Solid-state synthesis of undoped and Sr-doped K0.5Na0.5NbO3

Abstract

The solid-state synthesis of undoped K 0.5 Na 0.5-NbO 3 (KNN) and KNN doped with 1, 2 and 6 mol% Sr, from potassium, sodium and strontium carbonates with niobium pentoxide, was studied using thermal analysis and in situ high-temperature X-ray diffraction (HT-XRD). The thermogravimetry and the differential thermal analyses with evolved-gas analyses showed that the carbonates, which were previously reacted with the moisture in the air to form hydrogen carbonates, partly decomposed when heated to 200°C. In the temperature interval where the reaction was observed, i.e., between 200 and 750°C, all the samples exhibited the main mass loss in two steps. The first step starts at around 400°C and finishes at 540°C, and the second step has an onset at 540°C and finishes with the end of the reaction between 630 and 675°C, depending on the particle size distribution of the Nb 2 O 5 precursor. According to the HT-XRD analysis, the perovskite phase is formed at 450°C for all the samples, regardless of the Sr content. The formation of a polyniobate phase with a tetragonal tungsten bronze structure was detected by HT-XRD in the KNN with the largest amount of Sr dopant, i.e., 6 mol% of Sr, at 600°C.

Mechanochemically-Assisted Synthesis of Lead-Free Piezoelectric CaZrO3-Modified (K,Na,Li)(Nb,Ta)O3-Solid Solution

“…This value is in good agreement with the theoretical mass loss (10.18%) calculated for the complete chemical reaction between the carbonates and oxides forming the KNLNT-CZ solid solution. The EGA analysis shows a two-step release of CO 2 in the temperature interval between 400 and 700 • C, accompanied by a weak exothermic peak on the DTA curve at 489 • C. These observations suggest that the course of this reaction is similar to the well described thermal decomposition of carbonates during the solid-state synthesis of KNN [38,39] and to the thermal behavior of the nonactivated powder used for the mechanochemical synthesis of KNLNT [16].…”

“…This endothermic process is characteristic for the decomposition of alkali hydrogencarbonates, in this case most probably KHCO 3 [36,37]. Their presence in small amounts in the initial mixtures is frequently reported in the studies of KNN-based compositions and originates from the exposure of hygroscopic carbonates to the ambient humidity [16,34,38]. The total mass loss up to 240 • C amounts to 1.2%, while upon further heating up to 700 • C the sample loses additional 10.27%.…”

Tetragonal tungsten bronze phase thin films in the K–Na–Nb–O system: Pulsed laser deposition, structural and dielectric characterizations

“…This structure is also studied for optical properties [26] and catalytic activities [27]. In the KNN system, even though the TTB phase has been found as a secondary phase in bulk perovskite [28][29][30], the pure phase was neither reported nor properly characterized. Some previous works on a modified K x Na 1-x NbO 3 perovskite ceramic have reported that the TTB phase, as a secondary phase, could explain the increase of the piezoelectric properties [29].…”

Microstructure evolution and electromechanical properties of (K,Na) NbO3‐based thick films

“…The powder compacts were calcined at 800°C for 4 hours, milled and then re‐calcined at 750°C for 4 hours, as described in Ref. [24]. The as‐synthesized powder consisted of sub‐micrometer‐sized particles with a median particle size of 0.64 μm determined with a laser light‐scattering particle‐size analyzer (Microtrac S3500).…”