Ion track technology (track etching)
Encyclopedia

Selective ion track etching is closely related with the selective etching of grain boundaries and crystal dislocation
Dislocation
In materials science, a dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials...

s. The etch process must be sufficiently slow to discriminate between the irradiated and the pristine material. The resulting shape depends on the type of material, the concentration of the etchant and the temperature of the etch bath. In crystals and glasses, selective etching is due to the reduced density of the ion track. In polymers, selective etching is due to polymer fragmentation in the ion track core. The core zone is surrounded by a track halo in which cross-linking can impede track etching. After removal of the cross-linked track halo, the track radius grows linear in time. The result of selective etching is a trough, pore or channel.
Surfactant enhanced etching is used to modify ion track shapes. It is based on self-organized
Self-organization
Self-organization is the process where a structure or pattern appears in a system without a central authority or external element imposing it through planning...

 monolayers
Monolayer
- Chemistry :A Langmuir monolayer or insoluble monolayer is a one-molecule thick layer of an insoluble organic material spread onto an aqueous subphase. Traditional compounds used to prepare Langmuir monolayers are amphiphilic materials that possess a hydrophilic headgroup and a hydrophobic tail...

. The monolayers
Monolayer
- Chemistry :A Langmuir monolayer or insoluble monolayer is a one-molecule thick layer of an insoluble organic material spread onto an aqueous subphase. Traditional compounds used to prepare Langmuir monolayers are amphiphilic materials that possess a hydrophilic headgroup and a hydrophobic tail...

 are semipermeable for the solvated ions of the etch medium and reduce surface attack. Depending on the relative concentration of the surfactant and the etch medium, barrel or cylindrical shaped ion track pores are obtained. The technique can be used to increase the aspect ratio
Aspect ratio
The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...

.
Repeated irradiation and processing. A two step irradiation and etching process is used to create perforated wells.
Arbitrary irradiation angles enforce an anisotropy along one specific symmetry axis.
Multiangular channels are interpenetrating networks consisting of two or more channel arrays at different directions.


Track etching of common polymers
Material pH Wet etchant Sensitizer1) Desensitizer2) T/C3)Speed4)Selectivity5)
PC
Polycarbonate
PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...

basic NaOH UV Alcohols 50-80 Fast 100-10000
PET
Polyethylene terephthalate
Polyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...

basic NaOH UV, DMF Alcohols 50-90 Fast 10-1000
basic K2CO3 80 Slow 1000
PI
Polyimide
Polyimide is a polymer of imide monomers. The structure of imide is as shown. Polyimides have been in mass production since 1955...

basic NaOCl NaOH 50-80 Fast 100-1000
CR39
CR-39
CR-39, or allyl diglycol carbonate , is a plastic polymer commonly used in the manufacture of eyeglass lenses. The abbreviation stands for “Columbia Resin #39,” because it was the 39th formula of a thermosetting plastic developed by the Columbia Resins project in 1940.The first commercial use of...

basic NaOH UV 50-80 Fast 10-1000
PVDF
Polyvinylidene fluoride
Polyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...

6)
basic KMnO4 + NaOH 80 Medium 10-100
PMMA6) acidic KMnO4 + H2SO4 50-80 Medium 10
PP
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...

6)
acidic CrO3 + H2SO4 80 Fast 10-100

1) Sensitizers increase the track etch ratio by breaking bonds or by increasing the free volume.

2) Desensitizers decrease the track etch ratio. Alternatively ion tracks can be thermally annealed.

3) Typical etch bath temperature range. Etch rates increase strongly with concentration and temperature.

4) Axial etching depends on track etch speed vt, radial etching depends on general etch speed vg.

5) Selectivity (aspect ratio, track etch ratio) = track etch speed / general etch speed = vt / vg.

6) This method requires to remove remaining metal oxide deposits by aqueous HCl solutions.
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