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Bi-based Aurivillius family of compounds have received considerable attention as the materials for ferroelectric random access memory (FRAM) because of their low operating voltage, fast switching speed, large remnant polarization, low coercive field, superior polarization fatigue resistant characteristics and high Curie temperature. A large remnant polarization, low coercive field and high Curie temperature are required for better performance of FRAM devices. Majority of Aurivillius oxides are normal ferroelectrics, while only a few of them such as BaBi2Nb2O9, BaBi2Ta2O9, BaBi4Ti4O15 etc. exhibit relaxor behaviour. Relaxor ferroelectrics are attractive for a wide range of applications owing to their excellent high dielectric and piezoelectric responses over a wide range of temperatures. Ferroelectric properties of these compounds are often improved by chemical lattice site engineering. This is done by suitable atomic substitutions at 'A' and/or 'B' of the structure. Nb5+ substitution at 'B' site has been proved to be an most effective site engineering in 2 and 3 layered compounds. In the present study, Nb5+ has been substituted at 'B'-site and Na1+ at 'A'-site to compensate charge in the formulation Ba1-xNaxBi4Ti4-xNbxO15. Effect of the substitution on the structural, microstructural, dielectric and ferroelectric properties were evaluated. The AC complex impedance spectroscopy was used to analyze the change in dielectric conductivity of the ceramics. An improved permittivity, increased remnant polarization and decreased coercive field were found in the Nb-substituted compound.
Ferroelectric RAM (FeRAM or FRAM) is a random access memory similar in construction to DRAM but uses a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FRAM is one of a growing number of alternative non-volatile memory technologies that offer the same functionality as Flash memory. FRAM advantages over Flash include: lower power usage, Low leakage currents, faster write performance and a much greater maximum number (exceeding 1016 for 3.3 V devices) of write-erase cycles.
Bi-based Aurivillius family of compounds has received considerable attention as the materials for ferroelectric random access memory (FRAM) because of their low operating voltage. They have very fast switching speed. It has large remnant polarization and low coercive field. They have superior polarization fatigue resistant characteristics and high Curie temperature [1-7]. For better performance of FRAM devices a large remnant polarization is required. FRAM requires low coercive field for good performance. High Curie temperature is required for best performance of FRAM devices.
Majority of Aurivillius oxides are normal ferroelectrics, while only a few of them such as BaBi2Nb2O9, BaBi2Ta2O9, BaBi4Ti4O15 etc. exhibit relaxor behaviour [3, 8, 9]. Relaxor ferroelectrics are attractive for a wide range of applications owing to their excellent high dielectric and piezoelectric responses over a wide range of temperatures [10, 11].
Bi-layered structure Aurivillius compound is generalized as Bi2Am-1BmO3m+3. The structure consists of 'm' number of (Am-1BmO3m+1)2- slabs sandwiched between (Bi2O2)2+ layers, where 'A' represents monovalent, divalent or trivalent element and 'B' represents tetravalent, pentavalent or hexavalent metallic cations which are in 12-fold and 6-fold co-ordination respectively [4, 10, 11]. BaBi4Ti4O15 (BBT) is a four layered ('m'=4) compound with structural formula [(Bi2O2)2+ ((BaBi2)Ti4O13)2-], where the Ba- and Bi-ions occupies the A-site and Ti-ions resides in the B-site. Their intrinsic electrical properties are anisotropic, with the maximum value of conductivity and the major component of spontaneous polarization parallel to the (Bi2O2)2+ layers. As a result, properties of the polycrystalline materials are strongly affected by their microstructure, especially by the orientation of the plate-like grains and by the length-to-thickness ratio (aspect ratio) of the grains [12, 13].
Ferroelectric properties of these compounds are often improved by chemical lattice site engineering. This is done by suitable atomic substitutions at 'A' and/or 'B' of the structure. Nb5+ substitution at 'B' site has been proved to be an most effective site engineering in 2 and 3 layered compounds.
3. Literature Review:
3.1. General Introduction:
The family of bismuth oxides was discovered more than 50years ago by Aurivillius .Recently, interest in the properties of the Aurivillius phases as temperature-stable ferro-piezoelectrics has been renewed. Several bismuth-layered crystal structures and their properties have been investigated in detail. The majority of those materials are normal ferroelectrics with a fairly high Curie temperature, while only a few of them such as BaBi2Nb2O9,BaBi2Ta2O9, etc. exhibit relaxor behavior [15-17]. Relaxor materials are characterized by frequency dispersion having broad dielectric normally near the dielectric maximum. The latter properties are very use ful for a wide range of applications due to their extremely high dielectric and piezoelectric responses in a wide range of temperatures . Growing need for new lead-free materials for various applications such as Bi-based Aurivillius family of oxides. BaBi4Ti4O15 (BBiT) as a member of this large family of compounds is a promising candidate for high-temperature piezoelectric applications, memory application and ferroelectric non volatile memories (Fe-RAM) but a lot of aspects of the preparation and properties of barium bismuth titanate remain unexplored.
The lattice structure of the Aurivillius family of compounds is composed of n number of like perovskite (Anâˆ’1BnO3n+3)2âˆ’ unit cells sandwiched between (Bi2O2)2+ slabs along pseudo tetragonal c-axis (n is an integer between 1 and 5). The 12 coordinate perovskite like A-site is typically occupied by a large cation such as Na+, K+,Ca2+, Sr2+, Ba2+, Pb2+, Bi3+ or Ln3+ and the 6-coordinate perovskite like B-site by smaller cations such as Fe3+, Cr3+, Ti4+, Nb5+ or W6+.BBiT, as the n = 4 member of the Aurivillius family has Ba and Bi ions at the A sites and Ti ions at the B sites of the perovskite block [(Bi2O2)2+·((BaBi2)Ti4O13)2âˆ’] .
Fig.1. Crystal structures of BaBi4Ti4O15
3.2. Requirement of doping:
The disadvantage of the layer-structure perovskite materials for high-temperature piezoelectric applications is their relatively high conductivity. This conductivity is electronic p-type and ,therefore, can be suppressed by donor doping[19-22]. The p-type conductivity observed in layer-structure perovskite materials is related to the creation of electron holes that result from the compensation of intrinsic structural defects (cation vacancies) or acceptor impurities. The dominant structural defects producing electron holes in BaBi4Ti4O15 are not known. The concentration of electron holes and, consequently, the material's conductivity areIncreased by acceptor doping and decreased by donor doping [19-22] .However, the effect of aliovalent dopants is relatively small. For example the minimum conductivity in air sintered BaBi4Ti4O15 ceramics was obtained by the substitution of 5 mol % of the Ti4+ ions with Nb5+ donors, and this resulted in a conductivity decrease of 2 orders of magnitude.
3.3. Effect of dopants:
Nb doping decreases the grain size of the BaBi4Ti4O15 ceramics. Niobium doping suppresses the grain growth in BaBi4Ti4O15 ceramics . Nb5+donors incorporates into the BaBi4Ti4O15structure at the Ti4+sites cause an increase in the barium concentration and decreases the bismuth concentration. Excess donor charge is mainly compensated by a decrease in the average valency of the A sites in the perovskite (A3B4O13)2-block of the (Bi2O2)2-. ((BaBi2)Ti4O13)2+) layered structure with the incorporation of more 2-valent barium ions and fewer 3-valent bismuth ions.
Doping of BaBi4Ti4O15 with Nb changes the temperature of the dielectric constant maximum (Curie temperature,TC) ,strongly suggesting incorporation of the dopants into the BaBi4Ti4O15 structure. Substitution of the Ti ions in the BaBi4Ti4O15 structure by Nb ions decreases the Curie temperature (TC) by approximately 7 oC/mol% NbTi . Nb increases the room-temperature dielectric constant of BaBi4Ti4O15ceramics. The Nb doping changes the shape of the dielectric-constant maximum. Whereas the low Nb concentrations, up to 2 mol%, increase the dielectric constant at TC, the higher Nb concentrations suppressed the dielectric-constant maximum. Broadening of the dielectric-constant maximum can be related to the changes in the microstructure with Nb doping, e.g. the appearance of secondary phases, the non-homogeneous distribution of dopants and/or the decrease in grain size .
Compensatory doping of sodium in place of 12-fold co-ordinated Ba greatly affects the properties.The TC temperature decreases as a consequence of the substitution of larger 12-fold coordination Ba2+ cations for smaller cations such as Na+. For high Na contents, the microstructure is dominated by the presence of large rounded edges platelet-like grains. This grain shape is observed for many bismuth layered materials and is connected with the strong anisotropy of the crystal structure. The rounded edges are characteristic of liquid phase sintering. Curie temperature as well as the temperature of the maximum permittivity decrease regularly as the sodium concentration decreases. The decrease of the Curie temperature with increasing size of the 12-coordinated cations in the Aurivillius compounds has been attributed to the increase of the tolerance factor which leads to a less distorted structure .
The properties of ceramics are greatly affected by the characteristics of the powder, such as particle size, morphology, purity and chemical composition. Various chemical methods, e.g.co-precipitation, sol-gel, hydrothermal and colloid emulsion techniques are used to efficiently control the morphology and chemical composition of prepared powder. The citrate gel process offers a number of advantages for the preparation of fine powders of many complex oxides as quoted in the literature [8-11].The main drawback of this process is the possible formation of carbonate during decomposition of the polymeric gel.
Non-Conventional methods are used to get better homogeneous and reactive precursor powder compared to solid-state method. Usually Ti-alkoxides and Ti-chlorides are used as the Ti-metal source in chemical routes. Since Ti-alkoxides/nitrates are relatively costlier and solid state route is less time consuming and cheaper method so solid state route was selected.
3.5. Summary of literature:
From the literature review it was found that there is no report of Nb+5 substitutions in BaBi4Ti4O15 relaxor ferroelectrics.Niobium doping suppresses the grain growth in BaBi4Ti4O15 ceramics. Doping of BaBi4Ti4O15 with Nb changes the temperature of the dielectric constant maximum (Curie temperature,TC) ,strongly suggesting incorporation of the dopants into the BaBi4Ti4O15 structure. Substitution of the Ti ions in the BaBi4Ti4O15 structure by Nb ions decreases the Curie temperature (TC). Nb substitution increases the room-temperature dielectric constant of BaBi4Ti4O15ceramics. The TC temperature decreases as a consequence of the substitution of larger 12-fold coordination Ba2+ cations for smaller cations such as Na+.
3.6. Objective of work:
A & B site substitution to improve the relaxor ferroelectric property
Nb5+ substitution for Ti4+ because Nb5+ has been reported to enhance the relaxor property.
Na1+ substitution for Ba2+ to compensate Nb5+ substitution in the structure.Na1+ ahs also been reported as a useful substituent in aurivillus compound.
4.1. Experimental procedure:
Polycrystalline samples of Ba1-xNaxBi4Ti4-xNbxO15 (BTNX) with x = 0.1, 0.2, 0.4, 0.5, 0.6, 0.7 compositions (abbreviated as BTN1, BTN2, BTN4, BTN5, BTN6 and BTN7 respectively) were prepared by solid state reaction. Stoichiometric amounts of BaCO3, Bi2O3, TiO2, Na2CO3 and Nb2O5 were mixed in an agate mortar using isopropyl alcohol as the liquid media. The mixed powders were calcined at 800, 900, 1000 and 1050oC depending on the composition with intermittent grinding after each heating step. Phase identification of calcined powders was performed using a Cu KÎ± X-ray Diffractometer (PW-1830, Philips, Netherlands). The final BTNX powders were pelletized at a pressure of 220 MPa using poly vinyl alcohol as a binder. The pellets were sintered at 1100oC. The density of sintered pellets was determined by Archimedes' method. The microstructures of sintered specimens were studied using Scanning electron microscopy (JSM-6480LV). For dielectric measurement, sintered pellets were electroded with silver conductive electrode paste (Alfa Aesar) to provide the ohmic contacts. The dielectric properties and complex impedance were measured using an Impedance Analyzer (Solatron 1260). All the dielectric data were collected while heating at a rate of 1oC/min. A standard ferroelectric analyzer (PE loop tracer, Marine India Electronics) was used to trace P-E hysteresis loops.
4.2. Flow chart for preparation of BTNX:
Electroding with silver conductive paste
Sintering of pellets 1100oC for 4 hours
Pellet pressing at 220 Mpa
Mixing of powder with binder (PVA)
Calcination at 10500C for 8 hours
Calcination at 10000C for 8 hours
Calcination at 9000C for 8 hours
Calcination at 8000C for 8 hours
Grounded in Agate Mortar
Dried & scratched
Mixing with IPA
Fig.2. Flowchart of the experimental procedure for preparation of Ba1-xNaxBi4Ti4-xNbxO15
Results and discussion
5. Results and discussion
5.1. Phase formation analysis and lattice parameters
To understand the intermediate phase formation behaviour and reaction mechanism during synthesis of the ceramics, the powder was heated in the temperature range 800 oC to 1050 oC and each time, the calcined powder was analyzed by XRD. Fig. 3 shows the XRD patterns of BTN1 ceramics calcined at 800, 900, 1000 and 1050oC. Two intermediate phases Bi4Ti3O12 and BaTiO3 were observed in the specimen that was calcined at 800oC (Fig. 3(a)). BBT phase was formed at and above 900oC as shown in Fig. 3(b)-(d).
Fig.3. XRD pattern of BTN1 powder obtained after calcination at (a) 800oC, (b) 900oC, (c) 1000oC, and (d) 1050oC for 4 h.
Finally, a pure phase of BBT was found in the specimen after calcination at 1050oC for 4 hours as shown in Fig. 3(d). Hence, it can be concluded that the main mechanism of BBT formation is:
BaTiO3 + Bi4Ti3O12 = BaBi4Ti4O15 -------------------------------(1)
Similar mechanism was observed for all the other BTNX compositions. BTN6 and BTN7 specimens showed formation of Na2Ti3O7 secondary phase along with main BBT phase. Fig. 4 shows the phase formation sequences in BTN6 specimen. Fig 4(c) shows the presence of well crystallize Na2Ti3O7 phase in 1050 oC calcined product. So, it may be concluded that the solid solubility limit of Nb+5 in Ba1-xNaxBi4Ti4-xNbxO15 is upto x=0.5.
Fig.4. XRD pattern of BTN6 powder obtained after calcination at (a) 800oC, (b) 900oC, and (c) 1050oC.
The effect of substitution on the change in lattice parameters of the ceramics was evaluated based on orthorhombic symmetry of the structure. The lattice parameters were calculated based on the relation:
= + + ------------------------ (2)
The lattice parameters, mainly b and c were found to decrease with increasing substitution rate as shown in Fig. 5. This decrease may be due to the replacement of Ba2+ by Na1+, as the ionic radii of Na1+ is much smaller than Ba2+. The ionic radii of Ti4+ and Nb5+ are almost similar and their substitution is unlikely to affect the lattice parameter.
Fig.5. Lattice parameters with composition (x) for Ba1-xNaxBi4Ti4-xNbxO15 ceramics. Inset XRD pattern shows the marked (hkl) peaks which were used to calculate lattice parameters.
Microstructure of as-sintered sample was obtained by SEM. Fig. 6 (a) and (f) shows the microstructure of BTN1 and BTN7 ceramics. It consists of plate like grains with random orientation of plate faces. It is known that plate like grain formation is a typical characteristic of bismuth layer-structured ferroelectrics as they have highly anisotropic crystal structure. The microstructure is dominated by the presence of large rounded edges platelet-like grains. The rounded edges are characteristic of liquid phase sintering. The grain size is found to increase with increase in substitution (Table 1). Due to the plate like structure the length increases with substitution. So, it may be concluded that Nb and Na accelerates the densification and grain growth in the ceramics.
Table 1 : Grain size (µm)of Ba1-xNaxBi4Ti4-xNbxO15 for x=0.1,0.2,0.4,0.5,0.6,0.7
Fig.6. SEM Photographs of BTN1,BTN2, BTNX4, BTN5, BTNX6,BTNX7.
5.3. Dielectric study:
Fig. 7 shows the temperature dependence of dielectric constant (Îµ´) and dielectric loss (tan Î´) of Ba1-xNaxBi4Ti4-xNbxO15 ceramics measured at 100 kHz. The temperature of dielectric maximum (Tm) is found near ~400oC. The maximum dielectric constant (ÎµmÎ„) increases for x = 0.1 and 0.2 compared to BBT. With further substitution it is found to decrease for x â‰¥ 0.4 (Fig. 7(a)). This may be a result of the decrease in lattice parameter 'b', and associated 'A' type cation displacement. Smaller displacement leads to a decrease of polarization resulting in a reduction of dielectric constant. ÎµmÎ„, Tm and room temperature dielectric constant (Îµrm) at 100 kHz are shown in Table 2. It can be noted that Îµrm increases with substitution for x = 0.1 and 0.2 which can be due to the slight shifting of ÎµmÎ„ peak towards room temperature. Fig. 7(b) shows the tan Î´ vs temperature plots for the ceramics.
Fig.7. Temperature dependence of ÎµÎ„ and tan Î´ for Ba1-xNaxBi4Ti4-xNbxO15 ceramics at 100 kHz.
One of the important characteristics of a relaxor material is the frequency relaxation which is measured from the degree of relaxation parameter Î”Tm and is usually represented by
Î”Tm = Tm(1 kHz) - Tm(1 MHz) ------------------------------------- (3)
The Î”Tm for x = 0.1 and 0.2 are in the range 15-20oC. Fig. 8 shows that for x=0.2 composition. However, the Î”Tm was found to be zero for compositions x â‰¥ 0.4 (Fig. 9). This highlighted the fact that relaxor behavior was suppressed from x = 0.4 substitution.
Fig.8. Temperature dependence of ÎµÎ„ and tan Î´ for Ba1-xNaxBi4Ti4-xNbxO15 ceramics at various frequencies for x = 0.2.
Fig.9. Temperature dependence of ÎµÎ„ and tan Î´ for Ba1-xNaxBi4Ti4-xNbxO15 ceramics at various frequencies for x = 0.4.
Another important parameter; dielectric dispersion is frequently used for relaxor characterization. An empirical relation is proposed to describe the dielectric dispersion as
1/Îµâ€² - 1/ Îµmâ€² = (T - Tm)Î³/C1 ----------------------------(4)
Where Îµmâ€² is the maximum dielectric constant, Î³ and C1 are constants. The values of Î³ lies in the range; 1 < Î³ < 2. For an ideal ferroelectric to paraelectric phase transition Î³ = 1 and for ideal relaxors Î³ = 2. Fig. 10 shows the plot of log (1/Îµâ€²-1/ Îµmâ€²) versus log (T-Tm) for different compositions at 100 kHz and respective Î³ values. The Î³ values were found to decrease with increase in substitutions from 1.89 for BTN1 to 1.39 for BTN6. This may be due to the decrease in relaxor phenomena as stated above. The low Î³ value of BTN6 (1.39) suggests it a normal ferroelectrics. Structural studies of BBT reveal its relaxor effect is due to the structural disorder of Ba2+ and Bi3+ in the Bi2O2 layers . In the present substitution, Ba2+ is replaced by Na1+. As a result, the structural disorder of the system decreases, thus suppressing the relaxor effect of the system.
Fig.10. Plot of log (1/Îµ×³ - 1/ Îµm×³) versus log (T - Tm) at 100 kHz for different Ba1-xNaxBi4Ti4- xNbxO15 ceramics.
5.4. Ferroelectric Polarization versus Electric field study
The ferroelectric hysteresis loop of BTNX ceramics were obtained under a maximum applied electric field of 30 kV/cm. Fig. 13 shows the P-E loop for all the compositions recorded at room temperature and at a frequency of 100 Hz. With increase in substitution, the value of 2Pr increases up to x = 0.2. Similar trend is also observed for 2Ec (Fig. 10). This behaviour of 2Pr and 2Ec is exactly similar to the change in the value of lattice parameter 'b'. In this context it can be said that, 'b' is the ferroelectric axis of the system.
Fig.11. Plot of ferroelectric hysteresis loop measured at room temperature for different Ba1- xNaxBi4Ti4-xNbxO15 (x = 0.1, 0.2, 0.4, 0.5, 0.6) ceramics.
Fig.12. Plot of 2Pr and 2Ec measured at room temperature for different Ba1-xNaxBi4Ti4- xNbxO15 (x = 0.1, 0.2, 0.4, 0.5, 0.6) ceramics.
5.5. DC conductivity
Impedance spectroscopy may shed a new light on the dc conductivity of BTNX ceramics. Cole-Cole plots obtained from impedance spectra are shown in Fig.13. It is observed that two different semicircles can be traced, which represent the grain and grain boundary regions, respectively. To calculate the resistance (R) and capacitance (C) values, an equivalent circuit is used to model the electrical response comprising of two parallel resistor-capacitor (RC) elements connected in series representing grain and grain-boundary regions. The values of R and C are determined from Cole-Cole plot.
Fig.13. Cole-Cole plot for Ba1-xNaxBi4Ti4-xNbxO15 (x = 0.1, 0.2, 0.4, 0.5, 0.6) ceramics at
The dc conductivities of grain and grain boundary regions are evaluated from the impedance spectra using the relation,
Ïƒdc = d/RA ----------------------------------------(5)
where d is the thickness of the pellet, A is the electrode area and R is the grain or grain boundary resistance, which is derived by fitting the complex impedance data to the equivalent circuit model as described above. The values of dc conductivity at 600oC for both grain and grain boundary are listed in Table 2. It can be noted that the dc conductivity is minimum for x = 0.2 composition.
Table 2: Room temperature permittivity (Îµrm), Maximum relative permittivity (Îµ×³m) , maximum permittivity temperature (Tm) values at 100 kHz, dc conductivity for grain and grain boundary at 600 oC for different Ba1-xNaxBi4Ti4-xNbxO15 ceramics.
Ïƒdc (Î©-1cm-1) at 600 oC (grains)
Ïƒdc (Î©-1cm-1) at 600 oC (grain boundary)
In summary, Nb-substituted Ba1-xNaxBi4Ti4-xNbxO15 ceramics was successfully synthesized by simple solid oxide reaction route. The solid solubility limit of Nb+5 in Ba1-xNaxBi4Ti4-xNbxO15 is upto x=0.5. The substitution accelerates densification and grain growth in the ceramics due to low melting nature of Nb and Na. The relaxor behavior was suppressed from at x = 0.4 substitution due to the decrease in disorderness created by Ba in the structure. It can be said that, 'b' is the ferroelectric axis of the system. The room temperature permittivity, remnant polarization are maximum and coercive field, the dc conductivity are minimum at x = 0.2 composition. This composition is highly promising for FRAM capacitor application and satisfies the requirement of FRAM.