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Microstructural phenomena in low-firing ceramics
Materials Chemistry and Physics 79 (2003) 104–110
Microstructural phenomena in low-firing ceramics Matjaz Valant∗ , Danilo Suvorov “Jozef Stefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia
Abstract The contribution of a ceramic’s extrinsic phenomena to its microwave dielectric properties is reviewed. The extrinsic phenomena which are related to the microstructural characteristics, are grouped into four classes: grain characteristics, pore characteristics, phase composition and crystallite characteristics. The individual classes are discussed with the aim of minimizing their mainly detrimental influence on the dielectric losses. A number of examples are described. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Microstructure; Dielectric properties; LTCC; Extrinsic phenomena
1. Introduction The development of any new material, including microwave dielectrics, must go through three general phases. First, extensive work on the synthesis of possible candidates is needed to identify a material, the properties of which can in the next phase be further modified and adjusted to the technological requirements. In the last phase, the manufacturing parameters must be determined and optimized. This last phase includes the following: optimization of the synthesis; processing of the powder; development or adjustment of an appropriate shaping technique such as compacting, depositing, casting, etc.; optimization of the heat-treatment conditions and post-firing treatment; and machining and encapsulating if needed. All the steps related to the last phase of the material’s development have an impact on the microstructural characteristics of the ceramics and, consequently, also on the properties. In order to eliminate the detrimental effect of these so-called extrinsic factors on the properties, all the correlations relating to these factors must be known. This paper aims to review the influence of particular microstructural phenomena on microwave dielectric ceramics with the focus on how to minimize this influence. The microstructural characteristics can be grouped into following four classes: • • • •
grain characteristics; pore characteristics; phase composition; crystallite characteristics.
∗ Corresponding author. Tel.: +386-1-477-3547; fax: +386-1-426-3126. E-mail address: [email protected] (M. Valant).
The contributions of particular phenomena to the deviation of the actual material properties from the intrinsic properties are additive and, in the vast majority of cases, detrimental. Because they tend not to be mutually dependent, their existence or their influence can be individually assessed and then suppressed. In some cases, when the two or more features are interrelated (e.g. exaggerated grain growth and the presence of a liquid phase), it is necessary to eliminate the critical one in order to avoid the others.
2. Grain characteristics The most important grain characteristics for microwave dielectric properties are the average grain size, the grain size distribution, the grain morphology and the grain orientation. Several studies were performed to characterize the influence of the average grain size on the microwave dielectric properties, in particular on the dielectric losses [1–3]. A critical review of these studies shows that no general trend can be identified or, if it exists, that it can be ascribed to other microstructural features related to the intensive grain growth, such as the presence of a liquid phase, an increased concentration of structural defects or closed porosity, etc. The same is true for exaggerated grain growth. It is not the exaggeratedly grown grains themselves, but the reason for their appearance that is detrimental to the microwave dielectric properties. The primary reasons for exaggerated grain growth include the initial bimodal grain size distribution, preferential wetting, structural defects (e.g. polytypic sequences) and non-uniform pore pinning. However, all the primary reasons result from secondary mechanisms that are specific to a
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 2 4 8 - 1
M. Valant, D. Suvorov / Materials Chemistry and Physics 79 (2003) 104–110
particular material. These include decompositional and transformational mechanisms as well as eutectic melting. An example of exaggerated grain growth can be found in incorrectly processed AgNb1/2 Ta1/2 O3 ceramics [4]. After firing such ceramics under conditions that do not entirely suppress decomposition (a insufficiently high oxygen partial pressure or a temperature that is too high) a fragile spongy shell consisting of Ag8 (Nb, Ta)26 O69 and the hard sintered core consisting of AgNb1/2 Ta1/2 O3 , in addition to Ag8 (Nb, Ta)26 O69 grains are formed. On the core–shell boundary region where the extensive decomposition has taken place the exaggerated grain growth of the Ag8 (Nb, Ta)26 O69 phase was observed (Fig. 1). The primary reason for the appearance of the exaggerated grains was identified to be the localized formation of a liquid phase resulting from the partial decomposition of AgNb1/2 Ta1/2 O3 into Ag8 (Nb, Ta)26 O69 and metal Ag which under the firing conditions is in a liquid state. The increase in the sintering temperature of Bi12 TiO20 affects the mean grain size, which increases from ∼10 to ∼20 m for the samples sintered at 740 and 820 ◦ C. At a sintering temperature of 850 ◦ C the exaggeratedly grown grains with a size of up to 1 mm were observed (Fig. 2). The large size of these grains and the high concentration of closed porosity within the grains indicate fast growth kinetics that can be explained only by the presence of a liquid phase during the firing. It is apparent from the Bi2 O3 –TiO2 phase diagram [5] that the volatilization of Bi cannot be the reason for the formation of the liquid phase. The Bi-loss shifts the composition in the binary Bi12 TiO20 –Bi4 Ti3 O12 field to where no melting occurs up to 875 ◦ C. However, the firing temperature of 850 ◦ C is very close to the peritectic melting of Bi12 TiO20 (875 ◦ C) and the impurities present in the ceramics may allow the formation of the liquid phase
Fig. 1. Scanning electron microscope micrographs of the exaggerated grain growth of Ag8 (Nb, Ta)26 O69 on the core–shell boundary region of the ceramic sample with starting composition AgNb1/2 Ta1/2 O3 after firing in an oxygen atmosphere at 1220 ◦ C for 10 h (A· · · AgNb1/2 Ta1/2 O3 , C· · · Ag8 (Nb, Ta)26 O69 ).
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at the sintering temperature. Variations in the sintering temperature for Bi12 TiO20 did not affect the permittivity and τ f , which were measured to be 41.1 ± 0.4 and −32 ± 2, respectively. The only significant variation was found for the Qxf-value, which increases slightly with sintering temperature from 2200 GHz for 740 ◦ C to 3280 GHz for 820 ◦ C. This correlates well with the increase in the grain size. For the samples sintered at >820 ◦ C the Qxf-value decreases, most probably as a result of the presence of the liquid phase. A highly anisometric unit cell is the reason for the growth of grains with a high aspect ratio. As a rule, such compounds also exhibit a high (di)electric anisotropy. Well-known dielectric ceramics with such characteristics include the Ba6−3x R8+2x Ti18 O54 solid solutions (R: La–Gd). During the formation of green bodies from such a powder, the elongated crystallites become partially oriented and as a result the ceramic’s grains are preferentially oriented as well. This causes variations in the dielectric properties that depend on the calcination and sintering regimes as well as on the shaping technique [6,7]. Different force magnitudes and force directions of the shaping techniques—uniaxial or isostatic pressing, extrusion, tape casting, hot forging— induce a different alignment of the anisometric crystallites. Consequently, the dielectric properties of the ceramics produced from the same batch of the powder will also be different. The problem can be minimized by optimizing the calcination conditions in order to suppress the grain growth during the calcinations and arrive at the shaping stage with the smallest grains possible.
3. Pore characteristics The pore characteristics include the level of porosity, the type of porosity, the pore distribution and morphology, however, the presence of microcracks, cracks and voids can be considered within this group. The most investigated of these characteristics is the influence of the level of porosity on the dielectric properties. It is widely accepted that low levels of porosity (<5 vol.%) significantly decrease the permittivity but do not effect the microwave dielectric losses so dramatically; although some increase in dielectric losses is observed as well. Higher levels of porosity, associated with an increased pore size, significantly increase the dielectric losses. The temperature dependence of permittivity (or resonant frequency) is the least dependent on the level of porosity. A number of studies were published which by using different empirical relations, try to extrapolate dielectric properties, measured on the porous ceramics, to obtained values for the pore-free ceramics [1–3,8,9]. The accuracy of such calculations depends on the mathematical model, the density of the measured sample and the accuracy of the measurements of the dielectric properties and the ceramic’s density. All these factors together can contribute to a significant error, which might mislead researchers. From the standpoint of applied value, the most appropriate method is to densify the
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Fig. 2. Scanning electron microscope micrographs of the Bi12 TiO20 sintered at (a) 740 ◦ C; (b) 800 ◦ C; (c) 850 ◦ C (exaggerated grain growth).
ceramics and determine the properties of a sample with a relative density of ≥97%. All other approaches have lower applied, although some academic, value. Good powder characteristics are essential for the proper densification of ceramics. The agglomerates, if present in the initial powder, must not be too hard to deform during the compacting of the green body. If the agglomerates are too dense and do not deform they sinter faster than the matrix. This results in the formation of large pores or even voids that cannot be eliminated with a subsequent heat treatment. A similar effect occurs when the granulation of the powder results in a hard granulate in which granules do not deform during the compacting. A typical microstructure of such a ceramic shows dense, intra-granule regions and porous inter-granule regions (Fig. 3). The porosity can also appear as a result of an interaction at the interface of two phases. Such a porosity type is typical for multilayer structures. When there is a difference in the diffusion rates between the ions from the opposite sides
of the interface the net mass flow is not zero. Since the site number remains constant, the formation of vacancies is induced. When their concentration exceeds the critical concentration they coalesce and form so-called Kirkendall porosity, which in multilayer structures appears as a layer of porosity at a discrete distance from the interface (Fig. 4). Such porosity does not affect the dielectric properties as much as the mechanical properties. However, for the technological application of any kind of electronic multilayer modules, the mechanical characteristics are as important as the dielectric properties. The application of copper electrodes in LTCC modules demands that the firing takes place in an oxygen-free, usually nitrogen, atmosphere where the oxidation of copper is prevented. For such modules, materials are required that do not reduce during firing in a nitrogen atmosphere. However, such materials are not necessarily resistant towards oxidation. In the case of processing failures, when the oxygen partial pressure is slightly increased a damaging chemical
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Fig. 3. Scanning electron microscope image of CaTiO3 –NdAlO3 ceramics produced from the hard granulate and sintered at 1400 ◦ C for 5 h.
Fig. 4. Scanning electron microscope image showing the Kirkendall porosity on the interface of two LTCC layers.
transformation can also occur in the ceramics layers which, in the case of a significant change in the unit-cell parameters, can result in the formation of microcracks. An example of this is low-sinterable Bi6 Te2 O13 , which in the presence of oxygen oxidizes to Bi6 Te2 O15 . The formation of an oxidized form that exhibits a completely different crystal structure with significantly different unit-cell parameters generates the strains that relax with the formation of microcracks (Fig. 5).
4. Phase composition The phase composition of the microwave ceramics must be controlled on three levels: primary phase composition,
intentionally or unintentionally generated secondary phases and grain-boundary phases. Commercial LTCC tapes are mainly low-permittivity glass-ceramic composites that are used as a substrate material in LTCC modules. During the co-firing of an LTCC module two material properties must be carefully considered: the sintering curves of all the materials present in the LTCC module must match and all of the phases present must be mutually chemically compatible. The only expected chemical reaction during the co-firing is the recrystallization of the glass, however, the products of the recrystallization must again be chemically compatible with the whole material system. A typical LTCC tape after firing consists of the Al2 O3 phase, recrystallization products (e.g. anorthite), remaining glass and the phase for the
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Fig. 5. Scanning electron microscope image showing the microcracks generated as a result of the oxidation of the Bi6 Te2 O13 .
modification of the dielectric properties (SrTiO3 , TiO2 , . . . ) [8,9]. The presence of so many phases in the LTCC tape significantly increases the complexity of the system, especially in the case when more than one type of ceramic layer is present. It is believed that the complexity of such systems hinders the further development of LTCC modules in terms of integration of new types of ceramic layers with new functions. The role of the glass phase in commercial tapes is mainly in the enhancement of the sintering (and as a result, the mechanical properties) and through the recrystallization providing the phase with the appropriate dielectric properties. If sintering of the glass-free ceramic tape with the appropriate dielectric properties was possible at the required temperature then the mechanical properties would also be ensured
and there would be no need for the glass phase. By applying such an approach, the simplicity of the LTCC system is maintained but special attention must be devoted to the sintering behavior of the materials with all the other requirements for the LTCC modules being considered. If the chemistry of the primary phases is not well understood then secondary phases, precipitates, liquid phase, etc. can be generated during the heat treatment. The dielectric losses are particularly sensitive to the presence of small concentrations of other phases. The low concentration of a phase with high dielectric losses in a low-loss matrix would dominate the dielectric losses of such a ceramic. The consequence of this phenomenon is that the dielectric properties of LTCC modules are dominated by the phase with the highest dielectric losses, which in these systems is the conductor (Fig. 6). To minimize the conductor losses, the choice of the appropriate electrode is limited to highly conductive metals like copper and silver. The introduction of the conductor into the LTCC module generates a new restriction related to the chemistry of the ceramic phase(s). Any interaction between the conductor and the ceramics that results in either a chemical reaction and the formation of secondary phases or changes in the circuit structure due to, for example, diffusion processes or eutectic melting, significantly influences the dielectric properties of the module and, therefore, must be prevented. To investigate chemical compatibility a simple compatibility test [10] can be performed by mixing the ceramic powder with powdered Ag, or Ag2 O which at elevated temperatures decomposes into very reactive Ag0 , and firing the mixture at the LTCC heat-treatment temperature. The microstructural inspection and X-ray analysis of such composite ceramics must reveal either the presence of the metal phase and the nominal ceramic phase, which confirms the chemical compatibility, or the presence of new phases that appeared as a result of the chemical reaction.
Fig. 6. Scanning electron microscope image of the cross-section of an LTCC module showing the silver conductor co-fired with the ceramics layers.
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Fig. 7. Transmission electron microscope images of the defects in the crystal structure of the ceramics with the composition 0.7CaTiO3 –0.3NdAlO3 .
When the LTCC module is composed of more than one type of ceramic layer, all the different layers must be mutually compatible. This includes compatibility in terms of properties such as sintering parameters, thermal expansion coefficients as well as chemical compatibility. It is essential for the proper functioning of such an LTCC module that the mutual compatibility test performed between all the ceramic materials in the system shows the thermodynamic compatibility of the materials involved.
5. Crystallite characteristics Since the dielectric losses are related to the anharmonicity in the crystal-structure dynamics the presence of a structural disorder necessarily has a detrimental influence on the losses. The example that best illustrates this phenomenon is the difference in the Q-value between the ordered and disordered microwave perovskites. By inducing long-range cation order through extended high-temperature annealing the Q-value of Ba(Zn1/3 Ta2/3 )O3 and Ba(Mg1/3 Ta2/3 )O3 ceramics can be increased from ∼500 to >35,000 at 10 GHz [11–13]. Accordingly, due to the increased anharmonicity of the amorphous glass network the presence of the glass phase in the LTCC tape necessarily reduces the Q-value. In addition, glasses are known to a exhibit low permittivity, which makes them a parasitic phase in high-permittivity systems. All these facts, together with the problem related to the increased complexity of the LTCC systems, which was discussed in the previous section, suggest the elimination of the glass phase from the LTCC tapes. The alternative is the compositionally simple single- or two-phase glass-free LTCC systems.
Although it is widely assumed that defects in the crystal structure, such as antiphase boundaries and twins, significantly reduce the dielectric losses, the experimental evidence disproves this thesis. An example is the ceramic material based on the CaTiO3 –NdAlO3 solid solution. Although the concentration of defects is high (Fig. 7) this ceramic exhibits one of the lowest dielectric losses (Qxf ∼ 45.000 GHz) in its permittivity range (permittivity 45) [14]. As was pointed out by Schlömann [15], the effect of the compositional and/or charge disorder associated with the domain boundaries is almost negligible compared to the other microstructural phenomena. One such type of phenomena, which often accompanies structural defects—especially dislocations—are the strain and stress fields induced during sintering or cooling. The properties of such ceramics are usually sensitive to post-sintering conditions including the cooling rate and any subsequent annealing treatments. In many cases, the properties can be recovered by annealing but sometimes they relax as a result of the formation of microcracks and cracks.
6. Conclusion The contributions of particular microstructural phenomena to the deviation of the actual material properties from the intrinsic properties are additive and, in the vast majority of cases, detrimental. Understanding the reasons for their appearance and their mechanisms is crucial if we are to eliminate or at least suppress their influence. The processing of ceramic materials must be optimized with respect to microstructural characteristics, because only in this way can we produce manufactured ceramics with properties that approach the level of the intrinsic dielectric properties.
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References [1] S.J. Penn, N. McN Alford, A. Templeton, X. Wang, M. Xu, M. Reece, K. Schrapel, Effect of porosity and grain size on the microwave dielectric properties of sintered alumina, J. Am. Ceram. Soc. 80 (7) (1997) 1885–1888. [2] A. Templeton, X. Wang, S.J. Penn, S.J. Webb, L.F. Cohen, N. McN Alford, Microwave dielectric loss of titanium oxide, J. Am. Ceram. Soc. 83 (1) (2000) 95–100. [3] R. Heidiner, S. Nazare, Influence of porosity on the dielectric properties of AlN in the range of 30–40 GHz, Powder Metall. Int. 20 (6) (1988) 30–32. [4] M. Valant, D. Suvorov, A. Meden, AgNb1−x Tax O3 —new high permittivity microwave ceramics. Part I. Crystal structures and phase decomposition relations, J. Am. Ceram. Soc. 82 (1) (1999) 81–87. [5] T.M. Bruton, Study of the liquidus in the system Bi2 O3 –TiO2 , J. Solid State Chem. 9 (1974) 173–175. [6] T. Negas, P.K. Davies, Influence of chemistry and processing on the electrical properties of Ba6−3x Ln8+2x Ti18 O54 solid sloutions, Ceram. Trans. 53 (1995) 179–196. [7] C. Hoffmann, R. Waser, Hot-forging of Ba6−3x R8+2x Ti18 O54 , Ferroelectrics 201 (1997) 127–135. [8] H. Jantunen, R. Rautioaho, A. Uusimaki, S. Leppavuori, Preparing low-loss low-temperature cofired ceramic material without glass addition, J. Am. Ceram. Soc. 83 (2000) 2855–2857.
[9] K.B. Shim, N.T. Cho, S.W. Lee, Silver diffusion and microstructure in LTCC multilayer couplers for high frequency applications, J. Mater. Sci. 35 (2000) 813–820. [10] M. Valant, D. Suvorov, Chemical compatibility between silver electrode and low-firing binary oxide compounds—conceptual study, J. Am. Ceram. Soc. 83 (2000) 2721–2729. [11] S. Kawashima, M. Nishida, I. Ueda, H. Ouchi, Ba(Zn1/3 Ta2/3 )O3 ceramics with low dielectric losses at microwave frequencies, J. Am. Ceram. Soc. 66 (6) (1983) 421–423. [12] K. Matsumoto, T. Hiuga, K. Takata and H. Ichimura, Ba(Mg1/3 Ta2/3 )O3 ceramics with ultra-low loss at microwave frequencies, in: Proceedings of the Sixth IEEE International Symposium on Application of Ferroelectrics, Institute of Electronic Engineering, New York, 1986, pp. 118–121. [13] P.K. Davies, J. Tong, T. Negas, Effect of ordering-induced domain boundaries on low-loss Ba(Zn1/3 Ta2/3 )O3 –BaZrO3 perovskite microwave dielectrics, J. Am. Ceram. Soc. 80 (7) (1997) 1727– 1740. [14] B. Jancar, D. Suvorov, M. Valant, Microwave dielectric properties of CaTiO3 –NdAlO3 ceramics, J. Mater. Sci. Lett. 20 (1) (2001) 71– 72. [15] E. Schlömann, Dielectric losses in ionic crystals with disordered charge distribution, Phys. Rev. 135 (2A) (1964) 413–419.
Microstructural phenomena in low-firing ceramics Matjaz Valant∗ , Danilo Suvorov “Jozef Stefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia
Abstract The contribution of a ceramic’s extrinsic phenomena to its microwave dielectric properties is reviewed. The extrinsic phenomena which are related to the microstructural characteristics, are grouped into four classes: grain characteristics, pore characteristics, phase composition and crystallite characteristics. The individual classes are discussed with the aim of minimizing their mainly detrimental influence on the dielectric losses. A number of examples are described. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Microstructure; Dielectric properties; LTCC; Extrinsic phenomena
1. Introduction The development of any new material, including microwave dielectrics, must go through three general phases. First, extensive work on the synthesis of possible candidates is needed to identify a material, the properties of which can in the next phase be further modified and adjusted to the technological requirements. In the last phase, the manufacturing parameters must be determined and optimized. This last phase includes the following: optimization of the synthesis; processing of the powder; development or adjustment of an appropriate shaping technique such as compacting, depositing, casting, etc.; optimization of the heat-treatment conditions and post-firing treatment; and machining and encapsulating if needed. All the steps related to the last phase of the material’s development have an impact on the microstructural characteristics of the ceramics and, consequently, also on the properties. In order to eliminate the detrimental effect of these so-called extrinsic factors on the properties, all the correlations relating to these factors must be known. This paper aims to review the influence of particular microstructural phenomena on microwave dielectric ceramics with the focus on how to minimize this influence. The microstructural characteristics can be grouped into following four classes: • • • •
grain characteristics; pore characteristics; phase composition; crystallite characteristics.
∗ Corresponding author. Tel.: +386-1-477-3547; fax: +386-1-426-3126. E-mail address: [email protected] (M. Valant).
The contributions of particular phenomena to the deviation of the actual material properties from the intrinsic properties are additive and, in the vast majority of cases, detrimental. Because they tend not to be mutually dependent, their existence or their influence can be individually assessed and then suppressed. In some cases, when the two or more features are interrelated (e.g. exaggerated grain growth and the presence of a liquid phase), it is necessary to eliminate the critical one in order to avoid the others.
2. Grain characteristics The most important grain characteristics for microwave dielectric properties are the average grain size, the grain size distribution, the grain morphology and the grain orientation. Several studies were performed to characterize the influence of the average grain size on the microwave dielectric properties, in particular on the dielectric losses [1–3]. A critical review of these studies shows that no general trend can be identified or, if it exists, that it can be ascribed to other microstructural features related to the intensive grain growth, such as the presence of a liquid phase, an increased concentration of structural defects or closed porosity, etc. The same is true for exaggerated grain growth. It is not the exaggeratedly grown grains themselves, but the reason for their appearance that is detrimental to the microwave dielectric properties. The primary reasons for exaggerated grain growth include the initial bimodal grain size distribution, preferential wetting, structural defects (e.g. polytypic sequences) and non-uniform pore pinning. However, all the primary reasons result from secondary mechanisms that are specific to a
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 2 4 8 - 1
M. Valant, D. Suvorov / Materials Chemistry and Physics 79 (2003) 104–110
particular material. These include decompositional and transformational mechanisms as well as eutectic melting. An example of exaggerated grain growth can be found in incorrectly processed AgNb1/2 Ta1/2 O3 ceramics [4]. After firing such ceramics under conditions that do not entirely suppress decomposition (a insufficiently high oxygen partial pressure or a temperature that is too high) a fragile spongy shell consisting of Ag8 (Nb, Ta)26 O69 and the hard sintered core consisting of AgNb1/2 Ta1/2 O3 , in addition to Ag8 (Nb, Ta)26 O69 grains are formed. On the core–shell boundary region where the extensive decomposition has taken place the exaggerated grain growth of the Ag8 (Nb, Ta)26 O69 phase was observed (Fig. 1). The primary reason for the appearance of the exaggerated grains was identified to be the localized formation of a liquid phase resulting from the partial decomposition of AgNb1/2 Ta1/2 O3 into Ag8 (Nb, Ta)26 O69 and metal Ag which under the firing conditions is in a liquid state. The increase in the sintering temperature of Bi12 TiO20 affects the mean grain size, which increases from ∼10 to ∼20 m for the samples sintered at 740 and 820 ◦ C. At a sintering temperature of 850 ◦ C the exaggeratedly grown grains with a size of up to 1 mm were observed (Fig. 2). The large size of these grains and the high concentration of closed porosity within the grains indicate fast growth kinetics that can be explained only by the presence of a liquid phase during the firing. It is apparent from the Bi2 O3 –TiO2 phase diagram [5] that the volatilization of Bi cannot be the reason for the formation of the liquid phase. The Bi-loss shifts the composition in the binary Bi12 TiO20 –Bi4 Ti3 O12 field to where no melting occurs up to 875 ◦ C. However, the firing temperature of 850 ◦ C is very close to the peritectic melting of Bi12 TiO20 (875 ◦ C) and the impurities present in the ceramics may allow the formation of the liquid phase
Fig. 1. Scanning electron microscope micrographs of the exaggerated grain growth of Ag8 (Nb, Ta)26 O69 on the core–shell boundary region of the ceramic sample with starting composition AgNb1/2 Ta1/2 O3 after firing in an oxygen atmosphere at 1220 ◦ C for 10 h (A· · · AgNb1/2 Ta1/2 O3 , C· · · Ag8 (Nb, Ta)26 O69 ).
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at the sintering temperature. Variations in the sintering temperature for Bi12 TiO20 did not affect the permittivity and τ f , which were measured to be 41.1 ± 0.4 and −32 ± 2, respectively. The only significant variation was found for the Qxf-value, which increases slightly with sintering temperature from 2200 GHz for 740 ◦ C to 3280 GHz for 820 ◦ C. This correlates well with the increase in the grain size. For the samples sintered at >820 ◦ C the Qxf-value decreases, most probably as a result of the presence of the liquid phase. A highly anisometric unit cell is the reason for the growth of grains with a high aspect ratio. As a rule, such compounds also exhibit a high (di)electric anisotropy. Well-known dielectric ceramics with such characteristics include the Ba6−3x R8+2x Ti18 O54 solid solutions (R: La–Gd). During the formation of green bodies from such a powder, the elongated crystallites become partially oriented and as a result the ceramic’s grains are preferentially oriented as well. This causes variations in the dielectric properties that depend on the calcination and sintering regimes as well as on the shaping technique [6,7]. Different force magnitudes and force directions of the shaping techniques—uniaxial or isostatic pressing, extrusion, tape casting, hot forging— induce a different alignment of the anisometric crystallites. Consequently, the dielectric properties of the ceramics produced from the same batch of the powder will also be different. The problem can be minimized by optimizing the calcination conditions in order to suppress the grain growth during the calcinations and arrive at the shaping stage with the smallest grains possible.
3. Pore characteristics The pore characteristics include the level of porosity, the type of porosity, the pore distribution and morphology, however, the presence of microcracks, cracks and voids can be considered within this group. The most investigated of these characteristics is the influence of the level of porosity on the dielectric properties. It is widely accepted that low levels of porosity (<5 vol.%) significantly decrease the permittivity but do not effect the microwave dielectric losses so dramatically; although some increase in dielectric losses is observed as well. Higher levels of porosity, associated with an increased pore size, significantly increase the dielectric losses. The temperature dependence of permittivity (or resonant frequency) is the least dependent on the level of porosity. A number of studies were published which by using different empirical relations, try to extrapolate dielectric properties, measured on the porous ceramics, to obtained values for the pore-free ceramics [1–3,8,9]. The accuracy of such calculations depends on the mathematical model, the density of the measured sample and the accuracy of the measurements of the dielectric properties and the ceramic’s density. All these factors together can contribute to a significant error, which might mislead researchers. From the standpoint of applied value, the most appropriate method is to densify the
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Fig. 2. Scanning electron microscope micrographs of the Bi12 TiO20 sintered at (a) 740 ◦ C; (b) 800 ◦ C; (c) 850 ◦ C (exaggerated grain growth).
ceramics and determine the properties of a sample with a relative density of ≥97%. All other approaches have lower applied, although some academic, value. Good powder characteristics are essential for the proper densification of ceramics. The agglomerates, if present in the initial powder, must not be too hard to deform during the compacting of the green body. If the agglomerates are too dense and do not deform they sinter faster than the matrix. This results in the formation of large pores or even voids that cannot be eliminated with a subsequent heat treatment. A similar effect occurs when the granulation of the powder results in a hard granulate in which granules do not deform during the compacting. A typical microstructure of such a ceramic shows dense, intra-granule regions and porous inter-granule regions (Fig. 3). The porosity can also appear as a result of an interaction at the interface of two phases. Such a porosity type is typical for multilayer structures. When there is a difference in the diffusion rates between the ions from the opposite sides
of the interface the net mass flow is not zero. Since the site number remains constant, the formation of vacancies is induced. When their concentration exceeds the critical concentration they coalesce and form so-called Kirkendall porosity, which in multilayer structures appears as a layer of porosity at a discrete distance from the interface (Fig. 4). Such porosity does not affect the dielectric properties as much as the mechanical properties. However, for the technological application of any kind of electronic multilayer modules, the mechanical characteristics are as important as the dielectric properties. The application of copper electrodes in LTCC modules demands that the firing takes place in an oxygen-free, usually nitrogen, atmosphere where the oxidation of copper is prevented. For such modules, materials are required that do not reduce during firing in a nitrogen atmosphere. However, such materials are not necessarily resistant towards oxidation. In the case of processing failures, when the oxygen partial pressure is slightly increased a damaging chemical
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Fig. 3. Scanning electron microscope image of CaTiO3 –NdAlO3 ceramics produced from the hard granulate and sintered at 1400 ◦ C for 5 h.
Fig. 4. Scanning electron microscope image showing the Kirkendall porosity on the interface of two LTCC layers.
transformation can also occur in the ceramics layers which, in the case of a significant change in the unit-cell parameters, can result in the formation of microcracks. An example of this is low-sinterable Bi6 Te2 O13 , which in the presence of oxygen oxidizes to Bi6 Te2 O15 . The formation of an oxidized form that exhibits a completely different crystal structure with significantly different unit-cell parameters generates the strains that relax with the formation of microcracks (Fig. 5).
4. Phase composition The phase composition of the microwave ceramics must be controlled on three levels: primary phase composition,
intentionally or unintentionally generated secondary phases and grain-boundary phases. Commercial LTCC tapes are mainly low-permittivity glass-ceramic composites that are used as a substrate material in LTCC modules. During the co-firing of an LTCC module two material properties must be carefully considered: the sintering curves of all the materials present in the LTCC module must match and all of the phases present must be mutually chemically compatible. The only expected chemical reaction during the co-firing is the recrystallization of the glass, however, the products of the recrystallization must again be chemically compatible with the whole material system. A typical LTCC tape after firing consists of the Al2 O3 phase, recrystallization products (e.g. anorthite), remaining glass and the phase for the
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Fig. 5. Scanning electron microscope image showing the microcracks generated as a result of the oxidation of the Bi6 Te2 O13 .
modification of the dielectric properties (SrTiO3 , TiO2 , . . . ) [8,9]. The presence of so many phases in the LTCC tape significantly increases the complexity of the system, especially in the case when more than one type of ceramic layer is present. It is believed that the complexity of such systems hinders the further development of LTCC modules in terms of integration of new types of ceramic layers with new functions. The role of the glass phase in commercial tapes is mainly in the enhancement of the sintering (and as a result, the mechanical properties) and through the recrystallization providing the phase with the appropriate dielectric properties. If sintering of the glass-free ceramic tape with the appropriate dielectric properties was possible at the required temperature then the mechanical properties would also be ensured
and there would be no need for the glass phase. By applying such an approach, the simplicity of the LTCC system is maintained but special attention must be devoted to the sintering behavior of the materials with all the other requirements for the LTCC modules being considered. If the chemistry of the primary phases is not well understood then secondary phases, precipitates, liquid phase, etc. can be generated during the heat treatment. The dielectric losses are particularly sensitive to the presence of small concentrations of other phases. The low concentration of a phase with high dielectric losses in a low-loss matrix would dominate the dielectric losses of such a ceramic. The consequence of this phenomenon is that the dielectric properties of LTCC modules are dominated by the phase with the highest dielectric losses, which in these systems is the conductor (Fig. 6). To minimize the conductor losses, the choice of the appropriate electrode is limited to highly conductive metals like copper and silver. The introduction of the conductor into the LTCC module generates a new restriction related to the chemistry of the ceramic phase(s). Any interaction between the conductor and the ceramics that results in either a chemical reaction and the formation of secondary phases or changes in the circuit structure due to, for example, diffusion processes or eutectic melting, significantly influences the dielectric properties of the module and, therefore, must be prevented. To investigate chemical compatibility a simple compatibility test [10] can be performed by mixing the ceramic powder with powdered Ag, or Ag2 O which at elevated temperatures decomposes into very reactive Ag0 , and firing the mixture at the LTCC heat-treatment temperature. The microstructural inspection and X-ray analysis of such composite ceramics must reveal either the presence of the metal phase and the nominal ceramic phase, which confirms the chemical compatibility, or the presence of new phases that appeared as a result of the chemical reaction.
Fig. 6. Scanning electron microscope image of the cross-section of an LTCC module showing the silver conductor co-fired with the ceramics layers.
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Fig. 7. Transmission electron microscope images of the defects in the crystal structure of the ceramics with the composition 0.7CaTiO3 –0.3NdAlO3 .
When the LTCC module is composed of more than one type of ceramic layer, all the different layers must be mutually compatible. This includes compatibility in terms of properties such as sintering parameters, thermal expansion coefficients as well as chemical compatibility. It is essential for the proper functioning of such an LTCC module that the mutual compatibility test performed between all the ceramic materials in the system shows the thermodynamic compatibility of the materials involved.
5. Crystallite characteristics Since the dielectric losses are related to the anharmonicity in the crystal-structure dynamics the presence of a structural disorder necessarily has a detrimental influence on the losses. The example that best illustrates this phenomenon is the difference in the Q-value between the ordered and disordered microwave perovskites. By inducing long-range cation order through extended high-temperature annealing the Q-value of Ba(Zn1/3 Ta2/3 )O3 and Ba(Mg1/3 Ta2/3 )O3 ceramics can be increased from ∼500 to >35,000 at 10 GHz [11–13]. Accordingly, due to the increased anharmonicity of the amorphous glass network the presence of the glass phase in the LTCC tape necessarily reduces the Q-value. In addition, glasses are known to a exhibit low permittivity, which makes them a parasitic phase in high-permittivity systems. All these facts, together with the problem related to the increased complexity of the LTCC systems, which was discussed in the previous section, suggest the elimination of the glass phase from the LTCC tapes. The alternative is the compositionally simple single- or two-phase glass-free LTCC systems.
Although it is widely assumed that defects in the crystal structure, such as antiphase boundaries and twins, significantly reduce the dielectric losses, the experimental evidence disproves this thesis. An example is the ceramic material based on the CaTiO3 –NdAlO3 solid solution. Although the concentration of defects is high (Fig. 7) this ceramic exhibits one of the lowest dielectric losses (Qxf ∼ 45.000 GHz) in its permittivity range (permittivity 45) [14]. As was pointed out by Schlömann [15], the effect of the compositional and/or charge disorder associated with the domain boundaries is almost negligible compared to the other microstructural phenomena. One such type of phenomena, which often accompanies structural defects—especially dislocations—are the strain and stress fields induced during sintering or cooling. The properties of such ceramics are usually sensitive to post-sintering conditions including the cooling rate and any subsequent annealing treatments. In many cases, the properties can be recovered by annealing but sometimes they relax as a result of the formation of microcracks and cracks.
6. Conclusion The contributions of particular microstructural phenomena to the deviation of the actual material properties from the intrinsic properties are additive and, in the vast majority of cases, detrimental. Understanding the reasons for their appearance and their mechanisms is crucial if we are to eliminate or at least suppress their influence. The processing of ceramic materials must be optimized with respect to microstructural characteristics, because only in this way can we produce manufactured ceramics with properties that approach the level of the intrinsic dielectric properties.
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