【作者】 张鹏林；

【导师】 夏天东；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2008， 博士

【摘要】 自蔓延高温合成（Self-propagating High-temperature Synthesis,SHS）因其反应迅速、工艺简单、节约能源等优点成为制备陶瓷、金属间化合物、高性能涂层以及复合材料等的新技术。镁热剂反应自蔓延高温还原合成陶瓷,采用天然氧化物为原料,与普通的元素自蔓延高温合成工艺相比,具有成本低廉、产品性能优异等显著特点,近年来受到国内外研究人员的普遍关注。然而,镁热剂反应由于放热量大、反应温度高、反应过程迅速及反应难以控制等特殊性,迄今为止,对其反应过程及产物结构形成机理等的研究不够深入。因此,研究镁热剂反应自蔓延高温合成TiB2和ZrB₂的反应过程及产物结构形成机理,无论从自蔓延高温合成方法本身的完善还是从促进TiB₂和ZrB₂陶瓷材料的广泛应用来讲都具有重要意义。本文以B₂O₃、TiO₂、ZrO₂和还原性金属Mg为主要原料,采用自蔓延高温合成法成功制备了TiB₂和ZrB₂陶瓷,研究了Mg-TiO₂-B₂O₃和Mg-ZrO₂-B₂O₃两个三元体系的自蔓延过程及其化学反应特点,并从热力学及结构动力学两方面进行了系统研究。基于热力学理论,对Mg-TiO₂-B₂O₃和Mg-ZrO₂-B₂O₃体系的绝热温度及反应自由能进行了理论计算和分析。计算结果表明:两个体系的绝热温度都超过3000K,远远大1800K（反应能够自维持的温度）,并且随着稀释剂含量的增加绝热温度呈现逐渐降低的趋势;两个体系在所研究的温度范围（400-2000K）可能发生的反应的生成自由能均小于零,存在发生反应的可能性。探讨了工艺参数如原料配比、稀释剂等对镁热剂反应自蔓延高温合成TiB₂和ZrB₂陶瓷的合成过程、产物相组成及组织形貌的影响。研究结果表明,原料中Mg和B₂O₃的挥发对产物粉末纯度具有重要影响。随着Mg和B₂O₃含量的增加,产物纯度提高。在反应原料中加入适量的稀释剂MgO（0-5mol）,可调节燃烧温度,改善产物粉末的形貌和粒度,随稀释剂MgO含量的增加,产物粉末平均粒度降低。采用三种不同方法成功淬熄了镁热剂自蔓延高温合成TiB₂和ZrB₂陶瓷时的燃烧波,得到了不同反应程度的产物微区形貌。通过对不同的淬熄区XRD测试分析和扫描电镜观察,结合反应体系DSC分析,系统研究了Mg-TiO₂-B₂O₃和Mg-ZrO₂-B₂O₃体系自蔓延高温合成过程。结果表明:在Mg-ZrO₂-B₂O₃体系中,反应过程经由多个中间反应直至最后完成,B₂O₃在623K熔化,Mg在922K熔化,三相反应的发生始于1043K。首先发生的反应是ZrO₂和Mg的还原反应生成金属Zr,其次是B₂O₃和Mg的还原反应生成B,最后是Zr和B反应合成ZrB₂。Mg-ZrO₂-B₂O₃体系燃烧反应可划分为如下几个阶段:①预热阶段,B₂O₃、Mg熔化,在“毛细管”作用下,液态Mg渗透到熔融的B₂O₃和固态的ZrO₂颗粒间隙,形成空心熔体球。液态Mg、B₂O₃和固态ZrO₂颗粒混合物在熔体球表面形成薄壳,反应在此薄壳上发生;②反应初段,ZrO₂颗粒与Mg熔体以溶解-析出机制生成Zr和MgO,释放大量的反应热;③反应中段,反应初段放出的强热诱发了Mg-B₂O₃之间的反应,生成B和MgO;④反应末段,Zr和B结合生成ZrB₂。ZrO₂-B₂O₃-Mg之间的反应为复杂的固-液-液反应。Mg-TiO₂-B₂O₃体系的反应过程及产物结构转变与ZrO₂-B₂O₃-Mg体系具有相似性。最后,提出了Mg-TiO₂-B₂O₃和Mg-ZrO₂-B₂O₃体系自蔓延高温合成TiB₂/ZrB₂陶瓷的固相扩散-溶解-析出机制,并建立了相应的物理模型来进行描述。通过物理模型,最后得到了Mg-TiO₂-B₂O₃/Mg-ZrO₂-B₂O₃体系反应生成TiB₂/ZrB₂动力学本征方程。更多还原

【Abstract】 For its advantages of rapid synthesis, simple process, energy saving, etc., SHS （Self-Propagating High-Temperature Synthesis） is a novel technology for preparing the advanced materials, such as ceramics, intermetallic compounds, high performance coatings and composite materials. Synthesizing ceramics by SHS with Magnesiothermit reactions uses natural oxides as the raw material, so the cost is low and the property of the product is good. Compared with SHS with common element, this technique offers obvious advantages and has been given more and more attention by researchers at home and abroad recently. However, due to some particularities of Magnesiothermit reactions, such as the quantity of releasing heat is large, the reaction process is difficult to control, the reaction temperature is high, etc., the studies on its reaction process and the formation mechanism of structure of the products are far from systematical. Therefore, the studies on the mechanism of synthesizing TiB₂ and ZrB₂ by SHS with Magnesiothermit reactions are of significance to improve SHS itself and promote the wide application of TiB₂ and ZrB₂.In this paper, B₂O₃, TiO₂, ZrO₂ and Mg are used as the main raw material, the research on the characteristics of SHS process and chemical reaction process of Mg-TiO₂-B₂O₃ and Mg-ZrO₂-B₂O₃ system has been made from the perspective of thermodynamics and Structural Macro Kinetics/structure dynamics.According to the thermodynamics theory, the free energy of reaction and the adiabatic temperature of Mg-TiO₂-B₂O₃ and Mg-ZrO₂-B₂O₃ system have been theoretically calculated and analyzed. The calculation of the adiabatic temperature shows that the adiabatic temperatures of the two reaction systems are both higher than3000K, which are far higher than 1800K （the temperature meets the need of reaction itself）. And the adiabatic temperatures of the reaction are falling with the increasing amount of diluents. The calculation of the free energy of reaction shows that the free energy is below than zero reaction can take place.The technological parameters, that is, the join effect of material proportion, the pressure of green compact and the green compact diameter and the diluents upon the process of synthesizing TiB₂ and ZrB₂ by SHS with Magnesiothermit reactions are also studied in this thesis. The research on technological rules shows that the volatilization of Mg and B₂O₃ in raw material has a great effect on the purity of product. When the amount of Mg and B₂O₃ is increased, the purity of product becomes higher. In order to adjust the combustion temperature and change the morphology and particle size of product, appropriate amount of diluents MgO （0-5mol） can be added into the raw material. With the increasing amount of diluents MgO, the average particle size of product becomes smaller.The combustion wave of green compact in synthesizing TiB₂ and ZrB₂ by SHS with Magnesiothermit reactions can be successfully gained by means of combustion front quenching （CFQ） in cylindrical steel mould. And the microstructure of every region of the different reaction extent was observed. According to the relevant test and analysis on different product of different combustion front quenching areas combined with differential scanning calorimetry （DSC）, the physical and chemical changes of all regions in the quenched samples during the combustion synthesis were followed by scanning electron microscopy （SEM）, energy-dispersive spectroscopy （EDS） and x-ray diffraction （XRD）. The systematic research on SHS of Mg-TiO₂-B₂O₃ and Mg-ZrO₂-B₂O₃ system has been carried out. Studies shows that B₂O₃ is melting at623K, Mg at 923K, the reaction among them takes place from 1003K. The reaction between ZrO₂ and Mg to produce Zr happens first, and then B is gained from the reaction between B₂O₃ and Mg. At last, ZrB₂ is synthesized from the reaction between Zr and B. The results show that the reaction process of combustion synthesis ZrO₂-B₂O₃-Mg can be expressed as: （1） the preheat phase: B₂O₃ and Mg are melting, liquid Mg is permeable into molten B₂O₃ and solid ZrO₂ forming a hollow-cylinder molten ball, where the reaction takes place; （2） the early stage of reaction: the reaction between ZrO₂ particle and molten Mg occurs in the mechanism of dissolution and precipitation to produce Zr and MgO, releasing a great amount of reaction heat; （3） the middle stage of reaction: the great amount of reaction heat releasing in the early period leads to the reaction between B₂O₃ and Mg, which can form B and MgO; （4） the later stage of reaction: the reaction between Zr and B takes place and the ZrB₂ grain is formed. The reaction among ZrO₂, B₂O₃ and Mg is a complicated reaction of solid-liquid-liquid. The reaction process and structural change of the products of Mg-TiO₂-B₂O₃ system has the similarity with Mg-ZrO₂-B₂O₃ system.Thus, the diffusion-dissolution-precipitation mechanism of synthesizing TiB₂ and ZrB₂ by SHS with Magnesiothermit reactions is put forward and the relevant physics modeling is set. According to the physics modeling, we can finally get the original formula of kinetics in TiB₂ and ZrB₂ which is reacted and generated by the system of Mg-TiO₂-B₂O₃ and Mg-ZrO₂-B₂O₃.更多还原