ROOM TEMPERATURE HYDROGEN SENSOR BASED ON NANO-MICRO INTEGRATION FOR SPACE EXPLORATION
PI: S. Seal (University of Central Florida)
Co-PI: H.J. Cho (University of Central Florida) S. Shukla (University of Central Florida)
L. Ludwig (Kennedy Space Center) P. Zhang (University of Central Florida)
S. Deshpande (University of Central Florida) C. Drake (University of Central Florida)
Abstract
We are developing the sol-gel derived nanocrystalline indium oxide (In2O3)-doped tin oxide (SnO2) sensor, in the form of thin film/nanowires/nanofibers, for room temperature hydrogen (H2) sensing application, for NASA, under the atmospheric conditions existing on the surface of the Earth. The nanocrystalline thin film sensor is incorporated into the microelectromechanical system (MEMS) device to achieve high H2 sensitivity and selectivity with minimum detection and recovery time at room temperature. Effect of various test parameters such as the air pressure, the H2 concentration as well as the MEMS design parameters such as the finger spacing and the number of fingers on the room temperature H2 sensing characteristics of the present sensor has been demonstrated. The present nano-micro integrated sensor shows giant room temperature H2 sensitivity (S=103-105) with high selectivity over CO. The current H2 detection and recovery time at room temperature lie within the range of 100-250 sec and 150-200 sec respectively. New technological solutions for further reducing the response and the recovery time of the present nano-micro integrated sensor have been proposed. Sensor tests are underway to test the H2 sensitivity of the present nano-micro integrated sensor under the atmospheric conditions existing on the surface of Mars and Moon.
Introduction
Hydrogen (H2) is the most abundant element in the universe and one of the most abundant on Earth. Due to the rapid consumption of the fossil fuels, much attention has been paid towards H2 as an economical non-conventional energy source for the diversified industrial applications. For example, solid oxide fuel cell (SOFC) technology uses gaseous H2 for the generation of power and heat. H2 powered cars and buses are already in normal transit service in some of U.S. cities. Liquid H2 has been used by NASA for launching the space-shuttles. As summarized in Figure 1, H2 also finds applications in electronic, metallurgical, pharmaceutical, nuclear fuel, food and beverages, as well as glass and ceramic industries. Every day, millions of pounds of H2 are used by hundreds of industries around the world. Due to the realization of the potential use of H2 energy further interest has grown into the production of large quantity of different forms of H2, the enhancement of H2 storage capacity, and the development of safe transportation system for H2.
Depending on the quantities required, H2 can be transported by road tanker or pipeline. North America alone has at least 700 km H2 pipeline system. Pipelines for liquid H2 have also been built by NASA for direct delivery of H2 to the space vehicle at the launch pad. However, due to its very small size, H2 is the most susceptible for the leakage through
the pipelines; typically about 1-3 % of H2 in the existing systems is always lost, mostly through the joints in the pipes. If handled carelessly, H2 is as dangerous for transport, storage and use as many other fuels. As a result, safety remains a top priority in all the aspects of H2 energy and has been the prime motivation for the present work.
In the recent years, nanotechnology has emerged as an attractive field for the development of novel materials having unusual properties, which have provided different pathways to solve many unresolved issues in various other fields. We strongly believe that the application of nanotechnology to H2 sensors would help in advancing the science and the technology related to the development of sensor materials.
Background
Different experimental (metal-oxide-semiconductor (MOS)-based, catalytic resistor, acoustic wave and pyroelectric) and commercial sensors (catalytic combustion, electrochemical, semiconductor, and thermal conductivity) based on different principles are currently available but with major drawbacks as outlined in Figure 1. The very low sensitivity of these sensors at room temperature to low concentrations of H2 has been invariable associated with the poor response and recovery time, which insists further investigation in these areas. In addition to this, the poor H2 selectivity, which is a severe problem at room temperature, has been another major but pending issue, which needs to be delved in great detail. Particularly, attention must be paid in improving the H2 selectivity by a novel approach without sacrificing the room temperature H2 sensitivity and the response and recovery time of the sensor. Moreover, many of the experimental as well as commercially available sensors use the nanocrystalline materials, which are susceptible to changes in their physical properties (such as nanocrystallite size) if operated at highly temperature, which may reduce the potential life of the sensor. Such sensors, hence, must be operated at lower temperatures, where further research is still awaited.
In Table 1, we summarize the list of companies, which manufacture and sell the H2 sensor devices. Various characteristics of these H2 sensor devices have also been shown for comparison. It appears to us that, these commercially available sensor devices are claimed to sense H2 at room temperature within the concentration limits of 200 ppm-2%, which large enough for any practical application. Response time quoted for these sensor devices is less than 10 sec, but it is related to high H2 concentration as high as 90% and not for ppm-level H2 concentration. Although the sensor devices are claimed to operate at room temperature, the recovery time often is associated with higher temperature (70 oC). Moreover, the room temperature H2 sensitivity values for these devices are often not quoted. In addition to this, there are only few manufacturers, which guarantee the cross-sensitivity to other poisonous gases. No claims are made regarding the suitability of these sensor-deices for sensing H2 on other planetary conditions, which is an essential requirement for NASA. It is suggested that, the commercial sensors currently available in the market are designed only to meet the atmospheric conditions on the earth’s surface. Modifying the sensor material properties to meet the NASA’s over all requirements is imperative.
Since our efforts are mainly focused on improving the semiconductor oxide based sensors, the current status of this particular class of sensor has been summarized in Table 2. Various forms of sensors such as thin films, random network of nanowires and
WHY ARE HYDROGEN SENSORS NEEDED?
Hydrogen Applications
Metallurgical Industries • Reducing Atmosphere • Steel Making • Copper Brazing
Chemical and Petroleum Industries • Production of Ammonia and
Methanol • Sulfur Removal • Catalytic Converter
Pharmaceutical Industries • Production of sorbitols
used in cosmetics, adhesives, surfactants, vitamins a and C.
Food and Beverages Industries • Hydrogenating Liquid
Oils Converting them into Semisolid Materials
Aerospace Industries • Launching Space
Shuttles • Power Life Supports and
Computers • Produce Drinkable
Water as Byproduct
Glass and Ceramic Industries • Manufacturing Float
Glass to Prevent Oxidation of Large Tin Bath.
Electronic Industries • Carrier Gas for Active
Trace Elements as Arsine and Phospine for Manufacturing Semi-Conducting Layers in Integrated Circuits.
Nuclear Fuel Industries • Protective Atmosphere for
Fabrication of Fuel Rods
Hydrogen Production • Crude Oil, Coal,
Natural Gas • Nuclear • Solar, Hydro, Wind
Wave, Geothermal • Wood, Organic Waste,
Biomass
Hydrogen Transportation • Specially Designed Tanker
Trucks • Pipelines
Hydrogen Storage • Metal Hydrides • Liquid Hydrides • Carbon Nanotubes • Compressed Hydrogen
Figure 1. Chart summarizing the need for an immediate development of hydrogensensors based on innovative approaches for overcoming the limitations of the currentsensor technology and meeting the requirements of the hydrogen based industries.
STATUS OF HYDROGEN SENSORS
Exiting Hydrogen Sensors Experimental
• MOS Structures • Catalytic Resistors • Acoustic Wave • Pyroelectric
Commercial • Catalytic Combustion • Electrochemical • Semiconductor • Thermal Conductivity
Precincts • H2O Sensitive • Film Damage at High Hydrogen
Concentrations • Hydrogen Induced Drift • Insensitive to Low H2 Concentrations • Poor Response at Room Temperature • Limited Hydrogen Detection Range • Poor Selectivity • Cannot Operate in Helium and in Vacuum • Large Size
Table 1. Summary of characteristics of commercially available H2 sensors.
Company
Sensor
H2 Range
Temperature
Range (oC)
Humidity
%
Power
Response/ Recovery
(sec)
Fuel Cell-Sensor
-
0-5000 ppm 0-10000 ppm
80
<95
1 W
12 V DC
<10 sec
90% Concentration
H2 Scan LLC
Pd/Ni Thin Film
0.5%-10%
70 (Gas
Temp.)/1 atm 0-40
(Operating)
<95
At 40 oC
2 W
12-24 V DC
In sec
Depending on Concentration
Neodym
Technologies
MOS
<2%
80
10-95
At 40 oC
1 W
12 V DC
4 (Recovery 10) Alarm Point 2000 ppm
RKI
Instruments
MOS with Molecular
Sieve+Transmittor
2000 ppm-
2%
-9 to 43
5-95
11-30 V DC
20 sec for
90% Concentration
Applied Nanotech. Inc.
Pd Nanoparticles
0.5-2%
In Volume
110
-
microwatts
<10 sec
(Recovery <10 sec at
70 oC)
Arrgh! Manufacturing
Inc.
-
1%
-10 to 40
-
12-48 V DC
-
Industrial Sci.
Corp.
-
-
60
<99
3 V DC
-
Enmet Co.
-
200-2000
ppm
-
-
-
-
Nanotubes, as well as aligned or single nanowires of semiconductor oxides have been developed for the H2 sensing application. It seems that, the maximum H2 sensitivity at room temperature has been reported for the thin film form of the sensor with the response and the recovery time in few minutes. On the other hand, a single nanowire type sensor exhibits the response time in few seconds but with extremely low H2 sensitivity. The random network of nanowires, having large porous structure, may be a good choice for selecting the appropriate form of the sensor, which may compromise the H2 sensitivity to some extent for improving the response time. Recently, the single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) have been synthesized and utilized for the H2 (and other gases as well) sensing application. Some of the typical gas sensing results reported in the literature for these new gas sensing materials are tabulated in Table 3 and can be compared with the gas sensing properties of semiconductor oxides gas sensors, Table 2. Three different forms of carbon nanotubes such as single, parallely aligned, and
random network have been investigated for the gas sensing. Comparison reveals that, relative to the semiconductor oxides gas sensors, the carbon nanotubes exhibit very low gas sensitivity (<2) and high detection time (5-2700 sec). Moreover, the recovery time associated with the carbon nanotubes based gas sensor has been few hours.
Table 2. Typical gas sensing results reported recently for the various forms of
semiconductor oxides and our results
Sensor Material
Sensor Form
Synthesis
Method
Operating
Temperature(oC)
Gas
(Amount)
Sensitivity (Rair/Rgas)
Response
Time (Sec)
C2H5OH
(250 ppm)
2
SnO2
Single Nanobelt
Vapor Phase Evaporation
200
NO2
(0.5 ppm)
30
Few Seconds
SnO2/Pd
Single
Nanowire
Thermal
Evaporation
200
H2 (?)
2.5
2.5
Pd
Single
Nanowires
Electrochemical
Deposition
25
H2 (5 %)
3.5
75
(mSec)
In2O3
Random
Network of Nanowires
Carbothermal
Reduction
370
C2H5OH (1000 ppm)
30
10
ZnO
Random
Network of Nanowires
Thermal
Evaporation
300
C2H5OH
(200 ppm)
50
15
TiO2
Nanotubes
Array
Anodization
290
H2
(1000 ppm)
10000
200
In2O3-SnO2SEAL
Thin Film
Sol-Gel
20
H2
(900 ppm)
110000
100
Table 3. Typical gas sensing results reported recently for the various forms of carbon nanotubes.
Sensor Material
Sensor Form
Synthesis
Method
Operating
Temperature (oC)
Amount of Gas
Sensitivity (Rair/Rgas)
Response
Time (Sec)
SWCNT-
Pd
Single Tube
Patterned
CVD Growth
RT
H2
(400 ppm)
2
5-10
SWCNT
Parallely Aligned
PECVD
165
NO2
(100 ppb)
1.28
2700
SWCNT-
PABS
Random Network
Arc
Discharge
32
NH3 (100 ppm)
0.25
60
25
1.1
SWCNT -Pd
Random Network
Arc Discharge
250
H2 (0.5-2.0
%)
1.2
120
MWCNT
Random Network
Modified PECVD
25
NH3 (200 ppm)
0.7
180
CNT
Random Network
PECVD
165
NO2
(100 ppb)
0.56
Few
Minutes
Experimental
(a) H2 Sensing Tests The MEMS devices, which utilize an oxidized Si-wafer (Si-SiO2) as the platforms, were patterned with four interdigitated (Au) electrodes, using thermal evaporation, photolithography, and wet chemical etching techniques. The MEMS devices were designed with the different number of fingers (8 and 20) and different finger spacing (10 μm and 20 μm). The tin-isopropoxide solution in iso-propanol and toluene, corresponding to the concentration of 0.23 M of tin-isopropoxide, was used with the addition of calculated amount of indium(III)-isopropoxide to obtain the thin films of SnO2-6.5 mol% In2O3 via a sol-gel dip-coating process. The dried gel films were sputtered with a thin Pt-layer for 10 sec using a sputter-coater. The coated-MEMS devices were dried at 150 oC for
15-30 min in air. The dip-coating, the sputtering, and the drying processes were repeated to obtain a desired film thickness. Finally, the Pt-sputtered dried gel films were fired at 400 oC in air for 1 h and utilized for the characterization and H2 sensing tests. The coated and calcined MEMS devices were wire-bonded to an integrated circuit chip and installed in the 32 pin socket assembly, which was in turn placed centered over the sensor test-board designed using the LPKF CircuitCAM 4.0 software and cut using the LPKF Boardmaster 4.0 software on a single-sided copper clad prototype boards. All H2 sensing tests were conducted in the dynamic test condition at room temperature (22 oC with the relative humidity of 35-50%). In this type of sensor-testing, the air-pressure within the test-chamber was reduced and maintained at a desired level using the turbo-pumps. A mixture of appropriate amounts of nitrogen (N2) and H2 was admitted into the test-chamber through the respective mass-flow-controllers. The N2 (15000 ppm) was used as a carrier-gas. The amount of H2 in ppm was calculated using the ratio of number of moles of H2 admitted into the test-chamber per minute to the total number of moles of gas molecules (that is, the summation of number of moles of N2, H2, and air) within the test-chamber. Thus, in the dynamic test condition, a desired amount of H2 was continuously blown into the test-chamber per minute and simultaneously pumped out of the test-chamber throughout the test-duration. Thus, the dynamic test condition simulates the condition, which may be encountered in an actual service application, for example, H2 leakage through a pipe line. (b) Nanowires/Nanofibers Sensor Material Development
Two different techniques, electrospinning and thermal evaporation, have been utilized to synthesize the nanocrystalline SnO2-based nanowires/nanofibers. In the electrospinning Figure 2, tin (II) chloride (SnCl2) precursor was dissolved completely in a highly concentrated polymeric solution, which was then taken in a syringe placed on a syringe pump, that fed the syringe tip with the polymeric solution at a constant speed. A Cu-plate covered with Al-foil was placed in front of the syringe at a distance of 10 cm and a high voltage (15 kV) was applied in between the Cu-plate and the syringe tip. The
-+ 0-30 kV
Syringe Pump Syringe
Taylor Cone
Jet
Cu-Plate Covered with Al-Foil
High Voltage Power Supply
Polymer Solution
Metallic Syringe Tip
Bending Instability of Jet
Bending Instability of Jet
-+ 0-30 kV
Syringe Pump Syringe Metallic Syringe Tip
Taylor Cone
Jet
Cu-Plate Covered with Al-Foil
High Voltage Power Supply
Polymer Solution
Figure 2. Schematic of electrospinning process for nanowires/ nanofibers formation.
polymer fibers were drawn from the syringe tip, which were subsequently deposited on the Al foil (or the Si-SiO2 substrate). Since, the polymer fibers contain the Sn-precursor, low temperature calcination temperature burnt off the polymer leaving behind the inorganic SnO2-based nanowires/nanofibers. In the thermal evaporation technique, Figure 3, a high temperature furnace was utilized. The Sn-precursor powder and the Si-SiO2 substrate (with Pt catalyst) were taken into the Al2O3 crucible, which was placed in the center of the furnace. Argon (Ar) gas was blown continuously into the furnace. The furnace was ramped to 900 oC, held at that temperature for 5 h, then cooled naturally to room temperature.
Ar InAr Out
Furnace
Precursor PowderSubstrate
Ar InAr Out
Furnace
Precursor PowderSubstrate
Figure 3. Schematic diagram describing the thermal evaporation process fornanowires/nanofibers formation.
Results and Discussion (a) Room Temperature H2 Sensing Characteristics of Nano-Micro Integrated Sensor As shown in Figures 4 and 5, the present nano-micro integrated sensor shows very high H2 sensitivity at room temperature as high as 103-104. Moreover, the sensor being insensitive to CO at room temperature, it exhibits very high H2 selectivity over CO. As shown in Figure 6, the response time of the present sensor lies within the range of 100-250 sec and the recovery time lies within the range of 150-200 sec. The variation in the response kinetics of the present nano-micro integrated sensor is presented in Figure 7. Improved response kinetics with increasing H2 concentration within the range of 100-15000ppm is noted. Within this range room temperature H2 sensitivity lies within the range of 3-105. As demonstrated in Figure 8, the room temperature H2 sensitivity of the present sensor is almost insensitive to the air pressure level within the range of 50-760 Torr. The sensor is also sensitive to H2 at room temperature in the helium (He) atmosphere, Figure 9. The room temperature response kinetics of the present nano-micro integrated sensor is superior for smaller finger spacing, Figure 10.
Figure 5. Room temperature H2 selectivity over CO.
1.0E+04
1.0E+05
1.0E+06
1.0E+07
1.0E+08
1.0E+09
0 2000 4000 6000 8000 10000 12000
Sens
or R
esis
tanc
e (Ω
)
Air
H2
CO Air
104
105
106
107
108
109
Time (sec)0 4000 8000 12000
1.0E+04
1.0E+05
1.0E+06
1.0E+07
1.0E+08
1.0E+09
0 2000 4000 6000 8000 10000 12000
Sens
or R
esis
tanc
e (Ω
)
Air
H2
CO Air
104
105
106
107
108
109
Time (sec)0 4000 8000 12000
Time (sec)
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1000 2000 3000 4000 5000 6000 7000
H2
Air
H2H2 H2 H2 H2
Air AirAir Air AirSe
nsor
Res
ista
nce
(Ω)
106
107
108
109
2000 40000 6000
Time (sec)
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1000 2000 3000 4000 5000 6000 7000
H2
Air
H2H2 H2 H2 H2
Air AirAir Air AirSe
nsor
Res
ista
nce
(Ω)
106
107
108
109
2000 40000 6000
Figure 6. Low room temperature response (100-250 sec) and recovery time(150-200 sec). (Note: Data points are separated by 50 sec time interval.)
1.0E+05
1.0E+06
1.0E+07
1.0E+08
1.0E+09
0 4000 8000 12000 16000
Sens
or R
esis
tanc
e (Ω
)
Air
H2 H2
Air
Time (sec)
(b)
107
105
109
4000 8000 12000 1600001.0E+05
1.0E+06
1.0E+07
1.0E+08
1.0E+09
0 4000 8000 12000 16000
Sens
or R
esis
tanc
e (Ω
)
Air
H2 H2
Air
Time (sec)
(b)
107
105
109
4000 8000 12000 160000
Figure 4. Room temperature H2 (900 ppm) sensing with high sensitivity.
1.0E+04
1.0E+05
1.0E+06
1.0E+07
1.0E+08
1.0E+09
0 2000 4000 6000 8000 10000 12000
He Atmosphere
H2
AirSens
or R
esis
tanc
e (Ω
)
UV-ON
UV-OFF
H2
104
105
106
4000 8000 120000
107
108
109
Time (sec)
1.0E+04
1.0E+05
1.0E+06
1.0E+07
1.0E+08
1.0E+09
0 2000 4000 6000 8000 10000 12000
He Atmosphere
H2
AirSens
or R
esis
tanc
e (Ω
)
UV-ON
UV-OFF
H2
104
105
106
4000 8000 120000
107
108
109
Time (sec)
Figure 9. Room temperature H2 sensing in He atmosphere.
B B
1.0E-01
1.0E+01
1.0E+03
1.0E+05
1.0E+07
0 2000 4000 6000 8000
Sens
itivi
ty
Time (sec)
10-1
101
103
105
107
0 2000 4000 6000 8000
H2
Air
1.0E-01
1.0E+01
1.0E+03
1.0E+05
1.0E+07
0 2000 4000 6000 8000
Sens
itivi
ty
Time (sec)
10-1
101
103
105
107
0 2000 4000 6000 80001.0E-01
1.0E+01
1.0E+03
1.0E+05
1.0E+07
0 2000 4000 6000 8000
Sens
itivi
ty
Time (sec)
10-1
101
103
105
107
0 2000 4000 6000 8000
H2
Air
Figure 8. Room temperature H2 sensing for different air pressure levels(50-760 Torr).
Figure 7. Room temperature H2 sensing for different H2 concentration levels.
0.1
1
10
100
1000
10000
100000
1000000
0 2000 4000 6000 8000 10000 12000
100
300
500700
15000
100
102
104
106
0 2000 4000 6000 8000 10000 12000
Time (sec)
Sens
itivi
ty (R
air/R
gas)
Air
900
H20.1
1
10
100
1000
10000
100000
1000000
0 2000 4000 6000 8000 10000 12000
100
300
500700
15000
100
102
104
106
0 2000 4000 6000 8000 10000 12000
Time (sec)
Sens
itivi
ty (R
air/R
gas)
Air
900
H2
0.1
1
10
100
1000
10000
100000
0 2000 4000 6000 8000 10000 120000 2000 4000 6000 8000 10000 1200010-1
101
103
105
Time (sec)
Sens
itivi
ty (R
air/R
gas)
10 μm
H2
Air
20 μm
0.1
1
10
100
1000
10000
100000
0 2000 4000 6000 8000 10000 120000 2000 4000 6000 8000 10000 1200010-1
101
103
105
Time (sec)
Sens
itivi
ty (R
air/R
gas)
10 μm
H2
Air
20 μm
Figure 10. Comparison of room temperature H2 response kinetics for twodifferent finger spacing.
(b) Nanowire/Nanofiber Sensor Material Development The nanocrystalline SnO2based fibers deposited on the MEMS device via electrospinning technique are shown Figure 11; while, that derived using the thermal evaporation process are shown in Figure 12. Determining the room temperature H2 sensing characteristics of the present nanofibrous sensor is under investigation.
1 μm
(b)
500 μm
(a)
1 μm
(b)
1 μm
(b)
500 μm
(a)
500 μm
(a)
Figure 11. SEM images of SnO2 nanowires synthesized via electrospinning. In (a),microelectromechanical system (MEMS) device with interdigitated gold (Au)electrodes is seen.
2 μm2 μm
Figure 12. SnO2 nanowires synthesized via thermal evaporation technique.
Future Work
- Utilize nanoelectromechanical system (NEMS) design (instead of MEMS) for reducing the room temperature response and recovery time below 60 sec.
- Utilize the nanofibrous SnO2-based sensor for the room temperature H2 sensing.
- Utilize the thin film as well as nanofibrous sensor to sense H2 under the
atmospheric conditions existing on the surface of Moon and Mars.
- Develop a prototype H2 sensor device for NASA operating at room temperature.
March 2006
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