Low-temperature Magnetization Research Articles

Study on High-Voltage Cathode Li-Ni-Mn Spinel Oxides with Low-Temperature Magnetism

Lithium-ion rechargeable batteries have been attracting researcher’s interest, because they are candidate to address environmental issues, especially for application to electric vehicles and stationary energy storage systems. The battery operating voltage is one of the factor affecting the energy density of battery cell, and therefore positive electrode with high operating voltage is very attracting. A typical high-voltage cathode material, LiNi0.5Mn1.5O4 is an end member of LiNixMn2-xO4 solid solution compounds (0.00<x<0.50). From the previous study, it is known that LiNi0.5Mn1.5O4 spinel is a ferri-magnetic material with large magnetic moment and high transition temperature. In this work, the compounds of LiNixMn2-xO4 (0.40<x<0.50) are studied with both low-temperature magnetism and electrochemical charge-discharging. Additionally, the correlation between electrochemical and magnetic properties is also discussed.LiNixMn2-xO4 (0.40<x<0.50) were prepared by gelation of nitrate/citrate solution, thermal decomposition at 500 ℃ and annealing under various conditions (temperature and atmosphere). Under the preparation condition at relatively low temperature (700 ℃) in oxygen atmosphere, they were single phase of cubic spinel with fine particles. The lattice parameters monotonously decreased, as x increased. The charge-discharge profiles have twin plateaus around 4.7 V, to Ni2+/Ni3+ and Ni3+/Ni4+ redox signals, with a small signal at ~4.0 V. The capacity at >4.5 V formers increased with x and that at <4.5 V was reduced. From the magnetic evaluation, the saturation magnetization at 2 K (Ms) increased with x, and simultaneously, the ferri-magnetic/paramagnetic transition temperature (Tc), evaluated from temperature dependence of the saturation magnetization, shifted higher. These results exhibited the following relation-ships (presented in Fig. 1); the capacity at >4.5 V was almost proportional to Ms, and the operating voltage was proportional to Tc. However, it was found that the magnetic response under weak magnetic field was not usual (shown in Fig. 2) and it implied two-step phase transition around Tc. The anomaly was thought to be attributed to magnetic in-homogeneity, which is very sensitive to the specimen characters. The analysis of experimental results of specimens prepared under different conditions are being carried out and then presented and discussed at the conference. Figure 1

Coexistence of distinct Ce 4f-electron states at inequivalent crystallographic sites: CePtGe2 case

In CePtGe2, which has two crystallographically inequivalent Ce sites, the coexistence of a localized magnetic 4f-electron state at one Ce site and an itinerant heavy-electron state at the second one has been suggested. We prepared a series of pseudoternary (CexY1−x)PtGe2 compounds and investigated them by X-ray diffraction, magnetization, and specific heat measurements. The structure analysis revealed that the Y ions selectively occupy the Ce sites at the 4h position, which is more suited for smaller Ce ions with the itinerant heavy-electron state. The Néel temperature, low-temperature saturation magnetization, and magnetic entropy change weakly in the interval 1.0 ≥ x ≥ 0.5 while the linear specific-heat coefficient decreases rapidly with decreasing x. Further substitution of Y for Ce leads to a rapid decrease of the Néel temperature, low-temperature saturation magnetization, a remaining low linear specific-heat coefficient, and finally vanishing antiferromagnetism. These results corroborate the scenario of the coexistence of the magnetic Ce ions with localized 4f electrons responsible for the ordered magnetic state hosted by the 4i site, whereas the Ce ions at the 4h site are characterized by itinerant heavy 4f electrons.

Cooperative spin crossover leading to bistable and multi-inert system states in an iron(III) complex

Cooperativity among spin centres has long been the royal road in spin crossover (SCO) research to impose magnetic bistability in terms of thermal hysteresis. In this work we access magnetic multi-inert states of the iron(III) compound {FeL2[B(Ph)4]} ≡ FeB at low temperature, in addition to thermal bistability. The packing of the low-spin and high-spin forms of crystalline FeB differs only marginally what ultimately leads to structural conservatism. This indicates that the SCO-immanent breathing of the complex cation is almost fully compensated by the anion matrix. The unique cooling rate dependence of the residual low-temperature magnetisation in FeB unveils continuous switching between the trapped high-spin (ON) and the relaxed low-spin state (OFF). The macroscopic ratio of the spin states (ON:OFF) can be adjusted as a simple function of the cooling rate. That is, cooperative spin crossover can be the source of bistable and multi-inert system states in the very same material.

Elastic-plastic conductor damage evaluation at over 0.4% strain using a high-stress REBCO coil

Recent reports on screening current stress simulations of high-field REBCO magnets frequently present peak stresses over 1 GPa. However, this result is probably an unrealistic artifact of purely elastic calculations, considering the macroscopic yield and fracture stresses of approximately 900 MPa and less than 1.1 GPa for Hastelloy substrate-coated conductors. Here, we evaluate elastic-plastic conductor damage at over 0.4% strain using a high-stress REBCO coil exposed to a high field to explore this elastic-plastic regime. The coil was located off-center in a low-temperature superconductor magnet so as to induce a significant screening current in the enhanced radial field. Voltage taps, a Hall sensor, and two strain gauges were used for the instrumentation. We obtained strains exceeding 0.4% near the outward edge during the coil current charge from 350 A to 390 A, where the coil was exposed to external axial and radial magnetic fields of 13 T and 0.5 T. Post mortem results showed wavy plastic deformation, electrical damage, and REBCO defects. An elastic-plastic simulation reproduced the measured strains and predicted that ∼1 GPa stress is sufficient to induce ∼0.9% strain, thus validating our initial concerns with purely elastic models. This paper provides our experimental and simulation results.

(Invited) Magnetism of Tb3N@C80 Endofullerenes

Tb3N@C80 was one of the first trimetallic endofullerenes to be studied in view of its magnetization where the microscopic picture of 3 Tb magnetic moments non-collinearly aligned by the ligand field of the central nitrogen ion was proposed [1]. The triangular geometry results in a net molecular magnetization of two Tb3+ ions or 18 μB. Today, this is confirmed with 23 possible pseudospin arrangements of the 3 Tb magnetic moments, resulting in a frustrated magnetic ground state with fluctuating pseudospins [2]. This picture does not include coherent tunneling of the magnetization, which is predicted within an 8-dimensional Hamiltonian and can be tested with low temperature magnetization measurements. Experimentally, we find a low temperature hysteresis that identifies Tb3N@C80 as a single molecule magnet. More surprisingly, we observe room temperature ferromagnetism in cubic single crystallites of the molecules using SQUID magnetometry. Since the magnetic interaction between adjacent carbon cages is expected to be dipolar it is neglected in the explanation of magnetic order above 3 K [3] and it is challenging to explain this finding with endofullerene magnetism. We present a series of analytical chamistry results of samples with and without room temperature ferromagnetism.[1] M. Wolf et al. Angew. Chem. Int. Ed. 44, 3306 (2005).[2] R. Westerström et al. Phys. Rev. B, 89, 060406(R) (2014).[3] A. Kostanyan et al. Phys. Rev. B, 101, 134429 (2020).

Quantum magnetism in Fe2Cu2 polymeric branched chains: insights from exactly solved Ising-Heisenberg model

The spin-1/2 Ising-Heisenberg branched chain, inspired by the magnetic structure of three isostructural polymeric coordination compounds [(Tp)2Fe2(CN)6X (bdmap)Cu2(H2O)] ⋅ H2O to be further denoted as Fe2Cu2 (Tp = tris(pyrazolyl)hydroborate, bdmapH = 1,3-bis(dimethylamino)-2-propanol, HX = acetic acid, propionic acid or trifluoroacetic acid), is rigorously studied using the transfer-matrix method. The overall ground-state phase diagram reveals three distinct phases: a quantum antiferromagnetic phase, a quantum ferrimagnetic phase and a classical ferromagnetic phase. In the zero-temperature magnetization curve, two quantum ground states are manifested as intermediate plateaus at zero and half of the saturation magnetization, while the magnetization reaches its saturated value within the classical ferromagnetic phase. The bipartite entanglement between nearest-neighbor Heisenberg spins is more pronounced in the quantum ferrimagnetic phase compared to the quantum antiferromagnetic phase due to a fully polarized nature of the Ising spins. A reasonable agreement between theoretical predictions for the spin-1/2 Ising-Heisenberg branched chain and experimental data measured for a temperature dependence of the magnetic susceptibility and a low-temperature magnetization curve suggests strong antiferromagnetic coupling between nearest-neighbor Cu2+-Cu2+ magnetic ions and moderately strong ferromagnetic coupling between nearest-neighbor Cu2+-Fe3+ magnetic ions in the polymeric compounds Fe2Cu2. A thermal entanglement between nearest-neighbor Cu2+-Cu2+ magnetic ions persists up to a relatively high threshold temperature T ≈ 224 K and undergoes a transient magnetic-field-driven strengthening.

Elucidation of annealing effects on austenite–martensite and magnetic phase transitions in Mn2Ni1.6Sn0.4 ribbons synthesized from bulk alloys prepared by RF induction melting

We investigate the consequences of annealing on the structure, microstructure, magnetic, and electrical transport of Mn2Ni1.6Sn0.4 ribbons. The 40–60 μm thick ribbons were formed by melt spinning of the bulk ingot synthesized by RF induction melting. Ribbons were annealed at 600 °C for 12 hrs and the other at 800 °C for 2 hrs in vacuum. All ribbons crystallize into a cubic symmetry (space group F4¯3m). The orthorhombic phase appears as the secondary one. In ribbons annealed at 600 °C, the crystallite size decreases from ∼ 47 nm to ∼ 29 nm, and annealing at 800 °C doubles the crystallite size to ∼ 96 nm. The least amount of microstrains (17x10−4) in the as-synthesized ribbon is consistent with the occurrence of the superlattice peaks (111) and (200). Lower intensities of the superlattice peaks in 600 °C and 800 °C ribbons are consistent with higher microstrains (25x10−4 and 24x10−4). Hence, a higher degree of Ni-Mn ordering improves magnetic properties in annealed samples. The as-synthesized ribbon has an austenite-phase Curie temperature of TCA∼303K, which enhances to TCA∼320K and TCA∼310K after annealing at 600 °C and 800 °C. The ribbon annealed at 600 °C has the highest saturation moment. All the ribbons show the exchange-bias (EB) effect, and the most pronounced effect is seen in the 800 °C annealed ribbon. Microstructure uniformization and relative strengths of the Ni-Mn and Mn-Mn interactions improve the magnetics. Near TC, MT=M0TC-Tβ is satisfied (β = 0.31, 0.36, and 0.37, for as-synthesized, 600 °C and 800 °C annealed ribbons). The low-temperature spontaneous magnetization obeys the Bloch law MT=M01-AT32, with a deviation in 600 °C annealed ribbon due to spin freezing. The ρ-T has been fitted in the different temperature ranges using the various combinations in ρT=ρ0+AphT+BsdTα. At T < 85 K, the resistivity of the as-synthesized ribbon follows ρT=ρ0+BsdTα, with α≈1.47, while for the 600 °C annealed ribbon, α≈1.49. The exponent α∼1.5 supports the existence of the spin glass state in the lower temperature region of as-synthesized and 600 °C annealed ribbons. In the high-temperature range, the linear ρ-T behavior is described by ρT=ρ0+AphT and consistent with the electron–phonon scattering.

Inducing a tunable skyrmion-antiskyrmion system through ion beam modification of FeGe films

Skyrmions and antiskyrmions are nanoscale swirling textures of magnetic moments formed by chiral interactions between atomic spins in magnetic noncentrosymmetric materials and multilayer films with broken inversion symmetry. These quasiparticles are of interest for use as information carriers in next-generation, low-energy spintronic applications. To develop skyrmion-based memory and logic, we must understand skyrmion-defect interactions with two main goals—determining how skyrmions navigate intrinsic material defects and determining how to engineer disorder for optimal device operation. Here, we introduce a tunable means of creating a skyrmion-antiskyrmion system by engineering the disorder landscape in FeGe using ion irradiation. Specifically, we irradiate epitaxial B20-phase FeGe films with 2.8 MeV Au4+ ions at varying fluences, inducing amorphous regions within the crystalline matrix. Using low-temperature electrical transport and magnetization measurements, we observe a strong topological Hall effect with a double-peak feature that serves as a signature of skyrmions and antiskyrmions. These results are a step towards the development of information storage devices that use skyrmions and antiskyrmions as storage bits, and our system may serve as a testbed for theoretically predicted phenomena in skyrmion-antiskyrmion crystals.

Low-temperature magnetism of Ln2Ba (Ln = Nd, Er, Ho) furoate-based polymeric chains: Slow relaxation, magnetic anisotropy and interactions

The synthesis, structure and magnetothermal properties of three new lanthanide 1D polymeric complexes, {[Ln2Ba(α-fur)8(H2O)4]}n for Ln = Nd (1), Er (2) and {[Ho2Ba(α-fur)8(H2O)4]⋅2H2O}n (3), based on carboxylic α-fur = C4H3OCOO ligands is reported. The α-furoate ligands consolidate zigzag chains formed by Ln2 dimers separated by Ba ions. Ab initio calculations, in combination with the fit of experimental static magnetization and heat capacity, predict the magnetic ground state, energy levels and magnetic interactions in these heteronuclear nanomagnets. Nd2Ba (1) presents two different coordination sites for Nd, with an orthorhombic magnetic ground state. Nd ions are coupled along the chain through a weak antiferromagnetic (AF) interaction J″/kB = −0.08 K. Er(III) ions in Er2Ba (2) present a highly axial ground state, forming magnetic dimers with an interaction of J′/kB = −8.6 K, while interdimer coupling along the chain is J″/kB = −0.28 K. The Ho2Ba (3) complex consists of a highly anisotropic quasi-doublet with a ΔHo/kB = 0.7 K gap. Non-Kramers Ho ions form magnetic dimers within the Ho2Ba cluster, coupled by an AF intradimer interaction J′/kB = −2.5 K. The three complexes exhibit in-field slow relaxation of the magnetization: 1 relaxes through an Orbach process at high temperatures [Ueff/kB = 60(1) K] evolving to quantum tunneling below 3 K [τQT = 0.05(1) s]; 2 exhibits a rapid Orbach-like process [τ0 = 8(6)⋅10–8 s and Ueff/kB = 10(2) K] and 3 shows a direct process [τ = 0.4(1) s].

Advances in XRD Characterization of 2G HTS Wire for Low-Temperature Magnet Applications

Advances in XRD Characterization of 2G HTS Wire for Low-Temperature Magnet Applications

Superconductivity-Electron Count Relationship in Heusler Phases-the Case of LiPd2Si.

We report superconductivity in the full Heusler compound LiPd2Si (space group Fm3̅m, No. 225) at a critical temperature of Tc = 1.3 K and a normalized heat capacity jump at Tc, ΔC/γTc = 1.1. The low-temperature isothermal magnetization curves imply type-I superconductivity, as previously observed in LiPd2Ge. We show, based on density functional theory calculations and using the molecular orbital theory approach, that while LiPd2Si and LiPd2Ge share the Pd cubic cage motif that is found in most of the reported Heusler superconductors, they show distinctive features in the electronic structure. This is due to the fact that Li occupies the site which, in other compounds, is filled with an early transition metal or a rare-earth metal. Thus, while a simple valence electron count-property relationship is useful in predicting and tuning Heusler materials, inclusion of the symmetry of interacting frontier orbitals is also necessary for the best understanding.

The energy spectrum and magnetization profile of the decorated spin ladder systems

The peculiarities of the energy spectrum of the decorated mixed spin two-leg ladder system having three spin unit cells have been studied by the perturbation theory in the limit of dominant spin coupling in ladder rungs. It is shown, that despite the singlet ground state, this spin ladder may have an intermediate plateau in a low-temperature magnetization profile. The existence of a similar plateau was shown also in the case of some intermediate values of coupling parameters by means of exact diagonalization study of small ladder clusters and by the density-matrix renormalization group calculations. We also found that the Ising type of interactions in ladder rungs may lead to the disappearance of the above intermediate magnetization plateau for the corresponding isotropic spin ladder.

Vibronic effects on the quantum tunnelling of magnetisation in Kramers single-molecule magnets

Single-molecule magnets are among the most promising platforms for achieving molecular-scale data storage and processing. Their magnetisation dynamics are determined by the interplay between electronic and vibrational degrees of freedom, which can couple coherently, leading to complex vibronic dynamics. Building on an ab initio description of the electronic and vibrational Hamiltonians, we formulate a non-perturbative vibronic model of the low-energy magnetic degrees of freedom in monometallic single-molecule magnets. Describing their low-temperature magnetism in terms of magnetic polarons, we are able to quantify the vibronic contribution to the quantum tunnelling of the magnetisation, a process that is commonly assumed to be independent of spin-phonon coupling. We find that the formation of magnetic polarons lowers the tunnelling probability in both amorphous and crystalline systems by stabilising the low-lying spin states. This work, thus, shows that spin-phonon coupling subtly influences magnetic relaxation in single-molecule magnets even at extremely low temperatures where no vibrational excitations are present.

Magnetic studies of ultrafine CoFe2O4 nanoparticles with different molecular surface coatings.

Surface functionalized ultrafine CoFe2O4 nanoparticles (NPs), with mean diameter ∼5 nm, were investigated by means of DC magnetization and AC susceptibility over the temperature range of 4-400 K. All NPs present the same CoFe2O4 core, with different molecular surface coatings, increasing gradually the number of carbon atoms in the coating layer: glycine (C2H5NO2), alanine (C3H7NO2), aminobutanoic acid (C4H9NO2), aminohexanoic acid (C6H13NO2), and aminododecanoic acid (C12H25NO2). Samples were intentionally fabricated in order to modulate the core-core magnetic dipolar interaction, as the thickness of the coating layer increases with the number of carbon atoms in the coating molecule. The magnetic data of the uncoated CoFe2O4 NPs were also collected for comparison. All investigated CoFe2O4 NPs (coated and uncoated) are in a magnetically blocked state at room temperature as evidenced by ZFC/FC measurements and the presence of hysteresis with ∼700 Oe coercivity. Low temperature magnetization scans show slightly constricted hysteresis loops with coercivity decreasing systematically with a decreasing number of carbon atoms in the coating molecule, possibly resulting from differences in magnetic dipole coupling between NPs. Large thermomagnetic irreversibility, slow monotonic increase in the FC magnetization and non-saturation of the magnetization give evidence for the cluster glass (CG) nature in the CoFe2O4 NPs. The out of phase part (χ'') of AC susceptibility for all samples shows a clear frequency dependent hump which was analyzed to distinguish superparamagnetic (SPM), cluster glass (CG) and spin glass (SG) behavior by using Néel-Arrhenius, Vogel-Fulcher, and power law fittings. These analyses rule out the SPM state and suggest the presence of significant inter-cluster dipolar interaction, giving rise to CG cooperative freezing in the high-temperature region. In the low-temperature range, however, the disordered spins on the nanoparticle's surface play an important role in the formation of the SG-like state, as evidenced by Arrott plots and temperature dependency of dM/dH in the initial magnetization curves. In summary, the magnetic measurements showed that undercooling the system evolves from a SPM state of weakly interacting spin clusters, through the CG state induced by strong dipolar interaction, to the SG state resulting from the frustration of the disordered surface spins.

Fabrication and properties of nano-CeO&lt;sub&gt;2&lt;/sub&gt; doped Y-Ba-Cu-O bulk superconductors

Single-domain Y-Ba-Cu-O (YBCO) bulk superconductors can be widely used in the superconducting maglev, cryomagnets, motors/generators fields. In order to improve the performance of the YBCO bulk superconductors further, in this work, nano-CeO&lt;sub&gt;2&lt;/sub&gt; doped YBCO bulk superconductors are fabricated by two infiltration growth techniques (011-IG and 211-IG) respectively, in which two solid pellets of compositions Y&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;+1.15BaCuO&lt;sub&gt;2&lt;/sub&gt;+0.1CuO+1wt.% nano-CeO&lt;sub&gt;2&lt;/sub&gt; and Y&lt;sub&gt;2&lt;/sub&gt;BaCuO&lt;sub&gt;5&lt;/sub&gt; (Y-211)+1 wt.% nano-CeO&lt;sub&gt;2&lt;/sub&gt; are employed. And a novel pit-type seeding mode is used to prevent the film seed from moving in the heat treatment process, then the growth morphologies, microstructures and superconducting properties of the samples are investigated. The results show that at a low doping level (1 wt.%), the normal growth of the YBCO crystal is not affected, and fully-grown single-domain YBCO bulk superconductors can be successfully prepared by the two techniques. Furthermore, the positions of the seeds do not move at all, which proves the effectiveness of the new seeding mode. The perpendicular growth sector boundaries on the top surfaces of the samples and clear (00&lt;i&gt;l&lt;/i&gt;) series X-ray diffraction (XRD) peaks both prove the high &lt;i&gt;c&lt;/i&gt;-axis orientations and high growth quality of the samples. The scanning electron microscopy (SEM) results indicate that the nano-CeO&lt;sub&gt;2&lt;/sub&gt; doping can effectively refine the sizes of the Y-211 micro-particles in the bulk superconductors, and this method is applicable to both techniques. Low-temperature magnetization measurement shows that the nano-CeO&lt;sub&gt;2&lt;/sub&gt; doped sample prepared by the 011-IG method shows obviously better &lt;i&gt;J&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; property than the undoped sample at low fields, indicating that the refined Y-211 particles can effectively enhance the δ&lt;i&gt;l&lt;/i&gt;-type pinning. In addition, compared with the 211-IG-processed sample, the 011-IG-processed sample performs better in terms of levitation force, microstructure and &lt;i&gt;J&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; property, thus the 011-IG method is a more promising preparation process. The results of this study are important in improving the performance of the YBCO bulk superconductors and optimizing the fabrication technique further.

Current–voltage characteristics in single layer SrIrO3 films deposited on LaAlO3(100) substrate

Here, we investigate the structural and electrical properties on SrIrO3 films grown on LaAlO3 (100) substrate with varying thickness (18, 25 and 40 nm). The x-ray diffraction shows good quality epitaxial films without any chemical impurity. The out-of-plane lattice parameter increases with the film thickness. All the films show a semiconducting behavior where the resistivity increases with increasing thickness. Our analysis shows at high temperature the charge conduction mechanism follows Mott’s two-dimensional variable-range-hopping model. Detailed current–voltage (I–V) measurements show a linear Ohmic behavior at room temperature, however, a prominent deviation from linearity has been observed at low temperatures where the exponent n () increases with decreasing temperature, reaching 1.5 at low temperature. Analysis of I–V data indicates that the charge conduction has dominant contribution of Poole–Frenkel mechanism rather than Schottky behavior. This evolution of charge transport with temperature is quite intriguing which may be related to the development of low temperature magnetism in films of SrIrO3.

Spontaneous Hall effect in the magnetic Weyl semimetallic Eu2Ir2O7 (111) thin films

Here, we have carried out a low-temperature magnetization and magnetotransport study on epitaxial antiferromagnetic Eu2Ir2O7 (111) thin films with an all-in-all-out/all-out-all-in spin structure to delve into the possibility of realizing spontaneous Hall effect that can provide strong experimental evidence for the existence of the Weyl semimetallic phase. The temperature-dependent resistivity on Eu2Ir2O7 (111) thin films indicates a semimetallic transition below 90 K and the existence of a correlated metallic state in the high-temperature regime (90–300 K). Magnetoresistance measurements suggest that at a low temperature (below 10 K), the all-in-all-out (AIAO) spin structure is stable enough against the applied magnetic field, whereas above 10 K, application of magnetic field causes field-induced modification of the spin structure and plastic domain deformation. Hall resistivity shows a spontaneous Hall effect (SHE) in the low-temperature semimetallic phase of Eu2Ir2O7 (111) thin films with minimal magnetization (AIAO/all-out-all-in spin structure). The observed SHE provides strong evidence for the emergent Weyl semimetallic phase in the (111) thin film geometry of antiferromagnetic Eu2Ir2O7.

Controlled preparation of Gd2O2SO4:Eu3+ monospheres via hydrothermal precursor engineering for enhanced photoluminescence

Rare-earth oxysulfates (RE2O2SO4) attracted attention for large-capacity oxygen storage, low-temperature magnetism and luminescence, whose preparation mostly involves toxic SOx gases and/or complicated procedures. In the phosphor field, monospheres are preferred for a number of applications due to their better luminescence and high packing density. With the simple reactants of RE(NO3)3, Na2CO3 and (NH4)2SO4 for hydrothermal reaction, dispersed monospheres and nanoplates were selectively synthesized for the recently discovered RE2(OH)2CO3SO4·nH2O (RE = Gd0.95Eu0.05, REOCSH) layered compound precursor, from which RE2O2SO4 phosphors mostly retaining their precursor morphologies were facilely derived via calcination in air at 800 °C, without involving SOx. Solution pH was found to decisively determine the chemical composition and crystallization kinetics of the initial precipitate, and the possible mechanisms of phase/morphology evolution during hydrothermal treatment were proposed by comparatively investigating the temperature-course and time-course of REOCSH formation under the typical pH values of 6 and 7. It was revealed that (Gd0.95Eu0.05)2O2SO4 monospheres emit ∼1.35 times as strong as nanoplates (λex = 257 nm, λem = 617 nm) and may retain as high as ∼81% of the room-temperature intensity at 150 °C. The phosphor monospheres were also analyzed to have a fluorescence lifetime of ∼1.32 ms at room temperature and an activation energy of ∼0.19 eV for the thermal quenching of luminescence.

Vacuum Brazing and Performance Evaluation of T2 Copper Block and 316L Stainless Steel Tube

The International Thermonuclear Experiment Reactor (ITER) Thermal shield (TS) serves as a cryogenic heat exchanger to maintain the thermal stability of the ITER superconducting magnet coil, which is critical to the control of the plasma during the operation of the ITER device. The TS is composed of long-length 316L stainless steel (SS) and copper as brazed joints. In this case, a feasible fabrication design for the CCS TS is presented, accomplished by three kinds of joining processes (vacuum brazing, friction stir weld, and TIG weld). In the reliable fabrication design, the brazing quality of the as-brazed long-distance 316L SS and copper joints plays a critical role in the thermal conductivity performance of the ITER thermal shield. Therefore, a high-quality vacuum brazing process of long-length SS/Cu joints applied in a low-temperature superconductor magnet system was first studied. The macro metallography analysis demonstrates the braze ratio of the samples is 100%, and no crack or defect is observed in the samples. The microstructural characterization reveals the brazing seams are composed of silver-based Ag-rich eutectic. The micro-shear test indicates that the shear strength of the 316L tube and copper joint is 205 MPa, with the fracture position located on the copper side; this zone will be the most vulnerable zone of the joints. In addition, the SEM results illustrated that the shear fracture morphology displayed a ductile fracture feature. The test results demonstrated that the highly precise depth drilling employed in this paper ensured a good control of the brazing clearance, resulting in a 100% braze ratio for the long-length SS/Cu joints. Therefore, it can be concluded that the brazing process can be applied in the ITER TS for the good thermal conductivity performance of long-length SS/Cu-brazing joints.

Crystallization of RE2(OH)2CO3SO4·nH2O as a new family of layered hydroxides (RE = Gd−Lu lanthanides and Y), derivation of RE2O2SO4, photoluminescence and optical thermometry

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 SOx 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 SOx. 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+, λex = 275 nm) and 608 nm (Sm3+, λex = 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.