Ali Sayir

High temperature properties of BiScO3–PbTiO3 piezoelectric ceramics

Smart actuators and intelligent structures are sought after for aeronautical applications. As a result of high Curie temperature (430 °C) and piezoelectric coefficient (>200 pC/N), BiScO3–PbTiO3 (BS-PT) ceramics are prospective materials for high temperature actuators. This paper reports on the temperature dependent electrical, ferroelectric, and electromechanical properties of liquid phase sintered BS-PT ceramics. Compared to solid state sintered BS-PT, liquid phase sintered BS-PT with Bi2O3 showed improved electrical performance: (1) threefold reduction in loss tangent at elevated temperatures, (2) fivefold increase in dc resistivity at high electrical fields, and (3) 15% increase in high field piezoelectric coefficient. Hysteresis loops of the highly resistive ceramics were saturated and showed no major dependence on the magnitude and the frequency of the applied field. BS-PT ceramics exhibit depoling behavior at temperatures below (>350 °C) the Curie temperature (430 °C). Liquid phase sintering using ...

High temperature creep deformation of directionally solidified Al2O3/Er3Al5O12

Abstract The microstructure of directionally solidified Al 2 O 3 /Er 3 Al 5 O 12 (19.5 mol% Er 2 O 3 ) is analyzed and high temperature creep deformation studied using fibers in tension between 1400° C and 1550° C. The directionally solidified Al 2 O 3 /Er 3 Al 5 O 12 system is an in situ composite and has a fine eutectic- microstructure with sub-micron phase spacing. The microstructure is elongated in the direction of growth. Transmission electron microscopy observations revealed well-bonded interfaces and scatter within the crystallographic alignment of the constituent phases. The creep resistance of the system was very high, comparable to c-axis sapphire, and failure initiated at the lamella interfaces. The influence of the different elastic and plastic behaviors of the eutectic components on creep is examined. A critical discussion on the origin of the high stress dependence of the creep rate, the existence of steady state creep, and the relevant microscopic deformation mechanisms is presented.

Elastic constants of yttria (Y2O3) monocrystals to high temperatures

The single-crystal elastic moduli of yttria have been measured by Brillouin spectroscopy up to 1200 °C. The room temperature values obtained are C11=223.7±0.6 GPa, C44=74.6±0.7 GPa, and C12=112.4±1.1 GPa. The resulting bulk and (Voigt–Reuss–Hill) shear moduli are K=149.5±1.0 GPa and GVRH=66.3±0.8 GPa, respectively. These agree much more closely with experimental values reported for polycrystalline samples than do previous single-crystal measurements. Linear least squares regressions to the variation of bulk and shear moduli with temperature result in derivatives of dK/dT=−17±2 MPa/°C and dGVRH/dT=−8±2 MPa/°C. Elastic anisotropy was found to remain essentially constant over the temperature range studied.

Elastic constants of yttria (Y{sub 2}O{sub 3}) monocrystals to high temperatures

The single-crystal elastic moduli of yttria have been measured by Brillouin spectroscopy up to 1200 °C. The room temperature values obtained are C11=223.7±0.6 GPa, C44=74.6±0.7 GPa, and C12=112.4±1.1 GPa. The resulting bulk and (Voigt–Reuss–Hill) shear moduli are K=149.5±1.0 GPa and GVRH=66.3±0.8 GPa, respectively. These agree much more closely with experimental values reported for polycrystalline samples than do previous single-crystal measurements. Linear least squares regressions to the variation of bulk and shear moduli with temperature result in derivatives of dK/dT=−17±2 MPa/°C and dGVRH/dT=−8±2 MPa/°C. Elastic anisotropy was found to remain essentially constant over the temperature range studied.

Tensile strength and microstructure of Al2O3–ZrO2 hypo-eutectic fibers

Abstract Al 2 O 3 –ZrO 2 (Y 2 O 3 ) eutectic materials possess good fracture strengths and creep resistance. Increased Al 2 O 3 content is one means to further improve creep resistance. The objective of this study is to examine fracture strength of Al 2 O 3 -rich (hypoeutectic) compositions at varying Y 2 O 3 contents. Fibers 160–220 μm in diameter with 68 m/o Al 2 O 3 and 1.1–7.6 m/o Y 2 O 3 (30.5 to 16 m/o ZrO 2 ) were directionally solidified at 0.11 mm/s using the laser-heated float-zone process. Defect populations increased in size and severity with higher Y 2 O 3 contents. However, fibers maintained 1 GPa fracture strength in the presence of numerous pores and shrinkage cavities, which extend with crack-like morphology along the fiber axis.

High Temperature Mechanical Properties of Al2O3/ZrO2(Y2O3)Fibers

In-situ composite fibers produced by directional solidification of two phase oxide eutectics are one means of producing fibers with good strength and higher creep resistance than single crystal fibers. In this work, directionally solidified alumina-yttria stabilized zirconia eutectic fibers have been grown by the laser heated float zone (LHFZ) method at NASA Lewis. The average tensile strength of the alumina-zirconia (60.8 m/o Al 2 O 3 ; 39.2 m/o ZrO 2 (9.5 m/o Y 2 O 3 )) eutectic fibers was 1.2 GPa at room temperature. The high temperature tensile strength and creep resistance of the eutectic fiber were determined and compared to single crystal Al 2 O 3 .

Crystallographic characterization and indentation mechanical properties of LaB6-ZrB2 directionally solidified eutectics

AbstractLaB6-ZrB2 eutectics were directionally solidified by a zone melting process. The microstructure and crystallography were characterized by X-ray diffraction, optical microscopy and transmission electron microscopy. The anisotropies of hardness and indentation fracture toughness were assessed by making measurements along different crystallographic directions on both transverse and longitudinal sections. While the hardness did not vary appreciably between the two orientations, the fracture toughness was observed to be highly anisotropic. Toughening behavior was observed on longitudinal sections with crack deflection and bridging mechanisms apparent. Cracks were difficult to initiate on transverse sections along directions perpendicular to the growth direction, but for 20 N loads an indentation fracture toughness of 11 MPa

Creep resistance of directionally solidified ceramic eutectics of Al203/c-Zr02 with sub-micron columnar morphologies

\sqrt m