Interfaces and Space Charge in Polymeric Insulating Materials (invited)

In this paper, the influence of the interface between electrode and polymer or polymer and polymer on the space charge dynamics has been studied. Planar samples of low density polyethylene (LDPE) were subjected to high dc electric stresses for extended periods of time and space charge measurements were taken using the pulsed electroacoustic (PEA) technique. Common electrode materials used in either the laboratory or power cable industry were selected (i.e. aluminium (Al), gold (Au) and carbon loaded XLPE (Sc)). Experimental results demonstrated that charge injection processes take place in all cases once the applied stress has exceeded a threshold. However, the amount of charge and polarity of the dominant injected charges showed a significant dependence on the electrode materials (under the same applied electric stress). Having establishing the influence of the electrode material on charge accumulation, our attention was paid to the effect of polymer/polymer interface on charge dynamics. Unlike our previous approach where two different polymeric materials were used, this time the polymer/polymer interface was formed by using the two layer of LDPE films cut from the same sheet. Sc and Al were used as electrodes to form different combinations. The results clearly indicated that the interface between two layers of LDPE acts as traps for electrons but not for positive charge carriers. The charge distribution in the bulk of the sample strongly depends on the electrode materials

Space Charge in Solid Dielectrics (Keynote Lecture)

The importance of space charge in solid dielectrics has been recognized for a long time. Over the years many attempts have been made to observe its dynamics in solid dielectrics. Significant development in measurement techniques has emerged two decades ago. In this paper several popular non-destructive techniques are briefly reviewed, followed by description of some advanced space charge measurement facilities housed in our laboratory. The selected results obtained over the years will be presented. It has been found that the interfaces play important role in space charge formation in addition to the bulk materials. Electrical ageing of polymeric materials under dc condition has been extensively studied and the technique has been extended to XLPE power cables. Recent work on ac ageing of polymeric materials has clearly indicated that ageing under high ac electric field results in the formation of space charge and these charges are very stable, indicating the formation of deep traps due to ac ageing. The impurities in the materials have significant influence on space charge formation and dynamics while the morphological feature of the material however has less effect on space charge. Simulation based on a bipolar charge model shows a good agreement with experimental results

Space charge measurement as a diagnostic tool to monitor electrical ageing in polymeric materials

Space charge is a term used to describe the excessive charges in volume. Such excessive charges can be electrons, ions or mixture of both. In solid dielectrics space charge tends to build up at discontinuities, such as at the dielectric-electrode interface, at the grain boundaries in crystalline dielectrics, at the boundaries between crystalline and amorphous regions in semi-crystalline dielectrics, and at various molecular sites simply called charge traps. Under the influence of the electric field the trapped charges may begin to detrap and move. Space charge phenomena have long been observed and their importance has been recognized over many years. In fact, space charge is recognized as a major factor in dielectric behavior, such as leakage, high-voltage reliability, and dissipation factor. However, its role in dielectric ageing is still the subject of research. Researchers have shown a special interest in cross-linked polyethylene (XLPE) because of its widespread use as high voltage cable insulation in power transmission. Many efforts have been made to quantify charge in solid dielectrics with a various degree of success. Research on space charge in solid dielectrics has been intensified in last two decades due to new development in non-intrusive measurement techniques. In this paper, we report an investigation on charge dynamics in polymeric materials after aged under ac electric field using the pulsed electroacoustic (PEA) technique. The emphasis is placed on charge decay and it has been found that the charge decay rate in additive free low density polyethylene (LDPE) becomes much slower in the aged sample in comparison with the unaged sample. The charge dynamics of the aged samples under dc bias differ from the sample without ac ageing, indicating changes brought in by ac ageing. It is believed that a slow decay rate of charge in the ac aged sample is related to the formation of deep traps in the material. However, chemical analysis by infrared spectroscope (FTIR) and Raman microscope reveals no significant chemical changes taken place in the bulk of the material after ac ageing. Further experiments have revealed that the deep trap formation is related to the electrode material used during the ageing. Au electrodes have little effect on the charge decay rate, indicating that the cause of the deep traps is due to charge injection. The influence of electrode material on clearly suggests that the process of charge injection leads to the changes in trapping characteristics. The effect of ac ageing has also been carried out on XLPE and similar conclusions have been reached. The findings presented in this paper clearly indicate that space charge measurement can be used as a diagnostic tool to monitor electrical ageing in polymeric materials. Compared with the conventional methods, it seems that space charge technique is more sensitive to the change in either physical structure or chemical alternation due to electric stressing

Walking and Wandering: Reconstructing Diasporic Subjectivity in T. C. Huo\u27s Land of Smiles and Lê Thi Diem Thúy’s The Gangster We Are All Looking For

Diaspora has often been defined as the condition of dispersal and displacement in which its members express minimal connections with their host country and always look to return to their ancestral homelands. However, from the literary representations in T. C. Huo’s Land of Smiles and Lê Thi Diem Thúy’s The Gangster We Are All Looking For, it is clear that members of the Southeast Asian diaspora determine to set root in their host country and refuse to be treated as temporary guests. This determination is warranted by their desire to redefine the contentious idea of home beyond cultural ancestry and geopolitical boundaries. Both authors utilize the trope of walking and wandering to debunk the essentialist conception based on the premise that one’s subjectivity is static, especially when it applies to diasporic members’ supposed sense of homelessness and the longing to return to their native countries. Rather than perpetuating the systemic labels onto diasporic members, such as homelessness, passivity, and powerlessness, through walking and wandering, the Southeast Asian refugees in the novels demonstrate the psychosomatic connections with their host country, take part in the pursuit of success, and declare an active, visible presence in their new homeland as fervent subjects who embrace opportunities to obtain material security in the United States

State Taxation of Unitary Businesses

The income taxation of multistate businesses has created problems for tax administrators, primarily with regard to the question of how to divide the income taxation amongst the multiple states. To address this, the concepts of unitary business and formula apportionment have been created. However, the non-uniform state taxation practices create difficulties even with the existence of these concepts. Some states have adopted the Multistate Tax Compact, but for it to be completely effective there still must be a uniform view adopted on what constitutes a unitary business. This note examines the constitutional issues attendant to developing a standard definition of a unitary business, an in-depth analysis of the unitary business concept and its origins, and proposes a workable definition of a unitary business

State Taxation of Unitary Businesses

The income taxation of multistate businesses has created problems for tax administrators, primarily with regard to the question of how to divide the income taxation amongst the multiple states. To address this, the concepts of unitary business and formula apportionment have been created. However, the non-uniform state taxation practices create difficulties even with the existence of these concepts. Some states have adopted the Multistate Tax Compact, but for it to be completely effective there still must be a uniform view adopted on what constitutes a unitary business. This note examines the constitutional issues attendant to developing a standard definition of a unitary business, an in-depth analysis of the unitary business concept and its origins, and proposes a workable definition of a unitary business

Gibbsian Hypothesis in Turbulence

We show that Kolmogorov multipliers in turbulence cannot be statistically independent of others at adjacent scales (or even a finite range apart) by numerical simulation of a shell model and by theory. As the simplest generalization of independent distributions, we suppose that the steady-state statistics of multipliers in the shell model are given by a translation-invariant Gibbs measure with a short-range potential, when expressed in terms of suitable ``spin'' variables: real-valued spins that are logarithms of multipliers and XY-spins defined by local dynamical phases. Numerical evidence is presented in favor of the hypothesis for the shell model, in particular novel scaling laws and derivative relations predicted by the existence of a thermodynamic limit. The Gibbs measure appears to be in a high-temperature, unique-phase regime with ``paramagnetic'' spin order.Comment: 19 pages, 9 figures, greatly expanded content, accepted to appear in J. Stat. Phy

On the Inverse Problem Relative to Dynamics of the w Function

In this paper we shall study the inverse problem relative to dynamics of the w function which is a special arithmetic function and shall get some results.Comment: 11 page