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THERMAL SENSORS
指導教授 : 吳坤憲 老師
報告學生 : 蕭傑穎
OUTLINE
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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,
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 :
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Conduction
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Convection
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Radiation
Conduction
Conduction refer to heat transfer by diffusion
through solid material or non-moving fluid.
Conduction in solids:
(1) electron conduction
(2) 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
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 :
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Physical transduction process
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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
The principle of “ cold “ junction compensation for thermocouple-based
temperature measurements.
T.
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
(b)
(a)
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
 kT   I J A 
VBE  VBE1  VBE 2    ln  c1 s 2 e 2 
 q   I c 2 J s1 Ae1 
 kT 
VBE    ln  pr 
 q 
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
1. S.M.Sze, “ Semiconductor Sensors “, 1994
2. Gregory T.A. Kvacs, “ Micromachined Transducers SOURCEBOOK “,
1998
The End