Syntheses of Doped-LaCrO 3 Nanopowders by Hydrothermal Method

The effects of additives and precipitants on the syntheses of doped LaCrO3 (lanthanum chromites) were studied by hydrothermal reaction at temperature ranges of 100 ̊C to 230 ̊C. LaCrO3 nanopowders were synthesized by hydrothermal methods using several types of precipitants such as NaOH, KOH, NH4OH, and NH2CONH2. The influence of Sr, Ca and Co doping on the lanthanum chromites prepared by hydrothermal method were investigated. The synthesized nanopowders were characterized by means of XRD, SEM and densitometer. The electrical conductivity of the doped LaCrO3 was studied at 750 ̊C in air by a DC four point probe method. The particles size of undoped LaCrO3 nanopowder was approximately 100 nm when using KOH as a precipitant. The relative density of lanthanum chromite doped with calcium and cobalt is over 97%. The highest electrical conductivity of La0.62Ca0.38Co0.18Cr0.82O3 was 32.75 S/cm at 750 ̊C in air, which is 30 times higher than undoped LaCrO3. The density and electrical conductivity are increased by doping cobalt and calcium on the LaCrO3.


Introduction
The interconnect of a SOFC stack electrically and physically is connected to the anode of one unit cell with the cathode of the adjacent unit cell in the stack.The interconnect can be ceramic or metallic materials.The interconnect materials used for SOFC stacks must be electronically conductive, oxidation resistant, impermeable to the diffusion of gases, and chemically stable with fuel cell materials [1].There are suitable materials, such as ceramics and metals for interconnect in solid oxide fuel cell (SOFC).Each material has advantages, but neither is ideal.Metallic chromium-forming alloys are most commonly used, but the formation of chromium-containing vapor species can lead to poisoning of the cathode after long operation.The resistance to this degradation can be improved through a combination of the two materials by ceramic coating on the metallic interconnect [2].
Ceramic interconnects based upon ceramic oxides with perovskite structure have been subject of intensive study over the past several decades.It was found that only a few such oxides systems can fulfill the rigorous requirements for the interconnect materials in SOFC.Lantha-num chromites (LaCrO 3 ) is the most common candidate material since it demonstrates reasonably high electronic conductivity in both fuel and oxidant atmospheres, moderate stability in the fuel cell environments as well as fairly good compatibility with other cell components in terms of phase, microstructure and thermal expansion.In order to improve the electrical conductivity as well as modify the thermal expansion coefficient (TEC), LaCrO 3 is often doped on lanthanum site, chromium site or both sites of the perovskite for practical applications.Due to ionic radius similarity, strontium and calcium tend to replace La ions whereas magnesium, iron, nickel, copper and cobalt prefer to take over the site of Cr ions.As a matter of fact, in SOFC configuration, the doped LaCrO 3 is still the most widely used as an interconnect [3].
Nanotechnology is the key for enhancing the performance of fuel cell.It is being used to lower the operating temperature of solid oxide fuel cells (SOFC).Also it can improve durability and increase oxygen-ion conductivity in the low temperature.The interconnect materials prepared using nanopowders in SOFC could be decreased the sintering temperature due to higher surface area.Also, it is advantageous for the formation of the dense film to have high temperature durability [4].
The nanopowders have been synthesized by several methods such as glycine nitrate process [5], oxalic salt method [6], hydrazine [7], coprecipitation [8], and solgel [9].These chemical processes, however, involve a subsequent calcination at high temperatures beyond 700˚C, in order to obtain a stable crystalline phase.The LaCrO 3 is suitable for use as an interconnector, due to its high stability at high temperature.However, it has poor sinterability.The advantage of hydrothermal method is easy to control the particle size and shape, and it can be synthesized at low temperatures as a method of synthesis of crystalline nanopowders that depends on the solubility of the material under high pressure.
In this study, we synthesized doped LaCrO 3 nanopowder without secondary phase at low temperatures of 100˚C -230˚C using hydrothermal method for interconnect materials in solid oxide fuel cell (SOFC).In order to improve its poor sinterability and electrical properties, Ca, Sr and Co were doped on lanthanum chromites, the sintering and electrical properties behaviors were investigated.
The sintering of the nanopowders was conducted by a conventional firing in air.The nanopowders was mixed with poly vinyl alcohol as a binder and pressed into quadrilateral sheet with a diameter of 10 mm at 10 MPa, and then compressed by press (CARVER, 385L-0) at 1 psi for 5 min.The pellets were sintered in a covered alumina crucible at 1200˚C -1500˚C for 1 to 5 h in air.The pellet was heated at a constant heating rate of 5˚C/min up to the sintering temperature.

Characterizations
In order to determine the crystalline phase and the lattice parameter constants of the synthesized nanopowder, Xray diffraction analyses were carried out.The crystal structures of the sintered specimens were examined using an X-ray diffractometer (XRD, D/MAX 2500-V/PC, Rigaku, Japan) with graphite-monochromatized Cu Ka radiation at 40 kV and 100 mA.Diffraction patterns were taken from 10 to 80˚ at a scanning speed of 4˚/min.Morphological aspects of the nanopowders were examined by scanning electron microscope (FE-SEM, Supra 40, Zeiss) equipped with an energy dispersive X-ray spectroscopy (EDX).The particle size was measured by particle size analyzer (PSA, Beckman Coulter LS 13 320).Moreover, the relative densities of the hydrothermally synthesized nanopowders were calculated from the densitometer.The electrical conductivity of the samples was studied at 750˚C in air by a standard DC four point probe method.

Result and Discussion
The highly crystalline LaCrO 3 nanopowders were prepared by hydrothermal method using several species of precipitants such as urea, ammonia, potassium hydroxide and sodium hydroxide.Aqueous solutions with 0.05 M of La(NO 3 ) 3 to Cr(NO 3 ) 3 and dopants (Ca, Sr, Co) were prepared with deionized water.The aqueous solutions of precipitants which are KOH of 0.35 M, NaOH of 0.5 M, NH 2 CONH 2 of 0.25 M and NH 4 OH of 0.3 M were prepared.Table 1 shows the optimal hydrothermal synthesis condition of LaCrO 3 , according to the species of precipitants.Figure 1 shows X-ray diffraction patterns of the reaction product after hydrothermal reaction at the several conditions.LaCrO 3 phase formed when it treated hydrothermally in the autoclave at 250˚C for 30 h using urea and ammonia as the precipitant.However, precipitants such as KOH and NaOH result in the complete transformation to the perovskite structure of ABO 3 type at the low temperature, which is the reaction temperature of 220˚C to 230˚C.The XRD pattern of the specimen Copyright © 2013 SciRes.OJINM Table 1.The optimal hydrothermal syntheses conditions of LaCrO 3. prepared from the different ratio of KOH with nitrate precursor shows that it occurs small amounts of secondary phases, such as La(OH) 3 and La(CO 3 )OH.Small amounts of secondary phases were only observed when the NH 2 CONH 2 of 0.25 M was used as a precipitant.The preferential formation of La(CO 3 )OH phase might be attributed to CO 2 absorption in alkaline media [10].The presence of a small amount of La(OH) 3 might be due to an incongruent dissolution behavior of La 3+ in alkaline solvents [11].

Reaction conditions
Figure 2 shows the morphology of hydrothermally synthesized LaCrO 3 nanopowders observed by SEM and the particle size measured by PSA.It shows some differences on particle shape and size when various precipitants were used.When NaOH of 0.5 M as a precipitant was used, the particle size was approximately 230 nm and it exhibits clusters of round-shaped particles as shown in Figure 2(a).The LaCrO 3 nanopowder using KOH of 0.35 M has a regular morphology, which is oval shape and homogeneous size distribution as shown in Figure 2(b), the average particle size was less than 100 nm.The LaCrO 3 nanopowder, using NH 2 CONH 2 of 0.25 M had irregular morphology and wide particle size distribution in which particle size was about 380 nm, as shown in Figure 2(c).In contrast, the LaCrO 3 nanopowders, using NH4OH of 0.3 M formed plate type and small particle agglomerated together with larger cluster.The particle size was about 280 nm as shown in Figure 2(d).The shape of the nanoparticles seems to be affected by species of the precipitants.
We also synthesized the doped lanthanum chromites (La 1-x M x Cr 1-y Co y O 3 , M = Ca, Sr) by hydrothermal reaction at 220˚C.Potassium hydroxide was used as a precipitant, and then the mole ratio of KOH to nitrates precursor was 7:1 in all composition.The prepared specimens were La 1-x M x Cr 1-y Co y O 3 with x = 0 to 0.4 and y = 0 to 0.2.The nitrate precursor was mixed in distilled water.Figure 3 shows the X-ray diffraction patterns of doped LaCrO 3 nanopowders.Figure 3(a) shows X-ray diffraction patterns of the Ca-doped LaCrO 3 nanopowders.Most of Ca-doped LaCrO 3 nanopowders were obtained crystalline phase, but the La 0.85 Ca 0.15 CrO 3 was observed a small amount of secondary phase, CaCrO 4 .Generally, the X-ray diffraction pattern peaks is shifted to higher angle with increasing calcium concentration.It could be decreased the lattice constants of perovskite structure because the Ca ions is substituted to La ions, due to the Ca ionic radius (0.99Ả) smaller than La (1.15Ả) [12].pends on the amount of doping precursor.Sr-doped La-CrO 3 can synthesize homogeneous nanopowders at relatively low temperature.Additionally, secondary phases such as LaCrO 4 , SrCrO 4 and Sr(NO 3 ) 2 was observed.It is possible to the solubility limits of Sr in the LaCrO 3 .La-CrO 4 can be considered as an intermediate phase in the lanthanum chromites [14,15].We studied the influence of additives, which are calcium, strontium and cobalt, for the densification behavior of the doped LaCrO 3 nanopowders.The relative densities of doped LaCrO 3 sintered at temperature ranges of 1200˚C to 1500˚C are shown in Figure 4.The relative density of the undoped LaCrO 3 is 74.9% at 1400˚C.The densities of doped LaCrO 3 were increased with increasing the sintering temperature.The density of all the A-site doped LaCrO 3 was increased at all the temperature ranges.The density of (Ca,Co)-doped LaCrO 3 was higher than (Sr,Co)-doped LaCrO 3 .The La 0.62 Ca 0.38 Co 0.18 Cr 0.82 O 3 sintered at 1200˚C for 4 h exhibits the highest density.When the sintering temperature increased over 1500˚C, the relative density was hardly change because the doped LaCrO 3 was fully sintered at 1400˚C for 4 h.The sintering temperature of La 0.62 Ca 0.38 Co 0.18 Cr 0.82 O 3 is approximately 200˚C lower than undoped LaCrO 3 samples, due to enhanced sinterability by doping Ca and Co [16][17][18].
Figure 5 shows the microstructures of the nanopowders and sintered doped LaCrO 3 nanopowders at the several conditions.Figure 6 shows the electrical conductivity of doped LaCrO 3 measured at 750˚C in air.The electrical conductivity increased in all the doped LaCrO 3 .The La 0.62 Ca 0.38 -Co 0.18 Cr 0.82 O 3 sintered at 1200˚C had maximum conductivity of 32.73 S/cm at 750˚C in air, which is about 30 times as high as that of undoped LaCrO 3 .

Acknowledgements
This research was financially supported by the Ministry of Knowledge and Economy, Korea (grant no.10037 152).

Figure 5 (
a) shows micrograph of La-CrO 3 nanopowder and the particle size was ~100 nm.

Figure 4 (
b) shows the fracture surface of the LaCrO 3 sintered at 1400˚C for 4 h.

Figure 5 (
c) shows the microstructure of the La 0.62 Ca 0.38 Co 0.18 Cr 0.82 O 3 nanopowder which is the optimum composition, and the particle size

Figure 4 .
Figure 4. Relative density of doped LaCrO 3 at the several sintering temperature. 100nm

Figure 5 .
Figure 5. SEM micrographs of doped LaCrO 3 ; (a) LaCrO 3 nanopowder; (b) Fracture surface of (a) sintered at 1400˚C for 4 h; (c) La 0.62 Ca 0.38 Co 0.18 Cr 0.82 O 3 nanopowder; (d) Fracture surface of (c) sintered at 1200˚C for 4 h; (e) La 0.8 Sr 0.2 Co 0.15 Cr 0.85 O 3 nanopowder; and (f) Fracture surface of (e) sintered at 1500˚C for 4 h.was about 80 nm.Figure 5(d) shows the fracture surface of La 0.62 Ca 0.38 Co 0.18 Cr 0.82 O 3 sintered at 1200˚C for 4 h.Figure 5(e) shows microstructure of the La 0.8 Sr 0.2 Co 0.15 -Cr 0.85 O 3 nanopowder and the particle size was ~200 nm.Figure 5(f) shows the fracture surface of the La 0.8 Sr 0.2 -Co 0.15 Cr 0.85 O 3 sintered at 1500˚C for 4 h.When sintered at over 1200˚C, the doped LaCrO 3 were densely sintered.It indicated that sintering density is dependent on the

Figure 5 (
d) shows the fracture surface of La 0.62 Ca 0.38 Co 0.18 Cr 0.82 O 3 sintered at 1200˚C for 4 h.

Figure 5 (
e) shows microstructure of the La 0.8 Sr 0.2 Co 0.15 -Cr 0.85 O 3 nanopowder and the particle size was ~200 nm.

Figure 5 (
f) shows the fracture surface of the La 0.8 Sr 0.2 -Co 0.15 Cr 0.85 O 3 sintered at 1500˚C for 4 h.When sintered at over 1200˚C, the doped LaCrO 3 were densely sintered.It indicated that sintering density is dependent on the additives and sintering temperature.

LaCrO 3
nanopowders were synthesized by hydrothermal method at temperature ranges of 100˚C to 230˚C for 8 -30 hrs.The average particle size of undoped LaCrO 3 was approximately 80 nm when the KOH was used as a precipitant.The particles size of (Sr, Co)-doped LaCrO 3 nanopowder was approximately 200 nm.The (Ca,Co)doped LaCrO 3 nanopowders exhibit improved crystalline compared with the undoped LaCrO 3 nanopowders.The undoped LaCrO 3 nanopowder has a poor sinterability in air, but a highly dense La 0.62 Ca 0.38 Co 0.18 Cr 0.82 O 3 was obtained by doping Ca and Co.The sintering temperature of La 0.62 Ca 0.38 Co 0.18 Cr 0.82 O 3 is about 200˚C lower than undoped LaCrO 3 .The relative density increased over 97% by doping Ca on perovskite A site and Co on perovskite B site.The electrical conductivity of La 0.62 Ca 0.38 -Co 0.18 Cr 0.82 O 3 was 32.73 S/cm.However, Sr-doped lanthanum chromite nanopowders were observed the secondary phases of LaCrO 4 and SrCrO 4 for all the compositions.The electrical conductivity of La 0.8 Sr 0.2 Co 0.15 -Cr 0.85 O 3 was 29.4 S/Cm at 750˚C.The density and electrical conductivity were increased by doping calcium and cobalt on the LaCrO 3 .The (Ca, Co)-doped LaCrO 3 is adequate for applications as an interconnector in SOFC.

Figure 6 .
Figure 6.Electrical conductivity of doped LaCrO 3 measured at 750˚C in air.