Space charge induced luminescence in epoxy resin

Dielectric breakdown of epoxies is preceded by a light emission from the solid state material, so-called electroluminescence. Very little is known however on the luminescence properties of epoxy. The aim of this paper is to derive information that can be used as a basis to understand the nature of the excited states and their involvement in electrical degradation processes

Electroluminescence excitation mechanisms in an epoxy resin under divergent and uniform field

Electroluminescence excitation mechanisms have been investigated in epoxy resin under divergent and uniform field situations. Metallic wires embedded in the resin were used to produce field divergence whereas film samples were metallised to obtain a uniform field. Electroluminescence under divergent field was stimulated by an impulse voltage. Light was emitted on the positive and negative fronts of the square pulses when the field exceeded 20 kV/mm at the wire surface, with equal intensity and without polarity dependence. There was evidence of space charge accumulation around the wires in multiple-pulse experiments. Charge injection and extraction occurring at both fronts of the pulse provide the condition for EL excitation. Further excitation of the EL during the plateau of the voltage pulse is prevented by the opposite field of the trapped charge. Field computation with and without space charge supports the proposed interpretation. A DC voltage was used for the uniform field experiments. A continuous level of electroluminescence is found at 175 kV/mm. Charging/discharging current measurements and space charge profile analyses using the pulsed electro-acoustic (PEA) technique were performed at different fields up to the EL level. Dipolar orientation generates a long lasting transient current that prevents the conduction level being reached within the experimental protocol (one hour poling time). The continuous EL emission is nevertheless associated with a regime where the conduction becomes dominant over the orientational polarization. Polarization and space charge contribute to the PEA charge profiles. Homo-charge injection at anode and cathode is seen at 20 kV/mm and a penetration of positive space charge in the bulk is detected above 100 kV/mm, suggesting an excitation of the continuous EL by bipolar charge recombination

Raman spectroscopy of iodine-doped double-walled carbon nanotubes

We present a Raman spectroscopy study of iodine-intercalated (p-type-doped) double-walled carbon nanotubes. Double-walled carbon nanotubes (DWCNTs) are synthesized by catalytic chemical vapor deposition and characterized by Raman spectroscopy. The assignment of the radial breathing modes and the tangential modes of pristine DWCNTs is done in the framework of the bond polarization theory, using the spectral moment method. The changes in the Raman spectrum upon iodine doping are analyzed. Poly-iodine anions are identi- fied, and the Raman spectra reveal that the charge transfer between iodine and DWCNTs only involves the outer tubes

Photoluminescence, recombination induced luminescence and electroluminescence in epoxy resin

Dielectric breakdown of epoxies is preceded by light emission, or so-called electroluminescence, from the solid-state material. Very little is known about the luminescence properties of epoxies. The aim of this paper is to derive information that can be used as a basis to understand the nature of the excited states and their involvement in electrical degradation processes. Three different kinds of stimulation were used to excite the material luminescence. Photoluminescence was performed on the base resin, the hardener and the cured resin. Luminescence excited by a silent discharge has been analysed to identify which of the luminescent centres are optically active upon the recombination of electrical charges and could therefore act as charge traps. Finally, the electroluminescence spectrum has been acquired and compared with the previous ones. Although the identification of the origin of these emissions is far from being complete, it has been found that the photoluminescence from the cured resin is due to in-chain chromophores, which acts as trapping centres. The excited states involved in photoluminescence also seems to be involved in electroluminescence, but other components are detected as well, which could be due to the degradation of the resin molecule under the effect of the electric stress

The relationship between charge distribution, charge packet formation and electroluminescence in XLPE under DC

Different reports describing the internal distribution of space charge in cross-linked polyethylene (XLPE) under DC field have been published recently. The most striking fact observed is the organization of the space charge into charge packets that cross the insulation. All models for charge packet formation imply that carrier recombination will occur. As the recombination region is potentially a luminescence one it is of interest to record the electroluminescence in this regime. This topic is addressed in this paper

Isochoric, isobaric and ultrafast conductivities of aluminum, lithium and carbon in the warm dense matter (WDM) regime

We study the conductivities $\sigma$ of (i) the equilibrium isochoric state ($\sigma_{\rm is}$), (ii) the equilibrium isobaric state ($\sigma_{\rm ib}$), and also the (iii) non-equilibrium ultrafast matter (UFM) state ($\sigma_{\rm uf}$) with the ion temperature $T_i$ less than the the electron temperature $T_e$. Aluminum, lithium and carbon are considered, being increasingly complex warm dense matter (WDM) systems, with carbon having transient covalent bonds. First-principles calculations, i.e., neutral-pseudoatom (NPA) calculations and density-functional theory (DFT) with molecular-dynamics (MD) simulations, are compared where possible with experimental data to characterize $\sigma_{\rm ic}, \sigma_{\rm ib}$ and $\sigma_{\rm uf}$. The NPA $\sigma_{\rm ib}$ are closest to the available experimental data when compared to results from DFT+MD, where simulations of about 64-125 atoms are typically used. The published conductivities for Li are reviewed and the value at a temperature of 4.5 eV is examined using supporting X-ray Thomson scattering calculations. A physical picture of the variations of $\sigma$ with temperature and density applicable to these materials is given. The insensitivity of $\sigma$ to $T_e$ below 10 eV for carbon, compared to Al and Li, is clarified.Comment: 10 figure