Ti3GeC2 compound has excellent electrical and mechanical properties, which can be used as a potential reinforcement for Ag-based electrical contact materials. However, few reports have been addressed on the synthesis of its powders. The present study, therefore, presents a new route to fabricate high-purity Ti3GeC2 powders from Ti/Ge/TiC by pressureless sintering in vacuum. The influence of sintering temperature and Ge content on the powder purity was studied and the optimized conditions are found to be 1550 ℃ and Ti:Ge:TiC=1:1.1:2. In addition, the reaction path of Ti/Ge/TiC was revealed to be three steps: First, liquid Ge starts to react with Ti to form Ti5Ge3 compounds at around 1140 ℃; Then Ti5Ge3 consumes the liquid Ge and TiC to form Ti2GeC at around 1200 ℃; Finally, Ti2GeC transfers into Ti3GeC2 with the consumption of residual TiC. The present study lays the foundation for the subsequent applications of Ti3GeC2 powders in high performance composites.
Research Article
Synthesis of Ti3GeC2 Powders from Ti-Ge-TiC Mixtures by Pressureless Sintering
https://doi.org/10.21203/rs.3.rs-104533/v1
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MAX phases, with the formula of Mn+1AXn, where n = 1 ~ 3, M is an early transition metal, A is an A-group (mostly IIIA and IV A) element, and X is carbon or nitrogen, are a series of layered ternary carbides or nitrides[1, 2]. They were first reported by Jeitschko and Nowotny in the early 1960s [3]and initially named in 2000 by Barsoum[4]. The unique crystal structure and combined bonding of MAX phases endow them with both metal and ceramic characteristics, such as excellent electrical and thermal conductivities, even exceeding the corresponding transition metal elements[5, 6]. Accordingly, MAX phase materials have broad application prospects, including high-temperature structural materials, machinable ceramics, kiln furniture, wear and corrosion protection, heat exchangers, rotating parts etc[4, 7].
As a typical MAX phase, Ti3GeC2 has a theoretical density of 5.55 g/cm3 and a hexagonal crystal structure[8, 9]. Ti3GeC2 exhibits moderate hardness (2 ~ 6 GPa), high Young's modulus (~ 197 GPa), excellent machinability, high yield strength, and significant plasticity at 1300 ℃[7, 10]. It also shows excellent corrosion and oxidation resistance, remarkable thermal shock resistance, sound electrical and thermal conductivity[10, 11]. Wolfsgruber et al. first fabricated bulk Ti3GeC2[8] in 1967 by hot pressing at 1200 ℃ for 20 h from Ti, Ge, and C powder mixtures. In 1997, Barsoum et al.[7] synthesized Ti3GeC2 plates by hot pressing at 1500 °C for 4 h. However, the phase purity was not satisfied due to the high content of carbide impurities, which was attributed to the severe loss of Ge during sintering. To compensate for the loss of Ge, Kephart, et al. [12]added an excess of Ge in the starting materials during the preparation of Ti3GeC2 bulks by an arc melting ingot method, while several impurity phases like TiC, Ti5Ge3, and Ti2GeC were formed. To further improve the purity of Ti3GeC2, the same authors heated the reactants at 1200 °C for about 100 hours[12], but a large amount of TiC, unreacted Ge, and other Ti-Ge intermetallic compounds were still detected. Barsoum et al. [13]synthesized the bulk Ti3GeC2 and Ti3(SixGe1−x)C2 (x = 0.5, 0.75) solid solutions by hot isostatic pressing (HIP) in 2004. However, these samples still contains a large amount of binary carbides unexpected.
Since the first preparation of Ti3GeC2 in 1967, most studies focused on the synthesis of its bulk samples, but few reports concerned the direct production of Ti3GeC2 powders. Rahul et al. [14]synthesized Ti3GeC2 powder via pressureless sintering at 1500 ℃, containing Ti5Ge3 and TiC impurities. Generally, the Ti3GeC2 powders was fabricated from Ti/Ge/C raw materials[14], similar to the preparation of bulk samples[7, 8, 12, 13]. The motivation to further improve Ti3GeC2 powder purity was inspired by the report that using Ti/Si/TiC instead of Ti/Si/C powder mixtures will significantly increase the phase purity of Ti3SiC2 [15]. The synthesis of high-purity MAX phase powders will benefit to the production of MAX components with complicated structures via ceramic processing like slip casting, and also the development of MAX-reinforced composites. For instance, MAX phase can be used as the reinforcements for metal-based electrical contacts[16–21], where the synthesis of high-purity MAX powder is the prerequisite for the developments of high-performance Ag-MAX composites.
In the present paper, therefore, Ti3GeC2 powders were prepared by pressureless sintering using Ti, Ge, and TiC as raw materials, and the processing parameters were optimized to improve the phase purity. Differential scanning calorimetry and X-ray diffraction analysis were applied to identify the formation mechanism.
Powders of Ti (99.99%, ~ 45 µm), Ge(99.999%, ~ 75 µm), and TiC(99.95%, ~ 30 µm) were used as raw materials in this study. Ti/Ge/TiC powders with different molar ratios (1:0.9:2, 1:1:2, and 1:1.1:2) were mixed by Turbula mixer (T2F, Switzerland) for 24 h. These homogeneous mixtures were then respectively placed in Tungsten crucible and sintered at 1500–1550 ℃ at 10 ℃/min for 2 h in a high-temperature vacuum furnace (~ 10− 4 bar). Based on the preliminary experiments, the processing parameters such as temperature and composition were optimized. To determine the reaction route,differential scanning calorimetry (DSC, Netzsch 404C, Germany) was conducted on a 1Ti/1Ge/2TiC powder mixture under the Ar atmosphere at a scanning rate of 10℃/min. The phase constituents of Ti3GeC2 powders were determined by X-ray diffraction (XRD, Bruker-AXS D8, Germany) with Cu Kα radiation at a scanning step of 0.05° and scanning rate of 5°/min. Besides, The morphologies and chemical compositions of the synthesized Ti3GeC2 powders were characterized by a scanning electron microscope (SEM, FEI/Philips Sirion 2000, Netherlands) equipped with an energy dispersive spectrometer (EDS, Aztecs X-MAX 80).
3.1. Effect of temperature on the formation of Ti3GeC2
To eliminate these impurity phases, the sintering temperature of Ti3GeC2 powders was examined from 1500 ℃ to 1550℃. Figure 1a shows the XRD results of 1Ti/1Ge/2TiC powder sintered in vacuum for 2 h at different temperatures. At 1500 ℃, the main phase of Ti3GeC2 has been formed, but significant amount of TiC remains in the product, which is attributed to the relatively low reaction temperature and thus the residual TiC unreacted. With the increase of temperature to 1550 ℃, the content of Ti3GeC2 in the product continues to increase, and the content of TiC decreases significantly if compared with 1500 ℃. Therefore, the Ti3GeC2 sintered from 1Ti/1Ge/2TiC powder mixture at 1550 ℃ has relatively high phase purity.
For comparison, Fig. 1b presents the XRD results of 3Ti/1Ge/2C powder mixtures sintered in vacuum for 2 h at 1500 ℃ and 1550 ℃. Accordingly, the large amount of residual TiC at 1500℃ and the gradually increased phase purity of Ti3GeC2 at 1550 ℃ in the product sintered from 3Ti/1Ge/2C powder mixtures can be derived. Thses results is consitent with the literature. However, if compared with 1Ti/1Ge/2TiC specimens, the 3Ti/1Ge/2C ones produces higher TiC content in final products at the same sintering temperature according to the stronger peak intensity of TiC at 41.7º, which means the relativly lower phase purity. Therefore, Ti/Ge/TiC powder mixtures were utilized as starting materials to synthesize Ti3GeC2 powder in the following experiments.
3.2. Effect of Ge molar ratio on the formation of Ti 3 GeC 2
In addition to sintering temperature, the composition of raw materials also affects the phase purity of Ti3GeC2 powder. 1Ti/xGe/2TiC powder mixtures with different Ge contents were sintered at 1550 ℃ for 2 h to fabricate Ti3GeC2 powders. They were labled as S1 (1Ti/0.9Ge/2TiC), S2 (1Ti/1.0Ge/2TiC) and S3 (1Ti/1.1Ge/2TiC), respectively.
As shown in Fig. 2, the S1 sample consists of the main phase Ti3GeC2, with a small amount of TiC and Ti2GeC detected. When the content of Ge increases to 1.0 in the S2 sample, the amount of Ti3GeC2 increases and those of TiC and Ti2GeC decreases accordingly. With the further increase of Ge content to 1.1 in S3 sample, Ti3GeC2 dominates the phase constituents with trace Ti2GeC and TiC.
The Ge element exists as a liquid phase and promotes the formation of Ti3GeC2 phase during the sintering process. When the content of Ge is less than the stoichiometric value of 1.0, the liquid Ge phase may become insufficient due to the evaporation loss at high temperatures during the sintering of Ti/Ge/TiC mixtures. Therefore, raw materials like TiC is left in the final product. Meanwhile, a small amount of Ti2GeC as intermediate phase is remained, as shown in S1 and S2 samples. When the content of Ge element reaches 1.1, the excessive Ge compensates its loss, accompanying with the mostly consumed TiC and the promoted transformation from Ti2GeC to Ti3GeC2, as shown in S3 sample.
Figures 3(a)-(c) presents the respective morphologies of the synthesized powders of S1, S2, and S3 samples, all of which have the typically layered structures of MAX phases. EDS patterns suggest the presence of Ti, Ge and C elements and the Ti/Ge ratios change slightly from 3.18–3.28, indicating the high purity of Ti3GeC2 powders. The content of Ge in raw materials seems hardly to affect the microstructure of Ti3GeC2 powders. Therefore, the synthesized conditions of Ti3GeC2 powders are optimized to be the starting materials of 1Ti/1.1Ge/2TiC sintered at 1550 ℃ for 2 h.
3.3 Powder Synthesis
Figure 4 displays the DSC curve of 1Ti/1Ge/2TiC powder mixture heated from room temperature to 1350℃ at a rate of 10℃/min. There is a sharp endothermic peak near 942℃, which corresponds to the melting of Ge. The formation of liquid Ge benefits to the diffusion and reaction among the raw materials. The broad exothermic peak appears at 1140–1200℃, suggesting some chemical reactions take place within this temperature range.
To determine the chemical reactions and phase changes of the raw materials during the heating process, 1Ti/1Ge/2TiC powders were respectively sintered at 1140 °C, 1200 °C and 1250 °C for 2 h, which corresponds to the starting, the ending and a higher temperatures to the exothermic peak. Figure 5 shows the XRD patterns of three samples sintered at these temperatures.
As sintered at 1140 °C, the sample is composed of a large amount of Ti5Ge3 with a significant amount of Ge and TiC raw materials, as well as a small amount of Ti2GeC. This indicates that the reaction between Ti and liquid Ge to form Ti5Ge3 contributes mainly the appearance of exothermic peak at the starting temperature. For the sample sintered at 1200 ℃, most of the peaks can be indexed to Ti2GeC phase with a small amount of TiC and a trace of Ti5Ge3 and residual Ge. With the increase of sintering temperature, Ti5Ge3 reacts with Ge and TiC to form Ti2GeC, which explains the exothermic peak around the ending temperature. Note that these results provide a ready method to fabricate high-purity Ti2GeC powders. At temperatures higher than the exothermic peak, Ti3GeC2 phase appears with the decreasing amount of Ti2GeC and TiC, indicating the formation of Ti3GeC2 with the consumption of Ti2GeC and TiC. Therefore, the reaction route of Ti3GeC2 from Ti/Ge/TiC powder mixtures can be described as follows:
The present study provides a new route to fabricate high-purity Ti3GeC2 powders from Ti/Ge/TiC by pressureless sintering in vacuum. The influence of sintering temperature and Ge content on the Ti3GeC2 powder purity and the corresponding reaction route was investigated. The main conclusions are as follows:
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High-purity Ti3GeC2 powders were successfully synthesized by pressureless sintering from the starting powder mixture of 1Ti/1.1Ge/2TiC at 1550 ℃ for 2 h. The purity of Ti3GeC2 samples is mainly related to the sintering temperature and the Ge content in starting composition.
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The reaction route of Ti/Ge/TiC power mixtures to Ti3GeC2 was revealed to be three steps: First, Ge melts at 942 ℃ and starts to react with Ti to form Ti5Ge3 at 1140 ℃; Then, Ti2GeC phase is completely formed by consuming Ti5Ge3, Ge and TiC at 1200 ℃; finally, Ti2GeC reacts with residual TiC to form Ti3GeC2 gradually at higher temperatures.
Acknowledgments
The authors wish to acknowledge the financial support of National Natural Science Foundation (Nos. 51731004 and 51671054), the Natural Science Foundation of Jiangsu Province (BK20181285).
- Barsoum MW. Mechanical Properties of the MAX Phases. Encyclopedia of Materials: Science and Technology 2004: 1–16
- Sun ZM (2011) Progress in research and development on MAX phases: a family of layered ternary compounds. Int Mater Rev 56:143–166
- Jeitschko W, Nowotny H, Benesovsky F (1964) Carbides of formula T2MC. Journal of The Less-Common Metals 7:133–138
- Barsoum MW (2000) The MN+1AXN phases: A new class of solids. Prog Solid State Chem 28:201–281
- Finkel P, Barsoum MW, Hettinger JD et al. Low-temperature transport properties of nanolaminates Ti3AlC2 and Ti4AlN3. Physical Review B 2003, 67: 235108.1-235108.6
- Lofland S, Hettinger J, Meehan T et al (2006) Electron-phonon coupling in Mn+1 AXn-phase carbides. Phys Rev B 74:3840–3845
- Michel B (1997) Dmitri, et al. Layered machinable ceramics for high temperature applications. Scripta Mater 36:535–541
- Wolfsgruber H, Nowotny H, Benesovsky F et al (1967) Die Kristallstruktur von Ti3GeC2. Monatshefte Für Chemie Verwandte Tle Anderer Wissenschaften 98:2403–2405
- Magnuson M, Palmquist JP, Mattesini M et al (2005) Electronic structure investigation of Ti3AlC2, Ti3SiC2, and Ti3GeC2 by soft-X-ray emission spectroscopy. Phys Rev B 72:245101
- Radovic M, Barsoum MW, Ganguly A et al (2006) On the elastic properties and mechanical damping of Ti3SiC2, Ti3GeC2, Ti3Si0.5Al0.5C2 and Ti2AlC in the 300–1573 K temperature range. Acta Mater 54:2757–2767
- Mane RB, Ampolu H, Rohila S et al (2019) Oxidation kinetics of Ti3GeC2 MAX phase. Corros Sci 151:81–86
- Kephart JS, Carim AH (1998) Ternary Compounds and Phase Equilibria in Ti-Ge‐C and Ti‐Ge‐B. Cheminform 29:3253–3258
- Ganguly Z, Barsoum MW (2004) Synthesis and mechanical properties of Ti3GeC2 and Ti3(SixGe1–x)C2 (x = 0.5, 0.75) solid solutions. Journal of Alloys Compounds 376:287–295
- Mane RB, Haribabu A, Panigrahi BB (2017) Synthesis and Sintering of Ti3GeC2 MAX Phase Powders. Ceram Int 44:890–893
- Yang S, Sun ZM, Hashimoto H (2003) Synthesis of Single-Phase Ti3SiC2 Powder. J Eur Ceram Soc 23:3147–3152
- Zhang Y, Sun ZM, Zhou Y (1999) Cu/Ti3SiC2 composite: a new electrofriction material. Material Research Innovations 3:80–84
- Dudina DV, Mali VI, Anisimov AG et al (2013) Ti3SiC2-Cu composites by mechanical milling and spark plasma sintering: Possible microstructure formation scenarios. Met Mater Int 19:1235–1241
- Ding JX, Tian WB, Sun ZM (2018) Arc erosion behavior of Ag/Ti3AlC2 electrical contact materials. J Alloy Compd 740:669–676
- Ding JX, Tian WB, Zhang PG et al (2019) Preparation and arc erosion properties of Ag/Ti2SnC composites under electric arc discharging. Journal of Advanced Ceramics 8:90–101
- Ding JX, Tian WB, Wang DD, et al. Microstructure evolution, oxidation behavior and corrosion mechanism of Ag/Ti2SnC composite during dynamic electric arc discharging. Journal of Alloys & Compounds 2019, 785: 1086–1096.
- Zhang M, Tian WB, Zhang PG et al (2018) Microstructure and properties of Ag-Ti3SiC2 contact materials prepared by pressureless sintering. International Journal of Minerals Metallurgy Materials 25:810–816