Gas sensors are usually operated at high temperature to achieve fast response and reversibility and this leads to shorter lifetime of the sensor. Nanocrystalline SnO₂ thin films fabricated from the thermal decomposition of Langmuir-Blodgett (LB) film precursor, exhibit room temperature gas sensitivity comparable to that required for air quality monitoring. LB technique offers control over SnO₂ film thickness and crystallite size. By controlling the crystallite size and film thickness room temperature operation is achieved. The sensor is specific to NH₃ gas at room temperature and shows fast response and recovery without any carrier gas flow. The stability studies of the sensor indicated that these sensors are stable at least for a year with no significant change in sensitivity.

ADVANTAGES

• Room temperature operation, low power requirement and portable.
• Reliable over a period of 1 year.

SPECIFICATIONS

APPLICATIONS

• Heavy water plants
• Fertilizer plants
• Pollution monitoring agencies

NH₃ is one of the highly toxic gas, which is widely used in various industrial applications. It  causes irritation in nose and throat and  in extreme case it causes severe lungs problem (pulmonary edema). Its long-term (8 h) and short-term (10 minutes) exposure limits are 25 and 50 ppm respectively. The nanocrystalline tin oxide (SnO₂) thin film based sensor detects NH₃ with high sensitivity, is selective to NH₃ among other reducing gases, and is stable for more than 1 year. Commercially available SnO₂ gas sensors are usually operated at high temperature to achieve reversibility and fast response. High temperature operation causes shorter life time and not favorable in explosive environment.

WORKING PRINCIPLE

The sensor works on the principle of change in electrical resistance of nanocrystalline SnO₂ thin film upon chemical interaction with NH₃. This change is calibrated and displayed directly on a monitor as NH₃ concentration.

Preparation of SnO₂ thin film and sensing mechanism :

The sensor element is nanocrystalline SnO₂ thin film which is prepared by using Langmuir- Blodgett (LB) film precursors. Multilayer LB films (100 ± 20 layers) of ODA-stannate complex deposited on quartz are decomposed at 600 °C in the presence of ambient oxygen.

Nanocrystalline SnO₂ surface has a net negative charge due to the large number of surface species O^2-,O^- present on it. NH₃ which is a mild reducing gas, a polar molecule containing partial positive charge on its H atoms. When NH₃ gas is injected into the gas testing chamber containing SnO₂ thin film, it reduces the mobility of the electrons in SnO₂, thereby increasing the resistance. This makes the sensor unique and specific to ammonia at room temperature.

SPECIFICATIONS

Selectivity in presence of other gases

APPLICATIONS

• Heavy water plants
• Fertilizer plants
• Pollution monitoring agencies
  

REQUIREMENTS FOR SENSOR PREPARATION

A. Raw materials

1. Chemicals: Octadecyl amine, Choloroform and Sodium stannate
2. Distilled water for making sodium stannate solution
3. Quartz substrates (1cm x 2 cm) for film deposition
4. Gold (24 ct, for contact electrodes deposition)

B. Equipments

1. LB trough or stepper motor controlled deposition unit
2. Furnace for heating at 600°C
3. Thermal evaporation system for gold contact electrode deposition
4. Enclosure for keeping the film, connector cable
5. Calibration chamber
6. Gas canisters and syringes (Electronics for the sensor is not included)

C. Space

• The space requirement is approximately 12ft by 15ft to comfortably accommodate the process equipments such as LB trough, CVD system, Furnace, space for assembling and packaging, testing and calibration setup etc.

MANPOWER REQUIREMENT

2 Numbers :

• 1 person with B.Sc. degree in Physics/Chemistry
• 1 person with diploma in Electrical/Electronics