RSCE 2016.4

23rd Regional Symposium on Chemical Engineering (RSCE2016)

Innovation in Chemical Engineering towards the linkages among

education, academia, industry

27-28 Oct. 2016 in Vung Tau City, Vietnam

Effects of Sintering Temperature on Phase Formation

and Microstructure Development of a Porous

Cordierite Ceramic from Rice Husk

Nguyen Luong The Thinh, Vu Thi Ngoc Minh, Mai Van Vo

Department of Silicate Materials Technology, Hanoi University of Science and Technology

Hanoi, Vietnam

Abstract—Porous cordierite ceramic samples were synthesized

by firing a powder compact from clay, talc, alumina and rice husk

charcoal (RHC). Gasses released by the combustion of the RHC

and decomposition of the hydrates in the raw materials during

firing created a matrix of interconnected channels throughout the

samples. Cordierite (Mg2Al4Si5O18) was formed at 1160 degree

Celsius, but became a major phase only when the firing

temperature reached 1220 degree Celsius and above. The

apparent porosity of the samples decreased from 49.5 percent to

42.7 percent as the firing temperature increased from 1140 degree

Celsius to 1240 degree Celsius. At all firing temperatures, their

compressive strength were approximate 50 mega Pascal, and their

apparent densities were around 1.5 gram per cubic centimeter.

Keywords— porous ceramic, cordierite, sintering temperature,

phase formation, microstructure.

I. INTRODUCTION

Cordierite (2MgO.2Al2O3.5SiO2) was characterized by a very low thermal expansion, which was followed by a significantly higher thermal shock resistance in comparison to other ceramic materials [1]. In the porous form, cordierite has been proven effective as a thermal insulator in high temperature applications, and catalyst support in automotive exhaust treatment [2, 3].

Porous cordierite ceramic has been produced by a number of processing methods, including solid-state reaction, sol-gel, gel-casting, and foam replication [4-6]. Among these methods, solid-state sintering of a mixture of oxide precursors with one or more pore-forming agents allowed a better control on the shape, size and structure of the final product.

The present work synthesized porous cordierite ceramic at low sintering temperatures by utilizing rice husk, which is an

agricultural by-product of the rice milling process, and contains high amount of silica and potassium oxides after ignition [7]. The influence of the sintering temperature on the porosity and mineral phase formation was studied.

II. MATERIALS AND METHOD

The present work used Truc Thon white clay, YFA talc powder, Almatis CT9FG alumina powder, and Tien Hai rice husk (RH) as the starting materials. The RH was heat treated to form rice husk charcoal (RHC) for a better grindability according to a previous study [7], and then ground in a planetary ballmill until passed the 0.063mm sieve. Talc and clay were crushed until passed the 0.5mm sieve, and dried at 110oC until constant mass is attained.

A powder mixture, comprised 23% by weight (wt%) of the clay, 29 wt% talc, 17 wt% alumina, and 30 wt% RHC was prepared. The chemical compositions of these materials are presented in Table 1. The mixture was ground in 15 minutes to ensure efficient mixing. To estimate the behavior of the powder mixture upon heating, thermal analysis was performed in air at a heating rate of 10oC/min. Distilled water was added to the powder mixture to form a paste having a water content of 30wt%. The mixture was kept in a plastic bag for 48 hours before shaping.

After aging, the paste was pressed to form cylindrical pellets

with a dimension of 20mm x H 20mm. They were then dried at 110ºC overnight, and fired at different sintering temperatures: 1140ºC, 1160ºC, 1180ºC, 1200ºC, 1220ºC and 1240ºC. The heating rate was kept at 125ºC/h from room temperature to 600ºC, and at 180ºC/h from 600oC to the sintering temperature. A dwell time of 60 minutes at the sintering temperature was applied to all samples.





TABLE I. CHEMICAL COMPOSITION OF THE STARTING MATERIALS.

Material Chemical composition (wt%)

SiO2 Al2O3 TiO2 CaO MgO Fe2O3 FeO K2O Na2O LOI

Clay 59.14 26.90 0.95 0.41 0.61 1.30 - 3.05 0.43 7.19

Talc 59.40 3.27 0.15 0.07 31.95 3.27 0.02 0.08 0.09 3.36

Alumina 0.06 99.50 - - - 0.06 - - 0.30 -

Heat-treated rice husk 16.10 0.14 0.01 0.44 0.17 - 0.08 0.88 0.05 82.14

III. RESULTS AND DISCUSION

A. Thermal Analysis of the Rice Husk and Raw Mix

Thermal analysis of the RH was presented in Figure 1 and of the raw mix was presented in Figure 2. In case of the RH, below 120oC was the evolution of the absorbed water with an endothermic peak presented at 105oC. There was almost no weight change in the range of temperatures from 105oC to 240oC. A rapid outgassing occurred between 240oC and 490oC with two exothermic peaks presented at 355oC and 425oC. Further changes in the TG and DTA curves were not significant after the heating temperature crossed 490oC. This outgassing was expected to contribute to the formation of interconnected pores in the sintered ceramic.

The DTA curve of the raw mix behaved similarly to that of

the RH with two partially overlapped exothermic peaks, one

appeared at around 350oC and the other at around 680oC. It was

possible that gasses released by RHC decompostion and firing

were increased in concentration but partially blocked by the

inorganics in the raw mix. As a result, kinetics of the reaction

was reduced, and the exothermic effect on the DTA curve was

postponed to higher temperatures. An endothermic peak appear

immediately after the 680oC could be because of the

decomposition and phase transition of the raw mix minerals.

After crossing 700ºC, the exothermic effect appeared again

while the weight change was negligible. This significant

increase of exothermic effect above 700ºC majorly attributed to

the combustion of RHC, which was previously hindered.

B. Effect of the Sintering Temperature on the Physical

Properties

The physical properties of samples fired at six different temperatures were shown in Table 2. They all reached a compressive strength of around 50MPa and an apparent density of around 1.5g/cm3. As the temperature increased, the apparent porosity decreased slightly from 49.5% at 1140ºC to 42.7% at 1240ºC, the water absorption decreased from 34.1% to 28.2%, and the total shrinkage increased from 22.9% to 30%. Explanation for these observations could be that either void spaces were filled with glass phase or a better sintering occurred at elevated temperature.

The microstructure of the polished sample fired at 1240ºC was shown in Figure 3. The x200 SEM image indicated that the sample fired at 1240ºC had a high porosity, with interconnected void spaces. The smaller holes, which had a diameter of 10μm, was seen on the surface of the bigger ones, which had diameter of 100μm. The x1000 image showed that the grains were connected by a glassy phase, instead of a direct bonding.

Fig. 1. DTA and TG curves of rice husk.

Fig. 2. DTA and TG of the raw mix.

-50

0

50

100

150

0

20

40

60

80

100

0 200 400 600 800 1000

Hea

t fl

ow

(m

W)

Wei

ght

(%)

Temperature (oC)

TGDTA

0

10

20

30

40

50

0

20

40

60

80

100

0 200 400 600 800 1000

Hea

t fl

ow

(µ

V)

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(%)

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DTA





TABLE II. PHYSICAL PROPERTIES OF SAMPLES FIRED AT DIFFERENT TEMPERATURES.

Firing

temperature

(ºC)

Water

absorption

(wt%)

Apparent

density

(g/cm3)

Apparent

porosity

(%)

Linear

shrinkage

(%)

Compressive

strength

(MPa)

1140 34.10 1.452 49.49 22.90 52.83

1160 30.58 1.462 44.74 23.79 51.42

1180 29.90 1.535 45.83 24.81 48.58

1200 31.03 1.500 46.51 29.97 56.91

1220 29.71 1.535 45.59 29.34 52.23

1240 28.22 1.512 42.69 29.97 50.40

C. Effect of the Sintering Temperature on Phase Formation

Figure 4 presents the XRD patterns of samples fired at six different temperatures after being cooled down to room temperature. The presence of the crystalline phases became more complicated as the firing temperature increased. The typical diffraction peaks of mullite, a phase that was usually formed when synthesizing cordierite, were not seen in all cases. The diffraction peaks of cordierite were seen on sample fired at 1160oC, but very weak. They became more obvious only on samples fired at 1220oC and above. Corundum, quartz and cristobalite was seen on all samples.

-cristobalite, which is stable at high temperatures, was seen on samples fired at under 1220oC. α-cristobalite, which is stable at room temperature, was seen on samples fired at 1220ºC and above. These cristobalite phases were formed as the products of RHC combustion. The distribution of minor elements in the cristobalite matrix induced stresses and prevented the

transformation of -cristobalite to α-cristobalite. A higher sintering temperature and a longer dwell time led to the

recrystallization of silica and forced the impurities out toward the grain boundaries. Therefore, the stress was released and α-cristobalite was seen on samples fired at 1220ºC and above.

The presence of spinel could be explained by the reaction between the products of clay and talc decomposition as follow:

Kaolinite, an important mineral in clay, dehydrated to form metakaolinite at temperatures above 450oC:

Al2O3. 2SiO2. 2H2O450oC→ Al2O3. 2SiO2 + 2H2O

Talc decomposed at temperature above 950oC to form protoenstatite:

3MgO. 4SiO2. H2O950oC→ 3(MgO. SiO2) + SiO2 + H2O

Metakaolinite and protoenstatite reacted to form spinel and release SiO2:

Al2O3. 2SiO2 +MgO. SiO2 → MgO. Al2O3 + 2SiO2

Fig. 3. SEM images of the sample fired at 1240o: a) at x200 magnification, and b) at x1000 magnification.

a) b)





The transformation of metakaolinite at temperatures above

1000oC could form mullite as follow:

3(Al2O3. 2SiO2)>1000𝑜𝐶→ 3Al2O3. 2SiO2 + 4SiO2

The transformation of metakaolinite at temperatures above 1000oC could form mullite as follow:

3(Al2O3. 2SiO2)>1000𝑜𝐶→ 3Al2O3. 2SiO2 + 4SiO2

However, it was possible that its size was small and its structure was defective, causing mullite peaks to be unclear and easily be recognized as the background.

The typical diffraction peaks of cordierite were observed from 1160ºC, but its intensity increased slowly as the sintering temperature increased to 1200oC. Only after reaching 1220ºC

did cordierite perfect its structure and crystalize in a large enough amount to become a major phase.

Cordierite was possibly formed by the reaction between spinel and cristobalite and/or between mullite, cristobalite and protoenstatite as follow:

2(MgO. Al2O3) + 5SiO2 → 2MgO. 2Al2O3. 5SiO2

and

2(3Al2O3. 2SiO2 + 5SiO2 + 6(MgO. SiO2)→ 3(2MgO. 2Al2O3. 5SiO2)

Other major phases that presented on all sample was corundum and quartz with high intensity diffraction peaks. It was likely that these minerals were relatively inactive and did not participate much in cordierite formation.

Fig. 4. X-ray diffraction patterns of the sintered samples.





IV. SUMMARY

Rice husk charcoal played the role of a pore-forming agent in a porous cordierite ceramic fabrication. Its decomposition created a matrix of interconnected pores. The microstructure, crystalline phases and other properties including water absorption, and apparent porosity of the porous cordierite ceramic were strongly related to the firing temperature. As the temperature increased, the apparent porosity and water absorption decreased slightly while the apparent density and compressive strength remained at relatively stable values.

MgO-containing mineral phases presented as spinel and cordierite. Spinel was formed quite early while cordierite was formed later. The strongest peak of the cordierite phase (at 2θ=10.5º) first appeared on the sample fired at 1160ºC with the intensity increased slowly until a firing temperature of 1200ºC. Cordierite became a major phase only on samples fired at 1220ºC and 1240oC. Corundum presented on all studied samples as a major phase while quartz and cristobalite presented as the minor ones.

REFERENCES

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Source. Vietnam Journal of Chemistry, 2015: p. 192-196. [1]

Documents

RSCE 2016.4