6.2 Nanocrystalline Deposits of Metallic Elements
197
resistivity revealed a thickness dependence of the self-annealing kinetics that was
not reported for nc-Ag samples. Namely, no self-annealing was found for samples up
to a thickness of 0.9 μm, above which the rate of self-annealing increased with sample
thickness. Another difference as compared to nc-Ag samples was that the grain size
of crystals of various orientation, 111 and 200, all increased to the 150–200 nm level
upon self-annealing. In contrast to the self-annealing studies, the recrystallization
of other nc-Cu samples was achieved with annealing only [51]. The latter study
indicated that both the relaxation of microstrains and the grain growth has an onset
temperature of about 120 °C, and the two processes take place in parallel. Parabolic
grain growth kinetics was found for all annealing temperatures studied.
6.2.3 Nickel
Nickel was a key material in the study of electrodeposited metals. Research related to
nc-Ni started well in the twentieth century [54–64], and the first short review article
of the topic was published in 2000 [65]. The explanation for the role of Ni in the
research on nanocrystallinity was the relative ease of Ni deposition in nanocrystalline
form, the stability of the deposit, ferromagnetism of Ni and the importance of Ni in
corrosion protection.
The big majority of the works related to electrodeposited nc-Ni was based on
the Watts-type bath (containing nickel sulphate, nickel chloride and boric acid as
major components). Baths based on nickel sulphamate also proved to be suitable
[66–73], and a few studies were focused on the properties of other baths [17–19,
60]. Although there were some specific differences found between the deposits, all
bath types proved to be suitable in achievement of nanocrystallinity, and the grain
size was related rather to other aspects of the deposition process than the major bath
components. Grain size between 12 and 200 nm could be easily achieved with any
basic bath type. For decreasing the grain size below 10 nm, deep eutectic solvents
had to be used [74, 75], even though this grain size regime does not offer a further
property improvement and the deep eutectic solvents are considered to be very costly
as compared to aqueous baths.
Additives are often used for grain refinement with any major bath type, and the
variation in the additive concentration is an easy tool of grain size tuning. Saccharin
is a traditional and generally used grain refinement agent in nc-Ni deposition [17–
19, 62, 64, 72, 76–84], and it often leads to a tenfold or even higher degree of grain
refinement (depending also on other experimental conditions). It is applied up to 20
g dm
−3 concentration, although the maximal grain refinement efficiency is achieved
at the 3 g dm
−3 concentration (see [62, 82, 83] and references cited therein). The
impact of saccharin is partly based on its slow electrochemical reduction during which
sulphur atoms incorporate into the metallic deposit. Concerning the molecular-level
explanation of the impact of saccharin, a study performed with additive of similar
molecular structure can be recommended [85], while an attempt for a quantitative
description of the additive effect based on an adsorption model was offered by Rashidi
197
resistivity revealed a thickness dependence of the self-annealing kinetics that was
not reported for nc-Ag samples. Namely, no self-annealing was found for samples up
to a thickness of 0.9 μm, above which the rate of self-annealing increased with sample
thickness. Another difference as compared to nc-Ag samples was that the grain size
of crystals of various orientation, 111 and 200, all increased to the 150–200 nm level
upon self-annealing. In contrast to the self-annealing studies, the recrystallization
of other nc-Cu samples was achieved with annealing only [51]. The latter study
indicated that both the relaxation of microstrains and the grain growth has an onset
temperature of about 120 °C, and the two processes take place in parallel. Parabolic
grain growth kinetics was found for all annealing temperatures studied.
6.2.3 Nickel
Nickel was a key material in the study of electrodeposited metals. Research related to
nc-Ni started well in the twentieth century [54–64], and the first short review article
of the topic was published in 2000 [65]. The explanation for the role of Ni in the
research on nanocrystallinity was the relative ease of Ni deposition in nanocrystalline
form, the stability of the deposit, ferromagnetism of Ni and the importance of Ni in
corrosion protection.
The big majority of the works related to electrodeposited nc-Ni was based on
the Watts-type bath (containing nickel sulphate, nickel chloride and boric acid as
major components). Baths based on nickel sulphamate also proved to be suitable
[66–73], and a few studies were focused on the properties of other baths [17–19,
60]. Although there were some specific differences found between the deposits, all
bath types proved to be suitable in achievement of nanocrystallinity, and the grain
size was related rather to other aspects of the deposition process than the major bath
components. Grain size between 12 and 200 nm could be easily achieved with any
basic bath type. For decreasing the grain size below 10 nm, deep eutectic solvents
had to be used [74, 75], even though this grain size regime does not offer a further
property improvement and the deep eutectic solvents are considered to be very costly
as compared to aqueous baths.
Additives are often used for grain refinement with any major bath type, and the
variation in the additive concentration is an easy tool of grain size tuning. Saccharin
is a traditional and generally used grain refinement agent in nc-Ni deposition [17–
19, 62, 64, 72, 76–84], and it often leads to a tenfold or even higher degree of grain
refinement (depending also on other experimental conditions). It is applied up to 20
g dm
−3 concentration, although the maximal grain refinement efficiency is achieved
at the 3 g dm
−3 concentration (see [62, 82, 83] and references cited therein). The
impact of saccharin is partly based on its slow electrochemical reduction during which
sulphur atoms incorporate into the metallic deposit. Concerning the molecular-level
explanation of the impact of saccharin, a study performed with additive of similar
molecular structure can be recommended [85], while an attempt for a quantitative
description of the additive effect based on an adsorption model was offered by Rashidi