Ultrafast high-temperature sintering as an alternative technique for potassium-sodium niobate production

• Main Author: Miranda, João Francisco Lopes
• Publication Date: 2024
• Format: Master thesis
• Language: eng
• Source: Repositórios Científicos de Acesso Aberto de Portugal (RCAAP)
• Download full: http://hdl.handle.net/10773/41830
• topic: Ultrafast high-temperature sintering; Alternative sintering; Potassium-sodium niobate; Electroceramics; Lead-free piezoelectrics; Ferroelectrics

Ultrafast High-temperature Sintering (UHS) is a novel non-conventional sintering technology, with great potential for application in ceramic processing. One of its key characteristics is the use of heating rates estimated above 10³ °C/min, which can affect the structure and microstructure of the ceramics and reduce the necessary processing time. In this work, the UHS technology was applied and studied for the first time for processing of the perovskite type K₀.₅Na₀.₅NbO₃, KNN, an important ferroelectric material and a promising lead- free substitute for piezoelectrics in the market. The main sintering drawbacks of KNN include poor densification and stoichiometric control, caused by the necessary conditions of the sintering stage. This R&D master dissertation has a dual objective. Firstly, it seeks to contribute to the advancement in the implementation of a novel and potential alternative technology for the sintering of ceramic materials, particularly focusing on electroceramics. Secondly, the dissertation aims to explore alternative strategies for the sintering of critical lead-free ferroelectrics which face inherent sintering limitations. Through these dual avenues, this research endeavours to address challenges in the sintering process and pave way for innovative approaches in the field of electroceramic materials. To achieve these objectives, a UHS equipment was constructed, subjected to systematic testing, and optimized in terms of operational conditions. This process encompassed a comprehensive characterisation of the heating element, the carbon felt, to assess its structure, microstructure, and thermal stability under varying atmospheric conditions. This approach ensured the establishment of an effective UHS system, as the basis of this work for successful experimentation and material processing. KNN powders were synthesized and characterised regarding their crystal structure, particle morphology and size, thermal stability, and shrinkage. The powders were used to obtain bulk ceramics using various UHS conditions and conventional sintering, for comparison purposes. Varying the UHS time and current parameters allowed for the construction of a processing map, enabling the identification of optimal conditions for the application of UHS in KNN. Consequently, ceramics sintered using UHS with 23 A for 60 s, 20 A for 90 s or with a multistep regime, with equivalent densities, underwent thorough analysis of structure, microstructure, chemical composition, mechanical behaviour and electrical properties. KNN ceramics processed by UHS present fine microstructures, which impacted their electrical behaviour, showing equivalent performance to the conventional in most situations. With this investigation, correlations were established between the UHS processing of KNN and the final properties. Being an initial and pioneering investigation in the context of KNN by UHS, there are various aspects that require further improvement and more systematic exploration. These include enhancing the density of bulk materials, researching chemical composition variations, and optimizing the thermal annealing process. Despite the need for this improvement, it is noteworthy that KNN ceramics were successfully produced by UHS, showcasing significant reductions in total processing time while demonstrating comparable properties. This achievement represents a unique contribution not yet documented in the existing literature.