Printing Three‐Dimensional Refractory Metal Patterns in Ambient Air: Toward High Temperature Sensors

Abstract Refractory metals offer exceptional benefits for high temperature electronics including high‐temperature resistance, corrosion resistance and excellent mechanical strength, while their high melting temperature and poor processibility poses challenges to manufacturing. Here this work reports a direct ink writing and tar‐mediated laser sintering (DIW‐TMLS) technique to fabricate three‐dimensional (3D) refractory metal devices for high temperature applications. Metallic inks with high viscosity and enhanced light absorbance are designed by utilizing coal tar as binder. The printed patterns are sintered into oxidation‐free porous metallic structures using a low‐power (<10 W) laser in ambient environment, and 3D freestanding architectures can be rapidly fabricated by one step. Several applications are presented, including a fractal pattern‐based strain gauge, an electrically small antenna (ESA) patterned on a hemisphere, and a wireless temperature sensor that can work up to 350 °C and withstand burning flames. The DIW‐TMLS technique paves a viable route for rapid patterning of various metal materials with wide applicability, high flexibility, and 3D conformability, expanding the possibilities of harsh environment sensors.


DIW-TMLS Equipment
The direct ink writing and tar-mediated laser sintering processes are implemented on a 4-axis precision motion stage driven by stepper motors (Shinano Kenshi, Japan), as shown in Figure S1(a).The three translational axes (X/Y/Z) have a stroke of 100 mm and resolution of 1 μm, while the rotary axis (θ) has a resolution of 0.005˚.The syringe filled with metal-tar ink is connected to a pneumatic dispenser (DS-982A, Taiwan Tech.& Material, Figure S1(b)), which provides a maximum pressure of 0.7MPa.The extrusion speed can be estimated by the Poiseuille's Law: [1] where  ̃ is the extrapolated viscosity of the ink, ∆ is the pressure drop in the nozzle of length L and radius R. The inner diameter of the printing nozzle ranges from 150 μm to 300 μm, and the corresponding extrusion speed varies between 45 mm/min and 180 mm/min.The printing speed is set to match the estimated extrusion speed in order to obtain the desired printing linewidth.A 976 nm continuous wave diode laser (BWT Beijing Ltd., Figure S1(c)) with maximum power of 30 W is adopted to sinter the printed pattern.The laser beam is transported by an optical fiber and focused by the focusing lenses mounted next to the syringe.The printing nozzle and the laser spot are aligned by adjusting the manual microstage.The ceramic heating plate is placed under the substrate for preheating.The air nozzle is placed next to the motion stage and activated during the laser sintering process to blow away the spatter.by incorporating C2 or C2H2 units, introducing a mass difference gap of 24/26 Da). [2]There are also a large amount of subordinate peaks that have certain mass increments to the listed ones, which can also be attributed to PAHs that have the same aromatic core but additional functional groups or side chains. [3]The abundance of PAHs results in a high degree of unsaturation, making tar an ideal antioxidative medium for laser sintering.Padprinting [5] > 15 mins

Figure S3 .
Figure S3.SEM images and particle size distributions of a) Cu-tar ink and b) Mo-tar ink.

Figure S4 .
Figure S4.Rheological behavior of metal-tar inks.a) Table of ink compositions.In the first column, the Mo-tar inks differ in the weight percentage of metal powders.In the second column, the Cu-tar inks differ in the particle sizes.b,c) Comparison of rheological behavior of Mo-tar inks.d,e) Comparison of rheological behavior of Cu-tar inks.f) Schematic showing the packing of multi-sized powders.

Figure S5 .
Figure S5.Schematics showing the processing conditions of a) laser focusing and b) defocusing.SEM images showing the surface morphology of printed Cu-tar filament with c) focused scanning and d) defocused scanning.Laser parameters: 75 mm/min -7 W.

Figure S6 .
Figure S6.SEM images of sintered Cu-tar filaments at different laser powers and scanning speeds.Scale bar: 10 μm.

Figure S7 .
Figure S7.SEM images of sintered Mo-tar filaments at different laser powers and scanning speeds.Scale bar: 20 μm.

Figure
Figure S8.a) 3D view, b) top view and c) front view of the hemispherical antenna.d) Detailed design parameters.

Figure S9 .
Figure S9.a) Schematic showing the effect of laser-nozzle gap on the forming accuracy during mid-air sintering.b) Helical trajectory of the printing nozzle.c) Photograph showing the dimensions of the printed helical structure.

Figure S10 .
Figure S10.a) Photograph of the mid-air sintering process showing the volumn increase after sintering.b) Measurement results of the sintered linewidth.12 measurement points were evenly selected along the helix structure.c) SEM image showing the porous surface of the sintered structure.d) SEM image showing the cross-section of the sintered structure.

Figure S11 .
Figure S11.a) Schematic and b) photograph of the interrogation antenna.c) Detailed design parameters.d) Simulated reflection coefficient of the interrogation antenna.

Figure S12 .
Figure S12.a) Schematic and b) design parameters of the SRR sensor.c) Schematic of wireless interrogation in simulation.d) Simulated reflection coefficient of the SRR sensor.e) Simulated electric field distribution and f) magnetic field distribution in the sensor.
Table S1 lists 8 polycyclic aromatic hydrocarbon (PAH) molecules that are matched to the predominant peaks.Peaks with odd molecular weights marked in Figure S2 may contain nitrogen atoms and are not further considered.Since MALDI-TOF MS cannot provide specific structure information, only possible structures corresponding to the molecular formulas are provided.However, the results are credible because the differences in molecular weights between the listed PAH compounds are either 24 Da or 26 Da, satisfying the 24/26 rule (new aromatic cores are formed

Table S1 .
Molecular formulas and structures of PAH components identified from MALDI-

Table S2 .
Performance comparison of electrically small hemispherical antennas fabricated by different methods.