Materials Science Forum Vols. 480-481

Abstract: The transient conductivity in low-density polyethylene is studied. Isochronal currentvoltage measurements for 1800 s and 1 day time intervals are carried out under dry N2 atmosphere. When after every measurement the sample is fully discharged at high temperature the isochronal current - voltage characteristic reveals an ohmic behavior. When the next field increase is applied without sample discharging the current-voltage characteristic is super-quadratic. We explain this increase of the current assuming that a fraction of the previous injected charge is detrapped by the field and it contributes to current increase. Consequently the current – voltage characteristic is strongly dependent on the time lag between two successive rises in the field. Neither the Poole-Frenkel mechanism nor the Richardson – Schottky mechanism can by used to explain the experimental results. The isothermal charging and discharging currents are explained assuming the movement of injected/ejected charge in the resultant local field. The values obtained for the adjustable parameters of the model are in good agreement with the values in the literature.

Abstract: Space charge in electrically aged cross-linked polyethylene (XLPE) was studied using a procedure combining isothermal and non-isothermal measurements of charge and discharge currents. Aging is carried out using an AC field while immersing the disk-shaped samples in an ionic aqueous solution at constant temperature. After aging the samples were isothermally DC charged and discharged. Next a non-isothermal experiment with constant heating rate was performed (FTSDC). Finally the sample was kept at the highest temperature in order to completely discharge the polymer. The space charge introduced in the XLPE during aging can be analyzed from the study of the FTSDC spectra. The thermogram (FTSDC) shows a very broad peak. The peak is attributed to trapped space charge in traps with long relaxation times. It is possible to decompose it into three or four individual peaks and obtain the corresponding activation energies. The results were compared with previous ones obtained for LDPE (low density polyethylene) aged under similar conditions.

Abstract: A combined isothermal and non-isothermal measurement technique is described for analyzing space charge behavior in dielectric materials. The experimental conditions are chosen so that the dipolar effects are insignificant and the measured current is mainly due to space charge detrapping. For polyamide 11 four peaks have been identified above the poling temperature. As the difference between the peak temperature and the poling temperature increases, the apparent activation energies decrease suggesting that only the faster modes participate to the charging process. The charge associated to the apparent peak and the peak at 96 °C scales linearly with the polarization field. The injected space charge at 35 °C is very stable at 150 °C, an important fact with respect to technological applications of polyamide 11 as an electroactive material.

Abstract: It has long been recognized that the mechanical behaviour of materials under conditions of rapid loading and impact differs significantly from that under static load application [1].These differences are specially important for those materials as polymeric foams used as low energy impact absorbing materials[2]. An optimum energy absorbing material needs to dissipate the kinetic energy of the impact while keeping the force on it below some limit, thus resulting in a no-dangerous deceleration of the protected object[3]. The mechanical properties at room temperature of six polyethylene foams with closed cells and different densities have been evaluated in purely compressive impact loading conditions. The energy absorption characteristics have been evaluated through different parameters as the peak of deceleration, the load transmitted, the maximum strain and the impact time. The peak of deceleration is used to obtain the cushion diagrams at five different heights, useful to design energy absorption structures.

Abstract: In this paper, we report the characteristics of GaN heteroepitaxial films grown on Si(111)at 700oC using plasma-assisted metalorganic chemical vapour deposition (PA-MOCVD). In this growth technique, H2 plasma was used in addition to N2 plasma. Two sets of samples with different buffer layers were used, i.e. GaN and AlN buffer layers. In the infrared region both samples exhibit similar reststrahlen band shape. However the sample with GaN buffer layer exhibits better optical properties in the visible region compared with its counterpart. This is attributed to its better structural bulk and surface properties.

Abstract: GaN-based materials have been the subject of intensive research recently for blue and ultraviolet light emission and high temperature/high power electronic devices. Ohmic contacts with low contact resistance are essential in improving the electrical and optical performances of the devices. A wide variety of contact metallizations have been reported for p-GaN, including the standard Ni/Au as well as Ni. Different surface pretreatments have been investigated to lower the contact resistivity. To employ metal layers as a reliable ohmic contact on GaN, it is essential to understand the thermal stability of metal-GaN contact in addition to developing low resistance ohmic system. In this paper, we report on the characteristics of Ni/Ag bi-layer contacts on p-type GaN. The structural and electrical stability of the contacts at various annealing temperatures (480°C – 780°C)were investigated. Changes in the surface morphology of the contacts on annealing were examined using scanning electron microscopy (SEM). Specific contact resistivity and barrier height, determined using transmission line method (TLM) and current-voltage (I-V) measurements were calculated.

Abstract: Gallium nitride (GaN) is a highly promising wide band gap semiconductor with applications in high power electronic and optoelectronic devices. Thin films of GaN are most commonly grown in the hexagonal wurtzite structure on sapphire substrates. Growth of GaN onto silicon substrates offers a very attractive opportunity to incorporate GaN devices onto silicon based integrated circuits. Although direct epitaxial growth of GaN films on Si substrates is a difficult task (mainly due to the 17% lattice mismatch present), substantial progress in the crystal quality can be achieved using a buffer layer. A full characterization of the quality of the material needs to be assessed by a combination of different techniques. In this work, a detailed characterization study of GaN thin film grown on Si(111) with AlN buffer layer by low pressure metalorganic chemical vapor deposition (LP-MOCVD) was carried out. Post deposition analysis includes scanning electron microscopy (SEM), x-ray diffraction (XRD), Hall and infrared (IR) spectroscopy techniques. The IR spectra were compared to the calculated spectra generated with a damped single harmonic oscillator model. Through this method, a complete set of reststrahlen parameters (such as ε∞, S, wTO, γ) of the GaN epilayer were obtained. Our results show that the GaN film has a single crystalline structure. Current-voltage characteristics (I-V) of this GaN/Si heterojunction were measured at room temperature. Rectification behavior was observed for this anisotype heterojunction. The electrical characteristics of Ni Schottky barriers on this unintentionally doped n-type film were also investigated. The barrier height of Ni/GaN Schottky barriers has been determined to be 0.93 eV by I-V measurement.

Abstract: A time-dependent gas-phase isovolumetric desorption technique has been used to evaluate the diffusive transport parameters of hydrogen isotopes in polycrystalline tungsten in the temperatures range 673 to 1073 K and driving pressures from 1.3 104 to 105 Pa. Experiments have been run with both protium and deuterium obtaining their respective transport parameters diffusivity (D), Sieverts’ constant (Ks), the trap site density (Nt) and the trapping activation energy (Et). Isotope effects on these transport parameters are analysed and modelled. Because the classical isotope relation for diffusivity has not been fulfilled, quantum-statistical vibration theory has been applied to model the isotopic relation. A congruent isotopic variation of diffusion parameters related to the type of microstructure, bcc, has been confirmed.

Abstract: The research made in the field of recording materials for holographic applications has yielded to materials of improving quality. Photographic emulsions have played an important role in this investigation, since although this is the first material used for recording purposes, there are a series of characteristics that convert the photographic emulsion in a very interesting material for holographic recording. In this work we will present the optimisation of a reversal bleaching process. This process has been largely used for Agfa 8E75 HD emulsions, but there is still a significant lack of information about it when used with BB-640 emulsions. There are some differences between BB-640 and Agfa 8E75 HD emulsions such as the degree of hardening of the gelatin, the mean grain size or the emulsion thickness. These differences must be taken into account in order to use the same chemical procedures commonly used for Agfa plates with BB-640 ones. For instance, one of these differences, the mean size of the silver halide grains present in the emulsion, influences directly in the scattering of the final hologram recorded in the emulsion. It will be demonstrated that the use of ultra-fine grained emulsions, such as BB-640, allows obtaining holograms with high diffraction efficiencies and low absorption and scattering. In particular, in this work a reversal bleach procedure will be optimised for diffraction gratings recorded with different spatial frequencies on BB-640 emulsions.

Abstract: Zirconia based gels were prepared by the sol-gel route using sodium tris(glycozirconate)complex as precursor synthesized directly from the reaction of inexpensive and readily available compounds via the one step method. Rheological technique was used to investigate the sol-gel transition of zirconium based materials at different water/alkoxide ratios and reaction temperatures. The gelation time was determined from the evolution of the storage and loss moduli versus time at various frequencies using the Winter-Chambon criterion. The effect of an increase of the hydrolysis ratio and /or temperature was an enhancement of the kinetics of crosslink reactions, thus decreasing the gelation time. The variation of the gelation time versus temperature could be described by Arrhenius law. Furthermore, zirconia powders prepared from the thermal treatment at 500°C were analyzed by X-ray diffraction and the BET technique.