Applying Taguchi method for optimization of the synthesis condition of nano-porous alumina membrane by slip casting method

https://doi.org/10.1016/j.jallcom.2009.06.093Get rights and content

Abstract

In this work thin disc type pure alumina membranes have been prepared by slip casting technique. The colloidal stabilization of micro-sized alumina suspensions with different amount of 1,2-dihydroxy-3,5-benzenedisulfonic acid disodium salt (Tiron) at various suspension concentration were examined and the suspension stability was characterized by measuring sedimentation height. Also the necessary ball milling time (used as a deflocculating process) to prepare defect free membranes was investigated. A statistical experimental design method (Taguchi method with L9 orthogonal array design) was implemented to optimize experimental conditions for the preparation of Al2O3 nano-porous membrane. Sintering temperature, solid content and polyvinyl alcohol (PVA) content were recognized and selected as important effecting parameters. Also structural studies by means of isopropanol adsorption and scanning electron microscopy were carried out on membranes. As the result of Taguchi analysis in this study, sintering temperature was the most influencing parameter on the membrane porosity. Reasonable membrane characteristics were obtained at an optimum temperature of 1400 °C, 20% solid content and 20 cc PVA solution per 100 g of alumina powder.

Introduction

Ceramic membranes have been used in a number of industries including water purification or filtration, clarification and sterilization of beverages, concentration of proteins in the food and dairy industries, purification and recovery of lignin in paper industries. In addition they can be used in the petrochemical industry to separate high molecular hydrocarbons at elevated temperatures [1].

Some outstanding properties of ceramic membranes such as long lifetime, very good separation ability, constant filter quality and catalytic properties which are all due to their intrinsic nature have given considerable advantages to this group of membrane materials. These advantages along with some others such as high chemical, mechanical, thermal and microbiological stability and improvement in their fabrication techniques have caused a rapid growth in their use in recent years. Alumina, silica, titania and zirconia are different ceramic materials used for the synthesis of the porous membranes which among them alumina is the most popular [1], [2].

From the most important properties of alumina membranes can point to high pressure resistance, back flushable, permanent resistance to concentrated hot acidic and caustic solutions, high temperature resistance, sterilisable by steam, high permeation flux and their resistance to oxidation and solvents.

Long operating experience of users with ceramic membranes based on aluminum oxide, even under extreme conditions, has caused this type of membrane to be considered as a dependable filter medium with an excellent chemical, thermal and mechanical strength that is matched by almost no other materials [2].

High permeability property of ceramic membranes can only be achieved in an asymmetric configuration. The advantage of creating an asymmetric membrane is to produce a smaller pore size over a thinner region, thereby giving superior permeation properties. There are some different methods to synthesis asymmetric ceramic membrane known as: slip casting, sol–gel, gel casting, sedimentation, dip coating, etc. Slip casting is one of common techniques used to fabricate ceramics with simple to complex shapes from particle suspensions. In this method a ceramic powder suspension is poured into a porous plaster mould of the desired shape. The liquid of the initial slip is withdrawn into the pores of the mould by the capillary action. As the liquid penetrates through the mould, a cast or cake is simultaneously shaped on the plaster surface. The depth of liquid penetration into the mould and cake thickness both increase with time [2], [3].

Two driving forces due to the gravity force and the capillary suction of the plaster mould are responsible for particle segregation in the slip casting process. Both of these depend on other factors such as particle density, particle size distribution, particle interaction forces and solid content. In the slip casting process, the separation of particles by size (or density) under influence of these two driving forces can occur through two different segregation mechanisms: (i) settling of the coarser (denser) particles; (ii) clogging the cake by the finer (lighter) ones. In contrast with sedimentation that occurs only in one direction (vertical), the clogging effect can occur in any direction parallel to liquid flow, in processes like slip casting and filtration. Fine particles have a lower inertia momentum and are more easily transported by the fluid. Thus, they will be the first to be deposited at the mould wall. Some can even penetrate and clog the larger pores within the mould [4].

The key factor to gain asymmetric structure is to prepare colloidally unstable or met stable suspensions. This can be achieved by using powder with broad particle size distribution and dispersant, causing segregation based on particles diameter. This produces a functionally gradient or continuously enlarging mean pore diameter profile from bottom to top over the cross section of the structure [5], [6].

Binders are used in casting processes to provide sufficient strength to the body so that the green bodies can be molded and retained in the desired shape without breaking or damage, before and during sintering process. Also they cause to achieve higher porosity after they are burned out during sintering process [3], [6].

The objective of this study was to estimate the best synthesis conditions of alumina ultra filtration membrane based on slip casting route by using Taguchi experimental design. The main parameters were sintering temperature, solid content, and the binder content. Moreover, effect of different ball milling times on sintered body properties was probed. In Taguchi method, the results of experiments were analyzed to achieve the following objectives: (1) to find the best or optimal synthesis condition for the product or process, (2) to identify the contribution of individual factors and (3) to estimate the value of the defined response under optimal condition. Analysis of variance (ANOVA), was also used to analyze the results of experiments. By studying the main effects of each factor, the general trends of the influencing factors, can be characterized.

Section snippets

Selection of effecting factors and their levels

Three factors with three levels were chosen and studied in this work.

  • -

    Sintering temperature: 1400 °C, 1500 °C and 1600 °C.

  • -

    Solid content: 15%, 20% and 30%.

  • -

    Amount of PVA solution per 100 g of alumina powder: 100 cc, 200 cc and 300 cc.

Selection of orthogonal array (OA) and assignment of factors

Standard table known as L9 OA with three levels and three factors (shown in Table 1) was used for the design of the experiments. Each row in the table represents a trial condition with the factor levels. The columns correspond to the factors specified in this study and each

Sedimentation tests

As it is shown in Fig. 1, the optimum amount of Tiron tended to increase with an increase in solid content. The same trend was observed before by other researchers with another kind of dispersant [7]. Sedimentation height decreased till optimum amount of Tiron was reached and then remained constant or even increased which was in agreement with the findings of others [7], [8], [9], [10], [11]. When 15 wt% alumina suspensions was used, the proper amount of Tiron that led to the lowest sediment

Conclusions

In this work Taguchi method was implemented to find the optimum condition for synthesizing an alumina ceramic from porosity point of view. The results obtained from experiments revealed that:

  • 1.

    The optimum amount of dispersant was dependent on the solid content and it became larger for bigger value of the solid content.

  • 2.

    Ball milling had important effect on porosity and pore size but high milling times caused defects appeared in the surface of samples.

  • 3.

    Sintering temperature was the most effective

References (16)

  • J.M. Benito et al.

    J. Eur. Ceram. Soc.

    (2005)
  • J.M.F. Ferreira

    J. Eur. Ceram. Soc.

    (1998)
  • K. Darcovich et al.

    J. Membr. Sci.

    (2001)
  • A. Tsetsekou et al.

    J. Eur. Ceram. Soc.

    (2001)
  • C. Falamaki et al.

    J. Membr. Sci.

    (2006)
  • L. Jiang et al.

    Surf. A: Physicochem. Eng. Aspects

    (2002)
  • B.J. Briscoe et al.

    J. Eur. Ceram. Soc.

    (1998)
  • A. Seal et al.

    J. Eur. Ceram. Soc.

    (2004)
There are more references available in the full text version of this article.

Cited by (19)

  • Investigation of affecting parameters on slip casting of yttria-magnesia IR-transparent bodies

    2019, Ceramics International
    Citation Excerpt :

    So second method or the Taguchi experimental design method was used to reduce the number of experiments in order to obtain optimal conditions. The results of the designed experiments were analyzed to evaluate the impact of each factor on the main effects [30]. According to Taguchi method, 16 experiments were designed as listed in Table 4.

  • Highly porous α-Al<inf>2</inf>O<inf>3</inf> ceramics obtained by sintering atomic layer deposited inverse opals

    2017, Ceramics International
    Citation Excerpt :

    When compared to other materials, such as metals and polymers, they also present longer lifetimes, constant filter quality, and high chemical, microbiological and thermal stability [1]. The most common materials used for porous ceramics are alumina, zirconia, titania and silica [2]. Among their applications are filters, catalysts supports, bioreactors, light weight components, thermal insulators, sensors and bone substitutes [1,3–10].

  • Recycling of waste fly ash for production of porous mullite ceramic membrane supports with increased porosity

    2014, Journal of the European Ceramic Society
    Citation Excerpt :

    In particular, a decrease of the fabrication cost of the membrane support is an essential since it constitutes the majority of the cost of the whole membrane system mainly due to its very high mass ratio (support/layer)6 and high sintering cost.7,8 In order to reduce fabrication cost, several effective methods have been developed such as low-temperature sintering with low melting-point aids,9 adoption of simplified fabrication process (central casting, one-step co-sintering),10,11 and use of cheap raw materials such as natural clay minerals (kaolin,12 bauxite,13 and natural topaz,14 etc.), industrial solid wastes (coal fly ash6). Fly ash is a by-product generated during the combustion of raw coal in thermal-electric power plants, which causes serious problems related to land disposal and environmental pollution.

View all citing articles on Scopus
View full text