三元层状可加工导电MAX相陶瓷研究进展

三元层状可加工导电陶瓷是一类键合具有明显各向异性的层状碳化物或氮化物,它通常又被称为MAX相陶瓷。MAX相陶瓷具有优良的可加工性,良好的导电、导热能力,可观的高温强度,同时还具有良好的热稳定性、抗氧化性、抗热震性和耐腐蚀性能。本文介绍了MAX相陶瓷的结构、性能以及制备方法和应用前景。     

Synthesis of MAX phase-based ceramics from early transition metal hydride powders

MAX phase ceramics are typically prepared by the reactive sintering of elemental powders that are often coarse, expensive, and prone to oxidation. The temperature-driven dehydrogenation of metal hydride powders offers an alternative synthesis approach, as the hydrides decompose into phase-pure, dimensionally fine elemental powder particles. The increased reactivity of these in situ formed, fine powder particles drastically reduces the formation temperature of the antecedent intermetallic phases, without forming excess binary carbides or facilitating powder oxidation in the Ti-Al-C and Zr-Al-C systems. This work elucidates the effect of metal hydrides on the sequence of formation reactions in MAX phase ceramics. In the Zr-Al-C system, the use of coarse, oxidation-prone elemental Zr powders prevented MAX phase formation, whereas spark plasma sintering of ZrH₂ powders at 1500 °C produced ceramics containing 60 wt% Zr₃AlC₂. Similarly, in the Ti-Al-C system, spark plasma sintering of TiH₂ powders at 1200 °C produced phase-pure Ti₃AlC₂ ceramics.     

MAX phase Zr2SeC and its thermal conduction behavior

The elemental diversity is crucial to screen out ternary MAX phases with outstanding properties via tuning of bonding types and strength between constitutive atoms. As a matter of fact, the interactions between M and A atoms largely determine the physical and chemical properties of MAX phases. Herein, Se element was experimentally realized to occupy the A site of a MAX phase, Zr₂SeC, becoming a new member within this nanolaminated ternary carbide family. Comprehensive characterizations including Rietveld refinement of X-ray Diffraction and atom-resolved transmission electron microscopy techniques were employed to validate this novel MAX phase. The distinct thermal conduction behaviors emerged are attributed to the characteristic interactions between Zr and Se atoms.     

Thermal stability of metal-doped diamond-like carbon fabricated by dual plasma deposition

Metal incorporation into amorphous diamond-like carbon films can provide superior properties as metal nano-clusters or nanocrystalline metallic carbides can be embedded in the carbon network. In this work, a filtered metal plasma cathodic arc technique is used to generate a metal plasma and acetylene is introduced to the metal plasma plume to deposit metal-containing DLC (Me-DLC) films and form nanocrystalline carbide phases in the amorphous carbon matrix. The films exhibit high thermal stability up to annealing temperatures of 500 °C as revealed by X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. At treatment temperature over 500 °C, a large amount of hydrogen is lost from the Me-DLC films as shown by elastic recoil detection. Breakdown and structural collapse of the film at high temperature can be attributed to the breaking of C–H bonds. Consequently, the C–C networks become more graphite-like to facilitate the formation of volatile C–O and metal oxides phases.     

Characterization of Silicon Carbide Whiskers

SiC whiskers from six manufacturers were characterized by bulk chemical techniques, X-ray photoelectron spectroscopy, X-ray diffraction, and scanning transmission electron microscopy or scanning electron microscopy. Major component (C, Si, and O) surface chemistries of the whiskers fell into four general categories: high oxygen content with oxide resembling a SiO₂, high oxygen content with oxide resembling a Si-O-C glass, and hydrocarbon. Several whiskers exhibited significant surface impurities—in particular, Fe. From a morphological viewpoint, significant differences in diameter, debris level, straightness, and types and quantities of defects were observed from one manufacturer to another.     

Synthesis of Highly Crystalline Needle-Like Silicon Nanowires for Enhanced Field Emission Applications

Needle-like silicon nanowires (SiNWs) were successfully synthesized on gold-coated silicon substrates using a very high frequency plasma enhanced chemical vapor deposition method (VHF-PECVD). The prepared samples were characterized by field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy (EDX), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and photoluminescence (PL). XRD analysis confirmed formation of single crystalline SiNWs along (111) crystalline planes and microscopic studies revealed formation of NWs with diameters ranging from 10 to 100 nm and lengths of a few micrometers. Furthermore, the presence of gold nanoparticles on the tip of the NWs verified the vapor–liquid–solid growth mechanism of SiNWs. It was also demonstrated that SiNWs are composed of well-crystallized silicon cores and an amorphous shell. The obtained results verified potential application of such structures in field emission displays.     

Hybrid A–B–A type nanowires through cation exchange

Hybrid A–B–A type nanowires (NWs) with Ag5Te3–HgTe–Ag5Te3 composition have been created by the reaction of Hg2+ with Ag2Te NWs. The NW morphology of Ag2Te is preserved upon reaction with minor changes and the two separate phases formed are spatially separated within the same NW. The reaction of Hg2+ with Ag2Te NWs was monitored at different concentrations and the reactivity was attributed to cationic exchange depending on solubility products. Hybrid NWs were formed by partial cation exchange only at low concentrations (below 50 ppm) resulting in Ag5Te3 and HgTe within the same NW. However, at high concentrations (above 100 ppm), the HgTe phase alone was formed. These studies have been extended to other metal ions such as Pb2+, Cd2+, and Zn2+ whose reactivity towards Ag2Te NWs is different from that of Hg2+. These ions form a passivating Te oxide layer upon reaction with other metal ions. The mechanism of reactivity of Hg2+ is explained on the basis of free energy of formation of the ionic solid. Phase transition of Hg2+-reacted NWs occurs at a lower temperature than the parent (Ag2Te NWs) and other metal ions-reacted Ag2Te NWs. Details of the process were elucidated using microscopic and spectroscopic investigations. The physical and chemical properties of the individual components within a NW are expected to provide a novel functionality to the metal chalcogenide systems.     

利用菱镁矿制备氧化镁晶须

从镁盐产品的开发和解决镁资源综合利用的角度出发，以菱镁矿制备的氧化镁浆液为主要原料，加入沉淀剂碳酸钠，在室温下得到氧化镁晶须的前驱物（碳酸镁晶须）。通过控制碳酸镁的分解条件保持晶须形状不被破坏，在煅烧情况下转变为氧化镁晶须。采用X射线衍射仪和扫描电子显微镜观察氧化镁晶须的结构和形貌，结果表明所制备的氧化镁晶须表面光滑，直径分布均匀，结晶良好。     

Synthesis and characterization of monetite and hydroxyapatite whiskers obtained by a hydrothermal method

High temperature hydrothermal syntheses, using calcium nitrate tetrahydrate, sodium dihydrogen phosphate and urea as precursors, and characterization of hydroxyapatite (HAp) whiskers are reported herein. The morphology and chemical composition of the crystals from a monetite to a hydroxyapatite phase were controlled by varying the starting concentrations of the precursors and the solution pH through the amount of urea that is decomposed during heating. X-ray diffraction (XRD) analysis, infrared spectroscopy (IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were used to investigate the products of the syntheses in order to find the optimum reaction conditions for obtaining the desired morphology and phase composition. Different morphologies ranging from single crystals of monetite through rods and plates of hydroxyapatite with different size distribution to whisker-like single hydroxyapatite crystal were achieved by simply varying the starting concentration of urea. Structural refinement of the hydroxyapatite whiskers confirmed a strong preferential orientation along the c-axis direction of the hexagonal crystal structure, which was significantly different from the usually observed random crystal orientation. TEM and SEM analysis of the apatite whiskers confirmed single crystal structure with the a c-axis orientation parallel to the long axis of the whiskers, with sizes up to 150 μm in length, 10 μm in width and with a thickness of about 300 nm, that grew from the same centre of nucleation, forming flaky-like particles.     

Phase and microstructural evolution based on Al,Si and TiO2 reactions with a MgO-C resin-bonded refractory

Adding carbon to refractory products (i.e. MgO-C) usually results in unique properties that allow these materials to attain the performance level required for the steel-making process. Nevertheless, C oxidation is still a big concern and using antioxidant additives has become an important alternative to prevent carbon loss and induce the generation of further compounds (carbides, nitrides, etc.) in the fired microstructure, which will affect the overall thermo-mechanical behavior of the refractories. Aiming to better understand the main phase transformations derived from the interaction of polymeric binders with antioxidants and other refractory components in the MgO-C system, this work addresses the evaluation of mixtures containing novolak resin + antioxidants (Al, Si, and/or TiO₂) and: (i) fine MgO, (ii) ferrocene (catalyst for carbon graphitization), and (iii) coal-tar-based binder. X-ray diffraction, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were carried out in order to identify the main phases comprising the fired samples (kept at 1000 °C and 1400 °C for 5 h in a reducing environment) and observe their morphology and distribution in the resultant microstructure. Based on the attained results, a great variety of whiskers with different shapes, dimensions and distributions were also observed on the fractured surface of the prepared samples containing antioxidants. Furthermore, not only the carbon graphitization, but also the presence of MgO and other binders (Carbores® P) affected the phase evolution at a high temperature, which highlights the complexity of phase transformations and the many likely interactions derived from the combination of various raw materials.     

Preparation and growth mechanism of the flower-like whiskers of γ-, θ-, and α-Al2O3 phases by homogeneous precipitation/calcination method

The different alumina whiskers phases are extensively used to reinforce high-performance composite materials. The understanding of the mutual relationship of the different phases is key for simulating Al₂O₃ whiskers containing systems and is of general interest for the metal oxide and ceramic communities. In this paper, we used homogeneous precipitation to synthesized flower-like boehmite whisker precursors from aluminum sulfate and urea. The synthesized precursors were calcined at 600, 900, and 1200 °C to obtain flower-like whiskers of γ-, θ-, and α-Al₂O₃ phases, respectively. The precursor boehmite crystals prepared in closed, semi-closed, and open reactor systems were qualitatively analyzed using SEM. The results showed that the usea hydrolysis, which serves as a source of OH^? ions, is slow under closed-reactor conditions, which favors the growth of the boehmite crystal nuclei into whiskers. We adopted the L₁₆ (4³) orthogonal design of experiment to analyze the influence of reaction temperature, time, and the amount of urea (relative to the amount of Al₂(SO₄)₃) in the closed system. Synthesis temperature of 160 °C and the processing time of 8 h yielded optimal boehmite flower-like whiskers. Physicochemical properties of the precursor and series of alumina phases were characterized using thermogravimetric/differential thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) methods. Finally, the obtained results and the thorough analysis of the growth mechanism suggested the step-growth model of boehmite flower-like whiskers.     

Isolation and characterization of cellulose nanowhiskers from Acacia caesia plant

Acacia caesia (L.) Willd (soap bark) fiber is an abundant natural resource, that is rich in cellulose. The study reports the effective utilization of underutilized Acacia caesia fiber for the isolation of nanocellulose whiskers. The nanocellulose whiskers were isolated successfully from Acacia caesia fibers by following alkali, bleaching, and sulfuric acid treatment. The obtained nanocellulose whiskers were carefully investigated for its chemical composition, structure, morphology, crystallinity, and thermal stability. The chemical composition and Fourier transform infrared spectra of nanocellulose whiskers showed the elimination of the non-cellulosic parts present in the raw Acacia caesia fibers. The X-ray diffraction analysis showed an upsurge in the crystallinity of the cellulose fibers after the chemical treatments. The isolation of nanocellulose whiskers from Acacia caesia raw fiber was confirmed by electron microscopy analysis. The thermogravimetric analysis showed remarkably high char residue for the nanocellulose whiskers compared to raw fibers. Based on the properties of nanocellulose whiskers, it can be concluded that the nanocellulose whiskers produced from Acacia caesia raw fibers are potential reinforcing material for developing high-performance green composites.     

Dense tungsten carbide bulks by self-densification reaction sintering of nanodiamond with tungsten at 1700 °C

Tungsten carbide (WC) bulks from the sintering of WC powders possess excellent physical and chemical properties, widely used in high-precision optical molds, seals, nozzles and many other scenarios. To produce dense WC bulks, however, needs a very high sintering temperature (>1900 °C) and super fine WC powders due to the poor sinterability. The high sintering temperature required puts a huge strain on processing equipment, and the preparation of super fine WC powders involves many complex steps. In this work, dense WC bulks are prepared by spark plasmas sintering via self-densification reaction of nanodiamond with tungsten at 1700 °C. The use of nanodiamond enables dense WC bulks of a density of 98.1% and Vickers hardness of 21.8 GPa for a volume expansion associated with the change of diamond to graphite greatly reduces the porosity of as-prepared WC bulks, demonstrating a novel mechanism of self-densification reaction sintering. Besides, reaction between tungsten and diamond produces far less volume shrinkage than other allotropes of carbon, partly contributing to the densification of the as-prepared WC bulks. Our creative strategy has significant advantages over prevailing methods in preparing high-density WC bulks, facilitating the application of WC bulk materials in industrial production and providing a new insight into the preparation of dense bulks of other metal carbides.     

Extension of MAX phases from ternary carbides and nitrides (X = C and N) to ternary borides (X = B,C, and N): A general guideline

Although significant progress has been made on understanding the structure–property relationship of MAX phases, a clear answer has not been given to the outstanding question whether MAX phases can be extended from carbides and nitrides (X = C and N) to borides (X = B). Herein, based on the systematically investigations on the general trend of lattice constants and elastic properties of the experimentally found MX and M₂AX (X = B, C, and N) with the valence electron concentration (VEC), a guideline for the discovery of new MAX borides is given. The MX and M₂AX (X = B, C, and N) compounds are situated in a small range of VEC, and their lattice constants generally decrease with VEC. Second-order elastic constants c₁₁, c₃₃, and bulk modulus B of M₂AX (X = B, C, and N) increase with VEC due to enhanced bonding, c₄₄ and G, however, increase up to some critical values and then decline with the further increase of VEC. Based on the electronic structure–elastic property relationship and the presence of rock salt–structured transition metal monoborides, nine possible MAX phase borides M₂AB (M = Zr, Hf, Nb, A = P, As, and Sb) are predicted. Thus, MAX phases can be extended from X = C and N to X = B, C, and N.     

Structural and photoluminescence studies on catalytic growth of silicon/zinc oxide heterostructure nanowires

Silicon/zinc oxide (Si/ZnO) core-shell nanowires (NWs) were prepared on a p-type Si(111) substrate using a two-step growth process. First, indium seed-coated Si NWs (In/Si NWs) were synthesized using a plasma-assisted hot-wire chemical vapor deposition technique. This was then followed by the growth of a ZnO nanostructure shell layer using a vapor transport and condensation method. By varying the ZnO growth time from 0.5 to 2 h, different morphologies of ZnO nanostructures, such as ZnO nanoparticles, ZnO shell layer, and ZnO nanorods were grown on the In/Si NWs. The In seeds were believed to act as centers to attract the ZnO molecule vapors, further inducing the lateral growth of ZnO nanorods from the Si/ZnO core-shell NWs via a vapor-liquid-solid mechanism. The ZnO nanorods had a tendency to grow in the direction of [0001] as indicated by X-ray diffraction and high resolution transmission electron microscopy analyses. We showed that the Si/ZnO core-shell NWs exhibit a broad visible emission ranging from 400 to 750 nm due to the combination of emissions from oxygen vacancies in ZnO and In₂O₃ structures and nanocrystallite Si on the Si NWs. The hierarchical growth of straight ZnO nanorods on the core-shell NWs eventually reduced the defect (green) emission and enhanced the near band edge (ultraviolet) emission of the ZnO.     

Process development for the laser powder bed fusion of WC-Ni Cermets using sintered-agglomerated powder

Although ceramic particle-metal matrix materials (i.e., cermets) can offer superior performance, manufacturing these materials via conventional means is difficult compared to the manufacturing of metal alloys. This study leverages the laser powder bed fusion (LPBF) process to additively manufacture dense tungsten carbide (WC)-17 wt.% nickel (Ni) composite specimens using novel spherical, sintered-agglomerated composite powder. A range of processing parameters yielding high-density specimens was discovered using a sequential series of experiments comprised of single bead, multi-layer, and cylindrical builds. Cylinders with a relative density >99% were fabricated and characterized in terms of microstructure, chemical composition, and hardness. Scanning electron microscopy images show favorable wetting between the Ni binder and carbide particles without any phase segregation and laser processing increased the average carbide particle size. Energy dispersive X-ray and X-ray diffraction analyses detected traces of secondary products after laser processing. For samples processed at high energy densities, complex carbides and carbon agglomerate phases were detected. The maximum hardness of 60.38 Rockwell C is achieved in the printed samples. The successful builds in this study open the way for LPBF of dense WC-Ni parts with a large workable laser power-laser velocity processing window.     

莫来石晶须的制备

采用化学法制备Ａｌ２Ｏ３－ＳｉＯ２粉料,在氟化物气相存在条件下经过高温合成制得均匀且长径比大的莫来石晶须。研究结果表明,莫来石晶须形成与生长机理为气－固反应机制;铝硅氟化物气相、Ａｌ２Ｏ３－ＳｉＯ２粉料细度、烧成温度及保温时间是制备莫来石晶须的重要工艺条件。     

Review of phase stability in the group IVB and VB transition‐metal carbides

This review summarizes the phase stability in the group IVB (Ti‐C; Zr‐C; Hf‐C) and group VB (V‐C; Nb‐C; Ta‐C) transition‐metal carbides. The order parameter functional (OPF) method and density functional theory (DFT) method have been used to predict phase equilibria in these systems. Extensive experimental investigations have attempted to determine both phase stability as a function of composition as well the crystal structures present using X‐ray diffraction, neutron diffraction, electron backscatter detection, and selected area electron diffraction. These investigations have demonstrated that the structures that form are based on the close‐packing of the metal atoms and the arrangement of the carbon atoms in the octahedral interstices. In general, the rocksalt B1 phase is stable for all of the transition‐metal carbides, with their substoichiometry tolerance increasing with temperature; vanadium carbide is the exception due to its negative vacancy formation energy. Vacancy‐ordered M₆C₅ phases have been predicted and experimentally confirmed in both groups of carbides; however, kinetic limitations often inhibit the formation of vacancy‐ordered phases, which has contributed to controversy in phase identification. The vacancy‐ordered M₄C₃ phase has been predicted for select carbides and has only been observed in zirconium carbide. In contrast, the stacking fault phase ζ‐M₄C_3?x has been readily reported in the group VB carbides (but not in the group IVB carbides). The vacancy‐ordered M₃C₂ has been predicted by DFT for the group IVB carbides but not in the group VB carbides, whereas OPF predicts its stability in both carbides. Vacancy‐ordered M₃C₂ phases have been experimentally observed in the Ti‐C and Hf‐C systems. Finally, the M₂C phase has been predicted in both group carbides, except for hafnium carbide, with an order‐disorder transition with temperature. These factors result in phase diagrams that are similar among all the carbides, but each phase diagram is unique due to subtle differences in bonding that result in slight differences in thermodynamically stable phases.     

Distinguishing nanowire and nanotube formation by the deposition current transients

ABSTRACT: High aspect ratio Ni nanowires (NWs) and nanotubes (NTs) were electrodeposited inside ordered arrays of self-assembled pores (approximately 50 nm in diameter and approximately 50 um in length) in anodic alumina templates by a potentiostatic method. The current transients monitored during each process allowed us to distinguish between NW and NT formation. The depositions were long enough for the deposited metal to reach the top of the template and form a continuous Ni film. The overfilling process was found to occur in two steps when depositing NWs and in a single step in the case of NTs. A comparative study of the morphological, structural, and magnetic properties of the Ni NWs and NTs was performed using scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometry, respectively.     

Conversion of MAX phase single crystals in highly porous carbides by high temperature chlorination

We show that combining high temperature chlorination with the chemical sensitivity of the A-atom planes of single crystals of some nano-lamellar MAX phases allows one to synthesize highly porous and electrically conducting carbides. Focusing on the case of porous Cr₃C₂, we present the dependence of the layer morphology, structure and formation kinetics on processing parameters such as temperature and time. The determination of the Raman signature of Cr₃C₂ and Cr₇C₃ allows us to follow the creation and elimination of the various phases as a function of processing time. Electrical resistivity and magnetoresistance data versus temperature and magnetic field are also presented and analyzed.