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In this study, it was aimed to prepare a series of PVdF-co-HFP based electrolytes with different LiCl[O.sub.4] loadings and to investigate their chemical and electrical properties in detail. For this purpose, PVdF-co-HFP based electrolytes with different LiCl[O.sub.4] loadings (1-20 weight %) were prepared using solution casting method. X-ray diffraction (XRD), differential scanning calorimetry, and thermogravimetric (TGA) -differential thermal and dielectric spectroscopy analysis of PVdF-co-HFP/LiCl[O.sub.4] were performed to characterize their structural, thermal, and dielectric properties, respectively. XRD results showed that the diffraction peaks of PVdF-co-HFP/LiCl[O.sub.4] electrolytes broadened and decreased with LiCl[O.sub.4]. TGA patterns exhibited that PVdF-co-HFP/ LiCl[O.sub.4] electrolytes with 20 wt % of LiCl[O.sub.4] had the lowest thermal stability and it degraded above 473 K, which is highly applicable for solid polymer electrolytes. Dielectric constant, dielectric loss, and conductivities were calculated by measuring capacitance and dielectric loss factor of PVdF-co-HFP/LiCl[O.sub.4] in the range from 10 mHz to 20 MHz frequencies at room temperature. In consequence, conductivities of PVdF-co-HFP/LiCl[O.sub.4] increased significantly with frequency for low loading of LiCl[O.sub.4] while they only slightly changed with higher LiCl[O.sub.4] addition. On the other hand, dielectric constant values of PVdF-co-HFP/LiCl[O.sub.4] films decreased with frequency whereas they rose with LiCl[O.sub.4] addition. The dielectric studies showed an increase in dielectric constant and dielectric loss with decreasing frequency. This result was attributed to high contribution of charge accumulation at the electrode-electrolyte interface. The electrolyte showed the maximum conductivity of 8 x 10-2 S/cm at room temperature. Key words: conducting polymers, impedance spectroscopy, dielectric properties, ionic conductivity. Notre but ici est de preparer des electrolytes bases sur du polyfluorure de vinylidene-co-hexafluoropropylene (PFdVco-HFP/PVdF-co-HFP) avec differents dopages de LiCl[O.sub.4] et d'etudier leurs proprietes chimiques et electriques en detail. A cette fin, nous preparons nos electrolytes avec differentes additions de LiCl[O.sub.4] (1-20 % en poids) en utilisant la methode de coulage de solution. Les methodes de diffraction X (DX), de calorimetrie differentielle a balayage avec TG-DT et de spectroscopie dielectrique sont utilisees pour analyser les electrolytes PFdV-co-HFP/LiCl[O.sub.4], afin de caracteriser leurs proprietes structurelles, thermiques et dielectriques respectivement. Les resultats de DX indiquent que les pics de diffraction des electrolytes/LiCl[O.sub.4] s'elargissent et s'abaissent avec l'augmentation du dopage. Les patrons de thermogravimetrie montrent que les electrolytes avec dopage de 20 % en poids ont la plus basse stabilite thermique et se degradent au dessus de 473 K, ce qui les rend hautement desirables comme electrolytes polymeres solides. La constante dielectrique, la perte dielectrique et la conductivite sont ici calculees a partir de mesures de la capacitance et du facteur de perte dielectrique des electrolytes/LiCl[O.sub.4] dans le domaine de frequence de 10 mHz a 20 MHz a la temperature de la piece. Nous constatons que la conductivite augmente significativement avec la frequence, lorsque l'addition de LiCl[O.sub.4] est faible, mais qu'elle varie tres peu pour de fortes additions de LiCl[O.sub.4]. D'autre part, la constante dielectrique decroit avec la frequence, mais augmente avec l'addition de LiCl[O.sub.4]. Les etudes dielectriques indiquent une augmentation de la constante dielectrique et de la perte dielectrique lorsque la frequence diminue. Ce resultat est attribue a une haute contribution de l'accumulation de charges a l'interface electrode-electrolyte. La valeur maximale de conductivite electrique des echantillons est mesure etre 8 x [10.sup.-2] S/cm a la temperature de la piece. [Traduit par la Redaction] Mots-cles : polymeres conducteurs, conductivite ionique, proprietes dielectriques, spectroscopie d'impedance, PFdV-co-HPF/ LiCl[O.sub.4]., 1. Introduction Today, among some favorable polymers, polyvinylidene fluoridehexafluoropropylene (PVdF-HFP), based on poly(Vinylidenefluoride) (PVdF) and hexafluorpropylene (HFP), is a highly preferred copolymer matrix by researchers due to various interesting properties [...]

It has recently been established that the high-transition-temperature (high-[T.sub.c]) superconducting state coexists with short-range charge-density-wave order (1-11) and quenched disorder (12, 13) arising from dopants and strain (14-17). This complex, multiscale phase separation (18-21) invites the development of theories of high-temperature superconductivity that include complexity (22-25). The nature of the spatial interplay between charge and dopant order that provides a basis for nanoscale phase separation remains a key open question, because experiments have yet to probe the unknown spatial distribution at both the nanoscale and mesoscale (between atomic and macroscopic scale). Here we report micro X-ray diffraction imaging of the spatial distribution of both short-range charge-density-wave 'puddles' (domains with only a few wavelengths) and quenched disorder in Hg[Ba.sub.2]Cu[O.sub.4] + y the single-layer cuprate with the highest [T.sub.c], 95 kelvin (refs 26-28). We found that the charge-density-wave puddles, like the steam bubbles in boiling water, have a fat-tailed size distribution that is typical of self-organization near a critical point (19). However, the quenched disorder, which arises from oxygen interstitials, has a distribution that is contrary to the usually assumed random, uncorrelated distribution (12, 13). The interstitial-oxygen-rich domains are spatially anticorrelated with the charge-density-wave domains, because higher doping does not favour the stripy charge-density-wave puddles, leading to a complex emergent geometry of the spatial landscape for superconductivity., Although it is known that the incommensurate charge-density-wave (CDW) order in cuprates (copper oxides) is made of ordered, stripy, nanoscale 'puddles' with an average of only 3-4 oscillations, information about [...]

Poly(isobutyl methacrylate) (PiBMA) polymer doped in pure p-Ethoxybenzylidene p-Heptylaniline (EBHA) mesomaterial has been investigated in the present study. The dielectric and electro-optical parameters of EBHA having nematic phase were altered due to the addition of polymer in different concentration. The strong dopant concentration dependent nature of negative dielectric anisotropy and conductivity are the interesting results of the present polymer doped nematic liquid crystal (NLC) system. The present composite shows its useful application in low charge consumable devices due to the easy charge transportation phenomenon. The investigated polymer dopedNLC system exhibiting increased negative dielectric anisotropy and high conductivity may prove to be advantageous for video applications in vertically aligned cells., INTRODUCTION Nematic liquid crystals (NLCs) have been investigated for more than a century and their applications are omnipresent in our daily life. NLCs are very interesting mesophase materials for their [...]

The β phase of polyvinylidene fluoride (PVDF) crystals is polar and has very good piezoelectric and dielectric properties as compared with the nonpolar α phase. Benzyl triphenyl phosphonium chloride (BTPC) has been previously shown to directly nucleate the β phase from melt, instead of ot phase, and is an additive of practical importance. Different amounts of BTPC were melt mixed into PVDF using a micro twin screw extruder to study the rheology of the blends using oscillatory and steady shear viscometry. Data at different temperatures were found to superimpose onto a master curve using time-temperature superposition. The complex viscosity and steady shear viscosity increased significantly upon addition of 0.5% BTPC and decreased slightly with further addition of BTPC. The storage modulus exhibited a plateau at low frequencies indicating structure formation in the melt on addition of BTPC. The horizontal shift factors derived from the time-temperature superposition were found to follow an Arrhenius temperature dependence and the flow activation energy for each blend was obtained. Pure PVDF and PVDF films with 1 % and 3% BTPC were melt extruded using a laboratory twin screw extruder. The film containing 3% of BTPC gave the highest fraction of [1 phase crystals (75%). Small angle light scattering results showed that the size of spherulites decreased with increase in the weight fraction of BTPC. The dielectric constant and conductivity of the films at low frequencies increased significantly with concentration of BTPC, as did the dielectric loss and AC electrical conductivity., INTRODUCTION The polymer, polyvinylidene fluoride (PVDF), is chemically inert, melt processable and has good piezoelectric properties [1, 2], The applications of PVDF films thus include sensors, actuators, and finger pressure [...]

Metastability effects in hydrogenated microcrystalline silicon thin films due to air, high purity nitrogen, helium, argon, and oxygen were investigated using temperature-dependent dark conductivity, photoconductivity, and steady-state photocarrier grating methods. It was found that short-term air, nitrogen, and inert gases caused a small reversible increase of [σ.sub.Dark] and [σ.sub.photo] within a factor of two, but they did not affect the minority carrier μτ-products significantly. These changes are partially reduced by vacuum treatment and completely reduced after heat treatment at 430 K. However, oxygen gas treatment at 80°C resulted in more than an order of magnitude increase in both [σ.sub.Dark] and [σ.sub.photo] and an increase in the diffusion length, [L.sub.D], by 50% from that of the annealed-state value in highly crystalline samples, while no significant metastability is detected in amorphous and low crystalline silicon thin films. A following heat treatment partially recovers both [σ.sub.Dark] and [σ.sub.photo] to their annealed-state values, while [L.sub.D] decreases only slightly. Such increase in the [L.sub.D] values could be due to a decrease in the density of recombination centers for holes below the Fermi level, which may be related to passivation of defects by oxygen on the surface of crystalline grains. PACS Nos.: 73.61.Jc, 73.63.Bd, 73.50.Pz, 72.80.Ng. Nous etudions les effets de la metastabilite dans des films minces de silicium microcristallins hydrogenes du a de l'air, de l'azote tres pur, de l'helium, de l'argon et de l'oxygene, en utilisant la dependance en temperature de la conductivite en obscurite, la photoconductivite et la methode de photocarrier grating en regime stationnaire. Nous trouvons qu'a court terme, l'air, l'azote et les gaz inertes causent de petites augmentations reversibles de [σ.sub.Dark] et [σ.sub.photo] a l'interieur d'un facteur deux, mais n'affectent pas de facon significative les porteurs minoritaires μτ. Ces changements sont partiellement reduits par un traitement par le vide et le sont completement apres chauffage a 430 K. Cependant, le traitement a l'oxygene a 80°C cause une augmentation de plus d'un ordre de grandeur de [σ.sub.Dark] et [σ.sub.photo] et une augmentation de 50 % de la longueur de diffusion [L.sub.D], compare a la valeur d'echantillons cristallins recuits, alors qu'aucune metastabilite n'est detectee dans les films minces de silicium amorphe et faiblement cristallin. Un chauffage subsequent ramene partiellement [σ.sub.Dark] et [σ.sub.photo] a leur valeur de recuit, mais [L.sub.D] ne diminue qu'un peu. Une telle augmentation de [L.sub.D] pourrait etre due a une diminution de la densite des centres de recombinaison sous le niveau de Fermi et peut etre reliee a la passivation des defauts par l'oxygene sur les grains cristallins de la surface. [Traduit par la Redaction], 1. Introduction Hydrogenated microcrystalline silicon (μc-Si:H) has been extensively studied for the last two decades and successfully applied as an absorber layer in thin film silicon solar cells [1, 2]. [...]

The electronic properties of undoped microcrystalline silicon oxide films have been investigated by transient photocurrent (TPC) density of states (DOS) spectroscopy, supported by dark conductivity, steady-state photoconductivity, and constant-photocurrent measurements (CPM). Film compositions span the range from amorphous to microcrystalline and contain up to 10% oxygen content, yielding optical bandgap values [E.sub.04] (the photon energy at which the absorption depth equals one micrometre) between 1.85 and 2.11 eV. Carrier transport is consistent with multiple-trapping in a localised DOS, which depends upon film structure and oxygen content. TPC measurements indicate that both conduction band-tail energy and deep defect density increase with increasing oxygen content, accompanied by a reduction in majority carrier mobility-lifetime product. CPM measurements on amorphous films show a broadening of the Urbach tail with increasing oxygen content. Significantly higher oxygen incorporation without seriously compromising electronic quality appears possible in microcrystalline films. This suggests potential application as solar cell absorber layers offering increased optical bandgap and open-circuit voltage. PACS Nos.: 73.50.-h, 73.61.-r, 73.50.Pz, 71.23.Cq. Les proprietes electroniques de films d'oxyde de silicium microcristallins non dopes sont etudies ici a l'aide de la spectroscopie de photo-courant transitoire (TPC) a densite d'etat (DOS), supportee par des mesures de conductivite en obscurite, de photoconductivite stationnaire et de photo-courant constant (CPM). La composition des films varie d'amorphe a microcristallin, contenant jusqu'a 10 % d'oxygene, ce qui donne des gaps optiques ([E.sub.04]; l'energie du photon a laquelle la profondeur d'absorption atteint un micrometre) entre 1.85 et 2.11 eV. Le transport des porteurs est coherent avec un piegeage multiple dans une DOS localisee qui depend de la structure du film et de son contenu en oxygene. Les mesures TPC indiquent qu'a la fois, l'energie de la queue de bande de conduction et la densite de defauts profonds augmentent avec le contenu en oxygene, ce qui s'accompagne d'une reduction du produit de la mobilite fois le temps de vie de la majorite des porteurs. Des mesures CPM sur des films amorphes montrent un elargissement de la queue d'Urbach avec l'augmentation du contenu en oxygene. Il apparait tres possible d'incorporer significativement plus d'oxygene dans les films microcristallins, sans compromettre leurs qualities electroniques. [Traduit par la Redaction], 1. Introduction Silicon oxide (Si[O.sub.x]:H) films are of interest for use in optoelectronic applications primarily because the optical band-gap [E.sub.04] (the photon energy at which the absorption depth equals one [...]

Metal-semiconductor contacts are a vital part of semiconductor devices as they can form a Schottky barrier or an Ohmic contact. The nature of the contact plays an important role in determining the electrical and physical characteristics of the device and hence is of paramount importance in the operation of the device. In the current work we report the design, fabrication, and current-voltage (I-V) characteristics of microbolometers, a type of infrared detector where the change in temperature changes the resistance of the sensing layer. Eight different types of microbolometers were fabricated using a-[Si.sub.x][Ge.sub.1-x] or a-[Si.sub.x][Ge.sub.1-x][O.sub.y] sensing layers and Ti, Cr, Al, Au, Ni, or [Ni.sub.0.80][Cr.sub.0.20] metals contacts. It has been observed that bolometers with an a-[Si.sub.0.15] [Ge.sub.0.85] (Si was lightly p-doped) sensing layer formed a Schottky contact with Ti, Au, Cr, and Al contact metals, while bolometers with a-[Si.sub.0.15][Ge.sub.0.85] (Si was heavily n-doped) sensing layers formed an Ohmic contact with Au. For microbolometers with a [Si.sub.0.15][Ge.sub.0.85][O.sub.0.039] sensing layer, both Ni and [Ni.sub.0.80][Cr.sub.0.20] contact metals formed the Ohmic contact. For a-[Si.sub.x][Ge.sub.1-x] and a-[Si.sub.x][Ge.sub.1-x][O.sub.y] microbolometers, Au and [Ni.sub.0.80][Cr.sub.0.20] were used as the absorber layers, respectively. The I-V characteristics of the microbolometers were analyzed with a thermionic emission model. A linear dependence on the Ge composition was approximated to find the effective Richardson constant. The theory predicts Richardson constants of 112 and 50 A/[cm.sup.2][K.sup.2] for Si and Ge, respectively. Barrier heights of all devices are calculated and the reasons for the formation of the Ohmic and Schottky contacts are discussed. PACS No.: 07.57.Kp. Les contacts metal-semiconducteur sont une partie vitale des dispositifs semiconducteurs, parce qu'ils peuvent former des barrieres de Schottky ou des contacts ohmiques. La nature du contact est importante pour determiner les proprietes electriques et physiques du dispositif et donc est d'une importance capitale dans l'operation du dispositif. Nous presentons ici le design, la fabrication et les caracteristiques courant-voltage (I-V) de micro-bolometres, un type de detecteur infrarouge ou les variations en temperature changent la resistance de la couche sensible. Nous fabriquons huit differents types de bolometres qui utilisent une couche sensible de a-[Si.sub.x][Ge.sub.1-x] ou de a- [Si.sub.x][Ge.sub.1-x][O.sub.y] et des ports de contact metalliques de Ti, Cr, Al, Au, Ni ou [Ni.sub.0.8][Cr.sub.0.2]. Nous observons que les bolometres avec couche sensible de a-[Si.sub.0.15][Ge.sub.0.85] (le Si fortement dope p) forment des barrieres de Schottky avec des contacts de Ti, Au, Cr et Al, alors que les bolometres de a-[Si.sub.0.15][Ge.sub.0.85] (le Si est fortement dope n) forment des contacts ohmiques avec Au. Les bolometres de couche sensible de a-[Si.sub.0.15][Ge.sub.0.85][O.sub.0.039] forment des contacts ohmiques avec Ni et [Ni.sub.0.8][Cr.sub.0.2]. Avec les bolometres bases sur a-[Si.sub.x][Ge.sub.1-x] et a-[Si.sub.x][Ge.sub.1-x][O.sub.y], nous utilisons une couche absorbante de Au et de [Ni.sub.0.8][Cr.sub.0.2] respectivement. Les courbes I-V des micro-bolometres sont analysees a l'aide d'un modele d'emission thermoionique. Nous utilisons une approximation lineaire de la dependance sur la composition de Ge afin de determiner la constante efficace de Richardson. La theorie predit des constantes de Richardson de 112 et de 50 A/[cm.sup.2][K.sup.2] respectivement pour le Si et le Ge. Nous calculons les hauteurs de barriere de tous les dispositifs et discutons des raisons pour la formation de contact de Schottky ou ohmique. [Traduit par la Redaction], 1. Introduction Bolometers are infrared detectors whose resistance changes with temperature and are used for defense and security, search, rescue, and numerous other applications. Various materials have been used as [...]

In recent years a very large amount of data has been collected regarding devices formed by multiwalled carbon nanotubes (MWCNTs) in polymer matrix as well as single nanotubes. Conversely, very little investigation exists of nanotube networks composed of a small number of MWCNTs. A detailed investigation of the long time stability, adhesion to the surface, and topological structure of the interconnections between MWCNTs is reported here. Three different microscopy techniques, focused ion beam (FIB), scanning electron microscope (SEM), and atomic force microscope (AFM), were used to investigate the interconnection of MWCNTs deposited by electrophoresis on a thermally oxidized silicon wafer with aluminum microgap structures. SEM, AFM, and FIB imaging revealed an interesting interconnection morphology between the drop casted MWCNTs. In particular it was found that in some cases the MWCNTs were connected to each other in a geometry similar to a twisted structure. Furthermore a good stability of the sample in time has been found, proving a strong adhesion of the nanotubes to the oxide surface. Despite the fact that electrical contacts with aluminum to carbon nanotube--based devices are in general not very reliable, using the dielectrophoresis deposition technique with an adequate subsequent annealing procedure, long-term stable temperature sensors with carbon nanotube networks in the aluminum contact microgap were realized. Their temperature dependence can be explained by modeling the internanotube contact resistance. PACS Nos.: 73.63.Fg, 85.35.Kt, 68.37.Hk. Dans les dernieres annees, beaucoup de donnees ont ete accumulees concernant des dispositifs formes de nanotubes de carbone multicouches (MWCNT) dans des matrices de polymeres ou comme simples nanotubes. A contrario, peu est connu des reseaux de nanotubes constitues par un petit nombre de MWCNTs. Nous presentons ici une etude poussee de la stabilite a long terme, de l'adhesion a la surface et de la structure topologique des interconnexions entre les nanotubes multicouches. Trois differentes techniques microscopiques, le faisceau ionique focalise (FIB), le microscope electronique a balayage (SEM) et le microscope a force atomique (AFM), sont utilisees pour analyser les interconnexions de MWCNTs deposes par electrophorese sur la surface thermiquement oxydee d'un wafer de silicium avec des contacts micro-separes d'aluminium. Les techniques d'imagerie SEM, AFM et FIB revelent une interessante morphologie d'interconnexions entre les MWCNTs obtenus par drop casting. Dans certains cas particuliers, nous trouvons que les MWCNTs sont connectes les uns aux autres dans une geometrie similaire a une structure vrillee. Nous observons que l'echantillon a une bonne stabilite temporelle, ce qui prouve une forte adhesion entre les nanotubes et la surface d'oxyde. Malgre le fait que les contacts electriques d'aluminium avec des dispositifs bases sur les nanotubes de carbone sont en general peu fiables, l'utilisation d'une technique de depot bi-electrophorese avec une procedure adequate de recuits subsequents, permet d'obtenir une stabilite a long terme en temperature pour des senseurs faits de reseaux de nanotubes avec contact en aluminium micro-separes. [Traduit par la Redaction], 1. Introduction Carbon nanotube networks (CNNs) are very attractive for electronic and optoelectronic device applications. Besides single-walled carbon nanotube--based CNNs, that are, for example, used for the realization of thin [...]

The complex chalcogenides with excellent memory switching properties are mainly situated close to the border of glass formation domain. The simulation of the structural changes occurring during the memory switching process of a ternary chalcogenide composition has been carried out. The transition of a high resistivity Ge[As.sub.4][Te.sub.7] amorphous cluster with 120 atoms to a low resistivity crystalline cluster was analyzed. The coordination of atoms changes from that corresponding to 8-N coordination rule (two for tellurium, three for arsenic, and four for germanium) in the amorphous phase to six (the same for all atoms) in metastable crystalline phase. Because of spatial constraints exercised by the amorphous matrix, the amorphous cluster cannot expand. In these circumstances Te atoms seem to be over-coordinated (up to sixfold-coordinated). During the switching process, the atoms are moving on distances up to 4.0 Å. The average displacement is of 2.36 Å. PACS Nos.: 47.20.Hw, 77.84.Bw, 78.55.Qr, 77.80.Fm, 83.10.Tv. Les complexes chalcogenures avec d'excellentes proprietes de commutation de resistance a memoire se situent maintenant pres de la frontiere de la formation de verre. Nous faisons ici une simulation des changements structuraux qui se produisent lors de la commutation de memoire d'un compose chalcogenure tertiaire. Nous analysons la transition entre l'amas de 120 atomes de haute resistance de a-Ge[As.sub.4][Te.sub.7], vers l'amas cristallin de basse resistance. La coordinence des atomes change, de celle correspondant a un indice de coordination 8-N (deux pour le Te, trois pour l'As et quatre pour le Ge) dans la phase amorphe, a six (la meme pour tous les atomes) dans la phase cristalline metastable. A cause des contraintes spatiales exercees par la matrice amorphe, l'amas amorphe ne peut pas s'etendre. Dans ces circonstances, l'indice de coordination des atomes de Te est tres eleve (jusqu'il six fois). Pendant le processus de commutation, les atomes se deplacent sur des distances allant jusqu'il 4.0 Å. Le deplacement moyen est de 2.36 Å. [Traduit par la Redaction], 1. Introduction The complex chalcogenides with memory switching properties (phase change materials) are the core of the nonvolatile solid-state memory devices or phase change random access memory. The knowledge of [...]

It is known that deviation from the Drude law is dramatic in most electronically conductive nanomaterials. A brief review of this topic is given, and it is shown that a series sequence of transport involving grains and grain boundaries produces a type of Lorentz resonance (non-Drude behavior) in nanomaterials. The physical parameters deduced from this model are reasonably good. PACS Nos.: 73.63.Bd, 78.67.Bf, 73.63.-b, 62.25.Fg. Nous savons qu'il y a une deviation dramatique de la loi de Drude dans la plupart des nanomateriaux electroconducteurs. Nous presentons une breve revue du sujet et montrons qu'une sequence de transports en serie impliquant les grains et les bordures de grains produit un type de resonance de Lorentz (non Drude) dans les nanomateriaux. Les parametres physiques deduits de ce modele sont raisonnablement bons. [Traduit par la Redaction], 1. Introduction The interaction of terahertz (THz) radiation with charge carriers provides important information on carrier transport in a wide range of materials, in particular, in nanomaterials [1-5]. When the [...]

The present work reports a detailed study of some physical properties of some novel glasses of [Se.sub.98-x][Ag.sub.2][In.sub.x] (x = 0, 2, 4, 6) system. Measurements of thermal transport properties (i.e., thermal conductivity, κ, and thermal diffusivity, [x.sub.e]) have been carried out using the transient plane source technique. Specific heat measurements have been done by differential scanning calorimetry. Thermomechanical properties (i.e., Vickers hardness, [H.sub.v], and modulus of elasticity, E) have been evaluated by the indenter test. The minimal energy for formation of microvoids, [E.sub.h], and microvoids volume, [V.sub.h], of the previously mentioned glassy system are discussed in terms of microhardness, [H.sub.v]. Temperature and frequency dependence of dielectric constant, [ζ.sub.1], and dielectric loss, [ζ.sub.2], for the same system were measured in the frequency (50 Hz -1000 κHz) and temperature (303-338 K) range. The experimental results illustrate that the values of dielectric constant, ex and dielectric loss, [ζ.sub.2], are decreased with frequency and increased with temperature. The maximum barrier height, [W.sub.b], is calculated using the dielectric measurements according to the Guintini equation. The morphology and microstructural analysis of as-prepared alloys are confirmed by X-ray diffraction, scanning electron microscope, and transmission electron microscope. PACS Nos.: 81.05.Gc, 81.05.Kf, 61.41.Dq. Nous presentons ici une etude detaillee des proprietes physiques de nouveaux verres du systeme [Se.sub.98-x][Ag.sub.2][In.sub.x] (x = 0, 2, 4, 6). La technique de source plane transitoire permet de mesurer les proprietes thermiques de transport, donc la conductivite thermique κ et la diffusivite thermique [χ.sub.e]. La capacite calorifique est determinee par DSC, alors que les proprietes thermomecaniques, comme la durete de Vickers [H.sub.V] et le module d'elasticite E, sont evaluees par le test d'indentation. Nous analysons, l'energie minimale de formation de micropores [E.sub.h] et de volume de porosite [V.sub.h], en fonction de la micro-durete [H.sub.V]. Nous determinons la dependance en temperature et en frequence de la constante dielectrique[ζ.sub.1] et de la constante de perte dielectrique [ζ.sub.2], dans les domaines 303-338 K et 50 Hz -1000 κHz respectivement. Nous calculons la hauteur maximale de la barriere [W.sub.b] en utilisant l'equation de Guintini et les valeurs mesurees des proprietes dielectriques. Nous confirmons par techniques XRD, SEM et TEM la morphologie et la structure microscopique des alliages prepares ici. [Traduit par la Redaction], 1. Introduction Chalcogenide (ChG) glasses, mostly selenium based binary and ternary alloys, are valuable because of their greater hardness, high photosensitivity; higher crystallization temperature, smaller aging effect, and low viscosity [...]

We review the material properties that allowed amorphous silicon to become the dominant large area semiconductor and then point out how amorphous oxide semiconductors could displace a-Si in thin film transistors, and how phase change materials, such as GeSbTe alloys, have provided an optical storage technology and will provide a nonvolatile electrical storage technology based on their unique properties. PACS Nos.: 71.15.Mb, 71.23.Cq, 73.61.Jc. Nous passons en revue les proprietes du silicium amorphe qui lui ont permis de devenir un joueur si dominant dans les semi-conducteurs et nous soulignons comment les oxydes de semi-conducteurs amorphes pourraient remplacer a-Si dans les transistors en films minces et comment les materiaux a changement de phase, tel l'alliage GeSbTe, peuvent nous donner une technologie de stockage optique et nous fournir une technologie de stockage electrique non volatile, a cause de leurs uniques proprietes. [Traduit par la Redaction], Introduction This paper reviews the properties and usefulness of amorphous silicon (a-Si) versus other contender amorphous semiconductors over the history of the ICANS conference. Early ICANS conferences focused on the [...]

Untreated multiwalled carbon nanotubes (MWCNTs) were incorporated into a brittle epoxy matrix at 1 wt% by solvent evaporation and showed significant gains in tensile strength, strain at break, and electrical conductivity. However, only fractional improvement in fracture toughness was observed. To improve fracture toughness, 10 pm polyamide-12 (PA) preformed particles were added at 20 wt% loading. The PA particles were observed to aid in the dispersion of MWCNTs, reducing most of the large MWCNT clusters to submicron sizes. The epoxy/PA/MWCNT ternary composite exhibits an increased electrical percolation threshold, but nearly identical electrical scaling behavior as the epoxy/ MWCNT composite. The similarity in percolation parameters suggest conduction occurs by efficient electron transport through MWCNT clusters, rather than pathways composed of individual tubes. The ternary composite also shows significant improvements in strain at break and fracture toughness due to synergistic interaction between filler phases. The usefulness of this ternary composite approach in vacuum-assisted resin transfer molding for advanced composite applications is discussed. POLYM. ENG. SCI., 51:2245-2253, 2011. © 2011 Society of Plastics Engineers, INTRODUCTION Epoxy resins have been used as high performance polymers because of their excellent mechanical properties, chemical resistance, and thermal stability, but require a second phase conductive filler material in [...]

The salinity status of soil can be obtained through the measurement of the pore-water electrical conductivity [[sigma].sub.p]. In the present study the WET sensor's capability in predicting [[sigma].sub.p] through the simultaneous measurements of the soil bulk electrical conductivity ([[sigma].sub.b]) and the soil dielectric constant (K) is investigated. The estimation of [[sigma].sub.p] is based on a model by Hilhorst, which relates [[sigma].sub.p], to the two other quantities K and [[sigma].sub.b] and an extra fitting parameter [K.sub.0] which is incorporated in the software of the device. The study involved experimental measurements in the laboratory using four different soils with a large range in soil texture and volumetric water content [theta]. In each soil type four different electrical conductivity aqueous solutions were used. The results exhibited a rather strong linear relationship between K and [[sigma].sub.b]. The slope of this linear relationship appeared to depend on both the soil type and the electrical conductivity of the pore water. The value of [K.sub.0] seems to be soil specific and increases when the salinity level increases. The [[sigma].sub.p] prediction according to the linear model of Hilhorst deviates for almost all soils tested, except for the case of the sandy soil. Besides the linear model of Hilhorst, the Malicki and Walczak linear model was also tested for all soils under investigation and the model of Munoz-Carpena et al. was tested for sand. Malicki and Walczak model performance at predicting [[sigma].sub.p], values was approximately the same as the Hilhorst model for sand and slightly better for sandy loam. In general, one could argue that the linear models could predict [[sigma].sub.p] with some accuracy for the cases of coarse porous media. DOI: 10.1061/(ASCE)IR.1943-4774.0000218 CE Database subject headings: Probe instruments; Salinity; Soil water; Pore water; Electrical conductivity; Porous media. Author keywords: WET sensor; Salinization; Dielectric constant; Soil pore water electrical conductivity; Soil bulk electrical conductivity.

Time domain reflectometry (TDR) is commonly used to determine the soil bulk electrical conductivity. To obtain accurate measurements, the three parameters of a series resistor model (probe constant, [K.sub.p]; cable resistance, [R.sub.c]; and remaining resistance, [R.sub.0]) are typically calibrated using liquids with known electrical conductivity. Several studies have reported discrepancies between calibrated and directly measured parameters of the series resistor model. In this study, we examined the possibility that a technical issue with the TDR100 cable tester contributed to part of these inconsistencies. Our results showed that with an increasing level of waveform averaging, the reflection coefficient, as well as [K.sub.p], [R.sub.C], and [R.sub.0], approached a maximum value. A comparison with independently determined values indicated that a high level of waveform averaging provided the physically most plausible results. Based on our results, we propose averaging at least 16 waveforms, each consisting of at least 250 points. An oscilloscope-based signal analysis showed that the increase in the reflection coefficient with increasing waveform averaging in saline media is related to a capacitance associated with electrode polarization in combination with a change in the pulse period of the pulse train when the TDR100 starts collecting data points. This capacitance resulted in a slow change of the average voltage in the TDR pulse train until a stable average voltage was reached. Higher levels of waveform averaging cancel the impact of the first erroneous voltage measurements out. In practical applications, the errors in the determination of the bulk electrical conductivity can be as high as 5% for the low-conductivity range ( Abbreviations: RC, resistor--capacitor; TDR, time domain reflectometry; WavAvg, number of waveform averages. doi:10.2136/sssaj2009.0247

Studying solute transport in soils is hampered by a lack of technology for continuously monitoring ionic concentration of contaminants. The electrical conductivity of pore water is a strong indicator of ionic concentration of contamination in soil. Using the bulk electrical conductivity of a soil measured by lime domain reflectrometry (TDR) to predict the soil pore-water electrical conductivity appears to be a promising technique. This study presents a new method for estimating the pore-water electrical conductivity of saturated sandy soils using a single TDR test. The effects of pore-water electrical conductivity, temperature, porosity, and ionic types on the electrical conductivity of soil were studied. An average value of the exponent in the Archie's Law was found to be 1.457 for the saturated sandy soils used in this study. A laboratory model infiltration test was also conducted with continuous monitoring of the electrical conductivity of the pore water by TDR. The results showed that TDR is able to provide a reasonably accurate estimation of the electrical conductivity of pore water. Consequently, it may be possible to monitor the in situ ionic contamination in saturated sandy soils using TDR technology. Key words: sandy soils, electrical conductivity, time domain reflectometry (TDR), dielectric constant. L'etude du transport des solutes dans les sols est limilee par le manque de technologies pouvant mesurer en continu la concentration ionique des contaminants. La conductivity electrique de l'eau inlerstitielle est tin bon indicateur de la concentration ionique des contaminants dans les sols. L'utilisation de la conductivity electrique globale d'un sol a I'aide de la reflec tome trie en domaine temps (RDT) semble etre une technique prometteuse pour la prediction de la conductivity electrique de l'eau inlerstitielle contenue dans ce sol. Cette etude presente une nouvelle methode pour estimer la conductivity electrique de l'eau inlerstitielle de sols sablonneux satures a I'aide d'un seul essai RDT. Les effets de la conductivity electrique de l'eau interstitielle, de la temperature, de la porosite et des especes ioniques sur la conductivity electrique du sol ont etc etudies. La valeur moyenne de l'exposant de la loi d'Archie a ete evaluee a 1,457 pour les sols sablonneux satures utilises dans cette etude, Un essai en laboratoire d'infiltration, durant lequel la conductivity electrique de l'eau interstitielle a etc mesuree en continu par RDT, a aussi ete effect ue. Les resultats ont demontre qu'il est possible d'oblenir une estimation raisonnablement precise de la conductivity electrique de l'eau interstitielle a I'aide de la RDT, Ainsi, il serail possible de mesurer in situ la contamination ionique dans des sols sablonneux satures avec la technologie RDT. Mots-cies : sols sablonneux, conductivity electrique, reflectometrie en domaine temps (RDT), constante dielectrique. [Traduit par la Redaction], Introduction Determination of ionic concentrations in soils in the field is important for assessment of salinity (Rhoades 1989), study of solute transport (Kachanoski et al. 1992; Ward et al. 1994; [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2009.10.001 Byline: Philippe Grosso (a), Marc Le Menn (b), Jean-Louis De Bougrenet De La Tocnaye (a), Zong Yan Wu (a), Damien Malarde (a) Keywords: Salinity sensor; Absolute salinity; Refractive index; Density Abstract: Optical salinity sensors described here measure directly the seawater refractive index and thus enable a measurement of the seawater density and composition variation. We detail the measurement dependence to environmental parameters (in particular temperature and pressure) compared to conductivity sensors, and demonstrate that it may be advantageous to directly measure refractive index rather than electrical conductivity and so obtain a more direct route to density and absolute salinity. Author Affiliation: (a) Telecom-Bretagne, Departement Optique, UMR CNRS 6082 FOTON, CS 83818, 29238 Brest Cedex 3, France (b) Laboratoire de Metrologie et de Chimie Oceanographique, SHOM, 13 rue du ChAcentstelier, BP 30316, 29603 Brest Cedex, France Article History: Received 17 June 2008; Revised 23 September 2009; Accepted 2 October 2009

In soils with high electrical conductivities, time domain reflectometry (TDR) principles fail to measure the apparent dielectric constant by travel time analysis. This limits the application of water content measurement of existing TDR technologies on those materials. This paper describes a new approach for determining the dielectric constants in highly conductive soils from surface reflections of TDR signals. The multiple reflections at the interfaces of impedance mismatches in the ASTM standard probe were studied. Extension rods were used to avoid the overlap of the reflections along the probe. A relationship between the reflection coefficient at the soil surface and the apparent dielectric constant of the soil was established theoretically and validated by laboratory experiments. Results indicate that the dielectric constant can be determined with reasonable accuracy from the surface reflection coefficient even for soils with high electrical conductivities, where the conventional travel time analysis fails. Compared with alternative approaches, such as based on inversion model, this new method offers high time efficiency in extending TDR applications in highly conductive soils. DOI: 10.1061/(ASCE)GT.1943-5606.0000170 CE Database subject headings: Soil properties; Electrical conductivity; Coefficients; Reflection.

Electrical conductivity (EC) is used for monitoring different types of aqueous systems. The measured EC value at any temperature needs to be corrected for a standard temperature, for reporting or comparison, since EC is dependent on temperature. A temperature compensation factor has been derived for conductivity correction from anaerobic digester supernatant/centrate samples of five different wastewater treatment plants in western Canada. For a temperature compensation factor of 0.0198[degrees][C.sup.-1], corresponding to the standard temperature at 25[degrees]C, the estimated [EC.sub.25] values were found to be reasonably accurate, with a maximum error of 2.01%. The estimated EC values, based on any standard temperature, were found to be statistically similar to each other. Considering the temperature dependence of EC, a relationship between EC and ionic strength was developed in this study for anaerobic digester supernatant/centrate samples. This relationship can be used to estimate the ionic strength of the solution in a system associated with struvite formation from anaerobic digester supernatant/centrate and effectively monitor the system performance. DOI: 10.1061/(ASCE)0733-9372(2009)135:11(122l) CE Database subject headings: Electrical conductivity; Monitoring; Temperature; Wastewater management.

We present a simple and precise method to measure the azimuthal electrical conductivity in nonferromagnetic tubes and rods at low frequencies. The results are readily obtained by analyzing the eddy currents in the sample when it is subjected to an axial harmonic magnetic field. The information about the eddy currents can be obtained from a measurement of the phase of the induced voltage in a short probe coil located inside the tube or around the rod when the sample is axially excited by a long primary coil. At low frequencies, the eddy currents are distributed over the full depth of the sample and any irregularities can be detected when the probe coil is moved along the sample axis. [DOI: 10.1119/1.3184154]

A lysimeter study was conducted in the field in Karaj, Iran to investigate the effects of water table management on water quality of subsurface drainage effluents. Drain volumes, nitrate-N concentration, phosphorus concentration, and electrical conductivity of drain effluents were monitored during the growing seasons of alfalfa (Medicago scutellata). Totally 12 lysimeters consisted of four treatments were used in this study, of which nine of them were equipped with subirrigation (SI) and the other three with free drainage (FD) systems. Annual alfalfa (Medicago scutellata) was planted in all lysimeters. Water table levels were kept at 30 cm (SI30), 50 cm (SI50), and 70 cm (SI70) below the soil surface in SI-lysimeters and more than 100 cm below the soil surface in FD-lysimeters. The results of this 2-year study showed a significant reduction in nitrate-N concentrations in SI-lysimeters compared to FD-lysimeters. In 2005, the mean nitrate-N concentrations in drainage effluent were reduced by 84% in the SI30 and by 82% in the SI50, relative to FD. Similarly, in 2006, drain water depth and nitrate-N concentrations were significantly reduced relative to FD. The forage dry matter production from SI30 and SI50 were significantly higher than those from FD in both years. In 2006, the average of dry matter production was increased by 69 and 89% by the SI30 and SI50, respectively, relative to FD. The average electrical conductivity of drainage water was reduced in SI lysimeters compared to FD lysimeters that meet Iranian standard level (3 dS/m). There are no statistically significant differences in phosphorous concentration in drainage water of different treatments. Finally, the results of this 2-year study indicate that the water table management practices are economically and environmentally feasible in Iran in order to have a sustainable agriculture. DOI: 10.1061/(ASCE)IR.1943-4774.0000080 CE Database subject headings: Subirrigation; Effluent; Electric conductivity; Phosphorous; Water table; Water use; Iran; Arid lands; Drainage.

We developed soil-specific calibrations for a time domain reflectometry (TDR) sensor, a water content reflectometer (WCR), and an impedance sensor for 19 soil moisture monitoring sites in Wyoming. The main objective was to calibrate the WCR sensors that form a statewide distributed soil moisture network. A secondary objective was to use the TDR and impedance sensor data to better understand the WCR readings. All calibrations were conducted in the laboratory by mixing different amounts of water in the soils to obtain a range of soil water contents and by packing the soils in a bucket. The observed apparent permittivity as a function of soil water content was relatively high for the WCR and impedance sensors (up to 187 and 150% higher, respectively, than TDR). This was attributed to the relatively low operating frequency (< 175 and 50 MHz, respectively) of the WCR and impedance devices. Soil texture had little impact on sensor readings. Differences between soils for the WCR and impedance sensors were due to differences in bulk soil electrical conductivity (EC) and dielectric relaxation losses. Relaxation losses were found to be significant at 50 MHz for all soils. Regression equations (R2 > 0.643) were developed for the WCR and impedance sensors that relate the slope of the soil water content vs. permittivity relationship to the bulk soil EC (WCR and impedance sensors) or the imaginary permittivity (impedance sensor). Abbreviations: DC, direct current; EC, electrical conductivity; TDR, time domain reflectometry; WCR, water content reflectometer.

The current practice for monitoring of subsurface plumes involves the collection of water samples from sparsely distributed monitoring wells and laboratory analysis to determine chemical concentrations. In most field situations, cost and time constraints limit the number of samples that could be collected and analyzed for continuous monitoring of large, transient plumes. With the development of wireless sensor networks (WSNs), that allow sensors to be incorporated into a distributed wireless communication and processing system, the potential exists to develop new, efficient, economical, large-scale subsurface data collection and monitoring methods. This paper presents a proof-of-concept study conducted in a two-dimensional synthetic aquifer constructed in an intermediate scale test tank to demonstrate the feasibility of using WSN for subsurface plume monitoring. The tank was packed to represent a heterogeneous aquifer, and a sodium bromide tracer was used to create a plume. A set of ten wireless sensor nodes (motes) equipped with conductivity probes to measure electrical conductivity formed the network. Software for automated data acquisition was developed and tested. Results of two experiments conducted using this test system are presented. The lessons learned from the first experiment were used to make modifications to the way the sensors were placed, how they were calibrated and how the sensors were interfaced with the data acquisition system. The findings are used to identify future research directions and issues that need to be addressed before field implementations of a WSN based data collection system for plume monitoring. DOI: 10.1061/(ASCE)EE.1943-7870.0000045 CE Database subject headings: Groundwater quality; Groundwater pollution; Monitoring; Sampling; Probe instruments; Plumes.

A novel technique is developed based on the piezoresistivity of electrically filled polymers to measure the tensile strain in modified geosynthetics without the need for conventional instrumentation (e.g., strain gauges). This paper reports the development of the technique and the results obtained on high-density polyethylene and polypropylene (PP) geogrid specimens filled with carbon black and carbon nanotubes (NTs). It was found that except for NT-filled PP specimens all other composites exhibited significant strain sensitivity in their conductivity. The proof-of-concept study reported in this paper has two important features: (1) strain sensitivity of electrical conductivity was demonstrated in polyolefins used to manufacture geosynthetics; and (2) this strain sensitivity was obtained and demonstrated over the range of strain values that are important in geosynthetic engineering applications. DOI: 10.1061/(ASCE)GT.1943-5606.0000062 CE Database subject headings: Geosynthetics; Instrumentation; Sensors; Strain.

Electrohydrodynamic (EHD) conduction pumping is associated with the heterocharge layers of finite thickness in the vicinity of the electrodes, generated by the process of dissociation of the neutral electrolytic species and recombination of the generated ions. The theoretical formulation for EHD conduction pumping of liquid film is presented and fundamentally analyzed with the aid of numerical solutions. This model includes fluid dynamics governing equations under laminar and isothermal conditions which are modified to account for the presence of electric body force. The model also includes charge transport equations which are related to the dissociation/recombination phenomenon along with Maxwell's relations that govern the electric field distribution. This paper determines how liquid film flow is generated based on the electric conduction phenomenon. Specifically, the role of controlling dimensionless parameters on the heterocharge layers and flow structures along with the impact of liquid film velocity on charge distribution are illustrated and fundamentally analyzed. In addition, the contribution of unique electrode designs toward electric body force distribution and flow pattern is investigated followed by the effect of interaction between adjacent electrode pairs in multi-pair configurations on generated flow rate. Further, a brief discussion of the conduction pumping efficiency is presented. Finally, the numerical results are verified against experimental data. Index Terms--EHD Conduction, charge dissociation, liquid film pumping.

Dielectric materials used in space on satellite structure may have to cope with strong levels of charging under electron irradiation in space environment. This could lead to potential hazardous discharges and electric arcs and consequent anomalies on the satellite, such as electromagnetic disturbances or, in worst case, the destruction of some on-board systems. These materials need to be tested on-ground to assess their electric behaviour and predict any risk of failure in space environment. The SIRENE facility allows the simulation of geostationnary orbit electron environment and the evaluation of charging capabilities of material samples. We demonstrate, in this paper, that polyimide and polytetrafluoroethylene samples, commonly used in space, are submitted to potentially high radiation induced conductivity or resistivity and electrical ageing under radiation dose in this environment. These physical mechanisms can either reduce the charging level, as observed for polyimide, or dramatically enhance the electric surface potential and the charging kinetics, as seen on polytetrafluoroethylene, leading to high risk of discharge. Index Terms--Space materials, Electron irradiation, Dose Effect, Radiation Induced Conductivity (RIC)

Surface potential dynamics on corona charged surfaces of thick samples of EPDM and silicone rubbers have been studied and analyzed by examining the so called tdV/dt characteristics. This methodology allows for directly obtaining information about charge trap distributions in materials from measurements of potential decay without a-priori knowledge about trap parameters. More importantly, the combination of the above mentioned methodology in conjunction with measurements of other material parameters may provide information desired when analyzing the surface charge behavior on composite insulators for HVDC applications. Index Terms--Surface charging, composite insulation, charge carrier processes, charge measurement, conductivity measurement.

The radio planning of systems in lower frequency bands, such as LF and MF, makes use of ground conductivity at these frequencies, as it determines the attenuation of the surface wave, characteristic of the propagation in these bands. In this paper, three alternative methods to obtain an estimation of the conductivity are presented and compared. The first one is based on measurements of the electric field strength generated by AM transmitters. Although it is considered the most accurate, it is also the most expensive as it requires extensive and detailed measurements that are not always feasible. The second method relies on several terrain characteristics that are known to be related to the conductivity, and uses reference conductivity values from similar areas. The third method makes use of remote sensing data, information that is readily available worldwide at low cost. The methods are compared through their application in a research work carried out in Mexico. The results show that these approximate methods can yield results with reasonable accuracy, when compared with the results obtained from field strength measurements. Index Terms--Conductivity measurement, field strength measurement, ground conductivity, remote sensing, terrain characteristics.

Magnetic induction tomography (MIT) is an imaging modality that aims at mapping the distribution of the electrical conductivity inside the body. Eddy currents are induced in the body by magnetic induction and the resulting fields are measured by an array of receiver coils. In MIT, the location of the receivers affects the quality of the image reconstruction. In this paper, a fast deterministic algorithm was applied to obtain optimum receiver array designs for a given specific excitation. The design strategy is based on the iterative exclusion of receiver locations, which yield poor conductivity information, from the space spanning all possible locations until a feasible design is reached. The applicability of 'regionally focused' MIT designs that increase the image resolution at a particular region was demonstrated. Currently used design geometries and the corresponding reconstructed images were compared to the images obtained by optimized designs. The eigenvalue analysis of the Hessian matrix showed that the algorithm tends to maintain identical conductivity information content sensed by the receivers. Although the method does not guarantee finding the optimum design globally, the results demonstrate the practical usability of this algorithm in MIT experimental designs. Index Terms--Information measure, magnetic induction tomography (MIT), optimum design.

Two sets of silica particles have been synthesized and dispersed in both 1-propanol and toluene-ethanol mixture. The conductivity of the suspensions has been measured as a function of their concentration, by means of a device and experimental procedure developed by the authors. It has been observed that the conductivity increases linearly with the solid fraction for values smaller than 5%. Additionally, the water content has also been measured, to evaluate its effect on the conductivity of suspensions. Index Terms Conductivity measurement, dielectric liquids, interface phenomena, ions, particle charging, colloidal dispersions.

Lime kiln dust (LKD) is used for modifying pavement subgrades to expedite construction on wet clayey soils. This paper describes the short-term development (typically, over the first 3 to 7 days) of electrical conductivity and penetration resistance of LKD-modified soils. The normalized net change of electrical conductivity is solely related to the LKD dosage. The decrease of electrical conductivity with time coincides with the increase of penetration resistance with time. The correlations of electrical conductivity with strength gain in LKD and lime-modified soils suggest that electrical conductivity measurements can potentially be useful for quality control in field applications. CE Database subject headings: Dust; Electrical conductivity; Penetration resistance; Clays.

Numerical results of the complex effective permittivity and DC conductivity of heterogeneous samples with random constituents' distribution are analyzed to search for a suitable mixture law. The calculation method consists of the simulation of a time-domain reflectometry experiment by means of the finite-difference time-domain (FDTD) algorithm. The permittivity and conductivity of several series of samples with their constituents distributed at random are calculated. Numerical results, as a function of the volume fractions of the mixture's constituents, are compared with analytical mixture laws. For lossless binary media, it is found that both the Bruggeman law and a weighted geometric mean provide a good agreement. The last one can be generalized to composites with more than two constituents, with good results. On the other hand, for lossy binary mixtures it is shown that the weighted geometric mean of the constituents' complex permittivities matches the numerical results obtained for the effective complex permittivity of the mixture. PACS Nos: 72.20.-i, 77.84.Lf, 77.22.Gm, 72.80.Tm Dans la recherche d'une loi de melange adequate, nous avons analyse des resultats numeriques sur la permittivite complexe effective et la conductivite DC d'echantillons heterogenes dont les constituants sont distribues au hasard. La methode de calcul consiste a simuler une experience de reflectometrie dans un domaine de temps, en utilisant l'algorithme FDTD. Nous avons calcule la permittivite et la conductivite de plusieurs series d'echantillons dont les constituants etaient distributes au hasard. Nous comparons nos resultats numeriques, en tant que fonctions des fractions volumiques, avec les lois analytiques de melange. Pour les milieux binaires sans perte, nous trouvons que la loi de Brugeman et de la moyenne par poids geometriques sont en bon accord. Cette derniere peut etre generalisee, avec de bons resultats, a des echantillons de plus de deux constituants. De plus, pour des milieux binaires avec perte, nous trouvons que la moyenne a poids geometriques des permittivites complexes des constituants reproduit le resultat numerique obtenu pour la permittivite complexe du melange. [Traduit par la Redaction], 1. Introduction Finding the effective permittivity, DC conductivity, or thermal conductivity of a heterogeneous mixture of two substances constitute three analogue problems that have been thoroughly studied for more than [...]