Shuming Peng

Role of nanolaminated crystal structure on the radiation damage tolerance of Ti 3 SiC 2 : theoretical investigation of native point defects

Nanolaminated Ti3SiC2, a representative MAX phase, shows excellent tolerance to radiation damage. In this paper, first-principles calculations were used to investigate the mechanism of intrinsic point defects in order to explain this outstanding property. Formation energies of intrinsic point defects are calculated and compared; and the results establish a low-energy disorder mechanism in Ti3SiC2. In addition, the migration energy barriers of Si vacancy, Si interstitial, and TiSi antisite yield very low values: 0.9, 0.6, and 0.3 eV, respectively. The intercalation of Si atomic plane between Ti3C2 nanotwinning structures dominates the formation and migration of intrinsic native point defects in Ti3SiC2. The present study also highlights a novel method to improve radiation damage tolerance by developing nanoscale-layered structure which can serve as a sink or rapid recovery channel for point defects.

Ti3C2 MXene: a promising microwave absorbing material

In this work, we demonstrate the enhancement of microwave attenuation capability of Ti3C2 enabled microwave absorbing materials (MAMs) within a frequency range of 2–18 GHz. Ti3C2 nano-sheet/paraffin composites exhibit enhanced microwave absorbing performance with an effective absorbing bandwidth of 6.8 GHz (11.2–18 GHz) at 2 mm and an optimal reflection loss of −40 dB at 7.8 GHz. Moreover, mechanisms for the dielectric responses of the Ti3C2 MXene nanosheets are intensively discussed. Three typical electric polarizations of Ti3C2 are illustrated with the Cole–Cole diagram. The enhanced microwave absorbing properties can be ascribed to the high dielectric loss accompanied with the strong multi-reflections between MXene layers.

Dislocation-enhanced experimental-scale vacancy loop formation in hcp Zirconium in one single collision cascade.

Large defects are the main factor leading to the degradation of material properties under irradiation environments. It is commonly assumed that the large defects are mainly formed through cluster growth under continuous irradiations. Besides this mechanism, recent experiments and simulations show that sometimes an individual ion can also directly create a large defect. Here we report a novel mechanism for the formation of the large defects, as discovered by our Molecular Dynamics (MD) simulations of the collision cascades in hcp Zirconium (Zr): a pre-existing edge dislocation (ED) can significantly promote the nucleation of the vacancy clusters, and even facilitate the direct formation of an experimental-scale large vacancy loop (about 3 nm) in only one single displacement cascade. This dislocation-related mechanism may be the key for understanding the experimental results in the low-dose irradiated Zr where the high-density large dislocation loops are observed but difficult to be explained by the two mechanisms mentioned above. Considering that intrinsic dislocations exist in nearly all crystalline materials, our results provide a significant concept: pre-existing dislocations have a strong influence on the primary damage production, and taking them into account is indispensable for assessing and improving the material’s irradiation-resistance.

Lightweight graphene nanoplatelet/boron carbide composite with high EMI shielding effectiveness

Lightweight graphene nanoplatelet (GNP)/boron carbide (B4C) composites were prepared and the effect of GNPs loading on the electromagnetic interference (EMI) shielding effectiveness (SE) has been evaluated in the X-band frequency range. Results have shown that the EMI SE of GNP/B4C composite increases with increasing the GNPs loading. An EMI SE as high as 37 ∼ 39 dB has been achieved in composite with 5 vol% GNPs. The high EMI SE is mainly attributed to the high electrical conductivity, high dielectric loss as well as multiple reflections by aligned GNPs inside the composite. The GNP/B4C composite is demonstrated to be promising candidate of high-temperature microwave EMI shielding material.

Developments in hot pressing (HP) and hot isostatic pressing (HIP) of ceramic matrix composites

Abstract: This chapter introduces the hot pressing (HP) and hot isostatic pressing (HIP) methods and their application to produce advanced ceramics. Typical oxides, carbides, borides and nitrides are selected to describe the synthesis process. The conclusion is that HP and HIP are good technologies for fabricating dense or single-phase ceramics. Their advantages, disadvantages and future improvements are discussed.

Hydrogen production by methane decomposition over Ni–Cu–SiO2 catalysts: effect of temperature on catalyst deactivation

Catalytic decomposition of methane (CDM) is a simple process for the production of high-purity, COX-free (CO or CO2) hydrogen. The CDM is a moderately endothermic reaction, and high temperatures are thermodynamically favorable for achieving high methane conversion. However, Ni–Cu catalysts easily lose their activities at high temperature. To study the effect of temperature on the deactivation of Ni–Cu catalysts, a 65% Ni–15% Cu–SiO2 catalyst was prepared by the heterophase sol–gel method. A series of kinetic experiments (routes I, II, III) were designed to test the catalytic performance and generate by-product carbon structures. The effects of reaction temperature and methane dissociation rate on catalyst deactivation were studied. The phase transition temperature was estimated. Based on the kinetic experiments, TEM images, XRD data, TGA-DSC curves, and TEM-EDX data, a thoroughly deactivation study of the 65% Ni–15% Cu–SiO2 catalyst was carried out. The results of this study proved that high degree of graphitization was the key factor contributing to the deactivation of Ni–Cu catalysts. Fragmentation and phase separation at high temperature were both responsible for carbon atom enrichment and a high degree of graphitization, which in turn caused the 65% Ni–10% Cu–25% SiO2 catalyst to lose activity at high temperature.

Analysis of hydrogen isotopes with quadrupole mass spectrometry

Hydrogen isotope separation is one of the most critical technological problems in nuclear fusion research, and, in order to assess accurately the performance of hydrogen isotope separation, quantitative analysis of hydrogen isotopes takes priority and becomes the first essential problem to be addressed. However, since hydrogen isotopes have almost identical shape, size, and chemical properties, separation and analysis of hydrogen isotopes is really not an easy task. By using the thermal-desorption spectroscopy (TDS) method, a quadrupole mass spectrometer (MS) was calibrated for the quantitative analysis of hydrogen isotopes in this paper with a methodic error less than ±3% using titanium hydride and titanium deuteride as the calibration standards. The linear response range of MS was extracted. Deviations that originated from the H+/D+/HD+ species revealing a negligible influence on real H2/D2 mixture analysis were also discussed. Due to the mass discrimination of the ion source and the isotopic fractionation effect of the molecular pump, the actual sensitivity of MS towards H2 and D2 is not the same, revealing some deviation from theoretical results.

KD-S SiCf/SiC composites with BN interface fabricated by polymer infiltration and pyrolysis process

Continuous silicon carbide fiber reinforced silicon carbide matrix (SiCf/SiC) composites are attractive candidate materials for aerospace engine system and nuclear reactor system. In this paper, SiCf/SiC composites were fabricated by polymer infiltration and pyrolysis (PIP) process using KD-S fiber as the reinforcement and the LPVCS as the precursor, while the BN interface layer was introduced by chemical vapor deposition (CVD) process using borazine as the single precursor. The effect of the BN interface layer on the structure and properties of the SiCf/SiC composites was comprehensively investigated. The results showed that the BN interface layer significantly improved the mechanical properties of the KD-S SiCf/SiC composites. The flexure strength and fracture toughness of the KD-S SiCf/SiC composites were evidently improved from 314±44.8 to 818±39.6 MPa and 8.6± 0.5 to 23.0±2.2 MPa·m1/2, respectively. The observation of TEM analysis displayed a turbostratic structure of the CVD-BN interface layer that facilitated the improvement of the fracture toughness of the SiCf/SiC composites. The thermal conductivity of KD-S SiCf/SiC composites with BN interface layer was lower than that of KD-S SiCf/SiC composites without BN interface layer, which could be attributed to the relative low thermal conductivity of BN interface layer with low crystallinity.

Cathodic voltage-dependent composition, microstructure and corrosion resistance of plasma electrolytic oxidation coatings formed on Zr-4 alloy

Plasma electrolytic oxidation (PEO) coatings are fabricated on Zr-4 alloy by a pulsed bipolar power supply. When the anodic voltage remains constant, the variation of cathodic voltage exhibits a significant impact on the microstructure and corrosion resistance of the oxide coatings. Here we systematically investigate the influence of cathodic voltage on the phase composition, morphology, thickness, and elemental composition of the PEO coatings. Corrosion behaviors are evaluated by electrochemical impedance spectroscopy (EIS). The coating thickness and the electrolyte borne elements incorporated in the coatings both increase with the increase of cathodic voltage. It is interesting to note that the relative content of tetragonal ZrO2 and monoclinic ZrO2 also shows a strong cathodic voltage dependence. The coating formed at 50 V cathodic voltage shows the most compact microstructure with the largest amount of tetragonal ZrO2 and correspondingly exhibits the optimized corrosion resistance. The presence of t-ZrO2 is found to be beneficial for dense oxide coatings and better corrosion resistance. Therefore, cathodic voltage is an important parameter during PEO process to adjust the microstructure and the corrosion resistance performance of PEO coatings.

Dependences of microstructure on electromagnetic interference shielding properties of nano-layered Ti 3 AlC 2 ceramics

The microstructure dependent electromagnetic interference (EMI) shielding properties of nano-layered Ti3AlC2 ceramics were presented in this study by comparing the shielding properties of various Ti3AlC2 ceramics with distinct microstructures. Results indicate that Ti3AlC2 ceramics with dense microstructure and coarse grains are more favourable for superior EMI shielding efficiency. High EMI shielding effectiveness over 40 dB at the whole Ku-band frequency range was achieved in Ti3AlC2 ceramics by microstructure optimization, and the high shielding effectiveness were well maintained up to 600 °C. A further investigation reveals that only the absorption loss displays variations upon modifying microstructure by allowing more extensive multiple reflections in coarse layered grains. Moreover, the absorption loss of Ti3AlC2 was found to be much higher than those of highly conductive TiC ceramics without layered structure. These results demonstrate that nano-layered MAX phase ceramics are promising candidates of high-temperature structural EMI shielding materials and provide insightful suggestions for achieving high EMI shielding efficiency in other ceramic-based shielding materials.