32 Grain Porous Structure and Exploitation Properties of Humidity-Sensitive. . .
515
In spite of a small amount of transporting pores, ceramics sintered at 1300 ◦ C
are characterized by linear dependence of electrical resistance R vs. RH in the
entire studied region without significant hysteresis in absorption-desorption cycles
(Fig. 32.8b). But after degradation tests there is a drop in sensitivity down to
35%. However, studied characteristics before and after degradation does not change
substantially.
In contrast to other ceramic samples, ceramics sintered at 1400 ◦ C have a small
amount of macropores centered near r 3 = 380 nm. Humidity sensitivity of these
ceramics is characterized by linearity but with appreciable hysteresis (Fig. 32.8c).
Thus, humidity sensitivity in ceramics sintered at low 1200 ◦ C and recoverability
of electrical characteristic in adsorption-desorption cycles are obviously connected
with sufficient amount of open pores with different size from all pore size
distribution region. Increasing of humidity sensitivity and stability of ceramics
sintered at 1300 ◦ C results in increased amount of open water-exchange outsidedelivering macropores. They provide efficient sorption processes of water through
small amount of communication mesopores [16, 17].
Bimodal pore size distribution of ceramics sintered at 1400 ◦ C continues to
be modified. Capillary condensation processes effectively occur due to increasing
of amount of transferring nanopores. Hysteresis in absorption-desorption cycles
becomes larger due to the reduction of pores with radius r 3 .
32.5 Conclusions
It is shown that the structure of humidity-sensitive spinel MgAl 2 O 4 ceramics is
improved with the increase of the sintering temperature, which mainly results
in the transformation of the pore size distribution and decreasing of amount of
MgO/Al 2 O 3 phases located near grain boundaries. Positrons are trapped more
intensively in the spinel ceramics obtained at lower temperature. This is reflected
in the second component of the lifetime spectra. The third and the fourth longest
components of the spectra are due to “pick-off” annihilation of o-Ps atoms in
nanopores. Tao-Eldrup model can be applied in order to calculate of nanopore size
in ceramic materials.
It is established that the sintering temperature allows to change the porous structure of ceramic materials. Evolution of pore size distribution in humidity-sensitive
spinel MgAl 2 O 4 ceramics leads to corresponding changes in water-sorption processes in these materials. Degradation transformations at 40 ◦ C and RH = 95%
result in the increased humidity sensitivity of ceramics in all studied regions with
minimal hysteresis. Such changes confirm the active work of transporting pores after
the full saturation of some nanopores by water is reached.
The sintered temperatures allow to refine the most significant changes in freevolume (porous) structure of modified MgO-Al 2 O 3 ceramics and to decrease the
amount of additional phases located near grain boundaries. Evolution of pore size
distribution from tri- to bimodal in the studied ceramics leads to corresponding
515
In spite of a small amount of transporting pores, ceramics sintered at 1300 ◦ C
are characterized by linear dependence of electrical resistance R vs. RH in the
entire studied region without significant hysteresis in absorption-desorption cycles
(Fig. 32.8b). But after degradation tests there is a drop in sensitivity down to
35%. However, studied characteristics before and after degradation does not change
substantially.
In contrast to other ceramic samples, ceramics sintered at 1400 ◦ C have a small
amount of macropores centered near r 3 = 380 nm. Humidity sensitivity of these
ceramics is characterized by linearity but with appreciable hysteresis (Fig. 32.8c).
Thus, humidity sensitivity in ceramics sintered at low 1200 ◦ C and recoverability
of electrical characteristic in adsorption-desorption cycles are obviously connected
with sufficient amount of open pores with different size from all pore size
distribution region. Increasing of humidity sensitivity and stability of ceramics
sintered at 1300 ◦ C results in increased amount of open water-exchange outsidedelivering macropores. They provide efficient sorption processes of water through
small amount of communication mesopores [16, 17].
Bimodal pore size distribution of ceramics sintered at 1400 ◦ C continues to
be modified. Capillary condensation processes effectively occur due to increasing
of amount of transferring nanopores. Hysteresis in absorption-desorption cycles
becomes larger due to the reduction of pores with radius r 3 .
32.5 Conclusions
It is shown that the structure of humidity-sensitive spinel MgAl 2 O 4 ceramics is
improved with the increase of the sintering temperature, which mainly results
in the transformation of the pore size distribution and decreasing of amount of
MgO/Al 2 O 3 phases located near grain boundaries. Positrons are trapped more
intensively in the spinel ceramics obtained at lower temperature. This is reflected
in the second component of the lifetime spectra. The third and the fourth longest
components of the spectra are due to “pick-off” annihilation of o-Ps atoms in
nanopores. Tao-Eldrup model can be applied in order to calculate of nanopore size
in ceramic materials.
It is established that the sintering temperature allows to change the porous structure of ceramic materials. Evolution of pore size distribution in humidity-sensitive
spinel MgAl 2 O 4 ceramics leads to corresponding changes in water-sorption processes in these materials. Degradation transformations at 40 ◦ C and RH = 95%
result in the increased humidity sensitivity of ceramics in all studied regions with
minimal hysteresis. Such changes confirm the active work of transporting pores after
the full saturation of some nanopores by water is reached.
The sintered temperatures allow to refine the most significant changes in freevolume (porous) structure of modified MgO-Al 2 O 3 ceramics and to decrease the
amount of additional phases located near grain boundaries. Evolution of pore size
distribution from tri- to bimodal in the studied ceramics leads to corresponding