Modification Of Potassium Sodium Niobate-Based Lead-Free Piezoelectric Ceramics

Piezoelectric materials are important functional materials applying for the energy transitions between mechanical and electrical ones, and have been widely used in electronic and micro-electronic devices. Piezoelectric materials include piezoelectric single crystals, piezoelectric ceramics, piezoelectric polymers and piezoelectric composites, among which piezoelectric ceramics are the most widely used because of their many advantages, such as simple process, low cost, easy to get large size and easy to modify. For the past 50 years, Pb(Zr1-xTix)O3 (PZT) ceramics have been the mainstay for high performance actuators and transducers because of their excellent piezoelectric and dielectric properties.However, Lead-based ceramics have a high lead content (around 60% in weight), which is harmful to the human health and the environment. Thus, it is urgent to develop environmental friendly lead-free piezoelectric materials and components to replace PZT-based ceramics and products.Among the varieties of lead-free piezoelectric ceramics that are extensively investigated, potassium sodium niobate (KNN)ceramics have attracted considerable attentions for their good piezoelectric properties, ferroelectric properties and high Curie temperature. KNN-based ceramics have been considered as a promising alternative candidate for alternative PZT systems.However, there are some problems in the atmospherically sintered of KNN ceramics.For examples, the bulk is difficult to be densified;the stoichiometry is difficult to control due to the volatility of the K and Na elements; and piezoelectric properties are very low. Moreover, KNN ceramics are easy to show deliquescence once exposed to humidity. It is reported that the hot-pressed samples could be sufficiently densified and show better properties. Nevertheless, hot-pressing is not suitable for the large-scale industry productions.The goal of this thesis is to solve some of the above problems of atmospherically sintered KNN-based ceramics prepared by the conventional solid-state reaction route and to improve their properties. Many researchers consider that the low piezoelectric properties and low density of KNN-based ceramics are mainly cused by the volatility of K/Na component. They believe that excess A-site ions to compensate the volatility is helpful to the improvement of KNN electrical properties. However, the results obtained by other researchers indicate that A-site vacancies caused by excess B-site ions favor the improvement of the bulk density and electrical properties. In order to clarify the influence of excess A/B-site ions,(Na0.52K0.48)Nb1+x%O3(KNNx)ceramics are prepared and their properties are investigated. Dielectric loss of the KNNx ceramics is low (<3%)except for two A-site excess compositions, KNN-1 and KNN-0.5 (22% and 3.2%, respectively).The piezoelectric properties of A-site excess compositions are lower than that of B-site excess compositions, and the compositions around x=0 posess the best piezoelectric properties:KNN-0.1(d33-126 pC/N,kp-0.410), KNNO (d33/33-125 pC/N,kp～0-382),KNN0.05 (d33-121 pC/N,kp～0.370).Moreover, the ageing behavior of A-site excess compositions, especially KNN-1 and KNN-0.5,is much serious than other compositions.B-site excess compositions are denser than the A-site excess compositions, and the KNN0.1 ceramic shows the maximum density. The A/B-site excess ions show little influence on the two phase transition temperatures TC and TO-T.Besides density, the internal structure is an important factor that affecting the properties of the ceramics.The superfluous Na/K ions in the perovskite structure inhibited the densification of the bulk during the sintering process, leading to low density and low properties.Conversely, the A-site vacancies caused by the excess Nb5+ are helpful to the densification of the samples.However, when superfluous Nb5+ exceeds 1%,a second phase will be formed, and the piezoelectric properties will decrease.Therefore it is not difficult to understand that the KNNx compositions around x=0 with fewer interior flaws and no second phase show best piezoelectric properties. The excess A-site alkaline elements easily react with the moisture in the air, and the big crystal grains in A-site excess compositions helps relaxation of the domain wall.The above reasons make A-site excess compositions aging faster than other compositions.The hygroscopic nature of KNN-based ceramics has been pointed out hereinabove. This hygroscopic sensitivity was disadvantageous to usage. However, few researches on the hygroscopic characteristic have been conducted, and the physical mechanism is not clear yet. To solve the deliquescent problem, ScTaO4 is used to modify KNN ceramics, and (Nao.5K0.5NbO3-x mol%ScTaO4(KNN-STx) ceramics are prepared in Chapter 3.The KNN ceramics possess a pure perovskite structure. After the addition of ScTaO4, a hygroscopic second phase K6Nb10.8O30 is formed. The content of K6Nb10.8O30 decreases with x, and disappears when x=1.5.The relative density increases monotonously with the increasing ScTaO4 content, from 93.5% for x=0 to 97.3% for x=1.5.A number of pores can be observed in pure KNN composition. The amount of pores decreases considerably and the liquid phase can be observed after the addition of ScTaO4, indicating that the densification of the KNN-STx ceramics might be improved by liquid phase, which could lead to a higher density. The piezoelectric coefficient d33 of pure KNN decreases by 11% and the dielectric loss tgδincreases to 12 times of the initial value after 120 h of immersion in water, while the ScTaO4 modified ceramics show no obvious reduction in d33 and the increase of tgδis relatively low. Especially for the KNN-ST1.5 ceramics, d33 decreases by 1 pC/N and tanδ<3% after 120 h of immersion, indicating that the deliquescent problem can be effectively solved by ScTaO4 modification.The deliquescent problem can be linked to either the presence of hygroscopic second phase or the microstructure.The most deliquescent composition, KNN,possesses a single perovskite structure without deliquescent second phase, so the second phase should not be the major factors for deliquescence. It should be noted that the samples with high density and less pores are more stable against humidity. Therefore, the dense microstructure is an important factor for the KNN-based ceramics to resist the humidity in air.In chapter 4, Li+ and SbD+ are added into the KNN ceramics to improve the piezoelectric properties.Previous researches on (1-x%)KNN+x mol%LiSbO3 system focus on the low LiSbO3 content with x<6, investigations on high LiSbO3 modified KNN ceramics have not been carried out in details.In this work, KNN ceramics co-modified with 7 mol% lithium and x mol% antimony, (Na0.53K0.4Li0.07)Nbi1-x%Sbx%O3 (NKLNSx, x≥73) were prepared. The structure changes from pure tetragonal phase to the coexistence of orthorhombic and tetragonal phases, and the orthorhombic-tetragonal phase transition temperature TO-T increases with the increasing x. This TO-T increasing phenomenon, resulting from the formation of LiSbO3 second phase, is different from the previously reported results. The NKLNSx ceramics possess high piezoelectric properties with d33>250 pC/N and kp>46%, especially NKLNS8.8 and NKLNS9.1 (d33>290 pC/N, kp>0.48). The shift of TO-T down to room temperature should be the reason for the drastic enhancement in piezoelectric properties. However, the piezoelectric properties around room temperature show strong temperature dependence due to the phase transition near room temperature.The work in the Chapter 5 is to improve the thermal stability of the KNN based ceramics.It has been pointed out in the previous chapter that the poor thermal stability of modified KNN ceramics is ascribed to the phase transition temperature TO-T near room temperature. Therefore, the key approach to enhance thermal stability is to move TO-T away from room temperature. This is accomplished via two approaches in this chapter:1.(Na0.53K0.41Li0.06)Nb0.91Sbo.o903 is modified with MgTiO3, and its TO-T decreases from 57℃to 10℃, while the Curie temperature TC increases, which making tetragonal phase region expanded.As a result, the thermal stability of the piezoelectric, dielectric and ferroelectric properties of the MgTiO3 modified NKLNS ceramics is effectively improved in a wide temperature range of 0℃-150℃.2.Ag+ and Sb5+ are use to modify pure KNN ceramics (the chemical formula is (Na0.53K0.47-xAgx)Nb1-xSbxO3, abbreviated as NKNASx).The density and piezoelectric properties are effectively improved by the addition of AgSbO3. Moreover, the phase transition temperature TO-T keeps above 100℃for the composition with x<0.07.The planar electromechanical coupling factor kp keeps almost unchanged up to the TO-T temperature, indicating that the NKANS ceramics exhibit a much improved piezoelectric thermal stability. In addition, both the two phase transitions at TO-T and TC have changed into diffuse phase transitions.In other words, the phase transition from orthorhombic to tetragonal can not accomplish at a certain temperature, but occurs gradually in a wide temperature range around TO-T, and the two phases coexist quite stably in this temperature region. Consequently, the piezoelectric properties around TO-T exhibit weak temperature dependences. This is also helpful to the thermal stability of the NKANS ceramics.