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. 

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, Na_0.5Bi_0.5TiO₃, K_0.5Bi_0.5TiO₃, and BaZr_0.08Ti_0.92O₃), 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 (T_C) 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 (Ba_0.5Na_0.5TiO₃; 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.

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 Bi_0.5Na_0.5TiO₃(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 (Mg_0.5Ti_0.5) O₃) [BMT] is a rhombohedral ferroelectric perovskite similar to sodium bismuth titanate (Bi_0.5Na_0.5) 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 (Bi_0.5Na_0.5) TiO₃-(Bi_0.5(1+x) Na_0.5) TiO_3-0.75x. 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) Bi_0.5Na_0.5TiO_3-xSr_0.85Bi_0.1TiO₃ 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) Bi_1/2Na_1/2TiO_3–x BaTiO₃ (BNT-BT). Based on the enhanced property of piezoelectric, MPB existence becomes evident at this composition.

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 (ε_r), dielectric loss (tanδ), phase transition temperature (T_FA), remnant polarization (p_s), and coercive field (E_c) 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-BKT_x-BF_y 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-BKT_x-BF_y 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 (k_p = 0.26 and k_t = 0.57) piezoelectric constant (d₃₃ = 133 pC/N) which confirm good piezoelectric property and is presented in Table 4.

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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