Intercrystalline defects

Dybiec Cz. Eddy current method for defecting intercrystalline corrossion Materials XXIII KKBN, Szczyrk 1994. 

Despite improvement to synthesis methods used it is known that all membranes possess unavoidable defects due to intercrystalline porosity, which are formed during membrane growth [7, 8]. The goal of many characterization techniques is to determine the size and concentration of such defects and evaluate how their presence affects membrane performance. 

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

Other factors are temperature, ionic strength, rate of flow of corrosive fluid, etc. At pH >11 corrosion rate decreases due to the formation of a protective film of ferric hydroxide/oxide. Although general corrosion rate decreases above this pH, the metal becomes susceptible to intercrystalline attack (at defects in oxide film) and thus fails due to caustic embrittlement. 

Nevertheless, even for defective zeolite membranes (i.e., presence of cracks and pinholes), higher n-butane/isobutane separation factors [95] could be potentially achieved by intercrystalline adsorption and capdlary condensation. 

The third problem associated with the measurement of diffusivities in zeolite membranes is related to the fact that the membrane is a polycrystalline layer that possesses intercrystalline defects and diffusion also takes place through these random defects. 

The mechanical strength of sintered boron carbide is 15-20 % lower than that of hot-pressed samples this may be owing to intercrystalline defects in the former case, and transcrystalline ones in the latter case. 

Figure 11.3a and b shows, respectively, the XRD spectra of a supported NaA zeolite membrane and of the resulting powder after filtering and drying the liquid phase. SEM observations allow the evaluation of membrane thicknesses, shape, orientation and size of the crystals, homogeneity and uniformity of the zeolite layer, and a morphological analysis on the existence of intercrystalline defects. SEM-energy-dispersive x-ray analysis (EDX) can be used to measure qualitatively and quantitatively the atomic composition of... 

The lack of methods for a fast and reliable assessment of membrane quality is still one of the outstanding issues in zeolite-membrane development. The usual meaning of the term quality relates to the ability of the membrane to carry out a given separation with a reasonable flux therefore, a system-specific property and a universal membrane quality test do not exist. In general, specihc permeation measurements at different temperatures, either of single gases (or vapors) or of multicomponent mixtures in the gas or liquid (pervaporation) phase, provide extremely useful information on the effective pore structure of the membrane, on the existence of intercrystalline defects, and amorphous material and permeation fluxes, as well as information about the main transport controlling effect (adsorption or diffusion). 

The current commercial zeolite membranes, developed for pervaporation, are not yet useful in gas separations (H2/CO2 selectivity for NaA membranes of Mitsui and Inocermic are 6 and 5.6, respectively) because of the presence of large inter-crystalline defects. They, furthermore, participate in the separation process. During pervaporation the water fills the intra-crystalline and intercrystalline pathways. However, much effort is in progress to produce defect free zeolitic membrane also for gas separations. In this chapter the application of zeolite membranes in gas separations is reported and deeply discussed. The main strategic methods used for the membrane preparation and mass transport through zeolite membranes are also dealt with. 

Mordenite membranes were prepared by seeded hydrothermal synthesis onto commercial ceramic tubular supports by Casado et al. (2003) and nsed for the PV of alcohol-water mixtures. It was reported by them that selective adsorption of water on zeolite pores and small intercrystalline defects controlled the separation mechanism in the mordenite. 

It is known that zeolite membranes essentially contain intercrystalline non-zeolitic pores (defects). This irregular nature of zeolite membranes with intercrystalline pores adds to the complexity of the transport process in addition to the contribution of a support layer to the permeation resistance. For zeolite membranes, selectivity similar to that expected for Knudsen flow generally indicates the presence of intercrystalline pores. Separation based primarily on adsorption differences, which is generally true in the separation of liquid mixtures by pervaporation, may have tolerance to the intercrystalline pores. However, in order to obtain high perm-selectivity, the zeolite membranes must have negligible amounts of intercrystalline pores and pinholes of larger than 2nm so as to reduce the gas flux from these defects [3]. 

Knudsen diffusion can be applied to describe transport in the intercrystalline non-zeolitic pores or defects [9]. If the intercrystalline... 

Zeolite membranes generally have a thickness of 2-50 pm, formed on porous materials such as AI2O3 and stainless steel tubes or disks owing to their greater structural stability and reduced mass-transfer resistance. The quality of zeolite membranes in terms of permeability and selectivity is determined by their intercrystalline porosity (defects), the crystal orientation relative to the membrane layer, the size of zeolite crystals, and the thickness and uniformity of the zeolite layer. 

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