Piezoelectric ceramics are important ceramic materials for the conversion of mechanical and electrical energy and have an important function in our national economy and defense industry. Lead-containing ferroelectric piezoelectric materials are widely applied owing to their abundant storage capacity, good performance, and low cost. However, the current piezoelectric materials are lead zirconate titanate ceramics, which are harmful to the human body and environment. With the gradual advance of the green sustainable development concept, the improvement of environment-friendly lead-free piezoelectric ceramics has become an important research objective in the discipline of ferroelectric materials. Based on the requirements of human protection of ecological environment and the development of environmentally friendly materials, lead-free piezoelectric ceramics with potassium niobate have been widely used in science and technology, industry, military, aerospace, and aviation because of their properties, such as environmental friendliness, good electrical properties, and proper Curie temperature. They are considered to be one of the most likely materials to replace lead-containing piezoelectric ceramics, and have become one of the current focuses of piezoelectric ferroelectric materials research. The research progress of potassium sodium niobate (KNN) piezoelectric ceramic materials in recent years is reviewed in this paper, which summarizes the doping preparation of the KNN by metal and metal-oxide, rare earth ions, and other compound doping and discusses the hot pressing, microwave, and discharge plasma sintering technologies and the molding processes, such as laser pulse deposition technology and radio frequency (RF) magnetron sputtering. The results show that the sintering technology of KNN is not suitable for the sintering of sodium niobate. The doping of the KNN piezoelectric ceramics and advanced sintering techniques can significantly improve their electrical properties. The appropriate concentration of the metal ions in the doping promotes the grain growth of the KNN-based piezoelectric ceramics and constructs a multiphase coexistence structure at room temperature, which in turn improves the electrical properties of the KNN-based piezoelectric ceramics. Rare earth element doping generally occurs via an unequal substitution, reducing the internal stress and relieving the domain wall motion, thereby enhancing the electrical properties. Additionally, the use of other compounds in the doping permits the KNN ceramics to be applied in high temperature and medical environments, expanding their application field. Sintering can inhibit the volatilization of the alkali metals during the preparation of the ceramics, and hot-pressure sintering increases the driving force on the powder to obtain ceramics with better densities and well-developed grains, which effectively enhances the electrical properties of the ceramics. Microwave sintering shortens the sintering time and reduces the volatilization of the elements, which in turn improves the electrical properties of the ceramics. Spark plasma sintering, owing to its short sintering time, produces ceramic samples with a smaller grain size and increased density, which in turn improves the electrical properties of the ceramics. The surface morphology of the films prepared by the pulsed deposition technique is uniform and dense, and their electrical properties are good. Films with a dense and uniform microstructure are obtained by the sol–gel method after annealing at an appropriate temperature, and the electrical properties of the films are excellent. The preparation of thin films by the RF magnetron sputtering method requires an appropriate annealing temperature and atmosphere, which can promote grain growth, aid in the formation of dense films, and improve the electrical properties. The preparation of KNN piezoelectric ceramic films and coatings expands the range of applications of KNN in fields such as defense, aerospace, and communication. Finally, the contents of the paper and the development trend of KNN are summarized. This paper reviews the doping preparation, sintering technology, and molding processes of KNN ceramics, providing a theoretical basis and reference for the research of KNN piezoelectric ceramics in various fields. © 2023 Chinese Mechanical Engineering Society. All rights reserved.
