Emprendimiento

| Experimental Procedure 31

4.1. Optimization of Dispersant for Preparation of Stable Al2O3 Slurry

Darvan C has been used as dispersant in the present study. Darvan C has been used to prepare the aqueous alumina slurry (with 25 vol. % Al2O3 loading). The concentration of Darvan C has

been varied in the range 0.1 to 0.8wt% (based on the active matter) of dry alumina powder. The stability of the slurry has been evaluated by measuring the viscosity and the sedimentation height measurement. The sediment height has been measured after 24 hours of settling.

4.2. Processing of Porous Alumina Ceramics

Porous alumina ceramic samples have prepared by three different techniques. These are Starch Consolidation Casting (SCC) using corn, arrowroot and fine flour as the pore formers, Sponge Replica Technique (SRT) using polymer sponge as the template and combination of the SCC and SRT methods. The fabrication of porous alumina ceramics using the three techniques is given in Fig 4.1.

4.2.1. Starch Consolidation Casting (SCC)

An aqueous slurry of alumina has been prepared using 0.3wt% Darvan C (based on the active matter) at different solid loading (30, 35, 40, 45vol %). Different amount of starch (i.e., corn starch, arrowroot and fine flour) in the range 1-50 vol% (calculated on the basis of alumina loading) has been added to the alumina slurry. The slurry has been pot milled in polyethylene bottles for one hour using alumina grinding media. The rheological behavior of the stable slurries has been studied at room temperature.

The stabilized slurry has been cast in preheated cylindrical metal molds (20mm x 20mm). The molds were lubricated with grease for easy demolding. The cast samples were kept at 90o_{C for 2}

hours for hydration of starch and setting of the gel. After setting, the casts were de-molded and dried at room temperature for 24 hours for development of sufficient green strength. The dried samples have been heated at 1oC/min up to 850o_{C (with a dwell time of 1 hour) to burn out the}

pore former. Subsequently, the samples were heated at 3o_{C/min to the desired sintering}

temperature (1300o_C-1600o_{C) with a soaking time of 2 hours. The sintered samples have been}

characterized by porosity, pore size distribution, strength, and microstructure. 4.2.2. Sponge Replica Technique (SRT)

Alumina slurry containing different alumina content in the range 20-35vol% has been prepared by adding 0.3wt% Darvan C as a dispersant. Poly Vinyl Alcohol (PVA) in the range, 1-5% has

| Experimental Procedure 32

been added to the slurry as the binder to maintain the viscosity of the slurry. Sponge samples (2 x 2 x 2 cm) were impregnated with the as prepared alumina slurry, and the soaked sponge samples are dried at 90o_{C for 12 hours. The samples have been sintered in the temperature range 1400-}

1600o_{C. The firing schedule has been maintained as that of the SCC at two different}

temperatures. The sintered samples have been characterized by porosity, pore size distribution, strength and microstructure.

4.2.3. Combination of SCC and SRT

Alumina slurry containing 20 vol. % of Al2O3 has been prepared by adding 0.3 wt. % Darvan C

as a dispersant and 3 wt. % of PVA as the binder. Different amounts of corn starch (30 and 40vol. %) calculated on the basis of alumina loading were added to the slurry. Similar drying and firing schedule is maintained as that of the SRT at two different temperatures (1400 and 1600o_{C). The sintered samples are characterized by porosity, pore size distribution, strength, and}

microstructure.

| Experimental Procedure 33

4.3. Characterizations

4.3.1. Study of Swelling Behaviour of Starch

A small quantity of starch powder was taken in a 50ml measuring cylinder, and 45ml of deionized water was added to it. The setup has been placed in a preheated air oven. This experiment has been conducted at 80o_{C and 90}o_{C.The swelling of the starch has been recorded}

after every 15mins. The degree of swelling of starch corresponds to the percentage volume increase of starch observed as a function of time at two different temperatures. The physical appearance of the sample before and after swelling has been shown in the Fig. 4.2.

4.3.2. Rheology Study

The rheological behavior of the stable slurries has been studied using the Anton Parr Rheometer. The measurements were carried out with an increasing shear rate (1-100 s-1_{) at 25}o_{C. The}

different rheological properties such as viscosity, shear stress and non-Newtonian index of the slurry has been studied.

4.3.3. Porosity Measurement

The porosity of the prepared alumina samples has been measured by Archimedes principle. Dry weights of the samples were taken, and then the samples were kept in a beaker filled with water. The system was then placed in a vacuum desiccator for about one hour under vacuum. The suspended and soaked weight of the samples was recorded. Porosity and density of the samples have been calculated using the formulae given below

Before swelling After swelling

| Experimental Procedure 34 𝐴𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑃𝑜𝑟𝑜𝑠𝑖𝑡𝑦 = 𝑆𝑜𝑎𝑘𝑒𝑑 𝑊𝑒𝑖𝑔ℎ𝑡−𝐷𝑟𝑦 𝑊𝑒𝑖𝑔ℎ𝑡 𝑆𝑜𝑎𝑘𝑒𝑑 𝑊𝑒𝑖𝑔ℎ𝑡−𝑆𝑢𝑠𝑝𝑒𝑛𝑑𝑒𝑑 𝑊𝑒𝑖𝑔ℎ𝑡 (4.1) 𝐵𝑢𝑙𝑘 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 =_{𝑆𝑜𝑎𝑘𝑒𝑑 𝑊𝑒𝑖𝑔ℎ𝑡−𝑆𝑢𝑠𝑝𝑒𝑛𝑑𝑒𝑑 𝑊𝑒𝑖𝑔ℎ𝑡}𝐷𝑟𝑦 𝑊𝑒𝑖𝑔ℎ𝑡 𝑋 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑊𝑎𝑡𝑒𝑟 (4.2) 𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 𝐵𝑢𝑙𝑘 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑋 100 (4.3)

The theoretical density of Al2O3 has been taken as 3.98 gm/cc in the present study.

4.3.4. Compressive Strength of the Samples

The binding behavior of starch on Al2O3 has been studied from the green strength measurement

of the samples in the compression mode. The compressive strength of the sample (both green and sintered) was measured using Universal Testing Machine (Tinius Olsen HK 10S). The samples were kept in the two compression platen and broken in compression at a cross head speed of 0.5mm/min. The load cell was 1 KN.

4.3.5. Microstructural Study

High resolution scanning electron microscopy (FESEM) is a typical electron microscope by which images are formed by simply scanning it with a beam of electrons. The secondary electrons (SE), backscattered electrons (BSE) as well as the characteristic X-rays interact with the surface atoms and provide the relevant information about the samples microstructure. The microstructure of the porous alumina samples has been studied using scanning electron microscope (Nova Nano SEM - 450). The samples were gold coated to avoid charging.

| Theoretical Prediction of Porosity in SCC Samples 35

Chapter 5 Theoretical Prediction of Porosity