查看更多>>摘要：Ultra-fine Ta_4HfC_5 and TaHfC_2 powders were prepared through the pyrolysis of a precursor synthesised by using a facile one-pot reaction. HfCl_4, TaCl_5, and phenolic resins were used as the sources of hafnium (Hf), tantalum (Ta), and carbon (C), respectively. The as-synthesised precursors were further utilised to prepare Ta_4HfC_5- and TaHfC_2-modified carbon/carbon (C/C-Ta_4HfC_5 and C/C-TaHfC_2) composites through precursor infiltration and pyrolysis. The transformation of Ta_4HfC_5 and TaHfC_2 precursors into ultra-fine ceramic powders and anti-ablation performance of the C/C-Ta_4HfC_5 and C/C-TaHfC_2 composites were investigated. The carbothermal reduction of Ta_2O_5 and Hf_6Ta_2O_(17) and the solid solution reaction between HfC and TaC occur successively during the pyrolysis of Ta_4HfC_5 precursors. HfC and TaC undergo a sufficient solid solution reaction at 1800 °C to form the Ta_4HfC_5 solid solution with a particle size of 200-300 nm. The pyrolysis products of TaHfC_2 precursors acquired at 1400-1600 °C are composed of TaC, HfC and TaHfC_2. High-temperature pyrolysis promotes the solid solution reaction between TaC and HfC. The TaHfC_2 solid solution with a particle size of 100-150 nm is formed at 1800 °C. The introduction of TaHfC_2 and Ta_4HfC_5 solid solutions into the C/C composite substantially enhances their anti-ablation performance. The linear ablation rates of C/C-Ta_4HfC_5 and C/C-TaHfC_2 are 11.04 and 16.97 um/s, respectively, which are considerably lower than that of the C/C composite. The good anti-ablation performance of the C/C-Ta_4HfC_5 and C/C-TaHfC_2 composites can be attributed to the formation of the Ta_2O_5 and Hf_6Ta_2O_(17).
查看更多>>摘要：Poor dispersion of nanodiamonds (NDs) is the bottleneck to exploit their intrinsically excellent properties in metal matrix composites (MMCs). In this study, a strategy of surface modification was developed to uniformly disperse ND particles into 2024 aluminum alloy. Microstructure observations showed that individual NDs are closely contacted with the matrix, forming reaction-free interfaces. Consequently, a high-strength 0.8 vol% ND/2024A1 composite with an ultimate tensile strength of 741.0 MPa is developed. Theoretical and experimental measurements prove that the strength improvement of ND/2024A1 composite is mainly attributed to the Orowan strengthening, thermal mismatch strengthening and grain refinement strengthening. This work demonstrates the effective application of NDs as a reinforcement for MMCs towards high-specific-strength materials.
查看更多>>摘要：In this paper, polarization electric field (PEF) induced in GaN/In_xGa_(1-x)N superlattice (SL) was studied and found it can be treated as a reductant of thermal conductivity (k). Elastic properties and phonon group velocity are modified by the impact of interfacial PEF as by inverse piezoelectric effect. It improves scattering and thermal resistance at the boundaries because of dissimilarity in specific heat and group velocity, that decreases transmission coefficient of phonon and more acoustic mismatches resulting in reduction of in-plane (k_(ip)) as well as cross-plane (k_(cp)) thermal conductivities. k_(ip) of In_xGa_(1-x)N (5 nm)/ GaN (10 nm) SL with (without) interfacial PEF field are 7.807 (8.921), 7.350 (8.355) and 7.018 (8.090) Wm(-1)K~(-1) respectively, for x = 0.1,0.3 and 0.5; whereas k_(cp) for the similar compositions are respectively, 4.652 (5.710), 4.282 (5.221) and 4.081(5.185) Wm~(-1)K~(-1) at 300 K demonstrating the reduction exceeds 20%. It shows that the optimal k can be accomplished with the adaptation of nitride SL's electric field for thermoelectric improvements.
查看更多>>摘要：The struggle with "thermal load" is always an unavoidable challenge for photo-luminescent material. In this work, a novel red phosphor of ZnGa_2O_4: Eu~(3+) with spinel-type structure is prepared by high-temperature solid-phase method. Due to the charge imbalance during the replacement processes of Ga~(3+)→Zn~(2+) and Eu~(3+)→Zn~(2+) in mixed spinel structure, vacancy defects can be introduced into the lattice structure of ZnGa_2O_4: 0.02Eu~(3+). The results of Density Function Theory (DFT) calculation demonstrate that the existence of vacancy defects can construct defect energy levels in the energy band structure of the host crystal, which can assist enhancing the fluorescence emission intensity of the phosphor at high temperature. The strong thermal stability of its own structure combined with the assistance of defect energy levels enables the achievement of zero-thermal quenching in ZnGa_2O_4: 0.02Eu~(3+) under multi-wavelength excitation. Our results open up a new way for the exploration and utilization of zero-thermal quenching phosphors.
查看更多>>摘要：The tendency of AA3003 towards localized corrosion is one of the most challenging issues for its broader use. Intermetallic phases (IMPs) in AA3003 play dual contradictory roles: strengthening effect and initiating pitting. Solving this contradiction contributes to improvement of corrosion resistance and fully utilizing the potential of materials. Herein, a superhydrophobic surface with IMPs in-situ pinning effect was fabricated via a chemical etching approach combined with stearic acid (STA) modification. Thus, IMPs can provide sound support for the superhydrophobic surface and meanwhile eliminate the adverse effect of promoting localized corrosion. Formation mechanism of etching structure was attributed to the synchronous chemical reaction of IMPs and matrix phases in the etchant. Further modification with STA changed mico/nano hierarchical surface from hydrophilicity to superhydrophobicity. The as-prepared surface exhibited a maximum static contact angle of-169° and a sliding angle of ~1 °, possessing excellent superhydrophobicity. Besides, the wettability, self-cleaning, chemical stability, mechanical durability, and corrosion resistance properties of the superhydrophobic surface were comprehensively investigated. The results showed that the corrosion inhibition efficiency exceeded 99% in 3.5 wt% NaCl solution. The pinning direction of IMPs in superhydrophobic surface influenced its properties. It is anticipated that the construction of superhydrophobic surfaces will broaden its promising applications.
查看更多>>摘要：The strengthening effect of composites is rather limited in comparison with the excellent properties of graphene due to difficulty in acquiring strong interfacial bonding. To enhance the interfacial bonding and reduce the interface mismatch between the matrix and reduced graphene oxide (rGO), a novel strategy in this study is proposed through generating hybrid layered double oxides (LDO) nanoparticles on rGO (LDO@ rGO). The 2024A1 composites with heterogeneous structure were constructed by ball milling and spark plasma sintering (SPS), which was reinforced by flake-like LDO@rGO-rich zones contained LDO@rGO in the Al matrix with fine grain size of ~1 um. The yield strength, elongation and fracture energy of 1 vol% LDO@ rGO/Al composite with heterogeneous microstructure were 69.6%, 63.9% and 140.5% higher than those of the composite reinforced by uniformly distributed 0.67 vol% graphene oxide (GO), respectively, achieving an improvement in the strength-ductility synergy of the fabricated LDO@rGO/Al composite. The rationally spatial arrays of LDO@rGO-rich and LDO@rGO-free zones are beneficial for promoting the synergistic strengthening of Orowan, solid solution, thermal mismatch and load transfer and simultaneously toughening the composite through enhanced crack deflection and bridging effects. The proposed method offers a promising route for fabricating composite with optimized and improved material properties by coupling interface and heterogeneous structure.
查看更多>>摘要：Tungsten alloys are extensively applied in kinetic energy penetrators but these alloys exhibit poor self-sharping properties-connected with high thermal conductivity, large grain size, and low hardness-which limit their application. Ni_3Al is one of the most promising candidates as a binder phase for tungsten alloys used as kinetic energy penetrators because of its high hardness, low thermal conductivity, and low solubility of tungsten, which can refine the tungsten grain. In this study, a simple and scalable methodology, termed 'synergistic strengthening with flaky binder phase and ceramic nano-particles', has been developed to fabricate high-performing W-Ni_3Al alloys. The flaky Ni_3Al binder phase acted as a ductile layer to improve the mechanical properties of the alloy by crack deflection and tip blunting. The in-situ formed nano Al_2O_3 particles on the interface between the W and Ni_3Al phase inhibited the dislocation motion and restrained the dissolution and re-precipitation of the tungsten grain, leading to grain refining of the tungsten and alloy strengthening. As a result, a unique structural W-Ni_3Al alloy was prepared with an approximately 2 um tungsten grain size, and its bending strength was improved by 24.3% in comparison to conventional W-Ni_3Al alloys, and the hardness of greater than 70 HRA remained constant. Moreover, it was exciting that the higher bending strength could be obtained by regulating the W-Ni_3Al alloy microstructure. In short, we demonstrated that this structural design strategy was an effective method to fabricate refractory alloys, such as tungsten alloys, with good overall mechanical performance.
查看更多>>摘要：Copper Zinc Tin Selenide Cu_2ZnSnSe_4 (CZTSe) and Copper Zinc Tin Sulfide Cu_2ZnSnS_4 (CZTS) thin films were deposited from a multicomponent single target with different RF sputter powers and a constant annealing temperature of 450°C. One step deposition with in-situ annealing was performed in order to avoid the individual metal losses during the process. Effects of various deposition parameters were investigated to finely control the elemental ratio of the quaternary target which in-turn helps to obtain the desired stoi-chiometry suitable for solar absorber materials as confirmed from EDS analysis. The phase purity of thin films was investigated and confirmed using XRD and Raman spectroscopy. A thin film solar cell device with n-CdS, ZnO, ZnO:Al layers and Al grids over the prepared kesterite CZTX (X = S, Se) films were deposited on Mo coated soda lime glass using complete vacuum deposition technique. Even though the optical, electrical and structural results are promising, the conversion efficiency for these single step processed CZTSe and CZTS solar cell devices were limited to 3.72% and 2.6% due to possible interface effect and smaller grain size of the deposited films. Considering the industrial fabrication of kesterite thin film solar cells, this processes assures an environmental friendly approach with inherent purity leading to one-step reduction in the mass production process.
查看更多>>摘要：High-quality thermoelectric thin films formed on flexible substrates are important for the development of flexible thermoelectric generators as power supplies for wireless sensors and wearable devices. However, the thermoelectric properties of the films formed on flexible substrates are inferior to those of films formed on rigid substrates because flexible substates shrink during the processes of film deposition and post-thermal annealing. To overcome this problem, we propose a modification during these processes. Bismuth telluride thin films were deposited on polyimide substrates using radio-frequency magnetron sputtering. The substrates were bent to a convex or concave shape, followed by thermal annealing under the same bending conditions. The crystallite size and crystal orientation improved when the films were set to a concave shape during the film deposition and post-thermal annealing processes. This phenomenon occurred presumably because the substrate shrinkage was counteracted by the stretching of the substrate, and the strains did not accumulate much in the films, resulting in improved crystal growth. Consequently, the electrical conductivity was increased, and the resulting power factor of 16.1 μW/(cm K_2) is 1.4 times higher than that of the Bi_2Te_3 film formed on a flat substrate.
查看更多>>摘要：Heat treatment strategy has been initially used to prepare boron doped graphite carbon nitride nanodots (BCNDs)/carbon-rich g-C_3N_4 nanosheets (CNS) heterojunction catalysts. The overall procedure involved three stages. First, BCNDs were prepared by the molten salt method. Next, BCNDs were dispersed onto CNS via heat treatment. Finally, the prepared heterojunction catalysts were test for photocatalytic water splitting hydrogen evolution. The experiment results show that the highest photocatalytic hydrogen evolution activity reached 6260.5 umol-g~(-1)-h~(-1) which was approximately 15.7, 7.2 and 3.1 folds higher than that of bulk carbon nitride (bulk C_3N_4), CNS and BCNDs/CNS-H prepared by hydrothermal method, respectively. The enhanced photocatalytic performance may be attributed to the synergistic effect of the formation of heterojunction and the increase of C content in carbon nitride. This work provides reliable guideline for the design of a novel and simple method for composite photocatalyst.
NSTL
Elsevier