Benefiting with aerosol deposition in ceramics

In an interview Pylin Sarobol, a research and development materials scientist and engineer in the Materials Science Center at Sandia National Laboratories in Albuquerque, New Mexico with Chemical Today Magazine talks in depth about using aerosol deposition technique in ceramic coatings and how this technology can bridge the mesoscale gap between two established technologies — thin films and thermal spray coating technology.

Explain your current research.

We are currently making advances on a solid-state deposition of thick films, using the room-temperature process known as aerosol deposition (AD). Rather than heat, AD uses kinetic energy and special material properties found at a micro- and nano-scales to deposit up to ~100 µm thick metallic, ceramic, and composite films. The ability to put down ceramics, compounds, and/or high melting temperature materials at room temperature mean you can process and integrate these materials and lower-melting-temperature materials at the same time. Traditionally, a high ceramic processing temperature of about 700 degrees Celsius or more makes it difficult to consolidate and then integrate them into devices with other materials that have relatively low melting temperatures.

There are only a few places that work on such room-temperature, kinetic coating processes. Our initial research at Sandia National Laboratories started about 3 years ago by working to better understand the basic building blocks of coatings and the scientific fundamental mechanisms behind coating consolidation. In AD, a nozzle accelerates submicron particles suspended in a carrier gas toward the surface without an organic binder. Particles impact, deform and adhere, building up a coating layer-by-layer. A key is to use submicron particles (50 times smaller than the diameter of a human hair) that allow researchers to tap into materials properties found only at small scales and activate plastic deformation in the particles.

It’s the plasticity of submicron particles that causes consolidation of subsequent deposition layers and generates the continuous surface that layers are built upon. Our research focuses on expanding the materials suite, establishing the process-microstructure-properties relationship, and developing coatings towards potential applications. Many materials have been deposited, including copper, nickel, aluminium oxide, titanium dioxide, barium titanate, and carbide compounds.

Ceramic coatings benefit for the coatings industry.

Aerosol deposition bridges the mesoscale gap between two established technologies, thin films, and thermal spray coating technology. This frees us from conventional high-temperature processing and enables enticing new opportunities to consolidate exotic materials or brittle compounds into thick films at room temperature.

Almost 100 percent dense coatings of high and low melting temperature materials can be deposited in any order or co-deposited, providing agility and flexibility for design, processing, and device integration. It opens up new possibilities for depositing functional materials — electrical circuits combining hybrid materials or tiny capacitors or sensors — onto a circuit board rather than high-temperature processing, followed by tedious manual assembly.

Microelectronics, communication devices, energy generation and storage, medical devices among others, are some of the sectors that will benefit from ceramic coatings research.

Aerosols used in the ceramic coatings.

We use dry, submicron feedstock with tight particle size distribution. The particles should not clump together into large aggregates. No organic binders or surfactants are used. Different carrier gases can be selected for feedstock particle suspension and transport. The deposition is performed in a vacuum, which helps alleviate the effects of gases reflected from the substrate on the flying particles. Reflection of the high-velocity carrier gas from the deposition substrate creates a so-called bow shock, a gas boundary layer that is difficult for the smallest of particles to penetrate. But in a vacuum, reflected gases are diffused so the bow shock layer is thinner. The fast travelling smaller particles have high enough momentum and can get through the thin bow shock layer. Without a vacuum, the bow shock layer is large and particles don’t have enough momentum to penetrate to the substrate.

Advantages of “plastic deformation” process in your research.

We first captured the plastic deformation process in compressed submicron ceramic particles using in situ micro-indentation and molecular dynamic simulations. In the early experiments and simulations, we compressed ceramic particles and observed plastic deformation, along with fracturing. As part of the plastic deformation process, we recorded and reported dislocation nucleation and slip in brittle aluminium oxide ceramic particles. In later experiments, ceramic particles were compressed at a high rate by accelerating the particles in a carrier gas and impacting on a substrate during the deposition process. We observed very similar microstructural characteristics in the particles compressed by micro-indentation and by impact during deposition.

Maintaining the particle kinetic energy through the bow shock layer is critical to achieving material plastic deformation. Without plastic deformation, there’s no sticking and no coating. When a particle impacts the substrate or another layer, it plastically deforms and changes shape, fractures without fragmentation, and adheres. The next particle that hits and deforms tamps down the original layer, creating an even tighter bond. Those mechanisms make many layers possible, building up coatings that are tens of microns thick.

Aerosol deposition technique versus other ongoing research.

The aerosol deposition technique was first reported in the early 2000s out of Japan by a group of pioneering researchers including but not limited to J Akedo and Y Imanaka. Since then, aerosol deposition has advanced and been gaining interest, evidenced by many published papers from various research groups around the world such as D S Park, C Lee, D M Chun, R Moos, S Johnson, P Fuierer etc. Aerosol deposition is being used in industries in Asia and is being researched and developed in Europe and America. At Sandia National Laboratories, we are shifting our focus from understanding the process’ basic building blocks to expanding into the different materials systems, process optimization, materials integration, and potential applications. We believe that understanding more about the science of this cutting-edge deposition process will pay dividends.

Applications of ceramic coatings in the field of microelectronics.

Examples include capacitors, resistors, inductors, electrical contacts, hybrid circuits, actuators, waveguides, optical modulators, heat-spreaders or heat-sinks, as well as oxidation-resistant surfaces, corrosion-resistant surfaces, and wear-resistant surfaces.

Evolution of devices to become more effective.

The as-deposited films are highly strained and are nanocrystalline in nature — not ideal for many applications such as micro-actuators, micro-motors or capacitors that need large grain structure for better device function. When heat treatment is performed, strain relaxation occurs and, at the same time, these nanocrystals grow and the properties change. By controlling the crystallite size, we can tune the properties in predictable ways. For example, high dielectric permittivity barium titanate films are used in electronic circuits as capacitive devices. Aerosol-deposited barium titanate films, in the as-deposited state, are nanocrystalline with significant grain boundary area. This disordered grain boundary significantly dilutes the dielectric permittivity. Heat treatment that can grow these grains will decrease the grain boundary area, induce stress relaxation, improve crystal quality, and enable more effective domain switching, thus increasing the dielectric permittivity of the film for better device function. This is where we are currently building a wealth of understanding by performing some very exciting experiments — using in situ X-ray diffraction methods to monitor grain growth as a function of temperature and investigating different heat treatment methods to facilitate microstructure control.

Challenges faced during research.

We quickly learned the source of high-quality submicron particle feedstock has a significant impact on film quality. This continues to be one of the most challenging issues. Our feedstock particles must exhibit tight chemical stoichiometry, tight particle distribution, and high purity to achieve world-class material property values. In general, if the particles are of sufficient size (<1 micron), it is possible to deposit films. Those deposited films may not exhibit the properties required for your applications, however, if the feedstock is not well controlled.

Plans for future R&D.

We will continue advancing the technological readiness level of the aerosol deposition process at Sandia National Laboratories, such as a powder capturing and recycling process and feedstock particle treatment. We also will continue using Sandia’s state-of-the-art process diagnostics and advanced materials characterization methods (laser Doppler velocimetry for particle velocity, situ micro-indentation in the transmission electron microscope, in situ X-ray diffraction) to gain fundamental materials science insights into the process-microstructure-properties relationship for different potential applications.

© Chemical Today Magazine

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