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20MODIFYING OF THE STAINLESS STEEL POROUS SURFACE BY NANOSTRUCTURED ALUMINA
V. V. Shustov*", V. A. Zelensky", and A. B. Ankudinova
aBaikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, 119334 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10163
A number of experiments were carried out to modify the inner surface in order to create nanostructures on carriers that do not have a highly developed inner surface but have high permeability values. On samples made of X18H10T stainless steel powder the nanostructure was formed by impregnating a porous carrier with solutions of salts and their subsequent thermal decomposition.
The process of production a porous sample consisted in compacting a mixture of stainless steel powder (with an average particle size of 10-20 p,m) and a pore former (particle size of 250-320 p,m) with a single-sided pressure of 400 MPa in a cylindrical matrix. Burnout of the pore former and sintering at 700°C result to the formation of a porous structure in the material, which was characterized by a bimodal pore size distribution.
Figure 1 shows the image of the fracture surface of a sintered sample obtained with a raster electron microscope. In Fig. 1 b, for clarity, some large pores in the sample are circled in white, the size of which is about 200 p,m, and the presence of channels that are visible as black spots can also be noticed. These channels can connect pores throughout the sample volume, having outlets into the space of separate pores (in the figure such outlets are marked with arrows). When determining the pore sizes in permeable materials, it should be understood that it is the size and number of such channels that determine such functional characteristics as filtration fineness and permeability of the material.
(a) (b)
Fig. 1. Micrograph of the fracture of the stainless steel sample: (a) general view, (b) with the selection of macropores.
Modifying the surface of a porous carrier with aluminum oxide was carried out by impregnating it with an aqueous solution of an aluminum nitrate salt Al(NO3)3-9H2O. The porous sample was impregnated with solutions of salts of different concentrations from 0.5 to 1.5 mol/l. Full impregnation of the sample was achieved by immersing the pre-evacuated sample into the solution. It is known that aluminum nitrate Al(NO3)3 undergoes hydrolysis,
iSHS 2019
Moscow, Russia
when dissolved in water. The process of hydrolysis proceeds in three stages with the formation of intermediate compounds and ends with the appearance of aluminum hydroxide Al(OH) and nitric acid HNO3 in solution. As a result of drying and subsequent annealing. decomposition of the remaining salt and aluminum hydroxide to an oxide occurs. and nitric acid also decomposes. Annealing was carried out at a temperature of 450°C for 60 min in a flowing argon atmosphere.
The structure of the obtained material was studied using scanning electron microscopy. Pores of about 10 ^m in size are also present in the material. due to the fact that stainless steel powder with a particle size of 10-20 |im was used. The sintering mode led to the growth of necks between the particles. while the porosity value slightly changed as compared to compressed powder compact. Figure 2 clearly shows how a structure resembling a smooth thin film was formed on the surface of a porous carrier.
X-ray microanalysis showed the presence of aluminum and oxygen, with the aluminum content on the surface of the carrier particles being less than 1 wt % (Fig. 3, spectra S1, S2, S4), which can be explained by the nanometer thickness of the film formed on the surface of stainless steel. More voluminous aluminum oxide clusters were also found, which on SEM images in reflected electrons were seen as a translucent material located in the voids of the base material made of stainless steel (Fig. 4). Thus, the authors succeeded in creating a nanostructure on the surface of a material from stainless steel powders with a porous structure — a thin film of aluminum oxide. Such material is planned to be used for the manufacture of a catalytic cell in an ethanol catalysis unit.
i—I
Fig. 2. Micrograph of the sample after the application of aluminum oxide.
4Q|jm "*_Electron Image 1
Spectrum C O Al Cr Fe Ni
S1 4.8 10.1 0.65 17.3 55.4 11.8
S2 5.2 9.5 0.67 17.0 56.4 11.2
S3 7.5 18.4 62.0 12.1
S4 7.0 12.8 0.69 15.2 53.6 10.8
Fig.3. X-ray microanalysis of a stainless steel sample after the application of alumina.
Fig. 4. SEM image of a porous stainless steel material coated with aluminum oxide. V. V. Shustov et al. 447
The research was supported by the Russian Foundation for Basic Research (project no. 17-03-00867) with the involvement of state assignment number 075-00746-19-00.
