摘要
纳米陶瓷因优异的强度、韧性、抗氧化性、耐腐蚀及与金属类似的超塑性，展现出诱人的应用前景。然而，高致密度的纳米陶瓷的制备却非常困难。在高温烧结促进致密化的过程中，总是伴随着晶粒的快速长大。目前，最具商业价值的无压烧结工艺在这一领域中的进展并不如预想的那样迅速，极大地制约了纳米陶瓷的广泛应用。
本文考察了当前纳米陶瓷的致密化研究状况，梳理出无压烧结制备致密纳米陶瓷的关键环节，并有针对性地提出了解决问题的途径。作者认为，无压烧结制备致密纳米陶瓷应着重解决下述问题：
A．制备粒径细小、分散良好的无团聚纳米粉体。
B．制备出高密度、微观结构均匀的生坯。
C．采用动态控制烧结技术，利用晶粒长大和致密化的动力学差异，促使竞争向有利于致密化的方向转化。对于某些体系，相变辅助致密化效应尤其值得关注。
在此基础上，论文选择了粉体制备、凝胶注模成型、相变辅助致密化结合动态控制烧结三个问题为重点加以研究。并获得如下主要结论：
A．采用不同湿化学工艺，制备出Al_2O_3、Nb_2O_5、TiO_2纳米粉体。结果表明：碳酸铵铝(AACH)经1100℃煅烧1.5 h，可以获得平均晶粒尺寸为30nm的α-Al_2O_3纳米粉体，添加α-Al_2O_3籽晶后，煅烧温度可降至1050℃。AACH在煅烧中的相变历程：AACH→amorphous→γ→δ→θ→α，与γ-AlOOH中γ→α相变没有本质的区别，缺乏δ-Al_2O_3并非AACH相变历程的特征。使用粗晶氧化铌及浓硫酸为原料，采用沉淀法可低成本制备出各种晶型的纳米Nb_2O_5粉体。在煅烧过程中纳米Nb_2O_5粉体历经amorphous→δ→γ→β→α相变历程。
B．在水基凝胶注模中，分散剂PAA-Na有效降低氧化铝的等电点并增加Zeta电势，分散效果良好，分散剂的最佳用量为1.6 wt％。pH值对料浆颗粒表面的Zeta电势的影响非常大，当pH值在10.5左右时，料浆最稳定。悬浮体的粘度随着球磨时间的延长而降低，当球磨时间达到10h时，粘度最小，球磨时间继续延长时，粘度有增大的趋势。固相量的增大，使氧化铝陶瓷料浆的粘度迅速增大。采用水基凝胶注模工艺对于所用纳米氧化铝粉体所能达到的最高固相量仅为30 vol％。无法满足凝胶注模的需求。采用有机溶剂基凝胶注模工艺对所选纳米氧化铝粉体可以获得40 vol％固相量的料浆。提高料浆温度可以显著降低体系粘度，在阻聚剂0.5mmol／L对苯二酚协助下，70℃料浆固化诱导期延长至约60 min。所得料浆成功浇铸出结构致密均匀的生坯。尽管烧结样品结构均匀性良好，相对密度仍然偏低，进一步提高料浆固相量仍有待进一步开发。
C．论文采用动态控制烧结、相变辅助致密化效应，研究纳米γ-Nb_2O_5、β-Nb_2O_5、TiO_2陶瓷的致密化和晶粒长大过程。结果表明：动态控制烧结技术和相变辅助致密化效应有效利用了晶粒长大和致密化的动力学差异，促进致密化的同时，晶粒尺寸维持在纳米范畴；可以用无压烧结工艺获得致密的、晶粒尺寸分别为～80 nm和～40nm，接近完全致密的γ-Nb_2O_5和β-Nb_2O_5纳米陶瓷。在δ-Nb_2O_5→γ-Nb_2O_5相变温度附近，纳米Nb_2O_5致密化速率显著提高。在1000℃以前，随着相变的进行，Nb-O-Nb键强持续降低，键的松弛和断裂导致大量单空位尺度的缺陷，导致原子可迁移性大幅提高，是导致致密化速率提升的主要原因。在TiO_2烧结过程中，伴随着锐钛矿→金红石相变的是大量单空位尺度缺陷的生成，这是相变促进纳米TiO_2致密化的主要原因。“相变促进致密化”、“添加剂活化晶格”、“(位错)自激活烧结”等，均有着相似的活化烧结机制，可以划归为传统的强化烧结的范畴。在γ-Nb_2O_5→β-Nb_2O_5相变温度附近，晶界上高密度的缺陷数量急剧上升，意味着β-Nb_2O_5相拥有充足的形核位置，每个γ-Nb_2O_5晶粒可以生成若干β-Nb_2O_5晶粒，获得极为细小的β-Nb_2O_5晶粒结构。因此相变过程中新相较高的形核率对于产生细小的晶粒尺寸十分重要。
Fully dense speciments with nanosize features are most important for structural,magnetic or electronic applications. But there are currently some difficults inachieving fully dense nanocrystalline ceramics. Pressureless sintering is still a mostpromising method because of its low cost and easy technique. Unfortunately, theadvancement of this domain is not as rapidly as one might expect.
According to the current advancement in nanocrystalline ceramics processtechniques, especially for the pressureless sintering, there were three enlighten pointsmust be emphasized here:
A. The presence of agglomerates in the nanocrystalline particles was the mostimportant obstacle to obtain the dense nanocrystalline ceramics.
B. Extremely fine-structured, homogeneous green-body microstructures andhigher green body density will improve the sinterability.
C. The transformation-assisted consolidation is a useful method for consolidatingoxide-ceramic powders in selected systems. Dynamically controlled sintering is oneof the most promising methods to fabricate dense nanocrystalline ceramics by usingthe competition of sintering mechanisms on heating to minimize the grain growthwithout affecting the densification behavior.
Based upon the analyses mentioned above, the aim of this work is to synthesizeAl_2O_3, Nb_2O_5, TiO_2 non-agglomerated nanopowders by conventional chemicaltechniques, to synthesize nanocrystalline ceramics using gelcasting process andpressureless sintering method and to investigate the application of thetransformation-assisted consolidation method and the dynamically controlledsintering method in the selected systems. The main conclusions include:
A. Theα-Al_2O_3 nanoparticles with an average particle size of 30 nm could beprepared by thermal decomposition of the NH_4Al(OH)_2CO_3(AACH). The AACHundergoes the following phase transformations during heating:AACH→amorphous→γ→δ→θ→α, somewhat similar to that ofγ-AlOOH(γ-AlOOH→γ→δ→θ→α). The existence ofδ-Al_2O_3 in the phase transformationsequence is found, which is different from results reported in literatures. Theα-Al_2O_3seed crystals can reduce the formation temperature ofα-Al_2O_3 to 1050℃. Niobiumpentoxide nanopowders of amorphous,δ-Nb_2O_5,γ-Nb_2O_5,β-Nb_2O_5 andα-Nb_2O_5with grain size in the nanoscale using coarse-grain Nb_2O_5 as precursor was developed.Nanocrystalline titania crystals were prepared by a sol-gel method. Results showedthat the phase transformation from anatase to rutile will occur at 750～950℃
B. The dispersion and rheology of nano alumina aqueous suspension forgelcasting have been investigated. The results showed that both pH value and amountof dispersant can affect the dispersion effect and rheology, the optimum had beenachieved when pH10.5 and the amount of dispersant was 1.6 wt%.The solid contenteffect on rheology of alumina aqueous suspension has been investigated, and the solidcontent of alumina aqueous suspension with 30 vol % has been prepared which is not enough for gelcasting.
The dispersion and rheology of nano alumina nonaqueous suspension withvarious solid contents for gelcasting have been investigated. Nano aluminanonaqueous suspension with 40 vol % which fits into gelcasting has been prepared. Itwas found that the apparent viscosity decrease with the increase of the temperature.The inhibitor of hydroquinone can delay solidication of nano alumina slurry effectlyunder 70℃.
C. According to our research, the enhanced densification process of niobiumpentoxide was acquired around theγ-Nb_2O_5 toβ-Nb_2O_5 phase transformationtemperature. The bond order varied continually until 1000℃. The atoms were verymobile because of the bond laxation and breakage, and the large numbers ofcrystallite defects, thus an enhanced sintering can be expected. During the phasetransformation ofγ-Nb_2O_5 toβ-Nb_2O_5, large numbers of crystallite interfaces containshigh-density defects beneficial to nucleation inβ-Nb_2O_5 emerged. Theγ-Nb_2O_5 graincould be divided into several domains and the overall grain size can be refined. Inconclusion, near-denseγ-Nb_2O_5 andβ-Nb_2O_5 nanostructured ceramics with grain sizeof about 80 nm and 35 nm respectively were fabricated by exploiting the difference inkinetics between the competitive mechanism such as grain-growth, densification andphase transformation using a dynamically controlled sintering accompanied by atransformation-assisted consolidation method. The transformation-assistedconsolidation effect can be owed to the mobility of the atoms during the phasetransformation from anatase to rutile, with the occurrence of the bond laxation andbreakage, and a large numbers of crystallite defects.
引文
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