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MaMaSELF News : articles from alumni, students, professors

MaMaLINK  news from alumni, professors, students
  1. Expectation vs. reality by Brittany FOLEY
  2. Plasma catalysis: synergy between plasma and catalyst. by Chloé FROMENTIN
  3. 2D-ACAR: Using Positrons to Determine the Electronic Structure of Materials by Josef KETELS

Expectation vs. reality by Brittany FOLEY

When I arrived in Rennes to begin my first year of MaMaSELF, I did so with a head swimming with expectations and anxieties of what everything would be like. To put it lightly, most of them were pretty far off. 

 

Expectation: Customs at CDG will take forever
Reality:With my visa, I basically flew through customs and was even ushered through the EU line. Pretty sweet.

 

Expectation: When I arrive in Rennes, I will arrive at the train station I saw on Google Maps (République)
Reality:I arrived at a hollowed-out mess of a train station in the midst of serious renovations (Gare de Rennes). It was a disaster. There were people packed everywhere, a dismantled lobby swimming with flopping sardines. I was expecting a beautiful facade and was greeted with scaffolding. There was supposed to be a metro somewhere...but where? Good news though! Today, that mess of a train station has been transformed to one of the coolest stations I've ever seen! 
 

 

Expectation: I'll just take an Uber to campus from the train station
Reality:Uber does not exist in Rennes. Please try again. (Taxi, 12 euros). This is a blessing in disguise because after only a few weeks of getting used to it, I was comfortable walking all over! And Rennes also has an excellent public transportation system: super fast metro, bus routes all over, and even special lanes on the roads so the buses don't get stuck in traffic. With your Korrigo card you get unlimited access to the public transport system all month long. 
 

 

Expectation: I will be studying and living in the charming little city center I see in Google Images when I search "University of Rennes"
Reality:Trees. Everywhere. So much green. No ATMs in sight. Nearest restaurant: a pizza vending machine. My father: "Do they have Pinterest there?". This was an adjustment as someone coming from a major city. In the end, I loved it. The city center is really easily accessible from campus (sometimes we even just walked over) but being on a green campus space was lovely, especially when the weather got nice. Le Parc des Gayeulles is also awesome--we had big group campfires at night and I loved jogging through the woods and along the ponds on weekend mornings. 
 

 

Expectation: I don't have to pack the little necessities, like a toothbrush or shampoo or forks, I'll buy them all at the grocery store after I get settled in
Reality:Everything is closed before 7 PM. Please try again. It can be hard to adjust to the business of welcome week and living in a new place without the bare necessities! Make sure to pack the essentials: tooth brush, towel, shampoo, fork+knife+spoon, and water bottle. The residence will have sheets and a blanket for international students but you might find extra comfort in bringing a little pillow or a cozier blanket from home if you can fit it! I overpacked, so I just brought the blanket they gave me on the airplane for a little extra something cozy. It could be a few days before you have time to go to Ikea! 
 

 

Expectation: Welcome week will be fun picnics and awkward ice breakers and name tags written in sharpie and bus-rides to field trip locations
Reality:all of the above +  lectures + "On Thursday morning, we will have a brief exam...." Don't worry, it's not graded. (;
 Expectation: "Bring comfortable sport shoes for welcome week" = we will go on walking tours, maybe a scavenger hunt or hike
Reality:We will run across ice chasing soccerballs with broomsticks, we will run across the city solving complicated play-on-word riddles in a language only 10% of us understand,  we will canoe, kayak, and raft through rainstorms. Welcome to Survivor: Bretagne!
 

 

Expectation: Everyone else will be really smart and accomplished
Reality:"I speak English, French, German, Japanese, and am learning Italian," "oh, I studied at MIT for a while," "actually, I've already worked at a synchrotron," "I already have a master's degree, this is my second." (Yes, everyone is really smart and hard-working...and so are you! You've earned this, so keep working hard and don't doubt yourself!)
 
Expectation: Everyone will be really studious and serious
Reality:"We will divide into two teams, those who like pineapple on their pizza, and those who do not."
 

Expectation: We will have free time to get settled in
Reality:Whenever you arrive in Rennes, if there is a grocery store open, go go go! Welcome week is one of the most fun parts of MaMaSELF but you will be BUSY. That means little/no time for grocery shopping and, if you're like most of us were and fail to prepare, lots of sandwiches. Pick up a couple things from the grocery store when you arrive (maybe even a bit to share) and you'll be able to enjoy some nice late-night dinners after activities with your new friends. Otherwise, you'll be doomed to eat every meal from the sandwich bar (pictured above). It's pretty good, but one can only eat a jambon-beurre for so many meals before it starts to lose its luster...
 

Expectation: I will be in France so I will eat pain au chocolat and espresso every day
Reality:I ate pain au chocolat and espresso every day
 
Expectation: Everyone else will know all about synchrotrons even though I really have no clue
Reality:"This class says 'Electrons and Phonons...'" "Do they mean photons?". There will be lots of different topics tossed around in those first few weeks...don't panic! Don't get scared, don't give up. Even the subjects that are totally different from what you studied before will eventually become familiar. Studying so many different tough subjects takes time and hard work, but your peers and professors are there to support you. We all enter the program with different backgrounds but we all leave incredibly well-rounded. Don't be afraid to ask questions and don't get too self-conscious if you get a funny look for not knowing something. MaMaSELF is like a family, professors included, and they believe in you too! And remember, your classmates can be teachers too. Rely on each other, lift each other up, and you'll all be fine.
 

 

Best friends Mamaself and Brittany

Expectation: This is going to be a disaster
Reality:This is the greatest thing I've ever done in the most beautiful place I have ever lived with the most amazing people I have ever met.
 

Plasma catalysis: synergy between plasma and catalyst. by Chloé FROMENTIN

The interaction between catalysts and low-temperature plasmas may lead to synergetic effects that can improve the conversion and energy efficiency and selectivity of a process.  In the context of plasma catalysis, we can define synergy as an additional effect upon combining the plasma with a catalyst, in other words, the effect of putting together the plasma and the catalyst is greater than the sum of their individual effects. Plasma catalysis presents a variety of applications for gas conversion such as air pollution control (volatile organic compound (VOC) abatement) and greenhouse gas conversion into value-added chemicals or fuels (syngas). 
Catalysis is defined as the process in which the rate of a reaction is increased (by lowering the activation energy barrier) while the thermodynamics remain unchanged. Catalysis is divided into three types, homogeneous, enzymatic and heterogeneous. More specifically, in the case of plasma catalysis heterogeneous catalysis (in which the catalyst occupies a different phase from the reactants and products) is mostly used and brings the selectivity towards specific compounds. A heterogeneous catalyst is usually a nanostructured solid (presenting nanoparticles, nanopores, …) containing a high number of active sites (large surface area) responsible of its catalytic activity. Catalysts used in plasma catalysis are transition metals, metal oxide such as CoOx, MnOx, TiO2and zeolites (solid acids).  
Plasma (also referred to as the fourth state of matter) is described as a partially ionized gas composed of charged (electrons, ions) and neutral species (electronically or vibrationally excited) and radicals, exhibiting a collective behaviour. The plasma thus provides high energy and chemically activated species in the gas phase. Low-temperature plasmas (LTP) electrons have an energy  in the range of electron volts corresponding to typical bond dissociation and ionization energies of atoms and molecules while the heavy species (ions and neutrals) have a temperature close to the gas temperature (300−1000 K). The combination of reactivity, non-equilibrium situation, and low-temperature operation enables plasmas to be used for numerous applications such as catalysis. Typical examples of LTP are low pressure glow discharges, RF discharges, DBD, etc.

Massurement of oxygen atoms density and gas temperature using Two photons Absorption Laser induced fluorescence (TALIF) signal in low pressure CO2 direct current discharge

Synergism is a complex phenomenon as it results from the interdependence of the plasma-catalyst interactions and it may not always be observed. Typical effects of the catalyst on the plasma, mainly due to the nanofeatures and dielectric constant of the catalyst, are: the enhancement of the electric field, the formation of micro discharges inside of the pores, the change in discharge type, and the change in species concentration. Vice versa, the plasma triggers physicochemical and morphological changes in the catalyst. Plasma photons (generated by decaying excited plasma species) could activate photocatalyst and electrons could induce surface reactions. Finally, by impact of charged species, photons, metastable and excited neutrals and through exothermic surface reactions the plasma can heat up the surface. The plasma itself may change the energy barrier of activated process such as chemisorption by creating radicals, by modifying the morphology of the catalyst or by providing some of the energy necessary to surmount the activation barrier (e.g. by vibrational excitation of the reactants). In addition, the plasma enables one to modify the feedstock (gas or gas mixture) via elementary processes in the gas phase (electron impact ionization, excitation, dissociation…) and via the plasma/catalyst interactions. 
Despite being very successful for some applications, plasma catalysis remains an expensive process, thus, the major challenge is to improve the energy efficiency and reduce the energy cost as much as possible. However, the additional energy costs associated to the acquisition of plasma equipment and the generation and sustainment of the plasma may be counterbalanced exploiting the synergistic plasma-catalytic processes. Moreover, the possibility to switch on/off the plasma source may allow to use renewable energy sources. Finally, the various effects of the plasma on the catalyst, and inversely, have been investigated experimentally and the results as well as modeling efforts are already described in the literature [1-3]. Further research combining experimental, theoretical and modeling studies will ensure great advances in this field both in terms of understanding the underlying mechanisms involved and in terms of turning this fundamental knowledge into practical outcomes.

References
[1] Neyts, E., Ostrikov, K., Sunkara, M. and Bogaerts, A., (2015) ‘Plasma Catalysis: Synergistic Effects at the Nanoscale’ Chemical Reviews, 115(24), pp.13408-13446.
[2] Neyts, E., (2015) ‘Plasma-Surface Interactions in Plasma Catalysis’ Plasma Chemistry and Plasma Processing, 36(1), pp.185-212.
[3] Neyts, E. and Bogaerts, A., (2014) ‘Understanding plasma catalysis through modelling and simulation—a review’ Journal of Physics D: Applied Physics, 47(22), p.224010.

 

2D-ACAR: Using Positrons to Determine the Electronic Structure of Materials by Josef KETELS

2D-ACAR: Using Positrons to Determine the Electronic Structure of Materials

Quantum oscillations, which has high demands concerning the ambient conditions, and the mostly surface sensitive angle-resolved photoemission spectroscopy (ARPES) are two well known techniques for the measurement of the electronic structure of materials. One less common experimental technique is angular correlation of annihilation radiation (ACAR) which employs positrons, the antiparticle of electrons, to measure projections of the electron momentum density (EMD). ACAR is a pure bulk probe, with low experimental demands, which offers the possibility to e. g. investigate temperature effects like phase transitions.
 

A basic 2D-ACAR setup, as illustrated in figure 1, consists of two position sensitive 2D detectors (in our case so-called Anger cameras), a sample which is located exactly in the middle of the detector-detector-axis and a 22Na-source that produces positrons through β+-decay. After implantation into the sample the positrons thermalize within a few picoseconds and thus loose their momentum. After diffusing through the crystal (100 ps – 200 ps) the positron finally annihilates with an electron under the predominant emission of two gamma particles. Due to the momentum carried by the electron of the material (Pauli principle) and conservation of momentum, the gammas are  not emitted at exactly anti-parallel directions but with a slight deviation from 180°. Using small angle approximation, the deviation ΔΘ is proportional to the momentum of the electron. By neglecting the small influence of the positron and integrating over many such annihilation processes, we can finally measure a two-dimensional projection of the EMD. This basic principle is depicted in figure 2.

 

Figure 2: Basic principle of ACAR spectroscopy: annihilation of a thermalized positron with an electron



One additional advantage of 2D-ACAR is the possibility to conduct spin-resolved measurements which directly results from the usage of the22Na-source (see figure 3). Due to the parity violation of the weak interaction in the  β+-decay the positrons are spin-polarized. By magnetizing a ferromagnetic sample the majority and minority spin-channels can be probed separately as the annihilation predominantly occurs in the singlet state.

 

One approach on how to use this technique in condensed matter physics is the direct comparison of 2D-ACAR measurements to theoretical calculations. Two examples how this was employed are the benchmarking of different theoretical calculation schemes like LDA, LDA+U or LDA+DMFT on vanadium [1] or the determination of the electron-electron correlation strength in nickel [2]. The

 
second approach is based on taking several 2D projections of the EMD and reconstructing the 3D EMD from those. At the Fermi-surface the EMD shows sharp steps and thus this important surface in momentum space can be extracted from a set of measurements. This was e. g. shown in the well known Heusler alloy Cu2MnAl in which the minority and the three majority Fermi sheets could be measured (see figure 4) [3].

 

[1] J.A. Weber, et al.; PRB 95 (2017) 075119
[2] H. Ceeh, et al.; Sci. Rep. 6 (2016) 20898
[3] J.A. Weber, et al.; PRL 115 (2015) 206404

 

Last updated: 09/04/2020 - 15:18