TiC composite powders by a wet chemical route

    Synthesis and formation mechanism of W/TiC composite powders by a wet chemical route Xiao-Yue Tan, Lai-Ma Luo, Hong-Yu Chen, Ping Li, Guang-Nan Luo, Xiang Zan, Ji-Gui Cheng, Yu-Cheng Wu PII: DOI: Reference:

S0032-5910(15)00336-8 doi: 10.1016/j.powtec.2015.04.051 PTEC 10961

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

24 December 2014 7 March 2015 20 April 2015

Please cite this article as: Xiao-Yue Tan, Lai-Ma Luo, Hong-Yu Chen, Ping Li, GuangNan Luo, Xiang Zan, Ji-Gui Cheng, Yu-Cheng Wu, Synthesis and formation mechanism of W/TiC composite powders by a wet chemical route, Powder Technology (2015), doi: 10.1016/j.powtec.2015.04.051

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ACCEPTED MANUSCRIPT Synthesis and formation mechanism of W/TiC composite powders by a wet chemical route

School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009,

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People’s Republic of China b

Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of

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China c

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Ji–Gui Chenga, c, Yu–Cheng Wua, c*

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Xiao–Yue Tana, Lai–Ma Luoa, c*, Hong–Yu Chena, Ping Li a, c, Guang–Nan Luob, Xiang Zana, c,

National–Local Joint Engineering Research Centre of Nonferrous Metals and Processing

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Technology, Hefei 230009, People’s Republic of China

*Corresponding author: [email protected] (L.M. Luo), [email protected] (Y.C. Wu)

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Abstract: W/TiC composite powders were prepared through a novel wet chemical

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method using ammonium paratungstate and nanosized TiC powders as raw materials. The as-synthesized powders were characterized by X-ray diffraction, field emission

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scanning electron microscopy, high-resolution transmission electron microscopy, and energy-dispersive spectroscopy. The as-synthesized powders possessed a uniform

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diameter of about 30 nm to 100 nm and contained only nanosized W and TiC particles. TEM images and diffraction ring spectra confirmed that the “cluster” structure composite powders compose of TiC and nano-sized W particles, which locate on the surface of TiC particle. To observe the formation of this structure, we focused on the process of preparing W/TiC powder and proposed a reasonable mechanism for the formation of the second-phase doped in W matrix materials by wet chemical method. The method followed an ideal model that may be used to prepare other materials. Keywords: Wet chemical method; W/TiC composite powders; Forming mechanism; 1. Introduction Tungsten (W) and its alloys present high melting point, good thermal 1

ACCEPTED MANUSCRIPT conductivity, high strength at elevated temperatures, low sputtering yield in irradiated environments, and low tritium inventory [1–3]. These properties make W the most

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promising plasma facing material (PFM) in future fusion reactors [4, 5]. However, W

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becomes brittle under low-temperatures, high-temperatures, and radiation exposure [6,

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7]. W materials used as PFM need to withstand high temperatures and radiation that consists of electrons, protons, neutrons, and α-particles [8]. Therefore, a critical issue in

preparing

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materials

for

fusion

reactors

involves

improving

their

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high-temperature properties and radiation resistance.

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Nanostructured W materials were proposed to improve the performance of W matrix materials for use in fusion reactors. On one hand, nanostructured W materials

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can be efficiently fabricated by doping W with nanoparticles, such as TiC, La2O3, and

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Y2O3 [9–13]. These doped phase-dispersed W materials inhibit recrystallization and grain growth, as well as improve high-temperature strength and creep resistance, by

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hindering grain boundary sliding and stabilizing the microstructure [14]. On the other hand, nanostructures reduced by second-phase nanoparticles act as points of

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annihilation for radiation-induced defects, thereby improving the irradiation resistance of materials [15]. Therefore, doped second-phase nanoparticles could promote the irradiation embrittlement of W materials. However, nanosized second-phase particles aggregate on the surface of W nanoparticles because of their high surface free energies. In consequence, second-phase nanoparticles aggregate or disperse at the grain boundaries of sintered W materials and diminish their advantageous properties. Numerous studies have investigated the fabrication of dispersed TiC in W materials through mechanical alloying (top–down approach) [16–18]. However, this method causes particles to agglomerate and introduces a detrimental phase from milling equipment and medium wear [15, 19]. 2

ACCEPTED MANUSCRIPT A study [20] illustrated that core–shell structure with nanosized particles present excellent properties that could address the embrittlement problem. Xia et al. [15]

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synthesized TiC/W core–shell nanoparticles through a wet chemical method using

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ammonium metatungstate and hydrochloric acid. This method effectively fabricates

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complex nanostructured materials and shows considerable potential in preparing W/second-phase nanoparticles through molecular engineering design, yielding a defined, highly pure, and homogeneous composition [21, 22]. In our study, nanosized

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particles of W/TaC and W/TiC have been obtained by wet chemical method [2, 23].

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Thus, the preparation of nanosized powder by wet chemical processing (bottom–up approach) is an effective method to obtain a special structure in which the

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performance of materials.

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second-phase could disperse in the grain interior after sintering and improve the

We prepared W/TaC and W/TiC alloys following the same route of wet chemical

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method and SPS technology. In a recent work, we characterized the W/TiC alloy. Based on previous studies [2], we discovered that second-phase (TaC, TiC) could be

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detected in the grain interior and grain boundary upon focusing on the microstructures of these alloys. To conduct an in-depth study, we focus on the processes for preparing W/TiC alloy and characterize each step of sample treatment. We proposed an ideal model to explain the novel chemical method for fabricating nanosized second-phase doped W alloys. The proposed ideal model may also be used to prepare other materials. 2. Experimental procedure The detailed process of preparing the W/TiC alloy has been illustrated in our previous study [23]. The powder was prepared from the nanosized second-phase TiC particles (commercial powder, particle size about 50 nm, purity 99.9%, Cwnano CO. 3

ACCEPTED MANUSCRIPT Ltd., Shanghai, China) and ammonium paratungstate (NH4)10H2W12O42·XH2O (APT). The ratio of the dopant TiC 1.0 wt.%, which calculated by stoichiometry. Oxalic acid

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(C2H2O4·2H2O) acted as the precipitating agent to prepare the precursor. During the

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process that called as evaporating precipitation, meaning that the precursor was

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precipitated from the mixture solution, resulting by a reaction that with the concentration of oxalic acid increased which owing to the solution evaporation at 165 °C. After grinding, the precursor was placed in a tubular furnace with a highly

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pure hydrogen atmosphere (purity = 99.999%) for reduction. Reduction was

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performed by increasing the temperature to 200 °C for 30 min to exclude the low sublimation point of the residual inorganic material. Then, a two-step reduction

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process was carried out by heating to 500 °C for 60 min and then to 800 °C for 60 min,

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and finally the material was cooled.

The composition of the precursor and the reduced powder was analyzed by X-ray

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diffraction (XRD, X’Pert PRO, Holland). Field emission scanning electron microscopy (FE-SEM, SU8020, Japan) and transmission electron microscopy (TEM,

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JEM-2100F, Japan) were used to characterize the precursor and the reduced powder. The corresponding energy dispersive spectroscopy (EDS) was used to assist in verifying these structures. The particle size was confirmed from the FE-SEM images and TEM image. 3. Results and discussion 3.1 Characterization of precursor structure The XRD pattern of the W/TiC precursor is displayed in Fig. 1, which shows that the precursor powder consisted of hydrogen tungsten oxide hydrate and hydrogen oxalate hydrate derived from precipitated APT and residual oxalic acid. The XRD spectrum did not exhibit a TiC peak in Fig. 1, verifying the low ratio of the TiC 4

ACCEPTED MANUSCRIPT nanoparticles. Figures 2a and 2b show the FE-SEM images of the W/TiC precursor after the

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evaporation precipitation and grinding. The particles were caked together with an

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undefined irregular structure, as shown in Fig 2a. In general, the TiC nanoparticles

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and W precursor should present different contrasts under the FE-SEM. The second phase was not evident on the particle surface, which may imply that the TiC nanoparticles are coated with the W precursor. The high-magnification FE-SEM

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image shows several floccules that adhered together to form an aggregated “cake

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mass” on the surface of the precursor particles, as shown in Fig. 2b. Figure 2c shows the EDS spectra of the selected black square region in Fig. 2b, indicating that the

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region is composed of O, C, and W, which denotes the precursor. The precursor

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particle was further characterized by TEM. Figure 3a shows the TEM image of the floccules. The selected white square region shows a nanosized dark phase detected in

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the floccules. The dark phase was further observed under HRTEM. Figure 3b shows the HRTEM image of the dark phase, and the interplanar spacing corresponds well

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with the (200) plane of TiC. The region of the nanosized dark phase was also characterized by EDS (Fig. 3d), which verified the phase to be the TiC nanoparticle. Based on the above-mentioned results, we could boldly speculate that TiC nanoparticles were coated by the W precursor. 3.2 Characterization of reduced powder The precursors were subsequently reduced in a tube furnace and characterized by XRD and SEM. Figure 4 shows the XRD pattern of the reduced W/TiC powder, which only consisted of “pure” W peaks. All main peaks were consistent with the standard body-centered cubic structure of pure W (JCPDS#04-0806). No TiC peaks were detected in the XRD spectrum, which may be due to the doping of 1 wt.% TiC 5

ACCEPTED MANUSCRIPT content. The FE-SEM images of the W/TiC powder are shown in Figs. 5a and 5b. The powder exhibited a diameter range of about 30 nm to 100 nm, and all particles

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approximate a spherical shape. However, we observed that these nanoparticles

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showed certain loose “cluster” structures, which easily formed from the

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agglomeration of small nanosized W particles, as indicated by arrows in Fig. 5b. The nanoparticles easily reunite. Clusters are formed by two routes. One method is through nanosized W particles possibly nucleating on nanosized TiC particles and

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forming a “cluster”. In the other route, the reduced nanosized W particles situated

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away from nanosized TiC particles could gather together and form a “cluster”. Figure 5c shows the EDS spectrum of the entire view field from Fig. 5b, indicating that the

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powder contains W, C, and Ti. The content of Ti was low, about 0.24 wt.%, which

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could be attributed to several factors, namely, low TiC content, lower response of TiC to X-ray relative to W, and TiC being possibly covered by W particles.

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To illustrate the structural relationship between nanosized W particles and the doped TiC nanoparticles, the obtained reduced powders were studied by TEM. Figure

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6a shows the typical morphology of W/TiC particles, which presented a “cluster” structure. A large particle with a diameter of about 300 nm was discovered. We suggested that this large particle is formed from the clusters (as shown in Fig. 5b) and gathered

by

smaller

particles

through

adsorption.

Figure

6b

shows

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high-magnification image focused on particles in Fig. 6a. Evidently, several smaller particles with a diameter of about 20 nm were gathered together. These results matched well with the images in Figs. 5a and 5b. Figure 6c shows the EDS spectrum that was detected from the image in Fig. 6b. These particles were composed of W, Ti, C, and Cu, and the detected Cu was ascribed to the copper mesh. The figure indicates that TiC nanoparticles exist in these particles. Figure 6d shows a selected-area 6

ACCEPTED MANUSCRIPT electron diffraction spectrum, which is viewed from Fig. 6b, illustrating the polycrystalline diffraction ring spectrum and single-crystal diffraction bright spots.

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Pattern analysis revealed that the polycrystalline diffraction ring spectrum represented

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the polycrystalline W, whereas the diffraction bright spots comprise TiC spectrum. In

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empirical potential and based on the above findings, this W/TiC composite powder shown in Fig. 6a presents a “cluster” structure, wherein the interior was a TiC particle and the surface was composed of smaller-sized W particles.

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HRTEM images were used for detection to verify this “cluster” structure. Figures

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7a and 7b show the HRTEM images of the selected region from Fig. 6b. As seen in Fig. 7a, the center, which is marked by the red dashed line and surrounded by several

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W particles, was identified as TiC. Based on the light and dark contrast, TiC was

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located underneath several W particles. Notably, the HRTEM of these interfaces between W and TiC presented a foggy crystal lattice, which resulted from the effect of

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the interaction of the two crystal structures. In addition, a slight difference in lattice spacing and crystal plane angle exists in these particles because the incident electron

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could move with each zone axis of these gains. Figure 7b shows another HRTEM image of the selected region in Fig. 6b that more clearly shows the location of TiC below the W particle. In view of the results shown in Figs. 6 and 7, we verify that the obtained W/TiC powder presents “cluster” structure, wherein the interior was a TiC particle and the surface was composed of smaller-sized W particles. In our previous work [23], we used SPS technique to consolidate the obtained W/TiC powder, and we found a structure in which TiC particles exist in the grain interior and grain boundary. The same structure was detected in the W/TaC alloy through the same route [2]. 3.3 Modeling of formed “cluster” structure A reasonable mechanism for the formation of the “cluster” structured composite 7

ACCEPTED MANUSCRIPT powder was depicted in Fig. 8 based on the above-mentioned results and other papers [2, 23]. In the initial stage (Fig. 8a), second-phase nanoparticles uniformly mixed,

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assisted by ultrasonication, with the tungsten-containing solute. The C2H2O4·2H2O

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dope and oil bath conditions provided the set-up for an evaporating precipitation

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process, which is carried out obtain the precursor. During the process of evaporating precipitation (Fig. 8b), the W precursor had sufficient time to nucleate and grow on the surface of the nanosized second-phase particles (TiC). With increasing

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evaporation time, the tungsten-containing solute would aggregate and grow. The

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nanosized second-phase particles (TiC) could normally trap the surrounding tungsten-containing solute because of the high surface energy of the nanosized

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particles. After full evaporation (Fig. 8c), the nanosized second-phase nanoparticles

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were coated by the W precursor, based on the results shown in Figs. 2 and 3. An in situ reduction process, which was carried out in a tube furnace under an H2

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atmosphere, was observed in the precursor. We saw in the obtained reduced powder that the second-phase nanoparticles were coated with smaller nanosized W particles

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and form the “clusters” structures (Fig. 8d). The reduction procedure results could be obtained from the SEM images in Fig. 5 and TEM images in Figs. 6 and 7. Nucleation and growth would occur in these smaller nanosized W particles during sintering process, which will result in the disappearance of the nanosized W particles and the formation of a large grain. In addition, a structure in which second-phase nanoparticles are distributed in the grain interior and grain boundary is presented (Fig. 8e). This result can be obtained from our previous work displayed in the papers [2, 23]. In our viewpoint, the second-phase particles in the grain interior may be caused by the surrounding dense nanosized W particles, whereas those on the grain boundaries may be caused by surrounding loosely nanosized W particles. 8

ACCEPTED MANUSCRIPT 4. Conclusions We presented a process of preparing a W/TiC composite powder by wet chemical

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method, and performed detailed characterization of the sample at each step. We

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discovered that nanosized TiC particles were initially coated with a W precursor, and

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then with nanosized W particles through in situ reduction. The obtained powder presented diameters ranging from 30 nm to 100 nm after reduction. In addition, HRTEM images or diffraction ring spectrum verified that the central particle was the

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TiC phase, and that the surrounding particles were the W phase. Based on our works,

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we proposed a mechanism to illustrate the experimental procedures, and our ideal model may be used for instructing the preparation of other materials or for explaining

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the formation mechanism of this structure.

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Acknowledgement

This paper was supported by National Magnetic Confinement Fusion Program of

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China (Grant No. 2014GB121001) and National Natural Science Foundation of China (Grant No. 51204064 and 51474083).

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ACCEPTED MANUSCRIPT Figure capious Fig. 1 XRD spectrum of the precursor of W/TiC.

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Fig. 2 (a) (b) Low and high revolution SEM images of the precursor of W/TiC,

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respectively; (c) corresponding EDS spectrum of the selected black square region from the floccules in fig. 2b.

Fig. 3 (a) A TEM micrograph of the floccules; (b) (c) HRTEM image of the selected

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white square region from the fig. 3a and corresponding EDS spectrum, respectively.

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Fig. 4 XRD spectrum of the reduced powder of W/TiC. Fig. 5 (a) (b) Low and high revolution SEM images of the reduced powder of W/TiC,

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respectively, the arrows show the “cluster”; (c) the corresponding EDS spectrum

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which detected from the fig. 2b.

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Fig. 6 (a) A typical morphology of the W/TiC particles detected by TEM; (b) a high-magnification image focus on those particles of fig. 6a; (c) (d) the corresponding

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EDS spectrum and SAED spectrum, respectively, viewing from the fig. 6b. Fig. 7 (a) (b) Two typical HRTEM micrographs of the selected region from the Fig. 6b.

Fig. 8 Schematic diagram of the formation mechanism of the “cluster” structure.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights >A

wet chemical route was used to prepare W/TiC composite powders. >A

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mechanism is proposed to explain synthesis of W/TiC composite powders. >The

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method follows an ideal model may be used for other “cluster” structure materials.

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