1. Chang Liu MASS UIUC Thermal Sensors and Actuators: A Survey of Principles and Applications Chang Liu

2. MASS UIUC Outline General knowledge of heat and energy transfer Thermal actuators Thermal sensors –Thermal sensors: sensors for thermal phenomena or sensors that use thermal phenomena –Bimetallic cantilevers –Thermal resistors –Sensors that are based on thermal transfer principles

3. Chang Liu MASS UIUC Thermal Transfer Principles Heat will flow between two points of different temperatures. The heat transfer can take one of three forms Conduction Convection –Natural convection –Forced convection Radiation

4. Chang Liu MASS UIUC

5. Chang Liu MASS UIUC Thermal Resistance

6. Chang Liu MASS UIUC Example: Thermal Resistance of a Suspended Bridge

7. Chang Liu MASS UIUC

8. Chang Liu MASS UIUC Energy Storage The term sh is the specific heat (J/KgK) The term C th is heat capacity

9. Chang Liu MASS UIUC Actuation Methods Electrostatic Thermal actuation Magnetic actuation Piezoelectric actuation Pneumatic actuation

10. Chang Liu MASS UIUC Thermal Actuation Thermal expansion –Liquid –Air –Solid Phase change expansion –Vapor –Bubble generation –Solidification (volume contraction) Most famous example: ink jet nozzle

11. Chang Liu MASS UIUC Ink Jet Droplet Injector (TIJ 1.0, 1984) Bubble formation time: 1  s. Ink ejection time: 15  s. Peak pressure: 14 ATM Upon removal of heat, vapor cools and the bubble retreats. Refill at 24  s, lasts about 25  s. Surface temperature: 90% of critical temperature (vaporization temperature) which is 330 o C. Homogeneous boiling across the surface of the heater, made of tantalum-aluminum (Ta-Al) alloy. The heater has near zero TCR, so zero thermal expansion. HP Ink Jet Printer - Single Drop

12. Chang Liu MASS UIUC Comparison of Thermal Actuation and Electrostatic Actuation Electrostatic actuation –Power: low power due to voltage operation. –Response speed: high speed. –Construction and fabrication: relatively simple –range of motion: for parallel plate capacitor, range of motion relatively small. Thermal actuation –Relatively high power: due to current operation. –Lower response speed due to thermal time constant (dissipation and thermal charging) –Construction and fabrication: more complex due to material compatibility considerations. –Range of motion: relatively large.

13. Chang Liu MASS UIUC Thermal Sensor Principles Summary of major principles discussed in class –Thermal bimetallic bending induced by temperature change –Thermal resistive transducers measures change of resistance under temperature variation common materials include polysilicon and metal oxide. –Thermal couples Seebeck effect –Semiconductor type temperature sensors diode, transistors

14. Chang Liu MASS UIUC Basic Principle Thermal expansion coefficient: dimensional expansion of materials under elevated temperature. –Unit: 1/degC. Or

15. Chang Liu MASS UIUC Thermal Bimorph Actuator

16. Chang Liu MASS UIUC Example: T -> E A thermostat for home use Energy in thermal domain Translates into energy in mechanical domain (bimetallic bending) Translate into position of mercury balls in the tube Translate into electrical trigger for controlling the AC

17. Chang Liu MASS UIUC Scanning Probe Microscopy Probe for Nano Lithography

18. Chang Liu MASS UIUC Thermal Bimetallic Actuation

19. Chang Liu MASS UIUC

20. Chang Liu MASS UIUC Material Properties Rules for designing efficient thermal bimetallic actuator: maximize difference in  ease of fabrication material thermal stability

21. Chang Liu MASS UIUC Micro Ciliary Motion System - biomimetic micro motion system Biomimetic ciliary transport system –utilizing large number of distributed actuators to achieve macroscopic motion.

22. Chang Liu MASS UIUC Thermal Bimetallic Actuator Composite layer: polyimide (organic polymer) + metal (heater) Resistance of heater: 30-50 ohm current input: 25 mA (above which polyimide might be damaged) Cutoff frequency is 10 Hz. Beam bends upward due to intrinsic stress (tensile) in polyimide; Upon heating, the thermal expansion in the polyimide is more extensive - the beam therefore bends downwards.

23. Chang Liu MASS UIUC Thermal Resistive Transducers As temperature increases, the following variables change –electrical resistivity –dimensions, since The value of the resistance as a function of the temperature is generally referred to as –where R 0 is the value of resistance at room temperature T o –  R is called the TCR, or temperature coefficient of resistance, with unit being o C -1. –Note the equation is true for moderate temperature excursions.

24. Chang Liu MASS UIUC Common Materials for Thermal Resistor Doped silicon or polysilicon –Most commonly used in silicon micromachining for simplicity of fabrication –doping on the neighborhood of 1-2x10 19 cm -3 gives zero TCR –higher doping, TCR approximately 0.2-0.5%/ o C –lower doping, TCR approximately - 2%/ o C (10 18 cm -3 doping) or - 6%/ o C (2x1016 cm-3 doping) Pure metal –the value of  R is on the order of 5000 ppm/ o C, which is around 0.5%/ o C. –Usually positive (resistance increase with temperature) Semiconducting oxides of metal –oxide of Li, Cu, Co, Ti, Mn, Fe, Ni etc –value of is around negative 4-6%/ o C, or 4-6%/K.

25. Chang Liu MASS UIUC Known TCR of Polysilicon (Doped with Boron) High TCR at low concentration; but value is less stable over long term.

26. Chang Liu MASS UIUC A resistive temperature sensor may also serve as a ohmic heater Ohmic heating power P=I 2 R The heating power is partially used to raise the temperature of a resistor and partially lost to surroundings through –Conduction –Convection –Radiation

27. Chang Liu MASS UIUC IV-Characteristics The Current-Voltage Relationship for a resistor is obtained when the voltage is systematically varied and the current is recorded. –Automated machine (semiconductor curve tracer) –Manual data collection What is the IV characteristic of a thermal resistor? Why cann’t we just use an ohm-meter (multimeter) to measure the resistance?

28. Chang Liu MASS UIUC Thermal Insulation if Characterized by Thermal Resistance, R T Electrical vs. thermal analogy Voltage – temperature difference Current – thermal heat flux Resistance – thermal resistance For electrical R[Ohm]=V[Volt]/I[Amp] =electrical resistivity*length/area For thermal R T =(T1-T2)/power[W] =thermal resistivity*length/area I, or P

29. Chang Liu MASS UIUC Conclusions Know how to measure the TCR of a thermal resistor; Know how to measure the current-voltage characteristics (IV curve) of a thermal resistor; Know how to obtain resistance-power relationship based on the IV curve; Know how to obtain the temperature-power relationship based on the IV curve; Know how to calculate the thermal resistance based on the IV curve.

30. Chang Liu MASS UIUC Thermal Couples - Seebeck Effect Thermal electric effect refers to the generation of electrical potential when a temperature differential exist across a piece of material. At the high temperature end, more electron will be excited into the conduction band and starts diffusion into the colder region. The Seebeck effect (nameed after Seebeck), is commonly characterized by the Seeback coefficient which is expressed in the following form, for a single piece of metal: A working thermal couple with two different Seeback coefficients develop a voltage difference when subject to a temperature change of  T. ΔV High T Low T Why “thermal couple”?

31. Chang Liu MASS UIUC

32. Chang Liu MASS UIUC Seebeck Coefficient of Common Thermal Couple Materials The rule of thumb is to find materials with maximum different of Seebeck coefficients.

33. Chang Liu MASS UIUC Exercise Problem First calculate the radius

34. Chang Liu MASS UIUC Calculate Vertical Displacement  =l/r=0.08881 radian=5.0884 o d=r-rxcos  m   d r r

35. Chang Liu MASS UIUC MEMS Infra Red Sensor – Hybrid Sensing Thermal bimetallic material as sensing IR infrared beam heat the top plate temperature rise causes the zig- zagged beam to bend The distance between the parallel plate capacitor changes thermal isolation allows maximum temperature rise given the absorbed energy.