500
H. Klym et al.
to capillary condensation processes in ceramics. One of such methods is called
positron annihilation lifetime (PAL) spectroscopy [8, 9]. This is an experimental
technique used to study extended defects and nanosized free-volume entities in
solids despite their structural hierarchy [10].
It is well known that exploitation electrical properties of humidity-sensitive
elements based on MgAl 2 O 4 ceramics depend on sorption processes of active
materials [11]. Therefore, the actual task appears in relation to modification of
porous materials with controlled microstructure, firstly, with large specific surface
area, high open porosity, and controlled and optimal pore size distribution [12, 13].
In this work, we present an attempt to investigate grain porous structure of
humidity-sensitive MgAl 2 O 4 and modified MgO-Al 2 O 3 ceramics using combined
XRD, Hg-porosimetry, and SEM with PAL methods and influence of technological
modification on exploitation properties of the studied ceramics.
32.2 Preparation of Magnesium Aluminate Ceramics
The traditional ceramic technology was used for preparation of spinel MgAl 2 O 4 and
MgO-Al 2 O 3 ceramics, as was described in greater details elsewhere [14–18]. The
samples of ceramics under study were prepared at maximum sintering temperatures
T s of 1200, 1300, and 1400 ◦ C. Total treatment duration was 2 h. For preparation
of MgAl 2 O 4 ceramics, initial fine-dispersed powders of Al 2 O 3 with specific surface
area of 12.4 m 2 /g and MgO with specific surface area of 10.7 m 2 /g were used as
starting components in conventional ceramics technology route [14].
For preparation of modified MgO-Al 2 O 3 ceramics, equimolar amounts of initial
powders (Al 2 O 3 with specific surface area of 67 m 2 /g and 4MgCO 3 ·Mg(OH) 2 ·5H 2 O
with specific surface area of 12.8 m 2 /g) were mixed in a planetary ball mill for 96 h
in an environment with acetone to obtain mixture. The aqueous solution of polyvinyl
alcohol was used for obtaining of the molding powder. Bilateral compression was
performed in steel molds. After pressing, the samples were sintered also in a furnace
at 1200, 1300, and 1400 ◦ C for 2 h [16, 17].
Electrical contacts on the planar surface of all ceramics were formed by screen
printing using Ru-contained paste and Pt contacts [19, 20]. Pre-dried layers of paste
were sintered at 850 ◦ C with an exposure of 10 min [21, 22].
32.3 Experimental Details
The phase composition of the studied ceramics was determined by XRD method.
The XRD patterns were recorded at room temperature using HZG-4a powder
diffractometer with CuKα radiation. This equipment was calibrated with NIST
SRM-1976 and Si standards. The measurements were carried out with the 2θ step
of 0.05 ◦ with variable scanning rate, depending on the sample quality. The profile
H. Klym et al.
to capillary condensation processes in ceramics. One of such methods is called
positron annihilation lifetime (PAL) spectroscopy [8, 9]. This is an experimental
technique used to study extended defects and nanosized free-volume entities in
solids despite their structural hierarchy [10].
It is well known that exploitation electrical properties of humidity-sensitive
elements based on MgAl 2 O 4 ceramics depend on sorption processes of active
materials [11]. Therefore, the actual task appears in relation to modification of
porous materials with controlled microstructure, firstly, with large specific surface
area, high open porosity, and controlled and optimal pore size distribution [12, 13].
In this work, we present an attempt to investigate grain porous structure of
humidity-sensitive MgAl 2 O 4 and modified MgO-Al 2 O 3 ceramics using combined
XRD, Hg-porosimetry, and SEM with PAL methods and influence of technological
modification on exploitation properties of the studied ceramics.
32.2 Preparation of Magnesium Aluminate Ceramics
The traditional ceramic technology was used for preparation of spinel MgAl 2 O 4 and
MgO-Al 2 O 3 ceramics, as was described in greater details elsewhere [14–18]. The
samples of ceramics under study were prepared at maximum sintering temperatures
T s of 1200, 1300, and 1400 ◦ C. Total treatment duration was 2 h. For preparation
of MgAl 2 O 4 ceramics, initial fine-dispersed powders of Al 2 O 3 with specific surface
area of 12.4 m 2 /g and MgO with specific surface area of 10.7 m 2 /g were used as
starting components in conventional ceramics technology route [14].
For preparation of modified MgO-Al 2 O 3 ceramics, equimolar amounts of initial
powders (Al 2 O 3 with specific surface area of 67 m 2 /g and 4MgCO 3 ·Mg(OH) 2 ·5H 2 O
with specific surface area of 12.8 m 2 /g) were mixed in a planetary ball mill for 96 h
in an environment with acetone to obtain mixture. The aqueous solution of polyvinyl
alcohol was used for obtaining of the molding powder. Bilateral compression was
performed in steel molds. After pressing, the samples were sintered also in a furnace
at 1200, 1300, and 1400 ◦ C for 2 h [16, 17].
Electrical contacts on the planar surface of all ceramics were formed by screen
printing using Ru-contained paste and Pt contacts [19, 20]. Pre-dried layers of paste
were sintered at 850 ◦ C with an exposure of 10 min [21, 22].
32.3 Experimental Details
The phase composition of the studied ceramics was determined by XRD method.
The XRD patterns were recorded at room temperature using HZG-4a powder
diffractometer with CuKα radiation. This equipment was calibrated with NIST
SRM-1976 and Si standards. The measurements were carried out with the 2θ step
of 0.05 ◦ with variable scanning rate, depending on the sample quality. The profile