Modelation of the state of inelastic deformation induced by a rigid indenter in a brittle material

In microhardness measurements an irreversible deformation field is produced within a sample in a region beneath the indenter. For macroscopically brittle material this can be modelled on the basis of the concept of a generalised linear-elastic continuum theory. In discussing the results of structurally conditioned basic processes particulary the case of pressure-induced densification is treated.     

Structural relaxation in the glass: Evidence for a path dependence of the relaxation time

Using a recently developed novel temperature perturbation experiment, one of the primary assumptions underlying the phenomenological models of structural recovery, i.e., that the relaxation time depends on the instantaneous state of the material and not on its temperature history, is tested. The results of our experiments show that as the material gets close to equilibrium, this assumption is valid, but far from equilibrium, it is not.     

Exponential structural relaxation of a high purity silica glass

While most other glasses exhibit non-exponential structural relaxation characteristics even when the change of fictive temperature is small, a high purity silica glass exhibited exponential structural relaxation. This was demonstrated by showing that the non-exponential exponent or β value of the KWW function of the high purity silica glass approaches unity when the change of the fictive temperature approaches zero both from higher and lower temperature sides of the heat-treatment temperature. The non-exponentiality of the structural relaxation of this glass when fictive temperature change is finite is due to the change of relaxation time during the structural relaxation.     

Dielectric relaxation of water adsorbed on cellulose

Water is an essential part of the structure of biological materials. To estimate how the physical state of water adsorbed on cellulose affects various properties, the dielectric properties of moist cellulose were investigated. Three dielectric relaxations were obtained. The relaxation with the lowest frequency was due to electrode polarization. The direct-current conduction calculated from the dielectric loss leaving out the relaxations appearing in mid and high frequencies increased rapidly from 7% or more moisture content (MC). Free water, which can dissolve electrolytes, was increased in the adsorbed water of materials with 7% or more MC. The relaxation appearing in the mid-frequency range was due to interfacial polarization in the heterogeneous structure which consists of adsorbed water with large electrical conductivity within the insulating cellulose. The relaxation appearing at higher frequencies was due to the motion of adsorbed water in cellulose. The relaxation was investigated from relationships between activation enthalpy and entropy in the relaxation. The relationships in low MC cellulose were close to those of sugar, while the relationships in high MC cellulose were close to the relationships extrapolated from bulk water.     

Surface relaxation of a crystal plate with a step-like distribution of impurity atoms

The deformation of a crystal plate with a step-like distribution of dilatation centres is calculated according to the isotropic theory of elasticity. Analytical expressions for quantities which are interest for interpreting and calculating X-ray intensity distributions are derived and discussed.     

TSDC study of structural relaxation on polyethylene terephthalate

The influence of physical aging processes on the electrical and thermal properties of amorphous polyethylene terephthalate was investigated by means of the thermally stimulated depolarization current (TSDC), differential scanning calorimetry (DSC) and low frequency dielectric methods. A noticeable decrease in polarization and loss factor corresponding to the main α relaxation was observed with increasing aging times, especially for aging temperatures ranging from 10 to 20 °C below T_g. This behavior is compatible with the free-volume concept, and can be explained by a gradual decrease in segmental mobility following a shift of the relaxation time spectrum of molecular motions to longer time scales. The whole experimental evidence suggests that proper consideration should be given to thermal history and aging effects of polyethylene terephthalate. The thermally stimulated depolarization current was chosen due to its low equivalent frequency, useful to study systems exhibiting multiple relaxations.     

Simulation of mass transfer in a kinetic experiment based on migration of flat layers of a solution in a melt in a temperature gradient field

Mass transfer upon migration of flat liquid layers of a solution in a melt under conditions of slow change in the average temperature at a constant rate has been analyzed for the dislocation and nucleation mechanisms of interface processes. It is shown that the experimental data on the dependence of the migration rate of flat layers on their thickness can be adequately described within the theory developed for strictly steady thermal conditions. A technique is proposed for determining the overall limitations on the mass transfer at interfaces and the migration rate in the diffusion mode from the experimental results obtained in a mixed migration mode.

     

Load relaxation behavior of a Zr41.2Ti13.8Cu12.5Ni10 Be22.5 bulk metallic glass

The load relaxation behavior within the supercooled liquid region of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass has been investigated. To explain the relationship between normalized stress and relaxation time, two different stress relaxation modes such as a Kohlrausch-Williams-Watts (KWW) behavior and a simple power law were applied to the short and long relaxation time regimes, respectively. The apparent activation energy for stress relaxation is 126 ± 10 kJ/mol. Flow curves were obtained by converting load-displacement data into a flow stress-strain rate relation, resulting in three different deformation characteristics through a wide strain rate region interpreted in terms of strain rate sensitivity. A prediction of hot workability has also been attempted by constructing a power dissipation map based on a dynamic materials model.     

Outer space formation of a laser host glass

Classical crystallization calculations were performed to determine the possibility of forming a particular type of laser glass with the avoidance of devitrification in an outer space laboratory. Although the laser glass in question readily crystallizes in an earth environment, it is demonstrated that under the homogeneous nucleating conditions obtainable in a zero gravity laboratory this laser glass may be easily quenched to a virtually crystal-free product. Use of this material as a host in a neodymium glass laser would result in a more than 10% increase in efficiency when compared to laser glass rods of similar composition currently commercially available.     

Investigation of the stability of the flat crystallization front of a binary melt by a variational method

The stability of the flat crystallization front of a dilute binary melt is investigated within the two-dimensional model of solidification taking into account the latent heat of fusion and the difference between thermal conductivities of the solid and liquid phases by the method of integral functionals with an unknown region of integration. The stationary problem of impurity diffusion in a crystallizing melt and heat propagation in a limited region Ω is solved within the second-order approximation in the amplitude of deviation from a flat crystallization front. An expression for the functional is obtained whose value is proportional to the energy dissipated in the region Ω owing to heat and mass transfer processes. The results obtained are compared with the literature data on the stability of the flat crystallization front of a binary melt.     

Synthesis and structure of a HgI2 coordination polymer with novel rigid N-donor ligand

Solution reaction of mercury iodide with organic multifunctional ligand 2, 3-di (4-pyridyl)-2, 3-butanediol (dpb) generated a 1D polymer [(HgI₂)₂(dpb)₂(DMSO)₂] _n 1, and the crystal structure has been determined (C₁₈H₂₈HgI₂N₂O₄S₂), M _r=854.93, a=10.821(2), b=25.196(5), c=10.039(2)?, space group Pnma, Z=4, and V=2737.0(9)?³. In 1 the rigidity of HgI₂ and dpb make it a zigzag structure.     

Effect of fragility on relaxation of density fluctuations in glass

Density fluctuations play a crucial role in governing the optical, mechanical, thermodynamic, and kinetic properties of glass. The relaxation of density fluctuations displays an inherently nonmonotonic behavior, yielding a minimum in fluctuations following a quench and isothermal hold. Here we investigate the impact of liquid fragility on the relaxation of density fluctuations in the nonequilibrium glassy state. While fragility has a direct impact on the kinetics of the relaxation process, the minimum level of density fluctuations is unaffected by changes in fragility alone. The magnitude of density fluctuations can be minimized by tailoring the thermal history of the glass.     

Study on the gel casting of fused silica glass

Slip casting process is usually applied for the forming of fused silica products. Segregation always occurs and it will results in density deviation. By using gel casting process, green is fabricated by means of in situ polymerization with a three-dimensional network, holding the particles together and eliminating the tendency of migration. To prepare gel casting slurries, premix solutions were composed of acrylamide and N,N′-methylenebisacrylamide. Solid loading was kept 60% and the average particle size of silica powder 8–8.5 μm. Lactic acid was introduced as a dispersant to regulate the pH value 3–4. Mechanism of the dispersant was investigated by studying ζ-potential at different pH. Ammonium persulfate (NH₄)₂S₂O₄ was added as an initiator. Gelation took place with the help of initiator at 50–60 °C. Nanometer silica was introduced to boost sinterability so that the density and bending strength of fused silica ceramics have been increased to 2.1 g/cm³ and 40 Mpa, respectively.     

Distribution of hydroxyl groups on the silica gel surface

An experimental method for the investigation of the structure of surface hydroxyl coverage has been developed using the vapour of evaporable halides. It was found that the reaction of the surface hydroxyl groups with TiCl₄ proceeds to completion. A possibility for a quantitative characterization of the hydroxyl groups density was found on the basis of the proved applicability of the binomial distribution for the investigated.     

Effect of pressure on the water relaxation in glassy polyetherimide

A dielectric study of the water peak in glassy polyetherimide is reported. The peak is found to shift to higher frequency with increasing intensity (associated with increasing water content) and to shift to lower frequency with increasing pressure. The temperature dependence of the peak position is Arrhenius with activation energy 0.46 eV. The activation volume is approximately equal to the Van der Waals b coefficient which is about twice the volume of a water molecule. This implies that the relaxation mechanism involves more than simple reorientation of a water molecule. Modeling results suggest that water molecules can associate with the glassy polymer through hydrogen bonds to the imide carbonyl oxygen atoms. In the case of Ultem^® 1000, a water molecule is able to H-bond simultaneously with two carbonyl oxygens, however in Ultem^® 5000 the para-bisimide linkage leads to improved chain packing and the more extended geometry allows only one H-bond between each imide carbonyl unit and water molecule.     

Influence of gel inclusions on the mechanical behaviour of Gypsum

In a single test tube system, gypsum crystals grown in alcoholic gel comprise of interpenetration twinned crystals at the interface towards the gel, whereas untwinned tabular crystals are observed at greater depths inside the gel. By indentation of the crystals with a microhardness tool and recording the indentation impressions by means of a scanning electron microscope (SEM) and a transmission electron microscope (TEM) it is found that the hardness of the tabular crystals is of the same order of magnitude as those of natural crystals of gypsum, whereas interpenetration twinned crystals show a remarkable increase in hardness. On studying the morphology of cleavage faces and on carrying out energy dispersive X-ray analysis (EDXA), it is concluded that the increase in hardness of interpenetration twinned crystals with respect to tabular and natural crystals may be due to the role of the inclusion of gel particles during the growth. The implications are discussed.     

A detailed study of gold single crystal growth in a silica gel

Gold single microcrystals have been fabricated by electrochemical growth in a silica gel. Structural characterization of the single crystals by backscatter electron diffraction showed a preferred orientation of Au (1 1 1) and a minor orientation of Au (1 0 0). In addition, the influence of additives on the nucleation and growth of gold microcrystals has been studied. It was found that the inclusion of chemical additives in the growth solutions altered the characteristics of the gold crystals. Possible mechanisms for nucleation and growth of these crystals are discussed.     

Effect of cooling rate on enthalpy and volume relaxation of polystyrene

This paper presents the results of measurements of physical aging on polystyrene with a narrow molecular weight distribution (M_w/M_n = 1.03). The evolution of the aging process was followed by recording the relaxed enthalpy and the accompanying decrease in volume, using differential scanning calorimetry and mercury-in-glass dilatometry, respectively. The measurements were carried out after cooling the sample at constant rate to the aging temperature. The cooling rate varied between 0.0037 and 1 °C/min. The aging data were fitted with the KWW and the TNM functions. The differences in the time scales of enthalpy, h, and volume, v, relaxation were relatively small. From the h(v) graphs the derivative dh/dv was found to amount to about 1.8 GPa, a value significantly exceeding the predictions of the thermodynamic model containing an additional internal variable.