Original ArticleA compromise between piezoelectricity and transparency in KNN-based ceramics: The dual functions of Li2O addition
Introduction
Transparent ceramics can be applied in many emerging technologies, such as optical storage, infrared detection and so on, due to their high temperature resistance, high durability and high strength. [[1], [2], [3]] However, continuous technology evolution necessitates the combination of optical transparency with electrical properties, e.g., energy storage, ferroelectricity and piezoelectricity [[2], [3], [4], [5]]. Therefore, it is particularly urgent to enhance the electrical capabilities of transparent ceramics to ensure better transparency [[3], [4], [5], [6], [7]].
After extensive research on transparent ceramics, the factors that affect the transparency of ceramics can be summarized as follows. First, the search for a powder with high purity, low optical anisotropy, moderate fineness and suitable pre-sintering temperature is the prerequisite for preparing highly transparent ceramics. Second, during the sintering process, the optimum heating and cooling rate, sintering temperature and holding time are critical to achieving highly dense ceramics. Third, in the process of making transparent ceramics, the existence of pores should be excluded as much as possible to make the samples highly dense and reduce its influence on transparency and other properties. Fourth, it is necessary to avoid the formation of secondary phase, impurities and defects at grain boundaries to greatly improve the linear transmittance of ceramics. Moreover, the surface smoothness of the sample before the transmittance test will directly affect its value, so manual and mechanical polishing methods are used to reduce the unnecessary light loss to better reflect the intrinsic properties of the ceramic. [1,[5], [6], [7], [8]] In addition, according to the principle of light scattering, when the size of the scattering centre is approximately equal to the wavelength of the incident light, the scattering is the largest and the transmittance is the lowest. Therefore, reducing the grain size to avoid the wavelengths of incident light has been used extensively to improve the transparency of ceramics [5,[9], [10], [11]]. For example, the KNN-LB samples prepared by Yang et al. obtained the highest optical transparency at the component with the smallest grain size, reaching 72 % in the visible region. [12] Bi2O3-doped KNN-based transparent ceramics have also been reported by Liu et al.. The synthesized ceramics had a transmittance of about 62 % at 780 nm, and the improvement in transparency was accompanied by a deterioration in electrical properties. It is reported by Yang et al. that the (1-x)(K0.37Na0.63)NbO3-xCa(Sc0.5Nb0.5)O3 ceramics has a transparency of about 58 % at x = 0.09, but the piezoelectric constant of the sample is only 38 pC/N. [13] Apparently, the obtained transparent ceramics with small grain sizes have not been able to provide outstanding electrical properties [[10], [11], [12], [13], [14]]. So, regulation and control in order to obtain ceramics with superior optical and electrical properties are not yet to be determined. Actually, compared with grain size, the grain boundary has a greater effect on transmittance. Generally, the grain boundary should be adjusted as much as possible to make it thin, small, clear and free of impurities. Only in this way, it can have better light matching due to the depressed losses linked with refraction, scattering and reflection of light at the grain boundary. Overall, the grain size and the grain boundary are vital to the improvement of the electrical and optical properties of transparent piezoelectric ceramics [6,9,13,14].
In this work, a new design concept that increasing the grain size of ceramics to improve its light transmittance while taking into account its excellent electrical properties have been proposed. A kind of sintering aid Li2O, which can make grains growth, is used to be introduced into 0.93K0.5Na0.5NbO3-0.07SrZrO3 (KNN-SZ) ceramics to achieve our design goals. The reasons for choosing Li2O as additive are mainly due to the following reasons: on the one hand, Li+ can compensate for the volatilized K+ and Na+ during high temperature sintering for improving electrical properties; meanwhile, Li+ can also induce lattice distortion and lead to the coexistence of O–T phases, which is great benefit to the good piezoelectricity of ceramics. On the other hand, Li2O, as a sintering aid, may produce a small amount of a liquid phase, thus promoting the growth of grains, which favours good piezoelectric performance and high transmittance. [[15], [16], [17]] Meanwhile, the influence of the intrinsic defects originated from intrinsic cationic substituion in niobates formed by the introduction of Li+ on the optical properties of the sample cannot be ignored. [18,19] Finally, superior dielectric properties (εr ∼ 9643), good piezoelectricity (d33 ∼ 150 pC/N) and excellent transmittance (T ∼ 75 % in the near-infrared region) in 0.93K0.5Na0.5NbO3-0.07SrZrO3-0.03Li2O ceramics with a larger grain size (510 nm) are achieved. The realization of the combination of excellent optical and electronic properties in this material might provide useful clues for the future design of KNN-based multifunctional ceramics.
Section snippets
Sample preparation
Using a conventional solid-state method, ceramics with nominal compositions of 0.93K0.5Na0.5NbO3-0.07SrZrO3-xLi2O (0.93KNN-0.07SZ-xLi2O) (x = 0, 0.03, 0.07, 0.10, 0.13) were synthetized. [8] Powders of Nb2O5 (99.99 %), Na2CO3 (99.99 %), K2CO3 (99.99 %), ZrO2 (99.95 %), SrCO3 (99.99 %) and Li2O (99.99 %) were mixed with ethanol and ball milled with zirconia balls for 24 h, then dried in an oven at 80 °C. The obtained powders were ground and then calcined at 850 °C in air for 5 h. The calcined
Results and discussion
Fig. 1(a) shows the XRD patterns of 0.93KNN-0.07SZ-xLi2O ceramics at room temperature. For x ≤ 0.07 ceramics, they exhibit pure perovskite structure. When the Li2O content is greater than 0.07, the phase K3Li2Nb5O15 with tetragonal tungsten bronze structure is detected. Fig. 1(b) gives the (200) characteristic peak of 0.93KNN-0.07SZ-xLi2O ceramics, i.e., the amplified diffraction peaks at the 2θ range from 44-48°. The shape of the diffraction peak changes significantly as the Li2O content
Conclusion
We synthesized a series of 0.93KNN-0.07SZ-xLi2O ceramics that combine noteworthy optical properties with good piezoelectric performance, and systematically researched the microstructure and phase structure origin. The best optical transparency was obtained in the ceramic of x = 0.03 with a large average grain size (∼510 nm), which exhibits dense microstructure due to abnormal grain growth and reduced light scattering by grain boundaries. The maintaining of good piezoelectric properties can be
Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
Acknowledgments
This work was supported by National Science Foundation of China (NSFC) (Grant Nos. 51607108, 51872177, 51572163, 51577111), the Fundamental Research Funds for the Central Universities (Program No. GK201903017, GK201802007 and No. GK201701011), and the Information Materials and Devices Research Center of the Shanghai Institute of Ceramics of the Chinese Academy of Sciences (SICCAS) (KLIFMD-2015-04).
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