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 conductors 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 chemical/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 for 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 been 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 highly 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 distributions. 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 solution [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 powder-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 continuously 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 produced 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 total 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 performed 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, respectively, 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 hexagonal 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 between 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 maximum 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. Careful 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 has 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-rich 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 structure. 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 constant; 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 time, 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 change 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 temperature 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 with 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, TS2G8 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 solution 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 enriched 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 solutions 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 temperature 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, which 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 mechanically 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 micrographs 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) tig (min) ΔPressure (bar) ΔTemp. (K)TS 5:3:0 0.0124 40 +2.53 +109.6TS8G2 5:2.4:0.6 0.0114 40 +2.49 +117.5TS6G4 5:1.8:1.2 0.0106 34 +2.36 +119.5TS5G5 5:1.5:1.5 0.0102 41 +2.35 +124.0TS4G6 5:1.2:1.8 0.0099 30 +2.67 +145.0TS2G8 5:0.6:2.4 0.0093 34 +2.84 +164.3TG 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 Ti5Si3−xGe3 phases formed by MSR.

Sample a = b (Å) c (Å) c/a V (Å3)TS 7.4396 5.1450 0.692 246.613TS8G2 7.4781 5.1630 0.690 250.043TS6G4 7.4902 5.1829 0.692 251.820TS5G5 7.5000 5.1876 0.692 252.708TS4G6 7.5024 5.1849 0.691 252.739TS2G8 7.5152 5.1952 0.691 254.105TG 7.5349 5.2184 0.693 256.580

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5