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 ...

<ol>
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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.

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	<tbody>
		<tr height="19">
			<td height="19" style="height:19px;width:215px;">Boron carbide powder 1250F</td>
			<td style="width:253px;">Tetrabor Ceramics</td>
			<td style="width:215px;">Lot 211M419</td>
			<td style="width:363px;">&#62;96% purity</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Boron carbide powder 1500F</td>
			<td style="width:253px;">Tetrabor Ceramics</td>
			<td style="width:215px;">Lot 209M102/9</td>
			<td>&#62;96% purity</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Boron carbide powder 3000F</td>
			<td style="width:253px;">Tetrabor Ceramics</td>
			<td>Lot 111m53/9&#160;</td>
			<td>&#62;96% purity</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Polyethylene Imine (PEI)</td>
			<td style="width:253px;">Sigma Aldrich</td>
			<td style="width:215px;">MKBP3417V</td>
			<td>Averaged MW ~25,000 by L.S.&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Resorcinol</td>
			<td style="width:253px;">Sigma Aldrich</td>
			<td style="width:215px;">MKBG6751V</td>
			<td>BioXtra, &#8805;99%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Formaldehyde</td>
			<td style="width:253px;">Fisher Scientific</td>
			<td style="width:215px;">F79-1</td>
			<td>37% by weight; Stabilized with 10-15% Methanol</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Acetic Acid</td>
			<td style="width:253px;">Sigma Aldrich</td>
			<td style="width:215px;">SKU 695092</td>
			<td>Glacial &#8805;99.7%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Acetone</td>
			<td style="width:253px;">Sigma Aldrich</td>
			<td>SKU 179124</td>
			<td>ACS Reagent Grade &#8805;99.5%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Water</td>
			<td style="width:253px;">LLNL In-house (Milli-Q)</td>
			<td style="width:215px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Planetary Mixer</td>
			<td style="width:253px;">Thinky</td>
			<td style="width:215px;">AR-250</td>
			<td>Fits 150mL and 300mL Thinky containers</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:215px;">Acrylonitrile butadiene styrene (ABS) plastic filament</td>
			<td style="width:253px;">eSUN</td>
			<td style="width:215px;"></td>
			<td>Natural color</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Taz 6 (3D printer)</td>
			<td style="width:253px;">Lulzbot</td>
			<td style="width:215px;"></td>
			<td>FDM 3D printer</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">4%H2/96%Ar gas</td>
			<td style="width:253px;">Air Gas</td>
			<td style="width:215px;">UHP</td>
			<td>4% Hydrogen, balanced Argon</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Helium gas</td>
			<td style="width:253px;">Air Gas</td>
			<td style="width:215px;">UHP</td>
			<td>Helium</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Heating oven</td>
			<td style="width:253px;">Neytech</td>
			<td style="width:215px;">Vulcan 9493308</td>
			<td>Oven for 80 &#176;C curing</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Quartz tube furnace</td>
			<td style="width:253px;">Applied Test Systems, Inc.&#160;</td>
			<td style="width:215px;">LEA 05-000075</td>
			<td>Furnace for 1050 &#176;C carbonization</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Graphite furnace</td>
			<td style="width:253px;">Thermal Technology LLC</td>
			<td style="width:215px;"></td>
			<td>Sintering furnace</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:215px;">Scanning Electron Microscope (SEM)</td>
			<td style="width:253px;">Jeol</td>
			<td style="width:215px;">JSM-7401F</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">pH meter</td>
			<td style="width:253px;">Thermo Scientific</td>
			<td style="width:215px;">Orion 4 Star</td>
			<td>calibrated with buffer standards</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Rheometer</td>
			<td style="width:253px;">TA Instrument</td>
			<td style="width:215px;">AR2000ex</td>
			<td>For measurement of viscosity</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">X-ray Diffractometer (XRD)</td>
			<td style="width:253px;">Bruker</td>
			<td style="width:215px;">AX D8 Advanced</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Analytical balance</td>
			<td style="width:253px;">Mettler Toledo</td>
			<td style="width:215px;">XS104</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Bruker EVA&#160;</td>
			<td style="width:253px;"></td>
			<td style="width:215px;"></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...

Watch this Scientific Journal Video about Negative Additive Manufacturing of Complex Shaped Boron Carbides at JoVE.com

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, or AM, is a cost-effective method to produce ultra hard, dense ceramic parts into complex shapes, that would not have been possible by traditional casting methods. The use of an aerogel in this technique is unique. It serves both as a gelling agent, with little to no shrinkage during casting, and also is a well-dispersed carbon source for sintering after pyrolysis.Gel casting into 3D printed molds is a versatile technique that can be extended to other materials. The resorcinol formaldehyde gelling agent is best suited for carbon bearing systems. To begin, prepare a two part suspension to help prolong the shelf life of the suspensions before casting.To do so, create the R mix by dissolving 0.88 grams of polyethyleneimine in 25.00 grams of water, using a planetary mixer. To create a separate F mix, dissolve 0.88 grams of polyethyleneimine in 16.83 grams of water, using a planetary mixer. Now, dissolve 12.60 grams 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. Also, add 17.03 grams of formaldehyde solution to the f-mix, and ensure complete mixing. At this point, incrementally add 5.25 grams of boron carbide powder into both the R mix and F mix separately.Then, add 6.50 grams of acetic acid to the R mix and F mix, and ensure complete mixing in each. To prepare molds for casting, print the molds using a fused deposition modeling 3D printer with acrylonitrile butadiene styrene, or ABS, filaments. After thoroughly agitating the suspensions, combine the R mix and F mix to obtain the final suspension.Before casting, mix and apply vacuum to the final suspension mixture for about 10 minutes, to remove air bubbles without boiling the water. This can be a accomplished by using a stirring plate at 200 to 300 RPM, with a vacuum jar. This following step is the most critical, because the gelling agent will start to react, and cause the suspension's viscosity to rapidly increase.Being aware of this time-sensitive step is crucial for best results. Immediately pour the de-aired suspension into the 3D printed mold, and seal the glass container. Place the mold inside the sealable glass container to prevent moisture loss during the curing process.Place the sealed container with the mold into a 60 to 80 degree Celsius oven to initiate the curing process. Allow the cast to cure for at least eight hours for parts that are several centimeters in length scale. After eight hours, remove the sealed container with the mold from the oven, and allow it to cool to room temperature.Transfer to a new container, and then add enough acetone until the mold is fully submersed. This process might take up to two to four days, depending on the amount of volume that needs to be dissolved away. Minimal agitation of the acetone bath, or heating it slightly to 40 degrees C, may help speed up the process.Extract the free green body from the acetone bath, after the ABS plastic is dissolved away. Place the green body in an oven at 80 degrees Celsius to ensure complete drying and removal of all moisture. After drying, place each green body in a two-inch quartz tube lined with graphite foil.Place the quartz tubes into a furnace with flowing gas at 250 SCCM, consisting of four weight percent hydrogen gas, and 96 weight percent argon gas, to create a reducing atmosphere during the pyrolysis treatment. Heat the green body inside the furnace at five degrees Celsius per minute, until 1, 050 degrees Celsius, and hold for three hours. Ensure that the green body comes out uniformly darker in color, indicating the presence of carbon from the pyrolysis treatment.Now, place the carbonized parts in a graphite furnace with vacuum backfilled flowing helium gas for sintering. Heat up the furnace 2, 290 degrees Celsius, and hold for one hour to achieve optimal densification of the parts. Remove after one hour.Viscosity of the suspension is optimized for gelcasting at pH 2.8, to provide the longest working time before significant gelation occurs. There is approximately 20 minutes until a viscosity of 1.0 pascal seconds. Scanning electron microscope images show evidence of the spatially uniform carbon network that forms on the boron carbide particles after the pyrolysis of resorcinol formaldehyde.X-ray diffraction of the samples, at various processing stages, also confirms the growth of a graphite peak after pyrolysis. Additional microscopy of a cross-sectioned sample reveals the low porosity of the final product, after being sintered at 2, 280 degrees Celsius. Don't forget that formaldehyde is carcinogenic, and working with it can be extremely hazardous.Therefore, wearing proper personal protective equipment, and working in a well-ventilated area, is essential when handling the suspensions. After watching this video, hopefully you are now familiar with the negative additive manufacturing process, and can successfully gelcast boron carbide suspensions into complex shaped green bodies, and later sinter them to their full densities.

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Research

JoVE Journal

Chemistry

8.5K visualizações.  Lawrence Livermore National Laboratory. A fabricação negativa de aditivos, ou AM, é um método econômico para produzir peças cerâmicas ultra duras e densas em formas complexas, o que não teria sido possível pelos métodos tradicionais de fundição. O uso de um aerogel nesta técnica é único. Ele serve tanto como um agente de gelagem, com pouco ou nenhum encolhimento durante a fundição, e também é uma fonte de carbono bem dispersa para sinterização após pirólise.Fundição de gel em moldes impressos em 3D ?...