Chapter 1 Introduction
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1-1 Overview of Gas sensors
In recent age, interests in industry safety and environmental pollution have been growing in our life. The needs of prevention and control of air pollution and detection for toxic gases have gradually increased. Thus, researches and developments on the gas sensors are carried out rapidly. According to Frost and Sullivan, global gas sensor markets were worth $48.5 million in 2005; their forecast for 2012 is $80.6 million.
And based on a new technical market research report (BCC Research) (Fig. 1-1), the global market for gas sensors and gas metering is worth an estimated $3.9 billion in 2010, but is expected to increase to nearly $5.2 billion in 2015, for a 5-year compound annual growth rate (CAGR) of 5.9%.These data show how large, and still expanding, this market is. A gas sensor is a device which outputs the appropriate signals (the change of current, voltage and conductance) for detection and measurement when specific gas was released (Fig. 1-2). In our daily life, most of the gases are colorless and smell-less. Moreover, the sense of smelling of human beings was not able to identify the amount and content of certain gases. Hence, people always become conscious that some toxic and harmful gases run out due to the inhalation of overdose that causes uncomfortableness. Therefore, an accurate, fast-reacting and stable gas sensor plays an important role to improve the safety in our life.
In the analysis of gases, conventional analytical instruments have many usage restrictions while gas sensor was more convenient. As shown in Table. 1-1, the data
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precision of analytical instrument is absolute value, yet its data acquisition time is too long and the size is much larger, which means the low-cost gas sensor attracts much attention from researchers.
Generally, gas sensor was classified into the following kinds:
1) Catalytic combustion gas sensor, 2) Semiconductor-absorbing Gas sensor, 3) Electrochemical Gas sensor,
4) Field-effect transistor Gas sensor, 5) Infrared Gas sensor and
6) Gas ionization sensor.
The first three kinds of gas sensors must be heated to high temperature before detecting gas leakage. Besides, they have different reactive sensitivities and response time of distinct gases, which cause mistakes or inability in sensing specific gases.
The mechanism of Field-effect transistor Gas sensor is as follows. When target gas contacts with catalyst metals, chemical reaction occurs and the product (take H2 for example) will tunnel through the catalyst metal layer to affect charge density in the channel. Sander J.Tans et al. [1] and R.Marel et al. [2] have used carbon nanotubes (CNTs) as the channel of field-effect transistor (Fig. 1-3). By applying a voltage to a gate electrode, the nanotube can be switched from a conducting state to an insulating state. Generally, Source and Drain were fabricated by noble metals like Pt and Au, and doped Si was used as the back gate of transistor. Using the gate electrode, the conductance of a SWNT-FET could be modulated by more than 5 orders of magnitude.
The sensitivity of this kind of gas sensor is three times higher than that of common gas sensors and the former has a good response time of two to ten seconds. However, there are still many limitations on Field-effect transistor Gas sensor. For example, it
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takes high temperature to recover to the initial resistance after each gas sensing measurement. Also, the fabrication process is more complicated, and the improved response time is still too long to detect specific toxic gases.
The Infrared gas sensor, however, measures target gases by determining the absorption of an emitted infrared light source through a certain air sample [3].
Infrared gas analyzers usually have two chambers that one is a reference chamber while the other is a measurement chamber. Infrared light is emitted from some type of source on one end of the chamber, passing through a series of chambers that contain given quantities of the various gases in question. Target gases found in the atmosphere get excited under specific wavelengths found in the infrared range. The concept behind the technology can be understood when considering the greenhouse effect. As sunlight hits the earth surface, the incoming short wave radiation gets turned into long wave infrared radiation that is reflected back into space. If the planet has a thick atmosphere, much of this radiation is absorbed by the "greenhouse gases" in the atmosphere which acts as an isolative blanket. The infrared gas sensor operates on the similar principle. However, the response time of infrared gas sensor is still quite long.
A gas sensor with very short response time was required to detect some poison gases.
Among all of these sensors, Gas ionization sensor meets the requirement of response time with a stable discharge current in microseconds to ppm-level gases by its breakdown effect. It shows good sensitivity and selectivity, and is unaffected by extraneous factors such as temperature, humidity, and gas flow [4]. The fundamental operation principle of Gas ionization sensor is as follows. When applying a high voltage between two parallel electrodes, the electrical-field-induced band bending of the Vacuum level forces electrons to tunnel through the electrode material onto the Vacuum level which is the so-called Field emission effect. Since there are neutral molecules in the path of electrons which are moving to the positive electrode by the
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traction of electric field, impact ionization reaction would take place when the electrons receive enough kinetic energy to ionize the molecule from the electrical field. A positive gas ion and a negative electron would be generated after an effective impact ionization process, and the number of electrons gets doubled after each time of effective collision. The whole reaction could be described via (eq. 1-1) :
e− +X2 →X2+ +2e− (1-1) The number of charged particles between two electrodes would increase rapidly when the reaction repeats perpetually. Once it reaches a certain amount, the original poor electrical conducting gases would turn out to cause the electrical breakdown and an unusual high current could be measured afterward. Due to different molecular physical properties of different gases, distinct breakdown voltage could be obtained when breakdown effect occurs. Although gas ionization sensor work by fast response time and fingerprinting the ionization characteristics of distinct gases, they are, however, limited by their huge architecture, risky high-voltage operation and high power consumption. Hence, the miniaturization of device size, the reduction of breakdown voltage, and the improvement of power consumption are the most important issues for this kind of gas sensor.