Wide band gap materials and devices for NOx, H2 and O2 gas sensing applications

Im Rahmen dieser Arbeit sind Feldeffektgassensoren (Schottky Dioden, MOS Kapazitäten, und MOSFET Transistoren) auf der Basis von Halbleitern mit großer Bandlücke (Siliziumkarbid (SiC) und Gallium Nitrid (GaN), sowie resistive Gassensoren, die auf aktiven Indiumoxid-Schichten (In2O3) basieren, für die Detektion von reduzierenden Gasen (H2, D2) und oxidierenden Gasen (NOx, O2), entwickelt worden. Die Entwicklung der Sensoren ist am Institut für Mikro- und Nanoelektronik der Technischen Universität Ilmenau in Zusammenarbeit mit General Electric (GE) Global Research (USA) und der Umwelt- und Sensortechnik GmbH (Geschwenda) durchgeführt worden. Kapitel 1: dient als eine Einführung in das mit dieser Arbeit verbundene wissenschaftliche Feld. Die theoretischen Grundlagen der Festkörper-Gassensoren werden dargestellt. Zusätzlich werden in diesem Kapitel die relevanten Eigenschaften der Materialien mit großer Bandlücke (SiC und GaN) präsentiert. Kapitel 2: Pt/GaN Schottky Dioden mit verschiedener Dicke des katalytischen Metalls werden als Wasserstoffgasdetektoren vorgestellt. Die Fläche sowie die Dicke von Pt-gates wurden zwischen 250 × 250 µm2 und 1000 × 1000 µm2, 8 und 40 nm, systematisch variiert. Die Sensorantwort (Sensorsreaktion) auf 1 vol.% Wasserstoff in synthetischer Luft wurde in Abhängigkeit von der aktiven Fläche, der Pt-Dicke, und der Betriebstemperatur untersucht. Durch Anheben der Betriebstemperatur auf ca. 350°C und durch Reduzierung der Dicke des Pt auf 8 nm beobachteten wir eine beträchtliche Erhöhung der Empfindlichkeit sowie eine Verkürzung der Ansprech- und Erholzeiten. Untersuchungen am Elektronenmikroskop zeigten, dass das dünnere Platin eine höhere Korngrenzendichte aufwies. Die Erhöhung der Empfindlichkeit gemeinsam mit der Reduzierung der Dicke des Pt deuten auf die Dissoziierung von molekularem Wasserstoff an der Oberfläche, die Diffusion atomaren Wasserstoffs entlang der Korngrenzen des Platins und die Adsorption von Wasserstoff an der Pt/GaN Grenzfläche als ein möglicher Mechanismus der Detektion von Wasserstoff durch Schottky Dioden hin. Die Reaktion auf D2, NOx, and O2 von Metall-Oxid-Halbleiter (MOS) Strukturen mit Rhodium Schottky-Kontakten mit einer Dicke von 30 nm in Abhängigkeit von der Betriebstemperatur und der Gaspartialdrücke wurde in Kapitel 3 untersucht. Die Reaktion dieses Gates wurde als Verschiebung entlang der Spannungsachse in der Kapazität-Spannungs Kurve (C-V) nachgewiesen. Positive und negative Flachband-Verschiebungen jeweils bis zu 1 V wurden für oxidierende und reduzierende Gase beobachtet. Abhängig vom gewählten Typ des Isolators wurden Unterschiede in den Empfindlichkeiten beobachtet. In Kapitel 4: SiC-basierten FETs mit verschiedenen Materialien für das Gate (Gemisch aus Metalloxiden: Indiumoxide und Zinnoxid (InxSnyOz), Indiumoxid und Vanadiumoxid (InxVyOz) sowie ein Gemisch aus Metalloxiden mit Zugabe einer entsprechenden Menge Metallzusätzen) wurden als NOx, O2, und D2 Gasdetektoren untersucht. Die Reaktion auf diese Gase wurde in Abhängigkeit von der Betriebstemperatur und der Gaspartialdrücke untersucht. Die Zusammensetzung der aktiven Metalloxid-Schicht und die Mikrostruktur der sensitiven Gateelektrode sind die entscheidenden Parameter mit Einfluss auf den Messmechanismus und somit die entscheidenden Leistungsparameter des Sensors: Empfindlichkeit, Selektivität und Reaktionszeit. Durch die Optimierung der Temperatur und des richtigen Materials des Katalysators können Sensoren mit sehr hoher Empfindlichkeit gegenüber relevanten Gasen realisiert werden. Wird auch der Katalysator sorgfältig ausgewählt, können diese Erkenntnisse für eine Erhöhung der Selektivität des Sensors genutzt werden. In Kapitel 5: Polykristalle von 200 nm Dicke und 10 nm nanostrukturierten Dünnschichten aus In2O3, die durch MOCVD (metallorganische Gasphasenabscheidung) gewachsen sind, wurden untersucht, um Informationen über ihre Eigenschaften hinsichtlich der Detektion von NOx- and O2-Gasen zu erhalten. Die Reaktion auf diese Gase wurde in Abhängigkeit von der Betriebstemperatur und der Gaspartialdrücke untersucht. Die Experimente in Anwesenheit verschiedener Partialdrücke des NOx haben gezeigt, das beide Dünnschichten in der Lage sind, Stickoxide zu detektieren. Es wurde festgestellt, dass besonders die nanostrukturierte In2O3-Dünnschicht stärker auf NOx reagiert. Dieser Effekt wird durch das höhe Oberflächen-zu-Volumenverhältnis infolge der niedrigen Korngröße verbessert, so dass der relative interaktive Oberflächenbereich größer und die Dichte der Ladungsträger höher ist. Wir haben ermittelt, dass die Reduzierung der Korngröße des messenden Materials auf unter 10 nm erhebliche Auswirkung auf die Empfindlichkeit des Sensors hat. Die hinsichtlich der Empfindlichkeit und Reaktion optimalen Temperaturen des nanostrukturierten In2O3 für den Nachweis von NOx treten im Bereich von 100-175°C auf. In diesem Temperaturbereich ist die Reaktion auf O2 sehr schwach, was darauf hinweist, das der Sensor für die selektive Erkennung von NOx bei niedrigen Temperaturen sehr gut geeignet ist. Zudem wurde festgestellt, dass die nanostrukturierte In2O3-Dünnschicht für den Einsatz in der Erkennung niedriger Partialdrücke die optimale ist. Kapitel 6 enthält Schlussfolgerungen aus den gegenwärtigen Arbeiten. In diesem Kapitel vergleichen wir alle untersuchten Gassensoren in Bezug auf deren Empfindlichkeit, Selektivität und Reaktionszeit und stellen diese anschließend den entsprechenden Ergebnissen anderer, in der wissenschaftlichen Literatur zu findenden Autoren gegenüber.In this thesis, field effect gas sensors (Schottky diodes, MOS capacitors, and MOSFET transistors) based on wide band gap semiconductors like silicon carbide (SiC) and gallium nitride (GaN), as well as resistive gas sensors based on indium oxide (In2O3), have been developed for the detection of reducing gases (H2, D2) and oxidising gases (NOx, O2). The development of the sensors has been performed at the Institute for Micro- and Nanoelectronic, Technical University Ilmenau in co-operation with (GE) General Electric Global Research (USA) and Umwelt-Sensor-Technik GmbH (Geschwenda). Chapter 1: serves as an introduction into the scientific fields related to this work. The theoretical fundamentals of solid-state gas sensors are provided and the relevant properties of wide band gap materials (SiC and GaN) are summarized. In chapter 2: The performance of Pt/GaN Schottky diodes with different thickness of the catalytic metal were investigated as hydrogen gas detectors. The area as well as the thickness of the Pt were varied between 250 × 250 µm2 and 1000 × 1000 µm2, 8 and 40 nm, respectively. The response to hydrogen gas was investigated in dependence on the active area, the Pt thickness and the operating temperature for 1 vol.% hydrogen in synthetic air. We observed a significant increase of the sensitivity and a decrease of the response and recovery times by increasing the temperature of operation to about 350°C and by decreasing the Pt thickness down to 8 nm. Electron microscopy of the microstructure showed that the thinner platinum had a higher grain boundary density. The increase in sensitivity with decreasing Pt thickness points to the dissociation of molecular hydrogen on the surface, the diffusion of atomic hydrogen along the platinum grain boundaries and the adsorption of hydrogen at the Pt/GaN interface as a possible mechanism of sensing hydrogen by Schottky diodes. The response to deuterium D2, NOx, and O2 of metal-oxide-semiconductor (MOS) and metal-metal oxide-oxide-semiconductor (MMOOS) structures with rhodium (Rh) gate were investigated in dependence on the operating temperature and gas partial pressures was investigated in chapter 3. The response of the sensor was measured as a shift in the capacitance-voltge (C-V) curve along the voltage axis. Positive and negative flat-band voltage shifts up to 1 V were observed for oxidizing and reducing gases, respectively. Depending on the type of insulator that is chosen, differences in the sensitivity of the sensor were observed. In chapter 4: The performance of SiC-based field effect transistors (FETs) with different gate materials (mixture of metal oxides: indium oxide and tin oxide (InxSnyOz), indium oxide and vanadium oxide (InxVyOz), as well as mixtures of metal oxides with metal additives) were investigated as NOx, O2, and D2 gas detectors. The response to these gases was investigated in dependence on the operating temperature and gas partial pressures. The composition and microstructure of the sensing gate electrode are the key parameters that influence the sensing mechanism, and hence key performance parameters: sensitivity, selectivity, and response time. By choosing the appropriate temperature and catalyst material (gate material), devices that are significantly sensitive to certain gases may be realized. In addition, the temperature of maximum response varies dependent on the gas species being measured. This information, along with a careful choice of catalyst (gate material) can be used to enhance device selectivity. In chapter 5: Polycrystalline and nano-structured In2O3 thin films were investigated with the aim to obtain information about their NOx and O2 gas sensing properties. The response to these gases was investigated in dependence on the operating temperature and gas partial pressures. The analysis in the presence of different partial pressures of NOx has shown that both thin films are able to detect nitrogen oxide, but their responses exhibit different characteristics. In particular, nano-structured In2O3 thin films were found to have the higher response to NOx. This is most probably due to the enlarged overall active surface area of the sensing layer as a consequence of the small grain size (higher surface to volume ratio) so that the relative interactive surface area is larger, and the density of charged carriers per volume is higher. We have found that reducing the grain size of the sensing material to the ~10 nm regime can have a substantial effect on performance. The optimum detection temperatures of the nano-structured In2O3 occur in the range of 100-175°C for NOx considering the sensitivity as well as the response time. In this range of temperatures the response to O2 is very low indicating that the sensor is very suitable for selective detection of NOx at low temperatures In addition, nano-structured In2O3 thin films were found to be more suitable to be used in the field of application for detecting low partial pressures. Chapter 6: offers conclusions of the current work. In this chapter we compare also all studied gas sensors according to their sensitivity, selectivity, and response time and then we compare them with the related works by other authors available in the scientific literature

The Role of Mediational Discourse in Developing an EFL Teacher’s Oral Corrective Feedback Strategies: A Sociocultural Perspective

Taking sociocultural theory (SCT) as a theoretical framework, this qualitative phenomenological case study investigated the mediational discourse of a series of ten observation and post-observation conferences between a coach and an English language teacher teaching at Iran Language Institute (ILI) in Shiraz, Iran, in order to represent the development of student-focused oral feedback in an EFL teacher over time. Features of verbal mediation already identified by Wertsch (1998) such as shared definition of the task, inter-subjectivity, graduated help, and reasoning existed in the data, while they were insufficient in triggering the teacher to think conceptually about his use of oral feedback to students. Similarly, professional discourse, languaging motives, and teacher dynamicity, alongside the aforementioned meditational features, were revealed to be pivotal on the uptake of conceptual thinking by the teacher. The study was divided into three phases, namely, before, during, and after verbal mediations given by the coach to the teacher participant. The results of the study, along with the previous descriptive and empirical studies, accounted for an obvious role of mediational discourse in the development of teacher’s understanding of conceptual thinking through verbally-mediated activity. The results also found that negotiated help given by the teacher to learners was excessively pivotal in assisting learners to internalize their oral errors. The study concluded with some pedagogical implications for second language teachers to be reflective, dynamic, and evaluative in dealing with oral corrective feedback challenges in their classroom practices

Guanidinoacetic acid supplementation improves feed conversion in broilers subjected to heat stress associated with muscle creatine loading and arginine sparing

It was hypothesized that dietary guanidinoacetic acid (GAA), the precursor of creatine (Cr), would be beneficial to heat-stressed finisher broilers owing to improved cellular energy status and arginine sparing effects. A total of 720 one-day-old male Ross 308 broilers were allocated to 3 treatments, 0 (control), 0.6, or 1.2 g/kg of GAA added to complete corn–soybean meal diets, and were fed for 39 D, with 12 replicates (20 birds each) per treatment. A chronic cyclic heat stress model (at a temperature of 34°C and 50 to 60% relative humidity for 7 h daily) was applied in the finisher phase (day 25–39). Samples were taken on day 26 and 39 to determine thrombocyte, white blood cell, corticosterone, protein and amino acid levels in blood and Cr, phosphocreatine (PCr), and adenosine triphosphate levels in the breast muscle. Meat quality was assessed on day 40 after overnight fasting. Guanidinoacetic acid at a dose of 1.2 g/kg decreased feed-to-gain ratio compared with the control in the grower phase (1.32 vs. 1.35, respectively; P <0.05). In the finisher period, the supplementation of 1.2 g/kg of GAA reduced feed intake compared with the control (–3.3%, P <0.05), whereas both GAA supplementation levels improved feed efficiency markedly (1.76, 1.66, and 1.67 for 0 [control], 0.6, and 1.2 g/kg of GAA, respectively, P <0.05). Mortality outcomes highlight that GAA feeding improved survival during heat stress, supported by lower panting frequency (linear effect, P <0.05). Plasma arginine was higher with increase in dietary GAA concentration on day 26 (+18.3 and + 30.8% for 0.6 and 1.2 g/kg of GAA, respectively; P <0.05). This suggests enhanced availability of arginine for other metabolic purposes than de novo GAA formation. In the breast muscle, PCr (day 39, P <0.05), free Cr (day 39, P <0.05), total Cr (both days, P <0.05), and PCr-to-adenosine triphosphate ratio (day 39, P <0.05) levels were increased with higher GAA content in diet. Guanidinoacetic acid supplementation improved feed conversion and survival during chronic cyclic heat stress, which may be associated with enhanced breast muscle energy status and arginine sparing effect

Antibiotic affects the gut microbiota composition and expression of genes related to lipid metabolism and myofiber types in skeletal muscle of piglets

Background: Early-life antibiotic administration is known to affect gut microbiota and host adiposity, but the effects of antibiotic exposure on skeletal muscle properties remain unknown. The present study evaluated the changes in skeletal muscle properties including myofiber characteristics and composition, as well as intramuscular fat (IMF) content in skeletal muscle of piglets when exposed to a tylosin-containing diet. Results: A total of 18 piglets (28 days of age) were randomly allocated into two groups: control basal diet (Control) and Control + 100 mg tylosin phosphate/kg of feed (Antibiotic). The trial lasted for 39 days. High-throughput amplicon sequencing revealed that no significant difference in initial gut microbiota composition was existed between Control and Antibiotic groups. Antibiotic administration increased body weight and growth rate and decreased feed to gain ratio of pigs (P < 0.05). The carcass lean and fat volumes of pigs were increased by the tylosin administration (P < 0.05). Antibiotic treatment increased myofiber density and the expression of genes related to type I and type IIb myofibers in longissimus muscle (P < 0.05). The IMF content in longissimus muscle was increased by antibiotic exposure (P < 0.05). Antibiotic administration increased expression of genes related to fatty acid uptake and de novo synthesis, and decreased expression of genes related to triglyceride hydrolysis (P < 0.05). Tylosin administration affected taxonomic distribution and beta diversity of the caecal and colonic microbiota of piglets. Conclusion: These results confirm that the growth performance, myofiber composition and muscle lipid metabolism are affected by antibiotic administration, which may be associated with an altered gut microbiota, suggesting that the gut microbiota could be served as a potential target for modulating skeletal muscle properties of host

Effect of a carotenoid-producing Bacillus strain on intestinal barrier integrity and systemic delivery of carotenoids : a randomised trial in animals and humans

The aim of the present study was to investigate effects of the carotenoid-producing Bacillus indicus strain PD01 on intestinal barrier function and its ability to survive passage through the gastrointestinal tract and to assess systemic bioavailability of these carotenoids in vivo. As model for impaired barrier function, 16 early weaned piglets were randomly assigned to a control diet or control diet with PD01 for 23 days. In addition, 67 overweight/obese, otherwise healthy individuals were randomly assigned to groups receiving PD01 or placebo for 6 weeks. PD01 survived passage through the gastrointestinal tract in piglets and human subjects and resulted in significant accumulation of PD01 derived carotenoids (methyl-glycosyl-apo-8'-lycopenoate and glycosyl-apo-8'- lycopene) in human plasma after 3- and 6-weeks supplementation versus baseline (0.044 and 0.076 vs 0 mu M, respectively; p = 0.104). In summary, PD01 survived transit through the gastrointestinal tract, resulted in systemic carotenoid accumulation and improved compromised barrier function outcomes

Short-chain fructo-oligosaccharides supplementation to suckling piglets : assessment of pre- and post-weaning performance and gut health

Farmers face difficulties in redeeming their investment in larger litter sizes since this comes with larger litter heterogenicity, lower litter resilience and risk of higher mortality. Dietary oligosaccharides, given to the sow, proved beneficial for the offspring's performance. However, giving oligosaccharides to the suckling piglet is poorly explored. Therefore, this field trial studied the effect of dietary short-chain fructo-oligosaccharides (scFOS; 1g/day; drenched) supplementation to low (LBW, lower quartile), normal (NBW, two intermediate quartiles) and high (HBW, upper quartile) birth weight piglets from birth until 7 or 21 days of age. Performance parameters, gut microbiome and short-chain fatty acids profile of feces and digesta were assessed at birth (d 0), d 7, weaning (d 21.5) and 2 weeks post-weaning (d 36.5). Additional parameters reflecting gut health (intestinal integrity and morphology, mucosal immune system) were analysed at d 36.5. Most parameters changed with age or differed with the piglet's birth weight. Drenching with scFOS increased body weight by 1 kg in NBW suckling piglets and reduced the post-weaning mortality rate by a 100%. No clear difference in the IgG level, the microbiota composition and fermentative activity between the treatment groups was observed. Additionnally, intestinal integrity, determined by measuring intestinal permeability and regenerative capacity, was similar between the treatment groups. Also, intestinal architecture (villus lenght, crypt depth) was not affected by scFOS supplementation. The density of intra-epithelial lymphocytes and the expression profiles (real-time qPCR) for immune system-related genes (IL-10, IL-1ss, IL-6, TNF alpha and IFN gamma) were used to assess mucosal immunity. Only IFN gamma expression, was upregulated in piglets that received scFOS for 7 days. The improved body weight and the reduced post-weaning mortality seen in piglets supplemented with scFOS support the view that scFOS positively impact piglet's health and resilience. However, the modes of action for these effects are not yet fully elucidated and its potential to improve other performance parameters needs further investigation

Stress-strain behaviour of granular soils under monotonic and cyclic loading conditions

The stress-strain behaviour of granular soils under monotonic and cyclic loading conditions and small lateral strains was studied in this work. A simple cubic triaxial apparatus (SCTA), originally developed at Leeds University for monotonic stress-strain studies of sand under controlled and small lateral strains, was used in this investigation. The three principal stresses and strains can be independentLy controlled and measured in this apparatus. The SCTA was modified and further developed to allow cyclic stress-strain studies of granular soils-to be performed. To increase the stress-strain data available on granular soils similar tests to those previously carried out on the medium sand, were performed on fine and coarse sands under monotonic loading conditions. The samples tested were cubic of 150 mm side length and prepared with ranges of initial porosities in a dry condition. To study the cyclic stress-strain behaviour of sand at small strains, a series of new tests on similar cubic samples of the medium sand were performed under cyclic loading conditions. Cyclic loads with different frequencies, amplitudes and number of cycles were applied and the samples were prepared dry at the loosest and densest conditions. Values of the coefficients of active pressure, earth pressure at rest, constrained secant modulus, Young's modulus and Poisson's ratio for the fine, medium and coarse sands were obtained and compared for different conditions. The relationships between vertical and lateral stresses are found and the volume change behaviour of sands in different conditions are studied. Finally some comparisons are made between the results obtained from monotonic and cyclic loading conditions