Thermal and chemical nanofractal relaxation

We studied shape relaxation of nano-fractal islands, during annealing, after their growth from antimony cluster deposition on graphite surface. Annealing at 180^°C shows evidence of an increase of the fractal branch width with time followed by branch fragmentation, without changing the fractal dimension. The time evolution of the width of the arm suggests the surface self-diffusion mechanism as the main relaxation process. With Monte Carlo simulations, we confirmed the observed behavior. Comparison is done with our previous results on fragmentation of nano-fractal silver islands when impurity added to the incident cluster promotes rapid fragmentation by surface self-diffusion enhancement [1].     

Coarsening and pearling instabilities in silver nanofractal aggregates

We investigate the morphological changes of 3D supported fractal aggregates generated through the deposition of silver clusters on graphite. The fractal relaxation, activated after their formation by perturbing them either by thermal annealing or by using a surfactant, as oxide molecules, carried by silver clusters in a subsequent deposition, shows evidence of two distinct fragmentation patterns. The post coarsening, driven by thermal heating, gives a broad asymmetrical distribution of fragments in agreement with a random process, whereas the entire silver fractal pearling fragmentation is driven by chemical adjunction of the surfactant.     

Spin relaxation and chemical exchange in NMR simulations

Theory for describing the density matrix of a spin system experiencing chemical exchange and relaxation during the steps of an NMR experiment is presented in a form suitable for computation. Features in the theory that arise from exchange are discussed in detail, and comparisons to the exchange-free situation are made. A general computer program to carry out simulations of NMR experiments is described, and several examples of its performance are presented.     

Thermal relaxation of charm in hadronic matter

The thermal relaxation rate of open-charm (D) mesons in hot and dense hadronic matter is calculated using empirical elastic scattering amplitudes. D-meson interactions with thermal pions are approximated by D^? resonances, while scattering off other hadrons (K, η, ρ, ω, K^?, N, Δ) is evaluated using vacuum scattering amplitudes as available in the literature based on effective Lagrangians and constrained by realistic spectroscopy. The thermal relaxation time of D-mesons in a hot π gas is found to be around 25-50 fm/c for temperatures T=150-180 MeV, which reduces to 10-25 fm/c in a hadron-resonance gas. The latter values, argued to be conservative estimates, imply significant modifications of D-meson spectra in heavy-ion collisions. Close to the critical temperature (Tc), the spatial diffusion coefficient (Ds) is surprisingly similar to recent calculations for charm quarks in the Quark-Gluon Plasma using non-perturbative T-matrix interactions. This suggests a possibly continuous minimum structure of Ds around Tc.     

Structural relaxation and chemical decomposition in amorphous TM-M alloys

Small angle X-ray scattering experiments performed on amorphous two-component transition metal-metalloid alloys have shown that these materials decompose on annealing clearly before crystallization starts. This process influences the behaviour of physical properties in a similar way as structural relaxation does. To separate the two effects mainly composition dependent properties must be compared with others which are mainly sensitive to structural changes. In this paper it is shown that the ratio of the annealing-induced changes of the temperature coefficient of the resistivity and of the residual resistivity is determined by the underlying elementary process. The two values resulting for predominating structural relaxation and for predominating decomposition seem to be universal and are compatible with a theory of phonon-controlled conductivity in high-resistance conductors.     

Thermal and chemical equilibrium for vaporizing sources

Vaporized sources produced in collisions between ³⁶Ar and ⁵⁸Ni at 95 MeV per nucleon have been detected with the multidetector INDRA. Complete information concerning the deexcitation properties of quasi-projectiles, including second moments of chemical composition, is compared to a quantum statistical model describing a gas of fermions and bosons in thermal and chemical equilibrium. Inclusions in the calculation of all known discrete levels of nuclear species which deexcite into light particles and of a final state excluded volume interaction are found decisive to very well reproduce the experimental data, which strongly supports that thermodynamical equilibrium was achieved at freeze-out for such sources. Received: 6 April 1999     

Effects of viscosity and chemical relaxation on acoustic pulse propagation in seawater

This paper considers acoustic pulse distortion in seawater due to effects of viscosity and chemical relaxation. The acoustic medium is modelled as a linear, time-invariant, space-invariant filter. The deformation of initially Gaussian-shaped pulses is estimated. The combined effects of the dissipative processes are that as a pulse propagates through the medium, the maximum amplitude is decreased, the pulse is stretched in time and a delayed tail is developed. Input pulses with high-frequency energy contents can undergo a considerable deformation due to these phenomena.     

Transverse relaxation in the rotating frame induced by chemical exchange

In the presence of radiofrequency irradiation, relaxation of magnetization aligned with the effective magnetic field is characterized by the time constant T1rho. On the other hand, the time constant T2rho characterizes the relaxation of magnetization that is perpendicular to the effective field. Here, it is shown that T2rho can be measured directly with Carr-Purcell sequences composed of a train of adiabatic full-passage (AFP) pulses. During adiabatic rotation, T2rho characterizes the relaxation of the magnetization, which under adiabatic conditions remains approximately perpendicular to the time-dependent effective field. Theory is derived to describe the influence of chemical exchange on T2rho relaxation in the fast-exchange regime, with time constant defined as T2rho,ex. The derived theory predicts the rate constant R2rho,ex (= 1/T2rho,ex) to be dependent on the choice of amplitude- and frequency-modulation functions used in the AFP pulses. Measurements of R2rho,ex of the water/ethanol exchanging system confirm the predicted dependence on modulation functions. The described theoretical framework and adiabatic methods represent new tools to probe exchanging systems.     

Transverse field dependence of muonium relaxation: Spin exchange and chemical reaction

It has been shown experimentally that the muonium relaxation due to spin exchange is 1.5 times faster in intermediate transverse fields (say, atB=50 G, where the so-called two-frequency muonium signal is observed) than in low fields (say, atB=5 G), in agreement with an earlier theoretical prediction. It has also been confirmed experimentally that this distinct field dependence is totally absent in the case of chemical reaction.     

Dielectric relaxation of ZnO nanostructure synthesized by soft chemical method

Thioglycerol capped nanoparticles of ZnO have been prepared in methanol through chemical technique. Nanostructures of the prepared ZnO particles have been confirmed through X-ray diffraction measurement. The Debye–Scherrer formula is used to obtain the particle size. The average size of the prepared ZnO nanoparticles is found to be 50 nm. The frequency-dependent dielectric dispersion of the sample is investigated in the temperature range from 293 to 383 K and in a frequency range from 100 Hz to 1 MHz by impedance spectroscopy. An analysis of the complex permittivity (ε′ and ε′′) and loss tangent (tan δ) with frequency is performed assuming a distribution of relaxation times. The frequency-dependent maxima of the imaginary part of impedance are found to obey Arrhenius law with activation energy ∼1 eV. The scaling behavior of dielectric loss spectra suggests that the relaxation describes the same mechanism at various temperatures. The frequency-dependent electrical data are analyzed in the framework of conductivity and modulus formalisms. The frequency-dependent conductivity spectra obey the power law.     

A chemical relaxation study of protonation effects in amino acids

A modulation temperature perturbation method has been used to study protonation effects in amino acids. The results of this technique compare favourably with results obtained using NMR methods. A novel relationship has been found correlating various of the rate constants of protonation. Some activation energies have also been measured.     

Thermal expansion and relaxation of magnetostriction in a metastable state of dysprosium orthoaluminate

A study of thermal expansion and magnetostriction caused by the metamagnet phase transition in a rare earth sublattice was performed for a DyAlO₃ single crystal, which is the model object of a wide family of rare earth oxides with the structure of distorted perovskite. The sharp variation of the relative strains along the b axis 3.6 × 10⁻⁵ was found at T ≌ 2.18 K. The nature of the observed anomaly is caused by the occurrence of a metastable state of the magnetic subsystem achieved in the conditions of the experimental process. The relaxation of magnetostriction is studied and the exponential character is found. The possible mesoscopic nature of thermal and quantum mechanisms of magnetic relaxation is considered.     

Thermal rotations and relaxation of vibrational energy of excited dye molecules in solution

Measurements of polarization and decay time of fluorescence of dyes in solution as a function of excitation wavelength were performed. The rate of thermal rotational depolarization was found to be independent of vibrational energy excess. To explain the observed influence of excitation wavelength on the emission anisotropy, a change in the magnitude of torsional vibrations, in addition to possible mixing of states, is proposed.     

Thermal and chemical passivation of gallium-arsenide films deposited from ablation plasma

The electric and photoelectric properties of gallium-arsenide films deposited on a polycrystalline corundum substrate from the ablation plasma formed by a high-power ion beam are investigated. It is ascertained that vacuum and air annealing (in the former case, P = 10^?2 Pa and T = 300–1200 K) and sulfide chemical passivation in an alcoholic solution affect the characteristics of the dark conductivity and photo-conductivity of the film surfaces. The optimal conditions for thermal and chemical treatment, at which the most stable changes in film the properties are attained, are determined. Enhancement in the stability of the electrical and photoelectric characteristics of films, which is achieved after thermal treatment, arises from the annealing of defects and their clusterization. Sulfide passivation leads to changes in the characteristics and increases the stability of properties under air oxidation.     

Thermal and chemical diffusion in the rapid solidification of binary alloys

Solidification of binary alloys is characterized by the necessity to reject away from the advancing front two conserved quantities: the latent heat released at the solid-liquid interface and the solute atoms that cannot be accommodated in the solid phase. As thermal diffusion is much faster than chemical diffusion, the latter is generally assumed to be the rate limiting mechanism for the process, and the problem is addressed through the isothermal approximation. In the present paper we use the phase-field model to study the planar growth of a solid germ, nucleated in its undercooled melt. We focus on the effects of a noninstantaneous thermal relaxation. The steady growth predicted at large supersaturation in the isothermal limit is prevented. Depending on the value of the Lewis number the growth rate is limited by either mass or heat diffusion; in the latter case we observe a sharp transition between two different regimes, in which originates a nonmonotonic time dependence of the interface temperature. The effects of this transition reflect in the composition of the solidified alloy.     

Thermal diffusivity and nuclear spin relaxation: a continuous wave free precession NMR study

Continuous wave free precession (CWFP) nuclear magnetic resonance is capable of yielding quantitative and easily obtainable information concerning the kinetics of processes that change the relaxation rates of the nuclear spins through the action of some external agent. In the present application, heat flow from a natural rubber sample to a liquid nitrogen thermal bath caused a large temperature gradient leading to a non-equilibrium temperature distribution. The ensuing local changes in the relaxation rates could be monitored by the decay of the CWFP signals and, from the decays, it was possible to ascertain the prevalence of a diffusive process and to obtain an average value for the thermal diffusivity.     

Thermal conductivity and specific heat of amorphous Zr0.67Ni0.33 after structural relaxation

Low-temperature measurements of the thermal conductivity (0.3KT5K) and of the specific heatC (0.07KT3.5K) of splat-cooled amorphous superconducting Zr_0.67Ni_0.33(T _c 2.7K) after different annealing stages are reported. increases progressively (up to 55%) after annealing. An analysis of with the help of normal-state measurements belowT _c in an overcritical field shows that the phonon-electron scattering remains unaltered after annealing. Hence the increase in must be entirely attributed to structure-induced (intrinsic) scattering, i.e. by two-level tunneling states (TLS) at low temperatures (T1K). The specific heat shows a small decrease aboveT _c (by 8%) which is attributed to a small diminution of the electronic density of states at the Fermi level and to a small increase in the Debye temperature. ForTT _c where TLS dominate, the specific heatC decreases less upon annealing than expected from the increase of in the standard tunneling model. This points to a change in the TLS relaxation time spectrum upon annealing, as observed previously for Zr _x Cu_1–x glasses.