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Ceramics, Volume 2, Issue 3 (September 2019)

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Open AccessArticle
Crack Growth Tests in Air Plasma-Sprayed Yttria Coatings for Alumina Ceramic Matrix Composites
Ceramics 2019, 2(3), 514-524; https://doi.org/10.3390/ceramics2030039
Received: 26 June 2019 / Revised: 16 July 2019 / Accepted: 1 August 2019 / Published: 19 August 2019
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Abstract
Yttria coatings for all-oxide combustor walls were tested for their crack-growth behavior. These environmental and thermal barrier Y2O3-coatings were processed by atmospheric plasma spraying (APS). The stiffness and strength were measured for as-received and aged samples that were heat [...] Read more.
Yttria coatings for all-oxide combustor walls were tested for their crack-growth behavior. These environmental and thermal barrier Y2O3-coatings were processed by atmospheric plasma spraying (APS). The stiffness and strength were measured for as-received and aged samples that were heat treated at 1000 °C, 1100 °C and 1200 °C for a 10 h dwell time. The results show a clear development with respect to the aging conditions. The changes of the modulus and the bending strength indicate that the microstructural changes are not completed, even after aging at 1200 °C for 10 h. The fracture toughness was tested for different orientations on samples aged at 1200 °C. Bending tests as well as indentation experiments were conducted. Furthermore, a bending device was designed to observe the crack-growth in situ. The device had to be rigid and is driven by a piezo stack. The crack growth resistance shows differences in the rise of the R-curves for different orientations of the crack propagation. This is in agreement with the microstructure that results from the plasma spray process. Full article
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Open AccessArticle
Enhancement of the Ionic Conductivity in Electric Field-Assisted Pressureless Sintered BITIVOX Solid Electrolytes
Ceramics 2019, 2(3), 502-513; https://doi.org/10.3390/ceramics2030038
Received: 16 June 2019 / Revised: 12 July 2019 / Accepted: 25 July 2019 / Published: 11 August 2019
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Abstract
Bi4V1.8Ti0.2O11 (BITIVOX) ceramic pellets, prepared with powders obtained by a sol gel technique, were sintered either conventionally at 800 °C/8 h or by applying an AC electric voltage, limiting the electric current through the pellets. Electric [...] Read more.
Bi4V1.8Ti0.2O11 (BITIVOX) ceramic pellets, prepared with powders obtained by a sol gel technique, were sintered either conventionally at 800 °C/8 h or by applying an AC electric voltage, limiting the electric current through the pellets. Electric voltages were applied isothermally at 700 °C and 800 °C during 5 min in the green pellet positioned in the sample holder of a dilatometer for monitoring thickness variation. The BITIVOX pellets shrank 13.6% after applying 200 V cm−1 at 800 °C and 10.4% heating to 800 °C for 8 h. Thermal analysis and X-ray diffraction of the powders were performed to evaluate the crystallization temperature and the structural phase, respectively. The electrical behavior of the sintered BITIVOX pellets was analyzed by the impedance spectroscopy technique, showing that the sample flash sintered at 800 °C/5 min had lower bulk resistivity than the sample conventionally sintered at 800 °C/8 h. The surfaces of the sintered pellets were observed in a scanning electron microscope showing similar grain sizes and pore content in all sintered samples. Full article
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Open AccessArticle
The Influence of Kinematic Conditions and Variations in Component Positioning on the Severity of Edge Loading and Wear of Ceramic-on-Ceramic Hip Bearings
Ceramics 2019, 2(3), 488-501; https://doi.org/10.3390/ceramics2030037
Received: 24 May 2019 / Revised: 5 July 2019 / Accepted: 16 July 2019 / Published: 1 August 2019
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Abstract
Dynamic separation and direct edge loading of hip replacement bearings can be caused by many factors, including implant positioning, implant design, changes in device over time, surgical variations and patient variations. Such dynamic separation and direct edge loading can lead to increased wear. [...] Read more.
Dynamic separation and direct edge loading of hip replacement bearings can be caused by many factors, including implant positioning, implant design, changes in device over time, surgical variations and patient variations. Such dynamic separation and direct edge loading can lead to increased wear. Different input kinematic conditions have been used for experimental hip simulator studies to produce clinically relevant elliptical contact wear paths between the bearings during gait. The aim of this study was to investigate the influence of input kinematics (two axes of rotation simulation conditions (without abduction/adduction) and three axes of rotation simulation conditions (with abduction/adduction and different loading profiles) and variations in component positioning (different levels of medial-lateral translational mismatch at standard and steep cup inclination angles) on the occurrence, severity of edge loading, dynamic separation and wear of size 36 mm ceramic-on-ceramic hip bearings on an electromechanical hip joint simulator. The results showed that, overall, either of the two axes or three axes input profiles were equally valid in providing a suitable preclinical testing method for assessing the occurrence and severity of edge loading and wear under edge loading conditions. In terms of component positioning, as cup inclination and medial-lateral translational mismatch increased, so did dynamic separation, axial load at the rim, severity of edge loading and wear. Full article
(This article belongs to the Special Issue Ceramics for Biomedical Applications)
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Open AccessArticle
Towards In-Situ Electron Microscopy Studies of Flash Sintering
Ceramics 2019, 2(3), 472-487; https://doi.org/10.3390/ceramics2030036
Received: 7 June 2019 / Revised: 20 June 2019 / Accepted: 8 July 2019 / Published: 25 July 2019
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Abstract
Flash sintering, a special case of electric field-assisted sintering, results in accelerated densification at lower temperatures than conventional sintering methods. However, the mechanisms remain elusive despite the wide application potential. In-situ electron microscopy studies reveal shrinkage of ZnO green bodies due to both [...] Read more.
Flash sintering, a special case of electric field-assisted sintering, results in accelerated densification at lower temperatures than conventional sintering methods. However, the mechanisms remain elusive despite the wide application potential. In-situ electron microscopy studies reveal shrinkage of ZnO green bodies due to both heating and heating/biasing but show no obvious effect of the current on the behavior. In contrast, thin epitaxial ZnO films deposited on an Al2O3 substrate undergo a clear flash event during in-situ voltage application in the TEM, providing the first observation of flash sintering of a thin film. The specimen was captured in the high conductivity state where grain boundary motion was observed. The microscopic origins of the high conductivity state could not be detected, but may have the same underlying physical origin as the high conductivity memristive state. Full article
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Open AccessArticle
Hybrid Atmosphere Processing of Lead-Free Piezoelectric Sodium Potassium Niobate-Based Ceramics
Ceramics 2019, 2(3), 460-471; https://doi.org/10.3390/ceramics2030035
Received: 20 May 2019 / Revised: 11 June 2019 / Accepted: 5 July 2019 / Published: 17 July 2019
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Abstract
K0.5Na0.5NbO3-based ceramics, a promising group of lead-free piezoelectrics, are challenging to sinter dense while avoiding alkali evaporation. This work explores hybrid atmosphere processing, a new approach where reducing atmospheres is used during heating to avoid coarsening from [...] Read more.
K0.5Na0.5NbO3-based ceramics, a promising group of lead-free piezoelectrics, are challenging to sinter dense while avoiding alkali evaporation. This work explores hybrid atmosphere processing, a new approach where reducing atmospheres is used during heating to avoid coarsening from alkali carbonates and hydroxides, and oxidizing atmospheres is used during sintering to avoid alkali evaporation. Discs of Li0.06(K0.52Na0.48)0.94Nb0.71Ta0.29O3 with 0.25 mol% Mn (KNNLTM) were sintered in air, N2, 9% H2 in N2, or 9% H2 in N2 during heating and air during sintering (hybrid atmosphere processing). The highest density was obtained by sintering in 9% H2 in N2, but resulted in high alkali loss and decomposition of the surface, followed by low piezoelectric response. However, with the hybrid H2/air processing it was possible to both avoid surface decomposition and leakage currently associated with alkali evaporation during sintering in H2, and to obtain a denser, more phase-pure and small-grained KNNLTM ceramic with a higher piezoelectric response than obtained by sintering in air or N2. Full article
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Open AccessArticle
Chemical Durability of Thermal Insulating Materials in Hall-Héroult Electrolysis Cells
Ceramics 2019, 2(3), 441-459; https://doi.org/10.3390/ceramics2030034
Received: 20 May 2019 / Revised: 16 June 2019 / Accepted: 28 June 2019 / Published: 8 July 2019
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Abstract
The most common thermal insulating materials used in the cathode lining in aluminum electrolysis cells are Moler (diatomaceous earth), calcium silicate, or vermiculite based materials. The thermal insulation layer is critical for the overall thermal stability of the cell and is vulnerable to [...] Read more.
The most common thermal insulating materials used in the cathode lining in aluminum electrolysis cells are Moler (diatomaceous earth), calcium silicate, or vermiculite based materials. The thermal insulation layer is critical for the overall thermal stability of the cell and is vulnerable to volatile species, such as sodium vapor, that may penetrate through the carbon cathode and refractory layer. Here, we present an investigation of the chemical degradation of typical thermal insulating materials by exposure to sodium vapor in a laboratory test. Changes in microstructure and chemical and mineralogical composition of the exposed materials were characterized by electronic microscopy and powder X-ray diffraction. The materials possess different reaction patterns, ranging from deformation by creep to formation of a glassy layer reducing further sodium penetration. The results from the laboratory test were compared with chemical reactions with sodium predicted by computational thermodynamics and discussed with respect to relevant ternary phase diagrams. Full article
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Open AccessArticle
Static Fatigue of SiC Multifilament Tows at Temperatures up to 1200 °C in Air
Ceramics 2019, 2(3), 426-440; https://doi.org/10.3390/ceramics2030033
Received: 5 May 2019 / Revised: 23 May 2019 / Accepted: 25 June 2019 / Published: 1 July 2019
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Abstract
SiC-based fibers are sensitive to delayed failure under constant load at high temperatures in air. Static fatigue at intermediate temperatures < 800 °C was attributed to slow crack growth from flaws located at the surface of fibers, driven by the oxidation of free [...] Read more.
SiC-based fibers are sensitive to delayed failure under constant load at high temperatures in air. Static fatigue at intermediate temperatures < 800 °C was attributed to slow crack growth from flaws located at the surface of fibers, driven by the oxidation of free carbon at grain boundaries. The present paper examines the static fatigue behavior of SiC-based Hi-Nicalon fibers at high temperatures up to 1200 °C and Hi Nicalon S fibers at intermediate temperatures (500–800 °C). The degradation of stress- rupture time relation of multifilament tows with increasing temperature was investigated. Predictions of tow lifetime based on critical filament-based model of tow failure were compared to experimental stress-rupture time diagrams. Critical filaments are characterized by strength–probability relation. The critical filament-based model was found to describe satisfactorily the static fatigue behavior of fiber tows at these temperatures. The influence of various factors on lifetime as well as the origins of variability is analyzed. Full article
(This article belongs to the Special Issue Damage and Lifetime of Ceramic Matrix Composites)
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Ceramics EISSN 2571-6131 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
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