RU2614916C1 - Method for bismuth titanate thin layers production - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method relates to the technology of ferroelectric coatings manufacturing on conductive surfaces, in particular, thin layers of bismuth titanate on titanium, and can be used to create dielectric layers as a photorefractive material in devices for data recording and processing, in thin film capacitors, for piezoelectric ceramic manufacture, etc. The method comprises titanium product surface treatment by plasma electrolytic oxidation in an electrolyte containing 0.1-0.2 M of Na₂B₄O₇ tetraborate in galvanostatic mode at anode polarization for 10-15 minutes at an effective current density of 0.20-0.25 A/cm² to form a titanium oxide layer, which is then impregnated with a solution of primary bismuth nitrate with molten rosin, turpentine diluted, and calcined at a temperature of 650-700°C for 0.5-1.0 hours to provide a thin layer containing Bi₄Ti₃O₁₂.

EFFECT: simplified method technological scheme and reduced time spent on its implementation.

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Description

The method relates to the field of manufacturing technology of ferroelectric coatings on conductive surfaces, in particular, thin layers of bismuth titanate on titanium. Bismuth titanates Bi ₂ Ti ₂ O ₇ , Bi ₁₂ TiO ₂₀ , Bi ₄ Ti ₃ O ₁₂ ) exhibit photocatalytic properties in the optical range (in visible light). Due to the high dielectric constant, they are used to create dielectric layers, as photorefractive material in recording and processing devices, in thin-film capacitors, in the manufacture of piezoceramics, etc.

Known methods for producing bismuth titanate are for the most part expensive, require special sophisticated equipment, and do not always provide reproducible results.

A known method of obtaining a coating of bismuth titanate by two-stage synthesis (Chenn K., Hu R., Feng X., Xie K., Li Y., Gu H Bi ₄ Ti ₃ O ₁₂ / TiO ₂ heterostructure: Synthesis. // Ceram. Int 2013. V. 39. P. 9109-9114). First, an oxide coating on titanium is obtained by electrochemical method in a solution of ethylene glycol, additionally containing NH ₄ F. After that, the oxidized samples are placed in a solution of Bi (NO ₃ ) ₃ in KOH alkali and hydrothermal synthesis is carried out at 200 ° C for a certain time (up to 24 hours). The disadvantage of this method is the need to use autoclave equipment for the hydrothermal process and the duration of the hydrothermal treatment.

A known method of pulsed excimer laser deposition of ferroelectric films of bismuth titanate on substrates of silicon and magnesium oxide (Pulsed laser deposition and ferroelectric characterization of bismuth titanate films. Appl. Phys. Lett. 58 (14), April 8, 1991. P. 1470-1472) . The disadvantage of this method is the need to use complex expensive equipment, as well as the impossibility of its use for processing products with complex surface topography.

A known method of producing coatings from bismuth titanate by thermal decomposition of volatile organometallic compounds of bismuth and titanium (Ferroelectric bismuth titanate films from hotwall metalorganic chemical vapor deposition. Jie Si and Seshu B. Desua. J. Appl. Phys. 73 (II), 1 June 1993 P. 7910-7913) using triphenylbismuth and tetraethoxy titanium as starting materials. The disadvantage of this method is the need to use complex vacuum equipment, as well as the restrictions imposed on the mass-dimensional parameters of products and the geometric shape of their surface.

Closest to the claimed is a method for producing coatings of bismuth titanium on electrically conductive surfaces (RU 2278910, publ. 2006.06.27), including electrophoretic deposition of a ferroelectric charge containing bismuth titanate, which is pre-sintered at a speed of 100-150 degrees / hour, is kept at a temperature 1150 ° C for 1-3 hours and cooled at a speed of 150-300 deg / h, then grind it to obtain fractions from 0.5 to 100 microns in size and load into the electrolyte, providing uniform mixing with stirring powder division by volume of electrolyte. Electrophoretic deposition is carried out from electrolytes with a ferroelectric powder concentration of from 10 to 800 g / l under conditions of microarc discharges between the substrate and powder particles at a current density of 2 to 60 A / dm ² . After the formation of a thin layer of ferroelectric, the samples are washed in running water and dried for 30 min at a temperature of 180 ° C.

The disadvantages of this method: multi-stage, the need for long and multi-stage preliminary preparation of a ferroelectric mixture, including the sintering process at high temperature, as well as the need to prepare an electrolyte-suspension, complicating the method and increasing the time required for its implementation.

The objective of the invention is to provide a simple method for producing thin layers of bismuth titanate on titanium, which does not require a large investment of time.

The technical result of the proposed method is to simplify the technological scheme of the method and to reduce the time of its implementation.

The indicated result is achieved by the method of producing thin layers of bismuth titanate on a conductive surface using electrochemical treatment under microarc discharge conditions, in which, in contrast to the known, the conductive surface of a titanium product is treated by plasma electrolytic oxidation in a borate electrolyte in a galvanostatic mode with anodic polarization of titanium products for 10-15 minutes at an effective current density of 0.20-0.25 a / cm ² to form a titanium oxide layer, otorrhea then impregnated with a solution of basic bismuth nitrate melt rosin, turpentine diluted, and calcined at a temperature of 650-700 ° C for 0.5-1.0 hours.

In an advantageous embodiment of the method, a 0.10-0.20 M aqueous solution of tetraborate Na ₂ B ₄ O _{7 is} used as a borate electrolyte.

The method is as follows.

By plasma electrolytic oxidation (PEO) in a unipolar galvanostatic mode for 10-15 minutes at an effective current density i = 0.20-0.25 A / cm ² in an aqueous electrolyte containing 0.10-0.20 M sodium tetraborate Na ₂ B ₄ O ₇ , an oxide layer is formed on the anodically polarized titanium substrate under continuous plasma microdischarges in the surface region.

As shown by the data of x-ray spectral analysis (XRD), in the obtained using PEO layer (in the claimed mode using a borate electrolyte) contains oxygen and titanium.

Samples coated with an oxide coating with a thickness of up to 12 μm are washed from the electrolyte, rinsed with distilled water and dried in air at room temperature.

The preparation of an organic solution of bismuth is carried out according to the known method described in (Vizir V.A., Martynov M.A. Ceramic paints. Kiev, "Technique", 1964, p. 191). To do this, the main bismuth nitrate is dissolved in the molten rosin and the melt is diluted with turpentine.

The prepared solution is impregnated, for example, by immersion, washed and dried samples with an oxide-coated PEO method and pyrolyzed at 650-700 ° C for 0.5-1.0 hours.

In FIG. 1 shows X-ray diffraction patterns of the surface of a layer formed on titanium by the PEO method (Fig. 1a) and the layer obtained as a result of final processing by the proposed method (Fig. 1b).

In FIG. Figure 2 shows SEM images of the surface of the layer formed by the PEO method (Fig. 2a and 2b) and its surface after impregnation with an organic bismuth solution and heat treatment (Fig. 2c and 2d), while images 2a and 2c are given in the amplitude representation, 2b and 2 g - in phase.

In FIG. 3 are shown at various magnification images of individual sections of layers formed on the surface of titanium by the proposed method.

In the radiograph shown in FIG. 1a, there are fairly intense reflections of titanium and titanium dioxide in the rutile modification, as well as insignificant peaks of TiO ₂ in the anatase modification, which indicates that when conducting PEO under the claimed conditions, the electrolyte components are not embedded in the formed oxide layer: the x-ray reflects the presence of only material the substrate. After the oxide layer is impregnated with an organic bismuth solution, followed by pyrolysis, a significant change in the phase composition of the deposited layer occurs: intense peaks appear on the X-ray diffraction pattern (Fig. 1b), indicating a predominantly crystalline structure of the formed layer, and specifically peaks corresponding to Bi ₄ Ti ₃ O ₁₂ reflections orthorhombic modification.

This corresponds to the data shown in Table 1, which characterize the phase and elemental composition, as well as the thickness of the layers formed using PEO (1) and obtained after their impregnation with an organic bismuth solution followed by heat treatment (2). The data on the elemental composition of the coatings averaged over large surface areas were obtained by microprobe analysis.

The X-ray diffraction data (Fig. 1b) and Table 1 show that, as a result of impregnation of the formed PEO layer with an organic bismuth solution and subsequent heat treatment, bismuth is embedded in the oxide layer formed by PEO, interacting with it to form bismuth titanium.

The thickness of the obtained layers, the morphology of their surface are mainly determined by the composition of the electrolyte and the conditions of the PEO and practically do not change after impregnation and heat treatment. Bismuth is embedded in the formed layer, while the titanium content does not change, the oxygen content decreases.

A detailed study of the elemental composition (Table 2) of the bismuth-rich regions (light regions in Fig. 2) showed that the atomic ratio of Bi: Ti: O in the resulting coatings corresponds to the Bi ₄ Ti ₃ O ₁₂ phase and is thus consistent with the XRD data (Fig. one). Using high resolution scanning electron microscopy (Hitachi S-5500), it was found that the surface areas enriched with bismuth are coated with nanoscale crystals, the composition of which also corresponds to Bi ₄ Ti ₃ O ₁₂ .

The presence of carbon (up to 23 at.%) In the coating composition is most likely the result of the interaction of an organic bismuth solution with an oxide PEO layer with the formation of strong carbon-containing compounds that are stable after annealing at a temperature of 700 ° C, at which pyrolysis of the solution occurs.

Examples of specific implementation of the method

As samples, we used VT1-0 grade titanium plates measuring 0.5 × 2.5 × 0.1 cm. The samples were subjected to mechanical grinding and chemical polishing in a mixture of acids HF: HNO ₃ = 1: 3 at 60-80 C for 2-3 s, washed with distilled water and dried in air.

For the preparation of solutions, a commercial reagent Na ₂ B ₄ O ₇ ⋅ 10H ₂ O and distilled water were used.

As a current source during the formation of oxide coatings using plasma electrolytic oxidation, a TEP4-63 / 460H thyristor unit with a unipolar pulse current shape was used. Oxide coatings on anodized polarized titanium immersed in the electrolyte were formed in the galvanostatic mode. The counter electrode (cathode) was a coil made of a hollow (0.5 cm diameter) stainless steel tube through which cold water was passed to cool the electrolyte, which was noticeably heated during oxidation at high current densities used. During the oxidation process, the electrolyte was mixed using a magnetic stirrer.

The elemental composition of the coating surface was determined using a JEOL SUPERPROBE JXA-8100 microprobe X-ray spectral analyzer, on which surface images were simultaneously obtained. The surface of the electrodes was also studied using a Hitachi S-5500 scanning electron microscope (Hitachi, Japan) with an energy dispersive X-ray microanalysis (EDX) system manufactured by ThermoScientific.

The phase composition was determined by X-ray phase analysis on a D8 ADVANCE diffractometer (Germany) in Cuk _a radiation with the identification of compounds in the automatic EVA search mode using a data bank (PDF-2).

Example 1

On a prepared sample of titanium alloy using PEO in an electrolyte containing 38.2 g / l Na ₂ B ₄ O ₇ ⋅ 10H ₂ O, in the anode galvanostatic mode with an effective current density of 0.20 A / cm ² , an oxide coating was formed over 10 minutes. The initial temperature of the electrolyte is 18 ° C, the final 22 ° C.

After PEO, the samples were rinsed with distilled water and dried in air at room temperature.

In 10 g of molten rosin, 2 g of basic bismuth nitrate was dissolved and this melt was diluted with turpentine in a volume ratio of 1: 1. The resulting solution was immersed in an impregnated PEO-treated sample and calcined at 700 ° C for 0.5 hours. As a result, a coating was obtained on the surface of the sample containing, according to x-ray phase analysis, Bi ₄ Ti ₃ O ₁₂ .

Example 2

The process was carried out under the conditions of example 1 in an electrolyte containing 76.4 g / l Na ₂ B ₄ O ₇ ⋅ 10H ₂ O, with an effective current density of 0.25 A / cm ² for 15 minutes. The initial temperature of the electrolyte is 18 ° C, the final 28 ° C. Then, the washed and dried oxide-coated sample was impregnated with a solution of basic bismuth nitrate in rosin melt diluted with turpentine and calcined at 650 ° C for 1.0 hour. The result is similar to that obtained in example 1.

Claims (2)

1. A method of producing thin layers of bismuth titanate on a conductive surface using electrochemical treatment under microarc discharge conditions, characterized in that the conductive surface of the titanium product is treated by plasma electrolytic oxidation in a borate electrolyte in galvanostatic mode with anodic polarization of the product for 10- 15 minutes at an effective current density of 0.20-0.25 A / cm ² with the formation of a layer of titanium oxide, which is then impregnated with a solution of basic nitrogen bismuth oxide in a rosin melt diluted with turpentine, and calcined at a temperature of 650-700 ° C for 0.5-1.0 hours. 2. The method according to p. 1, characterized in that the formation of the oxide layer on titanium is carried out in an electrolyte containing 0.1-0.2 M tetraborate Na ₂ B ₄ O ₇ .

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