LaMeAl11O19 ceramics is a kind of thermal barrier coating (TBC) material with promising application prospect due to its unique crystal structure, excellent thermodynamic properties, low thermal conductivity, and high temperature phase stability. Here, LaMeAl11O19/YSZ (Me=Mg, Cu, Zn) thermal barrier coatings were prepared by atmospheric plasma spraying (APS). Failure analysis of the coating was carried out by burner rig test and other analysis techniques. The results show that LaMgAl11O19 (LMA), LaZnAl11O19 (LZA) and LaCuAl11O19 (LCA) powders are decomposed during the plasma spraying, resulting in different contents of magnetoplumbite phase in the coatings, which may be an important factor responsible for their distinction of thermal cycling lifetimes. The LaMeAl11O19 layer is delaminated upon YSZ layer due to mismatch of thermal expansion coefficient between LaMeAl11O19 layer and YSZ layer and volume shrinkage caused by recrystallization of amorphous phase. Then the YSZ layer is exposed high temperature, accelerating sintering and TGO growth, and promoting the delamination of the YSZ layer from the bond coat. At low temperature, with the increase of the atomic number of the divalent Me2+, the thermal conductivity of the LaMeAl11O19 decreases. At high temperature, LCA coating has better infrared emissivity (0.88, 600 ℃) than both LMA and LZA, which weakens the contribution of photon conduction to thermal conductivity and leads to the reduction of thermal conductivity. Therefore, LCA coating has potential application in high temperature infrared radiation coating.
Environmental barrier coating (EBC) is essential for protection of ceramic matrix composite hot-sections in future gas turbine engines with high thrust-to-weight ratio. Rare-earth silicates, such as Yb2SiO5 and Yb2Si2O7, have been developed for potential application as EBC. However, the corrosion behaviors and mechanisms of EBC in molten salt environment such as Na2SO4 at high temperature are not clear. In this work, the Yb2SiO5/Yb2Si2O7/Si coating was prepared by vacuum plasma spraying (VPS). The molten salt (Na2SO4+25% NaCl, in mass) corrosion behaviors and mechanisms of the coating at 900 ℃ for 60-240 h were investigated. Results showed that the Yb2SiO5/Yb2Si2O7/Si coating exhibited dense structure with good bonding between the triple ceramic layers. The molten salt of Na2SO4+25% NaCl penetrated the Yb2SiO5 top layer and enriched in the Yb2Si2O7 interlayer, while the interfacial bonding between the coating and substrate still remained good after corrosion for 240 h. The Yb2SiO5 phase in the top layer exhibited good stability, while the second phase of the Yb2O3 reacted with molten salt. The content of the Yb2O3 decreased with the increase of corrosion time. The Yb2Si2O7 phase in the interlayer reacted with molten salt to form apatite phase of NaYb9Si6O26 and sodium silicate as well as volatile species such as Cl2 and SO2, which might shorten the service life of the coating. Moreover, there was almost no molten salt in the silicon bond layer, which remained intact. The Yb2SiO5/Yb2Si2O7/Si coating exhibited good resistance to molten salt corrosion.
Joining technology has become an important part of the preparation and engineering application of large or complex-shaped Cf/SiC composites. In this study, the stable joining of Cf/SiC composites was achieved via the Si-C reaction joining method by using the phenolic resin as the carbon precursor. Effects of the bulk density and pore size of porous carbon blanks on the joining properties and microstructure of the joints were investigated. Effect of the content of inert fillers on the joining properties and microstructure of joint were discussed. Bulk density and pore size of the resin-based porous carbon blank are suitable to be set in the range of 0.71-0.90 g·cm-3 and 200-600 nm, respectively. The size of free silicon increases gradually with the increase of the pore size. The flexural strength of the joined specimens can reach (125±12) MPa at pore size of 190 nm. Addition of SiC inert filler is obviously beneficial to reduce the volume shrinkage of the porous carbon blank. Flexural strength of the joined specimens reached the highest value, i.e., (216±44) MPa at the inert filler content of 50%. Overall, this study provides theoretical guidance for the stable joining of Cf/SiC composites, which has significance for realizing the preparation and engineering application of complex-shaped or large Cf/SiC composites.
Silica aerogels have wide application prospect in high temperature heat insulation due to their low density and high porosity. However, the brittleness and high cost of supercritical drying restrict their application. In this study, spongy silicone aerogels with high flexibility were prepared via Sol-Gel polymerization and atmospheric pressure drying using vinyltrimethoxysilane (VTMS) and vinylmethyldimethoxysilane (VMDMS) as precursors. The effects of precursor molar ratio on the microstructure and compressive resilience of aerogels, as well as the inorganic transformation process of aerogels in high temperature aerobic and anaerobic environments were studied. The results show that with the increase of VTMS/VMDMS ratio in the precursor, the aerogel particles become smaller and more tightly packed, and the compression resilience of aerogels also decreased. In air at 800 ℃, aerogels were transformed into inorganic SiO2 by oxidation of organic side groups, fracture and rearrangement of main chain Si-O-Si. In N2 at 800 ℃, aerogels were transformed into the mixture of inorganic SiO2 and free carbon by pyrolysis reaction, and after further treatment at 1000-1400 ℃, SiO2 and free carbon were subjected to carbothermal reduction reaction to form amorphous Si-O-C structures such as SiO4, SiCO3, SiC2O2, and SiC3O, and a small amount of β-SiC nanowires. The Si-O-C structure formed by carbothermal reduction reaction at 1200 ℃ has optimal high temperature oxidation resistance, which can provide reference for the preparation of pyro-oxidation resistant Si-O-C aerogels.
Flash sintering is a sintering technology coupled with temperature field and electric field, with characteristics of rapid mass transfer at low temperature, showing significant advantages in the synthesis of high entropy ceramics. In this study, relatively dense high entropy oxide ceramic (MgCoNiCuZn)O was synthesized by flash sintering, which properties were compared with those of conventional sintered samples. Under flash sintering condition of room temperature, the electric field intensity of 50 V/cm and the current density of 300 mA/mm2, the time of phase transformation is only 10 s. The maximum relative density of flash sintered sample is 94%, which is 22.8% higher than that of conventional sintered sample. The maximum hardness of flash sintered sample is 5.05 GPa, which is 3.95 GPa higher than that of conventional sintered sample. When the frequency is lower than 2 Hz, the dielectric constant of flash sintering sample is one order of magnitude higher than that of conventional sintered sample. The property improvement of flash sintered samples is attributed to the acceleration of mass transfer by the critical electric field to increase the material density, and the extra defects introduced by the critical electric field.
It is an effective strengthened technique to introduce a coating containing compressive stress on the surface of ceramic. In this work, the mixed slurry of alumina and quartz powder was coated on the pre-sintered alumina body, then the mullite-alumina coating with lower thermal expansion coefficient was synthesized in-situ after pressureless co-sintering. The pre-stressed strengthening of alumina was achieved by the residual compressive stress formed in the coating during the cooling process. The results indicate that, with the increase of the doping content of quartz in the coating, the flexural strength of pre-stressed alumina increases firstly and then decreases. The flexural strength of specimen realizes the highest value when the doping mass fraction of quartz is 15%, and the interface between the coating and the substrate bonds tightly. Under this condition, the flexural strength of the pre-stressed alumina ceramic is (549.44±27.2) MPa, which is 37.19% higher than that of the common alumina. When the doping mass fraction of quartz is higher than 15%, the flexural strength decreases due to the shrinkage stress mismatch in the sintering process. The effect of prestress enhancement weakens gradually with the increase of temperature. As the testing temperature reaches and exceeds 1000 ℃, pre-stressed alumina and common alumina possess approximately equal flexural strength. Pre-stressed alumina also exhibits better thermal shock resistance and damage tolerance due to the compressive stress formed in the coating
MgAgSb is a promising room temperature thermoelectric material with relatively abundant element reserves for the construction of high-performance wearable thermoelectric batteries. In this study, Mg-Ag-Sb thin films were prepared on polyimide (PI) substrates by using magnetron sputtering, and the effects of annealing conditions on their thermoelectric properties were systematically investigated. The results showed that the MgAgSb flexible thermoelectric film composed of MgO, Ag3Sb and Sb2O4 multiphases instead of pure phase, in which Ag3Sb played main role of thermoelectric function. Different annealing atmospheres significantly improved the thermoelectric properties of MgO-Ag3Sb-Sb2O4 (Mg-Ag-Sb) flexible thin films, among which the vacuum treatment appeared the best performance. Under vacuum conditions, the thermoelectric properties firstly increased and then decreased with the annealing temperature increasing, and the best thermoelectric property was achieved when the annealing temperature was 573 K, with a power factor of 74.16 μW?m-1?K-2 near room temperature. Moreover, the film exhibited good flexibility, and the conductivity changed by only 14% after 900 bending times. This study provides a reference for the preparation of MgAgSb flexible thermoelectric films and their wearable applications.
Perovskite (ABO3) piezoceramics have been developed for several decades, and there are a lot of data available. It is of great significance to find relationships between structure and properties of materials from these data. In this work, experimental data of Curie temperature (Tc) of BiFeO3-PbTiO3-BaTiO3 solid solution of perovskite piezoelectric ceramics was collected to build the model to predict the Tc. From the perspective of thermodynamics, the quadratic polynomial relationship between Tc and reduced mass was introduced but the deviation was relatively large. More descriptors (including element information, physical quantities, space groups number) and SISSO (Sure Independence Screening and Sparsifying Operator) were used for machine learning to find the correlation between Tc and components. Comparing the root mean square error (RMSE) of different descriptors and dimensions, it's found that more descriptors, more fundamental the descriptors are, and larger dimension will result in smaller RMSE to be used. Meanwhile, RMSE of the same number of descriptors in the same dimension are compared. The optimal four-dimensional model is build using six descriptors: reduced mass, the ratio of A- and B-site ion radii, the ratio of A- and B-site unfilled electrons and element contents of Ba, Pb and Bi. RMSE and maximum absolute error (MaxAE) of our model are 0.59 ℃ and 1.38 ℃, respectively. The average relative error (MRE) of external test is 1.00%. Our results indicate that SISSO machine learning based on limited samples is suitable for the predication of Tc of perovskite piezoelectric ceramics.
Photocatalytic degradation of organic pollutants from water bodies can efficiently reduce the organic pollutants in wastewater, which has broad application prospects. In this study, using CoFe1.95Sm0.05O4 as a support, the Z-scheme heterojunction Ag2S/Ag/CoFe1.95Sm0.05O4 was synthesized through a facile in situ deposition method followed by photo-reduction. Microstructure, phase structure, optical and magnetic properties of the samples were analyzed. Ag2S/Ag/CoFe1.95Sm0.05O4 composite exhibited the highest catalytic activity, which dynamic constant (k) was 1.96, 2.71 and 7.24 times higher than those of Ag2S/Ag, Ag2S and CoFe1.95Sm0.05O4, respectively. Introduction of CoFe1.95Sm0.05O4 could ef?ciently promote the separation ef?ciency of photogenerated charge carriers in Ag2S/Ag. And ?O2-and ?OH- were proved to be the main active substances in the photocatalytic process. In addition, the as-prepared Ag2S/Ag/CoFe1.95Sm0.05O4 composite could be quickly separated from the solution by an extra magnetic field after the photocatalytic reaction. Cyclic photodegradation test showed that the Ag2S/Ag/CoFe1.95Sm0.05O4 hybrid materials had the stable degradation ability and crystal structures in the photodegradation process. This research provides a useful approach to develop photocatalysts with high efficiency, narrow band gap and magnetism.
Lithium (Li) metal is one of the most attractive anode materials for the development of high energy density batteries due to its high theoretical specific capacity and low electrochemical potential. However, during the repeated deposition/stripping of Li metal anode, irregular Li dendrite growth inevitably takes place, which seriously affects the cycle life and safety of Li metal batteries. In this study, a simple and mild strategy was developed to in-situ modify the carbon nanotubes with bismuth (Bi) nanoparticles, followed by coating the as-prepared materials on the surface of commercial copper foil as current collector for Li metal anode. It is demonstrated that the in-situ modified Bi nanoparticles promotes the uniform Li deposition, thereby inhibiting the growth of Li dendrites and improving the electrochemical performance of Li metal batteries. Under the current density of 1 mA·cm-2, Coulombic efficiency of Li|Cu cell based on the Bi@CNT/Cu current collector maintains 98% after 300 cycles. Meanwhile, the symmetric cell based on the Li@Bi@CNT/Cu anode can maintain the stable cycling for 1000 h. When it is applied in LiFePO4 (LFP) full cell, the Bi@CNT/Cu current collector also exhibits excellent electrochemical performance, which can retain the stable cycling for 700 cycles at the rate of 1C (170 mA·g-1). This study provides a new strategy for suppressing dendrite growth of Li metal anodes.