Laser sintering of electrohydrodynamic inkjet-printed silver in microgravity for in-space manufacturing of electronic devices

Abstract

The National Aeronautics and Space Administration (NASA) plans to deploy a fully functional 3D printer in low Earth orbit for in-space manufacturing (ISM) of semiconductor electronics. Earlier, our group demonstrated the successful application of high-resolution printing for semiconductor manufacturing using electrohydrodynamic printing. However, sintering of printed films is a significant challenge due to payload constraints. In this paper, we demonstrate laser sintering as a viable technology for ISM. The first experiments of successful demonstration of laser sintering of EHD printed silver on silicon in micro and lunar gravity are presented. The microstructures and conductivity of the samples are compared to those of the ground samples, and we observe that samples under microgravity exhibit increased heat transport compared to those under gravity. We hypothesize that the samples under microgravity experience lower convective heat transport, resulting in increased surface melting. A COMSOL heat transport model supports the laser sintering studies and hypotheses.

Introduction

Life sustenance and longevity in space open avenues for experimentation, processing, and testing new technologies and methods in the microgravity environment. In microgravity, the behavior of matter is unique, as weightlessness causes density separation in liquids to be absent¹. For example, convective heat flow through thermal processes caused by changes in density is no longer present^2,3. Liquids act under surface tension, adhesion, and cohesion as their driving forces to conglomerate in microgravity, rather than under the influence of gravity, which pulls and compresses the liquids toward Earth’s surface⁴. These characteristics of microgravity make in-space manufacturing (ISM) a unique, untested potential for improved devices^5–7. The National Aeronautics and Space Administration (NASA) plans to utilize the low Earth orbit (LEO) environment for manufacturing electronics for Earth-based applications for ISM and in its On-Demand Manufacturing of Electronics (ODME) Mission⁸. The ODME Mission involves producing electronics on the International Space Station (ISS) using additive manufacturing methods to replace and upgrade existing systems, as well as recycle materials for long-term space expeditions⁸. Printing methods are investigated as the primary additive manufacturing methods for the ODME Mission due to their low payload and ease of manufacturing⁹.

Unfortunately, printing technologies suffer from a major problem in that they do not provide a consolidated thin film of the desired material on the substrate; instead, they simply deliver the ink to the substrate, leaving the material in the form of wet NP agglomerates or clusters¹⁰. Thus, a post-sintering or post-annealing (thermal) step is mandatory to consolidate these NPs into a film, providing connectivity and realizing the desired film properties¹¹. This thermal step requires significant power for ISM, and a search for a low-power alternative has become increasingly important. Laser sintering is an attractive technology for creating the desired film due to its low power requirements and ability to selectively sinter features of a multilayer device¹². Additionally, a laser is ideal for sintering due to its ability to be incorporated directly into the printing tools.

Deposition methods for ISM of printed electronics include electrohydrodynamic (EHD) inkjet printing and direct ink write (DIW) printing, as both methods operate independently of gravity¹³. Both printing methods deposit inks comprised of nanoparticles (NPs), solvents, and binders¹⁴. In EHD printing, the ink is released from a nozzle in microscopic droplets and directed toward the substrate. The nozzle and subsequent ink at its tip are electrically charged while the substrate is grounded. The potential difference attracts and directs the ink from the nozzle to the substrate, reducing the reliance on gravity in the printing process¹⁵. EHD printing has a resolution of less than 1 µm and is used in semiconductor manufacturing¹⁶. In our earlier work, we demonstrated using parabolic flight EHD printing as the first proven technology capable of high-resolution electronics and semiconductor manufacturing in microgravity, offering strong potential for future ISM missions^17–19. Parabolic flight testing is a widely used method to simulate microgravity environments by flying an aircraft along a series of precisely controlled parabolic trajectories²⁰. The free-fall segment creates approximately 15 seconds of reduced gravity, including conditions simulating microgravity, lunar gravity, or Martian gravity, depending on the flight profile. In our previous parabolic flight campaigns, we successfully demonstrated EHD printing for materials including silver (Ag), barium titanate (BTO), and zinc oxide (ZnO), with line widths as narrow as 20 µm^17–19. DIW printing extrudes viscous ink directly onto the substrate through a nozzle by pneumatic or mechanically driven pressure. The ink deposition does not rely on gravity because the ink adheres to the substrate directly from the nozzle^21,22. DIW has a resolution of 100–1200 µm, depending on the nozzle dimensions, and is used for developing sensors and other electronic devices with larger feature sizes²³.

Laser sintering for printed electronics has been extensively investigated^24–36. Previous work has shown successful laser sintering for printing methods such as spin coating, aerosol jet printing (AJP), inkjet printing (IJP), DIW, and EHD printing. Through laser sintering, the resolution of the print size is decreased, as the laser path sinters a path through the print, and the excess ink is washed away using an organic solvent. This process decreases the line width from 100 µm to approximately 6 µm. Previous laser sintering work on EHD-printed samples investigated flexible and non-conductive substrates for a scaffold bioelectronic sensor, using a dual-nozzle system that prints polycaprolactone (PCL) microfibers and gold NPs³⁰. The gold NPs are EHD-printed and laser sintered on top of the EHD-printed PCL microfibers using a 532 nm continuous wave laser at 8 mW, so as not to melt the PCL. Additionally, laser sintering of EHD-printed Ag lines on glass was performed to investigate EHD printing and laser sintering on a non-conductive substrate³¹, resulting in 3 µm-wide sintered Ag lines. For laser sintering of these printing methods to be feasible, parameters such as laser wavelength, intensity, and scanning speed for each material must be optimized. The laser parameters vary based on the sample substrate, ink composition, and ink thickness. The variation in laser sintering parameters based on sample characteristics is demonstrated in our previous work, which investigated laser sintering for the first time of DIW-printed Ag on glass and polyimide (Kapton)³². The variation in laser sintering parameters needed to sinter due to the change in sample parameters was demonstrated in our previous laser sintering work, where we varied the ink composition, printing method, and substrate^32–36 for various materials such as Ag, silver-palladium (AgPd), BTO, gold (Au), platinum (Pt), and titanium dioxide (TiO₂). The laser wavelength required for sintering is determined by the interplay between the optical absorption of the ink and the thermal conductivity of the substrate. As the thickness of the print decreases, the substrate’s thermal properties become more significant in determining the required laser intensity. The higher the thermal conductivity of the substrate, the higher the intensity needed to sinter, as it dissipates the heat deposited on the ink.

In this work, we present the first microgravity-based laser sintering results for EHD-printed Ag. The Ag was laser sintered on Earth and during microgravity and lunar-gravity flights performed by the company Zero-G³⁷. To optimize the laser parameters and provide a control group for the in-flight samples, on-ground experiments were done, where EHD-printed Ag on glass and silicon (Si) wafers were laser sintered with a 445 nm laser. For microgravity and lunar gravity laser sintering testing, 20 parabolas were performed during a Zero-G flight campaign. During the flight, the optimized ground laser sintering parameters were applied to EHD-printed Ag on Si. A data-driven model was developed, similar to our previous simulation studies for Ag ink printed on selected substrates, such as glass, Kapton, and alumina³⁸, using the licensed version of the finite element-based software COMSOL Multiphysics 6.2, to gain an understanding of the laser sintering experiments on earth and under microgravity. The simulation process implements a transient heat transfer model by solving a two-dimensional heat conduction equation. The model investigates the effect of heat deposited on the Ag ink due to various important factors, including the effect of substrate composition, variations in laser power, laser speed, and laser spot size. Various important parameters, including temperature-dependent thermal conductivity, heat capacity, and wavelength-dependent absorbance values, were used as primary input parameters in the COMSOL model. The in-situ electrical conductance was measured using a two-point probe at various temperatures to calculate the temperature-dependent transient thermal conductivity using Weidmann-Franz^39,40. Our work demonstrates that the conductivity of the Ag samples, using optimized laser sintering parameters, achieved higher melting under microgravity and lunar-gravity conditions. Additionally, we describe our modeling process and results for laser sintering in microgravity. Modeling indicates that a lack of convective flow results in higher temperatures on the sample, leading to an increase in melting, as our experimental results demonstrate.

Results

Laser sintering the on-ground samples results

First, an experimental proof of concept was performed on Sample 1, EHD-printed Ag on a glass substrate, using the laboratory experimental setup. The resulting SEM images of the Ag particles and their conductivities are shown in Fig. 1. Figure 1a shows the results from using laser powers from 1 to 6 W (1.74–10.4 × 10³ W/cm²) at 0.5 mm/s scanning speed. The films sintered at 1 and 2 W achieved a conductivity of approximately 10⁷S/m. As the laser power increases, conductivity decreases to 10⁻²S/m as the higher power begins to ablate the sample. In Fig. 1b, the experiment was repeated with a scanning speed of 0.1 mm/s. The SEM images of Ag particles sintered with a laser power of 2 W and 0.5 mm/s scanning speed are compared to 2 W at 0.1 mm/s. The decrease in laser scanning speed increased the exposure time of the Ag particles to the laser beam, reducing the conductivity of the film to 10¹S/m.

Fig. 1. EHD-printed Ag on glass SEM images with corresponding laser power, scanning speed, and conductivity measurements.

a SEM images at 1–6 W laser power at 0.5 mm/s, (b) SEM images at 2 W at 0.5 mm/s and 0.1 mm/s.

The proof-of-concept laser sintering tests performed on EHD-printed Ag on glass had two results: sintered and over-sintered. As seen in our previous work with DIW films^26–30, the relationship between laser power and film conductivity has a common trend in that as laser power increases, the conductivity increases. However, the EHD films in this work experience a drop in conductivity using excessive laser power, even as low as 2 W at a lower scanning speed, due to over-sintering. As a result, the EHD films in this work must be sintered at the optimal laser power range to achieve the desired conductivity and avoid over-sintering. Over-sintered films are shown in Fig. 1a, where the laser power exceeds 2 W and the film conductivity decreases to 10⁻²S/m. Figure 2 shows laser sintering results when using 0.1 mm/s at powers exceeding 4 W (6.9 x 10³W/cm²). The NPs melt and conglomerate to form significant gaps in the film or are ablated from the substrate completely. This results in non-conductive traces of Ag. The scale of the SEM images is increased from 1 µm to 5 µm and 100 µm to show the ablation of NPs from the glass surface. The 100 µm scale SEM images of the nonconductive traces show that the outer edges of the printed film remain; however, the majority of the NPs are completely removed, revealing bare glass where the laser was directly exposed.

Fig. 2. SEM images of over-sintered Ag NPs at 1, 5, and 100 µm scale.

Over-sintering resulted in an ablation of NP material from the glass substrate after 4 W (7.56 × 10³ W/cm² laser intensity) was reached, as shown in Fig. 2.

Samples 2–6, EHD-printed Ag on Si placed on glass, were laser sintered using the laboratory experimental setup, ranging from 1 to 17 W (1.74 x 10³–3.21 x 10⁴W/cm²) at 2 mm/s scanning speed. The samples on Si were placed on a glass slide to enhance heat absorption from the laser to the Si substrate and the Ag particles. Initially, laser sintering without the glass slide underneath resulted in an unsintered film for the maximum laser output power of 30 W. Figure 3 shows the SEM images of the EHD Ag particles at increasing laser intensity and their corresponding conductivity.

Fig. 3. SEM images and conductivity of EHD-printed Ag on Si as the laser power increases.

Laser power increases from (a) 1–12 W and (b) 13–17 W. The scanning speed is 2 mm/s. From 1 to 12 W, the particles are under-sintered and conductivity is the same as unsintered particles. 13–15 W results in 10⁶S/m sintered particles, whereas >15 W results in over-sintered particles and reduced conductivity.

At laser powers higher than 15 W (2.84 x 10⁴W/cm²), the conductivity reduces due to the conglomeration of NPs and reduced conductive paths. Laser power of 13–15 W (2.46–2.84 x 10⁴W/cm²) results in conductive traces of 10⁶S/m. The laser intensity needed to sinter EHD-printed Ag on Si is 10x higher than on glass to achieve the desired 10⁶–10⁷S/m conductivity. Laser powers below 13 W (2.46 x 10⁴W/cm²) result in under-sintered Ag NPs that show no particle growth. As a result, Fig. 3 shows three result regions: under-sintered, sintered, and over-sintered.

Figure 4 shows the SEM images and conductivity of the EHD-printed Ag laser sintered when the scanning speed is varied from 0.25 – 4 mm/s for 21 W (3.97 x 10⁴W/cm²), which exceeds the previously specified range of power values (13–15 W, 2.46–2.84 x 10⁴W/cm²) that resulted in a conductivity of 10⁶S/m. As a result, the higher powers cause over-sintering, leading to the conglomeration and separation of NPs. Unlike the glass substrate, the NPs remain on the substrate rather than being completely ablated. The reduction in conductive paths results in a conductivity of 10⁻¹–10⁻³ S/m. As the scanning speed increases from 0.25 to 4 mm/s, the conductivity increases as melting is reduced. However, even at a higher speed of 4 mm/s, the sample yields conductivity like that of unsintered NPs.

Fig. 4. SEM images of over-sintered EHD-printed Ag as the scanning speed increases.

The scanning speed increases from (a) 0.25–1.5 mm/s and (b) 2–4 mm/s, and the laser power is maintained at 21 W (3.97 × 10⁴ W/cm²).

Further testing of the range of optimal intensity values to achieve conductivity was done utilizing ATS prior to the flight campaign. Figure 5 shows the SEM images and conductivity of EHD-printed Ag lines on Si placed on glass, from Samples 6 and 7, laser sintered at 13, 18, and 22 W (0.157, 2.17, and 2.65 x 10⁴W/cm² laser intensity) at 0.5, 1, and 4 mm/s, respectively. Grain growth and complete melting of the NPs without separation were achieved, resulting in a measured conductivity of 10⁷S/m. Note that the best conductivity wasn’t achieved with complete melting, but rather with 18 W at 1 mm/s, which is 3x better than that at 22 W. Interestingly, when we investigated the porosity using ImageJ software, the surface porosity of the 18 W and 22 W samples was approximately the same, between 10% and 11%. The sublayer porosity, visible in the 22 W sample SEM image, was not measured by the software; however, we believe it contributed to the lower conductivity at 22 W.

Fig. 5. Laser sintering on the advanced tool plate system.

SEM images of Samples 6 and 7 laser sintered in ATS with increasing laser powers of 13 W, 18 W, and 22 W.

Figure 6 shows a summary of the laser intensity and resulting conductivity using the laboratory and ATS for laser sintering, irrespective of the scanning speed. Approximately 2–3 x 10⁴W/cm² is the optimal laser intensity range for EHD-printed Ag on a Si substrate placed on glass. The three result regions are labeled for the Si substrate: under-sintered, sintered, and over-sintered.Sintered

Fig. 6. Summary of laser parameter effect.

On-ground laser sintering conductivity results prior to Zero-G flight campaign experiments with respect to laser intensity, irrespective of scanning speed the experiments utilized the laboratory experimental setup and ATS.

Samples 8 and 9, EHD-printed Ag on Si placed on glass, were laser sintered using ATS immediately before the Zero-G flight using laser intensities within the specified range of 2 – 3 x 10⁴W/cm², or 17–24 W.

Figure 7 shows the SEM images and conductivities of the laser powers and scanning speeds used: 18 W (2.17 x 10⁴W/cm²) at 1 and 4 mm/s, and 22 W (2.65 x 10⁴W/cm²) at 4 mm/s. The three laser sintering parameters yielded the expected magnitude of conductivity, 10⁷S/m, regardless of the scanning speed.

Fig. 7. Ground testing before the flight.

SEM images and conductivity of ground samples laser sintered in ATS immediately before the Zero-G flight at three laser sintering parameters within the optimal range: 18 W at 1 and 4 mm/s, and 22 W at 4 mm/s.

Laser sintering the in-flight sample results

Sample 10, EHD-printed Ag on Si placed on glass, was laser sintered during the Zero-G flight for twelve lunar-gravity and eight microgravity parabolas at the three laser sintering parameters: 18 W at 1 and 4 mm/s, and 22 W at 4 mm/s. The ATS mounted to the inside of the Zero-G plane was used to sinter the sample. The twenty parabolas were split into two groups of ten parabolas with the following number of EHD-printed Ag lines laser-sintered at the following parameters: four lines at 18 W at 1 mm/s, three at 18 W at 4 mm/s, and three at 22 W at 4 mm/s. Figure 8 shows the resulting SEM images for the in-flight Sample 10 for the first ten parabolas at lunar gravity compared to unsintered particles. Similar to the on-ground samples, particle growth can be seen for every laser-sintered sample for the lunar-gravity tests. The average particle diameter increased from approximately 170 nm for on-ground tests to 225 nm for lunar-gravity tests.

Fig. 8. Lunar gravity laser sintering results (unsintered NPs and parabolas 1–10).

a SEM images and conductivity results for trials at 18 W at 1 mm/s under lunar gravity. b SEM images and conductivity results for trials at 18 W at 4 mm/s and at 22 W at 4 mm/s under lunar gravity.

Figure 9 shows the resulting SEM images for the in-flight Sample 10 for the second group of ten parabolas compared to unsintered particles. The last eight parabolas were conducted in microgravity, while the first two were conducted at lunar gravity. The in-flight Sample 10 achieved a conductivity of 10⁶–10⁷S/m and an average particle diameter of approximately 285 nm for all microgravity laser sintering trials.

Fig. 9. Microgravity laser sintering results (unsintered NPs and parabolas 11–20).

a SEM images and conductivity results for trials at 18 W at 1 mm/s under microgravity. b SEM images and conductivity results for trials at 18 W at 4 mm/s and at 22 W at 4 mm/s under microgravity.

Laser sintering the after-flight sample results

Sample 11 was laser sintered immediately on the ground after the parabolic flights in ATS, using the following laser parameters: 18 W at 1 and 4 mm/s, and 22 W at 4 mm/s.

Figure 10 shows the SEM images grouped by the laser sintering parameters used. Conductivity of 10⁷S/m was achieved for all the samples, except for a couple at 10⁶S/m. Particle growth was seen for all of the laser-sintered samples. Figure 11 compares the flight campaign EHD-printed Ag SEM images at the three laser parameters: 18 W at 1 mm/s and 4 mm/s, and 18 W at 4 mm/s. The in-flight samples exhibit more considerable NP growth and reduced porosity, suggesting that melting rather than sintering was achieved due to the higher temperatures and lack of convective flow.

Fig. 10. On-ground results for laser sintering done immediately after the parabolic flights.

SEM images and conductivity results for trials at (a) 18 W at 1 mm/s after zero G flight, (b) 18 W at 4 mm/s after zero G flight, and (c) 22 W at 4 mm/s after zero G flight.

Fig. 11. Summary of laser sintering results before, during and after Zero G flight.

Comparison between SEM images for the flight campaign samples.

The conductivity of all the on-ground and in-flight samples grouped by the laser sintering parameters is shown in Fig. 12. The conductivity is plotted for each printed line that was measured. There is a slight variation in the conductivity between the samples, but all lie within the range 10⁶–10⁷S/m. The average conductivity of the on-ground samples for 18 W at 1, 18 W at 4 mm/s, and 22 W at 4 mm/s were 1.38 x 10⁷S/m, 1.05 x 10⁷S/m, and 1.72 x 10⁷S/m, respectively. The in-flight samples averaged 1.00 x 10⁷S/m, 4.97 x 10⁶S/m, and 9.67 x 10⁶S/m for the same three laser sintering parameters, respectively. The 18 W at 4 mm/s saw a 53% decrease, whereas the 18 W at 1 mm/s and 22 W at 4 mm/s had a 27% and 30% decrease in conductivity for the in-flight samples, respectively.

Fig. 12. Summary of conductivity plots.

Conductivity plots comparing the flight campaign samples and laser sintering parameters before, during, and after flight campaign.

Laser sintering modeling results

To develop data-driven simulation models, prior experiments were conducted to obtain the required input data, including temperature-dependent thermal conductivity measurements and heat capacity estimations.

First, the electrical conductivity of the printed Ag ink on the glass substrate was measured to estimate the thermal conductivity. After printing Ag ink on glass substrates, the samples were dried at room temperature overnight. After heating the samples at various temperatures, the electrical resistance was measured using a digital multimeter, and the electrical conductivity was calculated by implementing Eq. 3. Thermal conductivity was calculated via the Wiedemann-Franz law, which states that thermal conductivity is given by Eq. (1)³⁹:

$$Thermalconductivity\left(\Lambda \right)=\sigma \mathit{LT}$$ 1

Where T (K) is the temperature corresponding to the measured electrical conductivity, and L is the Lorentz number that is defined as follows⁴⁰:

$$L=\frac{{\pi }^2}{3}{\left(\frac{k_B}{e}\right)}^2=2.44\times {10}^{-8}{V}^2{K}^{-2}$$

Table 1 presents the measured values of electrical and thermal conductivity. Thermal conductivity values at a solidus temperature of 1075 K and liquidus temperature of 1273 K were based on bulk Ag values, such as 400 W/(m·K)and 419 W/(m·K), respectively, from the literature⁴¹.

Table 1.

List of the experimentally measured electrical resistance, electrical resistivity, electrical conductivity, and corresponding thermal conductivity at different temperatures for printed Ag ink on a glass substrate

Temperature (K)	Electrical Resistance (Ω)	Electrical Resistivity (Ω·m)	Electrical Conductivity (S/m)	Thermal Conductivity (W/(m·K))
323	6.4285	4.34E-06	2.31E + 05	1.8206
348	2.6571	1.79E-06	5.58E + 05	4.7381
373	1.3428	9.06E-07	1.10E + 06	10.0113
398	0.8285	5.59E-07	1.79E + 06	17.3830
423	0.7142	4.82E-07	2.07E + 06	21.3649

In the simulation, the printed samples are dry, similar to the experiment. The heat capacity of the printed sample depends on the porosity of the sample and is given by Eq. (2)⁴²:

$$C_{\mathit{sample}}=C_{\mathit{bulk}}\left(1-\mathrm{\varnothing }\right)$$ 2

Where $\varnothing$ denotes the porosity, $C_{\mathit{bulk}}$ is the heat capacity of bulk Ag, and $C_{\mathit{sample}}$ is the heat capacity of the printed Ag ink. For calculating the $\varnothing$ value, the surface morphology of the printed Ag inks was characterized by SEM images. Figure 13 shows the SEM images of the surface view of the printed Ag ink glass substrate sample. The porosity of the sample was analyzed by importing the SEM image shown in Fig. 13a using ImageJ software. The area of the sample used in porosity estimation was 2500 µm². Figure 13b shows a color map of the particle distribution in the unsintered sample of Ag. The porosity of the sample was estimated to be 0.13504, or 13.504%. The heat capacity of the printed Ag ink used in the simulation was taken to be, on average, 206.422 J/(kg·K).

Fig. 13. Porosity analysis used for laser sintering simulation input parameters.

a SEM image of the top surface of unsintered EHD-printed Ag. b ImageJ color map showing the porosity (in black) and particle distribution of the EHD-printed Ag.

The porosity of the sample was analyzed using the freely available software ImageJ. The porosity of the sample was estimated to be 0.06319 or 6.319%, and the heat capacity of the printed Ag ink for the simulation was estimated to be 223.567 J/(Kg·K).

The absorption coefficient of Ag nanoparticles not only depends on their size but also varies with the spectral wavelength. The value of the absorption coefficient of Ag nanoparticles decreases with the increasing particle size, and this also goes down with increasing value of the spectral wavelength⁴³. The laser sintering experiments were performed using aqueous Ag NP ink, Novacentrix JS-A19, which consists of spherical particles with an average diameter of 30 – 50 nm. The laser absorption coefficients of the Ag ink, with an average particle size of around 300 nm, for different laser wavelengths were taken from the reference⁴³ and used in the simulation model.

The first simulation was performed for laser sintering printed Ag ink on different substrates with a 445 nm laser wavelength, 1 W laser power, 230 μm laser spot size, and 1 mm/s laser sintering scanning speed. Figure 16 shows the 3D heat profiles and the maximum surface temperature profiles for three different substrate combinations. Figure 14a, d show the 3D heat profile and maximum temperature profile for Ag ink with a thickness of 290 nm printed on a glass substrate of 1.05 mm thickness at 0.025 seconds after the laser started scanning the sample at a speed of 1 mm/s. The inset of Fig. 14a is magnified to show the heat distribution throughout the Ag layer. Figure 14a predicts that the maximum surface temperature is 516 K at 0.025 seconds, and this further increases to 600 K at 0.05 seconds as the laser moves. Figure 14d shows the maximum surface temperature at the points K, L, M, N, O, and P that were considered to predict the transverse and radial heat distribution through the printed Ag layer. In Fig. 14d, the maximum surface temperature varies very similarly at all points, and this predicts that the heat is distributed uniformly through the Ag layer in both transverse and radial directions. Figure 14b shows the 3D temperature profile for 192 nm thick Ag printed on a Si substrate of 400 μm thickness at 0.025 seconds after the laser started scanning the sample. Figure 14e shows that the surface temperature of the Ag printed on Si attains its maximum value very quickly and becomes constant, but at the points M and N, the temperature is slightly lower than that of points K and L, and this can also be observed for the points O and P. This predicts that heat is distributed equally in the radial direction but decreases in the transverse direction because of the higher thermal conductivity of the Si substrate. The high thermal conductivity of the Si substrate results in a lower value of the maximum surface temperature when compared to glass, and it only reaches 298 K for the same laser power.

Fig. 16. Effect of convective flow on 3D heat profile and maximum surface temperature.

3D heat profiles and the maximum surface temperature plots of the sample of Ag (192 nm) printed on a Si substrate (400 μm) but placed on a glass substrate of thickness 1.05 mm for 1 W laser power at 0.025 seconds (a) with convective heat flow, and (b) without convective heat flow. The maximum surface temperature variation plots for the same samples: (c) with convective heat flow, and (d) without convective heat flow.

Fig. 14. 3D heat profiles at 0.025 seconds for different thicknesses of Ag printed on a variety of substrates.

3D Heat profile for (a) Ag (290 nm) printed on glass substrate (1.05 mm), (b) Heat profile for Ag (192 nm) printed on Si substrate (400 μm), and (c) sample of Ag (192 nm) printed on Si substrate (400 μm) placed on glass substrate (1.05 mm); (d–f) are the surface temperature plots for the combination shown in (a–c) respectively.

Figure 14c shows the 3D heat profile for 192 nm thick Ag ink printed on a 400 μm thick Si substrate placed on a glass substrate of 1.05 mm thickness at 0.025 seconds after the laser started scanning the sample. The inset of Fig. 14(c) magnifies the 3D heat profile to show the heat distribution throughout the Ag layer as well as the Si substrate. Figure 14c shows that the surface temperature reaches 348 K at 0.025 seconds after placing the glass substrate underneath the Ag printed on Si, and this further increases to 398 K after 0.05 seconds. After placing the sample on the glass substrate, heat is uniformly distributed throughout the Ag layer and Si substrate, resulting in uniform radial and transverse heat distribution, as shown in Fig. 14f.

To gain a deeper understanding of the effect of the substrate on the maximum sintering temperature, additional simulations were performed, as shown in Fig. 15. Figure 15 illustrates the surface temperature distribution at point K for all possible configurations. Figure 15a shows the results for using 1 W laser power, resulting in the maximum sintering temperature of 600 K at 0.05 seconds. Figure 15b shows the results for Ag printed on Si using varying laser powers from 1 W to 60 W, resulting in 60 W laser power needed to attain a surface temperature of 600 K. In our experiment, we observed that to achieve similar conductivity, the laser intensity used on glass was 5.3x lower than the intensity used on Ag on Si on glass.

Fig. 15. Effect of substrate and laser power on surface temperature.

The maximum surface temperature variation plot at point K during the simulation of Ag printed on various substrates and sintered at different laser powers. (a) Ag (290 nm) printed on glass substrate (1.05 mm) with 1 W laser power, (b) Ag (192 nm) printed on Si substrate (400 μm) with varying laser power between 1 and 60 W, (c) Ag (192 nm) printed on Si substrate (400 μm) but placed on glass substrate of thickness varying between 0.65 and 1.05 mm, (d) Ag (192 nm) printed on Si substrate (400 μm) but placed on glass substrate of thickness 1.05 mm with varying laser power between 1 and 3.1 W, and (e) Ag (192 nm) printed on Si substrate (400 μm) but placed on glass substrate of thickness 0.95 mm with varying laser power between 1 and 5.3 W.

To gain an understanding of how the glass thickness affects the temperature profile of the sample, the surface temperature distribution at point K is plotted in Fig. 15c for Ag printed on a Si substrate placed on glass with varying thicknesses of 0.65 mm to 1.35 mm. In Fig. 15c, the value of the surface temperature increases with the increasing thickness of glass. This is further validated by Fig. 15d and Fig. 15e. Figure 15d shows the sintering temperature distribution at point K for Ag printed on Si placed on 1.05 mm thick glass using varying laser powers. To achieve the value of maximum surface temperature up to 600 K, 3.1 W laser power is required. The glass thickness underneath the Si substrate was decreased to 0.95 mm, and the resulting temperature distribution at point K is shown in Fig. 15e. The laser power required to achieve the maximum surface temperature of up to 600 K is 5.3 W rather than 3.1 W for the 1.05 mm thick glass.

To understand the effect of gravity, we looked at the heat transport on the sample with and without convective heat flow. Figure 16a, b show the 3D heat profiles for 192 nm thick Ag printed on 400 μm thick Si on a 1.05 mm thick glass substrate, simulated with two different boundary conditions. Figure 16a presents the heat distribution for the sample simulated with free convective heat flow between the developed structure and the surrounding medium, considering the sample to be sintered on Earth. Figure 16b shows the heat distribution for the sample simulated without convective heat flow on the Ag layer to simulate the condition of microgravity.

In Fig. 16a, heat is uniformly distributed through the Ag layer and Si substrate because of the free convective heat flow between the sample and the surrounding medium, but Fig. 16b shows that most of the heat is localized in Ag layer because of the convective heat constraint applied on it. The localization of heat in the Ag layer increases the maximum surface temperature to 398 K at 0.025 seconds of sintering time, while the heat profile shown in Fig. 16a attains the maximum sintering temperature of 348 K at 0.025 seconds. Figure 16c, d show the surface temperature distribution at points K, L, M, N, O, and P with and without convective heat flow, respectively. Figure 19c shows the surface temperature distribution for the sample sintered under the condition of free convective heat flow and presents the uniform heat distribution in the radial and transverse directions. While Fig. 16d shows the surface temperature distribution for the sample sintered with the convective heat flow constraint applied on the Ag layer. Figure 16d shows that the heat is more localized at the center of the sample presented by point K, and this decreases in both transverse and radial directions, presented by the points L, M, N, O, and P.

Fig. 19. Schematic of the heat transfer simulation design considered in COMSOL.

The design is considered for the following configurations: (a) 290 nm thin Ag printed on a glass substrate of 1.05 mm thickness, (b) 192 nm thin Ag printed on a Si substrate of 400 μm, and (c) a sample of Ag printed on Si and placed on a glass substrate. d Schematic for selecting points for temperature distribution at the surface and in-depth of the Ag ink. e Schematic showing the dimensions for where the points for the temperature distribution are located in the Ag ink relative to each other.

Discussion

The laser sintering experiments of EHD-printed Ag performed on a glass substrate resulted in a conductivity of 9.44 x 10⁶S/m and 1.12 x 10⁷S/m using laser intensities of 1.74 x 10³W/cm² (1 W) and 3.78 x 10³W/cm² (2 W), respectively. Although the ink is the same, and there were only slight variations in print dimensions, the change in substrate from glass to Si on glass required at least 2.0 x 10⁴W/cm² to achieve 1.8 x 10⁷S/m, 7.25x increase in laser intensity. The scanning speed affected the EHD-printed Ag on the glass substrate, whereas the Si substrate samples achieved conductivity within an optimal intensity range regardless of the scanning speed. The over-sintering of the Ag on Si substrate placed on glass samples resulted in the complete melting of the Ag NPs, resulting in nanopores throughout the sublayer of the Ag film. Although melted, the nanopores exceed the porosity of the sintered film, resulting in slightly lower conductivity. As laser intensity increases, the conglomeration and melting of the particles continue, resulting in the complete separation of the Ag NPs, which achieves lower conductivity than the unsintered samples. In comparison, the material was removed entirely from the surface of the over-sintered glass substrate samples. The thermal conductivity of glass is significantly lower compared to that of Si. As a result, the heat from the laser is maintained on the surface of the glass where the printed sample is located. In contrast, the Si dissipates the heat from the laser, relocating the concentrated heat, requiring higher laser intensity to achieve the same sintering results. The COMSOL simulation model confirms the same finding when we compare surface temperature profiles for the printed Ag on different substrates. It was observed that the sample of Ag ink printed on a Si substrate requires 60x the intensity to attain the same value of maximum surface temperature in comparison to the sample of Ag ink printed on a glass substrate. This is because of the very large value of thermal conductivity of the Si substrate in comparison to the glass substrate. It was also observed that placing the glass substrate underneath the sample of Ag ink printed on a Si substrate reduced the required laser power to around 5x the intensity for achieving the same maximum surface temperature, due to heat dissipation from the Si to the glass interface. The heat dissipation is also affected by the thickness of the glass substrate.

The ground samples on Si substrates consistently achieved conductivity levels of 10⁶ – 10⁷ S/m within the optimal range of laser intensity values from approximately 2 – 3 x 10⁴W/cm². Within this range, examining the SEM images revealed consistent microstructures formed by grain growth or NPs expanding from 30 – 50 nm to 120 – 270 nm in diameter, with an average of approximately 170 nm. The Zero-G parabolic flight yielded similar results regarding conductivity; however, upon inspection, the SEM microstructures exhibit a distinct difference: there is higher coalescing of the nanoparticles as gravity is reduced, which is typically observed at higher power in Earth’s gravity. For lunar gravity, the NP diameter increased to 120 – 330 nm, and for microgravity, the diameter increased to 180 – 380 nm. Additionally, minimal gaps between the particles suggest that the particles have adhered or coalesced more closely together than in the on-ground samples. We hypothesize that the absence of convective flow improves heat transport in the printed sample. Convective flow relies on gravity, as denser material sinks and less dense material rises, as occurs during heating. With convective flow during the sintering process, the density of the components in the ink causes gaseous phase components to escape the print. In contrast, heat transfer relies on conductive and radiative heat transport, rather than convective flow, during in-flight experiments. The absence of convection heat transport leads to decreased heat dissipation. As a result, the sample may reach higher temperatures in lunar and microgravity conditions than in normal-gravity conditions. This hypothesis is confirmed by COMSOL simulations performed under both free and constrained convective heat flow conditions. The samples sintered under conditions of constrained convective heat over the printed Ag layer were simulated by isolating them from the surrounding medium. The model results of the constrained convective heat attain higher sintering temperatures than those simulated with free convective heat flow between the sample and the surrounding medium. The samples sintered without convective heat flow between the sample and the surrounding medium, like microgravity conditions, experience a localized heat distribution. The samples sintered with free convective heat flow between the sample and the surrounding medium, like on Earth gravity conditions, experience a uniform heat distribution both in radial and transverse direction, resulting in a lowering of the value of maximum sintering temperature. In conclusion, we showcase that laser sintering is a viable technology for ISM and is improved by microgravity.

Methods

Laser sintering experimental setup

The laser sintering experiments occurred in two different settings: 1) a laboratory setting and 2) an integrated system that incorporated both the EHD printhead and the laser system, called the Advanced Toolplate System (ATS), and was flown in Zero-G. The laboratory laser sintering setup is pictured in Fig. 17a. A fiber-coupled continuous wave (CW) 445 nm diode laser is mounted with its output directed downwards onto four stacked stages. The stages include three motorized stages in the X, Y, and Z directions and one hand-adjustable stage in the Y direction for minor adjustments outside the programmed movement. The printed sample is placed on the top stage and translated beneath the focused laser beam using Thorlabs Kinesis software⁴⁴. The laser light coming from the fiber end is collimated and focused using a 15 mm plano-convex lens and a bi-convex 20 mm lens, respectively. The focused laser spot diameter is 230 µm, with an 18.7 mm working distance from the 20 mm lens. Laser power and scanning speed were varied from 1 to 24 W (1.7 x 10³ to 4 x 10⁴ W/cm²) and 0.25 to 4 mm/s, respectively, to optimize the parameters for the flight campaign. The integrated system, referred to as ATS, was designed and built by Sciperio⁴⁵ and is shown in Fig. 17b, c. ATS was tested on the ground at Sciperio and on the Zero-G plane before, during, and after the flight campaign. ATS is comprised of a 445 nm laser mounted next to the EHD printer toolhead, which includes the ink cartridge and nozzle. The toolheads move together in the x, y, and z directions and are directed downwards onto a workpiece holder, where a workpiece with a sample mounted is interchanged according to the experiment performed. For laser sintering experiments, an EHD-printed Ag sample on a rectangularly diced Si wafer is mounted on a glass slide on the workpiece and placed in the workpiece holder beneath the laser output. The 445 nm laser spot diameter is 288 µm with a working distance of 17.8 mm.

Fig. 17. Pictures of the laboratory and the Advanced Toolplate System (ATS) laser sintering experimental setups.

a The laboratory laser sintering setup. b The inside of ATS showing the interchangeable toolheads that allow for multiple functions of fabrication, such as printing, laser sintering, and cameras for viewing the sample. c ATS from the outside.

During the flight, the Zero-G B-727 aircraft performs a series of parabolas to achieve the change from Earth’s gravity (1 G, 9.81 mm/s²) to microgravity and lunar gravity³⁷. Flying level, the plane experiences normal gravitational pull³⁷. For one parabola, the plane angles upwards and approaches the peak of the parabola and experiences 1.8 Gs, or hyper-gravity. The plane then reaches the apex of the parabola and angles downward. The plane experiences freefall, or microgravity, for about 15 seconds. The angle of decline determines whether total or partial gravity loss is experienced. As a result, microgravity ( ~ 0 G) or lunar gravity (0.166 Gs, 1.62 mm/s²) is experienced in the cabin where the experiments are performed³⁷. This concludes one parabola, and the process is repeated for a total of 20 parabolas for this work. For each parabola, a single line of EHD-printed Ag was laser sintered.

Sample preparation

A commercial Ag NP ink (Novacentrix JS-A191, aqueous dispersed ink, 40 wt% Ag content, 8 – 12 cP viscosity, 30 – 50 nm average particle diameter) was used in this study to fabricate the printed structures. The ink was chosen for its inkjet compatibility and aqueous formulation, as it is water-soluble and easily cleaned for ISM. A 75 mm × 25 mm p-type Si wafer served as the substrate. Before printing, the wafer was cleaned using ethanol and subsequently treated with oxygen plasma to remove residual surface contamination and increase surface energy, thereby enhancing ink adhesion and improving the uniformity of the printed Ag layers. The samples were fabricated using a custom-built EHD lab printer equipped with a glass nozzle having an inner diameter of 75 µm. In EHD printing, the applied voltage plays a critical role in both initiating droplet ejection and controlling the line width of printed features. For this experiment, the printing parameters were set as follows: applied voltage of 1000 VDC, printing speed of 1 mm/s, and a stand-off distance (nozzle-to-substrate gap) of 250 µm. After printing, the samples were heated and dried in a nitrogen-purged oven at 80 °C for 5 minutes to remove residual solvents and promote adhesion of the Ag ink to the substrate. A total of 24 Ag lines, each with a line width of 160 ± 10 µm, arranged in a 3 × 8 array, were printed on each Si wafer. To ensure sufficient conductivity and thickness, each line was printed in two layers. These printed samples were prepared and packaged for testing during the parabolic flight campaign.

Samples

Printed samples comprise three sets, totaling twelve samples: laboratory test samples utilizing the experimental setup, ATS samples collected before the flight campaign, and ATS samples collected during the flight campaign. The flight campaign includes laser sintering experiments immediately before, during, and after flying on the Zero-G aircraft. The laboratory tests include six EHD-printed Ag on glass and Si samples, two of which are pictured in Fig. 18a, b. Sample 1 utilizes a glass substrate and is used for proof-of-concept experiments in laser sintering. Samples 2 – 6 are EHD-printed Ag on circular Si wafers. The Si substrate samples were placed on a glass slide to enhance heat absorption from the laser to the Si substrate and the Ag particles. The average thickness and width of the Ag lines on glass were 0.288 µm and 95.1 µm, respectively. For Samples 2-6 on a Si substrate, the average thickness and width of the Ag lines were 0.192 µm and 96.9 µm, respectively. Figure 18c shows Samples 7 and 8 that were laser sintered in ATS on the ground before the flight campaign. These samples were cut into a rectangular shape to match the size of the glass slide placed underneath during laser sintering and to fit the ATS workpiece. One of the EHD-printed Ag on Si samples, Samples 9-12, tested in ATS during the flight campaign, is shown in Fig. 18d. Samples 9 and 10 were laser sintered in the airplane hangar immediately before the flight. Sample 11 was laser sintered during, and Sample 12 was laser sintered after the flight. A glass slide was placed under each Si substrate when laser sintered. The average thickness of the lines was 0.269 µm, and the average width was 159 µm.

Fig. 18. Pictures of the laser sintering samples.

EHD-printed Ag on (a) glass sample and (b) Si sample laser sintered on the ground using the laboratory laser sintering setup. Pictures of the EHD-printed Ag on Si samples cut in rectangles for laser sintering using ATS (c) before and (d) during the flight campaign.

Table 2 presents a summary of the 12 samples, along with a brief description of their sintering conditions. Note that the silicon substrate samples, Samples 2-12, were laser sintered with glass underneath.

Table 2.

Samples 1-12 experimental descriptions

Sample	Description	Laser Power (W)	Laser Scanning Speed (mm/s)
1	Glass substrate: Proof of concept	1–6	0.1–1.25
2-6	Silicon substrate on glass: Laboratory tests	1–21	0.25–4
7-8	Silicon substrate on glass: ATS on-ground tests	6–24	0.25–4
9-10	Silicon substrate on glass: ATS on-ground before Zero-G flight campaign tests	18, 22	1, 4
11	Silicon substrate on glass: ATS in-flight Zero-G flight campaign tests	18, 22	1, 4
12	Silicon substrate on glass: ATS on-ground after Zero-G flight campaign tests	18, 22	1, 4

Characterization

The microstructures of the unsintered and laser-sintered EHD-printed Ag on Si samples were characterized using scanning electron microscopy (SEM). A FEI Quanta 600 F Environmental SEM and Zeiss ULTRA-55 FEG SEM were used. The images were taken at various magnifications to capture both smaller and larger-scale microstructures, including NP growth, ablation or removal of the NPs, and conglomeration of NPs. The width and thickness of the Ag lines were measured using stylus profilometry, a method using a stylus to trace the topography of the sample’s surface. The stylus was traced at the middle of each line to estimate the cross-section dimensions throughout the entire line. The resistance of each Ag line was measured using a 2-point probe, subtracting the probe resistance from the measurements. The dimensions of each line, measured using profilometry, were used to calculate the conductivity, σ, in S/m using Eq. (3).

$$\sigma =\frac{l}{\mathit{RA}}$$ 3

Where l is the length, R is the resistance, and A is the cross-sectional area of the printed line. The line is labeled as ‘Nonconductive’ if conductivity is not achieved, and ‘Unsintered’ for Ag printed lines that were not laser sintered. Each SEM image is accompanied by a corresponding laser power, scanning speed, and conductivity value. The laser intensity is calculated using the laser optical power and the area of the laser beam spot size on the sample. The variation in the laser system used resulted in varying spot sizes; however, these were normalized by comparing the intensity rather than the laser power (W). Equation (4) is used to calculate the laser intensity, I, in W/cm².

$$I=\frac{\mathit{Average\; Power}}{\pi r^2}$$ 4

Where r is the radius of the laser beam spot size.

Modeling

For understanding the underlying physics behind the different laser power requirements to sinter the samples in different conditions, three model designs developed in COMSOL have been schematically shown in Fig. 19. Figure 19a shows the schematic of the sample for 290 nm thin Ag ink printed on a glass substrate with a thickness of 1.05 mm. Similarly, Fig. 19b is the schematic of the samples of 192 nm thin Ag printed on a Si substrate of thickness 400 μm. Figure 3c shows the condition when the sample of Ag printed on Si was placed on a glass substrate. The length and width of the printed Ag ink were 50 μm and 50 μm, respectively, on a substrate size of 100 μm², due to the limitation on computer memory required for the COMSOL simulation. Figure 19d represents the various selected points of interest on the printed Ag ink. These points were selected to plot the temperature profile at different positions and analyze the temperature variation caused by radial and transverse heat flow during laser sintering. The radial heat flow refers to the heat flow along the surface plane where the laser hits the sample, and the transverse heat flow refers to the heat flow through the thickness of the sample. Figure 19e shows the dimensions where the temperature points are located using two vertically parallel planes through the Ag layer. The first plane, with the red dots, is located at the center cross-section of the Ag layer, while the second plane, with the green dots, is at the front surface of the Ag layer. The thickness of the Ag layer is denoted by d, which varies for different modeling scenarios.

Points K and L show the radial heat flow on the surface of the printed Ag ink. K is on the surface and at the center of the printed Ag ink strip. Point L is also considered to be on the surface of the printed strip, but at 25 μm away from the center. Points M, N, O, and P present the transverse heat flow in the depth of the printed Ag ink. Points M and O are at the center of the Ag layer, but at ½ of the total depth and the full depth of the Ag layer from the surface, respectively. Similarly, points N and are also considered to be under point L, but at ½ of the total depth and full depth of the Ag layer from the surface, respectively. This indicates that points O and P are located at the interface between the printed Ag strip and the selective substrate. The simulation was performed for continuous wave laser sintering with a laser wavelength of 445 nm, a laser spot size of 230 μm in diameter, and a laser scanning speed of 1 mm/s.

Author contributions

E.S., the first author, wrote the main manuscript on laser sintering experiments and the integration of the manuscript. Dr. S.V. contributed to the modeling and its integration in the manuscript. L.J., P.Z., and Dr. H.Q. provided the information on the EHD printers. Dr. N.K. is the main PI of the project with contributions on the primary conception of the idea, editing, integration, and proofreading the manuscript, and advising Ellie Schlake and Sagar Verma.

Data availability

Data will be made available upon reasonable request to the corresponding author.

Code availability

Code will be made available upon reasonable request to the corresponding author.

Competing interests

The authors declare no competing interests.