Electrical characteristics of bismuth titanate glass-ceramics containing SiO2 and Nd2O3

Bismuth-titanate ceramics containing SiO2 and Nd2O3 as additives are synthesized at two different ways of cooling of the melts. The introduction of SiO2 and Nd2O3 leads to more complex crystallization with participation of several phases including Bi4Ti3O12. It is proved that the applied methods of synthesis are suitable for generation of different microstructures in the bulk doped bismuth titanate ceramics, which is promising basis for modification of their electrical properties. The increasing of SiO2 content improves the glass formation ability and addition of Nd2O3 stimulates the crystallization. The conductivity of selected samples is determined by impedance analyzer in the frequency range from 10 to 100 kHz and DC resistible bridge using two-terminal method. All investigated samples are dielectrics with conductivity 10-6–10-9 (Ω·cm)-1.


I. Introduction
The application of Aurivillius family of bismuthbased ferroelectric compounds with a layered structure [1] in capacitors, sensors, piezoelectric and electro-optic devices [2][3][4] is strongly influenced by the method of preparation.Between them the crystalliza tion from melts and glasses recently is also applied [5][6][7][8][9].This method gives possibility for doping with different cations to improve the properties.It allows also to control the particle size evolution during the transition from amorphous to crystalline state and to achieve suitable crystallographic orientation in the polycrystalline materials.
In our previous study, the phase formation in the system Bi 2 O 3 -TiO 2 -SiO 2 from fast quenched melts is investigated [10].It is established that the introduction of 20-40 mol% SiO 2 stimulates the partial amorphiza tion of the samples.It was proved that by melt quenching it is possible to modify the microstructure and to control the crystallization process in bismuth titanate ceramics.These results motivate our future investigations.
The purpose of the present study is to elucidate the influence of the cooling rate and additional heat treatment on the phase formation and microstructure of bismuth titanate ceramics with addition of SiO 2 , obtained by melting.Full replacement of SiO 2 with Nd 2 O 3 is also performed.The SiO 2 is introduced to change the melting conditions, while the reason for Nd 2 O 3 addition to bismuth titanate is that this oxide is more effective in improving the electrical properties [11][12][13][14][15][16][17][18][19].According to our knowledge, up to know, there are no reports for the preparation of glass-ceramics containing the phase Bi 4 Ti 3 O 12 by melt quenching with participation of SiO 2 and Nd 2 O 3 .

II. Experimental
Bismuth-titanate ceramics containing SiO 2 and Nd 2 O 3 as additives were synthesized by quenching from melt.The melting was made from the following pre-cursors: TiO 2 , Bi 2 O 3 , SiO 2 and Nd 2 O 3 in alumina crucibles at 1250-1500°C.The contact between the samples and crucible during the melting was about 15 min.as after longer exposition (3-5 hours) contamination with 3-5 mol% Al 2 O 3 was detected.The samples were prepared by two cooling rates of the melts: i) fast cooling to room temperature, performed by pouring of the melts between two cooper plates (samples with notation A) and ii) slow cooling of the melts in the crucibles (samples with notation B).The selected compositions are: 24Bi 2 O 3 •36TiO 2 •40SiO 2 , 30Bi 2 O 3 •50TiO 2 •20SiO 2 ,  50Bi 2 O 3 •30TiO 2 •20SiO 2 and 30Bi 2 O 3 •50TiO 2 •20Nd 2 O 3 (Table 1).
The conductivity of selected samples was measured by impedance analyzer in the 10-100 kHz frequency range (Hewlett-Packard HP4192A) and DC resistible bridge using two-terminal method and a suitable sample holder with graphite electrodes.

Structural characterization
Depending on the cooling rate, more or less well separated amorphous regions are visually observed.They are distinguished because the crystals are milklike, while the glass part is dark or transparent in some samples.For this reason all obtained samples may be considered as glass-ceramics.However, after the slow cooling higher crystallization of the prepared samples is observed (Fig. 1).
XRD analysis of the samples containing 20 mol% SiO 2 (Fig. 1) shows that the introduction of SiO  tion of solid solution with the bismuth-titanate phase or from the appearance of a new phase.Crystallization behaviour of the sample 24Bi 2 O 3 •36TiO 2 •40SiO 2 was additionally investigated.Thus, it is observed that after the slow cooling, the amorphous microstructure is transformed to glass-ceramic one and after additional heat treatment at 700°С for 5 hours the amount of crystalline phase increases.According to the EDS analysis (Fig. 2) the crystals and matrix differ in composition: the separated crystals are with decreased Bi 2 O 3 -content and increased TiO 2 -content in comparison with the nominal batch composition, as it was established also in our previous studies [20].
The addition of Nd 2 O 3 up to 20% decreases the glass formation trend and raises the melting temperature up to 1500°C.XRD results, presented in Fig. 3, show that addition of already 10 mol% stabilize Bi 4 Ti 3 O 12 phase, which is completely different behaviour in comparison to the samples containing 20 mol% SiO 2 .In the sample containing 20 mol% Nd 2 O 3 the phase Bi 4 Ti 3 O 12 texturing in the plane (00l) was detected after fast cooling (Fig. 3).According to EDS data (Fig. 4), the sample contains pure Bi 4 Ti 3 O 12 phase and crystals of the bismuth titanate with Bi 2 O 3 partially replaced by Nd 2 O 3 (Fig. 4).

Electrical properties
Conductivity of the samples depending on the composition, cooling method and method of measurement are presented in Table 1.The conductivity of all samples is in the rage 10 -6 -10 -9 (Ω•cm) -1 and the materials may be considered as dielectrics.The dependency Z"=f(Z') (Fig. 5) shows one not well distinguished semicircle.The semicircular in the complex plane yield to an arc, whose centre is displaced below the real axis, due to the presence of distributed elements and a relaxation process resulting from the trapped states.The semicircle is ascribed to originate from the grain boundary and nonzero intercept corresponding to the resistance of the grain, with no semicircle corresponding to electrode-sample interface [21].More detail analysis of the electrical data will be the subject in a new investigation.

IV. Conclusions
The investigation carried out shows that depending on the conditions of the melting and additional heat treatment of the super cooled samples, different polyphase glass-ceramics materials with various microstructures could be obtained in the systems Bi 2 O 3 -TiO 2 -SiO 2 and Bi 2 O 3 -TiO 2 -Nd 2 O 3 .The presence of several phases is established, mainly Bi 4 Ti 3 O 12 and Bi 2 Ti 2 O 7 .All investigated samples are dielectrics with conductivity in the rage 10 -6 -10 -9 (Ω•cm) -1 .These results are promising ba-sis for control and modification of the electrical properties of the bismuth titanate glass-ceramics doped with SiO 2 and Nd 2 O 3 .
2 leads to more complex crystallization with participation of several phases, including dominant Bi 2 Ti 2 O 7 (Bi 2 Ti 2 O 7 -JCPDS 32-0118) and Bi 4 Ti 3 O 12 (Bi 4 Ti 3 O 12 -JCPDS 73-2181).It can be seen (Fig.1) that the fast cooling even favorizes formation of the pyrochlore Bi 2 Ti 2 O 7 phase and stimulates the partial amorphization of the prepared samples.However, in our previous paper[20]  it was confirmed that for samples containing ≥ 40% SiO 2 XRD patterns were close but do not completely coincide with these ones of the phase Bi 4 Ti 3 O 12 .That can result from the forma-

Table 1 . Conductivity of the samples depending on the composition and method of the measurement Sample
Method of cooling and view of the sample σ, (Ω•cm) -1