Obtaining and Characterization of Alumina Ceramic

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Materials used as filters must present specific features such permeability or ability to .... Romanian Journal of Materials, 43(3), 306-311, 2013. 8. POPOVICI, C.
Obtaining and Characterization of Alumina Ceramic Filters STEFANIA STOLERIU, MIHAELA EMANUELA CRACIUN*, ADRIAN SORIN TABACARU, OVIDIU OPREA University Politehnica of Bucharest, Faculty of Applied Chemistry and Material Science, 1-7 Polizu Str., 011061, Bucharest, Romania

Present paper deals with obtaining and characterization of alumina ceramic filters. To create the required porosity, different proportion of glucose were used. To induce the photocatalytic property to ceramic filters, their surfaces was coated with nanoTiO2 (anatase). Porous alumina ceramics have been characterized from the ceramic and mechanical view point. Also, there were determined specific properties of alumina ceramic filters (electric conductivity measurements, filtration rate) The obtained alumina ceramic filters belong to category of microfiltration and are gravitational filters, which do not require applying of additional pressure. Key words: alumina ceramic filters, gravitational filters, microfiltration

Currently, many industrial processes involve the existence of a filtration process to remove impurities. Often, these filtration processes can be repetitive, in corrosive and temperature conditions. Thus, the filters must have mechanical, thermal and chemical resistance. Gravity filtration technique is often used due to its good efficiency and economic profitability. The necessary condition is that the pores size to be less than the impurities size, since flow of the fluid phase takes place perpendicular to the filter. All impurities are retained on the surface of entry in the filter, which can cause a rapid reduction of the permeability / filtration velocity because the filter can be clogged at the surface. [1-3] As examples of such kind of filtration can be mentioned: drinking water treatment, waste water treatment, recovery of solids from liquid suspensions, separations in food and drinks industry [1]. Materials used as filters must present specific features such permeability or ability to flow and filter capacity, allowing the possibility of easy regenerating of permeable structure by declogging. Also, its must provide sufficient mechanical strength at extern stress (working pressure of fluid, the mechanical stress during utilization) and resistance to the corrosive fluid. Any used filter it must be easy to be obtained and to have low cost manufacturing [1]. Alumina ceramic filters can present all the above mentioned properties, because alumina is characterized by good thermal and chemical stability, excellent thermal and mechanical resistance and acts as electrical insulator [5]. Experimental part In the present study, alumina (> 98% purity, SigmaAldrich) was used to obtain porous ceramic bodies. To create the required porosity was used a porogen agent in varying proportions 10, 20 and 30% (glucose, SigmaAldrich). The green bodies of alumina were cylinders obtained by uniaxial pressing with diameter of 20 mm and height of 10 mm. The sintering temperature was 1400°C. To induce the photocatalytic property to ceramic filters, their surfaces was coated with nanoTiO2 (anatase) [6-8]. The coating was made by brushing technique with

suspension of TiO2 in 12% polyvinyl alcohol (PVA) aqueous solution. Nano-TiO2 was obtained via the sol-gel technique from titanium butoxide [6, 7]. The flow chart used to obtained porous alumina filters is presented in figure 1.

Porous ceramic characterization Ceramic characterization Porous alumina ceramics have been characterized from the ceramic point of view and the results are shown in table 1. From table 1 one can be seen that a greater proportion of porogen agent induce, as was natural, a higher open porosity. The presence of porogen agent in a higher proportion prevents the increase of shrinkage. This is due to the combustion gases formed during the glucose combustion and due to the occurrence of pores. Compressive mechanical strength To determine the mechanical strength of ceramic samples, a Walter Bai AG Testing Machine Lfm 50 KN testing equipment was used. The results are summarized in table 2. It can be concluded that the decrease of compressive mechanical strength is directly proportional with the increase in the proportion of porogen agent. Microstructure study (scanning electronic microsocpy) Surfaces resulted after mechanical testing of the porous alumina samples were analyzed by scanning electron microscopy. Study of the samples microstructure was performed using scanning electron microscope HITACHI S-2600N model. From SEM images one can see the samples high porosity, due to the use of porogen agent porous. Also, due to the low temperature of sintering the samples present partially sintered aspect. One can observe unimodal grain size distribution of the granules with variable shape. Also, there is a tendency to create blocks, packets, and granular walls due to increasing of porogen agent proportion which allows the obtaining of a high porosity. Based on the SEM images one can determine the average size of the pores in the ceramic body. The pores size allows the classification of the ceramic filters.

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Fig. 1. Flow chart for obtaining of alumina porous bodies

Table 1 CERAMIC PROPERTIES* OF POROUS CERAMIC SAMPLES

Table 2 MECHANICAL PROPERTIES OF SINTERED CERAMICS*

a

b

Fig. 2. SEM images of porous samples with: a. 10% glucose; b. 20% glucose; c. 30% glucose

c

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Fig.4. Electric conductivity of AAS solution vs. porogen agent proportion and TiO2 coating

Fig.3. Pores size distribution into obtained ceramic samples

conductivity was measured again used. The results are shown in figure 4. From figure 4 it can be observed that after passing through obtained filters, the used solution has a lower electrical conductivity. One can say that this is a proof of the separation capacity of the filters. The presence of TiO2 increases the efficiency of filtration, since the coating is itself porous, with a much smaller pore size. It can be seen that the porosity and proportion of porogen agent used has a positive influence on the ability filtration. Rate of filtration of ceramic filters Filtration rate was determined by passing a volume of water through filters and quantifying the time during which it passes. The obtained values are presented in table 3. From table 3 it can be said that under normal pressure conditions the obtained filters are gravitational filters providing filtering without applying any additional pressure. Photocatalytic capacity of ceramic filters with TiO2 coating To estimate the photocatalytic activity of TiO2-coated ceramic filters it was used a 10-5% methylene blue solution. The filter was continuously irradiated with light by a white fluorescent lamp.

According to histograms from figure 3, alumina ceramic samples sintered at 1400°C can be used as ceramic membranes and, also, can be used for standard microfiltration, which implies an average pores size between 0.1 - 10 µm [1,2]

Specific properties of alumina ceramic filters with and without TiO2 coating Electric conductivity measurements All conductivity measurements were carried out using a solution of 20% ASS (complex of iron with the sulfosalicylic acid). This solution has an electric conductivity of 13.9 mS/cm. After the filtration, the solution Fig.5. Photocatalytic activity of TiO2-coated ceramic filters

Table 3 RATE OF FILTRATION OF CERAMIC FILTERS VS. POROGEN AGENT PROPORTION

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With the spectrophotometer Able Jasco 560 was measured the absorbance of methylene blue solution before and after filtration(fig. 5). The results are not very evidently because on one hand the filter surface area irradiated with white light was small and secondly using a gravitational filtration involving a very thick layer solution, thus the light reaching the TiO2 coated filter is insufficient. Conclusions The porous ceramics were obtained and studied in terms of specific properties through various techniques. In order to improve the characteristics of alumina filters, the surface was coated with a layer of titanium oxide nanopowder (for a better adherence on the filter surface was used with an aqueous solution of polyvinyl alcohol). From results, we can say that the proportion of porogen agent is an important factor of influence and the presence of TiO2 on the ceramic surface improves the filtration properties. Framing the filters into a filtering categor y was performed by measuring the pores size. The obtained alumina ceramic filters belong to categor y of microfiltration, they fulfilling the required average pore size. It has also been observed that the filtration rate is proportional to the apparent porosity and the amount of

porogen agent. This leads to the conclusion that the obtained filters are gravitational filters, which does not require applying of additional pressure. Reference 1. SIEGFRIED, R., G., & ALT, C., Filtration, 1. Fundamentals. Evaluation, 2, 1., Elsevier, 2012. 2. LEVY, R. V., JORNITZ M. W., Types of filtration. Sterile Filtration, Springer Berlin Heidelberg, 2006, p. 1-26. 3. *** http://www.smartmembranesolutions.co.nz, looked in 22.06.2015, hour19:56. 4. FLORESCU, C., ION, M., PODE, V., Studies and Researches for Water Resources and Environmental Protection Program, Rev. Chim. (Bucharest), 61, no.12, 2010, p. 1239 5. DAVIS, K., Material Review: Alumina (Al2O3), School of Doctoral Studies European Union Journal 2 (2010). 6. DONG S., K, SEUNG, The hydrothermal synthesis of mesoporous TiO2 with high crystallinity, thermal stability, large surface area, and enhanced photocatalytic activity, Applied Catalysis: A general, 323, 2008, p. 110-118 7. PARASCHIV, G-L, and STOLERIU, S., The influence of obtaining conditions upon the structure and morphology of anatas nanopowders, Romanian Journal of Materials, 43(3), 306-311, 2013. 8. POPOVICI, C., GIRTU, I., CHIRILA, E., CIUPINA, V., PRODAN G., HRTEM Study of nano-TiO2 Powder, Rev. Chim. (Bucharest), 59, no. 4, 2008, p. 413 Manuscript received: 23.10.2015

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