Ionic Materials

Master thesis

Are you interested in making your master thesis in the group Ionic Materials?

Research topics:

  • Materials science research, regarding the possible storage of highly radioactive waste in rocksalt
  • Development of a TL dating method for young sediments

Materials science research, regarding the possible storage of highly radioactive waste in rocksalt

"Exploding Salt"

Household salt is a very stabile compound, that’s what we have always learned during our highschool education. It has a high formation energy, a simple structure and the ions know exactly which position they are supposed to fill in their crystal lattice. We know how bonds are formed in NaCl and it ’s also old news that the energy of the lattice is further decreased by the neat way in which negative and positive ions are arranged. Seems to be very straightforward and not very challenging, you would say. We can prove you wrong!

Exactly the fact it is a stabile and simple material makes NaCl such a fascinating compound. NaCl is not always as stable as one would think: it can even be transformed into a highly volatile explosive! To get there the salt, with certain contaminations, has to be irradiated with high doses of ionizing radiation within a small temperature range. Because of the stringent parameters it is understandable that this effect, until recently, was unknown, even after years of dedicated research.

Some well-known explosives are very stable. They can only be initiated by a shockwave. Until the early 80’s scientists believed that ‘stable’ explosives could not be initiated by irradiation. However, in 1983 Miles and others discovered that the well-known explosive, HMX, became less stable under the influence of relatively low doses of ionizing irradiation. Even a dose of 0.1 MGy was enough to induce a spontaneous explosion in HMX.

During irradiation of NaCl, extremely small, metallic sodium and chlorine precipitates are formed that can be observed by calorimetry. Also large voids are formed that can be made visible with electronmicroscope techniques. At the meltingpoint of sodium (97° C) and chlorine (-100 ° C) the specific heat of the irradiated salt changes as a result of the melting processes. The back reaction between sodium and chlorine at about 400° C causes much larger effects and can also be measured by calorimetry.

In the figure below the result of a Differential Scanning Calorimetry (DSC) experiment is shown. In this experiment we increased the temperature of an irradiated sample and an unirradiated reference sample linearly in time. When we near the melting point of sodium, the temperature of the irradiated crystal has the tendency to lag behind because the melting process drains energy. Through an increase of heatflow to the irradiated sample, the experimental apparatus makes sure that the temperature of both samples remains equal. When all the sodium has melted, the heatflow decreases to the same level as when the experiment commenced.

Besides meltingpeaks, the next figure also shows an intense peak for the back reaction. The large difference in intensity can be explained by the fact that the process of reacting molecules involves much more energy than the melting processes. In addition the reaction between sodium and chlorine produces energy (exothermal reaction), while the melting process costs energy. Because the back reaction produces heat, the heatflow decreased as long as the back reaction takes place.

Differential Scanning Calorimetry experiment graph
Differential Scanning Calorimetry experiment graph
The DSC-peaks are usually proportional to one another. Equal numbers of sodium and chlorine atoms are formed and the back reaction generates an equal amount of energy for each molecule. Still in certain cases the meltingpeak and the reactionpeak may differ as much as 30-50% from what is expected. In all those cases sharp thermal "spikes" were visible during the DSC experiments. We have found that during such a sharp "spike" up to 30% of the sodium and chlorine present can revert back to NaCl within a fraction of a second. The typical result, shown in figure below, indicates this. While increasing the temperature an intense sodium meltingpeak was measured at about 100° C. After a further increase of the temperature a sudden change in the heat flow ("spike") occurred at 186° C. Exactly at the moment of this spike, the experiment was ceased and the temperature was allowed to drop. After this a second measurement was started. This second run showed a sodium meltingpeak that was 30% lower than the first one. Apparently the amount of sodium had instantaneously decreased as a result of the spike. This happened without any loss of sample mass. The only rational explanation can be that during the spike, more than 30% of the sodium and chlorine present reacted back to NaCl! Such experiments left the NaCl crystal entirely crushed to dust-like fragments. Occasionally, it was even seen that these processes blew away a relatively heavy platinum lid in the experimental setup.
Differential Scanning Calorimetry: thermal "spikes"
Differential Scanning Calorimetry: thermal "spikes"
During the heating of intensely irradiated salt explosive reactions occur. Our conclusion for the apparent instability of irradiated NaCl is based on a large number of important properties of the radiation-induced precipitates of sodium and chlorine and voids. Depending on the conditions up to 15% of the NaCl crystal was converted to sodium and chlorine. The void percentage is about the same. These voids play a central role in the initiation of the explosive reaction. When large voids are introduced into the sample, the material becomes unstable. In general the average size of radiation induced voids increases, in turn increasing the instability of the crystal. Figure below shows how the properties of the voids vary with the latent heat of melting of the Na colloids, which depends approximately linearly upon the irradiation dose. The upper graph shows the behavior of the average void size and the lower picture shows the relative void volume as a function of the latent heat of melting of the Na colloids.
Void properties versus Na colloids' latent heat of melting
Void properties versus Na colloids' latent heat of melting
We are certain that the percentage of radiation-induced defects will further increase with increasing doses of radiation, thereby further increasing both the instability and force of this new and mysterious explosive. Experiments to test this hypothesis are still in progress at the moment… the effects that are associated with this exploding salt are shown in the next figure. The initiation commences with a trigger: a hotspot that is formed by an unstable void (shown as a red spot). The shockwave that results from the initiation propagates through an energetic medium. Precisely at the moment the shockwave passes, the resulting high pressure instigates the correct mixture of sodium and chlorine because these particles are located within about 1 nm of each other. At that exact moment a local back reaction occurs, adding energy to the shockwave. This process continues as long as the shockwave is propelled through the energetic medium, up until the moment the sample explodes as a result of the extreme pressure buildup.
Effects of exploding salt
Effects of exploding salt

Within the framework of the Dutch research program, conducted to find possibilities to store radioactive waste in underground saltformations, the Ionic Materials Group of the Solid State Physics Department, especially in view of the facts mentioned above, has researched what the consequences of storage are for the salt directly surrounding the radioactive material. This work is mainly financed by the Ministry of Economic Affairs.

Possibilities for researching radiation damage within the group are:

  • Research on the explosive properties of heavily irradiated NaCl
    Approximately 10 years ago our group has shown that, under certain circumstances, NaCl can be so badly damaged by irradiation that it explodes spontaneously. These explosions have been studied by means of microscopy (in combination with fast video techniques), calorimetry and theoretical methods.
  • The melting properties of Na and Cl2 precipitates in heavily irradiated NaCl
    The melting properties of radiation induced Na and chlorine particles differ greatly from the corresponding properties of the bulk materials. The meltingpoint of the Na-particles in heavily irradiated NaCl varies between 80 and 140ºC (!), depending on the circumstances during and after irradiation. In some cases up to three different meltingpoints were observed for a single irradiated sample. Furthermore we have found that in many cases the meltingpoint of the sodium particles changes significantly by annealing the irradiated material at 180ºC. These effects are accounted to particles so small that adding only a few atoms causes large changes in their structure and properties.
  • Research on the optical properties of heavily irradiated NaCl
    Irradiation of NaCl causes, among other things, the formation of Na-particles. These particles are generally small (a typical diameter is 5 nm). At first the diameter of these particles increases with an increase of the radiation dose, but during extreme doses Quasi-1D structures of ultra-small Na-particles with diameters of only 1 nm are formed! These particles form during advanced stages of damage formation, in which more than 1% of the sodium-ions present in the crystal is converted to Na-metal atoms. Many physical properties, including the optical properties, of these Na-particles differ from the normal properties of Na-metal.
  • Magnetic properties of heavily irradiated NaCl
    Initially during irradiation of NaCl, small isolated Na-particles are formed. These particles do not show, as a normal metal would, Pauli paramagnetic behavior, but Curie paramagnetism. Such behavior has been mentioned in literature and can be traced back to the existence of very small particle dimensions. When, during continued irradiation, the particles become bigger, we often find a transition from Curie to Pauli paramagnetism. Materials with more than 1% metallic sodium, i.e. in compounds containing structures made up of extremely small Na-particles, behave in a very surprising manner. The individual particles within the structure show strong magnetic interactions, therefore we think of them as Quasi-1D conductors.
  • Light scattering properties of heavily irradiated NaCl
    Heavily irradiated NaCl is completely black, making standard optical experiments, such as optical absorption spectroscopy, practically impossible. An attractive method to obtain new information about the structure of heavily irradiated NaCl is Raman scattering. Using this technique many indications have been founds over the last couple of years that the super-small Na-particles in heavily irradiated NaCl form more or less fractal structures with Quasi-1D character. In addition this method also gives information on the form of the radiation-induced chlorine precipitates. Especially in very heavily irradiated NaCl strong bands have been associated with chlorine.
  • Theoretical research regarding radiolysis processes in NaCl
    Experimental research has shown that prolonged irradiation at moderate temperatures causes extremely small sodium and chlorine precipitates to be formed along with voids. The explosive properties, mentioned under a., are caused by a subtle collaboration of the aforementioned radiation induced defects. Therefore it is imperative that we understand how radiation damage in NaCl is formed. This is especially important because it is impossible to carry out irradiation experiments that even come close to the proposed storage conditions. That would make irradiation experiments necessary that last several hundred years. A reliable physical model, that is now being developed, using systematical experiments and extrapolations, should help us to get an idea of the problems we should expect from storage facilities for highly radioactive waste.

Development of a TL dating method for young sediments

The Dutch coastline contains many sandy beaches. The sand on these beaches has traveled over long distances before it was deposited here. When we consider the age of the sedimentation along our coasts, we usually consider the time that passed after the deposited material was permanently covered by other layers. It is important to determine this time, i.e. the age of the sediment, as it can teach us what we must do to protect our coastlines from the sea. So far large quantities of sand are regularly applied to maintain our coastline. If we could find out more about the mechanisms that govern the deposition of sand on the coast, it could be of great practical value. To contribute to this knowledge we are developing a special ThermoLuminescence (TL) dating method for young sediments (age plm. 10-100.000 years). This research is financed by the Foundation for Technical Sciences (STW).

Possibilities for research in this field are:

  • Separation and characterization of sand sediment
    The sand sediments of the Dutch coast, and many others, consist of a collection of minerals. A common material is natural quartz, but many other compounds are also present. That is very fortunate for us as the presence of zirconium-silicate (ZrSiO4), zircon for short, is of great importance to our research. This heavy mineral contains relatively many radioactive uranium and thorium impurities that internally bombard the grains with a -particles. We use several methods to isolate the grains. It has been found that the quartz grains, the dominant species in the sand, are significantly larger than the zircon grains. This means that, using an appropriate sieve, we can do an initial rough separation. The other separation steps, that follow, make use of specific properties of the zircon grains, need further investigation.
  • Dating of natural zircon (ZiSiO4) grains
    Natural zircon grains often contain relatively high concentrations of uranium and thorium. These are a -emitters that irradiate the grains internally. Because of this, the applied doses are relatively high; in principal making these compounds very useful for dating purposes. Another important advantage is that the dose caused by external effects is negligible. This means that all the information regarding the dose is present within the grain itself. This is not the case for quartz, which at the moment is frequently used for dating. In quartz the dose is strongly dependant on external effects, such as the possible presence of natural radioactive isotopes in nearby grains, the presence of water in the sediment, the intensity of cosmic radiation, etc. In spite of the many advantages that zircon has over quartz for dating methods, zircon is still only sporadically used in dating experiments because of the still numerous (mostly material/physical) problems that have to be solved.
  • TL experiments with natural zircon
    Irradiated zircon grains emit light when they are heated. This phenomenon is called thermoluminescence (TL). Radiation causes meta-stable defects to be formed. These defects recombine at high temperatures, emitting light that is characteristic for the aforementioned defects. The intensity is a measure for the dose, i.e. for the concentration of the radioactive isotopes and the duration of the irradiation. When the grains are exposed to sunlight, the meta-stable defects are ‘bleached’. This phenomenon is called ‘resetting’. Resetting is essential to age determination: the timeclock only starts running when the grains are covered definitively by other layers of sedimentation. This means that TL gives us information about the period of time in which the grains were no longer lit by the sun.
  • Fading effects
    To obtain information about the effect of a certain radiation dose on the intensity of TL light we can irradiate natural zircon grains by means of an artificial radiation source. Most obvious is the use of a g -source because it can be used to apply a homogeneous dose to the samples. On the other hand a -particles have important advantages, as these particles claim a dominant role in the actual dating process. For now we mostly use an intense g -source. In comparison to natural processes, the application of a desired dose happens very swiftly (usually a couple of days). This has consequences for the TL yield, as meta-stable defects tend to decay even at room temperature (fading). An important breakthrough for our new dating method will be the moment we will be able to simulate the fading processes that follow artificial irradiation.
  • Electron spin resonance (ESR) with irradiated zircon
    During irradiation of zircon, mainly oxygen ions are excited, making it possible for hole-centers to form on these ions. In many cases the electron will return very quickly to its original position, restoring the original situation. However, sometimes the electron is captured by another defect, leaving the hole-center in tact. This leads to the formation of relatively stable paramagnetic defects, which can be detected by means of ESR. The defect concentrations are a measure for the received dose (and of course the age). In addition ESR can be used to obtain information about the nature of the radiation induced defects.
Last modified:October 22, 2012 16:38