U.S. patent application number 11/529127 was filed with the patent office on 2007-03-15 for thin film ceramic thermocouples.
Invention is credited to Gustave Fralick, Otto Gregory, John Wrbanek, Tao You.
Application Number | 20070056624 11/529127 |
Document ID | / |
Family ID | 35394821 |
Filed Date | 2007-03-15 |
United States Patent
Application |
20070056624 |
Kind Code |
A1 |
Gregory; Otto ; et
al. |
March 15, 2007 |
Thin film ceramic thermocouples
Abstract
A thin film ceramic thermocouple (10) having two ceramic
thermocouple (12, 14) that are in contact with each other in at
least on point to form a junction, and wherein each element was
prepared in a different oxygen/nitrogen/argon plasma. Since each
element is prepared under different plasma conditions, they have
different electrical conductivity and different charge carrier
concentration. The thin film thermocouple (10) can be transparent.
A versatile ceramic sensor system having an RTD heat flux sensor
can be combined with a thermocouple and a strain sensor to yield a
multifunctional ceramic sensor array. The transparent ceramic
temperature sensor that could ultimately be used for calibration of
optical sensors.
Inventors: |
Gregory; Otto; (Wakefield,
RI) ; Fralick; Gustave; (Middleburg Heihts, OH)
; Wrbanek; John; (Sheffield Village, OH) ; You;
Tao; (Waukegan, IL) |
Correspondence
Address: |
Gauthier & Connors LLP
Suite 2300
225 Franklin Street
Boston
MA
02110
US
|
Family ID: |
35394821 |
Appl. No.: |
11/529127 |
Filed: |
September 28, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/US05/12004 |
Apr 12, 2005 |
|
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11529127 |
Sep 28, 2006 |
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60561393 |
Apr 12, 2004 |
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Current U.S.
Class: |
136/238 ;
136/201; 374/E7.008 |
Current CPC
Class: |
G01K 7/028 20130101;
G01K 7/04 20130101 |
Class at
Publication: |
136/238 ;
136/201 |
International
Class: |
H01L 35/16 20060101
H01L035/16; H01L 35/34 20060101 H01L035/34 |
Claims
1. A ceramic thermocouple, said thermocouple comprising: a
substrate; a first ceramic thermoelement formed on a substrate; and
a second ceramic thermoelement formed on a substrate, each element
having a different composition such that the first and second
thermoelements are in contact with each other in at least on point
to form a junction, and wherein each element was prepared in a
different oxygen/nitrogen/argon plasma.
2. The ceramic thermocouple of claim 1, wherein each ceramic
thermoelement has a different electrical conductivity and different
charge carrier concentration.
3. The thermocouple of claim 1, wherein the charge carrier
concentration in each element is controlled by the nitrogen and
oxygen partial pressures in the plasma from which said
thermoelements were prepared.
4. The thermocouple of claim 1, wherein the thermocouple is
transparent in the visible spectrum.
5. The thermocouple of claim 1, wherein the thermocouple may
operate at a temperature of greater than 1500.degree. C. for an
extended period of time.
6. The thermocouple of claim 1, wherein each ceramic thermoelement
is formed in plasma containing 10 mtorr or less of nitrogen, 0-10
mtorr of argon and 0-10 mtorr of oxygen.
7. The thermocouple of claim 1, wherein the substrates are
electrically insulating.
8. A ceramic sensor, said sensor comprising a ceramic thermocouple
comprising a substrate; a first ceramic thermoelement, a second
ceramic thermoelement, each element prepared in a different
oxygen/nitrogen/argon plasma, and fabricated in an immediate
vicinity of a ceramic strain gauge having the composition of either
the first or second thermoelements, wherein the strain gauge is
formed simultaneously of either the first or second
thermoelement.
9. The ceramic thermocouple of claim 1, wherein each ceramic
thermoelement is one of the following oxides or mixture thereof,
including at least one of indium oxide, antimony oxide, tin oxide
and aluminum-doped zinc oxide.
10. A method of preparing a thin film ceramic thermocouple, said
method comprises: providing a substrate; depositing
indium-tin-oxide onto said substrate in a plasma of argon, nitrogen
and oxygen; depositing indium-tin-oxide onto said substrate to form
a second ceramic thermocouple element in a different
oxygen/nitrogen/argon plasma than the first ceramic thermoelement
such that the two elements have different charge carrier
concentrations; coupling the two elements to form a thin film
ceramic thermocouple.
11. The method of claim 10, wherein the plasma contains 10 mtorr or
less of nitrogen, 0-10 mtorr of argon and 0-10 mtorr of oxygen.
Description
PRIORITY INFORMATION
[0001] This application claims priority to U.S. Provisional
Application No. 60/561,393 filed on Apr. 12, 2004.
BACKGROUND OF THE INVENTION
[0002] The evolutionary development of new engine materials and
designs has allowed turbines to be operated at much higher
temperature and thus, achieve higher efficiencies. In order to
evaluate engine performance, it is necessary to monitor the
temperature of all the static and dynamic components in the turbine
environment. Several techniques have been used to monitor the
surface temperature of blades and vanes, including wire
thermocouples, infrared photography, pyrometry and thermal paints.
One technique employs imbedded thermocouple wires in the blade wall
however this may cause serious structural and aerodynamic problems,
disturbing the flow of cooling air. Infrared photography has been
used for this purpose but is a non-contact method where the thermal
radiation patterns of an object are converted into a visible image.
These techniques are not easily transferable to the gas turbine
engine environment for temperature monitoring where smoke or other
particulates may scatter the light. The extreme temperatures and
velocities within a turbine gas engine make it difficult to produce
reliable infrared images. Pyrometry can be used at a reasonably
large distance from the object as long as the object can be
focused, however, it is required that the areas of engine are line
of sight accessible. It is important to note that adsorption by
dust, windows, flames, gases and other optical interferences can
produce errors. Another method to measure surface temperature is
the use of thermal paints. They are convenient to use and give a
visual display or thermal map of component, but these paints do not
exhibit the adhesion necessary to survive the harsh environment in
gas turbine engine.
SUMMARY OF THE INVENTION
[0003] As operating temperatures in gas turbine engines are pushed
to higher levels, engine designs must rely on complex cooling
systems and ceramic coatings to maintain the structural integrity
of the metallic blades and vanes. Embedded wire thermocouples are
frequently used for temperature measurement in the gas turbine
engine environment but as the blades get thinner, structural
integrity can be compromised. A thin film ceramic thermocouple
based on indium-tin-oxide (ITO) alloys may be used to measure the
surface temperature of both static and rotating engine components
employed in propulsion systems that operate at temperatures in
excess of 1300.degree. C. By fabricating two different ITO
elements, each having substantially different charge carrier
concentrations, it is possible to construct a robust ceramic
thermocouple. A thermoelectric power of 6.0 .mu.V/.degree. C., over
the temperature range 25-1250.degree. C. has been measured for an
unoptimized thin film ceramic thermocouple.
[0004] Testing in a computer controlled burner rig showed that ITO
thermocouples exhibited a linear voltage-temperature response over
the temperature range 25-1250.degree. C. Not only was the
thermoelectric power a critical measure of performance of
thermocouples in these applications but the electrical and chemical
stability was equally important in these harsh conditions, since
these temperature sensors must survive tens of hours of testing at
elevated temperatures. To enhance the carrier concentration
difference in the different legs of thermocouple, ITO thin films
were deposited by r.f. sputtering in different oxygen, nitrogen,
and argon plasmas. ITO thin films prepared in nitrogen rich plasmas
have survived temperatures in excess of 1575.degree. C. for tens of
hours. SEM micrographs revealed that the surfaces of the ITO thin
films after high temperature exposure exhibited a partially
sintered microstucture with a contiguous network of ITO
nanoparticles. In these films, nitrogen was metastably retained in
the individual ITO grains during deposition. Nitrogen diffused out
of the bulk grains at elevated temperature and eventually became
trapped at grain boundaries and triple junctions. Not only are
these ceramic thermocouples being considered for propulsion
applications, other applications such as glass melting and steel
making are also being considered. Thermal cycling of ITO thin films
in various oxygen partial pressures showed that the temperature
coefficient of resistance (TCR) was nearly independent of oxygen
partial pressure, with TCR's ranging from 1320 ppm/.degree. C. to
1804 ppm/.degree. C. at temperatures above 800.degree. C., and
eventually became independent of oxygen partial pressure after
repeated thermal cycling below 800.degree. C.
[0005] It is an object of the present invention to provide a
versatile ceramic sensor system having an RTD heat flux sensor
which can be combined with a thermocouple and a strain sensor to
yield a multifunctional ceramic sensor array.
[0006] It is another object of the invention to provide a ceramic
sensor array prepared under different plasma conditions, i.e.
different oxygen and nitrogen partial pressures in the argon plasma
and having very high temperature stability.
[0007] It is another object of the invention to provide a
transparent ceramic temperature sensor that could ultimately be
used for calibration of optical sensors.
[0008] It is still another object of the invention to provide an
ITO ceramic sensor which can be used in aerospace applications,
glass melting and steel making applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIGS. 2A and B are photographs of a high-temperature test of
a ceramic thermocouple on a quartz substrate and a ceramic
thermocouple fabricated on an alumina rod;
[0010] FIG. 3 is a graph of electrical resistivity of ITO in low
oxygen partial pressure wherein the films are sputtered in an
oxygen and argon plasma;
[0011] FIG. 4 is a of graph electrical resistivity of ITO in high
oxygen partial pressures wherein the films are sputtered in an
oxygen and argon plasma;
[0012] FIG. 5 is a of graph electrical resistivity of ITO in low
oxygen partial pressure wherein the films are sputtered in a
nitrogen rich plasma;
[0013] FIG. 6 is a of graph electrical resistivity of ITO in high
oxygen partial pressures wherein the films are sputtered in a
nitrogen plasma;
[0014] FIG. 7 is SEM micrograph of an ITO sensor prepared in an
oxygen/argon plasma and an ITO sensor prepared in a nitrogen rich
plasma;
[0015] FIG. 8 is a graph of resistivity of ITO sensors in various
nitrogen partial pressure;
[0016] FIG. 9 is a graph of response of ceramic thermocouple during
thermal cycling to 1200.degree. C.; and
[0017] FIG. 10 is a graph of response of ceramic thermocouple
during thermal cycling to 1000.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Generally shown in FIG. 1, is a thin film thermocouple 10
including a first and second element 12, 14 positioned on a
substrate 16. Thin film metallic leads are indicated at 18. Thin
film thermocouples deposited on the blades and vanes of gas turbine
engines can serve as an ideal means of measuring the surface
temperature of engine components during operation. The sensitivity
and response of thermocouples are based on the development of an
electromotive force (emf), which is dependent on the electrical
resistivity of the individual metals used to form the couple. Thin
film thermocouples can accurately measure the surface temperature
of engine components because they have low thermal mass and thus,
provide a more accurate measurement of the temperature at a
specific point. The small inertial mass of thin films also
translates into a negligible impact on vibration patterns. They are
also non-intrusive in that the thermocouple thickness is
considerably less than the gas phase boundary layer thickness.
Thus, the gas flow path through the engine will not be adversely
affected. Critical to implementation of thin film temperature
sensor technology in advanced aerospace application is the chemical
and electrical stability of the active sensor elements and the
magnitude of the thermoelectric power at elevated temperatures.
Ceramic thermocouples based on reactively sputtered
indium-tin-oxide (ITO) thin films can measure the surface
temperature of both stationary and rotating engine components
employed in propulsion systems that operate at temperatures in
excess of 1500.degree. C. ITO solid solutions dissociate in pure
nitrogen at temperatures above 1100.degree. C., but are stable in
pure oxygen atmosphere at temperature up to 1600.degree. C. The
sensor elements are oxidation resistant and do not undergo any
phase change when thermally cycled between room temperature and
1500.degree. C.
[0019] Currently used platinum based thermocouples are expensive,
have a limited temperature range, are prone to yield errors due to
catalytic effects and can give results that can deviate by as much
as 50 degree C. from the actual temperature. Platinum and rhodium
thermocouples are prone to creep and other metallurgical effects at
elevated termperature. The sensitivity and response of
thermocouples are based on the development of an electromotive
force (emf), which is dependent on the electrical properties of the
individual thermoelements, namely the density of free carriers. By
controlling ITO deposition conditions, a robust ceramic
thermocouple can be produced using two different ITO elements with
substantially different charge carrier concentrations and
resistivities.
[0020] High purity aluminum oxide substrates were used for all high
temperature electrical tests, since they provide excellent
electrical isolation and stability at high temperature. These
substrates were cleaned by rinsing in acetone, methanol and
deionized water, followed by a dry nitrogen blow dry. Shadow
masking techniques were used to fabricate all thin film
thermocouples. The ITO films were deposited by rf sputtering
whereas the platinum/rhodium (10%) films were deposited by rf
sputtering. A high density ITO target (12.7 cm in diameter) with a
nominal composition of 90 wt % In.sub.2O.sub.3 and 10 wt %
SnO.sub.2 was used to deposit ITO thermoelements and high purity
(99.9999%) platinum and platinum/rhodium targets (10.7 cm in
diameter) were used for all platinum depositions. The sputtering
chamber was evacuated to a background pressure
<1.times.10.sup.-6 torr prior to sputtering and semiconductor
grade argon, oxygen and nitrogen were leaked into the chamber to
establish a total gas pressure of 9 mtorr. The oxygen, argon and
nitrogen partial pressures were maintained in the deposition
chamber using MKS mass flow controllers and rf power density of 2.4
W/cm.sup.2 was used for all ITO sputtering runs. Platinum films (3
.mu.m thick) were used to form ohmic contacts to the active ITO
thermoelements and thin film leads to make electrical connection. A
computer controlled burner rig and a Deltech tube furnace with a
7-inch hot zone was used for high temperature experiments (FIG. 2).
The furnace was ramped at 3.degree. C./min to the desired
temperature in 50.degree. C. increments and held for at least 1
hour to establish thermal equilibrium. The corresponding resistance
changes were monitored with a 6-digit multimeter (Hewlett-Packard
34401A) and a programmable constant current source (Keithley 224).
A Hewlett-Packard multimeter and Keithley constant current source
were interfaced to an I/O board and an IBM 488 GPIB card for
continuous data acquisition using Lab windows software. A type S
thermocouple connected to a second multimeter was used to measure
the temperature inside the Deltech furnace.
[0021] Electrical and chemical stability of the sputtered ITO is
critical to the performance of these temperature sensors, since
these ceramic sensors must survive tens of hours of testing at
elevated temperature. Towards this end, high temperature stability
of ITO thin films was evaluated at temperatures up to 1250.degree.
C. under different oxygen partial pressures. The properties of the
ITO elements were measured continuously during thermal cycling to
establish the temperature coefficient of resistance (TCR). This was
used as an indirect measure of thermocouple stability from the
viewpoint of charge carrier concentration. The desired partial
pressures were established by mixing argon and oxygen in different
ratios followed by thermal cycling between 25.degree. C. and
1500.degree. C. Results of testing under low oxygen partial
pressures (FIG. 3) and high oxygen partial pressures (FIG. 4),
showed that temperature coefficient of resistance (TCR) was very
stable and not affected by partial pressure at temperatures above
700.degree. C. The change in resistance-temperature behavior was
not significant after several testing cycles at temperature above
700.degree. C. These results showed that ITO thin films prepared in
argon/oxygen plasmas exhibited reasonably good stability. A
temperature coefficient of resistance of 1388 ppm/.degree. C. and
2486 ppm/.degree. C. was observed under low and high oxygen partial
pressures, respectively.
[0022] A ceramic thermocouple was fabricated by depositing two
different ITO films (FIGS. 2A and 2B), each prepared with a very
different charge carrier concentration. To insure a reasonable the
charge carrier concentration difference in the different elements
of the thermocouple, ITO films were prepared by r.f. sputtering in
different oxygen/argon and oxygen/nitrogen/argon plasmas. The high
temperature stability of thin films prepared in nitrogen-rich
plasmas is shown in FIGS. 5 and 6. After the first thermal cycle,
sintering of these nitrogen doped films had occurred and thereafter
resulted in excellent stability at elevated temperature (almost
independent of oxygen partial pressure in the test
environment).
[0023] The different electrical conductivity in each thermoelement
is controlled by the amount of nitrogen in the plasma. It has been
determined that by utilizing nitrogen in the plasma, the
thermoelements are unexpectedly able to withstand much higher
temperatures. The plasma should include at least some and up to 10
mtorr of nitrogen, 0-10 mtorr of oxygen and 0-10 mtorr of argon.
One preferred combination of plasma components includes 6 mtorr of
argon, 3 mtorr of nitrogen and 1 mtorr of oxygen.
[0024] A temperature coefficient of resistance (TCR) of 1320
ppm/.degree. C. was observed in low oxygen partial pressure and
1748 ppm/.degree. C. was observed in pure oxygen environments.
Nitrogen-doped ITO films exhibited greater stability at high
temperatures with an almost linear response.
[0025] ITO temperature sensors were examined by SEM after high
temperature exposure. SEM micrographs indicated that a marked
change in microstructure had occurred in the ITO films after the
first thermal cycle. The SEM micrograph of an ITO sensor subjected
to a post-deposition heat treatment in air (FIG. 7) showed a
partially sintered microstructure with interconnected nanopores.
ITO films prepared in a nitrogen-rich plasma retained more
metastable nitrogen in the structure and thus, lead to a much finer
microstructure. The average ITO particle size was considerably
smaller in the nitrogen sputtered ITO films compared to the oxygen
sputtered films and the ITO particles exhibited a more angular and
faceted morphology.
[0026] In the case of the nitrogen doped ITO films, it appears that
more nitrogen was metastably retained in the individual ITO grains
during sputtering which later diffused out of the bulk grains at
elevated temperature, eventually becoming trapped at grain
boundaries and triple junctions. Under these conditions, sintering
and densification of the ITO particles containing nitrogen rich
grain boundaries was retarded and a contiguous network of
nanometer-sized ITO particles was established. In both cases, the
controlled microstructure developed in these sensors was achieved
by controlling the partial pressure of nitrogen in the
interconnected porosity during processing, such that a balance
between the rate of decomposition and the rate of sintering rate
was maintained. Since the decomposition of ITO alloys in pure
nitrogen atmospheres can occur at temperatures as low as
1100.degree. C., higher equilibrium (decomposition) pressures at
these higher temperatures occurs in the nitrogen sputtered films
and must be accommodated in the isolated pores to maintain
equilibrium. Continued sintering in these nitrogen sputtered films
will require even higher temperatures until a new equilibrium is
reached. Preliminary experiments indicate that a stable nitride may
have also formed on the surfaces of these particles, which can also
lead to the stabilization of the ITO nanoparticles.
[0027] To determine the resistivity and carrier concentration
difference in ITO elements comprising the thermocouples, a series
of ITO films was sputtered in different argon/oxygen/nitrogen
partial pressures. The reactively sputtered ITO films were
determined to be n-type and exhibited typical semiconductor-like
resistivities. The resistivity of the as-deposited ITO films was
dependent on the nitrogen partial pressure established in the
plasma, as shown in FIG. 8. Based on equation (1) below, the charge
carrier concentration can be estimated from the resistivity and
mobility of the ITO films: p = 1 q .times. .times. .mu. .times.
.times. N c ( 1 ) ##EQU1## where q is the charge of electron, .mu.
is the mobility and N.sub.e is the charge carrier concentration.
Generally, increasing the nitrogen partial pressure in the plasma
during sputtering resulted in lower resistivity. Increased
resistivity of ITO films as a function of oxygen partial pressure
is due to the decrease in the oxygen vacancy concentration in the
films, via compensation by molecular oxygen. However, when too much
nitrogen was incorporated in plasma, indium nitride may have
formed. In this case, ITO films will become degenerate when
nitrogen partial pressures exceed 2.35.times.10.sup.-a torr (FIG.
8).
[0028] The ITO thermocouples were tested from room temperature to
1250.degree. C., and a linear relationship between emf and
temperature was observed. As shown in FIG. 9, a thermoelectric
power of 6 .mu.V/.degree. C. was determined over this temperature
range. Other ceramic thermocouples prepared with higher nitrogen
partial pressure (1.853.times.10.sup.-3 torr and
2.43.times.10.sup.-3 torr) lost their linear response during high
temperature testing.
[0029] Other transparent conducting oxides include aluminum doped
zinc oxide, tin oxide, antimony oxide and antimony tin oxide.
[0030] To simulate the real engine operation environment, a
oxy-propane open flame burner rig was used to test the performance
of ITO ceramic thermal sensor (FIG. 10). ITO ceramic thermal sensor
successfully survived through this severe testing with almost same
thermoelectric power of 6 .mu.V/.degree. C. The burner rig test
further confirmed that ITO ceramic thermocouples were good
candidates for the gas turbine engine applications.
[0031] Although the present invention has been shown and described
with respect to several preferred embodiments thereof, various
changes, omissions and additions to the form and detail thereof,
may be made therein, without departing from the spirit and scope of
the invention.
* * * * *