Unidirectional freezing

Other semi-2-IPNs [52] processed by the freeze-drying method included IPN from 4,4 -bismaleimido diphenylmethane (BMI) and linear BTDA/3,4 -ODA polyamic acid that were dissolved in 1,3,5-trioxane (Fig. 16). The resulting semi-2-IPNs exhibited higher Tgs and reduced phase separation, and contained no plasticizing solvent. A comparison of unidirectional properties of composites prepared by the freeze-dry process to those by traditional solvent evaporation process is presented in Table 13. The freeze-drying method for the preparation of IPNs appears to be superior to previous technology. 

Table 3.10 Mechanical properties of freeze-gelled, unidirectional carbon-fibre-reinforced CMC made by hand lay-up73...

T. Fukasawa, Z, Y, Deng, and M. Ando, Synthesis of Porous Silicon Nitride with Unidirectionally Aligned Channels Using Freeze-drying Process, J. Am. Ceram. Soc., 85 [9] 2151-2155 (2002),... 

Honeycomb structures can be susceptible to water intrusion, which may affect, for e. am-ple. the weight and balance of an aircraft, and in the long term induce corrosion. During winter, or at high altitude in the case of aircraft, trapped water in such structures may freeze, and the subsequent expansion of ice causes cracks, breakage of the honeyeomb cells, and disbonds. Moisture absorption in unidirectional carbon fiber epoxy-matrix composites causes swelling and degradation of epoxy film adhesive joints. 

Because natural tissues have anisotropic mechanical properties, in order to better replicate the properties of soft tissue, specifically cardiovascular tissues such as heart valve leaflet and vascular conduits, these anisotropic mechanical properties should be incorporated into the design of PVA-C. PVA-C prepared by the standard freeze-thaw cycling process is isotropic, with mechanical properties independent of sample orientation. MiUon et al. [59] produced anisotropic PVA-C by subjecting the hydrogel to an initial controlled unidirectional strain after the PVA solution... 

Kim JW, Taki K, Nagamine S, Ohshima M. Preparation of poly(L-lactic acid) honeycomb monolith structure by unidirectional freezing and freeze-drying. Chem Eng Sci 2008 63 3858-63. 

Various Ti02 materials have been prepared by sol-gel method. For example, porous Ti02 fiber was prepared by unidirectional freezing of gel (Maki, 1985). Sakka et al. obtained a bundle ofporous Ti02 fibers ofabout 10 cm in length and 20-100 /rmin diameter, which contained small anatase crystals. They have not reported the photocatalytic activities, but in the light of applications, such morphological control will play more important role. 

Fig. 27 Schematic drawing of (a) the freezing apparatus for unidirectional growth of ice crystals (b) the porous Col/HAp composite obtained and the sites for SEM observation and (c) specimens for the mechanical tests and animal experiments. The container was composed of two parts a brass bottom with high thermal conductivity, and silicone sides. Only the brass bottom was cooled to generate ice growth along the vertical direction. Reprinted from [297] with permission from Wiley InterScience...

Unidirectional freezing of gels [202,208] produces discontinuous fibers with polygonal cross sections (see Fig. 15) and high surface areas (240-900 m /g). Potential applications of these fibers are as supports for catalysts and enzymes. 

Fibers prepared by unidirectional freezing of a TiOj gel. From Sakka (205], in Sol-Gel Technology for Thin Films, Preforms, Electronics, and Speciality Shapes, ed. L.C. Klein (Noyes Publications, Park Ridge, New Jersey, 1988). 

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