Porous carbons with complex 3D geometries via selective laser sintering of whey powder

doi:10.21203/rs.3.rs-4899246/v1

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Porous carbons with complex 3D geometries via selective laser sintering of whey powder

https://doi.org/10.21203/rs.3.rs-4899246/v1

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In addition to the inherent limitations of carbons to melt or flow, a vast majority of carbon precursors deforms during carbonisation, with stereolithography of thermoset resins being the preferred technology for 3D printing of carbons. An alternative is now presented with the possibility of using a melting-based technology, selective laser sintering (SLS), to fabricate 3D structures that withstand carbonisation. The key factor that makes this happen is whey powder, a natural, abundant and cheap by-product of the dairy industry. When heating the whey powder with a laser at 180-200 ºC for a few seconds, whey particles sinter, and 3D structures are obtained layer-by-layer. Carbonisation of the sintered whey structures brings about 3D porous carbons with excellent mechanical properties that preserve the SLS printed form albeit an isotropical shrinkage. Melanoidins are identified as responsible for both the sintering and the thermoset behaviour during carbonisation of the whey powder.

Physical sciences/Engineering

Physical sciences/Materials science

Additive manufacturing (AM) is having an impact on a number of applications, from analytical chemistry to biomedicine, electrochemistry and chemical reactors, due to its ability to provide materials with bespoke 3D structures [1-3]. There is a continuous quest for materials to be processed using the multiple AM technologies available, with polymers and metals showing a consolidated hegemony in the field [4]. Carbons, on the other hand, are a wide family of materials with unrivalled properties in terms of electrical and thermal conductivities, porosity, chemical inertness and mechanical performance. Carbons (including nanocarbons) have found a niche in AM mainly as fillers in polymeric formulations, either as fibres or particles, or as the active component in inks [5]. Yet the possibility of using a conventional route in which a carbon precursor would be 3D printed and then carbonised to obtain a carbon material is hampered by the inherent limitation of most of the AM technologies, namely the need for the printed material (i.e., the carbon precursor) to flow or melt [5].

3D printing of thermoset carbon precursors rather than thermoplastics is, in principle, required to obtain structures that withstand carbonisation. Stereolithography (SLA) based techniques are preferred for 3D printing thermosetting photopolymers [6], and few works have already explored the possibility of carbonising the resulting structures [7-11]. In spite of the unbeatable resolution that SLA-based technology provides, only photocurable resins with very specific formulations render 3D structures that preserve their geometry upon direct carbonisation. Alternatively, other SLA printed resins require a tedious post-processing involving impregnation with different chemicals to avoid the collapse of the structure during the heat treatment [12,13]. In addition to SLA, AM technologies such as Direct Ink Writing (DIW) and Binder Jetting has also been used to print carbon precursors that were subsequently carbonised [14-18]. DIW printing of thermoset polymers is however not straightforward [19,20], and the results obtained so far had limited success in terms of dimensions and printing precision of the final carbon 3D structures [14-16].

Selective laser sintering (SLS) is a high-speed AM technology popular for metals and polymers processing [21-23]. Successful SLS requires powders with good flowing properties, made of particles that melt together when heated for short time. Thermoset polymers are thus poor candidates for manufacturing 3D structures with this technology, with different strategies being currently proposed to improve their SLS printability [24,25]. Surprisingly though, one of the first works on AM of carbon materials relied on this technology to sinter carbon fibre/phenol-formaldehyde (PF) resin particles [26]. This approach was mimicking the sintering of carbon fibre/polyamide powders [27], which are nowadays very popular for SLS printing of composites. According to the authors of the study [26carbon], the PF resin used softens/melts at 102 ºC and cures at higher temperatures. In order to maintain the integrity of the 3D printed structures, a tight control of a lengthy (20 h) carbonisation process was necessary.

This work shows the possibility of using an alternative carbon precursor, whey powder, which can be SLS printed and then readily carbonised without losing its original shape. Recently, we have described the carbonisation of whey powder (600-1000 ºC) [28]. From a Carbon Science perspective, it was actually an unexpected result to discover that whey powder does not melt when heating under N₂ at temperatures above 200 ºC, thus behaving as a thermosetting precursor. Moreover, rather than melting whey powder particles sinter at relatively low temperatures (120-150 ºC). 3D carbon structures were then crafted by simply pouring whey powder in moulds and carbonising them. This triggered the possibility of using whey powder in a SLS printer to obtain 3D structures with complex geometries that could be easily transformed into carbon, which is the subject of the present communication.

SLS of whey powder

Figure 1a shows a scheme of the SLS processing of whey powder. This by-product of the dairy industry is made of particles with typical carbohydrate (mainly lactose) and protein (mainly β-lactoglobulin) contents of 65-80% and 10-15%, respectively (Table S1). Crystalline α-lactose monohydrate constitutes 80-95% of the lactose in non-hygroscopic whey powder [29]. Most whey particles are rounded, either spherical or potato-shaped, which is convenient for SLS sintering [30,31,22]. Whey particles size distribution (PSD) depend on the specific processing and origin of the liquid whey, with typical D₅₀ values of 70-110 µm (Table S1). Whey powder does not completely melt in the SLS process. Instead, necking of whey particles is ubiquitous in the resulting (macro)porous structures (Figure 1a). This effect makes possible the laser printing of 3D whey structures that preserve their shape after cooling down the powder bed and de-powdering. The non-enzymatic browning of the laser sintered whey pieces is also obvious.

The temperature window of SLS printability is defined by the DSC profile of the whey powder (Figure 1b). The first thermal event at temperatures ca. 50 ºC is related to the amorphous lactose glass transition (T_g) temperature, which depends on the origin and relative humidity of the whey powder [32,33]. This event was not identified in all the whey powder samples tested, thus pointing out whey powder samples with high (>95%) crystalline lactose contents. The endotherm peak at ca. 150 ºC corresponds to the release of the water of crystallisation of the α-lactose monohydrate present in whey [34], and marks the lowest temperature limit of the processing of whey powder with this technique. The next wide endothermal band allows the SLS of whey powder in air up to ca. 220 ºC, when whey powder combustion would eventually start. We anticipate that higher printing temperatures would be feasible if working under inert atmosphere.

We have tested the SLS of 5 whey powder samples (W1-W5, Table S1) from diverse origin, including sweet and acid whey powder. The values of the true density (ρ_He) of the whey powder range from 1.15-1.48 g/cm³ (Figure 1c). Whey powder is denser than PA12 powders (1.02 g/cm³), whereas the ρ_He values of W1 and W5 are very similar to those of cured RF resin xerogels (1.41-1.44 g/cm³) [35]. The studied whey powder samples present different compactability and flowability as determined by their bulk and tap densities, and the corresponding Hausner ratio and Carr index (Figure 1c). All whey powder samples except W3 present Carr indexes >15, the maximum expected value for a powder to have a good flowability. Moreover, W4 shows a Carr index >21, which is considered an indicative of passable (just before poor) flowability [36]. Nonetheless, the Hausner ratios of almost all whey powder samples correspond to the critical value (1.2) that guarantees the minimum powder viscosity of polymers [37]. Only W4 has a Hausner ratio significantly higher (1.31), although a minimal increase of powder viscosity would be expected [37]. In practice, the limitations to whey powder SLS coming from its flowability properties were negligible, and SLSed cubes were obtained from all whey samples tested, under similar operational conditions (Figure 1c). Using acid whey powder for SLS is especially relevant from the valorisation standpoint of this type of whey [38,39].

Optimisation of the operational conditions to determine the SLS printing precision was carried out with W5. A simple model (Figure 2a) was selected for this purpose. Two qualitative printing responses were considered for the optimisation, namely the sturdiness of the sintered piece and the easiness of depowdering. Once the piece was successfully SLSed, the dimensional accuracy of the printed model was determined. A very elemental printer equipped with a low power (2.3 W) blue (445 nm) laser was used in this work. Hence, only basic SLS parameters could be optimised and the main results of the optimisation are summarised in Table 1. Essentially, the printing speed is the variable showing a higher effect in the SLS of whey powder. Neither the printing surface temperature nor the PSD of the whey powder are so relevant within the variable range explored. The latter variable (PSD) affects though to the definition of the perimeters of the printed structure, with lower PSD bringing about surfaces with better finish and sharper details. The resulting pieces have dimensional deviations <3% of the nominal values, with no significant warpage or cracking, and easy handling overall. SEM analysis of the cross-sections of the printed models (Figure 2b) shows a very uniform porous structure with almost imperceptible layering [40], which highlights the homogeneous sintering of the whey powder in the z direction.

The printing conditions of experiment 8 were then selected for further studies. An artifact test (Figure 2c) was designed to determine the SLS printing precision of whey powder. Figure 2d shows an example of the printed test, which includes rods, bulges, grooves, surface and through-holes, bridges and steps of different dimensions. Most features are present in the sintered whey structure, whereas absent or poorly defined elements are either related to the post-processing (brushing off the slenderer rods is quite challenging) or to the limitations of the whey powder sintering (i.e., through-holes <2 mm). Whey reuse was also contemplated in this study, and results are shown in Figure 2e. In spite of the evident browning and caking of the spare whey powder remaining in the two printer reservoirs, a simple sieving procedure brings about a powder with adequate flowability and sintering behaviour to be successfully re-processed. Similar results were also obtained with powders processed for a second time, i.e., after two heating and cooling down cycles in the SLS printer.

From SLS printed whey powder to complex 3D pieces made of porous carbon

Once the whey powder is sintered, transformation into a porous carbon is straightforward as already described [28]. A SEM image of the cross-section of a SLS whey derived carbon (Figure 3a) shows a homogeneous porous structure made of carbonised sintered whey particles. The similarities between Figure 2b and Figure 3a confirm the thermosetting nature of the sintered whey, which does not melt during the carbonisation treatment. As a result, any 3D structure made of sintered whey does not deform during carbonisation and all the features present in the original are preserved albeit a 23% isotropic shrinkage (Figure 3b). The attainable resolution of the resulting SLS printed porous carbon structure is astonishing, comparable to that of carbons prepared by SLA of photocurable resins [8-11]. Figure 3c shows some examples of 3D structures made of whey-derived porous carbon. They are presented here as a proof of concept of the capabilities of the methodology. Some of those structures with intricate overhangs cannot be produced with other conventional AM technologies such as FDM or DIW. Typical manufacturing times are 6-8 h, i.e., 3-5 h of SLS printing and de-powdering, plus 3 h carbonisation (at 850 ºC, 1.5 h dwell time).

As for the properties of the SLSed porous carbons, they have similarities with those obtained by moulding [28] (Figure 4). Thus, typical elemental composition analysis indicates C and N contents of ca. 75 wt% and 3 wt%, respectively (Figure 3c). This relatively high N content of the 3D carbon materials is interesting for electrochemical and catalytic applications [41-43]. Whey powder-derived carbons have an initial ash content of 10-13 wt%. The mineral matter in these porous carbons comes from the soluble salts present in the liquid whey after dairy processing. Almost half of the mineral matter in the carbons is soluble in water. Furthermore, washing with HCl removes it to levels <3 wt% [44]. Another similarity of SLSed and moulded 3D carbon structures is that they do adsorb little to none quantities of N₂ at cryogenic temperatures (Figure 4a). This changes when the mineral matter is washed off with acid (sample SLS 850_D), rendering a carbon material with a relatively high surface area, which is further increased by thermal activation procedures (sample SLS 850_A) (Figure 4a) [44].

Dissimilarities between porous carbon materials obtained by SLS or moulding and further carbonisation of whey powder are, however, relevant in terms of the overall porosity of the resulting structures. The bulk density of the carbon pieces obtained by SLS carbons is considerably lower than that of moulded whey powder-carbons (Figure 4b). Accordingly, the porosity of the SLS structures is as high as 74%. The macropore size distributions of the SLS whey-derived carbons shift to higher pore sizes when compared to the moulded ones (Figure 4c), this being the reason for the increase of the porosity from 60% to 74%. The occurrence of wider macropores on the SLS printed carbons also affects the mechanical properties of the 3D structures, with a significant decrease of their compressive strength and modulus (Figure 4d). Still, the compressive performance of the SLSed carbon materials is very good considering their high porosity [44].

The processing of whey powder with SLS is related to previous studies using the same technique for 3D printing of carbohydrates. Thus, SLS of glucose and sucrose [45], and isomalt [46] has been already reported for biomedical applications. Also, even though SLS of pure lactose has not been reported, drug formulations including quantities of α-lactose monohydrate (up to a 12 wt%) have been SLSed for printlets manufacturing [47]. According to the different researchers, the fabrication of the 3D printed structures from these powders involves melting/caramelisation of the sugars. However, the presence of proteins in whey powder makes a substantial difference in the sintering of whey particles. Furthermore, that very same concomitance of lactose and proteins in whey is compulsory to preserve the SLS printed shape during carbonisation [28].

There is scarce information on the particle sintering of dairy powders at temperatures above 100 ºC. Three possible mechanisms are postulated here to be, in principle, responsible for the sintering of whey particles, namely caking, caramelisation and Maillard reactions. The two first are specific of lactose, the main constituent in whey, and the latter requires the coexistence of lactose and proteins. The sintering of whey powder depends on the temperature/time combination and sintered structures can be obtained from 85 ºC on (Figure S1), if enough time (at least 2 h) is given. On the other hand, much higher temperatures are required when the heating times (2 min) come close to the conditions of the SLS processing (Figure S2). At such short times, the lump obtained at 160 ºC was easily friable (Figure S2); whereas at 220 ºC, 2 min, the whey powder started to scorch, as predicted from Figure 1b. In all cases, non-enzymatic browning is evident, thus pointing out the occurrence of pigments characteristic of Maillard reactions and/or lactose caramelisation.

Caking (i.e., particle sintering) of lactose is well studied for its implications in the preservation of dairy powders and pharmaceutics [48]. The main mechanisms of lactose caking at temperatures well below 100 ºC include amorphous, humidity and mechanical caking [49,50], and the particle bridging observed in Figure 1a resembles that depicted in micrographs of caked whey powder [51]. However, the SLS conditions used in this work are far from those required for whey (lactose) caking, namely high amorphous lactose contents and/or high water activities in the whey powder [48-52]. As shown in Video S1, α-lactose monohydrate powders cake to a certain extent after 2 h at 160 ºC, whereas 2 min at such temperature has no apparent effect on powder compaction (Figure S3).

Lactose caramelisation is known to happen at temperatures close to 220 ºC (Figure S4) [53,54]. It involves the pyrolytic breakage and further polymerisation of the galactose and glucose molecules of lactose, and the physical transformation of the powder into a viscous glass [55]. This has obvious implications in the SLS of whey particles, which would stick together in a similar fashion to other SLSed carbohydrates, as mentioned above. Experimentally though, caramelisation of lactose requires relatively long times at high temperatures to be effective (Figure S5). When approaching to the sintering conditions on the SLS printer, i.e., 2 min at 200º C, α-lactose monohydrate powders remain loose and white (Figure S6).

Maillard reactions between lactose and whey proteins are responsible of the non-enzymatic browning of dairy powders at ambient temperature [55]. Whereas Maillard reactions and caking (particle sintering) are identified as damages occurring during storage of whey powders [56,57], a possible contribution of Maillard reaction products (e.g., melanoidins) to whey particle sintering has not been established. Melanoidins are brown coloured polymers formed in the so-called late-phase of the Maillard reactions [58,59]. In spite of a detailed structural characterisation, melanoidins are roughly classified attending to their size into low, intermediate and high molecular weight, with their molecular weight distribution (MWD) depending on a number of factors including temperature/time conditions of the reaction. It is now accepted that low molecular weight (1-10 kDa) melanoidins in reducing sugars/protein systems (e.g., whey) are chromophore aggregates covalently bonded to protein backbones [59-61]. These aggregates account for the colour of the melanoidins and come from the dehydration and condensation of the carbohydrates. Heating up the reducing sugars/protein systems shifts the MWD of melanoidins towards higher values, reaching values >100 kDa [59-61]. High molecular weight melanoidins include multiple protein backbones crosslinked with the chromophores, which, in addition, are known to polymerise with temperature [59-61]. It is also accepted that carbohydrate caramelisation products contribute in Maillard reactions at high temperatures [62]. Furthermore, characterisation of melanoidins in roasted malts demonstrates that there is a temperature threshold (ca. 160 ºC) for the formation of high molecular size melanoidins [63].

This melanoidin size-increase pattern in Maillard reactions resembles the curing mechanism of PF or RF polymeric resins, and it is postulated responsible for the sintering of whey powder with temperature. Thus, the sintering mechanism proposed here involves the progressive enlargement with temperature of melanoidins on the surface of the whey particles. It should be pointed out that the relative lactose/whey protein ratio concentration is considerably lower at the outermost surface of the whey particles [64], thus making this mechanism especially relevant. The completion of the melanoidins “curing” process, i.e., the maximisation of their MWD, would be required to attain a strong inter-particle bond that endures carbonisation and keeps the shape of the SLSed whey structure unaltered.

The final part of this discussion is devoted to explain the lower density of the SLS whey derived carbons when compared to the moulded ones. This result was unexpected in light of the minimum layering effect observed (Figure 2b and Figure 3a). To further investigate powder packing issues during SLS printing, carbon structures were prepared from SLSed whey using a higher printing layer height (200 µm). The bulk density of the resulting materials was 0.44 g/cm³, same as that of SLS_850 in Figure 4b. This discards or, at least, minimise the effect of eventual powder packing differences when moulding or SLS whey powders. A very relevant result was however found when re-used whey powders (Figure 2e) were poured in a mould and carbonised at 850 ºC (1.5 h). The bulk density of the resulting carbons was 0.48 g/cm³. In other words, it is not moulding or SLS of whey powders what makes the difference. Sintering of fresh whey powders brings about denser carbons than sintering brown whey powders.

It should be clear that SLS of whey powder does not actually sinter fresh powder, as it browns (Maillard reaction) due to the heating required to reach and maintain the bed and printing temperatures (80 ºC and 135 ºC, respectively) for approx. 4 h. As mentioned above, this brown powder is lightly sintered after the printing process. The browning process has little effect on the SLS processability parameters of the whey powder (Figure S7), but changes significantly its TG and DSC profiles (Figure S8). Specifically, the loss of crystalline water at ca. 150 ºC is much lower in the browned powder. This is expected and denotes the transformation of the α-lactose monohydrate present in the original whey powder into different polymorphs when dehydrated at high temperature (>120 ºC) [65-67]. Specifically, the loss of crystalline water at such temperature leads to two anhydrous α-lactose phases, one unstable (hygroscopic) and the other stable, and to anhydrous β-lactose, with the two latter polymorphs prone to cake after short times [67]. Actually, this transformation explains the caking of the lactose powders shown in Video S2. Furthermore, the caked lactose after dehydration is friable, a similar behaviour observed for the brown whey powder.

Since we do not expect different lactose polymorphs to affect the Maillard reactions products significantly, there are two possible explanations for the lower density of the brown whey derived carbons. The first one would imply the sintering of the fine particles, thus affecting the PSD of the browned whey. This was discarded when considering the flowability parameters (Figure S7) and by measuring the PSD (Figure S10) of the fresh and re-used W5 powders. The other, more plausible possibility is related to the behaviour of the melanoidins with temperature once they are formed at a given temperature and then cooled down. We have observed that if, after cooling down, we break a 3D structure of whey sintered in a mould at a given temperature (e.g., 120 ºC, 2h) and try to stick the two parts together by re-heating them, it is not possible. This, again, is a similar behaviour of conventional, thermoset resins. As a consequence, the whey interparticle bridging attained at a given temperature cannot be reconstructed if broken, thus reducing the contact points between whey particles and the density of the sintered pieces.

Whey powder of different companies was used in this study, namely, LaFuente (W1), Lemasa (W2 and W3), CAPSA FOOD (W4) and Lactalis (W5). Table S1 collects details of their composition, according to the supplier specifications. α-lactose monohydrate (>99.5%) powder from MERCK was used for reference studies.

Selective Laser Sintering (SLS) of the whey powder was carried out in a Sintratec Kit bench printer equipped with a low power (2.3 W) green (445 nm) laser. The printer is controlled by proprietary software that allow to control a few processing parameters, namely surface (printing) and chamber (bed) temperatures, speed, layer height, perimeter number, perimeter offset, hatch offset and hatch distance. Chamber (bed) temperature, perimeter number, perimeter offset, hatch offset and hatch distance were fixed in all experiments at 80 ºC, 1, 75 µm, 75 µm and 75 µm, respectively. Layer height was set at 100 µm for all experiments except for the study of the effect of whey powder packing (layer height of 200 µm). Surface (printing) temperature and speed were initially set at 135 ºC and 250 mm/s. These were the printing conditions used for the sintering of the whey cubes in Figure 1c. Optimisation of the surface (printing) temperature and speed was then carried out as explained in the text. The whey powder in the reservoir is conditioned at the chamber (bed) temperature for approx. 45 min before the laser starts to print. The surface (printing) temperature is set and maintained during approx. the last 30 min of the chamber conditioning. Once the printed work is finished, the chamber (bed) temperature is maintained for 30 min before cooling down. Depowdering of the printed structures was done manually by brushing-off the non-sintered powder. The SLSed whey parts were then transferred to a tubular oven (Carbolite) and heated up to the desired temperature (850 ºC), at 10 ºC/min under 150 ml/min of N₂. Dwell time at 850 ºC was 1.5 h. Video 1 in Supplementary Information shows a synopsis of the whole process. The STL files for the manufacturing of the artifact test (Figure 2c), the Ugrinsky wind turbine, the aluminothermic mould and one of the fusilli-like helicoid presented in Figure 3c are available at [68]. All these pieces were designed using Autodesk Inventor Professional 2015 (USA). The rest of 3D objects (the long fusilli-like helicoid, pitched-blade turbine and Big Ben replica) are common designs available in the web.

Additional treatments to the porous carbons obtained from the SLS of whey powder include acid washing and thermal (CO₂) activation. The acid washing protocol includes four cycles of 15 min into sonicated water at 80 °C, replacing the washing water after each cycle. Then, the structures were sonicated in a 37 wt% HCl solution for four cycles of 15 min. Finally, they underwent four cycles of sonication (15 min) with distilled water at 80 °C to remove the acid residues. For CO₂ activation, the demineralised structures were heated at 850 °C (10 ºC/min) under 150 ml/min of N₂. When reaching the set temperature, the N₂ flux was switched to 50 ml/min of CO₂ for 1 h. The activated structures were cooled down in N₂ (150 ml/min).

Characterisation of the whey powder and porous carbons include DSC in air and inert (N₂) atmospheres. Experiments were carried out in a DSC 2010 calorimeter (TA Instruments) under 50 ml/min of air or N₂. 25 (±0.1) mg were placed in closed aluminium pans and 5 ºC/min heating ramps were used to reach the set temperature. Similar conditions but with open ceramic pans were used in the TA/DTA experiments (SDT Q600 thermobalance, TA Instruments).

True and apparent densities were measured as detailed elsewhere [28].Tap densities were manually measured in accordance with ISO 23145-1:2007 by introducing 60 g of sample into a 100 ml test tube and subjecting it to 1000 taps in 10 minutes. Whey powder particle size distributions (PSD) were determined in a LS 13 320 Laser Diffraction Particle Size Analyser (Beckman Coulter) equipped with a Tornado dry powder system. N₂ adsorption isotherms (-196 ºC) and Hg intrusion characterisation are also described in previous works [28,44]. SEM images were obtained in a a Quanta FEG 650 microscope. Metal (Ir) coating was only required for the characterisation of sintered whey powder pieces. Compression tests of the porous carbons were carried out in a SMT1-100N device (MTS System Corporation), using a cell load of 5 kN and a test velocity of 0.5 mm/min, according to the ASTM C1424-15 Standard. The SLSed whey specimens were bulk cylinders made of 165 sintered layers. After carbonisation (850 ºC, 1.5 h) the diameter of each porous carbon cylinder (6.4 ± 0.1 mm) was the average of three measurements along the cylinder length. Length of the cylinders was 12.7 ± 0.1 mm. Porous carbon cylinders of similar dimensions made by whey powder moulding were also tested. The compressive strength value was obtained from the maximum load registered in the test. Elastic compressive modulus (E) was calculated from the slope of the linear elastic region. Compressive strengths and moduli reported are the average of three compressive tests ± standard deviation.

Acknowledgements

The authors acknowledge the financial support received from the Ministerio de Ciencia e Innovación (MCIN/AEI/10.13039/501100011033, Project PID2020-115334GB-I00) and Principado de Asturias (FICYT)-European Union (FEDER) (Project PCTI-Asturias IDI/2021/000015). J. Angel Menéndez and Miguel Montes are members of the PTI+SusPlast of the Spanish National Research Council (CSIC).

Data Availability

All data generated or analysed during this study are included in this published article [and its supplementary information files].

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Llamas-Unzueta, Raúl, Alejandro Reguera-García, Miguel A. Montes-Morán, and J. Angel Menéndez « Selective laser sintering of whey powder for producing porous carbon artifacts with complex geometries and high printing precision: Videos and STL files». http://hdl.handle.net/10261/356996.

Table 1 is available in the Supplementary Files section.

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Porous carbons with complex 3D geometries via selective laser sintering of whey powder

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