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 SO₂ gas at room temperature and shows fast response and recovery without any carrier gas flow. The stability studies 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

• Sulfur- Iodine cycle for Hydrogen production
• Coal based power plants
• Industries where sulfuric acid at high temperature is used
• Pollution monitoring agencies

SO₂ is one of the highly toxic gases, which poisons humans by inhalation. SO₂combines with water to form sulfuric acid and can irritate the throat by inhalation. Sulfur dioxide is released when compounds containing sulfur, such as fossil fuels like coal are burned. Its long-term (8 h) and short-term (10 minutes) exposure limits are 2 and 5 ppm respectively. The nanocrystalline SnO₂ thin film based sensor detects SO₂ with high sensitivity, selectivity, 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 decrease in electrical resistance upon a chemical interaction with SO₂. This change is calibrated and displayed directly on a digital monitor as SO₂ concentration in ppm.

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 net negative charge due to the large number of surface species O^2-,O^- present on the SnO₂ surface. SO₂ is a strong reducing gas. When SO₂ gas is injected into the gas testing chamber containing SnO₂ thin film, it reacts with surface oxygen species ( O^2-,O^-) forming SO₃ while releasing electrons to the SnO₂ conduction band, thereby decreasing the resistance. Ambient oxygen subsequently chemisorbs on the oxygen vacant SnO₂ surface attracting electrons from SnO₂ conduction band during the recovery phase. This makes the sensor specific to sulfur dioxide at room temperature.

SPECIFICATIONS

Selectivity in presence of other gases

APPLICATIONS

• Sulfur- Iodine cycle for Hydrogen production
• Coal based power plants
• Industries where sulfuric acid at high temperature is used
• 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 substrate (1cm x 2 cm) for SnO₂ film deposition
4. Gold (24 ct)

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 the SnO₂ 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