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1.
Layered -titanate materials, Na xM x/2Ti 1−x/2O 2 (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na 0.4Ni 0.2Ti 0.8O 2 were ca. 7×10 3 S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm −1 K −1 at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na 0.4Ni 0.2Ti 0.8O 2 and Na 0.4Co 0.2Ti 0.8O 2 was about 1.9×10 −4 and 1.2×10 −4 K −1 around 700 °C, respectively. 相似文献
2.
Preparations and physico-chemical characterizations of NASICON-type compounds in the system Li 1+xAl xA 2−xIV(PO 4) 3 (A IV=Ti or Ge) are described. Ceramics have been fabricated by sol-gel and co-grinding processes for use as ionosensitive membrane for Li + selective electrodes. The structural and electrical characteristics of the pellets have been examined. Solid solutions are obtained with Al/Ti and Al/Ge substitutions in the range 0≤ x≤0·6. A minimum of the rhombohedral c parameter appears for x about 0·1 for both solutions. The grain ionic conductivity has been characterized only in the case of Ge-based compounds. It is related to the carrier concentration and the structural properties of the NASICON covalent skeleton. The results confirm that the Ti-based framework is more calibrated to Li + migration than the Ge-based one. A grain conductivity of 10 −3 S cm −1 is obtained at 25°C in the case of Li 1·3Al 0·3Ti 1·7(PO 4) 3. A total conductivity of about 6×10 −5 S cm −1 is measured on sintered pellets because of grain boundary effects. The use of such ceramics in ISE devices has shown that the most confined unit cell (i.e. in Ge-based materials) is more appropriate for selectivity effect, although it is less conductive.© 相似文献
3.
The objective of this work was to study the promotional effect of Pt on Co-zeolite (viz. mordenite, ferrierite, ZSM-5 and Y-zeolite) and Co/Al 2O 3 on the selective catalytic reduction (SCR) of NO x with CH 4 under dry and wet reaction stream. After being reduced in H 2 at 350°C, the PtCo bimetallic zeolites showed higher NO to N 2 conversion and selectivity than the monometallic samples, as well as a combination of the latter samples such as mechanical mixtures or two-stage catalysts. After the same pretreatment, under wet reaction stream, the bimetallic samples were also more active. Among the other catalysts studied with 5% of water in the feed, (NO = CH 4 = 1000 ppm, O 2 = 2%), the NO conversion dropped to zero over Co 2.0Mor at 500°C and GHSV = 30,000 h −1, whereas it is 20% in Pt 0.5Co 2.0Mor. In Pt/Co/Al 2O 3 the NO x conversion dropped below 5% with only 2% of water under the same reaction conditions. The specific activity given as molecules of NO converted per total metal atom per second were 16.5 × 10 −4 s −1 for Pt 0.5Co 2.0Fer, 13 × 10 −4 s −1 for Pt 0.5Co 2.0Mor, 4.33 × 10 −4 s −1 for Pt 0.5Co 2.0ZSM-5 and 0.5 × 10 −4 s −1 for Pt/Co/Al 2O 3. The Y-zeolite-based samples were inactive in both mono and bimetallic samples. The species initially present in the solid were Pt° and Co°, together with Co 2+ and Pt 2+ at exchange positions. Co° seems not to participate as an active site in the SCR of NO x. Those species remained after the reaction but some reorganization occurred. A synergetic effect among the different species that enhances both the NO to NO 2 reaction, the activation of CH 4 and also the ability of the catalyst to adsorb NO, could be responsible for the high activity and selectivity of the bimetallic zeolites. 相似文献
4.
A new brownmillerite-related compound. Ca 2Cr 2O 5, has been prepared. It has been indexed according to an orthorhombic lattice a = 5·750 Å, b = 14·398 Å and c = 5·483 Å. A series of experiments was performed in order to find the appropriate firing temperature. The total conductivity was measured by a four-point method in the range of 690–911°C. Impedance spectroscopy was also employed in the temperature range 343–785°C. Conductivity measurements at different oxygen pressures at 500°C suggest that Ca 2Cr 2O 5 is a predominantly ionic conductor at P o2 = 1–10 −2atm. 相似文献
5.
The electrical conductivity of KI solutions in anhydrous acetonitrile has been determined at 0, 25 and 35°C in the concentration range 0·9– 600 × 10 −4 mole/l. The values of Λ 0, K and a calculated from the results are, respectively: 145·9 mho/cm, 0·95 × 10 −2 and 1·72 Å at 0°C; 186·2 mho/cm, 8·98 × 10 −2 and 4·6 Å at 25°C; and 204·8 mho/cm, 5·17 × 10 −2 and 3·5 Å at 35°C. The phoreograms at all the three temperatures are catabatic at lower concentration, but become anabatic at 0·017, 0·022 and 0·024 respectively, at 0, 25 and 35°C. 相似文献
6.
Nanosized particles dispersed uniformly on Al 2O 3 particles were prepared from the decomposition of precursor Cr(CO) 6 by metal organic chemical vapor deposition (MOCVD) in a fluidized chamber. These nanosized particles consisted of Cr 2O 3, CrC 1−x, and C. A solid solution of Al 2O 3–Cr 2O 3 and an Al 2O 3–Cr 2O 3/Cr 3C 2 nanocomposite were formed when these fluidized powders were pre-sintered at 1000 and 1150 °C before hot-pressing at 1400 °C, respectively. In addition, an Al 2O 3–Cr 2O 3/Cr-carbide (Cr 3C 2 and Cr 7C 3) nanocomposite was formed when the particles were directly hot pressed at 1400 °C. The interface between Cr 3C 2 and Al 2O 3 is non-coherent, while the interface between Cr 7C 3 and Al 2O 3 is semi-coherent. 相似文献
7.
Three compounds, K 2(H 2O) 4H 2SiMo 12O 40 · 7H 2O (1), K 2Na 2(H 2O) 4SiW 12O 40 · 4H 2O (2), and Na 4(H 2O) 8SiMo 12O 40 · 6H 2O (3) have been synthesized and structurally characterized by single-crystal X-ray analysis, IR, and thermogravimetry. Compounds 1 and 2 both show the high symmetry trigonal space group P3 221 and a novel 3D network structure. The Keggin anions [SiM 12O 40] 4−(M = Mo, W) are linked by potassium or sodium cations to generate hexagon-shaped channels along the c-axis, in which water molecules are accommodated. Compound 3 is tetragonal, space group P4/mnc constructed from [SiMo 12O 40] 4− anions and Na ions. 相似文献
8.
N 2O 5 reacts with O 2− ion in LiCl---KCl eutectic at 450° to give NO 3−. By analogy to the salts of the other oxides of Group V, NO 3− can be considered as metanitrate and is expected to give—under appropriate conditions—the corresponding pyro-salt. Experiments are described in which the O 2− ion in LiCl---KCl melt is potentiometrically titrated with KNO 3. The titration curves show an inflexion at the composition corresponding to pyronitrate, N 2O 74−. The formation of pyronitrate in KNO3 melts is also established. Strong oxide-ion donors, eg Na2O2 or NaOH, or electrolytically generated O2− ion, react slowly with the melt to produce a compound of less basic character. The reaction is zero-order with respect to O2− and has an activation energy of ca 6·17 Kcal/mole. Pyronitrate in molten KNO3 possesses a basicity comparable to that of the carbonate ion in the same melt. It readily lends its oxide ion to strong acids eg, Cr2O72− and PO3−. X-ray diffraction patterns of NO3−-N2O74− mixtures show peaks that can be correlated to the new anion. 相似文献
9.
The chemical composition of cristobalite, tridymite, glass, and accessory phases of different zones of used silica bricks taken from the roof of a glass tank was studied with a high resolution microprobe. Tridymite and cristobalite contain as impurities TiO 2 (≤ 0.36 wt%), Al 2O 3 (≤ 0.37 wt%), and Na 2O (≤ 0.27 wt%). Main constituents of the glass phase coexisting with crystalline silica are: SiO 2 (74 to 60 wt%), TiO 2 (0.4 to 9 wt%), Al 2O 3 (1 to 5 wt%), Fe 2O 3 (0.3 to 3 wt%), CaO (5 to 20 wt%), and Na 2O (8 to 17 wt%). Temperature curves within the bricks during operation of the glass tank have been estimated using direct temperature measurement at the hot front of the bricks, and the transition temperatures of cristobalite to tridymite ( 1450°C), and of - to β-wollastonite ( 1200°C). Microchemical data and supposed temperatures were correlated with the Nernst distribution law. The applicability of the Nernst law shows that local equilibrium conditions were reached during the use of the bricks; they have been preserved during cooling of bricks. The results of the Nernst law cation distribution imply that structural saturation with Al 2O 3, TiO 2, and Na 2O was not reached in the investigated composition range. Al 3+ is believed to substitute Si 4+ at tetrahedral lattice sites. Al 3+ substitution is favoured with decreasing temperature in relation to the Al 2O 3 content in the glass phase. Al 3+ → Si 4+ substitution produces charge deficiency, which is compensated by interstitial entry of Na + into structural channels and voids of tridymite and cristobalite. Ti 4+ incorporation into the cristobalite and tridymite structures is favoured at higher temperatures with respect to the TiO 2 content of the glass phase. The close reciprocal dependence between Al 3+ and Ti 4+ in silica may indicate that Ti 4+ is tetrahedrally incorporated as well. 相似文献
10.
A series of CuO–ZnO/Al 2O 3 solids were prepared by wet impregnation using Al(OH) 3 solid and zinc and copper nitrate solutions. The amounts of copper and zinc oxides were varied between 10.3 and 16.0 wt% CuO and between 0.83 and 7.71 wt% ZnO. The prepared solids were subjected to thermal treatment at 400–1000°C. The solid–solid interactions between the different constituents of the prepared solids were studied using XRD analysis of different calcined solids. The surface characteristics of various calcined adsorbents were investigated using nitrogen adsorption at −196°C and their catalytic activities were determined using CO-oxidation by O 2 at temperatures ranged between 125°C and 200°C. The results showed that CuO interacts with Al2O3 to produce copper aluminate at ≥600°C and the completion of this reaction requires heating at 1000°C. ZnO hinders the formation of CuAl2O4 at 600°C while stimulates its production at 800°C. The treatment of CuO/Al2O3 solids with different amounts of ZnO increases their specific surface area and total pore volume and hinders their sintering (the activation energy of sintering increases from 30 to 58 kJ mol−1 in presence of 7.71 wt% ZnO). This treatment resulted in a progressive decrease in the catalytic activities of the investigated solids but increased their catalytic durability. Zinc and copper oxides present did not modify the mechanism of the catalyzed reaction but changed the concentration of catalytically active constituents (surface CuO crystallites) without changing their energetic nature. 相似文献
11.
Electrical conductivity and thermal expansion of Bi 2O 3 doped with 15 and 25 mol% prasoeodymia have been studied, named BP15 and BP25 respectively. The crystal phases formed in the sintered oxides and their lattice parameters were determined by X-ray diffraction (XRD). The electrical conductivity was measured in air by means of both impedance spectroscopy and the four-point DC technique within the temperature range 100–850°C. Thermal expansion coefficient measurements were carried out using the dilatometry technique. The results derived show the formation of a single rhombohedral phase in both cases which appears to be stable up to 680°C. The electrical conductivity is thermally activated exceeding 1·0 and 0·7 S/cm at 850°C for the compositions BP15 and BP25 respectively. The thermal expansion coefficient is highly non-linear and varies between 6 and 13 × 10 −6 K, presenting an increase up to 500°C followed by a decrease above this temperature region. 相似文献
12.
The phase evolution and microstructural development of a series of geopolymer mixtures comprising SiO 2, Al 2O 3, Na 2O and H 2O prepared by alkali reaction of metakaolin, have been studied. The study also included the effects of cure duration and its impact on physical properties such as compressive strength. The characteristic molar ratios of the geopolymer mixtures were of the range SiO 2/Al 2O 3 [2.50–5.01] and Al 2O 3/Na 2O [0.60–1.70], respectively. The formulations were subjected to continuous curing at 40 °C for 7 months, and were analyzed periodically by XRD and SEM techniques. Amorphous Na–Al–Si phase(s), observed at early ages, gradually transformed to crystalline phase(s) with prolonged curing. The initial SiO2, Na2O, and Al2O3 contents of mixtures appeared to be critical factors governing the observed amorphous → crystalline transformation. Well-developed crystalline zeolitic phases, including chabazite, faujasite, zeolite A and zeolite P, were identified in some of the mixtures investigated. In most cases, with prolonged curing, some correlation emerged relating compressive strength development with corresponding phase changes. In essence, the mixture formulations that developed crystalline phases after prolonged curing tended to produce low strengths. The relevance of these findings on the phase development of mild- to warm-temperature prolonged curing of geopolymer systems is discussed. 相似文献
13.
Ceria doped with praseodymia and niobia has been studied in order to evaluate the possibility of applying these materials as cathodes for solid oxide fuel cells. The content of crystallographic phases and their lattice parameters have been determined by X-ray diffraction, the thermal expansion coefficient has been characterised by the dilatometry technique and the electrical conductivity has been measured by complex impedance spectroscopy and by the four probe DC technique. The results have shown the presence of one fluorite phase in binary compositions (CeO 2-PrO 2−x) for PrO 2−x concentrations up to about 20 mol% and two fluorite phases with different lattice parameters for higher PrO 2−x concentrations. The addition of 3 mol% NbO 2·5 has allowed the stabilisation of a single fluorite phase up to 50 mol% PrO 2−x. The thermal expansion coefficient varies between 0 and 30 × 10 −6/K depending on composition and temperature. The electrical conductivity is mainly electronic and thermally activated. The conductivity exceeds 0·1 S/cm at 800°C for compositions with 40 to 50 mol% PrO 2−x. 相似文献
14.
A series of the Ce 1−xCu xO 2−x/Al 2O 3/FeCrAl catalysts ( x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H 2-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce 1−xCu xO 2−x/Al 2O 3/FeCrAl catalysts the surface phase structure were the Ce 1−xCu xO 2−x solid solution, -Al 2O 3 and γ-Al 2O 3. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce 1−xCu xO 2−x solid solution. The Cu component of the Ce 1−xCu xO 2−x/Al 2O 3/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce 1−xCu xO 2−x solid solution and the Al 2O 3 washcoats and the FeCrAl support. 相似文献
15.
A method to quantify DRIFT spectral features associated with the in situ adsorption of gases on a NO x adsorber catalyst, Pt/K/Al 2O 3, is described. To implement this method, the multicomponent catalyst is analysed with DRIFT and chemisorption to determine that under operating conditions the surface comprised a Pt phase, a pure γ-Al 2O 3 phase with associated hydroxyl groups at the surface, and an alkalized-Al 2O 3 phase where the surface –OH groups are replaced by –OK groups. Both DRIFTS and chemisorption experiments show that 93–97% of the potassium exists in this form. The phases have a fractional surface area of 1.1% for the 1.7 nm-sized Pt, 34% for pure Al 2O 3 and 65% for the alkalized-Al 2O 3. NO 2 and CO 2 chemisorption at 250 °C is implemented to determine the saturation uptake value, which is observed with DRIFTS at 250 °C. Pt/Al 2O 3 adsorbs 0.087 μmol CO 2/m 2and 2.0 μmol NO 2/m 2, and Pt/K/Al 2O 3 adsorbs 2.0 μmol CO 2/m 2and 6.4 μmol NO 2/m 2. This method can be implemented to quantitatively monitor the formation of carboxylates and nitrates on Pt/K/Al 2O 3 during both lean and rich periods of the NO x adsorber catalyst cycle. 相似文献
16.
Stoichiometric polycrystalline samples of Mn xMg 1−xFe 2O 4 (0·5 ≤ x ≤ 0·66) have been synthesized by following a novel route using stabilized MnO and Fe 2O 3 at high temperatures. This route precludes the formation of large amounts of Mn 3+ and Fe 2+ and precipitation of MgO and -Fe 2O 3 which are generally observed during the usual route of preparation by conventional ceramic techniques. These samples have been characterized for their structural and magnetic properties using X-ray diffraction, Fe 57 Mössbauer spectroscopy and bulk magnetic properties such as initial permeability, loss factor, ferromagnetic transition temperature, remanance and coercivity. For X = 0·62, these ferrites exhibit the highest remanance ratio 0·96, suitable for square loop applications. 相似文献
17.
A novel catalyst based on copper-silver was developed to solve the contradiction between the high conversion temperature of Cu-based catalyst and low N 2 selectivity of Ag-based catalyst during selective oxidation of ammonium gas. The Cu-Ag-based catalyst (Cu 5 wt.%-Ag 5 wt.%/Al 2O 3) displayed a relatively low complete conversion temperature (<320 °C) with a high N 2 selectivity (>95%). Increasing loading of Cu and Ag decreases N 2 selectivity. The low N 2 selectivity of Ag-based catalyst is possibly related to the formation of Ag 2O crystals. Improvement of N 2 selectivity of Ag-based catalyst was obtained by doping Cu to decrease crystallized Ag 2O phase. The temperature programmed reaction (TPR) data show that N 2O is the main byproduct of oxidation of ammonia at temperature lower than 200 °C. Two bands of nitrate species at 1541 and 1302 cm −1 were observed on Ag 10 wt.%/Al 2O 3 at the temperature higher than 250 °C, which indicates the formation of NO x during the selective catalytic oxidation of ammonia. No nitrate species was observed on Cu 10 wt.%/Al 2O 3 and Cu 5 wt.%-Ag 5 wt.%/Al 2O 3, while only one nitrate species (1543 cm −1) existed on Cu 10 wt.%-Ag 10 wt.%/Al 2O 3. We proposed that mixing Ag with Cu inhibited the formation of NO x during the selective catalytic oxidation of ammonia over Cu-Ag/Al 2O 3. 相似文献
18.
In situ growth of needlelike LaAl 11O 18 grains reinforcing Al 2O 3 composites can be fabricated by a coprecipitation method using La(NO 3) 3√6H 2O and Al(NO 3) 3√9H 2O as starting materials. The new two-step process involved firstly preparing needlelike LaAl 11O 18 grains distributed homogeneously in Al 2O 3 powder and then pressureless sintering the composite powders. The Al 2O 3/25 vol.%LaAl 11O 18 samples pressureless sintered at 1550°C for 4 h achieve relative density up to 96.5% and exhibit a bending strength of 420±30 MPa and a fracture toughness of 4.3±0.4 MPa m 1/2. 相似文献
19.
Powders of pure and 5% ytterbium substituted strontium cerate (SrCeO 3/SrCe 0.95Yb 0.05O 3−δ) were prepared by spray pyrolysis of nitrate salt solutions. The powders were single phase after calcination in nitrogen atmosphere at 1100 °C (SrCeO 3) and 1200 °C (SrCe 0.95Yb 0.05O 3−δ). Dense SrCeO 3 and SrCe 0.95Yb 0.05O 3−δ materials were obtained by sintering at 1350–1400 °C in air. Heat treatment at 850 and 1000 °C, respectively, was necessary prior to sintering to obtain high density. The dense materials had homogenous microstructures with grain size in the range 6–10 μm for SrCeO 3 and 1–2 μm for SrCe 0.95Yb 0.05O 3−δ. The electrical conductivity of SrCe 0.95Yb 0.05O 3−δ was in good agreement with reported data, showing mixed ionic–electronic conduction. The ionic contribution was dominated by protons below 1000 °C and the proton conductivity reached a maximum of 0.005 S/cm above 900 °C. In oxidizing atmosphere the p-type electronic conduction was dominating above 700 °C, while the contribution from n-type electronic conduction only was significant above 1000 °C in reducing atmosphere. 相似文献
20.
Both NO decomposition and NO reduction by CH 4 over 4%Sr/La 2O 3 in the absence and presence of O 2 were examined between 773 and 973 K, and N 2O decomposition was also studied. The presence of CH 4 greatly increased the conversion of NO to N 2 and this activity was further enhanced by co-fed O 2. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH 4 in the absence of O 2, and reduction with 1% O 2 in the feed were 8.3·10 −4, 4.6·10 −3, and 1.3·10 −2 μmol N 2/s m 2, respectively. This oxygen-enhanced activity for NO reduction is attributed to the formation of methyl (and/or methylene) species on the oxide surface. NO decomposition on this catalyst occurred with an activation energy of 28 kcal/mol and the reaction order at 923 K with respect to NO was 1.1. The rate of N 2 formation by decomposition was inhibited by O 2 in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH 4 continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O 2 with equal activation energies of 26 kcal/mol. The addition of O 2 increased the reaction order in CH 4 at 923 K from 0.19 to 0.87, while it decreased the reaction order in NO from 0.73 to 0.55. The reaction order in O 2 was 0.26 up to 0.5% O 2 during which time the CH 4 concentration was not decreased significantly. N 2O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10 −4 μmol N 2/s m 2 at 623 K and 1220 ppm N 2O and an activation energy of 24 kcal/mol. The addition of CH 4 inhibits this decomposition reaction. Finally, the use of either CO or H 2 as the reductant (no O 2) produced specific activities at 773 K that were almost 5 times greater than that with CH 4 and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H 2 to reduce NO to N 2 over these REO catalysts. 相似文献
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