Preparation and Properties of Lightweight Aggregates from Discarded Al2O3-ZrO2-C Refractories.

Refractory materials are an important pillar for the stable development of the high-temperature industry. A large amount of waste refractories needs to be further disposed of every year, so it is of great significance to carry out research on the recycling of used refractories. In this work, lightweight composite aggregate was prepared by using discarded Al2O3-ZrO2-C refractories as the main raw material, and the performance of the prepared lightweight aggregate was improved by adjusting the calcination temperature and introducing light calcined magnesia additives. The results showed that the cold compressive strength and thermal shock resistance of the lightweight aggregates were significantly improved with increasing calcination temperature. Moreover, the introduction of light calcined magnesia can effectively improve the apparent porosity, cold compressive strength, and thermal shock resistance of the prepared lightweight aggregates at the calcination temperature of 1400 °C. Consequently, this work provides a useful reference for the resource utilization of used refractories, while the prepared lightweight aggregates are expected to be applied in the field of high-temperature insulation.

Study on Low-Velocity Impact and Residual Compressive Mechanical Properties of Carbon Fiber-Epoxy Resin Composites.

Room temperature drop hammer impact and compression after impact (CAI) experiments were conducted on carbon fiber-epoxy resin (CF/EP) composites to investigate the variation in impact load and absorbed energy, as well as to determine the residual compressive strength of CF/EP composites following impact damage. Industrial CT scanning was employed to observe the damage morphology after both impact and compression, aiding in the study of impact-damage and compression-failure mechanisms. The results indicate that, under the impact load, the surface of a CF/EP composite exhibits evident cratering as the impact energy increases, while cracks form along the length direction on the back surface. The residual compressive strength exhibits an inverse relationship with the impact energy. Impact damage occurring at an energy lower than 45 J results in end crushing during the compression of CF/EP composites, whereas energy exceeding 45 J leads to the formation of long cracks spanning the entire width of the specimen, primarily distributed symmetrically along the center of the specimen.

Effect of In Situ Mg-Sialon on the Oxidation Behavior of Low-Carbon MgO-C Refractories.

The in situ Mg-sialon in low-carbon MgO-C refractories was studied with respect to its oxidation behavior and mechanism at 1500 °C. The results indicated that the oxidation index and rate constant of low-carbon MgO-C refractories with Mg-sialon were 26.2% and 0.51 × 10-3 cm2/min at 1500 °C for 2 h, respectively. The formation of a dense MgO-Mg2SiO4-MgAl2O4 protective layer contributed to considerable oxidation resistance, and the generation of this thicker layer was due to the combined volume effect of Mg2SiO4 and MgAl2O4. The reduced porosity and more complex pore structure were also found in the refractories with Mg-sialon. Therefore, further oxidation was restricted as the oxygen diffusion path was effectively blocked. This work proves the potential application of Mg-sialon in improving the oxidation resistance of low-carbon MgO-C refractories.

Improvement of Mechanical Properties of Composites with Surface Modified B4C for Precision Machining.

In order to solve the problem of difficult sintering and high brittleness of B4C-based ceramics, B4C@ZrB2-TiB2 composite powder was synthesized by molten salt method, and B4C-(Zr, Ti)B2 composite ceramics were successfully prepared by spark plasma sintering. The effects of different raw material ratios on the composition, microstructure, and mechanical properties of the prepared composite ceramics were characterized by XRD, XPS, SEM, and TEM. The results show that ZrB2 and TiB2 were grown on the surface of B4C by template mechanism to form a dense nanocrystalline coating, and the original surface of B4C was exposed gradually with the decrease of the ratio of metal powder. When the composite powders were sintered at 1700 °C, ZrB2 and TiB2 formed a solid solution, which can refine grains and improve strength. When the raw material ratio is n(B4C): n(Zr): n(Ti) = 12:1:1, the composite ceramics have excellent comprehensive properties, the Vickers hardness reaches 41.2 GPa.

Liquid-Phase-Assisted Catalytic Nitridation of Silicon and In Situ Growth of α-Si3N4.

Si3N4 powders were synthesized with Fe, Co, or Ni as catalysts using Si powder at 1250 °C in a nitrogen atmosphere by liquid-phase-assisted catalytic nitridation synthesis (LPA-CNS). The catalytic effects of the metals on the nitridation of silicon powder were investigated by mixing the powder with 2 wt% by mass of Fe, Co, or Ni in a high-temperature liquid phase in flowing nitrogen. The α-Si3N4 micro-morphology could be effectively changed by adjusting the type of catalyst in the initial reaction mixtures. Fe, Co, and Ni promoted the formation of α-Si3N4 at 1250 °C and controlled the morphology of the α-Si3N4 particles. The hexagonal flakes of α-Si3N4 with a better defined morphology were obtained using Ni as the catalyst, compared to that obtained from the other two catalysts.

Observing dynamic molecular changes at single-molecule level in a cucurbituril based plasmonic molecular junction.

In recent years, surface enhanced Raman spectroscopy (SERS) has emerged as a prominent tool for probing molecular interaction and reaction with single-molecule sensitivity. Here we use SERS to investigate the dynamic changes of the cucurbit[7]uril (CB[7]) based plasmonic molecular junctions in solution, which are spontaneously formed by the adsorption of gold nanoparticles (GNPs) at the CB[7] modified gold nanoelectrode (GNE) surface. The typical fingerprint Raman peaks of CB[7] are very weak in the SERS spectra. However, chemically enhanced peaks are prominent in the spectra due to the charge transfer across the metal-molecule interface through specific noncovalent interactions between the gold atoms and CB[7] or its guest molecule. We first investigated the selectively enhanced and greatly shifted C[double bond, length as m-dash]O peak of CB[7] in the SERS spectra. Based on the bias-dependent changes of the C[double bond, length as m-dash]O peak, we found the gold-carbonyl interaction was strengthened by the positive bias applied to the GNE, resulting in stable CB[7] junctions. Next, we found the CB[7] junction could also be stabilized by the inclusion of a guest molecule amino-ferrocene, attributed to the interactions between gold adatoms and the cyclopentadienyl ring of the guest molecule. Because this interaction is sensitive to the orientation of the guest molecule in the cavity, we revealed the rotational motion of a guest molecule inside the CB[7] cavity based on the dynamic spectral changes of the cyclopentadienyl ring peak.

Low temperature synthesis via molten-salt method of r-BN nanoflakes, and their properties.

r-BN nanoflakes were synthesized using KBH4 and NH4Cl as the main raw material in a high-purity nitrogen atmosphere. The effects of salt and salt-free conditions and heating temperature on the synthesis of BN were studied. The molten-salt method was used to synthesize BN at 650 °C, which was 250 °C lower than the BN synthesis method without salt. Furthermore, at 1000 °C the prepared flake-like BN crystals showed good crystallinity, uniform morphology, a particle diameter of 200-300 nm, and a thickness of 40-70 nm. Moreover, the specific surface area of BN was 294.26 m2/g. In addition, the BN synthesized at 1100 °C had a large elastic modulus value and good oxidation resistance.