1,720,991 research outputs found
Development of nanocrystalline graphite for MEMS and membranes
No full textThin carbon films such as graphite and graphene are promising materials for micro- and nanoelectromechanical systems (MEMS and NEMS) applications, including resonators and switches, and also to be used as membranes for molecular sieving. The potential of these materials to be used in such applications is due to their electrical and mechanical properties, and also because they may be used as ultra-thin films. However, the most widely-used methods to synthesise these films have a tendency to cause poor performance in MEMS and NEMS. The most common technique, chemical vapour deposition (CVD) onto a catalyst, is followed by a physical transfer of the film to a separate substrate. This causes wrinkling and a variation in strain across the film, which leads to variable performance of MEMS and NEMS. Therefore, plasma-enhanced CVD (PECVD), which can be used to deposit nanocrystalline graphitic films directly onto large-area substrates, such as 6-inch silicon wafers, is a promising route to overcome this issue.Doubly-clamped beams and square micromechanical membranes were fabricated using nanographite films of 300 to 400 nm thickness, on silicon substrates. The compressive built-in stress of the film, measured as 436 MPa, caused these microstructures to buckle out-of-plane when they were released. The buckling behaviour of both structures was used as a characterisation tool to measure the Young’s modulus of nanographite, which is a key mechanical parameter of materials, and was measured as 23 GPa.To demonstrate the use of nanographite in a MEMS application, cantilever and doubly-clamped beam resonators of thickness around 300 nm were fabricated. Despite the built-in compressive stress of the film, the doubly-clamped beams were fabricated without buckling, and were under an effective tensile stress. This was achieved through the fabrication procedure, by using a 25 to 30 μm isotropic etch undercut of the 200 μm-wide anchors. The stress gradient in the film caused deflection of the anchors which ‘pulled’ the beams tight through the application of tensile stress. The devices were actuated electrostatically and the vibration response was measured using laser Doppler vibrometry. Cantilevers were measured with fundamental natural frequencies between 5 and 25 kHz, and for doubly-clamped beams natural frequencies were measured between 245 and 640 kHz. Quality factors under ambient pressure were around 5 to 10 for cantilevers and 20 to 30 for doubly-clamped beams, and around 1800 under 30 mTorr vacuum for a doubly-clamped beam.An ultrahigh vacuum system with mass spectrometer was constructed to measure the permeance of gases through nanographite thin films. The permeance of He, H2, Ne, CO2 and O2 through 350 nm thick nanographite membranes was measured. The permeance of He, H2 and Ne at 150 °C was 5.1, 4.0 and 0.08 × 10-10 mol / (m2 ·s · Pa), respectively, but the permeance of CO2 and O2 was below the limit of detection of the mass spectrometer. An estimation of the maximum permeance of CO2 and O2 respectively were 1.12 × 10-12 and 4.76 ×10-15 mol / (m2 ·s · Pa). The low permeance of Ne, CO2 and O2 relative to H2 and He showed that nanographite is a promising membrane material for molecular sieving
Fabrication and characterisation of resistive nanocrystalline graphite
No full textThis work demonstrates the feasibility of fabricating resistive nanocrystalline graphite (NCG) on a Si substrate. The NCG film thickness of 9 nm was deposited using metal-free plasma enhanced chemical vapour deposition (PECVD) on a 6-inch p-type silicon wafer. The surface and electrical properties of the resistors produced were investigated. The average grain size of the NCG thin film is 35 nm with 0.8 nm of surface roughness. The electrical characterization of the NCG strips show metal-like behaviour in which the resistance is proportional to the strip lengths. The sheet resistance is found to be 39 kohm/sq which is two orders of magnitude larger than graphene deposited using Chemical Vapour Deposition. This indicates the carrier transport across grain boundaries has a large influence on the overall resistance of the device. However, the nano-sized grains on the NCG material could be used to enhance the sensitivity of the material towards the environment
Characterisation of nanographite for MEMS resonators
No full textThin-film graphite and graphene are promising materials for nanoelectomechanical systems (NEMS) resonators, for sensors and signal processing applications. The high in-plane stiffness, low mass density and electrical conductivity of graphene are key properties to obtain NEMS resonators with high natural frequencies, sensitivities and tunability. Chemical vapor deposition (CVD) onto a copper catalyst is the most widely-used method to obtain large-scale graphene. However this requires transfer to a desired substrate which adds complexity and can cause wrinkling and polymer contamination. As an alternative, plasma-enhanced CVD (PECVD) has been used to deposit nanographene and nanographite films directly onto insulating substrates, such as SiO2. Such films have graphitic domains ~10 nm in diameter. In this work, we fabricate electrostatically actuated MEMS resonators from nanographite, establishing this as a route towards integration of nanographene/graphite using CMOS-compatible fabrication. To fabricate our devices, 300 nm thick nanographite is deposited by PECVD onto 6-inch silicon wafers with 200 nm SiO2 layer. Methane is the carbon precursor with hydrogen diluent in ratio 60:75 sccm and material characterisation is performed using Raman spectroscopy and atomic force microscopy. The film is patterned via optical lithography into 10 µm wide doubly-clamped and cantilever beams and etched using O2 based reactive ion etching. E-beam evaporated nickel pads are used as contacts, then the device is released by isotropically etching the underlying SiO2 using HF vapour. The nanographite is under a relatively high compressive stress which causes buckling of the doubly-clamped beam. However, we over-etch the SiO2 to achieve a ~30 µm undercut of the beam anchors. The stress gradient in the film creates an upward deflection of the anchors and imparts an effective tension to the suspended beam. Finite element simulation has been undertaken to take account of the added ‘length’ which is added to the beam. We then model the fundamental mode of vibration as a beam under tension. To measure the resonant frequency of the resonators, we apply DC bias plus a time varying AC voltage, between the beam and substrate, causing a varying force at the frequency of the AC voltage. The velocity of the beam is measured using laser Doppler vibrometry and becomes large at mechanical resonance. Natural frequency of vibration has been measured for a large number of devices: 257 kHz for 150 µm beams, 420 kHz for 100 µm, 595 kHz 75 µm beams and 15 kHz for 100 µm cantilevers. Quality factors have been calculated from a fitted Lorentzian curve and at ambient pressure are 20 and 1300 at 30 mTorr. Application of increasing DC Bias (up to 50 V maximum) enables tuning of the natural frequency by electrostatic spring softening, with an average tunability of 1.19 kHz per volt across this range
Mechanical characterisation of nanocrystalline graphite using micromechanical structures
Full text linkConductive nanocrystalline graphite has been deposited using plasma-enhanced chemical vapour deposition at 750 °C, directly onto silicon substrates without any catalyst and fabricated into micromechanical membrane and beam structures. Using the buckling profile of the membrane and beam structures, we measure a built-in strain of - 0.0142 and through wafer-bow measurement, a compressive stress of 436 MPa. From this we have calculated the Young's modulus of nanographite as 23.0 ± 2.7 GPa. This represents a scalable method for fabricating nanographite MEMS and NEMS devices via a microfabrication-compatible process and provides useful mechanical properties to enable design of future devices
Sensing performance of nanocrystalline graphite based humidity sensors
Full text linkEnvironmental sensors play a crucial role in a wide range of applications. Amongst them, humidity sensors that are stable and operational in harsh environments are incredibly important for process control and monitoring. Nanocrystalline graphite (NCG) is a type of carbon-based thin film material. Previous work has shown that NCG has excellent mechanical properties and is able to withstand high radiation doses. The granular structure of the NCG film makes it a good candidate for humidity sensing as the film consists of conductive graphitic grains with a high density of sp2 bonds and amorphous grain boundaries with high resistivity, adsorption of water molecule onto the film forms conductive pathways between grains through the Grotthuss mechanism which lowers the resistance of the film by a measurable amount. Here we report for the first time, a working humidity sensor with linear response, fabricated using NCG as the sensing material for harsh, real-world environments, which include exposure to weak acids via rainfall, UV radiation, mechanical wear, and high humidity environments. The calculated sensitivity of the best-fabricated sensor is S = 0.0334%, with a maximum resistance change of -4.4 kOhms, over the range of 15% RH to 85% RH. The response time of the sensor is 20ms with the current measurement setup. The baseline resistance value of the sensor at 15% RH is 210 kOhms. The sensor has the potential to be used as a humidity sensor for harsh environments due to the chemical, thermal and mechanical stability of the NCG film
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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