› journal › node › j2018-6 › 5_17-203_p_1061... electron microscopy investigation of the...

Ruzha Harizanova, Liliya Vladislavova, Georgi Avdeev, Christian Bocker, Ivailo Gugov, Christian Rüssel

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ELECTRON MICROSCOPY INVESTIGATION OF THE MICROSTRUCTURE AND ELEMENTAL COMPOSITION

OF BARIUM TITANATE PRECIPITATED IN OXIDE GLASSES

Ruzha Harizanova1, Liliya Vladislavova1,3, Georgi Avdeev2,Christian Bocker3, Ivailo Gugov1, Christian Rüssel3

ABSTRACT

The present work reports on the synthesis of glasses for two different ratios of alumina to sodium oxide in compo-sitions of the system Na2O/Al2O3/BaO/TiO2/B2O3/SiO2 with addition of Fe2O3. Barium titanate-based glass-ceramics of different average crystallite sizes, depending on the ratio Na/Al-oxides, are precipitated after appropriate thermal treatment of the glasses prepared. The phase composition analyses performed by X-ray diffraction show the formation of cubic barium titanate, BaTiO3 as a main phase and also, some BaTi0.75Fe0.25O2.888. Additional crystalline phases, such as nepheline, NaAlSiO4 appear in the course of thermal treatments at higher temperatures and longer anneal-ing times. The scanning electron micrographs of the prepared glass-ceramics verify the presence of interconnected spherulitic structures, which correspond to barium titanate crystals and which start to grow with crystallization time increase. The morphology of the spherulitic crystals is elucidated by means of transmission electron microscopy. The elemental analysis carried out by TEM verifies the precipitation of two main crystalline phases – barium titanate and barium titanium iron oxide.

Keywords: barium titanate, controlled crystallization, glass-ceramics, microstructure.

Received 15 November 2017Accepted 15 June 2018

Journal of Chemical Technology and Metallurgy, 53, 6, 2018, 1061-1066

1 Department of Physics, University of Chemical Technology and Metallurgy 8 Kl. Ohridski Blvd., 1756 Sofia, Bulgaria E-mail: [email protected] Institute of Physical Chemistry, Bulgarian Academy of Sciences Block 11, Acad. G. Bonchev Str., 1113 Sofia, Bulgaria3 Otto-Schott-Institut, University of Jena, Fraunhoferstr. 6, 07743 Jena, Germany

INTRODUCTION

The necessity to synthesize barium titanate con-taining oxide glass-ceramics is determined by their potential for applications as a part of resistive sensors, multilayered capacitors and optoelectronic devices [1 - 12]. The traditional approach to the preparation of barium titanate from oxide glasses is based on glass-ceramics in which the crystals are incorporated in an amorphous matrix [8, 12]. The latter offers a chemical as well as a mechanical support and protection of the obtained materials, prevents them from clustering and degradation during transportation, storage or operation. The preparation of glass-ceramics for the electronics requires environmentally friendly materials of a large

crystalline volume fraction, controllable crystallite size and advanced dielectric properties. Different types of such “green” materials have recently been prepared. They refer mainly to non-lead containing glass ceramics as well traditional ceramics [2, 5 - 8, 12]. There are also composite polymer-ceramic materials containing BaTiO3 [13]. BaTiO3 possesses several allotropic modifications which exhibit para- to ferroelectric properties [1, 3, 5 - 7, 9, 11] to a different extent. The type of BaTiO3 modifi-cation occurrence is determined by the temperature [1, 3], the preparation technique [1, 6, 9, 11], as well as by the crystallite size [1, 3, 6, 14]. In the past few years, attention is paid to the possibility to prepare BaTiO3 from oxide glasses by sintering or by applying appro-priate time-temperature annealing programs to the glass

Journal of Chemical Technology and Metallurgy, 53, 6, 2018

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already prepared. Thus materials of controllable crystal-lite sizes and volume fractions [2, 5 - 8, 12, 14 - 18, 20, 21] are obtained. The latter preparation methods provide BaTiO3 synthesis at lower temperatures when compared to those of pure barium titanate crystals synthesis.

This work reports on the preparation of two glass compositions of different Na2O/Al2O3 ratios using the oxide system Na2O/BaO/TiO2/SiO2/B2O3/Al2O3/Fe2O3. The prepared glasses are subjected to thermal treatment at a temperature above that of the glass-transition one, in order to crystallize BaTiO3. The phase composition and microstructure, as well as the elemental composi-tion of the resulting glass-ceramics are studied by X-ray diffraction and electron microscopy.

EXPERIMENTAL

Preparation of the glassesReagent grade Na2CO3, BaCO3, SiO2, B(OH)3,

Al(OH)3, Fe2O3 and TiO2 were used for the preparation of the glasses. Batches (60 g) were homogenized and melted in a Pt-crucible using a SiC-furnace and a melt-ing temperature of 1250°С. Furthermore, the melts were quenched on a Cu-block, transferred to a pre-heated graphite mould and annealed in a muffle furnace for 10 min at 480°C to release the mechanical stresses. Subse-quently, the furnace was switched off and the samples were allowed to cool. The glasses were heat treated at different temperatures above Tg, according to Tg- and Tc-values determined by DTA in our earlier investigations [15, 18], for the crystallization of BaTiO3.

Characterization methods The phase composition was analyzed by X-ray dif-

fraction, XRD (Siemens D5000) using CuKα-radiation, 2θ-values in the range from 10° to 60°. The microstruc-ture and crystal morphology were investigated by a scanning and transmission electron microscopy (SEM: JEOL 7001F, an accelerating voltage 15 kV and TEM: HITACHI H8100, an accelerating voltage 200 kV with EDX analysis system OXFORD ISIS 200).

RESULTS AND DISCUSSION

In both compositions, the bulk glasses are formed as previously reported [15, 17, 18-20]. The character-

istic temperatures determined by a differential thermal analysis are: Tg = 440°C, Tc = 560°C for 3 mol % Al2O3

and Tg = 480°C, Tc = 620°C for 7 mol % Al2O3. These are used to adjust the temperature-time programs. Dif-ferent crystallization schedules are applied to the glasses and the results for the two alumina concentrations are summarized in Figs. 1 and 2. The X-ray diffraction patterns show, in case of thermally treated samples with 3 mol % alumina, that for shorter heat-treatment

Fig. 1. XRD patterns of the parent glass (1) and glass-ceramic samples heat-treated at 560 °C for 5 min (2), 15 min (3), 30 min (4), 3 h (5), 5 h (6), 48 h (7) and 100 h (8) with 3 mol% Al2O3 concentration – crystallization of cu-bic BaTiO3 (B) and hexagonal BaTi0.75Fe0.25O2.888 (arrows).

Fig. 2. XRD patterns of glass-ceramic samples with 7 mol % Al2O3 crystallized at 600°C for 30 min (1), 1 h (2), 2 h (3), 3 h (4) and 5 h (5) – crystallization and growth of cubic BaTiO3 (B) and NaAlSiO4 (N).


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periods at 560°C cubic BaTiO3 (JCPDS 98-002-7970) and BaTi0.75Fe0.25O2.888 (JCPDS 98-005-0857) are formed as crystalline phases as reported in previous work [19, 20]. The crystallization for periods longer than 5 h re-sults in the occurrence of small peaks at 2θ = 23.2 and 34.5 in the XRD pattern which could not be attributed to any particular phase due to the very small intensity of the peaks. The XRD pattern of the 3 mol % alumina composition is given here for clarity and comparison with that of 7 mol % alumina. The situation with the latter sample is similar - a second crystalline phase, nepheline, NaAlSiO4 (JCPDS 19-1176), is well resolved after crystallization for 2 h at 600°C. It is recognized that nepheline is formed as a second crystalline phase [18] but the time-temperature dependency of this additional alumino-silicate phase formation which is observed in the course of the present study has not been reported. As expected, the longer crystallization times result for both compositions in more and larger crystals, although the peaks remain broad as observed also by other authors working with nanosized BaTiO3 [10]. The comparison between Figs. 1 and 2 shows that the 7 mol % Al2O3 glass-ceramics have broader peaks than those of the 3 mol % alumina containing ones. This should result in smaller barium titanate crystals. In order to examine this hypothesis, scanning electron microscopy investigations of the prepared glass-ceramics are performed. The typi-cal microstructure observed is shown in Figs. 3 and 4 and confirms the results from the X-ray diffraction analyses. This morphology is typical for other samples from the

same compositional series, besides heat-treated for dif-ferent times at different temperatures above Tg [14, 15, 17 - 20]. The comparison of the microstructures of the glass-ceramics containing 3 mol % and 7 mol % of alu-mina shows that the increase in the alumina concentra-tion leads to glass stabilization and increased viscosity, i.e. smaller crystals. The latter finding, together with the viscosity dependency on the compositions Na2O/Al2O3 ratio, is also reported for other sodium-aluminoborosil-icate glasses [21]. They are in correspondence with the results referring to the intermediate behaviour of Al3+ ions in the glass-network. The microstructures shown in Figs. 3 and 4 evidence that the heavier elements of the composition, Ba and Ti in this case, as well as Fe probably, are always incorporated in the crystals - the brighter contrast of the SEM micrographs is outlined. The morphologies of the crystals in the two samples imaged in Figs. 3 and 4 suggest that in the parent glass most probably phase separation occurs and in one of the phases, in which later BaTiO3 and BaTi0.75Fe0.25O2.888 crystallize, the heavier elements are concentrated, while the second phase contains mainly the rest of the elements from the composition which will later become the amor-phous matrix. The presence of Fe in the BaTiO3 crystals has been observed by other authors working with barium titanate and Fe2O3 [22]. The interconnected globular crystals formed and shown here are reported elsewhere [14, 15] but the SEM could not give information about their elemental analysis due to the small size of the crys-tals formed for both 3 and 7 mol % alumina containing

Fig. 3. SEM micrograph of a sample with composition 20.1Na2O/23.1BaO/23TiO2/17.4SiO2/7.6B2O3/3Al2O3/5.8Fe2O3 heat-treated for 5 h at 550°C.

Fig. 4. SEM micrograph of a sample with composition 16.1Na2O/23.1BaO/23TiO2/17.4SiO2/7.6B2O3/7Al2O3/5.8Fe2O3 heat-treated for 2 h at 610°C.


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compositions. This is reason to perform transmission electron microscopy investigations of the samples. They are illustrated in Fig. 5. The elemental analysis, EDX of the glass-matrix and the crystals for the sample contain-ing 3 mol % of alumina and 5.8 Fe as also verified by XRD are given in Figs. 6 and 7. TEM micrographs show that the separate spherulitic particles consist of stick-like formations of a length of some ten nanometers and a thickness of few nanometers. They have darker appear-ance which confirms again the suggestion made on the ground of SEM measurements, i.e. heavier elements like Ba, Ti and probably Fe are present in the composi-tion. The sticks seem to grow randomly with no specific orientation. The particles have a diameter of the order

of some hundred nm up to a micrometer. The idea that the darker contrast of the crystals is due to their higher average atomic number of the constituents is supported by the EDX analyses undertaken for the glass matrix and the particles, as seen in Figs. 6 and 7, respectively. Here the information is gathered from areas larger than 20 nm. The elemental analysis of the matrix shows the presence of mainly Na and Si, though some traces of Al, Ti and Ba are also seen in Fig. 6. The particles, on the contrary, contain mainly Ba, Ti and Fe. Detectable peaks of Al, Si and especially Na are also seen. The presence of the latter elements, which is not detected by XRD in the phase composition of the crystals, could be explained by the diffuse morphology of the crystals. There is obviously some glassy phase present between the sticks. Since boron is not be detected by TEM, it is impossible to conclude with certainty whether B enters the crystalline or the amorphous phase. The same is the situation with the samples containing 3 mol % of Al2O3 crystallized for 5 h at the same temperature, 550°C. As seen in Fig. 7 for this glass-ceramic sample, the spherical particles are again present. However, the stick-like crys-tals, which are larger in a diameter, have approximately the same size like those annealed for 3 h at 550°C. Fig. 7 also shows that the glassy phase is almost absent in the sample heat-treated for 5 h. Here the concentration of Fe in the matrix and in the particles is almost equal. Here, compared to the results from the EDX analyses from Figs. 6 and 7, the concentration of Fe in the matrix and particles is almost equal. However, almost no Si and Al are detected in the crystals, while those are enriched in Na in comparison to its concentration in the glass matrix.

Fig. 5. TEM micrograph of a sample with composition 20.1Na2O/23.1BaO/23TiO2/17.4SiO2/7.6B2O3/3Al2O3/ 5.8Fe2O3 – crystallized for 3 h at 550°C.

Fig. 6. EDX analysis results for the glassy matrix from Fig. 5.

Fig. 7. EDX analysis results for the crystals from Fig. 5.


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CONCLUSIONS

The synthesis of bulk glasses is possible in case of Al2O3 concentrations of 3 mol % and 7 mol%. The increase in the alumina concentration leads to increase in both glass-transition and crystallization temperatures. The applied time-temperature programs result in the crystallization of BaTiO3 or BaTiO3 and BaTi0.75Fe0.25O2.888 for the lower alumina concentration. Sodium-aluminosilicate, NaAlSiO4 is also formed in case of the sample containing 7 mol% of alumina. Electron microscopy results show that annealing leads first to phase separation and then to the crystallization of mainly BaTiO3 but also BaTi0.75Fe0.25O2.888 in the demixed regions. This is also witnessed by TEM-EDX. The increased annealing times in case of isothermal crys-tallization lead to crystal growth. Globular, randomly oriented and interconnected crystals are formed. The increasing alumina concentration results in finer crystals.

AcknowledgementsThe authors would like to express their gratitude to

NIS at UCTM and BNSF for the financial support under grants NIS 11633 and BNSF DN08/13.

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