目录结构

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参考文献

1 Introduction▲
2 Experimental▲
2.1 β-β"-Al2O3 preparation of electrolyte tube
2.2 Mensuration of sodium stannate’s formation free energy
3 Results and discussion▲
3.1 Phase characterization of precursor powders
3.2 Testing results of electrolyte electrochemical performance
3.3 Results of sodium stannate’s formation free energy
4 Conclusion▲
图表▲

Rare Metals2017年第11期

Formation free energy of sodium stannate measured using β-β″-Al₂O₃ ceramic electrolyte

Zhi-Meng Wang Hong-Wei Xie Yi Zhang Yu-Chun Zhai

School of Metallurgy and Materials, Northeastern University

Institute of Process Engineering, Chinese Academy of Sciences

收稿日期：13 January 2015

基金：financially supported by the National Natural Science Foundation of China (No.51234009);

Formation free energy of sodium stannate measured using β-β″-Al₂O₃ ceramic electrolyte

Zhi-Meng Wang Hong-Wei Xie Yi Zhang Yu-Chun Zhai

School of Metallurgy and Materials, Northeastern University

Institute of Process Engineering, Chinese Academy of Sciences

Abstract：

β-β″-Al₂O₃ precursor powder was successfully prepared by a solid-phase sintering method with Li₂CO₃,Na₂CO₃ (as the sources of Li₂O and Na₂O, respectively) and α-Al₂O₃ powder as the raw materials. The precursor was characterized by X-ray diffraction (XRD) and scan-ning electron microscope (SEM). The results indicate that the amount of Na₂O in the raw materials has a great effect on the formation of β″-Al₂O₃ in the β-β″-Al₂O₃ precursor.When Na₂O content is 10 wt%,the content of β″-Al₂O₃ phase reaches the maximum value of 86.24 wt% in the precursor. The β-β〃-Al₂O₃ ceramic was prepared from β-β″-Al₂O₃ precursor powder by isostatic pressing and burying sintering process. The conductive property of the β-β″-Al₂O₃ ceramic was examined by electrochemical impedance spectroscopy (EIS)method, and the density was measured by the Archimedes method. The results reveal that when 10 wt% Na₂O was added, the sample exhibits the best performance with the lowest resistivity of 4.51 Ω·cm and the highest density of 3.25 g·cm^-3. A solid electrolyte battery of Pt|SnO₂, Na₂SnO₃1|β-β″-Al₂O₃|Na-CrO₂, Cr₂O₃|Pt was assembled by the β-β″-Al₂O₃ elec-trolyte tube to measure the open potential of the resulting battery, and the formation free energy of sodium stannate was calculated. In the temperature range of 1273-773 K,the relationship between formation free energy of sodium stannate and temperature was generated as follows:ΔG_Na₂SnO₃⁰= —1040.83 + 0.2221T± 7.54.

Keyword：

β-β″-Al₂O₃ electrolyte; Electrochemistry; Sodium stannate; Formation free energy of sodium stannate;

Author： Hong-Wei Xie e-mail:xiehw@smm.neu.edu.cn;

Received： 13 January 2015

1 Introduction

Theβ-β"-Al₂O₃,composing ofβ-Al₂O₃ andβ"-Al₂O₃,is an excellent Na⁺solid conductor [ 1, 2] .The ideal chemical constructions ofβ-Al₂O₃ andβ"-Al₂O₃ are Na₂O-11Al₂O₃and Na₂O·5.33Al₂O₃,respectively.In theβ-β"-Al₂O₃ solid electrolyte,β-Al₂O₃ is stable at high temperature,but with poor conductivity.On the contrary,β"-Al₂O₃ has better conductivity,but worse high-temperature stability.It will transform fromβ"-Al₂O₃ intoβ-Al₂O₃ phase when temperature is higher than 1873 K.As a result,the preparation of a stableβ-β"-Al₂O₃ solid electrolyte with highβ"-A1₂O₃content has become the focus of intense interests [ 3, 4, 5, 6, 7, 8, 9, 10] .Theβ-β"-Al₂O₃ precursor powder was commonly made by a solid-phase reaction method or a sol-gel method at high temperature [ 11, 12] .Theβ"-Al₂O₃ content in theβ-β"-Al₂O₃ ceramic was strongly influenced by precursor powder preparation,green molding and sintering process.The Li₂O could improve the stability ofβ"-Al₂O₃ phase [ 13, 14, 15] .

Theβ-β"-Al₂O₃ is a necessary material for sodium sulfur batteries [ 16, 17] and also an ideal material for electromotive force (EMF) measurement of formation free energy of sodium metal oxide [ 18, 19, 20, 21] .Thermodynamic data of free energy of oxide sodium salt can help the understanding and improvement of the practical smelting process of the related system.However,the formation free energies of some oxide sodium salt are not listed in the documents such as Pb,Zn and Te oxide sodium salts.In this present,the formation free energy of sodium stannate was measured by the assembled battery usingβ-β"-Al₂O₃ceramic electrolyte tube [ 22] .It is of great significance to get thermodynamic properties of the metal oxide sodium for the thorough analysis and understanding of the alkaline smelting process.

2 Experimental

2.1 β-β"-Al2O3 preparation of electrolyte tube

2.1.1 Prepαration ofβ-β"-Al2O3 precursor powder

Na₂CO₃,Li₂CO₃ and ultrafineα-Al₂O₃ were used as raw materials.The content of Li₂O was fixed as 0.8 wt%,while the content of Na₂O was varied as 9.0 wt%,9.5 wt%,10.0 wt%and 10.5 wt%,respectively.The formula ofβ"-Al₂O₃ can be represented as Na_1.7Li_0.3Al_10.7O₁₇ when Li₂O is stable.The raw materials were mixed through ball milling (QM-3SP04-type planetary ball mill) for 24 h with alcohol as the medium,and the rotational speed of the ball milling was 750 r·min^-1.The mixed material was dried in vacuum oven (DZF-6050MBE-type vacuum drying oven)at 1523 K for 3 h and then was slowly cooled to room temperature.Finally,theβ-β"-Al₂O₃ precursor powder was obtained.The phase and morphology ofβ-β"Al₂O₃ precursor powder were studied through X-ray diffractometer(XRD,D/max-2500PC) and scanning electron microscopy(SEM,SSX-550),respectively.

2.1.2 Preparation of electrolyte tube

The above self-madeβ-β"-Al₂O₃ precursor powder was mixed uniformly with polyvinyl butyral solution used as the binder.The mixed material was shaped up through300 MPa isostatic pressing (CIP120/350-300Y cold isostatic press) into the green ceramic body.In order to ensure the Na₂O content,the burying sintering process was used.The ceramic green was compacted in the crucible withβ-β"-Al₂O₃ powder as the investment.And the crucible was sealed with high-temperature cement and put in the MoSi₂resistance furnace heating to 1873 K for 8 min.Finally,it was slowly cooled to room temperature.

2.1.3 Testing of electrolyte performance

2.1.3. 1 Density measurement The dry weight (ω1),buoyant weight (ω3) and wet weight (ω2) of the solid electrolyte cube were measured by Archimedes density method.The real density of the electrolyte cube was calculated according to the following equation:

whereρ_m andρ_m1 denote the densities of the sample and the used liquid medium (g·cm^-3),respectively;ω_l,ω₂ andω₃ correspond to the weights of the sample,the dipped sample in the air and the dipped sample in the liquid medium (g).In this experiment,kerosene was chosen andρ_m1 is 0.8 g·cm^-3.

2.1.3. 2 Testing of electrolyte electrical performance The conductivity of the preparedβ-β"-Al2O3 electrolyte was studied.Both sides of the electrolyte sheet were coated with platinum paste and then dried at 673 K for 10 h.The platinum wire was connected with the coated electrolyte sheet,and two corundum pieces with hole in the edge were used to clamp the sheet,and then those two corundum pieces were fixed together with high-temperature-resistant screws.The prepared sample was placed in the resistance furnace at 623 K for 2 h,after that the resistivity was measured through the electrochemical impedance spectroscopy (EIS) after connecting with the electrochemical workstation (AUTOLAB PGSTAT302N) with working frequency of 1 MHz-1 Hz.

2.2 Mensuration of sodium stannate’s formation free energy

Solid electrolyte battery was assembled by the self-made electrolyte tube to measure formation free energy of sodium stannate from the open-circuit voltage (OCV).NaCrO₂+Cr₂O₃ were used as the reference electrode material.The activity of Na₂O is similar to that of sodium stannate to reduce the influence of gaseous Na diffusion.

The battery used to measure formation free energy of sodium stannate can be described as:Pt|SnO₂,Na₂SnO₃|β-β"-Al₂O₃INaCrO₂,Cr₂O₃IPt.Firstly,SnO₂ and Na₂SnO₃were mixed together at mole ratio of 1:1,shaped up through compression molding.NaCrO₂ and Cr₂O₃ were mixed together with mole ratio of 1:1,shaped up through compression molding.The primary battery was assembled with the above two kinds of electrode materials andβ-β"-Al₂O₃ electrolyte tube,and the experimental device is shown in Fig.1.

Before formation free energy of sodium stannate was measured,the battery measurement system should be detected with the oxygen concentration cell as the reference cell.The battery was assembled by ZrO₂-Y₂O₃ solid electrolyte tube and can be described as:Pt|Fe,Fe_xO|ZrO₂-Y₂O₃|NiO,Ni|Pt.There are many reports about the reference cell [ 23] .The standard cell and the tested cell were put in the silicon carbon tube furnace at temperature of1273 K for 10 h.The electromotive force of the battery was measured every 50 K in the temperature range of1273-773 K,and the system was kept for 2 h before each measurement to reach the equilibrium state.The value of electromotive force would be recorded until the fluctuation was<0.1 mV.

Fig.1 Battery device diagram.1 Platinum electrode;2 corundum tube;3 corundum slice;4 spring;5 corundum tube;6β-β"-Al₂O₃electrolyte tube;7 SnO₂+Na₂SnO₃;8 Cr₂O₃+NaCrO₂

3 Results and discussion

3.1 Phase characterization of precursor powders

XRD patterns ofβ-β"-Al₂O₃ powder with different Na₂O contents are presented in Fig.2.As shown in Fig.2,the syntheticβ-β"-Al₂O₃ is composed ofβ"-Al₂O₃,β-Al₂O₃,minorα-Al₂O₃ and some mixtures of sodium aluminate.With the increase of Na₂O content,the diffraction intensity ofβ-Al₂O₃ andβ"-Al₂O₃ increases firstly and then decreases,while the diffraction intensity ofα-Al₂O₃ decreases all the time.It indicates that the addition amount of Na₂O has great influence on getting the relatively single componentβ-β"-Al₂O₃ precursor.Low addition amount of Na₂O can lead to excessiveα-Al₂O₃ existing in the product;however,high addition amount of Na₂O will result in the generation of other types of sodium aluminate.The optimizing addition amount of Na₂O is about 9.5 wt%-10.0 wt%.

The content ofβ"phase (f(β"))inβ-β"-Al₂O₃ can be described as following [ 24] :

where I_β(026)(Peak 1 in Fig.2) is the diffraction intensity ofβ-Al₂O₃ when 2θ=44.5°andI_β"(2010)(Peak 2 in Fig.2)is the diffraction intensity ofβ"-Al₂O₃ when 2θ=46.1°.The relative content ofβ"-Al₂O₃ in four kinds of mixture powders is presented in Table 1.

Fig.2 XRD patterns ofβ-β"-Al₂O₃ powder with different Na₂O contents:a 9.0 wt%,b 9.5 wt%,c 10.0 wt%,and d 10.5 wt%

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Table 1 Relative content ofβ"-A1₂O₃

Fig.3 SEM images of prepared powder with different Na₂O contents:a 9.0 wt%,b 9.5 wt%,c 10.0 wt%,and d 10.5 wt%

As shown in Table 1,theβ"-Al₂O₃ phase is the dominant component inβ-β"-Al₂O₃,with the content of about80 wt%.There is no proportional relation between the content ofβ"-Al₂O₃ and the dosage of Na₂O.Particularly,the proportion ofβ"-Al₂O₃ phase begins to decline when the amount of Na₂O is 10.5 wt%.It is interesting to know that when the Na₂O content is 10.0 wt%,β"-Al₂O₃ phase is up to 86.24 wt%.

SEM images of the preparedβ-β"-Al₂O₃ powder are shown in Fig.3.When the content of Na₂O is less than10.0 wt%,the particle size ofβ-β"-Al₂O₃ is almost in the range of 1-3μm.However,when the content of Na₂O increases to 10.5 wt%,the larger particles account for the major part in the precursor material,which is detrimental to the preparation of ceramic.Thus,Na₂O content should be controlled within 10.0 wt%.In addition,there is a clear layer structure in the electrolyte precursor powder,which is consistent with the result reported in previous literature [ 25] .

3.2 Testing results of electrolyte electrochemical performance

Figure 4 shows the enlarged and fitting alternating current(AC) impedance spectra of the solid electrolyte flake at high-and intermediate-frequency.The insets are the full spectrum diagram.Z'and Z"are real and imaginary parts of impedance plural expressions,respectively.As shown in Fig.4,electrochemical impedance spectra consist of highfrequency capacitive reactance arc and low-frequency diffusion impedance.The high frequency corresponds to polarization resistance and spatial electric double layer,and the diffusion impedance is due to the dispersion of Na⁺in the electrolyte.High-frequency part is a complete capacitive reactance arc.It can be concluded that grain resistance is the main body among the resistance of the solid electrolyte.This illustrates that the sintered ceramic completely forms Al-O skeletal structure under the condition of preparation.Based on the physical meaning and electrochemical impedance spectrum graph,equivalent circuit is shown in Fig.4,where R_s is electrolyte resistance,R_p is polarization resistance,C_dl is space charge electric double layer capacitors and W and S are Warburg impedance.

The resistivity of electrolyte formula is:

where h is thickness of electrolyte,S is cross-sectional area of the electrolyte film (wafer radius is 0.3 cm),R is the electrolyte resistance andρis electrolyte electrical resistivity.The calculation results are given in Table 2.The resistivity ofβ-β"-Al₂O₃ does not change linearly with the increase of Na₂O content.But when the addition amount of Na₂O is 10.0 wt%,it exhibits the minimum value of4.51Ω·cm.The results can match the relation betweenβ"-Al₂O₃ content and addition amount of Na₂O shown in XRD pattern in Fig.2.In addition,the density of the solid electrolyte made ofβ-β"-Al₂O₃ with 10.0 wt%Na₂O is also the highest,which is beneficial to improving the electrical conductivity of the solid electrolyte.The result further confirms the inference that the solid electrolyte sintered body is less concluded from the impedance spectra.Therefore,it can be determined that electrolyte tube with Na₂O addition amount of 10.0 wt%is applicable to measure formation free energy of sodium stannate.The performances of the electrolyte tube were tested in previous work [ 26] .

3.3 Results of sodium stannate’s formation free energy

The measured electromotive force values of the reference battery are shown in Fig.5a.In the temperature range of773-1273 K,the fitting curve equation of the data is:

where E is battery electromotive force (mV).The correlation coefficient is 0.982,which means that the measurement data are well agreement with those in Ref. [ 23] ,and it can be used as the reference in this experiment.

The battery used to measure formation free energy of sodium stannate can be described as Pt|SnO₂,Na₂SnO₃|β-β"-Al₂O₃|NaCrO₂,Cr₂O₃|Pt.The corresponding half-cell reaction formulae are as follows:

Fig.4 AC impedance spectra (experimental (exp.) and calculated (cal.)) of prepared powder with different Na₂O contents:a 9.0 wt%,b 9.5 wt%,c 10.0 wt%,and d 10.5 wt%;and e equivalent circuit

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Table 2 Physical parameters of electrolyte sheet with different Na₂O contents

The cell reaction is:

The simulation data between the measured electromotive force values of battery and the temperature are shown in Fig.5b.In the temperature range of 773-1273 K,the fitting equation with the data is:

The correlation coefficient is 0.958.The measured electromotive force values are converted into the change in the free energy of the cell reaction based on standard formation free energy:

whereΔG⁰ is the change in the free energy of the cell reaction,n is charge number of the cell reaction,E⁰ is standard EMF of the cell reaction which equals to the measured EMF of the cell (E) in this present,and F is Faraday constant.The equation is as follows:

Fig.5 Relationship between battery electromotive force (E) and temperature:a reference cell and b cell to be tested

In Reaction (7),each substance exists independently,so the equation can also be written as:

The formation free energies of SnO₂,Cr₂O₃ and NaCrO₂are 0.2691T[27], [27],respectively.All the data are substituted into Eq.(11).In the temperature range of 773-1273 K,the standard formation free energy of sodium stannate is as follows:

4 Conclusion

Theβ-β"-Al₂O₃ precursor powder with 80 wt%β"-Al₂O₃phase was prepared by a solid-phase sintering method.The particle size of theβ-β"-Al₂O₃ precursor powder is almost in the range of 1-3μm.The electrical resistivity of the electrolyte increases firstly and then decreases with the increase of the Na₂O content.When Na₂O content is 10 wt%,the electrical resistivity of the electrolyte reaches the minimum value of 4.51Ω·cm.In the temperature range of 773-1273 K,the relationship between formation free energy of sodium stannate and temperature measured by solid electrolyte battery assembled with theβ-β"-Al₂O₃ electrolytetube is .