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Inkjet printing of Al2O3 dots, lines, and films: From uniform dots to uniform films
Current Applied Physics 11 (2011) S359eS363
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Inkjet printing of Al2O3 dots, lines, and films: From uniform dots to uniform films Yeonjun Oh a, b, Jihoon Kim a, *, Young Joon Yoon a, Hyotae Kim a, Ho Gyu Yoon b, Sung-Nam Lee c, Jonghee Kim a a
Future Convergence Ceramic division, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Republic of Korea Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea c Department of Nano-Optical Engineering, Korea Polytechnic University, Gyeonggi 429-793, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2010 Received in revised form 2 November 2010 Accepted 24 November 2010 Available online 2 December 2010
We have investigated the inkjet-printing of Al2O3 ink on the solid surface. The deposit patterns of nanoparticles from ink droplets were controlled by adding a drying agent to the main ink solvent. This co-solvent ink system creates a circulating flow in the ink droplet during its drying process, resulting in a uniform deposit of nanoparticles from the ink droplet. The study on the spreading of Al2O3 ink droplets on the solid surface was implemented to identify an optimum condition for the inkjet printing of straight lines. With the inkjet-printed straight lines, uniform Al2O3 films were printed by adjusting the line-toline pitches and investigated by SEM and a surface profiler. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Inkjet printing Ceramic printing Al2O3 film Ink-droplet spreading Marangoni flow
1. Introduction Recently, inkjet printing has attracted significant attention because it is possible to selectively deposit various functional materials directly from design files. Inkjet printing is a non-contact process which discards possible contamination, resulting in better performance of printed functional materials. Inkjet printing is also considered as a potential replacement of conventional patterning processes which require high financial investment such as photolithography, etching, and vacuum deposition. So, inkjet printing is known as a non-contact, cost effective and direct additive technique for the formation of fine patterns. Since the inkjet printing deposits materials in the form of ink droplets which consist of nanoparticles in a liquid solvent, a uniform distribution of the nanoparticles over the entire contact area of the ink droplet on a substrate is critical to achieve high quality of the printed materials. Deegan et al. indicated that a ring-like deposit (coffee ring phenomenon) of nanoparticles from the ink droplet occurred as the solvent evaporated predominantly from the pinned contact line of the ink droplets [1]. This non-uniform deposit prevents inkjet printing from being widely applied to various functional devices which require well-defined patterns, uniform
* Corresponding author. E-mail address: [email protected] (J. Kim). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.065
coatings, etc. There were several reports regarding removing the coffee ring phenomenon by introducing a drying agent in the main ink solvent [2e4]. This co-solvent ink system induces both outward and inward flows in sequence within the ink droplet, leading to the circulation of nanoparticles inside the ink droplet during its drying process. This circulation flow results in a uniform deposit of the nanoparticles during the inkjet printing process. With the co-solvent ink system, we studied a direct writing of Al2O3 dots, lines, and films by inkjet printing. Since inkjet-printed films consist of the printed lines which are coalesced from a series of ink droplets, the mechanism of how ink droplets coalesce into a uniform line was examined by looking into the spreading of the ink droplets on a substrate. From this study, an optimal range of dropletto-droplet pitch for the printing of uniform lines was determined for the formation of uniform Al2O3 films by inkjet printing. 2. Experimental Procedure Al2O3 particles with D50 ¼ 200 nm from Denka (ASFP-20) were used in Al2O3 ink formulation. The Al2O3 solid content in the ink was fixed at 4 vol% in the entire experiment. Al2O3 inks were formulated using either single-solvent of water (boiling point: 100 C, surface tension: 72.8 dyn/cm) or a co-solvent of 90 vol% water and 10 vol% drying control agent. After considering the boiling points and surface tension values of many different solvents, N,N-dimethylformamide (DMF; boiling point: 153 C, surface tension: 40.4 dyn/cm) was
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Y. Oh et al. / Current Applied Physics 11 (2011) S359eS363
selected as a drying agent. In order to disperse Al2O3 nanoparticles in the both ink system, they were mixed in the solvent by 48 h ball-mill process. And high speed mixing at 2000 rpm was implemented for 8 min. The formulated Al2O3 inks were filtered through a 6 mm nylon mesh before they were used in order to remove agglomerated Al2O3 particles in the inks. Pt coated Si water of [100] orientation was used as substrates. The wafer was cleaned by acetone and ethanol before the inks were printed. UJ 200 inkjet printing unit (Unijet) was used in the experiment. The inkjet printing unit is equipped with a piezoelectric nozzle with 50 mm orifice from Microfab technology. The ink droplet ejection was monitored by charge-coupled device (CCD) camera. The distance between nozzle and Si substrate was maintained at 1 mm. The volume of ink droplet was fixed at 150 pl throughout the entire experiment. The ink ejection speed was 2.5e3.0 m/s. The morphology and thickness of the inkjet-printed Al2O3 dots, lines, and films were measured by Veeco Dektak 150 surface profiler, CCD camera in the inkjet printing unit, and scanning electron microscope (SEM; TOPCON SM-300). 3. Results and discussion 3.1. Nanoparticle deposit from ink droplets The morphologies of Al2O3 nanoparticles deposited from both single- and co-solvent ink droplets were presented in Fig. 1. Fig. 1(a) shows the deposit of nanoparticles from the single-solvent. Most of Al2O3 particles are accumulated along the perimeter of the dot due to the outward flow resulting from a preferential evaporation of water from the contact line of the ink droplet. In order to compensate the loss of water at the contact line, a radial outward flow is induced from the interior of the ink droplet. The resulting outward flow moves Al2O3 nanoparticles toward the contact line of the droplet. The width and height of Al2O3-accumulated ring are 20 mm and 4 mm, respectively. In order to remove this coffee ring phenomenon, a co-solvent ink system was designed by mixing a drying agent into the main ink solvent. The drying agent needs to have higher boiling point and lower surface tension compared to the main solvent. In this co-solvent ink system, the main solvent with a lower boiling point evaporates preferentially at the contact line and creates an outward flow. However, this outward flow is gradually retarded as the solvent composition at the contact line moves toward a larger fraction of the drying agent. This larger fraction of the drying agent at the contact line simultaneously develops a surface tension gradient across the ink droplet since the center of the ink droplet is still abundant with water which has
a higher surface tension. This surface tension gradient results in an inward flow toward the center of the ink droplet. These outward and inward flows consecutively circulate the nanoparticles inside the ink droplet and lead to a uniform deposit of nanoparticles [5,6]. In this experiment, DMF was mixed with water as a drying agent since it has a higher boiling point and a lower surface tension than water. Since DMF has higher boiling point and lower surface tension, the preferential evaporation of water occurs at the edge of ink droplet and induces a surface tension gradient across the ink droplet. This results in both outward and inward flows in sequence and uniformly distributes Al2O3 nanoparticles over the entire contact area of the ink droplet as illustrated in Fig. 1(b). This inward flow driven by a surface tension gradient is known as Marangoni flow. The magnitude of Marangoni flow can be defined with the Marangoni number, M [5,7]:
M ¼
DgL ; mDAB
(1)
where Dg denotes the surface tension difference between the center and edge of the drop, L is the radius of the ejected ink droplet on the substrate (66 mm), m is the viscosity of the co-solvent (10 vol% DMF þ 90 vol% water), and DAB is the diffusion coefficient in of solvent A (drying agent, DMF) in solvent B (main solvent, water). DAB can be calculated from the following equation [8]:
DAB ¼ 7:4 1012
TðfmB Þ1=2
mB V0:6 A
;
(2)
where T is the temperature (298 K), 4 is the association factor of the main solvent (2.6 for water), mB is the molecular weight of the main solvent (18 g/mol for water), mB is the viscosity of the main solvent (0.89 mPa s) and VA is the molar volume of the drying agent (77.43 cm3/g mol). The calculated DAB for our co-solvent system is 1.25 109 m2/s. The Marangoni number calculated for 10 vol% DMF þ 90 vol% water co-solvent system is 1.81 105. This number falls into the range reported by other literatures for the creation of Marangoni flow [5,7]. It means the magnitude of Marangoni flow in our co-solvent system is sufficient enough to counterbalance the outward flow and lead to a uniform deposit of Al2O3 nanoparticles in the ink droplet. 3.2. Ink droplet deposition mechanism The deposit mechanism of the ink droplets on the solid surface can be categorized into two stages: a gravity-driven stage where
Fig. 1. SEM images and surface profiles of Al2O3 ink droplets: (a) water single-solvent ink and (b) DMF þ water co-solvent ink.
Y. Oh et al. / Current Applied Physics 11 (2011) S359eS363
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Fig. 2. Geometric model of the ink droplet with spherical cap.
the impact kinetic energy is dissipated, and a surface tension driven stage where the ink droplet spreads to a diameter which is determined by the surface energy interaction between the ink and the solid surface. To describe which ink deposition mechanism dominates in Al2O3 ink system, the Bond number of the ceramic ink droplets was calculated by [9]:
B0 ¼
rgD2o ; s
(3)
where r is the ink density, g is the gravity acceleration, Do is the diameter of the ink droplet, and s is the surface tension. The calculated Bond number for Al2O3 co-solvent ink is 2.6 103, indicating the gravity effect is negligible compared to the surface tension in our experiment. It means that the cross-section of the printed Al2O3 ink droplets can be illustrated as the spherical cap with the contact angle of q which defines the angle between the ink surface and the solid substrate (Fig. 1). The expression for the ink spreading on the solid surface can be derived from the geometric parameters of the spherical cap described in Fig. 2. From the fact that the volumes of the initial ink droplet from the ink nozzle and the spherical ink cap on the substrate (Eqs. (4) and (5)) should be identical, the spreading ratio (b ¼ D/Do) can be expressed as in Eq. (6).
Vinitialink droplet ¼
VSphericalcap ¼
b¼
D ¼ Do
p 3
4 Do 3 p ; 2 3
(4) 3
hð3r hÞ ¼
p D 3 2
ð1 cosqÞð2 þ cosEÞ ; sinqð1 þ cosqÞ
4sinqð1 þ cosqÞ ; ð1 cosqÞð2 þ cosqÞ
Fig. 3. The dimensionless width of Al2O3 lines printed at various ink-droplet pitches, compared to the theoretically predicted values obtained from Eq. (7).
ratio can be related to the line formation mechanism. Stringer et al. proposed a model to co-relate the ink spreading ratio to the line printing [9,10]. They use an assumption that the volume of the inkjet-printed line is the same as the total volume of the consecutively dispensed ink droplets which coalesce into the line. With this assumption, a geometrical model was made for the inkjetprinted lines with a circular segment cross-section. From this model, a dimensionless width was defined in terms of Do, q, and ink-droplet pitch as in Eq. (7) where w is the width of the inkjet printed line and p is the pitch between ink droplets.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u w 2pDo u ; ¼ t 2 2 bDo q=sin q cosq=sinq 3pb
(7)
However, this expression is valid only if the printed line has straight and parallel contact lines at both sides of the printed line. When the printed line width is smaller than the diameter of individual ink droplet on the surface (D), the lines start to be rounded with scallop patterns. So, the maximum pitch (Pmax) between ink droplets maintaining the straight lines is determined when the width of the line is equal to D (Eq. (7)). In our Al2O3 ink system, the calculated Pmax was 110.6 mm.
(5)
(6)
Table 1 shows the comparison of the calculated and the experimentally measured ink spreading ratio, indicating there is a reasonable agreement between two spreading ratios. 3.3. Al2O3 line formation from ink droplets Since the line formation in the inkjet printing process is done by merging a series of ink droplets on the surface, the ink spreading Table 1 Rheological properties (density and surface tension) of Al2O3 ink system and its inkdroplet deposition factors (bond number and spreading ratio). Ink system
r
Bo s q b b (Kg/m3) (J/m2) (103) (degree) (calculated) (measured)
4 vol% Al2O3 ink 1116 (10 vol% DMF þ 90 vol% water)
0.018. 2.6
47.1
1.8
2.0
Fig. 4. Examples of the inkjet-printed ceramic lines printed at various ink-droplet pitches: (a) ink-droplet pitch at 50 mm showing the line bulges, (b) ink-droplet pitch at 90 mm showing a straight and uniform line formation, (c) ink-droplet pitch at 120 mm where the printed lines show non-uniform scallop patterns, and (d) ink-droplet pitch at 140 mm leading to the disconnected lines due to the extremely large ink-droplet pitch.
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Y. Oh et al. / Current Applied Physics 11 (2011) S359eS363
Fig. 5. Images of a inkjet-printed Al2O3 ceramic film layer: (a) optical image of Al2O3 films, (b) the surface image of the Al2O3 film by SEM, and (c) SEM cross-sectional image of the printed Al2O3 film.
Pmax ¼
3b
2
2pDo ; q=sin q cosq=sinq
2
(8)
Fig. 3 shows the dimensionless width values of Al2O3 lines printed at various ink-droplet pitches, plotted together with the theoretically prediction from Eq. (7). It indicates that the range of ink-droplet pitch where the straight contact lines are printed is well matched with the theoretical prediction. However, two extreme cases in Fig. 3 at relatively high and low pitches are deviated from the theoretical prediction. The images of Al2O3 lines printed at different pitches are shown in Fig. 4. As the printing pitch between Al2O3 ink droplets became less than 70 mm, bulges were observed along the printed line (Fig. 4(a)). The bulging effect is a commonly observed feature in the inkjet printing process, especially at relatively small ink-droplet pitches [11]. Duineveld gave a thorough consideration of this bulging effect in the printed lines [4]. The rounded lines with scallop patterns as shown in Fig. 4(c) were observed at the ink-droplet pitch wider than 100 mm which is close to the theoretical value of Pmax (110.6 mm) calculated from Eq. (8). 3.4. Inkjet printing of Al2O3 films from printed lines The optimized condition for the formation of straight Al2O3 lines by inkjet printing was discussed from the previous sections. We found that the straight lines were printed at the pitches ranging from 70 mm to 100 mm in the case of Al2O3 co-solvent ink. As the ink droplets coalesced into a straight line, we tried to overlap these straight lines to create uniform Al2O3 films by adjusting the line-toline pitches. It was found that the printing of uniform Al2O3 films was achieved with the line-to-line pitches between 25 mm and 50 mm.
However, the film with 25 mm pitch resulted in a smoother surface morphology than the film with 50 mm pitch. The surface roughness of the Al2O3 film printed with 25 mm pitch was 0.8 mm while the surface roughness of the film with 50 mm pitch was 1.2 mm. The surface roughness was measured by scanning the film across the printing direction. Since the inkjet-printed Al2O3 films did not go through a high temperature sintering process, we infiltrated a polymer resin into the Al2O3 films as a binding material. This resin-infiltration was also implemented by the same inkjet printing. The resin-infiltration process was discussed in detail elsewhere [12]. Fig. 5(a) and (b) show the surface image of the inkjet-printed Al2O3 films by optical and electron microscope, respectively. Cross-sectional image of this film is also shown in Fig. 5(c). The thickness is 12 mm. 4. Conclusions In this paper, inkjet printing of Al2O3 dots, lines, and films was investigated. Co-solvent ink system was designed to create a circulating flow within the ink droplet during its drying process. This results in a uniform deposit of nanoparticles over the inkjet-printed dots. The geometric analysis on the printed lines from a series of ink droplets leads to the prediction of the optimum range of the inkdroplet pitch for the straight line printing. This prediction was well matched with the experimental results. However, non-uniform lines with the patterns of either scallops or bulges occurred when the inkdroplet pitch becomes higher or smaller than the optimum pitch range (70 mme100 mm). With the optimum ink-droplet pitch condition, the inkjet printing of Al2O3 films was implemented by adjusting the line-to-line pitches. With the studies on the printing of uniform dots and lines, uniform Al2O3 films were successfully printed.
Y. Oh et al. / Current Applied Physics 11 (2011) S359eS363
Acknowledgements This research was supported by the core material development program, Ministry of Knowledge Economy, Korea under Grant No. M2007010011. References [1] R.D. Deegan, O. Bakajin, T.F. Dupont, G. Huber, S.R. Nagel, T.A. Witten, Nature 389 (1997) 1997. [2] R.D. Deegan, O. Bakajin, T.F. Dupont, G. Huber, S.R. Nagel, T.A. Witten, Phys. Rev. E 62 (2000) 827.
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[3] E. Tekin, B.J. de Gans, U.S. Schubert, J. Mater. Chem. 14 (2004) 2627. [4] E. Tekin, P.J. Smith, S. Hoeppener, A.M.J. van den Berg, A.S. Susha, A.L. Rogach, J. Feldmann, U.S. Schubert, Adv. Funct. Mater. 17 (2007) 23. [5] J. Park, J. Moon, Langmuir 22 (2006) 2224. [6] H. Hu, R.G. Larson, Langmuir 21 (2005) 3972. [7] E. Chow, J. Herrmann, C.S. Barton, B. Raguse, L. Wieczorek, Anal. Chim. Acta 632 (2009) 135. [8] R.H. Perry, D. Green, Perry’s Chemical Engineers’ Handbook. McGraw-Hill, Singapore, 1984. [9] P.C. Duineveld, J. Fluid Mech. 477 (2003) 175. [10] J. Stringer, B. Derby, J. Eur. Ceram. Soc. 29 (2009) 913. [11] S. Soltman, S. Vivek, Langmuir 24 (2008) 2224. [12] H.W. Jang, J. Kim, H. Kim, Y. Yoon, S. Lee, H. Hwang, J. Kim, Jpn. J. Appl. Phys. 49 (2010) 071501.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Inkjet printing of Al2O3 dots, lines, and films: From uniform dots to uniform films Yeonjun Oh a, b, Jihoon Kim a, *, Young Joon Yoon a, Hyotae Kim a, Ho Gyu Yoon b, Sung-Nam Lee c, Jonghee Kim a a
Future Convergence Ceramic division, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Republic of Korea Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea c Department of Nano-Optical Engineering, Korea Polytechnic University, Gyeonggi 429-793, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2010 Received in revised form 2 November 2010 Accepted 24 November 2010 Available online 2 December 2010
We have investigated the inkjet-printing of Al2O3 ink on the solid surface. The deposit patterns of nanoparticles from ink droplets were controlled by adding a drying agent to the main ink solvent. This co-solvent ink system creates a circulating flow in the ink droplet during its drying process, resulting in a uniform deposit of nanoparticles from the ink droplet. The study on the spreading of Al2O3 ink droplets on the solid surface was implemented to identify an optimum condition for the inkjet printing of straight lines. With the inkjet-printed straight lines, uniform Al2O3 films were printed by adjusting the line-toline pitches and investigated by SEM and a surface profiler. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Inkjet printing Ceramic printing Al2O3 film Ink-droplet spreading Marangoni flow
1. Introduction Recently, inkjet printing has attracted significant attention because it is possible to selectively deposit various functional materials directly from design files. Inkjet printing is a non-contact process which discards possible contamination, resulting in better performance of printed functional materials. Inkjet printing is also considered as a potential replacement of conventional patterning processes which require high financial investment such as photolithography, etching, and vacuum deposition. So, inkjet printing is known as a non-contact, cost effective and direct additive technique for the formation of fine patterns. Since the inkjet printing deposits materials in the form of ink droplets which consist of nanoparticles in a liquid solvent, a uniform distribution of the nanoparticles over the entire contact area of the ink droplet on a substrate is critical to achieve high quality of the printed materials. Deegan et al. indicated that a ring-like deposit (coffee ring phenomenon) of nanoparticles from the ink droplet occurred as the solvent evaporated predominantly from the pinned contact line of the ink droplets [1]. This non-uniform deposit prevents inkjet printing from being widely applied to various functional devices which require well-defined patterns, uniform
* Corresponding author. E-mail address: [email protected] (J. Kim). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.065
coatings, etc. There were several reports regarding removing the coffee ring phenomenon by introducing a drying agent in the main ink solvent [2e4]. This co-solvent ink system induces both outward and inward flows in sequence within the ink droplet, leading to the circulation of nanoparticles inside the ink droplet during its drying process. This circulation flow results in a uniform deposit of the nanoparticles during the inkjet printing process. With the co-solvent ink system, we studied a direct writing of Al2O3 dots, lines, and films by inkjet printing. Since inkjet-printed films consist of the printed lines which are coalesced from a series of ink droplets, the mechanism of how ink droplets coalesce into a uniform line was examined by looking into the spreading of the ink droplets on a substrate. From this study, an optimal range of dropletto-droplet pitch for the printing of uniform lines was determined for the formation of uniform Al2O3 films by inkjet printing. 2. Experimental Procedure Al2O3 particles with D50 ¼ 200 nm from Denka (ASFP-20) were used in Al2O3 ink formulation. The Al2O3 solid content in the ink was fixed at 4 vol% in the entire experiment. Al2O3 inks were formulated using either single-solvent of water (boiling point: 100 C, surface tension: 72.8 dyn/cm) or a co-solvent of 90 vol% water and 10 vol% drying control agent. After considering the boiling points and surface tension values of many different solvents, N,N-dimethylformamide (DMF; boiling point: 153 C, surface tension: 40.4 dyn/cm) was
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Y. Oh et al. / Current Applied Physics 11 (2011) S359eS363
selected as a drying agent. In order to disperse Al2O3 nanoparticles in the both ink system, they were mixed in the solvent by 48 h ball-mill process. And high speed mixing at 2000 rpm was implemented for 8 min. The formulated Al2O3 inks were filtered through a 6 mm nylon mesh before they were used in order to remove agglomerated Al2O3 particles in the inks. Pt coated Si water of [100] orientation was used as substrates. The wafer was cleaned by acetone and ethanol before the inks were printed. UJ 200 inkjet printing unit (Unijet) was used in the experiment. The inkjet printing unit is equipped with a piezoelectric nozzle with 50 mm orifice from Microfab technology. The ink droplet ejection was monitored by charge-coupled device (CCD) camera. The distance between nozzle and Si substrate was maintained at 1 mm. The volume of ink droplet was fixed at 150 pl throughout the entire experiment. The ink ejection speed was 2.5e3.0 m/s. The morphology and thickness of the inkjet-printed Al2O3 dots, lines, and films were measured by Veeco Dektak 150 surface profiler, CCD camera in the inkjet printing unit, and scanning electron microscope (SEM; TOPCON SM-300). 3. Results and discussion 3.1. Nanoparticle deposit from ink droplets The morphologies of Al2O3 nanoparticles deposited from both single- and co-solvent ink droplets were presented in Fig. 1. Fig. 1(a) shows the deposit of nanoparticles from the single-solvent. Most of Al2O3 particles are accumulated along the perimeter of the dot due to the outward flow resulting from a preferential evaporation of water from the contact line of the ink droplet. In order to compensate the loss of water at the contact line, a radial outward flow is induced from the interior of the ink droplet. The resulting outward flow moves Al2O3 nanoparticles toward the contact line of the droplet. The width and height of Al2O3-accumulated ring are 20 mm and 4 mm, respectively. In order to remove this coffee ring phenomenon, a co-solvent ink system was designed by mixing a drying agent into the main ink solvent. The drying agent needs to have higher boiling point and lower surface tension compared to the main solvent. In this co-solvent ink system, the main solvent with a lower boiling point evaporates preferentially at the contact line and creates an outward flow. However, this outward flow is gradually retarded as the solvent composition at the contact line moves toward a larger fraction of the drying agent. This larger fraction of the drying agent at the contact line simultaneously develops a surface tension gradient across the ink droplet since the center of the ink droplet is still abundant with water which has
a higher surface tension. This surface tension gradient results in an inward flow toward the center of the ink droplet. These outward and inward flows consecutively circulate the nanoparticles inside the ink droplet and lead to a uniform deposit of nanoparticles [5,6]. In this experiment, DMF was mixed with water as a drying agent since it has a higher boiling point and a lower surface tension than water. Since DMF has higher boiling point and lower surface tension, the preferential evaporation of water occurs at the edge of ink droplet and induces a surface tension gradient across the ink droplet. This results in both outward and inward flows in sequence and uniformly distributes Al2O3 nanoparticles over the entire contact area of the ink droplet as illustrated in Fig. 1(b). This inward flow driven by a surface tension gradient is known as Marangoni flow. The magnitude of Marangoni flow can be defined with the Marangoni number, M [5,7]:
M ¼
DgL ; mDAB
(1)
where Dg denotes the surface tension difference between the center and edge of the drop, L is the radius of the ejected ink droplet on the substrate (66 mm), m is the viscosity of the co-solvent (10 vol% DMF þ 90 vol% water), and DAB is the diffusion coefficient in of solvent A (drying agent, DMF) in solvent B (main solvent, water). DAB can be calculated from the following equation [8]:
DAB ¼ 7:4 1012
TðfmB Þ1=2
mB V0:6 A
;
(2)
where T is the temperature (298 K), 4 is the association factor of the main solvent (2.6 for water), mB is the molecular weight of the main solvent (18 g/mol for water), mB is the viscosity of the main solvent (0.89 mPa s) and VA is the molar volume of the drying agent (77.43 cm3/g mol). The calculated DAB for our co-solvent system is 1.25 109 m2/s. The Marangoni number calculated for 10 vol% DMF þ 90 vol% water co-solvent system is 1.81 105. This number falls into the range reported by other literatures for the creation of Marangoni flow [5,7]. It means the magnitude of Marangoni flow in our co-solvent system is sufficient enough to counterbalance the outward flow and lead to a uniform deposit of Al2O3 nanoparticles in the ink droplet. 3.2. Ink droplet deposition mechanism The deposit mechanism of the ink droplets on the solid surface can be categorized into two stages: a gravity-driven stage where
Fig. 1. SEM images and surface profiles of Al2O3 ink droplets: (a) water single-solvent ink and (b) DMF þ water co-solvent ink.
Y. Oh et al. / Current Applied Physics 11 (2011) S359eS363
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Fig. 2. Geometric model of the ink droplet with spherical cap.
the impact kinetic energy is dissipated, and a surface tension driven stage where the ink droplet spreads to a diameter which is determined by the surface energy interaction between the ink and the solid surface. To describe which ink deposition mechanism dominates in Al2O3 ink system, the Bond number of the ceramic ink droplets was calculated by [9]:
B0 ¼
rgD2o ; s
(3)
where r is the ink density, g is the gravity acceleration, Do is the diameter of the ink droplet, and s is the surface tension. The calculated Bond number for Al2O3 co-solvent ink is 2.6 103, indicating the gravity effect is negligible compared to the surface tension in our experiment. It means that the cross-section of the printed Al2O3 ink droplets can be illustrated as the spherical cap with the contact angle of q which defines the angle between the ink surface and the solid substrate (Fig. 1). The expression for the ink spreading on the solid surface can be derived from the geometric parameters of the spherical cap described in Fig. 2. From the fact that the volumes of the initial ink droplet from the ink nozzle and the spherical ink cap on the substrate (Eqs. (4) and (5)) should be identical, the spreading ratio (b ¼ D/Do) can be expressed as in Eq. (6).
Vinitialink droplet ¼
VSphericalcap ¼
b¼
D ¼ Do
p 3
4 Do 3 p ; 2 3
(4) 3
hð3r hÞ ¼
p D 3 2
ð1 cosqÞð2 þ cosEÞ ; sinqð1 þ cosqÞ
4sinqð1 þ cosqÞ ; ð1 cosqÞð2 þ cosqÞ
Fig. 3. The dimensionless width of Al2O3 lines printed at various ink-droplet pitches, compared to the theoretically predicted values obtained from Eq. (7).
ratio can be related to the line formation mechanism. Stringer et al. proposed a model to co-relate the ink spreading ratio to the line printing [9,10]. They use an assumption that the volume of the inkjet-printed line is the same as the total volume of the consecutively dispensed ink droplets which coalesce into the line. With this assumption, a geometrical model was made for the inkjetprinted lines with a circular segment cross-section. From this model, a dimensionless width was defined in terms of Do, q, and ink-droplet pitch as in Eq. (7) where w is the width of the inkjet printed line and p is the pitch between ink droplets.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u w 2pDo u ; ¼ t 2 2 bDo q=sin q cosq=sinq 3pb
(7)
However, this expression is valid only if the printed line has straight and parallel contact lines at both sides of the printed line. When the printed line width is smaller than the diameter of individual ink droplet on the surface (D), the lines start to be rounded with scallop patterns. So, the maximum pitch (Pmax) between ink droplets maintaining the straight lines is determined when the width of the line is equal to D (Eq. (7)). In our Al2O3 ink system, the calculated Pmax was 110.6 mm.
(5)
(6)
Table 1 shows the comparison of the calculated and the experimentally measured ink spreading ratio, indicating there is a reasonable agreement between two spreading ratios. 3.3. Al2O3 line formation from ink droplets Since the line formation in the inkjet printing process is done by merging a series of ink droplets on the surface, the ink spreading Table 1 Rheological properties (density and surface tension) of Al2O3 ink system and its inkdroplet deposition factors (bond number and spreading ratio). Ink system
r
Bo s q b b (Kg/m3) (J/m2) (103) (degree) (calculated) (measured)
4 vol% Al2O3 ink 1116 (10 vol% DMF þ 90 vol% water)
0.018. 2.6
47.1
1.8
2.0
Fig. 4. Examples of the inkjet-printed ceramic lines printed at various ink-droplet pitches: (a) ink-droplet pitch at 50 mm showing the line bulges, (b) ink-droplet pitch at 90 mm showing a straight and uniform line formation, (c) ink-droplet pitch at 120 mm where the printed lines show non-uniform scallop patterns, and (d) ink-droplet pitch at 140 mm leading to the disconnected lines due to the extremely large ink-droplet pitch.
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Fig. 5. Images of a inkjet-printed Al2O3 ceramic film layer: (a) optical image of Al2O3 films, (b) the surface image of the Al2O3 film by SEM, and (c) SEM cross-sectional image of the printed Al2O3 film.
Pmax ¼
3b
2
2pDo ; q=sin q cosq=sinq
2
(8)
Fig. 3 shows the dimensionless width values of Al2O3 lines printed at various ink-droplet pitches, plotted together with the theoretically prediction from Eq. (7). It indicates that the range of ink-droplet pitch where the straight contact lines are printed is well matched with the theoretical prediction. However, two extreme cases in Fig. 3 at relatively high and low pitches are deviated from the theoretical prediction. The images of Al2O3 lines printed at different pitches are shown in Fig. 4. As the printing pitch between Al2O3 ink droplets became less than 70 mm, bulges were observed along the printed line (Fig. 4(a)). The bulging effect is a commonly observed feature in the inkjet printing process, especially at relatively small ink-droplet pitches [11]. Duineveld gave a thorough consideration of this bulging effect in the printed lines [4]. The rounded lines with scallop patterns as shown in Fig. 4(c) were observed at the ink-droplet pitch wider than 100 mm which is close to the theoretical value of Pmax (110.6 mm) calculated from Eq. (8). 3.4. Inkjet printing of Al2O3 films from printed lines The optimized condition for the formation of straight Al2O3 lines by inkjet printing was discussed from the previous sections. We found that the straight lines were printed at the pitches ranging from 70 mm to 100 mm in the case of Al2O3 co-solvent ink. As the ink droplets coalesced into a straight line, we tried to overlap these straight lines to create uniform Al2O3 films by adjusting the line-toline pitches. It was found that the printing of uniform Al2O3 films was achieved with the line-to-line pitches between 25 mm and 50 mm.
However, the film with 25 mm pitch resulted in a smoother surface morphology than the film with 50 mm pitch. The surface roughness of the Al2O3 film printed with 25 mm pitch was 0.8 mm while the surface roughness of the film with 50 mm pitch was 1.2 mm. The surface roughness was measured by scanning the film across the printing direction. Since the inkjet-printed Al2O3 films did not go through a high temperature sintering process, we infiltrated a polymer resin into the Al2O3 films as a binding material. This resin-infiltration was also implemented by the same inkjet printing. The resin-infiltration process was discussed in detail elsewhere [12]. Fig. 5(a) and (b) show the surface image of the inkjet-printed Al2O3 films by optical and electron microscope, respectively. Cross-sectional image of this film is also shown in Fig. 5(c). The thickness is 12 mm. 4. Conclusions In this paper, inkjet printing of Al2O3 dots, lines, and films was investigated. Co-solvent ink system was designed to create a circulating flow within the ink droplet during its drying process. This results in a uniform deposit of nanoparticles over the inkjet-printed dots. The geometric analysis on the printed lines from a series of ink droplets leads to the prediction of the optimum range of the inkdroplet pitch for the straight line printing. This prediction was well matched with the experimental results. However, non-uniform lines with the patterns of either scallops or bulges occurred when the inkdroplet pitch becomes higher or smaller than the optimum pitch range (70 mme100 mm). With the optimum ink-droplet pitch condition, the inkjet printing of Al2O3 films was implemented by adjusting the line-to-line pitches. With the studies on the printing of uniform dots and lines, uniform Al2O3 films were successfully printed.
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Acknowledgements This research was supported by the core material development program, Ministry of Knowledge Economy, Korea under Grant No. M2007010011. References [1] R.D. Deegan, O. Bakajin, T.F. Dupont, G. Huber, S.R. Nagel, T.A. Witten, Nature 389 (1997) 1997. [2] R.D. Deegan, O. Bakajin, T.F. Dupont, G. Huber, S.R. Nagel, T.A. Witten, Phys. Rev. E 62 (2000) 827.
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