基于新元素和新多层结构的MAX相陶瓷材料研究进展

由于独特的层状结构和原子间特殊的化学键合,MAX相陶瓷材料(化学式为M_n+1AX_n)兼具金属和陶瓷材料的优异性能,在很多领域具有广泛的应用前景,自20世纪60年代问世以来就一直备受关注。至今已经发现了100多种MAX相陶瓷材料,其中包括80余种单相以及一系列固溶体。传统的MAX相局限于一定的元素范围和若干M₆X层与单A原子层交替堆垛的结构。最近含有Au、Ir、Cu、Zn等新元素的MAX相材料的成功合成大大丰富了MAX相家族,多A层和多MA层结构MAX相的发现也打开了新型MAX相研究的一扇大门。随着理论计算的发展和实验条件的进步,越来越多的新型MAX相陶瓷材料逐渐出现在人们的视野中。本文综述了基于新元素和新多层结构的MAX相的国内外实验合成和理论研究进展,并指出了后续研究需要克服的问题,最后对新型MAX相的研究方向和发展趋势进行了预测和展望。     

熔盐法制备三元层状MAX相陶瓷的研究进展

MAX相是一种兼具金属和陶瓷性能的新型三元层状过渡金属碳氮化物。传统合成MAX相的方法都有一定的局限性,如反应温度较高、合成时间过长、合成样品较少,且大部分无法直接一步制备所需MAX相。近些年来,采用熔盐法合成MAX相的报道越来越多,并且工艺持续改进。本文从传统熔盐法合成MAX相出发,分析并阐述了新熔盐法合成MAX相的研究进展。传统熔盐法利用较低熔点的熔盐作为反应溶剂,提高了反应效率;熔盐屏蔽法以熔盐作为反应溶剂的同时还可防止氧化,使得反应可以在空气中进行;路易斯酸盐法则是将熔盐作为反应原料来合成MAX新相;熔盐电化学法以电脱氧的方式,将合成原料由纯金属改为金属氧化物,降低了生产成本。熔盐法所合成MAX相产物较传统方法所合成产物的产量及纯度更高,所需要的温度、能耗以及成本更低。因此,熔盐合成法是未来大批量合成MAX相以及MAX新相合成的一个重要方法。     

一种新的具有非线性光学效应的固溶体MgxZn3-xBPO7

采用固相反应在820～1050℃合成了MgxZn3-xBPO7多晶样品,在x＝1.8时用自发成核法获得了单晶.所有样品具有相似的X射线粉末衍射花样和红外光谱,表明它们具有相同的结构,形成了固溶体.对红外光谱进行了解析,结果表明固溶体的晶体结构中存在BO3和PO4基团.由反射光谱得知,随着Mg逐步取代Zn,固溶体在紫外区的吸收逐渐减弱.固溶体系列的粉末倍频效应约是KDP(KH2PO4)的0.7倍.     

Ti3(ZnxAl1-x)C2固溶体热学、电学和力学性质的理论研究

采用第一性原理的密度泛函理论平面波赝势法, 通过投影缀加波(PAW)和广义梯度近似(GGA)系统地研究了Ti₃(Zn_xAl_1-x)C₂的结构、能量、声子性质、电子性质和弹性性质。对MAX相Ti₃AlC₂晶体中A位置的Al元素用Zn元素进行替换掺杂,构建出Ti₃(Zn_xAl_1-x)C₂(x=0,0.25,0.5,0.75,1)固溶体结构模型。计算分析表明:在所研究的掺杂浓度范围内Ti₃(Zn_xAl_1-x)C₂均是热力学、动力学和力学稳定的脆性材料;此外,Ti₃(Zn_xAl_1-x)C₂(x=0,0.25,0.5,0.75,1)均呈现金属性,在费米能级处的电子态密度主要贡献来自Ti-3d态,同时具有离子键、共价键和金属键的综合性质。随着Zn原子掺杂浓度的增加,在一定程度上其导电性和塑性均增强。     

TiO2对固相反应制备莫来石陶瓷性能影响

以天然红柱石和活性氧化铝为原料,固相反应烧结制备莫来石陶瓷,研究了TiO2对莫来石陶瓷的烧结性能和热震稳定性的影响,利用XRD、SEM及相关软件对莫来石陶瓷的晶相组成、晶胞参数和显微结构进行分析和讨论.研究结果表明:通过高温固相反应可以制备出以Al4.52Si1.48O9.74莫来石固溶体为主晶相的反应烧结莫来石陶瓷.引入2wt;的TiO2对于提高干压成型的反应烧结莫来石陶瓷热震稳定性有利.TiO2对于反应烧结莫来石陶瓷起到促烧结作用,掺杂Ti4+改变了主晶相莫来石固溶体晶胞参数,但未影响莫来石固溶体类型.     

络合法制备不同形貌的铈锆固溶体及其性能研究

采用不同的络合剂(草酸、谷氨酸、苹果酸和酒石酸),通过络合法制备出玫瑰花状、块状、多孔网状的Ce0.77Zr0.23O2铈锆固溶体,利用热分析(TG-DSC)、X射线衍射分析(XRD)、扫描电镜(SEM)、比表面积和氢气程序升温还原分析(H2-TPR)等对前驱体及固溶体进行了测试表征.前驱体在450℃下煅烧2h即可得到铈锆固溶体,700℃以后会发生晶粒长大的现象.其中以草酸为络合剂制备的玫瑰花状铈锆固溶体随着煅烧温度的升高,平均晶粒尺寸迅速增大,并发生分相,热稳定性较差;以酒石酸为络合剂制备的米粒状铈锆固溶体比表面积高达53.89 m2/g,900℃下煅烧2h,平均晶粒尺寸依然小于20 nm,具有优良的储氧性能.     

二氧化硅对锆酸钙材料结构及性能的影响

以分析纯氧化锆和碳酸钙为原料,成本相对低廉的二氧化硅为添加剂,利用固相反应烧结制备锆酸钙材料.研究了二氧化硅对锆酸钙材料相组成、晶胞参数、微观结构及常温性能的影响.用X射线衍射、扫描电镜及X 'pertplus分析软件分别对烧后锆酸钙试样的晶相组成、显微结构和晶胞参数进行表征和计算,并对试样的体积密度和显气孔率进行了测定.结果表明:利用碳酸钙和氧化锆在1600℃下保温3h,制备出的锆酸钙材料致密度较低;引入二氧化硅可以促进锆酸钙材料的烧结,当二氧化硅的加入量小于0.5mol;时,二氧化硅主要是通过在锆酸钙材料中形成间隙固溶体,产生晶格缺陷而促进材料的烧结;同时二氧化硅在锆酸钙材料中形成固溶体后,还导致锆酸钙相晶胞参数和晶胞体积的增大;由于O2-半径较大,难以进入间隙位置,使得二氧化硅在锆酸钙材料中的固溶量很小.当二氧化硅的加入量大于或等于0.5mol;时,锆酸钙的晶胞参数和晶包体积将几乎不发生变化,但是在锆酸钙材料中会有硅酸二钙相的出现和增加;二氧化硅最佳加入摩尔百分含量为0.25; ～ 0.5;.     

钛（IV）固溶氧化镁纳米粉体的制备与表征

采用液相共沉淀方法,以Mg（NO3）2.6H2O和Ti（OC4H9）4为起始原料,NH3.H2O为沉淀剂,PEG为分散剂,冰乙酸为修饰剂,制备了钛（IV）固溶氧化镁纳米粉体。利用XRD,XPS和TEM等分析手段对粉体的物相组成、结构和形貌等进行了表征。结果表明Ti4＋固溶到MgO的晶格中,物相只有立方晶系的方镁石结构MgO相组成,无其它相存在。所制得的粉体颗粒尺寸小（约40~50 nm）、粒度分布均匀,具有良好的分散性能。     

钛(Ⅳ)固溶氧化镁纳米粉体的制备与表征

采用液相共沉淀方法,以Mg(NO3)2·6H2O和Ti(OC4H9)4为起始原料,NH3·H2O为沉淀剂,PEG为分散剂,冰乙酸为修饰剂,制备了钛(Ⅳ)固溶氧化镁纳米粉体.利用XRD,XPS和TEM等分析手段对粉体的物相组成、结构和形貌等进行了表征.结果表明Ti4+固溶到Mgo的品格中,物相只有立方晶系的方镁石结构Mgo相组成,无其它相存在.所制得的粉体颗粒尺寸小(约40～50 nm)、粒度分布均匀,具有良好的分散性能.     

La2(Zr0.7Ce0.3)2O7纳米粉体的制备与表征

采用化学沉淀法制备了高性能热障涂层用La2(Zr0.7Ce0.3)2O7陶瓷粉体.通过TG-DSC、XRD、SEM及TEM对前驱体的烧结温度、粉体物相及微观结构进行了分析.结果表明:制备的粉体为纳米级,且颗粒分布均匀,粒径大约30 ～ 50 nm;La2(Zr0.7Ce0.3)2O7是由烧绿石结构的La2Zr2O7固溶体和萤石结构的La2Ce2O7固溶体复合组成.     

Heterophase states and a bridging phase in (1‐x)BiScO3 − xPbTiO3

Heterophase ferroelectric solid solutions of (1‐x)BiScO₃ − xPbTiO₃ are studied near the morphotrpic phase boundary (0.60 ≤ x ≤ 0.64). The role of non‐180° domain types of the P4mm and R3m phases in heterophase structures is studied in the context of the phase content. It is shown that the bridging Cm phase coexists with the adjacent phase (either P4mm or R3m), and different variants of an elastic matching ‘single‐domain Cm phase–polydomain P4mm phase’, ‘single‐domain Cm phase–polydomain R3m phase’ and ‘single‐domain Cm phase–near single‐domain R3m phase’ promote the complete stress relief. Based on these concepts and considering a minimum number of the domain types in the coexisting phases, we find a molar‐concentration dependence of the volume fraction of the Cm phase in the heterophase states. A correlation between the predicted volume fraction of the Cm phase and ratios of its unit‐cell parameters is first revealed. The role of the intermediate R3m phase in the phase sequence R3c − R3m − Pm$\overline 3$m and in the stress relief at x = 0.60 is discussed. Good agreement between the predicted and experimental data on the phase content is reached near the morphotrpic phase boundary.     

Structural relationships between single atom type crystal structures and intermetallic compound structures,as illustrated by Laves‐ and Hume‐Rothery phases

In comparison to the single atom type crystal structures, which are relatively simple and are concentrated in four types of structure (Cu, W, Mg, diamond), the crystal structures of intermetallic phases are manifold and often very complicated. At first sight there is no obvious connection between these types of crystal structures, however, a transition mechanism must exist for the solid‐state transition from the simple single atom type crystal structures to the intermetallic phase structures. Such a mechanism would show the inner connection between the structures. In this work the topic is dealt with from a crystallographic point of view. The transition between the structures can be described by the three steps (1) substitution of atoms, (2) systematically ejection of particular atoms from the emerging intermediate structure and (3) spatial relaxation especially of the atoms around the newly formed interstices. This general structural phase transition path will be discussed for two of the intermetallic classes, the Laves phases and the Hume‐Rothery phases. A suitable way of showing this is by making a comparison between the coordination polyhedra (atomic environments) of the single atom type crystal structures and those of the derived intermetallic crystal structures.     

Synthesis and electrical properties of Aurivillius phases in the Bi<Subscript>2</Subscript>MoO<Subscript>6</Subscript>-Bi<Subscript>2</Subscript>VO<Subscript>5.5</Subscript> system

To search for new oxygen conductive phases, a series of Bi₂V_{1 ? x} Mo_xO_{5.5 + x/2} ceramic samples have been synthesized in which the extreme compositions Bi₂VO_5.5 (x = 0) and Bi₂MoO₆ (x = 1) are single-layer Aurivillius phases having ferroelectric properties and high ionic oxide conductivity. It is established that single-layer Aurivillius phases exist in the composition ranges 0 ≤ x ≤ 0.1 and 0.9 ≤ x ≤ 1. Additional phases with BiVO₄ and Bi₆Mo₂O₁₅ structures are found in the range 0.2 ≤ x ≤ 0.8. The presence of molybdenum stabilizes the orthorhombic β-Bi₂VO_5.5 phase at room temperature. The conductivity of solid solutions based on Bi₂VO_5.5 differs only slightly from that of pure bismuth vanadate. Conductivity of solid solutions based on Bi₂MoO₆ is half an order of magnitude higher than that of pure bismuth molybdate.     

Compositional effects accompanying near equilibrium vapour growth of solid solution crystals of the types IV‐VI and II‐VI

Near equilibrium evaporation‐condensation in a sealed ampoule leads to almost full compositional reproduction of a solid solution if it consists of components having comparable vapour pressures; this can be qualitatively interpreted by domination of entropy increase. Nevertheless, even vestigial separation requires closer characteristics, since it may prove crucial – particularly for properties of semiconducting solid solutions. Maximum component separation allowed by a small temperature difference is described here in terms of thermodynamics and kinetics of solid‐vapour and vapour‐solid phase transitions. Theoretical models of the determining effects having different character are shortly described, and their applicability areas are determined. Experimental data collected for crystal growth of numerous semiconducting solid solutions of the II‐VI and IV‐VI type support the conclusion drawn from the models that the near equilibrium crystal growth from the vapour in a closed system ensures the highest degree of compositional uniformity. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Liquid phase separation in immiscible Ag-Ni-Nb alloy and formation of crystalline/amorphous composite

Phase decomposition in liquid miscibility gap presents a unique opportunity for designing amorphous/crystalline composites with a multilayer structure. One of typical monotectic systems, Ag-Ni alloy, is selected, and element Nb is added in the Ag-Ni alloy. The new ternary Ag-Ni-Nb monotectic alloys are rapidly quenched by using melt spinning technique. It is revealed that the ternary alloy undergoes a liquid-liquid phase decomposition and forms two immiscible Ag-rich and NiNb-rich liquids. Subsequently, the immiscible liquids solidify into Ag-rich crystalline and NiNb-based amorphous phases, respectively. Self-assembled amorphous/crystalline composites with double layer and sandwich type structures have been developed. The structure of the alloy ribbons with a dependence on the composition changes from a particle-dispersed structure to double layer structure and to sandwich type structure. The phase formation and thermal property of the quenched alloys are investigated. The formation mechanism of the crystalline/amorphous composite structures has been discussed.     

Nonstoichiometric fluorides—Solid electrolytes for electrochemical devices: A review

The solid electrolytes with fluorine-ion conductivity that were revealed during the analysis of the phase diagrams of the MF _m -RF _n systems within the program of search for new multicomponent fluoride crystalline materials carried out at the Shubnikov Institute of Crystallography, Russian Academy of Sciences, are described. The most widespread and promising materials are the nonstoichiometric phases with fluorite (CaF₂) and tysonite (LaF₃) structures, which are formed in the MF₂-RF₃ systems (M = Ca, Sr, Ba, Cd, or Pb; R = Sc, Y, or La-Lu). These phases have superionic fluorine conductivity due to the anion sublattice disorder. The ionic conductivity of crystals of both structure types has been studied and the limits of its change with composition and temperature are determined. Nonstoichiometric fluorides are used as solid electrolytes in chemical sensors, fluorine sources, and batteries. The prospects of the use of fluorine-ion conductors in solid-state electrochemical devices, principles of their operation, and the problems of optimization of their composition are discussed.     

Preparation and investigation of (CuInSe2)x(2ZnSe)1‐x and (CuInTe2)x(2ZnTe)1‐x solid solution crystals

The (CuInSe₂)_x(2ZnSe)_1‐x and (CuInTe₂)_x(2ZnTe)_1‐x solid solution crystals prepared by Bridgman method and chemical vapor transport have been studied. The nature of the crystalline phases, the local structure homogeneity and composition of these materials have been investigated by X‐ray diffraction (XRD) and Electron Probe Microanalysis (EPMA) methods. The analysis revealed the presence of chalcopyrite‐sphalerite phase transition between 0.6 ≤ X ≤ 0.7. Lattice constants, value of σ position parameter and bond length between atoms were also calculated. It was found that the lattice parameters exhibit a linear dependence versus composition. The transmission spectra of solid solution crystals in the region of the main absorption edge were studied. It was established that the optical band gap of these materials changes non‐linearly with the X composition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)