Potassium sodium niobate based lead-free piezoelectric ceramics
Niobate-based piezoelectric ceramics, particularly potassium sodium niobate based lead-free ceramic having a high Curie temperature (Tc) is regarded as one alternative to PZT based piezoelectric ceramic main object of selection. Advances from the current point of view, to get lead-free piezoelectric ceramics completely replace the traditional lead-based piezoelectric ceramics possibility is not large; but the analysis of the current situation shows that the piezoelectric applications, if the choice of lead-free piezoelectric ceramics materials and devices used in a large number of low-end applications (k33 and d33 parameters such as the indicator is not too high), or very likely. Preparation of uniform, dense, wide range of quasi-phase transition with lead-free piezoelectric ceramic grains are highly oriented main way to improve its range of applications.
Preparation * Ceramic Powders
Potassium sodium niobate based lead-free piezoelectric ceramic powder preparation methods generally include ordinary milling, sol - gel method, hydrothermal and molten salt method. From the point of view of industrial production, large scale synthesis of ordinary powder milling process is easy, but this method is time consuming and polluting; on ecological and environmental protection, calls for preparation techniques have low energy consumption, low pollution and other environmental characteristics coordination, research and developed hydrothermal method, electrochemical method and sol - gel method soft solution preparation techniques such as ceramic materials have the advantage of these technologies is considered a high-performance ferroelectric and piezoelectric ceramics prepared by advanced technologies of the 21st century.
MDAguas and IPParkin sol - gel synthesis has been NaNbO3 powder prepared; Christian microemulsion media such as alcohol use saline solution to prepare a (KxNa1-x) NbO3 nanopowders; Chen Qiang and other uses Pechini synthesized Nb5 +, Li + precursor solution at 650 ℃ for 2h get LiNbO3 relative proportion of> 97% of the powder; FanYa hong, such as molten salt method synthesized (K0.5Na0.5) NbO3 powder, and for the preparation of a piezoelectric ceramic ; and so on. However, these methods have their own deficiencies. For example, although some methods capable of producing high purity fine piezoelectric ceramic powder, but the equipment requirements too high and is limited to laboratory use; some methods which are not necessarily the most advanced technology, but the advantage prominent, but also of great value, such as high-energy ball milling method can be used as a more efficient and economical ceramic powder preparation techniques are widely used in industrial production.
From Table 1, the high-energy ball milling ceramic powder particle size than the conventional method were prepared by ball milling is reduced by several times to obtain a more uniform particles, the activity of large specific surface energy powder, in order to reduce the sintering temperature increase density and improve ceramic piezoelectric properties provide a guarantee. Furthermore, the literature also reports improved powder preparation techniques.
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* Ceramic molding
Forming a second ceramic basic process step, after the molding material into a certain shape, size, strength, and density of the semi-finished products. Currently, potassium sodium niobate based lead-free ceramic green mainly metal molding. YipingGuo et conventional forming process based on the combined isostatic pressing, to prepare a good piezoelectric properties (1-x) (K0.5Na0.5) NbO3-xLiNbO3 ceramics, when x = 0.06, the properties of the ceramic parameters ρ = 4.35g / cm3, d33 = 235pC / N, kp = 0.42, kt = 0.48. Cast molding process technology combined with the template grain growth can also prepare excellent performance based lead-free piezoelectric potassium sodium niobate.
* Ceramic sintering
Sintering is heating the pre-firing the ceramic blank in an appropriate type of environment or atmosphere to below the melting point of its fundamental component below the temperature of incubation, and then cooled to room temperature, the treatment process, through a series of physical and chemical changes, the ceramic blank powder particles are gathered into a combination of grains, porous body into a compact body, the rapid increase in body strength and density and other physical and mechanical properties has also been significantly improved, resulting in the desired physical and mechanical properties of the product.
The density of the piezoelectric ceramic important parameter reflecting the size of its quality. Good sintering process is to minimize porosity so as to increase the density of the ceramic body. Potassium sodium niobate based lead-free piezoelectric ceramic sintering process using mainly due to the traditional good is difficult to obtain a dense ceramic body is approximately at 900 ℃, Na2O, K2O began volatile; this situation and with the growth of the holding time increased insulation temperature and more obvious.
Measuring the deviation of theoretical chemistry makes ceramic body than the crystal phase occurs miscellaneous non-perovskite phase, the impact of firing the piezoelectric properties of the sample. General approach in the early stages of ingredients that adding an excess of Na, K to compensate for the amount of evaporation loss during the sintering process, but the method for improving potassium sodium niobate based lead-free piezoelectric ceramics little performance effectiveness. Increasing the density of sodium potassium niobate based lead-free piezoelectric ceramics, is key to improving their performance, but also the ceramic sintering process, the main focus of the research topics to be.
Conventional sintering need to select the appropriate sintering temperature and time, in order to obtain an appropriate grain size, the density of the sample is preferably a ceramic. In recent years, the country, and outside scholars use some novel sintering process, the preparation of a potassium sodium niobate based lead-free piezoelectric ceramics and other niobate-based lead-free piezoelectric ceramics. For example, Ken-ichiKakimoto etc. prepared by vacuum sintering a KBa2Nb5O15 ceramics; HongliangDu etc. has been prepared by using the same technology (K0.5Na0.5) NbO3 ceramics; TakahiroWada such as spark plasma sintering prepared a NaNbO3 ferroelectric ceramics, RuipingWang etc. Preparation has been using the same method (K1-xNax) NbO3 ceramics. DavidJ.Brooks such as the use of microwave-induced plasma-assisted sintering technology successfully prepared three yuan titanate niobate system and the system, while relatively conventional sintering sintering time is shortened, for potassium sodium niobate based lead-free piezoelectric ceramic sintering process opens a new effective way.
Liquid phase sintering method, which adds the appropriate flux in potassium sodium niobate based lead-free piezoelectric ceramics, can be produced using conventional ceramic technology have good performance ceramic body. If using K5.4Cu1.3Ta10O29 (KCT) was prepared as a flux of (K0.5Na0.5) NbO3 and (K0.5Na0.5) (Nb0.7Ta0.3) O3 ceramics; or use K4CuNb8O23 (KCN) prepared as a flux of the (K, Na) NbO3 ceramics. This is because the KCT, KCN, etc. at a certain sintering aids the sintering temperature of the melting ceramic, solid phase filling the pores between the grains of the analysis, so that the entire range can give the relative density of the sintered dense ceramic body larger.
* Ceramic grain orientation
In general, the use of conventional ceramic preparation technique is difficult to obtain a high-performance, sodium potassium niobate-based lead-free piezoelectric ceramics. In recent years, the use of grain-oriented techniques, such as template grain growth (TGG) technology, grain growth reaction of the template (RTGG) technology, forging (HF), so that the preferred crystal orientation, to obtain a good performance of lead-free piezoelectric ceramics. The preferred orientation of the ceramic structure, and similar natural fibers or fabric structure and texture of the arrangement state of aggregation rules called texture. This type of structure textured ceramics can be obtained applying an external field (mechanical force field, electric field, magnetic field, the microwave field) during synthesis. The technology is currently used more template grain growth (TGG) technique and reactive template grain growth (RTGG) technology.
* Template grain growth (TGG) technology
Template grain growth (TGG) technology is a polycrystalline ceramic material texturing method. Specific operation: First prepare a template with a certain degree of orientation of the grain, then by casting, extrusion or slip casting and other means whiskers, fibers, ceramic blank implanted in platelets, and then sintered at high temperatures to form crystals a greater degree of orientation of grains of ceramic material.
For the preparation of a ceramic high textured template particles must meet the following requirements: The template must exhibit a suitable shape by applying a shear force to obtain a high degree of orientation of the crystal arrangement; should be appropriate to the size of the template, the template and the substrate because of crystal the existence of differences in particle size to produce a driving force for growth; crystal structure of the template and the substrate particles to be similar; template has a relatively stable thermodynamic properties. Templates in the template grain growth technique has relatively chemically inert, and only to the grain growth during the sintering process to provide a carrier.
* Reaction template grain growth (RTGG) technology
Reaction template grain growth (RTGG) technology template with a chemical reactivity, can be directly involved in the synthesis of solid phase reaction ceramics. With potassium sodium niobate based lead-free piezoelectric ceramics as an example, since the alkali metal Na, K easily volatilizes at a high temperature, the use of conventional preparation process is difficult to obtain a dense ceramic is better, the better the performance of the piezoelectric ceramic, and the use of template for the reaction grain growth technology available to the excellent performance of alkali metal niobate ceramics.
Yasuy-oshiSatio such as the use of tabular grains as raw material preparation Bi2.5Na3.5Nb5O18 get NaNbO3 tabular grains as a template, supplemented NaN-bO3, KNbO3, KTaO3, LiSbO3 grains mixed by casting and sintering stack have a high degree of orientation (K0.44Na0.52Li0.04) (Nb0.84Ta0.10Sb0.06) ceramics. The portion of the piezoelectric ceramic and the performance is quite conventional PZT piezoelectric ceramics (Table 2). Takao still use history and other grain-oriented technology to prepare a recipe for {Lix (K1-yNay) 1-x} (Nb1-x-zTazSbw) O3 lead-free piezoelectric ceramics, in which the ratio of: 0 x 0.2; 0 y 1; 0 z 0.4; 0 w 0.2; x + z + w> 0.
