Amorphous Boundary Research Articles

Plastic deformation transfer through the amorphous intercrystallite phase in nanoceramics

A three-dimensional model is proposed for plastic deformation transfer through the amorphous intercrystallite phase in mechanically loaded nanoceramics. In this model, glide dislocation loops are pressed against amorphous intercrystallite boundaries by the applied local shear stress and initiate in them local longitudinal plastic shears, which causes emission of new glide dislocation loops into neighboring grains. The energy characteristics of these processes and the critical applied stress required for barrierless nucleation of grainboundary and intragrain loops are calculated. As an example, a nanoceramic based on cubic silicon carbide is considered. It is shown that plastic deformation transfer through the amorphous intercrystallite phase in such nanoceramics is energetically favorable and can occur athermically over wide ranges of values of the applied stress and the structural characteristics of the material.

Emission of partial dislocations from amorphous intergranular boundaries in deformed nanocrystalline ceramics

A theoretical model is suggested that describes emission of partial lattice dislocations from amorphous intergranular boundaries into crystalline grains in deformed nanocrystalline ceramics. Within the model, a dipole of immobile lattice dislocations is generated at an amorphous intergranular boundary through local shear events in this boundary. One of the dislocations then undergoes transformation, resulting in emission of a partial dislocation into a grain. It is shown that the emission process is energetically favorable in nanoceramic SiC deformed at high stresses.

Dislocation nucleation at amorphous intergrain boundaries in deformed nanoceramics

A theoretical model is proposed for lattice dislocation nucleation in deformed nanocrystalline ceramics with amorphous intergrain boundaries. According to the model, a lattice dislocation dipole nucleates at an amorphous intergrain boundary through a local plastic shear along the boundary cross section. The energy parameters of this nucleation process are calculated. It is demonstrated that the dislocation nucleation at amorphous intergrain boundaries is energetically favorable and can occur as an athermic process (without energy barrier) in the nanocrystalline phase of cubic silicon carbide 3C-SiC and in the TiN/a-Si3N4 nanocomposite over wide ranges of structural parameters and mechanical loads.

Photocurrent of hydrogenated nanocrystalline silicon thin film/crystalline silicon heterostructure

We report on the photocurrent properties of the hydrogenated nanocrystalline silicon (nc-Si:H) thin film/crystalline silicon (c-Si) n-p heterostructure. By comparison with the c-Si n-p homojunction, two Gaussian-type photocurrent peaks are observed in the nc-Si:H/c-Si heterostructure and attributed to be transitions from a tail band or discrete levels in quantum dots with localized states, and a miniband with extended states associated with the embedded nanometer crystallites in the amorphous boundaries of the nc-Si:H thin film. The observed strong photocurrent signals and temperature dependency have revealed the unique electronic states of the miniband in the nc-Si:H thin film. Our investigations into the photocurrent properties may help to realize nc-Si:H/c-Si heterostructure-based optoelectronic devices.

HRTEM and EELS Studies on Structural Defects in the Superconductor Sr2CuO2+δCl2−y Newly Synthesized Under High Pressure

Structural defects in the newly high-pressure synthesized superconductor Sr2CuO2+δCl2−y have been studied by means of high-resolution transmission electron microscopy and parallel electron energy-loss spectroscopy. Two types of grain boundaries were found in the material. One is the amorphous boundaries usually lying in the ab-planes. The other is the structurally intact grain boundaries, and most of them are large-angle boundaries. One of the large angles frequently observed is the 90°〈110〉 asymmetrical tilt boundary. In some domains, periodicity lengths were changed due to the lattice distortion usually associated with atom deficiency. These domains can be regarded as “second-phase particles” embedded in the perfect grains. O-K electron energy-loss spectroscopy results suggest that those defect domains are in the hole-poor states. Quantitative composition analysis from energy dispersive of X-ray suggests the hole-poorness is caused by oxygen deficiency. Dislocations, such as the \({\textstyle{1 \over 2}}\)a[110] and \({\textstyle{1 \over 2}}\)c[001] edge dislocations, were also observed in the material. In most cases, those dislocations are very complex and exhibit stacking faults.

Thermal Stability and Mechanical Properties of Plasma Sprayed Al<sub>2</sub>O<sub>3</sub>/ZrO<sub>2</sub> Nano-Composite Coating

Alumina/zirconia nano-composite coating was fabricated by plasma spraying using agglomerated feedstock from fine powders of about 100 nm. The coating was consisted of fine γ-alumina and zirconia crystals with size of several nano meter and some amorphous boundary layers. The amorphous phase was crystallized and disappeared after heat treatment at 930°C. However, the crystallite size was kept under 50 nm even after 1500°C-100hr heating, so the alumina-zirconia nano-composite showed good thermal stability against the grain growth.

Growth dynamics and strain relaxation mechanisms inBaTiO3pulsed laser deposited onSrRuO3∕SrTiO3

The atomic structure of epitaxial ${\mathrm{BaTiO}}_{3}$ films grown on ${\mathrm{SrRuO}}_{3}$-covered (001)${\mathrm{SrTiO}}_{3}$ substrates by pulsed laser deposition has been studied by transmission electron microscopy. It revealed a three-layered structure, each layer with a different morphology, remaining strain, and density of defects. These results pointed to the existence of two growth regimes with distinctive strain-relaxation mechanisms: (i) A first dislocation-free layer extending $3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ from the coherent interface with ${\mathrm{SrRuO}}_{3}$. (ii) Beyond it, a second $7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick semicoherent layer exhibiting a high density of misfit dislocations with a Burger vector $a⟨010⟩$. The structures of both layers are the outcome of a two-dimensional layer-by-layer growth regime. (iii) A third layer extending throughout the rest of the ${\mathrm{BaTiO}}_{3}$ film, showing a columnar structure of stoichiometric grains encapsulated by amorphous Ti-enriched boundaries, as disclosed by energy-dispersive x-ray spectroscopy. The structure of this third layer reflects its three-dimensional-featured growth habit. By considering both regimes as a single mode, the growth dynamics of the ${\mathrm{BaTiO}}_{3}$ films is discussed in relation to strain-relaxation mechanisms potentially responsible for the Stranski-Krastanov growth mode of this system.

Effect of Annealing Temperature on the Permeability and Magneto-Impedance Behaviors of Fe68.5Mn5Si13.5B9Nb3Cu1Amorphous Alloy

The effect of annealing temperature on the permeability and giant magneto-impedance (GMI) behaviors of <TEX>$Fe_{68.5}Mn_{5}Si_{13.5}B_9Nb_3Cu_1$</TEX> amorphous alloy has been systematically investigated. The nanocrystalline <TEX>$Fe_{68.5}Mn_{5}Si_{13.5}B_9Nb_3Cu_1$</TEX> alloys consisting of ultra-fine <TEX>$(Fe,Mn)_3Si$</TEX> grains embedded in an amorphous matrix were obtained by annealing their precursor alloy at the temperature range from <TEX>$500^{\circ}C\;to\;600^{\circ}C$</TEX> for 1 hour in vacuum. The permeability and GMI profiles were measured as a function of external magnetic field. It was found that the increase of both the permeability and the GMI effect with increasing annealing temperature up to <TEX>$535^{\circ}C$</TEX> was observed and ascribed to the ultrasoft magnetic properties in the sample, whereas an opposite tendency was found when annealed at <TEX>$600^{\circ}C$</TEX> which is due to the microstructural changes caused by high-temperature annealing. The study of temperature dependence on the permeability and GMI effect showed some insights into the nature of the magnetic exchange coupling between nanocrystallized grains through the amorphous boundaries in nanocrystalline magnetic materials.

Metastability in Hydrogenated Nanocrystalline Silicon Solar Cells

AbstractWe have recently reported that hydrogenated nanocrystalline silicon (nc-Si:H) solar cells exhibit metastability behavior that is significantly different from hydrogenated amorphous silicon (a-Si:H) solar cells. First, we studied the light-induced change of nc-Si:H cells under different light spectra, and observed that no light-induced degradation occurs when the photon energy is lower than the bandgap of a-Si:H, while light-induced degradation indeed occurs when the photon energy is higher than the bandgap of a-Si:H. Based on this observation, we conclude that the light-induced defect generation occurs mainly in the amorphous and/or grain boundary regions. Secondly, we found that forward-bias current injection in the dark does not cause any degradation in the performance of nc-Si:H cells, in contrast to observations in a-Si:H cells. This phenomenon can be explained by assuming transport percolation through crystallites, where excess carrier recombination does not cause degradation. Third, we found that a reverse electrical bias does not reduce, but rather enhances the light-induced degradation in the nc-Si:H cell performance. This enhancement increases with the magnitude of the reverse bias. We carried out a systematic study including measurements of quantum efficiency losses and fill factors under different color filters. In view of the heterogeneity of the material structure, we proposed a “back-to-back” diode model which explains most of the experimental results. Finally, we present results of the improvement of stability of nc-Si:H cells made using hydrogen dilution profiling. We find that the nc-Si:H cells with an optimized hydrogen dilution profile show very little light-induced degradation though these cells have a high amorphous volume fraction. This indicates that the amorphous volume fraction is not the only factor determining the degradation. Other factors, such as the structure and distribution of the amorphous phase, as well as the properties of the grain boundaries, apparently play important roles in the overall stability of nc-Si:H cells.

Generation of amorphous surface layers in LiNbO3 by ion-beam irradiation: thresholding and boundary propagation

The refractive-index profiles induced by high-energy (5 MeV, 7.5 MeV) silicon irradiation in LiNbO3 have been systematically determined as a function of ion fluence in the range 1013–1015 cm-2. At variance with irradiations at lower energies, an optically isotropic (‘amorphous’) homogeneous surface layer is generated whose thickness increases with fluence. These results have been associated with an electronic excitation mechanism. They are discussed in relation to the well-documented phenomenon of latent (amorphous) track generation under ion irradiation, requiring a threshold value Se,th for the electronic stopping power Se. Our optical data have yielded a value of ≈5 keV/nm for such a threshold, within the range reported by independent single-track measurements. The propagation of the amorphous boundary into the crystal during irradiation indicates that the threshold value decreases on increasing the fluence. Complementary Rutherford backscattering–channeling and micro-Raman (on samples irradiated at 30 MeV) experiments have been performed to monitor the induced structural changes.

Effect of annealing on the microstructure and magnetic properties of Fe-based nanocomposite materials

The influence of annealing on microstructure, magnetic properties including the giant magnetoimpedance (GMI) effect of a Fe-based nanocomposite has been investigated. The nanocomposite structure composed of ultra-fine Fe(Si) grains embedded in an amorphous matrix was attained by annealing the Fe-based amorphous alloy prepared by rapid quenching method. The GMI profiles were measured for samples annealed at different temperatures ranging from 350 to 650 °C in vacuum and for 30 min. It is found that the mean grain size of the α-Fe(Si) crystallites in the order of 12 nm remains almost unchanged until the annealing temperature reached 540 °C. A decrease of anisotropy field and an increase of GMI with increasing annealing temperature up to 540 °C were observed and ascribed to the increase of the magnetic permeability and the decrease of the coercivity, whereas the opposite tendency was found for the sample annealed above 600 °C which is likely due to the microstructural change caused by high-temperature annealing. This indicates that variation in the magnetic characteristic of the amorphous phase upon annealing changed the intergrain exchange coupling. This altered both the magnetic softness and the effective anisotropy and consequently modified the GMI features. The study of the temperature dependence of the GMI effect provides further understanding of the magnetic exchange between these crystallized grains through the amorphous boundaries in Fe-based nanocrystalline materials.

Optimum information in crackling noise

The crackling noise due to scratching superhard nanocomposite coatings was investigated by using a simple stick-slip model. The optimum information extracted from statistical analysis, in terms of the Akaike information criterion, is in good agreement with real tests. As a nanocomposite coating approaches an optimal performance, the acoustic emission energy follows a power-law distribution and its behavior is likely to be independent of microscopic and macroscopic details. The results imply that a peculiar deformation behavior, due to the competition between different deformation mechanisms such as dislocation pile-ups in nanocrystalline grains and grain sliding-grain rotation within amorphous boundaries, plays a vital role in the nanostructure with superhardness.

Microstructures and magnetic domain structures in Sm 2(Fe,Mn) 17N δ powders studied by analytical electron microscopy and Lorentz microscopy

Microstructures and magnetic domain structures of Sm 2(Fe,Mn) 17N δ ( δ = 0 , 3.4, 4.2, 5.0) powders are investigated by analytical electron microscopy and Lorentz microscopy, respectively. It is found that amorphous phases form with the addition of nitrogen and the area of the amorphous phases increases with the increase in nitrogen content. By elemental mapping with electron energy-loss spectroscopy (EELS), it is elucidated that the amorphous phase is enriched with N and Mn, while the crystalline phase is enriched with Fe. The analysis with Lorentz microscopy reveals that the size of the magnetic domains decreases with the increase in nitrogen content. Further, it is clarified that the domain walls exist on the Mn-enriched amorphous phases. Finally, the domain wall pinning in the amorphous boundary regions is considered to result in a large coercivity (1.04 MA/m) in Sm 2(Fe,Mn) 17N 5.0 powder.

Atomic Resolution Transmission Electron Microscopy of the Intergranular Structure of a Y2O3‐Containing Silicon Nitride Ceramic

High‐resolution transmission electron microscopy (HRTEM) employing focus‐variation phase‐reconstruction methods is used to image the atomic structure of grain boundaries in a silicon nitride ceramic at subangstrom resolution. Complementary energy‐dispersive X‐ray emission spectroscopy experiments revealed the presence of yttrium ions segregated to the 0.5–0.7‐nm thin amorphous boundary layers that separate individual grains. Our objective here is probing if yttrium ions attach to the prismatic planes of the Si3N4 at the interface toward the amorphous layer, using Scherzer and phase‐reconstruction imaging, as well as image simulation. Crystal structure images of grain boundaries in thin sample (&lt;100 Å) areas do not reveal the attachment of yttrium at these positions, although lattice images from thicker areas do suggest the presence of yttrium at these sites. It is concluded that most of the yttrium atoms are located in the amorphous phase and only a few atoms may attach to the terminating prism plane. In this case, the line concentrations of such yttrium in the latter location are estimated to be at most one yttrium atom every 17 Å.

Annealing radiation damage and the recovery of cathodoluminescence

The structural recovery upon heat treatment of a highly metamict, actinide-rich zircon (U≈6000 ppm) has been studied in detail using a range of techniques including X-ray powder diffraction, Raman spectroscopy, SHRIMP ion probe, electron microprobe, transmission electron microscopy and cathodoluminescence analysis. The structural regeneration of the amorphous starting material depends on random nucleation. It starts between 800 and 950 °C when amorphous ZrSiO 4 decomposes to form crystalline ZrO 2 and amorphous SiO 2. At around 1100 °C, well-crystallised ZrSiO 4 grows at the expense of the oxides. U has been retained in the newly grown zircon whereas Pb was evaporated during the heat treatment. This process is in marked opposition to the reconstitution of moderately metamict minerals, which experience a gradual recovery controlled by the epitaxial growth at the crystalline–amorphous boundaries. Both of these recovery processes are not the direct inverse of metamictisation. The structural regeneration was found to be connected with a significant increase in the emission of CL. In all cases (annealing heavily damaged zircon and moderately damaged zircon and monazite), we observe that the final, well-crystallised annealing products emit more intense CL than their radiation-damaged starting minerals, although having almost identical elemental composition. Our observations are taken as evidence that the CL is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres.

Structure and electrical properties of sputtered lithium cobaltite thin films

Li x CoO y films with x<1 and y>2 have been prepared by radio-frequency (rf) sputtering from high temperature (HT) LiCoO 2 targets. Their structures have been examined with high resolution electron microscopy. Conductivities have been studied between 77 and 400 K. The electrochemical behaviour of film electrodes have been investigated with Li/LiClO 4–PC/Li x CoO y cells. The annealed films consist of nanocrystalline domains with amorphous boundaries. Electrical conductivities appear to arise from variable-range hopping (VRH) of holes. The films form good electrodes with operating potentials between 2.7 and 3.8 V. The observations have been discussed on the basis of a tentative and heuristic molecular orbital based energy band diagram.

Manipulation of the equilibrium between diamond growth and renucleation to form a nanodiamond/amorphous carbon composite

Composite films of ∼10 nm nanodiamond particles embedded in an amorphous carbon matrix were formed using a double bias assisted hot filament chemical vapor deposition system with a feeding gas mixture of 1% CH4:99% H2. The structure was obtained via the equilibrium of a multistage process including: (1) bias enhanced nucleation of diamond in an amorphous carbon matrix, (2) growth of both amorphous carbon and diamond, (3) suppression of the diamond growth by the surrounding amorphous carbon matrix, and (4) bias enhanced renucleation of diamond on the new amorphous carbon boundaries. The work adds insight to the diamond nucleation and growth processes.

Low Leakage TiO2 Gate Insulator Formed by Ultrathin TiN Deposition and Low-Temperature Oxidation

A new method of forming TiO2 gate insulators is proposed. It was found that ultrathin TiN deposition on ultrathin SiO2 and low-temperature thermal oxidation resulted in smaller TiO2 grains surrounded by amorphous boundaries. The gate leakage current was effectively reduced by applying ultrathin TiO2/SiO2 stacked insulators to metal-oxide-semiconductor (MOS) capacitors and Damascene gate metal-oxide-semiconductor-field-effect-transistors (MOSFETs) with good characteristics were successfully fabricated.

High-temperature giant magnetoimpedance in Fe-based nanocrystalline alloy

Giant magnetoimpedance (GMI) was investigated from room temperature up to 823 K in an Fe-based nanocrystalline Fe73.0Cu1.0Nb2.5V1.0Si13.5B9.0 ribbon. With an increment of the measuring temperature (T), GMI shows notable enhancement followed by a declining dependence, yielding a maximum value around 603 K where the relative GMI is nearly four times that at room temperature. The field at the peak of the GMI vs Hdc curve decreases monotonically with T, but around T=603 K there superimposes a trough-shaped variation. The thermal evolution of the soft magnetic property and magnetic anisotropy is suggested to be responsible for the high-temperature GMI features. Discussion on the intergrain exchange magnetic coupling through the amorphous boundaries in the two-phase Fe-based nanocrystalline alloy is also given.

Tensile creep behavior in an advanced silicon nitride

Abstract Tensile creep behavior and changes in the microstructure of the advanced silicon nitride, SN 88M, were studied at temperatures from 1250 to 1400°C to reveal the creep resistance and lifetime-controlling processes. Assuming power law dependence of the minimum strain rate on stress, stress exponents from 6 to 8 and an apparent activation energy of 780 kJ/mol were obtained. Extensive electron microscopy observations revealed significant changes in the crystalline secondary phases and creep damage development. Creep damage was classified in two groups: ‘inter-granular’ defects in the amorphous boundary phases, and ‘intra-granular’ defects in silicon nitride grains. The inter-granular defects involved multigrain junction cavities, two-grain junction cavities, microcracks and cracks. The intra-granular defects included broken large grains, small symmetrical and asymmetrical cavities, and crack-like intragranular cavities. Cavities are generated continuously during the whole deformation starting from the threshold strain of ∼0.1%, and they contribute linearly to the tensile strain. Cavities produce more than 90% of the total tensile strain, and it is concluded that cavitation is the main creep mechanism in silicon nitride ceramics. The multigrain junction cavities are considered to be the most important for generating new volume and producing tensile strain. The Luecke and Wiederhorn (L&W) creep model, based on cavitation at multigrain junctions according to an exponential law, was proven to correspond to the stress dependence of the minimum strain rate. A qualitative model based on the L&W model was suggested and expanded to include intragranular cavitation. The basic mechanisms involve a repeating of the sequence grain boundary sliding (GBS) ⇒ cavitation at multigrain junctions ⇒ viscous flow and dissolution-precipitation. The oxynitride ⇒ disilicate secondary phase transformation also occurs during creep and may result in the reduction of the residual glass content. The suggested creep model considers the effects of phase transformation, oxidation, crystallization, etc. on the basic creep mechanisms due to changes of the properties of the residual glass. Control of the amount and viscosity of the residual glass provides the possibility for the reduction of cavitation and consequently, for more creep-resistant ceramics.