Structural property-induced different phonon-twin-boundary scattering in diamond.

Twin boundaries (TBs), as one kind of crystal defect, have often been observed in various material systems, and the (111)/[110] TB has been verified to show weak phonon scattering. However, it's still not clear whether other TBs can show similar thermal properties to the (111)/[110] TB. To solve this issue, in this work, we perform a systematic study of heat transport across six kinds of twin boundaries in diamond, including the (111)/[110], (221)/[110], (331)/[110], (113)/[110], (112)/[110] and (310)/[001] TBs, by both molecular dynamics simulations and first-principles calculations. The results indicate that the thermal boundary resistance of the six TBs ranges from 1.01 × 10-11 to 6.35 × 10-10 m2 K W-1; specifically, the (111)/[110] TB shows much weaker phonon scattering than the others. The different phonon scattering at TBs mainly depends on the transmission coefficients across the twin boundaries for boundaries with the same symmetry, as well as the combined action of group velocity and phonon mean free path. Furthermore, by analyzing the structural properties of TBs, it can be observed that TB thermal resistance varies significantly with the TB structure, and is strongly correlated with TB energy and bond difference parameter. Our findings will provide useful guidelines for designing efficient thermoelectric and thermal management materials based on phonon-TB scattering.

High bond difference parameter-induced low thermal transmission in carbon allotropes with sp2 and sp3 hybridization.

Carbon allotropes play an important role in the thermal transmission field, while there are huge thermal differences in their thermal conductivities. In this work, thermal transmission in three novel carbon allotropes with sp2 and sp3 hybridization has been studied, including T6-carbon, T10 and 3D-C5 by using non-equilibrium molecular dynamic simulations and phonon kinetic theory. Graphene and diamond with standard sp2 and sp3 hybridization, respectively, are also examined for comparison. Our results indicate that the thermal conductivities of T6-carbon, T10 and 3D-C5 at room temperature are much lower than those of diamond and graphene. Phonon kinetic theory analysis shows that the lower thermal conductivity of T6-carbon, T10 and 3D-C5 is caused by the combined action of their reduced phonon group velocities and relaxation time. Moreover, the bond difference parameter has been proposed to describe the relationship between bond structures and thermal conductivity in carbon allotropes, which presents a new and convenient method for in-depth understanding the thermal conductivity of carbon allotropes.

Study on the correlation between microstructures and corrosion properties of novel ZrTiAlV alloys.

In this work, microstructures and corrosion behaviors of novel ZrTiAl-χV (χ = 0, 1, 3, 5, 7 wt%) alloys have been investigated. Phase composition and microstructures of the specimens are characterized using X-ray diffraction, optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results show that phase composition of the alloys change by α' → α″ → α″ + ßâ€¯â†’â€¯ß as V is added gradually. Meanwhile, the mean size of the α' phase, α″ martensite and ß phase decreases as V content increases. In order to test the corrosion performance of the specimens, potentiodynamic polarization tests in NaCl and HCl solutions and immersion tests in HCl solution are performed. Analysis of potentiodynamic potential curves indicates that corrosion potential increases and corrosion current density decreases with adding V content in the examined alloys. From the results of weight loss tests, it can be observed that the weight loss of the examined alloys decreases along with the increase of V content. In addition, when the content of V is added from 3 to 7 wt%, the metastable corrosion pits are replaced with the stable corrosion pits. The phase composition as well as the grain size can be identified as the main factor affecting the corrosion performance of the alloys.

Thermal Conductivity of Diamond/SiC Nano-Polycrystalline Composites and Phonon Scattering at Interfaces.

To compare the thermal properties of heterogeneous and homogeneous interfaces, polycrystalline composites are proposed. Thermal properties of heterogeneous and homogeneous interfaces in the composites are investigated using molecular dynamics simulations. The results indicate that when the inflow of heat arises from the same material, phonon scattering at heterogeneous interfaces is stronger than that at homogeneous interfaces. The phonon wave packet simulations indicate that the stronger phonon scattering at heterogeneous interfaces is caused by the combined actions of transmission coefficients and transmission time.

Weak phonon scattering effect of twin boundaries on thermal transmission.

To study the effect of twin boundaries on thermal transmission, thermal conductivities of twinned diamond with different twin thicknesses have been studied by NEMD simulation. Results indicate that twin boundaries show a weak phonon scattering effect on thermal transmission, which is only caused by the additional twin boundaries' thermal resistance. Moreover, according to phonon kinetic theory, this weak phonon scattering effect of twin boundaries is mainly caused by a slightly reduced average group velocity.

Relative importance of grain boundaries and size effects in thermal conductivity of nanocrystalline materials.

A theoretical model for describing effective thermal conductivity (ETC) of nanocrystalline materials has been proposed, so that the ETC can be easily obtained from its grain size, single crystal thermal conductivity, single crystal phonon mean free path (PMFP), and the Kaptiza thermal resistance. In addition, the relative importance between grain boundaries (GBs) and size effects on the ETC of nanocrystalline diamond at 300 K has been studied. It has been demonstrated that with increasing grain size, both GBs and size effects become weaker, while size effects become stronger on thermal conductivity than GBs effects.

Improvement of thermal stability of polypropylene using DOPO-immobilized silica nanoparticles.

After the surface silylation with 3-methacryloxypropyltrimethoxysilane, silica nanoparticles were further modified by 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The immobilization of DOPO on silica nanoparticles was confirmed by Fourier transform infrared spectroscopy, UV-visible spectroscopy, magic angle spinning nuclear magnetic resonance, and thermogravimetric analysis. By incorporating the DOPO-immobilized silica nanoparticles (5 wt%) into polypropylene matrix, the thermal oxidative stability exhibited an improvement of 62 °C for the half weight loss temperature, while that was only 26 °C increment with incorporation of virgin silica nanoparticles (5 wt%). Apparent activation energies of the polymer nanocomposites were estimated via Flynn-Wall-Ozawa method. It was found that the incorporation of DOPO-immobilized silica nanoparticles improved activation energies of the degradation reaction. Based on the results, it was speculated that DOPO-immobilized silica nanoparticles could inhibit the degradation of polypropylene and catalyze the formation of carbonaceous char on the surface. Thus, thermal stability was significantly improved.