Materials Characterization 50 (2003) 349 – 352
Short communication
Pyroelectric properties of Pb0.88Ln0.08Ti0.98Mn0.02O3 (Ln=La, Sm, Eu) ferroelectric ceramic system Ernesto Suaste-Go´mez *, Rube´n Gonza´lez-Ballesteros, Victor Castillo-Rivas CINVESTAV-IPN, Departamento de Ingenierı´a Ele´ctrica, Seccio´n de Bioelectro´nica, Avenida Instituto Polite´cnico Nacional 2508, Me´xico, D.F. C.P. 07000, Mexico Received 3 April 2003; received in revised form 14 July 2003; accepted 17 July 2003
Abstract In this work, the pyroelectric properties of Pb0.88Ln0.08Ti0.98Mn0.02O3 (Ln = La, Sm, Eu) ferroelectric ceramic system are studied. This type of ferroelectric ceramic presents high values of the following characteristics: dielectric constant, Curie temperature, electromechanical anisotropy, and high frequencies of operation, which make them useful for applications such as ultrasonic transducers in biomedical applications. The relationship between dielectric constant and temperature measurements as well as pyroelectric measurements using the technique of Byer and Roundy were performed. Values of the pyroelectric coefficient and figure of merit for infrared detector materials were obtained to use these ceramics in the detection of infrared radiation, laser power measurements, and solar energy technology. D 2003 Elsevier Inc. All rights reserved. Keywords: Ferroelectric ceramic; Pyroelectricity; Rare-earth elements; Pyroelectric ceramic
1. Introduction The pyroelectric effect in some materials has been known since ancient ages. This effect has been studied in ferroelectric ceramics and crystals made with many formulations. Chynoweth [1] developed a dynamic method to study the pyroelectric effect in small single crystals of barium titanate. Glass [2] obtained the dielectric constant, specific heat, and pyroelectric coefficient in large single crystals of LiTaO3 using the method developed by Chynoweth. In addition, * Corresponding author. Tel.: +52-57-47-38-00; fax: +52-5747-70-80. E-mail addresses:
[email protected] (E. Suaste-Go´mez),
[email protected] (R. Gonza´lez-Ballesteros),
[email protected] (V. Castillo-Rivas). 1044-5803/$ - see front matter D 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1044-5803(03)00126-8
Glass [3] studied the pyroelectric properties in the Sr1 xBaxNb2O6 system using the dynamic pyroelectric technique of Chynoweth and a second pyroelectric technique, which involved the continuous integration of the charge developed between the crystal faces when its temperature was increased. Byer and Roundy [4] developed a direct technique to obtain the pyroelectric coefficient. At the same time, it was mentioned that a variation of temperature causes changes in the spontaneous polarization. This variation produces a displacement current I parallel to the polar axis described by I ¼ ApðT ÞdT=dt
ð1Þ
where p(T) is the pyroelectric coefficient evaluated at temperature T and A is the surface area normal to the polar axis.
E. Suaste-Go´mez et al. / Materials Characterization 50 (2003) 349–352
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The pyroelectric coefficient is given by pðtÞ ¼
I AðdT=dtÞ
ð2Þ
if dT/dt is held constant over a wide temperature range, a measurement of the current I gives a direct plot of p(t) over that temperature range [4]. Recently, ferroelectric ceramics of PbTiO3 modified with elements of the rare-earth group using the formulation Pb0.88Ln0.08Ti0.98Mn0.02O3 (Ln = La, Sm, Eu) have been studied. These ceramics have characteristics such as a large Curie temperature of high resonance frequency and ultrahigh electromechanical anisotropy. A particular case is when Eu substitutes Pb in the Pb0.88Eu0.08Ti0.98Mn0.02O3 ceramic system, obtaining with this an ultrahigh electromechanical anisotropy, i.e., kt/kp ! l where kt is the thickness dilatational vibration and kp is the planar extensional vibration (kp ! 0) [5– 7]. In this paper, we determine the pyroelectric properties of the Pb0.88Ln0.08Ti0.98Mn0.02O3 (Ln = La, Sm, Eu) ferroelectric ceramic system to use these ceramics in applications such as the detection of infrared radiation, laser power measurements, and solar energy technology.
2. Experimental The preparation of the Pb0.88Ln0.08Ti0.98Mn0.02O3 (Ln = La, Sm, Eu) ceramics was performed in our
Fig. 2. Graphs of the dielectric constants of the ferroelectric ceramics: (a) Pb0.88La0.08Ti0.98Mn0.02O3, (b) Pb0.88Sm0.08Ti0.98 Mn0.02O3, and (c) Pb0.88Eu0.08Ti0.98Mn0.02O3.
Fig. 1. Experimental setup used to measure the pyroelectric effect: (a) ceramic, (b) furnace, (c) temperature control, (d) temperature meter, (e) high sensitivity multimeter, and (f) computer with ADC card.
laboratory using the oxide-mixing method [8]. The raw powders PbO, TiO2, MnO2, and LnO3 were weighed in appropriate proportions, mixed, and calcined at 850 jC for 2 h in air. Then, the sample was crushed and milled into fine powder. The calcined powder was pressed at 3 108 Pa and formed into disks with 10 mm diameter and 1 mm thickness.
E. Suaste-Go´mez et al. / Materials Characterization 50 (2003) 349–352
These disks were sintered at 1240 jC for 2 h in a wellcovered platinum crucible. The sintering atmosphere was enriched in PbO vapor (PbO + ZrO2) to limit evaporation of PbO from the specimen [8]. The dielectric constant measurement was performed by placing the ceramic inside a well-insulated
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furnace that incremented its temperature gradually. The temperature was measured using a thermocouple type K, and the capacitance used to calculate the dielectric constant was measured at 1 kHz using a RCL bridge (LM22A Beckman). The measurement of the pyroelectric coefficient was performed using the Byer –Roundy technique. The ceramic was placed in a programmable furnace (Carbolite HTC1600), and the temperature was incremented with a rate of 1 K/min. The current generated by the ceramic was measured using a high sensibility multimeter (PM2525 Philips). Fig. 1 shows the experimental setup used to measure the pyroelectric coefficient. A pyroelectric figure of merit for infrared detector materials (Rv) was calculated for the samples. Rv is defined as Rv ¼ pi =er
ð3Þ
where pi is the peak pyroelectric coefficient and er is the permittivity at Tc.
3. Results Fig. 2 shows the results of the dielectric constant measurements for the Pb 0.88 Ln 0.08 Ti 0.98 Mn0.02O3 (Ln = La, Sm, Eu) ferroelectric ceramic system. Fig. 3 shows the pyroelectric coefficient response at different temperatures of the Pb0.88Ln0.08Ti0.98 Mn0.02O3 (Ln = La, Sm, Eu) ferroelectric ceramic system.
4. Discussion
Fig. 3. Graphs of the pyroelectric coefficients of each ferroelectric ceramic: (a) Pb0.88La0.08Ti0.98Mn0.02O3, (b) Pb0.88Sm0.08Ti0.98 Mn0.02O3, and (c) Pb0.88Eu0.08Ti0.98Mn0.02O3.
The studies mentioned above have revealed that the ferroelectric ceramics of the system Pb0.88Ln0.08 Ti0.98Mn0.02O3 (Ln = La, Sm, Eu) present a large Curie temperature (430, 390, and 470 jC, respectively) as well as a high dielectric constant (see Fig. 2) in accord with previous studies [5 –7]. Additionally, the values of the pyroelectric coefficient associated with high values of temperature were obtained (see Fig. 3).
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Table 1 Material characteristics Type
Tc (jC)
er at Tc ( 103)
pi [AC/(cm2 jC)]
Rv ( 10 10) [C/(cm2 jC)]
Pb0.88La0.08Ti0.98Mn0.02O3 Pb0.88Sm0.08Ti0.98Mn0.02O3 Pb0.88Eu0.08Ti0.98Mn0.02O3 PLZT (8/40/60)8) PbTiO3:Ca8) (Pb0.8Ba0.2)[(Zn1/3Nb2/3)0.7Ti0.3]O9) 3
455 394 480 145 106 39
13.236 6.14 5.51 16.25 7.4 5.467
2.358 0.422 0.773 3.22 2.98 5.31
1.78 0.687 1.402 1.98 4.03 9.7
Table 1 shows the characteristics of materials studied by other authors. Each of these materials has a high peak dielectric constant and high peak pyroelectric coefficient at Tc. Although the Rv values are below some of the compositions studied by other authors, in the same table, it can be seen that there is not a great deal of difference between the values. In this way, a good infrared detector to operate at high temperatures can be obtained from the Pb0.88Ln0.08Ti0.98Mn0.02O3 (Ln = La, Sm, Eu) ferroelectric ceramic system. Finally, we noted significant drops of the values of the pyroelectric coefficient that occur at certain temperatures (270 – 290 jC) in the Pb0.88Ln0.08Ti0.98 Mn0.02O3 (Ln = La, Sm, Eu) ferroelectric ceramic system (see Fig. 3). This phenomenon is actively under study.
Acknowledgements The authors acknowledge the Third World Academy of Science for financial support of the Latin American Network of Ferroelectric Materials (NET-43).
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