THERMAL THERMAL SENSORSSENSORS

指導教授 : 吳坤憲 老師

報告學生 : 蕭傑穎

OUTLINEOUTLINE

INTRODUCTIONINTRODUCTION HEAT TRANSFERHEAT TRANSFER THERMAL STRUCTURESTHERMAL STRUCTURES THERMAL-SENSING ELEMENTSTHERMAL-SENSING ELEMENTS THERMAL SENSORSTHERMAL SENSORS CONCLUSIONCONCLUSION

INTRODUCTIONINTRODUCTION

Heat , also called thermal energy, can Heat , also called thermal energy, can in a simple, intuitive way be viewed as thin a simple, intuitive way be viewed as the internal kinetic energy of a collection oe internal kinetic energy of a collection of molecules or atoms.f molecules or atoms.

What is HEAT ?

HEAT TRANSFERHEAT TRANSFER

For gases,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 latticeposition.

ConductionConduction

ConvectionConvection

RadiationRadiation

There are three modes of heat transfer to There are three modes of heat transfer to considerconsider : :

ConductionConduction

Conduction refer to heat transfer by diffusion Conduction refer to heat transfer by diffusion through solid material or non-moving fluid.through solid material or non-moving fluid.

Conduction in solids:

(1) electron conduction(2) phonons

Thermal Conductivity (W/(m·K))

ConvectionConvection

Convection refers to heat transfer by the Convection refers to heat transfer by the

movement of fluid or gas.movement of fluid or gas.

Velocity and temperature profiles in a boundary layer

adhere adopt

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.

RadiationRadiation

Wien’s displacement lawThe wavelength of the maximum of the curve is inversely proportional to the absolute temperature.

Spectral radiancy of a black body

Black bodyA body that absorbs all of the radiation is called a black body.

THERMAL STRUCTURESTHERMAL 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.

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

Self-heating of a cylindrical temperature sensor in a cylindrical hole

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

Floating MembranesFloating Membranes

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

Cantilever Beams and BridgesCantilever Beams and Bridges

Thermal-Sensing Thermal-Sensing ElementsElements

The basic operating principle of a thermal bimorph switch.

A latching thermal bimorph switch.

“latched"

ThermocoupleThermocouple

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

Physical electronic system for an SAW sensor.Resulting in a dependence of the delay time on temperature.

SAW (surface acousric waves) SensorsSAW (surface acousric waves) Sensors

IDT : Interdigital Transducer

The SAW sensor is composed of an acoustic sensing element and decided electronic circuits, forming a feedback loop which oscillates witha temperature-dependent frequency.

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

( a ) ( b )

Thermal flow sensorsThermal flow sensors

Humidity (Dew-Point) SensorsHumidity (Dew-Point) SensorsOperating 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.

Using a capacitive element detecting the change in capacitance between two electrode pairs.

transistor

capacitor

Micromachined CalorimetersMicromachined Calorimeters

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.

silicon-aluminum thermopile

Thermal SensorsThermal Sensors

• Thermal sensing elementsThermal sensing elements– ResistorResistor

• Integrated or thin-film resistor: NTCIntegrated or thin-film resistor: NTC• Pt 100 and Platinum resistor: PTCPt 100 and Platinum resistor: PTC

– ThermocouplesThermocouples• Seebeck effectSeebeck effect• ThermopilesThermopiles

– TransistorTransistor• IIcc = = AAee J Jss exp( exp(qVqVBEBE//kTkT))• VVBEBE = = ( (kT/q)ln(IkT/q)ln(Ic c /A/AeeJJss))

– Acoustic-Wave SensorAcoustic-Wave Sensor• SAW or PW (plate wave)SAW or PW (plate wave)• IDTIDT• Temp. change Temp. change freq. change freq. change

Temperature ICTemperature IC

• PTATPTAT– Output current or voltage Output current or voltage

PProportional roportional TTo the o the AAbsolubsolution tion TTempertaureempertaure

If If JJS1S1 = = JJS2S2, then, thenEmitter ratio: rEmitter ratio: r = = AAe2e2//AAe1e1 Current ratio: Current ratio: pp = = IIC1C1//IIC2C2

112

22121 ln

esc

escBEBEBE AJI

AJI

q

kTVVV

prq

kTVBE ln

CONCLUSIONCONCLUSION

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.

REFERENCESREFERENCES

1. S.M.Sze, “ Semiconductor Sensors “, 1994

2. Gregory T.A. Kvacs, “ Micromachined Transducers SOURCEBOOK “, 1998

The End