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Low-temperature sintered lead tantalate-based ceramics of high dielectric constants
Volume 5, number 3
February 1987
MATERIALS LETTERS
LOW-TEMPERATURE SINTERED LEAD TANTALATE-BASED OF HIGH DIELECTRIC CONSTANTS
CERAMICS
M. HALMI, G. DESGARDIN and B. RAVEAU Laboratoire de Cristallographie, Chimie et Physique des Solides, U.A. 251, ISMRa-Vniversitk, 14032 Caen Cedex, France
Received 26 November 1986
High grade tantalum perovskite, with composition 0.5 PFN + 0.5 PFf (0.5 PbFelizNbiiz03 + 0.5 PbFe,,,Ta,,,O,) can be sintered at low temperature in spite of the refractory character of tantalum, by using a small amount of lithium carbonate as a sintering agent. The dielectric properties of the ceramics sintered at 950- 1000°C are attractive: the dielectric constant is increased to reach 35000 and the dielectric losses at high temperature are considerably reduced with respect to the systems. These properties are interpreted in terms of the introduction of lithium in the perovskite matrix with creation of anionic vacancies; this model is in agreement with the ability of lithium to take both the octahedral and tetrahedral coordinations.
1. Introduction
Recent research in the field of ceramics for capacitors with high dielectric constants has been focused on with the of the PMN-PFN lead niobates perovskite structure systems called the (PbMg,,3Nb2,303-PbFe,,*~,,*O~) and PMN-PZN (PbMg,,31\Tb2,30,-PbZn,,3~2,303) [ l-91. This choice results from the high dielectric constants of these niobates and from their ability to be sintered at low temperatures. In this respect, the system PFN-PFT (PbFe,,zNb,,203-PbFe1/2Ta,,*O~) is a potential candidate, owing to the Curie temperatures of these oxides corresponding to 114°C and - 4O”C, respectively, and to their high dielectric constants of 20000 and 9000, respectively. An investigation of this system from the crystallographic point of view has shown the existence of a solid solution with dielectric constants ranging from 6000 to 12000 [ 71. However, no attempt of sintering such compositions was reported up to the present, most likely due to the refractory character of the lead tantalate PbFe1,2Ta1,203. Recent sintering studies of lead niobates in the presence of lithium salts [ 8-101 suggest a possibility to apply this method to the system PFN-PFT. The present study deals with the sintering study of the optimal composition in this system at low temperature in order to synthesize ceramics with high dielectric constants, i.e. with E> 20000, at room temperature. 2. Experimental Raw materials Fe203 (from Prolabo, France, quality rectapur), NbzOs, Ta,Os (both from Starck, Berlin, quality, HPO), PbO (from Merck, Darmstadt, pro analysis) and Li,C03 (from Prolabo, France, quality rectapur) were mixed in stoichiometric ratio by wet-milling in alcohol using a planar shaker-mill. Precursors FeNbO, and FeTa04 were obtained after calcination at 1200°C for 12 h. Final compositions were obtained by mixing in the same way FeNbO,, FeTaO, and PbO before calcination at 750°C 4 h. One to five mole % lithium salt where then added to the calcined perovskites. These products were then wet milled in alcohol, and then dried in an infrared radiator. An organic binder (aqueous solution of 2% PVA) was added before pressing at 1000 kg/cm2 to obtain disks 12 mm in diameter and 1.5 mm in thickness. The disks were then sintered in air in the temperature range 950- 1000’ C for different soak times, with heating and cooling rate of 150” C/h. The calcined materials as well as the sintered products were systematically analysed by X-ray diffraction. 0167-577x/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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Microstructure of the sintered disks was examined by SEM and their dielectric properties e=J?( T), tg 6 = f(T) andp(20”C) measured. 2.1. Synthesis of the optimal perovskitein the system PFN-PFT In the synthesis of lead niobates or tantalates PbFeti2M,,203 (M=Nb, Ta), there is the problem due to the formation of a second phase with the pyrochlore structure besides the perovskite form. This latter compound, which is non-stoichiometric and may exhibit a wide composition range, is characterized by a low dielectric constant and must be eliminated for capacitor applications. A method was recently developed by Schwartz and Shrout [ 31 for the PMN system in order to avoid the formation of pyrochlore and to obtain a pure perovskite after calcination of the oxides. It consists in using the columbite MgNbzOs as a precursor which then reacts with the lead oxide instead of using a mixture of the oxides MgO, Nb205 and PbO. The application of this method to the system PMN-PFN [ 81 allowed the pyrochlore phase to be decreased fom about 20-30°h to about l-2O/b.We have tried to transpose this method to the system PFN-PFT by using the wolframite type oxides FeNb04 and FeTaO, as precursors; these precursors which are first prepared at 1200’ C are then calcined in a second step in the presence of PbO according to the reactions: FeNbO, +2PbO~2Pb(Fe,,~~,,~)O~ FeTaO, +2Pb0+2Pb(Fe,,zTal,z)0,
, ,
(1-x)FeNb04+xFeTaO,+2Pb0+2Pb(Fe,,~Nbf,_,,,2Tatiz)03.
(1) (2)
(3)
This method is very successful in the case of the niobate: a pure perovskite is isolated for reaction (1). On the opposite, a pyrochlore phase, almost alone, is observed for reaction (2), whereas a mixture of pyrochlore and perovskite is always obtained for reaction (3). Thus from these results it is clear that FeTa04 is not an efficient precursor for the systems involving PFT’. This behaviour is similar to that observed for the PZN system for which the action of the columbite MgNbz06 on the lead oxide led mainly to the formation of the pyrochlore phase instead of the perovskite f 81. Our previous results concerning the system PMN-PFN [ 91 have shown that the addition of small amounts of lithium salts allowed the pyrochlore phase to be eliminated and favoured an excellent densification. The addition of different amounts of lithium carbonate to the calcined products of reactions (2) and (3) allowed the same phenomenon to be observed by heating at low temperature. The elimination of the pyrochlore phase could result from the introduction of lithium in the octahedral sites of the perovskite. In order to develop a dielectric material for capacitor applications, we have tried to synthesize a perovskite whose Tc was close to room temperature. Preliminary investigations allowed the curve Tc=f(x) to be established for the solid solution Pb (Fe 1,2Nb,, _nj,2 Ta,,*)03. Fig. 1 shows that the curve T,=f(x) deviates from linearity, and that the composition x= 0.50 which corresponds to a Tc close to 20°C is the optimal composition for application. 2.2. Sinteri~~ of the mixture 0.5’PFNi 0.5 PFT at 950” C The mixture of FeNbO, + FeTaO, was calcined in a first step at 750°C during 4 h as described above. The Xray diffraction spectra of the calcined products showed that for the composition 0.5 PFN+O.S PFT, a mixture of about 50°h pyrochlore and 50% perovskite was obtained. After addition of various amounts of lithium carbonate (ranging from 1 to 5 mole%) to this mixture, it was sintered at 950°C for 2 h. The main characteristics of the ceramics are summarized in table 1. It can be seen from the X-ray analysis that the pyrochlore phase has completely disappeared whatever the lithium content. In contrast, the shrinkage increases as the lithium content increases up to 4 mole% Li,CO,; this latter content is 104
February 1981
MATERIALS LETTERS
Volume 5, number 3
c
I
Fig. 1. Curie temperature versus composition for the system (1 -x)PFN+xPFT.
X
the optimal composition leading to a maximum of the shrinkage as shown from the evolution of the microstructure versus the lithium content (left part of fig. 2). For 19/oLi2COS a residual intergranular porosity is indeed observed with an heterogeneous distribution of the grain size. For 2W L&CO3 the grains are better joined and their size distribution is more homogeneous. The porosity has completely disappeared for 4% Li2C03. For this latter lithium content the fractures are only intergranular whereas one observes well-faceted grains with angular outlines. Moreover the diameter of the grains does not exceed 10 urn. The relationship of the dielectric constant at T, ( emax,of the ceramic) versus the lithium content is in agreement with the shrinkage results: the maximum value of emaxis reached for 4W Li2C03 and does not vary beyond this content. The dielectric curves e=f( T) (fig. 3a) show that Tc decreases as the lithium content increases. Thus, it is clear that lithium enters into the perovskite matrix during sintering. Table 1 0.5 PFN+O.5 PFT: characteristics observed after sintering at 950°C for 2 h versus lithium content Sintered cycle 95O”C, 2 h 0.5 PMN+O.S + 1%Li2COx 0.5 PMN+O.S + 2% LiIC03 0.5 PMN+O.S +4% Li2C03 0.5 PMN+O.S + 5% LizCO,
Shrinkage (%)
Tc( “C)
enl,x
tg 6 (at 20°C)
p(Rcm)
RX (P: perovskite)
PFT
a.33
23
19000
1%
7x 10”
pure P
PFT
8.70
18
29000
12
3x 10”
pure P
PFT
13.50
15
31500
1%
3x 10”
pure P
PFT
13.90
12
31500
1%
2x 10”
pure P
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Fig. 2. Evolution of the microstructure versus lithium content for the composition 0.5 PFN+O.S PFT sintered in the presence ofdifferent lithium carbonate contents at 950°C for 2 h (left), and at 1000°C for 2 h (right). Upper: 0.5 PFN+O.5 PFI’+ 1 mole % Li,CO,; middle: 0.5PEN+O.5PFT+2mole%Li,CO,;lower:0.5PFN+O.5PFT+ 4mole%Li2C03. The curves of the dielectric losses versus temperature (fig. 3b) are also characteristic of a well-densified ceramic: very low losses are observed between 20 and 100' C,and these dielectric losses are rather low even in the ferroelectric domain (tg 6 c 4%). However, a maximum of tg 6 near jr, is observed, and a slight increase of
106
DIELEC.
February 1987
MATERIALS LETTERS
Volume 5. number 3 CONSTANT
35000
OIELEC.
LOSSES %
7000
-60
-40
. -20
. 0
.
. 20
, 40
.
. 80
. 80
.
, 100
. . . I 120 140 TEMPERATURE
-00
-40
-28
0
20
40
W
88
Fig. 3. Dielectric characteristics: (a) t=f(T) and (b) tg S=./(T) for 0.5 PFN+O.5 PFT sintered at 950°C for 2 h. PFT+ 1 mole % Li&O,, - - - 0.5 PFN+O.S PFTf2 mole % Li2C03, - - 0.5 PFN+O.S PFT+4 mole % Li2C03, -.PFT+ 5 mole % Li2COj.
100
120 140 TEMPERATURE
0.5 PFN +0.5 0.5 PFN+O.5
the dielectric losses beyond 100°C; the fact that tg 6 increases much more drastically in the case of the sample with 1% Li2COS is likely due to the lower densification of this ceramic. Besides these good dielectric constants, the insulating resistances, measured under 60 V/mm at room temperature are quite acceptable (table 1) . 2.3. Sinteringofthe mixture 0.5 PFN+O.5 PFTat 1000°C The sintering of identical compositions for 2 h at 1000’ C leads to very similar characteristics as shown from table 2. The densification is scarcely better than at 950°C and the relationship of the microstructure versus the lithium content (right part of fig. 2) is very similar to the one observed at 950°C (left part of fig. 2). Nevertheless, the granulometry is slightly greater than at 950°C whatever the lithium content. Moreover, the fractures are mainly intragranular for 1% Li2C03. The insulating resistances do not change, whereas the curves c=f( T) are very similar for 2% and 4% Li203. However, it must be pointed out that E,,, is drastically increased for 1% Li203 (E = 26000) and 5% Li2C0, Table 2 0.5 PFN+O.S PFT: characteristics observed after sintering at 1000°C for 2 h versus lithium content Sintering cycle lOOO”C,2 h 0.5 pFN+o.s + I % L&CO3 0.5 pFN+o.s + 2% Li&O, 0.5 pFN+o.s + 4% Li2COj 0.5 pFN+o.s + 5% Li2C03
Shrinkage (%)
Tc(‘C)
tg 6 (at 20°C)
p(fi cm)
PFr
8.33
22
26000
1%
3x 10’
PFT
8.60
20
30000
1%
3x 10’
PFT
13.9
15
30000
1%
3x101
PFr
14.3
15
36000
II
3x10’
107
MATERIALS LETTERS
Volume 5, number 3 DIELEC.
February 1987
CONSTANT
40000
DIELEC.
LOSSES $Y,
20
b.
I
24000
-60
-40
-20
0
20
40
80
80
TEMPERATURE
Fig. 4. Dielectric characteristics: (a) e =fl7’) and (b) tg 6 =A 7’) for 0.5 PFN + 0.5 PFT sintered at 1000°C for 2 h. PFf+ 1 mole % Li2C03, - - - 0.5 PFN+O.S PFT+2 mole % Li2C03, - - 0.5 PFN+O.S PFT+4 mole % L&CO,, -.PFT + 5 mole % LizC09.
100
120 140 TEMPERATURE
0.5 PFN + 0.5 0.5 PFN+O.S
(Ex 36000)(fig. 4a). Moreover, the maximum of the dielectric losses at Tc has been increased, especially for 1% LizC09, and the dielectric losses at high temperature are drastically reduced (fig. 4b).
3. Discussion and conclusion The addition of Li2C03 to the composition 0.5 PPN-0.5 PFT allows the material to be sintered at low temperature in the form of a perovskite, in spite of the presence of tantalum which is a very refractory element. The dielectric properties of these ceramics, which are remarkable, result from the introduction of lithium in the perovskite matrix. The decrease of Tc as the lithium content increases is in agreement with this point of view. It can be explained by the creation of defects on the A sites and on the anionic sites of the perovskite AB03 according to the equation ( M=Nbo.sTa,,5):
Pb(Fel~&bdO~ +GC03
+U
+2~)Pbl,(l+2~)(Fel,2(l+2c)M1,2(l+2c)Li2~,(l+2c))C~,(1+2r)03(1+c),(,+2c)
.
The decrease of Tc can indeed be due to the introduction of lithium on the octahedral sites, but especially to the decrease of the Pb2+ content on the A sites. It is now established that the Pb2+ ion plays an important part in the ferroelectric properties of the oxides, owing to its lone pair of 6s2 electrons: the high T, value of PbTi03 [ 111 compared to BaTiOJ can be explained in this way, as well as the particular polar axis of the orthorhombic niobate PbNb20h [ 121 which exhibits the tetragonal tungsten bronze structure. The comparison of the PFN-PFT ceramic to the one of the system PMN-PPN [ 4,8] shows that the first one exhibits higher dielectric constants and that the value of E,,, is not lowered by increasing the lithium content in this case, contrary to PMN-PFN. Moreover, the variation of tg 6 with temperature is different in the two systems: the dielectric losses are much higher at low temperature for the PMN-PP’N ceramics in the ferroelectric domain, and increase regularly with 8 up to the Curie point whereas they exhibit only a small maximum in the PFN-PFT ceramics. The fact that good dielectric characteristics can be obtained for the PPN-PPT system 108
MATERIALS LETTERS
Volume 5. number 3
a
b
February 1987
Fig. 5. Elimination of rows oxygen atoms ( 0 ) along 110 leading to the formation of planes LiOa or FeO., tetrahedra labelled as triangles.
in spite of higher Fe3+ contents, compared to the PMN-PFN system, can be attributed for a great part to the replacement of Nb( V) by Ta( V). Ta( V), owing to its 5d orbitals, does not favour conduction contrary to Nb(V) whose 4d orbitals can form A bonds with p orbitals of the oxygen atoms, owing to their much lower energies. The rather high insulating resistance observed for the PFN-PFT ceramics is in agreement with this point of view. It is worth noting that the dielectric losses by conduction are weak at high temperature, in spite of the rather high Fe3+ content. This phenomenon suggests that the Li+ ions located on the octahedral sites have a compensation effect. At high temperature the conductivity in the absence of lithiuin would result from the creation of anionic vacancies, involving a partial reduction of Nb (V) to Nb( IV) according to the equilibrium 2Nb(v)=2Nb(Iv)+v;:.
(4)
The addition of lithium to the ceramic involves the formation of oxygen vacancies according to the reaction Nb(V)-iLi++2Vz,+.
(5)
This excess of anionic vacancies created by reaction (5) tends to displace the first equilibrium towards the left and avoids the presence of electrons in the 4d orbitals. It must be pointed out that the formation of a rather large amount of oxygen vacancies in those oxides is quite easily explained by the ability of Fe3+ and of Li+ to take both the octahedral and tetrahedral coordinations. Consequently, the FeO, or Li04 tetrahedra can easily be adapted to the octahedral framework (fig. 5) exactly as in the structures derived from the brownmillerite [ 131. The formation of such anionic vacancies in the perovskite structure may certainly play an important part in the diffusion of Li+ and other species through the lattice and may contribute to the good densification of these materials. As a conclusion, lead tantalate-based ceramics with the perovskite structure appear as potential materials for capacitors in spite of the refractory character of tantalum. The drastic increase of the dielectric cunstant ( f > 30000) for the ceramic 0.5 PFN+ 0.5 PFT is very promising. However, this too sharp peak makes that this ceramic does not belong to the Z5U class for capacitors. Substitutions either on the A sites or on the B sites will be necessary in order to obtain a more diffuse transition compatible with this class.
References [ I] K. Furukawa, S. Fujiwara, N. Kikachi, 0.1. Izawa and H. Tanaka, High Dielectric Constant Type Ceramic Compositions PFN-PMN, USPatent4,216,102 (1980). [2] G. Desgardin, H. Bali and B. Raveau, Proceedings Colloque International sur les Nouvelles Orientations des Composants Passifs, Paris, March 1982, pp. 176- 182.
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[ 31 S.L. Schwartz and T.R. Shrout, Mat. Res. Bull. 17 (1982) 1245. [4] H. Bali, Thesis, Caen (1982). [ 51 M. Lejeune and J.P. Boilot, Ceramics Intern. 9 (1983) 119. [ 61 G. Desgardin, H. Bali and B. Raveau, Mat. Chem. Phys. 8 (1983) 469. [7] L.I. Shvomeva, V.M. Petrov and Yu.N. Venetzev, Inorg. Mat. (1970) 1739. [ 81 G. Desgardin, M. Halmi, J.M. Haussonne and B. Raveau, Sciences of Ceramics XIII, Orleans, September 1985, J. Phys. Suppl. Cl (1985) 889. [9] M. Halmi, G. Desgardin and B. Raveau, French Patent no. 85.13257 (1985). [ lo] J.M. Haussonne, 0. Regrenny, J. Lostec, G. Desgardin, M. Halmi and B. Raveau, 6th CIMTECH High Tech Ceramics, Milan, June 1986, to be published. [ 111 G. Shirane, S. Hoshino and K. Suzuki, Phys. Rev. 80 (1950) 1105. [ 121 Ph. Labbe, M. Frey, B. Raveau and J.C. Monier, Acta Cryst. B33 (1977) 2201. [ 131 E.F. Bertaut, P. Blum and A. Sagnieres, Acta Cryst. 72 (1979) 149.
110
February 1987
MATERIALS LETTERS
LOW-TEMPERATURE SINTERED LEAD TANTALATE-BASED OF HIGH DIELECTRIC CONSTANTS
CERAMICS
M. HALMI, G. DESGARDIN and B. RAVEAU Laboratoire de Cristallographie, Chimie et Physique des Solides, U.A. 251, ISMRa-Vniversitk, 14032 Caen Cedex, France
Received 26 November 1986
High grade tantalum perovskite, with composition 0.5 PFN + 0.5 PFf (0.5 PbFelizNbiiz03 + 0.5 PbFe,,,Ta,,,O,) can be sintered at low temperature in spite of the refractory character of tantalum, by using a small amount of lithium carbonate as a sintering agent. The dielectric properties of the ceramics sintered at 950- 1000°C are attractive: the dielectric constant is increased to reach 35000 and the dielectric losses at high temperature are considerably reduced with respect to the systems. These properties are interpreted in terms of the introduction of lithium in the perovskite matrix with creation of anionic vacancies; this model is in agreement with the ability of lithium to take both the octahedral and tetrahedral coordinations.
1. Introduction
Recent research in the field of ceramics for capacitors with high dielectric constants has been focused on with the of the PMN-PFN lead niobates perovskite structure systems called the (PbMg,,3Nb2,303-PbFe,,*~,,*O~) and PMN-PZN (PbMg,,31\Tb2,30,-PbZn,,3~2,303) [ l-91. This choice results from the high dielectric constants of these niobates and from their ability to be sintered at low temperatures. In this respect, the system PFN-PFT (PbFe,,zNb,,203-PbFe1/2Ta,,*O~) is a potential candidate, owing to the Curie temperatures of these oxides corresponding to 114°C and - 4O”C, respectively, and to their high dielectric constants of 20000 and 9000, respectively. An investigation of this system from the crystallographic point of view has shown the existence of a solid solution with dielectric constants ranging from 6000 to 12000 [ 71. However, no attempt of sintering such compositions was reported up to the present, most likely due to the refractory character of the lead tantalate PbFe1,2Ta1,203. Recent sintering studies of lead niobates in the presence of lithium salts [ 8-101 suggest a possibility to apply this method to the system PFN-PFT. The present study deals with the sintering study of the optimal composition in this system at low temperature in order to synthesize ceramics with high dielectric constants, i.e. with E> 20000, at room temperature. 2. Experimental Raw materials Fe203 (from Prolabo, France, quality rectapur), NbzOs, Ta,Os (both from Starck, Berlin, quality, HPO), PbO (from Merck, Darmstadt, pro analysis) and Li,C03 (from Prolabo, France, quality rectapur) were mixed in stoichiometric ratio by wet-milling in alcohol using a planar shaker-mill. Precursors FeNbO, and FeTa04 were obtained after calcination at 1200°C for 12 h. Final compositions were obtained by mixing in the same way FeNbO,, FeTaO, and PbO before calcination at 750°C 4 h. One to five mole % lithium salt where then added to the calcined perovskites. These products were then wet milled in alcohol, and then dried in an infrared radiator. An organic binder (aqueous solution of 2% PVA) was added before pressing at 1000 kg/cm2 to obtain disks 12 mm in diameter and 1.5 mm in thickness. The disks were then sintered in air in the temperature range 950- 1000’ C for different soak times, with heating and cooling rate of 150” C/h. The calcined materials as well as the sintered products were systematically analysed by X-ray diffraction. 0167-577x/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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Microstructure of the sintered disks was examined by SEM and their dielectric properties e=J?( T), tg 6 = f(T) andp(20”C) measured. 2.1. Synthesis of the optimal perovskitein the system PFN-PFT In the synthesis of lead niobates or tantalates PbFeti2M,,203 (M=Nb, Ta), there is the problem due to the formation of a second phase with the pyrochlore structure besides the perovskite form. This latter compound, which is non-stoichiometric and may exhibit a wide composition range, is characterized by a low dielectric constant and must be eliminated for capacitor applications. A method was recently developed by Schwartz and Shrout [ 31 for the PMN system in order to avoid the formation of pyrochlore and to obtain a pure perovskite after calcination of the oxides. It consists in using the columbite MgNbzOs as a precursor which then reacts with the lead oxide instead of using a mixture of the oxides MgO, Nb205 and PbO. The application of this method to the system PMN-PFN [ 81 allowed the pyrochlore phase to be decreased fom about 20-30°h to about l-2O/b.We have tried to transpose this method to the system PFN-PFT by using the wolframite type oxides FeNb04 and FeTaO, as precursors; these precursors which are first prepared at 1200’ C are then calcined in a second step in the presence of PbO according to the reactions: FeNbO, +2PbO~2Pb(Fe,,~~,,~)O~ FeTaO, +2Pb0+2Pb(Fe,,zTal,z)0,
, ,
(1-x)FeNb04+xFeTaO,+2Pb0+2Pb(Fe,,~Nbf,_,,,2Tatiz)03.
(1) (2)
(3)
This method is very successful in the case of the niobate: a pure perovskite is isolated for reaction (1). On the opposite, a pyrochlore phase, almost alone, is observed for reaction (2), whereas a mixture of pyrochlore and perovskite is always obtained for reaction (3). Thus from these results it is clear that FeTa04 is not an efficient precursor for the systems involving PFT’. This behaviour is similar to that observed for the PZN system for which the action of the columbite MgNbz06 on the lead oxide led mainly to the formation of the pyrochlore phase instead of the perovskite f 81. Our previous results concerning the system PMN-PFN [ 91 have shown that the addition of small amounts of lithium salts allowed the pyrochlore phase to be eliminated and favoured an excellent densification. The addition of different amounts of lithium carbonate to the calcined products of reactions (2) and (3) allowed the same phenomenon to be observed by heating at low temperature. The elimination of the pyrochlore phase could result from the introduction of lithium in the octahedral sites of the perovskite. In order to develop a dielectric material for capacitor applications, we have tried to synthesize a perovskite whose Tc was close to room temperature. Preliminary investigations allowed the curve Tc=f(x) to be established for the solid solution Pb (Fe 1,2Nb,, _nj,2 Ta,,*)03. Fig. 1 shows that the curve T,=f(x) deviates from linearity, and that the composition x= 0.50 which corresponds to a Tc close to 20°C is the optimal composition for application. 2.2. Sinteri~~ of the mixture 0.5’PFNi 0.5 PFT at 950” C The mixture of FeNbO, + FeTaO, was calcined in a first step at 750°C during 4 h as described above. The Xray diffraction spectra of the calcined products showed that for the composition 0.5 PFN+O.S PFT, a mixture of about 50°h pyrochlore and 50% perovskite was obtained. After addition of various amounts of lithium carbonate (ranging from 1 to 5 mole%) to this mixture, it was sintered at 950°C for 2 h. The main characteristics of the ceramics are summarized in table 1. It can be seen from the X-ray analysis that the pyrochlore phase has completely disappeared whatever the lithium content. In contrast, the shrinkage increases as the lithium content increases up to 4 mole% Li,CO,; this latter content is 104
February 1981
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Volume 5, number 3
c
I
Fig. 1. Curie temperature versus composition for the system (1 -x)PFN+xPFT.
X
the optimal composition leading to a maximum of the shrinkage as shown from the evolution of the microstructure versus the lithium content (left part of fig. 2). For 19/oLi2COS a residual intergranular porosity is indeed observed with an heterogeneous distribution of the grain size. For 2W L&CO3 the grains are better joined and their size distribution is more homogeneous. The porosity has completely disappeared for 4% Li2C03. For this latter lithium content the fractures are only intergranular whereas one observes well-faceted grains with angular outlines. Moreover the diameter of the grains does not exceed 10 urn. The relationship of the dielectric constant at T, ( emax,of the ceramic) versus the lithium content is in agreement with the shrinkage results: the maximum value of emaxis reached for 4W Li2C03 and does not vary beyond this content. The dielectric curves e=f( T) (fig. 3a) show that Tc decreases as the lithium content increases. Thus, it is clear that lithium enters into the perovskite matrix during sintering. Table 1 0.5 PFN+O.5 PFT: characteristics observed after sintering at 950°C for 2 h versus lithium content Sintered cycle 95O”C, 2 h 0.5 PMN+O.S + 1%Li2COx 0.5 PMN+O.S + 2% LiIC03 0.5 PMN+O.S +4% Li2C03 0.5 PMN+O.S + 5% LizCO,
Shrinkage (%)
Tc( “C)
enl,x
tg 6 (at 20°C)
p(Rcm)
RX (P: perovskite)
PFT
a.33
23
19000
1%
7x 10”
pure P
PFT
8.70
18
29000
12
3x 10”
pure P
PFT
13.50
15
31500
1%
3x 10”
pure P
PFT
13.90
12
31500
1%
2x 10”
pure P
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Fig. 2. Evolution of the microstructure versus lithium content for the composition 0.5 PFN+O.S PFT sintered in the presence ofdifferent lithium carbonate contents at 950°C for 2 h (left), and at 1000°C for 2 h (right). Upper: 0.5 PFN+O.5 PFI’+ 1 mole % Li,CO,; middle: 0.5PEN+O.5PFT+2mole%Li,CO,;lower:0.5PFN+O.5PFT+ 4mole%Li2C03. The curves of the dielectric losses versus temperature (fig. 3b) are also characteristic of a well-densified ceramic: very low losses are observed between 20 and 100' C,and these dielectric losses are rather low even in the ferroelectric domain (tg 6 c 4%). However, a maximum of tg 6 near jr, is observed, and a slight increase of
106
DIELEC.
February 1987
MATERIALS LETTERS
Volume 5. number 3 CONSTANT
35000
OIELEC.
LOSSES %
7000
-60
-40
. -20
. 0
.
. 20
, 40
.
. 80
. 80
.
, 100
. . . I 120 140 TEMPERATURE
-00
-40
-28
0
20
40
W
88
Fig. 3. Dielectric characteristics: (a) t=f(T) and (b) tg S=./(T) for 0.5 PFN+O.5 PFT sintered at 950°C for 2 h. PFT+ 1 mole % Li&O,, - - - 0.5 PFN+O.S PFTf2 mole % Li2C03, - - 0.5 PFN+O.S PFT+4 mole % Li2C03, -.PFT+ 5 mole % Li2COj.
100
120 140 TEMPERATURE
0.5 PFN +0.5 0.5 PFN+O.5
the dielectric losses beyond 100°C; the fact that tg 6 increases much more drastically in the case of the sample with 1% Li2COS is likely due to the lower densification of this ceramic. Besides these good dielectric constants, the insulating resistances, measured under 60 V/mm at room temperature are quite acceptable (table 1) . 2.3. Sinteringofthe mixture 0.5 PFN+O.5 PFTat 1000°C The sintering of identical compositions for 2 h at 1000’ C leads to very similar characteristics as shown from table 2. The densification is scarcely better than at 950°C and the relationship of the microstructure versus the lithium content (right part of fig. 2) is very similar to the one observed at 950°C (left part of fig. 2). Nevertheless, the granulometry is slightly greater than at 950°C whatever the lithium content. Moreover, the fractures are mainly intragranular for 1% Li2C03. The insulating resistances do not change, whereas the curves c=f( T) are very similar for 2% and 4% Li203. However, it must be pointed out that E,,, is drastically increased for 1% Li203 (E = 26000) and 5% Li2C0, Table 2 0.5 PFN+O.S PFT: characteristics observed after sintering at 1000°C for 2 h versus lithium content Sintering cycle lOOO”C,2 h 0.5 pFN+o.s + I % L&CO3 0.5 pFN+o.s + 2% Li&O, 0.5 pFN+o.s + 4% Li2COj 0.5 pFN+o.s + 5% Li2C03
Shrinkage (%)
Tc(‘C)
tg 6 (at 20°C)
p(fi cm)
PFr
8.33
22
26000
1%
3x 10’
PFT
8.60
20
30000
1%
3x 10’
PFT
13.9
15
30000
1%
3x101
PFr
14.3
15
36000
II
3x10’
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Volume 5, number 3 DIELEC.
February 1987
CONSTANT
40000
DIELEC.
LOSSES $Y,
20
b.
I
24000
-60
-40
-20
0
20
40
80
80
TEMPERATURE
Fig. 4. Dielectric characteristics: (a) e =fl7’) and (b) tg 6 =A 7’) for 0.5 PFN + 0.5 PFT sintered at 1000°C for 2 h. PFf+ 1 mole % Li2C03, - - - 0.5 PFN+O.S PFT+2 mole % Li2C03, - - 0.5 PFN+O.S PFT+4 mole % L&CO,, -.PFT + 5 mole % LizC09.
100
120 140 TEMPERATURE
0.5 PFN + 0.5 0.5 PFN+O.S
(Ex 36000)(fig. 4a). Moreover, the maximum of the dielectric losses at Tc has been increased, especially for 1% LizC09, and the dielectric losses at high temperature are drastically reduced (fig. 4b).
3. Discussion and conclusion The addition of Li2C03 to the composition 0.5 PPN-0.5 PFT allows the material to be sintered at low temperature in the form of a perovskite, in spite of the presence of tantalum which is a very refractory element. The dielectric properties of these ceramics, which are remarkable, result from the introduction of lithium in the perovskite matrix. The decrease of Tc as the lithium content increases is in agreement with this point of view. It can be explained by the creation of defects on the A sites and on the anionic sites of the perovskite AB03 according to the equation ( M=Nbo.sTa,,5):
Pb(Fel~&bdO~ +GC03
+U
+2~)Pbl,(l+2~)(Fel,2(l+2c)M1,2(l+2c)Li2~,(l+2c))C~,(1+2r)03(1+c),(,+2c)
.
The decrease of Tc can indeed be due to the introduction of lithium on the octahedral sites, but especially to the decrease of the Pb2+ content on the A sites. It is now established that the Pb2+ ion plays an important part in the ferroelectric properties of the oxides, owing to its lone pair of 6s2 electrons: the high T, value of PbTi03 [ 111 compared to BaTiOJ can be explained in this way, as well as the particular polar axis of the orthorhombic niobate PbNb20h [ 121 which exhibits the tetragonal tungsten bronze structure. The comparison of the PFN-PFT ceramic to the one of the system PMN-PPN [ 4,8] shows that the first one exhibits higher dielectric constants and that the value of E,,, is not lowered by increasing the lithium content in this case, contrary to PMN-PFN. Moreover, the variation of tg 6 with temperature is different in the two systems: the dielectric losses are much higher at low temperature for the PMN-PP’N ceramics in the ferroelectric domain, and increase regularly with 8 up to the Curie point whereas they exhibit only a small maximum in the PFN-PFT ceramics. The fact that good dielectric characteristics can be obtained for the PPN-PPT system 108
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Volume 5. number 3
a
b
February 1987
Fig. 5. Elimination of rows oxygen atoms ( 0 ) along 110 leading to the formation of planes LiOa or FeO., tetrahedra labelled as triangles.
in spite of higher Fe3+ contents, compared to the PMN-PFN system, can be attributed for a great part to the replacement of Nb( V) by Ta( V). Ta( V), owing to its 5d orbitals, does not favour conduction contrary to Nb(V) whose 4d orbitals can form A bonds with p orbitals of the oxygen atoms, owing to their much lower energies. The rather high insulating resistance observed for the PFN-PFT ceramics is in agreement with this point of view. It is worth noting that the dielectric losses by conduction are weak at high temperature, in spite of the rather high Fe3+ content. This phenomenon suggests that the Li+ ions located on the octahedral sites have a compensation effect. At high temperature the conductivity in the absence of lithiuin would result from the creation of anionic vacancies, involving a partial reduction of Nb (V) to Nb( IV) according to the equilibrium 2Nb(v)=2Nb(Iv)+v;:.
(4)
The addition of lithium to the ceramic involves the formation of oxygen vacancies according to the reaction Nb(V)-iLi++2Vz,+.
(5)
This excess of anionic vacancies created by reaction (5) tends to displace the first equilibrium towards the left and avoids the presence of electrons in the 4d orbitals. It must be pointed out that the formation of a rather large amount of oxygen vacancies in those oxides is quite easily explained by the ability of Fe3+ and of Li+ to take both the octahedral and tetrahedral coordinations. Consequently, the FeO, or Li04 tetrahedra can easily be adapted to the octahedral framework (fig. 5) exactly as in the structures derived from the brownmillerite [ 131. The formation of such anionic vacancies in the perovskite structure may certainly play an important part in the diffusion of Li+ and other species through the lattice and may contribute to the good densification of these materials. As a conclusion, lead tantalate-based ceramics with the perovskite structure appear as potential materials for capacitors in spite of the refractory character of tantalum. The drastic increase of the dielectric cunstant ( f > 30000) for the ceramic 0.5 PFN+ 0.5 PFT is very promising. However, this too sharp peak makes that this ceramic does not belong to the Z5U class for capacitors. Substitutions either on the A sites or on the B sites will be necessary in order to obtain a more diffuse transition compatible with this class.
References [ I] K. Furukawa, S. Fujiwara, N. Kikachi, 0.1. Izawa and H. Tanaka, High Dielectric Constant Type Ceramic Compositions PFN-PMN, USPatent4,216,102 (1980). [2] G. Desgardin, H. Bali and B. Raveau, Proceedings Colloque International sur les Nouvelles Orientations des Composants Passifs, Paris, March 1982, pp. 176- 182.
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[ 31 S.L. Schwartz and T.R. Shrout, Mat. Res. Bull. 17 (1982) 1245. [4] H. Bali, Thesis, Caen (1982). [ 51 M. Lejeune and J.P. Boilot, Ceramics Intern. 9 (1983) 119. [ 61 G. Desgardin, H. Bali and B. Raveau, Mat. Chem. Phys. 8 (1983) 469. [7] L.I. Shvomeva, V.M. Petrov and Yu.N. Venetzev, Inorg. Mat. (1970) 1739. [ 81 G. Desgardin, M. Halmi, J.M. Haussonne and B. Raveau, Sciences of Ceramics XIII, Orleans, September 1985, J. Phys. Suppl. Cl (1985) 889. [9] M. Halmi, G. Desgardin and B. Raveau, French Patent no. 85.13257 (1985). [ lo] J.M. Haussonne, 0. Regrenny, J. Lostec, G. Desgardin, M. Halmi and B. Raveau, 6th CIMTECH High Tech Ceramics, Milan, June 1986, to be published. [ 111 G. Shirane, S. Hoshino and K. Suzuki, Phys. Rev. 80 (1950) 1105. [ 121 Ph. Labbe, M. Frey, B. Raveau and J.C. Monier, Acta Cryst. B33 (1977) 2201. [ 131 E.F. Bertaut, P. Blum and A. Sagnieres, Acta Cryst. 72 (1979) 149.
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