A compact tri-notched flexible UWB antenna based on an inkjet-printable and plasma-activated silver nano ink


Plasma-activated printable silver nano ink

For decades printable metal inks with high conductivity and operational stability have been successfully utilized in the fabrication of conductive patterns60,61,62,63. Silver nanoparticles possess high electrical conductivity and low melting point (caused by thermodynamic size effect64), making them suitable for ink applications. Silver-based conductive inks are less expensive than platinum and gold-based metal inks, and they are more stable than copper-based inks and less prone to oxidation. Thus, a commercial silver nano ink was chosen here for the designed antenna.

The morphology and size of the metallic particles in the silver ink were observed by TEM firstly. As shown in Fig. 3a, the silver particles in ink exhibit spherical or hexagonal morphologies, with sizes smaller than 30 nm, which is advantageous for the electrical property. This is because a conjunction of different shapes and sizes provides multiply contact points and improved packaging efficiency. In addition, it is beneficial to the inkjet printing since it can avoid clogging and blocking of print-head nozzles.

Figure 3
figure 3

(ac) TEM image, contact angle and surface tension of the silver nano ink; (d) the printed conductive patterns; (e,f) XRD patterns of the printed silver films with thermal sintering at 70, 110 and150°C and plasma sintering at 300W for 10, 20 and30 min; and (g) Resistivity values of the silver films against temperature or plasma time

For printing, the fluid properties of the ink are vital because they impact printing quality, which in turn influences the electrical and mechanical properties of the printed patterns. For antennas, their radiation performance is usually related to three parameters: effective length, effective area and impedance. The effective length characterizes the antenna’s efficiency in transmitting and receiving electromagnetic waves, while the effective area indicates the antenna’s capacity to capture and focus electromagnetic waves65. Impedance is the sum of resistance and reactance, which is governed by the ink used. During printing, if there are significant geometric differences between the printed and the designed antenna, the values of these three parameters will inevitably change, which will in turn cause changes in the antenna’s radiation performance. Therefore, the printing quality of the antenna pattern is crucial. In other words, the fluid properties of the ink are important. To jet smoothly, the rheological parameters of the ink, such as viscosity, surface tension and wettability, must be carefully evaluated. Favorable wettability is a prerequisite to ensure good adhesion between the ink and flexible substrates, while surface tension is useful in determining whether an ink will remain where it is deposited and how wide it will be after drying61. Both are critical for obtaining well-defined, high-resolution and conductive antenna patterns. Based on the Ref.60, in the case of the used piezoelectric print head, the ink viscosity should be in the range of 8-15 cP, while the surface tension should be in the range of 25–35 dyne/cm. As seen in Fig. 3b and c, the ink has a surface tension of 29.1 dyne/cm and a contact angle of 19.75° is measured on the PET substrate. The viscosity is measured to be 8.2 cP. These values are within the range required for piezoelectric print heads, suggesting that the ink is suitable for inkjet-printing. As expected, silver conductive patterns in different shapes were successfully fabricated (Fig. 3d) on flexible PET substrates, proving the good printability and applicability of the ink.

Metal nano inks, in general, do not have intrinsic conductivity and require additional post-processing to remove the solvent, stabilizing agents from the surface of the nanoparticle and induce a coalescence of the nanoparticles in order to achieve good conductivity. Together with drying and curing, sintering processes are considered post treatments. Once this process takes place a continuous percolating network is formed throughout the printed features, resulting in electrical property. Three dominant factors are mainly responsible for the electrical performance of the metal ink films after sintering: the sizes of the particles produced, the degree of organic residues and the film’s densification. This is the result of both the conductive channel effect and the tunneling effect, as proved by previous work66,67,68,69.

Here, thermal sintering and plasma sintering were adopted for the printed square films in order to identify a more efficient way. XRD, SEM, EDS and four-probe analyses were employed to investigate the physical phase, morphology and resistivity of the films produced by both sintering approaches.

Figure 3e and 3f show the crystalline structure of thermal and plasma sintered films, respectively. All samples exhibit peaks corresponded to (111), (200), (220), (311), and (222) planes of a face-centered cubic silver crystal, respectively, revealing the formation of metal silver films. The intensity of diffraction peaks of silver increases with the increase of sintering temperature or time, indicating an improved crystallinity. The resistivity of the sintered silver films against temperature or plasma time is given in Fig. 3g. A significant decrease in resistivity was found as the temperature or time increases, from 63.5 μΩ·cm of 70 °C to 20.2 μΩ·cm of 150 °C for thermal sintering and from 51.6 μΩ·cm of 5 min to 14.1 μΩ·cm of 30 min for plasma sintering. In comparison with thermal sintering, plasma sintering takes less time to achieve similar electrical performance, as proved by the resistivities obtained using thermal sintering at 150 °C for 60 min (20.2 μΩ·cm) and plasma sintering at 300 W for 10 minutes (19.7 μΩ·cm). The decrease in resistivity with the temperature or time can be easily understood because silver nanoparticles are better connected, and the solvents and organic layer evaporate or/and decompose adequately. The resistance values measured at various points on the film are basically the same, meaning that the printed film has homogeneous electrical properties.

Figure 4 shows the surface morphologies and chemical composition results of printed silver films with thermal sintering and plasma sintering. It is clearly visible that both treatments promote the coalescence of silver nanoparticles but there are some nuances. In the case of thermal sintering, at 70 °C, the silver nanoparticles are evenly dispersed and loosely connected. Upon sintering at 110 °C, these particles begin to densify, and some of them gather to prepare for diffusing each other. A temperature of 150 °C caused significant neck formation. For plasma sintering, the impact is more pronounced, especially for the silver film produced after 30 minutes. This can probably be attributed to the fast-heating speed and increased thermal energy produced in short time, which allows for a flowing of the structure and the coalescence of the nanoparticles. When plasma sintering silver nanoparticles, the excited high-energy plasma active substances can decompose the organic coating layer covering the outer layer of the nanoparticles and form small molecular compounds through chain scission. These small molecules are volatilized in the low-pressure plasma, leaving behind the uncoated silver nanoparticles, thereby promoting the connection between particles.

Figure 4
figure 4

Surface morphologies and EDS results of the printed silver films with thermal sintering at (a) 70 °C, (b) 110 °C and (c, g) 150 °C and plasma sintering at 300 W for (d) 10min, (e) 20 min and (f, h) 30 min; (i) Surface profile of the plasma-sintered silver film.

EDS analysis was employed to investigate the chemical composition of the silver films from both sintering methods. Three elements, C, O and Ag, were detected in the films, which is in accordance with the original chemical composition of the ink. In terms of silver content, the film formed by thermal sintering at 150 °C for 60 min has a value of 95wt%, which is almost the same as that obtained by plasma sintering at 300 W for 30 min. This indicates the possibility of utilizing a rapid plasma method to produce a conductive silver film. In addition, a plasma-activated ink is also beneficial to the usage of thermally sensitive plastic substrates, broadening options for antennas on flexible substrates.

The above analyses indicate that for the ink conversion, plasma sintering outperformed thermal sintering in terms of electrical performance, sintered film structure and time, as well as without causing damage to the substrates. Considering the PET substrate’s low thermostability and the electrical requirements of the antenna pattern, 300W for 15 minutes were finally chosen for the ink conversion.

Optimization of antenna parameters

Notch technology can generate a suppression effect in an antenna’s specific frequency band, which is similar to a band-stop filter. It not only solves the problem of interference between systems, but also reduces the demission of the systems. The adoption of three nested “C”-shaped slots is the key to the design of the proposed antenna, which is primarily used to realize the operating bandwidth, achieve three notch performance and decrease the size of metal patch. The influences of parameter values of “C”-shaped slots such as the width (N), the length (W3), the height (L1, L2, L3), and the opening length (W1, W2) on the performance of the antenna were investigated independently. Here, the influence of the width parameter N of the “C”-shaped slot on the notch performance of the antenna was taken as an example. During the optimization procedure, all other antenna parameters are kept constant. When the N value is between 0.2mm and 0.6mm, the ultra-wideband performance of the antenna suffers. Simultaneously, the center frequency of the notch shifts to the left, the coverage width narrows, and the WiMAX band cannot be effectively filtered, and the antenna fails to meet the design requirements. With the gradual increase of the N value to 0.4mm, the S11 curve of the antenna exhibits the best return loss performance. Therefore, we choose 0.4 mm as the optimal N parameter. The process for determining the optimal values of the other parameters is similar, but there is a sequential order, that is, the length and height of the outermost c-slots are determined first, and so on. After a series of simulation optimization, the ultimate antenna size was determined to be: 16 × 17.6 × 0.12 mm3, as shown in Table 1.

Table 1 Dimensions of the optimized antenna.

Performance of the proposed antenna

Figure 5a–c shows the simulation results of the antenna in terms of gain, radiation efficiency and S11 values. It can be seen that the antenna operates at 2.9–10.61 GHz, covering the desired UWB frequency band. Meanwhile, the antenna generates notches at 3.51–4.58 GHz, 5.42–5.96 GHz and 8.04–8.31 GHz with S11 values greater than – 10 dB in these areas, successfully shielding the interference from WiMAX, WLAN and X uplink frequency bands. In addition, the smaller return loss values indicate that the antenna has good matching performance and can reduce signal reflection and loss. The gain of the antenna is stable above 2dBi in the whole operating frequency band, with a peak value of 5.5dBi, and the radiation efficiency is stabilized around 60%, which indicates that the antenna has good radiation performance and can meet the application requirements of UWB communication systems. In the WiMAX, WLAN and X notch frequency bands, the gain and radiation efficiency of the antenna are significantly lowered, with minimum gain values of − 17.2 dBi, − 10.2 dBi and – 6 dBi respectively, demonstrating the good notch characteristics of the antenna in theses frequency bands.

Figure 5
figure 5

(a) Gain, (b) radiation efficiency, (c) S11 values, (d) radiation pattern and (e) surface current distribution of the designed flexible trip-notched UWB antenna.

The E-plane and H-plane patterns of the antenna at the 6 GHz frequency are given in the Fig. 5d. The E-plane presents an “8”-shaped bidirectional radiation characteristic, while the H-plane shows omnidirectional radiation. This can meet the requirements of a UWB monopole antenna.

The surface current distribution of the antenna was analyzed by HFSS at the notch frequencies of 4.1 GHz, 5.7 GHz and 8.1 GHz to illustrate its notch mechanism, as shown in Fig. 5e. The red and blue color indicates the maximum and the minimum distribution of the current. Obviously, the current is basically concentrated around the three C-shaped slots, indicating the creation of strong notch resonance at these notch frequencies. As can be seen from the first image of Fig. 5e, the flow of surface current on the radiating patch is mainly collected around the outermost “C”-shaped slot structure instead of radiating from the patch edges; consequently, the net radiation of the antenna is blocked at frequency of WiMAX band. Similarly, the middle and the inner “C”-shaped slots show the surface currents that are concentrated around the slot structure, which provides the band-rejection of the WLAN and X-uplink bands. In other words, the energy is focused in the notch structure and not radiated out, thus resulting in good notch characteristics and confirming the rationality of the antenna design.

Assessment of antenna prototype

Finally, an antenna prototype was fabricated on flexible PET substrates by inkjet printing of the silver nano ink. The S11 values of the prototype were measured using a Keysight E5063A vector network analyzer, and the results are given in the Fig. 6.

Figure 6
figure 6

S11 values of the fabricated antenna prototype before (a) and after (c) bendability test, and (d) radiation patterns of the antenna prototype at 6.7GHz when bent along the X-axe with a curvature radius of 10mm, 20mm and 30mm.

As can be seen from Fig. 6a, the changing trends of the measured S11 values are essentially identical with those of the simulated ones in the UWB frequency range. Simultaneously, the antenna prototype successfully suppresses narrow-band interferences from WiMAX, WLAN and X uplink frequency bands, with S11 values larger than-10dB in three frequency bands of 3.5-4.45 GHz, 5.5-6.4 GHz and 8.1-8.3 GHz. The difference between the measured and the simulation results is primarily reflected in the aspects of the right shift of the second notch center frequency and the downward shift or the right shift of some resonance points, which could be related to the errors introduced during the physical processing and the uneven application of conductive silver adhesive connecting the antenna and the RF connector during the measurements.

Figure 6c,d depicts the changes in return loss values and radiation patterns when the antenna prototype is bent along the X-axis with a curvature radius (R) of 10 mm, 20 mm and 30 mm, respectively. With the decrease of R value from 30 mm to 10 mm, the operating frequency range of the antenna varies slightly and the antenna maintains good notch properties, with S11 values less than − 10 dB. The radiation properties are satisfactory when R is 30 mm but distorted when the antenna is bent with a small radius of 10 mm.

From the above analyses, it can be seen that the effects of bending radius on the return loss values of the antenna are not significant but mainly affect the radiation patterns. As for the reason, it might be associated with the impedance matching characteristics of the antenna. Therefore, when designing a bendable antenna, one must consider this issue.

Besides analyzing the basic parameters, the interaction between human body and antenna was also investigated. A 40 × 60 mm2 body phantom consisting of muscle, fat, and skin layers was employed to simulate the antenna’s performance on the human body, as shown in Fig. 7a. The required human tissue model parameters were obtained from the reference70. The index parameter SAR (specific absorption rate) was used to evaluate the antenna security performance for human body application. According to the EU safety limits, when the maximum SAR value of 10g tissue is less than 2 W/Kg, human health will not be affected by electromagnetic radiation. During the simulation process, the antenna was placed at a height of 3 mm from human tissue to obtain its SAR value and radiation pattern. As shown in Fig. 7b, the proposed antenna has a maximum SAR value of 1.153 W/Kg at an input power of 0.093 W, which is within the safety limit. The radiation pattern results demonstrate that the radiation intensity of the side lobe near the human body is attenuated (Fig. 7c), which might be attributed to the absorption and reflection of electromagnetic waves by human tissues. The simulation results of SAR value and radiation pattern reveal that the influences of the antenna on human health are within an acceptable range and will not affect human health. The antenna prototype is tested on a human arm in free space (Fig. 7d). The measured S11 values (Fig. 7e) show that the antenna operates in a range of 2.4 to 12.0 GHz, which deviates somewhat from the simulated values of 2.9 and 10.61GHz, but both cover the desired UWB frequency band with desired triple notch properties. There is a slightly shift at the second notch center frequency, which might be associated with the absorption and reflection of electromagnetic waves by clothes and human tissue. On the whole, the proposed antenna satisfies the design requirements and achieves the notch characteristics in the targeted UWB frequency bands while having a certain of bendability and acceptable SAR values under on-body conditions.

Figure 7
figure 7

(a) The human body phantom utilized for antenna simulation, (b,c) Simulated SAR values and radiation patterns, (d) photo of the antenna prototype placed on the human arm under the measurement and (e) the tested S11 values

Here, we also compared the created antenna to those reported in the literatures in terms of dimensions, notch numbers, bendability and fabrication technique. As shown in Table 2, the majority of existing flexible UWB antennas were fabricated using an etching procedure, which is not environmentally friendly. Besides, UWB antennas with triple-notch characteristics are relatively large in size and not flexible, making integration on flexible/wearable devices difficult. Compared with the antennas listed in the Table 2, the proposed antenna has three advantages over them. For starters, it can be created on low-cost flexible substrates using a facile, high-efficiency and environmentally friendly ink-based printing process, which has cost and process advantages over those fabricated with traditional etching approach, which needs to go through a series of complex processing steps and produces a large amount of waste liquid, as well as over those fabricated with screen-printing, which needs to make a pattern plate in advance and is unable to modify the antenna pattern in real time. Secondly, it has the smallest size in comparison with others, making it suited for portable UWB system applications, as well as easy integration on flexible and wearable devices. Thirdly, it is bendable and safety for on-body use while maintaining favorable notch characteristics, making it a possible candidate for applications in high-performance UWB systems and flexible/wearable electronic devices. Finally, the utilization of plasma sintering broadens the application range of the antenna substrates and has obvious advantages in terms of time and efficiency in comparison with thermal sintering. A variety of flexible substrate materials, such as polymers, textiles, even paper materials could be possible for the antenna. If the hydrogen is used as the feed gas, the plasma can have a reducing property, which is also beneficial for the use of other metal-based inks such as copper inks. Of course, there are some other flexible UWB antennas with notch characteristics. Although these antennas have achieved structural flexibility, they are created via non-inkjet printing process. Kapton film, polyimide, is often used as a substrate for flexible electronics. It was not chosen as a substrate in this work because it is less compatible with the purchased silver ink than PET, and an additional hydrophilic surface treatment process is required for it during inkjet printing to ensure the precision of the printed antenna pattern in geometry. In addition, polyimide is almost opaque and has a wide thermal stability, and we aim to explore the antenna applications on transparent and temperature-sensitive flexible substrates and highlight the advantages of plasma sintering.

Table 2 Comparison of the proposed antenna with others reported in the literatures.

Overall, the usage of inkjet printing, flexible substrates and low temperature-activated conductive ink has provided a facile method for the fabrication of flexible/wearable UWB antennas. However, it concurrently brings challenges to the materials, printing process and antenna structure design. Future research will focus on innovative ideas in material science, process technology, creative engineering solutions in mechanical and electromagnetic designs, and the intelligent combination of these parts.



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