8. Gas sensors Introduction Market production of gas sensors Development of resistance gas sensors technology Influence of micromachining technology Non-resistance type gas sensors Measurement modules

Introduction Main fields of gas sensors applications : safety control (combustible and toxic gases) air quality (buildings, vehicles) process control in industry laboratory analytics burning control in vehicle engines Only in analytics and in industry one uses expensive gas analysers. In other cases there is a need of cheap, small size and easy in application gas sensors.

Market production Gas sensors manufactured by Figaro Eng. Inc.

Market production Lambda sensors used in car industry (fuel combustion control)

Gas sensors – historical review First reports: gas sensing properties of Ge: W.H.Brattain, J.Bardeen, Bell Syst. Tech. J. (1952) First reports on gas sensing properties of metal oxides: G. Heiland, Z. Physik (1954) A. Bielański, J. Dereń, J. Haber, Nature (1957) T. Seiyama et al., Anal. Chem. (1962) N. Taguchi – gas sensors as a commercial product (TGS sensors), production at the end of 1968. The dominating technology at present: deposition of thick films onto ceramic substrates. One observes increasing role of micromachining technology combined with technology of gas sensitive thin films. The most frequently used gas sensitive oxide layers: SnO2, ZnO, TiO2, WO3, In2O3.

Understanding resistance gas sensor Jonosorption reaction with a capture of electron: Reduction of oxygen ion e.g. ½ O2 + CO CO2 Change in conductance: (a), (c) where: W-width, H-height, L – length of a sample, x0 = LD Vs – barrier height NS – conc. of chemisorbed oxygen (b), (d)

Development of gas sensors technology, monocrystals (a) – upper view (b) – bottom view Structure of a monocrystalline sensor: 1 – alumina substrate, 2 – platinum electrodes, 3 – conducting paste, 4 – crystalline ZnO, 5 – heater (screen printing), 6 – leads

Development of gas sensors technology, sintered structures TGS 203 sensor for CO detection RH = 1.9 Ω VH = 0.8 V VC = 12V PH = 2 x 370 mW Sintered semiconductor powder with selected catalytic admixtures and a binder

Development of gas sensors technology, microsensors FIS microsensor SB series: (a) structure, (b) basic measurement circuit. Termination „1” is common for the heater voltage VH and the sensor voltage VC. Due to miniaturisation the consumed power does not exceed 130 mW. The sensor is used for detection of methane, propane and other inflammable and toxic gases.

Development of gas sensors technology Figaro TGS 800 series Basic measurement circuit Structure of the sensor: layer of SnO2 with a thickness of ca. 50 μm, alumina tube (inner diam. 1 mm, length ca. 4.2 mm), Pt wire leads (Φ = 0,08 mm), platinum heater (30 Ω), required working temp. ca. 300o C, power of the heater ca. 830 mW.

FIS sensor SP series working in a pulse modulated Thick films FIS sensor SP series working in a pulse modulated temperature regime. Advanced thick film technology, Au contacts, RuO or Pt heater, power consumption 300 – 400 mW.

Thick films Figaro sensor TGS 2400 series for detection of toxic gaes One-side configuration, power consumption 14 mW (avg value) Pulse control, power supply VC=VH= 5V

Influence of micromechining technology (a) closed membrane (b) suspended membrane ( hotplate type)

Influence of micromechining technology Motorola MGS 1100 sensor for CO detection Sensor in a package Sensors layout

Micromachined sensors manufactured in cooperation ITE Warsaw - AGH Schematic of the resitance – type gas sensor on the silicon membrane Power consumption below 100 mW

Micromachined sensors manufactured in cooperation ITE Warsaw – AGH, cont. Single sensor die, upper view 3” Si wafer with manufactured sensors

Micromachined sensors manufactured in cooperation ITE Warsaw – AGH, cont. Micromachined gas sensor in TO-5 encapsulation without a cap Sensor with a cap and a steel protecting mesh

Pulse power supply of the sensors Modulation of sensors working temperature 400 ppm NO2 1000 CO + 400 NO2 air 1000 ppm CO Pulse power supply of the sensors heater

Pellistor – type sensors Pellistor with a catalyst surface layer (diam. ca.1 mm) Pellistor fixed in an encapsulation Platinum coil is surrounded by ceramics on which surface a catalytic layer of a noble metal (Pt, Pd, Rh) is deposited. At elevated temperature an inflammable detected gas is catalytically oxidized which produces additional heat and increase in resistance of a Pt coil. Platinum coil serves also as a heater for initial heating of a pellistor to reach the required working temperature.

Thermoconductometric TCD sensors (catharometers) λG - heat conductivity of a gas, M0 – molecular weight d0 - molecule diameter Gas with small molecular weight and small particles, e.g. H2, will have high heat conductivity. μTAS with separation column and TCD detector for the analysis of chemical composition of a gas mixture. Intensity of cooling of a given resistor depends on gas heat conductivity SEM picture of the flow channel with suspended Pt thermoresistor (magnification 212 x)

Electrochemical sensors YSZ –solid state electrolyte Yttrium Stabilised Zirconia Oxygen Lambda sensor characteristics at a working temp. ca. 600oC. p- measured pressure, pR – reference presure Oxygen Lambda sensor of the finger type installed in a car exhaust pipe. When concentration p of exhaust oxygen increases, generated SEM tends to zero (according to the Nernst relation where pR is a partial pressure of oxygen in air). λ – air excess coefficient A – quantity of air F – quantity of a fuel

Electrochemical amperometric sensors Oxygen sensor with a current limit caused by a slit. Saturated current depends on gas concentration. Thin film amperometric sensor with a current limit due to chemical reactions.

Optical sensors based on absorption Methane absorption in a main abs. band 3200 – 3400 nm; 1 – diode LED1, 2 - diode LED2, 3 - detector Two-diode system for absorption measurement in the NIR region

Optical sensors based on fluorescence Absorption spectrum A and fluorescence emission of DPB compound in nitrogen atmosphere B and oxygen atmosphere C . Integrated fluorescence sensor coupled with a μTAS microchannel structure.

Structues basd on the field effect MISFET type transistor as a gas sensor. Gate metal – mostly Pd Change of the threshold voltage of a FET transistor as a consequence of interaction with a gas atmosphere.

Structues basd on the field effect Mechanism of detection of hydrogen molecules and other hydrocarbons by a thick palladium gate. Hydrogen molecules dissociate on Pd surface, diffuse as atoms through the metal and adsorb as dipoles on metal/insulator interface. This dipole layer is responsible for lowering of the threshold voltage and increase of the drain current.

MOSFET arrays Nordic Sensor Four MOSFET-s with cathalytic gate in a micromachined structure: 1 – MOSFET 2 – Al 3 – PECVD nitride 4 – LPCVD nitride Power consumption ca. 100 mW at 200oC.

Organic composites Cyrano Sciences Sensor Technology Thin layer of a composite: conducting carbon in a polimer matrix Swelling of a polymer after interaction with a gas causes breaking of carbon chains and increase of composite resistance.

Measurement with the help of sensor array (MSc thesis) Calculation block (microprocessor with software) Sensor 1 Gas mixture Calculated concentrations Sensor 2 Meter for measurements of CO and C3H8 concentrations in a mixture.