FIELD OF INVENTION:

The present invention relates to field of sensor for detecting ammonia gas. More particularly the present invention relates to a clad-modified fiber optic ammonia gas sensor for selective sensing of ammonia gas among various gas environment and method thereof.

BACKGROUND OF THE INVENTION:

Ammonia emissions from agricultural waste may exceed the normal level in the atmosphere and lead to pollution which is harmful to human beings and living things. Many types of sensors have been proposed for monitoring and controlling the ammonia pollution. The gas sensitivity and selectivity of the sensors are to be improved and have become an intense research.

Gas sensors using optical fibers are very attractive as they are simple, small and low cost. Gas detection and monitoring could be done at inaccessible and explosive environments [1-3]. The intensity of a transmitted light through an optical fiber is very sensitive to optical properties (refractive index, absorption etc.,) of a cladding and clad-modification technique is widely used in the fiber optic based gas sensor [4-8]. In this, a small portion of a passive cladding is removed and replaced with a gas sensing material whose optical properties change during the gas interaction.

Metal oxides (SnO2, ZnO and TiOa) based ammonia gas sensors are being developed extensively as they are more sensitive to NH3 and also easy to fabricate. These sensors are, traditionally, of electrical resistive-type whose resistance varies when they are exposed to the detecting gas. However, these sensors exhibit enhanced gas sensitivity only at high operating temperatures (above 300 °C) and also sufficient response to many gases (CO, methanol, NH3, CH4 ). Hence, the gas sensitivity of metal oxide sensors are being improved by changing their physical properties by doping, annealing or changing their size (nanomaterials) for ambient temperature operation as well as for gas selectivity [9-14].

The ZrtO is one of the promising gas sensing materials as it is more abundance in nature, low cost and nontoxic [15,16]. It is also a very good optical and optoelectronic material [17]. Optical fibers clad-modified with ZnO and SnO2 have been studied for sensing NH3 and butane gases at room temperature [18,19 & 6]. ZnO is suitable for doping with elements like Fe, Sb, Ce and Al. Doped ZnO is being widely studied in electrical-resistive type of gas sensors for improving the gas sensitivity and selectivity [20-23]. Dopant acts as a catalyst, produces structural changes, or introduces electronic energy bands, which enhance gas sensing properties [21]. Doping of ZnO with different materials may find potential applications in the clad- modified fiber optic gas sensor as the optical properties are different. However, the gas effects on optical properties of doped ZnO have been rarely reported.

TiO2 are widely used as gas sensing medium in electrical resistive type sensors because of its high gas sensitivity and thermal stability. Pure TiO2 show significant sensitivity to reducing gases like methanol, ethanol etc., and gas selectivity has been improved by doping with small amounts of Pt, Nb and La.

PRIOR ART:

Sensors &Transducers Journal 88(2), 2008,40-46, discloses a comparative study of annealed and irradiated ZnO thin films for room temperature ammonia gas sensing. The studies revealed that increased sensitivity of irradiated films for NH3 sensing.

Sensors and Actuators B 146(1) 2010, 331-336 discloses optical sensing of ammonia using ZnO nanostructure grown on a side polished optical-fiber. The nanostructured sensor element demonstrated a substantially higher sensitivity due to its structure compared to the only smooth and porous samples.

Sensors and Actuators B: 126(2) 2007, 368-374 discloses Cr203-activated ZnO thick film resistors for ammonia gas sensing operable at room temperature.

Physics and Biophysics, 2009, V, Part 7, 303-306, discloses ZnO films, undoped and doped with Er, Ta and Co, for sensitivity to exposure to NH3.

Sensors and Actuators B: 114(2), 2006, 910-915 discloses NH3 gas sensor based on Fe203-Zn0 nanocomposites at room temperature. The results of electrical and sensing measurement indicated that the sensor with Fe:Zn = 2% exhibited fairly excellent sensitivity and selectivity to NH3 at room temperature.

Colloids and Surfaces A: 276(1-3) 2006, 59-64 discloses effect of Pd2 + doping on ZnO nanotetrapods ammonia sensor and found that Pd2+ doped ZnO nanotetrapods showed high sensitivity to ammonia in the range of 30-1000 ppm.

Sensors and Actuators B: 115(1) 2006, 128-133 discloses pure and Ru02-doped zinc oxide for sensing ammonia.

Thin Solid Films 516 (2008) 3338-3345 discloses higher NH3 sensitivity of 4 at.% Ni doping of zinc oxide thin films.

Materials science & engineering. B, vol. 137(1-3), 2007, 53-58 discloses Preparation and gas-sensing properties of Ce-doped ZnO thin-film sensors by dip-coating. The paper concluded that 5 at% Ce-ZnO thin-film sensors show good selectivity to alcohol, and thus can serve as alcohol-sensing sensor.

Molecular and Quantum Acoustics vol. 27, (2006) 133 discloses investigations of thin film of titanium dioxide (TiO2) in a surface acoustic wave gas sensor system and observed a good result to ammonia gas.

Proceedings of IMECE2008 discloses dye doped clad modified evanescent optical fiber (CMEOF) sensor array for the detection of aqueous-ammonia. The dyes used were Bromocresol green and phenol red.

A poster presentation of International Conference on Optics and Photonics CSIO, 2009 discloses development of optical fiber chemical sensor for detection of ammonia. In the designing of the sensor cladding modification methodology was used. Ammonia sensitive layer of the PPy-PVS thin film has been used as an active coating on the optical fiber sensing probe. Interaction of ammonia with PPy-PVS coated sensing probe changes the optical power at the detector. Sensor shows linear response to (1-10 ppm) ammonia.

Sensors and Actuators B: 97(2-3), 2004,174-181 discloses evanescent sensing of alkaline (NH3) and acidic (HCI) vapors using a plastic clad silica fiber doped with poly(omethoxyaniline).

The prior art discussed above discloses various sensing materials for ammonia gas. Earlier ZnO was used as ammonia sensing material but it demands the need of higher temperature for sensing ammonia. Latter annealed and irradiated ZnO thin films were used for sensing ammonia gas at room temperature. Further ZnO was doped with various metals for sensing ammonia. Doped ZnO is being widely studied in electrical-resistive type of gas sensors for improving the gas sensitivity and selectivity. Since nanocrystalline materials exhibit greater surface activity, nanocrystalline zinc oxide was also used in sensing ammonia.

In the recent years, Fiber Optics based sensors are being preferred over conventional chemical sensors, because of their large dynamic range, selectivity, sensitivity, simplicity and cost effectiveness. Clad modifying technology for gas sensing in fiber optics is in the current trend. From the cited prior art it is evident that clad modified fiber optic gas sensor using dyes, PPy-PVS, etc as sensing material for ammonia detection is known.

To summarize thin film semiconducting metal oxide sensors suffers cross sensitivity from other gasses thereby limiting the selectivity. Thin film semiconducting metal oxides suffers cross-sensitivity at least to humidity and other vapors or gases due to different adsorption and reactivity properties of analytes. Further fiber optic based sensors with existing sensing materials suffer disadvantages, for example the use of polymer/volatile sensing materials necessitates relatively cool gas temperatures (i.e., generally <100.degree. C.).

Thus there still exists a need for a simple, economic and convenient method to make an optical fiber sensor for ammonia sensing by implanting a nanocrystalline semiconducting metal oxides or semiconducting doped metal oxides using clad-modification technology to achieve at least double the sensitivity and higher selectivity among other gases, compared to any of the techniques disclosed in the prior art.

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OBJECT OF THE INVENTION:

The object of the present invention is to develop a clad-modified fiber optic ammonia gas sensor which has advantages of small size, low cost, fast response, remote and distributed sensing capability, resistance to severe environments, and immunity to electrical noise for detecting ammonia.

Another object of the present invention is to use nanocrystalline semiconducting metal oxide as sensing material in clad-modified fiber optic sensor for sensing ammonia.

Yet another object of the present invention is to use doped semiconducting nanocrystalline metal oxide as sensing material in clad-modified fiber optic sensor for sensing ammonia.

Yet another object of the present invention is to develop clad-modified fiber optic gas sensor having high sensitivity for detecting ammonia using nanocrystalline semiconducting metal oxide or doped semiconducting nanocrystalline metal oxide as sensing material.

Yet another object of the present invention is to develop clad-modified fiber optic gas sensor having high selectivity for detecting ammonia among various gas environments using nanocrystalline semiconducting metal oxide or doped semiconducting nanocrystalline metal oxide as sensing material.

Yet another object of the present invention is to develop clad-modified fiber optic gas sensor having rapid response time and capable of rapid regeneration for detecting ammonia using nanocrystalline semiconducting metal oxide or doped semiconducting nanocrystalline metal oxide as sensing material.

Yet another object of the present invention is to use nanocrystalline TiO2 as sensing material in clad-modified fiber optic gas sensor

Yet another object of the present invention is to use nanocrystalline cerium doped ZnO as sensing material in clad-modified fiber optic gas sensor.

Yet another object of the present invention is to provide a sensitive and selective method for detecting ammonia gas among various gas environments utilizing the developed clad-modified fiber optic ammonia gas sensor.

SUMMARY OF THE INVENTION:

The present invention relates to a clad modified fiber optic type gas detecting apparatus for selectively detecting ammonia gas amongst plurality of gases. The apparatus comprising of a light source means for introducing an optical beam into the fiber optic core, a sensing means for selectively sensing ammonia and a detector means for providing transmission properties of the optical fiber which represents ammonia concentration. The sensing means is a selective sensing means which comprises of an optical transmission medium which includes plurality of layers arranged around the fiber optic core. The first layer which is arranged around the core is a first passive cladding layer and forms the surface of the transmission medium. A second nano crystalline selective sensing layer is coated around the core of the optic fiber in a removed portion of the passive cladding layer. Any change in the optical properties of the modified cladding material due to the presence of ammonia gas changes the transmission properties of the optical fiber which represents ammonia concentration. Nanocrystalline TiO2, and nanocrystalline cerium doped zinc oxide (Ce-ZnO) have been explored as second nanocrystalline selective sensing layer, for improving gas sensing properties as nanocrystalline materials exhibit a greater surface activity. In the present work, the gas sensing properties of a clad modified optical fiber with nanocrystalline cerium doped zinc oxide (Ce-ZnO) and titanium dioxide (TiO2) are studied with ammonia, methanol and ethanol gases at room temperature. The output characteristics of the sensor are analyzed with a model. The time response of the sensor is reported.

BRIEF DESCRIPTION OF DRAWINGS:

Figure 1 Illustrate the schematic diagram of a fiber optic gas sensor.

Fig.2a Illustrates the powder XRD pattern of Ce-ZnO

Fig.2b Illustrates the powder XRD pattern of TiO2

Fig. 3a Illustrates the SEM micrographs of Ce-ZnO sample

Fig. 3b Illustrates the SEM micrographs of TiO2 sample.

Fig.4a Illustrates the Spectral response of Ce-ZnO for ammonia.

Fig.4b Illustrates the Spectral response of TiO2 for ammonia.

Fig.5 (a) Illustrates the total internal reflection and evanescent field when ncore > nmClad

Fig 5 (b) Illustrates the partial reflection (leaky mode) when ncore< nmclad

Fig.6 Illustrates the gas sensing mechanism in the proposed fiber optic sensor

Fig.7 Illustrates the absorption spectra of Ce-ZnO in air, ammonia and methanol environments.

Fig.8a Illustrates the graph between a peak intensity and gas concentration for Ce-ZnO for ammonia, methanol and ethanol.

Fig.8b Illustrates the graph between a peak intensity and gas concentration for TiO2 for ammonia, methanol and ethanol.

Fig.9a Illustrates the time response of a sensor for Ce-ZnO with ammonia gas. Fig.9b Illustrates the time response of a sensor for TiO2 with ammonia gas.

DETAILED DESCRIPTION OF THE INVENTION:

The present invention relates to a clad modified fiber optic type gas detecting apparatus for selectively detecting ammonia gas amongst plurality of gases. The invention also discloses the method of detecting the presence of ammonia using clad modified fiber optic type gas detecting apparatus. The invention utilizes nanocrystalline semiconducting metal oxide or doped semiconducting nanocrystalline metal oxide as sensing material in clad-modified fiber optic sensor for sensing ammonia.

The schematic diagram of the optical fiber gas sensor is shown in Figure 1. It consists of a white light source (Model SL1, Stellar Net Inc., USA) with emission wavelengths from 100 to 2000 nm and a miniature fiber optic spectrometer (EPP-2000, Stellar Net Inc., USA) with spectral response from 100 to 1100 nm. The signal from the spectrometer is interfaced with a computer and the spectral graphs are recorded separately. Multimode plastic (PMMA) step index optical fiber (length 42 cm, diameter 750 micron and numerical aperture 0.51) is used with cleaved ends. The refractive index of the core is 1.492 and cladding 1.402. The refractive index of ZnO is 1.901. The refractive index of TiO2 is 2.38.

Sensor region was prepared by removing mechanically the cladding of the fiber up to the core surface for a length of 3 cm. The surface of the clad-removed region was polished with 1000 grit sheet by monitoring with optical microscope for uniformity. The polished surface was cleaned, coated with cerium (6 at%) doped zinc oxide and titanium dioxide by dip-coating method and then dried in air. The thickness of a coating was about 30 microns. A conical flask was used as a gas chamber in which the fiber optic sensor region was inserted (Fig.l). Different concentrations (0, 50,100,150 ...500 ppm) of ammonia, methanol and ethanol solutions were prepared separately and taken in a circular bottom flask for the study. Vapors produced from the flask were directly passed into the gas chamber through the gas inlet (Fig.l) without any carrier gas and gas regulation. The outlet of the gas chamber was exposed to the atmosphere. The spectral response of the sensor was recorded for each concentration after 10 minutes by allowing the solutions to produce sufficient vapors. Experiments were done in a dark room at room temperature and normal atmospheric pressure conditions. The level of relative humidity in the laboratory was around 71%.

Cerium doped ZnO samples was synthesized through hydrothermal method. Stochiometric amount of zinc (Zn (II) hydroxyl complex) and dopant precursor (cerium nitrate,) was taken. Ammonia solution was used as a precipitating agent and poly ethylene glycol (0.1 M) as a stabilizer for controlling the particle size and shape. The precursor and solution were stirred continuously for 20 min until a white slurry appeared and then transferred to teflon lined autoclave (100 ml capacity) and heated for 24 hours at 160 °C. The precipitates formed were washed with ethanol and distilled water repeatedly for removing unreacted chemicals. Finally the product was dried at room temperature and taken for various characterizations. The refractive index of ZnO (1.901) would be modified when it is doped with the cerium whose refractive index is 2.36. The change may be small as lower concentration of dopant (6 at %) is present. For the study, the refractive index of the doped ZnO is considered to be approximately as that of undoped ZnO.

TiO2 nanocrystalline materials were prepared by chemical method. Titanium (iv) n- butoxide (6 ml) was hydrolyzed using anhydrous ethanol (43 ml) as the solvent under constant stirring. After stirring for five minutes, hydrochloric acid (1 ml) and water added into the solution. The solution was successively stirred till a clear and transparent sol was obtained. The TiO2 sol forms gel immediately. The gel was allowed for some time (30 minutes) to settle. The excess solvent is evaporated by drying the gel in open air at 60 °C for 6 hours. The dried powders are denoted as as-prepared powders and are annealed to higher temperatures. The dried powders contain residual organic solvents which will be removed by the successive calcinations.

The phase and microstructure of the synthesized samples were studied using X-ray diffractometer (Model Ultima III, Rigaku, Japan) (XRD) and scanning electron microscope (Model 3000H, Hitachi, Japan) (SEM), respectively.

The XRD diffraction patterns of Ce doped zinc oxide and TiO2 are shown in the figure 2. The XRD patterns of doped ZnO sample is similar to pure ZnO, which could be indexed as a single phase hexagonal with wurtzite structure without any additional impurity peaks. It implies that the dopant atom Ce is substituted or replaced for Zn atom in the ZnO lattice. The synthesized doped ZnO powders are polycrystalline in nature (Fig 2a).

Powder X-ray diffraction (XRD) patterns of the synthesized TiO2 sample confirm that the sample crystallizes predominantly in anatase phase (Fig. 2b).

The crystallite size of doped ZnO and TiO2 samples could be estimated from the width of the XRD peak corresponding to 2 0 using the Debye-Scherrer formula, dma = where 8 peas 6 d^ is the average grain size, A is the wavelength of Cu Kal radiation, /? is the full width at half maximum (FWHM) and d is the diffraction angle. To eliminate the additional instrument broadening, FWHM is corrected using the FWHM from a large grained Si sample. The average crystallite size of Ce-ZnO and TiO2 samples are found to be about 29 and 10 nm, respectively.

Figure 3(a) shows the SEM images of Ce doped zinc oxide nanoparticle. It appears from the image that the particle sizes are larger than the above estimated crystallite sizes. This is because, the SEM gives images of the particles formed due to combination or aggregation of two or more crystals whereas the Debye-Scherrer formula gives an estimated average crystallite size of particles. The Ce doped ZnO exhibits hexagonal shaped nanoparticles and are highly aggregated.

The scanning electron micrograph of TiO2 is shown in Figure 3b, which shows that the particles are agglomerated.

The figures 4 (a & b) show the spectral responses of Ce doped zinc oxide and TiO2 exposed to ammonia of concentrations 0-500 ppm at room temperature. The spectra are found to be similar for doped zinc oxide and TiO2. The spectra exhibit three peaks around 689, 790 and 941 nm, which is a characteristic spectrum of the optical fiber used [6]. These characteristics suggest that the spectra undergo only intensity changes with doped zinc oxide and different gases. The peak intensity around 689 nm exhibits more variations with the concentration of the gas compared to others. It is found that the intensity of the spectra increases for ammonia gas with the increase in the concentration whereas it decreases for ethanol and methanol.

In clad-modified fiber optics sensors, the output light intensity variations with gas could generally be attributed to evanescent wave absorption in the modified cladding or changes in the refractive index of it [24,4,5]. When a light undergoes total internal reflection at the interface of core-modified cladding (Fig.5a), not all its intensity is reflected back at the interface. A part of a light intensity penetrates into the cladding material. This is called evanescent field and its intensity decays exponentially away from the interface [24]. The depth of penetration (dp) of evanescent field in the clad material is related to the angle of incidence 0, refractive index of a core ni and cladding n2 and a wavelength of the light X as When a gas interacts with the evanescent field, the intensity of the propagating light in the fiber undergoes changes and the output light intensity of the sensor varies.

When a change occurs in the refractive index of the modified cladding, the output light intensity variations depend upon the refractive index of the modified cladding (nmcladd) compared to the core (nCOre)- When nmcladd becomes lesser than ncore, total internal reflections occur at the core/cladding interface (Fig.5b) and the intensity of the light propagating through the fiber increases. When ncore< nmclad, the light undergoes partial reflection at the interface and light enters into the modified cladding (leaky mode) (Fig.5b). The percentage of reflected light at the interface depends upon the refractive indices of core and modified cladding [4 ]. It leads to a loss of intensity of the propagating light through the fiber leading to decrease in the sensor output [4,5].

The proposed fiber optic sensor is in the leaky mode condition as the refractive index of modified cladding (nmclad = 1.901) is higher than the core (eg. 1.492 (Ce-doped ZnO)). If the refractive index of the modified cladding changes, then it may decrease or increase from the value of 1.90 (nmClad). In the case of ammonia, for which the sensor output increases, the nmcladd should have decreased from 1.90 and become lesser than the core (ncore=1.492) with the increase in the concentration, which would produce total internal reflections. If this is the case, then the output light intensity would have decreased initially for some concentration as the decreased refractive index of modified cladding (nmcladd) would still be greater than ncore, (where the light leakage increases and reflectivity decreases (Case 1)) and have become minimum after some concentration when nCOre= nmcladd (Case 2), and then increased for ncore > nmcladd with the further increase in the concentration (Case 3) [5]. However, the output characteristics of ammonia show that the spectral intensity increases monotonically with the concentration.

The above analysis suggests that the output characteristics of the sensor may be related to the relative percentage of light reflectivity at the interface of core-modified cladding with and without gas. As the refractive index of the modified cladding starts decreasing with the concentration initially (Case 1), the percentage of reflectivity without gas (reference value) would be higher than that with gas. In general, when the gas presents, the percentage of light reflectivity may be higher or lower than the reference value depending upon the refractive index of the modified cladding. If it is higher than nmcladd (1.90), then the percentage of reflectivity would be higher than the reference value. Otherwise, it would be lesser. These suggest that there would be increase or decrease of the output light intensity when the sensor exposed to different gases.

The gas sensing mechanism in metal-oxide based gas sensors is related to a change in the electrical conductivity which is attributed to reaction between the gases and adsorbed oxygen on the surface of the metal-oxides. The reaction is surface controlled [21,25,26]. In an air environment, oxygen molecules are adsorbed on the surface of metal oxides as O and O2" ions. When the gases interact with the oxygen ions, they liberate electrons to the metal-oxides, which changes the electrical conductivity [21,16]. During the interaction, reaction products are formed which are in a gaseous phase. The gas sensitivity depends upon the concentration of the adsorbed oxygen ions on the surface [21].

It appears from the gas sensing characteristics of metal-oxides that the gas sensing by the modified cladding may occur at its outer surface (modified cladding-air interface) instead of at the interface of core-modified cladding as commonly observed. A gas sensing model is proposed as shown in the Fig. 6 for understanding the output characteristics of the sensor. A light enters into the clad modified region from the core as the sensor works in the leaky mode condition and re-enters the core through the interface of core-modified cladding (interface A) after reflection from the interface of modified cladding-air (interface B). Now the output light intensity of the sensor may be due to light propagating through the fiber (ray 1) and the reentered leaked light (ray 2). If any changes occur in the re-entered leaked ray, then the output light intensity may change.

The angles θ1,θ2, θ3 and θ4 represent the angle of incident of the light ray at the interface A, angle of refraction in the modified cladding, angle of incident at the interface B and angle of incident at the interface A in the clad modified region, respectively. The angle θ5 is the angle of refraction in the core region.

The leaked light may undergo total internal reflection at the interface B. The occurrence of total internal reflection depends upon the refractive indices of the outer medium (nom) and modified cladding region (nmcladd) as well as the incident angle [4, 27]. The calculation shows that the leaked light undergoes total internal reflection at the interface B. It is as follows, the critical angle of unmodified clad optical fiber is found to be 70 degree (ncore=1.492 and refractive index of unmodified clad (numclad) =1.402) and if it is assumed, for example, the angle of incident (θ1) is 75 degree above the critical angle, than the angles θ2, θ3, and θ4 become 49 degree in the clad modified region. Since, the angle θ3 (49 degree) is greater than the critical angle (32 degree) of the interface B (nmcladd =1.901 and nom =1.00 (refractive index of air)), total internal reflection occurs. The reflected ray from the interface B reaches the core of the fiber through the interface A, where it undergoes partial reflection as the incident angle 84 (49 degree) is lesser than the critical angle (52 degree) of this interface (nmcladd =1.901 and ncore =1.492).

The leaked light in the modified cladding may suffer losses by the absorption or may be completely attenuated. Since, the Ce doped zinc oxide exhibits high optical transmission (above 80%) in the UV-Visible region, the leaked light undergoes reflection at the interface B and re-enters the core (Fig.6). The occurrence of total internal reflections at the interface B suggests that the light ray 2 may undergo changes in its intensity due to evanescent fields that exist in the outer medium (air) near the interface (Fig.6) when the gas interacts with them, leading to changes in the light output intensity of the sensor. Initially, when no gas present, losses may occur to the light ray 2 in its intensity due to evanescent wave absorption by the air. This corresponds to an output intensity spectrum obtained when the sensor is not exposed to the gas (reference value). When the gas present the evanescent wave absorption may be higher or lower than the reference value. In the case of ammonia, the sensor output light intensity increases with the increase in the concentration. It indicates that evanescent wave absorption occurs at the interface during the interaction of ammonia with doped ZnO, however, the magnitude of absorption is lesser than the air environment. In the case of methanol and ethanol, the magnitude of absorption is higher than the air, which results in the decrease in the sensor output.

Absorption characteristics of Ce-doped ZnO were studied using spectrophotometer (Model UV-1700, Shimadzu, Japan) under air, ammonia and methanol environments for comparing the proposed model by stimulating the experimental condition for the sample. The Ce-doped ZnO was coated (around middle portion) on the inside surface of one of the sides of the cuvette and a small amount of solution (about 2 ml) was taken at the bottom of the cuvette for producing a vapour. The coating did not make a contact with the solution. Another similar cuvette with the same amount of solution was taken without any coating for a reference. The open ends of the cuvettes were closed with Teflon lids and the solutions were allowed to vaporize for 10 min.

The absorption spectra observed for Ce doped ZnO in various environments are shown in the figure 7. It shows that absorption of light occurs when doped ZnO is exposed to air and gases. It implies the occurrence of evanescent wave absorption at the interface B during the interaction of gases with modified cladding of the fiber sensor. The absorption spectra are broad and no characteristic absorption peaks are observed. The absorption of light is almost same in the spectral range of about 400-1000 nm where the sensor response is observed. It is seen that the absorption of light is lesser for ammonia and higher for ethanol compared to the air. These may be the reason for the increase in the output intensity of the sensor for ammonia and decrease for methanol and ethanol with the concentration. Reaction products are formed during the interaction between doped zinc oxides and ammonia/methane, which may result in the different absorption characteristics.

The absorption characteristics of Ce-doped ZnO were also studied by using a water alone. The spectra for air and water were found to be almost same. Similarly, the output characteristics of the sensor were also studied with water alone as studied for ammonia and methanol solutions. The results showed that the spectrum is similar to that of air and its intensity remained almost unchanged with water. The effects of humidity on the sensor output characteristics were also studied for an ammonia and methanol (300 ppm) by changing the humidity condition of the laboratory (about 52-72%) using an air conditioner. The temperature of the laboratory was found to be changed from 24 to 30°C. The outlet of the gas chamber was kept open to the atmosphere in the laboratory throughout the experiment. Again the spectral intensity was found to be almost unchanged for the humidity levels used.

Figures 8(a & b) give graphs between the peak intensity (689 nm) and concentration of various gases. It is seen that the peak intensity varies almost linearly with the concentration of the gas. Gas sensitivity of the sensor is defined as a change in the peak intensity of the spectral graph to the change in the gas concentration. For Ce doped ZnO, it is found to be about 58 counts/100 ppm for ammonia and -54 counts/100 ppm and -56 counts/100 ppm, for methanol and ethanol, respectively. In the case of TiO2, the gas sensitivity is 59 counts/100 ppm for ammonia and it is -35 counts/100 ppm and -29 counts/100 ppm for methanol and ethanol. The negative sign in the gas sensitivity indicates that the spectral intensity decreases with the concentration of the gas.

The proposed fiber optic sensor shows a positive slope in the output for ammonia and negative slope for methanol and ethanol gases. These behaviors of the sensor may be used for distinguishing ammonia or methanol/ethanol environments.

The time response characteristics of the sensor was studied with ammonia gas for a concentration of 300 ppm and spectral peak wavelength of 689 nm. The time response study was carried out by closing the gas outlet of the gas chamber and keeping the gas inlet open and the recovery time by reversing this process. The spectral intensity was noted at the intervals of 10 minutes. The figures 9 (a & b) give time response characteristic of the sensor for Ce-ZnO and TiO2. They show good reversibility. The response time was calculated by observing the time duration the sensor signal took for rising from 10% to 90% of the maximum and for recovery time, the intensity fall from 90% to 10% of the maximum. The response time and recovery time for Ce-ZnO were about 80 min. and 60 min., respectively. The response time for TiO2 is about 6 min. and recovery time about 8 min., respectively.

Gas sensing properties of a fiber optic sensor clad modified with Ce-ZnO and TiO2 as gas sensing media were studied at room temperature using ammonia, methanol and ethanol gases. The sensor exhibited good sensitivity for ammonia, methanol and ethanol gases. It was about 58, -54 and -56 counts/100 ppm, respectively, for Ce doped ZnO. It was about 59, -35 and -29 counts/100 ppm, respectively, forTiO2.

The sensor output light intensity increased for ammonia whereas it decreased for methanol and ethanol with the concentration. These properties could be used for selecting ammonia or methanol/ethanol environments. A model was proposed for understanding the output characteristics of the sensor. The time response study showed that the sensor exhibited good reversibility.

In one of the preferred embodiment the present invention shall discloses a clad modified fiber optic type gas detecting apparatus for selectively detecting ammonia gas among plurality of gases. The apparatus comprises of a light source means, a sensing means and a detector means. The light source means is provided for introducing an optical beam into the fiber optic core. The sensing means is provided for selectively sensing ammonia presence by absorbing part of the optical beam introduced by the light source and thereby changing the overall power in the optical fiber. The detector means is provided for providing the transmission properties of the optical fiber as an output which is a function of absorption level sensed by the sensing means which represents ammonia concentration. The sensing means is a selective sensing means which comprises of an optical transmission medium which includes plurality of layers arranged around the fiber optic core. The first layer which is arranged around the core is a first passive cladding layer and forms the surface of the transmission medium. A second nano crystalline selective sensing layer, is coated around the core of the optic fiber in a removed portion of the passive cladding layer. Any change in the optical properties of the modified cladding material due to the presence of ammonia gas changes the transmission properties of the optical fiber which represents ammonia concentration.

In another preferred embodiment the present invention shall discloses a method for selectively detecting ammonia gas amongst plurality of gases using a clad modified fiber optic type gas detecting apparatus. The method involves the following steps. An optical beam was introduced in to the fiber optic core from a light source means. Sensing the ammonia gas with a sensing means by absorbing a part of the optical beam introduced by the light source, thereby changing the overall power in the optical fiber. Providing transmission properties of the optical fiber as an output which is a function of absorption level sensed by the sensing means which represents ammonia concentration with a detector means. The sensing means is a selective sensing means which comprises of an optical transmission medium which includes plurality of layers arranged around the fiber optic core. The first layer which is arranged around the core is a first passive cladding layer and forms the surface of the transmission medium. A second nano crystalline selective sensing layer is coated around the core of the optic fiber in a removed portion of the passive cladding layer. Any change in the optical properties of the modified cladding material due to the presence of ammonia gas changes the transmission properties of the optical fiber which represents ammonia concentration.

As per the invention the nanocrystalline selective sensing layer is cerium doped Zinc oxide or Titanium dioxide. In accordance with the invention the detector means may be a spectrophotometer.

According to the invention the output of detector means provides an output curve as a function of ammonia concentration which is a sloping curve and a positive slope output curve representing the presence of ammonia. Further the detector means provides a first negative slope output curve representing the presence of methanol and a second negative slope output curve representing the presence of ethanol.

As per the invention the light source means is a white light source with emission wavelengths from 100 to 2000 nm. In accordance with the invention the refractive index of fiber optic core is preferably 1.492, the refractive index of the passive cladding is preferably 1.402 and the refractive index of Ce doped ZnO is approximately 1.901. The refractive index of Ce doped ZnO is found to be the same refractive index as of undoped ZnO. Further the the XRD pattern of doped ZnO is similar to pure ZnO and the synthesized doped ZnO powder is polycrystalline in nature.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive.

WE CLAIM :

1. A clad modified fiber optic type gas detecting apparatus for selectively detecting ammonia gas amongst plurality of gases, the said apparatus comprising of :

a. a light source means for introducing an optical beam into the fiber optic core,

b. a sensing means for selectively sensing ammonia presence by absorbing part of the optical beam introduced by the light source and thereby changing the overall power in the optical fiber, and

c. a detector means for providing transmission properties of the optical fiber as an output which is a function of absorption level sensed by the sensing means which represents ammonia concentration,

d. the sensing means characterized in the selective sensing means forming an optical transmission medium arranged around the fiber optic core with a plurality of layers thereon including atleast a first passive cladding layer and a second nano crystalline selective sensing layer, wherein the passive cladding layer is formed as the surface of the transmission medium and the second sensing layer is coated around the core of the optic fiber in a removed portion of the said passive cladding layer, whereby any change in the optical properties of the modified cladding material due to the presence of ammonia changes the transmission properties of the optical fiber.

2. The apparatus as claimed in claim 1, wherein the nanocrystalline selective sensing layer is cerium doped Zinc oxide.

3. The apparatus as claimed in claim 1 wherein the nanocrystalline selective sensing layer is Titanium dioxide.

4. The apparatus as claimed in claim 1, wherein the said light source means is a white light source with emission wavelengths from 100 to 2000 nm.

5. The apparatus as claimed in claim 1, wherein the said detector means may be a spectrophotometer.

6. The apparatus as claimed in claim 1, wherein the refractive index of fiber optic core is preferably 1.492.

7. The apparatus as claimed in claim 1, wherein the refractive index of the passive cladding is preferably 1.402.

8. The apparatus as claimed in claim 1, wherein the output of detector means is further characterized to provide an output curve as a function of ammonia concentration which is a sloping curve and a positive slope output curve in the detector means representing the presence of ammonia, and further provides a first negative slope output curve in the detector means representing the presence of methanol and a second negative slope output curve in the detector means representing the presence of ethanol.

9. The apparatus as claimed in claim 2, wherein the refractive index of Ce doped ZnO is approximately 1.901, the same refractive index as of undoped ZnO.

10. The apparatus as claimed in claim 2, wherein the XRD pattern of doped ZnO is similar to pure ZnO.

11. The apparatus as claimed in claim 2, wherein the synthesized doped ZnO powder is polycrystalline.

12. A method for selectively detecting ammonia gas amongst plurality of gases using a clad modified fiber optic type gas detecting apparatus, the said method comprising the steps of:

a. introducing an optical beam into the fiber optic core from a light source means,

b. selectively sensing ammonia presence with a sensing means by absorbing part of the optical beam introduced by the light source and thereby changing the overall power in the optical fiber, and

c. providing transmission properties of the optical fiber as an output which is a function of absorption level sensed by the sensing means which represents ammonia concentration with a detector means,

d. the method of sensing characterized in the selective sensing with an optical transmission medium arranged around the fiber optic core with a plurality of layers thereon including atleast a first passive cladding layer and a second nano crystalline selective sensing layer, wherein forming the passive cladding layer as the surface of the transmission medium and coating the second sensing layer around the core of the optic fiber in a removed portion of the said passive cladding layer, whereby any change in the optical properties of the modified cladding material due to the presence of ammonia changes the transmission properties of the optical fiber.

13. The method as claimed in claim 12, wherein the nanocrystalline selective sensing layer is cerium doped Zinc oxide.

14. The method as claimed in claim 12, wherein the nanocrystalline selective sensing layer is Titanium dioxide.

15. The method as claimed in claim 12, wherein the said light source means is a white light source with emission wavelengths from 100 to 2000 nm.

16. The method as claimed in claim 12, wherein the said detector means may be a spectrophotometer.

17. The method as claimed in claim 12, wherein the refractive index of fiber optic core is preferably 1.492.

18. The method as claimed in claim 12, wherein the refractive index of the passive cladding is preferably 1.402.

19. The method as claimed in claim 12, wherein the output of detector means is further characterized by providing an output curve as a function of ammonia concentration which is a sloping curve and a positive slope output curve in the detector means representing the presence of ammonia, and further providing a first negative slope output curve in the detector means representing the presence of methanol and a second negative slope output curve in the detector means representing the presence of ethanol.

20. The method as claimed in claim 13, wherein the refractive index of Ce doped ZnO is approximately 1.901, the same refractive index as of undoped ZnO.

21. The method as claimed in claim 13, wherein the XRD pattern of doped ZnO is similar to pure ZnO.

22. The method as claimed in claim 13, wherein the synthesized doped ZnO powder is polycrystalline.