High-purity Silicon Nitride Research Articles

Deposition and Thermal Stability of DLC/Si-N Composite Films Synthesized Using a Sputtering-PBII Hybrid System

Diamond-like carbon (DLC) is a metastable amorphous material that exhibits unique properties. However, there are many limitations regarding the use of this material due to factors such as its tribological characteristics at high temperature and limited thermal stability. In this study, the thermal stability and tribological properties of DLC/silicon-nitrogen (DLC/Si-N) composite films were investigated and compared to those of pure DLC films. All the films were synthesized using a combination of radio frequency (RF) magnetron sputtering and plasma-based ion implantation (PBII) (a so-called sputtering-PBII hybrid system) which is newly developed by us. A high purity silicon nitride (99.9%) disk was used as the target, applying an RF power in the range of 500 - 700 W and a negative pulsed bias voltage of 5 kV to the substrate. An Ar-CH4 mixture was used as the reactive gas. The CH4 partial pressure was varied between 0 and 0.15 Pa, while the total gas pressure and total gas flow were fixed at 0.30 Pa and 30 sccm, respectively. The structures of the resulting films were characterized using Raman spectroscopy, while the thermal stabilities were assessed using thermogravimetric-differential thermal analysis (TG-DTA) and friction coefficients were obtained via ball-on-disk friction tests. The results indicate that the DLC/Si-N composite films produced in this work exhibit improved thermal stability relative to that of pure DLC owing to the presence of thermally stable atomic-scale Si-N compound in the carbon main flame networks. A DLC/Si-N film containing approximately 11 at.%Si and 18.5 at.%N shows good thermal stability in air over 800°C up to 1100°C, together with excellent tribological performance at 500°C in air. Overall, the data demonstrate that DLC/Si-N composite films offer improved thermal stability and superior tribological performance at high temperatures.

Effects of residual radicals on compositional and structural stability of silicon nitride fibers

In this study, amorphous silicon nitride fibers were prepared through the nitridation of cured polycarbosilane fibers. It was observed that their composition and properties can be controlled by adjusting the flow of NH3 during the nitridation process. Based on their compositional and structural stability, the samples could be divided into two classes: stable fibers and unstable fibers. As indicated by the electron spin resonance spectra, the amount of the residual free radicals in the unstable fibers was significantly higher than that in the stable fibers. Because of the reactions of the radicals with the moisture in the air, the oxygen content of the unstable fibers increased day by day until the residual radicals were exhausted. Thus, to develop silicon nitride fibers with both low oxygen content and low carbon content, the amount of NH3 used should be optimized to eliminate the free radicals. These results suggest that it is possible to tailor the nitridation conditions for preparing high-purity silicon nitride materials so that they exhibit desirable properties, such as compositional and structural stability, good mechanical properties, and high electrical resistivity.

Direct determination of trace impurities in high-purity silicon nitride by axial viewed inductively coupled plasma optical emission spectrometry using a slurry nebulization technique

A simple, rapid and reliable method was developed for the determination of trace impurities in high-purity silicon nitride (nm- and μm-sized) by ICP-OES using a slurry nebulization technique.

Windows for small-angle X-ray scattering cryostats

To determine the suitability of commonly used windows for small-angle X-ray scattering, a range of materials, including Kapton, (aluminized) Mylar, beryllium, high-purity aluminium foil, mica and silicon nitride have been studied. At small wavevector transfers, Q, in the range 2 x 10(-3) to 0.2 nm(-1), the scattering from Kapton, mica and beryllium is reasonably well described by power laws in Q with exponents of -3.25, -3.6 and -3.9, respectively. There are large variations in the scattering from mica, but a freshly cleaved natural mica window was by far the weakest scatterer. For applications where radiation in the infrared or visible range should be blocked, aluminized Mylar is the most suitable material. Both Mylar and Kapton can be used to make very simple demountable superfluid-tight windows using indium O-ring seals.

Suppression of Viscous Grain-Boundary Sliding in a Dilute SiAION Ceramic

Aluminum nitride (AIN) and alumina (Al 2 O 3 ) additives were blended into a high-purity silicon nitride (Si 3 N 4 ) powder, and optimized processing conditions were applied for promoting a sintering process, which was shown to partly remove the amorphous silica-rich (SiO 2 -rich) remains from grain boundaries. Such an approach led to the formation of a dilute SiAION structure. The microstructure of the obtained SiAION material consisted of two distinct types of matrix phases, namely, a main network of equiaxed (dilute) β-SiAlON grains, with a minor fraction of (oxygen-rich) acicular O'-SiAION grains. As a consequence of the resulting solid solution of aluminum plus oxygen in the Si 3 N 4 lattice, many grain boundaries were free from any amorphous interlayer, as revealed by high-resolution electron microscopy (HREM) observation. Other than some boundaries that were still wetted, small amounts of residual glass were noted at some triple-grain pockets. However, the average size of such glass pockets was only a few nanometers. The observed microstructural characteristics were critical for the anelastic response of the present SiAION material, as compared with undoped Si 3 N 4 , whose grains were continuously encompassed by an amorphous SiO 2 film. Internal-friction data that were collected up to very high temperatures showed that, in the present SiAION material, no anelastic relaxation peak was monitored. Thus, viscous sliding along grain boundaries was inhibited. In addition, the partial elimination of the amorphous SiO 2 film also implied that no continuous path for oxygen diffusion was available along grain boundaries. Because of this important circumstance and despite the possible softening effect due to solid solution, the viscoelastic (background) component of the internal-friction data shifted toward higher temperatures and the creep resistance of the material was improved.

Fabrication of high performance 3D SiO2/Si3N4 composite via perhydropolysilazane infiltration and pyrolysis

Perhydropolysilazane, a low viscosity preceramic polymer with good wettability and high char yield, was used to fabricate three-dimensional silica fiber reinforced silicon nitride matrix composites through the repeated infiltration-curingpyrolysis cycles. With the increase of the pyrolysis temperature from T 1, T 2 to T 3, the density of the composites increased all through, but the flexural strength showed a maximum value at T 2 followed by a sharp decrease. The composite prepared at T 2 exhibited a good ceramization of the preceramic polymer, a high flexural strength of 144.9 MPa and excellent dielectric property. The high performance of the composite resulted from the good state of the silica fibers, controlled fiber/matrix interfacial microstructures and high-purity dense silicon nitride matrix.

Variation of Width and Composition of Grain-Boundary Film in a High-Purity Silicon Nitride with Minimal Silica

Contrary to the widely accepted observation that grain-boundary amorphous films for a given Si 3 N 4 composition have common (equilibrium) widths and compositions, a significant variation for both parameters from film to film was observed in an undoped high-purity Si 3 N 4 prepared using a hot isostatic pressing method. This material previously has been reported to have an equilibrium film width of 0.6 nm, as measured using a high-resolution electron microscopy (HREM) method; this value is significantly different from that which is typical for other high-purity Si 3 N 4 ceramics (1.0 nm). A total of four boundaries were analyzed, using spatially resolved electron energy-loss spectroscopy methods, which can give the chemical width and composition for the film. Widths of these grain-boundary films were substantially different from each other; only the thinnest matches the previous HREM observations. The nitrogen content in the film decreased concurrently as the film thickened. This material had many cavities and complicated configurations at triple pockets, because of the very low total-SiO 2 content (0.55 vol%). They created locally different equilibrium conditions for grain-boundary films, in comparison with other fully densified Si 3 N 4 , causing such strong variation in both film structure and chemistry. This observation reveals the importance of triple pockets in equilibrium film structures, providing new insight in evaluating the absorption and wetting models. The thinnest film may correspond to the amorphous structure that is required to bind two randomly oriented Si 3 N 4 grains under greater local stress.

On the Effect of Local Grain-Boundary Chemistry on the Macroscopic Mechanical Properties of a High Purity Y2O3-Al2O3-Containing Silicon Nitride Ceramic

ABSTRACTThe effects of grain-boundary chemistry on the mechanical properties of a high-purity silicon nitride ceramics were investigated, with specific emphasis on the role of oxygen. Variations in the grain-boundary oxygen content, through control of oxidizing heat treatments and sintering additives, was found to result in a transition in fracture mechanism from transgranular to intergranular fracture, with an associated increase in fracture toughness. This phenomenon is correlated to an oxygen-induced change in grain-boundary chemistry that appears to affect fracture by “weakening” the interface, facilitating debonding and crack advance along the boundaries, thereby enhancing the toughness by grain bridging. It is concluded that if the oxygen content in the thin grain-boundary films exceeds a lower limit, which is ∼0.87 equiv% oxygen content, then the interfacial structure and bonding characteristics favor intergranular debonding during crack propagation; otherwise, transgranular fracture ensues, with consequent low toughness.

Effects of the Amorphous Oxide Intergranular Layer Structure and Bonding on the Fracture Toughness of a High Purity Silicon Nitride

ABSTRACTThe microstructural evolution and structural characteristics and transitions in the thin grain-boundary oxide films in a silicon nitride ceramic, specifically between two adjacent grains and not the triple junctions, are investigated to find their effect on the macroscopic fracture properties. It is found that by heat treating a model Si3N4-2wt% Y2O3ceramic for ∼200 hr at 1400°C in air, the fracture toughness can be increased by ∼100%, coincident with a change in fracture mechanism from transgranular to intergranular. Structural phase transformations occur in the thin grain boundaries during oxidation that are revealed by XRD, EDX, Raman and EELS analyses. They affect the local bonding of atoms. It is concluded that only specific crystal “building blocks”, i.e., tetrahedra, are transformed along the grain boundary and the resulting difference in the atomic structure of the oxide interface is seen directly to alter the macroscopic fracture behavior.

Ultrahigh-temperature deformation of high-purity HIPed Si3N4

High-temperature tensile deformation behavior of high-purity HIPed silicon nitride material was investigated in the temperature range between 1600°C and 1750°C. Recoverable anelastic and non-recoverable deformation was observed in high-purity HIPed silicon nitride. A power-law deformation model analogous to rheological models was used to distinguish the different deformation components. A stress exponent n = 1.64 and an activation energy Q 1 = 708 kJ/mol was determined for the non-recoverable deformation. For the anelastic deformation a stress exponent p = 4 and an activation energy Q 3 = 619 kJ/mol was observed. Diffusional creep and grain boundary sliding with the accomodation process responsible for the anelastic component are discussed as deformation mechanisms.

Compressive Creep and Stress Relaxation Kinetics in a High Purity Silicon Nitride Ceramics in the 1400-1650 °C Range

Compressive Creep and Stress Relaxation Kinetics in a High Purity Silicon Nitride Ceramics in the 1400-1650 °C Range

The pyrolytic conversion of perhydropolysilazane into silicon nitride

Polymeric precursors to ceramics have the potential to enable production of ceramic articles in novel forms, with novel microstructures, and improved properties. The polymer-to-ceramic conversion process is not sufficiently understood for this approach to be widely utilised. Perhydropolysilazane converts to high purity silicon nitride (Si 3N 4), and has been shown to be a useful coating and matrix precursor. The conversion of PHPS to Si 3N 4 is being examined by Solid State, Magic Angle Spinning (MAS) NMR spectroscopy and other techniques. This paper summarises the results to date of this investigation.

Grain‐Boundary Relaxation in High‐Purity Silicon Nitride

Internal friction, torsional creep, and shear modulus relaxation experiments were conducted on a model Si3N4 polycrystalline material, which contained a continuous amorphous film of pure SiO2 at the grain boundary. Internal friction experiments were performed in the frequency range between 3 and 13 Hz, in 5 Pa of nitrogen atmosphere. Very high temperatures (up to 2000°C) could be applied for the first time by using a newly developed torsional pendulum apparatus. This apparatus was also capable of precise torsional strain measurements under static‐load conditions. The internal friction curves at various frequencies were generally found to consist of a grain‐boundary peak super‐imposed on an exponential‐like background. The peak, of anelastic diffusive origin, was centered in the temperature range of 1612–1710°C depending on the frequency of the measurement, namely within an interval of about 100°C below the nominal melting point of the pure SiO2 phase (i.e., ∼ 1730°C). The background was instead found to be of viscoelastic nature. A common micromechanical origin between the creep plastic strain and the internal friction background curves was identified and the data could be fitted by the same Arrhenius plot. Structural and chemical characterization of internal grain boundaries was performed by high‐resolution electron microscopy (HREM) in addition to electron energy‐loss spectroscopy (EELS). A small amount of nitrogen was detected within the amorphous residue along grain boundaries. According to the above set of microstructural/chemical and mechanical data, the viscosity properties of the intergranular phase were assessed and the sliding mechanism between adjacent Si3N4 grains was modeled.

Grain‐Boundary Microstructure and Chemistry of a Hot Isostatically Pressed High‐Purity Silicon Nitride

Two high‐purity Si3N4 materials were fabricated by hot isostatic pressing without the presence of sintering additives, using an amorphous laser‐derived Si3N4 powder with different oxygen contents. High‐resolution transmission electron microscopy and electron energy‐loss spectroscopy (EELS) analysis of the Si3N4 materials showed the presence of an amorphous SiO2 grain‐boundary phase in the three‐grain junctions. Spatially resolved EELS analysis indicated the presence of a chemistry similar to silicon oxynitride at the two‐grain junctions, which may be due to partial dissolution of nitrogen in the grain‐boundary film. The chemical composition of the grain‐boundary film was SiNxOy, (x ∼ 0.53 and y ∼ 1.23), and the triple pocket corresponded to the amorphous SiO2 containing ∼2 wt% nitrogen. The equilibrium grain‐boundary‐film thickness was measured and found to be smaller for the material with the lower oxygen content. This difference in thickness has been explained by the presence of the relatively larger calcium concentration in the material with the lower amount of SiO2 grain‐boundary phase, because the concentration of foreign ions has been shown to affect the grain‐boundary thickness.

Preparation and nitridation of silicon whiskers

The preparation of high-purity silicon nitride powder (useful as high-temperature ceramic material) from silicon tetrachloride has been studied. Zinc was chosen for reduction to Si, which involved vapour reaction in a two-zone horizontal furnace. The silicon nitride whiskers formed were characterized by x-ray diffraction (XRD) and electron microscopy. The whiskers were very thin (approx 0.5 mu m, length gt; 1 cm) at low reaction temperature (950 deg C). In situ nitration with increased nitrogen flow was tried without success, although 0.1 mu m whisker thickness was obtained, and dry ammonia gas was then used for separate nitration at 1200-1300 deg C. XRD and infrared absorption data revealed alpha -Si sub 3 N sub 4 as the product (thickness 0.5, length approx 12 mu m). Such whisker-like particles with uniform aspect ratio may also be useful in ceramic composites.

Chemistry of grain boundaries in calcia doped silicon nitride studied by spatially resolved electron energy-loss spectroscopy

Spatially resolved electron energy-loss spectroscopy has been used to interrogate the chemistry and bonding at grain boundaries in hot isostatically pressed (HIPed) high purity silicon nitride ceramics nominally doped with either 0, 100, 300 or 500 at. ppm calcium. Analysis of the nanometer thick glassy films between grains, shows that there is a monotonic increase in impurity segregation with dopant concentration. The predominant constituents of the grain boundary films are Si, O and N, forming a silicon oxynitride phase ranging in composition from SiO 2 to Si 2N 2O. The energy-loss near-edge structure indicates the structure of the glass is composed predominantly of tetrahedra but also suggests the presence of a small concentration of molecular N 2, possibly as sub-microscopic bubbles. There is still no data on the composition gradients on a sub-nanometer level, though it is shown that in the detection limit set by counting statistics, near single atom sensitivity with a spatial resolution of 0.22 nm is achievable.

Multi-element characterization of silicon nitride powders by atomic and mass spectrometric solution methods

A sample pretreatment technique for silicon nitride involving digestion and matrix/traces separation was developed by means of radiotracers and applied to analysis of this material by inductively coupled plasma atomic emission spectrometry, inductively coupled plasma mass spectrometry and graphite furnace atomic absorption spectrometry. The results obtained for a high purity silicon nitride material by these methods are compared each with the other and with those obtained by neutron activation analysis. The limits of detection and the capabilities of the methods are compared and discussed.

Fracture Toughness and Time‐Dependent Strength Behavior of Low‐Doped Silicon Nitrides for Applications at 1400°C

The influence of small additions of three selected oxides on the microstructure and the mechanical behavior of high‐purity silicon nitride was systematically investigated. Dense silicon nitride bodies doped respectively with SiO2, Y2O3, and Yb2O3 were fabricated by hot isostatic pressing (HIP). Two different compositions of the intergranular phase (nominal oxide additions of 0.7 and 1.7 vol%) were examined for Y2O3 and Yb2O3 in comparison with the same volume of pure SiO3. Only in the material with the higher Y2O3 and Yb2O3 content was an improved level of fracture toughness obtained. The mechanical properties at 1400°C were evaluated with emphasis placed on time‐dependent strength and deformation behavior. The materials containing only SiO2 or doped with the small amount of Y2O3 showed linear elastic KI‐controlled fracture behavior at 1400°C and the critical phenomenon for failure was subcritical crack growth (SCG) from preexisting defects. In the materials with additions of Yb2O3 or the larger amount of Y2O3, crack extension was governed by creep crack growth as a result of the exhibited strong creep effects. In the silicon nitride doped with 1.7 vol% Yb2O3, however, a considerably improved creep behavior as a consequence of crystallization processes in the intergranular phase (Yb2Si2O7) caused by both thermal treatment and stress‐initiated effects during the mechanical testing at 1400°C was found.

Determination of sub-ng g −1 concentrations of thorium and uranium in microelectronic materials by radiochemical neutron activation analysis

A radiochemical neutron activation analysis technique for the determination of thorium and uranium in various high-purity microelectronic materials was developed. It is based on the production of 233Pa and 239Np as indicator radionuclides for Th and U, respectively, and their simultaneous separation by anion exchange from 12 M hydrochloric acid followed by elution and 1 M ammonium fluoride. Using the radiotracer technique, separation yields of Np and Pa and 32 other elements were determined. The method was applied to the determination of Th and U at ng g −1 and sub-ng g −1 levels in high-purity quartz, silicon nitride, polyimide and aluminium oxide. Depending on the kind and purity grade of the sample, limits of detection of 10–100 pg g −1 were achieved. Comparison of the results obtained by this method with those obtained by instrumental neutron activation analysis and inductively coupled plasma mass spectrometry showed satisfactory agreement and illustrates the advantages of this technique when ultra-low concentrations of Th and U are to be determined.

Microstructure and grain-boundary characterization of HIP'ed high-purity silicon nitride

Laser synthesized, ultrafine silicon nitride was densified via hot isostatic pressing (HIP'ing) without any oxide-sintering-aid additions. HIP'ing was performed on exposed samples made from powder that had been exposed to the atmosphere, thereby picking up an oxide surface layer; and unexposed samples made from powders processed entirely under glove-box conditions, i.e. with minimal oxygen contamination. High-resolution transmission electron microscopy (TEM) studies indicate that the exposed Si3N4 powder samples, HIP'ed at temperatures in excess of the melting point of SiO2, densified via a solution-reprecipitation mechanism, with an intergranular glassy phase of high-purity SiO2. In contrast, samples of unexposed Si3N4 powder had to be HIP'ed to 2050°C to achieve a density of ∼ 70%ρTh (where ρTh is the theoretical density of silicon nitride). In this state, the sample consisted of equiaxed β-Si3N4 grains, with localized high-density regions. These regions had clean grain boundaries.