Effect of Water Vapor Amount in a Hydrogenous Atmosphere on Structure and Properties of Nickel-Zirconia Anode Materials for Solid Oxide Fuel Cells

Nickel-zirconia anode ceramics of YSZ-NiO system for solid oxide fuel cells (SOFCs) has been investigated. A series of specimens were singly reduced in hydrogenous atmosphere (the Ar-5 vol%H2 mixture) at 600°C under the pressure of 0.15 MPa or subjected to reduction-oxidation (redox) cyclic treatment at 600°C. Influence of water vapor concentration in hydrogenous atmosphere on structure and properties of the materials was studied. Based on structural changes in the as-received material it was revealed that a small amount of water vapor in Ar-5 vol% H2 mixture (water vapor pressure below 0.03 MPa) accelerates a reduction of the nickel phase at 600°C with formation of nanopores on tiny Ni particles. A higher concentration of water vapor (the pressure above 0.03-0.05 MPa) causes a converse change in the reduction kinetics. For as-received material, such an amount of water vapor in the mixture is an obstacle for its reduction. For the material treated by redox cycling, better physical and mechanical properties were revealed after dwelling at 600°C in a water depleted gas mixture. Based on the SEM microscopy and the data on the conductivity and strength, the dual effect of water vapor on durability of a nickel-zirconia anode is discussed

CORRELATION OF SHORT-TERM TO LONG-TERM OXIDATION TESTING FOR ALUMINA FORMING ALLOYS AND COATINGS

Engineering long cyclic oxidation life of high temperature materials requires success on two fronts. First a slow growing protective oxide scale must form during the elevated temperature exposure. To satisfy this aspect, alumina-forming alloys and coatings are widely accepted as leading materials for use in this environment and are the focus of this discussion. The second aspect is the formation of an adherent oxide that resists spallation during thermal cycling. The driving force for spallation is the stored elastic strain energy that develops from stresses in the oxide scale. Once this stored elastic strain energy exceeds the oxide-substrate interfacial toughness, cracking and subsequent spallation occurs followed by rapid oxidation of the substrate. With advances in alloy and coating development resulting in higher operating temperatures and increased service lives, researchers are faced with excessive laboratory time and cost required to perform a long-term cyclic oxidation test.The challenge is to predict long-term oxidation behavior from short-term experiments. Since the rate limiting step to high temperature oxidation is a thermally activated process, previous investigations were performed at increased exposure temperatures for rapid degradation of the alloys and coatings to rank material performance. Others have mechanically induced oxide spallation to give insight on the adherence of oxide scales prior to spontaneous failure. In this investigation, short-term testing is employed to gain insight on long-term performance and to determine inputs into a cyclic oxidation model for life-time prediction. This model operates in an iterative process where one iteration is a thermal cycle. The amount of oxide formed during the high temperature segment is calculated followed by the amount that is lost due to scale spallation during cooling. Retained oxide at the end of this cycle is used as the starting point for the following iteration. The two inputs into this model are the oxide scale growth and spallation behavior. Scale growth behavior corresponds to the isothermal growth kinetics that are experimentally determined by thermogravimetric analysis. Oxide scale spallation behavior is quantified by two short-term experiments of a novel acoustic emission experiment during a 24 hour exposure and the stress measurement of the scale after an exposure to the temperature of interest. Results from these short-term tests and modeled cyclic oxidation are compared to life-times from long-term cyclic oxidation tests

Bistability and Electrical Characterisation of Two Terminal Non-Volatile Polymer Memory Devices.

Polymer blended with nanoparticle and ferroelectric materials in two terminal memory devices has potential for electronic memory devices that may offer increased storage capacity and performance. Towards understanding the memory performance of a combination of an organic polymer with a ferroelectric or unpolarised material, this research is concerned with testing the memory programming and capacitance of these materials using two-terminal memory device structures. This research contributes to previous investigation into the internal working mechanisms of polymer memory devices and increases understanding and verifies the principles of these mechanisms through testing previously untested materials in different material compositions. This study makes a novel contribution by testing the electrical bistability of new materials; specifically, nickel oxide, barium titanate and methylammonium lead bromide and considers their properties which include nanoparticles, ferroelectric, perovskite structures and organic-inorganic composition. Due to their material properties which have different implications for internal switching and memory storage. Nanoparticles have a greater band gap between the valence band and conduction band compare to bulk material which is exploited for memory storage and ferroelectric properties and perovskite materials have non-volatile properties suitable for switching mechanisms. Specific attributes of memory function which include charging mechanism, device programming, capacitance and charge retention were tested for different material compositions which included, blend and layered with a PVAc polymer, and as a bulk material with a single crystal structure using MIM memory devices and MIS device structures. The results showed that nickel oxide was the most effective material as a blend with the polymer for memory performance, this was followed by barium titanate, however, methylammonium lead bromide performed poorly with polymer but showed promise as a single crystal structure. The results also showed that an increase in concentration of the tested material in a blend composition resulted in a corresponding increase in memory function, and that blend compositions were much more effective than layered compositions

Synthesis and gas sensing properties of inorganic semiconducting, p-n heterojunction nanomaterials

En aquesta tesis utilitzant principalment Aerosol Assited Chemical Vapor Deposition, AACVD, com a metodologia de síntesis d'òxid de tungstè nanoestructurat s'han fabricat diferents sensors de gasos. Per tal d'estudiar la millora en la selectivitat i la sensibilitat dels sensors de gasos basats en òxid de tungstè aquest s'han decorat, via AACVD, amb nanopartícules d'altres òxids metàl·lics per a crear heterojuncions per tal d'obtenir un increment en la sensibilitat electrònica, les propietats químiques del material o bé ambdues. En particular, s'han treballat en diferents sensors de nanofils d'òxid de tungstè decorats amb nanopartícules d'òxid de níquel, òxid de cobalt i òxid d'iridi resultant en sensors amb un gran increment de resposta i selectivitat cap al sulfur d'hidrogen, per a l'amoníac i per a l'òxid de nitrogen respectivament a concentracions traça. A més a més, s'han estudiat els mecanismes de reacció que tenen lloc entre les espècies d'oxigen adsorbides a la superfície del sensor quan interactua amb un gas. I també s'ha treballat en intentar controlar el potencial de superfície de les capes nanoestructurades per tal de controlar la deriva en la senyal al llarg del temps, quan el sensor està operant, a través d'un control de temperatura.En esta tesis utilizando principalmente Aerosol Assited Chemical Vapor Deposition, AACVD, como metodología de síntesis de óxido de tungsteno nanoestructurado se han fabricado diferentes sensores de gases. Para estudiar la mejora en la selectividad y la sensibilidad de los sensores de gases basados en óxido de tungsteno estos se han decorado, vía AACVD, con nanopartículas de otros óxidos metálicos para crear heterouniones para obtener un incremento en la sensibilidad electrónica, las propiedades químicas del material o bien ambas. En particular, se han trabajado en diferentes sensores de nanohilos de óxido de tungsteno decorados con nanopartículas de óxido de níquel, óxido de cobalto y óxido de iridio resultante en sensores con un gran incremento de respuesta y selectividad hacia el sulfuro de hidrógeno, para el amoníaco y para el óxido de nitrógeno respectivamente a concentraciones traza. Además, se han estudiado los mecanismos de reacción que tienen lugar entre las especies de oxígeno adsorbidas en la superficie del sensor cuando interactúa con un gas. Y también se ha trabajado en intentar controlar el potencial de superficie de las capas nanoestructuradas para controlar la deriva en la señal a lo largo del tiempo, cuando el sensor está trabajando, a través de un control de temperatura.In this thesis, using mainly Aerosol Assited Chemical Vapor Deposition, AACVD, as a synthesis methodology for nanostructured tungsten oxide, different gas sensors have been manufactured. To study the improvement in the selectivity and sensitivity of gas sensors based on tungsten oxide, they have been decorated, via AACVD, with nanoparticles of other metal oxides to create heterojunctions to obtain an increase in electronic sensitivity, in the chemical properties of the material or at the same time in both. Particularly, we have worked on different tungsten oxide nanowire sensors decorated with nanoparticles of nickel oxide, cobalt oxide and iridium oxide resulting in sensors with a large increase in response and selectivity towards hydrogen sulfide, for ammonia. and for nitrogen oxide respectively at trace concentrations. In addition, the reaction mechanisms that take place between oxygen species adsorbed on the sensor surface when it interacts with a gas have been also studied. Furthermore, efforts have been put on trying to control the surface potential of the nanostructured layers to control the drift in the signal over time, when operating the sensors, through temperature control

MODELLING AND SIMULATION OF FIELD EMISSION IN CARBON NANOTUBE BASED IONIZATION GAS SENSOR

Gas sensors are of main interest in the field of oil and gas industry. They are used to sense corrosive gases in the pipelines and leakage in the delivery system. One of the recently developed gas sensor that has become the focal point of research is the ionization gas sensor. This sensor technology is still in its infancy and much can be done to increase the efficiency of the sensor. In this research, a new model to study the gas detection mechanism of carbon nanotube (CNT) based ionization gas sensor has been developed. The model incorporates electron field emission property of the CNTs. The new model consists of three modules, i.e., CNT particle injection module, CNT density and aspect ratio variation module, and CNT velocity assignment module. These three modules are combined together and embedded in the standard Particle-In-Cell / Monte Carlo Collision (PIC-MCC) codes. The integrity of the enhanced PIC-MCC codes has been validated by calculating the field enhancement factor, β. Furthermore, the functionality of these codes is checked by running simulations of DC discharges in different gases and comparing the results with published experimental and simulated works. With the help of enhanced PIC-MCC codes the simulation of gas breakdown behavior with CNT field emission effects become possible for the first time. From the results, around one order of magnitude decrease in the breakdown voltages is observed when CNT is used in ionization gas sensor. The electrostatic screening effects are reduced to a minimum when inter-tube spacing is equal to the height of the CNT. Faster response time is also observed with the presence of CNT in ionization gas sensor. These results suggest that by properly controlling the growth of CNT structures, an optimized CNT based ionization gas sensor can be realized

Nano-derived sensors for high-temperature sensing of H2, SO2 and H2S

The emission of sulfur compounds from coal-fired power plants remains a significant concern for air quality. This environmental challenge must be overcome by controlling the emission of sulfur dioxide (SO2) and hydrogen sulfide (H2S) throughout the entire coal combustion process. One of the processes which could specifically benefit from robust, low cost, and high temperature compatible gas sensors is the coal gasification process which converts coal and/or biomass into syngas. Hydrogen (H2), carbon monoxide (CO) and sulfur compounds make up 33%, 43% and 2% of syngas, respectively. Therefore, development of a high temperature (\u3e500°C) chemical sensor for in-situ monitoring H2, H2S and SO2 levels during coal gasification is strongly desired. The selective detection of SO2/H2S in the presence of H2, is a formidable task for a sensor designer. In order to ensure effective operation of these chemical sensors, they must inexpensively function within the gasifier\u27s harsh temperature and chemical environment. Currently available sensing approaches, which are based on gas chromatography, electrochemistry, and IR-spectroscopy, do not satisfy the required cost and performance targets.;There is also a substantial necessity for microsensors that can be applied inexpensively, have quick response time and low power consumption for sustained operation at high temperature. In order to develop a high temperature compatible microsensor, this work will discourse issues related to sensor stability, selectivity, and miniaturization. It has been shown that the integration of nanomaterials as the sensing material within resistive-type chemical sensor platforms increase sensitivity. Unfortunately, nanomaterials are not stable at high temperatures due to sintering and coarsening processes that are driven by their high surface to volume ratio. Therefore, new hydrogen and sulfur selective nanomaterial systems with potentially highly selective and stable properties in the proposed harsh environment were investigated. Different tungstates and molybdates (WO3, MoO3, MgMoO4, NiMoO4, NiWO4, Sr2MgWO6 (SMW), Sr2MgMoO6 (SMM), SrMoO4, and SrWO4) were investigated at the micro- and nano-scale, due to their well-known properties as the reversible absorbents of sulfur compounds. Different morphologies of aforementioned compounds as well as microstructural alterations were also the subject of the investigation. The fabrication of the microsensors consisted of the deposition of the selective nanomaterial systems over metal based interconnects on an inert substrate. This work utilized the chemi-resistive (resistive-type) microsensor architecture where the chemically and structurally stable, high temperature compatible electrodes were sputtered onto a ceramic substrate. The nanomaterial sensing systems were deposited over the electrodes using a lost mold method patterned by conventional optical lithography.;Development of metal based high temperature compatible electrodes was crucial to the development of the high temperature sensor due to the instability of typically used noble metal (platinum) based electrode material over ceramic substrates. Therefore, the thermal stability limitations of platinum films with various adhesion layers (titanium (Ti), tantalum (Ta), zirconium (Zr), and hafnium (Hf)) were characterized at 800 and 1200°C. Platinum (Pt)-zirconium (Zr)-hafnium (Hf) were investigated. The high-temperature stable composite thin film architecture was developed by sequential sputter deposition of Hf, Zr and Pt. In addition to this multilayer architecture, further investigation was carried out by using an alternative DC sputtering deposition process, which led to the fabrication of a functionally-gradient platinum and zirconium composite microstructure with very promising high temperature properties. The final process investigated reduced labor, time and material consumption compared to methods for forming multilayer architectures previously discussed in literature.;In addition to electrical resistivity characterization of the different thin film electrode architectures, the chemical composition, and nano- and micro-structure of the developed nanomaterial films, as well as sensing mechanism, were characterized by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS), atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), Raman spectroscopy, temperature programmed reduction (TPR) and transmission electron microscopy (TEM). The macro-configurations of the sensors were tested and analyzed for sensitivity and cross-sensitivity, response time and recovery time, as well as long term stability. The microsensor configuration with optimized nanomaterial system was tested and compared to a millimeter-size sensor platform in terms of sensitivity and accuracy. Electrochemical relaxation (ECR) technique was also utilized to quantify the surface diffusion kinetics of SO2 over the chosen sensor material surface. The outcomes of this research will contribute to the economical application of sensor arrays for simultaneous sensing of H2, H2S, and SO2

A novel technique for evaluating the degradation of engine components non-destructively

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Impedance spectroscopy (IS) was used to evaluate the microstructural changes of a thermally-grown oxide (TGO) layer on a nickel-based superalloy or bond coat, with or without a thermal barrier coating (TBC) system, at various temperatures. TBC is used for hot section parts of gas turbine engine, such as turbine blades and vanes. However, spallation of TBC can take place due to long-term operation at high temperature. The spallation is mainly caused by both microstructural changes and thermal stresses as a result of oxide layer (mainly alumina) formation and growth at the interface between the TBC and bond coat. The electrical resistance and capacitance of the oxide layer, formed from oxidation of IN738LC superalloy at high temperature, were obtained from fitting the results of the measured impedance diagrams based on an equivalent circuit model. The equivalent circuit model should represent the features of the oxide layer or the TBC system. The electrical resistance of the oxide layer increased with increasing oxidation time for samples exposed to air at 900°C. Similar results were obtained for the NiCrCoAIY bond coat samples and the TBC systems. The capacitance decreased with increasing thickness of the alumina layer. The activation energy of electrical conduction was used to characterise the alumina layer formed on the bond coat at 900°C, 1000°C and 1100°C. The activation energy values for the alumina layer, formed at various temperatures, decrease with increasing impurity or porosity. Changes in the electrical properties of TGOs are correlated with those in their microstructure and microchemistry. The degradation of a TBC can be identified, when the electrical resistance of the TGO decreases with increasing oxidation time. The fast decrease in resistivity corresponds to the compositional change in the TGO from cc-Al2O3 to a mixture of a-Cr2O3 and (Ni or Co)(Cr or Al)2O4 spinel. The disappearance of a-A1203 in the TGO makes the scale non-protective and leads to cracks and spallation of TBCs. Non-destructive testing of the crack formation in a TBC system is essential for predicting the failure and lifetime of TBCs in service. IS was used to evaluate the crack formation in the TBC system due to thermal cycling. During the thermal cycling, cracks initiated and propagated along the interface between the TGO and the yttrium stabilised zirconia (YSZ), used as a TBC. This caused the spallation of the TBC eventually. The propagation of cracks at the interface of TGO/YSZ was found to contribute to an increase in the interfacial impedance. The interfacial area determines the interfacial resistance corresponding to the oxygen reaction. Therefore, the crack propagation induced an increase of the interfacial resistance, whereas the interfacial capacitance showed no trend in its alteration with the propagation of the cracks. As a result, the relaxation frequency of the interface moved towards a lower frequency during the propagation of the cracks. Therefore, impedance spectroscopy has been used to examine the crack formation in TBC system non-destructively. By using scanning electron microscopy and X-ray diffraction techniques, the composition and microstructure of the oxide scales were examined. It was found that their electrical properties were determined, not only by the microstructure of the oxide scales, but also by the composition of the oxide scales. By determining the relationship between the electrical properties, microstructure and composition of the oxide scales, IS could be used as a non-destructive technique for monitoring the oxidation of metallic alloys at high temperature in gas turbine engine components.This study is funded by the EPSRC and Alstom gas turbine Ltd