Editorial Feature

Analyzing the Progress on Lead-Free Piezoelectric Materials

Researchers are investigating how to substitute the PZT materials to protect the environment. Continuous research is going on to build up different lead-free materials, for eco-friendly applications. This article considers research conducted by Kuanar, B et al. 

piezoceramics, piezoelectric, ceramics, thin films, PZT

Image Credit: BossCo77/Shutterstock.com

Scientists have been focusing on the application of high dielectric constant capacitors, buzzers, sensors, ultrasonic motors, actuators, piezoelectric sonar, ultrasonic transducers, medical diagnostic transducers, ferroelectric thin films memories, multilayer capacitors, ferroelectric random-access memory (FERAM), and so on. Piezoceramic PZT used in functional devices is toxic due to the presence of lead. 

At present researchers are diverted towards lead-free piezoelectric ceramics, namely titanate-based perovskite material (BaTiO3, Na0.5Bi0.5TiO3, K0.5Bi0.5TiO3, and BaZr0.08Ti0.92O3), and its solid solutions.

The main drawback of natural piezoelectric materials is truncated piezoelectricity. Lead oxide is a PZT component and is highly toxic in behavior. The toxicity can be enhanced because of its volatility at a high temperature. 

Between the two different lead-free systems: (i) Perovskite materials are Bismuth Sodium Titanate (BNT), Barium Titanate Oxide (BaTiO3), Potassium Niobate (KNbO3), Sodium Titanate Oxide (NaTiO3), etc. (ii) Non-Perovskite materials such as Bismuth layer structure and Tungsten-Bronze have appeared as a suitable material for the substitution of the lead-based ceramics.

However, the main bottleneck includes low Curie temperature (TC) and high coercive field that make the ceramic strenuous for polarization state transition with remarkable conductivity and dielectric loss.

Due to the large remnant polarization (Pr∼38 μC/cm2), Bismuth sodium titanate (Ba0.5Na0.5TiO3; BNT) is assumed as a righteous candidate of lead-free piezoelectric ceramics at room temperature. BNT holds a complex perovskite structure and rhombohedral phase. If the transition temperature is 200 °C, it changes to the anti-ferroelectric phase. The Curie temperature (Tc) of such material is about 320 °C.

Because of the above advantages of BNT, the authors mainly focused on the changes in the transition of phases, electrical and dielectric behaviors of BNT, its solid solutions with other perovskite structures, and doping of di, tri, and tetravalent ions at A and/or B-site.

Methodology

The sample stoichiometric followed by various research groups is displayed in Figure 1. Out of various powder preparation methods available, most commonly the mixed oxide route is followed where appropriate stoichiometric proportions of desired oxides are used. To achieve the required chemical and optical homogeneity, ball milling is crucial. Milling time is based on powder characteristics and varies from 2 to 16 hours.

Steps for sample preparation and characterization: (a) Solid-state route, (b) Auto-combustion route, and (c) Sol-gel route.

Steps for sample preparation and characterization: (a) Solid-state route, (b) Auto-combustion route, and (c) Sol-gel route.

Steps for sample preparation and characterization: (a) Solid-state route, (b) Auto-combustion route, and (c) Sol-gel route.

Figure 1. Steps for sample preparation and characterization: (a) Solid-state route, (b) Auto-combustion route, and (c) Sol-gel route. Image Credit: Kuanar, et al., 2021

Generally, the mixtures are milled with either zirconia or alumina balls with non-flammable liquid-like trichloroethylene or distilled water. The milled powders are then dried and calcined (800 °C–900 °C) for 2 to 3 hours.

During calcination, volatile components like moisture are separated and then the required phase formation occurs. The organic vehicle generally includes a binder (ethylcellulose), a plasticizer (polyethylene glycol), a dispersing agent (butoxy ethoxy ethyl acetate), and a solvent (α-terpineol).

After the calcination process, polyvinyl alcohol (PVA) is used as a binder to press the granulated powder pellets. The pellets are then sintered at a temperature varying between 1050 °C-1200 °C. For the electrode, the sintered pellets are coated with silver paste, and for orientation, the ferroelectric domains in ceramic and electrical properties are measured by applying a strong DC field.

Dielectric measurement (dielectric loss and dielectric constant) was performed with the use of an automatic LCR Meter Bridge. A Sawyer-Tower circuit is used to perform ferroelectric hysteresis measurement. A constant tester of the piezoelectric is used to measure the piezoelectric coefficient d33. The other properties of materials are studied by using Raman, FTIR, PL, and UV by the researchers for various industrial applications.

Results and Discussion

The ferroelectric oxides with general formula ABO3, considered as a perovskite structure, such as Bi0.5Na0.5TiO3(BNT), BaTiO3(BT), KNbO3, NaTiO3, NaTaO3, etc. are the renowned lead-free piezoelectric materials. The physical properties of such structure are tuned easily with the use of several iso- and aliovalent substitutions either at A site or B site of ABO3 structure as per the requirement of its simple structure.

It has been observed that BNT-based compositions modified with BaTiO3, BiKTiO3, NaNbO3, BiFeO3, La2O3, Sc2O3, BaCO3, etc. showed improved properties and are easy to be poled.

The substitution of Zr in the B-site enhances the relative ionic displacement and electrical property by the expansion of the perovskite lattice. It can also be seen that the substitution of Zr suppresses the conduction in electronic hoping in an oxidation state of Ti4+ and Ti3+.

The addition of Gd at the Bi site shows distortion, and it is an amphoteric dopant. Therefore, there is a possibility of using this material as a photovoltaic cell.

Bismuth magnesium titanate (Bi (Mg0.5Ti0.5) O3) [BMT] is a rhombohedral ferroelectric perovskite similar to sodium bismuth titanate (Bi0.5Na0.5) TiO3 [BNT] ceramics. For improving the ferroelectric and piezoelectric properties, researchers have studied the Mg dopant’s effects as an additive. It has been concluded that the Rhombohedral structure is maintained till x = 0.04-mole fraction, and it changed to a cubic phase in higher mole fraction.

Piezoelectric and thermal depoling properties of Bismuth Sodium Titanate ceramic were investigated with a new group of compounds (Bi0.5Na0.5) TiO3-(Bi0.5(1+x) Na0.5) TiO3-0.75x. The ratios of assumed and measured densities were more than 97%. With the increase in Bi or decrease in Na in BNT, the resistivity increases.

Another interesting BNT based ceramic (1-x) NBT-xBT was studied by researchers. The transition of structural phase using XRD and Raman spectra with intermediate coexistence of the morphotropic phase boundary (MPB) has been reported. All the ferroelectric and dielectric properties are enlisted in Table 1.

Table 1. Properties of NBT-BT system. Source: Kuanar, et al., 2021

Compositions
(1-x) NBT-xBT
Tm (°C) TFR (°C) tanδ εr Pr (μC/cm2) Ec (kV/cm)
x = 0.00 335 170 0.01389 2100 71.8 58.6
x = 0.02 315 160 0.00571 2500 46.7 48.2
x = 0.05 305 137 0.01229 3800 15.4 34.7
x = 0.06 290 105 0.0355 4800 57.7 28.8
x = 0.07 270 109 0.04749 4100 37.1 22.1
x = 0.08 260 120 0.06313 2900 17.1 22.4
x = 0.10 255 133 0.0193 2400 13.94 22.6

 

The influence of a small amount of (2.0 wt.%) of Ba2+ ions on A- and B-site substitution in BNT was studied. With the increase in frequency, there was a rapid decrease in the dielectric constant. The dielectric loss reduces with the increasing frequency.

The highest piezoelectric coefficient (d33) of 1658 pC/N and large electrostrictive coefficient Q = 0.287 m4C-2 were obtained in the ceramics of (1-x) Bi0.5Na0.5TiO3-xSr0.85Bi0.1TiO3 sintered at 1260 °C and most importantly the P-E loops were hysteresis-free, indicating a suitable material for high-precision actuator application.

When x is closed to 0.06-0.08, the presence of MPB is revealed in Figure 2 reflecting binary system phase (1-x) Bi1/2Na1/2TiO3–x BaTiO3 (BNT-BT). Based on the enhanced property of piezoelectric, MPB existence becomes evident at this composition.

Morphotropic Phase boundaries in BNT-BT.

Figure 2. Morphotropic Phase boundaries in BNT-BT. Image Credit: Kuanar, et al., 2021

Researchers did a further investigation on the dielectric and ferroelectric properties of BNT-BT-xBZT (with 0 ≤ x (BZT) ≤ 0.10). They synthesized the ceramic by the traditional solid-state reaction method.

They also confirmed the co-existence of rhombohedral and tetragonal phases for all samples, and that the tetragonal phase increased with an increasing amount of BZT. In Table 2, the values of dielectric constant r), dielectric loss (tanδ), phase transition temperature (TFA), remnant polarization (ps), and coercive field (Ec) are presented.

Table 2. Properties of BNT-BT-xBZT system. Source: Kuanar, et al., 2021

Compositions (x)
[(0.935-x) BNT-0.065BT-xBZT]
εr
at TFA
tanδ
at TFA
TFA TSA Pr
(μC/cm2)
Ec
(kV/cm)
x = 0 6820 0.015 148 265 32 30
x = 0.02 6956 0.015 154 268 29 23
x = 0.04 7058 0.011 143 280 24 20
x = 0.06 6913 0.011 181 263 24 26
x = 0.08 7234 0.014 197 259 20 29
x = 0.10 8224 0.014 210 249 21 31

 

The role of BiFeO3 on the electrical and electrochemical properties as well as structure (microstructure) of the BNT-BKTx-BFy ternary system is presented in Table 3. The incorporated BF disseminates into the BNT-BKT lattice to come out a solid solution but decreases the tetragonal and rhombohedral distortions with a strong influence on properties concluding as a good piezoelectric material.

Table 3. Properties of BNT-BKTx-BFy system. Source: Kuanar, et al., 2021

Compositions (x,y) εr tan δ Pr (μC/cm2) Ec (kV/mm) d33 (pC/N)
(0.18, 0) 1344 0.047 33.5 3.20 150
(0.18, 0.03) 1233 0.048 28.9 3.16 125
(0.18, 0.07) 1187 0.052 27.5 2.73 70
(0.20, 0) 1743 0.053 37.5 2.90 195
(0.20, 0.03) 1411 0.053 33 2.36 155
(0.20, 0.07) 1235 0.055 28.9 2.00 120
(0.22, 0) 1518 0.051 34 1.64 160
(0.22, 0.03) 1375 0.054 7.6 0.83 20
(0.22, 0.07) 1274 0.056 5.4 0.78 8

 

Researchers also investigated morphotropic phase boundary in the BNT-BKT-BT system as shown in Figure 3. With the increase in BKT and BT amount, the value of the dielectric constant increases for all compositions. The ternary mixture, i.e., 0.865 BNT–0.035 BT–0.100 BKT morphotropic composition shows high electromechanical coupling factors (kp = 0.26 and kt = 0.57) piezoelectric constant (d33 = 133 pC/N) which confirm good piezoelectric property and is presented in Table 4.

Morphotropic phase boundary of BNT-BKT-BT System.

Figure 3. Morphotropic phase boundary of BNT-BKT-BT System. Image Credit: Kuanar, et al., 2021

Table 4. Properties of BNT-BKT-BT content. Source: Kuanar, et al., 2021

Compounds εr tanδ kp kt d33 (pC/N)
BNT 423 5.0 0.11 ± 0.01 0.49 ± 0.02 67 ± 10
0.96 BNT-0.04 BT 532 1.7 0.13 ± 0.01 0.43 _± 0.02 123 ± 10
0.94 BNT-0.06 BT 782 3.1 0.35 ± 0.02 0.53 ± 0.05 170 ± 4
0.935 BNT-0.065 BT 873 3.4 0.26 ± 0.01 0.49 ± 0.05 147 ± 9
0.93 BNT-0.07 BT 626 2.1 0.18 ± 0.02 0.45 ± 0.05 123 ± 3
0.88 BNT-0.12 BKT 541 4.1 0.22 ±0.01 0.43 ± 0.03 78 ± 9
0.84 BNT-0.16 BKT 710 3.6 0.29 ± 0.01 0.44 ± 0.02 130 ± 7
0.80 BNT-0.20 BKT 936 3.9 0.39 ± 0.01 0.54 ± 0.05 137 ± 7
0.892 BNT-0.054 BT-0.054 BKT 904 3.6 0.20 ± 0.02 0.43 ± 0.01 130 ± 10
0.89 BNT-0.03 BT-0.08 BKT 665 3.8 0.26 ±0.01 0.44 ± 0.02 112 ± 4
0.865 BNT-0.035 BT-0.100 BKT 891 4.2 0.26 ± 0.02 0.57 ± 0.05 133 ± 6

 

The result of NiNb2O6 content on the dielectric properties and structure of the BT-BNT system was observed. The result shows that doping of NiNb2O6 can suppress grain growth. Initially, the dielectric constant increased at a low temperature then decreased. Curie temperature has been increased slightly with doping concentration.

For 1.5 mol.% NiNb2O6 doped specimen, exceptional dielectric properties have been revealed at room temperature with a dielectric constant value 1652 and a dielectric loss value found in different pieces of literature, as mentioned in Table 5.

Table 5. Properties of some BNT-based solid solution systems. Source: Kuanar, et al., 2021

Compositions εr tanδ
(%)
d33
(pC/N)
Kp
(%)
Tc
(℃)
Td
(℃)
Pr
μC/ cm2
Ref.
(1-x) Na0.5Bi0.5TiO3-xBi (Mg0.5Ti0.5)O3     108         [52]
(Bi0.5Na0.5) TiO3-(Bi0.5(1+x) Na0.5)TiO3-0.75x 583   72.9, 81.3
(highest)
    187
(highest)
  [53]
(1-x) Bi0.5Na0.5TiO3-xSr0.85Bi0.1TiO3     1658         [55]
(K0.5Na0.5)1-3x BiNbO3 750 0.03 164 0.47 403   30.1 [56]
0.8(Na0.5Bi0.5) TiO3-0.2(K0.5Bi0.5) TiO3     215       56.7 [57]
NaBiTiZrO3(5%Zr) 1618       313     [50]
(Na0.5Bi0.5)1-x Bax TiO3 (6 mol%) 787 0.0709 120         [59]
0.92(Bi0.5Na0.5TiO3) +0.08 (BaTiO3) 2573 <0.1     301     [60]
0.912Ba0.97TiO3-0.088 (Bi0.5Na0.5) TiO3-xTa2O5         170     [61]
0.93(Bi0.5Na0.5) TiO3-0.07 BaTiO3     151 0.278       [62]
[(Bi0.5Na0.5)-TiO3]0.92-[BaTiO3]0.08 (BNT-BT0.08) 243           0.87 [63]
Bi0.465-xLax Na0.465 Ba0.07 TiO3     152         [65]
(Bi1-xLax)0.5 Na0.5 TiO3 (3%La doped)     209   365   27 [40]
Bi0.5Na0.5(Ti1-xScx) O3 (5%Sc doped)     155   370   20 [40]
0.77Bi0.5Na0.5TiO3-0.20 Bi0.5K0.5TiO3-0.03 K0.5 Na0.5 NbO3 1460   164       125 [66]
0.85Ba0.5Na0.5TiO3-0.11 Bi0.5K0.5-xRbxTiO3-0.04 BaTiO3     203         [70]
0.9BaTiO3-0.1 (Bi0.5 Na0.5) TiO3+NiNb2O6 1652 1.8     144.9     [72]

 

Conclusion

Lead-free ferroelectric ceramic oxides are highly used in modern technology for their high dielectric constant, piezoelectric coefficient, large remnant polarization, low coercive field, and high Curie temperature to substitute the extensively used lead-based ceramics, due to their pollution-free eco-friendly nature. However, these ferroelectric oxides suffer serious polarization problems.

Due to the large coercive field and high conductivity, PZT is ahead of BNT in industrial applications. For this reason, researchers intend to develop new lead-free ferroelectric oxides having suitable properties for multi-functional applications in a wide temperature and frequency range.

A novel type of application of the materials is the photostrictive actuator, which is the combined result of the photovoltaic effect and piezoelectric effect. One of the applications of piezoelectric composites is piezoelectric transducers. These ceramics-polymer composites have a broad range of applications in hydrophones, sensors and, medical ultrasonic.

Researchers have been investigating many dopants into bismuth sodium titanate ceramics to resolve industrial needs, and in the future it is expected that smart and very smart materials will be developed for a wide range of applications.

Journal Reference:

Kuanar, B., Mohanty, H. S., Behera, D., Nayak, P., Dalai, B. (2021) An elementary survey on structural, electrical, and optical properties of perovskite materials. Engineering and Applied Science Research. Available at: https://doi.org/10.14456/easr.2022.30.

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