Patent Application
20080216558
Embodiments are generally related to gas sensors. Embodiments are also related to the field of surface acoustic wave (SAW) based CO2 sensors. Embodiments are additionally related to CO2 sensors using carbon nanotubes as the sensitive layer, either alone or in conjunction with organic or inorganic materials to improve the sensitivity and selectivity.
Gas sensors are used in many industrial, medical, and commercial applications. For example, oxygen sensors are used in the monitoring of combustion engine environments to increase engine performance and reduce emission of green house gases. Ammonia sensors are important for monitoring ambient ammonia concentration since it is related to many environmental issues such as acidification, human health and climate change through particle formation. In addition to controlling industrial processes and monitoring air quality, CO2 sensors are also widely used in food and medicine packages as means of detecting spoilage. Most gas sensors available in the market today are either optical based or operate by measuring the impedance of a capacitor coated with a gas responsive polymer(s) or ceramic such as heteropolysiloxane, BaTiO3, CeO2/BaCO3/CuO, Ag2SO4, Na2CO3 and SnO2 etc.
Low cost CO2 sensors are very important for indoor air quality monitoring, medical applications and also for environmental CO2 monitoring. Most of the present day commercially available CO2 sensors are infrared sensors and are very expensive. Apart from these devices, there are reports about the fabrication of CO2 sensors based on metal oxide semiconductors, which operate only at high temperature. This shows the importance for an alternative technology for fabricating low cost CO2 sensors which operate at room temperature and/or at elevated temperature.
The application of multiwall carbon nanotubes for remote query detection of carbon dioxide, oxygen, and ammonia based upon the measured changes in multiwall nanotubes permittivity and conductivity with gas exposure has been reported. In the prior art the sensor consists of a printed inductor-capacitor resonant circuit that is first coated with a protective, electrically insulating SiO2 layer followed by a second layer of gas-responsive multiwall carbon nanotubes-SiO2 mixture with the SiO2 matrix acting to physically bind the multiwall carbon nanotubes to the sensor.
A CO2 sensor made up of a substrate and a nanostructure disposed over the substrate where in the nanostructure comprises of a carbon nanotube or a network of nanotubes is also reported. Here two conductive elements are disposed over the substrate and electrically connected to the nanotube. This sensor is fabricated using field-effect transistor based device design.
Gas sensors can be fabricated by combining surface acoustic wave (SAW) devices and nanotechnology. The SAW devices are known to be highly sensitive to the slightest changes happening on the piezoelectric substrate surface, and the carbon nanotubes are highly desirable for adsorption and detection of gases due to their hollow centre, nanometer size, and large surface area. Hence it is believed that the combination of these two technologies together with other organic or inorganic materials or a mixture of both organic and inorganic materials will result in highly sensitive devices.
The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide for an improved surface acoustic wave (SAW) based CO2 sensors that can operate at room temperature and/or at elevated temperature.
It is another aspect of the present invention to provide for CO2 sensors using carbon nanotube as the sensitive layer.
The aforementioned aspects and other objectives and advantages can now be achieved as described herein. Sensors are fabricated by combining SAW devices and nanotechnology. The device structure in general consists of the gas sensitive material between the input and output interdigital transducers (IDTs) of a SAW device. Carbon nanotubes are used as the gas sensitive material for CO2. The CO2 gas gets adsorbed on nanotubes when the carbon nanotube based SAW sensor is exposed to CO2 at room temperature, which in turn changes conductivity of the carbon nanotube. This conductivity change will affect the velocity of the SAW traveling across the nanotubes and will give a frequency change which corresponds to the percentage of the CO2 molecules adsorbed by the nanotubes.
To fabricate the prototype of the above mentioned devices, the first step in general is to deposit the nanotubes directly on to a piezoelectric substrate, for example LiNbO3 or Quartz. The nanotube deposition can be carried out by any of the methodologies available in the literature. After the deposition, the nanotubes strip can be patterned. Then the input and output IDTs can be deposited with nanotubes strips laying exactly between them. All these process can be carried out by the standard photolithographic technique using positive or negative photo resist. Characterization of the prototype device can be carried out using an oscillator circuit and impedance analyzer
The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein.
The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof.
Referring to
The CO2 sensor device 105 can be standardized by applying an electric signal 110 to the input inter digital transducer 120. The input signal causes a surface acoustic wave 140 to be induced in the piezoelectric substrate 160 and propagate towards the output inter digital transducer 150. The shape of the generated surface acoustic wave 140 depends on the electric signal applied to the input inter digital transducer 120, the design and orientation of inter digital transducer fingers, and the piezoelectric material used. When the wave reaches the output inter digital transducer 150 a voltage is induced across the fingers of the interdigital transducer 150 which is then represented by an output signal 170 from the device. The shape of the output wave is affected by the design of the output inter digital transducer 150. The CO2 gas gets adsorbed on the carbon nanotube material 130 when the CO2 sensor device 100 is exposed to CO2 at room temperature. As a result the velocity of the surface acoustic wave 140 traveling across the nanotubes can be changed either due to the mass loading effect or electro acoustic interaction which in turn gives rise to a change in output 170 from the device. This change corresponds to the percentage of the CO2 molecules adsorbed by the nanotubes.
A similar idea can be worked out with the nanotubes or wires of the oxide or non-oxide semiconductors. Other than carbon nanotubes, the nanotubes can be made of any one or a combination of: silicon, oxide, gallium, nitride, silver, and other known semiconductors. Alternatively the SAW based device 105 can be comprised of SiO2 protective layer only on top of the input and output inter digital transducers 120 and 150. This will help to prevent the shorting of the device with the nanotubes deposited between the input and output interdigital transducers and thereby improve the sensitivity. This may also help to deposit the nanotubes on all over the piezoelectric substrate without restricting them only between the interdigital transducers. This may also reduce an additional photolithographic step in the whole process.
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The nanotube material can be formed of nanotubes including at least one material from the group comprising: heteropolysiloxane, nano-crystalline, BaTiO3, CeO2, BaCO3, CuO, Ag2SO4, Na2CO3, SnO2, BaTiO3, La2O3, CaCO3, CuO—SrTiO3BaSnO3, CuO—BaTiO3, Sm2O3, and Y2O3. The nanotube material can also be formed of nanotubes including a combination of at least two materials from the group comprising: carbon, silicon, semiconducting oxides, gallium-nitride, and silver.
The surface acoustic wave CO2 gas sensor can include a SiO2 protective layer disposed on top of two metallic interdigital transducers to prevent them from shorting with each other or with said nanotube material deposited between said two metallic interdigital transducers. The surface acoustic wave CO2 gas sensor can be provided in the form of a Rayleigh SAW (R-SAW) device. The surface acoustic wave CO2 gas sensor can also be provided in the form of a shear horizontal SAW (SH-SAW) device
It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
