Chan, HLW

Choy, CL

Zhang, Q

Zhou, Q

PolyU Institutional Repository

656 STRUCTURE OF NANOCRYSTALLINE POWDER AND THIN FILMS OF LEAD LANTHANUM TITANATE! PREPARED BY THE SOL-GEL PROCESS Q.F. ZHOU, H.L.W. CHAN, Q.Q.ZHANG and C.L. CHOY Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. Abstract Ultrafine powder and thin films of lead lanthanum titanate (PLT) were prepared by the sol-gel process. The structural variation of the powder with increasing annealing temperature was monitored by differential thermal analysis, thermogravimetric analysis and x-ray diffraction. The lowest temperature for the formation of nanocrystals in the powder was found to be about 500°C. Crystallites of various sizes were obtained by controlling the annealing temperature and time. Thin films of PLT were fabricated by spin casting the sol-gel solution onto Si(l11) substrates and the microstructure was analyzed using a scanning electron microscope. By controlling the concentration of the precursor and the processing temperature, PLT thin films with uniform crystallite size could be prepared. 1. Introduction Lead titanate (PbTiO, or PT) is a perovskite-type ferroelectric ceramic with a Curie temperature (T,) of 49OoC. It is a good candidate for pyroelectric infiared detector applications as it has large pyroelectric coefficient and relatively low dielectric permittivity. However, poling of bulk PbTi03 requires a high electric field because of its high T, and the large tetragonal distortion [ 11. After dopmg with lanthanum (La), the permittivity of the resulting lead lanthanum titanate (PLT) increases while T, and the tetragonality decrease with increasing La content. [2] The coercive field required to polarize the ceramic also drops and a pyroelectric coefficient larger than that of PT is observed. [3] In particular, (Pbo goLao lO)Ti03 (PLTI 0) was reported to have good pyroelectric property. [4] Preparation of PLT thin films by sputtering, pulsed laser deposition and the sol-gel process have attracted considerable interest.[S] The present authors are interested in producing nanocrystalline PLT powder since it is planned to incorporate them into a pyroelectric vinylidene fluorideitrifluoroethylene copolymer (P(VDF-TrFE)) matrix to form nano-composites. In this study, PLTlO powder and thin films were prepared by the sol-gel process. The structural variation of the powder with annealing temperature was studied by differential thermal analysis (DTA), thermogravimetric analysis (TGA) and x-ray difiaction (XRD). From the XRD data, the lowest temperature for the formation of PLTlO nanocrystallites was found and the crystallite size was evaluated using the Scherrer’s equation. The activation energy required for the conversion of amorphous phase into nanocrystalline phase was calculated from the DTA results. By controlling the concentration of the precursor, the spinning speed and spinning time, homogeneous PLTlO thin films were produced. CH35808/96/0-7803-2695-4$04 00 Copyright 1996 IEEE Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 30, 2008 at 23:03 from IEEE Xplore.  Restrictions apply.657 Cooled to room temp. Add stoichiometric amount of lanthanum nitrate 2. Exuerimental procedure in a 2% HF solution for 1 min. in order to remove the surface oxide layer. Using a 0.2 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 30, 2008 at 23:03 from IEEE Xplore.  Restrictions apply.658 between 2SOoC and 400°C, together with the 0 (2) PLTlO Gel 334 about 500°C, not accompanied by a significant weight loss, is h (‘Umin.) T m  (“c> 20 5 17.7 15 514.7 9% 50 150 250 350 450 550 h (aC/min.) T, (“C) 10 509.2 5 500.7 assumed to be associated with Temperature (OC) the crystallization of the powder. According to the Kissinger equation [q enfi/T,,,’) = - E/(RT,,,) + C (1) 90 where h is the heating rate, T, is the temperature at the exothermic peak near SOOOC, E is the activation energy, R is the gas constant and C is a constant. The DTA results of PLTlO powder at different heating rates are shown in Table 1. From the slope of the ln(h/Tm2 ) vs UT, curve, the activation energy E was found to be 39.6 KcaVmol. PLTlO Gel Scan rate IO deg’min 60 40 Temperature (OC) Fig. 2 Presenting (a) a DTA curve of PLT powder and (b) a TGA curve of PLT powder. Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 30, 2008 at 23:03 from IEEE Xplore.  Restrictions apply.659 .- 0 v) C aJ C c -400°C Fig. 3 shows the XRD patterns of PLT10 powder annealed at various temperatures for lh. The samples annealed below 450°C exhibits the typical amorphous pattem. When the annealing temperature reaches 5OO0C, crystalline peaks of PLTlO powder appear, in agreement with the DTA result. The average crystallite size of the powder at various annealing temperatures were calculated from the full width at half maximum (FWIIM) of the (IOl), (111) and (200) x-ray diffraction peaks using the Scherrer's equation [9] D =: (KA)/(BcosB) (2) where D is the crystallite diameter, h is the wavelength, 8 is the difiaction angle, B is the FWKM of a diffraction peak and K is Scherrer's constant (= 0.89). By comparison with the diffraction peaks of a standard material, Si, the width due to the instrument can be evaluated and the true width arising from finite crystallite size can be obtained. The crystallite size is given as a function of annealing temperature in Table 2. Fig. 3 XRD pattems of PL,T powder annealed at various temperatures for 1 Ihour. Table 2 Relationship between the average crystallite diameter and the annealing temperature ofPL.Tl0 nanocrystalline powder I 500 I 600 1 700 I 1 21.6 I 25.4 I 28.2 1 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 30, 2008 at 23:03 from IEEE Xplore.  Restrictions apply.660 The SEM micrographs of PLTlO thin films (Fig. 4) show that the film is homogeneous and crack-free. It contains nanocrystalline particles of diameter 50 to 80 nm. Fig. 5 shows the cross section of a five-layer film deposited on silicon, it can be seen that the film thickness is about 1.2 pm. Research on the growth of orientated thin films and pyroelectric property measurements are in progress. Fig. 4 (a) and (b) SEM micrographs with different scales, showing homogeneous PLT film prepared by the sol-gel process. Fig. 5 SEM micrograph of the cross section of a PLT film on Si 4. Conclusion PLTlO ultrafiie powder and thin films have been prepared by the sol-gel process. The lowest annealing temperature for the formation of nanocrystals in the powder is 500°C. The average crystallite size is below 25 nm when the annealing temperature is below 600'C. The activation energy of crystallization determined from DTA is 39.6 Kcdmol. SEM results show that PLTlO thin films annealed at 600°C for Ih are homogeneous and crack-free. Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 30, 2008 at 23:03 from IEEE Xplore.  Restrictions apply.661 ACKNOWLEDGEMENT This research was supportt:d by the Hong Kong Research Grant Council. REFERENCES Y. Xu, “Ferroelectric Materials and Their Applications”, (Elsevier Science Publishers, Amsterdam, 1991) G.H. Haertling, “Iml~roved hot-pressed electrooptical ceramics in the (Pb, La)(Zr, Ti)3 system”, J. Am. Ceraun. Soc., Vol. 54, pp. 303-309 , 1971. R. Takayama, Y. TiDmita, K. Lijima and I. Ueda, “Pyroelectric linear array infrared sensors made of c-axis-oriented La-modified PbTi03 thin films”, J. Appl. Phys., Vol. 63, T. Futakuchi and K. Tanino, “Pyroelect,ic Properties of La-modified PbTiOs Thin Films Prepared by Screen Printing”, Jpn. J. Appl. Phys., Vol. 33, pp. 5294-5296, 1994. Y.M. Kang, J.K. KUI and S. Baik, “Crystallographic characterization of tetragonal (Pb, La)TiO3 epitaxial thin films grown by pulsed laser depositiion”, J. Appl. Phys., Vol. 78, Q.F. Zhou, A.X. Kuang and T.S. Zhou, “Study of preparation and properties of PZT ceramic by sol-gel method”, Ferroelectric Letter, Vol. 14, pp.73-78 , 1992. Q.F. Zhou, H.L.W. Chan and C.L. Choy, “Nanocrystalline powder and fibres of lead zirconate titanate prepared by the sol-gel process”, J. Mater Pro. Tech., in press. H.E. Kissinger, “Reaction kinetics in differential thermal analysis” Anal. Chem., Vol. L.S. Birks and H. Friiedman, “Particle Size Determination from X-ray Line Broadening”, pp. 5868-5872,1988. pp. 2601-2606, 1995. 29, pp. 1502-1506, 1951. J. Appl. Phys., Vol. 17, pp.687-692, 1946. Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 30, 2008 at 23:03 from IEEE Xplore.  Restrictions apply.

Structure of nanocrystalline powder and thin films of lead lanthanum titanate prepared by the sol-gel process

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