Thermal expansion of sintered polycrystals of hexacelsian solidsolution (Sr,Ba)Al2Si2O8

Yuichi KOBAYASHI,³ Jae-Hwan PEE,* Yorikazu MURABAYASHI and Masaki KATAYAMA

Aichi Institute of Technology, Yachigusa Yakusa-cho, Toyota, Aichi 470–0392, Japan*Korea Institute of Ceramic Engineering & Technology (KICET), Icheon, Gyeonggi-do, Korea

The purpose of this work is to clarify the thermal expansion properties of hexacelsian solid solutions (SrxBa1¹xAl2Si2O8, x =0­1.0) using high-temperature powder X-ray diffraction and dilatometry of sintered polycrystals. Hexacelsian solid solutionswere prepared from BaCO3, SrCO3 and finely purified kaolin powder (Al2O3·2SiO2·2H2O) by heat-treatment above 1000°C.Hexacelsian polycrystals showed more sluggish ¡-¢ phase transitions, and the transition temperature increased from 300 to700°C with the increase in Sr content. The average thermal expansion coefficient of hexacelsian polycrystals strongly depended onthe temperature range and on the Sr content because of their nonlinearity of expansion curves. The thermal expansion gap of theunit cell associated with the ¡-¢ hexacelsian phase transition was found to increase from 0.13 to 0.50% with the increase in Srcontent by high-temperature powder X-ray diffraction. The average linear thermal expansion coefficients of the ¡-hexacelsianand ¢-hexacelsian unit cells showed nearly constant values of 6 © 10¹6/K, though the expansion anisotropy of the crystal axes inunit cell altered depending on the chemical composition.©2016 The Ceramic Society of Japan. All rights reserved.

Key-words : Thermal expansion, Hexacelsian, Lattice parameter, Phase transition

[Received January 6, 2016; Accepted June 24, 2016]

1. Introduction

Four different polymorphs of BaAl2Si2O8 and SrAl2Si2O8 arewell-known, those being monoclinic celsian, paracelsian, and ¡-hexacelsian and ¢-hexacelsian. Monoclinic celsian, a frameworkaluminosilicate, is the thermodynamically stable phase underambient conditions. Since this phase has a lower thermal expan-sion coefficient,1) celsian ceramics are useful in many applicationsdesigned around thermal shock-resistant materials. Although ithas many advantageous physical properties, the direct preparationof monoclinic celsian ceramics through various means such asthe solid state reaction of Ba-bearing materials with silica andalumina,2) sol­gel synthesis,3) or the crystallization of Ba-bearingaluminosilicate glasses,4) is difficult to accomplish without the useof additives, since the initially formed hexacelsian transforms tomonoclinic celsian at a very slow rate5),6) even at high temper-atures. Hexacelsian is a metastable polymorph with a diphyllo-aluminosilicate structure7),8) and has a higher thermal expansioncoefficient.7) Upon heating, Ba-hexacelsian and Sr-hexacelsiantransform from the orthorhombic ¡-phase (pseudo-hexagonal) tothe hexagonal ¢-phase at 300°C7),9) and at 700°C,10) respectively.Relatively pure Ba-hexacelsian and Sr-hexacelsian reconstruc-tively transform to monoclinic celsian when heated above 13006)

and 1200°C,10) respectively. Since Ba-hexacelsian and Sr-hexacelsian have relatively high thermal expansion and low di-electric constants,10) they are potential candidate materials for sub-strates of multilayered ceramics. Control of the thermal expansioncoefficient is very important in order to conform to electronicpackaging materials such as metals or dielectric materials. How-ever, the effects of chemical composition on the thermal expan-sion properties of hexacelsian polycrystals and unit cell have not

yet been sufficiently examined. Especially, there are few reportsabout the thermal expansion property of Sr-hexacelsian except ourreport10) because of the difficulty to prepare single-crystalline Sr-hexacelsian ceramics.In this study, single-crystalline hexacelsian solid-solution

(SrxBa1¹xAl2Si2O8, x = 0­1.0) polycrystals could be preparedby the thermal reaction of BaCO3, SrCO3 and finely purifiedkaolin, and the effects of chemical composition on ¡-¢ transitiontemperatures and thermal expansion characteristics were clarifiedusing high temperature X-ray powder diffraction (XRD) anddilatometric measurements of sintered polycrystals.

2. Experimental procedures

2.1 Sample preparationMany preparation routes for hexacelsian have been reported,

such as solid state thermal reactions between barium carbonateand silica with alumina11),12) or kaolin,13),14) solution-polymeriza-tion routes15) and sol­gel methods.3),9) In this study, New Zealandkaolin (hereafter referred to kaolin) was used as a starting mate-rial. Kaolin was elutriated into particle size 1¯m or less by sedi-mentation in an aqueous suspension. Chemical analysis of thiskaolin is shown in Table 1, revealing that purified kaolin had amolar SiO2/Al2O3 ratio of 2.01, and had an almost ideal com-position for kaolinite (Al2O3·2SiO2·2H2O). The alkaline metaloxide (K2O, Na2O) content in the kaolin was extremely low.Reagent grade BaCO3 and SrCO3 were wet-milled into a meanparticle size of 1.5¯m in order to achieve sufficient thermalreactions with kaolin. After weighing BaCO3, SrCO3 and kaolinin different compositions (SrxBa1¹xAl2Si2O8, x = 0­1.0), themixtures were ultrasonically dispersed and gradually dried whilebeing stirred using a mortar. Powder mixtures were calcined at800°C for 10 h and then wet-milled for 20 h in ethanol. Thecalcined powders were uniaxially compressed at 49MPa to formdisks and then isostatically compressed at 198MPa. These green

³ Corresponding author: Y. Kobayashi; E-mail: y-kobayashi@aitech.ac.jp

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disks were heat-treated in air at a heating rate of 2.5°C/min.Samples were kept at their respective temperatures for 1 h, andthen left to cool down by simply turning the furnace off.

2.2 MeasurementsThe bulk density and apparent porosity of sintered samples

were determined by using the Archimedes immersion technique.The crystalline phases were identified by using XRD (ModelRINT2500/PC, Rigaku Denki, Tokyo, Japan). Linear thermalexpansions of the ¡-hexacelsian and ¢-hexacelsian unit cellswere carried out using high-temperature powder X-ray diffractionat a scanning speed of 0.25° (2ª)/min under argon gas. UltrapureSi powder was mixed with sample powders as an inner standardfor precise measurement.16) For better comparison, a hexagonalpseudocell was used for the ¡-hexagonal phase as well.The thermal expansion measurement of sintered samples was

carried out using a thermomechanical analysis apparatus (TMA,Model TAS-100, Rigaku Denki, Tokyo, Japan) at a heating rateof 10°C/min.

3. Results

3.1 Thermal reaction and crystallizationIn this study, all mixtures of kaolin with BaCO3 and/or SrCO3

calcined at 800°C for 10 h were confirmed to be amorphousaccording to X-ray diffraction measurements. Although thedecomposition temperatures of BaCO3 and SrCO3 were above1050°C, dehydrated kaolin reacted with decomposed BaO and/orSrO, generating amorphous materials without any trace of BaOand/or SrO.14) X-ray diffraction patterns of BaAl2Si2O8 speci-mens heat-treated at 900­1200°C for 1 h are shown in Fig. 1.Sintered specimens heat-treated at 900°C were amorphous,showing structures similar to the calcined powders. When heat-treated at 950°C, hexacelsian rapidly crystallized and consumedthe amorphous materials, as shown by the disappearance of thebroad X-ray diffraction peak around 20­35°. Above 950°C, sin-gle crystalline phase of hexacelsian was observed up to 1200°Cand all the baselines of X-ray diffraction patterns from 20 to 35°were straight.X-ray diffraction patterns of SrxBa1¹xAl2Si2O8 specimens (x =

0­1.0) heat-treated at 1000°C for 1 h are shown in Fig. 2. Siliconwere added in order to measure the diffraction peak positionsof the hexacelsian crystal accurately. No crystals other thanhexacelsian and silicon were observed in these diffraction pat-terns. Many of the diffraction peaks shifted to higher diffractionangles with the increase in Sr content. This result reveals theformation of Ba and Sr hexacelsian solid solutions over a widecomposition range. Lattice parameters of hexacelsian (hexagonalpseudocell) calculated from the precise measurement of manypeak positions are shown in Fig. 3. The lattice parameters a0 and

c0 decreased almost linearly with the increase in Sr content,because of the smaller ionic radius of Sr than that of Ba.Figure 4 shows relative densities and apparent porosity of the

sintered bodies of these hexacelsian solid solutions. The relativedensity of a given sample was calculated from the bulk densitymeasured using the Archimedes immersion method and the truedensity calculated from their unit cell volume with the molecularweight. The relative density of each body rapidly increased

Table 1. Chemical composition of N. Z. kaolin

Raw Purified

SiO2 48.86 45.72Al2O3 36.36 38.09Fe2O3 0.26 0.25TiO2 0.08 0.08CaO 0.01 0.01MgO 0.01 0.08K2O 0.01 0.04Na2O 0.04 0.06Ig. loss 13.97 14.05

Total 99.6 98.99

(mass%)

Fig. 1. X-ray diffraction patterns of specimens having chemicalcomposition of BaAl2Si2O8 heat-treated at 900­1200°C.

Fig. 2. X-ray diffraction patterns of specimens having chemicalcomposition of (Ba1¹x,Srx)Al2Si2O8 heat-treated at 1000°C for 1 h.

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around 900­950°C, reaching 94% or more in all compositions.This rapid densification is brought about by the viscous flow oftransiently formed amorphous materials as shown by the X-raydiffraction patterns in Fig. 1. This viscous transient sintering andrapid crystallization have already been reported for the mix-tures of kaolin with BaCO3, SrCO3

14) or CaCO3.17),18) Figure 5shows SEM photographs of BaAl2Si2O8 hexacelsian polycrystalsheat-treated at 1100°C for 1 h. Only a small amount of poreswas observed in Fig. 5(a) showing densely sintered polycrystals.Figure 5(b) shows many micron-size microcracks, probablycreated by HF-etching of the grain boundaries. SEM Photographsof Sr-hexacelsian polycrystals heat-treated at 1000°C showedalmost much the same microstructure with Ba-hexacelsian.

3.2 Comparison of thermal expansion character-istics of unit cell with sintered polycrystals

The dilatometric expansion curves (TMA) during heating areshown in Figs. 6­9 along with thermal expansion curves corre-sponding to the a-axis and c-axis lattice parameters of hexacelsian(SrxBa1¹xAl2Si2O8, x = 0­1.0). In this study, an average latticeexpansion value E was approximated by (2Ea+Ec)/3 assuming ahexagonal pseudocell. ¡-¢ hexacelsian transition temperaturechanged depending on the compositional ratio of Ba and Sr.Moreover, ¡-hexacelsian sluggishly transformed to ¢-phaseover a wide temperature range though it was a displacementtransition.Total thermal expansion of a crystal is the sum of the stretching

of atomic bond lengths and the structural expansion arising fromthe bonding angle changes among atoms induced by displace-

Fig. 3. Effect of molar ratio of Sr and Ba on lattice parameters ofunit cell.

Fig. 4. Relative density and apparent porosity of Ba­Sr hexacelsian.

Fig. 5. SEM photographs of BaAl2Si2O8 hexacelsian polycrystals heat-treated at 1100°C for 1 h.

Fig. 6. Thermal expansion curves of Ba:Sr = 10:0 hexacelsian calcu-lated from unit cell parameters.

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ment phase transitions. In the case of hexacelsian, the totalthermal expansion of the crystal is assumed to be the sum of (1)stretching of the ¡-hexacelsian crystal lattice from room temper-ature to transition temperature, (2) expansion induced by ¡-¢hexacelsian transition arising from the bonding angle changesamong atoms and (3) stretching of the ¢-hexacelsian crystal lat-tice as a function of temperature. In this study, the thermal expan-sion of ¡-hexacelsian is approximated by straight line below thetransition temperature and that of ¢-hexacelsian is approximatedby straight line above the transition temperature. Furthermore, thedifference in two extrapolating straight lines at a middle temper-ature is defined as the thermal expansion gap induced by ¡-¢transition. Thus, the whole thermal expansion of hexacelsian isthe sum of three.In Fig. 6, the dilatometric thermal expansion curve of the Ba-

hexacelsian (Ba:Sr = 10:0) adequately coincided with the aver-age expansion curves of the lattice parameters across a widetemperature range. At the ¡-¢ hexacelsian phase transition

around 300°C, Ea increased greatly while Ec remained small.Linear thermal expansion coefficients of the cell parameters of¡-hexacelsian (25­200°C) and ¢-hexacelsian (400­900°C) hadnearly identical values of ¡ = 6.2 © 10¹6/K. An average expan-sion gap associated with the ¡-¢ hexacelsian phase transitionaround 300°C was estimated to be about 0.13% as shown inFig. 6, and the volume change of the phase transition corre-sponded to 0.38 vol%. This volume change was mostly broughton by elongation of the a-axis in the unit cell of Ba-hexacelsian.These thermal expansion properties of Ba-hexacelsian are ingood agreement with results reported by Bumpei Yoshiki et al.7)

and Julius Schneider et al.19) Specimens having Ba:Sr ratio =8:2­6:4 showed nearly identical thermal expansion characteristicsas that of Ba-hexacelsian, and the volume change associated withthe phase transition remained constant.On the other hand, specimens having Ba:Sr ratios = 4:6­2:8

demonstrated considerably different characteristics from thosespecimens discussed above. For the Ba:Sr = 4:6 specimens asshown in Fig. 7, the dilatometric and cell dimensional expansiongaps associated with the ¡-¢ hexacelsian phase transition wereboth estimated to be approximately 0.32%, much larger thanthose of the Ba:Sr = 10:0­6:4 specimens. The volume change atthe displacement phase transition amounted to about 1.0%. Thislarge increase in cell volume was caused by the expansion of boththe a-axis and the c-axis of hexacelsian. Furthermore, the c-axiscell parameter of hexacelsian shows a critical change at 400°C,while the a-axis cell parameter shows a more gradual changefrom 200 to 500°C. Ba:Sr = 2:8 specimen demonstrated similarthermal expansion characteristics as those seen at Ba:Sr = 4:6,as shown in Fig. 8. Thermal expansion of the a-axis and c-axiscell parameters exhibited nearly identical values, and the averagecurve of the cell parameters departed from the dilatometric curve.The dilatometric and cell dimensional expansion gaps associatedwith the ¡-¢ hexacelsian phase transition were both estimatedto be approximately 0.37% and the volume change at the phasetransition reached 1.1%.The Ba:Sr = 0:10 specimen (SrAl2Si2O8) demonstrated much

different thermal expansion behavior from the other samples asshown in Fig. 9. The linear thermal expansion curve of the sin-tered body showed an extremely sluggish ¡-¢ transition from

Fig. 9. Thermal expansion curves of Ba:Sr = 0:10 specimen. Thermalexpansion of bulk specimen was measured by TMA, and that of unit cellwas measured by high temperature X-ray diffraction.

Fig. 7. Thermal expansion curves of Ba:Sr = 4:6 specimen. Thermalexpansion of bulk specimen was measured by TMA, and that of unit cellwas measured by high temperature X-ray diffraction.

Fig. 8. Thermal expansion curves of Ba:Sr = 2:8 specimen. Thermalexpansion of bulk specimen was measured by TMA, and that of unit cellwas measured by high temperature X-ray diffraction.

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300 to 800°C. However, the change in cell parameters along thea-axis and c-axis showed a critical phase transition at 650­750°C,making the thermal expansion curves of the sintered body andcell dimensions much different. The cell parameter changes at thephase transition amounted to 0.45 and 0.59% along the a-axisand c-axis, respectively.The thermal expansion of hexacelsian from room temperature

to higher temperature can be examined by dividing the changesin two factors; one is a linear thermal expansion of hexacelsianwith temperature and the other is the expansion gap associatedwith the ¡-¢ hexacelsian phase transition. The linear thermalexpansion coefficients of ¡- and ¢-hexacelsian (x = 0­1.0) werealmost constant at a value of 6 © 10¹6/K, though the thermalexpansion property differed along the crystal axes. On theother hand, the unit cell expansion gap associated with the ¡-¢hexacelsian phase transition increased with the increase in Srcontent. In particular, the lattice volume change associated withphase transition of Sr-hexacelsian reached a value of 1.5 vol%.These expansion properties of the unit cell affected the com-plicated dilatometric expansion of sintered hexacelsian solidsolution (Ba,Sr)Al2Si2O8. Though a large expansion gap wasobserved at ¡-¢ hexacelsian phase transition, any microcracks, asknown for ¡-¢ transition of quartz in porcelain body,20) were notobserved at the polished surfaces of Sr-hexacelsian polycrystals.Furthermore, the size of sintered specimens came back to thestarting length accurately at the thermal expansion measurementon heating and followed cooling. No Microcracks are thought tooccur because ¡-hexacelsian sluggishly transforms to ¢-phaseover a wide temperature range.

3.3 Thermal expansion of Ba, Sr-hexacelsiansintered polycrystals

In this study, all of the thermal expansion of sintered poly-crystals were not straight but sigmoidal curves, since they includ-ed a sluggish ¡-¢ transition between 300 and 700°C. Therefore,the thermal expansion coefficient changed depending on the Ba/Sr ratio. The thermal expansion coefficients of SrxBa1¹xAl2Si2O8

(x = 0­1.0) from 25 to 900°C are shown in Table 2. The thermalexpansion coefficient between 25 and 900°C almost linearlyincreased from 8.0 to 10.5 © 10¹6/K with the increase in Srcontent. Yen-Pei Fu et al.12) reported that the thermal expansioncoefficient of Sr-hexacelsian reached to 15 © 10¹6/K (25­400°C)and the appropriate replacement of Ba with Sr acted to lowerthe thermal expansion coefficient because of the enhancement ofthe hexacelsian-to-celsian transformation. In this paper, on theother hand, all specimens consisted of single-crystalline-phasehexacelsian, so that these results correspond to the true thermalexpansion property of (Ba­Sr) hexacelsian solid solutions.

4. Conclusions

Stoichiometric hexacelsian ceramics were prepared by the ther-mal reaction of BaCO3, SrCO3 and purified kaolin. The thermalexpansion properties of the unit cells and polycrystals wereexamined using high-temperature X-ray diffraction and dilatom-etry, respectively.(1) Single-crystalline-phase hexacelsian solid solutions

SrxBa1¹xAl2Si2O8 (x = 0­1.0) could be prepared by thethermal reaction of BaCO3 and SrCO3 with kaolin at1000°C.

(2) Densely sintered hexacelsian polycrystals heat-treated at1000°C showed gradual ¡-¢ phase transitions, and thetransition temperature increased from 300 to 700°C withthe increase in Sr content.

(3) The linear thermal expansion gap associated with the ¡-¢hexacelsian phase transition increased from 0.13 to 0.50%with the increase in Sr content. On the other hand, thethermal expansion coefficient of hexacelsian solid solu-tions SrxBa1¹xAl2Si2O8 (x = 0­1.0) remained constant atabout 6.0 © 10¹6/K over a wide range of composition.

(4) Lattice volume change associated with ¡-¢ phase transi-tion altered from 0.38 to 1.5 vol% with the increase in Srcontent.

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Table 2. Thermal expansion coefficient (T.E.C.) of Ba­Sr hexacelsianpolycrystals on heating

Ba:Sr T.E.C. (10¹6/K)

10:0 8.08:2 8.36:4 9.04:6 9.72:8 10.10:10 10.5

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