Stress analysis at the interface between Ni-P coating and SiCp/Al substrate of space mirror

This paper investigates the mechanical properties at the interface of the coating-substrate system, which comprises the electroless nickel-phosphorus (Ni-P) coating and the aluminum matrix composite substrate reinforced by the silicon carbide particles (SiC_p/Al), and is used for the space mirror. To estimate the adhesion of Ni-P coating on SiC_p/Al substrate, the scratch adhesion testing has been performed by drawing a spherically tipped diamond indenter with a radius of 200 μm over the coated surface. The influence of the coating thickness on the interfacial stress induced by the inertial accelerations, temperature gradients and thermal soaks has been evaluated by simulation analysis based on the finite element method. The results of the scratch testing indicate that the adhesion strength of Ni-P coating to SiC_p/Al composite is more than 3.0 GPa. Compared the maximum value of the interfacial stress obtained by simulation analysis with results of the scratch testing, it is can be seen that the mirror has enough safety margin. Furthermore, the most significant conclusion that can be drawn from this work is that the coating thickness should not exceed 45 μm in order to ensure the performance and reliability of Ni-P coating and SiC_p/Al substrate system for space applications.     

AC conductivity and dielectric behavior of polyaniline/sodium metavenadate (PANI/NaVO3) composites

The conducting polyaniline/sodium metavenadate (PANI/NaVO₃) composites were synthesized by single step in situ polymerization technique by placing finely grinded powder of NaVO₃ during the polymerization of aniline. The formation of mixed phases of the polymer together with the conducting emeraldine salt phase was confirmed by spectroscopic techniques like FTIR. SEM images indicated a systematic morphological variation of particles aggregated in the composite matrix as compared to the pristine PANI. AC conductivity and dielectric behavior of these composites were investigated in the frequency range 50 Hz to 5 MHz. It is found that AC conductivity obeyed the power law index and the variation of conductivity with wt% of NaVO₃ could be related to conductivity relaxation phenomenon. These composites have shown high dielectric constant, which is related to polarization. It is seen that both dielectric constant and dielectric loss decrease with increase in frequency. Variations in measured parameters of AC response with increasing frequency of these composites are found to follow systematic trends that are similar to those observed with temperature and doping.     

Optoelectronic properties of Cu1−xPtxFeO2 (0 ≤ x ≤ 0.05) delafossite for p-type transparent conducting oxide

The samples of Cu_1−xPt_xFeO₂ (0 ≤ x ≤ 0.05) delafossite have been synthesized by solid-state reaction method to investigate their optical and electrical properties. The properties of electrical resistivity and Seebeck coefficient were measured in the high temperature ranging from 300 to 960 K, and the Hall effect and the optical properties were measured at room temperature. The obtained results of Seebeck showed the samples are p-type conductor. The optical properties at room temperature exhibited the samples are transparent visible light material with optical direct gap 3.45 eV. The low electrical resistivity, hole mobility and carrier density at room temperature displayed value ranging from 0.29 to 0.08 Ω cm, 1.8 to 8.6 cm²/V s and 1.56 × 10¹⁸ to 4.04 × 10¹⁹ cm⁻³, respectively. The temperature range for transparent visible light is below 820 K because the direct energy gap contains value above 3.1 eV. Consequently, the Cu_1−xPt_xFeO₂ delafossite enhance performance for materials of p-type transparent conducting oxide (TCO) with low electrical resistivity.     

Fabrication, characterization and measurement of thermal conductivity of Fe3O4 nanofluids

Magnetite Fe₃O₄ nanoparticles were synthesized by a co-precipitation method at different pH values. The products were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and transmission electronic microscopy. Their magnetic properties were evaluated on a vibrating sample magnetometer. The results show that the shape of the particles is cubic and they are superparamagnetic at room temperature. Magnetic nanofluids were prepared by dispersing the Fe₃O₄ nanoparticles in water as a base fluid in the presence of tetramethyl ammonium hydroxide as a dispersant. The thermal conductivity of the nanofluids was measured as a function of volume fraction and temperature. The results show that the thermal conductivity ratio of the nanofluids increases with increase in temperature and volume fraction. The highest enhancement of thermal conductivity was 11.5% in the nanofluid of 3 vol% of nanoparticles at 40 °C. The experimental results were also compared with the theoretical models.     

Hall mobility of amorphous Ge2Sb2Te5

The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge₂Sb₂Te₅ have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm²/V s at room temperature).     

Electrical conductivity of MgCO3 at high pressures and high temperatures

The electrical conductivity of polycrystalline magnesite (MgCO₃) was measured at 3-6 GPa at high temperatures using complex impedance spectroscopy in a multi-anvil high-pressure apparatus. The electrical conductivity increased with increasing pressure. The activation enthalpy calculated in the temperature range 650-1000 K also increased with increasing pressure. The effect of pressure was interpreted as being the activation volume in the Arrhenius equation, and the fitted data gave an activation energy and volume of 1.76±0.03 eV and −3.95±0.78 cm³/mole, respectively. The negative activation volume and relatively large activation energy observed in this study suggests that the hopping of large polarons is the dominant mechanism for the electrical conductivity over the pressure and temperature range investigated.     

Model of electroless Ni deposition on SiCp/Al composites and study of the interfacial interaction of coatings with substrate surface

Metallization techniques based on electroless plating are used to coat SiC_p/Al composite materials. The directly palladium chloride (PdCl₂) solutions in HCl is used to render the surface of such non-conductive substrates catalytically active towards metal deposition in the electroless plating solution. The microstructures of Ni-coated composites provided by scanning electron microscope (SEM) bring light into the palladium activation and electroless coating process. Also, X-ray photoelectron spectroscopy (XPS) and Line-scan have allowed to monitor the chemical and compositional surface modifications of activated and coated SiC_p/Al composites, as well as to understand the mechanisms of the catalyst (palladium species) chemisorption on the composites surface and the interaction mechanisms of Ni layer with the SiC_p/Al composites. The experimental results show that a nickel-substrate bonding action takes place during plating. Ni atom existing on the surface of the composites can partially obtain electrons from metals Al of the SiC_p/Al composites when the substrate is embedded in the Ni layers, that is, the orbital interaction through the mutual overlap of the electronic orbits does exist in the interfacial regions between the coated Ni atoms and composites substrate instead of the mechanical-interlocked form. On the basis of the evidence, a model of electroless Ni deposition on SiC_p/Al composites is submitted including Pd activation and Ni deposition processes to describe the formation of catalytic centers and the growth of deposited layer. The deposition model reveals that metal-substrate bond plays an important role in the high adhesion strength between the Ni coatings and the composites.     

Terahertz time-domain reflection spectroscopy for high-Tc superconducting cuprates

We have developed a terahertz time-domain spectroscopy (THz-TDS) system for reflectivity measurement with a temperature-controllable cryostat. For emission and detection of THz radiation, a Ti:Sapphire pulsed laser and photoconductive antennas are used. Two wire-grid polarizers enable us to carry out the polarized reflectivity measurements. Using our THz-TDS system, we measured the c-axis polarized reflectivity spectra for La_2−xSr_xCuO₄ single crystals (x = 0.10 and 0.13) and observed sharp Josephson plasma edges for both samples below T_c. The reflectivity spectra of the x = 0.10 sample were in good agreement with those reported previously, which confirms the validity of the system. For the x = 0.13 sample, we discuss the anomolous features of the optical spectra.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Thermoelectric properties of SnzCo8Sb24 skutterudites

Sn-filled CoSb₃ skutterudite compounds were synthesized by the induction melting process. Formation of a single δ-phase of the synthesized materials was confirmed by X-ray diffraction analysis. The temperature dependences of the Seebeck coefficient, electrical resistivity and thermal conductivity were examined in the temperature range of 300-700 K. Positive Seebeck and Hall coefficients confirmed p-type conductivity. Electrical resistivity increased with increasing temperature, which shows that the Sn-filled CoSb₃ skutterudite is a degenerate semiconductor. The thermal conductivity was reduced by Sn-filling because the filler atoms acted as phonon scattering centers in the skutterudite lattice. The lowest thermal conductivity was achieved in the composition of Sn_0.25Co₈Sb₂₄.     

The effect of titanium diboride addition on the thermoelectric properties of β-FeSi2 semiconductors

β-FeSi₂-TiB₂ composites with various amounts of TiB₂, from 0 up to 30 vol%, were prepared by hot pressing. The electrical and thermal conductivities, and the Seebeck coefficient were measured as a function of temperature. The results show that the thermal and electrical transport behavior of the composites is different as the volume fraction of TiB₂ is below and above about 0.255. A 5 vol% TiB₂ added sample has higher figure of merit than one without TiB₂ for temperatures above 650 K. The influence of an additional phase, ε-FeSi, formed during the hot pressing, on the thermoelectric properties of the β-FeSi₂-TiB₂ composites was also discussed.     

Structure and electrical properties of Bi5GexSe95−x thin films

Bi₅Ge_xSe_95−x (30, 35, 40 and 45 at.%) thin films of thickness 200 nm were prepared on glass substrates by the thermal evaporation technique. The influence of composition and annealing temperature, on the structural and electrical properties of Bi₅Ge_xSe_95−x films was investigated systematically using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX). The XRD patterns showed that the as-prepared films were amorphous in nature with few tiny crystalline peaks of relatively low intensity for 30 and 45 at.% and the Bi₅Ge₄₀Se₅₅ annealed film was polycrystalline. The chemical composition of the Bi₅Ge₃₀Se₆₅ film has been checked using energy dispersive X-ray spectroscopy (EDX). The electrical conductivity was measured in the temperature range 300-430 K for the studied compositions. The effect of composition on the activation energy (ΔE) and the density of localized states at the Fermi level N(E_F) were studied, moreover the electrical conductivity was found to increase with increasing the annealing temperature and the activation energy was found to decrease with increasing the annealing temperature. The results were discussed on the basis of amorphous-crystalline transformations.     

Electrical relaxation in proton conductor composites based on (NH4)H2PO4/TiO2

We report on electrical relaxation measurements of (1 − x)NH₄H₂PO₄-xTiO₂ (x = 0.1) composites by admittance spectroscopy, in the 40-Hz–5-MHz frequency range and at temperatures between 303 and 563 K. Simultaneous thermal and electrical measurements on the composites identify a stable crystalline phase between 373 and 463 K. The real part of the conductivity, σ’, shows a power-law frequency dependence below 523 K, which is well described by Jonscher’s expression , where σ ₀ is the dc conductivity, ω _p /2π = f _p is a characteristic relaxation frequency, and n is a fractional exponent between 0 and 1. Both σ ₀ and f _p are thermally activated with nearly the same activation energy in the II region, indicating that the dispersive conductivity originates from the migration of protons. However, activation energies decrease from 0.55 to 0.35 eV and n increases toward 1.0, as the concentration of TiO₂ nanoparticles increases, thus, enhancing cooperative correlation among moving ions. The highest dc conductivity is obtained for the composite x = 0.05 concentration, with values above room temperature about three orders of magnitude higher than that of crystalline NH₄H₂PO₄ (ADP), reaching values on the order of 0.1 (Ω cm)^− 1 above 543 K.     

Electronic properties of carbon nanotube/fabric composites

Single walled carbon nanotube (SWNT)/fabric composite materials were manufactured using two simple manufacturing processes. The first method is direct deposition of SWNTs by either a spray method or by incubation; the other is a Quasi-Langmuir–Blodgett (QLB) transfer technique. The composite retains high mechanical strength (governed by the fabric), and good electrical properties (determined by the nanotubes). We measure the DC electrical conductivity of the composite fabric to be 5.33 S/cm for the sprayed tubes, 13.8 S/cm for the incubated SWNTs, and 8 S/cm for the QLB transferred tubes; these values are limited not by the nanotube network, but by the surface roughness of the fabric itself. Measurements of the conductivity up to 1 MHz reveal a transport process that proceeds along a random network, with barriers separating the various nanotubes. The material is resistive both to changes in temperature (range of 0–80 °C) and mechanical deformations. The conductivity of the composite decreases by less than 10% when bent around a cylinder of 1 cm diameter.     

Effect of composition and thermal annealing on the absorption and electrical conduction of Se85−xTe15Sbx glasses

The optical absorption of the as-prepared and thermally annealed Se_85−xTe₁₅Sb_x (0≤x≤9) thin films was measured. The mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap (E₀) decreased from 1.12 to 0.84 eV with increasing Sb content of the as-prepared films from 0 to 9 at.%. The as-prepared Se₇₆Te₁₅Sb₉ films showed an increase in (E₀) with increasing the temperature of annealing in the range above T_g (363 K). The electrical conductivity of the as-prepared and annealed films was found to be of Arrhenius type with temperature in the range 300-360 K. The activation energy for conduction was found to decrease with increasing both the Sb content and temperature of annealing. The results were discussed on the basis of the lone-pair electron effect and of amorphous crystalline transformation.     

Study of DC conductivity and relative magnetic permeability of nanoparticle NiZnFe2O4/PPy composites

The DC conductivity and the relative magnetic permeability have been measured as functions of temperature for five powder samples of nanoparticle ferrites (Ni_xZn_1−xFe₂O₄; x=0, 0.25, 0.5, 0.75 and 1), a pure polypyrrole (PPy) powder sample and many composite samples prepared by mixing different ratios of the ferrites to PPy. By comparing the results it is found that there is an obvious increase in DC conductivity of the ferrite/PPy composite samples compared to the corresponding pure ferrite samples, whereas compared to the pure PPy sample there is a decrease in DC conductivity. On the contrary, the magnetic permeability of the composites is higher than that of the pure PPy sample and lower than that of the pure ferrite samples as was expected.     

The temperature dependence of internal friction in SicPCS/Al composites

In this study the relationship between Q^-1 and the microstructural characteristics of SiC_PCS/Al composites was investigated. Seven specimens made by various processes were divided into three groups and their Q^-1-T spectra were compared. The temperature dependence of internal friction up to 500°C was measured by vacuum inverse torsional pendulum apparatus. For the SiC_PCS/Al composite fabricated by the liquid infiltration method, Q^-1 increased more and more rapidly with rising temperature when the extent of fiber-matrix interfacial reaction intensified. For hot-pressed plates fabricated from preform wires and Al(1050) foils, poor diffusion bonding between the preform wires and A1 foils was introduced; this new interface is primarily responsible for the temperature dependence of Q^-1.     

Metal-polymer PVDF/nickel composites and evaluation of their dielectric and thermal properties

Dielectric properties, thermal stability and sub-molecular structure of metal-polymer composites on the basis of polyvinylidene fluoride (PVDF) modified by sub-micro sized nickel powder were studied in this work by different methods such as dielectric spectroscopy, resistivity measurement, breakdown strength measurement, differential scanning calorimetry and optical microscopy. It was found that modification of PVDF by nickel particles results in the increase of both the volume resistivity by a factor of 1.24…15 and the melting and decomposition temperature by 5 °C and 48 °C for composite with an optimal nickel content compared with non-modified PVDF.     

Microwave absorption properties of core-shell structured FeCoNi@PMMA filled in composites

FeCoNi-coated PMMA (FeCoNi@PMMA) core-shell structures prepared by an electroless plating process were investigated in this study. Further, we fabricated FeCoNi@PMMA in polydimethylsiloxane sheets with volume fractions of fillers from 10 vol% to 25 vol% to evaluate electromagnetic wave absorption performance. Subsequently, results were compared with those of carbonyl iron particles (CIP). The maximum reflection loss (RL) of FeCoNi@PMMA composite (25 vol%) reached a value of −37.2 dB at 12.2 GHz with a thickness of 1.5 mm and a broad absorption bandwidth (RL ≤ −10 dB) of 4.5 GHz, attributed to strong dielectric and magnetic losses. Though the microwave absorption performance of the core-shell structured FeCoNi@PMMA composites is similar to that of CIP composites, the weight fraction of FeCoNi@PMMA is reduced about 38% compared with CIP with the same volume fraction (25 vol%). These results indicate that FeCoNi@PMMA is a lightweight material and a good candidate for high-performance microwave-absorbing devices.     

Low temperature electrical conductivity and magnetoconductivity of Fe39Ni39Mo4Si6B12 and Co58Ni10Fe5Si11B16 metallic glass alloys

A detailed study on the weak localization phenomenon vis-a-vis electron-electron interaction effects in magnetic metallic glasses has been carried out. We measured the electrical conductivity and magnetoconductivity within the temperature range 1.8≤T≤300K. A maximum on the conductivity versus temperature curve exists atT=T _m. The conductivity was observed to follow aT ^1/2 law forT<T _m andT ² law forT>T _m. Magnetoconductivity data of these alloys indicate the prominence of electron-electron interaction at low temperatures. The authors have determined the inelastic scattering field and spin-orbit scattering field from the magnetoconductivity data. The inelastic scattering field obeys aT ^p law (p=2) at low temperatures.