Patent Application
20240238871
The invention is generally directed to methods and materials for manufacturing magnetocaloric devices through polymer-assisted 3D printing. More particularly, the invention enables manufacturing the magnetocaloric devices that are compositionally graded, anisotropically aligned magnetocaloric architectures with designed pores and channels, to bring forth significant improvement in heat exchange efficiency.
Magnetocaloric materials are a special class of magnetic materials which exhibit what is termed the magnetocaloric effect (MCE). In such a material, when applying an external magnetic field, the originally random magnetic moments are aligned leading to heating of the material. Usually heat is transferred from the magnetocaloric material to a circulating medium (e.g., water) for the heating cycle. When the magnetic field is turned off, the aligned magnetic moments revert back to the random orientation, resulting in cooling of the material. Heat transfer is enabled from the circulating medium to the magnetocaloric materials for the cooling cycle.
A key deficiency of current magnetocaloric devices is the absence of effective shaping technologies for the production of a porous regenerator, the centerpiece of any caloric cooling device (an example of which is depicted in
To date, research efforts to produce magnetocaloric regenerators are predominantly focused on the production of packed particle beds and plane plates that can be stacked and separated by spacers to provide channels for the heat-exchanger fluid. Both structures possess inherent processing challenges that reduce their performance once integrated into a device. Packed beds tend to disintegrate leading to pressure drop along the length of the regenerator, and ultimately leads to flushing of small particles into the surrounding heat-exchange system. Subtractive manufacturing methods do not satisfy the strict geometrical requirements required to provide large thermal efficiency (thickness <0.1 mm; spacing: ˜0.075 mm) for parallel plates. Further, due to the issues related to mechanical brittleness, the regenerator plates are generally composites consisting of a caloric powder and a polymer binder. The binder dramatically reduces the amount of active material in the plates and dilutes the caloric response while decreasing the thermal conductivity. Powder bed fusion based additive manufacturing methods such as selective laser sintering are expensive and fraught with issues associated with melting and re-crystallizing of the particles.
In one aspect, the invention provides a method and materials that allow for directly printing porous, compositionally graded, magnetocaloric structures, with complex architectures.
In another aspect of the invention, the invention provides ink formulations which, when printed, produce magnetocaloric structures. Preferably, the ink formulation includes 20-95 wt % solids of magnetocaloric material; 5-80 wt % solids of polymeric binder; and one or more solvents, wherein the polymeric binder and one or more solvents have a polymer-to-solvent ratio of 0.01 g/mL-0.5 g/mL (the solvents being present to dissolve the polymer, thereby creating a printable ink formulation).
In yet another aspect of the invention, the invention provides a printing methodology whereby enhanced alignment is achieved in a magnetocaloric structure.
Although extrusion-based 3D printing is quite often used in making prototypes and functional devices, this invention differs in the roles of the polymers and additives in the final printed composites. The polymers act as binding agents for the magnetic particles, most of which are burned off during the post-treatment process. Additives such as graphene nanoplatelets enhance heat transfer and mechanical strength of the printed magnetocaloric devices, in addition to functioning as viscosity modifier for ink formulation. The alignment of magnetic particles by applying an external magnetic field during printing results in anisotropic magnetic crystallinity and enhances the magnetocaloric responses. One advantage of the invention lies in its capability to print a compositionally graded structure.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.
In one aspect of the invention, near-net shaped magnetocaloric structures are printed for modular integration into pre-existing magnetocaloric devices. In other aspects of the invention magnetocaloric structures are printed for integration into new designs and configurations of magnetocaloric devices.
A key component of this fabrication process is a unique ink formulation comprising magnetocaloric material (20-95 wt %), mixed with a polymer binder (5-80 wt %), and thermally conductive additives (0-10 wt %). The magnetocaloric materials, polymer, and thermally conductive additives are incorporated within a multi-solvent system. In the ink formulation, one or more polymers in the polymeric binder act as sacrificial binding agents for magnetic particles which make up the magnetocaloric material, and the solvents dissolve the polymer binders and regulate the volatility and viscosity of the ink. The additives, when included in the formulation, help achieve desirable shear-thinning properties as well as enhance thermal conductivity in the final printed devices. Enhanced thermal conductivity is preferred for a robust printing process.
Non-limiting examples of magnetocaloric powder materials may include: FeRh, Gd5Si2Ge2, Gd5(Si1−xGex)4 (where 0<x<1), Co- and H-doped La(Fe13−xSix) (where 0<x<1), MnAS1−xSbx (where 0<x<1), MnFe(P, As, Si), Co(S1−xSex)2 (where 0<x<1), AIT2B2 (T=Fe, Mn, Ni, Co), NiMnSn, MnM′X (M′=Fe, Co, Ni; X=Si, Ga, Ge,Al), or R1−xMxMnO3, (where 0<x<1, R=lanthanide, M=Ca, Sr and Ba).
In some embodiments, the magnetocaloric powder is mixed with a polymer or a polymer mixture in a 3D printing ink composition. Acting as binder materials, these polymers facilitate the 3D printing process and hold the magnetocaloric powder together in the printed structures. During the first stage of post treatment (usually less than 500° C.), the polymer binders will be removed from the printed structures through thermal decomposition. In some embodiments, the 3D printing ink composition comprises 5-80 wt %, preferably 5-50 wt %, more preferably 5-20 wt % of polymers. Examples of polymers, which may be mixed with the magnetocaloric powders in the ink composition may include, but are not limited to, polylactide (PLA) including poly-L-lactide (PLLA), poly(lactic-co-glycolic) acid (PLGA), polystyrene (PS), polyethylene oxide (PEO), poly-glycolide (PGA), or polycaprolactone, caprolactones, polydioxanones, polyanhydrides, polyorthocarbonates, polyphospliazenes, chitin, chitosan, poly(amino acids), and polyorthoesters, and copolymers, terpolymers and combinations and mixtures thereof. Additionally, a plurality of biodegradable polymers derived from natural sources such as modified polysaccharides (cellulose, chitin, chitosan, dextran) or modified proteins (fibrin, casein) may also be used. These examples of biodegradable polymers which may be utilized in addition to a preferred synthetic polymer and are not intended to be limiting or exhaustive but are intended to be illustrative of potential polymers which may be used. In these embodiments, the polymer of choice will be one that does not react with the magnetocaloric materials, and can be removed from the printed structures through thermal decomposition and leave minimum residues after thermal decomposition.
In some embodiments, solvents are added to the ink formulation to dissolve the polymer binders at about 0.01-0.5 g/mL concentration, or preferably 0.02-0.2 g/mL, or preferably 0.5-0.1 g/mL. Some exemplary solvents include, but are not limited to: dichloromethane (DCM), ethylene glycol butyl ether (EGBE), and dibutyl phthalate (DBP), 2-butoxyethanol (2-Bu), and polyethylene glycol (PEG). Further, additional filler materials may be incorporated into the composition to adjust the overall viscosity. Some exemplary viscosity modifiers include fumed silica, nanoclay and graphene nanoplatelets, metal nanoparticles (Ag, Au, Cu), layered carbides Mn+1AXn, (MAX) where (M=Ti; A=Al,Si, X=C; n=1 to 4) In some embodiments, 0.05-20 wt %, preferably 0.08-15 wt %, more preferably 0.1-10 wt % of one or more viscosity modifiers may be included.
One aspect of the invention provides the method of using the ink composition described above for printing magnetocaloric devices. The preferred printing method, in some embodiments, is direct ink writing (DIW, also known to be analogous to robocasting). Generally, but not exclusively, a printing ink of the present invention is loaded into the ink cartridge of existing and commercially available printing devices to realize 3D printing of solid structures and functional devices. In aspects of the invention, a method of manufacturing a magnetocaloric device comprises steps of: i) using an appropriate computer implemented software that is well known in the art to determine the three-dimensional design of the magnetocaloric device; ii) loading an appropriate amount of the 3D printing composition into an ink cartridge of a 3D printer; iii) receiving or retrieving a 3D model of a magnetocaloric device with a printing parameter; iv) printing the magnetocaloric device with the 3D printer. In some embodiments, an extrusion-based printing process (i.e. DIW) is used to extrude the ink material directly without melting of the magnetocaloric powder. In these embodiments, the printing ink exhibits shear-thinning property that has a low viscosity to maintain its fluidity while being extruded out of the nozzle but after the extrusion, a high viscosity is achieved for the material to maintain its shape on the printing bed.
The printing parameter may also include an extrusion pressure of 0.05-14 bar, preferably 0.08-12 bar, more preferably 0.1-10 bar. In some embodiments, the magnetocaloric composition is uniform within the printed device. Alternatively, multiple magnetocaloric inks may be used to manufacture the magnetocaloric devices with a compositional or functional gradient. In these embodiments, the inner portion of the device has a higher concentration of the composition than an outer portion of the device. In addition, in some embodiments, the printing method may further comprise a magnetic field around the ink cartridge to align the magnetocaloric materials during printing.
Another 3D solid forming method, in some embodiments, is referred to as a cold extrusion process. Generally, but not exclusively, an ink composite of the present invention replaces the typical materials of metals, polymers, ceramics, concrete, modeling clay of existing and commercially available cold extrusion devices, in particular, within the ink feeder. The ink material is pushed through a die of the desired cross-section, e.g. a cross-section with tubular arrays, being extruded into magnetocaloric devices of a fixed cross-sectional profile. The term “die” used herein refers to a specialized tool used in extrusion-based printing or manufacturing to form or cut material to a desired shape or profile. Alternatively, any other extrusion processes known in the art (e.g., microextrusion, friction extrusion, warm extrusion, hot extrusion, etc.) may also be used with an effective temperature and/or extrusion pressure known in the art, which may be adjusted accordingly based on the material and/or intended purposes of the 3D ink composition.
In preferred embodiments, the method of manufacturing a magnetocaloric device may further comprise additional steps of controlling porosity on multiple scale levels to generate a scaffold structure with varying porosity that includes macroporosity (50-1000 microns), microprosity (1-50 microns) and nanoporosity (less than 1.0 micron). In some embodiments, the heat transfer efficiency of the magnetocaloric devices is affected by creating compositionally and functionally graded magnetic structures. in particular, the transition temperature of magnetocaloric device increases gradually from cold region to hot region. In some embodiments, additional steps of designing spatial optimization of channels and pathways for the cooling water are included to improve the heat exchange efficiency.
Following printing, the magnetocaloric article will be subjected to a two-stage sintering process, optionally in the presence of an externally applied magnetic field to improve magnetic anisotropy. It will first be heat-treated to remove the sacrificial polymer binder. Heating at ˜300-450° C. for 30 minutes to 2 hours is generally sufficient. Then, the magnetocaloric article will be sintered at high temperatures for prolonged periods to promote grain growth in the printed structure and densification of the printed structure. A suitable sintering cycle may include temperatures ranging from 900-1500° C. for periods of 4 to 24 hours. During the sintering heat-treatment, inert gas such as nitrogen, argon, optionally with a small fraction of hydrogen will preferably flow through the furnace to prevent oxidization of the additives added to promote thermal conductivity and magnetic particles. All operating conditions will ultimately depend upon chemistry and composition of the ink formulation.
A particular advantage of the ink formulations and 3D printing method is that they allow for realization of functionally-graded magnetocaloric structures wherein the composition of the working material will be varied spatially along the direction of heat transfer such that the transition temperature at any given cross-section is close to the local average temperature during operation. That is, the printed structure may have more of one type of magnetocaloric material on one side, and less on another side, such that when subjected to a magnetic field, the structure becomes hotter on one side than on the other side. The printed structure may have different magnetocaloric formulation components in different areas to achieve a variation of different heating and cooling in different areas. This can be accomplished by different techniques.
First, using an extrusion-based 3D printer featuring a single nozzle, multiple articles can be printed with magnetocaloric materials with different transition temperatures. These can then be integrated and assembled in the order of their transition temperatures to maximize the heat exchange efficiency.
Second, as shown in
The magnetocaloric devices which are made using the inks and 3D printing methods described are wide ranging. For example, they can be used for manufacturing magnetic cooling devices, magnetic heat pumps, for home and industrial grade refrigerators, electronics cooling, etc. coolers, and heat exchangers or generators. The inventive inks and 3D printing methods have particular application for use in refrigerators.
For exemplary purposes, an ink formulation according to the invention has been used to print, via a 3D printing operation, magnetocaloric components and components having micro channeled architecture.
Ink formulation: The magnetic ink comprises magnetic particles (rare-earth and transition-metal based ferrous alloys, perovskites, spinel ferrites etc.) mixed with polyethylene oxide (PEO) incorporated within a tri-solvent system. For every cc of magnetic powder, 0.60 g ethylene glycol butyl ether (EGBE), 0.03 g dibutyl phthalate (DBP), and 0.70 g dichloromethane (DCM) are added and mixed by a planetary centrifugal mixer (Thinky USA) for 20 minutes at 2000 RPM to form a particle slurry. PEO is dissolved in 12.5 mL DCM per cc of polymer and mixed by a vortex mixer for 1 minute at 3000 RPM. The PEO solution is retained for 24 hours to ensure a uniform solution. Graphene nanoplatelets powder of 1% to 10% in weight with respect to the magnetic powder is added to the particle slurry and mixed by the Thinky mixer for 1 minute at 2000 RPM. The particle slurry and polymer are then combined and mixed by the Thinky mixer for 8 minutes at 2000 RPM to form the magnetic ink. The ink is transferred to a printing cartridge and stored at 4° C. when not in use.
Demonstration of the feasibility of printing samples with micro-channeled architecture: The above-mentioned magnetic ink is printed on a 3D printer (EnvisionTec, 3D Bioplotter). The printing speed is 5-10 mm/s depending on the extrusion pressure, with printing nozzles of 400 μm and 800 μm. This 3D printed sample is next subjected to a two-stage sintering process. It is first heat-treated at ˜400-450° C. for 30 minutes to remove the sacrificial polymer binder and then sintered at high temperatures ranging 900-1100° C.) for prolonged periods (4 to 24 hours depending upon sample chemistry and composition) to promote grain growth and densification of the printed structure. During heat-treatment, nitrogen is flown through the furnace to prevent oxidization of the graphene and magnetic particles. The printed sample isotropically shrinks approximately ˜15% by volume during post-processing.
Processes may also be used for aligning the magnetic particles during printing and post-processing to maximize the magnetocaloric response. Applying a magnetic field has been employed in various printing processes (inkjet printing (Song, et al., Journal of Applied Physics, 2014, 115, 17E308) and extrusion-based 3D printing (Kim, et al., Nature, 2018, 558, 274-280)) to align the magnetic particles along their easy axes leading to anisotropic magnetic properties. With the present process, it is the first demonstration of applying a magnetic field to align the magnetocaloric materials during the fabrication of the magnetocaloric devices.
Demonstration of the feasibility of printing magnetocaloric components:
Here, μ0 is the permeability of free space, ∂M/∂T is the temperature derivative of the magnetization and Hmax is the maximum applied magnetic field.
Controlling and optimizing the process parameters may be used to achieve desired mechanical properties, macrostructure, microstructure, and magnetofunctional responses. Magnetocaloric regenerators with favorable heat exchanger geometries (example: microchannels, pitot tubes, pin fins) can be down-selected for 3D printing for minimizing mechanical stress/strain at features while maximizing the caloric effect and heat extraction efficiency. In addition, modules of magnetocaloric devices may be printed using the magnetocaloric materials with different transition temperatures. These may be integrated and assembled to maximize the heat exchange efficiency in the order of their transition temperatures. Different modules and fluid channels may also be connected through matching pins and channels.
Conventional machining methods and powder bed fusion methods such as (example: milling, boring and drilling) allow limited control over the internal structure. By contrast, the 3D printing methodology described herein provides an unmatched capability to print a compositionally graded magnetocaloric structure.
This application is a divisional application of U.S. Ser. No. 17/319,554 filed on May 13, 2021, and the complete contents thereof is herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 17319554 | May 2021 | US |
| Child | 18440289 | US |
