Formation and phase transitions of methane hydrates under dynamic loadings: Compression rate dependent kinetics

AbstractUnderstanding the high-pressure kinetics associated with the formation of methane hydrates is critical to the practical use of the most abundant energy resource on earth. In this study, we have studied, for the first time, the compression rate dependence on the formation of methane hydrates under pressures, using dynamic-Diamond Anvil Cell (d-DAC) coupled with a high-speed microphotography and a confocal micro-Raman spectroscopy. The time-resolved optical images and Raman spectra indicate that the pressure-induced formation of methane hydrate depends on the compression rate and the peak pressure. At the compression rate of around 5 to 10 GPa/s, methane hydrate phase II (MH-II) forms from super-compressed water within the stability field of ice VI between 0.9 GPa and 2.0 GPa. This is due to a relatively slow rate of the hydrate formation below 0.9 GPa and a relatively fast rate of the water solidification above 2.0 GPa. The fact that methane hydrate forms from super-compressed water underscores a diffusion-controlled growth, which accelerates with pressure because of the enhanced miscibility between methane and super-compressed water.

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Dynamic Mechanical Behavior of Two Fiber-Reinforced Composites

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.525-526.261 ◽

2012 ◽

Vol 525-526 ◽

pp. 261-264

Author(s):

Y.Z. Guo ◽

X. Chen ◽

Xi Yun Wang ◽

S.G. Tan ◽

Z. Zeng ◽

...

Keyword(s):

Compressive Strength ◽

Strain Rate ◽

Mechanical Behavior ◽

High Speed ◽

Split Hopkinson Pressure Bar ◽

Hopkinson Pressure Bar ◽

Stress Strain ◽

Rate Dependent ◽

Dynamic Loadings ◽

Axial Splitting

The mechanical behavior of two composites, i.e., CF3031/QY8911 (CQ, hereafter in this paper) and EW100A/BA9916 (EB, hereafter in this paper), under dynamic loadings were carefully studied by using split Hopkinson pressure bar (SHPB) system. The results show that compressive strength of CQ increases with increasing strain-rates, while for EB the compressive strength at strain-rate 1500/s is lower then that at 800/s or 400/s. More interestingly, most of the stress strain curves of both of the two composites are not monotonous but exhibit double-peak shape. To identify this unusual phenominon, a high speed photographic system is introduced. The deformation as well as fracture characteristics of the composites under dynamic loadings were captured. The photoes indicate that two different failure mechanisms work during dynamic fracture process. The first one is axial splitting between the fiber and the matrix and the second one is overall shear. The interficial strength between the fiber and matrix, which is also strain rate dependent, determines the fracture modes and the shape of the stress/strain curves.

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A Thermo-Magneto-Mechanical Free Energy Model for NiMnGa Single Crystals

MRS Proceedings ◽

10.1557/proc-1050-bb03-04 ◽

2007 ◽

Vol 1050 ◽

Author(s):

Phillip Morrison ◽

Stefan Seelecke ◽

Manfred Kohl ◽

Berthold Krevet

Keyword(s):

Free Energy ◽

Phase Transitions ◽

Single Crystals ◽

Energy Landscape ◽

Stable States ◽

Temperature Dependent ◽

One Dimensional ◽

Rate Dependent ◽

Energy Equations ◽

Increasing Temperature

AbstractThe paper extends the authors' recent model for one-dimensional rate-dependent magneto-mechanical behavior of NiMnGa single crystals to account for temperature-dependent effects including austenite/martensite and ferro-/paramagnetic phase transitions. The magneto-mechanical model is based on the Helmholtz free energy landscape constructed for a meso-scale lattice element with strain and magnetization as order parameters. This two-dimensional energy landscape includes three paraboloidal wells representing the two easy-axis and one hard-axis martensite variants relevant for the structurally one-dimensional case. Phase transformations resulting from applied stresses and magnetic fields follow from a system of evolution laws based on the Gibbs free energy equations and the theory of thermally activated processes, which in the low-thermal-activation limit appropriately reproduce the athermal transformation behavior observed in these materials. The phase fractions subsequently determine the macroscopic strain and magnetization of a sample of NiMnGa by means of a standard averaging procedure. To account for the first-order phase transitions to austenite, additional temperature-dependent wells representing the stable states of austenitic NiMnGa are introduced into the Helmholtz energy landscape. The transition from ferromagnetic to paramagnetic states is modeled as a second order transformation based on the gradual degeneration of the ferromagnetic wells with increasing temperature.

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