高温等静压后处理液相烧结SiC陶瓷的结构与性能表征

本文研究了高温等静压（HIP）后处理工艺对液相烧结SiC陶瓷的显微结构及力学性能的影响．实验表明,HIP后处理的效果随烧结助剂的不同及液相烧结温度的变化而改变．Ar气氛条件下的HIP后处理可以提高液相烧结SiC的密度,减少或消除内部气孔等结构缺陷,但不引起晶粒的长大．N₂条件下的HIP后处理除了具有Ar－HIP后处理的优点外,由于表面SiC与N₂之间的反应生成的Si₃N₄可以有效地改善表面状态,从而达到表面改性,提高SiC陶瓷的力学性能．结构分析表明,经N₂-HIP后处理,表面氮化层中晶粒细小,结构致密．同时,HIP后处理的效果还受液相烧结SiC陶瓷显微结构的影响,当液相烧结SiC的烧结温度较低,晶粒较细时,经HIP后处理,尤其是N₂-HIP后处理,强度和韧性均有较大幅度的提高．     

热压烧结TiB2陶瓷的显微结构和力学性能研究

以Y₂O₃－Al₂O₃为烧结助剂,通过热压制备了TiB₂陶瓷,研究了烧结温度、烧结时间和晶化处理对材料的显微结构和力学性能的影响．实验结果表明,随着烧结温度的升高,烧结体失重增加,其抗弯强度和断裂韧性降低;烧结时间延长,其显微结构的均匀性降低,对力学性能不利．晶粒直径对TiB₂陶瓷的力学性能有重要影响．晶化处理能够导致晶界拆出YAG相,从而提高TiB₂陶瓷的高温抗弯强度．     

原位增强SiC陶瓷

实验采用β-SiC为起始原料,Y₂O₃、A1₂O₃为烧结助剂,通过适当的烧结控制,获得了具有长柱状晶粒结构的α－SiC陶瓷,材料以液相烧结机制密化,在烧结过程中发生了与柱状晶形成有关的SiC晶粒3C→4H相变．材料的力学性能与晶粒的形态即长径比存在一定的依从关系,并显示出原位增强的特性．在较佳工艺条件下,材料的强度和韧性最大值分别达到620MPa、6．1MPa.m^1/2．压痕裂纹扩展的途径表明,裂纹偏转和晶粒桥联是主要的增韧机理,这得益于其弱的界面结合．     

CaO-Y2O3添加剂对AlN陶瓷显微结构及性能的影响

研究了掺杂ＣａＯ－Ｙ_２Ｏ_３热压烧结和常压烧结ＡｌＮ陶瓷的性能和显微结构．结果表明：热压烧结ＡｌＮ陶瓷的第二相为Ｙ_３Ａｌ_５Ｏ_１２,常压烧结ＡｌＮ陶瓷的第二相为Ｙ_３Ａｌ_５Ｏ_１2和Ｃａ_３Ｙ_２Ｏ_６;热压烧结ＡｌＮ的第二相体积百分数和晶格氧含量均低于常压烧结;热压烧结ＡｌＮ陶瓷的微观结构良好,其热导率达到２００Ｗ／ｍ·Ｋ．     

微波烧结和常压烧结对Al2O3-ZrO2陶瓷磨损行为的影响

采用多模微波烧结系统和常压烧结,对Ａｌ_２Ｏ_３－ＺｒＯ_２复合材料的基本性能进行了研究,提出了相关的磨损机理．与常压烧结相比,微波烧结可以提高ＺＴＡ陶瓷的密度、强度和韧性,使其结构均匀,耐磨性提高．常压烧结样品的磨损主要是晶粒铲平、磨屑填充体内气孔形成光滑的磨损界面;而微波烧结ＺＴＡ陶瓷主要是界面晶粒剥离脱落磨损．载荷的增加使磨损量增大．     

以铝为助剂结合放电等离子烧结制备Ti3SiC2

以铝为助剂结合放电等离子烧结工艺,在较低温度下快速制备出高纯致密Ti₃SiC₂块体材料,掺加适量铝能加快Ti₃SiC₂的反应合成,提高制备材料的纯度,并促进Ti₃SiC₂晶体的生长和材料的快速烧结致密,在升温速率为80℃/min,z轴压力为30MPa时,材料制备的最佳温度为1200～1250℃,所制备材料经XRD、SEM和EDS分析表明不含TiC和SiC等杂质相,Ti₃SiC₂为5～25μm的板状结晶。     

Y和YF3掺杂钛酸钡系PTCR材料的结构及性能

在不同烧结气氛下制备了Ｙ和ＹＦ_３掺杂钛酸钡材料,借助于ＸＲＤ、ＳＥＭ、ＸＲＦ和阻温测试分析仪,研究了烧结气氛对Ｙ和ＹＦ_３掺杂钛酸钡材料结构和性能的影响．研究结果表明,低氧分压气氛可促进Ｙ和ＹＦ_３掺杂钛酸钡材料的烧结,晶粒长大,而且这二种掺杂钛酸钡材料都是ｎ型半导体．经过氩气气氛烧结的Ｙ掺杂钛酸钡材料ＰＴＣＲ效应较弱;而对在氩气气氛中烧结的０．３ｍｏｌ％ＹＦ_３掺杂钛酸钡材料却观察到了较好的ＰＴＣＲ效应,这种效应的产生可能与Ｆ元素取代Ｏ位而导致材料的价控半导有关．     

共沉淀法制备YAG-Al2O3纳米复合粉体

本文采用共沉淀法制备了ＹＡＧ－Ａｌ_２Ｏ_３纳米复合粉体．并通过ＸＲＤ、ＴＥＭ详细研究了粉体组成、形貌随煅烧温度的变化．研究表明,在１３００℃下煅烧可获得ＹＡＧ粒径约１００ｎｍ、分散均匀、无杂相的ＹＡＧ－Ａｌ_２Ｏ_３纳米复合粉体．粉体在１４５０℃可热压烧结致密,远低于文献报道的热压烧结温度１６００℃．用共沉淀法制备ＹＡＧ－Ａｌ_２Ｏ_３纳米复合粉体具有成本低、产量高和工艺简单的优点．     

碳化硅晶须补强氧化铝复合材料的制备及其力学性能

本论文利用商用γ-Al₂O₃粉体和上海硅酸盐所制备的碳化硅晶须,通过热压工艺来制备碳化硅晶须补强氧化铝复合材料．当晶须含量为30vol％时,室温下复合材料的抗弯强度为812±38MPa,断裂韧性为8．8±0．1MPa·m^1／2;在1200℃、Ar气氛下,分别为560±61MPa和6．1±0．4MPam^1／2．在氮气氛下,由于晶须的损伤易导致材料的力学性能下降．添加剂可降低复合材料的烧结温度,但不利于其力学性能．显微结构观察发现,不同温度下,AS复合材料的增韧机理有变化．     

β-Si3N4粉末烧结及其显微结构形成

由β－Ｓｉ_３Ｎ_４粉末通过一定的工艺条件得到致密的氨化硅陶瓷,试样的显微结构为短柱状和等轴状颗粒交织排列而成的均匀结构．材料的烧结过程分为重排、晶形转变、晶粒生长三个阶段,随烧结时间增加,烧结试样的显微结构开始阶段变化很明显,２ｈ后结构比较稳定．温度升高有利于柱状颗粒长径比的提高,添加剂量的增加使显微结构粗化．     

Single-wall carbon nanotubes as attractive toughening agents in alumina-based nanocomposites

The extraordinary mechanical, thermal and electrical properties of carbon nanotubes have prompted intense research into a wide range of applications in structural materials, electronics, chemical processing and energy management. Attempts have been made to develop advanced engineering materials with improved or novel properties through the incorporation of carbon nanotubes in selected matrices (polymers, metals and ceramics). But the use of carbon nanotubes to reinforce ceramic composites has not been very successful; for example, in alumina-based systems only a 24% increase in toughness has been obtained so far. Here we demonstrate their potential use in reinforcing nanocrystalline ceramics. We have fabricated fully dense nanocomposites of single-wall carbon nanotubes with nanocrystalline alumina (Al2O3) matrix at sintering temperatures as low as 1,150 degrees C by spark-plasma sintering. A fracture toughness of 9.7 MPa m 1/2, nearly three times that of pure nanocrystalline alumina, can be achieved.     

多孔氮化硅表面γ-Y_2Si_2O_7陶瓷涂层的制备及抗热震性能

采用浆料喷涂与高温熔渗结合的方法,以γ-Y_2Si_2O_7和Y_2O_3-Al_2O_3-SiO_2三元氧化物粉体为原料,在多孔Si_3N_4表面制备致密γ-Y_2Si_2O_7陶瓷涂层,采用XRD和SEM测试方法系统研究原料配比及烧结温度对涂层结构、组织和抗热震性能的影响。结果表明:SiO2与Al2O3物质的量之比较高时,涂层为致密γ-Y_2Si_2O_7层和过渡层的双层结构;SiO_2与Al_2O_3物质的量之比较低时,只能形成单层结构;双层结构涂层的抗热震性能优于单层结构涂层。     

The effect of polysiloxane on the properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-NbC composite material produced by pyrolysis process

Recently, studies have been developed in order to obtain Al₂O₃-NbC composite materials. The reinforced materials have shown good potential to be used as cutting tool materials at high-speed cutting and high temperature as a substitute to WC-Co material. The main disadvantage to produce these alumina-reinforced materials is the necessity to use pressure assisted sintering or high sintering temperatures to produce dense bodies. Manufacturing of composite ceramic materials derived from polymer reactive filler has been intensively investigated. Polymer pyrolysis is a relatively new and very promising method for obtaining ceramic material in complex shapes and lower sintering temperatures. This work investigated a ceramic composite matrix based in SiC_xO_y and Al₂O₃ and reinforced with NbC obtained by means of the active fillers pyrolysis process. The results obtained in this work demonstrate that using a mixture of polysiloxanes produces a composite material with better properties when compared to others polymer materials.     

Influence of nano-SiO2 on the Portland cement pastes

The evolution of nanotechnology provides materials with new properties and over the last years a lot of effort has been put to introduce nano-particles into cement pastes in order to improve their properties and to produce materials of better performance. In the present research work, nano-SiO₂ produced by pyrolysis and with specific area of 200 m²/g has been added at different percentages (0%, 0.5%, 1%, 2% and 5%) to high-strength cement pastes. These pastes were tested for their mechanical and structural properties at different ages. Nanoparticles act as nuclei for crystallization and large, idiomorphic crystals of Ca–Si composition were formed assisting, up to a certain percentage, in producing materials with dense structure, reduced porosity and improved strength.     

Sintering of YBa2Cu3O7−x compacts

Heating of YBa₂Cu₃O_7−x compacts above about 930°C is shown to induce liquid formation. Presence of the liquid phase results in excellent densification, but limited superconducting properties. Sintering below 930°C occurs primarily by solid-state diffusion. Although the density of these samples is low, the superconducting properties are similar to those of the dense materials produced via liquid-phase sintering. The highest current densities (≈ 500 A/cm²) have been obtained in these solid-state sintered samples.     

In-situ hot pressing/solid-liquid reaction synthesis of dense titanium silicon carbide bulk ceramics

 An in-situ hot pressing/solid-liquid reaction process was developed for the synthesis of dense polycrystalline Ti₃SiC₂ ceramics using Ti, Si, and graphite powders as starting materials. The present work demonstrated that this process was one of the most effective and simple methods for the preparation of dense bulk Ti₃SiC₂ materials. Lattice constants of a=3.068 and c=17.645 are calculated for Ti₃SiC₂ made through this process. The synthesis temperature influenced the phase composition, microstructure and mechanical properties of Ti₃SiC₂ prepared at different temperatures. And bulk materials with flexural strength of 480 MPa and fracture toughness of 7.88 MPa.m^1/2 were obtained at 1600°C. The high fracture toughness and strength are discussed based on microstructure analysis. Received: 31 July 1998 / Accepted: 28 August 1998     

Friction and erosive wear of controlled nucleation thermochemically deposited W-C alloys and SiC and chemically vapor-deposited Si3N4

The controlled nucleation thermochemical deposition (CNTD) process differs from the conventional chemical vapor deposition (CVD) process in that CNTD results in a fine-grained non-columnar deposit with superior mechanical properties. Materials made by this technique include CM 500 (a WC alloy) and CM 4000 (CNTD SiC). These two materials, together with CVD Si₃N₄, were evaluated for their erosion and sliding wear characteristics and the results were compared with those obtained for conventional refractory and ceramic materials. It is shown that the application of a dense CVD or CNTD coating significantly improves the erosion resistance of substrate materials.     

Preparation of silica thin films by novel wet process and study of their optical properties

Silicon dioxide (SiO2) thin films have gained considerable attention because of their various industrial applications. For example, SiO2 thin films are used in superhydrophilic self-cleaning surface glass, UV protection films, anti-reflection coatings, and insulating materials. Recently, many processes such as vacuum evaporation, sputtering, chemical vapor deposition, and spin coating have been widely applied to prepare thin films of functionally graded materials. However, these processes suffer from several engineering problems. For example, a special apparatus is required for the deposition of films, and conventional wet processes are not suitable for coating the surfaces of substrates with a large surface area and complex morphology. In this study, we investigated the film morphology and optical properties of SiO2 films prepared by a novel technique, namely, liquid phase deposition (LPD). Images of the SiO2 films were obtained by scanning electron microscopy (SEM) and atomic force microscopy (AFM) in order to study the surface morphology of these films: these images indicate that films deposited with different reaction times were uniform and dense and were composed of pure silica. Optical properties such as refractive index and transmittance were estimated by UV-vis spectroscopy and ellipsometry. SiO2 films with porous structures at the nanometer scale (100-250 nm) were successfully produced by LPD. The deposited film had excellent transmittance in the visible wavelength region.     

微波烧结制备ZrO2-AlN复合陶瓷的微观结构与性能研究

氧化锆(ZrO₂)陶瓷具有出色的机械性能, 但其应用受到低热导率(Thermal Conductivity, TC)的限制。本研究设计并通过微波烧结制备了高热导率氧化锆-氮化铝(AlN)复合陶瓷, 优化制备条件后, 抑制了两种物质之间的反应, 获得了致密的复合陶瓷(相对密度>99%), 详细研究了该复合陶瓷的组织演变、热学性能和力学性能。研究结果表明, 随着AlN含量的增加, 复合陶瓷的室温下热导率、热扩散系数和热容增加, 分别达到41.3 W/(m·K)、15.2 mm²/s和0.6 J/(g·K)。这种具有高热导率和抗热震性的ZrO₂-AlN复合复合陶瓷在能源系统的高温热交换材料领域具有广阔的应用前景。     

SiO2对铜基摩擦材料摩擦磨损性能的影响

为了提高铜基摩擦材料在高速干摩擦条件下的耐磨性能和具有稳定的摩擦系数,通过粉末冶金方法,制备了铜-SiO2摩擦材料.在摩擦速度为1.6～47.1 m/s条件下,研究了SiO2含量、粒度及摩擦方式对材料摩擦磨损性能的影响.研究表明:材料的摩擦系数和磨损率随着SiO2含量的增加而略有增大;SiO2颗粒尺寸的变化,对摩擦系数影响不显著;摩擦方式对材料的摩擦磨损性能的影响明显.SiO2含量增加有利于提高摩擦系数归因于增加了摩擦面高硬度质点的数量.干摩擦条件下影响材料摩擦磨损性能的一个重要因素是摩擦方式.在高速摩擦条件下,形成的第三体组织致密而连续,这种致密而连续的第三体有利于降低磨损量,并明显降低由于摩擦速度不同而导致的摩擦系数的波动程度.