Ultra capillary properties of composite liquid absorbing cores manufactured by laser powder bed melting additive manufacturing

Researchers from Sichuan University, the Key Laboratory of Advanced Special Materials and Preparation Processing Technology of the Ministry of Education, and the Nuclear Additive Manufacturing Laboratory of China Nuclear Power Research and Design Institute reported on the study of the ultra capillary performance of laser powder bed melting additive manufacturing composite structure liquid absorbing cores. The related paper titled "Super capillary performance of hybrid structured wicked additive manufactured via laser powder bed fusion" was published in Additive Manufacturing.

The capillary coefficient of performance (K/Reff) is a key performance indicator of the wick, which is a critical component of high-performance heat pipes. However, it is difficult to simultaneously increase permeability (K) and capillary pressure (∆ Pcap). A liquid absorbing core with channels and porous composite structure was manufactured using laser powder bed melting (LPBF) technology, achieving excellent capillary performance. The channel structure ensures excellent permeability (K), while the porous structure provides high capillary pressure, and the angular flow effect further enhances this pressure. The optimized structure with a 0.6mm square channel and a porosity of 70.99% achieved ultra capillary performance of up to 3.24 × 10 ⁻⁶ m, which is 106.3% higher than the previously reported optimal value. This study proposes a new design concept and preparation method for a novel high-performance heat pipe.

Keywords: liquid absorbing core; Capillary performance; Laser powder bed melting; Composite structure; heat pipe

Figure 1.316L alloy powder: (a) SEM morphology, (b) particle size distribution.

Figure 2. Model diagram of porous absorbent core.

Figure 4. Schematic diagram of capillary performance tester.

Figure 5. X-ray computed tomography (XCT) data before and after binarization at the optimal threshold.

Figure 6. Scanning electron microscopy images of the surface morphology of a planar porous liquid absorbing core under different laser energy densities. (a)P1，(b)P2，(c)P3，(d)P4，(e)P5。

Figure 7. Reconstructed image of P1 sample. (a) Overall view of the sample, (b) distribution of interconnected pores in the sample.

Figure 8. Three dimensional visualization of P1 sample obtained from micro computed tomography (μ - CT) images. (a) Solid (gray) and total porosity (blue); (b) Individually labeled pore volume data; (c) Pore network model.

Figure 9. Scanning electron microscopy images of Round-R and Square-S liquid absorbing cores. (a)S1，(b)R1，(c)S2，(d)R2，(e)S3，(f)R3，(g)S4，(h)R4，(i)S5，(j)R5。

Figure 10. Reconstructed image of S1 sample. (a) Overall view of S1 sample, (b) distribution of interconnected pores in the sample.

Figure 11. Three dimensional visualization of S1 sample obtained from micro computed tomography (μ - CT) images. (a) Solid (gray) and total porosity (blue); (b) Individually labeled pore volume data; (c) Pore network model.

Figure 12. Schematic diagram of the relationship between laser energy density and molten pool. (a) Pool model, (b) Pool evolution.

Figure 13. Transient analysis of water droplets in contact with a planar porous absorbent core. (a)P1，(b)P2，(c)P3，(d)P4，(e)P5。

Figure 14. Schematic diagram of tortuosity.

Figure 15. (a) Capillary rise in R-type porous wick with channels and (b) S-type porous wick with channels.

In this study, additive manufacturing technology was used for the first time to design and manufacture a composite porous structure that combines structural design with process optimization of pore formation, achieving ultra-high capillary performance. The use of laser powder bed melting technology to manufacture channel porous composite absorbent cores with adjustable capillary properties is achieved by fine-tuning manufacturing process parameters and adding channels to simultaneously increase capillary pressure and permeability. Capillary rise tests were conducted using anhydrous ethanol as the working fluid to investigate the effects of laser energy density, material porosity and morphology, channel addition, and channel morphology on capillary performance.

The research results indicate that lower laser energy density increases the porosity of porous liquid absorbing cores, thereby improving permeability and capillary performance. Although the addition of channels increases the effective pore radius, the significant increase in permeability leads to an overall increase in capillary performance coefficient. It is worth noting that due to the phenomenon of angular flow and the difference in channel size, the performance of square channels is better than that of circular channels. The S1 channel porous composite structure achieved the best capillary performance, with a capillary coefficient (K/Reff) of 3.24 × 10 ⁻⁶ m, an effective pore radius (Reff) of 3.24 × 10 ⁻⁴ m, and a permeability (K) of 1.05 × 10 ⁻⁹ m. This exceeds the best values reported in the literature, even including those sintered fiber absorbent cores that have undergone complex surface treatments.

This work highlights the enormous potential of porous composite structures as heat pipe wick materials with high heat transfer coefficients. Helps to develop more efficient and effective heat pipe designs, especially in applications that require high thermal performance, such as electronic equipment cooling, aerospace engineering, and renewable energy systems.

Source: Yangtze River Delta Laser Alliance