Influence of irradiation spectrum and implanted ions on the amorphization of ceramics Page: 2 of 24
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The loss of crystallinity in ceramics due to irradiation with energetic ions has been the topic
of numerous studies over the past three decades (see refs. 1-5 for reviews). The displacement
dose required to induce amorphization in ceramics at room temperature has been found to vary by
more than 3 orders of magnitude, depending on the type of ceramic. Different criteria have been
proposed to explain this wide variation in the susceptibility of ceramics to amorphization. These
criteria include bond type, free energy of formation, and degree of structural freedom for the
polytopes that describe the crystal stacking arrangement [1,4,5]. At the present time, none of
these models can completely explain all of the experimental data on amorphization of ceramics. In
fact, numerous examples exist where the experimental results on amorphization of a given
ceramic do not agree with each other in terms of the amount of displacement damage that is
required to produce amorphization [1-5].
It has been recognized for several years that implanted ions can have a significant effect on
the dose required to amorphize damage resistant materials. For example, the dose required to
amorphize A1203 at room temperature with low-energy Zr ions is about 50 displacements per
atom (dpa), whereas amorphization does not occur during irradiation with ions such as Nb and
Cr for doses in excess of 100 dpa [3,6,7]. Unfortunately, most of the available results on
amorphization of ceramics were obtained using ions with energies <300 keV. Experimental
results obtained with ion energies in this range must be treated with caution, due to the complete
overlapping of the displacement damage and implanted ion distributions. As discussed
elsewhere, the microstructure in the implanted ion region of ceramics has been found to be
affected by the implanted ions even for inert or "self-ions" .
A further potential complication with the existing experimental data is the uncertain
influence of irradiation spectrum (ionizing and displacive) on the amorphization process. The
primary knock-on atom (PKA) spectrum is expected to have some influence on the
amorphization process, particularly in ceramics that amorphize directly within displacement
cascades . In addition, differences in the defect production efficiency for bombarding
particles with different PKA energies  would affect the amorphization dose, particularly for
ceramics that amorphize by point defect accumulation.
There are numerous experimental indications that ionization enhanced diffusion [10-12]
may be affecting the microstructural development of ceramics. The high amount of ionization per
unit of displacement damage associated with light ion irradiation has been found to inhibit defect
cluster nucleation in MgO, MgAI204 and A1203 [8,13,14], and ionizing radiation has been
shown to induce annealing of preexisting point defect swelling in several ceramics at room
temperature [15-18]. Several studies on semiconductors have shown that the ionizing radiation
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Zinkle, S.J. & Snead, L.L. Influence of irradiation spectrum and implanted ions on the amorphization of ceramics, article, December 31, 1995; Tennessee. (digital.library.unt.edu/ark:/67531/metadc672334/m1/2/: accessed July 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.