Electromagnetic and microwave absorption properties of Er-Ho-Fe alloys

The Er_xHo_(2-x)Fe_(17)(x = 0.0, 0.1,0.2, 0.3, 0.4) powders were prepared by arc melting and high energy ball milling method. The influence of the Er substitution on phase structure, morphology, saturation magnetization,electromagnetic parameters was investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM), vibrating sample magnetometer(VSM) and vector network analyzer(VNA),respectively. The results show that the saturation magnetization increases and the average particle size increases with the increase of Er content. The minimum absorption peak frequency shifts towards a lower frequency region with the increase of Er content. The Er_(0.3)Ho_(1.7)Fe_(17) powder can achieve the minimum RL of-24.07 dB at 6.96 GHz with a thickness of 2.0 mm and the minimum RL is less than-20 dB at the thickness range from 2.0 to 3.0 mm. The minimum RL of Er_(0.3)Ho_(1.7)Fe_(17) is-37.26 dB at5.68 GHz and the frequency bandwidth of R-10 dB reaches about 1.2 GHz with a thickness of 2.4 mm.And the microwave absorbing properties of the composite with different weight ratios of Er_(0.3)Ho_(1.7)Fe_(17)/graphene were researched. The microwave absorbing peaks of the composite shift to lower frequency with the increase of graphene content. The values of the minimum RL of Er_(0.3)Ho_(1.7)Fe_(17)/graphene are close to-10 dB with absorbing coating thicknesses increased.     

Effects of Sm-doping on microstructure,magnetic and microwave absorption properties of BiFeO_3

In this paper,polycrystalline samples of Bi_(1-x)Sm_xFeO~3(x=0,0.05,0.1,0.15) were successfully synthesized by sol-gel method.The effects of Sm concentration on the crystal structure,morphology,chemical states,magnetic properties and microwave absorption performance were studied by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),a vibrating sample magnetometer(VSM) and a Vector network analyzer(VNA),respectively.The results show that the rare earth Sm doping causes the crystal structure to change.When x≤0.1,Bi_(1-x)Sm_xFeO_3 is the distorted rhombohedral structure with space group R3 c.With the increase of Sm doping amount to x=0.15,the phase structure of Bi_(1-x)Sm_xFeO_3 changes from rhombohedral structure to cubic structure with the space group Pm3 m.The particle size decreases with the increase of the Sm doping amount.The analysis results show that Sm doping can effectively reduce the oxygen vacancies and significantly improve its magnetic properties.The results exhibit that moderately doped rare earth Sm element can effectively improve microwave absorption properties of Bi_(1-x)Sm_xFeO_3 powders.When Sm doping amount of x is 0.1,the Bi_(0.9)Sm_(0.1) FeO_3 compound has good microwave absorption performance,and the minimum reflection loss value of Bi_(0.9)Sm_(0.1)FeO_3 powder reaches about-32.9 dB at11.7 GHz,and its effective absorption bandwidth(RL -10 dB) is 2.6 GHz with the optimal matching thickness of 2.0 mm.     

First-principles study of structure,mechanical and optical properties of La-and Sc-doped Y_2O_3

The electronic,mechanical and optical properties of La-and Sc-doped Y_2O_3 were investigated using firstprinciples calculations.Two doping sites of Sc and La in Y_2O_3 were modeled.The calculated values of the energy of formation show that the most energetically favorable site for a La atom in Y_2O_3 is a d-site Y atom,while for Sc a b-site Y atom is the more stable position.The calculated band gap shows a slight decrease with increasing La or Sc concentration.The calculated results for the mechanical and optical properties of Y_(2-x)R_xO_3(R = Sc or La,0x ≤ 0.1875)show that La-or Sc-doped Y_2O_3 would have enhanced strength,and thus an ability of resisting external shocks,and increased hardness and mechanical toughness.These improved mechanical properties are achieved without sacrificing the optical properties of the doped compounds.So the doping of La or Sc in Y_2O_3 is permissible in the preparation of Y_2O_3 transparent ceramics,of course,doping of La or Sc will benefit the sintering of transparent ceramics.     

Influence of A-site doping barium on structure,magnetic and microwave absorption properties of LaFeO3 ceramics powders

In this paper, the effect of Ba²⁺ ions A-site doping LaFeO₃ on structure, magnetic properties and microwave absorption properties was investigated by the sol–gel method. According to the TEM and FullProf refinement results, the structure of LaFeO₃ changes from orthogonal (SG: Pnma) to cubic (SG: Pm-3m) when the Ba doping amount is x = 0.4. The SEM image shows that the particles size tends to decrease with the increase of Ba content. The production of weak ferromagnetism indicates that Ba doping has a significant effect on the magnetic properties of LaFeO₃. The Neel temperature (T_N) decreases significantly with the increase of Ba doping amount. An appropriate amount of Ba doping can effectively increase the dielectric and magnetic loss of LaFeO₃ ceramics powders. The increase permittivity (ε′ and ε″) may be attributed to the hopping of the electrons between the Fe³⁺ and Fe⁴⁺ ions with the Ba²⁺ ions doping. The minimum reflection loss (RL_min) of La_0.9Ba_0.1FeO₃ at 6.72 GHz reaches ?30.04 dB, its effective bandwidth (RL ≤ ?10 dB) is 2.1 GHz, and the matching thickness is only 3.2 mm. These results indicate that Ba doping can effectively control the microwave absorption properties of LaFeO₃, especially in the C-band.     

Upconversion luminescence of Gd2O3:Er3+ and Gd2O3:Er3+/silica nanophosphors fabricated by EDTA combustion method

Gd_2O_3:Er~(3+) nanophosphors were fabricated by the combustion method in presence of Na_2 ethylene diamine tetra acetic acid(EDTA-Na_2) as fuel at not high temperature(≤350℃) within a very short time of 5 min.The added concentration of Er~(3+)ions in Gd_2O_3 matrix was changed from 0.5 mol% to 5.0 mol%.The X-ray diffraction pattern of samples indicates the monoclinic structure of Gd_2O_3:Er3+.The morphology and chemical composition analysis of the Gd_2O_3:Er~(3+) samples are characterized by a field emission scanning electron microscope(FESEM) and a Fourier-transform infrared spectrometer(FTIR).The photoluminescence(PL),photo luminescence excitation(PLE) and upconversion(UC) at room temperature of the prepared materials with different concentrations of Er~(3+) were investigated.The PL of Gd_2O_3:Er~(3+)nanomaterials are shown in visible at 545,594,623,648,688 nm under excitation at 275 nm.The emission bands from transitions of Er~(3+) from ~2P_(3/2) to ~4F_(9/2) are observed,UC luminescent spectra of the Gd_2O_3:Er~(3+)/silica nanocomposites under 976 nm excitation show the bands at 548 and 670 nm.The influence of excitation power at 980 nm for transitions were measured and calculated.The results indicate that the upconversion process of Gd_2O_3:Er~(3+)/silica is two photons absorption mechanism.The low temperature dependence of UC luminescent intensities of the main bands of Gd_2O_3:Er~(3+)was investigated towards development of a nanotemperature sensor in the range of 10-300 K.     

Magnetocaloric effect in Gd5(Si,Ge)4 based alloys and composites

The research on magnetocaloric materials for applications concentrates, among other, on two parameters: the ordering temperature and the value of the magnetocaloric effect (MCE). The optimization consists in tuning the former without significant drop in the latter. These studies report on the magnetic susceptibility, magnetization curves, heat capacity and magnetocaloric effect measurements for compositionally and structurally modified Gd₅Si₄ compound. The modifications are based on the doping of the parent compound with an excess Gd atoms and substitution of Si with B as well as on the dimensional effect studied by mechanical milling. Moreover, composite samples of the type Gd:Gd₅Si₂Ge₂ were investigated revealing the influence of the intergranular interactions on the magnetocaloric properties. It appears that these interventions enable a controllable steering of the ordering temperature shifting it towards the room temperature with, in some cases, minor reduction of the parameters characterizing MCE.     

Optical properties of NaY(MoO4)2:Eu3+ nanophosphors prepared by molten salt method

We synthesized NaY(MoO_4)_2:Eu~(3+)phosphors of different doping concentrations by a molten salt method.This facile way possesses advantages such as simple process,lower calcination temperature(350℃) and small particle size(70 nm).The crystal system is tetragonal phase and crystal lattice is body centered.The photo luminescence measurements including emission spectra,excitation spectra and fluorescence decay curves were carried out,elucidating that NaY(MoO_4)_2:Eu~(3+)can be effectively excited by near UV and blue light.Moreover,it can be concluded that Eu3+energy transfer type is exchange interaction.Huang-Rhys factor and the critical energy transfer distance(Rc) were calculated to be 0.043 and 0.995 nm,respectively.Auzel's model was used to obtain the intrinsic radiative transition lifetime of~5 D_0 level(τ_0=0.923 ms).Furthermore,a calculation method was used to calculate refractive index n of nontransparent NaY(MoO_4)_2:1 mol% Eu3+phosphor,and n was obtained to be 1.86.     

Shape control over microwave hydrothermally grown Y2O3:Eu by europium concentration adjustment

Two-step synthesis of Y2〇3:Eu nanostructures was performed. It includes microwave driven hydrothermal and calcination stages. Performed route results in crystallization of Y4〇(OH)g(N〇3):Eu crystals initially, then Y2〇3:Eu crystals after calcination. Arranged Eu contents in relation to overall cation quantity were set to 2 mol%, 10 mol% and 20 mol%. Varying europium concentrations influence habit of obtained Y4〇(OH)g(N〇3):Eu crystals from needle-like to plate-like and as a result, also shapes of final Y2〇3:Eu nanostructures. Additionally, certain amount of Eu2+ ions was detected in as-grown material using laser spectroscopy and decay kinetics measurements. Obtained material was calcined at 1200 °C in the air, which results in oxidation of Eu2+ ions and crystallization of small number of cubic EU2 O3 nanocrystals. Characterization of obtained materials was performed using XRD, SEM, TEM, EDX, CL,Raman and photoluminescence spectroscopy.     

Effect of dysprosium substitution on magnetic and structural properties of NiFe_2O_4 nanoparticles

This work investigated the effect of dysprosium(Dy)ions on the structural,microstructural and magnetic properties of nickel nanospinelferrite,NiFe_2 O_4.The nanoparticles(NPs)of NiDy_xFe_(2-x)O_4(0.0≤x≤0.1)were prepared via the hydrothermal method.The formation of cubic phase of Ni nanoferrite was confirmed through X-ray diffraction,field emission scanning and transmission electron microscopy.Moreover,the magnetic properties of NiDy_xFe_(2-x)P_4(0.01≤x≤0.10)NPs were discussed.The magnetization versus field,M(H)curves exhibit super paramagnetic nature at room temperature and ferrimagnetic nature at low temperature(10 K).A noticeable improvement in the different deduced magnetic parameters is obtained especially for the NiDy_(0.07)Fe_(1.93)O_4(x = 0.07)product.The obtained result is mostly derived from the substitution of Fe~(3+)ions of smaller ionic radii with Dy~(3+)ions of larger ionic radii that will strengthen the super exchange interactions among nanoparticles.The calculated squareness ratios are found to be much less than 0.5,due to the effect of spin disorder in the surface regions of NiDy_xFe_(2-x)O_4(0.01≤x≤0.10)NPs.The Dy~(3+) ions substitution increases the magnetic hardness(higher values of remanence M_r,coercivity H_c,and magnetic moment n_B)of nickel nanoferrite samples.     

La~(3+) doped LiCo_(0.25)Zn_(0.25)Fe_2O_4 spinel ferrite nanocrystals:Insights on structural,optical and magnetic properties

This paper addresses the manipulation of structural,morphology,optical and magnetic properties of LiCo_(0.25)Zn_(0.25)Fe_2 O_4 ferrite via incorporation of different proportions of La~(3+) at the expense of iron ions using a sol-gel method.The samples were characterized using the X-ray diffraction technique(XRD),Fourier transform infrared(FT-IR) spectroscopy,the energy dispersive X-ray spectra(EDX),inductively coupled plasma optical emission spectroscopy(ICP-OES),high resolution scanning electron microscopy(SEM),Brunauer-Emmett-Teller(BET) surface area analysis,ultraviolet-diffuse reflectance spectroscopy(UV-DRS),and vibrating sample magnetometer(VSM) technique.The Rietveld refinements of the samples indicate that at higher concentrations of La3+,nanostructures with dual phase,i.e.cubic spinel and orthorhombic LaFeO_3 perovskite with space group(Pbnm) appear.Optical studies show that the energy band gap(E_g) of the bare LiCo_(0.25)Zn_(0.25)Fe_2 O_4 ferrite sample(2.18 eV) reaches up to 2.47 eV at x=0.06 and above this concentration,it drops sharply to 2.00 eV.Although the saturation magnetization and the coercivity of LiCo_(0.25)Zn_(0.25)La_xFe_(2-x)O_4 are lower than that of LiCo_(0.25)Zn_(0.25)Fe_2 O_4 NPs.Overall,the superparamagnetic nature and low values of saturation magnetization and coercivity of LiCo_(0.25)Zn_(0.25)La_xFe_(2-x)O_4 NPs are suitable to be applied in transformers core.     

Crystal-field levels of Nd3+ in a new langasite compound Nd3CrGe3Be2O14

We report on the crystal-field (CF) levels of Nd³⁺ in Nd₃CrGe₃Be₂O₁₄, a representative of recently synthesized Be-containing langasites. They are the first in the family to contain two magnetic subsystems, 1D chromium and Kagome-like rare-earth ones. High-resolution broad-band temperature-dependent spectra allow us to find all five CF Kramers doublets of the ground ⁴I_9/2 CF multiplet and 38 levels of the excited ⁴I_{11/2, 13/2, 15/2}, ⁴F_3/2, ⁴F_5/2 + ²H_9/2, ⁴F_7/2 + ⁴S_3/2 and ⁴F_9/2 CF multiplets. These data form a basis for a future exploration of this interesting two-sublattice magnet with frustrated interactions. Optical transitions between Kramers doublets of Nd³⁺ indicate magnetic activity of 1D-chromium chains at T < 20 K.     

Magnetic properties and magnetic entropy change of perovskite manganites La_(0.9-x)Eu_xSr_(0.1)MnO_3(x=0.000,0.075) by experimental method and numerical fitting

Polycrystalline samples La_(0.9-x)EuxSr_(0.1)MnO_3(x = 0.000, 0.075) were prepared by the standard solid-state reaction method. The results show that the samples preform a characteristic of clusters spin-glass state at low temperature. The samples show a characteristic of ferromagnetism(FM) characteristic in the temperature range of 15-125 K and 15-150 K respectively; the samples show preformed clusters in the temperature range of 125-343 K and 150-325 K, respectively, the samples show paramagnetism(PM)characteristic above 343 and 325 K, respectively. The second-order transitions are found at 118 and 135 K for undoped and doped sample, respectively. When the applied magnetic field is 7 T, the maximum magnetic entropy change |△S_M| value of the samples is near the Curie temperature(Tc), and the value of|△S_M| reaches 2.76 and 3.03 J/(K kg), respectively. In addition, the relative cooling power(RCP) is found to be 425.28 and 443.53 J/kg. The numerical fitting data fit well with experimental data. These results indicate that both the samples have the potential to realize magnetic refrigeration in the high temperature region(T 77 K).     

Unique structural and magnetic traits of Nd3+-substituted Co–Zn nanoferrites

Because of the technological potential of magnetic spinel nanoferrites, we prepared neodymium ion (Nd³⁺)-substituted cobalt-zinc ferrites (CZFs) with the form Co_0.5Zn_0.5Nd_xFe_2–xO₄ (0.0 ≤ x ≤ 0.05) via a hydrothermal method. The as-prepared samples were thoroughly characterized using various analytical techniques. XRD, FTIR and FESEM analyses confirm the formation of a cubic spinel phase of the CZFNPs (CZF nanoparticles). A decrease in the lattice parameter due to the substitution of Fe³⁺ by Nd³⁺ in the lattice structures is manifested in the XRD refinement data. The magnetic properties of the proposed CZFNPs were evaluated in terms of the saturation magnetization, remanence, coercivity, squareness ratio and magnetic moment. These CZFNPs exhibit superparamagnetic behaviors at room temperature. Moreover, the Nd³⁺ inclusion does not significantly affect the measured magnetizations and coercivities of the CZFNPs. Samples containing 0.01 and 0.03 Nd³⁺ exhibit lower saturation magnetizations than that of the pristine product. The squareness ratios much less than 0.53 are ascribed to surface spin disordering. The unique magnetic traits of the synthesized CZFNPs are primarily attributed to the substitution of Fe³⁺ ions, with smaller ionic radii, by Nd³⁺ ions, with larger ionic radii. The proposed CZFNPs may be useful for diverse magneto-optic applications.     

Influence of lanthanum on enhancement of mechanical and electrical properties of Cu-Al_2O_3 composites

Two kinds of Cu-Al_2O_3 composites(with and without La) were prepared via mechanical alloying-spark plasma sintering(MA-SPS) method. Microstructure, mechanical properties and electrical resistivity were investigated systematically using metallography, scanning electron microscopy, transmission electron microscopy, mechanical and electrical properties testing. The results indicate that an appropriate amount of La can homogenize the distribution of Al_2O_3. As such, yield strength, ultimate tensile strength and elongation of Cu-Al_2O_3-La are greatly increased. Some semi-coherent interface between Cu and Al_2O_3 is found, which means a low interface energy. The grain shape of Cu changes to irregular band with the addition of La. This change results in a density decrease of grain boundary and reduces electrical resistance. Lanthanum may exist in the form of La_2O_3.     

Magnetic properties and microstructure of Fe_(69.5-x)Nd_7B_(21)Nb_(2.5)Ga_x(x=0-1)alloys

The Fe_(69.5-x)Nd_7 B_(21)Nb_(2.5)Ga_x(x = 0-1)permanent magnets in the form of rods were prepared by annealing the bulk amorphous alloys.The magnetic properties,phase evolution and microstructure of the alloys were investigated systematically.It is found that the glass forming ability(GFA), microstructure and magnetic properties are sensitive to Ga content for Fe_(69.5-x)Nd_7 B_(21)Nb_(2.5)Ga_x(x = 0-1)bulk alloys.The annealed alloys are mainly composed of soft α-Fe,hard Nd_2 Fe_(14)B and nonmagnetic Nd_(1.1)Fe_4 B_4 phases.When x = 0.3,the optimally annealed magnets exhibit magnetic properties of the remanence Br = 0.63 T,intrinsic coercivity H_(cj) = 368.68 kA/m and maximum energy product(BH)_(max) = 33.73 kJ/m~3.Furthermore,magnetic field heat treatment at the temperature close to Curie temperature of Nd_2 Fe_(14)B phase was applied to the annealed Fe_(69.2)Nd_7 B_(21)Nb_(2.5)Ga_(0.3) magnet.The results of X-ray diffraction(XRD)and transmission electron microscopy(TEM)indicate that the magnetic field heat treatment can be beneficial for the precipitation of α-Fe.Thus,the B_r,H_(cj) and(BH)_(max) are enhanced by 8.7%,6.3% and 16.3%,respectively.     

Photoluminescence properties of Eu~(3+)-doped MgAl_2O_4 nanoparticles in various surrounding media

Eu~(3+)-doped MgAl_2O_4 nanoparticles were prepared by modified Pechini method.X-ray diffraction analysis shows pure tetragonal spinel phase without any impurities.The average size of synthesized nanoparticles was determined to be about 50-60 nm using scanning electron microscopy and static light scattering techniques.Emission and excitation spectra as well as lifetimes of MgAl_2O_4:Eu~(3+) nanophosphors were explored in surrounding media with different refractive indexes.Surrounding medium does not affect luminescence bands position,whereas ~5D_0 lifetime monotonically decreases along with increase of medium refractive index.Effect of surrounding media on radiative and nonradiative decay rates,which were calculated using 4f-4f intensity theory,was studied and discussed.Filling factor of prepared nanoparticles is defined using both radiative and observed lifetimes.     

Effect of preparation method on physicochemical properties and catalytic performances of LaCoO_3 perovskite for CO oxidation

Perovskite oxides LaCoO_3 prepared by templating, co-precipitation and sol-gel method with different complexants were systematically characterized and its catalytic performances for CO oxidation were investigated. The samples were characterized by X-ray diffraction, thermogravimetry analysis and differential scanning calorimetry, N_2 physisorption, transmission electron microscopy, temperature program reduction of hydrogen, temperature program desorption of oxygen and X-ray photoelectron spectroscopy measurement, results of which show that the properties of LaCoO_3, such as surface morphology, surface area, surface compositions, redox capability, oxygen vacancy, as well as the calcination temperature and formation mechanism, depend intimately on the preparation method. Catalytic tests indicate that the sample prepared by carbon templating method shows the best activity for CO oxidation, with full CO conversion obtained at 135 ℃. In particular, the catalyst can be activated and significant increase of activity can be obtained with the increase of reaction time. The cyclic and longterm stability of catalysts were discussed and compared.