502
H. Klym et al.
where R is the empirically determined parameter (in the classical case R ≈
0.1656 nm), describing effective thickness of the electron layer responsible for the
“pick-off” annihilation of o-Ps in the hole [40].
The humidity sensitivity of ceramics was determined by measuring the dependence of electrical resistance R on relative humidity RH of environment. The
electrical resistance of the studied ceramics was measured in the heat and humidity
chamber PR-3E “TABAI” at 20 ◦ C in the region of RH = 20–99%. The electrodes
were attached to the connecting cables of M-Ohmmeter. Electrical measurements
were made at fixed frequency of 500 Hz to avoid of polarization of adsorbed water
molecules. The maximal overall uncertainties in the electrical measurements did not
exceed approximately ±(0.02–0.04) M-Ohm in electrical resistance. The confidence
interval in RH measuring bar restricted by equipment accuracy was no worse than
±1%. In addition, the degradation transformation at 40 ◦ C and RH = 95% for 240 h
was induced in samples in order to study their stability in time [14, 16, 17].
32.4 Results and Discussion
32.4.1 Spinel MgAl 2 O 4 Ceramics
With respect to our XRD measurements, the ceramics sintered at 1200 ◦ C has a
three-phase composition with the lattice parameter à = 8.07996(3) Š(see Fig. 32.1).
In contrast, the ceramics sintered at 1300 ◦ C contains only two phases, the main
spinel-type (lattice parameter à = 8.0822(2) Å) and MgO phases (the remainders
of α-Al 2 O 3 phase disappear). The ceramics sintered at T s = 1400 ◦ C in addition
to the main spinel phase with lattice parameter à = 8.0828(1) Šhas a small
quantity of MgO phase as compared to the ceramics sintered at T s = 1300 ◦ C
(corresponding contents of MgO phase in the above ceramics are 3.48% and 1.54%,
respectively) [14]. Thus, it can be concluded that there is a different amount of
additional MgO/Al 2 O 3 phases in the studied MgAl 2 O 4 ceramics (see Fig. 32.1 and
Table 32.1).
The obtained trimodal pore size distributions of spinel MgAl 2 O 4 ceramics
sintered at 1200, 1300, and 1400 ◦ C are shown in Fig. 32.2. These distributions
describe the charge-transferring micropores with radius r 1 depending on sintering
conditions, water-exchange inside-delivering or communication mesopores (r 2 ), and
water-exchange outside-delivering macropores (r 3 ) depending on specific surface
area of milled MgO-Al 2 O 3 powder [14, 26]. Maximum peak positions (r 1 , r 2 , and
r 3 ) and intensities (I r1 , I r2 , and I r3 ) of the pore size distribution for studied ceramics
are summarized in Table 32.2. The obtained results indicate that the sintering
temperature influences the porous structure of ceramics. It is shown that the radius
of micropores r 1 in MgAl 2 O 4 ceramics slightly increases from 3.2 nm for samples
sintered at 1200 ◦ C to 3.5 nm for ceramics obtained at 1300 ◦ C. Position of the first
peak I r1 decreases from 5% to 3% for ceramics sintered at 1100–1200 ◦ C and is
stabilized at that level in ceramics obtained at 1300–1400 ◦ C (Fig. 32.2b, c).
H. Klym et al.
where R is the empirically determined parameter (in the classical case R ≈
0.1656 nm), describing effective thickness of the electron layer responsible for the
“pick-off” annihilation of o-Ps in the hole [40].
The humidity sensitivity of ceramics was determined by measuring the dependence of electrical resistance R on relative humidity RH of environment. The
electrical resistance of the studied ceramics was measured in the heat and humidity
chamber PR-3E “TABAI” at 20 ◦ C in the region of RH = 20–99%. The electrodes
were attached to the connecting cables of M-Ohmmeter. Electrical measurements
were made at fixed frequency of 500 Hz to avoid of polarization of adsorbed water
molecules. The maximal overall uncertainties in the electrical measurements did not
exceed approximately ±(0.02–0.04) M-Ohm in electrical resistance. The confidence
interval in RH measuring bar restricted by equipment accuracy was no worse than
±1%. In addition, the degradation transformation at 40 ◦ C and RH = 95% for 240 h
was induced in samples in order to study their stability in time [14, 16, 17].
32.4 Results and Discussion
32.4.1 Spinel MgAl 2 O 4 Ceramics
With respect to our XRD measurements, the ceramics sintered at 1200 ◦ C has a
three-phase composition with the lattice parameter à = 8.07996(3) Š(see Fig. 32.1).
In contrast, the ceramics sintered at 1300 ◦ C contains only two phases, the main
spinel-type (lattice parameter à = 8.0822(2) Å) and MgO phases (the remainders
of α-Al 2 O 3 phase disappear). The ceramics sintered at T s = 1400 ◦ C in addition
to the main spinel phase with lattice parameter à = 8.0828(1) Šhas a small
quantity of MgO phase as compared to the ceramics sintered at T s = 1300 ◦ C
(corresponding contents of MgO phase in the above ceramics are 3.48% and 1.54%,
respectively) [14]. Thus, it can be concluded that there is a different amount of
additional MgO/Al 2 O 3 phases in the studied MgAl 2 O 4 ceramics (see Fig. 32.1 and
Table 32.1).
The obtained trimodal pore size distributions of spinel MgAl 2 O 4 ceramics
sintered at 1200, 1300, and 1400 ◦ C are shown in Fig. 32.2. These distributions
describe the charge-transferring micropores with radius r 1 depending on sintering
conditions, water-exchange inside-delivering or communication mesopores (r 2 ), and
water-exchange outside-delivering macropores (r 3 ) depending on specific surface
area of milled MgO-Al 2 O 3 powder [14, 26]. Maximum peak positions (r 1 , r 2 , and
r 3 ) and intensities (I r1 , I r2 , and I r3 ) of the pore size distribution for studied ceramics
are summarized in Table 32.2. The obtained results indicate that the sintering
temperature influences the porous structure of ceramics. It is shown that the radius
of micropores r 1 in MgAl 2 O 4 ceramics slightly increases from 3.2 nm for samples
sintered at 1200 ◦ C to 3.5 nm for ceramics obtained at 1300 ◦ C. Position of the first
peak I r1 decreases from 5% to 3% for ceramics sintered at 1100–1200 ◦ C and is
stabilized at that level in ceramics obtained at 1300–1400 ◦ C (Fig. 32.2b, c).