Electronically conducting metal oxide nanoparticles and films for optical sensing applications
Inventors
Ohodnicki, JR., Paul R. • Wang, Congjun • Andio, Mark A.
Assignees
Publication Number
US-8836945-B1
Publication Date
2014-09-16
Expiration Date
2033-06-26
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Abstract
The disclosure relates to a method of detecting a change in a chemical composition by contacting a conducting oxide material with a monitored stream, illuminating the conducting oxide material with incident light, collecting exiting light, monitoring an optical signal based on a comparison of the incident light and the exiting light, and detecting a shift in the optical signal. The conducting metal oxide has a carrier concentration of at least 1017/cm3, a bandgap of at least 2 eV, and an electronic conductivity of at least 10−1 S/cm, where parameters are specified at the gas stream temperature. The optical response of the conducting oxide materials is proposed to result from the high carrier concentration and electronic conductivity of the conducting metal oxide, and the resulting impact of changing gas atmospheres on that relatively high carrier concentration and electronic conductivity. These changes in effective carrier densities and electronic conductivity of conducting metal oxide films and nanoparticles are postulated to be responsible for the change in measured optical absorption associated with free carriers. Exemplary conducting metal oxides include but are not limited to Al-doped ZnO, Sn-doped In2O3, Nb-doped TiO2, and F-doped SnO2.
Core Innovation
The invention relates to a method of detecting a change in a chemical composition of a gas stream at elevated temperatures by using an electronically conducting metal oxide material. This material has a carrier concentration of at least 10¹⁷/cm³, a bandgap of at least 2 eV, and an electronic conductivity of at least 10⁻¹ S/cm at the gas stream temperature. The method involves contacting the conducting oxide material with a portion of the gas stream, illuminating it with incident light, collecting exiting light that is transmitted, reflected, or scattered by the material, and monitoring an optical signal based on the comparison of the incident and exiting light to detect a shift indicating a change in chemical composition.
The background describes the problem of needing sensors capable of operating in harsh environments at elevated temperatures (at least 100° C.) where conventional metal oxide sensors are limited by low temperature stability, weak optical signal dynamic range, and reliance on noble metal incorporation or complex sensor designs like fiber Bragg gratings. Existing metal oxide sensors also have issues with long response times, instability under reducing conditions, and temperature limitations below about 100° C. The invention addresses the need for improved optical sensing materials that exhibit measurable, irreversible, and monotonic optical responses at elevated temperatures without the need for noble metals or complex sensor enhancements.
The core innovation is based on the discovery that conducting metal oxides with high carrier concentration can produce significant shifts in optical signals in response to changing gas atmospheres at high temperatures. These shifts arise predominantly from changes in free carrier density and electronic conductivity of the material induced by varying reducing or oxidizing gas concentrations. Exemplary materials include doped oxides like aluminum-doped zinc oxide (AZO), indium tin oxide (ITO), Nb-doped TiO2, and F-doped SnO2. The optical response can be measured through transmittance, reflectance, or absorption changes and can be enhanced by factors such as surface roughness causing light scattering.
Claims Coverage
The patent claims include 20 claims focusing on methods for detecting changes in chemical composition using conducting oxide materials with specified physical properties. The claims cover various embodiments including composition, temperature conditions, optical signal characterization, and sensor configurations.
Use of conducting metal oxide with specified physical properties for gas composition detection
A method of detecting changes in chemical composition of a gas stream by contacting a conducting oxide material comprising a conducting metal oxide characterized by a carrier concentration of at least 10¹⁷/cm³, bandgap of at least 2 eV, and electronic conductivity of at least 10⁻¹ S/cm at the gas stream temperature, illuminating it, collecting exit light (transmitted, reflected, or scattered), monitoring an optical signal via optical spectroscopy, and detecting a shift in the optical signal to identify the composition change.
Conducting metal oxide compositions with crystalline lattice structures and doping
The conducting metal oxide has an empirical formula MaOb or AyBxOz with the first and second elements forming crystalline lattice structures such as cubic, hexagonal, tetragonal, orthorhombic, monoclinic, rhombohedral, or triclinic and occupying special lattice positions. Exemplary doped oxides include Zr1-xAlxO3, In2-xSnxO3, and Ti1-xNbxO2.
Operation at elevated temperatures with defined optical signal shift sensitivity
Methods specifying monitoring gas streams at temperatures at least 100° C. or preferably 200° C. and detecting optical signal shifts of at least 0.1% in signal-averaged optical signals. The optical signals include features such as shifts in optical signal edges between UV-visible (250-550 nm) and near to mid-infrared ranges (1000-3750 nm). The conducting oxide material may exhibit surface roughness of at least 15 nm to enhance scattering effects.
Sensing changes in reducing or oxidizing gas composition
Detecting changes in concentrations of reducing gases (e.g., H2, CO, NH3, hydrocarbons) or oxidizing gases (e.g., O2, O3, NOx, SOx, halogens, acids) by monitoring carrier concentration induced shifts in the optical signal of the conducting oxide material.
Use of a barrier layer to selectively contact the conducting oxide material
Incorporating a barrier layer between the gas stream and the conducting oxide material to filter or control gas species contacting the sensor, enhancing selectivity or stability.
Integration with waveguides for optical interrogation
Placing the conducting oxide material in contact with a waveguide core and illuminating it through the waveguide to monitor optical signal shifts for sensing.
Simultaneous monitoring of electrical resistance and optical signals
Measuring both electrical resistance and optical signals of the conducting oxide material to provide complementary information for improved accuracy in gas sensing.
Use of a sensing head and interrogator instrument for concentration determination
Using a sensing head comprising the conducting oxide material in data communication with an interrogator and meter to produce and display a measurand corresponding to the optical signal shift, thereby determining the concentration of chemical species in the monitored gas stream.
The claims cover a method of optical gas sensing using conducting metal oxide materials with defined carrier concentration, bandgap, and conductivity properties. The inventive features include the material compositions, temperature operating ranges, optical signal characteristics, and sensor configurations involving barriers, waveguides, and simultaneous electrical monitoring. These features collaboratively define a novel optical sensing methodology for detecting and quantifying changes in gas compositions at elevated temperatures.
Stated Advantages
Enables gas sensing optical responses at elevated temperatures (above 100° C.) where typical sensors fail.
Avoids the need for noble metal incorporation or complex sensor designs like fiber Bragg gratings, reducing cost and complexity.
Provides reversible, monotonic optical signal shifts corresponding to gas concentration changes, enabling quantitative analysis.
Allows simultaneous optical and electrical monitoring for improved sensor accuracy and functionality.
Surface roughness-mediated light scattering enhances optical signal sensitivity and dynamic range.
Documented Applications
Gas composition monitoring in harsh, high temperature environments such as fossil-fueled power plants, carbon capture technologies, coal gasification, solid oxide fuel cells, and advanced boiler systems.
Optical sensing of reducing gases such as hydrogen, carbon monoxide, ammonia, and hydrocarbons, as well as oxidizing gases including oxygen, ozone, nitrogen oxides, and sulfur oxides.
Use in optical fiber-based sensor devices and instruments for real-time monitoring of gaseous streams at elevated temperatures.
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