WO2023178026A2

WO2023178026A2 - Microwave-activated thermal curing for preparing a polymer composite containing dispersed inorganic particles

Info

Publication number: WO2023178026A2
Application number: PCT/US2023/064216
Authority: WO
Inventors: Benito ROMAN-MANSO; Robert D. WEEKS; Jennifer A. Lewis
Original assignee: President And Fellows Of Harvard College
Priority date: 2022-03-15
Filing date: 2023-03-13
Publication date: 2023-09-21
Also published as: WO2023178026A3

Abstract

A method of microwave-assisted or microwave-activated thermal curing includes forming a colloidal suspension into a predetermined shape, where the colloidal suspension comprises an aqueous solution including water, a crosslinkable polymer dissolved in the water, and inorganic particles dispersed in the aqueous solution at a concentration of at least about 10 vol.%. The colloidal suspension is then exposed to microwave irradiation for about two minutes or less, such that the crosslinkable polymer undergoes thermal curing, and a polymer composite having the predetermined shape is formed. The polymer composite comprises a crosslinked polymer matrix with the inorganic particles dispersed therein. The polymer composite may serve as a precursor for a densified or sintered inorganic (e.g., ceramic) part.

Description

MICROWAVE-ACTIVATED THERMAL CURING FOR PREPARING A POLYMER COMPOSITE CONTAINING DISPERSED INORGANIC PARTICLES

RELATED APPLICATION

[0001] The present patent document claims the benefit of priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application 63/319,938, which was filed on March 15, 2022, and is hereby incorporated by reference in its entirety.

FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0002] This invention was made with government support under grant number FA8650-16-D-5851 awarded by the Air Force Research Laboratory and under grant number N00014-16-2923 awarded by the Office of Naval Research. The government has certain rights in the invention.

TECHNICAL FIELD

[0003] The present disclosure is related generally to preparation and processing of colloidal suspensions and polymer composites, and more particularly to microwave-assisted or -activated thermal curing of the colloidal suspensions to form the polymer composites.

BACKGROUND

[0004] The manufacture of polymer networks or matrices containing dispersed inorganic particles has been a subject of study in the field of ceramics. In certain ceramic processing methods that allow near-net shape fabrication, such as gel casting, monomers are added to ceramic suspensions to form - after an adequate polymerization step - macromolecular polymer networks holding the suspended ceramic particles together as well as maintaining the desired shape, such as that of the mold in which the polymerization takes place. Typically, after mold removal, the part is dried in controlled conditions forming a dried green body that may be used as the precursor of a ceramic part after polymer burn-out and sintering.

[0005] However, in existing processing methods that involve the thermal formation of polymer networks containing inorganic particles, various shortcomings have been reported. For example, (i) certain monomers used are corrosive or toxic, ii) surface-exfoliation and/or spallation phenomena may affect the final part due to oxygen-related inhibition of the polymerization, and iii) most importantly, inner stresses may induce microcracks in the green body during the curing and drying steps, destroying the structural integrity of the cured networks. It would be advantageous to develop and implement methods that surmount the above-mentioned difficulties.

BRIEF SUMMARY

[0006] A colloidal suspension comprises an aqueous solution that includes water, a crosslinkable polymer dissolved in the water, and inorganic particles dispersed in the aqueous solution at a concentration of at least about 10 vol.%. [0007] A method of forming a colloidal suspension comprises: forming an aqueous solution by mixing together water, a crosslinkable polymer, and inorganic particles at a concentration of at least about 10 vol.%; during the mixing, the inorganic particles are dispersed in the aqueous solution.

[0008] A method of microwave-activated thermal curing includes forming a colloidal suspension into a predetermined shape, where the colloidal suspension comprises an aqueous solution including water, a crosslinkable polymer dissolved in the water, and inorganic particles dispersed in the aqueous solution at a concentration of at least about 10 vol.%. The colloidal suspension is then exposed to microwave irradiation for about two minutes or less, such that the crosslinkable polymer undergoes thermal curing, and a polymer composite having the predetermined shape is formed. The polymer composite comprises a crosslinked polymer matrix with the inorganic particles dispersed therein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 A is a schematic showing constituents of an exemplary colloidal suspension for use in preparing a functional polymer composite or densified component; the constituents include water, a crosslinkable polymer, and a significant volume fraction of dispersed inorganic particles. [0010] FIG. 1 B is a schematic showing the colloidal suspension of FIG. 1 A after microwave irradiation to form a polymer composite comprising a crosslinked polymer network or matrix and dispersed inorganic particles.

[0011] FIG. 1 C is a schematic showing a sintered inorganic (e.g., ceramic) body formed by pyrolysis and sintering of the polymer composite of FIG. 1 B. [0012] FIG. 2A is a schematic illustrating an exemplary embedded 3D printing process where a colloidal suspension, such as that shown schematically in FIG.

1 A, is forced through a moving nozzle and an extruded filament comprising the colloidal suspension is deposited within a non-solid support material, forming, in this example, a single-layer (effectively a two-dimensional) square lattice.

[0013] FIG. 2B is a schematic showing microwave irradiation on the 3D printed lattice of FIG. 2A to induce crosslinking to form a polymer composite, such as that shown schematically in FIG. 1 B.

[0014] FIG. 2C is a schematic of an infrared imaging process to obtain a thermal map to evaluate heating of the microwave irradiated lattice embedded in the support material.

[0015] FIG. 3 shows 2D thermal maps of the surface temperature of embedded printed lattices revealing the effects of compositional, morphological and irradiation parameters on heat generation due to microwave irradiation. [0016] FIG. 4 shows average surface temperature for the thermal maps of FIG. 3.

[0017] FIGs. 5A-5D show scanning electron microscopy (SEM) images of a polymer composite rapidly cured using microwave irradiation for two minutes (FIGs. 5C and 5D) and a polymer composite cured in a conventional oven for ten hours (FIGs. 5A and 5B), where the latter shows extensive surface cracking.

[0018] FIG. 6 shows micro-computed tomography (micro-CT) images of strut cross sections corresponding to cured YSZ-PEGDA composites and sintered YSZ ceramics revealing the effects of polymenwater ratio and microwave power on the structural integrity of these 3D printed architectures; the scale bars represent 1 mm. DETAILED DESCRIPTION

[0019] Described herein are a colloidal suspension and microwave-assisted thermal curing method that may be used to rapidly form a crack-free polymer composite that may be further processed, if desired, into a densified or sintered inorganic (e.g., ceramic) part. The terms “microwave-assisted” and “microwave- activated” may be used interchangeably throughout this disclosure.

[0020] Referring to the schematic of FIG. 1A, the colloidal suspension 100 may comprise an aqueous solution 102 that includes water 104, a crosslinkable polymer 106 dissolved in the water 104, and inorganic particles 108 at a concentration of at least about 10 vol.% dispersed in the aqueous solution 102. When exposed to microwave irradiation, the crosslinkable polymer 106 undergoes curing or crosslinking, thereby forming a polymer composite 110 comprising a crosslinked polymer matrix or network 112 with the inorganic particles 108 dispersed therein, as illustrated in the schematic of FIG. 1 B. Due to the use of microwave radiation instead of conventional heating to induce curing, the resulting crosslinked polymer matrix 112 is substantially devoid of cracks. In addition, the polymer composite 110 has sufficient structural integrity to be handled for further processing and/or for use as a composite part.

[0021] Advantageously, a substantial amount of inorganic particles 108 may be dispersed in the suspension 102. For example, the concentration of the inorganic particles 108 may be at least about 10 vol.%, at least about 20 vol. %, or at least about 30 vol. %, and/or as high as about 40 vol. %, as high as about 50 vol. %, or as high as about 60 vol. %. With a sufficient amount of inorganic particles 108, the polymer composite 110 that results from microwave irradiation may be further processed into a densified or sintered (e.g., ceramic) inorganic body 120, as illustrated schematically in FIG. 1 C. Notably, the physical dimensions of the sintered inorganic body 120 may be reduced compared to the polymer composite 110 due to shrinkage that may occur as a consequence of pyrolysis of the polymer matrix and sintering of the inorganic particles, as discussed below. In some examples, the polymer composite 110 may include a sufficient amount and/or type of inorganic particles 108 to exhibit certain functional properties for use as a composite part. For example, the inorganic particles 108 and consequently the polymer composite 110 may be electrically conductive, thermally conductive, semiconducting, dielectric, ferromagnetic, optically transparent, catalytically active, optically active, light-emitting, mechanically tough, high-strength, high-stiffness, and/or abrasion-resistant. If a densified inorganic body 120 is formed from the polymer composite 110, the inorganic body 120 may also exhibit one or more of these functional properties. The inorganic particles 108 may comprise a ceramic, a metal, an alloy, a composite, a semiconductor and/or carbon. In some examples, the inorganic particles 108 may comprise an oxide such as aluminum oxide, calcium oxide, chromium oxide, iron oxide, magnesium oxide, nickel oxide, titanium oxide, yttrium oxide, zinc oxide, and/or zirconium oxide. The inorganic particles 108 may have a nominal particle size in a range from about 10 nm to about 50,000 nm, from about 10 nm to about 1 ,000 nm, or from about 100 nm to about 500 nm. [0022] The crosslinkable polymer 106 may comprise a water-soluble polymer such as a polyacrylate (e.g., po ly(ethylene glycol) diacrylate (PEGDA)). Other water-soluble polymers that may be suitable for the crosslinkable polymer 106 include, for example, polyethylene glycols, polyacrylamide and copolymers, polyvinyl alcohol, polyacrylic acid and copolymers, polyamines, polyethyleneimines, quaternary ammonium compounds, polyvinylpyrrolidone and copolymers, and/or polyvinyl methyl ether/maleic anhydride. Typically, a volume ratio of water to crosslinkable polymer 106 in the aqueous solution is in a range from about 60:40 to about 10:90, or from about 30:70 to about 10:90. As discussed below, higher water ratios may be advantageous to reduce the likelihood of cracking during polymerization; thus, in some examples, the volume ratio of water to crosslinkable polymer may lie in the range from about 30:70 to about 20:80, and/or from 30:70 to about 25:75.

[0023] The aqueous solution 102 may further comprise a thermal initiator 114, preferably a water-soluble thermal initiator, to promote crosslinking, as illustrated in FIG. 1 A. In one example, the thermal initiator 114 comprises an azo polymerization initiator, such as 2,2'-azobis(isobutyronitrile), 2,2'-azobis(2,4- dimethyl valeronitrile), 2,2'-azobis(2-methyl butane nitrile), and/or 4, 4'-azobis(4- cyanovaleric acid). Such compounds include an azo group (R-N=N-R’) which decomposes with heat, forming a reactive carbon radical that may promote curing of the crosslinkable polymer 106. Preferably, the thermal initiator 114 promotes curing at a temperature greater than or equal to 70°C. The thermal initiator 114 may be present in the aqueous solution 102 in an amount ranging from about 2.5 to about 3.5 wt.%.

[0024] The colloidal suspension 100 may be processed or otherwise configured such that the inorganic particles 108 are well dispersed within the suspension 100,102 with minimal to no particle aggregation. Dispersion of the inorganic particles 102 may be promoted by mixing, stirring, shaking, and/or ultrasonication of the aqueous solution 102. Also or alternatively, the aqueous solution 102 may include a dispersing agent or dispersant capable of modifying the surface chemistry of the inorganic particles 108 to provide steric or electrostatic repulsion against aggregation. Suitable dispersants for the inorganic particles 108 may include, in some examples, sodium pyrophosphate, ammonium citrate, sodium citrate, sodium tartrate, sodium succinate, glyceryl trioleate, phosphate ester, poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), ammonium polyacrylate (APA), sodium polyacrylate, sodium polysulfonate, polyethylene imine), menhaden fish oil, a random copolymer, and/or a comb polymer. The dispersant may be effective in small amounts, such as at a concentration in a range from about 0.1 wt.% to about 5 wt.%. Table 1 provides examples of suitable dispersants for exemplary oxide particles.

Table 1 . Examples of Inorganic Particles and Dispersants for Colloidal Suspensions

[0025] Particle dispersion may also or alternatively be improved by including a sufficient amount of water 104 in the aqueous solution 102, such as at least about 10 vol.%, or at least about 20 vol.%; however, the robustness of the polymer composite 110 may be inhibited if the amount of water 104 is too high, and thus the suspension 102 preferably includes no more than about 60 vol. % water, or no more than about 45 vol. % water 104. In some examples, a viscosifier may be added to the colloidal suspension 100 to control (e.g., increase) the viscosity of the suspension 100,102. This may be particularly applicable when the colloidal suspension 100 undergoes extrusion-based 3D printing prior to crosslinking, as discussed below.

[0026] To prepare the colloidal suspension 100, an aqueous solution 102 may be formed by mixing together water 104, a crosslinkable polymer 106, and inorganic particles 108 at a concentration of at least about 10 vol. %. During the mixing, the inorganic particles 108 are dispersed in the aqueous solution 102. The mixing may comprise stirring, shaking, and/or ultrasonication of the aqueous solution 102. Preparation of the colloidal suspension 100 may be carried out under ambient conditions (e.g., room temperature (20-25°C) and/or atmospheric pressure). As described above, in some examples, the aqueous solution 102 may further comprise a thermal initiator 114, a dispersant, and/or a viscosifier. The colloidal suspension 100, and each of the optional and required components of the suspension 100,102, may have any or all of the characteristics described above.

[0027] The colloidal suspension 100 may be further processed using microwave irradiation into a polymer composite 110 and, if desired, a densified or sintered inorganic (e.g., ceramic) body 120. More specifically, the colloidal suspension 100 as described in any example or embodiment in this disclosure may be formed into a predetermined shape and then exposed to microwave irradiation; due to the irradiation, the crosslinkable polymer 106 in the colloidal suspension 100 undergoes crosslinking or curing to form a polymer composite 110 comprising a crosslinked polymer matrix 112 with the inorganic particles 108 dispersed therein. The polymer composite 110, which may be crack-free and mechanically robust (e.g., structurally rigid) has the predetermined shape. As explained below, during the exposure to microwave irradiation, heat is in-situ generated within the colloidal suspension 100; thus, the exposure to microwave irradiation may alternatively be referred to as microwave-assisted (-activated) heating or microwave-assisted (-activated) thermal curing. Generally speaking, the microwave radiation may have a frequency ranging from about 1 GHz to 170 GHz, and in particular the frequency of operation of conventional microwave ovens (2.45 GHz) may be employed. The irradiation typically occurs at a power level in a range from about 100 W to about 1000 W, or higher, e.g., up to about 2000 W. Lower power levels (e.g., from about 100 W to about 400 W) may be advantageous to reduce the likelihood of cracking by slowing the polymerization process.

[0028] The mechanism for microwave-activated thermal curing is generally understood to be the absorption of microwaves by polar molecules (e.g., water molecules) or ionic particles within the aqueous solution 102; upon irradiation, these molecules are subjected to continuous reorientation cycles, and heat is generated in-situdue to rotation and collision of the molecules. Accordingly, the presence of water 104 in the colloidal suspension 100 may be critical for enabling microwave-activated curing. Compared to non-thermal polymerization techniques, such as ultraviolet radiation (UV) based methods, microwave irradiation may allow for curing of the colloidal suspension 100 at greater penetration depths. In addition, curing may occur rapidly, and cracking of the polymer matrix 112 may be avoided. As indicated above, the crosslinked polymer matrix 112 may be substantially or completely devoid of cracks. In addition, the polymer composite 110 may have sufficient structural integrity to be handled for further processing and/or for use as a composite part.

[0029] The thermal curing may occur in a few minutes or less (e.g., about 2 minutes or less), and as quickly as a few seconds (e.g., about 10 seconds or more). The colloidal suspension may be continuously or discontinuously exposed to the microwaves for a short time duration, e.g., for a time duration of 180 s or less, 90 s or less, or about 60 s or less, to achieve thermal curing or crosslinking. In other words, the microwave radiation may continuously impinge on the colloidal suspension, or the microwave radiation may be pulsed. Pulsed microwave irradiation may comprise cycles of radiation pulses, where the microwave radiation is on, separated by inoperative intervals, where the microwave radiation is off. Preferably, during the continuous or discontinuous exposure to microwave radiation, the colloidal suspension is heated to a temperature or range of temperatures less than 100°C to prevent the liquid water in the suspension from transforming (explosively) to steam. The pulsed irradiation approach may allow for interruption of the heat transfer during inoperative intervals while the colloidal suspension remains above room temperature after the previous pulse or pulses. Thus, during the off intervals, locally high temperatures may tend to homogenize within the suspension and, importantly, still contribute to the curing of the polymer matrix. In this way, the extra curing time supplied by these intervals can contribute to the full curing of the polymer matrix before temperatures reach 100°C, and/or prevent microbursts of steam due to a very strengthened polymer matrix if 100°C were unintentionally reached. A suitable pulse pattern may comprise, for example, 10-50 s of irradiation (“on”) followed by 10-50 s of no irradiation (“off”), for a total pulsing cycle of 20 s to 100 s.

[0030] Forming the colloidal suspension into the predetermined shape prior to microwave irradiation may comprise delivering the colloidal suspension into a mold, where a cavity of the mold that contains the colloidal suspension comprises an inverse of the predetermined shape. Prior to crosslinking, the colloidal suspension may be flowable and thus able to fill the cavity and adopt the predetermined shape. The colloidal suspension may be exposed to the microwave irradiation in the mold and then removed after thermal curing, which may result in a mechanically robust polymer composite.

[0031] Alternatively, forming the colloidal suspension into the predetermined shape may comprise an extrusion-based 3D printing process, as illustrated in FIG. 2A. In such an example, the colloidal suspension may be configured to be viscoelastic with a strain rate dependent viscosity that allows extrusion through a nozzle and formation of an extruded filament that substantially maintains its shape upon deposition. More specifically, the colloidal suspension may be configured to be shear thinning, a characteristic that provides low viscosity at high shear rates (e.g., while passing through the nozzle) and high viscosity at low shear rates (e.g., when deposited on a substrate). The 3D printing process may include extruding the colloidal suspension through a nozzle while the nozzle moves relative to a substrate, and depositing one or more extruded filaments comprising the colloidal suspension onto or within the substrate, which may be a solid substrate or a non-solid support material. The one or more extruded filaments may be deposited along a 2D or 3D print path determined by the motion of the nozzle relative to the substrate, such that the colloidal suspension is formed into the predetermined shape, which may be any geometry that may be 3D printed. For example, the predetermined shape may comprise a lattice structure with voids between filaments, a solid structure formed layer by layer from the one or more extruded filaments, or another printable geometry. The motion of the nozzle relative to the substrate may be carried out by translation of the nozzle only, by translation of the substrate only, or by translation of both the nozzle and the substrate.

[0032] In examples where the substrate comprises a non-solid support material, during the extruding, the nozzle may be translated through the non-solid support material, and/or the non-solid support material may be forced around the nozzle, depending on whether the nozzle and/or the substrate is being moved. Such a process may be referred to as embedded 3D printing since the extruded filament(s) are embedded into the non-solid support material during printing. A suitable non-solid support material may comprise a gel or viscous liquid (e.g., a silicone polymer such as polydimethylsiloxane (PDMS)) having a viscosity low enough for passage of the nozzle therethrough but high enough to fulfill its function of providing support for the extruded filament(s) upon deposition. The non-solid support material may exhibit self-healing in the wake of the translating nozzle, i.e., may have rheological properties that are adequate to prevent the formation of crevices in the matrix during the passage of the nozzle therethrough, thus minimizing possible disruptions of existing printed features. As shown in FIG. 2B, the colloidal suspension/extruded filament(s) may be exposed to the microwave irradiation to undergo crosslinking while supported by the non-solid support material, which preferably does not increase in temperature during the irradiation. Upon exposure to microwave radiation, the polymer composite including the crosslinked polymer matrix with inorganic particles dispersed therein may be formed in the predetermined shape. After curing, the polymer composite may be removed from the non-solid support material.

[0033] As indicated above, the polymer composite may be further processed to form a densified inorganic body, such as a sintered ceramic body. After removal of the polymer composite from the mold or substrate, as described above, the polymer composite may be heated to a temperature or range of temperatures sufficient to pyrolyze the crosslinked polymer matrix and sinter together the inorganic particles. The temperature or range of temperatures may include a first (lower) temperature sufficient to pyrolyze the polymer matrix and a second (higher) temperature sufficient to sinter together the inorganic particles. The term “pyrolyze” may be used alternatively with “burn-out” or “calcine” in some examples, and may refer to a thermal decomposition process conducted in air or in a controlled atmosphere (e.g., inert gas or vacuum) for the purpose of decomposing and/or removing the crosslinked polymer matrix. It may be beneficial in some examples to dry the polymer composite before pyrolysis and sintering to remove water that may remain after microwave-activated curing. The drying may comprise, for example, air exposure for a suitable time duration (e.g., several hours, such as up to 24 h). Sintering of the inorganic particles and concomitant densification into an inorganic body may take place at a temperature or range of temperatures below the melting point of the inorganic particles. As with pyrolysis, sintering may be carried out in air or in a controlled environment, such as an inert gas atmosphere or vacuum. After sintering, the inorganic body may exhibit a relative density in a range from about 30% to 100%, from about 70% to about 100%, from about 80% to about 100%, from about 90% to about 100%, or, preferably, from about 95% to about 100%. Typically, the inorganic particles and the inorganic body comprise a ceramic, although, as indicated above, the inorganic particles and consequently the densified inorganic body may more broadly include a ceramic, a metal, an alloy, a composite, a semiconductor, and/or carbon.

[0034] EXAMPLES

[0035] The speed of microwave-activated thermal curing may be important for producing defect-free polymer composites from colloidal suspensions prepared as described above. To evaluate the kinetics of microwave-activated thermal curing, polymer composites are prepared by embedded 3D printing followed by exposure to continuous and pulsed microwave irradiation under various experimental conditions, and then thermal 2D maps are obtained immediately after irradiation. Referring again to FIGs. 2A and 2B, single-layer square lattices are embedded just under the surface of a PDMS support material and subjected to microwave irradiation. Following irradiation, an infrared camera is employed to generate the thermal 2D maps from the support material surface, as shown schematically in FIG. 2C. The polymer composites include yttria-stabilized (YSZ) particles in a crosslinked PEGDA matrix.

[0036] The thermal 2D maps reveal that the temperatures are dependent upon several microwave irradiation parameters (time, power, pulsing), compositional parameters (water content in the colloidal suspensions), and geometrical lattice parameters (packing fraction, extruded filament diameter). Consequently, a systematic study is carried out to facilitate individual evaluation of each of these parameters by varying one parameter at a time while keeping all others constant. In one test, the standard experimental conditions are defined as follows: irradiation time, 60 s; irradiation mode, continuous; irradiation power, 600 W; water to polymer volume ratio, 30:70; packing fraction, 0.32; filament diameter, 1 .4 mm.

[0037] Firstly, the effects of irradiation time are assessed. When heating three identical lattices following the standard experimental conditions during 30 s, 60 s and 90 s, respectively, the temperature maps shown in FIG. 3 primarily reveal that: /) the in-situ transfer of microwave energy occurs solely within the extruded filaments, due to the presence of water in the colloidal suspension, and //) temperature increases with increasing irradiation time, as expected. The thermally insulating nature of the PDMS support material prevents a fast redistribution of heat within the support material-lattice system, as put on view primarily in the thermal map corresponding to the longer irradiation test (90 s) where the higher temperatures can visibly be distinguished in and around the printed filaments. Therefore, most of the microwave energy transferred to the 3D printed polymer composite is contributing to the curing of the polymer matrix. ^ [0038] The implementation of pulsed microwave irradiation, where, as described above, microwave-activated heating is performed in cycles each consisting of a radiation pulse (in which irradiation is on) followed by an inoperative interval (in which irradiation is off), is tested in an experiment using 30 s pulses within each 60 s cycle. Three identical lattices prepared by embedded 3D printing are heated in pulsed mode under the standard conditions for one, two and three cycles (30 s, 60 s and 90 s, respectively). The temperature maps of FIG.3 (first row) reveal temperature differences in the extruded filaments between continuous versus pulsed irradiation. In particular, the thermal maps identify regions locally reaching ~100^oC after 90 s of continuous irradiation, and show that temperatures close to steam formation are substantially diminished in the pulsed mode for the same irradiation time. Moreover, dissipation of heat during “off” intervals out of the matrix/ceramic system is negligible, as average surface temperatures are similar with both protocols regardless of the heating mode, as shown in FIG.4. [0039] When delivering microwave irradiation onto identical specimens following the standard testing conditions during the same heating times, it can be expected that the energy transferred to the specimens – and, consequently, their increases in temperature – should be directly proportional to irradiation power. This trend is verified within the error range, as shown in FIG.4. It should be pointed out, however, that increases in irradiation time led to slightly higher temperatures than the equivalent increases in irradiation power; the inventors hypothesize that if the energy delivered by the microwave device is only partially transferred to the specimens, it is more energy-efficient to induce any extra heating by increasing irradiation time as opposed to irradiation power. [0040] The presence of water in the colloidal suspensions is critically important as water may influence the dispersing ability of particles in the suspension, affect the drying of cured polymer composites, and, most significantly, enable the microwave-activated thermal curing, as heat is in-situ generated due to rotation and collision of water molecules upon absorption of microwaves. Colloidal suspensions having different water to polymer volume ratios (10:90, 20:80, 30:70) are printed and irradiated under standard conditions. Increasing amounts of water in the colloidal suspensions leads to increasing average temperatures, as expected, as indicated in FIG. 4. Within the tested range, variations in the water to polymer volume ratio prompts a remarkably weaker temperature dependence than the irradiation variables, time and power.

[0041] Lastly, the main geometrical parameters (packing fraction, extruded filament diameter) of the printed 2D-lattices are also evaluated, revealing that heat transfer may be enhanced in lattices or other printed structures having increasing density, which (in these examples) may be achieved with either smaller unit cells or thicker extruded filaments.

[0042] When using suitable colloidal suspensions and microwave irradiation parameters, which can be determined through experiments such as the infrared imaging test proposed in FIG. 2C, the issues that are typically reported to emerge during conventional oven curing of polymer networks (matrices) containing inorganic particles - such as surface spallation or cracking - tend to disappear. Scanning electron microscope (SEM) images of 3D printed and microwave-cured polymer composites (YSZ particles in a PEGDA matrix) exposed to microwave radiation for 2 min (FIGs. 5C and 5D) in comparison with SEM images of polymer composites of the same composition cured in a conventional oven at 60°C for 10 hours (FIGs. 5A and 5B) reveal the favorable effects of rapid microwave curing, in particular, the elimination of cracking phenomena.

[0043] An accurate understanding of the parameters associated to the microwave-activated thermal curing of particle-containing polymer suspensions is vital to form defect-free polymer-matrix composites. Microwave-activated thermal curing can eliminate the formation of surface cracks, which can typically be created when using conventional heating to cure, as discussed above. Among the parameters involved in the kinetics of the curing process, microwave power and the water:polymer ratio in the suspension can be adjusted to control the structural consolidation (curing) of the suspensions. The geometric parameters (such as packing fraction or filament diameter in printed parts) are inherent to each structure, the heating mode is chosen to be pulsed due to a better homogenization of temperatures in the cured part, and the irradiation time is selected to be the minimum time until reaching full curing. The effects of microwave power and water:polymer ratio on the curing of embedded zirconia-containing PEGDA structures are investigated; specifically, four octet structures were 3D-printed from two colloidal suspensions containing different water:polymer volume ratios (10:90 and 30:70) and cured with two different microwave powers (360 and 600 W) each. Micro-CT scans of individual struts in the lattice reveal the presence of striations in the cross-sections of cured structures. The striations are seemingly more abundant for colloidal suspensions with water:polymer ratio of 10:90, as can be seen in FIG. 6. A shell structure (~40 pm thick) is visible around the circumference of the strut, regardless of the water:polymer volume ratio or microwave power.

[0044] These cured structures are then subjected to polymer burnout and sintering, then scanned once more. Any cracks present in the cured samples should be exacerbated following these steps. The presence of internal catastrophic cracks following sintering is confirmed for structures printed with colloidal suspensions having a 10:90 water:polymer ratio throughout the entire core of the strut except for the outer shell. These cracks are not visible using optical microscopy. It is theorized that the cracking phenomena for colloidal suspensions with low water content are due to internal stresses developed during the formation of a stiff network of covalent polymer bonds while hot water vapor diffuses out of the strut. Alternatively, a more elastic polymer network from colloidal suspensions containing a higher water content (30:70 ratios) results in an astonishing reduction of both the cracking and the apparent shell. Crack-free structures can be attained with a slower, more uniform polymerization by reducing microwave irradiation power to 360 W.

[0045] To clarify the use of and to hereby provide notice to the public, the phrases "at least one of <A>, <B>, ... and <N>" or "at least one of <A>, <B>, ... or <N>" or "at least one of <A>, <B>, ... <N>, or combinations thereof" or "<A>, <B>, ... and/or <N>" are defined by the Applicant in the broadest sense, superseding any other implied definitions hereinbefore or hereinafter unless expressly asserted by the Applicant to the contrary, to mean one or more elements selected from the group comprising A, B, ... and N. In other words, the phrases mean any combination of one or more of the elements A, B, ... or N including any one element alone or the one element in combination with one or more of the other elements which may also include, in combination, additional elements not listed. Unless otherwise indicated or the context suggests otherwise, as used herein, "a" or "an" means "at least one" or one or more. [0046] While various embodiments have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible. Accordingly, the embodiments described herein are examples, not the only possible embodiments and implementations.

[0047] The subject-matter of the disclosure may also relate, among others, to the following aspects:

[0048] A first aspect relates to a colloidal suspension comprising: an aqueous solution comprising: water; a crosslinkable polymer dissolved in the water; and inorganic particles dispersed in the aqueous solution at a concentration of at least about 10 vol.%.

[0049] A second aspect relates to the colloidal suspension of the first aspect, wherein the concentration of the inorganic particles is at least about 20 vol.% or at least about 30 vol.%, and/or as high as about 40 vol.%, as high as about 50 vol.%, or as high as about 60 vol.%.

[0050] A third aspect relates to the colloidal suspension of any preceding aspect, wherein the inorganic particles are electrically conductive, thermally conductive, semiconducting, dielectric, ferromagnetic, optically transparent, catalytically active, optically active, light-emitting, and/or abrasion-resistant.

[0051] A fourth aspect relates to the colloidal suspension of any preceding aspect, wherein the inorganic particles comprise a ceramic, a metal, an alloy, a composite, a semiconductor, and/or carbon.

[0052] A fifth aspect relates to the colloidal suspension of any preceding aspect, wherein the inorganic particles comprise one or more oxides selected from the group consisting of: aluminum oxide, calcium oxide, chromium oxide, iron oxide, magnesium oxide, nickel oxide, titanium oxide, yttrium oxide, zinc oxide, and zirconium oxide.

[0053] A sixth aspect relates to the colloidal suspension of any preceding aspect, wherein the inorganic particles have a nominal particle size in a range from about 10 nm to about 50,000 nm, from about 10 nm to about 1 ,000 nm, or from about 100 nm to about 500 nm

[0054] A seventh aspect relates to the colloidal suspension of any preceding aspect, wherein the crosslinkable polymer comprises a polyacrylate, polyethylene glycol, polyacrylamide or copolymer, polyvinyl alcohol, polyacrylic acid or copolymer, polyamine, polyethyleneimine, quaternary ammonium compound, polyvinylpyrrolidone or copolymer, and/or polyvinyl methyl ether/maleic anhydride. [0055] An eighth aspect relates to the colloidal suspension of any preceding aspect, wherein the aqueous solution further comprises a thermal initiator, and/or wherein the thermal initiator is a water-soluble thermal initiator.

[0056] A ninth aspect relates to the colloidal suspension of the eighth aspect, wherein the water-soluble thermal initiator comprises an azo polymerization initiator. [0057] A tenth aspect relates to the colloidal suspension of the ninth aspect, wherein the azo polymerization initiator is selected from the group consisting of: 2,2'- azobis(isobutyronitrile), 2,2'-azobis(2,4-dimethyl valeronitrile), 2,2'-azobis(2-methyl butane nitrile), and 4,4'-azobis(4-cyanovaleric acid).

[0058] An eleventh aspect relates to the colloidal suspension of any preceding aspect, wherein the aqueous solution comprises a concentration of water sufficient to facilitate dispersion of the inorganic particles.

[0059] A twelfth aspect relates to the colloidal suspension of the eleventh aspect, wherein the concentration of water in the aqueous solution is at least about 20 vol.%. [0060] A thirteenth aspect relates to the colloidal suspension of any preceding aspect, wherein a volume ratio of water to crosslinkable polymer in the aqueous solution is in a range from about 60:40 to about 10:90, from about 30:70 to about 10:90, from about 30:70 to about 20:80, and/or from about 30:70 to about 25:75.

[0061] A fourteenth aspect relates to the colloidal suspension of any preceding aspect, wherein the aqueous solution further comprises a dispersant configured to modify surface chemistry of the inorganic particles.

[0062] A fifteenth aspect relates to the colloidal suspension of the thirteenth or fourteenth aspect, wherein the dispersant is selected from the group consisting of: sodium pyrophosphate, ammonium citrate, sodium citrate, sodium tartrate, sodium succinate, glyceryl trioleate, phosphate ester, poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), ammonium polyacrylate, sodium polyacrylate, sodium polysulfonate, polyethylene imine), menhaden fish oil, random copolymers, and comb polymers.

[0063] A sixteenth aspect relates to the colloidal suspension of any preceding aspect, wherein the aqueous solution further comprises a viscosifier to control a viscosity of the aqueous solution. [0064] A seventeenth aspect relates to a method of forming a colloidal suspension, the method comprising: forming an aqueous solution by mixing together: water; a crosslinkable polymer; and inorganic particles at a concentration of at least about 10 vol.%, wherein, during the mixing, the inorganic particles are dispersed in the aqueous solution.

[0065] An eighteenth aspect relates to the method of the preceding aspect, wherein the mixing comprises stirring, shaking, and/or ultrasonication of the aqueous solution.

[0066] A nineteenth aspect relates to the method of any preceding aspect, wherein the aqueous solution further comprises a thermal initiator, a dispersant, and/or a viscosifier.

[0067] A twentieth aspect relates to the method of any preceding aspect, wherein the colloidal suspension includes any or all features recited in any preceding aspect. [0068] A twenty-first aspect relates to a method of microwave-activated or microwave-assisted thermal curing, the method comprising: forming the colloidal suspension of any preceding aspect into a predetermined shape; and after the forming, exposing the colloidal suspension to microwave irradiation for about two minutes or less, whereby the crosslinkable polymer undergoes thermal curing to form a polymer composite having the predetermined shape, the polymer composite comprising a crosslinked polymer matrix with the inorganic particles dispersed therein.

[0069] A twenty-second aspect relates to the method of the preceding aspect, wherein the crosslinked polymer matrix is substantially devoid of cracks and/or is mechanically robust.

[0070] A twenty-third aspect relates to the method of the twenty-first or twenty- second aspect, wherein the microwave irradiation has a frequency in a range from about 1 GHz to about 170 GHz.

[0071] A twenty-fourth aspect relates to the method of any of the twenty-first through the twenty-third aspects, wherein the microwave irradiation has a frequency of 2.45 GHz.

[0072] A twenty-fifth aspect relates to the method of any one of the twenty-first through the twenty-fourth aspects, wherein the exposure occurs at an irradiation power level in a range from about 100 W to about 2000 W, from about 100 W to about 1000 W, and/or from about 100 W to about 400 W.

[0073] A twenty-sixth aspect relates to the method of any one of the twenty-first through the twenty-fifty aspects, wherein the thermal curing occurs within about about 90 s or less, or about 60 s or less, and/or as rapidly as about 10 s or more. [0074] A twenty-seventh aspect relates to the method of any one of the twenty- first through the twenty-sixth aspects, wherein the exposure to the microwave irradiation occurs continuously.

[0075] A twenty-eighth aspect relates to the method of any one of the twenty-first through the twenty-seventh aspects, wherein the exposure to the microwave irradiation occurs discontinuously, the microwave radiation being pulsed.

[0076] A twenty-ninth aspect relates to the method of any one of the twenty-first through the twenty-eighth aspects, wherein during the exposure to microwave irradiation, the colloidal suspension is heated to a temperature or range of temperatures less than 100°C.

[0077] A thirtieth aspect relates to the method of any one of the twenty-first through the twenty-ninth aspects, wherein forming the colloidal suspension into the predetermined shape comprises delivering the colloidal suspension into a mold. [0078] A thirty-first aspect relates to the method of the thirtieth aspect, wherein the colloidal suspension is exposed to the microwave radiation in the mold and then removed from the mold after thermal curing.

[0079] A thirty-second aspect relates to the method of any one of the twenty-first through the thirty-first aspects, wherein forming the colloidal suspension into the predetermined shape comprises extrusion-based 3D printing.

[0080] A thirty-third aspect relates to the method of the thirty-second aspect, wherein the extrusion-based 3D printing comprises: extruding the colloidal suspension through a nozzle while the nozzle moves relative to a substrate; and depositing one or more extruded filaments comprising the colloidal suspension onto or within the substrate, the substrate being a solid substrate or a non-solid support material, the one or more extruded filaments being deposited along a 2D or 3D print path determined by the motion of the nozzle relative to the substrate, the colloidal suspension thereby being formed into the predetermined shape. [0081] A thirty-fourth aspect relates to the method of the thirty-third aspect, wherein the substrate comprises the non-solid support material, and wherein, during the extruding, the nozzle is translated through the non-solid support material and/or the non-solid support material is forced around the nozzle.

[0082] A thirty-fifth aspect relates to the method of the thirty-third or thirty-fourth aspect, wherein the non-solid support material comprises a gel or viscous liquid, such as a silicone polymer.

[0083] A thirty-sixth aspect relates to the method of any of the thirty-third through the thirty-fifth aspects, wherein the colloidal suspension is exposed to microwave irradiation while supported by the non-solid support material, and wherein, during the exposure to microwave irradiation, the non-solid support material does not substantially increase in temperature.

[0084] A thirty-seventh aspect relates to the method of any one of the thirty-third through the thirty-sixth aspects, wherein, after the exposure to microwave irradiation, the polymer composite is removed from the non-solid support material.

[0085] A thirty-eighth aspect relates to the method of any of the thirty-third through the thirty-seventh aspects, wherein the polymer composite is further processed to form a densified or sintered inorganic body.

[0086] A thirty-ninth aspect relates to the method of the thirty-eighth aspect, wherein the further processing to form a densified or sintered inorganic body includes: heating the polymer composite to a temperature or range of temperatures sufficient to pyrolyze the crosslinked polymer matrix and sinter together the inorganic particles.

[0087] A fortieth aspect relates to the method of the thirty-ninth aspect, further comprising, prior to the heating, drying the polymer composite.

[0088] A forty-first aspect relates to the method of the thirty-ninth or fortieth aspect, wherein the temperature or range of temperatures includes a first temperature sufficient to pyrolyze the polymer matrix and a second temperature higher than the first temperature sufficient to sinter together the inorganic particles. [0089] A forty-second aspect relates to the method any one of the thirty-ninth through the forty-first aspects, wherein, after sintering, the densified or sintered inorganic body exhibits a relative density in a range from about 80% to 100%, from about 90% to 100%, or, preferably, from 95% to about 100%. [0090] A forty-third aspect relates to the method of any one of the thirty-eighth through forty-second aspects, wherein the inorganic particles and consequently the densified or sintered inorganic body comprise a ceramic, a metal, an alloy, a composite, a semiconductor, and/or carbon.

[0091] In addition to the features mentioned in each of the independent aspects enumerated above, some examples may show, alone or in combination, the optional features mentioned in the dependent aspects and/or as disclosed in the description above and shown in the figures.

Claims

1 . A colloidal suspension comprising: an aqueous solution comprising: water; a crosslinkable polymer dissolved in the water; and inorganic particles dispersed in the aqueous solution at a concentration of at least about 10 vol.%.

2. The colloidal suspension of claim 1 , wherein the concentration of the inorganic particles is at least about 20 vol.%, and/or as high as about 60 vol.%.

3. The colloidal suspension of claim 1 , wherein the inorganic particles are electrically conductive, thermally conductive, semiconducting, dielectric, ferromagnetic, optically transparent, catalytically active, optically active, lightemitting, and/or abrasion-resistant.

4. The colloidal suspension of claim 1 , wherein the inorganic particles comprise a ceramic, a metal, an alloy, a composite, a semiconductor, and/or carbon.

5. The colloidal suspension of claim 1 , wherein the inorganic particles comprise one or more oxides selected from the group consisting of: aluminum oxide, calcium oxide, chromium oxide, iron oxide, magnesium oxide, nickel oxide, titanium oxide, yttrium oxide, zinc oxide, and zirconium oxide.

6. The colloidal suspension of claim 1 , wherein the inorganic particles have a nominal particle size in a range from about 10 nm to about 50,000 nm.

7. The colloidal suspension of claim 1 , wherein the crosslinkable polymer comprises a polyacrylate, polyethylene glycol, polyacrylamide or copolymer, polyvinyl alcohol, polyacrylic acid or copolymer, polyamine, polyethyleneimine, quaternary ammonium compound, polyvinylpyrrolidone or copolymer, and/or polyvinyl methyl ether/maleic anhydride.

8. The colloidal suspension of claim 1 , wherein the aqueous solution further comprises a thermal initiator.

9. The colloidal suspension of claim 8, wherein the thermal initiator is a water-soluble thermal initiator comprising an azo polymerization initiator.

10. The colloidal suspension of claim 9, wherein the azo polymerization initiator is selected from the group consisting of: 2,2'-azobis(isobutyronitrile), 2,2'- azobis(2,4-dimethyl valeronitrile), 2,2'-azobis(2-methyl butane nitrile), and 4,4'- azobis(4-cyanovaleric acid).

11 . The colloidal suspension of claim 1 , wherein the aqueous solution comprises a concentration of water sufficient to facilitate dispersion of the inorganic particles.

12. The colloidal suspension of claim 11 , wherein the concentration of water in the aqueous solution is at least about 20 vol.%.

13. The colloidal suspension of claim 1 , wherein a volume ratio of water to crosslinkable polymer in the aqueous solution is in a range from about 60:40 to about 10:90.

14. The colloidal suspension of claim 1 , wherein the aqueous solution further comprises a dispersant configured to modify surface chemistry of the inorganic particles.

15. The colloidal suspension of claim 14, wherein the dispersant is selected from the group consisting of: sodium pyrophosphate, ammonium citrate, sodium citrate, sodium tartrate, sodium succinate, glyceryl trioleate, phosphate ester, poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), ammonium polyacrylate, sodium polyacrylate, sodium polysulfonate, polyethylene imine), menhaden fish oil, random copolymers, and comb polymers.

16. The colloidal suspension of claim 1 , wherein the aqueous solution further comprises a viscosifier to control a viscosity of the aqueous solution.

17. A method of forming a colloidal suspension, the method comprising: forming an aqueous solution by mixing together: water; a crosslinkable polymer; and inorganic particles at a concentration of at least about 10 vol.%, wherein, during the mixing, the inorganic particles are dispersed in the aqueous solution.

18. The method of claim 17, wherein the mixing comprises stirring, shaking, and/or ultrasonication of the aqueous solution.

19. The method of claim 17, wherein the aqueous solution further comprises a thermal initiator, a dispersant, and/or a viscosifier.

20. The method of claim 17, wherein a volume ratio of water to crosslinkable polymer in the aqueous solution is in a range from about 60:40 to about 10:90.

21 . A method of microwave-activated thermal curing, the method comprising: forming the colloidal suspension of claim 1 into a predetermined shape; and after the forming, exposing the colloidal suspension to microwave irradiation for about two minutes or less, whereby the crosslinkable polymer undergoes thermal curing to form a polymer composite having the predetermined shape, the polymer composite comprising a crosslinked polymer matrix with the inorganic particles dispersed therein.

22. The method of claim 21 , wherein the crosslinked polymer matrix is substantially devoid of cracks and/or is mechanically robust.

23. The method of claim 21 , wherein the microwave irradiation has a frequency in a range from about 1 GHz to about 170 GHz.

24. The method of claim 21 , wherein the microwave irradiation has a frequency of 2.45 GHz.

25. The method of claim 21 , wherein the exposure occurs at an irradiation power level in a range from about 100 W to about 2000 W.

26. The method of claim 21 , wherein the thermal curing occurs within about about 180 s or less.

27. The method of claim 21 , wherein the exposure to the microwave irradiation occurs continuously.

28. The method of claim 21 , wherein the exposure to the microwave irradiation occurs discontinuously, the microwave radiation being pulsed.

29. The method of claim 21 , wherein during the exposure to microwave irradiation, the colloidal suspension is heated to a temperature or range of temperatures less than 100°C.

30. The method of claim 21 , wherein forming the colloidal suspension into the predetermined shape comprises delivering the colloidal suspension into a mold.

31 . The method of claim 30, wherein the colloidal suspension is exposed to the microwave radiation in the mold and then removed from the mold after thermal curing.

32. The method of claim 21 , wherein forming the colloidal suspension into the predetermined shape comprises extrusion-based 3D printing.

33. The method of claim 32, wherein the extrusion-based 3D printing comprises: extruding the colloidal suspension through a nozzle while the nozzle moves relative to a substrate; and depositing one or more extruded filaments comprising the colloidal suspension onto or within the substrate, the substrate being a solid substrate or a non-solid support material, the one or more extruded filaments being deposited along a 2D or 3D print path determined by the motion of the nozzle relative to the substrate, the colloidal suspension thereby being formed into the predetermined shape.

34. The method of claim 33, wherein the substrate comprises the nonsolid support material, and wherein, during the extruding, the nozzle is translated through the non-solid support material and/or the non-solid support material is forced around the nozzle.

35. The method of claim 33, wherein the non-solid support material comprises a gel or viscous liquid, such as a silicone polymer.

36. The method of claim 33, wherein the colloidal suspension is exposed to microwave irradiation while supported by the non-solid support material, and wherein, during the exposure to microwave irradiation, the non-solid support material does not substantially increase in temperature.

37. The method of claim 33, wherein, after the exposure to microwave irradiation, the polymer composite is removed from the non-solid support material.

38. The method of claim 33, wherein the polymer composite is further processed to form a densified or sintered inorganic body.

39. The method of claim 38, wherein the further processing to form a densified or sintered inorganic body includes: heating the polymer composite to a temperature or range of temperatures sufficient to pyrolyze the crosslinked polymer matrix and sinter together the inorganic particles.

40. The method of claim 39, further comprising, prior to the heating, drying the polymer composite.

41 . The method of claim 39, wherein the temperature or range of temperatures includes a first temperature sufficient to pyrolyze the polymer matrix and a second temperature higher than the first temperature sufficient to sinter together the inorganic particles.

42. The method of claim 39, wherein, after sintering, the densified or sintered inorganic body exhibits a relative density in a range from about 30% to 100%.

43. The method of claim 38, wherein the inorganic particles and the densified or sintered inorganic body comprise a ceramic, a metal, an alloy, a composite, a semiconductor, and/or carbon.