Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2001-07-02
2004-10-12
Bell, Bruce F. (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S047000, C429S047000, C429S047000, C427S115000, C427S331000, C427S372200, C427S379000, C427S383500, C427S421100, C427S422000, C427S427000, C204S421000
Reexamination Certificate
active
06803138
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for depositing dense coatings of a ceramic electrolyte material (e.g., yttrium-stabilized zirconia) onto porous substrates of a ceramic electrode material (e.g., lanthanum strontium manganite or nickel/zirconia) and products prepared by this process. This coating deposition process is useful in several electrochemical system applications, such as solid oxide fuel cells, ceramic oxygen generation systems, and ceramic membrane reactors. The invention also relates to processes for preparing an aqueous suspension of a ceramic electrolyte material, and an aqueous spray coating slurry including a ceramic electrolyte material.
BACKGROUND OF THE INVENTION
Ceramic oxide powders with fine particle sizes have an advantage over conventional ceramic powders in that their high surface area allows them to be densified at relatively low sintering temperatures. Their particulate nature allows them to be formed using inexpensive techniques such as dry pressing and slip casting. However, as particle size is reduced into the nanoscale range (i.e., <100 nm), the fine particle size can be problematic during ceramic processing and fabrication due to agglomeration. Agglomerates create density gradients in green ceramic compacts, resulting in inhomogeneous densification, sintering stresses and exaggerated grain growth during subsequent heat treatment.
Several methods have been demonstrated for the production of nanoscale ceramic powders, using spray pyrolysis and/or vapor condensation processes, which can result in strong aggregation of the product powder. Alternative methods, such as chemical precipitation, sol-gel, and hydrothermal synthesis processes, also result in agglomeration of the powder. Thus, suitable methods are required to achieve dispersion of nanoscale particles.
In suspension, nanoscale particles agglomerate because of short-range attractive (i.e., Van der Waals) forces. These short-range attractive forces between particles overcome the electrostatic repulsion of the electrostatic double layer that surrounds the particles. A cloud of ions and counter-ions surround the particle, creating the repulsive field. Particle-particle interactions can be manipulated by pH control. The magnitude of the particle electrostatic potential, known as the zeta potential, is controlled by the suspension pH. Increasing the zeta potential increases the repulsive force between particles. However, the effectiveness of pH adjustment is limited because adjusting pH also increases the ionic strength of the suspension. As the ionic strength of the suspension increases, the ion cloud surrounding the particle is compressed, allowing closer interparticle approach. Even at extreme values of pH, where the particle surfaces are highly charged, the compression of the ion cloud allows the particles to approach close enough for the short-range attractive forces to overcome the electrostatic repulsion, and agglomeration results.
An alternative method of dispersing ceramic powders is the addition of polymers that attach to the particle surface. The polymer coating prevents particle-particle contact, and agglomeration. Such steric hindrance methods have the disadvantage that they require complete coverage of the particle surfaces. For high surface area powders, the necessary amount of dispersant can be four times the amount of powder.
Well-dispersed nanoscale suspensions can be used in conventional slip and tape casting processes to make parts that sinter at low temperatures. Nanoscale suspensions can also be used in novel approaches, such as aerosol spraying. Functional membranes and corrosion resistant coatings can be sprayed onto substrates or parts and sintered at low temperatures. Depositing such oxide films using conventional powders requires high sintering temperatures to achieve high density. Significant interaction between the coating and the part can occur at high temperatures, in addition to grain growth; as the grains in the film grow, they push one another away, forming pinhole defects. Conventional powder particle sizes are also often near target film thicknesses, making it difficult to achieve films with good cohesion and sinterablility.
The use of suspensions of ceramic powders to produce dense and continuous coatings onto substrates using aerosol spray methods requires methods to circumvent high capillary stresses that can occur during drying. These stresses can become exceptionally high as the particle size of the ceramic particles in the deposited coating is reduced into the nanoscale regime. To avoid these stresses, modifications can be made to the starting suspension and deposited coating. The liquid/vapor interfacial energy of the solvent can be reduced, the packing density of the film can be homogenized and improved, and the strength of the interparticle bonds in the coating can be increased. Drying cracks occur during the falling rate period, where the air/solvent interface has moved into the capillaries of the coating. The adhesion of the solvent to the walls of the capillaries results in tensile forces being exerted on the film. The stress exerted can be expressed by the following formula,
p
R
=2(&ggr;
lv
, cos &thgr;)/
a
where: p
R
=capillary pressure, &ggr;
lv
=liquid-vapor interfacial energy, &thgr;=solid-liquid contact angle, and a=capillary radius of curvature. From this equation and the consideration that capillary radius is directly proportional to grain size, it is evident that a film composed of nanoscale materials will suffer large drying stresses. The drying stresses from capillary pressure can be lowered by decreasing the liquid-vapor interfacial energy, using alternative solvents (e.g., alcohols), or by modifying an aqueous solvent by the addition of surfactants. Examples of surfactants include alcohols such as octanol and butanol and anionic surfactants such as alkali sulfonates, lignosulfonates, carboxolates and phosphates. Sulfonates and phosphates can leave behind inorganic components that are detrimental to sintering and the electrical properties of the fired ceramic, but organic surfactants typically do not, and are favored for ceramic applications.
Development of a successful coating process also requires good particle packing and high green strength of the applied coating. As is well described in the art, bimodal distributions pack better than unimodal distributions in the green state. Green strength of the deposited films can be improved by the addition of binders to impart a degree of plasticity to the film during drying, thus avoiding brittle fracture. Polyvinyl alcohol and methylcellulose are examples of aqueous binder systems for use in ceramics. For nanoscale systems, short chain polymers including low molecular weight starches and proteins are candidate systems.
Solid oxide fuel cells are an excellent example of an application that requires novel coating deposition technologies. Fuel cells generate power by extracting the chemical energy of natural gas and other hydrogen containing fuels without combustion. Advantages include high efficiency and very low release of polluting gases (e.g., NO
X
) into the atmosphere. Of the various types of fuel cells, the solid oxide fuel cell (SOFC) offers advantages of high efficiency, low materials cost, minimal maintenance, and direct utilization of various hydrocarbon fuels without external reforming. Power is generated in a solid oxide fuel cell by the transport of oxygen ions (from air) through a ceramic electrolyte membrane where hydrogen from natural gas is consumed to form water. Although development of alternative materials continues, the same types of materials are used in most of the SOFC systems currently under development. The electrolyte membrane is a yttrium-stabilized zirconia (YSZ) ceramic, the air electrode (cathode) is a porous lanthanum strontium manganite ((La,Sr)MnO
3
) (LSM) ceramic, and the fuel electrode (anode) is a porous Ni-YSZ cermet. To obtain high efficiency and/or lower operating temperature, the YSZ ceramic e
Dawson William J.
McCormick Buddy E.
Seabaugh Matthew M.
Swartz Scott L.
Bell Bruce F.
NexTech Materials, Ltd.
Porter Wright Morris & Arthur LLP
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