1. Introduction
In the last decade, additive manufacturing (i.e., AM), also known as 3D printing (i.e., 3DP), has drawn strong attention from academic as well as industrial fields since it allows easy, quick, and thorough production of three-dimensional complicated structures with a wide range of sizes, shapes, and functional properties [
1]. This technology favors rapid prototyping since it does not require any molds as for conventionally machined parts, thus, offering different advantages in terms of ease of use, mass customization, tangible product testing, reliability, and cost-effectiveness [
2].
Among the different additive manufacturing approaches, such as selective laser sintering, inkjet 3D printing, and solvent-cast 3DP (SC3DP), the fused deposition modeling (FDM) has pervaded most of the different fields given its simplicity [
3,
4]. In fact, the feedstock is generally a thermoplastic polymer filament which, after being heated above its glass transition temperature (i.e.,
Tg), is extruded by means of a PC-controlled moving nozzle. This process allows to form, layer by layer, the desired 3D structure [
5].
However, a serious limitation of this technology is the possibility to print electrically conductive parts, since the choice of matrices is still limited to insulating ones (in general polyester, acrylonitrile butadiene styrene-ABS, nylon, polyvinyl alcohol-PVA, polycarbonate, and poly(lactic) acid-PLA). More recently, this issue is being addressed with the introduction of nanotechnology in the field of additive manufacturing the inclusion of suitable nanofillers into printable resins [
6]. This combination between additive manufacturing and nanotechnology paves the way for the development of 3D printable materials exhibiting multifunctionality and customized properties, thus leading to an expansion of AM application areas [
7,
8].
Along this stream, the possibility of using poly(vinyl alcohol) PVA filled with ultra-dispersed diamonds as new materials for 3D printing is investigated in [
9] where the main factors conditioning the quality of the final printed parts together with material design and preparation are discussed. The electromagnetic interference (EMI) shielding of various 3D printed polymeric composite structures based on commercially available filaments is analyzed in the so-called C-band of the electromagnetic spectrum (3.5–7.0 GHz) since it is generally exploited for long-distance radio telecommunications [
10]. Moreover, given the possibility to print electrically conductive parts, in recent years, 3D printing technology has been adopted for the design of affordable and versatile antennas [
11,
12], also made with combined technologies [
13], or for the realization of flexible and stretchable microelectrodes for interconnections in electronic and optoelectronic devices [
14]. Additive manufacturing has also been widely adopted in the aerospace sector for the rapid prototyping and subsequent production of complex components otherwise requiring particular technological solutions and long design times [
15]. The EM properties of poly(lactic) acid (PLA) reinforced with graphite nanoplatelets (GNP) have been evaluated in the microwave (26–37 GHz) and terahertz (0.2–1 THz) frequency ranges, finding positive results for their use as absorptive materials in electromagnetic (EM) shielding applications for electronic devices [
16]. The interesting possibilities of printing electrically conductive structures by using a non-conventional filament made with CNT- and graphene-based polybutylene terephthalate (PBT) are investigated in [
17] with due attention also paid to the mechanical stability of the analyzed materials before and after 3D printing. Mechanical properties of 3D-printed polymer specimens are also explored in [
18] with a particular focus on identifying the major printing factor that influences them. Even the build orientation in the additive manufacturing process has been considered since it can affect the overall component quality especially in terms of precision and surface finish [
19]. Apart from the interest in the electrical and mechanical properties of thermoplastic filaments suitable for the 3D printing, there are potential heat transfer applications of 3D-printed structures. In Reference [
20], the effective thermal conductivity of different suitable 3D-printed filaments based on neat polymer and carbon-based polymer composite was measured with an ad-hoc created apparatus after its validation. The effects of the 3D printer parameter setting on the thermal conductivity of filaments made with acrylonitrile butadiene styrene (ABS) are investigated in [
21]. First evaluations of the thermal conductivity and mechanical properties for some selected commercial 3D material samples are presented in [
22] in order to consider their potential applications in the cryogenic temperature regime.
Although several interesting results have been achieved with an intensive research work, different issues still remain to be solved for printing electrically conductive nanocomposites. In order to really benefit from the advantages due to the combination of AM and nanotechnology, it is necessary to enhance the knowledge of the different available nanomaterials and resultant nanocomposites in terms of general properties and manufacturing processes, as well as identifying the best suited AM techniques to realize the desired structures [
23]. Taking advantage of the broad variety of materials requires the development of specific implementations of 3DP. The most relevant ones available in the literature are summarized in [
24]. However, there are still uncertainties on the correct design parameters set, manufacturing guidelines, and reproducibility of achieved results. Moreover, further experimental characterizations are needed to enrich the knowledge in this field.
In [
25], two different manufacturing processes, i.e., solution blending and melt extrusion, have been compared in terms of rheological and electrical properties of the resulting materials (up to 6 wt % of total charge) based on polylactic acid (PLA) filled with multi-walled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), and a mixture of both fillers (MWCNTs/GNPs). Inferior electrical properties were observed for all composites produced with the solution blending technique. Therefore, in light of these findings, in [
26], an electrical characterization in terms of DC conductivity and impedance spectroscopy (magnitude and phase) at low frequency (up to 1 MHz) have been performed for nanocomposites (up to 12 wt % of loading) prepared by melt extrusion. Preliminary results on the thermal conductivity of monophase-composites, filled exclusively with MWCNTs or GNPs, have been reported in [
27]. Results on mechanical properties (e.g., nano-indentation and tensile characteristics) have also been discussed [
28,
29,
30].
In the present paper, in order to complete this extensive experimental characterization [
25,
26,
27,
28,
29,
30] and verify the applicability of 3D-manufactured composites in electromagnetic (EM) applications, the EM properties in terms of transmission, reflection, absorption coefficients, and complex permittivity in the frequency range from 26 GHz to 37 GHz (the so-called Ka band) and the thermal conductivities of different formulations are investigated. The behavior of either single-filler (mono-phase) or two-filler (multiphase) composites, differently from the analysis in [
27], is explored. The EM and thermal characterization is supported by the morphological investigation achieved by means of high-resolution images. In particular, such analysis allows us to observe the dispersion states of the fillers within the polymer and their mutual interaction, which is important in determining the observed EM and thermal properties. The physical mechanisms linking morphological structures with EM and thermal properties are discussed. The final goal is to apply such nanocomposites in electronics packaging applications requiring either electromagnetic interference (EMI) shielding capability or remarkable thermal properties to dispose the heat produced by the devices. In fact, since the microwave spectrum becomes more and more crowded, the need to electromagnetically shield more components coexisting in the same environment is a current problem that can be solved by introducing innovative materials with suitable properties and new technologies, as additive manufacturing, to produce tailored shapes.
2. Materials and Methods
In the present study, the adopted thermosetting polymer is the poly(lactic) acid (PLA) Ingeo™ Biopolymer PLA-3D850 (Nature Works) characterized by fast crystallizing, whereas graphene nanoplates (GNPs) and multiwall carbon nanotubes (MWCNTs) are provided from Times Nano, China. Details on basic properties of fillers and host polymer are summarized in
Table 1. PLA has been selected as base thermoplastic polymer in order to favor the development of sustainable and eco-friendly composite materials characterized by good stiffness, strength, and ductility [
31]. In [
32], electrically conductive nanocomposite filaments for FDM with remarkable electrical and mechanical properties are achieved by using similar fillers but a host polymer, i.e., polyetheretherketone (PEEK) whose recycling is not feasible. In fact, PEEK waste is not biodegradable.
2.1. Preparation of Nanocomposites
Three different types of PLA-based nanocomposites were prepared through the melt mixing technique. In particular, nanocomposites of GNP/PLA and MWCNT/PLA with filler concentrations variable from 0 up to 12 wt %, as well as multiphase systems based on GNP/MWCNT/PLA with ratio 50:50 of GNPs to MWCNTs have been produced. Both polymer and nanofillers were dried prior to their use. Firstly, as starting materials, two masterbatches of 12 wt % GNPs and MWCNTs, respectively, were prepared by melt mixing of the filler and the polymer in twin-screw extruder (COLLIN Teach-Line ZK25T) by setting a screw speed of 40 rpm and keeping the temperature in the range 170–180 °C. In fact, they are diluted with neat PLA in a subsequent melt mixing process (masterbatch dilution method) in order to produce mono-filler composite pellets of 1.5%, 3%, 6%, and 9% filler amounts, as well as multiphase composites with equal proportions of both fillers (1.5:1.5, 3:3, and 6:6). After that, the composite pellets were extruded by a single screw extruder (Friend Machinery Co., Zhangjiagang, China) in the temperature range 170–180 °C and a screw speed of 10 rpm for producing filament for 3D printing (FDM) with 1.75 mm in diameter.
2.2. 3D Printing (FDM) for Sample Preparation
A part of the test samples was prepared by 3D printing (FDM) using the PLA-based nanocomposite filaments, described above in 2.1. Disc specimens (see
Figure 1) with a thickness of 10 mm and a diameter of about 50 mm were modeled, and then 3D printed using the 3 types of nanocomposite filaments, GNP/PLA, MWCNT/PLA, and GNP/MWCNT/PLA, with filler contents of 3, 6, 9, and 12 wt %. The fused deposition modeling (FDM-FFF)-type 3D printer X400 PRO German RepRap with an extrusion nozzle with a diameter of 0.5 mm was used. Based on previous experimental tuning, the processing parameters of the 3D printing were a temperature of 200 °C, an extrusion speed of 100 mm/s, and the platform temperature of 60 °C. Samples were printed with a layer height of 0.2 mm and 100% infill, in a rectangular direction of one layer to another.
2.3. Experimental Methods
2.3.1. Scanning Electron Microscopy (SEM)
Field emission scanning electron microscopy (JSM-6700F, JEOL, Akishima, Japan) operating at 3 kV was used to get information on morphological features of the resulting nanocomposites. Suitable nanocomposites sections were cut with a cold treatment in liquid nitrogen (77 K, −196 °C) and some of them have undergone chemical etching before the observation by SEM, in order to also evaluate the worthiness of this treatment (being classically recommended for the thermosetting resins) for thermoplastic matrices. The etching procedure has been described in detail in [
33] and simply schematized in
Figure 2.
2.3.2. Thermal Measurements
Hot Disk
® thermal constants analyzer mod 2500 S (Hot-Disk AB TPS 2500 S, Gothenburg, Sweden) was used to perform thermal conductivity measurements based on the transient plane source technique (TPS) [
34,
35,
36] and according to the ISO 22007-2-2015 standard [
37]. More in detail, the TPS element that is placed between two smooth pieces of the sample under test, is an insulator nickel flat disk sensor, provided of concentric and circular line heat sources, which acts simultaneously as heater and temperature sensor (
Figure 3). Insulation is guaranteed by a thin layer of kapton, teflon, or mica covering both sides of the probe.
The TPS technique relies on the recording of the resistance changes (against the time) of the heat source serving as the measuring probe [
38].
Since the temperature coefficient of the resistance (i.e.,
β) of the sensor is well known (≈4.0 × 10
−3 K
−1 at room temperature), its resistance change (i.e.,
R(
t)) with respect to the initial value
R0 (≈4 Ω at room temperature) before the transient recording allows deriving information on its time-dependent temperature variation ∆
T(
t) in agreement with the following relationship:
By assuming that the sensor with the features described above is positioned in a sample of infinite dimensions, the thermal properties of the material are evaluated by recording temperature increase over time by means of the formula:
where the dimensionless time
τ = (t·α/r
2)
1/2 is a function of the measurement time
t, the thermal diffusivity α, and the radius of the sensor
r,
P0 is the input heating power,
λ is the thermal conductivity, and
is a geometric function including the modified Bessel function of the first kind
I0 with
n representing the number of concentric and equally spaced circular linear sources that make up the probe [
39].
On the basis of the above theory, first we must find the value of thermal diffusivity, which actualizes the best linear fit between the temperature increase of the TPS sensor and the geometric function; subsequently, the thermal conductivity is determined by the slope of this straight line, knowing the input heating power P0 and the sensor radius r.
The advantage of the TPS technique relies on the fact that the same TPS element acts as a heat source and a temperature sensor, thus ensuring better accuracy in determining the thermal transport properties compared to other transient-based techniques.
In our case, the measurement of the thermal conductivity was performed on disc-shaped specimens, produced by 3D printing, with a thickness of 10 mm and a diameter of about 50 mm.
A thermal power pulse of magnitude P = 0.1 W for a time measurement of 40 s at room temperature through the sensor (6.40 mm radius r) was used. These parameter values ensure that the TPS element is applied to a sample of infinite dimensions. For each sample, five thermal conductivity measurements were performed. The average value is obtained by neglecting the first 50 and the last 10 of 200 points of each recording.
2.3.3. Electrical Measurements
The DC bulk conductivity of the nanocomposites was measured by using circular-shaped specimens, prepared by hot pressing, of about 50 mm diameter and a thickness of 1 mm. Before carrying out the electrical measurements, the samples are thermally pre-treated at 40 °C for 24 h in order to ensure the evaporation of residual solvents and to avoid effects due to the humidity.
In order to reduce eventual surface roughness and ensure good electrical contacts between the measurement electrode and the specimen, both sides of the latter have been metalized (circular mask of 22 mm of diameter) with silver paint (RS 186-3600 characterized by a volume resistivity 0.001 Ω∙cm). For the DC electrical characterization, performed at room temperature, a multimeter Keithley 6517A configured as both a voltage generator (max ± 1000 V) and a voltmeter (max ± 200 V) and an ammeter HP34401A (min current 0.1 fA) have been adopted.
To obtain accurate results, five samples for each composition were prepared and then characterized. However, for graphics clarity, only their average values are reported as electrical data.
2.3.4. Electromagnetic Measurements
The electromagnetic response of the samples is evaluated in terms of scattering parameters. The test samples are films with thickness of 1 mm, prepared by hot pressing. In more detail, transmitted/input (S
21) and reflected/input (S
11) signals have been investigated in the so-called Ka-band frequency range (from 26 GHz to 37 GHz) by using a scalar network analyzer R2-408R (ELMIKA, Vilnius, Lithuania) equipped with a 7.2 × 3.4 mm waveguide system. A plane-parallel layer of material was placed in a waveguide cell perpendicular to the incident radiation (
Figure 4). They have been mechanically reduced in size in order to fit in the waveguide. Reflection (
R), transmission (
T), and absorption (
A) coefficients are derived from the measured S-parameters as
,
,
. The electromagnetic shielding efficiency (EMI) was computed as a sum of absorption and reflection (EMI =
A +
R, in %). The complex dielectric permittivity was calculated from the experimental data by the standard method (see Reference [
40] for calculation details).