Hochtemperaturstabile Mikroheizer für miniaturisierte Sensorsysteme

During the past decade the development of microheater devices has been an active field of investigation. Driver applications for such devices have been micromechanically fabricated metal oxide gas sensors. In the majority of these investigations standard silicon micro-machining technologies have been employed to produce thermally insulated membrane structures, which feature low levels of heating power consumption (~50 mW at 400°C) and small thermal response times (~10 ms). Up to the present time the heating of such structures is generally accomplished by passing currents through evaporated or sputtered films of noble metals, like platinum (Pt). Using such material combinations long-term stable device operation has been found feasible up to about 600°C. Over the years, an increasing demand of microheaters with higher operating temperatures for gas sensing systems as well as for thermal infrared (IR) sources in optical sensor systems arose. The currently employed metal heaters, however, have limited stability above 600°C and fail rapidly when operating at higher temperatures. The objective of the present thesis therefore was finding alternative heater materials and metallisation techniques to enable microsystems with prolonged operating temperatures of about 800°C and beyond. As a representative high-temperature application, micromachined thermal infrared (IR) emitters have been realised and their performance characterised. The electric heating of the emitter hotplate is achieved with electrically conducting heater structures employing different kinds of metallic and semiconductor materials. In order to assess the long-term performance of these heater elements, accelerated degradation tests had been performed and the results compared to each other. The results clearly revealed that doped semiconductors provide a high-temperature performance that is superior to metallic ones. The best results so far have been obtained with antimony-doped tin oxide (SnO2:Sb) heaters. This kind of heater metallisation combines the moderate electrical conductivity of doped semiconductors with the advantage of complete oxidation stability of stoichiometric oxides.In the second part of this thesis detailed investigations of this new kind of heater metallisation are presented to reveal the high-temperature degradation mechanisms that limit the lifetime of SnO2:Sb heater elements. The results show that - unlike most other heater materials - the high-temperature stability of SnO2:Sb is not limited by electromigration but rather by outdiffusion and evaporation of Sb impurities from the SnO2 bulk. Continuing such heating experiments towards higher temperature revealed that SnO2:Sb coated hotplates can be heated up to the melting temperature of silicon (~1410°C) before they get destroyed. Aiming at even higher temperatures microheaters based on silicon carbide (SiC) have been realised. Apart from a much higher mechanical strength, SiC has a substantially higher temperature stability (up to ~2300°C). The acquired knowledge leads to microheater devices with an estimated lifetime of 10 years at operation temperatures up to 950°C. Heater elements based on SnO2:Sb films therefore form a very useful complement to the existing silicon micromachining technology that opens up interesting new applications in the field of high-temperature MEMS devices.In den vergangenen Jahren wurden von Mikroheizern für die Gassensorik, aber auch für thermische Infrarotquellen zunehmend höhere Betriebstemperaturen gefordert. Bei Temperaturen von über 600°C stoßen die bisher verwendeten Heizermaterialien jedoch an ihre Leistungsgrenzen. Ziel dieser Doktorarbeit war es daher, alternative Materialien und Technologien zu erforschen, welche einen langzeitstabilen Betrieb dieser Mikro systeme auch bei Temperaturen von 800°C und mehr ermöglichen. Als repräsentative Hochtemperaturanwendung wurden thermische IR-Emitter mit unterschiedlichen Heizermetallisierungen mikromechanisch aus Silizium und Siliziumkarbid hergestellt und auf ihre Eigenschaften hin untersucht. Die gewonnenen Erkenntnisse führten schließlich zur Entwicklung einer hochtemperaturfähigen Heizermetallisierung aus Antimon-dotiertem Zinnoxid (SnO2:Sb), welche einen permanenten Betrieb bei Temperaturen von bis zu 950°C über einen Zeitraum von mindestens 10 Jahren ermöglicht. Diese Tatsache eröffnet der Mikrosystemtechnik ein weites Feld neuer Anwendungsgebiete im Hochtemperaturbereich

The effects of ozone on immune function.

A review of the literature reveals that ozone (O3) exposure can either suppress or enhance immune responsiveness. These disparate effects elicited by O3 exposure depend, in large part, on the experimental design used, the immune parameters examined as well as the animal species studied. Despite the apparent contradictions, a general pattern of response to O3 exposure can be recognized. Most studies indicate that continuous O3 exposure leads to an early (days 0-3) impairment of immune responsiveness followed, with continued exposures, by a form of adaptation to O3 that results in a re-establishment of the immune response. The effects of O3 exposure on the response to antigenic stimulation also depend on the time at which O3 exposure occurred. Whereas O3 exposure prior to immunization is without effect on the response to antigen, O3 exposure subsequent to immunization suppresses the response to antigen. Although most studies have focused on immune responses in the lung, numerous investigators have provided functional and anatomical evidence to support the hypothesis that O3 exposure can have profound effects on systemic immunity

The mechanism and mitigation of niacin-induced flushing

AimsTo summarise the metabolic responses to niacin that can lead to flushing and to critically evaluate flushing mitigation research.Methods and resultsThis comprehensive review of the mechanism of action of niacin-induced flushing critically evaluates research regarding flushing mitigating formulations and agents. Niacin induces flushing through dermal Langerhans cells where the activation of G protein-coupled receptor 109A (GPR109A) increases arachidonic acid and prostaglandins, such as prostaglandin D(2) (PGD(2)) and prostaglandin E(2) (PGE(2)), subsequently activating prostaglandin D(2) receptor (DP(1)), prostaglandin E(2) receptor (EP(2)) and prostaglandin E receptor 4 (EP(4)) in capillaries and causing cutaneous vasodilatation. Controlling niacin absorption rates, inhibiting prostaglandin production, or blocking DP(1), EP(2) and EP(4) receptors can inhibit flushing. Niacin extended-release (NER) formulations have reduced flushing incidence, duration and severity relative to crystalline immediate-release niacin with similar lipid efficacy. Non-steroidal anti-inflammatory drugs (NSAIDs), notably aspirin given 30 min before NER at bedtime, further reduce flushing. An antagonist to the DP(1) receptor (laropiprant) combined with an ER niacin formulation can reduce flushing; however, significant residual flushing occurs with clinically-relevant dosages.ConclusionsNiacin is an attractive option for treating dyslipidemic patients, and tolerance to niacin-induced flushing develops rapidly. Healthcare professionals should particularly address flushing during niacin dose titration

Microfabricated Formaldehyde Gas Sensors

Formaldehyde is a volatile organic compound that is widely used in textiles, paper, wood composites, and household materials. Formaldehyde will continuously outgas from manufactured wood products such as furniture, with adverse health effects resulting from prolonged low-level exposure. New, microfabricated sensors for formaldehyde have been developed to meet the need for portable, low-power gas detection. This paper reviews recent work including silicon microhotplates for metal oxide-based detection, enzyme-based electrochemical sensors, and nanowire-based sensors. This paper also investigates the promise of polymer-based sensors for low-temperature, low-power operation

Influence of a montmorency cherry juice blend on indices of exercise-induced stress and upper respiratory tract symptoms following marathon running—a pilot investigation

Background: Prolonged exercise, such as marathon running, has been associated with an increase in respiratory mucosal inflammation. The aim of this pilot study was to examine the effects of Montmorency cherry juice on markers of stress, immunity and inflammation following a Marathon. Methods: Twenty recreational Marathon runners consumed either cherry juice (CJ) or placebo (PL) before and after a Marathon race. Markers of mucosal immunity secretory immunoglobulin A (sIgA), immunoglobulin G (IgG), salivary cortisol, inflammation (CRP) and self-reported incidence and severity of upper respiratory tract symptoms (URTS) were measured before and following the race. Results: All variables except secretory IgA and IgG concentrations in saliva showed a significant time effect (P < 0.01). Serum CRP showed a significant interaction and treatment effect (P < 0.01). The CRP increase at 24 and 48 h post-Marathon was lower (P < 0.01) in the CJ group compared to PL group. Mucosal immunity and salivary cortisol showed no interaction effect or treatment effect. The incidence and severity of URTS was significantly greater than baseline at 24 h and 48 h following the race in the PL group and was also greater than the CJ group (P < 0.05). No URTS were reported in the CJ group whereas 50 % of runners in the PL group reported URTS at 24 h and 48 h post-Marathon. Conclusions: This is the first study that provides encouraging evidence of the potential role of Montmorency cherries in reducing the development of URTS post-Marathon possibly caused by exercise-induced hyperventilation trauma, and/or other infectious and non-infectious factors