Pseudo-steady-state solutions for solidification in a wedge

Polubarinova-Kochina's analytical differential equation methodis used to determine the pseudo-steady-state solution to problemsinvolving the freezing (solidification) of wedges of liquidwhich are initially at their fusion temperature. In particular,we consider four distinct problems for wedges which are: freezingwith the same constant boundary temperature, freezing with thesame constant boundary heat fluxes, freezing with distinct constantboundary temperatures and freezing with distinct constant fluxesat the boundaries. For the last two problems, a Heun's differentialequation with an unknown singularity is derived, which in bothcases admits a particularly elegant simple solution for thespecial case when the wedge angle is . The moving boundariesobtained are shown pictorially.     

Increased intensities of YOYO-1-labeled DNA oligomers near silver particles

DNA detection is usually performed using fluorescence probes. Using a DNA oligomer stained with the widely used dye 1,1'-[1,3-propanediylbis[(dimethylimino)-3,1-propanediyl]]bis[4-[(3-methyl-2(3H)-benzoxazolylidene)methyl]]-quinolinum tetraiodide (YOYO-1), we show that a substrate containing silver particles can lead to a greater than 10-fold increase in the fluorescence intensity. Proximity to silver particles also increases the photostability of YOYO-1-DNA. These results suggest that substrates or gels containing silver particles may be used for increased sensitivity in DNA detection.     

Structural and magnetic properties of Zn1−xSbxCr2−x/3Se4 (x=0.11, 0.16 and 0.20) single crystals

Single crystals of Zn_1−xSb_xCr_2−x/3Se₄ based on the ZnCr₂Se₄ spinel, which is known to exhibit interesting magnetic and electronic transport properties, have been prepared by solid state reaction from the appropriate selenides. Three compounds of different Sb content (x=0.11, 0.16, and 0.20) were studied by X-ray diffraction, X-ray photoelectron scattering technique and macroscopic magnetic measurements with the aim to determine (i) stability of the cubic symmetry and (ii) influence of the Sb admixture on the magnetic properties. The results show that the Sb³⁺ and Zn²⁺ ions share the tetrahedral sites in the spinel structure, while the Cr³⁺ions carrying magnetic moments, are located in the octahedral sites. The X-ray photoelectron spectroscopy (XPS) data indicate that in this series of compounds the chromium ions have a 3d³ electronic configuration. The three samples studied order antiferromagnetically at low temperatures, with the magnetic characteristics being hardly altered with respect to those reported for the parent ZnCr₂Se₄ compound.     

Luminescent blooming of dendronic carbon nanotubes through ion-pairing interactions with an Eu(III) complex

A multiwalled carbon nanotube (MWCNT) scaffold was covalently functionalized with a second-generation polyamidoamine (PAMAM) dendron, presenting four terminal amino groups per grafted aryl moiety. These reactive functions were alkylated to obtain a positively charged polycationic dendron/carbon nanotube system (d-MWCNTs?Cl), which eventually underwent anion exchange reaction with a negatively charged and highly luminescent Eu(III) complex ([EuL(4)]?NEt(4), in which L = (2-naphtoyltrifluoroacetonate)). This process afforded the target material d-MWCNTs?[EuL(4)], in which MWCNTs are combined with red-emitting Eu(III) centers through electrostatic interactions with the dendronic branches. Characterization of the novel MWCNT materials was accomplished by means of TGA and TEM, whereas d-MWCNTs?Cl and d-MWCNTs? [EuL(4)] further underwent XPS, SEM and Raman analyses. These studies demonstrate the integrity of the luminescent [EuL(4)](-) center in the luminescent hybrid, the massive load of the cationic binding sites, and the virtually complete anion-exchange into the final hybrid material. The occurrence of the ion-pairing interaction with MWCNTs was unambiguously demonstrated through DOSY NMR diffusion studies. Photophysical investigations show that MWCNTs?[EuL(4)] is a highly soluble and brightly luminescent red hybrid material in which MWCNTs act as photochemically inert scaffolds with negligible UV/Vis absorption, compared with the grafted Eu complex, and with no quenching activity. The high dispersibility of MWCNTs?[EuL(4)] in a polymer matrix makes it a promising luminophore for applications in material science.     

Determination of chemical composition of siderite in concretions by wavelength-dispersive X-ray spectrometry following selective dissolution

Determination of chemical composition of siderite (Fe, Me)CO₃ (where Me = Mg, Ca, Mn) present in siderite concretion is developed. An accurate and precise determination of Mg, Ca, Mn and Fe in siderite required complete separation of this mineral from other materials, e.g. calcite, quartz. For this purpose, selective dissolution in acetic acid (HAc) was applied. HAc concentration from 0.1 to 1 mol L⁻¹ and extraction time from 0.5 to 8 h were investigated. In each step of investigation of selective dissolution, the X-ray diffraction measurements (XRD) of the residues was performed and also calcium (complexometric titration) and iron (XRF) in solution were determined. HAc of concentration 0.25 mol L⁻¹ and extraction time of 2 h was adopted for siderite separation because in these conditions the siderite was not dissolved and, simultaneously, calcite was completely dissolved. In the next step, the nondissolved sample was digested in hydrochloric acid. The solution of the separated siderite was pipetted onto membrane filter and Mg, Ca, Mn and Fe were determined by wavelength-dispersive X-ray fluorescence (WDXRF) spectrometry. The calibration was performed using 11 certified reference materials of iron ores. Matrix effects were corrected using empirical coefficient model for intermediate-thickness samples.     

The effects of doping ferromagnetic spinel CdCr2Se4 with Sb ions

Semiconducting spinel CdCr₂Se₄ orders ferromagnetically below T_C=130 K. A series of single-crystals of CdCr₂Se₄ doped with Sb³⁺ ions has been synthesized in order to study an effect of the substitution on the cation distribution and the magnetic properties. The compounds of Cd_ySb_xCr_zSe₄ have been investigated by means of X-ray diffraction, magnetization measurements and electron spin resonance spectroscopy. Two selected samples of the composition (Cd_1−xSb_x)[Cr₂]Se₄ with x=0.13 and 0.44 retained cubic symmetry with space group . The unit cell parameter appeared to be sensitive to the concentration of Sb³⁺ admixture: it increases with x, despite a close similarity in the ionic radii of Cd²⁺ and Sb³⁺ in tetrahedral coordination. Upon partial substitution of Cd²⁺ by Sb³ no obvious change in the Curie temperature was observed, however, the effective magnetic moment slightly increased, what may result from the appearance of Cr²⁺ ions. The characteristic feature of the system studied is an extended range of short-range magnetic order, in which the magnetic properties in the paramagnetic state are governed by the formation of ferromagnetic clusters, as indicated by both the bulk magnetometric and spectroscopic data.