Jul 9, 2018 - Li + eucryptite superionic conductors thick films. H. PERTHUIS, Ph. COLOMBAN*. Groupe de Chimie du Solide, LPMC Ecole Polytechnique, ...
J O U R N A L OF M A T E R I A L S SCIENCE LETTERS 4 (1985) 344-346 Li +
eucryptite superionic conductors thick films
H. PERTHUIS, Ph. COLOMBAN* Groupe de Chimie du Solide, LPMC Ecole Polytechnique, 91128 Palaiseau, France
Among solid electrolytes, lithium conductors have specific advantages, due to the small size of the Li÷ cation. Thus they have been used for many years in microbatteries, and in the field of microinonic devices they are promising in new applications (for example, being associated with an electrochromic material such as WO3). The aim of this work was to demonstrate the feasibility of /3-eucryptite (LiA1SiO4) and spodumene (LiA1Si206) thick films. Thick films have been achieved onto a-alumina substrates by silk-screen printing, using powders synthesized via a sol-gel process, mainly developed for the synthesis of NASICON-type solid electrolytes [ 1]. Structural evolution was studied as a function of thermal treatments; conductivity of the thick films was determined using a complex impedance method and the results were compared with those obtained on ceramics having the same composition, or with other Li ÷ superionic-conductor thick films. The achievement of superionic-conductor thick films by silk-screen printing requires the use of fine homogeneous powders: a sol-gel route for powder synthesis has been shown to be particularly suitable to obtain these kinds of powders [2], and this involves hydrolysis-polycondensation reactions of metal alkoxides. In this process, an organic solution containing aluminum s-butoxide and tetraethoxysilane in the required proportions to obtain a stoichiometric compound was hydrolysed by an aqueous solution containing lithium nitrate at 80°C (pH 4.5), as a source of conducting ions (in general, only the network former needs to be introduced by means of metal-organic reagents, but a completely m e t a l organic route is possible [3, 4]). Mechanical stirring was achieved during the hydrolysis, so that the added aqueous solution always met the nongelled part of the organic solution. Polycondensation reactions occurred at the same time as hydrolysis, leading to the formation of a polym-
erized network in which the Li÷ cations were retained. The alcohol-gel transformed, after drying in ambient atmosphere and room temperature to release the major part of water and alcohols, into a fine white and amorphous powder (S 100m 2 g-X). Thermal evolution of the powders corresponding to the LiA1SiO4 and LiA1Si206 compositions showed that a sharp decrease of the powder surface area could be correlated with the crystallization zone (between 550 and 750°C, as evidenced from X-ray diffraction). Powders synthesized by the previous process were heated at 650°C (2h) in order to adjust the grain size; in fact, silk-screen printable powders must have an adapted reactivity, to limit the shrinkage after their deposition on the substrates. A dynamical sifting was performed (only the quantity of powder which passes immediately through a 40 gm sift was kept), in order to break out agglomerates allowing a unidimensional repartition of grain sizes. Inks were involved, besides these thermal treated powders, as a classical organic binder to adjust the rheological properties, and a volatile fluidifying agent (LiF, 5wt%), the aim of which was to promote a liquid phase during the film sintering step, allowing bulk sintering to occur in the deposits before their adhesion onto the substrates, to avoid a change in shape and to keep the desired lithium content. In order to make devices, cermet electrodes (Pt80%, ZrO2: Y 2 0 % in weight) were previously deposited onto potycrystalline aalumina substrates (Superstrates 996 A, Materials Research Inc.), and sintered 10' at 1350~'C in the form of rectangular shapes (Fig. 1). Then the solid electrolyte films were deposited as above also by silk-screen printing; in the case of these compounds, three layers were successively deposited and sintered 20' at 1000 to 1150°C (heating rate 300°Ch -1) in an Li+ enriched
*To whom correspondence should be addressed. 344
0261-8028/85 $03.00 + .12
© 1985 Chapman and Hall Ltd.
2
~3.25mm . ~
Li 123: To.
1050° C+ 9500C
-,4"--+--'4-/ Z r 02 :Y
l
:
..~
l ~ ! l ~2mm~
x
X
X
X
l
ii~::~iiiiii iii:.~: ~ : i ~ i 1:,: ::::::.::::~.:~.~:.:.:.:.:.,.:.:.:1 2mm~4.75mm
X
x
Substrate X X
t
h
x
x
X
X
\
I0 pm
1
0 Figure 1 Scheme of the structure elaborated for electrical
characterization. Pt/ZrO~:Y: cermet electrode; SIC: superionic conductor thick film, h ~ 50pm. The gold upper electrode was deposited and sintered last. atmosphere, to reduce the alkali loss. The technique of successive deposits through a 35/lm thick mask allowed improvement of the resulting film microstructure; in particular, their rugosity was very limited. Besides, the first layer could be fired 100°C higher than the following, to improve the adhesion with the substrate and the cermet electrodes. After firing, the resulting solid electrolyte films were about h ~ 5 0 g m in thickness. Commercial gold, platinum or WO3 upper electrodes could be deposited by the same process and sintered at 500 to 850 ° C.
1 ~0
211
220
E uc
1100Oc
÷
I~ 113
I
,o,o°c
~ 101
SPO
100
30 o
20 °
20 Cu
Figure 2 X-Ray diffraction patterns of thick films (CuKa
radiation). The sintering temperature is given for both LiA1SiO4 (Euc) and LiA1Si206 (SPO) compositions (dashed peaks correspond to the a alumina substrate); dh kl are given in hexagonal description.
c, c, = 0.1A[ EUCRYPTloTg --*-,,-,, 1150 C+1000°C SPODUHENE
~. ~'~ X '~~ \ ,
~'~',' -X
-**-" I°5°°c ÷I°°°°c
k
D o~
-1 '~x
X
-2
-3 700
i j 500
\\\ \\
i
i 300
l 200
i 100 (%)
Figure 3 Comparison of conductivity plots against temperature (log oT =f(103/T)) for different Li÷ conduc-
tors thick films. The sintering temperatures are given (Li123:Ta:Li0.sZrl.,Tao.2 (PO4)~; 0.1 AI: LiZrl.sTao.2 (PO4)2.9 (A104)0.i).
The thick films were structurally characterized using X-ray diffraction patterns (Fig. 2). LiA1SiO4 films appeared to consist of mainly a and /3 phases, LiA1Si206 films of spodumene and /~-eucryptite (or O type) structure [5-7]. The relative proportion of phases was dependent on the thermal cycles. The film conductivity was determined using the complex impedance method, by means of a Solartron (Schlumberger Inc.) impedance analyser, monitored by an Apple II microcomputer (an extended description of the apparatus used can be found in [1 ]). Complex capacitance diagrams were obtained in the 700 to 200°C temperature range from which the materials conductivity curves were derived at sufficiently high frequencies (Fig. 3) ( > 10kHz, so that only bulk conduction was taken into account). A suitable equivalent network consisted of a simple parallel network: electrolyte resistance, connection capacitance. Conductivity curves ( a T = f ( l O 3 / T ) ) are plotted in Fig. 3. The conductivity values were very similar to those of Lio.8Zrl.8Tao.2(PO4)3 (Li123:Ta) thick films synthesized using the same process [1]: (030o° c 4 x 10 -5 ~2-z cm-1, ORT ~ 10 -8 to 10 -9 ~-1 c m - 1 EA : 0.4 to 0.6 eV). These values were also similar to 345
those obtained by Tindwa et al. [8] on phosphorous substituted fl-eucryptite ceramics and correspond to the conduction paths perpendicular to the Li ÷ channel [6]. In the case of eucryptite thick films, the accident observed at about 450°C could be related to the previously evidenced phase transition [9]. Eucryptite-type superionic-conductors thick films have been achieved using powders issued from a sol-gel process. Their conductivity was similar to that of ceramics of the same composition. However 3-dimensional networks such as the NASICON type structure (Lio.sZrl.sTao.: (PO4)3) exhibit higher conducting properties [1].
2. H. PERTHUIS, Ph. COLOMBAN, J. P. BOILOT and G. VELASCO, "Ceramic Powders," Materials Science Monograph Vol. 16 edited by P. Vincenzini (Elsevier, Amsterdam, 1983) p. 575-582. 3. J. P. BOILOT, Ph. COLOMBANand N. BLANCHARD, Solid State Ionies 9/10 (1983) 639. 4. J. P. BOILOT and Ph. COLOMBAN, J. Mater. Sei. Lett. to be published. 5. C. -T. LI, Zeitsch. Kristallogr. 127 (1968) 327. 6. W. NAGEL and H. BOHM, Solid State Comrt~ 42 (1982) 628. 7. Powder Diffraction File, International Center for Diffraction Data, Pennsylvania, USA. 8. R. M. TINDWA, A. J. PERROTTA, P, JERUS and A. CLEARFIELD, Mater. Res. Bull. 17 (1982) 873. 9. U.V. ALPEN, E. SCHONHERR, H. SCHULZ and G. H. TALAT, Eleetroehim. Acta 22 (1977) 805.
Acknowledgements Dr G. Velasco is acknowledged for his comments and use of his laboratory's facilities.
References 1. H. PERTHUIS, G. VELASCO and Ph. COLOMBAN, Jpn. J. Appl. Phys. 23 (1984) 401.
346
Received 9 July and accepted 30 July 1984
