1
50
T. Kikuchi et al. / Journal of Alloys and Compounds 586 (2014) 148–154
reduction under the condition. In contrast, the complete reduction
of the zirconium oxide powder was observed when 50–100% ex-
cess calcium reductant was used (Fig. 2b and c), and it was clear
that a pure metallic zirconium could be produced under these
experimental conditions.
The electroless reduction of zirconium oxide in the present
investigation can be represented by the following electrochemical
reactions:
zirconium and its oxide is approximately 1.5 [27,28], and the mate-
rials should shrink to 67 vol% after electroless reduction. However,
the shrinkage of the materials was not observed after electroless
reduction (Fig. 3) because the reduced materials were sintered at
a high temperature (1173 K) during the electroless reduction.
The variation of the residual oxygen content in the reduced
materials, C
o
, for different calcium stoichiometries, e, is illustrated
decreases rapidly with an increasing in the calcium
in Fig. 4. The C
o
stoichiometry, and reaches 13.4 wt% at e = 100% because of the
incomplete reduction described in Figs. 2 and 3. Metallic zirconium
was produced by electroless reduction with e = 150% (Fig. 2b), and
2
Ca ¼ 2Ca þ þ 4eꢀ ðAnodic reactionÞ
2
ð4Þ
ð5Þ
ZrO
2
þ 4eꢀ ¼ Zr þ 2O2ꢀ ðCathodic reactionÞ
o
the C showed a small value of 1.33 wt%. When a large amount of
calcium reductant (e = 200%) was used, zirconium particles with
the lowest value of Co (0.16 wt% oxygen) were successfully ob-
tained. This suggests that the residual oxygen is also removed from
the reduced zirconium particles by the dissolved calcium in the
molten salt during the electroless reduction. In the metallic zirco-
nium reduced by calcium, the oxygen dissolved in the zirconium
lattice interstitially and formed a Zr–O solid solution. A small
amount of oxygen may have been captured as the byproduct
CaO, unreacted ZrO , and composite oxide CaZrO in the secondary
grains, although a thin native oxide film was formed on the mate-
rials after the investigations [17]. Metallic zirconium with low oxy-
gen content can be successfully obtained by electroless reduction
with a calcium reductant, and the lowest value of oxygen content
is acceptable for industrial materials.
The entire electrochemical reaction can be expressed as Eq. (3).
During electroless reduction, molten calcium metal (melting point:
1
1
115 K) may have floated to the surface of the molten CaCl
173 K because the density of molten calcium is lower than that
[16,26]. The floated molten calcium metal can be
easily dissolved in molten CaCl because the solubility is
.9 mol% calcium in CaCl molten salt at 1173 K [16]. The zirco-
nium oxide powder at the bottom of the titanium crucible can be
reduced by calcium metal dissolved in CaCl molten salt. A small
2
at
of molten CaCl
2
2
3
2
2
3
2
amount of molten calcium, however, may vaporize into the
SUS316L vessel during the electroless reduction because of its high
vapor pressure at high temperature [26]. Therefore, electroless
reduction using the calcium reductant with e = 100% resulted in
an incomplete reduction of the zirconium oxide powder, and a
lower zirconium oxide was formed during the electroless reduction
Electroless reduction of titanium dioxide powder was also per-
formed using a calcium reductant in CaCl molten salt at 1173 K for
2
(Fig. 2a). Approximately 20 mol% calcium oxide, a byproduct
1 h. A pure metallic titanium can be produced with excess calcium
formed by the reduction, can be dissolved in molten CaCl [16],
2
(e = 150–200%) in the same manner as the zirconium oxide reduc-
tion. Under these conditions, the analytical value of the oxygen
content in the reduced titanium particles was less than 1 wt%.
From XRD measurements for phase identification, titanium dioxide
was determined to have been reduced by the calcium reductant via
and the CaO formed on the materials dissolved rapidly in the mol-
ten salt. However, some of the calcium oxide will react with the
residual zirconium oxide powder by the following chemical
reaction:
a lower titanium oxide, Ti
6
O.
ZrO
2
þ CaO ¼ CaZrO
3
ð6Þ
3.2. Fabrication of Ti–Zr alloy from mixed oxides
Therefore, Zr
3
O and CaZrO
3
were observed by XRD (Fig. 2a) in
the incomplete reduction.
Powders of TiO
ball-milling and then reduced by calcium reductant in CaCl
2
and ZrO
2
with 70 at%ZrO
2
were mixed by wet
mol-
The excess calcium in the electroless reduction causes the com-
plete reduction of zirconium oxide, which is shown in Figs. 2b and
c, and a pure metallic zirconium can be obtained under this condi-
tion. A small amount of calcium still remains in the molten salt
after the electroless reduction. In fact, hydrogen gas evolution
can be observed while washing the molten salt with hot water;
the production of hydrogen gas can be expressed by the following
electrochemical reaction:
2
ten salt at 1173 K to obtain Ti–70 at%Zr alloy. Fig. 5 shows X-ray
diffraction patterns obtained from the mixed materials after elec-
troless reduction with e = 100–200% calcium reductant for 1 h. In
the case of the lower calcium content (e = 100%) (Fig. 5a), the
XRD pattern shows no peaks of TiO
nium and zirconium oxides (Ti O and Zr
as calcium zirconate, CaZrO . Conversely, the materials reduced by
2
2
and ZrO ; however, lower tita-
3
3
O) were observed, as well
3
Ca þ 2H
2
O ¼ CaðOHÞ þ H
2
ð7Þ
e = 150% and 200% were identified as metallic Ti–Zr alloy forma-
tions (Fig. 5b and c). The excess calcium reductant was also needed
for the complete reduction of the oxides because of the small
amount of liquid calcium that evaporated from the surface of mol-
ten salt. It is clear from Fig. 5 that the reduction behavior of the
2
Fig. 3a shows an SEM image of the surface of the zirconium
oxide micro-particles used as the starting materials in the electro-
less reduction. It is clear that the spherical shaped 5–10
ondary ZrO particles are agglomerations of few 100 nm primary
ZrO particles. Fig. 3b shows an SEM image of the material reduced
by calcium with e = 100% in CaCl molten salt at 1173 K for 1 h. Cal-
cium zirconate with well-defined cubic microstructures (approxi-
mately 5 m in diameter) and many small particles of lower
lm sec-
2
2 2
TiO and ZrO mixture was similar to the reduction behaviors de-
2
scribed in Fig. 2 for zirconium oxide. However, the residual oxygen
content in the materials reduced by e = 200% calcium for 1 h
showed a relatively high value of 1.8 wt% when measured by quan-
titative analysis, and the oxygen remained in the metallic Ti–Zr al-
loy after electroless reduction for 1 h. This result strongly suggests
that a longer reduction time is required to obtain a highly pure
Ti–Zr alloy.
2
l
zirconium oxide (a few hundred nm in diameter) are observed in
the SEM image and EPMA analysis. The particles and cubic struc-
tures are sintered together to form a sponge-like morphology with
a cluster size range from approximately 100 nm to 3
lm after long-
Fig. 6 shows the oxygen content in the reduced materials, C
o
, at
er electroless reduction (Figs. 3c and d). The appearance of these
two types of morphologies after longer electroless reduction is
attributable to the reduction of the zirconium oxide through inter-
different electroless reduction times, t, in CaCl molten salt at
2
1173 K. The oxygen content decreases rapidly as the reduction
time increased during the initial period for t = 2 h, for which the
oxygen content is 0.20 wt% oxygen. After the initial stage, the con-
tent decreases slightly with as the reduction time increased, and
3
mediate compounds, cubic CaZrO crystals and sub-micron spher-
ical Zr O particles. The Pilling–Bedworth ratio (P–B ratio) for
3
Kikuchi, Tatsuya
