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Mechanism of Molten-Salt-Controlled Thermite Reactions


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The present work was undertaken to study the chemistry and phase formation mechanism in the salt-controlled MoO3 + Mg + NaCl thermite reaction. It was found that the structure and phase formation mechanism in the studied system primarily depend on the salt content in the initial mixtures. In salt-poor mixtures, nucleation of product particles takes place in the molten magnesium, whereas under salt-rich conditions, products are mainly formed in molten sodium chloride. Analyses of combustion temperature profiles and product microstructures and thermal analysis of reacting mixtures suggested that the molybdenum oxide reacts with the salt at early stages of the process. The formed intermediate molybdenum oxychloride and sodium molybdate then react with magnesium, yielding Mo, MgO, and NaCl phases. The low value of the activation energy (50 kJ/mol) of the combustion process also suggests that gaseous (liquid) intermediates play an important role in the phase formation mechanism.
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Published: August 29, 2011
r2011 American Chemical Society 10982 |Ind. Eng. Chem. Res. 2011, 50, 1098210988
Mechanism of Molten-Salt-Controlled Thermite Reactions
Khachatur V. Manukyan,*
Khachatur G. Kirakosyan,
Yeva G. Grigoryan,
Ok M. Niazyan,
Armenuhi V. Yeghishyan,
Artavazd G. Kirakosyan,
and Suren L. Kharatyan
Laboratory of Kinetics of SHS Processes, A. B. Nalbandyan Institute of Chemical Physics, National Academy of Sciences of the Republic
of Armenia (NAS RA), 5/2, P. Sevak Street, Yerevan 0014, Armenia
Department of Inorganic Chemistry, Yerevan State University, 1, A. Manoogian Street, Yerevan 0025, Armenia
ABSTRACT: The present work was undertaken to study the chemistry and phase formation mechanism in the salt-controlled
+ Mg + NaCl thermite reaction. It was found that the structure and phase formation mechanism in the studied system
primarily depend on the salt content in the initial mixtures. In salt-poor mixtures, nucleation of product particles takes place in the
molten magnesium, whereas under salt-rich conditions, products are mainly formed in molten sodium chloride. Analyses of
combustion temperature proles and product microstructures and thermal analysis of reacting mixtures suggested that the
molybdenum oxide reacts with the salt at early stages of the process. The formed intermediate molybdenum oxychloride and sodium
molybdate then react with magnesium, yielding Mo, MgO, and NaCl phases. The low value of the activation energy (50 kJ/mol) of
the combustion process also suggests that gaseous (liquid) intermediates play an important role in the phase formation mechanism.
The term thermite reactionis used to describe a class of
reactions that involves a metal reacting with a metal or nonmetal
oxide. This form of oxidationreduction reaction can be written
in general form as
where M (typically Mg, Al, Ti, Zr, Zn, etc.) is a metal, A (MoO
, TiO
, SiO
, CuO, etc.) is either a metal or a
nonmetal, MO and AO are their corresponding oxides, and ΔHis
the heat generated by the reaction.
Because of the large ex-
othermic eect, thermite reactions can generally be initiated locally
and become self-sustaining, a feature that makes their use ex-
tremely energy-ecient. Many thermite reactions yield a molten
product consisting of a heavier metallic phase and a lighter oxide
phase that can be separated by gravity and surface tension forces.
The latter makes these reactions potentially useful in a variety of
metallurgical applications.
More recently, thermite reactions
have become important in the synthesis of refractory ceramics,
composite materials,
and metal powders.
However, in-
tense gas evolution due to the decomposition/vaporization of initial
oxides and/or reducing elements coupled with high reaction
temperatures make it dicult to control the microstructure of
the obtained materials. Therefore, some approaches have been
adopted to soften violent reaction conditions and tune the mor-
phology of the products. One of the most recognized methods is the
application of so-called inert diluents. Addition of diluents to
thermite mixtures eectively reduces the combustion tempera-
ture and reaction rate because of the production of less heat and
the longer transport distances between reactants. A modied pro-
cess of conventional thermite reactions with halide salt additives is
known as molten salt-controlled combustion synthesis.
The basic precursors for the process are known higher oxides
of transition metals such as WO
. Metallic
magnesium and zinc are frequently used as reduction agents.
For certain oxides (WO
), sodium azide (NaN
), and
sodium boron hydride (NaBH
) can also be used as reducing
Recently, it was shown that one can use this method to
synthesize not only nanopowders of pure metals but also dierent
carbides (e.g., TiC,
), silicides (e.g, MoSi
), and complex
compositions such as WCCo.
Two main factors are important in controlling the microstruc-
ture of the products in salt-controlled thermite reactions. The
rst factor is mild reaction conditions, such as low temperatures,
which prevent intense grain growth. Second is the presence of a
molten inert phasein the reaction zone. Because of the heat
generated by self-sustaining reaction, the salt melts at about
800 °C, and further nucleation of product particles occurs in the
molten salt environment, which protects them from agglomera-
tion and grain growth. In all published works, however, the eect
of sodium chloride on the chemistry of combustion process was
not studied, and salt was always considered as only an inert
diluent. This does not rule out the possibility that, in the initial
stages of the reaction, metal oxides might react with salt yielding
various intermediates. For instance, it is well-documented
that MoO
reacts intensely with NaCl at 400800 °C, forming
and Na
. Early research
on the interaction
of transition metal oxides, namely, Ta
with sodium chloride showed intense weight loss at 650950 °C,
which is conditioned by evaporation of sodium chloride, volatile
initial metal oxide, metal chlorides, and oxyclorides. The aqueous
solutions obtained after water treatment of the metal oxide
NaCl reaction products contains oxyanions and chloride species.
The concentrations of soluble metal species varied from several
Received: February 21, 2011
Accepted: August 23, 2011
Revised: August 2, 2011
10983 |Ind. Eng. Chem. Res. 2011, 50, 10982–10988
Industrial & Engineering Chemistry Research ARTICLE
hundred parts per million to several thousand parts per million.
The soluble metal species in the solutions were in the form of
either metal chloride or MeO
and MeO
. Products formed
from the MoO
+ NaCl interaction showed relatively high
concentrations of soluble species (e.g., Na
). Signicant
losses of tungsten conditioned by volatile tungsten oxychloride
vaporization during the WO
electroreduction in molten salt
media at 900 °C were also reported.
Therefore, studying the inuence of sodium chloride on the
initial stages of the process in the salt-controlled thermite re-
actions is of special interest. The present work focused on studying
the eects of sodium chloride on the chemistry and phase forma-
tion mechanism of salt-controlled combustion reaction in the
+ Mg + NaCl system. This system was selected because
earlier research
showed that molybdenum trioxide reacts with
sodium chloride more intensely than other transition metal oxides.
2.1. Combustion Experiments. The precursors used in this
study included MoO
(technical condition of manufacturing no.
6-09-4471-77, Pobedit Co., Vladikavkaz, Russia, purity 99.5%,
particle size <5 μm), magnesium (MPF-3, AVISMA, Verkhnaya
Salda, Russia, particle size 150300 μm), and sodium chloride
(high grade, Michailovskii Factory of Chemical Reagents, Michai-
losvk, Russia, purity 99.9%, particle size <20 μm) powders. The
precursors were thoroughly hand-mixed in the desired ratio in a
ceramic mortar for 1 h to ensure homogeneity of the reaction
medium, which was then uniaxially cold-pressed into cylindrical
pellets (diameter 20 mm, height 40 mm) at a pressure of 20 MPa
to relative densities in the range of 0.450.5. Two thermocouple
holes (2 mm in diameter, 10 mm deep) were drilled into each
specimen perpendicular to the cylinder axis at a spacing of 10 mm.
Combustion experiments were conducted in a laboratory constant-
pressure reactor (CPR-3 L, Sapphire Co., Abobyan, Armenia).
Before reaction initiation, the reaction chamber was sealed,
evacuated, and purged with argon (purity 99.8%, oxygen content
no more than 0.1%) for three cycles and finally filled with argon
to the desired pressure (1 MPa). A tungsten coil, positioned at
the upper surface of the sample, was electrically heated until the
reaction was initiated locally, after which the power was immedi-
ately turned off, as the reaction wave propagated along the sample.
The temperaturetime distributions (temperature prole)
at given points of the reacting samples were recorded by two
0.1-mm-diameter C-type thermocouples. To ensure the stability
of the measurements, the thermocouples rst were sputtered by a
thin layer of boron nitride. The output signals of the thermo-
couples were transformed by a multichannel data acquisition
board at a rate of 2 kHz and were recorded on a computer. The
maximum combustion temperatures (T
) were determined from
the maxima of the temperature proles. The average values of
combustion velocity (U
) were calculated from the physical dis-
tance between the thermocouples and the temporal distance be-
tween the signals of the thermocouples. All experimental data
points for the combustion parameters were determined as averages
of at least three measurements. The standard measurement
errors for T
and U
were (20 °C and 5%, respectively. Average
heating rates of initial reagents in the combustion wave were
determined from the temperaturetime proles.
2.2. Characterization of Materials. After the combustion
process, reacted samples were kept in the reactor to complete
cooling. Typical purification operations for crushed solid products
include treatment with warm deionized water and dilute hydro-
chloric acid (5 wt %). After the water treatment the content of
metals (Mo, Mg, Na, Ca, etc.) in the obtained solution were
analyzed by ELAN-9000 ICP mass spectrometer. The drying of
purified material was performed in a vacuum furnace at a tem-
perature of 80 °Cfor4h.
The combustion products were studied by XRD analysis with
Cu Kαradiation (diractometer DRON-3.0, Burevestnik, Russia).
XRD analyses of samples were performed at 25 kV and 10 μA.
Nova 230 and Hitachi S4800 eld-emission scanning electron
microscopes were used to study the microstructure and compo-
sition of the produced materials by the energy-dispersive spec-
troscopy (EDS) method. The spatial resolution of the EDS
analysis was 1.52μm. The specic surface areas of the ob-
tained molybdenum powders were determined by the Brunauer
EmmettTeller (BET) method using nitrogen adsorption
(Gasometer, GKh-1). The oxygen content in the molybdenum
powder was determined with a LECO TC400 analyzer.
Dierential thermal analysis (DTA) of initial reagents and
reacting mixtures was performed using a Derivatograph Q1500
instrument (MOM, Budapest, Hungary). DTA investigations
were conducted in argon ow (7 mL/s) at 20 °C/min heating
rate. Oxidation of the prepared molybdenum powder in air was
investigated by the DTA technique at a heating rate of 20 °C/min.
3.1. Thermodynamic Considerations. Before experiments,
was used to consider the thermo-
dynamics in the MoO
+ 3Mg + nNaCl (where nis the number of
moles of salt) system. This software is specially designed to
calculate adiabatic temperature (T
) and product equilibrium
compositions in heterogeneous chemical processes. The inert gas
pressure in these calculations was kept at 1 MPa. The results
(Table 1) suggest that the calculated T
value for the diluent-free
mixture (n= 0) is about 3500 °C. Increasing nto 12 decreases the
reaction temperature to 800 °C. The equilibrium products at T
2000 °C consist of Mo, MgO, andNaCl, as well as small amounts of
different molybdenum oxychlorides. At elevated temperatures, the
amounts of molybdenum oxychlorides (MoOCl
and MoO
in the products are relatively higher (Table 1). Some amounts of
and MoO
are also present in the products.
Table 1. Adiabatic (T
) and Measured Maximum (T
Combustion Temperatures, Amounts of Molybdenum Oxy-
chlorides Formed (C), Heating Rates of Reagents in the
Combustion Wave (V
), and Flame Propagation Velocities
) for MoO
+ 3Mg + nNaCl Mixtures
0 3500 0  
2 2260 0.025 2050 1500 0.33
3 1970 0.04 1760 350 0.15
4 1970 0.05 1330 50 0.07
4.5 1880 0 930 35 0.04
5 1780 0 770 20 0.03
6 1600 0 no combustion
8 1270 0
10 1030 0
12 800 0
10984 |Ind. Eng. Chem. Res. 2011, 50, 10982–10988
Industrial & Engineering Chemistry Research ARTICLE
Equilibrium concentrations of the products for the MoO
mixture as a function of temperature were calculated as well. Figure 1
shows that, at T<770°C, no reaction occurred between the salt and
molybdenum trioxide. In the 770 < T<1200°Crange,liquid
and gaseous MoO
were the main products of the
reaction between sodium chloride and molybdenum oxide. Thus,
these calculations predicted that, for molten-salt-assisted combustion
reactions, sodium chloride cannot be considered as an inert medium.
3.2. Combustion of MoO
+ Mg + NaCl Mixtures. Fol-
lowing the thermodynamic analysis, combustion processes in the
+ 3Mg + nNaCl mixtures were studied under 1 MPa inert
gas pressure. Combustion temperature profiles for these mix-
tures are shown in Figure 2. Inspection of all of the data suggests
(Table 1 and Figure 2) that the maximum combustion tempera-
ture (T
) decreased dramatically as the salt content grew. The salt
amount was found to have a critical point (n= 5) above which
the combustion wave did not propagate throughout the sample.
The measured maximum temperature at this point was about
770 °C. Note that the experimentally measured values for the
combustion temperature were significantly lower than the ther-
modynamically calculated adiabatic temperatures (Table 1). This
disagreement is a result of heat losses that usually occur during
the combustion process. In all cases, the combustion wave prop-
agated within the sample in the steady-state regime.
Another combustion feature that is signicantly aected by the
salt amount is the heating rate (V,°C/s) of the reagent in the
reacting zone, determined from the temperature proles
(Table 1). This parameter for n= 2 is about 1500 °C/s. For
n=3,V= 350 °C/s. Unusually low heating rates, from 20 to
50 °C/s, for combustion synthesis processes were observed at
high n. The temperature proles (Figure 2) recorded for most n
values contained constant-temperature plateaus at 450510,
650, and 790820 °C. As can be seen, these plateaus were
well-dened, especially at low heating rates (high nvalues).
The salt concentration strongly inuences the ame propagat-
ing velocity (U
) as well. U
fell by more than a factor of 10,
from 0.33 to 0.03 cm/s, as the salt content increased to 5 mol
(Table 1). U
exhibited a dramatic dependence on T
, which
provides a basis for the determination of the apparent activation
energy of the combustion process. The relationship correlating
combustion temperature, ame velocity, and activation energy is
expressed by the simplied equation
ln Uc
¼constant Eeff
Figure 1. Calculated equilibrium compositions of products for the
+ NaCl mixture.
Figure 2. Temperaturetime distributions at combustion of MoO
+ 3Mg + nNaCl mixtures for n= (a) 2, (b) 3, (c) 4, (d) 4.5, and (e) 5.
10985 |Ind. Eng. Chem. Res. 2011, 50, 10982–10988
Industrial & Engineering Chemistry Research ARTICLE
where E
is the eective value of activation energy of the re-
action responsible for the combustion wave propagation rate and
Ris the universal gas constant. As a result, the slope of a plot of
) versus 1/T
can provide the eective value of the
activation energy of the process, as depicted in Figure 3. From the
slope of the best-t line of all of the data in Figure 3, a value for
the activation energy of 50 kJ/mol was obtained.
3.3. Characterization of Combustion Products. XRD anal-
ysis showed that materials obtained from the MoO
+ 3Mg +
nNaCl mixtures always consisted of Mo, MgO, and NaCl. Figure 4
shows XRD patterns of products formed at n= 4 before and after
purification. The XRD pattern of the leached solid material
contains only the diffraction lines of molybdenum.
The molybdenum content in the solutions obtained by water
extraction of the reacted materials varied from 600 to 800 mg/L.
This result suggests that the reacted sample contained water-
soluble molybdenum species. The calculated amounts of water-
soluble molybdenum in the reacted samples varied from 1.25 to
1.45 wt %.
Most of the reacted samples contained dened zones formed
from the solidication of molten salt, as shown in Figure 5a.
Molybdenum, magnesium, and oxygen identied in these zones
by microanalysis suggest that at least some portion of the products
formed in the molten salt medium. Note that such zones were
most frequently observed for the salt-rich samples. The other
type of morphology observed in salt-poor samples is shown in the
Figure 5b. In the magnied image (Figure 5c) of the same area,
white particles with sizes of 0.31μm represent molybdenum.
It can be seen that these particles predominantly formed at the
grain boundaries of several-micrometer-sized magnesia crystals.
The combustion products also contained whiskers with dia-
meters of 0.51μm and lengths on the order of tens of
micrometers (Figure 5d). The whiskers consisted of molybde-
num, sodium, chlorine, magnesium, and oxygen, as estimated by
microanalysis. The high-magnication micrograph shows the
whiskers covered by particles with dimensions of 0.050.3 μm.
The morphology of the puried product obtained at n=4is
presented in Figure 5e,f. Here, two characteristic fractions of
particles can be clearly distinguished. The sizes of the particles in
the rst fraction vary from 0.3 to 1 μm, whereas the second
fraction contains ne particles with dimensions of 0.050.3 μm.
Note that the puried material did not contain whiskers.
The specic surface area of molybdenum powder was mea-
sured to be 1.35 m
/g. The oxygen content in the molybdenum
powder was about 0.2 wt %. The oxidation onset of the powder in
ambient air, as determined by the DTA method, was 400 °C.
3.4. DTA Investigation of MoO
+ Mg + NaCl Mixtures. To
track the dynamics of phase formation in the studied system, the
thermal behaviors of individual reagents and their various com-
binations were studied by DTA as well. The DTA trace for MoO
shows two endotherms (Figure 6). The first endotherm, ob-
served at 400450 °C, corresponds to the molybdenium trioxide
αfβpolymorphic trasformation.
The second endotherm
starting at 750 °C corresponds to the sublimation of MoO
TG curve displays intense weight loss. Figure 6 also displays DTA
traces for Mg and NaCl. The expressed endotherms at 650 °C
(for Mg) and 800 °C (for NaCl) correspond to the melting
points of the reagents. Sublimation of melted magnesium and salt
started at 770 and 880 °C, respectively.
The behaviors of the 3Mg + 5NaCl, MoO
+ 3Mg, and MoO
5NaCl binary mixtures were also studied by DTA. The DTA
trace for the 3Mg + 5NaCl mixture shows two endothermic
eects coinciding with melting of the individual reagents (Figure 7).
Intense exothermic reaction followed by Mg melting for binary
+ 3Mg mixture corresponds to Mg reduction of molybdnum
trioxide. The exothermic eect for the binary MoO
mixture accompanied by weight loss of the reacting sample at
450650 °C (Figure 7) is conditioned by the evaporation of the
formed MoO
Note that the XRD pattern of rapidly
quenched material recorded after this experiment showed dif-
fraction lines for NaCl, MoO
, and Na
pounds (Figure 8).
Finally, the results of DTA for the MoO
+ 3Mg + 5NaCl
mixture are displayed in Figure 9. Here, melting of the reducer
coincides with the start of the intense exothermic reduction of
by Mg. The DTA trace shows that the exothermic reaction
and melting of sodium clroride are slightly overlapped. Most
likely, the exothermic MoO
+ NaCl reaction was not detected in
this DTA curve because of reletively high concentration of sodium
chloride. Nevertheless, some small weight loss was recorded in the
TG curve at 470530 °C.
To date, numerous investigations of salt-controlled thermite
reactions have been performed using dierent metals and metal
oxides, studying burning rates, types and amounts of salt used,
inuences of particle size and gas pressure, and so on. However,
the mechanisms of salt-controlled thermite reactions are still far
from being completely understood.
Figure 3. ln(U
plot for calculation of the eective
activation energy.
Figure 4. XRD patterns of the (a) reaction products of MoO
+ 3Mg +
4NaCl mixture and (b) puried metal.
10986 |Ind. Eng. Chem. Res. 2011, 50, 10982–10988
Industrial & Engineering Chemistry Research ARTICLE
Addition of salts of alkali metals (e.g., NaF, KF, NaCl, and KCl),
alkaline earth metals (e.g., CaF
, MgF
), and cryolite (NaAlF
can increase the combustion rate of thermite mixtures with
aluminum reducers.
The highest combustion rate has been
found for compositions containing aluminum uoride and
It has been proposed that such salts reduce the
temperature at which the reaction between the oxide and the
aluminum commences. The oxide lm on the aluminum particle,
which acts as a barrier to the interaction, can be disintegrated by
uorides at temperatures signicantly lower than the ignition
temperature of the thermite, and consequently, the ignition
temperature of the thermite mixture with salt addition is notably
Alkali metal chlorides (e.g., NaCl) are mainly used as control-
ling agents in salt-controlled thermite reactions with magnesium
as the reducer.
In such processes, the heat generated by
the self-sustaining reaction causes the salt to melt at about 800 °C.
Further nucleation of product particles occurs in the molten salt,
which protects them from agglomeration and growth. In all cases,
salt addition signicantly reduces the combustion rate and max-
imum combustion temperature in the Mg-thermite reactions.
Therefore, the salt was always considered as an inert medium,
Figure 5. Microstructure of the products obtained: (a) product of the MoO
+ 3Mg + 4NaCl mixture, (b,c) product of the MoO
+ 3Mg + 2NaCl
mixture, (d) whiskers formed as the product of the MoO
+ 3Mg + 4NaCl, (e,f) puried Mo particles of the initial MoO
+ 3Mg + 4NaCl mixture.
10987 |Ind. Eng. Chem. Res. 2011, 50, 10982–10988
Industrial & Engineering Chemistry Research ARTICLE
and the eects of the salt on the reaction mechanism have not yet
been considered. The thermodynamic analysis in the present
study predicts that, under combustion mode, MoO
reacts with
NaCl, yielding Na
and MoO
The obtained experimental results suggest that the reaction
parameters in the MoO
+ Mg + NaCl system depend strongly
on the salt content. The process temperature can be reduced to
800 °C as the salt content in the reacting mixture is increased.
The salt amount also dramatically aects the heating rate of re-
agents in the combustion front. Temperature prole analysis
enables the observation of well-dened plateaus at 480550,
650, and 800 °C. Thermal analysis of the precursors and their
mixtures helps clearly identify these temperature plateaus. Parti-
cularly at 450650 °C, molybdenum oxide reacts exothermically
with salt according to the reaction
2MoO3ðsÞþ2NaClðsÞ¼MoO2Cl2ðgÞþNa2MoO4ðs, lÞ
The molybdenum detected in the water extracts of reacted
samples and XRD analysis of quenched samples also conrm the
mechanism of reaction 1. Temperature prole analyses and DTA
investigations suggest that the following reaction starts just after
melting of the reducer
Microstructure analysis shows that, under salt-poor condi-
tions, the nucleation of product particles mainly takes place in the
molten magnesium. It is obvious that molybdenum particles are
separated from the melt upon cooling and appear at the grain
boundaries of the formed magnesia crystals. In salt-rich mixtures,
the nucleation of product particles mainly takes place at molten
NaCl media. Whiskers formed by a vaporliquidsolid mechanism
indicate that the following reaction might occur during ame
Na2MoO4ðs, lÞþMoO2Cl2ðgÞþ6Mgðl, gÞ
¼2MoðsÞþ6MgOðsÞþ2NaClðg, lÞð3Þ
It is assumed that ne Mo particles (0.050.3 μm) in the
puried product (see Figure 5f) are mainly formed by reaction 3.
The low activation energy (50 kJ/mol) of the combustion
reaction might provide additional evidence of the important role
of gaseous (liquid) intermediates in phase formation processes.
The thermodynamic analysis and experimental results of this
work show that the main combustion parameters (temperature,
Figure 6. DTA and TG curves of initial precursors: (1,1*) MoO
, (2,2*)
NaCl, and (3,3*) Mg.
Figure 7. DTA analyses of (1,1*) MoO
+ 3Mg, (2,2*) 3Mg + 5NaCl,
and (3,3*) MoO
+ 5NaCl binary mixtures.
Figure 8. XRD pattern of the queched material obtained by DTA of the
+ 5NaCl mixture.
Figure 9. DTA results for the MoO
+ 3Mg + 5NaCl mixture.
10988 |Ind. Eng. Chem. Res. 2011, 50, 10982–10988
Industrial & Engineering Chemistry Research ARTICLE
velocity, heating rate) in the combustion of MoO
+ Mg + NaCl
mixtures depend strongly on the salt content. It was shown that
the structure of the combustion products also depends on the salt
content in the initial mixtures. In salt-poor mixtures, nucleation
of the product particles takes place in the molten magnesium,
whereas under salt-rich conditions products form in molten
sodium chloride. The results obtained suggest that the 2MoO
(s) +
2NaCl(s) = MoO
(g) + Na
(s,l) reaction proceeds at
early stages of the combustion process. The formed intermedi-
ates molybdenum oxychloride and sodium molybdate then react
with magnesium, yielding Mo, MgO, and NaCl phases. The cal-
culated low activation energy of the process also conrms
the dominant role of gaseous (liquid) intermediates in the phase
formation mechanism.
Corresponding Author
*E-mail: Tel.: 00374 10 28-16-10. Fax:
00374 10 28-16-34. Address: 1 Manoogian Street, Yerevan 0025,
The authors acknowledge the nancial support of the State
Committee of Science of the Republic of Armenia (Project 354).
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... When NaCl is added, a reduction of ∼300°C occurs in the transition temperature, placing the transition temperatures at 525°C, 523°C, 525°C and 514°C for MoO 3 -P, −30, −120 and −300, respectively. Additionally, another transition temperature becomes evident near 600°C for BM samples with NaCl added, which can be accounted for molybdenum oxyhalide species [67]. This high mass flux facilitates the increase of nucleation sites, achieving a quasi-homogeneous dispersion. ...
Single and few-layered MoS2 materials have attracted attention due to their outstanding physicochemical properties with potential applications in optoelectronics, catalysis, and energy storage. In the past, these materials have been produced using the chemical vapor deposition (CVD) method using MoO3 films and powders as Mo precursors. In this work, we demonstrate that the size and morphology of few-layered MoS2 nanostructures can be controlled, modifying the Mo precursor mechanically. We synthesized few-layered MoS2 materials using MoO3 powders previously exposed to a high-energy ball milling (BM) treatment by the salt-assisted CVD method. The MoO3 powders milled for 30, 120, and 300 min were used to synthesize sample MoS2-30, MoS2-120, and MoS2-300, respectively. We found morphologies mainly of hexagons (MoS2-30), triangles (MoS2-120), and fullerenes (MoS2-300). The MoS2 nanostructures and MoO3 powders were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). It was found that MoO3 milled powders exhibit oxygen loss and decrease in crystallite size as milling time increases. Oxygen deficiency in the Mo precursor prevents the growth of large MoS2 crystals and a large number of milled MoO3-x + NaCl promote greater nucleation sites for the formation of MoS2, achieving a high density of nanoflakes in the 2H and 3R phases, with diameter sizes in the range of ~30-600 nm with 1-12 layers. Photoluminescence characterization at room temperature revealed a direct bandgap and exciting trends for the different MoS2 samples. We envisage that our work provides a route for modifying the structure and optical properties for future device design via precursor engineering.
... Decreasing their melting point can contribute to the evaporation and the release of reactant atoms or clusters. Taking the growth of transition metal dichalcogenides (TMDs) as an example, molten salts (NaCl, KCl, etc.) can reduce the melting point of precursors to promote their evaporation (Manukyan et al., 2011;Gaune-Escard and Haarberg., 2014). A library of 2D TMDs has been successfully synthesized by molten-salt-assisted chemical vapor deposition (CVD) . ...
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Two-dimensional atomic single crystals (2DASCs) have drawn immense attention because of their potential for fundamental research and new technologies. Novel properties of 2DASCs are closely related to their atomic structures, and effective modulation of the structures allows for exploring various practical applications. Precise vapor-phase synthesis of 2DASCs with tunable thickness, selectable phase, and controllable chemical composition can be realized to adjust their band structures and electronic properties. This review highlights the latest advances in the precise vapor-phase synthesis of 2DASCs. We thoroughly elaborate on strategies toward the accurate control of layer number, phase, chemical composition of layered 2DASCs, and thickness of non-layered 2DASCs. Finally, we suggest forward-looking solutions to the challenges and directions of future developments in this emerging field. : Atomic Structure; Chemical Synthesis; Materials Characterization Subject Areas: Atomic Structure, Chemical Synthesis, Materials Characterization
2D materials are enabling disruptive advancements in electronic and photonic devices yielding to the development of sensing and wearable materials and in the field of energy production and storage as key components of photovoltaic technology and batteries. Nevertheless, little attention has been paid to TMDs and oxides that contain vanadium, as it is the case of vanadium disulfide (VS2) and vanadium dioxide (VO2). In this study we review the synthesis and characterization using Raman spectroscopy of VS2 and its oxidized states. Laser-induced oxidation occurring during the Raman experiments in ambient conditions is described and plateau values of laser power levels to induce oxidation are provided. Furthermore, tip-enhanced Raman spectroscopy (TERS) spectra and maps are conducted to reveal at the single flake level the onset of oxidation mechanisms at the surface of the 2D platelets.
Conference Paper
The emergence of atomically thin 2D materials of semiconducting (transition metal dichalcogenides, TMDs, e.g. MoS2, WS2) layers has opened new avenues for scientific and technological advancement in the area of ultrathin electronic and optoelectronic devices. Chemical vapor deposition (CVD) is known as effective method to synthesize monolayers of TMDs. One of the most important open questions of the synthesis of different TMDs monolayers is the reaction mechanism, which can be studied by computational modeling of the synthesis process. Here thermodynamic approach of modeling of precursor evolution during the synthesis of MoS2 has been developed. CVD with solid precursors under the influence of NaCl has been studied. The H2S gas precursor decomposition has been considered. Thermodynamic modeling provides analysis of the influence of chemical composition of the reactants, carrier gas, temperature and pressure on the reaction products. It has been found that the best evaporation of MoO3 in presence of the NaCl is realized at temperature about 770 °C; S2 amount increase with decreasing of pressure. In this way optimal parameters for the synthesis of MoS2 by CVD have been analyzed.
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Single atomic metal–N–C materials have attracted immense interest as promising candidates to replace noble metal‐based electrocatalysts for the oxygen reduction reaction (ORR). The coordination environment of metal–N–C active centers plays a critical role in determining their catalytic activity and durability, however, attention is focused only on the coordination of metal atoms. Herein, Fe single atoms and clusters co‐embedded in N‐doped carbon (Fe/NC) that deliver the synergistic enhancement in pH‐universal ORR catalysis via the four‐electron pathway are reported. Combining a series of experimental and computational analyses, the geometric and electronic structures of catalytic sites in Fe/NC are revealed and the neighboring Fe clusters are shown to weaken the binding energies of the ORR intermediates on Fe–N sites, hence enhancing both catalytic kinetics and thermodynamics. This strategy provides new insights into the understanding of the mechanism of single atom catalysis.
High-quality single-crystalline NbSe2 nanosheet arrays were successfully synthesized by a salt and hydrogen co-assisted CVD method, and the crystal structure and morphology of the material were investigated by XRD, Raman...
Solution combustion synthesis (SCS) utilizes exothermic self-propagating reactions to prepare nanoscale materials that can be used widely in energy, electronics, and biomedical technologies and other applications. SCS is a specific variety of a more general combustion synthesis (CS) method. Investigations of the thermodynamics, kinetics, and the mechanisms of SCS reactions, are not as well studied as the other CS processes. This work reports on a systematic study of the thermodynamics and kinetics of SCS reactions involving Ni(NO3)2, an oxidizer, and either glycine (C2H5NO2) or hexamethylenetetramine (HMT, C6H12N4) as fuels. A thermodynamic modeling approach, based on the Gibbs free energy minimization principle, is applied to the simultaneous calculations of the adiabatic temperatures and compositions of the equilibrium products. Our calculations reveal the influence of fuel-to-oxidizer ratio, amount of water, and the oxygen in air on the combustion temperature under adiabatic conditions and the composition of the resulting products. We have, in turn, measured the combustion temperature and phase composition of products and compared them with the calculations. Variations of drying times for the solutions yield precursor gels with varying water contents. This approach enables the manipulation of combustion parameters and confirms the use of calculated activation energies for reactions using the Merzhanov-Khaikin method. The results show that SCS reactions in fuel-lean solutions producing NiO have higher activation energy in contrast to reactions with fuel-rich solutions that form Ni. Reduction of activation energies due to the increase in the fuel-to-oxidizer ratio could be related to the observed change of the rate-limiting stages of the endothermic decomposition of the individual reactants to the exothermic decomposition of coordinate compounds formed between the reactants.
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Bulk 1T‐TaSe2 as a charge‐density‐wave (CDW) conductor is of special interest for CDW‐based nanodevice applications because of its high CDW transition temperature. Reduced dimensionality of the strongly correlated material is expected to result in significantly different collective properties. However, the growth of atomically thin 1T‐TaSe2 crystals remains elusive, thus hampering studies of dimensionality effects on the CDW of the material. Herein, chemical vapor deposition (CVD) of atomically thin TaSe2 crystals is reported with controlled 1T phase. Scanning transmission electron microscopy suggests the high crystallinity and the formation of CDW superlattice in the ultrathin 1T‐TaSe2 crystals. The commensurate–incommensurate CDW transition temperature of the grown 1T‐TaSe2 increases with decreasing film thickness and reaches a value of 570 K in a 3 nm thick layer, which is 97 K higher than that of previously reported bulk 1T‐TaSe2. This work enables the exploration of collective phenomena of 1T‐TaSe2 in the 2D limit, as well as offers the possibility of utilizing the high‐temperature CDW films in ultrathin phase‐change devices.
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2D transition metal dichalcogenides (TMDs) are well suited for energy storage and field–effect transistors because of their thickness‐dependent chemical and physical properties. However, as current synthetic methods for 2D TMDs cannot integrate both advantages of liquid‐phase syntheses (i.e., massive production and homogeneity) and chemical vapor deposition (i.e., high quality and large lateral size), it still remains a great challenge for mass production of high‐quality 2D TMDs. Here, a molten salt method to massively synthesize various high‐crystalline TMDs nanosheets (MoS2, WS2, MoSe2, and WSe2) with the thicknesses less than 5 nm is reported, with the production yield over 68% with the reaction time of only several minutes. Additionally, the thickness and size of the as‐synthesized nanosheets can be readily controlled through adjusting reaction time and temperature. The as‐synthesized MoSe2 nanosheets exhibit good electrochemical performance as pseudocapacitive materials. It is further anticipates that this work will provide a promising strategy for rapid mass production of high‐quality nonoxides nanosheets for energy‐related applications and beyond. High‐crystalline transition metal dichalcogenides nanosheets (including MoS2, WS2, MoSe2, and WSe2) can be massively synthesized with the reaction time of only several minutes through a molten salt method.
Powders of new CompoNiAl M5-3 heat-resistant alloy were prepared by mechanoactivated SHS from (Ni–Al–Cr–Co–Hf)–NaCl mixtures. Ni dissolution in Al melt was found to be the motive force of combustion. Unlike the binary Ni–Al system, NiAl is formed not in the melt but in the post-combustion zone as a result of diffusion-controlled processes. Conditions for MASHS were optimized toward fabrication of a superalloy with homogeneous composition/structure and low content of gas impurities. As-prepared combustion product was disintegrated into a powder and the latter was subjected to plasma spheroidization, keeping in mind the needs of additive manufacturing. The attained degree of spheroidization was 98%. The structure and phase/chemical compositions of spherical powders did not differ from those of the synthesized powders. After plasma treatment, the content of gas impurities (О2 and N2) decreased.
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Na2SO4 and NaCl are the two important ionic salts which are actively involved in inducing hot corrosion reactions. The aggressiveness of these salts towards superalloys at high temperatures is very well documented. Metallic carbides and/or oxides form the important constituents of high temperature materials; oxides are also integral constituents of protective scales formed on the superalloys. During hot corrosion reaction, these constituents react with molten Na2SO4 or molten NaCl to propagate the reaction. However, the chemistry of such hot corrosion reaction is not very well understood. This paper presents the results of important studies carried out on this subject in recent years. High temperature interactions of metal oxides, namely Co3O4, NiO, Al2O3, Cr2O3, Fe2O3, SiO2, TiO2, ZrO2, Nb2O5, Ta2O5, MoO3 and WO3, and carbides, namely Cr7C3, Fe3C, TiC, ZrC, NbC, TaC, MoC, WC, VC and HfC, separately with sodium sulphate (Na2SO4) and sodium chloride (NaCl) have been studied in the temperature range 900-1200K. At these temperatures, reaction kinetics (weight change vs time) and variation in weight with mole fraction of salt have been measured. The reaction products were identified by X-diffraction (XRD) and energy dispersive X-ray analysis (EDAX). The formation of products was also investigated by thermodynamic computation of free energies of the reactions and the study of relevant equilibrium phase diagrams. The multiphase structures of the reaction products were studied by metallographic analysis. The solubility measurements of the reaction products were carried out to determine the soluble species in aqueous solutions. The weight losses during interaction are interpreted in terms of evolution of gases like CO2/SO2 or Cl2 or in some cases to volatilisation of oxide or chloride, whereas weight gains are explained in terms of the formation of sodium metal oxide, metal sulphide or metal chloride. The mechanism(s) of the high temperature interaction of metal oxides and carbides with Na2SO4 and NaCl has been discussed, and experimental evidence has been presented to support the proposed mechanism. In general, NaCl appears to be more reactive than Na2SO4.
Preparation of TiB2–Al2O3 and NbB2–Al2O3 in situ composites with a broad range of phase composition was conducted by self-propagating high-temperature synthesis (SHS) involving thermite reactions of different types. Thermite mixtures of Al–TiO2 and Al–TiO2–B2O3 were incorporated with the Ti–B combustion system to produce the composites of TiB2–Al2O3, within which the increase of the thermite mixture for a higher content of Al2O3 decreased the reaction temperature and combustion wave velocity. This implies that the thermite reaction of Al with TiO2 reduces the exothermicity of the overall SHS process. In the synthesis of NbB2–Al2O3 composite, two thermite mixtures of Al–Nb2O5 and Al–Nb2O5–B2O3 were added to the Nb–B combustion system and both of which were found to increase the combustion temperature and propagation rate of the flame front. This is due to the highly exothermic nature of the thermite reaction between Al and Nb2O5. For both kinds of composites, it was found that adoption of B2O3 as one of the thermite reagents improved the product formation effectively. The XRD analysis shows that the final products composed of no more than TiB2 and Al2O3 are obtained from the powder compacts containing the thermite mixture of Al–TiO2–B2O3. On formation of the NbB2–Al2O3 composite, NbB2 is identified as the major boride phase in the products involving the thermite reactions of Al–Nb2O5–B2O3, while Nb3B4 dominates in the case of using Al and Nb2O5 as the thermite reagents.
The combustion process of TiO2–Mg and TiO2–Mg–C systems with sodium chloride as an inert diluent was investigated. The values of combustion parameters and temperature distribution on a high-temperature wave according to the amount of sodium chloride were obtained by the thermocoupling technique. The leading stages of combustion processes are found and the sizes of reactionary zones were estimated. It is shown that the introduction of NaCl in an initial mixture promotes the formation of a nanocrystalline structure of the final products. As a result, nanosized titanium, and titanium carbide powders have been successfully obtained.
Radial combustion experiments on Fe2O3/aluminum thermite thin circular samples were conducted. A stoichiometric (Fe2O3+2Al) and four over aluminized mixtures were tested. The combustion products were characterized by X-ray diffraction and Mössbauer spectroscopy and the influence of Fe2O3/aluminum ratio on their composition was assessed. The main products were identified as alumina (α-Al2O3) and iron (Fe). A significant amount of hercynite (FeAl2O4) was detected, decreasing with the aluminum excess in the reactants. Close to the sample/confinement interface, where reaction quenching occurs, a non-stoichiometric alumina (Al2.667O4) was observed, being its XRD intensity correlated to the hercynite amount. Fe3Al intermetallic phase was found in the products of over aluminized mixtures. A reaction mechanism was proposed comprising: (i) Fe2O3 reduction to Fe3O4 and FeO; (ii) Al oxidation to Al2O3; (iii) interaction of the remaining Al with Fe3O4 and FeO with formation of iron–aluminates (hercynite) and iron; (iv) for the over aluminized mixtures, incorporation of Al into the iron–aluminates takes place with the formation of iron and alumina and, in parallel, Al reacts with iron to produce intermetallics.
A study was conducted on the reaction process between Fe2O3 and liquid aluminium by intentionally slowing down the reaction process. The reaction sequences during the thermite process were analyzed by using DTA. The phases and compositions of the reaction products were identified using electron microscopy and X-ray diffraction techniques. Based on experimental results and thermodynamic analysis, the reaction mechanisms of the thermite process were proposed.
A new nano-sized NiAl reinforced FeNiCr–TiC composite coating was fabricated using a thermite reaction technique in centrifugal field. All of the consisted phases in the composite coating were obtained in situ from the reaction product melt. The microstructure investigation showed that the composite coating consisted of ferrite (α-FeNiCr), NiAl and TiC. The NiAl phase appeared in whisker with a diameter of about 100nm and it was coherent with the ferrite matrix. The present composite coating exhibited excellent oxidation resistance and the mass gain after 100h isothermal exposure at 1000°C in air was less than 1mg/cm2 which is about 1/130 of that for CrNi1Mo steel. The coating had a good wear resistance, its relative wear resistance was 30–40 to the CrNi1Mo steel substrate at room temperature. As compared to NiCr1Mo steel, a significant increase of the composite coating in wear resistance at high temperature was observed. The volumetric wear rate of the composite coating was only about 1/7 of that for the NiCr1Mo steel.