Visualisation and measurement of a laminar boundary layer on the surface of an isothermal section-triangular roof

Natural convection in air over a heated section-triangular roof with a fixed aspect ratio of 0.1 is experimentally investigated. The development of the flow over the roof subject to a range of temperatures is measured by digital interferometry and thermocouples. The experiments present distinct images of the thermal boundary layer, which changes from a quasi-steady to an unsteady state as the surface temperature of the triangular roof increases. Contrary to numerical simulations previously published, the observed flow becomes unsteady, which is very likely influenced by uncontrolled perturbations at the critical Rayleigh number where a pitchfork bifurcation of a steady flow is theoretically expected.

Phase Equilibrium in ZrO2-CeO2-Eu2O3 System at a Temperature of 1500 °C

The phase equilibria and structural transformations in the ternary ZrO2-CeO2-Eu2O3 system at 1500 °C were studied by X-ray diffraction and scanning electron microscopy in the overall concentration range. The system was found to constitute fields of solid solutions based on the tetragonal (Т) modification of ZrО2, cubic (С) and monoclinic (B) modifications of Eu2O3, cubic with a fluorite-type structure (F) modifications of СеО2 (ZrО2), and ordered intermediate phase with a pyrochlore-type structure of Eu2Zr2O7 (Py). The refined lattice parameters of the unit cells corresponding to the solid solutions and microstructures of the definite field of compositions for the systems were determined. The peculiarity of the isothermal section of the phase diagram in the ZrO2-СеO2-Eu2O3 system at 1500 °С is the formation of phase equilibria on the basis of the fluorite solid solutions of ZrO2(CeO2) along with other components. There are at least three homogeneous fields of cubic phases. The isothermal section of the ZrO2-CeO2-Eu2O3 system at 1500 °С was constituted of four three-phase regions (C-Eu2O3 + F-CeO2+Py, C-Eu2O3 + F-ZrO2+Py, Py + F-ZrO2 + T-ZrO2, Py + F-CeO2 + T-ZrO2).

Phase equilibria in the Gd–Cr–Ge system at 1070 K

The isothermal section of the phase diagram of the Gd–Cr–Ge ternary system was constructed at 1070 K over the whole concentration range using X-ray diffractometry, metallography and electron microprobe (EPM) analysis. Three ternary compounds are realized in the Gd–Cr–Ge system at the temperature of annealing: Gd117Cr52Ge112 (Tb117Fe52Ge112 structure type, space group Fm-3m, Pearson symbol cF1124, a = 2.8971(6) nm), GdCr6Ge6 (SmMn6Sn6 structure type, space group P6/mmm, Pearson symbol hP16, a = 0.51797(2), c = 0.82901(4) nm) and GdCr1-хGe2 (CeNiSi2 structure type, space group Cmcm, Pearson symbol oS16, a = 0.41569(1)-0.41593(8), b = 1.60895(6)-1.60738(3), c = 0.40318(1)-0.40305(8) nm). For the GdCr1-xGe2 compound the homogeneity range was determined (x=0.73 – 0,69).

Interaction of Components in the Lu-Ag-Si System at 500 ºC

Isothermal section of the Lu-Ag-Si system at 500ºC was studied by means of X-ray powder diffraction, microstructure and EDX-analyses in the whole concentration range. The existence of earlier reported binary compounds LuAg4, LuAg2, LuAg and LuSi2, LuSi, Lu5Si3, Lu5Si4 was confirmed. New binary compound Lu3Si5 (own str. type) was found. Almost none of the binary silicides dissolve more than 5 at.% of third component. The exception is the existence of the substitution type solid solutions based on LuAg2 (MoSi2-type structure), which dissolves up to 20 at.% Si, as well as on Lu5Si3 (Mn5Si3-type structure), which dissolves up to 15 at.% Ag. The crystal structure of the LuSi compound was redetermined by X-ray single crystal diffraction (TlI-type, space group Cmcm, a = 4.1493(3), b = 10.2641(7), c = 3.7518(2) Å, R = 0.0173, wR = 0.0415 for 173 independent reflections). No ternary compound is observed in the Lu-Ag-Si system.

ISOTHERMAL SECTION STRUCTURE THE ZrO2-La2O3-Gd2O3 SYSTEM AT 1500 °С

Using the methods of physicochemical analysis (XRD, petrography, scanning electron microscopy analyses) phase equilibria were firstly investigated in the ternary system ZrO2–La2O3–Gd2O3 system at 1500 ºС. It was established that in the system there exist fields of solid solutions based on hexagonal (A) modification of La2O3 and cubic with fluorite-type structure (F) and tetragonal (Т) modification ZrО2 , cubic (С) and monoclinic (M) modification Gd2O3 and ordered intermediate phase with pyrochlore-type structure lanthanum zirconate La2Zr2O7 (Py). No new phases were found. The refined lattice parameters of the unit cells for solid solutions for the systems were determined. In the zirconia-rich corner, the solid solutions based on tetragonal modification of ZrO2 are formed. The phase field T-ZrO2 is narrow and elongated (0–18 mol% CeO2) along the ZrO2–CeO2 side of the binary system. The solubility of La2O3 in the T-ZrO2 is low and amounts to ~ 0.5 mol%, as evidenced by XRD analysis results. It is worth noting that the solid solutions based on tetragonal modification of zirconia cannot be quenched from high temperatures due to low stability of T-ZrO2 under cooling with furnace conditions. The diffraction patterns recorded at room temperatures included the peaks of monoclinic phase M-ZrO2. The homogeneity field of solid solution based on A-La2O3 extends to 31 mol% Gd2O3 and 12 mol% ZrO2 in the corresponding binary systems and locates near the composition 6,7 mol % ZrO2–90 mol% La2O3–3.3 mol% Gd2O3 on the section La2O3–(67 mol % ZrO2–33 mol % Gd2O3). It should be noted that the samples with a higher lanthanum oxide content after annealing and cooling rapidly absorb water in humid air and become hydrated. Hence, according to XRD, the hexagonal A-La(OH)3 modification forms instead of the hexagonal A-La2O3 phase. The lattice parameters for A-La(OH)3 phase vary from а = 0.6513 nm, c = 0.3847 nm the sample containing 3.35 mol % ZrО2–95 mol % La2O3–1.65 mol % Gd2O3 to а = 0.6508 nm, c = 0.3847 nm in the two-phase sample (Py+А ) containing 6.7 mol % ZrО2–90 mol % La2O3–3.3 mol % Gd2O3 and to а = 0.6477 nm, c = 0.3725 nm in the three-phase sample (Py+F+А) containing 40.2 mol % ZrО2–40 mol % La2O3–19.8 mol % Gd2O3 The isothermal section of the ZrO2–La2O3–Gd2O3 system at 1500°C contains four three-phase regions (F+Py+A, F+B+A, F+C+B, T+F+Py) and ten two-phase regions (Py+A, A+F, A+B, F+B, B+C, C+F, F+Py, Py+T, T+F, Py+F).