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Fine-Tuning of Ceramic-Based Sensor

March 1, 2006 By: Peter Adrian

This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.

The vital need to develop fast responding sensors for toxic gases before their concentration reaches lethal levels is driven by events such as the Sago coal mine disaster, which took place on January 2, 2006, in Tallmansville, West Virginia, where 12 miners died. Key interests of Abdul-Majeed Azad, Chemical and Environmental Engineering, University of Toledo, in this area have lead to the development of a novel technique for detecting low levels of carbon monoxide (CO) using tungsten oxide (WO3) and molybdenum oxide (MoO3).

Challenges, such as high selectivity, enhanced sensitivity, and short response time, which are highly dependant on the nanofeatures of materials used in solid-state ceramic-based chemical sensors, are some of the aspects that have been addressed in this research. This has been augmented by the concept that nanofeatures, such as dimensionality and size, provide a better understanding of the material's behavior, such as its chemical, mechanical, and optical properties.

This technique is based on rigorous thermodynamic considerations of metal/metaloxide coexistence and has resulted in a novel redox technique to enhance sensor behavior. By modulating the oxygen partial pressure across the equilibrium metal/metal oxide (M/MO) proximity line, the formation and growth of new oxide surface on an atomic/submolecular level under conditions of 'oxygen deprivation' has been achieved in potential sensor materials.

Speaking to Sensor Technology Abdul said, "We believe that we have identified a technique based on sound thermodynamic principles whereby morphological and microstructural modifications can be brought out in a semiconducting oxide by using a novel gas phase redox scheme. We are not aware of such attempts in the published literature. By precisely modulating the oxygen potential slightly lower or slightly higher than that existing in the vicinity of a given oxide, we can cause atomic/molecular level reduction or oxidation of the given ceramic oxide. The concept and technique of 'oxygen deprivation' for such changes on the atomic/molecular level is new and novel and has not been exploited hitherto."

Some of the specific potential applications of the latest sensors developed are largely due to the innovative modifications done at the sensor material level. Such sensors become very useful in situations where sensing of noxious gases, such as CO, methane, nitrogen oxides, and hydrogen sulfide (H2S), in the shortest period of time and with the highest sensitivity is necessary. The sensitivity of sensors made with these systems is much higher than that of the same oxides prepared by traditional techniques because of the innovative modification done to the sensor material as well as to the presence of unique microstructural features, such as a large surface area, high-aspect ratio, and textured and oriented grains. This is one of the major drivers of this technology in the area of effective and timely hazardous gas detection. Complementing this are the results obtained in the case of tungsten oxide-based CO sensors (in the range of 14 ppm to 100 ppm CO) with enhanced sensing characteristics.

Abdul added, "We have shown by ample research that sensors with extraordinary functional features can be developed by incorporating in them nanofibrillar structures. Thus, sensors with such attributes would find applications in the areas of air quality control and monitoring, furnace environment, carburization processes, environmental detection of nitrogen oxide (NOx) and H2S in coal power plants, methane (CH4) and CO in coal mines (Sago coalmine disaster is an example, where fast responding sensors could have saved lives), and CH4 and H2S in solid oxide fuel cell anodes."

Currently, Abdul is collaborating with Sheikh Akbar, professor at the Ohio State University, on the research aspects. On the commercialization endeavor, he is working with Essential Research Inc., based in Cleveland, OH.

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