WO2024035344A1 - Continuous 3d printing for microstructured composites - Google Patents

Abstract

A method of continuous 3D printing to print an article. The method includes displacing a nozzle of a 3D printer along a nozzle displacement direction; concurrent with the displacing of the nozzle, extruding an ink from the nozzle to form an extrudate on a porous base; and concurrent with the displacing of the nozzle, displacing a rotating magnetic field along the nozzle displacement direction. The ink includes a plurality of magnetically responsive particles and a colloidal gel. The colloidal gel may include fumed silica in an aqueous solution.

Description

CONTINUOUS 3D PRINTING FOR MICRO STRUCTURED COMPOSITES

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims benefit of priority to the Singapore patent application no. 10202250710M filed August 11, 2022, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to the field of additive manufacturing, and more particularly to 3D printing of microstructures.

BACKGROUND

[0003] Direct ink writing is an extrusion-based additive manufacturing technique involving the extrusion of a viscoelastic ink out from a nozzle and deposition of the ink, layer-by- layer, to build up 3D structure on a computer-controlled translational stage. In conventional direct ink writing, the ink must be a paste with high viscosity and elasticity so that the printed material can have the shape-retention property required for building up layers. The conventional direct ink writing method is unable to produce 3D printed products with anisotropic properties.

SUMMARY

[0004] In one aspect, the present application discloses a method of continuous 3D printing to print an article. The method includes displacing a nozzle of a 3D printer along a nozzle displacement direction; concurrent with the displacing of the nozzle, extruding an ink from the nozzle to form an extrudate on a porous base; and concurrent with the displacing of the nozzle, displacing a rotating magnetic field along the nozzle displacement direction. The magnetic field is a rotating magnetic field characterized by a rotating speed faster than 1 Hz. The ink includes a plurality of magnetically responsive microparticles and a colloidal gel. The colloidal gel may include fumed silica nanoparticles in an aqueous solution.

[0005] In another aspect, the present application discloses an ink for use in the method of continuous 3D printing. The ink includes a plurality of magnetically responsive microparticles. The magnetically responsive particles include: a plurality of microplatelets; and a plurality of magnetically responsive nanoparticles adsorbed on respective surfaces of the plurality of microplatelets. The ink may be selected from a material characterized by a viscosity of up to 1000 Pa.s and a yield stress of up to 45 Pa for a preferred operational range of the alignment capability (of the magnetically responsive microparticles) and shearthinning properties. In some embodiments, the ink includes water; and 4 wt% fumed silica. In some embodiments, the ink is characterized by a viscosity of less than 1000 Pa-s and a yield stress of less than 45 Pa. In some embodiments, examples of the 3D printable ink include but is not limited to compositions including 20 wt% mAhCh, characterized by a viscosity of 200 Pa.s and 4 Pa yield stress.

[0006] In yet another aspect, the present application discloses an apparatus for use in the method of continuous 3D printing. The apparatus includes a 3D printer with a nozzle; and a magnet coupled to the nozzle. The magnet is rotatable relative to the nozzle to provide a rotating magnetic field. The magnet and the nozzle are displaceable together along the nozzle displacement path to provide a gradient magnetic field that is characterized by a magnetic field strength that decreases with increasing distance from the nozzle.

BRIEF DESCRIPTION OF DRAWINGS

[0007] FIG. 1A is a schematic diagram of an apparatus and method of continuous 3D printing according to one embodiment of the present disclosure;

[0008] FIG. IB is an electronic micrograph of an AI2O3 microplatelet modified with superparamagnetic iron oxide nanoparticles (mAhCh), as an example of a magnetically responsive particle in the ink, according to embodiments of the present disclosure;

[0009] FIG. 2A is a schematic diagram of a side view of the apparatus according to another embodiment;

[0010] FIG. 2B is a schematic diagram of a top view of the apparatus of FIG. 2 A;

[0011] FIG. 3 A is a schematic diagram of a perspective view of another embodiment of the apparatus; [0012] FIG. 3B is a schematic diagram of a top view of the apparatus of FIG. 3A;

[0013] FIG. 4 is a schematic diagram showing the time for magnetic alignment, the time for jamming, and the time related to the 3D printing as a function of the viscosity;

[0014] FIG. 5 is a schematic diagram showing the time for magnetic alignment, the time for jamming, and the time related to the 3D printing as a function of the printed layer number;

[0015] FIG. 6A shows the alignment dynamics of a single magnetic platelet in resins of various background viscosity (resin with increasing concentration of fumed SiCh of 0 wt%, 5 wt%, 6 wt%, 7 wt%, 7.5 wt%, and 8 wt%, respectively, in which the platelets shown an ability to align in the resins up to 7.5 wt% SiCh content;

[0016] FIG. 6B is a plot showing the effect of varying wt% of SiCh on the yield stress c_y of the ink;

[0017] FIG. 6C are plots showing the degree of alignment (DL/DW) and time for alignment t_m as a function of the FesC coating, in a polymer resin with 5 wt% SiCh;

[0018] FIG. 6D are plots showing the degree of alignment (DL/DW) and time for alignment t_m for microplatelets with 14 vol% FesC coating, as a function of the wt% of SiCh in the resin;

[0019] FIG. 6E are images showing the different alignment dynamics of a single magnetic microplatelet in resins of various background viscosity (resin with increasing concentration of fumed SiCh of 0 wt%, 5 wt%, 6 wt%, 7 wt%, 7.5 wt%, and 8 wt%), demonstrating that the microplatelet is able to align in the resin up to 7.5 wt% SiCh content;

[0020] FIG. 7A shows plots of viscosity as a function of the shear rate for aqueous inks containing 4 wt% SiCh and increasing concentrations of mAhCh;

[0021] FIG. 7B is a plot showing the effect of varying wt% of mAhCh on the yield stress of inks containing water and 4 wt% SiCh;

[0022] FIG. 7C shows plots of shear stress vs shear rate curves for the ink with concentrations of 15 wt% to 35 wt% of mAFCh; [0023] FIG. 7D shows plots of elastic and viscous moduli, G' and G", as a function of the shear stress for the inks of FIG. 7C;

[0024] FIG. 7E shows plots of alignment quality (DL/DW) and zero-shear viscosity of the ink as a function of the microplatelet loading;

[0025] FIG. 7F shows the theoretical calculation of t_m as a function of the viscosity of the inks, for a magnetic field of 50 mT and 100 mT;

[0026] FIG. 7G shows jamming time tj as a function of the printed layer number n when printing on a gypsum substrate at 23 °C, 40 °C, and 50 °C using the ink containing 20 wt% mAhCh;

[0027] FIG. 8 A to FIG. 8E are schematic diagrams and corresponding electron micrographs of 3D printed microstructures obtained using an aqueous ink containing 20 wt% m AI2O3 in 4 wt% SiCh;

[0028] FIG. 9 is an image of 3D printed samples in the shape of square, a star, and a circle;

[0029] FIG. 10 is an image of a porous 3D printed sample;

[0030] FIG. 11 is an image of a sintered sample possessing magnetically responsive properties;

[0031] FIG. 12 shows a 3D printed article with magnetically responsive shape-changing functionality;

[0032] FIG. 13 is an electron micrograph of a 3D printed graphite sample with vertically aligned microstructures;

[0033] FIG. 14 shows the electrical conductivity of 3D printed graphite composites measured on the surface of (a) sample with horizontal (transverse) alignment, (b) along the vertical (axial) alignment, and (c) perpendicular to vertical (axial) alignment, and the reduction of graphite composites by a high temperature of 1050 °C for 4 hours as shown in the shaded area;

[0034] FIG 15 shows an example of an application of the 3D printed magnetic and electrical composite for remote actuation; and

[0035] FIG. 16 shows an example of another application of the 3D printed graphite composite for electromagnetic shielding.

DETAILED DESCRIPTION

[0036] Reference throughout this specification to “one embodiment”, “another embodiment” or “an embodiment” (or the like) means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” or the like in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of embodiments. One skilled in the relevant art will recognize that the various embodiments may be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, some or all known structures, materials, or operations may not be shown or described in detail to avoid obfuscation.

[0037] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. As used herein, the singular ‘a’ and ‘an’ may be construed as including the plural “one or more” unless apparent from the context to be otherwise.

[0038] Terms such as “first” and “second” are used in the description and claims only for the sake of brevity and clarity, and do not necessarily imply a priority or order, unless required by the context. The terms "about" and "approximately" as applied to a stated numeric value encompasses the exact value and a reasonable variance as will be understood by one of ordinary skill in the art, and the terms “generally” and “substantially” are to be understood in a similar manner, unless otherwise specified.

[0039] In the present disclosure, the terms "three-dimensional printing" (3D printing) and additive manufacturing may be used interchangeably to refer methods in which the intended product is formed by adding materials, as opposed to conventional subtractive methods such as milling, etc. Examples of 3D printing methods include (and are not limited to) continuous 3D printing methods such as inkjet printing, direct ink writing, etc. Continuous 3D printing may be described as methods in which the material is provided in the form of an extrudate from a traveling nozzle (as opposed to being provided as a powder bed). Continuous 3D printing may also include methods in which the target shape of a solid article is substantially determined by one or more layers of materials deposited or printed according to a corresponding computer-aided design (CAD) model. Persons skilled in the art would appreciate that conventional molding processes have to address uneven distribution/flow of materials, voids or air pockets, formation of flash, into cavities of a mold, etc., and that conventional molding processes can be especially challenging for relatively complex shapes. In contrast, continuous 3D printing can provide a greater degree of freedom to create new shapes outside the constraints of a conventional mold.

[0040] For the sake of brevity, the terms "extrude", "eject", "deposit", "write", and "print" or the like, may be used interchangeably in referring to the delivery of a 3D printing material on a substrate or a previously printed layer, e.g., in the process of direct ink writing. For the sake of brevity, the term "porous base" as used herein refers to one or both of a substrate and a previously printed layer.

[0041] In describing a process or certain mechanisms, the present disclosure may refer to "steps" merely to aid understanding and for convenient reference. One skilled in the art would appreciate that, in actual practice and in various embodiments, various "steps" may overlap, be concurrent, or be different to distinguish into discrete sequential parts in various combinations or permutations.

[0042] Exemplary methods, articles made thereby, apparatus, materials, etc., according to various embodiments of the present disclosure are described herein with the aid of the appended figures and experimental data/results merely to aid understanding and not to be limiting.

[0043] 3D Printing Apparatus [0044] FIG. 1A is a schematic diagram illustrating an apparatus 100 and a method 500 according to one embodiment of the present disclosure. The apparatus 100 includes a 3D printer 110 with a nozzle 120. In operation, the apparatus 100 carries out continuous 3D printing by moving the nozzle 120 in a nozzle displacement direction 209 at a nozzle speed V_p in the presence of a rotating magnetic field (H) concurrent with the extrusion and deposition of an ink 300 from the nozzle 120 onto a substrate 150 or onto a previously printed layer. Preferably, the rotating magnetic field is characterized by a rotating speed higher/faster than 1 Hertz (Hz). Throughout the continuous 3D printing process, the apparatus 100 provides a stronger magnetic field proximal to the nozzle 120 relative to the magnetic field distal to the nozzle 120. The ink 300 include magnetically responsive particles 320. FIG. IB shows an image of an exemplary magnetically responsive particle 320. In this example, the magnetically responsive particle 320 includes FesC nanoparticles attached to the surfaces of an AI2O3 microplatelet to form a magnetic alumina particle (mAhCh). The method 500 includes continuous 3D printing on a substrate 150 / previously printed layer, in which the substrate / previously printed layer is sufficiently porous to aid or effect removal of water from the extrudate 200. The substrate is one selected with a porosity that facilitates relatively fast drying which in turn is found to enable the extrudate to retain its "as-deposited" shape. The deposited ink or extrudate 200 undergoes a time of magnetic alignment (510), followed by a time of jamming (520), and a subsequent time of drying or consolidation (530).

[0045] The apparatus 100 is further described with reference to FIG. 2A and FIG. 2B. The apparatus 100 of the present disclosure includes the 3D printer 110 with the nozzle 120 for extruding a continuous extrudate 200 of the ink. The 3D printer 110 is selected to be one that enables control of its nozzle 120 over a 3D nozzle displacement path. In the embodiments illustrated in FIGS. 2A to 3B, the proposed magnetic field distribution (a stronger magnetic field proximal to the nozzle 120 relative to the magnetic field distal to the nozzle 120) is provided by a magnet 130 coupled to the nozzle 120. The 3D printer may include a magnet 130 (e.g., a permanent magnet) coupled to a motor 140 and to the 3D printer 110 such that the magnet 130 can rotate (or stop rotating) while following the nozzle 120 along the nozzle displacement path. In some other embodiments, as illustrated schematically in FIG. 3 A and FIG. 3B, the nozzle 120 may be positioned behind the magnet 130 with reference to the nozzle displacement direction 209.

[0046] The motor 140 is preferably coupled to a part of the 3D printer 110 that supports the nozzle 120 such that the magnet 130 is displaced similarly to the nozzle 120 along the nozzle displacement path. The magnet 130 is preferably coupled to the 3D printer 110 such that an axis of magnet rotation (rotation axis 139) can be controllably oriented relative to a nozzle axis 129 (direction in which the nozzle extrudes the ink). The magnet 130 is preferably coupled to the 3D printer 110 with the magnet 130 spaced apart from the nozzle 120. The magnet 130 is preferably coupled to the 3D printer 110 such that the magnet 130 is spaced apart from the extrudate 200 by a spacing 137 that can be controllably varied, e.g., to vary the magnetic field to which the extrudate 200 is subjected. The magnet 130 is preferably coupled to the nozzle 120 such that the magnetic field strength at the nozzle 120 or nozzle tip 122 remains substantially unchanged throughout one run of the continuous 3D printing. The magnet 130 may be configured to be switchable between a non-rotating state and a rotating state, independent of the nozzle speed V_p, the nozzle displacement, and/or the rate of ink extrusion at the nozzle tip 122. The magnetic field distribution provided enables the application of a rotating magnetic field to align the microplatelets biaxially in the plane of the magnet 130. Preferably, the frequency of rotation of the magnetic field is sufficiently high to promote biaxial alignment resulting from a balance between viscous forces and magnetic torques. The position and orientation of the magnet 130 can be adjusted to tune the magnetic field strength and alignment direction as desired.

[0047] In one exemplary experiment, a neodymium magnet (e.g., having dimensions 35 mm x 10 mm x 10 mm) was fixed to a rotatable shaft of a direct current geared motor (e.g., motor 140). The magnet 130 was coupled to the 3D printer 110 with the magnet 130 spaced about 1 centimeter apart from a tip of the nozzle (nozzle tip) 122 of the 3D printer. The magnetic field experienced by the extruded ink was about 100 mT (millitesla). The printed thickness of each layer was about 0.2 mm. The printing speed was about 1 mm/s (millimeter per second). The flowrate of the ink was about 400%. About 5 mL of the ink was provided. The ink was extrudable via a needle or a nozzle of about 1.2 mm diameter.

[0048] Preferably, a porous substrate 150 is provided. That is, the substrate 150 provided to support the article being printed in the course of the 3D printing may be one selected to draw water away from the ink printed thereon. The substrate 150 may have a total porosity of 60% and density of about 1.2 g/mL. In one example, the substrate 150 is formed by dispersing 8 g of gypsum powder in 8 g of water, and dried in an oven at 48°C overnight. Preferably, the substrate 150 provides a substantially flat surface 502 (FIG. 1A / FIG. 2A) for the deposition of materials, so as to enable the continuous 3D printing process to take advantage of the freedom to build a wide variety of articles of various shapes/ structures/features without the expense and limitations of a mold.

[0049] Method and Product Thereof

[0050] For the sake of brevity, the present method 500 may be referred to as magnetic direct ink writing (M-DIW) although it would be appreciated by a skilled person in the art would believe that the ink proposed herein for M-DIW would render the conventional DIW inoperable.

[0051] The method 500 of 3D printing proposed herein includes extruding the ink through the nozzle of the 3D printer concurrently with movement of the nozzle and the magnet (or magnetic field distribution) along the nozzle displacement path. A first layer of extrudate may be deposited on a porous substrate (e.g., a gypsum substrate). Subsequent layers of extrudates may be deposited on previously formed layer(s) of the same 3D printed material. The 3D printing process can be continuous, i.e., multiple layers can be printed in one continuous process without stopping the printing. For example, it is not necessary to stop before printing a second layer on top of a first layer. That is, it is not necessary to stop printing at intervals throughout the printing of an article for the purpose of curing or drying the printed materials to enable the printing of a subsequent layer.

[0052] Concurrent with the printing of the ink proposed herein, the magnet field distribution tracks the nozzle along the nozzle displacement path, e.g., the magnet 130 and the nozzle 120 concurrently move along the same or similar path during printing. In some embodiments, relative to the nozzle displacement direction 209, the magnet may be behind the nozzle, in front of the nozzle, or alongside the nozzle. Depending on the intended local anisotropy, the magnet 130 may be made to rotate about a rotation axis 139 that is angular displaced relative to both the nozzle axis 129 and the nozzle displacement direction 209. The method 500 involves displacing a magnetic field along the nozzle displacement path of 3D printing nozzle 120, with the magnet 130 rotating relative to the nozzle 120. The method 500 may be described in terms of subjecting the printed ink or extrudate 200 to a magnetic field (W) that decreases at a rate corresponding to the nozzle velocity (V_p).

[0053] Referring again to FIG. 1 A or FIG. 2A, the printed ink may form a continuous line of extrudate 200, printed in accordance with the nozzle displacement path. Upon being printed, the printed ink may undergo a step of magnetic alignment 510, followed by a step of jamming 520, and a step of drying 530. The steps may overlap in some cases. The solidified material (i.e., the printed ink after drying) is observed to be anisotropic, with particles therein aligned or oriented in a similar direction. In contrast, the ink before extruding from the nozzle is a liquid with the magnetic particles in various random orientations.

[0054] Simultaneous with the ink 300 being extruded out from the nozzle and even before the ink 300 is deposited (before the ink reaches the substrate or a previously printed layer), magnetically responsive particles 320 in the ink 300 would be subjected to the magnetic field and start to orientate themselves such that they are generally similarly oriented or in alignment. That is, the magnetically responsive particles in the newly extruded ink undergo magnetic alignment 510 with the rotating magnetic field. Upon the extruded ink being deposited, the deposited ink continues to be subject to the magnetic field. The relative orientations of the rotating magnet 130 and the substrate 150 / previous layer enable the generally aligned magnetically responsive particles 320 to be oriented in a desired orientation. The desired orientation can be controlled by setting the orientation of the rotation axis 139 of the magnet 130 relative to any one of the nozzle axis 129, the nozzle displacement direction 209, or the nozzle velocity (V_p). As the nozzle 120 moves further away from a volume of deposited ink, the magnetically responsive particles 320 in the deposited ink are subject to a decreasing magnetic field strength.

[0055] The proposed method 500 facilitates or enables jamming 520 to occur concurrent with the loss of water from the deposited ink, e.g., to the porous substrate. Jamming refers to a phenomenon of similarly aligned particles packing themselves more closely as the water or the solution (in which the particles were distributed) is removed. The water may be lost from the solution or the ink, at least partially by the water being drawn by capillary action of the porous base. Jamming may include particles "dropping" as a result of gravity and a decrease in the surrounding/supporting water or solution in which the particles were distributed. Jamming may involve particles slotting between other similarly oriented particles as the surrounding water or solution is lost (e.g., via absorption, capillary action, evaporation, etc.).

[0056] Drying or consolidation 530 may continue after jamming. Preferably, drying is aided by a porous substrate 150 and/or heating. The printed article may be heated or sintered, i.e., after the article has been 3D printed (as opposed to sintering an intermediate workpiece after printing each layer and before printing the next layer). In one example, for an article made of a 3D printed alumina-based composite, the article may be sintered in air for about two hours at 1600 °C in a box furnace (e.g., box furnace available from Nabertherm, Switzerland). Preferably, the heating profile includes a heating rate of 2.5 °C/minute with an intermediate plateau of about five hours at 500 °C. In another example, for an article made of a 3D printed graphite, the graphite composites may be reduced at 1050 °C for four hours with a heating rate of 5 °C/minute, in a tube furnace in an Argon atmosphere (e.g., Protege XST split tube furnace). In the examples where a mineral-based additive (e.g., SiCh) is used, sintering can advantageously form a mineral matrix to bond the microplatelets.

[0057] The configuration of the microstructure in the present method 500 may be described in the following steps: (i) in the freshly deposited ink, the microplatelets align under the magnetic field (step 510); (ii) the water from the deposited ink is removed with the aid of the porous substrate, leading to a jamming of the microplatelets and facilitating shaperetention (step 520); and (iii) a consolidation of the layer of printed ink by drying (step 530). In the consolidation step, the jammed deposited layer is further dried by the porous base (e.g., a porous substrate or a previously printed and dried/solidified layer) to strengthen the assembly and allow subsequent layers to be deposited following the same process. For the subsequent layers, the underlying layer acts as the substrate and removes the water by capillary forces.

[0058] The Ink

[0059] Embodiments of the present disclosure include an ink 300 suitable for continuous 3D printing of an article, including a ceramic-based article. The ink is characterized by a viscosity or fluidity that permits a continuous delivery or extrusion of the ink via a nozzle and a viscosity that enables the extruded ink to substantially maintain an intended shape (e.g., in terms of a height of extrudate, contact angle, etc.) before the extrudate is fully solidified. In other words, the ink is characterized by a shape-retaining property or a shaperetention property after it is deposited on the porous substrate. The shape-retention property of the ink is further characterized by being magnetically responsive at a microstructure level while retaining the as-printed shape at a macro level. The ink preferably includes magnetically responsive particles that can bi-axially align, i.e., align in any of at least two directions, in a rotating magnetic field of a relatively low magnetic field strength (low magnetic field), e.g., microplatelets. Preferably, the magnetically responsive particles are microparticles coated with magnetic nanoparticles which have a surface charge opposite to that of the microparticles. That is, non-magnetic particles may be selected and rendered magnetically responsive by adsorption of magnetic nanoparticles thereon. The ink is characterized by a solidification time (after being extruded) that is long enough to permit alignment of the magnetically responsive particles and short enough to permit another layer of the ink to be printed on a previously printed ink, i.e., multilayer 3D printing. In the present disclosure, the term “microparticle” refers to a particle having linear dimensions in the order of micrometers (pm) as will be understood by a skilled person in the art, e.g., in a range from 0.1 pm to 100 pm. The term “microplatelets” refers to a relatively thin piece of material having two opposing major surfaces, e.g., substantially in the form of a flake or a thin piece of material, such that the microplatelets may be described as “two-dimensional” (2-D) or substantially planar in shape. The term "nanoparticle" refers to a particle having linear dimensions in the order of nanometers (nm) as will be understood by a skilled person in the art, e.g., in a range from 1 to 200 nm.

[0060] The ink composition may be determined or tuned based on time-based parameters and a viscosity within a range of printable and orientable viscosities, as shown schematically in the graphs of FIG. 4 and FIG. 5. FIG. 4 is a schematic representation of a time for magnetic alignment t_m, a time for jamming tj and a time related to the 3D printing t_p as a function of the viscosity of the ink (ink viscosity). The ink exhibits a decrease in the magnetic alignment time t_m with increasing magnetic susceptibility %. In order for the ink to be printable and orientable, the ink viscosity is tuned to be within the range of printable and orientable viscosities. FIG. 5 is a schematic representation of t_m, tj, and t_p as a function of the printed layer number n. The ink viscosity may be tuned (e.g., with the aid of fumed silica) such that (for the number of layers to be printed) the jamming time and the magnetic alignment time are within the printable and orientable ranges.

[0061] Advantageously, the particles may be selected from a wide variety of different materials, including non-magnetic materials. The magnetically responsive particles of the ink may be functionalized particles. Examples of the particles include and are not limited to metallic flakes, carbon-based materials, boron nitride microplatelets, microfibers, etc. The amount of magnetically responsive nanoparticles (e.g., SPIONs) can be tuned depending on the dimensions and other intrinsic properties of the microplatelets chosen.

[0062] One of ordinary skill in the art and without inventive effort would not select the ink of the present disclosure for conventional 3D printing because although the ink is extrudable, it is not buildable as the low yield stress will make it too flowable. That is, the printed ink will not be capable of shape-retention and has no ability to withstand the weight of multiple layers. In conventional direct ink writing, for example, the ink is required to be a paste with high viscosity and elasticity so that the printed material can have the shaperetention property required for 3D printing. In contrast, the present disclosure proposes a preferred ink composition for direct ink writing in which the ink is characterized by a high zero-shear viscosity of about 200 Pa s and a low yield stress of about 4 Pa. In some embodiments of the present disclosure, the ink composition for direct ink writing is characterized by a relatively high zero-shear viscosity, in which the zero-shear viscosity is less than 200 Pa-s. In some embodiments of the present disclosure, the ink composition for direct ink writing is characterized by a relatively low yield stress, in which the yield stress is less than 4 Pa.

[0063] In some embodiments, the concentrations of magnetically responsive microplatelets and a colloidal gel in the ink are balanced to obtain an ink that is viscous enough to be printable, but not too viscous so that the microplatelets are able to rotate and align with the magnetic field. The high zero-shear viscosity enables extrusion through a thin nozzle without developing severe die swell. Also, it ensures a homogeneous ink without sedimentation. The low yield stress enables the motion of microplatelets under the action of the magnetic torque and their magnetic orientation.

[0064] The present method of continuous 3D printing (also referred to as magnetic direct ink writing or M-DIW) includes, after extruding a magnetically responsive ink in a less viscous form, removing the low viscosity solvent, (e.g., water) from the ink in a controlled fashion during the printing. In some examples, the removal of water is achieved by printing onto a porous substrate (e.g., gypsum) that can remove water from the printed material by capillary suction. After water is removed, the microplatelets accumulate at the surface of the gypsum forming a percolated layer.

[0065] Preferably, the ink fulfills antagonistic requirements in terms of viscosity. At low viscosities, back-pressure prevents continuous extrusion, whereas at high viscosities, the forces required to extrude become too high for the apparatus. In addition, for the alignment of the microplatelets, the following temporal relationships are preferably fulfilled:

where t_m is the time required for the magnetic alignment of the microplatelets, t_p is a time related to the printing speed, and tj is the jamming time of the ink (FIG. 4). The time t_p is the time at which the magnetic field where the ink is just deposited is sufficiently strong to orient the microplatelets. Indeed, the magnet is moving with the nozzle. Therefore, the magnetic field in the deposited ink decreases as the nozzle moves away at a speed V_p. The time t_p is thus defined as follows:

where Z is the distance traveled by the nozzle where the magnetic field is high enough to align the microplatelets in the deposited ink. The magnetic field strength distribution is set by the magnet used. Preferably, a high printing speed is used, corresponding to a low t_p. This poses constraints on t_m as the magnetic field at one point in the deposited ink is preferably high enough for the microplatelets to align quickly. The time t_m is given by:

r where V is the volume of a microplatelet, — is the Perrin friction factor, J]_o is the viscosity fo of fluid, ₀ is the permittivity of vacuum, a and b are the average thickness and diameter of the microplatelet, respectively, d is the thickness of the magnetic coating, and % is the magnetic susceptibility of the microplatelets. For a given ink composition, t_m is proportional to — according to Eq. (3). A preferred way to decrease t_m and ensure t_m < t_p

X is to increase % by increasing the magnetic coating of the microplatelets. The jamming time tj after the ink is deposited onto the printed substrate is given by:

where R is the hydrostatic resistance, J is the ratio between the volume of the consolidated layer and of the extracted liquid, AP is the capillary suction and x is the distance to the substrate.

[0066] As shown in Equation (4), tj increases with r|o. In general, suspensions of particles have a long tj as compared to t_m. However, Equation (4) indicates that the first printed layers (small x and high AP) have a very short jamming time and likely cannot achieve tm < tj. The strong hydrodynamic forces that develop at the surface of the porous substrate inevitably lead to horizontally aligned structures.

[0067] Further, tj increases with x and therefore increases with the number of layers (n) (FIG. 5). Too long a tj would require setting a lower V_p as the layers need more time to jam before the next layer can be deposited. Too long a tj may mean inadequate shape retention when the next layers are 3D printed. It is therefore not intuitive to choose an ink with a high zero-shear viscosity and a low yield stress.

[0068] Ink Characterization

[0069] To aid understanding, the relationship between r|o and t_m for an ink based on mAhCh microplatelets is further described. In the experiments conducted, mAhCh of an average diameter of 5 pm was suspended at various concentrations in background fluids of viscosities ro. The time t_m taken for the microplatelets to align with a magnetic field of 100 mT were measured. The magnetic susceptibility % of the microplatelets was also varied to reduce t_m (FIG. 6 A to FIG. 6D). For the purpose of the experiment, a resin was used (zero drying time) instead of water so as to study a wider range of viscosities. The resin was a liquid epoxy with low zero-shear viscosity r|o of around 20 Pa-s. The addition of 5 to 8 wt% SiCh to the resin increased r|o from 30 to 700 Pa-s and its yield stress c_y from 0 to 45 Pa (FIG. 6A and FIG. 6B). At the same time, the elastic and viscous moduli G’ and G” increased by two orders of magnitude.

[0070] The AI2O3 microplatelets were coated with SPIONs up to a maximum of 18.6 vol% with respect to the microplatelets. To measure t_m, an inverted optical microscope was used to observe the dynamic response of the mAhCh when subjected to a magnetic field of 100 mT rotating at a frequency of 2 Hz. The alignment degree DL/DW was determined by taking the ratios of the long and short diameters at the full width of half-maximum of Lorentz fits, respectively, of the ellipse of the Fast Fourier Transforms (FFT) of the microscopic images. A DL/DW = 1 indicates no alignment, whereas DL/DW > 1 indicates magnetic alignment. As expected from Equation 3, increasing the amount of SPIONs on the microplatelets increased their magnetic response and resulted in a shorter alignment time t_m. In both the pure resin and the resin with 5 wt% SiO2 (FIG. 6C), t_m decreased from 6 to 0.1 seconds by increasing the SPIONs coating from 5 to 18 vol% while maintaining a high alignment quality with a DL/DW > 2. The results showed that t_m increased with the SiO2 content (FIG. 6D). In the resin with 7 wt% SiO2, optical micrograph images show that alignment could still take place, with a t_m of 4 seconds. In the resin with 8 wt% SiO2, optical micrograph images (FIG. 6E) show that no alignment occurred. Even with 18 vol% coating, the resin containing 8 wt% SiO2 did not allow for the magnetic alignment. This study of the influence of the background liquid on the magnetic orientation of the microplatelets therefore indicates that m AI2O3 can align with low magnetic fields when the yield stress of the liquid is c_y less than 45 Pa.

[0071] It was found that mAhCh microplatelets with a SPIONs coating of 14 vol% aligned with a t_m of 4 seconds only when suspended in background liquids of viscosities at zero shear less than 200 Pa.s and with a yield stress less than 4 Pa. [0072] The rheological properties of the ink and not of the background liquid have to be considered as the microplatelets will start interacting, thereby augmenting the ink viscosity. In the following, the colloidal gel or the background liquid in the ink was chosen to be water with 4 wt% of fumed silica. This background is very liquid and individual microplatelets can easily align. A SPIONS coating of 14 vol% for the mAhCh was chosen and various concentrations of mAhCh were tested (Table 1).

Table 1. Ink Compositions

[0073] The viscosity at zero shear of the inks increased from 10 Pa-s for 15 wt% loading of microplatelets to 3000 Pa-s for 35 wt% (FIG. 7A). All inks exhibit shear-thinning behavior (FIG. 7A) and their yield stress increases from 4 Pa for the ink containing 20 wt% mAhCh to 45 Pa for 35 wt% concentration (FIG. 7B and FIG. 7C). Additionally, the inks containing more than 15 wt% microplatelets present a solid-like behavior (G’ > G”) at low shear rates (FIG. 7D). This suggests that some shape-retention can be obtained for these inks. In turn, the inks containing 0 and 15 wt% mAhCh exhibit liquid-like behavior (G’ G”), indicating no strong interaction between SiCb nanoparticles and mAbOs microplatelets (FIG. 7C and FIG. 7D). These results reveal the formation of a percolating network structure in the ink starting at 20 wt% mAhCh.

[0074] Based on experimental results, it is expected that magnetic alignment will be possible for all inks with the yield stress lower than 45 Pa, except for 35 wt% concentration. At 35 wt%, the high yield stress indicates that the microplatelets strongly interact with each other, preventing their orientation with the low magnetic field as used in the experiments.

[0075] To verify that magnetic alignment occurred as predicted, the ratio DL/DW was measured from electron micrographs instead of optical images (FIG. 7E). The direction of alignment was set to orient the microplatelets vertically with respect to the horizontal substrate. This orientation was chosen since it is the least favorable to obtain as gravitational forces and hydrodynamic forces due to the capillary suction need to be overcome. As predicted, all inks could be aligned except the ink with 35 wt% microplatelets (see insert in FIG. 7E).

[0076] In some experiments, the magnetic field strength varied in space from 378 mT at the surface of the magnet to 6 mT at a distance of 3 cm. The magnet was placed so that the field strength at the nozzle tip is 100 mT. Continuously 3D printing at a constant speed V_p of 1 mm/s, the magnetic field applied to the deposited ink gradually decreases from 100 mT to 50 mT in 5 s. This printing speed is reasonable for 3D printing. Therefore, t_p was set to 5 seconds. Preferably, t_m < 5s. Faster printing speed to improve the printing efficiency can be possibly achieved by applying wider spatial distribution of magnetic field strength above 50 mT, for example using a magnet with a different geometry.

[0077] Since the high concentration of mAhCh in the ink makes it difficult to see the individual platelets under an optical microscope, the t_m was therefore calculated using Equation (3), for magnetic field strength of 100 mT and 50 mT (FIG. 7F). When the concentration of microplatelets is high, they are susceptible to interact and to hinder each other from rotation, therefore it is the viscosity of the ink, i.e., the background liquid and the other microplatelets, that is taken as r|o in Equation. (3). The prediction shows that the mAhCh microplatelets can be aligned in inks of viscosities 2000 Pa.s within 40 seconds for a magnetic field strength of 100 mT and 140 s for 50 mT. In practice, the calculated plot was used to find the ink viscosity required to obtain a t_m < t_p = 5 s. At 100 mT, inks of viscosities up to 400 Pa.s can be aligned. This corresponds to inks containing 20 to 25 wt% mAhCh. This means that the inks containing as much as 20 wt% mAhCh can be aligned during M-DIW.

[0078] The ink containing 20 wt% mAhCh exhibited a yield stress of 5 Pa and good shape retention using the present method. The jamming time tj was measured for this ink as a function of the number of layers printed (FIG. 7G). At 23 °C, tj increased with the number of layers, as expected from Equation 4. As more layers are deposited and the thickness of the print increases, the removal of the water slows due to the increased tortuosity. The first layer had tj < 2 s < t_m and therefore no alignment control can be obtained in the first layer, which corresponds to the experimental results. After 10 layers, tj ~ 1 min which makes it challenging for printing thicker parts. To increase the number of layers, tj could be reduced by half by adding heat to the substrate.

[0079] To demonstrate that microstructural control in the 3D printed part is effective, experiments were conducted using the ink containing 20 wt% mAbOs and adjusted the direction of the rotating magnetic field during printing (FIG. 8A to FIG. 8E). In printed layers above the first layer, horizontally aligned microplatelets and vertically aligned microplatelets with their basal plane perpendicular or parallel to the printing direction can be realized (FIG. 8A to FIG. 8C). The quality of the alignment was good, with a DL/DW of 1.9.

[0080] Local orientation can be obtained in the printed parts, between layers (FIG. 8D) as well as within a single layer (FIG. 8E). Some lightly misaligned segments of about 100 pm could be observed between well-aligned regions. These misaligned domains are attributed to the manual changing of orientation of the magnet. Therefore, the ink containing 20 wt% mAhCri is a good ink for M-DIW. Interestingly, the ink containing 15 wt% mAhCri could also be printed despite the very low viscosity at zero shear of about 10 Pa-s.

[0081] Preparation of the ink

[0082] In some examples, the ink includes magnetically responsive alumina microplatelets (mAhCri). A suspension may be prepared by dispersing 10 grams (g) of AI2O3 (alumina) microplatelets in 400 mL (milliliter) of distilled water for about 10 min (minutes). Examples of suitable AI2O3 microplatelets may have a lateral dimension (size) of 5 pm (micrometer) and a thickness of about 700 nm (nanometer), such as may be obtained from Kinsei Matec Co., Ltd. In some examples, the alumina particles have diameters in a range of 1 pm to 10 pm. A volume of a ferrofluid was diluted into 100 mL of distilled water and added to the suspension drop by drop. Examples of the ferrofluid (cationic) may include EMG 605 (available from Ferrotec Corporation, USA) containing 3.9% vol SPIONs (superparamagnetic iron oxide nanoparticles). The mixture may be left to stir for about three days or for a suitable period of time to allow electrostatic adsorption of the SPIONs to the AI2O3 microplatelets. Next, water may be filtered out of the mixture and the microplatelets may be washed with water and ethanol before being dried (e.g., overnight) in an oven (e.g., IKA Oven 125) at about 48 °C (degree Celsius).

[0083] In some other examples, the ink includes magnetically responsive graphite (mGr) microplatelets. The graphite particles may have diameters in a range of 1 pm to 10 pm. The graphite particles may be functionalized as described above. A suspension may be prepared by grinding 2 g of graphite in 20 mL of ethanol via ball milling. The ball milling may be carried out (e.g., Retsch PM 100 apparatus) with milling balls of diameters 5 mm (millimeter) and 3 mm, at a speed of 500 rpm (revolutions per hour) for 4 hours (in intervals of 5 minutes with breaks of 2 minutes). After filtering, the ball-milled graphite microplatelets may be again dispersed in water by adding diluted ferrofluid (diluted into 100 mL of water) dropwise. The mixture may be left to stir for about three days or a suitable period of time. Next, water may be filtered out of the mixture and the microplatelets may be washed with distilled water and ethanol before being dried in a freeze-dryer (e.g., Martin Christ / Alpha 2-4) and subsequently in an oven at about 48 °C.

[0084] In various examples of the present disclosure, the ink includes magnetically responsive particles (preferably, functionalized microplatelets) and a rheological modifier in aqueous solution (also referred to as a colloidal gel). Examples of the rheological modifier include but are not limited to a material that is essentially composed of fumed silica (SiCL) nanoparticles.

[0085] Exemplary Products

[0086] Being able to 3D print a colloidal gel or solution containing oriented microplatelets is useful for multiple reasons. The present method and ink advantageously enable the production of ceramics and composite materials with controllable properties, as was demonstrated using the ink containing 20 wt% mAbCf, for example. The printed material is an assembly of inorganic particles at high solid loading. Using the proposed printable and orientable ink, the relative density of the resulting product measured after printing was about 31%. In the solid part, the concentration of microplatelets was a very high concentration of 70 vol%, the rest being made of 20 vol% SiCL and 10 vol% FesCU In the final printed body, there was thus about 22 vol% microplatelets. Such concentration of oriented anisotropic microplatelets could not be realized in other 3D printing methods which typically achieve only 15 vol% and 4.7 vol%. A high concentration of inorganics can lead to highly reinforced composites after their infiltration with a polymer or can be turned into ceramics after sintering. In other words, the range of products that can be made using the present method and ink is very wide.

[0087] Additionally, the present disclosure enables control of the microstructure and enables the formation of local properties in different parts of the same article, to form multifunctional articles. For example, one section of an article could be printed with vertically oriented microplatelets to increase the hardness and the wear resistance, whereas another region of the article could have horizontally oriented microplatelets to confer higher strength.

[0088] Microstructured ceramics of various 3D shapes, porous lattices, and bulk samples were 3D printed using the ink and the proposed M-DIW process (FIG. 9 to FIG. 11). The ink contained 20 wt% alumina in 4 wt% fumed silica. The resolution of the layer height was 0.1± 0.02 mm, and the line width was 1.2 ± 0.3 mm. Up to 30 layers could be deposited, leading to samples of about 3 mm thickness. The printed parts were formed into ceramics by sintering at 1600 °C. The shape of the 3D printed samples with a unique orientation could be maintained with a good resolution after the sintering without deformation. The high temperature treatment maintained the iron oxide (FesC ) in a ceramic which possessed magnetic properties and could be easily attracted to a magnet. This is an interesting property that can be exploited to make actuated structures. For example, the sintered ceramics can be combined with a soft polymeric matrix to create a magnetically actuated gripper (FIG. 12).

[0089] Samples with a bilayer microstructure, such as with horizontal layers and vertical layers, morphed because of the anisotropic shrinkage between the two layers. This morphing ability can be used to fabricate ceramics with unusual shapes that could not be made otherwise.

[0090] The ability to 3D print self-shaping ceramics provides a larger range of structures and a greater design space. A self-shaping coil was 3D printed with two layers containing horizontally aligned microplatelets and eight layers containing vertically aligned microplatelets. A curved lattice structure could also be obtained after sintering of lattice structure with three layers of transversely-aligned microstructures alignment and three layers of axially-aligned microstructures. The ceramics obtained could deform to a much greater extent than the conventional self-shaping owing to the thinner dimensions and the larger shrinkage anisotropy enabled by the present disclosure. After sintering, the shrinkage of the printed ink with 20 wt% mAhCh perpendicular to the basal plane of the microplatelets was of 39.4 ± 1%, whereas the shrinkage parallel to the basal plane was 22.3 ± 2.9%. This high shrinkage may be attributed to the densification of ceramic and the melting of the fumed silica during the sintering.

[0091] In the samples made, the sintered ceramics had a density of 3 g/cm³ with a total porosity of 23%. The ceramic articles retained their microstructure after sintering. This demonstrates a capability to 3D print bioinspired ceramics, such as nacre-like alumina. A nacre-like ceramics made by the present continuous 3D printing exhibited a hardness of 7.86 ±1.12 GPa on axially-oriented (e.g., vertically-oriented) microplatelets and 7.38 ± 1.06 GPa on transversely-oriented (e.g., horizontally-oriented) microplatelets. The slight difference of hardness despite the anisotropic microstructure is likely due to the remaining porosity and glassy interfaces. The ceramics exhibited anisotropic flexural properties with a flexural strength of the transversely-aligned ceramics 36% higher than the axially-aligned ones, reaching a value of 134 ± 6.5 MPa. Likewise, the flexural modulus of the transversely- aligned samples was 50% higher than the axially-aligned ones, reaching a value of 152 ± 40 GPa. These good mechanical properties resulted from the densification, the well-aligned microstructures, as well as the strong interface between the alumina grains and the silica matrix. Element mapping showed that Fe and Si diffused in AI2O3 during sintering. Compared to other nano-silica-reinforced alumina ceramics obtained by a conventional 3D printing method, our ceramics had a bending strength about 3 times higher.

[0092] Even in examples where the flexural properties obtained using the present method are comparable with those of conventional methods, the present method has the additional benefit of enabling control of the orientation of the microstructure. High density alumina ceramics can be obtained using conventional methods, for example via vat photopolymerization and digital light processing. For example, a dense alumina ceramic with a flexural strength greater than 450 MPa can be obtained using conventional methods, but control of the microstructure is not possible. [0093] The proposed M-DIW printing method is not limited to alumina-based inks. Graphite microplatelets instead of alumina were used successfully to produce functional materials with anisotropic properties. It was found that the mGr microplatelets could align in a resin with up to 7 wt% SiCh, corresponding to a background viscosity of 100 Pa-s (FIG. 13). It took the mGr microplatelets about 25 s to align. This is quite different from the case with mAhCh microplatelets. The longer alignment time is a result of the lower SPIONs coating. Graphite had less ability to adsorb SPIONs when compared to AI2O3. Nonetheless, the mGr microplatelets could align in the same viscous solutions as the mAhCh microplatelets.

[0094] In some experiments, 4 wt% SiO2 was suspended in water as a background liquid for the ink and the wt% of mGr was varied. It was found that 10 wt% of mGr, corresponding to 4.3 vol%, yielded a suitable ink with a viscosity of 78 Pa-s as well as the yield stress of 2.5 Pa. The alignment time in 4 wt% SiO2 was less than 5 s, thereby satisfying the 3D printing temporal requirements of Equation 1. Similar to the alumina ink, lattice structures and other 3D shapes could be printed with high resolution and exhibited magnetic attraction.

[0095] Further, the graphite composites exhibited anisotropic electrical conductivities from the microstructure. FIG. 14 shows the electrical conductivity of 3D printed graphite composites measured on the surface of (a) sample with horizontal (transverse) alignment, (b) along the vertical (axial) alignment, and (c) perpendicular to vertical (axial) alignment, and the reduction of graphite composites by a high temperature of 1050 °C for 4 hours as shown in the shaded area. For instance, the electrical conductivity measured using four- point probes on the surface of transversely-aligned (e.g., horizontally aligned) 3D printed composite was 5 S/m, which was higher than on the axially-aligned (e.g., vertically aligned) samples. After reduction using heat treatment at 1050 °C in Argon, the electrical conductivity increased by almost 5 times, up to 26 S/m and showed high anisotropy. As compared to other 3D printed graphite materials, not only can M-DIW control the orientation, but the conductivity achieved was also higher and showed anisotropy.

[0096] Low electrical conductivity can be interesting for sensing applications whereas high conductivity can be used for electronic devices, batteries, etc. With tunable electrical and magnetic properties, the present method and ink enable the fabrication of magnetically controlled switches (FIG. 15) or electromagnetic shielding in electronic devices (FIG. 16).

A conductive composite was 3D printed to join two copper wires powering a LED.

[0097] The continuous 3D printing proposed herein enables an article to be formed in one run (e.g., one continuous 3D printing process using one ink to build up multiple layers to form a 3D article) and yet for the article to have different anisotropic properties in different parts of the same article. This opens up possibilities for a wide variety of products or articles to be manufactured in a fully automated production environment. A large range of applications can be found in various fields, e.g., robotics, actuators, sensors, electronics, supercapacitors, and batteries, etc. It can also reduce the number of parts that conventionally require separate production and subsequent assembly.

[0098] In the present disclosure, M-DIW is disclosed as a universal, facile, cost and timeefficient 3D printing method for creating microstructured composite and ceramic materials of various chemistries and functionalities. The alignment quality obtained was similar to that obtained using other magnetically oriented methods, with a misorientation of about 20°, depending on the aspect ratio of the microplatelets used.

[0099] The printing time is greatly improved as compared to other magnetic printing methods owing to the continuous deposition of the ink. In conventional magnetic printing, it took about 5 hours to print 51 layers. Using the present M-DIW method, it would take only 1.1 hours approximately. The aided removal of water (e.g., via a porous substrate) enable a relatively quick removal of water from the deposited ink, enabling faster solidification and good shape-retention, i.e., the deposited ink could maintain shape fidelity. It is possible to achieve a thickness of the print of a few centimeters.

[0100] Variations and modifications may be incorporated without going beyond the scope of the present disclosure. For example, for printed articles of a larger size / dimensions, an external source of heat can be applied in conjunction with the porous substrate. To further accelerate the printing, a larger magnet could be used to distribute the magnetic field over a larger area and to align the microplatelets over a larger area. Depending on the microstructural design desired, there may be a trade-off between achieving orientations in local areas of small dimensions and high printing speed. Consistent with the principles presented herein, inks compositions with other microplatelets chemistries and dimensions could be developed by tuning the magnetic coating, concentration, and viscosity of the background liquid. In addition to high solid loading materials, soft composites with microstructure could a fortiori be obtained as well by adding a polymer or hydrogel in the ink, or by using a low concentration in the ink and infiltrating the printed material after its drying. Alternatively, oxides acting as flux, such as CaO may be added. These optimizations could enable the 3D printing of nacre-like alumina with high strength and toughness. For the conductive inks, the shape and exfoliation of the graphite microplatelets may be tuned to improve their percolation.

[0101] Using SPION-coated particles provide an option of converting any one or more of a wide variety of particles to magnetically responsive particles. The magnetic property may be removed after the article has been formed, e.g., by dissolving / removing the iron oxide using orthophosphoric acid.

[0102] According to various embodiments, a method of continuous 3D printing to print an article, includes: displacing a nozzle of a 3D printer along a nozzle displacement direction at a nozzle velocity; concurrent with the displacing of the nozzle, extruding an ink from the nozzle to form an extrudate on a porous base, the ink including a plurality of magnetically responsive particles and a colloidal gel; and concurrent with the displacing of the nozzle, displacing a rotating magnetic field along the nozzle displacement direction.

[0103] According to various embodiments, the method includes: rotating a magnet relative to the nozzle to provide the rotating magnetic field, the magnet being disposed proximal to the nozzle; and concurrently displacing the magnet and the nozzle along the nozzle displacement direction at the nozzle velocity.

[0104] According to various embodiments, the rotating magnetic field may be characterized by a rotating speed higher than 1 Hertz.

[0105] According to various embodiments, the colloidal gel may include fumed silica nanoparticles in an aqueous solution.

[0106] According to various embodiments, the extrudate may undergo the following changes in sequence: a step of magnetic alignment over a time of magnetic alignment; a step of jamming over a time of jamming; and a step of drying over a time of drying, wherein a concentration of the magnetically responsive particles in the extrudate after the time of drying is higher than the concentration of the magnetically responsive particles in the extrudate before the time of magnetic alignment.

[0107] According to various embodiments, the step of magnetic alignment may include aligning the magnetically responsive particles in the extrudate proximal to the nozzle, wherein the aligning is in response to the magnetic field.

[0108] According to various embodiments, the magnetically responsive particles are microparticles.

[0109] According to various embodiments, the step of jamming may include similarly oriented ones of the magnetically responsive particles packing themselves more closely together as water is lost from the colloidal gel.

[0110] According to various embodiments, water may be lost from the colloidal gel at least partially by the water being drawn by capillary action of the porous base. According to various embodiments, the step of drying may include a loss of water from the extrudate distal to the nozzle.

[0111] According to various embodiments, the method further includes a step of sintering the article.

[0112] According to various embodiments, the method further includes determining a composition of the ink based on time-based parameters and a viscosity of the ink within a range of printable and orientable viscosities. According to various embodiments, the ink is characterized by a yield stress smaller than 45 Pa.

[0113] According to various embodiments, the ink includes magnetically responsive particles that are alignable in any one of two directions, in response to the magnetic field.

[0114] According to various embodiments, the magnetically responsive particles include superparamagnetic iron oxide nanoparticles adsorbed on alumina microplatelets. According to various embodiments, the ink includes AI2O3 microplatelets with 14 vol% superparamagnetic iron oxide nanoparticles adsorbed thereon, 4 wt% SiCh, and water. According to various embodiments, the ink is characterized by a viscosity of less than 1000 Pa-s and a yield stress of less than 45 Pa.

[0115] According to various embodiments, the ink further comprises graphite. According to various embodiments, the ink is characterized by 10 wt% of graphite microplatelets with superparamagnetic iron oxide nanoparticles adsorbed thereon, 4 wt% SiCh, and water. According to various embodiments, the ink is characterized with a viscosity of 78 Pa-s and a yield stress of 2.5 Pa.

[0116] According to various embodiments, an ink for use in the method of continuous 3D printing includes: a plurality of magnetically responsive particles, the magnetically responsive particles including: a plurality of microplatelets; and a plurality of magnetically responsive nanoparticles adsorbed on respective surfaces of the plurality of microplatelets; water; and 4 wt% fumed silica, wherein the ink is characterized by a viscosity of less than 200 Pa-s and a yield stress of less than 4 Pa.

[0117] According to various embodiments, an apparatus for use in the method of continuous 3D printing includes: a 3D printer including a nozzle, the nozzle being displaceable along a nozzle displacement direction; a porous substrate to receive an extrudate from the nozzle; and a magnet coupled to the nozzle, wherein the magnet is concurrently (i) rotatable relative to the nozzle to provide a rotating magnetic field and (ii) displaceable with the nozzle at a same displacement velocity along the nozzle displacement path.

[0118] All examples described herein, whether of apparatus, methods, materials, or products, are presented for the purpose of illustration and to aid understanding and are not intended to be limiting or exhaustive. Various changes and modifications may be made by one of ordinary skill in the art without departing from the scope of the invention as claimed.

Claims

1. A method of continuous 3D printing to print an article, comprising: displacing a nozzle of a 3D printer along a nozzle displacement direction at a nozzle velocity; concurrent with the displacing of the nozzle, extruding an ink from the nozzle to form an extrudate on a porous base, the ink including a plurality of magnetically responsive particles and a colloidal gel; and concurrent with the displacing of the nozzle, displacing a rotating magnetic field along the nozzle displacement direction.

2. The method according to claim 1, comprising: rotating a magnet relative to the nozzle to provide the rotating magnetic field, the magnet being disposed proximal to the nozzle; and concurrently displacing the magnet and the nozzle along the nozzle displacement direction at the nozzle velocity.

3. The method according to claim 1 or claim 2, wherein the rotating magnetic field is characterized by a rotating speed higher than 1 Hertz.

4. The method according to any one of claims 1 to 3, wherein the colloidal gel comprises fumed silica nanoparticles in an aqueous solution.

5. The method according to any one of claims 1 to 3, wherein the extrudate undergoes the following changes in sequence:

(i) a step of magnetic alignment over a time of magnetic alignment;

(ii) a step of j amming over a time of j amming; and

(iii) a step of drying over a time of drying, wherein a concentration of the magnetically responsive particles in the extrudate after the time of drying is higher than the concentration of the magnetically responsive particles in the extrudate before the time of magnetic alignment.

6. The method according to claim 4, wherein the step of magnetic alignment comprises aligning of the magnetically responsive particles in the extrudate proximal to the nozzle, wherein the aligning is in response to the magnetic field.

7. The method according to any one of claims 4 to 6 wherein the magnetically responsive particles are microparticles.

8. The method according to any one of claims 5 to 7, wherein the step of jamming comprises similarly oriented ones of the magnetically responsive particles packing themselves more closely together as water is lost from the colloidal gel.

9. The method according to claim 8, wherein water is lost from the colloidal gel at least partially by the water being drawn by capillary action of the porous base.

10. The method according to any one of claims 5 to 9, wherein the step of drying comprises a loss of water from the extrudate distal to the nozzle.

11. The method according to any one of claims 1 to 10, further comprising a step of sintering the article.

12. The method according to any one of claims 1 to 11, further comprising determining a composition of the ink based on time-based parameters and a viscosity of the ink within a range of printable and orientable viscosities.

13. The method according to claim 12, wherein the ink is characterized by a yield stress smaller than 45 Pa.

14. The method according to any one of claims 1 to 13, wherein the ink comprises magnetically responsive particles that are alignable in any one of two directions, in response to the magnetic field.

15. The method according to any one of claims 1 to 14, wherein the magnetically responsive particles comprise superparamagnetic iron oxide nanoparticles adsorbed on alumina microplatelets.

16. The method according to claim 15, wherein the ink comprises AI2O3 microplatelets with 14 vol% superparamagnetic iron oxide nanoparticles adsorbed thereon, 4 wt% SiCh, and water.

17. The method according to claim 16, wherein the ink is characterized by a viscosity of less than 1000 Pa-s and a yield stress of less than 45 Pa.

18. The method according to any one of claims 1 to 14, wherein the ink further comprises graphite.

19. The method according to claim 18, wherein the ink is characterized by 10 wt% of graphite microplatelets with superparamagnetic iron oxide nanoparticles adsorbed thereon, 4 wt% SiCh, and water.

20. The method according to claim 19, wherein the ink is characterized with a viscosity of 78 Pa-s and a yield stress of 2.5 Pa.

21. An ink for use in the method of continuous 3D printing as recited in claim 1, the ink comprising: a plurality of magnetically responsive particles, the magnetically responsive particles including: a plurality of microplatelets; and a plurality of magnetically responsive nanoparticles adsorbed on respective surfaces of the plurality of microplatelets; water; and

4 wt% fumed silica, wherein the ink is characterized by a viscosity of less than 200 Pa-s and a yield stress of less than 4 Pa.

22. An apparatus for use in the method of continuous 3D printing as recited in claim 1, the apparatus comprising: a 3D printer including a nozzle, the nozzle being displaceable along a nozzle displacement direction; a porous substrate to receive an extrudate from the nozzle; and a magnet coupled to the nozzle, wherein the magnet is concurrently (i) rotatable relative to the nozzle to provide a rotating magnetic field and (ii) displaceable with the nozzle at a same displacement velocity along the nozzle displacement path.