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5.3.3.5.1 natural gas detectors

The chemical products from complete combustion of a hydrocarbon fuel are mainly CO2 and H2O (vaper). Combustion of gaseous fuel in air can occur in two different modes - one where fuel and oxygen is mixed during the combustion process, and the other where fual and air are premixed(gas condensing boilers) and the fuel concetration must be within the flammability limits. In general the premixed situation allows the fuel to burn faster, I.e. more fuel is consumed per unit time.

Natural gas detectors should reliably detect the presence of gas concentrations far below the lower explosion limit of about 5% vol. In Germany many different cheap gas alarm systems can be purchased in DIY-market, but most of these devices are not based on any quality standard. Although high priced alarm and monitoring systems are available there is a lack of low cost reliable gas alarm systems for use in households which are based on appropriate standards.

The European norm EN 50194 contains a proposal concerning the construction of alarm systems for household gas combustion appliances, including norms for threshold limits, cross-sensitivities and long-term stability. One aspect of cross-sensitivity is the possibility of raising false alarms when the system is exposed to 2000 ppm ethanol. As ethanol is an ingredient of alcoholic beverages and cleaning chemicals, a sensor may well be exposed to it. Semiconductor metal oxide sensors are promising candidates to realize low-cost standardized natural gas alarm systems. In general these gas sensors are poorly selective and often show a considerable cross-sensitivity to ethanol. Apart from the measures mentioned in chapter 5.3.2.1. selectivity can also be improved by analyzing the sensor signal in a temperature-activated signal e-valuation mode which is called "power activation mode" by the authors [20]. The method is suitable to differentiate between ethanol and methane by e-valuating the signal slope when the operation temperature is raised. Fig. 5.42 shows the relevant logical operation digram. The power activation mode prevents false alarms caused by ethanol at natural gas detection levels in the range from 2 to 20% LEL.

5.3.3.6 Other appliances

Many appliances for gas sensors in households are still under development. For example, several attempts have been made in the past to include gas sensors in kichen ovens to control cooking and frying processes as well as to control the self-cleaning process (pyrolysis).

Fig5.42 "Power Activation Mode" - operation sequence for Ga2O3-natural gas sensors to avoid false alarm in the presence of ethanol and other interfering air components [20]. (With kind permission of D.kehl, D.Skiera and M.Lammer. Institute of Applied Physics. University of Giessen. Germany)

5.3.3.6.1 Cooking and Frying contral

Mainly two different approaches for the development of cooking controls have to be mentioned. The first, more empirical approach is based on the investigation of different non-selective sensors and/ or sensor arrays and their signals during the cooking of different goods. In this approach it is assumed that the cooking good emits specific volatile compounds which indicate when the good is done. The objective is to find a specific signal or signal pattern when the "point of interest " is reached. Because of their good overall stability and robustness also in hot and harsh environments, mostly semiconductor metal oxide sensors and sensor arrays are investigated [21]. One major problem which has to be solved is the considerable amount of humidity which is generated during the cooking process. Unfortunately metal oxide sensors are known to show strong cross-sensitivity to humidity. An invention by Panasonic for example even e-valuates the humidity signal of the sensor in a bread baker which increases strongly when the crust breaks.

The second, more theoretical approach is based on the chemical analysis of the volatile compounds which are specific for the current status of the cooking process. A possible disadvantage of this approach is that after the identification of volatile components it cannot be assured that suitable gas sensors are available or can be developed.

5.3.3.6.2 Sepf-cleaning of Ovens

The control of the self-cleaning procedure (pyrolysis) of especially equipped kitchen ovens is another focus of development. The underlying idea is to burn the organic residues at elevated temperatures(around 400'C) and to detect the emerging volatile compounds. In order to minimize energy consumption, the process time should be kept as short as possible. During this process considerable amounts of CO and CO2 are released. A decrease in concentration of these compounds can thus be taken is an indicator of the end of process. The most direct method would be the detection of CO2 in the flue gas. The most common CO2 sensors are based on electrochemical cells or optical NDIR-systems. Unifortunately, electrochemical cells are not stable at higher temperatures so that they can not be placed directly in the flue gas. Apossible solution coution could be the use of a by soiling. Other approaches use low-cost self-cleaning Ga2O3-metal oxide sensors which are stable at high temperatures, to detect CO prior its conversion into CO2. The ultimate ambition of developers could lie in the combination of cooking and pyrolysis control in one system.

5.3.4 References

5.4

UV sensors -Problems and Domestic Applications

Application areas for UV sensors in the household environment are introduced and the technological requirements and challenges of UV-sensing discussed. Different detection technologies with their strengths and weaknesses are explained. Finally, reasons that limit the use of UV sensors in household appliances are discusssed and way outs are lines out.

5.4.1

UV Radiation - A General Introduction

UV Radiation is the part of the electromagnetic spectrum that lies between visible light and x-rays at a wavelength below 400 nm. Wavelengths shorter than 200 nm(VUV radiation) do not figure in this chapter because they are absorbed by air. The relevant spectrum is divided into UVA (320nm-400nm), UVB(280nm-320nm) and UVC(200nm-280nm) radiation. Compared to visible light, UV radiation generally has a greater chemical and biological impact because the photon energy of UV radiation is higher than that of visible light.

5.4.2

UV Radiation in Household Environments

5.4.2.1 Natural UV Radiation

The most common UV-source is sunlight. The UV part of the sun spectrum is restricted mainly to UVA and its intensity strongly decreases below λ=320nm, see Fig. 5.43. The UV intensity ranges up to 6mW/cm2. UV radiation cent of the sun's intensity in the visible with up to 100mW/cm2. UV radiation has an impact on the human skin such as tanning or burning (erythema). Other sun UV-related effects are the degradation of organic materials and colors. The intensity and spectral distribution of UV changes with the azimuth of the sun, but also depends on the atmospheric weather condition. Especially the short-wave-length part below 330nm strongly depends on the condition of the atmospheric ozone layer.

Most light sources for artificial lighting have a UV component in their emission spectrum. Usually, the glass encapsulation absorbs all radiation below about 330nm. Light bulbs work as a blackbody radiator at a temperature of approx. 2800 K. Temperatures up to 3300 K can be reached by filling a bulb with halogenated gas. The spectral radiation distribution has a tall below λ =400nm that strongly decreases at shorter wavelengths. its magnitude rises with the temperature of the source of radiation.

Fluorescent lamps generate light through a low-pressure mercury vapor discharge that has strong emission lines in the UV, namely at λ=254nm and around 366nm. The fluorescent layer is excited by the UV radiation and emits in the visible part of the sprctrum. While remains of the 254nm line are efficiently rejected by the glass tube, some fraction of the 366nm radiation can be measured in the emission spectrum of the lamp.

Flames and related buring and combustion processes are an imprtant source of UV radiation. Intermadiate molecular species like OH, CO and CH groups are generated in an excited molecular state during the oxidation process of the fuel. Their lifetime is very short, usually in the range of nanoseconds, and they may emit UV-photons as they decay to their ground state. The intensity of combustion related UV radiation lies in the nW/cm2-range or below. Thererfore, the UV component is often ignored when thinking of a flame emission spectrum. However, the occurrence of combustion-originated UV may be used for flame detection or fire alarms. The emission spectrum lies significantly below 330nm, see Fig. 5.44 and can therefore be well distinguished from ambient UV radiation coming from the sun or artificial lighting.

5.4.2.2 Man-made UV Radiation

Sunbeds with fluorescent lamps that emit in the UVA and UVA are used for indoor tanning. They are supposed to simulate the solar UV spectrum and have therefore similar effects on the human skin. However, the intensity of radiation is often not monitored, and excessive exposure may cause serious dermatological health problems. A more detailed analysis reveals that the long-wave UV(UVA) is mainly responsible for the tanning while UVB radiation tends to be more dangerous.

UVC radiation can not only be used to sterilize tap water, but also for the treatment of air and sewage. Radiation between 250nm is passes through water and is strongly absorbed by nucleic acids, I.e. any living creature present. This kind of radiation therefore efficiently kills all microorganisms in the water. It is a lucky coincidence that at 254nm, the main emission line of mercury lamps lies within the range of effective UV sterilization.

5.4.3.

Principles of UV Detection

Apart from a few applications, such as UV disinfection and lacquer hardening the intensity of UV radiation is well below that of visible light in ambient daylight or indoor lighting. A UV sensor must therefore be insensitive to visible light, otherwise the detection signal would easily be drowned out by the visible fraction of the radiation spectrum. Sensors that fulfill this requirement have a selective spectral sensitivity in the UV range. There are two important selectivities, known as visible-blindness and solar-blindness.

A visible-blind UV sensor detects radiation only below λ=400nm and thus is sensitive to the UV radiation of sunlight. A solar-blind sensor does not react to sunlight and usually detects radiation below λ=300nm. An outside fire alarm sensor imposes one of the most stringent requirements for solar-blindness. It must be sensitive to 100pW/cm2 or less between 220nm and 300nm but should not react to direct sunlight that gives 100mW/cm2 between 320nm and 720nm.

5.4.3.1 UV-Enhanced Sl Photodiode

The most common photodetector is a photodiode with a p-n or p-i-n junction made from crystalline silicon (Si). Photons are absorbed in the semiconductor, and an electron-hole pair is generated if the photon energy is above the band gap. There is a built-in electric field close to the p-n junction and the electron-hole pairs created there may escape from recombination and generate a net current. The band gap of silicon is Eg=1.1eV, and Si photodetectors are thus sensitive to wavelengths below λ=1100nm. Quantum efficiencies above 80% can be reached, but below 400am sensitivity strongly decreases towards lower wavelengths. There, the absorption of Si increases and most of the photon are absorbed before they can reach the charge-density zone.

Special UV-enhanced Si photodiodes can be made by positioning the p-n junction close to the surrace. Then, quantum efficiencies of 50% can be achieved for between 200nm and 400nm. However, the photodiode remains sensitive in the visible. An inherently visible-blind photodiode cannot be made from silicon.

Visible- or solar-blind UV sensors can be made from a Si photodiode by additionally using an optical filter that transmits UV radiation only, see below. A more detailed explanation of the physics of UV photodiodes (made from Si as well as from other semiconductor materials) can be found in Ref. [1].

The most convincing argument for using Si photodiodes in UV detection the availability of strong expertise in electronic Si devices. Processing and performance of opto-electronic Si devices have been optimized for decades, and the UV-enhanced photodiode is a high-performance niche product that can be produced at a reasonable price, thanks to these efforts. Its probably most serious drawback is the necessity of using filters for visible-blind applications, which considerably increases the cost of sensors and reduces their otherwise optimum sensitivity.

5.4.3.2. Crystalline Wide Band-Gap Semiconductors

Semiconductor photodiodes can be made visible-blind by using a semiconductor with a sufficiently high band gap. Promising materials are SiC (Eg=3.1eV), GaN(Eg=3.3eV) and its related compound AlGaN (Eg=3.3-5 eV, depending on the Al/Ga ratio) and diamond (Eg=5.5 eV)[1]. Sometimes, GaP (Eg=2.3 eV) is also used, but due to the low Eg GaP remains sensitive in the blue and green range.

Similar to silicon, crystalline silicon carbide is grown as an ingot and cut into wafers. The market share of SiC is still low but a strong increase can be expected due to its superior properties including charge-carrier mobility, heat-conductivity and maximum usable temperature. This is of particular interest for high-power electronics and highly integrated circuits. Visible-blind UV photodiodes based on SiC with performances similar to Si can be found on the market. However, their price is considerably higher than for Si photodiodes, and especially detectors with large sensing surfaces are very expensive.

GaN as a semi-conducting material for electronics is about to be launched on the market, especially for the use in blue-and UV-emitting LEDs and laser diodes[2]. The material is deposited on crystalline substrates like sapphire using thin-film epitactical techniques. Often, metal-organic chemical vapor deposition (MOCVD) is used. The necessity for such technologies limits the production rate and pushes up costs.

Very promising indeed is the ternary compound AlGaN. By shifting the Al/Ga ratio its spectral sensitivity can be tailored. The cut-off wavelength can be shifted between 380nm and 310nm [3]. Quantum efficiencies up to 50% have been obtained for SiC as well as for GaN, Which is similar to the UV sensitivity of UV-enhanced Si photodiodes.

5.4.3.3 Polycrystalline wide band-gap semiconductors

Although visible-blind UV photodiodes with good performances can be made from crystalline SiC and GaN, their introduction to the market is hampered by their high production costs. In many market segments, a photodiode made with polycrystalline thin files as wide band-gap semiconductor material could be an alternative. Production costs could be lowered considerably, at the price of lower sensitivity and a longer response time.

Polycrystalline GaN UV detectors have been realized with 15% quantum efficiency [4]. This is about 1/4 of the quantum efficiency obtained by crystalline devices. Available at a fixed price, however, their increased detection range may well compensate their lack in sensitivity. Furthermore, new semiconductor materials with a matching band gap appear as promising candidates for UV detection if the presumption of the crystallinity is given up. Titanium dioxide, zinc sulfide and zinc oxide have to be mentioned. The opto-electronic properties and also low-cost production processes for these compound semiconductors have already been investigated to some extent for solar cell applications[5].

5.4.3.4 Fluorescent Converters

Perfect semiconductor photodiodes with a supposed quantum efficiency 1 would generate one electron-hole pair per absorbed photon, regardless of the photon energy. Since photon energy increases with lower wavelengths, conversion efficiency, in terms of photocurrent per incident radiation power, decreases with lower wavelengths. As an example, a photodiode with quantum efficiency 1 would generate a photocurrent of 480 mA/W λ=600nm but only 240 mA/W at λ=300nm. The sensitivity (photo current per light power ) of a photodiode may therefore get increased if the radiation is efficiently converted to a higher wavelength. This concept sounds particularly interesting if UV radiation is converted to visible light, which can be measured by a standard Si photodiode. Standard materials from fluorescent lamp production, such as rare-earth aluminates and yttrium vanadate compounds can be used as fluorescence converters. However, to obtain a visible-blind sensor a UV-transmitting filter is still indispensable.

5.4.3.5 Discharge Tubes

In contrast to semiconductor photodiodes, discharge tubes are photo-emissive devices. Photons hit the metallic surface of a cathode, and if the photon energy is above the ionization energy an electron is emitted. An electric field between the cathode and an anode accelerates the electron in a low-pressure gas atmosphere, and the number of electrons are multiplied by the avalanche effect. The breakdown rate is then used for measuring the radiation intensity. Suitable cathode materials for UV sensors are molybdenum, tungsten and nickel with a cut-off wavelength of 300nm, 275nm and 250nm, respectively.

Discharge tubes have an excellent detectivity and are solar-blind. These technological advantages have to be balanced against the need for acceleration voltage of some 100V and complicated readout electronics. These requirements make sensing systems based on discharge tubes much more expensive than those based on photodiodes. The major field of application is therefore fire alarm monitoring in industrial environments. Another problem is the poor reliability of discharge tubes.

5.4.3.6 Filters

Filters that block visible light (and sometimes 1R radiation) are necessary to make a broadband detector like a silicon photodiode visible-blind. UV-transmitting filters are made either of colored glass or as interference filters. Colored-glass filters like UG5 or UG11 of Schott have a maximum transmittance of typically between 320nm and 370nm. They block in the visible and radiation below 260nm. A problem in using them together with a Si-photodiode is their re-appearing transmittance below 660nm.

Interference filters consist of several evaporated dielectric layers on a glass or quarts substrate. Their transmittance can be tailored by choosing appropriate layers. A problem is their limited bandwidth of transmission which is usually above Δλ=30nm. Also, a substantial loss in sensitivity has to be accepted since the maximum transmission is limited to less than 40%.

5.4.3.7 The Entrance Window

The entrance window is a relevant part of the sensor. Common glasses as well as most transparent plastic block UV radiation. Standard UV-transmitting materials like quartz and sapphire are expensive. For UVA and to some extent the UVB special glasses but also plastics like some PMMA-derivatives or some silicon gels are available with a reasonable transmittance down to about 330nm. For UVC applications with the important 254nm line of Hg, quartz remains the standard solution.