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H YD R O GEN EN ER GY AN D TRANSPORT Gas analytical systems and hydrogen sensors
SEMICONDUCTOR METALOXIDE HYDROCARBON
GAS SENSORS
V. M. Aroutiounian
Member of International Editorial Board
Department of Physics of Semiconductors and Microelectronics at Yerevan State University
A. Manoukyan, 1, Yerevan, 0025, Armenia E-mail: kisahar@ysu.am
Vladimir M. Aroutiounian received his physics-engineer MS degree from Kiev Polytechnic Institute in 1964, the doctoral degree (Ph. D.) from Yerevan State University in 1970 and Dr. Sc. degree from Vilnius State University, Lithuania, in Physics of Semiconductors and Dielectrics in 1977. Academician of National Academy of Sciences and Engineering Academy of Armenia. He is currently the full professor, head of Department of Physics of Semiconductors and Microelectronics and the research supervisor of Research Laboratory of Physics of Semiconductor Materials and Devices at Yerevan State University. His current research interests include chemical sensors, physical properties of porous and quantum-well semiconductor structures, photoelectrochemical and photoelectrical conversion of solar energy.
Rugged chemical gas sensors are of great importance in industrial process monitoring and exhaust gas control applications. Detection of toxic and flammable gases is a subject of growing importance in both domestic and industrial environments. Methane and other hydrocarbon gases monitoring has got a new dimension because of the use of compressed natural gas (containing methane) as a fuel for automobiles.
Promising metal oxide semiconductors for detection of methane and other hydrocarbons are mentioned in review paper. Materials, the range of necessary temperatures, type of sensor and filters for them, time of response as well as corresponding references are presented in review.
Aroutiounian Vladimir M.
Introduction
Several metal oxides thin or thick films show a fast and sensitive response to varying gas atmospheres and are therefore appropriate for gas sensing systems. In particular, it is important to £ have the information about the concentration of a. methane and other hydrocarbon gases in air in the
5 range of 700-10000 ppm of gases [1]. Semicon-1 ductor gas sensors based on n-type semiconductor ¥ metal oxides, proposed first by Seiyama et al.[2] | and Taguchi [3], have been widely used for detec-° tion of different gases. Semiconductor gas sen-
6 sors have been widely studied during 30 years g and now there are several commercial products, ™ especially based on tin dioxide material (see, for 0 example [4-13]). Available in the market sensors
are based on a change of resistivity R after gas exposure. Some semiconductor gas sensor advantages are high sensitivity, simple design, low cost,
Статья поступила в редакцию 21.01.2007 г.
mass production and integration compatibility which can be obtained using different approaches.
Nevertheless, these types of sensors continue to suffer of lack of selectivity and also long-term stability which limit their applications. For example, it is difficult or even impossible to use existing cheap sensors for methane detection in the case of automobiles powered by natural gas without getting false signals from the presence of other chemicals such as gasoline vapors and solvents. It is important also the methane detection selectivity in the presence of alcohol and flue gases at houses and factories. Very stringent criteria were established for methane safe mine monitoring systems [14]. Existing platinum wire sensors need in its heating above 400 °C. Such detectors have drawback, which, apart from very high price of platinum, comes from a partial re-crystallization of Pt and a consecutive decrease of their sensitivity when such detectors are intensively used.
The article has entered in publishing office 21.01.2007.
Many research groups are always focused on studies on one hand on fundamental models to try to have a good control of the performances of these sensors, and on the other hand on signal treatment including multi-sensors devices and pattern recognition. Of course, the researches concern also the materials, especially the modification by doping and the addition of catalytic metals or oxides. The sensitivity and selectivity of gas sensors can be improved by the introduction of catalysts Pt, Pd, Rh, Ni etc., by the addition of active or passive filters (this solution will be discussed below), by alteration of their working temperatures and operation modes and by adequate treatment and processing of their signals [15].
The main metal oxide sensors made of tin dioxide SnO2 (see Table). Among other promising materials, TiO2, ZnO, different Ga2O3, metal oxide solid solutions, SnBaO3 etc. should be mentioned. Main drawback of all semiconductor sensors is rather high temperature of pre-heating of work body of all type of sensors.
SnO2 semiconductor sensors
Typical dependencies of sensitivity of SnO2-based sensor on concentration of methane are shown on Figs. 1 and 2. Sensing mechanisms of noble metal doped thick film devices for hydrocarbons have been proposed by several authors [38, 7679]. In the particular case of methane sensing, the mechanism involves the oxidation reaction of methane which produces CO2 and H2O via CHn or CHnO intermediates (0 < n< 4) and the reaction rate is promoted by noble metals such as Rh, Pt and Pd [80-84]. So far, the high sensitivities of these semiconductor sensors to methane have been explained as a result that the noble metal loaded effectively catalyzes the reaction between methane and adsorbed oxygen on semiconductor oxides, leaving oxygen vacancies or conduction electrons behind. In many cases the promoting effects of noble metals are clear but the mechanisms involved are not, due to a lack of direct supporting evidence in spite of many excellent researches on the subjects [38, 85-88]. The roles of noble metals in sensors should be made clear on the basis of the informa-
200 250 300 350 400
Operating temperature (°C)
450
500
Fig. 1. Sensitivity to 5000 ppm CH4 as a function of operating temperature for undoped SnO2 sensors annealed at 500-700 °C [41]
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_L_
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_L_
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2000
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il
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25
20
15
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500 ppm CH4 0% RH
500 ppm CH4 30 % RH
500 ppm CH4 60 % RH
A'''. . -V' ' -
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200 225 250 275 300 325 Operating temperature (°C)
350
375
4000 6000 8000 10000 12000 CH4 concentration (ppm)
Fig. 2. CH4 gas concentration characteristics for undoped and 0.1 wt. % Ca-doped SnO2 sensors operated at 400 °C [41]
tion on the dispersion states of metals, as well as the interactions between metals and supports.
It is seen from Table that various noble metals were incorporated in the tin oxide-based methane sensors to verify their catalytic roles (Fig. 3). Although several conventional methods have been applied for loading tin oxide sensors with noble metals, it is rather difficult to get high metal dispersion and to control the metal dispersion regardless of the loadings with these methods. Such difficulty was expected to be fairly serious in the present study in which commercial tin oxide powder having a small specific surface area was used. From these considerations, authors [38] adopted an unconventional method for noble metal loading, i. e. mixing the tin oxide powder with alumina (or silica) supported noble metal catalysts. The noble metal components supported there are known to be well defined and highly dispersed. This method turned out to be very effective for increasing the sensitivity of the tin oxide sensor to methane. For comparison, the conventional methods were also applied to get information on the effects of metal
Fig. 3. Sensitivity data from the 450 °C calcined tin oxide nanopowders, with a 2 % of Pt, as a function of the operating temperature of the sensor. In the figure the sensitivity to 500 ppm of CH4 is shown. Measurements are reported under synthetic air atmospheres with a relative humidity in the range between 0 % and 50 % [40]
5
0
Table of CH. sensors
Operating temperature range (°C) Sensor physical parameter
Material of work body Range of detection limit Sensing element form Response time Reference
SnO2 25-500 1000 ppm Films R 16, 17
320 1 % Paste R < 1 min 18
200-600 0.26 v/% T-ck F R ~ 1 min 19,20
RT to 700 10000 ppm TF R 5-10 ms 21, 22
SnO2-Pt 100-400 5000 ppm TF R < 3 min 23
SnO2-Pd 200-450 0.5 v/% Films R 1-2 min 23-27
350 Up 10000 ppm Paste R < 1 min 28
SnO2-Pd/Mo/ 450-500 1000 ppm TF R ~ 5 min 29, 30
Cu/Rh/Pd
SnO2(Pd/PdO) RT 40-1000 ppm Xls&TF R 31
SnO2(Sb2O3) 350-700 20-10000 ppm Semistor R 10-30min 32
20-300 1500 ppm Paste R 16
SnO2(MoO3) 20 20000 ppm Film R 33
SnO2(CdSnO3) 25 1000 ppm Paste R ~ 10 s 34
SnO2(Al2O3 - 220-320 50-10000 ppm Paste R < 1 min 18
PdCl2) R
SnO2<Fe>/Pd 350 1000 ppm Paste R 35
SnO2-PorPt 400-600 R 36
SnO2<Sb> 480 TF R 37
SnO2/Pt,Ca 300-385 0.03-1% R 38
SnO2/Pt,Au,Pd 350-500 Th-ckF R 39
325-350 Sol-gel R 40
SnO2/Ca 400 5000 ppm R 41
SnO2<F> 20 Up 100 % Paste R 42
Nb2O5 /SnO2 350-700 R 43
SnO2 - TiO2 300-640 10000 ppm Paste R 44, 45
Cr2O3 (Pt etc) 27 10-10000 ppm TF WF 2-4 min 46
CoOx 500-1000 5500 ppm Film R 30 ms 47
Ga2O3 /SnO2 500-950 10000 ppm Paste R < 1 min 48
540-899 25-50 ppm T-ck F R ~ 1 min 49
Ga2O3 (Rh, Ru) 350 1000 ppm TF R 5-90 s 50
In2O3 525-1075 200 ppm Xl R 51
Fe2O3 420 0.05-0.5 v/% Porous R ~ 30 s 52
Fe2O3 (AI2O3, 300-640 10000 ppm Paste 53
La2O3)
NiO(Pt etc) 46
Bi2O3, MoO3, Th-ckF R 54
PbO, MgO
Al2O3/Pd Wire/bead R 55
Al2O3/SnO2/Pd, F 200 R 56
ThO2/Pt 500-550 Pellistor R 57
SrFeO3-x 58
YBaCuO<F> 59
YSZ EMF 60
BaSnO3/Al2O3 500-1000 0.03-1% R 61
PorSiO2/Ga2O3 700 TF R 62
TiO2 600 R 63
TiO2 400 TF FET 64
TiO2(Pd/Pt) 700-1000 0.5 % Xl R ~ 10 min 52
Por Si-CHx 200 ppm 65
ZnO TF 66
ZnO 225 SchotBarr 67
25-320 TF R 68, 69
ZnO-Pt 300&400 0.5 v/% Bar R ~ 4 min 71
ZnO(Pt,Ru,Rh) 450 Up 8000 ppm T-ck F R 72
ZnO(Al,In,Ga) 100-400 0.2 % Paste R 73
ZnO(Li) 200-350 2-1000 ppm TF R 74
200-700 1 % Whisker R ~ 2 min 75
The following notations are made in table: R — resistance, WF transistor, SchotBarr — Schottky barrier
work function, EMF — electromotive force, FET — field effect
dispersion and the roles of metals in the tin oxide-based methane sensor.
The sensitivity of the sensor mixed with alumina supported Pd catalyst to methane at 658 K was higher than those of any other sensors which were mixed with supported Pt, Rh or Ni catalyst or loaded directly with Pd by conventional methods, when compared at the same metal loading.
This sensor responded to methane in the range 500-10000 ppm with sufficiently high sensitivity and response rates, though it also responded to many other gases. Such promoting effect of the supported Pd catalyst is considered to originate from the high dispersion of Pd (or PdO) particles supported in addition to the high intrinsic activity of Pd for the catalytic oxidation of methane. At a
lower operating temperature (300 °C), however, the above promotion by the supported Pd catalyst was not always sufficient. The addition of Ca and/or Pt (0.1 wt. %) to the parent tin oxide powder, which turned out to suppress the grain growth of tin oxide during calcination, was found to be effective for further promoting the methane sensitivity at 300 °C. Based on these results, methane sensing mechanism is considered to consist of two
to C3H8
and CH4 was
5 ' 4 '
ig «
в 3 ' К
250
300
350
Temperature (°C)
400
450
Fig. 4. Sensitivity of Pt/SnO2 to 1000 ppm CO, 800 ppm ethanol and 2.8 mol % CH4 in air versus temperature (with the active filter) [108] 4
steps, i. e. activation of methane molecules on the supported Pd catalyst and surface reaction of the activated molecules on the tin oxide particles. The presence of Pt on tin oxide would assist the latter step, especially at the lower temperature (Fig. 4).
Numerous efforts have so far been directed to improving the sensitivity of semiconductor gas sensors to methane by adding a small amount of a sensitizer [39, 89-91]. As was mentioned above, in most cases, the improved sensitivity is due to the chemical sensitization by noble metals [9], whereas the electronic sensitization is dominant in some cases of SnO2 loaded with Ag [89] or Pd [92]. Thus, the sensitivity is mainly controlled by the catalytic activity of a sensor material, or by the reactivity of an objective gas on the surface of the sensor material. Besides the reactivity, diffusivity of gases (more precisely, difference in diffusivity between an objective and oxygen gas through a porous sensor) was found to be another important factor determining the sensitivity of the interior region of sensors [93, 94]. In [39] the reason for the higher H2 sensitivity observed for the interior region of a porous thick film SnO2 sensor than the surface region has proved to be attributed to lower diffu-sivity of O2 than H2 into the interior region [93]. Note that CH4 is less reactive than H2, has larger molecular diameter and smaller mean free path in comparison with H2 and O2 species.
Enhancement in CH4 sensitivity of an SnO2 thin film sensor induced by the 1.0 wt. % Pt loading was explained by the chemical sensitization effect, namely by the effect of gas reactivity based on the catalytic activity of SnO2 [39].
The sensitivity of SnO2 markedly increased up to two cycles of the surface modification with a diethoxydimethylsilane sol solution, but decreased with further modification [95].
Filters for SnO2 sensors
It was mentioned that in the multicomponent gas mixtures of real applications, the metal oxides do not selectively react to the gas to be detected but they react to a wide range of gases with similar chemical behavior. Temperature step methods may help in certain applications, but their use is limited. To overcome these problems, intense work in the field of gas filters has been performed with high-temperature stable gas filters located directly at the surface of the sensing thin film. Selective CH4 sensors are reported in [62] using porous catalyst filter and sensitive layer made of Ga2O3 operated at 500-900 °C.
Filtering membranes [56] can separate interfering gas molecules of different size due to different diffusion rates [96] and also due to selective interactions with certain gas molecules [97, 98]. The concept of filtering upstream the sensor is now used in particular from several years for different commercial CO sensors using charcoal to avoid the interfering effects of hydrocarbons and alcohols [99, 100]. In this case, the active coal is placed in the packaging box and it acts as gaseous absorbent working at moderated temperature.
For the concept of filtering, two types of actions can be used: porous films or membranes with an action of physical separation based on the size of the pores and molecules, or catalytic films with an action on gas decomposition.
The membranes can increase stability of semiconductor gas sensors by filtering corrosive and irreversibly adsorbing gases. In some cases they can increase sensitivity of sensor element to a target gas [101]. This solution has been proposed from many years, firstly in 1986 by E. Logothetis to stop the interfering gases in the detection of CH4 by a porous platinum catalyst placed before a SnO2 sensor but not in contact with the sensing material and also in 1994 by the group of M. Egashira for H2 detection using a thin film of SiO2 deposited by sol-gel directly onto the SnO2 sensing film. The most used filters are thin films of catalytic metals or pure and doped ceramic membranes on the surface of sensor element but both have its limitations. Ceramic filters significantly reduce kinetics of interactions of target gas molecules with surface of sensing element decreasing its response time and sensitivity. Thin metal films can short circuit the sensing element and also metal atoms can diffuse into semiconductor oxide changing properties of the sensor. The deposition of pure and doped with catalytic metals dielectric thin films directly on the surface of semiconductor sensing element allows increasing the selectivity without considerable decrease of the sensitivity and increase of the response time. Such membranes also can protect gas sensitive materials from corrosive gases. Doping of the membranes by various
6
2
1
0
catalytic metals can improve filtering of certain gases due to selective interactions of doped membrane and gas phase molecules. Platinum films as well as SiO2 thick insulating layer were used in [101] for CH4 detection with the goal to reduce the cross sensitivities to CO and alcohol.
Thin porous films of pure and doped Al2O3 have been investigated as catalytic filters for SnO2 (Pd) sensors. Al2O3 membranes deposited on the SnO2 surface reduce sensitivity of SnO2 (Pd) to H2 and especially to CO and increase it to CH4 at 200 °C. The use of the membrane structures allowed detecting CH4 or H2 in presence of CO. Ru-doped membranes drastically reduced the sensitivity to H2 in comparison with other membranes. Selective detection of methane in the presence of CO and H2 was possible for Ru-doped membranes at 200 °C [56].
Zeolitic porous materials constitute interesting filters. According to their nature, these materials can present pores diameters ranging from 0.2 to 1 nm which is in very good agreement with the general sizes of some gaseous molecules: H2 = 0.218, CO = 0.380 or C6H6=0.6nm [101]. Different materials for filters and results obtained are discussed in [101] in details.
CH4 gas-sensing characteristics were investigated for undoped and 0.1 wt. % Ca-doped thin film SnO2 sensors formed by thermal oxidation of Si wafer and ion beam sputtering onto SiO2 [41]. Annealing and operating temperatures were varied to find best sensitivity. Long-term stability was also measured.
Highly sensitive and selective methane sensors were obtained by laminating the selective oxidation catalytic layer on the microstructure-controlled sensitive SnO2 thin films [20, 102]. Recently, research on the decrease of power consumption of a sensor heated by a micro thin film heater made by using the Micro Electro Mechanical Systems (MEMS) technology has been conducted [103-107]. CH4 sensing properties of a SnO2-based thin film sensor
й о ft
70 -
60 -
50 -
40 -
30 -
20
10
—■— 0 wt. % —▼— 0.1 wt. %
0.5 wt. %
300 320 340 360 380 400 420 440 460 480 500
Temperature (°C) Fig. 5. Percent response to 1000 ppm methane vs. temperature curves for tin dioxide coatings containing different amounts of iron oxide [35]
having low power consumption and fabricated by MEMS technology were reported in [20].
Fe-doped SnO2 sensors which can detect methane and butane through temperature modulation were reported in [35] (Fig. 5). Sensors can selectively detect methane at 350 °C and 425 °C correspondingly.
The LaCoO3 perovskite is used as an active filter for elimination of the sensor sensitivity to CO and ethanol in [108]. Both CO and ethanol are completely removed by the filter at temperatures as low as 190 °C. At 250 °C, the sensor sensitivity to ethanol dramatically decreased from 158 to 0.44 and that to CO declined from 2.2 to 0.9, when active filter is used. Only methane reaches the Pt/SnO2 sensor at temperatures higher than 190 °C, for which the sensor shows high sensitivity to methane. As a result, the LaCoO3 perovskite filter eliminates the sensor sensitivity to CO and etha-nol, making the sensor highly selective to methane in presence of CO and ethanol in air.
Sensors on other materials
It was mentioned above that most of the commercial gas detectors for methane available at present make use of SnO2 as a sensing element [44]. They operate at moderate temperatures and are considered as "surface sensors" because the gas-semiconductor interactions leading to the physical adsorption and chemisorption modify change in the conductivity of relatively shallow region near the surface. Other materials can be used for manufacture of methane sensors. Such materials are also listed in Table.
In titanium dioxide the bulk diffusion of defects determines the sensor response. Such thermo-dynamically controlled "bulk defect sensors" operate over a large range of oxygen partial pressures at much higher temperatures (1000-1200 °C). High sensitivity of TiO2 to oxygen has been so far successfully exploited in so called lambda sensors that control the air-to-fuel ratio in an internal combustion engine [44].
ZnO samples have the sensitivity to methane, hydrogen and carbon monoxide [66].
Mixed oxides have recently emerged as promising candidates for gas detection [44]. It has been realized that such systems may benefit from the combination of the best sensing properties of their pure components. Formation of mixed oxides leads to the modification of the electronic structure of the system. This includes the changes in the bulk as well as in the surface properties. Bulk electronic structure, the band gap, Fermi level position, transport properties, etc., are affected mostly in the case of compounds and solid solution. Surface properties are expected to be influenced by new boundaries between grains of different chemical composition. It is anticipated that all these phenomena will contribute advantageously to the gas sensing mechanism. For example, the mixed-oxide TiO2-SnO2 system has recently attracted considerable attention thanks to the structural analogy between TiO2 and SnO2. Both TiO2 and SnO2 crys-
tallize in the tetragonal rutile structure and form solid solutions over the entire range of compositions only above a certain temperature. This critical temperature is in turn a function of the chemical composition of the solution. SnO2 and TiO2 show many similarities in the electronic structure and yet they possess some fundamental differences (see [44]).
The gas sensing properties of TiO2-SnO2 have been studied [44]. Mixed oxides (SnO2-WO3, TiO2-WO3, and CdO-In2O3) are also promising for gas detection. In [45] the response of Sn-Ti mixed oxides to methane is remarkable since 200 °C and increase up to 1000 °C. Sn0 05Ti0 95O3 thin films have best sensitivity to methane at 700 °C. Sensitivity was decreased for systems containing more tin in comparison to titan (concentration of tin was increased up to 95 %), but it was possible to detect methane at pre-heating at 350 °C. Sensitivity to hydrogen was higher at 450 °C. Enhanced sensor responses towards methane (150-1000 ppm) and hydrogen (200-10000 ppm) and lower preheating temperature (400 °C) were obtained in [45] in the case of the rheotaxial growth and thermal oxidation of above-mentioned solid solution.
Detecting of other hydrocarbon gases
Situation with detecting of other hydrocarbon gases is far from satisfactory one. Anyway, there is small quantity of papers in this field part of which will be cited below. Sensor for unsaturated hydrocarbon gases made of zirconia was reported in [109] which produced rather large electromotive force after the contact with gases. Sensors detecting butane (C4H10) are reported in [35, 37, 51, 55, 104, 110, 111], C3H8 — [36, 42, 95, 112, 113], C3H6 — [113, 114]. Propane was detected in [115118], ethylene — [54, 115], propylene — [54, 60], C2H4— [118-120], toluene and benzene — [121, 122], isobutene — [61], acetylene C2H2 — [123], benzyl C6H6 — [101, 119, 124]. 2 2
Conclusion
Knowledge of a large variety of materials and their characteristics is important with regard to highly specialized applications. Detection of methane and other hydrocarbons is a subject of growing importance in both domestic and industrial environments. Monitoring of methane and different hydrocarbon gases has got a new dimension because of the use of compressed natural gas (containing methane) as a fuel for automobiles.
Promising metal oxide semiconductors for detection of methane and other hydrocarbons are mentioned in review paper. It is necessary to provide pre-heating of work body of sensors made of such materials in order to reach remarkable sensitivity of detectors to methane and other hydrocarbons. Materials, the range of necessary temperatures, type of sensor and filters for them, time of response as well as corresponding references are presented in review.
Main material for such sensors is still tin oxide.
Acknowlegments
This work was carried out in the framework of the ISTC A-1232 project and 041030 "Semiconductor Nanoelectronics" National Scientific Program.
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■ International Conference and Trade Fair on Hydrogen and Fuel Cell Technologies
Oct 22 - 23, 2008 http://www.hamburg-messe.de/H2Expo/h2_en/start_main.php
INTERNATIONAL CONFERENCE OF H2EXPO
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Congress Center Hamburg Patronage
Senator Dr. Michael Freytag, Department of Civil Engineering & Environment Conference Chair
Prof. Dr. Wolfgang Winkler (HAW - Hochschule für angewandte Wissenschaften, Hamburg) Dr. Subhash C. Singhal (Pacific Northwest National Laboratory)
Program committee
Honorary president
Rüdiger Kruse Hamburg state parliament
Dr. Etsuo Akiba Energy Technology Research Institute National Institute of Advanced Industrial Science and Technology (AIST) Dr. Gerd Michael Würsig Germanischer Lloyd AG Prof. Dr. K. Andreas Friedrich DLR - Deutsches Zentrum für Luft- und Raumfahrttechnik
Dr. Albert E. Hammerschmidt Siemens, Industrial Services and Solutions, Marine Solutions, PEM Fuel Cells Prof. Dr. Jobst Hapke TUHH Technische Universität Hamburg-Harburg
Volker R. Hiebel Airbus Deutschland GmbH Prof. Dr. Kevin Kendall University of Birmingham Dr. Subhash C. Singhal Pacific Northwest National Laboratory Dr. Georgios Tsotridis European Commission Joint Research Center Institute for Energy
Dr. Oliver Weinmann Vattenfall Europe AG Innovationsmanagement
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