Sol–gel preparation and characterization of nano-crystalline lithium–mica glass–ceramic

Abstract

The nano-crystalline lithium–mica glass–ceramic with separated crystallite size of 13 nm was prepared using sol–gel technique. In such a process, the structural evolutions and microstructural characteristics of the synthesized samples were investigated through X-ray diffraction, transmission electron microscopy, thermal analysis and Fourier transform infrared spectroscopy. It was found that the crystallite size of the mica obtained from sol–gel method is smaller than the one synthesized via conventional melted method. The XRD results also showed that the crystallization of mica occurred above 675 °C and it could originate from MgF₂ so that the next stage will also be the transformation from mica to norbergite and norbergite to chondrodite. The activation energy of the crystallization and Avrami factor were measured as 376.7 kJ mol⁻¹ and 2.3, respectively. It is found that the bulk crystallization could be considered as the predominant crystallization mechanism for the glass–ceramic.

Introduction

Glass–ceramics are polycrystalline ceramic materials, derived through the controlled nucleation and crystallization of glass, where the content of residual glassy phase is usually less than 50% [1], [2]. Glass–ceramics have several advantages over conventional types of powder processed ceramics. In addition to the ease of flexibility of forming the glassy state, glass–ceramics have a uniformity of microstructure and reproducibility of glass [1], [2], [3]. Of the many types of obtainable microstructures in glass–ceramics, those based on uniformly dispersed crystals <100 nm in size provide unique attributes for the current products and offer promise for many potential new applications [3].

Mica glass–ceramics are such typical machinable ceramics where the crystals of mica disperse within a glassy matrix. Apart from being machinable, the mica-based glass–ceramics exhibit heat resistance exceeding 800 °C, electrical insulating properties as well as high mechanical strength [4], [5], [6], [7], [8], [9].

Recently Taruta et al. [4], [7] successfully prepared a novel mica glass–ceramics where the separated micas are lithium–mica type with a mean crystallite size of 20 nm in which the interlayer cation is lithium ion. Thus, the novel mica glass–ceramics have potential applications not merely as mechinable ceramics and optical materials but also as lithium ion conductors, practically used in many application fields [4], [7].

In recent years, the glasses prepared through a sol–gel route are found to have advantages over conventional melted-quenching method such as: better homogeneity, higher level purity and lower stoichiometric losses [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. On the other hand, it is generally accepted that the sol–gel rout enables to provide the transparent bulk glass–ceramic which can be very useful technique in the optical devices fabrication. Since, there is a technological interest in luminescent properties of transparent lithium–mica glass–ceramic [8] their sol–gel synthesis method can open technological possibilities in the wide range of this purpose. In addition, the mica glass–ceramic prepared by the sol–gel process can possess homogeneity that is particularly necessary for the machinable glass–ceramics because an interlocked, randomly oriented uniform microstructure of mica crystals is responsible for their machinability and good strength [5], [6].

Although there are a few articles that show is possible to lithiated mica by ion exchange from different melts [22], [23], so far, no research has been reported in the area of lithium–mica sol–gel synthesis hence, the present investigation intends to prepare the nanocrystalline lithium–mica glass–ceramic applying the aqueous sol–gel process, to characterize the synthesized powders and compare it with the one provided by Taruta et al. through the conventional melted method.