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Gallium nitride sensors for hydrogen/nitrogen and hydrogen/carbon monoxide gas mixtures

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Title:
Gallium nitride sensors for hydrogen/nitrogen and hydrogen/carbon monoxide gas mixtures
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Monteparo, Christopher Nicholas
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University of South Florida
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Bandgap
Resistive sensors
Semiconductor
Syn-gas
Fischer-Tropsch synthesis
Dissertations, Academic -- Chemical Engineering -- Masters -- USF   ( lcsh )
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non-fiction   ( marcgt )

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Abstract:
ABSTRACT: As hydrogen is increasingly used as an energy carrier, gas sensors that can operate at high temperatures and in harsh environments are needed for fuel cell, aerospace, and automotive applications. The high temperature Fischer-Tropsch process also uses mixtures of hydrogen and carbon monoxide to generate synthetic fuels from non-fossil precursors. As the Fischer-Tropsch process depends upon particular gas mixtures to generate various fuels, a sensor which can determine the proper ratio of reactants is needed. To this end, gallium nitride (GaN) has been used to fabricate a resistive gas sensor. GaN is a suitable semiconductor to be used in hydrogen because of a wide, direct bandgap and greater stability than many other semiconductors. Additionally, resistive sensors offer several advantages in design compared to other types of sensors. Response time of resistive sensors is faster than those of other semiconductor sensors because catalytic and diffusion steps are not part of the response mechanism. Instead, a thermal detection mechanism is employed in resistive sensors. In this work, sensor response to changes in hydrogen concentration in nitrogen was measured at 200°C and 300°C. Sensor response was measured as change in current from a reference response to pure nitrogen at each temperature under a constant 2.5 V bias. Isothermal operation was achieved by controlling sensor temperature and pre-heating gas mixtures. Sensitivity to concentration increased upon an increase in temperature. Additionally, sensor response to concentration changes of H₂ in CO at 50 °C was demonstrated. Sensors show similar responses to nitrogen and carbon monoxide mixtures, which have similar thermal properties. Using the thermal detection mechanism of the sensors, a correlation was shown between sensor response and a gas mixture thermal conductivity.
Thesis:
Thesis (M.S.Ch.)--University of South Florida, 2009.
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by Christopher Nicholas Monteparo.
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Gallium Nitride Sensors for Hydrogen/Nitrogen and H ydrogen/Carbon Monoxide Gas Mixtures by Christopher Nicholas Monteparo A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering Department of Chemical and Biomedical Engineering College of Engineering University of South Florida Major Professor: John T. Wolan, Ph.D. Venkat Bhethanabolta, Ph.D. Scott Campbell, Ph.D. Jeffrey Cunningham, Ph.D. Date of Approval: March 6, 2009 Keywords: bandgap, resistive sensors, semiconductor syn-gas, Fischer-Tropsch synthesis Copyright 2009, Christopher Nicholas Monteparo

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Dedication This work is dedicated with love to my parents and grandparents. Thank you for everything you have given me, especia lly the gift of knowledge and the spark of curiosity.

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Acknowledgments This work is the culmination of ongoing research s ince my undergraduate years. Without the wisdom and guidance of others, I would not have been able to see it to completion. I would especially like to acknowledge Dr. John T. Wolan for his counsel and encouragement throughout my research and educat ion. Also, I thank Dr. Carlos Smith and Mr. Charles Gimbel for establishing my in terest in Chemical Engineering. I would also like to acknowledge the foundations o f my research put forth by Dr. Timothy Fawcett and Dr. Jodi Pope. I would like to thank Dr. Timothy Fawcett for following up on the project and providing invaluabl e assistance. I would also like to thank Dr. Hadis Morkoc for allowing me the use of t he GaN sensors and Dr. Babu Joseph for his support of my research. Over the years, I have had help from a variety of c olleagues and friends in the Applied Surface Science Laboratory. Dr. Benjamin G rayson, Ala'a Kababji, Aaron Black, Brad Ridder, Phil Saraneeyavongse, and Ali Syed-Gar dezi have been close friends and a great source of knowledge, experience, and camarade rie. I would also like to thank my fellow Master’s Program students Alex Page and Patr ick Brandon for their support and conviviality. Finally, I would like to thank my family and frien ds for their faith and encouragement. I would especially like to thank Ch ristine Bringes for her dedication and care for the last year and a half. Without them, I might have fallen down the quick and easy path. Instead, they encouraged me to do my be st and reach new heights.

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i Table of contents List of tables .................................... ................................................... ...............................iii List of figures ................................... ................................................... ..............................iv Abstract .......................................... ................................................... ..................................v Chapter 1: Introduction........................... ................................................... .........................1 1.1 Motivation................................... ................................................... ...................1 1.2 Research objectives.......................... ................................................... ..............2 1.3 Methodology.................................. ................................................... ................2 1.4 Expected contribution of the research........ ................................................... ....3 1.5 Thesis overview.............................. ................................................... ...............3 Chapter 2: Hydrogen technology and detection...... ................................................... .........4 2.1 Hydrogen energy technologies................. ................................................... .....4 2.2 Hydrogen sensors............................. ................................................... ..............5 2.3 Summary...................................... ................................................... ..................6 Chapter 3: Gallium nitride and semiconductor senso rs................................................. .....8 3.1 Applying gallium nitride properties to sensor s.................................................8 3.1.1 Bandgap structure of GaN.................. ................................................8 3.1.2 Thermal stability......................... ................................................... ..10 3.1.3 Electron mobility......................... ................................................... ..10 3.2 Comparison of semiconductor sensors.......... .................................................11 3.2.1 Gate sensors.............................. ................................................... ....12 3.2.2 MOS sensors............................... ................................................... ..13 3.2.3 Schottky diode sensors.................... .................................................13 3.2.4 Resistive sensors......................... ................................................... ..15 3.3 Summary...................................... ................................................... ................16 Chapter 4: Experimental methods................... ................................................... ...............18 4.1 Gas sensor setup............................. ................................................... ..............18 4.2 Experimental procedures ..................... ................................................... .......20 Chapter 5: Data and analysis...................... ................................................... ....................22 5.1 Hydrogen detection in nitrogen at 200 oC and 300 oC....................................22 5.2 Sensor response to H2/CO mixtures....................................... .........................23

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ii 5.3 Relationship between sensor response and ther mal conductivity...................25 5.4 Summary...................................... ................................................... ................28 Chapter 6: Conclusions and future work............ ................................................... ...........29 6.1 Conclusions ................................. ................................................... ................29 6.2 Future work ................................. ................................................... ................30 6.2.1 Further investigation into high temperatur e response......................30 6.2.2 Further investigation of synthesis gas sen sitivity............................30 6.2.3 Further investigation of thermal property relationships...................31 References ........................................ ................................................... ..............................32 Appendices......................................... ................................................... .............................36 Appendix A: Nomenclature......................... ................................................... ......37

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iii List of tables Table 2.1: Heating values of hydrogen and fossil f uels............................................... .......5 Table 2.2: Properties of odorless gases........... ................................................... .................6 Table 3.1: Semiconductor bandgap properties....... ................................................... ..........9 Table 3.2: Melting points of semiconductors....... ................................................... ..........10 Table 3.3: Electron mobility of semiconductors.... ................................................... ........11 Table 3.4: Summary of semiconductor sensors....... ................................................... ......17 Table 4.1: H2:CO experiment flow rates.......................... .................................................21

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iv List of figures Figure 3.1: Bandgap energy transitions............ ................................................... ...............9 Figure 3.2: Gate sensor mechanism................. ................................................... ..............12 Figure 3.3: MOS sensor structure.................. ................................................... ................13 Figure 3.4: Schottky diode sensor................. ................................................... .................14 Figure 3.5: GaN resistive sensor.................. ................................................... ..................15 Figure 4.1: Gas sensor setup...................... ................................................... ....................19 Figure 5.1: GaN sensor responses to H2 in N2 mixtures at 200 oC and 300 oC.................22 Figure 5.2: GaN sensor responses to H2 in CO mixtures at 50 oC....................................24 Figure 5.3: Thermal conductivity vs. current chang e for H2 in N2 mixtures....................27 Figure 5.4: Thermal conductivity vs. current chang e for H2 in CO mixtures at 50 oC.....28

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v Gallium Nitride Sensors for Hydrogen/Nitrogen and H ydrogen/Carbon Monoxide Gas Mixtures Christopher Nicholas Monteparo ABSTRACT As hydrogen is increasingly used as an energy carr ier, gas sensors that can operate at high temperatures and in harsh environments are needed for fuel cell, aerospace, and automotive applications. The high temperature Fisc her-Tropsch process also uses mixtures of hydrogen and carbon monoxide to generat e synthetic fuels from non-fossil precursors. As the Fischer-Tropsch process depends upon particular gas mixtures to generate various fuels, a sensor which can determin e the proper ratio of reactants is needed. To this end, gallium nitride (GaN) has been used t o fabricate a resistive gas sensor. GaN is a suitable semiconductor to be used in hydrogen because of a wide, direct bandgap and greater stability than many other semic onductors. Additionally, resistive sensors offer several advantages in design compared to other types of sensors. Response time of resistive sensors is faster than those of o ther semiconductor sensors because catalytic and diffusion steps are not part of the r esponse mechanism. Instead, a thermal detection mechanism is employed in resistive sensor s. In this work, sensor response to changes in hydrog en concentration in nitrogen was measured at 200oC and 300oC. Sensor response was measured as change in curre nt

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vi from a reference response to pure nitrogen at each temperature under a constant 2.5 V bias. Isothermal operation was achieved by controll ing sensor temperature and preheating gas mixtures. Sensitivity to concentration increased upon an increase in temperature. Additionally, sensor response to conc entration changes of H2 in CO at 50 oC was demonstrated. Sensors show similar responses t o nitrogen and carbon monoxide mixtures, which have similar thermal properties. U sing the thermal detection mechanism of the sensors, a correlation was shown between sen sor response and a gas mixture thermal conductivity.

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1 1.1 Motivation Hydrogen has generated interest as an environmenta lly clean alternative to fossil fuels. For instance, hydrogen (H2) has been used as fuel in jet aircrafts, and hightemperature fuel cells provided internal power to s pacecrafts during the Apollo Program [1,2]. Additionally, the Fischer-Tropsch synthesis (FTS) process uses synthesis gas (syngas), composed of hydrogen and carbon monoxide, to generate hydrocarbon fuels [3]. Production of fuels from FTS is of note because fos sil precursors are not required. The extreme conditions of these applications limit the materials which can be used for hydrogen detection. Design and implementation of H2 gas sensors have been improved through use of semiconductors, which yield compact and simple devi ces. Sensor size is reduced through the use of thin film semiconductor layers which are formed on the microscale [4-6]. Transmission of electrical signals between these la yers is influenced by exposure to hydrogen. Current and voltage characteristics of sensors can then be correlated with concentration. Hence, interfacing sensors with con trol systems is straightforward. Not all semiconductors are suitable for use in gas detection. Physical properties, such as melting point, constrain the temperature ra nge in which a material functions properly. Careful consideration of material attrib utes has led to the creation of gallium nitride sensors for H2 detection at elevated temperatures. Chapter 1: Introduction

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2 1.2 Research objectives The objective of this research is to demonstrate t he detection of hydrogen in nitrogen at elevated temperatures and the detection of H2:CO ratios in gas streams using gallium nitride (GaN) resistive gas sensors. Senso r responses to small and large changes in concentration were compared in the case of H2/N2 mixtures. The ability of GaN sensors to distinguish between H2:CO ratios was investigated with simulated syn-gas compositions. Correlations between the collected d ata to thermal conductivity were also investigated. 1.3 Methodology Resistive sensor responses were measured as curren t change from a baseline current response of pure N2. When voltage is constant, a current change is di rectly related to a resistance change. Sensitivity for re sistive sensors is change in current, (I), divided by change in hydrogen concentration, (CH2), shown in Equation 1.1. (Eqn 1.1) The sensitivity was investigated through analysis of sensor responses over a range of concentrations. Experiments consisted of sensor response to hydrogen-nitrogen mixtures that varied hydrogen concentration from 0100% at temperatures of 200oC and 300oC. First, the response to concentrations of pure n itrogen (N2) and pure hydrogen (H2) flow rates were measured. Next, sensor response to 10-100% H2 in N2 were measured at 10% intervals with a N2 purge between each interval to remove any adsorbed H2. Finally, response to 1-10% concentrations in 1% intervals with purge was recorded.

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3 Production of different fuels requires control of H2/CO ratio [3]. Hydrogen and carbon monoxide gas mixtures were used to simulate common synthesis gas compositions used in FTS. Experiments were conduct ed at 50 oC to determine if the response of GaN resistive sensors is affected by th e H2:CO composition. 1.4 Expected contribution of the research This research demonstrates the operation of GaN r esistive sensors over a range of temperatures. Previous experiments have explored t he response of GaN resistive sensors at a lower temperature [7]. The response of these sensors at temperatures above 200 oC demonstrates how sensor sensitivity is affected by temperature. An investigation into GaN sensor response to H2/CO mixtures was conducted for the first time. Sen sor function as a thermal conductivity detector was inv estigated using empirical thermal conductivity calculations. 1.5 Thesis overview In order to assist the reader, this thesis is orga nized as follows. Chapter 2 explains background information on hydrogen technologies to establish the importance of hydrogen detection. Chapter 3 presents GaN propert ies which are incorporated into sensor design and compares resistive sensors to oth er semiconductor sensors. In Chapter 4, the sensor test bed and experimental procedure a re explained. Chapter 5 presents data and results for experiments involving H2/N2 mixtures at 200 oC and 300 oC and H2:CO at 50 oC. Finally, Chapter 6 presents conclusions and rec ommendations for future work.

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4 2.1 Hydrogen energy technologies Hydrogen is a viable replacement for non-renewable fossil fuels. Unlike petroleum, H2 is renewable via the electrolysis of water, acidif ication of metals, steam reforming of natural gas, dissociation of ammonia a nd gasification of biomass or coal [8,9]. Hydrogen is already used as an energy carri er in fields such as aerospace and automotives [1,2,10-11]. The energy content of H2 is greater than that of many commonly used fossil fuels, including gasoline, as shown in Table 2.1. The low er heating value (LHV) and higher heating value (HHV) are measurements of the heat re leased from fuel combustion. Heating values of hydrogen are more than double tha t of gasoline on a weight basis. However, widespread use of H2 gas in automobiles is of limited practicality beca use conventional storage systems are highly pressurized and heavy[12]. H2 liquefaction and porous-material impregnation are developing alterna tives for hydrogen storage [14]. Hydrogen fuel cells offer several advantages over e ngines using hydrocarbon fuels. Since fuel cells are not heat engines, they are not limited by the Carnot efficiency. In addition, only water is produced instead of envi ronmentally harmful byproducts, such as CO2. Hydrogen is commonly used in fuel cells with ope rating temperatures from as low as 100 oC in polymer electrolyte membrane (PEM) fuel cells to higher than 1000 oC in solid oxide fuel cells (SOFC) [2]. Chapter 2: Hydrogen t echnology and d etection

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5 Table 2.1: Heating values of hydrogen and fossil f uels. [14] HHV is the amount of heat that is discharged as a quantity of fuel at 25 oC is combusted and the products return to 25 oC accounting for the latent heat of water produced. LHV neglects heat from water recovery and any product cooling below 150 oC. Fuel LHV(MJ/kg) HHV (MJ/kg) Hydrogen 119.9 141.6 Gasoline 44.5 47.3 Diesel 42.5 44.8 Hydrogen can also be used to alleviate demand for fossil fuels. Fischer-Tropsch synthesis (FTS) reactions of H2/CO synthesis gas (syn-gas) create synthetic hydroc arbons [3]. The products of these reactions are a distrib ution of hydrocarbons determined by variables including feedstock composition, catalyst selection, and operating temperatures [3,15]. Process control can then be improved by em ploying H2 sensors for measuring feedstock H2/CO ratios. 2.2 Hydrogen sensors Detection of hydrogen gas without sensors is a diff icult problem because hydrogen is odorless and colorless. The properties of hydrogen are compared to those of other gaseous species in Table 2.2. Other odorless gases can be laced with a foul smelling mercaptan, which diffuses with the gas to alert nea rby users of leaks before conditions become dangerous. H2 diffusivity is larger than any known mercaptan so conditions could become hazardous before the presence of a leak is k nown. Reliance on sensors is a necessary alternative and allows H2 concentration to be quantified.

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6 Table 2.2: Properties of odorless gases. [16, 17] Species MW (g/mol) DA-Air at 20 oC (cm2/s) H2 2.016 0.611 N2 28.02 ----O2 32.00 0.178 CO 28.01 0.208 CO2 44.01 0.138 H2 sensors serve a variety of purposes in industrial processes. Leak detectors are important from safety concerns due to hydrogen's ex plosion limit of 4% to75% in air [18,19]. Determination of hydrogen content in molt en metal is used to control porosity caused by dissolved gas [20]. Growth of anaerobic bacteria is monitored by hydrogen sensors in biological processes[21]. Sensor are us ed to control stream compositions in chemical reactions such as biomass gasification, FT S, and fuel cell redox [22,3]. Processes requiring H2 detection often take place under intense temperatu re and/or pressure, such as in industrial reactors, mi crobes, and outer space [22,23,2,10]. Proper material selection enables sensors to functi on in these harsh conditions. To that end, GaN has been chosen as an appropriate material for use in H2 sensors operating at elevated temperatures. 2.3 Summary Hydrogen as an energy carrier is an attractive alt ernative to fossil fuels because of greater energy content. Fuel cells use hydrogen as a fuel to generate power more efficiently than combustion engines. The Fischer-T ropsch process uses hydrogen to create synthetic hydrocarbons which alleviates dema nd for fossil fuels. Safe control of

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7 processes such as these requires hydrogen sensors. Sensor operating conditions are diverse. Selection of sensor materials needs to in tegrate operating variables into sensor design.

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8 3.1 Applying gallium nitride properties to sensors The electrical and mechanical properties of GaN ma ke it a suitable material for hydrogen sensing. GaN sensors are able to function at higher temperatures because GaN has a high melting point and a wide, direct bandgap Fabrication of GaN resistive sensors is more economical than many other sensors because of inexpensive materials and a reduced number of processing steps. 3.1.1 Bandgap structure of GaN GaN has a wide bandgap compared to other semicondu ctors, as shown in Table 3.1, which allows sensor operation at higher temper atures. Electrons occupy different energy bands within semiconductors. The bandgap en ergy (Eg) is the energy difference, in eV, between the valance and conduction bands of a semiconductor [24,25]. The bandgap is also called the forbidden gap because el ectrons cannot occupy the energy states within this region [24,25]. The band struct ure of semiconductors creates unique properties different from metals and insulators. T wo classifications exist for semiconductor bandgaps, direct and indirect, based on whether electron momentum changes between the conduction and valance bands [2 5]. Chapter 3: Gallium nitride and semiconductor sensor s

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9 Table 3.1: Semiconductor bandgap properties. [25] Material Eg (eV) Type GaN 3.42 Direct Si 1.1 Indirect SiC() 2.86 Indirect GaP 2.26 Indirect GaAs 1.43 Direct Due to the direct bandgap of GaN, electron transit ion between energy levels does not require a change in momentum, yielding better e nergy absorption. Wave functions () mathematically describe the electron wave propert ies, such as the direction of wave propagation called wave vector (k). Figure 3.1 shows that there is a change in k of electrons transitioning from the valance to conduct ion band of indirect bandgap, but momentum is unaltered in a direct bandgap. Transit ion across an indirect bandgap results in the emission of heat [25]. This heat loss negat ively impacts sensor operations, but is avoided with direct bandgap materials. Figure 3.1: Bandgap energy transitions. a) Direct bandgap b) Indirect bandgap

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10 3.1.2 Thermal stability Sensor materials should have a high melting point to withstand high temperature operations. Hydrogen processes like SOF Cs, FTS and coal gasification require operating temperatures greater than 1000 oC [2,3,26]. Sensor materials must be thermally stable under these conditions. Better th ermal stability directly corresponds to a higher melting point. GaN is stable at high temper atures because the melting point of GaN is 2700 oC, which is a temperature where other semiconductor s exist in the liquid phase, as shown in Table 3.2. Table 3.2: Melting points of semiconductors. [27] Material Melting Point (oC) GaN 2700 Si 937 GaAs 1421 3.1.3 Electron mobility Electron mobility (n), units m2/V/s, relates to how sensors respond to changes in conduction of electrons or electron holes in a mate rial. The electron drift velocity (vn), in m/s, is the average electron velocity caused by the application of an electric field (E), in V/m. The movement of electrons in electron drift v elocity is related to this field by the electron mobility, as shown in Equation 3.1 [24]. n nv E =m (Eqn 3.1)

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11 In sensors, electron mobility should be as large as possible for better conduction without risking voltage breakdown. Breakdown occurs when a small voltage change causes a rapid current increases [28]. Semiconduct ors with higher n breakdown at lower voltages. Gallium nitride has average electron mob ility when compared to many other semiconductors as illustrated in Table 3.3. Howeve r, n of GaN is large enough for sensor function without risking breakdown at 2.5V, which i s the voltage bias for these experiments. Table 3.3: Electron mobility of semiconductors. [25 ,27] Material n (cm2/V-s) GaN 440 Si 1350 SiC() 500 GaP 300 ZnS 110 3.2 Comparison of semiconductor sensors Many types of hydrogen sensors utilize gallium nit ride (GaN), including gate, MOS, Schottky diode and resistive sensors [6,7,29-3 3]. Semiconductor sensors measure an electrical property, such as resistance or capac itance which is changed due to the presence of hydrogen. Each sensor type has a disti nct structure and theory of operation, despite similar sensing mechanisms. GaN resistive sensors investigated in this research operate with reduced cost and stability problems pr evalent in other sensor designs.

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12 3.2.1 Gate sensors Gate sensors use platinum (Pt) or palladium (Pd) t o dissociate H atoms from adsorbed H2 molecules. The H atoms adsorb to the surface and t hen diffuse into the material between two electrodes. This diffusion ch anges the spacing between atoms in the material's lattice structure [34,35]. This mec hanism is illustrated in Figure 3.2. Changes to the lattice alter electrical properties which corresponds to gas concentration. Figure 3.2: Gate sensor mechanism. a) Dissociate d hydrogen atoms diffuse into the lattice. b) The presence of these atoms in the la ttice changes the electrical properties of the material. Gate sensors have some inherent design problems. Gate devices are reset by removing the hydrogen from the lattice, however som e irreversible hydrogen deposition will result in hysteresis over time [22]. Also, Pt and Pd are expensive materials. Response times for gate sensors are longer because the mechanism requires diffusion of dissociated hydrogen rather than surface interactio ns with H2 molecules.

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13 3.2.2 MOS sensors Metal-Oxide-Semiconductor (MOS) sensors were first developed by Lundstrum et al. in 1975 [36]. A metal oxide layer is deposi ted between a metal and semiconductor, as shown in Figure 3.3 [25,36]. The insulating oxi de layer causes charge to accumulate on the metal and semiconductor layers producing a c apacitive effect. Figure 3.3: MOS sensor structure. Similar to gate sensors, the dissociated hydrogen gas diffuses into the sensor layers. This causes a change the capacitance. Ele ctric current is passed through the sensor by connections to electrical contacts and th e catalytic metal. A voltage is applied and measured as capacitance changes, which correspo nds to a certain hydrogen concentration. MOS sensors are also hindered by many of the same issues as gate sensors, including longer response times and use of expensiv e metals [37]. The cost of the sensor is also increased due to the fabrication process wh ich requires precise control of thicknesses for multiple layers. 3.2.3 Schottky diode sensors In Schottky diode sensors, Pt and Pd are deposited on semiconductors such as SiC and GaN as shown in Figure3.4a [6,29-33,36]. Disso ciated hydrogen diffuses through

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14 the metal forming a layer between the metal and sem iconductor (Figure 3.4b). This layer increases the potential difference between the mate rials, called the Schottky barrier [29]. Variations in Schottky barrier height are shown by current-voltage (I-V) characteristics which correspond to gas concentration. Figure 3.4: Schottky diode sensor. a) H2 dissociates on contact with catalytic metal. b) H atoms diffuse and form a layer between the me tal and semiconductor, changing I-V characteristics. A disadvantage of the Schottky diode sensors for h ydrogen detection is the presence of a catalytic step to dissociate the hydr ogen molecules. Thermal instability in these sensors is caused by large breakdown current leaks. Sensor structures can be modified to reduce these leaks, as decreasing dopan t concentration or adding a guard ring structure [33,38,39].

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15 3.2.4 Resistive sensors Resistive gas sensors measure the change in curren t through the sensor under constant voltage applied across the sensor. An adv antage of resistive sensors is that these electrical properties can be accurately measured. The structure of resistive sensors, shown in Figure 3.5, is a semiconductor deposited o nto a substrate, such as sapphire. Metal contacts are used to induce electric charge a cross the semiconductor. Figure 3.5: GaN resistive sensor. The product of resistance (R), in and current (I), in A, is the voltage (V), in V, of a direct current system, V = IR. Electrical res istance depends upon the temperature of a semiconductor. Gas molecules adhere to and/or di ffuse into a semiconductor. Heat transfer based on thermal interactions with H2 changes the resistance of the semiconductor [40]. A particular composition of a gas stream at a particular temperature produces a particular resistance. Thin films depos ited onto the semiconductor surface increase the sensitivity of a sensor by preventing bonding of undesired species to the semiconductor surface. There are several advantages to resistive sensors in terms of sensor construction and response. Fabrication of GaN resistive sensors is more economical than many other sensors because of inexpensive materials and a redu ced number of processing steps.

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16 Additionally, there are no catalytic metals require d for sensor operations. Resistive sensor response times do not depend on diffusion in to semiconductor material as in the case of other semiconductor sensors. Instead, heat transfer properties influence sensors response based on a thermal detection mechanism [40 ]. Changes in gas composition are detected when there is a significant difference in thermal conductivities of gas species such as between H2 and N2 or CO, shown in Table 2.2. 3.3 Summary The properties of gallium nitride are more benefic ial to sensor design than many other semiconductors, particularly silicon. GaN is thermally stable which allows devices to take advantage of bandgap characteristics at hig h temperatures. At these elevated temperatures, current breakdowns are less common be cause of GaN's moderate electron mobility. Sensors using GaN take advantage of electrical pro perty changes induced by exposure to a gaseous chemical system. Voltage, Sc hottky barrier height, and resistance are a few of the electrical properties measured by sensors to determine chemical composition. A summary comparison of common semico nductor gas sensors is shown in Table 3.4. GaN resistive sensors are able to respo nd faster than many other sensor types with decreased fabrication cost.

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17 Table 3.4: Summary of semiconductor sensors. Sensor Type Property Measured Catalytic Metals Mechanism Steps Gate Sensor I and V Pt/Pd -Catalytic H2 dissociation -Diffusion into semiconductor -Lattice expansion MOS Capacitor C and V Pt/Pd -Catalytic H2 dissociation -Diffusion into semiconductor -Lattice expansion shifts electrical properties Schottky Diode I and V Pt/Pd -Catalytic H2 dissociation -Diffusion into semiconductor -Layer of H atoms changes Schottky barrier height Resistive Sensor I or R None -H2 adsorption to surface -Diffusion into semiconductor -Thermal interactions change resistance

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18 4.1 Gas sensor setup The sensor test-bed, shown in Figure 4.1, has an a utomated system for data collection and control of gas flows. To initializ e the sensor, a voltage is sent from the computer to the electrical housing and sensor bed. The circuit boards in the electrical housing relay information to and from the sensor. The temperature of the sensor heater is controlled in the same way with a user ramped volta ge until the desired temperature is obtained. Gas flows are input by the user into a c ustom Labview program which controls several mass flow controllers. Gas mixtures are he ated in a furnace before entering the sensor. This is done to achieve isothermal operati on during experiments and minimize the effects of heat transfer between the sensor and gases. Sensor temperature, voltage and current data is collected by computer. Chapter 4: Experimental methods

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19 Figure 4.1: Gas sensor setup.

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20 4.2 Experimental procedures GaN resistive semiconductors were previously fabri cated for testing, with silicon doped gallium nitride (n-type carrier concentration of 2x10-18) [7]. Experiments were conducted to determine if GaN resistive sensors can detect hydrogen in nitrogen at elevated temperatures and be used to determine the H2/CO ratio of a gas stream. Nitrogen and carbon monoxide are designated carrier gases because in both cases hydrogen is the species of interest. In all experi ments, sensor response to inert nitrogen (N2) at each experimental temperature was used to dete rmine a baseline current. The voltage applied to the sensor is directly related t o heat transfer between the sensor and gas streams, which affects sensitivity of the senso r [40]. Changes in the current, I (mA), across the sensor were measured under a constant vo ltage, V, (V), of 2.5V. As the gas mixture changes composition, the amount of heat required to keep the gas sensor at a constant temperature changes. Duri ng experimentation, voltage applied to a heater on the gas sensor was adjusted manually un til the desired temperature of the sensor was reached. The first set of experiments were conducted at 200 oC and 300 oC; pure gaseous N2 and H2 streams were used to establish baseline responses. Total volumetric flow rates for all of the experiments was 100 standard cubic c entimeters per minute (sccm). Once isothermal conditions were met at 200 oC and 300 oC respectively, sensors were then exposed to hydrogen concentration changes in 10% st eps over a range of 10-100% and in 1% steps over a range of 1-10%. Nitrogen purges to remove adsorbed H2 were pulsed between changing concentrations.

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21 In the second set of experiments, the sensor was e xposed to mixtures of hydrogen and carbon monoxide with nitrogen purges between di fferent concentrations. H2:CO ratios of 3:1, 3:2, 2:1, 1:1, 2:3 and 1:3 were test ed at 50 cC. These ratios were established based on typical synthesis gas compositions used to produce different fuels for the Fischer-Tropsch synthesis [3]. Volumetric flow rate for each mixture was 100 sccm. A summary of the H2/CO concentrations and their respective gas flow ra tes is given in Table 4.1. Table 4.1: H2:CO experiment flow rates. Ratio (H2:CO) H2 Flow Rate (sccm) CO Flow Rate (sccm) 3:1 75.0 25.0 2:1 66.6 33.3 3:2 60.0 40.0 1:1 50.0 50.0 2:3 40.0 60.0 1:2 33.3 66.6 1:3 25.0 75.0

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22 5.1 Hydrogen detection in nitrogen at 200 oC and 300 oC The electrical response of gallium nitride (GaN) r esistive sensors to increasing hydrogen concentrations in nitrogen at 200 oC and 300 oC is shown in Figure 5.1. Current change (I) was measured relative to the response of pure ni trogen at each temperature. I increased with an increase in hydrogen concentrat ion. Figure 5.1: GaN sensor responses to H2 in N2 mixtures at 200 oC and 300 oC. Error in concentration was + 0.1%. Chapter 5: Data and analysis

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23 Sensitivity analysis was performed using linear reg ression. Estimated sensitivity to changes in H2 concentrations in N2 was 5.98x10-3 mA/% H2 at 300oC and 3.86x10-3 mA/% H2 200oC. This indicates that sensitivity is temperature dependent. Heat transfer between the sensor and the gas has been minimized t hrough pre-heating of the gas. However, the sensor mechanism is affected by therma l characteristics of the gas stream [40,41]. At higher temperatures, the difference in thermal conductivities of hydrogen and nitrogen is greater which produces an increased I between concentrations. After exposure to hydrogen, the measured sensor re sponse to pure N2 increased by 1.3%. This shift from nitrogen’s original basel ine value may be caused by adsorption of hydrogen. Adsorption of hydrogen is known to d ecreases sensitivity to changes in concentrations [42]. Such a decrease in sensitivit y occurred for both high and low concentration experiments, as can be seen by slight curve in the data at higher concentrations in Figure. However, when the sensor was purged with pure nitrogen between temperature runs, sensor response returned to a linear trend, as shown in the responses to different temperatures and concentrati ons. 5.2 Sensor response to H2/CO mixtures The response of GaN resistive sensors to various c oncentrations of H2 in CO at 50 oC is shown in Figure 5.2. As with responses to H2 in N2, the baseline for sensor current response was pure nitrogen at the experimental temp erature. As with the previous test, error in concentration was + 0.1%. There is a linear trend sensor response to a changing H2:CO ratio, indicating that GaN sensors are able to detect changes in H2/CO ratios.

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24 At 300 K, thermal conductivity (k) is 25.0x10-3 W/m/K for CO and 25.6 x10-3 W/m/K for N2, whereas k = 186.9 x10-3 W/m/K for H2. [17]. Increases in H2 concentration in both CO and N2 follow comparable trends because the thermal characteristics of the gas mixtures are similar. T he response to pure CO and pure N2 gases varied less than 0.1%. Such a small differe nce is probably the result of similar thermal conductivities because resistive gas sensor s operate on a thermal mechanism [40,41]. Sensor responses appear to show slight, i f any, hydrogen adsorption, but the measured concentration span may be too small for ad sorption affects to manifest. Figure 5.2: GaN sensor responses to H2 in CO mixtures at 50 oC. Error in concentration was + 0.1%.

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25 Observed I measurements for H2/CO mixtures were much less than those for H2/N2 mixtures. Sensitivity of H2 in CO was 8.76x10-5 mA/% H2, several orders of magnitude lower than that of the higher temperature H2/N2 experiments in the previous section. This could be due to the temperature depe ndence of resistance and sensitivity. 5.3 Relationship between sensor response and therm al conductivity Previous investigations have proposed a gas sensor mechanism for resistive gas sensors [40]. Other semiconductor devices have bee n used as thermal conductivity detectors (TCD) utilizing a similar sensing mechani sm [43,44]. Sensor data collected during for H2/N2 and H2/CO mixtures has been compared to thermal conductiv ity to demonstrate use of GaN resistive sensors as a TCD. Binary gas mixtures at low pressure do not follow a linear trend based upon mole fractions (y) of gaseous species [45]. The Wassilj ewa Equation with the Mason and Saxena Modification (Aij), shown below in equations 5.1 and 5.2 respectivel y, was used to calculate values of thermal conductivity for gas mixtures (km) [45]. These equations are empirical relationships using physical properti es of pure gas species, thermal conductivity (k), molecular weight (MW) in g/mole a nd viscosity () in Pa*s. = ==n i n j ij j i iA y k y k1 1 m (Eqn 5.1)

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26 2 2 1 4 1 2 1 ij8(1 1 n r + n r n r + =j i i j j iMW MW MW MW Ah h (Eqn 5.2) TCDs compare signal response of an unknown mixture to a reference signal in the same way that sensor signals in the previous sectio ns were referenced to the response to pure nitrogen. A model was developed for the heat balance as gas mixtures flow over the sensor using Joule’s law and Newton’s law of coolin g. Equation 5.3 is the heat balance on the sensor where q is heat energy (in J), Qheater is rate of heat transferred from the heater to the gas (in J/s), t is time (in s), I is current (in mA), R is resistance (in ), h is the heat transfer coefficient (W/m2/K), A is sensor surface area (m2), and T (in K) is difference between sensor temperature and gas tempe rature. heaterQ T A h R I dt dq + D =2 610 (Eqn 5.3) From Ohm’s law, the resistance was equivalent to th e voltage divided by current. At steady state operation the change in heat energy was zero. The heat balance was then be solved for I(h,T) by subtracting the steady state balance for the reference gas from that of a particular gas concentration, which resul ted in Equation 5.4. A relationship between h and k is known using the Nusselt number. However, finding T as a function of k is beyond the scope of this work. However, kn owing that the relationship I theoretically exists is sufficient for analysis.

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27 ) ( 106 ref ref gas gasT A h T A h V ID D = D (Eqn 5.4) Comparison of thermal conductivities to current cha nge is shown in Figures 5.3 and 5.4. The data was suggested a power-law relati onship of I(k) shown in Equation 5.5. The model fit the data with high R2 values, which supported the use of these sensors for TCD applications. ( ) 2 /1 2/1 ref gask k I D (Eqn 5.5) Figure 5.3: Thermal conductivity vs. current chang e for H2 in N2 mixtures.

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28 Figure 5.4: Thermal conductivity vs. current chang e for H2 in CO mixtures at 50 oC. 5.4 Summary GaN sensors responses to H2/N2 and H2/CO mixtures were observed as a direct, linear response of I to change in H2 concentration in the carrier gases. Hydrogen adsorption was observed during H2/N2 experiments, but was reversible with nitrogen purges. Sensor responses to pure nitrogen and carb on monoxide were equivalent. Observed sensitivity was 5.22x10-3 mA/% H2 at 300oC and 3.64x10-3 mA/% H2 200oC for H2/N2 mixtures and 8.80x10-5 mA/% H2 for H2/CO mixtures at 50 oC. Sensor response also matches well with the empirical with empirical thermal conductivity correlations suggesting future TCD applications.

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29 6.1 Conclusions GaN resistive sensors have been tested for gas det ection. Detection of hydrogen concentrations in the presence of nitrogen was obse rved at temperatures up to 300 oC. Sensor response to H2/N2 mixtures indicates that temperature affects sensit ivity with limited changes to the reference baseline caused by H2 adsorption. Gas sensors exposure to varying ratios of H2/CO gas streams produced measureable current response to changing concentrat ions which was comparable to H2/N2 responses. At 50 oC sensors had a clear response to changing H2/CO mixtures. However, sensitivity was much lower than that of the H2/N2 mixtures which is probably caused by a reduced experimental temperature. The sensor thermal mechanism which is utilized for gas detection is demonstrated by thermal conductivity changes. Sensor data showe d a close match to the Wassiljewa Equation with the Mason and Saxena Modification for thermal conductivity. A logarithmic correlations between sensor current cha nge and thermal conductivity was observed. Chapter 6: Conclusions and future work

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30 6.2 Future work 6.2.1 Further investigation into high temperature response Testing sensor operation at temperatures above 300 oC could allow for a model of sensitivity dependence on temperature which could b e used to calibrate sensors for controls of hydrogen energy processes. Selectivity to hydrogen at higher temperatures and in the presence of impurities would also be a n oteworthy investigation. Impurities with similar thermal properties to hydrogen may inc rease current change by driving more heat transfer between the gas mixture and sensor. 6.2.2 Further investigation of synthesis gas sensi tivity High temperature sensor response would be an impor tant continuation of this research. Experiments could be conducted to determ ine if H2/CO mixtures followed the same trend of increased sensitivity in H2/N2 mixtures at higher temperatures. Elevated temperature investigations will also determine whet her GaN sensors can be used in high temperature industrial processes such as FTS. Investigations of the sensitivity of GaN sensors w ill also show how impurities in synthesis gas composition affect sensor response. Production of synthesis gas from coal gasification produces by products such as sulfides and carbon dioxide [26]. Irreversible adsorption or diffusion of these compounds into sen sors would impact sensor response. Additional modifications to sensors may be necessar y before they are practical for industrial use.

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31 6.2.3 Further investigation of thermal property re lationships Measurement of physical properties of gaseous mixt ures at high temperatures may be another practical application for GaN sensors. Change in thermodynamic properties and temperature dependent properties of gas mixture s other than thermal conductivity may be detectible with GaN sensors. Investigations may be conducted with correlate sensor response to properties such as gas mixture h eat capacity and viscosity.

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32 References [1] A. Contreras, S. Yigit, K. Ozay, and T.N. Vezir oglu, Hydrogen as aviation fuel: A comparison with hydrocarbon fuels, International Journal of Hydrogen Energy 22 (10-11), 1053-1060 (1997). [2] T.F. Fuller, and M.L. Perry, A historical perspective of fuel cell technology in the 20th century, Journal of the Electrochemical Society 149 (7), S59 -S67 (2002). [3] A. Steynberg and M. Dry, Fischer-Tropsch technology (Elsevier, Amsterdam, 2004). [4] H. Nanto, T. Minami, and S. Takata, Zinc oxide thin film ammonia gas sensors with high sensitivity and excellent selectivity Journal of Applied Physics 60, 482 (1986). [5] S. M. Sze, Semiconductor Sensors (John Wiley & Sons, New York, 1994). [6] F.K. Yam, Z. Hassan, and A.Y. Hudeish, The study of Pt Schottky contact on porous GaN for hydrogen sensing Thin Solid Films 515 (18), 7337-7341 (2007). [7] F. Yun, S. Chevtchenko, Y.-T Moon, H. Morkoc, T .J. Fawcett, and J.T. Wolan, GaN resistive hydrogen gas sensors, Applied Physics Letters 87 (7), 073507 (2005). [8] R.E. Krebs, The History and Use of Our Earth’s Chemical Element s (Greenwood Press, Westport, CT, 1998). [9] N.I. Sax, and R.J. Lewis, Hawley’s condensed chemical dictionary, 11th ed., (Van Nostrand Reinhold, New York, 1987). [10] G.W. Hunter, P.G. Neudeck, C.C. Liu, B. Ward, Q.H. Wu, P. Dutta, M. Frank, J. Trimbol, M. Fulkerson, B. Patton, D. Makel, and V. Thomas, Development of chemical sensor arrays for harsh environments and a erospace applications Sensors, 2002. Proceedings of IEEE 2, 1126-1133 (20 02).

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33 [11] J. M. Ogden, and M.M. Steinbugler, and T. G. K reutz, A comparison of hydrogen, methanol and gasoline as fuels for fuel c ell vehicles: implications for vehicle design and infrastructure development, Journal of Power Sources 79, 143–168 (1999). [12] R. Wengenmayr, and Thomas Buhrke, Renewable Energy R. Wengenmayr, Thomas Buhrke, (Wiley-vch Verlag GmbH & Co., 2008). [13] N. Takeichi, H. Senoh, T. Yokota, H. Tsuruta, K. Hamada, H.T. Takeshita, H. Tanaka, T. Kiyobayashi, T. Takano, and N. Kuriyama, Hybrid hydrogen storage vessel, a novel high-pressure hydrogen storage vess el combined with hydrogen storage material, International Journal of Hydrogen Energy 28, 1121 – 1129 (2003). [14] A. Lantz, J. Heffel, and C. Messer, Hydrogen fuel cell engines and related technologies (College of the Desert, Palm Desert, CA, 2001). [15] R.B. Anderson, The Fischer-Tropsch synthesis, (Academic Press, Inc., Orlando 1984). [16] Perry’s chemical engineer’s handbook ; 6th ed., edited by R. H. Perry, D. W. Green, and J. O. Maloney (McGraw-Hill, New York, 19 84). [17] CRC handbook of chemistry and physics 88th Ed., edited by D.R. Lide, (CRC Press, Boca Raton, 2008). [18] M. N. Carcassi, and F. Fineschi, Deflagrations of H2–air and CH4–air lean mixtures in a vented multi-compartment environment Energy 30, 1439–1451 (2005). [19] A. Trouillet, C. Veillas, E. Sigronde, H. Gagn aire, and M. Clement, Gaseous hydrogen leakage optical fibre detection system, Proceedings of the SPIE – The International Society for Optical Engineering 5502 (1), 247-250 (2004). [20] V. Krishnan, J.W. Fergus, and F. Fasoyinu, Solid Electrolyte Based Hydrogen Sensor for Molten Aluminum Advances in Aluminum Casting Technology II edited by M. Tiryakioglu and J. Campbell (ASM International, 2002). [21] L. Bjornsson, E.G. Hornsten, and B. Mattiasson Utilization of a palladium-metal oxide semiconductor (Pd-MOS) sensor for on-line mon itoring of dissolved hydrogen in anaerobic digestion, Biotechnology and Bioengineering 73 (1), 3543 (2001). [22] R.B. Gupta, Hydrogen Fuel, (CRC Press, Boca Raton, 2009).

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34 [23] O. Lenz, M. Bernhard, T. Buhrke, E. Schwartz, and B. Friedrich, The hydrogensensing apparatus in Ralstonia eutropha Journal of Molecular Microbiology and Biotechnology, 4 (3), 255–262 (2002). [24] S. M. Sze, Physics of semiconductor devices (John Wiley & Sons, New York, 1981). [25] B.G. Streetman, Solid State Electronic Devices, 4th Ed., (Prentice-Hall, Englewood Cliffs, 1995). [26] C. Higman, and M. van der Burgt, Gasification (Elsevier Science, New York, 2003). [27] http://www.ioffe.rssi.ru/SVA/NSM/Semicond/ [28] R.F. Coughlin, and F.F. Driscoll, Semiconductor Fundamentals (Prentice-Hall, Inc., Englewood Cliffs, 1976). [29] J. Schalwig, G. Muller, U. Karrer, M. Eickhoff O. Ambacher, M. Stutzmann, L. Gorgens, and G. Dollinger, Hydrogen response mechanism of Pt-GaN Schottky diodes Applied Physics Letter 80, 1222 (2002). [30] M. Ali, V. Cimalla, V. Lebedev, H. Romanus a V. Tilak, D. Merfeld, P. Sandvik, and O. Ambacher a, Pt/GaN Schottky diodes for hydrogen gas sensors, Sensors and Actuators B 113 (2), 797-804 (2006). [31] B. P. Luther, S. D. Wolter, and S. E. Mohney, High temperature Pt Schottky diode gas sensors on n-type GaN Sensors and Actuators B: Chemical 56 (1-2), 164-168 (1999). [32] B.S. Kang, S. Kim, F. Ren, B.P. Gila, C.R. Abe rnathy, and S.J. Pearton, Comparison of MOS and Schottky W/Pt–GaN diodes for hydrogen detection Sensors and Actuators B 104, 232–236 (2005). [33] S.W. Chunga, W.J. Hwanga, Chin C. Leeb, and M. W. Shina, The thermal effect of GaN Schottky diode on its I-V characteristics Journal of Crystal Growth 268, 607–611 (2004). [34] K.I. Lundstrom, M.S. Shivaraman, and C.M. Sven sson, A hydrogen-sensitive Pdgate MOS transistor, Journal of Applied Physics 46, 3876 (1975). [35] L. Mariucci, A. Pecora, C. Reita, G. Petrocco, and G. Fortunato, Pd-Gate a-Si:H thin film transistors as hydrogen sensors Japanese Journal of Applied Physics, 29 (12), (1990).

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35 [36] I. Lundstrom, S. Shivaraman, C. Svensson, and L. Lundkvist, A hydrogensensitive MOS fieldeffect transistor Applied Physics Letters, 26, 5557 (1975). [37] A.D. Brailsford, M. Yussouff, and E.M. Logothe tis, A first principles model of metal oxide gas sensors for measuring combustibles, Sensors and Actuators B 49, 93–100 (1998). [38] R. Weiss, L. Frey and H. Ryssel, Tungsten, nickel, and molybdenum Schottky diodes with different edge termination Applied Surface Science, 184 (1-4), 413418 (2001). [39] S-J. Kim, S. Kim, S-J. Yu, and S-G. Kim, Improvement in breakdown voltage characteristics of SiC Schottky barrier diode by in corporating a guard ringassisted field limiting ring and an internal ring, Physica Status Solidi A 203 (15), 3873-81 (2006). [40] T. J. Fawcett, Ph.D.Ch.E. Dissertation, Univer sity of South Florida, 2006. [41] T.J. Fawcett, J.T. Wolan, L. Spetz, A. Reyes, and S.E. Saddow, Thermal detection mechanism of SiC based hydrogen resistive gas sensors, Applied Physics Letters 89, 182102 (2006). [42] T. J. Fawcett, M.S.Ch.E. Thesis, University of South Florida, 2004. [43] P. Tardy, J.-R. Coulon, C. Lucat, and F. Menil, Dynamic thermal conductivity sensor for gas detection Sensors and Actuators B 98 (1), 63-68 (2004). [44] G. Pollak-Diener and E. Obermeier, Heat-conduction microsensor based on silicon technology for the analysis of twoand three-compo nent gas mixtures Sensors and Actuators B 13-14 (1-3), 345-347 (1993). [45] B.E. Poling, J.M. Prausnitz, and J. P. O’Connell, Properties of liquids and gases, 5th Ed, (McGraw-Hill, New York, 2001).

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36 Appendices

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37 Appendix A: Nomenclature Symbol Definition Units A surface area m2 Aij Mason-Saxena modification dimensionless C concentration moles/sscm DAB diffusivity in air cm2/s E electric field V/m Eg bandgap energy eV h heat transfer coefficient W/m2/K I current mA k thermal conductivity W/m/K k wave vector kg*m/s q heat energy J Q rate of heat transfer J/s R resistance t time S T temperature K vn electron drift velocity m/s y mole fraction moles species/total moles viscosity Pa*s n electron mobility cm2/V/s wave function dimensionless


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Gallium nitride sensors for hydrogen/nitrogen and hydrogen/carbon monoxide gas mixtures
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ABSTRACT: As hydrogen is increasingly used as an energy carrier, gas sensors that can operate at high temperatures and in harsh environments are needed for fuel cell, aerospace, and automotive applications. The high temperature Fischer-Tropsch process also uses mixtures of hydrogen and carbon monoxide to generate synthetic fuels from non-fossil precursors. As the Fischer-Tropsch process depends upon particular gas mixtures to generate various fuels, a sensor which can determine the proper ratio of reactants is needed. To this end, gallium nitride (GaN) has been used to fabricate a resistive gas sensor. GaN is a suitable semiconductor to be used in hydrogen because of a wide, direct bandgap and greater stability than many other semiconductors. Additionally, resistive sensors offer several advantages in design compared to other types of sensors. Response time of resistive sensors is faster than those of other semiconductor sensors because catalytic and diffusion steps are not part of the response mechanism. Instead, a thermal detection mechanism is employed in resistive sensors. In this work, sensor response to changes in hydrogen concentration in nitrogen was measured at 200C and 300C. Sensor response was measured as change in current from a reference response to pure nitrogen at each temperature under a constant 2.5 V bias. Isothermal operation was achieved by controlling sensor temperature and pre-heating gas mixtures. Sensitivity to concentration increased upon an increase in temperature. Additionally, sensor response to concentration changes of H in CO at 50 C was demonstrated. Sensors show similar responses to nitrogen and carbon monoxide mixtures, which have similar thermal properties. Using the thermal detection mechanism of the sensors, a correlation was shown between sensor response and a gas mixture thermal conductivity.
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