An Introduction to Plasma

Plasma, known as the fourth state of matter, is a discrete processing medium for the treatment and adaptation of surfaces. The following articles offers further explanation of the nature of plasma, its unique benefits, its interaction with surfaces, and the variety of process gases that can be applied.

Nature of Plasma

Plasma is a partially ionized gas made up of electrons, ions and neutral atoms or molecules. The plasma as a single unit is at near ambient temperature, despite the fact that plasma electrons are at a significantly greater temperature (around 104 K) than the neutral gas species. The plasma electron density is generally around 1010 cm-3.

Plasma Formation

Plasma is produced when a radio frequency (RF) oscillating electric field is made in the gas, either through the application of capacitive plates or through magnetic induction. At adequately low pressures, the collective effect of the electric field acceleration of electrons, and the elastic scattering of the electrons with neutral atoms or field lines results in the heating of the electrons.

When electrons gather kinetic energy beyond the first ionization threshold in the neutral gas species, electron-neutral collisions result in further ionization, producing added more electrons that are subsequently heated themselves.

Plasma Advantages

Plasma treatment primarily has an impact on the near surface of a material without changing the main material properties. Additionally, plasma is created at near-ambient temperature, minimizing the threat of harm to heat-sensitive materials.

Contingent on process gases and usage arrangement, plasma treatment can be used to clean, activate, or chemically adapt surfaces. Subsequently, plasma treatment can be used on a number of varying materials, as well as complex surface geometries, including polymer fibers and fibrous scaffolds, oxide and metal nanoparticles, glass coverslips and slides, semiconductor wafers, and porous membranes.

Plasma-Surface Interaction

The energy of plasma electrons and ions is adequate to ionize neutral atoms, break molecules apart to create reactive radical species, produce excited states in atoms or molecules, and locally increase the temperature of the surface.

Dependent on the process gases and parameters, plasmas can engage in both mechanical work, through physical ablation and high-energy ion bombardment of the surface, and chemical work, through the reaction of reactive radical species with the surface.

On the whole, plasmas can interact with and alter a surface through numerous methods: ablation, activation, deposition, cross-linking and grafting.

Ablation

Plasma ablation relates to the mechanical elimination of surface contaminants by energetic electron and ion bombardment. Ablation has an impact only on the contaminant layers and the outermost molecular layers of the substrate material.

Argon gas is frequently used for its high ablation efficiency and chemical inertness with the surface material.

Activation

Plasma surface activation relates to the formation of surface chemical functional groups via the application of plasma gases, including oxygen, hydrogen, nitrogen, and ammonia, which disconnect and react with the surface.

With polymers, plasma breaks down feeble surface bonds in the polymer and substitutes these surface polymer groups with chemical groups from the plasma gas, including carbonyl, carboxyl, and hydroxyl groups. This activation changes the surface chemistry and wettability of the surface, which can vastly improve adhesion and bonding to other surfaces.

Cross-Linking

Cross-linking refers to the bonding and linkage of molecular chains in a polymer. Plasma processing with inert gases can be applied to cross-link polymers and creates a sturdier and more rigid substrate micro surface. In certain conditions, crosslinking through plasma treatment can also offer extra wear or chemical resistance to a material.

Deposition

Plasma deposition relates to the creation of a thin polymer coating on the substrate surface because of polymerization of the process gas. The process gas is a combination of a vaporized monomer and inert carrier gas, to be applied for plasma formation. Plasma polymerization can be used to place a polymer layer with distinctive chemical functional groups.

This information has been sourced, reviewed and adapted from materials provided by Harrick Plasma.

For more information on this source, please visit Harrick Plasma.

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