Changes In Macroscopic Properties Research Articles

Role of porous structure in char oxidation

The role of the porous structure and its effect on reactivity was studied from changes of macroscopic properties. We present a review of our experimental-modeling effort that concerns porosity and reactivity. Shrinkage gave an understanding into the structure, and occurs becasue of the reduction of microcrystal dimensions. Detailed modeling for the porous structure was developed and reproduced the shrinkage. Fragmentation increased the understanding into the behavior of the porous structure; it depends on the structure geometry and threshold porosity, and has been shown to occur when macroporosity reaches a threshold. Tiny details of the structure were gained from thermal conductivity (TC). A five-time decrease was measured from 0 to 30% burnout, then TC remained constant up to 80% burnout and increased twofold to 100% burnout. Heat transfer calculations using the same model showed an excellent fit to experimental data, indicating that the extent of connectivity between microcrystals is the single most important parameter for TC. The behavior of TC at low conversions is consistent with the reactivity data, since steep changes in TC are accompanied by generating defects which are attributed to active sites.

New amorphous phase formation during amorphous state decay in soft magnetic amorphous alloy

Decay of amorphous alloys of metal-metalloid type from the initial state up to surface crystallization under thermal action has been studied for three different alloys based on Fe- Ni, Fe-Co and Fe-Ni-Co. Temperature dependences of their mechanical, magnetic, magneto-optical and heat properties have been investigated. Embrittlement is established as two stage process for all alloys under consideration. Nature of microscopic process determining the changes of the amorphous alloys macroscopic properties has been established from magnetic, magneto-optical and heat measurements. Experimental data obtained by above mentioned methods let us to argue the formation of new amorphous phase clusters in the embrittled layers.

Electrical impedance studies on a culture of a newly discovered strain of Streptomyces

Electrical impedance studies were performed in the frequency range of 10 Hz–13 MHz on a culture of a newly discovered strain of Streptomyces. Data obtained in low conductivity media using concentrated bacterial suspensions in water showed the existence of two dispersions for both the conductivity and permittivity: the low-frequency α-dispersion, ascribed to the ions in the diffuse layer of the cellular wall and the high frequency β-dispersion, ascribed to charging of the cytoplasmic membrane. It is in the “low-frequency range” of the β-dispersion that the conductance and capacitance measurements can provide biomass-selective information. From experiments performed in the culture broth, it was found that in the audio-frequency range, the capacitance of the suspension is dominated by the polarization term, and its modification with the incubation time is indicative of the changes in the growth medium due to the metabolic process. The changes of macroscopic electrical properties of the medium capacitance (measured in the audio-frequency range) and conductance (measured in the low radio-frequency range) vs. the incubation time were found to parallel the growth curve obtained by dry weight measurements. Direct information on biomass increase during fermentation was obtained from two types of measurements in the low RF frequency: the capacitance in the low RF range and the difference between the conductance of the growth medium and that of the suspension.

Amplitude Spectrum Analysis of Dynamic FT-IR Spectra of Uniaxially Drawn Poly(Ethylene Terephthalate) Film with a Variable-Temperature Polymer Stretcher

Dynamic FT-IR spectra of uniaxially 5×-drawn poly(ethylene terephthalate) (PET) film at a temperature of 30 to 150 °C were measured with the use of a variable-temperature polymer stretcher, and amplitude spectrum analyses of the dynamic spectra were carried out. A drastic decrease in dynamic variations of the trans C–O stretching band was observed above the glass-to-rubber transition temperature. This observation suggests that the trans C–O bond in the ethylene glycol unit is closely related to changes in macroscopic mechanical properties of PET film in the glass-to-rubber transition.

The reaction rate theory of radiation effects

In materials irradiated with energetic neutrons or charged particles, point defects, clusters, and extended-defect microstructures are produced. Because of the possibility of complete atom-by-atom rearrangement of the material under irradiation, significant changes may result in physical, mechanical, and electrical properties. The theoretical and computational approaches for describing these changes begin with the atomic displacement reactions and follow numerous reaction pathways to an evolved microstructure and the properties dictated by it. Rate theory based on defect reactions has been developed to understand these processes. It is the only approach that spans the large time, spatial, and energy ranges from defect production to changes in macroscopic properties. It has been most thoroughly applied to the phenomena of radiation-induced dimensional changes, particularly swelling and creep. This emphasis has derived both from the importance of these phenomena in technological applications and from their scientific challenge.

An associated solution model for albite-water melts

An associated solution model linking microscopic and macroscopic data is developed to describe the thermodynamic properties of NaAlSi 3O 8-H 2O melt mixture. The model is based upon the following two homogeneous melt speciation reactions: Na 0.75Al 0.75Si 2.25O 60.3Na 2Al 2SiO 6 + 0.15NaAlSi 2O 6 + 0.55Si 3O 6; Na 2Al 2SiO 6, + H 2ONa 2Al 2(OH) 2SiO 5, where the first reaction represents the formation of three-membered rings in albite melt and the second one represents hydrolysis of Al-O-Al linkages. The equilibrium constants of these two reactions are calibrated on the basis of available water speciation data, phase equilibrium data; thermodynamic data, solubility data, and P-V-T data. The calculated thermodynamic mixing properties show that relative to our standard states, albite and H 2O melt components exhibit strong negative deviation from ideality. The value for G ex for any given P and T reaches a minimum at approximately 33 mol% water, which coincides with the composition at which there is a significant change in many microscopic and macroscopic properties in this system. For all P, T conditions, it is calculated that hydroxyl-bearing species dominate over molecular water at low water contents but the latter becomes dominant after the total water content exceeds about 33 mol%. When the total water content in the albite melt increases, the abundance of molecular water increases and that of albite species decreases, while the abundances of both three-membered rings and hydroxyl-bearing species (Na 2Al 2(OH) 2SiO 5) first increase, then decrease. The calculated water solubility is positively correlated with P and negatively correlated with T for pressures lower than approximately 5 kb but positively correlated with T for pressures higher than this, consistent with the behavior suggested by previous workers. The water solubility has been readily modeled by hydrolysis of Al-O-Al linkages in spite of their very small abundance in these melts. The calculated hypersolidus phase relations are consistent with available experimental data. In addition, critical phenomena are predicted within the system, in agreement with the implications of available experimental data.

Reaction-induced changes in interfacial and macroscopic mechanical properties of SiC monofilament-reinforced titanium

Results are presented from single fibre push-out testing, simple tension and impact three-point bending of SiC/Ti composites based on Ti-6AI-4V alloy reinforced with W-cored monofilaments having duplex C/TiB 2 coatings. Specimens were tested after various heat treatments and it is shown that a progressive increase in the interfacial shear stress for frictional sliding is observed as reaction proceeds. This increase in the resistance to frictional sliding can be correlated with a decrease in the thickness of the graphitic layer. Testing of composites subjected to the same heat treatments also revealed progressive reduction in tensile strength with increased reaction layer thickness. Monitoring of Poisson's ratios during testing was used to confirm that matrix plasticity occurred during axial loading, but inelastic behaviour was due to interfacial damage under transverse load. Consistent with this, impact energies were found to be higher under axial loading. Although some fibre pull-out was observed for axially loaded specimens without thick reaction layers, it is suggested that neither this nor any other interfacial process was contributing significantly to the work associated with impact loadings.

M�ssbauer study of YBa2(Cu1?x Fex)3Oy from an aspect of distribution of Fe

Fe Mossbauer spectra have been measured systematically in YBa2(Cu1−xFex)3Oy prepared by two different heat treatments: the usual treatment (O) and the heat treatment including that under lower oxygen fugacity at high temperatures followed by the oxidation process at low temperatures (NO). From the analysis of the results in terms of binomial random probability, it has been found that the Fe distribution in the Cu(1) site is almost random in the heat treatment (O). Here, the D1 and D2 components in the Mossbauer spectra, which have been assigned to the 4 and 6 oxygen coordinated Fe ions in the Cu(1) site respectively, should correspond to the monomer and cluster Fe ions. In the heat treatment (NO), some of the Fe ions are transferred to the Cu(2) site and there exist only cluster Fe ions in the Cu(1) site, resulting in the characteristic change of macroscopic properties of this system.

Na and Ca tracer diffusion in plagioclase glasses and supercooled melts

The diffusion of Na and Ca was investigated in various glasses of plagioclase compositions by the residual activity method in the temperature range 400–1000°C at normal pressure. At anomalous diffusion was observed for both Na and Ca diffusion in high-calcic glasses. This behaviour is attributed to a partially interrupted contact between the sample and the activated surface layer. Na diffusivity decreases strongly with increasing CaAl2Si2O8 (An) content of the glass, especially for An-rich glasses. By contrast, the compositional dependence of Ca diffusion is less pronounced. For example, at 800°C the diffusivity of Ca in a glass containing 60 mole% An is lower only by a factor of 5 compared to pure NaAlSi3O8 (Ab) glass whereas it is by a factor of 60 for Na. Arrhenius plots of Na diffusivities in the An50 and An70 glasses showed marked changes of slope, probably caused by the transformation from a glass to a supercooled melt. Apparent transformation temperatures obtained from these plots are lower than those obtained calorimetrically. This indicates, that relaxation processes occur in these glasses at temperatures below the onset of non-linear changes in macroscopic properties.

Glass dispersed liquid crystals for electro-optical devices

The sol-gel process has been used to trap nematic liquid crystal droplets, thus producing thin-film and bulk gel-glass dispersed liquid crystals (GDLC). Marked changes of macroscopic optical properties were achieved by controlling pore size and chemical surface composition of GDLCs, thus facilitating the design of practical devices. Switching between opaque and transparent states and variable birefringence has been demonstrated. Voltages required for droplet reorientation, and electro-optical response times, are related to pore-surface chemical composition, droplet size and shape, liquid crystal concentration, and anchoring strength of the silica-cage inner surface.

Defect reactions and clustering in irradiated solids

Irradiation of solid materials with energetic neutrons or charged particles can lead to profound changes in defect structure, microcomposition, and macroscopic properties. Such changes occur by atomic and microstructural mechanisms, some of which are familiar in "classical" physical metallurgy and materials science. However, other cases appear to be unique to irradiation. Irradiation has considerably broadened and indeed provided an entirely new dimension in materials science, since the energetic displacement of atoms potentially reaches to every property or process. The initial damaging events leading to the creation of point defects are generally complete in times of order 10−11 s. Subsequent changes in structure, composition, and properties take place in a span of much longer time scales corresponding to interstitial and vacancy diffusion, clustering, solute segregation, and precipitation. An extensive theoretical framework has been developed to understand the kinetics of these processes. Emphasis has been placed on both steady cumulative processes and on fluctuations, and on the appropriate application of stochastic and deterministic descriptions. Parallel and interactive experimental activities for both applied and basic programs over the past two decades have increased the level of phenomenological knowledge enormously. Much of the work has emphasized either high-dose phenomena such as irradiation-induced swelling, creep, embrittlement, phase instability, and solute segregation relevant to materials applications, or the properties, structures, and interactions of defects, which underlie more fundamental issues.

The Concept of Production Bias and Its Possible Role in Defect Accumulation under Cascade Damage Conditions

AbstractIn the study of microstructural evolution and macroscopic property changes due to radiation damage, the point defects are usually modelled as being produced continuously in time and uniformly in space. Furthermore, the excess of vacancies required to sustain the void growth is assumed to arise entirely due to the preferential trapping of interstitial atoms at dislocations. It is shown that this approach may not be appropriate for cascade damage conditions where both vacancies and interstitials are produced in a highly localized and segregated fashion. A model is presented which takes into account the effects of both, vacancy and interstitial clustering in the cascade zones on the defect accumulation rate (e. g. swelling rate) under cascade damage conditions. The consideration of interstitial clustering in the cascade yields a production bias which is found to be a potent driving force for void swelling in addition to the one provided by the dislocation bias.

Stabilizer additives in ionizing radiation environments under oxidizing conditions

Very little work has been done with energy-scavenger additives in radiation environments under oxidizing conditions. In this study, the stabilizing effect of an energy scavenger (pyrene) and a radical scavenger (hindered phenol), which were incorporated in a polychloroprene rubber, has been evaluated in a γ-radiation environment in air under conditions which gave rise to oxidation throughout the sample. Both compounds were found to significantly inhibit changes in macroscopic tensile properties of the materials. The pyrene enhanced the material's radiation resistance when used alone and also when used in conjunction with the hindered phenol. The rate of radiation-induced chemical destruction of the two additives was monitored by gas chromatography after solvent-extraction from degraded samples. Following a dose of 3 × 10 5 Gy, only a few per cent of the hindered phenol remained, whereas more than 90% of the pyrene was still present. The rate of disappearance of the hindered phenol was slower in samples which also contained the pyrene.

The use of TGA to study thermal ageing effects of orientated polypropylene

The effective lifetimes of many orientated polymers like polypropylene films and fibres are defined in terms of the time required for a given environment to promote a sudden change in a macroscopic property ; often catastrophic failure may follow such a change. TGA may be used to investigate the microscopic physico-chemical changes which occur during the induction period of thermally-aged polypropylene. Marked shifts in the temperatures of the onset and maximum rates of mass loss accompanying post-fusion oxidation of aged orientated films are observed which reflect the slight changes in macroscopic properties that also occur during the induction period. The apparent kinetic parameters of unaged and aged film post-fusion oxidations are determined.

Effect of mechanical disturbances on stability of impregnated windings

The impregnation of magnet windings with epoxy resin is widely used in developing high-performance superconducting magnets. The purpose of the epoxy impregnation is to prevent wire motion, to enhance the rigidity of the windings, and to assemble the windings in one body. Composite beams simulating an impregnated coil have been examined in order to investigate the effect of mechanical disturbances such as bond failures or epoxy cracks on the stabilities. Since the elastic energy stored in impregnated coil could be released as heat, the sudden change of rigidity during deformation would induce the mechanical disturbances which could quench the superconducting windings. Mechanical disturbances simultaneously release acoustic emissions, hence, the disturbances were monitored by thermocouples together with an acoustic-emission measuring apparatus. The change of macroscopic mechanical properties was measured in terms to the load-displacement curve of the windings. The stress analysis was made using the finite-element method (FEM). It was confirmed experimentally that the epoxy crackings and bond failures could induce the rigidity degradation and cause the heat generation which could result in a premature quench. Such disturbances were successfully estimated by the FEM analysis.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Sintering and crystallization of titanium‐containing lead borate glasses

AbstractThe sintering and crystallization of glass powders with chemical composition 47 PbO—38 B2O3—15 TiO2 (mole %) was investigated under non‐isothermal conditions by shrinkage measurements, differential thermo‐analysis (DTA), X‐ray diffraction (XRD), dielectric measurement and positron annihilation (Doppler broadening). The correlation between changes of macroscopic properties and positron annihilation results recommends this method for the detection of structural changes in glasses.

Application of high fluence fast reactor data to fusion-relevant materials problems

In three recent comparative studies in HFIR and EBR-II where the effect of helium and solid transmutants could be assessed, it was found that in each case there was no significant perturbation of the macroscopic property change under consideration. These findings reinforce the belief that fast reactors can serve as a major tool for fusion materials studies and that the effects of helium and other transmutants can be treated as second-order perturbations to be studied by other methods. A number of new fusion-relevant insights derived from fast reactor studies are presented.

Studies on Macroscopic Property and Microstructural Changes in PLZT Ferroelectric Ceramics

The macroscopic property and microstructural changes correlated with the so-called α-β transition are presented with emphasis on the eletro-optic and TEM studies. The results of electro-optic measurement show that in the temperature range between the β-α transition point Tt and 10–15°C below Tt a definite reverse electric field can induce the ferroelectric β phase back to nonferroelectric α phase and the transmittance of the sample will rise sharply to near the transmittance of the thermally depolarized specimen. From TEM study microregions were observed in α phase PLZT grains, it would be a forceful verification of polar microregions concept for relaxor ferroelectrics.

Theory of relaxation in viscous liquids and glasses

A theory of relaxation (the time-dependent change of macroscopic properties) of nonpolymeric viscous liquids and glasses is presented. The fluid is described by a set of quasiequilibrium structures, and a master equation gives the transitions among these structures. Any structural change is presumed to require a cooperative rearrangement involving many atoms, and this rearrangement entails a fluctuation to a high-energy transition state. The structure of the fluid varies from point to point, and the rate of this transformation depends crucially on the local structure. The resulting kinetic equation describes very well the main features of observed relaxation—namely, the broad distribution of relaxation times and the nonlinearity (in ΔT) of relaxation following a temperature jump ΔT, where the apparent activation energy for relaxation depends on time. The kinetic equation is solved exactly, and the resulting solution is exhibited for a particular set of the parameters. The resulting relaxation function for energy relaxation goes as e−t1/4, except at the very shortest times. (The precise exponent— 1/4 in this case—will depend on the parameters used in the particular calculation.) A detailed comparison with other theories is made, and suggestions of how the theory can be used to develop a phenomenological model of relaxation of glass are given.

Combined environment aging effects: Radiation‐thermal degradation of polyvinylchloride and polyethylene

AbstractResults are presented for a case of polymer aging in which powerful synergisms are found between radiation and temperature. This effect was observed with formulations of polyvinylchloride and polyethylene and occurred in simultaneous and Sequential radiation‐thermal experiments. Dose rate dependencies, which appear to be mechanistically related to the synergism, were also found. The evidence indicates that these aging effects are mediated by a thermally induced breakdown of peroxides initially formed by the radiation. Similar effects could be important to material degradation in a variety of other types of combined‐stress environment. A new technique, which uses PH3 treatment of intact polymer specimens to test for the importance of peroxide is the pathway that leads to changes in macroscopic tensile properties, is described.