查看更多>>摘要：Rare earth elements(REEs)are irreplaceable materials supporting low-carbon technology and equip-ment,and their commercial demand and strategic position are becoming increasingly prominent.With the continuous depletion of rare earth(RE)resources,developing high-efficiency beneficiation and eco-friendly metallurgical processes has attracted widespread attention.This paper reviews the major minerals exploited for RE production and their deposits,as well as the beneficiation and metallurgical processes of RE minerals.Bastnaesite,monazite,mixed RE ores,and ion-adsorption clays are the main raw materials in the world to date.RE-bearing ores(except ion-adsorption minerals)are generally beneficiated by flotation,gravity and magnetic separation techniques.The mainstream metallurgical processes for bastnaesite,monazite and mixed RE concentrates are oxidation roasting-HCl leaching,caustic soda decomposition and high-temperature concentrated sulfuric acid roasting,respectively.Ion-adsorption clays are directly processed by in situ leaching-precipitation/solvent extraction.To achieve the sustainable development of RE resources,it is essential to further explore innovative techniques to achieve comprehensive utilization and cleaner production.
查看更多>>摘要：The thermoluminescence(TL)emission of synthetic and natural Ce-monazites was characterized here to determine the potential application in the identification of microscopic defects from a qualitative point of view trying to link each TL peak to a chemical-physical process.The kinetic parameters that lead the luminescence processes were calculated by means of variable heating rate and computing glow curve deconvolution methods and allow identifying three groups of components at～90,130 and 290 ℃(for the mineral sample)and～90,170,220,270 and 320 ℃(for the synthetic CePO4∶Nd0.20,La0.25).The main differences appreciated in these complex TL curves are mainly due to(i)the content of impurities(natural sample contains lanthanides as well as U 0.60％and Th 5.22％)and(ii)the degree of crystallinity of the samples which is directly related to the type of impurities(synthetic monazite relies only on Nd and La).The behavior of the dose response in the range of 1-8 Gy is similar for both samples;the TL intensity increases linearly as the dose increases without changes in the position of the maxima,denoting first-order kinetic luminescence mechanism.Each peak could be mostly associated with structural defects(i.e.,phase transitions),chemical reactions(i.e.,Ce3+⇄Ce4+ redox reaction,dehydra-tion or dehydroxylation processes)or intrinsic defects(i.e.,Frenkel defects,ODCs or NBOHCs).
查看更多>>摘要：The interaction between support and noble metal plays a crucial role in heterogeneous catalysis design.However,how to tune metal support interactions to optimize the activity still needs further exploration.CeO2 was introduced to promote CO oxidation over Ir/TiO2 by adjusting the interaction strength between iridium(Ir)and CeO2.The strong interaction between Ir and CeO2 blocks CO adsorption and causes low CO oxidation activity.However,introducing CeO2 on Ir/TiO2 produces localized interaction between Ir and CeO2,which can tune the surface electronic state of Ir,so a"volcano curve"relationship between CO oxidation activity and electronic state is built.Limited amount of CeO2 on Ir/TiO2(Ir/Ce0.2Ti)leads to CO complete oxidization at 22 ℃,and a new pathway for CO oxidation was explored.The study demon-strates that the utilization of tuning interaction strength between active metal and support is a potential method to increase the catalytic activity.
查看更多>>摘要：Several ternary oxides CuCeZrOy(CCZ)were synthesized by a facile grinding method followed by calcination at high temperatures,and used as catalysts for CO oxidation at low temperatures.The in-fluences of calcination temperature(400-600 ℃)on the physicochemical properties of the as-synthesized ternary oxides were investigated by thermogravimetric analysis/differential scanning calo-rimetry(TGA/DSC),X-ray diffraction(XRD),transmission electron microscopy(TEM),Raman,inductively coupled plasma-optical emission spectrometry(ICP-OES),N2 adsorption,H2-temperature programmed reduction(H2-TPR),and X-ray photoelectron spectroscopy(XPS)characterizations.The results show that the increase in calcination temperature from 400 to 500 ℃ is conducive to the high dispersion of CuOx on catalyst surface and the incorporation of Cu species into the support to form the Cu-Ce-Zr-O solid solution.Further raising of calcination temperature from 500 to 600 ℃,however,leads to the segregation of Cu species from the solid solution to aggregate on support surface and growth of highly dispersed CuOx nanoparticles.The highest catalytic activity is acquired over the CCZ calcined at 500 ℃,which can be ascribed to the largest contents of Cu+species and oxygen vacancies owing to the formation of the maximum amount of Cu-Ce-Zr-O solid solution.
查看更多>>摘要：A series of rare earth metals modified birnessite manganese dioxides(RE-Mn)was synthesized by hy-drothermal method and their catalytic degradation of toluene was investigated.Yb-Mn shows the best toluene oxidation activity with the conversion of T50％and T99％at 196 and 226 ℃,respectively.The physiochemical characterizations identified that the high catalytic performance and excellent stability of Yb-Mn are mainly associated with its higher molar ratios of Mn4+/Mn3+,more lattice oxygen(Olatt),and better mobility of surface adsorbed oxygen(Oads).Moreover,it is confirmed that introduction of Yb into birnessite MnO2 facilitates the production of acetic acid at lower temperature.These results can aid the design of high-efficiency manganese oxide catalysts for the catalytic combustion of VOCs.
查看更多>>摘要：The extremely high structural tolerance of ceria to oxygen vacancies(Ov)has made it a desirable catalytic material for the hydrocarbon oxidation to chemicals and pharmaceuticals and the reduction of gaseous pollutants.It is proposed that the formation and diffusion of Ov originate from its outstanding reduction property.However,the formation and diffusion process of Ov over the surface of ceria at the atomic level is still unknown.Herein,the structural and valence evolution of CeO2(111)surfaces in reductive,oxidative and vacuum environments from room temperature up to 700 ℃ was studied with in situ aberration-corrected environmental transmission electron microscopy(ETEM)experiments.Ov is found to form under a high vacuum at elevated temperatures;however,the surface can recover to the initial state through the adsorption of oxygen atoms in an oxygen-contained environment.Furthermore,in hydrogen environment,the step-CeO2(111)surface is not stable at elevated temperatures;thus,the steps tend to be eliminated with increasing temperature.Combined with first-principles density function calculations(DFT),it is proposed that O-terminated surfaces would develop in a hypoxic environment due to the dynamic diffusion of Ov from the outer surface to the subsurface.Furthermore,in a reductive environment,H2 facilitates the formation and diffusion of Ov while Ce-terminated surfaces develope.These results reveal dynamic atomic-scale interplay between the nanoceria surface and gas,thereby providing fundamental insights into the Ov-dependent reaction of nano-CeO2 during catalytic processes.
查看更多>>摘要：High magnetic protective fabrics with rare earth Nd doped Fe3O4 nanoparticles were fabricated via a grafting method.The structure,crystal form,and elemental composition of nanoparticles were investi-gated by transmission electron microscopy,energy dispersive X-ray,X-ray powder diffraction,and X-ray photoelectron spectroscopy.The obtained NdFe2O4 nanoparticles show spherical shape with fine dispersion and reasonable element composition.However,they demonstrate fine superparamagnetic properties with a magnetic saturation value of 29.25 A·m2/g and low coercivity of 1.902 mT through the vibrating sample magnetometry technique,which can be well developed in magnetic shielding pro-tective applications.Then,the cotton fabrics with plain weave were finished by a proofing rapier loom as a carrier.The NdFe2O4 nanoparticles underwent some appropriate surface modification and then were grafted onto the cotton fabrics by a bridge agent of N,N'-dissuccinimidyl carbonate.The morphology,structure,dispersion effect,and electromagnetic protective properties of the fabrics were observed through scanning electron microscopy,Fourier infrared spectroscopy,thermogravimetric analysis-differential scanning calorimetry(TG-DSC),and a vector network analyzer.The reliable fabrics with grafting reaction on the interface are expected to have potential applications in the field of electro-magnetic protection and biomedicine fields.
查看更多>>摘要：The effects of wheel speeds on the magnetic properties and microstructures of[(Nd,Pr)1-xCex]-Fe-B melt-spun ribbons were investigated.Compared to melt-spun ribbons with low cerium(Ce)content(x=0.2),amorphous formation can be suppressed at high wheel speed in the ribbons with a relatively high Ce content(x=0.8),and with the increase of the wheel speed,the magnetic properties of the ribbons with high Ce content raise continuously.At high wheel speed,the coercivity mechanism of melt-spun ribbons is mainly pinning field,and the high wheel speed introduces a large number of defects into the matrix as pinning sites.Furthermore,ribbons with high Ce content form a fine and uniform grain structure,so the grain boundary area enlarges,which further hinders the movement of domain walls.The combined effects of the above factors enable ribbons with high Ce content to have excellent magnetic properties(Hcj=0.977 T,Mr=64 m2 A/kg)at the wheel speed of 35 m/s.
查看更多>>摘要：Grain boundary diffusion process(GBDP)is a widely used method of increasing the coercivity of sintered NdFeB magnets.In this study,the effects of the GBDP on the bending strength and microhardness of sintered NdFeB magnets and the fracture mode were investigated.Results show that the bending strength of magnets is reduced by pickling and heat treatment and greatly recovers after heavy rare earth element(Tb)grain boundary diffusion.The pickling and the heat treatment cause a slight decrease in microhardness.Compared with the recovery of the bending strength,the hardness decreases after the GBDP.The fracture mode of bended magnets changes from intergranular to transgranular.This study helps in further improving the mechanical and magnetic properties of sintered NdFeB magnets.
查看更多>>摘要：We report the magnetostrictive performance of the Nd0.2Tb0.3Dy0.5Fe1.93/epoxy composites.Measure-ment of Mössbauer spectra and the XRD results indicate that the easy magnetic direction of the Nd0.2Tb0.3Dy0.5Fe1.93 compound is along<111>direction with spontaneous magnetostriction λ111 of 1900 ppm.The<111>-oriented Nd0.2Tb0.3Dy0.5Fe1.93/epoxy composites with particle volume fractions of 20 vol％-55 vol％were prepared.Magnetization measurements demonstrate that the composite is highly anisotropic.The raw material cost of Nd0.2Tb0.3Dy0.5Fe1.93 is about 85％of that of the Terfenol-D(Tb0.3Dy0.7Fe2)composition.Nd0.2Tb0.3Dy0.5Fe1.93/epoxy composites possess comparable magnetostric-tion with those composites made of Tb0.3Dy0.7Fe2 particles.At room temperature,the magnetostriction λⅡof the Nd0.2Tb0.3Dy0.5Fe1.93/epoxy composite reaches 420 ppm at a low field of 80 kA/m and about 1000 ppm at 239 kA/m.In addition,the composite exhibits a large saturation magnetostriction λⅡ over 1200 ppm from 300 to 100 K,suggesting its potential applications as a low-cost magnetostrictive ma-terial in a wide temperature range.Therefore,the current study offers an opportunity to widen the operating temperature range of the magnetostrictive composite.
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