This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM release LLNL-JRNL-750634.

CAUTION: Please consult with the safety data sheets (SDS) of all materials, and wear proper protective equipment (PPE) when handling materials before casting and curing. Resorcinol and polyethylene imine are known to be toxic. Formaldehyde is both toxic and carcinogenic20. Preparation of ceramic suspensions should be done in chemical fume hoods or other properly ventilated work environments.
1. Negative Additive Manufacturing
<ol>
	<li>Preparation of a 120 mL two-part suspension 
	NOTE: A two-part suspension will be prepared to help prolong the shelf life of the suspensions before casting. One suspension (R-mix) will contain the resorcinol component, and the other (F-mix) will contain the formaldehyde component. Both suspensions will be mixed together to form a final suspension that will initiate the gelation process.
	<ol>
		<li>To create the R-mix, begin by dissolving 0.88 g of polyethylene imine (PEI) in 25.00 g of water using a planetary mixer.</li>
		<li>To create a separate F-mix, dissolve 0.88 g of polyethylene imine (PEI) in 16.83 g of water using a planetary mixer. 
		NOTE: Using a planetary mixer at 2000 rpm for at least several minutes will provide sufficient shearing forces to help dissolve the viscous PEI, resorcinol, and formaldehyde, and to suspend the boron carbide particles. PEI serves as the dispersing agent for the B4C particles</li>
		<li>Dissolve 12.60 g of resorcinol powder into the R-mix. The solution should turn from a cloudy-white to a clear transparent solution after complete dissolution of the powder from mixing.</li>
		<li>Add 17.03 g of formaldehyde solution to the F-mix and ensure complete mixing.</li>
		<li>Incrementally add 5.25 g (12 increments until reaching 63.00 g) of boron carbide powder (1500F) into both the R-mix and F-mix separately.</li>
		<li>Add 6.50 g of acetic acid to the R-mix and F-mix and ensure complete mixing in each. 
		NOTE: At this point, the two-part suspensions will have 42 vol% of B4C and are ready to be combined for casting or stored for future use (if adequately sealed). Beware that if the suspensions sit for ~1 h or more, particle settling will occur. Ensure that the particles are resuspended by applying thorough agitation before using the suspensions. Also, three different commercial batches of boron carbides, 1250F, 1500F, and 3000F (named according to their approximate sieved mesh sizes), were originally tested. Each batch has a different particle size distribution, and the 1500F B4C batch was found to achieve the highest sintering density, as reported in Lu et al.8. Acetic acid can also be added before the B4C solids loading step as well, but adding at the end offers better ease of handling by limiting acetic acid odors.</li>
	</ol>
	</li>
	<li>Preparation of the 3D printed molds for casting
	<ol>
		<li>Prepare the mold design in a computer-aided design (CAD) software program.</li>
		<li>Print the molds using a Fused Deposition Modeling (FDM) 3D printer with acrylonitrile butadiene styrene (ABS) filaments. 
		NOTE: Acetone vapors can be used to smooth out the mold texture if desired21. The suggested nozzle and bed temperatures are 240 &#176;C and 110 &#176;C, respectively. Parameters such as layer thickness (0.2 mm), extrusion speed, and cooling rate are chosen to optimize the quality of the part with minimum deformations. This requires some trial and error with each unique printer system. A wall thickness of at least 1 mm is advised. The minimum feature size is 0.5 mm; however, it is suggested not to go below 1 mm. Molds from Lu et al.8 are available for download online in supporting material.</li>
	</ol>
	</li>
	<li>Combination of the two-part suspension to prepare for casting
	<ol>
		<li>Before combining, thoroughly agitate (by using a vortex or planetary mixer) the R-mix with the F-mix suspensions individually to ensure the B4C particles are well-suspended.</li>
		<li>Combine the R-mix and F-mix to obtain the final suspension. 
		NOTE: The pH of the combined suspension should be 2.8, which will provide about 30 minutes of working time to de-air and cast the final suspension before gelation starts occurring. The onset of gelation can be observed from the sharp increase in viscosity of the suspension.</li>
		<li>Before casting, mix and apply vacuum (20-200 torr or 2.7-27 kPa) to the final suspension mixture for about 10 minutes to remove air bubbles without boiling the water. This can be accomplished by using a stirring plate at 200-300 rpm with a vacuum jar.</li>
	</ol>
	</li>
	<li>Gelcasting
	<ol>
		<li>Immediately pour the de-aired suspension into the 3D-printed molds.</li>
		<li>Place the molds inside a sealed glass container to prevent moisture loss during the curing process.</li>
		<li>Place the sealed container with the molds into a 60-80 &#176;C oven to initiate the curing process.</li>
		<li>Allow the casts to cure for at least 8 hours for parts that are several centimeters in length scale or possibly longer for larger molds.</li>
	</ol>
	</li>
	<li>Dissolution of the molds to obtain green bodies
	<ol>
		<li>Remove the sealed container with the molds from the oven and allow it to cool to room temperature.</li>
		<li>Add enough acetone into the container until the mold is fully submersed. The amount will vary depending on the size and volume of the mold used (generally ~100 mL of acetone for a mold that is 50 cm3 in dimension). 
		NOTE: This process may take up to 2-4 days depending on the volume of plastic that needs to be dissolved away. Minimal agitation of the acetone bath or heating it slightly to 40 &#176;C may help speed up the process. Execute caution when heating acetone bath, as it is a flammable chemical and may become explosive when combined with air in certain composition ranges.</li>
		<li>Extract the free green body from the acetone bath after the ABS plastic is dissolved away. 
		NOTE: After the RF is cured, the mold can be dissolved away to obtain a solid green body shaped as a negative copy of the inner mold geometry. This green body should be strong enough to survive gentle and careful handling in the subsequent post-processing steps without breaking.</li>
		<li>Place the green bodies in an oven at 80 &#176;C to ensure complete drying and removal of all moisture. 
		NOTE: Drying time varies depending on the volume of the green body. Leaving the part to dry overnight (&#62; 8 hours) is sufficient for green body sizes less than 1000 cm3. There is no harm in over-drying.</li>
	</ol>
	</li>
</ol>
2. Carbonization
<ol>
	<li>After drying, place each green body in a 2-inch quartz tube lined with graphite foil and put them into a furnace with flowing gas [250 standard cubic centimeters of air (SCCM) consisting of 4 wt% H2(g) and 96 wt% Ar(g) to create a reducing atmosphere during the pyrolysis treatment].</li>
	<li>Heat the green bodies inside the furnace at 5 &#176;C/min until 1050 &#176;C and hold for 3 hours. 
	NOTE: The gel-cast green bodies will have 15 wt% of RF relative to the B4C and will provide about 7.5 wt% in carbon after the pyrolysis process. This process removes much of the resorcinol-formaldehyde residue and will severely contaminate the furnace if no trap is used.</li>
	<li>Ensure that the green bodies come out uniformly darker in color, indicating the presence of carbon from the pyrolysis treatment.</li>
</ol>
3. Sintering
NOTE: After sintering, the surface roughness of the samples will improve slightly compared to the surface roughness of the molds used. This is a consequence of the 57-58 vol% shrinkage of the samples from sintering.
<ol>
	<li>Place the carbonized parts in a graphite furnace with vacuum backfilled flowing helium gas (420 SCCM) for sintering. Apply 280 SCCM to the front and pyrometer windows and 140 SCCM directly into the sample chamber with an inlet pressure of ~170 kPa.</li>
	<li>Heat up the furnace to 2290 &#176;C (20 K/min to 2000 &#176;C then 3 K/min to 2290 &#176;C) and hold for 1 hour to achieve optimal densification of the parts. 
	NOTE: Archimedes density is a common and quick technique to measure the density of the sintered boron carbide parts. Archimedes density kits can be added onto analytical balance scales to measure the density of samples or manually determined22. Boron carbide with 7.5 wt% in carbon will have a theoretical maximum density (TMD) of 2.49 g/cm3. Parts sintered at 2290 &#176;C from this methodology will result in 2.43 &#177; 0.01 g/cm3 which is 97.6 &#177; 0.4% TMD.</li>
</ol>

<ol>
	<li>An, Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Prediction+of+superstrong+&#964;+-boron+carbide+phase+from+quantum+mechanics.">Prediction of superstrong &#964; -boron carbide phase from quantum mechanics.</a> Physical Review B. 95 (10), 100101 (2017).</li><li>Th&#233;venot, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Boron+carbide+-+A+comprehensive+review.">Boron carbide - A comprehensive review.</a> Journal of the European Ceramic Society. 6, 205-225 (1990).</li><li>Lee, H., Speyer, R. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pressureless+sintering+of+boron+carbide.">Pressureless sintering of boron carbide.</a> Journal of the American Ceramic Society. 86, 1468-1473 (2003).</li><li>Deng, J. X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Erosion+wear+of+boron+carbide+ceramic+nozzles+by+abrasive+air-jets.">Erosion wear of boron carbide ceramic nozzles by abrasive air-jets.</a> Materials Science and Engineering: A. 408, 227-233 (2005).</li><li>Schwetz, K. A., Grellner, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+influence+of+carbon+on+the+microstructure+and+mechanical+properties+of+sintered+boron-carbide.">The influence of carbon on the microstructure and mechanical properties of sintered boron-carbide.</a> Journal of Less-Common Metals. 82, 37-47 (1981).</li><li>Schwetz, K. A., Vogt, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Process+for+the+production+of+dense+sintered+shaped+articles+of+polycrystalline+boron+carbide+by+pressureless+sintering.">Process for the production of dense sintered shaped articles of polycrystalline boron carbide by pressureless sintering.</a> US. , (1980).</li><li>Suzuki, H., Hase, T., Maruyama, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+carbon+on+sintering+of+boron+carbide.">Effect of carbon on sintering of boron carbide.</a> Journal of the Ceramic Association, Japan. 87, 430-433 (1979).</li><li>Lu, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Complex+shaped+boron+carbides+from+negative+additive+manufacturing.">Complex shaped boron carbides from negative additive manufacturing.</a> Materials &amp; Design. 148, 8-16 (2018).</li><li>Omatete, O. O., Janney, M. A., Nunn, S. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gelcasting:+From+laboratory+development+toward+industrial+production.">Gelcasting: From laboratory development toward industrial production.</a> Journal of the European Ceramic Society. 17, 407-413 (1997).</li><li>Yang, J., Yu, J., Huang, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+developments+in+gelcasting+of+ceramics.">Recent developments in gelcasting of ceramics.</a> Journal of the European Ceramic Society. 31, 2569-2591 (2011).</li><li>Gilissen, R., Erauw, J. P., Smolders, A., Vanswijgenhoven, E., Luyten, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gelcasting+a+near+net+shape+technique.">Gelcasting a near net shape technique.</a> Materials &amp; Design. 21, 251-257 (2000).</li><li>Travitzky, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Additive+manufacturing+of+ceramic-based+materials.">Additive manufacturing of ceramic-based materials.</a> Advanced Engineering Materials. 16, 729-754 (2014).</li><li>Zocca, A., Colombo, P., Gomes, C. M., Gunster, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Additive+Manufacturing+of+Ceramics:+Issues,+Potentialities,+and+Opportunities.">Additive Manufacturing of Ceramics: Issues, Potentialities, and Opportunities.</a> Journal of the American Ceramic Society. 98, 1983-2001 (2015).</li><li>Deckers, J., Vleugels, J., Kruthl, J. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Additive+Manufacturing+of+Ceramics:+A+Review.">Additive Manufacturing of Ceramics: A Review.</a> Journal of Ceramic Science and Technology. 5, 245-260 (2014).</li><li>Costakis, W. J., Rueschhoff, L. M., Diaz-Cano, A. I., Youngblood, J. P., Trice, R. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Additive+manufacturing+of+boron+carbide+via+continuous+filament+direct+ink+writing+of+aqueous+ceramic+suspensions.">Additive manufacturing of boron carbide via continuous filament direct ink writing of aqueous ceramic suspensions.</a> Journal of the European Ceramic Society. 36, 3249-3256 (2016).</li><li>Colombo, P., Schmidt, J., Franchin, G., Zocca, A., Gunster, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Additive+manufacturing+techniques+for+fabricating+complex+ceramic+components+from+preceramic+polymers.">Additive manufacturing techniques for fabricating complex ceramic components from preceramic polymers.</a> American Ceramic Society Bulletin. 96, 16-23 (2017).</li><li>Olsson, A., Hellsing, M. S., Rennie, A. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=New+possibilities+using+additive+manufacturing+with+materials+that+are+difficult+to+process+and+with+complex+structures.">New possibilities using additive manufacturing with materials that are difficult to process and with complex structures.</a> Physica Scripta. 92, 053002 (2017).</li><li>Dhara, S., Kamboj, R. K., Pradhan, M., Bhargava, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shape+forming+of+ceramics+via+gelcasting+of+aqueous+particulate+slurries.">Shape forming of ceramics via gelcasting of aqueous particulate slurries.</a> Bulletin of Materials Science. 25, 565-568 (2002).</li><li>Lewicki, J. P., Fox, C. A., Worsley, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+synthesis+and+structure+of+resorcinol-formaldehyde+polymeric+networks+-+Precursors+to+3D-carbon+macroassemblies.">On the synthesis and structure of resorcinol-formaldehyde polymeric networks - Precursors to 3D-carbon macroassemblies.</a> Polymer. 69, 45-51 (2015).</li><li>Swenberg, J. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Formaldehyde+Carcinogenicity+Research:+30+Years+and+Counting+for+Mode+of+Action,+Epidemiology,+and+Cancer+Risk+Assessment.">Formaldehyde Carcinogenicity Research: 30 Years and Counting for Mode of Action, Epidemiology, and Cancer Risk Assessment.</a> Toxicologic Pathology. 41, 181-189 (2013).</li><li>Kuo, C. C., Chen, C. M., Chang, S. X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Polishing+mechanism+for+ABS+parts+fabricated+by+additive+manufacturing.">Polishing mechanism for ABS parts fabricated by additive manufacturing.</a> The International Journal of Advanced Manufacturing Technology. 91, 1473-1479 (2017).</li><li>Kires, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Archimedes'+principle+in+action.">Archimedes' principle in action.</a> Physics Education. 42, 484-487 (2007).</li><li>Zheng, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Design+and+optimization+of+a+light-emitting+diode+projection+micro-stereolithography+three-dimensional+manufacturing+system.">Design and optimization of a light-emitting diode projection micro-stereolithography three-dimensional manufacturing system.</a> Review of Scientific Instruments. 83, 125001 (2012).</li><li>Franchin, G., Colombo, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Porous+Geopolymer+Components+through+Inverse+Replica+of+3D+Printed+Sacrificial+Templates.">Porous Geopolymer Components through Inverse Replica of 3D Printed Sacrificial Templates.</a> Journal of Ceramic Science and Technology. 6, 105-111 (2015).</li></ol>

The authors have nothing to disclose.

The methodology of negative additive manufacturing described in the protocol allows complex shaped boron carbide parts to be produced at nearly full density after sintering at an optimal temperature of 2290 &#176;C. The first several steps related to preparation and casting are the most critical for generating a high-quality cast with minimal defects. If the viscosity of the suspension is too high, poor mixing will occur. The porosity of the sintered part is also affected since increased viscosity hinders air bubble remo...

Boron carbide (B4C), with a Vickers hardness of about 38 GPa, is known as the third hardest commercially available material, behind diamond (~115 GPa) and cubic boron nitride (~48 GPa). This particular property, along with a low density (2.52 g/cm3), makes it attractive for defense applications such as armors1. B4C also has a high melting point, superior wear resistance, and high neutron absorption cross section2,3,4. However, utilization of these favorable mechanical properties typically requires B4C to be sintered to a high density. Hot pressing is a conventional method for sintering B4C to full densification. This technique is often limited to simple geometries with limited curvature and fairly uniform thickness. Expensive and labor-intensive machining with polycrystalline diamond tooling or laser cutting is required to introduce finer or more complex features.
Alternatively, colloidal forming techniques with pressure-less sintering can produce near-full density parts that require minimal to no machining. Due to a lack of external pressure during consolidation, sintering aids are normally added to the ceramic medium to increase the effectiveness of pressureless sintering. Carbon is commonly used as a sintering aid for B4C5,6,7.Various carbon sources, such as nanoparticle powders or carbonized organics from pyrolysis, can be used. Homogeneous distribution of the carbon sintering aid along grain boundaries is an important factor for obtaining uniform sintering of B4C. Therefore, carbon concentration and B4C particle size are also important and interrelated factors for sintering parts to high density8.
One of the most promising colloidal forming techniques for obtaining complex shaped ceramic parts is gelcasting. This technique involves casting a ceramic suspension with an organic monomer into a mold which polymerizes in situ to act as a gel9,10,11. The gel serves as a binder to form a green body in the shape of the mold that is strong enough to be handled without breakage in subsequent processing steps. Previously impossible 3D mold geometries can now be produced through low-cost polymer-based additive manufacturing (AM) techniques such as stereolithography (SLA) and fused deposition modeling (FDM)12. The recent availability of 3D printers has opened new possibilities for designing ceramics with highly complex geometries.
Negative additive manufacturing is a technique that combines gelcasting with sacrificial 3D-printed molds. The complexity of the ceramic part is directly related to the complexity of the mold design. Mold designs can now be incredibly sophisticated with the advent of high resolution plastic 3D printers. For example, 3D scanning tools can be used to capture an individual&#39;s contours and be incorporated into molds. By using negative AM, lightweight ceramic armors tailored to the individual&#39;s body size and shape can be created. Such design customizations can provide lighter weight armors with enhanced mobility for users.
Other common ceramic AM techniques such as direct ink write (DIW), selective laser sintering (SLS), and binder jetting (BJ) are also effective in producing complex shaped ceramic parts. However, most of these techniques are only useful for producing fine porous structures and are not efficient when scaling up to large parts, such as armor applications13,14,15,16,17. Moreover, most of these techniques are not feasible for high volume production due to high expenses. Therefore, negative AM is a preferred and relatively inexpensive route for industrial-level production of large-scale parts.
The B4C suspensions used for gelcasting must be low in viscosity and contain a gelling agent and sintering aid. Resorcinol and formaldehyde are chosen for their ability to undergo polycondensation reactions to form a resorcinol-formaldehyde (RF) network, which helps to bind the B4C particles together. Traditional hydrogels used for gelcasting are limited to molds with hollow cores due to the high inward shrinkage experienced during the drying process18. Since RF is commonly used as an aerogel, there is little to no shrinkage, which permits the use of more intricately shaped molds. Another advantage of using RF is that the gelation rate can be controlled by altering the pH of the suspension&#160;(Figure 3). Additionally, suspensions containing either resorcinol or formaldehyde can be prepared in advanced and stored separately until they are ready for casting. Most importantly, the RF gel can be pyrolyzed to leave behind 50 wt% carbon19. This highly homogenous distribution of carbon can aid the densification of B4C to near-full densities during sintering. 15 wt% of RF relative to boron carbide is used in the formulation of the suspension to provide 7.5 wt% of carbon after pyrolysis of the cast parts.
The overall goal of this work is to combine traditional gelcasting techniques with inexpensive 3D printing capabilities and a unique gelling agent to obtain near-full density boron carbide parts with highly complex geometries. In addition to ceramics, negative AM can be applied to other material fields to create entirely new geometries of multi-material systems. The methodology described here expands on the work presented in Lu et al.8&#160;and aims to provide a more detailed protocol for reproducing those results.

<table><tbody><tr><td>Boron carbide powder 1250F</td><td>Tetrabor Ceramics</td><td>Lot 211M419</td><td>&gt;96% purity</td></tr><tr><td>Boron carbide powder 1500F</td><td>Tetrabor Ceramics</td><td>Lot 209M102/9</td><td>&gt;96% purity</td></tr><tr><td>Boron carbide powder 3000F</td><td>Tetrabor Ceramics</td><td>Lot 111m53/9&amp;nbsp;</td><td>&gt;96% purity</td></tr><tr><td>Polyethylene Imine (PEI)</td><td>Sigma Aldrich</td><td>MKBP3417V</td><td>Averaged MW ~25,000 by L.S.&amp;nbsp;</td></tr><tr><td>Resorcinol</td><td>Sigma Aldrich</td><td>MKBG6751V</td><td>BioXtra, &amp;ge;99%</td></tr><tr><td>Formaldehyde</td><td>Fisher Scientific</td><td>F79-1</td><td>37% by weight; Stabilized with 10-15% Methanol</td></tr><tr><td>Acetic Acid</td><td>Sigma Aldrich</td><td>SKU 695092</td><td>Glacial &amp;ge;99.7%</td></tr><tr><td>Acetone</td><td>Sigma Aldrich</td><td>SKU 179124</td><td>ACS Reagent Grade &amp;ge;99.5%</td></tr><tr><td>Water</td><td>LLNL In-house (Milli-Q)</td><td></td><td></td></tr><tr><td>Planetary Mixer</td><td>Thinky</td><td>AR-250</td><td>Fits 150mL and 300mL Thinky containers</td></tr><tr><td>Acrylonitrile butadiene styrene (ABS) plastic filament</td><td>eSUN</td><td></td><td>Natural color</td></tr><tr><td>Taz 6 (3D printer)</td><td>Lulzbot</td><td></td><td>FDM 3D printer</td></tr><tr><td>4%H2/96%Ar gas</td><td>Air Gas</td><td>UHP</td><td>4% Hydrogen, balanced Argon</td></tr><tr><td>Helium gas</td><td>Air Gas</td><td>UHP</td><td>Helium</td></tr><tr><td>Heating oven</td><td>Neytech</td><td>Vulcan 9493308</td><td>Oven for 80 &amp;deg;C curing</td></tr><tr><td>Quartz tube furnace</td><td>Applied Test Systems, Inc.&amp;nbsp;</td><td>LEA 05-000075</td><td>Furnace for 1050 &amp;deg;C carbonization</td></tr><tr><td>Graphite furnace</td><td>Thermal Technology LLC</td><td></td><td>Sintering furnace</td></tr><tr><td>Scanning Electron Microscope (SEM)</td><td>Jeol</td><td>JSM-7401F</td><td></td></tr><tr><td>pH meter</td><td>Thermo Scientific</td><td>Orion 4 Star</td><td>calibrated with buffer standards</td></tr><tr><td>Rheometer</td><td>TA Instrument</td><td>AR2000ex</td><td>For measurement of viscosity</td></tr><tr><td>X-ray Diffractometer (XRD)</td><td>Bruker</td><td>AX D8 Advanced</td><td></td></tr><tr><td>Analytical balance</td><td>Mettler Toledo</td><td>XS104</td><td></td></tr><tr><td>Bruker EVA&amp;nbsp;</td><td></td><td></td><td>XRD Analysis Software</td></tr></tbody></table>

negative additive manufacturing of complex shaped boron carbides

Boron carbide (B4C) is one of the hardest materials in existence. However, this attractive property also limits its machineability into complex shapes for high wear, high hardness, and lightweight material applications such as armors. To overcome this challenge, negative additive manufacturing (AM) is employed to produce complex geometries of boron carbides at various length scales. Negative AM first involves gelcasting a suspension into a 3D-printed plastic mold. The mold is then dissolved away, leaving behind a green body as a negative copy. Resorcinol-formaldehyde (RF) is used as a novel gelling agent because unlike traditional hydrogels, there is little to no shrinkage, which allows for extremely complex molds to be used. Furthermore, this gelling agent can be pyrolyzed to leave behind ~50 wt% carbon, which is a highly effective sintering aid for B4C. Due to this highly homogenous distribution of in situ carbon within the B4C matrix, less than 2% porosity can be achieved after sintering. This protocol highlights in detail the methodology for creating near fully dense boron carbide parts with highly complex geometries.

Following the outlined procedure (Figure 1), complex shaped boron carbide parts with carbon (B4C/C) can be sintered up to 97.6 &#177; 0.4% of theoretical max density with a Vicker&#39;s hardness of 23.0 &#177; 1.8 GPa8. Several possible examples of sintered B4C/C parts are demonstrated (Figure 2). These examples show the fine textural features that can be copied by the gelcasting tech...

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Negative Additive Manufacturing of Complex Shaped Boron Carbides

A method called negative additive manufacturing is used to produce near fully dense complex shaped boron carbide parts of various length scales. This technique is possible via the formulation of a novel suspension involving resorcinol-formaldehyde as a unique gelling agent that leaves behind a homogenous carbon sintering aid after pyrolysis.

Boron carbide (B4C) is one of the hardest materials in existence. However, this attractive property also limits its machineability into complex shapes for ...

negative-additive-manufacturing-of-complex-shaped-boron-carbides

Universidad San Sebastian

Research

JoVE Journal

Chemistry

8.6K Views.  Lawrence Livermore National Laboratory. A method called negative additive manufacturing is used to produce near fully dense complex shaped boron carbide parts of various length scales. This technique is possible via the formulation of a novel suspension involving resorcinol-formaldehyde as a unique gelling agent that leaves behind a homogenous carbon sintering aid after pyrolysis.

Video: Negative Additive Manufacturing of Complex Shaped Boron Carbides

Fabrication, Densification, and Replica Molding of 3D Carbon Nanotube Microstructures

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip devices, and their applications. In particular, capabilities for cost-effective fabrication of polymer microstructures were transformed by the advent of soft lithography and other micromolding techniques 1, 2, and this led a revolution in applications of microfabrication to biomedical engineering and biology. Nevertheless, it remains challenging to fabricate microstructures with well-defined nanoscale surface textures, and to fabricate arbitrary 3D shapes at the micro-scale. Robustness of master molds and maintenance of shape integrity is especially important to achieve high fidelity replication of complex structures and preserving their nanoscale surface texture. The combination of hierarchical textures, and heterogeneous shapes, is a profound challenge to existing microfabrication methods that largely rely upon top-down etching using fixed mask templates. On the other hand, the bottom-up synthesis of nanostructures such as nanotubes and nanowires can offer new capabilities to microfabrication, in particular by taking advantage of the collective self-organization of nanostructures, and local control of their growth behavior with respect to microfabricated patterns. 
Our goal is to introduce vertically aligned carbon nanotubes (CNTs), which we refer to as CNT "forests", as a new microfabrication material. We present details of a suite of related methods recently developed by our group: fabrication of CNT forest microstructures by thermal CVD from lithographically patterned catalyst thin films; self-directed elastocapillary densification of CNT microstructures; and replica molding of polymer microstructures using CNT composite master molds. In particular, our work shows that self-directed capillary densification ("capillary forming"), which is performed by condensation of a solvent onto the substrate with CNT microstructures, significantly increases the packing density of CNTs. This process enables directed transformation of vertical CNT microstructures into straight, inclined, and twisted shapes, which have robust mechanical properties exceeding those of typical microfabrication polymers. This in turn enables formation of nanocomposite CNT master molds by capillary-driven infiltration of polymers. The replica structures exhibit the anisotropic nanoscale texture of the aligned CNTs, and can have walls with sub-micron thickness and aspect ratios exceeding 50:1. Integration of CNT microstructures in fabrication offers further opportunity to exploit the electrical and thermal properties of CNTs, and diverse capabilities for chemical and biochemical functionalization 3.

We present methods for fabrication of patterned microstructures of vertically aligned carbon nanotubes (CNTs), and their use as master molds for production of polymer microstructures with organized nanoscale surface texture. The CNT forests are densified by condensation of solvent onto the substrate, which significantly increases their packing density and enables self-directed formation of 3D shapes.

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip ...

Engineering

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from ...

Supercritical Nitrogen Processing for the Purification of Reactive Porous Materials

Supercritical fluid extraction and drying methods are well established in numerous applications for the synthesis and processing of porous materials. Herein, nitrogen is presented as a novel supercritical drying fluid for specialized applications such as in the processing of reactive porous materials, where carbon dioxide and other fluids are not appropriate due to their higher chemical reactivity. Nitrogen exhibits similar physical properties in the near-critical region of its phase diagram as compared to carbon dioxide: a widely tunable density up to ~1 g ml-1, modest critical pressure (3.4 MPa), and small molecular diameter of ~3.6 &#197;. The key to achieving a high solvation power of nitrogen is to apply a processing temperature in the range of 80-150 K, where the density of nitrogen is an order of magnitude higher than at similar pressures near ambient temperature. The detailed solvation properties of nitrogen, and especially its selectivity, across a wide range of common target species of extraction still require further investigation. Herein we describe a protocol for the supercritical nitrogen processing of porous magnesium borohydride.

Nitrogen is an effective supercritical fluid for extraction or drying processes due to its small molecular size, high density in the near-liquid supercritical regime, and chemical inertness. We present a supercritical nitrogen drying protocol for the purification treatment of reactive, porous materials.

Supercritical fluid extraction and drying methods are well established in numerous applications for the synthesis and processing of porous materials. ...

Reverse Microemulsion-mediated Synthesis of Monometallic and Bimetallic Early Transition Metal Carbide and Nitride Nanoparticles

A reverse microemulsion is used to encapsulate monometallic or bimetallic early transition metal oxide nanoparticles in microporous silica shells. The silica-encapsulated metal oxide nanoparticles are then carburized in a methane/hydrogen atmosphere at temperatures over 800 &#176;C to form silica-encapsulated early transition metal carbide nanoparticles. During the carburization process, the silica shells prevent the sintering of adjacent carbide nanoparticles while also preventing the deposition of excess surface carbon. Alternatively, the silica-encapsulated metal oxide nanoparticles can be nitridized in an ammonia atmosphere at temperatures over 800 &#176;C to form silica-encapsulated early transition metal nitride nanoparticles. By adjusting the reverse microemulsion parameters, the thickness of the silica shells, and the carburization/nitridation conditions, the transition metal carbide or nitride nanoparticles can be tuned to various sizes, compositions, and crystal phases. After carburization or nitridation, the silica shells are then removed using either a room-temperature aqueous ammonium bifluoride solution or a 0.1 to 0.5 M NaOH solution at 40-60 &#176;C. While the silica shells are dissolving, a high surface area support, such as carbon black, can be added to these solutions to obtain supported early transition metal carbide or nitride nanoparticles. If no high surface area support is added, then the nanoparticles can be stored as a nanodispersion or centrifuged to obtain a nanopowder.

A &#8220;removable ceramic coating method&#8221; is presented in visual format for the synthesis of non-sintered and metal-terminated monometallic and bimetallic early transition metal carbide and nitride nanoparticles with tunable sizes and crystal structures.

A reverse microemulsion is used to encapsulate monometallic or bimetallic early transition metal oxide nanoparticles in microporous silica shells. The ...

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees of freedom to control electronic devices. In particular, the behavior of graphene&#8217;s charge carriers in response to potentials from charged Coulomb impurities is predicted to differ significantly from that of most materials. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) can provide detailed information on both the spatial and energy dependence of graphene&#39;s electronic structure in the presence of a charged impurity.&#160;The design of a hybrid impurity-graphene device, fabricated using controlled deposition of impurities onto a back-gated graphene surface, has enabled several novel methods for controllably tuning graphene&#8217;s electronic properties.1-8 Electrostatic gating enables control of the charge carrier density in graphene and the ability to reversibly tune the charge2 and/or molecular5 states of an impurity. This paper outlines the process of fabricating a gate-tunable graphene device decorated with individual Coulomb impurities for combined STM/STS studies.2-5 These studies provide valuable insights into the underlying physics, as well as signposts for designing hybrid graphene devices.

This paper details the fabrication process of a gate-tunable graphene device, decorated with Coulomb impurities for scanning tunneling microscopy studies. Mapping the spatially dependent electronic structure of graphene in the presence of charged impurities unveils the unique behavior of its relativistic charge carriers in response to a local Coulomb potential.

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees ...

Assessment of Boron Doped Diamond Electrode Quality and Application to In Situ Modification of Local pH by Water Electrolysis

Boron doped diamond (BDD) electrodes have shown considerable promise as an electrode material where many of their reported properties such as extended solvent window, low background currents, corrosion resistance, etc., arise from the catalytically inert nature of the surface. However, if during the growth process, non-diamond-carbon (NDC) becomes incorporated into the electrode matrix, the electrochemical properties will change as the surface becomes more catalytically active. As such it is important that the electrochemist is aware of the quality and resulting key electrochemical properties of the BDD electrode prior to use. This paper describes a series of characterization steps, including Raman microscopy, capacitance, solvent window and redox electrochemistry, to ascertain whether the BDD electrode contains negligible NDC i.e. negligible sp2 carbon. One application is highlighted which takes advantage of the catalytically inert and corrosion resistant nature of an NDC-free surface i.e. stable and quantifiable local proton and hydroxide production due to water electrolysis at a BDD electrode. An approach to measuring the local pH change induced by water electrolysis using iridium oxide coated BDD electrodes is also described in detail.

Assessment of Boron Doped Diamond Electrode Quality and Application to In Situ Modification of Local pH by Water Electrolysis

A protocol is described for the characterization of the key electrochemical parameters of a boron doped diamond (BDD) electrode and subsequent application for in situ pH generation experiments.

Boron doped diamond (BDD) electrodes have shown considerable promise as an electrode material where many of their reported properties such as extended ...

Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film synthesis) for the purpose of exploratory materials research. Certain materials pose a challenge wherein the traditional bulk Hall bar device fabrication method is insufficient to produce a measureable device for sample transport measurement, principally because the single crystal size is too small to attach wire leads to the sample in a Hall bar configuration. This can be, for example, because the first batch of a new material synthesized yields very small single crystals or because flakes of samples of one to very few monolayers are desired. In order to enable rapid characterization of materials that may be carried out in parallel with improvements to their growth methodology, a method of device fabrication for very small samples has been devised to permit the characterization of novel materials as soon as a preliminary batch has been produced. A slight variation of this methodology is applicable to producing devices using exfoliated samples of two-dimensional materials such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs), as well as multilayer heterostructures of such materials. Here we present detailed protocols for the experimental device fabrication of fragments and flakes of novel materials with micron-sized dimensions onto substrate and subsequent measurement in a commercial superconducting magnet, dry helium close-cycle cryostat magnetotransport system at temperatures down to 0.300 K and magnetic fields up to 12 T.

We describe the methodology of mechanical exfoliation and deposition of flakes of novel materials with micron-sized dimensions onto substrate, fabrication of experimental device structures for transport experimentation, and the magnetotransport measurement in a dry helium close-cycle cryostat at temperatures down to 0.300 K and magnetic fields up to 12 T.

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film ...

Probing C84-embedded Si Substrate Using Scanning Probe Microscopy and Molecular Dynamics

This paper reports an array-designed C84-embedded Si substrate fabricated using a controlled self-assembly method in an ultra-high vacuum chamber. The characteristics of the C84-embedded Si surface, such as atomic resolution topography, local electronic density of states, band gap energy, field emission properties, nanomechanical stiffness, and surface magnetism, were examined using a variety of surface analysis techniques under ultra, high vacuum (UHV) conditions as well as in an atmospheric system. Experimental results demonstrate the high uniformity of the C84-embedded Si surface fabricated using a controlled self-assembly nanotechnology mechanism, represents an important development in the application of field emission display (FED), optoelectronic device fabrication, MEMS cutting tools, and in efforts to find a suitable replacement for carbide semiconductors. Molecular dynamics (MD) method with semi-empirical potential can be used to study the nanoindentation of C84-embedded Si substrate. A detailed description for performing MD simulation is presented here. Details for a comprehensive study on mechanical analysis of MD simulation such as indentation force, Young&#39;s modulus, surface stiffness, atomic stress, and atomic strain are included. The atomic stress and von-Mises strain distributions of the indentation model can be calculated to monitor deformation mechanism with time evaluation in atomistic level.

Probing C84-embedded Si Substrate Using Scanning Probe Microscopy and Molecular Dynamics

This paper reports the nanomaterial fabrication of a fullerene Si substrate inspected and verified by nanomeasurements and molecular dynamic simulation.

This paper reports an array-designed C84-embedded Si substrate fabricated using a controlled self-assembly method in an ultra-high vacuum chamber. The ...

Chemical Synthesis of Porous Barium Titanate Thin Film and Thermal Stabilization of Ferroelectric Phase by Porosity-Induced Strain

Barium titanate (BaTiO3, hereafter BT) is an established ferroelectric material first discovered in the 1940s and still widely used because of its well-balanced ferroelectricity, piezoelectricity, and dielectric constant. In addition, BT does not contain any toxic elements. Therefore, it is considered to be an eco-friendly material, which has attracted considerable interest as a replacement for lead zirconate titanate (PZT). However, bulk BT loses its ferroelectricity at approximately 130 &#176;C, thus, it cannot be used at high temperatures. Because of the growing demand for high-temperature ferroelectric materials, it is important to enhance the thermal stability of ferroelectricity in BT. In previous studies, strain originating from the lattice mismatch at hetero-interfaces has been used. However, the sample preparation in this approach requires complicated and expensive physical processes, which are undesirable for practical applications.
In this study, we propose a chemical synthesis of a porous material as an alternative means of introducing strain. We synthesized a porous BT thin film using a surfactant-assisted sol-gel method, in which self-assembled amphipathic surfactant micelles were used as an organic template. Through a series of studies, we clarified that the introduction of pores had a similar effect on distorting the BT crystal lattice, to that of a hetero-interface, leading to the enhancement and stabilization of ferroelectricity. Owing to its simplicity and cost effectiveness, this fabrication process has considerable advantages over conventional methods.

Here, we present a protocol for the synthesis of porous barium titanate (BaTiO3) thin film by a surfactant-assisted sol-gel method, in which self-assembled amphipathic surfactant micelles are used as an organic template.

Barium titanate (BaTiO3, hereafter BT) is an established ferroelectric material first discovered in the 1940s and still widely used because of its ...

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

The synthesis of high quality bulk and thin film (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O entropy-stabilized oxides is presented.

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and ...

Negative Additive Manufacturing of Complex Shaped Boron Carbides

Summary

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Chapters in this video

Fabrication, Densification, and Replica Molding of 3D Carbon Nanotube Microstructures

Fabrication of Spatially Confined Complex Oxides

Supercritical Nitrogen Processing for the Purification of Reactive Porous Materials

Reverse Microemulsion-mediated Synthesis of Monometallic and Bimetallic Early Transition Metal Carbide and Nitride Nanoparticles

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

Assessment of Boron Doped Diamond Electrode Quality and Application to In Situ Modification of Local pH by Water Electrolysis

Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials

Probing C₈₄-embedded Si Substrate Using Scanning Probe Microscopy and Molecular Dynamics

Chemical Synthesis of Porous Barium Titanate Thin Film and Thermal Stabilization of Ferroelectric Phase by Porosity-Induced Strain

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides