WO2014003591A1

WO2014003591A1 - Process for manufacturing a solid oxide fuel cell electrolyte using microwave sintering

Info

Publication number: WO2014003591A1
Application number: PCT/PT2013/000039
Authority: WO
Inventors: Bruno Miguel QUELHAS DE SACADURA CABRAL TRINDADE; Albano Augusto CAVALEIRO RODRIGUES DE CARVALHO; Fernando DE ALMEIDA COSTA OLIVEIRA; João Manuel GREGÓRIO MASCARENHAS; Teresa Maria ROSADO CORTES SIMÕES MARCELO; Cátia Alexandra PODENCE ALVES
Original assignee: Lneg - Laboratório Nacional De Energia E Geologia; Universidade De Coimbra
Priority date: 2012-06-29
Filing date: 2013-06-25
Publication date: 2014-01-03

Abstract

The present invention relates to a new process for the production of a solid oxide electrolyte for fuel cells and comprises the following steps of: a) mixing powders of La203, Si02, and Ge02 by high energy milling, in a controlled atmosphere, using variable rotational speeds between 150 and 350 rpm and milling times between 15 h and 35 h, in order to obtain the desired oxyapatite phase; b) compaction of the resulting mixture either by uniaxial pressing, with pressures in the range 390-885 MPa or by cold isostatic pressing, with pressures ranging from 200 to 320 MPa, in order to obtain compacts with green densities between 65 and 75% of the theoretical density; c) sintering of the compacts in a microwave oven with frequencies between 2 and 3 GHz, power rating equal or higher than lkW, temperatures up to 1350 °C and sintering times from 5 minutes to 60 minutes, using silicon carbide susceptors, capable of absorving the microwaves from the room temperature and d) cooling of the sintered compacts in the furnace, under an argon atmosphere with a flowing rate of 2-10 L/min.

Description

DESCRIPTION

"PROCESS FOR MANUFACTURING A SOLID OXIDE FUEL CELL ELECTROLYTE

USING MICROWAVE SINTERING"

Field of the Invention The present invention is directed to a novel process for obtaining dense ceramics based on lanthanum oxides doped with silicon and germanium. These materials are applied as electrolytes in solid oxide fuel cells and have similar ionic conductivity (in the range 500-750 °C) than the ones based on yttria-stabilized zirconia, commonly used in this type of application .

Background of the Invention Solid oxide fuel cells (SOFC) are electrochemical devices capable of directly convert chemical energy into electrical energy with high efficiency and low pollutants emission [1] . This type of cells has the particularity of its electrolyte being formed by solid oxides. The advantages of these cells include high efficiency, long term stability, flexibility for using different fuels and reduced emissions of polluting gases. The main disadvantage of this technology is its high operating temperature, resulting in longer times for starting operation, materials degradation and chemical and mechanical compatibility problems at high temperatures, in both oxidizing and reducing atmospheres. Unlike proton exchange membrane fuel cells (PEMFC) that lead the transport of positive hydrogen ions (protons) from the anode to the cathode through a polymer electrolyte, the solid oxide electrolyte present in SOFCs carries negative oxygen ions from the cathode to the anode. Electrochemical oxidation of the oxygen ions with hydrogen or with carbon monoxide takes place on the anode side.

SOFCs typically operate at temperatures between 500 and 1000 °C. At these high temperatures the use of expensive platinum catalysts is not necessary, contrary to what occurs with cells operating at lower temperatures (such as PEMFC) . Accordingly, PEMFC are vulnerable to catalyst poisoning, for instance by carbon monoxide.

SOFCs can be used in a wide range of applications, including auxiliary electricity generating units for power fuel cell vehicles and stationary power stations with powers in the range of 100 W to 2000 MW. The efficiency of a solid oxide fuel cell can reach up to 80%, especially in cases that it can be used together with heat recovery or cogeneration systems. Unlike other types of fuel cells, SOFCs can have multiple design geometries. The planar cell geometry consists basically in stacks where the electrolyte is placed between two electrodes. The SOFCs can also present tubular type geometries in which air and the gaseous fuel circulate in different channels, thus preventing them to mix. However, planar cell stacks have currently a better performance than the tubular ones owing to their greater mechanical resistance.

Each cell consists basically of a dense electrolyte in direct contact with two porous electrodes. It is essential that the electrolyte is dense to guarantee gas tightness in order to prevent mixing of the gases that are widespread through the electrodes. A solid oxide fuel cell is, therefore, made up of four layers (including the interconnector) with a total thickness typically not exceeding 1 mm, three of which are ceramics. These cells are connected in series being commonly referred to as "S.OFC stacks". The anion oxide conductivity of the ceramic electrolyte used in these cells increases with increasing temperature in the range of 500 to 1000 °G. The reduction of oxygen atoms in oxygen ions occurs at the cathode. These ions diffuse through the solid oxide electrolyte to the anode, where occurs the oxidation of the fuel, releasing heat. The reaction gives rise to a stream of electrons and water as a by-product. These electrons then flow in an external circuit.

The anode layer must be porous enough to allow the fuel to flow towards the electrolyte. In addition, it must, like the cathode, conduct electrons and have an adequate ionic conductivity. The most widely used anode material is a nickel- based cermet (which acts as a catalyst) mixed with yttria- stabilized zirconia (YSZ), whose main function is to prevent nickel particles to sinter to each other. The anode is typically the thickest and most resistant layer of a cell owing to the fact that it has the smallest polarization losses and must serve as mechanical support. On the other hand, the electrolyte is a dense ceramic layer that conducts oxygen ions. Its electronic conductivity must be as low as possible in order to avoid current losses and its ionic conductivity the highest possible. The most common materials used as electrolytes are the aforementioned YSZ and cerium oxide doped with gadolinium (GDC) or samarium (SDC) . In turn, the cathode of SOFCs, also known as air electrode, is a thin porous layer where oxygen reduction takes place. For this purpose, the cathode must have a suitable electrical conductivity, being the most used the LaGa0₃-based materials or lanthanum strontium manganite (LSM) , due to their good compatibility with zirconia-based electrolytes. Finally, the interconnection of multiple cells is usually made up of a ceramic (e.g. Lai_x(Sr or Ca)_xCr0₃) or a metallic layer (either a superalloy or stainless steel) placed between the various individual cells.

The main problem associated with the existing technology lies in finding advanced materials with high durability performance at high temperatures, which can be manufactured by innovative techniques at low cost.

Oxide ion conductors, such as apatite-type oxides, have been attracting considerable interest as potential candidates in a variety of technological applications, including electrolyte materials in solid oxide fuel cells (SOFCs) [2] . The solid electrolyte is the main critical component of these devices, in terms of high efficiency, low production costs and greater operating stability over the long term. An electrolyte must have a high ionic conductivity (typically > 1 x lO^-3 S cm^"1 at temperatures above 500 °C) , low electronic conductivity, high thermal shock resistance and chemical stability, as well as an adequate gas-tightness and compatibility (both thermal and chemical) with the electrodes. There are still several technical issues that need to be overcome before the large-scale commercialisation of these devices becomes a reality, especially at the level of the development of new materials whose properties would allow a better performance and greater durability of these cells.

At present, electrolytes based on YSZ which have high ionic conductivity at high temperature (850-1000 °C) are the most widely used, despite the problems associated with sealing and the consequent short service lifetime of these devices. To increase components durability it is necessary to reduce the working temperature. One way of reaching this objective would be to use thinner YSZ electrolytes, which is not an ideal solution and thence new materials are being sought to replace the YSZ electrolyte. Summary of the invention

The present invention deals with a novel method to fabricate an electrolyte for solid oxide fuel cells based on lanthanum oxide doped with silicon and germanium, consisting of mechanical synthesis of the apatite phase through high energy ball milling followed by densification of the obtained powders through sintering in a 2.45 GHz microwave furnace with multimodal resonant cavity. The process comprises selection of the precursor powders and correspondent grain size distributions, powders compaction either by unidirectional pressing or cold isostatic pressing and non-conventional sintering in a microwave furnace using silicon carbide susceptors. The materials can be used as electrolytes of solid oxide fuel cells, their ionic conductivities are similar to those of the YSZ based electrolytes (between 500 and 750 °C) usually used in this type of application. Detailed description of the invention

Recent research works have focused on materials that can replace YSZ electrolytes based on apatite-type phase (oxyapatite) , such as the doped lanthanum oxides of general formula Laio (M0₄) ξ0₂, M being Ge, Si, Al or P. These oxides have high ionic conductivity and low activation energy at intermediate temperatures (500 - 750 °G) when compared to those of the YSZ electrolytes. In the crystal structure of the apatite-type phase, channels are formed through which transport of the oxygen Ions is possible and this can justify its high ionic conductivity. The ionic conductivity is dependent on the deficiency of La atoms (non-stoichiometric composition) , the larger the lack of La atoms, the higher the ionic conductivity, the limit of deviation from the stoichiometry depending on the system under consideration.

One of the main limitations associated to the fabrication of oxyapatite-based materials is their poor sinterability and considerable efforts have been dedicated for obtaining apatite-based materials through several processing routes, such as solid state reactions, sol-gel synthesis, hot pressing, high energy milling associated to a process of azeotropic distillation or colloidal processing. Several problems associated to the manufacturing of very thin hermetic electrolytes were also found and states-of-the-art on this subject have been recently published [3, 4] . Difficulties in controlling grain growth and grain morphology as well as Ge0₂ volatilization above 1250 °C [5, 6] require a careful analysis since besides high ionic conductivity, these materials must also exhibit adequate thermo-mechanical properties to be inserted in a fuel cell. Mechanical alloying (MA) has been considered an alternative route for processing these lanthanum oxide-based materials at lower cost [7] . During the MA, the particles are hardly deformed, the density of defects is increased and the diffusion path is decreased, thus diffusion is accelerated. As a consequence, these highly disordered structures can be densified at temperatures lower than those needed in conventional routes (several days at temperatures above 1600 °C) . The main drawbacks of the M are (a) Fe contamination which can increase the materials sinterability but decrease their ionic conductivity [8] and (b) inadequate chemical homogeneity which can lead to formation of undesired low ionic conductivity secondary phases. In previous works, Si and Ge doped lanthanum oxide based materials with apatite-type phase have been produced by MA and further conventional oven annealing [9] . The influence of some MA parameters on the apatite/type phase formation has been observed. Hence, the higher the rotation speed the lower the annealing temperature. In optimized conditions, it was possible to obtain the Si/Ge doped apatite phase at 350 rpm upon 15 h milling at room temperature.

The actual invention reveals the possibility of densifying the obtained MA powders through microwave sintering i a 2.45 GHz microwave furnace together with silicon carbide susceptors at temperatures about 250 °C lower when compared to conventional solid state sintering processes (1600 °C) . Lower temperatures processing minimizes germanium oxide volatilization that occurs at temperatures above 1250 °C.

One can infer from the obtained results that it will be mandatory to optimize powder mixtures preparation conditions in order to maximize green density, taking into account especially powder particles sizes and their distribution. Hence, higher green density favours higher final density of the components with the desired properties, as the ceramic particles heating occurs mainly through a volumetric resistive heating mechanism, from its core to the surface.

From the interaction between materials and microwaves, it is possible, in certain temperature and frequency conditions, to convert electromagnetic energy in heat. This conversion depends, amongst other factors, on the material's dielectric constant and electric conductivity (in the case of rather low conductivity materials) . In a conventional furnace, the material is heated by heat transfer through conduction, convection and radiation mechanisms (mainly at high temperatures, i.e. above 700 °C) , whilst in a microwave furnace the electromagnetic field power is converted into heat within the material.

Radiation penetration depth (D) , that relates the power dissipated as heat with the distance traveled by the wave of incident radiation within a dielectric material, is defined as the depth at which the incident power is reduced by one half, as follows:

D = 3λ₀/8.686π tan δ(ε ) ^{1 2} where λ₀ is the incident radiation wavelength, tan6 is the loss tangent, 6 is the loss angle and ε_Γ' is the real dielectric constant .

By means of the electric field, dipoles rotation is induced thereby producing friction which in turn generates heat. There are dielectric materials which absorb MW radiation from room temperature, e.g. powdered graphite, silicon carbide, ferrites and titanates, amongst others. Most ceramic materials have low loss tangent and only absorb MW radiation above a certain temperature, known as critical temperature above which tan δ increases (0.1-20) converting the absorbed energy into heat. These materials densification is mainly assigned by green density and by the possible occurrence of "hot spots" that can produce an abnormal grain size growing and/or localized melting¹. In order to avoid these phenomena it is necessary to ensure a homogeneous electric field distribution within the resonant cavity. This can be achieved through susceptors that absorb MW radiation from room temperature. The susceptors heating allows the components heating up to a temperature from which they couple with MW radiation, absorbing it. Another way to ensure that the electric field is uniform consists of using spreaders or a turntable. When susceptors are used the heating process is called "hybrid". In the present case, the Si/Ge doped lanthanum oxides were sintered with the aid of susceptors owing to their low electrical conductivity at low temperatures. This non-conventional electromagnetic field heating method has some advantages over the conventional electric heating, allowing much faster heating rates and shorter sintering cycles (3.5 h from starting RT to final RT after cooling down, whilst in a conventional furnace it takes 12 h) . These are very interesting characteristics for Ge0₂ containing materials, which are prone to volatilization above 1250 °C.

Object of the Invention The object of the invention consists in a process for manufacturing an electrolyte for solid oxide fuel cells comprising the following steps: a) Mixing La₂03, Ge0₂ and Si0₂ powders by high energy milling, in a controlled atmosphere of argon or nitrogen, using variable rotating speeds from 150 up to 350 rpm and milling times between 15 h and 35 h until the desired oxyapatite phase Lac,.3₃Si₂Ge4026 s obtained; b) Compaction of the said resulting mixture by uniaxial pressing, using pressures in the range of 390-885 MPa, or by cold isostatic pressing, using pressures in the range of 200- 320 MPa, in order to achieve compacts with green densities between 65 and 75% of the theoretical density; c) Sintering of the said compacts in a microwave oven with 2.45 GHz frequency, with a nominal power higher than or equal to lkW at temperatures up to 1350 °C and dwell times between 5 min and 60 min, using silicon carbide susceptors, enabling microwaves to be absorbed from room temperature; and d) Cooling of the sais sintered compacts in argon atmosphere with a flow rate in the range of 2-10 L/min.

The raw materials (La₂0 , Ge0₂ and Si0₂ powders) are previously selected in order to have purity higher than 99% and particle size between. 0.1 and 10 μπι, and they are weighed (with an accuracy down to 0.01 g) in the suitable proportions to achieve the envisaged oxyapatite phase.

Milling is usually performed in a planetary ball mill made of heat treated steel, under argon or nitrogen atmosphere .

After milling the mixtures are characterized in relation to particles size distribution (laser dispersion), morphology (scanning electron microscopy) and structure (X-ray diffraction) to check if the desired oxyapatite phase has been achieved.

Sintering has to be carried out in a ceramic fiber "pod" transparent to microwaves in the temperatures range to be used. The temperature control has to be performed by optical pyrometry or by a S-type thermocouple, embedded in a platinum foil to avoid interferences with electromagnetic field, placed at. a minimum distance of 5 mm of the compacts to be sintered. Cooling conditions in the furnace have to be selected in order not to damage the equipment (door sealing and deterioration of the electronic devices of the furnace) . Next step is the determination of the densities by the Archimedes method which is performed to verify if the values, are in the range from 95 to 100% of the theoretical density. Then, the structural and morphological characterization is carried out to ensure that undesirable phases are not occurring, i.e. those with an ionic conductivity lower than the one of oxyapatite phase (oeoo°c=6 · 0*10^~2 Scirf¹ ) . Finally, the ionic conductivity of the samples, with surfaces coated with platinum paste, calcined at 900°C during 1 h in order to promote the contact between the samples and the electrodes, will be determined by complex impedance spectroscopy, in the range from 400 °C to 1000 °C, the values shall be in the range from 1.0 to 6.0*10^~2 Scirf¹ between 500 and 800 °C. Example

A La9.₃₃Si2Ge/j0₂6 powdered phase was obtained from La₂0₃ (99.9%), Si0₂ (99.4%) and Ge0₂ (99.9%) powders by mechanical alloying (MA) in a Fritsch planetary ball mill. The constituent powders were placed in a 250 ml hardened steel vial together with 20 mm diameter balls made with the same material, as to keep the ball-to-powder weight ratio of 20:1. Dry mechanical milling was carried out in a protective atmosphere (argon at 200 kPa) by using a rotating speed of 350 rpm for 15h. Prior to the synthesis of the La₉.₃₃Si₂Ge₄0₂6 phase, the starting materials were milled separately at 350 rpm for 30 min in argon at 50 kPa in order to achieve low particle size distributions with a greater ability for sintering. After mechanical alloying particle size distribution of the La₉.33Si₂Ge40₂6 powder was measured by laser scattering (CILAS 1064) from a powder suspension in water under mechanical agitation after a 60 s sonication. Scanning Electron Microscopy with Electron Dispersive Spectrometry (SEM/EDS Philips XL30FEG/EDAX) was used for characterization of the MA particles morphology. X-ray diffraction (XRD) analysis was performed for phase evaluation in a Philips X' Pert PW3020 equipment. The measurements were performed using CoK_a radiation (λ = 1.78897 A) within the 2Θ range of 20-100° using a step size of 0.04° (2Θ) and an acquisition time of 3 s per step. Phase identification was carried out with reference to the ICDD cards database.

The MA powder was compacted by uniaxial pressing at 390 MPa followed by sintering for 1 h at temperatures of 1300 and 1350 °G (hereafter referred to as samples SI and S2, respectively) under static air in a 2.45 GHz microwave furnace (M ) (Microwave Research Applications Inc (USA) ) with a nominal power of lk . Temperature was controlled by using a S thermocouple (Pt-Ptl0%Rh) placed near the samples. The geometric density (weight/volume) of the green compacted samples was measured. For the sintered samples, density and open porosity were determined using the boiling test method. SEM/EDS, XRD and atomic force microscopy (AFM) were also used for samples characterization. SEM observations were carried out on polished cross sections, coated with a thin layer of Au-Pd to provide surface electrical conductivity (Sputter Coater Emitech K575X) .The samples were transversely broken and the fracture surfaces were also analyzed by SEM/EDS coated with a thin layer of Or, to avoid overlapping of the AuL_a and SiK_a lines. Concerning AFM analysis, a Kruge.r Innova microscope, capable of performing scans up to 90 mm was used. The scans were performed using a MPP-12 Cantilever and a MPP- 12200-10 Probe with 150 kHz resonance frequency and a constant force of 5 N/m. Scans were obtained by intermittent contact., known as tapping mode. A modular multiplatform software for profilometric data analysis, Gwyddion v2.28 was used for image analysis .

Tests performed

Nanoindentation tests were performed on the polished surfaces of both samples using a NanoTest Platform from MicroMaterials, equipped with a Berkovich indenter. Corrections of the geometrical imperfections of the tip indenter, thermal drift of the equipment and zero indentation- depth position were performed. A series of indentations at different loads (from 10 mN to 100 mN) were conducted and used for the determination of nanohardness (H) and Young's Modulus (E) according to a method described elsewhere. Two creep periods of 30 s were programmed during tests in order to stabilize the penetration depth before unloading, for thermal drift correction. In order to obtain representative average values for the evaluated properties, 100 indentation tests were performed on each sample.

A Shimadzu indentation equipment with a Vickers diamond indenter was used for the determination of Vickers hardness (HV1), fracture toughness (K_c) and yield stress (a_y) .

This technique was used to perform measurement at loads higher than 100 mN and the HVl values were converted in GPa for comparison with the hardness measured by the depth sensing indentation technique. 200, 300 and 500 m loads were performed in order to analyze the . indentation size effect (ISE) contribution. The nanoindentation results were also used for the analysis. The fracture toughness (K_c) of the samples was determined by the Palmqvist method from the results of the indentation tests. A 4.9 N load was chosen to assure the appearance of cracks at the corners of the indentations. Ten measurements from randomly selected areas of each sample were performed, according to the AST standard. The total crack lengths emanating from the Vickers indentation fracture tests were measured by AFM. Finally, yield stress (a_y) was determined from the results of the indentation tests by means of the reverse analysis approach proposed by Antunes et al.

Results

Mechanical alloying

The particle size distribution (differential and cumulative curves) , obtained from the MA powders, is multimodal and quite large with characteristic particle diameters dlO, d50 and d90 of 0.38, 3.30 and 28,57 μπ, respectively.

Microstructural characterization by SEM of a MA particle shows that the resulting particles consist of agglomerates of smaller particles, some of which are nanosized. This is typical of the MA process in which repeated cold welding, fracturing and re-welding of powders occur, giving rise to new particles with different compositions.

XRD pattern of the MA powder shows, as expected, that the diffraction peaks correspond to a nanometric apatite phase (crystallite size of about 20 rim, determined by the Sher.rer equation) . No ICDD card is available for the composition La9.33Si₂Ge₄0₂₆. However, taking into account the two cards existing in the literature for L ₉.33Si₆0₂6 (ICDD card no. 49- 0443) and La₉.3₆Si₃Ge₃026 (ICDD card no. 75-3458),. the diffraction peaks position of apatite formed during MA match better with this latter card, corresponding to the Ge-doped apatite. In fact, when compared to the ICDD card of apatite with no germanium, the diffraction peaks of apatite La_9-33Si₂Ge₄0₂₆ are shifted to lower angles (higher interplanar distances) as a result of the larger size of the Ge atom when compared to the Si one. This observation corroborates the results obtained in previous work. The lattice parameters of the apatite La9.₃₃Si₂Ge₄026 formed during milling are the following: a = 9.91 A and c = 7.26 A. In addition, traces of

La₂0₃ (peak at 2Θ ~ 33°) are also evident, suggesting that the reaction was not fully completed.

Sintering behaviour

The green density of the compacts as well as the bulk densities of the pellets sintered for 1 h at 1300 and 1350 °.C under static air in a microwave furnace, together with the open porosities of the pellets are presented in Table 1.

Table 1 - Green and sintered density of the pellets and open porosity measured on sintered pellets

Sample S2 sintered at 1350 °C showed higher density and, consequently, lower porosity than sample SI sintered at 1300 °C. These samples are denser than the ones obtained in previous works, conventionally sintered in an electric furnace at the same temperatures (5.04 against 4.81 g.cirf³ at 1300 °C and 5.25 against 5.13 g.crrf³at 1350°C) .

In a conventional furnace, radiation is the dominant heat transfer mechanism over conduction and convection at temperatures above 700 °C, while in an microwave furnace the electromagnetic field power is converted into heat within the material. In this work, susceptors of silicon carbide were used, which are capable of absorbing radiation from room temperature. This allows to heat the material up to the temperature of which it starts to absorb radiation. This process is referred in the literature as hybrid sintering. This advanced heating method by electromagnetic field has some advantages over conventional electrical heating, such as much higher heating rates and shorter sintering cycles (about 3._· 5 h when compared to 12 h in a conventional furnace) . This is a very important aspect in particular for the Ge-doped apatites, since shorter times at high temperatures may contribute for minimizing Ge02 volatilization occurring at temperatures above 1250 °C. A good relationship between the results of open porosity and the bulk density of the sintered apatite La9.33Si₂Ge<026 pellets was observed. Taking into account the results of previous works for conventional sintering and the ones of this work for hybrid microwave sintering a linear trend is obtained. Based on this correlation, a value of 5.45 g.cm^-3 is estimated for fully dense Lao.33Si₂Ge₄026 materials. This value is slightly lower than the one reported for the apatite La9.3₆Si₃Ge30₂6 (5.4.8 g.cm^-3, from ICDD card no. 75-3458). This is attributed to the differences in chemical composition and unit volume cells of both apatites. Therefore, considering this reference value, samples Si and S2 present relative densities close to 92 and 96%, respectively. Sample S2 presents fewer pores than sample SI confirming the results obtained for both bulk density and open porosity. In both cases, the pores are typically smaller than 5 ]i (mean equivalent diameter of 3.2 and 2.5 pm were determined from samples SI and S2, respectively) and are mostly round and of closed type indicating that the sintering stage was close to full densification. EDS analysis performed on different areas of these two samples did not reveal any differences in chemical composition, neither on each sample nor between the two samples. The volatilization of the element Ge was prevented by the sintering method/cycle used in this work .

The XRD patterns of the sintered samples show that the two sintered temperatures used in this work do not seem to have a clear effect oh the phases formed, the apatite-type structure (ICDD card 75-3458) being the major phase in both samples. Besides, two peaks in the 2Θ range 30 to 35° were also detected and ascribed to the phase La₄Ge0₈ (ICDD card 040- 1185) . This phase was also observed in previous works for the conventional sintered Lag.33Si2Ge₄0₂6 pellets.

The pellets experienced a mass loss of around 5%, which can be attributed to Ge loss, since this is the most volatile component, once germanium enters the hexagonal apatite lattice, the problem of Ge loss is not eradicated but judging from intensity of the peaks observed for the La₄Ge0₈ phase, the amount of Ge loss appears to be rather small in the temperature range investigated. Therefore, keeping sintering temperatures as low as possible seems to adequate to improve these materials further. Mechanical Properties

Indentation size effect (ISE) was analyzed for both samples. ISE denotes the general tendency for hardness to vary with load and this is especially apparent in the case of hard and brittle ceramics.

As expected, hardness of both samples increased with the decreasing load. This is especially noticeable for lower indentation loads (nanoindentation tests) where no fracture occurred at the corners of the indentations. Local cracking can affect both depth and size of the indentation, since part of the transferred energy to the material surface is spent in the initiation and propagation of the cracks rather than in the indentation process.

For loads higher than 100 mN, there is a clear separation of the two curves, this phenomenon being more evident for higher applied loads. This is attributed to the microporosity of the samples. Indeed, sample SI is more porous than sample S2 (8.33% against 3.59%) and therefore its hardness is more influenced by porosity at higher applied loads .

The Young's modulus, E, was calculated from nanoindentation tests with different applied loads, from 10 to 100 mN. Mean values of 123 and 133 GPa were obtained for samples SI and S2, respectively. As expected, Young's modulus decreased when the porosity increased.

As mentioned in the experimental section, fracture toughness of the sintered samples was determined by the Palmqvist method from indentation tests with a 4.9 N load. This load was chosen to assure the formation of cracks at the indentation corners. Table 2 summarizes the a_y and σ_Γ values obtained for the sintered samples.

Table 2 - σ_ν and σ_Γ values of the samples obtained by two distinct methods.

Antunes et al.

Dao et at. [29]

Sample [24]

o_y{MPa) Or(MPa) o_v(MPa) o_r( Pa)

51 1807 2009 1790 1874

52 2073 2305 2058 2154

As observed for hardness and Young's modulus, the increase in the sintering temperature from 1300 to 1350°C is responsible for higher yield stress values. These values are significantly higher than the ones obtained for samples with the same chemical composition, sintered at the same temperature by conventional sintering. This can be explained by the higher density of the samples produced by hybrid microwave sintering.

Conclusions

In the present invention, hybrid microwave sintering was Used for the densification of mechanically alloyed La₉.3₃Si2Ge4G26 powders at temperatures of 1300 and 1350°C. The results showed that the combination of these techniques (MA and MW) allows obtaining samples of high density, with an apatite structure. The pellets sintered at these two temperatures present relative densities of 92 and 96%, respectively. The mechanical properties showed a direct dependence on the sintering temperature. Sample sintered at 1350°C presented the higher values of hardness (7,97 against 7,06 GPa) , Young's modulus (133 against 122 GPa) and yield strength (2073 against 1807 MPa) , respectively. Contrarily, fracture toughness decreased from 4.4 to 3.8 MPa.m^{1 2}, with increasing sintering temperature.

References

1. K. Huang, J. B. Goodenough, Solid oxide fuel cell technology

Principles, performance and operations, oodhead PublishingLtd. , Cambridge, UK, 2009.

2. R.J. Ruka, J. Weissnart, A solid electrolyte fuel cell, US patent 3, 400, 054, filled on July 24, _.1961.

3. A. Orera, P.R. Slater, New Chemical Systems for solid oxide fuel cells, Chem. Mater. 22 (2010) 675-690.

4. N. H. Menzler, F. Tietz, S. Uhlenbruck, H.P. Buchkremer, D. Stover, Materials and manufacturing technologies for solid oxide fuel cells, J. Mater. Sci. 45 (2010) 3109-3135.

5. E.J. Abram, C.A. Kirk, D.C. Sinclair, A.R. West, Synthesis and characterisation of lanthanum . germanate-based apatite phases, Solid State Ionics 176 (2005) 1941-1947.

6. J.E.H. Sansom, P.R. Slater, Oxide ion conductivity in the mixed Si/Ge apatite-type phases La₉,33Si6-xGe_x026f Solid State Ionics 167 (2004) 23-27.

7. A. F. Fuentes, E. Rodriguez-Reyna, L.G. Martinez-Gonzalez, M. Maczka, J. Hanuza, U. Amador, Room temperature synthesis of apatite-type lanthanum silicates by mechanically milling constituent oxides, Solid State Ionics 177 (2006) 1869-1873.

8. A.L. Shaula, V.V. Kharton, J.C. Waerenborgh, D.P. Rojas, F.M.B. Marques, Oxygen ionic and electronic transport in apatite ceramics, J. Eur. Cerarn. Soc. 25 (2005) 2583-2586.

9. M.M. Vieira, J.C. Oliveira, A. Cavaleiro, B. Trindade, Synthesis of La₉.₃₃ (Si0₄) ₆0₂ apatite-type by mechanical alloying, Rev. Adv. Mater. Sci. 18 (2008) 344-348.

Claims

1. A process for manufacturing an electrolyte for solid oxide fuel cells comprising the following steps of: a) mixing of La₂0₃, Ge0₂ and . Si0₂ poedersby high energy milling, in a controlled atmosphere of argon or nitrogen, using variable rotation speeds between 150 and 350 rpm and milling times between 15 h and 35 h until the desired oxyapatite phase La9,33Si₂Ge₄0₂6 is obtained; b) compaction of the said resulting mixture by uniaxial pressing using pressures in the range of 390-885 MPa and by cold isostatic pressing using pressures in the range of 200- 320 MPa in order to achieve compacts with green densities between 65 and 75% of the theoretical density; c) sintering of the said compacts in a microwave oven with 2· 45 GHz frequency, with a nominal power higher than or equal to lkW at temperatures- up to 1350 °C and dwell times between 5 min and 60 min, using silicon carbide susceptors, enabling microwaves to be absorbed from room temperature; and d) cooling of the said sintered compacts in argon atmosphere at a flow rate in the range of 2-10 L/min.

2. A process in accordance with claim 1, wherein the powders of La₂0₃, Ge0₂ and Si0₂ have purity higher than 99% and a particle size in the range of 0.1 and 10 pm.

3. A process in accordance with claim 1 or 2, wherein the step of high energy milling is performed in a planetary ball mill made of heat treated steel.