自蔓延高温合成/快速加压法制备二硼化钛陶瓷的致密化机理

用自蔓延高温合成/快速加压(self-propagating high temperacre synthesis/quick pressing,SHS/QP)法制备了 TiB2陶瓷.用扫描电镜及透射电镜观察陶瓷产品的微观形貌.对TiB2陶瓷致密化的动力学过程及结构形成过程进行了分析,并提出SHS/QP陶瓷烧结的致密化机理.结果表明:在SHS/QP过程中TiB2陶瓷的致密化是晶粒的重排及高温塑性变形共同作用的结果.     

SHS结合密实化技术制备材料进展

SHS结合密实化技术可以一步完成材料的合成与致密化过程,这是该领域研究的最新方向.本文介绍了国内外正在研究的几种典型的SHS结合密实化技术,对它们的优劣进行了分析,认为SHS/QP方法是一种有希望的材料制备技术.     

陶瓷内衬复合管的自蔓延高温合成技术

介绍了自蔓延高温合成(SHS)的离心技术和重力分离技术,着重阐述了陶瓷内衬复合钢管的合成机理。认为陶瓷内衬的自蔓延高温合成过程需要经历铝热燃烧过程、液相分离过程和陶瓷内衬凝固过程,合理地控制这 3个基本过程,就可以极大地改善复合管的组织结构。同时,还对影响合成陶瓷内衬致密化的因素 (预热温度、燃烧合成温度、稀释剂和化学激活剂)进行了分析、探讨。     

反应合成非氧化物陶瓷材料

反应合成是陶瓷材料的重要制备工艺之一,在制备块体陶瓷、纤维增强复合材料基体、复合材料的陶瓷防护涂层等方面具有广泛的应用。本文简要介绍了作者多年来在反应合成复相陶瓷方面的研究结果,包括硼化钛体系、硼化锆体系、碳化物体系、氮化硼体系以及最近发展起来的高熵陶瓷体系。不同的反应体系包含有化学性质完全不同的反应物、化学反应中可能存在较大的放热现象、反应物存在分解后再反应等中间过程、产物中可能存在气相副产物等不同特征,后续致密化过程与前期反应过程的控制具有复杂多样的密切关系,所以深入研究前期反应过程和后期致密化及显微结构形成机理对反应合成具备特殊显微结构和性能的陶瓷材料具有重要的意义。高温化学反应不仅可以用于合成陶瓷材料,在采用非反应合成的常规工艺制备陶瓷材料特别是非氧化物陶瓷的过程中也普遍存在,了解这些化学反应进行的条件有利于设计合理的工艺制度,从而获得性能优越的材料。     

自蔓延高温合成法制备Al2O3-Cr金属陶瓷

研究了稀释剂Cr2O3对Al-Cr2O3体系自蔓延高温合成（SHS）反应过程及反应产物的影响,并讨论了用自蔓延加压法制备致密的Al2O3-Cr金属陶瓷的可能性。实验结果表明：随稀释剂Cr2O3加入量的增加,Al-Cr2O3体系SHS反应温度和反应产物的孔隙度逐渐降低,激发反应所面的加热时间延长。稀释剂Cr2O3的加入能改善Al2O3与Cr之间的润湿性,避免在反应产物中出现Cr偏聚和分层现象。在一定稀释剂Cr2O3加入量下,通过采用在SHS反应结束后迅速施加－压力将可能制备出致密的Al2O3-Cr金属陶瓷。     

碳化物复相陶瓷及高温等静压氮化表面改性

采用高温等静压氮化表面改性技术制备高性能复相陶瓷。首先实施无包封致密化工艺技术,提出了Ｒｅａｃｔｉｏｎ－ＨＩＰ模型从理论上给予解释;成功地制备出Ｓｉ３Ｎ４－ＳｉＣ梯度功能材料,并推广到一系列碳化物或含碳化物复焙,使我国碳化物复相陶瓷研究和表面改性技术处于国际领先地位。     

SHS转炉补炉料的物相组成及显微结构特征

利用自蔓延高温合成(SHS)原理,选择工业铝粉作为发热剂,菱镁矿为供氧剂,通过铝热反应,研制出了一种以镁砂为骨料,以镁铝尖晶石和炭质材料作为结合相的新型SHS转炉补炉料.对不同环境温度下补炉料的物相组成及显微结构进行了研究,并对合成尖晶石的固相反应原理及SHS产物相的烧结机制进行了探讨.结果表明补炉料中颗粒状的骨料方镁石与SHS反应产物尖晶石、非晶质碳、少量刚玉相和硅酸盐玻璃相共同构成含有气孔的交织结构,形成尖晶石、碳桥和陶瓷相与方镁石骨料的多重结合;SHS反应过程分碳酸盐矿物的分解反应、铝热氧化还原反应(即SHS反应)和合成尖晶石的固相烧结反应三步进行,其烧结受扩散机理控制.     

Ti3SiC2/4SiC复相陶瓷的高温力学性能研究

为了进一步了解Ti3SiC2/nSiC复合材料优良的综合性能,特别是其高温力学性能,本文以热等静压原位合成技术制备的Ti3SiC2/4SiC复相陶瓷为试验材料,对其高温拉伸和高温弯曲行为进行研究。结果表明:Ti3SiC2/4SiC复相陶瓷的高温抗拉强度比室温抗拉强度高;Ti3SiC2/4SiC复相陶瓷的高温抗弯强度在900℃出现一极大值,1000℃后具有好的高温塑性。     

钛铁矿原位合成金属陶瓷复合材料的研究

钛铁矿资源储量丰富,充分利用钛铁矿中的钛和铁来制备金属陶瓷复合材料是天然钛铁矿综合开发利用中的一个重要方面.通过原位铝热、碳热还原法,以天然钛铁矿为主要原料,合成制备出了TiC-Al2O3/Fe陶瓷复合材料.研究了产物的物相、结构和性能,分析了铝热、碳热的反应过程.实验表明:反应产物主要为TiC相、Al2O3相和Fe相,还有少量Fe-Al相.大部分晶体颗粒尺寸约为3～5μm,分布较为均匀.金属添加剂改善了Fe对陶瓷相的润湿性,同时抑制了Al2O3和TiC间的高温化学反应,减少了气体的放出,界面结合得到了改善,促进了陶瓷材料的烧结致密.     

自蔓延高温合成技术焊接制备(TiB_2 Fe)/Fe结构材料

SHS焊接陶瓷与金属是一种新的陶瓷 /金属连接工艺 ,本文采用SHS工艺制备了 (TiB2 Fe) /Fe结构材料 ;通过SEM测试及显微结构分析表明TiB2 Fe陶瓷金属层结构致密 ,Ti从TiB2 Fe侧向Fe基片侧进行扩散 ,以及TiB2 Fe层与Fe基片结合良好。接头断裂时 ,断裂位置发生在TiB2 Fe金属陶瓷层 ,而不是沿着TiB2 Fe层与Fe基片的界面断裂。     

Ultra-fast densification of boron carbide ceramics under high heating rate and high pressure

Boron carbide ceramics were obtained in 2 min by a method based on self-propagating high-temperature synthesis plus quick pressing (SHS/QP). The samples were densified to 98% of theoretical density under a large mechanical pressure (120 MPa) and a fast heating rate (2300 °C/min). The microstructure and mechanical properties were studied. The sample obtained at this heating rate presents an average grain size of 3 μm and a hardness of 34 ± 0.2 GPa.     

Sintering and densification mechanisms of tantalum carbide ceramics

Dense tantalum carbide (TaC) ceramics were prepared using TaC nanopowder via spark plasma sintering (SPS). The effects of the sintering temperature and applied pressure on the densification and grain growth behaviour of TaC ceramics were investigated. The results showed that high temperature and pressure promoted sintering densification, while their increase caused an increase in the grain size of TaC ceramics. A highly dense TaC ceramic (∼97.19%) with a fine grain size of 2.67 μm was obtained by sintering at 1800 °C for 10 min under 80 MPa. The Vickers hardness, Young's modulus and fracture toughness were 15.60 GPa, 512.66 GPa and 3.59 MPa·m^1/2, respectively. The densification kinetics were investigated using a creep deformation model. Diffusion and grain boundary sliding were proven to be the dominant densification mechanisms based on the stress and grain size exponents combined with the microstructural characteristics. The apparent activation energy of the mechanism controlling densification was 252.94 kJ/mol.     

Two-Step Sintering of Ceramics with Constant Grain-Size, II: BaTiO3 and Ni–Cu–Zn Ferrite

We investigated the preparation of bulk dense nanocrystalline BaTiO₃ and Ni–Cu–Zn ferrite ceramics using an unconventional two-step sintering strategy, which offers the advantage of not having grain growth while increasing density from about 75% to above 96%. Using nanosized powders, dense ferrite ceramics with a grain size of 200 nm and BaTiO₃ with a grain size of 35 nm were obtained by two-step sintering. Like the previous studies on Y₂O₃, the different kinetics between densification diffusion and grain boundary network mobility leaves a kinetic window that can be utilized in the second-step sintering. Evidence indicates that low symmetry, ferroelectric structures still exist in nanograin BaTiO₃ ceramics, and that saturation magnetization is the same in nanograin and coarse grain ferrite ceramics.     

Alumina ceramics based on seeded boehmite and electrophoretic deposition

It is shown that seeding a commercial boehmite sol with crystallographically suitable seeds reduces both the crystallization temperature for the final -A1₂O₃ phase and the sintering densification temperature. The seeding component was a tailored combination of δ- and -alumina particles in nanometre and micrometer ranges, respectively, dispersed in water. Electrophoretic deposition (EPD) provided a quick, simple and cost-effective processing route to prepare dense monolithic alumina ceramics from the seeded boehmite suspensions. EPD-formed green bodies could be sintered and densified at temperatures as low as 1250 °C for 2 h.     

Reactive hot pressing route for dense ZrB2-SiC and ZrB2-SiC-CNT ultra-high temperature ceramics

The in-situ exothermic reactions between ZrC_0.8, B₄C and Si have assisted densification and allowed to obtain fully dense ZrB₂-31 wt.%SiC ultra-high temperature ceramics within 6 min at 1750 °C. The use of zirconium carbide instead of metallic zirconium in the green body obviated the possibility of in-situ SHS process and allowed to apply the pressure at low temperatures. The latter provided a first densification stage just above 1050 °C. A slight carbon excess was created in the green body to preserve the carbon nanotubes. The developed reactive hot pressing route (1830 °C, 3 min, 30 MPa) has been successfully used to obtain ZrB₂-SiC ceramics containing 8 vol.% of multi-wall carbon nanotubes (MW-CNT). The carbon nanotubes survived the thermal cycle and could be clearly observed in the sintered ceramics. The CNT addition improved the fracture toughness of the composite from 4.3 MPa m^1/2 for ZrB₂-31 wt.%SiC to 6.8 MPa m^1/2 for ZrB₂-29 wt.%SiC-CNT.     

Flash Spark Plasma Sintering of 3YSZ: Modified sintering pathway and impact on grain boundary formation

Yttria stabilized zirconia (3 mol% YSZ) ceramics were prepared by Flash-SPS, while allowing high heating rates up to 200 °C/s, which led to the extremely fast densification within a few seconds. The high heating rates had strong impact on sintering mechanisms, in terms of densification and grain growth. While the specimens ended with 5–15 vol% porosity and limited grain growth (< 350 nm), their hardness is higher than fully dense counterpart SPSed ceramics. Using the sintering trajectories, microstructural observations, and impedance spectroscopy, we highlight altered sintering mechanism which resulted in very thin grain boundaries compared to SPS. It appears that densification is largely advanced at grain boundary interfaces, with no residual nano-pores at the grain junctions, where some pores with size comparable to grain size were present. This opens up opportunities for the fabrication of porous lightweight ceramics with good mechanical properties.     

Translucent Mg-α-Sialon Ceramics Prepared by Spark Plasma Sintering

The translucent Mg-α-sialon ceramics have been prepared by spark plasma sintering (SPS) α-Si₃N₄ powder with AlN and MgO as the additives at 1850°C for 5 min. The sample possesses a uniform, dense microstructure under the rapid densification of SPS process. The translucent Mg-α-sialon ceramics achieve the maximum transmittance of 66.4% for the sample of 0.5 mm in thickness in the medium infrared region, which could be attributed to the equiaxed microstructure and few glassy phase confirmed by the observation of transmission electron microscopy. The material also exhibits good mechanical properties of high hardness (21.4±0.3 GPa) and fracture toughness (6.1±0.1 MPa·m^1/2).     

La2O3对镁铝尖晶石透明陶瓷致密化及其性能的影响

镁铝尖晶石透明陶瓷兼具了良好的光学和力学性能,在军、民两用领域有着广泛的实际和潜在应用前景。由于其致密化速率低,在烧结过程中往往需要引入烧结助剂。稀土倍半氧化物熔点高,高温不易挥发,近些年被证实可以促进镁铝尖晶石陶瓷的致密化,但其促烧机理尚不明确。本文以高纯商业化镁铝尖晶石粉体为原料,La₂O₃为烧结助剂,采用无压预烧结合热等静压烧结,制备镁铝尖晶石透明陶瓷,通过XRD、SEM、紫外-可见分光光度计、万能试验机等测试手段对其致密化过程及其力学和光学性能进行表征和分析,研究了La₂O₃对镁铝尖晶石透明陶瓷致密化过程的影响规律和作用机制。结果表明,La₂O₃通过与尖晶石反应或固溶产生晶格畸变,增加缺陷浓度,从而起到促进致密化的作用,一定程度上降低了预烧温度和热等静压温度。对于190 MPa、1 500 ℃热等静压烧结3 h的样品,La₂O₃掺杂可以显著提高紫外区域的透过率;同时,La偏析到晶粒表面,抑制了尖晶石晶粒的生长,从而提高了样品的力学强度。掺杂0.05%(质量分数)La₂O₃样品较未掺的样品,400 nm处透过率从63%提高到81%,弯曲强度从263.7 MPa提高至319.0 MPa,断裂韧性从1.69 MPa·m^1/2提高至1.82 MPa·m^1/2。