Pyroelectric current spectroscopy: example of application on Nb doped Pb(Zr0.92Ti0.08)O3 ceramics for infrared detection
Introduction
Lead zirconate–titanate (PZT) ceramics are known to possess very good pyroelectric properties, making them attractive for the infrared (IR) detection [1]. The main parameter defining the quality of such a material is the pyroelectric coefficient [1] that is defined as the temperature derivative of the spontaneous polarization (p=dPS/dT). In case of the PZT system the value of the pyroelectric coefficient is strongly dependent on the composition. Materials with different Zr/Ti ratio will have different transition temperatures, thus different dependencies of spontaneous polarization versus temperature will be obtained. That leads to different pyroelectric coefficients. The doping elements such as La or Nb also affect the ferroelectric transition temperature and the pyroelectric coefficient. A non-homogeneous distribution of the Zr/Ti ratio or of the doping elements inside a PZT ceramic sample can lead to different properties than those expected when the ceramic was prepared. The problem arises from the fact that different compositions mean different transition temperatures, thus different values of the pyroelectric coefficient in different regions of the samples [2]. All these mean that pyroelectric IR detectors manufactured from the same wafer of bulk ceramic could have different detection characteristics. Thus, such non-homogeneity is not desirable in case of mass production because it can decrease the yield and increase the costs.
In this work, we present a simple method to investigate the quality of a PZT ceramic from the point of view of its pyroelectric properties. The method is based on pyroelectric current measurement and is called Pyroelectric Current Spectroscopy (PCS). The samples selected for this study are PZT ceramics with a Zr/Ti ratio of 92/8 doped with 2 and 6 at.% Nb. This specific Zr/Ti ratio was chosen because it presents two ferroelectric phase transitions: (1) a secondary transition between the two rhombohedral phases of the PZT system (FRH(LT)–FRH(HT)), that should be at around 65 °C; (2) the main transition from the high temperature rhombohedral ferroelectric phase to the cubic paraelectric one (FRH(HT)–PC), that it is at about 245 °C [2]. Due to the low temperature secondary phase transition it is expected to obtain higher values of the pyroelectric coefficient at room temperature. The Nb was added to the PZT system in order to improve the densification of the ceramic [3]. It was thought that the Pb vacancies introduced by the Nb doping would facilitate the solid-state reactions, leading to ceramics with higher density. Also, the Nb doping seems to alter both transition temperatures [4], [5].
Section snippets
Theory
The basic formula for pyroelectric current measurements is [6]where p is the pyroelectric coefficient, A the electrode area, and dT/dt is the heating rate. Assuming a second order phase transition [7], with a continuous temperature decreasing of the spontaneous polarization (it becomes zero at the transition temperature), then the temperature variation of the pyroelectric coefficient is [1]where γ and β are the coefficients of the free energy polynomial expansion in the
Experiments
The ceramic samples were prepared by using the standard ceramic technology. The basic formula was Pb1−(y/2)(Zr0.92Ti0.08)1−yNbyO3, where y is the Nb content in at.% (y=2 and 6). The starting materials were PbCO3, TiO2, ZrO2, and Nb2O5. No lead excess was used. The powders were calcinated at 875 °C for 2 h. The sintering of the pressed pellets was performed at 1200 °C for 40 min. The calcination and sintering parameters were intently chosen to obtain ceramics with non-homogeneous composition.
The
Results
The result of the XRD analysis are presented in Fig. 1. The XRD spectra reveal the presence of the perovskite phase peaks in all the samples. A secondary fluorite phase is also present. The amount of this phase increases with increasing Nb content.
Pyroelectric current measurements were then performed in a temperature range covering the entire transition spectrum of the PZT system. The results are presented in Fig. 2.
Both spectra presented in Fig. 2 can be divided in two parts. The low
Discussions
The discussion will focus only on the correlation between composition analysis and pyroelectric current measurements. It is not the purpose of this paper to conclude over the quality of the ceramic samples and their IR detection properties.
According with the nominal composition two ferroelectric transitions were expected to occur in the pyroelectric current spectra. Some results already published on the same composition show that for 2 at.% Nb the temperature of the FRH(LT)–FRH(HT) transition
Conclusions
The basic principles of the pyroelectric current spectroscopy are presented and the method is applied for exemplification on PZT(92/8) ceramics doped with 2 and 6 at.% Nb. The ceramic samples were intently prepared at shorter calcination times in order to obtain “islands” with different compositions. It is shown that information about the ceramic composition homogeneity can be extracted by analyzing the pyroelectric current spectra. Combined SEM-EDAX analysis was performed to support the above
Acknowledgements
The authors acknowledge the financial support from FCT-Portugal (project POCTI/CTM/12140/1998). I. Boerasu thanks FCT for the grant PRAXIS XXI/BD/21539/99. L. Pintilie acknowledges the INVOTAN grant at the University of Minho.
Lucian Pintilie received Graduation degree from the Faculty of Physics, Bucharest University, in 1984. Doctor in Physics, specialty Solid State Physics, since 1995. Senior researcher rank 1 at the National Institute of Materials Physics, Bucharest, Romania. Head of semiconductor physics laboratory since 1997. Specialized in ferroelectric materials characterization, electric/photoelectric characterization of complex dielectric–semiconductor heterostructures, trap investigation (DLTS, TSC, OCS),
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Lucian Pintilie received Graduation degree from the Faculty of Physics, Bucharest University, in 1984. Doctor in Physics, specialty Solid State Physics, since 1995. Senior researcher rank 1 at the National Institute of Materials Physics, Bucharest, Romania. Head of semiconductor physics laboratory since 1997. Specialized in ferroelectric materials characterization, electric/photoelectric characterization of complex dielectric–semiconductor heterostructures, trap investigation (DLTS, TSC, OCS), and IR detectors (pyroelectric, PbS). Director of several national and international projects. NATO fellowship at University do Minho, Braga, Portugal (2002–2003). Work stages as invited researcher at MPI, Halle, Germany (1997), and IKZ, Berlin, Germany (2001–2002). Member of the Romanian Society of Crystal Growth-Advanced Materials. Author or co-author of over 70 publications in refereed journals or conference proceedings.
Mario Pereira is assistant professor in the Physics Department at the University of Minho (Portugal). He received his Bachelor and PhD degrees in Science and Materials Engineering from University of Limoges (France). His research focus is in developing ceramic materials, particularly as oriented thin films and improving the chemical processing of ceramic materials. Research in the pyroelectrics area has included piezoelectric and ferroelectric materials and improvement of fatigue resistance.
Maria J.M. Gomes received her Bachelor degree from the Faculty of Science and Technology of University of Coimbra (Portugal) in 1980. Doctor in Physics of Solids from the University Louis Pasteur (Strasbourg I—France), in 1990. Associate Professor since 1997 at the Physics Department of the University of Minho. Her research interests are in semiconductor nanocrystals and ferroelectric ceramic materials, with a research program emphasizing two major independent areas: growth of semiconductor-doped glass and ceramic films using PVD (rf-magnetron sputtering and laser ablation) and sol–gel techniques, as well as optical, photoelectrical, and electrical investigation. Coordinator of several national scientific projects and several international collaborative research projects. Member of the Portuguese Physical Society. Co-author of about 65 publications in referred international journals or conference proceedings.
Iulian Boerasu graduated from the Faculty of Physics, Bucharest University, in 1996. PhD degree in Physics from the University of Minho, in 2003. Currently, junior researcher at the National Institute of Materials Physics, Bucharest, Romania. Staff member of the Semiconductor Physics Laboratory since 1997. Specialized in design and preparation of ferroelectric thin films, materials characterization, and electric/photoelectric characterization of metal–ferroelectric structures. Active member of several national and international projects. Individual PhD grant from FCT-Portugal to prepare the PhD thesis at University of Minho, Braga (since 2000). Work stages at JSI, Ljubljana, Slovenia (1999). Board member of the Romanian Society of Crystal Growth-Advanced Materials. Author or co-author of 20 publications in refereed journals or conference proceedings.