Internal Gelation Technique Research Articles

Crystallization of Chromium Oxide Prepared by Sol-Gel Route

Chromia finds large applications as a catalyst base. The aim of the present investigation was to prepare chemically, mechanically and thermally stable porous Cr2O3 microspheres. Chromia microspheres (500–700 μm) were prepared by internal gelation technique of the sol-gel process. These microspheres were dried in air at various temperatures between room temperature and 300°C for further thermal studies in air/oxygen/argon. These gel microspheres were subjected to X-ray diffraction analysis (XRD) and thermal analysis. Crystallization of chromia takes place at 440°C in air and at 710°C in argon. It is reported that in air amorphous Cr(III) oxide partially gets oxidized to Cr(VI) below 350°C and reduces back to Cr(III) during crystallization. Maximum oxidation of about 30% could be achieved by heating chromia microspheres in pure oxygen at 300°C. The present study deals with a probable mechanism for the crystallization of chromia in presence of Cr(VI).

External versus internal source of calcium during the gelation of alginate beads for DNA encapsulation

Alginate gels produced by an external or internal gelation technique were studied so as to determine the optimal bead matrix within which DNA can be immobilized for in vivo application. Alginates were characterized for guluronic/mannuronic acid (G/M) content and average molecular weight using 1H-NMR and LALLS analysis, respectively. Nonhomogeneous calcium, alginate, and DNA distributions were found within gels made by the external gelation method because of the external calcium source used. In contrast, the internal gelation method produces more uniform gels. Sodium was determined to exchange for calcium ions at a ratio of 2:1 and the levels of calcium complexation with alginate appears related to bead strength and integrity. The encapsulation yield of double-stranded DNA was over 97% and 80%, respectively, for beads formed using external and internal calcium gelation methods, regardless of the composition of alginate. Homogeneous gels formed by internal gelation absorbed half as much DNAse as compared with heterogeneous gels formed by external gelation. Testing of bead weight changes during formation, storage, and simulated gastrointestinal (GI) conditions (pH 1.2 and 7.0) showed that high alginate concentration, high G content, and homogeneous gels (internal gelation) result in the lowest bead shrinkage and alginate leakage. These characteristics appear best suited for stabilizing DNA during GI transit.

External versus internal source of calcium during the gelation of alginate beads for DNA encapsulation

Alginate gels produced by an external or internal gelation technique were studied so as to determine the optimal bead matrix within which DNA can be immobilized for in vivo application. Alginates were characterized for guluronic/mannuronic acid (G/M) content and average molecular weight using 1H-NMR and LALLS analysis, respectively. Nonhomogeneous calcium, alginate, and DNA distributions were found within gels made by the external gelation method because of the external calcium source used. In contrast, the internal gelation method produces more uniform gels. Sodium was determined to exchange for calcium ions at a ratio of 2:1 and the levels of calcium complexation with alginate appears related to bead strength and integrity. The encapsulation yield of double-stranded DNA was over 97% and 80%, respectively, for beads formed using external and internal calcium gelation methods, regardless of the composition of alginate. Homogeneous gels formed by internal gelation absorbed half as much DNAse as compared with heterogeneous gels formed by external gelation. Testing of bead weight changes during formation, storage, and simulated gastrointestinal (GI) conditions (pH 1.2 and 7.0) showed that high alginate concentration, high G content, and homogeneous gels (internal gelation) result in the lowest bead shrinkage and alginate leakage. These characteristics appear best suited for stabilizing DNA during GI transit. ©1998 John Wiley & Sons, Inc. Biotechnol Bioeng 57: 438-446, 1998.

Application of emanation thermal analysis for characterization of intermediate products of urania and thoria ceramics

Emanation thermal analysis (ETA) is demonstrated as a tool for checking the processes taking place in the preparation of urania ceramics from uranyl gel and urania powder. By means of this method solid phases arizing during heat treatment of the uranyl gel microspheres prepared by internal gelation technique were characterized. The differences in thermal behaviour of various uranyl gel samples were shown, the samples differing in the concentration of uranium and gelation additives (carbamide and urotropine), in the way of drying, washing and aging of the gel. The differences in the sinterability of urania powders (delivered commercially) were tested by means of ETA during the heat treatment in reducing atmosphere. For thoria powder, obtained by the decomposition of thorium oxalate, the kinetics of the changes taking place at early sintering stages, was determined by means of emanation thermal analysis. The emanation thermal analysis has been recommended as the method for characterization of intermediate products of urania and thoria ceramics.

Emanation Thermal Analysis—Results from Characterizing Intermediate Steps and Intermediate Products for Urania Spheres

Emanation thermal analysis (ETA) is demonstrated as a tool for the objective characterization of solid phases arising during treatment of the uranyl gel microspheres up to the final UO2 product. The method is able to monitor very sensible differences in the morphology, surface area, and structure of a solid, as well as their changes when heated. The ETA results for seven samples are discussed and correlated with results from other independent methods (differential thermal analysis, thermogravimetry, thermodilatometry, and density measurements). Various preparation conditions of the internal gelation technique, such as uranium concentration, concentration of gelation additives, the way of leaching, etc., are reflected. The ETA is proposed as a method for simulating thermal processes of UO2 fuel preparation.