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Thermal Energy Domain

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					    Thermal Energy Domain

 Could be an entire course
 Qualitative experience from everyday life
 Our concern is with rates of heat transfer




                                               1
Why Heat Transfer in a MEMS Course?
• Thermal Actuators




                                       Amplified Thermomechanical
                                      In-Plane Microactuator (ATIM)

Micromechanical Switch

http://www-g.eng.cam.ac.uk/edm/images/research/mems/thermal_actuators_1.jpg
                                                                         2
Why Heat Transfer in a MEMS Course?
• Undesirable effects of Joule Heating




     Drain Failure at High Current (Xiaomin Yan)
                                                   3
Why Heat Transfer in a MEMS Course?
• Testing material properties at high
  temperatures (Juan Aceros)




                                        4
Why Heat Transfer in a MEMS Course?
• Testing material properties
• Steady-State Temperature Response




   (a) Un-Released    (b) Released
                                      5
Why Heat Transfer in a MEMS Course?
• Testing material properties
• Transient Temperature Response
                                            Hotplate Resistance vs. Time
                                                                                                                                   Hotplate Resistance vs. Time
                                                                                                            900
                               750
                                                                                                            800
                               700
   Hotplate Resistance (Ohm)




                                                                                Hotplate Resistance (Ohm)
                               650                                                                          700

                               600                                                                          600

                               550
                                                                                                            500
                               500

                               450                                                                          400


                               400                                                                          300

                               350
                                                                                                            200
                                                                                                                  0   100    200       300     400    500         600   700   800
                               300
                                       20              40                  60                                                            Time (microsec)
                                                 Time (microsec)

                                     (a) Un-Released                                                                        (b) Released
Why Heat Transfer in a MEMS Course?
• MEMS Temperature Measurement Devices
    1113°C   1118°C   1124.5°C   1130.5°C 1136°C   1142°C   1149°C




                        neck




            SEM micrograph of a breaking T-MEMS thermometer
   array of shape # 3 after fabrication and before heating (Patricia Nieva)
                                                                              7
Why Heat Transfer in a MEMS Course?
• Residual thermal stress produces a
  curvature




SEM micrographs bending T-MEMS thermometer array successfully fabricated with a
            1 m SiO2 bottom layer and a 7500 Å polysilicon top layer
                                                                          8
Why Heat Transfer in a MEMS Course?

• Designing and Modeling of thermal actuation and
  thermal processes.
• Sensing of a substance may require an elevated
  temperature.
• Testing material properties as a function of
  temperature and/or temperature variation.
• Issues involve: power needed, temperature
  uniformity, time response.



                                               9
    Modes of Heat Transfer
• Conduction, Radiation, Convection

• Conduction will be studied first
• Heat transfer within a solid occurs by
  conduction. Also occurs in a fluid.
• Heat is transferred by lattice vibrations.
• In a metal by the flow of free electrons.
• Conduction can also occur in a gas due to
  molecular collisions
                                               10
MTMG260/ECEG244   11
MTMG260/ECEG244   12
MTMG260/ECEG244   13
MTMG260/ECEG244   14
                  Or try
                  www.memsnet.org




MTMG260/ECEG244               15
MTMG260/ECEG244   16
MTMG260/ECEG244   17
18
MTMG260/ECEG244   19
MTMG260/ECEG244   20
MTMG260/ECEG244   21
Transient Temperature Profiles of a Slab




                    t/t c 1




                                     22
       Heat Transfer by Convection
 Heat Transfer by convection is quite complicated !
 Some empirical formulae exist, especially at the macroscale

  Nu is the Nusselt Number which characterizes the
  convective Heat Transfer coefficient, i.e.
            hL
     Nu 
            

 • Unfortunately most of the macroscale data is not applicable
   to the microscale.
 • Some recent work at the microscale in research journals.

                                                                 23
MTMG260/ECEG244   24
        Thermal Radiation
• Electromagnetic radiation




• Important range is for wavelengths
  between 0.1-100 µm.                  25
Thermal radiation occurs between two bodies at different
temperatures (T1 and T2)
If the electromagnetic radiation does not interact with the
medium (e.g. air) surrounding the bodies, then the heat flux

 J Q   SB F12 T14  T24    SB is the Stefan-Bolzmann constant
                                    (5.67×10-8 W/m2K4)
                               T1, T2 are absolute temperature

  • F12 is a factor (0 ≤ F12 ≤ 1) accounting for the efficiency of
  the radiant heat transfer process.
  • F12 includes geometry and the emissivity (e; 0 ≤ ei ≤ 1) of
  the bodies.
                                                   1
  •For closely spaced parallel plates F12 
                                                  1 1
                                                     1
                                                  e1 e2          26

				
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posted:9/9/2012
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