Enhanced sinterability of mechanical alloyed La9.33Si2Ge4O26 oxyapatite powders for IT-SOFC electrolytes

Dense oxyapatite-based La_9.33Si₂Ge₄O₂₆ electrolytes have been successfully prepared by electrical sintering at 1400 °C in static air for 1 h from dry milling La₂O₃, SiO₂ and GeO₂ powders, in adequate atomic proportions, at 350 rpm for 15 h, under controlled environmental conditions, in a planetary ball mill. The densification behaviour of apatite-type phase La_9.33Si₂Ge₄O₂₆ powders synthesized by mechanical alloying was investigated through microstructural evolution with sintering temperature by means of XRD and SEM/EDS analyses. The content of germanium in the sintered samples remained almost constant, suggesting that its incorporation in the apatite phase hinders the high temperature (>1250 °C) volatilization process.     

Electrical characterization of La9.33Si2Ge4O26 oxyapatite for prospective intermediate-temperature solid oxide fuel cells

La_9.33Si₂Ge₄O₂₆ materials have been fabricated from La₂O₃, SiO₂ and GeO₂ powders by high speed mechanical alloying followed by conventional and microwave hybrid sintering at different temperatures and holding times. XRD data showed that the apatite phase is formed after 1 h of mechanical alloying at 850 rpm. This phase remained stable after conventional sintering in an electric furnace with density increasing as sintering temperatures and holding times were increased. However, the highest density was achieved for samples sintered in the microwave furnace (5.44 g cm⁻³), corresponding to a relative density of 98%. The electrical conductivity of the samples microwave sintered at 700 and 800 ºC are 4.72×10⁻³ and 1.93×10⁻² S.cm⁻¹, respectively, with a correspondent activation energy of 0.952 eV.     

Sintering mechanisms of Yttria with different additives

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B₂O₃, YF₃, Al₂O₃, ZrO₂, and TiO₂, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B₂O₃ and YF₃ could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO₂, Al₂O₃ and ZrO₂ appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La₂O₃ added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.     

Synthesis and densification of lanthanum silicate apatite electrolyte for intermediate temperature solid oxide fuel cell via co-precipitation method

Lanthanum silicate apatite (LSA, La_9.33+xSi₆O_26+1.5x, x = 0–0.67) has been widely investigated as a promising electrolyte material for intermediate temperature solid oxide fuel cell (SOFC). In this work, a facile and low-cost co-precipitation method is used to synthesize LSA precursor powders. The well dispersed nanopowders (ca. 70 nm) with pure hexagonal LSA phase are obtained by calcining the precursor at 900 °C. Impurity of La₂SiO₅, caused by the different precipitation productivities of La(NO₃)₃ and TEOS, can be eliminated through lowering the La/Si ratio in the starting mixtures. The dispersant (PEG200) plays a crucial role in co-precipitation processes, which can effectively mitigate the agglomeration and therefore significantly improve the sinterability of the nanoparticles. Dense LSA ceramic with relative density of 98% is obtained after sintering at 1550 °C, which exhibits a conductivity of 0.13 mS cm⁻¹ at 500 °C.     

Microstructure and mechanical properties of tantalum carbide ceramics: Effects of Si3N4 as sintering aid

Stoichiometric Tantalum carbide (TaC) ceramics were prepared by reaction spark plasma sintering using 0.333–2.50 mol% Si₃N₄ as sintering aid. Effects of the Si₃N₄ addition on densification, microstructure and mechanical properties of the TaC ceramics were investigated. Si₃N₄ reacted with TaC and tantalum oxides such as Ta₂O₅ to form a small concentration of tantalum silicides, SiC and SiO₂, with significant decrease in oxygen content in the consolidated TaC ceramics. Dense TaC ceramics having relative densities >97% could be obtained at 0.667% Si₃N₄ addition and above. Average grain size in the consolidated TaC ceramics decreased from 11 µm at 0.333 mol% Si₃N₄ to 4 µm at 2.50 mol% Si₃N₄ addition. The Young's modulus, Vickers hardness and flexural strength at room temperature of the TaC ceramic with 2.50 mol% Si₃N₄ addition was 508 GPa, 15.5 GPa and 605 MPa, respectively. A slight decrease in bending strength was observed at 1200 °C due to oxidation of the samples.     

Oxy-apatite reaction sintering of colloidal and classic ceramic processed powders

Oxy-apatites are promising electrolyte materials for intermediate-temperature solid oxide fuel cells, IT–SOFC. However the requirements of electrolytes make necessary the preparation of dense films with the appropriated composition to show good electrical properties; in this way, colloidal processing is a key issue.This work involves the application of colloidal processing for four oxygen-excess oxy-apatites, La_9.67(Si₆O₂₄)O_2.5, La₁₀(Si₆O₂₄)O₃, La₁₀(Si_5.5Al_0.5O₂₄)O_2.75 and La₁₀(Si₅Al₁O₂₄)O_2.5 and their characterization (phases, microstructure and electrical properties). The results have been compared with those obtained by classical ceramic method to assure the same composition without loosing properties. Samples with the desired compositions were obtained by reaction sintering of La₂O₃, SiO₂ and Al₂O₃. La₁₀(Si_5.5Al_0.5O₂₄)O_2.75 prepared by colloidal processing and heated at 1923 K showed the highest conductivity value, 3.0 × 10^?2 S cm^?1 at 973 K. Furthermore, its residual porosity was very low. On the other hand, La₁₀(Si₆O₂₄)O₃ stoichiometry was tried by colloidal and ceramic methods under several experimental conditions. Unfortunately, the obtained oxy-apatite seems to have slightly lower lanthanum content. In spite of previous reports claiming the preparation of stoichiometric La₁₀(Si₆O₂₄)O₃, this study cannot support these findings.     

Spark plasma sintering of silicon nitride/barium aluminum silicate composite

Si₃N₄ based composites were successfully sintered by spark plasma sintering using low cost BaCO₃, SiO₂ and Al₂O₃ as additives. Powder mixtures were sintered at 1600–1800 °C for 5 and 10 min. Displacement-temperature-time (DTT) diagrams were used to evaluate the sintering behavior. Shrinkage curve revealed that densification was performed between 1100 and 1700 °C. The specimen sintered at 1700 °C showed the maximum relative density (99.8±0.1%), flexural strength (352±16 MPa), Vickers harness (11±0.1 GPa) and toughness (5.6±0.05 MPa m^1/2).     

Single step reactive sintering and chemical compatibility between La9Sr1Si6O26.5 and selected cathode materials

Apatite-type silicates are considered as promising electrolytes for solid oxide fuel cells (SOFC). However more studies on the chemical compatibility of these materials with common SOFC electrodes are required. Here, we report the synthesis of single phase La₉Sr₁Si₆O_26.5 composition by reactive sintering at 1650 °C for 10 h. Fully dense pellets showed very high oxide-anion conductivity, 25 mS cm^?1 at 700 °C. Furthermore, the chemical compatibility of La₉Sr₁Si₆O_26.5 with some selected cathode materials has also been investigated. The lowest reaction temperatures were determined to be 1100 °C, 1000 °C and 900 °C for La_0.8Sr_0.2MnO_3?δ, La₂Ni_0.8Cu_0.2O₄ and La_0.6Sr_0.4Co_0.8Fe_0.2O₃, respectively. The segregation of minor amounts of SiO₂ seems to be a key limiting factor that must be overcome. Finally, these cathode materials were deposited over dense oxy-apatite pellets and the area specific resistances in symmetrical cells were determined. These values, at 700 °C, were 14.4 and 2.6 Ω cm² for La_0.8Sr_0.2MnO_3?δ and La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ, respectively. Furthermore, the area specific resistances are notably improved 0.6 Ω cm² when a 50 wt.% composite of La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ and Ce_0.8Gd_0.2O_1.9 is used.     

Diamond dispersed Si3N4 composites obtained by pulsed electric current sintering

30 vol.% 2 and 30 μm diamond dispersed Si₃N₄ matrix composites were prepared by pulsed electric current sintering (PECS) for 4 min at 100 MPa in the 1550–1750 °C range. The densification behaviour, microstructure, Si₃N₄ phase transformation and stiffness of the composites were assessed, as well as the thermal stability of the dispersed diamond phase. Monolithic Si₃N₄ with 4 wt% Al₂O₃ and 5 wt% Y₂O₃ sintering additives was fully densified at 1550 °C for 4 min and 60 MPa. The densification and α to β-Si₃N₄ transformation were substantially suppressed upon adding 30 vol.% diamond particles. Diamond graphitisation in the Si₃N₄ matrix was closely correlated to the sintering temperature and grit size. The dispersed coarse grained diamonds significantly improved the fracture toughness of the diamond composite, whereas the Vickers hardness was comparable to that of the Si₃N₄ matrix ceramic. The Elastic modulus measurements were found to be an excellent tool to assess diamond graphitisation in a Si₃N₄ matrix.     

Synthesis of nano-crystalline apatite type electrolyte powders for solid oxide fuel cells

Rare earth silicates with apatite structure are being actively studied as an alternative electrolyte material for solid oxide fuel cells (SOFC) operating in the intermediate temperature range. In this paper we report on the synthesis of La_9.33+x/3Al_xSi_6?xO_26+δ (with x = 0–1.5) and La_9.83Fe_1.5Si_4.5O₂₆ powders using a modified sol–gel process. The parameters involved in the process have been optimized for preparing phase pure, homogeneous and nanometer sized powders. The obtained powders were characterized using scanning electron microscopy, X-ray diffraction and thermal analysis. Pressureless sintering experiments were performed and pellets having relative densities greater than 96% could be obtained after 5 h dwelling in the temperature range between 1500 and 1550 °C.     

Rare-earth element doped Si3N4/SiC micro/nano-composites—RT and HT mechanical properties

Dense Si₃N₄/SiC micro/nano-composites with varying grain boundary phase composition were fabricated by hot-pressing under the same conditions. Six different sintering aids (Lu₂O₃, Yb₂O₃, Y₂O₃, Sm₂O₃, Nd₂O₃ and La₂O₃) were used. The formation of SiC nano-inclusions was achieved by in situ carbothermal reduction of SiO₂ by C during the sintering process. Room temperature, fracture toughness, hardness and strength tended to increase when the cation radius of the rare-earth element used in the oxide additive decreased (i.e. from La³⁺ to Lu³⁺). The composite material with Lu₂O₃ sintering additive showed the highest hardness and had reasonably high fracture toughness and strength. The same micro/nano-composite also possessed the highest creep resistance in the temperature range from 1250 °C to 1400 °C and with loads in the range 50–150 MPa.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.     

Mixed-potential type NO2 sensor based on La10Si6O27 electrolyte and WO3 sensing electrode with different morphologies

A novel mixed-potential type NO₂ sensor was fabricated using La₁₀Si₆O₂₇ electrolyte and WO₃ sensing electrode (SE). The sinterability of La₁₀Si₆O₂₇ was significantly improved by the introduction of Y₂O₃ as sintering aid. WO₃ with different morphologies prepared by the citric acid (CA) assisted hydrothermal method was examined as the sensing electrodes of the mixed-potential type NO₂ sensors based on La₁₀Si₆O₂₇ electrolyte. The results showed that 6 wt% Y₂O₃ added La₁₀Si₆O₂₇ electrolyte sample could get quite dense at a temperature as low as 1500 °C. The morphologies and phase constituents of WO₃ were influenced by the CA content. The sensor showed good response–recovery characteristics. Compared with the sensor based on the irregular WO₃ particles or nanorods, the sensor using WO₃ nanosheets-SE with hexagonal structure exhibited much higher sensitivity (195.6 mV/decade) to NO₂ at 550 °C. The response signals of the sensor were slightly affected by coexistent O₂ varying from 5 to 20 vol%.     

Structural modifications of lanthanum silicate oxyapatite exposed to high water pressure

Dense lanthanum silicate oxyapatite samples La_9.33 + x(SiO₄)₆O_2 + 3x/2 were exposed to high water pressure in autoclave, in order to study the effects of oxygen stoichiometry and treatment duration on the protonation of these materials. TG analyses showed that protonic species were successfully introduced into the bulk of the material, especially for La_9.60(SiO₄)₆O_2.4 sample after 84 h of treatment at 550 °C under 40 bar water pressure. It was shown that the mass loss compares well with perovskite materials and increases when protonation time rises (0.66% above 600 °C for 408 h), with a time limit beyond which the microstructure is no more stable.Rietveld refinement and Raman spectroscopy studies confirmed some structural modifications which could be linked to the incorporation of protonic species. It was shown that autoclave treatment induced an increase of the cell volume related to an increase of the a and b lattice parameters (enlargement of the characteristic channels of oxyapatite).     

Poly(N-isopropylacrylamide) assisted microwave synthesis of magnesium aluminate spinel oxide for production of highly transparent films by hot compression

Magnesium aluminate spinel oxides have been prepared via poly(N-isopropylacrylamide) assisted microwave technique. The prepared MgAl₂O₄ powders showed a crystalline cubic structure with spinel phase after calcination at 600 °C only. The poly(N-isopropylacrylamide) amount showed a high effect on the crystallite size and the densification behavior of MgAl₂O₄. The increase of the amount of poly(N-isopropylacrylamide) reduced the sintering temperature of MgAl₂O₄ from 1400 °C to 1050 °C. The hot-pressed of MgAl₂O₄ powders in the presence of 3 wt% of poly(N-isopropylacrylamide) exhibited a full density at sintering temperature 1100 °C in 15 min only. The sintered films showed high transparency (81 ± 2%) in the wavelength range 500–1000 nm.     

Microstructure and properties of mullite–ZrO2 ceramics with silicon nitride additive prepared by spark plasma sintering

The process of densification and development of the microstructure of mullite–ZrO₂/Y₂O₃ ceramics from mixture of Al₂O₃, SiO₂, ZrO₂ and Y₂O₃ by gradually adding of α–β Si₃N₄ nanopowder from 1 to 5 wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400 °C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si₃N₄ additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33 g/cm³, while for traditionally sintered samples, the value is 2.55 g/cm³, and the compressive strength for SPS grows with Si₃N₄ additives, reaching 600 N/mm². In the case of traditional sintering, it decreases to approximately 100 N/mm². The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO₂ grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si₃N₄ additive, are filled with mullite crystalline formations.     

Assessment of thermochemically stable apatite La10(SiO4)6O3 as electrolyte for solid oxide fuel cells

Apatite-type lanthanum silicate with a formula of La₁₀(SiO₄)₆O₃ is a potential candidate electrolyte for SOFC system because of its high ionic conductivity and low activation energy. Pure La₁₀(SiO₄)₆O₃ powder was prepared by solid state reaction and using a suitable thermal pretreatment (1000 °C/5 h) of the as-purchased La₂O₃ powder. Materials characterization, thermal behaviors, and electrical properties of La₁₀(SiO₄)₆O₃ samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and two-point probe DC conductivity. A pure La₁₀(SiO₄)₆O₃ pellet was prepared at the sintering process of 1600 °C and 2.5 h with a high relative sintered density of 96.91%. The existence of secondary phases in La₁₀(SiO₄)₆O₃ pellet resulted in a lower conductivity than that of pure La₁₀(SiO₄)₆O₃ pellet. Eight-hour reduction-resistant stability tests under reducing atmosphere at the elevated temperature of pure La₁₀(SiO₄)₆O₃ prepared in this study shows a good thermochemical stability as compared to the well-developed 8 mol% Y₂O₃ stabilized ZrO₂ (8YSZ).     

Tape casting of new electrolyte and anode materials for SOFCs operated at intermediate temperature

An important task in SOFC research is the reduction of the operating temperature to 700 °C, to avoid premature ageing of the cell components and the use of expensive interconnect materials. This requires the development of new electrolytes and electrodes materials.Oxyapatite electrolytes have recently attracted considerable attention and we have already reported some interesting performances for the La₉Sr₁Si₆O_26.5 composition (σ = 8.8 × 10⁻³ S cm⁻¹ at 700 °C) [Beaudet Savignat, S., Lima, A., Barthet, C. and Henry, A., In Proceedings of the International Symposium Solide Oxide Fuel Cells VIII, Vol. 2003–2007, pp. 372–378].This study was centered on the manufacturing of an apatite electrolyte and a Ni/apatite cermet anode by the tape casting process, with a view to the development of anode/electrolyte 1/2 cells.Slurries compositions were first optimized to adjust green tapes characteristics. Secondly, we focused on the binder burnout and the sintering. Dense electrolytes were synthesized. The influence of the particle size of the apatite powder, with a fixed NiO powder particle size, and the influence of the addition of pyrolyzable organic particles on the sintering and the microstructure of the anode material were studied.     

Liquid phase sintering of Al2O3/SiC nanocomposites

Al₂O₃/SiC nanocomposites are usually prepared by hot pressing or using high sintering temperatures, viz. 1700°C. This is due to the strong inhibiting effect of the nano-sized SiC particles on the densification of the material. Liquid phase sintering (LPS) can be used to improve densification. This work explored two eutectic additive systems, namely MnO₂.SiO₂ (MS) and CaO.ZnO.SiO₂ (CZS). The additive content in Al₂O₃/5 wt% SiC nanocomposite material varied from 2 to 10 wt%. Densities of up to 99% of the theoretical value were achieved at temperatures as low as 1300°C. Characterisation of the materials by XRD, indicated the formation of secondary crystalline phases in addition to Al₂O₃ and SiC. SEM and TEM analysis showed the presence of a residual glassy phase in the grain boundaries, and an increase in the average grain size when compared to nanocomposites processed without LPS additives.     

Microstructure and electrical conductivity of fast fired Sr- and Mg-doped lanthanum gallate

La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ solid electrolytes were consolidated by fast firing aiming to investigate the effects of the sintering method on densification, microstructure and ionic conductivity. Powder mixtures were prepared by solid state reaction at 1250 and 1350 °C for 12 h, and fast fired at 1450 and 1500 °C temperatures for 5 and 10 min. The content of impurity phases was found to be quite low with this sintering method. Relatively high density (>90% of the theoretical value) and low porosity (<1.5%) were readily obtained for powder mixtures calcined at 1250 °C. The activation energy for conduction was approximately 1 eV. Specimens fast fired at 1450 °C for 10 min with a mean grain size of 2.26 µm reached the highest value of total ionic conductivity, 22 mS cm⁻¹, at 600 °C.