Hot-Pressed ceramics of ultradisperse composite powders

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.     

Electrical Properties and the Structure of Glasses in the xNa2O–(1 – x)2PbO · B2O3 System

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⁺[BO_4/2]^–, Na⁺[O^–BO_2/2] Na⁺[O^–BO_2/2], Pb²⁺ _1/2[BO_4/2]^–, Pb²⁺ _1/2[O^–BO_2/2], and [BO_3/2] types.     

Synthesis of Sn-Incorporated Folded Sheets Mesoporous Materials (Sn-FSM-16)

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 Si_2p, 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).     

Chemical Transformations in the Course of Solid-Phase Synthesis of Fibrous Alkali-Free Fluoroamphibole

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.     

Active-oxygen species on non-reducible rare-earth-oxide-based catalysts in oxidative coupling of methane

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 O^2– 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.     

Comparative studies on direct conversion of methane to methanol/formaldehyde over La–Co–O and ZrO2 supported molybdenum oxide catalysts

The selective oxidation of methane with molecular oxygen over MoO_x/La–Co–O and MoO_x/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. MoO_x/La–Co–O catalysts showed high selectivity to methanol formation while MoO_x/ZrO₂ revealed the property for the formation of formaldehyde in the selective oxidation of methane. 7 wt MoO_x/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 MoO_x/ZrO₂ catalyst at 400 °C and 5.0MPa. 7MoO_x/La–Co–O catalyst showed higher modified H₂-consumption than 12MoO_x/ZrO₂ catalyst. The reducibility and the O^–/O^2– ratio of the catalysts may play important roles on the catalytic performance. The proper reducibility and the O^–/O^2– ratio enhanced the production of methanol in selective oxidation of methane. [MoO₄]^2– species in MoO_x/ZrO₂ catalysts enable selective oxidation of methane to formaldehyde.     

Infrared Spectra and Structure of Si3N4, Si2ON2, and Sialons

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.     

Formation ofβ′-sialon in the system Si3N4-Al2O3-AlN

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.     

The oxidative coupling of methane and the activation of molecular O2 on CeO2/BaF2

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 (O^2–,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 3d_5/2 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.     

The temperature dependence of the electrochemistry of the lithium-sulphur dioxide system

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.     

Sialon based on natural clay raw materials

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.     

On the Possibility of Forming Complexes with the Participation of O2– Ions in Melts and Glasses of the Na2O–SiO2–Cu2O System

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 O^2– 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⁾⁺O_4/2]^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.     

First In Situ Raman Study of Vanadium Oxide Based SO2 Oxidation Supported Molten Salt Catalysts

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 V^V complex V^VO₂(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 V^V dimer (V^VO)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 V^V to V^IV (with the (V=O) shifting to 1024 cm^–1) and to V^IV precipitation below 420°C.     

Raman and Si NMR spectroscopic characterization of lanthanum silicate electrolytes: Emphasis on sintering temperature to enhance the oxide-ion conductivity

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.     

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 O^2– (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.     

Synthesis and sol-gel polymerization of [Si8O12](OCH3)8

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.     

Exploration of molten hydroxide electrochemistry for thermal battery applications

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 + O^2– where H₂O is the Lux-Flood acid (oxide ion acceptor) and O^2– 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⁺ + O^2–. 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] [O^2–]) and the reduction of Na⁺ or K⁺ in basic melts. The direct reduction of OH^– to hydrogen and O^2– 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.     

Co2 conversion and oxalate stability on alkali promoted metal surfaces: Sodium modified Al(100)

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.     

Perovskite-type oxide ACo0.8Bi0.2O2.87 (A=La0.8Ba0.2): a catalyst for low-temperature CO oxidation

Perovskite-type oxide ACo_0.8Bi_0.2O_2.87 (A=La_0.8Ba_0.2) 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 ACo_0.8Bi_0.2O_2.87, bismuth is pentavalent whereas cobalt is trivalent as well as bivalent; in La_0.8Ba_0.2CoO_2.94, 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 ACo_0.8Bi_0.2O_2.87, 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 La_0.8Ba_0.2CoO_3– 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.     

Synthesis and magnetic properties of nanoporous Co3O4 nanoflowers

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.