Structural Relaxation Mechanisms Research Articles

Acoustic spectra of some polypropylene glycols. mechanism of structural relaxation

The molecular mechanisms of structural relaxation in polypropylene glycols are reviewed. The rate constants and the activational parameters of the reactions of rupture of the intermolecular bonds have been determined. In the high molecular mass sample of polypropylene glycol structural relaxation is governed by the relaxation only of shear viscosity. The high and low frequency elasticity moduli have been calculated.

Calorimetric investigation of structural relaxation in supercooledNi75Al22Zr2B amorphous alloy

A differential scanning calorimetric (DSC) study is performed on a supercooled ${\mathrm{Ni}}_{75}$${\mathrm{Al}}_{22}$${\mathrm{Zr}}_{2}$B alloy to present an example for identification between amorphous and microcrystalline structures of a structurally disordered solid. The supercooled ${\mathrm{Ni}}_{75}$${\mathrm{Al}}_{22}$${\mathrm{Zr}}_{2}$B alloy exhibits a weak and broad exo- therm (which is followed by the pronounced crystallization exotherm) due to enthalpy or structural relaxation, which masks the glass-liquid transition (${\mathit{T}}_{\mathit{g}}$) characteristic of an amorphous solid. Thermal treatment of the alloy (performed at T=${\mathit{T}}_{\mathit{g}}$) removes the exothermic step, and the DSC output recorded after the thermal treatment showed a characteristic ${\mathit{T}}_{\mathit{g}}$ at around 228 \ifmmode^\circ\else\textdegree\fi{}C. The signal presents a monotonically decreasing enthalpy release (\ensuremath{\partial}H/\ensuremath{\partial}t${)}_{\mathit{T}}$ in an isothermal calorimetric measurement, which is modeled successfully by using the existing mechanism of structural relaxation. The microstructure of the specimens studied before and after the thermal treatments confirms the origin of the DSC exotherm and isotherm signal to be structural relaxation.

Point defect populations in amorphous and crystalline silicon

Abstract Crystalline Si ( c -Si) and relaxed amorphous Si ( a -Si) have been ion-bombarded. The kinetics and temperature dependence of the heat released on annealing of these materials are found to be qualitatively similar for temperatures lower than epitaxial crystallization temperatures (550°C). This behavior suggests a close similarity between the mechanisms of structural relaxation in a -Si and defect annihilation in c -Si. The heat release measurements imply that ion bombardment generates a variety of point defects in both a -Si and c -Si.

Changing the Structural State of Amorphous Silicon by Ion Irradiation

ABSTRACTIon beams of keV and MeV energies have been used to bombard amorphous Si (a-Si), which had previously been annealed (‘relaxed’). Analysis by Raman spectroscopy and differential scanning calorimetry shows that when 1 out of every 20 Si atoms is displaced by a nuclear collision, the a-Si returns to its unrelaxed state and cannot be distinguished from as implanted a-Si. Moreover, the kinetics of the heat release on annealing of similarly bombarded crystalline Si (c-Si) are qualitatively identical to those of structural relaxation in a-Si. This implies that the population of ion beam induced defects in a-Si is very similar to that in c-Si. It also shows that defect annihilation is an important ingredient in the mechanism of structural relaxation of a-Si.

Observation of Two Structurally Distinct States in Ni‐P Glasses Using EXAFS

Evidence is presented for the existence of at least two structurally distinct states in Ni‒P glasses. X‐ray absorption spectra above the Ni K‐edge of samples prepared by pulsed electrodeposition (which we call type I) and electrodeposition (type II) display distinct differences which may be related to a rearrangement of Ni‒P and Ni‒Ni bonds. The short range order in the type II glass appears to be similar in character to that of crystalline , while an analogous relationship may be drawn between the type I glass and . The transitions between these states cannot be understood within the bounds of a continuous structural relaxation mechanism.

Structural relaxation mechanisms in liquid Eugenol. A depolarized light scattering study

A depolarized light scattering study of liquid Eugenol, over a large temperature range including the supercooled region, is proposed. Comparisons with shear mechanical impedance measurements, obtained at lower frequencies, lead to more precise information on the viscoelastic parameters in the supercooled region. The structural relaxation process measurements by means of the photon correlation technique are compared to the dielectric and mechanical measurements. Molecular mechanisms are proposed.

Water under high pressure

The behaviour of water under high pressure was studied using Monte Carlo NPT simulation methods. The results of the simulations were compared to previous experimental and theoretical works. The TIP4P water model reproduced calculated densities for pressures ranging from 0·1 MPa to 1·0 GPa to within 2 per cent of the experimental densities. The trend of a slight decrease in energy as the pressure increases at constant temperature appears to reproduce experimental findings but is found to be model and method dependent. Hydrogen bond and angular distribution functions analyses were performed in an effort to uncover the structural relaxation mechanism of water at high pressures.

A COMPUTER SIMULATION ON LOCAL STRUCTURE AND ATOMIC-LEVEL STRESS VARIATIONS IN AMORPHOUS IRON UNDER HIGH PRESSURE

Using the static simulation method, the variations of local structure (Voronoi polyhedra statistics) and atomic-level stress in a model of amorphous iron under pressure are examined in this paper. The results show that external pressure increases the local stress and energy on each atom in the system, extends their distribution range of values and enhances the extent of fluctuation, though a homogenization of Voronoi polyhedra is promoted by the same procedure. The calculation shows that the local stresses in the structure studied divided into compressed and expansed regions, embeded each other with some percolative character. The compressed region which covers about 50 per cent atoms of the system at normal pressure and plays a role of rigid framwork of the body, will grow on increasing pressure at the expense of cosuming the expanded region. It is suggested that as the oppositely stressed regions are essentially in distinct physical conditions, they are expected to show different behaviours and give different contributions in a thermo-activated transport process. For this reason, it would be more reasonable to treat a metglas as an inhomogeneous medium on analyzing its structural relaxation mechanisms.

The Brillouin Scattering of Highly Viscous Mixtures

Abstract The sound velocities of mixtures of tri-o-tolyl phosphate and diisobutyl phthalate were measured by means of the Brillouin scattering method in the temperature range from 214 to 333 K. The velocity dispersion was observed in the mixtures at temperatures from 273 to 323 K in the gigahertz region. The velocity dispersion was attributed to the structural relaxation, as in the case of pure highly viscous liquids. The reduced variable method proposed by Lamb et al. could not be applied to the storage moduli over wide frequency and temperature ranges. The relaxation curves of the reduced storage modulus obtained at room temperature, where the viscosity obeyed the Arrhenius equation, were well expressed by a single relaxation equation. The structural relaxation mechanism of the highly viscous liquids investigated here is related to the weak cooperative motions of molecules in the liquid state. The activation energy, ΔEr, obtained from the temperature dependences of the relaxation times was about a third of those estimated from the Arrhenius plots of the shear viscosities of the mixtures.

Mechanism of structural relaxation in a metallic glass

In the present paper, the relaxation phenomena in a multicomponent metallic glass, Ni 45 Fe 5 Co 20 Cr 10 Mo 4 B 16 , has been investigated through electrical resistance, acoustic emission and Mössbauer spectroscopy to understand the mechanism of structural relaxation in the amorphous matrix. Isothermal electrical resistance measurements clearly show that the structural relaxation occurs discontinuously through a number of discrete steps. There exists a good deal of correspondence between the variations of the acoustic emissions and the electrical resistance with time and temperature. It is shown that the stepwise change in the electrical resistance arises due to the viscous flow in the matrix which is associated with the acoustic emissions. Fourier deconvolution analysis of the Mössbauer spectra recorded at room temperature was carried out for the as-received sample as well as those relaxed at various annealing temperatures. The QS-distributions and QS-IS correlation, so obtained, clearly show that the structural relaxation induces a small but definite change in the short-range order. Present results indicate that the viscoelastic flow is the primary unit process responsible for structural relaxations in the matrix.

Atomic transport and structural relaxation in metallic glasses

Structural relaxation effects in metal-metalloid glasses are reviewed. Data on viscous flow and atomic diffusion are presented, and possible mechanisms of structural relaxation are discussed.

Magnetic Relaxation in Amorphous (Fe1-xNix)77Si10B13 Alloys

Disaccommodation (DA) and the magnetic annealing effect in amorphous (Fe1-xNix)77Si10B13, 0≦x≦0.8, alloys have been studied as a means of investigating the mechanism of structural relaxation in amorphous solids. The relaxation process is divided into two stages. The first stage, bolow about 200°C, is caused by the movement of metalloid atoms and the second, above 200°C, by diffusion of the constituent atoms. It is shown that the origin of DA is the same as that of induced magnetic anisotropy.

Structural relaxation in (Fe-Cr)-based glass

The Mössbauer effect of 57Fe and Curie temperature in amorphous Fe65Cr20B13Si2 and Fe16Ni62B14Si8 were measured to investigate the microscopic mechanism of structural relaxation. Broadening of the Mössbauer spectra appears around 100 and 250°C of the annealing temperature in these alloys. Corresponding to the broadenings, Tc also changes after the annealing.

Laser Anneal and the Amorphous-Crystalline Transition in the Cu60-Zr40 Alloy System

ABSTRACTRecently, significant advances have been made in establishing a theoretical basis for explaining the many structural-sensitive physical properties of amorphous materials. Viewed from both the theoretical and experimental realms, reversible annealing dependency and the structural relaxation mechanism of the amorphous solids are conceptually deducible from the concept of localized structural fluctuations. In the case of annealing, however, the limiting temperature below which no permanent atomiclevel rearrangements occur is not known and may well be dependent on several intrinsic properties of the amorphous state. In an effort to establish this limiting temperature, amorphous Cu60-Zr 40 samples have been annealed using a 1kv pulsed laser. Samples annealed during 1200, 800, 600 400, and 200 μ sec. have been analyzed by using Differential Scanning Calorimetry (DSC), High Resolution Electron Microscopy (HREM), and microanalysis. Analysis of the DSC indicates variations in both height and the shape of the exotherms from the samples annealed above 600 μ sec. duration. The results from HREM of these samples indicate the onset of amorphous-crystalline transition evidenced during 200 μ sec. annealing.

Mössbauer study of structural relaxation in amorphous YFe2 and Fe16Ni62B14Si8

Mössbauer effect of 57Fe in the paramagnetic phase of amorphous YFe2 and Fe16Ni62B14Si8 was measured to investigate a microscopic mechanism of structural relaxation. The Mössbauer spectrum broadens after annealing around 100 °C for YFe2. On the other hand, broadening appears around 100 and 250 °C in Fe16Ni62B14Si8. These broadenings are attributed to disturbances of environment around iron caused by movements of constituent atoms. The quadrupole splitting becomes small owing to a development of topological atomic short range ordering by the annealing in both samples. From these results it is concluded that the structural relaxation of metal-metalloid glass is divided into two stages. The first stage, below ∼250 °C, is an alloying process between metal atoms. The second stage, above ∼250 °C, is an incipient process of crystallization which is caused by a movement of metalloid atoms.

On mechanisms of structural relaxation in a Pd48Ni32P20 glass

Abstract Structural relaxation processes are investigated calorimetrically for a pre-conditioned Pd48Ni32P20 glass over a wide temperature range from well below to just above the glass transition. The low temperature anneals further stabilize the glassy structure. Upon heating, the annealed sample shows an excess endothermic specific heat ΔCp above the annealing temperature and completely recovers the initial enthalpy before any manifestation of glass transition, Tg. Significantly the ΔCp peak evolves in a continuous manner with annealing time. A physically reasonable activation energy spectrum N 0 ∗ (Q) is obtained with the proper choice of coupling constants which are dependent on annealing temperature. Results suggest the existence of localized relaxation modes which do not contribute to macroscopic flow. A concept of distribution in glass transition temperatures H(Tg,m) is conceived to account for the reversible relaxation with temperature. A model glass transition based on percolation theory is proposed and is found to reproduce the calorimetric relaxation phenomena well.

Ultrasonic studies on a basalt melt

Compressional wave attenuation, Qp−1, and velocity, Vp, for an alkalic olivine basalt have been measured from 1250° to 1450°C and from 5 to 10 MHz by an ultrasonic interferometric technique. Vp shows no significant frequency or temperature dependence, and Qp−1 increases with increasing frequency and decreasing temperature. For the conditions of this study, the melt can be modeled as a viscoelastic material with a single thermally activated structural relaxation mechanism having an activation energy of 2.2 eV and a relaxation time on the order of 1 ns. The data presented here fall in the region of the relaxation spectrum where the frequency, f, is much smaller than the relaxation frequency, and the absorption coefficient is proportional to f². Analysis of the data suggests that (1) compressional wave losses will be negligible in a basalt melt at seismic frequencies, and (2) the volume viscosity of basalt melt is comparable in magnitude to shear viscosity.

Biomolecular absorption of ultrasound

Understanding of the mechanisms of the interaction of ultrasound and biomolecules is essential to a thorough understanding of the present biophysical and medical applications of ultrasound and to the systematic and efficient accomplishment of future advances. Experimental evidence indicates that the biomolecules present in tissues play an important role in the ultrasonic propagaTion properties of tissues. Classical absorption mechanisms apparently play no significant role at the biomolecular level. Chemical and structural relaxation mechanisms have been proposed to explain the acoustic behavior observed, although generalized dispersion relationships have been reported to apply as well. Specific mechanisms of absorption for biomolecules in suspension at neutral pH have not been determined. Molecular size plays a role (in a “threshold” sense) in absorption but not in propagation speed. Molecular conformation appears to be important in some cases but not in others. Biomolecular‐solvent equilibria and biomolecule‐biomolecule interactions apparently are involved in the absorption processes.

Static and dynamic Kerr-effect studies of glycerol in its highly viscous state

Static and dynamic Kerr-effect studies have been made for glycerol in the range 205–255 K. The static measurements indicate extensive association of the molecules and that this increases with decreasing temperature, in accord with the results of dielectric studies. The dynamic Kerr effect shows one, near single-relaxation time, process at all temperatures with the rise and decay correlation times being related according to τK,r=(1.45 ± 0.20)τK,d. Dielectric relaxation measurements made concurrently with the Kerr-effect relaxation measurements show that τD≈(½)τK,d. The relationships between the relaxation of the Kerr effect, dielectric ultrasonic and dynamic light-scattering processes are discussed in terms of the structural relaxation mechanism and cross-correlation functions which appear for each technique. The Kerr-effect relaxation is slower than that for dielectric relaxation at a given temperature which is, in turn, slower than that for the light-scattering relaxation. The importance of mixed Rayleigh and Mountain relaxation for the latter is discussed in relation to the theory of Demoulin and co-workers.

Mechanism of structural dielectric relaxation in associated liquids

A mechanism of structural dielectric relaxation is proposed, and on this basis the kinetic equations taking into account tunneling of protons from the clusters are written and investigated. The solution, together with the results of dielectric measurements, is used to determine the structural parameters of low-concentration aqueous solutions.