Rapid Thermal Quench Research Articles

Formation, preservation and extinction of high-pressure minerals in meteorites: temperature effects in shock metamorphism and shock classification

The goal of classifying shock metamorphic features in meteorites is to estimate the corresponding shock pressure conditions. However, the temperature variability of shock metamorphism is equally important and can result in a diverse and heterogeneous set of shock features in samples with a common overall shock pressure. In particular, high-pressure (HP) minerals, which were previously used as a solid indicator of high shock pressure in meteorites, require complex pressure–temperature–time (P–T–t) histories to form and survive. First, parts of the sample must be heated to melting temperatures, at high pressure, to enable rapid formation of HP minerals before pressure release. Second, the HP minerals must be rapidly cooled to below a critical temperature, before the pressure returns to ambient conditions, to avoid retrograde transformation to their low-pressure polymorphs. These two constraints require the sample to contain large temperature heterogeneities, e.g. melt veins in a cooler groundmass, during shock. In this study, we calculated shock temperatures and possible P–T paths of chondritic and differentiated mafic–ultramafic rocks for various shock pressures. These P–T conditions and paths, combined with observations from shocked meteorites, are used to constrain shock conditions and P–T–t histories of HP-mineral bearing samples. The need for rapid thermal quench of HP phases requires a relatively low bulk-shock temperature and therefore moderate shock pressures below ~ 30 GPa, which matches the stabilities of these HP minerals. The low-temperature moderate-pressure host rock generally shows moderate shock-deformation features consistent with S4 and, less commonly, S5 shock stages. Shock pressures in excess of 50 GPa in meteorites result in melt breccias with high overall post-shock temperatures that anneal out HP-mineral signatures. The presence of ringwoodite, which is commonly considered an indicator of the S6 shock stage, is inconsistent with pressures in excess of 30 GPa and does not represent shock conditions different from S4 shock conditions. Indeed, ringwoodite and coexisting HP minerals should be considered as robust evidence for moderate shock pressures (S4) rather than extreme shock (S6) near whole-rock melting.

Thermal quench in ITER locked mode disruptions

Simulations and theory are presented of an ITER locked mode thermal quench (TQ). In present experiments, locked mode disruptions have a long precursor phase, followed by a rapid termination and thermal quench, which can be identified with a resistive wall tearing mode (RWTM). In ITER, the RWTM will be slowed by the highly conductive vacuum vessel. The rapid termination might be absent, and the plasma could remain in the precursor phase. If the edge temperature is in the collisional regime, the TQ would proceed on a long timescale, limited by the RWTM to almost 100 ms. This is an important self-mitigating effect.

Rapid production of large Li7 Bose-Einstein condensates using D1 gray molasses

We demonstrate the production of large $^7$Li Bose-Einstein condensates in an optical dipole trap using $D_1$ gray molasses. The sub-Doppler cooling technique reduces the temperature of $4\times10^9$ atoms to $25~\mu{}$K in 3~ms. After microwave evaporation cooling in a magnetic quadrupole trap, we transfer the atoms to a crossed optical dipole trap, where we employ a magnetic Feshbach resonance on the $|F=1,m_F=1\rangle$ state. Fast evaporation cooling is achieved by tilting the optical potential using a magnetic field gradient on the top of the Feshbach field. Our setup produces pure condensates with $2.7\times10^6$ atoms in the optical potential for every 11~s. The trap tilt evaporation allows rapid thermal quench, and spontaneous vortices are observed in the condensates as a result of the Kibble-Zurek mechanism.

The influence of periodic shear on structural relaxation and pore redistribution in binary glasses

The evolution of porous structure and local density, and associated changes of potential energy in binary glasses under oscillatory shear deformation, are investigated using molecular dynamics simulations. The porous glasses were initially prepared via a rapid thermal quench from the liquid state across the glass transition temperature and allowed to phase separate and solidify at constant volume, thus producing an extended porous network in an amorphous solid material. We find that under periodic shear, the potential energy decreases over consecutive cycles due to gradual rearrangement of the glassy material, and the minimum of the potential energy after thousands of shear cycles is lower at larger strain amplitudes. Moreover, with increasing cycle number, the pore size distributions become more skewed toward larger length scales, where a distinct peak is developed and the peak intensity is enhanced at larger strain amplitudes. The numerical analysis of the local density distribution functions demonstrates that cyclic loading leads to formation of higher density solid domains and homogenization of the regions with reduced density.

Structural relaxation of porous glasses due to internal stresses and deformation under tensile loading at constant pressure

The time evolution of the pore size distributions and mechanical properties of amorphous solids at constant pressure are studied using molecular dynamics simulations. The porous glasses were initially prepared at constant volume conditions via a rapid thermal quench from the liquid to the glassy state and allowing for simultaneous phase separation and material solidification. We found that at constant pressure and low temperature, the porous network becomes more compact and the glassy systems relocate to progressively lower levels of the potential energy. Although the elastic modulus and the average glass density both increase with the waiting time, their dependence is described by the power-law function with the same exponent. Moreover, the results of numerical simulations demonstrated that, under tensile loading at constant pressure, low-density porous samples become significantly deformed and break up into separate domains at high strain, while dense glasses form a nearly homogeneous solid material.

Domain glasses: Twin planes, Bloch lines, and Bloch points

Ferroelastic materials can develop complex domain structures, which have properties of glassy systems (non‐ergodicity, glass dynamics, glass transitions, and freezing). Four characteristic temperatures are defined for such domain glasses: the dynamical nucleation temperature Td where local correlated clusters can form glass states within a (tweed‐) nano structure, To the Vogel–Fulcher temperature of these precursor nano‐structures, Tpt the phase transition temperature where the (ferroelastic) transition occurs, and TK the Kauzmann temperature where the complex domain structure freezes. Td exists in most ferroelastic materials whereas the other transitions depend on the complexity of the domain patterns and hence on their thermal history. Shear collapse and rapid thermal quench of ferroelastic crystals preferentially lead to domain glasses whereas slow anneal produces mostly highly correlated pattern such as stripe patterns or single domain crystals. Domain glasses are compared with structural glasses and several examples for domain glass features are discussed.

Nucleation of spontaneous vortices in trapped Fermi gases undergoing a BCS-BEC crossover

We study the spontaneous formation of vortices during the superfluid condensation in a trapped fermionic gas subjected to a rapid thermal quench via evaporative cooling. Our work is based on the numerical solution of the time dependent crossover Ginzburg-Landau equation coupled to the heat diffusion equation. We quantify the evolution of condensate density and vortex length as a function of a crossover phase parameter from BCS to BEC. The more interesting phenomena occur somewhat nearer to the BEC regime and should be experimentally observable; during the propagation of the cold front, the increase in condensate density leads to the formation of supercurrents towards the center of the condensate as well as possible condensate volume oscillations.

Spontaneous crystallization of skyrmions and fractional vortices in fast-rotating and rapidly quenched spin-1 Bose-Einstein condensates

We investigate the spontaneous generation of crystallized topological defects via the combining effects of fast rotation and rapid thermal quench on the spin-1 Bose-Einstein condensates. By solving the stochastic projected Gross-Pitaevskii equation, we show that, when the system reaches equilibrium, a hexagonal lattice of skyrmions, and a square lattice of half-quantized vortices can be formed in a ferromagnetic and antiferromagnetic spinor BEC, respetively, which can be imaged by using the polarization-dependent phase-contrast method.

Gold Seed Removal from the Tips of Silicon Nanorods

A chemical method was developed to remove the gold (Au) seed particles from the tips of solution-liquid-solid (SLS) grown silicon (Si) nanorods. The nanorods are capped with hydrophobic ligands during the synthesis, which made it necessary to perform the Au etching in an aqua regia and chloroform emulsion. Preliminary etching experiments revealed that a thin Si shell coated the Au seeds and prevented Au removal. Therefore, a rapid thermal quench of the reaction mixture was needed to crack this shell and provide etchant access to the Au seed. More than 95% of the Au seeds could be removed from the tips of thermally quenched samples without damaging the crystalline Si nanorods.

Dynamics of vortex nucleation by rapid thermal quench

By numerical and analytical studies of the time-dependent Ginzburg-Landau model we show that vortex nucleation in superfluid ${}^{3}\mathrm{He}$ by rapid thermal quench in the presence of superflow is dominated by a transverse instability of the moving normal-superfluid interface. The instability threshold is found analytically as a function of supercurrent density and the front velocity. The dynamics of vortex evolution at long times after the quench is investigated.

Nucleation of vortices in superfluid 3He–B by thermal quench

We show that nucleation of vortex loops in superfluid 3He by rapid thermal quench in the presence of superflow is dominated by a transverse instability of the moving normal–superfluid interface. Exact expressions for the instability threshold as a function of supercurrent density and the front velocity are found. The results are verified by numerical solution of the Ginzburg–Landau equation.

Nucleation of Vortices by Rapid Thermal Quench

We show that vortex nucleation in superfluid $^3$He by rapid thermal quench in the presence of superflow is dominated by a transverse instability of the moving normal-superfluid interface. Exact expressions for the instability threshold as a function of supercurrent density and the front velocity are found. The results are verified by numerical solution of the Ginzburg-Landau equation.

Oxidative pyrolysis of natural gas in a spouted-bed reactor: reaction stoichiometry and experimental reactor design

A simple reaction network is developed to understand methane oxidative pyrolysis in a spouted-bed reactor. Oxidative pyrolysis for converting natural gas is attractive because it gives high methane conversions (80–90% per pass) at high selectivities to C 2+ (40%) and carbon monoxide (40%). Based on results reported in a British Petroleum patent, our analysis shows that oxidative pyrolysis is a combination of methane partial oxidation to carbon monoxide which produces heat and of methane pyrolysis to C 2+ (mostly ethylene and acetylene) which consumes heat. The methane pyrolysis reaction is thermodynamically limited and requires temperatures above 1000°C for significant conversion. A pilot-plant oxidative pyrolysis reactor was designed from cold flow studies to verify the BP results obtained at atmospheric pressure and to establish process feasibility at higher pressures. Scoping economic studies show that higher pressure operation is needed to avoid costly product compression in downstream upgrading processes. The pilot-plant spouted-bed reactor evolved from a fragile quartz tube to a more robust one of stainless steel. The stainless steel performs similarly to the quartz reactor when the walls are coated with an aluminum oxide layer to passivate inherent catalytic activity. Although a number of experimental challenges remain, initial runs have been successful in demonstrating that our method of preheating the feed successfully ignites the reaction and that our analytical technique gives excellent closure in detailed material balances. Based on results presented in this paper ways are needed to improve on BP C 2+ yields, such as by process variable optimization and rapid thermal quench to minimize undesirable secondary pyrolysis reactions.

N-Alkylnitrilium Cations in Zeolites: A Study Using Theoretical Chemistry and in Situ NMR with the Pulse-Quench Reactor

Stepanov and Luzgin recently applied solid-state NMR with magic-angle spinning (MAS) to the reactions of acetonitrile with olefins or alcohols in zeolite HZSM-5 at 296 K and observed signals they assigned to N-alkylnitrilium cations (Chem. Eur. J. 1997, 3, 47). Here we report strong verification of their assignment and show that these cations can also form in catalytic flow reactors. We used the recently developed pulse-quench reactor to contact acetonitrile and either tert-butyl alcohol or isobutylene for several seconds at temperatures up to 433 K followed by a rapid thermal quench. 13C and 15N MAS NMR measurements were performed at 77−298 K to identify the species present in the zeolite and to permit determination of the principal components of the chemical shift tensors. We optimized the geometry of the N-tert-butylacetonitrilium cation and acetonitrile using density functional theory at B3LYP/DZVP2 and then calculated theoretical chemical shift tensors using the GIAO-MP2 level of theory and two basis...

Strain energy effects on the ordering process in diblock styrene-butadiene copolymer

The technique of time-resolved small-angle x-ray scattering is used to monitor the disorder-order transformation occurring in asymmetric, diblock styrene-butadiene. A rapid thermal quench is applied to drive the system from its initial high-temperature disordered state to a low-temperature ordered structure, a body-centered cubic lattice of styrene spheres characterized by a series of concentric Debye rings in the two-dimensional scattering profile. At relatively late times in the microdomain ordering process, the uniform Debye rings were seen to rapidly develop nonuniformities indicating the existence of preferred orientation with a fiber texture, the 110] axis of the bcc structure defining the fiber axis. These results are interpreted in the context of a developing transformation strain energy during the ordering process and shown to be consistent with accompanying changes in lattice spacing as well as earlier observations of apparent fluctuations in the ordering process.

A solid solution pumping mechanism for the nitrogenation of titanium during mechanical deformation in air

The formation of TiN during ball milling of Ti metal in air has been investigated by XRD, laser raman microscopy/spectroscopy, and x-ray photoelectron spectroscopy. The results cannot be readily explained by a conventional gas - metal reaction mechanism. Instead, a new model is proposed - solid solution pumping - which involves gas absorption into the -Ti phase during the thermal spike induced by collisions, retention of absorbed nitrogen after the rapid thermal quench, homogenous nucleation of TiN, and the growth of the TiN phase at the expense of nitrided -Ti, due to additional nitrogen being `pumped' into the bulk by the sequence of collision events. The presence of Fe in small amounts is required due either to it acting as an oxygen scavenger, or as a substrate for dissociative chemisorption of nitrogen. These results are of some technical interest, since they demonstrate that TiN can be synthesized in ambient air by simple ball milling.

Shock synthesis of amino acids in simulated primitive environments.

A mixture of gases roughly simulating the primitive terrestrial atmosphere has been subjected to shock heating followed by a rapid thermal quench. Under strictly homogeneous conditions there is a very high efficiency of 5 x 10(10) molecules per erg of shock-injected energy for production of alpha-amino acids. Calculations suggest that rapid quenching bypasses the usual thermochemical barrier. The product of energy flux and efficiency implies the unexpected conclusion that shocks occurring on atmospheric entry of cometary meteors and micrometeorites and from thunder may have been the principal energy sources for pre-biological organic synthesis on the primitive earth.