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1.
Single crystals of phenacite (Be 2SiO 4), bromellite (BeO), and tridymite (SiO 2) were grown from an Li 2MoO 4-MoO 3 flux. Phenacite, with rhombohedral symmetry, grew in three distinct shapes with aspect ratios (length/width) as follows: needles (>3), rods (>1.1 to 1.5), and rhombohedral-faced crystals (=1). The latter grew as single crystals; the others were twinned on the . For most experiments the temperature was held constant at 1165°C and the Li 2MoO 4/MoO 3 ratio at 1/16. The growth mechanism for crystallization was the evaporation of MoO 3. The system produced one to three phases, depending on the BeO/SiO 2 ratio. Bromellite grew until a BeO/SiO 2 ratio of 0.8 was attained. It grew as a hemipyramidal crystal having a short prism with a curved top or as a hexagonal plate. The pyramid- and prism-shaped crystals were twinned, although a few hexagonal plates were single. Tridymite grew in small hexagonal plates when the BeO/SiO 2 ratio was less than 1.5. The effect of temperature, nucleation, and flux composition on crystal shape, twinning, and occurrence is discussed. 相似文献
2.
A new proton-conductive composite of NH 4PO 3–(NH 4) 2Mn(PO 3) 4 was synthesized and characterized as a potential electrolyte for intermediate temperature fuel cells that operated around 250 °C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH 4) 2Mn(PO 3) 4 was stable as a supporting matrix for NH 4PO 3. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH 4PO 3 in both dry and wet atmospheres. A conductivity of 7 mS cm −1 was obtained at 250 °C in wet hydrogen. Electromotive forces measured by hydrogen concentration cells showed that the composite was nearly a pure protonic conductor with hydrogen partial pressure in the range of 10 2–10 5 Pa. The proton transference number was determined to be 0.95 at 250 °C for 2NH 4PO 3–(NH 4) 2Mn(PO 3) 4 electrolyte. Fuel cells using 2NH 4PO 3–(NH 4) 2Mn(PO 3) 4 as an electrolyte and the Pt–C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm 2 was achieved at 250 °C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH 4PO 3–(NH 4) 2Mn(PO 3) 4 electrolyte is not compatible with the Pt–C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel cells. 相似文献
3.
Co 3O 4/CeO 2 composite oxides with different cobalt loading (5, 15, 30, 50, 70 wt.% as Co 3O 4) were prepared by co-precipitation method and investigated for the oxidation of methane under stoichiometric conditions. Pure oxides, Co 3O 4 and CeO 2 were used as reference. Characterization studies by X-ray diffraction (XRD), BET, temperature programmed reduction/oxidation (TPR/TPO) and X-ray photoelectron spectroscopy (XPS) were carried out. An improvement of the catalytic activity and thermal stability of the composite oxides was observed with respect to pure Co3O4 in correspondence of Co3O4–CeO2 containing 30% by weight of Co3O4. The combined effect of cobalt oxide and ceria, at this composition, strongly influences the morphological and redox properties of the composite oxides, by dispersing the Co3O4 phase and promoting the efficiency of the Co3+–Co2+ redox couple. The presence in the sample Co3O4(30 wt.%)–CeO2 of a high relative amount of Ce3+/(Ce4+ + Ce3+) as detected by XPS confirms the enhanced oxygen mobility. The catalysts stability under reaction conditions was investigated by XRD and XPS analysis of the used samples, paying particular attention to the Co3O4 phase decomposition. Methane oxidation tests were performed over fresh (as prepared) and thermal aged samples (after ageing at 750 °C for 7 h, in furnace). The resistance to water vapour poisoning was evaluated for pure Co3O4 and Co3O4(30 wt.%)–CeO2, performing the tests in the presence of 5 vol.% H2O. A methane oxidation test upon hydrothermal ageing (flowing at 600 °C for 16 h a mixture 5 vol.% H2O + 5 vol.%O2 in He) of the Co3O4(30 wt.%)–CeO2 sample was also performed. All the results confirm the superiority of this composite oxide. 相似文献
4.
The influence of catalyst pre-treatment temperature (650 and 750 °C) and oxygen concentration ( λ = 8 and 1) on the light-off temperature of methane combustion has been investigated over two composite oxides, Co 3O 4/CeO 2 and Co 3O 4/CeO 2–ZrO 2 containing 30 wt.% of Co 3O 4. The catalytic materials prepared by the co-precipitation method were calcined at 650 °C for 5 h (fresh samples); a portion of them was further treated at 750 °C for 7 h, in a furnace in static air (aged samples). Tests of methane combustion were carried out on fresh and aged catalysts at two different WHSV values (12 000 and 60 000 mL g−1 h−1). The catalytic performance of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 were compared with those of two pure Co3O4 oxides, a sample obtained by the precipitation method and a commercial reference. Characterization studies by X-ray diffraction (XRD), BET and temperature-programmed reduction (TPR) show that the catalytic activity is related to the dispersion of crystalline phases, Co3O4/CeO2 and Co3O4/CeO2–ZrO2 as well as to their reducibility. Particular attention was paid to the thermal stability of the Co3O4 phase in the temperature range of 750–800 °C, in both static (in a furnace) and dynamic conditions (continuous flow). The results indicate that the thermal stability of the phase Co3O4 heated up to 800 °C depends on the size of the cobalt oxide crystallites (fresh or aged samples) and on the oxygen content (excess λ = 8, stoichiometric λ = 1) in the reaction mixture. A stabilizing effect due to the presence of ceria or ceria–zirconia against Co3O4 decomposition into CoO was observed. Moreover, the role of ceria and ceria–zirconia is to maintain a good combustion activity of the cobalt composite oxides by dispersing the active phase Co3O4 and by promoting the reduction at low temperature. 相似文献
5.
Silicoaluminophosphate (SAPO) membranes with Si/Al gel ratios from 0.05 to 0.3 were synthesized by in situ crystallization onto porous, tubular stainless steel support. Pure SAPO-34 membranes were obtained when the Si/Al ratio was 0.15 or higher. The adsorbate polarizability correlated with the adsorption capacity on SAPO-34, and the amounts of gases adsorbed were in the order: CO 2 > CH 4 > N 2 > H 2. The Si/Al ratio did not affect the pore volume significantly, but it changed the CO 2 and CH 4 adsorption equilibrium constants. The SAPO-34 membranes effectively separated CO 2 from CH 4 for feed pressures up to 7 MPa. At 295 K, for a pressure drop of 138 kPa and a 50/50 feed, the CO 2/CH 4 selectivity was 170 for a membrane with a Si/Al gel ratio of 0.15. At 7 MPa, the CO 2/CH 4 selectivity was 100 and the CO 2 permeance was 4 × 10 −8 mol/(m 2 · s · Pa) at 295 K. This membrane was also separated CO 2/N 2 (selectivity = 21) and H 2/CH 4 (selectivity = 32) mixtures at 295 K and a pressure drop of 138 kPa. Competitive adsorption and difference in diffusivities are responsible for CO 2/CH 4 and CO 2/N 2 separations, whereas the H 2/CH 4 separation was due to diffusivity differences. For a membrane with Si/Al gel ratio of 0.1, a mixture of SAPO-34 and SAPO-5 formed, and the CO 2/CH 4 selectivity was lower. 相似文献
6.
(Mg 1−xZn x) 2SiO 4 ceramics were prepared and characterized. The densification temperatures of the present ceramics are much lower than those for Mg 2SiO 4 and Zn 2SiO 4 end-members. Small solid solution limits of Zn in Mg 2SiO 4 and Mg in Zn 2SiO 4 are observed, and the bi-phase structure is confirmed in (Mg 1−xZn x) 2SiO 4 ceramics with x = 0.1–0.9. Even though, it is clear that the Qf value of Zn 2SiO 4 ceramics can be significantly improved together with a suppressed temperature coefficient of resonant frequency τf by substituting Mg for Zn. (Mg 0.4Zn 0.6) 2SiO 4 ceramics indicate a good combination of microwave dielectric characteristics: r = 6.6 Qf = 95,650 GHz, and τf = −60 ppm/°C. 相似文献
7.
The NiSO 4 supported on Fe 2O 3-promoted ZrO 2 catalysts were prepared by the impregnation method. Fe 2O 3-promoted ZrO 2 was prepared by the coprecipitation method using a mixed aqueous solution of zirconium oxychloride and iron nitrate solution followed by adding an aqueous ammonia solution. No diffraction line of nickel sulfate was observed up to 20 wt.%, indicating good dispersion of nickel sulfate on the surface of Fe 2O 3–ZrO 2. The addition of nickel sulfate (or Fe 2O 3) to ZrO 2 shifted the phase transition of ZrO 2 (from amorphous to tetragonal) to higher temperatures because of the interaction between nickel sulfate (or Fe 2O 3) and ZrO 2. 15-NiSO 4/5-Fe 2O 3–ZrO 2 containing 15 wt.% NiSO 4 and 5 mol% Fe 2O 3, and calcined at 500 °C exhibited a maximum catalytic activity for ethylene dimerization. NiSO 4/Fe 2O 3–ZrO 2 catalysts was very effective for ethylene dimerization even at room temperature, but Fe 2O 3–ZrO 2 without NiSO 4 did not exhibit any catalytic activity at all. The catalytic activities were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The addition of Fe 2O 3 up to 5 mol% enhanced the acidity, surface area, thermal property, and catalytic activities of catalysts gradually, due to the interaction between Fe 2O 3 and ZrO 2 and due to consequent formation of Fe–O–Zr bond. 相似文献
8.
CO and CH 4 combined oxidation tests were performed over a Pd (70 g/ft 3)/Co 3O 4 monolithic catalyst in conditions of GHSV = 100,000 h −1 and feed composition close to that of emission from bi-fuel vehicles. The effect of SO 2 (5 ppm) on CO and CH 4 oxidation activity under lean condition ( λ = 2) was investigated. The presence of sulphur strongly deactivated the catalyst towards methane oxidation, while the poisoning effect was less drastic in the oxidation of CO. Saturation of the Pd/Co 3O 4 catalytic sites via chemisorbed SO 3 and/or sulphates occurred upon exposure to SO 2. A treatment of regeneration to remove sulphate species was attempted by performing a heating/cooling cycle up to 900 °C in oxidizing atmosphere. Decomposition of PdO and Co 3O 4 phases at high temperature, above 750 °C, was observed. Moreover, sintering of Pd 0 and PdO particles along with of CoO crystallites takes place. 相似文献
9.
The synthesis conditions of SiC ultrafine particles from SiH 4 and C 2H 4 using a CO 2 laser were studied and a comparison was made between SiH 4---C 2H 4 and SiH 2Cl 2---C 2H 4 systems. Ultrafine SiC particles were synthesized by irradiating a SiH 4 and C 2H 4 gas mixture with a CO 2 laser at atmospheric pressure. SiC particles were obtained at a laser power of more than 0·92 kW/cm 2. The behavior of the reaction flame temperature and the extent of the laser light absorption by SiH4---C2H4 was different from that of SiH2Cl2---C2H4, although an abrupt temperature increase was observed in both cases. In the case of SiH4---C2H4 an abrupt increase in the laser light absorption was not observed, whilst it was observed in the case of SiH2Cl2---C2H4. This difference resulted from the difference in liability to form solid carbon particles. 相似文献
10.
Coupled semiconductor (CS) Cu/CdS–TiO 2/SiO 2 photocatalyst was prepared using a mutli-step impregnation method. Its optical property was characterized by UV–vis spectra. BET, XRD, Raman and IR were used to study the structure of the photocatalyst. Fine CdS was found dispersed over the surface of anatase TiO 2/SiO 2 substrate. Chemisorption and IR analysis showed methane absorbed in the molecular state interacted weakly with the surface of catalyst, and the interaction of CO 2 with CS produced various forms of absorbed CO 2 species that were primarily present in the form of formate, bidentate and linear absorption species. Photocatalytic direct conversion of CH 4 and CO 2 was performed under the operation conditions: 373 K, 1:1 of CO 2/CH 4, 1 atm, space velocity of 200 h −1 and UV intensity of 20.0 mW/cm 2. The conversion was 1.47% for CH 4 and 0.74% for CO 2 with a selectivity of acetone up to 92.3%. The reaction mechanisms were proposed based on the experimental observations. 相似文献
11.
LuLiF 4 single crystals codoped with thulium (Tm) and holmium (Ho) were successfully grown by the Czochralski method. The excellent laser performance, in terms of highest slope efficiency, and highest optical-to-optical efficiency and lower laser threshold, was studied in comparison with 5% Tm, 0.5% Ho-codoped YLiF 4, and Lu 3Al 5O 12 under the same conditions. 相似文献
12.
Conversion of NO x with reducing agents H 2, CO and CH 4, with and without O 2, H 2O, and CO 2 were studied with catalysts based on MOR zeolite loaded with palladium and cerium. The catalysts reached high NO x to N 2 conversion with H 2 and CO (>90% conversion and N 2 selectivity) range under lean conditions. The formation of N 2O is absent in the presence of both H 2 and CO together with oxygen in the feed, which will be the case in lean engine exhaust. PdMOR shows synergic co-operation between H 2 and CO at 450–500 K. The positive effect of cerium is significant in the case of H 2 and CH 4 reducing agent but is less obvious with H 2/CO mixture and under lean conditions. Cerium lowers the reducibility of Pd species in the zeolite micropores. The catalysts showed excellent stability at temperatures up to 673 K in a feed with 2500 ppm CH 4, 500 ppm NO, 5% O 2, 10% H 2O (0–1% H 2), N 2 balance but deactivation is noticed at higher temperatures. Combining results of the present study with those of previous studies it shows that the PdMOR-based catalysts are good catalysts for NO x reduction with H 2, CO, hydrocarbons, alcohols and aldehydes under lean conditions at temperatures up to 673 K. 相似文献
13.
Tetrahydroborate sodalite formation was investigated in the system Na 2O–SiO 2–Al 2O 3–NaBH 4–H 2O under mild hydrothermal conditions. Due to the high degree of decomposition of hydroborates in aqueous solutions synthesis conditions were tuned by variation of the parameters alkalinity, liquid/solid ratio, reaction temperature and reaction time. The insertion of 8–16 molar NaOH solution was crucial for the higher stability of pure tetrahydroborate salt under strong alkaline conditions. Synthesis at 393 K and 24 h reaction time reveal tetrahydroborate sodalite Na 8[AlSiO 4] 6(BH 4) 2 beside a small amount of amorphous material within the total batch. Structure, composition and thermal stability of this new sodalite was investigated using XRD, NMR, infrared and TG/DTA methods. The crystal structure of tetrahydroborate sodalite has been refined in the space group P-43n with a = 891.61(2) pm. The Si- and Al-atoms of the aluminosilicate framework are completely ordered. The boron atoms of the tetrahydroborate anions are located at the centre of the sodalite cage whereas the hydrogen atoms are positionally disordered. Na 8[AlSiO 4] 6(BH 4) 2 shows a high stability under inert gas conditions. At atmospheric conditions the group can be oxidized to borate and boroxide anions suggesting the formation of hydrogen which leaves the sodalite cages. Future investigation of reloading properties of the oxidized form could be highly interesting for the hydrogen storage capabilities of these sodalites. 相似文献
14.
LaPO 4/Al 2O 3 composites were fabricated by spark plasma sintering. The effects of LaPO 4 contents on the mechanical properties of the composites were investigated. The bending strength and fracture toughness can reach the maximum value of 568.2±30 MPa and 4.8±0.5 MPa·m 1/2 for the composite with 16.4 vol% LaPO 4 addition, respectively. The elastic moduli and hardness of the composites decreased with increasing LaPO 4 content. Furthermore, the experimental results show that the composites can be machined by a tungsten carbide drill as the LaPO 4 volume fraction is higher than 34.4 vol%. 相似文献
15.
Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr 2(PO 4) 3─KTi 2(PO 4) 3 (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10 −6/°C with increasing x in KZr 2 − xTi x (PO 4) 3 (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface. 相似文献
16.
Submicronic CoAl 2O 4 powders were prepared by double decomposition reaction between solid LiAlO 2 and molten KCoCl 3 at 500 °C for 24 h. The reaction mechanism involves the dissolution of LiAlO 2 shifted by the precipitation of CoAl 2O 4 until complete transformation and the reaction leads to powders with a very homogeneous chemical composition. The powders obtained were mainly characterized by XRD, FTIR, ICP, X.EDS, electron microscopy and diffraction and diffuse reflexion. The blue pigments obtained exhibit a high thermic stability allowing their use for the colouring of ceramic tiles. 相似文献
17.
The oxidation of CH 4 over Pt–NiO/δ-Al 2O 3 has been studied in a fluidised bed reactor as part of a major project on an autothermal (combined oxidation–steam reforming) system for CH 4 conversion. The kinetic data were collected between 773 and 893 K and 101 kPa total pressure using CH 4 and O 2 compositions of 10–35% and 8–30%, respectively. Rate–temperature data were also obtained over alumina-supported monometallic catalysts, Pt and NiO. The bimetallic Pt–NiO system has a lower activation energy (80.8 kJ mol −1) than either Pt (86.45 kJ mol −1) and NiO (103.73 kJ mol −1). The superior performance of the bimetallic catalyst was attributed to chemical synergy. The reaction rate over the Pt–NiO catalyst increased monotonically with CH 4 partial pressure but was inhibited by O 2. At low partial pressures (<30 kPa), H 2O has a detrimental effect on CH 4 conversion, whilst above 30 kPa, the rate increased dramatically with water content. 相似文献
18.
A novel sol–gel method based on V 2O 5· nH 2O hydro-gel was developed to synthesize nanocrystalline Li 3V 2(PO 4) 3/carbon composite material. In this route, V 2O 5· nH 2O hydro-gel, NH 4H 2PO 4, Li 2CO 3 and high-surface-area carbon were used as starting materials to prepare precursor, and the Li 3V 2(PO 4) 3/carbon was obtained by sintering precursor at 750 °C for 4 h in flowing argon. The sol–gel synthesis ensures homogeneity of the precursors and improved reactivity. The sample was characterized by XRD, SEM and TEM. X-ray diffraction results show Li 3V 2(PO 4) 3 sample is monoclinic structure with the space group of P2 1/ n. The TEM image indicates that the Li 3V 2(PO 4) 3 particles modified by conductive carbon are about 70 nm in diameter. The Li 3V 2(PO 4) 3/carbon system showed that the discharge capacities in the first and 50th cycle are about 155.3 and 143.6 mAh/g, respectively, in the range of 3.0–4.8 V. The sol–gel method is fit for the preparation of Li 3V 2(PO 4) 3/carbon composite material which may offer some favorable properties for commercial application. 相似文献
19.
YTaO 4:Eu 3+ powders were synthesized by a flux method with LiCl. The use of a flux in the synthesis of M′–YTaO 4 facilitated the reaction of raw materials, leading to lowering the heating temperature, but not effective at the high firing temperature. The red emission peaks were observed around 613 nm with an excitation wavelength of 254 nm. Emission peaks were composed of two sets around 613 nm and 590 nm, which originated from 5D 0– 7F 2 and 5D 0– 7F 1, respectively. PL intensity of YTaO 4:Eu 3+ prepared with a small amount of LiCl (10 wt%) significantly depended on the firing temperatures, while that with a larger amount of LiCl (40 wt%) slightly relied on firing temperatures. The highest PL intensity could be obtained at the firing conditions of 1300 °C and 10 wt% LiCl. 相似文献
20.
Homogeneous-eutectic microstructure of Y 3Al 5O 12–Al 2O 3 system without coarse primary crystals was formed at an off-eutectic composition. This method utilizes a low migration rate in an amorphous phase. A mixture of Y 2O 3 and Al 2O 3 having the off-eutectic composition was melted and quenched rapidly to form an amorphous phase. A heat-treatment of the amorphous phase at 1000 °C and 1300 °C for 30 min formed Y 3Al 5O 12 and Al 2O 3 phases. SEM observation of this material, which was formed from the amorphous phase at 1300 °C for 30 min, showed homogeneous eutectic-like microstructure. The formation of the primary crystals (coarse Al 2O 3), which are always observed in the off-eutectic compositions by ordinary method, was completely suppressed. 相似文献
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