Challenges in Rheological Characterization of Highly Concentrated Suspensions &amp;#8212; A Case Study for Screen-printing Silver Pastes

Yüce, Ceren; Willenbacher, Norbert

doi:10.3791/55377

Cited by 20 publications

(32 citation statements)

References 29 publications

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“…The first consisted of the rheological investigations performed at the Institute for Mechanical Engineering and Mechanics at KIT (Karlsruhe Institute of Technology) in Karlsruhe, Germany. A thorough rheological characterization was performed, including steady and oscillatory rotational shear rheometry, capillary viscometry and filament stretching elongational rheometry, as described in [24]. Secondary flow effects, such as wall slip, shear banding and sample spillage, were carefully considered.…”

Section: Experimental Methodsmentioning

confidence: 99%

“…We performed the measurements in a stepwise controlled stress mode. The shear stress was varied from 1 to 5000 Pa, in 41 steps, equally separated on a logarithmic scale, and each stress was applied for 30 s. Yield stress values were determined from the measured deformation using the tangent intersection point method [24,32,33].…”

Section: Rheological Measurementsmentioning

confidence: 99%

“…A rotational rheometer with parallel-plate measuring system (Physica MCR501, Anton Paar Germany GmbH, Ostfildern, Germany) and a customized capillary rheometer (Göttfert Rheograph 2000, Göttfert Werkstoff-Prüfmaschinen GmbH, Buchen, Germany) were used for viscosity measurements to cover paste behavior at low and high shear rates. Choosing a plate roughness of R q = 2-4 µm avoided wall slip effects and/or plug flow in the measuring gap by video imaging of the gap edge described in [24]. The measuring gap height was h = 1 mm and the plate diameter, d plate = 25 mm.…”

Section: Rheological Measurementsmentioning

confidence: 99%

“…Information about structural recovery of model vehicles and silver pastes was obtained from oscillatory time interval tests as described in [16,24]. The measurement protocol comprised three sections with different deformation, but constant frequency f = 1 Hz, to mimic the paste deformation during the screen-printing process.…”

Section: Rheological Measurementsmentioning

confidence: 99%

“…Rheological measurements are limited by wall slip and shear banding phenomena, and sample spill is strongly dependent on the employed experimental set up, and particularly on the surface energy and roughness of the sample fixtures, and the details of the experimental protocol. Recently, we have presented detailed experimental protocols allowing for a meaningful, comprehensive characterization of paste flow properties addressing all of the aspects mentioned above [24]. Silver pastes for solar cell metallization consist of silver particles, which finally provide the conductivity of the printed and sintered structures, as well as glass frits opening the passivation layer during sintering [25].…”

Section: Introductionmentioning

confidence: 99%

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Rheology and Screen-Printing Performance of Model Silver Pastes for Metallization of Si-Solar Cells

2018

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Further strong growth of solar energy conversion based on PV (photovoltaic) technology requires constant improvement to increase solar cell efficiency. The challenge in front-side metallization of Si-solar cells is to print uniform fine lines with a high aspect ratio to achieve higher efficiencies simultaneously with a reduced consumption of raw materials. An in-depth understanding of the relationship between paste composition, rheology and screen-printed line morphology is essential. Three model pastes with similar silver content and corresponding vehicles differing in their thixotropic agent content were investigated. Rheological properties (yield stress, viscosity, wall slip velocity, structural recovery, and fracture strain) were determined using steady and oscillatory shear, as well as elongational flow rheometry. Pastes were screen-printed at various speeds through a layout screen including line widths between 20 and 55 µm. Dried fingers were analyzed with respect to line width, aspect ratio (AR) and cross-sectional area. Our investigations reveal that minor changes of thixotropic agent result in substantial variations of the paste’s flow properties. However, this only weakly affects the line morphology. Irrespective of printing speed or finger opening, AR is slightly increasing; i.e., the screen-printing process is robust against changes in paste rheology.

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Section: Experimental Methodsmentioning

confidence: 99%

Section: Rheological Measurementsmentioning

confidence: 99%

Section: Rheological Measurementsmentioning

confidence: 99%

Section: Rheological Measurementsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Rheology and Screen-Printing Performance of Model Silver Pastes for Metallization of Si-Solar Cells

2018

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Phase‐Change‐Enabled, Rapid, High‐Resolution Direct Ink Writing of Soft Silicone

Wang

Willenbacher

2022

Advanced Materials

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Soft silicone is an ideal flexible material for application, e.g., in soft robotics, flexible electronics, bionics, or implantable biomedical devices. However, gravity‐driven sagging, filament stretching, and deformation can cause inevitable defects during rapid manufacturing, making it hard to obtain complex, high‐resolution 3D silicone structures with direct ink writing (DIW) technology. Here, rapid DIW of soft silicone enabled by a phase‐change‐induced, reversible change of the ink's hierarchical microstructure is presented. During printing, the silicone‐based ink, containing silica nanoparticles and wax microparticles, is extruded from a heated nozzle into a cold environment under controlled stress. The wax phase change (solid–liquid–solid) during printing rapidly destroys and rebuilds the particle networks, realizing fast control of the ink flow behavior and printability. This high‐operating‐temperature DIW method is fast (maximum speed ≈3100 mm min−1) and extends the DIW scale range of soft silicone. The extruded filaments have small diameters (50 ± 5 µm), and allow for large spans (≈13‐fold filament diameter) and high aspect ratios (≈1), setting a new benchmark in the DIW of soft silicone. Printed silicone structures exhibit excellent performance as flexible sensors, superhydrophobic surfaces, and shape‐memory bionic devices, illustrating the potential of the new 3D printing strategy.

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The Link between Ag‐Paste Rheology and Screen‐Printed Solar Cell Metallization

Tepner

Wengenmeyr

Linse

et al. 2020

Adv Materials Technologies

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printing machine technology with the goal of continuously increasing production throughput while improving alignment precision. Additionally, further paste development is conducted to enhance printed line geometry and contact formation. Over the last 15 years, screen printing of Ag-pastes for Si-solar cell metallization has become a success story by decreasing the printed electrode width from ≈120 µm in 2007 reported by Mette [2] toward only 20 µm reported by Tepner et al. in 2020. [3] Therefore, flatbed screen printing is catching up with other fine-line printing approaches for solar cell metallization. Recent studies reported finger widths down to 17 µm by using the parallel dispensing approach [4] and 20 µm by using pattern transfer printing. [5] Both results demonstrated homogenous line widths at competitive printing speeds. These achievements of screen printing have their origin in the continuous efforts of the industry around metallization to improve their technology and products and the scientific community providing an in-depth insight into the underlying physics around that topic. Over the years, the latter provided various research studies, focusing on understanding the impact of paste rheology, [6-10] optimizing the electrical performance of Ag-contacts, [11-14] and the impact of screen design on printing results. [6,15-19] Commonly available Ag-pastes for front-side PERC are highly filled suspensions containing up to 95 wt% spherical Ag-particles with diameters below 5 µm. [2,20] They show non-Newtonian flow characteristics with strong shear thinning behavior after exhibiting significant yield stress. [21,22] A fast and reproducible screen printing process imposes a few crucial requirements on the rheology because the paste is first excessively sheared when being transferred through a fine mesh. [23] Afterward, it is further pushed onto the substrate through the screen opening width w n and the height EOM (emulsion over mesh). During this motion, shearing should already be minimized because the recovery of the zero shear viscosity is highly time-dependent (thixotropic behavior). [6,24] Usually, this minimization of shearing within the screen opening is achieved by the paste's ability to slip at the emulsion surface. This effect has been the focus of several research studies, which showed how slip effects are enhancing screen-printing performance. [25-27] After the paste has been pushed through the screen opening onto the substrate, the Today's photovoltaic production chain is moving into a material crisis as the use of silver for front-side metallization of passivated emitter and rear contact solar cells remains a crucial requirement. The shared effort of the scientific and industrial community to further reduce Ag-consumption as much as possible without compromising cell efficiency has become more challenging in recent years. Further improvements require a deep understanding on the paste-screen interaction at narrow line widths. This study presents the impact of Ag-paste rheology on fine line screen ...

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Challenges in Rheological Characterization of Highly Concentrated Suspensions — A Case Study for Screen-printing Silver Pastes

Cited by 20 publications

References 29 publications

Rheology and Screen-Printing Performance of Model Silver Pastes for Metallization of Si-Solar Cells

Rheology and Screen-Printing Performance of Model Silver Pastes for Metallization of Si-Solar Cells

Phase‐Change‐Enabled, Rapid, High‐Resolution Direct Ink Writing of Soft Silicone

The Link between Ag‐Paste Rheology and Screen‐Printed Solar Cell Metallization

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