Chemistry Reference
In-Depth Information
important observation was that all precipitates obtained under conditions of re-
duction of metals in the aqueous solution were characterized by significant incor-
poration of metal boride. Later it will be shown that cobalt or nickel coatings ob-
tained by heterogeneous reduction in the bulk of polyimide modified layer do not
contain boron.
The reaction of sodium borohydride with silver results in the formation of pure
metal, independent of the acidity of the reduction medium and presence of ligands
[26, 27].
8Ag
+
+ BH
4
-
+ 8OH
-
→ 8Ag + H
2
BO
3
-
+ 5H
2
O (3)
Reduction of Cu
2+
/PAA2 films is complicated and indefinite. The process of
Cu
2+
→ Cu
0
in solutions goes via different intermediate products and depends on
media conditions as well as reagents ratio. Moreover, reduction of the films is
complicated by the heterogeneous nature of the process, i.e. it is limited by diffu-
sion of the BH
4
-
ions into the film. Previously [28] it was stated that interaction of
NaBH
4
with CuCl
2
resulted in different products depending on the ratio (α =
CuCl
2
:NaBH
4
) of the reagents and pH of the solution:
-
at α = 0.1-0.25; pH 9-9.5
the reaction proceeds as:
CuCl
2
+ 2NaBH
4
+ 6H
2
→ Cu + 2H
3
BO
3
+ 2NaCl + 7H
2
(4)
- at α = 0.25-0.5; pH 7-9:
CuCl
2
+ 2NaBH
4
+ 6H
2
O → CuH + 2H
3
BO
3
+ 2NaCl + 6.5H
2
(5)
- at α = 1; pH 5:
2CuCl
2
+ 2NaBH
4
+ 6H
2
O → CuCl + CuH + 2H
3
BO
3
+ 2NaCl + HCl + 6H
2
(6)
According to reaction (4) metallic copper can be formed in the film under con-
ditions of large excess of BH
4
-
ions (α = 0.1 to 0.25). This is not acceptable in
conditions of heterogeneous process because the reaction proceeds at large excess
of Cu
2+
ions in the bulk of chelated modified layer and is limited by diffusion of
BH
4
-
ions into the film. The black film formed corresponds to CuH/PAA2 film
according to reaction (5). This is confirmed by chemical conversions of CuH/
PAA2 film which are similar to the conversions of CuH: gas liberation (H
2
) is ob-
served in the immersion of the CuH/PAA2 film into NaOH solution; in ammonia
solution the film takes on its initial slightly blue color (Cu
2+
/PAA2); CuH repre-
sents itself as a strong reducing agent. During immersion of CuH/PAA2 black
film into a solution of AgNO
3
the intense process of H
2
liberation and formation
of metallized silver surface with mirror reflection takes place.
3.3. Metallized polyimide films
Polyimide films (Kapton and Upilex) were metallized by similar methodology:
hydrolysis, chelation and reduction. All films were covered with metallic lustre
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