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Microwave sintering of lead-based perovskite ceramics for capacitors
MATERIALS LETTERS
Volume 5, number 9
MICROWAVE
SINTERING
OF LEAD-BASED
PEROVSKITE
August 1987
CERAMICS
FOR CAPACITORS
M. ALLIOUAT, L. MAZO, G. DESGARDIN and B. RAVEAU Laboratoire de Cristallographie et Sciences des MatPriaux, Universitk de Caen, ISkfRa, Campus 2, Bd du Mar&ha1 Juin, 14032 Caen Cedex, France
Received 20 May I987
The dielectric perovskites of the systems PMN-PFN (Pb(Mg,,3Nbz,3)03-Pb( Fe1,2Nb,,2)03) and PMN-PNZ (Pb(Mg,,,Nb2,,)0,-Pb(Zn,,3Nb2i,)03) have been sintered by microwave heating at 2.45 GHz. The microwave heating process is described. This study is completed by the study of the behaviour of the ferroelectric perovskite PbTiO, under microwaves. The interaction of the microwaves with these materials is interpreted in terms of the influence ofthe nature of the A and B ions on the polarization, ionic conduction and electron conduction losses.
1. Introduction The sintering of ceramics in conventional furnaces requires generally rather long times owing to the inertia of the furnaces and much energy is lost by radiation. Those disadvantages could be avoided by using microwaves provided that the dielectric losses of the materials be high enough for the penetration of the waves to be allowed. It is also worth pointing out that the use of microwaves allows heating to be started at the core of the sample contrary to classical heating which starts at the surface; this could change the microstructure of the ceramics and consequently their physical properties. Moreover the quenching of the ceramics in order to stabilize a high-temperature property should be easier with microwave devices. For these reasons microwave sintering of barium titanate was undertaken recently [ 1,2]. It was shown that this ferroelectric material could be sintered in the presence of small amounts of LiF or BaLiF, in spite of its low dielectric losses. The lead-based perovskites previously studied in our laboratory [ 3-51 are like BaTiO, important as ceramics for type II capacitors. The present study deals with the microwave sintering of the complex perovskites belonging to the systems: Pb(Mg,,,Nb2,~)03-Pb(Fel/zNbl/z)O~ (PMN-PFN) 328
and
(PZN-PMN). This study is completed by the study of the interaction of microwaves with the ferroelectric perovskite PbTiO,.
2. Experimental 2.1. Powder synthesis and sintering conditions Starting materials consist of oxides or carbonates wet mixed in proper proportions in alcohol, using a shaker-mill, then dried in an infrared radiator and then calcined using a classical method at 800°C during 4 h. The compositions studied correspond to the formulations: (1 -x)Pb(Mg,,,Nbz,3)03 +xPb(Fel12Nbl12)03
(PMN-PFN),
with x=0.23 and (1 -x)Pb(Mg,,,Nb,,,)Ox +xPb(Zn,,3Nb2,3)03
(PMN-PZN),
MATERIALS LETTERS
Volume 5, number 9
August 1987
Resonant
Microwave Generator
mbvable
Adaptated
piston
load
Fig. 1. Experimental device: assembly line.
with x=0.25 which lead by sintering in a classical furnace to Curie temperatures close to room temperature. A sintering agent (lithium salt) is eventually added in small amount (l-8% weight) into the calcined powder using the wet mixing technique described above. The samples are then pressed in the form of cylinders (I=40 mm, 0=7 mm> using an isostatic press (P= 3000 bar). The so pressed cylindrical samples are positioned in a resonant applicator linked to a 1 kW-2.45 GHz microwave generator (fig. 1). The temperature conditions are controlled by modifying the coupling of the cavity and can reach 1500°C in the core of the sample without any formation of a “plasma”.
The structure of the so sintered materials is determined by X-ray diffraction analysis and the microstructure observed with a scanning electron microscope. Lithium content is obtained by atomic absorption spectroscopy and dielectric measurements E=f( T) and tan 6 =f( 7’) are performed at 1 kHz using disks of 1 mm thickness.
3. Results
The behaviour of the PMN-PFN sintering is shown as an example in fig. 2. The PMN-PZN system exhibits a similar behaviour. Five zones can be distinguished during the sintering process. In the first area I we have to decrease the length of the cavity f, and the reflected power P, decreases, i.e. the ab-
sorbed power Pa increases and the sample starts heating. In zone IE, the reflected power P, does not vary any more as 1decreases. At the end of this latter zone, P, decreases rather rapidly if resonance of the cavity is maintained (dotted lines zone III), involving a glow of the sample and leading finally to a thermal racing. In order to avoid this latter phenomenon we have to move away from the cavity resonance conditions, i.e. to increase 1 slightly in zone III, and then to decrease it again in order to reach a constant P, value. This behaviour of the lead-based perovskite is rather similar to the one observed for BaTiO, f l-61, except that the thermal racing is not so important as for BaTiO,. Then P, is almost constant (zone IV) without changing 1. An important difference with BaTiO, is that the phenomenon described here is only obtained for a limited region of the sample (region A, fig. 2). After 5 to 10 min heating P, increases again rapidly and it is no longer possible to carry on heating in this region; no resonance of the cavity can be reached any more; moreover the region A which could be heated is limited to the central area of the bar. Then another region B can be heated and sintered by a new research of the cavity resonance (zone V), without any displacement of the bar in the cavity. Finally the whole of the bar can be sintered, by repeating successively the heating sequence described here in each region of the bar. Using an incident power of 100 W, an absorbed power of 20 W alIows each region of the bar to be sintered. The energy dissipated by microwave interaction in a given region, by thermal conduction allows the sintering of the neighbouring regions to be achieved, and at the end of the process a well sintered bar (fig. 3) can be obtained whose dielectric 329
Volume 5, number 9
MATERIALS LETTERS
August 1987
T
length of the cavt
gton
A of the
Bar
Fig. 2. Microwave sintering of PMN-PFN compositions in the presence of 2 wt% LiF. Evolution of the reflected power versus time correlated with the evolution of the resonance of the cavity. 10000
I
DIELECTRIC
CONST.
,/---.
e000
I
/I
-60 DIELECTRIC
-40
‘\
I’
‘. ‘\ ‘\
-20
0
20
40
BE
80
100
120 140 TEMPERATURE
0
20
40
60
80
100
120 140 TEMPERATURE
LOSSES%
10
of a composition Fig. 3. Microstructure + 0.23PFN + 0.06LiF after microwave sintering.
0.77PMN
Fig. 4. Dielectric properties e=A T) and tan S=f( T) for the k composition 0.77PMN + 0.23PFN sintered in a microwave applicator with P,= 100 W, P,= 30 W: - 8 wt% LiF, 30 min; --- 8 wt% LiF, 60 min; --- 1wt%LiF, 30 min. 330
e
/
2’ 40
-40
-20
Volume 5, number 9
MATERIALS LETTERS
August 1987
Fig. 5. Microwave sintering of PbTiO,+Z wt% LiF: evolution of the reflected power versus time correlated with the evolution of the resonance of the cavity.
constant is however weaker than that observed by classical heating (fig. 4). This different behaviour of the PMN-PPN and PMN-PZN perovskites compared to BaTi03 could be due either to the A ions which are very different - Pb*+ being a lone-pair cation compared to Ba2+ or to the B ions, Nb( V) and Mg( II), Zn( II), Fe( II) compared to Ti(IV). For this reason we have studied in a second step the interaction of the microwaves with the ferroelectric perovskite PbTi03. One observes a behaviour which is different from that of BaTi03 and of the lead niobate perovskites, at the beginning of the heating (fig. 5).It is worth pointing out that for PbTi03 the absorbed power can be stabilized whatever the length I may be, i.e. whatever P, may be. For an incident power of 180W, an absorbed power of 50 W is obtained at cavity resonance. For this latter power, a glow of the sample is observed corresponding to a heating of the entire bar. Nevertheless a prolongated heating leads to a cracking of the bar preventing a correct sintering of the material.
4. Discussion It is not possible at this stage of the investigation to give a detailed inte~retation of the different behaviour of PbTi03, PMN-Pi and PMN-PZN, and BaTiO, during the beginning of the heating. Nevertheless, it is obvious that at low temperature (Tc SOO'C) the polarization and ionic conduction losses will have a greater influence whereas at high temperature the electron transport losses will prevail. From these considerations it appears that the particular behaviour of PbTiOl can be explained by its high Curie temperature ( T,=49O"C): in this material the polarization losses are higher and allow higher temperatures to be reached without playing on the cavity resonance. On the other hand, for BaTiO, and lead niobate systems, the T, is rather low (Tc< 150°C), so that heating is governed, mainly in a first step by losses induced by porosity, and by ionic conduction due to the presence of lithium, the polarization losses being much weaker. The main difference between these materials deals
331
Volume 5, number 9
MATERIALS LETTERS
August 1987
\ TI
\ \ I
\
\
I
\
I \
I
\
\
I
\
\
\ \ \ \
b
Fig. 6. Band model of the super-exchange in perovskite-type compositions showing an easy overlap of the 3d orbital of titanium and the px orbital of oxygen contrary to the case of niobium.
with their behaviour at high temperature, where electronic conduction losses govern the heating process. It is indeed clear that the titanates BaTiO, and PbTi03 are rather similar and very different from the systems PMN-PFN and PMN-PZN for which the dissipation of energy is stopped in the sintered area. This can be explained by using the classical band model based on super-exchange in perovskite-type conductors [ 71. For the titanates a potential band model can be proposed resulting from the overlap of the 3d orbitals of titanium and px orbitals of oxygen (fig. 6a); this leads to an empty conduction band x* and a gap which is rather small. On the other hand, for the niobates this band model resulting from the overlap of the 4d orbitals of niobium and pn orbitals of oxygen (fig. 6b) is characterized by a much larger gap due to the higher energy of the 4d level. Thus it appears that in the first case the polarization losses will be high enough to induce excitation of electrons in the 7c*band and thus to carry on the heating by electron conduction losses, whereas in the case of niobates this process will be stopped by the too large gap. Moreover in those latter compounds, ions like Mg2+ and Zn2+ are present in the network which do not exhibit d-orbitals available and thus tend to break the conduction process. 332
A detailed study of barium titanate, and of other titanates such as the non-ferroelectric SrTi03 will be necessary to get a better understanding of the different factors influencing the microwave heating process. However, it is now clear that the electronic structure of the B ions of the octahedral sites, play an important role in the sintering of the ABOJ perovskites in the high-temperature region where the electron conduction predominates.
References [ 1] L. Quemeneur, G. Desgardin and B. Raveau, Silic. Ind. 1 (1985) 7. [2] G. Desgardin, M. Maze, L. Quemeneur and B. Raveau, J. Phys. (Paris) 47 (1986) 397. [3] G. Desgardin, H. Bali and B. Raveau, Mat. Chem. Phys. 8 (1983) 469. [4] G. Desgardin, M. Halmi, J.M. Haussonne and B. Raveau, J. Phys. (Paris) Cl-47 (1986) 889. [ 51 G. Desgardin, B. Raveau and M. Halmi, Nouvel agent de frittage pour perovskites au plomb, patent no. 85-13257. 16] G. Desgardin, M. Aliouat, M. Mazo and B. Raveau, in: Proceedings of the Conference on Applications of Energetic Microwaves, Paris (1987), to be published. 17] J.B. Goodenough, Les oxydes des metaux de transitions (Gauthier Villars, Paris, 1973).
Volume 5, number 9
MICROWAVE
SINTERING
OF LEAD-BASED
PEROVSKITE
August 1987
CERAMICS
FOR CAPACITORS
M. ALLIOUAT, L. MAZO, G. DESGARDIN and B. RAVEAU Laboratoire de Cristallographie et Sciences des MatPriaux, Universitk de Caen, ISkfRa, Campus 2, Bd du Mar&ha1 Juin, 14032 Caen Cedex, France
Received 20 May I987
The dielectric perovskites of the systems PMN-PFN (Pb(Mg,,3Nbz,3)03-Pb( Fe1,2Nb,,2)03) and PMN-PNZ (Pb(Mg,,,Nb2,,)0,-Pb(Zn,,3Nb2i,)03) have been sintered by microwave heating at 2.45 GHz. The microwave heating process is described. This study is completed by the study of the behaviour of the ferroelectric perovskite PbTiO, under microwaves. The interaction of the microwaves with these materials is interpreted in terms of the influence ofthe nature of the A and B ions on the polarization, ionic conduction and electron conduction losses.
1. Introduction The sintering of ceramics in conventional furnaces requires generally rather long times owing to the inertia of the furnaces and much energy is lost by radiation. Those disadvantages could be avoided by using microwaves provided that the dielectric losses of the materials be high enough for the penetration of the waves to be allowed. It is also worth pointing out that the use of microwaves allows heating to be started at the core of the sample contrary to classical heating which starts at the surface; this could change the microstructure of the ceramics and consequently their physical properties. Moreover the quenching of the ceramics in order to stabilize a high-temperature property should be easier with microwave devices. For these reasons microwave sintering of barium titanate was undertaken recently [ 1,2]. It was shown that this ferroelectric material could be sintered in the presence of small amounts of LiF or BaLiF, in spite of its low dielectric losses. The lead-based perovskites previously studied in our laboratory [ 3-51 are like BaTiO, important as ceramics for type II capacitors. The present study deals with the microwave sintering of the complex perovskites belonging to the systems: Pb(Mg,,,Nb2,~)03-Pb(Fel/zNbl/z)O~ (PMN-PFN) 328
and
(PZN-PMN). This study is completed by the study of the interaction of microwaves with the ferroelectric perovskite PbTiO,.
2. Experimental 2.1. Powder synthesis and sintering conditions Starting materials consist of oxides or carbonates wet mixed in proper proportions in alcohol, using a shaker-mill, then dried in an infrared radiator and then calcined using a classical method at 800°C during 4 h. The compositions studied correspond to the formulations: (1 -x)Pb(Mg,,,Nbz,3)03 +xPb(Fel12Nbl12)03
(PMN-PFN),
with x=0.23 and (1 -x)Pb(Mg,,,Nb,,,)Ox +xPb(Zn,,3Nb2,3)03
(PMN-PZN),
MATERIALS LETTERS
Volume 5, number 9
August 1987
Resonant
Microwave Generator
mbvable
Adaptated
piston
load
Fig. 1. Experimental device: assembly line.
with x=0.25 which lead by sintering in a classical furnace to Curie temperatures close to room temperature. A sintering agent (lithium salt) is eventually added in small amount (l-8% weight) into the calcined powder using the wet mixing technique described above. The samples are then pressed in the form of cylinders (I=40 mm, 0=7 mm> using an isostatic press (P= 3000 bar). The so pressed cylindrical samples are positioned in a resonant applicator linked to a 1 kW-2.45 GHz microwave generator (fig. 1). The temperature conditions are controlled by modifying the coupling of the cavity and can reach 1500°C in the core of the sample without any formation of a “plasma”.
The structure of the so sintered materials is determined by X-ray diffraction analysis and the microstructure observed with a scanning electron microscope. Lithium content is obtained by atomic absorption spectroscopy and dielectric measurements E=f( T) and tan 6 =f( 7’) are performed at 1 kHz using disks of 1 mm thickness.
3. Results
The behaviour of the PMN-PFN sintering is shown as an example in fig. 2. The PMN-PZN system exhibits a similar behaviour. Five zones can be distinguished during the sintering process. In the first area I we have to decrease the length of the cavity f, and the reflected power P, decreases, i.e. the ab-
sorbed power Pa increases and the sample starts heating. In zone IE, the reflected power P, does not vary any more as 1decreases. At the end of this latter zone, P, decreases rather rapidly if resonance of the cavity is maintained (dotted lines zone III), involving a glow of the sample and leading finally to a thermal racing. In order to avoid this latter phenomenon we have to move away from the cavity resonance conditions, i.e. to increase 1 slightly in zone III, and then to decrease it again in order to reach a constant P, value. This behaviour of the lead-based perovskite is rather similar to the one observed for BaTiO, f l-61, except that the thermal racing is not so important as for BaTiO,. Then P, is almost constant (zone IV) without changing 1. An important difference with BaTiO, is that the phenomenon described here is only obtained for a limited region of the sample (region A, fig. 2). After 5 to 10 min heating P, increases again rapidly and it is no longer possible to carry on heating in this region; no resonance of the cavity can be reached any more; moreover the region A which could be heated is limited to the central area of the bar. Then another region B can be heated and sintered by a new research of the cavity resonance (zone V), without any displacement of the bar in the cavity. Finally the whole of the bar can be sintered, by repeating successively the heating sequence described here in each region of the bar. Using an incident power of 100 W, an absorbed power of 20 W alIows each region of the bar to be sintered. The energy dissipated by microwave interaction in a given region, by thermal conduction allows the sintering of the neighbouring regions to be achieved, and at the end of the process a well sintered bar (fig. 3) can be obtained whose dielectric 329
Volume 5, number 9
MATERIALS LETTERS
August 1987
T
length of the cavt
gton
A of the
Bar
Fig. 2. Microwave sintering of PMN-PFN compositions in the presence of 2 wt% LiF. Evolution of the reflected power versus time correlated with the evolution of the resonance of the cavity. 10000
I
DIELECTRIC
CONST.
,/---.
e000
I
/I
-60 DIELECTRIC
-40
‘\
I’
‘. ‘\ ‘\
-20
0
20
40
BE
80
100
120 140 TEMPERATURE
0
20
40
60
80
100
120 140 TEMPERATURE
LOSSES%
10
of a composition Fig. 3. Microstructure + 0.23PFN + 0.06LiF after microwave sintering.
0.77PMN
Fig. 4. Dielectric properties e=A T) and tan S=f( T) for the k composition 0.77PMN + 0.23PFN sintered in a microwave applicator with P,= 100 W, P,= 30 W: - 8 wt% LiF, 30 min; --- 8 wt% LiF, 60 min; --- 1wt%LiF, 30 min. 330
e
/
2’ 40
-40
-20
Volume 5, number 9
MATERIALS LETTERS
August 1987
Fig. 5. Microwave sintering of PbTiO,+Z wt% LiF: evolution of the reflected power versus time correlated with the evolution of the resonance of the cavity.
constant is however weaker than that observed by classical heating (fig. 4). This different behaviour of the PMN-PPN and PMN-PZN perovskites compared to BaTi03 could be due either to the A ions which are very different - Pb*+ being a lone-pair cation compared to Ba2+ or to the B ions, Nb( V) and Mg( II), Zn( II), Fe( II) compared to Ti(IV). For this reason we have studied in a second step the interaction of the microwaves with the ferroelectric perovskite PbTi03. One observes a behaviour which is different from that of BaTi03 and of the lead niobate perovskites, at the beginning of the heating (fig. 5).It is worth pointing out that for PbTi03 the absorbed power can be stabilized whatever the length I may be, i.e. whatever P, may be. For an incident power of 180W, an absorbed power of 50 W is obtained at cavity resonance. For this latter power, a glow of the sample is observed corresponding to a heating of the entire bar. Nevertheless a prolongated heating leads to a cracking of the bar preventing a correct sintering of the material.
4. Discussion It is not possible at this stage of the investigation to give a detailed inte~retation of the different behaviour of PbTi03, PMN-Pi and PMN-PZN, and BaTiO, during the beginning of the heating. Nevertheless, it is obvious that at low temperature (Tc SOO'C) the polarization and ionic conduction losses will have a greater influence whereas at high temperature the electron transport losses will prevail. From these considerations it appears that the particular behaviour of PbTiOl can be explained by its high Curie temperature ( T,=49O"C): in this material the polarization losses are higher and allow higher temperatures to be reached without playing on the cavity resonance. On the other hand, for BaTiO, and lead niobate systems, the T, is rather low (Tc< 150°C), so that heating is governed, mainly in a first step by losses induced by porosity, and by ionic conduction due to the presence of lithium, the polarization losses being much weaker. The main difference between these materials deals
331
Volume 5, number 9
MATERIALS LETTERS
August 1987
\ TI
\ \ I
\
\
I
\
I \
I
\
\
I
\
\
\ \ \ \
b
Fig. 6. Band model of the super-exchange in perovskite-type compositions showing an easy overlap of the 3d orbital of titanium and the px orbital of oxygen contrary to the case of niobium.
with their behaviour at high temperature, where electronic conduction losses govern the heating process. It is indeed clear that the titanates BaTiO, and PbTi03 are rather similar and very different from the systems PMN-PFN and PMN-PZN for which the dissipation of energy is stopped in the sintered area. This can be explained by using the classical band model based on super-exchange in perovskite-type conductors [ 71. For the titanates a potential band model can be proposed resulting from the overlap of the 3d orbitals of titanium and px orbitals of oxygen (fig. 6a); this leads to an empty conduction band x* and a gap which is rather small. On the other hand, for the niobates this band model resulting from the overlap of the 4d orbitals of niobium and pn orbitals of oxygen (fig. 6b) is characterized by a much larger gap due to the higher energy of the 4d level. Thus it appears that in the first case the polarization losses will be high enough to induce excitation of electrons in the 7c*band and thus to carry on the heating by electron conduction losses, whereas in the case of niobates this process will be stopped by the too large gap. Moreover in those latter compounds, ions like Mg2+ and Zn2+ are present in the network which do not exhibit d-orbitals available and thus tend to break the conduction process. 332
A detailed study of barium titanate, and of other titanates such as the non-ferroelectric SrTi03 will be necessary to get a better understanding of the different factors influencing the microwave heating process. However, it is now clear that the electronic structure of the B ions of the octahedral sites, play an important role in the sintering of the ABOJ perovskites in the high-temperature region where the electron conduction predominates.
References [ 1] L. Quemeneur, G. Desgardin and B. Raveau, Silic. Ind. 1 (1985) 7. [2] G. Desgardin, M. Maze, L. Quemeneur and B. Raveau, J. Phys. (Paris) 47 (1986) 397. [3] G. Desgardin, H. Bali and B. Raveau, Mat. Chem. Phys. 8 (1983) 469. [4] G. Desgardin, M. Halmi, J.M. Haussonne and B. Raveau, J. Phys. (Paris) Cl-47 (1986) 889. [ 51 G. Desgardin, B. Raveau and M. Halmi, Nouvel agent de frittage pour perovskites au plomb, patent no. 85-13257. 16] G. Desgardin, M. Aliouat, M. Mazo and B. Raveau, in: Proceedings of the Conference on Applications of Energetic Microwaves, Paris (1987), to be published. 17] J.B. Goodenough, Les oxydes des metaux de transitions (Gauthier Villars, Paris, 1973).