In-situ formation of BN layers by nitriding boron powders in BN/Si3N4-based composite ceramics

Boron nitride/silicon nitride (BN/Si₃N₄) composite ceramics were fabricated via the in-situ nitridation of boron (B) and silicon (Si) powders in forming gas (95%N₂/5%H₂) at 1390?°C. The effect of the B content on the phase composition, microstructure, density/porosity, machinability as well as mechanical properties of nitridized BN/Si₃N₄ composite ceramics was investigated. The addition of B slightly increased the nitridation degree of the Si and B powders mixture, and improved the ratio of the β-Si₃N₄ phase significantly at low B contents. B powders may have acted as a nucleating agent to promote the formation of β-Si₃N₄ crystals. A core-shell Si₃N₄/BN structure was revealed by the TEM technique, and the number of BN layers increased with the increase of the B content. The in-situ BN formed by the nitridation of B played a similar role with the BN directly added in enhancing the machinability of the BN/Si₃N₄ composite ceramics. The method of the in-situ nitridation of B is also effective to prepare SiC fiber-reforced BN/Si₃N₄ ceramic matrix composites.     

Phase,microstructure and sintering of aluminum nitride powder by the carbothermal reduction-nitridation process with Y2O3 addition

Aluminum nitride powders were synthesized by carbothermal reduction-nitridation method using Al(OH)₃, carbon black and Y₂O₃ as raw materials. The change of phase, microstructure and densification during the AlN synthesis and sintering process were investigated and the effects of Y₂O₃ was discussed. The results showed that Y₂O₃ reacted with Al₂O₃ to form yttrium aluminates of YAlO₃ (orthorhombic and hexagonal phases), Y₄Al₂O₉ and Y₃Al₅O₁₂ at the low temperature of 1350 °C. YAlO₃ could firstly be transformed into Y₂O₃ and then completely into YN when the firing temperature and holding time increased. However, YN could be oxidized into Y₂O₃ again after the carbon removal at 700 °C in the air atmosphere. There were two ways generating AlN when adding Y₂O₃ and the possible mechanism was proposed. Y₂O₃ from YN oxidation favored the densification of AlN ceramics because the liquid had better flowability and distribution in the sintering process at 1800 °C.     

Enhanced energy storage properties of Ba0.4Sr0.6TiO3 lead-free ceramics with Bi2O3-B2O3-SiO2 glass addition

In this study, we present an effective strategy to enhance the energy storage properties of Ba_0.4Sr_0.6TiO₃ (BST) lead-free ceramics by the addition of Bi₂O₃-B₂O₃-SiO₂ (BBS) glass, which were prepared by the conventional solid state sintering method. The phase structure, microstructure and energy storage properties were investigated in detail. It can be found that the Ba_0.4Sr_0.6TiO₃-x wt%(Bi₂O₃-B₂O₃-SiO₂) (BST- x wt%BBS, 0 ≤ x ≤ 12) ceramics possess large maximum polarization (P_max), low remanent polarization (P_r) and slim polarization electric field (P-E) hysteresis loops. The breakdown strength (BDS), recoverable energy storage density (W_rec) and energy storage efficiency (η) are enhanced obviously with the addition of BBS glass. The BST-9 wt%BBS ceramic is found to exhibit excellent energy storage properties with a W_rec of 1.98 J/cm³ and a η of 90.57% at 279 kV/cm. These results indicate that the BST-x wt%BBS ceramics might be good candidates for high energy storage applications.     

Li2AGeO4 (A = Zn,Mg): Two novel low-permittivity microwave dielectric ceramics with olivine structure

Two low-permittivity dielectric materials Li₂AGeO₄ (A?=?Zn, Mg) were prepared via the solid-state reaction method. X-ray diffraction analysis and Rietveld refinement indicated that both ceramics crystallize in an orthorhombic olivine structure with a space group Pmn2₁. Dense ceramics with high relative density and homogeneous microstructure were obtained. Li₂ZnGeO₄ densified at 1200?°C possessed a relative permittivity ε_r?=?6.5, a quality factor Q?×?f?=?35,400?GHz, and a temperature coefficient of resonant frequency. Li₂MgGeO₄ exhibited ε_r?=?6.1, Q?×?f?=?28,500?GHz, and τ_f?=?–74.7?ppm/°C when sintered at 1220?°C. Additionally, the large negative τ_f values of Li₂AGeO₄ (A?=?Zn, Mg) ceramics were successfully adjusted compensated by forming composite ceramics with CaTiO₃ and near-zero τ_f values of +2.9?ppm/°C and +5.8?ppm/°C were achieved in 0.92Li₂ZnGeO₄-0.08CaTiO₃ and 0.90Li₂MgGeO₄-0.10CaTiO₃, respectively.     

New approaches to determine the interface height of fire smoke based on a three-layer zone model and its verification in large confined space

Large confined space has high incidence of fires, which seriously threatens the safety of people working there. Understanding the distribution of smoke in such large space is critical to fire development prediction and smoke control. Three improved methods for the stratification interface prediction of fire smoke are developed, including of improved intra-variance, integral ratio and N-percentage methods. In these methods, the interface height is determined by the vertical temperature distribution based on a three-layer smoke zone model, which is an improvement of a two-layer zone model. Thereafter, the three improved methods are applied to several typical fire cases simulated CFD to predict the smoke interface, and their applicability and reliability are verified by comparison of the smoke stratification results with the filed simulation results. Results show that the three improved methods can effectively determine the location of the three-layer zone model's interface, and they have the ability to predict smoke interface for fires with different fire source types and ventilation conditions.     

Fast Chemo‐Responsive Shape Memory of Stretchable Polymer Nanocomposite Aerogels Fabricated by One‐Step Method

A new type of stretchable poly(caprolactone)/graphene oxide (PCL/GO) aerogel with fast chemo‐responsive shape memory effect is fabricated by one‐step method of a sol‐gel procedure. The PCL/GO aerogels show uniformly circular and interconnected pores formed by twisted PCL nanolayers. GO platelets improve the crystallinity of PCL and increase the fracture stress and strain by 150% and 300% respectively, although the GO loading is only 0.5%. The dramatic increment of break strain is attributed to the uniform and circular pores that can afford large deformation and the interaction of GO and PCL. The aerogels can be programmed by external stress at ambient temperature without heating and recover upon ethyl acetate (EA) in 1 s. The fast chemo‐responsive shape recovery is ascribed to the fast wrinkle of the PCL nanolayers that decrease the diffusion time greatly and the interconnected micrometer pores that are in favor of penetrating for EA molecules.     

Preparation of YSZ porous ceramics with precise porosity control

Conventional preparation of porous ceramics often hard to accurately control the porosity of the samples. In this paper, by improving the foaming method, the foaming process is placed in the limited space of the ball mill tank, and YSZ porous ceramics with different porosity are obtained by changing the amount of slurry. Initial results showed that the porosity of the sample varies linearly with the increasing of addition of slurry. From the microscopic morphology, it can be seen that the increase in slurry content will reduce the number of pore with little changed size, which lead to the decrease in porosity. Therefore, with simple calculation YSZ porous ceramics with 85%, 80%, 75%, 70%, and 65% porosity can be obtained using the method of foaming in limited space by ball milling.     

Image and Feature Space Based Domain Adaptation for Vehicle Detection

The application of deep learning in the field of object detection has experienced much progress. However, due to the domain shift problem, applying an off-the-shelf detector to another domain leads to a significant performance drop. A large number of ground truth labels are required when using another domain to train models, demanding a large amount of human and financial resources. In order to avoid excessive resource requirements and performance drop caused by domain shift, this paper proposes a new domain adaptive approach to cross-domain vehicle detection. Our approach improves the cross-domain vehicle detection model from image space and feature space. We employ objectives of the generative adversarial network and cycle consistency loss for image style transfer in image space. For feature space, we align feature distributions between the source domain and the target domain to improve the detection accuracy. Experiments are carried out using the method with two different datasets, proving that this technique effectively improves the accuracy of vehicle detection in the target domain.     

The Regulatory Functionality of Exosomes Derived from hUMSCs in 3D Culture for Alzheimer's Disease Therapy

Reducing amyloid‐β (Aβ) accumulation could be a potential therapeutic approach for Alzheimer's disease (AD). Particular functional biomolecules in exosomes vested by the microenvironment in which the original cells resided can be transferred to recipient cells to improve pathological conditions. However, there are few reports addressing whether exosomes derived from cells cultured on scaffolds with varying dimension can reduce Aβ deposition or ameliorate cognitive decline for AD therapy. Herein, both 3D graphene scaffold and 2D graphene film are used as the matrix for human umbilical cord mesenchymal stem cell culture, from which the supernatants are obtained to isolate exosomes. The levels of 195 kinds of miRNAs and proteins, including neprilysin, insulin‐degrading enzyme and heat shock protein 70, in 3D‐cultured exosomes (3D‐Exo) are dramatically different from those obtained from 2D culture. Hence, 3D‐Exo could up‐regulate the expression of α‐secretase and down‐regulate the β‐secretase to reduce Aβ production in both AD pathology cells and transgenic mice, through their special cargo. With rescuing Aβ pathology, 3D‐Exo exerts enhanced therapeutic effects on ameliorating the memory and cognitive deficits in AD mice. These findings provide a novel clinical application for scaffold materials and functional exosomes derived from stem cells.