Journal of Rare Earths最新文献

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.

This work reports the synthesis, characterization, and energy focused applications of the novel lanthanides co-doped tantalum pentoxide hetero-system (Sm³⁺-Eu³⁺-Tm³⁺:Ta₂O₅). Ln³⁺-doped Ta₂O₅ express excellent opto-electronic features reflected by the narrow band gap energy of 3.87 eV. Different vibrations confirm the presence of Ta–O–Ta and Ta–O bonds. The synthesized system possesses orthorhombic geometry with 59.46 nm particle size. With the smoother and compact morphology, the synthesized material succeeds in augmenting the performance of different systems aimed at energy applications. Fully ambient perovskite solar cell device fabricated with the Ln³⁺-doped Ta₂O₅ as an electron transport layer excels in achieving an efficiency and fill factor of 14.17% and 76% under artificial sun. This device was marked by the negligible hysteresis behavior showing profound photovoltaic performance. The electrochemical activity of the Ln³⁺-doped Ta₂O₅ decorated electrode was evaluated for electrical charge storage potential with pseudocapacitive behavior. With the highest specific capacitance of 355.39 F/g and quicker ionic diffusion rate, the designed electrode excels conventionally used materials. Electro-catalysis of water with Ln³⁺-doped Ta₂O₅ material indicates its capacity for H₂ production with the lowest overpotential and Tafel slope values of 148 and 121.2 mV/dec, while the O₂ generation is comparatively lower. With the stable electrochemical output, this rare earth modified material has the potential to replace conventionally used environmentally perilous and costly materials.

The magnetic properties, magnetic phase transition and magnetocaloric effects (MCE) of Er₃Si₂C₂ compound were investigated based on theoretical calculations and experimental analysis. Based on the first principles calculations, the antiferromagnetic (AFM) ground state type in Er₃Si₂C₂ compound was predicted and its electronic structure was investigated. The experimental results show that Er₃Si₂C₂ compound is an AFM compound with the Néel temperature (T_N) of 7 K and undergoes a field-induced first-order magnetic phase transition from AFM to ferromagnetic (FM) under magnetic fields exceeding 0.6 T at 2 K. The magnetic transition process of Er₃Si₂C₂ compound was investigated and discussed. The values of the maximum magnetic entropy change (${-\Delta S}_M^{\max })$ and the refrigeration capacity (RC) are 17 J/(kg·K) and 193 J/kg under changing magnetic fields of 0–5 T, respectively. As a potential cryogenic magnetic refrigerant, the Er₃Si₂C₂ compound also provides an interesting research medium to study the magnetic phase transition process.

In perovskite EuTiO₃, the magnetic characteristics and magnetocaloric effect (MCE) can be flexibly regulated by converting the magnetism from antiferromagnetic to ferromagnetic. In the present work, a series of Eu(Ti,Nb,Mn)O₃ compounds, abbreviated as ETNMO for convenience of description, was fabricated and their crystallography, magnetism together with cryogenic magnetocaloric effects were systematically investigated. The crystallographic results demonstrate the cubic perovskite structure for all the compounds, with the space group of Pm3m. Two magnetic phase transitions are observed in these second-order phase transition (SOPT) materials. The joint substitution of elements Mn and Nb can considerably manipulate the magnetic phase transition process and magnetocaloric performance of the ETNMO compounds. As the Mn content increases, gradually widened –ΔS_M-T curves are obtained, and two peaks with a broad shoulder are observed in the –ΔS_M-T curves for Δμ₀H≤0–1 T. Under a field change of 0–5 T, the values of maximum magnetic entropy change (−Δ$S_M^{\text{max}}$) and refrigeration capacity (RC) are evaluated to be 34.7 J/(kg·K) and 364.9 J/kg for EuTi_0.8625Nb_0.0625Mn_0.075O₃, 27.8 J/(kg·K) and 367.6 J/kg for EuTi_0.8375Nb_0.0625Mn_0.1O₃, 23.2 J/(kg·K) and 369.2 J/kg for EuTi_0.8125Nb_0.0625Mn_0.125O₃, 17.1 J/(kg·K) and 357.6 J/kg for EuTi_0.7875Nb_0.0625Mn_0.15O₃, respectively. The considerable MCE parameters make the ETNMO compounds potential candidates for cryogenic magnetic refrigeration.

Phosphor-converted near-infrared light-emitting diodes (NIR pc-LEDs) are finding applications in various fields including food quality analysis, biomedical imaging, night vision, and biomedicine. The crucial factor in the development of NIR pc-LEDs devices lies in the advancement of high-performance broadband NIR phosphors. In this work, novel Cr³⁺-activated silicate phosphors NaRESi₃O₉ (RE = Y, Lu, Sc) are reported. This silicate has a special 3D network structure in which RE has four different sites, forming four octahedrons and providing suitable occupation sites for Cr³⁺. The phosphors demonstrate a wide emission spectrum ranging from 750 to 1450 nm when excited by light at 468 nm. The full width at half maximum (FWHM), which benefits from the presence of Cr³⁺ ions occupying multiple sites, is measured to be 203 nm. Notably, the strongest emission peak is observed at a longer wavelength of 984 nm compared to most other systems activated by Cr³⁺. The Na₃ScSi₃O₉ lattice provides a weak crystal field (Dq/B = 1.97) and weak phonon-photon coupling for Cr³⁺, and the integrated emission intensity of Na₃ScSi₃O₉:0.03Cr³⁺ is 4.66 times stronger than that of Na₃YSi₃O₉:0.03Cr³⁺.

Thermal barrier coatings (TBCs) materials with lowered thermal and oxygen ion conductivity can provide thermal and oxidative protection for high temperature hot-end components in aeronautical engines and gas turbines. The rare-earth tantalate RETaO₄ (RE = Dy, Gd and Sm) ceramics with monoclinic (m) phase were successfully synthesized via spark plasma sintering. Oxygen vacancies responsible for the thermal and oxygen ion conductivities of RETaO₄ were demonstrated by atomic-resolution energy dispersive X-ray and X-ray photoelectron spectroscopy. Among the three samples, DyTaO₄ has excellent oxygen/thermal barrier performance. Compared to the current service thermal barrier coating material ZrO₂-8 wt% Y₂O₃ (8 YSZ), DyTaO₄ has an ultra-low oxygen ion conductivity benefiting from low oxygen vacancy concentration and strong stretching force constants. The intrinsic thermal conductivity of DyTaO₄ is 68.2% less than that of 8 YSZ. Additionally, the thermal expansion rate curves indicate that the phase transformation does not happen from room temperature to 1200 °C. The above results demonstrate that high-growth rate thermally grown oxide can be retarded by creating dense DyTaO₄ coating with lowered thermal and oxygen ion conductivity.

Lanthanum doped nickel–cobalt nano ferrites with chemical formula Ni_0.5Co_0.5La_xFe_2–xO₄ (x = 0.05, 0.10, 0.15 and 0.20) were prepared using a simple sol–gel auto combustion method. The basic structural properties were determined by X-ray diffraction method and the formation of single phased spinel ferrite was confirmed. The crystalline size decreased from 25 to 11 nm and lattice parameter a increases with increase of La doping. The surface morphology of these ferrites was observed by field-emission scanning electron microscopy (FESEM) and agglomerated irregular grains are observed with increase of the rare earth element La doping. Energy-dispersive X-ray spectroscopy (EDX) result confirms the presence of the required elements. The Fourier transform infrared spectroscopy (FTIR) spectrum indicates the formation of the spinel ferrite structure with M−O bonds. Optical direct band measurements from ultraviolet–visible spectroscopy (UV-Vis) spectroscopy indicate that the direct band gap decreases from 1.39 to 1.19 eV for x = 0.05 to x = 0.15, then increases to 1.28 eV for x = 0.20. The room temperature magnetic properties of these ferrites were studied by a vibrating sample magnetometer (VSM). The enhanced saturation magnetization of 49.73 emu/g is observed for x = 0.10 and then saturation magnetizations are gradually decreased for x = 0.15 and x = 0.20. Interestingly the remanent magnetization and coercivity also follow the same trend.