Fabrication and contact resistivity of W–Si3N4/TiB2–Si3N4/p–SiGe thermoelectric joints

The design and fabrication of silicon germanium (SiGe) thermoelectric elements, typically including the selection of electrode and intermediate materials, the process of joining electrode and intermediate layer onto thermoelectric materials, are the major challenge for SiGe thermoelectric device technology. In this study, W–Si₃N₄ and TiB₂–Si₃N₄ composites are designed as the electrode and intermediate layer, respectively, and the W–i₃N₄/TiB₂–Si₃N₄/p–Si₈₀Ge₂₀B_0.6 joints are fabricated by a one-step spark plasma sintering process. The influences of the composition of TiB₂–Si₃N₄ intermediate layer on the interfacial structure, contact resistivity and interfacial thermal stability are investigated. The interfacial thermal stability is improved with increasing Si₃N₄ content in TiB₂–Si₃N₄ intermediate layer due to the reduced mismatch of coefficients of thermal expansion between the intermediate layer and SiGe. On the other hand, the contact resistivity increases with the rising of Si₃N₄ content due to the weakened TiB₂/SiGe ohmic contact, which degrades the device efficiency. As the balanced point, the intermediate layer with the composition of 80 vol% TiB₂+20 vol% Si₃N₄ provides good interfacial thermal stability and moderately small contact resistivity (~75 μΩ cm² after aging at 1000 °C for 120 h) simultaneously, which is an optimized intermediate layer composition for W–Si₃N₄/TiB₂–Si₃N₄/p–Si₈₀Ge₂₀B_0.6 thermoelectric element. The TiB₂–Si₃N₄ intermediate layer has excellent chemical stability to both W–Si₃N₄ electrode and SiGe thermoelectric material at high temperatures, which contributes to the sharp interface of the joint and effectively prevents the inter-diffusion between the electrode and the SiGe.     

Long wavelength Eu2+ emission in Eu-doped Y–Si–Al–O–N glasses

A number of (Eu,Y)–Si–Al–O–N glasses have been prepared and their optical properties examined. Eu was found to be present in the divalent instead of in the trivalent state due to a reaction between the Eu³⁺ and the chemically incorporated N^3- during the preparation of the glasses. The luminescence characteristics were found to be negligibly influenced by the O/N and Si/Al ratio, but appear to be strongly dependent on the concentration and type of modifying cations (Eu,Y). As a function of the cationic composition the emission shifts from wavelengths below 500 nm to wavelengths as long as 640 nm, which is very unusual for Eu²⁺ containing compounds. This shift to longer wavelengths is ascribed to a combination of energy transfer between the different sites and change of the Eu²⁺ site distribution.     

Quantitative EFTEM study of precursor-derived Si–B–C–N ceramics

Ceramic materials derived from a boron modified polysilazane were investigated by means of energy-filtering transmission electron microscopy (EFTEM). After cross-linking of the polymer and subsequent thermolysis, a coarse powder with average composition Si_24.0B_8.0C_44.0N_24.0 is obtained. For further investigation, monolithic particles with sizes of several millimeters were heat treated in crucibles under a flowing nitrogen atmosphere at 1800 °C for 10 h. During thermolysis, the particles developed internal cracks on the macroscopic scale. At the crack surfaces, a layer of pure carbon was found. In the crack-free region, the material is composed of Si₃N₄ and SiC nano crystallites which are embedded in a turbostratic BNC-matrix. Quantitative electron spectroscopic imaging (ESI) shows an atomic ratio of the elements B:C:N of 1.0:4.0:1.1 in this matrix. In the vicinity of the cracks, silicon nitride locally decomposes with formation of silicon carbide because of its reaction with excess carbon. A detailed EFTEM study of the phase distribution near the crack surfaces showed that the first Si₃N₄ crystallites occur at a distance of approx. 5 μm from the carbon covered crack surface. In additional experiments the composition of the BNC-layers as a function of the distance from the crack surface was investigated.     

多孔Si3N4–SiO2复相陶瓷及其性能

采用近净尺寸成型制备工艺–氧化烧结结合溶胶浸渍再烧结法,制备了多孔Si3N4–SiO2复相陶瓷。讨论了制备工艺对材料的成分、微结构和性能的影响规律。研究表明：随着硅溶胶浸渍量的增加,材料的抗弯强度、硬度、断裂韧性、密度和介电常数均增大。分别采用压痕法和单边切口梁法对材料的断裂韧性进行了测定和比较。结果表明：采用压痕法测定断裂韧性时,多孔Si3N4–SiO2复相陶瓷的增韧机理有裂纹偏转、裂纹分叉、裂纹桥接以及孔的钝化。采用单边切口梁法测定断裂韧性时,多孔Si3N4–SiO2复相陶瓷的增韧机理只有裂纹偏转。     

SiC nanograins stabilized Si–C–B–N fibers with ultrahigh-temperature resistance

Continuous ceramic fibers with ultrahigh-temperature stability are in high demand for applications in advanced space propulsion and thermal protection systems. In this study, SiC nanograins stabilized Si–C–B–N ceramic fibers were prepared using chemically modified polyborosilazane via a polymer-derived method. The fabricated Si–C–B–N fibers exhibited a rather high tensile strength of approximately 1.8 GPa and a high strength retention of approximately 90% after annealing at 2100°C for 0.5 h under a nitrogen atmosphere. The ultrahigh-temperature stability can be contributed to the presence of thermodynamically stable SiC nanograins and the encapsulation of SiC nanograins by the BN(C) phase and amorphous Si–C–B–N matrix. Our work offers a convenient strategy for preparing Si-based ceramic fibers with ultrahigh-temperature stability at beyond 2000°C.     

Mechanical properties and microstructure of porous BN–SiO2–Si3N4 composite ceramics

Porous silicon nitride (Si₃N₄) ceramics incorporated with hexagonal boron nitride (h-BN) and silica (SiO₂) nanoparticles were fabricated by pressureless-sintering at relatively low temperature, in which stearic acid was used as pore-making agent. Bending strength at room and high temperatures, thermal shock resistance, fracture toughness, elastic modulus, porosity and microstructure were investigated in detail. The mechanical properties and thermal shock resistance behavior of porous Si₃N₄ ceramics were greatly influenced by incorporation of BN and SiO₂ nanoparticles. Porous BN–SiO₂–Si₃N₄ composites were successfully obtained with good critical thermal shock temperature of 800 °C, high bending strength (130 MPa at room temperature and 60 MPa at 1000 °C) and high porosity.     

Pyrolysis synthesis of Si–B–C–N ceramics and their thermal stability

Si–B–C–N ceramics were synthesized by co-pyrolyzing hybrid polymeric precursors of polycarbosilane (PCS) and polyborazine (PBN). The pyrolysis behavior and structural evolution of the hybrid precursor, the microstructure and composition of the prepared Si–B–C–N ceramics were fully investigated. It was found that the copyrolysis of hybrid polymeric precursors in Ar led to the release of CH₄, CH₃NH₂ and CH₃CN gases at temperatures ranging from 200 to 1100 °C, and finally resulted in the formation of amorphous Si–B–C–N ceramics. In particular, the Si–B–C–N ceramics formed from the hybrid precursor with PBN/PCS mass ratio of 1 could keep amorphous state up to the annealing temperature of 1800 °C with weight change of only 2.08%. But this amorphous ceramics would decompose to form crystalline SiC, BN and Si₃N₄ at 2000 °C. Additionally, compared with PCS-derived SiC ceramics, the Si–B–C–N ceramics showed improved anti-oxidation performance up to 1300 °C due to the formation of borosilicate layers covering the ceramics.     

含Si–O–C界面层的C/Si–C–N复合材料的抗氧化性能(英文)

为了研究利用Si–O–C界面层来提高碳纤维增强陶瓷基复合材料的抗氧化性能,利用化学气相浸渗和聚合物浸渗裂解工艺制备了以Si–O–C为界面的碳纤维增强Si–C–N陶瓷基复合材料（C/Si–O–C/Si–C–N）和无界面层的碳纤维增强Si–C–N陶瓷基复合材料（C/Si–C–N）。研究了C/Si–O–C/Si–C–N和C/Si–C–N在600、900℃和1 200℃空气环境中的氧化行为。结果表明：采用Si–O–C界面层后可提高复合材料的抗氧化性能;Si–O–C界面层较高的氧化抗力是碳纤维增强Si–C–N复合材料抗氧化性能提高的主要原因。     

A novel polyborosilazane for high-temperature amorphous Si–B–N–C ceramic fibres

Polyborosilazane synthesised from BCl₃, HMeSiCl₂, and Me₃SiNHSiMe₃ is easy to cross-link for dehydrogenation of Si–H and N–H, which limits its practical applications for Si–B–N–C fibres on an industrial scale. Therefore, in this context, MeSiCl₃ was used instead of HMeSiCl₂ to synthesise a novel polyborosilazane with limited cross-linking density to fabricate Si–B–N–C fibres. The polyborosilazane synthesised from BCl₃, MeSiCl₃, and Me₃SiNHSiMe₃ exhibits good melt-processability and 1 km long polyborosilazane fibre can be obtained by melt spinning. Prior to pyrolysis, chemical curing with vapour HSiCl₃ at 80 °C was utilised to make the λ green fibres infusible. The as-cured fibres were subsequently pyrolyzed at 1200 °C in nitrogen atmospheres to provide Si–B–N–C ceramic fibres with ca. 1.5 GPa in tensile strength, ca. 160 GPa in Young's modulus, ca. 12 μm in diameter and keeping amorphous up to 1700 °C, which makes them to be promising reinforcements in ceramic matrix composites for high temperature applications.     

Preparation of Si–C–N–Fe magnetic ceramic derived from iron-modified polysilazane

In this paper, Si–C–N–Fe magnetoceramics were obtained by pyrolysis of iron-modified polysilazane (PFSZ) precursors which were synthesized by using polysilazane (PSZ) and iron (III) acetylacetonate (Fe(acac)₃) as starting materials. The as-synthesized PFSZ precursors were characterized by Fourier transform infrared spectroscopy (FT-IR) and gel permeation chromatography. The polymer-to-ceramic conversion of the PFSZ was studied by FT-IR and thermal gravimetric analysis. It is found that the ceramic yield of the PFSZ precursor is ca. 25% higher than that of the original PSZ. The crystallization behavior, microstructures and magnetic properties of the PFSZ-derived Si–C–N–Fe magnetoceramics were studied by techniques such as X-ray diffraction, transmission electron microscopy and vibrating sample magnetometer. The results indicate that the formed α-Fe nanoparticles are uniformly dispersed in amorphous Si–C–N(O) matrix, leading to the soft magnetization of the resultant Si–C–N–Fe ceramics. Moreover, the iron content and the magnetic properties of the Si–C–N–Fe ceramic could be easily controlled by the amount of Fe(acac)₃ in the precursor.     

Nb–Si–N纳米复合薄膜中界面的结构与力学性能

为了考察Nb–Si–N纳米复合薄膜中的界面结构形式,采用基于密度泛函理论(DFT)的第一性原理方法,计算了单个Si原子在NbN晶体中固溶结构形式,以及Nb–Si–N中由Si原子形成的界面结构形式,并分别考察NbN、Nb–Si–N固溶结构及其界面结构形式的力学性能。计算结果表明:单个Si原子在NbN晶体中可以实现间隙固溶或置换固溶;Nb–Si–N中存在置换型界面与间隙型界面两类结构形式;Nb–Si–N固溶结构及其界面结构形式的弹性常数、体模量和剪切模量均低于NbN晶体的力学性能;较NbN的弹性模量各向异性有明显差异不同,置换型与间隙型界面的弹性模量各向异性并不十分突出,但是置换型与间隙型界面的弹性模量的最大值下降非常明显。     

丙烯酸–2–((N–甲基–N–氰乙基)胺基)乙酯的合成

以2–甲氨基乙醇和丙烯腈为原料,通过Michael加成反应合成中间体2–((N–甲基–N–氰乙基)胺基)乙醇,进一步与丙烯酸乙酯发生酯交换反应合成目标产物丙烯酸–2–((N–甲基–N–氰乙基)胺基)乙酯,优化了中间体及产品的合成工艺,使其收率分别达到98.5%、85.3%,并采用FT–IR、1H–NMR、元素分析等手段对其结构进行表征。目标化合物具有氧阴离子聚合活性,可用于制备结构易控的新型聚合物键合剂。     

Si2N2O结合SiC棚板的研制

本文研究分析了SiO2细粉加入量与泥料的颗粒组成对Si2N2O结合SiC材料常规性能与使用性能的影响,并通过X-射线衍射和扫描电镜分析了材料的矿物组成和显微结构。试验表明,Si2N2O结合SiC材料具有比较高的常温与高温抗折强度,并具有良好的抗热震性与抗氧化性能。制作出440×460×12mm的试验板,用于烧卫生洁具隧道窑的棚板。     

Correlation of boron content and high temperature stability in Si–B–C–N ceramics

The preparation and characterization of precursor derived Si–B–C–N ceramics with similar Si/C/N ratios but variable boron content are reported. The polymeric precursors were prepared via hydroboration of poly(methylvinylsilazane) using different BH₃·SMe₂/polymer stoichiometries. High temperature thermogravimetric analysis of as-pyrolysed ceramics as well as XRD studies of post-annealed samples display a retarding effect of boron on both crystallization of SiC and Si₃N₄ and stabilization of crystalline β-Si₃N₄.     

Thermal shock resistance of Si3N4 and Si3N4–SiC ceramics with rare-earth oxide sintering additives

The influence of various rare-earth oxide additives and the addition of SiC nanoparticles on the thermal shock resistance of the Si₃N₄ based materials was investigated. The location of SiC particles inside the Si₃N₄ grains contributed to a higher level of residual stresses, which caused a failure at the lower temperature difference compared to the composites with a preferential location of the SiC at the grain boundaries. A critical temperature difference increased with an increasing ionic radius of RE³⁺ for both the composites and the monoliths. The critical temperature difference for the composite (580 °C) and the monolith (680 °C) sintered with La₂O₃ was significantly higher compared to the composite and the monolith doped with Lu₂O₃ (430 °C). A good agreement was found between the results of the critical temperature difference estimated by the indentation quench test and that obtained by the strength retention method.     

The preparation and properties of zirconia-doped Y–Si–Al–O–N oxynitride glasses and glass-ceramics

Two series (N-9 and N-18 series) of zirconia-doped Y–Si–Al–O–N oxynitride glasses and glass-ceramics were designed. Nominal compositions of the glass samples in equivalent percent (eq%) are xZr: (24–0.25x)Y: (15–0.25x)Al: (61–0.5x)Si: 91O: 9 N and xZr: (24–0.25x)Y: (15–0.25x)Al: (61–0.5x)Si: 82O: 18 N (x=0, 2, 4, 6), respectively. The obtained samples were characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Densities, Vickers hardness, fracture toughness, glass transition temperature, and thermal expansion coefficient data were established for each sample. Effect of Zr and N content on glass network structure, thermal and mechanical properties was investigated. It was found that the addition of zirconia is effective in preparing Y–Si–Al–O–N oxynitride glasses with lower glass transition temperature and higher hardness.     

High-temperature oxidation induced layering process for Si–C–B–N ceramic fibers with SiC nanograins

High-temperature oxidation resistance is important for Si–C–B–N ceramic fibers when reinforcing ceramic matrix composites with superior reliability and faulting tolerance. At present, few studies have investigated on the high-temperature oxidation behavior of Si–C–B–N fibers, limiting their further applications. In this work, we analyzed the high-temperature oxidation process of Si–C–B–N ceramic fibers with SiC nanograins (SiBCN-SiCn fibers) at 1000–1500 °C in air. SiBCN-SiCn fibers stated to be oxidized at 1000 °C, with the formation of thin oxide layer. After oxidizing at 1300 °C, obvious oxide layer that mainly consisted of amorphous SiO₂ could be detected. Further oxidizing at 1500 °C caused the thickness increment of oxide layer, which could inhibit the oxidation products (CO, N₂) to release away from the fibers. The remained CO and N₂ may react with SiC nanograins to form SiO₂ and graphite-like g-C₃N₄, causing the formation of additional transition layer. Our finding may support useful information for the applications of SiBCN-SiCn fibers under harsh environment.     

Oxidation behaviour of ceramic fibres from the Si–C–N–O system and related sub-systems

The oxidation of Si–C–N–O fibres has been investigated. The oxidation rates and the activation energies for the Si–C–O system are similar to those for crystalline SiC. The oxygen and the free carbon concentrations in the ceramics have a limited influence on the oxidation behaviour. As long as the formed silica scale is protective, oxidation kinetics are essentially controlled by the diffusion of oxygen through SiO₂. The parabolic rates in the Si–C–N–O and Si–N–O systems are lower and their activation energies higher than those for SiC. Their values strongly depend on the ratios of C and N bonds to Si and continuously vary from those for SiC (E_a=110−140kJ mol⁻¹) to Si₃N₄ (E_a=330–490 kJ mol⁻¹). The oxidation mechanism might be related to a complex diffusion/reaction regime via the formation of an intermediate silicon-oxynitride (like for Si₃N₄) or silicon-oxycarbonitride layer. The oxidation behaviour of such complex systems is not significantly influenced by the oxygen nor the free carbon contents. It might be governed by the C/Si and N/Si ratios, limiting the nitrogen concentration gradient of the silicon-oxy(carbo)nitride sub-layer and therefore affecting the diffusion/reaction rates.     

SrO对Si3N4–ZrO2–SrO系统反应合成ZrN的影响EI北大核心CSCD

通过引入SrO,利用Si3N4–ZrO2–SrO三元系统反应合成ZrN,使得SiO2–Si3N4–ZrO2–ZrN四元系统形成互易关系,并结合热力学计算,对在N2气氛无压条件下Si3N4–ZrO2–SrO三元系统分别在1 500和1 700℃时的反应途径进行了研究,并采用XRD进行物相分析。结果表明：当烧结温度为1 500℃时,Si3N4–ZrO2–SrO系统可生成ZrN＋SrSi2O2N2和ZrN＋Sr3SiO5＋SrZrO3的复合相;当烧结温度提高到1 700℃时,反应产物中与ZrN复合的物相不仅有SrSi2O2N2和Sr3SiO5＋SrZrO3,还生成了Sr2SiO4＋SrZrO3和Sr7ZrSi6O（21）＋SrZrO3,其中,SrZrO3为ZrO2–SrO二元系统反应的结果。在1 500和1 700℃时,Si3N4–ZrO2–SrO三元系统中可生成ZrN＋X（＋SrZrO3）复合相,且ZrN–X共存关系在扩展的SiO2–Si3N4–SrO–ZrO2–ZrN五元系统中给出。Si3N4–ZrO2–SrO三元系统的数个反应可被用于在较低温度下一步反应制备ZrN复相陶瓷。     

MoSi2增韧Si3N4陶瓷的研究

以Ｓｉ粉和Ｍｏ粉为原料，采用反应烧结－热压吉二步法得到了Ｓｉ３Ｎ４－ＭｏＳｉ２复合材料，并对材料进行了力学性能和Ｘ－射线衍射（ＸＲＤ）及扫描电子显微镜（ＳＥＭ）研究。认为：Ｓｉ３Ｎ４－ＭｏＳｉ２复合材料的高韧性与材料的相组成、显向下 结构、裂纹扩展方式及应力诱导开裂等有关。