Electric field-induced nucleation and growth of focal-conic and stripe domains in a smectic A liquid crystal

We have studied the electric field-induced first order transition from a homeotropic smectic A structure to a polydomain texture that occurs through the nucleation of toric focal-conic domains (TFCDs). The process involves two steps: first, nucleation of TFCDs of a size larger than a critical radius a*, and then a steady growth of TFCDs to a secondary critical radius a**, when surface anchoring effects become dominant and cause a transition from a circular TFCD to an elongated stripe domain (SD). Studies were performed for pure smectic A materials and for smectic A doped with kunipia nanoparticles. Non-destructive 3D imaging with fluorescence confocal polarizing microscopy showed that field-induced TFCDs can nucleate in the smectic A bulk. Clay particles reduce the energy barrier for nucleation as they distort the smectic A layers and thus increase the ground state energy. Simple elastic models of the TFCD and SD allow us to describe the qualitative features of the observed phenomena.     

Electrical and ionic conductivity effects on magic-angle spinning nuclear magnetic resonance parameters of CuI

We investigate experimentally and theoretically the effects of two different types of conductivity, electrical and ionic, upon magic-angle spinning NMR spectra. The experimental demonstration of these effects involves (63)Cu, (65)Cu, and (127)I variable temperature MAS-NMR experiments on samples of γ-CuI, a Cu(+)-ion conductor at elevated temperatures as well as a wide bandgap semiconductor. We extend previous observations that the chemical shifts depend very strongly upon the square of the spinning-speed as well as the particular sample studied and the magnetic field strength. By using the (207)Pb resonance of lead nitrate mixed with the γ-CuI as an internal chemical shift thermometer we show that frictional heating effects of the rotor do not account for the observations. Instead, we find that spinning bulk CuI, a p-type semiconductor due to Cu(+) vacancies in nonstoichiometric samples, in a magnetic field generates induced AC electric currents from the Lorentz force that can resistively heat the sample by over 200 °C. These induced currents oscillate along the rotor spinning axis at the spinning speed. Their associated heating effects are disrupted in samples containing inert filler material, indicating the existence of macroscopic current pathways between micron-sized crystallites. Accurate measurements of the temperature-dependence of the (63)Cu and (127)I chemical shifts in such diluted samples reveal that they are of similar magnitude (ca. 0.27 ppm/K) but opposite sign (being negative for (63)Cu), and appear to depend slightly upon the particular sample. This relationship is identical to the corresponding slopes of the chemical shifts versus square of the spinning speed, again consistent with sample heating as the source of the observed large shift changes. Higher drive-gas pressures are required to spin samples that have higher effective electrical conductivities, indicating the presence of a braking effect arising from the induced currents produced by rotating a conductor in a homogeneous magnetic field. We present a theoretical analysis and finite-element simulations that account for the magnitude and rapid time-scale of the resistive heating effects and the quadratic spinning speed dependence of the chemical shift observed experimentally. Known thermophysical properties are used as inputs to the model, the sole adjustable parameter being a scaling of the bulk thermal conductivity of CuI in order to account for the effective thermal conductivity of the rotating powdered sample. In addition to the dramatic consequences of electrical conductivity in the sample, ionic conductivity also influences the spectra. All three nuclei exhibit quadrupolar satellite transitions extending over several hundred kilohertz that reflect defects perturbing the cubic symmetry of the zincblende lattice. Broadening of these satellite transitions with increasing temperature arises from the onset of Cu(+) ion jumps to sites with different electric field gradients, a process that interferes with the formation of rotational echoes. This broadening has been quantitatively analyzed for the (63)Cu and (65)Cu nuclei using a simple model in the literature to yield an activation barrier of 0.64 eV (61.7 kJ/mole) for the Cu(+) ion jumping motion responsible for the ionic conductivity that agrees with earlier results based on (63)Cu NMR relaxation times of static samples.     

Distributions of conduction electrons as manifested in MAS NMR of gallium nitride

A strategy is demonstrated for identifying unambiguously and characterizing quantitatively the effects of distributions of conduction electron concentrations arising from intentional or unintentional dopants in semiconductors by magic-angle spinning (MAS) NMR. The 71Ga MAS NMR spectra of a number of chemically synthesized GaN samples with no intentional doping show inhomogeneously broadened absorptions to high frequency of the main peak. These broad signals are shown, from spin-lattice relaxation time measurements as a function of shift position in a single sample, to be due to Knight shifts arising from degenerate conduction electrons. For a GaN sample with Ge as an intentional dopant at the 0.13% (wt) level, the spectrum is dramatically broadened and shifted to high frequency by up to several hundred parts per million. Analysis of the inhomogeneously broadened line shape yields a quantitative probability density function for electron carrier concentration in the bulk sample that reflects significant compositional heterogeneity due to a variety of possible sources.     

The coulometric determination of salicylate in serum

Constant-current coulometry can be used for the determination of salicylate in 100 μg of blood serum. The titration is preceded by extraction of acidified serum with ethylene dichloride followed by removal of the salicylate to an aqueous solution of pH 7.5 tris(hydroxymethyl)aminomethane buffer which also contains copper(II) sulfate to enhance recovery. A residual titration with bromine as titrant followed by addition of potassium thiocyanate is used because of the slow bromination of salicylate. Serum samples containing salicylate in the range 4–60 mg/100 ml were analyzed. Recoveries of about 90% were obtained.     

The application of substoichiometric radioisotopic dilution principles to controlled-potential coulometry and solvent extraction

Radioisotope dilution principles are applied to controlled-potential electrolysis and solvent extraction with dithizone for the determination of trace amounts of cadmium. A method was developed to verify whether or not the substoichiometric principle was obeyed. If the substoichiometric principle was not obeyed, analysis was still possible by means of calibration curves. In order to obtain independent verification of the series controlled-potential method and to establish a means of comparison with the radioisotope dilution technique a current integration procedure was also employed. A microtechnique was used to extend the sensitivity of the solvent extraction system for cadmium. Standard zinc spelter and high-purity zinc were analyzed for cadmium after separation by a method of standard addition.     

Kinetic-coulometric determination of mercury in biological samples

A simple approximate calculation method is given which permits determina-tion of the maximal scan rates of potential allowable for distortion-free recording of current-voltage curves with X-Y recorders. For the calculations only the response time of the recorder, the wave shape and the scan rate of potential need be known. Stationary mercury electrodes and rapid polarography with a dropping mercury electrode at controlled drop times were examined. Electroanalytical implications are discussed, with particular emphasis on the rapid a.c. polarographic method with short controlled drop times and on stationary-electrode fundamental and second harmonic a.c. voltammetry. Theoretically and experimentally it has been shown that an X-Y recorder with 0.5–1.0-s response time can be used for scan rates up to about (50/nt') mV s^-1 with a.c. techniques and about (100/nt') mV s^-1 with d.c. polarography (t'= response time of recorder, n = number of electrons).