Investigation on thermal conductivity and mechanical properties of Si3N4 ceramics via one-step sintering

High thermal conductivity Si₃N₄ ceramics were fabricated using a one-step method consisting of reaction-bonded Si₃N₄ (RBSN) and post-sintering. The influence of Si content on nitridation rate, β/(α+β) phase rate, thermal conductivity and mechanical properties was investigated in this work. It is of special interest to note that the thermal conductivity showed a tendency to increase first and then decrease with increasing Si content. This experimental result shows that the optimal thermal conductivity and fracture toughness were obtained to be 66 W (m K)^-1 and 12.0 MPa m^1/2, respectively. As a comparison, the nitridation rate and β/(α+β) phase rate in a static pressure nitriding system, i.e., 97% (MS10), 97% (MS15), 97% (MS20) and 8.3% (MS10), 8.3% (MS15), 8.9% (MS20), respectively, have obvious advantages over those in a flowing nitriding system, i.e., 91% (MS10), 91% (MS15), 93% (MS20) and 3.1% (MS10), 3.3% (MS15), 3.3% (MS20), respectively. Moreover, high lattice integrity of the β-Si₃N₄ phase was observed, which can effectively confine O atoms into the β-Si₃N₄ lattice using MgO as a sintering additive. This result indicates that one-step sintering can provide a new route to prepare Si₃N₄ ceramics with a good combination of thermal conductivity and mechanical properties.     

Nitridation Kinetics of Silicon Monocrystal and Morphology of Nitride Film

Basing on TGA (thermal gravimetric analysis) of thermal nitridation at l200, l250, l300℃, respectively,analysis of high temperature kinetics for nitridation of silicon monocrystal has been carried out. According tothe theory for kinetics of reaction of vapour with solid phase a nitridation kinetic model, from which it can beshown thal the rate of nitridation reaction of silicon crystal should be controlled by three stage limiting factors,was proposed. These limiting factors are chemical reaction, chemical reaction mixed with diffusion and diffu-sion. Using this model to treat our experimental data, satisfactory correlation coefficient and apparentactivation energy of nitridation of p-type (lll) silicon crystal have been obtained. The nitride film was identi'fied to be a-Si_3N_4 (Hexagonal, a=0.7758nm,c_o=0.5623nm) by X-ray diffraction analysis. Morphology ofthe nitride films formed in different nitridation duration was observed in both planar andcross-sectional viewsby SEM (scanning electron microscope).     

Synthesis and spark plasma sintering of the α-Si3N4 nanopowder

A two-step process (milling and then heat treatment) was used for the preparation of α-Si₃N₄ nanopowder. The influence of the milling time and heat treatment temperature as processing parameters were investigated on the formation of α-Si₃N₄. Silicon nitride ceramic was produced by spark plasma sintering at 1700 °C for 15 min, using MgSiN₂ additive. The optimum sample was produced in a 30 h milling time, heat treatment at 1300 °C, and a 22 °C/min heating rate conditions. X-ray fluorescence analysis showed that the purity of the final product is above 98%. Nanoindentation hardness and Young’s modulus of the SPS-ed sample were measured as 17±2.0 GPa and 290±11.0 GPa, respectively.     

Synthesis of AlN nanopowder coated with a thin layer of C,using a thermal plasma reactor

High-purity nanocrystalline aluminum nitride powders were synthesized by using a 12?kW non-transferred arc plasma. The synthesis was conducted in a versatile, new designed, one-chamber thermal plasma reactor (TPR). The novel experimental assembly incorporated better working conditions like: high temperature gradient between the crucible and reactor's wall, and high super-saturation of the system by nitrogen and carbon. Thermodynamic modelling of the synthesis was conducted in order to achieve the best conditions for AlN formation. In this study, aluminum discs of Al 1100 were used as precursor material and pure nitrogen was the only gas used as reagent and plasmogenic gas.Nanopowders collected from reactor's wall were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-Ray diffraction (XRD). Synthesized h-AlN nano-powders were found to be free of oxides and aluminum metal. A thin carbon-layer around the particles was detected. TEM results indicated that the carbon-layer was around 5 and 10?nm. This outcome could make a significant difference with other synthesis reported in the literature since the occurrence of the carbon-layer, could delay AlN oxidation, prevent hydration, and could avoid the agglomeration of the particles.     

Fabrication of Si3N4 ceramics by post-reaction sintering using Si–Y2O3–Al2O3 nanocomposite particles prepared by mechanical treatment

Post-reaction sintering of a powder compact of Si and sintering aids is a useful technique for fabricating silicon nitride (Si₃N₄) ceramics at low costs. In order to inhibit the inhomogeneous and uncontrollable exothermic nitridation of Si in the powder compact, Si–Y₂O₃–Al₂O₃ nanocomposite particles are designed as an aid for post-reaction sintering. These Si–Y₂O₃–Al₂O₃ nanocomposite particles are prepared via mechanical treatment applying high shear stress. Scanning electron microscopy (SEM) observations show that Y₂O₃ and Al₂O₃ particles are homogenously dispersed, and fixed to the Si particles. A green compact prepared using the Si–Y₂O₃–Al₂O₃ nanocomposite particles results in lower electrical resistivity than that prepared using a powder mixed by wet ball-milling, which suggests that Si particles in the green compact prepared using the nanocomposite particles are isolated by Y₂O₃ and Al₂O₃ particles. The isolation of Si particles by the sintering aids successfully prevents the Si particles from melting and agglomerating during the nitridation process, resulting in a higher nitridation ratio and higher α-Si₃N₄ phase content due to the inhibition of rapid heat transfer caused by the exothermic reaction. The nitridation ratio also increases with the applied power during mechanical treatment. As a result of firing the homogeneously nitrided powder compacts at high temperatures, Si₃N₄ ceramics with homogeneous microstructure and improved density are successfully fabricated in this manner.     

Preparation of H-bar cross-sectional specimen for in situ TEM straining experiments: A FIB-based method applied to a nitrided Ti-6Al-4V alloy

The in situ tensile straining of cross-sectional specimens inside a TEM is intrinsically very difficult to perform despite its obvious interest to study interfaces of surface treated materials. We have combined a FIB-based method to produce H-bar specimens of a nitrided Ti-6Al-4V alloy and in situ TEM straining stage, to successfully study the plastic deformation mechanisms that are activated close to the nitrided surface in the Ti-based alloy.     

Impact of preanneal process on threshold voltage of MOS transistors for trench DRAM

As the first step of DRAM manufacture, preanneal process plays an important role in determining the threshold voltage variation. It is found that the higher trans-1,2-dichloroethene flow in pad oxide growth and the higher nitrogen flow in high-temperature annealing step would respectively engender a lower boron segregation coefficient and higher nitridation of the oxide, both modify the boron distribution in the substrate and consequently the behavior of the threshold voltage. As the feature size of DRAM devices enter nanometer regime, besides gate oxidation, ion implantation and related thermal processes, the impact of preanneal process condition should be prudentially taken into consideration for rigorous control of the threshold voltage in the advanced DRAM production.     

Rapid thermal nitridation of thin SiO2 films

An alternative to SiO₂ for gate dielectric applications in MIS devices is nitrided silicon dioxide. A study of this material is presented in this paper. Thin SiO₂ layers (10 nm minimum thickness) were grown on silicon substrates and subsequently nitrided in ammonia at 1 atm using a rapid thermal processing system. Nitridation times ranged from 3 sec to 60 sec at temperatures from 900 to 1200‡ C. The resulting films were then characterized using a variety of techniques including high resolution TEM, XPS, AES, SIMS, and electrical measurements (C-V). Higher temperatures and longer processing times resulted in the accumulation of nitrogen at the film surface and at the Si/SiO₂ interface. As expected, the electrical characteristics of the nitrided films were strongly influenced by the processing conditions. The morphology of the interface, as revealed by high-resolution TEM, was also altered by the nitridation process, especially for high processing temperatures (>1000° C).     

A novel family of solid basic catalysts obtained by nitridation of crystalline microporous aluminosilicates and aluminophosphates

Novel basic catalysts are obtained by ammonia treatment of crystalline, microporous aluminosilicates (zeolites) and aluminophosphates at temperatures above 800°C. The resulting materials are active catalysts in the Knoevenagel condensation of benzaldehyde with malononitrile, presumably due to the presence of nitrogen-containing species bound to the crystalline framework. While nitridation of zeolite NaY at temperatures around or below 800°C does not result in an increase of the catalytic activity, ammonia treatment at 850–875°C produces a significantly more active material. Further typical experimental results are presented which suggest that the activity gain seems to depend not only on the temperature of ammonia treatment but also on the structure and the chemical composition of the parent material. The novel microporous catalysts with their reasonable base strength offer the principle possibility to perform base catalyzed reactions in a shape selective manner.     

Room temperature deposition of self-assembled Al nanoclusters on stepped sapphire (0001) surface and subsequent nitridation

Self-assembled growth and nitridation of ultrathin Al nanoclusters on a stepped sapphire (0001) surface were studied by high-resolution X-ray photoemission spectroscopy, atomic force microscopy and low-energy electron diffraction (LEED). Upon room temperature deposition, in the coverage range of ∼ 0.79 to 2.3 monolayer (ML), Al nanoclusters were uniformly nucleated over the entire surface of defect-free atomically smooth terraces as well as step edges. Subsequent nitridation at elevated temperatures by ammonia did not alter the morphology of the nanoclusters. The global morphology of the stepped sapphire (0001) surface such as terrace width, step height and facet orientation had no obvious influence on the nucleation morphology of the nanoclusters in the given Al coverage range. However, local structural defects at the joints of short facets and step edges played a noticeable role on the local morphology of the nanoclusters and subsequently the nitridation chemistry. The Al nanoclusters were uniformly nitridated from surface and downwards through the 3D structures. The LEED pattern indicated a certain degree of crystallinity on the nitridated surface at a nominal Al coverage less than 2 ML, whereas at 2.3 ML Al coverage, the nitridated surface became amorphous. Thus there is a critical coverage for good surface order.