Journal of Rare Earths最新文献

A novel microstructure of magnetically anisotropic SmCo-based magnet with high-performance is reported. The magnet consists of SmCo₇-H (TbCu₇-hexagonal type), Sm₂Co₁₇–H (Th₂Ni₁₇-hexagonal type) and SmCo₃-R (SmCo₃-rhombohedral type) phases. The maximum magnetic energy product of the magnet is 231.69 kJ/m³, and the intrinsic coercivity is 1005.47 kA/m. An outstanding intrinsic coercivity temperature coefficient (β) of −0.125%/K between 298 and 773 K is obtained, which is very close to the β of commercial high-temperature Sm₂Co₁₇-based sintered magnets. The initial magnetization curve indicates that the coercivity mechanism is controlled by a domain wall pinning mechanism. The SmCo₃-R lamellar phase may be a potential pinning center or self-pinning center. The microstructure of the magnet is different from that of any previous SmCo-based magnets. These findings provide a new idea for preparing high-performance SmCo-based permanent magnets.

Highly pure red phosphors LiM(PO₃)₃:Eu³⁺ (M = Sr, Ca) doped with Eu³⁺ (1 mol%) were synthesized via solution combustion method and their crystal structure and luminescence dynamics were studied to explore its suitability in white light emitting diodes. The Rietveld refinement analysis of the powder X-ray diffraction patterns reveals that the phosphors belong to the pure triclinic phase of LiSr(PO₃)₃ and LiCa(PO₃) with space group P-$\overline{1}$ (2). The scanning electron microscopy images showed the agglomerated morphology. The photoluminescence emission spectra under 393 nm show an orange band at 594 nm and a red band at 613 nm ascribed to ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ transitions of Eu³⁺ ion in both the phosphors. Moreover, the spectroscopic properties such as luminescence behaviour, and Stark splitting were used to examine the symmetry of Eu³⁺ ions in LiM(PO₃)₃:Eu³⁺ (M = Sr, Ca) phosphors in terms of distortion induced upon doping. The Stark splitting shows that the actual site symmetry for Eu³⁺ ion was estimated to be D₂ type for both phosphors. The photometric properties of LiCa(PO₃)₃:Eu³⁺ such as Commission International de l’Eclairage coordinates (x = 0.64, y = 0.36) near to the standard one (red), high color purity (95%) and higher brightness reveal that the phosphor has the capability of acting as a red component in n-UV white light emitting diodes.

Bastnaesite ((Ce,La,Pr,Nd)CO₃F) is a significant light rare earth mineral found in nature, known for its fine-grained properties. Flotation is commonly employed for the recovery of fine-grained bastnaesite particles. Collectors serve as an essential flotation reagent that enhance the surface hydrophobicity of target minerals. A novel collector, N-hydroxy-9-octadecenamide (N-OH-9-ODA), was synthesised in this study. N-OH-9-ODA exhibits superior selectivity compared to the traditional collector oleic acid in the flotation separation of bastnaesite and fluorite. The experimental and computational results indicate that N-OH-9-ODA exhibits superior selectivity due to its higher adsorption affinity for bastnaesite surface compared to fluorite surface. The zeta potential and the binding energies of the Ce 3d peaks in the X-ray photoelectron spectrum (XPS) of bastnaesite surface exhibit significant shifts. Conversely, fluorite surface demonstrates minimal alterations in its zeta potential and the binding energies of the Ca 2p peaks in its XPS after its interaction with N-OH-9-ODA.

CeCO₃OH has a unique crystal structure and excellent optical, electronic and catalytic properties, which has been widely investigated for many applications. Interestingly, ceria obtained from CeCO₃OH has a morphology that is similar to that of the precursor, and the CeO₂-based products obtained from CeCO₃OH exhibit outstanding properties, such as catalytic performances, owing to their designed morphology and oxygen vacancies (OVs). To introduce CeCO₃OH into a wider range of potential researchers, we first systematically review the physico-chemical properties, synthesis, reaction and morphology tuning mechanism of CeCO₃OH, and summarize the conversion behavior from CeCO₃OH to ceria. Then, we thoroughly survey the applications of CeCO₃OH and its conversion products. Suggestions for further investigations of CeCO₃OH are also made in this review. It is hoped that the exhaustive compilation of the valuable properties and considerable potential investigations of CeCO₃OH will promote further applications of CeCO₃OH and CeO₂-based functional materials.

Dual-excitation and dual-emission Y₄GeO₈:Bi³⁺,Sm³⁺ phosphors were manufactured by traditional solid-phase sintering technique. The X-ray diffraction, morphology, photoluminescence, energy transfer process and temperature sensing properties of Y₄GeO₈:Bi³⁺,Sm³⁺ samples were comprehensively evaluated. The Y₄GeO₈:Bi³⁺,Sm³⁺ phosphors exhibit characteristic emissions of Bi³⁺ (³P₁→¹S₀) and Sm³⁺ (⁴G_5/2→⁶H) under both 290 and 347 nm excitations. In fluorescence intensity ratio and Commission International de L'Eclairage coordinates modes, Y₄GeO₈:Bi³⁺,Sm³⁺ samples present excellent temperature measurement performance. The maximum relative sensitivity (S_r-max) values of the former are 1.55%/K (460 K, 290 nm excitation) and 0.82%/K (506 K, 347 nm excitation). The S_r-max(x) values of the latter are 0.21%/K (437 K, 290 nm excitation) and 0.15%/K (513 K, 347 nm excitation). These results illustrate that Y₄GeO₈:Bi³⁺,Sm³⁺ phosphors can be used as a candidate material for a dual-mode optical thermometer under dual-excitation.

In current report, the structural, magnetic, and thermoelectric properties of RE doped MgPm₂X₄ (X = S, Se) spinels were investigated. The energy difference in ferromagnetic and antiferromagnetic states reveals the stability of MgPm₂(S/Se)₄ in the ferromagnetic states. The computation of enthalpy of formation also ascertains thermodynamic stability of crystal structure. Spin-dependent band structure and density of states analysis reveal ferromagnetic semiconducting character showing different electronic behavior in both spin channels. The room temperature ferromagnetism, spin polarization and Curie temperature are estimated from exchange energies analysis. In addition, exchange constants (N₀α and N₀β), exchange energy Δ_x(pd), crystal field energy, and double exchange mechanism were studied to explore the magnetic response. Likewise, the electrical conductivity, thermal conductivity, Seebeck co-efficient, and power factor show effect on electrons spin and their potential for thermoelectric devices.

The substitution of Fe by Co in the 2:14:1 phase is an effective method to increase the Curie temperature and enhance the thermal stability of the Nd-Fe-B magnets. However, the accumulation of Co element at the grain boundaries (GBs) changes the GBs from nonmagnetic to ferromagnetic and causes the thin-layer GBs to become rare. In this paper, the method of diffusing Tb element was chosen to improve the microstructure and temperature stability of high-Co magnets. Three original sintered Nd_28.5Dy₃-Co_xFe_balM_0.6B₁ (x = 0, 6 wt%, 12 wt%; M = Cu, Al, Zr) magnets with different Co contents were diffused with Tb by grain boundary diffusion (GBD). After GBD, high-Co magnets exhibit more continuously distributed thin-layer GBs, and their thermal stability is significantly improved. In high-Co magnets (x = 6 wt%), the absolute value of the temperature coefficient of coercivity decreases from 0.603%/K to 0.508%/K in the temperature range of 293–413 K, that of remanence decreases from 0.099%/K to 0.091%/K, and the coercivity increases from 18.44 to 25.04 kOe. Transmission electron microscopy (TEM) characterization reveals that there are both the 1:2 phase and the amorphous phase in the high-Co magnet before and after GBD. EDS elemental analysis shows that Tb element is more likely to preferentially replace the rare earth elements in the 2:14:1 main phase than in the 1:2 phase and the amorphous phase. The concentration of Tb at the edge of the main phase is much higher than that in the 1:2 phase and amorphous phase, which is beneficial to the improvement of the microstructure. The preferential replacement of Tb elements at the edge of the 2:14:1 phase and thin-layer GBs with a more continuous distribution are synergistically responsible for improving the thermal stability of high-Co magnets. The study indicates that GBD is an effective method to improve the microstructure and thermal stability of high-Co magnets.

Layered rare-earth hydroxides (LREHs) draw wide research interest because of their peculiar crystal structure, rich interlayer chemistry and abundant functionality of the RE element, but are limited to the two categories of RE₂(OH)₅A·nH₂O (A: typical of Cl⁻ or NO₃⁻) and RE₂(OH)₄SO₄·nH₂O. On the other hand, rare-earth oxysulfates (RE₂O₂SO₄) have attracted attention due to their properties of large-capacity oxygen storage, low-temperature magnetism and luminescence, but their preparation procedure mostly involves toxic SO_x gases and/or complicated procedures. In this work, RE₂(OH)₂CO₃SO₄·nH₂O as a new family of LREHs (RE = Gd‒Lu lanthanides and Y) were produced via hydrothermal reaction, from which phase-pure RE₂O₂SO₄ was derived via subsequent annealing at 800 °C in air without the involvement of SO_x. The compounds were thoroughly characterized to reveal the intrinsic influence of lanthanide contraction (RE³⁺ radius) on crystal structure, thermal behavior (dehydroxylation/decarbonation/desulfurization), vibrational property and crystallite morphology. Through analyzing the photoluminescence of Eu³⁺ and Sm³⁺ in the Gd₂O₂SO₄ typical host it is found that the 617 nm (Eu³⁺, λ_ex = 275 nm) and 608 nm (Sm³⁺, λ_ex = 407 nm) main emissions can retain as high as ∼79.6% and 85.5% of their room-temperature intensities at 423 K, with activation energies of ∼0.19 and 0.21 eV for thermal quenching, respectively. Application also indicates that both the phosphors have the potential for optical temperature sensing via the fluorescence intensity ratio (FIR) technology, whose maximum relative sensitivity reaches ∼2.70%/K for Eu³⁺ and 1.73%/K for Sm³⁺ at 298 K.

Ce/BEA has the potential to be applied as a novel passive NO_x absorber (PNA) in the after-treatment of vehicles due to its considerable NO_x storage capacity. However, as a vehicle exhaust after-treatment material, it must withstand the test of long-term hydrothermal aging. This work examined the deactivation mechanism of Ce/BEA during hydrothermal aging. 3.0 wt% Ce/BEA was prepared using the ion-exchange method, and then subjected to hydrothermal treatment at 650 °C with 10% H₂O for 1–12 h to obtain samples with different aging extent. For comparison, the H-BEA support was aged under the same conditions. Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), NH₃ temperature programmed reduction (NH₃-TPD), ²⁷Al MAS nuclear magnetic resonance (²⁷Al MAS NMR), H₂ temperature programmed reduction (H₂-TPR), and high resolution-transmission electron microscopy (HR-TEM) were performed to characterize the changes in PNA performance, structure, Ce species, and acidity. The HR-TEM and H₂-TPR results show that CeO_x particles appear after hydrothermal aging, which results from the detachment and aggregation of active Ce species. Based on the ²⁷Al MAS NMR results, we conclude that BEA zeolite dealumination leads to the loss of acidic sites and the transformation of active Ce species on the acidic sites into the less active CeO_x. This is the primary reason for the hydrothermal aging deactivation of Ce/BEA.

Two novel phosphors LiBa_4(1‒x)Eu_4xTa₃O₁₂ (H-LBTO:xEu³⁺) and Li_0.25Ba_1‒xEu_xTa_0.75O₃ (C-LBTO:xEu³⁺) were prepared successfully by a molten salt method. The transformation between these two structures was realized by changing the sintering temperature or changing the Eu³⁺ ions concentration, which was also demonstrated by the X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectra (DRS), and photoluminescence excitation (PLE) analyses. Both the sintering temperature and the Eu³⁺ ions doping concentration have significant impact on the formation of the crystal phase. All these phosphors sintered at 1023 K exhibit two major luminescence lines at 594 and 614 nm under near-UV light of 395 nm excitation, corresponding to Eu³⁺ ions typical transitions of ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂. The optimum concentration of Eu³⁺ ions is 9 mol% for C-LBTO:xEu³⁺ samples and the quenching interaction type is the nearest-neighbor ion interaction. The thermal stability of the C-LBTO:0.09Eu³⁺ sample was investigated in detail and the device application further suggests that C-LBTO:0.09Eu³⁺ can be used as a red phosphor for near-UV excited w-LEDs in lighting.