Ce、Eu、Tb、Sm在CaO-B2O3-CaCl2中的光谱行为

采用高温固相法制备Ce、Eu、Tb、Sm单双激活CaO-B2O3-CaCl2(CBC)的系列荧光材料,研究它们的光谱和Stokes位移.Eu、Tb单掺杂的发光体分别在468 nm和550 nm处有蓝光和绿光发射,但是Eu、Tb的猝灭浓度较大.双掺杂Ce/Tb、Sm/Eu的发射光谱分别归属Eu2 的4f65d1→8S7/2和Tb3 的5D4→7F5特征跃迁发射.在CBC中,Ce、Sm分别是Tb、Eu的高效敏化剂,双掺Ce/Tb和Sm/Eu的荧光体发光强度比单掺Tb、Eu提高4~12倍,产品成本降低了25%~35%.     

The magnetic ordering in the Ho5Si3 and Ho5Ge3 compounds

A neutron diffraction investigation has been performed on the Ho₅Si₃ and Ho₅Ge₃ compounds (hexagonal Mn₅Si₃-type, hP16, P6₃/mcm) to study their magnetic structure. The results prove that these intermetallic phases show a complex sine-modulated type magnetic ordering, both presenting on cooling two subsequent antiferromagnetic orderings (T_N1 and T_N2).Between T_N1 = 24(2) and T_N2 = 16(4) K Ho₅Si₃ shows a first antiferromagnetic ordering of the sine-modulated type with a propagation vector K₁ = [0, 0, ±0.284(1)]; then, below T_N2 the sine-modulated type ordering is conserved, but by the two propagation vectors K₁ = [0, 0, ±0.2773(7)] and K₂ = [±1/5, ±1/5, 0].For Ho₅Ge₃, between T_N1 = 27(2) and T_N2 = 18(4) K, the sine-modulated ordering is realized through the propagation vectors K₁ = [0, 0, ±3/10] and K₂ = [0, 1/2, 0], whilst below T_N2 the ordering takes place with propagation vectors K₁ = [0, 0, ±3/10], K₂ = [0, 1/2, 0], K₃ = [0, 0, ±2/5] and K₄ = [±1/5, ±1/5, 0]. For both the Ho₅Si₃ and Ho₅Ge₃ compounds, the dimensions of the magnetic unit cell are 5a × 5a × 10c times the crystal unit cell of Mn₅Si₃-type.     

Influence of rare-earth addition on microstructure and dielectric behavior of Ba0.6Sr0.4TiO3 ceramics

Ba_0.6Sr_0.4TiO₃ (BST) ceramics with 0.5 mol% various trivalent rare-earth additions prepared by a solid-state route are investigated. A strong correlation is observed between the microstructure, dielectric properties and rare-earth element dopant. The results display that comparing with the lattice constants of undoped and doped rare-earth BST, the structure transforms from cubic to tetragonal structure. In addition, the dopant improves the tetragonal distortion with the ionic radius of rare earth decreasing, and then deteriorates it with further decreasing. Large ions rare-earth additions effectively suppress the grain growth of BST. It is found that the temperature-permittivity characteristics for the BSTR (R, namely, rare earth) system could be controlled using various rare-earth elements. Especially, such as Sm, Eu, Gd dopants are effective to satisfy the tunable microwave devices application due to the decrease of permittivity and the improvement of dissipation factors of BST ceramic with the accompanying high-tunability.     

21世纪高技术功能材料—稀土精密陶瓷

陶瓷、金属、高分子材料是构成现代材料三个重要的支柱,在陶瓷中掺入少量的稀土元素将賦子陶瓷在机械、热电、电子、磁学、光学、化学、生物诸方面新的特性,从而在高、新技术中获得来些特殊的应用。     

X-ray photoelectron spectroscopy and magnetic properties of CeCo7Mn5 and CeCo8Mn4 isostructural ThMn12 type compounds

The magnetic properties of CeCo₇Mn₅ and CeCo₈Mn₄ compounds have been investigated by combining X-ray photoelectron spectroscopy (XPS) and magnetic measurements in a wide temperature range (4–550) K and magnetic field up to 12 T. X-ray powder diffraction (XRD) measurements showed that CeCo₇Mn₅ and CeCo₈Mn₄ compounds are isostructural and crystallize in the ThMn₁₂ structure type. XPS spectra pointed out the intermediate valence state of Ce atoms and that both Co and Mn atoms carry magnetic moments. The complex magnetic structure of CeCo₇Mn₅ and CeCo₈Mn₄ is determined by the competition between the ferromagnetic (Co–Co pairs) and antiferromagnetic (Co–Mn and Mn–Mn pairs) interactions. Two different ordering temperatures T_N and T_C correlated to antiferromagnetic and ferromagnetic coupling of 3d magnetic moments, respectively, are evidenced. Magnetic moments of about 1.6 μ_B/Co and 3.2 μ_B/Mn atoms were determined by correlating the magnetic data of the two compounds, in good agreement with the exchange splitting of XPS Co 3s and Mn 3s core levels.     

Comparison between Y-doped ceria and Ho-doped ceria: Electrical conduction and microstructures

The electrical conduction and microstructure in Y-doped ceria and Ho-doped ceria were investigated. Visible differences between the two doped ceria were observed. The plots of conductivity of Y-doped ceria are well fit to the Arrhenius behavior, while curvature and kinks were observed on the plots of Ho-doped ceria. Such deviation from the Arrhenius behavior could be related to the existence of precipitates in Ho-doped ceria. It indicates that the electrolytic conductivity of doped ceria could be significantly influenced by microstructure features.     

Effect of multi-component rare-earth doping on maintaining structure stability of RE2Zr2O7 (RE = La,Sm, Gd,Y, Yb) coatings under thermal cycling

Inspired by the high entropy effects of high-entropy components, a novel high-entropy rare-earth zirconate (La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇ (HEC-LZ) was designed and successfully synthesized in this work. In addition, two binary rare-earth doped zirconates (RE-LZ), (La_1/3Sm_1/3Yb_1/3)₂Zr₂O₇ (LSYZ) and (La_1/3Gd_1/3Y_1/3)₂Zr₂O₇ (LGYZ), were proposed using the same rare-earth elements for comparison. The thermal barrier coatings with LZ-based ceramic top layer were prepared by spray granulation, solid-phase synthesis and atmospheric plasma spraying techniques. The as-synthesized LZ-based ceramics are all dominated by the pyrochlore phase. Under 1000 °C, the thermal cycling performances of the three coatings were studied. The microstructure evolution and crack expansion during the failure process were investigated in detail. The strengthening mechanism and the cause of coating spallation are proposed in combination with mechanical properties and thermal matching analysis. The results showed that compared with the undoped LZ coating, the thermal shock life of LGYZ coating, LSYZ coating and HEC-LZ coating is improved by nearly 46%, 27% and 57%, respectively. Due to the characteristics of high randomness, HEC-LZ ceramic has a large lattice distortion than RE-LZ ceramics, resulting in a higher coefficient of thermal expansion and fracture toughness, which contributes to maintaining the structure stability of coatings under thermal stress.     

The influence of Gd/Ce substitution on the magnetic properties,electronic structure and the valence of cerium in the solid solution CexGd1−xNi3

The polycrystalline compounds Ce_xGd_1−xNi₃ with a rhombohedral PuNi₃ (x ≤ 0.2) and hexagonal CeNi₃ (x ≥ 0.8) – type of crystal structures have been obtained. The change of the lattice parameters may suggest an unstable valence of cerium in the studied system. The effect of partial substitution of Gd by Ce is reflected in a change of the cell volume V(x), the Curie temperature T_C(x) and in the temperature dependence of the magnetic susceptibility and electrical resistivity. Thus, T_C(x) decreases from 115 K (x = 0) to ∼6.8 K (x = 0.8). The XPS spectra have been measured at room temperature. The valence band spectra (VB) as well as the core level lines have been analysed as influence of the Gd/Ce substitution on the electronic structure. The VB near the Fermi level (E_F) is dominated by Ni3d states. In the VB region some slight effects of hybridization and changes of intensity of states on E_F have been noticed. The analysis of Ce core level lines reveals the occurrence of possible intermediate valence. The values of the Ce4f - state occupation parameter (n_f) and the hybridization energy (Δ) have been estimated. The gradual filling of the Ni3d band is revealed by a reduction of the 6 eV satellite intensities in the Ni2p core level spectrum. All these effects can modify the magnetic properties of the investigated compounds.     

Synthesis and thermal stability of Re2Zr2O7, (Re = La,Gd) and La2(Zr1−xCex)2O7−δ compounds under reducing and oxidant atmospheres for thermal barrier coatings

Rare-earth zirconates and cerates have attracted particular interest for thermal barrier coating (TBC) applications due to their advantageous thermal properties, such as a low conductivity and efficient phase stability at elevated temperatures. This study focuses on synthesising La₂Zr₂O₇, Gd₂Zr₂O₇, La₂Ce₂O_7?γ and La₂(Zr_0.7Ce_0.3)O_7?γ compounds via two soft chemistry processes, alkoxide and citrate synthesis. Thermal analysis, X-ray diffraction (XRD) and scanning electron microscope observations were used to analyse the powder after calcinations under air. Chemical reactivity tests under a reducing atmosphere were performed at 1400 °C and investigated by XRD analysis. It was found that the lanthanum and gadolinium zirconates are the most stable and interesting materials under an Ar_(g)/3%H_2(g) atmosphere.