mechanically induced self-sustained reactions

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Post
-
print of:
Intermetallics Volume 19, Issue 11, November 2011, Pages 1688

1692




Formation of the complete range of Ti5Si3−xGex solid solutions via
mechanically induced self
-
sustained reactions


José M. Córdoba,


Ernesto Chicardi,

Miguel A. Avilés,

Francisco J. Gotor

Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC
-
US, Américo
Vespucio 49, 41092 Sevilla, Spain


Abstract

The complete range of Ti5Si3

Ti5Ge3 solid solutions was synthesised from elemental
mixtures of Ti, Si, and Ge

under an inert atmosphere via mechanically induced self
-
sustaining reactions (MSR). The stoichiometry of Ti5Si3−xGex solid solutions was
controlled by adjusting the Si/Ge ratio of the initial mixture. The chemical composition
and lattice parameters of the

materials confirmed that Ti5Si3

Ti5Ge3 solid solutions
with good chemical homogeneity could be produced via MSR.

Keywords

A. Titanium silicides;


A. Silicides, various;


A. Ternary alloy systems;


C. Mechanical
alloying and milling;

C. Reaction synthesis

1. Introduction

Titanium silicides and germanides have attracted a significant amount of attention in
recent years due to their specific physical and chemical properties [1], [2] and [3]. In
general, titanium silicides and germanides are good electrical co
nductors and have
similar resistivities to metals and alloys. The potential use of silicides as conductors has
motivated thin
-
film silicide research [4] and [5], including applications such as (a)
Schottky barriers and ohmic contacts [6] and [7], (b) gates

and interconnection metals,
and (c) epitaxial conductors in heterostructures [8].

Ti5Si3 and Ti5Ge3 have high melting points (2130 °C and 1960 °C, respectively) and
are difficult to process. Common methods for the synthesis of Ti5Si3 and Ti5Ge3
include ch
emical/physical vapour deposition, rapid thermal processing, and traditional
casting or powder metallurgical processes [9], [10], [11] and [12]. Due to the highly
exothermic heat of formation of these intermetallic compounds (−579 kJ/mol and −566
kJ/mol fo
r Ti5Si3 and Ti5Ge3, respectively) [13] and [14], self
-
propagating high
-
temperature syntheses (SHS) have also been proposed.

Mechanically induced self
-
sustained reactions (MSR) [15] are a promising alternative to
conventional synthetic methods and have bee
n used to produce advanced materials such
as borides, carbides, nitrides, carbonitrides, hydrides, silicides, aluminides, and other
intermetallic compounds [16], [17], [18], [19], [20], [21], [22] and [23]. MSR is a self
-
sustaining process due to the highl
y exothermic nature of the reactions; thus, MSR is
low cost, has low energy requirements, and yields highly pure products [24]. Moreover,
mechanochemical processes have shown several advantages such as small particles
sizes [25] and narrow size distributio
ns. In general, mechanochemical syntheses refine
the microstructure of the product, which improves the superparamagnetic,
electromagnetic, electrical, optical (i.e., refractive index, transparency), and mechanical
properties (hardness, strength, toughness,

plasticity) of the material.

Materials with superior or tailored properties can be produced from solid solutions [26]
and [27]. Ti5Ge3 and Ti5Si3 have the same structure, nearly equal lattice parameters
(both are hexagonal (P63/mcm) with a = b = 7.5370 Å,

c = 5.2230 Å, and a = b =
7.4440 Å, c = 5.1430 Å, respectively), and similar atomic radii (Si, 1.32 Å; Ge, 1.37 Å)
and electronegativities (Si, 1.90; Ge 2.02); thus, Ti5Ge3 and Ti5Si3 should be totally
soluble and form a homogenous and complete solid solu
tion [28].

Due to the great potential of titanium silicides/germanides, the objective of the present
study was to synthesise the complete range of Ti5Si3−xGex from stoichiometric
elemental powder mixtures via mechanically induced self
-
sustaining reactions.

2. Materials and methods

Titanium (99.98% pure, 325 mesh, Sigma Aldrich), silicon (99% pure, 325 mesh,
Sigma Aldrich), germanium (99.999% pure, 100 mesh, Sigma Aldrich), and high
-
purity
helium gas (99.999%, H2O

3 ppm, O2

2 ppm, CnHm 0.5 ppm; Air Liquide
) were
used in the present study. The reagents were combined in the appropriate proportions
(see Table 1) by hand grinding in an agate mortar, sonicated for 5 min in ethanol, and
ball milled under 6 bar of highly pure helium gas using a modified planetary
ball
-
mill
(model Micro Mill Puverisette 7, Fritsch).

In each milling experiment, six tempered steel balls and 4 g of powder were placed in a
50
-
cc tempered steel vial (67Rc). The diameter and mass of the balls were 15 mm and
13.3 g, respectively, and the p
owder
-
to
-
ball mass ratio (PBR) was equal to 1/20. The
vial was purged with helium gas several times, and the desired helium pressure was
selected prior to milling. During the grinding experiments, the vial was permanently
connected to the gas cylinder by a

rotary valve and a flexible polyamide tube. The
powder mixture was milled at a spinning rate of 600 rpm. The aforementioned rotation
rate was applied to both the supporting disc and the vial, which was spun in the opposite
direction. The pressure was cont
inuously monitored with an SMC Solenoid Valve
(model EVT307
-
5DO
-
01F
-
Q, SMC) connected to an ADAM
-
4000 series data
acquisition system (Esis Pty Ltd.). When MSR was conducted, the temperature
increased due to the occurrence of exothermic reactions, which pro
duced an
instantaneous increase in the total pressure of the system. The ignition time was
determined according to the time

pressure record, and the temperature inside the vial
was estimated from the change in pressure by applying the ideal gas law. The to
tal
volume of the system was calibrated prior to the calculation. Upon ignition, milling was
prolonged for 10 min to ensure complete conversion.

X
-
ray diffraction patterns of the powders were obtained with a Philips X’Pert Pro
instrument equipped with a Θ/
Θ goniometer. Cu Kα radiation (40 kV, 40 mA), a
secondary Kβ filter, and an X’Celerator detector were employed. The diffraction
patterns were scanned from 25° to 75° (2Θ) in step
-
scan mode, and a step of 0.05 and a
counting time of 120 s/step were applied.

Using Fullprof computer program [29] and
assuming hexagonal symmetry, the lattice parameters (a and c) were calculated from the
entire set of peaks in the XRD diagram.

Scanning electron microscopy (SEM) and energy
-
dispersive X
-
ray (EDX) analysis were
perf
ormed with a Hitachi FEG S
-
4800 microscope equipped with an EDS detector
(EDAX Inc.) for chemical analysis. Powder samples were dispersed in ethanol and were
deposited onto a holey carbon grid.

3. Results and discussion

Table 1 shows the elemental analysis

of mixtures submitted to high
-
energy ball milling
for the synthesis of Ti5Si3

Ti5Ge3 solid solutions with different Si/Ge atomic ratios
(2.4/0.6, 1.8/1.2, 1.5/1.5, 1.2/1.8, and 0.6/2.4). Samples TS and TG, which contained
pure Ti5Si3 and Ti5Ge3 phases, re
spectively, were synthesised for comparative
purposes. In all of the experiments, the effects of MSR were detected during the milling
process. The ignition times (tig) were determined from the time

pressure records and
are shown in Table 1. Similar values
were obtained for all of the mixtures, except the
binary Ti/Ge sample, which presented a considerably shorter ignition time.

The XRD diagrams of the products after MSR were indicative of the formation of a
P63/mcm hexagonal phase (Fig. 1). The position of
the XRD reflections in samples TS
and TG were identical to those in JCPDS data file 29
-
1362 (Ti5Si3) and 05
-
0684
(Ti5Ge3), respectively. However, when ternary mixtures of Ti, Si, and Ge were milled,
the XRD reflections of the resulting phases were between
those of binary Ti5Si3 and
Ti5Ge3. The dotted red lines in Fig. 1 over the (002), (210), (311), (222), and (213)
characteristic reflections were indicative of a shift from pure Ti5Si3 (sample TS) to pure
Ti5Ge3 (sample TG). Therefore, the synthesised hexag
onal phases can be described as a
solid solution with a molecular formula of Ti5Si3−xGex. Moreover, the continuous shift
in the XRD reflections (Fig. 1), which was attributed to different compositions in the
solid solution, proved that the stoichiometry of

the solid solution could be controlled by
adjusting the initial Si/Ge atomic ratio.

The lattice parameters of the hexagonal phases of the Ti5Si3

Ti5Ge3 solid solution
were calculated from the XRD patterns and are shown in Table 2, along with the c/a
ratio

and the cell volume. Fig. 2 shows the relationship between the initial composition
and lattice parameters of the milled samples. The similar trend followed by the lattice
parameters, a and c, in Fig. 2, suggests a random and homogeneous substitution
betwe
en both species independently of the atomic position in the lattice. It is remarkable
the asymmetrical character of the deviation of the Vergard’s law in comparison with the
approximately quadratic deviation in most binary alloy systems [30] and [31]. For
silicon
-
rich and germanium
-
rich compositions, a positive and negative deviation from
Vegard’s model was observed, respectively, producing an S
-
shaped variation of lattice
parameters with composition. The maximum deviation in a(=b) was 0.14%, and the
maximu
m deviation in c was 0.37%. The presence of secondary phases or unreacted
elements, which could account for the deviation between the presumed composition of
the solid solution and the estimated composition according to Vegard’s law, was not
detected. Care
ful examination of the XRD diagrams confirmed the formation of a
monophasic product. Both positive and negative deviations from linearity near the two
terminal compositions of the same system that leads to the anomalous S
-
shaped
violation of Vegard’s law h
as been found in some covalent pseudobinary alloys, and has
been attributed to the effect of bond
-
bending forces [32]. Moreover, this same S
-
shaped
deviation has also been predicted for alloy systems with large size
-
mismatched
constituents [33]. For Si
-
ric
h compositions, the Ti5Si3 lattice acts as a host structure for
Ge atoms. In contrast, for Ge
-
rich compositions, the host lattice is Ti5Ge3, and Si can
be considered an impurity. The substitution of Si or Ge generates local deformations in
the host structu
re. Because the atomic radii of Si and Ge are different, the host Ti5Ge3
lattice shrinks upon Si
-
substitution, and the Ti5Si3 lattice expands around Ge atoms.

In Fig. 3, the temperature increase inside the vial due to MSR is plotted against the
initial Si/
Ge atomic ratio. As shown in the figure, the final temperature inside the vial
increased with an increase in the germanium content of the initial mixture. Due to the
fact that the heats of formation of the products in the solid solution (Ti5Si3, ΔH°for =

579 kJ/mol and Ti5Ge3, ΔH°for = −566 kJ/mol) are nearly identical, the Ti5Si3−xGex
solid solutions have similar enthalpies of formation. Moreover, to obtain identical PBR
values for all of the milling runs, the total powder charge in the vial was held cons
tant;
thus, the initial molar content decreased with an increase in the germanium content
(which is a heavier element than silicon), as shown in Fig. 3. Therefore, the maximum
temperature increase due to the substitution of silicon cannot be attributed to
an increase
in the amount of heat released during MSR.

The aforementioned behaviour can be justified by considering that the germanium
reacts relatively fast, as indicated by the short ignition time, thus all the reaction heat is
released in a very short t
ime, leading to a large temperature increase. The reaction is
more sluggish with silicon. Thus by the time all the reaction heat is available, a large
fraction is lost to the balls and the milling container, thus the temperature maximum is
lower. The chang
e in the slope of the temperature profile observed in Fig. 3 from the
sample TS5G5 can be justified taking into account the specific heat (Cp) of the
Ti5Si3−xGex phases. The specific heat of the phases at different temperatures is
presented in Fig. 4. The
Cp equation of Ti5Si3 and Ti5Ge3 was obtained from the
literature [13] and [34], and the Cp of the Ti5Si3−xGex solid solution phases was
determined by applying the law of mixtures. Fig. 4 shows how the Cp of the material
changes its trend as the temperatur
e increases and the composition changes from Ti5Si3
to Ti5Ge3. At low temperatures, the Ge
-
rich phases have higher specific heat than Si
-
rich phases and by contrast from

400 K the specific heat of the Si
-
rich phases is
higher. This temperature matched wit
h that reached by the sample TS5G5 during its
reaction (Fig. 3, Te = ΔT + 298K = 422K), from which an increment in the temperature
slope was observed.

Fig. 5 shows the SEM micrographs of the powder samples after milling (samples TS,
TS8G2, TS6G4, TS4G6, TS
2G8 and TG) and illustrates the morphological aspects of
the products. The SEM images revealed that the microstructures of the products are
similar and are characteristic of samples obtained by MSR [23], [24], [25] and [26].
Powdered Ti5Si3−xGe3 solid solu
tion phases consisted of highly agglomerated sub
-
micrometric particles, and aggregates ranging from 1 to 3 μm were observed. Although
Ti5Si3 and Ti5Ge3 phases have similar fusion temperatures, the particles appeared
sintered when the solid solution was enr
iched in germanium due to the locally high
temperatures attained during the self
-
sustaining reaction and the lower specific heat of
germanium
-
rich phases, which induces relatively high local temperatures.

4. Conclusions

Monophasic Ti5Si3−xGe3 solid solutio
ns were synthesised from blends of titanium,
silicon and germanium under helium atmosphere via mechanically induced self
-
sustaining reactions, and the stoichiometry of the Ti5Si3−xGe3 phases was controlled
by adjusting the initial Si/Ge atomic ratio. The t
emperature of the combustion process
was determined by applying the ideal gas law, and different behaviour was observed,
depending on the initial composition.

Acknowledgements

This work was supported by Spanish government under grant No. MAT2010
-
17046,
whi
ch is financed in part by European Regional Development Fund of 2007

2013. E.
Chicardi and J. M. Córdoba were supported by CSIC through JAE
-
Pre and JAE
-
Doc
grants, respectively, which are financed in part by European Social Fund (ESF).



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Figure captions


Figure 1.

X
-
ray powder diffraction diagrams of products obtained from the initial
mixtures shown in Table 1 via mechanically induced self
-
sustaining reactions.

Figure 2.

The lattice parameters (a and c) of the P63/mcm hexagonal Ti5Si3−xGe3
phases obtained by mechan
ically induced self
-
sustaining reactions.

Figure 3.

Temperature inside the vial during MSR and initial molar quantities versus
the Si/Ge atomic ratio.

Figure 4.

Specific heat (Cp) of the Ti5Si3

Ti5Ge3 system at different temperatures.

Figure 5.

SEM microgr
aphs showing the morphology of Ti5Si3−xGe3 samples. (a) TS;
(b) TS8G2; (c) TS6G4; (d) TS4G6; (e) TS2G8; (f) TG.



Table 1



Table

1.
Summary of elemental mixtures submitted to milling, ignition times and pressure
changes observed during MSR.
The change in
temperature was calculated from the pressure.

Sample

Ti:Si:Ge Atomic Ratio

Powder charge (mol)

t
ig

(min)

ΔPressure (bar)

ΔTemp. (K)

TS

5:3:0

0.0124

40

+2.53

+109.6

TS8G2

5:2.4:0.6

0.0114

40

+2.49

+117.5

TS6G4

5:1.8:1.2

0.0106

34

+2.36

+119.5

TS5G5

5:1.5:1.5

0.0102

41

+2.35

+124.0

TS4G6

5:1.2:1.8

0.0099

30

+2.67

+145.0

TS2G8

5:0.6:2.4

0.0093

34

+2.84

+164.3

TG

5:0:3

0.0087

19

+3.10

+191.3




Table 2


Table

2.
Lattice parameters (
a

and
c
),
c
/
a

ratio, and cell volume (
V
) of hexagonal
Ti
5
Si
3−
x
Ge
3

phases formed by MSR.

Sample

a

=

b

(Å)

c

(Å)

c
/
a

V


3
)

TS

7.4396

5.1450

0.692

246.613

TS8G2

7.4781

5.1630

0.690

250.043

TS6G4

7.4902

5.1829

0.692

251.820

TS5G5

7.5000

5.1876

0.692

252.708

TS4G6

7.5024

5.1849

0.691

252.739

TS2G8

7.5152

5.1952

0.691

254.105

TG

7.5349

5.2184

0.693

256.580




Figure 1





Figure 2





Figure 3




Figure 4





Figure 5