# Ultralow UV absorber content in 3D printed nanocomposites: maximizing printability and microwave absorption efficiency

David Tilve-Martínez, Wilfrid Neri, Jeanne Lessire, Benjamin Dulucq, Nicolas Vukadinovic, Benoit Berton, Jinkai Yuan, Philippe Poulin

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## To cite this version:

David Tilve-Martínez, Wilfrid Neri, Jeanne Lessire, Benjamin Dulucq, Nicolas Vukadinovic, et al., Ultralow UV absorber content in 3D printed nanocomposites: maximizing printability and microwave absorption efficiency. Journal of Physics: Materials, 2025, 8 (1), pp.015010. 10.1088/2515-7639/ada82a hal-05125502

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# Ultralow UV absorber content in 3D printed nanocomposites: maximizing printability and microwave absorption efficiency

To cite this article: David Tilve-Martínez et al 2025 *J. Phys. Mater.* 8 015010

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# Ultralow UV absorber content in 3D printed nanocomposites: maximizing printability and microwave absorption efficiency

David Tilve-Martínez<fnref n="2" /> ID, Wilfrid Neri<fnref n="1" />, Jeanne Lessire<fnref n="1" />, Benjamin Dulucq<fnref n="3" />, Nicolas Vukadinovic<fnref n="4" />, Benoit Berton<fnref n="4" />, Jinkai Yuan<fnref n="1" /> ID and Philippe Poulin<fnref n="1" />,* ID

E-mail: philippe.poulin@crpp.cnrs.fr

**Keywords:** microwave absorption, VAT photopolymerization, nanocomposite, graphene oxide, carbon nanotubes

Supplementary material for this article is available [online](#)

## Abstract

The increasing prevalence of electromagnetic wave exposure in our daily life, particularly within the 100 MHz–300 GHz range, necessitates advancements in microwave absorption materials. This study explores the utilization of 3D printing and VAT photopolymerization to optimize material properties for efficient microwave absorption. While electrically conductive nanocomposites comprising dielectric matrices and conductive fillers have shown promise, their 3D printability poses challenges particularly because of strong UV absorption by conductive fillers. This work addresses this challenge by employing weakly UV absorbing graphene oxide (GO) as a functional surfactant to stabilize single-walled carbon nanotubes (SWCNTs) in an acrylic polymer matrix. The GO particles adsorb at the SWCNT interface. GO remains insulating until *in-situ* thermal reduction to reduced GO (rGO). After reduction, rGO at the SWCNT interface minimize electrical contact resistance between nanotubes, promoting thereby high conductivity of the nanotube network. The high aspect ratio and conductivity of SWCNTs, combined with the transparency and amphiphilic nature of GO, result in nanocomposites with enhanced electrical conductivity and minimal UV absorption. This allows for the 3D printing of conductive formulations with SWCNT contents as low as 0.03 wt%. This ultralow UV absorber content ensures excellent printability, with maximum cure depths exceeding 100 µm within seconds of UV irradiation. Moreover, the resulting nanocomposites exhibit promising microwave absorption properties in the S and Ku bands (2–4 and 12–18 GHz, respectively). Their reflexion losses, are below –10 dB over a 2.5 GHz bandwidth for a 4.75 mm thick layer. Emphasis is placed on the remarkable printability achieved in this study, as the microwave absorption properties remain unoptimized for specific applications.

## 1. Introduction

Nowadays, exposure to electromagnetic waves (EM), notably between 100 MHz and 300 GHz, is a significant part of our daily life [1–3]. This exposure is primarily associated with the widespread use of mobile phones, geolocation services, Wi-Fi networks, and RADAR waves (Radio Detection And Ranging). Effectiveness of microwave absorbers is quantified by the frequency bandwidth of high absorption. Absorption is generally considered substantial when the so-called reflexion losses (RL) fall below –10 dB. There are mainly two ways to optimize microwave absorbers: the geometric design of components relative to the incidence of radar waves, involving factors like the angle and surface of incidence [4], and the manipulation of the properties of materials that interact with EM [5, 6].

3D printing and VAT photopolymerization of electrically conductive nanocomposites could address the above two objectives [7–11]. On one hand, nanocomposite materials formed by a dielectric matrix and a

<footnote n="1">¹ University Bordeaux, CNRS, CRPP, UMR5031, 115 Avenue Dr Albert Schweitzer, Pessac 33600, France</footnote>

<footnote n="2">² IMDEA Materials Institute, C/Eric Kandel 2, Getafe, 28906 Madrid, Spain</footnote>

<footnote n="3">³ Dassault Aviation, 54 Av. Marcel Dassault, Mérignac 33700, France</footnote>

<footnote n="4">⁴ Dassault Aviation, 78 quai Marcel Dassault, Saint-Cloud 98552, France</footnote>

<footnote>* Author to whom any correspondence should be addressed.</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 1.** Scheme of the formulation and 3D printing of GO/SWCNT hybrid composite. After thermal annealing, the nanocomposite effectively absorbs EM microwaves in the S and Ku bands.</figcaption></figure>

conductive filler [12–15] have aroused great interest as microwave absorption materials (MAMs) because they combine lightness, corrosion resistance, high dielectric and conduction losses. On the other hand, digital light processing (DLP) [16–18] allows rapid additive manufacturing of complex geometries with high resolution. Furthermore, the precursor resin used by the DLP technique is in a liquid state, facilitating the loading of conductive fillers into the matrix [19–22].

However, filler loading comes with its own set of drawbacks. It not only adds to the composite's weight but also renders it more brittle. More critically, magnetic and conductive fillers tend to absorb and scatter UV light, which poses challenges for 3D printing using photopolymerization. This weakness is actually the main limitation to fast and high-resolution 3D printing of conductive nanocomposites by photopolymerization. Finally, incorporating large quantities of fillers can lead to a notable rise in resin viscosity, ultimately making the 3D printing process considerably more difficult, if not impossible.

The development of conductive nanocomposites with low content of UV absorbing materials is therefore a key objective for the efficient implementation of photopolymerization in 3D printing of MAM. Progresses in this direction have been recently achieved, in particular using light and anisotropic conductive particles such as graphene or carbon nanotubes. These anisotropic particles display a low percolation threshold and can render polymers conductive at low carbon content.

Zuo et al [19, 20] mixed magnetic oxide nanoparticles and graphene nanoplatelets in an acrylic resin, achieving single-layer absorbers in X band with RL below −10 dB over a narrow bandwidth of 2 GHz, for a 2.1 mm thick material. Microwave absorption results from the combination of magnetic and dielectric losses in such materials. However, this approach necessitates formulating highly dense magnetic particles. Meanwhile, Zhang et al [21] dispersed multi-walled carbon nanotubes (MWCNTs) in an acrylic resin with loading up to 1.5 wt.% to attain a 1 GHz bandwidth efficient absorption for a 6 mm thick layer in the Ku band. Xiao et al [22] successfully functionalized SiC nanowires and dispersed them in an acrylic matrix. By adding 2–4 wt.% of filler, they produced composites with high conduction and polarization losses in the 4–18 GHz range. RL below −10 dB were obtained over a bandwidth of 4 GHz for 2 mm thick materials. It is worth noting that SiC is 1.5–2 times denser than carbon nanoparticles.

In order to circumvent these limitations (figure 1), we presently use graphene oxide (GO) as a functional surfactant to stabilize single walled carbon nanotubes (SWCNTs) in an acrylic polymer matrix. Such nanotubes display a high conductivity and a very low percolation threshold because of their high aspect ratio, generally much greater than that of MWCNTs. The pristine GO is insulating nanosheet that displays an amphiphilic character, enabling it to effectively adsorb SWCNTs at its surfaces and establish a strong affinity with the acrylic medium. It allows for maintaining the particles optimally dispersed during 3D printing. Importantly, GO does not strongly absorb UV light. It is therefore an additive that does not impede high printability.

Furthermore, by contrast to conventional surfactant molecules, GO can be in-situ thermally reduced into reduced GO (rGO) once the matrix is cross-linked rGO is electrically conductive and expected to provide a much lower contact resistance compared to conventional surfactants confined at the interfaces of SWCNTs.

Our present approach originally combines the high aspect ratio and high conductivity of SWCNTs with the transparency and amphiphilic character of GO to maximize electrical conductivity while keeping the amount of UV-absorbing species as low as possible. After printing and GO reduction, nanocomposites with conductivities above 0.1 S.m⁻¹ are achieved with SWCNT weight fractions of only 0.036 wt% SWCNTs, the sole UV absorber in our 3D printing formulations. This amount is one to two orders of magnitude smaller
than that reported in similar formulations [19–23]. Because of this ultralow content of the UV absorber, the present formulations display excellent printability with a maximum cure depth that far exceeds 100 µm for UV irradiation of a few seconds. These formulations also show promising microwave absorption properties in the so-called S and Ku bands, in frequency ranges of respectively 2–4 GHz and 12–18 GHz. For example, a 4.75 mm thick layer of the most conductive materials investigated in this work displays RL below −10 dB over a bandwidth of 2.5 GHz in the Ku band. Anyway, it is primarily the very high printability that should be emphasized, as the microwave absorption properties have not been optimized or tailored for a specific application here.

## 2. Results and discussion

### 2.1. Rheological and optical properties of GO/SWCNT resins

The stabilization of particles in a matrix is crucial for 3D printing of nanocomposites. As demonstrated in previous work, pre-suspending GO in ethanol (EtOH) easily allows for the homogenization of GO monolayers in an acrylic matrix [24–26]. We took advantage of GO stability to disperse SWCNTs using tip sonication. Figure S1 shows optical micrographs of the resultant suspensions. A dark texture emerges, indicating the presence of large but light SWCNT aggregates. These aggregates coexist with a uniform nanotube phase of individualized nanotubes, appearing as a lighter background in the micrographs. This coexistence results from weak attractive interactions between the nanotubes [27]. These weak interactions presumably resulting from an excess of GO in the solution are known as depletion interactions. They have been already reported in nanotube-based suspensions [27]. They play here a positive role to promote clustering and lower the percolation threshold [28]. The depletion induced aggregates, by contrast to dense clusters, are light and do not sediment during the printing process.

Optical and rheological characterizations are important to assess the 3D printability of the composite resins. Adding GO to the resin changes the rheological behavior from that of a Newtonian fluid to that of a shear-thinning one. This effect intensifies when SWCNTs are added. SWCNTs at three different weight fractions have been added to the dispersions in this study: 0.01, 0.026 and 0.036 wt.%. The recommended viscosity range for ceramics in DLP is generally 3–5 Pa.s at a shear rate of 10 s⁻¹ [29–31]. For MWCNT resins [32–34], it is advised to keep viscosity under 10 Pa.s at a shear rate of 1 s⁻¹. Our GO and SWCNT based formulations (figure 2(a)) slightly exceed these values. Nonetheless, they remain close and the formulations are still 3D printable.

We have also studied the attenuation of UV reticulation caused by the addition of GO and SWCNTs to the resin. This attenuation can be quantified by the so-called Jacobs working curves [16, 35, 36]. In this model, C<sub>d</sub>, the maximum cure depth (µm), is plotted as a function of E<sub>max</sub> (mJ cm<sup>-2</sup>), the UV irradiation energy. The data are fitted according to equation (1),

$$C_d = \frac{1}{\alpha} \ln \left( \frac{E_{\max}}{E_c} \right) \quad (1)$$

where α is the attenuation coefficient (mm⁻¹) and E<sub>c</sub> is the critical energy required to initiate polymerization (mJ cm<sup>-2</sup>). We observe that the attenuation is nearly doubled with the addition of 0.2 wt.% GO to the neat resin (figure 2(b)). Moreover, even slight increases in the amount of SWCNTs have a strong impact on the attenuation compared to GO. However, these formulations remain sufficiently transparent for efficient and high-resolution 3D printing. For instance, with 0.2 wt.% GO/0.036 wt.% SWCNTs and an exposure time of 4 s, a C<sub>d</sub> of over 100 µm is achieved.

### 2.2. Electrical conductivity

As detailed in the Experimental section, DC electrical conductivity of the samples is measured both parallel and perpendicular to the printed layers. Results as function of the total weight fraction of fillers are shown in figure 3. As previously observed in other nanocomposites [37, 38], a clear conductivity anisotropy is observed. This anisotropy results from the contact resistances between printed layers [38].

The weight fraction of conductive species is the sum of the respective weight fractions of 0.2 wt.% GO and SWCNTs, resulting in total values of 0.210, 0.226 and 0.236 wt.%. The composites of GO and SWCNTs show a percolation type behavior with a strong increase of conductivity obtained for a small concentration of SWCNTs. We will use in the following the most conductive samples that contain a weight fraction of conductive species of 0.236 wt%. These samples are indeed expected to display the greater conductive and polarization losses for enhanced microwave absorption.
<figure><img src="image_8.png" /><figcaption>Figure 2. (a) Rheological properties (viscosity vs shear rate) of the neat acrylate resin, and resin containing 0.2 wt.% of GO and different weight fractions of SWCNTs. (b) Jacobs working curves for the neat acrylate resin, resin containing 0.2 wt.% of GO and different weight fractions of SWCNTs for a 1.27 mW cm⁻² UV irradiation at 405 nm. Note that T stands for SWCNTs in the inset indicating the weight fractions.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Figure 3. Electrical conductivity, parallel to the layers (grey) and perpendicular to the layers (orange), as a function of the total weight fraction of GO and SWCNTs for nanocomposites that have been thermally annealed at 200 °C for 2 h. Bold lines are a guide to the eye.</figcaption></figure>

## 2.3. Dielectric properties

The present materials are diamagnetic, so we just have to consider their dielectric permittivity to assess their microwave absorption potential. The complex permittivity is given by:

$$ \epsilon = \epsilon_r \epsilon_0 = \epsilon' - j\epsilon'' \quad (2) $$

where $\epsilon_0$ is the vacuum permittivity, $\epsilon_r$ is the complex relative permittivity of the material. The imaginary part of permittivity reflects the combined effects of losses attributed to bond charges and conduction associated to free charges. The absorbing character of a material can be evaluated by the so-called loss tangent ($\tan \delta$) which is given by the ratio of dissipated energy to the stored energy:

$$ \tan \delta = \frac{\epsilon'''}{\epsilon''}. \quad (3) $$

Samples at the APC7 [39] format were fabricated using 3D printing, and thermal annealing at 200 °C for 2 h. As previously mentioned, for optimal measurement of dielectric properties, the samples were
<figure><img src="image_10.png" /><figcaption>**Figure 4.** Dielectric properties of printed GO/SWCNT composites thermally annealed at 200 °C for 2 h: (a) real permittivity (bold squares) and imaginary permittivity (empty squares); (b) loss tangent (bold circles) versus frequency in the S band. Note that T stands for SWCNTs in weight fraction.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 5.** Dielectric characterization of 0.2 wt.% GO/0.036 wt.% SWCNT nanocomposite in the Ku band: (a) real permittivity (solid blue line), imaginary permittivity (dashed blue line), and loss tangent (yellow); (b) ohmic (dashed blue line) and polarization (solid blue line) losses normalized by the dielectric losses.</figcaption></figure>

manufactured with layers perpendicular to the normal axis of the cylinder (figure S4). The APC7 samples have been used to characterize dielectric properties in the 2–4 GHz (S band) range (figure 4).

The real and imaginary parts of the relative permittivity (ε') are respectively 3 and 0.05 for the neat industrial blend (IB) resin. The resultant loss tangent is 0.015. These values are characteristic of an insulating polymer, indicating that EM waves can easily penetrate the material without significant losses. In other words, the resin exhibits transparent behavior to EM waves.

Addition of 0.2 wt.% of GO and small amounts of SWCNTs has a noticeable impact on the dielectric properties. The real and imaginary permittivity gradually increase with the weight fraction of SWCNTs tan δ also increases to 0.30, 0.35, and 0.40 at 3 GHz.

The sample containing 0.2 wt.% GO and 0.036 wt% SWCNTs displays the higher dielectric losses. This material was shaped into a 15.80 × 7.90 × 1 mm³ volume for further characterization using a wave-guide in the Ku-band from 12 to 18 GHz. The results are shown in figure 5 for this nanocomposite. The dielectric properties in the Ku-band are constant in this frequency range, and tan δ is about 0.35.

For comparisons with other nanocomposites 3D printed by DLP/Stereolithography, Zhang *et al* [21] have dispersed MWCNTs in an acrylic matrix. The obtained nanocomposites have a tan δ of 0.14 between 4–18 GHz with a weight fraction of 1.5 wt.% of MWCNTs. Losses are lower than that of the present nanocomposite, despite their 42 times greater weight fraction of SWCNTs. Besides, Xiao *et al* [22] have stabilized functionalized SiC nanowires in a commercial acrylic resin with a weight fraction from 2 to 4 wt.%. They obtained tan δ values ranging from 0.35 to 0.55 between 4–18 GHz. This positive outcome
makes the approach very interesting for MAM development. However, for applications in the aerospace sector where density is crucial, SiC is twice denser than SWCNTs. More critically, regarding printability and resolution, the amount of SiC in such materials is up to two orders of magnitude greater than the amount of SWCNTs currently used.

The synergy between GO and SWCNTs can be observed by comparing the mechanisms of dielectric losses that come into play in the composite compared to separate GO and SWCNTs (figure S5). The dielectric losses ($\epsilon''$) are the sum of losses due to polarization ($\epsilon''_p$) and losses due to ohmic conduction ($\epsilon''_c$). These can be represented as follows:

$$ \epsilon'' = \epsilon''_p + \epsilon''_c = \epsilon''_p + \frac{\sigma_{DC}}{2\pi f \epsilon_0} \quad (4) $$

where $\sigma_{DC}$ represents direct current electrical conductivity and f stands for frequency. Imaginary permittivities of the GO-SWCNT composites are shown in figure 5(b) dissected into bound charges and polarization-related on one side and free charges and ohmic losses on the other. Despite the high electrical conductivity (3.1 ± 0.7) 10⁻¹ S m⁻¹, ohmic losses alone do not explain the high imaginary permittivity. The contribution of bound charges proves to be more significant. This phenomenon can likely be explained by the strong interfacial polarization of the GO and SWCNT particles within the matrix [40, 41].

## 2.4. Microwave absorption properties

An effective microwave absorber material should be capable of absorbing over 90% of EM waves across a wide frequency range. Absorption is quantified by RL. More than 90% EM energy absorption corresponds to RL ≤ −10 dB. According to the transmit-line theory [42–44], and for normal incidence, the RL of a single-layer material next to a metal plane is given by:

$$ \text{RL (dB)} = 20 \log_{10} \left| \frac{Z_{\text{in}} - Z_0}{Z_{\text{in}} + Z_0} \right| \quad (5) $$

$$ Z_{\text{in}} = Z_0 \sqrt{\frac{1}{\varepsilon_r} \tanh \left( j \frac{2\pi f d}{c} \sqrt{\varepsilon_r} \right)} \quad (6) $$

where $Z_{\text{in}}$ is the input impedance of the material, $Z_0$ is the impedance of the free space, d is the thickness of the material, c is the speed of light. $Z_{\text{in}}$ should be as close as possible to that of air to ensure low reflection and high absorption of the EM wave.

Figure 6 illustrates the results for the material with high dielectric losses, showcasing the dependence of RL on frequency and thickness. Color maps to represent RL values in relation to frequency (f) and thickness (d) have been generated by using equations (5) and (6). In the S band (figure 6(a)), the first absorption band is observed at 3.6 GHz for a 5 mm thickness. As indicated in figures 6(b) and (a) maximum absorption (RLmax) of −40 dB (equivalent to 99.99% of electromagnetic energy absorption) is achieved with a 7.52 mm thickness at 2.28 GHz. Besides, the bandwidth with RL values below −10 dB define as Δf−10 dB, spans 0.5 GHz and lies between thicknesses of 5.5 and 8 mm. Moving to the Ku band (figure 6(c)), the RL color map clearly exhibits three absorption bands with different curvature in the (frequency, thickness) plane. The intermediate band displays the larger absorption in absolute value in dB. For this one, the initial absorption band is located at 4.1 mm for a frequency of 18 GHz. The RLmax values (figure 6(d)) are larger compared to the S band results, yet they exhibit absorption bands within 4.5–6.5 mm in thickness. Although the absorption is less pronounced, the bandwidth is notably wider, Δf−10 dB covers a range of 2.5 GHz at a thickness of 4.75 mm.

As shown in figures 6(a) and (c), the absorption bands shift towards lower frequencies as the thickness increases, This can be explained by the 1/4 wavelength cancellation equation (6):

$$ d_m = \frac{nc}{4f_m \sqrt{\varepsilon_r}}; (n = 1, 3, 5 \dots) \quad (7) $$

where $d_m$ is the matching thickness, $f_m$ is the matching frequency, which gives information of how the absorption band shifts, c is the speed of light, $\varepsilon_r$ is the relative permittivity and n is an odd natural number. Figure 7 shows the evolution of the matching thickness and the matching frequency of the S-band and Ku-band. In both cases, the thickness decreases with frequency. This behavior is consistent with the absorption band shifting in the color maps of figure 6. Figure 7 shows the RLmax calculated previously (stars) that matches with the 1/4 wavelength cancellation equation (7) (solid line). This indicates that that the EM wave will be attenuated by the composite by microwave absorption or reflected backward with the opposite phase.
<figure><img src="image_12.png" /><figcaption>**Figure 6.** Color maps of the reflection loss versus the thickness and the frequency for a 0.2 wt.% GO/0.036 wt.% SWCNTs material (a) S-band and (c) Ku-band. Maximum reflection loss ($RL_{max}$) and frequency bandwidth with a reflexion loss below -10 dB ($\Delta f_{-10\,\text{dB}}$) for a 0.2 wt.% GO/0.036 wt.% SWCNTs material in the (b) S-band and (d) Ku-band.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 7.** 1/4 wavelength cancelation with the experimental thickness obtained from $RL_{max}$ (stars) for a 0.2 wt.% GO/0.036 wt.% SWCNTs at the (a) S-band and (b) Ku-band.</figcaption></figure>

### 3. Conclusion

GO and SWCNTs have been dispersed successfully in 3D printable acrylic resins. Carbon nanotubes are stabilized by GO flakes adsorbed at their interface. GO is a weak UV absorber and does not hinder 3D printing by photopolymerization. Neat GO is insulating. However, it becomes conductive after thermal reduction into rGO promotes a lowering of the contact resistance between nanotubes. As a consequence, the conductivity of the whole nanotube network is enhanced by the present approach. The resultant materials display conductivity values up to (3.1 ± 0.7) $10^{-8}$ S m$^{-1}$, a value that makes the present materials promising for microwave absorption. To validate this potential, the dielectric properties of composites manufactured by DLP within the S and Ku bands have been characterized. These characterizations have been used to simulate single-layer S-band and Ku-band absorber screens, confirming a significant potential for microwave
absorption. The present nanocomposites utilize a conductive material weight fraction below 0.3%, a lower amount than in previously related nanocomposites. This minimal concentration enables rapid 3D printing with high resolution. Consequently, these materials show strong potential for applications in electronics, particularly in EM absorption and shielding of sensitive components. Further enhancements are likely achievable by incorporating magnetic particles into the resins, a method known to increase microwave absorption through magnetic loss [45]. Current work is in progress to validate this perspective.

Although the current cost of high-quality SWCNTs, similar to those used here, remains high—ranging from $2000 to $100 000 per kilogram [46]—the economic feasibility of this approach may still be viable. SWCNT costs have decreased substantially since their introduction and are expected to decline further. Moreover, the target applications are high-value, and the nanotube content required is minimal. Assuming a cost of $5000 per kilogram and a maximum concentration of 0.036 wt% as used in this study, the SWCNT cost would be only $1.80 per kilogram of resin—an acceptable level for electronic applications. Additionally, the cost of GO is manageable, as it can now be produced cost-effectively and at scale from natural graphite.

# 4. Experimental section

## 4.1. Materials

Aqueous GO solutions at 0.4 wt.% and 2.5 wt.% concentrations were obtained from Graphenea (Spain). Tuball SWCNTs powder was sourced from OCSiAl (Luxembourg). According to the product datasheet, these SWCNTs have an average diameter of 1.6 nm and an average length of approximately 5 µm. The resin used for 3D printing was an IB resin, supplied by FunTodo (Netherlands). This resin consists of various acrylic and/or methacrylic monomers and oligomers, though its exact formulation is proprietary. The manufacturer's website indicates that the resin remains dimensionally stable and functional within a temperature range of -45 °C-225 °C. It has a density of 1.05 g cm⁻³ and a UV curing wavelength range from 225 to 415 nm, with a viscosity of 95 mPa·s at 25 °C. EtOH and isopropanol (IPA) were obtained from Carlo Erba Reagents and were used without further purification.

## 4.2. Formulation

The GO resin dispersion is followed as the procedure explained elsewhere [24]: EtOH is added to the 2.5 wt.% GO solution to dilute it down to a 0.4 wt.% concentration. The resulting solution is homogenized via tip sonication using a Branson digital sonifier 450. Sonication is performed with a 13 mm exponential horn attachment for 30 min at 25% amplitude, with pulses set to 0.5 s on and 0.2 s off. After sonication, the solution is centrifuged at 30 000 g for 30 min in an ultracentrifuge (Beckman Coulter). This process yields a concentrated GO slurry at 2.40 wt.% with high monolayer content (as measured by dry extract) in an EtOH-rich medium (85 wt.% EtOH). EtOH, unlike pure water, is soluble in the IB resin, making it an effective transfer medium for directly dispersing GO within the resin, eliminating the need for drying. For this, the slurry is directly mixed with the resin in a light-blocking container. The mixture is homogenized by sonication with a 3.2 mm microtip for 30 min at an amplitude of 30%, with 0.5-second on pulses and 0.2-second off pulses. The GO-CNT hybrid composites were prepared as follows: first, GO EtOH rich solutions at 2.4 wt.% is prepared. Then, it is added directly to the IB resin at 0.2 wt.% of GO and homogenized by tip sonication using a Branson digital sonifier 450 for 30 min, amplitude set to 40% with pulses of 0.5 s on, and 0.2 s off. Once GO is dispersed, the SWCNTs are added by also using a Branson digital sonifier 450 with a sonication tip tapped exponential horn 13 mm for 45 min, amplitude set to 40% with pulses of 0.5 s on, and 0.2 s off.

## 4.3. 3D printing

A Phrozen Sonic Mini 4k DLP 3D printer, operating at 405 nm UV wavelength, is used for printing. Characterization of the Jacobs working curves, as described below, guided the selection of a 100 µm layer thickness for printing. The UV exposure time varies depending on the weight fraction of GO/SWCNTs. The platform's lifting and retraction speed is set at 60 mm min⁻¹, with a lift height of 5 mm. After printing, the GO/SWCNT composite is immersed in an IPA bath for 15 min to remove any unreacted resin. The part is then placed in a UV curing chamber (UltraV360) at 405 nm for 1 h to finalize the resin cross-linking. Following UV curing, the 3D-printed GO/SWCNT samples undergo thermal annealing in air at 200 °C for 2 h.

In order to obtain the maximum conductivity containing a total weight fraction of 0.236 wt% is 2.60 10⁻² S m⁻¹. After the thermal annealing, the conductive network is highly improved due to the combination of two phenomena. Firstly, the GO is reduced into rGO [24, 47, 48], transforming it into conductive material. Secondly, the matrix is heated above the $T_g$ and exhibits enhanced chain mobility. This allows the
reorganization of the nanotubes that can approach more intimately in response to strong van der Waals interactions [49–53].

## 4.4. Characterization

The distribution of GO/CNTs within the matrix was qualitatively examined through optical microscopy (figure S1) and TEM imaging (figure S3). The thermal stability of the resin was assessed using thermogravimetric analysis (TGA) with a TA TGA 5500 system. Approximately 5 mg of sample were placed in an aluminum pan, then heated from room temperature to 200 °C, where an isothermal hold at 200 °C was maintained for 120 min followed by a ramp of 300 °C and hold for 2 h. Finally, the sample is cooled down to room temperature. All experiments were conducted under airflow.

Jacobs working curves were established by irradiating 2 × 2 mm² square patterns as explained in previous works [24, 38]. The output power of the LCD 3D printer was measured with a Thorlab PM1000A power meter and determined to be 1.27 mW cm⁻². Non-crosslinked ink was rinsed away using 2-propanol following UV exposure, and a caliper was used to measure the thickness of the residual polymerized resin.

Rheological properties were measured at 25 °C with a controlled strain rheometer (TA AR2000), using a 40 mm diameter, 2° cone-plate geometry. Rheological testing occurred within the linear viscoelastic range as identified from steady-state flow experiments, with frequencies spanning 0.1–100 s⁻¹.

The electrical conductivity of the composites was evaluated using a two-point probe method with a 7260 Impedance Analyzer (Materials Mates Italia), applying a voltage of 5 V across a frequency range from 1 to 10⁶ Hz. Five 3D-printed cubes, each measuring 2 × 2 × 2 mm³, were tested. Silver paint was applied to ensure effective electrical contact between the samples and electrodes.

The dielectric and electromagnetic properties of nanocomposites manufactured by DLP in the hyperfrequency range were measured using a microwave network analyzer PNA Agilent E8362B. Microwave scattering parameters (S) have been obtained with two different waveguides. For S-band measurements, we used an APC7 coaxial connector and a cylindrical toroidal sample with outer diameter 7.00 mm, inner diameter 3.00 mm, and 1 mm thickness. In the Ku-band, we used a rectangular waveguide with dimensions 15.8 × 7.90 × 1.00 mm.

## Data availability statement

All data that support the findings of this study are included within the article (and any supplementary files).

## Acknowledgment

Funding from the AID (Agence Innovation Défense) and the Nouvelle Aquitaine Region are acknowledged.

## ORCID iDs

David Tilve-Martínez https://orcid.org/0000-0002-5550-4631

Jinkai Yuan https://orcid.org/0000-0003-2347-5191

Philippe Poulin https://orcid.org/0000-0001-7748-8671

## References

[1] Chung D D L 2020 *Mater. Chem. Phys.* 255 123587

[2] Zeranska K, Filak K, Wilczyński K, Siemion A, Palka N, Godziszewski K, Yashchyshyn Y and Zdrojek M 2022 *ACS Appl. Electron. Mater.* 4 4463

[3] Paton K R and Windle A H 2008 *Carbon* 46 1935

[4] Ahmad H et al 2019 *Polym. Compos.* 40 4457

[5] Green M and Chen X 2019 *J. Mater.* 5 503

[6] Qin F and Brosseau C 2012 *J. Appl. Phys.* 111 061301

[7] Gong P, Hao L, Li Y, Li Z and Xiong W 2021 *Carbon* 185 272

[8] Lee K P M, Baum T, Shanks R and Daver F 2021 *Addit. Manuf.* 43 102020

[9] Lim D D, Park J, Lee J, Noh D, Lee J, Choi J and Choi W 2022 *Addit. Manuf.* 55 102856

[10] Meisak D, Gurnevich E, Plyushch A, Bychanok D, Georgiev V, Kotsilkova R and Kuzhir P 2020 *J. Phys. D: Appl. Phys.* 53 305301

[11] Yao L, Yang W, Zhou S, Mei H, Cheng L and Zhang L 2022 *Carbon* 196 961

[12] Abbasi H, Antunes M and Velasco J I 2019 *Prog. Mater. Sci.* 103 319

[13] Bagotia N, Choudhary V and Sharma D K 2018 *Polym. Adv. Technol.* 29 1547

[14] Wu N, Hu Q, Wei R, Mai X, Naik N, Pan D, Guo Z and Shi Z 2021 *Carbon* 176 88

[15] Huo J, Wang L and Yu H 2009 *J. Mater. Sci.* 44 3917

[16] Kodama H 1981 *Rev. Sci. Instrum.* 52 1770

[17] Farahani R D, Dubé M and Therriault D 2016 *Adv. Mater.* 28 5794

[18] Truby R L and Lewis J A 2016 *Nature* 540 371
[19] Zuo Y, Yao Z, Lin H, Zhou J, Lu J and Ding J 2019 *Composites* B 179 107533

[20] Zuo Y, Su X, Li X, Yao Z, Yu T, Zhou J, Li J, Lu J and Ding J 2020 *Carbon* 167 62

[21] Zhang Y, Li H, Yang X, Zhang T, Zhu K, Si W, Liu Z and Sun H 2018 *Polym. Compos.* 39 E671

[22] Xiao S, Mei H, Han D and Cheng L 2019 *Ceram. Int.* 45 11475

[23] Xin C, Zhang J, Peijs T and Li Y 2022 *Mater. Today Commun.* 31 103865

[24] Tilve-Martinez D, Neri W, Horaud D, Vukadinovic N, Berton B, Desmedt A, Yuan J and Poulin P 2023 *Adv. Funct. Mater.* 33 2214954

[25] Liu Y, Xu Y, Yang M, Hamon A-L, Haghi-Ashtiani P, He D and Bai J 2018 *Chem. Phys. Lett.* 713 189

[26] Liu Y, Hamon A-L, Fan B, He D, Haghi-Ashtiani P, Reiss T and Bai J 2018 *J. Appl. Polym. Sci.* 135 46748

[27] Vigolo B 2005 *Science* 309 920

[28] Kyrylyuk A V, Hermant M C, Schilling T, Klumperman B, Koning C E and van der Schoot P 2011 *Nat. Nanotechnol.* 6 364

[29] Griffith M L and Halloran J W 1996 *J. Am. Ceram. Soc.* 79 2061

[30] Hinczewski C, Corbel S and Chartier T 1998 *J. Eur. Ceram. Soc.* 18 583

[31] Zakeri S, Vippola M and Levänen E 2020 *Addit. Manuf.* 35 101177

[32] Cortés A, Cosola A, Sangermano M, Campo M, González Prolongo S, Pirri C F, Jiménez-Suárez A and Chiappone A 2021 *Adv. Funct. Mater.* 31 2106774

[33] Gonzalez G, Chiappone A, Roppolo I, Fantino E, Bertana V, Perrucci F, Scaltrito L, Pirri F and Sangermano M 2017 *Polymer* 109 246

[34] Mu Q, Wang L, Dunn C K, Kuang X, Duan F, Zhang Z, Qi H J and Wang T 2017 *Addit. Manuf.* 18 74

[35] Bennett J 2017 *Addit. Manuf.* 18 203

[36] Jacobs P F 1992 *Rapid Prototyping & Manufacturing: Fundamentals of Stereolithography* (Society Of Manufacturing Engineers)

[37] Mendes-Felipe C, Cofano R, Garcia A, Sangermano M and Lanceros-Mendez S 2023 *Addit. Manuf.* 78 103867

[38] Tilve-Martinez D, Neri W, Vukadinovic N, Berton B, Pénicaud A, Yuan J and Poulin P 2024 *Addit. Manuf.* 86 104221

[39] Adous M, Quéffélec P and Laguerre L 2006 *Meas. Sci. Technol.* 17 2241

[40] Liu Y, He D, Dubrunfaut O, Zhang A, Zhang H, Pichon L and Bai J 2020 *Compos. Sci. Technol.* 200 108450

[41] Liao X, Ye W, Chen L, Jiang S, Wang G, Zhang L and Hou H 2017 *Composites A* 101 50

[42] Bai X, Zhai Y and Zhang Y 2011 *J. Phys. Chem. C* 115 11673

[43] Gao X, Wang X, Cai J, Zhang Y, Zhang J, Bi S and Hou Z-L 2023 *Carbon* 211 118083

[44] Micheli D, Vricella A, Pastore R and Marchetti M 2014 *Carbon* 77 756

[45] Morrot-Woisard I, Nguyen E K, Vukadinovic N and Boero M 2024 *Carbon Trends* 17 100410

[46] Taylor L W, Dewey O S, Headrick R J, Komatsu N, Peraca N M, Wehmeyer G, Kono J and Pasquali M 2021 *Carbon* 171 689

[47] Stankovich S, Dikin D A, Dommett G H, Kohlhaas K M, Zimney E J, Stach E A, Piner R D, Nguyen S T and Ruoff R S 2006 *Nature* 442 282

[48] Pei S and Cheng H-M 2012 *Carbon* 50 3210

[49] Bartholome C, Miaudet P, Derre A, Maugey M, Roubeau O, Zakri C and Poulin P 2008 *Compos. Sci. Technol.* 68 2568

[50] Kilbride B E, Coleman J N, Fraysse J, Fournet P, Cadek M, Drury A, Hutzler S, Roth S and Blau W J 2002 *J. Appl. Phys.* 92 4024

[51] Kymakis E and Amaratunga G A J 2006 *J. Appl. Phys.* 99 084302

[52] Miaudet P, Badaire S, Maugey M, Derré A, Pichot V, Launois P, Poulin P and Zakri C 2005 *Nano Lett.* 5 2212

[53] Miaudet P, Bartholome C, Derre A, Maugey M, Sigaud G, Zakri C and Poulin P 2007 *Polymer* 48 4068