Magnetism in nanocrystalline SiC films

Magnetism been studied in two series of nanocrystalline SiC films obtained by the method of direct deposition of ions with an energy of ~100 eV at temperatures 1150 °С and 1200 °С. There were separated the contributions of diamagnetism, paramagnetism and superparamagnetism+ferromagnetism. Magnetization value of the films correlates with the deposition temperature. In the films deposited at higher temperatures the value of magnetization was by 1.5 times lower. It was concluded that induced magnetism in nanocrystalline SiC films is caused by interaction of magnetic moments of neutral V_SiV_C divacancies in separate nanocrystals. The estimated concentration of neutral V_SiV_C divacancies in nanocrystalline SiC films is ~10²⁰ сm⁻³.     

Magnetism of nanocrystalline Finemet alloy: experiment and simulation

Mössbauer spectrometry and magnetic measurements are employed to experimentally investigate the magnetic behavior of nanocrystalline Fe_73.5Cu₁Nb₃Si_13.5B₉ ribbons obtained by appropriate annealing of the amorphous precursor. A detailed analysis of the correlation between the microstructure of annealed samples and their magnetic properties is provided. Thermomagnetic data allow the Curie temperatures of both amorphous residual matrix and nanocrystalline phase to be estimated. The differences between Curie temperatures of amorphous residual matrix and amorphous precursor are investigated and explained in terms of magnetic polarization of the matrix by exchange fields arising from the nanocrystalline grains. Theoretical systems of spins consisting of a single ferromagnetic nanocrystalline grain immersed in weakly ferromagnetic environment, quite similar to our real samples, are considered and their magnetic behavior is investigated by Monte Carlo simulation of low temperature spin ordering, with emphasize on the matrix-nanocrystalline grain interface which is shown to exhibit peculiar magnetic behavior. The magnetic features of the matrix-nanocrystalline grain interface are studied, as depending on matrix-nanocrystalline grain exchange coupling as well as crystalline fraction of the nanocrystalline systems.Received: 10 April 2003, Published online: 4 August 2003PACS: 81.07.Bc Nanocrystalline materials - 75.30.-m Intrinsic properties of magnetically ordered materials - 75.75.+a Magnetic properties of nanostructures     

Mechanosynthesis of nanocrystalline materials

Mechanical alloying is a method of synthesis of advanced materials, often with non-equilibrium structures, and, remarkably, of crystalline materials with nanometersized grains. Grinding is also a way of inducing or activating chemical reactions. After having described some general characteristics and some applications of high-energy ball-milling, we sketch out some contributions which Mössbauer spectroscopy has made to gaining a deeper understanding of synthesis mechanisms and of mechanosynthesized materials.     

Structural-diffraction analysis of nanocrystalline materials

We have studied the structure and phases of boron nitride and zirconium dioxide (both have a wide spectrum of crystalline sizes) using x-ray analysis and electron diffraction microscopy. We show that, even when the crystallites are of order 100 nm, their diffraction pattern is similar to that of other nanocrystalline materials. This pattern is dictated by the high degree of dispersion and the lattice distortions of the crystallites' periphery. The distortions in these materials are caused by internal stresses. In boron nitride the stress relaxes by polygonization of the peripheral regions. This destroys the long-range translational order and leads to the formation of intercrystallite regions due to incoherent binding between crystals. In zirconium oxide short-range order is destroyed by variations in the lattice constants in the regions near the crystal faces.Tomsk' State Architectural—Structural Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika No. 1, pp. 107–113, January, 1994.     

Grain-size-dependent thermal conductivity of nanocrystalline materials

In order to study the influence of grain size and lattice strain on the thermal conductivity of nanocrystalline (NC) materials, both experimental and theoretical studies were carried out on NC copper. The NC copper samples were prepared by hot isostatic pressing of nano-sized powder particles with mean grain size of 30 nm. The thermal behaviors of the samples were measured to be 175.63–233.37 W (m K)^?1 by using a laser method at 300 K, which is 45.6 and 60.6 % of the coarse-grained copper, respectively. The average grain size lies in the range of 56–187 nm, and the lattice strain is in the range of ?0.21 to ?0.45 % (in the direction of 111) and ?0.09 to 0.92 % (in the direction of 200). In addition, a modified Kapitza resistance model was developed to study the thermal transport in NC copper. The theoretical calculations based on the presented theoretical model were in good agreement with our experimental results, and it demonstrated that the thermal conductivity of NC materials show obvious size effect. It is also evident that the decrease in the thermal conductivity of NC material can be mainly attributed to the nano-size effect rather than the lattice strain effect.     

Advances in amorphous and nanocrystalline materials

A new amorphous alloy has been recently introduced which shows a saturation magnetic induction B_s of 1.64 T which is compared with B_s=1.57 T for a currently available Fe-based amorphous alloy and decreased magnetic losses. Such a combination is rare but can be explained in terms of induced magnetic anisotropy being reduced by the alloy's chemistry and its heat treatment. It has been found that the region of magnetization rotation in the new alloy is considerably narrowed, resulting in reduced exciting power in the magnetic devices utilizing the material. Efforts to increase B_s also have been made for nanocrystalline alloys. For example, a nanocrystalline alloy having a composition of Fe_80.5Cu_1.5Si₄B₁₄ shows B_s exceeding 1.8 T. The iron loss at 50 Hz and at 1.6 T induction in a toroidal core of this material is 0.46 W/kg which is 2/3 that of a grain-oriented silicon steel. At 20 kHz/0.2 T excitation, the iron loss is about 60% of that in an Fe-based amorphous alloy which is widely used in power electronics. Another example is a Fe₈₅Si₂B₈P₄Cu₁ nanocrystalline alloy with a B_s of 1.8 T, which is reported to exhibit a magnetic core loss of about 0.2 W/kg at 50 Hz and at 1.5 T induction. This article is a review of these new developments and their impacts on energy efficient magnetic devices.     

Structural methods for studying nanocrystalline materials

The most important methods for determining the grain size, the grain size distribution and also the actual microstructure of nanocrystalline materials are: X-ray diffraction line profile analysis, transmission electron microscopy, and small-angle neutron scattering. For each of these three methods their specific advantages and disadvantages are discussed and an experimental example is given.     

Structural mechanisms of fracture of nanocrystalline materials

Structural mechanisms and features of brittle and quasi-brittle fracture of nanocrystalline materials are theoretically analyzed. The role of size effects and internal stresses caused by a nonequilibrium structure during brittle trans-and intercrystallite fracture is studied. The dependence of the nanocrystalline material durability on the working stress and grain size is calculated. The conditions for certain mechanisms of plastic deformation to be operative in nanocrystalline materials are analyzed. The influence of the grain-boundary and dislocation mechanisms of plastic deformation on the conditions of nanocrack formation is studied. The dependence of the fracture toughness of nanomaterials on structure parameters is calculated.     

High strength and superplasticity of nanocrystalline materials

The part played by grain boundaries in nanocrystalline materials produced by equal-channel angular pressing is considered. The tensile strength and tensile and compressive yield stresses of various materials and alloys were studied over a broad low-temperature range. It was found that, at close-to-liquid helium temperatures, the strength is the highest possible for the given material and that strain localization is better pronounced than in conventional materials. The results obtained are interpreted in terms of the influence of boundaries, which act as the dominant hardening factor increasing the resistance to dislocation motion. By contrast, experiments conducted at elevated temperatures showed the boundaries to become mobile, thus imparting superplasticity to materials in some cases. The actual maximum tensile and torsional shears are compared. It is demonstrated that, despite the closeness between the tensile-and torsional-deformation activation energies, the stress-strain curves and the shears differ strongly, which implies that these characteristics are affected by the actual type of deformation involved.     

Advances in amorphous and nanocrystalline magnetic materials

Recent advances made in the area of amorphous and nanocrystalline alloys exhibiting high saturation inductions are reviewed. A new chemical composition was identified that achieves a saturation induction of 1.64 T in an iron-based amorphous alloy. This alloy, when used in electrical transformers, shows a much improved performance over the existing amorphous alloy. Nanocrystalline FeCoCuNbSiB alloys are found to have saturation induction levels reaching 1.7 T. These materials are suited for use in sensors and inductors carrying large currents. Some of these nanocrystalline alloys show a BH squareness ratio exceeding 90%, which can be utilized in pulse power devices. Recent developments in the applications of these materials are also pointed out.     

Stability map for nanocrystalline and amorphous materials

We present a stability map which predicts the domains of shear instability due to grain rotation in nanocrystalline materials. The onset of this mode of instability is influenced by grain-size-dependent mechanisms and the length-scale of intergranular interaction. The map shows the grain size regimes that are inherently susceptible to this mode for a range of materials. In the amorphous limit, the model predicts embryonic nuclei sizes of about 10-50 nm, which agrees well with the shear band thicknesses for many metallic glasses.     

Cooperative grain boundary sliding in nanocrystalline materials

Superplastic behaviour of microcrystalline materials is now believed to be controlled by cooperative grain boundary sliding (CGBS). An increasing role of grain boundary mediated plasticity with decreasing grain size down to the nanoscale was predicted leading to the prospect of enhanced superplasticity in nanocrystalline materials. Nevertheless, materials with nanosized grains have revealed a significant decrease in plasticity contrary to theoretical prediction. Direct evidence of CGBS in nanocrystalline Ni₃Al alloy from SEM surface analysis and in-situ TEM tensile testing was detected, confirming one similarity in the rheology of deformation processes between micro- and nanomaterials. Thus, differences in deformation behaviour of materials at these two length scales are related to the probability of sliding surface formation, sliding distance and related accommodation mechanisms.     

Structural mechanisms of plastic deformation in nanocrystalline materials

A model of the initial stage of plastic deformation in nanomaterials is proposed. Within this model, the plastic deformation occurs through grain boundary microsliding (GBM). The accommodation processes accompanying the formation of GBM regions are considered. The relationships describing the regularities in the deformation behavior of nanomaterials and the dependence of the flow stress on the grain size are derived, and the temperature dependence of the GBM resistance stress is calculated. It is demonstrated that the results obtained are in good agreement with the experimental data.     

Diffusion model of internal friction in nanocrystalline materials

The stress normal and tangenital components arise in grain-boundary segments differently oriented with respect to an external periodic load, causing the fluxes of vacancies and impurity atoms between neighboring segments. By solving the diffusion problem, one can find the velocity of mutual displacement of grains, the stress distribution in the segments with allowance for stress adjustment, and the amount of internal friction. The frequency dependence of the internal friction shows peaks associated with the redistribution of impurity atoms over the segments, grain sliding, and a high-temperature background.     

Plasticity and strength of micro- and nanocrystalline materials

This review is devoted to the effect of grain boundaries on the deformational and strength properties of poly-, micro-, and nanocrystalline materials (predominantly metals). The main experimental facts and mechanisms concerning the dislocation structure and mechanical behavior of these materials over wide ranges of temperatures and grain sizes are presented. The experimentally established regularities are analyzed theoretically in terms of equations of dislocation kinetics taking into account the properties of grain boundaries as barriers, sources, and sinks for dislocations and as places where dislocations annihilate. The origin of the Hall-Petch relations for the yield stress and the flow stress as functions of the grain size, as well as the deviations from these relations observed in nano- and microcrystalline materials, is discussed in detail in terms of the dislocation-kinetics approach. Embrittlement of micro- and nanocrystalline materials at low temperatures and superplasticity of these materials at elevated temperatures are also analyzed in terms of the dislocation-kinetics approach.     

Disclination mechanism for plastic deformation of nanocrystalline materials

A model is developed for the plastic deformation of nanocrystalline materials in terms of the evolution of a spatial grid of disclinations located at the triple junctions of grains. Plastic deformation takes place as the result of plastic rotation of grains, the mismatch of whose rotations causes the nucleation of partial disclinations at the junctions of intergrain boundaries. It is shown that the distinctive feature of the mechanical behavior of nanocrystals is a deviation from the Hall-Petch law up to a critical grain size D _cr⩽25 nm. Fiz. Tverd. Tela (St. Petersburg) 39, 2023–2028 (November 1997)     

Generation of dislocation loops in deformed nanocrystalline materials

A theoretical model is suggested which describes the generation of lattice perfect, lattice partial and grain boundary dislocation loops (DLs) at pre-existent DLs in mechanically loaded nanocrystalline materials (NCMs). The energy characteristics of various modes of the DL generation are calculated and compared. With these calculations and comparison, the two basic ranges of the grain size in NCMs are revealed each is characterized by its specific set of effectively operating modes of the DL generation and plastic deformation. The role of the DL generation in plastic and superplastic deformation processes in NCMs is discussed.