1,721,246 research outputs found
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
Multimode memtransistors as optoelectronic synapses for neuromorphic computing
No full textRecently, a neuromorphic approach to electronics has gained attention by bringing a fundamentally different approach to existing computing architectures for pattern recognition and learning applications. Emulating complex neural behavior for a synapse through conventional Si-based devices requires many elements, increasing fabrication complexity and bringing challenges to connectivity and energy consumption. Thus, there is a need to investigate alternative material systems and device architectures for emulating neural behavior. The abrupt switching physics of most two-terminal memristors (memory + resistor) limits the number of addressable states to two/binary, limiting their plasticity and storage capacity that adversely affects the trainability of artificial neural networks. The coupling of more than one control terminal is indispensable to elicit multiple programmable conductance states and non-abrupt state transitions as weighted connections to store and update weights necessary for learning algorithms. By adopting multimodal electro-optical schemes that ‘gates’ the memconductance, one can realize multi-state optoelectronic synapses with higher order weight changes that standard two-terminal devices fail to address. This thesis explores various interfacial strategies to exploit three-terminal gate-tunable memristors (aka Memtransistors) for emulating higher-order synaptic functions.Doctor of Philosoph
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
Toxic-free inorganic coating materials for flame retardation and prevention of flame spreading on underground infrastructure
No full textChemically bonded phosphate ceramic (CBPC) is an inorganic solid that has intermediate properties between cement and conventional ceramics. As CBPCs are produced by solution processing at low temperature, this type of ceramics can be utilized as functional coating. Low temperature setting aluminium based phosphate ceramics were produced by two different methods. One involved direct mixing of α-alumina powder with phosphoric acid while the other involved formation of AlH3(PO4)3•3H2O gel phase before mixing with α-alumina. Both methods required heat treatment between 155-175°C for at least 24hrs in order to facilitate the reaction. X-ray diffraction studies showed that the major evolved phosphate peaks in all samples belonged to a water soluble phase Al(H2PO4). The second method yielded insoluble berlinite (AlPO4) phase while AlPO4 only appeared in samples produced by first method after heating at 400°C. DSC from 40-400°C showed that phosphates produced by the second method (separate reaction) were more thermally stable compared to direct reaction method. Furthermore, an endothermic peak was observed in DSC of most samples at 240°C. It was suggested to be the decomposition of Al(H2PO4) into AlPO4 through the XRD analysis of samples that went through a simulated DSC heating process. By analysing the amount of heat absorbed during the endothermic reaction, it was also suggested to be the loss of crystal water within the structure. Overall the separate reaction method is concluded to be better and more practical as the reaction kinetic can be better controlled to produce berlinite and the product has better thermal stability.Bachelor of Engineering (Materials Engineering
Self-healing materials for flexible electronic devices
No full textWith the onset of flexible and wearable electronics, devices are now put on places like the human body and various curved surfaces which doesn’t have any regular shape and sizes. Therefore, these devices need to be flexible, conformable and stretchable to fulfil all these applications. However, the continuous exposure of these devices is always subjected to higher mechanical stress, wear and tear when compared to conventional electronics. Hence the ability to recover and repair upon damage is a necessity and not a luxury for flexible electronic devices. These applications not only require good electrical conductivity but demand better mechanical properties like stretchability, flexibility and robustness where the conventional electrodes, for example, indium tin oxide (ITO) fail to perform. Hence, self-healing, flexible, transparent electrodes could transform the way of fabrication of electronic devices in future. Challenges associated with mechanical fracture of electrical conductors has hindered the realization of truly flexible, robust and high-performance wearable electronics. The remarkable achievements in transparent and flexible electrodes have raised widespread interest in research groups in flexible electronics, owing to their low-cost fabrication, easy scale-up, and unique properties. However, they still suffer from innate problems like mechanical rupture, scratching and bending torsion because of their “soft” and flexible nature related to their solid counterparts. Hence, self-healing capability would be highly desirable for these electrodes in flexible electronic devices.
In this dissertation, the demonstration of versatile transparent healable electrodes has been developed and examined to alleviate these problems. The composite electrode features a layer of interconnecting AgNWs network on a polyurethane film modified with Diels–Alder adducts (PU-DA). The PU based DA polymeric electrode can heal multiple times due to the presence of DA and retro-DA mechanism, which are based on thermo-reversibility. Surface modification using hydrophilic molecules improved adhesion of the AgNWs and resulted in mechanically robust flexible electrodes with a high figure of merit; showing low sheet resistance with good transmittance in the visible spectrum. Transparent and flexible healable heaters (TFHH) with good mechanical and thermal stability were fabricated using these electrodes for potential applications in thermochromics, electrically driven displays and defrosters. The PU-DA based healable heaters exhibited high Joule heating temperatures with a low operation voltage, rapid thermal response and enhanced robustness to withstand large repeated mechanical strain for over 500 bending cycles with small variance in resistance. After deliberate damage by a knife cut, the electrodes healed and recovered back to its original conductivity via a simple heat treatment at 120 °C. Uniquely, the healing process can also be triggered by utilizing electrical power.
The self-healing polymer with the addition of ionic liquids (ILs) may have great potential for future electronic materials. ILs would make it possible to integrate functional excipients to recognize multifunctional electrical applications; owing to the intrinsic advantages of suppressed volatility, high ionic mobility, thermal stability, and a wide range of electrochemical window. Thermally-reversible Diels-Alder (DA) mechanism for self-healing are promising but have only been demonstrated healing at high temperatures (~120 °C). However, theoretically, the DA mechanism can be triggered at temperatures as low as 50 °C, indicating that the self-healing mechanism is limited by the thermal mobility of the polymeric chains. Next, the effect of ionic liquid as a plasticizer was investigated in PU-DA in order to minimize the healing temperature and increase the mechanical properties of the polymeric composite. The incorporation of ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMITFS) alleviates this challenge, rapidly accelerating healing, while concomitantly improving the dielectric constant and the mechanical properties of a polyurethane derivative based on the DA chemistry for PU. For optimized compositions, the healing temperature reduced from 120 °C to 60 °C and the maximum strain to failure significantly increased from 17.1% to 102.1%. Owing to the ionic polarizability of EMITFS, the composite exhibited highly attractive dielectric properties with the dielectric constant enhanced from 2.7 to 12.9. Finally, a highly flexible, healable and fully solution-processed electroluminescent device was demonstrated.
To tailor specific multi-property materials, usually, sophisticated multi-step synthesis processes are employed and will affect the device fabrication method. With the increase in concertation of ionic liquid, the polymeric films have increased in mechanical, electrical as well as healing properties as it was observed in the case of PU-DA. Taking that concept as a motivation, a composite system with healing behaviours in polymeric systems without DA recovery mechanism was investigated. The incorporation of ionic liquid (EMITFS) in amorphous polymer like PVP, rapidly accelerates the healing, while improving the mechanical properties. The PVP with ILs composite films were compared with IL composites with another amorphous polymer, PMMA, as well as semi-crystalline polymers like PVA and PVDF-HFP to mimic the healing behaviours and mechanical properties, while incorporating EMITFS in these polymers. The effect of cation and anion present in ionic liquids was investigated, on the best polymer system (PVP), in terms of healing behaviour and mechanical properties. The different ionic liquids (EMITFB and EMITFSI) was used as compared to EMITFS since these ionic liquids have the same cation but different anions. This chapter depicts the study of the influence of ionic liquids on the different class of polymer structure and its effects on the healing properties. In the end, a novel healing mechanism with ionic liquids was demonstrated in PVP polymer without DA recovery mechanisms.Doctor of Philosoph
Oxalic acid leaching and eggshell wastes adsorption for recycling of solar cells
No full textIncreasing climate change and global warming have risen concerns and drive the search
for alternative energy sources, preferably greener and renewable. Photovoltaics (PV) is
one of the promising candidates as it is clean and generates no hazardous emission. The
industry has grown drastically ever since, as many switch from traditional energy source
to PV to exploit its benefits. Combined with government policies to promote PV as an
energy alternative, the growth has been further catalysed. With the increasing number of
installations and the limited lifespan of 25-30 years, it is essential that proper waste
management for End-of-Life (EoL) panels is in place to deal with the large, expected
volume of wastes. Many raw materials are present in the solar panel wastes and it is
crucial to extract and repurpose these materials. Metal extraction/dissolution has been
usually carried out with strong acids and/or bases such as nitric acid (HNO3), hydrofluoric
acid (HF) and sodium hydroxide (NaOH). The utilisation of these harsh chemicals can
have detrimental effects on the users and environment. Noxious gases like NOx can get
released in the process of extracting metal ions using HNO3, while HF and NaOH are
highly corrosive and have adverse effects to environment if not properly treated before
discharge. Therefore, it is necessary to strategize an environmentally friendlier leaching
approach. This project focus on the utilization of oxalic acid in replacement for strong
inorganic acids for leaching, and eggshell as biosorbent to remove aluminium (Al) from
the leachate solutions. Herein, we evaluated the potential of oxalic acid as leaching reagent
for solar cell waste as well as demonstrated eggshell as biosorbent to remove Al from the
collected leachate solution.Bachelor of Engineering (Materials Engineering
Direct ink writing for electroresponsive human machine interfaces
No full textTo aid with efficient and reliable communication with machines, Human-Machine Interfaces (HMIs) are crucial. Current approaches for HMIs rely on rigid, non-compliant devices. This structural non-compliance with inherently soft, curvilinear human body makes interfaces non-intuitive and limits widespread applications of HMIs. Hence, it is necessary to develop newer, compliant form factors for HMIs via development of soft sensors and responsive devices.
Existing methods for fabrication of soft devices require high temperature processing and are hence not suitable for large scale devices made with soft polymeric materials. Extrusion based additive manufacturing methods such as Direct Ink Writing (DIW) show great potential for this application. This work focuses on optimization and modification of DIW system to handle various viscosities of inks, and special functional materials, to fabricate all-printed HMI devices.
Initially, a custom DIW setup was assembled in-house. 3 axis motion stage was used for moving the dispensing head. Syringe pump and peristaltic pumps were used for low viscosity inks. Pneumatic ink dispenser was used for highly viscous shear thinning inks. The Syringe-pump system was modified to accommodate Phase Change Material (PCM) ink for tactile response devices. Formulations of various composite material systems were optimized for printing via DIW systems. First, a low viscosity conducting ink of PEDOT:PSS and MWCNT aqueous suspensions was prepared. 2 different dilutions of MWCNT, 0.5mg/ml and 1mg/ml were prepared and mixed with PEDOT:PSS in varying volume ratios. 1:3 ratio of PEDOT:PSS to 0.5 mg/ml MWCNT suspension was found out to have lowest resistivity. Subsequently, Acetylene black nanoparticles were mixed with PDMS to form viscous conducting composite ink. Percolation threshold for the network is found out to be at ~13%(w/w). To maintain printability, loading was limited to 15% and ink was prepared by adding crosslinker and thinner to the mixture. Further, Polyethylene glycol (PEG) was dispensed using a modified setup to keep it molten. Ti3C2 MXene was intercalated using LiBr and DMSO and delaminated to prepare 10mg/ml suspension which was added to PEG. The PEG-MXene composite had higher amount of nucleation sites. This helped in speeding up the phase transformation of printed patterns. For the sensing part of HMI, various tactile sensors were printed using the developed conducting inks. PEDOT:PSS-MWCNT composite was used to fabricate bending angle sensor, and strain sensor. The bending angle sensor shows fast, highly linear response with
up to 10° resolution, and a rage of 0-180°. The strain sensor successfully measures small strains with ΔR/R0=15 measured for 5% strain. Further, Acetylene black – PDMS ink was utilized for fabrication of proximity sensor. Material differentiation and water content detection using these sensors is demonstrated. Different sensor geometries were tested and upto 2% of ΔC/C0 was achieved for metallic object. PEG-MXene PCM ink was used along with silver joule heater to fabricate flexible hardness modulation based
tactile response devices. A thermally modulated transition resulting in 10 times change in hardness of the material was obtained as a response. A 3*3 array of such devices was printed to demonstrate display of letters via hardness modulation
Finally, future scope of this project is discussed. Challenges for printing inks such as MXene dispersions and Liquid Metal are presented. Results of initial experiments carried out for newer printing paradigms such as 3D printing of stretchable materials and multimaterial printing is discussed. Further, newer devices such as all printed stretchable electroluminescent devices and printed active devices such as TFTs are proposed.Master of Engineerin
Metal oxide memristors for neuromorphic electronics
No full textNeuromorphic electronics aim to emulate the functionalities of the brain and enable the next generation of power efficient devices for artificial intelligence applications. Neuromorphic devices emulate various neural features including synaptic plasticity- which is the brain’s ability to re-wire itself during training and is credited for the brain’s efficiency in processing information and learning. Although silicon CMOS based approaches exist, efficient implementation of neuromorphic behavior necessitates the development of memristive devices based on novel materials and device architectures. Present memristive approaches for neuromorphic electronics utilize conventional two- terminal oxide memristors which operate based on filamentary formation and rupture. These conventional devices, although suitable for non-volatile memory applications are less suited for neuromorphic electronics since they do not natively demonstrate multiple states and critically show poor control over the temporal response. Essentially, these devices fall behind in terms of the required analogue programmability due to its stochastic nature. As memristors are devices that encode information in conductivity levels, it is important to study the properties not only of the bulk materials but also the interfaces to induce richer neuro-emulative behavior. Alternative programming modes and device structures which go beyond electrical biasing of 2 electrodes need to be explored. The overarching aim of this thesis is to explore such approaches to improve memristive device performances by tuning the number of states and temporal response. This can only be achieved through a mechanistic understanding of the charge transfer, charge transport and chemical changes in the oxide memristor device. The work presented in the thesis was able to address various challenges in the area of memristive devices for neuromorphic electronics by – (1) inducing multiple states in a memristor device by control over filament formation, (2) tuning the temporal (transient) response of the devices by utilization of electrical and optical stimulation and (3) incorporating simultaneous state and temporal control via a third electrode and optical input. Based on these approaches, devices with multi-state programmability and temporal tunability has been shown. Lastly, these properties were utilized to emulate functionalities inspired by biological systems demonstrating synaptic
Abstract
plasticity, the phenomenon of inhibition in memory formation and sensory adaptation to intensities similar to the eye.
In the first work that focuses on filamentary memristors - the controllability of intermediate switching is targeted. Resistive switching is often explained by the formation and rupturing of conductive filaments comprised of oxygen vacancies. With an ion-blocking electrode impeding the oxidative process, the suppressed anodic reaction can lead to poor controllability of the resistive switching process. By employing electrodes of high reduction potential or oxygen affinity, the memristor is shown to have improved multi-state programmability. Through spectroscopic analysis, the formation of Sn(0) species during the resistive switching process was verified ex situ. With multi-state programmability, synaptic functionalities such as short-term plasticity, long-term plasticity and spike-timing dependent plasticity are emulated. While intermediate states can be achieved, the level of temporal response in filamentary switching is insufficient for neuromorphic application.
Temporal response similar to a biological synapse was targeted using light as an additional means to modulate the conductance of the memristive systems. In the second work that focuses on photo-modulation – the inhibition of long-term weight changes is explored. Here, a photomemristor device based on the Schottky ITO/SnOx interface is fabricated. With a low-temperature low-oxidative thermal atomic layer deposition process, the SnOx thin film with high oxygen vacancies acts as the active switching layer. Through impedance measurements, the photomodulation mechanism is compared with the interfacial switching process. The switching behaviour of optical devices can be explained by the de-trapping of electrons at the oxide-oxide interface via photo-excitation. However, the same interface is also responsible for electrical based memristor switching. By applying electrical pulses before the optical pulses, we have shown that the memory retention of optical trainings can be tuned on demand via inhibition and facilitation. Lastly, with electrical pre-exposure, a self-filtering function known as latent inhibition can be demonstrated in associative learning by the synapse.
Abstract
Finally, in the third work a system which allows for simultaneous control of states and temporal decay was demonstrated. While a strong dual mode coupling can be demonstrated in photomemristors, they lack selectivity in terms of photoresponse. With a third electrode as a gating option, doping/de-doping of the electrochromic MoO3 semiconductor with Li+ can be used to tune the optoelectronic properties and hence the photoresponse. Through spectroscopic analysis, the alteration in band structure of MoO3 can be verified. This explains how both the optical and charge transport properties can be modulated with electrochemical doping/de-doping. While electrochemical transistors have been explored as synaptic devices, the tunability of optoelectronic properties is not yet utilized. Here, with reversible electrochromic switching, multiple state programmability and temporal response tunability can be demonstrated in the electrochromic transistor device. Lastly, with the tunability in photoresponsivity, adaptation features such as scotopic and photopic vision in the eye can be emulated.
The thesis concludes with how further improvement on analog properties and the emulation of neuronal features can be explored. Lastly, with the development of functional memristors in processing and sensing, the opportunities in memristor-enabled computing and sensing platforms are discussed.Doctor of Philosoph
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