12. - 14. 10. 2010, Olomouc, Czech Republic, EU
DETERMINATION OF OXYGEN SPECIES IN Ta, Mo AND Re NANOPOWDERS
Sergey S. Shibaev, Konstantin V. Grigorovich
Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii pr. 49, 119991
Moscow, Russia, E-mail [email protected]
Abstract
In the present work, a method for the determination of oxygen species in nanopowders by inert gas fusion
technique has been developed. The method allows us to determine the content of oxygen in Ta, Mo and Re
nanopowders, which is present in the form of oxides, H2O and chemisorbed oxygen. The accuracy of oxygen
species identification was proved by thermo extraction and X-ray diffraction methods. Oxygen was found to
present in Ta, Mo, and Re nanopowders mainly as the Ta2O5, MoO2, and ReO2 oxides, respectively. The
content of oxygen in the form of H2O is shown to reach 50 % of the total oxygen concentration in the
nanopowders. The developed technique can be used for the quality control of nanopowders and optimization
of their production technology.
1.
INTRODUCTION
Modern science is characterized by evergrowing extent of investigations into nanoindustry. The search for
new applications of nanomaterials and goods produced from them continue. At the same time, a lot of works
are carried out to develop methods for the quality control of nanopowders.
Metallic nanopowders are known to be highly reactive owing to their large specific surface area. Thus,
oxygen is a unavoidable impurity in processing and storing nanomaterials.
The determination of both total content of oxygen and its species in nanopowders is of interest for
investigators. Information about oxygen species inside particles and on their surface is necessary for the
quality control of nanomaterials and optimization of their production technology. The oxygen content can
indicate the contamination of product with particles of another material or the completion of transformation
process (for example, of oxide into metal) [1].
Owing to the urgency of the problem, the purpose of this work was to develop a technique for the control of
oxygen species in Mo, Re and Ta nanopowders.
2.
EXPERIMENTAL
It was shown by authors [2] that water begins converting to CO on the hot walls of graphite crucible only at
1400 K and completes at 2000 K. In [2] authors succeeded in the determination of oxygen in the form of
adsorbed water, carbon–oxygen complexes weakly fixed on the surface, and surface oxides (WOx and
CoOx) in W-C-Co nanopowders.
In the present study, the size of nanopowder particles was measured by atomic-force microscopy. The
maximum size of particles is found to be less than 50 nm.
The oxygen contents in Mo, Re and Ta nanopowders were determined by infrared method after fusion under
an inert gas atmosphere using a LECO TC-600 analyzer. The determination was performed at the modes of
total and fractional analysis. The fractional gas analysis (FGA) is a modification of inert gas reducing fusion
12. - 14. 10. 2010, Olomouc, Czech Republic, EU
method at set temperature rate [3]. FGA is based on different thermodynamic stabilities and, hence, different
temperatures of the onset of carbon reduction in a melt. Thus, when setting conditions of monotonic heating
of a sample in a graphite analyzer crucible, curves of oxygen evolution in the form of CO and CO2 can be
recorded. These curves are spectra of peaks corresponding to oxide and other oxygen species reduction.
The peaks are characterized by specific temperatures of the onset and maximum of reduction, based on
which compounds can be identified [4].
Since the linearity of IR cell graduation for TC-600 was shown in the previous studies, the cells were
calibrated for a certified reference pure CaCO3 sample. Moreover, the calibration was verified by state
certified reference samples 502-201 and 501-646 produced by LECO with total oxygen contents of 0.296 ±
0.009 and 0.0421 ± 0.0014 wt %, respectively.
The nickel bath was not used in case of fractional gas analysis of oxygen in the Mo and Re nanopowders,
whereas the FGA of Ta samples was performed using the nickel bath. In the mode of total oxygen analysis,
all samples were placed in nickel caps and after that droped down in a hot crucible through a load device.
Before analysis, the caps were preliminarily held in analyzer's furnace at 1273 K for 90 s for removing the
chemisorbed oxygen and oxygen in the form of thin oxide films from the nickel surface. The conditions of
fractional and total analysis of nanopowders by the TC-600 analyzer are given in Table 1.
Table 1
Conditions of fractional and total analysis of nanopowders by TC-600 analyzer
Sample
Beginning
temperature,
K
Finishing
temperature,
K
Та
Мо
Re
673
473
673
2523
2173
2273
Та
Мо
Re
2473
2173
2173
2473
2173
2173
Heating
rate, K/s
Ni caps
FGA mode
3,2
+
3,5
2,7
Total analysis mode
+
+
+
Mean
sample
mass, g
Analysis
time, s
0,01
0,01
0,15
4
-
0,01
0,01
0,01
4
4
4
80
100
80
The water content in the nanopowder samples was also determined by RC-412 LECO analyzer. The IR-cell
was calibrated in using the state reference sample 509-091 (calcium oxalate, LECO) with a certified water
content of 12.21 ± 0.01 wt %. Before analysis, the samples were evenly distributed on the surface of quartz
boats preliminarily annealed in a resistance tube furnace at 1473 K for 15 min. The temperature and analysis
time were 448 K and 1200 s, respectively. Such a low analysis temperature allowed us on the one hand to
prevent the oxidizing of nanopowder particles with water oxygen and on the other hand to perform the
complete release of H2O from the samples.
3.
RESULTS AND DISCUSSION
The typical curves of oxygen extraction from the nanopowder samples during fractional gas analysis are
shown in Fig. 1. It was observed that oxygen releases from the Ta and Mo samples mainly in the form of
CO, but in analyzing Re up to 873 K – mainly in the form of CO2.
12. - 14. 10. 2010, Olomouc, Czech Republic, EU
Fig. 1. Typical curves of oxygen extraction from the nanopowder samples during fractional gas analysis:
a) Ta – without Ni bath; b) Ta – with Ni bath; c) Mo; d) Re. Straight lines show the calculated temperatures of
the onset of reduction of Ta2O5 and MoO2.
As it seen from Fig. 1a, surface oxygen releases from the Ta sample at start of the heating up to 1273 –
1373 K; oxygen in the form of oxides begins extracting at 1673 – 1873 K, progresses slowly and is not
completed right up to 2573 K. Since the Ta nanopowder was analyzed with the Ni bath, which made it
possible to decrease the reduction temperature of tantalum oxides to 1573 – 1673 K (Fig. 1b) and to realize
the complete oxygen extraction from the samples.
As of molybdenum, it should be noted there are three main stages of oxygen release from the samples
during FGA (Fig. 1c): 1 – surface oxygen extraction up to 973 K, 2 – evolving of oxygen present in the form
of molybdenum oxides consequently the solid-phase reaction at the contact region of the nanopowder
particles with a graphite crucible at 1023 – 1123 K, 3 – oxygen extraction as result of the oxides dissociation
at about 1373 K. It is known from the literature [5] that MoO2 dissociates at 1273 K.
The curve of oxygen released from Re nanopowders is characterized by extraction of both CO and CO2 at
low temperatures. According to x-ray phase analysis data, oxygen is present in Re samples in the form of
ReO2, which begins reducing during the FGA at 923 – 1023 K.
For the purpose of identification of the oxygen species in nanopowders, physicochemical calculations of
temperatures of the onset of reduction of tantalum and molybdenum oxides with a carbon crucible in crucible
in a flow helium atmosphere were performed. Previously, we studied the possibility of estimation of the
temperatures of iron oxides reduction during the FGA in using IVTANTHERMO data [6]. As a result,
12. - 14. 10. 2010, Olomouc, Czech Republic, EU
parameters of a thermodynamic model, which allow the analogous calculations to be performed, were
obtained. Unfortunately there are no thermodynamic functions for the rhenium oxides in the IVTANTHERMO
data base. For example, the initial conditions for reducing temperature of MoO2 (reaction (1)) were found to
be the following:
-3
-8
•
starting composition, mole: MoO2 – 10 ; He – 2·10 ; C – 0.1
•
equilibrium parameters: P = 1.85 atm.
MoO2(c) + 2C(c, graphite) + He(g) → Mo(c) + 2CO(g) + He(g)
(1)
The calculation results are shown in
Fig. 2. As is see from Fig. 2 the
beginning temperature of realization of
the reaction of solid-phase reducing of
MoO2 under the FGA conditions is
1050 K. In much the same way, the
temperature of Ta2O5 reduction was
estimated to be 1780 K. The presence
of Ta2O5 in nanopowder sample was
confirmed by x-ray phase analysis.
The results of gas analysis of the
nanopowder samples are given in
Table 2.
Fig. 2. Results of thermodynamic calculation of temperature of
the onset of carbon reduction of MoO2 in helium.
It was obtained that the total oxygen
contents
determined
by
TC-600
analyzer at the modes of total and
fractional analysis do not coincide. It follows from the quite low beginning temperature of FGA (473 – 673 K)
to transform water extracting from the samples to CO [2]. This effect allowed us to calculate the oxygen
content in the form of H2O ( O HTCO−600 ) as the difference between the oxygen content determined by TC-600 in
2
the mode of total analysis and the total oxygen content obtained by FGA (Table 2). In this study probable
chemisorbed oxygen except water was ascribed to surface oxygen. Oxide oxygen is oxygen in the form of
tantalum, molybdenum and rhenium oxides both on the surface of particles and inside them.
Table 2 Results of gas analysis of the nanopowder samples
Sample
Ta
Mo
Re
Surface
0.04 / 0
0.02 / 0
0.05 / 0
Oxygen content / Standart deviation, wt %
Analyzer
ТС-600
Total
FGA
O HTC2O−600
analysis
Oxide
Total
mode
1.81 / 0.02
1.85 / 0.02
1.95 / 0.04
0.10
1.07 / 0.01
1.09 / 0.01
1.23 / 0.02
0.14
0.31 / 0
0.36 / 0
0.67 / 0.01
0.31
RC-412
− 412
O RC
H2O
0.23 / 0.05
0.18 / 0.02
0.16 / 0.02
12. - 14. 10. 2010, Olomouc, Czech Republic, EU
The water content in nanopowder samples determined by thermo extraction technique using the RC-412
− 412
analyzer was recalculated to the oxygen content in the form of H2O ( O RC
). As is seen from Table 2,
H2O
− 412
O HTC2O−600 values are in agreement with O RC
values only in the order of magnitude. It results from both the
H2O
significant deviation in the case of low concentration determination of water by RC-412 and a IR-cell
calibration in using the state reference sample with a certified water content of 12.21 ± 0.01 wt %. Therefore
− 412
one should consider the O HTC2O−600 values as more accurate than the O RC
values.
H2O
4.
CONCLUSIONS
In the present work, the method for the oxygen species determination in nanopowders by inert gas fusion
technique has been developed. The method allowed us to determine the content of oxygen in tantalum,
molybdenum and rhenium nanopowders, which is present in the form of: 1) oxides, 2) H2O, 3) chemisorbed
oxygen. The accuracy of oxygen species identification was proved by thermo extraction and X-ray diffraction
methods.
It was shown that thermodynamic methods can be used for the estimation of the temperatures of solid phase
reduction of oxides under the fractional gas analysis conditions. The temperature of the onset of reaction of
solid-phase reduction of MoO2 was calculated using IVTANTHERMO data; it is equal to 1050 K; for Ta2O5,
the reduction temperature was estimated to be 1780 K.
It was obtained that, in Ta nanopowder, oxygen presents mainly in the form of Ta2O5; in Mo powder, it is
found as MoO2; in Re powder, oxygen is in the form of ReO2. The oxygen content in the form of H2O is
shown to reach 50 % of the total oxygen content in the nanopowders.
The developed technique can be used for the quality control of nanopowders and optimization of their
production technology.
ACKNOWLEDGEMENTS
This work was supported by the Russian Foundation for Basic Research, project no. 10-03-00826-а.
LITERATURE REFERENCES
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[2]
KRASOVSKII P.V., BLAGOVESHCHENSKII YU.V., AND GRIGOROVICH K.V. Determination of oxygen in W-C-Co
nanopowders. Inorganic materials, 2008, vol. 44, nr. 9, pp. 954-959.
[3]
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Int. Conf. of Analyt. Chem. in the Steel and Metal Industr. Luxembourg, 1994, pp. 527-532.
[4]
GRIGOROVITCH K.V. Fractional gas analysis – a new trend in quality control. Analitika i control, 2000, 4 (3), pp. 244-251.
[5]
MINERALOGY DATABASE: http://webmineral.com
[6]
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