Mechanical, electrical and electro-mechanical properties of thermoplastic elastomer styrene–butadiene–styrene/multiwall carbon nanotubes composites

Composites of styrene–butadiene–styrene (SBS) block copolymer with multiwall carbon nanotubes were processed by solution casting to investigate the influence of filler content, the different ratios of styrene/butadiene in the copolymer and the architecture of the SBS matrix on the electrical, mechanical and electro-mechanical properties of the composites. It was found that filler content and elastomer matrix architecture influence the percolation threshold and consequently the overall composite electrical conductivity. The mechanical properties are mainly affected by the styrene and filler content. Hopping between nearest fillers is proposed as the main mechanism for the composite conduction. The variation of the electrical resistivity is linear with the deformation. This fact, together with the gauge factor values in the range of 2–18, results in appropriate composites to be used as (large) deformation sensors.

The spin polarization of CrO2 revisited

Here, we use Andreev reflection spectroscopy to study the spin polarization of high quality CrO2 films. We study the spin polarization as a function of growth temperature, resulting in grain size and electrical resistivity. In these films low temperature growth appears to be a necessary but not sufficient condition to guarantee the observation of high spin polarization, and this is only observed in conjunction with suppressed superconducting gap values and anomalously low interface properties. We suggest that this combination of observations is a manifestation of the long range spin triplet proximity effect. (C) 2007 American Institute of Physics.

Electrical transport properties of CuS single crystals

Electrical resistivity, transverse magnetoresistance and thermoelectric power measurements were performed on CuS high quality single crystals in the range 1.2-300 K and under fields of up to 16 T. The zero field resistivity data are well described below 55 K by a quasi-2D model, consistent with a carrier confinement at lower temperatures, before the transition to the superconducting state. The transverse magnetoresistance develops mainly below 30 K and attains values as large as 470% for a 16 T field at 5 K, this behaviour being ascribed to a band effect mechanism, with a possible magnetic field induced DOS change at the Fermi level. The transverse magnetoresistance shows no signs of saturation, following a power law with field Delta rho/rho(0) proportional to H(1.4), suggesting the existence of open orbits for carriers at the Fermi surface. The thermoelectric power shows an unusual temperature dependence, probably as a result of the complex band structure of CuS.

Study of Annealed Indium Tin Oxide-Films prepared by RF Reactive Magnetron Sputtering

Tin doped indium oxide (ITO) films were deposited on glass substrates by rf reactive magnetron sputtering using a metallic alloy target (In-Sn, 90-10). The post-deposition annealing has been done for ITO films in air and the effect of annealing temperature on the electrical, optical and structural properties of ITO films was studied. It has been found that the increase of the annealing temperature will improve the film electrical properties. The resistivity of as deposited film is about 1.3 x 10(-1) Omega*cm and decreases down to 6.9 x 10(-3) Omega*cm as the annealing temperature is increased up to 500 degrees C. In addition, the annealing will also increase the film surface roughness which can improve the efficiency of amorphous silicon solar cells by increasing the amount of light trapping.

A study of ternary Cu2SnS3 and Cu3SnS4 thin films prepared by sulfurizing stacked metal precursors

Thin films of Cu2SnS3 and Cu3SnS4 were grown by sulfurization of dc magnetron sputtered Sn–Cu metallic precursors in a S2 atmosphere. Different maximum sulfurization temperatures were tested which allowed the study of the Cu2SnS3 phase changes. For a temperature of 350 ◦C the films were composed of tetragonal (I -42m) Cu2SnS3. The films sulfurized at a maximum temperature of 400 ◦C presented a cubic (F-43m) Cu2SnS3 phase. On increasing the temperature up to 520 ◦C, the Sn content of the layer decreased and orthorhombic (Pmn21) Cu3SnS4 was formed. The phase identification and structural analysis were performed using x-ray diffraction (XRD) and electron backscattered diffraction (EBSD) analysis. Raman scattering analysis was also performed and a comparison with XRD and EBSD data allowed the assignment of peaks at 336 and 351 cm−1 for tetragonal Cu2SnS3, 303 and 355 cm−1 for cubic Cu2SnS3, and 318, 348 and 295 cm−1 for the Cu3SnS4 phase. Compositional analysis was done using energy dispersive spectroscopy and induced coupled plasma analysis. Scanning electron microscopy was used to study the morphology of the layers. Transmittance and reflectance measurements permitted the estimation of absorbance and band gap. These ternary compounds present a high absorbance value close to 104 cm−1. The estimated band gap energy was 1.35 eV for tetragonal (I -42m) Cu2SnS3, 0.96 eV for cubic (F-43m) Cu2SnS3 and 1.60 eV for orthorhombic (Pmn21) Cu3SnS4. A hot point probe was used for the determination of semiconductor conductivity type. The results show that all the samples are p-type semiconductors. A four-point probe was used to obtain the resistivity of these samples. The resistivities for tetragonal Cu2SnS3, cubic Cu2SnS3 and orthorhombic (Pmn21) Cu3SnS4 are 4.59 × 10−2 cm, 1.26 × 10−2 cm, 7.40 × 10−4 cm, respectively.

Mo bilayer for thin film photovoltaics revisited

Thin film solar cells based on Cu(In,Ga)Se2 as an absorber layer use Mo as the back contact. This metal is widely used in research and in industry but despite this, there are only a few published studies on the properties of Mo. Properties such as low resistivity and good adhesion to soda lime glass are hard to obtain at the same time. These properties are dependent on the deposition conditions and are associated with the overall stress state of the film. In this report, a study of the deposition of a Mo bilayer is carried out by analysing first single and then bilayers. The best properties of the bilayer were achieved when the bottom layer was deposited at 10 × 10−3 mbar with a thickness of 500 nm and the top layer deposited at 1 × 10−3 mbar with a thickness of 300 nm. The films deposited under these conditions showed good adhesion and a sheet resistivity lower than 0.8 .

Incorporação de resíduo da indústria petrolífera em argamassas de cal hidráulica para a reabilitação de edifícios

Trabalho Final de Mestrado para obtenção do grau de Mestre em Engenharia Civil na Área de Especialização de Edificações

In-situ X-ray diffraction studies during growth of Ni-Ti shape memory alloy films and their complementary ex-situ characterization

Shape Memory Alloy (SMA) Ni-Ti films have attracted much interest as functional and smart materials due to their unique properties. However, there are still important issues unresolved like formation of film texture and its control as well as substrate effects. Thus, the main challenge is not only the control of the microstructure, including stoichiometry and precipitates, but also the identification and control of the preferential orientation since it is a crucial factor in determining the shape memory behaviour. The aim of this PhD thesis is to study the optimisation of the deposition conditions of films of Ni-Ti in order to obtain the material fully crystallized at the end of the deposition, and to establish a clear relationship between the substrates and texture development. In order to achieve this objective, a two-magnetron sputter deposition chamber has been used allowing to heat and to apply a bias voltage to the substrate. It can be mounted into the six-circle diffractometer of the Rossendorf Beamline (ROBL) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France, enabling an in-situ characterization by X-ray diffraction(XRD) of the films during their growth and annealing. The in-situ studies enable us to identify the different steps of the structural evolution during deposition with a set of parameters as well as to evaluate the effect of changing parameters on the structural characteristics of the deposited film. Besides the in-situ studies, other complementary ex-situ characterization techniques such as XRD at a laboratory source, Rutherford backscattering spectroscopy(RBS), Auger electron spectroscopy (AES), cross-sectional transmission electron microscopy (X-TEM), scanning electron microscopy (SEM), and electrical resistivity (ER) measurements during temperature cycling have been used for a fine structural characterization. In this study, mainly naturally and thermally oxidized Si(100) substrates, TiN buffer layers with different thicknesses (i.e. the TiN topmost layer crystallographic orientation is thickness dependent) and MgO(100) single crystals were used as substrates. The chosen experimental procedure led to a controlled composition and preferential orientation of the films. The type of substrate plays an important role for the texture of the sputtered Ni-Ti films and according to the ER results, the distinct crystallographic orientations of the Ni-Ti films influence their phase transformation characteristics.

Zinc oxide as an ozone sensor

Journal of Applied Physics, Vol. 96, nº3

Effect of different dopant elements on the properties of ZnO thin films

Vacuum, Vol. 64

Influence of the deposition pressure on the properties of transparent and conductive ZnO: Ga thin-film produced by r.f. sputtering at room temperature

Thin Solid Films, vol. 427, nº 1-2

Durability performance of self-compacting concrete (SCC) with binary and ternary mixes of fly ash and limestone filler

The basic objective of this work is to evaluate the durability of self-compacting concrete (SCC) produced in binary and ternary mixes using fly ash (FA) and limestone filler (LF) as partial replacement of cement. The main characteristics that set SCC apart from conventional concrete (fundamentally its fresh state behaviour) essentially depend on the greater or lesser content of various constituents, namely: greater mortar volume (more ultrafine material in the form of cement and mineral additions); proper control of the maximum size of the coarse aggregate; use of admixtures such as superplasticizers. Significant amounts of mineral additions are thus incorporated to partially replace cement, in order to improve the workability of the concrete. These mineral additions necessarily affect the concrete’s microstructure and its durability. Therefore, notwithstanding the many well-documented and acknowledged advantages of SCC, a better understanding its behaviour is still required, in particular when its composition includes significant amounts of mineral additions. An ambitious working plan was devised: first, the SCC’s microstructure was studied and characterized and afterwards the main transport and degradation mechanisms of the SCC produced were studied and characterized by means of SEM image analysis, chloride migration, electrical resistivity, and carbonation tests. It was then possible to draw conclusions about the SCC’s durability. The properties studied are strongly affected by the type and content of the additions. Also, the use of ternary mixes proved to be extremely favourable, confirming the expected beneficial effect of the synergy between LF and FA. © 2015 RILEM.

Sistema para o estudo da condutividade térmica a baixa temperatura aplicado à espuma de cobre

O desenvolvimento de novos materiais e a sua caracterização é de extrema importância no dimensionamento e construção de equipamentos criogénicos. A empresa Versarien desenvolveu uma técnica capaz de produzir cobre poroso, conseguindo controlar a porosidade e o tamanho de poros. Os materiais porosos são de especial interesse para dispositivos criogénicos em aplicações espaciais. Um exemplo desta aplicação são as unidades de armazenamento de energia (Energy Storage Units-ESU), onde um material poroso é usado em ausência de gravidade para reter um líquido criogénico por capilaridade, de modo a manter dispositivos a uma temperatura baixa e constante. Neste caso, um material poroso de elevada condutividade térmica, como o cobre, seria de grande interesse uma vez que permite obter uma boa homogeneidade de temperatura na célula. Neste trabalho foi desenvolvido um sistema para medir a condutividade térmica deste material, entre 15 e 260 K, para porosidades entre 50% e 80%, utilizando um criorrefrigerador 2 W @ 20 K. Estas medições permitiram determinar que a pureza do cobre poroso se encontra entre RRR20 (RRR: Residual-resistivity ratio) e RRR10, apresentando uma tortuosidade que se encontra de acordo com um modelo simples descrito nesta dissertação. Foi ainda desenhado, construído e testado um criostato portátil, que apenas necessita de azoto líquido e de bombeamento primário para que se possam realizar medições de condutividade térmica entre 77 e 300 K.

Application of superconducting bulks and stacks of tapes in electrical machines

The present dissertation focuses on the research of the recent approach of innovative high-temperature superconducting stacked tapes in electrical ma-chines applications, taking into account their potential benefits as an alternative for the massive superconducting bulks, mainly related with geometric and me-chanical flexibility. This work was developed in collaboration with Institut de Ciència de Ma-terials de Barcelona (ICMAB), and is related with evaluation of electrical and magnetic properties of the mentioned superconducting materials, namely: analysis of magnetization of a bulk sample through simulations carried out in the finite elements COMSOL software; measurement of superconducting tape resistivity at liquid nitrogen and room temperatures; and, finally, development and testing of a frequency controlled superconducting motor with rotor built by superconducting tapes. In the superconducting state, results showed a critical current density of 140.3 MA/m2 (or current of 51.15 A) on the tape and a 1 N∙m developed motor torque, independent from the rotor position angle, typical in hysteresis motors.

Cellulose-based composites as functional conductive materials for printed electronics

In this work, cellulose-based electro and ionic conductive composites were developed for application in cellulose based printed electronics. Electroconductive inks were successfully formulated for screen-printing using carbon fibers (CFs) and multi-walled carbon nanotubes (MWCNTs) as conductive functional material and cellulose derivatives working as binder. The formulated inks were used to fabricate conductive flexible and disposable electrodes on paper-based substrates. Interesting results were obtained after 10 printing passes and drying at RT of the ink with 10 % wt. of pristine CFs and 3% wt. of carboxymethyl cellulose (CMC), exhibiting a resistivity of 1.03 Ωcm and a resolution of 400 μm. Also, a resistivity of 0.57 Ωcm was obtained for only one printing pass using an ink based on 0.5 % wt. MWCNTs and 3 % wt. CMC. It was also demonstrated that ionic conductive cellulose matrix hydrogel can be used in electrolyte-gated transistors (EGTs). The electrolytes revealed a double layer capacitance of 12.10 μFcm-2 and ionic conductivity of 3.56x10-7 Scm-1. EGTs with a planar configuration, using sputtered GIZO as semiconducting layer, reached an ON/OFF ratio of 3.47x105, a VON of 0.2 V and a charge carrier mobility of 2.32 cm2V-1s-1.