Effect of Ball Milling Parameters on Properties of Nano-Sized Tungsten Powder via Mechanochemical Processing

Abstract

Nano-sized tungsten exhibits superior properties due to its high-density grain boundaries’ strengthening. The high-quality nano-sized powder is essential for sintering nano-sized tungsten bulks through powder metallurgy techniques. In this study, nano-sized tungsten powder was successfully synthesized by mechanochemical methods using mixed WO₃ and Mg powders. The effects of processing parameters on the morphology and microstructure of synthesized powder were thoroughly investigated. The results reveal that the thermite reaction of WO₃ and Mg is almost complete after 5 min of ball milling at a speed of 300 rpm. The average grain size of the tungsten powder decreases with the increasing milling duration and speed. Optimal average grain size and purity were achieved at a milling speed of 300 rpm and a milling duration ranging from 30 to 120 min. Moreover, centrifugation sieving further reduces the average grain size of tungsten powder to 19.5 nm. In addition, the entire mechanochemical process can be divided into two stages: the reaction stage and the grain size refinement stage.

1. Introduction

Tungsten, known for its high hardness and excellent electrical and thermal conductivity, is extensively utilized in powder metallurgy, electronics, cutting tools, and filaments [1,2,3]. Recently, tungsten-based alloys have been considered as the most important candidates for plasma facing materials (PFMs) due to their resistance in extreme environments, including high thermal load, high-energy fusion neutron irradiation, and high-flux plasma bombardment [4,5,6]. Consequently, many studies on the responses of tungsten under extreme conditions have been conducted. The results show that nano-sized tungsten exhibits superior properties when exposed to high-energy ions due to its high-density grain boundaries, which serve as sink sites to capture and annihilate irradiation defects [7,8,9]. However, the use of excellent pristine nano-sized powder is essential for obtaining high-quality nano-sized tungsten bulk via powder metallurgy methods.

In general, nano-sized powder is primarily prepared using the wet-chemical method [10,11,12,13,14] and high-energy ball milling [15,16,17,18,19,20]. Tungsten powder prepared by wet-chemical processing typically exhibits effective dispersion without severe agglomeration. However, the average grain size is comparatively larger with diameters ranging from ~50 to ~200 nm [10,11,12], and the wet-chemical processing is a complex and dangerous process. Because the reduction process in the wet-chemical method is usually conducted in a hydrogen atmosphere at high temperature, this greatly influences its application [10,11,12,13,14]. High-energy ball milling is a simple and effective method to prepare nano-sized tungsten powder. However, it has always been conducted at high rotation speeds and long durations, which inherently introduces many impurities and exacerbates powder agglomeration, directly impacting the sintering behaviors [21,22,23,24,25,26].

Mechanochemical methods using the mixed powders of WO₃/CaWO₄/MgWO₄ and Mg have been proven as effective in producing nano-sized tungsten powder [27,28,29,30]. Compared to the wet-chemical method and high-energy ball milling, the solid reactions are activated by mechanical energy at room temperature in the mechanochemical process, which seems to be more energy-efficient and less time-consuming according to previous reports [1,2,3]. However, former reports often involved either high rotation speed (e.g., 1200 rpm) [2] or long milling duration (e.g., 100 h) [27], which may increase the impurity content. Moderate milling conditions, such as processing with low speeds and short durations, may reduce impurity content, whereas it remains uncertain whether the chemical reaction can be ignited and the properties of obtained powder meet requirement. Additionally, the process of the mechanochemical method is complex, and the relations between properties of obtained powder (such as powder size, agglomeration, impurity) and milling parameters are still ambiguous.

Motivated by the above questions, systematic research has been conducted by the mechanochemical method using the mixed powders of WO₃ and Mg. The effects of experimental parameters, such as milling speed, milling duration, and pristine powder states, on the properties of obtained tungsten powder have been investigated in detail.

2. Materials and Methods

Commercial pure tungsten trioxide powder (purity > 99.9%, 300 mesh) and magnesium powder (purity > 99.9%, 300 mesh) were used as the initial material. The experiments were conducted in a planetary ball mill (PM400, Retsch, Haan, Germany) at room temperature according to reaction (1). The reaction process was performed in the vessels made by WC/Co (250 mL in volume) with argon atmospheres. To remove residual Mg and produced MgO, the milled powder was leached with a hydrochloric acid (4 mol/L) solution for 10 h. Then, the powder was washed with deionized water until the pH value of the solution appeared as neutral. Finally, the powder was centrifuged with anhydrous ethanol and dried under vacuum.

$$W{O}_3+3Mg\to W+3MgO$$

(1)

The phases of the obtained powder were analyzed by X-Ray Diffraction (XRD; DX-2700BH, Dandong, China) with a step size of 0.02°/s. The average grain size and microstrain of the prepared tungsten powder were also estimated by the Hall method, as referenced elsewhere [8]. And the Hall plot with the calculation for prepared powder is listed in the Supplementary Information. The Mg content in the leached tungsten powder was accurately measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 5110, Mulgrave, Australia) spectrometer. The milled and leached tungsten powder samples with 1 mL anhydrous ethanol were ultrasonically dispersed for 10 min. Then, the solution was dripped on the conductive glue and ultra-thin copper wire to make SEM and TKD test samples, respectively. Finally, the morphology of the milled/leached powder samples was characterized by a field-emission scanning electron microscope (FE-SEM, Zeiss Sigma, Oberkochen, Germany) with 15 kV. Additionally, the band contrast (BC), inverse pole figure (IPF), and grain size of the leached powder samples were further analyzed using transmission Kikuchi diffraction (TKD; thermoscientific, Helios 5CX, Czech Republic) with 30 kV and the step size of 5–10 nm. And the obtained TKD data were then analyzed by using the AZtecCrystal 2.1 software.

3. Results

3.1. Phases and Morphology of Tungsten Powder Prepared by Mechanochemical Method

The XRD patterns in Figure 1a show the phases of the powder prepared by the mechanochemical method. After 30 min of ball milling with a speed of 300 rpm, the presence of W and MgO phases can be confirmed, while the diffraction peaks of WO₃ and Mg have disappeared. This indicates that the reaction described in Equation (1) has occurred successfully. Subsequently, the milled powder was leached with hydrochloric acid solutions, and the MgO was removed completely according to the XRD diffraction peak analysis. The result from the inductively coupled plasma spectrometer in Figure 1b also shows that when the leaching time is 4 h, the content of the Mg element in the leached powder has decreased to about 100 ppm. Therefore, single-phase tungsten powder was obtained using the mechanochemical method at a relative moderate speed and short duration.

Figure 2 shows the SEM results of one milled and one leached powder type at different magnifications. It can be found that the milled powder contains numerous irregular agglomerations with a size range from several to hundreds of micrometers (Figure 2a,b). The EDS results shown in Figure 2c–e demonstrate a uniform element distribution of W, Mg, and O, indicating that the W and Mg elements are well mixed and the size of powder is fine. The morphologies of pure tungsten powder after acid leaching are shown in Figure 2f,g. Contrary to the intensely welded powder typically produced by high-energy ball milling [4,21], the leached powder manifests loosely aggregated morphology. It can also be noted that there are some larger particles with a size more than 100 nm in the leached powder as denoted by the blue arrows (Figure 2g). To enhance the uniformity and further reduce the average size of obtained nano-sized powder, systematical studies were conducted focusing on the ball milling parameters.

3.2. The Effect of Milling Parameters on Powder Properties

The XRD results of the milled powder under different milling parameters are presented in Figure 3. In terms of the phase evolution of mixed powder, the reaction of WO₃ with Mg is nearly complete after 5 min of ball milling at a speed of 300 rpm, with only a small amount of WO₃ remaining in the synthesized powder (Figure 3a,c). As the milling time increases to 30 min, the diffraction peaks of WO₃ gradually disappear, and only diffraction peaks of W and MgO are found in the milled powder. When the milling time reaches 60 min and 120 min, WC impurity (ICDD Card No: 51–0939) [31] is detected and the content increases with milling duration. Besides that, the reaction state is also impacted by the milling speed. As shown in Figure 3b, the lower the milling speed, such as 150 rpm and 200 rpm, the stronger the diffraction peaks of unreacted WO₃. Moreover, the duration of unreacted WO₃ increases as the milling speed decreases (Figure 3c). Regarding the impurity phase, the milling condition of 300 rpm and 30 min is the most suitable parameter for obtaining tungsten powder with minimal WC abrasion and unreacted WO₃ according to Figure 3c. And regarding average grain size and microstrain, it can be observed that they exhibit an inverse evolution tendency based on the XRD data analysis depicted in Figure 3d. The former decreases with the increasing in milling duration and speed, while the latter increases. When the milling speed is 300 rpm, the average grain size of the obtained tungsten powder decreases from 98.0 nm to 41.2 nm as the milling duration increases from 5 to 120 min. Meanwhile, the microstrain increases from approximately 0.216% to 0.507%.

Transmission Kikuchi diffraction (TKD) was employed to further analyze the uniformity of the leached powder, and the results are shown in Figure 4. The uniformity of grain size in the powder milled for 30 min and 120 min is notably different. After 30 min of ball milling at 300 rpm, the average grain size is 46.83 ± 32.93 nm, with 90% of the grains being smaller than 86.09 nm (Figure 4b). Besides that, agreeing with the SEM results shown in Figure 2g, some large grains with a size more than 100 nm exist in the 30 min milled powder. In the 120 min milled powder, finer and more uniform grains are observed compared to the 30 min milled powder. Over 90% of the grains are lower than 58.57 nm, and there are no grains larger than 100 nm (Figure 4d). Therefore, the increase in milling time can refine the powder grains and enhance the uniformity of the powder. Although 120 min of ball milling inevitably introduces WC impurity, the impurity level is acceptable when compared to those found in high-energy ball milling processes [8].

3.3. Effects of the Properties of WO₃ Powder and Sieving on the Prepared Tungsten Powder

The comparisons between two systems, which are as-received coarse WO₃ and grain-refined WO₃, were investigated. Grain-refined WO₃ powder was milled for 120 min with a speed of 300 rpm before Mg powder was added. Compared to as-received coarse WO₃ powder, the diffraction peaks of grain-refined WO₃ are intensively broadened, as shown in Figure 5a. Subsequently, the Mg powder was mixed with the grain-refined WO₃ powder for 30 min of ball milling with the milling speed of 300 rpm. Figure 5a shows that W and MgO are the predominant phases in the milled powder. Notably, there is no significant difference in the average grain size and microstrain between those derived from grain-refined and as-received coarse WO₃ powder (Figure 5b). Furthermore, this observation is further characterized by TKD results shown in Figure 5d and Figure 4b, which suggest that grain-refined WO₃ powder has a negligible effect on the uniformity and average grain size of obtained tungsten powder.

Based on the above studies, the optimal milling process should be conducted at a milling speed of 300 rpm and a milling time of 30 to 120 min. However, the average grain size of produced tungsten powder is more than 60 nm, and prolonging the milling duration or increasing milling speed does not seem feasible. This is due to that the relative proportion of WC impurity will increase simultaneously. However, it should be noted that the median value of obtained W powder is ~30 nm, and the powder is highly dispersed according to the TKD results shown in Figure 4b,d. Thus, removing the larger particles in the powder might be effective to further diminish grain size and improve uniformity. Therefore, sieving experiments were conducted using the centrifugation method in anhydrous ethanol. Figure 6a,b show the phase and grain size of nano-sized powder before and after sieving. After centrifugation, the average grain size of leached tungsten powder decreases from 60.7 nm to 19.5 nm successfully.

4. Discussion

The mechanochemical solid-state reaction of WO₃ with Mg has been performed according to Equation (1), and nano-sized tungsten powder was obtained successfully. Since the mechanochemical method involves both a chemical reaction process and mechanical ball milling, the final obtained powder exhibits the advantages of the powders produced by wet-chemical and ball milling methods. As illustrated in Figure 2f,g, the tungsten powder derived from mechanochemical processing displays a loosely aggregated morphology with the average grain size of approximately 60 nm (Figure 3d). This unique property should be attributed to the uniform mixing of MgO and W powders. Furthermore, the presence of MgO acts as a dispersant to reduce the welding of tungsten particles during the ball milling process [3].

Analogous to high-energy ball milling, the grain size and the relative scale of WC impurity of the obtained nano-sized powder are highly influenced by milling parameters. Consequently, this study primarily aimed at optimizing the milling durations and speeds to obtain tungsten powder with higher purity and finer grain size. It can be found that the reaction process between Mg and WO₃ is almost finished after 5 min of ball milling with the speed of 300 rpm. When milling duration further increases, the average grain size of obtained tungsten powder will continue to decrease due to the repeated collision between the powders and balls. This refinement process is dominated by high-energy ball milling and would inevitably introduce impurity such as the WC phase due to the wear and tear of the milling balls and vial [32,33]. Actually, it is apparent that the longer the milling duration, the stronger the intensity of WC diffraction peaks, as depicted in Figure 3a. Based on the above discussions, it should be noted that the whole mechanochemical process includes two stages at least in the moderate milling conditions, which are the reaction stage and grain size refinement stage. (1) Reaction stage: the thermite reaction of WO₃ with Mg is carried out by converting mechanical energy, and tungsten powder is produced; (2) grain size refinement stage: due to the repeated collisions between powders and balls, the average grain size of the obtained tungsten powder decreases, and the uniformity of tungsten powder improves with the increase in milling duration.

The effect of the average grain size of pristine reactant WO₃ on the obtained tungsten powder product remains unknown. If the reduction reaction happens in the form of particles between WO₃ and Mg, the final obtained tungsten powder should have a relation with the pristine reactant powders. Furthermore, a finer reactant size implies larger reaction surfaces, potentially enhancing reaction rates [34,35]. Nevertheless, there is no discernible difference in the average grain size and microstrain of the two final tungsten powders between those derived from grain-refined and as-received coarse WO₃ powders (Figure 5). As previous research reported [29,32], during the mechanochemical process, the solid-state reaction is initiated by the collisions of surface atoms between WO₃ and Mg particles in the spot contact regions of mechanical action, and the reaction productions can nucleate in the spot contact regions between colliding particles. Hence, a solid-state reaction typically occurs at collision zones in the atomic scale instead of two types of whole particles as illustrated in Figure 6c; Then, the effect of grain refinement of WO₃ mainly plays a role in increasing reaction areas and collision possibility. However, the whole mechanochemical process can be separated into the reaction stage and milling-induced grain size refinement stage. And the reaction process is almost completed after several minutes of ball milling, which means that there is no obvious distinction between the two systems. Therefore, the products of tungsten powders in two systems undergo similar milling-induced grain refinement duration and show no obvious difference in grain size.

Encouragingly, due to a superior dispersion property, large particles can be easily removed by the centrifugation method, and tungsten powder with an average grain size of 19.5 nm was obtained successfully in this work. Comprehensively, the comparisons of this work and former studies are listed in

Table 1. The comparisons of tungsten powder in this and reported works.

Powder	Milling Parameters	Average Grain Size/nm	Impurities
WO3 + Mg	300 rpm—30 min—Sieved	60.7	Undetected
		19.5
WO3 + Mg [2]	1200 rpm—10 min	83.33	Unreacted WO3
WO3 + Mg [3]	Unknown rpm—3 h	<100	W-Fe compounds
CaWO4 + Mg [27]	165 rpm—100 h	13.5 ± 4.5	~0.6%Fe, ~0.4%Cr
WO3 + MgO + Mg [30]	1000 rpm—8 min	100	Unknown

, and it can be obtained that the refined nano-sized powder can be produced at relatively moderate milling conditions with short milling duration and low milling speed.

5. Conclusions

In this study, systematic research has been conducted by the mechanochemical method using the mixed powders of WO₃ and Mg. And the effects of processing parameters on the effects of the synthesized powder were systematically investigated. The key findings are as follows:

(1)

Highly dispersed nano-sized tungsten powder can be produced by the mechanochemical solid-state reaction of WO₃ with Mg at the moderate milling conditions. The obtained nano-sized tungsten powder possesses different average gran size depending on the milling parameters. Moreover, the optimized milling process should be conducted at a milling speed of 300 rpm for a milling duration of 30 to 120 min.

(2)

The entire mechanochemical process can be separated into the reaction stage and grain size refinement stage. In the case of 300 rpm ball milling, the reaction stage is almost completed after several minutes of ball milling. The subsequent stage is dominated by the ball milling process, and the refinement degree and uniformity of obtained tungsten powder increase with the milling duration. Although the introduction of WC impurity is inevitable in this stage, the impurity level is acceptable compared to those found in the high-energy ball milling processes.

(3)

There is no discernible difference in the average grain size and microstrain of the final tungsten powders between those derived from grain-refined and as-received coarse WO₃ powders. Furthermore, the sieving experiments conducted using the centrifugation method can obtain nano-sized tungsten powder with the average grain size of ~19.5 nm.