**3.2 Single crystals (as reference materials)**

The single crystals are selected for the purpose of providing reference materials, which will be compared to the sintered alumina. Two types of single crystals taken from a Verneuilgrown rod are considered (Table 1). The first of very low impurity content (about 15 ppm) is provided by Pi-Kem Co. (U.K.). The second, manufactured by RSA Co. (France), contains about 380 ppm of various impurities (with 290 ppm of silicon). The samples were polished to flat mirror surface finishes, using successively finer grades of diamond pastes down to 1 µm. In order to anneal the defects induced by machining and polishing, a thermal treatment in air at 1773 K during 4 hours was performed.

#### **3.3 Characterization of defects in sintered alumina using positron annihilation lifetime spectroscopy**

In this section, complementary investigations concerning the use of Positron Annihilation Lifetime Spectroscopy "PALS" for the characterization of defects in alumina will be summarized (Moya et al., 2003; Si Ahmed et al., 2005). After a short description of the procedure, we will spotlight the results relevant to the charging properties.

#### **3.3.1 Experimental procedure**

558 Sintering of Ceramics – New Emerging Techniques

**sintering temperature, Ts (K)** 

Table 2. Sintering temperature, dwelling time at the sintering temperature and

Fig. 2. Microstructure of the 1.2 µm grain diameter of the impure polycrystalline alumina

The single crystals are selected for the purpose of providing reference materials, which will be compared to the sintered alumina. Two types of single crystals taken from a Verneuilgrown rod are considered (Table 1). The first of very low impurity content (about 15 ppm) is provided by Pi-Kem Co. (U.K.). The second, manufactured by RSA Co. (France), contains about 380 ppm of various impurities (with 290 ppm of silicon). The samples were polished to flat mirror surface finishes, using successively finer grades of diamond pastes down to 1 µm. In order to anneal the defects induced by machining and polishing, a thermal

corresponding grain diameters and densities (Liebault, 1999).

1.7 1863 100 98.7 2.7 1893 120 98.9 4.5 1923 120 99.1

1.2 1773 180 95.7 2 1823 180 98 4 1923 360 95.4

**dwelling time, ts (min)** **density (in % of theoretical)** 

**grain diameter, d (µm)** 

**"pure" samples** 

**"impure" samples** 

sample (SEM image).

**3.2 Single crystals (as reference materials)** 

treatment in air at 1773 K during 4 hours was performed.

The positron lifetime spectra of the samples, identical to those described in Table 1, were recorded at room temperature using a conventional fast-fast coincidence system with γ detectors consisting of plastic scintillators characterized by a time resolution of 270 ps. The 22Na positron source, 10 μCi sealed by 0.75 μm thick nickel foils, was sandwiched between two identical samples (of 20 mm diameter and 2 mm thickness). The spectra were measured in 2000 channels, with a calibration of 27 ps/channel, collecting 6.9×106 to 20×106 counts.
