تهیه سرامیک TiO2بر اساس جریان سریع غشاء امولسیون
|کد مقاله||سال انتشار||تعداد صفحات مقاله انگلیسی||ترجمه فارسی|
|10159||2007||3 صفحه PDF||سفارش دهید|
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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Chinese Journal of Chemical Engineering, Volume 15, Issue 4, August 2007, Pages 616–618
A novel method to prepare macroporous TiO2 ceramic, based on membrane emulsification was reported. To solve the paradox between the instability of nonaqueous emulsion and long emulsification time required by the membrane emulsification, a two-stage ceramic membrane jet-flow emulsification was proposed. Discussion was conducted on the evolution of droplet size with time, which followed the Ostwald ripening theory. And a monodispersed nonaqueous emulsion with an average droplet size of 1.6μm could be prepared. Using the emulsion as a template, TiO2 ceramics with an average pore size of 1.1μ.m were obtained. The material could be prospectively used for preparation of catalysts, adsorbents, and membranes.
Porous materials have recently received much attention because of their application in a wide variety of fields, such as biosensors, catalysts, adsorbents, chromatographic materials, and photonic crystals. Several approaches are currently available for the preparation of ordered structures with different length scales. Mesostructures can be obtained by the self-organization of the surfactant molecules, and colloidal templates are usually used to prepare ordered macroporous materials[ 1,2]. Imhof and Pine have proposed a famous method - the emulsion templating approach, for preparing highly ordered macroporous materials. In this approach, the sol-gel process is used to deposit an inorganic material on the exterior of the droplets in a nonaqueous emulsion. The preparation of monodispersed nonaqueous emulsion is one of the key processes. Membrane emulsification (ME) is a new emulsification technique based on membrane structure, which has received increasing attention over the last 10 years, because it consumes less energy, has low shearing stress and controllable droplet size of the emulsions. However, low flux of the dispersed phase, resulting in a long emulsification time, has hindered the application of this technology. Especially in the preparation of nonaqueous emulsions, the problem has become more obvious because of nonaqueous emulsions being unstable. In the previous study, a membrane jet-flow emulsification to resolve the paradox between the flux and the droplet size, in conventional membrane emulsification, was proposed. A monodispersed OW emulsion could be obtained under jet flow by a two-stage membrane jet-flow emulsification process. The main purpose of this study was to prepare amonodispersed nonaqueous emulsion with the help of membrane emulsification under jet-flow, to investigate the stability of the nonaqueous emulsion, and to prepare an ordered macroporous Ti02 ceramic based on this method.
نتیجه گیری انگلیسی
In these experiments, the stable emulsions were prepared with formamide as the continuous phase, isooctane as the dispersed phase, an emulsifier concentrate of 2%, and a Zr02 membrane with nominal size 0.16y.m was used in the first stage. Fig.1 shows the emulsion prepared at a pressure of 0.09MPa, and the rotate speed of 300rmin-'. As shown in this figure, the average droplet size of the emulsion was 3.2ym and the a was 0.563. The emulsion prepared was not monodispersed because of the higher pressure, however'the flux could reach 201.4LK2.h-'. Using the secondary jet-flow membrane emulsification, monodispersed emulsion with small droplet size could be obtained. Fig.2 shows the droplet size of the emulsion prepared by using an a-A1203 ceramic membrane, having a nominal pore size of 1.5ym, at 15OkPa pressure, with 300rmin - I stirring speed. The average droplet size of the emulsion was 1.6ym, which was slightly bigger than the membrane pore size. The a of the emulsion was 0.22, which meant the emulsion was monodispersed. The flux reached 176.4L.m-2.hp I , which was attributed to the larger pore size and higherpressure. The stability of the emulsion is shown in Fig.3. The cube of average droplet diameter of the emulsion linearly increased with time, which followed the famous Ostwald ripening theory. And the coefficient of dispersion (a) had little changes from 0.22 to 0.24, which meant the emulsion had always been kept monodispersed during the 960min. Therefore, the stability of the emulsion could meet the demands of the sol-gel process.A light-scattering particle sizer (Zetasizer 3000, Malvern Instruments, Malvern, UK), determined the particle size of the sol. The evolution of the sol with time is shown in Fig.4. The particle size of the sol was about 4nm initially, and it became more than lOnm after 10 days. The result indicated that the particle size increased with time because of coalescence. Generally the sol was stable and no precipitation emerged in several weeks, however, it was recommended to use the sol in three days to obtain the ordered porous material.The emulsion was concentrated by centrifuging and then dispersed in the sol, and the stable gel could be obtained by adding 30% ammonia into the mixture. The template was removed after heating to a temperature of 40°C for 24h, and then the gel was calcined at a temperature of 400°C in a furnace, for 4h, to obtain macroporous ceramic. Fig.5 shows the scanning electron micrographs of the surface and cross-section. The Ti02 ceramic with the pore size of 1.1pm was obtained by the calcining process, which was about 0.69times of the droplet size of the emulsion before ther- 2 mal treatment. In this study the droplet size of the emulsion template was controlled by the pore structure of the memtemplate determined the macroporous structure of the Ti02 ceramic. Therefore, it was possible to control the macroporous structure of the material by choosing the membranes with different pore structures. Further study on the control of porous structure is still in progress.