جستجوی مقالات مرتبط با کلیدواژه « میکرو » در نشریات گروه « مکانیک »

Since resonant micro/nanoresonators are very delicate devices with very small dimensions, therefore, any defects and faults caused by the process of manufacturing and laboratory implementation can lead to fundamental changes in their vibration behaviors. Therefore, the effects of the mentioned disadvantages should be considered as much as possible to obtain more accurate sensors with higher efficiency. In this study, a general model of a doubly clamped microbeam (nanotube) with an asymmetric cross-section with external excitation is considered. Then, linear and non-linear behaviors of an ideal nanotube with a circular cross-section are investigated. The results of the simulations indicate a good agreement with the experimental references available in the literature. Then, taking into account the asymmetry in the resonator cross-section, the system is moved away from an ideal model to a more real model, and the possible effects of the asymmetric cross-section in adjustment, reduction, and vanish of internal resonance are investigated and studied. Finally, the advantages and disadvantages caused by asymmetries and the optimal use of such an opportunity to obtain more innovative and complete models with higher efficiency are explained.

Currently, electronic components are very thin as well as fragile and their assembly on PCB boards is highly sensitive. The components may be damaged and broken in response to an excessive force. Therefore, it is necessary to use a mechanism with force control ability. A mechanism with a large member (macro) for long movements and a small member (micro) for short and delicate movements, is selected to provide accurate force control as well as high speed in installation. The macro member is rapidly approaches the surface of the PCB with the position control and then the micro member initiate its movement. As the micro member contacts the surface of the PCB, the position control is switched to force control and enables the installation of the device with controlled force. At the first attempt, PID controller is used. Thereafter, the fuzzy controller is selected to control the system. The fuzzy controller has a better performance and reduces the disturbances during installation. The relation between maximum contact force and contact time with velocity is also investigated and optimal velocity is determined.

Understanding the effects of thermoelastic damping on the vibration parameters such as natural frequency and frequency-sensitive is essential for the design of micro-nano-electromechanical systems. In this paper, the effects of thermoelastic damping in micro- and nanomechanical resonators beam with a rectangular cross section will be analyzed. The governing equations in present system are coupled of heat conduction equation and equation of motion where this methodology has been applied to three- dimensional analyses. To solve these governing equations analytically with considering suitable assumption, first the coupled heat conduction equation is solved for the thermoelastic temperature field by considering three-dimensional (3-D) heat conduction along the length, width and thickness of the beam.‫ Next; thermoelastic coupling is modeled into the equation of motion for flexural vibrations through a temperature-dependent moment of temperature distribution. Frequency shifts and quality factor due to thermoelastic damping are analyzed. For special cases, the obtained frequency shift is compared with the result of the frequency shifts computed using 2-D heat conduction; also the obtained quality factor is compared with exact 1-D heat conduction. The results obtained showed that the model presented in this paper were in good agreement with the other models and it predicted the effects of thermoelastic damping on the behavior of micro-nano resonators accurately.

Superhydrophobic surfaces received many applications in various industries such as desalinization, heat exchanger, anti-fog and self-cleaning surface production. In this study a wet etching process were used to produce superhydrophobic copper surfaces. The specimens were etched by multiple ferric chloride and deionized water solutions to create micro-nano structures on their surfaces. The electronic scanning electron microscopy (SEM) images of the resulted surfaces show a formation of micro-nano structures with specific templates. Contact and sliding angle measurement of surfaces after etching process show that contact angles of specimens improve to nearly 140o while sliding angle of all samples were 180o , which is the same as a rose petal property. In the next step, to promote hydrophobicity of surfaces, increase contact angle and decrease sliding angle, specimens were immersed in an ethanol and stearic acid solution with a specific concentration. As well as, effects of etching time and etchant concentration on the sliding and contact angles with/without stearic acid modification were investigated. Results show that contact angles increased and sliding angles decreased remarkably so that it reduced to lower than 10o in some cases and lotus effect were achieved.

The equations of Lamb wave propagation in an infinite isotropic micro plate with finite thickness on the basis of consistent coupled stress theory is presented in this study. By employing the characteristic length scale parameter, the effect of micro-plate size is considered, and thereby the effects of different plate dimensions on the dispersion of Lamb waves is illustrated. Lamb wave propagation velocity in aluminum nitride micro-plates has received many interests due to its applications in surface acoustic resonators. In the current work, at first, the dimensionless relations are developed through the definition of dimensionless parameters where the extracted curves can be applied to all thicknesses, propagation wavelengths and characteristic length scale parameters of a micro-plate. In addition, using the quasi-static approximation, the Lamb wave dispersion curves in both symmetric and asymmetric modes for an aluminum nitride micro plate are plotted and compared with the results from the classical theory. The integrity of the present formulation is verified by comparing the obtained results with the experimental data in the literature. Finally, by employing the dispersion curves and the reported experimental data, a novel method has been proposed to determine the size of characteristic length parameter in the consistent coupled stress theory.

Boiling is a remarkably efficient heat transfer method and is commonly used in daily life and industrial applications. Changing the physical and chemical structure of hot surface in some methods as making a porosity in a manner of enhancing boiling process is an interesting topic in recent decay. In this paper, porous metal micro/nano structural surfaces is produced in order to augmentation of boiling heat transfer on copper surface by the one- and two-stage electrodeposition method. The pictures in micro and nanoscale are captured to identification of structure and surface characteristics as porosity and capillarity are estimated. Next, the effects of structures in enhancing the pool boiling are measured experimentally. So then, boiling heat transfer profiles that demonstrate heat flux versus wall superheat, are derived for water fluid. Pool boiling curves of enhanced surfaces is compared with polished surface and results of other researchers to determine the efficiency improvement. Furthermore, comparison the effect of electrodeposition process time on obtained structures shows higher porosity, capillary and strength of structure with lower process time (30 sec) lead to further enhancement of pool boiling.