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We studied transient chemical speciation during high-temperature solid material pyrolysis and combustion in air. Our objective was to develop a database of chemical burn signatures. The material tested was the thermoplastic PMMA. Material samples were heated in an infrared furnace until they pyrolyzed, ignited, and combusted in air. Time-resolved quantitative measurements of the exhaust species CO2, O2, hydrocarbons, and CO along with exhaust gas temperature were obtained. Two categories of experiments were conducted: (1) pyrolysis tests in which there was no combustion; (2) combustion tests with chemical reaction and heat release. During heating, the sample underwent numerous processes that appear as diagnostic sequences. In the pyrolysis tests, as the furnace temperature was raised, the CO and hydrocarbon (HC) signals underwent transition from one peak to two peaks. In the combustion tests, spontaneous ignition occurred at higher test temperatures as evidenced by an exothermic reaction reported by the thermocouples, leading to three-peak CO and HC profiles. The measured O2/CO2 ratio of 1.3 ± 0.1 agreed with stoichiometric methyl-methacrylate monomer decomposition. Calculations of the power output using two independent methods supported (1) that combustion was experimentally observed in the furnace, and (2) the accuracy of the combustion gas analysis. Copyright © 2012 John Wiley & Sons, Ltd.

We have developed a high-temperature gas sensing system for the detection of combustion products under harsh conditions, such as an energy plant. The sensor, based on the wide band gap semiconductor silicon carbide (SiC), is a catalytic gate field-effect device (Pt–SiO 2 –SiC) that can detect hydrogen-containing species in chemically reactive, high-temperature environments. The hydrogen response of the device in an industrially robust module was determined under both laboratory and industrial conditions (1000 sccm of 350 °C gas) from 52 ppm to 50% H 2 , with the sensor held at 620 °C. From our data we find that the hydrogen adsorption kinetics at the catalyst–oxide interface are well fitted by the linearized Langmuir adsorption isotherm. For hydrogen monitoring in a coal gasification application, we investigated the effect of common interferants on the device response to a 20% H 2 gas stream. Within our signal to noise ratio, 40% CO and 5% CH 4 had no measurable effect and a 2000 ppm pulse of H 2 S did not poison the Pt sensing film. We have demonstrated the long-term reliability of our SiC sensor and the robustness of the sensor packaging techniques, as all the data are from a single device, obtained during 5 days of industrial measurements in addition to ∼480 continuous hours of operation under laboratory conditions.

Excitation of hexanuclear molybdenum complexes such as Mo6Cl12 and its derivatives in the ultraviolet results in a strongly red-shifted luminescence centered at 750nm. Since oxygen efficiently quenches the luminescence, these thermally stable inorganic complexes are candidate lumophores for real-time, high temperature optical fiber based sensing of oxygen. Sol-gel films containing the acetonitrile complex of Mo6Cl12 were deposited on quartz substrates by dip coating. After drying, the films were heated at 200∘C for 1 h. The luminescence lineshapes of films before and after heating were unchanged, indicating that heating did not adversely affect the cluster photophysics. Compared to solutions of the acetonitrile complex, quenching by oxygen was smaller in the as-prepared films, but heating at 200∘C for 1 h increased the quenching, apparently due to increased oxygen permeability resulting from the loss of water or other small molecules from the matrix. These results confirm the potential of hexanuclear molybdenum complexes such as Mo6Cl12⋅2CH3CN as the lumophores in fiber optic oxygen sensors that can operate up to 200∘C.

The ability to identify, via remote sensing, the identity of materials prior to self-ignition is important for fire safety. We have developed a system to monitor, in real time, the thermal and chemical properties of specific materials under well controlled heating conditions. Our goal is to develop a library of the pre-combustion “signature” of materials commonly present in buildings. These signatures focus on the gases produced by combustion: CO, CO 2 and total hydrocarbons; as well as oxygen, an essential reactant. An infrared furnace is used to rapidly (few seconds) heat (up to 800 °C) a small sample of combustible material, while output gases are monitored, with 1 s resolution, using a multi-component gas analyzer. Systematic measurements of PMMA, a thermoplastic, are presented. Ultimately these results will be used to provide first responders with remote knowledge of the type of fire prior to entering a site.

Oxygen plays a ubiquitous role in terrestrial biological processes such as respiration and chemical process, i.e. oxidation-reduction reactions. We have developed an optical technique for monitoring oxygen in both gas and liquid phases utilizing nanoscale metal-halide clusters immobilized in a porous matrix. The sensor monitors the luminescence intensity or lifetime from inorganic hexanuclear molybdenum chloride clusters (Mo 6 Cl 12 ) which is efficiently quenched by ground state 3O 2 . Control of the material properties of the matrix at the nanometer scale is requires for fast quantitative output from our reflection mode fiber based sensor. For aqueous applications we can monitor dissolved oxygen in the 0.2–17 mg/L range at temperatures of 10–25 °C. The sensor output was validated using a commercial electro-chemical sensor and the Stern-Volmer fit is linear over the entire measurement range. In the gas phase our device can monitor oxygen with a response time, t 95 , of 30s in the 0.1 – 21 % range.

The goal of this research is to obtain quantitative information on chemical speciation over time during high temperature material thermal decomposition. The long term goal of the research will be to impact structural fire safety by developing a data base of characteristic “burn signatures” for combustible structural materials. In order to establish procedure and to generate data for benchmark materials, the first material tested in these preliminary tests is poly-methyl-methacrylate (PMMA). Material samples are heated in an infrared (IR) heating chamber until they undergo pyrolysis. Time resolved quantitative measurements of the exhaust species CO2 , O2 , HC, and CO were obtained. During heating the PMMA sample undergoes two distinct processes. First, pre-combustion pyrolysis is characterized by the appearance a peak in the THC signal between 600–650 °C. Secondly, at about 900 °C flaming combustion occurs as evidenced by an exothermic reaction reported by the thermocouples. The time sequence of the production of HC, O2 depletion and CO2 production are consistent with combustion in an excess-oxidizer environment.Copyright © 2009 by ASME

We report on a reflection-mode fiber-optic oxygen sensor based on the 3O2 quenching of the red emission from hexanuclear molybdenum chloride clusters. Measurements of the probe operating in a 0%–21% gaseous oxygen environment have been obtained, a range suitable for biological and automotive applications. The luminescence signal increases with decreasing oxygen concentration in accordance with theory. We observe clearly resolvable steps in the sensor response for changes of 0.1% absolute oxygen concentration in the 0%–1.0% range. The response time of the fiber probe is theoretically predicted to be 1 s.

Dissolved oxygen concentration is considered the most important water quality variable in fish culture. Reliable and continuous (24/7) oxygen monitoring of dissolved oxygen (DO) in the 1-11 mg/L range would be of great benefit to the aquaculture industry. We briefly review selected DO sensors from both the Clark and optical sensor categories as well a comparing their differences, both advantages and disadvantages. We introduce our fiber optic technique for continuous monitoring of DO levels in a flowing aqueous stream. An inorganic molybdenum chloride compound was used as the optical indicator in the oxygen sensitive material. The reflection mode fiber sensor is based on the ability of oxygen to quench the red luminescence of the molybdenum chloride indicator. The advantage of our broad-band, optical dissolved oxygen sensor is shown and discussed.

We present a new apparatus for measuring the response times of a gas sensor with millisecond resolution, while also capturing the slower components of the response such as the steady state value. Laser induced fluorescence (LIF) imaging was used to quantify the exchange rate of the sensor's ambient gas. The millisecond response of high temperature (up to 950 K) field effect SiC sensors for detection of hydrogen containing gases was characterized.

We report on the development of an optical sensor suitable for remote monitoring of oxygen in high temperature environments. The sensor is based on /sup 3/O/sub 2/ quenching of the red emission from hexanuclear molybdenum chloride clusters. A reflection mode fiber optic probe is being developed by immobilizing Mo/sub 6/Cl/sub 12/ in a porous sol-gel matrix at the far end of a high temperature silica fiber. The sensor signal as a function of temperature was determined from in-situ spectroscopic measurements, demonstrating operation up to at least 200/spl deg/C. One of the advantages of the inorganic Mo-cluster is the lack of sensitivity to gases typically present in a coal-fired boiler due to the triplet nature of the transition from the excited to ground state.

A sensor based on the wide bandgap semiconductor, silicon carbide (SiC), has been developed for the detection of combustion products in power plant environments. The sensor is a catalytic gate field effect device that can detect hydrogen containing species in chemically reactive, high temperature environments. For these capacitive sensors we have determined that the optimum sensor operating point in terms of sensor lifetime and response time is at midgap. Detailed measurements of the oxide leakage current as a function of temperature were performed to investigate the high temperature reliability of the devices. In addition, robust metallization and electrical contacting techniques have been developed for device operation at elevated temperatures. To characterize the time response of the sensor responses in the millisecond range, a conceptually new apparatus has been built. Using laser induced fluorescence imaging techniques we have shown that the gas underneath the sensor can be completely exchanged with a time constant under 1 millisecond. Ultrahigh vacuum studies of the surface chemistry of the platinum gate have shown that sensor deactivation by adsorbed sulfur is a possible problem. Investigations on the chemical removal of sulfur by catalytic oxidation or reduction are continuing.

A reflection mode optical sensing system for monitoring oxygen in high temperature gas flows, 25-220degC and 500 sccm, is presented. The sensor is based on 3O2 quenching of the red emission from K2Mo6Cl14, one member of a family of hexanuclear molybdenum chloride clusters. One advantage of using inorganic Mo-clusters as the lumophore is the lack of sensitivity to gases typically present in a coal-fired boiler due to the triplet nature of the transition from the excited to ground state. The sensor is prepared by dip-coating fiber tip in a sol-gel solution containing K2Mo6Cl14 particle clusters. Drying and aging yields a composite film with K2Mo6Cl14 immobilized in a porous silica matrix. The composite film deposited on a planar substrate was characterized via in-situ spectroscopic measurement up to 200degC. The emission intensity is constant following 56 cumulative hours of heating at 200degC, demonstrating long-term stability at high temperature

We show that SiC based metal-insulator-semiconductor (MIS) sensors above 700 K respond to reducing/oxidizing gases by two independent mechanisms: a chemically induced shift in the metal-insulator barrier height and the passivation/creation of charged states at the SiO/sub 2/-SiC interface. The relative magnitude of the two phenomena is determined by the position of the Fermi energy at the insulator-semiconductor interface, i.e. the capacitor bias. The interface state contribution is much slower than the work function shift. For short response times and high sensor-to-sensor reliability, we find that the optimal operating bias of a gas sensor is near midgap. The combination of photoemission measurements and capacitance measurements can discriminate between the two sensor mechanisms.

We report on the development of a fiber based oxygen sensor for power plant applications. The sensor utilizes quenching of the bright red fluorescence from inorganic molybdenum chloride (Mo/sub 6/Cl/sub 12/) clusters by ground state /sup 3/O/sub 2/. We are developing a reflection mode fiber sensor by immobilizing the Mo/sub 6/Cl/sub 12/ clusters in an oxygen permeable sol-gel matrix at the far end of a Au clad high temperature silica fiber (continuous operation to 700 /spl deg/C). In contrast to organic indicators these metal halide compounds are well-suited for oxygen sensing schemes in harsh environments. In this paper we report on the high temperature photophysical parameters of the Mo/sub 6/Cl/sub 12/ clusters obtained via absorption and emission spectroscopy in a controlled gas environment.

Mo{sub 6}Cl{sub 12}, a cluster compound whose luminescence depends on the ambient concentration of oxygen, is the basis for a real-time oxygen sensor for combustion applications. Previously, the properties of Mo{sub 6}Cl{sub 12} have largely been studied at room temperature; these studies have now been extended to 200 C. Optical microscopy shows that Mo{sub 6}Cl{sub 12} undergoes a steady change in color as it is heated from room temperature to 200 C, changing from canary yellow to crimson and then back to canary yellow. Concurrent thermal gravimetric analyses show a small weight loss for Mo{sub 6}Cl{sub 12} that is consistent with loss of water or HCl from the clusters. These changes are reversible. Absorption and fluorescence emission spectroscopy of Mo{sub 6}Cl{sub 12} heated to 200 C for two hours shows no change in the photophysical parameters compared to the control sample that was not heat cycled.

We describe the use of HNQ (2-hydroxy-1,4-naphthoquinone or Lawsone) as a potential sweat sensor material to detect the hydration levels of human beings. We have conducted optical measurements using both artificial and human sweat to validate our approach. We have determined that the dominant compound that affects HNQ absorbance in artificial sweat is sodium. The presence of lactate decreases the reactivity of HNQ while urea promotes more interactions of sodium and potassium ions with HNQ. The interactions between the hydroxyl group of HNQ and the artificial sweat components (salts, lactic acid, and urea) were investigated comprehensively. We have also proposed and developed a portable diode laser absorption sensor system that converts the absorbance at a particular wavelength range (at 455 ± 5 nm, where HNQ has an absorbance peak) into light intensity measurements via a photocell. The absorbance intensity values obtained from our portable sensor system agrees within 10.4% with measurements from a laboratory based ultraviolet-visible spectrometer. Findings of this research will provide significant information for researchers who are focusing on real-time, in-situ hydration level detection.

The implementation of SiC based sensors and electronics for operation in chemically harsh, high temperature environments depends on understanding the SiO2/SiC interface in field effect devices. We have developed a technique to fabricate wedge polished samples (angle ∼ 1×10−4 rad) that provides access to the SiO2/SiC interface via a surface sensitive probe such as xray photoelectron spectroscopy (XPS). Lateral scanning along the wedge is equivalent to depth profiling. Spatially resolved XPS images of the O 1s and Si 2p core levels were obtained of the interfacial region. Samples consist of device-quality thermally grown oxides on 4H-SiC single crystal substrates. The C 1s spectrum suggests the presence of a graphitic layer on the nominally bare SiC surface following thermal oxidation.

We report on an optical oxygen sensor for aqueous media. The phosphorescent signal from the indicator, K2Mo6Cl14, immobilized in a polymer matrix, is quenched by ground state O32. Continuous measurements (Δt=10 s) over 36 h in oxygen atmospheres (0%–21%) were obtained with a signal to noise ratio better than 150. Photobleaching was not observed over ∼13 000 measurements. The senor response at 10, 22, and 37 °C water is governed by bimolecular collisional quenching, as evidenced by a linear fit to the Stern–Volmer equation for dissolved oxygen in the range 0