Three-Dimensional Printing of Abrasive, Hard, and Thermally

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Applications of Polymer, Composite, and Coating Materials

3D printing of abrasive, hard and thermally conductive synthetic micro-diamond-polymer composite using low-cost fused deposition modelling printer Sidra Waheed, Joan-Marc Cabot, Petr Smejkal, Syamak Farajikhah, Sepidar Sayyar, Peter C. Innis, Stephen Beirne, Grant Barnsley, Trevor William Lewis, Michael C. Breadmore, and Brett Paull ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18232 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

3D printing of abrasive, hard and thermally conductive synthetic micro-diamond-polymer composite using low-cost fused deposition modelling printer Sidra Waheed,a,b Joan M. Cabot,a,b Petr Smejkal,b Syamak Farajikhah,c Sepidar Sayyar,c Peter C. Innis,c Stephen Beirne,c Grant Barnsley,c Trevor W. Lewis,a Michael C. Breadmore,a,b and Brett Paull*a,b

(a) ARC

Centre of Excellence for Electromaterials Science (ACES), School of Natural

Sciences, Faculty of Science, Engineering and Technology, University of Tasmania, Hobart, 7001, Australia.

(b) Australian

Centre for Research on Separation Science (ACROSS), School of Natural

Sciences, Faculty of Science, Engineering and Technology, University of Tasmania, Hobart, 7001, Australia.

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(c)

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ARC Centre of Excellence for Electromaterials Science (ACES), AIIM Facility, Innovation Campus, University of Wollongong, NSW 2500, Australia.

Corresponding author: Brett Paull

E-mail: [email protected]; Fax: +61 3 6226 2858; Tel: +61 3 6226 6680


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Abstract

A relative lack of printable materials with tailored functional properties limits the applicability of 3D printing. In this work, a diamond - acrylonitrile butadiene styrene (ABS) composite filament for use in 3D printing was created through incorporation of high pressure and high temperature (HPHT) synthetic micro-diamonds as a filler. Homogenously distributed diamond composite filaments, containing either 37.5 % wt or 60 % wt micro-diamonds, were formed through pre-blending the diamond powder with ABS, followed by subsequent multiple fibre extrusions. The thermal conductivity of the ABS base material increased from 0.17 to 0.94 W./m.K, more than 5-fold following incorporation of the micro-diamonds. The elastic modulus for the 60 % wt micro-diamond containing composite material increased by 41.9 % with respect to pure ABS, from 1050 to 1490 MPa. The hydrophilicity also increased from by 32 %. A low-cost fused deposition modelling (FDM) printer was customised to handle the highly abrasive composite filament by replacing the conventional (stainless steel) filament feeding gear with a harder titanium gear. To demonstrate improved thermal performance of 3D printed devices using the new


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composite filament, a number of composite heat sinks were printed and characterised. Heat dissipation measurements demonstrated that 3D printed heat sinks containing 60 % (wt) diamond increased the thermal dissipation by 42 %.

Keywords: 3D printing; Fused deposition modelling; Composite; Micro-diamonds; Thermal conductivity; Heat sinks; Hydrophilicity; Recyclable


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1. Introduction

Additive manufacturing, commonly referred to as 3D printing (3DP), has emerged as a powerful

and

dynamic

technology

to

produce

a

wide

range

of

complex

structures/components, already enabling rapid prototyping, and beginning to impact industrial production significantly.1-4 3DP can provide a route to the rapid production of highly customised structures, tailored towards specific applications, whilst simultaneously reducing the cost and time associated with traditional subtractive fabrication techniques.56

Among all of the 3DP technologies, fused deposition modelling (FDM) is currently the most common applied print technology, essentially due to simplicity and low-cost, together with the availability of a wide variety of base print materials, which nowadays include metal alloys, a wide variety of polymers, ceramics and even foodstuffs.7-9 Polymer FDM printing involves the forced extrusion of a thermoplastic filament through a pinch


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roller mechanism into a heated nozzle, which is capable of 3-directional movement. This movement allows the nozzle to build a 3D structure in a layer-by-layer process.10 Acrylonitrile butadiene styrene (ABS) is one of the most commonly used thermoplastic polymer filaments in FDM printing. ABS has a relatively low glass transition temperature with excellent processability. Additionally, the non-crystalline nature of ABS reduces its shrinkage ratio during cooling process and enables high precision printing and dimensional stability.11 However, ABS has no specific functional advantages per se, and has an intrinsically low thermal conductivity, which hinders its use in many applications, including high power miniaturised electronic devices.12 There is general consensus amongst users that new print-compatible materials, including composite and functional materials, are required to expand the applicability of 3D printing.13-15 This need has seen considerable effort placed into the investigation of so-called ‘fillers’ to provide enhanced functionality to resultant polymer based composites, whilst remaining amenable to 3D printing. Recent developments in this area include the incorporation of carbon based nanomaterials into printable polymer composites, such as carbon nanotubes (CNT),16 carbon blacks,17-18 and graphene,19-20 which together have


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seen

the

creation

of

devices

with

improved

electrical

conductivity,

electromechanical/chemical sensitivity and mechanical strength.21 Amongst the various carbonaceous fillers available, high pressure and high temperature (HTHP) nano/micro-diamond possesses several attractive features, which have practical utility in a range of printable devices. These include its hydrophilicity, biocompatibility and non-toxicity, thermal stability, thermal conductivity, electrical insulation, (or indeed electrical conductivity in boron doped variants), hardness, wear resistance and low-cost. Hence, several studies have already reported the advantages of nano and micro-diamonds as reinforcing enhancers in polymer composites.22-24 In terms of fabrication approaches applied when using such composites, Su et al., recently reported upon hot pressing techniques applied to polylactic acid (PLA) microdiamond composites, which delivered a 10-fold increase in thermal conductivity as compared to pure PLA.25 More recently, Angjellari et al., reported upon a simple syringe based method to ‘print’ nano-diamond composite materials directly in the form of a polyvinyl alcohol (PVA) slurry.26 In our own previous work, we have reported upon an acrylate based composite material containing a maximum of 30% (wt) of micro-


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diamonds compatible with stereolithography (SLA) based printing.27 Although this approach proved reasonably successful, using the SLA based printing limited the concentration of filler which could be used, as this significantly impacted the photocuring of the resin as higher loadings. In addition, it was observed that the stability of the resin-filler suspension, if not carefully controlled, could lead to non-uniform distribution of the filler within the final printed material. More recently, to overcome such difficulties and achieve higher loadings, we have applied a ‘print, cast and peel’ approach23, which utilised a simple 3D printed microfluidic template within which a 60% (wt) micro-diamond-polydimethylsiloxane composite resin could be cast, cured, and readily removed, for subsequent testing and application. However, obviously in this case the composite material was not actually being printed. To create composite filaments containing the above significant % wt of functional or structural fillers, which are compatible with FDM printing, there are several challenges which need to be overcome.28 Firstly, the nano- or micro-materials must be homogenously distributed within the filament, otherwise the structure and function of the printed object will be compromised. Secondly, the formed composite filament must be


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suitably flexible, strong, smooth, and have a consistent diameter, such that it can be reliably fed into the extrusion nozzle of an FDM printer without interruption or stress, allowing continuous printing.29 Thirdly, the increase in viscosity (or other related properties) of the composite material when melted, caused through the addition of the particles to the base polymer, should not be so high as to cause a blockage or damage to the print head (nozzle). 30 With the above challenges in-mind, herein we report upon the development of new diamond composite filaments, which can be used with standard low-cost FDM printing, and which contain up to 60% (wt) of synthetic micro-diamonds. The ability to produce a homogenous distribution of micro-diamonds within the composite filaments was investigated, and the process optimised using a multiple filament extrusion protocol. The same multi-extrusion approach was investigated with regard to improved filament smoothness, reduced variations in filament diameter, and reduced filament porosity. Various micro-diamond composite filaments were characterised for mechanical stability and thermal conductivity. The print performance of the new composite filaments and their


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practical utility was evaluated via the printing and characterisation of a number of test heat sink designs.

2. Experimental Section 2.1. Materials ABS pellets were supplied by Aurarum Ptv Ltd (VIC, Australia). Industrial non-porous high pressure and high temperature micro-diamond powder (2–4 µm) was obtained from Hunan Real Tech Super-abrasive and Tool Co. Ltd. (Changsha, Hunan, China). Acetone LR was from Chem-Supply Pty Ltd (VIC, Australia).

2.2. Pellet preparation To form a 37.5% (wt) filament, 200 g of ABS pellets and 120 g of micro-diamonds were dissolved in acetone separately and sonicated for 80 min. Both solutions were then mixed and sonicated for a further 80 min. After sonication, a smooth slurry of ABS containing micro-diamond was formed (Fig. 1 (A)). Using this slurry, pellets were formed manually ranging in size between 5 to 10 mm. All pellets were left to dry at room temperature


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overnight, followed by drying in an oven at 100°C for 2 hr. All pellets containing microdiamonds were stored in a desiccator until used. Pellets containing 60% (wt) microdiamonds were formed following the same procedure, here incorporating 40 g of microdiamonds within 26.5 g of ABS.

Fig. 1(A) Slurry containing 37.5 wt% synthetic micro-diamond in ABS (B) Pellets of composite material were formed from the slurry. (C) Filament after the first extrusion. (D) Porous core after second extrusion. (E) A solid core was achieved after sixth extrusion. (F) Coil of composite filament with diameter between 1.60 mm and 1.90 mm.

2.3. Extrusion of the composite filament


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A benchtop single screw Wellzoom filament extruder (Shenzhen Mistar Technology Co., Ltd., China) with independent dual temperature zones was used at a power setting of 240W, with fibre extrusion at 1800 – 2500 mm/min. The extruder line was connected with a Wellzoom water-cooling tractor (Shenzhen Mistar Technology Co.), equipped with a stepping motor for controlling the diameter of the filament (shown in electronic supplementary information as Fig. S1). The melting and extrusion temperature were set at 180 °C and 185 °C, respectively. The filament was pulled from the nozzle at a constant speed via the water-cooling tractor. Molten filament was passed through a moving water sink, within a larger water bath, in which the water was continuously circulated using a pump, for controlled cooling of the filament, helping to deliver a uniform filament diameter.31 Filaments extruded only once were significantly ruptured, with a rough and brittle surface (Fig. 1(C)), reflecting the non-uniform distribution of micro-diamonds within the feed pellets. To decrease porosity and enhance mechanical properties, the filament was cut into ~ 1.5 cm lengths and re-extruded. This process was repeated multiple times. As can be seen in Fig. 1(D), the second extruded filament was unruptured but displayed large pores across the filament diameter. After the sixth extrusion, the filament was


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smooth and dense with no visible pores as shown in Fig. 1(E). Through carefully controlling the extrusion speed, an ABS composite filament containing 37.5% (wt) microdiamond (named as D-ABS(37.5%)) was produced with a diameter between 1.6-1.9 mm, as shown as whirl in Fig. 1(F). Following similar procedures, filament containing 60 % (wt) was similarly produced with matching diameter uniformity, hereafter named as DABS(60%).

2.4. 3D printer conditions A desktop FELIX 3D FDM printer (version 3.1; Nieuwegein, The Netherlands) with dual heads was used for printing. SolidWorks 2015−2016 (Dassault System̀esSE, France) was used for designing the various heat sinks. A wing shape heat sink design was downloaded from the GrabCAD Community Library.32 KISSlicer V1.5 was used for digital slicing and creation of the G-code using the following parameters: layer thickness, 250 μm; extrusion width, 500 μm; infill, 100 %; 2 loop stroke. A second nozzle was used to extrude PLA at 190 °C as a support material and the diamond composite was extruded at a temperature of 230 °C for the first layer and 220 °C for the rest. The build platform


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was set at 70 °C throughout the print. Once the print was completed, the support material was detached from the printed structure.

2.5. 3D printer modifications Due to the highly abrasive nature of the micro-diamond composite fibre, rapid abrasion of the FELIX printer stainless steel corrugated gears for driving the fibre into the heating elements was observed when first attempting to print the new composite material. The extent of this abrasion can be seen within electronic supplementary information (Fig.S2), and repeatedly occurred within a short time of printing with the abrasive composite. The eroded gears led to failure in the pinch roller feeding mechanism of the printer and so halted the delivery of the filament through the extrusion nozzle. To address this issue several different gears produced from various materials were tested, all of which are shown in supplementary information (Fig.S2). The majority of materials failed to withstand the abrasive nature of the composite fibre, however a custom-made titanium alloy gear, produced using a Realizer SLM-50 metal 3D printer (Realizer GmbH, Germany) using titanium grade 5 powder (Ti6Al4V, TLS Technik GmbH, Bitterfeld, Germany) proved


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sufficiently resistant with a hardness of 392 HV. This exceptionally hard gear (as shown in Fig 2) displayed the best performance and enabled the printer to provide a constant feed of the composite fibre without further abrasion-based problems.

Fig. 2 Schematic illustration of mechanism of fused deposition modelling (FDM) based printing. A feedstock thermoplastic filament is feed into the heating element and to the


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extrusion nozzle by the serrated gear. The commercially supplied stainless steel gear was replaced by 3D printed titanium alloy gear of matching dimensions (inset).

2.6. Characterisation Scanning electron microscopy (SEM) imaging was performed with an analytical UHR Schottky emission scanning electron microscope (Model SU-70, Hitachi, High Technologies America, USA). The filament was cut and placed on carbon tape on Al SEM stubs. Samples were sputtered with a thin (approx. 4 nm) layer of platinum to avoid the accumulation of charge. The dispersion was observed at 1.5 kV. The effect of microdiamond concentration on the thermal behaviour of the ABS composites was measured using a Q500 thermogravimetric analyser (TA Instruments, New Castle, DE, USA). Each sample was weighed and placed in an aluminium crucible. The temperature was increased from 20 to 600 °C at a heating rate of 10 °C/min in an atmosphere of nitrogen gas. Differential scanning calorimetry (DSC) measurements were performed using a Q200 calorimeter (TA Instruments) under nitrogen flow. The samples were heated from 35 °C to 200 °C.


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The thermal diffusivities and specific heat capacities of microdiamond-containing ABS composite filaments were measured using a LFA 467 HyperFlash® light flash apparatus at 25°C (NETZSCH GmbH, Selb, Germany). The filaments were melted and pressed to form a film with thickness of ca. 1.5 mm using a hot press (Carver Inc., Wabash, IN, USA). An optical microscope (Leica Microsystems Pty Ltd, Sydney, NSW, Australia) was used to measure the thickness of samples. Uniform samples of size 10×10 mm2 were cut using a laser cutter (Universal Laser Systems Inc., Scottsdale, AS, USA). Finally, both sides of the films were coated with graphite spray. The bottom surface of each square shaped sample was heated using a xenon flash lamp with energy controlled by voltage and pulselength. The resulting temperature increase on the rear surface was measured using an IR-detector (InSb). Data acquisition and evaluation was via Proteus 6.0 software (NETZSCH GmbH). Specific heat capacities of samples were determined using pyroceram as the standard reference sample. Thermal conductivity of samples was calculated using the following equation: λ = α · Cp· ρ


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Where λ represents thermal conductivity [W (m·K)-1], α represents thermal diffusivity [mm² s-1], Cp is specific heat capacity [J (g·K)-1] and ρ shows sample bulk density [g cm3].

The hydrophilicity of the composite was observed by measuring the apparent contact angle of water droplets deposited on the material. ABS and composite filaments were thermally compressed to yield an ultra-flat surface using a hot embosser (Tetraderon, San Diego, CA, USA). Briefly, a 6 μL droplet of Milli-Q water was dispensed from an automated 50 μL eVol XR digital analytical syringe (Trajan Scientific and Medical, Melbourne, Australia) onto the sample surface. The image of the drop on the surface was captured using a Canon DSLR camera (Canon Inc, Tokyo, Japan) with a 70 mm F2.8 EX DG Macro lens (Sigma Corp, Kawasaki, Japan). Apparent contact angles were measured using ImageJ software with the DropSnake plugin.33 The elastic moduli of the ABS and composite filaments was measured by using a Monsanto Type W Tensometer (Melbourne, VIC, Australia). The grips for holding the filament were prepared using PVC sleeves, thrust washers and cyanoacrylate, as shown


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in electronic supplementary information (Fig.S3). The load and extension data were measured using LabVIEW. Heat dissipation measurements were performed using a computer controlled Peltier thermoelectric heater/cooler system.34 The system operates from room temperature to 100 °C. 3D printed heat sinks were placed on the Peltier module and heated for 10 minutes. Heat dissipation across the external surface was monitored using an infrared camera (FLIR Systems, VIC, Australia).

3. Results and Discussions 3.1. Surface morphology and Porosity The novel ABS composite filaments impregnated with either 37.5 and 60% (wt) of synthetic micro-diamond were developed for the use with low cost FDM printers. To produce the desired functional enhancements in printed devices, the distribution of the filler materials within the composite filament is of crucial importance. Herein, to confirm the uniform distribution of the micro-diamonds within the composite, the morphology of the various extruded filaments was observed using SEM. Images were taken following


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multiple consecutive extrusions of the composite (Fig. 3), to evaluate the increasing filament density (loss of voids) and composite homogeneity (diamond distribution) (Fig. 4). Filaments extruded only once were significantly ruptured, with inconsistent diameter (Fig. 1(C)), reflecting the non-uniform distribution of micro-diamonds within the feed pellets. Highly voided structures were evident in the filaments extruded for the 2nd and 3rd time (Fig. 3A) and (B)), although these reduced in size and number drastically following the 4th extrusion and were essentially no-longer detectable in the filaments from the 5th and 6th extrusions (Fig. 3D) and (E)). After the final 6th extrusion, the filament was very compact with a well distributed morphology and consistent filament diameter.


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Fig. 3(A-E) SEM images of cross section of 2nd to 6th-time extruded D-ABS(37.5%) filament. (F) Apparent void area of 2nd to 6th-time extruded filaments.

The apparent percentage of cross-sectional void area (%) of the 3rd to 6th-time extruded filaments was measured by processing SEM images through ImageJ (1.51j8, USA) software as described elsewhere.35 SEM Images were binarized based on threshold (shown in electronic supplementary information (Fig. S4)) and the percentage of apparent void area was calculated. Fig. 3(F) shows how the cross-sectional void area (%) of the extruded filaments decreased exponentially to 1 %, as number of extrusion increased. Internal voids within the filament act to decrease the overall thermal conductivity, as pores not only disrupt the interconnectivity of the micro-diamonds but also act as a thermal insulator within the polymer matrix. Large voids can also negatively impact upon the resolution of the resultant 3D print. Fig. 4 (A-D) shows high resolution SEM images of the cross-sections of cut D-ABS(37.5%) and D-ABS(60%) filaments (following the 6th extrusion). Fig. 4(A-B) shows the incorporation of 37.5 %(wt) of diamonds, sized between 2 and 4 μm, that are clearly mostly isolated from


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each other whilst embedded within the polymer matrix. The images reveal the lack of any visible agglomerations of the particles following the multiple extrusion process. However, the SEM analysis reveals how the micro-diamond is embedded physically within the polymer matrix, but is not chemically bonded, or in other ways coated, with the polymer. The dislodging of the micro-diamonds from the polymer upon cutting, leaving obvious and numerous voids (see Fig. 4(A and B)), together with clear voids around the embedded diamonds, show how the diamond is not physically connected to the surrounded polymer. Fig. 4(A and B) also reveals the presence of small pores within the ABS matrix for the DABS(37.5%) composite, which are far less obvious in the D-ABS(60%) filament (Fig. 4 (C-D). The images of the D-ABS(60%) composite reveal a significant amount of connectivity between micro-particles and a very much more dense and less porous overall composition. At 60 % (wt) loading, the edges of the diamond particles are connected with surounding particles, held in place by the thinner (~0.5 to 1.5 um) cementing polymer matrix, as observed in Fig.4(D).


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(B)

(A)

(10 μm)

(C)

(5

μm)

(D)

(B)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10μm)

(5 μm)

Fig. 4(A-B) SEM images of D-ABS(37.5%) and (C-D) D-ABS(60%) filaments showing the homogenous distribution of synthetic micro-diamonds.

3.2. Thermogravimetric analysis Thermogravimetric analysis (TGA) was performed in order to observe the impact of micro-diamond content on the thermal stability of the ABS based composite. The thermal stability of pure ABS pellets, D-ABS(37.5%) and D-ABS(60%) was investigated using TGA and the results are shown in Fig. 5(A). Differences in onset degradation temperature of


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composites (T5% = temperature at which mass is reduced by 5 % (wt)) is often used to measure changes in thermal stability.36-39 Here TGA (Fig. 5(A)) revealed that for ABS, DABS(37.5%) and D-ABS(60%), T5% increased from 374.1 °C to 378.7 °C to 391.5 °C, respectively. The increased onset degradation temperature is more significant with the greater diamond loading, with the rise in T5% of only 4.6 °C for D-ABS(37.5%) as compared to pure ABS, whilst for D-ABS(60%) a rise of 17.4 °C was observed. This reflects the structure of the two composites, with the D-ABS(37.5%) composite effectively segregated diamond particles within the bulk ABS polymer, as opposed to the D-ABS(60%) material, which as observed in Fig.4, is a considerably denser connected diamond particle structure. Upon combustion, the greater density of diamond particles at the combustion surface acts as a more resistant barrier to progression of decomposition through the polymer matrix. Pure ABS showed a sigmoidal curve with a 95.7% mass loss across the temperature range 379–476 °C, consistent with previous reports.38 The final residue containing char at 600 °C was 4.3 %. The D-ABS(37.5%) filament showed a significant weight loss between 388-455°C, again corresponding to the structural decomposition of the ABS polymer. The


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final residue containing char and micro-diamonds was equal to 40.2 % at 600 °C, thus confirming the diamond content of the composite at (or within 2% of) the nominal 37.5 %. The final residue at 600 °C for the D-ABS(60%) composite was also as expected for the nominal composition, equal to 58.6 %. These results are listed together within Table 1.


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Fig. 5(A) Thermogravimetric analysis ABS, D-ABS(37.5%) and D-ABS(60%). (B) Differential scanning calorimetric (DSC) of ABS, D-ABS(37.5%) and D-ABS(60%). (C) Thermal conductivity of ABS, D-ABS(37.5%) and D-ABS(60%).

3.3. Differential scanning calorimetry The thermal properties of the composite materials were further investigated using differential scanning calorimetry. Fig. 5(B) shows the resultant DSC thermograms for ABS, D-ABS(37.5%) and D-ABS(60%). In the case of ABS, two transitions peaks are clearly visible i.e. the glass transition temperature (Tg) of styrene at about 108 °C, followed by an endothermic peak at about 138 °C associated with the melting of acrylonitrile.41 It is apparent from the DSC curves for D-ABS(37.5%) that the transition peaks become broader and less distinct. The unbound micro-diamond particles within the polymer matrix as observed under SEM (Fig. 4(B)) contribute to the less distinct Tg and Tm peaks.42 In the response recorded for the D-ABS(60%) filament, the Tg peak for styrene moves towards a higher temperature of 114 °C, followed by an endothermic peak at about 146 °C for the acrylonitrile melt. The D-ABS(60%) filament has significantly inter-connected micro-


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diamonds that entrap the polymeric matrix and provides a physical interlock to the mobility of polymeric matrix imparting, albeit marginal increase of 6 °C in Tg.37. Wei et al., have made similar observations while incorporating graphene into ABS.20

Table 1: Summary of physical-chemical properties of ABS, D-ABS(37.5%) and D-ABS(60%) filaments.

Sample

T5%

Wtres

Tg

Tm (°C)

Thermal Diffusivity (mm2/S)

Thermal Conductivity (W./m.K)

Contact angle ()

Elastic Modulus (MPa)

Elongation at break (%)

(°C)

(%)

(°C)

4.3

108

138

0.13-0.1843

0.1740

94±2.0

1050±109

4.32

40.2

-

-

0.27±0.01

0.37

65±4.0

1380 ±242

2.65

58.6

114

146

0.46±0.01

0.94

63±1.2

1490 ±374

1.7

374. 1

ABS DABS(37.5%)

378.

DABS(60%)

391.5

7

3.4. Thermal conductivity


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The thermal conductivity of the majority of polymers ranges between 0.1-0.25 W./m.K.44 As shown in Fig. 5(C) the D-ABS(60%) composite exhibited a 5.2 fold increase in thermal conductivity, as compared to the pure ABS, equal to 0.94 W./m.K, resulting from the high diamond loading. Diamond has intrinsically highest thermal conductivity equal to 10002200 W./m.K and act as an excellent heat conductor.45-47 With an increase in loading of micro-diamond, there is a corresponding increase in conductive pathways within the composite, which delivers the significant increase in thermal conductivity for the D-ABS(60

%)

material. To place the current composite fibre in context, Table 2 presents the thermal

conductivity of various ‘3D printable’ composites reported to-date within the literature. For example, recently Goa et al., developed a 3D printed fabric containing boron nitride with a thermal conductivity of 0.078 W./m.K.48 Nam et al., reported a hybrid filler of carbon fibre and graphite flakes incorporated within printable ink, which increased thermal conductivity up to 2 W./m.K.49 However, in this case the approach required careful optimisation of rheological properties of the ink as well as post-curing after printing. Jia et

al., demonstrated the highest through plane thermal conductivity (5.5 W./m.K.) for a 3D


28

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printed object by vertically aligning graphite flakes during printing within blend of polyamide 6 (PA6), maleic anhydride grafted poly(ethylene-1-octene) and polystyrene [PA6/POE-g-MAH/PS]50 However, obtaining such high thermal conductivity was dependent on printing direction and orientation of flakes, which was achievable with lowcost commercial FDM printers.

Table 2: Thermal conductivity and elastic modulus of 3D printable composites.

Filler

Thermal

Applied in

content

conductivit

3D

(Wt%)

y(W./m.K)

printing

Elastic Limitation

Modulus

Polymer

Filler

Ref.

ABS

-

-

0.15

FDM

-

ABS

-

-

-

FDM

-

1100-2900

51

ABSplus

-

-

-

FDM

-

2200

52

ABS

Mont-

5.0

-

FDM

-

3600

53

35.0

0.93

FDM

-

(MPa)

40

morillonite ABS

Boron

54

nitride ABS

Copper

50.0

0.912

FDM

-

930a

55

ABS

Iron

30.0

-

FDM

-

978.5

55


29


Epoxy

Graphene

3.0

0.75

oxide PVA

Boron

Page 30 of 48

Direct ink -

-

56

-

48

20

57

-

50

-

49

writing 5.0

0.078

nitride

Direct ink Single writing

layer

was

printed PLGA

Boron

63.2

2.1

nitride

Direct ink Tri-solvent writing

system for ink formulation develops microstructures

PA6/POE

Graphite

50.0

5.5

FDM

Dependent

-g-

on printing

MAH/PS

direction

Epoxy

Carbon+

7.60 +9.30 ≈2

graphite

Direct ink Postwriting

processing at elevated temperatur e

ABS

Micro-

60.0

0.94

FDM

-

1490

This

diamond

study

s a= Elastic modulus for 30%(wt) of copper.

3.5. Hydrophilicity and elasticity of the composite filament


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The diamond particles used herein have been previously characterised confirming their hydrophilic and ion-exchange surface properties.

58

Incorporation of such a hydrophilic

particle as a filler within the polymer matrix should obviously have a significant effect upon surface properties and the wettability (surface hydrophilicity) of the resultant composite. The apparent contact angles of the ABS, D-ABS(37.5%) and D-ABS(60%) filaments were measured to determine these changes. The apparent contact angle data recorded (θ°), shown in Fig. 6(A), reveals the significant impact the incorporation of synthetic microdiamonds had upon the surface hydrophilicity of the composite, showing a 32% decrease in the measured contact angle for the D-ABS(60%) filament. The results confirm the diamond is sufficiently present at the surface of the fibre to effect this change in surface properties, and not fully enclosed or coated. This enhanced hydrophilicity may also contribute to developed thermal properties, as previous studies indicates that hydrophilic surfaces tend to exhibit higher heat transfer coefficients in comparison to more hydrophobic surfaces.59


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Fig. 6(A) Apparent contact angle and (B) Elastic Modulus of D-ABS(37.5%) and D-ABS(60%) filament.

The elastic modulus of a material is a measure of its resistance to deformation or stiffness. For functional 3D printed devices inherent resistance to deformation under stress can be a useful additional property. Fig. 6(B). shows the elastic modulus of the ABS, D-ABS(37.5%) and D-ABS(60%) filaments. Incorporation of the micro-diamond at the 60 wt% level (D-ABS(60%)) saw a 41.9 % increase in the elastic modulus with respect to pure ABS filament. Elastic modulus of composite is dependent upon the interaction between filler and polymer as well as on dispersion pattern of filler. The high elastic modulus of ABS(60%) filament is attributed to well dispersed and inter-connected micro-


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diamond particles within the composite as witnessed in SEM images (Fig.4C-D). As mentioned above, the micro-diamond inter-locks the polymer matrix and restrict its mobility, resulting in higher elastic modulus. This significant increase in the elastic modulus is also in agreement with similar previous studies.24,60 The reported elastic modulus of various 3D printable polymers and composites is also given within Table 2. The measured elongation at break is an indication of the plasticity (ductility) of the polymer. The percentage of elongation at break values for ABS, D-ABS(37.5%) and DABS(60%) filaments are given in Table 1. Herein it was observed that elongation at break (%) decreases with an increase in the micro-diamond content. These results further confirm the incorporation of micro-diamond within the polymeric structure restricts the mobility of the ABS chains, resulting in the observed higher stiffness, elastic modulus and less elongation.61 High elastic modulus is required in certain application fields, such high performance engineering and medical applications, and again can be a useful additional property of such printable functional materials.

3.6. Heat dissipation within 3D printed heat sinks


33


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Over recent years it has become very evident that electronic devices have been getting smaller and more powerful. Within such devices there is often the need for efficient and low-cost thermally conductive materials, such as heat sinks, which can dissipate or distribute unwanted heat from areas within the electronic devices and thus enhance their performance or function. 3D printable miniature polymer heat sinks can improve the performance of electronic devices, while potentially dramatically reducing production costs. To demonstrate the potential of our 3D printable diamond composite material, various conventional and nonconventional heat sinks were test printed as shown in Fig.7(A-E). A detailed comparison of heat sinks designs and their performance is given elsewhere.62-64 However, here using a low cost commercial 3D printer, we were able to reproduce a variety of these designs, and additionally design and print heat sinks with complex structures that would not be easily achieved (or indeed possible at all) using either traditional direct milling or embossing.


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(B)

(A)

15 mm

(D)

(C)

15 mm

15 mm

(E)

15 mm

15 mm

Fig.7 (A-E) Photographs of conventional and non-conventional 3D printable heat sink designs.

In order to evaluate heat dissipation through the new composite material following its printing, a set of winged heat sinks were printed from the ABS, D-ABS(37.5%) and DABS(60%) filaments, using the customised FDM printer. The resultant 3D printed winged heat sinks (30mm wide and 16 mm high) are shown in Fig.8 (A-C). A Peltier heater module set at 100 °C was used to examine heat transfer performance of the various printed heat sinks. The temperature differences and heat distributions of the three heat sinks were determined using a thermal imaging camera. Thermal images of the winged heat sinks printed from the ABS, D-ABS(37.5%) and D-ABS(60%) are shown in Fig. 8(D-F). The heat sinks were heated for 10 min and the temperature was recorded at three points. The IR images for the D-ABS(60%) heat sink showed 6-14 °C higher temperatures for all


35


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three points. The highest temperature difference was observed at the top surface, although significant differences were also recorded at the point closest to the Peltier heater. At the bottom of 3D printed winged heat sink the temperature was 79.2, 83.0 and 85.0 °C for ABS, D-ABS(37.5%) and D-ABS(60%) respectively. At the top surface of heat sink, the increase in temperature for the D-ABS(60%) was equal to +42 % in comparison to the pure ABS heat sink, representing a considerable difference over a relatively short distance.

Fig. 8(A-F) 3D printed wing heat sinks with ABS, D-ABS(37.5%) and D-ABS(60%) filament with IR photograph showing temperature distribution after heating for 10 mins. (G) Temperature variation at the top surface of 3D printed heat sink with respect to time.


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The heat transfer capability of 3D printed winged shape heat sinks with varying concentration of micro-diamonds was further evaluated by measuring surface temperature with respect to time. The winged heat sinks was heated by using heating system set at 100°C. The temperature at the top surface was measured for 20 mins using IR camera. Fig.8 (G) shows a plot of time versus the top surface temperature for the 3D printed ABS and composite heat sinks. The highest temperature recorded after 20 mins for ABS, D-ABS(37.5%) and D-ABS(60%) heat sinks were 38.1 °C, 46.0 °C and 49.1 °C respectively. The recording of the highest surface temperatures of these printed devices with increasing concentration of micro-diamonds is consistent with the above thermal conductivity measurements for the composite fibres.

3.7. Recycling of waste print During optimisation of the 3D printer with the new composite fibres, various unsuccessful/waste prints were obtained. The waste printed parts (D-ABS(37.5%)) were dissolved in acetone, pellets were reformed and the filament was extruded just once. Following this simple process, it was noted that thermal conductivity was identical to the


37


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original filament, while the elastic modulus decreased by 16% as compared to the original filament. However, it was therefore possible to use the recycled fibres to print further devices, whilst essentially maintaining their functional properties.

4. Conclusions Homogenous and uniform ABS filaments containing 37.5 % (wt) and 60 % (wt) synthetic micro-diamonds were produced following a multiple extrusion procedure. The incorporation of synthetic micro-diamonds was shown to significantly improve thermal conductivity by more than 5-fold compared to ABS itself. TGA analysis also confirmed increased thermal stability for the D-ABS(60%) material. The elastic modulus for the 60 % wt micro-diamond containing composite material increased by 41.9 % with respect to pure ABS, from 1050 to 1490 MPa. The hydrophilicity also increased from by 32 %. The apparent contact angles for pure ABS and D-ABS(60%) filaments were 94 and 63, respectively A customised low-cost desktop FDM printer was used to print the composite fibres into 3D polymer-based heat sinks for evaluation of performance. A D-ABS(60%)


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printed heat sink demonstrated enhanced heat dissipation as compared to that of pure ABS. As electronics become smaller and more powerful, thermal management has become one of the key limiting factors in device performance. With an increasing demand for thermally conductive but electrically insulating materials for cooling of miniaturised microelectronic devices, e.g. within portable/wearable medical devices, this novel low cost micro-diamond containing ABS composite provides a potential practical solution. In addition, its compatibility with FDM printing for production of complex geometries at lowcost could be of significant advantage in miniaturised and compact thermal management applications.

ASSOCIATED CONTENT

Supporting Information. The electronic supporting information contains (i) Extruder line provided with cooling tractor. (ii) Various gear use for printing. (iii) Holding filament in PVC sleeves using cyano-acrylate and thrust washer. (iv) Comparison of SEM images and binarized images obtained through imageJ.


39


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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources

This work is funded by the ARC Centre of Excellence for Electromaterials Science (ACES) (Project Number CE 140100012). MCB would like to thank the ARC for a Future Fellowship (FT130100101).

ACKNOWLEDGMENT

We would like to thank the Central Science Laboratory (CSL) at the University of Tasmania (UTAS), in particular Dr. Sandrin T. Feig for SEM. Mr. Calverly Gerard from the Engineering workshop at the University of Tasmania (UTAS) is thanked for measuring mechanical properties. ABBREVIATIONS


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AM, Additive manufacturing; HPHT, High Pressure and High Temperature; ABS, Acrylonitrile Butadiene Styrene; FDM, Fused Deposition Modelling; D-ABS(60%), Acrylonitrile Butadiene Styrene containing 60% (wt) micro-diamonds; D-ABS(37.5%), Acrylonitrile Butadiene Styrene containing 37.5% (wt) micro-diamonds; SLM, Selective Laser Melting; SEM, Scanning electron microscopy; DSC, Differential Scanning Calorimetry; Tg, Transition Temperature; Tm, Melting Temperature; Wtres , Weight of residue; T5%, Temperature at 5 % weight loss; PA6/POE-g-MAH/PS, Polyamide 6, Maleic anhydride grafted Poly (ethylene 1-octene) and Polystyrene; PVA, Poly vinyl alcohol; PLGA, Poly lactic-co-glycolic acid;

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Three-Dimensional Printing of Abrasive, Hard, and Thermally

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