Constant-Phase-Element Behavior Caused by Coupled Resistivity and Permittivity Distributions in Films

A recent proposed model showed that the impedance of a film with a uniform permittivity and a resistivity that varies along its thickness according to a power-law is in the form of a constant phase element (CPE). This model is further considered in order to assess the effect of non-uniform permittivity profiles. It is shown that a power-law permittivity profile is also compatible with a CPE behavior when resistivity and permittivity vary in opposite ways along the film thickness. This work shows that, for important classes of materials which show CPE behavior, relaxation of the assumption of a uniform permittivity does not alter the conclusions developed in the earlier work, and the formula relating film properties to CPE parameters is shown to apply

The Apparent Constant-Phase-Element Behavior of an Ideally Polarized Blocking Electrode

Two numerical methods were used to calculate the influence of geometry-induced current and potential distributions on the impedance response of an ideally polarized disk electrode. A coherent notation is proposed for local and global impedance which accounts for global, local, local interfacial, and both global and local ohmic impedances. The local and ohmic impedances are shown to provide insight into the frequency dispersion associated with the geometry of disk electrodes. The high-frequency global impedance response has the appearance of a constant-phase element CPE but can be considered to be only an apparent CPE because the CPE exponent is a function of frequency

The Apparent Constant-Phase-Element Behavior of a Disk Electrode with Faradaic Reactions

Geometry-induced current and potential distributions modify the global impedance response of a disk electrode subject to faradaic reactions. The problem was treated for both linear and Tafel kinetic regimes. The apparent capacity of a disk electrode embedded in an insulating plane was shown to vary considerably with frequency. At frequencies above the characteristic frequency for the faradaic reaction, the global impedance response has a quasi-constant-phase element (CPE) character, but with a CPE coefficient alpha that is a function of both dimensionless frequency K and dimensionless current density J. For small values of J, alpha approached unity, whereas, for larger values of J, alpha reached values near 0.78. The calculated values of alpha are typical of those obtained in impedance measurements on disk electrodes. For determining the interfacial capacitance, the influence of current and potential distributions on the impedance response cannot be neglected, even if the apparent CPE exponent alpha has values close to unity. Several methods taken from the literature were tested to determine their suitability for extracting interfacial capacitance values from impedance data on disk electrodes. The best results were obtained using a formula which accounted for both ohmic and charge-transfer resistances

Community leaders as intermediaries: how everyday practices create and sustain leadership in five informal settlements in Cape Town

Community leaders are expected to navigate between different social and institutional contexts, but they must do so without the direction, authority or legitimacy available to leaders within formal organizations. In this article, we draw on qualitative data from a participation initiative to explore how community leaders get involved in everyday maintenance of public services in informal settlements in Cape Town, in order to understand how they fulfil this intermediary role. Applying the lens of leadership-as-practice (L-A-P), we identify four practices that connect the communities and city, and which facilitate access to public services. We unpack how these practices emerge in and are shaped by the service maintenance system and material conditions of informality. We argue that community leaders fulfil their intermediary role through everyday improvisations to find ‘what works’, and in the process, they also create and sustain relations of dependence and interdependence that reinforce those very roles

The origin of the complex character of the ohmic impedance

The local and global Ohmic response for an electrode exhibiting geometry-induced potential and/or current distributions has recently been shown to be represented by a frequency-dependent complex impedance. A physical explanation for this result is provided in terms of the radial contribution to local current density and the decrease in current density along the current lines. Experiments performed with Cu/Al and Mg/Al galvanic couples show that, in regions where a radial current density does not exist, the local Ohmic impedance is independent of position; whereas, in regions where the radial current density cannot be neglected, the local Ohmic impedance is a function of position. Simulations performed on recessed electrodes show that, even in the absence of a radial current, an axial variation of current density gives rise to a complex Ohmic impedance. The complex character of the Ohmic impedance shows that an equivalent circuit, using the usual two-terminal resistor to represent the Ohmic contribution of the electrolyte, provides an inadequate representation of an electrode with geometry-induced current and potential distributions

Local electrochemical impedance spectroscopy: A review and some recent developments

Local electrochemical impedance spectroscopy (LEIS), which provides a powerful tool for exploration of electrode heterogeneity, has its roots in the development of electrochemical techniques employing scanning of microelectrodes. The historical development of local impedance spectroscopy measurements is reviewed, and guidelines are presented for implementation of LEIS. The factors which control the limiting spatial resolution of the technique are identified. The mathematical foundation for the technique is reviewed, including definitions of interfacial and local Ohmic impedances on both local and global scales. Experimental results for the reduction of ferricyanide show the correspondence between local and global impedances. Simulations for a single Faradaic reaction on a disk electrode embedded in an insulator are used to show that the Ohmic contribution, traditionally considered to be a real value, can have complex character in certain frequency ranges

Place leadership in social accountability initiatives

This paper explores how social movement and civic actors enact and contribute to place leadership. It does so by examining how social movement organizations in South Africa use social audits to investigate and challenge government accountability and service delivery failures. The paper describes the meaning-making practices evident in social audit reports, and detail how social audit actors construct issues and positions through three framings – rights, regulations and lived realities. In this process, they leverage rights discourses and governance arrangements to legitimize their place leadership, and draw on multiple aspects and experiences of place to expose failures of governance and in the realization of rights. Through the dynamic interplay between legitimizing and exposing, they translate embodied realities and relations in and of place into a sense of purpose and direction for mobilizing a wider network of governance actors. On this basis, the paper contributes a social accountability perspective to place leadership studies

'A tough line to work through': ethical ambiguities in a South African SME

The unique contexts and experiences of SMEs have recently been taken up as a gap within business ethics scholarship. In this exploratory paper, I aim to contribute to the knowledge of ethics within SMEs through an in-depth case study comprising interviews, site visits and document analysis of a South African SME. The paper examines the SMEs’ three owners’ perceptions of ethics and corporate social responsibility, as well as the ethical challenges arising from their employee relations. The case illuminates the deep uncertainties and contradictions that permeate the owners’ values and practices and points to ethical decision-making as a process of reflection and moral imagination

Effect of time to onset on clinical features and prognosis of post-sternotomy mediastinitis

AbstractIncubation time affects the clinical features and outcome of many nosocomial infections. However, its role in the setting of post-sternotomy mediastinitis (PSM) has not been specifically studied. The present study aimed to evaluate the impact of time to onset of PSM on the clinical presentation and outcomes of patients. Hospital records of 197 patients who developed PSM over a 10-year period and were treated by closed drainage using Redon catheters were reviewed retrospectively. Follow-up was complete for all included patients (median of 19 months); 98 patients developed early-onset PSM (time from initial operation to PSM <14 days) and 99 patients had late-onset PSM (≥14 days). Patients with late-onset PSM had a higher rate of internal thoracic artery harvest and mediastinal re-exploration after initial operation. Patients with early-onset PSM presented more frequently with septic shock. Microbiological findings differed between early- and late-onset PSM by a higher incidence of Enterococcus species in the former and of Staphylococcus aureus in the latter. Overall mortality reached 34% (n = 66). Rates of superinfection, treatment failure, mediastinitis-related death, mortality at 1 year and overall mortality were all significantly higher in patients with early-onset PSM. Multiple regression procedures identified early-onset PSM as a significant and independent risk factor for both 1-year (OR 2.40; 95% CI 1.12-5.11) and overall (OR 2.11; 95% 1.26-3.53) mortality. In conclusion, the results obtained in the present study support the distinction between early- and late-onset PSM with different clinical and pathophysiological features. Early-onset PSM is associated with a significantly higher morbidity and mortality compared to late-onset PSM

Speaking or being Chinese: the case of South African-born Chinese

Language is often seen as an important symbol and marker of identity. The relationship between the two especially comes to the fore in the experience of immigrants who often must negotiate competing pressures on their identities and language usage. In this article we examine the link between language and identity through an exploration of the lived experiences of four Chinese individuals in South Africa. Drawing on interview data, we examine their language and identity shifts and the factors driving such shifts (or lack thereof). Our analysis reveals multiple degrees and expressions of Chinese identity, of which language is sometimes but not always relevant. We find that social and historical contexts shape the needs and motivations of the individual, who often uses language strategically and flexibly to emphasise or understate particular identities. We conclude that the situational view of the language-identity relation is more appropriate to explain the weak and strong language-identity links that occur at the micro-level