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| What Ions Do to Materials |
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What Ions Do to Materials
When high-energy ions affect materials, solid-state physicists and materials
engineers sit up and take note. The range of ion types and ion energies
available at GSI is also opening up a world of new opportunities and
exciting new potential technical applications for materials research. In this
respect, successful collaborations have already been set up with both
industry and neighboring centers of higher education.
When an energy-rich ion penetrates a solid, the material along the trajectory of the ion beam is
modified, atoms are pushed out of their normal positions, molecules are split into pieces, and
ordered structures - such as that of the crystal lattice - are destroyed. In this process, a so-
called latent track is created by the ion, whereby the diameter and length of this track depend
on the type of ion and its energy, as well as on the structure and chemical composition of the
irradiated material. Should the radiation dose be so high that ion tracks overlap, then the
physical and chemical properties of the material can also be altered on a macroscopic level to
such an extent that it can be considered a new material with new properties.
Materials research carried out at GSI covers both basic research and application-oriented
topics. Much of the research is aimed at a better understanding of the changes produced in
solids by ion beams. For, in the long term, the directed development of improved materials -
not to mention completely new materials - will only be possible through a more complete
understanding of the processes involved here. However, activity in this area has already
spawned a wide range of interesting technical applications. Examples include the creation of
micro-structures through galvanic replication of etched ion tracks, the production of optical
waveguides, and the increase of the maximum current density in superconductors through ion
bombardment.
When swift, heavy ions pass through a solid, their speed is sharply reduced. Within a very short
period of time, a considerable amount of energy is transferred as collisions occur with the
electrons of the material along the trajectory of the ion. The result is high local energy densities
along the ion path. The knocked-out electrons, in turn, transfer the energy they have received
to the neighboring atoms or molecules. If the irradiated material is a crystal, the crystalline
structure may be destroyed by these processes.
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Cross-section of the ion track of a high-energy uranium ion in germanium sulfide, recorded
using a transmission electron microscope. The elliptical form, with axis lengths 21.8 and
15.0 millionths of a millimeter (nm), is a result of the anisotropy of the crystal. The
four "butterfly-like" structures in the picture show the stress created in the crystal
lattice by the track.
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In the direct vicinity of the ion trajectory the material passes into a disordered state which is
generally referred to as amorphous. A cylindrical track measuring approximately one hundred
thousandth of a millimeter across is the result. The length of such a track can be up to several
millimeters given a sufficiently high initial energy of the ion. Studying the detailed
characteristics of an ion track tells us a great deal about the processes giving rise to its
formation. It is for precisely this reason that various track features are being studied in a wide
variety of different materials, ranging from metals to semi-conductors and insulators.
Characteristics currently under investigation include the size, form, and structure of ion tracks,
as well as the transition zone from the central area of damage to the intact surroundings.
A wide variety of experimental techniques are used in this kind of materials research. These
range from high resolution microscopy to X-ray and neutron scattering, infra-red
spectroscopy, and track-selective etching.
By combining the information produced by
experiments with that thrown up by computational simulations, researchers are able to develop
models of track formation. The predictions of these models are then once again compared with
experimental results obtained for different materials. As a consequence, researchers are able to
improve their understanding of the processes involved in a step-by-step manner.
Ion tracks are also being put to good use in numerous practical applications. Given their great
length with respect to diameter, and their selective behavior during etching, ion tracks are
especially well suited to the production of very fine structures and objects, the dimensions of
which may be less than a thousandth of a millimeter in some cases. Acids and bases may be
selected in such a way that the damaged substance in the inner region of the ion track is
attacked more aggressively than the undamaged surroundings. This phenomenon is particularly
pronounced in organic substances. In this way, depending on the susceptibility of the
undamaged material to the etching process, very long, almost cylindrical channels can be
produced along the ion tracks. No other method of creating structures can produce comparable
results.
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Microscopically-fine copper needles, produced by galvanic replication, with a length of
about 0.1 mm and a diameter of 2.2 thousandths of a millimeter and a tip radius of 0.17
thousandths of a millimeter.
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It is thus possible to make a galvanic replication of the etched ion tracks. If the deposition
process is terminated after the channels have been fully filled and the substrate material
subsequently dissolved, parallel microscopic metal needles are produced on a metal base. This
is only possible because the ion beam has a high degree of parallelism. By varying the geometry
of the etched ion track channels, the electrolytes, and the deposition conditions, it is possible to
produce bars, cones, pins, and tubes with microscopic dimensions. The galvanic replication
gives an undistorted reproduction of the inside of the etched ion track, thus also allowing an
"endoscopic" view of the channels.
The creation of micro-structures using the galvanic process would seem to have considerable
potential with regard to technical applications. It is possible to imagine galvanically-produced
long, thin columns that could be used as microscopic antenna for electromagnetic radiation, or
needles that could be used as sources for field emission currents.
By firing
individual ions in a directed manner, regular point grids can be formed. After etching these to
the desired diameter, a galvanic replication can be made, yielding a regular structure of
microscopic objects. In general, possible applications extend over a wide spectrum, ranging
from microelectronics to micromechanics to biology and medicine.
"Intelligent" filter membranes for medicine and biology
Another broad field of applications is associated with the use of ion-irradiated polymer films as
membranes with a uniform pore size. The pore diameter can be precisely determined through
the choice of irradiated material, the type of ion, its energy, and the etching process used. In
order to control the flow of particles or liquids through ion-track membranes, the following
trick is used: the etched membrane is coated with a layer of expandable hydrogel which covers
the film and the walls of the pores. Here the etched membrane acts as a mechanically stable
carrier material. Through the choice of a suitable substrate polymer, it is possible to apply a
hydrogel layer that, for example, expands when the temperature decreases and shrinks when
the temperature increases. In this way, the pores can be opened and closed in time with
temperature variation.
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Temperature-controlled opening and closing of a grafted ion track channel in a polymer
foil, demonstrated by changes in the electrical conductivity.
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Waveguide branch in lucite, produced by increasing the refractive index through ion
bombardment. The lucite is irradiated with ions through a mask. The Y-branch guides the
light signal entering from the glass fiber to two receivers. The diameter of each of
the two spots where the light exits is only 7.7 thousandths of a millimeter.
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Alternatively, the diameter of the pores can be controlled by
the pH value of the surrounding liquid. It is possible to imagine many applications of such
membranes such as, for example, the controlled release of medication when it is needed.
In telecommunications and related areas, optical waveguides and their corresponding
components, such as couplers and distributors, are steadily gaining in importance.
It
is extremely likely that they will be required in large numbers and at economical prices in
the
near future.
Over the last few years, component designers have concentrated more and more on polymer
compounds because of their low material costs. In addition to photochemical methods, ion
implantation is a very promising technique for producing waveguide structures in polymers.
The technique is based on the following effect: the implanted ions break the chemical bonds in
the polymer as they propagate along their paths, thereby causing low-molecular-weight
fragments to be given off. As a consequence, the local density of the material is raised and the
optical refractive index increases. The penetration of the ions, and thus also the thickness and
depth of the modified layer, can be controlled via the initial ion energy. The ion dose must be
selected in such a way that the change in the refractive index is sufficient for light to be
directed in curved waveguide structures too. Irradiation is carried out using a self-supporting
mask or a correspondingly structured layer of photosensitive resist.
Through ion implantation - the introduction of additional atoms of a particular chemical
element by ion irradiation - the properties of metallic materials can be changed in a controlled
way. Let us consider a concrete example of this phenomenon. The ball and shaft of artificial
hip-joints are at present predominantly made from an alloy consisting mainly of the light
element titanium, whilst a polymer is used for the artificial socket. The long term resistance to
wear and tear of this system is limited by the formation of titanium oxide particles at the
interface between the ball of the joint and the polymer socket. However, researchers are
currently investigating the implantation of various ions of different energies in a thin surface
layer of the titanium alloy as a means of reducing the amount of material abrasion. In
laboratory experiments, the amount of material abrasion was clearly reduced following the
implantation of certain elements.
A completely different application is the testing of electronic circuits. Cosmic rays can distort
or destroy stored data in the on-board computers of spacecraft, satellites, and high-altitude
aircraft. Such processes can be simulated using a heavy ion micro-probe, allowing the
radiation-sensitive regions of microelectronic components to be identified. Here a narrow beam
is separated from the main ion beam and focused by an ion-optical lens to a width of one
micrometer. Using a magnetic deflection device, the focused beam is scanned across the object
under investigation—line by line. When an ion strikes the electronic circuit, secondary particles
(ions and electrons) are released. These then set off a detector which emits a signal that
switches the micro-beam off within a fraction of a millisecond. A computer then searches the
integrated circuit for changes to the stored information - so called "bit flips" caused by the ion
hit. The coordinates of the position of the micro-probe beam at which bit flips occur are
recorded and a map of the ion-beam sensitive areas of the circuit drawn up.
Ion tracks improve superconductors
One of the most important properties of superconductors - particularly when it comes to
practical applications - is the critical current density. This determines the limit up to which a
superconductor can carry current without loss. Of great technological significance are the new
high-temperature superconductors, some of which function at temperatures of more than 100
degrees above absolute zero. These superconductors are penetrated by external magnetic fields
in the form of so called flux tubes which migrate in the material of the superconductor and
cause energy loss through the production of heat. It is thus the movement of these flux tubes
which determines the upper limit of the transport current. An effective method of fixing flux
tubes is currently an important research and development task.
It is known that inhomogeneities such as grain boundaries, dislocations, and pores in a material
can bind flux tubes so that the current density is carried without loss. Heavy ion experiments
exploit this situation and utilize the cylindrical tracks of the high-energy ions as binding centers
for the flux tubes. In this way, it is possible to obtain a significant increase in the critical
current.
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