Developments in Ceramic Materials Research

Optical Fluoride and Oxysulfide Ceramics: Preparation and Characterization 61
When employing the fluoride ceramics as an active laser media the doping of the matrix
is supposed primarily with the rare – earth activator ions. The first experiments show that a
mixture of powders is unacceptable for preparation of solid solution as an optical ceramics.
Distribution of cations in the Ca0,9Er0,1F2,1 ceramic sample prepared by hot pressing method
of a mixture of powders is presented in Figure 6. It is seen that conditions of the experiment
are not suitable for the diffusion homogenization of the components. The erbium
concentration in points 1 and 2 in Figure 6 differs 30 times. The variations of the components
of solid solution lead to the variation of the refraction index. This leads to the optical
inhomogeneity of the ceramic sample and makes it unsuitable for the use in photonics.
Figure 6. Microphotographs of the Ca 1-xEr xF 2+x (x=0,1) ceramics prepared from the CaF 2 : ErF 3 mixture
in the rays of Er. Shoot made at “Camebax”.
For the preparation of the precursors for optical fluoride ceramics the co-precipitation
method of fluorides of the alkaline – earth and rare - earth elements from the water solution is
provided [31]. With the reverse dropping of the solution of the mixture of nitrates into the
hydrofluoric acid it is managed by trial and error to select the conditions for co-precipitation,
washing, and drying. This leads to the preparation of the raw materials with uniform chemical
composition (see Figure 7).
The ceramics samples of the Ca1-xRxF2-x solid solutions prepared are presented in Figures
8 and 9. The microhardness H and fracture toughness K1C of the ceramic samples and
comparison with single crystals is provided by micro-indentation method [30]. The ceramic
samples are cut by the diamond saw, and then they are ground and mechanically polished.
The mechanical tests of single crystals are performed at cleaved [111] facets. The strain is
induced at room temperature by the diamond Vickers pyramid. Indentation is preformed
using a microhardness meter PMT-3. For the scratch-hardness tests, we use a Martens
sclerometer. The indentation load (P = 0.5 -1 H) is chosen so, as to reliably observe not only
the impressions themselves but also radial cracks outgoing from them.