THERMAL SENSORS 指導教授 : 吳坤憲 老師 報告學生 : 蕭傑穎

OUTLINE INTRODUCTION HEAT TRANSFER THERMAL STRUCTURES THERMAL-SENSING ELEMENTS THERMAL SENSORS CONCLUSION

INTRODUCTION What is HEAT ? Heat , also called thermal energy, can in a simple, intuitive way be viewed as the internal kinetic energy of a collection of molecules or atoms.

HEAT TRANSFER For gases, For liquids, For solids, The heat is closely to the average velocity of the molecules. For liquids, The situation is similar to that of gases. For solids, The molecules cannot move freely, the internal kinetic energy is stored as so-called “PHONONS”, which are the coordinated movements ( vibrations ) of the atoms about their fixed lattice position.

There are three modes of heat transfer to consider : Conduction Convection Radiation

Conduction Conduction refer to heat transfer by diffusion through solid material or non-moving fluid. Conduction in solids: electron conduction phonons

Thermal Conductivity (W/(m·K))

Convection Convection refers to heat transfer by the movement of fluid or gas. adhere adopt Velocity and temperature profiles in a boundary layer

When a free stream of fluid encounters a heat plate , laminar heat transfer from a flat plate with and without an initial cold length. The thickness of the thermal and mechanical boundary layers increase with the distance from the leading edge. The thermal boundary layer develops only after the unheated layer.

Radiation Wien’s displacement law Black body The wavelength of the maximum of the curve is inversely proportional to the absolute temperature. Black body A body that absorbs all of the radiation is called a black body. Spectral radiancy of a black body

THERMAL STRUCTURES Important design aspect : Physical transduction process Packaging

Positive temperature coefficient ( PTC ) For most common materials, their resistance increases with temperature raising. Negative temperature coefficient ( NTC ) Some materials, like carbon and ceramics, the thermistors decrease with temperature raising.

Self-heating of a cylindrical temperature sensor in a cylindrical hole A cylindrical temperature sensor mounted in the hole of a body to measure its temperature.

Floating Membranes Floating-membrane structure with a large floating membrane suspended from the wafer-thick rim by long and narrow cantilever beams.

Cantilever Beams and Bridges Cantilever-beam structure with thermopile and a hot region beyond the thermopile.

Thermal-Sensing Elements “latched" The basic operating principle of a thermal bimorph switch. A latching thermal bimorph switch.

Thermocouple The Seebeck effect: An electrical voltage V is generated due to a temperature difference T. The principle of “ cold “ junction compensation for thermocouple-based temperature measurements.

The basic thermoelectric effects

SAW (surface acousric waves) Sensors The SAW sensor is composed of an acoustic sensing element and decided electronic circuits, forming a feedback loop which oscillates with a temperature-dependent frequency. Physical electronic system for an SAW sensor. Resulting in a dependence of the delay time on temperature. IDT : Interdigital Transducer

Thermal flow sensors ( a ) ( b ) Integrated flow sensors with thermopiles measuring the flow-induced temperature difference in two directions : ( a ) wafer thick sensor ( b ) floating-membrane sensor

Humidity (Dew-Point) Sensors Operating principles : 1. Cooling the gas. 2. AS a gas bearing a vapor is cooled, condenses on to the sensor to measure the temperature at which dew forms. capacitor transistor Using a capacitive element detecting the change in capacitance between two electrode pairs.

Micromachined Calorimeters silicon-aluminum thermopile The catalyst is deposited on the sensitive interaction area of the thermal sensor. The device were tested by measuring the concentration of glucose in water with An enzyme membrane.

Thermal Sensors Thermal sensing elements Resistor Integrated or thin-film resistor: NTC Pt 100 and Platinum resistor: PTC Thermocouples Seebeck effect Thermopiles Transistor Ic = Ae Js exp(qVBE/kT) VBE = (kT/q)ln(Ic /AeJs) Acoustic-Wave Sensor SAW or PW (plate wave) IDT Temp. change  freq. change

Temperature IC PTAT Output current or voltage Proportional To the Absolution Tempertaure If JS1 = JS2, then Emitter ratio: r = Ae2/Ae1 Current ratio: p = IC1/IC2

CONCLUSION The operation of thermal sensors generally can be described in three steps : 1. First, a non-thermal signal is transduced into a heat flow. 2. Second, the heat flow is converted, within the thermal signal domain, into a temperature difference. 3. Third, the temperature difference is transduced into an electrical signal with a temperature sensor.

REFERENCES The End 1. S.M.Sze, “ Semiconductor Sensors “, 1994 2. Gregory T.A. Kvacs, “ Micromachined Transducers SOURCEBOOK “, 1998 The End