INVESTIGATION OF RADIATION STABILITY OF HIGH
TEMPERATURE PROTON CONDUCTORS
I.V. K h ro m u sh in , Zh.R . Z h otab aev, T.I. A k sen ova,
P.A . K orn ien k o , K .K . M u n a sb a ev a
Institute o f Nuclear Physics, National Nuclear Center o f RK, Almaty, 050032, Kazakhstan
Fax: 7-(727) 3 8 6 5 2 6 0 ; e-mail: aksenova@inp.kz khrom@inp.kz
1. INTRODUCTION
Complex oxides with perovskite structures are very interesting, both from scientific and applied point o f views, because o f their unique properties such as high temperature protonic conductivity [1]. These materials are topical for practical use as membranes o f fuel cells and hydrogen sensors [2]. The purpose of this work was study of possibility of these materials application for hydrogen sensors in atomic industry.
In spite o f significant progress in understanding o f the nature o f proton conductivity, some aspects o f this phenomenon are subjects o f further investigation. The information on behavior o f these materials under irradiation is absent.
It is known that the properties o f proton conductors depend dramatically on states o f hydroxide, hydrogen and especially oxygen ions in the oxide lattice. In accordance with widely accepted proton transfer in high temperature proton perovskites, oxygen positions serve both as a trap for proton and a pathway for proton movement. That is why it is very important to study the state o f oxygen in the lattice and its mobility and the proton and oxygen behavior under irradiation.
In this work influence o f reactor irradiation on doped and undoped barium (BaC e03) and strontium (SrCe03) cerates was investigated.
The samples were synthesized by ceramic and inductive melting technology. After synthesis the samples
were annealed at different conditions: in air (650-750°C, PO2~0 , 2 atm) and in “wet” nitrogen (625°C,
PO2~1,3x10'3 atm).
The samples were irradiated at 100 and 620°C in ‘dry’ and ‘w et’ channels o f the WWR-K reactor o f the Institute o f Nuclear Physics RK. Special heating cell was used for high temperature irradiation.
The method o f thermodesorption spectroscopy (TDS) was used in this study, involving measurement o f gas desorption during sample heating with known rate in vacuum. This method allows identification of desorbing gases and gives information about chemical states and mobility o f gases “dissolved” in solids.
Special device shown in fig. 1 was used for gas thermodesorption study. This device consists o f the following main parts: high vacuum chamber; heating device; radio frequency mass-spectrometer [3].
One by one samples (up to 7) are thrown into the heating device and thermodesorption spectra o f necessary gases are measured. Temperature of samples is measured by W-Re thermocouple.
Experiments are completely automated. Personal computer with Advantech PCI-1710HG module is used for samples heating, choice o f necessary gas masses, temperature and thermodesorption spectra measurements.
This automation scheme allows recording up to 6 desorption spectra o f different gases simultaneously.
Fig. 1. Principal Scheme o f the device for gas thermo desorption researches:
1- cell for samples, 2 - heater, MX-7304 - radio frequency mass-spectrometer, MS - static mass- spectrometer, GIN-0,5 - heteroionic pump, H l-C - steam jet pump, HBP-5D - preliminary vacuum pump, TR - high current temperature regulator, PC - personal computer.
It was found that after annealing o f doped barium cerate samples in air oxygen and water molecules release from the samples take place, and shifts o f the temperature corresponding to the maximum of desorption rate
are observed for different kind o f sample treatments. Only water molecule release was observed in
thermodesorption spectra o f the doped strontium cerates after different types annealing, no oxygen release occurred from SrCeo^Yo^Cb samples.
It was noticed that additional intercalation o f water and oxygen molecules took place after samples irradiation at 100°C. As an example in fig. 2, 3 the dose dependences o f oxygen and water molecules releases are shown.
— • — Ft = 4*1018 n/cm2
Fig. 3. Desorption o f H20 from BaCe0 gsNdo 1 2O3 .5 after reactor irradiation.
It is known that carbon dioxide has negative influence on properties o f these materials. Significant increase o f carbon dioxide content in these samples took also place with increase o f irradiation dose (fig. 4). Thus the low temperature reactor irradiation has influence on intercalation processes o f water, oxygen and carbon dioxide molecules in barium and strontium cerates.
— Ft = 4*10 n/cm
—a— Ft = 1.4*1019 n/cm2 — Ft = 2.8*1019 n/cm2
Fig. 4. Desorption of C 02 from BaCe0 gsNdo 1 2O3 .5 after reactor irradiation.
The investigations o f low temperature effect on strontium and barium cerates are interesting from science point o f view, but for practical purposes it is more important to know the behavior o f oxides under irradiation at work temperatures (T ~ 600 - 700 °C). So influence o f high temperature reactor irradiation on barium and strontium cerates was also studied.
It was found that high temperature reactor irradiation (620 °C) in contrast to low temperature irradiation calls not only quantitative but also qualitative changes in gas thermo desorption spectra from barium and strontium cerates.
It is necessary to note that amount o f oxygen molecules released from the samples irradiated at high temperature is significantly higher then from initial ones. An additional sharp oxygen desorption pick is appeared at 540 °C. This peak disappears after samples annealing in laboratory conditions in air at 620°C
— initial
Fig. 5. Desorption o f oxygen molecules from polycrystalline BaCe0.9Nd0.iO3 _5
after high temperature reactor irradiation.
It was noticed that intensity o f the sharp oxygen desorption peak from single crystal samples is significantly
higher then from ceramic ones (fig.6).
Fig. 6. Desorption o f oxygen molecules from single and polycrystalline
BaCeo.çNdo 1O3 .5 samples after high temperature reactor irradiation.
The low temperature oxygen peaks were observed both on barium and strontium cerates irradiated at high temperatures. It is known that strontium cerates are pure proton conductors and no oxygen is released from the samples during heating. It is also important that the low temperature peaks o f oxygen molecules release from undoped cerates irradiated at high temperatures were also found though oxygen desorption from undoped cerates without irradiation never took place .
Also a change in desorption spectra o f water molecules release from the irradiated samples was found at 540
— BaCe0^9Nd0JO3-y initial
Temperature, C
Fig. 7. Desorption o f water molecules from BaCe0.9Ndo_i0 3 _ 5 after high
temperature reactor irradiation.
Analysis o f the low temperature oxygen peak shape together with the change in water desorption spectra shows that phase transformation takes place in the irradiated oxides during hitting.
X - Ray spectroscopy o f the cerates confirmed our suggestion. It is appeared that partial disordering in the lattice o f the oxides irradiated at high temperatures took place. Annealing o f the oxides after irradiation
brings to the ordering o f their structures accompanied by crystallization o f C e02 phase.
It is necessary to carry out further investigations to make a final conclusion on possibility o f these materials use in atomic industry.
2. REFERENCES
1. Iwahara H., Uchida H., Tanaka S. High temperature proton conducting solids oxide fuel cells using
various fuels // J. Appl. Electrochem. 1986.-V. 16.-P.663-668.
2. Shober T. Applications o f oxidic high-temperature proton conductors // Solid State Ionics.-2003.- v.162-
163, P.277-281. 3
3. T.I Aksenova, I.V Khromushin, Zh.R. Zhotabaev, K.D. Bukenov, A.K Berdauletov, Z.V Medvedeva.
Thermodesorption study of barium and strontium cerates // Solid State Ionics. 2003. V. 162-163. P. 31 36.
