Stress measurements in ZrB2–SiC composites using Raman spectroscopy and neutron diffraction

Raman spectroscopy and neutron diffraction were used to study the stresses generated in zirconium diboride–silicon carbide (ZrB₂–SiC) ceramics. Dense, hot pressed samples were prepared from ZrB₂ containing 30 vol% α-SiC particles. Raman patterns were acquired from the dispersed SiC particulate phase within the composite and stress values were calculated to be 810 MPa. Neutron diffraction patterns were acquired for the ZrB₂–SiC composite, as well as pure ZrB₂ and SiC powders during cooling from ～1800 °C to room temperature. A residual stress of 775 MPa was calculated as a function of temperature by comparing the lattice parameter values for ZrB₂ and SiC within the composite to those of the individual powders. The temperature at which stresses began to accumulate on cooling was found to be ～1400 °C based on observing the deviation in lattice parameters between pure powder samples and those of the composite.     

Structural importance of Stone–Thrower–Wales defects in rolled and flat graphenes from surface-enhanced Raman scattering

We first survey the historical aspects of the term Stone–Thrower–Wales (STW) defect and its experimental identification. Physicochemical properties associated with the STW defect have been extensively investigated theoretically as well. However, it is difficult to verify the predicted properties by means of experiments. Here we demonstrate an experimental way to probe the vibrational properties of STW defects in single-wall carbon nanotubes (SWCNTs) using surface-enhanced Raman scattering (SERS). We also performed density functional theory calculations to support our interpretation of the SERS spectra. The characteristic fluctuations of peak intensities and frequencies are ascribed to dynamic motion of an STW defect in the hexagonal SWCNT lattice. The role of an STW defect at edges is also discussed in terms of its relevance to the stability and O₂ reactivity of flat and curved graphene structures.     

Wetting and interfacial behavior of Al–Ti/4H–SiC system:A combined study of experiment and DFT simulation

Wetting and interfacial behavior of molten Al-(10, 20, 30, 40) at.%Ti alloys on C-terminated 4H–SiC at 1500 and 1550 °C were investigated experimentally, and theoretical bonding strength, structure stability and electronic structure of interfacial reaction products/C-terminated 4H–SiC interfaces were evaluated by first-principle calculations. The wetting experiments show that the Al–Ti/SiC systems present excellent wettability with contact angle of less than 15° except the Al–40Ti/SiC system performed at 1500 °C × 30 min. The SEM-EDS and TEM analyses demonstrate that the reaction products are mainly composed of Al₄C₃, TiC, Ti₃SiC₂, Ti₅Si₃C_X and τ phase, and their formation and evolution can be mainly affected by the Ti concentration in the Al–Ti alloys and wetting temperature. Moreover, the calculated results show that the SiC/C-terminated TiC interface presents the highest work of separation and its electronic property reveals that the localization of electrons and formation of covalent bond between interfacial C atoms lead to the excellent bonding strength of SiC/TiC interface.     

Confocal micro-Raman scattering and Rutherford backscattering characterization of lattice damage in aluminum implanted 6H–SiC

High energy (MeV) and low dose aluminum implants were performed in p-type 6H–SiC at room temperature. The material was characterized by means of Rutherford backscattering in channeling configuration and confocal micro-Raman scattering. Information on the damage-induced changes in the absorption coefficient of the implanted layer can be extracted from the depth profiling of the first-order Raman intensity of the undamaged portion of the sample, using a confocal microprobe set-up. Optical modeling indicates the formation of two layers: an outermost, low absorbing, layer with thickness proportional to the energy of the bombarding ions; and a deeper, more damaged, and absorbing layer.     

Mechanical properties and indentation-induced phase transformation in 4H–SiC implanted by hydrogen ions

This paper aims to improve machining efficiency, suppress surface cracking, and reduce subsurface damage of silicon carbide (SiC). Hydrogen ions were implanted into SiC to study mechanical properties at nano and macro scales. Nanoindentation experiments were conducted using a Berkovich indenter. Firstly, the effect of ion implantation on the load-displacement curves at different indentation depths was investigated using molecular dynamics (MD) simulations. Elastic-plasticity at nanoscale was analyzed, and the values of material properties were obtained. Secondly, variability of surface morphology, phase transformation, and coordination number induced by nanoindentation with and without ion implantation was evaluated. Although ion implantation induced damage to the SiC model, the damage after nanoindentation was lower than that without ion implantation. Additionally, nanoindentation experiments were performed for small loads and high loads, respectively. The small load experiments were employed to derive material properties of the ion-implanted SiC. Improvement mechanisms of ion implantation on crack extension, fracture toughness, and elastic recovery rate were investigated under the high-load experiments. The results indicate that the amorphous structure induced by ion implantation can successfully prevent crack propagation and improve fracture toughness. The modification technology of SiC by ion implantation significantly improves the machining efficiency and the non-damage of its surface and subsurface.     

Mechanistic difference between Si-face and C-face polishing of 4H–SiC substrates in aqueous and non-aqueous slurries

The traditional aqueous-based polishing slurries have been extensively used in the ultra-precision machining process of SiC substrates, but their processing efficiency remains a major challenge in making SiC wafers with high surface quality. SiC polishing slurries based on non-aqueous solvents have been explored and reported, however, the mechanism for the accelerated SiC material removal rate (MRR) remains unknown. In this work, the Si-face and C-face of the SiC wafer were polished with water and methanol as polishing liquid carriers, respectively. The MRR of Si-face using the methanol-based slurry, can reach 260.9 nm/h, and the polished Si-face surface roughness Ra reduces to 0.150 nm. In contrast, the MRR of Si-face by using the aqueous-based slurry, is 66.8 nm/h, the polished Si-face surface roughness Ra is 0.691 nm. However, the results of MRR and Ra for C-face are opposite. The reaction between the polishing liquid carriers and the atomic structures of Si-face and C-face lead to differences of the MRRs by analyzing contact angle, XPS, and molecular dynamics (MD) simulation results. The newly revealed polishing mechanisms shined light for speeding up the development of SiC polishing slurries based on the specific aspects of the polishing surface of SiC.     

Structural changes in C–S–H gel during dissolution: Small-angle neutron scattering and Si-NMR characterization

Flow-through experiments were conducted to study the calcium–silicate–hydrate (C–S–H) gel dissolution kinetics. During C–S–H gel dissolution the initial aqueous Ca/Si ratio decreases to reach the stoichiometric value of the Ca/Si ratio of a tobermorite-like phase (Ca/Si = 0.83). As the Ca/Si ratio decreases, the solid C–S–H dissolution rate increases from (4.5 × 10^− 14 to 6.7 × 10^− 12) mol m^− 2 s^− 1. The changes in the microstructure of the dissolving C–S–H gel were characterized by small-angle neutron scattering (SANS) and ²⁹Si magic-angle-spinning nuclear magnetic resonance (²⁹Si-MAS NMR). The SANS data were fitted using a fractal model. The SANS specific surface area tends to increase with time and the obtained fit parameters reflect the changes in the nanostructure of the dissolving solid C–S–H within the gel. The ²⁹Si MAS NMR analyses show that with dissolution the solid C–S–H structure tends to a more ordered tobermorite structure, in agreement with the Ca/Si ratio evolution.     

Microwave assisted combustion synthesis in the system Ti–Si–C for the joining of SiC: Experimental and numerical simulation results

Microwaves at 2.45 GHz have been applied to ignite the combustion synthesis of compacted Ti–Si–C powders mixtures, having 1:1:1 atomic ratio, in order to join SiC-based components. A mixture of different refractory phases such as TiC and TiSi₂ were obtained. Depending on the synthesis conditions, no residual silicon in the joint was detected, suggesting the suitability of the here proposed experimental joining approach for nuclear plants and high temperature applications. A simplified model was developed with the aim of obtaining a deeper understanding of the here proposed rapid, almost pressure-less and localized heating joining method. Experimental and numerical simulation results demonstrate that joining of SiC can be rapidly obtained with minimization of heat affected zones in the SiC substrates. Maximum apparent shear strength values of the joints ranged from 9.9 to 45.1 MPa, depending on the process conditions.     

Papain from papaya (Carica papaya L.) fruit and latex: Preliminary characterization in alcoholic–acidic buffer for wine application

Papain from ripe fruit and from papaya latex was characterised, in comparison with stem bromelain, under wine-like conditions, with the aim to evaluate their applicability for white wine stabilization.Papains proteolytic activity was investigated, in McIlvaine buffer toward different synthetic peptide substrates and Bz-Phe-Val-Arg-pNA appeared the most suitable one for detecting proteolytic effect at wine average minimum pH (3.2). Kinetic parameters estimated in McIlvaine (as reference) and in tartaric buffer (mimicking wine medium), indicated a good hydrolytic activity toward selected substrate, at wine average minimum pH value (3.2) in spite of ethanol presence, in both mediums. Papain from latex showed a significantly higher and stable catalytic activity respect to fruit papain and stem bromelain, retaining after 7 days, about 50% of its initial activity.     

Tumor-Suppressor p53TAD1–60 Forms a Fuzzy Complex with Metastasis-Associated S100A4: Structural Insights and Dynamics by an NMR/MD Approach

Conformationally flexible protein complexes represent a major challenge for structural and dynamical studies. We present herein a method based on a hybrid NMR/MD approach to characterize the complex formed between the disordered p53TAD^1–60 and the metastasis-associated S100A4. Disorder-to-order transitions of both TAD1 and TAD2 subdomains upon interaction is detected. Still, p53TAD^1–60 remains highly flexible in the bound form, with residues L26, M40, and W53 being anchored to identical hydrophobic pockets of the S100A4 monomer chains. In the resulting “fuzzy” complex, the clamp-like binding of p53TAD^1–60 relies on specific hydrophobic anchors and on the existence of extended flexible segments. Our results demonstrate that structural and dynamical NMR parameters (cumulative Δδ, SSP, temperature coefficients, relaxation time, hetNOE) combined with MD simulations can be used to build a structural model even if, due to high flexibility, the classical solution structure calculation is not possible.     

Phase evolution and microstructure of SiC–MgAl2O4–Al composites sintered in argon atmosphere at 1450 °C: Reactions in Al–O–C system

SiC–MgAl₂O₄–Al composites were prepared using SiC, MgAl₂O₄, corundum and Al powder as raw materials by firing at 1450 °C in flowing argon. The effects of binders on phase composition and microstructure evolution were investigated. The results show that the external of specimens with thermosetting phenolic resin and water soluble resin as binders both contained SiC, MgAl₂O₄, corundum and Al₄O₄C phases. In center of the specimens, Al₄C₃ and Al₂OC phases appeared in the thermosetting phenolic resin bonded specimens while Al₄O₄C and Al₂OC phases appeared in the water soluble resin bonded specimens. The microstructure shows that MgAl₂O₄ whiskers generated at surface of both specimens through gas-gas reaction. As for the specimens with thermosetting phenolic resin as binder, the residual carbon (decomposed from phenolic resin) and Al in the external of the specimen react with O₂ to form CO and Al₂O, which promoting the formation of plate-like Al₄O₄C. In the center of the specimen, Al₄C₃, Al₂O₃ and Mg(g) formed. Mg(g) migrates to outside and transforms to MgAl₂O₄ whiskers, and a part of Al₄C₃ transform to columnar Al₂OC. Al₂OC whiskers can also be generated through the reaction among Al₂O, CO and C. Compared with the thermosetting phenolic resin bonded specimens, Al at external of the water soluble resin bonded specimen reacts with O₂ to form Al₂O₃, which further transforms to particle Al₄O₄C. In the center of water soluble resin bonded specimen, a little Al₄C₃ formed and can totally transform to Al₄O₄C and Al₂OC.     

Many-body effects in some thermodynamic properties of supercritical CO2, CO2–Ar,and CO2–CH4 using HFD-like potentials from molecular dynamics simulation

Molecular dynamics simulation has been performed to obtain the pressure and self-diffusion coefficient of supercritical carbon dioxide using a two-body HFD (Hartree–Fock dispersion)-like potential determined via the inversion of reduced viscosity collision integrals at zero pressure. We have also obtained pressures of CO₂–Ar and CO₂–CH₄ fluid mixtures at constant temperatures at different densities using new accurate two-body HFD-like potential functions. To take many-body forces into account, the three-body potentials of Hauschild and Prausnitz [27], Wang and Sadus [30], [38], Oakley et al. [3], and Guzman et al. [33] have been used with the two-body potentials. The significance of this work is that the modified many-body potential of Hauschild and Prausnitz (extended as a function of density, temperature, and molar fraction) has been used with the two-body HFD-like potentials of CO₂, CO₂–Ar, and CO₂–CH₄ systems to improve the prediction of the pressure values without requiring an expensive three-body calculation. The results are in good agreement with experimental values.     

Abnormal upconversion luminescence induced by defects and Er3+–Yb3+ distance change in Cs3GdGe3O9:Er3+/Yb3+ phosphors

A series of Er³⁺/Yb³⁺ co-doped Cs₃GdGe₃O₉ (CGG) phosphors were prepared by solid-phase sintering method, and the microstructure and upconversion luminescence (UCL) properties were tested by variable-temperature X-ray diffractometry and variable-temperature spectrometer. Abnormal UCL phenomena were found, which include UCL intensity continuously increasing under 980 nm laser continuous irradiation and UCL thermal enhancement. After 10 min of continuous irradiation by 980 nm laser at 513 K, the UCL intensity increased 2.91 times compared with the initial UCL intensity. The phenomenon is due to the electron releasing of host defects. The green UCL intensity of CGG:0.1Er³⁺/0.2Yb³⁺ decreases at 303–423 K and increases at 423–723 K, which reaches 13.23 times compared with that at 423 K. The phenomenon is due to Er³⁺–Yb³⁺ distance change by temperature and phonon-assisted transitions. In addition, the absolute temperature sensitivities of samples are calculated by luminescence intensity ratio technology, the maximum absolute sensitivity of CGG:0.1Er³⁺/0.4Yb³⁺ is 0.00691 K⁻¹ at 546 K, and the maximum relative sensitivity of CGG:0.1Er³⁺/0.1Yb³⁺ is 0.01224 K⁻¹ at 303 K. These results indicate that CGG:Er³⁺/Yb³⁺ phosphors can be used as a high-temperature optical thermometer.     

The direct influence of the support on the electronic structure of the active sites in supported metal catalysts: evidence from Pt–H anti-bonding shape resonance and Pt–CO FTIR data

The catalytic activity and spectroscopic properties of supported noble metal catalysts are strongly influenced by support properties such as the presence of protons, type of charge compensating cations, Si/Al ratio and/or presence of extra-framework Al. The metal–support interaction is relatively independent of the metal (Pd or Pt) or the type of support (microporous zeolites such as LTL and Y or macroporous supports such as SiO₂). As the alkalinity of the support increases (i.e., with increasing electronic charge on the support oxygen ions), the TOF of the metal particles for neopentane hydrogenolysis decreases. At the same time, there is a systematic shift from linear to bridge bonded CO as indicated by the IR spectra. This is a strong indication of a change in the electronic structure of the catalytically active Pt surface atoms. Analysis of the Pt–H anti-bonding shape resonance present in the Pt X-ray absorption spectra of the L₃ edge indicates that the difference in energy between the Pt–H anti-bonding orbital and the Fermi level decreases as the alkalinity of the support increases. The results from the IR and Pt–H shape resonance data directly show that the support influences the position in energy of the metal valence orbitals. The ionisation potential of the catalytically active Pt surface atoms decreases with increasing support alkalinity, i.e., with increasing electron charge on the support oxygen ions. This shift leads to a weakening of the Pt–H bond.     

In situ growth of LaSr(Fe,Mo)O4 ceramic anodes with exsolved Fe–Ni nanoparticles for SOFCs: Electrochemical performance and stability in H2, CO,and syngas

Fe–Ni nanoparticle–decorated LaSr(Fe,Mo)O₄ Ruddlesden–Popper (R–P) perovskite anodes, named R–LSFMNx, were prepared in situ by reducing perovskites La_0.5Sr_0.5Fe_0.9Mo_0.1–xNi_xO_3–δ (LSFMNx; x = 0.03–0.07) under SOFC anode operating conditions. Electrolyte–supported single cells with a configuration of R–LSFMNx|La_0.9Sr_0.1Ga_0.8Mg_0.2O_3–δ (LSGM)|Ba_0.5Sr_0.5Co_0.9Nb_0.1O_3–δ were used to evaluate the electrochemical performances and redox/long–term stability of the R–LSFMNx anodes fuelled by H₂, CO, and simulated syngases (x% H₂/CO; x = 50–10). EIS analyses indicated that the increased Ni level in the exsolved Fe–Ni nanocatalysts significantly promotes fuel diffusion/adsorption/dissociation, which plays a rate–limiting role in the anode fuel oxidation. Furthermore, the incremental Ni in Fe–Ni alloy also enhances the anode redox/long–term stability and carbon resistance/tolerance, and the R–LSFMN0.07 anode, i.e., Ni level in Fe–Ni alloy attaining ～14 mol.%, displays the optimal stability and carbon resistance/tolerance. Finally, the potential of the R–LSFMN0.07 anode for direct utilization of syngas was demonstrated by the characterization of the electrochemical performance and stability based on the R–LSFMN0.07 anode cell.     

New photosensitive and optically active organo-soluble poly(amide–imide)s from N,N′-(bicyclo[2,2,2]oct-7-ene-tetracarboxylic)-bis-L-amino acids and 1,5-bis(4-aminophenyl)penta-1,4-dien-3-one: synthesis and characterization

A new series of N,N′-(bicyclo[2,2,2]oct-7-ene-tetracarboxylic)-bis-L-amino acids 3a–g were synthesized by the condensation reaction of bicyclo[2,2,2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride 1 with two equimolars of various amino acids such as L-alanine 2a, L-valine 2b, L-leucine 2c, L-isoleucine 2d, L-phenyl alanine 2e, L-2-aminobutyric acid 2f and L-histidine 2g in an acetic acid solution. Also 1,5-bis(4-aminophenyl)penta-1,4-dien-3-one 7 was synthesized by using a two-step reaction. At first 1,5-bis(4-nitrophenyl)penta-1,4-dien-3-one 6 was prepared from the reaction of two equimolars 4-nitrobenzaldehyde 5 and one equimolar acetone 4 in ethanol and NaHCO₃ and dinitro compound 6 was reduced by using Na₂S. Then seven new photosensitive and optically active organo-soluble poly(amide–imide)s (PAIs) 8a–g with good inherent viscosities were synthesized from the direct polycondensation reaction of new N,N′-(bicyclo[2,2,2]oct-7-ene-tetracarboxylic)-bis-L-amino acids 3a–g with 1,5-bis(4-aminophenyl)penta-1,4-dien-3-one 7 by two different methods such as direct polycondensation in a medium consisting of N-methyl-2-pyrrolidone (NMP)/triphenyl phosphite (TPP)/calcium chloride (CaCl₂)/pyridine (py) and direct polycondensation in a tosyl chloride (TsCl)/pyridine (py)/N,N-dimethylformamide (DMF) system. The polymerization reactions produced a series of photosensitive and optically active organo-soluble PAIs with high yield and good inherent viscosity. The resulted polymers were fully characterized by means of FTIR and ¹H-NMR spectroscopy, elemental analyses, inherent viscosity, specific rotation, solubility tests, UV-vis spectroscopy, differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), and derivative of thermaogravimetric (DTG). These macromolecules exhibited maximum UV-vis absorption at around 370 and 265 nm in a DMF solution.     

Insights in the sol–gel processing of Pb(Mg1/3Nb2/3)O3. The synthesis and crown structure of a new lead magnesium cluster: Pb6Mg12(μ-OAc)6(μ2,η2-OAc)18(μ3,η2-OC2H4OPri)12

The synthesis and molecular structure of a Pb–Mg bimetallic acetatoalkoxide (Pb₆Mg₁₂(μ-OAc)₆(μ₂,η²-OAc)₁₈(μ₃,η²-OC₂H₄OPrⁱ)₁₂, space group R-3, a=b=30.032(2), c=18.855(2) Å, α=β=90°, γ=120°) are discussed in this article. This compound was isolated as an intermediate during the elaboration of Pb(Mg_1/3Nb_2/3)O₃ (PMN) using sol–gel process. It results from the reaction of a bimetallic Mg/Nb species with lead acetate in 2-isopropoxyethanol.