Cristian Pantea

Bio: Cristian Pantea is an academic researcher from Los Alamos National Laboratory. The author has contributed to research in topics: Diamond & Acoustic wave. The author has an hindex of 24, co-authored 96 publications receiving 1674 citations. Previous affiliations of Cristian Pantea include State University of New York System & Texas Christian University.

Enhancement of fracture toughness in nanostructured diamond-SiC composites

Abstract: We synthesized diamond–SiC nanocomposites with superhardness and greatly enhanced fracture toughness through a synthetic approach based on high-energy ball milling to form amorphous Si precursors followed by rapid reactive sintering at high pressure (P) and high temperature (T). We show how the simultaneous P–T application allows for better control of the reactive sintering of a nanocrystalline SiC matrix in which diamond crystals are embedded. The measured fracture toughness KIC of the synthesized composites has been enhanced greatly, as much as 50% from 8.2 to 12.0 MPa m1/2, as the crystal size of the SiC matrix decreases from 10 μm to 20 nm. Our result contradicts a commonly held belief of an inverse correlation between hardness and fracture toughness. We demonstrate the importance of nanostructure for the enhancement of mechanical properties of the composite materials.

Thermal equations of state of the α, β, and ω phases of zirconium

Abstract: We have conducted synchrotron x-ray diffraction studies on high purity zirconium metal at pressures (P) up to 17 GPa and temperatures (T) up to 973 K. Unit cell volumes (V) were derived from the refinements of x-ray diffraction data for the {alpha}, {beta}, and {omega} phases of zirconium and fitted to a Birch-Murnaghan equation of state with the pressure derivative of the bulk modulus, K{sub 0}{sup '}, fixed at 4.0. The derived thermoelastic parameters for {alpha} zirconium are isothermal bulk modulus K{sub 0}=92(3) GPa, temperature derivative of bulk modulus ({partial_derivative}K/{partial_derivative}T){sub P}=-2.3(8)x10{sup -2} GPa/K, volumetric thermal expansivity {alpha}{sub T}=a+bT with a=1.5({+-}0.8)x10{sup -5} K{sup -1} and b=1.7({+-}1.4)x10{sup -8} K{sup -2}, and the pressure derivative of thermal expansion ({partial_derivative}{alpha}/{partial_derivative}P){sub T}=-2.7(9)x10{sup -6} GPa{sup -1} K{sup -1}. For the {beta} phase we obtained an isothermal bulk modulus of K{sub T}=66(3) GPa at 973 K and a unit-cell volume of V(973 K)=47.7(3) A{sup 3} at ambient pressure. For the {omega} zirconium we obtained K{sub 0}=90(5) GPa. Within the experimental errors, the K{sub 0} values we determined for the {alpha} and {omega} phases and volumetric thermal expansion for the {alpha} phase are in agreement with previous experimental results, whereas all other thermoelastic parameters represent the first determinationsmore » for the three crystalline phases of zirconium metal.« less

Manipulation of diamond nanoparticles using bulk acoustic waves

Abstract: We investigate the manipulation of 5 nm diamond nanoparticles in a user-defined pattern on a substrate using the acoustic radiation force associated with a bulk acoustic standing wave. Both concentric and rectangular patterns are studied and the experimental results are compared with theoretical predictions. The effect of drag force acting on a nanoparticle is evaluated and limits for particle speed and particle size that can be moved by acoustic radiation force are determined. We found good agreement between our experimental results and existing theoretical models and demonstrate that nanosized particles can be manipulated effectively by means of bulk wave acoustic radiation force.

Colossal Positive and Negative Thermal Expansion in the Framework Material Ag3[Co(CN)6]

Abstract: We show that silver(I) hexacyanocobaltate(III), Ag3[Co(CN)6], exhibits positive and negative thermal expansion an order of magnitude greater than that seen in other crystalline materials. This framework material expands along one set of directions at a rate comparable to the most weakly bound solids known. By flexing like lattice fencing, the framework couples this to a contraction along a perpendicular direction. This gives negative thermal expansion that is 14 times larger than in ZrW2O8. Density functional theory calculations quantify both the low energy associated with this flexibility and the role of argentophilic (Ag+...Ag+) interactions. This study illustrates how the mechanical properties of a van der Waals solid might be engineered into a rigid, useable framework.

Liquid-liquid phase transition in supercooled silicon.

Abstract: Silicon in its liquid and amorphous forms occupies a unique position among amorphous materials. Obviously important in its own right, the amorphous form is structurally close to the group of 4–4, 3–5 and 2–6 amorphous semiconductors that have been found to have interesting pressure-induced semiconductor-to-metal phase transitions1,2. On the other hand, its liquid form has much in common, thermodynamically, with water and other ‘tetrahedral network’ liquids that show density maxima3,4,5,6,7. Proper study of the ‘liquid–amorphous transition’, documented for non-crystalline silicon by both experimental and computer simulation studies8,9,10,11,12,13,14,15,16,17, may therefore also shed light on phase behaviour in these related materials. Here, we provide detailed and unambiguous simulation evidence that the transition in supercooled liquid silicon, in the Stillinger–Weber potential18, is thermodynamically of first order and indeed occurs between two liquid states, as originally predicted by Aptekar10. In addition we present evidence to support the relevance of spinodal divergences near such a transition, and the prediction3 that the transition marks a change in the liquid dynamic character from that of a fragile liquid to that of a strong liquid.