Microwave absorption studies of MgB2 superconductor

Microwave absorption studies have been carried out on MgB₂ superconductor using a standard X-band EPR spectrometer. The modulated low-field microwave absorption signals recorded for polycrystalline (grain size ∼ 10 μm) samples suggested the absence of weak-link character. The field dependent direct microwave absorption has been found to obey a ✓H dependence with two different slopes, which indicated a transition from strongly pinned lattice to flux flow regime.     

A superior microwave absorption material: Ni2+-Zr4+ Co-Doped barium ferrite ceramics with large reflection loss and broad bandwidth

Large reflection loss and wide bandwidth are significant targets, determining the microwave absorption ability. However, it is still a challenge to simultaneously satisfy the two conditions. As a multifunctional material, BaFe₁₂O₁₉ possess excellent electromagnetic properties in the microwave frequency band. Due to the natural resonance phenomenon of the material, BaFe₁₂O₁₉ can produce a large magnetic loss which correlates with Fe³⁺ content, and the microwave absorption characteristics of barium ferrite can be modulated by ion doping. As a typical magnetic metal, Ni coupled with high-valence state Zr⁴⁺ doping helps to produce double resonance peaks. In this work, Ni²⁺-Zr⁴⁺ co-doping M-type barium ferrites (BaFe_12-2xNi_xZr_xO₁₉, BNZFO-x, x = 0–0.8) were prepared conveniently by solid-state reaction method. Several necessary measurements to characterize its microwave absorption property have been operated such as morphology, magnetic performance and electromagnetic parameters. The results show that reflection loss and bandwidth can be simply tuned by tailoring Ni²⁺-Zr⁴⁺ content. The reflection loss peak drifts from 18 GHz to 9.76 GHz, which involves a half of the studied frequency range. The maximum reflection loss achieves −60.6 dB and the corresponding bandwidth over −10 dB is 7.68 GHz for BNZFO-0.6 ceramic with only 2.1 mm thickness. Thus, the doping of Ni²⁺-Zr⁴⁺ ion pairs is beneficial to improve the absorbing properties of the material, and the superior microwave absorption property may originate from its inner double natural resonance in micro-scale. The excellent microwave absorption properties suggest that BNZFO-x is a promising candidate applied for designing electromagnetic shielding devices.     

关于MgB2超导多层膜间临界厚度的研究

利用多级的阶梯状势垒近似方法,应用于MgB<,2>作为超导体的SSeS结,对其临界厚度从基本原理入手并进行理论推导,获得超导多层膜的临界厚度公式,并对该公式展开讨论,取得一系列较有价值的结论.     

MgB2超导多层膜的制备方案

简要叙述了MgB2的发展和制备历史,通过对不同衬底材料上MgB2的反应情况的总结概括以及对不同衬底材料和MgB2的晶体结构和晶格常量的比较,提出一种MgB2超导多层膜的制备方案.     

MgB2超导薄膜制备方法对比研究

文章结合MgB2超导薄膜在电子器件中的应用,简介了混合物理化学气相沉积法、脉冲激光沉积法、磁控溅射法、分子束外延法和电子束蒸发法等MgB2超导薄膜制备方法和研究进展,并对主要的MgB2超导薄膜制备方法进行了比较,提出了改进意见.对MgB2超导薄膜的应用前景作了展望.     

Enhanced microwave absorption of Fe flakes with magnesium ferrite cladding

Surface cladding of Magnesium ferrite (MgFe₂O₄) was found to be helpful for effectively decreasing the permittivity of Fe flakes. So the electromagnetic impedance matching was improved, resulting in a good microwave absorption. Careful characterizations showed that the giant decreases of permittivity were ascribed to the high resistivity of surface due to the MgFe₂O₄ cladding. A high frequency microwave absorption property with thin absorber thickness was obtained for the Fe flakes with 10 wt% MgFe₂O₄.     

Electronic structure of MgB2

Results of ab initio electronic structure calculations on the compound MgB₂ using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables us to estimate the equilibrium volume, c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter by using our calculated D (E _F) and the experimental specific heat data. The T _c is found to be 24.7 K.     

Effect of shape of Fe3Al particles on their microwave permeability and absorption properties

Flake-shaped and sphere-shaped Fe₃Al powder-paraffine composites have been studied in the microwave frequency range. Mössbauer results show that the flake-shaped Fe₃Al particles have easy magnetization plane, which indicates that it is planar-anisotropy. Enhanced permeability is achieved in flake-shaped Fe₃Al compared with the sphere-shaped Fe₃Al. The permeability is further enhanced by using a rotational orientation method. The complex permeability can be characterized by the superposition of two types of magnetic resonance. The resonance peak at high frequency is attributed to the natural resonance, while the peak at low frequency is attributed to the domain-wall resonance. By employing the shape effect and the rotational orientation, the peak frequency of reflection loss for the oriented sample was adjusted to L-band. The planar-anisotropy Fe₃Al powder-paraffine composite can be attractive candidates for thinner microwave absorbers in L-band (1-2 GHz).     

Preparation and microwave absorption properties of electroless Co-P-coated nickel hollow spheres

Co-P-coated nickel hollow spheres (NHSs) were prepared by electroless plating technology. The morphology and component content of Co-P coating varies with the change of sodium citrate concentration in elctroless plating solution. And as phosphorus content increases in coatings, resulting in smaller grain, coercivity of microspheres decreases. The microwave absorption properties of spheres-wax composite were investigated in the range of 2-18 GHz. Both permittivity and permeability increase with an increase of cobalt content in coatings. For composite layer, a minimal reflection loss (RL, −36.9 dB) of was predicted at 8.1 GHz with a thickness of 3 mm.     

Characterization of magnetic materials by low-field microwave absorption techniques

A low-field non-resonant microwave absorption has recently been observed in a variety of magnetically ordered materials at low DC fields (−1000 Oe H_DC+1000 Oe), which is known as low-field microwave absorption (LFA). It has been shown that LFA is essentially similar to giant magnetoimpedance (GMI), and clearly different from ferromagnetic resonance (FMR). LFA strongly depends on the anisotropy field of the sample. In contrast with FMR (which can be described as the homogeneous precession of spins in the saturated state), LFA can be thought as a spin rotation process occurring during the magnetic saturation. In this work, we present a detailed study of the basic features of LFA in several types of materials: ferrites and amorphous microwires and ribbons; in particular the effects sample shape, temperature up to the Curie transition, the influence of easy axis and the effects of annealings. These examples show that once LFA is fully understood, it can become a powerful characterization tool.     

Remanence of the interparticle interactions and its influence on the microwave absorption in Co-ferrite

Cobalt ferrite nanoparticles were obtained by the sol–gel method at several annealing temperatures: 400, 450, 500, 550 and 600 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed the formation of the spinel phase with a nanoparticle size in the 17–26 nm range as function of the annealing temperature. The Mössbauer spectra at room temperature showed the presence of a partial inverse spinel structure. Saturation magnetization and the coercive field are strongly dependent on the annealing temperature and they can be associated with variations of the nanoparticles size. Microwave power absorption (MPA) (dP/dH) measurements were carried out as a function of DC field (H_DC) in asymmetric sweeps in the 0 kOeH_DC9 kOe range, at X-band (9.4 GHz), for all annealing temperatures. The large hysteresis in the MPA is due to interparticle interaction associated with its demagnetizing-like nature.     

Attractive microwave absorption and the impedance match effect in zinc oxide and carbonyl iron composite

The flower-like ZnO and ZnO/carbonyl-iron composite have been prepared by a sonochemical route and ball-milling process, respectively. For ZnO/carbonyl-iron composite, a reflection loss (RL) exceeding −20 dB was obtained in a broad frequency range of 8.4-17.9 GHz with a thin thickness of 1.2-2.3 mm. An optimal RL of −61 dB was found at 11.7 GHz for an absorber thickness of 1.91 mm. It is demonstrated that the attractive microwave-absorption properties are a consequence of a proper electro-magnetic impedance match and geometrical cancellation at the air-material interface. In addition, an impedance mismatch function was proposed, which provides an effective method to determine the microwave absorbing properties from the intrinsic materials constants. The calculated value of matching frequency and thickness is well consistent with the experimental data. The method also provides a simple theoretical graphic aid for determining the absorption characteristics and the location of the matching conditions in the frequency domain.     

Effect of 3d transition metal substitution on microstructure and microwave absorption properties of FeSiB nanocrystalline flakes

Effect of 3d transition metal substitution (Co and Ni) is studied on microstructure and absorption properties of FeSiB flakes in GHz. Fe_78−xM_xSi₉B₁₃ (M=Co or Ni, x=0, 5, 10 and 15) nanocrystalline flakes with nanocrystal/amorphous two-phase structure were prepared by ball milling. Substitution decreases the nanocrystal size and affects microwave behavior both compositionally and structurally according to the substitution mechanism. Compositional influence is contributing to the change of intrinsic magnetization parameters; structural influence works on exchange coupling. Absorption band of Fe₇₈Si₉B₁₃ flakes shifts towards lower frequency by the substitution, and Fe₆₃Co₁₅Si₉B₁₃ absorber shows promising absorption at C-band.     

Preparation and microwave absorption properties of loose nanoscale Fe3O4 spheres

Magnetite nanoparticles are found to assemble into randomly dispersed loose nanoscale spheres with diameters ∼300 nm in ethylene glycol in the presence of polyethylene and a small quantity of polyethyleneimine. Modern analysis methods are employed to provide structure information of the magnetic loose spheres. The ferromagnetic saturation magnetization is ∼80.0 emu g⁻¹, and the coercive force is 209 Oe. The microwave electromagnetic parameters are measured by a vector network analyzer. The synthesized loose spheres exhibit novel microwave properties compared with the conventional Fe₃O₄ nanoparticles. An additional microwave loss peak appears in the Ku band, which is attributed to the loose structure.     

The influence of temperature on magnetic and microwave absorption properties of Fe/graphite oxide nanocomposites

Fe/graphite oxide nanocomposites were prepared by inserting Fe³⁺ into layers of graphite oxide and then reducing Fe³⁺/graphite oxide compound at different reduced reaction temperatures in H₂. The composition, crystal structure, magnetic and microwave absorption properties of Fe/graphite oxide nanocomposites were investigated using elemental analysis, transmission electron microscope (TEM), X-ray diffraction (XRD), magnetic hysteresis curve and electromagnetic parameter analysis. The results show that the densities of samples are 2.43–2.47 g/cm³ and the nanocomposites are soft magnetic materials. The optimum reduced reaction temperature for preparing Fe/graphite oxide nanocomposites is 600 °C. With the increase of the thickness of the sample, the matching frequency tends to shift to the lower frequency region, and theoretical reflection loss becomes less at the matching frequency. Microwave absorption property of Fe/graphite oxide nanocomposites prepared at 600  °C (FeGO600) is the best. When the thickness is 1 mm, the maximum theoretical reflection loss of FeGO600 is −9 dB and the frequency region in which the maximum reflection loss is more than −6.0 dB is 11–18 GHz. In conclusion, FeGO600 is a good candidate for microwave absorbent due to its low density, wide frequency region for microwave absorption and large reflection loss.     

Preparation and microwave absorption properties of electroless Co-Ni-P coated strontium ferrite powder

A new type of Co-Ni-P coated strontium ferrite nanocomposite was prepared with electroless plating enhanced by ultrasonic wave at room temperature. The plating process was studied carefully. The morphology, crystal structure and microwave absorption properties of the Co-Ni-P coated powder were studied with field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDX) and vector network analyzer. The results show that the strontium ferrite powder was successfully coated with Co-Ni-P alloy and possesses excellent microwave absorption properties. The maximum microwave loss of the composite powder reaches −44.12 dB. The bandwidth with the loss above −10 dB exceeds 13.8 GHz.     

MgB2超导体掺Au的研究

分析不同烧结温度及时间和不同掺金量的名义组分为Mg1-xAuxB2样品的X射线衍射图后发现，很难得到单相的Mg1-xAuxB2样品，名义组分为Mg1-xAuxB2（x=1/16,2/16,0.146)的样品都是多相混合物，X射线衍射图显示主要是Au和Mg的化合物，样品的起始超导转变温度仍然由MgB2决定。     

Substitution Effects on MgB2 Superconductivity

With the help of supercell method, the first-principle calculations were performed for the study of doping crystal Mg_1-xAl_xB₂ and Mg(B_1-yC_y)₂. Analyzing the variations of the charge distribution and the partial densities of states, we found that the compounds with doping Al to MgB₂ compound and/or replacing boron by carbon exhibit new covalent bond effects and unexpected electronic properties, related to superconductivity. The study of the density of states indicates that superconductivity decreases with the increase of Al fraction and carbon concentration. There exists a transition of superconductor to non-superconductor with the change of Al doping fraction. The substitution of boron by carbon results in the decrease of the transition temperature since the decrease of the electron concentration and the lattice constant. The theoretical predictions agree with experimental observations.     

Influence of particle size on the electromagnetic and microwave absorption properties of FeSi/paraffin composites

The electromagnetic and microwave absorption properties of the composites employing FeSi alloy powders with different particle sizes as absorbent and paraffin as matrix were investigated. The results showed that the particle size had significant influence on the electromagnetic and microwave absorption properties of the composites in the 2-7 GHz frequency range. By decreasing the particle size of FeSi alloy powders, both the complex permittivity and permeability of the composites increased to a certain extent. In addition, the microwave absorption properties were improved, and the frequency of absorption peak shifted towards lower frequency range. In other words, the micron-grade FeSi alloy powders with smaller particle size were more suitable to be used as absorbent in measured frequency region.     

Hollow glass microspheres coated with CoFe2O4 and its microwave absorption property

Spinel CoFe₂O₄ coating on the surface of hollow glass microspheres of low density was synthesized by co-precipitation method. The phase structures, morphologies, particle size, shell thickness, chemical compositions of the composites have been characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDS). The results show that CoFe₂O₄ coating on hollow glass microspheres can be achieved, and the coating layers are constituted by CoFe₂O₄ nanoparticles of mean size ca. 10 nm. The as-synthesized powder materials were uniformly dispersed into the phenolic cement, then the mixture was pasted on metal plate with the area of 200 mm×200 mm as the test plate. The test of microwave absorption was carried out by the radar-absorbing materials (RAM) reflectivity far field radar cross-section (RCS) method. The results indicate that the coated CoFe₂O₄/hollow glass microspheres composites can be applied in lightweight and strong absorption microwave absorbers.