Study of the Crystal Structures of New Buffer Materials Bi1?xYxO1.5

Bismuth based oxides, formed by bismuth oxide doped with the yttrium or lanthanum element, have been widely studied for their high ionic conductivities and other applicable properties. Recently, they were used as buffer layers for film superconductors and have shown a promising future. But the detailed information of their structures and properties has not been fully understood. In this work, the series samples of Bi_1−x Y _x O_1.5 (x=0.2, 0.3, 0.4, and 0.5, respectively) were prepared. The crystalline structures of them were determined by X-ray powder diffraction and the data were analyzed by Rietveld refinement method. Two phases, face centered cubic (FCC) and rhombohedral structures, were observed. When annealed at 1,000 °C, the samples are FCC structure. But for Bi_0.8Y_0.2O_1.5, annealed at 650 °C, the sample shows a rhombohedral structure. The change of the structures of the samples were studied and discussed.     

Synthesis, and study of magnetic properties, of Bi1?xCdxFeO3

Ceramic Bi_1−x Cd _x FeO₃ (x = 0.0, 0.05, and 0.1) samples were prepared by a citrate-gel method. The as-prepared compounds were calcined at 600 °C for 3 h to obtain nearly single-phase materials. The crystal structure, examined by X-ray powder diffraction (XRD) and Rietveld analysis, confirmed that the samples crystallize in a rhombohedral (space group, R-3c no. 161) structure. Magnetic measurements were carried out on the resultant powders from 300 to ~2.5 K. Magnetic hysteresis loops showed a significant increase in magnetization as a result of Cd doping in BiFeO₃.     

Development of Some Fundamental Research of SiC_W/Al Composites in HIT PartⅠ Microstructure and SiC/Al Interface of SiC_W/Al Composites

The microstructure of SiC whisker reinforced aluminium alloy (SiC_w/Al) composite is reviewed,andthe SiC-Al interface in SiC_w/Al composite is especially discussed,The main contents aremorphology of the aluminium matrix in SiC_w/Al composite;microstructures and defects of SiCwhiskers in SiC_w/Al composite and bonding mechanisms of the SiC-Al interface in SiC_w/Al com-posite.     

The Effects of Substitution of Sn for Cu in Bi1.75Pb0.25Sr2CaCu2.3?xSnxSnxOy

The influence of Sn doping on superconductivity in the Bi-based 2212 phase is studied in this paper. For the samples R–T relations and magnetic hysteresis loops were measured. X-ray powder diffraction analysis was also performed. For Bi_1.75Pb_0.25Sr₂CaCu_2.3–x Sn _x O _y , the experimental results show that by adding the proper amount of Sn the superconductivity of the samples can be improved. As x = 0.15, the critical temperature T _c, the critical current density J _c, and the magnetic pinning force density F reach a maximum. At T = 11 K, the critical state parameters H _c1, H _c2, , , and are calculated and compared with the results reported by other researchers. The experimental results also show that the Sn doping is able to speed up the growth of the 2223 phase. In brief, Sn doping is an effective way of improving the superconductivity in Bi-based superconductors.     

The novel behavior of real and imaginary part of impedance,modulus, and AC conductivity of Au/PPy–MWCNTs/TiO2/Al2O3/n-Si/Al

Journal of Materials Science: Materials in Electronics - This work presents a novel structure of Au/PPy-MWCNTs/TiO2/Al2O3/n-Si/Al which hasn't been explored before. This structure is applied in...     

Stabilization of the anatase phase of Ti1?xSnxO2 (x < 0.5) nanofibers

We experimentally investigate the stabilization of the anatase phase of Ti_1−x Sn _x O₂ (x < 0.5) nanofibers when synthesized by an electrospinning method. The as-spun nanofibers became nano-grained, polycrystalline nanofibers after calcination and the diameters of the nanofibers depend on Sn content. Stabilization of the anatase phase in Ti-rich compositions and incorporation of Sn ions were confirmed by X-ray diffraction, Raman, X-ray absorption near-edge structure, and photoluminescence (PL) spectroscopies. Results from the PL study also demonstrated the tunable nature of the optical properties, with the emission maximum shifting towards higher wavelength with increasing Sn concentration.      

Electrical Characterization of p-Type Pb1?xEuxTe

The transport properties of p-type Pb_1–x Eu _x Te epitaxial layers were studied as a function of Eu content, temperature, and magnetic field. The low-temperature hole mobility is drastically reduced when the Eu concentration is increased from 0 to 6%, while the hole concentration remains almost constant. A metal-insulator transition was observed for x 0.04, which is probably induced by the disorder caused by the introduction of Eu. For temperatures down to 10 K, only positive magnetoresistance has been observed at low magnetic fields. An anomalous behavior of the resistivity as a function of temperature has been detected for a Eu content about 5%, which is attributed to the resonance between the localized Eu 4f level and the valence band maximum.     

Magnetic Phase Diagram of Ca1?xMnxO

Alternating current susceptibility and direct current magnetization have been studied for polycrystalline Ca_1–x Mn _x O. On increasing the Mn content, magnetic ordering changes from spin glass behavior for 0.25 x 0.4 to antiferromagnetic order. The paramagnetic/antiferromagnetic transition is of second order for 0.5 x 0.65 and of first order for x 0.7. For low Mn concentrations, the high-temperature alternating current susceptibility can be described by a diluted Heisenberg magnet model developed for diluted magnetic semiconductors.     

Survey of microwave surface impedance data of high-Tc superconductors—Evidence for nonpairing charge carriers

The microwave surface impedance of the high-T _c oxide superconductors has been measured at many laboratories around the world. A survey of their data between 100 MHz and 150 GHz for polycrystalline as well as single crystalline samples is given, focusing on YBa₂Cu₃O_7– . In comparison to the classical superconductors, these results reveal a very similar temperature dependence of the surface impedance close toT _c but an anomalous high residual surface resistance at lower temperatures. Both features can be explained by the assumption that oxide superconductors contain a significant number of nonpairing charge carriers. Within the framework of a properly extended two-fluid model, this is shown by analysis of our best thin-film data. Moreover, the enhanced losses in polycrystalline material especially for superposed magnetostatic fields result, to a large extent, from the deeper penetration depth. The possible origin of the nonpairing charge carriers and their impact on the applicability of the oxide superconductors is briefly discussed.     

Obtainment of the Fe0.70?xCrxAl0.3/Al2O3 nanocomposite by the method of SHS of mechanoactivated Cr2O3 + Fe + Al mixtures

Applying M?ssbauer spectroscopy methods, we have studied the structure of nanocomposites obtained by a technique combining the preliminary mechanical activation of an 8.1 wt % Cr₂O₃ + 65.9 wt % Fe + 25 wt % Al mixture and self-propagating high-temperature synthesis (SHS). It has been found that, at the stage of mechanical activation, an Fe/Al/Cr₂O₃ composite with a low impurity of the Fe₂Al₅ intermetallide is formed. At the stage of SHS, the interaction between activated components of the mixture leads to the formation of the Fe_{0.7 − x} Cr _x Al_0.3 (x = 0 − 0.2)/Al₂O₃ composite. Original Russian Text ? T.Yu. Kiseleva, A.A. Novakova, T.L. Talako, T.F. Grigor’eva, A.N. Falkova, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 7, pp. 892–896.     

Processing and thermal stability of a Cfiber/SiCfiller/Si–C–Nmatrix composite: effect of oligomer vaporization and the surface oxide layer of SiC filler

The processing of carbon fiber-reinforced ceramic matrix composites (CMC) made by the precursor impregnation and pyrolysis (PIP) method was improved, and factors which deteriorate the thermal stability of the CMC were investigated. The processing time for cross-linking of a precursor polymer was substantially reduced by the application of a sealed metal container due to the suppression of the vaporization of oligomers. The strength of the as-fabricated CMC was 286 MPa and 77% of the original strength was retained after a heating at 1350 °C for 24 h in Ar. The reduction of the strength after the heating was due to the decomposition of SiO₂ which remained at the surface of the SiC filler particles. The decomposition reaction induced deterioration of carbon fibers and the matrix of the CMC at high temperature.     

A Study of the Magnetic Properties of?Bi1.64?xPb0.36CdxSr2Ca2Cu3Oy Superconductor

The flux pinning energy and magnetic properties of Bi_1.64−x Pb_0.36Cd _x Sr₂Ca₂Cu₃O _y (BPCSCCO) with x=0.0, 0.02, 0.04 and 0.06 were studied. A series of Bi-2223 superconductor samples with a nominal composition of BPCSCCO was synthesized and the effect of Cd substitution for Bi was investigated. As a result, Cd addition has been found to improve the superconducting properties of the Bi-Pb-Sr-Ca-Cu-O system. The effects of the annealing time and the amount of Cd doping on the structure, AC magnetic susceptibility, ρ–T curves and flux pinning energy were investigated. Also, for all samples the relation between the current and voltage in the mixed state was found to follow the model relationship V=α I ^β . The maximum value of β is 22.30, which is obtained for the sample with an annealing time of 270 h and a Cd content of 0.04.     

Growth of a Cellular/Dendritic Array

This paper describes a time dependent numerical model for the steady and non-steady growthof a cellular/dendritic array in a moving linear temperature field. The model gives fully selfconsistent solutions for axisymmetric interface shapes, predicts cellular and dendritic spacings,undercoolings and the transition between structures. An important feature of the model is thatthe spacing selection mechanism has been treated. The model predicts two different dendriticgrowth regions. One occurs at low dimensionless velocities where the dendrite array is uniform:a small stable range of spacings is predicted and the prediction agrees very well with existing experiments. The other occurs at high dimensionless velocities where the dendrite array is irregular, its minimum spacing is inversely proportional to velocity Experiment was carried out to verify the prediction. The irregular dendrite array was observed and the measured minimum spacing fitted extremely well with the prediction. By fitting the numerical results. Relatively simple analytic expressions are obtained which provide an insight into the cellular and dendritic growth processes and are useful for comparing theory with experiment.     

The influence of BaGeO3 on the properties of Ba(Ti1?xSnx)O3 ceramics on the basis of sol-gel powders

Ba(Ti_1−x−y Sn _x Ge _y )O₃ (BTSG-x-y; x = 0, 0.05; y = 0–0.05) powders were synthesized by a sol-gel (SG) method and for comparative purposes also by a mixed-oxide (MO) method. In this system, BaGeO₃ functions as sintering additive. Due to smaller particle sizes of the SG powders a higher sintering activity was found, which resulted in reduced grain growth and in a more homogenous grain size distribution for the corresponding ceramics. The dependence on the paraelectric ⇆ ferroelectric phase transition, i.e. the phase transition temperature, the width of the transition region and completeness were examined by dielectric measurements, DTA as well as by SEM, EDX and XRD investigations with respect to the BaGeO₃ content, synthesis method and sintering temperature. The phase transition temperatures of the SG ceramics are remarkably higher than those of the MO ceramics with the same nominal compositions. The reason is a lower tin concentration within the grains of SG ceramics as confirmed by EDX and XRD investigations. The presence of BaGeO₃ in barium titanate–stannate system on the basis of a SG method caused an improved incorporation of tin in the BaTiO₃ lattice.     

Dielectric behaviour of thin films of β-PVDF/PZT and β-PVDF/BaTiO3 composites

Thin films of the composites formed between poly(vinylidene fluoride) (PVDF) and lead zirconium titanate (PZT) and also barium titanate with 0–3 connectivity, have been obtained by dispersion of the ceramic powder in a solution of PVDF in dimethylacetamide DMA. Evaporation of the solvent at 65 °C allowed crystallization of PVDF predominantly in the polar phase, regardless of the amount of PZT or BaTiO₃ powder added upto 40 vol %. The relative permittivity and loss index values were determined for the pure components and for the composites with different ceramic contents, in the frequency range of 100 Hz to 13 MHz. An increase in PZT or BaTiO₃ content resulted in an increase in the relative permittivity of the composites, and the experimental results are shown to be in good agreement with those calculated from the theoretical expression of Yamada et al. [1]. The de electrical conductivity of composites with different compositions was also determined.     

Wear Behaviour of CTC/FeCrCSi Composite

To bring the excellent abrasion-resistant po-tential of carbide into full play,composites of casttungsten carbide(CTC)grains based on mainlyplate martensite and high Cr,W white cast iron aremade on substrate of grey iron by cast-in-placehardfacing process.Results of high-stresstwo-body and three-body abrasion show that:(1)Base alloy microstructures have overwhelmingdominating effects on abrasion resistances of thecomposites in two-body and especially three-bodyabrasion systems.(2)All composites have very ex-cellent abrasion-resistances which increase drasti-cally with the increase of CTC grain size.In case ofthe same CTC grain size,composite based onmartensite white cast iron is much morewear-resistant than that based on mainly platemartensite although the volume fraction of CTCgrains in the latter is considerably larger than thatin the former.In three-body abrasion system,loadhas little effect on wear rate for composites basedon martensite white cast iron,but wear rate ofconventional wear-resistant materials increaseslinearly in a very steep slope as the nominal stressincreases.     

Thermal Properties of PHB/PEG Blends

Thermal properties of PHB and PHB/PEG blends were investigated by differential scanning calorimetry (DSC) and melt index (Ml) test. DSC thermograms indicate that two components of blend are miscible. From the DSC thermogram, we can also conclude that the melting point of PHB descends with the increased content of PEG, this can improve PHB processing properties.From Ml data, we may draw the same conclusion.     

Room-temperature oxidation of La2?xSrxCuO4

A series of oxidized La_2–x Sr _x CuO_4+y compounds has been prepared by treating the starting materials with a solution of Br₂ in NaOH at room temperature. The structural modifications due to the oxidation of the materials have been studied by X-ray diffraction. Some of the observed changes are: (i) a large increase in the long parameter of the unit cell for samples with Sr content and (ii) a slight decrease in a along the whole range ofx. Interesting features have been observed regarding the critical temperatures of these materials: transition temperatures are higher for those containing lower Sr amounts (some of them were even nonsuperconducting before the oxidation treatment) in contrast to materials with x0.15, whoseT _c's do not change very much. The influence of both oxygen and strontium contents on the structural modifications and the superconducting properties of the oxidized materials will be discussed.     

Development of Fundamental Research of SiC_w / Al Composites in HIT Part Ⅱ: Mechanical Behaviour of Squeeze-cast SiC_w / Al Composites

Studies on the mechanical behaviour of squeeze-cast SiCw / Al composites have been revjewed. The results show that SiCw / Al composites exhibit improved mechan ical properties and cyclic hard ening. The reasons leading to the above results are discussed. Localized deformation near SiC whiskers plays an important role in the initiation of microcracks in the composites, and the fracture of the composites is caused by the abrupt linking of microcracks     

Applicability of the polymeric precursor method to the synthesis of nanometric single- and multi-layers of Zn1?xMnxO (x?=?0–0.3)

Polymeric precursor method (Pechini) was employed to fabricate single- and multilayers of Zn_1−x Mn _x O (x = 0–0.3) on glass substrates. X-ray diffraction measurements revealed that crystal structure of Zn_1−x Mn _x O multilayers is the typical hexagonal würzite structure of pristine ZnO. A reduced peak intensity and widened full width half maximum (FWHM) value of prominent peaks suggested that the Mn²⁺ ions have substituted the Zn²⁺ ion without changing the würzite structure of pristine ZnO up to Mn concentrations x ≤ 0.2. A distinct redshift of the absorption edge was observed as the Mn concentration x was increased. Additionally, the absorption edge was less sharp due, probably, to s–d and p–d interactions, which give rise to band gap bowing. Nevertheless, amorphous states appearing in the band gap as a consequence of reduced crystallinity may also be responsible for the shrinking of the band gap in this material. Interestingly, the field dependence of the magnetization showed typical paramagnetic behavior for all the chosen Mn concentrations with no evidence of ferromagnetic ordering. Probably, the absence of ferromagnetism in the studied Zn_1−x Mn _x O films is strongly related to defects (say Mn impurities at the interface between nano-crystallites) in ZnO due to partial substitution of host Zn ions by Mn ions.