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Nanocrystalline barium titanate powder with applications in the field of electronic components

Luminita Elena STIRBU*, Cosmin STIRBU*, Monica Anca CHITA*,

Marioara ABRUDEANU*

* University of Pitesti, Str. Târgul din Vale, Nr. 1, Piteşti, Argeş, 110040, Romania

Abstract. Barium titanate is widely used in electronic (multilayer chip condenser –MLCC, transducer, thermister) and electro-optical components. Nanocrystalline (n-) BaTiO3 has gained interest due to the modifications of the dielectric constant, the temperatures of phase transitions and the crystal structures that are associated with the small crystallite size. BaTiO3 – a perovskite type compound was prepared by splat cooling method using solar energy that permits to obtain nanomaterials, in the laboratories of Materials and Proceedings Institute Odeillo, France. We used X-ray diffraction and high-resolution microscopy to characterize the material.

1 Introduction

Barium titanate is widely used in microelectronic (multilayer chip condenser –MLCC, transducer, thermister, varistors in protection circuits in order to prevent thermal overload), electro-optical components and electroceramic industry. Nanocrystalline (n-) BaTiO3 has gained interest due to the modifications of the dielectric constant, the temperatures of phase transitions and the crystal structures that are associated with the small crystallite size. BaTiO3 is a typical and long studied ABO3 perovskite crystal compound. Above room temperature it has two kinds of phase configuration, cubic structure (when T > Tc) and tetragonal structure (when T < Tc), which respectively corresponds to paraelectricity and ferroelectricity[1,2].

Figure 1: The transition of tetragonal – cubic phase of BaTiO3 In BaTiO3, Ti is deplaced from its site to create a dipole. In the paraelectric BaTiO3 there is a random dipole orientations, and in the feroelectric form there is an aligned dipole orientations. Under an applied electric field dipole orientations can be reserved, i.e. the structure is polarisable. Dipoles tend to be “frozen in ” at room temperature, as increase temperature, thermal vibration increase the polarisability.

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2 Methods of synthesis

BaTiO3 – a perovskite type compound, was prepared by splat cooling method using solar energy, that permits to obtain nanomaterials, in the laboratories of Materials and Proceedings Institute Odeillo, France. The BaTiO3 has been prepared the splat-cooling method leads to nanomaterials (stripes or rough powders) depending on the viscosity of the melt that can be amorphous or nanostructured. We made a mixture from BaCO3 and TiO2 that was melted in the air at 900˚C. Nanosize powders can be obtained from that by ball milling. Bulk-nanomaterials can be prepared by sintering the powders. The method uses a 2 kW solar reactor. The splat-cooling method uses only the balls. The fig.2 below shows the working of solar furnace:

Figure 2: 2 kW solar furnance

With: P-paraboloid; F-focus; PR-Gate reactor (mobile); SM-Mobile support; OP-Obturator with palettes; PO-Gate obturator; L-Field glass; H-Heliostat; S- Support of heliostat; RI-Incident rays; 1-vertical movement (z); 2-horizontal movement (x); 3- horizontal movement (y).

By the three types of movement the material is bringed very easily in the focus zone. The measured energy is around 0,9-1kW/m2 in the middle of the day at the captivation plan of mirror. When the experiment is finished, it is a gliding trap with a gliding obturator that closes the access of solar rays to paraboloid.

We have prepared BaTiO3 nanomaterial by splat cooling – a method that use solar energy. A hammer makes the splat-cooling device, which shocks under air an oxide drop melted in the solar furnace focus (fig.3). The material is collected as ribbons chips or rough powders depending on the viscosity and thermal conductivity of the melted material. Nanostructured ultra fine powders can be obtained by ball milling of such quenched materials. By sintering in appropriate conditions, massive nanomaterials can be obtained.

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The splat cooling process has a high cooling rate of about 104 K˚/s. So, during the short cooling time all the mechanisms of mass transfers (mass diffusion, crystal growth…), structure transformations (amorphous-crystal structure, high temperature-low temperature transformations…) and chemical reactions (oxidation, demixion of phases...) are possibly blocked up.

Figure 3: Splat –cooling device Figure 4: Splat – cooling priciple

3 The analyze of the material

X-ray diffraction and high-resolution microscopy analyzed the material. a) Analyze by X-ray diffraction

The diffraction instrument is a Philips generator whose performances are remarkable. It offers a grand stability and allows obtaining clear spectrum for acquisition time. The compounds are ball milling to make X-ray diffraction analyze.

The crystalline structure is known, so we recognize the crystalline plans for every peak. So, it’s possible to determine crystalline parameter with Bragg law and interreticular distance[3].

Figure. 5: The X-ray diffraction spectrum of BaTiO3

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b) Characterization by High Resolution Electron Microscopy

The most precise method to determinate the diameters of the particles, is analyze by Transmission Electron Microscopy. We can observe, also, the distribution of the size, the morphology, faults of the structure [4]. The figure below shows the allure of the populations of particles for the compound obtained by splat cooling:

Figure 6: TEM image of BaTiO3 nano-powder

4. Conclusions

• This new solar method permits to obtain nanomaterial BaTiO3 without to add inhibitors to prohibit the grain grow. This fact is very important because the electrical properties of this material depend strongly on its microstructure(for example reaches its highest value as its grain size is around 1µm)

• The electrical resistivity, Curie temperature and the relative permitivity at Curie temperature are all affected by its microstructure.

• The splat cooling method offers a very good control of the chemical composition of the material. This technique is the first phase of syntheses followed, then, by a mechanical treatment (type ball milling) to tailor the size of the grains.

• The phase transition from cubic to tetragonal will not occur even at room temperature if the crystallites are smaller than about 120nm (like in our case). The lattice constants of tetragonal BaTiO3 also, depend on the annealing temperature. That adds another reason to obtain this nanomaterial by a solar method.

References [1] H. Hyuga, Y. Hayashi, T. Sekino, Fabrication process and electrical properties of BaTiO3/Ni

nanocomposites, Nanostructured Materials, Acta Metalurgica S.U.A., vol. 9, pp. 547-550. [2] Z. Mao, K. Knowles, Microstructural studies of strontium titanate dielectric ceramics, Plenum Press,

New York, 1998 [3] H.P. Beck, F. Muller, R. Haberkorn, D. Wilhelm. Synthesis of perovskite type compounds via different

routes and their X-ray characterization, Nanostructured Materials, Acta Metalurgica S.U.A., vol.6, 1995, pp. 659-662.

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[4] B. Gissibi, D. Wihelm, R. Wurschum, H. Herrig, F. Muller, M. Keisch, K. Reimann. Electron microscopy of nanocrystalline BaTiO3, Nanostructured Materials, Acta Metalurgica S.U.A., vol.9, 1997, pp. 619-622.

[5] A.V. Ragulya. Rate-controlled synthesis and sintering of sintering of nanocrystalline barium titanate powder, Nanostructured Materials, Acta Metalurgica S.U.A., vol.10, 1998 pp. 349-355.

[6] E. Brzozowski, M.S. Castro. Conduction mechanism of barium titanate ceramics, Ceramics International 26, 200, pp. 265-269.

Acknowledgments - The authors thank to the Materials Science Institute and Engineering of Proceedings ODEILLO and Perpignan University, FRANCE