Search results for "Mechanical engineering"
showing 10 items of 4245 documents
XAFS studies of local atomic structure of niobium tellurides
1997
Abstract We present XAFS (K-edge) studies of the local atomic structure around the Nb and Te ions in niobium tellurides (NbTe 2 , Nb 3 Te 4 and NbTe 4 ) in comparison with ZrTe 2 . Using a multi-shell best-fit analysis procedure, we have reconstructed local environments of the Nb and Te ions and compared them with the existing structural models. We found that at the Nb K-edge the XAFS data of NbTe 4 are extremely sensitive to metal clustering and to the subsequent departure from the average crystallographic positions. Moreover, at the Te K-edge the XAFS data allowed us to distinguished the different type of TeTe bonds, for instance: the dimer (TeTe: 2.9 A) and the squares (TeTe: 3.3 A) i…
Nonlinear optical spectra of conjugated polymers: Effect of long-range coulomb interactions
1993
Abstract Nonlinear optical spectra of conjugated polymers are theoretically studied in the Su-Schrieffer-Heeger model supplemented by long-range Coulomb interactions. Excitonic correlation for electron-hole excitations in explicitly taken into account by a standard method. Nonlinear susceptibilities χ (3) are calculated numerically with a standard sum-over-states method for finite chains (up to 1000 sites). Using moderate interaction strength, our calculations can reproduce many distinct features observed in the linear and nonlinear spectra (two-photon absorption, third-harmonic generation, and electroabsorption) of polydiacetylenes and some other polymers. Especially, a hump in the spectru…
Extraction of the fluoride-, chloride- and bromide complexes of the elements Nb, Ta, Pa, and 105 into aliphatic amines
1998
Abstract Previous studies of the halide complex formation of element 105 in HCl–HF mixtures and extractions into triisooctyl amine (TIOA) have been performed with the Automated Rapid Chemistry Apparatus, ARCA II. Element 105 was shown to be absorbed on the column from 12 M HCl–0.02 M HF together with its lighter homologues Nb, Ta and the pseudohomologue Pa. In elutions with 10 M HCl–0.025 M HF, 4 M HCl–0.02 M HF, and 0.5 M HCl–0.01 M HF, the extraction sequence Ta>Nb>105>Pa was observed and element 105 behaved very differently from its closest homologue Ta. As it is not possible within reasonable effort to model the many presumably mixed fluoride–chloride complexes involved in these studies…
Inhibitoren der Korrosion (8) Über die Abhängigkeit der Korrosionsinhibierung des Eisens in Salzsäure von der Struktur quartärer Phosphoniumsalze
1971
Bei quartaren Phosphoniumsalzen be-steht zwischen der Inhibitorwirkung im sauren Medium unter Auschlus von Luft und den polarographisch ermittelten Re-duktionspotentialen keine direkte Bezie-hung. Die sterischen Faktoren der nach Aufnahme von zwei Elektronen entste-henden und an der Eisenoberflache fixierten Fragmente sind offenbar fur die Inhibitorwirkung hauptsachlich ver-antwortlich. Bisphoniumsalze mit gesattigten oder ungesattigten Brucken sind unter den oben genannten Bedingungen hervor-ragende Inhibitoren. Am wirksamsten sind Bisphosphoniumsalze mit Ketten von 4 bis 10 Methylengruppen zwischen den Phosphoniumzentren. Der starre p-Phenylenrest ist als Verknupfungs-prinzip weniger geei…
Kinetics of nitrogen removal in a MBR nutrient removal activated sludge system
2006
The application of membrane bioreactor (MBR) technology to biological nutrient removal (BNR) activated sludge (AS) systems is limited and uncertainty exists as to the impact of the conditions induced by the membranes on the biologically mediated processes of nutrient removal. Two main conditions associated with MBRs are (1) high total suspended solids (TSS) concentrations (8–20 g/L) and (2) different selection pressures due to biomass retention not being based on settleability, compared with conventional systems with secondary settling tanks (SSTs). Recently, Ramphao et al. [1] concluded that incorporating membranes in BNR AS makes a profound difference to the design of the system. Previous…
On the synthesis and characterization of layered antimony(III) phosphate and its interaction with moist ammonia and amines
1998
Samples of antimony(III) phosphate, a layered material, were prepared at room temperature, using the two allotropic forms of Sb2O3, layered valentinite and cubic senarmontite. The compounds were characterized by infrared spectroscopy, X-ray diffraction, and thermogravimetric–differential thermal analysis. Solids exposed to ammonia, ethylenediamine, or hydrazine vapors also were studied. The interaction between SbPO4 and the bases always led to the formation of stoichiometric amounts of Sb2O3 and hydrogen phosphate(–2) salts of the bases. With ammonia, the SbPO4 raw materials gave rise to valentinite or senarmontite, depending on which Sb2O3 allotropic form was used to prepare antimony phosp…
Anwendung der Mößbauer-Spektroskopie auf die Untersuchung von Deckschichten in Naturumlaufkesseln
1978
57Fe-Mosbauer-spektroskopische Transmissions- und Reflexionsmessungen wurden zur Aufklarung der qualitativen und quantitativen Zusammensetzung von Deckschichten auf der Innenseite von Rohren aus Naturumlaufkesseln vorgenommen. Die Deckschichten waren bei 120 bis 174 atu, 320 bis 353 °C, einem pH-Wert zwischen 9,5 und 10,5 und einem Gehalt an P2O5 und SiO2 von < 10 bzw. < 4 mg/l auf den Rohrinnenflachen (Werkstoff 15 Mo 3: 0,12%C, 0,15% Si, 0,5% Mn, 0,04% P, 0,04% S, ⩽ 0,3% Cr, 0,25% Mo, Rest Fe; St. 45. 8. III: < 0,22% C, 0,1% Si, ⩾ 0,45% Mn, 0,05% P, 0,05% S, ⩽ 0,3% Cr, Rest Fe) innerhalb von maximal 127000 h entstanden. Die Mosbauer-spektroskopischen Untersuchungen, unterstutzt durch eine…
Magnetic LB films based upon polyoxometalate clusters and single molecule nanomagnets
1999
Abstract By using the adsorption properties of polyoxometalates such as the ferromagnetic cluster [Co 4 (H 2 O) 2 (PW 9 O 34 ] 10− ) along a positively charged monolayer, we have prepared well organized monolayers of the magnetic polyanions. A similar procedure allowed us to obtain Langmuir-Blodgett films (LB films) based on Mn 12 clusters which show a marked hysteresis
Determination of the first ionization potential of einsteinium by resonance ionization mass spectroscopy (RIMS)
1998
Abstract The first ionization potential of einsteinium (IP Es ) was determined by resonance ionization mass spectroscopy (RIMS) using samples with ≤10 12 atoms of 254 Es ( T 1/2 =276 days). This method is based on the measurement of photoionization thresholds as a function of applied electric field strength, followed by extrapolation to zero field strength to yield IP Es . An atomic beam of Es was created by heating a filament on which einsteinium was electrodeposited from an aqueous solution onto a tantalum backing and covered with titanium metal. Es atoms were ionized via a three-step excitation scheme, and the ions mass-selectively detected in a time-of-flight (TOF) mass spectrometer. Th…
Morphological, electrical and optical properties of organic light-emitting diodes with a LiF/Al cathode and an Al-hydroxyquinoline/diamine junction
2004
Abstract We report the results of the morphological, electrical and optical characterisation of double-layer Alq3-based organic emitting diodes with a lithium fluoride (LiF)/Al cathode. A detailed electron microscopy investigation of their cross-section shows the presence of LiF isolated grains underneath the Al film. Due to the introduction of the LiF layer, luminance was larger than 30,000 cd/m2 at a bias voltage VB=25 V with a maximum external luminous efficiency as large as 46 lm/W at VB=20 V. Performing on/off VB cycles at a very low frequency, each time a recovery of the initial electric and luminous performance was observed. The non-exponential decay of both electric current and lumi…