三元层状碳氮化合物(MAX相)及其衍生二维纳米材料(MXene)研究趋势与展望

近年来,三元层状碳氮化合物(MAX相)及其衍生二维纳米材料MXene受到了科学界的广泛关注。MAX相的晶体结构由M_n+1X_n结构单元与A元素单原子面交替堆垛排列而成,兼具金属和陶瓷的诸多优点,在高温结构材料、摩擦磨损器件、核能结构材料等领域有较大的应用潜力。MAX相的A层原子被刻蚀之后获得成分为M_n+1X_nT_x(T_x为表面基团)的二维纳米材料,即MXene,具有丰富的成分组合以及可调谐的物理化学性质,在储能器件、电磁屏蔽、电子器件等领域表现出良好的应用前景。本文简要介绍近年来国内外MAX相和MXene材料领域在成分与结构、合成方法、性能与应用研究等方面的研究动态,据此展望未来几年该类新颖材料的发展方向。     

熔盐法合成纳米层状Sc2SnC MAX相

MAX相是一类兼具金属和陶瓷特性的三元层状材料,也是合成二维MXenes的前驱体材料.理论预测稳定的三元层状MAX相材料约有600余种,目前实验合成的三元层状MAX相材料已有80余种,但M位主要为前过渡族金属,而对M为稀土元素的三元MAX相鲜有报道.本研究以Sc、Sn,和C元素粉为原料,通过熔盐法合成了M位为稀土元素S...     

三元层状材料结构调控及性能研究进展

MAX/MAB相是一类非范德华三元层状材料,具有丰富的元素组成和晶体结构,兼具陶瓷和金属的物理性质,在高温、强腐蚀、辐照等极端环境中极具应用潜力。近年来,由MAX/MAB相衍生的二维(2D)材料(MXene和MBene)在材料物理与材料化学领域引起了广泛兴趣,已经成为继石墨烯和过渡金属硫族化合物之后最受关注的二维范德华材料。MAX/MAB相材料结构调控不仅对这类非范德华层状材料本征性能产生重要影响,而且对其衍生的二维范德华材料结构功能特性研究也具有重要价值。本文归纳和总结了MAX/MAB相层状材料在结构调控、理论计算和应用基础研究等方向的最新科研进展,并展望了该类层状材料未来发展方向。     

CFRP-钢层状复合结构的表面划痕损伤容限

碳纤维增强树脂复合材料(CFRP)-钢层状结构在实际运营过程中,脆性碳纤维层容易出现划痕等表面损伤,因此为了保障损伤后复合结构的安全运行,需要对其进行损伤容限研究。基于边界效应理论模型(Boundary effect model,BEM),建立了表面划痕损伤后的CFRP-钢层状结构三点弯曲断裂强度解析模型,并在CFRP表面分别预制了0.2 mm和0.4 mm深度的表面初始划痕缺陷,通过三点弯曲梁的成组试验验证了理论模型的可行性。研究结果表明：(1)利用金相显微镜观测了CFRP-钢层状结构三点弯曲极限荷载时的断裂特征,确定了表面划痕损伤后CFRP的结构特征参数C_ch,代入解析模型获得了CFRP层的拉伸强度,并与CFRP直接拉伸试验测试的拉伸强度对比,两者偏差小于10%;(2)该解析模型为“断裂荷载=拉伸强度×等效面积”的线性方程形式,“等效面积”仅与CFRP-钢层状结构和表面裂纹的几何参数有关,因此,通过CFRP的直接拉伸强度可以预测表面损伤后CFRP-钢层状结构的断裂强度,实现损伤容限设计。     

MAX相陶瓷增强金属基复合材料:制备、性能与仿生设计

由于原子间存在共价键、金属键与离子键的混合键合状态,MAX相陶瓷兼具金属和陶瓷材料的性能特点,并且常与金属之间表现出良好的润湿性,有助于形成强界面结合,独特的层状原子结构使MAX相陶瓷表现出良好的断裂韧性、阻尼与自润滑性能。因此,作为金属基复合材料的增强相, MAX相陶瓷具有显著优势,本文着重介绍相关研究进展。目前,MAX相陶瓷增强金属基复合材料主要通过搅拌铸造、粉末冶金和熔体浸渗等途径制备,得到的复合材料表现出优于金属基体的强度、硬度与模量,同时还具备良好的耐磨、导电、抗电弧侵蚀等性能。此外,借助真空抽滤、冰模板等工艺可实现超细片状MAX相陶瓷粉体的择优定向排列,然后利用金属熔体浸渗多孔陶瓷骨架,可获得具有类贝壳结构的MAX相陶瓷增强金属基仿生复合材料,进一步提升材料的强韧性能。MAX相陶瓷增强金属基复合材料在承载、电接触等应用领域具有显著优势和广阔前景。     

三元层状陶瓷Ti3SiC2的高温氧化行为

研究了以自蔓延准热等静压和热处理工艺制备出的高纯、致密的Ti3SiC2块体材料的高温氧化性能.采用XRD,SEM和EMPA分析了氧化膜表面形貌、组织结构和成分.研究结果表明,在1000℃的空气中,Ti3SiC2陶瓷材料具有优异的抗氧化性能,且恒温氧化行为与循环氧化行为相似.其在120h内的氧化动力学曲线趋近于直线-抛物线规律.随着氧化时间的延长,氧化膜由TiO2和SiO2混合物组成的单层结构逐渐过渡到双层结构.双层结构的内层为固溶了SiO2的TiO2层,而外层为纯金红石相.     

纳米Fe_2O_3从FCC相到HCP相的转变过程及相变温度宽化

藉助于实验中所得纳米γ－Ｆｅ２Ｏ３及其典型热处理样品的Ｘ射线衍射（ＸＲＤ）谱和有关数据，我们研究了纳米γ－Ｆｅ２Ｏ３结构相变的信息，揭示了这一相变的具体过程，对相变温度的宽化现象提出了解释     

TEM investigations on layered ternary ceramics

Layered ternary ceramics represent a new class of solids that combine the merits of both metals and ceramics.These unique properties are strongly related to their layered crystal structures and microstructures. The combination of atomic-resolution Z-contrast scanning transmission electron microscopy （STEM） and transmission electron microscopy （TEM）, selected area electron diffraction （SAED）, convergent beam electron diffraction （CBED） represents a powerful method to link microstructures of materials to macroscopic properties, allowing layered ternary ceramics to be investigated in an unprecedented detail. Vicrostructural information obtained using TEM is useful in understanding the formation mechanism, layered stacking characteristics, and defect structures for layered ternary ceramics down to atomic-scale level; and thus provides insight into understanding the ＂Processing-Structure-Property＂ relationship of layered ternary ceramics. Transmission electron microscopic characterizations of layered ternary ceramics in Ti-Si-C, Ti-Al-C, Cr-Al-C, Zr-Al-C, Ta-Al-C and Ti-Al-N systems are reviewed.     

New ternary nitride ceramics: CaSiN2

The synthesis of CaSiN₂ powder, starting from metal nitrides, is described. Fully dense CaSiN₂ ceramics can be sintered from the prepared powder at 1700 °C. The samples were sintered in a closed Mo vessel to prevent evaporation of calcium nitride. The phase composition, and the chemical and mechanical properties of the as-prepared ceramics are described. The thermal conductivity at room temperature was evaluated as 2.4 Wm^–1 K^–1. A reasonable strength of 179 MPa and a fairly good fracture toughness of about 2.1 MPam^1/2 were found. A hardness of 9.6 GPa and a Young's modulus of 174 GPa were measured. The value of the relative dielectric constant was measured as 13.3. The bandgap for CaSiN₂ at room temperature was estimated from diffuse-reflectance spectra to be 4.5 eV. Considerable improvement in the properties are expected when the processing conditions which is well within reach are optimized.     

Self-lubricate and anisotropic wear behavior of AZ91D magnesium alloy reinforced with ternary Ti2AlC MAX phases

The dry sliding wear behavior of Ti₂AlC reinforced AZ91 magnesium composites was investigated at sliding velocity of 0.5 m/s under loads of 10, 20, 40 and 80 N using pin-on-disk configuration against a Cr15 steel disc. Wear rates and friction coefficients were registered during wear tests. Worn tracks and wear debris were examined by scanning electron microscopy, energy dispersive X-ray spectrometry and transmission electron microscopy in order to obtain the wear mechanisms of the studied materials. The main mechanisms were characterized as the magnesium matrix oxidation and self-lubrication of Ti₂AlC MAX phase. In all conditions, the composites exhibit superior wear resistance and self-lubricated ability than the AZ91 Mg alloy. In addition, the anisotropic mechanisms in tribological properties of textured Ti₂AlC-Mg composites were confirmed and discussed.     

Synthesis of nanolayered ternary borides powders (MAB phases) by sustainable molten salt shielded synthesis/sintering (MS3) process

Journal of Materials Science - In this paper, we report the synthesis of Fe2AlB2, Mn2AlB2, and MoAlB powders by using molten salt shielded synthesis/sintering (MS3)—a sustainable processing...     

Electric response to pulse thermal impact from layered magnetoelectric composites of PZT-NiZn-ferrite ceramics

Electric responses of magnetoelectric composite structures with 2–2 coherence to thermal impact are presented. The detected electric response consists of two components having different dependences on the modulation frequency of heat flow. It is shown that in the piezoelectric ceramics PZT-NiZn-ferrite two-layered composite system electric signal forms as a result of the pyroelectric effect and mechanical interaction of ferrite and PZT plate that have expanded from heating.     

Thermodynamic properties of ternary phases in the Cu-Tl-Se system

The Cu-Tl-Se system has been studied at temperatures from 300 to 420 K using emf measurements with Cu₄RbCl₃I₂ as a Cu+ ion conducting solid electrolyte. The emf data have been used to map out the subsolidus phase diagram of the Cu-Tl-Se system in the composition region Tl₂Se-CuTlSe-CuSe-Se. We have calculated the partial molar thermodynamic functions of the copper in the alloys and the standard thermodynamic functions of formation and standard entropies of the ternary compounds CuTlSe₂, CuTlSe, and Cu₂TlSe₂. The results confirm that the thermodynamic properties of copper-containing ternary systems can be studied using the approach in question even when they contain an element (thallium in this study) located to the left of copper in the electrochemical series.     

Mechanical properties of materials based on MAX phases of the Ti-Al-C system

By studies of materials based on the Ti₃AlC₂ MAX phase containing inclusions of titanium carbide it has been shown that as a titanium carbide content increases from 2 to 99%, the nanohardness and Young modulus of the material increase from 2.0 ± 0.4 to 23.6 ± 1.2 GPa and from 137 ± 21 to 447 ± 11 GPa, respectively. The exponent in the equation of creep for these samples has been found to vary from 104 to 140, which indicates that mechanical properties of the material and, hence, of the Ti₃AlC₂ MAX phase depend on the strain rate only slightly. The formation of broad hysteresis loops has been observed in the cyclic loading/unloading of the indenter for samples consisting mainly of the Ti₃AlC₂ MAX phase. This points to serious losses in elastic energy of the MAX phase in strain cycling and, hence, the prospects of the MAX phase application as a damping material. It has been found that the microhardness of a sample consisting of 98% Ti₃AlC₂ produced by sintering under a load of 4.9 N was 2.1 GPa and its fracture toughness was high (no cracks from the indent corners were observed even under a load of 149 N). Microhardness and fracture toughness of the material consisting of 71% Ti₃AlC, 6% Ti₂AlC, and 23% TiC were 3.0 ± 0.6 GPa and 4.3 ± 1.4 MPa·m^1/2, respectively.     

Initial stages of the Pd-GaAs reaction: Formation and decomposition of ternary phases

Investigations of the metallurgical reactions occuring at metal-compound semiconductor interfaces are essential for the understanding of ohmic contact and Schottky barrier formation mechanisms. The resulting film and interface morphologies are also important for the performance of discrete devices and integrated circuits.In this study the reactions occurring during thermal annealing of the Pd-GaAs system are investigated. Reported here are the initial stages of the Pd-GaAs reactions as studied by high resolution transmission electron microscopy, electron diffraction and energy-dispersive analysis of X-rays. The first Pd-GaAs reaction product, observed in the low temperature range (less than about 315°C), is the hexagonal ternary phase Pd₅(GaAs)₂ (phase I with a₀ = 0.673 nm and c₀ = 0.338 nm). The second stage of the reaction leads to the formation of a second hexagonal ternary phase, Pd₄GaAs (phase II with a₀ = 0.92 nm and c₀ = 0.370 nm), and the binary phases Pd₂Ga and Pd₂As. The concurrent application of energy-dispersive X-ray analysis and high resolution cross-sectional transmission electron microscopy leads to the unambiguous identification of the ternary Pd₅(GaAs)₂ and Pd₄GaAs phases. X-ray diffraction data alone could erroneously lead to the identification of Pd₅(GaAs)₂ as PdGa and of Pd₄GaAs as PdAs₂. In view of this source of ambiguity in previous work, the sequence of reactions in the Pd-GaAs system is re-evaluated and the evolution of the film and interface morphologies is discussed.     

Investigation on crystalline phases and mechanical properties of TZP ceramics prepared from sol-gel powders

Y₂O₃-stabilized tetragonal zirconia polycrystalline (TZP) ceramics containing 1–5 mol% Y₂O₃ were prepared by hot pressing and pressureless sintering of sol-gel-derived powders. Sintered ceramics were evaluated for their density, grain and crystallite size, width of transformation zone, crystalline phases and mechanical properties. Variation in the values of fracture toughness and flexural strength has been explained on the basis of crystallite size and proportion of transformable tetragonal phase, which are influenced by the concentration of Y₂O₃ in TZP ceramics. Correlation of the data has indicated that the transformable tetragonal phase is the key factor in controlling the fracture toughness and strength of ceramics.