Effects of vanadium doping on sintering conditions and functional properties of NbLi co-doped PZT ceramics. Comments on Li location

Highlights

• Effects of 1 and 2 mol % vanadium in NbLi co-doped PZT ceramics were studied.

• V decreases the sintering temperature and promotes a harder piezoelectric behaviour.

• V increases the lattice tetragonality and grain size.

• Partial reduction of V⁵⁺ to V⁴⁺ ions and the occurrence of defects are observed.

• Li location was indirectly deduced from the experimental results.

Abstract

The effect of 1 and 2 mol % V addition on sintering conditions, structural and functional properties of NbLi co-doped PZT ceramics, prepared by conventional ceramic method, was systematically studied, by different experimental techniques. The reported results show that the increasing amount of vanadium, as a substitute for niobium, decreases the optimal sintering temperature with 100–150 °C, but slightly reduces the piezoelectric, ferroelectric, and electromechanical properties of these ceramics. In addition, the presence of vanadium increases the lattice tetragonality and grain size, promoting a harder piezoelectric behaviour, mostly in the absence of niobium. EPR spectra exhibit well-defined hyperfine splitting characteristics for V⁴⁺ state, dispersed in the lattice as VO²⁺ ion. Additionally, complex paramagnetic defects (charged oxygen vacancies and associated defects) are evidenced for 2 mol % vanadium addition. The results were explained by assuming that vanadium promotes the B-site Li substitution, compensated by oxygen vacancies, while niobium facilitates the A-site Li substitution with lead vacancies formation and partial reduction of V⁵⁺ to V⁴⁺. Thus, indirect information about lithium location is provided.

Introduction

Lead zirconate titanate (PZT) based ceramics are well known in the field of material science and engineering, due to their excellent piezoelectric properties, reflected in various fields of applications, such as: non-destructive testing, underwater detection, medical imaging and therapy, micro-motors, micro-positioning systems, ultrasonic cleaning and welding, etc.

Their highest piezoelectric response occurs in doped compositions, situated at the morphotropic phase boundary, where both tetragonal and rhombohedral ferroelectric phases coexist and allow enhanced domain reorientation during poling [1].

The conventional sintering of PZT, at relatively high temperatures of 1200–1300 °C, is confronted with the serious issue of lead oxide volatilization, which can alter the stoichiometry of the compound and contaminates the environment. Therefore it is mandatory to reduce the lead loss during sintering, by decreasing the holding temperature. Thus, it is possible to reduce or even to avoid the necessity of excess lead oxide addition, or of lead rich atmosphere during sintering. The reducing of the sintering temperature could be approached by addition of small amounts of low melting point compounds which promote a liquid phase sintering. Such a sintering aid is V₂O₅, widely used in many systems as a mineralizer agent [2], [3], [4]. The effect of vanadium on the properties and decreasing sintering temperature of un-doped 53/47 PZT was already reported [3], but it should be investigated in terms of different preparation methods, as it strongly depends on particle size and impurities of the raw powders, synthesis route, sintering conditions, etc. Besides, the presence of other co-dopants could change the influence of vanadium, therefore, in this regard, we found useful to investigate its effect in NbLi co-doped PZT ceramic. Niobium doped PZT is a soft piezoelectric material well known for the remarkable piezoelectric performances (high dielectric/piezoelectric constants and coupling factors), reported in the literature [3], [5], [6], for which it was successfully used in commercial ceramics, for numerous applications. Addition of very small amounts of Li preserved these characteristics [7] and contributed also to the softening effect by mutual valence compensation [6], between acceptor and donor ions and vacancies. The effect of lithium on material properties could be influenced by the presence of other co-dopants, as reported for KNN-Li/Ta [8] and KNN-Li/Ta/Sb [9], but also by its location in the lattice. It is known that Li¹⁺ ion can enter either the A-site (in KNN [8], [9]) or the B-site (in PZN-PZT [10], BT [11], [12], [13], [14] and BNT [14]) of a perovskite lattice, generating oxygen vacancies with hardening effect on the piezoelectric behaviour of the ceramic. A multisite (both A and B) occupation of Li, related to its doping level, was also reported, for BNT and BT [14]. Lithium location could depend on the type and concentration of other co-dopants, but no such explicit statements were found in references. However, there are some reported results, referring to the different locations of Li in BT [14] and BT co-doped with Ca and Sn [11], without being directly related to the presence of the other co-dopants. Direct information about lithium location is difficult to obtain, even by using techniques like TEM-EELS, XAFS, NMR. However, it was possible to be indirectly deduced by microstructure and temperature-dependent conductivity analysis and investigation of the derivative effects on the dielectric, piezoelectric and ferroelectric response [11], [12], [14]. In this regard, we also focused on the correlation of structural, piezoelectric and ferroelectric data to provide indirect information about Li location in the lattice. For decreasing the sintering temperature of NbLi co-doped PZT ceramic, we used addition of small amounts (1 and 2 mol %) of V₂O₅, as it proved to promote rapid densification of PZT, below 1000 °C, by enhanced surface activation and liquid phase sintering [3]. Vanadium addition also proved benefic effects in PZT thin films, as it significantly increased the remnant and saturation polarization, decreased their ferroelectric leakage current and improved the fatigue characteristics [15]. Another reason for choosing V₂O₅ is the multivalence state of vanadium (2+, 3+, 4+, 5+), scarcely investigated in PZT lattice, which could influence the morpho-structural, piezoelectric and ferroelectric properties of PZT ceramic. Moreover, in the presence of Nb co-dopant, the valence of vanadium could be reduced, by mutual valence compensation, as it was reported for NbMn co-doped PZT [16] and NiNbMn co-doped lead titanate [17]. It is possible, since the most stable oxidation state of vanadium, under normal conditions, is +4, while for Nb is +5, even if V and Nb are both in the same pentavalent group of transition elements [4].

In this work, we investigated the effects of vanadium on sintering conditions, structural, electromechanical, ferroelectric and dielectric properties of NbLi co-doped PZT, providing indirect information about lithium location in the lattice. The valence state and location of vanadium paramagnetic ions were also examined.