Anomalous formation of micrometer-thick amorphous oxide surficial layers during high-temperature oxidation of ZrAl2

The thermal oxidation of ZrAl₂ in the temperature range of 550–750 °C in pure oxygen has been investigated by a combinational experimental approach using X-ray diffraction, scanning electron microscopy/energy dispersive spectrometer, Auger electron spectroscopy and cross-sectional transmission electron microscopy. The thermal oxidation leads to the growth of anomalously thick (up to 4.5 μm) amorphous (Zr_0.33Al_0.67)O_1.66 surficial layers at temperatures as high as 750 °C. The oxidation kinetics obeys a parabolic law with an activation energy of 143 kJ/mol. The underlying mechanism for the formation of such micrometer-thick amorphous oxide surficial layers has been discussed on the basis of interface thermodynamics and the occurrence of high interface stability associated with a synchronous oxidation of Al and Zr elements.     

Stress rupture properties and deformation mechanisms of K4750 alloy at the range of 650 °C to 800 °C

The stress rupture properties and deformation mechanisms of K4750 alloy at 650 °C, 700 °C, 750 °C and 800 °C were investigated. As the decrease of temperature and stress, the stress rupture life gradually increased. A Larson-Miller Parameter (LMP) method was used for analyzing the stress rupture life under different conditions. The linear fitting formula between stress (σ) and LMP was derived as σ = 3166.455 − 119.969 × LMP and the fitting coefficient was 0.98. After testing, the dislocation configurations of all stress rupture samples were investigated by transmission electron microscopy (TEM). The temperature and stress had a significant impact on the deformation mechanism, thereby affected the stress rupture life of K4750 alloy. As the increasing stress at a given temperature, the deformation mechanism gradually transformed from Orowan looping to stacking fault shearing. Based on experimental results, the threshold stress at 650 °C, 700 °C, 750 °C and 800 °C for the transition of deformation mechanism was estimated to be about 650 MPa, 530 MPa, 430 MPa and 350 MPa, respectively. Below the threshold stress, γ' phase effectively hindered dislocation motion by Orowan looping mechanism, K4750 alloy had a long stress rupture life. Slightly above the threshold stress, Orowan looping combining stacking fault shearing was the dominant mechanism, the stress rupture life decreased. As the further increase of stress, stacking fault shearing acted as the dominant deformation mechanism, the resistance to dislocation motion decreased rapidly, so the stress rupture life reduced significantly.     

Optimizing the low-pressure carburizing process of 16Cr3NiWMoVNbE gear steel

Compared with the traditional atmospheric carburization, low-pressure carburization has the benefits of producing no surface oxidation and leaving fine, uniformly dispersed carbides in the carburized layer. However, the process parameters for low-pressure carburization of 16Cr3NiWMoVNbE steel have yet to be optimized. Thus, we use the saturation-value method to optimize these parameters for aviation-gear materials. Toward this end, the microstructure and properties of 16Cr3NiWMoVNbE steel after different carburization processes are studied by optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron probe microanalysis. Considering the saturated austenite carbon concentration, we propose a model of carbon flux and an alloy coefficient for low-pressure carburization to reduce the carbon concentration in austenite and avoid the surface carbide network. At the early stage of carburization (˜30 s), the gas-solid interface has a higher concentration gradient. The averaging method is not ideal in practical applications, but the carbon flux measured by using the segmented average method is 2.5 times that measured by the overall average method, which is ideal in practical applications. The corresponding carburization time is reduced by 60%. By using the integral average method, the actual carburization time increases, which leads to the rapid formation of carbide on the surface and affects the entire carburization process. Nb and W combine with C to form carbides, which hinders carbon diffusion and consumes carbon, resulting in a sharp decrease in the rate of C diffusion in austenite (the diffusion rate is reduced by ˜52% for 16Cr3NiWMoVNbE steel). By changing the diffusion coefficient model and comparing the hardness gradient of different processes, the depth of the actual layer is found to be very similar to the design depth.     

Electron mobility of rare earth complexes measured by transient electroluminescence method

Electron mobility of gadolinium/europium (dibenzoylmethanato)₃(bathophenanthroline) (Gd/Eu(DBM)₃ bath) was measured by transient electroluminescence (EL) method. Although electron mobility of the two complexes were expected to be same, the value of mobility (1.2 × 10⁻⁴ cm²/Vs at electric field of 1 MV/cm) of Eu(DBM)₃ bath complex was bigger than that (8 × 10⁻⁵ cm²/Vs at electric field of 1 MV/cm) of Gd(DBM)₃ bath complex. It was found to be related to the different luminescent mechanisms of active materials and recombination zones in the devices. According to this, penetration length of hole injected into electron transport layer of Eu(DBM)₃ bath was estimated.     

Unusual heating rates,dose responses and kinetic parameters detected on thermoluminescence from YAl3(BO3)4:Sm3+ phosphors

This study focused on the thermoluminescence (TL) characteristics of YAl₃(BO₃)₄ (YAB) phosphors modified with various contents of Sm³⁺ ions. The TL response of the YAB: Sm³⁺ phosphors exposed to beta irradiation was measured across the temperature range of 25–500 °C, exhibiting three TL maxima at 70, 235, and 408 °C. Preheating protocol was also carried out to remove the low temperature TL peak followed by further experiments with the two-remaining high-temperature peaks. In the TL measurements conducted with variable heating rates (HR) between 0.1 and 5 °Cs^-1, an anomalous heating rate behaviour was observed. A semi-localized transition model was used to address this feature. There was a standard deviation of less than 5% in the reusability measurements. The results of the kinetic parameters obtained by initial rise (IR) and various heating rate (VHR) methods were compared with those obtained by glow curve deconvolution (GCD) method. T_m-T_stop analysis revealed a continuous distribution of trap levels with a trap depth ranging from 1.35 to 2.20 eV. Through the use of GCD, the glow curve was found to demonstrate general order kinetics and consist of seven superimposed traps. The values of the kinetic parameters obtained for the glow curve agreed with those obtained by other methods excluding the VHR method encountering an anomalous impact. The results obtained from these tests showed that the sample could be successfully used for TL dosimetry applications.     

碳纳米管的功能化及其在聚合物结构复合材料中的应用

总结了可用于复合材料制备的碳纳米管的功能化形式和内容,简要评述了功能化碳纳米管在聚合物结构复合材料制备中的应用情况,并展望了今后的研究方向。     

Chemical and structural induced ductile-to-brittle transition in electrospun silica nanofiber membranes

Electrospun silica nanofiber membranes show a high potential in many advanced environmental applications. However, little is known about their mechanical performance which could be a limiting factor for further innovation. It is shown in this work that silica nanofiber membranes have a completely different deformation behavior compared to conventional polymeric/thermoplastic nanofiber membranes, resulting from their significant differences in chemical and physical properties such as fiber interactions and porosity. Furthermore, storage at room temperature initiates remarkable changes in failure mechanisms, depending on the storage humidity, which can be accelerated via a thermal treatment. These changes are linked to the structural changes of the membrane resulting from its chemical reactivity towards moisture in the air. Additional interactions and crosslinks are observed, leading to fiber shrinkage and rearrangement. As a result, more contact points are created between nanofibers, creating additional friction forces and, as such, a complete shift in mechanical properties towards a stronger, stiffer, and more brittle material (tensile strength of 14.0 ± 3.8 MPa vs. 3.1 ± 0.4 MPa and failure strain of 0.9 ± 0.2% vs. 24.2 ± 1.0%). The silica nanofiber membranes thus allow mechanical tunability via altering the storage or treatment conditions.