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Diffusive sintering diffusion path

While microscopic techniques like PFG NMR and QENS measure diffusion paths that are no longer than dimensions of individual crystallites, macroscopic measurements like zero length column (ZLC) and Fourrier Transform infrared (FTIR) cover beds of zeolite crystals [18, 23]. In the case of the popular ZLC technique, desorption rate is measured from a small sample (thin layer, placed between two porous sinter discs) of previously equilibrated adsorbent subjected to a step change in the partial pressure of the sorbate. The slope of the semi-log plot of sorbate concentration versus time under an inert carrier stream then gives D/R. Provided micropore resistance dominates all other mass transfer resistances, D becomes equal to intracrystalline diffusivity while R is the crystal radius. It has been reported that the presence of other mass transfer resistances have been the most common cause of the discrepancies among intracrystaUine diffusivities measured by various techniques [18]. [Pg.419]

Note y = surface energy, D = volume diffusivity, Ds = surface diffusivity, Db = grain boundary diffusivity, ij = viscosity, b = Burgers vector, k = Boltzman s constant, p = density, S = width of grain boundary diffusion path, P = pressure, M = molecular weight, and 2 = atomic volume. Source From R. M. German, Sintering Theory and Practice (New York Wiley, 1996). Reprinted with permission of John Wiley Sons, Inc. [Pg.146]

Other types of diffusive sampler have been less widely applied for indoor air studies. These include tube type samplers that are solvent desorbed and radial type samplers consisting of a cylindrical adsorbent surface that has a short diffusive path resulting in an effective uptake rate that is typically 100 times that of the tube type sampler (Cocheo, Boaretto and Sacco, 1996). One type of radial sampler developed for measuring carbonyl compounds in indoor air comprises silica gel coated with 2,4-DNPH as the adsorbent within a sintered polyethylene tube that acts as a diffusive membrane (Uchiyamaa, Aoyagi and Ando, 2004). [Pg.51]

Despite some positive results of experimental verification of earlier macroscopic--fiow models, diffusion of vacancies is, nowadays, predominantly thought of as the controlling transport mechanism in the sintering of solid oxides. Solution of the problem depends on the determination of diffusion paths. [Pg.141]

Fig, 5.48 depicts schematically the sintering processes that can occur between two spherical particles. The numbers correspond to those in Tab. 5.7. It can be concluded that several mechanisms causing material transport (= diffusion paths) do not produce densification. While sintering which results in densification is well researched (see Section 9.1), diffusion phenomena which do not cause shrinkage and are, therefore, important for the production of porous products are less known. [Pg.96]

Fig. 5.48 Model of the sintering processes that can occur between two particles [B.61]. (a) Depicts schematically bonding. X, Y, a, and p are bridge radius, penetration depth, particle radius, and neck radius, respectively, (b) Is the bonding area in more detail showing the diffusion paths (Tab. 5.7). Fig. 5.48 Model of the sintering processes that can occur between two particles [B.61]. (a) Depicts schematically bonding. X, Y, a, and p are bridge radius, penetration depth, particle radius, and neck radius, respectively, (b) Is the bonding area in more detail showing the diffusion paths (Tab. 5.7).
FIGURE 24.8 Sintering three spheres showing the diffusion paths from the GB to the neck surface and the development of a pore... [Pg.431]

When considering ceramic sintering processes, you should always remember that the oxidation state of the cations may vary depending on the environment. For example, when CeOi is sintered at high temperatures the Ce may be partially reduced to Ce, which can cause cracking in the compact. Such cracking will, of course, change the available diffusion paths. [Pg.433]

The complex behavior described in the preceding section discourages direct tests of sintering models for crystalline materials. Nevertheless, the theory clearly indicates the parameters that must be controlled to get rapid densification. Small particles provide short diffusion paths from the pore to the point... [Pg.370]

Change in diffusion path for sintering when particles sit on a substrate or surround an inclusion. To fill in the neck at B, material must diffuse from the neck at A. From Bordia and Scherer [131]. [Pg.376]

The length scale plays a vital role in heterogeneous catalysis to control the structure, active phase and overall framework of the catalyst system. In the CMR, catalysis by nano-scale materials will improve the physico-chemical properties of the catalytic membrane. A uniform nanostructured catalytic membrane was prepared using AAO and atomic layer deposition techniques and tested in oxidative dehydrogenation of cyclohexane. Nanostructured CMs have excellent properties, such as uniform pores, contact time control, uniform diffusion paths and active site isolation (no sintering), compared... [Pg.409]

The dominant mechanism and transport path—or combinations thereof—depend upon material properties such as the diffusivity spectrum, surface tension, temperature, chemistry, and atmosphere. The dominant mechanism may also change as the microstructure evolves from one sintering stage to another. Sintering maps that indicate dominant kinetic mechanisms for different microstructural scales and environmental conditions are discussed in Section 16.3.5. [Pg.401]

When the width of the channels between loosely bound grains in the sintered body decreases, the vaporization rate decreases and the rate of capture of SnO increases. This will favor the densification of TTO. The diffusion rate of SnO gas molecules is stopped when the width of the effusion channel is smaller than the mean free path of SnO gas. [Pg.136]

In the case of lower specific surface areas, the type of diffusion is not so certain. If the material is made up of particles of known size and if these particles have not been greatly deformed—such as by sintering processes —appraisal is again possible, inasmuch as it can be assumed that the magnitude of the channel dimensions are not far from those of the particles. With the mean-free-path at atmospheric pressure being of the order of 1000 A., a particle size of 10,000 A, = 1 n or more would indicate normal diffusion conditions. Specific pore volume V, and specific surface... [Pg.191]

Whether a pore remains attached to the boundary or becomes separated is vital to microstructural control during sintering. Pores that become separated are difficult to shrink in some cases the matter transport path has been eliminated (e.g., diffusion along the grain boundary), whereas at other times the rate of matter transport is very slow as a result of a large increase in the matter transport path. In general, separated pores remain as residual porosity in the sintered article and must be avoided if high density is a requirement. [Pg.83]

Tab. 5.7 Description of path routes of diffusion during sintering. Tab. 5.7 Description of path routes of diffusion during sintering.

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See also in sourсe #XX -- [ Pg.720 ]




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