Double layer thermal barrier coatings (TBCs) consisting of a Gd2Zr2O7 (GZO) top and an ytrria stabilized zirconia (YSZ) interlayer have been tested in a burner rig facility and the results compared ...to the ones of conventional YSZ single layers. In order to gain insight in the high temperature capability of the alternative TBC material, high surface temperatures of up to 1550 °C have been chosen while keeping the bond coat temperature similar. It turned out that the performance of all systems is largely depending on the microstructure of the coatings especially reduced porosity levels of GZO being detrimental. In addition, it was more difficult in GZO than in YSZ coatings to obtain highly porous and still properly bonded microstructures. Another finding was the reduced lifetime with increasing surface temperatures, the amount of reduction is depending on the investigated system. The reasons for this behavior are analyzed and discussed in detail.
Uniaxial viscous Poisson's ratio, bulk and shear viscosities were measured by discontinuous sinter-forging without and with alternating current electric fields under the same conditions, i.e. by ...keeping the sample temperature constant. This precaution enables the exclusion of any macroscopic Joule heating. Major findings are: (i) viscous Poisson's ratio increases with the relative density, with and without electric field; (ii) viscous Poisson's ratio increases with the presence of electrical fields; (iii) changes in the viscous Poisson's ratio can be correlated to a clear decrease in shear viscosity, which can be attributed to an easier grain boundary sliding under electric field.
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The influence of sample size on microstructure, density, and onset temperature of the flash event was investigated for 3 mol% yttria-stabilised zirconia (3YSZ). Pellet samples with different height ...and constant diameter were flash sintered under an AC electric field. The larger the samples were, the lower was their flash onset temperature. Furthermore, a more heterogeneous microstructure was verified on larger samples followed by lower density. The influence of the samples' height on the thermal runaway and the thermal gradient within the samples are discussed as responsible for these effects.
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This part is focused on the effect of surface bound water on the densification behavior and defect stoichiometry of zinc oxide. The second part doi: 10.1016/j.jeurceramsoc.2015.12.008 concentrates on ...the effect of hydroxide complexions on the microstructural development, texture formation and anisotropic grain morphology. Nanocrystalline zinc oxide powder was humidified or dried followed by quick heating (100K/min) with field-assisted sintering technique/spark plasma sintering (FAST/SPS). Densification is strongly enhanced due to hydroxide-ion-diffusion mechanism, which shows species with lower valence and ionic radius in comparison to oxygen ions. The lowered activation energy for densification exhibits no impact of the sintering electric current on this enhanced densification behavior. The defect stoichiometry and structure of sintered zinc oxide was analyzed by several spectroscopic methods, indicating the formation of hydrogen-related defects for sintering in presence of bound water, while no hydrogen was detected for sintering of dried powder.
Constrained sintering: A delicate balance of scales Green, David J.; Guillon, Olivier; Rödel, Jürgen
Journal of the European Ceramic Society,
2008, 2008-1-00, 20080101, Letnik:
28, Številka:
7
Journal Article, Conference Proceeding
Recenzirano
Sintering is a common technique for the production of ceramic, metallic and composite bodies and components. During this process it is known that internal stresses can arise from external constraint ...or from differential densification. These stresses can give rise to significant strain rate changes, distortions and damage in the fired parts. For quality control of sintered bodies, it is necessary to measure or predict the changes in deformation behaviour that occurs as the porous starting bodies undergo densification. Contributions of the continuum mechanical approach to this problem of constrained sintering are reviewed. Theoretical predictions as well as experimental techniques and results are detailed. Special emphasis is given to the development of anisotropy that can develop in the microstructure of a sintering body in response to the internal stresses.
Garnet-based Li
7
La
3
Zr
2
O
12
(LLZO) is considered one of the most promising oxide-ceramic solid electrolyte materials for inorganic all-solid-state batteries. Dopants and substituents like Al, ...Ta, Nb, Ga, and W were shown to have a high impact on the total ionic conductivity, increasing it from 10
−6
S/cm up to 10
−3
S/cm. However, natural zirconium sources always contain a small amount of hafnium which could also act as dopant, but the separation of these two elements is complicated and expensive. In this work, we investigate the influence of various Hf-impurity concentrations on the performance of tantalum-doped LLZO. We synthesised Li
6.45
Al
0.05
La
3
Zr
1.6−
x
Hf
x
Ta
0.4
O
12
(LLZHO with
x
= 0 – 1.6) via conventional solid-state synthesis and have demonstrated that up to
x
= 0.1, hafnium impurities do not have a significant impact on the performance of the material. Above this concentration, the Li-ion conductivity is steadily reduced to around 70% when zirconium is fully substituted by hafnium resulting in Li
6.45
Al
0.05
La
3
Hf
1.6
Ta
0.4
O
12
. As the purity of Zr precursors has a great impact on their price, these findings can help to reduce the price of LLZO in general, as lower grade zirconium can be used in industrial scale applications.
We report on state-of-the-art JÜLICH (Forschungszentrum Jülich) stack with anode-supported solid oxide fuel cells (AS-SOFCs) that have been tested in bio-syngas derived from wood pellets. The sulfur ...and chlorine were removed after gasification, but the tars were not reformed in the bio-syngas to study the influence of these tars on the degradation of SOFC stack. The total tar content during test was 3.5 g/Nm3 including benzene, toluene, phenol, m-cresol, naphthalene, and minor traces of undefined tars. The test result shows considerable performance degradation in tar-contaminated syngas. Moreover, the test was stopped after 5 hours of operation due to an increase of the pressure drop in the stack. A post-test analysis was carried out, and heavy carbon deposition was found at the cell anode-support surface and the Ni mesh current collector. Carbon was identified by SEM as numerous carbon fibers. The change of support microstructure was also observed near and under the carbon deposition area, and the dusting of Ni metal was observed in the support and Ni mesh current collector.
The development of high-performance Li 7 La 3 Zr 2 O 12 (LLZO)-based all-solid-state lithium batteries (SSLB) is usually hampered by highly resistive interfaces due to the need for sintering at ...elevated temperatures to form ionic diffusion paths through the grains. Many strategies have been proposed to solve the problem but the achievements have been limited. Herein, a new design principle is introduced, based on co-sintering crystalline LCO and Ta-substituted LLZO instead of using the more reactive Li–Co–O precursors and Al-substituted LLZO, which allows the fabrication of high specific areal density and low cell area resistance without the interface modification of LLZO-based SSLB. Detailed studies using micro-Raman and EDS mapping revealed that the well-sintered interfaces are free from detrimental secondary phases. To demonstrate that a true bulk-type SSLB can be constructed by this straightforward strategy, the material loading for a composite positive electrode was increased to about 10 times that in previous reports, which resulted in a high areal capacity of 1.63 mA h cm −2 ( i.e. 110 mA h g −1 ) when discharged with a current density of 50 μA cm −2 . It also allows one to discharge the fabricated SSLB at a very high current density of 500 μA cm −2 at 50 °C due to the minimized cell areal resistance. The new fabrication strategy for the LLZO-based SSLB paves the way for achieving SSLB with high safety and energy density.
Anode-supported solid oxide fuel cells (SOFCs) with a state-of-the-art Ni/YSZ anode have been tested in simulated bio-syngas with controlled addition of phenol as a model molecule to study the ...influence of tars on the degradation of SOFCs operated with gasified biomass. The post-test analysis results of SOFCs are described after operation with different concentrations of phenol. The tests with pure syngas and up to 2 g/Nm3 of phenol show a relatively stable performance in a short-term period of 500 h, but the test with 8 g/Nm3 phenol shows drastic degradation. The microstructural changes of anode and support layers, phase changes, and carbon deposition were analyzed and discussed based on performance degradation and post-test analysis. No structural changes were found after tests with pure syngas. On the other hand, the addition of phenol causes macro- and micro-scale structural changes in the support, spreading from the fuel inlet. The support shows an erosion pattern and both Ni and YSZ were found as dust after the test. In these eroded areas, carbon fibers were observed by SEM and it was more pronounced with higher phenol content. There was no material phase transformation related to syngas or phenol, but surface carbon deposition was confirmed by Raman spectroscopy in the support and anode layers.
•Phenol in simulated bio-syngas leads to severe damage to the Ni/YSZ anode support.•Phenol leads to coking and carbon formation in spite of a sufficiently high steam content.•The electrochemical performance of the cell degrades with high phenol concentrations.
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