Meishuan Li

In-situ hot pressing/solid-liquid reaction synthesis of bulk Cr2AlC

Abstract Single-phase bulk Cr2AlC, a non-Ti-containing carbide with superior expected properties, was successfully fabricated by in-situ hot pressing/solid-liquid reaction process. The reaction process was investigated using differential scanning calorimetry. The products were characterized using X-ray diffraction and scanning electron microscopy. Four distinguished stages of the reactions from Cr, Al, and C elemental powders were discussed. The obtained X-ray diffraction data are in good agreement with calculated ones and those from JCPDS card No. 29–0017. A new set of X-ray diffraction data comprising reflections, 2θ, and intensities of Cr2AlC is presented. Lattice parameters obtained by Rietveld XRD refinement of Cr2AlC are a = 2.858 A and c = 12.818 A, respectively. The measured Vickers hardness of the as-synthesized Cr2AlC is 5.5 ± 0.4 GPa, which is twice that of Ti2AlC. Cr2AlC displays excellent oxidation resistance. The parabolic rate constant of Cr2AlC was decreased by 3–4 orders of magnitude comp...

Atomic scale characterization of layered ternary Cr2AlC ceramic

Cr2AlC is a recently developed layered ternary carbide. In this work, the atomic scale microstructure is reported. The layer stacking sequence of Cr and Al atoms has been clearly resolved. The atomic scale characterizations were realized by means of high resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. Furthermore, electron energy loss spectroscopic analysis revealed that the Cr-C bonds in Cr2AlC are characterized by a strong sigma bonding. (c) 2006 American Institute of Physics.

Recent progress on synthesis, multi-scale structure, and properties of Y–Si–O oxides

Abstract Yttrium silicates (Y–Si–O oxides), including Y2Si2O7, Y2SiO5, and Y4·67(SiO4)3O apatite, have attracted wide attentions from material scientists and engineers, because of their extensive polymorphisms and important roles as grain boundary phases in improving the high-temperature mechanical/thermal properties of Si3N4 and SiC ceramics. Recent interest in these materials has been renewed by their potential applications as high-temperature structural ceramics, oxidation protective coatings, and environmental barrier coatings (EBCs). The salient properties of Y–Si–O oxides are strongly related to their unique chemical bonds and microstructure features. An in-depth understanding on the synthesis – multi-scale structure-property relationships of the Y–Si–O oxides will shine a light on their performance and potential applications. In this review, recent progress of the synthesis, multi-scale structures, and properties of the Y–Si–O oxides are summarised. First, various methods for the synthesis of Y–Si–O ceramics in the forms of powders, bulks, and thin films/coatings are reviewed. Then, the crystal structures, chemical bonds, and atomic microstructures of the polymorphs in the Y–Si–O system are summarised. The third section focuses on the properties of Y–Si–O oxides, involving the mechanical, thermal, dielectric, and tribological properties, their environmental stability, and their structure–property relationships. The outlook for potential applications of Y–Si–O oxides is also highlighted.

Effect of LiYO2 on the synthesis and pressureless sintering of Y2SiO5

Y2SiO5 has potential applications as a high-temperature structural ceramic and environmental/thermal barrier coating. In this work, we synthesized single-phase Y2SiO5 powders utilizing a solid-liquid reaction method with LiYO2 as an additive. The reaction path of the Y2O3/SiO2/LiYO2 mixture with variation in temperatures and the role of the LiYO2 additive on preparation process were investigated in detail. The powders obtained by this method have good sinterability. Through a pressureless sintering process, almost fully dense Y2SiO5 bulk material was achieved with a very high density of 99.7% theoretical.

Oxidation and Degradation of EB–PVD Thermal–Barrier Coatings

Thermal–barrier coatings (TBCs) consist of a magnetron-sputtered Ni–30Cr–12Al–0.3Y (wt.%) bond coat to protect the substrate superalloy from oxidation/hot corrosion and an electron-beam physical-vapor deposited (EB–PVD) 7 wt.% yttria partially stabilized zirconia (YPSZ) top coat. The thermal cyclic life of the TBC system was assessed by furnace cycling at 1050°C. The oxidation kinetics were evaluated by thermogravimetric analysis (TGA) at 900, 1000, and 1100°C for up to 100 hr. The results showed that the weight gain of the specimens at 1100°C was the smallest in the initial 20 hr, and the oxide scale formed on the sputtered Ni–Cr–Al–Y bond coat is only Al2O3 at the early stage of oxidation. With aluminum depletion in the bond coat, NiO, Ni(Cr,Al)2O4, and other spinel formed near the bond coat. During thermal cycling, microcracks were initiated preferentially in the YPSZ top coat along columnar grain boundaries and then extended through and along the top coat. The growth stress of TGO added to the thermal stress imposed by cycling, lead to the separation at the bond coat–TGO interface. The ceramic top coat spalled with the oxide scale still adhering to the YPSZ after specimens had been cycled at 1050°C for 300 cycles. The failure mode of the EB–PVD ZrO2–7 wt.% Y2O3 sputtered Ni–Cr–Al–Y thermal-barrier coating was spallation at the bond coat–TGO interface.

In-situ reaction synthesis and decomposition of Ta2AlC

Abstract Dense bulk Ta2AlC ceramic was fabricated by in-situ reaction/hot pressing of Ta, Al and C powders. The reaction path and effects of initial composition on the purity were investigated. It was found that Ta2AlC formed through the reactions between AlTa2 and graphite, or between Ta5Al3C, TaC and graphite at 1500–1550°C. By modifying the molar ratio of the initial Ta, Al, and C powders, single-phase Ta2AlC was prepared at 1550°C under an Ar atmosphere with an optimized composition of Ta: Al: C = 2: 1.2: 0.9. The lattice parameter and a new set of X-ray diffraction data of Ta2AlC were obtained. In addition, Ta2AlC was reported unstable above 1600°C and decomposed to Ta4AlC3, and then to TaCx.

Hot corrosion of Ti3SiC2-based ceramics coated with Na2SO4 at 900 and 1000 °C in air

The hot corrosion behavior of polycrystalline Ti3SiC2 under thin films of Na2SO4 was studied at 900 and 1000 degreesC in air. The microstructure and composition of the scales were investigated by scanning electron microscope/energy dispersive spectroscope and X-ray diffraction. The results demonstrated that Ti3SiC2 suffered from serious attack during hot corrosion at 900 and 1000 degreesC. The corroded scale had a duplex microstructure, the outer layer consisted of coarse grains with pores, the inner layer consisted of fine grains and was compact. The whole corroded layer consisted of a mixture of TiO2 and SiO2 after hot corrosion attack, which was different from the scale formed during the oxidation of Ti3SiC2 in air

Cracking behavior of oxide scale formed on Ti3SiC2-based ceramic

The cyclic oxidation and acoustic emission (AE) tests were carried out for studying cracking behavior of oxide scales formed on Ti3SiC2-based ceramic at 1100 degreesC. A duplex oxide scale with an outer layer of pure TiO2 and an inner layer of a mixture of TiO2 and SiO2 was formed. The oxide scale did not spall from substrate during the cyclic oxidation at 1100 degreesC for 360 times. However, a great number of micro-cracks penetrating whole inner oxide layer were detected. AE test showed that the oxide scale did not crack during the isothermal oxidation at 1100 degreesC for 1 h, however, the scale cracked during the cooling stage. Comparing the growth rate and thickness between the oxide layers formed during the isothermal oxidation and cyclic oxidation, respectively, indicated that cracks in the inner oxide layer served as paths mainly for outward diffusion of titanium and for inward diffusion of oxygen, resulting in increased growth rate of the outer oxide layer. Because of entire and compact TiO2 consisted of outer oxide layer, and low thermal stress resulting from small mismatch of thermal expansion coefficients between the oxides and the substrate, Ti3SiC2 exhibited excellent cyclic oxidation resistance at 1100 degreesC for 360 cycles

Corrosion behavior and strength degradation of Ti3SiC2 exposed to a eutectic K2CO3 and Li2CO3 mixture

The corrosion of polycrystalline Ti3SiC2 was studied in the eutectic Li2CO3 (68 at.%) and K2CO3 (32 at.%) Mixture at 650-850degreesC. Ti3SiC2 exhibited better corrosion resistance at 650 degreesC. However, the mass loss was fast when temperature was above 700 degreesC. It was demonstrated that the surface chemical reaction-controlled shrinking core model could be applied to describe the relationship between the degree of the corrosion and reaction time for the corrosion of Ti3SiC2 in the 700-850 degreesC temperature range. The corresponding apparent activation energy was 206 kJ/mol. Corrosion resulted in roughness of specimen surface. The fracture strength of the corroded samples was evaluated by a three-point bending test. The results showed that the degradation of the fracture strength was about 25% of the original values for the corroded specimens tip to 10% weight loss. The mechanism of the strength degradation was discussed based on the analysis of the microstructure and composition of the corroded sample

Investigation on the properties of Ta doped Ti3SiC2 as solid oxide fuel cell interconnects

The oxidation behaviours and electrical properties of 5 at% Ta doped Ti3SiC2 solid solution have been investigated at 800 °C in air for up to 500 h. The oxidation kinetics of (Ti0.95Ta0.05)3SiC2 follows a parabolic law. The oxidation rate constant is 7.33 × 10−14 g2 cm−4 s−1, which is lower than those of Ti3SiC2, (Ti0.95Nb0.05)3SiC2 and Crofer 22 APU. Ta doped r-TiO2 formed (Ti0.95Ta0.05)3SiC2 during the oxidation process. Ta doping can limit the outward diffusion of Ti by decreasing the native Ti interstitials concentration and simultaneously restraining the inward diffusion of oxygen by decreasing the O vacancy concentration. As a result, the oxidation resistance is significantly improved and the oxide scale structure of Ti3SiC2 changes from a double-layer to a monolithic layer. The ASR of (Ti0.95Ta0.05)3SiC2 after oxidation at 800 °C in air for 500 h is 29.5 mΩ cm2, which is much lower than that of Ti3SiC2. Ta doping can increase the electron concentration in r-TiO2 and thereby increase the electrical conductivity of r-TiO2. Therefore, the ASR of (Ti0.95Ta0.05)3SiC2 after oxidation is lower compared to that of Ti3SiC2.