Integrated Ferroelectrics Publication details, including instructions for authors and subscription information:

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1 This article was downloaded by: [University of South Carolina] On: 17 November 2011, At: 10:04 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Integrated Ferroelectrics Publication details, including instructions for authors and subscription information: INTEGRATION OF FERROELECTRIC BaTiO 3 THIN FILMS DIRECTLY ON NI AND TI METALLIC TAPES FOR STRUCTURAL HEALTH MONITORING SYSTEMS AND ENERGY HARVEST APPLICATIONS J. WEAVER a, Z. YUAN a, J. LIU a, G. COLLINS a, C. L. CHEN a b, J. C. JIANG c, J. HE c, E. I. MELETIS c, R. Y. GUO d, A. BHALLA d, B. LIN e, V. GIURGIUTIU e & M. W. COLE f a Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas, 78249, USA b The TcSUH, the University of Houston, Houston, TX, 77204, USA c Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, Texas, 76019, USA d Department of Electrical & Computer Engineering, University of Texas at San Antonio, San Antonio, Texas, 78249, USA e Department of Mechanical Engineering, University of South Carolina, Columbia, SC, 29208, USA f US Army Research Laboratory, Aberdeen Proving Ground, MD, 21005, USA

2 Available online: 20 Sep 2010 To cite this article: J. WEAVER, Z. YUAN, J. LIU, G. COLLINS, C. L. CHEN, J. C. JIANG, J. HE, E. I. MELETIS, R. Y. GUO, A. BHALLA, B. LIN, V. GIURGIUTIU & M. W. COLE (2008): INTEGRATION OF FERROELECTRIC BaTiO 3 THIN FILMS DIRECTLY ON NI AND TI METALLIC TAPES FOR STRUCTURAL HEALTH MONITORING SYSTEMS AND ENERGY HARVEST APPLICATIONS, Integrated Ferroelectrics, 100:1, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

3 Integrated Ferroelectrics, 100: 61 71, 2008 Copyright Taylor & Francis Group, LLC ISSN print / online DOI: / Integration of Ferroelectric BaTiO 3 Thin Films Directly on Ni and Ti Metallic Tapes for Structural Health Monitoring Systems and Energy Harvest Applications J. Weaver, 1 Z. Yuan, 1 J. Liu, 1 G. Collins, 1 C. L. Chen, 2, J. C. Jiang, 3 J. He, 3 E. I. Meletis, 3 R. Y. Guo, 4 A. Bhalla, 4 B. Lin, 5 V. Giurgiutiu, 5 and M. W. Cole 6 1 Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249, USA 2 Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249, USA, and the TcSUH, the University of Houston, Houston, TX 77204, USA 3 Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, Texas 76019, USA 4 Department of Electrical & Computer Engineering, University of Texas at San Antonio, San Antonio, Texas 78249, USA 5 Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA 6 US Army Research Laboratory, Aberdeen Proving Ground, MD 21005, USA ABSTRACT Ferroelectric BaTiO 3 thin films were fabricated directly on various metallic substrate materials such as Ni and Ti by using the pulsed laser for the development of structural health monitoring systems and energy harvest applications. Microstructure studies from x-ray diffraction and electron microscopy indicate that the as-grown BaTiO 3 thin films have pure BaTiO 3 crystal phase. The TEM studies indicate that the BaTiO 3 films are composed of crystalline assemblage of nanopillars with average cross sections from 100 nm to 200 nm and can be directly integrated on the Ni tapes without the formation of NiO interlayer suggesting that this system can be developed for super-capacitor devices. The BaTiO 3 films have good interface structures and strong adhesion with respect to Ni and Ti substrates. Dielectric measurements have shown the hysteresis loop at room temperature in the film with a large remanent polarization, indicating that the Received August 26, Corresponding author. cl.chen@utsa.edu 61

4 62 J. Weaver et al. ferroelectric domains have been created in the as-deposited BTO films. The successful integration of ferroelectric thin films directly on metallic materials has promised for the development of the structural health monitoring systems and energy harvest devices. Keywords: ferroelectric films, metallic substrate, microstructure, interface, integration INTRODUCTION Barium titanate, BaTiO 3 (BTO), is one of the most important ferroelectric materials, and has attracted much attention for its remarkable properties such as high dielectric constant, good ferroelectric properties, and large electrooptic and non-linear optic coefficients. Especially, this material has excellent piezoelectric properties resulting in the broad applications in the control systems. BTO thin films have been deposited on various substrates using a broad spectrum of techniques to provide a platform for many device applications as ferroelectric random access memories, optical modulators, waveguides and microelectromagnetic systems (MEMS). On the other hand, ferroelectric BTO thin film displayed excellent piezoelectric properties, similar to its single crystal ferroelectrics. These unique properties result in BTO thin films becoming great candidates for the development of the unobtrusive piezoelectric wafer active sensor arrays for structural health monitoring. However, major challenges exist in extending this technology to structural health monitoring since the fabrication of the ferroelectric BTO thin films with promised physical property on structural materials (steel, nickel, aluminum, etc.) has not been achieved yet, which will raise a number of crystallinity and interface-related challenges. Recently, piezoelectric wafer active sensors with the ability for sending and receiving guided Lamb/Rayleigh waves to detect the presence of incipient cracks and structural damage demonstrates a valuable tool in structural health monitoring [1, 2] which can be used in various applications from aerospace to civil infrastructure. Ferroelectric BTO has remarkable properties such as high dielectric constant, good ferroelectric properties, large electro-optic and non-linear optic coefficients [3 7]. Ferroelectric BTO thin films are considered promising materials for many device applications such as ferroelectric random access memories, optical modulators and switches, waveguides and microelectromechanical systems [8 10]. Especially, their excellent piezoelectric properties make them great candidates for developing unobtrusive piezoelectric wafer active sensor arrays for structural health monitoring [11, 12]. However, methods for installing the piezoelectric wafer active sensors on metallic structures are adhesively bonding the pre-manufactured piezoelectric wafers to the structural surface. The downside of these techniques is that the bonding layer is likely to be influenced by environmental ingression that may lead to loss of contact with the structural substrate, as well as inducing acoustic impedance mismatch with detrimental effects on damage detection. To overcome such drawbacks, we have

5 BaTiO 3 Thin Films on Ni and Ti 63 developed a unique technique to directly integrate ferroelectric/piezoelectric thin films for embedded active sensors on metallic structures. BTO thin films have been deposited on various substrates including oxide single crystal and semiconductor substrates using a variety of techniques such as pulsed laser deposition (PLD), hydrothermal method, sol-gel processing, solid-state reactions, and metal-organic chemical vapor deposition [5, 13 18]. However, many challenges, especially the interface-related issue, exist in fabricating ferroelectric thin films on structural materials (steel, aluminum, titanium, etc.) for structural health monitoring. Literature regarding the fabrication of ferroelectric thin films on structural materials were not available until the reports of our recent achievements of in-situ fabrication of BTO on the typical structural material Ni using PLD system [19, 20]. In this paper, we report our recent achievements on the fabrication of ferroelectric BTO thin films directly on both Ni and Ti substrates, and the studies of the crystallinity, microstructures, interfaces, and physical properties of BaTiO 3 thin films on Ni and Ti substrates. EXPERIMENTAL BaTiO 3 thin films were deposited on Ni and Ti substrates in a PLD system using a KrF excimer laser with a wavelength of 248 nm with an energy density of about 2.5 J/cm 2 and a laser repetition rate of 5Hz. The BTO thin films were fabricated with various conditions, details can be found from the literatures [19, 20]. X-ray diffraction (XRD) was employed to understand the crystal phases and the transmission electron microscopy (TEM), plan-view and cross-section, were employed to study the microstructure of the as-grown films and interfacial layers. The dielectric properties were characterized by using a Radiant RT6000 for understanding the physical properties of the as-grown films. RESULTS AND DISCUSSIONS Figure 1 (a) is the XRD θ 2θ pattern from the as-deposited BTO thin film on Ni which shows all the peaks are from the polycrystalline BTO phases and polycrystalline Ni substrate. These peak positions suggest that the Ni substrate is cubic phase and the BTO layer belongs the tetragonal phase. The relative stronger intensity from the (200) diffraction in the BTO film indicates than that the BTO film has a slightly preferred c-axis oriented rather than a randomly oriented grains. Figure 1 (a) is a θ 2θ XRD pattern of the as-deposited BTO thin film on Ti exhibiting peaks from BTO, Rutile TiO 2 and Ti. The BTO films on Ti were found to have the tetragonal structure with lattice constant a = 4.00Å and c = 4.03 Å. The rutile TiO 2 has a lattice constant a = 4.61 Å and c = 2.97 Å. The Ti substrate has a structure of α-ti and the lattice constant was found to be a = 2.97 Å and c = 4.78 Å, which is about 1% larger than the

6 64 J. Weaver et al. Figure 1. θ 2θ XRD pattern of the BTO films (a) on Ni and (b) on Ti substrate. lattice constant of pure Ti (a = 2.95 Å and c = 4.68 Å). Such a lattice expansion might be understood as the induction of the O atoms into the Ti lattice. The interface structures of the BTO films on Ni and Ti substrates were studied by using both cross sectional and plan view TEM techniques. Figure 2(a) is a bright-field TEM image with an inset of the selected-area electron diffraction (SAED) pattern of a plan-view BTO/NiO/Ni sample showing a crystalline grain structure of the films. The inset SAED pattern of the film shows sharp diffraction rings indicating that all the grains are randomly oriented crystalline whose structure can be identified as a tetragonal structure with a

7 BaTiO 3 Thin Films on Ni and Ti 65 Figure 2. (a) Cross-section TEM image of a cross-section TEM sample of BTO/Ni. (b) space group of p4 mm and lattice parameter of a = 3.992Å and c = Å. For example, the inner 6 diffraction rings 1, 2, 3, 4, 5 and 6 have a lattice spacing of 4.0 Å, 2.8 Å, 2.3 Å, 2.0 Å, 1.8 Å, 1.64 Å and 1.4 Å, respectively, which can be identified as the (001), (101), (111), (002), (102) and (112) reflection

8 66 J. Weaver et al. of tetragonal BTO. Figure 2(b) shows a cross-sectional TEM image of the BTO/NiO/Ni sample (inset) and the area near the interface. The BTO film has a thickness of about 500 nm and consists of nanopillar structures. Most of the nanopillars extend from the film/substrate interface to the film surface with a length of about 500 nm and show a lateral width from 30 nm to 100 nm (inset), which is close to the value obtained from the plan-view TEM. Near the interface area, an intermediate layer that can be identified as NiO was observed between the BTO film and Ni substrate (Figure 2b) indicating that a NiO oxidized layer was successfully produced during the prior to the deposition of BTO film. The BTO film is found to be very well bound the NiO layer with a sharp interface in between. The NiO layer has a thickness of about 100 nm and a clear interface with respect to the Ni substrate. It is interesting to note that the ferroelectric BTO films were found to be directly grown on the nano Ni surface without the NiO interlayer by nano fabrication technique. As seen in Figure 3(a), a cross-sectional TEM image shows the interface structure of the BTO films on Ni tapes. The BTO film has a thickness of about 200 nm and consists of nanopillar structures with lateral dimensions of about 100 nm, which is close to the value obtained from the plan-view TEM. The intermediate (IM) layer between the BTO film and Ni substrate shows nanocrystalline structure. Figure 3(b) is a plan-view TEM image and the SAED pattern (inset) of the IM layer. The grain size of the nanostructures varies from 30 to 100 nm in diameter and is smaller than that of the BTO grains. The nanostructures in the IM layer were found to be pure Ni (fcc, a = 3.52 Å) as identified by the electron diffraction analysis. For example, diffraction rings 1, 2, 3 and 4 have a lattice spacing of 2.03 Å, 1.76 Å, 1.25 Å and 1.06 Å, respectively, which can be identified as the (111), (200), (220) and (311) reflection of Ni. This great achievement suggests that the BTO films directly on Ni tape with no NiO dead layer has paved a way to develop supercapacitance devices for the energy harvest applications. Figure 4(a) is a cross-section TEM image of the BTO/Ti clearly showing a very rough surface of the film and the nanopillar structures in the films, which is very similar to the structures as seen in the Ni substrates. The BTO film has a thickness of about 700 nm that is close to the data obtained using SEM. The nanopillars have a lateral dimension of about 200 nm. Figure 4(b) shows a planview TEM image with an inset of a SAED pattern of the BTO film exhibiting formation and distribution of nanopillars in the film plane. The cross-sections of the nanopillars in this film vary from 100 nm to 200 nm. Presence of sharp diffraction rings in the SAED pattern demonstrates well-crystallized nanopillars in the BTO film. Electron diffraction analysis confirmed that the structure of the nanopillars is a tetragonal structure, the same as that determined by the XRD. The ferroelectric property measurements were also performed at room temperature, as seen Figure 5, for the BTO films on both Ni and Ti substrates. It is surprisingly found that the as-grown BTO films on Ni metal tapes with a NiO buffered layer exhibit very high resistivity of cm. The ferro-

9 BaTiO 3 Thin Films on Ni and Ti 67 Figure 3. (a) Cross-section and (b) plan-view TEM image of the BTO films on Ni with no NiO interlayer. Inset is SAED pattern of (b). electricity of the BTO films was evidenced from the hysteresis loop. The room temperature spontaneous polarization, remnant polarization, and coercive field from the as-deposited BTO layer can be obtained to be about 2.0 µc/cm 2 and 1.0 µc/cm 2, respectively, with a coercive field of 25 kv/cm. It is known that the

10 68 J. Weaver et al. Figure 4. (a) Cross-section and (b) plan-view TEM image of the BTO films on Ti. Inset is SAED pattern of (b). lattice dipole along the c-axis for a tetragonal perovskite structure is the origin of the ferroelectric properties associated with BTO. In other words, the ferroelectric dipole originates from ionic displacement in the c-axis direction, only c-axis oriented BTO thin films exhibit ferroelectricity [11]. The a-axis oriented BTO film cannot show ferroelectric hysteresis due to the randomly oriented polarization [4], the large spontaneous polarization obtained in the as-deposited film is consistent with the result of the microstructure measurement that the film has highly c-axis oriented texture structure. The piezoelectric response of the as-deposited BTO film was surprisingly found to be 130 ( C/N)

11 BaTiO 3 Thin Films on Ni and Ti 69 Figure 5. Dielectric property measurement showing the hysteresis loop achieved on the ferroelectric BaTiO 3 thin films on Ni and Ti. which is about 30% larger than the values ( C/N) of BTO single crystalline and polycrystalline bulk materials [12]. The large piezoelectric response might result from the uniform nanodomain structures [13]. The nature of the mechanisms is under investigation and will be reported later on. However, the break down voltage for the BTO films on Ti seems to be much lower than that on the Ni. This probably is due to the formation of a thin TiO layer. As seen in Figure 5 (b), the dielectric property measurements performed at room temperature indicates the ferroelectricity of the as-grown BTO films with the spontaneous polarization of 13 µc/cm 2 and the maximum breakdown voltage of only 1.3 KV/cm. Overall, these results suggest that the BTO films

12 70 J. Weaver et al. on Ni tapes can have better dielectric property than that on Ti substrate for the device developments. In summary, we have demonstrated achievability to grow ferroelectric BaTiO 3 thinfilms directly on both Ni and Ti substrates by optimizing the growth parameters and conditions. The as-deposited BTO films have nanopillar, crystalline tetragonal structures with a good interface with respect to the substrate. TEM studies reveal that BaTiO 3 films are composed of crystalline assemblage of nanopillars with average cross sections from 100 nm to 200 nm. The BaTiO 3 films have good interface structures and strong adhesion with respect to both Ni and Ti substrates. The room temperature ferroelectric polarization measurements show that the ferroelectric domains have been created in the as-deposited BTO films. Successful fabrication of such ferroelectric films on the metallic substrates has significant importance for the development of new applications such as structural health monitoring spanning from aerospace to civil infrastructures. The work can be extended to integrate other ferroelectric oxide films with various promising properties to monitor the structural health materials and the energy harvest applications. This work is partially supported by the National Science Foundation under Award Number NSF/CMS and NSF/CMMI , the Texas Advanced Program of the State of Texas, and the State of Texas through the Texas Center for Superconductivity at the University of Houston. REFERENCES 1. H. W. Park, H. Sohn, K. H. Law, and C. R. Farrar, J. Sound and Vibration 302, 50 (2007). 2. V. Giurgiutiu and A. Cuc, The Shock and Vibration Digest 37, 83 (2005). 3. K. Iijima, T. Terashima, K. Yamamoto, K. Hirata, and Y. Bando, Appl. Phys. Lett. 56, 527 (1990). 4. D. Roy and S. B. Krupanidhi, Appl. Phys. Lett. 61, 2057 (1992). 5. K. Nashimoto, D. K. Fork, F. A. Ponce, and J. C. Tramontana, Jpn. J. Appl. Phys. 32, 4099 (1992). 6. H. A. Lu, L. A. Wills, and B. W. Wessels, Appl. Phys. Lett. 64, 2973 (1994). 7. T. M. Shaw, S. Trolier-McKinstry, and P. C. McIntyre, Annu. Rev. Mater. Sci. 30, 263 (2000). 8. D. L. Polla and L. F. Francis, Annu. Rev. Mater. Sci. 28, 563 (1998). 9. L. R. Dalton, Nature 359, 269 (1992). 10. P. Muralt, J. Micromech. Microeng 10, 136 (2000). 11. B. Fan, G. Song, and F. Hussain, Smart Materials & Structures 14, 400 (2005). 12. V. Sethi and G. Song, J. Vibration and Control, 11, 671 (2005). 13. G. M. Davis and M.C. Gower, Appl. Phys. Lett. 55, 112 (1989).

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