Zhijun Lin

Nanocrystalline tungsten carbide: As incompressible as diamond

We investigate the compressibility of nanocrystalline tungsten carbide (nano-WC) using synchrotron x-ray diffraction. Nano-WC displays a bulk modulus (452 GPa) comparable to that of diamond; it is 10%–15% larger than previously reported values for bulk WC. This finding is consistent with a generalized model of nanocrystal with a compressed surface layer. The linear bulk moduli of nano-WC along a- and c-axes were determined to be 407 and 546 GPa, respectively. First-principles density functional theory (DFT) calculations confirm the experimental observations of an anisotropic linear compressibility and a lower bulk modulus for microsized WC.

First-principles prediction of mechanical properties of gamma-boron

The structural and mechanical properties of γ-B28 are investigated using first-principles density functional calculations. The single-crystal elastic constants calculations show that γ-B28 is mechanically stable under ambient conditions. The predicted bulk and shear moduli of γ-B28 are comparable to those of boron suboxide, suggesting that γ-B28 can be a superhard material. We also obtained the ideal tensile strength for γ-B28 through deformation from the elastic regime to structural instability. We find that the breaking of B1–B1 and B1–B2 bonds is responsible for the failure of γ-B28 under ⟨100⟩ and ⟨010⟩ tensile deformation, respectively.

Phase Transition and Compressibility in Silicon Nanowires

Silicon nanowires (Si NWs), one-dimensional single crystalline, have recently drawn extensive attention, thanks to their robust applications in electrical and optical devices as well as in the strengthening of diamond/SiC superhard composites. Here, we conducted high-pressure synchrotron diffraction experiments in a diamond anvil cell to study phase transitions and compressibility of Si NWs. Our results revealed that the onset pressure for the Si I-II transformation in Si NWs is approximately 2.0 GPa lower than previously determined values for bulk Si, a trend that is consistent with the analysis of misfit in strain energy. The bulk modulus of Si-I NWs derived from the pressure-volume measurements is 123 GPa, which is comparable to that of Si-V NWs but 25% larger than the reported values for bulk silicon. The reduced compressibility in Si NWs indicates that the unique wire-like structure in nanoscale plays vital roles in the elastic behavior of condensed matter.

Constitutive Law and Flow Mechanism in Diamond Deformation

Constitutive laws and crystal plasticity in diamond deformation have been the subjects of substantial interest since synthetic diamond was made in 1950s. To date, however, little is known quantitatively regarding its brittle-ductile properties and yield strength at high temperatures. Here we report, for the first time, the strain-stress constitutive relations and experimental demonstration of deformation mechanisms under confined high pressure. The deformation at room temperature is essentially brittle, cataclastic, and mostly accommodated by fracturing on {111} plane with no plastic yielding at uniaxial strains up to 15%. At elevated temperatures of 1000°C and 1200°C diamond crystals exhibit significant ductile flow with corresponding yield strength of 7.9 and 6.3 GPa, indicating that diamond starts to weaken when temperature is over 1000°C. At high temperature the plastic deformation and ductile flow is meditated by the <110>{111} dislocation glide and a very active {111} micro-twinning.

Thermal equation of state of copper studied by high P-T synchrotron x-ray diffraction

At ambient conditions, Cu has a face-centered cubic structure with space group Fm3m 225. The starting Cu powders with grain sizes smaller than 10 m were commercially obtained from Sigma‐Aldrich. The high P-T experiments were conducted using a cubic anvil apparatus at beam line X17B2 of the National Synchrotron Light Source, Brookhaven National Laboratory. The white radiation from the superconducting wiggler magnet was used for energydispersive measurements. The diffracted x-rays were collected with a 13-element detector at a fixed Bragg angle of 2=6.4900°. The cell assembly used in experiments has been described elsewhere. 8

High pressure neutron and synchrotron X-ray diffraction studies of tetragonal LaFeAsO0.9F0.1

We studied polycrystalline tetragonal LaFeAsO0.9F0.1 up to 9 GPa using time-of-flight neutron and synchrotron X-ray diffraction. Consistent with previous studies, the layer-structured LaFeAsO0.9F0.1 has a bulk modulus of 74–79 GPa, with the c−axis being twice as compressible as the a−axis. The refined structural parameters under pressure show non-monotonic variation with kink points at 4 GPa, well correlating with a maximum superconducting temperature (Tc) of 43 K previously reported for LaFeAsO0.9F0.1. At this pressure, however, both the As–Fe–As bond angle and anion height from the Fe layers deviate substantially from the respective ideal values of 109.47° and 1.38 Å for a regular FeAs4 tetrahedron. These findings indicate that the correlation between the maximum Tc and the geometry of conductive Fe–As layers is material dependent and may also be sensitive to atomic doping in the parent Fe-based superconductors.