206
6 Nanocrystalline Deposits
of view, a suitable complexing agent is a must for achieving the codeposition. In the
overwhelming majority of the relevant studies, citric acid (or its sodium or ammonium
salt) is applied. The mechanistic explanation of the codeposition is based on the mixed
citrate complex of the Ig
2+ and TrO 4
n− ions (Ig: iron group metal, Tr: oxoanionforming transition metal). Without the complex facilitating the electron transfer, the
reduction of the oxoanions stops at an intermediate level and a non-conducting oxide
is formed, usually with a composition near to the TrO 2 formula.
From the viewpoint of the deposition process itself, plating of any binary Ig–Tr
alloy fits to the following scheme: (i) If an Ig
2+ -containing solution is enriched in the
Tr compound, the Tr mole fraction increases in the deposit. (ii) As the Tr mole fraction
in the deposit increases, the current efficiency drops. (iii) The impact of the increase
in current density strongly depends on the deposition conditions. For baths relatively
rich for the iron group metal ions, the increase in current density often leads to a larger
Tr content of the deposit, while an inverse relationship is reported for baths rich in the
transition metal oxoanion. (iv) The pH of the baths used may vary in a wide range.
For metal-rich baths (in which c Citrate < c Ig ), the pH is between 2 and 6, while for
citrate-rich baths, the pH range is extended to 10. The pH itself is not a determining
factor for the deposition process but has to be considered with all other plating
conditions. (v) In parallel to the increase of the Tr content of the deposit, the grain
size decreases drastically, and the deposit structure becomes amorphous (while the
composition related to the structural turnover is system-specific). (vi) The variation
in temperature was often found to be controversial since the increase in temperature
can lead to a grain refinement, in contrast to the general trend found for many other
alloys. Such irregularities are due to the variation of the deposit composition with
temperature where the temperature elevation leads to a Tr enrichment and hence, a
concomitant structural transition. (vi) Additives are necessary to improve the wetting
of the surface (especially at low current efficiency when bubble formation is intense)
and to avoid the formation of cracks in the deposits.
The grain refinement with the increase in the Tr content of the alloy make this alloy
family an ideal candidate for structural studies, and the number of relevant studies
amounts to several hundreds. The reason for the easy achievement of the grain size
range below 10 nm probably lies in the fact that the self-diffusion coefficient of
the alloy components is relatively small, and the metastable structure being formed
cannot be rearranged at the temperature of the deposit formation. A representative
collection of some relevant papers is listed in Table 6.1.
The outstanding feature of the Ig-Tr alloys is that the grain size range of <10 nm
can be achieved fairly easily. In this range, the Hall–Petch relationship is inversed
since the deformation mechanism is no longer based on dislocation slipping, and
hence, hardness decreases as grains are further refined. This makes the Ig–Tr alloy
group an ideal candidate to study these grain size dependent mechanical properties
with samples of nearly identical composition. Another peculiar property of the Ig–Tr
alloy group is that the fine-grained structure can be retained as the alloys are annealed.
It is common that the grain growth is hindered by the segregation of the transition
metal element and the grain structure is composed of crystals smaller than 50 nm
even upon a heat treatment at 1000 K.
6 Nanocrystalline Deposits
of view, a suitable complexing agent is a must for achieving the codeposition. In the
overwhelming majority of the relevant studies, citric acid (or its sodium or ammonium
salt) is applied. The mechanistic explanation of the codeposition is based on the mixed
citrate complex of the Ig
2+ and TrO 4
n− ions (Ig: iron group metal, Tr: oxoanionforming transition metal). Without the complex facilitating the electron transfer, the
reduction of the oxoanions stops at an intermediate level and a non-conducting oxide
is formed, usually with a composition near to the TrO 2 formula.
From the viewpoint of the deposition process itself, plating of any binary Ig–Tr
alloy fits to the following scheme: (i) If an Ig
2+ -containing solution is enriched in the
Tr compound, the Tr mole fraction increases in the deposit. (ii) As the Tr mole fraction
in the deposit increases, the current efficiency drops. (iii) The impact of the increase
in current density strongly depends on the deposition conditions. For baths relatively
rich for the iron group metal ions, the increase in current density often leads to a larger
Tr content of the deposit, while an inverse relationship is reported for baths rich in the
transition metal oxoanion. (iv) The pH of the baths used may vary in a wide range.
For metal-rich baths (in which c Citrate < c Ig ), the pH is between 2 and 6, while for
citrate-rich baths, the pH range is extended to 10. The pH itself is not a determining
factor for the deposition process but has to be considered with all other plating
conditions. (v) In parallel to the increase of the Tr content of the deposit, the grain
size decreases drastically, and the deposit structure becomes amorphous (while the
composition related to the structural turnover is system-specific). (vi) The variation
in temperature was often found to be controversial since the increase in temperature
can lead to a grain refinement, in contrast to the general trend found for many other
alloys. Such irregularities are due to the variation of the deposit composition with
temperature where the temperature elevation leads to a Tr enrichment and hence, a
concomitant structural transition. (vi) Additives are necessary to improve the wetting
of the surface (especially at low current efficiency when bubble formation is intense)
and to avoid the formation of cracks in the deposits.
The grain refinement with the increase in the Tr content of the alloy make this alloy
family an ideal candidate for structural studies, and the number of relevant studies
amounts to several hundreds. The reason for the easy achievement of the grain size
range below 10 nm probably lies in the fact that the self-diffusion coefficient of
the alloy components is relatively small, and the metastable structure being formed
cannot be rearranged at the temperature of the deposit formation. A representative
collection of some relevant papers is listed in Table 6.1.
The outstanding feature of the Ig-Tr alloys is that the grain size range of <10 nm
can be achieved fairly easily. In this range, the Hall–Petch relationship is inversed
since the deformation mechanism is no longer based on dislocation slipping, and
hence, hardness decreases as grains are further refined. This makes the Ig–Tr alloy
group an ideal candidate to study these grain size dependent mechanical properties
with samples of nearly identical composition. Another peculiar property of the Ig–Tr
alloy group is that the fine-grained structure can be retained as the alloys are annealed.
It is common that the grain growth is hindered by the segregation of the transition
metal element and the grain structure is composed of crystals smaller than 50 nm
even upon a heat treatment at 1000 K.