Microwave-absorbing properties of NiCoZn spinel ferrites

In this paper, the microwave-absorbing properties of (Ni_1−x−yCo_xZn_y)Fe₂O₄ spinel ferrites have been investigated within the frequency range of 0.5–14 GHz. There are two kinds of resonance peaks observed in the permeability spectra: domain-wall resonances at lower frequency and spin-rotation resonances at higher frequency. The reflection loss (RL) calculations show that the prepared NiCoZn spinel ferrites are good electromagnetic (EM) wave absorbers in microwave range. In terms of the absorbing frequency band (AFB) and the matching thickness (t_m), (Ni_0.407Co_0.207Zn_0.386)Fe₂O₄ shows the best performances: t_m=3.15 mm and the AFB is 8.64–11.2 GHz. Decreasing the weight ratio of NiCoZn ferrites in ferrites/wax composites, the matching thickness decreases and the AFB shifts to higher frequencies. Compared with the absorbers with single-layer ferrites, the absorbers with double-layers ferrites have better absorbing performances, such as a thinner matching thickness and a wider EM wave AFB.     

Characterization Method of Polycrystalline Materials Using Conductive Atomic Force Microscopy

An apparatus for characterization of polycrystalline materials based on conductive atomic torce microscopy （cAFM） is developed and a quantitative measurement of electrical characteristics of individual grains in polycrystalline ZnO ceramic is demonstrated. Improvement of the experimental method is presented. Experimental results illuminate unambiguously the different electrical characteristics between individual grains, suggesting the suitability and maneuverability of this method in the study of local structure or properties and their relationship in polycrystalline materials such as semi-conducting ceramics.     

Properties of poly(2-hydroxyethyl acrylate)-silica nanocomposites obtained by the sol-gel process

Poly(2-hydroxyethyl acrylate) swollen in water forms a hydrogel with good biological acceptance but poor mechanical properties. Reinforcement by a nanometric silica phase obtained by acid catalyzed sol-gel reaction of tetraethoxysilane occurring simultaneously with the polymerization of the organic polymer leads to a hybrid material and is a way to improve the mechanical properties of the homopolymer. Dynamic mechanical measurements show a stiffness increase in the rubbery state as the silica content is increased, for both the xerogel (dry) and the swollen samples. Density measurements of the hybrid materials show that the silica phase has an apparent density close to that of vitreous silica, thus giving an indication of the intimate interpenetration of the organic and the inorganic phases in these nanocomposites. Thermogravimetric analysis has been used to probe the dependence of the thermal stability of the samples on the silica content, and to determine the actual silica amounts in each sample. Information about the percolation threshold of the silica phase could be gained from measurements of water uptake and mechanical moduli. All the results pointed out that the co-continuity of matrix and reinforcement starts at around 15 wt% of silica content.     

Structural analysis of Cd-Te-O films prepared by RF reactive sputtering

Crystalline and microcrystalline Cd-Te-O samples have been obtained by RF reactive sputtering from a CdTe target using N₂O as oxidant. The growth conditions were substrate temperatures of 323 K, 573 K and 773 K and cathode voltage of −400 V, corresponding to 30 W of forward power. The samples were studied by micro-Raman spectroscopy, X-ray diffraction and optical transmittance. The films are remarkably transparent in the visible range, with transmittances about 88% at 400 nm and band gap energies above the absorption edge of the glass substrates. Although only the samples prepared at 773 K present defined diffraction peaks, the analysis of the Raman spectra indicate that samples prepared at 323 K and 573 K have a defined microstructure indeed. The spectra fitting performed by comparison with pattern compounds demonstrate that Cd-Te-O films are formed of Te-O units similar to those present in metal oxide-doped tellurite glasses, such as TeO₃ and TeO_3 + 1 linked through Cd-O bonds. As the substrate temperature increases the microstructure evolves from a γ-TeO₂ richer state to Cd_xTe_yO_z. In the crystalline sample the main phase identified was CdTeO₃ even though evidence of other phases was observed.     

The effect of thermal annealing on the structural and mechanical properties of a-C:H thin films prepared by the CFUBM magnetron sputtering method

a-C:H films were prepared by closed-field unbalanced magnetron (CFUBM) sputtering on silicon substrates using argon (Ar) and acetylene (C₂H₂) gases, and the effects of post-annealing temperature on structural and mechanical properties were investigated. Films were annealed at temperatures ranging from 300 °C to 700 °C in increments of 200 °C using rapid thermal annealing equipment in vacuum ambient. Variations in microstructure were examined using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Surface and mechanical properties were investigated by atomic force microscopy (AFM), nano-indentation, residual stress tester, and nano-scratch tester. We found that the mechanical properties of a-C:H films deteriorated with increased annealing temperature.     

Structural and Thermodynamic Properties of M3W3N （M=Fe,Co, Ni）

Ternary transition metal nitrides, Fe3 W3N, Coa W3N, and Nia WaN~ are studied by the use of interatomic potentials acquired from lattice inversion. The study indicates that Fe3 WaN would be more stable than the other compounds in the family of intermetallic tungsten nitrides. The investigation of phonon density of states indi- cates that the lower frequency modes are mostly excited by the metal atoms, and the higher frequency modes are mostly excited by the nitrogen atoms. A qualitative analysis is carried out with the relevant potentials for the phase stability and vibrational modes.     

Hydrogen-induced magnetic and structural transformations of GdCu

The magnetization of GdCu induced by hydrogen uptake was measured within the temperature range of 4.2 to 300 K, occurring phase changes were followed by X-ray diffraction measurements at ambient temperature. The prepared GdCu powder of CsCl-type structure readily absorbed hydrogen at ambient temperature, where hydrogen pressure was below 100 kPa. Hydrogenation changed the magnetism of GdCu in a complex manner from an antiferromagnetic-like type to a paramagnetic-like one. The changes in magnetic properties of GdCu by hydrogenation are governed by hydrogen-induced disproportionation. Within the composition range 0<[H]/[GdCu]<1, GdCu disproportionated according to 2GdCu+H₂→GdH₂+GdCu₂ . The magnetization was evaluated by the expression χ_total=(1-x)χ_GdCu+(x/2)(χ_GdH2+χ_GdCu2). GdCu hydride was not observed. Hydrogenation beyond [H]/[GdCu]>1 gave rise to the disproportionation of GdCu₂ causing the change in magnetization.     

Synthesis, structure and luminescence of LaSi3N5:Ce phosphor

In this work, new LaSi₃N₅:Ce³⁺ phosphors have been synthesized by solid-state reaction. Rietveld refinement of the crystal structure of La_1−xCe_xSi₃N₅ reveals that Ce atoms substituted for La atoms occupy 4a crystallographic positions. Broad emission and excitation bands observed were attributed to the transitions between the doublet ground state of the 4f¹ configuration and the crystal field components of the 5d¹ excited state. At 77 K, the centroid and crystal field splitting ε_cfs of the 5d levels of Ce³⁺ in LaSi₃N₅:Ce³⁺ compounds were valuated at 33.4×10³ and 11.3×10³ cm⁻¹, respectively. The zero-phonon line and the Stokes shift were measured to be 26.0×10³ and 5.0×10³ cm⁻¹, respectively.