Both thermal stability and electrical conductivity of alkali ion conducting Na2CO3 and Na2SO4, were improved by adding alkaline earth carbonates and sulfates, respectively, as well as insulating materials like ¿-Al2O3. The admixing of divalent compounds causes two effects. First a more or less extended solution can exist depending on the radius of the alkaline earth ion and is accompanied by an increase in electrical conductivity. Secondly, a phase mixture with an excess of dopant was observed that shows an enhancement in conductivity and mechanical stability. This phenomenon known as composite effect was observed in the following systems: Na2CO3-BaCO3, Na2CO3-SrCO3, Na2SO4-BaSO4, Na2SO4-¿-Al2O3

Bouwmeester, H.J.M.

Brosda, S.

Guth, U.

English

Bouwmeester, Henricus J.M.

NARCIS 

Electrical conductivity and thermal behavior of solid electrolytes based on alkali carbonates and sulfates

University of Twente Research Information

ELSEVIER Solid State Ionics 101-103 (1997) 1201-1205 Electrical conductivity and thermal behavior of solid electrolytes based on alkali carbonates and sulfates S. Brosdaa**, H.J.M. Bouwrneesterb, U. Guth” “Institute of Physical Chemistry, University of Greifswald, Greifswald, Germany bUniversity of Twente, Enschede, The Netherlands Abstract Both thermal stability and electrical conductivity of alkali ion conducting Na,CO, and Na,SO,, were improved by adding alkaline earth carbonates and sulfates, respectively, as well as insulating materials like y-Al,O,. The admixing of divalent compounds causes two effects. First a more or less extended solution can exist depending on the radius of the alkaline earth ion and is accompanied by an increase in electrical conductivity. Secondly, a phase mixture with an excess of dopant was observed that shows an enhancement in conductivity and mechanical stability. This phenomenon known as composite effect was observed in the following systems: Na,CO,-BaCO,, Na,CO,-SrCO,, Na,SO,-BaSO,, Na,SO,-y-Al,O,. Keywords: Alkali carbonates; Alkali sulfates; Composite; Conductivity; Thermal stability bfaterials: Na,CO,; BaCO,; &CO,; CaCO,; Na,SO,; SrSO,; CaSO,; BaSO,; ~-Al,o, 1. Introduction Solid sodium carbonate and sulfate are fast ionic conductors with negligible electronic conductivity at higher temperatures. They can be used as solid electrolytes in potentiometric cells for rapid determi- nation of CO, and SO, [l]. The pure salts exhibit a phase transition into a high temperature phase ac- companied by a change of thermal expansion coeffi- cient so that the production of long-term thermally stable sintered layers or discs is not possible. Therefore, much work was devoted to stabilize the high temperature phase to ambient temperature by *Corresponding author. Tel.: +49-3834 864 426; fax: +49- 3834 864 303; e-mail: brosda@grypsl.rz.uni-greifswald.de well-known methods of doping. Additionally, the electrical conductivity is enhanced. Considerable effort is being made to develop in such a way Li’-based electrolytes in respect of high energy density batteries [2]. This paper intends to compare the results obtained from two new systems with those described in previous papers in order to show the different kinds of doping and their resulting electrical effects. 2. Experimental Mixtures of carbonates and sulfates (1 - x)M,CO, + xMeCO,, (1 - x)M,SO, + xMeS0, and M,SO, +y-Al,O, with M=K, Na; Me=Ca, Sr, 0167.2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved, PII SO1 67-2738(97)003 17-2 1202 S. Brosda et al. I Solid State Ba were prepared in a ball mill, pressed into pellets and sintered for 1.5 h at 50 K below their melting temperature. The electrical conductivity measure- ments were carried out using the four electrode dc method as well as by impedance spectroscopy. The results obtained were found to be equal for both methods. The experimental details were published elsewhere [3,4]. DSC (differential scanning calorimetry) measurements were performed on pres- sed compact samples in the temperature range 20- 500°C at a heating rate of 5 K/min using o-alumina as a reference. The procedure of heating and cooling was carried out threefold. For SEM/EDX investigations the equipment of Zeiss, Oberkochen, Germany was used. 3. Results and discussion In the Arrhenius plots of conductivity, mostly three major regions can be distinguished with respect to their different slopes (Fig. 1). The high tempera- ture region is determined due to formation of Frenkel defects (intrinsic disorder) according to: Na,, + V@Nai + Vk,. (1) In the medium temperature region the extrinsic disorder caused by doping with aliovalent ions or by impurities can be described by: 12001000 800 600 400 lvM”c - 200 0 M -5 - s . -6 - -K SO 2 4 ! Fig. 1. Temperature dependence of conductivity of undoped alkaline carbonates and sulfates. s 101-103 (1997) 1201-1205 MeXeMe,, + Vk, + X, (X = so;-, CO:-; Me = Ca, Sr, Ba). (2) The low temperature region is governed by a much lower solubility of aliovalent ions in the low tem- perature phase, in some cases additional by forma- tion of clusters. 3.1. Homogeneous doping The solubility of divalent ions in sodium carbonate and sulfates depends on the ionic radii of the dopants as expected and occurs to be quite different. In the case of comparable radii, about 7 mol% of the dopant dissolves homogeneously. This behavior can be shown on the system K,SO,-BaSO, [4]. This is in good agreement with results on Li,SO,-CaSO, reported by Singh and Bhoga [5]. On the other hand, in Na,CO, only less than 0.005 mol% are dissolved, as can be seen in the extrinsic region represented in Fig. 2. This small amount is sufficient for preparing long-term gas-tight discs that can be used as a solid electrolyte for the determination of CO, concen- trations. Cell voltages could be measured with values according to the Nemst equation in the temperature region 380-650°C for 7 vol.ppm to 10 vol.% without cross sensitivity against water vapor [6]. It seems to be an influence of defects on the kinetics of the electrode reaction as was shown in our previous paper [71. 800 600 400 WC - 200 I -1.8. I I 1 I I B . r tJ-2 - -3 - t . D -4- 2 -5 - -6 - - ~a*co,)0.995 (BaCOJo.oos --c oITazCO,),,,,_ (BaCO&,, -71 I ’ ’ ’ ’ ’ ’ ’ ’ ’ . ’ ’ 8 10 12 14 16 18 20 22 104w-r - Fig. 2. Temperature dependence of conductivity of BaCO,-doped Na,CO,. S. Brosda et al. I Solid Stare Ionics 101-103 (1997) 1201-1205 1203 3.2. Homogeneous and heterogeneous (mixed) doping A phase mixture consisting of both, the homoge- neous phase and the excess of dopant, is formed, by adding a dopant amount over the limit of solubility. The conductivity increases with regard to the extrin- sic disorder as well as by defects formed at the phase boundary [ 81 NaNa + V,uNai + Vk,. (3) A maximum in conductivity was found to occur at about 20 mol% BaSO, as illustrated previously [4]. Although the enhancement in conductivity in these composites is not very large, it should be noted that the grain size of the dispersoid depends mainly on the preparation conditions and influences the electri- cal properties. The comparison of the activation energy for Na,CO,-SrCO, and Na,CO,-BaCO, listed in Table 1 and shown in Fig. 3 represents both kinds of doping. In these mixtures, the mechanism in IT enhancement appears to be similar to that in alumina dispersed solid electrolytes first described by Liang [2]. Furthermore, the conductivity behavior is in- fluenced by formation of a new compound as could be observed in the Na,CO,-SrCO, system with the composition Na,Sr,(CO,), [3]. This phenomenon in mixed doped phases detected Table 1 Comparison of a, and activation energy E, of doped Na,CO, (in italic extrinsic region) x (mole) cp (“C) a, (S cm-‘) (Na,CO,), -,(BaCO,)X [this paper] 0 <400 57.124.7 400.. ,600 0.222.3 >600 (3.6?0.1).102 0.01 280...700 (1223.7) lo3 0.03 300...700 2.6.10’ 0.2 250...600 (l.O?O.Ol). lo3 0.5 380...650 12.522.1 E, (H mol-‘) 7628 45_f5 lOlk20 10028 83 8822 8825 (N&O,), -.(SrCO,)x 131 0.01 620... 785 6.420.2 3422 0.03 29.7kO.02 0.2 630...680 (5Z3).102 682 10 0.5 470...700 (18?3).10’ 8322 0.0 0.2 0.4 0.6 0.8 8,. , , t,, 7- I 6 1 5- : . 5 4- < 3- D 2- 0 :.?.., ___.._ __..................... 10 Jj ____.._________...__... 8 “. . ..-.............--.----...o : 13:‘. d = 600°C -G’$CO$,_, (B”COJx -“a*Cos),, wq 100 I 80 2 60 2 40 w” 0.0 0.2 0.4 0.6 0.8 1.0 x(SrCOJ, (BaCOJ - Fig. 3. Comparison of conductivities and activation energies between BaCO,- and SrCO,-doped Na,CO,. by electrical conductivity measurements could be shown semiquantitatively by SEM/EDX studies as well. In the system Na,SO,-BaSO, it can be distinguished between crystals from a homogeneous solid solution of BaSO, in Na,SO, and a second BaSO, phase. The mechanism of ion transport in this material is not well understood. The ‘paddle wheel’ mechanism introduced by Lunden [9] and a percolation model proposed by Secco et al. [lo] have been discussed for cubic ar-Li,SO,. According to our previous measurements of activation energies, e.g., for K,SO,-BaSO, and the obtained effect of mixed doping, we found the percolation type transport mechanism [ 111 to be the more suitable model. 3.3. Heterogeneous doping y-Al,O, as the most effective dispersoid was used to prepare Na,SO,-based composites. The conduc- tivity u was enhanced by 1 to 2 orders of magnitude and its maximum was found to be at about 40 vol.% as reported in literature [2]. Furthermore after break- ing, the microstructure of composites shows well- formed sodium sulfate crystals embedded in small alumina particles (Fig. 4). By adding alumina to mixtures of Na,SO,-BaSO, an enhancement in (T occurs as compared with those which contain no alumina [4]. Such composites look like porcelain and behave very well mechanically. Kinetics of phase transformation Na,SO,(III)+ 1204 S. Brosda et al. I Solid State Ionics 101-103 (1997) 1201-1205 Fig. 4. SEM picture of the surfaces after breaking of a y-Al,O,- doped Na,SO, sample. Na,SO,(I) and vice-versa [12] were studied under dynamic conditions by DSC. For this reason, a pressed unsintered sample of Na,SO,-y-Al,O, (60:40 vol.%) was heated at the linear rate of 5 K/mm up to 500°C and cooled at the same rate. A shift in the transition temperature occurred different- ly during heating and cooling towards a lower temperature as compared to pure Na,SO,. Surpris- ingly, by threefold repetition of this procedure the transition heat was diminished. In the first run, an almost appropriate amount of heat was measured whereas in the second run, much less heat was determined. The sintered samples showed the same behavior during the first run. Obviously, the more composite was formed by sintering the less intense the phase transition could be observed. The phase transition seems to be restrained kinetically. The 800 600 400 WC c 200 ;;{_+JMesA4 22 2 4 _ I -4 - -MgSO 4 -cCaso 4 -5 - --v-w30 4 +BaSO 4 -6 ” ” ” 8 10 12 14 x 16 18 20 22 24 lO%T - Fig. 5. Temperature dependence of conductivity of alkaline earth doped Na,SO,. results of DSC measurements are collected in Table 2. The influence of dopants on the phase transition in equilibrium or steady state is studied in Fig. 5 for (Na,S0,),,,,(MeS0,),,03. The transition tempera- ture (241°C) of the Na,SO,(III)+Na,SO,(I) pure phase transition was shifted to the lower value with decreasing ionic radius of Me=Ba, Sr, Ca, Mg. The samples exhibited an excellent thermal shock resis- tivity. 4. Conclusion In the systems Na,CO,-MeCO,, Na,SO,-BaSO, and Na,SO,-y-A&O, three main types of doping Table 2 Results of DSC measurements obtained on different Na,SO,-y-Al,O, composition Composition Phase Heat of Heat flow transition transition in mJ ss’ temperature inmJmg_’ at heating-up in “C Na,SO, pure 235.2 60.4 12.2 Na,SO,-y-Al,O, 1st run 233.4 32.9 6.6 (60:40) 2nd run 227.7 18.5 5.2 3rd run 227.5 18.5 5.4 Na,SO,-y-Al,O, 227.5 6.2 1.0 (sintered for 20 h) 224.7 8.3 1.0 Phase Heat flow transition in mJ ss’ temperatures at cooling- down in “C 205.4 196.9 6.65 206.5 188.5 4.78 206.5 187.7 4.01 207.4 188.6 4.13 207.8 _ 202.8 0.90 S. Brosda et al. I Solid State lonics 101-103 (1997) 1201-1205 1205 effects could be observed accompanied by an en- hancement in conductivity: (i) Homogeneous doping depending on solubility. (ii) Homogeneous and heterogeneous (mixed) doping. This effect can be accompanied by the formation of a compound. (iii) Heterogeneous doping with y-Al,O,. This effect occurs in systems of (ii)-type as well. Besides the enhancement in electrical conductiv- ity, mixed and heterogeneous doping leads to both a better sinterability and, therefore, mechanical strength and an improved thermal shock resistance. The temperature of phase transition shifts towards lower values from higher to lower temperatures and the kinetic rate of transition becomes very slow as observed with the Na,SO,(III)--+Na,SO,(I) system. Acknowledgements The authors wish to thank F. Barwisch, P. Schmidt, A. Simmich, B. Loscher, H. Wulff, H.-H. Mobius, P.J. Gellings and E. Nadler. 111 121 [31 [41 PI WI [71 PI 191 1101 [Ill [I21 M. Gauthier, A. Chamberland, J. Electrochem. Sot. 124 (1977) 1579. CC. Liang, J. Electrochem Sot. 120 (1979) 1289. U. Guth, F. Barwisch, H. Wulff, P. Schmidt, H.-H. Mobius, Cryst. Res. Technol. 22 (1987) 141. U. Guth, S. Brosda, B. Loscher, A. Simmich, P. Schmidt, H.-H. Mobius, Mater. Sci. Forum 76 (1991) 137. K. Singh, S.S. Bhoga, Solid State Ionics 39 (1990) 205. H.-D. Wiemhofer, S. Brosda, H. Keller, U. Guth, W. Gopel, Proc. Int. Symposium on Environmental Sensing ‘92, 22-26 June 1992, Berlin. U. Guth, P. Schmidt, R. Jahn, S. Brosda, H.-H. Mobius, Solid State Ionics 36 (1989) 127. J.B. Wagner Jr., Composite solid electrolyte conductors, in: T. Takahashi (Eds.), High Conductivity Solid Ionic Conduc- tors, Singapore, 1989, p. 146. A. Lunden, Solid State Ionics 28-30 (1988) 163. E.A. Secco, Solid State Ionics 28-30 (1988) 168. U. Guth, F. Barwisch, K. Elsner, J. Rose&ram, P. Schmidt, H.-H. Mobius, Ionentransportprozesse in Festkorpem, Uni- versitat Jena, 1986, p, 176. R.M. Murray, E.A. Secco, Can. J. Chem. 56 (1978) 2616. 

Electrical conductivity and thermal behavior of solid electrolytes based on alkali carbonates and sulfates

Abstract

Similar works

Full text

Available Versions

NARCIS

University of Twente Research Information