Fabrication of Nanoceramic Thin-Wall Tubes By Magnetic Pulsed Compaction and Thermal Sintering

Gasproof thin-wall tubes of fine-grained ceramics based on zirconia and gadolinia have been produced by magnetic pulsed compaction and thermal sintering. Data on their structures and electric characteristics are presented. The tubes with a diameter of ~ 15 mm, wall thickness of ~ 0.7 mm, and length up to 80 mm are characterized by an uniform porousfree structure with a mean grain size in the range of 100 – 300 nm. The obtained ceramics possess high electrolytic properties.


Introduction
A number of modern technical applications requires fabrication of long-length ceramic tubes with improved properties.In particular, thin-wall ceramic tubes are necessary to create solid oxide fuel cells (SOFC) of tube design.Such ceramics should be characterized with high oxygen-ion conductivity, gas impermeability, and mechanical strength.Yttria stabilized zirconia and gadolinia doped ceria ceramics with a fine submicron structure made using nanopowders are promising electrolytes for SOFC [1,2].Magnetic pulsed compaction (MPC) followed by sintering is one of the perspective approaches for ceramic tube fabrication.It has certain advantages over the known alternative methods for manufacturing ceramic tubes such as slip casting, plasma sputtering, and hydrostatic compaction.MPC represents dry intense densification of powders that guarantees chemical cleanness, high density and uniformity of compacts.MPC has previously been used for product formation from micron-sized powders of metals and composites [3,4].The principles of pulsed uniaxial and radial compaction of nanopowders have been developed in previous work [5][6][7][8].
In this article an experimental investigation of thin-wall tube fabrication from nanopowders based on zirconia and gadolinia using magnetic pulse compaction and thermal sintering is described.Data about microstructures of compacts and ceramic samples are presented.Electrolytic properties of the sintered ceramics are demonstrated.[9,10].Detailed characterization of the powders including the chemical composition, the specific surface S BEТ and the particle size d x is presented in table I.Both powders are singlephase solid solutions having a cubic fluorite-like lattice with parameters of 5,1459 Å (9.8YSZ) and 5,424 Å (GDC).TEM analysis showed that particles of the 9.8YSZ powder have a mainly spherical shape with weak faceting (fig.1a) and particles of the GDC powder have both spherical and cubical shapes (fig. 1b).The shell was a soft copper tube with an external diameter of 22 mm and a wall thickness of 1 mm.Powder filling was carried out using a vibration exciter.The relative density of the filled powder layer was about 0.2.Degassing was applied to remove the adsorbed substances and gases from the powder.This procedure was executed by means of evacuation and heating up to temperatures of 400 -450 °C during 1 -2 hours.For capsulation of degassed powder a gasproof sealing of the die was made using steel plugs (4) and polyethylene gaskets (5).
The thus prepared die was connected to a capacitive energy storage [8] as shown on fig. 2. Discharging of this storage generates a power pulsed current that flows along the copper shell (1) in one direction and along the rigid massive conductor (6) in the reverse direction.At the same time the pulsed current creates a strong magnetic field in a gap between these parts.Under action of this magnetic field the copper shell is compressed and presses the powder inside.
Thermal sintering of the tube-like compacts was carried out inside a resistive furnace in air atmosphere at temperatures from 1300 -1360 °C.The heating and cooling rates were 2 °C/min, and the holding time was 0 -60 min.
The structural and phase compositions of the materials were investigated by the X-ray diffraction (XRD) method using a DRON-4 diffractometer with filtered CuK α radiation.The mean crystallite size d x was determined from broadening of X-ray diffraction peaks by means of the Sherer-Seljakov's procedure.Structures of the compacts and sintered ceramics were studied by scanning electron microscopy (LEO 982) and atomic force microscopy (Solver 47).Densities of the samples were measured hydrostatically.

Results and discussion
Characteristics of the compacts and the sintered tubes made of 9.8YSZ and GDC ceramics are collected in Table II.The data were averaged over a large number of experiments.For both powders a relative density of the tubular compacts in the range of 40 -55 % is enough for obtaining nearly full density of the ceramics after thermal sintering at given temperatures.
Tab. II Characteristics of the compacts and the sintered ceramic tubes.Fracture images of the 9.8YSZ and GDC compacts (Fig. 3) show clearly uniform packing of particles without agglomerates.The pores are distributed uniformly in the sample volume and have a character size comparable with the particle diameter.

Type of ceramics
Fig. 4 presents the fracture images of the 9.8YSZ and GDC ceramics.It can be seen that dense uniform structures without large pores have formed during sintering.The average grain sizes of the 9.8YSZ and GDC ceramics are about of 200 -300 nm and 100 -200 nm, correspondingly.
An experimental fuel cell was fabricated using the 9.8YSZ ceramic tube obtained.Electrolytic characteristics investigated in detail elsewhere [12] demonstrated high effectiveness of the YSZ electrolyte.The power density amounted to 470 mW/cm 2 and the current density was up to 910 mA/cm 2 at the test temperature of 950 °C.At comparable conditions these data are 1.5 times higher than the characteristics of fuel cells made of a tubular electrolyte produced using conventional slip casting.The specific resistance of sintered GDC ceramic tubes has been studied by the impedance-spectroscopy method [13].Fig. 5 presents a comparison of these data (curve 1) with total specific resistances for several electrolytes based on gadolinia synthesized by different authors.It is seen that the total conductivity of our material is comparable with the best results with the exception of Ce10Gd ceramics (curve 3) sintered by Steele [14].

Conclusions
1. Conditions of magnetic pulsed compaction and thermal sintering for fabricating thin-wall ceramic tubes using weekly agglomerated 9.8YSZ and GDC nanopowders have been found.The tubes have a wall thickness of about of 0.6 mm, a diameter approximately of 14 mm and are up to 80 mm in a length.2. The sintered 9.8YSZ and GDC ceramics have dense uniform porous-free structures with average grain sizes of 200-300 nm and 100-200 nm, respectively.3. Characteristics of the fine-grained 9.8YSZ and GDC ceramic electrolytes are comparable with the best data obtained earlier for microstructured materials of the same compositions.17.V.P. Gorelov, V.V. Ivanov, Y.A. Kotov