ISSN 10637834, Physics of the Solid State, 2012, Vol. 54, No. 5, pp. 955–957. © Pleiades Publishing, Ltd., 2012. Original Russian Text © G.S. Grigoryan, A.M. Solodukha, 2012, published in Fizika Tverdogo Tela, 2012, Vol. 54, No. 5, pp. 898–900.
PROCEEDINGS OF THE XIX ALLRUSSIAN CONFERENCE ON PHYSICS OF FERROELECTRICS (VKSXIX) (Moscow, Russia, June 19–23, 2011)
Effect of Pulsed Laser Radiation on the Structure and Electrical Properties of Ferroelectric Ceramics G. S. Grigoryan and A. M. Solodukha Voronezh State University, Universitetskaya pl. 1, Voronezh, 394006 Russia email:
[email protected] @
Abstract—Ceramic samples of barium titanate and solid solution of barium–strontium titanate subjected to pulsed laser irradiation have been studied by impedance spectroscopy. The measurements have been carried out in the frequency range 102–106 Hz and at temperatures in the range from 20 to 450°C. The experimental data are represented by the dispersion of the electric modulus. The activation energy of relaxation processes in the paraelectric phase has been calculated. Scanning electron microscopy has been used to obtain micro graphs of the sample surfaces and the data on the elemental composition. DOI: 10.1134/S1063783412050149
RLC meter in the temperature range 300–700 K and in the frequency range 0.1 kHz–1.0 MHz. After the primary measurement of the dielectric properties of the samples (1.5mmthick pellets 10 mm in diame ter), the In–Ga electrodes were removed, and the samples were subjected to pulsed irradiation by a GOS301 laser with a wavelength of 1.06 μm. In this case, the power density was found by the formula [1]
Optical quantum generators have been widely used in technology and materials science. The study of the results of physicochemical processes occurring under laser radiation on a ferroelectric ceramics allow one to find optimal conditions of radiation and to choice the materials for further efficient application. In this work, the electrical properties of ceramic samples of barium titanate BaTiO3 and solid solutions of barium–strontium titanate Ba0.67Sr0.33TiO3 are studied by impedance spectroscopy using a WK4270
(a)
3 0.5
4 1
0.5 1 3 2
2 0 2
(b)
1.0
ImM/ImMmax
1.0
ImM/ImMmax
2
QF P = 2 , 2 πtR ( F – L )
4 3
4 log(ν, Hz)
5
6
0 2
3
4 log(ν, Hz)
5
6
Fig. 1. Isotherms of the frequency dependences of the reduced values of the imaginary part of the electric modulus for the ceramic samples of (a) barium titanate and (b) solid solution of barium–strontium titanate Ba0.67Sr0.33TiO3 measured (1, 2) in the initial state and (3, 4) after irradiation of the samples at temperatures of (1, 3) 500 K and (2, 4) 530 K.
955
956
GRIGORYAN, SOLODUKHA 4.5
log(ν, Hz)
4.0
3.5
4 3 1
3.0
2 μm
(а) 2 2.5 1.7
1.8
1.9 103/T, K−1
2.0
2.1
Fig. 2. Temperature dependences of the frequencies of the maxima of the imaginary part of the electric modulus rep resented in the Arrhenius coordinates for the ceramic sam ples (1, 2) BaTiO3 and (3, 4) Ba0.67Sr0.33TiO3 measured (1, 3) in the initial state and (2, 4) after irradiation of the sam ples.
where Q is the pulse energy; t is pulse time; F is the lens focal distance; R is the lens radius; and L is the dis tance between the lens and the target. In our case, t = 0.8 ms, F = 1 m, R = 3 cm, L = 0.67 m, Q = 250 J, and, as a result, the power is P = 4.1 × 105 W/cm2. After the laser irradiation, the electrodes were again deposited on the sample surface, and the repeated measurements were performed. We measured the impedance modulus |Z| and the phase shift between its active and reactive components. We present the data on the electric modulus M that well reflects the relax ation processes in ceramic grains, and it is expressed through the impedances as follows: M = iωC0Z, where i is the imaginary unit; C0 is the geometric capacity; and M and Z are complex quantities. First, we consider the effect of the radiation on the bulk properties of the ceramics. Figure 1 depicts the isotherms of the imaginary part of the electric modulus measured before and after the action of the laser pulse on the sample for two temperatures. An analysis of the shape of the curves obtained for BaTiO3 shows that the maximum is broadened as com pared to the Debye maximum, and its symmetry is slightly distorted; i.e., the maximum shape is closer to the Davidson–Cole curve [2]. After irradiation, the
2 μm
(b)
Fig. 3. Micrographs of the surfaces of (a) BaTiO3 and (b) Ba0.67Sr0.33TiO3 samples after their irradiation.
maximum width I increases, and the maxima shift toward lower frequencies. For Ba0.67Sr0.33TiO3, the frequency dependence of ImM is close to the Debye shape with a slight devia tion from the symmetry. The irradiation does not change the shape; however, a new relaxation mecha nism appears near 2 MHz. Similar behavior for these samples was observed after usual thermal quenching, i.e., when they were held at a temperature of 1000°C for an hour and then rapidly cooled to room tempera ture [3]. This circumstance indicates that the conver sion of the radiation energy to the internal energy of the sample increases its temperature by several hun dred kelvins at thermal gradient from the back to face surface facing the radiation source [4]. The appear ance of a new group of relaxers is likely due to the for mation of additional layers at the grain boundaries.
PHYSICS OF THE SOLID STATE
Vol. 54
No. 5
2012
EFFECT OF PULSED LASER RADIATION
The frequencies ν at which ImM becomes maxi mum allow the estimation of the activation energy E if the dependence of the relaxation time τ on tempera ture T has the Arrhenius shape (τ = τ0 exp(E/kT), ν = 1/2πτ). These data are presented in Fig. 2. It is found that, after the irradiation, E of BaTiO3 is unchanged within the error of measurement (±0.02 eV), and it is 1.12 eV, and E of Ba0.67Sr0.33TiO3 decreases from 1.32 to 0.98 eV.
(NBa + NSr)/NTi = 0.93, whereas, initially, the ratio is unit. The heterogeneity of the radiation energy density causes the local heating leading to the recrystallization of the material in nonequilibrium conditions. These changes in the structure can play a dominant role in the modification of the properties of thinfilm objects with the crystal lattice of the perovskite type.
The study of the surfaces of the samples performed using a JSM6380 LV scanning electron microscope showed that there are many dropletlike formations about 100 μm in size with craters about 8 μm in diam eter. Near the craters, a characteristic texture (Fig. 3) is formed, and it is substantially different for the com positions under study. For BaTiO3, the ratios of con centrations N atoms at such portions are deviated from the initial values. For example, NBa/NTi = 0.83, while, for nonirradiated portions, the ratio is close to unity. In the irradiated Ba0.67Sr0.33TiO3, NBa/NSr = 1.06 that is half as that in the nonirradiated portions, and
REFERENCES 1. Lasers in Technology, Ed. by M. F. Stel’makh (Energiya, Moscow, 1975) [in Russian]. 2. V. A. Stefanovich, M. D. Glinchuk, B. Khilcher, and E. V. Kirichenko, Phys. Solid State 44 (5), 946 (2002). 3. G. S. Grigoryan and A. M. Solodukha, Abstracts of Papers of the XXII International Scientific Conference “Relaxation Phenomena in Solids”, Voronezh, Russia, September 14–18, 2010 (Voronezh, 2010), p. 89. 4. M. Musella and H. R. Tschudi, Int. J. Thermophys. 26, 981 (2005).
SPELL: OK
PHYSICS OF THE SOLID STATE
Vol. 54
No. 5
957
2012
Translated by Yu. Ryzhkov
