1,721,008 research outputs found
MEMS actuators and sensors: observations on their performance and selection for purpose
No full textThis paper presents an exercise in comparing the performance of microelectromechanical systems (MEMS) actuators and sensors as a function of operating principle. Data have been obtained from the literature for the mechanical performance characteristics of actuators, force sensors and displacement sensors. On-chip and off-chip actuators and sensors are each sub-grouped into families, classes and members according to their principle of operation. The performance of MEMS sharing common operating principles is compared with each other and with equivalent macroscopic devices. The data are used to construct performance maps showing the capability of existing actuators and sensors in terms of maximum force and displacement capability, resolution and frequency. These can also be used as a preliminary design tool, as shown in a case study on the design of an on-chip tensile test machine for materials in thin-film form
Gelatinization rheological parameters of taro starches and their correlations with the pasting properties, gelatinization thermal properties and molecular structure.
No full textComputational simulation of thermally sprayed WC–Co powder
No full textWC–Co cemented carbides are a class of hard composite materials of great technological importance. They are widely used as tool materials in a large variety of applications that have high demands on hardness and toughness, including mining, turning, cutting and milling. The HVOF (high velocity oxygen fuel) technology has been very successful in spraying wear resistant WC–Co coatings with higher density, superior bond strengths and less decarburization than many other thermal spray processes, attributed mainly to its high particle impact velocities and relatively low peak particle temperatures. The degree of decomposition and bond strength is directly related to relevant particle parameters such as velocity, temperature and state of melting or solidification. These are consecutively related to process parameters such as powder particle size distribution, carrier gas flow rate, and fuel type employed. To obtain detailed particle data important for thermal spraying, mathematical models are developed in the present paper to predict the particle dynamic behavior in a liquid fuelled HVOF thermal spray gun. The particle transport equations are coupled with the three-dimensional, chemically reacting, turbulent gas flow, and solved in a Lagrangian manner. The melting and solidification within the particles as a result of heat exchange with the surrounding gas flow is solved numerically. The in-flight characteristics of WC–Co particles are studied and the effects of carrier gas parameters on particle behavior are examined. The results demonstrate that WC–Co particles smaller than 5 ?m in diameter undergo melting and solidification prior to impact while most particles never reach liquid state during the HVOF thermal spraying. The flow rate of carrier gas has considerable influence on particle dynamics as well as deposition on substrate. At higher flow rate the powder particles are redirected further away from the substrate center, while smaller flow rate results in better heating, higher impact velocity and deposition closer to the substrate center
Effect of granular characteristics on the viscoelastic properties of composites of amylose and waxy starches.
No full textNumerical modelling of sequential droplet impingements
No full textDuring the process of thermal spray coating, molten powders are sprayed and deposited on substrates to generate protective coatings. It is essential to have a clear understanding of the physics of droplet impingement on the surface of substrates for better control of the generation of splats and the structure of coating. A numerical model is developed in this paper to simulate the dynamics of transient flow during the impingement process, including spreading, break-up, air entrapment and solidification. The computation is achieved using the technique of volume of fluid surface tracking within a fixed Eulerian structured mesh. The three-dimensional simulation is able to accurately give a demonstration of dynamic flow patterns such as the generation of fingers, satellite droplets and pores during impingement. The numerical model is validated with experimental data from the tin droplet measurement and excellent agreement is found between the simulation and the experiment.<br/
On the design of two-dimensional Cellular metals for combined heat dissipation and structural load capacity
No full textSandwich panels with two-dimensional metal cores can be used to carry structural load as well as dissipate heat through solid conduction and forced convection. This work attempts to uncover the nature of heat transfer in these lightweight systems, with emphasis on the effects of varying cell morphologies and cell arrangements. The types of cell shape and cell arrangement considered include regular hexagon, square with connectivity 4 or 3, and triangle with connectivity 6 or 4. Two analytical models are developed: corrugated wall and effective medium. The former models the cellular structure in detail whilst, the latter models the fluid saturated porous structure using volume averaging techniques. The overall heat transfer coefficient and pressure drop are obtained as functions of relative density, cell shape, cell arrangement, fluid properties, and overall dimensions of the heat sink. A two-stage optimization is subsequently carried out to identify cell morphologies that optimize the structural and heat transfer performance at specified pumping power and at lowest weight. In the first stage, the overall heat transfer performance is optimized against relative density. Regular hexagonal cells are found to provide the highest levels of heat dissipation. In the second stage, a constraint on stiffness is added. It is then found that, for panels with thin cores, triangular cells constitute the most compact and yet stiff heat sink design; however, for high heat flux scenarios, hexagonal cells outperform triangular and square cells
Numerical modelling the bonding mechanism of HVOF sprayed particles
No full textDuring high velocity oxy-fuel (HVOF) thermal spraying, most powder particles remain in solid state prior to the formation of coating. A finite element (FE) model is developed to study the impact of thermally sprayed solid particles on substrates and to establish the critical particle impact parameters needed for adequate bonding. The particles are given the properties of widely used WC-Co powder for HVOF thermally sprayed coatings. The numerical results indicate that in HVOF process the kinetic energy of the particle prior to impact plays the most dominant role on particle stress localization and melting of the particle/substrate interfacial region. Both the shear-instability theory and an energy-based method are used to establish the critical impact parameters for HVOF sprayed particles, and it is found that only WC-Co particles smaller than 40 ?m have sufficient kinetic and thermal energy for successful bonding.<br/
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
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