Intriguing ZnO dendritic nanostructures have been synthesized by a two-step chemical vapor deposition process. Regular nanorods grow uniformly to the presynthesized ZnO nanowires on silicon ...substrate, the secondary nanorods are single-crystal hexagonal ZnO, and each nanorod grows along the 0001 direction. The relationship between the secondary-grown nanorods and the primary ZnO nanowire is not epitaxial due to the high temperature-increasing rate during the rapid grown process. The size and morphology of branches can be controlled by adjusting the temperature and duration of growth. Room temperature photoluminescence (PL) and mircrowave absorption properties of the ZnO dendritic nanostructures have been investigated in detail. The value of minimum reflection loss for the composite with 50 vol % ZnO dendritic nanostructures is −42 dB at 3.6 GHz with a thickness of 5.0 mm. Hierarchical nanostructures of this type are ideal objects for the fabrication of nanoscale functional devices.
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The Mg-xGd (x = 4–16, wt%) alloys with columnar crystals were prepared by directional solidification, and the effect of Gd content on the crystal orientation, microstructure, and tensile properties ...at room temperature were studied. The microstructure of the Mg-4Gd alloy is single α-Mg solid solution, and the microstructure of the Mg-6Gd alloy is composed of α-Mg and Mg24Gd distributed along the longitudinal grain boundaries of columnar crystals, while the microstructure of the Mg-(8−16)Gd alloys consists of α-Mg and lamellar eutectic phase. The Mg-4Gd alloy with growth orientation concentrated in< 112(_)0 > (hard orientation) has high yield strength (186 MPa) and moderate elongation (12 %), because {101(_)1} compression twinning are activated to coordinate strain. However, the Mg-6Gd alloy with growth orientation concentrated in<1(_)21(_)2 > (soft orientation) has excellent elongation (38 %), as a result of activated {101(_)2} tension twinning. When the Gd content exceeds 12 %, plenty of eutectic phases will cause low strength and poor plasticity of the Mg-Gd alloys.
•The Mg-xGd alloys with columnar crystals were prepared by directional solidification.•The research further discussed the correlation of crystal orientation and mechanical properties of the Mg-Gd alloys.•{101(_)1} compression twinning and {101(_)2} tension twinning are important to improve grain boundary strain coordination.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Simultaneous enhancement of the strength and ductility in metal-matrix nanocomposites with high content nano-reinforcements is crucial for the development of novel composite materials. Here SiC/Al ...nanocomposites with ultrafine Al grains and intragranular 10 vol% SiC nanoparticle were prepared by mechanical alloying and hot pressure sintering followed by hot extrusion. The results show that nanocrystalline Al grains and uniformly distributed SiC nanoparticles (SiCnp) in the composite powders are obtained after ball-milling. The prior particle boundary (PPB) in the composite powders cannot be fully removed by sintering at 580 °C under 20 MPa, but can be eliminated by subsequent hot extrusion at 450 °C with a ratio 25:1. The extruded nanocomposites possess ultrafine Al grains with diameter 320 nm and intragranular SiCnp created by the dynamic recrystallizations (DRX). Such optimal microstructure favors a significant improvement of 284% in yield strength (YS) and 259% in ultimate tensile strength (UTS) compared with monolithic Al, and more interestingly, maintains an elongation up to 12% due to the homogeneous nanoparticles in fine Al grains. Theoretical analysis shows that the dominant strengthening factor is Orowan strengthening related to the high content intragranular SiCnp, with second factor grain boundary strengthening due to ultrafine Al grains.
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Ti6Al4V matrix composites reinforced with needle-like Ti5Si3 particles distributed in the secondary-scale network and TiB whiskers in the first-scale network were in-situ synthesized using reaction ...hot pressing. In order to adjust Ti5Si3 characteristics and improve the mechanical properties of the composites, effects of sintering parameters on microstructure and tensile properties were systematically investigated. The size, distribution and morphology of the Ti5Si3 precipitation can be adjusted by controlling the sintering parameters. The quantity of the Ti5Si3 precipitation in the secondary-scale network (β-Ti phase) increased significantly with increasing temperatures. Moreover, compared with the sintering time, the sintering temperature plays a more dominant role in controlling the microstructure and tensile properties of the composites. The composites fabricated at 1300°C for 1.5h exhibited enhanced tensile properties at both room and elevated temperatures with more and finer Ti5Si3 distributed in the secondary-scale network.
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Ejectors can be used in Joule–Thomson (JT) cooling systems to create sub-atmospheric pressures and to recover expansion work, and thus achieve lower temperatures and higher system efficiencies. As ...the key components of ejector-equipped JT cryocoolers, ejectors and counter flow heat exchangers (CFHXs) have significant effects on the cryocooler performance. In order to optimize the cryocooler performance, a 1-D ejector model, a 1-D finite-element model of CFHXs and a cryocooler model are developed. Results show that the variation of the ejector entrainment ratio with the primary flow inlet temperature depends on the ejector operation modes. Two tubes-in-tube CFHXs are designed to realize heat recovery with a relatively low pressure drop along the low-pressure channel of the CFHXs. The performance of CFHX I determines the operating conditions of the ejector, and there is an optimal CFHX I length corresponding to the maximum system COP. Differing from CFHX I, the COP increases with the length of CFHX II. For the designed cryocooler with nitrogen as the working fluid, a cooling capacity of 10.89 W at 71.8 K and COP of 0.042 can be obtained, and the COP increases by 13.1% compared to the basic ideal JT cooling cycle with the same cold-end temperature.
•A model of ejector-equipped Joule–Thomson cryocooler is developed.•Effects of the ejector and heat exchangers on cryocooler performance are studied.•Heat exchangers are designed to realize heat recovery with low pressure drops.•COP improvement of 13.1% can be achieved at a cold-end temperature of 71.8 K.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A series of laminated Ti-(TiBw/Ti) composites fabricated at different diffusion welding temperatures of 1100 °C, 1200 °C and 1300 °C were subjected to tensile and bending tests. The results showed ...that the interfacial bonding strength, interfacial toughness and residual stress gradually increased with the increasing fabrication temperature. Therefore, along the transversal direction, the laminated composites fabricated at 1300 °C revealed the highest tensile strength and fracture elongation. However, the highest tensile strength (694 MPa) and elongation (22.7%) along the longitudinal direction, was recorded with the laminated composites fabricated at 1200 °C due to reasonable interfacial bonding and residual stress. Interestingly, the laminated composites fabricated at 1100 °C exhibited the highest bending fracture toughness and fracture work along arrester orientation. Due to the weak interfaces, the crack propagation path was displaced by delamination cracks and re-nucleated multiple tunnel cracks, which in turn lead to reduction in stress intensity of the main crack. This was beneficial to the toughening of the laminated Ti-(TiBw/Ti) composites.
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Studies on nanoscale materials have received great interest in both fundamental and applied aspects in recent years. In this letter, we report the synthesis of CoOOH nanorods and their possible ...applications as coating materials on nickel hydroxide for high-temperature nickel−metal hydride (Ni−MH) cells. The morphology and structure of CoOOH nanorods and coated nickel hydroxide particles are investigated by transmission electron microscopy, X-ray diffraction, and scanning electron microscopy, respectively. The electrochemical properties in the cylindrical AA size Ni−MH cells are evaluated. Our results show that the Ni−MH cells, where the positive electrodes are composed of such nanometer sized CoOOH coatings, have a higher capacity available and good performance at elevated temperatures of >50 °C.
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A novel laminated Ti–TiBw/Ti composites composed of Ti layers and TiB whisker reinforced Ti (TiBw/Ti) composite layers were successfully fabricated by reaction hot-pressing. Herein, the TiBw/Ti ...composite layers were in situ synthesized by reaction in the system of Ti and TiB2 powders during the hot pressing process. The composite layer can effectively refine grains of Ti layer. Moreover, in-situ synthesized TiB whiskers obviously restrain the growth of Ti matrix grains of the TiBw/Ti composite layers. It is worth pointing out that the in situ synthesized TiBw can strengthen the interface between Ti layer and TiBw/Ti layer. The tensile testing results show that the ultimate strength and the ductility of laminated composites can be increased to 617MPa and 20.5%, respectively, from 546MPa and 17.5% of pure Ti, indicating that the strengthening effect and the toughening effect of the composites were effectively exploited by tailoring the novel laminated structure of Ti–TiBw/Ti composites.
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A range of laminated Ti–TiBw/Ti composites composed of Ti layer and TiBw/Ti composite layer with different volume fractions of TiB whisker (TiBw) were successfully fabricated by diffusion welding. ...TiBw reinforcement exhibits different morphologies, such as needle-like, self-joining, claw-like and cluster structures due to the increasing volume fractions of TiB. The elastic modulus and yield strength of the laminated composites with different volume fractions of TiB are consistent with the HT equation and shear lag model, respectively. The ultimate tensile strength of the laminated composites with 5vol%, 8vol%, 10vol% and 12vol% TiBw/Ti composite layer is 604MPa, 689MPa, 744MPa and 789MPa, along with tensile elongation of 19.7%, 14.1%, 10.4% and 5.5%, respectively. The fracture characteristics of the laminated composites reveal a ductile to brittle transition (DBT) behavior with the increase of TiB volume fraction: fractograph in TiBw/Ti composite layer changes from tortuous into flatter, and Ti layer changes from ductile fracture accompanying with many dimples into brittle fracture with many lath-like fracture morphologies, which are attributed to the decreasing size of plastic zone at the tunnel crack tip.
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A novel laminated Ti–TiBw/Ti composite containing Ti layers and TiBw/Ti composite layers were successfully fabricated by the reaction hot-pressing technique. The TiB whiskers and their network ...distribution structure play an effective role in microstructural evolution of the Ti matrix during the sintering and cooling processes. Interestingly, the morphology and size of grains in the laminated Ti–TiBw/Ti composites exhibit gradient distribution. The Ti layer comprises coarse lath-like grains while the TiBw/Ti composite layer contains two different sizes of equiaxed grains. A combination of high strength and ductility can be obtained by incorporating a low volume fraction of in-situ TiBw through the grain refinement mechanism and the laminated structure. The TiBw/Ti composite layer with high strain hardening rate delays localized necking of the Ti layer. The novel laminated structure of Ti–TiBw/Ti composite allocates stress and strain reasonably well and reveals a prolonged uniform plastic deformation capability.
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