460
H. Klym et al.
These pores along with specific vacancy-type defects within individual crystalline
grains and grain boundaries represent free-volume structure of ceramics. In addition, previous investigations have shown that in temperature-sensitive spinel-type
ceramics, the quantity of the additional phase and its distribution in bulk and on the
surface of ceramic samples are essentially influenced by temperature-time sintering
regimes [5, 11].
But technological modification of spinel ceramics results in stabilization of
functional properties of ceramics. So, in Cu 0.1 Ni 0.8 Co 0.2 Mn 1.9 O 4 ceramics, metallic
additives located in intergranular regions closely to grain boundaries diminish
thermally activated aging phenomena owing by stabilizing the cationic distribution
within individual ceramic grains. As a result, the chemically modified ceramics
show higher stability in comparison with non-modified ones [4, 12].
Because of significant complications in the structure of spinel-type ceramics
revealed at the levels of individual grains, grain boundaries, and pores [13], the
further progress in this field is dependent to a great extent on the development of new
characterization techniques, which can be used in addition to traditional ones (scanning/tunneling electron microscopy, porosimetry methods, etc.). This concerns, in
part, the positron annihilation lifetime (PAL) spectroscopy, the alternative method
only recently applied to fine-grained powders, nanostructured glasses, and ceramics
[14, 15]. In general, the PAL reflects a so-called free-volume distribution within
structural network of solids.
It was shown previously [16–18] that in the case of ceramics, PAL data are
determined mainly by crystallographic features of individual grains, while structural
disturbances due to grain contacts inside ceramics were a subject for additional
complications. This is why the measured positron lifetime spectra for sintered
ceramics can be adequately explained within a unified positron annihilation model
involving discrete positron trapping and ortho-positronium (o-Ps) decay modes,
the best fitting being achieved using at least three independent components in the
resolved lifetime spectra (two for positron trapping and one for o-Ps decaying)
[11, 16–19]. In terms of this model, the second positron trapping component with
lifetime τ 2 is attributed to free-volume defects such as neutral or negatively charged
vacancies especially near grain boundaries. The shortest component named the
reduced bulk positron lifetime τ 1 is mainly due to annihilation in defect-free bulk
with small mixing from other positron trapping channels. The largest component
at the level of a few nanoseconds is responsible for a so-called o-Ps “pick-off”
annihilation [18, 20].
The main aim of this work is structural study of the technologically modified
Cu 0.4 Co 0.4 Ni 0.4 Mn 1.8 O 4 and Cu 0.1 Ni 0.8 Co 0.2 Mn 1.9 O 4 ceramics using alternative
PAL technique in comparison with scanning electron microscopy (SEM) and
porosimetry methods [8, 21].
H. Klym et al.
These pores along with specific vacancy-type defects within individual crystalline
grains and grain boundaries represent free-volume structure of ceramics. In addition, previous investigations have shown that in temperature-sensitive spinel-type
ceramics, the quantity of the additional phase and its distribution in bulk and on the
surface of ceramic samples are essentially influenced by temperature-time sintering
regimes [5, 11].
But technological modification of spinel ceramics results in stabilization of
functional properties of ceramics. So, in Cu 0.1 Ni 0.8 Co 0.2 Mn 1.9 O 4 ceramics, metallic
additives located in intergranular regions closely to grain boundaries diminish
thermally activated aging phenomena owing by stabilizing the cationic distribution
within individual ceramic grains. As a result, the chemically modified ceramics
show higher stability in comparison with non-modified ones [4, 12].
Because of significant complications in the structure of spinel-type ceramics
revealed at the levels of individual grains, grain boundaries, and pores [13], the
further progress in this field is dependent to a great extent on the development of new
characterization techniques, which can be used in addition to traditional ones (scanning/tunneling electron microscopy, porosimetry methods, etc.). This concerns, in
part, the positron annihilation lifetime (PAL) spectroscopy, the alternative method
only recently applied to fine-grained powders, nanostructured glasses, and ceramics
[14, 15]. In general, the PAL reflects a so-called free-volume distribution within
structural network of solids.
It was shown previously [16–18] that in the case of ceramics, PAL data are
determined mainly by crystallographic features of individual grains, while structural
disturbances due to grain contacts inside ceramics were a subject for additional
complications. This is why the measured positron lifetime spectra for sintered
ceramics can be adequately explained within a unified positron annihilation model
involving discrete positron trapping and ortho-positronium (o-Ps) decay modes,
the best fitting being achieved using at least three independent components in the
resolved lifetime spectra (two for positron trapping and one for o-Ps decaying)
[11, 16–19]. In terms of this model, the second positron trapping component with
lifetime τ 2 is attributed to free-volume defects such as neutral or negatively charged
vacancies especially near grain boundaries. The shortest component named the
reduced bulk positron lifetime τ 1 is mainly due to annihilation in defect-free bulk
with small mixing from other positron trapping channels. The largest component
at the level of a few nanoseconds is responsible for a so-called o-Ps “pick-off”
annihilation [18, 20].
The main aim of this work is structural study of the technologically modified
Cu 0.4 Co 0.4 Ni 0.4 Mn 1.8 O 4 and Cu 0.1 Ni 0.8 Co 0.2 Mn 1.9 O 4 ceramics using alternative
PAL technique in comparison with scanning electron microscopy (SEM) and
porosimetry methods [8, 21].