**1. Introduction**

120 Sintering of Ceramics – New Emerging Techniques

Tsai, C. T.; Kung, G.T.C. & Hwang, C. L. (2010). Use of High Performance Concrete on Rigid

Tsai, C.T.; Li, L.S. & Hwang, C.L. (March 2006). The Effect of Aggregate Gradation on

*International*, 09.03.2006, Available from http://www.astm.org

Vol.24, No.5, pp. 732-740, ISSN 0950-0618

Pavement Construction for Exclusive Bus Lanes. *Construction and Building Materials*,

Engineering Properties of High Performance Concrete. In: *Journal of ASTM* 

Thin film solid oxide fuel cells (SOFC) operating in the intermediate temperature (IT) range are now considered as promising for distributed, mobile, standby or auxiliary power generation. At present one of the most important scientific aims in design of solid oxide fuel cells is to lower the operating temperatures to 600-800С. In this temperature range, majority of problems inherent to SOFC operating at high (950-1000C) are alleviated. Thus, cations interdiffusion and solid state reactions between electrolyte and electrodes are hampered and thermal stresses are decreased which prevent degradation of the functional layers [Yamamoto, 2004 ]. Hence, design of thin film SOFC requires also elaboration of nanostructured electrodes compatible with electrolytes from chemical and thermophysical points of view and providing a developed three-phase boundary (TPB). In this respect, broad options are provided by design of nanocomposite mixed ionic-electronic conducting (MIEC) functional layers – (Sadykov et al., 2010; Sadykov et al., 2009; Sadykov et al., 2008).

One of the most demanding problem in solid oxide fuel cells design is caused by the necessity of co-sintering of thin layers (electrolyte, functionally graded nanocomposite cathode) to provide required density without degradation of their transport, electrochemical and thermo-mechanical properties. The most developed and cost-effective are methods based upon supporting electrolyte powders with addition of organic binders and dispersants via screen-printing (Souza et al., 1998), tape casting (Kobayashi et al., 2002) or slurry coating technique (Jung et al., 2007). Thus supported «green» layers are sintered at

<sup>\*</sup> Vladimir Usoltsev1, Yulia Fedorova1, Natalia Mezentseva1, Tamara Krieger1, Nikita Eremeev1, Marina Arapova1, Arcady Ishchenko1, Alexey Salanov1, Vitaly Pelipenko1, Vitaly Muzykantov1, Artem Ulikhin2, Nikolai Uvarov2, Oleg Bobrenok3, Alexander Vlasov4, Mikhail Korobeynikov4, Aleksei Bryazgin4,

Andrei Arzhannikov4, Petr Kalinin4, Oleg Smorygo5 and Manfred Thumm6

*<sup>1</sup>Boreskov Institute of Catalysis, Novosibirsk State University, Novosibirsk, Russia 2Institute of Solid State Chemistry and Mechanochemistry, Novosibirsk, Russia*

*<sup>3</sup>Institute of Thermal Physics, Novosibirsk, Russia 4Budker Institute of Nuclear Physics, Novosibirsk, Russia*

*<sup>5</sup>Institute of Powder Metallurgy, Minsk, Belarus* 

*<sup>6</sup>Karlsruhe Inst. Technology, Karlsruhe, Germany* 

Advanced Sintering Techniques in Design of

successfully used (Chen et al, 1992).

(Annenkov, 1996)

SOFC and oxygen separation membranes.

Planar IT SOFC and Supported Oxygen Separation Membranes 123

For sintering of cathode or membrane perovskite layers, silver or Ag-Pd alloy aids were

Advanced sintering techniques are based upon application of high-energy irradiation that could be focused directly on the sample or even functional layer (as described in selective laser sintering studies (Kumar, 2003). Electron beams as PVD modification were used for deposition of alumina or composite cermets functional layers of carbides/nitrides (Singh & Wolfe, 2005). Only a few works mention e-beam as a promising approach to deposit thin and dense YSZ or GDC electrolyte layer on electrode (mainly NiO/YSZ anode ) (Laukaitis et al., 2007; Lemkey et al., 2005). Deposition through the vapor (PVD or CVD) ensures thin uniform layers but, unfortunately, this approach requires high vacuum that increases costs and reduces commercial usage. Nevertheless, e-beam technique was used for direct sintering, including application of high-energy e-beams to produce layered metal composites (Zaeh&Kahnert, 2009), sintering of zirconia and zirconia-corundum composites etc. (Annenkov, 1996). Enhanced sintering of ceramics under e-beams action is explained by dissipation of radiation energy in heterogeneous structures, thermal-diffusion stimulation of mass transfer due to point defect formation and increase of thermal vibrations of the lattice

Compared with radiation-induced sintering techniques, microwave heating at frequency 2.45 GHz is more broadly applied for sintering of zirconia-based electroceramics (Alexeff & Meek, 2011; Mazaheri et al., 2008). Zirconia or ceria- based materials have a low microwave absorption at temperatures below 400C (Jiao, 2011). Jump-like change of their dielectric properties at 400-1000C requires to use special susceptors with a better absorption for preheating samples and smoothing the heating curve. As a susceptor usually disks of SiC were used to provide so called hybrid microwave sintering. Microwave heating produces samples sintered to density 90-95% of the theoretical value at temperatures lower by 200C than conventional heating (Mazaheri et al., 2008 ; Charmond et al., 2010). However, the maximum density obtained for doped zirconia samples in these conditions was ca 98%, and to produce fully dense samples the hybrid mode heating was used with dwelling about 1 hour at 1350C (Charmond et al., 2010). Microwave heating uses also significantly higher heating rates 5-50 (and even 200)/min as compared with conventional sintering carried out at a heating rate 2-5/min with dwelling at the maximum temperature for 5-10 hours. This becomes possible due to very uniform heating without space gradients of temperature in the sample. A higher heating rate provides a lower average grain size (2.3 μm at 5/min vs 0.9 μm at 50o/min) (Mazaheri et al., 2008 ; Charmond et al., 2010). Despite all these apparent advantages, up to date these advanced techniques have not been systematically applied for sintering functional layers comprised of mixed ionic –electronic conductors –perovskites and their nanocomposites with different electrolytes as required in design of thin film IT

This work presents results of research aimed at filling such a gap and providing verification of these advanced sintering techniques (e-beam and microwave heating) for such an application. Functional layers were comprised of oxides with ionic conductivity (doped ceria, doped zirconia and δ-Bi2O3, doped La silicate with apatite structure) and mixed ionicelectronic conductivity (perovskites and their nanocomposites with electrolytes). Effect of sintering parameters on the real structure/microstructure and functional properties of these

temperatures ~ 1400С. As a rule, high sintering temperatures required for electrolyte densification impose restrictions on the nature of electrode materials. Thus, at temperatures of sintering exceeding 1200C, application of cathode substrates comprised of Sr-substituted perovskites could result in formation of isolating La(Sr)-zirconate layers (Chiba et al., 2008). Similarly, in the case of metal substrates high-temperature sintering is possible only in vacuum or in reducing gas atmospheres (Tucker, 2010). Another approach to supporting thin layers of electrolytes is based upon application of vapor deposition methods operating at substrate temperatures below 1000С. This prevents formation of undesired phases in the course of synthesis, thus broadening the scope of electrode materials which can be used as electrolyte substrates. Among these methods, the most developed are different versions of Chemical Vapor Deposition (CVD) and Plasma Deposition (either in air or in vacuum) (Yamane & Hirai, 1987; Schiller et al., 2000; Minh, 1993).

Traditional sintering approaches requiring too high temperatures often failed in this respect. In design of supported oxygen-separation membranes, similar problems appear due to necessity of sintering thin dense mixed ionic-electronic conducting nanocomposite layers supported on macroporous ceramic (cermet or metal) substrates. For example to get Gddoped ceria (GDC) pellet with density 97% from agglomerated commercial powders by simple heating in the furnace, required temperature is 1600-1700C (Ma et al., 2004). Densification rate of GDC powder was found to increase only after 1100C being accompanied by grain growth (Zhang & Ma, 2004). By using weakly agglomerated GDC nanopowders (average grain size 10 nm), the density of 94% from the theoretical value was achieved after sintering at 1100C for several hours, and after 108 hours the relative density increased up to 99%. Increasing sintering temperature to 1200-1300C allows to obtain 97-99 % density samples after 5 hours dwelling (Ivanov et al., 2007). Sintering of composites of ceria with strontium-doped lanthanum manganites was also studied using conventional techniques. Samples with 95-98% density could be obtained at temperatures 1350-1400C. Sintering dynamics was found to depend upon samples composition, the best sinterability being observed for samples containing 30-70% ceria (Cutler et al., 2003). Together with application of nanosized powders, a popular approach in conventional sintering aiming at lowering sintering temperature is based upon using special sintering additives. Some of these additives form melts which enhance mass transfer between the particles of materials with a higher melting point. Bismuth oxide melts at 820C and can substitute rare earth cations in the functional layers improving electrochemical performance. Sintering of GDC was enhanced by adding small amounts of Bi2O3: samples of GDC containing 1 wt% bismuth sintered at 1200-1400C for 2 - 4 h were dense bodies (98-99% of theoretical density) which is achieved at temperatures lower by 250-300C than required for sintering of undoped GDC (Gil at al., 2007). Moreover, even refractory additives can improve sintering via affecting defect chemistry at interfaces. Thus, the density and grain size of sintered Gddoped CeO2 increased with adding up to 2 mol. % Al2O3 while decreasing at a higher content of this additive (Lee et al., 2004 ; Liu at al., 2008). Similarly, iron oxide addition via mechanical activation treatment also provides a higher density of GDC (Zhang at al., 2004). The maximum density was obtained by adding 5 mol.% Ga2O3 to GDC at sintering temperature 1400C (Lee et al., 2004). Combination of nanosized freeze-dried powders with addition of 1-3 mol.% cobalt to GDC allowed to get 94% density at 1000 C [Perez-Coll et al., 2003].

temperatures ~ 1400С. As a rule, high sintering temperatures required for electrolyte densification impose restrictions on the nature of electrode materials. Thus, at temperatures of sintering exceeding 1200C, application of cathode substrates comprised of Sr-substituted perovskites could result in formation of isolating La(Sr)-zirconate layers (Chiba et al., 2008). Similarly, in the case of metal substrates high-temperature sintering is possible only in vacuum or in reducing gas atmospheres (Tucker, 2010). Another approach to supporting thin layers of electrolytes is based upon application of vapor deposition methods operating at substrate temperatures below 1000С. This prevents formation of undesired phases in the course of synthesis, thus broadening the scope of electrode materials which can be used as electrolyte substrates. Among these methods, the most developed are different versions of Chemical Vapor Deposition (CVD) and Plasma Deposition (either in air or in vacuum)

Traditional sintering approaches requiring too high temperatures often failed in this respect. In design of supported oxygen-separation membranes, similar problems appear due to necessity of sintering thin dense mixed ionic-electronic conducting nanocomposite layers supported on macroporous ceramic (cermet or metal) substrates. For example to get Gddoped ceria (GDC) pellet with density 97% from agglomerated commercial powders by simple heating in the furnace, required temperature is 1600-1700C (Ma et al., 2004). Densification rate of GDC powder was found to increase only after 1100C being accompanied by grain growth (Zhang & Ma, 2004). By using weakly agglomerated GDC nanopowders (average grain size 10 nm), the density of 94% from the theoretical value was achieved after sintering at 1100C for several hours, and after 108 hours the relative density increased up to 99%. Increasing sintering temperature to 1200-1300C allows to obtain 97-99 % density samples after 5 hours dwelling (Ivanov et al., 2007). Sintering of composites of ceria with strontium-doped lanthanum manganites was also studied using conventional techniques. Samples with 95-98% density could be obtained at temperatures 1350-1400C. Sintering dynamics was found to depend upon samples composition, the best sinterability being observed for samples containing 30-70% ceria (Cutler et al., 2003). Together with application of nanosized powders, a popular approach in conventional sintering aiming at lowering sintering temperature is based upon using special sintering additives. Some of these additives form melts which enhance mass transfer between the particles of materials with a higher melting point. Bismuth oxide melts at 820C and can substitute rare earth cations in the functional layers improving electrochemical performance. Sintering of GDC was enhanced by adding small amounts of Bi2O3: samples of GDC containing 1 wt% bismuth sintered at 1200-1400C for 2 - 4 h were dense bodies (98-99% of theoretical density) which is achieved at temperatures lower by 250-300C than required for sintering of undoped GDC (Gil at al., 2007). Moreover, even refractory additives can improve sintering via affecting defect chemistry at interfaces. Thus, the density and grain size of sintered Gddoped CeO2 increased with adding up to 2 mol. % Al2O3 while decreasing at a higher content of this additive (Lee et al., 2004 ; Liu at al., 2008). Similarly, iron oxide addition via mechanical activation treatment also provides a higher density of GDC (Zhang at al., 2004). The maximum density was obtained by adding 5 mol.% Ga2O3 to GDC at sintering temperature 1400C (Lee et al., 2004). Combination of nanosized freeze-dried powders with addition of 1-3 mol.% cobalt to GDC allowed to get 94% density at 1000 C [Perez-Coll et al.,

(Yamane & Hirai, 1987; Schiller et al., 2000; Minh, 1993).

2003].

For sintering of cathode or membrane perovskite layers, silver or Ag-Pd alloy aids were successfully used (Chen et al, 1992).

Advanced sintering techniques are based upon application of high-energy irradiation that could be focused directly on the sample or even functional layer (as described in selective laser sintering studies (Kumar, 2003). Electron beams as PVD modification were used for deposition of alumina or composite cermets functional layers of carbides/nitrides (Singh & Wolfe, 2005). Only a few works mention e-beam as a promising approach to deposit thin and dense YSZ or GDC electrolyte layer on electrode (mainly NiO/YSZ anode ) (Laukaitis et al., 2007; Lemkey et al., 2005). Deposition through the vapor (PVD or CVD) ensures thin uniform layers but, unfortunately, this approach requires high vacuum that increases costs and reduces commercial usage. Nevertheless, e-beam technique was used for direct sintering, including application of high-energy e-beams to produce layered metal composites (Zaeh&Kahnert, 2009), sintering of zirconia and zirconia-corundum composites etc. (Annenkov, 1996). Enhanced sintering of ceramics under e-beams action is explained by dissipation of radiation energy in heterogeneous structures, thermal-diffusion stimulation of mass transfer due to point defect formation and increase of thermal vibrations of the lattice (Annenkov, 1996)

Compared with radiation-induced sintering techniques, microwave heating at frequency 2.45 GHz is more broadly applied for sintering of zirconia-based electroceramics (Alexeff & Meek, 2011; Mazaheri et al., 2008). Zirconia or ceria- based materials have a low microwave absorption at temperatures below 400C (Jiao, 2011). Jump-like change of their dielectric properties at 400-1000C requires to use special susceptors with a better absorption for preheating samples and smoothing the heating curve. As a susceptor usually disks of SiC were used to provide so called hybrid microwave sintering. Microwave heating produces samples sintered to density 90-95% of the theoretical value at temperatures lower by 200C than conventional heating (Mazaheri et al., 2008 ; Charmond et al., 2010). However, the maximum density obtained for doped zirconia samples in these conditions was ca 98%, and to produce fully dense samples the hybrid mode heating was used with dwelling about 1 hour at 1350C (Charmond et al., 2010). Microwave heating uses also significantly higher heating rates 5-50 (and even 200)/min as compared with conventional sintering carried out at a heating rate 2-5/min with dwelling at the maximum temperature for 5-10 hours. This becomes possible due to very uniform heating without space gradients of temperature in the sample. A higher heating rate provides a lower average grain size (2.3 μm at 5/min vs 0.9 μm at 50o/min) (Mazaheri et al., 2008 ; Charmond et al., 2010). Despite all these apparent advantages, up to date these advanced techniques have not been systematically applied for sintering functional layers comprised of mixed ionic –electronic conductors –perovskites and their nanocomposites with different electrolytes as required in design of thin film IT SOFC and oxygen separation membranes.

This work presents results of research aimed at filling such a gap and providing verification of these advanced sintering techniques (e-beam and microwave heating) for such an application. Functional layers were comprised of oxides with ionic conductivity (doped ceria, doped zirconia and δ-Bi2O3, doped La silicate with apatite structure) and mixed ionicelectronic conductivity (perovskites and their nanocomposites with electrolytes). Effect of sintering parameters on the real structure/microstructure and functional properties of these

Advanced Sintering Techniques in Design of

designed for heating in the argon flow was used.

into He stream (Sadykov et al, 2010, 2011).

**3. Results and discussion** 

**Electrolytes** 

temperature for 30 min and then cooling to room temperature.

**2.4 Characterization of the real structure and functional properties** 

EDX spectrometer equipped with Si(Li) detector (energy resolution 130 eV).

**3.1 Effect of sintering stage on the real structure of SOFC materials** 

**2.3 Sintering techniques** 

under electron beam action.

10-240 min.

Planar IT SOFC and Supported Oxygen Separation Membranes 125

Functional layers and uniaxially pressed pellets were sintered by different techniques: conventional heating (CH) in the furnace at temperatures up to 1300C (nanocomposites) or 1400C (electrolytes) with or without sintering aids (Bi or Ag nitrates, etc) in different gas atmospheres (air, Ar, etc); microwave heating (MH) or radiation-thermal sintering (RTS)

For conventional heating, a high-temperature oven equipped with a quartz reactor specially

MH was carried out using a system based on gyrotron with frequency 24 GHz specially designed for heating of materials. Samples were heated by the focused radiation (power 0.5- 1.5 kW) up to 1000-1200C with heating rate 50/min followed by dwelling at the final

RTS was carried out on an ILU-6 accelerator that gives electron pulses with a high (2.4 MeV) energy. Temperature was varied in range of 900-1200 C (heating rate 30-40 /min) by changing the frequency in the range of 8-20 Hz. Time of treatment was varied in the range of

Genesis of the real structure of sintered functional layers and pellets was studied in details by X-ray powder diffraction, high resolution transmission electron microscopy and scanning electron microscopy with local elemental analysis by EDX. XRD patterns were obtained with an ARLX'TRA diffractometer (Thermo, Switzerland) using CuKα monochromatic radiation (λ=1.5418 Å) in 2θ range 5-90o. Transmission Electron Microscopy (TEM) micrographs were obtained with a JEM-2010 instrument (lattice resolution 1.4 Å, acceleration voltage 200 kV). Analysis of the local elemental composition was carried out by using an energy-dispersive

Transport properties were characterized by impedance spectroscopy and oxygen isotope heteroexchange. Conductivity was measured in air with a MO-10 Micro-Ohmmeter at 25 Hz frequency with four Ag paste electrodes placed symmetrically at a perimeter of the pellet. The oxygen mobility was characterized by the temperature -programmed oxygen isotope exchange (a static installation with MS control of the gas phase isotope composition at pO2 4 Torr). Performance of functional layers sintered by using different techniques was tested using respective devices: button-size fuel cells operating with wet H2–air feeds and reactors equipped with supported membranes operating in the mode of oxygen separation from air

Fresh (after synthesis) powders of GDC and BE are characterized by cubic fluorite –like structure, with some tetragonal distortions for YSZ. After sintering stage (radiation-thermal

materials was elucidated. Experimental samples of button-size fuel cells and oxygen separation membranes were manufactured using advanced sintering techniques and successfully tested showing promising performance.
