Amorphous state dynamics

Although optical spectra of lanthanide-doped insulating nanociystals embedded in amorphous matrices are very similar to the free-standing nanocrystal counterparts, their excited state dynamics behaves very differently from that in simple nanocrystals. Some distinct dynamic properties have recently been found for nanocrystals embedded in polymers or glasses. Simple models for the interaction between lanthanide ions and the matrices were also proposed. However, further studies are needed in order to quantitatively understand the observed size-dependence and dynamic mechanisms. 

The dose required for amorphization is a function of the kinetics of simultaneous dynamic recovery processes. The recovery process is accelerated at elevated temperatures and, in many cases, is greatly increased by radiation-enhanced defect migration. These simultaneous recovery processes may be associated with defect recombination or annihilation, epitaxial recrystallization at crystalline-amorphous interfaces (Carter and Nobes 1991), or nucleation and growth recrystallization in the bulk of the amorphous state. For any crystalline solid, there is a critical temperature, above which the rate of amorphization is less than the rate of recovery, thus amorphization cannot occur. However, Tc also depends on the energy and mass of the incident beam, as well as the dose rate. 

In this chapter we present a survey of our current understanding of interrelations between the electronic and ionic structure in late-transition-polyvalent-element metallic glasses. Evidence of a strong influence of conduction electrons on the ionic structure, and vice versa, of the ionic structure on the conduction electrons, is presented. We discuss as well the consequences to phase stability, the electronic density of states, dynamic properties, electronic transport, and magnetism. A scaling behaviour of many properties versus Z, the mean electron number per atom, is the most characteristic feature of these alloys. Crystalline alloys which are also strongly dominated by the conduction electrons are often called electron phases or Hume-Rothery phases. The amorphous alloys under consideration are consequently described as an Electron Phase or Hume-Rothery Phase with Amorphous Structure. Similar theoretical concepts as applied to crystalline Hume-Rothery alloys are used for the present amorphous samples. 

We now present temperature measurements of the vibrational properties of the T) phase. Type II diamonds were used for mid-IR measurements to avoid interference with the characteristic absorption of the sample. The representative absorption spectra at different temperatures (see Fig. 14) clearly show the presence of a broad 1700 cm IR band (compare with Fig. 12). Its presence was also observed in the sample heated to 495 K at 117 GPa (see below). The position of the band and its damping (if fitted as one band) does not depend on pressure and temperature within the error bars. The Raman spectrum of the Tj phase obtained on heating (see below) does not show any trace of the molecular phase (see Fig. 12(b)). Careful examination of the spectrum in this case showed a weak broad band at 640 cm and a shoulder near 1750 cm (both indicated by arrows in Fig. 12(b)). For an amorphous state, the vibrational spectrum would closely resemble a density of phonon states [63] with the maxima corresponding roughly to the zone boundary acoustic and optic vibrations of an underlying structure [3-5, 55], which is consistent with our observations. The only lattice dynamics... 

Amorphous silica has also been studied recently using first principle molecular dynamics (Samthein et al., 1995). Such simulations are currently restricted to rather small unit cells (24 Si02 units) and short time spans ( 10 ps), unlike the simulations using interatomic potentials described in Chapter 9. Nevertheless, the direct simulation of the liquid phase followed by a rapid quench produces an amorphous state with structural and electronic properties in reasonable agreement with neutron diffraction and spectroscopic studies. QM simulations are able to treat a wide range of structural environments on an equal footing while providing information on both structural and electronic properties. 

In MD, too, demands for the future should be obvious from the discussion in Sections 3, 6, and 7.1. To obtain realistic simulations of the dynamics, not only is it essential to simulate considerably longer real-time relaxations but also to include an adequate number of particles in the simulation box. This is particularly important for modeling amorphous states of polymer systems for which the demands of computer storage and speed are critical and at present inadequately met. 

Crystallinity of PLA has a strong impact on its mechanical properties. Suryanegara et al. have prepared PLA/MFC nanocomposites in both fully amorphous and crystallized states. The tensile modulus and strength of pristine PLA were improved with an increase of MFC content in both amorphous and crystallized states. Dynamic mechanical analysis (DMA) has been used to study the effect of MFC reinforcement on the thermomechanical properties of PLA in both states and the results are shown in Figure 9.5. In the amorphous state, the storage modulus of pristine PLA below Tg is almost constant at around 3 GPa. Above Tg, the modulus drops to 4 MPa at 80 °C, and then increases to 200 MPa at 100 °C owing to the cold... 

In order to describe the static structure of the amorphous state as well as its temporal fluctuations, correlation functions are introdnced, which specify the manner in which atoms are distributed or the manner in which fluctuations in physical properties are correlated. The correlation fimctions are related to various macroscopic mechanical and thermodynamic properties. The pair correlation function g r) contains information on the thermal density fluctuations, which in turn are governed by the isothermal compressibility k T) and the absolute temperature for an amorphous system in thermodynamic equilibrium. Thus the correlation function g r) relates to the static properties of the density fluctuations. The fluctuations can be separated into an isobaric and an adiabatic component, with respect to a thermodynamic as well as a dynamic point of view. The adiabatic part is due to propagating fluctuations (hypersonic soimd waves) and the isobaric part consists of nonpropagating fluctuations (entropy fluctuations). By using inelastic light scattering it is possible to separate the total fluctuations into these components. 

The amorphous state is of eminent importance for polymeric systems. Its spedflc static and dynamic properties are presented in Chapter 1.08 by Andreas Schonhals and Friedrich Kremer. 

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