Benthic Foraminiferal Assemblages in a Restricted Environment - An Example from the Mljet Lakes (Adriatic Sea, Croatia)

Benthic foraminiferal assemblages from a peculiar restricted marine environment, the Mljet Lakes (Mljet Island, Adriatic Sea, Croatia) have been studied. These lakes are drowned karst dolines, which are connected with the Adriatic Sea through a narrow, shallow channel. Occasional stagnant conditions in the marine lakes cause hypoxic and anoxic conditions in the bottom waters. Such stressed conditions are reflected in oligospecific benthic foraminiferal assemblages with a Shannon-Wiener species diversity index (H) ranging from 0.8 to 1.0 and equitability index (E) ranging from 0.18 to 0.26, identified in samples from each marine lake. In the more dysoxic Malo Jezero, Haynesina depressula dominates an assemblage of 12 benthic foraminiferal species. In the less (and less frequently) hypoxic Veliko Jezero, we found an Asterigerinata mamilla assemblage with 18 foraminiferal species. A more diverse assemblage containing 55 different benthic foraminiferal species occupies an adjacent open-sea station. Long-term salinity measurements indicate that H. depressula tolerates higher salinity than formerly presumed (up to 38‰), and is well adapted to stressed hypoxic conditions

Electrochemical Redox Processes of Uranium In Aqueous Solutions of Acetylacetone

Electrochemical redox processes of uranium (VI) and uranium (V) in aqueous solutions of acetylacetone have been studied by means of various electrochemical techniques. The presence of acetylacetone accelerates the rate of disproportionation of uranium (V), even under conditions such that uranium (VI) is not in the form of the acetylacetonato complex. The corresponding rate constants of disproportionation were determined. The influence of the acetylacetonato ion concentration on the potential and on the rate of the uranium (VI) - uranium (V) electron transfer was investigated by means of cyclic voltammetry and squa,re-wave polarography. Both reduction and oxidation of uranium (V) were investigated by using the Kalousek commutator technique. A mechanism for those processes is proposed. It was proved by electrocapillary measurements that besides the adsorption of acetylacetone, adsorption of several uranium acetylacetonato species plays an important role in the overall mechanism. Taking into account the experimental results, the redox processes of uranium (VI) and uranium (V) in the presence of acetylacetone can be explained in terms of an ECE mechanism

Electrical Conductivity of Lignocellulose Composites Loaded with Electrodeposited Copper Powders

Composites based on polymers with conductive fillers have been gaining more and more significant roles in a variety of technological domains and are in the research focus of numerous studies as a part of growing research trend. Natural polymers based on renewable materials with addition of chosen materials can be directly used as contemporary materials by electrochemical methods. This article is concerned with the preparation characterization of the basic components: electrodeposited copper powder and lignocellulose as well as composite materials prepared by the compression molding of lignocellulose and galvanostatically obtained copper powder mixtures. Analysis of the most significant properties of individual components and prepared composites included quantitative structural analysis, morphological analysis, determination of density and porosity and measurements of electrical conductivity. Different investigation techniques including SEM, TGA, DSC, X-ray, FTIR, particle size distribution and conductivity measurements were used. The electrical conductivity of the composites is LT 10(-15) MS/m, unless the metal content reaches the percolation threshold of 14.4% (v/v), beyond which the conductivity increases markedly by as much as 14 orders of magnitude. It was found that this transition occurs at lower volume fractions than stated in the literature which can be due to the filler with high specific area

Structure and magnetic properties of nanocrystalline NiFe2O 4 prepared via precipitation route

Nanocrystalline Ni-ferrite was synthesized by modified precipitation method in which soluble starch is used as dispersing agent and Na2CO 3 as a precipitating agent. NiSO4-6H2O and Fe(NO3)3-9H2O were used as precursors for nickel and ferric oxide, respectively. The obtained nanocrystalline Ni-ferrite was analysed and discussed through structural, compositional and magnetic characterization. Formation of pure NiFe2O4 phase with average crystallite size of 21 nm has been con rmed by X-ray di raction analysis (XRD). The determined phase composition was additionally supported by results of 57Fe Mössbauer phase (MS) analysis and material's nanocrystalline structure by field emission scanning electron microscopy (FE-SEM). Thermomagnetic behaviour was studied up to 800 C. The obtained room temperature magnetic hysteresis loop, recorded by means of a vibrating sample magnetometer (VSM), exhibits characteristic "S" shape of the soft magnetic material with the measured coercivity of about 10 kA/m and the specific moment up to 40 Am2/kg

Methods of characterization of multiphase Nd-Fe-B melt-spun alloys

Nanocomposite permanent magnetic materials based on Nd-Fe-B alloys with a low Nd content are a new type of permanent magnetic material. The microstructure of these nanocomposite permanent magnets is composed of a mixture of magnetically soft and hard phases providing the so called exchange coupling effect. Beside the optimization process parameters, methods of characterization have a very important role in the design of an optimal magnetic matrix of multiphase melt-spun Nd-Fe-B alloys. Different methods and techniques of characterization were used for observation and study of the microstructure evolution during crystallization. A summary results of measurements using different methods of characterization are presented to enable a better insight into relations between the microstructure and magnetic properties of the investigated melt-spun Nd-Fe-B alloys. .Nanokompozitni permanentni magnetni materijali zasnovani na Nd-Fe B legurama sa niskim sadržajem neodijuma predstavljaju novi tip permanentnih magnetnih materijala. Mikrostruktura ovih nanokompozitnih permanentnih magneta sastoji se iz smeše magnetno meke i magnetno tvrde faze između kojih se javlja "exchange coupling" efekat. Osim optimizacije procesnih parametara, metode karakterizacije imaju veoma veliku ulogu u dizajniranju optimalnog magnetnog matriksa višefaznih melt-spun Nd-Fe-B legura. Različite metode i tehnike karakterizacije korišćene su za posmatranje i proučavanje evolucije miktrostrukture tokom kristalizacije. Sumarni rezultati merenja, dobijeni primenom različitih metoda karakterizacije, prikazani su radi boljeg uvida u povezanost između miktrostrukture i magnetnih svojstava istraživane melt-spun Nd-Fe-B legure.

Electrical Conductivity of Lignocellulose Composites Loaded with Electrodeposited Copper Powders. Part II. Influence of Particle Size on Percolation Threshold

This article is concerned with synthesis and characterization of electroconductive composite materials prepared by the compression molding of mixtures of lignocellulose and electrochemically deposited copper powder under different pressures, and investigation of the influence of particle size on conductivity and percolation threshold of obtained composites. Electrodeposited copper powder content was varied from 2.0-29.8 vol%. Analysis of the most significant properties of individual components and prepared composites included structural and morphological analysis and measurements of hardness and electrical conductivity. Hardness of the investigated composites, as expected, increased with the increase of the processing pressure, as well as lowering the particle size compared to previous work. The significant increase of the electrical conductivity can be observed as the copper powder content reaches the percolation threshold. The packaging effect and more pronounced interpartical contact with smaller, highly porous, highly dendritic particles with high values of specific area lead to "movement" of percolation threshold towards lower filler content, which for the particles LT 45 mu m and highest processing pressure of 27 MPa was 7.2% (v/v). In the investigated range of electrodeposited copper powder concentrations and applied pressures the increase of the electrical conductivity of composites is as much as fourteen orders of magnitude. It was found that this transition occurs at lower volume fractions than stated in the literature which can be due to the filler with high specific area

Influence of Electrolytic Copper Powder Particle Morphology on Electrical Conductivity of Lignocellulose Composites and Formation of Conductive Pathways

Composites based on polymers with conductive fillers have been gaining more significant roles in a variety of technological domains and they are getting in the research focus of numerous studies as a part of growing research trend. Galvanostatically produced copper powder with high values of specific area was used as filler for synthesis of electroconductive composite materials prepared by the compression molding of mixtures of lignocellulose and electrochemically deposited copper powder. This article is concerned with characterization of these composites. Analysis of the most significant properties of prepared composites and its components included measurements of electrical conductivity, impedance spectroscopy (IS) behavior and structural and morphological analysis. Volume fraction of the copper powder was varied from 2.0-29.8% (v/v). The significant increase of the electrical conductivity can be observed as the copper powder content reaches the percolation threshold (PT). It was shown that PT depends on both particle shape and type of spatial distribution. IS measurements have shown that particle morphology having pronounced grain boundaries has great effect on appearance of electric conductive layers. The packaging effect and more pronounced interpartical contact with copper powder particles lead to "movement" of PT, which for the particles LT 45 mu m and highest processing pressure of 27 MPa was 7.2% (v/v). IS response of the composites showed existence of electrical conductive layers, each having different resistivity which increases towards interior of the composite

An overstoichiometric Nd–Fe–B hard magnetic material

A commercial Nd-rich Nd–Fe–B-based hard magnetic material was studied. The obtained results were compared before and after recording of the thermomagnetic curve up to 800 °C. The curve itself showed clearly besides Curie points of the Nd2Fe14B phase and α-Fe also another critical temperature. Mössbauer spectroscopic (MS) phase analysis and X-ray diffraction analysis (XRD) showed in addition to the commonly known phases Nd2Fe14B and NdFe4B4 also some paramagnetic and ferromagnetic iron atoms (MS) and Fe17Nd2 intermetallics (XRD). During the exerted thermal treatment, the content of the Nd2Fe14B and NdFe4B4 phases remained almost unchanged, while iron atoms from remnant minor phases built a separate α-Fe phase. The XRD pattern also showed the presence of some minor Nd phase. The results of Squid magnetic measurements suggest a nanocrystalline decoupled structure of the Nd-rich alloy in the optimized magnetic state. Measurement of the magnetization loop showed, in spite of small changes in the phase composition, that magnetic properties of the quality material deteriorated during the thermal treatment

Structural and mechanical properties of some aluminum-based binary alloys

The purpose of this study is to investigate structural and mechanical properties of some aluminum-based alloys. Microstructures of the Al-Sn and Al-Zn binary alloys were examined using optic microscopy. The mechanical properties - Vickers hardness and micro hardness tests were investigated according to standard procedure