Enhanced thermoelectric performance of UV-curable silver (I) selenide-based composite for energy harvesting

Thermoelectric (TE) composites, with photocured resin as the matrix and Ag2Se (AS) as the filler, are synthesized by a digital-light-processing (DLP) based 3D printer. The mixture of diurethane dimethacrylate (DUDMA) and isobornyl acrylate (IBOA) is used as a UV-curable resin because of their low viscosity and high miscibility. Scanning electron microscopy (FE-SEM) images confirm that the filler retains its shape and remains after the UV-curing process. After completing curing, the mechanical and thermoelectric properties of the composite with different AS contents were measured. The addition of the AS filler increases the thermoelectric properties of the cured resin. When the AS contents increase by 30 wt.%, the maximum power factor was obtained (~ 51.5 μW/m·K2 at room temperature). Additionally, due to the phonon scattering effect between the interfaces, the thermal conductivity of composite is lower than that of pristine photoresin. The maximum thermoelectric figure of merit (ZT) is ~ 0.12, which is achieved with 30 wt.% of AS at 300 K with the enhanced power factor and reduced thermal conductivity. This study presents a novel manufacturing method for a thermoelectric composite using 3D printing.


Scientific Reports
| (2021) 11:16683 | https://doi.org/10.1038/s41598-021-96267-x www.nature.com/scientificreports/ (FDM), selective laser sintering (SLS), stereolithography apparatus (SLA), and digital light processing 26,27 . Yang et al. 28 fabricated a BiSbTe-based TE generator using an inkjet 3D printing technique and reported a high ZT with ~ 1.1. Wang et al. 29 achieved a maximum power factor of 11.3 μW/m·K 2 with a wire-like PLA/MWCNT/ BiSbTe composite, which was printed with 3D printing. DLP is considered a promising additive manufacturing technology among the many types of 3D printers due to its unique characteristic. DLP can print many samples within a single printing session. Also, by making photocured resins containing various fillers, composites for specific purposes can be printed.
In this study, we suggest a strategy to fabricate various shapes of thermoelectric materials using the 3D printer of the DLP method. The UV-curable resin used in this study is composed of diurethane dimethacrylate (DUDMA) and isobornyl acrylate (IBOA), and silver (I) selenide (Ag 2 Se) is chosen as the TE fillers. We designed a CAD design with various structures, and UV-curable resin with various Ag 2 Se contents is printed with this design using a 3D printer. After that, the mechanical and TE properties (S, σ, κ, PF and ZT) of our 3D-printed composite are measured. To the best of our knowledge, fabricating TE materials using a DLP-type 3D printer is novel and significant for manufacturing various shapes of TE devices.

Results and discussion
The composite sample preparation process is illustrated in Fig. 1. The formulation of the photoresin used in this study consists of a urethane monomer of DUDMA, a crosslinker of IBOA, and a photoinitiator of BAPO. At first, photoresin with varied Ag 2 Se contents (0, 10, 20, and 30 wt.%) are fabricated. The composition of each composite resin is listed in Table S1. It has been shown that when the AS content is more than 30 wt.%, the dispersion between the photoresin and AS fillers decreases, and the composite resin was not well-formed. After synthesizing the photoresin containing Ag 2 Se, all composite resins were printed with a DLP-based 3D printer. The exposure time was set at 5 s, and each layer of composite sample was set at 0.05 mm. The computer-aided design (CAD) model file was entered into the software of an ASIGA 3D printer. Then the printing proceeds through the composite resin in the tank. Composite resins can be cured into a variety of shapes and sizes. In Fig. S1, the various CAD designs and the 3D printed samples are illustrated, which can confirm the successful printing process. Finally, the cured composites were thermally annealed in N 2 gas in a tubular furnace at 123 °C, for 5 h.
XRD analysis was conducted to confirm the curing of the composite resin samples and to analyze the crystalline structure. The XRD patterns of the pristine Ag 2 Se NW and printed sample, with various Ag 2 Se contents (0, 10, 20, and 30 wt.%), are shown in Fig. 2a. The peaks in the XRD patterns of Ag 2 Se can be indexed to the orthorhombic phase of Ag 2 Se (JCPDS no. 24-1041). These results confirm that pristine Ag 2 Se was successfully synthesized. For the printed composite samples, it was found that the peaks related Ag 2 Se increased as the content of Ag 2 Se increases. However, since the intensity of the pristine photoresin peak is relatively weaker than that of Ag 2 Se, it is difficult to meaningful confirm the change of the peak according to the contents of the fillers.
The thermal degradation behavior of an Ag 2 Se, cured photoresin, and cured composite resin (AS30) is shown in Fig. 2b. A TGA was performed under N 2 gas to estimate the composition of the cured composites. The pristine Ag 2 Se showed thermal stability up to ~ 800 °C. However, the weight loss of the cured photoresin appeared around ~ 270 °C and ~ 380 °C. This characteristic is believed to be due to the different temperatures at which the masses of the two materials (DUDMA and IBOA) were used in photoresin. The TGA curve of two pristine materials with BAPO is shown in Fig. S2. The two weight loss points of the photoresin coincide with the weight loss point of each material. From these TGA data, the weight fraction of Ag 2 Se was confirmed (~ 28 wt.%). The average length and diameter of Ag 2 Se are ~ 20 nm and ~ 10 μm. After Ag 2 Se mixed with the photoresin and photoinitiator, the mixture was then printed into a various sample. Figure 3a,b shows the morphology cured photoresin. This image confirmed that the layered structure of a sample was cured by the DLP 3D printer. The low-and high magnification FE-SEM images of the AS30 sample are illustrated in Fig. 3c,d. The FE-SEM image (Fig. 3c,d) demonstrates the well-dispersion of Ag 2 Se, showing the morphology of the Ag 2 Se NW similar to Fig S2. Thus, the AS30 hybrid composite was successfully fabricated with a resin mixing and 3D printing process. Also, the surface FE-SEM images of AS30 are shown in Figs. S4a-b. In these figures, the dispersion of Ag 2 Se fillers is clearly shown.
The successful curing of resins containing an Ag 2 Se filler is in good agreement with the results found in the EDS analysis. Figure 4a-d shows the FE-SEM images and corresponding EDS mappings of the AS30 composite sample. No visible images other than Ag 2 Se were detected in the filler part of the composite, due to the strong signal of the Ag 2 Se filler compared to the composite resin. However, in other parts of the resin, image of C was relatively strong.
The mechanical properties of AS10, AS20, AS30, and the cured resin were studied. The dog-bone-shaped composite (Fig. S1) were used to analyze the mechanical properties of composite. The tensile strength of the various composite samples is plotted in Fig. 5. The tensile strength of the photoresin, AS10, AS20, and AS30 composite is 40.3, 31.5, 23.0, and 16.7 MPa. These mechanical properties allow the composites to withstand harsh mechanical environments.
For TE property measurement, the composites were printed with disks, which have 1 mm thickness and 25.4 mm diameter. Pristine Ag 2 Se samples were also prepared with a disk sample of the same size, which was prepared with hot-press method.
The room-temperature electrical conductivity of 4 composite samples (cured photoresin, AS10, AS20, and AS30) was shown in Fig. 6a. The electrical conductivity of pristine Ag 2 Se is ~ 680 S/cm, which was similar to the previous reports which analyzed the electrical conductivity of Ag 2 Se 30,31 . The composite's electrical conductivity was increased with 44.3 S/m to 98.9 S/cm with an increase of Ag 2 Se contents from 10 to 30 wt.%. This increase in electrical conductivity is because of the relatively higher electrical conductivity of pristine Ag 2 Se. Figure 6b shows the Seebeck coefficient of 4 composite samples at room temperature. Similar to the electrical conductivity, the absolute value of Seebeck coefficient also tends to increase with increasing Ag 2 Se content. This trend is also caused by the difference in the Seebeck coefficient of the filler and resin. The Seebeck coefficient of Ag 2 Se is ~ − 145 μV/K, which is similar to previous studies 32,33 . Also, the Seebeck coefficients of AS10, AS20, and AS30 are negative values, which indicates that the composite materials exhibit an N-type electrical transport behavior. Thus, most of the charge carriers in the composite are electrons.
The difference in electrical conductivity and Seebeck coefficient is further confirmed by the following relations. σ = n · e · µ S = 8 · π 2 · k B 2 3 · e · h 2 · m * · T · π 3 · n 2 3   Fig. S5. As the contents of Ag 2 Se increases, the carrier conductivity tends to decrease, and the carrier mobility shows the increasing trend. The room-temperature power factor (PF = S 2 · σ) of the 4 composite samples is illustrated in Fig. 6c. The maximum power factor of the composites is ~ 51.5 μW/m·K 2 at room temperature, which is seen in the AS30 sample. This increase is because of the increase of the Seebeck coefficient and electrical conductivities of the composite sample.
The total thermal conductivities of composite samples are shown in Fig. 7. The total thermal conductivity of the cured resin was 0.22 at room temperature, and the thermal conductivity of the AS10, AS20, and AS30 composite samples shows a lower value than that of the pristine resin. The thermal conductivity is composed of two terms, which are the electrical term (κ e ) and the lattice term (κ l ) (κ = κ e + κ l ). The electronic thermal conductivity can be estimated from the Wiedemann-Franz law; к e = L · T · σ, where L is the Lorentz number (L = 2.44 × 10 -8 V 2 /K 2 ) 34,35 . The total thermal conductivity is hardly affected by the electronic contribution due to its low electrical conductivity. Thus it mainly depends on the lattice term and the lattice thermal conductivity which, in turn, are mainly dependent on lattice phonon scattering. The composite sample contains Ag 2 Se, while the fillers were introduced into uncured photoresin. This addition makes more heterointerfaces between different materials. These boundaries generated from different materials increase phonon scattering, leading to a decrease in κ l . The results on the phonon scattering effect of different interfaces between these composite materials are supported by previous study 36 .
Based on this measured power factor and thermal conductivity, the thermoelectric figure of merits (ZT = S 2 · σ · T / к) was calculated, and these values are shown in Fig. 8. These ZT values show an increasing tendency with increasing Ag 2 Se contents. The AS30 composite sample shows a maximum ZT value which is ~ 0.12 at room temperature. Additionally, temperature-dependent thermoelectric properties of composite samples are shown in Fig. S6 and S7. The electrical conductivity is seen to gradually increases with increasing temperature, while the Seebeck coefficient initially increase until a temperature of ~ 450 K is reached, after thatm it  www.nature.com/scientificreports/ subsequently decreases with further increase in temperature. This tendency is similar to other previous studies using 3D-printed thermoelectric materials 37 . In this work, we demonstrated the technology of manufacturing thermoelectric materials through a 3D printing technique. Photoresins containing Ag 2 Se were successfully synthesized and cured with a 3D printer. The thermoelectric properties of the composites are lower than that of the pristine Ag 2 Se, but the use of 3D printing technology enables the creation of various shapes of thermoelectric materials that are difficult to obtain with conventional methods, which can greatly expand the field of thermoelectric applications.

Conclusions
The purpose of this study is to fabricate a thermoelectric composite with improved mechanically and thermoelectric properties. For this purpose, we synthesized the UV-curable resin containing DUDMA and IBOA because of their low viscosity and high miscibility. These composite resins showed high mechanical strength. Then, various contents of AS fillers were added to the composite resins and cured. The maximum filler content is 30 wt.% of AS. However, due to the light penetration issue during 3D printing and high viscosity, filler loading over 30 wt.% could not be processed. After curing, the composite's microstructure and morphology were analyzed with XRD, FE-SEM, and EDS analysis. Through these analyses, it was confirmed that the composite resin retains its composition after curing. The mechanical and thermoelectric properties were measured after successful printing. With increasing filler contents, the Seebeck coefficient and electrical conductivity of the printed sample showed an increasing trend. Composite sample with 30 wt.% of AS shows the maximum power factor of ~ 51.5 μW/m·K 2 at 300 K after curing. The results of this study show that thermoelectric composites can easily be synthesized  www.nature.com/scientificreports/ using 3D printing. We also believe that expanding these approaches will allow the versatility of 3D printing to be applied to the production of electronic and energy materials.

Date availability
All data generated or analyzed during this study are included in this paper. Raw datasets are available from the corresponding author on reasonable request.