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ВОДОРОДНАЯ ЭНЕРГЕТИКА И ТРАНСПОРТ
Газоаналитические системы и сенсоры водорода HYDROGEN ENERGY AND TRANSPORT
Gas analitycal systems and hydrogen sensors
METAL OXIDE HYDROGEN, OXYGEN AND CARBON MONOOXIDE SENSORS
(REVIEW PAPER)
V. M. Aroutiounian
Member of International Editorial Board
Yerevan State University Yerevan-25, Armenia
Use of the hydrogen, oxygen and carbon oxide metal oxide semiconductor sensors allows electronically controlling the content of these gases during the work of many hydrogen setups, cells and devices. Present review-paper adumbrates results of achievements in this field.
Introduction
Last 20-30 years, research and development of gas-sensing devices is in the focus of activity of scientists and engineers in many countries. Such detectors (including smoke-detectors) can be used for different applications — continuous monitoring of the concentration of gases in the environment and premises, detection of toxic dangerous gases, drugs, smoke and fire, health, control of automotive and industrial emissions as well as precious technological processes in industry, energy saving, anti-terrorist defense. Oxygen (O2), hydrogen (H2), carbon monoxide (CO), nitrous oxide (NO), nitrogen dioxide (N02), carbon dioxide (CO2), methane (CH4), ammonia (NH3), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3), smoke and many others are among the important gaseous species.
Gas sensors can be manufactured using different materials, technologies and phenomena [19]. Sensing devices should be smaller and cheaper than the analytical devices currently used and have sensitivities in the 10-several hundred parts per million (ppm) ranges for many cases. We considered below only one type of gas detectors-metal oxide semiconductor sensors, which found soon remarkable positions in science and technology as they allow producing fast, reliable, non-expensive, low-maintenance devices using modern electron technologies. Some sensors and very expensive and cumbersome electronic nose systems made of them are available soon on market. However many gas-sensing micro-systems have not yet reached commercial viability due to high price, consumed electric power and working temperatures, inaccuracies and inherent characteristics of the sensors
themselves. So, suitable semiconductor materials with the required surface and bulk having higher sensitivity, stability and selectivity are demanding today.
Concentration levels of typical gas components are shown in Fig.1 [8]. Star marks indicate the standards of the gases legislated in Japan by (1) Environmental Standard, (2) Ordinance on Health Standards in the Office, (3) Offensive Odor Control Law, (4) Working Environment Measurement Law, and (5) Ordinance by Ministry of Health, Labor and Welfare [8]. Gases like O2 and N2 and humidity should be kept at adequate levels in living atmospheres, while hazardous gases should be controlled to be under the designated levels. 1/10 of lower explosion limit (LEL) for lower hydrocarbons and H2 gases is usually taken as an alarming level. Standards for toxic gases, volatile organic compounds (VOCs), odors, and other air pollutants are indicated by star marks in the figure. The full line shown for each gas indicates the range of concentration safely covered by a commercial gas sensor, while the broken line indicates results of laboratory tests. Some standards of VOCs such as benzene are seen to be less than 0.1 ppm, far out of reach by the present gas sensors.
Current trends and promising materials for the manufacture of metal oxide hydrogen, oxygen and carbon monoxide sensors and different technologies are described below in more details after some general comments.
Semiconductor materials for gas sensors
A sensing element should fulfill many requirements-have high sensitivity and selectivity, small response and recovery times, room temperature
Статья поступила в редакцию 07.08.2006 г.
The article has entered in publishing office 07.08.2006
Gas concentration, ppm
0.001 0.01 ч-\
100 1000 10000 100000 \-1-1-ь
ф *
; © C4H9OH
■ ■ ■ ■ ^^f^™ ■ ■ ■ ■ ■ ■
■ ■■■
-Л
H2 CO
O2
Established
H2O (Water vapor)
H2S
NH3 (CH3)3N CH3SH Alcohol
VOCs
SO2 NO2 CO2
O3
Flon
1
Fig. 1. Concentration levels of typical gas components [8]
operation, low power consumption, drift of parameters, robustness and be used without using expensive noble metals/materials. But reality is rather far from such ideal demands.
Sensing performances, especially sensitivity, are controlled by three independent factors: the receptor function, transducer function and utility. Receptor function concerns the ability of the oxide surface to interact with the target gas. Chemical properties of the surface oxygen of the oxide itself is responsible for this function in a oxide device, but this function can be largely modified to induce a large change in sensitivity when an additive (noble metals, acidic or basic oxides) is usually loaded on the oxide surface. Transducer function concerns the ability to convert the signal caused by chemical interaction of the oxide surface (work function change) into electrical signal. This function is played by each boundary between grains, to which a double-Schottky barrier model can be applied. The utility depends on the barrier height, pore size of sensing work body, diffusion depth, film thickness, doping of the material and the concentration of the target gas. Decrease in the grain size (diameter) below a critical value can lead to appearing quantum-size effects and dramatically increase in the sensitivity of the sensor [5].
Main materials are semiconductor metal oxides which are stable physically and chemically. Semiconductor metal oxides have been widely investigated for gas and humidity detection. Differ-
ent materials such as semiconductor oxides (Sn02, ZnO, TiO2, Fe203), catalytic oxides (V2O5, MoO3, CuO, NiO), metals deposited on oxide supports (Pt/Sn02, Pt/ZnO, Pd/SnO2), and mixed (or complex) oxides exhibit different physical properties on exposure to different gas species. Even though the semiconductor oxides used for gas sensors are catalytically active, a small amount of a catalytic metal or metal oxide is often added to improve their selectivity and sensitivity. In practical applications, till now oxidation catalysts employed for sensing have had a transition metal or a noble metal supported on an oxide. Less attention is given to traditional single- and multi crystalline semiconductors like Si, Ge etc. Silicon carbide, porous silicon and GaN sensors are more promising in this range of materials [7, 10-15]. For example, investigations of influence of hydrogen gas on properties of FETs based on SiC with Ru or Pt were carried out in [16-17]. It was shown that SiC sensors can operate at rather high ambient temperatures. Pt/Ga2O3/SiC metal-reactive insulator-silicon carbide devices operated as Schottky diodes. The sensors have been tested towards different concentrations of hydrogen gas as a function of operating temperature. This study showed advantages of this structure compared to the pure thin film (90 nm) Ga2O3 conductometric sensor. Hydrogen sensing characteristics of a Pt-oxide-Al0 3Ga0 7As MOS Schottky diode are reported in
1
CH C3H C4H Natural Gas
CH2=CHCN
[18]. Investigations of high-temperature sensors made of SiC and GaN are continued.
The search of new materials and sensitive devices made of them by different technologies is very important. Note that gas sensors in the form of thin or thick films are more promising detectors in comparison with the pellet ones as the main processes in gas sensors take place actually at the surface and near-surface thin layer of the sensing material. Remain volume part of semiconductor leads often to an increase in Ohmic losses only and serves as a substrate for a sensor and a volume where a heater is placed. The development of gas sensors in the form of thin films allows preparing sensors with small size and low power consumption; they can be in some cases integrated in an array. Different deposition methods like physical vapor deposition, chemical vapor deposition and sol-gel techniques are preferable for the manufacture of thin-films. Sputtering is rather promising technology due to a wider choice of working materials and better step-by-step coverage. The target material is sputtered away mainly as neutral atoms which are deposited onto the substrate placed on the anode. SiO2, A12O3 and other ceramic substrates are currently being used as substrate materials to deposit these gas-sensing materials because of high temperature stability.
Mechanisms of sensitivity of semiconductor sensors to different gases. Measurement of sensitivity
Physics, chemistry, and technology of semiconductor sensors require a better understanding both the bulk and surface properties of the sensing materials. The sensing material interact the gas and changed it fully or partially. The adsorption of the gas or vapor by the sensor surface can be associated with decomposition or dissociation. A reducing molecule (CO, H2 etc) adsorbed on the sensor surface acts as a surface donor, injecting electrons into it. The opposite phenomenon occurs during exposure to oxidizing gases like that of NO. In the case of n-type semiconductors, the resistance of the sensor decreases when the sensor is in contact with reducing gas or vapor whereas the resistance increases when the sensor is in contact with an oxidizing gas or vapor in the case of p-type semiconductor. Beside the conductivity or resistivity R, values of the capacitance and thermal electromotive force (EMF) of sensors, their current-voltage characteristic can be changed and measured.
Physical-chemical processes can be rather complicate, which lead to changes of many parameters of a sensing device. For example, most often the sensor element gets covered with decomposition products like carbon, CO2, and H2O causing a gradual decrease in the sensitivity at the operating temperature. A periodic change of the sensor temperature removes all the adsorbed species and unburnt organic contaminants from the surface. Oxidation of hydrocarbons generally proceeds through partially oxidized intermediates. The hydrocarbons (CnHm) are adsorbed on the sensor sur-
face and react with the surface oxygen species. In this process, many intermediates formed and adsorbed hydrogen is also present. These intermediates are finally oxidized partially or completely to CO2 and H2O. Catalysts like Pt and Pd increase the rate of oxidation at lower temperatures. As the surface conductivity depends on the density of donors (adsorbed hydrogen atoms or oxygen vacancies) and acceptors (chemisorbed oxygen), therefore the density of these surface species varies due to interaction with the intermediates.
Integrated gas sensors
It is evident that single crystal silicon (Si) wafers, pre-oxidized on both sides for electrical isolation, can serve as a better substrate for the gas sensor micro systems whereas silicon can be used as a construction material for transducer integrated circuits (IC). Integration minimizes mismatch among sensors and decreases power consumed by sensor and the transducer signal-to-noise ratio. As the signal processing is carried out in sensors themselves, less noise is generated. Note that work function-based active devices like field effect transistors (FET) operated at room temperature, whereby the total power consumption is reduced. In such FET-based device, work function is reversible changed on exposure to different gases. Often noble metal oxides are using as sensitive layers. So, it is possible to combine sensors and ICs on a single chip.
Among successful silicon sensor devices the ion-sensitive field effect transistor (ISFET) [3, 4, 9, and 19] and palladium (Pd) gate MOSFET device should be mentioned [3, 4, 9, and 20]. Note that the ISFET is a simple form of MOSFET without a gate contact whereas a Pd metal in the Pd-gate MOSFET replaces the usual metal gate. Pd catalytically decomposes hydrogen molecules into hydrogen atoms and they diffuse to the Pd-SiO2 interface and alter the device drain current. Si FET-based sensors have been the objects of intensive research activity by using different metals as gate materials and different combination of metal oxides. A different type of device structure can also be used. Here the electrical resistance or the dielectric constant changes are possible to measure when they are exposed to the gases leading to devices like chemo-resistors and chemo-capacitors. Other structures are p-n junctions, metal-insulator-silicon (MIS), and Schottky diode where the electrical characteristics change on exposure to different gaseous species.
There are some problems in the process of the manufacture of integral sensors. It is well known that there is an operational limitation depending on the physical state of the material and the temperature at which it is sensitive to gas species. Using material directly on silicon substrates limits the temperatures to 150 °C but with the advent of silicon-on-insulator techniques the temperature can go up to 350 °C on silicon surfaces. With the help of micromachining it is possible to create a localized high temperature zone of the order of 450 to
500 °C to operate the sensing element. Hybrid microcircuit fabrication, screen-printing of the gas-sensing materials and electrodes on high temperature ceramics have some additional advantages and allow increasing temperature of work body.
Some important data on hydrogen, oxygen and carbon monoxide sensors are collected in Table 1—
3, in columns of which material of work body, operating temperature range (°C), range of detection limit, sensing element form, controlled physical parameter, their response time as well as numbers of corresponding references are given. Note again that only data about metal oxide sensors are given. If a semiconductor oxide and the change in
Hydrogen sensors
Table 1
Material of work body Operating temperature range (°C) Range of detection limit Sensing element form Sensor physical parameter Response time Reference
Al2Ü3, Bi2Ü3 450 9 % Thin film Electrical < 1 min 21
Cr2O3, CuÜ, (TF) resistance (R)
Fe2Ü3, NiO,
TiÜ2,
Fe2Ü3 420 0.05-0.5 v/% Porous R 30 s 22
Cr2Ü3, NiÜ (with 300-640 1000 ppm Paste R — 23
noble —
metals) < 1 min
NiÜx 30 — TF (MBE) WorkFunc-Tion (WF) < 1 min 24
CdÜ 450 2.1 % TF R 5-90 s 21
CeÜ 500 2.1 % TF R 10-60 s 21
In2Ü3 350 1000 ppm TF R — 25
SnÜ2 25-575 50-1000 ppm Paste onto Pt wire coil R 12-25 s 26-28
SnÜ2(Sb2Ü5) 20-300 1500 ppm Paste onto R 30 s 29
SnÜ2(CdÜ) 150-450 0.1-1000ppm Al2O3 tube — 30
SnÜ2(Bi2Ü3) 200-700 1 % 1 min 31
SnÜ2 250-400 500 ppm Pellets R 6-20 s 31-32
SnÜ2 200-600 0.32 v/% TF R 1-2 min 33-34
SnÜ2-Sn 150-250 100-5000 ppm TF R — 35
SnÜ2 -Pd 200-450 0.5 v/% Films R 5-7 s 36,37
SnÜ2<Cu> — 435 ppm TF R 10-20 s 38
SnÜ2<Cu> 270-320 400-1000 ppm TF R — 39
SnÜ2 <In> 50-250 500-3000 ppm TF R 40
SnÜ2 RT 40-1000 ppm TF R 5-20 min 41-43
(Pd/PdÜ) < 1 min
SnÜ2 (TiÜ2) 450-650 500-10000 ppm Bulk & TF R 44
SnO2-Bi2O3- 220-320 100 ppm Paste R 45
K2PtCl4
TiÜ2(Pd/Pt) 225 — — R — 23, 37
25-60 14 ppm-1.5% TF SchD 2 min 46
150-300 100-1000 ppm TF SchD 2 min 47
Au-WÜ3 250-350 200-5000 ppm TF R 1 min 48
ZnÜ 267-600 1-100 ppm Pellets, Xls R 2 min 49-52
200-400 1.0 % Disks R 30 s 28,31,52
ZnÜ/Pt 300-450 up 8000 ppm Thick F — 53
ZnÜ(Ru, Ag) 100-400 0.2 % Paste R — 239
ZnÜ(Al,In) 200-350 2-1000 ppm TF R 2-5 min 55-57
SrGeo.95 1000 3-100% Nano TF R 60-500 s 58
Y0.05 ü3
SnÜ2 150-400 500-3000 ppm TF,S-G R 7-85s 59-63
SnÜ2/Pt,Pd 300-600 1-2% TF R 64
200-400 up to 2% TF R 65-67
SnÜ2 150-250 TF R, WF 68,69
SnÜ2/Si 250 10.6-100 ppm TF WF, CVC 1-4 m 70
SnO2-Co3O4 200-250 1000 ppm TF R 71
TiO2-Nb, Pd 200-250 1000 ppm TF,S-G R 72
ZnÜ 300 TF R 73
SnÜ2-ZnÜ- HT R 74
CuÜ R
Fe2O3-6 wt% 320 500-2000 ppm TF R 4 s 75
Ag2Ü R
In2Ü3 350 Por TF 10-60 s 76
Ga2Ü3-SiÜ2 700 12.5-500 ppm TF R 30 min 77
Ga2Ü3 600 500 ppm TF R 78
300-900 PorFilterT R 79
NiO-TiOx 250-300 1000 ppm F R 2-2.3 m 80
MoO3-SiO2-Si 300 2000-9000 ppm TF R 10-40 s 81
MoO3-V2O5 150 1000-10000 ppm TF R 20 s 82
TiÜ2 <2% Al> 200-400 500-1000 ppm Porous TF R 5 s 83
TiÜ2-WÜ3 200 S-G TF R 1-20 m 84
Pt-SnÜ2 RT-50 100-4 vol % S-G Por R 85
<In2Ü3> Th-k F
SnÜ2<F> n-Xl TF R 86
TiÜ2 nanotube 25-250 1000 ppm Por TF R 30 s 87
Table 2
Oxygen sensors
Operating Range of detection limit Sensing Sensor Response time
Material of work body temperature range (°C) element form physical parameter Reference
AI2O3, Bi2Ü3, 450 2.1 % TF R < 1 min 21
CdO, Cr2O3,CoO,
CuO, Fe2O3, NiO,
ZnO, TiO2
CeO2 700-1100 TF R 5-10 ms 52,88
CeO2 (Pt) 615-1002 103-105 Pa n TF R 5-11 s 53
CeO2 on TiO2 700 n TF R 56
CeO2-ZrO2-TiO2 400-600 6%-5 TF EMF 89
ß-Ga2O3 480-821 7x10-5 Pa TF R 91
Ga2O3-AlVO4 600-1000 10000 ppm TF R < 1 min 92
Ga2O3 (ZrO2, 900-1200 0.002-0.02 bar Sandwich R ~ 2 min 93
TiO2, MgO) R
Ga2O3 (CeO2, 500-900 1-20 % T-ck F ~ 2 min 94
La2O3, Mn2O3) R
MoO3-TiO2 150-370 100-1000 ppm TF ~ 1 min 95
SnO2 27-650 2-10 ppm Disks R 10-20 m 96
150-300 300 mbar TF R 97
RT 100 ppb TF R < 1 min 98
200-585 50-700 ppm TF R 99
120 5x10 "9 mb TF R 100
SnO2 -Pd 225-500 R 101,102
ZnO<Al> 10"2-10"06 atm Pellet R 103
TF R 104
ZnO(SnO2,Ga2O3) 127 10-7-104 Pa TF R 105
ZnO-Cu 27 Pellet MISFET 106
TiO2-x 200-800 Up 100 % TF R 107
TiO2 (Pd/Pt) 225 SchD 101
TiO2 (Cr) 600-800 400-1200 Disk R 5-12 s 108
TiO2-Cr2O3 350-400 100-10000pmm TF R 1-3 min 90
TiO2 -Pt 130-800 50 ppm TF SchD ~ 2 min 109,110
Fe2O3<Zn, Au> 350-450 10-10000ppm TF R 10 min 111
ZrO2 253 ElChCell EMF 112
ZrO2 -Pd-PdO 350-500 < 10-6 atm ElChCell EMF 1-190 s 113
ZrO2-Ga2O3 900-1200 0.002-0.2 bar LayerStr R ~ 2 min 93
ZrO2-9% Y2O3 200-330 0.1-100% ElChCell EMF 1 min 114-115
ZrO2-Au,Pt,Cr 300-700 0-6% AmpCell Current 116
MoOx-SnO2 150-400 10 ppm Pellets R 10 min 117
SrTi1.yNb0.05O3 1000 3-100% n TF R 60-500 s 118
SrTiO3 40 20 % n Th-cF R 119
SrTixFeyO3_s 700 0.8-300 mTorr TF R 120
BiCuVOx 300-700 10-100% TF EMF 121
SmCoO3<Ba> 373-410 100 % Th-cF R 43 s 122
the resistivity R (conductivity) as controlled parameter are indicated, it means that the working material has Ohmic contacts and linear current-voltage characteristics (CVC). Schottky diode is marked as SchD. The doping with a metal and another metal oxide is shown in brackets, additional layer(s)-via dash, thin catalytic islands-via flesh. The corresponding parameter' absence in columns in the reference is marked by a dash. TF means thin film, Th-cF means thick film, HJ means hetero junction, Xl means crystal, por means porous, S-G means sol-gel, ElChCell means electrochemical cell.
It is seen from Tables, that pre-heating of the work body of the sensor is necessary in most cases, sometimes-dramatically high one. Problem of the selectivity of the work body of sensor does not solve in most cases.
Hydrogen sensors
Hydrogen is the most attractive and ultimate candidate for a future fuel and an energy carrier. Its generation can be realized by a variety of methods, including reforming of natural gas and alcohols (methanol etc.), electrolysis and photoelec-trolysis of water and biomass generation, as well
as chemical decomposition of hydrogen containing compounds. Result of hydrogen' burning is water, which is decomposed again in hydrogen and oxygen. In hydrogen-based fuel cells, the electrical energy will be derived from the reaction of hydrogen and oxygen gases within the fuel cell to make water. Hydrogen does not require a fuel processor in fuel cells, and hydrogen is producible from renewable energy resources using the electricity from solar cells or wind. The biggest barri- ¡5 ers to this hydrogen vision are the associated costs a. and the establishment of a safe and effective infrastructure. A common need in this technology | area is the ability to detect and monitor gaseous J hydrogen. But hydrogen gas sensors that can quick- 5 ly and reliably detect hydrogen over a wide range ° of oxygen and moisture concentrations are not & currently available. As hydrogen diffuses easily g through most materials, containment of hydrogen 8 is difficult. 0
There are many versions of hydrogen bulk sensors, for example, semiconductor oxide type or hot-wire type, where the resistance of the sensor materials changes dramatically as sensor surface reduced or the temperature of the sensor materials increased, respectively. Most of them are saturat-
Table 3
Carbon monoxide (CO) sensors
Material of work body Operating temperature range (°C) Range of detection limit Sensing element form Sensor physical parameter Response time Reference
Bi2O3 on SnO2 200-350 Up to 500 ppm TF R 80-90 s 123
CoOx 50-10000 ppm TF WF 2-4 min 124
CuO 350 1-1000 ppm Catal.Me R 125
mbr
CuO<Na>/ZnO 200-400 4000 ppm Disks Hetero Junct 1.5 min 126
CuO/ZnO 300 10000 ppm Disks CVC 30-120 m 127
Ga2O3-SnO2 500-950 10000 ppm Paste Hetero Junct 128, 129
I^O, 350 1000 ppm TF CVC < 1 min 25, 77
I^O, (Rb) 300-600 1000 ppm Paste R 5-90 s 130
In2O3/SnO2 RT- 400 1-3 ppm TF R 2 min 131
In2O3/Co3O4 250 2000 ppm Block R 10s-10m 132
Fe2O3 525-1075 200 ppm Xl R 21, 178
Fe2O3 (Au,Zn) 150-400 50-400 ppm TF R < 1 min 133
Fe2O3 (TiO2) 300 10-400 ppm TF R 2-5 min 134
MoO3/WO3 150-370 30 ppm TF R 50-250 s 95
NiOx 30 1000 ppm TF MBE R 2 min 24
Nb2O5 400-500 100-1000 ppm T-ckF R 135,136
SnO2 200-500 1-100 ppm T-ckF WF 3-4 min 137
130-310 1-20 ppm TF R < 1 min 138, 181
25-400 200-3000 ppm TF R 59-63, 139
575 TF R ~1 h 28, 140
RT 1000 ppm TF R < 1 min 98
150-350 10 ppm Paste R 141
20-300 1500 ppm T-ckF R < 1 min 29
500-1000 200-500 ppm Paste R 28, 129, 142
200 1 % Pellets R 45, 182
250-400 500 ppm TF R 2 s 32, 177
300-500 50-500 ppm TF R < 1 min 64, 143
80-450 10-200 ppm TF R 144
RT-700 100 ppm TF R < 1 min 34
SnO2-Pt 50-250 10-1000 ppm MES R ~0.1 s 145,146
27-100 54-500 ppm FET R 5-10 ms 147
TF R ~ 200 s
150-300 400-1000 ppm TF CVC Few min 148,149
RT-400 1-60 ppm T-ckF 150
SnO2-Pd/Pt RT-523 250&1000ppm50- TF R 13 s 151
SnO2-Pd 200-450 10000 TF R 5-10 s 36, 37, 152-4
SnO2<Cu> 50-350 15 ppm TF R 2-200 s 38
270-320 400-1000 ppm TF R 1-2 min 39
SnO2<In> 50-250 500-3000ppm TF R 40
SnO2<Au> 220-550 25-250 ppm Xl R 5-7 s 155, 156
SnO2<Sb> 500-1000 200 ppm R 10-20 s 156
~ 20 s
350 Up 1000ppm Paste R 157
SnO2(Pd) 50-450 50-300 ppm T-ckF R 158
450-500 10-1000 ppm TF < 1 min 135
80-280 100-300 ppm Paste R 3 min 109/159
100-300 50-800 ppm TF R 3 min 160
50-350 300 ppm TF R < 1 min 161
SnO2(Pd/Pt) 240-300 300 ppm TF R 50-850 s 43
75-450 200 ppm Pellet R 162
SnO2(ZnO) 27-800 50-10000 ppm Paste R ~ 5 min 45, 52, 163
SnO2(Bi2O3) 20-300 1500 ppm Paste R < 1 s 29
SnO2(Sb2O5) 20 20000 ppm Films n-n HetJ < 1 min 164
SnO2(MoO3) 100 -250 10-1000 ppm T-ck F R 71
SnO2(Co3O4) 20-280 up 1000 ppm TF R 165
Pt, Au, Ni, Mo
TFs with SnO2 50-500 up 10 torr TF R 166
Coating
Pt-SnO2- 200-450 50 ppm TF CVC Diode 109, 176
diamond 167
SnO2(Pt) 300 up 100 ppm MNOS SchD ~ 2 s 168
400 100 ppm
SnO2 700-1000 JFET < 3 min 156
225 200 ppm Xl FET ~ 10 min
227 R 37
TiO2 600 50 ppm TF SchD 47, 109
CrxTi1-xOy up 1000 ppm TF R 169
TiO2 (Pd/Pt) 420 30 ppm TF R 95
TiO2 (Pt) 300-500 TF 179
TiO2 (CuO) 267-600 20-100 ppm Xl R ~ 1 min 49
TiO2 (MoO3) 300&400 Pellets R ~ 2 min 50
TiO2 (Nb) 450 1-100 ppm Bars R 28, 52
ZnO 200-700 Up 8000 ppm T-ckF R 53
358-564 Whisker R ~ 2 min 57
200 1 % El-ChCell R
ZnO (Pt) 500 100-1000 ppm PorCeram EMF 170, 171
ZnO (Li) 400 1 % TF R 172
ZrO2(Y2O3) 100 ppm TF R
BiTiO3(La) < 80 ppm R 8-10 s 183
BiSnO3 3 min 180
Mo-W-O and min
Ti-W-Mo-O
ed and useless for high hydrogen concentration. There are thin film sensors like field effect transistors (FETs) with Pd catalyst or chemo-resistive Pd alloy resistor, where hydrogen at the Pd-gate interface results in a change of FET channel' current or the sensor resistance. There are other hydrogen sensors with different operating principles and type, based on different materials. Among them — optical fiber type, piezoelectric type, thermoelectric sensor, wire coated by Pd, Schottky and MIS diodes, solid electrolyte, polymers, am-perometric and potentiometric sensors, carbon na-notube and fullerene sensors, graphite oxide, different metal oxides etc. (see, for example, review-papers and books [1-9] and the references therein). In particular, different hydrogen sensors are considered in our review-paper published in this journal in 2005 [7]. But, as it was mentioned above, we collected below in Table 1 only data of metal oxide semiconductor hydrogen sensors.
At present, commercial hydrogen detectors are not suitable for widespread use, particularly in transportation, because they are mainly too bulky, expensive, and some are dangerous. The sensor working at high temperatures itself becomes a possible trigger of explosion, due to its enough high input electrical energy of sensor operation. The concentration monitoring of hydrogen is very important for the application of fuel cell as well as for the case that hydrogen being an undesirable contaminant, for example, chemical industry. Not only the leak detection but also the concentration monitoring is important application of hydrogen sensor. From the standpoint of the safety with the global environment, it is necessary today to develop new hydrogen gas sensors working at or near room temperature without any power source. In addition, they should be small, cheap and easy to be implanted into microelectronic integrated circuits.
Oxygen sensors
Oxygen sensors are producing in a large scale exceeding several ten million sets yearly. Oxygen sensors are widely used as sensors for automotive applications. To decrease the exhaust emissions from gasoline internal combustion engine automobiles, the air/fuel ratio is monitored with lambda oxygen sensors usually made of a metal oxide. Each car powered by a gasoline engine should be equipped with at least one lambda sensor measuring the air-to-fuel ratio to control engine operation. Second lambda sensor downstream catalyst is used for on-board continuous diagnosis. However, the air/fuel ratio cannot be detected during engine warm-up phase, because today oxygen sensors in car cannot operate at room temperature, they should be heated from room temperature to much higher temperatures. As most emissions are released in the warm-up phase, the so-called lightoff time, oxygen sensor requires with a very short light-off time. Planar zirconia sensors have been reported to have fast light-off characteristics. Together with short light-off time, sensor should have small size, simple structure and low cost. Oxygen
gas sensors must respond quickly to changes in oxygen partial pressure and allow controlling the combustion in each cylinder. For it, the response time of oxygen sensors must be 10 ms or less.
Solid-state potentiometric electrolyte sensors made of yttrium stabilized zirconia (YSZ) having fast and stable response characteristics are present today on the market for oxygen sensors. But a complex technological process is needed for manufacturing of such devices. Therefore, the semiconductor thin film sensors made of TiO2, SnO2, CeO2, Ga2O3, ZnO, SrTiO3, BaTa08Fe02O3, SrTi1_^FexO3_5, La-doped SrTi^Fe^g, La2CuO4 and other materials have been reported in the literature (see Table 2). For upcoming sensors, temperature-independent sensor characteristics are of high interest. Although the excellent temperature-independent properties of lanthanum cuprate are known, this material is not stable at high temperatures and under reducing conditions.
Investigations of oxygen sensors are still very important. Of course, the manufacture and use of oxygen sensors for fuel cells, energetic and other applications is demanded.
Carbon monoxide sensors
It is well known that CO is produced due to the incomplete combustion of fuels, it is commonly found in the emission of automobile exhaust. Such toxic and dangerous is colorless and odorless. The demand for better environmental control and safety has increased research activities of solid-state gas sensors as hydrogen and toxic gases have caused major accidents during the past several decades (including Chernobyl).
The CO detection is one of the most important. For CO detection, the German legislation fixes the maximum tolerance concentration in working place at 30 ppm and alarm level at 60 ppm, while in Italy an attention level is 12.9 ppm and alarm level is 25.8 ppm [180].
Promising materials for manufacture of CO sensors and their parameters are given in Table 3.
Arrays containing several gas sensors are often needed in order to sense the presence of smoke and such gases. The selective detection of CO in the presence of H2, however, is very difficult, because the working mechanism of SnO2 and other ceramic gas-sensing materials is based on the catalytic oxidation of gas molecules. H2 undergoes facile oxidation and, thus, most types of ceramic gas sensors are more sensitive to H2 than to CO.
New results on hydrogen and carbon monoxide sensors obtained in Yerevan State University
Investigations of tin oxide thin-film hydrogen sensors made using sol-gel technique were carried out [184]. A solution of Na2SnO3 was used as a precursor. The analysis of the surface morphology of specimens obtained by TEM images have shown that the dimensions of SnO2 particles were about 4-6 nm and practically do not increase even after calcinations at 750 °C.
The measurements of the gas sensors performances were carried out by means of the developed by us automated DAQ system allowing changing the target gas partial pressure in the measurement chamber, sensors operating temperature and other parameters. It was established that though our sensors are working also at room temperature without pre-heating of work body, the maximal response was registered at temperatures 100-125 °C. The high sensitivity to hydrogen at 100-5000 ppm concentration range was revealed. The response and recovery times were less than 1 s and about 1012 s, correspondingly. The sensors performance in high humidity conditions was realized, which was very important for the work in fuel cells.
Hydrogen gas sensor working at room temperature and made of TiO2-x/porous silicon structure have been recently realized [185]. The current-voltage characteristics of such hetero-junc-tion diode structure, their sensitivity to different hydrogen concentrations and the resistance change versus time have been studied.
New method of evaluation of sensitivity and selectivity of gas sensors using noise measurements was developed recently [186]. CVC and noise spectral density of the Al/porous silicon/single-crystalline Si/Al sandwich structures were measured in different gas media. It was shown that the spectrum of low-frequency noise varied with frequency f as f-8, where 8 is equal to ~1 ("classical" 1/f noise) for the air and ~1.6 for the mix of the air and CO. The presence of CO led to an increase in the electro conductivity and a reduction of the low-frequency noise level. The more was the percentage of a gas - oxidant in the mix with the air, the more were these changes. Value of spectral density is changed on several orders of magnitude in different gas media and in the same medium with a change in the concentration of the same gas in air.
Conclusion
1. Basic semiconductor materials for the manufacturing of hydrogen and carbon monoxide sensors are tin dioxide and indium oxide. Zirconia is main material for oxygen sensors. TiO2, ZnO, Fe2O3, NiO are promising oxides for sensors considered in the review.
2. Many of metal oxide sensors need in preheating and high operating temperatures, which lead to high consumed power.
3. It is very difficult to envisage high selectivity of metal oxide sensors although some success is detected.
4. New powerful method of noise spectrosco-py developed for measurements of sensitivity and selectivity of gas sensors.
5. High-response thin film metal oxide sensors working in the temperature range: room temperature-150 °C can work at high humidity and without remarkable power consumption. Such sensors can be recommended for large scale production after corresponding development.
Acknowledgemnts
This work is supported by ISTC A-1232 and CRDF-IPP ARE-10838-YE-05 Projects. Author is thankful to R. Abelyan, H. Hovhannisyan and
A. Poghosyan for their help.
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