Dynamic doping and Cottrell atmosphere optimize the thermoelectric performance of n-type PbTe

High thermoelectric energy conversion efficiency requires a large figure-of-merit, zT, over a broad temperature range. To achieve this, we optimize the carrier concentrations of n-type PbTe from room up to hot-end temperatures by co-doping Bi and Ag. Bi is an efficient n-type dopant in PbTe, often leading to excessive carrier concentration at room temperature. As revealed by density functional theory calculations, the formation of Bi and Ag defect complexes is exploited to optimize the room temperature carrier concentration. At elevated temperatures, we demonstrate the dynamic dissolution of Ag2Te precipitates in PbTe in situ by heating in a scanning transmission electron microscope. The release of n-type Ag interstitials with increasing temperature fulfills the requirement of higher carrier concentrations at the hot end. Moreover, as characterized by atom probe tomography, Ag atoms aggregate along parallel dislocation arrays to form Cottrell atmospheres. This results in enhanced phonon scattering and leads to a low lattice thermal conductivity. As a result of the synergy of dynamic doping and phonon scattering at decorated dislocations, an average zT of 1.0 is achieved in n-type Bi/Ag-codoped PbTe between 400 and 825 K. Introducing dopants with temperature-dependent solubility and strong interaction with dislocation cores enables simultaneous optimization of the average power factor and thermal conductivity, providing a new concept to exploit in the field of thermoelectrics

Reactive air wetting and brazing of Al2O3 ceramics using Ag–Nb2O5 filler: Performance and interfacial behavior

We firstly performed the reactive air wetting and brazing of Al2O3 ceramics using Ag–(0.5‒12)Nb2O5 fillers, where Nb2O5 can react with liquid Ag and O2 from air to generate AgNbO3. The contact angle of the Ag–Nb2O5/Al2O3 system almost linearly decreases from ~71.6° to 32.5° with the Nb2O5 content increasing, and the joint shear strength reaches the maximum of ~65.1 MPa while employing the Ag–4Nb2O5 filler, which are mainly related to the formation and distribution of the AgNbO3 phase at the interface. Moreover, the interfacial bonding and electronic properties of related interfaces were investigated by first-principles calculations. The calculated works of adhesion (Wa) of Ag(111)/Ag–O–AgNbO3(001) and AgNbO3(001)/Al2O3(100) interfaces are higher than that of the Ag(111)/Al2O3(110) interface, indicating good reliability of the Ag/AgNbO3/Al2O3 structure. The relatively large interfacial charge transfer indicates the formation of Ag–Ag, Al–O, and Ag–O bonds in the Ag/AgNbO3/Al2O3 structure, which can contribute to the interfacial bonding

Recent advances in joining of SiC-based materials (monolithic SiC and SiCf/SiC composites): Joining processes, joint strength, and interfacial behavior

Abstract Silicon carbide (SiC) has been widely concerned for its excellent overall mechanical and physical properties, such as low density, good thermal-shock behavior, high temperature oxidation resistance, and radiation resistance; as a result, the SiC-based materials have been or are being widely used in most advanced fields involving aerospace, aviation, military, and nuclear power. Joining of SiC-based materials (monolithic SiC and SiCf/SiC composites) can resolve the problems on poor processing performance and difficulty of fabrication of large-sized and complex-shaped components to a certain extent, which are originated from their high inherent brittleness and low impact toughness. Starting from the introduction to SiC-based materials, joining of ceramics, and joint strength characterization, the joining of SiC-based materials is reviewed by classifying the as-received interlayer materials, involving no interlayer, metallic, glass-ceramic, and organic interlayers. In particular, joining processes (involving joining techniques and parameter conditions), joint strength, interfacial microstructures, and/or reaction products are highlighted for understanding interfacial behavior and for supporting development of application-oriented joining techniques

TranSegNet: Hybrid CNN-Vision Transformers Encoder for Retina Segmentation of Optical Coherence Tomography

Optical coherence tomography (OCT) provides unique advantages in ophthalmic examinations owing to its noncontact, high-resolution, and noninvasive features, which have evolved into one of the most crucial modalities for identifying and evaluating retinal abnormalities. Segmentation of laminar structures and lesion tissues in retinal OCT images can provide quantitative information on retinal morphology and reliable guidance for clinical diagnosis and treatment. Convolutional neural networks (CNNs) have achieved success in various medical image segmentation tasks. However, the receptive field of convolution has inherent locality constraints, resulting in limitations of mainstream frameworks based on CNNs, which is still evident in recognizing the morphological changes of retina OCT. In this study, we proposed an end-to-end network, TranSegNet, which incorporates a hybrid encoder that combines the advantages of a lightweight vision transformer (ViT) and the U-shaped network. The CNN features under multiscale resolution are extracted based on the improved U-net backbone, and a ViT with the multi-head convolutional attention is introduced to capture the feature information in a global view, realizing accurate localization and segmentation of retinal layers and lesion tissues. The experimental results illustrate that hybrid CNN-ViT is a strong encoder for retinal OCT image segmentation tasks and the lightweight design reduces its parameter size and computational complexity while maintaining its outstanding performance. By applying TranSegNet to healthy and diseased retinal OCT datasets separately, TranSegNet demonstrated superior efficiency, accuracy, and robustness in the segmentation results of retinal layers and accumulated fluid than the four advanced segmentation methods, such as FCN, SegNet, Unet and TransUnet