Article pubs.acs.org/JPCC
Size Effects Observed via the Electrical Response of BaTiO3 Nanoparticles in a Cavity Joseph F. Lomax,*,† John J. Fontanella,‡ Charles A. Edmondson,‡ Mary C. Wintersgill,‡ Mark A. Westgate,‡ and Sitki Eker§ †
Chemistry Department, U.S. Naval Academy, Annapolis, Maryland 21402, United States Physics Department, U.S. Naval Academy, Annapolis, Maryland 21402, United States § Physics Department, Ahi Evran University, Kirşehir, Turkey ‡
ABSTRACT: Barium titanate (BaTiO3) is one of the most widely used electronic materials. The origin of changing structural and electronic behavior between larger and smaller nanoparticles in BaTiO3 has been extensively investigated. Here, the dielectric loss measured for BaTiO3 or BaZrO3 particles in a cavity at audio frequencies (10−105 Hz) and low temperatures (5.5−350 K) is reported. Distinct differences were found between small (50−100 nm diameter/cubic) and large (>200 nm diameter/tetragonal at room temperature) BaTiO3 particles. Isochronal relaxation features observed around 20 K showed differing shape and dynamics (50−100 nm particles: broad peak, Eact ≈ 0.04 eV, log10( f pre(s)) ≈ 14.7; >200 nm particles: sharp peak, Eact ≈ 0.025 eV and log10(f pre(s)) ≈ 12, with a shoulder). The low-temperature relaxations are attributed to the motion of off-center titanium ions. In support of this identification, low-temperature relaxations were not observed in BaZrO3 nanoparticles. Finally, structural change features were observed at about 180 and 270 K in the larger BaTiO3 particles (rhombohedral at low temperature) but not in the smaller particles (cubic at all temperatures). The results provide a new set of testable bounds that should contribute to the understanding of nanoparticle-sized BaTiO3, in particular, and ferroelectric behavior in perovskites, in general. work of Ohno et al.12 and Hoshina et al.13 However, the remaining eight references quoted by Yen et al. suggest that the critical size for the c → t transformation is much larger (on the order of 100 nm). This is supported by the more recent work of Tsunekawa.14 Yen et al. propose that the larger critical size is attributable to higher lattice strain that can be induced by different preparation methods. However, Wada et al.15 and Cho16 show that differences can also be explained by different measurement techniques in that they find that the structure of 17−100 nm particles can be assigned to cubic m-3m by X-ray diffraction and tetragonal 4mm by Raman spectroscopy. (Differences in the critical size due to measurement techniques can also be seen in Table 1 of Yen et al.) This suggests an inhomogeneous structure. Finally, Aoyama et al.17 observe bulklike behavior for 100−200 nm diameter nanoparticles. They label the results for 60−90 nm diameter nanoparticles as a transient region where translational symmetry is broken. For the 60−90 nm diameter nanoparticles, they point out that the usual phase transitions cannot be observed. Instead, they find that a new structure, R′, appears. They go on to point out that a highly distorted structure appears for nanoparticles with
1. INTRODUCTION Barium titanate (BaTiO3) is a widely used dielectric in the electronics industry in applications such as multilayer capacitors, thermistors, optoelectronic devices, electromechanical transducers, and memory storage.1−5 Primarily because of the increased interest in nanoscale phenomena, there has recently been a great deal of work on BaTiO3 nanoparticles. To date, the primary focus of much of the effort has been the ferroelectric phase transition and a recent introduction to and a summary of much of that work is given in Szwarcman et al.6 A related reason for studying these materials is the relatively high dielectric constant that BaTiO3 exhibits. Consequently, BaTiO3 nanoparticles are of interest as a high dielectric constant constituent in nanocomposites or nanodielectrics. Often, the goal of the nanodielectric work is to add nanoparticles to a matrix, usually a polymer, in an attempt to form a material with improved dielectric properties.7−10 Work on the physical properties of BaTiO3 nanoparticles has found changes as the particle diameter falls below a critical size. Table 1 of an early paper by Yen et al.11 (1995) contains the results from 11 other references for the critical crystallite size for the cubic (c) to tetragonal (t) phase transformation. The work of Yen et al. and three of the references that they quote suggest that the critical size for the c → t transformation is on the order of 30 nm. This result is supported by the more recent © 2012 American Chemical Society
Received: July 20, 2012 Revised: October 10, 2012 Published: October 11, 2012 23742
dx.doi.org/10.1021/jp307218v | J. Phys. Chem. C 2012, 116, 23742−23748
The Journal of Physical Chemistry C
Article
2.2. Measurements. The polycrystalline BaTiO3 was ground into wafers on the order of 1.5 mm thick using corundum paper. Some of the wafers were approximately square about 6 mm on each edge while others were irregularly shaped. Aluminum electrodes were vacuum evaporated onto the flat surfaces of the films and the polycrystalline BaTiO3 wafers to form a parallel plate configuration. In addition, the nanoparticles were placed in a cavity for dielectric measurements as described elsewhere.18,19 Complex conductance measurements were carried out using a CGA-85 Capacitance Measuring Assembly, which operates at 17 frequencies from 10 to 105 Hz in approximately equal intervals of the logarithm of the frequency. The equivalent parallel capacitance, C, and conductance divided by the angular frequency, G/ω, were measured. The measurements were carried out in vacuum from 5.5 to 350 K in a Precision Cryogenics CT-14 dewar and the temperature was controlled using a Lake Shore Cryotronics DR92 temperature controller. The data were transformed to dielectric loss, tan δ, using
diameters less than 50 nm. As a consequence of the wide range of values and interpretations of the critical crystallite size, further work is warranted. There is also ambiguity concerning the real part of the relative permittivity. For example, Ohno et al.12 find that, as the diameter of the nanoparticles is decreased below about 50 nm, the dielectric constant begins to increase. By contrast, Hoshina et al.13 find that the dielectric constant undergoes a maximum at a nanoparticle diameter of 140 nm, and they attribute this to the size at which surface effects become important. According to Hoshina et al., below 140 nm, the particle size affects the crystal structure of the whole particle. Further, Wada et al. find a maximum in the dielectric constant of over 15 000 at 70 nm,14 which led them to conclude that 70 nm diameter particles would be best for use in nanodielectrics. Recently, the authors carried out dielectric studies of 50−70 nm BaTiO3 nanoparticles in poly(ether imide).8 In the course of that work on the nanocomposite, a broad low-temperature relaxation region was observed. Comparing the results of the dielectric studies of the 50−70 nm nanoparticles of BaTiO3 nanoparticles in poly(etherimide) with the results for the particles in a cavity revealed that the relaxation region was in fact caused by the particles. Subsequently, studies were carried out on different size BaTiO3 nanoparticles and the results are reported in the present work. A dramatic difference in the lowtemperature relaxation spectrum is observed for small (cubic phase) and large (tetragonal phase at room temperature) nanoparticles. In addition, higher temperature electrical measurements were carried out and the well-known structural phase transitions at 180 and 270 K for the larger BaTiO3 particles were observed. Once again, a difference is found for the small and large nanoparticles. Next, changes are found in the relaxation spectrum and in the higher temperature dielectric loss after the nanoparticles are heated to 800 °C in air. Finally, all results are compared with those for BaZrO3 and polycrystalline (ceramic) BaTiO3. In this paper, the interconnected properties of structure, energetics, and electronics are explored and features are found that clearly distinguish between the smaller particle sizes and the larger. This provides new, testable bounds to models of the size effect in BaTiO3.
tan δ =
G ε″ = ωC ε′
(1)
Also included in eq 1 is the microscopic interpretation of tan δ. In this case, ε′ and ε″ are the effective real and imaginary parts of the relative permittivity of the cavity containing the nanoparticles. The fact that the nanoparticles were in a cavity for the measurements should be emphasized. The consequence of this is that the dielectric properties of individual nanoparticles were not determined. Rather, what was measured was the electrical response of the cavity containing the nanoparticles and the results are reported as tan δ.
3. RESULTS AND DISCUSSION 3.1. BaTiO 3 Nanoparticles. 3.1.1. Dielectric Loss Features. The low-temperature relaxation spectra at 104 Hz for the 50 and 500 nm nanoparticles are shown in Figure 1. The
2. MATERIALS AND METHODS 2.1. Materials. Three different forms of barium titanate (BaTiO3) were obtained from Alfa Aesar: 99+% (metals basis) BaTiO3 (Stock No. 41633, Ba/Ti 1.006 mole ratio) was in the form of 50−70 nm diameter particles (AA50-70), 99.7%+ (metals basis) BaTiO3 (Stock No. 88267) was in the form of
