Thermal Sensors and Actuators A Survey of Principles

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

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Thermal Resistance Chang Liu MASS UIUC

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

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Energy Storage • The term sh is the specific heat (J/Kg. K) • The term Cth is heat capacity Chang Liu MASS UIUC

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

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

Ink Jet Droplet Injector (TIJ 1. 0, 1984) • • Chang Liu Bubble formation time: 1 ms. Ink ejection time: 15 ms. Peak pressure: 14 ATM Upon removal of heat, vapor cools and the bubble retreats. Refill at 24 ms, lasts about 25 ms. 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 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. Chang Liu • 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. 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 Chang Liu MASS UIUC

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

Thermal Bimorph Actuator 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 Chang Liu MASS UIUC

Scanning Probe Microscopy Probe for Nano Lithography Chang Liu MASS UIUC

Thermal Bimetallic Actuation Chang Liu MASS UIUC

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

Thermal Bimetallic Actuator • Composite layer: polyimide (organic polymer) + metal (heater) • Resistance of heater: 30 -50 ohm • current input: 25 m. A (above which polyimide might be damaged) • Cutoff frequency is 10 Hz. • Beam bends upward due to intrinsic stress (tensile) in polyimide; • Upon heating, thermal expansion in the polyimide is more extensive - the beam therefore bends downwards. 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 To – a. 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. 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 -2 x 1019 cm-3 gives zero TCR – higher doping, TCR approximately 0. 2 -0. 5%/o. C – lower doping, TCR approximately - 2%/o. C (1018 cm-3 doping) or 6%/o. C (2 x 1016 cm-3 doping) • Pure metal – the value of a. 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. Chang Liu MASS UIUC

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

Thermal Insulation if Characterized by Thermal Resistance, RT Electrical vs. thermal analogy Voltage – temperature difference Current – thermal heat flux Resistance – thermal resistance I, or P For electrical R[Ohm]=V[Volt]/I[Amp] =electrical resistivity*length/area For thermal RT=(T 1 -T 2)/power[W] =thermal resistivity*length/area 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 thermal resistance based on the IV curve. 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 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 DT. High T ΔV Low T Chang Liu Why “thermal couple”? MASS UIUC

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

Exercise Problem • First calculate the radius Chang Liu MASS UIUC

Calculate Vertical Displacement • q=l/r=0. 08881 radian=5. 0884 o • d=r-rxcosq=44. 3758 mm. q r r d 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 zigzagged beam to bend • The distance between the parallel plate capacitor changes • thermal isolation allows maximum temperature rise given the absorbed energy. Chang Liu MASS UIUC