Properties of compounds of type NaxSi46 and NaxSi136

A qualitative account is given of the electrical and magnetic properties of compounds of the type Na_xSi₁₃₆. It is suggested that a metal-insulator transition takes place with decreasing x. It is conjectured that in sintered powders the resistance is due to amorphous contacts between grains, leading to a resistivity proportional to $\text{A}\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$.     

Design and evaluation of a Dean vortex-based micromixer

A mixer, based on the Dean vortex, is fabricated and tested in an on-chip format. When fluid is directed around a curve under pressure driven flow, the high velocity streams in the center of the channel experience a greater centripetal force and so are deflected outward. This creates a pair of counter-rotating vortices moving fluid toward the inner wall at the top and bottom of the channel and toward the outer wall in the center. For the geometries studied, the vortices were first seen at Reynolds numbers between 1 and 10 and became stronger as the flow velocity is increased. Vortex formation was monitored in channels with depth/width ratios of 0.5, 1.0, and 2.0. The lowest aspect ratio strongly suppressed vortex formation. Increasing the aspect ratio above 1 appeared to provide improved mixing. This design has the advantages of easy fabrication and low surface area.     

The metal-insulator transition in extrinsic semiconductors

The main purpose of this article is to describe the electrical and magnetic properties of extrinsic semiconductors when the concentration of donors varies, particularly for concentrations near that for which a metal-insulator transition takes place. Since the donor centres are distributed at random in space, the combined effects of correlation and Anderson localization have to be considered. As an introduction, in § 1 we give an outline of our present understanding of the transition in some crystalline materials, particularly V₂O₃. For this the metallic phase, a highly correlated electron gas, is discussed, as is also the question of a discontinuous change in the number of current carriers. In § 2 we discuss Si : P and similar materials. For uncompensated samples, the material near the transition is to be described by overlapping Hubbard bands, and the metal-insulator transition occurs when the states at the Fermi energy show Anderson localization. Just on the insulator side of the transition conduction is by variable-range hopping for uncompensated as for compensated materials, and a variation of log (resistivity) as T ^?1/4 is observed. The existence or otherwise of moments in the metallic phase is discussed, as is also the magnetoresistance, Hall effect and Knight shift. In § 3 the transition is described for narrow-band semiconductors, particularly La_1?x Sr _x VO₃. In § 4 some compounds of rare earths are treated, including Sm_1?x Nd _x Se.     

Role of base in the formation of silver nanoparticles synthesized using sodium acrylate as a dual reducing and encapsulating agent

The formation mechanism of Ag nanoparticles (NPs) synthesized with a wet-chemical reduction method using sodium acrylate as a dual reducing and capping agent was investigated with various analytical techniques. The time course of the state of the reaction solution was investigated using UV-vis and XAFS spectroscopies which showed that the NP formation rate increased with increasing concentration of sodium hydroxide (NaOH). The detailed kinetic analyses reveal that both the reduction rate of Ag ions and the nucleation rate of Ag NPs are dramatically increased with increasing NaOH concentration. XANES analyses imply that another reaction pathway via alternative Ag(+) species, such as Ag(OH)(x), was developed in the presence of NaOH. Consequently, NaOH is found to play an important role not only in creating specific intermediates in the reduction of Ag(+) to Ag(0), but also in accelerating the reduction and nucleation rates by enhancing the oxidation of sodium acrylate, thereby increasing the rate of formation of the Ag NPs.     

The bipolaron theory of high-temperature superconductors

An outline is given for the bipolaron theory of high-temperature superconductivity, both for the cubic bismuth oxide and the copper oxide materials. It is emphasized that the BCS theory takes this form for strong interaction. The possible role of spin polarons in the latter is also discussed.     

Strong S-domains

S-domains and strong S-rings are studied extensively with special emphasis on integral and polynomial ring extensions. The main theorem of this paper is that for a Prüfer domain R, the polynomial ring R[X₁,…X_n] in finitely many indeterminates is a strong S-domain. We also prove that any Prüfer υ-multiplication domain is an S-domain.