IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,
vol. 61, no. 4,
April
2014
561
Pyroelectric Properties of Rare-Earth Calcium Oxyborate Crystals: ReCa4O(BO3)3 (Re: Y, Gd, Nd, and Pr) Shuai Hou, Fapeng Yu, Yanxue Tang, Shujun Zhang, and Xian Zhao Abstract—Pyroelectric properties of the monoclinic ReCa4O(BO3)3 (ReCOB, where Re is rare earth: Y, Gd, Nd, and Pr) single crystals were investigated by the charge integration method in the temperature range of 20°C to 180°C. The two independent pyroelectric coefficients p1 and p3 were measured and their temperature-dependent properties were studied; the pyroelectric p1 was found to be negative and decrease with increasing temperature, varying from −59.3, −64.2, −65.5, and −59.5 μC/(m2·°C) at 30°C to −52.9, −53.3, −46.6, and −50.5 μC/(m2·°C) at 180°C for YCOB, GdCOB, NdCOB, and PrCOB, respectively, whereas the positive coefficient p3 was observed to decrease from 11.6, 13.6, 23.5, and 31.0 μC/(m2·°C) at 30°C to 2.5, 7.1, 4.9, and 10.1 μC/(m2·°C) at 180°C, respectively. In addition, the ReCOB crystals were found to possess relatively high detectivity Fd, being 7.6 to 11.4 × 10−5 Pa−1/2 at 30°C, approximately two times that of commercial triglycine sulfate (TGS) and LiTaO3 crystals, with minimal variations up to 180°C, indicating the potential for use in thermal imaging applications.
I. Introduction
R
are-earth calcium oxyborate crystals ReCa4O(BO3)3 (ReCOB, Re = Y, Gd, Nd, and Pr) are multi-functional materials. These crystals possess congruent melting points around ~1400°C to 1510°C, have no phase transformations before their melting points, and can be readily grown by the Czochralski pulling method [1]–[12]. In the last two decades, extensive studies have been carried out on ReCOB crystals, focusing on nonlinear optical and laser applications because of their wide transparency range, reasonable nonlinear optic coefficient, and high laser damage threshold [3]–[6]. Recently, ReCOB crystals have been explored for high-temperature piezoelectric applications, among which, high-temperature accelerometer sensors and Manuscript received July 10, 2013; accepted January 20, 2014. This work was financially supported by the National Natural Science Foundation of China (grant numbers 91022034 and 51202129), Natural Science Foundation of Shandong Province (ZR2012EMQ004), the Postdoctoral Innovation Foundation Funded Project of Shandong Province (grant number 201203066) and China Postdoctoral Science Foundation (2012M511019). S. Hou, F. Yu, and X. Zhao are with the State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan, P. R. China (e-mail:
[email protected]). S. Hou is also with the Advanced Research Center for Optics, Shandong University, Jinan, P. R. China. Y. Tang and S. Zhang are with the Materials Research Institute, Pennsylvania State University, University Park, PA (e-mail:
[email protected]). Y. Tang is also with the Key Laboratory of Optoelectronic Material and Devices, Shanghai Normal University, Shanghai, P. R. China. DOI http://dx.doi.org/10.1109/TUFFC.2014.2944
0885–3010
ultrasonic transducers have been fabricated using YCOB crystals, taking advantage of its high electrical resistivity and temperature stability of electromechanical properties over the temperature range of 20°C to 1000°C [13]–[16]. Because of their crystal symmetry (space group Cm), ReCOB crystals also possess the pyroelectric effect, in addition to nonlinear optical and piezoelectric properties. However, reports on the pyroelectric properties of ReCOB crystals are limited. In this work, the pyroelectric properties of ReCOB crystals, including YCOB, GdCOB, NdCOB, and PrCOB, are investigated for potential thermal imaging applications. II. Experimental Crystals were oriented along the crystallographic 〈010〉, 〈−201〉, and 〈101〉 using an X-ray orientation system, from which the crystallographic axes a, b, and c could be confirmed [17]. The physical axes X, Y, and Z, and the sign of pyroelectric coefficients were determined according to the IEEE Standard on Piezoelectricity [18]. The sample dimension ratios, t (thickness):w (width):l (length), ranged from 1:6:6 to 1:8:8. All of the samples were vacuum sputtered with 100-nm Pt film on the parallel faces for pyroelectric measurements. The pyroelectric properties were investigated by the charge integration method [19], using a pyroelectric test system consisting of a Keithley model 642 electrometer (Keithley Instruments Inc., Cleveland, OH) and a precision temperature control system. The pyroelectric coefficient pi was calculated by
pi =
C 0d v , (1) AdT
where dv is the measured pyroelectric voltage, A is the electrode area, and C0 is the capacitance of integrating capacitor, which is 1 μF. In this work, the pyroelectric coefficient pi was measured under constant strain by using a specially designed fixture, and was considered to be the primary pyroelectric coefficient [20]. The temperature step during measurement was on the order of dT = 10 ± 0.5°C, with temperature increasing from 20°C to 200°C. The secondary pyroelectric coefficient was calculated following the thermodynamic relation:
© 2014 IEEE
Psec = PiX − p ix = α jkc jklmd jlm , (2)
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IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,
where PiX is the unclamped pyroelectric coefficient, p ix is the primary pyroeletric coefficient, and αjk, cjklm, and djlm are the thermal expansion coefficient, elastic stiffness, and piezoelectric coefficient, respectively. The pyroelectric performance was evaluated by figures of merit (FOMs) [21], including current responsivity Fi, voltage responsivity Fv, and detectivity Fd: Fi =
pi (4) (C vεii )
Fv = Fd =
pi (3) Cv
pi , (5) C v (εii tan δ)
where Cv is the volume specific heat, εii is the dielectric permittivity, and tan δ is the corresponding dielectric loss. The dielectric permittivity was determined by an LCR bridge (4263B Agilent Technologies Inc., Santa Clara, CA), using
εii =
C pt , (6) A
where Cp is the capacitance measured under 100 kHz, t is the thickness of samples, and A is the electrode area. III. Results and Discussion A. Primary and Secondary Pyroelectric Properties at Room Temperature Pyroelectricity is a first-rank polar tensor property, associated with a change of polarization P to a change of temperature T. Using charge integration method, the primary pyroelectric coefficients p1 and p3 for ReCOB crystals were measured at room temperature and found to be on the order of −59.3 and 11.6, −64.2 and 13.6, −59.5 and 23.5, and −65.5 and 31.0 μC/(m2·°C) for YCOB, GdCOB, NdCOB, and PrCOB, respectively; results are given in Table I. Fig. 1 presents the variation of pyroelectric coefficients p1 and p3 as function of rotation angle about the y-axis
vol. 61, no. 4,
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2014
for YCOB, GdCOB, NdCOB, and PrCOB crystals from 0° to 360°, from which the maximum and minimum p1 and p3 values were obtained. According to the calculations, the maximum primary pyroelectric coefficients obtained were on the order of −60.4, −65.6, −69.6, and −67.1 µC/ (m2·°C) for YCOB, GdCOB, NdCOB, and PrCOB, respectively, corresponding to Z-cut samples with rotation angles (around the y-axis) of 78.9°, 78°, 71.2°, and 62.5°, respectively. The so-called primary pyroelectric coefficient (p ix ) is measured at constant strain and represents the rearrangement of polarization changes inside the unit cell, whereas the secondary pyroeletric coefficient contribution Psec(PiX − p ix ) comes from the dimensional change of the unit cell with temperature. The unclamped pyroelectric coefficient PiX measured at constant stress is equal to the sum of the pyroelectric effects caused by rearrangement of polarization changes inside the unit cell and the piezoelectric contribution resulting from the thermal expansion. Using (2) and combining with thermal expansion αij for YCOB, GdCOB, NdCOB, and LaCOB crystals, the secondary pyroelectric coefficients for X- and Z-cut crystals were calculated; the results are listed in Table II, where the secondary pyroelectric coefficients were found to be on the order of −1.47 and −1.53, 3.81 and 0.2, 1.41 and −5.78, and 1.38 and −0.44 μC/(m2·°C), respectively, comparable to those of tourmaline crystals [−3.52 μC/ (m2·°C)], and two orders of magnitude lower than triglycine sulfate (TGS) [−330 μC/(m2·°C)] [25]. The secondary pyroelectric coefficients were much smaller than the measured primary pyroelectric coefficients; thus, the total pyroelectric coefficients (PX = px + Psec) p1 and p3 are mainly attributed to the primary pyroelectric effect for ReCOB crystals. B. Temperature Dependence of the Primary Pyroelectric Properties of ReCOB Temperature dependence of pyroelectric properties for ReCOB crystals were investigated, where the variation of primary pyroelectric coefficients p1 and p3 (absolute value) as a function of temperature for YCOB and GdCOB are shown in Fig. 2 (small insets show the schematic of X and Z crystal cuts). It is observed that the pyroelectric coef-
TABLE I. Properties of Different Pyroelectric Materials at Room Temperature.
TC pi, µC/(m2·°C) Cv εr/ε0 tan δ Fi, pm/V Fv, m2/C Fd, 10−5/Pa1/2
YCOB (X/Z)
GdCOB (X/Z)
PrCOB (X/Z)
NdCOB (X/Z)
TGS [22]
PVDF [23]
LiTaO3 [24]
— −59.3/11.6 2.66 9.57/9.52
