Synthesis of zirconia nanoparticles on carbon nanotubes and their potential for enhancing the fracture toughness of alumina ceramics

Nanoparticles of zirconia (ZrO₂) were in situ synthesized on the surface of carbon nanotubes by means of liquid phase reactions and a proper heat treatment process. The size of the nanoparticles could be controlled by the amount of zirconium source materials in a solution and its reaction times. In this study, the size of the nanoparticles ranged from several nanometers to twenty nanometers. It was particularly noted that the synthesized zirconia possessed a cubic structure (c-phase) which generally existed as a stable form of zirconia crystals at high temperatures (above 2370 °C) as well as a form of zirconia that could be used for enhancing the fracture toughness of alumina ceramics. Experimental results showed that the mechanical properties of alumina ceramics mixed with in situ synthesized nanoparticles on the surface of carbon nanotubes were much better than that of pristine nanotubes or zirconia nanoparticles alone. The existence of the nanoparticles on the surface of nanotubes results in improving the dispersion and bonding properties of the nanotubes in alumina matrix environment. The fracture toughness of CNT/ZrO₂ alumina ceramics was also improved by the mechanism of bridging effect.     

A novel structure for carbon nanotube reinforced alumina composites with improved mechanical properties

Engineering ceramics have high stiffness, excellent thermostability, and relatively low density, but their brittleness impedes their use as structural materials. Incorporating carbon nanotubes (CNTs) into a brittle ceramic might be expected to provide CNT/ceramic composites with both high toughness and high temperature stability. Until now, however, materials fabrication difficulties have limited research on CNT/ceramic composites. The mechanical failure of CNT/ceramic composites reported previously is primarily attributed to poor CNT-matrix connectivity and severe phase segregation. Here we show that a novel processing approach based on the precursor method can diminish the phase segregation of multi-walled carbon nanotubes (MWCNTs), and render MWCNT/alumina composites highly homogeneous. The MWCNTs used in this study are modified with an acid treatment. Combined with a mechanical interlock induced by the chemically modified MWCNTs, this approach leads to improved mechanical properties. Mechanical measurements reveal that only 0.9?vol% acid-treated MWCNT addition results in?27% and?25% simultaneous increases in bending strength (689.6 ± 29.1?MPa) and fracture toughness (5.90 ± 0.27?MPa?m(1/2)), respectively.     

燃烧反应超快升温热压制备碳纳米管/氧化铝复合材料

报道一种制备碳纳米管增强氧化铝基复合材料的新方法,用燃烧反应所产生的热量为热源代替传统烧结炉,在燃烧反应完成的同时施加机械压力来实现快速烧结,当碳纳米管掺量为1%(质量分数)时复合材料的断裂韧性同比提高了50%,该方法有利于避免碳纳米管的高温破坏,复合材料的增韧作用主要来自于碳纳米管的拔出效应和桥联机制.     

纳米晶添加氧化铝粉体的低温烧结研究

以湿化学法制备的纳米α-Al₂O₃粉体作为添加剂,MgO和SiO₂为烧结助剂,对商用γ-Al₂O₃粉体预处理后,采用无压烧结工艺,有效的降低了烧结温度.在1450℃制备了高性能的氧化铝陶瓷,并对添加α-Al₂O₃纳米晶的作用机理进行了研究.     

纳米铌酸钾钠陶瓷的制备及性能

以新型溶胶-凝胶法制备的平均晶粒尺寸为30 nm的铌酸钾钠粉体为原料, 采用放电等离子体烧结工艺, 在烧结温度为900℃, 压力30 MPa, 烧结时间1 min的条件下, 制备得到纯正交相, 相对密度高达99%以上, 平均晶粒尺寸为40 nm的纳米铌酸钾钠陶瓷, 并对该陶瓷的相结构、微观形貌、介电性能和铁电性能进行了研究。结果表明, 与普通微米晶陶瓷不同, 纳米铌酸钾钠陶瓷的室温介电常数仅为341, 并且随温度变化不明显, 表现出明显的介电弛豫现象, 弥散因子γ为1.60, 并具有明显的电滞回线, 矫顽场强度为13.5 kV/cm, 剩余极化为1.5 μC/cm²。尺寸降低所引起的纳米铌酸钾钠陶瓷中晶界相所占的比例增大是其性能变化的主要原因, 并且可以推断, 如果铌酸钾钠陶瓷具有“临界尺寸”, 那么其值应该在40 nm以下。     

Bioinspired Nacre‐Like Alumina with a Metallic Nickel Compliant Phase Fabricated by Spark‐Plasma Sintering

Many natural materials present an ideal “recipe” for the development of future damage‐tolerant lightweight structural materials. One notable example is the brick‐and‐mortar structure of nacre, found in mollusk shells, which produces high‐toughness, bioinspired ceramics using polymeric mortars as a compliant phase. Theoretical modeling has predicted that use of metallic mortars could lead to even higher damage‐tolerance in these materials, although it is difficult to melt‐infiltrate metals into ceramic scaffolds as they cannot readily wet ceramics. To avoid this problem, an alternative (“bottom‐up”) approach to synthesize “nacre‐like” ceramics containing a small fraction of nickel mortar is developed. These materials are fabricated using nickel‐coated alumina platelets that are aligned using slip‐casting and rapidly sintered using spark‐plasma sintering. Dewetting of the nickel mortar during sintering is prevented by using NiO‐coated as well as Ni‐coated platelets. As a result, a “nacre‐like” alumina ceramic displaying a resistance‐curve toughness up to ≈16 MPa m^½ with a flexural strength of ≈300 MPa is produced.     

片状氧化铝晶种对氧化铝陶瓷断裂韧性的影响

以片状氧化铝晶种作为第二相,采用无压烧结制备了氧化铝陶瓷,分析了片状氧化铝含量对氧化铝陶瓷微观结构的影响,采用扫描电子显微镜（SEM）观察分析试样的断口形貌;采用压痕法计算试样的断裂韧性（KIC）值;研究了不同含量的晶种引入量对氧化铝陶瓷断裂韧性的影响。结果表明烧结温度为1575℃时,相对致密度可以达到96.7%;片状氧化铝晶种的引入能够显著提高氧化铝陶瓷的断裂韧性;其片晶的裂纹偏转、片晶拔出效应等增韧机制发挥了主导作用;随着片状氧化铝含量的提高,氧化铝陶瓷的力学性能逐渐提高,当掺杂含量达到35%（质量分数）时,KIC达到6.4MPa.m1/2,当含量继续增加,KIC呈现逐渐降低的趋势。     

Prospects of microwave processing: An overview

Microwave processing has been emerging as an innovative sintering method for many traditional ceramics, advanced ceramics, specialty ceramics and ceramic composites as well as polymer and polymer composites. Development of functionally gradient materials: joining; melting; fibre drawing; reaction synthesis of ceramics; synthesis of ceramic powder, phosphor materials, whiskers, microtubes and nanotubes; sintering of zinc oxide varistors; glazing of coating surface and coating development have been performed using microwave heating. In addition, microwave energy is being explored for the sintering of metal powders also. Ceramic and metal nanopowders have been sintered in microwave. Furthermore, initiatives have been taken to process the amorphous materials (e.g. glass) by microwave heating. Besides this, attempt has been made to study the heating behaviour of materials in the electric and magnetic fields at microwave frequencies. The research is now focused on the use of microwave processing for industrial applications.     

Contact-damage-resistant ceramic/single-wall carbon nanotubes and ceramic/graphite composites

There has been growing interest in incorporating single-wall carbon nanotubes (SWNTs) as toughening agents in brittle ceramics. Here we have prepared dense Al(2)O(3)/SWNT composites using the spark-plasma sintering (SPS) method. Vickers (sharp) and Hertzian (blunt) indentation tests reveal that these composites are highly contact-damage resistant, as shown by the lack of crack formation. However, direct toughness measurements, using the single-edge V-notch beam method, show that these composites are as brittle as dense Al(2)O(3) (having a toughness of 3.22 MPa m(0.5)). This type of unusual mechanical behaviour was also observed in SPS-processed, dense Al(2)O(3)/graphite composites. We argue that the highly shear-deformable SWNTs or graphite heterogeneities in the composites help redistribute the stress field under indentation, imparting the composites with contact-damage resistance. These composites may find use in engineering and biomedical applications where contact loading is important.     

Prospects of microwave processing: An overview

Microwave processing has been emerging as an innovative sintering method for many traditional ceramics, advanced ceramics, specialty ceramics and ceramic composites as well as polymer and polymer composites. Development of functionally gradient materials, joining, melting, fibre drawing, reaction synthesis of ceramics, synthesis of ceramic powder, phosphor materials, whiskers, microtubes and nanotubes, sintering of zinc oxide varistors, glazing of coating surface and coating development have been performed using microwave heating. In addition, microwave energy is being explored for the sintering of metal powders also. Ceramic and metal nanopowders have been sintered in microwave. Furthermore, initiatives have been taken to process the amorphous materials (e.g. glass) by microwave heating. Besides this, an attempt has been made to study the heating behaviour of materials in the electric and magnetic fields at microwave frequencies. The research is now focused on the use of microwave processing for industrial applications.     

Hot shock compaction of nanocrystalline alumina

An experimental investigation of hot shock compaction of a nanocrystalline alumina powder was performed. The effects of variations in shock pressure and compaction temperature on the properties of the compacted materials were studied. It was found that the bulk density and hardness of the compacted material increased with shock pressure. Increasing compaction temperature resulted in increases in compact hardness and bonding, and reductions in cracking within the compacted specimens. The results suggest that dense, well bonded, crack free nanocrystalline ceramics may be fabricated more effectively using hot shock compaction, than by room temperature shock compaction followed by sintering or room temperature static compaction followed by sintering.     

The sintering and grain growth behaviour of ceramic–carbon nanotube nanocomposites

The sintering and grain growth behaviour of alumina + 2, 3.5 and 5 wt.% carbon nanotubes (CNTs) and alumina + 2 wt.% carbon black nanocomposites prepared by Spark Plasma Sintering (SPS) were studied. The addition of CNTs to ceramics produces a large reduction in the sintering temperature required for their complete densification and a significant grain size refinement by a previously unreported mechanism. The CNTs form a strong entangled network around the grains, which constrains the normal and abnormal grain growth. An alumina/alumina + 2 wt.% CNT/alumina laminate structure was prepared to demonstrate directly the large grain-growth retardation effect of CNTs. These effects open up the possibility of using CNTs as a sintering aid to control the sintering behaviour and microstructures of ceramics in bulk, laminate and functionally gradient (FGM) form.     

Zirconia-MWCNT nanocomposites for biomedical applications obtained by colloidal processing

Zirconia ceramics are widely used as femoral heads, but case studies show that delayed failure can occur in vivo due to crack propagation. The addition of carbon nanotubes (CNT) is aimed to avoid the slow crack propagation and to enhance the toughness of the ceramic material used for prostheses. However, to really enhance the mechanical properties of the material it is necessary to achieve a uniform distribution of the CNT in the zirconia matrix. Colloidal processing has demonstrated to be suitable for obtaining ceramic-based composites with homogeneous distribution of the phases and high green density. This work compares the colloidal behavior of the as-received multi wall carbon nanotubes (ar-MWCNT) and the partially coated MWCNT (pc-MWCNT) when immersed in a nanozirconia matrix. With pc-MWCNT an improvement in the dispersion is proved. Moreover, the sintered samples that contain pc-MWCNT show higher density, lower grain size, improved toughness and enhanced hardness under the same sintering cycle when compared to the samples with ar-MWCNT.     

Synthesis, microstructure and mechanical properties of ceria stabilized tetragonal zirconia prepared by spray drying technique

Ceria stabilized zirconia powders with ceria concentration varying from 6 to 16 mol% were synthesized using spray drying technique. Powders were characterized for their particle size distribution and specific surface area. The dense sintered ceramics fabricated using these powders were characterized for their microstructure, crystallite size and phase composition. The flexural strength, fracture toughness and microhardness of sintered ceramics were measured. High fracture toughness and flexural strength were obtained for sintered bodies with 12 mol% of CeO₂. Flexural strength and fracture toughness were dependent on CeO₂ concentration, crystallite size and phase composition of sintered bodies. Correlation of data has indicated that the transformable tetragonal phase is the key factor in controlling the fracture toughness and strength of ceramics. It has been demonstrated that the synthesis method is effective to prepare nanocrystalline tetragonal ceria stabilized zirconia powders with improved mechanical properties. Ce-ZrO₂ with 20 wt% alumina was also prepared with flexural strength, 1200 MPa and fracture toughness, 9.2 MPa√m.     

Two-step sintering of dense, nanostructural forsterite

This paper investigates a method for preparing the nanocrystalline forsterite dense ceramic via sintering the forsterite nanopowder. The two-step sintering (TSS) method has been applied to suppress the accelerated grain growth of forsterite nanopowder compacts. The effects of sintering parameters on the mechanical properties and the microstructure characteristics of the forsterite ceramic were studied. Results verified the applicability of this method to suppress the final stage of grain growth in the system. The grain size of the high density compacts (~ 98.5% TD) of the forsterite was 60-75 nm. The optimal hardness (940 Hv) and fracture toughness (3.61 MPa.m^1/2) of the prepared nanocrystalline forsterite were found to be higher than those of the currently available hydroxyapatite bioceramics. It was concluded that the two-step sintering method can be used to fabricate improved forsterite dense ceramics with desired bioactivity and mechanical properties that might be suitable for hard tissue repair and biomedical applications.     

Effect of additives on the microstructure and mechanical properties of commercial alumina ceramics

The effect of impurities or additives on the microstructure of some commercial alumina ceramic samples have been characterized by scanning electron microscope-energy dispersive x-ray spectrometer and the mechanical properties investigated. Both the hardness and the toughness have been found to increase as the SiO₂ content decreases, and the theoretical density is reached in these materials. Grain size and distribution seem to be important factors in the mechanical properties of alumina ceramics. If the additives are controlled carefully, finer grain size and distribution can be obtained, together with a high relative density in terms of pore distribution characteristics and consequently enhanced mechanical properties.     

Formation of translucent hydroxyapatite ceramics by sintering in carbon dioxide atmospheres

Hydroxyapatite is used in a variety of clinical applications as a result of the apparent adherence to and mild reaction of bone and soft tissue to it owing to its structural similarity with bone mineral. Transparent hydroxyapatite has previously been fabricated by either or both of two methods; namely the application of pressure during sintering and/or the use of fine particle sized apatite prepared by either a sol-gel process or aqueous precipitation. Recently it has been shown that translucent carbonate hydroxyapatite may be formed by sintering nanocrystalline gels of carbonate hydroxyapatite in a wet carbon dioxide atmosphere. In this study we report for the first time that this atmosphere can be used to sinter microcrystalline powder compacts of hydroxyapatite to form translucent ceramics at ambient pressure. The effect of water partial pressure and sintering time at 1300°C on the optical transmission and microstructure of the ceramic was investigated. It was found that translucent ceramics were formed in all carbon dioxide atmospheres and that optical transmission varied with sintering time. Maximum transmission (13%) of 2 mm thick ceramic was obtained in materials sintered for four hours at 1300°C in a mixture of carbon dioxide containing water at a partial pressure of 4.6 kPa.     

Electrophoretic deposition of alumina,yttria, yttrium aluminium garnet and lutetium aluminium garnet

The electrophoretic deposition (EPD) method has been adapted for the deposition of ceramic green bodies from aqueous nanodispersions of alumina, yttria, yttrium aluminium garnet and lutetium aluminium garnet. These materials have been selected, since they are promising candidates for optically transparent ceramics. Films as well as cylindrical bodies have been successfully prepared by application of pulsed direct current (pDC) EPD. To guarantee constant deposition yield during pDC, a variant with variable pulse widths and pulse heights has been developed. The obtained green bodies were studied by surface area analysis, scanning electron microscopy, optical transmittance measurements, determination of pycnometric density and sintering behaviour. The effect of colloid-chemical dispersion properties on green and sintered ceramics is discussed as well. The green ceramics received are nanoporous and dense, providing excellent properties for further processing under mild conditions to optical materials. For comparison, EPD-formed green bodies were either processed directly to ceramic bodies or after additional compacting by hot-pressing in a piston-cylinder apparatus.     

Investigation of nanostructured oxides: synthesis, structure and properties

The process of forming three-component nanocrystalline fibers and powders of zirconia, yttria and alumina is studied depending on the component ratio and heat treatment temperature. It has been found that in the investigated system at 500-600 degrees C a nanocrystalline triple solid solution is formed, which exists up to 1200 degrees C. Beyond the above temperature, the triple solid solution decomposes into individual components. Specific regularities of changes in the crystalline structure and size of nanograins of oxides of triple solid solutions in the ZrO2(Y2O3)-Al2O3 system are established depending on the composition and thermal action. The structure--crystallite size--physical-chemical property relationship is also considered. The proposed synthesis method enables preparing nanocrystalline fibers and powders with a high degree of dispersion and reactive activity, whose use in composite materials and ceramics improves their service properties.     

Prediction of crack-tip toughness of alumina for given residual stresses with parallel-bonded-particle model

Distinct element method (DEM) is a powerful method to simulate the behavior of the heterogeneous materials. In this article the effect of average grain size on the crack-tip toughness of alumina ceramics is investigated by using a commercial computer program PFC^2D with the inclusion of residual stresses caused by thermal expansion anisotropy (TEA) of alumina grains into a DEM model. This is the first time that the residual stresses are included into DEM model and the residual stresses will be included as deviation to the strength of parallel bonds. Since the sintering technology and modeling of sintering is not the main subject of this article, the experimentally measured microlevel residual stress values will be used in order to predict a macrolevel behavior (fracture) of alumina. Comparison of the numerical results with the experimental ones showed that the default parallel-bonded-particle model in program PFC^2D gives particle-size dependent results when the fracture of alumina is simulated. With the help of the introduced normalization procedure, the crack-tip toughness of alumina with any average grain size can be predicted if the residual stresses are known and vice versa.