Effects Of Doping On Alkaline Niobate Lead-Free Piezoelectric Ceramics

Piezoelectric ceramics is one of the important functional materials, which has been widely used in electronic and micro-electronic devices. Since the lead zirconate titanate piezoceramics has been discovered, most of applied piezoceramics are based on PZT or PSM piezoelectric materials. However, the content of PbO in PZT is as high as 70 wt%, the PbO evaporation during the sintering of PZT and the discard of PZT product wastes pollute the environment and are harmful to human health. Therefore, it is an urgent task to search and develop environmentally-friendly new industrial piezoceramic products. In 2001, the European Parliament adopted the decree on the "Restricted hazardous substances in the electrical and electronic equipment" implemented in 2008. China's Ministry of Information Industry also developed "Management regulations to prevent pollution of electronic and information products," which has been implemented since July 1, 2006. Among these restricted hazardous substances, lead ranks in the front of the list. Therefore, European, USA, Japan and China begin to search and develop environmentally-friendly lead-free piezoceramics materials, and give more support to research lead-free piezoceramics year after year. However, the research basis of lead-free piezoceramics is very poor, and the performances of lead-free piezoceramics are still much lower than PZT piezoceramics. Therefore, searching and developing lead-free piezoelectric ceramics is an urgent task, which is related to the sustainable development of China's electronic technology.In chapter 1, the author first introduces the piezoelectric material development history, its classification, and gives a detailed exposition of the mechanism of piezoelectric effect. As all of piezoelectric ceramic materials are derived from the ferroelectric materials, the micro mechanisms and features about the ferroelectric materials are introduced in detail. Piezoelectric ceramics is the ferroelectric ceramics undergoing the artificial poling. Since the poling is very important for piezoelectric ceramics, this article made a more exposition in the poling electric field, temperature and time. The article then reviews the lead-free piezoelectric ceramic materials research progress, and makes the following classification for piezoelectric ceramics: niobate system lead-free piezoceramics (NKN or KNN), bismuth-layered structure lead-free piezoceramics, sodium bismuth titanate system lead-free piezoceramics (NBT), tungsten bronze structure system lead-free piezoceramics and BaTiO₃-based lead-free piezoceramics. Among the lead-free piezoceramics materials, the NKN system lead-free piezoceramics has attracted many attentions for its highest piezoelectric activity. However, NKN is very hard to be sintered into well-densified ceramics. As the investigation into the NKN lead-free piezoceramics conducted systematically, many methods to get well-densified NKN ceramics have been found, such as controlling the calcining and sintering temperature, improving the preparation process, changing the componets in stoichiometric ratio, controlling the size of raw materials, et al. With NKN densification, its piezoelectric properties were obviously improved. In these improvement experiments, the most common method is the substitution with the elements having the same valance, that is, substituting Li for A-site elements or substituting (Ta, Sb) for B-site element. In recent years researchers have done a lot of research work in this approach, and a great progress has been made. We have also done a part of work in this approach. An important feature of perovskite structure is that its A-site and B-site ions can be singly or complexly replaced by other ions with different valances and ionic radius in a wide concentration range, which can make the materials performance adjusted in a wide range to meet the requirements in different occasions. Research in this substitution method is still very less, most of researches mainly focused on the donor doping of alkaline-earth elements substitution for A-site ions or alkaline-earth titanate doping. Furthermore, the research work on rare earth elements doping and variable valance elements doping for NKN-based system lead-free piezoceramics is less reported.In this paper, the effects of substituting Li for A-site ions and substituting (Sb, Ta) for B-site ions have been investigated. NKN samples with high Curie temperature and high piezoelectric activity are obtained. In view of the difficulty in sintering NKN-based lead-free piezoceramics materials, this paper is concentrated on studying the effects of (Ce, Pr, Nd, W)-doping into NKN on the preparation process, physical properties, piezoelectric properties and dielectric properties. A method optimizing NKN-based lead-free piezoceramics sintering conditions was found, which extended its sintering temperature range from 10℃to 40℃, and significantly improved its time-stability. This work provides the theoretical and experimental basis for the industrial production of NKN-based lead-free piezoceramics.In chapter 2, the effects of substituting Li for A-site ions with same valance on microstructure, piezoelectric and dielectric properties of NKN-based lead-free piezoceramics have been investigated. The experimental confirms that there exists a morphotropic phase boundary (MPB) between orthorhombic and tetragonal phase inNKN-based lead-free piezoceramics, and the second phase, tungsten bronze structurephase appears at 6.3mol% Li content, in Li-doped NKN-based materials (NKLN). Outstanding performances of NKN-based lead-free piezoceramics samples are obtained, its piezoelectric constant d₃₃ values reach 202-212pC / N, electromechanical coupling coefficients k_p reach 44.4%-46.8%, Curie temperature T_C reach 490℃-510℃, dielectric losses tgδrange in 1.7%-1.9%. High Curie temperature and excellent piezoelectric properties show that NKLN lead-free piezoceramics material is a potential candidate for high-temperature lead-free piezoceramics materials. Then the effects of substituting Li for A-site ions and substituting Ta for B-site ions with same valance on piezoelectric and dielectric properties of NKN-based lead-free piezoceramics have been investigated. Combining X-ray diffraction with dielectricity-temperature diagram, the changes in the phase structure was emphatically analyzed, and a second phase with tungsten bronze structure phase appears at 5.3mol% Li content. The relation of the MPB dependence on Li content is figured out. The research shows that only small amount of Ta can obviously enhance the effect of Li doping, and improve the piezoelectric properties of NKN-based lead-free piezoceramics. It was found that the MPB region of NKN-based lead-free piezoceramics locates at the range of 0.053≤Li≤0.057. Around the MPB, the samples have highest piezoelectric properties, and its piezoelectric constant, the typical electromechanical coupling coefficients and dielectric loss are d₃₃=248pC/N, k_p=49.3%, k_t=49.1% and tgδ=1.96%, respectively.In chapter 3, the effects of substituting Sb for B-site ions with same valance on phase structure, piezoelectric and dielectric properties of NKN-based lead-free piezoceramics have been investigated. Through changing Sb content an MPB-like region appears between orthorhombic and psudo-cubic phase. In this region, the (Li, Sb, Ta)-doping NKN-based lead-free piezoceramics (NKLNST) samples with excellent performances are obtained, with d₃₃>300pC/N, relative densityρ≥94.6%, tgδ≤4.4%, planar electromechanical coupling coefficient k_p≥49% and thick electromechanical coupling coefficient k_t≥43%. Excellent piezoelectric properties show that NKLNST lead-free piezoceramics material is another potential substitute of PZT piezoceramics.Based on the substitute with same valance ions, the research work in chapter 4 on rare-earth-doping into NKN-based lead-free piezoceramics has been done. The effects on microstructure, properties and piezoelectric properties, as well as the aging rate, the non-deliquescence of 95NKN-5LiSbO₃ lead-free piezoceramics (NKLNS) have been investigated. Some experiment results with good guiding significance have been obtained in the sintering temperature, the mass loss before and after sintering, piezoelectric properties, microstructure, etc. It is found that doping different amounts of CeO₂, a large difference has been shown from preparation process to electrical properties in NKLNS samples. Beginning with the microstructure scanning, this paper makes a detailed analysis for the physical mechanism causing these differences. NKLNS lead-free piezoceramics samples are prepared, which has high piezoelectric constant (d₃₃=248pC/N), low aging rate and strong performance against deliquescence.In chapter 5, the effects of Ce-doping and excess (Na, K) on the sintering temperature, physical properties, dielectric and piezoelectric properties of NKLN lead-free piezoceramics have been investigated. The results show that the approach of Ce-doping and excess (Na, K) can improve the density and piezoelectric, dielectric stability of NKLN samples. Adding proper excess (Na, K) and doping CeO₂ can greatly extend the optimal sintering temperature range, from the initial 1070℃-1080℃extending to 1070℃-1110℃. Then, the effects of rare earth element (Pr, Nd)-doping and variable valance element W-doping on the sintering temperature, physical properties, piezoelectric properties, dielectric properties of NKLN lead-free piezoceramics have been investigated. This paper investigates the relationship between piezoelectricity and dielectric loss before poling, optimizes the preparation process, analyzes the origin of breakdown and its effects on piezoelectric and dielectric properties, and discusses the effect of space charge on dielectric property. Experiments show that doping proper amount of WO₃ can extend the optimal sintering temperature range of NKLN lead-free piezoceramics. The samples doped with WO₃ which are prepared at optimal sintering temperature have a high piezoelectric and dielectric stability, d₃₃=170±7pC/N,ε_r=634±17, tgδ=2.0%±0.3%. In addition, the study shows that W-doped samples can reduce the dielectric constant of NKLN lead-free piezoceramics, making this material have a very high piezoelectric voltage constant, which is expected to have potential applications in the ignition and blasting field. Finally, in view of the shortcomings of being hard to sintering, being undensified and easy breakdown, a tentative plan of multi-layer structure is proposed. Double-layer structure samples are prepared. Experiments show that the double-layer structure samples can reach their optimal performances even at a wide sintering temperature range. The double-layer structure NKN-based samples prepared in this paper show high piezoelectric constants, and high relative density, low dielectric loss and strong dielectric strength, which proves that double-layer structure is a potential preparation process.Based on the research on composition, structure and piezoelectric, dielectric and ferroelectric properties of NKN-based and doped NKN-based lead-free piezoceramics materials, the paper supplies a reliable experimental and theoretical basis for the research and developmental applications of NKN-based lead-free piezoceramics materials, enriching the knowledge system of material sciences and piezoelectric physics, promoting the practical applying of lead-free piezoceramics. It should be stated that, the preparation process method used in the paper is suitable for large-scale industrial production of electronic ceramics in order to make research results into the actual production as soon as possible.