WO2016164729A1 - Inline laser sintering of metallic inks - Google Patents

Abstract

The present disclosure relates to a device for inline sintering of an ink during 3D printing. The device comprises a nozzle having an outlet for extrusion of a filament comprising an ink, where the nozzle is configured to move relative to an underlying substrate; a laser generator to output a laser beam; and a light guide in optical connection with the laser generator to receive the laser beam from the laser generator and focus the laser beam on a focal area determined by a path of the nozzle over the substrate.

Description

INLINE LASER SINTERING OF METALLIC INKS

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under contract number DE-SC0001293 awarded by the Department of Energy. The government has certain rights in the invention.

PRIORITY STATEMENT & RELATED APPLICATIONS

[0002] This application claims priority to US Provisional Application

62/144,706, filed April 8, 2015, the entirety of which is incorporated therein by reference. Further, the following patent applications are also hereby incorporated by reference in their entirety: PCT/US2014/043860, filed June

24, 2014, PCT/US2014/063810, filed November 4, 2014,

PCT/US2014/065899, filed November 17, 2014, and PCT/US2015/015148, filed February 10, 2015.

TECH N ICAL FIELD

[0003] The present disclosure generally relates to three-dimensional (3D) printing technology. Specifically, the present disclosure relates to inline laser sintering of metallic filaments deposited by 3D printing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Fig. 1 is a schematic diagram illustrating a 3D printer for inline laser sintering of metallic inks according to exemplary embodiments of the present application;

[0005] Fig. 2 is a schematic diagram illustrating an exemplary

embodiment of an electronic controller of the 3D printer;

[0006] Fig. 3 is a flowchart illustrating a method for inline laser sintering of metallic inks;

[0007] Fig. 4 illustrates an example of focusing a laser beam over a filament of an ink; [0008] Fig. 5 illustrates another example of focusing the laser beam over the filament of the ink;

[0009] Fig. 6 illustrates an example of focusing the laser beam over the filament of the ink; and

[0010] Fig. 7 illustrates a relationship between a linear motion of a laser beam and a rotational motion of a substrate;

[0011] Figs. 8a and 8b illustrate effects of sintering laser frequency to material properties of sintered silver filaments; and

[0012] Figs. 9a and 9b illustrate effects of the sintering laser power to the material properties of the sintered silver filaments;

[0013] Figs. 10a and 10b illustrate an actual laser direct ink writing apparatus;

[0014] Figs. 1 1 a - 1 1 i illustrate basic settings of the laser direct ink writing apparatus in an example experiment and experimental results thereof;

[0015] Figs. 12a - 12c illustrate a method of printing curvilinear structure using a rotating state;

[0016] Figs. 13a and 13b Illustrate effects of pulse repetition rates on microstructure of a printed filaments;

[0017] Figs. 14a - 14i illustrate electrical conductivity and nanostructure of laser-annealed silver wires;

[0018] Figs. 15a and 15b illustrate effects of different pulse repetition rates on annealed silver microstructure.

DETAILED DESCRIPTION

[0019] The present disclosure describes an apparatus for 3D printing that allows a printed filament to undergo localized laser heating during deposition. For example, silver or other metallic inks may be sintered inline during 3D printing. With the methods and apparatus described herein, a sintered conductive filament having a distinct grain structure and/or lower thermal and/or electrical resistivity than achievable with conventional 3D printing may be obtained.

[0020] Fig. 1 is a schematic diagram illustrating a 3D printer 100 for inline laser sintering of metallic inks. The 3D printer 100 may include a controller 102, a laser generator 104, a waveform generator 108, a light guide 120, a printing nozzle 140 ("the nozzle"), an actuator 150, and a substrate 160.

[0021] The laser generator 104 may be any type of laser generator that meets the power and wavelength requirements of the present disclosure. For example, the laser generator may be an 805 nm diode laser generator configured to generate an infrared laser beam 1 10 near 805 nm. Emission of the laser beam 1 10 may be controlled by a waveform generator 108, which is electrically connected with the laser generator 104. Further, the waveform generator 108 may be in electronic communication with and under the control of the controller 102. During operation, the controller 102 may be able to control the waveform generator 108 to input a

predetermined laser actuation signal to the laser generator 104. The predetermined laser actuation signal may be any form. For example, the laser actuation signal may include periodic impulses at a predetermined frequency, a predetermined impulse duration, and predetermined amplitude. The predetermined frequency, duration and amplitude may be constants or may vary with respect of time and space. In response to the laser actuation signal, the laser generator 104 may emit laser impulses with the predetermined amplitude at the predetermined frequency.

Similarly, the laser actuation signal may also be a sinusoidal signal, a periodic triangular signal, or a signal with constant input amplitude.

[0022] The laser generator 104 may be optically connected to the light guide 120. For example, the laser generator 104 may be connected to the light guide 120 via an optical fiber 106, so that the emitted laser beam 1 10 may be directed into the light guide 120. [0023] The light guide 120 may include a housing 122 having a light path therein. For example, the light path may include a fiber optic collimator 136, a first mirror 130, a second mirror 132, a first lens 124, a second lens 126, and a third lens (objective lens) 128, which are all mounted on the housing 122. The fiber optic collimator 136 may be connected to the optical fiber 106 to introduce the laser beam 1 10 from the optical fiber 106 into the light path. The optical fiber may be multi-mode fiber (or single mode fiber). Upon exiting the multimode fiber optic, the laser beam has a uniform, top-hat intensity distribution. The narrower the beam, the greater the dispersion, i.e., the smaller the diameter the quicker the laser beam expands when traveling through space. To place the beam diameter expansion under control, the fiber optic collimator 136 first may actively expand the diameter of the laser beam 1 10 to reduce the rate of diameter expansion. Then the laser beam 1 10 may pass through the first lens 124 and the second lens 126 to further adjust the diameter for filling the back aperture of the third lens (objective lens) 128. To this end, the first mirror 130 and the second mirror 132 may be placed between the first lens 124 and the second lens 126 to reflect the laser beam 1 10. As a result, the length and direction of the light path between the first lens 124 and the second lens 126 may be adjusted so that when the laser beam passes through the second lens 126, the laser beam is a collimated beam with a proper diameter towards the correct direction. Finally, the third lens 128 may focus the laser beam 1 10 at a focal area A. For example, the focal area may be directly below the light guide 120 at a distance I.

According to an exemplary embodiment, the first lens 124, the second lens 126, and the third lens 128 are convex achromatic doublet lenses. The first lens 124, the second lens 126, and the third lens 128 may also be designed as a combination of other types of lenses to expand the laser beam 1 10 from the optical fiber 106 and then focus the laser beam 1 10 to the focal area A. [0024] The light guide 120 may be configured to select a mode of the laser beam 110 or shape of a cross section of the laser beam 110. For example, the light guide 120 may select and/or shape the laser beam 110 to have an annular cross section ("annular laser beam"). The annular beam may be formed by use of a light-filtering annular slit (not shown) placed in a conjugate image plane, through the use of a conical lens and spherical lens (not shown) in the light path, through use of a phase spatial light modulator (not shown) in the light path, use of a hollow optical fiber or a fiber that selects only a high-order mode (such as LP₀,_n where n>5) as the optical fiber 106.

[0025] Additionally, the light guide 120 may also include an observer 134 mounted on the housing 122 to observe a small area around the focal area A. For example, the second mirror 132 may be a semi-transparent mirror, such as a pellicle mirror, or a wavelength-selective dichroic mirror that reflects infrared light while transmitting visible light, and the observer 134 may be a camera above the semi-transparent or dichroic mirror 132 to take pictures or videos over the small area around the focal area A.

[0026] The nozzle 140 may include a chamber 146 to hold an ink. The chamber 144 may be of a cylindrical or prismatic shape such as a syringe. The nozzle 140 may include an outlet 142 at one end of the chamber 146. The size of the outlet 142 may vary depending on needs. For example, in an exemplary embodiment the outlet 142 may have a diameter of 10 microns. Further, the nozzle 140 may be in communication with the controller 102 and may be actuated by the controller 102. For example, the controller 102 may increase a pressure to the chamber 146 through a gas conduit 148. Consequently, the nozzle 140 may be able to extrude a filament 146 of the ink through the outlet 142. Further, with the control of the controller 102, the nozzle 140 may be able to extrude the filament 146 at a predetermined flow rate. For example, the controller 102 may control a pressure actuator, or a fixed flow actuator such as a syringe pump to control the filament 146 flow rate. Typical flow rates used may range from 10^"18 cm³/s to 10^"8 cm³/s.

[0027] The extruded filament 146 comprising the ink 144 may be deposited on a substrate 160. The outlet 142 of the nozzle 120 may be positioned substantially and/or sufficiently close to the focal area A, so that the filament 146 may pass through the focal area A when being extruded from the outlet 142. For example, the outlet 142 may be placed within the focal area A. Alternatively, the outlet 142 may be placed at a distance d away from the focal area. When the ink 144 is a metallic ink (e.g., a fluid with metal or alloy particles) and/or ceramic ink (e.g., a fluid with ceramic particles), the infrared laser beam 1 10 may heat the filament 146 to a temperature equal to or higher than a sintering temperature of the ink. Alternatively, the infrared laser beam 1 10 may also heat the filament 146 to an annealing temperature high enough to solidify the ink but lower than the sintering temperature. The power of the infrared laser, or the

temperature the ink is heated by the laser, may be determined by specific characteristics of the heated ink. For example, when the laser heats a silver ink filament into a silver wire, conductivity of the silver wire may depend on the temperature of the silver ink filament being heated. A sintered silver ink filament may have higher conductivity than an annealed silver ink filament. Accordingly, depending on the requirement of

conductivity, the power of the laser may be adjusted to sinter the silver wires, which results in a higher conductivity to the silver wire, or simply raise the temperature to an annealing temperature, which results a lower conductivity to the silver wire. Consequently, when the filament 146 passes through the focal area A, or when the focal area A passes through the filament 146, the filament 146 may be selectively heated to form a sintered filament of the corresponding metal, alloy or ceramic. Prior to sintering, the particles of the ink may be weakly bonded by van der Waals, electrostatic, or dipole forces. After sintering, adjacent particles in the filament may be strongly bonded or fused together. Consequently, the sintered filament may exhibit improved electrical conductivity as well as increased physical stiffness, yield strength, resilience, and toughness. In the present disclosure, the filament is considered sintered when it is completely sintered, or partially sintered, or when either some polymer binder has been burnt out or some particles have coalesced.

[0028] An unsintered portion of the filament may extend a distance d between the focal area A and the outlet of the nozzle. The distance d may be made sufficiently small so that the viscosity and/or surface tension of the unsintered portion of the filament are able to sustain its shape prior to sintering or reaching the substrate. For example, to print a bridge-like structure that is unsupported between two points, the nozzle may first be moved to extrude a supported portion of the filament 146 that is deposited directly on the substrate 160. Then, to form the unsupported portion of the structure, the nozzle may be moved in a direction away from the substrate while continuing to extrude the filament. If the rheological properties of the ink and the distance d are properly chosen, the unsintered portion of the filament can sustain its shape without collapse and/or buckle and/or deformation prior to sintering and/or reaching the substrate.

[0029] The nozzle 140 may be mounted on an actuator 150, which is controlled by the controller 102. The actuator 150 may be any type of mechanical structure that can provide linear and/or rotational motion to the nozzle 140. For example, the actuator 130 may be a carriage rail structure typically used in inkjet printers. The nozzle 140 may be mechanically mounted on a carriage rail 152, or on an air bearing. A carriage motor 154 may be configured to drive a belt or a thread to move the nozzle 140 along the x, y, z direction, or rotate around the z axis in Fig. 1 . By moving the nozzle 140 with a predetermined and controlled path, the 3D printer may be able to move relative to the underlying substrate 160 at a controlled and predetermined speed to deposit the ink 144 that flows out of the outlet 142 in a predetermined pattern and at a predetermined print speed on the substrate 160.

[0030] Similarly, the light guide 120 and the substrate 160 may also be respectively mounted on the actuator 150 and may move together with or independently from the nozzle 140.

[0031] Fig. 2 is a schematic diagram illustrating an exemplary

embodiment of the electronic controller 102. The electronic controller may be a specially designed electronic device for controlling the 3D printer 100 or may be a computer implementing special applications for controlling the 3D printer 100. The controller may be configured for wired or wireless communication with the 3D printer 100. The controller 102 may vary widely in configuration or capabilities, but it may include one or more central processing units 222 and memory 232, at least one medium 230 (such as one or more transitory and/or non-transitory mass storage devices) for storing application programs 242 or data 244 that may control components of the 3D printer 100. The processing units 222 may execute the application programs 242 or data 244 to perform the controlling methods disclosed in the present disclosure.

[0032] The controller 102 may further include one or more power supplies 226, one or more wired or wireless network interfaces 250, one or more input/output interfaces 258, and/or one or more operating systems 241 , such as Windows Server™, Mac OS X™, Unix™, Linux™,

FreeBSD™, or the like. Thus a controller 102 may include, as examples, industrial programmable motor controllers with or without a graphical user interface, dedicated rack-mounted servers, desktop computers, laptop computers, set top boxes, mobile computational devices such as smart phones, integrated devices combining various features, such as two or more features of the foregoing devices, or the like.

[0033] Fig. 3 is a flowchart illustrating a method for inline laser sintering of metallic inks. The method may be implemented using the 3D printer 100. For example, the method may be implemented as a set of instructions stored in the storage medium 230 and may be executed by the processor 222 of the controller 102. The method may include the following operations:

[0034] In 302: extruding the filament 146 of an ink 144 from the outlet 142 of the nozzle 140 at a predetermined speed. After extrusion, the filament 146 may be deposited on an underlying substrate 160. The substrate 160 may be a hard substrate, such as a piece of glass, a silicon wafer. Alternatively, the substrate may be a soft and/or flexible substrate, such as a polymer surface.

[0035] The ink 144 may be a ceramic ink and/or a metallic ink

associated with the sintering temperature. For example, the ink may be a silver ink, with a sintering temperature of 150-600°C. Based on a design of the structure being printed, the 3D printer 100 may extrude the filament 146 at a predetermined speed.

[0036] In 304: moving the nozzle 140 relative to the underlying substrate 160 to deposit the extruded filament 146 on the substrate 160 along a predetermined path.

[0037] In 306: focusing a laser beam on or above the focal area A over the filament of the ink.

[0038] The laser beam may be an infrared laser beam capable of heating the filament 146 to a temperature equal to or higher than the sintering temperature of the ink. Consequently, the portion of the filament passing through the focal area may be heated. When the temperature of the portion is higher than the sintering temperature, sintering may occur such that the portion of the filament contains fused metal or ceramic particles.

[0039] When printing on flexible substrates such as a polymeric substrate, step 306 may further include selecting a wavelength of the laser beam so that the polymer substrate is transparent or substantially transparent to the laser beam (i.e., the laser beam is not substantially absorbed by the polymer substrate). The properly selected wavelength may prevent overheating or ablating of the polymer, and ensure that the laser energy is predominantly absorbed by the silver filament. For example, PET has low absorptivity at a laser wavelength of 808 nm.

Therefore, when the PET is used as a substrate, the 3D printer may select an 808 nm laser as the sintering laser. Consequently, the PET substrate may not be heated or may just be slightly heated, whereas the silver filament may absorb the energy from the laser beam and be sintered.

[0040] In 308: moving the focal area A so that the filament 146 of the ink extruded from the outlet 142 always passes through the focal area.

[0041] In 310: heating a portion of the filament that passes through the focal area A to a temperature equal to or higher than the sintering temperature of the ink before the portion of the filament moves out of the focal area.

[0042] To this end, the 3D printer may position the focal area A of the laser beam 110 over the outlet 142 and may move the light guide 120, as shown in Fig. 4. Because the laser beam 110 is always focused on the outlet 142, the filament 146 may be heated by the laser beam 110 immediately when it flows out of the outlet 142. Because the filament 146 is extruded out of the outlet 142 at the predetermined speed, the filament 146 may only be heated by the laser beam for a limited period of time. This limited period of time may include the time when a portion of the filament is in the focal area A. It may also include a short period of time after the portion of the filament moves out of the focal area when the portion is continued to be continues being heated due to heat transferred from the focal area A. The faster the predetermined speed is the shorter the time that the filament 146 is heated by the laser beam 110. The heat from the laser beam 110 may cause physical or chemical effect on the portion of the filament 146. For example, the laser beam 110 may be tuned to a power strong enough to raise the portion of filament 146 within the focal area A to or above the sintering temperature before and/or after the portion of the filament passes through the focal area A, thereby sintering the filament 146.

[0043] Because the outlet 142 may be very close to or within the focal area A, precautions may be taken to avoid overheating the nozzle 142 and prematurely sintering the ink. For example, the controller 102 of the 3D printer 100 may select to output an annular laser beam from the laser generator 104. As shown in Fig. 4, the annular laser beam may have an annular cross section where the power of the laser beam 110 becomes weaker towards the center of the focal area. In an exemplary embodiment, the outlet 142 of the nozzle 140 may be placed at the center of the annular laser beam 110. The laser power at the center of the focal area A may be tuned low enough that the outlet 142 is not overly heated. Alternatively, the laser may be pulsed such that the time-averaged heat transfer to the silver is reduced, while the maximum temperature reached by the filament during a laser pulse remains high enough for sintering to occur. Alternatively, the print speed and/or extrusion rate may be increased, increasing the convection of heat away from the nozzle, preventing upstream heat diffusion from sintering the nozzle. Alternatively, the nozzle may be heat sinked either passively or actively, by a high thermal conductivity material to limit the temperature of the silver at the nozzle.

[0044] Because the focal area A is always positioned over the outlet 142 in this embodiment, the 3D printer 100 may linearly move the light guide 120 and the nozzle 140 together when printing the filament 146 along the predetermined path, regardless of whether the predetermined path is a straight line or curved line.

[0045] When printing on flexible substrates such as a polymeric substrate, and when the wavelength of the laser beam is properly selected to prevent overheating or ablating the polymer, the silver may be heated by the laser beam and the hot silver may generate a localized heat-affected zone (HAZ) in the underlying substrate. The silver may be heated hot enough so that the temperature of the HAZ is high enough to melt the polymeric substrate. Consequently, the laser beam may weld the HAZ with the heated silver filament, thereby providing excellent adhesion properties between the silver filament and the substrate.

[0046] For example, when PET is used as the substrate, the 3D printer may select an 808 nm laser as the sintering laser. With the 808 nm laser beam, the PET substrate may not be heated or may just be slightly heated. The silver filament, however, may absorb the energy from the laser beam and be sintered. As a result, the laser-heated silver filament may generate a localized heat-affected zone (HAZ) in the underlying PET substrate.

Typically the sintering temperature of silver may be 150-600 °C, which overlaps with the melting temperature of PET. Accordingly, the

temperature of the HAZ may be high enough to effectively weld the silver to the PET, yielding a mechanical robust connection therebetween.

[0047] In 312, conducting post sintering of the portion of the filament after the portion of the filament moves out of the focal area.

[0048] The post sintering (e.g., annealing) may be conducted after the 3D printer finishes printing a 3D structure comprising one or more of the filaments. For example, the 3D printer may include a conveying belt or a robot to send the printed 3D structure to a furnace to anneal the 3D structure. The 3D printer may also include a second laser that to carry out the post sintering by heats the portion of the filament again, after the portion of the filament moves out of the focal area. For a 3D structure formed from one or more sintered silver filaments, the annealing

temperature may be higher than 300 °C (such as 500 °C). The annealing may improve density and bending cyclability of the 3D structure.

[0049] Fig. 5 illustrates another example of focusing a laser beam 100 over the filament 146. The focal area A may be positioned over a portion of the filament at a distance d away from the outlet 142. The 3D printer 100 may move the nozzle 140 to print the filament 146 along a

predetermined path. In Fig. 5, the predetermined path is an L-shaped line. Thus the nozzle 140 may turn left in order to print an L-shaped the filament (shown by the arrow in Fig. 5). If the 3D printer 100 linearly moves the light guide 120 and the nozzle 140 together, the laser beam 110 may turn left together with the nozzle 140. But because the laser focal area A is distance d away from the outlet 142, by turning left together with the outlet 142 the focal area A may move out of the filament 146. Thus, according to an exemplary embodiment, the controller 102 may control the 3D printer 100 to linearly move the nozzle 140 and the light guide 120independently, so that the focal area A of the laser beam 110 moves along the extruded filament 146 rather than linearly moving together with the outlet 142.

[0050] The distance d may be selected to be greater than the diameter of the focal area A, so that the outlet 142 may be outside the focal area A, thereby avoiding being overheated. Meanwhile, the distance d may be sufficiently small that the unsintered portion of the filament 146 between the outlet 142 and the focal area A can maintain its shape and/or integrity prior to sintering and/or being deposited on the substrate. For example, the ink may be a silver ink, and the laser beam 110 may be focused on the focal area A at distance d away from the outlet 142. As the nozzle 140 moves and the focal area A follows, the laser beam 110 may sinter the silver filament that passes through the focal area into a conductive silver wire. As explained above, the distance d may be short enough so that when the filament 146 is extruded from the outlet 142, it maintains its shape until it is fully sintered by the laser beam 110, yet far enough such that the material is not sintered inside the nozzle.

[0051] Fig. 6 illustrates another example of focusing a laser beam 100 over the filament 146. Similar to Fig. 5, the focal area A may be positioned over a portion of the filament 146 at a distance d away from the outlet 142. The distance d may be selected to be greater than the diameter of the focal area A, so that the outlet 142 may be outside the focal area A, thereby avoiding being overheated. Meanwhile, the distance d may be sufficiently small enough, so that the unsintered portion of the filament 146 between the outlet 142 and the focal area A may maintain sustain itself without collapse.

[0052] Further, the 3D printer 100 may keep the nozzle 140 and the light guide 120 stationary with respective each other, while linearly moving the nozzle 140 and the light guide 120 and rotating the substrate 160 as needed, so that the nozzle 140 may move along the predetermined path relative to the substrate and the focal area A of the laser beam 1 10 may move along the extruded filament 146 on the substrate 160.

[0053] For example, in Fig. 6, the predetermined path is an L-shaped line on the substrate 160. Thus the outlet 142 may first move a straight line over the substrate 160. The focal area A of the laser beam 1 10 may also linearly move together with the outlet 142 at the distance d away, so that the focal area maintains on the straight line of filament 146 and the light guide 120 maintains stationary with respect to the nozzle 140. When the outlet 142 needs to print the right angle of the predetermined L-shape path, both the nozzle 140 and the light guide 120 may maintain a linear motion and maintain stationary with respect to each other, and the substrate 160 may rotate for Θ degree around a rotation center. The combination of the linear motion and rotational motion may result in a left turn of the outlet 142 with respect to the substrate 142 while at the same time the motion of the focal area A may follows the printed L-shaped filament 146.

[0054] The controller 102 may determine the linear motion of the laser beam 1 10 and/or the nozzle 140 and the rotational motion of the substrate 160 based on a relationship shown in Fig. 7. In Fig. 7, S(x, y) may be a predetermined path traced on or above a substrate and may be a function of two coordinates x, y that define a plan where the substrate 160 locates. At point A the predetermined path S(x, y) may have a positive or negative curvature κ, the laser beam 1 10 and nozzle 140 may move at a velocity v_p m/s (in vector form) with respect to the substrate 160 vertically below the nozzle, and at a velocity v_xy with respect to a stationary reference, such as a laboratory frame, and the substrate 160 may rotate at an angular velocity Ω = ΘΩ in rad/s (in vector form, where Ώ denotes the unit vector of Ω and Θ is the angle that the substrate rotates, θ = κν_ρ) around a rotation center 0 with respect to the nozzle 1 10. r is the vector between 0 and A. To move along the predetermined path S(x, y) at velocity v_p = dS/dt (in vector form), the 3D printer gantry may move the nozzle and laser beam at velocity v_xy and the rotary stage may rotate with angular velocity Ω such that

Ω = κν_ρΩ,

where v_v is the magnitude of the vector v_p. Thus once the path S(x, y) and desired print speed v_p are determined, the controller 102 may be able to select a pair of nozzle and laser velocity v_xy and substrate angular velocity Ω in order for the printed features to follow the predetermined path

S(x, y) .

[0055] It should be noted that in the above equation Ω is a relative angular velocity of the substrate 160 with respect to the laboratory frame around a rotation center O , and v_xy is the relative linear velocity of the laser beam 1 10 with respect to the laboratory, and may be generated by the linear translation of the 3D printer x- and y-axes. Therefore, the same equation may apply to any scenario where the laser beam 1 10 (or the outlet 142) has a relative linear velocity and a relative angular velocity with respect to the substrate 160.

[0056] In the above equation the predetermined path S(x, y) is a function with respect to the x-y plan. It should also be noted that the 3D printer may determine a pair of 3D nozzle and laser velocity and substrate angular velocity to print a 3D predetermined path, using the same principle of the 2D model. For example, the predetermined path may be a 3D function S(x, y, z) in the x-y-z space. Correspondingly, the curvature κ may be out of the plane of the substrate. Applying the same principle into 3D, the printer may determine the needed translational motion of the nozzle and laser velocity along x-y plane, y-z plane, and z-x plan. Similarly, the 3D printer may also determine the needed angular speed of the substrate along x-axis, y-axis, and z-axis, so that the combination motion

(translational motion and rotational motion) of the nozzle, laser beam, and the substrate would enable the laser beam to follow the predetermined path S(x, y, z). .

[0057] Accordingly, with the above equation, as an alternative to the exemplary embodiment in Fig. 6, the substrate 160 may be stationary during printing, and the nozzle 140 and the light guide 120 may be moved linearly together and rotated together around an axis, such that the outlet 142 prints the filament 146 on or above the substrate 160 along the predetermined path, and the focal area A moves along the filament 146.

[0058] Alternatively, with the above equation, the nozzle 140 and the light guide 120 may be rotated together around an axis, and the substrate 160 may be moved linearly, so that the outlet 142 prints the filament 146 on the substrate 160 along the predetermined path and the focal area A moves along the filament 146.

[0059] Alternatively, with the above equation, the nozzle 140 and the light guide 120 may remain stationary, and the substrate 160 may be moved linearly and rotated around an axis, so that the outlet 142 prints the filament 146 on the substrate 160 along the predetermined path and the focal area A moves along the filament 146.

[0060] To heat the filament 146, the laser beam 110 may be a waveform laser with predetermined amplitude and frequency. To this end, the controller 102 may control the waveform generator 108 to generate a waveform actuation signal at the predetermined frequency. Accordingly, when the waveform generator 108 actuates the laser generator 104, the emitted laser beam 1 10 may have waveform amplitude at the

predetermined frequency.

[0061] The laser beam 1 10 may have any amplitude of any waveform. For example, the laser beam 1 10 may be pulsed laser beam comprising laser pulses. The power of the laser beam 1 10 may be 0.1 mJ/pulse, 1 mJ/pulse, 5mJ/pulse, 20mJ/pulse, or 200mJ/pulse. Further, the frequency may be any number. For example, the frequency may be 5 Hz, 20 Hz, 200, or 1000 Hz.

[0062] Experiments show that the laser pulse power and frequency may have certain effects on the material properties of the sintered filament. An example composition for the silver ink in its unsintered state is a colloidal silver nanoparticle solution, stabilized with chains of linear poly(acrylic acid), in a solution of diethanolamine, water, and ethanol.

[0063] For example, Fig. 8A illustrates effects of the laser beam pulse frequency on the density and microstructure of sintered silver filaments. The silver filament may be sintered by the laser beam through burnout of a polymeric binder and/or polymerization. Here, the silver filament may be a conductive electrode of a micro scale electronic device, such as an electrode of integrated circuit on a silicon chip. The scale bar in Fig. 8A is 5 μιτι. As can be seen, the sintered silver filament exhibits a denser structure when sintered at a higher laser frequency and a more porous structure when sintered at a lower laser frequency. Fig. 8B is a chart of resistivity of the sintered silver filament versus sintering frequency of the laser. It shows that the resistivity of the silver filament generally decreases when the sintering frequency increases.

[0064] Fig. 9A illustrates effects of the laser beam pulse power on the nanostructure of sintered silver filaments. The scale bar in the top pictures of Fig. 8A is 100 nm, and the scale bar in the bottom pictures of Fig. 8A is 5 pm. As can be seen, the sintered silver filament exhibits a different nanostructure when sintered at different laser pulse powers. Fig. 8B is a chart of resistivity of the sintered silver filament versus pulse power of the laser. It shows that the resistivity of the silver filament dramatically decreases when the pulse frequency increases.

[0065] Based on the experimental results of Figs. 8-9, the controller 102 of the 3D printer may be able to select the power and frequency of the laser beam 110. For example, when a printed structure requires lower resistivity and higher mechanical strength, the controller 102 may select the laser beam 110 to be higher than 200 Hz and 200 mJ/pulse.

[0066] The controller may also dynamically vary the laser power, pulse frequency, or pulse duration settings during printing to spatially pattern electrical and/or mechanical properties. For example, a temporary reduction in laser power during printing can produce a length of filament with a higher electrical resistivity, thus producing a 3D printed electrical resistor.

[0067] The following embodiments are examples of direct ink writing (DIW) using inline laser sintering of a silver ink.

[0068] Fig. 10 illustrate an actual Laser-DIW apparatus, wherein Fig. 10a is a photograph showing the Laser-DIW printhead that combines a laser microscope and silver ink extrusion system, both mounted on a 3- axis printer, and the substrate on top of a rotating stage; and Fig. 10b is a photograph of the housing of the objective lens and nozzle above a silicon wafer substrate. In the apparatus, an 808 nm diode laser (Shanghai-Laser & Optics Century Co. Ltd.) is coupled via an SMA connector to a 200 μηι multimode optical fiber. The laser is collimated at the outlet of the multimode fiber by a collimating lens (A), and reflected by a 750 nm short- pass dichroic mirror (Edmund Optics Inc.) (C). The dichroic mirror was selected to transmit a small amount (< 1 %) of the laser light to enable visualization of the reflected laser spot by the microscope camera (Imaging Developing Systems, GmbH) for alignment purposes (B). The laser power, pulse duration and pulse frequency is modulated by a square wave from a waveform generator (Keysight). The laser beam is expanded 2X via a pair of achromatic doublet lenses (D) before being focused by a 0.16 NA, 15.29 mm focal distance aspheric objective lens (Thorlabs Inc.) (G). The silver ink is loaded into a syringe (F) which is extruded via a high-pressure piston (Nordson EFD) (E), pressurized via a pressure controller (Nordson EFD). For the purpose of alignment, the nozzle may be moved relative to the laser focus by means of a three-axis micrometer (I). The alignment and printing is aided by a side view camera and lens system (Imaging

Developing Systems, GmbH) (H). All the systems described above are mounted onto a 3-axis printing stage (Aerotech Inc.). The substrate (J) is mounted on a rotary stage (Aerotech Inc.) (L). An x-y leveling stage (K) lies in between the rotary stage and the substrate to ensure that the substrate x-y plane is aligned parallel to the x-y plane of the 3-axis printer.

[0069] Fig. 1 1 illustrates the basic settings of the example, i.e., a laser sintered DIW (Laser-DIW). During the Laser-DIW, an 808 nm IR laser is focused to a 100 μηι spot adjacent to the aperture of the glass nozzle through which a concentrated silver nanoparticle ink (85 wt% solids) is deposited (Fig. 1 1 a, Schematic of the Laser-DIW printhead, which consists of the laser microscope, silver ink syringe and nozzle. Fig. 10). Upon exiting the nozzle, the patterned features are rapidly heated by the focused laser to form a mechanically robust, electrically conductive wire.

Depending on the requirement of conductivity, the power of the laser may be adjusted to sinter the silver wires, which results in a higher conductivity to the silver wire, or simply raise the temperature to an annealing temperature, which results a lower conductivity to the silver wire. Here, the laser heating process is referred to as annealing process. The printed silver wires vary in diameter from < 1 m to 20 μηι depending on the nozzle diameter, extrusion pressure, and printing speeds used. The in-line laser annealing process induces a visible change of emissivity (dull to shiny) of the printed wires at the macroscale (Fig. 1 1 b, Side and top views showing the IR laser (the zigzag arrow) focused immediately downstream of the nozzle. Note the change in emissivity of the silver ink upstream and downstream of the laser.) as well as the densification of individual silver nanoparticles into larger grains at the microscale (Fig. 1 1 c, Nanostructure of (i) downstream laser-annealed silver and (ii) upstream as-printed silver features. Scale bars = 100 nm.).

[0070] To print curvilinear features via Laser-DIW, the sample must be rotated relative to the laser-nozzle axis using a rotary stage, such that the curvilinear wire is always patterned in a direction parallel to the laser- nozzle axis (Fig. 1 1 d, A side-view showing freeform 3D printing of a metal hemispherical spiral. Fig. 1 1 e, A photograph showing the objective lens positioned directly above the nozzle with the 3D metallic structures printed below.).

[0071] Fig. 12 illustrates a method of printing curvilinear structure using a rotating state, wherein Fig. 12a is a 3D spline created in Solidworks and Fig. 12b shows that the 3D spline is uploaded to a custom MATLAB script; Fig. 12c is a diagram showing the rotating stage and a curvilinear wire being printed above. To generate a wire of arbitrary geometry in 3D space, a path is first drawn up using Bezier curves (Fig. 12a) and split up into piece-wise linear segments (Fig. 12b). Studying an arbitrary curve to be printed (Fig. 12c), let dS represent a short segment that lies

instantaneously tangent to the wire being printed at a velocity v_p relative to the motion of a point P on the substrate that lies immediately below the segment. In this case, as the wire is being printed, dS is related to v_p by the equation:

dS = v_pdt (1 )

The instantaneous x-y curvature of the wire being printed, K_xy , is defined by:

where Θ represents the instantaneous x-y bearing of the wire segment, or equivalently, the current angle of the rotary stage. Thus, to print a wire with local curvature κ_χγ at a constant speed of

, the rotary stage must be rotated at a rate equal to:

In addition to rotating the stage, the printer must be translated in x, y and z to map both the translation of the point P on the substrate as the stage rotates, and generate the net print velocity v_p relative to the motion of P. If r represents the vector connecting the center of rotation of the rotary stage, 0, with point P, then the printer must be translated with velocity v_xyz governed by the following equation of motion:

where Θ denotes the unit vector along the rotation axis of the rotary stage. Using equations 3 and 4) any arbitrarily shaped piece-wise linear wire with G-code commands [x_n, y_n, z_n] can be converted into new 4-dimensional commands [x_n, y_n, z_n, θ_η]. A custom script that implements the mathematical conversion necessary for Laser-DIW has been made available at MATLAB Central.

[0072] Referring back to Fig. 11 , as the rotary stage moves, there exists a minimum radius of curvature of the metal trace p_min such that the wire passes through the laser spot, which depends upon the separation distance I between the nozzle and the perimeter of the laser spot of diameter D (Fig. 11f, a schematic showing key parameters involved in printing curvilinear structures.):

[0073] To create sharp turns, the laser must be placed as close to the ink deposition nozzle as possible (Fig. 11 g, Examples of printing

curvilinear shapes when the laser focus spot is placed (i) at an appropriate distance, or (ii) too far from the nozzle, wherein the silver ink does not impinge on the laser focus spot, equation 5). However, when the laser is positioned too close, heat is conducted upstream through the silver ink and into the nozzle, resulting in cessation of ink flow due to densification.

[0074] To optimize the nozzle-to-laser separation distance, the exemplary embodiment uses a simplified one-dimensional heat transfer model to study the temperature distribution along the silver wire during printing at a speed v_p, which accounts for the input laser energy (¾) as well as convective (c/_c) and radiative (c/_R) heat loss. The temperature distribution is modeled by the following convection-diffusion equation: k(x,t)

VT T + [q_L{x, t) - q_c{x, i) - q_R{x, t)], p(x,t)c_p(x,t) p(x,t)Cp(x,t) (6)

where the density p, the specific heat capacity c_p , and the thermal conductivity k of printed wire are a function of its thermal history. The exemplary embodiment numerically solves this partial differential equation using a finite difference method. The upstream heat transfer is reduced by three key mechanisms. First, using laser flash thermal analysis, the heat- annealed silver features have a 50-fold higher thermal diffusivity compared to the as-printed silver ink (20 mm2s-1 and 0.4 mm2s-1 , respectively). Hence, the laser-annealed regions of the printed silver wires serve as a downstream heat sink, limiting upstream heat transfer to the ink reservoir within the nozzle. Second, the printing speed limits upstream heat flow due to downstream heat advection, as shown by steady-state temperature curves generated by the simulation during printing at various speeds under continuous-wave (CW) laser illumination (Fig. 11 h, predicted silver ink temperature distribution at various print speeds annealed via a 6 kW/cm² continuous-wave illumination.). As an approximation, a characteristic upstream heating distance, L_c, is the length at which upstream conductive heat flux is balanced by downstream heat advection resulting from the printing velocity, i.e., the length for which Peclet number is equal to unity: Pe = ^ = 1 (7)

[0075] For example, using the thermal diffusivity for unannealed silver ink (as-printed), the characteristic upstream heating distance at a print speed of 1 mm s-1 is 400 μηι. Notably, this printing speed is more than 1000 times higher than meniscus printing. Finally, operating the laser in a pulsed mode instead of CW allows one to achieve high maximum

annealing temperatures, while limiting the total heat transfer to the wire, with low pulse repetition rates (PRR) resulting in very limited upstream heat conduction (Fig. 1 1 i, predicted silver ink temperature distribution using various laser pulse repetition rates with a pulse duration of 1 ms and a peak laser intensity of 30 kW/cm².). To achieve a uniform densification through the thickness of the printed wires, the laser pulse duration should be sufficiently long such that the characteristic thermal diffusion distance is large compared with the wire diameter:

x_c = 2Λ/ΟΓ » d_wire (8)

[0076] For a pulse duration of 1 ms, this characteristic length is 40 μηι, which is significantly larger than the thickest (-20 μηι) wires being printed. The simulation further predicts that for low PRR, the maximum temperature reached along the wire becomes non-uniform, even when all segments of the wire receive an equal laser exposure. This manifests in low PRR producing macroscopically heterogeneous wires with a non-uniform nanostructure. Fig. 13 shows how the PRR affects the microstructure of the printed filaments. At low PRR, the microstructure is highly porous throughout the thickness of the filament, whereas operating at a high PRR produces filaments that are uniform (Fig. 13a, wire porosity decreases with increasing PRR.). Filaments that are annealed using low PRR exhibit a periodic, heterogeneous microstructure at a spatial frequency that is related to the print velocity and the PRR (Fig. 13b, wires annealed at low PRR exhibit periodic heterogeneity. Scale bars: 10 μηι.). [0077] Conversely, heating the wire at a 1 ms pulse duration at 100 Hz generates a silver wire with a uniform nano- and microstructure, as predicted by the more uniform thermal history (Fig. 1 1 i). Importantly, the convection-diffusion equation (equation 6) used to generate the curves in Fig. 1 1 h - Fig. 1 1 i assumes that the wire is being printed in mid-air, and therefore does not incorporate a substrate conduction-loss term. Thus, equation 6 models a worst-case scenario for thermal management, as a substrate would serve as a heat-sink that limits upstream heat transfer. Printing directly onto a substrate would also significantly reduce the maximum annealing temperature, particularly when the wire is annealed by CW laser exposure, as a more uniform through-thickness heating would result in more heat-loss to the substrate.

[0078] Next, as shown in Fig. 14, the effect of laser intensity is studied, both continuous-wave and high frequency pulses (100 Hz, 1 ms pulse duration), on the electrical conductivity of Laser-DIW silver wires printed on a glass substrate (Fig. 14a, silver resistivity decreases with increasing laser intensity. The solid line at bottom represents the resistivity of bulk silver metal.). By modulating the incident laser intensity over an order of magnitude, the silver resistivity can be varied by more than three orders of magnitude. Annealing via a continuous wave laser achieved slightly lower electrical resistivity than that of a pulsed laser of the same peak intensity, reaching a minimum resistivity of 5.4 x 10-6 Q^»cm, compared with that of bulk silver at 1 .6 x 10-6 Q^»cm. The microstructure of printed wires produced by continuous-wave and pulsed laser operation show a similar progression with increasing laser intensity, i.e. they undergo both densification and grain growth as expected during thermal annealing (Fig. 14b, microstructure of silver wires annealed by continuous wave laser illumination. Numbers indicate peak illumination intensity in kW/cm². Scale bars: 10 μηι (i), 100 nm (ii). Fig. 14c, microstructure of silver wires annealed by pulsed (1 ms, 100 Hz) laser illumination. Numbers indicate peak illumination intensity in kW/cm². Scale bars: 10 μηι (i), 100 nm (ii).). [0079] Unlike bulk thermal annealing methods, Laser-DIW enables one to create patterned regions of low-to-high resistance simply by modulating the local laser intensity during silver ink printing. For example, when the laser power is modulated to varying degrees during printing, a series of 500 μηι silver segments with graded resistivity are created in-line within the same conductive silver wire, as visualized by gradations in the infrared signatures upon passing a constant current along the printed wire (Fig. 14d, a varying laser intensity profile (top) results in corresponding infrared emissions from resistive elements as current is passed through the wire. Each resistor is approximately 500 μηι in length.). The l-V characteristics of resistors created in this manner are initially linear until a critical current is reached, beyond which Joule-heating from resistive losses results in auto- annealing of the resistors causing a large deviation from Ohmic behavior (Fig. 14e, l-V characteristics of two resistors formed at different laser intensities. Current is stepped up (unfilled) to a certain level, and then stepped down (filled).). If, after the onset of auto-annealing, the current is stepped back down, a new linear characteristic is observed with a lower value of resistance, representing the increased conductivity from auto- annealing. This behavior is characteristic of a write-once read-many (WORM) memory element, or 'anti-fuse'. It is noted that the critical current or potential difference at which the anti-fuse anneals can be programmed via careful choice of laser power.

[0080] Importantly, Laser-DIW enables conductive silver wires to be patterned on flexible, low-cost plastic substrates, such as poly (ethylene terephthalate), PET (Fig. 14f, SEM images of various diameter wires laser annealed onto PET films. Top row: top view, the grey areas around the light colored printed wires reveals the HAZ extending from the printed wires, scale bars = 5 μηι; Middle row: oblique views of silver wires; Bottom row: magnified images of the interface between the silver (Ag) and PET substrate.). PET is of particular interest for flexible electronic and

photovoltaic applications owing to its high transparency and chemical stability. While PET exhibits low absorptivity at the laser wavelength of 808 nm, and hence is not directly heated by the 100 μηι laser spot, the laser- heated silver ink generates a localized heat-affected zone (HAZ) in the underlying plastic substrate. This HAZ effectively welds the silver to the PET (Fig. 14f, bottom row), yielding mechanical robust electrodes that can withstand a tape peel test. For printing on different polymeric substrates, the laser wavelength could be selected to ensure maximal optical transparency. As the laser energy is predominantly absorbed by the silver wire, and not the PET itself, the HAZ width increases with the diameter of the wire (Fig. 14g, the width of the HAZ (the monotonous increasing line) and the ratio of the HAZ width and the silver wire width (the monotonous decreasing line) vary with the diameter of the printed and annealed wires, N = 5 wires for each diameter.). Conversely the ratio of the HAZ width to the wire diameter decreases with increasing wire diameter. Despite the formation of a HAZ, PET films with printed submicron silver wires with a center-to-center separation distance of 500 μηι exhibit exceptional optical transparency (Fig. 14h, an array of silver wires, with submicron widths, are printed onto a transparent PET film across a 1 cm² area, using a wire spacing distance of 500 μηι. The resulting film, indicated by the white dashed line, remains transparent.). Next, cyclic tests were performed on wires printed onto a 20 μηι PET film by varying the radius of curvature between 7 mm and 2 mm while monitoring changes in resistance. The resulting change in resistance (Fig. 14i, cyclic testing of the electrical resistance of 10 m and 3 μηι silver wires printed onto a 20 μηι thick PET film. The radius of curvature alternates between 7 mm and 2 mm. The graph shows the variation of resistance after n cycles (R_n) normalized to the initial resistance before cyclic testing (R₀). N = 3 wires for each condition.) and per-cycle variation in resistance (Fig. 15a, wire porosity decreases with increasing PRR. Fig. 15b, wires annealed at low PRR exhibit periodic heterogeneity. Scale bars: 10 μηι.) were minimal (< 1 %) throughout the 1 ,000 cycle test. However, the thick wires, due to the presence of microscopic defects and porosity, are not able to withstand the same degree of extension when compared with defect-free thin-film approaches, which can reach a 50% strain.

[0081] In the above exemplary embodiments, the silver nanoparticle ink is synthesized using a protocol similar to that previously described . Briefly, 0.9 g of a 25% wt/v solution of 50 kDa poly(acrylic acid) (Polysciences Inc., Warrington, PA), in water and 1 .8 g of a 50% wt/v 5 kDa poly(acrylic acid) (Polysciences Inc) are dissolved into 50 g of distilled water in a clean 500 ml Erlenmeyer flask. 40 g of diethanolamine (Sigma Aldrich, St. Louis, Ml) is then added, and the solution is allowed to return to room temperature while stirring at 300 rpm using a 1 -inch stir bar. In a second clean flask, 20 g of silver nitrate (Sigma Aldrich) is dissolved in 20 g of distilled water and the solution is allowed to return to room temperature, before adding the solution to the contents of the first flask while stirring. The solution is covered and stirred for 24 h. During this time, the solution turns from colorless to a clear, pale brown color as silver nanoparticles precipitate out under the reducing conditions. After 24 h, the nanoparticles are ripened by increasing the temperature to 75°C and for 2 h. The solution is then cooled to room temperature. Next, 300 ml of ethanol is rapidly added to the solution, while stirring, to precipitate the nanoparticles. After 5 min of additional stirring, the nanoparticles are allowed to settle under quiescent conditions. The supernatant is decanted away, and the silver nanoparticle sediment is transferred quickly via a spatula into a separate 50 ml conical tube, ensuring that the suspension does not dry. The nanoparticles are then centrifuged at -13,000 g for 20 min into a dense pellet, and the supernatant is discarded. The nanoparticles are then suspended again by adding 15 ml of water followed by vigorous vortexing. The suspension is filtered through a 5 μιτι syringe filter by splitting the solution into two 50 ml conical tubes and adding 35 ml of ethanol into each tube. The

nanoparticles are allowed to settle for 20 min before decanting the supernatant. The nanoparticle suspension in one conical tube is transferred to the other by use of a spatula before compacting the silver nanoparticles by centrifugation at 13,000 g for 20 min. The nanoparticle pellet is then transferred out of the conical tube using a spatula, and placed in a jar to be mixed in a planetary mixer (Thinky Corp., Laguna Hills, CA 92653). The ink is then transferred via spatula into a 3 ml syringe (Nordson EFD, East Providence, Rl) and centrifuged for 10 min at 4,000 g to remove trapped air. The syringe is then placed into an HP3 high- pressure dispensing adaptor (Nordson EFD), connected to a variable pressure supply (Nordson EFD) and a 2-inch long glass nozzle with either a 10 m or 1 μηι inner diameter added to the syringe (World Precision Instruments). The syringe and high pressure adaptor are mounted onto the laser microscope and aligned with the focused laser spot.

[0082] The focused laser and ink printhead are of the following designs. A 5 W 808 nm CW diode laser (Shanghai Laser & Optics Century Co. Ltd., Shanghai, China) is connected to the laser microscope by means of a 200 μηι multimode optical fiber (Thorlabs Inc., Newton, NJ), and collimated via a 0.26 NA, 1 1 .07 mm collimating lens. The collimated beam is magnified 2X by a telescope consisting of a pair of IR anti-reflective achromatic doublet lenses (Thorlabs Inc.), and focused to a 100 μηι spot using a 0.16 NA, 12.43 mm working distance aspheric objective lens. The laser focus spot possesses a uniform, top-hat intensity distribution. The laser path includes a short-pass 750 nm dichroic mirror (Edmund Optics) that reflects the IR laser light, and transmits visible light collected by the objective to a CMOS camera (IDS, Obersulm, Germany) that serves as an alignment microscope. Importantly, to facilitate laser-nozzle alignment, the dichroic mirror is selected to enable a small amount of IR light to be transmitted (< 1 %) to enable visualization of the substrate-reflected laser light with the top alignment camera. The entire optical setup is mounted on an optical breadboard, which itself is mounted onto an x-y-z translating 3D printing gantry (Aerotech Inc., Pittsburg, PA). The laser diode driver is modulated by a square-wave signal derived from either a waveform generator (Keysight) or a National Instruments NI-6211 , to enable the precise variation of the laser pulse power, frequency and duration.

[0083] To align the laser and ink deposition nozzle for printing, the laser is first focused onto the substrate by observing the laser spot on the alignment camera. Next, x, y, and z micrometers are used to move the silver ink syringe and nozzle relative to the focused laser spot. A separate side camera (Imaging Development Systems) is used to aid this process. For omnidirectional printing, the nozzle is typically placed -100 μηι away from the laser spot. To begin printing, the silver ink is extruded through the nozzle by applying pressure (typically 150-200 psi), and the printer is translated in x, y, and z.

[0084] The rotary stage alignment is achieved by the following. For freeform 3D printing, a rotary stage is used for mounting the sample to enable the construction of curved lines. The rotary stage is positioned directly underneath the laser microscope. To identify the center of rotation, the laser is focused on the top surface of a glass slide colored with a permanent marker. As the stage is rotated through 360°, the laser spot ablates the permanent marker, tracing a circle, whose center lies at the center of rotation. The center of rotation is then supplied to a custom

MATLAB script that converts a series of x-y-z Gcode commands into a new set of commands in x-y-z, and Θ.

[0085] The conductivity is measured as follows. Four 75 mm electrodes, consisting of gold stripes, are patterned onto a 75 mm x 25 mm glass slide by sputter coating. Next, linear wires are printed perpendicular to the stripes and a four-point probe is used to measure resistance between the center two stripes, separated by 6.35 mm. To calculate electrical resistivity, their cross-sectional areas are measured by image analysis of scanning electron micrographs of cut wires.

[0086] For cyclic testing, 3 cm long silver wires were printed using Laser-DIW onto a 20 μηι thick PET substrate. After printing, one end of the film was affixed to a stationary point and the other end was affixed to a point on an x-y-z translating gantry. Alligator clips and a resistance meter were used to monitor changes in resistance. Before cyclic testing, the gantry was moved towards the stationary point, imparting a small initial curvature on the wire, and the initial resistance, R0 was recorded. Next, the gantry was translated towards and away from the stationary point to flex the film to a radius of curvature of 2 mm while continuously monitoring the resistance.

[0087] While exemplary embodiments of the present disclosure relate to devices and methods for sintering metallic inks extruded from a 3D printer, the devices and methods may also be applied to other applications. For example, in addition to sintering metallic inks, the devices and methods may also be used to sinter ceramic inks. When the laser beam is an ultraviolet laser, the devices and systems may also be used to real-time cure a printed structure made by photosensitive epoxy or acrylate ink. The present disclosure intends to cover the broadest scope of systems and methods in 3D printing field.

[0088] Thus, example embodiments illustrated in Figures 1 -9 serve only as examples to illustrate several ways of implementation of the present disclosure. They should not be construed as to limit the spirit and scope of the example embodiments of the present disclosure. It should be noted that those skilled in the art may still make various modifications or variations without departing from the spirit and scope of the example embodiments. Such modifications and variations shall fall within the protection scope of the example embodiments, as defined in attached claims.

Claims

Claims:

1. A device for inline sintering of an ink during printing, the device comprising:

a substrate;

a nozzle above the substrate, having an outlet for extrusion of a filament comprising an ink, the nozzle and substrate being configured to move relative to each other; and

a laser generator to output a laser beam,

wherein the laser beam imparts energy on a focal area determined by a path of the nozzle over the substrate.

2. The device of claim 1 , further comprising a light guide configured to focus the laser beam onto the focal area.

3. The device of claim 1 or 2, wherein the light guide is connected to the laser generator via an optical fiber.

4. The device of claim 2 or 3, wherein the light guide comprises a housing; and

a light path in the housing to focus the laser beam to the focal area, the light path comprising:

a fiber optic collimator mounted on the housing and connected with the optical fiber,

a mirror mounted in the housing to reflect the laser beam to a lens; and

the lens mounted in the housing to focus the laser beam on the focal area.

5. The device of claim 4, wherein the mirror is a wavelength- selective or semi-transparent mirror, and the light guide further comprises a camera, mounted on the housing and behind the mirror, to record visible and infrared images of the focal area through the mirror.

6. The device of any one of claims 1 to 5, further comprising an actuator mechanically connected to at least one of the nozzle, the light guide, and the substrate.

7. The device of claim 6, wherein the actuator is programmed to: deposit the filament on the substrate along the path.

8. The device of claim 6 or 7, wherein the actuator is programmed to linearly translate the nozzle and the light guide together and rotate the nozzle and the light guide together around an axis so that the focal area moves along the filament.

9. The device of claim 6 or 7, wherein the actuator is programmed to linearly translate the nozzle and the light guide together and rotate the substrate so that the focal area moves along the filament.

10. The device of claim 6 or 7, wherein the actuator is programmed to rotate the nozzle and the light guide together about an axis and linearly translate the substrate so that the focal area moves along the filament.

11. The device of claim 6 or 7, wherein the actuator is programmed to rotate and linearly translate the substrate so that the focal area moves along the filament.

12. The device of any one of claims 1 to 11 , wherein the laser generator is configured to output a laser beam comprising an annular cross section.

13. The device of any one of claims 1 to 12, wherein the outlet of the nozzle is within the focal area.

14. The device of any one of claims 1 to 13, further comprising a waveform generator electronically connected to the laser generator to control amplitude of the laser beam.

15. A method for inline sintering of an ink during printing, the method comprising:

extruding a filament comprising an ink from an outlet of a nozzle and depositing the filament on or above a substrate, the nozzle moving at a predetermined speed with respect to the substrate; and

focusing a laser beam on a focal area adjacent to the nozzle outlet, the focal area being moved with the nozzle during extrusion of the filament from the outlet,

wherein a portion of the filament passes through the focal area during deposition, thereby being heated by the laser beam.

16. The method of claim 15, further comprising heating the portion of the filament inline by the laser to a temperature equal to or higher than a sintering temperature thereof before or after the portion of the filament moves out of the focal area.

17. The method of claim 15 or 16, wherein the ink comprises at least one of a metallic ink, polymeric ink, and ceramic ink.

18. The method of any one of claims 15 to 17, wherein the focal area is sufficiently close to the outlet of the nozzle, so that an unsintered portion of the filament between the outlet and the focal area does not collapse, buckle, or deform.

19. The method of any one of claims 15 to 18, wherein the printing device further comprises:

a laser generator configured to emit the laser beam; and

a light guide configured to focus the laser beam to the focal area.

20. The method of any one of claims 15 to 18, further comprising depositing the filament on the substrate along a predetermined path.

21. The method of claim 20, wherein the depositing of the filament comprises linearly moving the nozzle and the light guide together and rotating the nozzle and the light guide together around an axis so that the focal area moves along the filament.

22. The method of claim 20, wherein the depositing of the filament comprises linearly moving the nozzle and the light guide together and rotating the substrate so that the focal area moves along the filament.

23. The method of claim 20, wherein the depositing of the filament comprises rotating the nozzle and the light guide together around an axis and linearly moving the substrate so that the focal area moves along the filament.

24. The method of claim 20, wherein the depositing of the filament of the ink comprises rotating and linearly moving the substrate so that the focal area moves along the filament.

25. The method of claim 20, wherein the depositing of the filament of the ink comprises:

linearly moving the nozzle to deposit the filament of ink along the predetermined path; and linearly moving the focal area of the laser beam along the extruded filament of ink.

26. The method of any one of claims 15 to 25, wherein the laser beam comprises an annular cross section.

27. The method of claim 26, wherein the outlet of the nozzle is within the focal area.

28. The method of any one of claims 15 to 26, wherein amplitude of the laser beam comprises a waveform associated with a predetermined frequency.

29. The method of claim 28, further comprising changing at least one of amplitude, pulse intensity shape, pulse frequency, and pulse duration of the laser beam during printing.

30. The method of any one of claims 15 to 29, wherein the predetermined speed is a variable with respect to at least one of time and space.

31. The method of any one of claims 15 to 30, further comprising changing the ink extrusion rate during printing.

32. The method of any one of claims 15 to 31 , further comprising, when the substrate is a polymer substrate,

selecting a wavelength of the laser beam so that the laser beam is not absorbed by the polymer substrate.

33. The method of claim 32, wherein, when the portion of the filament passes through the focal area, the portion of the filament is heated by the laser beam to weld to the substrate.

34. The method of any one of claims 15 to 33, further comprising, after the portion of the filament moves out of the focal area, annealing the portion of the filament.

35. A method for localized laser heating of an ink during 3D printing, the method comprising:

extruding a filament comprising an ink from an outlet of a nozzle and depositing the filament on a substrate, the nozzle moving at a

predetermined speed with respect to the substrate; and

focusing a laser beam on a focal area adjacent to the nozzle outlet, the focal area being moved with the nozzle during extrusion of the filament from the outlet,

wherein a portion of the filament passes through the focal area during deposition, the laser beam thereby causing a physical or chemical reaction in the portion of the filament.

36. The method of claim 35, wherein the reaction comprises a sintering effect.

37. The method of claim 35, wherein the reaction comprises the burnout of a polymeric binder.

38. The method of claim 35, wherein the reaction comprises a polymerization.

39. The method of any one of claims 35 to 38, further comprising, when the substrate is a polymer substrate, selecting a wavelength of the laser so that the laser is not absorbed by the polymer substrate.

40. The method of claim 39, wherein when the portion of the filament passes through the focal area, the portion of the filament is heated by the laser to weld to the substrate.

41. The method of any one of claims 35 to 40, further comprising conducting a post sinter to the portion of the filament that passes through the focal area.