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71.
This paper describes the cylindrical agglomerate model for oxygen/alkali gas diffusion electrodes fabricated from platinum, PTFE and porous nickel. Corrections for the increase in hydroxyl ion concentration with increasing current density have been made to the original model of Brown and Horve. Changes in performance by variation of the bulk structural parameters, e.g. agglomerate radius, porosity and tortuosity, have been studied. Theoretical modes of electrode decay have been explored.List of symbols
Transfer coefficient
-
C
Concentration of O2 in elec trolyte mol cm–3
-
C
i
Concentration of O2 atr = R mol cm–3
-
C
o
Concentration of O2 in electrolyte atr = mol cm–3
-
Diffusion coefficient of O2 in KOH cm2 sec–1
-
Film thickness cm
-
E
Overpotential of the electrode V
-
F
Faraday's constant
-
i
Electrode current density A cm–2
-
i
a
Current per agglomerate A
-
I
1(Z)
First order Bessel function
-
I
0(Z)
Zero order Bessel function
-
j
Local current density A cm–2
-
j
o
Exchange current density A cm–2
-
L
Agglomerate length (catalyst thickness) cm
-
N
Number of electrons in rate determining step
-
N
a
Number of agglomerates per cm2 of electrode
-
Potential drop along ag glomerate V
-
L
Potential drop at La V
-
r
Radial direction
-
R
Radius of agglomerate cm
-
R
o
Gas constant
-
Density of platinum g cm–3
-
S
g
Surface area per gram cm2 g–1
-
Solubility coefficient of O2 mol cm–3
- m
Electrolyte conductivity (ohm cm)–1
-
T
Absolute temperature °K
- a
Axial tortuosity
-
Porosity of platinum in the agglomerate
- r
Aadial tortuosity of the agglomerate
-
W
Catalyst loading g cm–2
-
x
Axial direction 相似文献
72.
The indigo method developed by Bader and Hoigné for aqueous ozone analysis was modified to allow for both gaseous and aqueous ozone determination. Gas or water samples were extracted with a gas-tight syringe containing a known volume of indigo reagent. The modified procedure provided a more consistent basis for gaseous and aqueous ozone determination allowing for more accurate ozone mass balance calculations. Direct gaseous ozone UV absorbance with molar absorptivity of 3,000 M?1cm?1 at 258 nm was used as primary standard to determine the molar absorptivity of the indigo reagent. The molar absorptivity of indigo reagent, assuming a 1:1 stoichiometric ratio for the reaction between indigo and ozone, was determined to be 23,150 ± 80 M?1cm?1, or approximately 16 percent higher than that of 20,000 M?1cm?1 suggested by Bader and Hoigné. An independently calibrated membrane-electrode ozone monitor showed good correlation with indigo method results using the molar absorptivity value determined in this study. The apparent molar absorptivity of aqueous ozone at the wavelength of 258 nm measured by the modified indigo method increased from 2,400 to 3,600 M?1cm?1 in the investigated ozone concentration range of 0.4 to 11.0 mg/L. This variation might have been caused by the inherent interference of unidentified ozone byproducts, which presence was supported with scanning spectra in the wavelength range of 200 to 300 nm. 相似文献
73.
Fred J. Parker 《Journal of the American Ceramic Society》1990,73(4):929-932
The characterization and properties of ceramic composites containing the phases Al2 TiO5 , ZrTiO4 , and ZrO2 are described. The range of compositions investigated gives very low average thermal expansions (α24–1000°C as low as −2.0 × 10−6 °C−1 ) and excellent high-temperature stability. The low thermal expansions are apparently due to a combination of microcracking by the titanate phases and a contractive phase transformation by the ZrO2 . The crystal chemistry and microstructure of the product are processing dependent. Although the composites represent a complex microcracking system, the low thermal expansions and high-temperature stability make them potential candidates for commercial applications requiring thermal shock resistance. 相似文献
74.
Effect of Zwitterionic Surfactants on Interparticle Forces, Rheology, and Particle Packing of Silicon Nitride Slurries 总被引:1,自引:0,他引:1
William A. Ducker Erik P. Luther David R. Clarke Fred F. Lange 《Journal of the American Ceramic Society》1997,80(3):575-583
Phosphocholine (PC) zwitterionic surfactants, with different hydrocarbon chain lengths (C6 C6 PC to C9 C9 PC), were absorbed on the surface of silicon nitride near the isoelectric point (pH 6). Adsorption of the surfactants changed the lateral and normal surface forces, the rheology, and the consolidation behavior of the particles. The normal force between two silicon nitride surfaces as a function of separation and the lateral (friction) forces were measured using an atomic force microscope (AFM). These measurements indicated that surfactant adsorption reduced the magnitude of the long-range attractive van der Waals force and produced a repulsive short-range force. Although the adsorbed layers provided a barrier to particle contact, they could be ejected with a critical force that increased with the hydrocarbon chain length. The effect of an adsorbed layer on the viscosity and consolidation of slurries was also measured. The viscosity of all slurries decreased with increasing shear rate, indicative of attractive particle networks. The highest viscosity was observed for slurries formulated at the isoelectric point without added surfactant. Much lower viscosities were observed when the surfactant concentration was greater than the critical micelle concentration (cmc). A relative density of 0.46 was obtained via pressure filtration at 4 MPa without a surfactant, and between 0.46 to 0.59 (C6 C6 PC to C9 C9 PC, respectively) for surfactant concentrations greater than the cmc. Comparing force measurements with rheology and packing density provides a basis for discussing the role of interparticle forces in ceramic powder processing via colloidal routes. 相似文献
75.
An efficient, reliable and easily programmed method for solving systems of equilibrium stage separation devices is described. The method is an extension of an algorithm developed by Ishii and Otto(9) for single columns. It solves all equations simultaneously, takes advantage of the basic tridiagonal matrix form form of the linearized steady state equations and can handle configurations which include distillation columns, absorbers and reboiled absorbers. The solution for a thermally coupled distillation system is presented. 相似文献
76.
Timothy J. Bunning Hong G. Jeon Ajit K. Roy Kristen M. Kearns Barry L. Farmer Walter W. Adams 《应用聚合物科学杂志》2003,87(14):2348-2355
The thermal and mechanical properties of polyurethane‐infiltrated carbon foam of various densities were investigated. By combining the high thermal conductivity of the carbon foam with the mechanical toughness of the pure polyurethane, a mechanically tough composite (relative to the unfilled foam) that could be used at higher temperatures than the polyurethane's degradation was formed. Both the tensile strength and the modulus increased by an order of magnitude for the composites compared to unfilled foam, while the compressive and shear strengths and moduli of the composites approached values exhibited by pure polyurethane. At both 300 and 400°C, the rectangular blocks of pure polyurethane lost their mechanical integrity due to decomposition in air. Thermogravimetric analysis confirms substantial initial weight loss above 290°C. Filled carbon foam blocks, however, maintain their mechanical integrity at both 300 and 400°C indefinitely, although the bulk of the rectangular block mass is polyurethane. Three different carbon foam densities are examined. As expected, the higher density foams show greater heat transfer. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2348–2355, 2003 相似文献
77.
Erik E Luther Fred F. Lange Dale S. Pearson 《Journal of the American Ceramic Society》1995,78(8):2009-2014
Two different methods are used to coat silicon nitride particles with an alumina precursor to make Si3 N4 behave like Al2 O3 in aqueous slurries. The first method involves the precipitation of an aluminum hydroxycarbonate from dissolved Al(NO3 )3 during the decomposition of urea. In the second method, dry silicon nitride powder is reacted with aluminum tri- sec -butoxide in hexane at room temperature. Both methods produce a coated powder in which the electrophoretic and rheological properties of aqueous slurries mimic those of alumina. When salt is added to slurries consisting of coated Si3 N4 powder, all rheological evidence suggests the presence of a short-range repulsive potential that produces a weakly attractive particle network similar to that previously reported for Al2 O3 powder. Although electrophoretic and rheological data showed that the coated powder behaved like Al2 O3 , consolidation data indicated that slurries of coated powder with added salt did not pack to high density. In addition, these bodies were not plastic as found for bodies consolidated from dispersed and salt-added Al2 O3 slurries. 相似文献
78.
Processing, Microstructure, and Wear Behavior of Silicon Nitride Hot-Pressed with Alumina and Yttria 总被引:3,自引:0,他引:3
Ajoy Zutshi Richard A. Haber Dale E. Niesz Jane W. Adams John B. Wachtman Mattison K. Ferber Stephen M. Hsu 《Journal of the American Ceramic Society》1994,77(4):883-890
Commercial silicon nitride powder with A12 O3 and Y2 O3 additives was hot-pressed to complete density. The resulting microstructure contained elongated grains with no trace of remaining α-Si3 N4 . The aspect ratio of the elongated grains increased with increasing soak time at a fixed hot-pressing temperature. X-ray diffraction analysis showed that the crystalline phase in the hot-pressed samples was β-sialon (Si6−z Alz Oz N8−z ) with z values that increased with soak time. The fracture strength and fracture toughness of the samples increased as the aspect ratio of the grains increased. The Vickers hardness decreased slightly as the soak time was increased, which was attributed to a grain size effect. Wear tests of silicon nitride against silicon nitride were conducted on a reciprocating pin-on-disk apparatus with paraffin oil as a lubricant. Correlation studies of wear with microstructure and mechanical properties were performed. The wear rate increased rapidly with increasing soak time in spite of the increased strength and toughness. This was attributed to increased third-body wear caused by pullout of pieces from the wear surface. The pullout mechanism was not conclusively identified. However, TEM examination showed clear evidence of dislocation motion under the wear scar. Grain boundary microstresses caused by the anisotropic thermal expansion and elastic properties of the elongated grains may have contributed to the observed pullout. 相似文献
79.
Mari Lou Balmer Fred F. Lange Carlos G. Levi 《Journal of the American Ceramic Society》1994,77(8):2069-2075
Aqueous solutions of zirconium acetate and aluminum nitrate were spray pyrolyzed at 250°C and upquenched to different temperatures to yield metastable solid solutions of composition Zr(1− x ) Alx O(2− x /2) . An amorphous oxide forms first during pyrolysis which subsequently crystallizes as a single phase for x ≤ 0.57 (≤40 mol% Al2 O3 ). The crystallization temperature increased with Al2 O3 content. Electron diffraction, supported by Raman spectroscopy, indicates that the initial phase is tetragonal. At higher temperatures, the initial solid solation partitions to other metastable phases, viz., t -ZrO2 +γ-Al2 O3 , prior to achieving their equilibrium phase assemblage, m -ZrO2 +α-Al2 O3 . Partitioning yields a nanocomposite microstructure with grain sizes of 20–100 nm, compared to the 3 to 5 nm in the initial, single phase. Compositions containing 45 to 50 mol% Al2 O3 concurrently crystallize and partition. The structure selected during crystallization and the partitioning phenomena are discussed in terms of diffusional constraints during crystallization, which are conceptually similar to those operating during rapid solidification. 相似文献
80.
The fluid sheet thickness and velocity at the tip of a spray nozzle have previously been identified as being important to the ultimate droplet size distribution. In this investigation both the fluid sheet thickness and the velocity of the top surface just beyond the rim of a splashplate nozzle have been measured as functions of nozzle exit velocity and fluid viscosity. Measurements were made on two Babcock & Wilcox Co. splashplate nozzles, a 12-49 and a 15-52, specifically designed for black liquor, a by-product of wood pulping. Comparisons of the ratio of the sheet velocity to the nozzle velocity are shown to be very similar to previous data for the ratio of droplet velocity to nozzle velocity. Likewise, the dependence of the sheet thickness on nozzle exit velocity closely matches the previously measured dependence of the ultimate median droplet size on nozzle exit velocity. Both of these results are consistent with current theories of droplet formation mechanisms. Analysis of continuity and viscous drag on the splashplate are used to develop correlation equations for the data 相似文献