The unexpected formation of MB2 layers (M = refractory metal) on metal surfaces

Article abstract of DOI:10.1002/ejic.201001064

The reactions between refractory metals (Ti, Zr, V, Nb, Ta, Mo, W) and BCl₃ vapour at high temperatures have been studied. For this purpose, the metal wires were heated up by an electrical current in a BCl₃ atmosphere for a couple of hours. Optical and X-ray diffraction methods were used to analyze the solid products, whereas mass spectrometry was used to study the gas phase composition. In the reactions with Ti, Zr, V, Nb and Ta, we observed the formation of MB₂ layers on the metal surfaces. Molybdenum reacted with formation of MoB, with tungsten no reaction was observed at all. This is contrary to the thermodynamic expectations. The mechanism of the coating process is discussed from a thermodynamic viewpoint. In the course of reactions between refractory metals (Ti, Zr, V, Nb, Ta) with boron(III) chloride at temperatures of about 1500 K, layers of the diborides of these metals are formed. The coexistence of the refractory metal with its diboride is impossible from a thermodynamic viewpoint. The formation of metal diborides in the presence of the metal is discussed. Copyright

Full text of DOI:10.1002/ejic.201001064

FULL PAPER
DOI: 10.1002/ejic.201001064
The Unexpected Formation of MB Layers (M = Refractory Metal) on Metal
2
Surfaces
Wibke Friedhoff,[a][‡] Edgar Milke, and Michael Binnewies*
[a]
[a]
Keywords: Refractory metals / Boron / Synthesis design / Transition metals / High-temperature chemistry
The reactions between refractory metals (Ti, Zr, V, Nb, Ta,
Mo, W) and BCl vapour at high temperatures have been
studied. For this purpose, the metal wires were heated up by
an electrical current in a BCl atmosphere for a couple of
hours. Optical and X-ray diffraction methods were used to
analyze the solid products, whereas mass spectrometry was
used to study the gas phase composition. In the reactions
with Ti, Zr, V, Nb and Ta, we observed the formation of MB
layers on the metal surfaces. Molybdenum reacted with for-
mation of MoB, with tungsten no reaction was observed at
all. This is contrary to the thermodynamic expectations. The
mechanism of the coating process is discussed from a
thermodynamic viewpoint.
2
3
3
Introduction
is between 10 and 50 nm. Somewhat thicker layers can be
[
18]
formed by sputtering (500 nm)
tion.
and electrodeposi-
Borides of several transition metals, especially refractory
[19–23]
metal diborides, MB , are under discussion for application
2
We report here on a simple chemical process to coat re-
fractory metals with layers of metal diborides by using
chemical reactions at the solid/gas interface. The layer
thickness is in the magnitude of up to some hundred μm.
The thermodynamic background of the processes will be
discussed.
in high-temperature processes.[
1–3]
For example, TiB , ZrB2
2
and CrB are used in turbine blades, high-temperature fur-
2
naces or rocket nozzles. Several of these diborides have high
specific electrical conductivities and chemical resistivity
against molten salts, molten metals and oxygen. Because of
this, they are ideal materials for high-temperature chemis-
try, especially high-temperature electrochemistry, and in
particular as cathode materials in aluminium electrolysis.
These materials are difficult to prepare and difficult to
process and shape, as a result of their hardness and high
melting points. Bulk metal borides are usually prepared
from the elements by solid-state reaction at very high tem-
peratures, even though this method has a lot of disadvan-
Results and Discussion
Spirally wound metal wires were heated by an electrical
current up to high temperatures of 2000 K and exposed to
a static atmosphere of boron chloride, [p(BCl ) ≈ 1 bar].
3
Typically, the reaction was performed for a couple of hours.
The wires had a diameter of 0.5–1 mm and a length of
about 10 cm. The temperature was measured pyrometri-
cally. A sketch of the specific reaction apparatus is shown
in Figure 1. These experiments are a little similar to the old
experiments of Moers in 1931. He reduced gaseous BBr3
with hydrogen on the surfaces of hot metal wires and de-
scribed the formation of metal borides, but no further infor-
mation about the composition of these borides has been
[
4]
tages. The main problems are low reaction rates, side reac-
tions and reactions with the crucible material. Therefore,
alternative preparation methods at lower temperatures have
[
5–7]
been developed by using molten metals as solvents.
For some applications, these diborides are used as coat-
ings on different substrates. Up to now several techniques
are available to build up such coatings. Different techniques
are used to form thin films of refractory metal diborides
[
24]
given.
In parallel experiments, we investigated the reac-
like TiB . Thin layers mostly used as diffusion barrier for
2
tions between the metals and boron chloride by mass spec-
trometry in order to gather information on the composition
of the gas phase. For this purpose, a spirally wound metal
wire (outer diameter about 4 mm) was placed in a quartz
semiconductor applications can be prepared by different
[
8–16]
CVD methods.
refractory metal boride thin films is the PLD technique.
The typical thickness of layers formed by these techniques
An alternative method of formation of
[
17]
tube and heated by an electrical current. BCl (p Ͻ1 mbar)
3
was passed over the wire, and the gaseous reaction products
were detected by a specifically equipped mass spectrome-
ter, whereas the solid products were analyzed by optical
methods, that is, by electron microscopy and X-ray powder
diffraction.
[
a] Institut für Anorganische Chemie,
Callinstr. 9, 30167 Hannover, Germany
Fax: +49-511-762-2254
[25]
E-mail: michael.binnewies@aca.uni-hannover.de
‡] Current address: An der Questenhorst 2, 30173 Hannover,
Germany
[
3398
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2
The Unexpected Formation of MB Layers on Metal Surfaces
data set under discussion here (Figure 2). Discussions on
the consistency of thermodynamic data sets of solid com-
[
28,29]
pounds in binary systems are given in ref.
Figure 1. Schematic representation of the apparatus.
–
1
Figure 2. Gibbs energies of vanadium borides in kJgatom .
The reactions were carried out with the elements Ti, Zr,
The systems Ti/B, Zr/B, Nb/B, Ta/B, Mo/B and W/B will
be reviewed much more briefly, because up to now these
systems have not been characterized from a thermodynamic
viewpoint. The observations and conclusions for these sys-
tems are similar to those in the vanadium system.
V, Nb, Ta, Mo and W in a BCl atmosphere. With Ti, Zr,
3
V, Nb and Ta, we observed the formation of MB layers on
2
the surface of the metals. Molybdenum formed a MoB
layer, but tungsten did not react under these conditions at
all. The formation of VB will be discussed in detail from
2
a thermodynamic viewpoint. We selected this system in
contrast to the other ones, because the thermodynamic data
for all relevant borides, V B , VB, V B , V B , V B and
The V/BCl System
3
3
2
5
6
3
4
2 3
VB , are available in critical compilations of thermochemi-
Figure 3 shows the experimentally determined phase dia-
2
[
26,27]
[30]
cal data.
The plot of the Gibbs energies of these bor- gram of the V/B system. There are six vanadium borides
ides versus composition shows clearly the consistency of the from V B to VB . We exposed a vanadium wire (d = 1 mm)
3
2
2
Figure 3. Vanadium/boron phase diagram.[30] Reprinted with permission of ASM International.
Eur. J. Inorg. Chem. 2011, 3398–3402
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3399

W. Friedhoff, E. Milke, M. Binnewies
FULL PAPER
to a BCl atmosphere at 1200 °C for one hour. During this ity of the thermodynamically favoured solid–solid reaction
3
time, a layer of VB (d = 0.22 mm) was formed. The follow- between these two high-melting solids [Equation (5)] is ex-
2
ing ions were found in a mass spectrometric investigation: pected to be very low.
+
+
+
+
+
+
Cl (25), HCl (100), BCl (6), BCl (55), BCl (17), V (1.1),
2
3
+
+
+
+
2 V (s) + VB
2
(s) Ǟ V
3
B
2
(s)
(5)
VCl (1.4), VCl (4.3), VCl (3.8), VCl (1.2). This indi-
2
3
4
cates the presence of HCl, BCl , VCl (and probably VCl )
in the gas phase. Additionally, the formation of solid VCl3
3
4
3
Δ
R
G01500 = –125.9 kJmol–1
could be proven by means of X-ray powder diffraction mea-
In the Ti/BCl
3
system, we have observed the formation
in addition to the boron-
surements. Obviously, vanadium reacts with gaseous boron of the boron-richest boride TiB
2
chloride with formation of a layer of VB on the metal core. poorest boride TiB. This is one indication for a step-by-step
2
Reaction (1) seems to be favoured from a thermodynamic reaction course (see below).
viewpoint, because of the negative Gibbs energy.
5/2 V (s) + 2 BCl
3
(g) Ǟ VB
2
(s) + 3/2 VCl
4
(g)
(1)
The Nb/BCl System
3
Δ
R
G01500 = –112.5 kJmol–1
The following phases are known in the phase diagram of
[
30]
Nb/B: Nb B , NbB, Nb B , Nb B and NbB .
We ex-
3
2
5
6
3
4
2
Nevertheless, the formation of VB as a coating in con-
2
posed a niobium wire (d = 1 mm) to a BCl atmosphere at
3
tact with vanadium metal is unexpected, because vanadium
and vanadium diboride cannot coexist in a thermodynamic
equilibrium. In addition, VB exhibits the lowest thermo-
dynamic stability among the vanadium borides, as shown
in Figure 2. Therefore, the question arises: What is the
1
500 °C for three hours. During this period of time, a layer
of NbB (d = 0.4 mm) was formed. In addition, a small
2
2
amount of a yellow condensate, probably NbCl , was found
5
in the reaction vessel. The following ions were found in a
+
+
mass spectrometric investigation: Cl (28), HCl (100),
reason for the formation of VB instead of V B ? To answer
2
3
2
+
+
+
+
+
+
BCl (1), BCl (9), BCl (4), Nb (0.4), NbO (0.5) . This
2
3
this question we consider a possible step-by-step reaction
course. The reaction of vanadium with boron chloride with
formation of solid boron and gaseous vanadium(IV) chlo-
ride is strongly endergonic (Δ G
cannot occur. Therefore, in a first step one would expect
the reaction given in Equation (2).
indicates the presence of HCl, BCl and very small amounts
3
of unidentified gaseous niobium species. Figure 4 shows an
edge of a fracture of the wire after the reaction. Only a little
bit of the niobium metal remained at the core of the wire
after the reaction time. In the end, a porous wire of NbB2
0
–1
= 222.7 kJmol ) and
R
1500
is formed by this method in a “shape-controlling synthe-
(2) sis”.[
32]
9/2 V (s) + 2 BCl
3
(g) Ǟ V
3
B
2
(s) + 3/2 VCl
4
(g)
Δ
R
G01500 = –202.6 kJmol–1
From a thermodynamic point of view, reaction (2) is pos-
sible. In addition, V B is the only boride phase that is pre-
3
2
dicted to be able to coexist with metallic vanadium. The
step-by-step formation of boron-richer borides by reaction
of a boron-poorer boride with gaseous boron chloride is
endergonic in all cases. This is shown by the two reactions
given in Equations (3) and (4).
V
3
B
2
(s) + 4/7 BCl
3
(g) Ǟ 18/7 VB (s) + 3/7 VCl
4
(g)
(3)
V
2
B
3
(s) + 4/10 BCl
3
(g) Ǟ 17/10 VB
2
(s) + 3/10 VCl (g)
4
(4)
G01500 = 30.3 kJmol , K = 0.09
–1
Δ
R
R
G01500 = 24.2 kJmol , K = 0.14
–1
Δ
2
Figure 4. NbB layer on a niobium wire.
Nevertheless, such reactions are possible. The reasons for
this are the experimental conditions: gaseous vanadium
chloride, which is formed during the reactions, is continu-
ously removed from the equilibria by condensation within
the reaction vessel. Therefore, equilibria like (3) or (4) are
The Ta/BCl System
3
The following phases are known in the phase diagram of
[
30]
shifted to the sides of the reaction products, and so the Ta/B: Ta B, Ta B , TaB, Ta B and TaB . We exposed a
2
3
2
3
4
2
boron-richest boride VB can be formed step by step. In an tantalum wire (d = 0.5 mm) to a BCl3 atmosphere at
2
alternative route, VB may be formed in one reaction step 1400 °C for 1.8 h. During this period of time, a layer of
2
between the metal and the boron-rich gas phase. In both TaB2 (d = 0.09 mm) was formed. In addition, a small
cases, the result is a nonequilibrium situation: vanadium in amount of a yellow condensate, probably TaCl , was found
5
contact with VB . This becomes possible, because the veloc- in the reaction vessel. No gaseous tantalum species could
2
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Eur. J. Inorg. Chem. 2011, 3398–3402

2
The Unexpected Formation of MB Layers on Metal Surfaces
+
+
+
+
+
be identified by means of mass spectrometric investigations. BCl (11), Zr (12.5), ZrCl (1.3), ZrCl (2), ZrCl (6.4),
3
2
3
+
Figure 5 shows the polished face of a wire after the reac- ZrCl (2.3). This indicates the presence of HCl, BCl and
4
3
tion. It is clearly visible that the layer is much more com- ZrCl . The reaction [Equation (7)] between zirconium and
4
pact than that obtained as a result of the reaction of ni- boron chloride has the highest exergonic potential, but the
obium with BCl3.
thickness of the resulting layer is the lowest of all discussed
here.
2
Figure 6. Powder diffraction diagram of ZrB .
5
/2 Zr (s) + 2 BCl
3
(g) Ǟ ZrB
2
(s) + 3/2 ZrCl
4
(g)
(7)
2
Figure 5. Polished face of a tantalum wire coated with TaB .
Δ
R
G01500 = –686.3 kJmol–1
A boride that is able to coexist with the metal was
formed only in the case of zirconium.
The Ti/BCl System
3
The following phases are known in the phase diagram of
[30]
Ti/B: TiB, Ti B and TiB . We exposed titanium wire (d
3
4
2
The Mo/BCl and W/BCl Systems
3
3
=
0.5 mm) to a BCl atmosphere at 1200 °C for two hours.
3
During this time, a layer of titanium borides (d = 0.07 mm),
The following phases are listed in the phase diagram of
[
30]
which could be identified as TiB , was formed in addition Mo/B: Mo B, MoB, MoB , Mo B and MoB
(recently
2
2
2
2
5
4
to TiB. Furthermore, a violet condensate, probably TiCl3, published structural investigations on “MoB2” and
[
31]
was found in the reaction vessel. The presence of TiB “Mo B ”; see ref. ). We exposed a molybdenum wire (d =
2
5
among the products indicates the step-by-step formation of 0.5 mm) to a BCl atmosphere at 1500 °C for 30 min. Dur-
3
boron-rich metal borides. The following ions were found ing this time, a nonporous thin layer of MoB (d =
+
+
in a mass spectrometric investigation: Cl (23), HCl (100), 0.09 mm), particularly adhering on the surface of the metal,
+
+
+
+
+
+
BCl (8), BCl (88), BCl (34), Ti (5), TiCl (6), TiCl (17), was formed (Figure 7). Mass spectrometric investigation
2
3
2
+
+
TiCl (13), TiCl (2). This indicates the presence of HCl, did not show any species including molybdenum and chlo-
3
4
BCl and titanium chlorides (TiCl , TiCl ). The reaction rine.
3
3
4
[
Equation (6)] between titanium and boron chloride is a lot
more exergonic than the reactions discussed above, but the
overall reaction behaviour is very similar.
5/2Ti (s) + 2 BCl
3
(g) Ǟ TiB
2
(s) + 3/2 TiCl
4
(g)
(6)
Δ
R
G01500 = –509.8 kJmol–1
The Zr/BCl System
3
The following phases are listed in the phase diagram of
[30]
Zr/B: ZrB and ZrB .
We exposed a zirconium wire (d
2
12
=
1.1 mm) to a BCl atmosphere at 1300 °C for 75 min.
3
During this time, a thin layer of ZrB (d = 0.05 mm) was
2
formed. As an example, the X-ray powder diffraction dia-
gram of ZrB is given in Figure 6. In addition, a colourless
2
condensate, probably ZrCl , was found in the reaction ves-
4
Figure 7. Fracture edge of a molybdenum wire coated with MoB
the arrowhead indicates the phase boundary between Mo and
sel. The following ions were found in a mass spectrometric
(
+
+
+
+
investigation: Cl (22), HCl (100), BCl (7), BCl (50), MoB).
2
Eur. J. Inorg. Chem. 2011, 3398–3402
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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W. Friedhoff, E. Milke, M. Binnewies
FULL PAPER
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3
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Published Online: June 28, 2011
3402
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Eur. J. Inorg. Chem. 2011, 3398–3402