Dielectric and magnetic response of Fe3O4/epoxy composites

Magnetic and dielectric properties of Fe₃O₄/epoxy resin composites were studied as a function of Fe₃O₄ concentration. The Fe₃O₄ powder was milled using a planetary ball-mill in order to reduce the particle size. B.E.T. area of these particles was determined, and a structural characterization was performed by X-ray diffraction (XRD). Fe₃O₄/epoxy composites were prepared mixing the raw materials and pouring them into suitable moulds. Dielectric measurements were performed at different frequencies and temperatures, while magnetic properties were assessed at different temperatures. It was found that permittivity was strongly dependent on the filler concentration and frequency. Maxwell–Wagner–Sillars interfacial polarization, Intermediate Dipolar Polarization (IDE), and α relaxation process were responsible for the observed behavior. Magnetic measurements revealed the presence of magnetite nanoparticles in the composites, with a blocking temperature close to 170 K.     

Preparation of TiC reinforced steel composites and their characterisation

Abstract

Carbide reinforced steel composites are useful in extensive wear resistance applications. Titanium carbide reinforced steel composites have been prepared by dissolving a TiC rich Fe–TiC master alloy in a liquid steel. The composites have been characterised by optical microscopy, energy dispersive X-ray scanning electron microscope analysis, image analysis, and X-ray diffraction studies. Tensile strength measurements showed that the ultimate tensile strengths varied between 790 and 880 MPa for composites containing 0·7–0·34 wt-%Ti. Some composites show better wear resistance properties in comparison with low alloy steels.     

Thermal conductivity of diamond/Ag composites with chromium carbide coated diamonds for the building materials of high power modules

Abstract

This paper reports on a study of the preparation and characterisation of diamond/Ag composites for the building materials of high power modules. The Cr₇C₃ coated diamond particles are utilised to improve the interfacial bonding between the Ag and diamond and composites are prepared by hot pressing technique. The characteristics of Cr₇C₃ coating layers were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the Cr₇C₃ coatings on the diamonds result in a strong interfacial bonding and a greatly enhanced thermal conductivity of the composites. A largely enhanced thermal conductivity of 768 W m^?1 K^?1 is obtained in Cr₇C₃ coated composites, which increases 168% relative to that of uncoated composites at 65% diamond volume fraction. The measured thermal conductivity agrees reasonably well with the predictions by a differential effective-medium (DEM) model.     

Texture of Metal-Matrix Composites by Ultrasonic Velocity Measurements

Abstract

An ultrasonic method is developed for the nondestructive characterization of texture in metal-matrix composites. In this approach, it is assumed that the presence of reinforcement particles changes the elastic properties of the composite but only the texture of the matrix. The method utilizes the measurements of the six independent ultrasonic velocities V _ij and the formulation given by Bunge. The examined composites are the silicon carbide (SiC)-particle-reinforced aluminum 8091, 7064, and 6061 metal-matrix composites. The fourth-order expansion coefficients of the orientation distribution function are determined as a function of the SiC content in these composites. The results show that the expansion coefficients change with the presence of SiC where the coefficients C₄ ¹¹ and C₄ ¹³ increase as the volume fraction of SiC is increased and the coefficient C₄ ¹² is zero in all composites examined. The analysis of these results indicates that ultrasonics can provide a promising technique for the texture characterization of metal-matrix composites.     

Investigation of thermal properties of Al–Si matrix reinforced fine SiCp composites

Abstract

The characterisation of thermal expansion coefficient and thermal conductivity of Al–Si matrix alloy and Al–Si alloy reinforced with fine SiC_p (5 and 20 wt-%) composites fabricated by stir casting process are investigated. The results show that with increasing temperature up to 350°C, thermal expansion of composites increases and slowly reduces when the temperature reaches to 500°C. The values of both thermal expansion and conductivity of composites are less than those for Al–Si matrix. Microstructure and particles/matrix interface properties play an important role in the thermal properties of composites. Thermal properties of composites are strongly dependent on the weight percentage of SiC_p.     

Humidity sensing behaviour of polyaniline/magnesium chromate (MgCrO<Subscript><Emphasis Type="Bold">4</Emphasis></Subscript>) composite

‘In situ’ polymerization of polyaniline (PANI) was carried out in the presence of magnesium chromate (MgCrO ₄ ) to synthesize PANI/ceramic (MgCrO ₄ ) composite. These prepared composites were characterized by XRD, FTIR and SEM, which confirm the presence of MgCrO ₄ in polyaniline matrix. The temperature dependent conductivity measurement shows the thermally activated exponential behaviour of PANI / MgCrO ₄ composites. The decrease in electrical resistance was observed when the polymer composites were exposed to the broad range of relative humidity (ranging between 20 and 95% RH). This decrease is due to increase in surface electrical conductivity resulting from moisture absorption and due to capillary condensation of water causing change in conductivity within the sensing materials. PANI / MgCrO ₄ composites are found to be sensitive to low humidity ranging from 20 to 50 % RH.     

Influence of heat treatment and particle shape on mechanical properties of infiltrated Al2O3 particle reinforced Al-2 wt-%Cu

Abstract

Al-2 wt-%Cu composites were produced by gas pressure infiltration of powder beds with a high volume fraction (45 to 60 vol.-%) of angular or polygonal alumina particles. The tensile behaviour and fracture toughness of the composites were characterised in as cast, solutionised and peak aged (T6) conditions. It was shown that coarse intermetallics that are formed during solidification and located preferentially at the particle/matrix interface lead to lower toughness compared with the same composites in solutionised and T6 conditions. The particle nature and shape exert a strong influence on the properties of the composites: polygonal particles are intrinsically stronger than angular particles and yield stronger, tougher, and more ductile composites. Composite toughness variations are explained in terms of fracture micromechanisms.     

Effect of C/C preform density on microstructure and mechanical properties of C/C–SiC composites prepared by alloyed reactive melt infiltration

Abstract

Low cost C/C–SiC composites were prepared by alloyed reactive melt infiltration. Effects of the density of C/C preforms on mechanical properties and microstructure of the C/C–SiC composites are reviewed. The results show that with increasing the density of C/C preforms, the flexural strength of the resulting composites increases, while the density of the composites decreases. The flexural strength can reach 341 MPa for the composite produced from the C/C preform of 1·3 g cm^?3. The phases in the composites produced from low density C/C preforms are Si, SiC, ZrSi₂ and carbon, while no Si phase is found in the composites with high density C/C preforms. Furthermore, the mechanism of the microstructure evolution of the C/C–SiC composites is proposed.     

The relationship between sensitisation and small punch test behaviour in a high-nitrogen austenitic stainless steel at cryogenic temperatures

Austenitic stainless components used in nuclear fusion reactors must be capable of maintaining reasonable mechanical properties to thermal ageing caused by welding and in‐service. Recently, high‐nitrogen (High‐N) austenitic stainless steels (SS) are receiving increased attention because of their strength advantages, but they have been found to be susceptible to dichromium nitride (Cr₂N) precipitation during thermal exposure at 823–1073 K. The susceptibility to sensitisation at thermal ageing temperature for high‐N austenitic SS is examined using the single‐loop electrochemical potentiokinetic reactivation (EPR) test. High‐N SS were found to be susceptible to sensitisation caused by grain boundary precipitation of Cr₂N, with the degree of sensitisation increasing systematically with ageing time and temperature. In particular, it was found that the precipitates, which effected sensitisation, were changed from carbides (M₂₃C₆) to nitrides (Cr₂N) with increasing ageing time and temperature. The deterioration of mechanical properties associated with thermal ageing in high‐N SS was investigated by a small punch (SP) test using miniature specimens at cryogenic temperatures. Results indicated that the degradation of mechanical properties in this alloy was caused by a decrease of cohesive strength resulting from carbides (Cr₂₃C₆) and nitrides (Cr₂N) precipitated in grain boundaries.     

Continuous observation of microstructural degradation during tensile loading of particle reinforced aluminium matrix composites

Abstract

The microstructural degradation of aluminium alloy composites by external tensile loading was continuously observed by in situ scanning electron microscopy tensile testing. The composites, which contained spherical Al₂O₃ and angular SiC particles, were prepared by the powder extrusion method. Some microcracks were initiated at small plastic strains after yielding in both composites by inteliace debonding and particle cracking. Angular particles generate microvoids at smaller strains than spherical particles. The microcracks do not propagate with increasing external loading because of the ductility of the matrix, but a number of new microcracks developed just before failure. Most microcracks are due to interface debonding rather than particle cracking. Many dimples on the matrix aluminium alloy are observed on some particles seen in the fracture surface, which proves that there is relatively strong bonding between the matrix and the particles in the composite produced by powder extrusion. However, part of the original surface observed on the debonded particles indicates that an incompletely bonded interface also exists in the composites.

MST /3111     

Dielectric, ferromagnetic and ferroelectric properties of the (1 ? x)Ba0.8Sr0.2TiO3–xCoFe2O4 multiferroic particulate ceramic composites

The (1 − x)Ba_0.8Sr_0.2TiO₃–xCoFe₂O₄ ceramic composites (x = 0–1) were prepared by standard solid state reaction method. X-ray diffraction and SEM indicate the Ba_0.8Sr_0.2TiO₃ (BST) phase and CoFe₂O₄ (CFO) phase coexist in the composites. The dielectric constant and dielectric loss for the composites were studied as a function of frequency (40 Hz–1 MHz) and temperature (30–600 °C). Magnetic and ferroelectric tests show that the ceramic composites display ferromagnetic and ferroelectric properties simultaneously. The saturated polarization of the composites decrease with ferrite concentration increasing, while the remnant polarization of the composites increase with increasing ferrite concentration. The enhanced ferroelectricity of composites may be attributed to space charge contribution in the composites.     

A simulation study on the effect of spring-shaped fillers on the viscoelasticity of rubber nanocomposite

Background/PurposeRubber nanocomposites have been widely used in many engineering fields due to their unique properties such as high elasticity and viscoelasticity. Much attention has been paid to the viscoelasticity of rubbers because it directly relates to the performance of the rubber products.MethodsBased on the micromechanical theory, the finite element method is used to analyze the effect of elastic modulus and volume content of spring-shape nanofillers on the dynamic viscosity of composites.ResultsThe simulation results show that there is an optimal elastic modulus of spring-shape nanofillers to make the loss factor a minimum. There is a threshold value of spring-shape nanofiller content for the dissipation energy density of composite.ConclusionThe elastic modulus of spring-shape nanofillers has a large effect on the loss factor of composites. The selection of elastic modulus of spring-shape nanofillers is critical for applications of composites. The efficiency of spring-shape nanofillers in reducing the dynamic viscosity of composites is so high that volume content of spring-shape nanofillers as low as 0.1% can greatly reduce the loss factor of composites with bonding interface.     

One-step facile fabrication of Ag/γ-Fe2O3 composite microspheres

Ag/γ-Fe₂O₃ composite microspheres were successfully prepared via a simple solvothermal reduction method under mild conditions. Electron and X-ray diffraction data revealed that these composites consisted of silver and maghemite. Through the optimization of processing conditions, Ag/γ-Fe₂O₃ composites with a spherical shape were successfully produced. The results from the transmission and scanning electron microscopy revealed that the composites were spherical with a diameter in the range of 200–300 nm. Magnetic measurements showed that the mixed microspheres exhibited a typical ferromagnetic behavior, a specific saturation magnetization of 56 emu/g and an intrinsic coercivity of 38 Oe at room temperature. The presence of Ag nanoparticles dispersed into γ-Fe₂O₃ microspheres was also confirmed by UV–Vis absorption. These composites with microspherical morphologies can be applied in a variety of areas, including catalysis, medicine, photonics, and new functional device assemblies.     

Tensile properties of as fabricated,aged, and stretched SiC whisker and particle reinforced 2009 aluminium alloy

Abstract

Tensile tests were carried out using specimens of 2009 aluminium alloy reinforced by either SiC whiskers or particles. The size distributions of the whiskers and particles in the matrix were obtained by image analysis. It was found that failure was a result of uniform void nucleation and coalescence in the as fabricated composites, or a result of fast crack propagation initiated by a flaw developed at clusters of SiC in the aged or stretched and aged composites. The strengths of the as fabricated composites were estimated based on the results of image analysis using continuum mechanics and dislocation theories. The estimation indicated that the tensile strengths are largely contributed to by composite strengthening, supplemented by residual dislocation strengthening and work hardening. Owing to the flaw controlled failure, the tensile strengths of the aged or stretched and aged composites were independent of aging time, aging temperature, and the amount of stretching. The elastic moduli of the composites were estimated using the Halpin–Tsai model and a good correlation was found between the measured and estimated moduli.

MST/3438     

Wear behaviour of aluminium matrix TiB2 composite prepared by in situ processing

Abstract

The wear behaviour and microstructure of aluminium and Al-12Si alloy (A413) matrix composites containing 1 and 5 vol.-%TiB₂ particles have been investigated. The composites were prepared by an in situ reactive slag technique. The wear surfaces and wear products were studied after reciprocating and rolling - sliding tests. Wear resistance increased with increasing particle content, and the Al-12Si composites were more wear resistant than those with Al matrixes. The wear mechanisms are briefly discussed.     

Dielectric properties of piezoelectric 3-0 composites of lithium ferrite/barium titanate

Piezoelectric 3-0 composite ceramics are prepared from a mixture of barium titanate and lithium ferrite phase constituents. Dielectric properties of composites are affected by a number of parameters that include electrical properties, size, shape and amount of constituent phases. The frequency dependent measurements can provide additional insight into mechanisms controlling electrical response. Frequency dependence of dielectric constant plots of lithium ferrite/barium titanate composites will be given and the relevance of trends seen in them will be discussed. Connectivity in composites developed is studied.     

Synthesis, characterization and conductivity studies of polypyrrole-fly ash composites

In situ polymerization of pyrrole was carried out in the presence of fly ash (FA) to synthesize polypyrrole-fly ash composites (PPy/FA) by chemical oxidation method. The PPy/FA composites have been synthesized with various compositions (10, 20, 30, 40 and 50 wt%) of fly ash in pyrrole. The surface morphology of these composites was studied with scanning electron micrograph (SEM). The polypyrrole-fly ash composites were also characterized by employing X-ray diffractometry (XRD) and infrared spectroscopy (IR). The a.c. conductivity behaviour has been investigated in the frequency range 10²–10⁶ Hz. The d.c. conductivity was studied in the temperature range from 40–200°C. The dimensions of fly ash in the matrix have a greater influence on the observed conductivity values. The results obtained for these composites are of greater scientific and technological interest.     

Magnetic levitation above the superconducting YBa2Cu3O7−x /polymer 0-3 and 2-2 composites

Superconducting YBa₂Cu₃O_7–x /polyurethane 0-3 composites have been fabricated with several volume fraction fillers. Magnetic forces between a small permanent magnet and the composites have been measured at 77 K and show hysteretic behaviour as a function of their separation distances. Such behaviour indicates multiple equilibrium heights of the magnet levitating above the composites. It is shown that the levitation height increases with increasing volume fraction of superconductor filler. The parallel YBa₂Cu₃O_7–x /polyurethane and series YBa₂Cu₃O_7–x /polypropylene 2-2 composites were also prepared and only tested for magnetic levitation.     

Synthesis, characterization and a.c. conductivity of polypyrrole/Y2O3 composites

Conducting polymer composites of polypyrrole/yttrium oxide (PPy/Y₂O₃) were synthesized byin situ polymerization of pyrrole with Y₂O₃ using FeCl₃ as an oxidant. The Y₂O₃ is varied in five different weight percentages of PPy in PPy/Y₂O₃ composites. The synthesized polymer composites are characterized by infrared and X-ray diffraction techniques. The surface morphology of the composite is studied by scanning electron microscopy. The glass transition temperature of the polymer and its composite is discussed by DSC. Electrical conductivity of the compressed pellets depends on the concentration of Y₂O₃ in PPy. The frequency dependent a.c. conductivity reveals that the Y₂O₃ concentration in PPy is responsible for the variation of conductivity of the composites. Frequency dependent dielectric constant at room temperature for different composites are due to interfacial space charge (Maxwell Wagner) polarization leading to the large value of dielectric constant. Frequency dependent dielectric loss, as well as variation of dielectric loss as a function of mass percentage of Y₂O₃ is also presented and discussed.     

Microstructure and mechanical properties of SiC Whisker reinforced ZrO2–Y2O3 composites

Abstract

The mechanical properties of 20 vol.-%SiC whisker reinforced ZrO₂?V₂O₃ composites containing 2 and 6 mol.-% Y₂O₃ were measured at room temperature and the fracture surface was examined. The results indicate that the mechanical behaviour of the composites is strongly influenced by the Y₂O₃ content. The magnitude of the enhancement of the toughness in composites containing 2 mol.-% Y₂O₃ compared with unreinforced ZrO₂?Y₂O₃ matrix is larger than that for the composites containing 6 mol.-% Y₂O₃. Crack propagation modes were characterised by crack deflection, whisker bridging, and whisker pullout. High resolution electron microscopic observations show that in composites containing 2 mol.-% Y₂O₃ the whiskers are directly bonded to the matrix. However, in composites containing 6 mol.-% Y₂O₃ there is always a thick amorphous layer at the interface, indicating that the high Y₂O₃ content has promoted the formation of interfacial amorphous layers. These interfacial amorphous layers strengthen the interfacial bonding, resulting in a composite with a low fracture toughness.

MST/2043