TECHNICAL FIELD

The present invention concerns a method for the deposition of films of mixed oxides on composite material substrates.

In particular, the invention concerns a method for the deposition of films of mixed oxides on composite material substrates for the growth of thin electrolytic or cathodic layers for solid oxide fuel cells (SOFC) , where the films of mixed oxides are deposited on metal/oxide or oxidel/oxide2 substrates, the latter already covered if necessary with an electrolytic layer.

In greater detail, the invention refers to the formation, on said composite substrates typically having the function of anode electrode, of a thin layer having the function of electrolyte or, if the thin layer is deposited on an electrolyte, of cathode electrode.

BACKGROUND ART

Fuel cells based on membranes of solid oxides (SOFC) constitute highly efficient eco-compatible systems for the generation of electrical energy. However, their use on a medium and large scale has so far been impeded by generally high costs and problems with performance, which occur above all in configurations where cost limitation is sought.

The greatest difficulties encountered in developing efficient SOFCs concern the requirements to be met by the electrolytic layer which:

must have a high density;

must have a uniform chemical composition in order to have good ionic conduction at operating temperatures;

- must be thin to reduce the ionic resistance;

must be extensive in order to maximise the current density;

must be resistant to thermal shocks.

The current techniques used at industrial level to produce these layers (for example pressing and annealing of very fine powders, screen-printing, tape-casting and others) do not allow the manufacture of sufficiently fine layers extending over vast areas, and this has induced the international scientific community to investigate the effectiveness of advanced physical technologies (Physical Vapour Deposition, PVD) for the deposition of thin films (thicknesses lower than or in the order of 10 μπι) of the various active components of the SOFCs. Another critical aspect for the development of good SOFCs is the operating temperature: ideally it is desirable to obtain cells that can operate at temperatures much lower than the current 850-1000°C. A reduction in the operating temperatures would also allow, among other things, the use of low cost metal interconnection materials compared to those currently required, and the use of simpler gas supporting and retaining structures for the cell stacks.

In short, the development of SOFCs requires the identification of appropriate and particular electrolytic ceramic materials and the consequent set-up of reliable procedures for the deposition of said materials in the form of thin films.

DISCLOSURE OF INVENTION

One object of the present invention is to provide a method for the deposition of a mixed oxide film able to meet the above- mentioned requirements.

Said object is achieved by the present invention, since it concerns a method according to claim 1.

The method of the invention is, as a whole, particularly simple and reliable and allows, above all, perfectly functional thin electrolytic or cathodic films to be obtained, for example, for use within a SOFC. Furthermore, the equipment necessary for implementing the method according to the present invention is relatively simpler and cheaper than that required for other vacuum deposition techniques. The deposition technique by means of electron gun has proved to be surprisingly flexible and very suited to said application.

BRIEF DECRIPTION OF THE DRAWINGS

The invention is further described in the following non- limiting implementation examples, with reference to the accompanying figures in which:

- figure 1 is an X ray diffractogram of a thin film of mixed oxide obtained according to the method of the present invention, and

- figure 2 is a SEM micrograph of a section of a mixed oxide film of zirconium and yttrium represented by the formula (Y2O3) 0.08 (ZrC>2) 0.92 (8-YSZ), obtained according to the method of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention concerns a method for the deposition of a film of mixed oxide on a composite material substrate (composite substrate) .

In particular, the film of mixed oxide deposited on the substrate constitutes a thin electrolytic layer of a SOFC, and the substrate is an anodic composite substrate of said SOFC.

The substrate is preferably a planar substrate, for example of circular or polygonal shape and in any case preferably already shaped for the final application, of a composite material typically used as anode in SOFCs . In particular, the substrate consists of a metal/oxide, oxidel/oxide2 composite material, or one of the latter already covered with an electrolytic layer for SOFCs .

For example, the substrate is made of a composite material of grains of nickel oxide and grains of an oxide of transition elements, even the same as the mixed oxide to deposit. The substrate is characterised by a certain degree of porosity and surface roughness. The mixed oxide with which the film is produced on the substrate is preferably selected from the group consisting of: BaCeo.gYo.iOs-d (BCY-10), BaCe ₀ . ₈ o.20 ₃ -d (BCY-20),

(La ₀ . ₈ Sr ₀ .2) 0.95C0O3-CI (LSC), (Y ₂ 0 ₃ ) o.os (Zr0 ₂ ) 0.92 (8-YSZ), Gd ₀ . iCe ₀ . ₉ 0 ₂ - _d (10-GDC), Gdo.2Ce ₀ .80 ₂ -d (20-GDC) , Sm ₀ . i ₅ Ce ₀ .8 ₅ 0 ₂ -d (15-SDC), Sm ₀ .2Ceo. ₈ 02-d (20-SDC) , (Sc ₂ 0 ₃ ) 0.1 (Zr0 ₂ ) 0.9 (10-ScSZ), Ba(Ce _x Zri- x) o.9Yo.i0 ₃ -d (BZY-10), Ba ₂ Ini. ₈ Tio.20 ₅₊ d (BIT-02), BaIn ₀ . ₃ Ti ₀ .7O ₃ - _d (BIT-07), Nd ₂ Ni0 ₄ +5, Pr ₂ Ni0 ₄ + ₅ , Er ₀ . ₄ Bii. ₆ O ₃ (ESB) ,

Dyo.O8Wo.0 Bio.88O1.56 (DWSB) . In particular, the substrate has (data from surface analysis by atomic force microscopy - AFM) the following characteristics :

• in an area of 50μπι x 50μπι:

maximum height of grains: <4500 nm;

- mean surface roughness: <600 nm;

• in said area, enlarged to 6μπι x 6μπι:

maximum height of grains: <1800 nm;

mean surface roughness: <300 nm;

• in said area ( 6μπι x 6μπι) , further enlarged to 800nm x 800nm:

maximum height of grains: <400 nm;

mean surface roughness: <70 nm;

• size of pores: less than 600 nm. In general, the method of the invention comprises:

- a step of providing in a deposition chamber, in which a vacuum can be created, for example by means of a vacuum pump: a substrate made of composite material on which a film of mixed oxide is to be deposited; and a target of mixed oxide made of a material having the same stoichiometric composition as the film to deposit;

- an evaporation-deposition step of the target material (mixed oxide) , in which the target is bombarded, by means of an electron gun (EB-PVD technique) , with an electron beam of a certain power so as to cause the evaporation of material from the target and deposition of the evaporated material on the substrate to form an oxide film;

a step of in situ heat treatment and annealing of the film of mixed oxide, to further densify the thin film obtained and/or to guarantee the right oxygen content in the thin film.

The target is for example in the form of solid densified pellets, placed in a crucible (evaporation source) positioned opposite the electron gun in the deposition chamber. Preferably, the pellets are made of the mixed oxide with purity of at least 99.00%.

In the evaporation-deposition step by means of electron beam, the mixed oxide is evaporated under a high vacuum and is deposited on the substrate in a quantity sufficient to guarantee substantial uniformity in terms of composition and thickness of the film.

Preferably, the evaporation-deposition step in turn comprises the steps of:

- reducing the pressure within the deposition chamber to below 10 ^~4 mbar;

heating the substrate to a temperature in the range between 300 °C and 800 "inactivating the electron gun so as to evaporate and deposit a film of mixed oxide on the substrate, obtaining a pre-treated substrate. Preferably, the substrate is heated to a temperature in the range between 450 and 750°C, more preferably to a temperature of around 700°C.

Obtaining of an adequate uniformity in terms of composition can be further regulated by modifying the distance between the target and the substrate, taking into account that an increase in said distance promotes uniformity at the expense of less efficient exploitation of the target material, a part of which deposits on the walls of the deposition chamber.

Clearly, when operating with electron guns with incorporated crucible, the contents have to be re-established over time, and this can be easily obtained by providing, within the deposition chamber, tanks and vibration conveyors or other types of conveyors for the transport of new material.

In practice, when the substrate has been heated to the desired temperature, the electron gun is activated causing an appropriate electric current to flow through a tungsten filament. By applying an appropriate potential difference between the above-mentioned filament and an earthed metal part in the vicinity thereof, the electron beam is extracted. Appropriate magnetic fields deflect the electron beam, causing it to impact on the target of mixed oxide, thus causing the heating and evaporation thereof. It is clear that the material of the crucible, containing the solid densified starting pellets of mixed oxide, must be chemically inert vis-a-vis the material to evaporate and deposit.

According to the invention, the evaporation-deposition step is carried out at an increasing deposition rate, measured on the substrate; in particular, the evaporation-deposition step comprises at least one first step of growth carried out at a deposition rate (measured on the substrate) lower than or equal to a predetermined threshold, and at least one second step of growth carried out at a deposition rate greater than said threshold. Said first step of growth, carried out at a deposition rate lower than or equal to the predetermined threshold, has the purpose of lowering the surface roughness and porosity level of the substrate and thus preparing the substrate for a homogeneous growth of the film of mixed oxide, during the following steps of growth. In particular, the threshold is 0.03 nm/sec.

The increase in the deposition rate can be obtained in different ways, in particular by means of discrete or progressive increments or combinations of progressive increments and discrete increments.

For example, the evaporation-deposition step comprises:

- two or more steps of growth in which the deposition rate is constant, and the deposition rate increases from at least one step of growth to the next; and/or

- at least one step of growth in which the deposition rate increases gradually; and/or

a plurality of steps of growth in some of which the deposition rate is maintained constant, and in others it increases, optionally increasing in a different manner in respective steps of growth;

different combinations of steps of growth in which the deposition rate is constant, increases at a constant gradient, increases at a different gradient in different steps of growth.

The number of steps of growth necessary is selected as a function of the desired final thickness of the film of mixed oxide .

To control the deposition rate, a feedback control system can be provided based on monitoring, for example by means of oscillating crystals, the thickness of the layer deposited on the substrate. It should be noted that the evaporation rate of the metal species is proportional to the density of the vapour and to the speed of propagation of the atoms in the vapour which in turn depend substantially on the temperature of the evaporative source. Said parameters can be controlled by adjusting the power of the electron gun, and therefore the values of the electric current emitted and the electric extraction/acceleration voltage of the electron beam produced. It is therefore possible to appropriately calibrate, over time, the evaporation rate and therefore the deposition rate and, consequently, the thickness of material which is deposited on the substrate.

Using appropriate control instruments for controlling the power of the electron gun, it is possible to automatically set power profiles over time (and therefore vary the deposition speed) .

The evaporation-deposition step is carried out for a time sufficient to form, on the substrate to cover, a surface layer of the desired thickness. It is furthermore possible to provide insertion of a flow of process gas from the outside, for example oxygen, such that the pressure in the deposition chamber does not increase beyond 2xlCT ⁴ mbar.

Two preferred embodiments of the method of the invention are described below, purely by way of non-limiting example. The two embodiments described differ in the evaporation-deposition step, more precisely at the rate at which the film of mixed oxide is deposited.

In a first embodiment, given the state of porosity and surface roughness of the composite anode substrates typically used in SOFCs, to obtain compact homogeneous layers of mixed oxides with thickness greater than 1.5 μπι, the evaporation-deposition step comprises four steps of growth, in each of which the deposition rate is maintained constant for a predetermined time ranging from approximately 60 to approximately 90 minutes. The deposition rate in each step of growth is greater than or equal to the deposition rate of the previous step of growth and lower than or equal to the deposition rate of the following step of growth, and the deposition rate increases between at least one step of growth and the next.

In particular, the evaporation-deposition step comprises:

a first step of growth with deposition rate constant in the range between 0.01 and 0.03 nm/sec;

a second step of growth with deposition rate constant in the range between 0.01 and 0.03 nm/sec;

a third step of growth with deposition rate constant in the range between 0.04 and 0.06 nm/sec;

a fourth step of growth with deposition rate constant in the range between 0.09 and 0.11 nm/sec.

To obtain compact homogeneous layers of mixed oxides having thickness less than or equal to 1.0 μπι, the evaporation- deposition step comprises a first and a second step of growth, in each of which the deposition rate is maintained constant for a predetermined time in the range between approximately 60 and approximately 90 minutes. The deposition rate is constant in the range between 0.01 and 0.03 nm/sec in the first step of growth; and it is constant in the range between 0.09 and 0.15 nm/sec in the second step of growth.

In both cases, the first step of growth is carried out at a deposition rate no higher than 0.03 nm/sec.

In general, according to the desired final thickness of the film, the number of steps of growth necessary are set each time, where the first step of growth is characterised by a deposition rate on the substrate no higher than 0.03 nm/sec.

In a second embodiment, to obtain on the substrate a compact homogeneous film of mixed oxide with thickness greater than 1.5 μπι, the evaporation-deposition step comprises four steps of growth, in each of which the evaporation-deposition rate is maintained constant for a predetermined time ranging between approximately 10 and 15 minutes. The deposition rate in each step of growth is greater than or equal to the deposition rate of the previous step of growth and lower than or equal to the deposition rate of the following step of growth, and the deposition rate increases between at least one step of growth and the next . In particular, the evaporation-deposition step comprises:

a first step of growth with deposition rate constant in the range between 0.01 and 0.03 nm/sec;

a second step of growth with deposition rate constant in the range between 0.2 and 0.4 nm/sec;

- a third step of growth with deposition rate constant in the range between 0.7 and 1.0 nm/sec;

a fourth step of growth with deposition rate constant in the range between 1.6 and 2.0 nm/sec. To obtain compact homogeneous layers with thickness less than or equal to 1.0 μπι, the evaporation-deposition step comprises two steps of growth, during each of which the deposition rate is maintained constant for a time ranging from approximately 10 to 15 minutes. The deposition rate is constant in the range between 0.01 and 0.03 nm/sec in the first step of growth; and it is constant in the range between 2.0 and 2.5 nm/sec in the second (following) step of growth.

In general, according to the desired final thickness of the film, the number of steps of growth necessary are set each time, where the first step of growth is characterised by a deposition rate on the substrate no higher than 0.03 nm/sec.

In all the embodiments, the evaporation-deposition step comprises optionally and advantageously, between two successive steps of growth, a step of re-establishment of the target, in which the mixed oxide material of the target is re ^¬ established (i.e. the crucible - the evaporation source - is re-supplied with pellets of mixed oxide) . In all the embodiments of the invention, the evaporation- deposition step is followed by the step of heat treatment and annealing which, preferably, is carried out in situ, in the same deposition chamber in which the evaporation-deposition step takes place, in an inert atmosphere (for example of argon or nitrogen), or in an oxidising atmosphere (for example in pure oxygen) .

For example, the in situ heat treatment and annealing step comprises in turn the steps of: bringing the temperature of the pre-treated substrate to a temperature in the range between 500°C and 800°C, at the same time introducing a gas flow into the deposition chamber, at least in the vicinity of the substrate, until reaching in the deposition chamber a partial pressure in the range between 200 mbar and 1000 mbar; and then maintaining constant pressure and temperature in the deposition chamber for a time ranging from approximately 30 to 60 minutes.

In practice, once transfer of the mixed oxide onto the substrate has been completed, the vacuum pump and the electron gun are switched off.

While the step of heat treatment and annealing of the film of mixed oxide is carried out in situ (without needing to open the deposition chamber) , the temperature values of the substrate and the (partial) pressure values of the treatment gas in the deposition chamber, once adjusted, are maintained substantially constant for a time ranging from approximately 30 to 60 minutes. Preferably, the pre-treated substrate obtained from the evaporation-deposition step is brought to a temperature in the range between 650 and 750°C, more preferably to a temperature of around 700°C. The final pressure of the treatment gas within the deposition chamber is preferably in the range between 500 and 1000 mbar.

The invention is further described in the following non- limiting implementation examples.

EXAMPLE 1

The electron gun crucible was filled with 4 grams of pellets of sintered material (Y203 ) ₀ . os ( Zr02 ) ₀ .92 (8-YSZ), having dimensions: diameter 2 mm, height 5 mm.

An NiO/8-YSZ planar composite substrate was provided with diameter of 80 mm and thickness of 0.3 mm.

The substrate was arranged in a deposition chamber in which the pressure was brought to approximately 2.0xl0 ^~5 mbar, below a heating device arranged at a height of approximately 25 cm above the electron gun crucible.

The deposition rate of 8-YSZ was kept under control by adjusting the power of the electron beam, in four steps of growth of 90 minutes each, according to the following values, measured on the substrate:

step 1) 0.02 nm/sec;

step 2) 0.02 nm/sec;

step 3) 0.1 nm/sec;

step 4) 0.1 nm/sec. During the first two steps of growth, no flow of gas was sent to the deposition chamber from the outside, whereas during the third and fourth step of growth, a flow of oxygen was sent so that the pressure inside the deposition chamber was approximately 1CT ⁴ mbar constant.

The following operating parameters were used:

substrate temperature: 700°C

deposition time: 90 min/step

The following operating parameters were used for the heat treatment and annealing step:

substrate temperature: 700°C

oxygen pressure: 500 mbar

annealing time: 60 min

On the sample produced according to the methods described above, structural analyses were carried out using X-ray diffractometry directly on the thin film deposited (figure 1) . Analyses performed using the scanning electron microscope revealed (figure 2) a high degree of compactness, particularly suitable for the electrolytic function scheduled within a SOFC.

FURTHER EXAMPLES

Using methods analogous to those described previously, samples of 20-GDC and BCY-20 were prepared.

Equally satisfactory results were obtained, both in terms of compositional purity obtained and in terms of compactness of the covering layer.

Further samples were prepared, varying the procedure for carrying out the method of the invention according to the preceding description, always obtaining satisfactory results. Lastly it is understood that further modifications and variations that do not depart from the scope of the attached claims can be made to the method for the deposition of films of mixed oxide on composite material substrates described and illustrated here.