Optimizing Tungsten Powder Fluidization: Applications for Atomic Layer Deposition

Tungsten is a refractory metal which possesses unique characteristics: high melting point, high density, a low thermal expansion coefficient, high tensile strength, high thermal conductivity and favorable corrosion resistance. This set of attributes makes it ideal for high-temperature applications in hypersonics, cermet fuel elements in nuclear thermal propulsion and advanced radiation shielding. Traditional powder metallurgical methods of manufacturing tungsten parts do not scale well with highly customizable and complex geometries. Emerging alternatives like additive manufacturing (AM) methods of powder bed fusion, binder jetting and material extrusion, face limitations such as cracking and difficult post-processing. Introducing metal additives allows for favorable fine-tuning of enhanced properties to withstand these demanding applications. Incorporating metal additives is accomplished via atomic layer deposition (ALD), a gas-phase chemical process that utilizes alternating reactions to deposit conformal thin films upon a substrate. ALD is ideal for refractory metals, as melting them is impractical and energy intensive. Powder ALD typically relies on a Fluidized Bed Reactor, where the minimum fluidization velocity is the critical threshold for effective powder fluidization.

This work investigated the fluidization of several different sizes of tungsten powder: 1–5-micron, 12-16 micron and 50-70 micron. Fluidization of incredibly dense powders such as tungsten (19300 kg/m³) proves difficult as the powder does not fall within the conventional Geldart classifications of particle diameter and densities. Literature on the subject is incredibly scarce, especially for particles that are smaller than 50-70 micron in size. The range explored was chosen based on previous literature and allowed for direct comparisons of fluidization behavior. While the smaller size range of 1-5 micron and 12-16 micron is most suitable for additive manufacturing applications and has no existing literature on its fluidization regimes or behavior. This work has compiled different thresholds of fluidization of this powder size with both mechanical agitation and the lack thereof. In addition to this, the effects of temperature during fluidization and that of powder bed height relative to reactor diameter (H/D) were investigated.

(H/D) ranges of 0.8-1.4 were explored and suggested that greater ratios resulted in more uniform fluidization with smaller bubble formation, greater pressure drops were also observed alongside greater minimum fluidization velocities. A temperature range of 200-400 °C was selected to account for the great majority of chemical precursors used in ALD for refractory metals. As predicted by the Ergun equation one would expect increased fluidization temperature to result in increased minimum fluidization velocities because of increased gas viscosity.

The findings on fluidization thresholds, temperature effects, and the impact of bed height (H/D) ratios extend our understanding of how to manipulate the processing conditions for more uniform powder dispersion, which is essential for enhancing the performance and precision of tungsten parts in high-temperature, high-stress applications. Importantly, the work addresses a gap in literature for finer tungsten powders (1-16 micron), which are pivotal for AM, while also laying the groundwork for the development of more efficient ALD techniques. Ultimately, these insights could advance the scalability and reliability of tungsten-based materials for aerospace, nuclear propulsion, and radiation shielding, enabling their use in increasingly demanding environments where traditional manufacturing methods fall short.