KR20050016431A - Solid oxide electrolyte with ion conductivity enhancement by dislocation - Google Patents

Abstract

In order to improve the ionic conductivity, a dislocation structure is formed in the electrolyte membrane. Ion rays and / or electron beams grow vacant clusters within the thin film and convert them into Frank potential loops that exhibit high ion conductivity. Spatial rearrangement of the Frank dislocation loops during subsequent thin film heat treatment maximizes the ion conductivity. In this case, the potential loops form a face-to-face continuous potential structure, and along this potential structure, ions can pass through the membrane with minimal activation energy. By irradiating ceramics such as YSZ and DC in a conventional manner, dislocation densities of about 10 ⁸ to 10 ¹⁴ cm / cm 3 can be obtained.

Description

Solid oxide electrolyte with improved ion conductivity by dislocation structure {SOLID OXIDE ELECTROLYTE WITH ION CONDUCTIVITY ENHANCEMENT BY DISLOCATION}

TECHNICAL FIELD The present invention relates to electrochemical devices and methods, and more particularly, to a solid oxide ion conductive electrolyte in a solid state ion device such as a fuel cell or a gas sensor using a potential structure.

Fuel cells are electrochemical devices that generate electrical current through chemical reactions. The basic structure of a fuel cell is the insertion of an ion conductive electrolyte between two electrodes supported by a fuel / oxidant distributor. The catalyst on one electrode promotes separation of ions and electrons on the oxidant side. Only ions pass through the electrolyte and recombine with the electrons on the fuel side. When these electrons flow through an external circuit, electric power is supplied. The solid oxide fuel cell includes an ion conductive metal oxide film as an electrolyte layer. Oxygen molecules are separated into individual electrons and oxygen ions on the air side. Oxygen ions pass through the electrolyte membrane and combine with electrons and hydrogen molecules into water. The basic structure of the gas sensor is the same, and current is produced by the difference in concentration of gas.

In fuel cells, efficiency is increased by minimizing the known electron conductivity of the electrolyte and maximizing the known ion conductivity. At the same time, fuel cells are thermodynamically more efficient at low temperatures and have less entropy losses, resulting in higher voltages.

Solid oxide fuel cell [SOFC; Solid oxide fuel cells have the following advantages:

-No special humidity conditions for ion exchange

-No clogging with generated water

-Expensive metal catalyst is unnecessary or small

High CO Tolerance

-Using waste heat

However, SOFCs also have some problems. One problem to be solved is to form a sealing seal. Lowering the operating temperature below 1000 ° C. to 600 ° C. can solve this problem by allowing the metal material to be used for sealing. Despite the large output losses, many attempts have been made to lower the operating temperature of the SOFC below 600 ° C. However, this operating temperature is still too high for the mobile type.

In particular, there is a need for an electrolyte layer that can be manufactured at low cost and has a structure in which a fuel cell operates effectively even at 500 ° C or lower. The present invention reflects these conditions.

1 is an isotherm graph showing the ion conductivity for mol% Y ₂ O ₃ of YSZ;

2 is an isotherm graph showing ion conductivity versus dislocation density;

3 is a block diagram showing a film manufacturing process having a face-to-face dislocation structure;

4 is an enlarged cross-sectional photograph of a multilayer structure including a layer having a dislocation structure;

5 is a schematic diagram showing that the face-to-face dislocation structure functions as an ion path;

FIG. 6 is a graph showing different ion conductivities with temperature for several potential densities, for example YSZ and SDC;

7 is an example of a device having a thin film having a dislocation structure according to the present invention.

Summary of the Invention

The present invention provides a solid oxide electrolyte thin film having a dislocation structure, wherein the dislocation structure spreads from the top surface to the bottom surface of the electrolyte. The present invention forms an electrolyte thin film using heat treatment in addition to ion irradiation. The basic structure of one preferred embodiment of the present invention is as follows:

1. Conventional ion conductive materials such as, but not limited to, yttria stabilized zirconia (YTS) or doped ceria (DC) prepared with an electrolyte.

2. A potential structure formed in an electrolyte material using high energy electron beams and / or ion beams.

3. Change the shape and direction of dislocation structure to heat treatment.

Some of the advantages of the present invention over existing devices and methods are as follows:

1. High ion conductivity, high power density / efficient fuel cell and high sensitivity gas sensor.

2. The low operating temperature solves the problems caused by the difference in coefficient of thermal expansion between the electrode and the electrolyte, as well as improves the utilization of materials including metals and polymers.

Ceramics with high ionic conductivity, such as doped ceria (DC) such as samarium doped ceria (SDC) and yttria stabilized zirconia (YAZ), are suitable for the electrolyte. Such a ceramic can be made into a single crystal, polycrystalline, or amorphous state to produce a fluid impermeable thin film layer. Dislocation structures may be formed in single crystals or polycrystalline ceramics. In general, dislocation structures may be formed by plastic deformation, rapid cooling, or ion beams, electron beams, and neutron beams.

Plastic deformation can cause dislocation densities of 10 ¹⁰ cm / cm 3 or less. However, plastic deformation of the ceramic can only occur at high temperatures. In YSZ, plastic deformation occurs at a considerably high rate at temperatures above 1000 ° C. Plastic deformation at high temperatures requires complex steps to fabricate an efficient electrolyte membrane to the appropriate thickness.

Dislocations may be caused by quenching or quenching rather than plastic deformation. During this process, the ceramic film is heated to a temperature of at least 1000 ° C. and then rapidly cooled. Heating / cooling creates high density vacancies in the atomic lattice structure. For best results, make the cooling as short as possible. Many short cooling cycles with high temperature gradients lead to mechanical deformation such as cracks in the ceramic. In electrolyte membranes, cracks should be avoided to prevent fluid permeation.

Irradiation is a preferred method of forming a thin film having a dislocation density of 10 ¹² cm / cm 3 or more. Ions, electrons and / or neutrons can be irradiated to the ceramic. Although neutron irradiation may cause radioisotopes to remain, ion irradiation and electron irradiation are environmentally safe, simple, and inexpensive, making it easily available on the market.

Ion / electron beams cause the growth of vacant clusters at the depth of irradiation of the ceramic. When the vacancies cluster reaches a critical size, the surrounding atomic lattice structure collapses and the vacancies cluster is converted into a known Frank dislocation loop. In the heat treatment following the irradiation, the ceramic is heated to a predetermined temperature and maintained at this temperature while the Frank Dislocation Loop itself is spatially rearranged to form a continuous dislocation structure. The heat treatment factors are adjusted in a known manner to keep the recombination of the potential loop to a minimum.

A preferred ceramic for forming a continuous dislocation structure by irradiation treatment is YSZ. The natural ionic conductivity of YSZ depends on the content of iridium oxide Y ₂ O ₃ . As can be seen from the isotherm of Fig. 1, the natural ion conductivity is the maximum at 4-8 mol% Y ₂ O ₃ . These isotherms mark the temperatures of the YSZ material. As the temperature of YSZ decreases, the maximum natural ion conductivity is concentrated around 4 mol% Y ₂ O ₃ . The natural ionic conductivity shown in FIG. 1 has no potential.

The reference line 60 is based on the ion conductivity of the YSZ thin film 1 having a thickness of 500 nm (see FIG. 3). For further details on the relationship between ionic conductivity, thin film thickness and overall ionic resistivity of thin films, see our co-filed "Sub-micron Electrolyte Thin Film on Nano-Porous Substrate by Oxidation of Metal Film".

FIG. 2 shows the ionic conductivity of YSZ [8YSZ] of 8 mol% Y ₂ O ₃ over dislocation densities at various temperatures. These various temperatures are near the operating temperature of the electrolyte membrane of the fuel cell. In a fuel cell operating at a maximum temperature of about 500 ° C. or less, it is desirable to reduce the known operation inhibiting effect. Such inhibitory effects include selecting materials of structural parts, seals and / or design features as high temperature materials to suit thermal expansion, heat dissipation, heat transfer, and the like.

As shown in Fig. 2, the ion conductivity was considerably lowered in the dislocation density range 21 of 10 ¹⁰ cm / cm 3 to 10 ¹⁴ cm / cm 3. Due to the continuous dislocation structure, the ion conductivity can rise from the second grade in the high temperature region to the eighth grade in the low temperature region. Therefore, as the operating temperature of the fuel cell and the gas sensor is lowered, weights for efficient electrolyte membrane formation due to the continuous potential structure are obtained.

The ionic conductivity of a solid material having a dislocation structure is as follows: Dislocation density is measured for the dislocation pipe defined by ρ _pipe and its unit is [mm-2] as dislocation length for unit volume. If the dislocation direction is the thickness direction of the material, the conductive area ratio through the dislocation becomes equal to the volume ratio of the dislocation as follows.

f _pipe = ρ _pipe * πb ²

Here, each potential extends through an area of πb ² , and b is 1-2 atomic in size as a burgers vector of potentials. The total conductivity of the sample is calculated using the rule of mixture logic. Basically, the total conductivity is the sum of the conductivity of the whole material measured by the volume ratio of the whole material and the conductivity of the potential pipe measured by the volume ratio of the potential pipe.

Finally, the conductivity of the dislocations is considered to be improved compared to that of the entire material. The reason why the conductivity of the potential portion is improved is that the conduction active energy Ea is lowered in the vicinity of the potential pipe. In the case of YSZ, the lattice expands near the dislocation structure and the bond strength between oxygen and Zr is weakened. As the bond strength weakens, the oxygen ion transfer (active) energy from the oxygen side to the oxygen vacancy is lowered. The active energy of the potential pipe is given below for the active energy of the material as a whole:

ρ_bulk = Ae^{-Ea / kT} ρ_pipe = Ae^{-Ea / 2kT}

3 shows the steps of forming a continuous dislocation structure. In the first step, the thin film 1 previously formed to a predetermined thickness 11 is exposed to ions or electron beams. The thickness 11 is selected as an irradiation parameter such that ions penetrating the surface are dispersed through the thickness 11. For example, the thickness of the YSZ thin film is about 140 nm. For further details on the formation of a fluid impermeable YSZ thin film, see the aforementioned patent application "Sub-micron Electrolyte Thin Film on Nano-Porous Substrate by Oxidation of Metal Film".

If the parameter is 5 × 10 ¹⁵ ions / cm 2 Xe ³⁺ at 450 kW, the thickness 11 is selected so that the dislocation density is about 10 ¹² cm / cm 3. In the first step, vacancy clusters 61 start to form in the crystal structure of the thin film 1. If irradiation continues, the empty cluster 61 grows to a crystal size. In addition to xenon ions, other ions such as argon ions may be used. The use of xenon ions is for conventional transmission electron microscope (TEM) observation. On the contrary, when argon ions are used, the dislocation density decreases but penetrates deeply, since argon ions are smaller and lighter than xenon ions. In the case of electron beams, the penetration depth is much deeper because the electrons are much smaller than the ions. The penetration depth can be measured using the commercially available IBM software "SPIM-2000.40". For example, in the case of 450 keV argon ion irradiation, the maximum penetration depth in the measured YSZ may be about 340 nm. Irradiation intensity should be kept at maximum for maximum penetration depth and high dislocation density. As will be appreciated by those skilled in the art, the irradiation intensity is limited such that structural breakdown of the thin film 1 does not occur. In some cases, irradiation may be performed simultaneously on both surfaces 12 and 13 of the thin film 1, in which case the maximum penetration depth is doubled.

If the irradiation continues beyond the critical size, the surrounding atomic lattice structure collapses and the vacant cluster 61 is changed into a known Frank dislocation loop 62. During this second step, the dipoles of the frank potential loop 62 are at any position inside the thin film 1. The ion conductivity can be improved only by irradiation while the Frank dislocation loop is formed in any direction.

The ion conductivity can be a maximum for a certain dislocation density, in which case the Frank dislocation loops are spatially redirected so that both dipoles coincide with the upper and lower surfaces 12, 13 (see FIG. 5). In this way, a continuous face-to-face dislocation 63 is formed, and along this dislocation structure ions can pass between both sides 12, 13 with minimal active energy.

The Frank dislocation loop is spatially reoriented during the separate heat treatment process for the thin film 1 after the irradiation is completed. In the case of the YSZ thin film 1, the heat treatment is to expose to a temperature of about 800 ℃ for about 3 hours. This temperature is chosen to initiate growth and spatial reorientation of the frank potential loop 62 without a decrease in potential density due to improper recombination of the potential loops. At the end of the heat treatment, the sample is cooled slowly to prevent cracking.

FIG. 4 is an enlarged TEM image of a cross section of an YSZ multilayer structure consisting of a platinum layer 54, a gold layer 53, an irradiated YSZ layer 52, and an unirradiated YSZ bulk layer 51 in order from the top. The white portion of layer 52 shows a dislocation structure with a dislocation density of about 10 ¹² cm / cm 3. In the sample of FIG. 4, there are layers 53 and 54 of gold and platinum on the irradiation treatment layer 52. These layers 53 and 54 are deposited after the irradiation of the sample is finished. In order to prepare this sample, the irradiation treatment layer 52 was bonded to the unirradiated layer 51. Frank dislocation loops that appear white in layer 52 have not yet been spatially redirected. The sample of FIG. 4 is for observation only.

5 shows the effect of the continuous face-to-face dislocation structure in which ions penetrate from one side 13 to the opposite side 12 of the thin film.

In addition to YSZ, a preferred ceramic material for the electrolyte membrane is SDC. The chemical formula of SDC is Sm _0.2 Ce _0.8 O _1.9 [20 SDC]. 6 is a graph showing the conductivity as a function of temperature, with natural 8YSZ (curve 81), 20 SDC (curve 83), as well as dislocation densities of 10 ¹¹ cm / cm 3 (curve 82) and 10 ¹⁴ cm / cm 3 (curve 84 ) 8YSZ, 20SDC having a dislocation density of 10 ¹¹ cm / cm 3 (curve 85) and 10 ¹⁴ cm / cm 3 (curve 86).

7 shows a device 100 having a thin film 1 in which a dislocation structure is formed according to the present invention. The device 100 may be a fuel cell or a gas sensor.

As will be appreciated by those skilled in the art, the various embodiments described above may be variously modified without departing from the scope of the present invention. For example, a fluorite material such as Ca stable zirconia and Sc stable zirconia may be used as the electrolyte material, but is not limited thereto. In addition, a known Perovskite ion conductive material may be used as the electrolyte material.

Claims (15)

An ion conductive oxide layer having a dislocation structure of dislocation density of 10 ¹⁰ cm / cm 3 or more. The ion conducting oxide layer according to claim 1, wherein the dislocation structure is continuous between the top and bottom surfaces. The ion conductive oxide layer according to claim 1, wherein the thickness is 350 nm or less. The ion conductive oxide layer according to claim 1, which is composed of YSZ. The ion conductive oxide layer according to claim 3, wherein the ion conductivity is 10 ^-6 S / cm or more at a temperature of 200 ° C. The ion conducting oxide layer of claim 1, wherein Sm is composed of doped cerium oxide. The ion conductive oxide layer according to claim 6, wherein the ion conductivity is 5x10 ^-4 S / cm or more at a temperature of 200 占 폚. The ion conductive oxide layer according to claim 1, which is an electrolyte membrane of a fuel cell. The ion conductive oxide layer according to claim 1, which is an electrolyte membrane of a gas sensor. In the method for forming an ion conductive oxide layer having a dislocation structure with a density of 10 ¹⁰ cm / cm 3 or more: a. Forming an oxide thin film having a predetermined thickness; And b. Irradiating irradiated radiation on the thin film to grow a vacant cluster in a crystal size in the thin film, but to collapse the atomic lattice structure around the vacant cluster to form a Frank dislocation loop; Wherein the thickness is determined such that the radiation penetrates the thin film. The thin film heat treatment step of claim 10, wherein the thin film heat treatment step for spatially reorienting the frank potential loop to form a continuous face-to-face dislocation structure is followed by the irradiation step, and the frank dislocation loops are spatially grown and redirected to form the continuous plane. And the thickness is determined such that it becomes a facing dislocation structure. The method according to claim 10, wherein said radiation is ion beam. 13. The method of claim 12, wherein the irradiated ions are argon or xenon. The method of claim 10, wherein the thickness is 350 nm or less. A fuel cell comprising an oxide electrolyte membrane, wherein the continuous internal operating temperature is 200 ° C. or less.