**4. Conclusions**

indicates the evaporation of water in the sample and the beginning of the organic material evaporation. An additional weight loss of 55% at 400°C indicates the organic matter evapora‐ tion from the cotton fiber which forms the porous body [31]. A mass loss of 11% and a final weight of 83% in relation to the green body which indicates the total organic matter evapora‐

Figure 4b shows morphological features of the porous ceramic of Al2O3-ZrO2, in which grain growth was possible due to the presence of ZrO2, causing the formation of diffusion barriers to control the growth and formation of the grain[32]. Furthermore, it was possible to verify the solid phase sintering, where the densification and grain growth are controlled by the diffusion across the grain boundary [33]. Therefore, it is reasonable to affirm that the formation of porosity in the structure is beneficial to decrease the thermal conductivity of the sample.

Table 1 illustrates physical and geometric properties of Al2O3 and Al2O3-ZrO2 porous bodies.

For the porosity calculation by the average of 3 samples, the Al2O3 theoretical density value was assumed to be 3940 kg.m-3; the actual density was 2336 kg.m-3. A porosity average value

Table 2 shows the results of thermophysical property determinations of Al2O3 and Al2O3-

**s-1) (Kg.m-3) Cp (J.Kg-1.K-1) K (W.m-1.K-1)**

of 40.7% and 77.9% was obtained for the Al2O3 and Al2O3-ZrO2 porous, respectively.

Al2O3 1.24 2336 561.8 1.63 Al2O3-ZrO2 1.09 2696 547.3 1.61

**Table 2.** Analysis of thermal conductivity by laser flash method of the Al2O3 and Al2O3-ZrO2 porous bodies.

In the literature very few data about the thermal conductivity of the Al2O3 and ZrO2 system is available. However the effect of the porosity to reduce the thermal conductivity of ceramic materials is a well recognized phenomenon, that has been widely applied for Al2O3 and ZrO2 [3,10,34]. In addition to the absolute value of the porosity, the interconnection of grain size and pore shape have a significant influence on the final thermal conductivity. The high purity Al2O3 with no pores and with average grain size ~1 μm shows a thermal conductivity of approximately 33 W.m-1.K-1 at room temperature. High purity ZrO2 without porosity and average grain size of ~1 μm has a thermal conductivity of approximately 3.3 W.m-1.K-1 [35] at

**/g) Pore Diameter (Å) Porosity (%)**

**/g) Pore Volume (cm3**

Al2O3 14.33 0.01 42.47 40.7 Al2O3-ZrO2 8.45 0.006 4.19 77.9

**Table 1.** Physical and geometrical properties of the Al2O3 and Al2O3-ZrO2 porous ceramics.

tion and the formation of the defined porous body.

**Sample Surface area (m2**

44 Sintering Techniques of Materials

ZrO2 porous bodies.

**Sample a (106**

**m2**

Al2O3 porous bodies composed of ceramic fibers were successfully obtained by the embedded fibrous organic matrix method with cotton as a template. SEM at different temperatures during heat treatment along with thermogravimetric analysis data indicates a step-by-step method for the complete formation of the ceramic fiber porous body. The sintering temperature, low heating rate and the use of cotton as template had a strong effect on the surface area, pore size and distribution of the synthesized fibers. Thermal conductivity data show excellent results when compared to the literature, due to the direct influence of the organic template as a shapemodel and the efficient method of synthesis. The results show that the Al2O3 and Al2O3-ZrO2 porous body are an excellent thermal insulator with direct application for refractories. A higher porosity and lower densification of the porous body is made possible with the addition of ZrO2 to the Al2O3 matrix. However, there was no difference in thermal conductivity due to the characteristic values of density, specific heat and microstructure observed in both materials.
