NMR characterization of injection-moulded alumina green compacts

Injection-moulded alumina green compacts containing polypropylene, wax, and stearic acid were studied for binder distribution by proton nuclear magnetic resonance (¹H NMR) imaging. The solid imaging technique of nuclear spin-spin relaxation time,T ₂, weighted imaging at 400 MHz was used. This imaging technique utilizes a multiple pulse sequence ofD ₀-(/2)_±x --D ₁₀-D ₇-() _x -D ₇-D ₁₀-AQ-D ₀ for echo detection and phase encoding. Two-and three-dimensional images were constructed from the intensities of these nuclear echo signals. Spatially resolved two-dimensional images obtained by the application of this technique indicated that the green compacts fabricated from the same nominal binder composition did not contain the same amounts of binder. This observation agrees well with our previous conclusion drawn from nuclear spin echo studies by Hahn's pulse sequence. A 64×64×64 three-dimensional imaging revealed that the inhomogeneity of binder distribution and internal imperfection do exist at certain locations of the samples. A binder-rich folding line was also detected in one of these green compacts.     

Nuclear magnetic properties of aluminium

We have demagnetized a 2.92 mole Al sample of 6 N purity and measured its nuclear specific heat at temperatures 21 μK≤T≤20 mK and in magnetic fields of 11.4 mT≤B≤69.0 mT. In addition, we have performed nuclear magnetic resonance experiments on it atT≤175 μK as well as on an Al powder and an AuAl₂ sample at 200 μK≤T≤22 mK. No anomalies could be detected in these temperature and field regions. This shows that Al is a good candidate for a nuclear demagnetization stage and that its nuclear specific heat can be used as a primary thermometer in the μK- and mK-temperature regimes. The NMR data indicate that the nuclear dipole-dipole interaction is dominant in Al. The absence of any anomalies in the spin lattice relaxation time τ₁ toT=34 μK indicates that a measurement of τ₁ also can be used for primary thermometry.     

Electron Spin Coherence of Phosphorus Donors in Isotopically Purified 29Si

We investigate spin coherence time of electrons bound to phosphorus donors in silicon single crystals, employing a pulsed electron paramagnetic resonance technique. The samples were isotopically controlled so that they may possess different concentrations (about 5% and 100%) of ²⁹Si, which is the only non-zero-spin (spin-1/2) stable isotope of Si. Both ²⁹Si-concentration dependence and orientation dependence of the electron spin coherence time demonstrate that the decoherence is caused by spectral diffusion due to mutual flip-flops of the environmental nuclear spins. The detail analysis of spin echo decay curves enables the unique assignment of the host sites responsible for electron spin echo envelope modulation.     

Electron Spin Coherence of Phosphorus Donors in Isotopically Purified <Superscript>29</Superscript>Si

We investigate spin coherence time of electrons bound to phosphorus donors in silicon single crystals, employing a pulsed electron paramagnetic resonance technique. The samples were isotopically controlled so that they may possess different concentrations (about 5% and 100%) of ²⁹Si, which is the only non-zero-spin (spin-1/2) stable isotope of Si. Both ²⁹Si-concentration dependence and orientation dependence of the electron spin coherence time demonstrate that the decoherence is caused by spectral diffusion due to mutual flip-flops of the environmental nuclear spins. The detail analysis of spin echo decay curves enables the unique assignment of the host sites responsible for electron spin echo envelope modulation.     

Determination of the uniformity of the carrier lifetime in a material from the profile of the amplitude spectrum of an ion detector

An analysis is made of charge transport in the neutral base of a p ⁺-n structure as a result of its diffusion to the boundary of the p-n junction under conditions where nonequilibrium carriers are generated by single α-particles. It is assumed that nonuniformity of the carrier lifetime (τ), described by a Gaussian distribution, exists over the area of the structure. The profile of the transported charge spectrum is calculated for these conditions and its correlation with the measure of nonuniformity τ is obtained. Since the tracks of the diffusing α-particles occupy an extremely small volume, recording them is equivalent to local probing of the material for τ. It is suggested that the calculated function should be used as a calibration function to determine the spread of τ values in materials. The method is tested on Si for a surface-barrier structure by recording 8.78 MeV π-particles. Pis’ma Zh. Tekh. Fiz. 24, 44–50 (March 12, 1998)     

NMR Evidence for C<Subscript>60</Subscript> Configurational Fluctuations Around Na Sites in Na<Subscript>2</Subscript>CsC<Subscript>60</Subscript>

The ²³Na nuclear magnetic resonance (NMR) spectrum, spin-lattice and spin–spin relaxation, as well as spin echo double resonance (SEDOR) are investigated in the ternary alkali fulleride compound Na₂CsC₆₀ in the temperature range of 10–300 K. The NMR line associated with the tetrahedral sodium site is split below 170 K (T and T′ lines) similarly to Rb₃C₆₀ although the crystal structures of these two materials are different. SEDOR measurements prove that the T and T′ sites are microscopically close. The merger of the two lines at about 170 K is attributed to motional narrowing resulting from a site exchange due to angular reorientations of the C₆₀ molecules. The exchange dynamics inferred from the spectra, spin–spin relaxation, and spin-lattice relaxation are all consistent and agree with inelastic neutron scattering, supporting our proposal that the observed T-T′ splitting originates from different local fullerene configurations around the tetrahedral alkaline sites.     

Formation of a fine-grained structure of iron-nickel alloys by reversible martensite transformation

We have investigated the influence of multiple γ-α-γ transformations proceeding by the shear mechanism on the structural-phase state of austenite in iron-nickel alloys N28T2Yu2 and N32. It has been shown that it is possible to form, by cyclic γ-α-γ transformations, a fine-grained structure of austenite due to the fragmentation of the initial structure that develops under the conditions of progressing disorientation of the crystal lattice of the γ-phase. In the process of thermocycling, the reversed austenite was continuously strengthened under the condition of the development of disorientation of its lattice. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 3, pp. 602–606, May–June, 2008.     

Design and Synthesis of a Novel Organic Triradical as a Model Compound for Generalized Ferrimagnets

We report magnetic properties of novel nitronyl nitroxide triradicals, ptriNN (1) and m-triNN (2), in which a π-conjugated biradical with a singlet ground state and a doublet monoradical are united byσ-bonds of an ether bridge. Two kinds of intramolecular exchange interactions, one between the biradical and monoradical entities J(σ) through theσ-bonds and the other within the biradical moiety J(π) through the π-conjugation, were examined by paramagnetic susceptibility χm for the sample dispersed in an organic polymer film. The magnitude of J(σ) was found to be much weaker than that of J(π). This result indicates that the triradicals 1 and 2 can be used as a building block for generalized ferrimagnets, which from our theoretical calculations have been predicted to exhibit an exotic spin alignment.     

Microstructural characterization of magnesias derived from different sources and their influence on the structure of ceramic films formed on a 3% silicon steel surface

A study on the influence of the type of magnesium oxide on the structure of the ceramic coating and consequently on the magnetic properties of a 3% Si steel was carried out. Decarburized samples coming from industrial equipment were coated with magnesia slurries and submitted to final annealing at 1200°C for 15 hours under H₂ atmosphere. We used magnesia synthesized from magnesium chloride and dolomite. The magnesia samples were characterized using granulometry measurements, scanning electron microscopy (SEM), nitrogen adsorption-desorption analyses, helium pycnometry, thermal analyses (TG and DTA), reactivity tests and calcination mass loss. The magnesia sintered from brine has the highest density, surface area and porosity. The rheological behavior of magnesia slurries, the magnetic properties of the steel and the ceramic film structure were correlated with the type of magnesia used. The slurry made with MgO from dolomite has the highest viscosity and the slurry made with oxide from brine the smallest one. Brine magnesia films are darker, thicker and rougher and sea water magnesia films are light gray, thinner and smoother.     

Multifunctional FeCo-graphitic carbon nanocrystals for combined imaging, drug delivery and tumor-specific photothermal therapy in mice

Ultrasmall FeCo-graphitic carbon shell nanocrystals (FeCo/GC) are promising multifunctional materials capable of highly efficient drug delivery in vitro and magnetic resonance imaging in vivo. In this work, we demonstrate the use of FeCo/GC for highly effective cancer therapy through combined drug delivery, tumor-selective near-infrared photothermal therapy, and cancer imaging of a 4T1 syngeneic breast cancer model. The graphitic carbon shell of the ∼4 nm FeCo/GC readily loads doxorubicin (DOX) via π-π stacking and absorbs near-infrared light giving photothermal heating. When used for cancer treatment, intravenously administrated FeCo/GC-DOX led to complete tumor regression in 45% of mice when combined with 20 min of near-infrared laser irradiation selectively heating the tumor to 43–45 °C. In addition, the use of FeCo/GC-DOX results in reduced systemic toxicity compared with free DOX and appears to be safe in mice monitored for over 1 yr. FeCo/GC-DOX is shown to be a highly integrated nanoparticle system for synergistic cancer therapy leading to tumor regression of a highly aggressive tumor model.      

The effect of a pulsed magnetic field on the nuclear spin echo signal in ferrite

A pulsed magnetic field produces shifts of domain walls in a magnetically ordered material (ferrite) and thus affects the nuclear spin echo signal in this material. The suppression and restoration of a response signal have been observed using exciting sequences with pulses of different polarities. The established dependence of the spin echo suppression coefficient on the external constant magnetic field confirms the hypothesis that the observed phenomena are related to the domain walls.     

Switching and hysteresis on the I–V curves of submicron BSCCO (2212) bridges

In thin BSCCO (2212) whisker-based superconducting bridges with width and length ≤1 μm we observe random telegraph noise at discrete values of current. We show that the noise is associated with spontaneous processes of addition and removal of discrete vortex trains and reflects the regular structure of steps on the I–V curves. The average lifetime of the trains τ falls down with increasing temperature T in a complex way: intervals of steep drop (∼an order of magnitude per Kelvin) are separated with a plateau of τ(T) with τ≈10⁻² s. The 70 GHz irradiation with 100 μW power results in the growth of the average switching frequency by 5 orders of magnitude; thus the HTSC bridges could be efficient detectors of microwave radiation. We discuss the peculiar features of τ(T).     

Studying electric field profiles in GaAs-based detector structures by Kelvin probe force microscopy

The method of scanning Kelvin probe force microscopy has been used to study the electric field distribution in GaAs-based p ⁺-π-n-n ⁺ detector structures. In the active layer volume, two maxima in the field strength profiles have been found, which are localized in the regions of p ⁺-π and π-n junctions. A volt-age drop on the π-n junction expands the region of collection of nonequilibrium holes, thus increasing the charge collection efficiency for the absorption of γ photons with an energy of 59.5 keV.     

Preparation of coal slurry with 2-propanol

This study explored the feasibility of using waste organic solvents as substitutes for water to prepare coal slurries. The rheological properties of coal-2-propanol slurries were examined and compared with that of coal-water slurry (CWS). The good compatibility between coal particles and 2-propanol resulted in stable particle suspension in slurry which usually exhibit Newtonian behaviour. However, coal-2-propanol slurries usually shown higher viscosities comparing to CWSs at a fixed solid loading due to swelling of coal by 2-propanol. In addition, coal-2-propanol slurries demonstrated lower settling rates (higher stability) compared to CWSs presumably due to good compatibility between coal particles and 2-propanol. Finally, coal-2-propanol slurries formed bulky sediment with loose structure even coal particles suspended in 2-propanol were more stable than coal particles in CWSs.     

The quinacridones: structure and colour: a study by powder diffraction

The known structure of 21 industrial gamma-quinacridone pigments served as a basis for the measurement of powder patterns with and without fluorophlogopite by a Bragg–Brentano diffractometer. Calibration and cell refinement were used to determine the cell parameters and to fit the profiles of the intensities. The unit cells vary systematically and the importance of the short b-axis is revealed. Averaging of all data leads to idealized unit cells. The corresponding criss-cross angles, τc, of the 21 pigments vary in a systematic way. It is possible to determine the relative frequency of the positions of the molecules inside the unit cell. The criss-cross angle of gamma-quinacridone increases from 2×26.0° (hydrogen-bond: 0.266–0.268 nm) to 2×28.0° (hydrogen-bond: 0.268–0.274 nm). The pigments are understood and described as physical mixtures of crystallographically similar structures. The colour of these pigments is one of the most important industrial properties. It varies from reddish purple to yellowish purple apparently depending on the structure. The difference between the gamma- and gamma′-form is described. Alpha-quinacridone is a special form of gamma-quinacridone and fits perfectly in the system of the latter. Evaluating the criss-cross angle of all known quinacridones, it is possible to classify their structure and colour, which are attributed to the cooperative effect of hydrogen-bonds and double bonds in the lattice (π–σ-correlation). This revised version was published online in November 2006 with corrections to the Cover Date.     

3He Anomalously High Spin Diffusion in Solid Para-Hydrogen

NMR measurements of ³He spin diffusion coefficient in solid para-H₂ are carried out at the temperatures 0.45–1.5 K. The crystals have been grown under constant pressure 20–130 bar. The ³He concentrations in the initial para-H₂–³He gas mixtures were 0.1% and 0.3%. It is found out that the decay of echo amplitude vs both magnetic field gradient G and time interval τ between RF pulses is of non-exponential character, typical of one-dimensional diffusion in restricted geometry. The values of true spin-diffusion coefficient D _S measured are found to be ∼10⁻⁴ cm²/s at 20 bar. At 108 bar D _S value is one order of magnitude less. D _S does not depend on temperature. Such spin diffusion coefficient values seem to be anomalously high in comparison with well-known values of D _S =10⁻⁵ cm²/s for bulk liquid ³He at 27 bar and D _S =10⁻⁸ cm²/s for bulk solid ³He at 108 bar. The special experiments with the crystal annealing make it clear that the high spin diffusion here is connected with fast diffusion along dislocation lines.     

NMR Spin-Lattice Relaxation Rates in Cuprates

An analysis of nuclear spin-lattice relaxation data in the normal state of cuprates that appropriately accounts for the highly anisotropic structures shows no contrasting temperature dependence of the Cu, O, and Y relaxations, which suggests that all nuclei relax by the same mechanism of the spin liquid. To investigate the temperature dependence of this mechanism, the model of fluctuating fields is used in which the rates are expressed in terms of hyperfine interaction energies and an effective correlation time τ _eff characterizing the dynamics of the spin fluid. The former contain the effects of the antiferromagnetic static spin correlations, which cause the hyperfine field constants to be added more coherently at low temperature but incoherently at high temperature. At low temperatures, τ _eff grows linearly with temperature as in ordinary metals. At high temperatures, however, the nuclear spin-lattice relaxation rates in various cuprates unequivocally reflect local moment features. This behavior is similar to that observed for the magnetic transition metals Fe, Co, and Ni, where also some properties show a cross-over from an itinerant behavior of delocalized electrons at low to that of localized moments at high temperatures.     

Development of single frame X-ray framing camera for pulsed plasma experiments

A single-frame X-ray framing camera has been set up for fast imaging of X-ray emissions from pulsed plasma sources. It consists of two parts, viz. an X-ray pin-hole camera using an open-ended microchannel plate (MCP) detector coupled to a CCD camera, and a high voltage short duration gate pulse for the MCP. The camera uses a 10-Μm pin-hole aperture for imaging on the MCP detector with a magnification of 6 X. The high voltage pulser circuit generates a pulse of variable duration from 5 to 30 ns (at 70% of peak amplitude) with variable amplitude from 800 V to 1.25 kV, and is triggered through a laser pulse synchronized with the event to be recorded. The performance of the system has been checked by recording X-ray emission from a laser-produced copper plasma. A reduction factor of ∼ 6.5 is seen in the dark current contribution as the MCP gate pulse is decreased from 250Μs to 5 ns duration.     

An oilwell cement slurry additivated with bisphenol diglycidil ether/isophoronediamine—Kinetic analysis and multivariate modelings at slurry/HCl interfaces

Loss of zonal isolation in oilwell cementing operations leads to safety and environmental problems. The use of new cement slurries can help to solve this problem. In this paper, an epoxy-modified cement slurry was synthesized and characterized. The features of the modified slurries were evaluated in relation to a standard cement slurry (w/c = 0.50). A kinetic study of HCl interaction with the slurries was carried out using cubic molds. The Avrami kinetic model appears to be the most efficient in describing kinetic isotherms obtained from 25 to 55 °C. Type of slurry, HCl concentration and temperature effects were also evaluated in HCl adsorption onto cement slurries considering a 2³ full factorial design. From the statistical analysis, it is inferred that the factor “HCl concentration” has shown a profound influence on the numerical values of the Avrami kinetic constants. However, the best statistical fits were found using binary and tertiary interactive effects. It was found that the epoxy-modified cement slurry presents a good potential to be used in environmental-friendly oilwell operations.     

Correlation between the morphology and photo-physical properties of P3HT:fullerene blends

The photo-induced charge transfer and optoelectronic efficiency of the solar cells correlated to the morphology and the structure of P3HT:C₆₀ blend was studied by means of photoluminescence and electron spin resonance (ESR). The occurrence of photo-induced charge transfer, well-known for blends of P3HT with fullerenes, was evidenced in blends of P3HT:C₆₀ (1:1 wt ratio) by a strong partially quenching of the P3HT luminescence. The ESR measurements allowed one to quantify the charge transfer between P3HT and C₆₀, which resulted in positive P3HT polarons. Wide-angle X-ray scattering (WAXS) and UV–vis spectroscopy showed that an inclusion of a C₆₀ fullerene in the P3HT matrix lead to lower peak intensities and dark Debye rings and a blue shift on the π–π* interband transition of the P3HT as well as a reduction in the absorption coefficient. Selected area electron diffraction patterns of a well-ordered sample of P3HT exhibit distinct diffraction rings indicating that the P3HT forms a polycrystalline film. The large-scale phase separation of P3HT and C₆₀ resulted from large C₆₀ agglomerations during spin coating lead to a low power conversion efficiency of 0.2 × 10⁻⁴%.