Powders of chromium and chromium carbides of different morphology and narrow size distribution

Article abstract of DOI:10.1016/S0025-5408(98)00062-2

Powders of Cr and Cr carbides (Cr₂C, Cr₇C₃, and Cr₂₃C₆) have been prepared by hydrogen reduction of CrO_1.9 and Cr₃C₂ powders of different morphology, texture and specific surface area. The Cr and Cr carbides particles have the morphology of the parent compound and a narrow size distribution. Depending on the shape, the average size is of the order of some micrometers or some tens of micrometers. Starting with Cr₃C₂ allows the preparation of Cr powders at a temperature (1000 °C) markedly lower than that required when CrO_1.9 is used as the precursor (1300 °C). However, a Cr₃C₂ specific surface area of the order of 10 m²/g is necessary to achieve this within a reasonably short time (6 to 12 h). Powders of the lower carbides are prepared by using a milder hydrogen thermal treatment.

Full text of DOI:10.1016/S0025-5408(98)00062-2

Materials Research Bulletin, Vol. 33, No. 6, pp. 935–944, 1998
Copyright © 1998 Elsevier Science Ltd
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0025-5408/98 $19.00 ϩ .00
PII S0025-5408(98)00062-2
POWDERS OF CHROMIUM AND CHROMIUM CARBIDES OF DIFFERENT
MORPHOLOGY AND NARROW SIZE DISTRIBUTION
S. Loubie`re, Ch. Laurent,* J.P. Bonino, and A. Rousset
Laboratoire de Chimie des Mate´riaux Inorganiques, ESA CNRS 5070,
Universite´ Paul-Sabatier, 31062 Toulouse Cedex 4, France
(Refereed)
(Received October 16, 1997; Accepted October 20, 1997)
ABSTRACT
Powders of Cr and Cr carbides (Cr2C, Cr7C3, and Cr23C6) have been prepared
by hydrogen reduction of CrO1.9 and Cr3C2 powders of different morphology,
texture and specific surface area. The Cr and Cr carbides particles have the
morphology of the parent compound and a narrow size distribution. Depend-
ing on the shape, the average size is of the order of some micrometers or some
tens of micrometers. Starting with Cr3C2 allows the preparation of Cr powders
at a temperature (1000°C) markedly lower than that required when CrO1.9 is
used as the precursor (1300°C). However, a Cr3C2 specific surface area of the
order of 10 m2/g is necessary to achieve this within a reasonably short time (6
to 12 h). Powders of the lower carbides are prepared by using a milder
hydrogen thermal treatment. © 1998 Elsevier Science Ltd
KEYWORDS: A. carbides, A. oxides
INTRODUCTION
Micrometric metal and carbide powders of controlled morphology and uniform size distri-
bution are of considerable interest for industrial applications such as conductive inks or
pastes for electronic materials and also for powder metallurgy [1–4]. Metals such as Fe, Co,
Ni, Cu, and Ru and the noble metals have been prepared by reduction of the appropriate
precursor compounds, using notably the so-called polyol process [5], the precipitation from
homogeneous solutions [6], and the microwave-hydrothermal method [7]. An alternative to
*To whom correspondence should be addressed.
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these wet routes is the reduction in H2 or CO atmosphere of solid precursors which already
are monodispersed and of a definite shape. As pointed out by Matijevic [6], such conversions
can take place without a loss in the morphology or the dispersion of the final products only
under carefully controlled conditions. Thus, in contrast to the above-mentioned metals, a Cr
powder is very difficult to obtain by such methods, because of the difficulty in reducing
Cr2O3. Thibaudon et al. [8] have shown that decarburization in hydrogen of Cr3C2 yields the
lower carbides Cr7C3 and Cr23C6 and even Cr if the treatment is severe enough (1000°C,
150 h); however, they did not investigate powder characteristics such as morphology and
particle size.
Lerch and Rousset [9] have shown that the pseudomorphic decomposition in air of
chromium oxalates of controlled morphology (needles, rods, polyhedra, and spheres) could
lead to metastable chromium oxides of general formula CrO1.9 with the same morphology.
These products have a specific surface area in the 200–350 m2/g range and it has been shown
in previous works [10,11] that they are interesting precursors for the synthesis of the stable
Cr3C2 and metastable Cr3C2-x phases of controlled morphology while combining the advan-
tages of moderate preparation temperature (700°C) and high yield. The aim of the present
paper is to investigate the synthesis of Cr2C, Cr7C3, Cr23C6, and Cr powders of different
morphology and narrow size distribution using CrO1.9 and Cr3C2 as starting materials.
EXPERIMENTAL
The synthesis of the oxalates and CrO1.9 oxides was described in detail in previous papers
[9,10]. Briefly, ammonium trioxalatochromate(III) (NH4)3[Cr(C2O4)3] was prepared by mix-
ing the appropriate amounts of ammonium oxalate (NH4)2(C2O4)⅐2H2O and chromium(III)
nitrate Cr(NO3)3⅐9H2O in an aqueous solution heated at 60°C. The clear solution obtained
was cooled to room temperature and rapidly added to an organic solution (equivolumic
mixture of either acetone and methanol or ethanol and ethyleneglycol), in which precipitation
occurred immediately. Oxalates of different morphology, namely, needles, rods, polyhedra,
and spheres (more precisely, spherical agglomerates of platelets), were obtained depending
TABLE 1
Morphological and Microstructural Characteristics of the
Starting Oxide and Carbide Powders
Nature
Shape
Sw (m2/g)
Average sizea (␮m)
CrO1.9
CrO1.9
Cr3C2
Cr3C2
Cr3C2
Cr3C2
Cr3C2
Cr3C2
polyhedra
spheres
needles
rods
polyhedra
spheres
spheres
spheres
185
320
10.0
10.0
9.1
12.0
4.0
1.0
30
9
70 ϫ 10
70 ϫ 20
20
8
8
8
Sw: specific surface area.
aDetermined from measurements of SEM images.

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CHROMIUM CARBIDES
937
FIG. 1
SEM images showing Cr particles obtained from the polyhedral (a) and spherical (b) CrO1.9
oxides after a H2 reduction at 1300°C for 6 h.
on the relative proportions of aqueous and organic solutions and on the nature of the latter.
After filtering and ethanol washing, the powders were oven dried at 90°C for 48 h.
Powders of the metastable chromium oxide CrO1.9 were prepared by decomposition in air
at 300°C of the chromium oxalates of different morphology [9,10]. The so-obtained oxides,
which have the same morphology as the parent oxalates, were heat-treated for 2 h at 700°C
in an H2-CH4 atmosphere (10 mol% CH4) to yield Cr3C2 powders with the same morphology
and a specific surface area of the order of 10 m2/g as described elsewhere [10]. The gas flow
was dried on P2O5 and its composition was monitored using mass flow controllers. Other
specimens of the spherical Cr3C2 were prepared by treatments at 800 and 1000°C to obtain
powders with a lower specific surface area (4 and 1 m2/g, respectively).
The Cr3C2 powders as well as the polyhedral and spherical CrO1.9 specimens were then
submitted to a treatment in pure, dry H2 at different temperatures and for different times as
required for the study. The morphological and microstructural features of all the starting
powders are reported in Table 1.
TABLE 2
Phases Detected by XRD After Treating the Spherical
Cr3C2 Powders in H2 for 2 h at Different Temperatures
Temperature (°C)
S1
S4
S12
500
600
650
700
800
850
900
950
1000
Cr3C2
Cr3C2
Cr3C2
Cr3C2
Cr3C2
Cr3C2
Cr3C2
Cr3C2
Cr3C2
Cr3C2 ϩ Cr2C
Cr2C
Cr2C ϩ Cr7C3
Cr2C ϩ Cr7C3
Cr7C3
Cr7C3 ϩ Cr23C6
Cr7C3 ϩ Cr23C6
Cr23C6
Cr3C2
Cr3C2 ϩ Cr7C3
Cr3C2 ϩ Cr7C3
Cr3C2 ϩ Cr7C3
Cr7C3
Cr3C2
Cr3C2 ϩ Cr7C3
Cr3C2 ϩ Cr7C3
Cr3C2 ϩ Cr7C3
Cr7C3

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FIG. 2
XRD patterns of the products obtained from the Cr3C2 powder S12 heat-treated in H2 for 2 h
at (a) 500°C (Cr3C2), (b) 650°C (Cr2C), (c) 850°C (Cr7C3), and (d) 1000°C (Cr23C6).
The resulting materials were studied by scanning electron microscopy (SEM) and X-ray
diffraction (XRD) using Co K␣ radiation (␭ ϭ 0.17902 nm). The specific surface areas were
measured by the BET method using N2 adsorption at liquid N2 temperature. Chemical
analyses were performed by atomic absorption.
RESULTS AND DISCUSSION
Starting from the CrO1.9 oxides, it was only possible to obtain Cr particles containing less
than 0.5 wt% of oxygen with a H2 reduction after 6 h at a temperature as high as 1300°C.

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CHROMIUM CARBIDES
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TABLE 3
Phases Detected by XRD after Treating the Cr3C2 Powders
in H2 at 1000°C for Different Times
Needles and
Dwell time (h)
Spheres
Cr23C6
rods
Polyhedra
1
2
4
5
6
8
10
12
Cr7C3
Cr7C3 ϩ Cr23C6
Cr7C3
Cr7C3 ϩ Cr23C6
Cr7C3 ϩ Cr23C6
Cr7C3 ϩ Cr23C6
Cr23C6
Cr23C6 ϩ Cr
Cr23C6 ϩ Cr
Cr
Cr23C6
Cr23C6 ϩ Cr Cr7C3 ϩ Cr23C6
Cr23C6 ϩ Cr
Cr23C6
Cr23C6 ϩ Cr
Cr23C6 ϩ Cr
Cr
Cr
—
—
—
—
Furthermore, SEM observations (Fig. 1) show that the so-obtained Cr particles exhibit an
ill-defined morphology, which is due to some sintering during the treatment. No difference
in the reduction behavior was observed between the polyhedral and spherical powders despite
the difference in specific surface area (185 and 320 m2/g, respectively).
The spherical Cr3C2 powders of different specific surface area (1, 4, and 12 m2/g, hereafter
denoted as powders S1, S4, and S12, respectively) were heat-treated in H2 for 2 h at different
temperatures ranging between 500 and 1000°C. The phases detected by XRD patterns
analysis are reported in Table 2. When starting from the S1 Cr3C2 powder, Cr3C2 is still the
only phase detected in the XRD patterns of products reduced a temperatures lower than
900°C. A mixture of Cr3C2 and Cr7C3 is detected for specimens reduced at 900, 950, and
1000°C. Starting from S4, Cr7C3 is detected in addition to Cr3C2 after the 800, 850, and
900°C treatments and is the sole phase detected after the 950 and 1000°C treatments. As
expected, the reactivity of the S12 powder is higher than that of S1 or S4. Indeed, the S12
Cr3C2 (Fig. 2a) is gradually transformed into Cr2C, which is the sole product detected in the
powder treated at 650°C (Fig. 2b). Cr2C is then transformed into Cr7C3 (obtained at 850°C,
Fig. 2c), which in turn yields Cr23C6 at 1000°C (Fig. 2d).
In agreement with Leclercq et al. [12], we observed that decarburization occurs before a
complete removal of the excess free carbon; indeed, the amount of free carbon in the S12
Cr3C2 was found to be 1.5 wt%, whereas it decreases to 0.8 wt% in the product treated at
500°C, 0.3 wt% in Cr2C, and 0 wt% in Cr7C3 and Cr23C6. It should be noted that the
proportion of extra carbon in our Cr3C2 powder is much lower than that reported by Leclercq
et al. [12].
To achieve the obtention of Cr at 1000°C, the S12 spherical Cr3C2 and the differently
shaped Cr3C2 powders (needles, rods and polyhedra), which have a similar specific surface
area (ca. 10 m2/g), were heat-treated in H2 for different times ranging between 1 and 12 h.
The results are summarized in Table 3 and the XRD patterns of the powders obtained from
the spherical carbide are shown in Figure 3.
Whatever the morphology of the starting Cr3C2 powder, it is possible to prepare Cr7C3,
Cr23C6, and Cr, or to obtain mixtures of these compounds, depending on the dwell time. It
appears that the spheres and polyhedra are the most and least reactive powders, respectively,
and that the needles and rods have an intermediate behavior upon reduction, similar to each
other. Indeed, Cr23C6 is obtained after treatments of 1 h (spheres), 5 h (needles and rods), and

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Vol. 33, No. 6
FIG. 3
XRD patterns of some of the products obtained from the spherical Cr3C2 powder S12
heat-treated in H2 at 1000°C for (a) 1 h (Cr23C6), (b) 4 h (Cr23C6 ϩ Cr), and (c) 6 h (Cr).
6 h (polyhedra), and Cr is obtained after treatments of 6 h (spheres), 10 h (needles and rods),
and 12 h (polyhedra).
The pseudomorphic nature of the transformation is evidenced by SEM observations.
Indeed, we obtained Cr7C3, Cr23C6, and Cr powders in the shape of needles, rods, polyhedra,
and spheres (Fig. 4). The Cr needles and rods are, however, very similar to each other. The
size distribution seems rather narrow and the powders are not aggregated. The particle size
distributions (in number) were derived from measurements of SEM images (Fig. 5). De-
pending on the shape, the average particle size is of the order of some micrometers
(polyhedra, spheres) or some tens of micrometers (needles, rods). The length distribution of
the Cr3C2 and Cr powders in the form of needles and rods (Figs. 5a, b, e, and f) is, in fact,

Vol. 33, No. 6
CHROMIUM CARBIDES
941
FIG. 4
SEM images of Cr3C2 powders showing different shapes—needles (a), rods (b), polyhedra
(c), and spheres (d)—and the corresponding Cr powders (e, f, g, and h, respectively). The Cr
needles and rods are very similar.

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Vol. 33, No. 6
FIG. 5
Size distribution of the different Cr3C2 powders—needles (a), rods (b), polyhedra (c), and
spheres (d)—and the corresponding Cr powders (e, f, g, and h, respectively).
large and bimodal because some needles/rods have been broken at some point during the
manipulations. The size distribution is much narrower for the polyhedra and, in particular, for
the spheres (Figs. 5c, d, g, h).
The Cr particles are slightly smaller than those of the parent Cr3C2, because the reduction
treatment provokes a coalescence of the primary crystallites, in addition to the decarburiza-
tion, on the one hand, and a shrinkage of the grains, on the other hand, as shown in the case

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CHROMIUM CARBIDES
943
FIG. 6
Higher magnification SEM images showing the spherical Cr3C2 (a) and Cr powders (b).
of spheres by SEM observations (Fig. 6). It should be noted that the average size of a given
powder (CrO1.9, Cr3C2, Cr2C, Cr7C3, Cr23C6, or Cr) depends on that of the parent Cr oxalate,
which can be varied by minor modifications of the precipitation process.
The difference in reactivities among the various Cr3C2 powders is not due to differences
in their specific surface areas, which are similar to each other (ca. 10 m2/g, Table 1), but
rather in their texture. Indeed, in contrast to the other specimens, which are more monolithi-
cal, the spherical grains are made up of fine crystallites (100 nm) forming a kind of “gypsum
flower” texture (Fig. 6a), favor H2 diffusion and removal of the gaseous species in this
material. Similar results were observed for the reoxidation behavior of the Cr3C2 powders
[10].
CONCLUSIONS
Powders of different morphology (needles, rods, polyhedra and spheres) of the metastable
chromium oxide CrO1.9 and of Cr3C2 have been prepared by the appropriate heat treatments
of the corresponding Cr oxalate precursors. The hydrogen treatment of Cr3C2 allows prep-
aration of Cr powders at a temperature (1000°C) markedly lower than that required when
CrO1.9 is used as the starting material (1300°C). A carbide specific surface area of the order
of 10 m2/g is necessary to achieve this within a reasonably short time (6–12 h). Due to these
relatively mild experimental conditions, it is possible to obtain Cr powders of tailored
morphology and with a narrow size distribution. Depending on the shape, the average particle
size is of the order of some micrometers or some tens of micrometers. Similar powders of the
lower carbides (Cr2C, Cr7C3, and Cr23C6) can be prepared using still milder thermal
treatments. A controlled morphology would be essential if these powders were to be used for
preparing coatings by electrodeposition.
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