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Journal of the European Ceramic Society 35 (2015) 2313–2320

Glass free, non-aqueous LTCC tapes of Bi4(SiO4)3 with high solid loading

ullanchiyodan Abhilash a, Mailadil Thomas Sebastian b, Kuzhichalil Peethambharan Surendran a,∗a Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST-CSIR), Thiruvananthapuram 695019, India

b Microelectronics and Materials Physics Laboratories, University of Oulu, PO BOX 4500, FIN-90014 Oulu, Finland

Received 22 December 2014; received in revised form 30 January 2015; accepted 5 February 2015Available online 25 February 2015

bstract

lass free Bi4(SiO4)3 ceramic tapes were prepared by non-aqueous solvent based tape casting technique. A higher powder loading above 65 wt.%as achieved in a silicate system through a judicious selection of suitable dispersants and binder. The amount of dispersant was optimized at.0 wt.% based on viscosity and sedimentation analysis. The surface morphology and mechanical strength of the green tapes were investigated.he as-prepared green tape of thickness 70 �m has an average surface roughness of 320 nm and a tensile strength of 0.37 MPa. The microwaveielectric properties of the green tape (ε = 6.2 and tanδ = 0.0390) and sintered tape (ε = 13.3 and tanδ = 0.0007) were measured at 15 GHz by using

r r

PDR technique. 2015 Elsevier Ltd. All rights reserved.

eywords: Silicates; Tape casting; Rheology; Microstructure; Dielectric properties

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. Introduction

The ever-growing advancements in microwave telecommu-ication systems demand development of new materials andethods for circuit miniaturization. The multilayer ceramicodule (MCM) is one of the important milestones in this direc-

ion. The low temperature co-fired ceramic (LTCC) technologys emerged as the real backbone of MCMs [1]. The LTCCechnique plays a vital role in miniaturization and hybridiza-ion of microwave devices by embedding passive componentsike resistors, capacitors and other functional parts within it,hereby creating a lot of open spaces over the surface for activeomponents [1–3].

The prime requirements for a dielectric material for LTCCpplications are relatively low dielectric constant, high qualityactor, near zero temperature coefficient of resonant frequency,hemical inertness with silver (Ag) and low sintering temper-

ture [1,3–5]. The major challenge in LTCC technology is theeduction in sintering temperature of the ceramic well below theelting point of Ag (961 ◦C), without sacrificing its dielectric

∗ Corresponding author. Tel.: +91 471 2515258; fax: +91 471 2491712.E-mail address: kpsurendran@niist.res.in (K.P. Surendran).

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ttp://dx.doi.org/10.1016/j.jeurceramsoc.2015.02.002955-2219/© 2015 Elsevier Ltd. All rights reserved.

roperties. Among various methods, the addition of low meltinglasses, chemical processing of ceramics, use of smaller parti-le sized precursors etc. are some of the widely used techniques6–8]. Among them, addition of low melting glasses is rela-ively easier but has some major drawbacks that it may result inhemical interaction with the metal electrode, forming compli-ated phases in the LTCC system. Furthermore, glass additionan cause cracks during the soldering due to the different ther-al expansion coefficient between glasses and ceramics [1,9].ence, an intense search is going on to develop glass free LTCCaterials and tapes [1].Tape casting is a well-established and cost effective method

or the production of thin ceramic sheets at industrial scale,hich has a prominent role in LTCC fabrication technology [10].epending on the solvents used, tape casting can be broadly clas-

ified into (i) aqueous and (ii) non-aqueous (organic) based. Inrder to transform the ceramic powder into homogeneous slurry,ifferent additives are used such as solvents, dispersants, plas-icizers, homogenizers and binders. Each of these additives hasistinct roles in tape casting process that can dictate the finalroperties of the tape. The organic solvent based tape casting

as many advantages, like suitability for large scale roll to rollroduction of ceramic sheets, ease of drying, enhanced smooth-ess and denseness of the film surface etc. On the other hand,

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314 P. Abhilash et al. / Journal of the Europ

n case of aqueous tape casting, the addition of ceramic powdernto water creates flocculation very easily, that make it hard toorm stable slurry with good dispersion. The binary mixture ofolvents, say a mixture of xylene and ethanol, is more preferredor non-aqueous based tape casting due to better solubility ofndividual constituents. In any form of tape casting, achieving

higher powder loading above 60 wt.% is not easy due to theifficulties in mixing. However, with a judicious selection ofuitable dispersants and binder, it must be possible to achieveigher solid loading which is demonstrated in the present work.

Glass free LTCC materials are emerging nowadays, sincelass added compositions may produce undesired variations inhe final tape properties [9]. Recently, Thomas et al. reported

glass free LTCC tape of LiMgPO4 having good microwaveielectric properties (εr = 6.4, tanδ = 0.0002) [1]. Prior to this,everal Chinese researchers have reported the development of aew other glass free LTCC materials. For example, Zhou et al.eported some glass free LTCC compositions such as Zn3B2O6εr = 6.7, Q × f = 58,500 GHz, τf = −58 ppm/◦C), La2Mo3O12εr = 10.1, Q × f = 60,000 GHz, τf = −80 ppm/◦C) andd2Mo3O12 (εr = 8.0, Q × f = 80,000 GHz, τf = −60 ppm/◦C)

3,11]. Wu and Bian also reported a series of LTCC com-osition of AW1−xTexO4 (A = Ca, Sr, Zn) where x variesrom 0.05 to 0.2, possessing interesting microwave dielec-ric properties [12]. A glass free complex oxide based onLa0.5Na0.5)1−x(Li0.5Nd0.5)xWO4 with a low sintering tem-erature of 800 ◦C was investigated by Bian and Wang [6].chieving a low value of temperature coefficient of resonant

requency is always a desirable property for an LTCC substrate.ur interest was drawn to a material, Bi4(SiO4)3 which was

eported to possess very low temperature coefficient of resonantrequency (τf = −9.42 ppm/◦C) with a sintering temperaturef 900 ◦C and good microwave dielectric properties (εr = 14.9,

× f = 36,101 GHz) [13]. It is interesting to note that mostf the research works on glass free LTCC materials wereerformed at the ceramic level and only a few reports arevailable on the literature about the tape casting of glass freeTCC materials. In the present paper we discuss the tapeasting process and optimization of different variables to get

good mechanical and dielectric properties for the glass freeTCC material based on Bi4(SiO4)3.

. Experimental

The Bi4(SiO4)3 (BSO) ceramic was prepared through con-entional solid state ceramic route. Stoichiometric amount ofhe precursors, Bi2O3 (<10 micron, 99.9%, Sigma Aldrich, St.ouis, MO, USA) and SiO2 (325 mesh, 99.6% metal basis,igma Aldrich) were weighed accurately and mixed by ballilling in ethanol medium using yttria stabilized zirconia balls

s the grinding media. The resultant mixture was dried overnightn a hot air oven. The dried powder was calcined at differentemperature (750–850 ◦C) for 4 h, and was subsequently ground

nto fine powder by ball milling for 72 h using ethanol andttria stabilized zirconia balls. The phase formation and crys-al structure of the ceramic were identified by X-ray diffractionnalysis (PANalytical X’Pert PRO diffractometer having Nickel

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eramic Society 35 (2015) 2313–2320

ltered Cu K�, Netherlands). The average particle size distribu-ion of the ground powder was determined by Dynamic Lightcattering (DLS) technique (Zetasizer Nanoseries: ZEN 3600,alvern,Worcestershire, UK). The BET surface area of the

owder was measured from nitrogen adsorption isotherm usingurface area analyzer (Gemini 2375, Micromeritics, Norcross,SA). The thermal properties such as thermal conductivity, ther-al diffusivity and specific heat constant of the sintered ceramic

ellets were measured using laser flash thermal property ana-yzer (FlashLine 2000, Anter Corporation, Pittsburgh, USA).he coefficient of thermal expansion (CTE) was measured using

hermo mechanical analyzer (TMA/SS7300, SII NanoTechnol-gy Inc.).

For the rheological studies, the ceramic powder was madento viscous slurry using 50:50 mixture of ethanol–xylenes sol-ent system. In order to get well-flocculated slurry in tape castingrocess, dispersant plays a crucial role. In the present study, wesed fish oil (Arjuna Natural Extracts, Kerala, India) as the dis-ersant. In order to optimize the dispersant amount, we preparederamic slurry with varying amount of dispersant (0.5–2.0 wt.%)ith respect to powder while keeping the powder loading at axed high value of 70 wt.%. The viscosity of the slurry was mea-ured using a rheometer (Brookfield, R/S Plus, MA, USA), andhe sedimentation analysis was carried out in a 10 ml graduated

easuring cylinder. The initial height (H0) and sediment heightH) at different time interval were noted. The final tape castinglurry was prepared in two stages. In the first step, precalculatedmount of ceramic powder was mixed with solvent and disper-ant which was ball milled for 24 h. In the second stage, a binderpolyvinyl butyral (PVB), Butvar B-98, Sigma–Aldrich), plas-icizers (butyl benzyl phthalate, Sigma–Aldrich, polyethylenelycol, Sigma–Aldrich), and a homogenizer (cyclohexanone,igma–Aldrich) were added to it and further ball milled for 24 h.he final homogenized slurry was degassed and casted into thin

apes using a tape-casting machine (Keko equipment, Zuzem-erk, Slovenia). The casting was carried out on a silicon coatedylar® film using double doctor blade technique. The casted

ape was then allowed to dry at room temperature for 24 h [1].Tensile strength of the green tape was measured using a

niversal Testing Machine (Hounsfield, H5K-S UTM, Red-ill, UK) at a crosshead speed of 1 mm/min. The specimenauge length was 40 mm, width 10 mm and thickness 0.07 mm.he thermogravimetric analysis (TGA) of the green tape wasonducted using a thermogravimetric analyzer (PerkinElmer,altham, USA). For further characterization, square pieces of

rea 40 mm × 40 mm were cut from the green tape and lam-nated together. The lamination was done in a uniaxial pressy applying a pressure of 5 MPa and a temperature of 70 ◦Cor 15 min. The laminated tape was sintered at 900 ◦C for 4 h.

hile sintering, the laminate was kept in between two thinlatinum plates in order to avoid warping and also to con-rol the lateral (x, y) shrinkage. The dimensional stability ofTCC material is a fundamental prerequisite for substrate appli-

ation. The stresses arising from various components duringo-firing can lead to uneven shrinkage and material breakdown.ence one of the widely used techniques to avoid dimensional

hrinkage is to employ constrained sintering technique [14]. The

ean Ceramic Society 35 (2015) 2313–2320 2315

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P. Abhilash et al. / Journal of the Europ

ensity of tapes was measured using Archimedes method. Theicrostructure of the tapes was analyzed using scanning elec-

ron microscope (SEM) (JEOL-JSM 5600 LV, Tokyo, Japan).he surface roughness of the green tape was measured usingn atomic force microscope (AFM) (Bruker Nano, Inc., USA)perating in the tapping mode regime. Images were acquiredith a scan size of 50 �m × 50 �m and scan rate of 1.0 Hz.he microwave dielectric properties were measured in a splitost dielectric resonator operating at 15.155 GHz with the aidf a vector network analyzer (Model No. E5071C ENA series;gilent Technologies, Santa Clara, CA).

. Results and discussions

A typical colloidal dispersion is characterized by extremelyigh area of interface and hence a substantial amount of interfa-ial energy is prompting them to form larger aggregates. Gravitys one force which may destabilize the dispersion; if the dis-ersed particles are less dense than the dispersing liquid, theyill tend to rise to the surface whereas a denser dispersed phase,

n contrast, will tend to sediment. Hence it becomes evident thatispersing a ceramic phase with high density is rather difficult,hich can be achieved only by exploiting electrostatic forces ory steric repulsion. In the present case, the density of Bi4(SiO4)3s 6.78 g/cm3 which is stabilized through a judicious selectionf dispersants and binders at their optimal concentrations. Theigh density of the filler material is helpful in achieving highowder loading in the initial stage of the slurry preparation.

The filler used in the present study, Bi4(SiO4)3 (BSO) is aulytite-type compound that belongs to the cubic space group4̄3d with four molecules per unit cell [15]. The phase formationnd calcination temperature of the BSO ceramic were confirmedy XRD analysis. Fig. 1 represents the powder XRD patterns ofSO ceramics calcined at different temperature (750–850 ◦C).

t is clear from the figure that the BSO powder calcined at 850 ◦C

hows complete phase formation with cubic symmetry. All theeaks in the XRD pattern could be well indexed using standardCPDS file 35-1007. The average particle size distribution and

Fig. 1. XRD patterns of BSO powder calcined at different temperatures.

ottstcttatr

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Fig. 2. Plot of CTE of BSO ceramics.

ET surface area of the ground powder were measured as 1.2 �mnd 1.0 m2/g respectively.

The thermal expansion of ceramic material plays a crucialole in LTCC hybrid technology wherein the ceramic mate-ial is expected to have a CTE value comparable to that ofuried components, in order to avoid deformations like cracksnd delamination, originating from thermal mismatch duringo-firing [16–18]. Fig. 2 represents a plot of linear thermalxpansion of BSO ceramic from ambient temperature to 500 ◦C.he material shows an average CTE value of 7.09 ppm/◦C,easured over the entire temperature range. Additionally, the

ubstrate should have a good thermal conductivity in the oper-ting temperature. Table 1 represents the values of thermalonductivity, specific heat capacity and diffusivity of sinteredSO ceramic pellet for a temperature range of 29–257 ◦C. The

hermal property measurements were made by keeping aluminaAl2O3) as the reference material. The specific heat capacityf alumina with varying temperature is taken from the litera-ure [19]. It is obvious from the table that BSO have a roomemperature thermal conductivity of 2.82 W/m K that shows ateady decrease with increasing temperature. This decreasingrend of thermal conductivity as a function of temperature isommonly found in typical ceramic insulators. This behavior iso be understood against the fact that in an ideal ceramic dielec-ric, heat is transferred by lattice vibrations (or thermal phonons)nd the mean free path of the thermal phonons decrease at higher

emperature due to the scattering of thermal phonons, therebyesults in a decrease of thermal conductivity [20,21]. Most of

able 1hermal properties of sintered BSO ceramic at different temperatures.

emperature (◦C) Thermal conductivity(W/m K)

Specific heat(J/kg K)

Diffusivity(cm2/S)

29 2.82 346.8 0.012752 2.66 348.0 0.011603 2.58 392.4 0.010054 2.34 401.3 0.008804 2.31 440.9 0.007957 2.11 438.1 0.0073

2316 P. Abhilash et al. / Journal of the European Ceramic Society 35 (2015) 2313–2320

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Table 2Optimized slurry composition of BSO tapes.

Material Amount Role

(wt.%) (vol.%)

Part 1Bi4(SiO4)3 powder 65.6 19.5 FillerFish oil 0.7 1.4 DispersantXylenes 14.1 32.8 SolventEthanol 14.1 35.9 Solvent

Part 2Polyvinyl butyral, Butvar B-98 2.0 3.7 BinderBenzyl butyl phthalate 1.6 3.2 Plasticizer (Type 1)Polyethylene glycol 1.6 2.9 Plasticizer (Type 2)C

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ig. 3. Plot of viscosity against shear rate of BSO slurry with varying amountf dispersant.

he LTCC materials have thermal conductivity value within theange of 2–3 W/m K, which is obviously higher compared to thatf commonly used printed circuit board [22]. Since the ther-al conductivity of BSO ceramic also falls in this preferred

ange, it can be a suitable candidate for substrate applications. similar trend was observed in the case of thermal diffusivity

lso, whereas the specific heat capacity shows an opposite trend.hermal diffusivity is a measure of how fast the heat propagate

hrough the material, and hence a decrease in thermal diffusivityith temperature may be due to the increased phonon scattering

23,24].Controlling the fluidity of tape casting slurry is important in

ape casting process. The additives added in the tapes influencehe quality of tapes either directly or indirectly [25]. Ideally,he ceramic slurry should have minimum viscosity and lowettling rate with good packing density, for efficient tape cast-ng. In aqueous tape casting, the powder loading is usually low∼50 wt.%) since it accommodates more binder which in turnesults in voids and poor shrinkage. On the other hand, rela-ively higher powder loading is desired in non-aqueous tapeasting which in turn increase the drying rate and simultane-usly minimize drying stresses in the tape. In order to achieveigh initial solid loading (say >55 wt.%), a common practices to suitably control or modify the polymeric binder. However,ecreasing the binder content may result in anisotropic shrinkagef the tapes despite the advantage of a lower slip viscosity. Onepproach to eliminate the polymeric binder is polymerizationf the solvent and dispersant after casting which can sometimesomplicate the procedure [26]. In the present research, through

meticulous control of the binder and solvent mixture, highowder loading is achieved. Fig. 3 represents a plot of viscos-ty against shear rate of BSO slurry, having an initial powderoading of 70 wt.% with respect to solvent as a function of disper-ant (0.5–2.0 wt.%). From the figure one could clearly observehat as the shear rate increases from 0 to 100 s−1, the viscosityf the slurry shows an exponential decrease. It is believed that

he lowering of viscosity with shear rate is due to the breakupf soft agglomerates present in the slurry [27]. It is also obvi-us from Fig. 3 that, as dispersant concentration increases from

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yclohexanone 0.3 0.7 Homogenizer

.5 to 1.0 wt.%, the viscosity of the slurry showed a noticeableecrease. On the other hand, further increase in the dispersantontent to 2.0 wt.% resulted in an increase in the slurry vis-osity. The initial decrease in viscosity may be due to bettereflocculation of ceramic particle by the fish oil. The fish oilmparts stability to a colloidal suspension by steric stabiliza-ion. The large number of different fatty acids presents in thesh oil surrounds the individual ceramic particle which prevents

he agglomeration by steric hindrance. On the other hand, thencrease in viscosity above 1.0 wt.% fish oil addition may beue to the bridging flocculation resulted by the individual fishil molecules [28].

Fig. 4 represents the sedimentation analysis of BSO slurryith varying amount of dispersant. Here, the ratio of sedimenteight to initial height (H/H0) is plotted against time (min). It islear from the figure that, a slurry with 1.0 wt.% fish oil shows

slow settling rate compared to all other dispersant concentra-ions. This result is evidently corroborating with the viscositynalysis. Hence, from these two results, one can conclude thathe optimum level of fish oil required for the preparation ofighly dispersed homogeneous slurry is 1.0 wt.% with respecto BSO ceramic powder.

The extent of solid loading of the suspension affects the dry-ng behavior of the casted tape as well as its microstructure inhe green state. As seen in preceding discussion, achieving aigh solids loading (>60 wt.%) in traditional tape casting is nottraightforward, especially for high surface area powders likeilicates. The present study demonstrates that, with a carefulhoice of dispersant, plasticizer and binder in a suitable solventixture, one can effectively increase loading limit of powder

>65 wt.%), which eventually resulted in higher sintered densitys well (>96.5%). Table 2 represents the optimized composi-ion of BSO tape casting slurry. The maximum powder loadingchieved was 65.6 wt.% (19.5 vol.%) with respect to the totallurry. The high solid loading is expected to have fast dryingate that simultaneously minimize the drying stresses in the tape,esulting in crack free microstructure [28,29]. The rigorous pro-ess of slurry optimization was performed as two parts where

art 1 of the slurry was optimized from rheological studies andhe part 2 (binder, plasticizers and homogenizer) from viscositynd green tape properties. The viscosity of ready-to-cast slurry

P. Abhilash et al. / Journal of the European Ceramic Society 35 (2015) 2313–2320 2317

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Fig. 4. Sedimentation analysis of BSO

s plotted in Fig. 5 which also shows a shear thinning behaviorn the measured range. This type of pseudo plastic behavior is

desirable property for an optimal tape casting slurry, in ordero avoid unprecedented flow after casting [30]. The mechanicalexibility, surface smoothness and good binder burn out of green

apes are some of the important factors to consider in LTCC fab-ication. Since most of the device processing like printing thelectrode, via formation, cavities, thick film resistors etc. is donen the green tape, the tape should be highly flexible that doesot undergo any damage during this process. The single layer of

SO green tape having thickness 70 �m shows an average ten-

ile strength of 0.37 MPa which in the same range of commercialTCC tapes.

Fig. 5. Viscosity analysis of final tape casting slurry.

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y with varying amount of dispersant.

Fig. 6 depicts the TGA curve of BSO green tape, mea-ured from ambient temperature to 700 ◦C. During the initialtage of heating from room temperature to 175 ◦C, the tapehows very low (around 1%) weight loss, which may be dueo the evaporation of low volatile solvent molecules. On thether hand, above 175 ◦C the material have sweeping loss ineight which is believed to be due to the decomposition of non-olatile constituents like binder and plasticizers present in theape. Obviously, the tape encounters a maximum weight loss ofbout 15% up to 375 ◦C, and above that temperature, the weightoss was almost constant within the limits of experimental error.

ence from TGA analysis, it can be concluded that the green tape

hows a complete removal of all the organic additives present inhe tape below 400 ◦C. This information was helpful to program

Fig. 6. TGA analysis of BSO green tape.

2318 P. Abhilash et al. / Journal of the European Ceramic Society 35 (2015) 2313–2320

Table 3Microwave dielectric properties of BSO tapes at 15 GHz.

Material No. of layers Thickness (�m) Relative density (%) Dielectric properties (at 15 GHz)

εr tanδ

BSO green tape 1 70 (±1) 68.0 (±0.2) 6.2 0.0390BB

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SO laminated tape 4 240 (±1)

SO sintered 4 170 (±2)

two-step binder burn out dwells at 300 and 600 ◦C respec-ively, prior to sintering. It should be noted that in the presetase, the sintering was performed by keeping the ceramic sheetsept between two platinum plates. Obviously, the loading on theample can restrict the x–y shrinkage of the tapes during sinter-ng while the z-shrinkage will be greatly improved with it [31].n spite of the abovementioned limitations, constrained sinterings believed to be a very efficient means to avoid warping.

The microwave dielectric properties of the green, thermo lam-nated and sintered BSO tape at 15 GHz are given in Table 3.t is interesting to note that the LTCC technology is mainlyominated by alumina based substrates where the outcome ofost of the research done by industries which are protected by

atent laws. Because of this, the publication in the open lit-rature is very limited. Thus, there exists a surprising scarcityf LTCC tapes with dielectric constants between 6.0 and 7.0,hich if available, will give a lot of flexibility for hybrid cir-

uit designers. The development of an alumina free green tapeith low dielectric constant is relevant due to the above cited

eason. The present green tape has a dielectric constant of 6.2ith a dielectric loss of 0.0390. The high dielectric loss of thereen tape is due to the presence organic contents in the tape.

he dielectric constant of laminated tape is a little higher com-ared to green tape which is obviously because of the improvedensity after thermolamination [1]. The dense sintered tape

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Fig. 7. AFM images of BSO green tape (A)

69.4 (±0.2) 7.8 0.053696.8 (±0.1) 13.3 0.0007

densification = 96.8%) has a dielectric constant of 13.3 and veryow dielectric loss of 0.0007 at 15.155 GHz. This improvementn the dielectric properties is a consequence of the better den-ification and control of microstructural grain growth duringintering.

The AFM surface morphology of BSO green tape-recordedn the tapping mode is given in Fig. 7 where A and B representD and 3D respectively. The sample shows an average surfaceoughness (Sa) of 320 nm. This surface roughness is in line withhe reported surface roughness of commercial LTCC tapes. Theurtosis of the topography height distribution (Sku) is found toe nearly 2.88 which indicate the presence of uneven surfaces ofhe tape relative to a normal distribution. The bumpy nature with

ountains and valleys of the surface of cast tape is more evi-ent in the 3D image in Fig. 7(B). In the presented tape patterns,he skewness which is a measure of asymmetry of the surface,s −0.011. The negative value of skewness generally indicateshat the surface distribution has a longer tail at the lower sidef the reference plane. A more detailed microstructural mor-hology of the surface could be revealed from an SEM analysishich is given in Fig. 8. Fig. 8(A) is the cross sectional viewf a thermolaminated green tape a lower magnification whereas

ig. 8(B) represents the surface morphology of the green tape.he green tape shows more uniformly distributed ceramic parti-les with very less porosity. The surface has very good packing

2-dimensional and (B) 3-dimensional.

P. Abhilash et al. / Journal of the European Ceramic Society 35 (2015) 2313–2320 2319

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ig. 8. SEM images of (A) and (C) cross section of thermo-laminated tape (4 lape respectively.

ensity, which reiterates the increased filler loading (65 wt.%)n the tape casting slurry. A close look at the morphology of

sintered tape (Fig. 8(D)) reveals the presence of well-formedrains during sintering, where the grains are appeared to be verylosely packed with minimal porosity. The average grain sizeomes around 3–4 �m. The delamination of the laminated tapes the challenging issue in the LTCC processing. The cross sec-ional SEM images of the laminated tape is recorded in ordero confirm the absence of individual layer delamination whichs presented in Fig. 8(A) and (C). From both images with lowernd higher magnification (Fig. 8(A) and (C) respectively) arack-free and debonding free microstructure could be observed,hich clearly points out that the reported laminating conditions

re good enough to form a non-delaminating stacks for LTCCabrication.

. Conclusions

Glass free Bi4(SiO4)3 ceramic powder suitable for LTCCpplications, was prepared through conventional solid stateeramic route, and the complete phase formation was con-rmed by XRD analysis. The finely ground powder has anverage particle size distribution of 1.2 �m and a BET sur-ace area of 1.0 m2/g respectively. The constituents of the finalape casting slurry were optimized from its rheology as well asrom the properties of the green tape. With a systematic con-rol of the vehicle components, a maximum powder loadingf 65.6 wt.% (19.5 vol.%) was achieved that resulted in highlyensified (densification = 96.8%) and crack-free microstructure

pon sintering. The green tapes with an approximate thicknessf 70 �m were prepared using double doctor blade tape castingechnique. The surface roughness and mechanical strength ofhe green tape were 320 nm and 0.37 MPa respectively, which

[

) having different magnification, (B) and (D) are surface of green and sintered

re consistent with those of commercial LTCC tapes. The BSOeramic has promising thermal properties, with a thermal con-uctivity of 2.82 W/m K and an average CTE of 7.09 ppm/◦C.he microwave dielectric properties of the green tape (εr = 6.2,

anδ = 0.0390), thermo laminated (εr = 7.8, tanδ = 0.0536) andintered tape (εr = 13.3, tanδ = 0.0007) at 15 GHz were mea-ured. The thermal, mechanical, surface and dielectric propertiesoint out that the newly developed glass free BSO tapes can beruitfully used as substrates in LTCC technology.

cknowledgements

The authors are grateful to Department of Science and Tech-ology, New Delhi for financial support. One of the authors. Abhilash acknowledges Council of Scientific and Industrialesearch, New Delhi for research fellowship.

eferences

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4]. Rama Rao SD, Roopas Kiran S, Murthy VRK. Correlation betweenstructural characteristics and microwave dielectric properties of scheel-ite Ca1−xCdxMoO4 solid solution. J Am Ceram Soc 2012;95:3532–7,http://dx.doi.org/10.1111/j.1551-2916.2012.05317.x.

5]. Zhang Q-L, Zou D, Yang H. Microwave dielectric prop-erties of Ba3Ti4−x(Zn1/3Nb2/3)xNb4O21 for low temperatureco-fired ceramics. J Eur Ceram Soc 2011;31:265–72,http://dx.doi.org/10.1016/j.jeurceramsoc.2010.09.018.

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