Project “Hydrogen Detectors” funded partially by Office of Proliferation
Threat Reduction of the Department of State
Here are the some excerpts taken from the full project's description and texts, those are substantially more fundamental in definitions. In these topics we have more experience and interest to make some input.
The project suggests that a complex of research works should be conducted on the development of laboratory models of low-size and reliable hydrogen detectors operating within broad ranges of the temperature T = -70 +55 and the pressure P=1105 Pa. Another major problem, which will be solved within the framework of this project, is the development and research of new materials for low-temperature hydrogen absorbers.
Work novelty – For the first time, a matter has been stated on the development of low-temperature hydrogen detectors functioning within broad pressure and concentrations ranges.
This work is devoted to theoretical and experimental investigations of hydrogen detectors functioning within broad temperature and pressure ranges, including the low-temperature region at reduced pressure and in vacuum conditions.
The objective of the project is to conduct the following research works:
1. Investigation of new sensor materials: transition metal oxides, graphite materials containing catalytic additives of transition metals and alloys, carbon nanomaterials.
2. Investigation of changes of the speed of sound in the hydrogen medium at various concentrations, pressure and temperature.
3. Development and research continuous operating hydrogen leak sensors in various gas mixtures in a broader ranges of the pressure and the temperature as compared with well-known analogues.
4. Investigation of a possibility of development of hydrogen absorbers in order to liquidate emergency situations.
Specifications of sensors being developed. In the course of research works, it is planned to develop hydrogen sensors with the following specifications:
1. Operating temperature range: T = -70+55
2. Operating pressure range P = 1105 Pa.
3. Hydrogen concentration being measured: within the range of 0,2% 3,1% with the accuracy of ±1%; within the range of 3,1% 20 % with the accuracy of ±0,2%; within the range of 20% 95% with the accuracy of ±0,5%.
Advantages as compared with well-known analogues. At the implementation of the project, well-known and new sensor materials will be investigated and technologies will be developed for the manufacture of sensors reliably operating within broadened temperature and pressure ranges.
1. Sensor materials will be developed on the basis of transition metal oxides containing catalytic additives as well as carbon nanomaterials.
2. A database including the dependence of heat, electronic and acoustic parameters of sensor materials upon the concentration of hydrogen.
3. A laboratory sensor model will be delivered.
4. Compositions and constructions of hydrogen absorbers will be developed.
At the development of hydrogen sensor and absorber designs, a special attention will be paid to the material preparation methods, including formation of the optimal porous structure, introduction of metallic and organic bonding materials as well as components having membrane properties to remove water being a reaction product.
The results of research are supposed to be used in order to increase the safety degree at nuclear power plants, hydrogen mainline pipelines and reservoirs, at explosion-hazard and harmful productions, at major cryogen complexes, in motor road and aviation vehicles using hydrogen as fuel. The development is also supposed to be used at testing production processes in cryo-vacuum heat-insulation cavities.
For long-distance hydrogen mainlines and, as a rule, numerous closed volumes with hydrogen those being monitored, cheap safe simple and reliable sensors are needed.
Most frequently, well-known hydrogen sensors are sufficiently complicated electronic-and-mechanical and optical-and-electronic systems. Due to a high value of such sensors, their use is not always economically justified.
The development of selective gas sensors, in particular, hydrogen sensors, is a relevant problem due to its broad use in cryogenic and hydrogen engineering.
During the project implementation, research works on the development of a low-size and cheap laboratory model of the continuously operating hydrogen detector based on transition metal oxides and graphite materials containing catalytic additives of transition metals and alloys as well as carbon nanomaterials will be carried out. Such detectors are capable to monitor both emergency hydrogen leaks and also the volumetric concentration of hydrogen in various gas mixtures in broader pressure and temperature ranges as compared with well-known methods. In addition, some of these materials can absorb considerable amounts of hydrogen, especially in case when emergency situations arise.
Specialists from the Russian Federal Nuclear Center – All-Russian Research Institute of Experimental Physics traditionally deal with matters of testing and disposal of hydrogen, tritium and deuterium. At this project, it is supposed to use their knowledge in modern physics and engineering, in particular, the results of research in the field of physical chemistry of semiconductor surfaces, to attract scientists and specialists who formerly were engaged in the development of mass destruction weapons, for the development of new hydrogen sensors of high and medium resolution, manufactured both in stationary and portable designs, as well as in the form of individual indicator tubes.
In order to solve the problem being stated, the use of solid-state sensors is prospective, in which the effects of interaction between a solid body and a gaseous medium are used. Phenomena of chemisorption and a deeper chemical interaction, which are used in semiconductor sensors, can run on the solid body surface (reduction, oxidation, etc.). These processes are accompanied by essential heat effects as well as by the change of electronic properties of solid body, on which basis new types of sensors can be developed.
In order to enhance the sensitivity of sensors, it is necessary to optimise the surface process running conditions by means of introduction of catalytic additives. Sensors, in which catalytic processes of hydrogen oxidation on the surface are used, are a new prospective but still low-developed type of selective sensors.
The selectivity provision in the case of catalytic sensors can be reached by the following methods:
1) use of catalysts applied onto a semiconductor substrate,
2) use of catalytic “filters”,
3) use of high-selective surface additives paying the role of catalysts of topochemical processes.
The technological base providing production of especially pure sample gas mixtures, a great experience of work with hydrogen mixtures and hydrogen as well as the availability of process equipment and specialists in miniature device production (in particular, miniature roentgen tubes, miniature radioactive sources, various kinds of sensors) will allow to develop and manufacture a high-level laboratory model of the hydrogen sensor.
In order to conduct this research as well as other important investigations, testing facilities will be manufactured. As hydrogen sources, it also is planned to use laboratory accumulators based on intermetallic compound hydrides.
The results of research and specific hydrogen sensors being developed are supposed to use for enhancement of the safety at nuclear power plants, hydrogen mainline pipelines, at explosion-hazard and harmful production. They are much needed at major cryogenic complexes, in motor road and aviation vehicles using hydrogen as fuel.
At the present time, there are no hydrogen sensors functioning below the temperature of –20°C.
The most optimistic forecasts for normal and increased pressures have been obtained with an acoustic sensor having practically a linear response characteristic in a broad concentration range depending upon concentration values. This sensor has not been investigated at low and cryogen temperatures. However the acoustic sensor has some limitations: it does not function in rarefied media. At the same time, sensors based on metal-oxide materials (thermochemical, chemical, piezoelectric or chemical vibro-laser ones) will have a high degree of degradation being operated in media with normal or increased pressure.
In the project, one will have to determine an optimal sensor design. Most likely, the sensor will be a symbiosis of an acoustic sensor and a metal-oxide sensor ((thermochemical, chemical, piezoelectric or chemical vibro-laser one) or only polymer proton-exchange one (ECG-principle).
In this works, an investigation on the combination of features of acoustic sensors and metal-oxide sensors will be carried out.
A number of metals, alloys and intermetallic compounds can reversibly interact with hydrogen, which allows to use them as detectors. The hydration process is accompanied by the crystalline lattice change and the heat release . So detectors fixing the change of various features (for example, electrical and magnetic ones) as well as the temperature can be developed on the basis.
The following reaction has been laid as the basis of metal-hydride hydrogen accumulators being proposed:
RTn + x H2 <===> RTnH2x + Q,
where R is one or several metals of group II, III or IV, T is a 3d-, 4d- or 5d-transition metal, n = 1-5. As a bonding in composite materials, 10-30 mass. % of polyethylene, teflon or metal (Al, Cu, Ni) are used.
By the amount of hydrogen being reversibly absorbed, fullerenes and their derivatives (for example, metal fullerides) are also very attractive as hydrogen absorbers:
С60 + х Н2 <==> С60Н2х
A reverse degradation reaction requires an increased temperature. Therefore, in order to use then as accumulators, it is necessary to increase the hydration rate and decrease the dehydration temperature.
The reaction of interaction of metalfullerenes with hydrogen runs at lower temperatures and can be used for the development of hydrogen sensors.
Another type of carbon nanomaterials is carbon nanotubes and nanofibres capable to absorb record-high amounts of hydrogen can have applied prospects in hydrogen power engineering. According to the data by R. Smolli et al. (Appl.Phys.Lett.1999, 74, p.2307), one-wall nanotubes adsorb up to 8.5 mass % of hydrogen. N. Rodrigez et al. (J. Phys.Chem.1998, 102, p.4253) observed the adsorption of 10-20 l of hydrogen per one gram of filamentary graphite structures obtained by catalytic hydrocarbon pyrolysis. P Chen with colleagues (Science.1999, 285, p.91) reported on the absorption of more than 5 mass % of hydrogen on carbon nanotubes doped by alkaline metals. It is evident that the hydrogen adsorption by such materials can be used for the development of hydrogen detectors.
So far it has been hindered by the lack of a cheap method of production of such materials. Perhaps, one will unlikely manage to derive a cheap hydrogen sorbent by the laser evaporation of a metal-graphite target described in the literature. Perhaps one would obtain comparatively cheap carbon nanomaterials being prospective for the storage of hydrogen and light hydrocarbons by an improvement of the method of electric arc evaporation of a metal-graphite electrode as well as by chemical removal of non-carbon atoms from polycrystalline carbides or by catalytic hydrocarbon pyrolisis.
It has been planned to do the following:
· Conduct an investigation of the correlation between the change of the heat flux being generated by the hybrid sensor and the hydrogen content in the volume being under investigation.
· The study of kinetics of the heat flux function generated by the hydrid sensor in dependence on the hydrogen content in the volume being under investigation in order to evaluate probable ranges of concentration, operating sensor temperature and hydrogen pressure being accessible for monitoring.
· Make a conclusion on prospects of the use of hybrid sensors as a low-temperature hydrogen sensor.
· the investigation of the curve behaviour kinetics due to heat flux being generated by the hydrid sensor depending on hydrogen content in the volume being under evaluation. The physical and mathematical models of process will be developed allowing to predict the accuracy of hydrogen concentration measurements within various measurement ranges.
By the investigation of a possibility of the use of carbon nanomaterials, it has been planned to do the following:
· Conduct an investigation of the correlation between the amount of hydrogen adsorbed by carbon nanomaterial and resistance of nanoporous carbon (NPC).
· Study the kinetics of hydrogen sorption by carbon nanomaterial in order to evaluate probable ranges of sensor operating temperature and hydrogen pressure being accessible for monitoring.
· Make a conclusion on prospects of the use of carbon nanomaterial in hydrogen sensors.
· the investigation of the kinetics of hydrogen sorption by carbon nanomaterial in the form of a model and a set of parameters allowing to predict its accumulation and the sensor resistance change connected with it;
· a demonstration software package of programs emulating sensor operation.
At the investigation of phase-generation processes at reduction and thermal decomposition of complex compounds of platinum group metals, it has been established that solutions of certain Pd and Pt complexes in polymer materials (films) are reduced by molecular hydrogen at high rates and at relatively low temperatures (20-30°C). In some cases the reduction rates are so high that the reaction runs in the diffusion mode and its rate is determined by the hydrogen pressure over the film surface. This circumstance can be used for the development of hydrogen pressure indicators and sensors by the change of optical properties of polymer films. By a selection of complexes and polymers, polymer films changing the transparency can be developed depending upon the hydrogen pressure within a preset pressure range and with a preset operating resource, which is determined by the polymer film thickness and the complex concentration in the polymer. It is expedient to use the effect of change of the metallic cluster concentration in the polymer matrix at high values of the hydrogen pressure and the effect of change of the chromoform group concentration at the complex reduction – at low values.
The sensitivity and the resource of sensors can be increased with the help of untrodusing substances forming intensively painted compounds into the polymer under catalytic hydration reactions. The project’s authors have received the positive results during investigation of the diazo combination reaction with the formation of red-coloured dyes; at the contact with molecular hydrogen, it is possible to create polymer films forming black pigments of the “aniline black” type. One can easily implement the registration of the film’s optical property changes with the use of simple photocells.
It has been planned to do the following:
· Conduct an investigation of the correlation between the amount of hydrogen adsorbed by polymer films and optical properties of the film.
· Make a conclusion on prospects of the use of special polymer films in hydrogen sensors.
· Develop a laboratory polymer hydrogen sensor.
1.9. Investigation of prospects of use of nanoporous carbon as a hydrogen detector
Nanoporous carbon (NPC) is a porous graphite-like structure consisting of micro-fragments of amorphous carbon connected by a pyrocarbon bonding. The material has the porosity of about 60%, including about 30% falling on nanopores with the average size of 0.8 nm. The specific surface evaluated by the nitrogen adsorption equals to 1200 m2/g.
During the last years, the hydrogen sorption by this material has been studied intensively. It has been established experimentally that at increased temperatures hydrogen is adsorbed by NPC dissociatively, which implies the electronic exchange between the adsorbate and the absorbing substance. With the account of a low internal conductivity of the material (the charge carrier density is approximately comparable with graphites), a supposition arises that hydrogen adsorbed by nanoporous carbon should change its conductivity. This supposition has been proved experimentally, which make NPC to be a prospective material to be used as a hydrogen sensor.
It has been planned to do the following:
· Conduct an investigation of the correlation between the amount of hydrogen adsorbed by NPC and resistance to NPC.
· Study the kinetics of hydrogen sorption by NPC in order to evaluate probable ranges of operating sensor temperature and hydrogen pressure being accessible for monitoring.
· Obtain a conclusion on prospects of the use of NPC in hydrogen sensors.
· the investigation of the kinetics of hydrogen sorption by NPC with the help of a model and a set of parameters allowing to predict the hydrogen accumulation and the sensor resistance change connected with it.
· a demonstration software package of programs emulating sensor operation.