BaTiO 3 thick fi lms obtained by tape casting from powders prepared by the oxalate route

BaTiO3 powders were prepared by co-precipitation via oxalate route. The size, morphology and particle size distribution of the oxalate powders have been optimized by the control of different synthesis parameters during the precipitation reaction (nature of salts, concentration of different solutions, aging time). The single phase BaTiO3 oxide particles were obtained after a thermal decomposition of the as-synthesized powders at 850°C for 4 hours under air atmosphere. Oxide powders with a suitable specifi c surface area were selected in order to obtain thick fi lms by the tape casting technique. The microstructure and dielectric properties of the thick fi lms varied obviously depending on the deposition-calcination-sintering cycle used. A double depositioncalcination cycle followed by sintering, as well as a two step deposition-calcination-sintering procedure was used in order to improve the compactness and therefore, the dielectric behaviour. A higher dielectric constant value (~ 750) and lower dielectric losses (~ 2 %) were achieved at room temperature and at 1 kHz frequency for the dense, double-deposited fi lm obtained after two deposition-calcination-sintering cycles. For this fi lm, a superior value of the dielectric constant (~ 1100), almost frequency independent in the frequency range of 100 Hz – 10 kHz was gained also at the ferroelectric-paraelectric phase transition temperature of 130°C.


I. Introduction
BaTiO 3 electronic ceramics are well-known for their high permittivity.BaTiO 3 compound is usually obtained by reaction of BaCO 3 with TiO 2 .This traditional way of processing, i.e. the solid/solid method, offers the advantage of its simplicity.However, high temperatures (T > 1000°C) are required to complete the reaction and prepare a single-phase material which has large particles, low surface area and uncontrolled microstructure.In order to overcome these disadvantages the low temperature, i.e. "soft chemistry" techniques have been developed.These methods permit to synthesize -at lower temperature -the same materials as the one observed at high temperature.Moreover, highly pure, submicronic, homogeneous and stoichio-metric powders can be obtained.The advantagеs of using powders with reduced grain size to prepare thick fi lms are increased specifi c area, improved reactivity and the possibility of lowering the sintering temperature.The chemical homogeneity, characterizing the powders prepared by low-temperature synthesis techniques, is a very important parameter for achieving reproducible properties.
The ongoing trend for higher integration in power electronic applications leads to the increase of the density of the components.While this operation can be achieved for power semiconductors, it is not the same for passive components.Obviously, their integration would be benefi cial for the increase of reliability and thermal and electrical performances, and decrease of the costs while having an improved design.This technology may be achieved using a multilayer ceramic construction combining different functional ceramics together in a 3D planar architecture [1][2][3][4][5][6].Such approach needs multidisciplinary competence in circuit design, materials manufacturing and characteri zation.We show in this paper the fi rst results obtained in this fi eld.These results are presented and discussed in details from a physical and chemical point of view to an electrical one.The role played by the different constituents and the infl uence of the synthesis process on the electrical properties of the fi nal products are also discussed.

Powder and thick fi lms preparation
The oxide powders were prepared by co-precipitation via oxalate route.The "precursor" powder (the mixed barium titanyl oxalate), was synthesized by pouring an aqueous solution of titanium chloride (TiCl 3 ) and barium chloride (BaCl 2 •2H 2 O) into an alcoholic solution of oxalic acid (H 2 C 2 O 4 ).Unlike the classical Clabauigh [7] procedure, when an aqueous solution of oxalic acid was used as precipitation agent, in this work the solvent was ethanol in order to increase the dielectric constant of the solution, leading to smaller particles [8].The ageing time has to be long enough (~ 5h 30 min) in order to complete the oxidation of the Ti 3+ ions into Ti 4+ .The precipitated oxalate was separated from the liquid phase by centrifugation in a HERAUS Sepatech centrifuge for 10 min at 3500 rot/min, then washed several times with ethanol and dried at 100°C for 16 hours.The oxalate powder was annealed in air with a plateau of 4h 30 min at various temperatures in the temperature range of 750-1050°C, in order to select the powder with the optimal characteristics for thick fi lm deposition.The synthesis fl owchart of the BaTiO 3 powders is presented in Fig. 1.
The fi rst step in depositing BaTiO 3 thick fi lms was preparation of a suspension with optimal viscosity (η ~ 2000 cP) by mixing suitable amount of BaTiO 3 powder with some adequate organic components (solvent, dispersant and plasticizer).The as-prepared suspension is deposited in two successive layers by tape-casting on Pt electrode-coated alumina substrate.After the drying of the deposit at 80°C for 15 min, three different thermal cycles were tried in order to obtain superior dielectric characteristics.The fi rst procedure consists of a calcination step of the deposit at 400°C, followed by sintering at 1200°C for 2 hours.The second procedure involves two successive deposition-calcination cycles followed by sintering in the same conditions and fi - nally the third procedure consists of two successive deposition-calcination-sintering cycles.After the deposition and thermal cycles, Ag top electrodes were deposited by screen-printing in order to perform dielectric measurements.

Powder and thick fi lms characterization
The morphology of the oxide powders as well as the surface and cross-section morphology of the thick fi lms were observed with a JEOL JSM-6400 scanning electron microscope (SEM) and with a JEOL 200CX transmission electron microscope (TEM).Particle size distribution was investigated by means of Malvern 2000 particle size analyzer.The phase composition of both oxide powder and thick fi lms was determined by X-ray diffraction analyses with a Brucker D500 diffractometer equipped with a Peltier effect counter and using Nifi ltered CuKα radiation (λ = 0.15418 nm) with a scan step increment of 0.03° and counting time of 1 s/step, for 2θ ranged between 20-80°.To estimate the structural characteristics (unit cell parameters), X-ray measurements were performed with a Seifert diffractometer, using a step increment of 0.02 o with a counting time of 10 s/step for 2θ ranged between 20-120°.
The Ba/Ti ratio was measured by means of X-ray fl uorescence using a Tracor X-Ray Spectrance 5000, in order to estimate the stoichiometry of the samples.The specifi c surface area was determined by the Brunauer, Emmet and Teller (BET) method using a Micrometrics Accusorb 2100 E .
Electrical measurements at various frequencies were performed on the thick fi lms by using a Hewlett Packard 4284 A LCR-meter.Temperature dependence of the dielectric constant in the temperature range 20-200°C was determined from capacitance measure ments performed on thick fi lms placed in a small oven and heated in air.The bias dependence of the capacitance was investigated in order to estimate the ferroelectric state of the fi lms. 1 V signal was used to measure the capacitance at 50 Hz as the bias voltage was ramped between -40 and +40 V.

Characteristics of BaTiO 3 powders
The mixed oxalate precursor powder, obtained by using ethanol as solvent, has a specifi c surface area of 14 m 2 /g and consists of small, uniform (in shape and size) and almost amorphous particles, with an average size of 70-80 nm.It has to be mentioned that for a given solvent there are also other processing factors, like aging time and synthesis temperature, which could affect the precursor particle size [9,10].
The thermal treatment parameters for the decomposition of the mixed barium titanyl oxalate precursor, mainly the thermal treatment temperature, can be tailored in order to obtain single phase, pure, fi ne, uni- For lower annealing temperatures (650-700°C), small amounts of secondary barium-or titanium-rich phases (BaCO 3 and BaTi 2 O 5 ) were detected beside the major BaTiO 3 phase with pseudocubic symmetry, proving that the solid state reactions are not complete in this temperature range.
The annealing temperature strongly infl uenced the average particle size, which varied from ~ 60 nm (for the powder annealed at 650°C) and ~ 100 nm (for the powders thermally treated at 750 and 850°C) to ~2 μm after annealing at 1050°C.The fi ne powders processed at lower temperatures exhibit a slight agglomeration tendency.It has to be mentioned also the bimodal particle distribution of BaTiO 3 powder, resulted after annealing at 950°C, which consists of fi ner particles of ~ 700 nm and larger particles with an average size of ~ 1.8 μm (Fig. 4).The linear decrease of the specifi c surface area against the annealing temperature for the analyzed powders is presented in Fig. 5.
The SEM and TEM images presented in Figs. 6 and 7a show the uniform morphology of the BaTiO 3 powder obtained after annealing at 850°C for 4 h 30 min in static air atmosphere.The particle size of ~ 100 nm, estimated from the TEM image, is in good agreement with the average particle size determined by the particle size scattering analysis (Fig. 4).The HRTEM analysis coupled with SAED (selected area diffraction electron) investigation pointed out the single-crystal nature of the powder particles (Fig. 7b,c).

Characteristics of BaTiO 3 thick fi lms
In order to prepare homogeneous suspensions and, therefore, to deposit thick fi lms with uniform thickness the single phase, stoichiometric BaTiO 3 powder with tetragonal structure, obtained after annealing in air at 850°C was considered as optimal.This powder had Ba/Ti ratio of 1.001 and specifi c surface area of ~ 6.5 m 2 /g.All thick fi lms prepared by three different procedures already mentioned above, were single phase.Thus, in the X-ray diffraction pattern of the thick fi lm deposited by the procedure no. 3, beside some peaks belonging to Al 2 O 3 substrate and Ag electrode, only the well-crystallized perovskite BaTiO 3 phase was identifi ed (Fig. 8).
Unlike the phase composition, the morphology and porosity of the BaTiO 3 thick fi lms were strongly infl uenced by the type of the deposition-calcination-sintering cycle.Thus, concerning the surface morphology, the SEM images presented in Fig. 9a-c indicated a clear tendency towards more dense microstructure and decreased porosity starting from the procedure no. 1 (with two deposition steps followed by calcination and sintering) to the procedure no. 3 (by two deposition -calcination -sintering cycles).The fi lms prepared by the procedure no. 1 and 2 show almost similar average grain size (of ~ 0.60 μm and 0.65 μm, respectively) (Fig. 9a,b).However, a higher compactness and a lower porosity were observed in the case of the thick fi lm obtained by two different deposition -calcination cycles, followed by sintering (the procedure no.2).Surprisingly, for the very compact and poreless thick fi lm obtained by the procedure no. 3, a lower average grain size (of only 0.29 μm) was estimated on the fi lm surface (Fig. 9c).In order to elucidate this aspect, further cross-section SEM investigations were performed.The cross-section SEM images showed that the transversal morphology of the BaTiO 3 thick fi lms prepared by the procedure no. 1 and 2 was uniform (Fig. 10a,b, respectively), so we concluded that the double calcination acts only in the sense of the porosity decrease and does not infl uence the average grain size.Unlike these samples, in the case of the Ba-TiO 3 thick fi lms obtained by two successive deposition -calcination -sintering cycles (the procedure no.3), a non-uniform transversal microstructure with a hetero- geneity of the grain size across the fi lm was pointed out.This microstructure induced by the double sintering process consists of two different layers clearly delimited by the different porosity and grain size, which decreased starting from the bottom layer to the top one (Fig. 10c).The presence of fi ner grains in the surface layer of the thick fi lm obtained by the procedure no.
3 suggests that the interface between the two deposits may act as an active, discontinuous nucleation surface for the crystallization of the second layer.In other words, the nucleation process is more favourable than the crystal growth, at least for the crystallization of the top deposit of this thick fi lm.This feature was also noticed in the case of the multilayer BaTiO 3 thin fi lms prepared by several rf-sputtering deposition -annealing cycles [11].The higher compactness induced by the particular transversal microstructure of the thick fi lm prepared by the procedure no. 3 determines the improvement of the dielectric behaviour.Thus, the room temperature dielectric losses at 1 kHz frequency decrease from ~ 25 % for sample 1 to 11 % for the sample 2 and to ~ 2 % for the sample 3. The evolution of the relative permittivity against the temperature at 1 kHz is quite similar for the samples 1 and 2 (Fig. 11).However, slightly higher values of the dielectric constant were obtained for the sample 2, mainly at the ferroelectricparaelectric phase transition temperature (ε r = 576 for the sample 1 in comparison with ε r = 652 for the sample 2).At the same frequency, a sharper ε r -T dependence with a well-marked maximum at the transition temperature and signifi cantly higher values of the dielectric constant in whole temperature range were recorded for the thick fi lm obtained by the procedure no. 3 (Fig. 11).In this case, the room temperature value of the dielectric permittivity is ε r = 740 and the relative permittivity at the transition temperature is ε r ~ 1100.No change in the value of the phase transition temperature (T C = 130°C) was noticed for three thick fi lms analyzed in this work.The capacitance variation with the dc bias is associated with the domain reorientation process [12].Fig. 12 shows the voltage-dependent capacitance measured at 50 Hz for the BaTiO 3 thick fi lm obtained by the procedure no. 3. A typical "butterfl y" hysteresis in the C -V plot, specifi c for the up-and down-ramped dc bias was recorded for this sample, indicating the ferroelectric nature of the thick fi lm.

IV. Conclusions
Single phase, stoichiometric BaTiO 3 powders were prepared by co-precipitation via oxalate route after the thermal treatment of the mixed barium titanyl oxalate precursor in air, at temperatures higher than 750°C, with 4 h 30 min plateau.Oxide powders with a suitable specifi c surface area were selected in order to obtain thick fi lms by the tape casting technique.The microstructure and dielectric properties of the thick fi lms varied obviously depending on deposition-calcination-sintering cycle used.A double deposition-calcination cycle followed by sintering, as well as two step depositioncalcination-sintering procedure were used in order to improve the compactness and therefore, dielectric behaviour.The double sintering process rather than the double calcination has a signifi cant favourable effect on dielectric properties of the BaTiO 3 thick fi lms.A higher dielectric constant value (of ~ 740) and lower dielectric losses (of ~ 2 %) were obtained at room temperature and at 1 kHz frequency for the dense, double-deposited fi lm obtained after two deposition-calcination-sintering cycles.For this fi lm, a superior value of the dielectric constant (of ~ 1100), almost frequency independent in the frequency range of 100 Hz -10 kHz was also obtained at the ferroelectric-paraelectric phase transition temperature of 130°C.

FigureFigure 4 .
Figure 2. Room-temperature X-ray diffraction patterns for BaTiO 3 powders thermally treated in air for 4 h 30 min at various temperatures

Figure 5 .Figure 7 .
Figure 5. Evolution of the specifi c surface area of BaTiO 3 powders versus the thermal treatment temperature