Synthesis of Ti3GeC2 Powders from Ti-Ge-TiC Mixtures by Pressureless Sintering

Ti 3 GeC 2 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 Ti 3 GeC 2 powders from Ti/Ge/TiC by pressureless sintering in vacuum. The inuence 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 Ti 5 Ge 3 compounds at around 1140 ℃ ; Then Ti 5 Ge 3 consumes the liquid Ge and TiC to form Ti 2 GeC at around 1200 ℃ ; Finally, Ti 2 GeC transfers into Ti 3 GeC 2 with the consumption of residual TiC. The present study lays the foundation for the subsequent applications of Ti 3 GeC 2 powders in high performance composites.


Introduction
MAX phases, with the formula of M n+1 AX n , 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 rst 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, Ti 3 GeC 2 has a theoretical density of 5.55 g/cm 3 and a hexagonal crystal structure [8,9]. Ti 3 GeC 2 exhibits moderate hardness (2 ~ 6 GPa), high Young's modulus (~ 197 GPa), excellent machinability, high yield strength, and signi cant 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. rst fabricated bulk Ti 3 GeC 2 [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 Ti 3 GeC 2 plates by hot pressing at 1500 °C for 4 h. However, the phase purity was not satis ed 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 Ti 3 GeC 2 bulks by an arc melting ingot method, while several impurity phases like TiC, Ti 5 Ge 3, and Ti 2 GeC were formed. To further improve the purity of Ti 3 GeC 2 , 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 Ti 3 GeC 2 and Ti 3 (Si x Ge 1−x )C 2 (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 rst preparation of Ti 3 GeC 2 in 1967, most studies focused on the synthesis of its bulk samples, but few reports concerned the direct production of Ti 3 GeC 2 powders. Rahul et al. [14]synthesized Ti 3 GeC 2 powder via pressureless sintering at 1500 ℃, containing Ti 5 Ge 3 and TiC impurities. Generally, the Ti 3 GeC 2 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 Ti 3 GeC 2 powder purity was inspired by the report that using Ti/Si/TiC instead of Ti/Si/C powder mixtures will signi cantly increase the phase purity of Ti 3 SiC 2 [15]. The synthesis of high-purity MAX phase powders will bene t 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][17][18][19][20][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, Ti 3 GeC 2 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. To eliminate these impurity phases, the sintering temperature of Ti 3 GeC 2 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 Ti 3 GeC 2 has been formed, but signi cant 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 Ti 3 GeC 2 in the product continues to increase, and the content of TiC decreases signi cantly if compared with 1500 ℃. Therefore, the Ti 3 GeC 2 sintered from 1Ti/1Ge/2TiC powder mixture at 1550 ℃ has relatively high phase purity.

Experimental Procedures
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 Ti 3 GeC 2 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 nal 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 Ti 3 GeC 2 powder in the following experiments.
As shown in Fig. 2, the S1 sample consists of the main phase Ti 3 GeC 2 , with a small amount of TiC and The Ge element exists as a liquid phase and promotes the formation of Ti 3 GeC 2 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 insu cient 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 nal product. Meanwhile, a small amount of Ti 2 GeC 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 Ti 2 GeC to Ti 3 GeC 2 , 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 Ti 3 GeC 2 powders. The content of Ge in raw materials seems hardly to affect the microstructure of Ti 3 GeC 2 powders. Therefore, the synthesized conditions of Ti 3 GeC 2 powders are optimized to be the starting materials of 1Ti/1.1Ge/2TiC sintered at 1550 ℃ for 2 h. 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 bene ts to the diffusion and reaction among the raw materials.

Powder Synthesis
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 Ti 5 Ge 3 with a signi cant amount of Ge and TiC raw materials, as well as a small amount of Ti 2 GeC. This indicates that the reaction between Ti and liquid Ge to form Ti 5 Ge 3 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 Ti 2 GeC phase with a small amount of TiC and a trace of Ti 5 Ge 3 and residual Ge. With the increase of sintering temperature, Ti 5 Ge 3 reacts with Ge and TiC to form Ti 2 GeC, which explains the exothermic peak around the ending temperature. Note that these results provide a ready method to fabricate high-purity Ti 2 GeC powders. At temperatures higher than the exothermic peak, Ti 3 GeC 2 phase appears with the decreasing amount of Ti 2 GeC and TiC, indicating the formation of Ti 3 GeC 2 with the consumption of Ti 2 GeC and TiC. Therefore, the reaction route of Ti 3 GeC 2 from Ti/Ge/TiC powder mixtures can be described as follows:

Conclusions
The present study provides a new route to fabricate high-purity Ti 3 GeC 2 powders from Ti/Ge/TiC by pressureless sintering in vacuum. The in uence of sintering temperature and Ge content on the Ti 3 GeC 2 powder purity and the corresponding reaction route was investigated. The main conclusions are as follows: 1. High-purity Ti 3 GeC 2 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 Ti 3 GeC 2 samples is mainly related to the sintering temperature and the Ge content in starting composition.
2. The reaction route of Ti/Ge/TiC power mixtures to Ti 3 GeC 2 was revealed to be three steps: First, Ge melts at 942 ℃ and starts to react with Ti to form Ti 5 Ge 3 at 1140 ℃; Then, Ti 2 GeC phase is completely formed by consuming Ti 5 Ge 3 , Ge and TiC at 1200 ℃; nally, Ti 2 GeC reacts with residual TiC to form Ti 3 GeC 2 gradually at higher temperatures. Figure 1 XRD patterns of (a) 1Ti/1Ge/2TiC and (b) 3Ti/1Ge/2C powder mixtures sintered at different temperatures for 2h. XRD patterns of 1Ti/1Ge/2TiC powder mixtures sintered from 1140 °C to 1250°C.