Formulation of SolâGel Derived Bismuth Silicate Dielectric Ink for

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Formulation of sol-gel derived bismuth silicate dielectric ink for flexible electronics applications Abhilash Pullanchiyodan, and Kuzhichalil Peethambharan Surendran Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00871 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 22, 2016

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Industrial & Engineering Chemistry Research

Formulation of sol-gel derived bismuth silicate dielectric ink for flexible electronics applications Abhilash Pullanchiyodan 1, 2 and Kuzhichalil P. Surendran1*, 2 1

Materials Science and Technology Division,

National Institute for Interdisciplinary Science and Technology (NIIST- CSIR) Thiruvananthapuram, India – 695019 2

∗

Academy of Scientific and Innovative Research (AcSIR), New Delhi, India

Corresponding author: Tel. +91 471 2515258, Fax +91 471 2491712,

Email: [email protected]

Abstract An agile formulation for the development of a low dielectric constant ink of nanostructured bismuth silicate (nBSO), synthesized by sol-gel method, was demonstrated. The viscous screen printable ink was formulated by controlling the organic additives. The viscosity, as well as dynamic viscoelastic properties of the formulated ink, was suitably tuned, to yield ideal flow characteristics. Microstructure and surface roughness of the printed film were characterized, and the film shows a minimum surface roughness values (Ra = 80.3 nm and Rq = 102 nm). Microwave dielectric properties of the printed dielectric film were ɛr = 4.36 and tanδ = 1.6×10-2 at 10 GHz. The apparent leakage current density of the film is of the order of 10-5 A/cm2. The improved dielectric and thermal properties, combined with low leakage current makes nBSO dielectric ink a suitable candidate for printed electronics.

Keywords: dielectric ink; sol-gel synthesis; microwave dielectric properties; rheology; screen printing

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Introduction Printed electronics serve as one of the most versatile emerging technologies which contain low cost and high volume for many products that we depend on everyday life. One of the added advantage of printed technology compared to the photolithography driven conventional electronics is its rather simplicity in the fabrication1. Within short time since the last decade, printed electronics has made rapid strides both in low and high performance domains like flexible displays, smart labels, decorative paints2, antennas3, RFIDs3, fuel cells4, sensors5,6, solar cells7, thin film transistors (TFT)8,9, multilayer ceramic capacitors (MLCC)10 etc. Different techniques such as screen printing, ink jet printing and gravure printing are commonly adopted for imprinting conducting, semiconducting and dielectric ink onto various substrates11. Screen printing technique, formerly known as serigraphy is a stenciling process, and it was widely used in textiles and printing cards in early days. Later, the process proliferated slowly into niche areas like electronics. Currently it is the most widely used technique for fabricating ceramic thick films in printed electronics.11,12,13 In screen printing process, viscous ink is squeezed through a finely woven mesh having the desired geometry, on to a substrate, by applying force with a rubber squeegee. A typical ink consists of a solid filler (> 60%), vehicle (typically, organic solvents), dispersant and binder in ideal proportion to enable uniform printing and better adhesion. In screen printing inks, drying or curing is usually achieved with the help of ultraviolet (UV), infrared (IR) rays or local heating. In UV curable inks, the vehicle is composed of a mixture of fluid oligomers, monomers, and initiators that, when exposed to UV light release free radicals that cause the polymerization of the vehicle, which hardens to a dry ink film. In the case of IR assisted curing, the energy emitted by IR radiation is directly used for heating the wet ink film. Nevertheless, a vast majority commercial functional inks avoid either of these radiations assisted curing, but employs local heating with the help of suitably designed flash driers and ovens. But, the oven drying of the ink above 100 oC11 preclude the use of a number of flexible substrates including paper. Therefore, the drying efficiency of the screen printing ink is one of the key requisites in these days in order to simplify the fully automatic printing techniques. A lot of research is going on this domain, aimed to develop room temperature evaporating screen printing inks to use in high performance applications.11,13 These room temperature evaporation drying rely primarily on


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the vehicle, where a tricky combination of organic solvents evaporates soon after the printing. However, the binder left behind in the printed film, so as to achieve maximum rub-off resistance. Developing such a fast curing ink has always been a challenge since the drying process should be initiated soon after ink is delivered to the substrates and not before the passage of ink through screen grids that can result in clogging. The development of conducting and semiconducting inks have seen an appreciable improvement in the last several years14. A few industries like Creative Materials Inc. (USA), Henkel AG & Co (Germany) etc. markets low κ dielectric insulating inks. At the same time, there are only a few reports available in literature on dielectric inks15. Wu et al. reported the development of nanocomposite dielectric ink for TFT application in 20148. Recently, Jobin et al. reported the development of silica and zircon based inks for printed electronic applications11,13. High κ dielectric ink has a critical role in the large area electronics since it is a vital component in TFTs8. The other major areas where high κ inks have a potential role include dye-sensitized solar cells16, field effect transistors17, piezo and ferroelectric devices18,19,20 and in SOFCs21. The low κ ink on the other hand, is used as insulation in flex circuits and thick films, for improving the moisture, arc track and weathering resistance of electrical/electronic devices. Since the cumbersome arrangements needed for curing can be eliminated, the development of a dielectric ink free of thermal and UV/IR curing receives a wider attention. Additionally, room temperature curing enables one to employ a galaxy of substrates. Bismuth silicate is a material which finds applications in many sectors due to its excellent dielectric, thermoluminescent and photocatalytic properties22,23,24. The microwave dielectric properties of bismuth silicate and suitability for low temperature co-fired ceramics (LTCC) applications were explored recently25,26 and the results indicate that this material has promising microwave dielectric properties for LTCC applications. Bismuth silicate is also successfully used in ferroelectric materials to tune the P-E characteristic by altering the crystal structure and bond distance27,28. However there were no efforts so far, to explore the possibility of this material for low κ screen printable electronic applications. In this paper, we discuss the sol gel synthesis of nano bismuth silicate, Bi4(SiO4)3 (abbreviated as nBSO hereafter), and the subsequent formulation of a fast curable low κ ink employing sol gel 3 ACS Paragon Plus Environment


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derived nBSO. The novel ink has low dielectric constant, dielectric loss and leakage currents, with enhanced rub-off resistance that are suitable for screen printed flexible electronic applications. Experimental Section Preparation of nBSO through sol-gel synthesis Bismuth (III) nitrate pentahydrate, Bi(NO3)3.5H2O, 99% (Merck, Mumbai, India) and tetraethyl orthosilicate, TEOS, 98% (Sigma-Aldrich, St. Louis, MO, USA) were used as the precursors for nBSO preparation. Initially, Bi(NO3)3.5H2O was dissolved in acetic acid under continuous stirring. After that, stoichiometric amount of TEOS was added drop by drop to the solution and continued the stirring for 1h. The obtained gel was dried in a hot air oven at 70 oC overnight. The dried gel was crushed, ground to powder, and then calcined at 850 oC for 4h to complete the phase formation. It was further ground to fine powder using a high energy planetary ball mill (FRITSCH, Pulverisette 5 classic line, Germany) in water with tungsten carbide as the grinding media. The TG-DTA analysis of the dried gel was carried out from ambient temperature to 900 o

C in air atmosphere using a thermogravimetric analyzer (Shimadzu TGA/DTA Instrument,

Japan). The phase purity and crystal structure of the calcined nBSO ceramic were studied using XRD (Cu Kα radiation, PANalytical X’Pert PRO diffractometer, Netherlands) analysis. The FTIR analysis of calcined nBSO powder was carried out using an FTIR spectrometer (FTIR spectrum II, Perkin Elmer, USA) in the range 400-4000 cm-1. The morphology and particle size of the nBSO powder was more systematically analyzed using HRTEM (FEI Tecnai G2 30STWIN, FEI Company, Hillsboro, OR). For this purpose, nBSO powder was well dispersed in acetone using ultrasonication and then drop casted onto a carbon coated copper grid and dried. The BET surface area of the calcined powder was measured by nitrogen adsorption using surface area analyzer (Gemini 2375, Micromeritics, Norcross, USA). Formulation and screen printing of nBSO ink A dielectric ink essentially comprises of ceramic filler, solvents, dispersant and binder. In the present case an equal volume of xylenes (Sigma-Aldrich, St. Louis, MO, USA) and anhydrous ethanol (Merck, Mumbai, India) were used as the solvent. In the first step, the dispersant (2 wt. % with respect to filler) - Triton X-100 (TCI, Tokyo, Japan) was dissolved in the solvent mixture 4 ACS Paragon Plus Environment

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using an ultrasonic bath for 10 minutes. At this stage, the filler, nBSO powder (60 wt. % with respect to solvent) was added to the mixture and stirred continuously for 6h using a magnetic stirrer. The binder (5 wt. % with respect to filler); ethyl cellulose (Sigma-Aldrich, St. Louis, MO, USA) was then added to it and continued the stirring for another 24h to get a homogeneous viscous ink. The viscosity of the ink was measured using a rheometer (Rheo plus32, Anton paar, USA) operating in rotational and oscillation methods. The variation of viscosity with the shear rate from 0 to 100 S-1 was measured in rotational mode where the storage modulus (G’) and shear modulus (G”) were measured in oscillation mode. The as prepared colloidal ink was screen printed on a clean Mylar® film using a semi-automatic screen printer XPRT2 (EKRA, Germany). The screen used for the present study was a homemade one. A 350 count mesh was used in the screen preparation. The screens are cleaned thoroughly in order to remove any dust or ink residue trapped in the mesh. The desired shape for printing was fabricated on the mesh by conventional photo resistive technique13. The adhesion strength of the printed ink on the substrate was carried out by scotch test analysis based on modified ASTM standard D3359-09. The surface homogeneity and quality of the printed film were characterized using optical microscopy (Leica, MRDX, Germany). The surface roughness of the printed pattern was analyzed using Atomic Force Microscopy, AFM (Bruker Multimode, Germany). The analysis was carried out in different areas of 10 µm×10 µm and 4 µm×4 µm respectively operating in tapping mode. The microstructural analysis of printed film was characterized by Scanning Electron Microscopic technique, SEM, (JEOL JSM 5600LV, Tokyo, Japan). The dielectric properties in the low frequency region (300 Hz-3 MHz) were studied using an LCR meter (Hioki model, 3532-50; Nagano, Japan). The microwave dielectric properties of the printed film were measured using split post dielectric resonator, SPDR (QWED, Warsaw, Poland) technique at different frequencies (5 GHz, 10 GHz and 15 GHz) with the aid of ENA series vector network analyzer (E5071C, Agilent Technologies, Santa Clara, CA). The I-V characteristic of the printed film was measured using a source meter (Keithley 2410, 1100V source meter). For this measurement the prepared ink was printed on a thin copper film and Au-metal was sputtered on the top of it as the front electrode.



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Results and Discussions In order to elucidate an exact idea of the calcination temperature and the different phase transition, the TG-DTA analysis of the prepared wet gel were carried out. Figure 1(a) shows the TG-DTA curve of the gel up to 900 oC in air atmosphere.

Figure 1: (a) TG-DTA analysis of the nBSO gel and (b) FTIR spectra of nBSO calcined at 850 o

C for 4h

In the TGA graph, it is clearly indicating four distinct stages of weight losses. In the first stage, the weight loss starts from 40 oC and show sharp decrease up to 110 oC, and the total weight loss is around 6%. This may be due to the removal of planar water and ethyl alcohol. In the DTA curve, corresponding sharp exothermic peak is observed around 54 oC. In the second stage, there is slow and steady weight loss of almost 3% occurs up to 240 oC. This is believed to be due to the dehydration of Bi(OH)3 to BiOOH, and representative endotherm is clearly visible around 236 oC in DTA curve. The sharp weight loss was observed after 240 oC to around 312 oC, and it indicates the decomposition of Si(OH)4. The thermogravimetric weight loss continued to 530 oC and there after no considerable weight loss is observed. The loss in this region implies the decomposition of the residual bismuth nitrate present in the system. From the entire TGA process we could observe a total of 24% weight loss from room temperature to 530 oC. The typical exotherms at 286.6 oC, 466.8 oC, 514.9 oC and 637.1 oC for the decomposition of Si(OH)4, Bi2O3 phase transition, decomposition of Bi(NO3)3, and the formation of Bi2SiO5 meta stable phase respectively are also evident in DTA curve. The sharp endotherm at 702 oC corresponds to the crystallization of Bi12SiO20 and a small endotherm around 860 oC indicating the formation of Bi4(SiO4)3.29,25,30. Based on the TG-DTA results, the prepared nBSO powder 6 ACS Paragon Plus Environment

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was calcined at 850 oC/4h. The FTIR spectroscopic technique can be used as a powerful tool for structural studies of colloidal compounds, to identify their constituent chemical species. Figure 1 (b) depicts the room temperature FTIR spectrum of the calcined nBSO powder. The two sharp bands at ̴ 482 cm-1 and ̴ 540 cm-1 in the spectra indicate the characteristic bending mode of Bi-O bond in the [BiO6] octahedral. The strong band centered at ̴ 887 cm-1 is assigned to the stretching vibration of Bi-O bond in [BiO6] octahedral unit. A weak diffuse band centered at ̴ 1100 cm-1 is the characteristic of Si-O-Si unsymmetrical stretching mode in [SiO4] tetrahedral unit31,32,33. These results indicate that the basic structural unit in the nBSO comprises of [BiO6]9- octahedral and [SiO4]4- tetrahedral groups, which is in concurrence with earlier observations34.

Figure 2: (a) XRD pattern of nBSO and (b) W-H plot of nBSO calcined at 850 oC for 4h The XRD pattern of the calcined nBSO powder was shown in figure 2(a). All the peaks in the XRD pattern were indexed using standard JCPDS file 35-1007. The result indicates that the phase formation nBSO was complete at 850 oC. The crystal structure belongs to I43d space group having cubic symmetry. The lattice strain and stress is believed to have a great influence on the physical properties of nanomaterial. The XRD profile analysis is one of the well-executed techniques in estimating the micro strain. From the available models, the Williamson-Hall (W-H) analysis is the most simplified and extensively studied technique35for analyzing the crystallite size and lattice strain. The instrumental broadening effect was rectified using silicon as the standard. W-H analysis is an integrated method, in which the crystallite size and strain induced line broadening of the XRD profile were calculated from the peak width using the following equations.35 7 ACS Paragon Plus Environment


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=

+ 4

(where D is the crystallite size, K is the shape factor (̴ 1), β is the peak width (FWHM), λ is the wave length of Cu Kα radiation used and ε is the strain in the crystal). From the plot of βcosθ/λ against 4sinθ/λ, we are able to calculate the strain as well as the size of a crystallite. In principle the slope of the graph gives the strain whereas the reciprocal of the intercept should be the crystallite size36. Figure 2(b) represents the W-H plot of nBSO calcined powder. A negative strain of around -2.64498×10-4 is obvious from the curve. This implies that the crystal undergo compressive strain in its lattice that may be mainly due to the lattice shrinkage37,38. The crystallite size calculated from the intercept is around 67 nm. The average crystallite size was also calculated from Debye-Scherrer’s formula ( = 0.9 /) by considering the five highest intensity peaks having (hkl) value (211), (310), (321), (422) and (431) respectively, and obtained an average value of 69 nm. The particle size, as we discussed, has a crucial role in the ink formulation including rheology and microstructure of the printed film. For example, smaller particle size can improve the viscosity as well as the colloidal stability of the ink. However in screen printing it is not smaller particles, but uniform particle size distribution is more advantageous to yield homogeneous microstructure of the printed film. In order to homogenize the particle size of the nBSO powder and deagglomerate them, the as prepared powder was ball milled for 4h. The actual nano crystalline nature of the nBSO particles could be tested using TEM analysis. Figure 3(a) represents the morphology and particle size of the calcined nBSO powder after ball milling for 4h. It is clear from the figures that nBSO has an average particle size of around 70-80 nm. From figure 3(b) the lattice fringes of the crystal are clearly visible, which attests a highly crystalline nature of the nanoparticles. The d- spacing value of the two adjacent fringes in this crystal lattice is measured to be around 2.72 Å. The result indicate that these fringes are of (321) plane, having a lattice constant of 10.18 Å. The FFT pattern (figure 3(c)) specifies the reflections from two crystalline planes. The d-spacing values of these two reflections were measured and results showed that these reflections were originated respectively from [310] and [321] planes of the crystal.


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Figure 3 (a) and (b) The HR-TEM image and (c) FFT pattern of nBSO calcined at 850 oC for 4h Ideally, the functional inks require a great control over their rheological properties, in order to facilitate smooth printing and efficient functioning of the end devices. It could be achieved only by the judicious selection of different additives and precise control of their physical and chemical properties. The quantity and quality of organic additives such as solvent, dispersant, binder etc. are to be chosen, so as to control the rheology of the ink within printable range.

Figure 4: (a) Variation of viscosity and shear stress with shear rate (b) dynamic viscoelastic properties and (c) stress-strain curve of nBSO ink The rheological analysis of the formulated ink is given figure 4. From figure 4(a), one can observe that the viscosity of ink decreases with increase in shear rate. Such a shear thinning



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behavior is essential for screen printing ink. This is because, when the shear rate increases, the ink become thinner, causing it to flow freely. One of the crucial step in screen printing process is the pushing of the ink through the mesh opening with a squeegee and subsequently registering the pattern over the carrier substrate with good leveling39. The shear stress curve also shows the typical nature of the pseudoplasticity, which help to prevent the ‘bleeding’ i.e., the tendency of ink to flow beyond the printed boundaries after printing. In fact, it was the organic binders and dispersants that play a crucial role to convert the viscosity of ceramic filler to follow a pseudoplastic behavior. Pseudoplasticity is a result of aggregation of the suspended particles and is caused by the attractive Van der Waals forces as well as the repulsive steric effect on the particles. The combination of these forces prevents the particles to join together, rather than offering a separation among the particles with weak physical bonding. When a shear force is applied, the particle–to-particle network is broken and as a consequence, the viscosity gets decreased40. In addition, the storage modulus and the loss modulus also have an influence on screen printing where the former indicates the elastic (solid like) component while the latter symbolizes the contribution of viscous (liquid like) component in the fluid39. For effective printing, it is likely that the ink has more viscous like character than elastic nature. The dynamic viscoelastic properties of the nBSO ink are given in figure 4(b). A better understanding of the nature G’ and G” will be useful in untying the discrepancy between the influence of organic and inorganic segments on the viscosity of the screen printing ink. From figure 4(b) it is obvious that the value of loss modulus (G”) is higher than that of storage modulus (G’) in the entire range of strain. This reveals that the developed ink has more liquid like behavior, which is an essential factor to prevent slumping and also to guarantee better homogeneity and high aspect ratio in printing39. The variation of stress against strain of the formulated ink is also shown in figure 4(c), which is a characteristic of pseudoplastic fluid and gives more insight into the role of polymer binder in controlling the rheology of formulated ink.


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Figure 5: (a) Photograph of CSIR-NIIST emblem printed on a Mylar® film (b) optical micrograph of the printed pattern (c) surface SEM image of printed film and (d) the cross sectional image of printed film Figure 5(a) shows the NIIST-CSIR emblem screen printed on flexible Mylar® film and figure 5(b) is the optical micrograph of the printed film. Obviously, in screen printing process, the ink is deposited on the substrate as tiny rectangles and once the screen is removed it flow out to form a smooth surface. Leveling is greatly controlled by a number of factors including flow time, surface tension, viscosity and thixotropic nature of the ink, mesh size, quality of the squeegee and screen etc41. In the present study, double stroke printing was adopted in order to mitigate the island formation. It is evident from figure 5(b) that the printing was more or less uniform, without much porosity. However, some mesh patterns were also visible in the printed film which may be due to the rheological variations at high curing rate, leading to leveling imperfections. In fact, the fast curing nature of the developed ink will tend to increase its viscosity very rapidly, soon after the printing. This in turn, will restrict the ink materials to get a more homogenized leveling before curing. The scanning electron micrograph given in figure 5(c) is also recorded from the same printed film, which obviously looks smoother only because the scanning area is smaller. A uniform distribution of the nBSO particles with minimum porosity was observed in the microstructure. A minor amount of porosity is unavoidable, since it is arising from the volatilization of the solvents used in the ink formulation. The cross sectional microstructure of the ink printed on Mylar® film is shown in figure 5(d). From the figure, we can clearly 11 ACS Paragon Plus Environment


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distinguish the printed pattern from Mylar® substrate. The thickness of the printed pattern is measured from the micrograph and it is almost 6.3 µm, more or less uniform throughout, signifying the homogeneity of the printed pattern. The surface smoothness is of prime concern in printed patterns which to a greater extent depend on the particle size distribution of the filler, the porosity created by the volatilization of the organic vehicle and also the leveling behavior of the ink during printing. A couple of typical AFM micrographs of the printed pattern, after double stroke printing is depicted in figure 6. The average surface roughness (Ra) and the root mean square roughness (Rq) of the printed film is around 80.3 nm and 102 nm respectively. These values were lower compared to earlier reports on room temperature curable dielectric inks11,13, indicating that the developed ink can be used to generate good quality printed patterns. The 3D images of the printed film show the bumps and valley nature of the printing. The kurtosis topography, which is the measure of unevenness of the printed film, was around 2.99. If the kurtosis value is less than 3, which usually called platykurtoic, it can be taken as a testimonial of the surface that has relatively a few high mountains and low valleys.13 This bumps and valley nature is mainly due to the presence of unavoidable porosity created through the fast evaporation of the solvent. However, this has no major significance in the quality of the printed film since the roughness values are comparatively lower. The skewness (the measure of asymmetry of the surface) value of the printed film is around -0.237. The negative value indicates that the surface has more number of longer tails with compared to the reference plane11.

Figure 6: (a) and (b) 3D AFM images of the printed film, after double stroke printing, with different scan area.


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The printed ink is found to be cured within seconds after printing. Even though drying process of evaporation drying type inks can be accelerated by blowing hot air over the surface to be dried, no such cumbersome arrangements need to be arranged in the present case. The rub-off resistance of the printed film on the substrate was characterized by adhesion test using scotch tape® 3M. In order to measure the peel off strength, an equal distant cross cut of six lines were made on the film and a scotch® 3M tape is stuck over that and after sometime it removed abruptly at an angle perpendicular to the surface of the printed pattern. Figure 7(a) shows the photograph of the film and scotch tape after testing. It is clear from the photographs that no portion of the printed ink is removed during this adhesion test, which indicate a strong adhesion of the formulated ink on the carrier substrate. To get a closer idea, the optical and SEM analysis of the film after the test were carried out. Figure 7(b) shows an optical image of the film taken exactly along the cut line. It is very evident from the image that no part of the ink is removed during the test, which is further confirmed by the SEM image as shown in figure7(c).

Figure 7: (a) Photographs of the printed film and scotch tape after testing (b) the optical micrograph and (c) the SEM image of film after the scotch test.



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Table 1: Microwave dielectric properties of Mylar® substrate and printed nBSO film Material

Thickness

Microwave dielectric properties

(µm)

5 GHz

10 GHz

15 GHz

ɛr

tanδ

ɛr

tanδ

ɛr

tanδ

Mylar®

73

3.31

0.0067

3.15

0.0073

3.13

0.0066

Printed film

6.3

4.46

0.0131

4.36

0.0160

4.23

0.0181

The dielectric properties of the printed film at microwave frequency were measured and the results are tabulated in Table 1. The printed film has a dielectric constant (ɛr) of 4.46 and tanδ of 0.0131 at 5 GHz. It is interesting to note that, with an increase in frequency the dielectric constant shows a slight decrease, whereas the tanδ have a reverse trend, the difference being within the experimental error limit. The tanδ of material is highly influenced by the external factors like humidity, porosity, impurity etc42 at this frequency range. For practical purposes, it is desirable that the dielectric layer should have a stable and low tanδ (< 0.05)8 value. In the present case, the dielectric loss of the printed film for all the measured frequencies comes within this range, reiterating the suitability of the developed ink in printed flexible circuit application. Additionally, it should be noted that the ɛr of the nBSO printed film was comparatively lower while tanδ was higher as that of bulk ceramic (ɛr=13.3 and tanδ=0.0007 at 15 GHz)26. This divergence is believed to be due to the presence of the organic binder, combined with the inherent porosity of prepared film. Likewise, the low ɛr value of the developed ink also indicates its aptness for the application in inter layer dielectric as well as protective coating for microelectronic devices. The fast curing nature is an added advantage for the developed ink from the currently available UV curable inks, besides, other advantages like durability, compatibility to all kinds of substrates, resistivity to abrasion and high flexibility. The stability of the dielectric properties with temperature as well as mechanical deformation is one of the major concerns in printed electronics. With this objective we have performed the reliability test of the printed film with temperature and mechanical bending. The results are shown in figure. 8.


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Figure 8: Variation of microwave dielectric properties (a) with temperature (b) with mechanical bending at 5 GHz Figure 8 (a) represents the variation of dielectric constant and tanδ at 5 GHz with temperature from ambient up to 60 oC. As obvious from the figure, dielectric constant and tanδ have a negligible variation with temperature; the variation is being linear. The results indicate that the dielectric properties of the film are quite stable in the operating temperatures. The variation of the dielectric properties with increasing bending cycle (up to 500 bending cycle) also emphasis that the dielectric properties are independent of the bending deformation.

Figure 9: (a) I-V characteristic of the nBSO printed film (b) and (c) Variation of ɛr, tanδ and conductivity with log frequency.



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The representative I-V plot of the printed film is shown in figure 9(a). The result indicates that with electric field the current density shows an exponential increase that is typical for a dielectric material43. Furthermore one can notice that the printed pattern has a low leakage current density. In conjunction with the other results discussed above, we can infer that the formulated nBSO ink was found to be a suitable candidate for creating a thin dielectric layer for electronic applications. The variation of dielectric and electrical properties of the developed ink in the low frequency region was shown in figure 9(b) and 9(c) respectively. It was obvious from the figure 9(b) that, the ɛr and tanδ decrease with increased frequency. This is because at lower frequency, all the polarization mechanisms that are responsible for the dielectric constant of the materials are active, yielding high dielectric constants. With increase in frequency, the effects of interfacial and dipolar polarization decayed, resulting in the lowering of dielectric constant44. The ac conductivity of the ink was presented in figure 9(c). The material has a very low conductivity in the order of 10-8 S/cm. At lower frequency, the conductivity almost remains constant and after a particular frequency the conductivity starts to increase exponentially. This can be well explained using Jonscher’s power law. = + where ‘ω’ is the frequency, ‘σdc’ is the frequency independent conductivity (low frequency region), ‘A’ is the temperature dependent pre exponential factor and ‘n’ is frequency exponent45. As per the equation, at low frequency the conductivity is more like frequency independent (dc conductivity), and with increase in frequency the second term in the power law equation starts dominating. On the other hand at higher frequencies, the ac conductivity is directly proportional to ωn, which denotes that the conductivity increases exponentially. Conclusions Low permittivity dielectric inks have potential application in printed electronics due to their ability to isolate printed conductive patterns in multilayer circuits, besides possessing flexibility and mechanical robustness. With the objective of formulating a fast curing dielectric ink, nBSO powder was prepared through sol-gel synthesis. The calcination temperature of the prepared powder was optimized using TG-DTA analysis. The phase formation of the nBSO powder was revealed using XRD and TEM analysis. Crystallite size of the nBSO was measured by W-H 16 ACS Paragon Plus Environment

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analysis and it was around 67 nm. FTIR studies were also carried out to under the structural units present in the prepared ceramic. The stable viscous ink of nBSO was formulated using the xylene-ethanol mixture as the solvent system, while Triton X-100 and ethyl cellulose were used as dispersant and binder respectively. The microstructure, surface roughness and dielectric properties of the developed ink were measured. The printed film has a dielectric constant of 4.41 and tanδ of 0.0131 at 5 GHz. The reliability test of the printed film for temperature variation and mechanical bending was studied, and results conclude that the film is stable for temperature and mechanical (bending) changes. The newly developed low κ ink can be used as insulation in flex circuits and thick films, for improving moisture, arc track and weathering resistance of electrical/electronic devices. Acknowledgements The authors acknowledge Department of Science and Technology (DST-SERB), New Delhi for financial assistance. One of the authors P. Abhilash thanks Council of Scientific and Industrial Research, New Delhi for his research fellowship. The authors are thankful to Dr. P. Prabhakar Rao for extending XRD and SEM facility. The authors would like to acknowledge Mr. A. Peer Mohammed for recording rheology and Mr. Kiran for HRTEM.



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List of Figures Figure 1: (a) TG-DTA analysis of the nBSO gel and (b) FTIR spectra of nBSO calcined at 850 o

C for 4h

Figure 2: (a) XRD pattern of nBSO and (b) W-H plot of nBSO calcined at 850 oC for 4h Figure 3: (a) and (b) The HR-TEM image and (c) FFT pattern of nBSO calcined at 850 oC for 4h Figure 4: (a) Variation of viscosity and shear stress with shear rate (b) dynamic viscoelastic properties and (c) stress-strain curve of nBSO ink Figure 5: (a) Photograph of CSIR-NIIST emblem printed on a Mylar® film (b) optical micrograph of the printed pattern (c) surface SEM image of printed film and (d) the cross sectional image of printed film Figure 6: (a) and (b) 3D AFM images of the printed film, after double stroke printing, with different scan area. Figure 7: (a) Photographs of the printed film and scotch tape after testing (b) the optical micrograph and (c) the SEM image of film after the scotch test. Figure 8: Variation of microwave dielectric properties (a) with temperature (b) with mechanical bending at 5 GHz Figure 9: (a) I-V characteristic of the nBSO printed film (b) and (c) Variation of ɛr, tanδ and conductivity with log frequency.

List of Tables Table 1: Microwave dielectric properties of Mylar® substrate and printed nBSO film


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For Table of Contents Only



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Figure 1: (a) TG-DTA analysis of the nBSO gel and (b) FTIR spectra of nBSO calcined at 850 oC for 4h 48x18mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2: (a) XRD pattern of nBSO and (b) W-H plot of nBSO calcined at 850 oC for 4h 49x20mm (300 x 300 DPI)



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Figure 3 (a) and (b) The HR-TEM image and (c) FFT pattern of nBSO calcined at 850 oC for 4h 18x6mm (300 x 300 DPI)


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Figure 4: (a) Variation of viscosity and shear stress with shear rate (b) dynamic viscoelastic properties and (c) stress-strain curve of nBSO ink 95x71mm (300 x 300 DPI)



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Figure 5: (a) Photograph of CSIR-NIIST emblem printed on a Mylar® film (b) optical micrograph of the printed pattern (c) surface SEM image of printed film and (d) the cross sectional image of printed film 27x18mm (300 x 300 DPI)


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Figure 6: (a) and (b) 3D AFM images of the printed film, after double stroke printing, with different scan area. 20x9mm (300 x 300 DPI)



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Figure 7: (a) Photographs of the printed film and scotch tape after testing (b) the optical micrograph and (c) the SEM image of film after the scotch test. 90x64mm (300 x 300 DPI)


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Figure 8: Variation of microwave dielectric properties (a) with temperature (b) with mechanical bending at 5 GHz 46x22mm (300 x 300 DPI)



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Figure 9: (a) I-V characteristic of the nBSO printed film (b) and (c) Variation of ɛr, tanδ and conductivity with log frequency 95x69mm (300 x 300 DPI)


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For Table of Contents Only 27x12mm (300 x 300 DPI)


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