Impact of Sintering Method on Certain Properties of Titanium Dioxide Nanopowder Materials

Titanium dioxide nanopowder samples consolidated by method of cold uniaxial compaction at 200 MPa and conventionally sintered in air at 1300 С with isothermal tempering during 60 minutes or spark-plasma sintering at 1300 С and 30 MPа were studied using the method of light combination scattering spectroscopy (Raman spectroscopy) and scanning electron microscopy. The samples were found to differ significantly in terms of color, density, phase composition and microstructure.


Introduction
Presently in Russia and abroad it has become strategic to develop nanoindustry, i.e. industrial application of nanotechnology.To this end, Russia needs specialists in nanomaterials and nanotechnologies, as well as the nanomaterials and nanotechnologies of domestic origin.Nowadays, owing to their extensive research, many metallic and ceramic nanopowders turn from exception to norm as they are used not only in laboratories but also in industrial processes [1][2][3][4].
It is crucial to preserve the nanosize structure in the process of nanopowders consolidation.Different methods of sintering are used, from conventional sintering in a standard oven [5] to spark-plasma sintering SPS-1050b, intended for experimental research to create a wide array of new materials [6].Unlike the conventional methods, spark-plasma sintering allows to produce sintered materials at lower temperatures and in less time by way of simultaneous effect of mechanical pressure and high power pulse current [7].In that case, the role of self-organization in the process of materials consolidation before achievement of active diffusion onset temperature can be revised.[8].
The purpose of this paper is to compare the processes of structure and phase formation in the course of titanium dioxide nanopowder sintering using conventional method and spark-plasma sintering.

Experimental
TiO 2 powder was synthetized using a proprietary methodology from ethanol-water solution with polymeric additives [9].In order to conduct the research, well-crystallized anatase powder with specific surface area of 70 m 2 /g (BET method) and average particle size 20-22 nm (BET method and Scherrer method).Particles size defined by means of scanning electron microscope ULTRA 55 (Carl Zeiss, Germany), is 40-45 nm.As the measurements have shown, the powder particle normally consists of 2-3 wafer crystallites, i.e. the nanoparticles coherent bonding occurs and primary aggregates form.The latter, going by the data [10] received for CeO 2 and Ba 4 Bi 3 F 17 nanoparticles, are the initial stage of crystallization.In the course of powders synthesis, particles form agglomerates with sized up to several microns.As the preshaping occurs, the agglomerates play the role of structural units.
In order to improve compacting [11] the powder was processed during 0.5 h in the SUND (САНД) planetary mill in chalcedonic drums with chalcedonic grinding bodies at the rotation rate of 160 rpm.Activation was performed in water (at mass ratio «balls : powder : water» = 2 : 1 : 1) adding 0.5 % (weight) agar-agar in form of prefabricated water solution.The samples were consolidated by method of cold uniaxial compaction.The compaction was performed in the closed press mould at 200 MPa.
In the first scenario, the compacted samples were annealed and sintered in air at 1300 o С with isothermal tempering during 60 minutes.In the second scenario, the sintering was performed in the Dr. Synter SPS-1050b unit (SPS Syntex, Japan) in graphite press mould with molybdenum circular panel at 1300 o С, 30 MPа, and isothermal tempering 10 min.Average heating rate was 80 o С/min.Current strength for 20 mm samples was 1200 А.
Phase composition of samples was researched by way of light combination scattering spectroscopy (Raman spectroscopy).Raman spectra were received with multipurpose Raman spectrometer «SENTERRA» (Bruker) at emitting lased wavelength of 532 nm and emission intensity of 5-10 mV.
Microstructure of materials was researched at microsections and fractures by the method of scanning electron microscopy (SEM) with the analytical field emission raster electronic microscope ULTRA 55 (Carl Zeiss, Germany) and scanning electron microscope Hitachi (Japan).
The samples density was researched in line with GOST 18898-89 which stipulates application of calculation and hydrostatic methods.

Experimental results and discussion
Conventionally sintered samples were light brown (close to yellow at fractures) and 3.7 g/cm 3 density, i.e. 14 % porosity.The porosity value corresponds to the like value of nearly any ceramic material from compacted submicron powder.
After spark-plasma sintering, the samples turned light grey with signature metallic shine both on the surface and at the fractures, with density only 2.2 g/cm 3 .As per instrumentation data, the material shrinkage ended at 1100 o С.The samples were easily disintegrated upon removal of molybdenum circular panel.
Fig. 1a shows a Raman spectrum of titanium dioxide sample after conventional sintering in air at 1300 o С.All detectable peaks are characteristic of high temperature rutile titan dioxide variant [12] and, generally, the material was rather well-crystallized.
Fig. 1b presents a Raman spectrum for titanium dioxide after spark-plasma sintering.First of all, it is notable that peaks intensity is significantly (by an order of ten times or more) lower.Peaks at wavenumbers 608, 443, 274 cm -1 are undoubtedly related with titanium dioxide, although the correlation between their intensities and peak position at 274 cm -1 instead of 234 cm -1 precludes from classifying the picture as a regular rutile spectrum.The similar spectrum is presented in studies [13,14], whereof the authors referred to it as nanostructured (virtually noncrystalline) rutile spectrum.Samples [13,14] were received by impact compression of anatase nanopowder compressed tablets.Peaks at higher wavenumbers might refer to carboniferous structures.Particularly, at 1688 cm -1 an sp 2 -variant carbon peak is detected.Carbon pikes occurrence may be a consequence of graphite current conducting instrumentation use in the equipment for sparkplasma sintering.Apparently, carbon penetrated the titanium dioxide inner structure during sintering (Raman spectra were measured at fractures).After additional round of conventional sintering in air (T treat =1300 o С) the material Raman spectrum was typical of well-crystallized rutile (Fig. 2).
The resulting sample microstructure was studied at microsections and fractures.Microsections were pretreated with high temperature etching.Fig. 3 presents SEM-images of microsection and fracture in conventionally sintered samples.Material microstructure at low magnification is similar to normal ceramic microstructure and consists of tight material with "grains" up to 5 µm and pores.At higher magnification (Fig. 3а) it is clear that the "grains" have complex columnar structure and columns inside the grains are aligned.The material structure at 50000 magnifications resembles the structure of natural material mother-of-pearl, while the size and shape of its components match the size and shape of initial powder particles.The same picture was observed at fractures.On Fig. 3d the distance between wafers and their thickness is 75-80 nm.Width of individual 'rods' constituting the wafers is about 40 nm, i.e. matching the initial powder particle size defined previously using the same method.The possibility of this segregation proves that diffusion process, being an indispensable condition of classical sintering, occurs virtually around the edges of agglomerates.Agglomerate surfaces sintering process gradually penetrates into the inner layered areas.Layers are positioned in parallel to each other within the agglomerates, while in the whole sample they are disordered.The agglomerate inner structure largely depends on the particles self-organization as discotic liquid crystals.Fig. 4 presents SEM images of samples microsections and fractures after sparkplasma sintering.The research was performed with scanning electronic microscope Hitachi (Japan) without preliminary application of carbon conductive coating.On material microsection, aligned columnar structural elements about 5 µm thick and 15-20 µm long were found.Their inner surface was porous, pores were spherical, with comparable diameter, pore size less than 500 nm.
At fractures, the material was expressly layered and cracked perpendicular to layers.The crack inner surface was (Fig. 4d) layered, probably splintered, with indefinite orientation, multicolor, which can probably be explained by varying carbon content.

Conclusion
The samples sintered according to the two scenarios proposed above significantly differ in color, density, phase composition and microstructure.Samples shrinkage in the course of spark-plasma sintering ends at 1100 o С, but material density received from compacted nanopowder amounts to merely 2.2 g/cm 3 .
After conventional sintering in air, the material is well-crystallized and consists entirely of rutile titanium dioxide high temperature variant.After spark-plasma sintering, the peak intensity at Raman spectrum is significantly lower, the resultant spectrum can be classified as nanostructured (virtually noncrystalline) rutile spectrum with structured carbon additives.
Microstructure of materials researched on microsections and fractures is expressly layered.Layers in samples sintered conventionally are located in parallel to one another within the agglomerates, but are disordered throughout the sample in general.The structure of material received by spark-plasma sintering is coarser and can be easily disintegrated at the margins of columnar elements.It may be assumed that the outcome of spark-plasma sintering is the so-called "black titanium oxide" [15], resistant to heating in air up to 1000 o С and above.