Thermal MEMS

The heat transfer rate, dQjdt, depends on the cross-sectional area A of the beam and its thermal conductivity k according to Fourier s Law  

Heat transfer from convection occurs in a medium due to molecular motion in a fluid or gas. The flow can be natural, from changes in density 

Flow Heat flux J — dQjdt Electrical current I = dqjdt 

The heat transfer is proportional to the area of the surface-fluid interface and the temperature difference between the surface, T urface, and the fluid, Tfluid- The constant of proportionality is called the surfaee heat transfer coefficient h  

Finally, another important heat transfer meehanism is through radiation. Here the heat transfer is determined by the Stephan-Boltzmann Law that relates the heat flow to the temperature of the radiating body  

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Blondy P, et al. Packaged mm-wave thermal MEMS switches. In 31st European micro-... 

Thermal MEMS make use of heat transfer for their operation. Some of the modes of heat transfer that are used include conduction through a solid, convection through a gas or a liquid, and optical radiation at high temperatures. Heat flow through a long, thin beam is driven by a temperature gradient AT, where heat Q flows from the hot end of the beam toward the cold end, as shown schematically in Figure 5.1. 

Table 5.1 Lumped parameter model for thermal MEMS...

Lin Q, Jiang F, Wang X-Q, Han Z, Tai Y-C, Lew J, Ho C-M (2000) MEMS Thermal Shear-Stress Sensors Experiments, Theory and Modehng, Technical Digest, Solid State Sensors and Actuators Workshop, Hilton Head, SC, 4—8 June 2000, pp 304-307 Lin TY, Yang CY (2007) An experimental investigation of forced convection heat transfer performance in micro-tubes by the method of hquid crystal thermography. Int. J. Heat Mass Transfer 50 4736-4742... 

In order to prevent the irrevisible adhesion of MEMS microstructures, several studies have been performed to alter the surface of MEMS, either chemically or physically. Chemical alterations have focused on the use of organosilane self-assembled monolayers (SAMs), which prevent the adsorption of ambient moisture and also reduce the inherent attractive forces between the microstructures. Although SAMs are very effective at reducing irreversible adhesion in MEMs, drawbacks include irreproducibility, excess solvent use, and thermal stability. More recent efforts have shifted towards physical alterations in order to increase the surface roughness of MEMS devices. 

M. von Arx, O. Paul, and H. Baltes. Process-dependent thin-film thermal conductivities of thermal CMOS MEMS , Journal of Microelectromechanical Systems 9, (2000), 136-145. 

Fig. 8-28. Cathodic polarization curves for several redox reactions of hydrated redox particles at an n-type semiconductor electrode of zinc oxide in aqueous solutions (1) = 1x10- MCe at pH 1.5 (2) = 1x10 M Ag(NH3) atpH12 (3) = 1x10- M Fe(CN)6 at pH 3.8 (4)= 1x10- M Mn04- at pH 4.5 IE = thermal emission of electrons as a function of the potential barrier E-Et, of the space charge layer. [From Memming, 1987.]...

The dichloro compound can be converted to the bicyclic molecule 6 on treatment with MeaSiNSNSiMea (Figure 6). Related bicyclic compounds with fluorine (or Ph and F) substituents on phosphorus have previously been obtained from the reaction of PF5 (or PhPF ) with MeaSiNSNSiMea (I8, IS) The thermal decomposition of 6 in toluene at ca. 900C for 3h regenerates the six-mem-bered ring (l). 

Hydrogenated GNF and SWNT powders with a mass of several milligrams were chosen to determine their thermal stability, hydrogen content and to study mem by X-ray diffraction, IR spectroscopy and so on. 

A. J., Schmidt, M. A., Jensen, K. F., A suspended-tube microreactor for thermally-effident fuel processing, in Proceedings of the 6th International Conference on Microreaction Technology, IMRET 6 (11-14 March 2002), AIChE Pub. No. 164, New Orleans, 2002,147-158. Tanaka, S., Chang, K.-S., Min, K.-B., Satoh, D., Yoshida, K., Esashi, M., MEMS-based components of a miniature fuel cell/fuel reformer system, Chem. 

Chung, J.W., Grigoropoulos, C.P., Greif, R., Infrared thermal velocimetry in MEMS-based fluidic devices. J. Microelectromech. Syst. 2003, 12, 365-372. 

Static Disorder and Thermal Motion of Cations Significant modifications of the thermal motion of cations, corresponding to the loss of degrees of freedom on cooling, are observed in both MEM(TCNQ)2 and TEA(TCNQ)2, near 335 K and 210 K, respectively. Such cation rearrangements appear better as the cause rather than as the consequence of the correlative 2k and 4kv electronic transitions. In effect, structural changes are so important that they can hardly be attributed to instabilities of the electronic subsystems. 

In both salts, cations are found, from x-ray studies, to be disordered between two preferential orientations. In MEM(TCNQ)2, the occupancies, x and 1 - x, of these two orientations are found to be T dependent. Below 113 K, x = 1, and from 113 to 243 K, x varies from 1 to 0.5 [24,25]. In TEA(TCNQ)2, the two occupancies are almost identical x = 1 - x = 0.5 [22]. This twofold disorder is definitely static at low temperature. However, due to thermal factors, it becomes more and more difficult to resolve the two oreintations at high temperature and also to conclude whether the disorder is static or dynamic in nature [22,24]. 

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