Two-phase Nanocomposites Research Articles

Electrical properties of ceria/carbonate nanocomposites

Ceria-based composites are developed as potential electrolytes for intermediate solid oxide fuel cell applications and a better understanding of the ionic conduction mechanism serves this purpose. Ceria-based composites were produced by several processing routes using a ceria-based ceramic host (Sm, Gd doped ceria) and various Na and Li carbonates. Simple ceria/carbonate two-phase nanocomposites were synthesized by the carbonate precipitation method followed by a thermal treatment. The decomposition course of the precursor, the thermal stability, morphology and the composite formation mechanism were studied by thermogravimetric–differential thermal analyses (TG–DTA), IR, UV, SEM and XRD analyses. The conductivity temperature dependence in the 300–800°C temperature range, performed on composite pellets, exhibits conductivity values one-two higher magnitude order and a lower activation energy of 0.809±0.001eV, with respect to 1.693±0.005eV for pure ceria obtained under the same conditions. This enhancement of the composite conductivity, corroborated with the blue shift of the gap energy, as deduced from the UV spectra, was attributed to the oxygen ion transport through an interface mechanism between the two phases of the ceria/carbonate composite.

Isothermal entropy changes in nanocomposite Co:Ni67Cu33

The temperature-dependent magnetic properties of artificial rare-earth, free-magnetic nanostructures are investigated for magnetic cooling. We consider two-phase nanocomposites, where 2 nm nanoclusters of cobalt are embedded in a Ni67Cu33 matrix. Several composite films were produced by cluster deposition. The average Co nanocluster size can be tuned by varying the deposition conditions. Isothermal magnetization curves were measured at various temperatures 150 K < T < 340 K in steps of 10 K. The isothermal entropy changes ΔS were calculated using the Maxwell relation. The entropy changes measured were, –ΔS = 0.15 J/kg·K in a field change of 1 T at 260 K and 0.72 J/kg·K in a field change of 7 T at 270 K.

Doped zirconia and ceria-based electrolytes for solid oxide fuel cells: a review

Zirconia and ceria possess fluorite-type structure and conduct electricity by means of oxide ion transport through the lattice. On doping, vacancies created at the oxygen site make way for the neighbouring oxygen atoms to ‘hop’ in the direction of electric field. Among doped zirconia, 8 mol% yttria-stabilized zirconia and 9–11 mol% scandia-stabilized zirconia exhibit the highest conductivity. Ceria electrolytes require lower operational temperature (~600–800°C) compared to that of zirconia electrolytes (800–1000°C). With improvement in the processing and fabrication techniques, researchers have developed thinner electrolytes to minimize ohmic polarization and enhance the ionic conductivity enabling operation at lower temperatures (~400–600°C). But such electrolytes are required to be supported on electrodes (anode or cathode) and in later times heterostructured bi-, tri- and multi-layered electrolyte films have been constructed. Since the last decade, development of submicron grain size electrolytes and ultimately two-phase materials with nanodimension grain size has shown improvement in ionic conductivity as well as low-temperature workability. The article reviews the turning points in the technological development of fluorite-based solid oxide fuel cell electrolytes from single-phase micrometer dimension grain size to recently developed two-phase nanocomposites and nanowires, and the successful achievement of its workability at low temperatures (~450°C).

Nanostructure model of thermal conductivity for high thermoelectric performance

The effective medium theory of thermal conductivity of two-phase composites studied by Nan et al. has been extended to investigate concentrated nanocomposites. Due to the presence of inter-particle phonon scattering processes in concentrated nanocomposites, the effective lattice thermal conductivity keff varies more rapidly with the volume fraction of second-phase inclusions in the composite. Applying the new keff expressions to monolithic material systems, the results are found to capture the experimental trend of monolithic nanostructured materials. In particular, it is noted that the dimensionless figure of merit, ZT, is nearly doubled by only reducing the lattice thermal conductivity. Two-phase nanocomposites have also been evaluated, demonstrating that these latter systems are very suited for high thermoelectric performance. Present study leads to several strategies for obtaining ZT ∼ 2 or higher in nanocomposites.

Nanocomposites for Advanced Fuel Cell Technology

NANOCOFC (Nanocomposites for advanced fuel cell technology) is a research platform/network established based on the FP6 EC-China project www.nanocofc.org. This paper reviews major achievements on two-phase nanocomposites for advanced low temperature (300-600 degrees C) solid oxide fuel cells (SOFCs), where the ceria-salt and ceria-oxide composites are common. A typical functional nanocomposite structure is a core-shell type, in which the ceria forms a core and the salt or another oxide form the shell layer. Both of them are in the nano-scale and the functional components. The high resolution TEM analysis has proven a clear interface in the ceria-based two-phase nanocomposites. Such interface and interfacial function has resulted in superionic conductivity, above 0.1 S/cm at around 300 degrees C, being comparable to that of conventional SOFC YSZ at 1000 degrees C. Against conventional material design from the structure the advanced nanocomposites are designed by non-structure factors, i.e., the interfaces, and by creating interfacial functionalities between the two constituent phases. These new functional materials show indeed a breakthrough in the SOFC materials with great potential.

A hybrid averaging approach to predict overall properties of nanocomposites

A new method is proposed to consider the characterizations of the fillers in overall properties of heterogeneous media, and in particular, nanocomposites. According to this method, named ‘Hybrid averaging,’ the effective properties of a general nanocomposite containing several phases and orientations can be found using the results from a number of simple unidirectional two-phase nanocomposites. The results for the simple media might be obtained from any roots including analytical approaches, numerical simulations, and experimental analyses. In this article, M—T as the analytical and FE as the numerical approaches are combined with the proposed hybrid method. To perform FE analysis, RRVE with periodic boundary conditions is employed. In addition, the influential parameters consisting of RRVE size and mesh density on the numerical results are justified by satisfying certain criteria. Numerical investigations are then done for two case studies of nanocomposites in which boron nitride nanotubes, as fillers, are distributed using two different functions. Furthermore, comparisons are made between the results obtained from different approaches for these materials.

Magnetic entropy changes in nanogranular Fe:Ni61Cu39

Artificial environment-friendly Gd-free magnetic nanostructures for magnetic cooling are investigated by temperature-dependent magnetic measurements. We consider two-phase nanocomposites where nanoclusters (Fe) are embedded in a Ni61Cu39 matrix. Several composite films are produced by cluster deposition. The average Fe cluster size depends on the deposition conditions and can be tuned by varying the deposition conditions. The quasiequilibrium Curie temperature of the Fe particles is high, but slightly lower than that of bulk Fe due to finite-size effects. Our experiments have focused on ensembles of 7.7 nm Fe clusters in a matrix with a composition close to Ni61Cu39, which has a TC of 180 K. The materials are magnetically soft, with coercivities of order 16 Oe even at relatively low temperature of 100 K. The entropy changes are modest, −ΔS = 0.05 J/kg K in a field change of 1 T and 0.30 J/kg K in a field change of 7 T at a temperature of 180 K, which should improve if the cluster size is reduced.

Synthesis of Bulk Nanocrystalline Materials and Bulk Metallic Glasses by Repeated Cold Rolling and Folding (RCR)

Repeated cold rolling with intermediate folding (RCR) represents a technique to obtain severe plastic deformation that avoids excessive heating at the internal interfaces and that proceeds without the simultaneous action of a high pressure in the range of several GPa. Aside from the opportunity to obtain amorphous bulk samples, the processing pathway also allows for synthesizing dense, bulk nanocrystalline materials. The sequential combination of different processing routes that drive a material to a different extent -, with different rates - and by different means from thermodynamic equilibrium present new and attractive processing opportunities to obtain bulk nanocrystalline or massive ultrafine grained materials that are widely unexplored. Here, an overview is presented concerning the sequential application of different deformation methods with largely different strain and pressure levels. The basic underlying mechanisms that can lead to ultrafine grained or nanocrystalline microstructures for pure metals or to two-phase nanocomposites or bulk metallic glasses for alloys are discussed and the current state of nanostructure control is highlighted by selected examples.

Novel approach to measuring interface stress of phase boundaries: The case of Ag/Ni

Based on the generalization of a capillary equation for solids, we develop a method for measuring the absolute value of phase-boundary stress in a particulate two-phase nanocomposite. Extracting interface stress requires precise measurement of residual-strain-free lattice parameters and determination of specific interfacial areas as well as volume fractions of phases. We find a value of 3.5 ± 0.2 N m −1 for the isotropic interface stress of the Ag/Ni phase boundary.

Spin wave spectroscopy and microwave losses in granular two-phase magnetic nanocomposites

We investigate the composition dependence of microwave properties of a series of cold-pressed powder compacts prepared from nanoparticles of ZnO, Ni, Co, and γ-Fe2O3 using the microstrip line method and spin wave spectroscopy (SWS). The microwave spectra of these magnetic nanocomposites (NCs) are found to possess a double-peak behavior in the losses over the 2–16GHz frequency range. The observed effect is most likely due to oxygen-containg species that were adsorbed at the surface of the NC leading to core/shell structured nanoparticles. The relative change of the SW group velocity induced by the samples, probed by SWS, is observed to depend significantly on the chemical composition and volume fraction of magnetic species contained in the NC. It is argued that the peaks in the losses have a magnetic character and are due to spin excitations of magnetic nanoparticles. Combined, the microwave characteristics of NCs are strongly influenced by the nature of the magnetic species and reveal opportunities for efficient nanomaterials in the realm of microwave magnetoelectric devices.

Electromagnetism and magnetization in granular two-phase nanocomposites: A comparative microwave study

Cold-pressed powder compacts in our experiments were prepared from commercial nanopowders of ZnO, Ni, Co and γ-Fe2O3. A systematic study of the room temperature effective permeability tensor of composite samples made of these nanophases is performed and provides a signature for the nonreciprocity of wave propagation in these nanostructures. Our measurements which cover a broad range of frequency in the microwave region provide a wealth of information leading to a much better understanding of the electromagnetic wave transport in nanogranular materials throughout this frequency range. We report our observations on the frequency and composition dependences of the permeability tensor components of a large set of nanocomposites (NCs) at different magnetic fields. It is found that mixing Ni nanoparticles with ZnO nanoparticles results in a smaller linewidth of the gyromagnetic resonance and an increased coercivity compared to a sample consisting solely of Ni nanoparticles. On the contrary, mixing of Co nanoparticles with ZnO nanoparticles resulted in the disappearance of the off-diagonal component of the permeability tensor and an increase in coercivity. Deviations of the saturation magnetization of Ni and Co in the Ni∕ZnO and Co∕ZnO NCs from bulklike values were observed. It is believed that the different microwave magnetic behaviors of the Ni∕ZnO and Co∕ZnO NCs are related to the difference in magnetic anisotropy of the Ni and Co particles. It is argued that surface and boundaries in the samples can play a significant role in the microwave magnetic response of these nanostructures. These NCs are promising for implementing the nonreciprocal functionality employed in many microwave devices, including isolators and circulators.

Effect of Mini Ga Doping in Nanocrystalline Nd<sub>4.5</sub>(Fe,Co)<sub>77.5</sub>B<sub>18</sub> Magnet on Coercivity

The microstructure and the coercivity of nanocomposite Nd4.5Fe76.5-xGaxCo1.0B18 (x=0~0.6at%)bonded magnets were studied. The results showed that addition of small amount of Ga in Nd4.5Fe76.5-xGaxCo1.0B18 made the grains finer and more uniform. A notable peak of coercivity was observed in the variational curve of coercivity with annealing time when Ga content was low (x=0.2). This phenomenon could be explained by the microstructure and the exchange coupling interaction of two-phase nanocomposite.

Magnetically soft phase in magnetization reversal processes of nanocompositeSm2Fe15Ga2Cx/α−Fepermanent magnetic materials

The magnetic properties and magnetization reversal processes of nanocomposite ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{15}{\mathrm{Ga}}_{2}{\mathrm{C}}_{x}/\ensuremath{\alpha}\ensuremath{-}\mathrm{Fe}$ permanent magnet materials have been investigated. Remanence increases and intrinsic coercivity decreases drastically with increasing magnetically soft phase \ensuremath{\alpha}-Fe content. About 5 vol. % excess of \ensuremath{\alpha}-Fe results in a significant increase in maximum energy product from 5.6 MG Oe for single-phase nanocrystalline alloys to 8.2 MG Oe for two-phase nanocomposites. A further increase in \ensuremath{\alpha}-Fe content leads to a decrease of the maximum energy product owing to the rapid decrease of coercivity and the deterioration of the squareness of the hysteresis loop. The nucleation field for reversing the magnetically hard phase decreases monotonically with increasing \ensuremath{\alpha}-Fe content. Remanence enhancement, a high degree of reversibility, and high ratios of remanence coercivity to intrinsic coercivity are observed in two-phase nanocomposites.

Influence of nanostructure and nitrogen content on the optical and electrical properties of reactively sputtered FeSiAl(N) films

In this study, the optical properties and dc resistivity of a series of FeSiAl(N) films reactively sputtered with different partial pressures of N were investigated. Spectroscopic ellipsometry was used to measure the real and imaginary parts of the complex dielectric functions. There is a distinct micro/nanostructural transition from single-phase columnar body-centered-cubic (bcc) grains for partial pressure (pp) of nitrogen in sputtering gas ⩽4% to a two-phase nanocomposite of equiaxed bcc nanograins in an amorphous matrix for films deposited with ⩾5% pp N. To assess the effect of surface oxidation on the optical properties, optical measurements were repeated on the 2 and 5% pp N films (representative of the two different types of films with different structures) after they were sputter etched in situ while performing depth profiling of the chemical composition using x-ray photoelectron spectroscopy. The low-nitrogen films (⩽4% pp N) showed a dielectric function typical of a metal whose charge carrier contribution can be described by a classical free electron Drude model. The nanostructured films (⩾5% pp N) showed a positive real part of the dielectric function ε1 and no evidence of free-carrier plasmon excitation. The optical conductivity decreased and the dc resistivity increased by about a factor of 2.5 as the film structure changed from a single phase columnar structure to the two-phase material that consisted of nanograins in an amorphous matrix.

Nonuniform magnetic structure in Nd2Fe14B/Fe3B nanocomposite materials

The magnetic structure of a Nd4Fe77Co3B16 two-phase nanocomposite is studied by Lorentz microscopy and electron holography in this article. The big domains consisting several grains show strong intergranular exchange interaction. Some bright spots inside domains reflect a nonuniform magnetic structure that is characterized as a magnetic vortex using electron holography. It results from the strong intergranular exchange interaction and the randomly oriented Nd2Fe14B particles. This nonuniform magnetic structure is extensive in two-phase nanocomposites, and has an important influence on the magnetic properties. The internal stray field is dramatically weakened because of the zero divergence of magnetization in the magnetic vortex, and small Neff is observed. With the numerical simulation, it is found that the formation of the magnetic vortex induces a concave demagnetization curve and decay of the coercivity. This is the reason why the coercivity of practical magnets deviates greatly from that of theoretical results.

Application of atomic force microscopy in microstructure analysis of mechanically alloyed Nd 2Fe 14B/α-Fe-type nanocomposites

The microstructure of the two-phase nanocomposites Nd 12.6Fe 81.4B 6/37.5 vol.% α-Fe and Nd 12.6Fe 80.9Zr 0.5B 6/37.5 vol.% α-Fe magnets has been investigated by atomic force microscopy. The required structure was achieved by mechanical alloying of elemental powders and further heat treatment at different temperatures. The nanocomposites with addition of Zr are characterized by smaller grains and a narrower grain size distribution than without Zr. Differences in grain size after different conditions of experiments were clearly and easily identified by AFM. A Nd 12.6Fe 80.9Zr 0.5B 6/37.5 vol.% α-Fe nanocomposite with an average grain size of ∼19 nm has been produced. The grains combine together in agglomerates with a size of ∼250 nm. The AFM height mode image presentation is more accurate for agglomerate analysis, deflection for grain analysis.

Analytical modeling of the electrical conductivity of metal matrix composites: application to Ag–Cu and Cu–Nb

Modeling the electrical conductivity of composite materials is a complex matter, particularly if microstructural features lead to a size effect in electronic conduction or create an anisotropy. The method described in this article accounts for these circumstances by incorporation of appropriate phenomenological models. It provides a means to analyze and estimate the resistive behavior of two-phase or multi-phase micro-composites and nano-composites. A comparison with experimental data is carried out for two high-strength conductors, Ag–Cu and Cu–Nb, both of which have undergone extensive quantitative microstructure analysis to provide the required input for the model.

Magnetron sputtering of hard nanocomposite coatings and their properties

The article reports on the present status of knowledge in the field of hard and superhard nanocomposite coatings prepared by magnetron sputtering. Special attention is devoted to the two-phase nanocomposites composed of one hard and one soft phase. Trends of the next development are outlined.

Fe-Cu two-phase nanocomposites: application of a modified rule of mixtures

Metallic glass (MG)-particle-reinforced nanocrystalline CuCrZr alloys with ultrahigh strengths and good conductivities are designed and synthesized via mechanical milling and spark plasma sintering (SPS). The particle morphology and dispersion state of the CuZrAl MG reinforcement in the CuCrZr alloy matrix are optimized by regulating the milling time of the composite powders prior to SPS consolidation. Outstanding properties are achieved by 5 h and 10 h milling processes with ultimate compressive strengths of 1099 ± 33 and 1185 ± 29 MPa and high conductivities of 29.9 ± 0.8% IACS and 25.2 ± 0.6% IACS, respectively. According to the quantified results of strengthening and conductivity mechanisms, MG reinforcements and ultrafine grains are responsible for the balance of superior strength and conductivity of the composites.

Grain boundary strength in non-cubic ceramic polycrystals with misfitting intragranular inclusions (nanocomposites)

The residual stress distribution in polycrystalline ceramics with thermal expansion anisotropy and misfitting intragranular dispersion is studied through micromechanical simulation. The effective grain boundary strength under remote tension is derived from the stability of a grain-boundary microcrack with thermal elastic residual stresses. This result is then applied to the strength and fracture properties of a two-phase nanocomposite (5 vol% SiC-Al2O3). The residual stresses from misfitting dispersion increase the effective grain boundary strength of the nanocomposite from 1.5 to 5 times more than that of the single-phase polycrystal, depending on the grain size of the matrix phase. The residual stresses reduce the instability range of microcrack precursors at grain junctions and increase the initial level of driving force for critical microcrack extension. Predicted strengthening of grain boundaries leads, in turn, to the superior inert strength of unnotched nanocomposite.