Developments in Ceramic Materials Research

Optical Fluoride and Oxysulfide Ceramics: Preparation and Characterization 55
the sintering process leads to the development of two fundamental methods of the preparation
of optical polycrystalline materials.
First, is a classic ceramic preparation which consists in sintering of the previously formed
green compact at the temperature slightly below melting temperature of the material.
Sintering process realizes in vacuum or controlled atmosphere. Application of nanopowders
allows to decrease a temperature of a process for several hundred degrees.
Second way is to obtain optical ceramics from different materials by a hot pressing (HP)
assisted sintering method. This method consists of application of sintering process with both
the temperature (0.5 – 0.8 from Tmel.) and the pressure (up to 300 Mpa) that allows decrease
both the sintering temperature and the recrystallization rate. Bulk diffusion, plastic
deformation and twinning take place in this situation, i.e. mass transfer mechanisms, which
set conditions for obtaining the dense, microcrystalline and mechanically rigid and
transparent ceramics.
In last years glass ceramics is used as an optical material. In this ceramics nanosized
crystalline grains form purposefully in glass matrix. Glass ceramics can keep optical
transparency with content of crystalline phase up to 90 % [12, 13].
Preparation of laser optical ceramics is a special process, which is more complicated in
practice. For laser generation the loss factor at specific wavelength must not exceed 10 -3 – 10 -
2 cm -1 . Laser ceramics as a material has some advantages over the crystals [4, 5] such as
possibility of preparation of large samples with an enhanced content and uniform distribution
of activator, improved mechanical characteristics as well as a synthesis of laser materials
when the process of single crystal growth is difficult (decomposition under heating,
incongruent melting, phase transitions).
In present time laser ceramics development is successfully realized for the row of oxide
materials [1, 2, 14 - 18]. As for the optical ceramics for the photonics based on the other
classes of materials the level of their development decelerates, though a first ceramic fluoride
laser (СaF2:Dy 3+ ) was developed in the beginning of laser era (in the middle of 1960s) by
Kodak (USA) [19, 20].
In the chapter preparation of fluoride and oxysulfide optical ceramics by hot pressing
assisted sintering method and its properties will be considered.
FLUORIDE OPTICAL CERAMICS PREPARATION AND
INITIAL CHARACTERIZATION
Specific physicochemical properties of the fluorides such as water vapor interaction
during a heating process places a specific requirements upon the technology of ceramics
preparation and upon the equipment. Production of colorless transparent fluoride ceramics
from fluorides precursors meets with some difficulties. Fluoride ceramics with nominally
100% density prepared by hot pressing assisted sintering method every so often has a black
color [21]. Even good samples have typical defect such as grey or yellowish disseminations,
which decreases the transparency. Annealing in fluorinating atmosphere leads to higher
transparency of the samples [22]. Preparation of optical ceramics with complex chemical
composition results in a problem with optical homogeneity [23, 24].