Article pubs.acs.org/crystal
Growth and Piezoelectric Properties of Melilite ABC3O7 Crystals Yuanyuan Zhang,† Xin Yin,† Haohai Yu,† Hengjiang Cong,† Huaijin Zhang,*,† Jiyang Wang,† and Robert I. Boughton‡ †
State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, 27 Shanda Nanlu, Jinan, Shandong 250100, China ‡ Department of Physics and Astronomy, Bowling Green State University, Bowling Green, Ohio 43403, United States
ABSTRACT: Single ABC3O7 crystals with the melilite structure, including SrLaGa3O7, SrGdGa3O7, and BaLaGa3O7, have been successfully grown by the Czochralski method. A complete set of room temperature elastic, dielectric, and piezoelectric constants was determined using resonance techniques and impedance analysis. Our results indicate that the crystals exhibit superior piezoelectric and elastic properties. The electrical resistivity was found to be on the order of 6 × 1013 ohm·cm at room temperature. The dielectric permittivity ε11, piezoelectric constant d14, and electromechanical coupling coefficient k′12 were found to be on the order of 15−17, 12−15 pC/N, and 16−19%, respectively. For an ABC3O7 group member such as SrLaGa3O7, the melting temperature was measured to be 1588 °C, below which the crystal maintains good thermal stability. Consequently, the piezoelectric properties were studied as a function of temperature over the range of −50 to 120 °C. The frequency vs temperature coefficient was determined to be −16 to −55 ppm/K, depending on orientation. The dielectric permittivity, piezoelectric constant, and electromechanical coupling coefficient showed good temperature stability. The absence of any phase transitions below their melting points (on the order of 1600 °C) makes the ABC3O7 family promising candidate materials for high temperature piezoelectric applications.
I. INTRODUCTION Piezoelectric materials are used as actuators, sensors, and transducers in a great number of devices. Advances in electronics and computer control have led to the incorporation of piezoelectric materials in active control devices and smart systems.1−3 Operation at elevated temperatures is desired for applications in smart structures, such as in aircraft, turbine engine components, space vehicles, etc.4−7 The commercial piezoelectric materials, α-quartz (SiO2) and lithium niobate (LiNbO3), cannot be operated at temperatures exceeding 600 °C. The highest application temperature of about 350 °C is reached with quartz resonators, above which high losses and twinning prevent its use. On the other hand, the upper operational temperature limit for the ferroelectric crystal LiNbO3 is caused by its high conductivity and thermal instability.8 Crystals with high melting points, such as La3Ga5SiO14, GaPO4, and ReCa4O(BO3)3 (Re: rare earth elements such as Gd, La, and Y) crystals, merit more research in high temperature piezoelectric crystal applications. Their advantages and disadvantages include, for example, high piezoelectric and electromechanical © 2011 American Chemical Society
constants in La3Ga5SiO14, but low electrical resistivity and high dielectric loss at high temperatures that limit its application to below 800 °C.9,10 GaPO4 is found to have high electrical resistivity, but a phase transformation around 970 °C limits its high temperature use.11,12 The high temperature piezoelectric behavior of ReCa4O(BO3)3 crystals is reported to be independent of temperature, and there is no phase transition up to the melting point at ∼1500 °C, but attention should be paid to possible pyroelectric cross-talk for pressure-sensing applications.13,14 Thus, design and synthesis of good high temperature piezoelectric materials are a challenge and therefore of particular interest. ABC3O7 crystals belong to the tetragonal system with space group P4̅21m. Here, A = Ca, Sr, Ba; B = La, Gd; and C = Al, Ga. Rare earth-ion doped ABC3O7 crystals have been studied extensively in laser applications.15−18 Thermal, optical, and laser properties of Nd: SrGdGa3O7 and Nd: SrLaGa3O7 crystals were Received: June 8, 2011 Revised: December 3, 2011 Published: December 9, 2011 622
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characterized by our group.19,20 Nd: ABC3O7 crystals have broad absorption and emission spectra, which are promising disordered crystalline laser materials. However, few studies have been reported on the elastic and piezoelectric properties of this system, with the exception of BaLaGa3O7.21 Since the reported melting points are higher (about 1650 °C for SrLaGa3O7,15 1600 °C for SrGdGa3O7,18 1560 °C for BaLaGa3O716) and there are no phase transitions below the melting point, these crystals could possibly be used in high temperature piezoelectric applications. In this paper, high quality single crystals with the ABC3O7 melilite structure, including Nd: SrGdGa3O7 (Nd: SGGM), Nd: SrLaGa3O7 (Nd: SLGM), and Nd: BaLaGa3O7 (Nd: BLGM), were grown using the Czochralski technique. According to the Power Diffraction Standard Data files (number 24-110, 22-1436) and Inorganic Crystal Structure Database file (number 109451), Nd: ABC3O7 crystals have the same tetragonal structure with ABC3O7 crystals. The space group is P4̅21m. The Nd ion dopant will not influence the piezoelectric properties. Nd ion-doped ABC3O7 rather than the pure crystals were grown because they can also be used as diode-pumped laser materials. By analyzing the crystal symmetry of ABC3O7 crystals, the piezoelectric coordinate system was defined according to the IEEE standard. The appropriate samples for determining the dielectric, piezoelectric, and elastic properties at room temperature were fabricated by using the coordinate rotation and transmission methods in detail. In addition, the dielectric, piezoelectric, and electromechanical properties were investigated over a wide temperature range and are discussed in detail. The results show that ABC3O7 crystals are promising high temperature piezoelectric materials.
thermal anomalies in SLGM during growth, simultaneous thermogravimetric and differential thermal analysis (TG/DTA) measurements were performed using a TG/DTA thermal analyzer (SETSYS-2400 CS Evolution, France). Samples were held in a tungsten crucible under a 99.999% pure argon atmosphere with a flow rate of 20 mL min−1. SLGM powder weighing 14.9 mg was heated from 283.67 to 2131.55 K and then cooled down to room temperature at a linear rate of 20 K min−1. Electrical Characterization. ABC3O7 has a tetragonal crystal structure with a P4̅21m space group. ABC3O7 exhibits 10 independent nonzero constants: two independent dielectric constants ε11 and ε33, six independent nonzero elastic constants s11, s12, s13, s33, s44, and s66, and two independent nonzero piezoelectric constants d14 and d36. A schematic of the six samples with different orientations is shown in Figure 1. Four transverse length extensional vibration mode and two
Figure 1. Orientations of specimens: (1) (xyt) 5° bar, (2) (xyt) 45° bar, (3) (xyt) 85° bar, (4) (zxt) 45° bar, (5) X-cut square plate, (6) Zcut square plate. face shear vibration mode samples were cut to dimensions according to the IEEE standard,22 where the X, Y, and Z-axes are parallel to the a, b, and c-axes, respectively. For circumgyrate samples, the first letter denotes the thickness direction, the second letter denotes the length direction, and the third letter denotes the direction about which the rotation is made during the coordinate transformation. The rotation angles are also given. The dimensions of the specimen were 8 × 8 × 2 mm3 for the X- and Z-cut square plates and 12 × 4 × 2 mm3 for the rectangular bars. All the investigated samples were sputter coated with silver to serve as electrodes on the parallel faces along the thickness direction. The electrical resistivity was determined by a pA meter/DC voltage source (Agilent 4140B) using the two probe method from the measured current intensity and voltage values. The dielectric properties were determined using a multifrequency LCR meter (Agilent 4274A). The frequency vs temperature behavior was investigated by the standard resonance-antiresonance techniques23 using an Agilent 4294A precision impedance analyzer connected to a specially designed sample holder in a furnace. Samples were first placed into a container filled with dry ice and then put into the furnace. This system enables samples to be cooled down to −50 °C. The temperature of the sample was measured by a Pt−Rh thermocouple and controlled by a Shimaden FP23 controller/programmer connected to a thyristor. The heating rate was maintained at 0.5 °C/min. The temperature was maintained for 2 min at each measurement point. Table 1 lists the specimen orientations and shapes, and the elastic and piezoelectric constants. The piezoelectric ABC3O7 crystals were
II. EXPERIMENTAL SECTION Crystal Growth. Raw materials for Nd: ABC3O7 crystal growth, including Nd: SLGM, Nd: SGGM, and Nd: BLGM, were 99.99% pure Nd2O3, SrCO3 (BaCO3), La2O3 (Gd2O3), and Ga2O3 powders. The compounds were dried and then weighed out according to the compositional formula Nd0.01AB0.99C3O7. Taking the evaporation of Ga2O3 during the growth process into account, an excess of Ga2O3 (1.0% of the total mass) was added to the starting components. The mixtures were ground, mixed, and heated at 1050 °C for 10 h in a platinum crucible to completely decompose the SrCO3 and BaCO3. The calcined powders were milled again and pressed into tablets and sintered at 1100 °C for 10 h to synthesize polycrystalline compounds. Crystals were grown using the conventional RF-heating Czochralski method in an atmosphere of N2 + O2 (2% by volume), using an iridium crucible. This oxygen content is great enough to suppress the thermal volatilization of Ga2O3 during the growth process, but on the other hand it is low enough to prevent oxidation of the iridium crucible. The Nd: ABC3O7 compounds were melted and kept for 3 h at a temperature 30 °C above the melting point, to ensure homogeneity of the melt. A ⟨001⟩-oriented Nd: ABC3O7 crystal seed was lowered to a position a little above the melt surface to reach thermal equilibrium, then slowly introduced into the melt to minimize thermal shock and to obtain a stable liquid meniscus between the seed and melt. To prevent dislocations in the seed from propagating into the crystal, a thin growth neck extending from the seed is important, so the pulling rate was initially set at 3.0 mm/h. After obtaining the expected neck diameter (3−4 mm), the pulling rate was reduced to 1.0 mm/h with a rotation rate of 20 rpm for the crystal growth. After growth was completed, the crystal was cooled to room temperature at a rate of 25 K/h, to avoid cracking.19 Thermal Stability Characterization. The SLGM crystal is reported to have the highest melting temperature of the group, so it was selected to investigate the general thermal stability of the ABC3O7 crystal class. In order to obtain in situ detection of any possible
Table 1. Effective Elastic and Piezoelectric Constants for Different Specimens electric field direction
specimen x-cut square plate z-cut square plate (xyt)5° (xyt)45° and (xyt)85° bars (zxt)45° 623
x z x z
coefficients ε11, s44 = s55 ε33, s66 s11, s33, 2s13 + s55, d14, k′12 2s12 + s66, d36, k′31
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studied over the range of −50 to 120 °C. The main formulas used for analysis are24−26
s′ =
1 4ρ(lfr )2
(1)
s′22 (θ) = s33 sin4 θ + s11 cos4 θ + (2s13 + s55) cos2 θ sin2 θ
(2)
s′11 = s11(sin4 θ + cos4 θ) + (2s12 + s66) cos2 θ sin 2 θ
(3)
s44 F2
=
1 4ρ(lfr )2
F = 1.2916 − 0.0458
(4) s22 + s33 s44
(5)
where s′ is the elastic constant in the new coordinate system, ρ is the Archimedes-method density, l is the length, f r is the resonance frequency, and F is a factor which is related to the contour-shear mode. The electromechanical coupling coefficients k′12(xyt)45° and k′31(zxt)45° were determined on (xyt)45° and (zxt)45° bars, respectively, from the following formula:
π fa ⎛ π fa − fr ⎞ ⎟⎟ tg ⎜⎜ = 2 fr ⎝ 2 fr ⎠ 1 − k′2 k′2
(6)
where fa is the antiresonance frequency. The piezoelectric coefficients can be calculated as
d′ = k′(s′ε)1/2
(7)
d14 = 2d′12
(8)
d36 = 2d′31
(9)
Figure 2. ABC3O7 crystals grown along the c-direction.
In d′ and k′, the superscript apostrophe denotes a coefficient in the new coordinate system. The shear mode electromechanical coupling coefficients can be calculated from21
e2 c εs
(kl)2 =
structure determination. It was found that the as-grown crystals consist of a single phase belonging to the P42̅ 1m group. The concentrations of Nd ions in these-grown Nd: SGGM, Nd: SLGM, and Nd: BLGM crystals were measured using X-ray fluorescence analysis.18 The result values are 1.36 atom %, 1 atom %, and 1.12 atom %, respectively. The melting temperature of SLGM, so far as we know, is the highest among the ABC3O7 crystals. Compared with Nd: SGGM and Nd: BLGM, great difficulty was encountered in the growth of Nd: SLGM because there is no suitable seed. The feed rod method was used to pull the first crystal. Polycrystalline materials were synthesized by the solid-state reaction method. Then they were ground up and loaded into a rubber membrane to fabricate the feed rod. The feed rod was vacuum treated and hydrostatically pressed to about 68 kN. The rod obtained has a diameter of 9 mm and a length of 80 mm. It was sintered at 1100 °C for 5 h in air. The following crystals were pilled on oriented seeds cut from the crystals pulled on the feed rod. The growth process of ABC3O7 single crystals from the melt also produced a serious problem regarding volatility. The main volatile component is generally regarded to be gallium oxide. The initial amount of Ga2O3 exceeded 1% of the required mass to compensate for volatilization. This condition induces the melt to substantially deviate from stoichiometry and thus may affect the ABC3O7 crystallization process. Thus, it is necessary to examine the high-temperature thermal behavior that governs the growth process. Figure 3 shows high-temperature TG-DTA results for SLGM single crystal. In Figure 3a, TG analysis shows a negligible mass loss up to 1530 °C, indicating that the loss of Ga2O3 by
(10)
where kl is the rod extensional coupling factor with transverse excitation, and εs is the clamped dielectric permittivity.
After obtaining the values of dij and sij, the elastic stiffness coefficients cij and piezoelectric stress constants eij can be obtained by using the following conversion formulas:27
c = s−1
(11)
e=d×c
(12)
The temperature characteristics of the resonance frequency shift can be expressed by the first few terms of the following power series:
f − fr0 Δf = r = fr0 fr0 T (f n) =
∑ T (fn)(T − T0)n n
n 1 ⎛ ∂ fr ⎞ ⎜⎜ n ⎟⎟ n! fr0 ⎝ ∂t ⎠
T = T0
(13)
(14)
where Tf(n) is the temperature coefficient of frequency (TCF) of nth order, f r0 is the resonance frequency at the temperature T0, and f r is the resonance frequency at temperature T.
III. RESULTS AND DISCUSSION Crystal Growth and Thermal Stability. Figure 2 shows the as-grown 1 atom % Nd: ABC3O7 crystals with dimensions of about Φ30 × 50 mm2. X-ray powder diffraction was used for 624
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Table 2. Room Temperature Electro-Elastic Properties of ABC3O7 Crystals Dielectric Constants εij ε11 SrGdGa3O7 SrLaGa3O7 BaLaGa3O7 α-SiO2 La3Ga5SiO14 YCa4O(BO3)3
d36
d14 SrGdGa3O7 SrLaGa3O7 BaLaGa3O7 α-SiO2 La3Ga5SiO14 YCa4O(BO3)3
ε33
ε22
17.28 11.59 14.76 10.39 14.84 10.96 4.51 4.60 19.2 50.7 9.57 9.52 Piezoelectric Constants dij (pC/N)
11.4 d11
d26
14.5 2.9 0 13.7 3.6 0 12.3 4.0 0 −0.7 0 −2.3 −6.0 0 6.2 0 0 1.4 Piezoelectric Stress Constants eij (C/m2)
10 e36
e14 SrGdGa3O7 0.52 SrLaGa3O7 0.54 BaLaGa3O7 0.49 Elastic Compliance Constants sij (pm2 /N) s11
s44
s66
27.6 25.5 25.0 20.0 20.3
16.1 16.6 17.7 29.1 26.2
c44
c66
16.5 7.7 5.8 16.1 3.6 17.3 8.2 6.3 16.8 3.9 16.1 7.2 5.5 15.1 4.0 Electromechanical Coupling Coefficients kij (%)
6.2 6.0 5.6
c11
Figure 3. High temperature TG-DTA results for Nd: SLGM: (a) TG curve; (b) DTA curve.
vaporization at low temperature is insignificant. However, when the temperature exceeds 1530 °C, there is a detectable mass loss at a rate of about 48.6% h−1 in Nd: SLGM. Then the evaporation rate tends to decrease beyond about 1632 °C and the resulting total weight loss is 51.34%. From Figure 3b, we can see that there is a sharp endothermic peak beginning at 1528.98 °C, which corresponds quite well with the TG analysis results. In this temperature range, there is large mass loss due to Ga2O3 vaporization. The endothermic peak located at 1588.27 °C corresponds to the melting point of Nd: SLGM in Ar atmosphere. These results show that Nd: SLGM maintains good thermal stability up to 1528.98 °C. There is another small broad endothermic peak located at 1705.58 °C. We conclude that as Ga2O3 volatilizes, the composition deviates from stoichiometry, and as a result, a new compound is formed. Room Temperature Piezoelectric Properties. Table 2 summarizes the electroelastic properties of ABC3O7 crystals at room temperature, as compared to the well-studied SiO2, La3Ga5SiO14, and YCa4O(BO3)3 single crystals. The ABC3O7 family exhibits comparable and even more favorable dielectric and piezoelectric properties than the other piezoelectric crystals. The dielectric permittivity ε11, piezoelectric constant
s13
s33
8.2 −3.2 −1.8 7.5 7.9 −3.1 −1.8 7.3 8.2 −3.0 −1.9 8.0 12.8 −1.8 −1.2 9.6 8.9 −4.2 −1.8 5.2 Elastic Stiffness Constants cij (1010 N/m2)
SrGdGa3O7 SrLaGa3O7 BaLaGa3O7 α-SiO2 La3Ga5SiO14
SrGdGa3O7 SrLaGa3O7 BaLaGa3O7
s12
0.18 0.22 0.22
SrGdGa3O7 SrLaGa3O7 BaLaGa3O7 La3Ga5SiO14 YCa4O(BO3)3
c12
c13
c33
k′12(xyt) 45°
k′31(zxt) 45°
kl14
kl36
18.6 19.6 16.4
5.7 7.4 7.7
22.2 23.9 21.4
7.1 9.4 9.4
k12 (xcut)
k26(zxw) 30°
16 22
d14, and electromechanical coupling coefficient k12′ of the crystals were found to be on the order of 15−17, 12−15 pC/N, and 16−19%, respectively. The shear mode electromechanical coupling coefficients kl14 at room temperature for SGGM, SLGM, and BLGM were found to be 22.2%, 23.9%, and 21.4%, respectively. The piezoelectric stress constants e14 and e36 in BLGM were measured to be 0.49 C/m2 and 0.22 C/m2, respectively. These values are larger than the previously reported values in ref 21 (0.29 C/m2 and 0.10 C/m2). For SGGM, the relative dielectric constants are ε11 = 17.28 and ε33 = 11.59. The piezoelectric constant (d14 = 14.5 pC/N) is greater than that of YCa4O(BO3)3 (d26 = 10 pC/N),28 being about 2 times that of La3Ga5SiO14 (d11 = 6.2 pC/N).10 The electrical resistivity was determined to be 6 × 1013 ohm·cm for BLGM at room temperature. Knowledge of the thermal expansion coefficients, which were determined to be on the 625
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order of 5−6 × 10−6/K over the temperature range of 300−770 K for the ABC3O7 crystals can enable precise tailoring of constant frequency transducers. The detailed measure method is given in ref 20. Good piezoelectric properties are closely related to the structure of the crystal. As shown in Figure 4, the melilite structure
Figure 5. Relative dielectric permittivity as a function of temperature along the a- and c-axes for ABC3O7.
Figure 4. Projection of the average crystal structure of SrLaGa3O7 approximately along the c-axis.
has been performed.29,30 The ABC3O7 crystal is built up from CO4 layers formed in the a−b plane. Between the layers, A2+ and B3+ ions are distributed randomly in eight coordinated sites with Cs symmetry. The ratio is 1:1. Because of Jahn−Teller distortion, both the A2+ and B3+ cations are in asymmetrically coordinated environments. There are two different types of tetrahedral CO4 and one type of 8-fold B(C)O8 antiprisms in the lattice. For crystals of the 4̅2m point group, there is no spontaneous polarization. When mechanical stress is applied along a crystallographic direction, deformation occurs and the overall polarization becomes nonzero, which leads to the piezoelectric effect. When SGGM, SLGM, and BLGM crystals are subjected to stress, the shifts of the positive and negative ions from their original positions result in large polarization vectors that make the crystals exhibit strong piezoelectric properties. The Temperature Stability of Piezoelectric Properties. Figure 5 presents the dielectric permittivity measured at 1 kHz as a function of temperature for the crystals along the X- and Z- axes. It can be seen that the dielectric permittivity values increase slightly with increasing temperature. The dielectric permittivity ε11 for SGGM was observed to be 18.36 at 120 °C, which is an increase of 18%, when compared to the −50 °C temperature value. This behavior demonstrates that the crystal possesses high dielectric stability at higher temperature. It should be noted that other ABC3O7 crystals, such as SLGM and BLGM, also show a similar temperature dependence in their dielectric properties. The temperature dependence of the resonance frequency shift ( f r − f r0)/f r0 over the temperature range of −50 to 120 °C for differently oriented SGGM samples is shown in Figure 6. As can be observed, the change in resonance frequency shifts downward with increasing temperature, exhibiting a linear frequency shift vs temperature behavior. The temperature coefficient of frequency (TCF) was found to be on the order of −16 ppm/K for the (xyt)85°-cut, while a value of −55 ppm/K was determined for the(zxt) 55°-cut sample.
Figure 6. Temperature characteristics of the resonance frequency for SGGM with different cuts (a) (xyt)5° (xyt)45° and (xyt)85° bars (b) (zxt)45° bar, X- and Z-cut square plate.
Knowledge of the temperature dependence of the elastic constants is also quite important for the application of ABC3O7 crystals. We determined the change in elastic constants as a 626
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function of temperature. Figure 7 shows the fitting curves of sij for SGGM. The elastic constants were found to maintain their
Figure 8. Electromechanical coupling factors k12′ and k31′ as a function of temperature. Figure 7. Temperature dependence of the elastic constants of SGGM.
value up to 120 °C, exhibiting a very stable value in this temperature range. First-, second-, and third-order temperature coefficients of the elastic coefficients have been calculated using eqs 12 and 13 (Table 3). Figure 8 shows the electromechanical coupling factors k12′ (xyt) 45° and k31′ (zxt) 45° as a function of temperature. The highest value of k12′ (xyt) 45° at room temperature was found to be 18.6% for SGGM, increasing linearly to 20.3% at 120 °C. The value of k12′ for SLGM was determined to be 16.5% at 200 °C, with a variation of the piezoelectric coefficient of only 0.5% over the temperature range from −50−120 °C. Other crystals studied in this work exhibited a similar temperature dependence of the electromechanical coupling factor, with
