Optical Properties of Nano-Glass Ceramics

It is believed that the most important properties of nano-glass ceramics are their optical properties. In this chapter, the structure, processing, properties, and application of the most important nano-glass ceramics are thoroughly discussed, with an emphasis on the experimental and practical aspects of the subject. The luminescent glass ceramics, which have attracted considerable attention, in recent years, such as mullite and spinel glass ceramics doped with transition metal ions, and especially oxyfluoride glass ceramics, containing rare-earth-doped fluoride nano-crystals, have been given extensive coverage. Structure properties and application of more conventional low thermal expansion glass ceramics, such as the stuffed β-quartzss nano-glass ceramics, and the most recent applications of nano-glass ceramics such as transparent YAG glass ceramics, have also been discussed.

Keywords

Optical glass ceramics; mullite glass ceramics; spinel glass ceramics; passive Q-switchers; oxyfluoride glass ceramics; luminescence; up-conversion; down-conversion; YAG glass ceramics

Chapter Outline

The most important optical property of nano-glass ceramics is their transparency, i.e., the ability to transmit light (electromagnetic waves), in certain range of wavelengths according to their specific application. The transparency, however, is not the sole requirement to be fulfilled by these glass ceramics as promising candidates for various current applications, or potential applications in the near future. The great attention that has been attracted in recent years by nano-glass ceramic is mainly because of their ability to combine transparency with other desired properties, such as mechanical, thermal, chemical, and electromagnetic.

2.1 Theoretical Background of Transparency

There are two main mechanisms that may hinder the travel of light through a glass ceramic, which are as follows:

Effect of the first mechanism, the light scattering, which is the far more effective obstacle for the transmission of light through glass ceramics, can be minimized by (a) trying to achieve closely matched indexes of refraction between the two (or more) phases existing in the glass ceramic and low birefringence in the crystals or (b) by reducing the size of the dispersed crystalline particles to much smaller sizes than the wavelength of the incident light (Beall and Pinckney, 1999).

MgZn stuffed β-quartz solid solution is an example for the criterion (a) in which, despite crystal sizes of up to 10 μm, good transparency can be achieved, whereas the nano-glass ceramics should satisfy the second criterion (small crystallite size).

Among several scattering theories, there are two theories which could better describe the mechanism of scattering in nano-glass ceramics. The first theory, known as Rayleigh–Gans model (Kerker, 1969) assumes the existence of widely separated independent scatterers in a glass matrix. In this case, σp, the total turbidity or attenuation due to scattering, is given as p≈(2/3)NVk4a3(nΔn)2, where N is the particle number density, V the particle volume, a the particle radius, =(2π/λ) (where λ is the wavelength), n the refractive index of the crystal, and Δn the index difference between the crystal and the matrix. For practical purposes, transparency is achieved here with particle radii of <15 nm and a refractive index difference of <0.1 between the glass and the dispersed crystals (Beall and Pinckney, 1999).

The other scattering model assumes the existence of small particles that are more closely spaced; the distance between particles should be no smaller than the particle radius but can be up to 6 times the particle radius. In this condition, the turbidity is given by the equation as developed by Hopper (1985):

c≈[(23×10−3)k4θ3](nΔn)2

where =[a+(W/2)] is the mean phase width, in which a and W are the particle radius and the inter-particle spacing, respectively. In this case, improved transparency is allowed with particle sizes <30 nm at larger refractive index differences, up to Δn=0.3 (Beall and Pinckney, 1999).

2.2 Application of Optical Nano-Glass Ceramics

As stated above, nano-glass ceramics as good candidates for various diversified applications are expected to have the capability of combining good transparency with other desired properties regarding the given application. One of the classification methods of these materials is according to the main characteristics (other than transparency) that determine their application. In this way, the transparent nano-glass ceramics can be classified into several groups.

In this chapter, the processing, properties, and application of some of the most important transparent nano-glass ceramics are discussed.

2.2.1 Low Thermal Expansion Glass Ceramics

These glass ceramics usually combine the transparency with low thermal expansion and high mechanical strength. Originally developed for use in the high-precision optical applications such as telescope mirror blanks, these glass ceramics have become known and entered the domestic market in applications such as cooker tops, cookware, and as reflectors for digital projectors. The most important materials of this group are stuffed β-quartzss glass ceramics, the detailed description of the structure and processing of which is discussed in the following sections.

2.2.1.1 Structure, Properties, and Application of Stuffed β-Quartzss Glass Ceramics

Buerger (1954) was the first investigator who recognized that certain aluminosilicate crystals, composed of three-dimensional networks of SiO4 and A1O4 tetrahedra, are similar in structure to crystalline forms of silica. These so-called stuffed derivatives of silica polymorphs may be imagined of as derived from silica networks by replacement of Si4+ by Al3+, accompanied by the filling of structural vacancies by monovalent cations.

β-Eucryptite, LiA1SiO4, and β-spodumene LiA1Si2O6, which are respectively similar to β-quartz and keatite structures, are such stuffed derivatives.

Monovalent Li+, divalent Mg2+, and, to a smaller extent, divalent Zn2+ ions can also randomly fill the interstitial vacancies in the β-quartz structure when Al3+ replaces Si4+ producing stuffed β-quartz solid solutions (β-quartzss).

The aforementioned materials are well known for their very low thermal expansion coefficients over considerable temperature intervals. The two unique properties of these glass ceramics, the ultralow thermal expansion and the ability to be polished similar to a glass, allow these transparent glass ceramics to be very suitable for some optical applications such as mirror blank materials.

For such uses, the thermal expansion of the material is very important, since any change in ambient temperature in the neighborhood of the mirror during use may result in a change of focus and lost time on the telescope.

On the other hand, since most of the ceramic materials have low thermal conductivities, there is the possibility of large thermal gradient buildup during the dissipation of frictional heat. In order to prevent the distortion of glass bodies, often a long and tedious procedure should be used during the finishing process. The use of ultralow expansion materials would help in minimizing the distortion problems and accelerating the finishing process (Duke and Chase, 1968).

As stated previously, the very fine microstructure of these transparent glass ceramics is also an important property. Since glass ceramics are polyphase crystalline assemblages, with significant residual glass, polishing problems could arise due to differential hardness between the phases. It was found that when the grain size was kept smaller than the wavelength of visible light, an optical finish, similar to that possible with glasses, could be obtained with no evidence of relief polishing (Duke and Chase, 1968).

Their transparent nature also allows inspection of the mirror blanks for residual stress and...