Commercial ZrO2 paints as coatings for SiGe thermoelectric materials

The use of ZrO₂ paints to coat SiGe materials used in radioisotopic thermoelectric generators was studied. The best results were obtained when the SiGe alloys were double coated with a 200 h anneal at 1000° C after each coating. The thermoelectric properties of these coated samples were about the same as for the SiGe alloys coated by SiO₂ or Si₃N₄. The vapoursupression properties of the best ZrO₂ coatings fell between those of SiO₂ and Si₃N₄. In the SiGe doped with GaP alloys, the interface between the oxide coating and alloy is enriched with Ga₂O₃.     

Si3N4 ceramics formed by HIP using different oxide additions — relation between microstructure and properties

Si₃N₄-based ceramic materials formed by glass encapsulation and hot isostatic pressing (HIP) using different additions of Al₂O₃, Y₂O₃ and ZrO₂ have been characterized by analytical electron microscopy and X-ray diffractometry. The microstructures have been related to formation process and to room temperature hardness and fracture toughness of the ceramics. A high volume fraction of retained -Si₃N₄ after processing at 1550 °C gave the Si₃N₄ ceramics high hardness. The equi-axed grain morphology of the Si₃N₄ matrices in these materials, which contained only small amounts of residual glass, resulted in comparatively low fracture toughness values. Processing at 1750 °C reduced the amount of retained -Si₃N₄ substantially. When Y₂O₃ was added, the microstructure contained a comparatively large volume fraction of residual glass, and the Si₃N₄ was present mainly as high aspect ratio -Si₃N₄ grains. This type of microstructure gave an Si₃N₄ ceramic material with high fracture toughness combined with a lower hardness. Additions of ZrO₂ and/or Al₂O₃ resulted also at 1750 °C in an extremely small volume fraction of residual glass, and a major part of the Si₃N₄ was present as equi-axed grains. These ceramics exhibited medium hardness and toughness values, however, larger additions of ZrO₂ appeared to slightly increase toughness.     

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.     

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.     

Large-scale synthesis of ear-like Si3N4 dendrites from SiO2/Fe composites and Si powders

Large-scale ear-like Si₃N₄ dendrites were prepared by the reaction of SiO₂/Fe composites and Si powders in N₂ atmosphere. The product was characterized by field emission scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The results reveal that the product mainly consists of ear-like Si₃N₄ dendrites with crystal structures, which have a length of several microns and a diameter of 100-200 nm. Nanosized ladder-like Si₃N₄ was also obtained when changing the Fe content in the SiO₂/Fe composites. The Si₃N₄ nanoladders have a length of hundreds nanometers to several microns and a width of 100-300 nm. The ear-like Si₃N₄ dendrites are formed from a two-step growth process, the formation of inner stem structures followed by the epitaxial growth of secondary branches.     

Charakterisierung des tribologischen Verhaltens von keramischen Gleitpaarungen mit moderner Oberflächenanalytik

Characterization of the Tribological Behaviour of Ceramic Sliding Couples with Modern Surface Analysis Methods Dry friction and wear tests were performed in ambient atmosphere with various self-mated couples of SiC, Al₂O₃, Si₃N₄ and ZrO₂. The temperature was varied between 22 °C and 1000 °C and the sliding velocity between 0.03 m/s and 5 m/s. Normal force, temperature and sliding velocity are kept constant during the tests. The materials and wear mechanisms were characterized by various surface analytic methods – optical microscopy, scanning electron microscopy, transmission electron microscopy, electron microprobe analysis, IR-spectroscopy, X-ray diffraction, auger electron spectroscopy and X-ray photoelectron spectroscopy. High wear/low wear transitions could be explained. On the basis of these results new ceramic materials with lower friction and/or wear were selected or successful developed.     

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.     

Microwave sintering of Si3N4 with LiYO2 and ZrO2 as sintering additives

Phase transformation, microstructure development and mechanical properties of 2.45 GHz microwave-sintered silicon nitride (Si₃N₄) with lithium yttrium oxide (LiYO₂) and zirconia (ZrO₂) sintering additives were investigated. It was found that α to β phase transformation completed at a lower temperature of 1500 °C. Scanning electron microscopy (SEM) micrographs revealed a bimodal microstructure with a large number of elongated β-Si₃N₄ grains in addition to smaller grains. Surface residual porosity was observed in all sintered samples due to selective localized over heating of grain-boundary glassy phase. The high aspect-ratio of β-Si₃N₄ grains exhibited significant crack deflection, debonding and pull-out. It was observed that Vickers hardness and indentation fracture toughness increased with increasing sintering temperature.     

BN/Si3N4 nanocomposite with high strength and good machinability

BN/Si₃N₄ nanocomposite was prepared using BN/Si₃N₄ powder obtained by nitriding Si₃N₄/NH₄HB₄O₇ mixture in ammonia gas as the starting powder. Microstructural investigations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that BN particles were homogeneously distributed within the matrix grains as well as at the matrix grain boundaries, and the growth of Si₃N₄ matrix grain was significantly retarded by BN particles. The BN/Si₃N₄ nanocomposite showed a higher strength than the conventional BN/Si₃N₄ microcomposite due to the formation of fine and homogeneous microstructure in it. BN/Si₃N₄ nanocomposite with a BN content of 20 vol% and above showed excellent machinability, because of the formation of weak BN/Si₃N₄ interfaces and the cleavage behavior of BN particles.     

Interfacial microstructure of Si3N4/Si3N4 brazing joint with Cu–Zn–Ti filler alloy

In this study, Si₃N₄ ceramic was jointed by a brazing technique with a Cu–Zn–Ti filler alloy. The interfacial microstructure between Si₃N₄ ceramic and filler alloy in the Si₃N₄/Si₃N₄ joint was observed and analyzed by using electron-probe microanalysis, X-ray diffraction and transmission electron microscopy. The results indicate that there are two reaction layers at the ceramic/filler interface in the joint, which was obtained by brazing at a temperature and holding time of 1223 K and 15 min, respectively. The layer nearby the Si₃N₄ ceramic is a TiN layer with an average grain size of 100 nm, and the layer nearby the filler alloy is a Ti₅Si₃N_x layer with an average grain size of 1–2 μm. Thickness of the TiN and Ti₅Si₃N_x layers is about 1 μm and 10 μm, respectively. The formation mechanism of the reaction layers was discussed. A model showing the microstructure from Si₃N₄ ceramic to filler alloy in the Si₃N₄/Si₃N₄ joint was provided as: Si₃N₄ ceramic/TiN reaction layer/Ti₅Si₃N_x reaction layer/Cu–Zn solution.     

Effect of Cr and Ni on diffusion bonding of Fe3Al with steel

Microstructure at the diffusion bonding interface between Fe₃Al and steel including Q235 low carbon steel and Cr18-Ni8 stainless steel was analysed and compared by means of scanning electron microscopy and transmission electron microscopy. The effect of Cr and Ni on microstructure at the Fe₃Al/steel diffusion bonding interface was discussed. The experimental results indicate that it is favourable for the diffusion of Cr and Ni at the interface to accelerate combination of Fe₃Al and steel during bonding. Therefore, the width of Fe₃Al/Cr18-Ni8 interface transition zone is more than that of Fe₃Al/Q235. And Fe₃Al dislocation couples with different distances, even dislocation net occurs at the Fe₃Al/Cr18-Ni8 interface because of the dispersive distribution of Cr and Ni in Fe₃Al phase.     

Liquid-phase bonding of silicon nitride ceramics using Y2O3–Al2O3–SiO2–TiO2 mixtures

Hot-pressure sintered β-Si₃N₄ ceramic was bonded to itself using Y₂O₃–Al₂O₃–SiO₂–TiO₂ mixtures. Reactive behavior at interface between Si₃N₄ and Y₂O₃–Al₂O₃–SiO₂–TiO₂ mixtures during silicon nitride ceramic joining was studied by means of scanning electron microscopy (SEM), electron probe microanalyses (EPMA), X-ray diffraction (XRD) and auger electron spectroscopy (AES). The joint strength under different bonding conditions was measured by four-point bending tests. The results of EPMA, AES and XRD analyses show that the liquid glass solder reacts with silicon nitride at interface, forming the Si₃N₄/Y–Si–Al–Ti–O–N glass/TiN/Y–Si–Al–O glass gradient interface. From the results of four-point bending tests, it is known that with increase of bonding temperature and holding time, the joint strength increased reaching a peak, and then decreased. The maximum joint strength of 200 MPa measured by the four-point bending tests is obtained for silicon nitride bonded at 1823 K for 30 min.     

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.     

The deposition of group III nitrides on silicon substrates

The reaction between silicon substrates and ammonia present in the systems GaCl (AlCl₃)-NH₃-H₂ (He) during the vapour phase epitaxy of Group III nitrides has been investigated. Silicon was treated with an NH₃-HCl-H₂ mixture at a constant temperature in the range873–1273 K. Using electron spectroscopy for chemical analysis measurements layers of Si₃N₄ and Si₃N₄-SiO₂ were detected at the silicon surface. Reflection high energy electron diffraction tests indicated the amorphous nature of this passivating layer which was found partially or completely to prevent epitaxial deposition of the Group III nitrides. The deposits of GaN on Si(111) and Si(100) were either polycrystalline or textured and an amorphous Si₃N₄-SiO₂ interface was found.     

Fabrication and microstructure of ZrO2-Ni functional gradient material by powder metallurgy

Functional gradient material (FGM) of the ZrO₂-Ni system was developed by a powder metallurgical process, and investigated for its microstructure by means of X-ray diffractometry (XRD), electron probe microanalysis (EPMA), transmission electron microscopy (TEM) and optical microscopy. It was shown that the sintered body of ZrO₂-Ni FGM is almost fully densified, and its chemical composition and microstructure have the expected gradient distribution. With composition variation, the microstructure changes gradually from zirconia particles dispersed in a nickel matrix to the converse with nickel particles dispersed in a zirconia matrix, with network structures in the intermediate composition range. Therefore, no distinct interfaces appear in the FGM due to the gradient change of components, that is, both zirconia and nickel are present everywhere in the microstructure. In phase composition, the sintered ZrO₂-Ni FGM consists of nickel, tetragonal zirconia and a little monoclinic zirconia. No reaction between nickel and zirconia has been detected. The substructure of nickel and monoclinic zirconia are twins, and strain fringes can also be found in zirconia.     

Joining of silicon nitride to silicon nitride and to Invar alloy using an aluminium interlayer

Si₃N₄ has been bonded to Si₃N₄ and to the Invar alloy using an aluminium interlayer at temperatures above the melting point of aluminium. Reaction was hardly observed at the interface between Si₃N₄ and aluminium up to 1223 K. The highest strength of the Si₃N₄-Al-Si₃N₄ joints was beyond 500 M Pa. In the Si₃N₄-Al-Invar joint, two main intermetallic compound layers were formed at the AI-Invar interface. The strength of the joints was between 150 and 200 MPa. It is expected that the aluminium layer and the reaction layer with the fine cracks growing perpendicular to the interface play an important role to compensate for the thermal expansion mismatch.     

AEM investigation of ceramic/lncoloy 909 diffusional reactions after joining by HIP

Diffusion bonding by hot isostatic pressing (HIP) was performed between Incoloy 909 and five different ceramics. Two of the ceramics were composites made from powder mixtures of Si₃N₄ and either 60 vol% TiN or 50 vol% TiB₂, while three were monolithic materials, namely Si₃N₄ with 2.5 wt% Y₂O₃ as a sintering additive, Si₃N₄ without additives, and Si₂ N₂O without additives. A diffusion couple geometry was developed to facilitate the preparation of thin-foil specimens for examination by analytical electron microscopy (AEM). Diffusion bonding was performed by HIP at 927°C (1200K) and 200 MPa for 4 h. The formation of reaction layers was very limited, being less than 1 m in total layer thickness. Two reaction products were found by AEM; a continuous, very thin, (100 nm) layer of fine TiN crystals at the initial ceramic/metal interface, and larger grains extending about 100–500 nm into the superalloy and forming a semi-continuous layer of a G-phase suicide containing mainly nickel, silicon and niobium.     

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.     

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.     

Surface oxidation kinetics of Si3N4-4%Y2O3 powders studied by Bremsstrahlung-excited Auger spectroscopy

Samples of silicon nitride powder containing 4.0% Y₂O₃ in weight were heated in air at temperatures between 900 and 1000 °C. The average SiO₂ layer thickness on the Si₃N₄ powder particles, as a function of time at a particular temperature, was measured by Bremsstrahlung-excited Auger electron spectroscopy. Oxidation was found to follow a linear rate law with an activation energy of 56±1.5 kcal mol^–1. The yttrium level measured by X-ray photoelectron spectroscopy was also found to decrease as a function of the oxide layer thickness. This suggests that there is a reaction between the Si₃N₄ and Y₂O₃ particles which results in the formation of an yttrium-rich phase at the interface between the surface SiO₂ layer and the underlying Si₃N₄ particle.