crystal growth and dielectric properties of batio3 obtained in aluminoborosilicate glasses

Journal of Non-Crystalline Solids xxx (2013) xxx–xxx

NOC-16767; No of Pages 6

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Journal of Non-Crystalline Solids

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Crystal growth and dielectric properties of BaTiO3 obtained inaluminoborosilicate glasses

Ruzha Harizanova a,⁎, Christian Bocker b, Georgi Avdeev c, Christian Rüssel b, Ivailo Gugov a

a University of Chemical Technology and Metallurgy, 8 Kl. Ohridski Blvd., 1756 Sofia, Bulgariab Otto-Schott-Institut, Jena University, Fraunhoferstr. 6, 07743 Jena, Germanyc Institute of Physical Chemistry, Bulgarian Academy of Sciences, Block 11, Acad. G. Bonchev Str., 1113 Sofia, Bulgaria

⁎ Corresponding author. Tel.: +359 2 8163 449.E-mail address: [email protected] (R. Hariz

0022-3093/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.jnoncrysol.2013.11.027

Please cite this article as: R. Harizanova, et alCryst. Solids (2013), http://dx.doi.org/10.101

a b s t r a c t
a r t i c l e i n f o
Article history:Received 30 August 2013Received in revised form 7 November 2013Available online xxxx

Keywords:Barium titanate;Invert glass;Crystallization;Impedance spectroscopy;Dielectric constant

The synthesis of invert glasses in the system Na2O/TiO2/BaO/Al2O3/B2O3/SiO2/Fe2O3 is reported for different[Na2O]/[Al2O3]-ratios and glass-former concentration less than 30 mol%. During annealing in the glasses cubicand tetragonal BaTiO3 crystals are simultaneously crystallized as proved by X-ray diffraction. Scanning electronmicroscopy micrographs show that during isothermal heat-treatment globular crystals with sizes from100 nm to 1 μm are formed. The dielectric properties of the glass-ceramics are studied at room temperatureby impedance spectroscopy using two contact points measurement and gold electrodes. The impedance andthe phase angle are measured in the frequency range from 0.1 Hz to 1 MHz. The obtained impedance spectraare simulated using an appropriate equivalent circuit in order to extract information on the bulk resistanceand capacitance of the sample. Measurements of the capacitance at fixed frequencies are carried out and thedielectric constants are calculated. High dielectric constants in the order of 1000 at a frequency of f = 13 Hzare estimated, which increase with the increasing annealing times and decrease with increasing frequencies.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In the early 1960s the concept of the invert oxide glasses was intro-duced by Stevels and Trap [1]. These are glasses with less than 50 mol%glass-forming oxides which however preserve their amorphous state.Such glasses show a high tendency towards crystallization [1–4]. Inthem can easily be precipitated one or more crystalline phases withlarge volume fractions. An appropriate thermal treatment may lead tocontrolled crystallization of a particular phase with tailored crystallitesizes [2,4] and advanced electrical and/or magnetic properties [2–9].Glass compositions containing both alkaline earth and transition metaloxides are an example for systems in which invert glasses with highcrystallization tendency and interesting transport propertiesmaybe ob-tained [1–3]. The controlled crystallization also allows to obtain from anoxide glass crystalline phases with extremely high melting points atlower temperatures. This technique can be applied for the synthesis ofdielectric materials such as barium titanate, BaTiO3. It is a well-knowndielectric with numerous allotropic modifications. The tetragonalBaTiO3 is ferroelectric up to about 120 °C (Curie temperature) and dueto its high dielectric constant is used for the preparation of capacitors,thus finding application in electronics, sensor technology and could bean efficient substitute of the magnetic RAM, e.g. as ferroelectric RAM(FRAM) [10–18]. The cubic BaTiO3 also has a high dielectric constantand due to the lack of ferroelectricity it exhibits isotropic dielectric

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., Crystal growth and dielectri6/j.jnoncrysol.2013.11.027

properties [10,13,14]which determine its use in the preparation of elec-tronic elements for energy storage [10,12,13]. Depending on its opticalproperties, the cubic modification may also be a promising candidatefor optoelectronic applications [14]. Many experimental techniquesexist for the preparation of BaTiO3 as bulk material, [10,11,13,14,18] aswell as in the form of thin films [12,15,19] — among these are differentchemical routes [16–20] or sintering of powdered precursors [11,21].The addition of 3d-transition metal oxides, for example Fe, to systemsin which BaTiO3 is crystallized was also reported [2,11]. The growth ofcore-shell nanoparticleswith high volume fraction and tailored size dis-tribution composed of BaTiO3 and magnetite, Fe3O4 in an oxide glassmight lead to materials for application in spintronics (see Ref. [2]).One well-studied approach to control the crystal size and the volumefraction of the crystalline phase is to adjust the viscosity of the glassby varying the ratio of alkaline and aluminum oxides [22–27].

The present work reports on the crystallization of BaTiO3 in glasseswith invert compositions (less than 30 mol% glass-forming oxides):(23.1-x)Na2O/23.1BaO/23TiO2/7.6B2O3/17.4SiO2/5.8Fe2O3/xAl2O3 andthe resulting microstructures. Further, it shows the dependence of thecrystal type, size and morphology on the [Na2O]/[Al2O3] ratio. Thedielectric constants of the synthesized glass-ceramic materials aredetermined by impedance spectroscopy.

2. Experimental

The ratio [Na2O]/[Al2O3] is varied and a series of compositions withthe formula (23.1-x)Na2O/23.1BaO/23TiO2/7.6B2O3/17.4SiO2/5.8Fe2O3/

c properties of BaTiO3 obtained in aluminoborosilicate glasses, J. Non-

http://dx.doi.org/10.1016/j.jnoncrysol.2013.11.027

mailto:[email protected]

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Fig. 2. XRD patterns of samples: (1) —with 0 mol% Al2O3 annealed for 4 h at 550 ° C, (2)3 mol% Al2O3 annealed for 3 h at 550 °C and (3) 7 mol% Al2O3 annealed for 3 h at 610 °C— crystallization of BaTiO3 and a second (Na, Al)-silicate phase (6 main peaks designatedby arrows).

2 R. Harizanova et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx

xAl2O3 with x = 0, 3 and 7 mol% are melted from reagent grade rawmaterials: Na2CO3, BaCO3, TiO2, Al(OH)3, B(OH)3, SiO2 and Fe2O3. Allglasses aremelted in 60 g batches for 1 h at 1250 °C in air using a Pt cru-cible in a furnace with SiC heating elements. The melts are quenched(without pressing) on a copper block and transferred to a pre-heatedC-mold, held for 10–15 min at 450 °C in a muffle furnace and afterswitching off the furnace, allowed to cool down.

The phase compositions of the samples were studied by X-ray dif-fraction (XRD), Siemens D5000, using Cu-Kα radiation (λ = 1.5406 Å)and Ni filter. The microstructure and the elemental composition of theglasses and the crystallized specimens were further analyzed byscanning electron microscopy (SEM), combined with energy-dispersiveX-ray spectroscopy (EDS), (JSM-7001 F, JEOL Ltd., Japan). Imaging ofthe crystallized samples was performed on both polished and if nogood contrast is achieved, on etched surfaces (5 s in 1% HCl solutionand then rinsing with distilled water). Cylindrical samples with diame-ter 10 mm and thickness of about 5 mmwere prepared for the electri-cal measurements. Both plane surfaces were roughly polished and Aulayers were sputtered on them to prepare the electrodes. Then the ob-tained samples were contacted by copper wires to the impedance ana-lyzer. The dielectric properties were investigated by impedancespectroscopy at room temperature in the frequency range from 0.1 Hzto 1 MHz. The amplitude of the perturbing ac signal was always50 mV, thus achieving the best ratio between linear response of theperturbed system and low noise due to the high impedances at roomtemperature. The impedance and phase angle were measured asfunction of the frequency and an appropriate equivalent circuit wasproposed (impedancemeter Zahner IM6,Kronach,Germany). The capac-itance derived from the equivalent circuit was determined at severalfrequencies and for known sample geometry the correspondingdielectric constants were calculated.

3. Results

The results from the differential thermal analyses of the obtainedglasses are shown in Fig. 1. Both glass transition and crystallizationpeak temperatures increase with the increasing alumina concentration:Tg = 430 °C, Tc = 540 °C for 0 mol%; Tg = 440 °C, Tc = 560 °C for3 mol% and Tg = 480 °C, Tc = 630 °C for 7 mol% alumina. The XRDpatterns of glasses with 0, 3 and 7 mol% Al2O3 treated for 3–4 h at550 °C and 610 °C are shown in Fig. 2. Here, in the glasses with 0 and3 mol% Al2O3 only BaTiO3 (JCPDS 1-74-1962) is observed while for thesample with 7 mol% alumina also the presence of some quantity ofNaAlSiO4 (JCPDS 19-1176) is detected. In Fig. 3, the comparison of theXRD patterns of the as melted glass with samples annealed at 550 °Cfor periods of time from 5 min to 7 h for 3 mol% Al2O3 is shown.For the sample annealed for 10 min already the main peaks ofBaTiO3 appear. The crystallization of only one phase — cubic BaTiO3

Fig. 1. DTA patterns of the three glasses.

Please cite this article as: R. Harizanova, et al., Crystal growth and dielectriCryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.11.027

(JCPDS 1-74-1962) is established. In Fig. 4, the Rietveld refinement ofthe peak at 45.3°, used to distinguish between tetragonal and cubicBaTiO3, for sample with 3 mol% Al2O3 annealed at 550 °C for 15 minsuggests that it can be described as a superposition of two overlappingpeaks. This indicates that simultaneously the cubic and the tetragonalBaTiO3 modifications are present. Increasing annealing time at a con-stant crystallization temperature of 550 °C for the same composition re-sults in crystal growth (see Fig. 3). In Fig. 5, an SEM-micrograph of asample without Al2O3 is shown with large needle-like crystals growingduring quenching of the melt and globular ones precipitating after an-nealing the sample above Tg. In this sample, similar crystal morphologyis observed as in the samples with 3 mol% Al2O3 annealed at 550 °C fordifferent times [3,28] — as shown in Fig. 6. Figs. 7a and b reveal theoverall crystallization behavior of the sample with 7 mol% Al2O3 asshown for the specimen annealed for 5 h at 610 °C (near the DTAcrystallization peak maximum). In this sample BaTiO3 crystals showalmost spherical shape with notably rounded edges and are generallymuch smaller than those in the samples with smaller alumina concen-tration. However, in the same sample with 7 mol% Al2O3 also largecrystals of a Na, Al-silicate phase are randomly distributed, as evidencedby EDS analyses, cf. Fig. 7b. Fig. 7b shows point analyses spectrarecorded from this crystal and the surrounding matrix (which consistsof the BaTiO3 crystal phase and the residual glassy phase). It cannot beexcluded that the analysis of the second crystal phase is affected by

Fig. 3. XRD patterns of untreated glass (1) and annealed samples with 3 mol% Al2O3:5 min (2), 10 min (3), 15 min (4), 30 min (5), 1 h (6), 2 h (7), 3 h (8), 5 h (9) and 7 h(10) at 550 °C— crystallization of BaTiO3.


image of Fig.�1

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image of Fig.�3


Fig. 4. XRD pattern of a sample with 3 mol% Al2O3 and 20.1 mol% Na2O annealed for15 min at 550 °C — crystallization of BaTiO3 (JCPDS 1-74-1962); inset: magnified peakat 45.3° with peaks of the tetragonal phase fitted.

Fig. 6. SEM of a C-covered sample with 3 mol% Al2O3 and 20.1 mol% Na2O, annealed for5 h at 550 °C — crystallization of globular BaTiO3.

3R. Harizanova et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx

the glass matrix, due to the too large interaction volume of several μm(the crystal might be not dense enough) and it is observed that thecrystal is enriched in sodium and depleted in all other components.

The XRD patterns for the annealed samples with 7 mol% Al2O3 showvery weak peaks of a second crystalline phase, occurring in addition toBaTiO3. These mostly coincide with the main reflexes of the BaTiO3

and are attributed to a (Na, Al)-silicate phase.The impedance measurements on samples without Al2O3 showed

poor reproducibility of the results, attributed to the simultaneous pres-ence of large needle-like and small, nearly spherical crystals of BaTiO3

and multiple cracking which occur in the samples (see Fig. 5). In thesamples with 3 mol% Al2O3 the electrical measurements show higherreproducibility. Typical Bode plot and the attributed equivalent circuitare shown in Fig. 8 for a sample annealed for 3 h at 550 °C. Impedancemeasurements are also performed at samples annealed for 15 min at550 °C and similar type of change in both the impedance and thephase angle with increasing frequency is observed. The dielectric con-stants for these samples with globular BaTiO3 crystals were calculatedfrom the sample geometry. The dependence of the dielectric constanton the frequency for the samples annealed for 15 min and 3 h at550 °C is shown in Fig. 9. Annealing for 15 min leads to dielectricconstants in the range from 400 to 1100, while annealing for 3 h resultsin dielectric constants in the range from 900 to 1050. The dielectricconstants decrease with increasing frequency. This decrease is strongerin the sample annealed for 15 min than that in the sample thermallytreated for 3 h.

Fig. 5. SEM of a polished and C-covered sample without Al2O3 — large crystals of BaTiO3

occur and growth of spherical BaTiO3 crystals is observed after annealing for 4 h at 550 °C.


4. Discussion

TheDTA data in Fig. 1 for the characteristic temperatures of the threeglasses shows that the increase of the Al2O3 concentration leads to anincrease in the difference between the glass transition and crystalliza-tion temperatures. It can be assumed that the higher alumina concen-tration stabilizes the glass network which will lead to higher viscosityand lower crystal growth rate, i.e. the formation of smaller crystals —as also seen in Figs. 5–7a. Furthermore, the samplewith 7 mol% aluminashows a pronounced crystallization peak and increased glass transitiontemperatures in comparison to the glasses with 0 and 3 mol% Al2O3.

Fig. 7. a. SEM of a C-covered sample with 7 mol% Al2O3 and 16.1 mol% Na2O, annealed for5 h at 610 °C — crystallization of BaTiO3. b. EDS analysis of the second type of crystalspresent.


image of Fig.�4

image of Fig.�5

image of Fig.�6

image of Fig.�7


Fig. 8. Bode plot of a sample with 3 mol% Al2O3 annealed for 3 h at 550 °C — impedancemodulus and phase angle as function of frequency at room temperature.


This might be attributed to changes in the incorporation of Al3+ in theglass network. Fig. 2 shows that an Al2O3 concentration of ≤3 mol%leads to the formation of only one crystalline phase— BaTiO3. However,the increase in the Al2O3 concentration leads additionally to an increasein the crystallization temperature and also to the appearance of asecond crystalline phase — a phenomenon, though undesirable, oftenobserved in multicomponent oxide systems fromwhich the crystalliza-tion of BaTiO3 is envisaged [29]. Reference [29] shows that in the systemBaO–TiO2–B2O3 additional crystallization of a barium borate phase andeven of TiO2 occurs, depending on the particular sample composition,i.e. concentration of the glass former and also, on the heat treatmentschedule supplied. In our multicomponent system only formation ofBaTiO3 is observed for Al2O3 concentrations of ≤3 mol% — as seen inFig. 3. Here the growth of BaTiO3 starts at an annealing time of 10 minas shown in the XRD patterns. With increasing annealing time, thesize of the crystals increases. Additional problem in the case of crystalli-zation of BaTiO3 could be the identification of the exact crystallinemodification formed. To decide whether tetragonal, cubic or someother modifications of BaTiO3 crystallizes are crucial for the applicationof the obtainedmaterials [10–14]. Usually, the two competingmodifica-tions during the crystallization of BaTiO3 using different synthesistechniques [10,12,13,20], are the tetragonal and the cubic ones. Thepeak at 2θ = 45.3° in the XRD pattern is used to decide whether cubicor tetragonal BaTiO3 is formed [10,16]. In Fig. 4 a slight splitting of thepeak at 45.3° is observed which might serve as indication that simulta-neously tetragonal and cubic BaTiO3 are present [10]. Identification ofthe BaTiO3modification is done also by using IR- and Raman spectrosco-py [10,16,30] which in our case might be difficult or even impossible

Fig. 9.Dielectric constant as a function of frequency at room temperature for sampleswith3 mol% Al2O3 — annealed for 15 min (■) and 3 h (Δ) at 550 °C.


due to the numerous constituents of the initial glass. One possibleexplanation for the simultaneous occurrence of cubic and tetragonalbarium titanate is that the samples are annealed between 550 and610 °C, i.e. above the Curie temperature for BaTiO3 and then quenchedto room temperature. So, cubic BaTiO3 should be the phase formed athigh temperatures. During cooling, pure cubic BaTiO3 should transformto tetragonal BaTiO3 below the Curie temperature (~120 °C). Hereby,the crystal size may be decisive whether tetragonal or cubic BaTiO3 isfinally formed [10]. Furthermore, the formed crystals may containimpurities such as iron which affect the Curie temperature and hencethe phase transformation. It is reported in [31] that sol–gel derivedBa0.9Fe0.1TiO3 is not ferroelectric and hence iron doping strongly affectsthe dielectric properties. In the present investigation, however, the de-termined dielectric constants are much higher than these reported in[31] and comparable to those reported for ferroelectric BaTiO3 [32] —see Fig. 9. This evidences that either no Fe or only a minor quantity ofiron is incorporated in the BaTiO3 crystals. So, the iron present couldpartly influence the crystalline structure butwill not affect the dielectricconstant values, as observed in Figs. 2–4 and 9. Then, depending on thecrystal size and the quantity of iron incorporated into the lattice, andpossibly the cooling rate, both the tetragonal and the cubic phases ofBaTiO3 might be observed. Furthermore, it should be stated that the te-tragonal phase exists only in samples annealed for a comparably shortperiod of time. For longer annealing times it disappears again aswitnessed by the XRD patterns in Figs. 2–4 — the splitting of the samepeak is even less pronounced or missing. Since after longer crystalliza-tion times, the formed crystals should be larger, more and more tetrag-onal BaTiO3 should be formed [10,21]. However, this explanation ishighly improbable in the present case and the incorporation of ironafter long annealing times could be decisive. During annealing, the ma-trix around the crystal is more and more depleted in barium and titani-um and enriched in the other glass components. At a certain time, whenmost of the crystal forming components are consumed, one can assumethat the only possibility for further crystal growth is the incorporation ofiron which however is not supported up to now from the data availablein the present study.

The microstructure of all annealed samples is studied by SEM. Thedecrease in the ratio [Na2O]/[Al2O3] leads to the formation of smallercrystals as seen from Figs. 5–7. This may be explained, as observed insimilar oxide glass systems [22–27], with the changing viscosity forglasses with different Na2O to Al2O3 ratio. As stated in Refs. [25–27]for [Na2O] N [Al2O3] the viscosity of the melt is comparatively smalland the diffusion of the crystallizing species will be fast, so that the nu-cleation rate is high and the crystal growth velocity large. When theAl2O3 concentration increases, the viscosity of the melt also increasesand the diffusion of the Ba and Ti ions in the glass is decelerated, thusmaking the crystal growth more difficult and slow. The sample with23.1 mol% NaO2 and no Al2O3 tend to crystallize even during coolingof themelt and in Fig. 5 two types of crystals are seen. The large ones re-sult from the spontaneous nucleation and crystal growth during coolingfrom melt temperature while the subsequent annealing at 550 °C re-sults in further nucleation and crystallization of small spherical BaTiO3

crystals which grow between the large initially formed ones. The sizesof the globular crystals vary for this sample from some hundred nm'sto about 1 μm. For the whole set of reported compositions the globularBaTiO3 crystals tend to grow together and form interconnected struc-tures. This observation can be attributed to a phenomenon alreadyobserved in other oxide glasses and glass-ceramics with similar compo-sition and higher iron concentration [33,34] which tend to liquid/liquidphase separation. After the substitution of 3 mol% Na2O against Al2O3,amorphous products are obtained during cooling the melt. Annealingof this glass at 550 °C for different periods of time always leads to nucle-ation and growth of globular BaTiO3, cf. Fig. 6. The average size of theformed crystals depends on the annealing time and for the samplefrom Fig. 6 is about 600 nm as estimated from the SEM micrographs.Something which is also noteworthy to mention is that the crystals for


image of Fig.�8

image of Fig.�9


5R. Harizanova et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx

annealing time 5 h both show a tendency towards clustering and theformation of aggregates — this supports the hypothesis that in thepresent system phase separation at some stages of the glass formationmight occur [33,34]. This phase separation leads to further growth ofthe barium titanate crystals in the regions enriched in Ba and Ti andmay thoroughly explain the occurrence of the interconnected crystal-line structures. The SEM micrograph of the annealed sample with16.1 mol% Na2O and 7 mol% Al2O3 shown in Fig. 7 again displays thepredominant formation of interconnected BaTiO3 crystals. Here, howev-er, themorphology of the crystals could scarcely be recognized. The con-trast is mainly material contrast, i.e. due to the detected backscatteredelectrons (BSE). Because of the comparable large information volumeof the BSE in the range of several 50 to 100 nm, the small structuresaround the spheres were not resolved. This did not allow concludingon the shape of the formed BaTiO3. Within the three studied composi-tions a dependency of the formed crystalline phases and resulting mi-crostructures on the presence of Fe-oxides in the composition up tonow has not been established. The initial idea that the combination offairly high BaO, TiO2 and Fe2O3 concentrations in an oxide glass maylead to formation of crystals composed by both BaTiO3 and magnetite,Fe3O4 and to multiferroic properties [2,18], was not confirmed in ourstudy.

The electrical properties of selected samples from the compositionswith 0 and 3 mol% Al2O3 are studied using impedance analyzer ZahnerIM6. The impedance spectra were recorded at room temperature as afunction of the frequency and illustrated by Bode plots. The presenceof two morphologically different types of BaTiO3 crystals in case of0 mol% Al2O3 leads, as mentioned in Section 3, to poor reproducibilityin the results from electrical measurements. So, our investigations ofthe dielectric properties in the following are focused on the compositionwith 20.1 mol% Na2O and 3 mol% Al2O3 where glassy samples were ob-tained after quenching the melt and only globular BaTiO3 crystals areobserved in all samples annealed at 550 °C. Hence, the dielectric proper-ties of selected samples with 3 mol% Al2O3 annealed at 550 °C are stud-ied. The impedance spectrawere recorded as a function of the frequencyand illustrated by Bode plots at room temperature. In Fig. 8, the Bodeplot allows to describe the behavior of the sample annealed for 3 h at550 °C by an equivalent circuit consisting of a serial connection of twoRC (resistivity and capacitance) in parallel. One of the RC-parallel is at-tributed to the behavior of the crystalline BaTiO3 phase and dominateschanges in impedance and phase angle at higher frequencies. Thesecond RC-parallel describes the electrical properties of the sample atlower frequencies and is attributed to the dielectric properties of theglassy phase. The resistance of the sample at room temperature is veryhigh – more than 1 GΩ at about 1 Hz. Such impedances are at thelimit of accuracy of the used Zahner impedance analyzer and the at-tempt to simulate the experimental spectrum and extract informationabout the numerical values of the resistances and the capacitancesattributed to the sample behavior was unsatisfactory. For this reasonthe shape of the obtained spectra is only used to get an overview onthe behavior of the sample and to choose the frequency range for themeasurement of the capacitance, dominating at higher frequencies, ata fixed frequency. The values of the phase angle for the sample annealedfor 3 h at 550 °C vary from −89 to −87° in the frequency range from100 kHz to about 100 Hz and for this reason an ideal capacitancecould be used in the equivalent circuit to describe the behavior of thesample in this frequency range. However, since the relaxation phenom-ena in glassy and glass-ceramic materials are usually regarded as non-Debye relaxation processes [35], in the equivalent circuit both capaci-tances are chosen to be CPE, i.e. loss-capacitances. The same Bode plotdeconvolution was done for the sample annealed 15 min at 550 °Cand again an equivalent circuit composed by a serial connection of 2RC circuits in parallel is used, where the second RCwhich is responsiblefor the glassy phase is attributed to larger phase angle, varying from −10 to −78 deg. This behavior can be explained by the higher volumefraction of the glassy phase in this sample and the additional presence


of Fe in the glass composition. Fe if present in two valence statesmight also contribute to the conduction process thus leading to polari-zation of the glass and the occurrence of a loss capacitance in the imped-ance spectrum. Measurements of the capacitance for some fixedfrequencies in the range from 1 Hz to 150 kHz have been carried out.The dielectric constants are calculated and shown in Fig. 9 for two sam-ples of interest — the first annealed for 15 min at 550 °C, mean crystal-lite size of BaTiO3 about 300 nm and the second annealed for 3 h at550 °C with mean crystallite size about 550 nm, i.e. comparable withthe average crystallite size shown in Fig. 6. The dielectric constant ofthe sample annealed for 3 h is always higher than that for the crystal-lized for 15 min. This is attributed to the higher quantity of the crystal-line phase and larger crystallite size in the sample annealed for a longertime since it is known that the dielectric constant depends on the crystalsize [10,21]. In [21] the crystal sizes, however, are much larger thanthose in the present study and possess solely tetragonal symmetryand thus much higher dielectric constants. Other authors report thesynthesis of BaTiO3 thin films by sol–gel method [19]. The resulting di-electric constants are notably smaller (around 200) than ours becausethe BaTiO3 particles are with sizes of some 10 nm and cubic symmetry.The dielectric constants of our glass-ceramic samples are comparable todata of other authors who synthesize purely ferroelectric BaTiO3 [32].

The dielectric constants of our samples decrease with the increasingfrequency as typical for all dielectric materials, as reported e.g. in ourearlier study of LiNbO3 silicate glass-ceramics [4]. Besides, the decreaseof the dielectric constant for the sample annealed for 3 h is lesspronounced than that of the one annealed for 15 min which might bedue to larger dielectric losses in the second sample.

5. Conclusion

For all studied compositions, the quenching of the melts resultsin glass formation. Annealing of samples from the bulk leads totheprecipitation of interconnected mostly spherical particles of cubicBaTiO3 for all [Na2O]/[Al2O3] ratios. The decrease in the ratio Na/Al is at-tributed to increasing viscosities, glass-transition and crystallizationtemperatures. This results in formation of smaller crystals. The Rietveldrefinement of the X-raydiffraction patterns suggests simultaneous crys-tallization of tetragonal and cubic BaTiO3 phase. The recorded X-raydiffraction patterns and the EDS analyses allow to conclude that thepresence of Fe in the composition has no influence on the phase compo-sition and symmetry of the BaTiO3 crystals. The dielectric constant,calculated for the samples with 3 mol% Al2O3 at room temperature, isabout 1000 at lower frequencies and decreaseswith increasing frequen-cy and increases if the annealing time is increased while the annealingtemperature is kept constant.

Acknowledgments

Thisworkwasfinancially supported bymeans of contract D02-180/14.02.2013, index: P-6-07/13, project: BG051PO001-3.3-05/0001«Science-business», funded by Operational program «Development ofhuman resources».

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crystal growth and dielectric properties of batio3 obtained in aluminoborosilicate glasses

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