查看更多>>摘要：Giant magnetocaloric effect(MCE)materials in the liquid helium temperature region have attracted a lot of attention in the field of low-temperature magnetic refrigeration(MR).In this study,a series of niobium(Nb)and iron(Fe)co-substituted EuTiO3 perovskites with cubic structure(space group pm3m)was successfully fabricated,and their magnetic properties as well as cryogenic magnetocaloric effects were investigated in detail.As expected,the introduction of Nb and Fe can significantly modulate the magnetic phase transition and magnetocaloric effect of the EuTiO3 compounds.With increasing Fe concentration,two local minima corresponding to the AFM-FM magnetic phase transition near 5.0 K and FM-PM transition near 10 K with no hysteresis in the thermomagnetic curves are observed,which is attrib-uted to an enhancement of FM coupling.At the same time,the gradually widened-△SM-T curves and the two peaks with a broad shoulder lead to considerable refrigeration capacity(RC).With the field change of △H=2 T,the calculated values of-△SmaxM for the EuTi0.9375-xNb0.0625FexO3(x=0.075,0.1,0.125,0.15)compounds are 24.2,17.6,14.5 and 14.0 J/(kg·K),respectively.The corresponding RC values were calculated to be 144.6,138.3,151.2 and 159 J/kg,respectively.Especially,the values of-△SmaxM for EuTi0.8625Nb0.0625Fe0.07503 are 8.6 and 15.1 J/(kg-K)under low field changes of 0.5 and 1 T,respectively.The giant low-field reversible magnetocaloric effect makes them attractive candidates for magnetic refrigeration in the liquid helium temperature region.
查看更多>>摘要：In this paper,nitrogen-doped graphene quantum dots(N-GQDs)were combined with gadolinium ions(Gd3+)by a surface modification to obtain magneto-optical dual-functional N-GQDs/Gd3+nanoparticles.The morphology of obtained composite was characterized by field emission scanning electron micro-scopy and transmission electron microscopy.Luminescence and magnetic properties were measured by a fluorescence spectrophotometer and a vibrating sample magnetometer,respectively.Results indicate that well-dispersed spherical N-GQDs/Gd3+nanoparticles have an average diameter of 7 nm.N-doping significantly increases the luminescence of particles with an optimal luminescence intensity at 20 ℃ and pH=9.X-ray photoelectron spectroscopy results indicate that the N-doping introduces pyrrolic N as an electron donor,enhancing fluorescence by increasing the surface electron cloud density of N-GQDs.In addition,density functional theory calculation results reveal that N-doping reduces the band gap of N-GQDs/Gd3+,enabling electronic transitions to higher energy levels and generating more activation sites,thereby enhancing luminescence.Compared to N-GQDs/Gd3+prepared at 20 ℃,the saturated magne-tization of particles prepared at 40 ℃ is 0.85 emu/g,indicating a better magnetic response.The above results suggest that bifunctional nanomaterials N-GQDs/Gd3+with excellent optical properties and magnetism can be better used for fluorescence and magnetic resonance imaging.
查看更多>>摘要：Five perovskite ceramics samples of Ba1-xErxSnO3 with x=0.00,0.05,0.10,0.15,and 0.20 were synthesized using the conventional solid-state reaction method.The prepared samples were characterized by X-ray diffraction(XRD),Fourier transforms infrared(FTIR),and UV-Vis spectroscopy.The shielding properties against ionizing radiation were also investigated.The XRD analysis shows the perovskite cubic structure as the major phase in all samples.The FTIR reveals a distinctive band around 614-620 cm-1 ascribed to the antisymmetric O-Sn-O vibration.UV-Vis spectroscopy was used to determine the bandgap according to diffuse reflectance,and the results display a fixed enhancement in the bandgap from 3.2 to 3.3 eV for all samples.Gamma-ray shielding properties of the synthesized samples were experimentally measured and compared with XCOM computational results.The relative differences(A,％)between experimental and theoretical values are low and fall within the 0.446％-7.10％range.The addition of Er leads to the enhanced density,neutron,and gamma shielding features.In contrast,charged particles'shielding parameters gradually reduce with rising Er contents.These results suggest that Ba1-xErxSnO3 samples can be used in different radiation shielding applications.
查看更多>>摘要：LaMnO3 modified Co3O4 nanocomposites were prepared by simple hydrothermal method combined with sol-gel method.The gas sensitivity properties of pure Co3O4 and LaMnO3/Co3O4 with different com-posite proportions are compared.It is found that 0.6-LMO/Co3O4 sensor has higher sensitivity to trie-thylamine(TEA)than pure Co3O4 sensor,which is improved by 9.27 times.And the working temperature is reduced from 150 to 130 ℃.Besides,it has excellent gas selectivity and repeatability.The improvement of the gas sensitivity of LaMnO3/Co3O4 sensor may be due to the fact that LaMnO3 is an effective catalyst,and the catalytic performance perhaps is beneficial to improving the sensing performance.In addition,the formation of p-p heterojunctions may be the key factor to improve the gas sensing performance.This work provides a new Co3O4-based gas sensing material for the detection of TEA.
查看更多>>摘要：While the metal to insulator transition(MIT)of d-band correlated perovskite nickelates(RENiO3)are widely adjustable via their rare-earth composition,the roles of potential valence variabilities associated with the rare-earth elements were rarely concerned.Herein,we demonstrate the material synthesis and MIT properties of RENiO3 containing valence variable rare-earth compositions,such as Ce,Pr,Sm,Eu and Tb.The metastable perovskite structure of SmNiO3 and EuNiO3 with a rare-earth valence states variable towards+2 can be effectively synthesized under high oxygen pressures as it is necessary to reduce their formation free energies.This is in contrast to Ce and Tb,in which situations the variable rare-earth valence state towards+4 reduces their ionic radius and prohibits their occupation or co-occupation of the rare-earth site within the perovskite structured RENiO3.Nevertheless,PrNiO3 with MIT properties can be effectively synthesized at lower oxygen pressures,owing to the higher stability to form a fully occupied 6s orbit associated Pr3+compared to the half-filled one related to Pr4+.The present work provides guidance for regulating the MIT properties of RENiO3.
查看更多>>摘要：Although coal fly ash(CFA)contains a high content of rare earth elements(REEs),the related extraction methods have limitations because of their low efficiencies,high levels of energy consumption,and other drawbacks.To address these problems,in this study,we examined the coextraction of REEs and Al2O3 from two types of Al2O3-rich CFA,pulverized CFA(PCFA)and circulating fluidized bed fly ash(CFBFA)using low-temperature calcination in the presence of K2S2O7.The total REEs,heavy REEs(HREEs),and light REEs(LREEs)extraction efficiencies were determined using different K2S2O7/Al2O3 molar ratios and calcination temperatures and correlated with the Al2O3 extraction efficiency using Pearson correlation coefficient analysis.The REEs are concentrated within CFA particles encapsulated in an aluminosilicate glass phase,and the REEs extraction efficiency is related to the form of Al in CFA.The extraction effi-ciencies of Al2O3 and REEs increase as the K2S2O7/Al2O3 molar ratio and calcination temperature increase,and the extraction selectivity of the more industrially valuable HREEs from CFBFA is higher.At high K2S2O7/Al2O3 molar ratios,the extraction of REEs from PCFA is more efficient than that from CFBFA with the regeneration of the highly active Al-O-Si bonds in CFBFA.The Al2O3 extraction efficiencies of PCFA as well as CFBFA correlate strongly with the total REEs,HREEs,and LREEs extraction efficiencies.The developed extraction technology has the potential to promote CFA valorization and expand REEs re-sources,thus mitigating the bottlenecks of REEs procurement.
查看更多>>摘要：In order to lower the usage of expensive Cyanex 923 and increase the extraction capacity of the system of Cextrant 230,the synergistic extraction of thorium from chloride media by a mixture of Cextrant 230 and Cyanex 923 was investigated.The maximum synergistic enhancement coefficient(R)of 1.53 is obtained at 1∶1 molar ratio of Cextrant 230/Cyanex 923.The synergistic extracted species of Th4+is determined as ThC14-2Cextrant 230-Cyanex 923.The synergistic extraction of Th4+is an entropy-driven exothermic process.The loading capacity of 0.60 mol/L mixed extractant for thorium is about 17.10 g/L(calculated as ThO2),and the loaded thorium in the organic phase can be effectively stripped by distilled water.For comparison,rare earth cations are barely extracted under the similar conditions,suggesting that the mixtures can be applied to separate thorium from rare earths.A cascade extraction process was devel-oped based on the synergistic extraction system to separate thorium from the hydrochloric acid leaching of bastnaesite.The content of thorium in the leaching solution decreases obviously from 19.90 mg/L to 1.4 μg/L by 3 stages of extraction,which is superior to sole Cextrant 230 or Cyanex 923.The introduction of Cextrant 230 into the extraction system not only lowers the usage of Cyanex 923 but also enhances the selective extraction of thorium at low acidity,implying that the synergistic extraction system can selectively extract thorium more efficiently and economically than the sole systems.
查看更多>>摘要：Rational design of extractant structure to improve the extraction and separation capacity of rare earth elements(REEs)is a long-standing challenge.Herein,a new strategy was proposed to improve the stability of the chelating structure formed by malonamide and REEs.The stability of the chelating structure is greatly improved by using a double-ring framework structure which makes the two carbonyl groups of malonamide unable to rotate freely and both of them point to the metal ion position.Three benzyl groups were used to construct the hydrophobicity of the extractant and the organic shell of the extracted species.Tribenzylhexahydro-pyrrolo-pyridine-dione(THPPD)was designed and synthesized.The structure reduces the energy consumed by rotating carbonyl group in the coordination with metal ions and then improves the extraction ability of extractant.The crystal structure was preorganized as expected,with the two carbonyls pointing in a favorable coordination direction to the structural com-plement of the metal ion.The extraction behaviors of REEs with THPPD in a nitric acid medium were studied.Compared with N,N'-dibenzyl-N,N'-dimethylmalonamide(DBDM-MA)with a chain structure,the extraction capacity of THPPD is 360 times higher than that of DBDM-MA at 5.0 mol/L sodium nitrate.Furthermore,the binding energy and Gibbs free energy were investigated by density functional theory(DFT)in conjunction with the B3LYP.The theoretical results show that THPPD has more effective interaction with Pr(NO3)3 than DBDM-MA.The construction of chelating groups conformation is a worthy direction to improve the coordination ability and even selectivity of extractant.
查看更多>>摘要：Chitin powder and chitin-aerogel were prepared from shrimp wastes and used to uptake Y3+from aqueous solutions and concentrate this rare earth element from phosphogypsum(PG).Chitin aerogel displays a specific surface area of 945 m2/g,while chitin powder is 3.6 m2/g,which largely influences its adsorption ability.Regarding the adsorption in synthetic solutions,the effect of pH on Y3+removal is strong for chitin powder adsorbent.In contrast,no big pH influence was detected for chitin aerogel.Electrostatic interactions and chelation can highlight the proposed mechanism of Y3+on chitin adsor-bents for the powder and aerogel.Furthermore,in addition to these interactions,pore filling/pore diffusion is the main mechanism of Y3+removal in the chitin aerogel.Chitin aerogel is efficient in concentrating 8 times the Y3+from PG,a very complex matrix.The complex chitin aerogel-Y3+can be a secondary source of rare earth elements for other applications.
查看更多>>摘要：The effect of cerium(Ce)on the solidification microstructure of Cr4Mo4V bearing steel was investigated via a combined experimental and theoretical method.With a trace amount(0.056 wt％)of Ce addition,the coarse columnar grains in as-cast microstructure transform into equiaxed ones,and the average diameter is reduced from 56 to 27 μm.The network-like and bulky primary MC and M2C carbides at the interdendritic regions become disconnected and refined,and their volume percentage decreases from 4.15 vol％to 2.1 vol％.Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries,both contribute to the refinement of grains.Thermody-namic calculations reveal that primary carbides are precipitated after γ-austenite forms near the end of the solidification process.The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refined γ-austenite grains in which the nucleation of carbides is promoted,while the growth is restrained owing to the less segregation of alloying elements.
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