Ion-beam-defect processes in group-III nitrides and ZnO

Recently, there has been much interest in wide band-gap wurtzite semiconductors such as group-III nitrides (GaN, AlGaN, and InGaN) and ZnO. Ion-beam-defect processes are considerably more complex in these wurtzite semiconductors than in the case of both elemental and group-III-V cubic semiconductors. This brief review focuses on our recent studies of the following aspects of ion-beam-defect processes: (i) effects of implanted species and the density of collision cascades, (ii) the nature of ion-beam-produced planar defects in GaN, (iii) defect production in GaN by swift heavy ions, (iv) blistering of H-implanted GaN, (v) electrical isolation of GaN and ZnO, (vi) the effect of Al and In content on defect processes in III-nitrides, and (vii) structural damage in ZnO with an intriguing effect of the formation of an anomalous defect peak. Emphasis is given to unusual ion-beam-defect processes and to the physical mechanisms underlying them.     

In-line holographic electron microscopy in the presence of external magnetic fields

It is now a well-known fact that the phase of electron waves is altered by external magnetic fields via the Aharonov-Bohm effect. This implies that any electron interference effects will be to some degree affected by the presence of such fields. In this study we examine the distortion effects of external (constant and variable) magnetic fields on electron interference and holography. For digital holography, the reconstruction of the object is done via numerical calculations and this leaves the door open for correcting phase distortions in the hologram reconstruction. We design and quantitatively assess such correction schemes, which decidedly depend on our knowledge of the magnetic field values in the holographic recording process. For constant fields of known value we are able to correct for magnetic distortions to a great extent. We find that variable fields are more destructive to the holographic process than constant fields. We define two criteria, related respectively to global and local contrast of the hologram to establish the maximum allowed external field which does not significantly hinder the accuracy of in-line holographic microscopy with electrons.     

Application of Cayley-Klein parameters in the problem of strapdown navigation system stochastic state vector synthesis for a self-contained space vehicle

The state vector and observer equations are obtained for a strapdown (gimballess) navigation system with three accelerometers, three angular rate sensors, and a barometric altimeter. These equations are based on the nonlinear filtration methods that ensure required estimation accuracy under arbitrary space maneuver of a vehicle.     

Calculation of the stability ratio of suspensions using Emulsion Stability Simulations

According to Emulsion Stability Simulations (ESS), the flocculation of two non-deformable drops in the primary minimum of the interaction potential, necessarily leads to their coalescence. This property is used here for the evaluation of the stability ratio (W) of solid particles, interacting with the same inter-particle potential as the one of non-deformable droplets. Two different methodologies are used. The first one consists on the repeated evaluation of the coalescence time between two particles. The second one consists on the estimation of the time required for a decrease in the number of aggregates of the dispersion equal to n₀/2 (where n₀ is the initial number of aggregates). The results of the simulations are contrasted with the stability ratio of an anionic latex suspension subject to several ionic strengths (400-1000 mM). The first methodology is far more efficient for the evaluation of W although it misses the development of the aggregates and their growth. Absolute coagulation rates (kf) can also be obtained using one N-particle simulation for the calculation of the fast flocculation rate , and several two-particle simulations for the evaluation of W. This combined procedure is also more efficient than the N-particle evaluation of .     

Image motion compensation at charge-coupled device photographing in delay-integration mode

In this paper, a problem of image motion compensation caused by the camera motion concerning the photographed surface is considered. The camera is set on a mobile carrier, and image registration is conducted by a matrix photoreceiver on basis of charge-coupled devices (CCD) in delay-integration mode. It was shown that together with the traditional electron compensation of longitudinal motion of image, the compensation of transversal velocity of image motion is possible.     

Radiation processing with the Messina electron linac

In the last decades radiation processing has been more and more applied in several fields of industrial treatments and scientific research as a safe, reliable and economic technique. In order to improve existing industrial techniques and to develop new applications of this technology, at the Physics Department of Messina University a high power 5 MeV electron linac has been studied and set-up.The main features of the accelerating structure will be described together with the distinctive features of the delivered beam and several results obtained by electron beam irradiations, such as improvement of the characteristics of polymers and polymer composite materials, synthesis of new hydrogels for pharmaceutical and biomedical applications, reclaim of culture ground, sterilization of medical devices, development of new dosimeters for very high doses and dose rates required for monitoring of industrial irradiations.     

Symmetry breaking and electrical conductivity of La0.7Sr0.3Cr0.4Mn0.6O3−δ perovskite as SOFC anode material

This work is focused on nanocrystalline solid oxide fuel cell synthesis and characterization (SOFC) anodes of La_0.7Sr_0.3Cr_0.4Mn_0.6O_3−δ (perovskite-type) with Nickel. Perovskite-type oxide chemical reactivity, nucleation kinetics and phase composition related with La_0.7Sr_0.3Cr_0.4Mn_0.6O_3−δ–NiO to La_0.7Sr_0.3Cr_0.4Mn_0.6O_3−δ–Ni transformation have been analyzed. SOFC anode powders were obtained by sol–gel synthesis, using polyvinyl alcohol as an organic precursor to get a porous cermet electrode after sintering at 1365 °C and oxide reduction by hydrogen at 800 °C/1050 °C for 8 h in a horizontal tubular reactor furnace under 10% H₂/N₂ atmosphere. Composite powders were compressed into 10-mm diameter discs with 25–75 wt% Ni.     

Template preparation of strontium hexaferrite (SrFe12O19) micro/nanostructures: Characterization, synthesis mechanism and magnetic properties

Using electrospun thermoplastic ester elastomer (TPEE) micro/nanofibers as the template, strontium hexaferrite micro/nanostructures with different morphologies were prepared successfully by a combination process composed of sol-gel precursor coating technique and subsequent calcination. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) were used to characterize the morphology, crystalloid structure and the magnetic properties of samples. The relation between the fabricated structure morphologies and TPEE micro/nanofibers properties was studied. It was believed that the hydrogen bond effect competition between TPEE/precursors and precursors/precursors played an important role on the formation of the obtained structures. In addition, the influence of annealing time and annealing rate on the magnetic properties of the samples obtained was investigated. The structures thus fabricated can be potential materials in many fields, drug delivery and photocatalysts for example.     

Influence of fluorosurfactants on the codeposition of ceramic nanoparticles and the morphology of electroless NiP coatings

Electroless NiP composite coatings with SiC or Si₃N₄ nanoparticles were prepared from an additive-free bath. Cationic, anionic and non-ionic fluorosurfactants were used to try to avoid the agglomeration and the particle incorporation into the coating. Particle size and zeta potential were used to characterize the dispersion. The drawbacks of measuring these properties at the plating conditions (high ionic strength media and temperature) were discussed. Results indicated that the addition of low concentrations of fluorosurfactants did not modify the zeta potential of the particles in the bath but increased the amount of embedded particles. Moreover, the presence of the fluorosurfactants, mainly the non-ionic one, had a positive effect on the surface roughness and performance of NiP composite coatings.