Narrow Results

Materials and mechanical characteristics of the low temperature PECVD silicon nitrides have been investigated using various analytical and testing techniques. TEM and SEM examinations reveal that there is no distinct microstructural difference existing between the films deposited under different conditions. However, their mechanical properties determined by nanoindentation indicate otherwise. The variations in mechanical properties with deposition conditions are found to be strongly correlated to the change in silicon-to-nitrogen ratio in the film.

In this study, explosive indentations were used to characterize the dynamic damage behavior of silicon nitride ceramics with different microstructures. Three grades of silicon nitrides with different grain size and shape were prepared. Subsurface damage of specimens was characterized extensively using optical and electron microscopy. It was found that the dynamic damage response depends strongly on the microstructure of specimens, particularly on the glassy grain boundary phase.

Strength degradations in silicon nitride ceramics subject to damage from contact with hard spheres are investigated. Strengths against indentation load, number of cycles in contact, or stress-rate parameter are reported and compared with theoretical models. Silicon nitride ceramics are prepared by nitride pressureless sintering (NPS) process, which process is the continuous process of nitridation reaction of Si metal combined with subsequent pressureless sintering. Microstructure characterizations reveal silicon nitride fabricated by NPS process exhibits a quasi-plastic mode, with continuous strength loss beyond a load above the onset of yield, and falloff at high number of cycles, > 10⁵ at contact load, P = 950 N, using WC sphere r = 1.98mm. The strength degradation is substantially faster by dynamic fatigue. Failures originated from contact damages, quasi-plastic microcrack zones, with developing radial cracks during strength test. The implication is that quasi-plastic damage of NPS silicon nitride itself can preserve benefits from the inherent higher damage tolerance at lower number of cycles of contacts, but fatigue susceptibility at multicycle contacts and lower stressing rate.

Porous Si₃N₄-SiO₂-BN composites were prepared by adding starch as both pore former and consolidator. Bruggeman effective-medium model, Maxwell-Garnett model and logarithmic model were used to describe and predict the dielectric constant of porous Si₃N₄-SiO₂-BN ceramics. Relative dielectric constant of porous Si₃N₄-SiO₂-BN composites decreases with the increase of apparent porosity within limits, and these models can forecast the change of the dielectric constant of the porous ceramics quite well. The minimum relative dielectric constant is 2.5 at the apparent porosity of 0.555 at room-temperature. The relationship between dielectric constant and temperature were investigated. It was found dielectric constant varied a lot with the increase of temperature, and Debye relaxation theory was employed to explain the variation of the dielectric constant with temperature increment. But the Debye relaxation theory can not explain the reason of variation of dielectric constant at the temperature range from 300°C to 900°C. To ascertain the cause of changes of dielectric constant at this temperature region, differential scanning calorimentry (DSC) measurement was performed. In this temperature region, phase transition behavior occurs at nearly 300°C in the porous composites. The new phase probably has a tidy large dielectric constant, and the dielectric constant increases sharply.

The friction and wear of silicon nitride (Si₃N₄) against silicon nitride (Si₃N₄) and zirconia (Y–TZP) and chilled cast iron and Alumina sliding under dry friction at room temperature conditions were investigated with pin-on-disk tribometer at sliding speed of 0.56ms^-1 and normal load of 50N, 80N, respectively. Based on the variety regulation of the wear maps, the wear mechanisms of the two couples were analyzed. Get the result of friction coefficient and maps of wear Rate of the Pin and the Disk. The results of comparing this couple is Si₃N₄/ chilled cast iron < Si₃N₄/ ZrO₂< Si₃N₄/ Si₃N₄< Si₃N₄/ Al₂O₃.

In the present work, a series of samples were prepared by pressureless sintering from the starting materials of Si₃N₄ (α and/or β phases), SiO₂ and Li₂CO₃. The phase transformation was studied with emphasis on the influence of sintering temperature and Li₂CO₃ content. Related phase transformation was observed and analyzed by SEM and XRD and probable mechanisms were given, which will be helpful for the further explanation to the oxidation mechanism of Si₃N₄ based ceramics at elevated temperature.

Sn-doped silicon nanocrystals (Si-NCs) embedded in silicon nitride was synthesized by post-annealing of silicon-rich silicon nitride (SRN) films deposited by co-sputtering. The effects of Sn impurity on the Si crystallization behavior and photoluminescence (PL) in SRN films were investigated. Doping with Sn dopants enhances phase separation in SRN and thus accelerates the crystallization of silicon nanoparticles. It was also found that the existence of Sn decreases crystallization temperature, and could be as low as 800°C. In the Sn-doped samples, the crystalline volume fraction increases monotonically from 16.2% to 78.5% with increasing annealing temperature and the area densities of about ~2.2×10¹² cm^-2 can be achieved after annealing at 1100°C. In addition, the intensity of PL of the SRN film could be enhanced by adding Sn impurity.

Silicon nitride (Si₃N₄) has attracted substantial interest because of its extreme chemical and physical properties, but the sintering densification of Si₃N₄ is difficult, and it is easily oxidized in the high-temperature air to impact high-temperature strength, which restricts its applied range. In order to decrease the oxidization and improve the strength of Si₃N₄ at high temperature, the surface of Si₃N₄ is coated with Al(OH)₃ and Y(OH)₃ to synthesis Si₃N₄@Al(OH)₃–Y(OH)₃ core-shell composite particles. Through TEM, XRD, and BET analysis, when pH is about 9, Si₃N₄@Al(OH)₃–Y(OH)₃ core-shell composite particles are successfully synthesized by co-precipitation methods. Coating layer is about 200 nm, which is compaction and conformability. Dispersion of coated Si₃N₄ with Al(OH)₃ and Y(OH)₃ particles are very good. Synthesis of Si₃N₄@Al(OH)₃–Y(OH)₃ core-shell composite powder will lay the foundation for preparing high-performance YAG/Si₃N₄ multiphase ceramic materials.

The elementary jumps in the electron current through conducting filaments of two nanometer-sized virtual memristor structures consisting of a contact of a conductive atomic force microscope probe to the Si₃N₄ layer with the thickness of 6nm deposited onto the $n^{++}$-Si(001) conductive substrates are investigated. These structures are: (S1) the Si₃N₄/Si film; (S2) the Si₃N₄/SiO₂/Si stack, a similar structure with 2nm SiO₂ sublayer between the film and the substrate. Usually, such investigations are performed by the analysis of the waveform of this current with the aim of extracting the random telegraph noise (RTN). Here, we develop a new indirect method, which is based on the measurement of the spectrum of the low-frequency flicker noise in the current without extracting the RTN. We propose that the flicker noise is caused by the motion (drift/diffusion) of nitrogen ions via vacancies within and around the filament. The number of these ions is estimated by taking into account the geometrical parameters of the filament. This allows us to estimate the root mean square magnitude $i_0$ of the current jumps, which are caused by random jumps of nitrogen ions, and the number M of these ions. This is fundamental for understanding the elementary mechanisms of the electron current flowing through the filament and the resistive switching in memristor devices.

The concept of elastic-modulus-graded ceramics for improved resistance to quasi-static contact damage (Hertzian-indentation), sliding-contact damage, and wear is reviewed. In these graded materials, the in-plane elastic modulus (E) is low at the contact surface and high in the interior (substrate) with a continuous, or step-wise continuous, E-gradation in-between. Processing strategies for fabricating such E-graded ceramic composites in the Al₂O₃-glass, the Si₃N₄-glass, and the Si₃N₄-SiC systems are described. The Hertzian indentation (quasi-static and sliding) behavior of these composites, along with some results from wear tests, are reviewed. Computational modeling (finite-element analysis or FEA) results are also reviewed, and are used to discuss the role of E-gradients in imparting contact-damage resistance to these materials. The use of calibrated FEA models as predictive tools for the design of next-generation graded materials is also discussed.

Poly(methyl methacrylate)-b-poly(vinyltriethoxysilane) (PMMA-b-PVTES) are synthesized using atom transfer radical polymerization (ATRP) and used as macromolecular coupling agents to modify silicon nitride nanoparticles (nano-Si₃N₄). The chemical composition of copolymers PMMA-b-PVTES and modified nano-Si₃N₄ are confirmed by various characterization techniques. The modified nano-Si₃N₄ shows excellent hydrophobic nature, which make nanoparticles (NPs) stably disperse in organic solvent. Transmission electron microscope (TEM), particle size testing, contact angle measuring and sedimentation experiment are employed to examine the dispersion of modified nano-Si₃N₄ in chloroform. By comparing the effects of copolymers with varied number-average molecular weight (M_n) and block length ratio on stabilizing nano-Si₃N₄, we discussed the mechanism of macromolecular coupling agent stabilizing NPs. In our experiments, the copolymer PMMA₈₈-b-PVTES₁₇ is found to be the most effective macromolecular coupling agent for stabilizing nano-Si₃N₄.

Good breakdown strength is an important feature for the selection of dielectric materials, especially in high-voltage engineering. Although nanocomposites have been shown to possess many promising dielectric properties, the breakdown strength of nanocomposites is often found to be negatively affected. Recently, imposing nonisothermal crystallization processes on polyethylene blends has been demonstrated to be favorable for breakdown strength improvements of dielectric materials. In an attempt to increase nanocomposites’ voltage rating, this work reports on the effects of nonisothermal crystallization (fast, moderate and slow crystallizations) on the structure and dielectric properties of a polyethylene blend (PE) composed of 80% low density polyethylene and 20% high density polyethylene, added with silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) nanofillers. Through breakdown testing, the breakdown performance of Si₃N₄-based nanocomposites was better than SiO₂-based nanocomposites. Since nanofiller dispersion within both nanocomposite systems was comparable, the enhanced breakdown performance of Si₃N₄-based nanocomposites is attributed to the surface chemistry of Si₃N₄ containing less hydroxyl groups than SiO₂. Furthermore, the breakdown strength of SiO₂-based nanocomposites and Si₃N₄-based nanocomposites improved, with the DC breakdown strength increasing by at least 12% when both the nanocomposites were subjected to moderate crystallization rather than fast and slow crystallizations. This is attributed to changes in the underlying molecular conformation of PE in addition to water-related effects. These results suggest that apart from changes in the nanofiller surface chemistry, changes in the underlying molecular conformation of polymers are also important to improve the breakdown performance of nanocomposites.

In this work, we present an investigation of the photovoltaic properties of low-temperature (700°C annealing temperature) prepared P-doped Silicon nanocrystals (Si NCs) in silicon nitride by ammonia sputtering followed by rapid thermal annealing (RTA). We examined how the flow rate of NH₃ influenced the structural properties of the annealed films by using Raman scattering, grazing incidence X-ray diffraction (GI XRD) and transmission electron microscopy (TEM), it was found that the appropriate flow rate of NH₃ is 3 sccm. For the sample deposited at the flow rate of 3 sccm, TEM image showed that Si NCs were formed with a mean size about 3.7 nm and the density of ~ 2.1 × 10¹² cm^-2; X-ray photoelectron spectroscopy (XPS) characterization showed the existence of Si–P bonds, indicating effective P doping; the average absorptance of higher than 65% and a significant amount of photocurrent makes it suitable for photoactive. Moreover, the experimental P-doped Si NCs:Si₃N₄/p-Si heterojunction solar cell has been fabricated, and the device performance was studied. The photovoltaic device fabricated exhibits an open-circuit voltage (V_OC) and a short-circuit current density (J_SC) of 470 mV and 3.25 mA/cm², respectively.

Si₃N₄–SiC composites were fabricated by spark plasma sintering at 1700${}^{\circ }$C for 480 S with MgSiN₂ and Y₂O₃ as additives. The morphology and phase characterization of the composites were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The values of $n$ parameter indicate that the grain boundary reaction is the rate controller at 1500${}^{\circ }$C and diffusion becomes the controlling step at 1550${}^{\circ }$C. The nanohardness and Young’s modulus attained the maximum value of 18.5 and 316GPa, respectively.

The promising role of structural ceramics in many useful devices and structures has in turn led to realization of the importance of developing ceramic joining techniques, which is perhaps one of the most important areas in the ceramic field for research and development. The increasing demand for joining ceramics arises from: a) fabrication of ceramic components of complicated shape or large size in one piece can be very expensive and in some cases impractical, and b) in many cases, because of the limitation of ceramics resulting from poor impact properties and lack of tensile ductility they have to be attached to metals, which must withstand stresses or temperature gradients too great for ceramics. This paper covers methods and problems of joining of ceramics with emphasis on brazing technique. This also includes examples of joining silicon nitride ceramic to itself and to molybdenum.

Improved of chemical corrosion resistance of silicon nitride (Si₃N₄) ceramics is strongly desired. However, there is not enough information on chemical corrosion at present. In this paper, corrosion resistance of two kinds of Si₃N₄ based ceramics with different additives developed for bearing material was investigated. One of the specimens is composed of Si₃N₄-Y₂O₃-Al₂O₃-AlN-TiO₂, the other of Si₃N₄-SiC-MgAl₂O₄-SiO₂-TiO₂. A corrosion test was conducted under conditions of 240h-soaking at 30°C and 100h-soaking at 80°C in acid and base solutions. The weights of both specimens after soaking were confirmed to decrease with decreasing concentration of sulfuric acid. The bending strength also decreased in proportional to the weight change. Contrary to the results in acid solutions, the weights of the specimens decreased with increasing concentration of sodium hydroxide solutions. The weight changes at 80°C were much larger than those at 30°C. The extent of corrosion was found to depend on the composition of the ceramics. Specimens with additives of Y₂O₃, Al₂O₃, AlN, and TiO₂ tended to corrode more easily than those with SiC, MgAlO₄, SiO₂ and TiO₂. A correlation between the weight loss and the bending strength was established.