Sodium ion incorporated alumina - A versatile anisotropic ceramic

Journal of the European Ceramic Society 39 (2019) 4473–4486

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Review article

Sodium ion incorporated alumina - A versatile anisotropic ceramic a,c

a

b

c

Pavan Pujar , Bikesh Gupta , Pradyut Sengupta , Dipti Gupta , Saumen Mandal a b c

a,⁎

T

Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Surathkal, 575025, India Advanced Materials Technology Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, 751013, India Plastic Electronics and Energy Laboratory, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, 400 076, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Sodium ion Alumina Ionic conductivity Dielectric Thin film Battery

The present article is a review of crystal structure dependent anisotropic properties of β and β″-phases of sodium ion incorporated alumina. The anisotropy in electrical properties such as ionic conductivity and dielectric permittivity is due to the layered structure. Conducting plane between two consecutive spinel aluminas constituting loosely bound mobile sodium ions, promote ionic conductivity in the parallel direction. In contrary, the restricted movement of ions in the orthogonal direction brings about polarization giving it directional dielectric property. High ionic conductivity of 1.3 S/cm and large dielectric constant of ˜ 200 are reported. Exchanging sodium ions with different cations, such as potassium and lithium, results in similar anisotropy. The processing of β and β″-phases along with metastability of intermediate mullite phase is described in the current review. In addition, the applications of sodium ion incorporated aluminas, such as solid electrolyte in batteries, thin film transistors and gas sensors are discussed.

1. Introduction The directional dependent properties in a single material give rise to a phenomenon called anisotropy, depicting a significant variation of properties in different directions. Composite materials are known to possess anisotropic mechanical properties based on reinforcement type, orientation, etc. [1]. Also, the specific orientation of grains in a polycrystalline material show anisotropic properties such as mechanical strength, ionic conductivity, etc. [2,3]. One such anisotropic nature composed of two distinct electrical functionalities, namely ionic conductivity and dielectric permittivity is observed in alkali metal ion incorporated alumina. The observed anisotropy arises at the unit cell level, which is distinct from composites or textured grains of a polycrystalline material. The presence of mobile alkali metal ions in the crystal results in the ionic conductivity in a specific direction, while the restricted movement in the orthogonal direction provides dielectric polarization. The alkali metal ion incorporated alumina, especially the beta (β) phase of sodium ion incorporated alumina, has drawn remarkable attention of the scientific community from the early 20th century. The existence of the β-phase was first reported in 1916 by Rankin and Merwin [4]. With significant advancements in the chemical and structural analysis, the exact structure of sodium β-alumina was revealed, where sodium ions are loosely situated between defective alumina

⁎

spinels (Fig. 1). The plane at which the free sodium ions reside is called a conduction plane, where these ions are free to move. The conduction planes are parallel to the alumina spinel blocks. Thus, the ionic conductivity in the material is the result of the movement of sodium ions, which has been leading the technology of solid electrolytes for batteries today. Apart from this, the sodium β-alumina has shown an excellent charge storage capacity with strong polarization in the direction perpendicular to the conduction plane, the realization of which has come into existence in recent years. Initially, Pal and co-workers [5] have demonstrated a thin film transistor (TFT) using the dielectric sodium βalumina. The performance of the TFT has greatly enhanced, especially in terms of low voltage operation, because of the high dielectric constant (ε) of sodium β-alumina. Additionally, the widespread application of sodium β-alumina in sensing gases opened up new opportunities. The standard galvanic cell employed with sodium β-alumina as an electrolyte interacts with gases like carbon dioxide (CO2) [6,7] and vapors of sodium [8], aid the movement of ions. Regardless of the type of application, obtaining pure β-phase of sodium ion incorporated alumina is crucial. In order to ensure the same, the selection of processing techniques plays a crucial role. The β-phase is stable at high temperatures and requires externally added stabilizers [9] such as magnesium oxide (MgO) and lithium oxide (Li2O). Several processing techniques have been adopted to produce powders of sodium β-alumina, namely sol-gel [10], spray pyrolysis [11], vapor phase

Corresponding author. E-mail address: [email protected] (S. Mandal).

https://doi.org/10.1016/j.jeurceramsoc.2019.08.001 Received 25 January 2019; Received in revised form 30 July 2019; Accepted 1 August 2019 Available online 02 August 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.

Journal of the European Ceramic Society 39 (2019) 4473–4486

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Fig. 1. The crystal structure of sodium β-alumina (space group: P63/mmc) with a composition, NaAl11O17 (yellow: sodium ions, blue: aluminum and red: oxygen ions) and lattice parameters, a = b =0.56418 nm and c =2.2782 nm (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

sites, such as Beevers-Ross (BR), anti-Beevers-Ross (aBR) and mid oxygen (mO) [19,22,24,25]. It was reported that stoichiometric sodium ions occupied BR site (2/3 1/3 1/4) and non-stoichiometric sodium ions occupied aBR site (0 0 1/4). The symmetry exhibited by the BR, aBR and mO sites are two-fold, two-fold and six-fold, respectively. In 1978, G. J. May reported that 75% of sodium ion density cloud is present close to BR site and the rest (25%) is found in the mO position. However, the aBR site in the unit cell of β-alumina is unoccupied [26]. Peter et al. (1971) observed that sodium β-alumina contains 15–30 % of excess sodium, which is off-stoichiometric [22]. The composition of sodium β-alumina is confirmed through nuclear magnetic resonance spectroscopy [27] and ion exchange [28] experiments and further compared with pseudo-binary Na2O-Al2O3 systems. Despite of excess cations originated from the non-stoichiometric composition of β-alumina, the charge neutrality is maintained due to the existence of point defects and oxygen interstitials. Neutron diffraction studies revealed that one oxygen interstitial is coordinated with two Frenkel defect sites (VAl Ali − O2i − − Ali VAl ) in order to accommodate an excess positive charge [29]. However, there is still some controversy regarding the fact that whether β and β″-aluminas are two distinct phases or they differ in chemical composition [30,31]. In this article, it is assumed that β -alumina is a generic name of a group of oxides that is stable with different phases (β, β′, β″, β‴, β⁗) over a wide range of composition and temperature in pseudobinary Na2O-Al2O3 and ternary Na2O-MgOAl2O3 systems. The non-stoichiometry, structure and intergrowth of a βalumina group of oxides have been reported in the literature [32–34]. Often, both the hexagonal (β) and rhombohedral (β″) polymorphs of βalumina are obtained simultaneously, while synthesizing sodium βalumina. This may be attributed to syntaxial intergrowth of β″-alumina and β-alumina in the basal plane due to similar lattice parameters. Yamaguchi reported the existence of two phases β′-alumina (Na2O∙8.1Al2O3) and β″-alumina (Na2O∙5.18Al2O3) [35]. In 1986, Schmid presented the structural characteristics of β-alumina and β″alumina using CuKα (2θ = 5°-67°) and CoKα (2θ = 17° - 55°) radiations [36]. The characteristic peaks of sodium β-alumina (017, 022, 025, 026, 027) and sodium β″-alumina (0111, 118, 027, 0210) were determined using Schmid technique. If β-alumina and β″-alumina polymorphs are present simultaneously in the structure, then the phase ratio β″ can be obtained from integrated intensity of (0111) peak of β″-

conversion technique [12], freeze-drying [13], sol-gel combustion [14], and so on. In order to deposit thin films of sodium β-alumina, spin coating of sol-gel precursors [5] and spray coating of aqueous combustion precursors [15] have been employed. Also, the low-temperature synthesis techniques of sodium β-alumina have been reported [5,15]. In the current review, a series of important aspects of sodium β-alumina is discussed. Also, the crystal structure, effect of alkali addition, various processing techniques and applications such as solid electrolyte, gas sensor and dielectric for microelectronics are addressed. 2. Crystal structure The thermodynamically stable form of alumina (Al2O3) is corundum, where the oxygen ions form a close-packed hexagonal structure with 2/3rd octahedral sites filled with aluminum ions and falls in a space group of R-3c. Also, alumina exists in a range of different phases namely, γ and η (cubic), χ (hexagonal), κ (orthorhombic), δ (tetragonal and/or orthorhombic) and β. Initially, the β-phase was manifested to be NaAl11O17 [16,17] and it misinterpreted as a polymorph of Al2O3. In the early 20th century, chemical analysis results were far from accuracy and consequently, the exact chemical structure of sodium β-alumina could not be determined accurately. Later with advancements in the chemical and structural analysis, many unknown facts regarding the structure and properties of sodium β-alumina were unveiled. Diffraction studies have shown that sodium ions play a dominating role in determining the chemical composition and electrical properties. Bragg, Gottfried and West published the crystal structure of β-Al2O3 in 1931 [18], which is later known to be sodium β-alumina. In 1937, the structure was reported by Beevers and Ross with the help of x-ray crystallography [19]. They reported the structure of potassium β-alumina as well, which is an isomorph of sodium β-alumina. The idea of non-stoichiometry was first presented by Saalfeld [20]. The spinel-type blocks resemble the cubic close-packed structure of MgAl2O4 [21]. The conduction plane is composed of loosely bound oxygen and sodium ions. However, an in-depth investigation of the crystal structure of sodium β-alumina was difficult, until the exact position of sodium ions was identified. Extensive work has been carried out by several research groups across the globe to determine the exact location of sodium ions in the unit cell of β-alumina [19,22–24]. Chemical analysis and diffraction studies disclosed that sodium ions are present in the conduction plane of sodium β-alumina at three probable

β″ +β

alumina and (017) peak of β-alumina with an accuracy of ± 1% [36]. X-ray diffraction studies established that the crystal structure of sodium 4474

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P. Pujar, et al.

blocks (2B), the thickness of each block is 1.59 nm) and β″-alumina (three spinel blocks (3B), the thickness of each spinel is 1.59 nm) respectively. From the equilibrium diagram of a Na2O-Al2O3 system, it can be interpreted that NaAlO2 is stable in Na2O rich regions and predominantly exists in various phases such as β, δ and γ. The low-temperature phase β-NaAlO2 transforms into γ-NaAlO2 at 470 °C. Similarly, δ-NaAlO2 is a high-temperature phase which is thermodynamically stable at a temperature higher than 1410 °C. β-alumina is stable in alumina rich regions of the Na2O-Al2O3 system and present as β and β″ polymorphs. As the ionic conductivity of β″-alumina is higher than βalumina, the former is a preferred candidate for electrolyte application [42]. However, β″-alumina loses its structural integrity at high temperature (> 1600 °C) due to the loss of Na2O [43]. The transformation of β″ to β above 1550 °C is irreversible, as stable β″-alumina phase cannot be obtained by cooling β-alumina below 1550 °C. β″-phase is a metastable phase present in the Na2O-Al2O3 system. High-temperature stability of β″-phase can be improved by doping with MgO as magnesium ions substitute for aluminum ions in spinel lattice. Introduction of small monovalent (Li+), divalent (Mg2+, Ni2+, Mn2+, Cu2+, Cd2+, Co2+, Zn2+) and tetravalent (Ti4+) cations with an ionic radius less than 0.097 nm in the structure stabilize the β″-phase, as cations occupy tetrahedral or octahedral sites of the spinel block. Non-stoichiometric compositions of monovalent (Li+), divalent (Mg2+) cation doped βalumina have been reported in the literature. In the unit cell of β-alumina, ‘c’ parameter is more sensitive to the concentration of Na2O as compared to ‘a’ parameter. G. B. Telnovaet. al., (2015) reported that ‘c’ parameter shrinks by 0.14-0.35% and ‘a’ parameter changes by 0.07% with an increase in the concentration of Na2O [44]. Fig. 2b shows the variation of the lattice parameter of β-alumina with a change in concentration of Na2O. The overall chemical composition of non-stoichiometric β and β″-aluminas can be reported as (Na2O)1+x·11Al2O3, where x varies from 0.25 to 0.55 for β-phase and ranges between 0.55 and 0.65 for β″-alumina [42].

β-alumina is hexagonal (space group: P63/mmc) [18,19]. In this review, the crystal structure of NaAl11O17 is reconstructed using VESTA software and presented in Fig. 1, which is composed of alternately stacked non-conducting close-packed oxide slabs and loosely packed conduction layer [37]. The close-packed oxide slabs resemble the structure of spinel in terms of occupancy of aluminum ions in tetrahedral and octahedral voids. The non-conducting spinel blocks of aluminum and oxygen ions are separated by an intermediate conducting layer comprising of sodium and oxygen ions. In other words, the conduction plane of sodium β-alumina is sandwiched between two adjacent nonconducting spinel blocks in such a manner that the two-fold screw axis is perpendicular to the conduction plane. In β-alumina, adjacent spinel blocks are separated by a distance of 1.123 nm, exhibiting mirror symmetry with conduction plane. In 1968, Yamaguchi and Suzuki reported that the crystal structure of sodium β-alumina is rhombohedral (space group: R 3¯ m) [35,38]; these forms exhibit distinct chemical stoichiometry, ionic (Na+) conductivity and occupancy of sodium ions. The unit cell of β″-alumina is composed of three spinel blocks with intermediate conduction planes in such a fashion that three-fold screw axis is perpendicular to the conduction plane without any mirror symmetry between the spinel blocks of β″-alumina. Owing to the difference in oxygen stacking sequence, ‘c’ axis in the unit cell of β″-alumina is 50% larger than that of β-alumina. The unit cell parameters of sodium β and β″-aluminas are presented in Table 1. The β″ is rhombohedral with space group R 3¯ m, but it is considered as a hexagonal unit cell with three spinel blocks rotated by 120° and the value of -h + k+l becomes ‘3n’ where n is an integer, when it is indexed as a hexagonal unit cell [38]. Sodium ions move in the conduction plane under applied electrical field. As β-alumina exhibits high ionic conductivity in ‘ab’ plane or the plane perpendicular to ‘c’ axis, thus it is known to be an anisotropic ionic conductor [21].

2.1. Effect of alkali

3. Anisotropic properties of sodium β-alumina

The presence of impurity ions dictates physical properties of sodium β-alumina. Fig. 2a shows alumina rich portion of the pseudo-binary Na2O-Al2O3 system proposed by De Vries and Roth [39]. They proposed that sodium deficient β″-alumina dissociates into β-alumina and δNaAlO2 at a temperature ˜ 1550 °C. Above 1550 °C, β″-alumina loses its structural integrity and β-alumina becomes stable. Kale studied the activity of Na2O in Na2O-Al2O3 system [40]. β and β″-aluminas are stable over a range (85–90 mol % Al2O3) in the pseudo-binary Na2OAl2O3 system. The equilibrium sub-solidus relations in the ternary Na2O-MgO-Al2O3 system shows that β-alumina is present as four distinct phases, such as β, β″, β‴ and β⁗ [21,41]. However, the presence of β‴ and β⁗ phases cannot be found in the Na2O-Al2O3 system. The crystal structures of β‴ and β⁗ are similar to that of β (two spinel

The structure of sodium β-alumina gives rise to distinct properties in two different directions. Along the conduction plane or parallel to the alumina blocks, the ions are mobile, but in the perpendicular direction, the movement is greatly restricted and gives rise to polarization under an applied electric field. The movement of sodium ions is characterized by ionic conductivity; Table 2 shows reported ionic conductivities of sodium β-alumina. The ionic conductivity of sodium ion incorporated alumina depends on a series of factors such as phase, processing route, density, grain size, grain orientation and sintering parameters. The β″alumina is known for its higher ionic conductivity due to the presence of a large fraction of sodium ions compared to β-phase. The implication

Table 1 Crystal structure, space group and unit cell dimensions of sodium β/β″-alumina. Type of β-alumina

Crystal structure

Space group

Sodium β-alumina

Hexagonal

P63/mmc

Sodium β″-alumina

Rhombohedral

R3¯ m

Lattice parameter (nm)

4475

Ref.

a

c

0.5560 0.5584 0.5596 0.5593 0.5590 0.5609± 0.0004 0.5604± 0.0005 0.5595 0.5607± 0.0003 0.5604± 0.0005 0.5600 0.5587

2.2550 2.2450 2.2520 2.2610 2.2610 2.235±0.002 2.252±0.004 6.7860 3.365±0.005 3.396±0.007 3.395 3.342

[18] [19] [23] [35] [18,19] [26] [26] [35] [26] [26] [35,38] [44]

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Fig. 2. (a) Phase diagram of Na2O·Al2O3-Al2O3 system (Reproduced from [39] by permission of The American Ceramic Society), where ‘B’ stands for spinel block (2B: two spinel blocks and 3B: three spinel blocks) and (b) effect of Na2O content on the lattice parameter of sodium β-alumina (Reproduced from [44] by permission of Springer).

multi-stage process [65]. As an example, the direct exchange of Li+ with Na+ [24] can be realized through an intermediate exchange of Ag+ ions. Moreover, divalent cations like Eu2+, Zn2+, Ni2+, Sr2+, Ba2+, Ca2+, Cd2+, Cu2+, Pb2+ [70–74] and trivalent cations like Gd3+, Nd3+, Eu3+, Yb3+, Sm3+, Tb3+, Dy3+, Bi3+, Pr3+ [63] have been successfully exchanged with sodium ions. Gd3+, Nd3+, Eu3+ have been completely exchanged by sodium ions, while other trivalent cations were partially exchanged with sodium ions. This exchange with various cations has tended to alter the ionic conductivity as well as the lattice parameter of the β-alumina system due to their relative sizes. In general, the alkali ion incorporated aluminas with general formula M2O·Al2O3 where ‘M’ is the alkali cation can show ionic conductivity [75] due to their free movement in the conduction plane parallel to the closely packed alumina spinels. A higher concentration of alkali ions can result in greater ionic conductivity. Thus, incorporation of excess alkali cations, sodium in particular, becomes crucial. The fundamental difference between β and β″ phases is due to the concentration of sodium ions. β″-phase accommodates larger concentration of sodium ions and thereby depicts higher ionic conductivity, while both β and β″ are non-stochiometric in nature [76–78]. Presence of alkali cations not only affect the ionic conduction but also the lattice parameters due to their relative sizes. It is important to know that the selection of alkali ions should be made such that it should not shatter the unit cell [73]. Tables 3 and 4 summarize the reported ionic conductivity and the lattice parameters, respectively. Ionic conductivity mainly depends on factors such as (a) geometry of the product, (b) added additives, (c) formation of secondary phases and (d) grain boundary segregation. The method of incorporation of excess sodium ions to increase the ionic conductivity needs higher solubility of sodium ions in the alumina crystal. A general composition of sodium-alumina (Na2O)1+x·Al2O3 with x = 0.57 results in the composition Na1.67Al10.33Mg0.67O17 and x = 0.67 gives rise to a composition Na1.67Al10.67Li0.33O17 which depicts β and β″ phases respectively. The latter accommodates larger composition of sodium ions, termed as β″ and the former is β-alumina. There are two mechanisms by which excess sodium ions can be accommodated into the β-alumina structure. First, interstitial oxygen ions as Frenkel defects can be stabilized by aluminum cation from the spinel block which leaves vacant octahedron/tetrahedron sites and allows extra sodium ions to fill the vacant positions. Thus, the presence of each interstitial oxygen ion allows two sodium ions making the crystal more conductive. Secondly the doped β-alumina is reported to be more ionically conductive compared to undoped one when it is doped with mono or divalent ions (Li+ and Mg2+). Mg and/or Li-doped β-alumina allows more sodium ions into the structure to stabilize the β″-phase. The illustration in Fig. 3 shows the method and effect of incorporation of

of this can be visualized from the magnitudes of ionic conductivities; many reports in the literature have shown a mixture of β and β″ phases (Table 2). With an increase in the fraction of β″, the ionic conductivity shot up. Also, the presence of secondary phases like MgAl2O4 reduces the ionic conductivity to a greater extent [45]. Thus, various processing techniques have been employed, which are aimed in synthesizing phase pure β″ or a mixture of β and β″, with a larger fraction of β″. Further, sintering temperature and time are the greater concerns in realizing high ionic conductivity. The pellets of phase pure β″ with sintering temperature and time of 1500 °C and 10 h respectively, have shown 10% higher ionic conductivity compared to 3 h sintered pellets at same temperature [46]. The variation in ionic conductivity as a function of different sintering temperatures and time can be ascribed to higher densification [46]. Dense bodies with phase pure (β″) are expected to possess higher ionic conductivity. The maximum densification is reported with spark plasma sintering, where 99.3% of the theoretical density was achieved at 1400 °C, with an exposure time of 180 min [47]. In addition, larger grain size and the orientation sensibly affect the ionic conductivity. The orientation has a considerable effect on the resistance to the flow of sodium ions. As an example, uniaxially pressed flake like β″-alumina resulted in the formation of parallelly oriented conduction planes in the direction of pressing. Thus, for β″-alumina, the ionic conductivity in the direction of the press is found to be higher (5 × 10−2 S/cm) than perpendicular to it (1.3 × 10-2 S/cm) [48]. Another aspect of varied ionic conductivity is attributed to the measurement conditions like temperature (Table 2). The higher temperature of measurement (generally 300 °C) results in the betterment of ionic conductivity due to improved mobility of sodium ions [46]. In β-alumina, sodium ions in the conduction plane can move freely along the plane parallel to the alumina spinel blocks. Sodium can be replaced by several other alkali ions which can contribute ionic conductivity to the crystal. 3.1. Ion exchange in β-alumina Owing to the high mobility of sodium ions in sodium β-alumina, ion exchange with other cations [24] has opened up a new door for the synthesis of different cation based β-alumina and its exploration as a high mobility solid-state electrolyte. Exchange with cations is possible from molten salts, aqueous solution and organic solvents [58]. It has also been reported that ion exchange is also possible in vapor state [59,60]. Various monovalent [61], divalent [62] and trivalent [63] cations have already been exchanged with Na+ in sodium β-alumina from their melt. Monovalent cations like, K+, Rb+, Ag+, NH4+, In+, Li+, NO+, Ga+, Cu+, H3O+ [24,64–69] have shown complete exchange with sodium ions. Ion exchange may also possible through the 4476

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Ref.

[49] [50] [51] [52] [53] [54] [55] [46] [56] [45] [57] [47] 13.88 × 10−3 0.18 3 × 10−3 to 6 × 10−3 1.3 ± 0.6 30 × 10−3 1.2 × 10−3 24 × 10−3 0.38 1.7 × 10−4 0.14 10−6 1.9 × 10−2

Incorporated cation +

Na Pb2+ Ca2+ Ca2+ Sr2+ Ba2+ Ti4+ Fe3+ Ti4+

10 1 4 4 3 / / / / /

/2 /–

β″ β″ β/β″ β″ β″ β/β″ β″ β/β″ β″ β/β″ β/β″ β″ – 0.84 Na2O·0.67 MgO·5.2 A12O3 – Nal.67MgO0.67Al10.33O17 Al0.33Mg0.68O17Na1.66 – – – Na1.0Mg0.67Al10.33O17 Na1.67Li0.33Al10.67O17 Na0.825Mg0.35Al4.825O8 Na1.67Mg0.67Al10.33O17

−2

1.3 × 10 4.6 × 10−3 3.9 × 10−6 – 2.4 × 10−6 2.8 × 10−6 – – –

σ at 300 °C (S/cm) −1

9.1 × 10 1.5 × 10−1 3.9 × 10−2 3.9 × 10−3 2.4 × 10−2 1.9 × 10−2 0.3 1.4 × 10−1 1.6 × 10−1

Ref. [50] [79] [79] [80] [79] [79] [81] [82] [82]

Incorporated cation

a (nm)

c (nm)

Ref.

Na+ K+ Na+ - K+ (50 %) Li+ Ag+ Rb+ NH4+ NO+ Ga+ Cd2+ Ca2+ Sr2+ Pb2+ Ba2+ Na+

0.5594 0.5596 0.5595 0.5596 0.5594 0.5597 0.5596 0.5597 0.5600 0.5620 0.5613 0.5610 0.5610 0.5619 0.5601

2.2530 2.2729 2.2606 2.2570 2.2498 2.2883 2.2888 2.2711 2.2718 3.3146 3.3270 3.3720 3.3976 3.4084 2.2520

[24] [24] [24] [64] [24] [24] [24] [65] [66] [90] [90] [90] [90] [90] [15]

excess sodium ions in the β-alumina. The presence of stabilizers helps in accommodating a larger fraction of sodium ions for every Mg2+ dopant, there is an extra sodium ion to be added [83]. Also, the added stabilizers such as Ni2+, Co2+ [84], Cu2+ [41], Cd2+ [85], Zn2+ [86] along with Mg2+ should occupy either octahedral or tetrahedral voids in the alumina spinel [21], but divalent ions such as Ca2+, Pb2+ and Sr2+ are not capable of entering the spinel, and thus they are not classified under stabilizers. However, among all existing stabilizers, Mg2+and Li+ is mostly preferred because other stabilizers are of variable valency [87,88]. Recently, the Li precursors were used for the stabilization of films of sodium β/β″-alumina using laser-chemical vapor deposition [89]. Mg2+ is more favorable to tetrahedral void forming a spinel of MgAl2O4, but lithium occupies octahedral vacant position forming inverse spinel (LiAl5O8). The occupancy of octahedral vacant position at the center of the alumina spinel is most favorable for conduction than Mg2+ tetrahedral occupancy nearer to the conduction plane. Another interesting electrical functionality of sodium β-alumina observed in the direction perpendicular to the conduction plane along which the movement of the sodium ions is restricted. Thus, under the application of electrical field, the polarization of the sodium ions takes place and results in the high capacitance (2.0 μFcm−2 at 50 Hz) and/or dielectric constant (ε ˜ 170) [5], which is superior to that of pure alumina (ε = 9.2). Similar dielectric property is also observed in amorphous sodium β-alumina (ε ˜ 200). Incorporation of different ions such as potassium and lithium have shown a varied dielectric response. Fig. 4 shows the capacitance behavior of metal-insulator-metal devices with K+, Li+ and Na+ incorporated ˜ 80 nm thick amorphous alumina, passive insulating layers sandwiched between ITO (bottom electrode) and thermally evaporated top aluminum electrodes [91]. Also, it has been commented that the time of response of these dielectrics is comparatively lower because of the slow movement of ions [92].

SBA : Sodium β-alumina, TS : Sintering temperature, tS : Sintering time.

–/– –/– 1360 1700 –/– –/– –/– 1500 1700 1550 1550 1400 Ambient – – – – – – Excess SBA powder, ambient Excess SBA powder, ambient Excess SBA powder, ambient Ambient Vacuum Solid state sintering – – – – – – Solid state sintering Solid state sintering Solid state sintering Liquid phase sintering Spark plasma sintering Powder metallurgy Cooling the eutectic melt of Al2O3 and Na2O Liquid-feed flame spray pyrolysis Crystal growth Single crystal growth from the melt Single crystal growth from the melt Single crystal growth Sol-gel citrate Extusion Solid state reaction Solid state reaction Solid state reaction

TS (ºC) / tS (h) Atm. Method

σ at 40 °C (S/cm)

Table 4 Lattice constants of ion exchanged sodium β and β″-aluminas.

– MgO MgO MgO MgO – – MgO MgO Li2O MgO MgO

Stabilizer Phase Chemical formula Sintering Synthesis method

Table 2 Reported ionic conductivities of sodium β/β″-alumina through different processing techniques.

Table 3 Ionic conductivities of β″ aluminas at 40 °C and 300 °C with different cations.

25 °C 25 °C 25 °C 25 °C 25 °C 25 °C 25 °C 300 °C 300 °C 300 °C 300 °C 25 °C

Measurement condition

Σ (S/cm)

P. Pujar, et al.

4477

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Fig. 3. Schematic representation of the effect of excess sodium ion incorporation in β-alumina (CP: conduction plane; ABC signifies stacking sequence).

and an oxidizer produces nanoparticles of sodium β-alumina at considerably less-supplied energy [42]. In literature, authors employed a citrate-nitrate method to synthesize nanopowders of sodium β″-alumina. Citric acid and nitrates acted as the fuel and the oxidizer respectively along with the stabilizer MgO. The stabilization of sodium β″-alumina with either lithium and/or magnesium is essential to avoid decomposition of sodium β″-alumina at high sintering temperature due to its low vapor pressure. The product constituted a mixture of β and β″phases with the majority of β″ with compositions, (Na2O)1+x∙11Al2O3, where x = 0.25-0.55 and x = 0.55-0.65 for β and β″ phases respectively. Also, the bulk density of the sintered product was observed as 3.01 g/cm3 (approx. 93% theoretical) and high-temperature sintering showed significant grain growth. Combustion is known for its low processing temperature; the internal heating due to high exothermicity gives rise to product-oxide. Another well-known technique called sol-gel is found to yield a mixture of β″ and β phases [10]. High purity gel is prepared using sodium citrate and nitrates salts of aluminum and lithium along with stabilizer-nitric acid, in addition to this ammonium hydroxide was used to maintain the pH of the system between 6–7. The high viscous gel is obtained when the solution is heated at 80 °C. The solvent evaporation step is followed by pre-firing of the system at 500 °C to obtain the product. The powder was then heated at 700–1100 °C for six hours to ensure the formation of sodium β / β″ alumina. Formation of β and β″ phases is observed at 1100 °C; whereas at 700 °C there is a formation of mullite-alumina (m-Al2O3). At 700 °C, synthesized particles have a size of around ˜ 30 nm. Also, during the processing of β / β″ aluminas, the mullite phase is observed prior to the sodium β / β″ alumina phase. The m-Al2O3 composed of close-packed alumina layers with sodium ions occupying the interstitials. Further, spray-freeze/freeze-drying synthesis techniques are investigated in the literature [13]. The precursor solution of Al (SO4)3∙16H2O, Na2CO3, MgSO4 is utilized for the technique. A jet of the precursor solution is dried to form particles of MgO stabilized β / β″ alumina. The fraction of β alumina (f(β)) is determined from the

Fig. 4. Capacitance as a function of frequency of K+ (PA), Li+ (LA) and Na+ (SA) incorporated amorphous aluminas (Reproduced frrom [91] by permission of The American Chemical Society).

4. Processing of sodium β-alumina There are a series of techniques to synthesize sodium β-alumina such as gel to crystallite conversion technique, sol-gel combustion, solgel technique, spray-freeze/freeze-drying, co-precipitation, solid state reaction and so on. Firstly, the gel to crystallite conversion technique which involves washing of hydrated alumina gel to make it free from sulphate ion impurities. The source of sulphate ions in the hydrated gel is aluminum sulphate solution which is used as the starting material along with ammonium hydroxide. The washed product on calcination (at 1473 K) yields sodium β-alumina [31]. The process is also extended to the synthesis of potassium β-alumina [32]. The sol-gel combustion is a well-known and largely used technique to synthesize the nanoparticles of sodium β-alumina. The exothermic reaction between the constituents of the combustible mixture of a fuel 4478

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intensity of x-ray diffraction peaks of both phases, β″ and β. The freezedried powder is sintered in two different atmospheres, namely air and the atmosphere of powder of the same composition. It is observed that there is a decrease in the ionic conductivity with increase in the f(β) irrespective of the sintering atmosphere. However, there is a drastic change in the f(β) (0.17 to 0.14) when the sintering atmosphere is altered from air to powder of the same composition while the density remained unaffected with the sintering atmosphere [10]. Irrespective of the type of synthesis process, polymorphs of alumina such as m-Al2O3 is formed prior to β″ and β. Thus, it is essential to understand the metastable m-Al2O3. The following report focuses on the synthesis, structural characterization of mullite like alumina phase, which is metastable [93]. The metastability of the m-Al2O3 is studied for three different compositions (Na2O:Al2O3 = 1:5, 1:6 and 1:7) in Na2O-Al2O3 pseudobinary system. Combustion synthesis technique is employed and found that there is no significant change in the surface area of the product powder (approximately ˜ 10 m2/g). Interestingly, the same fuel used in all the cases did not much affect the surface area of the synthesized powder. It is observed that the mullite structure is stable between 750 to 1000 °C, beyond 1100 °C new phases β and β″ started forming and transformation from β to β″ took place above 1165 °C, which is a stable high-temperature phase. Fig. 5 shows the sequence of phases as a function of temperature. Following are the few observations on the phase evolution

solid-state reaction needs additional processing such as post-sinter annealing, zeta process and addition of pre-reacted β″-alumina [37,101]. However, the solution-based precursors method emerged with advantages such as the production of chemically homogeneous powders with high surface area, thereby reducing the temperature of sintering. Recently, phase pure sodium β-alumina has been effectuated using lowtemperature aqueous solution combustion using urea as the fuel [15]. Generally, the microstructure of sodium β/β″-aluminas show layered hexagonal morphology (Fig. 6a and 6b); the c-axis perpendicular to the ab plane of the hexagon is shown in the schematic (Fig. 6c) [15] [42]. Further, the sintered pellets of sodium β/β″-aluminas should possess uniform grain sizes to achieve high ionic conductivity with maintained mechanical strength. Fig. 6d to f show uniform grain sizes of sodium β/ β″-aluminas sintered at different temperatures [48]. In contrary, the deposited films of sodium β/β″-aluminas via laser chemical vapor deposition show columnar structures with polygonal facets (Fig. 7a and b) [89]. On the other hand, irrespective of synthesis routes, the produced powder needs to be well sintered to achieve desired properties such as mechanical strength, fracture toughness, ionic conductivity and so on. The factors associated with sintering of sodium β-alumina are (i) suppressing evaporation of sodium at high temperatures [37], (ii) avoiding duplex microstructure composed of two district ranges of grain sizes and (iii) phase transformation. As the temperature of sintering is generally higher than 1480 °C, with the increase in the sintering temperature, the loss of sodium gets pronounced which can be eradicated by lowering sintering time and platinum encapsulation of the pellet [37]. Additionally, the duplex microstructure which imposes serious restrictions on the ionic conductivity, relative density and other mechanical properties needs considerable care in designing sintering process parameters. Duplex microstructure consists of large grains (< 500 μm) in the matrix of fine grains (˜ 10 μm) [78,102]. Uniformity in the grain size distribution is possible through short sintering times (˜ 30 min) at high temperatures of 1600 °C [37]. It is important to note that the microstructure of β/β″-aluminas has a significant impact on the ionic conductivity and the mechanical properties. Additives such as calcium, a common impurity found in ceramic powders in trace quantities affects the microstructure of β/β″. Presence of calcium promotes abnormal grain growth in β/β″ aluminas [103]. Whereas, the presence of small content of silicates along with β-alumina prohibits uncontrolled and abnormal grain growth [104]. Further, 3 wt. % of TiO2 in the thin films of β″-alumina via flame synthesized powders show dense microstructure with minimum porosity when it is processed at a temperature of 1360 °C for 2 h [51]. The microstructure with the coarser grains of pure β″-alumina is found to possess a resistivity of 2.81 Ω-cm, whereas the fine-grained ones have shown a resistivity of 4.80 Ω-cm measured at 300 °C [99]. Besides the variation in the properties of both fine-grained and coarse-grained β/β″ aluminas, the effect of microstructure in initiating electromechanical degradation under cell cycling of Na/S battery is studied. The reported critical current densities are 145 and 640 mA/cm3 for coarser and finer grains of β″-alumina respectively [105]. Further, the densification is another concern and results of which have direct implications on physical, mechanical and electrochemical properties. The properties can be enhanced by the addition of yttria stabilized zirconia (YSZ) to sodium β-alumina [106,107]. As an example, the fracture strength of YSZ/sodium β-alumina (300–380 MN/ m2) is double than that of pure sodium β-alumina (< 245 MN/m2) [107,108]. Recently, by the addition of YSZ in the lithium stabilized sodium β-alumina has shown a greater improvement in the

• Mullite is an intermediate phase which ubiquitously appears when • • •

the processing technique involves molecular level mixing of precursors. Sodium β and β″-aluminas are high-temperature phases. Low-temperature synthesis of sodium ion incorporated alumina composed of epitaxial mixtures of β and β″ [94] Sodium β and β″ aluminas can be stabilized through the formation of spinel with magnesium and/or lithium.

It is interesting to note that, β phase can be formed from mullite [95–97] using a complex system containing aluminum and sodium namely, trioxalatoaluminate (TXA: Nax(NH4)3-x·[Al(C2O4)3]·yH2O) where, 0.091 < x < 0.333 and y = 3. TXA undergoes thermal decomposition and forms fine powders of mullite phase. Beyond 1000 °C, the powder is found to be composed of mullite and β phases. Spray-frozen/ freeze-drying techniques were employed to achieve pure β″ phases at 1600 °C; an extra amount of Na2O is added to compensate the relative loss of the Na+ at higher temperatures and magnesium (Mg2+) is utilized as the β″ stabilizer [98]. The tetrahedral sites occupied by Mg2+ ions in the spinels of alumina stabilizes the β″-phase at higher temperatures. The phase fraction of β is found to be around 0.1 at 1600 °C. Besides the above-mentioned solution-based methods, the solidstate reaction is widely used to synthesize sodium β-alumina [99,100]. This technique comprises physical mixing of starting materials, such as α-Al2O3, NaNO3, Na2CO3 and NaOH in the presence of stabilizers Mg (OH)2, MgO, Li2O, LiOH and so on. The mixture of starting materials is subjected to ball milling and sintering at considerably high temperatures of around 1600 °C. Solution-based techniques are beneficial compared to solid-state reactions in terms of phase purity and chemical homogeneity. The challenges with solid-state reactions include (i) grain boundary segregation of moisture-sensitive secondary phase such as NaAlO2, (ii) loss of sodium, (iii) pronounced grain growth due to hightemperature annealing and (iv) the formation of a mixture of phases β and β″. Thus, to achieve phase pure sodium β-alumina, the product of

Fig. 5. Phase evolution of sodium ion incorporated alumina as a function of the processing temperature. 4479

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Fig. 6. (a) and (b) powders of sodium β/β″-aluminas (Reproduced from [42,15] by permission of Elsevier). (c) Schematic representation of hexagonal layered structure and morphology of pellets of sodium β/β″-aluminas sintered at (d) 1400 °C (e) 1475 °C and (f) 1525 °C (Reproduced from [48] by permission of The American Ceramic Society).

Fig. 7. (a) Cross sectional view and (b) surface morphology of laser chemical vapor deposited sodium β/β″-aluminas (Reproduced from [89] by permission of Elsevier).

of reports on the improvement of mechanical properties by the addition of YSZ, the effect of the same on the microstructural growth has not been firmly studied. In this regard, it is proposed that the microstructural evolution of the YSZ added sodium β-alumina is guided by

densification even at temperatures higher than 1540 °C (relative density: 0.90 to 0.95) [88]. Also, the occurrence of a duplex structure is shifted to high temperatures (1560–1620 °C) in the case of YSZ added lithium stabilized sodium β-alumina [88]. Although there exists a series 4480

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aluminas at low temperatures. Aqueous spray combustion derived sodium β-alumina at 600 °C have revealed pure β-phase with urea as the fuel and aluminum nitrate as the oxidizer and the source of aluminum [15]. Also, both crystalline and amorphous sodium β-alumina films have been fabricated at around 800 °C and 200 °C respectively using conventional spin coating technique [5]. For high temperature processed crystalline sodium β-alumina, aluminum secondary butoxide/ sodium acetates are used as precursors. Similarly, for amorphous lowtemperature sodium β-alumina, aluminum nitrate/ sodium acetate precursors are utilized. Apart from combustion technique, the conventional sol-gel technique has also been employed in the literature to fabricate thin films of sodium β-alumina; where, aluminum nitrate and the sodium bisulfate are utilized as precursors for aluminum and sodium respectively and the resulting films are annealed at 300 °C [91]. It is interesting to note that the thin films of sodium β-aluminas are more promising in terms of phase purity at significantly low processing temperatures than their bulk counterparts. The reaction could be due to a high surface to volume ratio [111] of as-deposited thin films and varied reaction path ways compared to bulk synthesis [112].

pore-grain boundary interaction [88]. The interdependency between velocities of pore migration and the grain boundary migration guides the microstructural evolution. The high relative density achievement is due to improved pore migration velocity in the presence of YSZ. Thus, YSZ added sodium β-alumina has achieved higher relative density and reduced duplex microstructure at high sintering temperatures. Besides the improved properties, YSZ added lithium stabilized sodium β-alumina has shown a small decrement in the ionic conductivity compared to lithium stabilized sodium β-alumina (0.13 to 0.16 S/cm). This can be attributed to the hindered movement of sodium ions across the grain boundaries [88]. Besides the various processing techniques reported in the literature aiming towards phase pure sodium ion incorporated aluminas, an additional factor called grain orientation or texture plays a key role in the ionic conductivity. The resistance to the flow of sodium ions is generally impeded by the grain boundaries or mismatch in grain orientations. The degree of orientation determines the resistance to the flow of sodium ions. Texturing in sodium ion incorporated β-aluminas is more frequently observed in sintered or hot-pressed bodies of β and β″-aluminas, whose green forms are prepared via slip casting, isostatic pressing and extrusion. Origin of the textured microstructure is investigated in the literature for tubular shaped bodies of polycrystalline β and β″-aluminas. A large number of grains of the polycrystalline β and β″ aluminas are oriented in such a way that their basal plane becomes parallel to the axis tube [3]. The degree of texturing is quantified in the literature by Lotgering’s method (Eq. (1))

f=

(P − Po) , (1 − P )

5. Applications 5.1. A solid electrolyte for batteries Applications of β and β″ phases of alumina are numerous such as solid electrolyte, high ε-dielectrics based devices and gas sensors. The requirement of a suitable electrolyte comes from the fact that the material should be an excellent ionic conductor with negligible electronic conduction. It should conduct the ions at a very faster rate. Sodium βalumina is known for its two-dimensional conductivity resulting from free sodium ions between the spinel layers of alumina. The presence of these oxide layers hinders the moment of sodium ions in the perpendicular direction. The higher ionic conductivity of sodium β-alumina parallel to the alumina layers makes it strong solid electrolyte. β″ is known to be more conductive because of high Na+ ion concentration in the crystal and can be stabilized by adding MgO and Li2O [37,46]. Thus, sodium β-alumina finds a suitable application as a solid electrolyte in sodium-sulphur (Na/S) batteries [46,113–115]. Schematic (Fig. 8a) shows the Na/S battery with sodium β-alumina as the solid electrolyte, where the sodium ions from the positive electrode diffuse towards the negative electrode and result as current in the external circuit. It has also been reported that nano-sized alumina has more ionic conductivity than micron-sized [116]. Ionic conductivities of sodium βalumina values are reported in Table 2. Owning to the high ionic conductivity and mature technology of producing bulky sodium βaluminas, commercialization of sodium β-alumina-based Na/S batteries was succeeded in the early 2000s [48,117] due to the successful developments in the previous two decades especially the joint program of NGK INSULATORS, LTD. and the Tokyo Electric Power Company (TEPCO) in Japan in the mid-1980s [48]. After the first commercialization in early 2000s, NGK Na/S batteries with sodium β-alumina have achieved a large output/storage capacity. In the recent data (March 2018), the United Nations Industrial Development Organization showed an output/storage of 360 MW/2510 MW h, 37 MW/260 MW h and 20 MW/140 MW h in Japan, Europe and North America respectively. Moreover, the conductivity of β and β″ phases are greatly affected by the presence of water. Hydration and dehydration can greatly affect the conductivity as it increases resistance. Effect of hydration plays an important role in the storage of solid electrolyte. An investigation, where hydration and dehydration tests are conducted on the tubes of β and β″-aluminas using flowing air with 2.7 kPa partial pressure of water vapor and it is found that there is an increase in the resistance only in the radial direction of the tube, but not in the axial direction [118]. Recently, the improvement in the battery performance is achieved by effectively eliminating the oxide and moisture layer from the solid

(1)

where, f is the degree of orientation, which varies between 0 (no preferred orientation) and 1 (perfectly orientated) and the value of P is estimated from Eq. (2).

P=

∑ I (00l) ∑ I (hkl)

(2)

The value of Po is for standard sample, where grains are not oriented in a particular direction. The c-axis orientation of the grains of polycrystalline β-aluminas greatly affects the ionic conductivity [3]. The preferentially oriented grains or the textured microstructure of β″-aluminas show considerable variation in the ionic conductivity. The ratio of longitudinal to the transverse resistivity (ρL/ρT) for an isostatically pressed green body of pure β″-alumina, estimated to be 1.4 (for smaller grains ˜ 2 μm) and 1.8 (for larger grains ˜ 100 μm) [109]. Further, these values predominantly increase for slip casted tubes up to a factor of 40 (measured at room temperature). While the hot pressed ones have depicted the ratio lying within a range of 3.9 to 5.4 for similar grain size measured at 400 °C [109,110]. The investigations mentioned above are reported at a fixed density of 3.28 g cm−3 [109]. The variation of the ionic conductivity in two orthogonal directions is mainly guided by the method of formation of green bodies rather than the type of sintering. Even spark plasma sintered bodies with shorter sintering times (˜15 min) show the preferential orientation of the grains [47]. It is worth mentioning that the anisotropy in the ionic conductivity due to orientated grains is different from the anisotropy at the atomic level. The grain orientation is a strong function of type of processing, whereas the anisotropy at the atomic level results in directional electrical functionalities namely, ionic conductivity and dielectric permittivity. Owing to the inherent anisotropic property of sodium β-alumina, it is important to fabricate the material in the form of thin films for application in thin film devices. The focus in the fabrication of low temperature sodium β-alumina is to realize the application in flexible electronic devices. However, the concern in the present scenario is to retain β/β″ phases at considerably low temperatures of annealing. In this view, various solution processing techniques such as solution combustion, sol-gel have been addressed to fabricate sodium β4481

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Fig. 8. Applications of sodium ion incorporated aluminas as a (a) solid electrolyte in Na/S batteries (b) gas sensor and (c) dielectric in thin film transistors.

into three types depending on the interaction of gas molecules with the electrolyte [133] namely, (a) the direct measurement of sodium ions (b) the indirect measurement of static or immobile components and (c) the analysis of surface modifications by auxiliary solid phases. The galvanic cell has been reported to sense partial pressure of gases like Cl2, NO2 [6,134], O2 [6], SO2 [129] efficiently. Sodium β-alumina based galvanic cell has also been adopted as CO2 [6,132,135] and sodium-vapor sensor [8]. Two different mechanisms have been adopted to accomplish sensing using solid-state galvanic cell, where sodium β-alumina is used as the electrolyte. The first mechanism is based on the inclusion of gaseous species in the cell reaction, where the change in Gibbs free energy quantifies the electrochemical potential. This mechanism is used by Weppner et al. for CO2 sensing [136] as shown in Fig. 8b. Due to the reaction of sodium ion with CO2 in the presence of oxygen forms sodium carbonate, and the resultant Gibbs free energy change is accounted for the generation of electrochemical potential (Eq. (4))

electrolyte through in situ heat treatment followed by deposition of bismuth [119,120]. Also, betterment in the performance of the batteries is achieved by reducing the interfacial resistance [121,122]. Due to hazardous and flammable nature of pure sodium under moisture and air, it is essential to search other alkali metals such as lithium (Li+), which shows a considerable ionic conductivity of 2.7 × 10−3 S/cm at 25 °C [123]. β-alumina has proved itself as a better ionic conductor than well-known sodium superionic conductor NASICON (Na1+xZr2SixP3-xO12, with 0 < x < 3) [124,125]. In addition to that, it is stable with many cathode materials such as solid, liquid or gaseous sodium [126]. However, NASICON faces the problem of instability while in contact with molten sodium. The pronounced transport of sodium ions at elevated temperatures imposes the challenge of high operating temperature. Reduction in the operating temperatures can call for new organic electrodes such as tetracyanoethylene and ionic liquids with better performance at low or room temperatures and at low cost [37]. In improving the performance of existing Na/S batteries, the surface area of the electrolyte plays a significant role. The traditional design of sodium β-aluminas is tubular. The enhancement area of the electrolyte is another challenge and needs scientific limelight. It is worth mentioning that the leaf structured cross-section of sodium βalumina electrolyte has significantly improved the performance [127].

ΔGr = −nqE

ΔGr, n, q and E are Gibbs free energy change, number of electrons involved in the reaction, elementary charge and electrochemical potential respectively. The second mechanism relates the change in partial pressure across the electrode with the electrochemical potential of the cell according to the Nernst equation (Eq. (5))

5.2. Gas sensor

(

) ( )

E = RT nF ln P1 P2

The initial attempts in developing electrochemical gas sensors are made in sensing gaseous species, such as SO3 [128,129] and CO2 [130] by using electrolytes like Na2SO4, K2SO4, and K2CO3 respectively. The low ionic conductivity of potassium and sodium in these systems have imposed limitations on the performance in terms of sensing activity. However, the usage of high ionically conductive sodium β-alumina along with gas-sensitive aides such as Na2SO4 and Na2CO3 have significantly improved the sensing activity [131]. The additional advantages of using sodium β-aluminas are (i) it does not undergo a phase change and (ii) it can be molded into complex shapes [132]. A conventional electrochemical cell for sensing CO2 can be presented as “Na | sodium β/β″-alumina | Na2CO3, O2, CO2, Au or Pt.” The underlying mechanism of sensing is due to Na2CO3 accompanied by O2 which equilibrates with gaseous species CO2 according to Eq. (3) Na2CO3 → 2 Na + CO2 + 0.5 O2

(4)

(5)

where R, T, F and n are gas constant, absolute temperature, Faraday’s constant and the number of charges required respectively. P1 and P2 are sodium vapor pressures across the electrode. Nernst equation is the fundamental concept in electrochemistry which must be satisfied. In this regard, sodium β-alumina imposes a certain degree of uncertainty with the cell voltage. In addition, the reproducibility of cell voltages for identical cells with the same experimental conditions is yet to be achieved. These are the few challenges that the sodium β-alumina based electrochemical sensors are facing and need a revisit, but it has a great potential for the improvement [132]. 5.3. Devices based on dielectrics with large ε The fundamental requirement of a dielectric material is the high dielectric constant, low leakage and high breakdown strength. The thin films of amorphous metal oxides are the good candidates. The dense

(3)

According to Weppner et al., (1987) gas sensors can be classified 4482

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Table 5 A comparative performance of various thin film transistor with alkali ion incorporated alumina as dielectric layer. Device configuration ITO/SBA/ZTO/Al Si/SBA/pentacene/Au ITO/SBA/ZTO/Al Si/SBA/ZnO/Al Si/SBA/IZO/Al ITO/PA/ZTO/Al ITO/LA/ZTO/Al Si/SBA/IZTO/Al

Operation voltage (V) < < < – – < < <

2 -2 2

2 2 3

Dielectric constant 200 200 170 200 200 – – 21

Thickness of SBA layer 75 75 80 75 75 80 80 106

Frequency 50 Hz 50 Hz 50 Hz 50 Hz 50 Hz – – 100 kHz

Mobility (cm2V−1s−1) 28 0.14 18 2.50 12.25 16.1 19.6 4.21

Ion:Ioff ratio 4

2 × 10 104 104 3 × 104 8 × 103 103 2 × 104 1.4 × 102

Threshold voltage (V)

Ref.

0.0 0.5 ˜ 0.0 0.0 −0.25 −0.007 −0.3 0.47

[5] [5] [92] [5] [5] [91] [91] [146]

SBA: sodium β-alumina, PA: Potassium alumina, LA: Lithium alumina.

magnitudes vary from 2.0 μF/cm2 (at 50 Hz) to 350 nF/cm2 (at 1 MHz) [5]. The observed variation of capacitance as the function of frequency typically follows ion-dependent electric double-layer, which is directed by the factors such as binding between ion and oxygen, the interaction between the ions, ions and electrode, density of ions at the interface and polarizability [91,143]. Alongside sodium β-alumina, its amorphous state is processed at a temperature of 200 °C, whereas the crystalline phase is prepared at 800 °C. As mentioned in the previous discussions, a high temperature of processing is required to evolve the β-phase. Since there is a considerable deviation in the film formation compared to the bulk counterpart [111,112,144], the films of sodium β-alumina crystallize at a temperature lower than the temperature depicted in bulk powders. Recently low-temperature deposition of crystalline sodium βalumina is reported using spray combustion synthesis. Around 800 nm thick film of sodium β-alumina processed at 600 °C, has shown a dielectric constant of ˜ 21 [15] which is three times higher than the conventional alumina. Additionally, potassium (PA) and lithium (LA) incorporated aluminas have been assessed for their dielectric properties in solution processed TFTs. Owing to its high dielectric constant, sodium β-alumina finds its application as a component of thin film transistors (TFTs) for low voltage operation. Both n- and p-type TFTs have been investigated with sodium β-alumina as the dielectric. Table 5 summarizes the performance parameters of various TFTs with different semiconductors such as zinc tin oxide (ZTO), zinc oxide, pentacene, IZO (indium zinc oxide), IZTO (indium zinc tin oxide) and indium gallium zinc oxide (IGZO). The highest mobility of 28 cm2V−1s−1 was observed with ZTO as the semiconductor and amorphous sodium β-alumina as the dielectric having zero hysteresis. It is worth mentioning that in literature, there are numerous articles presenting low voltage operation of TFTs enabled with high-ε dielectrics. The added advantage of sodium β-alumina is its zero-hysteresis nature. To give a comprehensive understanding of the above statement a comparative study of the performance of TFTs based on ZTO semiconductor and the other high-ε dielectrics is presented in Table 6. Though the TFTs based on sodium βalumina have shown appreciable TFT performance, it needs more scientific understanding in terms of microstructure and mechanism of dielectric response of amorphous sodium β-alumina as the passive layer in TFTs with fairly large dielectric constant. The dielectric layer favored amorphous, to achieve high-quality semiconductor-dielectric interface and low leakage currents. Thus, the amorphous sodium ion incorporated aluminas needs limelight of the scientific community to understand its mechanism as a passive layer with high dielectric constant and minimum or zero hysteresis. Finally, it is essential to note that the sodium β/β″-aluminas have conferred a wide spectrum of applications, as mentioned above. Apart from the discrete class of properties such as ionic conductivity, dielectric property, gas sensing ability, the sodium β-alumna can also be employed in the synthesis of two-dimensional metallic silver through a process of ion exchange. An ion exchange process between sodium ion and the silver ion resulted in the nanosheets of silver within β-alumina [145]. An initial composite of gel with β-alumina crystallites are used as precursors for the growth of silver nanosheets of the thickness ˜

thin film of metal oxides is capable of withstanding high electrical stresses. In the case of sodium ion incorporated alumina, the absence of conductivity of sodium ions in the direction along c-axis i.e., due to the presence of insulating aluminas on either side of the conducting plane [137] found to possess dielectric polarization. It is important to understand the origin of the polarization in the sodium ion incorporated aluminas. Further, when a condenser plate is connected across the sodium β-alumina pellet and simultaneously voltage is applied across it, then the conducting sheet becomes polarized because of the accumulation of charges at the boundary of conducting and the insulating layers. Relaxation of polarization takes place under the short circuit condition. The time constant is a linear function of the resistivity of conducting sheet as expressed by Maxwell (Eq. (6))

τ= ερ 4π

(

)

(6)

where τ is the time constant for the first order relaxation of the charge on the condenser plates, ε is the dielectric constant of the conducting sheet and ρ is the specific resistivity. Maxwell theory was extended by Wagner et al., (1914) where he proposed a modified equation by the inclusion of ‘concept of conducting spheres.’ Wagner’s modification is presented in Eq. (7), which accounts for the dispersion of small ionically conducting spheres in an insulating dielectric [138]. Conducting sphere is attributed to the presence of conducting particles in the insulating material. In the case of sodium β-alumina, conducting spheres are mobile sodium ions, whose movement is constrained in a particular direction by insulating defective alumina spinel blocks. Later, Wagner realized that the presence of conducting spheres in an insulting material greatly affects the relaxation time and the factor ‘2εr’ in Eq. (7) accounts for the same.

τ= ρ 4π (2εr+ε)

(

)

(7)

where εr is the dielectric constant of the conducting spheres and ε that of the insulator. Thus, Eq. (7) depicts the inter-dependency between the time constant of the relaxation, dielectric constants of both conducting sphere and the insulating material. Beek et al., (1965) later proved the hypothesis [139]. Extending Wagner theory, Sillars et al., (1937) studied the effect of different shapes of ionically conducting particles [140] and hypothesized that there exists a greatest deviation from the earlier theory if the ionically conducting particles are long needle-like. In recent days, it has been reported that sodium β-alumina can be used as a dielectric due to its high dielectric constant at lower frequencies. Solution-processed, both amorphous and crystalline sodium β-aluminas, are being used as passive dielectric layers in thin film transistors. Schematic of thin film transistor with sodium β-alumina dielectric is shown in Fig. 8c. The crystalline sodium β-alumina film has shown a dielectric constant of ˜170 and the amorphous one has shown the considerably higher value of ˜ 200 [5] at low frequencies. The previously reported single crystal sodium β-alumina has shown a dielectric constant of 30 [141]. The increment in the dielectric permittivity can be attributed to an increase in the internal stress under the reduced grain size [142]. In addition, the observed capacitance 4483

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Table 6 A comparative performance of ZTO thin film transistors with different high-ε dielectrics. Dielectric Al2O3 HfOx Al2O3 ZrO2 AlZrO HfOx ZrO2

Thickness (nm)

Dielectric constant

131 65 170 35 60 25 90

– 14 6.3 20.9 8.3 12.7 24

Mobility (cm2V−1s−1)

Ion:Ioff ratio 5

112 1.09 33 4 37 13.2 2.6

10 105 108 107 106 108 106

Operation voltage (V)

Ref.

< < < < < < <

[147] [148] [149] [150] [151] [152] [153]

5 5 5 3 5 3 5

References

0.6 nm. Thus, between alumina spinels, a percolation path can be achieved.

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6. Conclusion Sodium ion incorporated alumina is an anisotropic ceramic with directional properties such as ionic conductivity and dielectric polarization. The alternate layers of spinel blocks of alumina and the conducting planes of sodium result in the ionic conduction, parallel to the spinels of alumina. The phases β and β″ are conducting (ionic) in nature, whereas β″ phase exhibits higher conductivity because of the presence of a high concentration of sodium ions. The high ionic conductivity of the order ˜ 1.3 S/cm have been reported in β″-aluminas processed at a temperature more than 1700 °C. Whereas, the high dielectric constant of 200 has been reported for amorphous sodium doped aluminas thin films and 170 for its crystalline counterparts processed at a temperature of 800 °C at low frequencies, which is much higher than the single crystal sodium β-alumina (˜ 30). The variation of capacitance as a function of frequency follows ion dependent electric double-layer. At higher frequencies, the variation in the capacitance ceases to differ from pure alumina due to the hindered movement of ions. The synthesis of β and β″-aluminas is always associated with an intermediate metastable phase called mullite, which is stable in between the temperature range, 700 to 1000 °C. However high-temperature processing results in a mixture of β and β″ phase beyond 1100 °C, which transforms into β″ beyond 1165 °C. A series of different processes such as sol-gel, gel to crystallization, solid state reaction, solution combustion and freeze-drying have been addressed to synthesize phase pure β and β″aluminas. The major drawback of solid-state reaction is the formation of secondary phases, undesired grain growth, loss of sodium and formation of mixture of phases. These challenges are successfully addressed by chemical solution-based processes, which ensure the phase purity and chemical homogeneity. Further, the sodium can be replaced by a series of alkali cations, such as potassium, lithium, calcium, barium, cadmium, lead and gallium. Due to size variation of these cations, both the lattice parameters and ionic conductivity change significantly. The metal cation incorporated aluminas find their suitable applications in batteries as solid electrolytes due to their ionic movement in the conduction plane. Dielectric property is due to restricted movement of ions perpendicular to the spinel blocks of alumina. Gas sensing is due to the influence of gaseous environment on the movement of cations in the conduction plane. Although sodium β-alumina based Na/S batteries are commercialized but the electrochemical cells based on it are yet to be improved for gas sensing applications. Clearly, sodium ion incorporated alumina is a promising candidate for wide range of applications.

Acknowledgment This work is supported by Science and Engineering Research Board (ECR/2015/000339). Authors thank Mr. Robbi Vivek Vardhan, Mr. Komalkrishna Hadagalli and Mr. Mainak Dutta for the valuable suggestions in revising the article. 4484

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