Influences of milling media on the fabricating process and thermoelectric properties of silicon germanium alloys

In this work, Si₈₀Ge₂₀P₂ alloys were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). SiO₂ and ZrO₂ were taken as milling media respectively to investigate the effect of milling media on the fabricating process and the thermoelectric properties of SiGe alloys. The results show that, compared with the zirconia ball, though the agate one contains less kinetic energy, the solid solution of SiGe taken it as the milling media forms more quickly and at the same time has smaller average grain size through the whole milling time. The solid solution of Ge to Si crystal of samples under ZrO₂ media remains stably around 98% and is much higher than that of specimens with agate as the media. In the sintering process, the previous application of ZrO₂ easily causes the enrichment of elementary P and the loss of Si and leads to serious deviation from the original stoichiometric proportion of the compounds which is beneficial to the following machining and finally does harm to the thermoelectric properties of the Si₈₀Ge₂₀P₂ alloys.     

Preparation and interface modification of Si_3N_(4f)/SiO_2 composites

In order to modify the interface, SiON coating was introduced on the surface of silicon nitride fiber by perhydropolysilazane conversion method. Si₃N_4f/SiO₂ and Si₃N_4f/SiON_c/SiO₂ composites were prepared by sol-gel method to explore the influence of SiON coating on the mechanical properties of composites. The results show that with the protection of SiON coating, Si₃N₄ fiber enjoys a strength increase of up to 24.1% and Si₃N_4f/SiON_c/SiO₂ composites have a tensile strength of 170.5 MPa and a modulus of 26.9 GPa, respectively. After 1000 °C annealing in air for 1 h, Si₃N_4f/SiON_c/SiO₂ composites retain 65.0% of their original strength and show a better toughness than Si₃N_4f/SiO₂ composites. The improvement of mechanical properties is attributing to the healing effect of SiON coating as well as its intermediate coefficient of thermal expansion between Si₃N₄ fiber and SiO₂ matrix.     

A comparison of SiO2 and Si3N4 coatings on GaAs using transmission electron microscopy

Ion-implanted GaAs which had been encapsulated with either SiO₂ or Si₃N₄ layers was examined after heat treatment using transmission electron microscopy. Results indicate that β-Ga₂O₃ forms at the SiO₂-GaAs interface after annealing above about 500°C whereas no second-phase formation was discovered when Si₃N₄ layers were used. Thus Si₃N₄ is a superior coating to SiO₂. Heat treatment in the temperature range 600–750°C produced a large density of dislocation loops in specimens implanted with Te, Cd, Sn, Se and Ar ions irrespective of the encapsulant.     

ZrO2-Si3N4陶瓷复合材料中的化学不相容性

The chemical reaction between ZrO₂ and Si₃N₄ can lead to the formation of ZrN, Zr oxynitride, and/or N-stabilized ZrO₂. In this paper, Chemical incompatibility of the ZrO₂-Si₃N₄ ceramic composites and its elimination methods were reviewed.     

Alkali corrosion resistant coatings for Si3N4 ceramics

The use of ceramic oxide coatings on silicon nitride is one method to improve its alkali corrosion resistance. Four oxide coatings, including (Ca_0.6, Mg_0.4) Zr₄(PO₄)₆ (CMZP), zirconia, mullite and alumina, were examined. These coatings were applied on Si₃N₄ using both sol–gel and dip coating techniques. The coated and uncoated samples were exposed to sodium molten-salt and sodium-containing atmospheres at 1000 °C for 50 h. The weight loss of all the coated samples was less than that of the uncoated Si₃N₄ with CMZP-coated samples exhibiting the smallest weight loss. There was no decrease in the flexural strength of Si₃N₄ after coating with zirconia and CMZP, and a decrease in strength after coating with either mullite or alumina. After alkali exposure, the strength of the CMZP and zirconia coated samples were significantly higher than those of the mullite-coated, alumina-coated, and uncoated Si₃N₄. The observed behaviour is explained in terms of the microstructure and protection mechanisms. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of double passivation for optimize DC properties in gamma-gate AlGaN/GaN high electron mobility transistor by plasma enhanced chemical vapor deposition

Two different materials for double passivation layers have been implemented to an AlGaN/GaN high electron mobility transistor on Si (111) substrate and the improved DC properties are demonstrated. Si₃N₄ and SiO₂ passivation materials are deposited on the gamma gate upper and bottom layers by plasma enhanced chemical vapor deposition. The gamma shape gate can be made by selectively accurate Si₃N₄ or SiO₂ first passivation dry etching with wet etching. The second passivation on gamma gate effectively increases the DC properties. The effects of DC properties of Si₃N₄ or SiO₂ single passivation and Si₃N₄/Si₃N₄ or SiO₂/SiO₂ double passivations are compared. The Si₃N₄/Si₃N₄ double passivation shows the maximum saturation current density and the peak extrinsic transconductance which increases up to 72% and 18%, respectively, more than Si₃N₄ single passivation and also up to 18% and 5% than SiO₂/SiO₂ double passivation.     

Structure of double interfaces system of Si3N4/SiO2/Si irradiated by γ-rays

The interfacial structures of double interfaces system of Si₃N₄/SiO₂/Si were examined using X-ray photoelectron spectroscopy (XPS) before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of Si₃N₄ and SiO₂, while another was made of Si and SiO₂, the interface between SiO₂ and Si was extended towards the interface of the Si₃N₄/SiO₂ meanwhile the center of the former interface was removed in the direction of the latter interface by ⁶⁰Co. The concentration of silicon in the Si₃N₄ state (BE 101.8 eV) was decreased with the variation of radiation dosage as well as bias field within the SiO₂-Si interface, remarkably. The mechanism for the experimental results is analyzed.     

Densification behaviour of oxide-coated silicon nitride powders

Powder coating has been explored as a method of incorporating sintering additives into a ceramic powder. This procedure has been explored in the case of Si₃N₄ powders coated with thin layers of MgO.The effectiveness of the powder coating technique has been evaluated by comparing the powder properties, densification behaviour, microstructure and mechanical properties of coated Si₃N₄ powders with identical powders in which the additive oxide has been added in particulate form. It is concluded that the powder coating technique is an excellent method of homogeneously incorporating minor amounts of sintering additive into a powder. The coated powder exhibited improved homogeneity, and gave good green compact density, high green strength, and faster densification rate. Moreover, coated powders densified more easily by pressureless sintering and showed a more homogeneous microstructure, higher strength and faster densification rates, compared with materials prepared using mixed oxide powders. Significant improvements in hardness and fracture toughness were observed for the coated powders.     

Effects of ZrO2 and Y2O3 dissolved in zyttrite on the densification and theα/β phase transformation of Si3N4 in Si3N4-ZrO2 composite

In Si₃N₄-ZrO₂ composite, the effects of zirconia and Y₂O₃ dissolved in zyttrite on the densification and the/ phase transformation of Si₃N₄ were studied using hot-pressing of Si₃N₄ with the addition of pure, 3, 6, and 8 mol% Y₂O₃-doped zirconia. Reaction couples between Si₃N₄ and ZrO₂ of zyttrite were made to observe the reaction phenomena. The addition of pure zirconia was not effective to obtain full density of the Si₃N₄-ZrO₂ composite. However, Y₂O₃ diffused from the added zyttrite promoted densification; the density of Si₃N₄ with 5 vol% pure ZrO₂ composite was 71% theoretical, and nearly full density (>97%) could be obtained in Si₃N₄ with 5 vol% 6, 8 mol% Y₂O₃-doped ZrO₂ composite. On the basis of observations of the Si₃N₄-pure ZrO₂ reaction couple, the reaction between Si₃N₄ and ZrO₂ resulted in the formation of Si₂N₂O phase, and the/ phase transformation of Si₃N₄ occurred via this Si₂N₂O phase. From the XRD analysis of the reaction layer between Si₃N₄ and zyttrite, it is suggested that the reaction products, Y₂Si₂O₇ and Y₂Si₃N₄O₃ phases, play an important role in the densification of Si₃N₄-zyttrite composite.     

Oxidation bonding of porous silicon nitride ceramics with high strength and low dielectric constant

Silica (SiO₂) bonded porous silicon nitride (Si₃N₄) ceramics were fabricated from α-Si₃N₄ powder in air at 1200–1500 °C by the oxidation bonding process. Si₃N₄ particles are bonded by the oxidation-derive SiO₂ and the pores derived from the stack of Si₃N₄ particles and the release of N₂ and SiO gas during sintering. The influence of the sintering temperature and holding time on the Si₃N₄ oxidation degree, porosity, flexural strength and dielectric properties of porous Si₃N₄ ceramics was investigated. A high flexural strength of 136.9 MPa was obtained by avoiding the crystallization of silica and forming the well-developed necks between Si₃N₄ particles. Due to the high porosity and Si₃N₄ oxidation degree, the dielectric constant (at 1 GHz) reaches as low as 3.1.     

High Temperature Behavior of Si3N4 and Yb2SiO5 Coated Carbon Fibers for Silicon‐Nitride CMC

For oxide‐free ceramic matrix composites (CMC), with Si₃N₄ matrix and carbon fiber reinforcement, for extreme high temperature applications, protective coatings of the C‐fibers are investigated. Two different coatings are compared: reactive CVD‐derived pure Si₃N₄ coatings to investigate C‐fiber‐matrix reactions and powder based Yb‐silicate coatings to reveal potential reactions with the Yb‐silicate additive serving as sintering aid for Si₃N₄. The reactivity toward carbon in nitrogen atmosphere is studied in the temperature interval from 20 °C up to 1700 °C. A new ceramic phase – an Yb‐carbido‐nitiridosilicate, Yb₂Si₄CN₆–is found as product of carbothermal reduction of the Yb‐silicate. The carbothermal reduction occurs also with other RE‐silicates, RE = Yb, Er, Y, Gd, and Sm while SiC is found as reaction product on carbon fibers coated with pure Si₃N₄. The oxidation resistance of the coated fibers in air was investigated in the temperature interval up to 1000 °C, and the apparent activation energy of oxidation was analyzed based on DTA‐EGA results. The oxidation kinetic reveals a significant increase of onset point of oxidation temperature by up to 150 K for Si₃N₄ coated short carbon fibers obtained from the reactive CVD coating process. Such fibers have a high application potential for carbon‐fiber reinforced Si₃N₄‐CMC. The role of Yb₂Si₄CN₆ as reinforcement for Si₃N₄‐CMC is discussed based on bond strength comparison of carbides (SiC), nitride silicates (SiAlON), and nitrides (Si₃N₄).     

Plasmons in double interfaces system of Si3N4/SiO2/Si irradiated by Co

The first level plasmons of Si in the pure Si state, in the SiO₂ state and in the Si₃N₄ state (corresponding to bonding energy 116.95, 122.0 and 127.0 eV) were investigated directly with X-ray photoelectron spectroscopy before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of plasmons of Si in the Si₃N₄ and SiO₂ states, while another was made of plasmons of Si in the pure Si state and in the SiO₂ state. When the Si₃N₄-SiO₂-Si samples were irradiated by ⁶⁰Co, the interface at Si₃N₄/SiO₂ was extended and at the same time the center of this interface moved towards the surface of Si₃N₄. The concentration of plasmon for silicon in the SiO₂ state is decreased at the SiO₂-Si interface, and the effects of radiation bias field on plasmons in the SiO₂-Si interface are observable. Finally, the mechanism of experimental results is analyzed by the quantum effect of plasmon excited by the photoelectron.     

An investigation into a multilayered BN/Si3N4/BN interfacial coating

In order to prevent environmental degradation of the interface, a triplex coating was employed as the interface in ceramic matrix composites (CMC). This interface consists of an initial BN layer followed by a Si₃N₄ layer and lastly another BN layer. Single strand unidirectional mini-composites using BN/Si₃N₄/BN coated ceramic grade Nicalon? fibers as the reinforcement and chemical vapor infiltrated (CVI) SiC as the matrix were fabricated to understand the initial properties of the interfacial coating. Field emission scanning electron microscopy (FE-SEM) confirmed the thickness of the triplex coating before and after mini-composite fabrication. FE-SEM micrographs after mechanical and environmental testing of the single strand unidirectional mini-composites showed the consequences of using the triplex interfacial coating. Finally, eight ply continuous fiber reinforced (CFR) CMCs with the BN/Si₃N₄/BN triplex interface and the traditional BN/Si₃N₄ duplex interface were fabricated using the polymer impregnation and pyrolysis (PIP) process. The PIP process has gained popularity in recent years and this allows for the fabrication of larger CMC panels as compared with the CVI process. Mechanical testing for the PIP-fabricated CFR-CMC panels showed that the composites using the triplex interface had better mechanical properties than those fabricated with a BN/Si₃N₄ duplex interface after environmental testing.     

Thermal stability of nano-layered structure and hardness of TiAlN/Si3N4 nanoscale multilayered coating

Thermal stability of the TiAlN/Si₃N₄ nanoscale multilayered coating that was reported to show excellent hardness and toughness, has been investigated in terms of the nano-layered structure and hardness. TiAlN/Si₃N₄ nanoscale multilayered coatings with various thickness of Si₃N₄ layer were prepared by alternating deposition of TiAlN and Si₃N₄. In contrast to other nanoscale multilayered coating system such as AlN/CrN in which the intensity of the low angle XRD peaks decreases with increasing annealing temperature by interdiffusion between adjacent layers, the low angle XRD peak intensity of the nanoscale multilayered TiAlN/Si₃N₄ coatings increased after heat-treatment in an N₂ atmosphere up to 800 °C. Such a thermal stability of the nano-layered structure is believed to be due to spinodal type phase separation of TiAlN and Si₃N₄, which increased the hardness value of the TiAlN/Si₃N₄ coating at high temperatures.     

Carbothermal reduction and nitridation synthesis of silicon nitride by using solution combustion synthesized precursors

Carbothermal reduction and nitridation synthesis of Si₃N₄ was investigated by using precursor powders prepared by a solution combustion synthesis method. Glycine or urea (fuel), ammonium nitrate (oxidizer), silicic acid (Si source), and sucrose (major carbon source) were dissolved completely in water. This solution was dried and then heated to undergo the solution combustion synthesis reaction, resulting in a homogeneous mixture of nano-sized carbon and SiO₂ particles, which was used as the precursor powder for the carbothermal reduction and nitridation synthesis of Si₃N₄. When the carbothermal reduction and nitridation reaction was carried out at 1,425–1,450 °C for 4 h, formation of Si₃N₄ can be detected only when the C/SiO₂ weight ratio is greater than ~2.0. The Si₃N₄ yield increases rapidly as the C/SiO₂ weight ratio is increased from ~2.0 to 2.8 and decreases with further increase in the C/SiO₂ ratio. The α-phase content increases with increasing C/SiO₂ weight ratio and decreases with increasing temperature. Depending on the C/SiO₂ ratio, a Si₃N₄ yield of ~80 wt% and an α-phase content of ~90 wt% could be obtained.     

Thermal Noise Sensor Insulation Using Ultra-thin Ceramic Coatings

The insulation resistance and interference rejection of several prototype thermal noise sensors were measured. It was found that twisting the connecting wires as a pair optimizes the magnetic interference rejection. High-temperature-resistant wire insulation was developed by vapor depositing a few micrometer thick ceramic coating. The best insulation was obtained with a coating of 1.25μm SiO₂ + 1.5μm TiO₂ + 0.25μm ZrO₂. The insulation of four such coated pairs reached stable levels of 18 ± 3kΩ during a testing period of 1 week at 950°C under atmospheric conditions.     

Synthesis of monodisperse and high-purity α-Si3N4 powder by carbothermal reduction and nitridation

Carbothermal reduction-nitridation method is an effective means for synthesizing Si₃N₄ powder. Herein, spherical monodisperse silica was used as silicon source. The effects of reaction temperature, nitrogen flow rate and Si₃N₄ seeds content on the products were studied. It was found that high-purity α-Si₃N₄ (>99.0 wt%) was synthesized from C/SiO₂ = 3:1 at 1400 °C, reaction time of 6 h and nitrogen flow rate of 800 ml/min. The powder, with an average size of 0.5 μm, showed good dispersity and regular morphology because spherical monodisperse silica could be completely coated with carbon. The more contact sites between SiO₂ and C, the higher concentration of SiO(g) would be produced in the initial stage. It also indicated that the nucleation rate of α-Si₃N₄ increased, thereby inhibiting the formation of an agglomerate phase and suppressing the grain growth of α-Si₃N₄. Furthermore, higher nitriding temperature and Si₃N₄ seeds content both decreased the grain size and increased β-Si₃N₄ content. The forming mechanism of non-agglomerated and submicron-sized α-Si₃N₄ was clarified.     

Thermal barrier coatings for thermal insulation and corrosion resistance in industrial gas turbine engines

Four thermal barrier coatings were subjected to a 500 h gas turbine engine test. The coatings were two Y₂O₃-stabilized ZrO₂ composites, Ca₂SiO₄ and CaTiO₃. The Ca₂SiO₄ coating exhibited significant spalling. Y₂O₃-stabilized ZrO₂ and CaTiO₃ coatings showed little degradation except in blade leading edge areas. Post- test examination showed variations in the coating due to manual application techniques. Improved process control is required if engineering quality coatings are to be developed. The results indicate that some leading edge loss of the coating can be expected near the tip.     

Analysis of coating interlayer between silicon nitride cutting tools and titanium carbide and titanium nitride coatings

Silicon nitride (Si₃N₄) cutting tools exhibit excellent thermal stability and wear resistance in the high-speed machining of cast irons, but show poor chemical wear resistance in the machining of steel. Conventional chemical vapour deposition (CVD) coating of Si₃N₄ tools has not been very successful because of thermal expansion mismatch between coatings and the substrate. This problem was overcome by developing a CVD process to tailor the interface for titanium carbide (TiC) and titanium nitride (TiN) coatings. Computer modelling of the CVD process was done to predict which phases would form at the interface, and the results compared with analyses of the interface. Three Si₃N₄ compositions were considered, including pure Si₃N₄, Si₃N₄ with a glass phase binder, and Si₃N₄ + TiC composite with a glass phase binder. Results of machining tests on coated tools show that the formation of an interlayer provides superior wear resistance and tool life in the machining of steel as compared to uncoated and conventionally coated Si₃N₄ tools.     

Pressure-assisted direct bonding of copper to silicon nitride for high thermal conductivity and strong interfacial bonding strength

To achieve superior thermal and mechanical properties of copper-bonded (Cu-bonded) Si₃N₄ substrate, a pressure-assisted direct bonded Cu (DBC) technique was applied to bond Cu foil with Si₃N₄ plate. The effects of oxide layer (SiO₂) thickness of Si₃N₄ plate on the microstructure, thermal and mechanical properties of the Si₃N₄-DBC samples were investigated. The successful bonding of Cu foil to Si₃N₄ plate was confirmed by the presence of the interfacial products of Cu₂MgSiO₄ and CuYO₂. Additionally, it was demonstrated that a thin SiO₂ layer can result in a discontinuous distribution of interfacial products while a thick one can lead to the formation of pores in SiO₂ layer. Notably, the sample prepared by Si₃N₄ plate with 5-μm-thickness SiO₂ layer and Cu foil with 5.9-μm-thickness oxide layer (Cu₂O) exhibited the optimally comprehensive properties with thermal conductivity of 92 W·m^?1·K^?1 and shearing strength of 102 MPa, which demonstrates significant promise for application in power electronic modules.

Graphical abstract