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  • 1
    ISSN: 1662-8985
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Compositionally graded Ba(1-x)SrxTiO3 thick films were fabricated by the airflow deposition technique. Films displayed hysteresis translation along the polarization axis when driven by an alternating electric field. The trend of the hysteresis shift as a function of temperature is similar to the difference of DC currents measured when a positive and negative DC electric field is applied. The study suggests that the origin of this phenomenon lies in an asymmetry of the film conductance due to the presence of asymmetric Schottky contacts at the BaTiO3/Ag and Ag/Ba0.xSr1-0.xTiO3 interfaces
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Industrial & engineering chemistry 2 (1910), S. 124-127 
    ISSN: 1520-5045
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0894-3230
    Keywords: intramolecular thermal (4σ + 2π) dyotropy ; kinetic isotope effects ; limiting barrier parameters ; quantum tunnelling ; 2H transfer processes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Primary deuterium kinetic isotope effects (PDKIE) in parazoline-annelated syn-sesquinorbornenes exhibiting irreversible intramolecular (4σ + 2π) thermal dyotropy reveal unambiguous evidence for a tunnelling contribution to the kinetics in one instance but not for a close analogue. For analogous dyotropy of a cyclohexadiene-annelated syn-sesquinorbornene, the tunnelling components of the kinetic behaviour is small by comparison. The H atom traverse between alternative loci for the pyrazolines, deduced from x-ray and neutron diffraction data, is in agreement with approximate barrier parameters obtained by fitting of the PDKIE data to the Bell equation; barrier penetration is 3.22 kcal below the computed barrier corrected for the tunnelling contribution. The relative kinetic effect of systematic variation of the π-donor/acceptor groups on aryl ring substituents at C and N in the pyrazoline ring is consistent with a pericyclic process for dyotropy of these compounds, but not with rearrangement mediated by biradicals resulting from single H atom transfer in the rate-limiting step. Computer modelling of the transition state for dyotropy of these compounds is also consistent with a thermal, orbital symmetry conserved pericyclic reaction. Copyright © 1998 John Wiley & Sons, Ltd.
    Additional Material: 6 Ill.
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  • 4
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    Oxford : Periodicals Archive Online (PAO)
    Economic Inquiry. 6:5 (1968:Dec.) 395 
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  • 5
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Key engineering materials Vol. 352 (Aug. 2007), p. 153-163 
    ISSN: 1013-9826
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: An important aspect of previous sialon research in NZ has been the development of newsynthesis methods, including refinements in carbothermal reduction and nitridation (CRN) methodsand the use of mechanochemical activation of sialon precursors (either Al and Si nitrides and oxidesor CRN mixtures). Mechanochemical activation of CRN mixtures of clay and carbon heated in N2formed β-sialon (z = 2) at 1300oC (100oC lower than in unground mixtures) but 21R polytypoid andcorundum were also formed. More recently, our attention has focussed on the technique ofsilicothermal reduction and nitridation (SRN) to synthesise other sialons, including the AlNpolytypoids and Na and Li α-sialons. The interest in the polytypoids springs from their expectedphysical properties (thermal conductivity and good electrical insulation similar to AlN), theircovalent bonding and relatively light weight arising from their high Al and N contents and theirelongated crystal morphology which may improve the crack resistance of polytypoid compositeswith α-sialon. This paper describes the development of SRN single-step synthesis of high-puritydense 15R sialon from clay, Si and AlN, and the effect of additives on the synthesis and sintering ofthe product. A method is also described for SRN synthesis of Na and Li α-sialons from clay, Si andAlN using fluoride additives. Fluorides have the advantage of small size, high electronegativity,leading to their known facilitation of AlN synthesis. Furthermore, they do not readily enter thesialon structure but may toughen it by formation of glassy phases. Fluorides allow use of clay inthis SRN synthesis by introducing M+ without additional oxygen, but have the disadvantage ofgenerating SiF4 as a byproduct. The reaction using LiF proceeds readily at the very low temperatureof 1200oC via an O-sialon intermediate by a mechanism which probably involves Si migrationassisted by the formation of SiF4.The effect of mechanochemical activation (high energy grinding)on the SRN formation and sintering of Na and Li α-sialons, O and β-sialon has also beenstudied.Grinding the SRN O-sialon precursor promotes O-sialon formation in powders but not inpellets due to pre-reaction sintering, which is facilitated by the smaller particle size. Grinding Naand Li α-sialon SRN precursors forms a mixture of sialons rather than the target monophaseproduct, while sintering of all the sialons is assisted by grinding their SRN precursors
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  • 6
    ISSN: 1013-9826
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: AlN powders were synthesized by gas-reduction- nitridation of γ-Al2O3 powders usingNH3 and C3H8 as reactant gases. AlN was identified from the products that synthesized at1100-1400 oC for 120 min in the NH3-C3H8 gas flow, and it was confirmed that AlN can be easilyfabricated by the gas-reduction-nitridation of γ-Al2O3. The products synthesized at 1100oC for120min contained unreacted γ-Al2O3. By the 27A1 MAS NMR spectra, Al-N bonding in the productincreased with an increase in the nitridation ratio of the tetrahedral AlO4 shoulder which decreasedprior to that of the octahedral AlO6 shoulder. It seems that γ-Al2O3 was preferentially nitrided fromAlO4 rather than AlO6. AlN nano particles were easily converted directly from γ -Al2O3 at a lowtemperature because the AlO4 within γ-Al2O3was preferentially nitrided
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Early-stage thermal oxidation (below 1100°C) of carbothermally synthesized β-sialon powder was monitored by X-ray powder diffraction, solid-state 29Si and 27Al MAS NMR spectroscopy, and thermogravimetry. No crystalline oxidation products were detected by XRD but 29Si and 27Al MAS NMR indicated the early formation of amorphous silica, followed by the formation of an amorphous aluminosilicate with an atomic environment similar to that of mullite. The initial oxidation was described by a linear kinetic law with an activation energy of 170 kJmol−1, suggesting the rate-limiting step to be due to dissolution of O2 in an amorphous silica surface layer on the β-sialon particles.
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  • 8
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The order–disorder of the tetrahedrally coordinated aluminum and silicon atoms in mullite has been investigated by means of 29Si nuclear magnetic resonance (NMR) spectroscopy. Sinter (3/2) and fused (2/1) mullites in the as-received state and reheated at 1750°C, and a reference sillimanite were used for this study. All mullites display similar 29Si NMR spectra: The strongest peak occurs at about −88 ppm, with two subpeaks close to −92 and −96 ppm. The −88 ppm signal is assigned to a sillimanite-type environment with three aluminum oxygen tetrahedra as next nearest neighbors of the silicon oxygen tetrahedra. The two 29Si NMR signals near −92 and −96 ppm are assigned to silicon oxygen tetrahedra surrounded by two aluminum oxygen and one silicon oxygen tetrahedra, and one aluminum oxygen and two silicon oxygen tetrahedra, respectively. 29Si NMR spectra with different short-range-order parameters were simulated by an array of 2 × 10 000 tetrahedral positions by means of an adapted random generator. The comparison between measured and simulated mullite and sillimanite 29Si NMR spectra yields a moderate degree of tetrahedral aluminum–silicon order, with no tendency toward cation demixing.
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  • 9
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The effect of SiO2 addition on the anatase-to-rutile phase transition was investigated by DTA, XRD, FTIR, and XPS. TiO2 xerogels containing SiO2 up to 20 mol% were prepared by mixing and hydrolyzing titanium tetraisopropoxide (TTIP) and tetraethylorthosilicate (TEOS) with HNO3 as a catalyst. With increased amounts of SiO2 in the xerogels, the following results were obtained: (1) the crystallization temperature of anatase increased from 415°C in pure TiO2 to 609°C in 20-mol%-SiO2-containing xerogel in the DTA curves; (2) the formation temperature of rutile, according to quantitative XRD analysis, increased with increased SiO2 content up to 5 mol% SiO2 but became constant at higher SiO2 contents; (3) the crystallinity of anatase became lower; and (4) the lattice parameter a of the anatase decreased slightly, but the parameter c decreased greatly up to 20 mol% SiO2. Although the added silicon atoms were considered from these results to be incorporated into the amorphous TiO2 and anatase structures, the 29Si MAS NMR spectra of the xerogels containing 10 mol% SiO2 showed only tetrahedral silicon, with no indication of silicon in octahedral coordination. When calcined at higher temperatures, the xerogel showed polymerization of the SiO4 tetrahedra in the NMR spectra and the Si–O–Si vibration in the FTIR spectra. The chemical composition of the xerogel surfaces, measured using XPS, showed increased SiO2 content with increased calcining temperature, indicating the expulsion of silicon from inside the particles to form an amorphous SiO2 surface layer. The formation of this amorphous SiO2 surface layer was considered to be important in retarding the anatase-to-rutile phase transition by suppressing diffusion between anatase particles in direct contact and limiting their ability to act as surface nucleation sites for rutile. These effects of silicon additions were similar to those observed in the γ-Al2O3- to-α-Al2O3 transition.
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  • 10
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The formation of a poorly crystalline layer-lattice type of magnesium silicate has been demonstrated in mixtures of magnesium hydroxide with hydrated silicic acid that has been intimately combined via grinding and then aged at room temperature for six months. X-ray diffractometry, Fourier transform infrared spectroscopy, and 29Si solid-state magic-angle spinning nuclear magnetic resonance show the product to be similar to that which has been obtained from the same mixture via hydrothermal treatment at a temperature of 80°C for 24 h. This reaction does not occur to any appreciable extent when the silicic acid is replaced by silica gel, which suggests that the hydration water has an essential role, probably in the dissolution of the silica in the alkaline environment of the magnesium hydroxide.
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