Fluorine Sensors Based on Superionic Conductors and Heterostructures
New Materials and Heterostructures for Conductometric Sensors of Fluorine and Hydrogen Fluoride
Tech Area / Field
- CHE-SYN/Basic and Synthetic Chemistry/Chemistry
- ENV-MIN/Monitoring and Instrumentation/Environment
- MAT-SYN/Materials Synthesis and Processing/Materials
3 Approved without Funding
Russian Academy of Sciences / Physical Technical Institute, Russia, St Petersburg
- Institute of Microelectronics Technology and High Purity Materials, Russia, Moscow reg., Chernogolovka\nKurchatov Research Center, Russia, Moscow\nRussian Academy of Sciences / Institute of Crystallography, Russia, Moscow
- TASC-INFM, Italy, Trieste
Project summaryFluorine and hydrogen fluoride are toxic volatile compounds of a high chemical activity. These substances or are used as reagents, or released as by-products in a large number of the important industrial productions. Both in that, and in the other case, the problem of their timely detection and quantitative determination has been stood for a long time. The electrode for the analysis of solutions has been offered on the basis of lanthanum fluoride (Frant M., Ross J., Science, 1966, v.154, No.3736, p.1553). Other type of sensors uses absorption of radiation by analyzed (harmful) impurities in the atmosphere. Analytical devices of this type usually perform measurement of a wide spectrum of impurities, and they are expensive. These devices have own consumer "niche" which is not overlapped with highly specialized (selective) sensors considered here. The majority of productions with use of fluorine-containing reagents require inexpensive selective analyzers with short response time.
An alternative way of the solution of a monitoring problem of fluorine content in the atmosphere is a direct (conductometric) method of determination of fluorine concentration in gases using the effect of superionic conductivity of fluorine ions in non-stoichiometric fluoride crystals. The sensitive element (the chemical sensor) is a solid electrolyte film. Since the electrolyte with conductivity of fluorine ions only is chosen, this analysis is direct and selective. Project participants have obtained fluorine-conducting single crystals of non-stoichiometric fluorides R1-yMyF3-y with the tysonite structure (LaF3) during studies of phase diagrams of fluoride systems such as MF2 - RF3 [1, 2]. The fluorine model sensor has been developed on the basis of these crystals together with Saint-Petersburg University, GIPKH  and later with NPO "Chimavtomatika" . NPO "Chimavtomatika" has begun fabrication of the chemical sensor DF-1 for zones of industrial production since 1998. For its direct calibration using the same fluorine-conducting crystals the generator of testing mixtures of fluorine and hydrogen fluoride "Sting" has been created. Operating experience of several such analytical complexes has confirmed efficiency of the conductometric principle of the fluorine chemical sensor and has allowed defining tasks of the present project.
The chemical sensor DF-1 has a number of the shortcomings determined by parameters of solid electrolyte incorporated in its basis more than 15 years ago and since then not improved. Insufficiently high conductivity of the electrolyte demands heating the sensitive element up to 150-180°С. It complicates the device due to introduction of the heating block and power supply system, increases the general energy consumption, slows down measurement time (due to necessity to achieve thermal balance of the sensor) and adversely influences accuracy (temperature drift). There is also an unresolved problem of degradation of the sensor sensitive element, limiting its life time for one year. At the same time the sensor of conductometric type possesses selectivity on fluorine (in comparison with the potentiometric type), sufficiently rapid analysis, relatively low cost and other advantages. Pre-production models of this type sensors have shown efficiency of the conductometric method in principle and have been manufactured by the domestic industry (such type of sensors is not made abroad) till now. They form the basis for statement of the problem and preparing the research plan of the present project.
The purpose of the project is creation of conditions for a radical improvement of monitoring tools of industrial zones and an environment over fluorine and hydrogen fluoride, based on the conductometric principle of the chemical sensor. The key point is searching for new solid electrolytes - non-stoichiometric fluoride crystals and heterostructures of multicomponent composition - possessing higher conductivity in comparison with the materials now used in the chemical sensors. Such superionic conductors with improved electrophysical and other performance data will allow modernization of batch-produced analyzers and their promotion in new areas of chemical sensor engineering (creation of independent monitoring tools, including personal ones). Results of the investigations along the project will create premises for the further development of chemical sensor engineering for fluorine and hydrogen fluoride detection and will enable forseeing the most effective directions of instrument manufacturing in this area.
General working plan for all term of the project activity:
- Production of single crystals of bulk fluorine-conducting solid electrolytes with tysonite structure (LaF3) on the basis of phases R1-yMyF3-y formed in systems of the MF2 - RF3 type (M = Ca, Sr, Ba, R - rare-earth elements - REE).
- Growth of epitaxial layers and polycrystalline films as well as their characterization (chemical and phase composition, structure, morphology, etc.).
- Growth and study of multilayered heterostructures consisting of fluoride layers with superionic properties.
- Fabrication of test structures - elements of a fluorine and hydrogen fluoride sensor - and studying their performance data.
During realization of the project the following techniques will be applied: Bridgeman-Stockbarger method for growing bulk single crystals of multicomponent fluorides in fluorinating atmosphere; molecular-beam epitaxy to grow fluoride films and epitaxial heterostructures; reflection high energy electron diffraction to monitor crystal structure of the films and multilayered epitaxial heterostructures during their growth; X-ray diffractometry, including application of synchrotron radiation sources in RSC KI (Moscow) and Trieste (Italy) for after-growth phase and structural analysis of epitaxial films and heterostructures; neutron-diffraction measurements using sources of thermal neutrons in Grenoble and the Sacley (France) to analyze defect structure and thermal vibrations connected to superionic conductivity; atomic-force microscopy to analyze surface morphology of crystals and heterostructures to study growth processes and optimize growth conditions; impedance conductometry to measure ionic conductivity; photoelectronic spectroscopy with application of synchrotron radiation to control non-stoichiometry and spatial distribution of fluorine in heterostructures; optical spectroscopy of rare-earth ions to evaluate local concentration of fluorine ions.
Realization of the project will result in fabrication of multicomponent fluoride single crystals, epitaxial and polycrystalline films, and also epitaxial multilayered heterostructures with fluorine-ionic conductivity at the level of 10-3 Ohm-1см-1 at room temperatures. To achieve the intended parameters it is planned to apply methods of “defect engineering” - creation of structural defects of the certain type and concentration (anion vacancies and anion-cation clusters of nm-size) in crystals, films and epitaxial multilayered heterostructures, governing superionic conductivity. This approach involves four main research stages. In the first stage of the project, crystals are synthesized and their defect (atomic) structure is systematically studied for the first time. Epitaxial layers and polycrystalline films of tysonite solid solutions R1-yMyF3-y are grown and their characterization (chemical and phase composition, structure, morphology, etc.) are carried out at the second stage. Multilayered heterostructures of fluoride layers with superionic properties are grown and studied at the third stage, including electrophysical characterization (conductivity, activation energy, temperature dependence, etc.) of heterostructures with the optimized compositions, which determine operational characteristics of recommended materials. Test structures - film elements of fluorine and hydrogen fluoride sensors - are fabricated and studying their performance data is carried out at the fourth stage. The project outputs the development of recommendations on creation of devices on the basis of new compositions for sensitive elements of fluorine gas chemical sensor.
Searching for new non-stoichiometric crystals and films of multicomponent composition on their basis, that partially has been carried out by the project authors earlier and is planned in the project, will provide enlargement of the range of solid electrolytes with fluorine superionic conductivity, suitable for chemical sensors. It is supposed that electrolytes with conductivity, sufficient for sensor operation at room temperature, will be developed. It will increase speed, simplify power supply of the sensor, decrease dimensions and energy consumption, reduce manufacture price, increase time of off-line work and will open a way to a device of personal fluorine monitoring.
MAIN PUBLICATIONS OF THE PROJECT PARTICIPANTS ON THE SUBJECT
- Sobolev B.P., The Rare Earth Trifluorides, Part 1. The High Temperature Chemistry of the Rare Earth Trifluorides, Ed. Institut d'Estudis Catalans, Barсelona, Spain, (in English) 2000, 520 p.
- Murin I.V., Glumov O.V., Sobolev B.P., Electrical conductivity of solid electrolytes on the basis of CeF3, Bull. LSU, Phys. Chem. Ser., 1980, No 10, Issue 2, P.84–88.
- Sobolev B.P., Krivandina E.A., Murin I.V., Glumov O.V., Samusik D.B., Bogoljubov V.B., Besprozvannyj B.S., Lugina T.A., Avinnikov V.K., “The device for determination of fluorine in air ”, the certificate of authorship No 311414, priority of 31.11.1988.
- Sudakova E.F., Alksnis E.Ya., Perlovskii R.S., Krivandina E.A., Sobolev B.P., Chemical sensor for F2 and HF determination in gaseous atmospheres, Symposium on Solid State Ionics, Nov 30 – Dec 4. 1992, Boston, Section U-6.5, P. 160
- A.K. Kaveev, O.V. Anisimov, A.G. Banshchikov, N.F. Kartenko, V.P. Ulin, N.S. Sokolov, Epitaxial growth on silicon and characterization of MnF2 and ZnF2 layers with metastable orthorhombic structure, J. Appl. Phys., 98 (2005) 013519 (1-8)
- L. Pasquali, S. M. Suturin, V. P. Ulin, N. S. Sokolov, G. Selvaggi, A. Giglia, N. Mahne, M. Pedio, and S. Nannarone, Calcium fluoride on Si(001): Adsorption mechanisms and epitaxial growth modes, Phys. Rev. B 72, (2005) 045448 (1-15)
- L. Pasquali, S.M. Suturin, A.K. Kaveev, V.P. Ulin, N.S. Sokolov, B.P. Doyle, and S. Nannarone, Interface chemistry and epitaxial growth modes of SrF2 on Si(001), Phys. Rev. B 75 (2007) 075403 (1-15)
- I.V. Golosovsky, R. G. Delaplane, A. N. Naberezhnov and Y. A. Kumzerov. Thermal motion in lead confined within a porous glass, Phys. Rev. B 69, 132301-1-4, 2004.