Rapid In‑Situ Synthesis of Tungsten‑Copper Bimetallic Nanoparticles Using Reactive RF Thermal Plasma

Abstract

We report an in‑situ synthesis of tungsten‑x wt% copper (x = 5, 10, 20) bimetallic nanoparticles via inductively coupled radio‑frequency (RF) thermal plasma. The process reduces tungsten trioxide (WO₃) and cupric oxide (CuO) to metallic W and Cu under a hydrogen atmosphere, yielding uniformly dispersed composite particles.

Background

W–Cu composites are prized for their combined high‑temperature stability, low thermal expansion, and superior electrical conductivity, making them ideal for automotive, aerospace, power‑generation, and electronic applications. However, their markedly different melting points—W at 3683 K and Cu at 1353 K—combined with negligible mutual solubility and high contact angles, pose significant challenges to conventional liquid‑phase sintering and densification. Adjusting the W/Cu ratio (e.g., <20 wt% Cu for thermal management, <80 wt% Cu for high‑power contacts) allows tailoring of mechanical and transport properties.

Current nanoparticle fabrication routes—mechanical milling, thermochemical synthesis, and chemical reduction—often yield irregularly shaped particles with limited uniformity and poor densification during subsequent sintering. The immiscibility of W and Cu hinders particle coalescence, resulting in sub‑optimal microstructures. Recent studies suggest that coating W micropowders with Cu nanoparticles can enhance densification via liquid‑phase sintering, as molten Cu infiltrates pores and promotes mass transport.

Building on these insights, we employed inductively coupled RF thermal plasma to produce W‑x wt% Cu composite nanoparticles. The plasma’s high temperature and rapid quenching promote heterogeneous nucleation and collision‑coalescence, potentially overcoming the immiscibility barrier and yielding uniform bimetallic particles.

Methods

Feedstock powders were prepared by blending WO₃ and CuO micro‑particles (purity > 99.99 %) in 5, 10, and 20 wt% Cu ratios. The blend was dried at 423 K for one hour. These oxides were chosen for their low boiling points (WO₃ = 1973 K; CuO = 2273 K) relative to elemental W (5828 K) and Cu (2835 K), facilitating vaporization in the plasma. The 30 kW RF induction plasma (Tekna, Quebec) vaporized the feedstock; hydrogen gas reduced the vapors to W and Cu metal, while argon and nitrogen gases provided quenching and controlled nucleation kinetics. Full vaporization and reduction conditions are summarized in Table 1.

Results

Scanning electron microscopy coupled with energy‑dispersive spectroscopy (SEM‑EDS) confirmed that the synthesized nanoparticles matched the nominal W‑x wt% Cu compositions (Fig. 1). X‑ray diffraction (XRD) revealed the presence of α‑W (bcc, Im-3m), β‑W (A15, Pm-3n), and Cu (fcc, Fm-3m) phases, indicating complete reduction of the oxides and partial persistence of metastable β‑W (Fig. 2). Transmission electron microscopy (TEM) showed both cuboid and spherical particles with average diameters of 28.2 nm (5 wt% Cu), 33.7 nm (10 wt% Cu), and 40.2 nm (20 wt% Cu) (Fig. 3). Elemental mapping by high‑angle annular dark‑field STEM (HAADF‑STEM) confirmed uniform dispersion of W and Cu within individual nanoparticles (Fig. 4). Structural analysis of the 20 wt% Cu sample distinguished α‑W cuboids from β‑W and Cu spheres (Fig. 5).

Heat treatment at 1073 K in hydrogen eliminated the β‑W phase, as evidenced by XRD and selected area diffraction patterns (Fig. 6), confirming that the plasma‑synthesized particles can be fully reduced during sintering.

Discussion

The use of WO₃ and CuO as precursors enables efficient vaporization and selective reduction. W atoms nucleate at higher temperatures due to their lower vapor pressure, followed by Cu nucleation as the gas cools. Heterogeneous condensation and collision‑coalescence between W and Cu atoms produce composite nanoparticles. Poor wettability of Cu on W favors island growth, but the immiscibility prevents full intermixing, resulting in distinct W and Cu domains within each particle—an advantageous architecture for controlling electrical and thermal pathways.

Residual β‑W, a metastable phase, can be fully eliminated by post‑sintering heat treatment, ensuring a stable α‑W matrix for optimal mechanical performance.

Conclusions

We demonstrated a rapid, in‑situ RF thermal plasma method for producing W‑x wt% Cu (x = 5, 10, 20) bimetallic nanoparticles. The process achieves full reduction of WO₃ and CuO, uniform dispersion of W and Cu domains, and tunable particle morphology. These findings provide a scalable route to fabricate immiscible metal composites for advanced thermal‑electrical applications.