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International audience; Essential oils (EO) of nutmeg (Myristica fragan Houtt) have a different chemical composition according to their origin. So it is important that laboratories of quality control have a fast and effective method of analysis to prevent frauds and adulteration. The analytical method generally employed [1], is time consuming and two pairs of compounds were not resolved (sabinene/β-pinene, p-cimene/limonene). In order to optimize the GC separation, while reducing the analysis time, we have identified the influential analytical factors among 11 by using a screening of factors: 5 factors are influent. After, we studied the interactions between these 5 factors by using a fractional factorial experiment matrix 2 5 – 1 in 16 experiments [2] and by considering the interactions effect of first order b ij. This optimization allowed to reduce analysis time (20 min instead of 55 min) and to improve the separation between the unresolved compounds (Resolution > 1.5). This method was applied to differentiate EO from ripe nutmeg and those from maces: a chemometric treatment (PCA) of GC data was carried out. The repartition of samples in a 3D score plot (PC1; PC2; PC4 explained variance 81%) show two groups: the maces and the ripe nutmeg. The study of loading associated to these compounds showed that the EO of maces are different on the basis of their content of phenylpropenic compounds (eugenol, myristicin, safrol...) while the EO of ripe nutmegs is characterized by the content of terpenic compounds (α and γ-terpinene, sabinene, α and β-pinene...). However, an overlap of the different classes exists. So the supervised classification method SIMCA [3] was used. Four samples of each origin were used to build the disjoint PCA and three from the same origin to test the models: all the samples are well classified. This study allowed to reduce analysis time and to improve the separation between all the compounds, and to differentiate EO from their origin: this method can be used as quality control.

International audience; Gamma radiolysis was investigated on a nuclear grade mixed bed ion exchange resin and its pure components under different irradiation conditions. Screening designs were performed to identify the factors influencing gas production after their γ-radiolysis and to compare their γ-degradation stability. Only hydrogen and trimethylamine quantities were considered as the response in the experimental designs. The other detected gases and water-soluble products were used to improve the resins degradation. Aerobic irradiation atmosphere decreased the H 2g production of AmbOH, MB400, and amines. The water presence increased the H 2g quantities for AmbH and decreased those for MB400 resin. Liquid water had no effect on H 2g production from AmbOH but was favorable to increased amine production. The H 2g production of AmbH increased with the absorbed dose that had little effect on the AmbOH resin. No impact of dose on the H 2g production was detected for MB400 that appeared to be less degraded.

Some spiro[indoline-naphthoxazines] I and spiro[indoline-pyridobenzoxazines] II substituted by different groups were synthesized in order to evaluate their photochromic properties1 and to optimize the preparation of spiroheterocyclic systems. In addition, some spiro[indoline-naphthopyrans] III were also prepared as reference compounds. Their physical and spectroscopic characteristics (UV, 1H and 13C NMR) were determined. A study using the experimental design methodology allowed to point out the most important factors for improving the reaction yields.

High mobility two-dimensional electron systems in GaN/AlGaN heterostructures have been realized by plasma assisted molecular beam epitaxy on GaN templates. In the density range of 10 11 cm -2 to 10 12 cm -2 , mobility values exceeding 160000 cm 2 /Vs have been achieved. Scattering mechanisms that presently limit the production of higher mobility samples are discussed. We present results of a systematic study of the weak localization and antilocalization corrections to the classical conductivity at very low magnetic fields. The unambiguous observation of a conductivity maximum at B = 0 suggests that spin-orbit scattering is not negligible in GaN heterostructures as one might expect for a wide-bandgap system. We have recently realized electron transport through GaN nanostructures. We report on the transport properties of the first quantum point contacts (QPCs) in GaN. These devices are used to study one-dimensional transport in the Nitride system.

We demonstrate GaAs-based, metal-oxide-semiconductor field-effect transistors (MOSFETs) with excellent performance using an Al2O3 gate dielectric, deposited by atomic layer deposition (ALD). This achievement is very significant because Al2O3 possesses highly desirable physical and electrical properties as a gate dielectric. These MOSFET devices exhibit extremely low gate-leakage current, high transconductance, and high dielectric breakdown strength. A short-circuit, current-gain, cutoff frequency (fT) of 14 GHz and a maximum oscillation frequency (fmax) of 25.2 GHz have been achieved from a 0.65-µm gate-length device. The interface trap density (Dit) of Al2O3/GaAs is evaluated by the hysteresis of drain-source current, Ids, versus gate-source bias, Vgs, and the frequency dispersion of transconductance, gm.

Thermal degradation of polystyrene and polyvinylchloride was studied by thermomanometric analysis. A design of experiments was used to quantify the influence of 8 physico-chemical parameters on the responses of the analyser and on the different types of degradation compounds.

The study of the thermal degradation of the fireproof polystyrene—butadiene copolymer by an intumescent system: ammonium polyphosphate (APP)—pentaerythritol (PER)—talc (TAL), was done through an experimental process. Thermogravimetry under air sweeping was used. We observed for the degradation rate of the mixture with fireproofing agents, in the field of 20 to 50% mass loss, a better linear increasing when this mixture contains more TAL and the ratio APP/PER is smaller.

The oxytribromide Ag0.37WOBr3 crystallizes in the triclinic system: a = 3.766(3), b = 7.101(7), c = 10.49(1) A, α = 101.80(5)°, β = 100.25(5)°, γ = 105.34(5)°, with the space group P 1 . The crystal structure determined from single crystal X-ray diffraction data is based on W2Br6 units, with short W-W distances, joined by W-O-W bonds to form [W2Br6O4/2]1∞ infinite chains developing along the a direction. Along the c direction, the cohesion between the chains is achieved by silver atoms which exhibit statistical distribution; along the b direction exist only van der Waals Br-Br contacts between adjacent chains. These weak linkages between the chains in two directions of the space give a strong one-dimensional character to this new tungsten oxybromide.

The aim of this study was to prepare, in a reproducible manner, poly( d , l -lactide-co-glycolide) (PLGA) biodegradable microparticles with high encapsulation yield of nerve growth factor (NGF), a neurotrophic factor of interest in the treatment of neurodegenerative diseases. Human serum albumin (HSA) was co-encapsulated with NGF by the w/o/w emulsion solvent evaporation/extraction method to stabilize the primary emulsion and to protect NGF. The encapsulation yield optimization was first carried out with HSA alone since it was the major component in the final microspheres, as compared to NGF. The effects of ten process factors on HSA entrapment in PLGA microspheres were examined using a fractional factorial design. Four major factors were identified. The presence of carboxymethylcellulose sodium or mannitol in the internal aqueous phase, the increase of the internal aqueous volume and the concentration of HSA led to a decrease of the encapsulation yield. Two main factors, namely the internal aqueous phase volume and the proportion of acetone in the organic phase, previously identified as a predominant factor, were studied through a response surface methodology. The observed range of HSA encapsulation yield was 87.4–100%. NGF was encapsulated according to the optimal conditions found for the entrapment of HSA. The encapsulation yield was then 97.3±5.0%.

International audience; During cleaning steps, ultrafiltration membranes are mechanically and chemically stressed. This may result in membrane degradations and failures. In this paper, polysulfone membranes were used to evaluate membrane deteriorations by commercial detergents in static conditions. Ageing of the membrane was simulated by immersing samples in solutions containing commercial detergents with various concentrations, temperatures and times defined by experimental designs. Indeed, an innovative approach in the chemical membranes ageing researches, based on methodological tools, was used in order to achieve significant ageing experiments without using an accelerated ageing protocol. The macroscopic changes were monitored by permeability measurements and mechanical strength tests coupled with a microscopic characterization by ATR-FTIR and HRSEM. The present work details results obtained for three commercial detergents: an alkaline, an acidic and an enzymatic detergent. It was found that the detergents used in the industrial advised conditions (concentration, temperature and time of contact) were not detrimental for membrane properties (permeability and elongation at break) and so for the quality of the produced water. Over the industrial cumulated time of contact, different ageing effects can be observed and compared with the ones induced by NaOCl.

An alternative for the GC determination of permanent gases in Titan's atmosphere, given the severe constraints of space instrumentation, is the use of a 100–120-mesh molecular sieve 5A micropacked column. Because of its high efficiency, small pressure drop and short analysis time, such a column is fully compatible with the experimental constraints of the Cassini-Huygens mission. In order to optimize the GC conditions associated with this column, a methodological approach was supported by a Doehlert experimental design. With such optimized conditions, the column provides a rapid separation of the selected products under isothermal conditions, with about 900 theoretical plates per metre at 70°C for most of the solutes studied.

Our objective was to optimize a medium for preadipocyte differentiation into adipocytes. Methods: The differentiation medium contains fixed components as well as 7 variable ones. To perform this study, different experiments were designed and the study was carried out in 4 stages. The first two stages tested the influence of serum, dexam- ethasone, hydrocortisone and an cAMP activator. In the third stage, two new variables were added: rosiglitazone and insulin. In the final stage, the medium selected in stage 3 was validated. The differentiation selection criteria consisted of the number of mature adipocytes and adiponectin secretion. Results: We have shown that each variable was indispensable and that positive interactions occurred between some variables. No negative interactions were found and it was possible to optimize the concentration of each variable. Conclusions: We selected the following medium, which provides optimal adipocyte size and adiponectin secretion: DMEM/HAMF12 + 10% Foetal Clone Serum (FCS) + 2 nM triiodothyronine + 10 nM hydrocortisone + 0.5 mM IsoButyl Methyl Xanthine (IBMX) + 500 nM dexamethasone + 1 µM rosiglitazone + 0.15 UI/ml insulin + antibiotics.

We report on growth and characterization of both epitaxial and amorphous films Gd2O3 of (e=14) and Y2O3(e=18) as the gate dielectrics for Si prepared by ultrahigh vacuum vapor deposition. The use of vicinal Si (100) substrates is key to the growth of (110) oriented, single-domain films in the Mn2O3 structure. Typical electrical leakage results are 10−3 A/cm2 at 1 V for single domain epitaxial Gd2O3 and Y2O3 films with an equivalent SiO2 thickness, teq of 15 A, and 10−6 A/cm2 at 1 V for smooth amorphous Y2O3 films (e=18) with a teq of only 10 A. For all the Gd2O3 films, the absence of SiO2 segregation at the interface is established from infrared absorption measurements.

We present new models which provide satisfactory and low cost modeling of infrared absorption spectra of gases from low to high pressures. The first is a line-by-line approach based on simple and empirical corrections of the lorentzian shape which account for the effects of line-mixing and finite duration of collisions. It leads to satisfactory agreement with experimental infrared spectra in a wide density range. It is degraded for the deduction of a Generalized Narrow-Band Statistical Model (GNBSM) which enables very rapid computation of medium resolution radiative properties. The latter is an extension of the low density (i.e., when lines are much narrower than the considered spectral resolution) approach developed by Goody [1] ; it is based on a random representation of line-positions and statistical description of line integrated-intensities and leads to a very simple expression of spectrally averaged absorption properties of gases. It is consistent with previous low density models and requires computer times several orders of magnitude shorter than line-by-line calculations. Comparisons between degraded approaches, line-by-line models, and experimental results demonstrate the accuracy of the present models in cases where previously proposed approaches are very inaccurate.

Single crystals of SrxMo6S8 with different strontium concentrations (x 10 K under 20 kbar). The structural transition (Ts) and superconducting critical (Tc) temperatures under pressure for stoichiometric and substoichiometric samples can be described by the same curve, if chemical pressure is taken into account. Comparison is made with EuMo6S8 and related compounds where the cations are magnetic.

N 2 -broadened linewidths have been measured for 27 rotational transitions (from J = 6 to J = 32) in the ground state of hydrogen cyanide. The data were recorded at room temperature using a Fourier transform spectrometer operating in the spectral range 10–100 cm −1 , with an apodized resolution of 0.060 cm −1 . A comparison is made with other experimental data, and with the results obtained from two semiclassical impact theories of collisional broadening.

Solid-phase extraction (SPE) of trace elements before their analysis has become a conventional pretreatment step of analytes because of their frequent low concentrations in numerous samples. Additionally, interfering compounds often accompagny analytes of interest, thus requiring a clean-up step. The preconcentration step and/or matrix removal can be efficiently improved by chemometric approaches allowing obtention of reliable results. Single variable approach is often used but is time and cost consuming, and may be the source of mistakes; multivariable approach allows to overcome these problems and increases the probability of global optimum finding. In order to obtain a set of experimental conditions for the selective extraction of Al(III) in water samples, onto a modified organic support (salicylic acid grafted on XAD-4), a multicriteria approach (response surface methodology) has been applied. The extraction method was optimized by the aid of a factorial design and a uniform shell Doehlert design for six variables: sample percolation flow rate, trace metal amount, sample volume, concentration and volume of HCl used for elution of aluminium. Results demonstrate the synergic effects of four factors and allow us to define working ranges for each parameter tested. The designed SPE procedure was then sucessfully applied to synthetic and real samples, issued from a potable water treatment unit.

We demonstrate high-reflectivity crack-free Al 0.18 Ga 0.82 N/Al 0.8 Ga 0.2 N distributed Bragg reflectors (DBR) and monolithic microcavities grown by molecular beam epitaxy on thick c-axis GaN templates. The elastic strain energy in the epilayer is minimized by compensating the compressive and tensile stress in every period of the DBR structure. A 25 period DBR mirror provides a 26nm-wide stop band centered at 347 nm with the maximum reflectivity higher than 99%. The high-reflectivity DBRs can be used to form high Q -factor monolithic AlGaN/AlGaN microcavities. INTRODUCTION High-reflectivity distributed Bragg reflect ors (DBR) in the ultraviolet region ar e essential for the development of GaN-based optical devices. In particular, the region around 350 nm is important for devices containing pure GaN as an active medium. The growth of DBRs remains challenging in the nitrid e system. The difficulty lies in maintaining the structural integrity of a relatively thick structure that contains materials with a large lattice constant mismatch and different thermal expansion coefficients. Only a few groups have reported high-reflectivity Al

In this work, Amberlite XAD-4 resin functionalized with salicylic acid was synthetized, characterized and applied as a new packing material for an on-line system to iron determination in aqueous samples. The detection method is based on the sorption of Fe(III) ions in a minicolumn containing the synthesized resin, followed by a desorption step using an acid solution and measurement of iron by vis-spectrophotometry (CAS method). The optimization of the solid-phase extraction system was performed using factorial design and Doehlert matrix considering six variables: sample percolation rate (0.5–9 ml min −1 ), sample metal concentration (20–200 μg l −1 ), flow-through sample volume (0–5 ml) (all three directly linked to the extraction step), elution flow-rate (0.5–9 ml min −1 ), concentration and volume of eluent (HCl 0.1–0.5 M) (all three directly linked to the elution step). The aim of this study was to obtain a set of operating ranges for the six variables tested in order to obtain – by means of a mathematical function allowing maximisation of each response (desirability function) – at least 90% of iron recovery rates. Using the experimental conditions defined in the optimization, the method allowed iron determination with achieved detection limit of 2.3 μg l −1 and precision (assessed as the relative standard deviation) of 9.3–2.8% for iron solutions of 10.0–150 μg l −1 . Real samples (coming from a water treatment unit) were used successfully when evaluating potentialities of the developed SPE procedure coupled to a spectrophotometric determination.

In this paper, a multivariable approach has been applied for the selective removing of Fe(III) and Al(III), in the range 0–200 μg l−1, in water samples onto a modified organic support (salicylic acid grafted on XAD-4). An empirical mathematical model was designed which establishes the relationship between the variation of the responses (extraction yields), and the variation of three factors (sample volume, sample percolation flow rate and amount of metallic ions present in the sample). To estimate the coefficients of the developed model, an uniform shell Doehlert design has been applied; these experiments consisted in GF-AAS determination of aluminium and iron amounts in eluates after percolation of samples through modified support. Results show a similar behaviour of the resin towards aluminium and iron with a preponderant effect of the percolation flow rate value; however this one is crucial for aluminium extraction and should be maintained below to 0.55 ml min−1 to reach a 95% Al3+ extraction yield (versus 2.25 ml min−1 for Fe3+). The optima determined by this experimental design approach have been further applied to the selective extraction of aluminium and iron from multielement synthetic samples and from real samples at the outlet of potable water treatment units.

The aim of this research is to demonstrate the effect of variations in certain parameters of the oily phase (OP) in water-in-oil (W/O) emulsions on rheological and texture properties of finished products. The formulated emulsions were selected according to an optimal experimental procedure. The applied variations were nature of the OP, its volume fraction, the hydrophilic-lipophilic balance (HLB) value, and the surfactant proportion. Results are presented for the followed tests carried out on the emulsions: texture analysis, rheology, and particle size analysis. The oils used in the study were sweet almond oil, liquid paraffin, maize oil, cyclomethicone, dimethicone, and wheat germ oil. The resulting data demonstrate a notable influence of the volume fraction oil on hardness, viscosity, adhesiveness, and cohesiveness of W/O emulsions. Emulsion hardness and viscosity increased as the OP percentage increased; this effect being even more pronounced for the vegetable oils. In contrast, emulsion adhesiveness and cohesiveness decreased as the volume fraction oil increased. The HLB value of the surfactant mixture of the emulsion also influenced hardness, adhesiveness, and elasticity, increasing or decreasing as HLB value did.

In this paper, we demonstrate for the first time GaAs-based MOSFETs with excellent performance using of CVD is a very robust, highly manufacturable process.

Pyrolyser-gas chromatography-mass spectrometry was used to analyze polycyclic aromatic hydrocarbons in contaminated soil without preliminary extraction. Experimental research methodology was used to obtain optimal performance of the system. After determination of the main factors (desorption time, Curie point temperature, carrier gas flow), modeling was done using a Box-Behnken matrix. Study of the response surface led to factor values that optimize the experimental response and achieve better chromatographic results.

DC characteristics of a depletion-mode (D-mode) GaAs MOSFET with a thin Ga/sub 2/O/sub 3/(Gd/sub 2/O/sub 3/) gate dielectric layer (74 /spl Aring/) show low gate leakage current, negligible drain current hysteresis and higher than 10 V gale-drain two-terminal breakdown voltage. Compared to MESFET with the same gate length, channel material and fabricated by the same process, the GaAs MOSFET shows higher unity current gain cutoff frequency (Ft). The higher Ft for the MOSFET than that of the MESFET agrees with earlier theoretical predictions.

Employing Ga/sub 2/O/sub 3/(Gd/sub 2/O/sub 3/) as gate dielectric on GaAs, prepared in a multi-chamber MBE system, has resulted in a low interfacial density of states (D/sub it/). The gate oxide is subjected to photoresists, solvents, water, and air before metallization, and as a consequence, contamination of the gate oxide is inevitable. The authors have studied the effects of gate oxide cleaning and etching before metallization on the DC and RF characteristics of depletion-mode GaAs MOSFETs with Ga/sub 2/O/sub 3/(Gd/sub 2/O/sub 3/) as gate dielectric.

Ga2O3(Gd2O3) film was previously discovered to effectively passivate GaAs surface, and the employment of this oxide as gate dielectric has led to the first demonstration of the enhancement-mode GaAs metal oxide semiconductor field effect transistors (MOSFETs) with inversion. In order to gain insight into the passivation mechanism and elucidate the role of Gd2O3, we have carried out a systematic study of the dependence of the structural and dielectric properties of (Ga2O3)1−x(Gd2O3) on the Gd (x) content. Our studies indicate that it is necessary to have the Gd addition exceeding 14% in order to form an electrically insulating dielectric with low interfacial state density. Furthermore, we found that pure Gd2O3 film grows epitaxially on GaAs in the single domain, (110) oriented Mn2O3 structure. This new crystalline dielectric has a dielectric constant ∼10, and shows excellent dielectric properties with low leakage and high breakdown strength even for films as thin as 2.5 nm. We expect that epitaxial growth of the Mn2O3 structure can be extended to other rare earth oxides, and to other semiconductor substrates like Si.

Single crystal Gd2O3 dielectric thin films were epitaxially grown on GaAs. The Gd2O3 film has a cubic structure isomorphic to Mn2O3, and is (110) oriented in single domain on the (100) GaAs surface. The oxide film has low leakage current densities ˜ 10–9 – 10–10 A/cmT2 at zero bias. Typical breakdown field is 4 MV/cm for an oxide film 185 Å thick, and >10 MV/cm for an oxide less than 50 Å thick. Both accumulation and inversion layers were observed in the Gd2O3-GaAs metal oxide semiconductor (MOS) diodes using capacitance-voltage (C-V) measurements, with an interfacial density of states around 1011 cm–2 eV–1.

The critical current densities Jc of Chevrel phase wires with niobium as an antidiffusion barrier were measured in magnetic fields up to 24 T. At 20 T and 1.9 K, Jc reaches 5.4×108 A/m2 and decreases slightly down to 3.1×108 A/m2 at 24 T. A wire with a 20% superconducting cross section has been successfully drawn and its overall critical current density Jcov exceeds 100 A/mm2 at 1.9 K up to a magnetic field slightly above 20 T. This demonstrates the ability of Chevrel phase wires to be used in high magnetic field applications. Moreover, some parts of the coil have certainly higher Jc, since Jc is very often limited by a thermal excursion of the entire coil. The effective upper critical field μ0Hc2*, deduced from the magnetic field dependence of Jc, is too low compared to the expected bulk value, indicating that superconducting properties at the grain boundaries are still degraded. If the bulk μ0Hc2 can be restored at the grain surfaces, Jcov should be higher than 100 A/mm2 up to at least 30 T.

The rare earth molybdenum sulfides RE x Mo6S8 (RE = rare earth) have been synthesized and are found to be superconducting with a few exceptions. This result, which is surprising in view of the high concentration of magnetic ions, is correlated with the particular structure of these compounds based on on units of Mo6S8.

Molecular recognition mechanisms for cooperative chiral stationary phases such as cellulose triacetate are still poorly understood. Here studies on the effect of the position and type of the subtituent on the separation of N-arylthiazoline-2-thione and N-arylthiazolin-2-one atropisomers on cellulose triacetate (CTA1) are described.

Rare-earth-based Chevrel phases constitute model systems to study both superconductivity and magnetic order but results may be hindered by the quality of the sample. Materials elaboration is thus essential to separate out extrinsic effects (secondary phases) from those due to the Mo 6 S 8 clusters and the rare-earth contribution. Crystal growth, transport and magnetic properties of the quaternary systems (RE, Eu)Mo 6 S 8 (RE-Sm, Yb) are presented. Different RE/Eu nominal ratios allowed to cover a large range of concentrations for single crystals. Superconducting transitions are narrow and as high as 9 K for ytterbium-rich samples, while the structural transition of the ternary EuMo 6 S 8 ( T s = 110 K) is largely diminished by the partial substitution of Eu by RE.

Anaerobic sludge digestion may be divided into two stages: hydrolysis ‐acidogenesis and methanogenesis. These phases may be separated to improve the efficiency of the digestion. The hydrolysis of sludge was studied in laboratory fermenters with the aim of improving volatile fatty acids production during the first phase. The influence of pH, temperature and agitation speed was studied. A mathematical approach to this problem was used by means of a statistical tool : methodology experimental research (M.E.R.). The best running conditions for hydrolysis were found to be : pH = 6.8, temperature = 50°C and agitation speed = 545 rpm.

Resume L'influence des quatre constituants principaux (ciment, chaux, polystyrene expanse et eau) sur la densite et les proprietes mecaniques d'un beton de polystyrene expanse est etudiee a l'aide d'une matrice d'experience originale. On peut ainsi definir rapidement des compositions qui conduisent a des betons de faible densite (

A methodological approach was used to optimize experimental conditions leading to complete saponification of cod liver oil without loss of polyunsaturated fatty acids. Each experiment, using conditions imposed by a Doehlert design, was analyzed for the percentages of fatty acids with different chain lengths and degrees of unsaturation: Y1=%C22:6ω3, Y2=%C20:5ω3, Y3=%C18:4ω3, Y4=%C18:1ω9+ω7, Y5=%C16:0. The difference between values obtained in each experiment and values obtained by transesterification allowed the extent of fatty acid degradation to be quantified. The experimental results were expressed by response surfaces allowing the choice of saponification conditions for each type of fatty acid. An optimum zone for saponification was observed which was larger when carried out under nitrogen. Nitrogen did not eliminate degradation, but degradation was less marked under nitrogen than under air. The fatty acid having the greatest degree of unsaturation in this study, C22:6ω3, was the first to be degraded. The extension of the Doehlert design to four factors showed that under air, alkali concentration and reaction time were the major factors governing loss of polyunsaturated fatty acids.

Using slowly pulsed magnetic fields up to 510 kG it is shown that some lead-rich ternary molybdenum sulphides have critical fields above 500 kG, whereas the Sn-rich compounds have critical fields of the order of 300-400 kG.

The rare earth molybdenum sulfides RE x Mo6S8 (RE = rare earth) have been synthesized and are found to be superconducting with a few exceptions. This result, which is surprising in view of the high concentration of magnetic ions, is correlated with the particular structure of these compounds based on on units of Mo6S8.

The quaternary chloride KLuNb6Cl18 and the ternary chloride LuNb6Cl18 were synthesized. They crystallize in the rhombohedral lattice R 3 and exhibit ¦Nb6Cl12¦2+ and ¦Nb6Cl12¦3+ units respectively. In the first case, the ¦Nb6Cl12¦2+ unit has 16 e− per Nb6 and the cluster is non-magnetic; in the second case, the ¦Nb6Cl12¦3+ unit has 15 e− per Nb6 and the cluster is paramagnetic owing to one unpaired electron. We present here the synthesis of these compounds, the structural results and the magnetic properties which depend on the variation in the oxidation state of the cluster.

Recent progress of GaAs-on-Si technology is reviewed and the importance of initial nucleation is emphasized. Growth initiation with a gallium prelayer at a suitable low temperature combined with migration enhanced epitaxy and in-situ thermal annealing may be able to give substantially improved material quality. During the initial growth, use of the minimum necessary As4:Ga flux ration is found to be critical. Patterned growth or post-growth patterning releases tensile stress only if the growth edges are free. GaAs-on-Si was almost completely relaxed from stress by post-growth patterning to 5 μm × 5 μm size patterns. Steps on the surface owing to substrate misorientation do not affect the two-dimensional electron gas (2DEG) transport properties in AlGaAs/GaAs selectively doped heterojunction transistor (SDHT) structures. For a sheet density of 1012 cm−2, a 2DEG mobility greater than 50 000 cm2 V−1 s−1 at 77 K was obtained on silicon substrates, which was found to be adequate for the fabrication of the state-of-the-art devices. For 1 μm-gate-length SDHTs, maximum transconductances of 220 and 365 mS mm−1 were measured at 300 K and 77 K, respectively. A minimum propagation delay time of 28 ps stage−1 was measured at 300 K for direct coupled field effect transistor logic (DCFL) ring oscillators with a power dissipation of 1.1 mW stage−1. The propagation delay time reduced to 17.6 ps stage−1 at 77 K. These results are comparable to the SDHT technology on GaAs substrates.

We present a study of the properties of the series Mo 6 X 8− x Y x ( X = S, Se, Te; Y = Br, I) having the hexagonal rhombohedral structure of the PbMo 6 S 8 type. For X = S we have found two new superconducting compounds Mo 6 S 6 Br 2 and Mo 6 S 6 I 2 , having critical temperatures of 13.8 and 14.0°K, respectively. We further find that Mo 6 Te 8 becomes superconducting ( T c ≈ 2.6°K) upon substitution of Te by small quantities of iodine, and that in the case of Mo 6 Se 8 substitution of a Se atom by a halogen, raises T c up to about 7.6°K.

We have studied the correlation between the lattice parameters and the superconducting critical temperature T c in PbMo 6 S 8 and the series M x -PbMo 6 S 8 and M x Pb 1− x Mo 6 S 8 ( M = Cu , Ag, Ba, Sn, Eu, La, Gd, Lu, Y, Th, U). A net correlation was found between the volume V H and T c when M is trivalent or tetravalent. We also found that the high T c modification of PbMo 6 S 8 has a maximal occupation of the lattice site centred about the origin of the rhombohedral lattice cell and that this can be obtained by insertion of trivalent and tetravalent elements.