Fabrication of transparent lead-free KNN glass ceramics by incorporation method
© Yongsiri et al; licensee Springer. 2012
Received: 21 September 2011
Accepted: 16 February 2012
Published: 16 February 2012
The incorporation method was employed to produce potassium sodium niobate [KNN] (K0.5Na0.5NbO3) glass ceramics from the KNN-SiO2 system. This incorporation method combines a simple mixed-oxide technique for producing KNN powder and a conventional melt-quenching technique to form the resulting glass. KNN was calcined at 800° C and subsequently mixed with SiO2 in the KNN:SiO2 ratio of 75:25 (mol%). The successfully produced optically transparent glass was then subjected to a heat treatment schedule at temperatures ranging from 525° C -575° C for crystallization. All glass ceramics of more than 40% transmittance crystallized into KNN nanocrystals that were rectangular in shape and dispersed well throughout the glass matrix. The crystal size and crystallinity were found to increase with increasing heat treatment temperature, which in turn plays an important role in controlling the properties of the glass ceramics, including physical, optical, and dielectric properties. The transparency of the glass samples decreased with increasing crystal size. The maximum room temperature dielectric constant (ε r ) was as high as 474 at 10 kHz with an acceptable low loss (tanδ) around 0.02 at 10 kHz.
Potassium sodium niobate [KNN] (K0.5Na0.5NbO3) which has a complex perovskite structure was first reported in 1960 by Egerton and Dillon . It also has a high curie temperature of 420° C, piezoelectric constant (d33) of 80 pC/N, and coupling factor coefficient (kp) of 0.35. The crystal structure of KNN is dependent on the temperature , where an increase from room temperature to 200° C causes an orthorhombic-tetragonal phase transformation, and when the temperature is higher than 420° C, the tetragonal phase changes to a cubic phase or becomes paraelectric.
The phase diagram of (1-x) KNbO3-xNaNbO3 by Jaffe et al.  shows that the morphotropic phase boundary of this system occurs at an × approximately 0.5, at which the two orthorhombic phases separate. The as-fired ceramics from this system were found to posses the maximum piezoelectric value even though it is still far from that of the lead-based materials, such as PZT. KNN has recently been subjected to intensive studies as a promising lead-free ferroelectric to replace the toxic PZT.
Ferroelectric glass ceramics were first developed in order to combine the electrical properties of ferroelectric crystals and the transparency of a glass matrix, which makes them suitable for electro-optic applications especially electronic parts, such as electro-optical, high-power lasers, optical integrated circuits, adaptive optics, optical resonator, microwave, and pyroelectric devices [4, 5]. KNN ferroelectric glass ceramics have been investigated and attracted much attention since the 1970s . Many research projects have reported that the main problem producing KNN glass ceramics concerns the difficulty in generating the crystallization of the glass ceramics with a pure KNN phase. A secondary phase always occurs in the heat-treated samples.
In this work, the incorporation method was integrated into the glass-ceramic fabrication process. This method modifies the production process by aiming to crystallize only the KNN single phase and reduce the chance of any unwanted second phase which frequently occurred in the conventional method. In this method, starting powders of simple oxides were mixed to form glass batches which could then be subjected to a heat treatment schedule for crystallization, as described in the report of Prapitpongwanich et al. . They also reported that a glass ceramic containing a single LiNbO3 phase was achieved in the LiNbO3-SiO2 system when using the incorporation method. In addition, they were able to make nanocrystals of LiNbO3 with improved dielectric property and higher transparency.
In this work, KNN powders were first prepared by calcination, then mixed with SiO2 in a Pt crucible, and melted at a suitable temperature. The quench and heat treatment processes were then followed by the crystallization of the KNN crystals, respectively. Here, we report on the physical and electrical properties of the prepared KNN glass ceramics generated using silicate glass modified via the incorporation method. Phase identification, thermal analysis, and microstructures of the prepared glass and glass ceramics were also investigated by X-ray diffraction [XRD], differential thermal analysis [DTA], and scanning electron microscopy [SEM], respectively.
Materials and methods
Results and discussion
The resulting glass product was light yellow transparent and mechanically robust. To prepare the KNN glass ceramics, many past research projects have used less than 30 mol% of SiO2 because it produces a suitable ratio to exhibit transparent regions, therefore, 25 mol% SiO2 was chosen to prepare the glass in this work according to Yongsiri et al.  However, it has also been reported that the KNN glass with a low content of SiO2 possessed low mechanical strength.
Considering previous works of Petrovskii et al. and Zhilin et al. [11, 12] which studied the phase separation and crystallization in glasses of the Na2O-K2O-Nb2O5-SiO2 system using the conventional glass-ceramic method, the high temperature of about 700°C was used to precipitate a high-niobate phase which caused small angle light scattering. By using the incorporation method, a lower temperature of 525°C could be used to promote crystallization without any trace of a secondary phase.
The overall dielectric loss values of all glass ceramics were low, between 0.014 and 0.023 depending on the frequency. This KNN glass ceramic is a promising lead-free ferroelectric glass ceramic which may be applied to many applications replacing other materials, such as LiNbO3 and BaTiO3 glass ceramics which have lower dielectric constants. However, further investigation into such properties as the nonlinear optical effects should also be carried out.
This research shows that heat treatment temperature plays a significant role in controlling the microstructure, crystallite sizes, and crystal quantity of the glass ceramics. Highly transparent KNN glass ceramics can be obtained using the incorporation method with a low heat treatment temperature of 525°C-550°C. The dielectric properties of these glass ceramics are improved, while the transparency value dropped with an increase in the heat treatment temperature. The maximum dielectric constant obtained for these samples was 474 with a low loss (tanδ) of 0.02 from the glass-ceramic sample heat treated at 550°C, however, this sample also had the lowest transparency. The optimum values of the dielectric properties in this work promise a bright future for this KNN glass ceramic in electro-optical applications.
The authors would like to express their sincere gratitude to the Thailand Research Fund, National Metal and Materials Technology Center, and Faculty of Science, Chiang Mai University for financial support. We wish to thank the Graduate School Chiang Mai University and The National Research University Project under Thailand's Office of the Higher Education Commission for financial support. P. Yongsiri would like to thank the Thailand Graduate Institute of Science and Technology for her scholarship.
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