DESC:FIELD OF THE INVENTION
The present invention relates to semiconductor material used for hazardous gas detection sensors. More particularly, the present invention relates to a vanadium dioxide (VO2) based sensor and an arrangement facilitated by said sensor for sensing and measurement at room temperature.
BACKGROUND OF THE INVENTION
Rapid increase in atmospheric pollution is adversely affecting health and environment. Hence there is a requirement for the development of inexpensive, small and simple measurement devices for measuring or monitoring the harmful gas levels. The research and development of various types of gas sensors are in progress for the continuous monitoring of harmful gas concentration in a highly sensitive and selective manner.
Nitrogen dioxide (NO2) is one of the most known air pollutant in the environment from industrial wastage, automobiles and power plants. The continuous inhalation of NO2 gives rise to throat irritation, lung infections and reduces the capability of the respiratory system. Also, NO2 causes photo-chemical smog and acid rain. Increasing concentration of NO2 is a major concern for air quality in cities. Therefore, the development of highly sensitive NO2 gas sensors is important for timely detection in order to protect the ecosystem as well as human beings.
Various types of solid-state gas sensors have been developed using an appropriate sensor material in bulk or in thin film form. Thin film metal oxide semiconductor materials were found to have high sensitivity towards gas measurement. Metal oxide semiconductors, such as SnO2, TiO2, ZnO, V2O5, WO3 and In2O3 have been widely used for detection of toxic gases. The working principle of the metal oxide semiconductor gas detector is based on change in electrical parameter like resistance of the sensor material due to adsorption of gas. The main problem associated with these types of sensors is that the operating temperature should be of few hundred degrees Celsius and so a heater is essentially required to keep the sensing film at a predefined temperature. Hence, the design and development of these sensors becomes complicated due to the heating requirement of the sensing material and need of thermal isolation between the sensor and the electronic circuit.
For example, Prajapati et al. developed highly sensitive NO2 gas sensor using RF sputtered WO3 (tungsten oxide) thin films at the operating temperature of 150 ºC. Wojcik et al. achieved very good NO2 response with drop cast synthesized WO3 material at 300 ºC operating temperature. Schneider et al. showed V2O5 thin films has high sensitivity to NO2 in the operating temperature of few hundred degree Celsius. Cho et al. disclosed ZnO nanostructures-based gas sensors proved to be an excellent material for NO2 gas sensing at temperature above 200 ºC. Though the synthesis of the above sensing materials is simple, highly reproducible and less cost, the cost of the sensor built with these materials will become costly due to the addition of heaters. Even though the gas sensing materials can be decorated with reduced graphene oxide or gold nanoparticles to give very good response at room temperature, the process of synthesis will become difficult, less reproducible and costly referring to Jyoti et al. and Wang et al.
Further, US patent 5567622 discloses a fiber optic chemical dosimeter system for the detection of NO2 gas. In this invention, calorimetric sensors that react selectively with the target gas is used. These devices are large, expensive and are not suitable for commercial gas detector applications and are mainly used for gas sensing at rocket launch sites.
Another US patent 5866075 discloses a gas sensing device for sensing ammonia and nitrogen oxide gases at room temperature. In the invention, cuprate material Y:Ba:Cu:O (YBCO) and Bi:Sr:Ca:Cu:O (BSCCO) is used as gas sensing material. But the sensor materials showed instability in sensor response and the sensitivity is less for low NO2 concentration of less than 5 ppm.
Yet another publication WO95/00836 discloses the selectivity of various gas sensing materials to specific gases. In this invention, WO3 thin films were used for the NO2 gas sensing. WO3 thin films showed high sensitivity to NO2 at operating temperature of 500ºC.
Thus, various attempts have been made to develop high sensitive NO2 gas sensors using different types of materials. But these sensors show high sensitivity only at elevated operating temperature of more than 150 ºC and needs heater for the operation of the sensor thereby increasing the complexity in the design and fabrication of the sensor. So, there is a dire need of gas sensors having high sensitivity towards the target gas at room temperature.
OBJECTIVE OF THE INVENTION
These objectives are provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This objective are not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
An important objective of the present invention aims at providing a compact solution for the shortcomings of the above mentioned sensors.
Another objective of the current invention is to synthesis VO2 based metal oxide sensor for room temperature gas sensing.
Further objective of the present invention is to eliminate the requirement of heater and the power consumption.
Yet another objective of the current invention is to provide an arrangement capable of measuring low concentration of NO2.
Another objective of the current invention is to provide a compact sensor of very small device size (1mm x 3 mm).
Object of the present invention is not limited to the above mentioned problem. Other technical problems that are not mentioned will become apparent to those skilled in the art from the following description.
SUMMARY OF THE INVENTION
According to an aspect of the invention, a sensor for sensing nitrogen dioxide (NO2) gas at room temperature is provided. The said sensor comprises a semiconducting sensing material deposited on a substrate. The sensing material is a vanadium dioxide (VO2) and the substrate is a silicon (Si).
According to another aspect of the invention, an arrangement for measuring nitrogen dioxide (NO2) gas at room temperature is provided. The said arrangement comprises a sensor for sensing nitrogen dioxide gas, said sensor including a semiconducting sensing material deposited on a substrate, wherein said sensing material is vanadium dioxide (VO2) and said substrate is silicon (Si). The arrangement includes a sensor holder that supports the sensor and a sensor chamber that houses both the sensor and the sensor holder. The arrangement further comprises an air supply and a nitrogen dioxide gas supply each having a mass flow controller connected to said sensor chamber, said mass flow controller controls the flow of said air and said nitrogen dioxide gas to said sensor chamber. The arrangement includes a detection circuit that communicates with the sensor and measures output from said sensor in response to exposure of said sensor to said nitrogen dioxide gas. A display and buzzer (32) connected to said detection circuit (30) for displaying and alarming a concentration of said nitrogen dioxide gas based on said output from said sensor (10).
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawing, illustrating by way of example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in greater detail with reference to an embodiment which is illustrated in the drawing figures:
Figure 1 is a flow chart shows synthesis of vanadium dioxide (VO2) based thin film sensor, according to an embodiment of the present invention;
Figure 2 shows X-ray diffraction (XRD) graph of VO2 thin films on Si substrate by Pulsed laser deposition (PLD);
Figure 3 shows a sensor for detection of nitrogen dioxide (NO2), according to an embodiment of the present invention;
Figure 4 is a block diagram showing an arrangement for measuring nitrogen dioxide (NO2) gas at room temperature, according to an embodiment of the present invention;
Figure 5 shows a graph between sensitivity of VO2 thin films versus a concentration of NO2; and
Figure 6 is a flow chart shows sensor signal measurement using electronic circuit, according to an embodiment of the present invention.
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.
DETAILED DESCRIPTION OF THE INVENTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
Referring to figure 1 to 6, a sensor (10) and an arrangement (20) for sensing and measuring nitrogen dioxide (NO2) gas at room temperature is shown. The sensor (10) comprises a semiconducting sensing material (14) deposited on a substrate (12). The sensing material (14) is a vanadium dioxide (VO2) and the substrate (12) is a silicon (Si). In the present invention, vanadium dioxide based thin film sensors (10) are synthesised by Pulsed laser deposition (PLD) technique. The sensing material (14) has a thickness in the range of 90 - 100 nm. The optimised deposition parameters of vanadium dioxide thin film deposition by PLD are substrate temperature 600 °C, target to substrate distance 3.5 cm, base pressure = 5 x 10 -6 mbar, working pressure = 2.6 x 10 -2 mbar (20 mtorr), laser energy= 185 mJ, number of shots = 5000 and laser frequency = 5 Hz.
Referring to Figure 2, a graphical representation of an X- ray diffraction (XRD) plot of the as deposited vanadium dioxide thin film deposited on silicon is shown. It is seen that the XRD peaks are perfectly aligned with the monoclinic phase of VO2 (14). Also, it is observed that the deposited VO2 thin films (14) were of polycrystalline in nature.
The gas sensing of metal oxide semiconductors (MOS) like VO2 (14) depends on the type of target gas (oxidizing or reducing) and the type of the material itself (n or p-type semiconductors). The chemical reaction in gas sensing mechanism can be described by the surface adsorption and desorption of target gas molecules and its reaction with chemisorbed oxygen ions (O2-, O2-). VO2 thin film (14) behaves as a metal oxide semiconducting material. When VO2 thin films (14) are exposed to air, the chemisorbed oxygen ions (O2-, O2-) are created on the sensor's surface, due to the extraction of free electrons. When NO2 gas is exposed on the sensor’s (10) surface, it reacts with chemisorbed oxygen ions, and takes more numbers of electrons from VO2 semiconductor film (14) due to oxidizing nature of NO2 gas. This result in a decrease in resistance of the VO2 thin film (14).
Referring to Figure. 3, a pictorial representation of the sensor (10) is shown wherein the VO2 gas sensing material (14) deposited on Si substrate (12) with a silver paste or gold interdigitated electrode (IDE) pattern (16) with contact pad realised on the VO2 thin film. It is observed that covering the surface of the sensor material (10) with metal coating (16) does not affect the sensitivity. Sensors (10) with the different types of metal coatings (16) showed similar response to NO2 gas, that is the response of the VO2 sensor (10) is same when the contacts are taken from the film by silver or gold. In an embodiment, the size of the sensor (10) can be of 1mm x 3mm. However, the size of the sensor (10) may vary in other alternative embodiments of the present invention.
Referring to Figure 4, a block diagram of an arrangement for measuring nitrogen dioxide (NO2) gas at room temperature is shown. The arrangement (20) comprises the sensor (10), a sensor holder (22), a sensor chamber (24), synthetic air (79 % Nitrogen and 21 % Oxygen) and a nitrogen dioxide gas supply cylinders (26a, 26b), a detection circuit (30), a display and buzzer device (32), and a Keithley signal measurement unit (34). The sensor (10) is fixed on the sensor holder (22) and positioned inside the sensor chamber (24). During measurement mixture of synthetic air and NO2 gas from the supply cylinders (26a, 26b) is passed through the sensor chamber (24). The sensor (10) having contact pads that are connected to the Keithley signal measurement unit (34) through contact probes. The detection circuit (30) is connected to the contact pads of the VO2 sensor using copper wires. The sensitivity or response of the sensor (10) to the target gas NO2 is measured as the change in resistance. The mixer of synthetic air and NO2 is passed through the sensor chamber (24) and the measurement was carried out by switching on and off the NO2 gas through the sensor chamber (24). The flow rates of gases are independently controlled by mass flow controllers (MFCs) (28). The gas sensing measurements are performed at room temperature in the range of 25 ºC to 45 ºC and more preferably at 30 ºC and NO2 gas concentration of 1 ppm to 4.5 ppm. The responsivity of the sensor (S) is defined as the ratio of the relative change in resistance in presence of air and nitrogen dioxide to the resistance in the air.
In operation, the polycrystalline VO2 thin film (14) in the sensor (10) acts as a metal oxide semiconductor and its resistance decreases when exposed to NO2. The change in resistance is measured using Keithley signal measurement unit (34). The response of the sensor (10) to NO2 gas at room temperature at different concentrations of NO2 is shown in Figure 5.
Also, the change in resistance can be detected by the microcontroller based electronic circuit which can display the concentration of NO2 gas concentration in ppm and give alarm when the concentration reaches Threshold Limit Value (TLV) which is 3 ppm for NO2 gas. The process of detecting gas sensor signal using electronic circuit is shown in the flow chart in figure 6.
To check the selectivity of the VO2 sensor, the gas sensing measurement is carried out with other gases like H2 and NH3. It is observed that the sensor showed high selectivity to NO2 compared to H2 and NH3. The response of sensor was found to be less than 0.01 % for H2 and NH3.
For the VO2 sensor the response time is found to be in the range of 80 – 90sec, particularly 85 sec, and recovery time is found to be in the range of 85 -95sec, particularly 90 sec. The response time is measured as the time required for obtaining 90 % of saturation value during loading of NO2 gas into the sensor chamber and the recovery time is measured when the sensor response gets 10 % from the saturation value during deloading of NO2 gas.
The sensor according to the present invention which is made of VO2 thin film on Si substrate can sense at room temperature eliminating the requirement of heater and power consumption. This sensor is even capable of sensing low concentration of NO2. The room temperature sensing capability of the sensor makes it possible to fabricate the sensor and detection circuits on the same semiconductor chip, hence the sensor is very compact. The synthesis of VO2 thin films is simple, highly reproducible and cost effective. VO2 thin films deposited on Si substrate don’t need any other heat treatment or decorative processes. This NO2 gas detector can be successfully used for security and industrial safety applications.
While the preferred embodiment of the invention has been illustrated, it will be obvious to those skilled in this art that other embodiments may be readily designed within the scope and teachings thereof.
Although there has been shown and described the preferred embodiment of the present invention, it will be readily apparent to those skilled in the art that modifications may be made thereto which do not exceed the scope of the appended claims. Therefore, the scope of the invention is only to be limited by the following claims.

,CLAIMS:
1. A sensor (10) for sensing nitrogen dioxide (NO2) gas at a room temperature, comprising:
a semiconducting sensing material (14) deposited on a substrate (12), said semiconducting sensing material (14) forming a thin layer on the substrate (12) by pulsed laser deposition (PLD) technique, said thin layer of sensing material (14) is provided with a metal coating (16), wherein
said sensing material (14) is vanadium dioxide (VO2), and
said substrate (12) is silicon (Si).
2. The sensor (10) as claimed in claim 1, wherein said thin layer has a thickness in the range of 90 - 100 nm.
3. The sensor (10) as claimed in claim 1 or 2, wherein the metal coating (16) is selected from the group consisting of gold, and silver.
4. The sensor (10) as claimed in claim 1, wherein the size of the sensor (10) is 1mm x 3mm.
5. The sensor (10) as claimed in claim 1, wherein the room temperature is in the range of 25 ºC to 45 ºC and preferably at 30 ºC.
6. An arrangement (20) for measuring nitrogen dioxide (NO2) gas at room temperature, the arrangement comprising:
a) a sensor (10) for sensing nitrogen dioxide gas, said sensor (10) comprising a semiconducting sensing material (14) deposited on a substrate (12);
b) a sensor holder (22) for supporting said sensor (10);
c) a sensor chamber (24) housing said sensor (10) and sensor holder (22);
d) an air supply (26a) and a nitrogen dioxide gas supply (26b) each having a mass flow controller (28) connected to said sensor chamber (24);
e) a detection circuit (30) communicating with said sensor (10) for measuring output from said sensor (10) in response to exposure of said sensor (10) to said nitrogen dioxide gas; and
f) a display and a buzzer (32) connected to said detection circuit (30) for displaying and alarming a concentration of said nitrogen dioxide gas based on said output from said sensor (10).
7. The arrangement (20) as claimed in claim 6, further comprises a Keithley signal measurement unit (34) configured to measure the change in resistance of the sensor (10).
8. The arrangement (20) as claimed in claim 6, wherein said semiconductor sensing material (14) is vanadium dioxide (VO2) and said substrate (12) is silicon (Si).
9. The arrangement (20) as claimed in claim 6, wherein said mass flow controller (28) controls the flow of said air and said nitrogen dioxide gas to said sensor chamber (24);
10. The arrangement (20) as claimed in claim 6, wherein said arrangement (20) facilitates measurement of concentration of nitrogen dioxide gas as low as 1 ppm.
11. The arrangement (20) as claimed in claim 6, wherein said sensor (10) has a response time in the range of 80 – 90 sec and a recovery time in the range of 85 - 95 sec.
12. The arrangement (20) as claimed in claim 10, wherein the response time is measured as the time required for obtaining 90 % of saturation value during loading of nitrogen dioxide gas into the sensor chamber (24) and the recovery time is measured when the sensor response gets 10 % from the saturation value during deloading of nitrogen dioxide gas.