Search results for "Thermal"

Design of stable mixed-metal MIL-101(Cr/Fe) materials with enhanced catalytic activity for the Prins reaction

2020

[EN] This work highlights the benefit of designing mixed-metal (Cr/Fe) MOFs for enhanced chemical stability and catalytic activity. A robust and stable mixed-metal MIL-101(Cr/Fe) was prepared through a HF-free direct hydrothermal route with Fe(3+)content up to 21 wt%. The incorporation of Fe(3+)cations in the crystal structure was confirmed by(57)Fe Mossbauer spectrometry. The catalytic performance of the mixed metal MIL-101(Cr/Fe) was evaluated in the Prins reaction. MIL-101(Cr/Fe) exhibited a higher catalytic activity compared to MIL-101(Cr), improved chemical stability compared to MIL-101(Fe) and a higher catalytic activity for bulky substrates compared to MIL-100(Fe).In situinfra-red sp…

X‐irradiation induced photo‐ and thermostimulated luminescence of CsCdF 3 :Mn crystals

2005

Photo- and thermostimulated luminescence (PSL and TSL respectively) of previously X-irradiated CsCdF3 crystal doped with Mn were investigated. After X-irradiation of CsCdF3 crystal at 8 K PSL bands at about 300 nm and 550 nm appear. Several stimulation bands can be revealed for luminescence at 300 nm and 550 nm. The stimulation band at 340 nm is related to an F-type centre absorption band in accordance with the Mollwo-Ivey relation for halide crystals. Subsequent heating of the crystal after X-irradiation at 8 K shows TSL peaks in the temperature regions 8 K – 90 K and 200 K – 300 K. The spectral composition of the TSL involves both bands at 300 nm and 550 nm. Experiments performed allow us…

Tracks induced in TeO2 by heavy ions at low velocities

2000

On the basis of its thermal properties, TeO2 crystal was selected as an insulator with low threshold electronic stopping power for track formation Set. The crystals were irradiated by S, Zn, Mo, Kr, Te and Pb ions and the optical absorption and track formation were studied. Comparison is made with the published results on LiNbO3 ,Y 3Fe5O12 and SiO2 quartz. Good quantitative agreement is found with the predictions of the thermal spike model of Szenes with respect to Set and the variation of the track size with the electronic stopping power Se. It is shown that TeO2 has a high eAciency g at low ion velocities, which is a characteristic feature of the damage cross-section velocity eAect. ” 200…

Average Structure vs. Real Structure: Molecular Dynamics Studies of Silica

2003

The microscopic structure of a crystal and thermal fluctuations of the atoms constituting the crystal are intimately connected with the macroscopic elastic properties including mechanical stability. In some cases, however, the picture is more complex than that which is drawn in text books on solid state physics. (i) The instantaneous microscopic structure can deviate in a non-Gaussian way from the average structure even when domain disorder and/or crystal defects are absent. Quasi harmonic approximations may then turn out to be meaningless. (ii) The crystal is subject to external pressures that are sufficiently large in order to render the definition of elastic constants non unique. These t…

Über die intensitätsabhängigkeit der ultrarotbanden von der temperatur und vom kristallisationsgrad

1961

Beim Erhitzen von Polyathylenen und Paraffin geben die amorphen Banden bei 7,68μ und 7,38μ die Abnahme des Kristallisationsgrades richtig wieder, die kristalline Bande bei 13,7μ zeigt dagegen eine uber das partielle Schmelzen hinausgehende, durch Torsionsschwingungen der Ketten im Kristallgitter bedingte Intensitatsabnahme. Beim Terylen zeigen alle von uns untersuchten kristallinen und amorphen Banden eine verschiedene Abhangigkeit von der Temperaturbewegung, wobei einige amorphe Banden beim Erhitzen sogar verschwinden. Bei Messungen bei konstanter Temperatur zeigen alle Banden im Verlauf einer Kristallisation eine ubereinstimmende Veranderung mit zunehmendem Kristallisationsgrad. At the he…

Schmelz- und kristallisationserscheinungen bei makromolekularen substanzen. VI. Ergebnisse der röntgenklein- und weitwinkeluntersuchungen an polyglyk…

1962

Rontgenographische Untersuchungen ergaben, das die Kleinwinkelperiode im Verlauf der Nachkristallisation mit zunehmendem Kristallisationsgrad abnimmt. Ferner steigt nach dem Erhitzen einer bei 140°C auskristallisierten Probe auf eine hohere Temperatur die Kleinwinkelperiode zunachst stark an und nimmt wahrend der anschliesenden Neukristallisation wieder etwas ab. Die Deutung dieser experimentellen Ergebnisse wird diskutiert, wobei der Einflus des Bausteinfaktors berucksichtigt wird. Weitwinkeluntersuchungen zeigten, das bei der isothermen Kristallisation der Kristallisationsgrad zwar zunimmt, die Ordnung in Kettenrichtung aber nicht verbessert wird; diese hangt offenbar nur von der Kristall…

Thermal stability and crystallization kinetics of Al4Mn quasicrystals as studied by exoelectron emission and DTA methods

1995

Schmelz- und kristallisationserscheinungen bei makromolekularen substanzen. IV. Haupt- und nachkristallisation von terylen aus dem glaszustand

1960

Eine genaue Analyse der Kristallisationsisothermen zeigt, das innerhalb der bereits gewachsenen Spharolithe eine Nachkristallisation stattfindet. Unter der Annahme, das bei diesem Vorgang die die vollkommene Kristallisation hemmenden Hindernisse im Rahmen der Warmebewegung allmahlich uberwunden werden, wurde ein allgemeines Zeitgesetz fur die Nachkristallisation abgeleitet. Dabei wurden verschiedenartige Hindernisse und damit auch ein ganzes Spektrum von freien Aktivierungsenthalpien vorausgesetzt. In diesem Modell last sich die Abhangigkeit des erreichbaren Kristallisationsgrades von der Kristallisationstemperatur sowie der zeitliche Verlauf der Nachkristallisation verstehen. The exact ana…

Formation of complex defects in Mn c Mg 1–c O and Ni c Mg 1–c O single solid solution

2005

The results of investigation of the fast neutron irradiation and thermal treatment on absorption spectra of single solid solution of MncMg1–cO, NicMg1–cO and MgO crystals are presented. It is shown that at impurity ion concentration larger than 0.1 mass.% a probability of formation of aggregate centers consisting of more than three F+- or F-centers is small. After thermal treatment of MncMg1–cO and NicMg1–cO single solid solution irradiated by fast neutrons additional wide bands are observed. We assume that these additional bands belong to complex centers of Mn3+-VMg and Ni3+-VMg. The mechanisms of bands formation are discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystal chemistry and redox behaviour of antimony strontium calcium perovskites

2000

The compound Sr2Sb1.4Ca0.6O6 and their reduced forms Sr2Sb1.4Ca0.6O5.17 and Sr2Sb1.4Ca0.6O4.84 have been prepared and characterized by powder X-ray diffraction, electron diffraction, iodometric analyses and thermogravimetric analysis. The three phases with different oxygen stoichiometries are structurally related to the perovskite and show symmetry distortions from the ideal cubic structure (with cell parameter ap). The crystal structure of Sr2Sb1.4Ca0.6O6 may be refined by the Rietveld method from powder X-ray diffraction data using the space group P21/n, and the cell parameters a=5.776(2), b=5.7837(2), c=8.1718(3) A, β=90.039(3)° with the same structural model than for previously studied …