Enhancement of the Ionic Conductivity in Electric Field-Assisted Pressureless Sintered BITIVOX Solid Electrolytes

Abstract

Bi₄V_1.8Ti_0.2O₁₁ (BITIVOX) ceramic pellets, prepared with powders obtained by a sol gel technique, were sintered either conventionally at 800 °C/8 h or by applying an AC electric voltage, limiting the electric current through the pellets. Electric voltages were applied isothermally at 700 °C and 800 °C during 5 min in the green pellet positioned in the sample holder of a dilatometer for monitoring thickness variation. The BITIVOX pellets shrank 13.6% after applying 200 V cm⁻¹ at 800 °C and 10.4% heating to 800 °C for 8 h. Thermal analysis and X-ray diffraction of the powders were performed to evaluate the crystallization temperature and the structural phase, respectively. The electrical behavior of the sintered BITIVOX pellets was analyzed by the impedance spectroscopy technique, showing that the sample flash sintered at 800 °C/5 min had lower bulk resistivity than the sample conventionally sintered at 800 °C/8 h. The surfaces of the sintered pellets were observed in a scanning electron microscope showing similar grain sizes and pore content in all sintered samples.

1. Introduction

Oxide ion ceramic solid electrolytes find uses in many commercial devices, e.g., solid oxide fuel cells (SOFCs), oxygen sensors, and oxygen electrochemical pumps [1,2,3,4,5,6]. The most important ceramic solid electrolyte is the cubic fully stabilized zirconia ZrO₂: 8 mol% Y₂O₃ (8YSZ), which is a component of SOFCs with 0.01 S cm⁻¹ electrical conductivity at 800 °C to ensure high power densities [7]. Since 1982 solid electrolytes with oxide ion conductivity higher than that of 8YSZ have been proposed: CeO₂:20 mol% Gd₂O₃ (GDC, gadolinia-doped ceria) in 1982 [8], Bi₄V_2-xMe_xO₁₁ (BIMEVOX) in 1990 [9], CeO₂:20 mol% Sm₂O₃ (SDC, samaria-doped ceria) in 1992 [10], La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ (LSGM, strontium- and magnesium-doped lanthanum gallate) in 1996 [11], and La₂Mo₂O₉ (LAMOX, lanthanum molybdenum oxide) in 2000 [12]. A solid electrolyte with ionic conductivity higher than that of 8YSZ would allow for designing SOFCs operating at lower temperatures, thus reducing misfit problems between solid electrolyte and other components in those devices, like anode and cathode.

BIMEVOX solid electrolytes are obtained by partially replacing V⁵⁺ with pentavalent or aliovalent ions in the Bi₄V₂O₁₁ compound. Several BIMEVOX compounds have been synthesized with single or double replacements with several different metallic ions, Li⁺, Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, Fe³⁺, Al³⁺, Ti⁴⁺, Zr⁴⁺, Ge⁴⁺, Sn⁴⁺, Pb⁴⁺, Nb⁵⁺ [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]. The structural phases of Bi₄V₂O₁₁ are monoclinic (named alpha), orthorhombic from 445 °C (beta) and tetragonal from 567 °C (gamma), the gamma phase with the highest oxygen ion conductivity [23,32]. The tetragonal gamma phase may be partially or fully stabilized at room temperature by aliovalent replacements for V⁵⁺. Therefore, bismuth oxide and doped bismuth oxide systems exhibit a complex set of crystallographic structures and electrical properties, which depend upon temperature, atmosphere, dopant type, and concentration. The best bismuth-based ionic conductor is the Bi₄V_1.8Ti_0.2O_10.9 compound.

Flash sintering (FS) is an electric field-assisted pressureless sintering technique accomplished in short times, from seconds to minutes, in comparison with the time required in conventional sintering (hours). That technique has been widely applied to sinter several electroceramics at temperatures lower than those used in conventional sintering, producing consolidated bodies without considerable grain growth [33,34,35,36,37,38]. Sintering to near full density with that technique may be achieved by applying moderate electric fields (tens of Volts) with limiting currents (mA to few A range) to green ceramic pellets isothermally at temperatures below the conventional temperature for sintering, saving time and energy consumption.

The synthesis by the complex polymerization chemical method of BITIVOX powders and the conformation by electric field-assisted (flash) pressureless sintering at temperatures similar to and lower than those previously conventionally used for sintering (700 °C and 800 °C) are here reported for the first time. Microstructural analysis and impedance spectroscopy measurements show the possibilities of electric field-assisted pressureless sintering BITIVOX ceramic ionic conductors with improved electrical behavior.

2. Materials and Methods

Polycrystalline Bi₄V_1.8Ti_0.2O₁₁ powders were synthesized by the complex polymerization method with bismuth trioxide (Bi₂O₃, Riedel-de-Haen), vanadium pentoxide (V₂O₅, Alfa Aesar 99.2%), titanium (IV) isopropoxide [Ti(OCH(CH₃)₂)₄, Sigma Aldrich 99.999%], citric acid (C₆H₈O₇, CAAL) and ethane-1,2-diol (C₂H₄(OH)₂, ethylene glycol, Vetec, Sigma-Aldrich, St. Louis, MI, USA) according to the Schema shown below. To prepare a solution with titanium, stoichiometric amounts of Bi₂O₃ were dissolved in nitric acid (HNO₃, Vetec, Sigma-Aldrich, St. Louis, MI, USA), V₂O₅ in citric acid, and [Ti(OCH(CH₃)₂)₄] was dropwise dissolved in citric acid. The TiO₂ content was evaluated by gravimetric analysis. The citric acid/metal molar ratio was kept at 4:1. The solution was mixed and stirred at ~100 °C until it turned homogeneous; ammonium hydroxide (NH₄OH, Synth) was used to adjust the pH to 7. Afterwards, ethylene glycol was added to promote the citrate polymerization, fixing at 60:40 the mass ratio of the citric acid/ethylene glycol. The solution was then kept under magnetic stirring for several hours up to obtaining a gel. The gel was analyzed by simultaneous thermogravimetric (TG) and differential thermal analysis (DTA) from room temperature to 1000 °C at 10 °C min⁻¹ rate under flowing synthetic air in a simultaneous thermal analyzer (Netzsch STA 409E, Selb, Germany).

The powder, after calcination at 750 °C for removing organics, was pressed uniaxially (50 MPa) into φ 5.0 × 2.5 mm pellets and isostatically (200 MPa).

X-ray diffraction analyses were carried out in the synthesized ceramic powders and in ground powders of sintered BITIVOX pellets in a diffractometer (D8 Advance, Bruker-AXS, Karlsruhe, Germany) in Bragg-Brentano configuration with Cu-k_α radiation, scintillation detector, in the 25–60° 2θ range, 0.05° step size, 5 s per step.

For the flash sintering experiments, BITIVOX cylindrical pellets were positioned into the sample holder of a vertical dilatometer (Unitherm 1161, Anter, Pittsburgh, PA, USA). Platinum grids, placed on both parallel surfaces of the sample, were connected with platinum wires to a custom-made power supply operating at 50–60 V, 1.0 A, 1.1 kHz. The applied voltage and the electric current through the specimen were collected with two Fluke 8050 A multimeters and stored in a computer [39]. The experimental procedure for flash sintering consisted on applying an electric voltage during 5 min when the temperature of the specimen reached 700 °C or 800 °C, monitoring voltage and current pulses with the multimeters, and specimen thickness shrinkage (±1 µm) with the dilatometer gauge. Typical electric field and current profiles are shown in Figure 1, with initial field and current peaks, the voltage decreasing to a value dependent on the electrical resistance of the sample, to keep constant the pre-set current. The inset shows that the incubation time, i.e., the elapsed time to the occurrence of the electric current pulse, is 3.2 s and the half-width of the electric current pulse is approximately 0.5 s.

The apparent hydrostatic density of the sintered samples was evaluated by the Archimedes method in a Mettler Toledo AG245 analytical balance (Columbus, OH, USA) with a special density determination kit. The evaluated values were in the 76%−83% T.D. range (T.D.: theoretical density, 7.74 g cm⁻³ apud [31]): 5.86 g cm⁻³ (76.2% T.D.), 6.00 g cm⁻³ (77.5) and 6.43 g cm⁻³ (83.1) for the specimens flash sintered at 700 °C, heat-treated at 800 °C/8 h and flash sintered at 800 °C, respectively. Denser specimens could be obtained by stripping the external surfaces of the flash sintered specimen.

Impedance spectroscopy measurements were carried out with a Hewlett Packard 4192 A impedance analyzer (Yokogawa-Hewlett Packard, Tokyo, Japan) in the 275 °C−400 °C temperature range from 5 Hz to 13 MHz, 16 points per decade, with 200 mV input AC voltage. Three cylindrical samples, with their parallel surfaces covered with silver electrodes, were spring-loaded in a sample chamber with platinum disk electrodes and leads, which was positioned inside a programmable furnace. [-Z”(ω) × Z’(ω)] impedance data were collected with a special software [40].

Scanning electron microscopy images were observed with a FEG-SEM microscope (Inspect F50, FEI, Brno, Czech Republic) in the flat surfaces of the sintered pellets.

3. Results and Discussion

The thermogravimetric and differential thermal analyses of the synthesized BITIVOX gel are shown in Figure 2. These curves exhibit two main thermal events: a weight loss due to dehydration and evaporation of volatiles, besides the decomposition of free citric acid (added in excess during synthesis) and initial decomposition of the organic matter from room temperature to approximately 200 °C and an exothermic peak related to organic decomposition from 200 °C to approximately 480 °C. The inflection of the peak close to 550 °C points out to the beginning of the crystallization process. Based on these results, calcination at 750 °C was then carried out for producing the BITIVOX powders.

Figure 3 shows dilatometric curves of BITIVOX compounds with the following temperature profile: room temperature to 700 °C or 800 °C for a 5 min dwelling and back to room temperature. Figure 3a refers to with and without the application of 200 V cm⁻¹ during 5 min for a programmed 800 °C for 30 min. Figure 3b with the application of 200 V cm⁻¹ during 5 min when the samples reached 700 °C or 800 °C.

The application of the electric field for 5 min when the BITIVOX temperature reached 800 °C Figure 3a) promoted additional 3.2% shrinkage of the sample thickness (13.6–10.4%. The difference may be due to the Joule heating caused by the electric current pulse limited to 1 A through the ceramic pellet. Figure 3b shows the difference in densification when the electric field was applied at two temperatures, 700 °C and 800 °C: the higher the temperature the higher the shrinkage upon applying the same electric field for the same time and electric current limit. A simple explanation is that increasing the temperature decreases the electrical resistivity of the ceramic pellet [ρ = ρ₀ exp (H/kT), ρ is the resistivity at the absolute temperature T, ρ₀ the pre-exponential factor, H the activation energy and k the Boltzmann constant], allowing for an increase in the electric current J for a constant applied voltage. The temperature T resulting from the application of the electric field E may be estimated by using the black-body radiation equation $\mathrm{T}={[{\mathrm{T}}_0^4+\mathrm{W}/\mathrm{A}\mathsf{\sigma }]}^{1/4}$ [41]. T₀ is the temperature the electric current pulse starts, A = 1.82 × 10⁻⁵ m² the pellet parallel surface area, σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴ the Stefan-Boltzmann constant, W = V × J × $\sqrt{2}/2$ (V = E × pellet thickness): 2170 °C is the evaluated temperature when the voltage V is applied (time = 0 s in Figure 1) and 1090 °C at 700 °C and 1120 °C at 800 °C at subsequent times, when the power supply voltage drops to keep constant the electric current.

Figure 4 shows the results of the impedance spectroscopy measurements at 400 °C of the BITIVOX ceramic pellets sintered according to Figure 3. The BITIVOX sample sintered by applying 200 V cm⁻¹ at 800 °C had bulk electrical resistivity of 0.37 kOhm.cm, while it was 0.76 kOhm.cm without the application of the electric field (Figure 4a). This means that besides the effect of the furnace temperature heating the sample from its outside surface towards the bulk, Joule heating inside the sample due to electric current pulses helps to improve densification. By decreasing the temperature the electric field is applied, there is an increase in the value of the electrical resistivity (Figure 4b). This result has already been reported for other solid electrolytes and the explanation is simple: higher temperature means lower electrical resistivity and higher amplitude of the electric current, with consequent increased Joule heating, promoting therefore higher densification. An extrapolation to 400 °C of the reported value for the electrical conductivity of the Bi₄V_1.8Ti_0.2O₁₁ composition sintered at 800 °C for 12 h (electrical conductivity at 320 °C, σ₃₂₀ = 2.56 × 10⁻⁵ S cm⁻¹, activation energy E = 0.61 eV [32]) leads to σ₄₀₀ = 1.1 × 10⁻⁴ S cm⁻¹, which is 25 times lower than the electrical conductivity of our sample flash sintered at 800 °C (2.7 × 10⁻³ S cm⁻¹).

Figure 5 shows impedance and Bode diagrams of BITIVOX samples flash sintered at 700 °C and at 800 °C, measured at four temperatures in the 275 °C−400 °C range, for comparison purpose. The diagrams are composed of a skewed semicircle at high frequencies due to the bulk resistivity [42] and a spike at lower frequencies due to electrode polarization [43]. A comparison of the electrical conductivity was possible due to the similar crystallographic phase(s) of all sintered BITIVOX samples (cf. X-ray diffraction results). The sample flash sintered at 800 °C is a better electric conductor (lower electrical resistivity) than the one flash sintered at 700 °C: 4 kOhm.com (50), 2 (20) kOhm.cm, 0.8 (5.8) kOhm.cm and 0.6 (1.9) kOhm.cm for the flash (conventionally) sintered BITIVOX pellets at 275 °C, 300 °C, 350 °C and 400 °C, respectively.

Figure 6 shows the Arrhenius plots of BITIVOX samples sintered at 800 °C/8 h and flash sintered at 700 °C and 800 °C. The activation energy for the conductive process was determined as 0.78 eV, in agreement with reported values [17,30]. A comparison with reported values for the electrical conductivity and the activation energy is shown in

Table 1. Reported and this work values of electrical conductivity σ and activation energy E.

Composition	T (°C)	σ (S cm−1)	E (eV)	Reference
Bi2V0.9Cu0.1O9	227	2.9 × 10−4	0.66	[44]
Bi2V0.9Ni0.1O9	227	3.05 × 10−4	0.71	[45]
Bi2V0.9Zn0.1O9	227	1.27 × 10−4	0.73	[45]
Bi2V0.8Ti0.2O9	320	1.1 × 10−4	0.61	[32]
Bi2V0.8Ti0.2O9	400	3.7 × 10−2	0.78	this work

.

Figure 7 shows the scanning electron microscopy images of surfaces of BITIVOX pellets sintered following different procedures: (a) conventionally in air at 800 °C during 8 h in a resistive furnace; inside a dilatometer by applying 200 V cm⁻¹ at 700 °C (b) and 800 °C (c) during 5 min. The FEG-SEM micrographs indicate a broad distribution of irregular grains of average sizes higher than 1 μm. The sample flash sintered at 800 °C had larger average grain size than the sample flash sintered at 700 °C, probably due to the lower electrical resistivity, which is exponentially dependent on temperature. The sample flash sintered at 700 °C for 5 min shows grains similar to those of the sample sintered conventionally for 8 h. This means that the electric current was able to produce Joule heating leading to grain growth similar to the sample heated at 800 °C/8 h. The electric current provides also higher mobility of the charge carriers, increasing local Joule heating, promoting grain growth, decreasing porosity, and consequently improving the total conductivity, as shown in Figure 4a.

Intergranular and intragranular pores were observed in the polished surfaces, evidenced by the large contribution to the bulk electrical resistivity (cf. Figure 4).

After the scanning electron microscopy and electrochemical impedance measurements, the sintered pellets were ground in an agate mortar for using the powders for the X-ray diffraction analyses. The results are shown in Figure 8. All diffraction patterns are similar with minor residuals of the BiVO₄ parent phase and were indexed for tetragonal the majority phase (PDF 86-0104) [46].

4. Conclusions

Bi₄V_1.8Ti_0.2O₁₁ (BITIVOX) ceramic powders were successfully synthesized by a complex polymerization method followed by calcination at 750 °C. Compacts of the powders sintered by applying AC electric voltages (flash sintering) at 800 °C showed enhanced shrinkage and higher electrical conductivity than compacts flash sintered at 700 °C. Moreover, samples flash sintered at 800 °C for only 5 min achieved larger shrinkage and higher ionic conductivity than samples conventionally sintered by heating at 800 °C for 8 h. Flash sintering is an effective technique for obtaining BITIVOX solid electrolyte pellets in short times with improved electrical conductivity. Further research work is necessary for achieving higher density samples.