949

15-20, 3-7

The sintered density increases as the calcination temperature decreases. This can be explained as a result of the change in the powder morphology with calcination temperature. With increasing calcination temperature, the powder agglomerates become harder, denser and more angular. This results in a retardation of the particle flow and hence in the plastic deformation during pressing; this produces a more porous sintered ceramic.

Increasing the sintering temperature produces an increase in the sintered density of the material, however the microstructure is also affected. Fig. 1 shows the microstructure of (CeO2)0.8(GdO 1.5)0.2 sintered at 1600°C/15 hrs., while Fig. 2 shows the microstructure of (CeO2)0.8(GdO 1.5)0.2 sintered at 1650°C/15 hrs. Fig. 1 indicates that the sample has a regular grain size (approximately 3 - 5pm in size), with a number of closed pores at the grain boundary intersects, which indicates that the sample has reached the final stage of sintering [9]. Fig. 2, however, shows a microstructure containing a mixture of coarse grains (20 pm in size) and fine grains (3-7 pm). The sample contains fewer pores than the sample in Fig.l, but appears to have been oversintered [10].

Fig. 1 SEM micrograph of a polished sample of (CeO2)0.8(GdO 1.5)0.2. sintered at 1600"C/15 hrs.

Fig. 2 SEM micrograph of a polished sample of (Ce02)o,8(GdO].5)o,2. sintered at 165CTC/15 hrs.

The room temperature modulus of rupture (MOR) of the samples described in Table 1 was examined, and is shown in Fig. 3. At a sintering temperature of 1600*C, the MOR variation, as a function of calcination temperature, is very small compared to the samples sintered at the other temperatures. This appears to indicate that the powder morphology plays a minor role in the initial fracture mechanism of the sintered sample, and thus the variation can be postulated as being primarily due to the sintering temperature. Under-sintering (at 1550"C, for example) produces a sample with a large number of pores, because it has not yet developed into a fully dense body and may only be in the intermediate stages of sintering [9]. Thus, failure due to the pores can be postulated as being the main fracture mechanism. Over sintering (1650*C, for example) causes a reduction in the MOR via the presence of large grains. The higher temperature also causes the pores to grow via Ostwald ripening, and thus can act as fracture initiation sites. Thus, the optimum sintering temperature for this sample was found to be 1600"C.

The modulus of rupture of (CeO2)0.8(GdO[.5)0.2, sintered at 1600"C, was examined as a function of temperature and is shown in Fig. 4.

1540 1560 1580 1600 1620 1640 1660 Sintering Temperature (Degrees Celeius)

Fig. 3 The modulus of rupture of (CeO2)0.8(GdO l .5)0.2 as a function of sintering temperature, at various calcination temperatures.

1540 1560 1580 1600 1620 1640 1660 Sintering Temperature (Degrees Celeius)

Fig. 3 The modulus of rupture of (CeO2)0.8(GdO l .5)0.2 as a function of sintering temperature, at various calcination temperatures.

In general, Fig.4 shows that the modulus of rupture between room temperature and 500°C changes by very little; the increase is possibly an experimental artefact. However, there is an obvious drop in the modulus of rupture between 500"C and 800°C for this particular sample. On examination of the fracture surface at the lower test temperatures, it was apparent that the fracture was due to the extension of flaws and was, therefore independent of temperature, as discussed by Davidge and Evans [11]. At the higher test temperature, the modulus of rupture decreased. At this temperature, various plastic processes (dislocation motion, grain boundary sliding and plastic flow in the second phase) lead to the initial fracture [8], This is more fully described in [12].

Test Temperature (°Q

Fig. 4 The effect of test temperature on the modulus of rupture of (CeO2)0.8(GdO i .5)0.2

Test Temperature (°Q

Fig. 4 The effect of test temperature on the modulus of rupture of (CeO2)0.8(GdO i .5)0.2

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