Separation of dielectric and space charge polarizations in Ca Cu 3 Ti 4 O 12 Ca Ti O 3 composite polycrystalline systems Paulo R. Bueno, William C. Ribeiro, Miguel A. Ramírez, José A. Varela, and Elson Longo Citation: Applied Physics Letters 90, 142912 (2007); doi: 10.1063/1.2720301 View online: http://dx.doi.org/10.1063/1.2720301 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/14?ver=pdfcov Published by the AIP Publishing This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 186.217.234.225 On: Tue, 14 Jan 2014 12:18:13 http://scitation.aip.org/content/aip/journal/apl?ver=pdfcov http://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/2079205716/x01/AIP-PT/APL_ArticleDL_1213/aipToCAlerts_Large.png/5532386d4f314a53757a6b4144615953?x http://scitation.aip.org/search?value1=Paulo+R.+Bueno&option1=author http://scitation.aip.org/search?value1=William+C.+Ribeiro&option1=author http://scitation.aip.org/search?value1=Miguel+A.+Ram�rez&option1=author http://scitation.aip.org/search?value1=Jos�+A.+Varela&option1=author http://scitation.aip.org/search?value1=Elson+Longo&option1=author http://scitation.aip.org/content/aip/journal/apl?ver=pdfcov http://dx.doi.org/10.1063/1.2720301 http://scitation.aip.org/content/aip/journal/apl/90/14?ver=pdfcov http://scitation.aip.org/content/aip?ver=pdfcov Separation of dielectric and space charge polarizations in CaCu3Ti4O12/CaTiO3 composite polycrystalline systems Paulo R. Bueno,a� William C. Ribeiro, Miguel A. Ramírez, José A. Varela, and Elson Longo Departamento de Físico-Química, Instituto de Química, Universidade Estadual Paulista, C. Postal 355, Araraquara, Sao Paulo 14800-900, Brazil �Received 22 January 2007; accepted 5 March 2007; published online 5 April 2007� The complex analysis of dielectric/capacitance is a very useful approach to separate different polarization contributions existing in polycrystalline ceramics. In this letter, the authors use this type of spectroscopic analysis to separate the bulk’s dielectric dipolar relaxation contributions from the polarization contribution due to space charge in the grain boundaries of a CaCu3Ti4O12/CaTiO3 polycrystalline composite system. The bulk dielectric dipolar relaxation was attributed to the self-intertwined domain structures from the CaCu3Ti4O12 phase coupled to the dipole relaxation from the CaTiO3 phase, while the space charge relaxation was attributed to the Schottky-type potential barrier responsible for the highly non-Ohmic properties observed in this composite polycrystalline system. © 2007 American Institute of Physics. �DOI: 10.1063/1.2720301� The huge dielectric constant ��10.000 over a wide tem- perature range of 100–400 K� of a CaCu3Ti4O12 perovskite- like material was discovered by Subramanian et al.1 and, as foreseen, the discovery of such an intriguing dielectric property soon led CaCu3Ti4O12 materials to become impor- tant candidates for ceramic capacitors, attracting the technological2–4 and scientific2,5–9 interest of many research- ers. At present, it is generally accepted that the ultrahigh dielectric response is not an intrinsic behavior.10,11 Rather, the dielectric response is due to barrier-layer capacitances associated with one or more of the following: grain bound- aries, twin boundaries �or domain boundaries�, dislocation networks, etc.10–12 Specifically, the grain boundary barrier- layer capacitances are generally associated with non-Ohmic properties in metal oxide polycrystalline semi- conductors.13–16 Indeed, in addition to the remarkable and intriguing dielectric property, Chung et al.12 observed that potential barrier exists intrinsically in the grain boundary re- gion, likely possessing a Schottky-type nature, according to Refs. 12 and 17. This Schottky-type barrier or the grain boundary interfacial polarization effects also contribute to the total dielectric response, and recently, based on imped- ance and dielectric �or capacitance� complex analysis, our research group18,19 demonstrated that the grain boundary contribution can be responsible for up to 25% of the total dielectric response of this type of material.18,19 However, most results have led to the conclusion that the correlation between non-Ohmic and dielectric properties �i.e., the higher the non-Ohmic property, the higher the dielectric response� is not easily established mainly because of the presence of other kinds of barrier-layer capacitances inside the grain, e.g., self-intertwined domain structures inside the grains of polycrystalline CaCu3Ti4O12 materials.10,11,18,19 Therefore, although the grain boundary contributes to a large extent to the total dielectric constant value at ambient temperature, it is not the major effect contributing to this property. CaCu3Ti4O12/CaTiO3 composites are also very promis- ing Ba-/Pb-free dielectric materials20 due to their lower dis- sipation factor at room temperature compared with CaCu3Ti4O12 pure and stoichiometric materials, even though their dielectric constant at ambient temperature is much lower, i.e., around 1300–1800. Recently, we have also shown that such composites also present remarkable non-Ohmic properties19 �i.e., a nonlinear coefficient value � of �65 in the traditional current density range of 1–10 mA/cm2 and a�Author to whom correspondence should be addressed; electronic mail: prbueno@iq.unesp.br FIG. 1. �a� Complex capacitance diagrams for the CCTO-CTO composite, showing the whole relaxation pattern in this polycrystalline material. The higher frequency region exhibits a near-Debye relaxation pattern relating to the bulk dielectric features. �b� Bode capacitive diagrams, real �C�� and imaginary �C�� parts. �o� Experimental data and �continuous line� the result of the fitting to the theoretical function. APPLIED PHYSICS LETTERS 90, 142912 �2007� 0003-6951/2007/90�14�/142912/3/$23.00 © 2007 American Institute of Physics90, 142912-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 186.217.234.225 On: Tue, 14 Jan 2014 12:18:13 http://dx.doi.org/10.1063/1.2720301 http://dx.doi.org/10.1063/1.2720301 http://dx.doi.org/10.1063/1.2720301 �1500 in the current range of 3–30 mA, the same range used by Chung et al.12�. This letter proposes a methodology to separate the di- electric polarization response elicited by the grain’s internal capacitance barriers �i.e., self-intertwined domains� and di- polar relaxation from that of the grain boundary space charge region, using dielectric/capacitance spectroscopy.3 Based on such a physical analysis, we also propose an adequate equivalent circuit that can take into account the capacitance- voltage dependence of the Schottky-type barrier existing in metal oxide dielectric materials with highly non-Ohmic properties.19 The Mott-Schottky straight-line behavior was constructed from a complex capacitance analysis and dis- played via Cgb −2 �grain boundary capacitance� versus Vdc, proving the existence of potential barriers in the grain bound- ary region. The CaCu3Ti4O12/CaTiO3 polycrystalline composite was prepared based on traditional oxide mixture processing. All the precursors were of analytical grade: CaCO3 �J. T. Baker, 99.99%�, TiO2 �Aldrich, 99.8%�, and CuO �Riedel, 99%�. The mixed oxides were ball milled for 24 h in isopropylic alcohol using a polyethylene bottle and zirconium balls, followed by drying at 110 °C and heat treat- ing at 900 °C in an ambient atmosphere for 12 h. The CaCu3Ti4O12/CaTiO3 polycrystalline composite system �CCTO-CTO� was first slightly pressed into disk shaped pel- lets and then isostatically pressed under 210 MPa. The pel- lets were sintered at 1050 °C for 30 min at a heating rate of 230 °C/min in a domestic microwave oven �CCE, M301, 2.45 GHz, 900 W, GL107 magnetron�. Gold contacts were deposited by sputtering on the samples’ surfaces for the electrical measurements. Current- tension measurements were taken using a high-voltage source-measure unit �Keithley model 237�. The breakdown electric field �Eb� was obtained at a current density of 1 mA cm−2. Numerical values for the nonlinear coefficient � were obtained by a linear regression of the log J vs log E plot within the range of 1–10 mA cm−2, and the � values were calculated at around 62–68 in this current density range. The dielectric spectroscopy measurements were taken with a fre- quency response analyzer �HP 4294 A�, at frequencies rang- ing from 40 Hz to 110 MHz, with an amplitude voltage of 500 mV. For the capacitance-voltage analysis, a bias dc po- tential of 0–40 V was applied. It is widely known that dielectric relaxation can be em- pirically described by Debye-type or Cole-Cole relaxation functions,21 C*���=C�+�C /1+ ����1−�, in which � ap- proaches the zero value for pure Debye relaxation. Figure 1�a� shows the typical dielectric or capacitance complex dia- grams for the CCTO-CTO polycrystalline composite. Note the Debye-type relaxation21 ���0.1� in the frequency range of 0.1–110 MHz, which we attribute to the grain dielectric response; i.e., it is likely due to the dipolar relaxation of CCTO-CTO composite grains related to self-intertwined domains10,11 from the CCTO phase coupled to the dipole relaxation from the CTO phase. The characteristic frequency of this dipolar relaxation was found to be around 4.2 MHz, as indicated in Figs. 1�a� and 1�b�. The complex analysis of capacitance is useful to separate distinct relaxation processes and can sometimes be useful to separate different polarization effects that contribute to the global frequency response or to the total relaxation response.22–24 Based on this approach, an example of the usefulness of this methodology is the SnO2 non-Ohmic basic varistor system,13,14,22–24 in which the influence of trapping activity associated with the conductance term can be elimi- nated, as observed via the depression angle of a semicircular relaxation in the complex capacitance plane.13,22–24 This- methodology allows one to construct the “true” Mott- Schottky pattern,13,22–24 i.e., a frequency-independent Mott- FIG. 2. Comparison of impedance and capacitance complex diagrams in the low frequency region �from about 0.5 MHz to 40 Hz� of the frequency response data for the SnO2-based varistor system and the CCTO-CTO composite. �a� and �c� are, respectively, the impedance and capacitance complex diagrams for the SnO2-based varistor system. �b� and �d� are, re- spectively, the impedance and capacitance complex diagrams for the CCTO-CTO composite. 142912-2 Bueno et al. Appl. Phys. Lett. 90, 142912 �2007� This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 186.217.234.225 On: Tue, 14 Jan 2014 12:18:13 Schottky representation, which requires that the maximum slope of the straight-line behavior be attained. This implies reaching the minimum value of the Cgb, which is due only to the net/total geometric capacitance. As previously discussed, the CCTO-CTO composite possesses both non-Ohmic and good dielectric properties.19 Therefore, if one eliminates the dipolar dielectric response from the whole frequency response data �using an appropri- ate software after fitting to the theoretical function, see Fig. 1�, one can better visualize the response due only to the nonlinear electrical response �grain boundary response asso- ciated with the space charge polarization and Schottky-type potential barriers responsible for the highly non-Ohmic prop- erties�. The complex capacitance plane without the dielectric response of the CCTO-CTO composite is shown in Fig. 2�d� and compared with the usual non-Ohmic response of the SnO2-based polycrystalline system shown in Fig. 2�c�. The impedance diagrams in Figs. 2�a� and 2�b� correspond to the SnO2 and CCTO-CTO systems, respectively. The similarities between Figs. 2�c� and 2�d� lead one to infer that the range of frequencies of around 40 Hz to 0.1 MHz corresponds to the frequency region in which space charge relaxation occurs in the CCTO-CTO systems and that this region is equivalent to the one observed in SnO2-based non-Ohmic systems.13,15 This space charge relaxation is coupled to the dc grain boundary resistance Rgb; i.e., at lower frequencies, one finds that the conductive term �G /�� is related to Rgb �see Fig. 3�b��. At this point, it is important to emphasize that in ZnO- or SnO2-based varistor systems, at higher frequencies �usu- ally 106 to 109�, a resonance phenomenon emerges as a circle possessing negative values13,25 of the terminal parallel ca- pacitance in the C* plane. A detailed description of resonance events is given in Ref. 25. However, as stated previously, in the case of metal-oxide varistor/dielectric systems such as the CCTO-CTO composite described here, the high fre- quency region is dominated by the dipolar dielectric relax- ation of the grains, which is related to self-intertwined do- main structures.10,11 Figure 3 depicts the C-V or Mott-Schottky pattern con- structed from the grain boundary capacitance value as a function of bias voltage. The grain boundary capacitance val- ues were extracted from the fitting of the data to the theoret- ical relaxation function �see Fig. 1 to visualize this fitting�. This Mott-Schottky-type response of the C-V pattern proves the existence of the charge space region relating to potential barriers. Note that the equivalent circuit for this region con- siders the existence of trap effects �i.e., energy levels existing in the gap�, which is quite acceptable for ATiO3-based oxides,2 since low concentrations of oxygen vacancies �lower than 0.0002� cause a huge decrease in the grain’s resistivity.2 Figures 1�a� and 1�b� compare the fitting of the theory and the experimental data. In conclusion, dielectric/capacitance or impedance spec- troscopy can be used to separate the different types of relax- ation existing in CCTO-based polycrystalline ceramics, i.e., separate the contributions from dipolar relaxation and the space charge region. Dipolar relaxations likely occur due to self-intertwined domain structures in the bulk region of the CCTO phase coupled to the dipole’s relaxation in the bulk of the CTO phase, while space charge polarizations likely come from grain boundary regions due to Schottky-type potential barriers. It is important to stress that due to the composite’s low dielectric constant, the influence of self-intertwined do- main structures in the relaxation is not as strong as that ex- pected for the CCTO pure phase. This will be the subject of a future investigation. The financial support of this research project by the Bra- zilian research funding agencies CNPq and FAPESP is grate- fully acknowledged. 1M. A. Subramanian, D. Li, N. Duan, B. A. Reisner, and A. W. Sleight, J. Solid State Chem. 151, 323 �2000�. 2T. B. Adams, D. C. Sinclair, and A. R. West, Adv. Mater. �Weinheim, Ger.� 14, 1321 �2002�. 3A. F. L. Almeida, P. B. A. Fechine, J. C. Goes, M. A. Valente, M. A. R. Miranda, and A. S. B. Sombra, Mater. Sci. Eng., B 111, 113 �2004�. 4J. C. Jiang, E. I. Meletis, C. L. Chen, Y. Lin, Z. Zhang, and W. K. Chu, Philos. Mag. Lett. 84, 443 �2004�. 5T. B. Adams, D. C. Sinclair, and A. R. West, Phys. Rev. B 73, 094124 �2006�. 6G. L. Li, Z. Yin, and M. S. Zhang, Phys. Lett. A 344, 238 �2005�. 7J. 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Cilense, J. A. Varela, E. Longo, and E. R. Leite, Solid State Commun. 138, 1 �2006�. 18P. R. Bueno, M. A. Ramírez, J. A. Varela, and E. Longo, Appl. Phys. Lett. 89, 191117 �2006�. 19M. A. Ramírez, P. R. Bueno, J. A. Varela, and E. Longo, Appl. Phys. Lett. 89, 212102 �2006�. 20W. Kobayashi and I. Terasaki, Appl. Phys. Lett. 87, 032902 �2005�. 21C. J. F. Böttcher and P. Bordewijk, Theory of Electric Polarization: Di- electric in time-Dependent Fields �Elsevier, Amsterdam, 1992�, Vol. II, pp. 45–137. 22P. R. Bueno, J. A. Varela, and E. Longo, J. Eur. Ceram. Soc. �in press�. 23M. A. Alim, M. A. Seitz, and R. W. Hirthe, J. Appl. Phys. 63, 2337 �1988�. 24M. A. Alim, J. Appl. Phys. 78, 4776 �1995�. 25M. A. Alim, J. Appl. Phys. 74, 5850 �1993�. FIG. 3. �a� Mott-Schottky representation of the CCTO-CTO composite, which is an indication of the potential barrier in the grain boundary region responsible for the non-Ohmic properties found in this system. Cgb,0 is the grain boundary capacitance at zero potential bias. �b� Equivalent circuit representing the grain boundary region considering the influence of trapping states. GDR accounts for grain dielectric relaxation. Q is the constant phase element representing the distribution of trapping states or energy levels in the gap of the semiconductor. 142912-3 Bueno et al. Appl. Phys. Lett. 90, 142912 �2007� This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 186.217.234.225 On: Tue, 14 Jan 2014 12:18:13