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31.
The chemical versatility of 2,4(3H,5H)-furandione (β-tetronic acid, 1) and its synthetic applicability are of considerable current interest1b,2a-k. The multifunctional character of this small molecule confers an intriguing synthetic potential which has prompted us to develop synthetic schemes utilizing it as a synthon for certain heterocyclic systems. 相似文献
32.
Zimmer O Baumann K Fertl M Franke B Mironov S Plonka C Rich D Schmidt-Wellenburg P Wirth HF van den Brandt B 《Physical review letters》2007,99(10):104801
We report the first successful extraction of accumulated ultracold neutrons (UCN) from a converter of superfluid helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. Windowless UCN extraction is performed in vertical direction through a mechanical cold valve. This prototype of a versatile UCN source is comprised of a novel cryostat designed to keep the source portable and to allow for rapid cooldown. We measured time constants for UCN storage and extraction into a detector at room temperature, with the converter held at various temperatures between 0.7 and 1.3 K. The UCN production rate inferred from the count rate of extracted UCN is close to the theoretical expectation. 相似文献
33.
34.
Cem B. Yildiz Kinga I. Leszczyska Sandra Gonzlez‐Gallardo Michael Zimmer Akin Azizoglu Till Biskup Christopher W. M. Kay Volker Huch Henry S. Rzepa David Scheschkewitz 《Angewandte Chemie (International ed. in English)》2020,59(35):15087-15092
Main group analogues of cyclobutane‐1,3‐diyls are fascinating due to their unique reactivity and electronic properties. So far only heteronuclear examples have been isolated. Here we report the isolation and characterization of all‐silicon 1,3‐cyclobutanediyls as stable closed‐shell singlet species from the reversible reactions of cyclotrisilene c‐Si3Tip4 (Tip=2,4,6‐triisopropylphenyl) with the N‐heterocyclic silylenes c‐[(CR2CH2)(NtBu)2]Si: (R=H or methyl) with saturated backbones. At elevated temperatures, tetrasilacyclobutenes are obtained from these equilibrium mixtures. The corresponding reaction with the unsaturated N‐heterocyclic silylene c‐(CH)2(NtBu)2Si: proceeds directly to the corresponding tetrasilacyclobutene without detection of the assumed 1,3‐cyclobutanediyl intermediate. 相似文献
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38.
Using the tracer-standard sectioning technique the impurity diffusion of indium in copper has been investigated in the temperature range from 798.1 to 1081.0°C. For the frequency factor and the activation energy, respectively, the following values were determined: D02 = 1.87 cm2 · s−1; Q2 = 2.034 eV. The results are compared with predictions of theoretical models of the impurity diffusion in metals. 相似文献
39.
K. Zimmer R. Böhme B. Rauschenbach 《Applied Physics A: Materials Science & Processing》2007,86(3):409-414
Laser-induced backside etching of fused silica with gallium as highly absorbing liquid is demonstrated using pulsed infrared
laser radiation. The influences of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched
surface topography were studied and the results are compared with these of excimer laser etching. The high reflectivity of
the fused silica-gallium interface at IR wavelengths results in the measured high threshold fluences for etching of about
3 J/cm2 and 7 J/cm2 for 18 ns and 73 ns pulses, respectively. For both pulse lengths the etch rate rises almost linearly with laser fluence and
reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm2, respectively. The etching process is almost free from incubation processes because etching with the first laser pulse and
a constant etch rate were observed. The etched surfaces are well-defined with clear edges and a Gaussian-curved, smooth bottom.
A roughness of about 1.5 nm rms was measured by AFM at an etch depth of 0.95 μm.
The normalization of the etch rates with respect to the reflectivity and the pulse length results in similar etch rates and
threshold fluence for the different pulse widths and wavelengths. It is concluded that etching is a thermal process including
the laser heating, the materials melting, and the materials etching by mechanical forces. The backside etching of fused silica
with IR-Nd:YAG laser can be a promising approach for the industrial usage of the backside etching of a wide range of materials.
PACS 81.65.C; 81.05.J; 79.20.D; 61.80.B; 42.55.L 相似文献
40.
We show that a certain class of measures arising from generalized Riesz products is singular. In particular, cutting and stacking
(i.e. rank one) transformations whose cuts do not grow too rapidly, have singular maximal spectral type. The precise condition
is
, wherew
h is the number of cuts at stagen.
Supported by a Canada NSERC grant.
Supported by an NSF grant. 相似文献