Epitaxial bilayered Nb–Mo(001) films: growth, characterization and size effect in electron conductivity

Mo–Nb and Nb–Mo epitaxial thin (10–200 nm) films growth on the r- sapphire plane under ultra high vacuum by laser ablation deposition at the growth temperature 750°C were tested. Grown films were characterized by reflected high energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning atomic force microscopy and electron transport measurements. It was found that bilayered films are high quality heteroepitaxial single-crystalline (001) films with low surface roughness (0.3–0.5 nm) and large residual electron mean free path, compared to the film thickness. The superconducting transition temperature of Nb–Mo films with equal layer thickness varies from 5 up to 9 K as the thickness increases from 10 to 100 nm. Physical properties of the films are close to each other both for Nb/Mo and Mo/Nb films of equal layer thickness and do not depend on the alternation of Mo and Nb layers. This suggests the same film structure quality and mutual epitaxy of the metals. The size dependence of electron conductivity of bilayered Mo–Nb films showed that it is determined by the dimension of individual layers, constituting the film, where the effect of ‘contact’ potential formed at the interface between Mo and Nb layers and a fluctuation of film bending is important.     

Conservation en congélation et réfrigération de la viande hachée de boeuf et de porc: évolution de la flore microbienne et des propriétés physico-chimiques en cours de stockageFrozen and chilled minced meat (beef and pork): microbiol and physico-chemical evolution during storage

The minced meat market in Belgium is growing steadily; its share of the market has increased from 10 to 30% in three years. However, the mincing of meat can produce intense microbiol proliferation and thus cause food putrification.

Any changes occurring in meat, especially minced meat, are essentially of microbial origin, hence refrigeration is particularly important. With chilling, minimum temperatures must be maintained (0±1°C), since only temperatures below +3°C avoid any health risk linked with the growth of pathogens and production of toxins. Under chilled conditions the storage use of minced meat is four to five days. With freezing, temperatures of about - 18°C are needed to avoid the growth of bacteria, yeasts and mould. At this temperature micro-organisms are progressively rendered inactive, but destruction is never total. The kinetics of reducing micro-organism populations depends on the nature of the micro-organisms, the storage temperature (reduction is more intense at − 12°C that at − 18°C or at − 30°C) and the freezing rate (micro-organisms are better able to resist rapid freezing than slow freezing). One years storage at − 18°C will destroy 90 to 99% of the micro-organisms which were initially present in the product. Nevertheless, the chilled storage (+4°C) of thawed minced meat must not exceed three days.

The physico-chemical evolution of cold stored minced meat is explained. In chilled storage, the onset of flavour defects can be related to the measurement of volatile nitrogen. In frozen storage, the crystallisation of the water causes cellular breakdown. The role of packaging is important in avoiding superficial freeze-drying which is accompanied by browning. In addition, fatty matter can deteriorate by oxidation even at freezing temperatures. The degree of alteration of fats can be appreciated by the acidity index and peroxide index (showing that it is undesirable to store minced pork at − 18°C for more than nine months).

Use of a partial vacuum or modified atmosphere in combination with impermeable packaging will prolong the storage life of minced meat provided that the initial contamination is as low as possible and the maintenance of the cold chain is strictly respected.

Résumé

Le marché de la viande hachée progresse en Europe, du fait de plusieurs facteurs socio-économiques. Le hachage diminue considérablement la durée de conservation de la viande; sa consommation peut être dangereuse.

L'article analyse les points suivants: emballage, réfrigération et congélation (notamment vitesse de congélation), température et durée de conservation, atmosphères modifiées et leur influence sur les propriétés physico-chimiques de la viande et sur le comportement microbien.     

Crystallographic texture and morphology of obliquely deposited Co-Cr magnetic thin films on flexible polymeric substrates

Thin magnetic films of Co-Cr have been obliquely deposited by evaporation and sputtering at a fixed incidence angle of 45° using tilted sample holders. The layers exhibit a columnar microstructure with the column axes at an angle in the range 15°–35° with respect to the film normal. The tilt angle strongly depends on the type of substrate used, the deposition temperature and the substrate pretreatment. The c axes of the h.c.p. crystallites always coincide with the column axes. A Ti underlayer forces the columns and crystallites of the Co-Cr film to grow vertically to the film plane despite the oblique incidence deposition process.     

Étude électrique de couches minces de polymère obtenues dans une décharge luminescente II: Caractérisation électrique sous champ alternatif

The aim of this study was to investigate the electrical characteristics under alternating stress of a metal-polymer-metal structure, the dielectric of which was a thin layer created by a glow discharge in styrene.

The study, carried out in the 250 Hz–100 kHz range between −90° and 150°C, indicates two different domains for the variations of ε_r and ε_r'. Using bi-logarithmic coordinates the variation of the a.c. conductivity σ_ac(Ω) obeys the law σ_ac(Ω)∝Ω for temperatures lower than 50°C.

In order to explain this result, two hypotheses are proposed: that dipolar fluctuations occur and that a hopping process takes place.     

Graphite formation on Ni film by chemical vapor deposition

Thin film formation of graphite by chemical vapor deposition using 2-methyl-1,2′-naphthyl ketone as a starting material was carried out on Ni film substrates. On Ni films directly deposited on quartz glass, the graphite films were obtained when the Ni film thickness was above 1 000 Å and above 5 000 Å at 700 °C and 1 000 °C, respectively. Depositions on thinner Ni film substrates comprise amorphous carbon (a-C) or graphite tubes which was owing to the thermal coagulation of the Ni film into droplets. On the other hand, graphite film was obtained on the Ni film with thickness 10 Å when a-C was inserted between the Ni film and the quartz glass. The coagulation of the Ni film is considered to be avoided by inserting a-C layer.     

Temperature-dependent behavior of Langmuir monolayers of an amphiphilic spiropyran

The effect of subphase temperature on behaviors and morphologies of an amphiphilic spiropyran, 1′,3′-dihydro-3′,3′-dimethyl-6-nitro-1′-octadecyl-8-docosanoyloxymethylspiro[2H-1-benzopyran-2,2′[2H]indol] onto pure water was investigated by measurements of surface pressure–area isotherms and Brewster angle microscopy, respectively. Both depend strongly on the subphase temperature. The isotherms at 30 °C show a distinct phase transition at 7.5 mN m⁻¹, while no such phase transition is observed at 7 °C. The morphology changes drastically accompanying the phase transition at 7.5 mN m⁻¹, at which many large circular domains are formed. Furthermore, we have demonstrated that the morphology changes reversibly accompanying appreciable hysteresis in the isotherms upon continuous compression and expansion cycles.     

Effet de la structure sur les propriétés optiques de couches minces de composés de tungstène obtenues par décomposition en phase vapeur: Influence sur la sélectivité spectrale

Black or reflective tungsten thin films were deposited at 400°C onto fused quartz substrates by chemical vapour deposition (CVD) of tungsten hexacarbonyl in argon at atmospheric pressure. The structure was investigated by X-ray and electronic diffraction.

This analysis shows that the structure and composition of thin films depend on the preparation conditions and annealing. Optical properties and selectivity characteristics are also presented.     

Structural, dielectric and electrical conducting properties of CsB′B′′O6 (B′=Nb, Ta; B′′=W, Mo) ceramics

Polycrystalline samples of the defect pyrochlore-type CsB′B′′O₆ (B′=Nb, Ta; B′′=W, Mo) compounds were prepared by solid-state reaction technique. The formation of the compounds was checked by X-ray diffraction (XRD) method. All the compounds were found to have a cubic crystal structure at room temperature. Dielectric studies of the tungstate compounds, CsNbWO₆ and CsTaWO₆ indicated a small dielectric anomaly at (183±2) and (328±2) K, respectively, whereas the molybdate compounds, CsNbMoO₆ and CsTaMoO₆ did not exhibit such anomaly. The conductivity measurements indicated ionic transports (t_ion≥0.80) in the high temperature region (≥573 K).     

Chemical vapour deposition of molybdenum carbides: aspects of phase stability

Thin films of different molybdenum carbides (δ-MoC_1−x, γ′-MoC_1−x and Mo₂C) have been deposited from a gas mixture of MoCl₅/H₂/C₂H₄ at 800°C by CVD. The H₂ content in the vapour has a strong influence on the phase composition and microstructure. Typically, high H₂ contents lead to the formation of nanocrystalline δ-MoC_1−x films while coarse-grained γ′-MoC_1−x is formed with an H₂-free gas mixture. This phase has previously only been synthesized by carburization of Mo in a CO atmosphere and it has therefore been considered as an oxycarbide phase stabilized by the presence of oxygen in the lattice. Our results, however, show that γ′-MoC_1−x films containing only trace amounts of oxygen can be deposited by CVD. Stability calculations using a FP-LMTO method confirmed that the γ′-MoC_1−x phase is stabilized by oxygen but that the difference in energy between e.g. δ-MoC_0.75 and oxygen-free γ′-MoC_0.75 is small enough to allow the synthesis of the latter phase in the absence of kinetic constraints. Annealing experiments of metastable δ-MoC_1−x and γ′-MoC_1−x films showed two different reaction products suggesting that kinetic effects play an important role in the decomposition of these phases.     

Facing targets type of sputtering method for deposition of magnetic metal films at low temperature and high rate

A new type of cathode sputtering apparatus with two targets facing each other has been developed to prepare magnetic films at a high deposition rate without the extreme rise of the substrate temperature. When two disks of iron and nickel were used as targets, the maximum deposition rates obtained were approximately 4000 and 5000 Å/min, respectively. The substrate temperature was not elevated above 200°C during sputtering. The high rate deposition of Mo permalloy films also was attempted by co-sputtering of two facing targets composed of disks of iron and nickel and chips of molybdenum. The Vicker's hardness of the obtained Mo permalloy films was about 900 and the typical values of permeability at 1 MHz magnetic field and coercive force at dc magnetic field of them were about 2500 and 0.16 Oe, respectively.     

Epitaxial growth of AlN on single crystal Mo substrates

We have grown AlN films on single-crystalline Mo(110), (100), and (111) substrates using a low temperature pulsed laser deposition (PLD) growth technique and investigated their structural properties. Although c-axis oriented AlN films grow on Mo(100), the films contain 30° rotated domains due to the difference in the rotational symmetry between AlN(0001) and Mo(100). AlN films with only poor crystalline quality grow on Mo(111) substrates, probably due to the poor surface morphology and high reactivity of the substrates. On the other hand, single crystal AlN films grow epitaxially on Mo(110) substrates with an in-plane relationship of AlN[11-20] // Mo[001]. Reflection high-energy electron diffraction or electron backscattered diffraction analysis has revealed that neither in-plane 30° rotated domains nor cubic phase domains exist in the AlN films. X-ray reflectivity measurements have revealed that the heterointerface between AlN and Mo prepared by PLD at 450 °C is quite abrupt. These results indicate that PLD epitaxial growth of AlN on single crystal Mo substrates is quite promising for the fabrication of future high frequency filter devices.     

Propriétés des films étain-azote amorphes réalisés par pulvérisation cathodique réactive-nature de la liaison métal-azote

Les propriétés électriques, optiques et structurales des films étain-azote amorphes réalisés par pulvérisation cathodique réactive sont étudiées. La variation de la conductivité électrique en fonction de la température indique un comportement semiconducteur et permet de distinguer deux processus de conduction. Le caractère diélectrique et la largeur de bande interdite (3,5–3,6 eV) du nitrure d'étain Sn₃N₄ amorphe, principal constituant des films, sont déduits de l'étude de l'absorption optique. La longueur de la liaison Sn---N (2,25±0.04 Å) est calculée à partir des données de spectroscopie infrarouge. La considération des longueurs de liaison et des largeurs de bande interdite de Ge₃N₄, Sn₃N₄ et Pb₃N₄ permet de conclure que la liaison métal-azote est de type monoélectronique.     

Annealing effects on the thermoelectric power of thin copper and silver films

The effect of annealing on the thermoelectric power of thin copper and silver films has been investigated. Using Matthiessen's rule, the thermoelectric power is separated into three components: S₀ due to bulk lattice scattering, S_s due to surface scattering and S_i due to scattering by imperfections. The values of S₀ and S_s are independent of the film thickness, whereas S_i varies with film thickness. However, S_i approaches a constant value for both copper and silver when the film thickness is larger than 1000 Å. The values of S_s obtained for copper and silver films are 1.33 μV/°K and 3.23 μV/°K respectively. For thicker films (t>1000 Å), the values of S_i for copper and silver films are 3.89 μV/°K and 9.63 μV/°K respectively.     

Evolution of hole size and shape in {100}, {110} and {111} monocrystalline thin films of gold

Monocrystalline thin films of gold, containing controlled distributions of small holes, were produced by an epitaxial flash deposition process on heated {100}, {110} and {111} monocrystalline substrates of sodium chloride. These films, ranging from 10 to 20 nm in thickness, were then removed from their substrates, annealed for various periods at temperatures ranging from 180 to 290 °C and subsequently examined by transmission electron microscopy in order to record the evolution of hole size and shape as a function of crystallographic orientation and annealing conditions. During annealing, these holes either grow or shrink, depending on the ratio of hole diameter to film thickness, with growing holes developing clearly defined crystallographic facets aligned normal to the film surface. The evolution of hole size is in satisfactory agreement with a kinetic analysis based on atomic surface mobility, whereas the evolution of hole shape is consistent with anisotropy of the surface energy, as computed from a nearest neighbor bond model.     

Formation of second phase particles in aluminum-copper alloy films

The formation of second phase particles in Al+3% Cu alloy films was examined in order to find differences of the precipitation in thin films from that occurring in bulk alloys. Compared to bulk alloys in the thin film material the formation of Guinier-Preston zones is retarded. In well quenched thin films 60 h at 100°C or 6 h at 130°C are necessary to produce G.P. zones. θ′ particles are nucleated after approximately 100 h at 120°C without formation of θ″. The stable θ-Al₂Cu precipitate was found at temperatures as low as 150°C on the film surface. The results show that excess vacancies cannot be retained in thin films and that the film surfaces provide preferential nucleation sites for the equilibrium precipitate θ-CuAl₂.     

Alkoxide route to La0.5Sr0.5CoO3 epitaxial thin films on SrTiO3

An all alkoxide based sol–gel route was investigated for preparation of epitaxial La_0.5Sr_0.5CoO₃ (LSCO) films on 100 SrTiO₃ (STO) substrates. Films with 20–30 to 80–100 nm thickness were prepared by spin-coating 0.2–0.6 M (metal) solutions on the STO substrates and heat treatment to 800 °C at 2 °C min^− 1, 30 min, in air. The films were epitaxial with a cube-on-cube alignment and the LSCO cell was strained to match the STO substrate of 3.905 Å closely; a and b = 3.894 Å and 3.897 Å for the 20–30 and 80–100 nm films, respectively. The c-axis was compressed to 3.789 Å and 3.782 Å for the 20–30 and 80–100 nm films, respectively, which resulted in an almost unchanged cell volume as compared to polycrystalline film and nano-phase powders prepared in the same way. The SEM study showed mainly very smooth, featureless surfaces, but also some defects. A film prepared in the same way on an -Al₂O₃ substrate was dense and polycrystalline with crystallite sizes in the range 10–50 nm and gave cubic cell dimensions of a_c = 3.825 Å. The conductivity of the ca 30–40 nm thick polycrystalline film was 1.7 mΩcm, while the epitaxial 80–100 nm film had a conductivity of around 1.9 mΩcm.     

Émission jaune dans la cathodoluminescence des couches minces de ZnS/Cu, Cl

We have observed a yellow emission in the cathodoluminescence spectrum of ZnS/Cu, Cl thin films annealed at 250°C. The association Cu-Cl responsible for this emission is transient and unstable, since annealing at 400°C transforms the yellow centre into blue and green centres.

Thin films appear to be a useful form in which to study and follow the formation of luminescent centres.     

Influence de la réfrigération et de la congélation sur la qualité et l'aptitude à la germination des grainesInfluence of chilling and freezing on seed quality and germination ability

The high dehydration level of seeds provides them with an exceptional survival period, nevertheless seeds are living organs which can die. Refrigeration can prevent the germination of seeds which are destined for human or animal consumption or, on the contrary, produce germination in a required time. The article first describes some of the biological particularities of seeds which can be considered as survival or reproductive organs. As a survival organ, refrigeration can be used to preserve seeds to: Prevent germination: the storage of seeds of tropical and sub-tropical origin at +5°C: the maintenance of a cold and dry atmosphere for the bulk storage of seeds in silos. Prevent parasitic attack: a low temperature and low relative humidity to protect against bacterial or fungal growth: it is more difficult to combat insect invation. Protect product viability: the drier the seed and the lower the temperature, the longer the seed life. Refrigeration can also be used to facilitate germination by eliminating dormancy. It is already well known that humid cold promotes germination by eliminating embryonic dormancy (in general, from one to a few months at a temperature of about +5°C). Furthermore, good germination of ‘hard seeds’ (with teguments impermeable to water) can be obtained by immersing them in liquid nitrogen and repeating the operation if necessary.

Résumé

Les graines sont des organes très particuliers en ce sens qu'elles prérentent le plus souvent un état de déshydration très poussé. Les problèmes de stockage qui en résultent sont de ce fait très différents de ceux que l'on recontre généralement avec les autres produits végétaux. Les basses températures empêchent leur germination pendant la conservation, limitent les altérations dues aux parasites et prolongent leur durée de vie. La réfrigération et la congélation des graines ont permis la mise au point d'applications originales. Quelques graines ne peuvent germer qu'après un séjour au froid en milieu humide. Des graines imperméables à l'eau deviennent capables de germer facilement aprés traitement à très basse température (azote liquide).     

The effect of copper addition on the structure and strength of an Al–Li alloy

In this study the effect of copper addition on the structure, precipitation kinetics and hardness in the Al–Li and Al–Li–Cu alloys aged at 200°C was investigated. The structures of precipitates were studied using X-ray-small-angle-scattering (XSAS) and transmission electron microscopy (TEM) methods. The changes in the structure parameter (Rg) of both alloys was calculated using two methods, the Guinier approximation and correlation function γ(r). By use of a plot of r γ(r) the distribution law of the T₁ disc thickness was obtained and the coexisting spherical particles of δ′ were estimated. Two types of δ′ precipitates of approximately 2 nm size and above 8 nm and the T₁ precipitates of thickness between 3 and 4 nm were observed.     

Magnetic Properties of Single Crystalline Co2MnAl Heusler Alloy Thin Films

Ferromagnetic resonance measurements were carried out for single crystalline Co₂MnAl thin films as a function of the angle between applied external magnetic field and normal of the film surface. In- and out-of-plane angular dependence of the resonance field and only out-of-plane angular dependence of the linewidth of FMR spectra were analyzed using the Landau?CLifshitz?CGilbert equation. The easy and hard axes were determined by analyzing the in-plane angular dependence of resonant field. The films annealed at various temperatures showed four-fold magnetic anisotropy symmetry and the damping factor was estimated from the analysis of the angular dependency of FMR spectra.