Porosity-dependence of elastic properties and ultrasonic velocity in polycrystalline alumina — a model based on cylindrical pores

Effective elastic moduli and ultrasonic velocity of materials having aligned cylindrical pores have been derived using a series expansion in terms of the difference between the upper and lower bounds of elastic moduli obtained by the variational method. The theoretical results for polycrystalline alumina agree well with the experimental data, confirming the suggestion of previous researchers that a matrix containing parallel cylindrical pores orientated perpendicular to the applied stress, provide a better model than a spherical one in describing the porosity-dependence of elastic moduli in sintered specimens.     

Elastic properties of porous thermosetting polymers

A new equationE =E ₀ (1 –bp) ⁿ has been derived semi-empirically to describe the porosity dependence of elastic properties of thermosetting polymers. The material constantb is defined as a pore distribution geometry factor and the other material constantn is dependent on pore geometry. The equation shows good agreement with the data on porous polyester and epoxy resins.     

Evaluation of Concurrent Flaw Populations in Silicon Carbide in Terms of a Modified Weibull Distribution Function

Concurrent surface- and edge-flaw populations are often observed in strength testing of ceramics The overall strength distribution in such ceramic samples has been analyzed in terms of a modified Weibull distribution function. The junction provides an upper and lower strength limit and is characterized by two shape and location parameters. To evaluate the parameters of the distribution function, the type of flaw that causes failure need not be identified. The applicability of the function has been evaluated in terms of experimental results on SiC specimens. The analysis also shows that the relative values of shape parameters determine which flaw type dominates.     

Selective detection of dopamine using a functionalised polyaniline composite electrode

The behaviour of a poly (aniline boronic acid) (PABA) modified glassy carbon electrode (GCE) for the detection of dopamine (DA) in the presence of excess of ascorbic acid (AA) using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques is investigated. On bare GCE, both DA and AA are oxidized at ~0.16 V, whereas on PABA modified GCE they are oxidized at 0.2 and 0.054 V, respectively. Though PABA favours DA oxidation through ester formation with boronic acid motif, the AA oxidation is also promoted by polyaniline backbone through the involvement of AA in the redox of polyaniline. Since both DA and AA undergo oxidation at closely spaced potentials at a PABA electrode, Nafion®-incorporation into the PABA film was examined for selective determination of DA in the presence of AA. The selectivity was due to accumulation of DA on the electrode surface through ester formation with the boronic acid group and suppression of AA oxidative current through charge discrimination by Nafion.     

Disruption of ergosterol biosynthesis,growth, and the morphological transition in <Emphasis Type="Italic">Candida albicans</Emphasis> by sterol methyltransferase inhibitors containing sulfur at C-25 in the sterol side chain

The sterol substrate analog 25-thialanosterol and its corresponding sulfonium salt were evaluated for their ability to serve as antifungal agents and to inhibit sterol methyltransferase (SMT) activity in Candida albicans. Both compounds inhibited cell proliferation, were fungistatic, interrupted the yeastlike-form to germ-tube-form transition, and resulted in the accumulation of zymosterol and related Δ²⁴-sterols concurrent with a decrease in ergosterol, as was expected for the specific inhibition of SMT activity. Feedback on sterol synthesis was evidenced by elevated levels of cellular sterols in treated vs. control cultures. However, neither farnesol nor squalene accumulated in significant amounts in treated cultures, suggesting that carbon flux is channeled from the isoprenoid pathway to the sterol pathway with minor interruption or redirection until blockage at the C-methylation step. Activity assays using solubilized C. albicans SMT confirmed the inhibitors impair SMT action. Kinetic analysis indicated that 25-thialanosterol inhibited SMT with the properties of a time-dependent mechanismbased inactivator K _i of 5 =gmM and apparent k _inact of 0.013 min⁻¹, whereas the corresponding sulfonium salt was a reversible-type transition state analog exhibiting a K _i of 20 nM. The results are interpreted to imply changes in ergosterol homeostasis as influenced by SMT activity can control growth and the morphological transition in C. albicans, possibly affecting disease development.     

Methanol tolerant oxygen-reduction activity of carbon supported platinum–bismuth bimetallic nanoparticles

The oxygen reduction activity and methanol tolerance of Pt–Bi/C electrocatalysts were studied using electrochemical voltammetric techniques including rotating ring-disk electrode. The Pt–Bi/C catalyst was prepared via a polyol method and subjected to heat treatment to increase the degree of alloying. X-ray diffraction studies revealed the unalloyed character of the as-prepared catalyst and alloy formation upon heat treatment. The electrochemical behaviour of both catalysts showed different behaviour in dilute acid electrolytes, namely sulphuric and perchloric acids. In both electrolytes, the oxygen reduction reaction was found to occur via the four-electron process revealing that the mechanism of oxygen reduction is unaltered even in the presence of excess of methanol. Pt–Bi/C catalyst material showed dramatically different properties and reactivity with respect to oxygen reduction activity and methanol tolerance in perchloric and sulphuric acids. The onset potential for oxygen reduction reaction (ORR) significantly shifted by about 100 mV to more negative values and at the same time the current density was significantly enhanced. This type of non-ideal methanol-tolerant behaviour among Pt bimetallics and a “trade off” is common with all the known so-called methanol tolerant combinations of Pt. In general, the Pt–Bi surface appeared to have a negligibly lesser sensitivity towards methanol activity compared to pure platinum.     

A simple stochastic model for yielding in specimens with limited number of dislocations

A simple statistical model is developed based on a random distribution and orientation of dislocations in order to explain recent experimental observations of the strength of small specimens containing a limited number of dislocations. Two different types of randomness are introduced, namely, randomness in the spatial location of the dislocations and randomness in the stress needed to activate them. For convenience, the randomness in the activation stress is modeled by assigning a random Schmid factor to the dislocations. In contrast to previous stochastic models, the current model predicts the yield strength not only in the presence of dislocations but also in their absence. Furthermore, the model predicts the scatter in the yield strength in addition to the mean. The model is found to quantitatively explain the yield strength and scatter in micro-compression/tension tests of Mo-alloy fibers using dislocation densities and arrangements measured by transmission electron microscopy. The results of Brenner’s classic tensile tests on metallic whiskers are qualitatively reconciled. The model adds credence to the notion that “smaller is stronger” from a purely statistical point of view.     

Pyramid coding based rate control for constant bit rate video streaming

In this paper, a novel pyramid coding based rate control scheme is proposed for video streaming applications constrained by a constant channel bandwidth. To achieve the target bit rate with the best quality, the initial quantization parameter (QP) is determined by the average spatio-temporal complexity of the sequence, its resolution and the target bit rate. Simple linear estimation models are then used to predict the number of bits that would be necessary to encode a frame for a given complexity and QP. The experimental results demonstrate that the proposed rate control scheme significantly outperforms the existing rate control scheme in the Joint Model (JM) reference software in terms of Peak Signal to Noise Ratio (PSNR) and consistent perceptual visual quality while achieving the target bit rate. Finally, the proposed scheme is validated through experimental evaluation over a miniature test-bed.

     

The relations between the shear modulus, the bulk modulus and Young's modulus for porous isotropic ceramic materials

A relation between the shear modulus and Young's modulus of isotropic porous ceramics has been derived based on the Mori–Tanaka mean-field approach. The applicability of the relation has been evaluated using the experimental values available in the literature for the shear modulus, the bulk modulus and Young's modulus of porous ceramics prepared using various processing techniques and powder sizes. It is also shown that the ratio of the shear to Young's modulus of porous ceramics can be approximated by a constant value of 0.391.