Thermal Sensors and Actuators

Thermal Sensors and Actuators

Part I Fundamentals of heat transfer Heat transfer occurs where there is a temperature gradient until an equilibrium is reached. Four major mechanism Thermal conduction Natural and forced convection Radiation Thermal resistance Heating and cooling of microstructures Achieving good thermal isolation and conduction Examples of thermal-mechanical actuators Bimorph thermal actuators Lateral driven thermal actuators

Four Mechanisms of Heat Transfer Thermal conduction Transfer of heat in a media (solid, liquid, or gas) in the presence of a temperature gradient. Rely on the transfer of kinetic energy of adjacent molecules/atoms in the media '' dt q cond = k (1D case) dx q cond conduction heat flux (W/m 2 ) along the direction of x k thermal conductivity (W/m K) - the direction of heat flux is from high temperature to low temperature High T Low T

Four Mechanisms of Heat Transfer Natural and forced convection Natural convection: transfer of heat from a surface into a stationary body of fluid (liquid or gas). Temperature gradient sets up the convective fluid flow. Forced convection: transfer of heat to a body of fluid that moves with respect to a solid. The bulk fluid movement provides enhanced heat transfer compared with that of natural thermal convection. q = h( T T ) '' conv s q conv convection heat flux (W/m 2 ) from solid surface to fluid h convective heat transfer coefficient (W/m 2 K) T s and T the temperature of the solid surface and the fluid. Usually T is considered as a constant. T s can be higher or lower than T Low T Low T High T s High T s

Four Mechanisms of Heat Transfer Natural and forced convective heat transfer coefficients Liquids have higher h than gases. Forced convection has higher h than natural. Phase change has large hs.

Four Mechanisms of Heat Transfer Radiation No media required Radiation spectrum shifts from low frequency (infrared) to high frequency (visible light) when the temperature of the radiating surface increases. The absorption of radiation results in the rising of temperature of an object. 4 E = εσt R E emissive power flux (W/m 2 ) ε radiance emissivity (0 ε 1) σ Stefan-Boltzman constrant (5.67 10-8 W/(m 2 K 4 )) T R the temperature of the radiating surface

A Thermal Resistance A parameter used to measure the ease of heat transfer in the presence of a temperature gradient. Analogous to the electrical resistance A large thermal resistance means Smaller k, h, ε Better thermal isolation Cooler: thick plastic form wall and tight lid Down coat For the 1D thermal conduction High T l Low T q R cond thermal = q '' = cond ΔT q cond A = = k 1 k dt dx l A A = ΔT k l A

Heating and Cooling of Microstructures To heat up or cool down a microstructure, the amount of heat necessary is determined by Q = sh m ΔT = C th ΔT sh (J/KgK) specific heat, the amount of heat per unit mass required to raise the temperature of an object by one degree Celsius or Kelvin C th (J/K) thermal capacity Time constant for heating or cooling τ = R th C th = R th sh m To obtain a desirable temperature changing profile, both heating and cooling must be controlled. Microscale structures has very small mass and also thermal capacity. Can be heated up very quickly with low power if there is good thermal isolation. Can be cooled down very quickly if there is good heat dissipation. Faster response

To Achieve Good Thermal Isolation and Dissipation Thermal element Low T-cond material High T-cond material

Thermal Expansion Thermal expansion coefficient (TCE) Volumetric: α = (ΔV/V)/(ΔT/T) Linear: β = (ΔL/L)/(ΔT/T) α = 3β Metals have higher TCE than Si-based materials. Thermal expansion can be amplified or accumulated to generate larger bending or linear motion. Thermal expansion of gas PV = nrt Can be used for pneumatic actuation

Thermal Bimorph Principle Stack of layers with different TCEs. Bending will occur when temperature changes due to mismatch of TCEs. Vertical displacement of the free end of the bimorph beam will be much larger than the thermal expansion itself.

Thermal Bimorph Principle d d 2 1 2 1 l 1 2 = r r cos θ rθ = = l k l ( θ = ) 2 2 r 2 r 1 2 2 = l k l Δα ΔT 2 Longer composite beams with larger Δα will generate larger movement. Increasing the thickness will increase the bending stiffness, resulting in smaller movement. At small movement, d is linear to the change of temperature, suitable for both sensing and actuation.

Artificial Cilia Array for Object Transport Reference 4-8, Ataka, et al, 1993

Bent-beam Thermal-mechanical Actuator Device material: 3.7μm p++ doped Si (Reference 4-11, L. Que, et al, 2001)

Lateral Driven Thermal-mechanical Actuators http://www.sfu.ca/adm/heatuator.html Reference 4-16, Comtois, et al, 1997)

Part II Thermal-electric effects Seebeck effect and thermal couple Thermal resistor Examples of applications Accelerometers based on thermal effects Hot-wire anemometers Parylene deposition thickness sensor

Seebeck Effect ΔV High T Low T Seebeck effect When a temperature difference ΔT is applied to a piece of solid material (e.g. metal and semiconductor), it will be accompanied by a electrical voltage ΔV. Seebeck coefficient: α s = ΔV / ΔT. Also called thermoelectric power or thermopower Semiconductors usually have larger seebeck coefficient than metals. The seebeck coefficients of silicon and polysilicon is a function of doping level and resistivity. Si: ~1000μV/K Polysilicon: ~100μV/K @ 10μΩ m α s = (26k/q)ln(ρ/ρ 0 ) ρ 0 : 5x10-4 Ω, k: Boltzmann constant, q: charge of electron

Thermal Couple Thermal couple Two pieces of dissimilar metals are put together at the hot (cold) junction. An open circuit voltage occurs at the reference junction. Seebeck coefficient of a thermal couple α ab = α a - α b. When connected in series ΔV = n α ab ΔT. Advantages Directly transform temperature variation into voltage in a linear fashion. No need to provide electric bias. Disadvantages Need accurate reference temperature Not very accurate for small temperature variations

Industrial Thermal Couple Configurations The type of thermal couple and connector should match each other. In MEMS devices, thermal couple can be formed by the deposition and patterning of different metal materials (Ni/W, Al/Pt, Au/Ni)

Thermal Resistor R T = R 0 (1 + α R (T-T 0 )) α R temperature coefficient of resistance (TCR) Thermal resistors can be made of metal or semiconductors. Metal Positive TCR due to enhanced vibration of atom lattice Semiconductor TCR can be positive, negative or zero, depending on Doping type and concentration Vibration of atom lattice Effective mass and mobility of electrons and holes ΔR = R 0 α R ΔT ΔR is more critical for sensing applications. As a result, R 0 can not be too small. Metals: use long, thin and zigzag designs Semiconductor: use low doping concentration

Example: Thermal to Electrical A thermostat temperature sensor using a bimorph coil. A thermostat temperature sensor using a thermal resistor.

Parylene Deposition Monitoring Sensor Based on Thermal Conduction The heater and the sensor (thermal resistor) is thermally isolated due to low chamber pressure (30mT). When the gap is filled with parylene, a thermal conduction path is created between the heater and the sensor.

Parylene Deposition Monitoring Sensor Based on Thermal Conduction The sudden resistance increase of the sensor represent the sealing of the gap. Since parylene deposition is conformal, half of the width of the gap is equal to the deposition thickness. Only end-point detection is possible.

Self-heating of Thermal Resistor I α R < 0 α R > 0 V R T = R 0 (1 + α R (T-T 0 )) At considerable V and I input, the temperature of the thermal resistor will increase due to ohmic heating. Depending on the sign of α R, R T will increase or decrease with V.

Application of Self-heating I Stronger air flow (or steel) Air flow (or rubber) No air flow (contact) Air flow V Flow sensing At steady state, fluid flow carries more heat away from the thermal resistor via force convection, reducing its temperature and changing R T. Measuring R T gives the flow speed. Tactile sensing At steady state, contact with other objects brings more heat from the thermal resistor via enhance thermal conduction, reducing its temperature and changing R T. Measuring R T gives possible material of the object.

Hot-wire Anemometer I Stronger air flow Air flow No air flow Conventional anemometer Hot wire individually mounted Non-uniform performance Difficult to achieve dense array to characterize the flow field Micromachined anemometer Batch-scale fabrication Uniform performance Enable sensor array to generate entire picture of the flow field V

Micromachined Hot-wire Anemometer Material usage Pt/Ni/Pt as hot wire Polyimide as support prong J. Chen, et al, JMEMS 2003

Micromachined Hot-wire Anemometer Fabrication process Combination of surface micromachining and post-fabrication self-assembly Batch-scale, efficient and high-yield

Accelerometers Based on Thermal Effects Acceleration causes the steady-state temperature distribution, which is picked up by the temperature sensors. Advantages Resistant to electromagnetic interference Simple fabrication process for the second example Disadvantages MEMSIC.com Power consumption, low sensitivity and slow response.