Standard Hot Pressing as a Possible Solution to Obtain Dense K0.5Na0.5NbO3 Ceramic Doped by Er2O3 and Yb2O3

Abstract

In this study, the influence of the addition of rare earth oxides on the phase composition and density of KNN piezoelectric ceramics was investigated. The initial powders of Na 2 CO 3 and K 2 CO 3 were dried at 150 • C for 2 h. Then, a powder mixture for synthesis was prepared by adding a stoichiometric amount of Nb 2 O 5 and 5 and 10 wt % overabundance of Na 2 CO 3 . All powders were mixed by ball-milling for 24 h and synthesized at 950 • C. The phase composition of the reaction bed was checked by means of X-ray diffraction (XRD). It had an appearance of tetragonal and monoclinic K 0.5 Na 0.5 NbO 3 (KNN) phases. Then, 1 and 2 wt % of Er 2 O 3 and Yb 2 O 3 , were added to the mixture. Green samples of 25 mm diameter and 3 mm thickness were prepared and sintered by hot pressing at 1000 • C for 2 h under 25 MPa pressure. The final samples were investigated via scanning electron microscopy (SEM)-energy-dispersive X-ray spectroscopy (EDS), XRD, Rietveld, and ultrasonic methods. Phase analysis showed tetragonal and orthorhombic KNN phases, and a contamination of (K 2 CO 3 ·1.5H 2 O) was present. The obtained KNN polycrystals had a relative density above 95%. Texturing of the material was confirmed as a result of hot pressing.Materials 2019, 12, 4171 2 of 12 lead-based piezoelectrics. Due to the significant piezoelectric response obtained by numerous teams [5,[7][8][9], KNN-based materials have been widely researched, including in this study.In a perovskite-type ABO 3 structure, A-site cations are either alkaline earth elements or rare-earth elements, with B-site cations being transition metals. Theoretical structure consists of A cations occupying cuboctahedral coordination positions in the middle of eight corner-sharing BO 6 octahedrons. Due to the specific structure and composition, perovskites have plenty of conductive properties used in material sciences, including significant piezoelectricity. The piezoelectric effect is directly derived from the noncentrosymmetric structure of materials. Such structures are electrically neutral, but lack symmetrical arrangement. Therefore, when mechanical stress is applied, and such structure shifts, it does not maintain a neutral charge [4]. The high magnitude of electromechanical effects obtained in perovskites is derived from the ordering tendency present in those materials, mainly the shift of B cations [4]. K 0.5 Na 0.5 NbO 3 has a similar structure to BaTiO 3 , which consists of two phases, ferroelectric K 1-x NbO 3 and antiferroelectric Na x NbO 3 . Both phases have different ferroelectric transitions, crystallizing in a perovskite structure, with different symmetries. The phase transitions of KNN depend on the K/Na ratio [10]. To synthesize the basic structure of KNN, K, and Na carbonates are used in conjunction with Nb 2 O 5 [5]: K 2 CO 3 + Na 2 CO 3 + 2Nb 2 O 5 → 4K 0.5 Na 0.5 NbO 3 + 2CO ↑ 2 .

Na2co3 and Er2o3 Modified Lead Free Knn Piezoelectric Ceramic Obtained by Hot Pressing

“…The atomic ratio of Na/K for all observed grains is about 0.3 which further con rmed Na depletion due to its evaporation from material during the thermal processing. The nucleation rate of ne grains was previously con rmed to be enhanced by excess amount of Na 2 CO 3 introduced to the system [27]. The observed texturization leads to improved piezoelectric properties of material.…”

“…In order to obtain materials containing K 0.5 Na 0.5 NbO 3 , Er 2 O 3 sintering activator was added because of its positive effect in formation of both orthorhombic and tetragonal structures which enhance piezoelectric properties of obtained sinters [27]. The hot pressed KNN samples containing 5 wt.…”

“…However in both samples N5E1 and N5E2 there are same ne grains existed between those larger particles. Based on the result of our previous work [27] it can be concluded that those ne grains of tetragonal K x Na 1−x NbO 3 were formed due to higher sodium content which was enhanced by excess amount of Na 2 CO 3 introduced to the system [27]. The elemental concentration analysis was conducted in selected areas for each material.…”

“…Those studies showed that Yb 3+ substitution in perovskite structure could reduce coupling factor of materials, but greatly improve its other piezoelectric properties [26]. In case of KNN materials, coupling factor is considered high, and therefore such dopant was considered in our previous studies prior to this paper [27] alongside Er 2 O 3 which was investigated by Zhao et al and shown that it could result in creation of highly desired liquid phase during sintering [28].…”

Non-Destructive Examination for Cavitation Resistance of Talc-Based Refractories with Different Zeolite Types Intended for Protective Coatings

“…Hot pressing is based on using a uniaxial pressure (10–30 MPa) through a mold at the sintering temperature, which accelerates the densification, thus improving the overall material’s density and even getting close to the theoretical value in a reasonable time as well as providing a significant grain size reduction [ 30 , 31 ]. Hot isostatic pressing implies concurrent application of high temperature (up to 2200 °C) and a gas pressure (200–500 MPa) identically in all directions, usually through an impermeable membrane to compact the ceramic component.…”