Anne Dauscher

Thermal characterization and analysis of phase change random access memory

The cross-plane thermal conductivity of Ge2Sb2Te5, either in its amorphous state or fcc crystallized state, and titanium nitride (TiN) thin films has been measured at room temperature by the 3ω method. These materials are involved in the fabrication of phase change random access memory (PC-RAM), Ge2Sb2Te5 and TiN being the PC and pseudoelectrode materials, respectively. The thermal conductivity of insulating SiO2 and ZnS:SiO2 layers was determined too. Each thermal conductivity measurement was performed by the means of at least two strip widths in order to check both the measurement self-consistency and the measurement accuracy. The performance of PC-RAM cells, i.e., the time needed to reach the melting temperature of the PC material and the cooling speed, has been evaluated as a function of both the measured thermal conductivity of the PC material and the reset current intensity independently of the thermal properties of the pseudoelectrodes by the way of analytical formula. The influence of the thicknes...

Experimental setup for the measurement of the electrical resistivity and thermopower of thin films and bulk materials

An experimental setup was developed for the simultaneous measurements of the electrical resistivity and the thermopower in the temperature range going from 77 up to 330K. The system combines accuracy, rapidity, few efforts for the sample mounting, and is fully automated. Pressure contacts are used for contacting the thermocouples to the sample and the sample to the heat sink, making possible the measurement of both thin films and bulk materials with a great flexibility on their geometrical dimensions. Tests of the system have been made on a 99.98% pure Ni bulk sample and on PbTe∕BaF2 thin films for illustration.An experimental setup was developed for the simultaneous measurements of the electrical resistivity and the thermopower in the temperature range going from 77 up to 330K. The system combines accuracy, rapidity, few efforts for the sample mounting, and is fully automated. Pressure contacts are used for contacting the thermocouples to the sample and the sample to the heat sink, making possible the measurement of both thin films and bulk materials with a great flexibility on their geometrical dimensions. Tests of the system have been made on a 99.98% pure Ni bulk sample and on PbTe∕BaF2 thin films for illustration.

X-ray Characterization, Electronic Band Structure, and Thermoelectric Properties of the Cluster Compound Ag2Tl2Mo9Se11

We report on a detailed investigation of the crystal and electronic band structures and of the transport and thermodynamic properties of the Mo-based cluster compound Ag2Tl2Mo9Se11. This novel structure type crystallizes in the trigonal space group R3̅c and is built of a three-dimensional network of interconnected Mo9Se11 units. Single-crystal X-ray diffraction indicates that the Ag and Tl atoms are distributed in the voids of the cluster framework, both of which show unusually large anisotropic thermal ellipsoids indicative of strong local disorder. First-principles calculations show a weakly dispersive band structure around the Fermi level as well as a semiconducting ground state. The former feature naturally explains the presence of both hole-like and electron-like signals observed in Hall effect. Of particular interest is the very low thermal conductivity that remains quasi-constant between 150 and 800 K at a value of approximately 0.6 W·m(-1)·K(-1). The lattice thermal conductivity is close to its minimum possible value, that is, in a regime where the phonon mean free path nears the mean interatomic distance. Such extremely low values likely originate from the disorder induced by the Ag and Tl atoms giving rise to strong anharmonicity of the lattice vibrations. The strongly limited ability of this compound to transport heat is the key feature that leads to a dimensionless thermoelectric figure of merit ZT of 0.6 at 800 K.

Cu Insertion Into the Mo12 Cluster Compound Cs2Mo12Se14: Synthesis, Crystal and Electronic Structures, and Physical Properties.

Mo-based cluster compounds are promising materials for high-temperature thermoelectric applications due to their intrinsic, extremely low thermal conductivity values. In this study, polycrystalline cluster compounds Cs2CuxMo12Se14 were prepared for a wide range of Cu contents (0 ≤ x ≤ 2). All samples crystallize isostructurally in the trigonal space group R3̅. The position of the Cu atoms in the unit cell was determined by X-ray diffraction on a single-crystalline specimen indicating that these atoms fill the empty space between the Mo-Se clusters. Density functional theory calculations predict a metallic ground state for all compositions, in good agreement with the experimental findings. Magnetization measurements indicate a rapid suppression of the superconducting state that develops in the x = 0.0 sample upon Cu insertion. Transport properties measurements, performed in a wide temperature range (2-630 K) on the two end-member compounds x = 0 and x = 2, revealed a multiband electrical conduction as shown by sign reversal of the thermopower as a function of temperature.

Neutron Diffraction, Electronic Band Structure, and Electrical Resistivity of Mo3-xRuxSb7

Neutron diffraction experiments and Korringa-Kohn-Rostoker with coherent potential approximation electronic band structure calculations as well as electrical resistivity measurements have been performed on polycrystalline Mo(3-x)Ru(x)Sb(7) samples for 0 < or = x < or = 1. Neutron diffraction studies have been undertaken at room temperature and extended down to 4 K to get a better understanding of the crystalline structure modifications as the Ru content increases. Both structural and chemical characterizations have unambiguously revealed a solubility limit of the Ru atoms close to 0.8. Electronic band structure calculations have provided theoretical evidence of a progressive transition from a metalliclike state (x = 0) toward a semiconducting-like character as x = 1 is approached, although the solubility limit of Ru precludes a crossover to a semiconducting behavior. The theoretical prediction has been experimentally confirmed by low-temperature electrical resistivity measurements from 2 up to 350 K.

Crystal Structure and Transport Properties of the Homologous Compounds (PbSe)5(Bi2Se3)3m (m = 2, 3)

We report on a detailed investigation of the crystal structure and transport properties in a broad temperature range (2-723 K) of the homologous compounds (PbSe)5(Bi2Se3)3m for m = 2, 3. Single-crystal X-ray diffraction data indicate that the m = 2, 3 compounds crystallize in the monoclinic space groups C2/m (No. 12) and P21/m (No. 11), respectively. In agreement with diffraction data, high-resolution transmission electron microscopy analyses carried out on single crystals show that the three-dimensional crystal structures are built from alternating Pb-Se and m Bi-Se layers stacked along the a axis in both compounds. Scanning electron microcopy and electron-probe microanalyses reveal deviations from the nominal stoichiometry, suggesting a domain of existence in the pseudo binary phase diagram at 873 K. The complex atomic-scale structures of these compounds lead to very low lattice thermal conductivities κL that approach the glassy limit at high temperatures. A comparison of the κL values across this series unveiled an unexpected increase with increasing m from m = 1 to m = 3, in contrast to the expectation that increasing the structural complexity should tend to lower the thermal transport. This result points to a decisive role played by the Pb-Se/Bi-Se interfaces in limiting κL in this series. Both compounds behave as heavily doped n-type semiconductors with relatively low electrical resistivity and thermopower values. As a result, moderate peak ZT values of 0.25 and 0.20 at 700 K were achieved in the m = 2, 3 compounds, respectively. The inherent poor ability of these structures to conduct heat suggests that these homologous compounds may show interesting thermoelectric properties when properly optimized by extrinsic dopants.

Transversal growth microstructures of quasicrystalline Ti–Zr–Ni films

Abstract Pulsed laser deposition from a Nd:YAG laser was employed in the production of hundreds of nanometer thick quasicrystalline Ti–Zr–Ni films on glass substrate. The influence of deposition temperature Ts on the structure, morphology and microstructure of the films across their thickness was investigated. The morphology and microstructure features were evaluated by X-ray diffraction and transmission electron microscopy techniques. The low deposition temperatures were found to produce films with nanometer sized grains embedded in an amorphous matrix. The grains exhibit quasicrystalline order. The higher deposition temperatures lead to films whose structure is not uniform all along the growth direction. The layer in contact with the substrate is a very thin amorphous layer. The main part of the film consists of crystallized columns. The columns have grown from a nanocrystallized layer where the size of crystallites increases with increasing thickness.

Thermoelectric materials of p and n type from the (Bi,Sb,Te) phase diagram

It seems that the lower figure of merit for sintered n type telluride compounds is due to the selenium segregation at the grain boundaries. We present the transport properties of the Bi/sub 2/Te/sub 3/ and Sb/sub 2/Te/sub 3/ binary compounds in terms of stoichiometric deviation. We discuss these properties along the pseudo-binary phase diagram to determine the domain of existence of Bi/sub 2/Te/sub 3/-Sb/sub 2/Te/sub 3/ solid solutions. The study of the (Bi,Sb,Te) phase diagram allowed us to identify a zone where n-type materials can be obtained.

Influence de la teneur en ciment sur les propriétés thermomécaniques des blocs d’argile comprimée et stabilisée

La presente etude porte sur l’evaluation de la charge polluante des rivieres dans la ville de Bukavu. Les parametres de pollution, mesures dans cette etude sont la demande chimique en oxygene (DCO), l’azote total et le phosphore total, et la demande biochimique en oxygene (DBO 5 ). Les valeurs de la DBO5 des rivieres de Bukavu varient d’une riviere a l’autre dont la moyenne est comprise entre 620 mg/L et 1063 mg/L. Les valeurs moyennes de DCO varient entre 517 mg/L et 1747 mg/L. Ces valeurs de DCO et DBO 5 sont tres largement superieures a la norme, qui indique une forte pollution car les resultats sont loin de la situation critique. Le debit a ete mesure systematiquement, ce qui a permis d’evaluer la charge polluante deversee dans le lac Kivu soit de 0,195 a 9,899 kg/j de DBO, de 0,393 a 22,581 kg/j de DCO, de 0,0045 a 0,3217 kg/j d’azote et 0,00015 a 0,0422 kg/j de phosphore. Une grande partie de matiere organique est non biodegradable dans la riviere Wesha suite au rapport DCO/DBO 5 = 2,4. Neanmoins, pour les restes des rivieres leurs matieres organiques sont biodegradables, ce qui indique que les rivieres sont capables de se depolluer pourvu que la quantite de decharges soit reduite. Pour la riviere Wesha il faudra investiguer la source de la pollution. Mots-cles : charge polluante, eaux usees menageres, rivieres, Bukavu, lac Kivu. Assessment of the charge polluting of the rivers, wast domestic flowing waters and vain streaning in the Bukavu city south kivu, Democratic Republic of Congo The present survey is about the assessment of the pollution load of the rivers in the city of Bukavu. The parameters of pollution, measured in this survey are the chemical demand in oxygen (DCO), the total nitrogen and the total phosphor, and the biochemical demand in oxygen (DBO 5 ). The values of the DBO5 of the rivers of Bukavu vary from a river to the one of whose average is consisted between 620 mg/L and 1063 mg/L. The middle values of DCO vary between 517 mg/L and 1747 mg/L. These values of DCO and DBO 5 are very extensively superior to the norm, which indicates a strong pollution because the results are far from the critical situation. The flow has been measured systematically, what permitted to value the pollution load poured in the Kivu lake is from 0,195 to 9,899 kg/j of DBO, of 0,393 to 22,581 kg/j of DCO, of 0,0045 to 0,3217 kg/j of nitrogen and 0,00015 to 0,0422 kg/j of phosphor. A big part of organic matter is non-biodegradable in the Wesha river following the DCO/DBO 5 report = 2,4. Nevertheless, for the rests of the rivers their organic matters are biodegradable, what indicates that the rivers are capable of himself depolluted so long as the quantity of discharges is reduced. For the Wesha river it will need to study the source of the pollution. Keywords : pollution load, waters worn-out housewives, rivers, Bukavu, lake Kivu.Dans le cadre des changements climatiques nous nous proposons pour la station meteorologique de Yaounde, situee dans la region du climat equatorial classique, d’etudier le comportement des pluies interannuelles. A ce jour, cette station est confrontee a diverses perturbations pluviometriques : Sur le plan de la pluviometrie mensuelle, les mois de decembre, de janvier et de fevrier certes appartenant a la grande saison seche n’enregistrent parfois aucune pluie. Or cette situation n’est pas normale entre 1926 et 2006 avec des moyennes de 21,4 mm pour le mois de janvier, 51 mm pour le mois de fevrier et 135.7 mm pour le mois de mars. Certains de ces mois enregistrent des precipitations largement superieures a la normale. C’est le cas de janvier 1975 avec 98 mm ; de fevrier 1969 avec 125,9 mm et de decembre 1964 avec 97,5 mm. Sur le plan saisonnier, les repits pluviometriques des grandes saisons seches s’erigent dans la plupart des cas a de veritables secheresses. C’est le cas de 1987 qui a enregistre que 24,6 mm de precipitations durant les mois de decembre, janvier et fevrier au lieu de 94,1mm en moyenne au total. Dans le meme ordre, nous notons une modification dans la distribution des precipitations saisonnieres. En outre, la petite saison seche semble enregistrer de plus en plus des precipitations que jadis. Quant aux precipitations interannuelles, il ressort qu’il y a non seulement une diminution, mais aussi un prolongement des annees seches et, un dephasage entre l’arrivee des premieres et la fin des dernieres pluies. Mots-cles: Yaounde, precipitations, saisons, secheresse, El Nino. Analysis of the annual rainfalls in the Yaounde station from 1895 to 2006 In the context of climate change, we propose for the Yaounde weather station, in the area of the classic equatorial climate, to study the behavior of interannual rainfalls. It follows a variety rainfall interference. In terms of monthly rainfall, the months of December, January and February certainly belonging to the long dry season, sometimes recorded no rain. But this situation is not normal between 1926 and 2006 with averages of 21.4 mm for the month of January, 51 mm for the month of February and 135.7 mm for the month of March. Some of these months are above normal precipitations. This is the case of January 1975 with 98 mm; February 1969 with 125.9 mm and December 1964 with 97.5 mm. At the seasonal level, rainfall respites large dry seasons are erected in most cases to real drought. This was the case from 1987 which recorded 24.6 mm rainfall during the months of December, January and February instead of average total 94,1mm. Similarly, we note a change in the distribution of seasonal rainfall. In addition, the short dry season record seems more rainfall than before. As for interannual rainfall, it is apparent that there has not only reduced but also an extension between the arrival of the first and the end of the last rains. Keywords: Yaounde, rainfalls, seasons, drought, El Nino.

Development of a micro-generator based on Bi 2 Te 3 thin films

In this study, we fabricated in-plane thermoelectric micro-generators (4 mm times 4 mm) based on bismuth telluride thin films by using flash evaporation method. The thermoelectric properties of as-grown thin films are lower than those of bulk materials. Therefore the as-grown thin films were annealed in hydrogen at atmospheric pressure for 1 hour in a temperature range of 200 degC. to 400degC. By optimizing the annealing temperature, thin films with high thermoelectric power factors of 8.8 muW/(cmldrK2) in n-type and 13.8 muW/(cmldr K2) in p-type are obtained. To evaluate the figure of merit of the thin film, the thermal conductivity of the n-type thin film is measured by the 3omega method. The thin film annealed at 200degC exhibited a cross-plane thermal conductivity of 1.2 W/(mldrK). Micro-generators of flash-evaporated bismuth-telluride thin films are fabricated using three shadow masks. The shadow masks are prepared by standard micro-fabrication processes such as nitridation of Si, dry etching, and wet etching. Thermoelectric power of the as-grown thin film devices with 16 pairs of p-n legs are measured by YAG laser heating at the center of the devices. The maximum open circuit voltage is 6.7 mV under 2.5 K temperature difference.