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Ways to search for new functional materials — alternatives to ferroactive PZT compositions

S. I. Dudkina

https://orcid.org/0000-0003-3525-6973

Research Institute of Physics, Southern Federal University, Rostov-on-Don 344090, Russia

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E. V. Glazunova

https://orcid.org/0000-0002-2596-2471

Research Institute of Physics, Southern Federal University, Rostov-on-Don 344090, Russia

E-mail Address: kate93g@mail.ru

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K. P. Andryushin

https://orcid.org/0000-0003-0147-8359

Research Institute of Physics, Southern Federal University, Rostov-on-Don 344090, Russia

Complex Research Institute named after H.I. Ibragimov RAS, Grozny 364051, Russia

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I. N. Andryushina

https://orcid.org/0000-0001-8058-4381

Research Institute of Physics, Southern Federal University, Rostov-on-Don 344090, Russia

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L. A. Shilkina

https://orcid.org/0000-0002-8048-3617

Research Institute of Physics, Southern Federal University, Rostov-on-Don 344090, Russia

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I. A. Verbenko

https://orcid.org/0000-0001-6229-9691

Research Institute of Physics, Southern Federal University, Rostov-on-Don 344090, Russia

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, and

L. A. Reznichenko

https://orcid.org/0000-0001-5202-1610

Research Institute of Physics, Southern Federal University, Rostov-on-Don 344090, Russia

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https://doi.org/10.1142/S2010135X2440023XCited by:0 (Source: Crossref)

Abstract

This paper discusses ways to search for lead-free functional materials for various applications. Using the example of the solid solution systems based on alkali metal niobates, the influence of the position on the phase diagram of the corresponding systems, the number of the components in them, and modification with mono- and combined metal oxides on their characteristics is shown. It has been established that the most effective in terms of piezoelectric characteristics are the solid solution systems in or near the morphotropic region, with 3 or 4 components. A number of materials have been developed to create highly sensitive electromechanical transducers, ultrasonic delay lines and other applications.

Keywords:

1. Introduction

A characteristic feature of the modern component base of electronic instrument making is the limited possibilities for creating new functional materials used in it. This was a consequence of the almost complete exhaustion of the existing chemical bases and methods for their construction. In addition, the “screaming” need for greening all sectors of the world economy makes it necessary to abandon the industrial production of toxic ferroactive materials containing lead,^1,2 and transition to products that do not harm the environment and humans. All this has become an incentive to search for new functional materials — alternatives to the ferroactive PZT (Pb(Ti, Zr)O₃) compositions, the recognized foundation of the piezotechnics and microelectronics.

The search for such materials is based on the well-known triad: Choice of cationic composition — position on the phase diagram — number of components. The most successful was the direction associated with the development of the materials based on the sodium niobate (NaNbO₃, NN) and its solid solutions due to wide polymorphism. The largest number of distortion phase transitions among the crystals of the perovskite family (there are six³ in NN) is caused by both atomic displacements and rotations of the oxygen octahedra. Such a “cascade” of the phase transitions contributes to the emergence of the morphotropic boundaries in NN-based solid solutions, at which structures with complex distortions come into contact, which is reflected in the unique macroscopic properties. The possibility of the appearance in the NN (in the absence of external influences) already at room temperature, except for the antiferroelectric, ferroelectric Q-phase, similar to that induced in it by an electric field or formed with the introduction of small amounts of ferroelectric components (for example, potassium, lithium niobates), allows, indeed, to consider this object as the basis or component of functional materials of a new generation. In accordance with the above, the aim of the work was to select a promising cationic composition of lead-free functional materials, taking into account its position on the phase diagram of the corresponding system and the number of components in it.

2. Research Methods

2.1. Objects and sample preparation methods

Analysis of bibliographic information and patent literature showed that the most suitable for the implementation of these plans is the NaNbO₃–KNbO₃ (NKN) system, which is characterized by the highest piezoelectric parameters, comparable to those in PZT compositions.^4,5,6,7,8 Table 1 shows the parameters of the solid solutions of the systems (Na $_{1 - x}$ K_x)-NbO₃ and Pb(Zr $_{1 - x}$ Ti_x)O₃ with $x = 0.5$ according to the data.^7,8

**Table 1. Electrophysical parameters of the solid solutions composition (Na $_{0.5}$ K $_{0.5})$ NbO₃ and Pb(Zr $_{0.5}$ Ti $_{0.5}$ )O₃.**
Parameters*
$ε_{33}^{T} ∕ ε_{0}$	$K_{p}$	$d_{33}$ , pC/N	$\| d_{31} \|$ , pC/N	tg $δ \cdot 1 0^{2}$ $E = 50$ V/cm	$Q_{M}$	$V_{1}^{E}$ , km/s

(Na $_{0.5}$ K $_{0.5}$ )NbO₃
∼330	0.25	70	40	3.0	∼50	5.3
Pb(Zr $_{0.5}$ Ti $_{0.5}$ )O₃
700	0.25	150	30	2.0	200	3.8

* $ε_{33}^{T} ∕ ε_{0}$ – relative dielectric constant of polarized samples; $K_{p}$ – coefficient of electromechanical coupling of planar vibration mode; $d_{33}$ , $| d_{31} |$ – piezomodules; tg $δ$ – dielectric loss tangent; $Q_{M}$ – mechanical quality factor; $V_{1}^{E}$ – sound velocity.

As one would expect, extreme characteristics in systems are realized in SS from morphotropic regions, localized on the phase diagrams of both systems in the vicinity of $x \sim 0.5$ .^7,8

Regarding the appropriate number of components in NKN-based systems, as noted in Ref. 9, the most efficient in terms of piezoelectric characteristics $K_{p}$ , $d_{i j}$ and $g_{i j}$ are systems with the number of components, n, equal to 3; 4, in contrast to the PZT system, with additional related Pb-containing perovskite components, among which systems with $n = 5$ ¹⁰ have optimal parameters. At the same time, the wide isomorphism of the sodium–potassium niobates, due to favorable factors of the size and electronegativity of the alkali cations, as well as crystal chemical features of the structure, makes it possible to modify them iso- and heterovalently with a large number of the elements introduced according to various schemes. In this case, the main factor determining the physical properties of the solid solution is the mobility of non $18 0^{\circ}$ domain walls, determined by spontaneous deformation and their interaction with the resulting vacancies.¹¹

Great possibilities for the modification are due to the existence of a significant amount of the metal monoxides and their combinations that can dissolve in NKN over a wide range of concentrations; oxides of the elements that form “fluxes” in the initial objects (B₂O₃, SiO₂, V₂O₅, GeO $_{2} \dots$ ); additives of the combined action that affect the properties of the original systems due to the formation of the liquid phases and cation exchange interaction with the base (modifying glasses). The advantage of the modification method is the ability, through a targeted selection of the modifiers, their concentrations, method of the introduction and the nature of the distribution of vacancies, to purposefully change some (or all) parameters of the solid solutions, forming new phase states and, as a consequence, macro-responses of the objects.¹¹

Using this method and using the binary NKN system as a basis and the three- and four-component systems constructed with its participation, numerous materials with special electrical properties for various applications have been developed and obtained, described in a huge number of articles (see, for example, Refs. 12–40).

In this work, we chose the ternary system of sodium–potassium–lithium niobates as the base one, modifying it with a combined modifier $CdO + {GeO}_{2}$ .

The synthesis of the solid solutions was carried out by double firing at $T_{1} = 1073$ K, $T_{2} = 1123$ K, duration $τ_{1} = τ_{2} = 5$ h. Sintering was carried out by the hot pressing method⁴¹ at a pressure of 100–200kg/cm², the duration of isothermal exposures at sintering temperatures was from 40 min until 6–8h. Lower pressure values ( $< 200$ kg/cm²) and extended isothermal exposures 6–8h were used in the manufacture of the materials in the form of large blocks with dimensions of $110 \times 110 \times 25$ mm or $Ø 70 \times 25$ mm.

In some cases, we used the hot molding method,⁴² developed by us, which ensures the strengthening of ceramics and achieving a density of the hot-formed workpieces from synthesized products that are not ( $50 \dots 60$ )% of the theoretical density ( $ρ_{theor}$ ) as with conventional molding of the workpieces, but more than 90% $ρ_{theor}$ . This turned out to be decisive in the production of high-density, high-strength ceramic “sinters”.

The samples were polarized in silicone oil at 413K for 45min in a field of 55kV/cm, followed by cooling under the field to 363K.

2.2. Experimental methods

The study of the electrophysical characteristics of objects was carried out in accordance with Ref. 43. In this case, $ε ∕ ε_{0}$ , $ε_{33}^{T} ∕ ε_{0}$ – the relative dielectric constant of unpolarized and polarized samples; tg $δ$ – dielectric loss tangent; $K_{p}$ , $K_{15}$ – coefficients of electromechanical coupling of planar and shear vibration modes; $| d_{31} |$ , $d_{33}$ – piezomodules; $| g_{31} |$ , g $_{33}$ – piezosensitivity; $Q_{M}$ – mechanical quality factor; $V_{1}^{E}$ – sound velocity were estimated. The Curie temperature, $T_{C}$ , was determined from the maximum on the $ε ∕ ε_{0} (T)$ dependences and was refined using X-ray data.

3. Results and Discussion

In Table 2, the parameters of the best-developed material based on NKN with additions of lithium methaniobate, cadmium and germanium oxides are shown. Based on the combination of the characteristics, the developed material can be used in high-frequency acoustoelectric converters due to low-dielectric constant, $ε_{33}^{T} ∕ ε_{0}$ , and high-sound velocity, $V_{1}^{E}$ .

**Table 2. Electrophysical parameters of the developed ferroelectric piezoceramic material based on NKN with additions of lithium methaniobate, cadmium and germanium oxides.**
Parameters
$ε_{33}^{T} ∕ ε_{0}$	tg $δ \cdot 1 0^{2}$ $E = 50$ V/cm	$K_{P}$	$K_{15}$	$\| d_{31} \|$ , pC/N	$d_{33}$ , pC/N

460	2.5	0.42	0.60	45	100

Parameters

$\| g_{31} \|$ , mV⋅m/N	$g_{33}$ , mV⋅m/N	$Q_{M}$	$V_{1}^{E}$ . km/s	$T_{C}$ , K

10.0	35	150	5.4	693

At the same time, the high speed of the sound makes it possible to simplify the technology for manufacturing elements by increasing their thickness when operating at high frequencies and also ensures good matching of the elements with the external circuit in terms of the electrical resistance. A high shear electromechanical coupling coefficient, $K_{15}$ , with relatively low $ε_{i j} ∕ ε_{0}$ and $Q_{M}$ is of great importance when using materials in body wave ultrasonic delay lines. Our material has the specified combination of the parameters in which it is not inferior to the well-known foreign analogues PZT-2 and PCM-75 (Table 3). The material can also be used to create highly sensitive electromechanical transducers, in particular, for devices for measuring mechanical influences, such as pressure. The high Curie temperature, $T_{C}$ , of the material ensures increased temperature stability of its parameters, and its low-specific gravity ( $\sim 4.5$ g/cm³) allows it to be used in devices in which weight characteristics are decisive.

**Table 3. Electrophysical parameters of analogues of the piezoceramic materials with low-dielectric constant.**
	Parameters
Material	$T_{C}$ , K	$ε_{33}^{T} ∕ ε_{0}$	$K_{P}$	$K_{15}$

PZT-2	643	450	0.47	0.70
PCM-75*	632	390	0.23	—

	Parameters

Material	$\| d_{31} \|$ , pC/N	$\| g_{31} \|$ , mV⋅m/N	tg $δ \cdot 1 0^{2}$ $E = 50$ V/cm	$Q_{M}$

PZT-2	60.0	15.1	0.50	680
PCM-75*	23.5	6.8	0.25	4520

*PCM-75 – industrial material, firm “Matsushita”, Japan.

By introducing the above modifiers, it was possible to reduce the sintering temperature, $T_{sint}$ , of the material from 1493K to 1263K. Additional reduction in $T_{sint}$ was possible to implement using a technological method — combining the processes of the synthesis and the sintering by the introducing additional isothermal exposure at a temperature on (150–200) degrees below the maximum temperature.

Decrease in $T_{sint}$ helps to improve the reproducibility of the properties of the resulting materials due to the preservation of a given stoichiometric composition as a result of reducing the sublimation of components at low $T_{sint}$ . In addition, it helps to increase the durability and reliability of the process equipment and saves the metals used in the manufacturing of the process equipment.

In the course of this research, ferro-piezoceramic materials based on other non-PZT compositions were also developed:

•	Of the oxides of sodium, potassium, lithium, tantalum, niobium, bismuth, iron, antimony, which have a high coefficient of the electromechanical coupling of the planar vibration mode against the background of low-dielectric constant, for high-frequency ultrasonic piezoceramic transducers intended to work as emitters and receivers in remote systems control, in devices for measuring gas flow velocity;
•	Of the sodium, lithium, strontium, niobium, aluminum, bismuth, iron oxides, having a low-dielectric constant, an increased coefficient of the electromechanical coupling of the planar vibration mode while maintaining sufficiently high values of the mechanical quality factor — for high-frequency acoustoelectric converters;
•	Of the bismuth, iron, titanium oxides, with low- dielectric constant and dielectric loss tangent with increased stability of the dielectric characteristics — for creating capacitive magnetoelectric elements of the recording and reading heads;
•	Of the strontium, barium, titanium and silicon oxides, with high-dielectric constant and electromechanical coupling coefficient of the thickness vibration mode, reduced dielectric losses and mechanical quality factor — for medical diagnostic equipment and nondestructive testing devices;
•	Of the barium, strontium, germanium and other oxides, with high values of the mechanical quality factor, electromechanical coupling coefficient of the thickness mode vibration, piezoelectric modulus at average values of the relative dielectric constant — for acoustoelectronic devices, in particular, mid-frequency ultrasonic delay lines used in radar equipment, automatic control systems and long-distance communication technique.

The parameters of some developed materials are given in Tables 4–6.

**Table 4. Electrophysical parameters of the modified piezoceramic materials based on (Na, K, Li)NbO₃ and (Na, Li)NbO₃.**
	Parameters
No.	$ε_{33}^{T} ∕ ε_{0}$	$K_{P}$	$\| d_{31} \|$ , pC/N	$d_{33}$ , pC/N
1 (NKLN)	603	0.51	63	162
2 (NLN)	155	0.18	—	47

	Parameters

No.	$g_{33}$ , mV⋅m/N	$Q_{M}$	$V_{1}^{E}$ , km/s

1 (NKLN)	30.4	115	4.74
2 (NLN)	—	744	5.59

**Table 5. Electrophysical parameters of the modified piezoceramic materials based on (Ba, Sr)TiO₃ with additions of SiO₂ and GeO₂.**
	Parameters
No.	$ε_{33}^{T} ∕ ε_{0}$	tg $δ \cdot 1 0^{2}$ $E = 50$ V/cm	$K_{t}$	$K_{P}$

1 (SiO $_{2})$	405	0.61	0.50	0.08
2 (SiO $_{2})$	449	0.52	0.49	0.09
3 (GeO $_{2})$	223	1.12	0.26	0.07
4 (GeO $_{2})$	227	0.89	0.38	0.07

	Parameters

No.	$K_{t}$ / $K_{P}$	$d_{33}$ , pC/N	$Q_{M}$	$T_{C}$ , K

1 (SiO₂)	6.2	47	590	—
2 (SiO₂)	5.4	50	670	—
3 (GeO₂)	3.7	10	2070	617
4 (GeO₂)	5.4	19	1539	578

**Table 6. Electrophysical parameters of modified piezoceramic materials based on BiFeO₃.**
	Parameters
No.	$ε$ / $ε_{0}$ (298K)	$t g δ \cdot 1 0^{2}$ ( $E = 10$ V/cm)	( $ε_{473 K}$ – $ε_{298 K})$ / $ε_{473 K}$	( $ε_{LF}$ – $ε_{HF}) ∕ ε_{LF}$

1	115	0.17	0.03	0.02
2	116	0.31	0.06	0.05

4. Conclusion

Modified solid solutions of the multicomponent systems based on alkali metal niobates were obtained by double solid-phase synthesis followed by sintering hot pressing (at a pressure of 100–200kg/cm², the duration of isothermal exposures at sintering temperatures from 40min to (6–8)h).

It has been shown that the most effective in terms of piezoelectric characteristics $(K_{P}, K_{t}, d_{33}, g_{33})$ are solid solutions of systems with the number of components equal to 3 and 4, located in the morphotopic region or near it.

Materials with various combinations of the characteristics have been developed, for example, low-dielectric constant, increased electromechanical coupling coefficient of the planar vibration mode while maintaining sufficiently high values of mechanical quality factor for the creation of high-frequency electroacoustic transducers, etc.

The obtained results are advisable to use in the development of similar ferroelectric piezoelectric ceramics and devices with their participation.

Acknowledgments

This study was carried out with the financial support of the Ministry of Science and Higher Education of the Russian Federation. Project No. FENW-2023-0010/GZ0110/23-11-IF. This report is presented at the International Conference on “Physics and Mechanics of New Materials and Their Applications” (PHENMA 2024), Indore, India, November 6–11, 2024.

ORCID

S. I. Dudkina https://orcid.org/0000-0003-3525-6973

E. V. Glazunova https://orcid.org/0000-0002-2596-2471

K. P. Andryushin https://orcid.org/0000-0003-0147-8359

I. N. Andryushina https://orcid.org/0000-0001-8058-4381

L. A. Shilkina https://orcid.org/0000-0002-8048-3617

I. A. Verbenko https://orcid.org/0000-0001-6229-9691

L. A. Reznichenko https://orcid.org/0000-0001-5202-1610

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Received 14 November 2024

Revised 6 December 2024

Accepted 23 December 2024

Published: 5 February 2025

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This is an Open Access article published by World Scientific Publishing Company. It is distributed under the terms of the Creative Commons Attribution 4.0 (CC BY) License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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