【作者】 宋谋胜；

【导师】 李建国；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2009， 博士

【摘要】 自20世纪60年代Merzhanov等系统研究燃烧合成(Combustion synthesis, CS)技术以来,CS技术已成为当今材料制备方法中极具潜力的技术之一。CS的基本原理就是反应物在外部能源的诱发下而引燃自身充足而自持续的(或热爆式的)化学放热反应从而合成预期的产物。作为材料的一种原位合成(In situ)方法,CS技术已被广泛地用来制备金属间化合物、功能梯度材料、复合材料、陶瓷材料等,然而人们对原位合成材料的力学性能和应用研究较多,而对其形成机制、尤其是生长机理的研究却相对有限。因此,本文设计并采用燃烧反应技术合成了TiC、ZrC陶瓷颗粒,并利用差热(DTA),X射线衍射(XRD),微区X衍射,场发射扫描电镜(FE-SEM),能谱(EDS)以及透射电镜(TEM)等现代分析手段,研究了原位合成TiC、ZrC的反应特征、组织形貌、形成过程以及生长动力学机制。热力学分析表明,TiC、ZrC分别是Al-Ti-C和Al-Zr-C体系反应热力学上最稳定、最易形成的相。研究结果发现,在SHS(Self-propagating high-temperature synthesis)反应制备TiC时,随着Al-Ti-C体系中Al含量的增加,体系的绝热温度、反应温度和燃烧波速均明显下降,合成TiC晶粒的尺寸显著细化,而一旦Al含量超过50 mass.%时,体系的SHS反应进行得较不充分而致使中间相滞留在反应产物中。通过SHS合成TiC的反应过程特征并结合DTA实验以及淬熄处理,研究了TiC的形成机制。首先,Al粉与Ti粉经一定的预热后相互间发生固-固反应而形成少量的TiAl3相并放出适当热量促使Al熔化,然后由于部分Al的熔化而导致了更剧烈的Al-Ti固-液反应而生成大量TiAl3相,同时释放大量热量使得温度急剧升高而致使已生成的TiAl3相熔化,随后,C溶解到Al-Ti熔液中并与之反应而生成热力学上更稳定的TiC相。最后,该TiC相从熔体中形核、析出并生长为不同形态的TiC晶粒。当在Al-Zr-C粉末中SHS合成ZrC时,随着混合粉末中Al含量的增多,体系燃烧温度、反应波速也均明显降低,ZrC晶粒尺寸进一步细化。当Al含量超过50 mass.%时,体系的反应温度低至1460 K以致完全不能诱发ZrC合成反应。而在Al含量超过30 mass.%时,可以合成了纳米级(≤150 nm) ZrC颗粒,尤其是Al含量为40 mass.%时,ZrC晶粒尺寸竟小于50 nm。SHS反应合成ZrC相的燃烧过程、DTA分析以及燃烧波峰淬熄实验均表明,Al-Zr-C混合粉末中的添加剂Al粉不仅作为稀释剂降低整个反应温度而抑制ZrC晶粒的生长与粗化,更重要的是作为中间反应物参与整个SHS反应过程从而影响ZrC晶粒的形成机制和生长形态。实验还发现,在所有锆铝化合物中,ZrAl3化合物是最有利形成的、且唯一的相,这与其具有L12、D022亚稳态和D023稳定态等三种晶体结构紧密相关,还与D023-ZrAl3的点阵结构与fccα-Al的点阵结构相似有关。热爆(Thermal explosion, TE)合成TiC、ZrC时,由于其反应过程相对于SHS过程来说具有更快的反应速度和随后的冷却速度,使得整个反应进行得较不完全从而导致较多的中间过渡相如Al4C3、ZrAl3和Zr3Al3C5相驻留在最终产物中。但TE合成TiC、ZrC颗粒尺寸更细小,尤其在10 mass.%Zn-Ti-C粉末中TE合成了TiC纳米颗粒(< 200 nm),只不过TE合成TiC、ZrC晶体发育、生长较不完全。TiC、ZrC晶体均表现出强烈的小平面光滑生长趋势。实验结果发现,Al含量、点燃方式和稀释剂种类对TiC、ZrC晶体的生长形貌影响明显,但不影响二者的生长动力学机制,即二者的生长机制均表现为二维形核台阶侧向层状生长模式。燃烧合成过程极高的过饱和度与过冷度的差异使得TiC、ZrC晶核在凝固和析出生长过程中速度不同,从而导致TiC、ZrC晶体生长形态各异。特别是添加10 mass.%Zn到10 mass.%Al-Zr-C混合粉末中可TE合成中空的四方ZrC晶粒。当Al-Ti-C粉末中的Al含量超过40 mass.%时,SHS合成的TiC晶体形态呈规则的八面体,其显露的八个面为面网密度最大的{111}晶面,该{111}晶面具有最小的表面吸附能。TiC晶体的最小生长单元为六配位的Ti-C6八面体基元,正是Ti-C6基元通过二维形核方式不断进入{111}面并产生新的台阶,后续的基元通过这些台阶堆垛而最终生长成粗大而规则的TiC八面体。由于几何对称性和能量稳定性,TiC八面体联接时以棱边联接为稳定的联接方式。而在Al-Zr-C体系中,当Al含量为20 mass.%时,SHS反应合成的ZrC晶体生长成规则的、基面为{111}的六方体形态。根据分析可知,单层ZrC晶片在[111]方向的生长速度被抑制而在[110]方向的生长速度却得到促进。结果,ZrC晶核在{111}晶面内通过二维形核侧向生长模式沿[110]方向生长成单层ZrC六方形薄片,然后单层ZrC六方形薄片通过层状台阶生长机制沿[111]方向不断堆积而生长成为一个多层的、完整六方体形貌的ZrC晶体。更多还原

【Abstract】 CS (Combustion synthesis) technology has been systematically investigated by Merzhanov et al. since the 1960s and has gradually become one of potential technologies for the materials-fabricating. CS method is essentially based on the sufficiently and self-sustainingly (or thermal explosion) chemical exothermic reaction of starting reactant mixtures once ignited by external energy supply to synthesize the designed products. As an in-situ method of materials fabrication, CS has been widely applied to synthesize intermetallic compounds, functional gradient materials, composites and ceramic particles. However, the mechanical properties and applications of in-situ materials have been paid more and more attention, while few work or report has been focused on its formation mechanism, especially its growth mode. So, in this paper, TiC and ZrC ceramic particulates were designed and fabricated by the CS technology. The reaction characteristics, crystal morphologies, formation mechanisms and the growth kinetics modes of the in-situ TiC and ZrC ceramics were investigated in detail by using the modern analyzing methods such as differential thermal analyzer (DTA), X-ray diffractometer (XRD) and micro-diffractometer, field emission scanning electron microscopy (FE-SEM), energy dispersive spectrometer (EDS), as well as transmission electron microscopy (TEM).The thermodynamic analysis showed that TiC and ZrC are the most thermodynamically stable and favorable phase in Al-Ti-C and Al-Zr-C reaction systems, respectively. The experimental results exhibited that when fabricating TiC ceramic by SHS (Self-propagating high-temperature synthesis), with Al contents increasing in Al-Ti-C mixtures, the adiabatic temperature Tad, reaction temperature Tc and combustion wave rate Vc all reduced evidently, and also the synthesized particles size became more finer. Once the Al content exceeds 50 mass.%, SHS reaction will become insufficient and the intermediate phase will retain in the final products. The formation mechanism of TiC was studied by combining the reaction characteristic of SHS with the DTA experiment and quenching experiment. After being preheated, the solid-state reaction between Al and Ti initially occurred and produced a few TiAl3 phase, and released some heat to make Al melt. Owing to some Al melting, the drastic solid-liquid reaction took place to produce more TiAl3 compound and to liberate a mass of heat, and thus leading the temperature abruptly to rise and the formed TiAl3 to melt. Subsequently, C dissolved into the Al-Ti melt and reacted with it to synthesize the thermodynamically stable TiC phase. As a result, the TiC phase nucleated in the melt, precipitated and grew as the TiC grains with various morphologies.When synthesizing ZrC by SHS in the Al-Zr-C powder mixtures, with Al contents increasing, the combustion temperature, reaction wave rate, as well as the ZrC particles size all decreased obviously. But once Al content is higher than 50 mass.%, the combustion temperature was so low (1460 K) that the ZrC-forming reaction was hardly induced. It is noted that once Al exceeds 30 mass.%, the nano-scaled ZrC particles (≤150 nm) were produced successfully, especially the ZrC size is less than 50 nm for 40 mass.% Al addition. The combustion process of SHS, DTA analysis and combustion wave front quenched experiment all indicate that, the Al additive in mixtures serves not only as a diluent to reduce the reaction temperature and to inhibit the ZrC particle from growing and coarsening, but importantly as an intermediate reactant to participate in the total SHS process and thus to control the formation mechanism and growth morphologies of ZrC grains. It was also found that, among all Zr-Al compounds, the SHS-synthesized ZrAl3 phase was the most favorable and only phase, which is greatly influenced by its three lattice structures such as L12 and D022 metastable structures and D023 stable structure, and also the similarity of lattice structure between D023-ZrAl3 and fccα-Al is responsible for.When synthesizing TiC and ZrC by TE(Thermal explosion), because TE processing has more rapid reaction rate and cooling rate than SHS processing, the total reaction proceeded insufficiently and resulted in more intermediate phase such as Al4C3, ZrAl3 and Zr3Al3C5 retaining in the products. Moreover, the TE-synthesized ZrC and TiC particles sizes are more finer, especially the nano-sized TiC particles were synthesized during TE processing in 10 mass.%Zn-Ti-C mixtures. However, the TE-synthesized TiC and ZrC crystals were of rather insufficient growth.TiC and ZrC crystals exhibit the strong faceting and smooth growth tendency. It showed that Al contents, ignition method, as well as the diluent species have evident influence on the growth morphology rather than on the growth kinetics mechanisms of TiC and ZrC crystals, namely both growth mechanisms are of the layer-by layer growth mode through the two-dimensional (2D) nucleation method. Due to the high supersaturation and undercooling, TiC and ZrC nuclei have different solidification, precipitation and growth rate, and hence resulting in the diverse growth morphologies of TiC and ZrC crystals. In particular, the hollow and square ZrC grains were synthesized by TE when 10 mass.%Zn was added into 10 mass.%Al-Zr-C mixtures. Once Al content in Al-Ti-C compact exceeds 40 mass.%, the SHS-synthesized TiC crystal appears as a regular octahedral morphology, and its unfolded eight planes are of the {111} facets with the biggest face density and the lowest surface attachment energy. The growth cell of TiC crystal is Ti-C6 octahedron unit. The Ti-C6 units continuously enter into the {111} planes and generate the new steps through 2D nucleation mode, and thus the octahedron units grew as the big and regular TiC octahedron through this layer-by-layer growth mode. TiC octahedra are linked through an edge-shared manner in order to favorably meet the geometry symmetry and energy stability.For Al-Zr-C mixtures, when Al content is 20 mass.%, the SHS-synthesized ZrC crystal grew as a well-developed hexagonal morphology with the {111} basal planes. According to the analysis, for the monolayer ZrC crystal, the rate in the [111] direction was suppressed and the rate in the [110] direction was prompted. As a result, the ZrC growth units in the (111) facet grew as a monolayer hexagonal platelet via 2D growth mode along [110] direction, and then the thick ZrC hexagonal multilayers were formed through the layer-by-layer growth mechanism of monolayer ZrC hexagonal platelets.更多还原