M. A. Borik

Synthesis of Refractory Materials by Skull Melting Technique

This chapter discusses methods of growing refractory oxide single crystals and synthesis of refractory glasses by skull melting technique in a cold crucible. It shows the advantages of radiofrequency (RF) heating of dielectric materials in a cold crucible and points out some specific problems regarding the process of growing crystals by directional crystallization from the melt and by pulling on a seed from the melt. The distinctive features of the method of directional crystallization from the melt are discussed in detail on the example of technology of materials based on zirconia, i.e., cubic single crystals and partly stabilized single crystals. It is shown that the size and quality of crystals are functions of the process conditions, such as thermal conditions under crystallization, growth rate, and chemical composition. We provide an overview of research on the structure, phase composition, and physicochemical properties of crystals based on zirconia. The optical, mechanical, and electric properties of these crystals make them suitable for a number of technical and industrial applications in optics, electronics, materials processing, and medicine. In this chapter, we also consider some problems regarding the synthesis of refractory glasses by skull melting technique. The physicochemical and optical properties of glasses are given and their practical applications in technology are discussed. We note that one of the better developed and most promising applications of skull melting technique is the immobilization of liquid and solid waste (also radioactive waste) into solid-state materials by vitrification.

Structure and mechanical properties of crystals of partially stabilized zirconia after thermal treatment

The phase composition and morphology of the twin structure of the Y2O3-stabilized zirconia crystals (from 2.8 to 4.0 mol %) after the thermal treatment at 1600°C have been investigated by X-ray diffractometry and transmission electron microscopy. It is shown that as the concentration of the stabilizing Y2O3 impurity increases, the character of the twin structure changes, and the amount of the untransformed phase t′ increases. The dependence of the hardness and crack resistance of the crystals of partially stabilized zirconia on the Y2O3 concentration and the indenter orientation is investigated using the microindentation method. The sample with the lowest concentration of the stabilizing Y2O3 impurity turned out the most crack resistant. This can be explained by a high content of tetragonal phase t in it, which provides the transformation strengthening mechanism of the material, and by a more multilevel character of twinning.

Structure and phase composition studies of partially stabilized zirconia

Zirconia single crystals doped with 2.8, 3.2, 3.7, and 4.0 mol % of Y2O3 have been studied. The phase composition and structure have been studied by X-ray diffraction analysis and transmission electron microscopy. It has been established that all investigated samples has two ZrO2 tetragonal phases with tetragonality c/a = 1.006–1.007 and c/a = 1.014–1.015 independent of the stabilizing impurity content. The former phase is not transformed, whereas the latter is transformed into a monoclinic one. In all cases the samples have a developed twin structure. Twinning hierarchy is observed: there are first-, second-, third-order, etc., twins, each of them containing the next-order twins inside them. Elastic stress relaxation occurs by twinning rather than by generation of dislocations. The stabilizing impurity content affects the structure nonmonotonically; the minimum dimensions of the twin lamellas refer to the Y2O3 concentration of 3.2 mol %.

Spectral-luminescence characteristics of alkali-free silicate glasses with a high concentration of terbium and tin

We discovered and investigated sensitization of the luminescence of Tb3+ ions by Sn(II) oxocomplexes in highly activated alkali-free silicate glass. We show that the quantum yield of the luminescence sensitization in this glass can be increased by UV irradiation. The possibility for the use of such glasses as luminescing light filters for YAG-Nd laser quantrons is noted.

Preparation and properties of Y2O3 partially stabilized ZrO2 crystals

We have studied the effect of composition and growth conditions on the structure and properties of 2.5–5 mol % Y2O3 partially stabilized ZrO2 crystals grown by directional solidification in a cold-wall crucible. The phase composition and density of the crystals have been determined. The crystals are shown to be uniform in composition, with local changes in Y2O3 content within ±0.5 mol %. The dimensions and quality of the single crystals are influenced by the growth conditions.

Mechanical properties of partially stabilized zirconia crystals studied by kinetic microindentation

Using kinetic microindentation, we have studied the strength and elastic characteristics of partially stabilized zirconia (PSZ) crystals. The results demonstrate that the mechanical properties of the crystals depend on yttria stabilizer concentration. Additional doping with ceria leads to an increase in the kinetic hardness, kinetic Young’s modulus, and plasticity of the crystals, thus improving their mechanical characteristics. The PSZ crystals have high plasticity for nonmetallic materials.

Thermal conductivity of single-crystal ZrO2-Y2O3 solid solutions in the temperature range 50–300 K

The thermal conductivity of ZrO2−xY2O3 single crystals (x = 0.5, 1.5, 2.0, 2.5, 3.0, 8.0 mol %) has been studied experimentally in the temperature range 50–300 K. The influence of high-temperature annealings on the thermal conductivity has been analyzed.

Spectroscopy of optical centers of Eu3+ ions in partially stabilized and stabilized zirconium crystals

The structure of the optical centers of Eu3+ ions in tetragonal (ZrO2)1–x–y(Y2O3)x(Eu2O3)y (х = 2.7–3.6; y = 0.1) and cubic (ZrO2)1–x–y(Y2O3)x(Eu2O3)y (х = 8–38; y = 0.1–0.5) crystals of solid solutions on the basis of zirconium dioxide is studied using the methods of optical and Raman-scattering spectroscopy. Characteristic optical centers of Eu3+ ions with different crystalline environments are revealed in the above compounds.

Structure, phase composition, and spectral-luminescent properties of ZrO2-Y2O3-Er2O3 crystals

This paper presents the results of investigations of the structure, phase composition, and spectral-luminescent properties of Er2O3-doped zirconia crystals with different Y2O3 concentrations: 99.7 mol % ZrO2-0.3 mol % Er2O3, 97.2 mol % ZrO2-1.0 mol % Y2O3-1.8 mol % Er2O3, 97.2 mol % ZrO2-2.0 mol % Y2O3-0.8 mol % Er2O3, 97.2 mol % ZrO2-2.5 mol % Y2O3-0.3 mol % Er2O3, 96.3 mol % ZrO2-3.4 mol % Y2O3-0.3 mol % Er2O3, and 86 mol % ZrO2-13.4 mol % Y2O3-0.6 mol % Er2O3. The transmission electron microscopy investigation has revealed the presence of twins in the 99.7 mol % ZrO2-0.3 mol % Er2O3 crystals with a monoclinic structure and in the 97.2 mol % ZrO2-2.5 mol % Y2O3-0.3 mol % Er2O3 and 96.3 mol % ZrO2-3.4 mol % Y2O3-0.3 mol % Er2O3 crystals with a tetragonal structure. No twins have been found in the 86 mol % ZrO2-13.4 mol % Y2O3-0.6 mol % Er2O3 crystals with a cubic structure. The phase composition of these crystals has been investigated using X-ray diffraction. It has been found that the 97.2 mol % ZrO2-1.0 mol % Y2O3-1.8 mol % Er2O3 crystals contain only the transformable tetragonal phase t, whereas the 97.2 mol % ZrO2-2.0 mol % Y2O3-0.8 mol % Er2O3, 97.2 mol % ZrO2-2.5 mol % Y2O3-0.3 mol % Er2O3, and 96.3 mol % ZrO2-3.4 mol % Y2O3-0.3 mol % Er2O3 crystals include two tetragonal phases with different degrees of tetragonality c/a: the transformable phase t and nontransformable phase t′. Specific features of the formation of Er3+ optical centers in zirconia crystals with different concentrations of stabilizing oxides have been revealed using the results of experiments on optical spectroscopy.

Phase composition and spectral-luminescent properties of yttrium partially stabilized zirconia crystals doped with Nd 2 O 3 and CeO 2

The phase composition and spectral-luminescent characteristics of yttrium partially stabilized zirconia (YSZ) crystals doped with Nd2O3 and CeO2 (ZrO2-Y2O3-Nd2O3-CeO2) are studied. X-ray diffraction shows that YSZ ZrO2-2.5 mol %Y2O3-0.2 mol %CeO2-0.1 mol %Nd2O3, ZrO2-2.03 mol %Y2O3-0.435 mol %CeO2-0.334 mol %Nd2O3, and ZrO2-2.03 mol %Y2O3-0.653 mol %CeO2-0.111 mol %Nd2O2 YSZ crystals contain two different tetragonal phases (transformable t and nontransformable ’t). The spectral-luminescent properties of these crystals indicate that Nd3+ ions occupy sites mainly in the transformable (t) phase.