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6 Nanocrystalline Deposits
6.1.2 About the History of the Research on Nanocrystalline
Materials
It has long been known that the atomic arrangement at the grain boundary greatly
differs from that in the bulk of a large crystal [1]. This difference leads to the idea
to produce materials in which the majority of the atoms are at, or at least very near
to, the boundary of a crystallite. This material family was called “all-grain-boundary
materials” or “bulk interfacial material” by the pioneering researcher of the field,
Gleiter, already before the extensive research in the field started [2, 3]. The research
on nanocrystalline materials is one of the scarce cases when the desire for a new type
of material was anticipated before any specific discovery, and the synthesis followed
the announcement of the pursuit.
The early synthetic approaches to obtain nanocrystalline materials were based on a
two-step technique involving vapour condensation followed by particle compacting.
When the starting material, usually a metal, is evaporated either thermally or by
an electron beam, the atoms are condensed already in the gas phase to form relatively small crystals. Therefore, the small crystals as the building blocks of a bulk
nanocrystalline material are obtained with a bottom-up method, which provides
that the nanocrystals themselves are relatively free from defects, unlike in the case
when a bulk material is milled. By collecting these crystals and compacting them, a
cluster-assembled bulk nanocrystalline material was hoped to synthesize.
However, it turned out that methods based on gas condensation coupled with
powder consolidation suffer from effects that are likely to lead to false conclusions
on nanocrystallinity in general. For this reason, some differences have been identified
between nanomaterials processed by electrodeposition and powder consolidation [4].
The reason for the misleading results was at least twofold. First, the compactness of
the thus obtained materials was not always sufficient to provide that the grains are in
direct contact with each other to such extent that the role of the voids is negligible.
Later, an approximately 98% compactness level could be achieved. Secondly, the
inherent difficulty of the gas condensation method is that the boundary of the crystals
to be compacted is always loaded with the typical impurities of the vacuum chamber.
The compacting of the crystals with the adsorbates on their surface, however, leads to
different materials than simply nanocrystalline ones without contaminated internal
boundaries. Also, the porosity originating from the imperfect compacting methods
allows the adsorption of foreign materials after the completion of the synthesis when
a specimen is exposed to ambient atmosphere.
As compared to synthesis methods involving vacuum processes or mechanical
treatments, electrodeposition proved to be fairly easy and technically much less
demanding [5], ensuring a sufficient level of compactness of the deposits. Therefore, electrodeposited nanocrystalline materials played a decisive role in scientific
debates on the properties of nanocrystalline materials by providing suitable specimens to be tested. Although the first wave of the research of nanocrystalline materials
decayed until about 2005, electrodeposition of nanocrystalline materials remained
a fashionable field to provide specimens for studying specific properties related to
6 Nanocrystalline Deposits
6.1.2 About the History of the Research on Nanocrystalline
Materials
It has long been known that the atomic arrangement at the grain boundary greatly
differs from that in the bulk of a large crystal [1]. This difference leads to the idea
to produce materials in which the majority of the atoms are at, or at least very near
to, the boundary of a crystallite. This material family was called “all-grain-boundary
materials” or “bulk interfacial material” by the pioneering researcher of the field,
Gleiter, already before the extensive research in the field started [2, 3]. The research
on nanocrystalline materials is one of the scarce cases when the desire for a new type
of material was anticipated before any specific discovery, and the synthesis followed
the announcement of the pursuit.
The early synthetic approaches to obtain nanocrystalline materials were based on a
two-step technique involving vapour condensation followed by particle compacting.
When the starting material, usually a metal, is evaporated either thermally or by
an electron beam, the atoms are condensed already in the gas phase to form relatively small crystals. Therefore, the small crystals as the building blocks of a bulk
nanocrystalline material are obtained with a bottom-up method, which provides
that the nanocrystals themselves are relatively free from defects, unlike in the case
when a bulk material is milled. By collecting these crystals and compacting them, a
cluster-assembled bulk nanocrystalline material was hoped to synthesize.
However, it turned out that methods based on gas condensation coupled with
powder consolidation suffer from effects that are likely to lead to false conclusions
on nanocrystallinity in general. For this reason, some differences have been identified
between nanomaterials processed by electrodeposition and powder consolidation [4].
The reason for the misleading results was at least twofold. First, the compactness of
the thus obtained materials was not always sufficient to provide that the grains are in
direct contact with each other to such extent that the role of the voids is negligible.
Later, an approximately 98% compactness level could be achieved. Secondly, the
inherent difficulty of the gas condensation method is that the boundary of the crystals
to be compacted is always loaded with the typical impurities of the vacuum chamber.
The compacting of the crystals with the adsorbates on their surface, however, leads to
different materials than simply nanocrystalline ones without contaminated internal
boundaries. Also, the porosity originating from the imperfect compacting methods
allows the adsorption of foreign materials after the completion of the synthesis when
a specimen is exposed to ambient atmosphere.
As compared to synthesis methods involving vacuum processes or mechanical
treatments, electrodeposition proved to be fairly easy and technically much less
demanding [5], ensuring a sufficient level of compactness of the deposits. Therefore, electrodeposited nanocrystalline materials played a decisive role in scientific
debates on the properties of nanocrystalline materials by providing suitable specimens to be tested. Although the first wave of the research of nanocrystalline materials
decayed until about 2005, electrodeposition of nanocrystalline materials remained
a fashionable field to provide specimens for studying specific properties related to