Name: additive manufacturing of functionally graded ceramic materials by stereolithography
Uploaded: 2019-01-25T13:45:00
Duration: 6 min 53 s
Description: Watch this Scientific Journal Video about Additive Manufacturing of Functionally Graded Ceramic Materials by Stereolithography at JoVE.com

This project has received funding from the European Union&#39;s Horizon 2020 Research and Innovation Program under Grant Agreement No 678503.

1. Development of Photocurable Ceramic Suspensions
<ol>
	<li>Selection of ceramic powders
	<ol>
		<li>Use high-purity ceramic powders (e.g., aluminum oxide powder of 99.9% purity or higher).</li>
		<li>Choose powders with (1) a narrow particle size distribution for poor viscosity, (2) a mean particle size of &#60; 0.5 &#181;m for good sinterability, and (3) a specific surface in the vicinity of 7 m2/g for a low viscosity.</li>
	</ol>
	</li>
	<li>Powder specification 
	<ol>
		<li>Characterize the powders concerning shape, surface area, and particle size distribution if needed (Table of Materials).</li>
		<li>Characterize the particle shape using, for instance, FESEM analyses. In order to do so, take (a few milligrams) of powder with a spatula and deposit it on a carbon tape square with an area size of approximately 100 mm2. Metalize the ensemble prior to the introduction in the microscope chamber.</li>
		<li>Assess the particle size distribution of the utilized powders with, for instance, a laser diffraction method. Put (a few milligrams) of the sample with a spatula into the mixing chamber of the machine and deagglomerate it using high-frequency ultrasound waves 5x for 5 min each time.</li>
		<li>Measure the specific surface properties of the used powders using the Brunauer-Emmet-Teller (BET) approach. Collect the adsorption/desorption isotherms in liquid nitrogen. Degas the samples at 150 &#176;C prior to the measurements.</li>
	</ol>
	</li>
	<li>Selection of polymeric resin 
	<ol>
		<li>Choose, for example, a monofunctional binder (1; see Table of Materials) together with a di(2)- and tetra(3)-functional crosslinker (see Table of Materials) and a photoinitiator (4; see Table of Materials) active in the wavelength of the used printing device&#8217;s light engine, in this case at 405 nm.</li>
		<li>For a more flexible polymer network, use a plasticizing fluid (5; see Table of Materials).</li>
	</ol>
	</li>
	<li>Preparation of ceramic suspensions
	<ol>
		<li>If necessary, deagglomerate the alumina powders using a volatile solvent, such as ethanol absolute, together with a dispersing agent (see Table of Materials) and alumina milling balls.
		<ol>
			<li>For this, mix 80 wt.% of powder with 20 wt.% solvent together with the same absolute mass-like powder of mill balls with a diameter of 1 - 2 mm, and add dispersing agent in a range of 0.5 to 2.0 wt.% based on the powder content.</li>
			<li>Mill the mixture for 2 h in a planetary ball mill (see Table of Materials) to deagglomerate the powder in order to achieve the primary particle size.</li>
			<li>After milling, separate the powder mass from the mill balls by using a sieve (with a mesh of 500 &#181;m) and dry the suspension in a fume hood for 12 h at room temperature and, subsequently, in a stove dryer for 24 h at 110 &#176;C.</li>
			<li>Grind the dried powder through a sieve (100 - 500 &#181;m) to get the deagglomerated and functionalized powder. 
			Note: The surface of the particles is now functionalized with the dispersing agent necessary for a stable and low-viscous suspension.</li>
		</ol>
		</li>
		<li>Adapt the properties of the developed suspensions, especially the dynamic viscosity, to the printing process. Here, four different compounds were prepared and characterized in terms of dynamic viscosity and their curing behavior. Four different compounds (I, II, III, and IV) were created by changing the compositions.
		<ol>
			<li>In compound I, use a ratio of 1.5 between the di- and tetra-functional crosslinkers. Use a ratio between the complete crosslinker and the monofunctional binder of 1.2. The content of the photoinitiator was 1.3 wt.% to the reactive resin, and the content of the plasticizer was 30 wt.% of the total. Within compound I, use a powder content of 78 wt.%.</li>
			<li>In compound II, increase the powder content to 82 wt.%.</li>
			<li>In compound III, increase the amount of tetra-functional crosslinker by changing the ratio of the di- and tetra-functional crosslinkers to 1.8.</li>
			<li>In compound IV, reduce the powder content to 75 wt.% and change the ratio of the crosslinker to the monofunctional binder to 1.0.</li>
		</ol>
		</li>
		<li>Mix the different organic and photoreactive components based on the compounds I to IV described in section 1.4.2. Introduce the components into a can of a high-speed planetary ball mill (see Table of Materials) and homogenize the mixture for 4 min at a speed of 1,000 rpm. Additionally, a plasticizer can be added to get a higher flexibility of the polymer after curing.</li>
	</ol>
	</li>
	<li>Adding powder into the polymer mixture</li>
	<li>Homogenize the mixture at three levels: for 4 min at 1,000 rpm, for 45 s at 1,500 rpm, and for 30 s at 2,000 rpm. 
	Note: In case of an increased temperature, cool down the can with water. If necessary, repeat the mixing a second time.</li>
	<li>Characterization of the suspension
	<ol>
		<li>Characterize the rheological behavior, especially the dynamic viscosity as a characteristic value of the flow behavior. The measurement set-up should be based on the printing process parameters, especially the casting speed.
		<ol>
			<li>Use a rheometer with a cone/plate measuring system (25 mm in diameter), adjustable between -25 &#176;C to 200 &#176;C (see Table of Materials).</li>
			<li>Put a sample (approximately 1 mL) of the suspension on the plate and follow the measuring instructions of the rheometer for a rotational measurement.</li>
			<li>Analyze the dynamic viscosity by increasing the shear rate of 0.01 to 1000 s-1 at a constant temperature of 20 &#176;C and measuring the torque. 
			Note: During the process, the suspension is cast with a velocity of 40 mm/s. Therefore, the shear rate is approximately 200 s-1, lower for the movement of the printed component, and fixed at the building platform, within the coated suspension. Consequently, the set-up of the rheological measurement is defined.</li>
			<li>Make sure that the suspension shows a shear thinning behavior with a dynamic viscosity below 600 Pa&#183;s for a shear rate of 0.1 s-1 and below 10 Pa&#183;s for shear rates of 10 to 300 s-1.</li>
		</ol>
		</li>
		<li>Characterize the curing behavior of the developed suspensions. Analyze the curing behavior by oscillating measurements before, during, and after exposure with light (with a wavelength of 300 to 500 nm).
		<ol>
			<li>Use a rheometer (see Table of Materials), e.g., adjustable between -25 &#176;C to 200 &#176;C, with a plate/(glass)plate measuring system (25 mm in diameter) with a gap of 50 &#181;m, in combination with a blue LED light source (with a wavelength of 405 nm).</li>
			<li>Fix the LED below the (glass) plate and adjust the intensity conform to the printing intensity (approximately 33 mW/cm2) by using a photometer.</li>
			<li>Put a suspension sample of approximately 1 mL onto the (glass) plate and move the plate of the measurement system to the measurement position using a gap of 50 &#181;m.</li>
			<li>Measure the storage modulus G&#180;&#8212;a part of the complex shear modulus G*&#8212;by using a constant deformation amplitude (e.g., 0.1% [0.09&#176;]) with a frequency of 10 rad/s.</li>
			<li>Before exposure, measure G&#180; in 10 s intervals for 60 s. This represents a first plateau of G&#180; for the liquid suspension.</li>
			<li>Once completed, start the exposure after 60 s by using the blue LED (see Table of Materials) for a defined duration (e.g., 1 - 4 s). Measure G&#180; during and after the exposure. G&#180; increases due to the exposure, which indicates the polymerization process. Depending on the exposure time and suspension properties, G&#180; will increase to a second plateau during the polymerization.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
2. Manufacturing of Single-graded and FGM Components by Ceramic SLA
<ol>
	<li>Use a ceramic DLP-SLA printing device. See Discussion for the apparatus description.
	<ol>
		<li>Investigate the depth of the curing. This step is necessary to determine the curing capabilities of the slurry (i.e., the penetration depth of the light and the subsequent polymerization process). For this:
		<ol>
			<li>Apply approximately 1 mL of the ceramic-filled resin slurry (prepared in step 1.4) on a piece of transparent foil (see Discussion) with the help of a spatula. Use a polymer spatula that has a high chemical resistance (e.g., a nylon-glass fiber spatula).</li>
			<li>Place the foil with the slurry flush on the printing glass plate.</li>
			<li>Project, with the DLP-SLA printing device, a light masked test exposure for a fixed number of seconds in a range of 0.5 to 4 s.</li>
			<li>Remove the excess uncured slurry.</li>
			<li>Measure the cured layer with the help of a micrometer. The cured thickness must be at least the same as that of the chosen building layers, although it is recommended to reach several times the thickness of the layer in order to provide enough light penetration.</li>
			<li>Repeat steps 2.1.1.1. to 2.1.1.5 until the desired cured thickness is reached.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Manufacture the functionally graded material parts as follows.
	<ol>
		<li>Generate a 3-D model of the desired part using CAD software.</li>
		<li>Slice the 3-D file to layers of the required thickness with the help of a slicing software. Typical layer thickness of the printing system ranges from 25 to 100 &#181;m. Save the sliced file in *.slc format.</li>
		<li>Transfer the *.slc file to the device via USB or network connection.</li>
		<li>Create a printing program and adjust the printing parameters (e.g., curing time per layer, transport speed [casting speed], and building platform speeds).</li>
		<li>Fill the reservoir of the printing device to half its capacity with the ceramic slurry (approximately 200 g).</li>
		<li>Transport the slurry to fill the pump system until the slurry starts to be pumped back into the reservoir. Make sure that the generated slurry layer is several times thicker as the targeted file slice layer thickness.</li>
		<li>Attach a printing metal plate to the building platform using vacuum pressure from the vacuum pump integrated into the printing device.</li>
		<li>Start the printing program. 
		Note: The printing device will automatically transport the slurry layer. Refill the slurry reservoir during printing if necessary.</li>
		<li>When the printing program is complete, remove the printing metal plate with the product. Switch off the vacuum pump and hold the plate at the same time.</li>
		<li>Clean the remaining slurry attached to the product surface with a mild organic solvent (e.g., isopropanol). A thin layer of slurry may remain adhered to the surface of the parts, accentuated with products with a large surface.</li>
		<li>Dry the rinsed products at room temperature under a fume hood.</li>
	</ol>
	</li>
</ol>
3. Co-debinding and Co-sintering of Single-graded and FGM Components
<ol>
	<li>Debind the green samples as described in the following steps.
	<ol>
		<li>First, put the samples on a special kiln furniture which was sintered at a temperature at least 50 &#176;C higher than the final sintering temperature of the printed components. By doing this, transferring the debound components to another kiln furniture is not necessary.</li>
		<li>Perform a debinding program with a low heating rate in a furnace (see Table of Materials) under air atmosphere up to 600 &#176;C (e.g., with a heating rate of 7.5 &#176;C/h). Use a dwell time at 200 &#176;C, 400 &#176;C, and 600 &#176;C of 10 h. Increase the heating rate at 600 &#176;C to 60 &#176;C/h up to 900 &#176;C and use a dwell of 2 h. Cool down with a rate of 3 - 5 &#176;C/min. 
		Note: This cycle is based on prior characterization by TGA-DSC; however, a different set of polymer resin composition will require an updated debinding program. This is a crucial step in ceramic manufacturing and should not be ignored. 
		Note: All organic binder materials are, at this stage, thermally removed, while in the same step a presintering of the alumina particles is initiated to safely enable the subsequent transfer of the samples to a sintering kiln.</li>
		<li>Transfer the samples with the carrier plate to a sintering furnace (see Table of Materials).</li>
		<li>Sinter the samples under air atmosphere at 1,600 &#176;C for 2 h in the furnace. Use a heating rate of 3 &#176;C/min up to 900 &#176;C, followed by 1 &#176;C/min up to the final temperature of 1,600 &#176;C. 
		Note: The expected linear shrinkage of the components is about 20% - 25% in the x,y-direction and 25% - 30% in the z-direction.</li>
	</ol>
	</li>
</ol>
4. Characterization of Single-graded and Functionally Graded Components
<ol>
	<li>Cut the samples with a diamond saw and polish the surface using ceramographic methods.
	<ol>
		<li>Investigate the microstructure by using FESEM (see Table of Materials). 
		Note: Visually inspect the porosity of the two functionally graded phases and at the boundary interface of the used materials. To obtain a more detailed result, perform an interface analysis. If the porosity is too high, optimize the suspension composition (section 1), the printing parameters (section 2.2) and/or the thermal treatment (section 3). The targeted porosity is below 1%.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

For medical implants, the raw material has to be of high purity, ideally of 99.9% and higher. In this project, a non-commercial alumina powder with a narrow particle size distribution, an average particle size &#60; 0.5 &#181;m, and a specific surface of approximately 7 m2/g is used. Alternatively, it is also possible to use commercial material compositions.
In order to achieve the most appropriate handling conditions for these particular ceramic-polymer slurries, use the aforementioned printing technology. This technology is equipped with a transport foil system that carries the slurry from a reservoir to the printing area. The printing area is composed of a transparent glass surface at the bottom, under which there is a light source that projects the sliced layers. At the top of the printing area, there is a building platform that can move vertically up and down thanks to a z-axis slide. The product, then, hangs on the surface of the metal printing plate that can be attached by vacuum suction, above the printing area. The unused slurry is then collected by a wiper, reconditioned, and pumped back to the original reservoir, thus creating a closed circuit which allows researchers to reuse the slurry that was not consumed for the construction of the 3-D model. Different software parameters can be changed in order to adapt the process to different slurry compositions and ceramic fillers. The printer must be placed in a room with controlled light, temperature, and humidity settings. The room must be equipped with a UV-filter for the outside light; in addition, it is recommended to have a temperature of around 20 - 24 &#176;C and a relative humidity below 40%. The FESEM imaging shows an apparent larger average particle size of alumina powder after deagglomeration, compared to the theoretical 0.45-&#181;m alumina material analyses by the supplier. This can be explained in terms of agglomeration. During drying, after the deagglomeration step, the particles re-agglomerate, as seen in Figure 1D. During the suspension preparation, the re-agglomerated particles can be dispersed thanks to the surface functionalization step. A smaller apparent particle size can be seen in the FESEM imaging of the slurry in Figure 3.
Concerning the rheological behavior, an ideal slurry for ceramic SLA technology (e.g., Admaflex technology) should have a shear thinning behavior (i.e., decreasing dynamic viscosity at higher shear rates). For an optimal cast on supporting foil or use within a dispensing unit, the dynamic viscosity should be kept at an ideal range at low shear rates. In case of too high dynamic viscosity at low shear rates, the casting of a slurry layer of 200 &#181;m might be hampered by the lack of flow to fill the gap under the doctor blade. If the dynamic viscosity it is too low, the suspension may flow by itself from the reservoir below the blade or away from the support foil due to natural flow (gravity). For all investigated suspensions, the dynamic viscosity decreases with an increasing shear rate. The optimal suspension flow behavior is given by composition 1 (Figure 2). Different changes in the slurry composition affect the rheological behavior of the suspension. The optimal flow behavior with a low dynamic viscosity in the required range was achieved by suspension compound 1. An increase of the powder content or a non-optimal content of the dispersing agent (compound 2) and a change of the binder-crosslinker ratio using a higher amount of multifunctional crosslinker (composition 3) led to an increase of the dynamic viscosity, disadvantageously for the process. If the powder content is lower, together with a lower content of multifunctional crosslinker and in combination with a non-optimal content of the dispersing agent (composition 4), the dynamic viscosity is strongly reduced, possibly leading to an unstable suspension.
The change in storage modulus G&#180; of the slurries upon light irradiation can help to learn more about the curing behavior of the suspensions. This is complemented by experimental tests on the depth of curing at the printing device itself. The curing behavior at different curing times was characterized for an alumina suspension with an optimal rheological behavior. Before curing starts, the suspension shows a low level of G&#180; and presents values below 100 Pa. When curing starts, a polymerization of the photoreactive organics can be inferred by an increase of G&#180; to a higher level. With an increasing curing time, the slope of G&#180; increases to a maximum in a range of 105 to 107 Pa which depends on the composition. A curing time of 1 s led to a final G&#180; below 106 Pa, which is not enough for a minimum necessary strength. With an increasing curing time, more energy (photons) is supplied to the suspension, which leads to a higher G&#180; as a result of a faster and higher degree of conversion (higher slope). The optimal curing time for the developed alumina suspension should be in a range of 2 to 3 s. With a curing time of 4 s, the final level of G&#180; and the curing slope have large values, above 2 x 106 Pa. The conversion is almost complete and nearly no uncured polymers exist. Further energy supply may result in overcuring the slurry and an excessive hardening of the polymer, resulting in a brittle structure which has an adverse effect on the attachment of the product with the building platform.
The single-FGM test component chosen for this manuscript is a hemi-maxillary implant structure that contains a dense outer shell and a porous bone-like central core, as can be seen in Figure 5. This model could be additively manufactured and sintered defect-free, as seen by the FESEM imaging. Fine structures and wall thicknesses (less than 0.1 mm) can be realized and no apparent deformation during sintering occurred. It was found that the microstructure of the single alumina components is typical for the ceramic processing of alumina at the given sintering temperatures, with a homogeneous grain size. The porosity in the bulk areas is very low (&#60; 1%), and a density &#62; 99%, compared to the theoretical density, was achieved.

High-complex technical ceramics are increasingly in demand in almost every field of application, including many industrial areas. The field of human healthcare finds more and more applications as a result of the ease of individualization of the products for each patient. In the last decade, additive manufacturing has enhanced the options of individual medical treatments.
Additive manufacturing (AM) is a processing technology that allows the translation of a computer-generated 3-D model into a physical product by sequenced addition of material. In general, a series of 2-D layers form a stack that results in a 3-D shape, allowing the production of components with a, so far, unprecedented freedom of design. This is considered to be state-of-the-art shaping technology for polymers and metals. The first industrial technologies for ceramic processing are available1,2, and nearly all known AM technologies are used for AM of single-material ceramics in laboratories all over the world3,4,5. AM, especially stereolithography, began in the 1980s and was developed by Hull6. Different manufacturing approaches and materials lead to a variety of product properties, such as size, roughness, or mechanical properties. All additive manufacturing techniques can be classified into two groups: direct additive manufacturing technologies5, which are based on the selective deposition of the material (e.g., material jetting processes like Direct Inkjet Printing or Thermoplastic 3-D Printing [T3DP])7,8,9,10, and indirect additive manufacturing technologies, which are based on the selective consolidation of the material which is deposited on the whole layer (e.g., ceramic stereolithography [SLA]).
The complexity and readiness of the new applications demand an improvement of the AM ceramic processing technologies. For example, special innovative industrial or medical applications have to include different properties within the very same component, which leads to Functionally Graded Materials (FGMs). These materials include a variety of properties concerning transitions in the microstructure or in the material11. These transitions can be discrete or continuous. Different kinds of FGMs are known, such as components with material gradients or graded porosity, as well as multi-colored components. FGM components can be manufactured by single conventional shaping technologies12,13,14,15,16,17 or by a combination of these technologies, for example, by in-mold labeling as a combination of tape casting and injection molding18,19.
To combine the benefits of AM with the advantages of FGMs to ceramic-based 4-D components20 (three dimensions for the geometry and one degree of freedom concerning the material properties at each position), Admatec Europe has developed a stereolithography-based 3-D printing device within the &#34;CerAMfacturing&#34; European research project for the AM of multi-functional or multi-material components.
The technology adapted for FGM components is a stereolithography-based approach that employs a digital light processor (DLP) as light source containing a digital micromirror device chip (DMD), used to polymerize a resin which can be mixed with different powders. The DMD chip has an array of several hundred thousand microscopic mirrors, which correspond to the pixels in the image to be displayed. The mirrors can be individually rotated to set an on-off position of the pixel. The most commonly employed resins are based on mixtures of acrylate and/or urethane monomers. In these mixtures, we also found other additives, such as light-absorbing photoinitiator molecules and dyes. The resin mixture is typically poured into a container or bath, also called vat. The polymerization is induced by the reaction of a photoinitiator molecule (PI), with the light photons generated by the DMD chip. Different resin monomer structures may result in different polymerization rates, shrinkage, and final structure. For example, the use of monofunctional monomers vs. polyfunctional monomers has an effect in the cross-linking of the polymeric network.
One of the most important parameters to take into account with ceramic SLA is the light-scattering effect produced when light (photons) traverses through different materials. This is highly impacting; in this case, the resins are combined with an amount of powder to generate a suspension or slurry. The slurry is, then, composed of materials that present a different refractive index to the light. A large difference between the refractive index values of the resin and the powder affects the dimensional accuracy of the layers, the polymerization rates, and the total light dose to trigger the polymerization reaction. When light enters the suspension, the powder particles (i.e., ceramic, metal, or other polymers) diffract the light path. This effect induces a change in the original path of the (irradiated) photons. If the photons have a trajectory oblique to the exposure direction, they may generate a polymerization reaction in a location that can be transversal to the original direction. This phenomenon results in overexposure when the area of the cured slurry is larger than the exposed area. Likewise, it will under-expose, when the cured slurry layer is smaller than the originally exposed area.
Within the manuscript, the research for the AM of alumina components combining a dense and macroporous structure, realized by using the Admaflex technology, is described. As explained in the &#34;CerAMfacturing&#34; European research project, the production of FGM ceramic parts requires a high resolution and good surface properties to meet the demanding applications. DLP stereolithographic technologies, such as the one described here, allows researchers to obtain such ceramic-based, fully functional components.

<table>
	<tbody>
		<tr>
			<td>Taimicron (TM-100D)</td>
			<td>Taimei Chemicals Co Ltd., Japan</td>
			<td>&#8230;</td>
			<td>alumina (commercial)</td>
		</tr>
		<tr>
			<td>BYK LP C22124</td>
			<td>BYK-Chemie GmbH, Germany&#160;</td>
			<td>&#8230;</td>
			<td>dispersant&#160;</td>
		</tr>
		<tr>
			<td>Mastersizer 2000</td>
			<td>Malvern Instruments Ltd., United Kingdom</td>
			<td>&#8230;</td>
			<td>laser diffractometer</td>
		</tr>
		<tr>
			<td>TriStar 3000</td>
			<td>Micromeritics Instrument Corp., USA</td>
			<td>&#8230;</td>
			<td>adsorption/desorption</td>
		</tr>
		<tr>
			<td>Pulverisette 5/4 classic line</td>
			<td>Fritsch GmbH, Germany</td>
			<td>&#8230;</td>
			<td>planetary ball mill</td>
		</tr>
		<tr>
			<td>Thinky ARV-310</td>
			<td>C3-Prozesstechnik, Germany</td>
			<td>&#8230;</td>
			<td>high-speed planetary ball mill</td>
		</tr>
		<tr>
			<td>Modular Compact Rheometer MCR 302&#160;</td>
			<td>Anton Paar, Graz, Austria</td>
			<td>&#8230;</td>
			<td>rheometer</td>
		</tr>
		<tr>
			<td>UV-LED Smart</td>
			<td>Opsytec Dr. Gr&#246;bel GmbH, Germany</td>
			<td></td>
			<td>blue LED&#160;</td>
		</tr>
		<tr>
			<td>prototype</td>
			<td>Admatec, Netherland</td>
			<td>&#8230;</td>
			<td>Admaflex</td>
		</tr>
		<tr>
			<td>NA120/45</td>
			<td>Nabertherm, Germany</td>
			<td>&#8230;</td>
			<td>debinding furnace</td>
		</tr>
		<tr>
			<td>LH 15/12</td>
			<td>Nabertherm, Germany&#160;</td>
			<td>&#8230;</td>
			<td>sintering furnace</td>
		</tr>
		<tr>
			<td>Gemini 982&#160;</td>
			<td>Zeiss, Germany&#160;</td>
			<td>&#8230;</td>
			<td>FESEM</td>
		</tr>
	</tbody>
</table>

additive manufacturing of functionally graded ceramic materials by stereolithography

An additive manufacturing technology is applied to obtain functionally graded ceramic parts. This technology, based on digital light processing/stereolithography, is developed within the scope of the CerAMfacturing European research project. A three-dimensional (3-D) hemi-maxillary bone-like structure is 3-D printed using custom aluminum oxide polymeric mixtures. The powders and mixtures are fully analyzed in terms of rheological behavior in order to ensure proper material handling during the printing process. The possibility to print functionally graded materials using the Admaflex technology is explained in this document. Field-emission scanning electron microscopy (FESEM) show that the sintered aluminum oxide ceramic part has a porosity lower than 1% and no remainder of the original layered structure is found after analysis.

For the production of single-material components and, eventually, functionally graded structures by means of a combination of dense and porous sections in a macroscopic range, only suspensions based on the alumina have been used.
The measurement result of the average particle diameter (D50) of the used alumina powder after dispersion was 0.47 &#181;m. This result correlates with the given information of an actual particle size of 0.45 to 0.5 &#181;m from the supplier. Figure 1A shows the FESEM analysis of the alumina powder before preparation and Figure 1B a FESEM image of a granulate surface in detail. Figure 1C and Figure 1D show the same for the deagglomerated alumina in a dried state. The untreated powders are not present as single primary particles, but as big spherical granules (with a diameter up to 100 &#181;m), which is a typical condition for dry pressing raw materials. The FESEM images of the granulate surfaces show the primary particles of the alumina untreated (Figure 1B) and deagglomerated (Figure 1D) with an actual particle size of approximately 0.45 &#181;m.
Figure 2 shows the dynamic viscosity of the developed suspensions based on the alumina powder as a function of the shear rate&#8212;logarithmic presentation&#8212;and depending on different compositions concerning varied powder content, binder-crosslinker ratio, and content of the dispersing agent. All suspension compositions show a shear thinning behavior, but different levels of dynamic viscosities.
The suspension homogeneity is shown in Figure 3 with a FESEM image of a thin slice of ceramic-polymeric resin. The ceramic primary particles appear clearly while the polymeric resin is to some extent not detected by the electron detector.
The measurement of the storage modulus G&#180; as a function of time to characterize the curing behavior as depending on time is shown in Figure 4. The adjustable parameter of the printing device helps to evaluate the curing time during printing. Generally, the suspension shows a constant level of G&#180; below 1,000 Pa for a steady deformation. During the exposure of the suspensions, which starts after 60 s, G&#180; increases depending on the exposure time&#8212;varied in a range of 1 to 20 s&#8212;to a higher level of G&#180;, above 105 Pa. Within the diagram, the curves represent different exposure times of a suspension to show the influence on the strength of the cured polymer-ceramic-composite.
The ceramic SLA printing equipment, using the Admaflex technology, can handle high viscosity ceramic slurries thanks to the transport system. The FGM parts can be conceived by a pixel-by-pixel control that directs the irradiated light for each section of the network. The under- and overexposure effects can be compensated for by the same pixel-by-pixel control feature. In addition, this is complemented by a developed software suite identifying the different sections&#8212;porous and dense&#8212;in order to compensate the light behavior differences per exposed area. This proprietary technology provides adapted light-curing strategies to such sections.
By using a suspension with the dynamic viscosity behavior as presented in composition 1 (Figure 2), single-component FGMs with 3-D structures were manufactured after the empirical determination of the device parameters. Figure 5A shows a complex 3-D model and Figure 5B shows the sintered test structure based on the alumina suspensions additively manufactured within the research program.
Figure 6 shows FESEM images of the microstructure of a single-material FGM component within the dense part; the porosity is in a macroscopic range.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57943/57943fig1.jpg" /> 
Figure 1: FESEM Images. The first two panels show field-emission scanning electron microscope images of (A) the original alumina powder and (B) surface detail. The next two panels show field-emission scanning microscope images of (C) the powder particles after deagglomeration and (D) surface detail. <a href="https://www.jove.com/files/ftp_upload/57943/57943fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57943/57943fig2.jpg" /> 
Figure 2: Dynamic viscosity as a function of the shear rate for different developed suspensions as depending on composition. <a href="https://www.jove.com/files/ftp_upload/57943/57943fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57943/57943fig3.jpg" /> 
Figure 3: Field-emission scanning electron microscope image of a ceramic-resin suspension. The figure shows the powder suspension homogeneity on the polymeric resin.
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57943/57943fig4.jpg" /> 
Figure 4: Storage modulus G&#180; as a function of time for several suspensions with different compositions.
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57943/57943fig5.jpg" /> 
Figure 5: 3-D Modeling and Printing. (A) This panel shows a 3-D model of a single-material functionally graded ceramic material component. (B) This panel shows the sintered result of the printing process.
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/57943/57943fig6.jpg" /> 
Figure 6: Field-emission scanning electron microscope images of a sintered alumina structure. (A) This panel shows an overview. (B) This panel shows a detailed image. <a href="https://www.jove.com/files/ftp_upload/57943/57943fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Additive Manufacturing of Functionally Graded Ceramic Materials by Stereolithography at JoVE.com

Additive Manufacturing of Functionally Graded Ceramic Materials by Stereolithography

This manuscript describes the processing of single multifunctional ceramic components (e.g., combinations of dense-porous structures) additively manufactured by stereolithography.

An additive manufacturing technology is applied to obtain functionally graded ceramic parts. This technology, based on digital light ...

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Micro-masonry for 3D Additive Micromanufacturing

Transfer printing is a method to transfer solid micro/nanoscale materials (herein called &#8216;inks&#8217;) from a substrate where they are generated to a different substrate by utilizing elastomeric stamps. Transfer printing enables the integration of heterogeneous materials to fabricate unexampled structures or functional systems that are found in recent advanced devices such as flexible and stretchable solar cells and LED arrays. While transfer printing exhibits unique features in material assembly capability, the use of adhesive layers or the surface modification such as deposition of self-assembled monolayer (SAM) on substrates for enhancing printing processes hinders its wide adaptation in microassembly of microelectromechanical system (MEMS) structures and devices. To overcome this shortcoming, we developed an advanced mode of transfer printing which deterministically assembles individual microscale objects solely through controlling surface contact area without any surface alteration. The absence of an adhesive layer or other modification and the subsequent material bonding processes ensure not only mechanical bonding, but also thermal and electrical connection between assembled materials, which further opens various applications in adaptation in building unusual MEMS devices.

This paper introduces a 3D additive micromanufacturing strategy (termed &#8216;micro-masonry&#8217;) for the flexible fabrication of microelectromechanical system (MEMS) structures and devices. This approach involves transfer printing-based assembly of micro/nanoscale materials in conjunction with rapid thermal annealing-enabled material bonding techniques.

Transfer printing is a method to transfer solid micro/nanoscale materials (herein called &#8216;inks&#8217;) from a substrate where they are generated to ...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Synthesis and Characterization of High c-axis ZnO Thin Film by Plasma Enhanced Chemical Vapor Deposition System and its UV Photodetector Application

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto silicon (Si) substrates via different synthesized temperatures by using plasma enhanced chemical vapor deposition (PECVD) system. The effects of different synthesized temperatures on the crystal structure, surface morphologies and optical properties have been investigated. The X-ray diffraction (XRD) patterns indicated that the intensity of (0002) diffraction peak became stronger with increasing synthesized temperature until 400 oC. The diffraction intensity of (0002) peak gradually became weaker accompanying with appearance of (10-10) diffraction peak as the synthesized temperature up to excess of 400 oC. The RT photoluminescence (PL) spectra exhibited a strong near-band-edge (NBE) emission observed at around 375 nm and a negligible deep-level (DL) emission located at around 575 nm under high c-axis ZnO thin films. Field emission scanning electron microscopy (FE-SEM) images revealed the homogeneous surface and with small grain size distribution. The ZnO thin films have also been synthesized onto glass substrates under the same parameters for measuring the transmittance.
For the purpose of ultraviolet (UV) photodetector application, the interdigitated platinum (Pt) thin film (thickness ~100 nm) fabricated via conventional optical lithography process and radio frequency (RF) magnetron sputtering. In order to reach Ohmic contact, the device was annealed in argon circumstances at 450 oC by rapid thermal annealing (RTA) system for 10 min. After the systematic measurements, the current-voltage (I-V) curve of photo and dark current and time-dependent photocurrent response results exhibited a good responsivity and reliability, indicating that the high c-axis ZnO thin film is a suitable sensing layer for UV photodetector application.

We offered a method to directly synthesize high c-axis (0002) ZnO thin film by plasma enhanced chemical vapor deposition. The as-synthesized ZnO thin film combined with Pt interdigitated electrode was used as sensing layer for ultraviolet photodetector, showing a high performance through a combination of its good responsivity and reliability.

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto ...

Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications of semiconductor materials. However, it is still under debate how the defect structure determines the band structure, and therefore, the recombination behavior of electron-hole pairs responsible for the optical and electrical properties of the extended defects. The present paper is a survey of procedures for the spatially resolved investigation of structural and of physical properties of extended defects in semiconductor materials with a scanning electron microscope (SEM). Representative examples are given for crystalline silicon. The luminescence behavior of extended defects can be investigated by cathodoluminescence (CL) measurements. They are particularly valuable because spectrally and spatially resolved information can be obtained simultaneously. For silicon, with an indirect electronic band structure, CL measurements should be carried out at low temperatures down to 5 K due to the low fraction of radiative recombination processes in comparison to non-radiative transitions at room temperature. For the study of the electrical properties of extended defects, the electron beam induced current (EBIC) technique can be applied. The EBIC image reflects the local distribution of defects due to the increased charge-carrier recombination in their vicinity. The procedure for EBIC investigations is described for measurements at room temperature and at low temperatures. Internal strain fields arising from extended defects can be determined quantitatively by cross-correlation electron backscatter diffraction (ccEBSD). This method is challenging because of the necessary preparation of the sample surface and because of the quality of the diffraction patterns which are recorded during the mapping of the sample. The spatial resolution of the three experimental techniques is compared.

The optical, electrical, and structural properties of dislocations and of grain boundaries in semiconductor materials can be determined by experiments performed in a scanning electron microscope. Electron microscopy has been used to investigate cathodoluminescence, electron beam induced current, and diffraction of backscattered electrons.

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications ...

Digital Printing of Titanium Dioxide for Dye Sensitized Solar Cells

Silicon solar cell manufacturing is an expensive and high energy consuming process. In contrast, dye sensitized solar cell production is less environmentally damaging with lower processing temperatures presenting a viable and low cost alternative to conventional production. This paper further enhances these environmental credentials by evaluating the digital printing and therefore additive production route for these cells. This is achieved here by investigating the formation and performance of a metal oxide photoelectrode using nanoparticle sized titanium dioxide. An ink-jettable material was formulated, characterized and printed with a piezoelectric inkjet head to produce a 2.6 &#181;m thick layer. The resultant printed layer was fabricated into a functioning cell with an active area of 0.25 cm2 and a power conversion efficiency of 3.5%. The binder-free formulation resulted in a reduced processing temperature of 250 &#176;C, compatible with flexible polyamide substrates which are stable up to temperatures of 350 &#730;C. The authors are continuing to develop this process route by investigating inkjet printing of other layers within dye sensitized solar cells.

This paper investigates the suitability of inkjet printing for the manufacturing of dye-sensitized solar cells. A binder-free TiO2 nanoparticle ink was formulated and printed onto a FTO glass substrate. The printed layer was fabricated into a cell with an active area of 0.25 cm2 and an efficiency of 3.5%.

Silicon solar cell manufacturing is an expensive and high energy consuming process. In contrast, dye sensitized solar cell production is less ...

Quantitative Hardness Measurement by Instrumented AFM-indentation

In this work, a combination of amplitude-modulated non-contact atomic force microscopy and atomic force spectroscopy is applied for instrumented hardness measurements on an Au(111) surface with atomistic resolution of single plasticity events. A careful experimental procedure is described that includes the force sensor selection, its calibration, the calibration of the cantilever deflection detection system, and the minimization of instrumental drift for accurate and reproducible force-distance measurements. Also, a method for the data analysis is presented that allows the extraction of force-penetration curves from recorded force-distance curves. A typical curve displays a clear elastic deformation regime up to the first plasticity event, or pop-in, with a length in the range of one to two Burger's vectors. Later plasticity events exhibit the same magnitude. The work of plasticity is further extracted from the measurements. Finally, the hardness is determined in combination with the indentation curve using non-contact atomic force microscopy images of the remaining indents.

This experimental protocol describes how atomic force microscopy can be used to measure hardness at the true nanometer scale and to detect single atomistic plasticity events.

In this work, a combination of amplitude-modulated non-contact atomic force microscopy and atomic force spectroscopy is applied for instrumented hardness ...

Scanning SQUID Study of Vortex Manipulation by Local Contact

Local, deterministic manipulation of individual vortices in type 2 superconductors is challenging. The ability to control the position of individual vortices is necessary in order to study how vortices interact with each other, with the lattice, and with other magnetic objects. Here, we present a protocol for vortex manipulation in thin superconducting films by local contact, without applying current or magnetic field. Vortices are imaged using a scanning superconducting quantum interference device (SQUID), and vertical stress is applied to the sample by pushing the tip of a silicon chip into the sample, using a piezoelectric element. Vortices are moved by tapping the sample or sweeping it with the silicon tip. Our method allows for effective manipulation of individual vortices, without damaging the film or affecting its topography. We demonstrate how vortices were relocated to distances of up to 0.8 mm. The vortices remained stable at their new location up to five days. With this method, we can control vortices and move them to form complex configurations. This technique for vortex manipulation could also be implemented in applications such as vortex based logic devices.

We present a protocol for manipulation of individual vortices in thin superconducting films, using local mechanical contact. The method does not include applying current, magnetic field or additional fabrication steps.

Local, deterministic manipulation of individual vortices in type 2 superconductors is challenging. The ability to control the position of individual ...

Multi-material Ceramic-Based Components &#8211; Additive Manufacturing of Black-and-white Zirconia Components by Thermoplastic 3D-Printing (CerAM - T3DP)

To combine the benefits of Additive Manufacturing (AM) with the benefits of Functionally Graded Materials (FGM) to ceramic-based 4D components (three dimensions for the geometry and one degree of freedom concerning the material properties at each position) the Thermoplastic 3D-Printing (CerAM -&#160;T3DP) was developed. It is a direct AM technology which allows the AM of multi-material components. To demonstrate the advantages of this technology black-and-white zirconia components were additively manufactured and co-sintered defect-free.
Two different pairs of black and white zirconia powders were used to prepare different thermoplastic suspensions. Appropriate dispensing parameters were investigated to manufacture single-material test components and adjusted for the additive manufacturing of multi-color zirconia components.

Here we describe a protocol for additively manufacturing black-and-white zirconia components by Thermoplastic 3D-Printing (CerAM -&#160;T3DP) and co-sintering defect-free.

To combine the benefits of Additive Manufacturing (AM) with the benefits of Functionally Graded Materials (FGM) to ceramic-based 4D components (three ...

Fused Filament Fabrication (FFF) of Metal-Ceramic Components

Technical ceramics are widely used for industrial and research applications, as well as for consumer goods. Today, the demand for complex geometries with diverse customization options and favorable production methods is increasing continuously. With fused filament fabrication (FFF), it is possible to produce large and complex components quickly with high material efficiency. In FFF, a continuous thermoplastic filament is melted in a heated nozzle and deposited below. The computer-controlled print head is moved in order to build up the desired shape layer by layer. Investigations regarding printing of metals or ceramics are increasing more and more in research and industry. This study focuses on additive manufacturing (AM) with a multi-material approach to combine a metal (stainless steel) with a technical ceramic (zirconia: ZrO2). Combining these materials offers a broad variety of applications due to their different electrical and mechanical properties. The paper shows the main issues in preparation of the material and feedstock, device development, and printing of these composites.

Fused Filament Fabrication FFF of Metal-Ceramic Components

This study shows multi-material additive manufacturing (AM) using fused filament fabrication (FFF) of stainless steel and zirconia.

Technical ceramics are widely used for industrial and research applications, as well as for consumer goods. Today, the demand for complex geometries with ...

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in Cu(In,Ga)Se2 Thin-film Solar Cells

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes normally result in very poor cell performance. Embedding or sandwiching silver nanowires using moderately conductive transparent materials, such as indium tin oxide or zinc oxide, can improve cell performance. However, the solution-processed matrix layers can cause a significant number of interfacial defects between transparent electrodes and the CdS buffer, which can eventually result in low cell performance. This manuscript describes how to fabricate robust electrical contact between a silver nanowire electrode and the underlying CdS buffer layer in a Cu(In,Ga)Se2 solar cell, enabling high cell performance using matrix-free silver nanowire transparent electrodes. The matrix-free silver nanowire electrode fabricated by our method proves that the charge-carrier collection capability of silver nanowire electrode-based cells is as good as that of standard cells with sputtered ZnO:Al/i-ZnO as long as the silver nanowires and CdS have high-quality electrical contact. The high-quality electrical contact was achieved by depositing an additional CdS layer as thin as 10 nm onto the silver nanowire surface.

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in CuIn,GaSe2 Thin-film Solar Cells

In this protocol, we describe the detailed experimental procedure for the fabrication of a robust nanoscale contact between a silver nanowire network and CdS buffer layer in a CIGS thin-film solar cell.