Morphology of chemical vapor deposited titanium diboride

Titanium diboride has been chemical vapor deposited from TiCl₄, BCl₃, and H₃ gases on a heated graphite substrate. The morphologies of the deposits were correlated with the deposition parameters of substrate temperature, total pressure, B: Ti atomic ratio, Cl: H atomic ratio, and total flow rate. The diboride deposits exhibited a variety of morphologies which include coherent coatings with nodular or faceted surfaces, plate-like crystallites, dendritic crystallites, and irregularly shaped, faceted crystallites. The deposit morphology was found to be most sensitive to deposition temperature, B: Ti atomic ratio, and Cl: H atomic ratio. Variations in both the total pressure and total flow rate did not significantly affect the diboride morphology. All the TiB₂ deposits except those produced under a large excess of H₂ had a preferred orientation such that the hexagonal c-direction was parallel to the substrate surface.     

Electronic transport and localization in low mobility solids and liquids

This article is concerned with the electronic transport properties of the large group of solids (and also some liquids) which possess carrier mobilities from 10 cm²V^-1sec^-1 to many orders below that value. Its main aim is to trace the close relation between transport and localization. To this end the physical basis of three important localization models is discussed and the predicted transport mechanisms are reviewed in the light of the experimental information. After a brief summary of the experimental methods, the first subject is the localization arising from strong electron-phonon interaction. Here we consider in some detail the formation of a small polaron in a molecular crystal and its transport by phonon-assisted intermolecular hopping. The second part of the paper deals with the localization of an electron in a so-called electronic bubble, which occurs for example in simple liquids such as Ne or He. The stability and transport of this entity is discussed. The third section is devoted to non-crystalline solids. In these localization is associated with the lack of long-range order and does not depend on phonon interaction. The transport through the various parts of the density of states spectrum is summarized and the suggested models are reviewed on the basis of recent experimental work on amorphous silicon. Finally, the possibility of polaron formation and transport in some chalcogenide alloy glasses is briefly considered. The paper leads to the conclusion that available experimental evidence supports the general validity of the above three models, although it must be emphasized that this is based on an as yet limited range of experimental information.     

Octupole vibrational states in the even-mass barium isotopes

A low-lying state of ^{132, 134, 136, 138}Ba, strongly excited in the scattering of 20 MeV α-particles at 175°, is identified as the first 3^? octupole vibrational state in each case. Angular distribution measurements for α-particle scattering to this state and the well-known first 2⁺ state in ^{132, 134, 136, 138}Ba are in agreement with harmonic-vibration coupled-channel predictions. Deformation parameters are obtained, and the deduced isoscalar transition rates are compared with measured electromagnetic transition rates. The first 3^? states of ^{132, 134, 136, 138}Ba are found to be at excitation energies of 2.070, 2.251, 2.529 and 2.879 MeV, respectively. A state in ¹³⁰Ba at an excitation energy of 1.948 MeV is tentatively identified as the first octupole-vibrational state in this nucleus.     

The thermal decomposition of some primary explosives as studied by differential scanning calorimetry

Most currently used stab- and percussion-sensitive mixes consist of lead azide, lead styphnate or lead dinitroresorcinate in admixture with 2–5% tetrazene, which functions as an energetic sensitizer. A major problem with the use of such compositions is the poor thermal stability of tetrazene, depletion of which leads to loss of activity. The thermal decomposition of some 24 compounds, all identified as potential replacements for tetrazene in stab and percussion mixes, has been studied by non-isothermal differential scanning calorimetry, and the physicochemical parameters which characterise these explosive reactions have been determined and compared with data from temperature of ignition measurements.     

Quadrupole moments of the first excited states of 204,206Pb

The static quadrupole moments of the first excited states of ^204,206Pb are measured to be +0.23 ± 0.09 and +0.05 ± 0.09 eb, respectively. These results, together with measured B(E2; 0⁺ → 2⁺) values, are interpreted in terms of the systematics of nuclear deformation in the A ≈ 200 mass region.     

Measurement of regional cerebral glucose uptake by magnetic resonance spin-lock imaging

Purpose

The regional uptake of glucose in rat brain in vivo was measured at high resolution using spin-lock magnetic resonance imaging after infusion of the glucose analogue 2-deoxy-d-glucose (2DG). Previous studies of glucose metabolism have used ¹³C-labeled 2DG and NMR spectroscopy, ¹⁸F-labeled fluorodeoxyglucose (FDG) and PET, or chemical exchange saturation transfer (CEST) MRI, all of which have practical limitations. Our goal was to explore the ability of spin-lock sequences to detect specific chemically-exchanging species in vivo and to compare the effects of 2DG in brain tissue on CEST images.

Methods

Numerical simulations of R_1p and CEST contrasts for a variety of sample parameters were performed to evaluate the potential specificity of each method for detecting the exchange contributions of 2DG. Experimental measurements were made in tissue phantoms and in rat brain in vivo which demonstrated the ability of spin-lock sequences for detecting 2DG.

Results

R_1p contrast acquired with appropriate spin-lock sequences can isolate the contribution of exchanging protons in 2DG in vivo and appears to have better sensitivity and more specificity to 2DG–water exchange effects than CEST.

Conclusion

Spin-lock imaging provides a novel approach to the detection and measurement of glucose uptake in brain in vivo.     

Templated growth of calcium phosphate on tyrosine derived microtubules and their biocompatibility

Microtubular structures were self-assembled in aqueous media from a newly synthesized bolaamphiphile, bis(N-alpha-amido-tyrosyl-tyrosyl-tyrosine)-1,5-pentane dicarboxylate. In order to increase the biocompatibility of the microtubules, they were functionalized with the peptide sequence GRGDSP. Further, calcium phosphate nanocrystals were grown on the microtubules. In some cases, collagen was added in order to mimic the components of natural bone tissue. The biomaterials obtained were characterized via transmission electron microscopy (TEM), atomic force microscopy (AFM), IR, and energy dispersive X-ray spectroscopy (EDX) analyses. The biocompatibility of the calcium phosphate-coated microtubules was studied by conducting in vitro cell-attachment, cell-proliferation and cytotoxicity studies using mouse embryonic fibroblast (MEF) cells. The studies revealed that the biomaterials were found to be non-toxic and biocompatible. The functionalized tubular assemblies coated with calcium phosphate nanocrystals mimic the nanoscale composition of natural bone and may potentially support bone in-growth and osseointegration when used in orthopaedic or dental applications.     

Cross section measurements and thermonuclear reaction rates for 48Ca(p, γ)49Sc and 48Ca(p,n)48Sc

Absolute cross sections have been measured for the reaction ⁴⁸Ca(p, γ)⁴⁹Sc for 0.579 MeV ≦ E_{p, lab} ≦ 2.670 MeV and for the reaction ⁴⁸Ca(p, n)⁴⁸Sc for 0.956 MeV ≦ E_{p, lab} ≦ 2.670 MeV. Substantial competition effects in the cross section for ⁴⁸Ca(p, γ)⁴⁹Sc were observed at the thresholds for neutron emission to the 623 keV (3⁺), 1143 keV (2⁺) and 1402 keV (2^?) excited states of ⁴⁸Sc. Thermonuclear reaction rates were calculated from the measured cross sections for 0.1 ≦ T₉ ≦ 10.0. The new rates differ considerably from those used in earlier calculations of the production of the rare, neutron-rich intermediate mass nuclides during explosive carbon burning. In particular, the new rates may change the predicted abundances for ⁴⁸Ca, ^{49, 50}Ti and ⁵⁰V substantially. The good agreement between current global Hauser-Feshbach models and the experimental data indicates that Hauser-Feshbach calculations can provide sufficiently reliable rates for astrophysical calculations in cases where experimental data are non-existent.     

Mass spectrometric studies–VI: Aziridines

The mass spectra of C-alkylaziridines show that generally β-cleavage is the most important fragmentation, although α-cleavages also occur as do γ-, and δ- and ?-cleavages in appropriate examples. A surprising rearrangement of the β-cleavage ions has been explained as the result of a four-centre cyclization followed by fragmentation. Other decompositions occur via transannular cleavages with and without hydrogen transfer. α-Cleavage appears to be the most important fragmentation of N-alkylaziridines. N-Phenylaziridine shows very marked 1,2-phenyl-migration before fragmentation.