WO1991008996A1

WO1991008996A1 - Process for depositing a layer on a substrate

Info

Publication number: WO1991008996A1
Application number: PCT/NL1990/000183
Authority: WO
Inventors: Johannes Schoonman; Adrianus Mackor
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno
Priority date: 1989-12-20
Filing date: 1990-12-19
Publication date: 1991-06-27
Also published as: NL8903125A

Abstract

Process for depositing a thin ceramic layer on a porous substrate comprising two periods. During a first period a first reactant is fed in gas phase to the front of the substrate and a second reactant is fed in the gas phase to the back of the substrate. The first reactant diffuses as such through the substrate or proceeds after gastight sealing as ion through the deposited layer in order to react with the second reactant and to produce a gastight layer. During the second period, following after said first period, reactants are fed in the gas phase exclusively to the back of the substrate.

Description

Process for depositing a layer on a substrate

The invention relates to a process for depositing a thin ceramic layer on a porous substrate, comprising a first period in which a first reactant is fed in the gas phase to the front of the substrate and a second reactant is fed in the gas phase to the back, the first reactant diffusing as such through the substrate or, after gastight sealing, proceeding as an ion through the depo¬ sited layer in order to react with the second reactant and to produce a gastight layer.

Such a process is known from US-A-4,609,562. In that case, the first reactant diffuses as such or, after reaction with the layer of material formed, as an ion through the porous substrate or as an ion through the layer in order to react with the second reactant and to produce a layer of a pure, mixed or doped metal oxide if oxygen-containing compounds such as oxygen gas, water vapour, alcohols or ethers are used as first reactant. This process can also be carried out with compounds of other elements, such as fluorides, which are able to diffuse as an ion through the layer. In this period, all the pores of the substrate are sealed in a gastight manner with the pure or doped metal oxide or metal fluori¬ de. The reactants are then no longer in direct contact with each other. The layer formation continues, however, as a result of diffusion of oxygen (or fluorine) ions through the layer formed. This diffusion arises as a result of the difference in oxygen (or fluorine) concentration across the substrate. Such a process is relatively slow. The process from the US Patent specification is ^• used to manufacture electrochemical cells composed of porous elec¬ trodes on a gastight solid electrolyte. In this case it is not possible to use traditional thermally activated CVD because the electrode would be completely coated with the material and the system would no longer be able to operate as an electrochemical cell. Such cells are used, inter alia, as fuel cells and as cells for detecting the presence of gases (sensors) . Deposition with CVD in the normal way produces a polycrystalline layer having random grain orientation and a non-uniform thickness because the depo¬ sition rate depends on the local temperature and the reactant concentration. An additional problem is that some of the material deposits not on the substrate but on other parts situated in the reactor or on the walls of the reactor. For this reason, the pro¬ cess described above has been proposed, in which a reactant diffu¬ ses as such through the porous substrate or as an ion through the sealed substrate and reacts with the vapour situated at the other side of the substrate in order to obtain the desired layer forma- tion on the substrate.

Although this method appears to be completely satisfactory from the point of view of the result, it has the disadvantage that very much time is necessary to build up a layer of adequate mecha¬ nical strength. As a result, the costs of electrochemical cells so manufactured are so high that it is hardly attractive commercially to manufacture such cells in this way. This is due to the fact that the process described above is governed by diffusion in the solid state. Such processes proceed inherently slowly for that reason. As an example, a treatment time of eight hours at high temperature, such as 1,000-1,100°C, may be mentioned. The prolonged use of said high temperature also has the disadvantage that solid-state reac¬ tions may occur at the electrode-electrolyte interfaces, as a result of which an electrochemical cell which functions well would not be produced. In practice, there appears to be a loss of more than 50 .

The object of the present invention is to provide a process in which the growth rate of the layer which has to be deposited is increased and the substrate is also exposed for a shorter time to the high temperature mentioned above. This object is achieved in a process described above in that, during a second period following the said first period, reactants are fed in the gas phase exclusively to the back of the ^"substrate. That is to say, a modified CVD process is used after using the process, described above, during the first period. Surprisingly, it has been found that it is only necessary to bring about a very thin layer in the process mentioned above with the aid of diffusion through the substrate, after which the necessary thickness can be brought about by depositing further metal oxide in a much faster way by means of modified CVD. The modified process also proceeds at substantially lower temperature, with the result that the loss percentage will be substantially lower. According to a further embodiment of the invention, a parti¬ cular embodiment of the CVD process is used during the second period. This entails that particles are already precipitated in the gas flow. This may optionally be accompanied by cooling of the substrate. A process of this type is known as such from EP-A-0,186,910. The so-called thermophoresis phenomenon is describ¬ ed therein, in which very fine particles are produced in the vapour phase by homogeneous nucleation. In the process according to the invention, a conventional CVD process is continued on the layer obtained in the first period simultaneously with homogeneous nu- cleation in the gas phase. Because the substrate can be at a lower temperature than the vapour phase, the very fine particles can be deposited on the substrate by directed diffusion in the temperature gradient possibly present in the vapour phase. The process can be carried out in a manner such that these powder particles will form part of the gastight CVD layer. It has been found that a much higher growth rate of the metal oxide layer on the substrate can be brought about by this process. This process is of importance for manufacturing electrochemical cells such as solid oxide fuel cells SOFC, high-temperature cells for the electrolysis of water, (gas) sensors, solar cells and batteries based on originally porous substrates.

The invention will be explained below in greater detail with reference to a non limiting example. Example: The electrolyte of an electrochemical cell was manufactured by depositing a metal oxide layer on a substrate using the appara¬ tus according to US-A-4,60 , 62. In the process according to the invention, an yttrium-oxide-stabilised zirconium oxide (YSZ) was deposited on a porous aluminium oxide (AI2O0) substrate. The reac¬ tion has also been carried out on calcium-stabilised zirconium oxide CSZ and on YSZ instead of aluminium oxide. The first reactant, i.e. the one which diffuses through the substrate, is oxygen, optionally mixed with water vapour. In an embodiment of the process, use was made of a gas mixture which was composed of 6θ# water vapour and 0% oxygen gas. The second reac¬ tant in these experiments was 80-100 mol-# zirconium tetrachloride which had been mixed with 20-0 mol-J. yttrium trichloride in various ratios, which were determined by the vapour tension of the metal chlorides. Said vapour tensions were obtained by passing an inert gas, such as argon or nitrogen, over the heated solid metal chlori¬ des, for example at 200°C for ZrCl^ and at 850°C for YCLg. At the surface of the porous substrate, a sealing layer of a metal oxide or mixture of metal oxides was formed by reacting the oxygen gas (mixture) diffused through the substrate with the metal reactant. In the case of a mixture of metal oxides, the ratio of the latter in the layer is determined by the ratio of the metal chlorides in the vapour phase. After the layer had been sealed, the first period was terminated and the substrate was cooled. A mixture of metal halide(s) and oxygen, optionally in a composition which was identi¬ cal to that of the first period, was introduced above the cooled substrate, in particular at the side where the very thin layer of metal oxide was now present. At the same time, the process was carried out in a manner such that the conversion to metal oxide already took place in the gas. The observed growth rate was found to be a factor of ^100 higher than for conventional CVD. Surpris¬ ingly, the layer obtained as a result of the first period was found to have an optimum structure for an electrolyte in a fuel cell, while the growth of said layer by the modified CVD described above using thermophore≤is in the second period did not adversely affect the properties. In the second period, the thickness of the layer increased appreciably, it being possible to bring about the entire layer deposition in a relatively short time.

Although the invention has been described above with referen¬ ce to an example for use in a fuel cell, it must be understood that it is not limited thereto. The. present invention can be used in manufacturing the electrochemical cells mentioned earlier, such as in the manufacture of selective membranes for sensors and layered solar cells.

Claims

1. Process for depositing a thin ceramic layer on a porous substrate, comprising a first period in which a first reactant is fed in the gas phase to the front of the substrate and a second reactant is fed in the gas phase to the back, the first reactant diffusing as such through the substrate or, after gastight sealing, proceeding as an ion through the deposited layer in order to react with the second reactant and to produce a gastight layer, charac¬ terised in that, during a second period following the said first period, reactants are fed in the gas phase exclusively to the back of the substrate.

2. Process according to Claim 1, in which the process en¬ tails, during the second period, the precipitation of particles in the gas flow before deposition on the substrate.

3- Process according to Claim 2, in which the substrate is cooled.

4. Process according to one of Claims 1-3, characterised in that the first reactant is ^* an oxygen-containing compound or a mixture of an oxygen-containing compound and water vapour, and the second reactant is a metal chloride or a mixture of the chlorides of a plurality of metals, the deposited layer being a metal oxide or mixture of metal oxides.

5- Process according to one of Claims 1-3, characterised in that the first reactant is a fluorine-containing compound, the second reactant is a metal compound or mixture of compounds of a plurality of metals, and the deposited layer is a metal fluoride or mixture of metal fluorides.

6. Process according to one of the preceding Claims 1-3, characterised in that the second reactant is a mixture of 8-100 mol-Jit zirconium tetrachloride and 20-0 mol-# yttrium trichloride and the deposited layer is composed of yttrium-oxide-stabilised zirconium oxide (YSZ) .

7. Substrate provided with the ceramic layer produced by the process according to one of Claims 1-6.