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1.
I. I. Tkacheva 《Refractories and Industrial Ceramics》1994,35(1-2):55-63
The regularities of variation of the microstructure, phase composition, degree of crystallinity, and properties of hot-pressed ceramics prepared from ultradisperse composite powders of the Si3N4 - Y2O3 system depending on the / phase ratio in Si3N4 and the concentration of Y2O3 are discussed. An increase in the concentration of Y2O3 in composite powders up to 15 – 18% and a decrease in the / ratio of Si3N4 lead to a change of the composition of the secondary crystalline phases with an increase in their refractoriness (from silicon oxynitride and yttrium silicates to complex yttrium — silicon oxynitrides), a lower concentration of the vitreous phase, a higher degree of crystallinity, and a higher content of stretched grains in the structure of the ceramics. The presence of the high-melting phase of yttrium — silicon oxynitrides improves the phase and granular homogeneity of the structure that forms by the type of solid solutions, with an essentially similar composition of the grains and the intergranular phase. The formation of a high-melting fine-grained self-toughening structure with a coefficient of grain elongation up to 8 – 9 ensures a high and practically stable level of the mechanical properties of the synthesized materials at temperatures up to 1400 – 1500°C.Translated from Ogneupory, No. 2, pp. 13 – 20, February, 1994. 相似文献
2.
I. A. Sokolov I. V. Murin N. A. Mel'nikova A. A. Pronkin 《Glass Physics and Chemistry》2002,28(4):239-245
The temperature–concentration dependences of the electrical conductivity and the activation energy for electrical conduction of glasses in the Na2O–B2O3 and Na2O–2PbO · B2O3 systems are studied. The investigation into the nature of the electrical conduction in these glasses reveals that the contribution from the electronic component (10–3%) of the conductivity is within the sensitivity of the Liang–Wagner technique. A considerable alkali conductivity is observed upon introduction of more than 12 mol % Na2O. The true transport number of sodium Na is as large as unity at [Na2O] 15 mol %. It is shown that the observed temperature–concentration dependences of the electrical and transport properties are governed by the ratio between the concentrations of polar and nonpolar structural–chemical units of the Na+[BO4/2]–, Na+[O–BO2/2] Na+[O–BO2/2], Pb2+
1/2[BO4/2]–, Pb2+
1/2[O–BO2/2], and [BO3/2] types. 相似文献
3.
Sn-incorporated folded sheets mesoporous materials (Sn-FSM-16) with various contents of Sn were synthesized by using a mixture of water glass, SnCl4 and NaOH as starting materials. Hexadecyltrimethyl-ammonium chloride (surfactant) was used to intercalate into the layered silicate. The reaction process was followed by measurements of XRD patterns of intermediates. The Sn-FSM-16 was formed via the following mechanism: (1) layered silicates such as - - and -Na2Si2O5 were formed as intermediates by the calcination of the mixture of the starting materials; (2) the surfactant was intercalated into the layered silicates; (3) the surfactant-silicate complex with hexagonal structure was obtained as a precursor of Sn-FSM-16; (4) the precursor was calcined to decompose the surfactant in the interlayer and was changed to Sn-FSM-16. The structural aspect of Sn in Sn-FSM-16 was studied by XPS profiles of Sn 3d
5/2 and Si2p, 29 Si MAS NMR and FTIR. The content of Sn in Sn-FSM increased with increasing concentrations of both Sn and NaOH in the starting materials. The surface area of Sn-FSM-16 decreased with an increase of Sn content in Sn-FSM (1160–620 m2/g). 相似文献
4.
This paper reports on the results of investigations into the possible chemical transformations occurring in the course of the solid-phase synthesis of fibrous alkali-free fluoroamphibole from synthetic magnesium silicate dihydrate (MSDH) MgO · SiO2 · 2H2O in the MSDH–MgF2–NaCl model system in the concentration range of compositions corresponding to Mg-fluorocupfferite (Mg7Si8O22F2) at temperatures ranging from 60 to 1000°C. It is found that the formation of Mg-fluorocupfferite from MSDH under these conditions is a complex multistage chemical process. In the temperature range 60–700°C, dehydration, dehydroxylation of MSDH, and pyrohydrolysis of fluorides (MgF2, SiF4, etc.) proceed near the lower temperature boundary of the fluoroamphibole formation region (T 750°C). These processes result in the formation of compounds containing monomeric and simple polymeric silicon–oxygen anions [SiO4]4– and [SiO3]2–
(forsterite, fluoronorbergite, enstatite, etc.). In the temperature range 850–900°C, the transformation of intermediate compounds leads to the formation of the fluoroamphibole structure. The main elements of this structure are silicon–oxygen anions of the [Si4O11]6–
type. 相似文献
5.
Active-oxygen species on non-reducible rare-earth-oxide-based catalysts in oxidative coupling of methane 总被引:1,自引:0,他引:1
H.B. Zhang G.D. Lin H.L. Wan Y.D. Liu W.Z. Weng J.X. Cai Y.F. Shen K.R. Tsai 《Catalysis Letters》2001,73(2-4):141-147
From supplementary in situ Raman spectroscopic studies of active-oxygen species on non-reducible rare-earth-oxide-based catalysts in the oxidative coupling of methane (OCM) and structural adaptability considerations, further support has been obtained for our proposal that there may be an active and elusive precursor (of O2
– and O2
2– adspecies), most probably O3
2– formed from reversible redox coupling of an O2 adspecies at an anionic vacancy with a neighboring O2– in the surface lattice. This active precursor may initiate H abstraction from CH4 and be itself converted to OH–+O2
–, or it may abstract an electron from the oxide lattice and be converted to O2
2–+O–. The prospect of developing this type of OCM catalysts is discussed. 相似文献
6.
The selective oxidation of methane with molecular oxygen over MoOx/La–Co–O and MoOx/ZrO2 catalysts to methanol/formaldehyde has been investigated in a specially designed high-pressure continuous-flow reactor. The properties of the catalysts, such as crystal phase, structure, reducibility, ion oxidation state, surface composition and the specific surface area have been characterized with the use of XRD, LRS, TPR, XPS and BET methods. MoOx/La–Co–O catalysts showed high selectivity to methanol formation while MoOx/ZrO2 revealed the property for the formation of formaldehyde in the selective oxidation of methane. 7 wt MoOx/La–Co–O catalyst gave 6.7 methanol yield (ca. 60 methanol selectivity) at 420°C and 4.2 MPa. On the other hand, the maximal yield of formaldehyde ca. 4 (47.8 formaldehyde selectivity) was obtained over 12wt MoOx/ZrO2 catalyst at 400 °C and 5.0MPa. 7MoOx/La–Co–O catalyst showed higher modified H2-consumption than 12MoOx/ZrO2 catalyst. The reducibility and the O–/O2– ratio of the catalysts may play important roles on the catalytic performance. The proper reducibility and the O–/O2– ratio enhanced the production of methanol in selective oxidation of methane. [MoO4]2– species in MoOx/ZrO2 catalysts enable selective oxidation of methane to formaldehyde. 相似文献
7.
V. N. Antsiferov V. G. Gilev V. I. Karmanov 《Refractories and Industrial Ceramics》2003,44(2):108-114
Sialon powders are prepared from Kyshtymskoe kaolin with 12 – 28 wt.% carbon mixtures by the carbothermal reduction-nitridation method. The infrared spectra of -, O-, C- and 15R-phases in the Si – Al – O – N system are recorded and compared with the IR spectra of Si3N4, Si2ON2, and kaolin. The IR data are shown to correlate with the chemical composition of sialons and provide information supplementary to x-ray phase analysis data. The spectra of sialon powders differ from each other and from the spectrum of silicon nitride. 相似文献
8.
A. M. Gavrish A. B. Puchkov I. L. Boyarina Z. D. Zhukova É. V. Degtyareva 《Refractories and Industrial Ceramics》1988,29(7-8):477-479
Conclusions X-ray and crystal-optical methods were used to study the formation and structure of'-sialon, synthesized in the system Si3N4-Al2O3-AlN by hot pressing methods, using mixtures of the stated composition.It was established that in this system there is formed a monophase solid solution x(Al2O3· A1N) + (1–x) Si3N4 (-sialon) with a hexagonal structure, the region of existence of which (in molar parts) is located in the value range x = 0–0.8.Concentration relationships are stated for the parameters, volumes of the elementary cells, and the refractive indices of these solutions.Translated from Ogneupory, No. 8, pp. 28–29, August, 1988. 相似文献
9.
X. P. Zhou Z. S. Chao W. Z. Weng W. D. Zhang S. J. Wang H. L. Wan K. R. Tsai 《Catalysis Letters》1994,29(1-2):177-188
CeO2/BaF2 was used as the catalyst for the oxidative coupling of methane (OCM). At 800°C and CH4O2=2.71,CH4 conversion of 34% with C2 hydrocarbon selectivity of 54.3% was obtained. XRD measurement showed that partial anion (O2–,F–) and/or cation (Ce4+,Ba2+) exchange between CeO2 and BaF2 lattices occurred. ESR study showed that O– species existed on degassed catalyst. XPS study revealed that, when BaF2 was added to CeO2, the binding energy of Be 3d5/2 was 2.2 eV lower than that in CeO2, and the electron-enriched lattice oxygen species was detected. XPS, ESR and Raman study showed that, under O2 adsorbing conditions, O
2
2–
and O
–
2
species were detected on CeO2/BaF2.This work was supported by the State Key Laboratory for Physical Chemistry of the solid surface and the National Science Foundation of China. 相似文献
10.
N. F. Harman N. A. Hampson S. A. G. R. Karunathilaka D. Eyre 《Journal of Applied Electrochemistry》1984,14(6):765-768
Value for the activation energy, U
act, and the entropy change, S, for the reaction 2Li + S2O
4
2–
Li2S2O4+2e in acetonitrile have been found to be 72 kJ mol–11 and — 0.3 kJ mol–1 K–1, respectively, by a combination of impedance techniques and the use of a temperature-controlled environment on commercially manufactured cells which acted as constant volume containers. 相似文献
11.
I. L. Boyarina A. B. Puchkov A. M. Gavrish Z. D. Zhukova É. V. Degtyareva 《Refractories and Industrial Ceramics》1982,23(3-4):191-196
Conclusions Conditions were worked out for the synthesis of '-sialon on the basis of natural clay raw materials. It was established that the maximum yield of sialon is obtained from nitriding nonbriquetted mixtures of Novoselitsk kaolin and graphite. It was found that the yield of sialon is increased with a rise in the Al2O3SiO2 ratio in the clay material.We studied the structure of the resulting sialon and also the kinetics of milling. It was shown that at 1450°C there forms 'sialon of variable composition: 0.58 (Al2O3·AlN)–0.42 Si3N4, and moreover a rise in temperature to 1700–1750°C leads to the displacement of the composition into the region adjacent to the AlN corner in the phase diagram for the Si-Al-O-N system and the appearance of the sialon with the structure of the polytype 15RAlN.Translated from Ogneupory, No. 4, pp. 44–48, April, 1982. 相似文献
12.
N. V. Borisova V. G. Konakov T. G. Kostyreva M. M. Shultz 《Glass Physics and Chemistry》2003,29(1):28-34
The structural role of copper ions in melts (glasses) of the Na2O–SiO2–Cu2O–CuO system is analyzed in the framework of the acid–base concept with due regard for the geometric (the radius ratio for Cu2(1)+ and O2– ions) and energy (the mean enthalpies of the Cu2(1)+–O bonds) factors. It is demonstrated that copper ions in the structure fulfill the function of modifier cations. In these melts, the Cu1+–Cu2+ redox equilibrium can be described without regard for the formation of [Cu2(1)+O4/2]2(3)– ionic complexes (which could be incorporated into the structure of silicon–oxygen anions) and [Cu2+O
b/k
]2 – b/k
polyhedra providing the interaction between Cu2+ ions and anions. The influence of the formation of these polyhedra on the redox equilibrium is considered within the formalism of chemical thermodynamics. The composition dependence of the oxygen ion exponent pO is measured by an electromotive force (emf) technique. The ratio between the numbers of copper atoms with different valences is determined by chemical analysis. The experimental data obtained are in agreement with the theoretical inferences. 相似文献
13.
In situ Raman spectroscopy at temperatures up to 500°C is used for the first time to identify vanadium species on the surface of a vanadium oxide based supported molten salt catalyst during SO2 oxidation. Vanadia/silica catalysts impregnated with Cs2SO4 were exposed to various SO2/O2/SO3 atmospheres and in situ Raman spectra were obtained and compared to Raman spectra of unsupported model V2O5–Cs2SO4 and V2O5–Cs2S2O7 molten salts. The data indicate that (1) the VV complex VVO2(SO4)2
3– (with characteristic bands at 1034 cm–1 due to (V=O) and 940 cm–1 due to sulfate) and Cs2SO4 dominate the catalyst surface after calcination; (2) upon admission of SO3/O2 the excess sulfate is converted to pyrosulfate and the VV dimer (VVO)2O(SO4)4
4– (with characteristic bands at 1046 cm–1 due to (V=O), 830 cm–1 due to bridging S–O along S–O–V and 770 cm–1 due to V–O–V) is formed and (3) admission of SO2 causes reduction of VV to VIV (with the (V=O) shifting to 1024 cm–1) and to VIV precipitation below 420°C. 相似文献
14.
Enhancement of oxide-ion conductivity has been investigated with emphasis on the high sintering temperature of apatite-type structure lanthanum silicate (La10Si6O27) as a potential electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The influence of the sintering temperatures 1500, 1550, 1600 and 1650 °C as a function of ionic conductivity of the La10Si6O27 electrolyte synthesized via a diethylamine (DEA) precipitation process has been characterized using impedance spectroscopy. The ionic conductivity of the La10Si6O27 electrolyte sintered at 1650 °C revealed a higher value (1.22 × 10−2 S cm−1 at 700 °C) of one order of magnitude than the pellets sintered at lower temperatures. The sintered La10Si6O27 pellets have been characterized by 29Si NMR and Raman spectroscopy. The 29Si NMR data showed the characteristic secondary peak at 81.2 ppm, which confirmed the interstitial oxygen content contributing to high oxide-ion conduction. The Raman spectra revealed the appearance of a new resolved band centered at 861 cm−1 for the pellet sintered at 1650 °C compared with lower temperatures sintered pellets. The results confirmed the possibility of local structural distortion to create additional pathways for interstitial oxide-ion conduction between channels leading to higher conductivity for the pellets sintered above 1600 °C. Thus, the conduction pathway may be determined by the co-operative displacements of the SiO4 substructure units formed at elevated sintering temperatures for high oxide-ion conductivity. 相似文献
15.
Computer modelling of the kinetics of the coadsorption of ammonia and dioxygen at a Mg(0001) surface
The activity of a surface oxygen transient, O–(s), for H-abstraction from an ammonia molecule undergoing surface diffusion at a Mg(0001) surface at 298 K has been simulated by computer modelling. The time dependence of the chemisorbed products, NH2(a), OH(a) and O2– (a), has been determined making reasonable assumptions of the activation energy for the surface diffusion of physisorbed ammonia, the surface life-time of O–(s) and the surface stoichiometry determined from XPS studies.Paper presented in part at the Royal Society-USSR Academy Workshop on Surfaces and Catalysis, Oxford, April 8–10, 1989.We acknowledge gratefully the support of SERC. 相似文献
16.
C. S. Brevett P. C. Cagle W. G. Klemperer D. M. Millar G. C. Ruben 《Journal of Inorganic and Organometallic Polymers》1991,1(3):335-342
Sol-gel polymerization of [Si8O12](OCH3)8 in an inert solvent under neutral conditions has been shown to yield very high-surface area silica xerogels [P. C. Cagleet al., Mater. Res. Soc. Symp. Proc.
180, 29 (1990)]. Two recent results relevant to this process are described here. First, a practical synthetic route to [Si8O12](OCH3)8 is described that allows multigram quantities to be conveniently prepared. Second, TEM studies are reported that confirm retention of the [Si8O12] cubic core structure in high-surface area, [Si8O12](OCH3)8
– derived xerogels.This paper is from the Second International Topical Workshop, Advances in Silicon-Based Science. 相似文献
17.
M.H. Miles 《Journal of Applied Electrochemistry》2003,33(11):1011-1016
The electrochemistry of molten LiOH–NaOH, LiOH–KOH, and NaOH–KOH was investigated using platinum, palladium, nickel, silver, aluminum and other electrodes. The fast kinetics of the Ag+/Ag electrode reaction suggests its use as a reference electrode in molten hydroxides. The key equilibrium reaction in each of these melts is 2 OH– = H2O + O2– where H2O is the Lux-Flood acid (oxide ion acceptor) and O2– is the Lux–Flood base. This reaction dictates the minimum H2O content attainable in the melt. Extensive heating at 500 °C simply converts more of the alkali metal hydroxide into the corresponding oxide, that is, Li2O, Na2O or K2O. Thermodynamic calculations suggest that Li2O acts as a Lux–Flood acid in molten NaOH–KOH via the dissolution reaction Li2O(s) + 2 OH– = 2 LiO– + H2O whereas Na2O acts as a Lux–Flood base, Na2O(s) = 2 Na+ + O2–. The dominant limiting anodic reaction on platinum in all three melts is the oxidation of OH– to yield oxygen, that is 2 OH– 1/2 O2 + H2O + 2 e–. The limiting cathodic reaction in these melts is the reduction of water in acidic melts ([H2O] [O2–]) and the reduction of Na+ or K+ in basic melts. The direct reduction of OH– to hydrogen and O2– is thermodynamically impossible in molten hydroxides. The electrostability window for thermal battery applications in molten hydroxides at 250–300 °C is 1.5 V in acidic melts and 2.5 V in basic melts. The use of aluminum substrates could possibly extend this window to 3 V or higher. Preliminary tests of the Li–Fe (LAN) anode in molten LiOH–KOH and NaOH–KOH show that this anode is not stable in these melts at acidic conditions. The presence of superoxide ions in these acidic melts likely contributes to this instability of lithium anodes. Thermal battery development using molten hydroxides will likely require less active anode materials such as Li–Al alloys or the use of more basic melts. It is well established that sodium metal is both soluble and stable in basic NaOH–KOH melts and has been used as a reference electrode for this system. 相似文献
18.
CO2 conversion on alkali promoted metals in aprotic systems has been followed with surface sensitive spectroscopies. New results on sodium modified aluminum(100) are presented and compared with previous studies on magnesium [1], aluminum [2], and bulk alkali metals [3]. Electron energy loss spectra reveal two different states of CO2 adsorption at 100 K and monolayer sodium coverage. Vibrational bands at 650 cm–1 and 2325 cm–1 correspond to weakly bound molecular CO2 and a multitude of bands between 2300 cm–1 and 460 cm–1 to oxalate ions with low, possibly unidentate, coordination. Gentle annealing increases the coordination as apparent by vibrational shifts. This corresponds to oxalate to carbonate conversion, a process which is completed around room temperature. CO desorption was detected at 285 K and Auger measurements reveal a 13 C/O stoichiometry after high temperature annealing. We observe no release of CO2 above 110 K but an additional weak state of CO desorption around 470 K. High temperature annealing causes decomposition of all intermediates and leaves the aluminum surface covered with an irreducible carbide and oxide overlayer. We suggest that CO2 reduction and dimerization to C2O4
–2 is a common path to yield carbon deposition on all alkali promoted surfaces in hydrogen deficient systems. In contrast, oxalate decomposition is related to the specific chemistry of each substrate. 相似文献
19.
Perovskite-type oxide ACo0.8Bi0.2O2.87 (A=La0.8Ba0.2): a catalyst for low-temperature CO oxidation 总被引:1,自引:0,他引:1
Perovskite-type oxide ACo0.8Bi0.2O2.87 (A=La0.8Ba0.2) has been investigated as a catalyst for the oxidation of carbon monoxide. X-ray diffraction results revealed that the catalyst is single-phase and cubic in structure. The results of chemical analysis indicated that in ACo0.8Bi0.2O2.87, bismuth is pentavalent whereas cobalt is trivalent as well as bivalent; in La0.8Ba0.2CoO2.94, cobalt ions exist as Co3+ and Co4+. The substitution of Bi for Co enhanced the catalytic activity of the perovskite-type oxide significantly. Over the Bi-incorporated catalyst, at equal space velocities and with the rise in CO/O2 molar ratio, the temperature for 100% CO conversion shifted to a higher range; at a typical space velocity of 30000 h–1 and a CO/O2 molar ratio of 0.67/1.00, 100% CO conversion was observed at 250°C. Over ACo0.8Bi0.2O2.87, at equal CO/O2 molar ratio, the temperature for 100% CO conversion decreased with a drop in space velocity; the lowest being 190°C at a space velocity of 5000 h–1. The result of O2-TPD study illustrated that the presence of Bi ions caused the lattice oxygen of La0.8Ba0.2CoO3– to desorb at a lower temperature. The results of TPR, 18O/16O isotopic exchange, and CO-pulsing investigations demonstrated that the lattice oxygen of the Bi-doped catalyst is highly mobile. 相似文献
20.
Yuanguang Zhang Youcun Chen Tao Wang Juhong Zhou Yinguo Zhao 《Microporous and mesoporous materials》2008,114(1-3):257-261
Nanoporous Co3O4 hierarchical nanoflowers have been prepared through sequential process of a hydrothermal reaction and heat treatment. These nanoflowers consisting of a great deal of Co3O4 nanofibers have bimodal pore structures and Brunauer–Emmett–Teller surface area of 34.61 m2/g. The temperature dependence curves of magnetization in zero-field-cooled and field-cooled exhibit main antiferromagnet and weak ferromagnet of Co3O4 nanoflowers at blocking temperature of 34 K, respectively. In addition, analysis of their optic properties obviously indicates red shift of absorption peaks, exhibiting quantum-confined effect and traits of semiconductor. 相似文献