Polycrystalline Si3N4 Research Articles

Sputtering of (111) highly-oriented nanotwinned Ag on polycrystalline Si3N4 substrates for high-power electronic packaging

Nanotwinned silver (NT-Ag) exhibits excellent mechanical and electrical properties, attributed to its high-density and (111) highly-oriented twinning structure in nanoscale. Therefore, it has recently drawn much attention in the field of electronic packaging, where it can be utilized as interconnection or metallization materials. However, it has been a critical challenge to fabricate the high-density and highly-oriented NT-Ag onto polycrystalline ceramic substrates, whose crystal structure does not support the epitaxial growth of NT-Ag films. In the current work, a high-density and highly-oriented NT-Ag film has been fabricated onto a polycrystalline ceramic substrate, i.e., NT-Ag on silicon nitride (NT-Ag@Si3N4), using the magnetron sputtering method. As results, a close-packed arrangement of high-density NT-Ag has been observed within columnar grains aligned along its growth direction, whereas the nanoindentation hardness of the NT-Ag film reached 1.82 GPa, with an electrical resistivity of 2.06 μΩ‧cm. Moreover, this study has discussed and explained the growth kinetics and mechanism of NT-Ag films, by controlling magnetron sputtering process parameters, such as sputtering power and argon flow rate. Additionally, the differences in the growth mechanism of NT-Ag on (100) Si and polycrystalline Si3N4 have also been investigated. With its superior material properties, NT-Ag@Si3N4 holds great promise in the applications of advanced packaging technology for high-power electronics.

Properties of superconducting MgB2 spherical shells deposited on 2 mm and 1 mm diameter Si3N4 spheres

Superconducting spherical shells may not only provide an appealing platform for exploring superconductivity in three-dimensional (3D) geometries with curved surfaces, but also be practically applied in important fields, such as in gyroscopes and gravimeters. In inertial confinement fusion (ICF), the perturbation on the target capsule caused by traditional contact support methods is recognized as one of the major contributors to performance degradation in ICF implosions. It was proposed recently that coating the capsule with a thin superconducting MgB2 spherical shell to realize non-contact support by means of magnetic levitation at the required temperatures K might offer a promising solution to this longstanding problem. To simulate the coating of ICF capsules, we deposited MgB2 spherical shells onto polycrystalline Si3N4 spheres (diameter d = 2 mm and 1 mm) via a hybrid physical–chemical vapour deposition technique. For both diameters, the spherical shells, of about 1 µm in thickness and covering the whole spheres completely, were polycrystalline with thin plate-shaped MgB2 crystallites oriented randomly and closely connected. The spherical shells exhibited superconducting transition at a zero resistance temperature of 38.5–39.4 K in four-probe resistance measurements and ideal diamagnetism at low temperatures in magnetization measurements, suggesting the good crystallinity and homogeneity of the shells. The upper critical field , the irreversibility field and the lower critical field were characterized and showed similar magnitudes and temperature dependencies for the d = 2 mm and 1 mm shells, yielding of 7–8 T and of 7–12 mT at 20 K. The superconducting critical current density values, evaluated from the magnetization hysteresis loops, were A cm−2 and A cm−2 at 20 K under zero and 2 T applied field, respectively, for the d = 1 mm shell. The occurrence of small flux jumps was observed at low temperatures up to 12 K and in low fields below 0.3 T. The results demonstrate the feasibility of fabricating MgB2 spherical shells with properties applicable to potential areas employing magnetic levitation such as in ICF and that MgB2 spherical shells may be exploited as an experimental system to study superconductivity in 3D curvilinear geometry.

High-Energy Heavy Ion Tracks in Nanocrystalline Silicon Nitride

At present, silicon nitride is the only nitride ceramic in which latent ion tracks resulting from swift heavy ion irradiation have been observed. Data related to the effects of SHIs on the nanocrystalline form of Si3N4 are sparse. The size of grains is known to play a role in the formation of latent ion tracks and other defects that result from SHI irradiation. In this investigation, the effects of irradiation with high-energy heavy ions on nanocrystalline silicon nitride is studied, using transmission electron microscopy techniques. The results suggest that threshold electronic stopping power, Set, lies within the range 12.3 ± 0.8 keV/nm to 15.2 ± 1.0 keV/nm, based on measurements of track radii. We compared the results to findings for polycrystalline Si3N4 irradiated under similar conditions. Our findings suggest that the radiation stability of silicon nitride is independent of grain size.

Polymer-derived SiHfN ceramics: From amorphous bulk ceramics with excellent mechanical properties to high temperature resistant ceramic nanocomposites

Within the present work, additive-free amorphous bulk SiHfN ceramics with excellent mechanical properties were prepared by a resource-efficient low-temperature molding method, namely warm-pressing. As densification mechanism viscous flow has been identified based on cross-linking reaction. The critical problems concerning gas evolution and crystallization inducing bloating and cracking are addressed through controlled thermolysis and pressure. The microstructural evolution of the SiHfN ceramics indicates that the incorporation of Hf in perhydropolysilazane not only increases the ceramic yield (97.4 wt%) and crystallization resistance (1300 °C), but also suppresses the transformation from α-Si3N4 to β-Si3N4 at high temperatures (1700 °C). Especially, HfN/α-Si3N4 nanocomposites converted by the SiHfN ceramics at 1500 °C show a slight weight loss of 3.13 wt%, indicating the high temperature resistance of the ceramic nanocomposites. The method proposed in this work opens a new strategy to fabricate additive-free polycrystalline Si3N4- and amorphous Si3N4-based (nano)composites.

Raman Study of Polycrystalline Si3N4 Irradiated with Swift Heavy Ions

A depth-resolved Raman spectroscopy technique was used to study the residual stress profiles in polycrystalline silicon nitride that was irradiated with Xe (167 MeV, 1 × 1011 cm−2 ÷ 4.87 × 1013 cm−2) and Bi (710 MeV, 1 × 1011 cm−2 ÷ 1 × 1013 cm−2) ions. It was shown that both the compressive and tensile stress fields were formed in the irradiated specimen, separated by a buffer zone that was located at a depth that coincided with the thickness of layer, amorphized due to multiple overlapping track regions. The compressive stresses were registered in a subsurface region, while at a greater depth, the tensile stresses were recorded and their levels reached the maximum value at the end of ion range. The size of the amorphous layer was evaluated from the dose dependence of the full width at half maximum (FWHM) (FWHM of the dominant 204 cm−1 line in the Raman spectra and scanning electron microscopy.

New technique to synthesize silicon nitride nanopowder by discharge-assisted reaction of silane and ammonia

In this work, the chemical reaction of silane and ammonia gases was induced and assisted by electric power transferred to the reaction volume throughout argon discharge at pressures of about 3 mbar. This power was applied to the reaction volume as short pulses with different durations to produce highly-pure polycrystalline Si3N4 nanoparticles. The number of crystal planes was decreased by using shorter pulses of discharge power. The minimum nanoparticle size was 26.10 nm for the sample prepared using discharge pulse duration of 0.1 ms. This is the first attempt to employ fast discharge pulses to induce and assist the chemical reaction of silane and ammonia to produce highly-pure silicon nitride nanopowders.

Latent tracks of swift Bi ions in Si3N4

Parameters such as track diameter and microstruture of latent tracks in polycrystalline Si3N4 induced by 710 MeV Bi ions were studied using TEM and XRD techniques, and MD simulation. Experimental results are considered in terms of the framework of a ‘core–shell’ inelastic thermal spike (i-TS) model. The average track radius determined by means of electron microscopy coincides with that deduced from computer modelling and is similar to the track core size predicted by the i-TS model using a boiling criterion. Indirect (XRD) techniques give a larger average latent track radius which is consistent with the integral nature of the signal collected from the probed volume of irradiated material.

Analysis of the microstructural evolution of silicon nitride irradiated with swift Xe ions

The evolution of 220 MeV Xe ion induced radiation damage in polycrystalline Si3N4 is studied, within the fluence range 5 × 1011– 2 × 1014 cm−2, using transmission electron microscopy techniques. These irradiation conditions allow for the study of track morphology in both crystalline and in radiation-amorphized Si3N4. The average track size in the polycrystalline samples is 1.9 ± 0.4 nm and 3.1 ± 0.5 nm in the radiation-amorphized samples. The larger track sizes in the radiation-amorphized material is in agreement with predictions of the inelastic thermal spike model and the role of thermal conductivity in latent track formation.

Investigation of the surface generation mechanism of mechanical polishing engineering ceramics using discrete element method

Machining technology about ceramics has been developed very fast over recent years due to the growing industrial demand of higher machining accuracy and better surface quality of ceramic elements, while the nature of hard and brittle ceramics makes it difficult to acquire damage-free and ultra-smooth surface. Ceramic bulk can be treated as an assemblage of discrete particles bonded together randomly as the micro-structure of ceramics consists of crystal particles and pores, and the inter-granular fracture of the ceramics can be naturally represented by the separation of particles due to breakage of bonds. Discrete element method (DEM) provides a promising approach for constructing an effective model to describe the tool–workpiece interaction and can serve as a predicting simulation tool in analyzing the complicated surface generation mechanism and is employed in this research to simulate the mechanical polishing process of ceramics and surface integrity. In this work, a densely packed particle assembly system of the polycrystalline Si3N4 has been generated using bonded-particle model to represent the ceramic workpiece numerically. The simulation results justify that the common critical depth of cut cannot be used as the effective parameters for evaluating brittle to ductile transformation in ceramic polishing process. Therefore, a generalized criterion of defining the range of ductile regime machining has been developed based on the numerical results. Furthermore, different distribution of pressure chain is observed with different depth of cut which ought to have intense relationship with special structure of ceramics. This study also justified the advantage of DEM model in its capability of revealing the mechanical behaviors of ceramics at micro-scale.

Fabrication of translucent boron nitride dispersed polycrystalline silicon nitride ceramics

Optical transparency was achieved at infrared region and overall translucent silicon nitride was fabricated using hot press sintering (HPS). The increase in h-BN content decreased the optical transparency. Microstructral observations shows that the optical, mechanical and tribological properties of BN dispersed polycrystalline Si3N4 ceramics were affected by the density, α:β-phase ratio and content of h-BN in sintered ceramics. The hot pressed samples were prepared from the mixture of α-Si3N4, AlN, MgO and h-BN at 1850°C. The composite contained from 0.25 to 2 mass % BN powder with sintering aids (9% AlN + 3% MgO). Maximum transmittance of 57% was achieved for 0.25 mass % BN doped Si3N4 ceramics. Fracture toughness was increased and wear volume and friction coefficient were decreased with increase in BN content.

Boron Nitride Doped Transparent Polycrystalline Silicon Nitride Ceramics

The addition of h- BN to a polycrystalline Si3N4 was to increase the fracture toughness and other mechanical properties such as flexural strength and hardness of the material. The hot pressed samples were prepared from the mixture of α-Si3N4, AlN, MgO and h-BN. The composite contained from 0 to 2 wt.% BN powder with sintering aids (9% AlN + 3% MgO). The transparency, mechanical properties and microstructure of hot pressed polycrystalline Si3N4-BN composite materials were investigated by UV/VIS spectrophotometer, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The transparency decreased with increasing the content of h-BN into Si3N4.

Thermal conductivity and heat capacity of Si3N4/BN fiber monoliths

This paper reports on measurements within the 5–300-K temperature interval of the thermal conductivity of Si3N4 and BN polycrystalline ceramic samples and Si3N4/BN fiber monoliths (FM) with different fiber arrangement architecture, [0], [90], and [0/90], with fibers arranged, accordingly, along and across the sample axis and the [0] and [90] layers stacked alternately. In the 3.5–300-K interval, the heat capacity at constant pressure, and at 77 K, the sound velocity have been measured in polycrystalline Si3N4 and BN samples and in Si3N4/BN [0] fiber monoliths. Our studies suggest that, with a high enough degree of confidence, but for some compositions—with minor assumptions, it can be maintained that, in the case of the Si3N4/BN fiber monoliths, one can use for calculation of their thermal conductivities and heat capacities within certain temperature intervals simple models considering mixtures of the Si3N4 and BN components with due account of their contributions to formation of the Si3N4/BN FM. It has been established that in the low-temperature domain (5–25 K), phonons in Si3N4/BN [0], [90], and [0/90] fiber monoliths scatter primarily from dislocations. This effect is not observed in ceramic Si3N4 and BN samples. The experimental data obtained on the thermal conductivity, heat capacity, and sound velocity have been used to calculate phonon mean free path lengths in polycrystalline Si3N4 and BN samples and the effective mean free path length in the Si3N4/BN [0] FM.

Nanoscale amorphization at disclination quadrupoles in deformed nanomaterials and polycrystals

Special micromechanism of nanoscale amorphization (crystal-to-glass transition in nanoscale regions) in deformed nano- and polycrystalline materials is suggested and theoretically described. The nanoscale amorphization occurs through transformation of disclination quadrupoles generated during plastic deformation. It is shown that the nanoscale amorphization at disclination quadrupoles is energetically favorable in nano- and polycrystalline Si and Si3N4 deformed at high applied stresses in certain ranges of their structural parameters.

Raman Analysis of Microscopic Residual Stresses Stored in the Secondary Phase of Sc<sub>2</sub>O<sub>3</sub>-Doped Si<sub>3</sub>N<sub>4</sub>

Residual stress studies were performed on the intergranular phase of a Sc2O3-doped Si3N4 polycrystal. Sc2O3 additive represents an ideal substance for residual stress analysis because of the intense Raman spectrum of its related silicate (intergranular) phases, which form during sintering. Crystallization of Sc2O3 to Sc2SiO7 during the cooling process after sintering causes a negative volume change at triple pockets of polycrystalline Si3N4 which overlaps mismatches in thermal expansion coefficients between secondary phase and the Si3N4 matrix. Conventional piezo-spectroscopic (PS) stress analyses have usually been limited to measurements in the Si3N4 matrix. In this paper, we show for the first time measurements of residual stresses stored within the secondary phase of Si3N4, using the Raman spectrum of Sc2O3. A change in crack propagation mode from intergranular to transgranular was found after annealing the Sc2O3-doped Si3N4 material, such a difference leading to an embrittlement of the material. Residual stress analysis of the annealed sample revealed that the tensile residual stress stored within the secondary phase was removed, thus explaining why the rising R-curve behavior of the material was substantially suppressed as compared to the as-sintered sample.

Migration of Ion Implanted Hydrogen in Amorphous and Polycrystalline Si<sub>3</sub>N<sub>4</sub>:H Films: Experiments and Numerical Simulations

The tracer diffusion of ion implanted deuterium is studied in amorphous and polycrystalline magnetron-sputtered Si3N4:H films with secondary ion mass spectrometry (SIMS). The experimentally obtained diffusion profiles are numerically simulated by computer calculations based on the concept of trap-limited diffusion where the tracer atoms form immobile complexes with: (a) intrinsic film defects like dangling bonds and (b) extrinsic defects caused by the implantation damage. For amorphous Si3N4:H films a moderately high dissociation rate of intrinsic complexes (dangling bonds) is present and time independent effective diffusivities are observed, which obey an Arrhenius law with an activation energy of DE = 3.4 eV and a pre-exponential factor of D0 = 4 x 10-4 m2/s. For polycrystalline Si3N4 films non-Gaussian depth profiles and strongly time dependent diffusivities are observed, which have their reason in the presence of intrinsic traps with negligible dissociation, presumably located at the grain boundaries.

Local Viscosity of Si-O-C-N Nanoscale Amorphous Phases at Ceramic Grain Boundaries

Internal friction characterization has been used to quantitatively assess the viscosity characteristics of Si-O-C-N glasses segregated to nanometer-sized grain boundaries of polycrystalline Si3N4 and SiC ceramics. A relaxation peak of internal friction, which arises with rising temperature from the viscous sliding of glassy grain boundaries, was systematically collected and analyzed with respect to its shift upon changing the oscillation frequency. As a result of such an analysis, both activation energy for viscous grain-boundary flow and inherent viscosity of the intergranular glass film could be quantitatively evaluated. Two main features are shown: (i) the presence of N and/or C greatly affects the viscosity characteristics of SiO2 phases at Si3N4 and SiC grain boundaries; and (ii) the internal friction method has potential as a unique experimental tool for understanding the local properties of nanoscale amorphous phases in new ceramic materials.

Arrangement of rare-earth elements at prismatic grain boundaries in silicon nitride

The arrangement of rare-earth atoms at {100} prism planes of La- and Lu-containing polycrystalline Si3N4 specimens is studied using high-angle annular dark-field scanning transmission electron microscopy. For both systems, the attachment sites of rare-earth atoms are well-defined and largely conform to the periodicity of the terminating plane of the Si3N4 grain. We observe significant differences between the structural arrangement of La and Lu atoms at the interface.

Electronic Structure and Bonding of All Crystalline Phases in the Silica–Yttria–Silicon Nitride Phase Equilibrium Diagram

This paper reviews the structures and properties of 10 binary, ternary, and quaternary crystals within the equilibrium phase diagram of the SiO2–Y2O3–Si3N4system. They are binary compounds SiO2, Y2O3, Si3N4; ternary compounds Si2N2O, Y2Si2O7, and YSi2O5; and quaternary crystals Y2Si3N4O3(M‐melilite), Y4Si2O7N2, (N‐YAM), YSiO2N (wallastonite), and Y10(SiO4)6N2(N‐apatite, N‐APT). Although the binary compounds are well‐known and extensively studied, the ternary and the quaternary crystals are not. Most of the ternary and the quaternary crystals simply have been referenced as secondary phases in the processing of nitrogen ceramics. Their crystal structures are complex and not precisely determined. In the quaternary crystals, there exists O/N disorder in that the exact atomic positions of the anions cannot be uniquely determined. It is envisioned that a variety of cation–anion bonding configurations exist in these complex crystals. The electronic structure and bonding in these crystals are, therefore, of great interest and are indispensable for a fundamental understanding of structural ceramics. We have usedab initiomethods to study the structure and bonding properties of these 10 crystals. For crystals with unknown or incomplete structural information, we use an accurate total energy relaxation scheme to obtain the most likely atomic positions. Based on the theoretically modeled structures, the electronic structure and bonding in these crystals are investigated and related to various local cation–anion bonding configurations. These results are presented in the form of atom‐resolved partial density of states, Mulliken effective charges, and bond order values. It is shown that Y–O and Y–N bonding are not negligible and should be a part of the discussion of the overall bonding schemes in these crystals. Spectroscopic properties in the form of complex, frequency‐dependent dielectric functions, X‐ray absorption near‐edge structure (XANES), and the electron energy‐loss near‐edge structure (ELNES) spectra in these crystals also are calculated and compared. These results are discussed in the context of specific bonding configurations between cations (silicon and yttrium) and anions (oxygen and nitrogen) and their implications on intergranular thin films in polycrystalline Si3N4containing rare‐earth elements.

Preparation of Highly Oriented Aluminum Nitride Thin Films on Polycrystalline Substrates by Helicon Plasma Sputtering and Annealing

Highly c‐axis‐oriented aluminum nitride (AlN) thin films were prepared on polycrystalline Si3N4 and SiC substrates by helicon plasma sputtering. The difference in the substrate materials scarcely influenced the crystal structures of the films. The full width at half‐maximum (FWHM) of the X‐ray rocking curves of the films was 3.2°, which is the smallest value for AlN thin films deposited on polycrystalline substrates to our knowledge. Annealing at 800°C in vacuum decreased the FWHM from 3.2° to 2.8°. The structure of the films was densely packed and consisted of many fibrous grains. The films developed c‐axis orientation on the polycrystalline substrates, because the interaction between the films and substrate surfaces was small, and (100) and (110) AlN planes grew preferentially. The films were piezoelectric and generated electrical signals in response to mechanical stress.

Nanoindentation of a Polymer‐Derived Amorphous Silicon Carbonitride Ceramic

Nanoindentation has revealed significant scatter of the microscale hardness and elastic modulus of an amorphous SiCN ceramic, because of structural inhomogeneities (nanopores and clusters of free carbon within the material). As a consequence of the common feature, an amorphous nature, SiCN, in regard to its mechanical properties, resembles SiO2glass more than SiC or Si3N4. However, because of the stronger Si—C and Si—N covalent bonding, SiCN is harder and stiffer than SiO2. The mean hardness—13 ± 2 GPa, measured at a load of 250 mN for SiCN—is approximately half that of polycrystalline Si3N4(24.9 ± 0.6 GPa) but higher than that of SiO2glass (8.9 ± 0.04 GPa). The elastic modulus of the SiCN, measured at a load of 250 mN, is 121 ± 10 GPa.