H.C. Starck company has developed a special treated niobium-oxide, which has been introduced into the direct route and the columbite route of lead complex perovskite synthesis. The preparation-property relationship of the relaxor ferroelctrics lead magnesium niobate (Pb3MgNb2O9, PMN) and PMN-PbTiO3 (PMN-PT) and the modified piezoelectric PMN-Pb3NiNb2O9-PT (PMN-PNN-PT) have been studied.
An excellent quality concerning reactivity and homogeneity of the H.C. Starck precursors has been found. This correlates with a reduction of sintering temperature by 200 K, typically.
Main results concerning the obtained electrical material data are:
Table 1: Niobium and tantalum oxides in functional ceramics with perovskite struture - materials, basic properties and applications
composition properties of major applications interest
Pb(Mg1/3Nb2/3)O3 PMN dielectric, additive in lead ferroelectric, complex ceramics electro-optic
PMN-PbTiO3 PMN-PT dielectric, capacitor, actuator, ferroelectric, light valve, display electro-optic
LiNbO3 LN ferroelectric, SAW device, piezoelectric, pyro-detector, pyroelectric optical components
KNbO3 KN electro-optic wave guide, modulator, frequency doubler, memory
K(Ta,Nb)O3 KTN pyroelectric, pyrodetector, wave electro-optic guide
Ba(Mg1/3Ta2/3)O3 BMT dielectric resonator, microwave dielectric
Ba(Zn1/3Ta2/3)O3 BZT dielectric microwave dielectric
One of the challanges of future developments is seen in integration within the framework of the basic technologies in electronic industry. This needs materials and technolgy solutions for chip-devices (hybrid technology), thin and thick films, coatings (microsystem technology), multifunctional and smart materials, and composites.
Essentially, advanced technical applications involve both a material and manufactoring process development together with its evaluation on actual devices.
H.C. Starck company has developed a special treated niobium-oxide, which has been introduced into direct synthesis or the columbite route of Pb-complex perovskite fabrication (4a,b). We focussed our interest upon the relation between niobium oxide raw material quality and synthesis route and the attainable performance level of ceramics for some different device applications.
Three different material systems has been selected for the present study, the relaxor ferrolectrics Pb3MgNb2O9, (PMN) and PMN-PbTiO3 (PMN-PT) and the modified piezoelectric PMN-Pb3NiNb2O9-PT (PMN-PNN-PT).
Using the columbite method binary oxides expressed by the formula MIINb2O6 are formed in a preliminary reaction. They are then converted with help of lead compounds according to the reaction
Nb2O5 + MIIO --> MgNb2O6
3PbO + MIINb2O6 --> Pb3MIINb2O9.
Synthesis according to the present study follwed the flow diagrams in figure 1a and 1b.
Pb3O4 Nb2O5 MgCO3 Nb2O5 MgCO3Figure 1: Flow chart for synthesis of PMN-ceramics| | | | | |------|--------| |--------------| | <-- mixing --> |
| <-- calcining --> |
| | | MgNb2O6
| |
| | | mixing --> |---- Pb3O4
| calcining --> |
| |
PMN- PMN-
Powder Powder
| |
| <-- pressing --> |
| |
Pellets Pellets
| |
| <-- sintering --> |
| |
PMN- PMN-
Ceramic Ceramic
a) direct synthesis b) columbite method
The starting powders are calcined (800deg.C/2h), pressed into pellets (diameter: 10mm, thickness: 1..1,3mm) and sintered at various temperatures (range 850deg.C to 1200deg.C for 2h) in a closed platinium capsule. In order to suppress chemical decomposition during sintering, a defined PbO atmosphere is maintained. In some cases sintering is carried out under atmospheric conditions without using a special PbO atmosphere denoted by „o“ (1000deg.C o, for example see table 5 and figure 8).
The sintered pellets have been characterized usually by density, chemical composition, phase content, ceramic microstructure and dielectric properties (room temperature permittivity [[epsilon]]25 is measured with HP-LCR Meter 4275A, 1MHz). Depending on the material systems specific functional properties have been measured, additionally, as for example temperature behaviour of permittivity (measuring frequency 100KHz), ferroelectric hysteresis loop (Sawyer-Tower circuit, 50Hz) and strain (laser-interferometer).
The relaxor ferroelectric compound PMN is mainly used as chemical component in Pb- complex ferroelectric ceramics (for example PMN-PT, PMN-PZT). Main problems are connected with the occurence of pyrochlore phases, which deteriorate the functional properties. They are expected to be avoidable by choosing the right processing route.
Pyrochlore phases are formed by different mechanisms. Firstly, pyrochlore is the stabil low temperature phase. It is transformed into perovskite phase at raised temperatures. Thus the final phase content of the ceramic depends on reactivity and homogeneity of the preformend powder mixture. Secondly, perovskite to pyrochlore phase transformation may occure due to PbO-evaporation at sintering at temperatures >900deg.C. Consequently, the starting powder mixture should be very reactive to obtain densification at temperatures as low as possible.
PMN-powders and ceramics have been prepared by the following four routes:
route A direct synthesis using conventional Nb oxide (5)
route B direct synthesis using H.C. Starck Nb-oxide (H.C. Starck process - 4a)
route C columbite route using conventional Nb-oxide (5)
route D columbite route using H.C. Starck Nb-oxide (H.C. Starck process - 4b).
Results are given in figures 2-8 and tables 2-5.
XRD pattern of calcined powders with different MgO contents - figures 2-3 direct synthesis; figures 5-6 columbite synthesis - show pyrochlore phases and intermediate reaction products, clearly. Highest pyrochlore content is obtained by route A. Using H.C. Starck process, the pyrochlore phase volume fractions are obviously reduced and the perovskite volume fraction amounts to more than 82% (route B).
At low concentration level of pyrochlores x-ray diffraction fails to classify the phase purity of reaction products (route C and D). Therefore measurement of dielectric properties (permittivity at Curie-temperature, dielectric loss) has been used as a very sensitive method.
The different phase compositions of the starting powders are reflected by variations of sintering behaviour and dielectric properties of the sintered ceramic. Typically, sintering temperatures ³1000deg.C are necessary to obtain dense ceramic bodies using route A and B. Permittivity, as measure of quality, is raised with growing sintering temperature. At 1200deg.C/2h saturation is obtained.
Generally, ceramics with H.C. Starck Nb-oxide become more dense and have higher permittivity as compared with conventional oxides at corresponding technological conditions. The results concerning permittivity of PMN-ceramics prepared by conventional (A) and H.C. Starck route (B) are compared in figures 3 and 4. The H.C. Starck route is superior, as can be seen by the higher level of permittivity, especially at lower sintering temperatures. Additionally, MgO-excess has minor influence (figure 4a,b,c) corresponding to the already high sinter-activity of the stoichiometric mixture.
Table 2: Properties of PMN ceramics by direct synthesis, conventional route
Ts PMN stoichiometric PMN 5 Mol-% MgO excess PMN 10 Mol-% MgO excess
eps25 density del m eps25 density del m eps25 density del m
1000 C 1923 7.40 -1.96 2881 7.43 -2.35 2642 7.25 -2.36
1100 C 3804 7.66 -2.35 5916 7.69 -2.48 5953 7.64 -2.18
1200 C 7855 7.77 -3.30 10928 7.71 -3.07 11543 7.73 -2.77
eps25 - room temperature permittivity del m - weight loss during sintering
Table 3: Properties of PMN ceramics by direct synthesis, H.C. Starck route
Ts PMN stoichiometric PMN 5 Mol-% MgO excess PMN 10 Mol-% MgO excess
eps25 density del m eps25 density del m eps25 density del m
1000 C 6417 7.63 -0.63 5893 7.54 -0.55 5625 7.51 -0.52
1100 C 9312 7.84 -0.59 8624 7.63 -0.35 8813 7.54 -0.35
1200 C 9309 7.81 -0.60 10060 7.84 -0.62 10029 7.80 -0.65
eps25 - room temperature permittivity del m - weight loss during sintering
Ceramics with 10 Mol-% MgO excess show highest quality in the low sintering temperature range. For example, 97,5% th. density is obtained at 900deg.C/2h. This is due to MgO, which enhances sintering of PMN powders.
The H.C. Starck-PMN powders are showing very high sinter-activity already at stoichiometric composition and results in 97,5% th. density at 900deg.C/2h (table 5). As expected, raising sintering temperature gives higher permittivity, as seen in figure 8a. Also MgO-excess improves densification and dielectric properties. For example, 98.5% th. density is obtained with 5 Mol-% MgO excess at 850deg.C/2h, thereby permittivity is as high as 10800 (room temperature, 1MHz).
Another point of interest of H.C. Starck powders concerns atmospheric sintering without PbO supply. As shown in table 5, weight loss during sintering is comparable with that of sintering with controlled PbO atmosphere. Permittivity is even better (see figure 8).
Table 4: Properties of PMN ceramics by columbite synthesis, conventional route
Ts PMN stoichiometric PMN 5 Mol-% MgO excess PMN 10 Mol-% MgO excess
eps25 density del m eps25 density del m eps25 density del m
900 C n.d. -0.30 3862 7.66 -0.32 10072 7.80 -0.46
1000 C 5040 7.75 -0.29 5231 7.82 -0.24 11631 7.89 -0.58
1100 C 6059 7.88 -0.20 6693 7.91 -0.27 11174 7.73 -0.96
1200 C 7346 7.73 -0.49 9084 7.77 -1.19 9869 7.74 -1.40
eps25 - room temperature permittivity del m - weight loss during sintering
n.d. - not dense
Table 5: Properties of PMN ceramics by columbite synthesis, H.C. Starck route
Ts PMN stoichiometric PMN 5 Mol-% MgO excess
eps25 density del m eps25 density del m
850 C n.d. -0.31 10827 8.00 -0.55
900 C 8328 7.94 -0.42 12151 7.96 -0.41
1000 C 8166 7.98 -0.54 11909 8.00 -0.93
1100 C 10241 7.96 -0.64 13099 7.91 -0.98
1200 C 11865 7.90 -1.07 10800 7.91 -1.10
850 C n.d. 11251 8.00 -0.74
1000 C 9404 7.93 -0.79 12671 7.98 -1.05
eps25 - room temperature permittivity del m - weight loss during sintering
n.d. - not dense
PT additions are used to shift Curie-temperatur and the operation temperature range. The composition 0.9PMN-0.1PT has been intensively studied for use as actuator material for room temperature operation. The columbite synthesis of PMN using H.C. Starck-MgNb2O6 precursor (route D) has been applied for PMN-PT ceramic preparation in the present study.
The properties of prepared ceramics are seen in figures 9-10 and table 5. Similar to PMN attractive functional properties (permittivity, strain and ferroelctric hysteresis) are already obtained at reduced sintering temperatures.
Under special sintering conditions 0.9PMN-0.1PT powders have been found to densify to >99% of theoretical density already at 1000deg.C/2h. Comparable microstructure and the high level of functional properties have only been obtained with HIP-sintering or at higher sintering temperature and longer sintering times, respectively (6).
Table 6: Properties of PMN-PT ceramics
Ts 0.9PMN-0.1PT 0.8PMN-0.2PT
eps25 density del m eps25 density del m
1000 9698 7.95 -0.36 1985 7.99 0.29
1100 9945 7.97 -0.43 2190 7.92 -0.36
1200 8864 7.92 -0.52 3010 7.82 -0.40
eps25 - room temperature permittivity del m - weight loss during sintering
Complex compositions within the system PMN-PNN-PT allow the fabrication of materials with an intermediate electrostrictive and piezoelectric character. The application of such PMN-PNN-PT materials to fabricate MLC’s is described (7).
As shown experimentally, Curie-temperature and peak level of permittivity can be variied in wide ranges by composition (see figure 12). This helps to adjust application needs. Materials have beeen prepared with excellent strain and soft ferroelectric behaviour. Application as actuator material and for ferroelectric imaging (8) are now under consideration (see figure 13 and 14).
H.C. Starck powders (columbites, perovskites) were found to be highly sinter-actice. Lead perovskite ceramics of different complex systems, PMN-PT and PMN-PNN-PT, have been prepared showing the possibility to obtain a high level of functional properties at drastically reduced sintering temperatures. These functional ceramics may find new applications in integrated systems. Use as micropositioner and ferroelectric imaging material is actually under consideration.
(1) L.E.Cross: „RELAXOR FERROELECTRICS : AN OVERVIEW”; Ferroelectrics, 1994, vol. 151., pp. 305-320
(2) K.Uchino: „RELAXOR FERROELECTRICS DEVICES”; Ferroelectrics, 1994, vol. 151., pp. 321-330
(3) L.E.Cross: „BOUNDARY CONDITIONS FOR SHAPE MEMORY IN CERAMIC MATERIAL SYSTEMS” J. of Intelligent Materials, Systems and Structures, 6 (1995) 1, 55-61
(4a) H. C. Starck GmbH & Co KG, DE P 4217819 (1992)
(4b) H. C. Starck GmbH & Co KG, DE P 4217817 (1992)
(5) S.L.Swartz, T.R.Shrout, W.A.Schulze and L.E.Cross: „DIELECTRIC PROPERTIES OF LEAD MAGNESIUM NIOBATE CERAMICS” J. Am. Ceram. Soc., 67(5) 311-315, (1984)
(6) K.Reichert, F.Schlenkrich: „DIELECTRIC PROPERTIES OF LEAD PEROVSKITES AS A FUNCTION OF PROCESSING AND PRECURSORS“ Ferroelectrics, 1994, vol. 154., pp.213-218
(7) J.-H.Oh, J.-H.Lee, S.-H.Cho: „BROADENING OF DIELECTRIC CONSTANT BY A CONTROL OF COMPOSITINAL FLUCTUATION IN (1-x)PMN-xPT SYSTEM“ Ferroelectrics, 1994, vol. 158., pp.241-246
(8) A.Hirth, R.Weis: „PRINTING WITH FERROELECTRIC MATERIAL“ Proceedings of the Conference Soc. Imaging Science and Technology 9, 1993, pp181-184,
Properties of PMN powders and ceramics by direct synthesis
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Figure 1: XRD pattern of PMN powders by direct synthesis, conventional route
a) stoichiometric composition b) 5 Mol-% MgO excess
c) 10 Mol-% MgO excess
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Figure 2: XRD pattern of PMN powders by direct synthesis, H.C. Starck route
a) stoichiometric composition b) 5 Mol-% MgO excess
c) 10 Mol-% MgO excess
a) b) c)Figure 3: Temperature behaviour of permittivity of PMN ceramic with powders by direct synthesis, conventional route
a) stoichiometric composition b) 5 Mol-% MgO excess
c) 10 Mol-% MgO excess
a) b) c)Figure 4: Temperature behaviour of permittivity of PMN ceramic with powders by direct synthesis, H.C. Starck route
a) stoichiometric composition b) 5 Mol-% MgO excess
c) 10 Mol-% MgO excess
Properties of PMN powders and ceramics by columbite synthesis
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Figure 5: XRD pattern of PMN powders by columbite synthesis, conventional route
a) stoichiometric composition b) 5 Mol-% MgO excess
c) 10 Mol-% MgO excess
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Figure 6: XRD pattern of PMN powders by columbite synthesis, H.C. Starck route
a) stoichiometric composition b) 5 Mol-% MgO excess
a) b) c)Figure 7: Temperature behaviour of permittivity of PMN ceramic with powders by columbite synthesis, conventional route
a) stoichiometric composition b) 5 Mol-% MgO excess
c) 10 Mol-% MgO excess
a) b)Figure 8: Temperature behaviour of permittivity of PMN ceramic with powders by columbite synthesis, H.C. Starck route
a) stoichiometric composition b) 5 Mol-% MgO excess
Properties of PMN-PT ceramics by columbite synthesis, H.C.Starck route
0.9PMN-0.1PT 0.8PMN-0.2PTFigure 9: Temperature behaviour of permittivity of PMN-PT ceramics
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Figure 10: Strain of a 0.9PMN-0.1PT ceramic (sintering temperature: 1000deg.C/2h)
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Figure 11: Hysteresis loop of 0.8PMN-0.2PT (sintering temperature: 1000deg.C/2h)
Properties of PMN-PNN-PT ceramics by columbite synthesis, H.C. Starck route
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Figure 12: Temperature behaviour of permittivity of PMN-PNN-PT ceramics with different compositions
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Figure 13: Strain of a PMN-PNN-PT ceramic
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Figure 14: Hysteresis loop of a PMN-PNN-PT ceramic
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