5B₈Zr_0.5 were produced using the hydrogen decrepitation process. Sintered magnets prepared under specific processing conditions from the zirconium-free alloy exhibited excellent remanence (1.22 T), intrinsic coercivity (1.22 T) and energy product (278 kJm⁻³). The squareness factor of magnets prepared from the Pr₁₆Fe_75.5B₈Zr_0.5 alloy was improved considerably (0.96). This investigation also shows the remarkable influence of zirconium addition on the intrinsic coercivity of these permanent magnets.     

Structure and magnetic properties of 300-nm-thick FePt films with Hf underlayer

Structure, microstructure, magnetic properties of 300-nm-thick FePt films with 10-nm-thick Hf underlayer have been studied. The experimental results showed that the very thin Hf underlayer could promote the ordering at reduced temperatures by facilitating the nucleation of the order phase, leading to refined grain size and magnetic domain size. Therefore, the permanent magnetic properties of FePt films were enhanced. (BH)_max and H_c of FePt films were greatly enhanced from 5.0–21.0 MGOe and 1.4–11.0 kOe for single layer to 10.2–23.6 MGOe and 4.5–13.2 kOe for Hf-underlayered films annealed in T_a region of 400–600 °C, respectively. Nevertheless, the severe interdiffusion between the Hf and FePt layers at T_a=800 °C resulted in the decreased S, coarsened surface morphology, grain and magnetic domain sizes, and therefore the slightly decreased (BH)_max to 18.0 MGOe.     

Structure and electrochemical properties of Sm0.5Sr0.5Co1 − xFexO3 − δ cathodes for solid oxide fuel cells

Crystal structure, thermal expansion coefficient, electrical conductivity and cathodic polarization of compositions in the system Sm_0.5Sr_0.5Co_1 − xFe_xO_3 − δ with 0 ≤ x ≤ 0.9 were studied as function of Co / Fe ratio and temperature, in air. Two phases, including an Orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9, were observed in samples of Sm_0.5Sr_0.5Co_1 − xFe_xO_3 − δ at room temperature. The adjustment of thermal expansion coefficient (TEC) to electrolyte, which is one of the main problems of SSC, could be achieved to lower TEC values with more Fe substitution. High electrical conductivity above 100 S/cm at 800 °C was obtained for all specimens, so they could be good conductors as cathodes of IT-SOFC. The polarization behavior of SSCF as a function of Fe content was evaluated by means of AC impedance using LSGM electrolyte. It was discovered that the Area Specific Resistance (ASR) of SSCF increased as the amount of substitution of Fe for Co increased. When the amount of Fe reached to 0.4, the highest ASR was obtained and then the resistance started decreasing above that. The electrode with a composition of Sm_0.5Sr_0.5Co_0.2Fe_0.8O_3 − δ showed high catalytic activity for oxygen reduction operating at temperature ranging from 700 to 800 °C.     

Effect of Cu and Ga additions on the anisotropy of R2Fe14B/α-Fe nanocomposite die-upset magnets (R=Pr,Nd)

Fully dense nanocomposite magnets containing hard R₂Fe₁₄B and soft α-Fe phases were produced from both melt-spun and mechanically milled alloys by hot pressing and subsequent die upsetting. Although R-lean R–Fe–B alloys that do not contain the grain-boundary R-rich phase are known not to be susceptible to texture development by means of die upsetting, we found that small additions of Cu make the texturing possible. The resulting microstructure of oriented platelet grains is similar to that of the R-rich die-upset magnets. Properties of the Cu-containing R₂Fe₁₄B/α-Fe die-upset magnets can be further improved by adding Ga. The anisotropic Pr₁₂Fe₈₀Cu₁Ga₁B₆ magnet made from mechanically milled alloy and containing 17.2 wt% α-Fe had a remanence of 13 kG and a maximum energy product of 23.4 MG Oe. The Pr_11.25Fe_80.75Cu₁Ga₁B₆ magnet made from melt-spun alloy and containing 16.2 wt% α-Fe had a maximum energy product of 19.9 MG Oe. The low coercivity of 3–4 kOe typical for the Cu-containing R₂Fe₁₄B/α-Fe die-upset magnets is due to the relatively coarse α-Fe grains. The latter grains are too large for intergranular exchange interaction, but, nevertheless, they are well coupled with the R₂Fe₁₄B grains by a long-range magnetostatic interaction.     

Developments in the processing and properties of NdFeb-type permanent magnets

The composition, microstructure and processing of NdFeB-type permanent magnets are all critical factors for the successful production of high performance magnet components. Three common fabrication routes can be used to categorize these NdFeB-based bulk magnets: sintering, polymer bonding and hot deformation. Generally, the former type of magnet has a high-energy product (30–50 MGOe), full density and a relatively simple shape. Bonded magnets have intermediate energy products (10–18 MGOe), lower density and can be formed into intricate net-shapes. Hot deformed magnets possess full density, intermediate to high-energy products (15–46 MGOe), isotropic or anisotropic properties and have the potential to be formed into net shapes. This article discusses the critical issues of improved magnetic performance, environmental stability, net-shape formability and magnetization behavior for the main categories of NdFeB magnets.     

High coercivity Nd2(Fe,Co)14C-type ribbons prepared by melt spinning

Melt-spun Nd₁₃Dy₂Fe_77−xCo_xC₆B₂ (x=0, 5, 10, 15, 20) ribbons with a high coercivity more than 2 T have been obtained. It was found that the ribbons quenched at the optimum wheel speed 15 m/s (as-spun ribbons) mainly consist of ferromagnetic 2 : 14 : 1 phase and paramagnetic NdC₂ phase, and the ribbons spun at 25 m/s and subsequently annealed at 973 K for 15 min (as-annealed ribbons) are primarily composed of the magnetic 2 : 14 : 1 and 2 : 17 phases. The magnetization process of as-spun ribbons controlled by a pinning of the domain wall is different from that of as-annealed ribbons determined by a nucleation of the reverse domain. This significant difference originates possibly from the existence of paramagnetic NdC₂ phase acting as a pinning center in as-spun ribbons. In the as-annealed ribbons, the substitution of Co for Fe leads to increase of remanence (μ₀M_r), maximum energy product ((BH)_max) from 0.67 T, 9.7 MGOe for x=0 to 0.84 T, 14.4 MGOe for x=10, respectively. A coercivity of 2.74 T is obtained for as-quenched Nd₁₃Dy₂Fe_77−xCo_xC₆B₂ (x=0) ribbons.     

Microstructure refinement and significant improvements of magnetic properties in Pr2Fe14B/α-Fe nanocomposites

Exchange coupled (Pr,Tb)₂(Fe,Nb,Zr)₁₄B/α-Fe nanocomposites have been produced by melt spinning. A trend for perpendicular and planar c-axis orientation of the 2:14:1 phase was observed in the free surface of ribbons spun at speeds below 10 m/s and at optimal speeds, respectively. Higher wheel speeds led to the formation of an amorphous phase that transformed to 2:14:1 phase around 680°C. Optimum magnetic properties were found in samples spun at 14–17 m/s and annealed at 700°C for 20 min. The loop squareness was also found to depend mainly on the microstructure that is very sensitive to the sample composition. A few percentage of Nb and Zr suppressed the grain growth, resulting in a drastic improvement of magnetic properties, with approximate 50% enhancement in the intrinsic coercivity and an increase in maximum energy product from 5.6 kOe and 14.7 MGOe for the (Nb,Zr)-free sample to 8.2 kOe and 20.3 MGOe for the (Nb,Zr)-substituted samples, respectively. The significant improvement in magnetic properties originated from a much finer and homogeneous nanocomposite microstructure with an average grain size of 20 nm, leading to a high remanence of 0.73 M_s. Henkel plots indicate the enhancement of exchange coupling between hard and soft magnetic phases.     

High-temperature oxidation resistance and magnetic properties of Si-doped Sm2Co17-type magnets at 500 °C

The high-temperature oxidation resistance and magnetic properties of Si-doped Sm₂Co₁₇-type magnets at 500 °C were systematically investigated. The Sm(Co_0.76, Fe_0.1, Cu_0.1, Zr_0.04)₇Si_x (x=0–0.6) magnets were prepared by the conventional powder metallurgical technique. It is found that the addition of silicon in the Sm₂Co₁₇-type magnet can remarkably improve its oxidation resistance. Moreover, a small amount of silicon addition can also increase its high-temperature intrinsic coercivity. A maximum intrinsic coercivity of 6.7 kOe at 500 °C was obtained for the Sm₂Co₁₇-type magnet with Si content x=0.4, whose high-temperature maximum energy product loss was about 2.5 times smaller than pure Sm₂Co₁₇-type magnet after oxidation at 500 °C for 100 h, indicating the enhanced oxidation resistance. Its corresponding Curie temperature and saturation magnetization are about 723.9 °C and 7.4 kG, respectively.     

The magnetic properties,thermal stability and microstructure of Nd–Fe–B/Ga sintered magnets prepared by blending method

The effect of gallium added by blending method on the magnetic properties, thermal stability and microstructure of Nd_16.5Dy_16.0Fe_53.45Co_13.0B_1.05 (wt%) sintered magnets was investigated. The experimental results show that an appropriate Ga addition can markedly increase the coercivity, reduce the irreversible loss and slightly enhance the remanence. For instance, by adding 0.5 wt% Ga, the coercivity is increased from 1232 to 1819 kA/m; the irreversible loss after being exposed at 200°C for 0.5 h is reduced from above 33% to below 5%. Microstructure analyses show that the grain boundaries of the magnets with and without Ga addition are substantially different. The grain boundaries of the Ga-free magnet are meandrous. On the other hand, most of the boundaries of Ga-containing magnets are straight and smooth. These characteristics can be explained by the appearance of new phases during sintering process.     

Mixed conductivity and oxygen permeability of doped Pr2NiO4-based oxide

Pr₂NiO₄-based oxide was studied as a new mixed electronic and oxide ionic conductor for the oxygen permeation membrane. It was found that Pr₂NiO₄ doped with Cu and Fe for Ni site exhibits the relatively high oxygen permeation rate. Doping second cation to Ni site is effective for improving the oxygen permeation rate and the trivalent cation seems to be effective for increasing the oxygen permeation rate. Among the examined cation, the highest oxygen permeation rate was obtained by doping 5 mol% Fe. The oxygen permeation rate was also significantly affected by the surface catalyst and the highest oxygen permeation rate of 80 μmol·min^− 1·cm^− 2 at 1273 K was achieved by using La_0.1Sr_0.9Co_0.9Fe_0.1O₃ for the surface catalyst. Since the electrical conductivity slightly decreased with decreasing P_O2 and it dropped significantly at P_O2 = 10^− 19 atm, chemical stability of Pr₂NiO₄-based oxide seems to be reasonably high. Application of this new mixed conductor for the oxygen permeation membrane under the CH₄ partial oxidation was also studied and it was confirmed that the oxygen permeation rate much improved under the CH₄ oxidation condition and this Pr₂NiO₄ can be used for the oxygen permeation membrane for the CH₄ partial oxidation.     

Resonant energy transfer and cross relaxation between Sm3+ ions in LiYF4 crystals

Luminescence decay curves of Sm³⁺ ions in LiYF₄ crystals doped with 1, 5 and 10 mol% Sm³⁺ are multi-exponential, whereas that in a LiYF₄ crystal doped with 0.1 mol% Sm³⁺ is well approximated by a single exponential function with a decay time of 4.8 ms. The average luminescence decay times decrease from 4.8 to 0.60 ms with the increasing Sm³⁺ concentrations between 1 and 10 mol%. The decay curves for all crystals are found to be almost independent of the temperature between 15 and 300 K. The decrease of the decay times for the higher Sm³⁺ concentrations indicates energy transfer between two Sm³⁺ ions. Taking the crystal structure of LiYF₄ into account, it is deduced that a single-step energy transfer process for the 1 and 5 mol% Sm³⁺ concentrations occurs from a Sm³⁺ ion at the origin of (0 0 0) to one of the Sm³⁺ ions substituting for the first nearest neighbor Y³⁺ sites and beyond within a sphere with an approximate radius of less than 0.7 nm. On the other hand, a multi-step energy transfer process dominates for the highest concentration (10 mol%) because the calculated average distance between two Sm³⁺ ions in the 10 mol% Sm³⁺ sample is comparable with the migration length of the single-step energy transfer process estimated from the 1 and 5 mol% Sm³⁺ samples.     

Microstructures and soft magnetic properties of as-deposited Fe–Sm–O thin films

The influences of O₂ partial pressure on saturation magnetization, coercivity and effective permeability of the as-deposited Fe–Sm–O thin films, which were fabricated by RF magnetron reactive sputtering method, were investigated. The nanocrystalline Fe_83.4Sm_3.4O_13.2 thin film fabricated at O₂ partial pressure of 5% exhibited the best magnetic softness with a saturation magnetization of 1.43 MA/m, coercivity of 65.2 A/m and effective permeability of about 2600 in the frequency range from 0.5 to 100 MHz. The electrical resistivity of Fe_83.4Sm_3.4O_13.2 was 130 μΩ cm. The microstructures and electrical resistivity were investigated in this work.     

Corrosion behaviour of hot-pressed and die-upset nanocrystalline NdFeB-based magnets

Isotropic and anisotropic nanocrystalline Nd₁₄Fe₈₀B₆ and Nd₁₂Dy₂Fe_73.2Co_6.6Ga_0.6B_5.6 magnets have been produced from melt-spun materials by hot pressing and subsequent die-upsetting. The microstructure has been characterized using XRD, scanning electron microscope and energy dispersive X-ray analysis. The corrosion behaviour of die-upset NdFeB-based magnets has been studied in 0.1 M H₂SO₄ by inductively coupled plasma solution analysis and electrochemical polarization techniques and compared with their hot-pressed counterparts. Texturing of hot-pressed (isotropic) NdFeB-based magnets via die-upsetting significantly modifies their corrosion performance. Textured Nd₁₂Dy₂Fe_73.2Co_6.6Ga_0.6B_5.6 magnets exhibit the highest corrosion resistance in this study. The low effective diffusivity of corrosion hydrogen inside the bulk magnet and the reduction in the strength of galvanic coupling between magnet phases are the main reasons for the observed improvement in the corrosion resistance. The corrosion behaviour of the magnets in relation to their phase composition and phase distribution is discussed in terms of dissolution, hydrogenation and pulverization. Pulverization trends are correlated with hydrides formation and hydrogen-trapping sites using thermal desorption analysis.     

The production and characterisation of highly anisotropic PrFeCoB-type HDDR powders

A study of the processing of highly anisotropic HDDR powder and PTFE-bonded magnets with the composition Pr_13.7Fe_63.5Co_16.7B₆M_0.1 (M=Zr or Nb) has been undertaken. These alloys were processed by an homogenising heat treatment for a range of times, and a subsequent HDDR treatment employing a range of disproportionation times. The optimum time for the homogenisation of the as-cast structure was found to be 20 h at 1100°C, while the optimum disproportionation time in the HDDR treatment was found to be 10 min at 860°C. Zr-additions appear to inhibit grain growth during the heat treatment process, whereas Nb-additions appeared to control more effectively the grain growth during the disproportionation and recombination stages of the HDDR process.     

Synthesis and luminescence of La2BaZnO5 phosphors

A modified synthesis of La₂BaZnO₅ phosphors activated with rare earths Eu³⁺, Tb³⁺, Pr³⁺ and Sm³⁺, and ns² ion Bi³⁺ is reported. RE₂BaZnO₅ compounds are conventionally prepared by two step solid state reaction. In the first step, carbonates or similar precursors are intimately mixed and heated at 900 °C to decompose the precursors to oxides. To eliminate the unwanted phases like BaRE₂O₄, the resulting powders are reheated at 1100 °C for long time. We prepared La₂BaZnO₅ phosphors activated with various activators by replacing the first step by combustion synthesis. Results on photoluminescence are presented. PL results on Eu³⁺ and Tb³⁺ are in good agreement with the literature reports. PL emission from Sm³⁺, Pr³⁺ and Bi³⁺ had not been reported earlier. Excitation spectrum of Eu³⁺ is dominated by a charge transfer band around 318 nm, while for the other rare earths a band at 240 nm is always present. This is attributed to the host absorption.     

Effect of Boron substitution on the superconductivity of non-oxide perovskite MgCNi3

We report synthesis, structural and magnetic (DC and AC) properties of Boron substituted MgCNi₃ superconductor. A series of polycrystalline bulk samples Mg_1.2C_1.6?xB_xNi₃ (x=0.0, 0.08 and 0.16) is synthesized through standard solid-state reaction route, which are found to crystallize in cubic perovskite structure with space group Pm3m. Rietveld analysis of observed XRD data show that lattice parameters expand from a=3.8106 (4) Å for pure, to 3.8164 (2) Å and 3.8173 (5) Å for 5% and 10% Boron substituted samples respectively. DC magnetization exhibited superconducting transition (T_c) at around 7.3 K for pure sample, and the same decreases slightly with Boron substitution. The lower critical field (H_c1) at 2 K is around 150 Oe for pure sample, which increases slightly with Boron substitution. For pure sample the upper critical field (H_c2) being determined from AC susceptibility measurements is 11.6 kOe and 91.70 kOe with 50% and 90% diamagnetism criteria respectively, which decreases to 5.57 kOe and 42.5 kOe respectively for 10% Boron substituted sample. 10% Boron substitution at Carbon site has decreased both the H_c2 and T_c. On the other hand lower critical field (H_c1) at 2 K is slightly increased from around 150 Oe for pure sample, to 200 Oe for 10% Boron substituted sample. Seemingly, the Carbon site Boron substitution induced disorder though has increased slightly the H_c1 but with simultaneous decrease in superconducting transition temperature (T_c) and upper critical field (H_c2). The high relative proportion of Ni in studied MgCNi₃ suggests that magnetic interactions are important and non-oxide perovskite structure make it interesting.     

Effect of the substitution of Pr for Nd on microstructure and magnetic properties of nanocomposite Nd2Fe14B/α-Fe magnets

Microstructure and magnetic properties of melt-spun nanocomposite magnets with nominal compositions of (Nd_1−xPr_x)₉Fe₈₆B₅ (x=0–1) were investigated. Substitution of Nd by Pr could significantly improve the hard magnetic properties of the nanocomposite magnets; the intrinsic coercivity (_iH_c) and the maximum magnetic energy product ((BH)_max) increase from 414 kA/m and 124 kJ/m³ for x=0 to 493 kA/m and 152 kJ/m³ for x=0.6, respectively. Further substituting Nd by Pr (x>0.6) strongly weakens exchange-coupling interaction between magnetically hard and soft phases.