Purpose –An integral component in heat pipes (HPs) and vapor chambers (VCs) is a porous wicking structure. Traditional methods for manufacturing wicking structures within HPs and VCs involve secondary manufacturing processes and are generally limited to simple geometries.This work aims to leverage the unprecedented level of design freedom of laser powder bed fusion (LPBF) additive manufacturing (AM) to produceintegrated wicking structures for HPs and VCs. Design/methodology/approach –Copper wicking structures are fabricated through LPBF via partial sintering and via the formation of square, hexagonal and rectangular arrangements of micro-pins and micro-grooves, produced in multiple build directions. Wicks are characterized byconducting capillary performance analysis through the measurement of porosity, permeability and capillary rate-of-rise. Findings –Copper wicking structures were successfully fabricated with capillary performance, K/reff, ranging from 0.186 –1.74 mm. The rectangular-arrangement micro-pin wick presented the highest performance. Originality/value –This work represents the ﬁrst published report on LPBF AM of copper wicking structures for HPs/VCs applications and represents foundational knowledge for fabricating complete assemblies of copper VCs and HPs through LPBF AM. Keywords Additive manufacturing, Copper, Vapor chamber, Porous materials, Thermal management, Wick Paper type Research paper 1. Introduction Thermal management of high heat- ﬂux electronic devices poses a signi ﬁcant engineering challenge. As an alternative to conventional heat exchangers, heat pipes (HPs) and vapor chambers (VCs) have gained signi ﬁcant attention in the past several decades, given their very high thermal conductance andpassive cooling capability ( Elnaggar and Edwan, 2016 ;Lips et al.,2 0 1 6 ;Reay et al., 2014 ). Integral to the operation of these two-phase thermal management systems is a conformal, porouswick, which transports liquid from the heat sink to the heatsource via capillary pressure. Once evaporated, the vaporrapidly travels toward the heat sink. Heat transfer is, thus,achieved through cyclic evaporation and condensation. Conventional HPs and VCs are limited in geometry and complexity to pipe- or plate-like geometries and involvesecondary manufacturing processes for porous wick productionand assembly (e.g. sintering, welding, electroplating and vaporor chemical deposition). Laser powder bed fusion (LPBF)additive manufacturing (AM) offers the potential forcomplexity and customization in HP and VC geometries and function, and eliminating secondary wick manufacturing steps. Here, we demonstrate, for the ﬁrst time, LPBF of sintered copper wicks. Moreover, the fabrication of micro-pin andgrooved copper wicking structures via LPBF for capillarywicking applications is reported. This work, thus, demonstratesthe feasibility of producing pure copper wicking structures viaLPBF, provides reproducible processing parameters and recommends build-up geometries for production. 1.1 Quantifying capillary performance of wicking structures Within a wick, liquid transport is driven by a pressure gradient in the direction of lower liquid saturation. At the liquid frontillustrated in Figure 1 , three pressure gradients are in equilibrium: Pc¼Ph1Pf (1) The current issue and full text archive of this journal is available on Emerald Insight at: https://www.emerald.com/insight/1355-2546.htm Rapid Prototyping Journal 27/6 (2021) 1181 –1188 © Emerald Publishing Limited [ISSN 1355-2546][DOI 10.1108/RPJ-01-2021-0016 ]Funding : This work was supported by SET Group, LLC though prime contract number 80NSSC18C0125 from the National Aeronautics and Space Administration under FY 2017 SBIR topic S3.03. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the of ﬁcial policies, either expressed or implied, of the government. Received 18 January 2021 Revised 21 April 2021Accepted 26 April 2021 1181 The capillary pressure ( Pc), which drives the liquid transport, is counteracted by hydrostatic pressure ( Ph) and pressure generated from viscous friction ( Pf). An ideal wick exhibits a balance of high capillary pressure as well as high permeability. Capillary pressureincreases as the size of pores within the wick decreases. However,decreasing pore size generally decreases permeability and thusincreases viscous friction. To capture the effects of these twoopposing parameters, capillary performance parameters aregenerally used; these include the ratio of permeability to effective pore radius ( K/r eff) and the product of capillary pressure and permeability ( Pc·K)–see Jafari et al. (Jafari et al.,2 0 1 8 )f o r derivations of these parameters. Here, we aim to demonstrateAM copper wicks with a capillary performance ( K/r eff)o np a r with reported literature ( Ameli et al.,2 0 1 3 ;Esarte et al.,2 0 1 7 ; Jafari et al.,2 0 1 8 ;Richard et al.,2 0 1 7 ). 1.2 Manufacturing process of porous wicks via laser powder bed fusion Recently, AM of wicks has received signi ﬁcant attention (Ameli et al. ,2 0 1 3 ;Esarte et al. ,2 0 1 7 ;Jafari et al. ,2 0 1 8 ; Jafari and Wits, 2018 ;Richard et al. , 2017 ). Most studies have investigated easy-to-print AM alloys, including 316 L stainless steel ( Esarte et al.,2 0 1 7 ;Jafari et al., 2018 ;Richard et al. ,2 0 1 7 ) and the AlSi10Mg aluminum alloy ( Ameli et al. , 2013 ).Ameli et al. (2013) demonstrated one-step fabrication, via LPBF, of a complete assembly of analuminum HP. Jafari et al. (2018) successfully fabricated, via LPBF, and characterized octahedral lattice-structure wicks from 316 L stainless steel of unit length of 0.5 mm, creating periodic and ordered pores of approximately 0.23 mm indiameter. However, the printability of wicking structuresfrom pure copper via LPBF has not been reported, possiblybecause AM of pure copper is considered more dif ﬁcult thanother material systems (e.g. stainless steel, titanium and Inconel), due to the signi ﬁcantly higher re ﬂectivity of the laser and thermal conductivity of pure copper. High reﬂectivity and thermal conductivity can lead to instability in the melt pool during processing, which leads to more defects.Despite the increased dif ﬁculty, AM of pure copper has great potential for next-generation thermal management devices due to copper ’s superior thermal conductivity ( Jafari and Wits, 2018 ). Somewhat analogous to traditional sintered wick fabrication, AM of partially melted wicks via adjustment of laser power and translation speed has been demonstrated in 316 L steel by Richard et al. (2017) . A similar approach has been used for loop HP wick production by Esarte et al. (2017) . AM of single-layer partially melted wicks has been demonstrated by Jafari et al. (2020) by using pulse wave emission. Non-AM wick fabrication processes for periodic micro- structures, such as electroplating of square, hexagonal andrectangular pin arrangements ( Cho et al.,2 0 1 8 ), as well as micro machining of v-grooves, Zeng et al. (2017) have been demonstrated. Cho et al. (2018) found that, among silica-plated copper micro-pin structures with 45 –80% porosity, rectangular arrangement provided the highest permeability, capillary performance and directionality in ﬂuidﬂow (highest permeability along widest tracks). Other works have con ﬁrmed these ﬁndings (Hale et al., 2014a ;Hale et al., 2014b ), showing that rectangular arrangement provide higher capillary performance than the square arrangement due to a higher permeability with similarcapillary pressure. However, rectangular pins show signi ﬁcant anisotropy in ﬂuidﬂow; in comparison, hexagonal arrangement performed most similarly to sintered wicks in isotropy of capillary ﬂow. AM of AlSi10Mg pillar-like micrometer/millimeter size structures via LPBF has been investigated for applications inhigh-temperature surface cooling ( Wang et al.,2 0 1 8 ). Thus far, the capillary performance of AM pillar-like microstructures has not been investigated. In applications where directional ﬂow is more suited (e.g. HPs) and gravity forces are low (i.e. space applications) ( Reay et al.,2 0 1 4 ), micro grooves are an easy-to-fabricate alternative to pin arrangements. Groove cross-section and compositioncan be used to improve capillary performance, with triangular(Zeng et al. ,2 0 1 7 ) and composite ( Deng et al. , 2013 ;Tang et al.,2 0 1 0 ) micro v-grooves, the latter containing a sintered powder layer that provides higher capillary pressures thansquare grooves. The addition of reentrant structures on the v- groove channels, which increased the achieved capillary pressure of the groove surface, further improves performance(Zeng et al., 2017 ). Here, methods for fabricating partially sintered and micro-pin arrangements via LPBF are presented (Section 2.1). Wick porosity, permeability and capillary performance are quanti ﬁed (Section 3). Lastly, generalizable strategies for production of copper wickingstructures for VC and HP applications are discussed (Section 4), providing a pathway for future development. 2. Material and methods 2.1 Sample design and fabrication To represent a cross-section of a VC, 1-mm-thick wicks were printed atop 1-mm-thick bases, with sides measuringFigure 1 Capillary ﬂow of water (blue) in a homogeneous sintered wick and a micro-structured wick Fabrication and capillary characterization Adnen Mezghani et al.Rapid Prototyping Journal Volume 27 · Number 6 · 2021 · 1181 –1188 1182 25.4/C225.4 mm ( Figure 2(a) ). Micro-pins, micro-grooves and sintering wicks were produced. Micro-pin arrangementsincluded square, hexagonal and rectangular. Figure 3 lists and illustrates these arrangements with an intended feature diameter/width of approximately 200 mm. Square-arrangement micro- pins were also built horizontally, vertically, inclined at 45° withthe pins facing upward ( Figure 2(b) ). The micro-pin and micro-groove samples of this study were processed with a laser power of 360 W 65%, a laser scan speed of 590 mm/s 65% and a single-hatch strategy (i.e. a single laser pass). Studies with great insight on laser processing parametersfor AM of pure copper and copper alloys, which provide minimum energy density requirements and typical nominalparameters, can be found in several studies ( Colopi et al., 2019 ; El-Wardany et al.,2 0 1 8 ;Pogson et al., 2003 ;Popovich et al., 2016 ;Robinson et al.,2 0 2 1 ;Zhang et al.,2 0 2 0 ). However, it is important to be aware of the differences in setup (e.g. laser beam diameter, layer height, build plate material), the presence of alloying elements and limitations of the system(e.g. maximum laser power output). Sintered powder wicks were produced by lowering laser energy density (LED = power/speed) to sinter, rather than meltthe copper powder. A laser power of 100 W was selected, andlaser scan speeds were increased to 1.6 and 2.1 times the nominal value, achieving LED reductions of six and eight times the nominal value. Hatching was conducted with a hatchspacing of 0.13 mm and a 90° rotation between layers.Processing parameters are provided in Table 1 along with build orientations: horizontal, vertical and inclined at 45°. Samples were fabricated on an EOS M280 LPBF system, equipped with a 400 W maximum power ytterbium-dopedﬁber laser focused to an ISO 11146 –1 measured second moment diameter of 70 mm. Feedstock copper powder (99.6% Cu) was procured from Elementum 3D with particle sizedistribution (PSD) presented in Figure 4 . The D10, D50, D90, from the PSD analysis are 13.5, 30.5 and 56.1 mm, respectively. All processing occurred in an argon environmentwith a maximum oxygen concentration less than 1,000 ppm. Aconstant layer thickness of 40 mm was used together with a ﬂexible silicon recoater blade. After fabrication, samples were brushed to remove powder and stress relieved at 650°C for 2 h, then furnace cooled underFigure 2 Illustration of (a) sample geometry, and (b) build orientations: horizontal (left), vertical (middle), and inclined at 45°(right) Figure 3 Implemented micro-structure arrangements strategies. Intended build geometries are illustrated in the last column, where pin/groove diameter/width is set to be approximately 200 mm µm. Sample Micro-structure arrangementunitsize(μm) Buildorientation Illustrationshowingpinorgroove arrangement P-S-400-H Square (S)I = 400 Horizontal P-S-400-V I = 400 Vertical (V) P-S-400-IU I = 400Inclined 45˚ with pins up (IU) P-H-350-H Hexagonal (H) I = 350 Horizontal P-R-450-600-H Rectangular (R)I = 450; S = 600 Horizontal P-R-450-800-H I = 450; S = 800 Horizontal G-600-H Groove (G)I = 600 Horizontal G-800-H I = 800 Horizontal Fabrication and capillary characterization Adnen Mezghani et al.Rapid Prototyping Journal Volume 27 · Number 6 · 2021 · 1181 –1188 1183 nitrogen. Finally, samples were removed from the build plate via electrical discharge machining (EDM). Prior to testing, samples were cleaned and dried following (Reay et al.,2 0 1 4 ;Singh et al., 2009 ;Tang et al.,2 0 1 7 ) with minor adjustments, as follows. Samples were submerged in 2 M hydrochloric acid for 1 min, then rinsed with deionized water to remove carbon and oxides residue. To remove remaining oils, contaminants or loose powder, samples were agitated in an ultrasonic acetone bath for 15 min, then rinsed in an ultrasonic DI water bath for 10 min. Finally, samples were dried in a furnace at 150°C for 30 min, followed by air cooling for 5 min. The samples were subsequently tested for rate-of-rise, porosityand permeability, respectively. 2.2 Characterization and data analysis 2.2.1 Porosity measurement Porosity was measured using the density method: comparing the theoretical density of copper to the apparent density of the wick: «a¼1/C0mw Vwrs(2) where «a,mw,Vwandrsare the wick ’s apparent porosity, mass, external volume and the density of copper, respectively. Porosity measurements from the density method were juxtaposed with true porosity ( «tÞ, measured using the Archimedes method, in which the wick is saturated with a wetting liquid through: «t¼1 11mwrl mlrs(3)where mlandrlare the mass and density of the in ﬁltrating liquid, respectively. The Archimedes method only measuresporosity interconnected to the surface, while the densitymethod includes interconnected as well as trapped porosity inthe wick. By comparing the porosity measurements of these twomethods, the percent amount of trapped porosity in a wick canbe calculated through % «trapped =(«a/C0«t)/C2100%. 2.2.2 Permeability measurement Permeability was measured using a form of Darcy ’s law (i.e. forced- ﬂow method) describing the pressure drop for steady- state laminar ﬂuidﬂow through porous media ( Reay et al. , 2014 ) through: DP¼ml_m rKA(4) where mis the viscosity, ris the density of the ﬂowing liquid, K is the permeability, lis the length of the wick along the ﬂuid ﬂow,Ais the cross-sectional area of the wick perpendicular to ﬂuidﬂow and mis the ﬂuid mass ﬂow rate. Though forced- ﬂow methods are common ( Ameli et al., 2013 ;Choi et al., 2013 ; Deng et al.,2 0 1 3 ;Esarte et al., 2017 ;Espinosa et al.,2 0 1 2 ; Jafari et al.,2 0 1 8 ;Semenic et al.,2 0 0 8 ;Singh et al.,2 0 0 9 ), we note that high ﬂuidﬂow velocities lead to failure of Darcy ’sl a w and result in unreliable permeability measurements. In thiswork, we ensured ﬂow velocity was below the transition velocities found in similar studies (0.07 –0.1 m/s) ( Boomsma and Poulikakos, 2002 ;Jafari et al., 2018 ). Samples were placed in a ﬂow housing, schematically shown in Figure 5 . Thin rubber strips were placed around the sample to ensure awatertight ﬁt and eliminate any ﬂow bypassing the sample. Before pressure and ﬂow rate measurements, samples were ﬂushed with water for 5 min to eject air bubbles. A scale was also used to validate the measured ﬂow rate. 2.2.3 Capillary rate-of-rise test An infrared (IR) camera was used to visualize the rising DI water meniscus and measure the liquid rate of risedh dt/C16/C17 ,a s illustrated in Figure 6 , following Tang et al. (2010) . To reduce evaporation from the reservoir and wick, tests were conductedwithin an enclosure with a relative humidity of 70 –95%. Capillary performance ( K/reff) was derived using rate-of-rise, wetting liquid properties and wick porosity as follows. The rateof rise expression being: dh dt¼2s m«tK reff1 h/C0rgK m«t(5)Table 1 Processing parameters used to fabricate the sintered-powder wick samples Sample IDProcessing parameters LED (J/mm) LED reduction multiplier Power (W) Scanning speed (mm/s) Hatch spacing (mm) Build orientation S-100-504-I 100 504 0.16 Inclined 45° 0.198 6 S-100 –672-H 100 672 0.16 Horizontal 0.149 8 S-100 –672-V 100 672 0.16 Vertical 0.149 8 S-100 –672-I 100 672 0.16 Inclined 45° 0.149 8 Figure 4 PSD of copper powder stock Fabrication and capillary characterization Adnen Mezghani et al.Rapid Prototyping Journal Volume 27 · Number 6 · 2021 · 1181 –1188 1184 where sis surface tension of the liquid, gis the gravitational acceleration and h,dh dt,m,r,«t,Kare de ﬁned previously. Substitutingdh dtwith yand1 hwith x,equation (5) becomes a linear equation as: y¼2s m«tK reffx/C0rgK m«t(6) having a slope of2s m«tK reff/C16/C17 that contains the capillary performance parameterK reff/C16/C17 and known liquid and wick properties2s m«t/C16/C17 . An explicit solution to equation (5) is found byﬁtting height vs time data to a natural log curve of the form: h¼alntðÞ1b (7) From which, the inverse height ( x=1 /h) and rate of rise (y = dh/dt) are calculated for the initial capillary rise duration. Finally, the slope of a linear ﬁtt oequation (6) is determined and the capillary performanceK reff/C16/C17 is calculated. 3. Results Several types of copper wicking structures were successfully additively manufactured and characterized. Figure 7 presents top-view optical images of fabricated (a,c,d) micro-pin, (b) micro-groove and (e) sintered wicks. Vertical and inclined wicks were also successfully fabricated ( Figure 7(f) ). The capillary rate-of-rise was determined from IR video, exempli ﬁed in Figure 8 ,a n d ﬁtted to a natural log curve with a coefﬁcient of determination, i.e. goodness-of- ﬁt, (R2)o f 0.98 –0.99. The micro-structured wick samples of this study exhibited capillary performance K/r eff, ranging from 0.530 –1.744 mm, comparable to other AM micro-structured wicks, e.g. K/r eff= 1.04 mm(Jafari et al., 2018 ) and K/r eff= 2.04 mm(Ameli et al., 2013 ). Figure 9 presents a plot of the capillary performance K/r eff against true porosity of the fabricated samples and other AM samples. Measured permeability is presented in Figure 10 . 4. Discussion 4.1 Capillary performance comparison Micro-pin wicks with rectangular pin arrangement and 450 – 600mm pin spacing showed the best capillary performance and were similar to the performance of electroplated copper micro- pins ( Cho et al. , 2018 ). Micro-pins in a rectangular arrangement are also more permeable compared to other pinarrangements and sintered-powder wicks, shown in Figure 11 . This is due to the presence of larger ﬂow channels in a rectangular pin arrangement, which results in larger permeability to liquid ﬂow, and with the combination of small pin-to-pin spacing, a higher overall capillary performance is achieved –this also reinforces the ﬁndings of Hale et al. (2014a ) andHale et al.(2014b ). Sintered-powder wicks exhibited K/r eff, from 0.186 – 0.247 mm, comparable to traditionally sintered-powder wicks, namely, K/r eff= 0.192 mm(Semenic et al. ,2 0 0 8 ),Figure 5 A schematic of the permeability test setup Figure 6 Capillary rate-of-rise test setup used to capture IR videos of the capillary rise of water through the wick samples Figure 7 Optical images of some of the fabricated wick samples Fabrication and capillary characterization Adnen Mezghani et al.Rapid Prototyping Journal Volume 27 · Number 6 · 2021 · 1181 –1188 1185 K/reff= 0.405 mm(Singh et al., 2009 ) and K/r eff= 0.603 mm (Deng et al., 2013 ). A positive correlation exists between the true porosity of sintered-powder wicks and capillary performance, shown in Figure 11 . This indicates that further increasing the capillary performance is possible by increasingporosity (e.g. by further reducing LED). Because the density ofthe copper built with nominal LED resulted in >99% density,any inherent porosity in the 1-mm-thick solid base that is fabricated with the wicking structure is unlikely to affect wickperformance. 4.2 Interconnectedness of the porosity Lowering LED increased interconnectedness of the porosity in sintered-powder wicks ( Figure 12(a) ). Thus, low LED, is desirable and leads to higher permeability. Micro-structuredsamples with larger spacing also exhibited less trapped porositycompared to samples with smaller spacing between pins orgrooves ( Figure 12(b) ). This correlation is most evident in micro-grooved wicks. We also note that irregularities in the shape and surface roughness of micro structures could cause adjacent pins/grooves to fuse and trap pores. 4.3 The effect of sample build orientation For the sintered-powder wicks, the vertical and inclined samples are 9 –16% less porous (in terms of true porosity, «t)Figure 8 Snapshots of the rate-of-rise IR video at different time stamps in seconds Figure 9 Capillary performance (K/r eff) and porosity of successfully tested samples, with comparison to other AM wicks as follows Figure 10 Measured permeability of ﬁve wicking samples Figure 11 Plot of the capillary performance parameter (K/r eff) against porosity of the sintered-powder wicks in the current study and a numberof non-AM sintered wicks as follows Fabrication and capillary characterization Adnen Mezghani et al.Rapid Prototyping Journal Volume 27 · Number 6 · 2021 · 1181 –1188 1186 compared to the horizontally fabricated sample. We speculate that this is due to the difference in heat dissipation. Most heatgenerated during fabrication is dissipated through the wick ’s dense base and into the build plate. Vertical and inclined samples cool slower because the cross-sectional area available for heat dissipation is signi ﬁcantly smaller, compared to the horizontal orientation. This phenomenon has been describedby Krauss et al.(2015) . For the square-arrangement micro-pin wicks, vertically fabricated samples had more trapped porosity compared to the horizontal and inclined samples. The micro-pins in the vertically fabricated sample are essentially overhanging with anangle of 0° from the horizontal plane. Lacking support, suchoverhanging structures can de ﬂect (e.g. sag or curl upwards) and may fuse to adjacent pins, which can result in trapping pores (i.e. less interconnected porosity). 4.4 Challenges in the development of laser powder bed fusion additive manufacturing of Cu wicks Production of the reported samples proved challenging due to inherent limitations of existing computer-aided design (CAD) software and AM digital work ﬂows. CAD modeling of sintered- powder and micro-structured wicks requires signi ﬁcantly different strategies. Whereas, modeling of sintered structures ispossible using a single conformal body bordering a solid surface, micro-structured wicks require modeling of individual pin or groove structures. Hence, for micro-structured wicks, thenumber of modeled elements, and consequently the CAD ﬁle size, is signi ﬁcantly larger than that of the sintered-powder wick. Translation of CAD ﬁles containing thousands of cylindrical elements to the standard STL format, used by AM datapreparation software, exceeds the capabilities of mostcommercial workstations and would not have been possible for the micro-structures presented here. Overcoming this hurdle requires the use of proprietary software tools –in this work, suchtools were produced by the researchers and are beyond the scope of the current work –which, rather than create surface representations of pins and grooves, encapsulate simpli ﬁed laser paths directly within slice or native build plan formats. It isrecommended for future research efforts into micro-structured wicks to pursue a similar path. It is the authors ’perspective that the potential bottlenecks in the adoption and implementation of AM wicks are mostly inthe digital work ﬂow. Currently, sintered wicks are much easier to implement than micro-pin and micro-groove wicks, and hence merit further research to improve wick performance. Nonetheless, all types of wicks are additively manufacturable. 5. Conclusions This work provides experimental validation of the printability ofporous copper wicks in the form of sintered-powder, micro-pinand micro-groove wicking structures using LPBF AM in multiple build orientations. The achieved capillary performance, K/r eff,o f sintered-powder and micro-structured wicks ranged from 0.186 – 1.744 mm. The micro-pin wick with rectangular pin arrangement of 450 –600mm pin spacing provided the best capillary performance and permeability compared to other pin arrangements. Capillary performance for almost all micro-structured samples, nonetheless, were comparable to the bestperformance reported in previous publications on AM wicks, i.e. K/r eff=1 . 0 4 mm(Jafari et al., 2018 )a n dK / r eff=2 . 0 4 mm(Ameli et al.,2 0 1 3 ). Sintered-powder and micro-pin wicks were printed vertically and inclined at 45° without additional supportstructures, indicating the possibility of fabricating VCs/HPs assemblies in various orientations and inclination angles without requiring additional internal support structures. The successful fabrication of these sintered-powder and micro- structured wicking samples in multiple build orientation via LPBFAM indicate the feasibility of additively manufactured copper HPs and VCs with integrated wicking structures. Ongoing work seeks to further optimize processing parameters. In particular, thedetermination of ideal parameters and ranges, as functions of wickorientation and structure is an appropriate area of future research, as well as the investigation of other self-supporting micro- structured wick designs for other build inclination angles.