Measuring and testing – Gas analysis – Detector detail
Reexamination Certificate
1999-11-23
2001-03-20
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Detector detail
Reexamination Certificate
active
06202473
ABSTRACT:
FIELD OF THE INVENTION
The invention is related to gas sensors. In particular, the invention is related to a gas sensor with a protective gate.
DISCUSSION OF RELATED ART
Gas and vapor sensors have many industrial applications. These applications include the detection of hazardous gas in the workplace for worker safety, enhanced control of the air-fuel mixture and feed in combustion of fuel, increased product yield and reduced waste in a product stream. These applications generally rely on individual sensors, which are installed to detect a single gas or vapor whose concentration may approach a harmful level. The examples of these gases include, but are not limited to, hydrogen sulfide, carbon monoxide, chlorine, ammonia, hydrogen, and methane.
Methods for industrial gas and vapor sensing have evolved basically along two technology paths. The first path involves use of complex analytical instruments, for example infrared spectroscopy, gas chromatography, and mass spectrometry. The development of microcontrollers and microcomputers has led to smaller versions of these analytical instruments. These instruments are very powerful, but they have significant disadvantages, such as being very maintenance intensive. Further, these instruments are sensitive to adverse, corrosive environments and must be located far from the gas or vapor source in a climate controlled enclosure. Thus, a gas (vapor) sample must be transported to the analyzer, as a result real time information is not available. Also, these instruments are expensive, and are thus not affordable candidates for real-time, in-situ applications.
The second technology path for industrial gas sensors developed with chemical sensor technologies. The applications of chemical sensors in chemical processes are well established, and these chemical sensors may be located in process streams to enhance process efficiency and yield, and to reduce waste. For example, water-concentration amount determination in a silicone process feed stream can be enhanced by using chemical sensors to sense water concentrations.
The ability of sensors to identify a target gas depends on several factors. These factors include the sensitivity of the sensor to other interfering gases and vapors, and a concentration of the target gas. The ability to resolve the target gas from other gases is called the selectivity. There are very few known sensors that are highly selective where a sensor has greater than about a tenfold difference in gas detection between sensing states and non-sensing states. Further, within these very few sensors there are even fewer that are relatively reliable to accurately detect individual gases. For example, an unreliable sensor does not provide correct indication of a gas amount when the concentration of an interferant is high. In practice, this limitation is avoided by using a sensor only where a high concentration of interferant is unlikely. This solution, however, limits the effectiveness and uses of a sensor.
Gas sensors have been used in detection of particular undesirable gases in oil-filled electrical transformers. Faults in an oil-filled transformer include arcing (electrical), corona discharge (electrical), low energy sparking (electrical), severe overloading (electrical), pump motor failure (electrical and thermal) and overheating (electrical and thermal) in an insulation system. Faults can generate undesirable gases, such as hydrogen (H
2
), acetylene (C
2
H
2
), ethylene (C
2
H
4
), and carbon monoxide (CO). These fault conditions result in a transformer malfunctioning or indicate an impending malfunction, which, if not corrected, may lead to failure of the transformer. A statistical correlation exists between transformer malfunction and fault gases generated by the transformer. This correlation has use of gas to detect precursors of possible transformer malfunctioning. Accordingly, if an accurate detection of potentially dangerous gases in a transformer is achieved, possible malfunction and failure of the transformer can be addressed and often avoided.
One class of gas detection sensors normally comprise a semiconductor substrate, a thin insulator layer mounted, for example grown, on the semiconductor substrate, and a catalytic metallic gate mounted, for example deposited, on the thin insulator layer. Sensors of this nature are known. The sensors level of sensitivity is different for each gas depending on the gate material. The gate material determines what designated gas will be detected, in other words, the gate material tunes the sensor for a designated gas. Accordingly, while an individual sensor may be useful to detect a single gas to which it is tuned, it will not be as useful to detect other gases. If the sensor is not appropriately tuned, the designated gas may not accurately detected, which of course is undesirable.
Further, known sensors do not provide protection of the sensor components from harmful environments. For example, corrosive gases are often present in a transformer, and adversely affect operation of a sensor. These corrosive gases in a transformer must be kept away from a sensor's catalytic gate in order for the sensor to operate accurately and properly. Also, these known sensors are not protected from water or particulate foreign matter interference, which are often found in transformers.
Some gas sensors have proposed the use of membranes, sieves and discontinuous layers of material to protect the sensor. However, these proposals are not seen to protect the sensor from corrosion, water and foreign particulate materials. The membranes, sieves and discontinuous layers are not secured to the sensor components so they do not adhere to and protect the sensor, especially the gate.
Therefore, it is desirable to provide a sensor that includes protection from corrosive environments, water, and foreign matter. It is also desirable to provide a sensor that can be designed to be tuned so as to selectively pass and detect a designated gas. Also, it is desirable to provide a system for monitoring concentrations of gases in various applications, for example in a manufacturing apparatus and failure mode monitoring of equipment process to power transformers.
SUMMARY OF THE INVENTION
Accordingly, the invention overcomes the above noted deficiencies in known gas sensors. A gas sensor that determines the presence of at least one designated gas in the ambient environment is disclosed in an embodiment of the invention. The gas sensor comprises a semiconductor substrate; a thin insulator layer mounted on the semiconductor substrate; a catalytic metallic gate mounted on the thin insulator layer; and a protective layer mounted on the catalytic metallic gate layer. The protective layer changes at least one of a surface chemical and physical property of the sensor. The protective layer is formed from a material that protects the sensor from corrosive environments and interference from at least one of particulates (minute separate particles that often result when oil degrades) and water. The protective layer enhances the sensitivity of the sensor to a designated gas, and permits the designated gas to pass through to the catalytic metallic gate. The catalytic metallic gate alters a designated gas, for example by heterolytic cleavage (decomposition into charged ions) of a C—H or H—H bond in the gas (herein “alters”). The resultant ionic, atomic hydrogen (H
.
) diffuses through the gate and varies electrical sensitivity of the sensor.
Also, in accordance with another embodiment of the invention, a method for sensing a designated gas in a gaseous state comprises providing a sensor, where the sensor includes a semiconductor substrate; an insulator layer disposed on the semiconductor substrate; a catalytic metallic gate layer disposed on the insulator layer; and a protective layer disposed on the catalytic metal gate. The protective layer changes at least one of the surface chemical and physical properties of the sensor. The protective layer is formed from a material that provides protection of foreign matter and water. The pr
Gui John Yupeng
Stokes Edward Brittain
Bennett Bernadette M.
General Electric Company
Johnson Noreen C.
Politzer Jay L.
Williams Hezron
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