Study of the growth and switching kinetics on ferroelectric nanocrystals of copolymer vinylidene fluoride and trifluoroethylene on an atomic force microscope

The growth of nanocrystals obtained from Langmuir-Blodgett films of ferroelectric copolymer consisting of 70% vinylidene fluoride and 30% trifluoroethylene has been investigated by atomic force microscopy (AFM). The radius and concentration of nanocrystals are found to depend on the annealing time of the film. A model for nanocrystal growth is proposed which yields adequate time dependences for nanocrystal size parameters. The switching kinetics of individual ferroelectric nanocrystals with an average diameter of 100–200 nm and a height of 15–20 nm has been investigated in the piezoelectric response mode. It is shown that the switching of nanocrystals has an activation character.     

In situ Neutron Powder Diffraction on Intermediate Hydrides of MgPd3 in a Novel Sapphire Gas Pressure Cell

A novel gas pressure cell for in situ neutron powder diffraction has been developed. It is based on a single crystal sapphire tube as a sample holder, allows a 360° unobstructed access by the neutron beam and has little background contribution. This device was used to study the hydrogenation of α‐MgPd₃, which undergoes a hydrogen driven rearrangement from a ZrAl₃ to a AuCu₃ type structure. Deuterium could be located and a strong preference of [Pd₆] voids was found in α‐MgPd₃D_0.79 under 5 bar and in α‐MgPd₃D_0.94 under 20 bar deuterium pressure. The crystal structure may be described as a new defect superstructure variant of the NaCl type. In situ thermal analysis under 5 bar hydrogen pressure showed that both the hydrogen uptake of α‐MgPd₃, which is complete at temperatures below 450 K, and the transformation to the hydride of cubic β‐MgPd₃, starting around 550 K, are exothermic. This completion of the hydrogenation‐dehydrogenation series of MgPd₃ suggests, that the rearrangement of the metal structure proceeds by a hydrogen assisted gliding mechanism with a shift vector of [110]. This is also supported by quantum chemical calculations, which show a decohesion of the intermetallic structure upon hydrogenation accompanied by the appearance of Pd–H bonding interactions.     

Thinking wants to be organized

Can the describable complexity of test problems concerning mathematical thinking and the empirical results of their dealing with be put into a relation? Can graded test problems be constructed which lead to results which can basically be predicted? Empirical studies give interesting and helpful answers which lead to didactically important consequences, just like the evaluation of the PISA results.     

Un test d'adéquation global pour la fonction de répartition conditionnelle

We propose a global test of goodness-of-fit to assess the validity of an entertained statistical model by testing simultaneously all the assumptions made about it. This test is based on a local polynomial estimator of the conditional distribution function and on the standard paradigm relating the distance between the nonparametric estimator and the theoretical parametric model. We derive the asymptotic distribution of the resulting test statistic under both the null hypothesis and local alternatives. To cite this article: S. Ferrigno, G.R. Ducharme, C. R. Acad. Sci. Paris, Ser. I 341 (2005).     

Simulation einer Fabrik für synthetischen Gummi — eine Fallstudie

Zusammenfassung Die Fallstudie behandelt das Problem der Dimensionierung der verschiedenen Teile einer sehr komplexen Anlage für mehrstufige Produktion. Die Programmierung erfolgte in FORTRAN. Anhand der Fallstudie werden einige wesentliche Punkte herausgearbeitet, die für den Erfolg von Simulationsprojekten wesentlich sind, nämlich: klare Zielsetzung, Simulation von Teilbereichen, so daß in kürzester Zeit Resultate vorliegen.

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Summary The case-study deals with the problem of the size of various parts of a very complex plant lay-out for multi-step production. The programming was done in FORTAN. Based on the case-study a few points were worked out which are essential for the success of simulation-projects, to wit: clear statement of purpose; simulation of partial scopes, which produce short-term results.

     

Intrinsic ferroelectric coercive field

The Landau-Ginzburg theory of ferroelectricity predicts the intrinsic coercive field for polarization reversal, but the observed extrinsic coercive field is always much smaller as a result of nucleation, dynamic processes not covered by the static theory. We have realized the intrinsic coercive field for the first time, in two-dimensional Langmuir-Blodgett polymer films as thin as 1 nm. The measured coercive field is in good agreement with the theoretical intrinsic value, exhibits the expected dependence on temperature, and does not depend on thickness below 15 nm.     

Bidirectional asymmetric allylboration. A convenient asymmetric synthesis of C(2)-symmetric 3-methylenepentane-1,5-diols and rapid access to C(2)-symmetric spiroketals

The double allylboration of aldehydes using 1, 3-bis(diisopinocampheylboryl)-2-methylenepropanes (R,R)-3 and (S, S)-3 under Brown's salt-free conditions provides C(2)-symmetric 3-methylenepentane-1,5-diols 1 in excellent enantiomeric excess. The absolute stereochemistry of the products was confirmed by a single-crystal X-ray study of bis-Mosher ester 6g. Desymmetrization and further functionalization of diol 1a were achieved by treatment of the bis-BOC carbonate 13 with IBr in toluene at -80 degrees C to give cyclic iodocarbonate 14 as a single diastereomer. This methodology is also applicable in natural product synthesis; enantiomerically pure spiroketals 1,7-dioxaspiro[5.5]undecanes 18 and 25, the latter representing an expedient synthesis of the AB ring system of the spongistatins 20, were easily accessed from simple starting materials in excellent yields and selectivities.     

Synthesis and Crystal Structure of Tellurium(II) bis(2‐methoxycarbonylethanethiolate): A Novel Coordination Mode of Tellurium

The tellurium(II) dithiolates Te[SCH₂CH₂C(O)OCH₃]₂, ( 1 ), Te[SCH₂CH₂CH₂SC(O)CH₃]₂, ( 2 ), and Te[SCH₂CH₂CH₂CH₂SC(O)CH₃]₂, ( 3 ) were synthesized from Te(S^tBu)₂ and the corresponding thiol. All compounds are sensitive toward higher temperatures and light and decompose to elemental tellurium and the disulfide. In the solid state, the Te atom of 1 exhibits the novel Te(S₂Te₂) coordination mode. Additionally to the two Te—S bonds, each Te atom forms two long Te···Te contacts to neighboring molecules, leading to a coordination number of four and a distorted sawhorse configuration. No intramolecular Te···O interactions are present in the solid state, in accordance with ab initio calculations (MP2/ecp‐basis) for the isolated molecule. ¹²⁵Te NMR shifts of all compounds lay within a narrow range and close to the respective shift of other Te(SCH₂R)₂ compounds. VT ¹²⁵Te NMR spectra gave no hint to donor acceptor interactions in solution for any of the compounds and thus corroborate results from IR‐spectroscopy, ab initio geometry optimizations, and thermochemical calculations.     

SAMBADENA Hyperpolarization of 13C-Succinate in an MRI: Singlet-Triplet Mixing Causes Polarization Loss

The signal enhancement provided by the hyperpolarization of nuclear spins of biological molecules is a highly promising technique for diagnostic imaging. To date, most ¹³C-contrast agents had to be polarized in an extra, complex or cost intensive polarizer. Recently, the in situ hyperpolarization of a ¹³C contrast agent to >20 % was demonstrated without a polarizer but within the bore of an MRI system. This approach addresses some of the challenges of MRI with hyperpolarized tracers, i. e. elevated cost, long production times, and loss of polarization during transfer to the detection site. Here, we demonstrate the first hyperpolarization of a biomolecule in aqueous solution in the bore of an MRI at field strength of 7 T within seconds. The ¹³C nucleus of 1-¹³C, 2,3-²H₂-succinate was polarized to 11 % corresponding to a signal enhancement of approximately 18.000. Interesting effects during the process of the hydrogenation reaction which lead to a significant loss of polarization have been observed.