1,721,052 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Nanotribology of ionic liquid-solvent mixtures
Get PDFIonic liquids (ILs) have gained notoriety within the field of liquid matter, specifically as a lubricant. Fascination with ILs as lubricants is due to their physio-chemical stability and strong interactions with solid surfaces, marking an improvement over industrial lubricants. IL lubricants can therefore operate in high-pressure regimes without compromising the integrity of the surfaces.
While studies have demonstrated that pure ILs are excellent lubricants, using large amounts is costly and unfeasible for industry. A route around this problem is to use trace amounts of IL as an additive to make an industrially efficient lubricant.
This thesis explores the interfacial structure and frictional response across a range of IL-solvent mixtures to explore their performance as lubricant additives. Using the surface forces balance (SFB), the structural and frictional forces at the nanoscale can be simultaneously measured while the applied load is controlled. Macroscale friction behaviour is also studied using a ball-on-disc tribometer.
Both nanoscale and macroscale measurements across a range of ILs, mixed with either a polar solvent or a model base oil, showed that friction is influenced by the liquid film structure. Using the SFB, we were able to understand the molecular mechanisms underlying the friction, as we could probe the simultaneous confined structure. Varying the composition of these mixtures revealed that while macroscale performance could be improved with trace additions of ILs, the nanoscale behaviour was not as reliable.
The SFB measurements of oil-soluble ILs showed a lack of layering and ineffective lubrication when compared to the base oil, while the water-soluble IL showed reliable and superlubric performance across both length scales, with the nanoscale studies showing that a switch in the structural forces was reflected in the frictional forces. These results indicate that while ILs can become lubricant additives, a wide-scale systematic study over multiple length scales and surfaces is required
Interfacial nanostructure of solvate ionic liquids and ionic liquid solutions
Get PDFThe technology employed by human beings for the generation, storage and usage of energy is presently undergoing the fastest and most profound change since the industrial revolution. The changes in the generation and usage of energy necessitate the development of new methods of energy storage. In these systems, electrochemical energy storage will play a crucial role and to this end new electrolytes need to be explored to complement these changes.
One such class of liquids is ionic liquids, a class of salts that are molten at room temperature. These liquids have a broad applicability to batteries and supercapacitors. This thesis details work where molecular dynamics simulations have been used to explore the nanostructure of ionic liquids and their mixtures with various molecular solvents at simplistic electrodes.
The thesis has two broad sections. The first is covered in Chapter 3, and explores the nanostructure of ionic liquid propylene carbonate solutions, developing a framework through which these nanostructures can be understood. The section concludes that the increasing dilution of ionic liquids decreases the surface charge at which the characteristic ionic liquid oscillatory interfacial structure gives way to a different structure featuring monotonic charge decay. The behaviour of ionic liquids at interfaces is found to be correlated to ion size and type, as well as concentration. A wide divergence in the observed behaviour is shown at positive and negative electrodes due to the asymmetry of propylene carbonate.
The second section, consisting of two chapters, explores the interfacial nanostructure of solvate ionic liquids using two different boundary conditions to model the electrode. This work is the first simulation of solvate ionic liquids at electrified interfaces. This section will explore the effect of electrode model on the behaviour of these ionic liquids at the electrode. Chapter 4 uses a fixed charge electrode, whereas Chapter 5 uses one with a fixed potential. The section concludes that regardless of electrode model, the idealised portrait of a solvate ionic liquid - one where the liquid behaves exactly as an aprotic ionic liquid - is not applicable. In Chapter 4's exploration of fixed charged electrodes, the formation of 2 glyme to lithium complexes contradicts the idealised portrait of the liquid. A different change is observed in Chapter 5's exploration of fixed potential electrodes, with both lithium glyme and lithium anion clusters forming at the interface. The key difference between the two studies is that lithium does not coordinate to the electrode in the fixed charge simulations, while in the fixed potential case it does. At the end of Chapter 5 the results are compared against experimental data, with the efficacy of the two models discussed.
The aim of both studies is to look at the nanostructure of ionic liquids, when the symmetry between co-ion and cation repulsion - and related effects - is broken by the presence of a non ionic constituent in the liquid.</p
Nanostructure, self-assembly and friction in confined liquids
Get PDFThe work presented in this thesis uses a custom-built surface force balance with extreme sensitivity and resolution to understand at the molecular level how the structure of confined liquid films relates to their frictional and lubricating properties. The experiments involve shearing two identical and atomically smooth mica surfaces past one another with sub-nanometer control of the film thickness and ultrasensitive force resolution. With this, molecular mechanistic details relevant to boundary lubrication are uncovered for several systems. Friction modifiers are commonly used in engine oil formulations and adsorb as monolayers to surfaces, preventing surface contact and reducing friction between the surfaces. Here it is shown that the shape of additive surfactant molecules affects both the confined film structure and the lubricating behavior, with more upright monolayers exhibiting lower friction. Interestingly, mixing different surfactant molecules can give rise to friction much higher than for either molecule. This result is significant given that lubricant formulations typically contain many different types of additive molecules with different functions. Measurements made with ionic liquids of varying alkyl chain length reveal a dramatic cross-over in the interfacial layering structure, from alternating cation- and anion-enriched monolayers for ionic liquids with short alkyl chains, to bilayer formation for more amphiphilic ions. Their structural and dynamic properties in confinement are pertinent to applications ranging from electrolytes in nanoporous electrodes to specialist lubricants in extreme environments. The ionic liquids show clear evidence for ‘quantized friction’, where multiple friction-load regimes with different friction coefficients are measured for different numbers of confined ion layers for the same ionic liquid. Most significantly, the results of these experiments allow elucidation of shear mechanisms and sliding interfaces for monolayer and bilayer-forming ionic liquids which differ markedly to those of molecular liquids
Surface forces and structures in complex electrolytes
Get PDFElectrolytes, environments in which ions are free to move, are found throughout the natural and the human-centred world. The biochemistry that allows for life, alongside many of the innovations that make modern life so convenient, rely on the physical chemistry of these substances. Despite this, the behaviours of many complex and high concentration electrolytes are relatively poorly understood. This thesis investigates the forces and structures present in three examples of complex, high concentration electrolytes in order to better the understanding of these fascinating systems.
First, we present measurements of surface forces present in water-in-salt electrolytes, high concentration aqueous electrolytes with potential applications in next generation energy storage devices. We find evidence for a layered structure of aqueous and non-aqueous domains within the confined electrolyte films. These domains are maintained in the bulk liquid, and our analysis of the clustering of the domains suggests consequences for the mechanism of ion transport in water-in-salt electrolytes. We also studied ionic liquids, liquid electrolytes consisting solely of ions. We show that different layered structures can form in ionic liquids confined between surfaces, where the structure adopted depends on the nature of the ions. On studying mixtures of ionic liquids that form different interfacial structures, we find that the mathematical expressions describing the mixtures can be written as a linear combination of the terms describing pure ionic liquids. These results have important implications for the interpretation of correlation lengths within liquid mixtures, and for the optimisation of surface properties by creating liquid mixtures. Finally, we investigate the modifications of surface forces that occur in the presence of the high-concentration protein denaturant guanidinium chloride. We find varying behaviour across a broad concentration range that suggests strong time and concentration dependence in the action of guanidinium chloride at protein surfaces.
Together, the results of this thesis highlight the complexities of high concentration and poorly understood electrolyte solutions, and shed light on their interactions and behaviours. This work will be important in fully understanding the role these complex systems play, both in the devices and conveniences that allow modern life to function, and in the complex chemistries that govern life at a molecular level
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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