Microstructure characterization of CVI-densified carbon/carbon composites with various fiber distributions

Mechanical behavior of multi-phase composites is crucially influenced by volume fractions, orientation distributions and geometries of microconstituents. In the case of carbon–carbon composites manufactured by chemical vapor infiltration, the microconstituents are carbon fibers, pyrolytic carbon matrix, and pores. The local variable thickness of the pyrolytic carbon coating, distribution of the fibers and porosity are the main factors influencing the properties of these materials. Two types of fiber arrangements are considered in this paper: 2D laminated preform and random felt. The materials are characterized by determining their densities and their fiber distribution functions, by establishing types of pyrolytic carbon matrix present in the composites, and by studying the porosity. A technique utilizing X-ray computed tomography for estimation of the orientation distribution of the fibers and pores with arbitrary shapes is developed. A methodology based on the processing of microstructure images with subsequent numerical simulation of the coating growth around the fibers is proposed for estimation of the local thickness of the coating. The obtained information is appropriate for micromechanical modeling and prediction of the overall thermo-mechanical properties of the studied composites.     

Effect of carrier screening on ZnO-based resistive switching memory devices

The carrier screening effect occurs commonly in dielectric materials. It reduces the electric potential gradient, thus negatively affecting the functionality of resistive random access memory (RRAM) devices. An Au/ZnO film/Al-doped ZnO device fabricated in this work exhibited no resistive switching (RS), which was attributed to the carrier screening effect. Therefore, annealing was used for alleviating the screening effect, significantly enhancing the RS property. In addition, different on/off ratios were obtained for various bias values, and the screening effect was accounted for by investigating electron transport mechanisms. Furthermore, different annealing temperatures were employed to modulate the free carrier concentration in ZnO films to alleviate the screening effect. The maximal on/off ratio reached 10⁵ at an annealing temperature of 600 °C, yielding the lowest number of free carriers and the weakest screening effect in ZnO films. This work investigates the screening effect in RS devices. The screening effect not only modulates the characteristics of memory devices but also provides insight into the mechanism of RS in these devices.

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Structural,magnetic and photocatalytic characterization of Bi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FeO<Subscript>3</Subscript> nanoparticles synthesized by thermal decomposition method

Single-phase La-substituted bismuth ferrite (Bi\(_{\boldsymbol {1-x}}\)La\(_{\boldsymbol {x}}\)FeO\(_{\mathbf {3}}\)) nanoparticles have been synthesized by thermal decomposition of a glyoxylate precursor. The crystal structure transition of BiFeO\(_{\mathbf {3}}\) from the rhombohedral (R3c) to the cubic \(\boldsymbol {Pm}\bar {\mathbf {3}}\boldsymbol {m}\) structure by La addition was confirmed by X-ray diffraction and infrared spectrometry methods. Furthermore, the Bi\(_{\boldsymbol {1-x}}\)La\(_{\boldsymbol {x}}\)FeO\(_{\mathbf {3}}\) nanoparticles showed a weak ferrimagnetism behaviour, while the magnetization increased from 0.18 to 0.48 emu g\(^{\mathbf {-1}}\) with La substitution. The Bi\(_{\boldsymbol {1-x}}\)La\(_{\boldsymbol {x}}\)FeO\(_{\mathbf {3}}\) nanoparticles exhibited strong absorption in the visible region with the optical band gap calculated from Tauc’s plot in the range of 2.19–2.15 eV. Furthermore, the effects of La substitution on the photodegradation of the methylene blue (MB) under visible light were also studied. The photodegradation of MB dye was enhanced from 64 to \(\sim \)99% with increasing La substitution from \(\boldsymbol {x =}\) 0 to 0.1 and then decreased to 8% for \(\boldsymbol {x =}\) 0.15.     

Poly (vinyl alcohol) hydrogels for pH dependent colon targeted drug delivery

Oral drug administration is convenient with pH dependent drug delivery system since the drug has to pass through different pH environments in gastro intestinal (GI) tract. The pH dependent swelling/shrinking behavior of hydrogel drug carrier controls the drug release without affecting the function of drug. pH dependent hydrogels of poly (vinyl alcohol) (PVA) were prepared by cross linking with maleic acid (MA). The hydrogels were characterized by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, DSC, porosimetry, SEM, TEM, biocompatibility study and by measuring their swelling behavior in water, simulated gastric fluid (SGF) and intestinal fluid (SIF). Swelling of the hydrogels was found to be highest in SIF (pH: 7.5) and lowest in SGF (pH: 1.2) resembling that required in colon targeted drug delivery systems. Since the swelling behavior of the gel is pH dependent, these hydrogels were studied for colon targeted drug delivery in an in-vitro set-up resembling the condition of GI tract. The ratio of PVA and MA in the hydrogel was varied to study the effect on the drug diffusion rate. For drug delivery study, vitamin B₁₂ and salicylic acid were used as model drugs. The hydrogel, loaded with model drugs vitamin B₁₂ and salicylic acid also demonstrated colon specific drug release with a relatively higher drug release in SIF (pH: 7.5) than that in SGF (pH: 1.2).     

Poloidal and toroidal flows in tokamak plasma near magnetic islands

Technical Physics Letters - The radial electric field and toroidal rotation of tokamak plasma near a magnetic island have been calculated. Outside the magnetic island, the radial electric field...     

Formation of structure of the CdTe film, recrystallized on Mo/glass substrate under high temperature and mechanical pressure

Large-grained and 7 μm thick CdTe film has been fabricated on top of Mo coated soda-lime glass substrate. As a new approach the dynamic recrystallization process (DRC) was used to form the structure of films. For the characterization of the structure and composition of the films a scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDS) and X-ray diffraction (XRD) methods were used. The changes in the structure of films were studied in accordance with the process temperature, pressure and as-deposited film thickness. Significant changes in the CdTe film were observed after DRC of deposited films at the process temperatures between 450 °C and 550 °C. EDS quantitative analysis showed that during the recrystallization the Mo and CdTe films composition remained stable for all studied samples. The XRD results showed that the increase in the process temperature caused improvement in orientation of the films along direction of (111). The DRC temperature above 550 °C reduced the orientation again. The limits of the temperature and pressure in application of soda-lime glass in DRC were found and discussed.     

Quantitative Analysis of the Degradation Behavior of Silicon Solar Cell Irradiated by 1 MeV Electron Beams Using Photocarrier Radiometry Combined with Lock-in Carrierography

Silicon solar cells with cover glass irradiated by 1 MeV electron beams at various fluences were investigated using photocarrier radiometry (PCR) combined with lock-in carrierography (LIC, spectrally gated dynamic photoluminescence). The minority carrier transport properties (i.e., minority carrier lifetime τ, diffusion coefficient D, surface recombination velocities S) and the degradation of these properties were studied using PCR. The relative damage coefficient obtained by LIC was consistent with the PCR measurement. The local series resistance of the solar cell before and after irradiation was characterized by LIC. The results showed that the series resistance increased with electron fluences.     

Evaluation of Fatigue Crack Orientation Using Non-collinear Shear Wave Mixing Method

In this paper, a non-collinear shear wave mixing technique is proposed for evaluation of fatigue crack orientation. Numerical analysis of the nonlinear interaction of two shear waves with crack is performed using two-dimensional finite-element simulations. The simulation results show that the nonlinear interaction of the two shears waves with cracks leads to the generation of transmitted and reflected sum-frequency longitudinal waves (SFLW), moreover the propagation direction of reflected SFLW is correlated with the orientation of crack, which can be used for crack orientation evaluation. Non-collinear wave-mixing experiments were conducted on specimens with fatigue crack. The experimental results show that the directivity of the generated SFLW agrees well with the simulation results, and non-collinear shear wave mixing has potential use in fatigue crack orientation evaluation.     

Detection of Corn Starch Irradiated with Low Doses of Gamma Rays

A modification of the existing thiobarbituric acid test for irradiated starch has been devised which eliminates interference caused by side reactions of the thiobarbituric acid and which does not require a sample of unirradiated starch as a standard for comparison. Doses as low as 10 krad have been successfully detected.