1,720,984 research outputs found
Fabrication of Silicon Nanowire Forests for Thermoelectric Applications by Metal-Assisted Chemical Etching
Full text linkSilicon nanowires, whose thermal conductivity is strongly reduced with respect to that of the bulk silicon, are very promising for high-efficient thermoelectric conversion. This work focuses on the development of a technique for the fabrication of thermoelectric generators which are based on vertical silicon nanowire forests, achieved through a metal-assisted chemical etch. As heavily doped nanowires are essential in thermoelectric applications, this chemical process has been applied both on lightly and on highly doped (> 1019 cm−3) silicon substrates. A comparison of the results shows that the etch behaves in a completely distinct way when applied to the differently doped substrates. The results of this comparison and a preliminary insight into the diverse behavior occurred are reported. The different initial nucleation of silver, which determines the hole injection, essential to the etching of silicon, seems to be the key point of this different behavior
Silicon de novo: Energy filtering and enhanced thermoelectric performances of nanocrystalline silicon and silicon alloys
No full textSecond-phase precipitation in nanocrystalline silicon may lead to improved thermoelectric power factors (up to ≈15 mW K-2 m-1 at room temperature) and high figures of merit. In this review, energy filtering is shown to provide an explanatory framework for these findings, helping predict strategies to further improve nanocrystalline silicon efficiency in thermal harvesters
Large increase of the thermoelectric power factor in multi-barrier nanodevices
No full textImproving the power factor (PF) of thermoelectric materials, crucial for enhancing the power output of thermoelectric generators, is challenging due to the adverse interdependence of the Seebeck coefficient and the electrical conductivity on carrier density. We introduce a novel strategy employing energy filtering via built-in potential barriers to alleviate this dependency, significantly enhancing the PF. Utilizing electron-beam lithography, we developed a Si-based nanodevice featuring a multiple well/barrier design. Measurements yielded a PF of 11 mW m−1 K−2, more than doubling the optimal PF achievable in bulk silicon. Experimental findings align well with theoretical models, affirming the efficacy of the approach. Leveraging established silicon technologies in device fabrication unveils pathways for on-chip micro-energy harvesters and localized Peltier coolers. Moreover, the results validate a material-agnostic energy filtering model, guiding the creation of PF-enhanced devices across various thermoelectric materials
High figures of merit in degenerate semiconductors. Energy filtering by grain boundaries in heavily doped polycrystalline silicon
No full textHeavily boron-doped polycrystalline silicon has been reported to be characterized by somewhat unexpectedly high power factor. High Seebeck coefficients are however unexpected in materials with high carrier densities. A semi-quantitative model was proposed, showing that the potential barrier structure at grain boundaries, along with the nanometric grain size, leads to an unusual mechanism of carrier filtering, named adiabatic energy filtering. Actually, the presence of potential barriers associated with segregated boron disables charge transport by holes in the band deep tail. This leads to a decrease of the actual carrier density, as in the case of standard energy filtering. However, the nanometric grain size along with the inefficiency of the hole-hole relaxation mechanism in degenerate semiconductors actually prevents carriers from relaxing, causing an increase of the average (macroscopic) drift mobility. Thus, in spite of the decrease of drifting hole density the electrical conductivity is found to increase. In this communication a refinement of the model is presented, that will be discussed and corroborated with an extended body of experimental data gathered by several authors on degenerate polycrystalline silicon films
Nanovoid Formation and Dynamics in He+-Implanted Nanocrystalline Silicon
No full textHelium implantation in single crystal silicon is known to lead, after a proper thermal treatment, to the formation of voids with diameters ranging between 10 nm and 30 nm. Formation of voids is governed by the coalescence of vacancies created by implantation, initially trapping helium atoms. At high temperatures (), helium leaves the nanobubbles and outdiffuses, while the now empty voids grow in size and eventually change their shape to form tetrakaidecahedra (Wulff construction). In this communication, we report how He+ implantation in heavily boron-doped nanocrystalline silicon shows a completely different dynamics. Annealing at leads to the formation of large voids, located around grain boundaries, along with a large number of nanovoids with an average diameter of 2-4 nm and an estimated density of distributed throughout the grains. Annealing at higher temperature (up to ) also induces a decrease of the void size with a change in their density, finally accounting to . The high temperature annealing also causes vacancy evaporation down to a depth of 80-100 nm from the outer surface. The possibility of obtaining a stable, uniform distribution of nanometer-sized voids is of major relevance as a novel tool for phonon and electron engineering in thermoelectric materials
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
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