Due to a high propensity of nano-particles to agglomerate, making aluminium matrix composites with a uniform dispersion of the nano-particles using liquid routes is an exceptionally difficult task. In this study, an innovative approach was utilised to prevent agglomeration of nano-particle by encapsulating SiC nano-particles using graphene sheets during ball milling. Subsequently, the milled mixture was incorporated into A356 molten alloy using non-contact ultrasonic vibration method. Two different shapes for graphene sheets were characterised using HRTEM, including onion-like shells encapsulating SiC particles and disk-shaped graphene nanosheets. This resulted in 45% and 84% improvement in yield strength and tensile ductility, respectively. The former was ascribed to the Orowan strengthening mechanism, while the latter is due primarily to the fiber pull-out mechanism, brought about by the alteration of the solidification mechanism from particle pushing to particle engulfment during solidification as a consequence of high thermal conductive graphene sheets encapsulating SiC particles.

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Enhanced tensile properties of aluminium matrix composites reinforced with graphene encapsulated SiC nanoparticles

Thermal diffusivity and specific heat of 4H-SiC crystals as a function of temperature are measured, respectively, from room temperature to 600 °C. The thermal conductivity normal to c-axis was calculated from the measured data for both N-type and V-doped semi-insulating (SI) 4H-SiC single crystals. The thermal conductivity of N-type sample normal to c axis is proportional to T−1.26. It is approximately 280 W/mK at the room temperature. For V-doped SI sample, the thermal conductivity is proportional to T−1.256 and it is about 347 W/mK at room temperature, bigger than that of N-type sample. For semiconductor materials, total thermal conductivity is the sum of the contributions of lattice and carrier thermal conductivities. Temperature dependent Raman spectrum showed that the life time of phonons for N-type sample is shorter than that for SI sample. Accordingly thermal conductivity contributions from both lattice and carrier components are relatively small for N-type sample.

Thermal conductivity of 4H-SiC single crystals

HfB{sub 2} and ZrB{sub 2} are of interest for thermal protection materials because of favorable thermal stability, mechanical properties, and oxidation resistance. We have made dense diboride ceramics with 2 to 20 % SiC by hot pressing at 2000 C and 5000 psi. High-resolution transmission electron microscopy (TEM) shows very thin grain boundary phases that suggest liquid phase sintering. Fracture toughness measurements give RT values of 4 to 6 MPam{sup 1/2}. Four-pt flexure strengths measured in air up to 1450 C were as high as 450-500 MPa. Thermal diffusivities were measured to 2000 C for ZrB{sub 2} and HfB{sub 2} ceramics with SiC contents from 2 to 20%. Thermal conductivities were calculated from thermal diffusivities and measured heat capacities. Thermal diffusivities were modeled using different two-phase composite models. These materials exhibit excellent high temperature properties and are attractive for further development for thermal protection systems.

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Ultra High Temperature Ceramics for Hypersonic Vehicle Applications

Although estrogen loss can alter skeletal muscle recovery from disuse, the specific components of muscle regrowth that are estrogen sensitive have not been described. The primary purpose of this study was to determine the components of skeletal muscle mass recovery that are biological targets of estrogen. Intact, ovariectomized (OVX), and ovariectomized with 17beta-estradiol replacement (OVX+E2) female rats were subjected to hindlimb suspension for 10 days and then returned to normal cage ambulation for the duration of recovery. Soleus muscle mass returned to control levels by day 7 of recovery in the intact animals, whereas OVX soleus mass did not recover until day 14. Intact rats recovered soleus mean myofiber cross-sectional area (CSA) by day 14 of recovery, whereas the OVX soleus remained decreased (42%) at day 14. OVX mean fiber CSA did return to control levels by day 28 of recovery. The OVX+E2 treatment group recovered mean CSA at day 14, as in the intact animals. Myofibers demonstrating central nuclei were increased at day 14 in the OVX group, but not in intact or OVX+E2 animals. The percent noncontractile tissue was also increased 29% in OVX muscle at day 14, but not in either intact or OVX+E2 groups. In addition, collagen 1a mRNA was increased 45% in OVX muscle at day 14 of recovery. These results suggest that myofiber growth, myofiber regeneration, and extracellular matrix remodeling are estrogen-sensitive components of soleus muscle mass recovery from disuse atrophy.

Estrogen status and skeletal muscle recovery from disuse atrophy.

Strengthening mechanisms of graphene sheets in aluminium matrix nanocomposites

The temperature dependent thermal conductivity of silicon carbide has been calculated taking into account the various phonon scattering mechanisms. The results compared very well with available experimental data. The inclusion of four-phonon processes is shown to be necessary for obtaining a good match. Several important phonon scattering parameters have been extracted in this study. Dislocations are shown to have a strong effect at 300 K, but not as much at the higher temperatures.

P. G. Neudeck

Papers

Analysis of the temperature dependent thermal conductivity of silicon carbide for high temperature applications