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Journal of Materials Science

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Open Access 08-03-2017 | Original Paper

Large pyroelectric properties at reduced depolarization temperature in A-site nonstoichiometry composition of lead-free 0.94Na_xBi_yTiO₃–0.06Ba_zTiO₃ ceramics

Authors: A. M. Balakt, C. P. Shaw, Qi Zhang

Published in: Journal of Materials Science | Issue 12/2017

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Abstract

Nonstoichiometry lead-free 0.94Na_xBi_yTiO₃–0.06Ba_zTiO₃ (N_xB_yT–0.06B_zT) (from x = y = 0.5, z = 1.00 to x = 0.5, y = 0.534, z = 1.02) ceramic compositions were prepared by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. The depolarization temperature (T _d) can be lowered by modifying x, y and z. The pyroelectric coefficient (p) of nonstoichiometry N_xB_yT–0.06B_zT compositions is greatly increased, compared with stoichiometry NBT–0.06BT composition, from 3.15 × 10⁻⁴ C m⁻² °C⁻¹ at room temperature (RT) and 23.9 × 10⁻⁴ C m⁻² °C⁻¹ at T _d, and reaches maxima of 6.99 × 10⁻⁴ C m⁻² °C⁻¹ at RT and 75.3 × 10⁻⁴ C m⁻² °C⁻¹ at T _d for x = y = 0.52 and z = 1. The figures of merits, F _i, F _v, and F _D, also have been improved from 1.12 × 10⁻¹⁰ m v⁻¹ and 0.021 m² C⁻¹ to 2.50 × 10⁻¹⁰ m v⁻¹, 0.047 m² C⁻¹ and 16.63 × 10⁻⁶ Pa^−1/2, respectively, for N_0.52B_0.52T–0.06BT composition at RT. Furthermore, N_0.52B_0.52T–0.06BT composition shows a huge enhancement in F _i, F _v and F _D to 26.9 × 10⁻¹⁰ m v⁻¹, 0.39 × 10⁻¹⁰ m² C⁻¹ and 138.7 × 10⁻⁶ Pa^−1/2, respectively, at T _d. The same composition also presents F _C values which are ~2.58 and ~2.86 (×10⁻⁹ C cm⁻² °C⁻¹) at RT at 100 and 1000 (Hz). N_0.5B_0.534T–0.06BT and N_0.5B_0.534T–0.06B_1.02T compositions show a large p values at a wide temperature range. The enhanced pyroelectric properties make nonstoichiometry N_0.52B_0.52T–0.06BT composition a promising candidate for pyroelectric and other applications at wide temperatures range.

Introduction

Pyroelectric ceramic materials have been used as an active element in various electronic devices [1, 2]. These materials are playing a major role in diverse applications such as sensors, infrared detectors, thermal cameras, remote medical diagnostic devices, gas detectors, and many other applications. PZT-based multicomponent ceramic materials are the most widely utilized in electronic applications due to their superior ferroelectric, pyroelectric, and piezoelectric properties, but environmental issues relating to the hazardous nature of lead-containing materials and its disposal are driving the need to find lead-free ceramic materials as an alternative to fill the industrial gap [1‐3].

Lead zirconate titanate (PZT) and PZT-based materials exhibit exceptional ferroelectric properties around the morphotropic phase boundary (MPB). Consequently, effort has focused on alternative lead-free materials that possess an MPB region as replacements for PZT-based materials, and such materials have been explored in the past few decades [2]. (1 − x)(Na_0.5Bi_0.5)TiO₃‐xBaTiO₃ (NBT–xBT) as a lead-free system has attracted interest of numerous researchers due to its high ferroelectric properties at the MPB at x = 0.06–0.07 [1, 2, 4, 5], and the lower coercive field here which makes poling easier [5]. The phase diagram of NBT–0.06BT is complex and presents many phases. Firstly, from ferroelectric (FE) to antiferroelectric (AFE) or to relaxor at depolarization temperature (T _d), it is reported at around 100 to ~165 °C [6‐13]. Secondly, from antiferroelectric (AFE) to nonpolar (PE) phases, it happens at around 225–292 °C, at either Curie temperature (T _c) or the temperature at the maximum dielectric permittivity (T _m) [6‐10, 12]. The NBT–0.06BT phase transitions are also associated with structural changes, which can be either from rhombohedral (Rh) and tetragonal (Tr), if the composition is at the MPB, or from rhombohedral (Rh), if the composition is out of MPB, to tetragonal (Tr), and from tetragonal (Tr) to cubic (C) [9, 11, 12]. These structural phase changes are very important to the pyroelectric applications because large pyroelectric effects can be induced at these transitions.

NBT–0.06BT has been extensively investigated for dielectric, ferroelectric, and piezoelectric properties but to a lesser extent in terms of its pyroelectric properties [7, 9, 14, 15]. In previous work, we investigated the effects of Ba²⁺ content on the pyroelectric properties of NBT–0.06B_(1+x)T ceramics [2] and found that p increased from 2.90 to 3.54 (×10⁻⁴ C m⁻² C⁻¹) at RT when x = 1.02. In addition, p showed huge enhancement from 55.3 to 740.7 (×10⁻⁴ C m⁻² C⁻¹⁾ at T _d (85 °C) for the composition NBT–0.06B_1.02TiO₃ [16]. Follow on studies on this work, reported the effects of Ta doping in NBT–0.06BT with the pyroelectric coefficient highly enhanced from 3.14 to 7.14 (×10⁻⁴ C m⁻² C⁻¹) at RT at Ta = 0.2% with the same composition showing a maximum value of 146.10 × 10⁻⁴ C m⁻² C⁻¹ at T _d (78.8 °C) [16]. Furthermore, our recent investigation of NBT–0.06BT doped with La reveals that the pyroelectric properties of NBT–0.06BT were greatly improved by La doping from 3.14 to 7.42 (×10⁻⁴ C m⁻² °C⁻¹) at RT at La = 0.5% and to 105.40 × 10⁻⁴ C m⁻² °C⁻¹ at T _d (67.9 °C) at La = 0.2% [17].

Pyroelectric properties usually reach a peak value at phase transition temperatures, such as T _d and T _c. These two phase transition temperatures in NBT–0.06BT are far too high for practical use at room temperature, so it would be beneficial if these phase transition temperatures can be lowered. In this study, we expect to bring down the depolarization temperature by modifying the stoichiometric state of the compositional contents x, y, and z of NBT–0.06BT, to simultaneously improve the pyroelectric properties of these compositions at both RT and T _d.

Experimental

A solid-state synthesis technique was selected to prepare the stoichiometry 0.94NBT–0.06BT (NBT–0.06BT) and nonstoichiometry 0.94Na_xBi_yTiO₃–0.06Ba_zTiO₃ (Na_xBi_yT–0.06Ba_zT) compositions. The reagents used in this project were bismuth oxide (Bi₂O₃), 99.999%, sodium carbonate (Na₂CO₃), 99.5%, barium carbonate (BaCO₃), 99.98%, and titanium dioxide (TiO₂), >99.8% (all Sigma-Aldrich).

The amounts of powders were calculated according to the chemical formula of 0.94Na_xBi_yTiO₃–0.06Ba_zTiO₃, with the x, y, and z values for each composition listed in Table 1, where the start composition of B is modified by receive an extra 0.02 from Na(x) and Bi(y) concentrations and the composition of C is also changed by modify the y/x ratio to be >1, while the start composition of D is as same as composition C with extra Ba (z) 0.02 concentration, as given in Table 1. The raw materials were ball-milled (zirconia milling media) in acetone in polyethylene pots for 24 h in order to mix and mill the powders. The resultant slurries were dried overnight at 50 °C, and the dried powder cakes were ground in a mortar for 10 min and sieved through a 250-μm mesh in order to aid the calcination step. The powders were calcined at 850 °C for 180 min in a closed alumina crucible with a heating ramp rate of 1 °C per minute and cooling rate 5 °C per minute in a furnace (Pyro Therm, ITEMP 14/6). After calcination, the powders were re-milled in acetone for 24 h, and then, 2% (wt.) of poly vinyl alcohol (PVA) was added as an organic binder to the dried powders to enhance the mechanical strength of the pellets during pressing. After that, the powders were fully dried in oven at 80 °C. The dried powders were ground and sieved and subsequently pressed into green pellets with a diameter of 10 mm under an uniaxial compaction with a load of ~78MPs for 5 min at RT. The pellets were sintered at temperatures up to 1150 °C for 120 min in closed crucibles in order to minimize the loss of volatile Na⁺ and Bi³⁺. The pellets were lapped/polished on both sides using silicon carbide paper. Silver conductive paint (RS limited) was used to electrode the pellets, and electrical poling at 6.5 kV/mm for 10 min at RT in mineral oil was carried out using a Keithley (6517 electrometer/high resistance) DC power supply. Afterwards, the poled samples were washed thoroughly by using isopropanol in order to remove the mineral oil and then left in air for 3 h to dry. The poled samples were placed at RT for 60 min with their electrodes short-circuited to eliminate any unwanted trapped charges.

Table 1

Lattice parameters (Å), cell volume (Å³), tolerance factor, grain size, and density (gm cm⁻³) of NBT–0.06BT and N_xB_yT–0.06B_zT ceramics

Composition, name	Lattice parameter (Å)		c/a ratio	Average grain size (µm)	Density (ρ) (gm cm⁻³)
Composition, name	a	c	c/a ratio	Average grain size (µm)	Density (ρ) (gm cm⁻³)
NBT–0.06BT, A	5.5113	13.4744	2.4449	1.79	5.757
N_0.52B_0.52T–0.06BT, B	5.4914	13.5048	2.4595	1.67	5.754
N_0.5B_0.534T–0.06BT, C	5.5113	13.4902	2.4516	1.55	5.541
N_0.5B_0.534T–0.06B_1.02T, D	5.4957	13.5048	2.4586	1.41	5.704

SEM (FEI XL30 SFEG) was used to look at the grain morphology of the sintered samples, and X-ray diffraction (XRD) [Siemens Ltd Model: D500) was used to investigate crystallization and phase. Dielectric measurements were performed on an impedance analyser (Wayne kerr Electronics Ltd. Model 3245 and Hewlett Packard HP4092A)] over a temperature range from RT to 150 °C using a custom-built temperature-controlled hotplate in the frequency range of 0.1–10 kHz. Pyroelectric measurements were made using the Byer–Roundy [18] method on a custom-built computer-controlled rig which used thermoelectric heaters to ramp the temperature between 20 and 90 °C while under vacuum and collect the pyroelectric current response from a Keithley electrometer (Model 6217). Dielectric and pyroelectric data were then used to study the phase changes such as depolarization temperatures (T _d) and to determine figure of merit values F _i, F _v, F _D, and F _C.

Results and discussion

Effects of nonstoichiometry composition on microstructure

Figure 1a depicts the XRD spectra of stoichiometry and nonstoichiometry ceramic powders calcined at 850 °C for 3 h. All the compositions present a single-crystalline perovskite structure with no evidence of any secondary phases indicating that the excess Na⁺, Bi³⁺, and Ba²⁺ diffused into the NBT–0.06BT main lattice and formed the new nonstoichiometry N_xB_yT–0.06B_zT ceramic solid solutions [19, 20].

Figure 1b illustrates a splitting of the peak [111] into [003] and [021] at 39.0°–41.0°, for all the compositions showing the existence of the rhombohedral (Rh) phase. This split coincides with that observed in the X-ray diffraction spectra of the unmodified or undoped NBT [11, 19].

A similar splitting was identified in all compositions for the [200] peak into [200] and [002] at 46.0°–48.0° as shown in Fig. 1c, which indicates the presence of tetragonal (Tr) phase in the N_xB_yT–0.06B_zT lattice [11, 20, 21]. The coexistence of both Rh and Tr phases in the N_xB_yT–0.06B_zT ceramics confirms that the structures of stoichiometry composition (A) are at the MPB and also the nonstoichiometry compositions (B, C, and D) remain at MPB area. Although the stoichiometric state of B, C, and D has been changed by modifying the elements molar concentrations (x, y, and z) in this study as mention before, XRD patterns show no significant changes in the phase structure of nonstoichiometry N_xB_yT–0.06B_zT compositions compared with the stoichiometry composition which is in agreement with the literature [11, 19, 21, 22].

According to the ionic matching relationship [23], the ionic radius of all elements in the compositions is Na⁺ (1.39 Å, CN 12) (CN = coordination number), Bi³⁺ (1.31 Å, CN 12), Ba²⁺ (1.61 Å, CN12), and for Ti⁴⁺ (0.61 Å, CN 6). The excess in the elements content will compensate the loss of elements due to evaporation at the calcining and sintering stages and may also substitute other cations in the composition [24]. For instance, the excess in the barium content in Composition D may compensate the deficiency in the Ba²⁺, which will improve the tetragonality in the N_0.5B_0.534TiO₃–0.06B_1.02T composition [2], simultaneously being more likely to substitute the sodium than replace the bismuth in NBT due to the larger radial difference (>15% for Bi) [19].

To determine the stability of the perovskite structure, Goldschmidt tolerance factor (t) relation was calculated by using Eq. 1 [25‐28]:

$$ t \, = \frac{{R_{\text{A}} + R_{\text{O}} }}{{\sqrt 2 (R_{{{\text{B}} }} + R_{\text{O}} )}} $$

(1)

where R _A, R _B, and R _o are the ionic radius of ions which will occupy A, B sites, and the oxygen anion. The tolerance factor value of stable perovskite structure is in the range of 0.77–0.99 [27]. The average t value which has been calculated for the stoichiometry and nonstoichiometry compositions in this study is around 0.982, suggesting that all compositions illustrate stable structure.

Table 1 illustrates the lattice parameters (Å), the c/a ratio, the average grains size (µm), and samples density (g cm⁻³) of all compositions. The lattice parameters are “a = b” and “c” vary by modifying x, y, and z contents. The c parameter shows a slight increase from composition A to D. Compositions B, C, and D have higher c/a ratio than Composition A indicating that the nonstoichiometry Compositions B, C, and D have more tetragonality in their structure. This result is in agreement with the Yoon et al. [29] results.

Figure 2 shows the SEM surface morphology of the Compositions A, B, C, and D. All the compositions display a dense microstructure with an average density of around 5.689 ± 0.264 g cm⁻³. Although B, C, and D have nonstoichiometry compositions with excess sodium, bismuth, and barium contents, they present a lower density value than Composition A (Table 1). Stoichiometry and nonstoichiometry compositions show a granule grain shape with various grain sizes. The average grain size reduces gradually from Compositions A to D (Table 1). The reason behind the reduction in the average grain size may link to the compositional changes that happened during calcining and sintering at high temperature. The loss of Na⁺ is more likely higher than that of Bi³⁺ due to the larger volatility of Na⁺. Thus, the composition may shift to a bismuth-nonstoichiometry state after calcining and sintering. Generally, the bismuth behaves as an inhibitor of the grain growth in the NBT structure. Hence, the excess in bismuth content in Composition B can reduce the grain size [11].

Both Compositions C and D are bismuth nonstoichiometric. Therefore, the effect of the Bi³⁺ on grain size is more pronounced in these compositions. Composition D presents the smallest grain size, which is due to the influence of Bi³⁺ and Ba²⁺ nonstoichiometric (excess content), and both of which may boost the prevention of the grain growth [2, 19, 29, 30]. In PZT ceramics, similar results (grain size reduction) have been reported where lead was substituted with barium [2, 19, 29, 30].

Effects of nonstoichiometry composition on dielectric properties

Modifying the stoichiometric state of the starting composition has a big effect on the magnitude of the dielectric properties of the N_xB_yT–0.06B_zT material. Figure 3a–d presents the change of the dielectric properties of poled compositions from A to D as a function of temperature from 20 to 150 °C at 1, 10, and 100 (kHz). The relative permittivity (ɛ _r) of all the compositions increases with temperature, which behaves like a typical ferroelectric material [31].

The relative permittivity (ɛ _r) shows a significant increase with the increase in the x, y, and z contents, with values of 396, 597, 893, and 933, respectively, for the Compositions A to D at RT and 1 kHz; therefore, the small variation in start compositions can be responsible for the significant increase in the relative permittivity value [24].

The dielectric loss (tanδ) also increases with the temperature up to a certain point and then decreases. The tanδ value of Composition A is ~0.0436 at RT and 1 kHz (Fig. 3a). It increases for nonstoichiometry Compositions C and D to around 0.0531 and 0.0561, respectively (Fig. 3b–d). However, Composition B presents unexpected dielectric loss value (~0.0427) which is the lowest value among all other compositions (Fig. 3b).

The temperature at which tanδ shows a maximum value is widely known as the depolarization temperature (T _d) [13, 32, 33]. The tanδ values at T _d are around 0.0510, 0.0548, 0.0599, and 0.0625, and their corresponding temperatures estimated at ~115, 85, 55, and 65 (°C) for Compositions A, B, C, and D, respectively (Fig. 4). The tanδ results at T _d reveal that B shows the lowest value among all other compositions. The ɛ _r values at T _d are ~582, 792, 1055, and 1105 for Compositions A, B, C, and D, respectively (Fig. 3a–d). The ɛ _r values at T _d clearly present that the change in the nonstoichiometric state of the composition has a significant effect on the dielectric properties, but the dramatic effect is on the relative permittivity at wide temperature range.

Effects of nonstoichiometry composition on depolarization temperature

Changing the stoichiometric state of the composition can also make a large effect on the depolarization temperature (T _d) of the N_xB_yT–0.06B_zT compositions, where the Na_0.49Bi_0.50TiO₃ shows lower T _d than Na_0.50Bi_0.50TiO₃ [24]. Although depolarization temperature is an important parameter in pyroelectric study, its origin has not been fully understood for NBT–xBT. It has more than one definition [27, 32, 34‐37], which includes the phase transition temperature from ferroelectric (FE) to antiferroelectric (AFE) phases or from ferroelectric to relaxor [33, 34, 38, 39].

In this study, the T _d is subjectively identified by two methods: (1) dielectric method, from the dielectric loss peak value versus temperature plots (Fig. 4), and (2) pyroelectric method, the pyroelectric coefficient (p) peak value versus temperature plots (Fig. 5) [14]. For both methods, the T _d is strongly dependent on modifying x, y, and z concentrations of the compositions. Figure 4 reveals that T _d decreased from 115 °C in Composition A to around 55 °C for Composition C, respectively. Composition D illustrates a higher T _d, which is more than 10 °C higher than the T _d for Composition C (Fig. 4), whereas a shift to lower temperature for Ba²⁺-nonstoichiometric composition was expected according to our previous study [2].

T _d can also be obtained from the maximum pyroelectric coefficient values. It is widely known that the maximum p can be achieved at the phase transition temperature [40‐43]. In this study, T _d values identified by this method showed a decrease from more than 90 °C to ~82.7, 60.0, and 60.6 °C for Compositions A to D, respectively. The exact T _d could not be identified for Composition A by the pyroelectric method due to the upper temperature limit of equipment (Fig. 5). In addition, Composition D shows a similar T _d to Composition C although as mentioned above, a lower value with increasing the Ba²⁺ content [2] was expected. The Bi³⁺ nonstoichiometric (excess content) in Composition D may be the reason for unexpected high T _d in both methods.

The decrease in T _d for nonstoichiometry compositions is probably caused by the excess amount of bismuth and/or deficiency of sodium and also by extra barium concentration in the case of Composition D. According to Yoon et al. [29], Guo et al. [33], and Li et al. [44], the T _d coincides with phase transition from ferroelectric to a so-called intermediate phase phase, and this transition shows a shift of T _d towards lower temperature for compositions with excess Bi³⁺ and/or deficit of Na⁺. Excess Bi³⁺ concentration in the Compositions B, C, and D is more likely to replace the A-site ions, Na⁺, Bi³⁺, and Ba²⁺ rather than B-site Ti⁴⁺ [29, 44] as discussed before, which may result in the generation of some of the A-site vacancies when Bi³⁺ replaces Na⁺. These vacancies can reduce ferroelectric long-range ordering and weaken the coupling between A-site ions and TiO₆ octahedron, as a result inducing the phase change at lower temperature [29, 44]. Our results agree with the literature but also reveal that the T _d can be reduced and shifted towards lower temperature not only by changing the Bi³⁺ /Na⁺ ratio but also by modifying all the A-site ions (Na⁺, Bi³⁺, and Ba²⁺) contents [2, 24, 33].

Comparing T _d values obtained by the two methods, the pyroelectric method shows T _d values slightly lower than the dielectric method, which is in agreement with the literature [32].

Effects of nonstoichiometry composition on pyroelectric properties

Modifying the stoichiometry state of the start composition can significantly affect the pyroelectric coefficient (p) of NBT–0.06BT material. However, the efficient way for evaluating the pyroelectric material performance is to calculate the figure of merits (FOMs) that represent the essential parameters for the pyroelectric applications [32]. The calculation of the FOMs is based on the values of the pyroelectric coefficient, relative dielectric permittivity, dielectric loss, and the specific heat [29]. There are several types of FOMs depending on special types of the pyroelectric applications such as infrared or thermal imaging [21, 45]. The types of FOMs depend on the input of thermal or electrical circuits, current, or voltage [8, 14, 32].

In this study, the pyroelectric coefficient (p) of NBT–0.06BT and N_xB_yT–0.06B_zT was measured from 20 to 90 °C for stoichiometric Composition (A) or T _d for nonstoichiometric Compositions B, C, and D. The calculations of the pyroelectric coefficient (p) and FOMs were carried out using Eqs. 2–6 [9, 15, 18].

$$ I_{\text{p}} = pA\frac{{{\text{d}}T}}{{{\text{d}}t}} $$

(2)

$$ F_{\text{i}} = \frac{p}{Cv} $$

(3)

$$ F_{\text{v}} = \frac{p}{{Cv\varepsilon_{\text{o}} \varepsilon_{\text{r}} }} $$

(4)

$$ F_{\text{D}} = \frac{p}{{Cv\sqrt {\varepsilon_{\text{o}} \varepsilon_{\text{r}} \tan \delta } }} $$

(5)

$$ F_{\text{C}} = \frac{p}{{\sqrt {\varepsilon_{\text{r}} } }} $$

(6)

where T is absolute temperature, t is the time, p is the pyroelectric coefficient, I _p is the pyroelectric current, F _i is the high current (i) detectivity, F _v is the high voltage ($ {\text{v}} $) detectivity, F _D is high the detectivity and F _C is the pyroelectric figure of merit. $ C_{v} $ * is the specific heat (2.8 J K⁻¹ cm⁻³, quoted from Ref. [14]), ɛ _r is the relative dielectric permittivity, and ɛ _o is the permittivity of free space.

Figure 5 shows the change of p versus T for all compositions from 20 to 90 °C, with Composition B showing the highest p value at RT. In the temperature range from 33 to 70 °C, Compositions C and D have similar pyroelectric coefficient values with a very close maximum value of p. In contrast, Composition B shows a very characteristic peak p value. Generally, pyroelectric materials show one sharp peak at a phase transition temperature as with Composition B. However, it is very evident that the broad flat profiles for Compositions C and D in the temperature range (33–70 °C) must be linked to a different compositional structure. The composition of both samples is located at the MPB. At T _d, their structure has been changed to tetragonal with antiferroelectric or relaxor state. However, with the increase in temperature, the structure and polarization states possibly changed from rhombohedral and tetragonal coexisting (MPB) ferroelectric to tetragonal ferroelectric and then to tetragonal antiferroelectric or relaxor. This phase transition may give the pyroelectric coefficient with a broad flat peak temperature region.

The p, and FOMs F _i, F _v, and F _D values for all compositions are listed in Table 2 at RT, while those values at 90 °C or T _d are listed in Table 3.

Table 2

Pyroelectric coefficient (p) and FOMs (F _i, F _v, and F _D) results at RT of NBT–0.06BT and N_xB_yT–0.06B_zT in the present study and literature

Composition, name	$ p $ × 10⁻⁴ (C m⁻² °C⁻¹) at RT	F _i × 10⁻¹⁰ (m.V⁻¹) at RT	F _v (m² C⁻¹) at 1 kHz at RT	F _D × 10⁻⁶(Pa^−1/2) at 1 kHz at RT	References
NBT–0.06BT, A	3.14	1.12	0.021	9.08	This study
N_0.52B_0.52T–0.06BT, B	6.99	2.50	0.047	16.63	This study
N_0.5B_0.534T–0.06BT, C	4.77	1.71	0.022	8.32	This study
N_0.5B_0.534T–0.06B_1.02T, D	4.20	1.50	0.018	6.96	This study
PZT	4.14	1.42	0.008	9.01	[9, 19]
NBT–0.065BT	4.60	–	–	–	[15]
NBT–0.06BT–0.002Mn	3.51	–	–	–	[14, 30]
NBT–0.07BZT	5.73	2.03	0.022	10.52	[7, 9, 19]
BNKBT	3.25	1.95	0.026	13.43	[9, 19]
KNLNTS	1.94	0.93	0.007	11.51	[9, 19]
NKLBT–0.05BT	3.61	1.27	0.017	–	[32]

Table 3

Pyroelectric coefficient (p) and FOMs (F _i, F _v, and F _D) results at T _d of NBT–0.06BT and N_xB_yT–0.06B_zT ceramics in the present study and literature

Composition, name	$ p $ × 10⁻⁴ (C m⁻² °C⁻¹) at 90 °C or T _d	F _i × 10⁻¹⁰ (m V⁻¹) at 90 °C or T _d	F _v (m² C⁻¹) at 1 kHz at 90 °C or T _d	F _D × 10⁻⁶(Pa^−1/2) at 1 kHz at 90 °C or T _d	References
NBT–0.06BT, A	23.91 at 90 °C	8.60 at 90 °C	0.19 at 90 °C	65.51 at 90 °C	This study
N_0.52B_0.52T–0.06BT, B	75.33 at 82.7 °C	26.92 at 82.7 °C	0.39 at 82.7 °C	138.72 at 82.7 °C	This study
N_0.5B_0.534T–0.06BT, C	35.24 at 60.0 °C	12.61 at 60.0 °C	0.13 at 60.0 °C	52.34 at 60.0 °C	This study
N_0.5B_0.534T–0.06B_1.02T, D	34.93 at 60.6 °C	12.54 at 60.6 °C	0.12 at 60.6 °C	49.80 at 60.6 °C	This study
NBT–0.07BZT	20.01 at 50 °C	7.33 at 50 °C	0.07 at 50 °C	33.62 at 50 °C	[9, 19]
NBT–0.07BZT	22.10 at 87 °C	–	–	–	[9, 19]

The pyroelectric coefficients of nonstoichiometric compositions vary but are higher than Composition A at RT and T _d.

Composition B shows the optimum p value at both RT and T _d (Tables 2 and 3). This may be linked to the microstructural change due to the change of the stoichiometric state of this composition. Composition B has a bigger c/a ratio than all other compositions (Table 1), indicating that Composition B has more tetragonality in its structure than all other compositions [13, 43, 44, 46, 47]. Importantly, at RT, the p values for nonstoichiometry compositions are higher than that of PZT and other lead-free compositions (Table 2) [7, 19, 27, 32].

The F _i, F _v, and F _D show also an increasing trend as p at RT. Compared with PZT and other lead-free compositions (Table 2) [7, 9, 29, 30, 42], the nonstoichiometry compositions have higher F _i, F _v, and F _D values at RT with Composition B showing the highest value at RT and T _d (Tables 2 and 3) from this study. The optimum F _v and F _D values (Tables 2 and 3) were observed for Composition B at both RT and T _d, which can be linked to the lower ɛ _r and the higher p values observed at these temperatures compared to Compositions C and D. Also the stoichiometry composition A has a better F _D value than C and D due mainly to the increases in ɛ _r and tanδ in this nonstoichiometry compositions.

The F _C values of stoichiometry and nonstoichiometry compositions at RT and at two frequencies 100 and 1000 (Hz) were calculated (Table 4). The F _C values revealed a strong dependence upon x, y, and z contents. Generally, the materials suitable for infrared detectors should have a F _C threshold value ~3 ± 1 × 10⁻⁹ C cm⁻² °C⁻¹ [15]. Composition B shows F _C values which are within the threshold range. Compositions A, C, and D present lower and close to that the RodrÍguez-Ruiz et al. [15] founded for a 0.935Na_0.5Bi_0.5TiO₃–0.065BaTiO₃ composition which is around 1.44 × 10⁻⁹ C cm⁻² °C⁻¹. Thus, the composition of Composition B is a promising candidate for this particular pyroelectric application.

Table 4

The figure of merit F _C, results at room temperature and at 100 and 1000 (Hz) of NBT–0.06BT and N_xB_yT–0.06B_zT ceramics

Composition, name	FOM, F _C (×10⁻⁹ C cm⁻² °C⁻¹) at RT
Composition, name	100 Hz	1000 Hz
NBT–0.06BT, A	1.53	1.58
N_0.52B_0.52T–0.06BT, B	2.58	2.86
N_0.5B_0.534T–0.06BT, C	1.52	1.60
N_0.5B_0.534T–0.06B_1.02T, D	1.31	1.37

Conclusion

The stoichiometry NBT–0.06BT and nonstoichiometry N_xB_yT–0.06B_zT ceramic compositions were prepared by a conventional solid-state technique. The pyroelectric properties of these compositions were investigated, and their values were better than those in PZT and other lead-free ceramic compositions.

XRD indicates that all compositions phase structures are at the MPB region. The lattice parameter values and c/a ratio slightly change in nonstoichiometry compositions. The average grain size reduces from 1.79 (A) to 1.41 µm (D). Moreover, the stoichiometry composition shows a higher density than the nonstoichiometry compositions. Both relative permittivity (ɛ _r) and dielectric loss (tanδ) are affected by change in the stoichiometric state of the compositions and show an increase with the increase in x, y, and z contents.

The T _d was subjectively identified by two different methods and decreases from ~115 °C in Composition A to around 55 °C in Composition C, as predicted by the dielectric method. However, the T _d value for Composition C was ~60 °C according to pyroelectric coefficient method which questions the accuracy of the two techniques to give a definitive answer. However, it was evident that certain compositions showed much clearer maxima to pinpoint the T _d, compared to others, which suggests that the techniques may still be valid.

Composition B has the optimum p at RT and T _d. The F _i, F _v , and F _D increase with the increase in Na⁺ (x) and Bi³⁺ (y) contents up to x = y = 0.52 at both RT and T _d. The F _C shows the optimum values in Composition B. The results presented in this study demonstrate that the nonstoichiometry N_0.52B_0.52T–0.06BT composition is a promising material for infrared detectors and other pyroelectric applications in a wide range of temperatures and frequencies.

Compliance with ethical standards

Conflict of interest

We declare that we have no conflict of interest because there is no funding received for this work.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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Title: Large pyroelectric properties at reduced depolarization temperature in A-site nonstoichiometry composition of lead-free 0.94Na x Bi y TiO3–0.06Ba z TiO3 ceramics
Authors: A. M. Balakt
C. P. Shaw
Qi Zhang
Publication date: 08-03-2017
Publisher: Springer US
Published in: Journal of Materials Science / Issue 12/2017
Print ISSN: 0022-2461
Electronic ISSN: 1573-4803
DOI: https://doi.org/10.1007/s10853-017-0973-1

Springer Professional

Large pyroelectric properties at reduced depolarization temperature in A-site nonstoichiometry composition of lead-free 0.94Na_xBi_yTiO₃–0.06Ba_zTiO₃ ceramics

Abstract

Introduction

Experimental

Results and discussion

Effects of nonstoichiometry composition on microstructure

Effects of nonstoichiometry composition on dielectric properties

Effects of nonstoichiometry composition on depolarization temperature

Effects of nonstoichiometry composition on pyroelectric properties

Conclusion

Compliance with ethical standards

Conflict of interest

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Springer Professional

Abstract

Introduction

Experimental

Results and discussion

Effects of nonstoichiometry composition on microstructure

Effects of nonstoichiometry composition on dielectric properties

Effects of nonstoichiometry composition on depolarization temperature

Effects of nonstoichiometry composition on pyroelectric properties

Conclusion

Compliance with ethical standards

Conflict of interest

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