Search

A novel lightweight organic aerogel-impregnated low-density carbon/carbon (C/C) composite was produced through vacuum impregnation using carbon-bonded carbon fibre (CBCF) as substrate and resorcinol-furfural (RF) aerogel as infiltrant. Microstructural analysis showed that fibres in CBCF were uniformly coated with a thin layer of aerogels, acting as the supporting skeletons, to strengthen the aerogel matrix. The as prepared RF-C/C possessed low densities between 0.26 and 0.37g/cm3, relatively high compressive strength, ranging from 0.45 to 3.27MPa, and low thermal conductivities of 0.105 to 0.350W/(mK) at room temperature. Furthermore, good thermal ablative and insulative properties (recession rates as low as 0.082mm/s and internal temperature peaks below 90°C at 38mm in-depth position as the surface temperature exceeded 2000°C) under an atmospheric re-entry condition. From mentioned above, RF-C/C present huge application prospects in heat preservation and thermal protection field, especially in energy-saving and aerospace. Keywords: Aerogel, Carbon/carbon, Thermal properties, Mechanical properties, Ablation

Through design hierarchical architectures and interlayer interaction, multifunctional macroscopic graphene oxide papers tend to attain outstanding mechanical properties. Three dimensional architectures functionalized graphene oxide papers are self-assembled with zirconium diboride particles and poly(vinyl alcohol, PVA) using vacuum assisted filtration method. Firstly, ternary graphene oxide micro-scale structures are achieved by the addition of zirconium diboride particles. Then, PVA is introduced to enhance the interface interaction between graphene oxide nanosheets and zirconium diboride particles. The results show significant enhancements in fracture strength (~200%) and tensile modulus (20–100%) are attained through synergistic effect of ternary graphene oxide structures and interlayer interaction between PVA, graphene oxide and zirconium diboride particles. Keywords: Graphene oxide paper, Zirconium diboride, Mechanical behaviors

Lightweight carbon-bonded carbon fiber (CBCF) composites were fabricated with chopped carbon fibers and dilute phenolic resin solution by pressure filtration, followed by carbonization at 1000 °C in argon. The as-prepared CBCF composites had a homogenous fiber network distribution in xy direction and quasi-layered structure in z direction. The pyrolytic carbon derived from phenolic resin was mainly accumulated at the intersections and surfaces of chopped carbon fibers. The composites possessed compressive strengths ranged from 0.93–6.63 MPa in xy direction to 0.30–2.01 MPa in z direction with a density of 0.162–0.381 g cm−3. The thermal conductivity increased from 0.314–0.505 to 0.139–0.368 Wm−1 K−1 in xy and z directions, respectively. The experimental results indicate that the CBCF composites prepared by this technique can significantly contribute to improve the thermal insulation and mechanical properties at high temperature. Keywords: Carbon fiber, Phenolic resin, Microstructure, Compressive strength, Thermal properties

ZrB 2 –SiC nanocomposite ceramics toughened by ZrO 2 fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO 2 fiber incorporated into the ZrB 2 –SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO 2 fiber exhibited remarkable effects on the fracture toughness of the ZrO 2(f) /ZrB 2 –SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m 1/2 ± 0.3 MPa m 1/2 by the addition of 15 mass% of ZrO 2 fiber. The microstructure of the ZrO 2 fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO 2 to m-ZrO 2 , which was also an important reason behind the improvement in toughness.

The effects of cooling and heating velocity on the microstructure and mechanical properties of the hot-pressed ZrB2–20 vol.% SiC ceramics added with 15 vol.% Zirconia fiber were investigated in details. It was indicated that the appropriate cooling velocity could reduce the residual stress concentration, and the proper heating velocity could allowed the green compacts to get densified, which was favorable to enhance the flexural strength and fracture toughness to 1117 ± 98 MPa and 7.1 ± 0.5 MPa m1/2, respectively. Due to the introduction of the Zirconia fiber, the observed toughening mechanisms were attributed to the fiber debonding, fiber pull-out and phase transformation toughening. The results presented here point out a promising way for improving the mechanical properties of ZrB2-based ultra-high temperature ceramics.

HfB 2 –20 vol.% MoSi 2 ultra high temperature ceramic composites were prepared through aqueous gelcasting route. The stability of HfB 2 and MoSi 2 suspensions were studied by zeta potential measurements, sedimentation tests and apparent viscosity measurements. The solids loading had significant effects on the green and sintered densities, microstructure and mechanical properties of HfB 2 –MoSi 2 composites. The values of flexural strength of the green and sintered bodies ranged from 18.3 to 38.7, and 111.5 to 415.9 MPa, respectively, which were strongly dependent on the solids loading. The values of fracture toughness of the sintered bodies ranged from 2.18 to 4.24 MPa m 1/2 . The highest relative density, mechanical properties and the most homogeneous microstructure was obtained when the solids loading was 45 vol.%. The highest green strength, flexural strength and fracture toughness were 38.7 ± 5.3 MPa, 415.9 ± 17.0 MPa and 4.24 ± 0.22 MPa m 1/2 , respectively.

The ZrB 2 –SiC–Graphite ceramic was immerged in the supercritical water for different times. The microcracks appeared on the surface of the specimen and the composition of the microcracks was confirmed by EDS analysis to be ZrO 2 . The obvious corrosion of SiC and graphite flake on the surface of the specimen was not observed, which was attributed to the dissolution of the oxides of silicon and carbon (graphite) into the supercritical water. The corrosion of the specimen was accelerated as the pressure and temperature of the water increased. XPS analysis was carried out on the specimen corroded in water of 40 ± 1 MPa and 500 ± 10 °C for 75 min, and the significant peak of B 1s was also measured, indicating the presence of the ZrB 2 phase on the surface of the specimen. For the specimen immerged in all conditions, the Vickers’ hardness did not reduce, the fracture toughness was improved and the minimal strength of the immerged specimen was still higher than 90% of the original strength of 480 MPa, which indicated that the ZrB 2 –SiC–Graphite ceramic has excellent resistance to corrosion of the supercritical water applied to reactor.

Fully dense Zirconium Diboride (ZrB2)-based ceramics with Zirconium Silicide (ZrSi2) and Silicon Carbide platelet (SiCpl) additive were fabricated by hot-press sintering in flowing argon atmosphere. Their mechanical properties and microstructure were examined. The contents of ZrSi2 and SiCpl were optimized. The defined addition of ZrSi2 was beneficial for densification and improving the mechanical properties of the ZrB2-based ceramics. The addition of SiCpl obviously improved the fracture toughness of the ZrB2-based ceramics. The range of fracture toughness of ZrB2–SiCpl–ZrSi2 composite was 6.7–9.0 MPa m1/2. The flexural strength of the composites varied a little and the maximum was 597 MPa when the content of ZrSi2 and SiCpl was both 5 vol.%. XRD results showed that a small amount of Zirconium Carbide (ZrC) formed during sintering. The microstructure of the composites was fine, compact and homogeneous.

The critical thermal shock temperature difference to represent the thermal shock resistance of ceramic matrix composites was commonly predicted by the thermal shock parameter R TSF = σ ( 1 - υ ) E α . It can be usually measured experimentally by quenching specimens from various elevated temperatures and determining the quenching temperature difference that results in a reduction of strength for a given specimen geometry. However, the large difference between the measured (ΔTc) and calculated values always exists. In this study, a modified thermal shock parameter (Rm) of ceramic matrix composites was proposed based on theoretical derivation and experimental verification, although the new coefficient k was introduced. The value of coefficient k in the Rm was validated by our present and previous experiment data. Furthermore, the Rm was verified using the literature data by comparing its predictions with the ΔTc reported in the literatures. The results showed that the calculated Rm was very close to the ΔTc measured in the literatures.

The crack-healing behavior of Zr 2 Al 4 C 5 was investigated by oxidizing the pre-cracked sample. A Vickers indentation (∼80 μm in size) was introduced and the crack was healed under different temperature. Cracks could heal completely in air above 1000 °C, with oxidation products consisting primarily of t-ZrO 2 and α-Al 2 O 3 . The bending strength of crack-healed specimen (188–283 MPa) is higher than that of cracked samples (182 MPa), but all of them are lower than the strength of as-sintered and polished specimens (420 MPa). This composite has self crack-healing ability at temperatures ranging from 800 to 1200 °C, with 1000 °C being the optimum healing temperature.

The major problems associated with zirconium diboride-based ceramics are lack of damage tolerance as structure components, so ZrB 2 /20 vol% SiC based ceramics toughened by 20 vol% short carbon fibers were produced by hot pressing at 2000 °C under 30 MPa for 1 h. SEM results showed that short carbon fibers could disperse uniformly in powder mixtures and sintered sample through milling. Three-point bend strength and fracture toughness were measured. The decrease in strength from 448 MPa to 397 MPa as the addition of short fibers was attributed to the chemical reaction between fiber and matrix. The fracture toughness increased from 4.25 MPa m 1/2 for ZrB 2 /SiC to 6.35 MPa m 1/2 for short fibers toughened ZrB 2 /SiC, which was led by the crack deflection and fiber fracture.