An Environmental Friendly Approach for the Development of Ultra Low

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An Environmental Friendly Approach for the Development of Ultra Low Firing LiWO Ceramic Tapes 2

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Arun Sasidharanpillai, Chi Heon Kim, Chang Hyun Lee, Mailadil Thomas Sebastian, and Hyo Tae Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00656 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Sustainable Chemistry & Engineering

An Environmental Friendly Approach for the Development of Ultra Low Firing Li2WO4 Ceramic Tapes Arun Sasidharanpillai, Chi H Kim, Chang H Lee, Mailadil T Sebastian and Hyo T Kim* Korea Institute of Ceramic Engineering and Technology, Jinju-si, 52851, Republic of Korea *Email: [email protected]

Abstract Ultra low temperature co-firable Li2WO4 substrate has been developed using an environmental friendly tape casting technique. The non-aqueous tape casting slurry comprised of an ecofriendly binder-solvent system consisting of polypropylene carbonate as binder and dimethyl carbonate as solvent. The structural, thermal, morphological, rheological and electrical properties of the sintered substrate is investigated. The bulk ceramics sintered at 650 oC posses a relatively high thermal expansion coefficient of about 16 ppm/oC. The thermal conductivity of sintered tape measured at room temperature was about 2.6 W/m⋅K. The sintered substrate exhibits excellent microwave dielectric properties with a relative permittivity of 5.4 and a very low dielectric loss of 9.21×10-5 at 5 GHz and is co-fireable with Ag electrode. Keywords: ULTCC, tape casting, dielectric properties, thermal conductivity, thermal expansion

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Introduction Low temperature co-fired ceramic (LTCC) is a viable substrate technology, which has extensive applications in wireless communication, automotive, military, space and medical industries. Unlike the other ceramic processing technologies, LTCC technology provides the freedom for 3D integration of passive microwave components such as strip lines, antennas, filters, resonators etc. in a single module.1,2 An ideal microwave substrate should have a low relative permittivity with a very high quality factor, for fast and lossless signal transmission.3 Most of the available ceramic materials having excellent dielectric properties are high temperature co-fireable. Various methods are developed to reduce the firing temperature of ceramic materials to less than 900 oC in order to co-fire with highly conducting Ag, Cu and Au. One approach to reduce the sintering temperature of ceramic is mixing it with low melting oxides. However, this method is not preferred, since it seriously affects the dielectric properties of the parent material. Intensive research is going on to find new materials having inherent ultralow sintering temperature (ULTCC systems). Recent works in this field shows that most of the ULTCC systems are composed of one of the TeO2, MoO3, Bi2O3, V2O5 and WO3 based compounds.4–9 Tungstate based compositions are reported to have excellent microwave dielectric properties with ultra low sintering temperature.3,10-13 However most of them are not useful for substrate application, due to their incompatibility with electrode metals (Ag, Al) 11,13 and the use of toxic raw materials.14 Among them lithium based tungstate was reported to have excellent microwave dielectric properties and good chemical compatibility with Ag and Al. The reported microwave dielectric properties of Li2WO4 ceramic sintered at 640o-660oC were, εr = 5.5, Q×f = 62,000 GHz, and a TCF of -146 ppm/oC at 15.7 GHz.10 2 ACS Paragon Plus Environment

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Tape casting is a well-established and cost-effective ceramic fabrication technique mainly for the pilot scale production of LTCC substrates, multilayer ceramic capacitors (MLCC) and solid oxide fuel cells.15–17 The slurry formulation is the prime factor in tape casting process. Recently, the environmental impact of the chemicals used in a tape casting slurry formulation has received comprehensive attention. Generally, aqueous and organic solvent based systems are practiced to prepare tape casting slurry. A variety of organic solvents, such as alcohols, ketones or hydrocarbons are widely used for suspensions with reproducible rheological properties and drying behavior.18,19 Organic solvents have the advantage of high volatility, faster drying and prevent the ceramic powder from hydration. The decomposition of the highly toxic organic moieties creates drastic environmental and health hazards, which lead researchers to think about aqueous-based formulations.20 However, the slow evaporation rate and agglomeration due to strong hydrogen bonding of water-based system limit its application in tape casting in spite of its significant environmental impact and low cost.21 Recent researches in green chemistry for environmentally benign compounds has resulted in the development of dimethyl carbonate, a new non-toxic, versatile chemical that exhibits high reactivity.22,23 Dimethyl carbonate has broadly used as an effective solvent in the development of inks,24 lithium batteries,25 supercapacitors,26 pharmaceuticals and other energy storage devices. Polypropylene carbonate (PPC) is a biodegradable and biocompatible compound,27 widely used for sacrificial polymer applications in microelectronics due to its low decomposition temperature (∼250 oC). PPC can be successfully used as a binder in tape casting technique due to low residue level during burnout compared to other organic binders.28–30 This green binder-solvent combination was applied for ULTCC fabrication of Al2O3-BBSZ and BaTiO3-BBSZ tapes.31



In this work, we emphasize the casting and characterization of ultra-low temperature sinterable Li2WO4 tape by a purely environmental friendly slurry formulation. The structural, thermal, morphological and electrical properties of the prepared substrate were conducted and report it as a promising material for microelectronic substrate applications.

Experimental Preparation of LWO ceramic The polycrystalline ceramic compound, Li2WO4 (LWO) was prepared by mixed oxide ceramic route using high purity chemicals of Li2CO3 and WO3 (>99.9%, Kojundo Chemical Laboratory Co. Ltd., Japan). The stoichiometric amount of the chemicals were weighed, and ball milled together in ethanol medium using yttria stabilized zirconia balls for 12 h. The dried powder was calcined at 600 oC in a box furnace for 4 h. The calcined powder was milled in a high energy planetary mill for 3 h to reduce the particle size necessary for tape casting. The structural analysis was conducted by X-ray diffraction spectroscopy using Cu K-α radiation (RIGAKU, RINT 2000). The particle size of the milled powder was measured by laser scattering particle size distribution analyzer (LA-950V2, HORIBA Ltd., Kyoto, Japan) in ethanol medium. The milled powder was made into cylindrical pellets under a uniaxial pressure of 200 MPa. The pellets were sintered at a temperature of 650 oC for 4 h in air atmosphere. The density of the sintered ceramic was measured using Archimedes’ method. The coefficient of linear thermal expansion (CTE) of the LWO ceramic sintered at 650 oC was analyzed using thermo mechanical analyzer (TMA Q 400, TA Instruments).


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Preparation of LWO tape The viscosity of the slurry was optimized to yield maximum colloidal stability and to obtain a homogenous tape (see supporting information for the procedure of slurry preparation). The viscosity analysis of the final slurry was carried out by Rheometer (Digital Viscometer, BROOKFIELD DV-II+Pro, USA). To remove the air bubbles formed during mixing, the slurry was decompressed for 20 min with a rotary pump. The de-aired slurry was cast into green tapes with a thickness of 81 µm using a tape casting machine (TCA-2000, Techgen, Korea). The thermal decomposition analysis of the green tape was conducted by simultaneous DTA-TG apparatus (DTG-60H, Shimadzu, Kyoto, Japan). The ultimate tensile strength of the green tape was measured using Universal Testing Machine (Instron 5544, North America) (ASTM D638M). The dried green tape was cut and thermolaminated (12 layers) at a temperature of 60 oC and under a pressure of 5 MPa using thermolaminator. The thermolaminated sheets were sintered at a temperature of 625 oC for 4 h in a box furnace under air atmosphere. The microstructure of the green, thermolaminated and sintered LWO tapes were characterized using a field emission scanning electron microscopy (FE-SEM) (JEOL/JSM-6700F, Oxford). The surface roughness of the green and sintered samples was examined by atomic force microscopy (AFM) (WITEC, Focus Innovations, Germany) operating in the tapping mode. For determining the thermal conductivity, the stacked tape was cut into cylindrical shape using circular punch having a diameter of 15 mm (ASTM E1461) and was measured using laser flash analysis (LFA 427, NETZSCH). The dielectric properties in low frequency region were analyzed using a precision impedance analyzer (4294A, Agilent Technologies, Santa Clara, CA). The microwave dielectric properties of the green and sintered tape at 5 GHz were measured in a split post dielectric 5 ACS Paragon Plus Environment


resonator (QWED, Warsaw, Poland) with the aid of a Network Analyzer (8720ES, Agilent Technologies, Santa Clara, CA). The breakdown strength of the sintered tape was measured in silicone oil at room temperature using a breakdown voltage measurement system (TOS 5101, Kikusui, Japan) with maximum DC voltage of 10 kV.

Results and discussion Powder Characterization LWO possesses polymorphic behavior, with four different crystal structures in which phenakite type structure (Li2WO4-I) is the most stable, under atmospheric pressure.32,33 Zachariasen in 1926, first reported the structural behavior of Li2WO4 and later the phenakite type structure was confirmed by Moon.33 Yamaoka et al. confirmed the presence of three additional phases of Li2WO4 at high pressure and temperature.34 XRD pattern and crystal structure of LWO powder calcined at 600 oC is shown in Figure S1 and Figure 1 respectively, which is well matched with the rhombohedral structure suggested by Zachariasen having space group R3̅(148). The XRD pattern of sample was similar to the calcined powder. All the peaks can be indexed using JCPDS file 79-2006. The theoretical density calculated from XRD data and the apparent density of the 650 °C sintered LWO ceramics are 4.56 and 4.38 g/cm3 respectively. A relative density of 96% is achieved at a sintering temperature of 650 oC for a holding time of 4 h. The average particle size of the starting powder is one of the important parameters to be monitored in accordance with desired characteristics of the final sintered product. The particle size distribution shown in Figure S2 clearly indicates that the calcined powder of LWO may 6 ACS Paragon Plus Environment

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consist of agglomerates or group of particles, which will oppose the formation of a good dispersion and enhances the sedimentation of ceramic particles, which is highly undesirable for a tape casting slurry. To obtain a well-dispersed slurry, the particle size of the starting powder should be reduced to few micrometers.35 Apart from that, particle size distribution has a greater influence on the properties of the final product. Wide particle size distribution is shown to exhibit better packing and also yield shear thinning slurries.36,37 Yan et al. had experimentally verified that a narrow particle size distribution is suitable for better fired properties.38,39 Figure S3 displays the size distribution of calcined powder of LWO after 3 h high energy planetary milling. It illustrates an intermediate distribution with an average particle size of 0.9 µm, compromising between the advantage of the wide and narrow distribution.36

Figure 1: Crystal structure of LWO powder calcined at 600 oC



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Characterization for green tape Viscosity Measurement In the tape casting process, slurries with high solid loading and low viscosity are crucial for obtaining green tape with high density and homogeneous microstructure. Figure 2 depicts the flow characteristic of the ready-to be-casted slurry. This slurry with an optimum composition clearly follows a shear thinning behavior due to the gradual breaking of the network formed while the slurry was in repose, which is an indication of the need of a higher shear stress to break them.21 It should be noted that this trend in the viscosity is a primary requirement for effective tape casting.40 Table 1 shows the composition of the final tape casting slurry. As prepared green tape has an ultimate tensile strength (UTS) of 0.518 MPa. Table 1: Tape casting slurry composition Component

Composition (wt. %)

Function

Li2WO4

51.00

Filler

Nopco 092

0.51

Dispersant

Dimethyl Carbonate

42.38

Solvent

Polypropylene Carbonate

5.09

Binder

Dibutyl Phthalate

0.49

Plasticiser (Type I)

Poly Ethylene Glycol-300

0.17

Plasticiser (Type II)

[First Stage]

[Second Stage]


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Figure 2: Viscosity analysis of the final tape casting slurry Thermogravimetric Analysis (TGA) The thermal analysis of the green tape (TGA) was carried out under an N2 atmosphere at a heating rate of 10 oC/min. It is clear from the Figure 3 that initially the material was stable until 110 oC. A weight loss of 8 % occurred between 110 oC and 240 oC, which is attributed mainly due to the burning of dispersant, plasticisers, and solvent present in it. Further investigation tells us that between 240 oC and 290 oC, where the complete burnout of binder is expected to occur in a slow rate and it accounts almost 3% of the remaining weight of the material. This result is in agreement with the thermal decomposition analysis of polypropylene carbonate (QPAC 40) binder based green tape studied by McAndrew et al.41 The TG graph shows that there is no considerable weight loss after 350 oC. The role of the additives such as binders and plasticisers are only to impart strength and flexibility to the ceramic green tape for workability before sintering. The pyrolysis of these additives should be carried out in the initial stages of sintering. Otherwise, the residues will alter the desired properties of the final product.42 If the decomposition rate of the additives exceeds the gas diffusion, there is a higher probability for delamination and cracks to occur in the sintered tape.43–45 Hence a properly designed heating 9 ACS Paragon Plus Environment


schedule and enough holding time at every intermediate stage are required to prevent stresses and defects developed in the green sheet. The binder burnout must be controlled to yield a sintered body without flaws and minimum carbon residues, which may inhibit sintering. The clean burnout property of PPC binder through an unzipping mechanism and its relatively low decomposition temperature allows it to be selected as an alternative to the conventional high temperature decomposing binders in ultra-low temperature co-fired ceramic technology (ULTCC), where the sintering temperature is less than 700 oC but greater than 300 oC.31,46 Figure S4 shows the sintering profile for LWO tape for sintering at 625 oC, included with two intermediate holding at 250 oC for binder burnout and at 625 oC for full densification.

Figure 3: Thermogravimetric analysis of LWO green tape

Morphological characterization The FE-SEM image of the surface of the 12-layer stack (green tape) before and after thermolamination is shown in Figure 4(a) and Figure S5, respectively. An uneven porous distribution is observed due to the evaporation of solvent from the tape, and the strong action of the binder molecules are clearly observed in the green surface as shown in Figure 4(a). After 10 ACS Paragon Plus Environment

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thermolamination, as visible from Figure S5, there is an increase in density due to the lamination pressure and temperature. The cross-sectional SEM images of the thermolaminated stack shown in Figure 4(b) (magnified image as inset) demonstrates that the laminate is defect free with no delamination and crack, which reveals that the above-given condition for thermo lamination is good enough to diffuse the individual layers into a single homogenous module. The surface and cross-section images of LWO sintered at 625 oC are shown in Figure 4(c) and 4(d) respectively. It is observed from the figure that LWO laminate is well densified at 625 oC without much porosity.

Figure 4: SEM images of LWO: (a) surface morphology of the green tape (backscattered), (b) cross section of the thermolaminated stack (inset, its magnified image), (c) surface of the sintered stack, and (d) fractured surface of the sintered stack One of the most important processes in any LTCC package manufacturing involves the co-firing of the ceramic with highly conducting metals such as Ag, Au, and Al.

A very

restrictive requirement for LTCC device fabrication is the chemical compatibility of the ceramic with the electrode material. Any kind of chemical reaction between the ceramic and metal can 11 ACS Paragon Plus Environment


destroy the functionality of the module.47 In addition to that, the shrinkage mismatch between the electrode and substrate as well as diffusion of Ag ions into the ceramic layer can also affect the reliability and performance of the module.48,49 Hence it is necessary to verify the co-fireability of a ceramic with the metal (Ag). FE-SEM analysis together with elemental mapping can provide valuable information on this. Figure 5 (a) shows the cross-sectional view of the co-fired LWO. The elemental mapping of the cross-section (Figure 5(b)) clarifies the fact that there is no considerable diffusion of Ag into the ceramic, and also there is no observable evidence for the reactivity of Ag with the ceramic at the interface.

Fig 5: (a) FE-SEM image of the cross section of LWO co-fired with Ag at 625 oC and (b) elemental mapping of Ag layer The quality and functionality of a printed metallization have a strong dependence on the surface properties of the substrates, especially on its surface roughness. A tape surface with flatness at the microscopic level is highly recommended for better characteristics. The mobility of the electrons in the screen printed patterns depends on the roughness of the substrates, and it was reported that the carrier trapping is more pronounced on rougher substrates.50,51 Figure 6 and Figure S6 shows the AFM image of the surface of green and sintered tape of LWO. The green tape has an average surface roughness of 181.6 nm, which is relatively small compared to the roughness of commercial LTCC green tapes and the sintered tape has a roughness of 135.1 nm.52 12 ACS Paragon Plus Environment

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Fig 6: AFM image of the surface of LWO green tape

Electrical characterization The low-frequency dispersion of relative permittivity and dielectric loss of the LWO sintered multilayer is shown in Figure S7. Microwave dielectric properties of the single green layer, thermolaminated and sintered tape of LWO at 5 GHz are shown in Table 2. In comparison, it is observed that single layer of LWO green tape has the lowest permittivity and highest loss, which is mainly due to a relatively low density evident from (Figure 4(a)) and the presence of organics. After thermolamination, the permittivity of the green stack increased to 4.6 and loss reduced to 1.29×10-2, and is expected due to the increase in density (decrease in thickness) offered by the applied pressure and temperature. Upon sintering at 625 oC, a drastic decrease in dielectric loss is observed from 1.29×10-2 to 9.21×10-5 with a measurable change in relative permittivity from 4.6 to 5.4, mainly due to the increase in densification and removal of low 13 ACS Paragon Plus Environment


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permittivity polymer by sintering. The photograph of the sintered LWO tape is shown in Figure S8. Table 2: Microwave dielectric properties of green (single layer), thermolaminated and sintered stack of LWO Material

Thickness

Dielectric properties

(mm)

at 5 GHz ɛr

tanδ

0.081

3.3

1.3×10-2

0.66

4.6

1.29×10-2

0.61

5.4

9.21×10-5

Li2WO4 – Green tape Single layer Li2WO4 – Green tape 12 layer stack Li2WO4 – sintered at 625oC/4h 12 layer stack

LTCC substrates are generally designed for applications in high voltage environment, and typical breakdown strength of commercial substrates are greater than10 kV/mm.53 It is well known that the breakdown strength varies in accordance with the thickness and relative permittivity. Neusal et al. studied the dependence of thickness and permittivity on breakdown strength of Al2O3 and BaTiO3 and confirmed that the probability of breakdown increases with sample thickness.54 The average value of the breakdown strength measurement conducted on


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0.84 mm thick LWO laminate sintered at 625 oC is 2.2 kV/mm. The space charge limited conduction (SCLC) may be the dominating mechanism for the breakdown in LWO multilayer.55

Thermal properties Coefficient of Linear Thermal Expansion Ceramic materials with a wide range of thermal expansion values are used in various applications including heat engine parts, electronic circuits, cookware and automotive. In a typical LTCC substrate surface, various semiconducting devices are coupled using metal wiring, in addition to that LTCC modules can also be an integrated component in many printed wiring boards (PWB).56,57 Any kind of thermal mismatch between the host substrate and the embedded device will initiate substantial cracking or electrical failure in the substrate. Ota et al. had proposed a series of framework silicates with high thermal expansion ranging from 16 to 28 ppm/oC for stress free bonding with Fe, Al, Cu and Ag.58–61 Park et al. developed calcium zinc borosilicate (50CaO. 20ZnO. 20B2O3. 10SiO2,) glass having a CTE value of 12.8 ppm/oC for LTCC applications.62 Arun et al. had recently reported, Li2ZnTi3O8 based glass ceramic LTCC system with a CTE value of 11.97 ppm/oC for hybrid circuit applications.17 Figure 7 shows the variation of the linear dimension of LWO with temperature from ambient temperature to 300 oC. A nearly linear relationship is shown to exist between the sample length and temperature, which is common in almost all ceramic materials.17 From the graph, the linear coefficient of thermal expansion can be obtained by using the following equation, =

− 1 −



Figure 7: Thermal expansion characteristics of LWO sintered at 650oC

where L1 and L2 are the lengths of the specimen at temperature T1 and T2, respectively. LWO possesses relatively high average CTE of 16 ppm/oC, which enables the successful cofiring with Ag and other metallic materials without causing substantial thermal stress. The temperature coefficient of the permittivity, τε of LWO as calculated from , and known value of TCF (τf = −146 ppm/oC, Zhou et al.,) is 260 ppm/oC.63 Thermal Conductivity The thermal conductivity of a material can be obtained indirectly by measuring thermal diffusivity using laser flash technique proposed by Parker et al.64 The thermal conductivity and diffusivity are related by the simple mathematical rule, κ = ρDC 2


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Fig 8: Variation of thermal diffusivity, specific heat, and thermal conductivity with temperature Where κ is the thermal conductivity, ρ is the density, D is the thermal diffusivity and Cp the specific heat. Figure 8 shows the variation of thermal diffusivity, specific heat and thermal conductivity of sintered tape of LWO as a function of temperature from room temperature to 200 oC. Most of the LTCC materials have low thermal conductivity in the range 2-4 W/m⋅K compared to HTCC materials.65 Recently a few reports appeared in literature for high thermal conducting glass ceramic LTCC materials. Yuan et al obtained a thermal conductivity of 5.9 W/m⋅K for the composite CaO–BaO–Al2O3–B2O3–SiO2/AlN with 40 wt.% AlN content.66 Induja et al. reported Al2O3-BBSZ glass/ceramic system having a thermal conductivity of 7.2 W/m⋅K at room temperature.67 The decrease in thermal conductivity with the rise in temperature is consistent with the phonon conduction mechanism.68,69 LWO sintered at 625 oC (Figure 8) has a thermal conductivity of 2.6 W/m⋅K, specific heat of 0.517 J/g⋅K and thermal diffusivity of 1.149 mm2/s at room temperature. 17 ACS Paragon Plus Environment


In this work, we have developed LiWO4 ULTCC tapes for the first time. The tapes sintered at 650 oC has low dielectric constant of 5.4 and very low loss of the order 10-5 in the microwave frequency range. There are only a couple of reports on ULTCC tapes (ZnTe3O870 and SiO2 based glass71), and the present sintered LiWO4 ULTCC tape has the lowest loss and lowest dielectric constant which is important for fast and efficient signal transmission. The fabricated Li2WO4 tapes shows good chemical compatibility with Ag and Al at 640 oC. Moreover, the raw materials are inexpensive. Hence LiWO4 tapes may be very useful for commercial applications.

Conclusion In this work, the development of an ultra low temperature co-firable Li2WO4 (LWO) substrate using an environmental friendly tape casting technique is proposed. The rheology, thermal, and electrical behavior of LWO were studied in detail. The LWO tape exhibited good chemical compatibility with Ag metal and is experimentally verified by SEM analysis. The green tape shows a tensile strength of about 0.518 MPa. The bulk ceramics sintered at 650 oC demonstrated a very high thermal expansion coefficient of about 16 ppm/oC. The LWO tape sintered at 625 oC possessed a thermal conductivity of 2.6 W/m⋅K and an excellent microwave dielectric properties with a relative permittivity of 5.4 and dielectric loss of 9.21×10-5 at 5 GHz. The eco-friendly nature of this method will help large-scale production and commercialization of LWO ceramic tapes.


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Acknowledgements The authors are thankful to the financial support from the brain pool programme by KOFST (171S-2-1-1853, 2017) and ceramic strategic technology development programme by KICET (KPP17004-1, 2017).

Supporting Information Figure S1: XRD of LWO powder calcined at 600 oC; Figure S2: Particle size analysis of LWO calcined at 600oC before milling; Figure S3: Particle size analysis of LWO calcined at 600oC after milling; Figure S4: Sintering profile of LWO laminate; Figure S5: Backscattered FE-SEM image of the surface of the green tape after thermolamination; Figure S6: AFM images of the surface of sintered tape of LWO; Figure S7: Variation of relative permittivity with frequency of LWO sintered at 625 oC; Figure S8: Image of sintered LWO tape.

Author information Corresponding Author *Dr. Hyo Tae Kim Email: [email protected]

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Synopsis The use of non-toxic and biodegradable chemicals is promising alternative against conventional carcinogenic organic additives in tape casting.


An Environmental Friendly Approach for the Development of Ultra Low

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