to perovskite structure in which half of all A-site positions are filled with bismuth (3+)

Transcription

1 2.1 LITERATURE SURVEY Structure and Phase Transitions of Bismuth Sodium Titanate Bismuth sodium titanate, Bi 0.5 Na 0.5 TiO 3 (BNT ), with the general formula A'A''BO 3 belong to perovskite structure in which half of all A-site positions are filled with bismuth (3+) ions, and the other half with sodium (1+) ions. The B-site positions are filled by titanium (4+) ions [25-27] as shown in figure 2.1. Figure 2.1: Representation of an A'A"BO 3 perovskite shown as cubic BNT. Many structural investigations have been performed on BNT since it was discovered by (Smolenskii et al., 1961) [28]. These studies used the best methods of the time to determine the phases and phase transitions of BNT [29-32], Phase transitions 14

2 determined from these earlier studies have been clarified by an extensive neutron powder diffraction study of BNT. This study definitively determined the phase transitions and crystal structures at various temperatures (John and Thomas, 2000). The results of tthe study are shown in Table 2.1 from which it can be seen that with decreasing temperature, BNT transforms from cubic to tetragonal and then to rhombohedral with coexistence regions in between. BNT in rhombohedral phase is polar due to the cation displacement which allows the crystal to have a spontaneous polarization ( ); vital for a material to be ferroelectric. The tetragonal phase is also a polar phase, but it has a weaker polarization response than the rhombohedral phase; this is evident by the decrease in relative permittivity above the Curie temperature 320 C [33, 34]. At room temperature and ambient pressure, BNT is a relaxor like [35] ferroelectric in the rhombohedral [36] phase and it exhibits diffuse phase transitions between each of the phases (Park and Chung, 1994; Chu et al., 2002b; Jones and Thomas, 2002, Walsh and Schulze, 2004) [37-40]. Diffuse phase transitions are evident in the dielectric response of BNT since the relative permittivity peaks versus temperature become broad instead of sharp peak, like those seen in materials with Curie Weiss behavior. Broad relative permittivity peaks are important in solid solutions and in disordered structures. The Curie range, rather than the Curie temperature can be used to describe the broad peaks that are caused by the diffuse phase transitions [33]. 15

3 Table 2.1: Phase Transitions in BNT Measured by Jones and Thomas, Temperature ( C) Phase(s) Lattice Parameters (Å) -268 to 255 Rhombohedral R3c = (2) = Glazer Tilt System = (2) = to 400 Rhombohedral/Tetragonal Variable Variable 400 to 500 Tetragonal P4bm = (2) = (2) = = to 540 Tetragonal/Cubic Variable Variable Above 540 = (3) = (3) Cubic Prototype Pm3m Synthesis Methods of Bismuth Sodium Titanate Pure BNT can be synthesized as a polycrystalline ceramic [41], thin film [42, 43] or as a single crystal [44, 45]. When using a standard mixed oxide route, the stoichiometrically mixed and milled powders are usually calcined at around 800 C to 900 C from two to four hours [26, 46, 47]. Longer calcination times as well as repeated milling between multiple calcinations can improve sample quality if bismuth oxide vaporization is prevented [48]. Thermogravimetric analysis of BNT powder has shown that bismuth oxide vaporization occurs above 1130 C [46]. However, to achieve acceptable densities, sintering temperatures of at least 1200 C are required [49], which is close to its melting point of 1225 C. Therefore, loss of bismuth oxide needs to be suppressed, or compensated and temperatures have to be well controlled [46]. BNT based materials, which can be sintered below 1100 C, require no suppression or compensation of bismuth [50, 51]. Temperature and time dependence of grain growth in BNT during 16

4 sintering has been found to be relatively low [52]. (Nagata et al., 2004) illustrated a near linear increase in relative density of BNT with temperature from 90 % at 1150 C to 98% at 1225 C with a sintering time of two hours. At temperatures above 1225 C, the samples partially melted. At 1100 C, a two-hour sintering time resulted in 87% relative density, which can be improved to 96% relative density after long time sintering [53]. Besides from the standard mixed oxide route, sol-gel-type processing has been successfully applied to BNT [42, 43, 48, 54-56]. Nano-sized powders derived from solgel can be prepared by dissolving bismuth oxide and sodium carbonate in 70% nitric acid. Once dissolved, ethylene glycol is added and the remaining water evaporated at 80 C. A stoichiometric amount of titanium tetra isopropoxide is added and stirred for two hours at 70 C. Calcination temperatures between 600 C and 700 C of the BNT sol for six hours lead to 100 to 200 nm sized crystals of BNT [48]. A steric acid gel synthesis route shows particle sizes of 10 to 40 nm with a calcination time of one hour at 600 C to 800 C [54]. Particles of 100 to 200 nm can also be prepared by hydrothermal synthesis at 200 C for 24 hours in 12 Molar sodium hydroxide [57, 58] or potassium hydroxide [59]. Chemical vapour deposition produces thin film of BNT up to 162 nm thickness with dielectric constant being about half that of other fabrication techniques [38]. Single crystals of BNT are also easily grown via a flux or Czochralski method [45]. The stoichiometrically weighed powders are mixed and placed in a closed platinum crucible. Spontaneous-crystallisation occurs during cooling in air. Unfortunately, either the starting temperature or the cooling rates are rarely specified [53, 61]. 17

5 2.1.3 Electrical Properties of Bismuth Sodium Titanate (Suchanicz.1998) investigated the temperature-dependent real and imaginary parts of the dielectric constant (ε' and ε") [62]. The real part showed a linear increase from 80 C up to 250 C where the slope peaking at 320 C. (Roleder et al., 1989) also showed the frequency dependence of the dielectric constant at different temperatures [44]. (Vakhurshev et al., 1985) determined a depolarization temperature of 190 C from pyroelectric current measurements during heating [25]. (Suchanicz et al., 1988) found a second pyroelectric current peak at 320 C [53]. They also reported a small but finite piezoelectric resonance above 200 C [63]. Pure BNT has a coercive field of 73kV/cm, which is higher than the usual breakdown strength in the polycrystalline ceramic and conductivity is also an issue [15]. (Li et al., 2005) showed a hysteresis of almost fully poled BNT with =29.0 µc/cm² and = 61.1 kv/cm [64]. A study of the effects of sintering time and temperature variation showed only statistical fluctuations of the dielectric and piezoelectric properties as well as grain size [57]. Data on the piezoelectric and the pyroelectric properties of pure BNT ceramics are scarce due to the difficulties encountered with the poling process (Takenaka et al., 1991) [47]. According to (Park et al., 1994; Wang et al., 2003, 2005b; and Yan et al., 2005) it is difficult to pole pure BNT ceramics, due to a combination of factors including the high coercive field, high conductivity and high leakage current [65-68]. 18

6 2.1.4 Dopant studies on Bismuth Sodium Titanate In BNT the desired improvements are a lower coercive field, lower dielectric loss, higher dielectric constant, and higher piezoelectric coefficient with better sintering behavior. Improvements of room temperature dielectric and piezoelectric properties of BNT-related materials are directly correlated with a reduction in depolarisation temperature [70]. Strontium titanate (SrTiO 3 ) was added to BNT initially in an attempt to lower the phase transition temperature T d [30]. (Hiruma et al.2008) reported that mol% SrTiO 3 moves the transition temperature T d close to room temperature which accompanied by a large normalized strain d 33 of 488 pm/v [71]. Addition of 0.2wt.% Manganese carbonate (MnCO 3 ) reduces the Curie temperature to 310 C with improving resistivity by more than three orders of magnitude to 3x10 14 Ωm [72]. (Chu et al.2002 & Sung et al. 2010) investigated nonstoichiometry of BNT [53,54], either by creating deficiency of (Bi 1/2 Na 1/2 ) 2+ or excess on the A site of BNT and improves d 33 but lowers T d, T C and ε 33. It has been reported that BNT based compositions modified with CeO 2, Eu 2 O 3, MnO 2, La 2 O 3 etc. showed improved properties and easier treatment in poling process when compared with pure BNT ceramics [75-77,52]. BNT was also doped with bismuth[78], iron[79], niobium[80], calcium[81] and perovskite materials such as BaTiO 3 [82], NaNbO 3 [83,84], BiFeO 3 [85], BiScO 3 [86], NaTaO 3 [87], Ba(Cu 1/2 W 1/2 )O 3 [88] and K 0.5 Na 0.5 NbO 3 [89]. Among the various modified BNT, that are studied, bismuth sodium barium titanate (Bi 0.5 Na 0.5 ) 1-x Ba x TiO 3 () was found to form a rhombohderal- 19

7 tetragonal morphotropic phase boundary (MPB) near x = 0.06 with enhanced piezoelectric properties [24] Dopant studies on Bismuth Sodium Barium Titanate Based on the knowledge that piezoelectric properties are increased at the morphotropic phase boundary, (Takenaka et al. in 1991) observed the morphotropic phase boundary between rhombohedral BNT and tetragonal barium titanate (BaTiO 3 or BT) at 6 mol% BT addition [24]. They constructed a temperature and composition dependent phase diagram based on x-ray diffraction, dielectric and piezoelectric data up to 30 mol% barium titanate (figure 2.2). At room temperature, the morphotropic phase boundary is clearly defined by a sharp increase in the dielectric constant of poled and unpoled samples to ~1600 and ~950, respectively. Coupling factors remain relatively constant around 0.20, 0.48 and 0.15, respectively, up to 30 mol% BT addition to BNT, whereas increases from 0.35 to above 0.50 near the MPB. The depolarisation temperature, values have been reported in between 100 C and 130 C [24, 90]. Preparation of BNT-BT via mixed oxide route is identical to that of BNT but achievable densification is higher [24, 90-91]. The remanent polarisation ( ) peaks at the MPB is 38.8 µc/cm² [74]. The coercive field ( ) is continuously reduced with increasing BT content up to its lowest value of 32.5 kv/cm at 8 mol% BT. Compositional dependence of and tanδ are reported up to 12 mol% BT., and tanδ show their optimum values of 155 pc/n, 36.7 %, and approximately 2.5 %, respectively, at the MPB [24]. (Ranjan et al., 2005) provide a detailed powder x-ray diffraction 20

8 analysis for compositions around the MPB [92]. They show that there are no measurable tetragonal lattice distortions or tetragonal superstructure reflexes in BNT-BT between the MPB (6 mol% BT) and 10 mol% BT. They instead claim a nearly cubic phase. (Xu et al., 2008) however, show clear tetragonal distortion above 6 mol% added BT [93]. (Xu et al., 2008) also measured the diffuseness of the phase transition at Td. The gamma value of morphotropic is reported as 2.02, making it a pure relaxor, as reported earlier [94]. The relaxor nature is attributed to the A-site cation disorder of BNT-BT. (Qu et al., 2005) introduced up to 20 % BT in BNT and showed continuously improved density with BT content. These investigations claim that there is phase coexistence between trigonal and tetragonal above Td. Combined with a thermal hysteresis in the dielectric constant; this is treated as evidence for the existence of polar micro-domains above Td [91]. Dopant-free modifications such as deficiency of titanium cations in BNT-BT improve and worsens tanδ, excess titanium cations deteriorate all of the above mentioned parameters [95]. Substitutions of BNT-BT with metallic cations such as cerium [75, 96], tantalum [97], Manganese [98] Mn + Ca [99], Ce + Sn [100], improve some combination of d 33, tanδ, ε 33 by improving densification and/or controlling grain size. However, dielectric loss or tanδ is still a problem and the conductivity becomes a problem in polling. Table 2.2 gives a compilation of the properties reported for pure and some modified for the above mentioned cations. 21

9 Figure 2.2. Phase diagram for BNT with increasing mol % BT [25]. Table 2.2 Dopants for ceramics and resulting electromechanical properties reported in literature. System Additive A or B site lattice Reported parameters % change Reference Sintering Temperature = (pC/N) (Takenaka et al.,1991) = 34.1 µcm C, 2h Tan = 1.3-4% Nil = µcm 2 =

10 = 288 o C = = 120 o C 0.4 wt% CeO2 A & B = 152(pC/N) 21.6%, -1.9% (Wang et al.,2003) = % Tan = 1.2% -8%,-70% 1200 o C,2 hr = 37.7µcm %, % = %, -23.3% = % 0.4 wt% CeO2 A &B = 116(pC/N) 7.2%, -25% = 22.6 kv/cm +13% Tan = 1.8% +38.5%,-55% = 34.2 µc/cm 2 +71%, -11.9% (Fan et al.,2008)) 1600 o C, 1hr = %, +8.24% = 94 o C -21.7% = % 0.4wt%Sn & 0.4wt% Ce A & B = 92 (pc/n) 26.4%,,-40.7% (Liu et al.,2006)) = 33.4kV/cm -2.05% 23

11 Tan = 2% +54%, -50% = 28.8µC/cm 2 +44%, -25.8% = %,,+16.4% 1600 o C, 1hr T d = 92.6 o C -22.8% = % k t = wt % CaO & 0.01 wt.% MnO A &B = 179 pc/n 43.2%, +16% (Yoon et al.,2009)) = 26.7 kv/cm -22% Tan = ~2% +54%,-50% = 38.7µC/cm %, -2.6% 1170 o C, 2hr ε T 33 = 1137 = % 0.5mol%Ta2O5 B d 33 = 170 pc/n 36%,,+10% (Zuo et al.,2008)) Tan = 5.03% +287%,+26% = %, % = % o C, 1-4 hr = 269 o C = 160pC/N 28% (Li et al., 2008) 0.3wt% MnO2 = 29% -20.9% = %,+3.4% = 2.6% +100%,-35% 1600 o C~40 minutes 24

12 2. 2.RESEARCH OBJECTIVES Since the discovery of BaTiO 3, several ferroelectric and piezoelectric structural families have been investigated for use in a wide variety of solid state devices. The scientific search led to the development of lead based PZT materials with the best piezoelectric properties known. However, lead being a known environmental and health hazard, there has been a demand over the past few decades to remove it from all electronics and other consumer products. In the later part of the 1950's, a lead-free perovskite compound, Bi 0.5 Na 0.5 TiO 3 (BNT), was first reported as having piezoelectric properties. In the last two or three decades BNT has gained popularity and become widely accepted as an environmentally friendly alternative to the classical" lead-based perovskites. Although BNT is one of the most promising alternatives, its piezoelectric properties are inferior to the lead-based ceramics. Many additives to BNT have been studied in the ongoing effort to improve the piezoelectric properties of this ceramic. Typically, small amounts of additives that replace either or both of the A-site ions (Bi 3+ and Na + ), or the B-site ions (Ti 4+ ) are added to achieve this. Among the various solid solutions that are reported, bismuth sodium barium titanate, (Bi 0.5 Na 0.5 ) 0.94 Ba 0.06 TiO 3 () was found to exhibit a morphotropic phase boundary with enhanced piezoelectric properties (Takenaka et al., 1991)[24]. This field is still relatively young, and there is no widely accepted theory which would allow one to predict final properties of BNT doped with a specific set of additives. As a result much of the work to date has been largely experimental in nature. The improvements in the piezoelectric properties of these new 25

13 materials have not yet fully met today's requirements for industrial use (Sasaki et al., 1999). These requirements are evolving and becoming more demanding as new applications for the materials developed. Much has been published on lead-based piezoelectric ceramics. Two of the most commonly used systems are Pb(Zr 1-x Ti x )O 3 and (1-x)Pb(Mg 1/3 Nb 2/3 )O 3 -xpbtio 3, abbreviated as PZT and PMN-PT respectively. These are PbTiO 3 ceramics with Zr 4+ and Mg 1/3 Nb 2/3 replacing some of the Ti 4+ in the B-site of the perovskite structure. The general objective of the current research was to investigate four different sets of additives (Zr 4, Sn 4+, Nb 5+ and (Mg 1/3 Nb 2/3 ) 4+ which replace the Ti 4+ atoms in the structure, and form compositions that are not reported in the literature. The systems were abbreviated as Zx, Sx, Nx and (MN)x respectively. Specific research objectives of this study are to: 1. Determine the optimum amounts of additives for each system. 2. Investigate the influence of dopant on microstructure of. 3. Assess the performance of the additives through measurement of the dielectric constant and dielectric loss at different temperatures and frequencies. 4. Determine the transformation temperature and piezoelectric coefficient (d 33 ). 5. Find out the major causes for the dielectric leakage in. 26