Some considerations on equation of state and phase relations: Polymer solutions and blends

We review briefly the general assumptions underlying hole theories of the configurational thermodynamic functions for single and multiconstituent systems. From the original Simha-Somcynsky theory several important modifications have recently evolved. First, there is a revision of the combinatorial entropy originating from the mixing of segments and holes in the spirit of Huggins's theory. With consistent additional modifications of the configurational free energy, quantitatively significant consequences for certain aspects of phase equilibria can arise. Finally, allowance for nonrandom mixing of constituents species and holes has been made. We illustrate the theory's potentials first in terms of pressure-volume-temperature (PVT) data for high and low molar mass species and their miscible mixtures. The influence of PVT properties on the miscibility behavior of solutions and blends is examined next. Of particular concern are the connections between predictability of lower from upper mixing spinodals, pressure effects, and the binary interaction χ-function.     

Influence of preparation method on the catalytic properties of acid-activated tetramethylammonium-exchanged clays

Two montmorillonites STx-1 (Texas) and SWy-2 (Wyoming) were first activated with different amounts of 12 M HCl and then exchanged with a fixed amount of 1 M tetramethylammonium (TMA⁺) chloride solution (H/TMA samples) at room temperature. TMA⁺-exchanged samples and then acid activated (TMA/H samples) were also prepared to evaluate the resistance to displacement of TMA⁺ by protons. The surface area and the acidity were determined by BET and cyclohexylamine adsorption methods, respectively. The catalytic activity of these acid activated organoclays was directly measured using the isomerization of 1-butene at 300 °C to yield cis- and trans-2-butene. The total conversion for the isomerization of 1-butene was higher for the TMA/H-samples (70 and 47% for SWy-2 and STx-1, respectively) than for the H/TMA-samples (51 and 25% for SWy-2 and STx-1, respectively). TMA⁺ cations adsorbed on the clays were extremely resistant to exchange by protons, but protons were easily displaced by TMA⁺ cations.     

On the windowed Cholesky factorization of the time-varyingasynchronous CDMA channel

In this letter we present a simplified proof for the convergence of the windowed Cholesky factorization of the time-varying asynchronous code-division multiple-access (CDMA) channel     

Effect of Friction on Edge Singularities in Slip Bands

Resistance to dislocation glide in slip bands due to the presence of other dislocations may be represented by an additional term in the integral equation formulation of the equilibrium state. As a consequence, the asymptotic behaviour of dislocation density at the edge of interval is modified from the conventional square root singularity. We propose an efficient and fast numerical method that is suitable for solving the resulting singular equations of the second kind based on the Gauss-Jacobi quadrature. The quadrature formulae involve fixed nodal points and provide exact results for polynomials up to degree 2n-1, where n is the number of nodes. A numerical example of the application of the Gauss-Jacobi rule to a slip band problem is provided as a demonstration of the validity of the method.     

Fabrication of Transparent Silicon Nitride from Nanosize Particles

Compaction of ultrafine silicon nitride (Si₃N₄) powder at high pressures and various temperatures followed by pressureless sintering was investigated. The powder, consisting of nearly spherical particles (16 nm in diameter) of amorphous stoichiometric Si₃N₄, was pressed in a diamond anvil cell under pressures up to 5 GPa and temperatures ranging from liquid nitrogen to 500°C. Quality of compaction, evaluated by visual transparency and hardness of the produced compacts, depended on the amount of adsorbed gases on the surface of the particles and on the temperature of compaction. Visually transparent compacts were produced by pressing the starting powder without outgassing in liquid nitrogen under 5 GPa. The transparent compacts exhibited a hardness of 1200 kg/mm² after pressing in the diamond anvil cell at 500°C for 3 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 1400°C in a tube furnace under nitrogen, the hardness of these samples increased to over 2000 kg/mm² and the visual transparency was maintained. The results demonstrated that transparency was maintained. The results demonstrated that transparent compacts of nanosize amorphous Si₃N₄ particles could be sintered to high hardness at relatively low temperatures without using sintering aids or applying pressure during sintering.     

Separation of cholesterol,and fatty acylglycerols,acids and amides by thin-layer chromatography

A rapid unidimensional thin-layer chromatographic (TLC) method for the separation of neutral lipids is described, using two sequential solvent systems of different polarity. Excellent separations of mono-, di- and triglycerides, fatty acids, fatty amides, and cholesterol are thereby achieved. Separation is accomplished at room temperature and requires 25 min. Reference to brand of firm name does not constitute endorsement by the United States Department of Agriculture over others of a similar nature not mentioned.     

Shape-factor effect on melting in an elliptic capsule

An approximate mathematical model of contact melting of an unfixed material in an elliptical capsule is developed. The main characteristic scales and non-dimensional parameters which describe the principal features of the melting process are found. Choosing a special heat flux distribution on the wall of the capsule allows us to derive a closed-form evolution equation for the motion of the solid accounting for the energy convection in the liquid, expressed through the non-linearity of the temperature distribution across the molten layer. It is shown that the melting rate of the solid depends on the shape of the capsule. Generally, elliptical capsules show higher rate of melting than circular ones. Elongated capsules provide more effective melting than oblate ones, even though they have the same aspect ratios and vertical cross-sectional areas. This phenomenon is caused by the fact, that the pressure necessary to support the solid is larger for the elongated capsules than that for oblate ones, which leads to thinning of the molten layer along with the increase of the heat flux across it. The time required for complete melting can be achieved by the right choice of the shape of the capsule, which is specified by the value of the aspect ratio. The found influence of the capsule shape on the melting rate can be used for design and optimization of practical latent-heat-thermal-energy systems.     

Effect of a Third Invariant on the Effective Reaction of Ductile Porous Bodies. Part 2. Loading Surface of Porous Bodies Whose Properties Are Sensitive to a Triaxial Stressed State

The lower and upper boundary approximation for describing the yield condition of porous bodies whose behavior is sensitive to a third strain rate invariant is obtained. Together with porosity the constitutive equations contain a material parameter that is responsible for the change in pore shape. This approach makes it possible to present the macroscopic yield condition in a compact form convenient for analysis and various applications. In particular, generalization of well-known plasticity conditions of Gurson and Green-Shima is presented. The lower estimate generalizes the Gurson model and the upper estimate corresponds to the Green-Shima model. Octahedral contours of the loading surface are constructed that agree with experimental data for loose materials and porous powder bodies.