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Livro - Introduction to Ceramics processing - HEINRICH

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85. Cawley, J.D.; Heuer, A.H.; Newman, W.S.; Mathewson, B.B.: Computer-aided 
manufacturing of laminated engineering materials. Am. Ceram. Soc. Bull. 75, 1996, 75- 
86. Argawala, M.K.; Jamalabad; V.R.; Langrana, N.A.; Safari, A.; Whalen, P.J.; Danforth, 
S.C.: Structural Quality of Parts Processed by Fused Deposition. Rapid Prototyping 
Journal 2, 1996, 4-19. 
87. Heinrich, J.G.; Ries, C.; Görke, R.; Krause, T.: Lasersintern keramischer Werkstoffe. 
In: J. Heinrich, G. Ziegler, W. Hermel, H. Riedel (Hrsg.): Keramik. Werkstoffwoche 
1998, DGM Informationsgesellschaft mbH, Verlag der Deutschen Gesellschaft für 
Materialkunde e.V., Frankfurt. 
 
 
 
 
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5. Thermal processes 
5.1 Drying 
After shaping of the ceramic parts, the parts follow to drying. During drying, the particles that 
have been surrounded by more or less water, depending on the forming techniques get 
closer and the green body smaller, what is called shrinkage (Fig. 5.1.1). Shrinkage is finished 
as soon as the powder particles touch each other. The remaining water now has to leave the 
system through the small pore or channels. In the Bourry diagram (Fig. 5.1.2) the water 
content is plotted versus the drying time. Water content diminishes at the beginning of the 
drying process and the part shrinks. After about 72 hours, water can only be found in the 
pores. Water content further decreases, but shrinkage does not change any more. The dry 
bending strength increases as the green bodies’ water content decreases (Fig. 5.1.3). As a 
result the particles can be handled and, for example, be set on conveyor bands mechanically 
or by hand. 
 
Fig. 5.1.1: Illustration of the drying stages in ceramic compounds [1]. 
 
Fig. 5.1.2: Bourry drying diagram for clay components. 
 
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Fig. 5.1.3: Dry bending strenght as a function of the drying temperature [1]. 
 
The graphic of the mass loss (Fig. 5.1.4) shows the moisture content as a function of the 
drying time. At the beginning, the reduction on the moisture content is linear. In this case, the 
water has direct access to the surface of the tested pieces. When the water is trapped into 
the pores, the curve deviates from the straight line. The differentiated mass loss graph (Fig. 
5.1.4b) shows the area of constant drying rates even clearer. Drying speed decreases with 
the water’s retraction into the pores. The drying speed in dependence of the moisture content 
(Fig. 5.1.4c) is also a standard presentation in the literature. The diagram has to be read 
from right to the left. If the moisture content is high, the drying speed at first is constant, and 
decreases as the water is trapped into the pores. 
 Mass loss curve differentiated mass loss curve drying curve 
 
Fig. 5.1.4: Drying diagram according to Scholz. 
 
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Drying rate can be increased by raising drying air speed (Fig. 5.1.5) and temperature (Fig. 
5.1.6). Furthermore, the drying speed is affected by specimen geometry and density of the 
components (Fig. 5.1.7). If the total porosity is high (decreasing density), the water can leave 
the system relatively quickly. At very low moisture degrees we see again a deviation in the 
curve. This is the point at which only OH groups are found at the surface. Now, if 
temperature is further increased, drying speed is again reduced and the OH groups leave the 
surface of the test sample. This is the third drying stage which, however, is not relevant in 
practice. 
 
Fig. 5.1.5: Drying curve for clay minerals, two-stage drying of samples with thickness of 3 
cm, in a air humidity of 53.7% and temperature of 25°C according to Kamei. 
 
Fig. 5.1.6: Drying velocity of granulates used for floor-ware tile compounds according to 
Schrader. 
 
 
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Fig. 5.1.7: Drying curves of different samples of brick tiles according to Schmidt. 
 
Several drying plants will be presented in the following. When the drying process begins the 
relative moisture is high. As the drying process proceeds and water starts to get trapped into 
the pores, temperature is increased and humidity reduced (Fig. 5.1.8). These complex 
correlations are discussed in detail in other lectures and shall not be further explained in this 
one. 
 
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Fig. 5.1.8: Variation of the air humidity and temperature in a drying chamber according to 
Krause. 
 
Transitions in a real furnace aggregate are more sliding (Fig. 5.1.9). In a progressive tunnel 
dryer, the ware is brought in from the right side. Temperature is relatively low at the 
beginning while humidity is high. In the course through the furnace, temperature increases 
and humidity decreases. Therefore, an almost constant drying speed can be adjusted for the 
complete drying process. 
 
Fig. 5.1.9: Combined contra and parelell air current in a tunnel dryer according to Krause. 
 
 
 
 
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Porcelain articles are sometimes dried together with the plaster form used for shaping. 
Plates, cups or bowls still lying on their plaster forms are, for example, directly blown on and 
dried at the same time (Fig. 5.1.10). 
 
Fig. 5.1.10: Principle of the blowing dryer. 
 
Bricks are dried in big chamber dryers by blowing the air directly between the bricks (Fig. 
5.1.11). Chamber dryers are discontinuous aggregates. 
 
 cross-section top view 
 
Fig. 5.1.11: Rotomixair (according to Thoma). 
 
A roller dryer (Fig. 5.1.12) can perform the drying continuously. In a flat chamber, the 
temperature across the whole channel can be maintained constant. For example, tiles are 
transported on the rollers without firing support. This drying process is particularly qualified 
for mass production. 
 
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Fig. 5.1.12: Roller dryer (Heimsoth). 
 
 
5.2 Sintering 
Drying is followed by a further thermal treatment, considerably below melting point of the 
ceramics, the so-called sintering. Sintering promotes consolidation combined with further 
shrinkage. Now the ceramic products get their typical structure and their characteristics. 
 
5.2.1 Sintering of porcelain, silicate and oxide ceramics 
Porcelain is made from the natural raw materials kaolin, feldspar and quartz (50:25:25 
weight-%). Before chemical reactions between these raw materials take place, some 
physical and chemically bonded water is dehydrated (Fig. 5.2.1). Furthermore, some phase 
transformation can happen, which is in most of the cases related to volume changes. This 
may cause stresses during cooling and, if the temperature is inappropriately adjusted, lead to 
cracks in the components. 
 
 
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Fig. 5.2.1.1: DTA analysis of a kaolin [1]. 
 
When quartz is used as raw material, at 573°C there is the low to high quartz temperature 
modification. This is combined with a volume expansion. At 870°C the formation of tridymite 
which at 1470°C is transformed into cristobalite. Both transformations are combined with 
volume changes. Tridymite can be transformed during cooling into its low temperature 
modification, as well as cristobalite (Fig. 5.2.1.2). To avoid stresses or crack formation, 
threshold times are adjusted in the T-t program during cooling. 
 
Fig. 5.2.1.2: Phase transformation diagram in the SiO2-system with the temperature. 
 
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Crystal transformations and also chemical reactions may occur during the sintering of 
porcelain. After the water dehydration, kaolin reacts with quartz to build mullite. The mullite 
content grows as the temperature increases (Fig. 5.2.1.3). Part of the quartz disperses in the 
melted feldspar, therefore the quartz content of the structure decreases as the temperature is 
increased (Fig. 5.2.1.4).