Novel flower-like hyperbranched ZnTe nanostructures prepared via catalyst-assisted vacuum thermal evaporation

Novel flower-like hyperbranched ZnTe nanostructures were prepared by catalyst-assisted vacuum thermal evaporation method. Various analysis techniques including X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electronic diffraction (SAED) were conducted on the as-prepared products. The XRD analysis demonstrates the ZnTe nanostructures are high pure single crystalline of zinc-blende structure. These novel ZnTe nanostructures are grown by a combination of the vapor–liquid–solid (VLS) growth mechanism and screw-dislocation-driven mechanism. The nanostructures formed by VLS mechanism have smaller sizes of several to ten micrometers and secondary branches with diameters of 100 nm. The nanostructures combined VLS and screw-dislocation-driven growth mechanism have a diagonal size of 40 µm and they consist of quartic branches, among which the secondary branches are spiral. The stems of the nanostructures with no-spiral secondary branches and their branches of the two kinds of nanostructures are all capped by a spherical catalyst particle, which is an indication of VLS growth mechanism. These as-prepared ZnTe nanostructures perhaps have potential applications in optoelectronics due to their unique geometric configurations.     

Facile Synthesis and Tensile Behavior of TiO<Subscript>2</Subscript> One-Dimensional Nanostructures

High-yield synthesis of TiO₂ one-dimensional (1D) nanostructures was realized by a simple annealing of Ni-coated Ti grids in an argon atmosphere at 950 °C and 760 torr. The as-synthesized 1D nanostructures were single crystalline rutile TiO₂ with the preferred growth direction close to [210]. The growth of these nanostructures was enhanced by using catalytic materials, higher reaction temperature, and longer reaction time. Nanoscale tensile testing performed on individual 1D nanostructures showed that the nanostructures appeared to fracture in a brittle manner. The measured Young’s modulus and fracture strength are ~56.3 and 1.4 GPa, respectively.     

Unusually-high growth rate (∼2.8 μm/s) of germania nanowires and its hierarchical structures by an in-situ continuous precursor supply

Low-dimensional nanostructured semiconductors are becoming the promising materials for high-performance nanophotonics, nanoelectronics, and quantum devices. To enable these applications, it requires an efficient methodology to control the dimension of these materials during synthesis processes, and to achieve mass production of these materials with high reproducibility, perfect crystallinity, and low production cost. In this study, an ultra-fast, facile synthesis strategy is presented for reproducible monocrystalline hexagonal germania (GeO₂) nanowires (NWs) and hierarchical structures. These GeO₂ nanostructures were grown by one-step-annealing of the Ni film-covered GeSn epilayers in a rapid thermal annealing (RTA) system without any gaseous or liquid Ge sources. It was found that after short annealing for 60 s at 675 °C, the long GeO₂ NWs of more than 170 μm are obtained, indicating that the growth rate is several magnitude orders higher than that of the common chemical vapor deposition (CVD) methods. The mechanism of the growth was studied by changing the growth temperature, catalyst type, and surface oxidation. The results indicate that this record-fast growth (>2.8 μm/s) of NW is due to the continuously generated in-situ GeO vapors from the Ni-catalyst decomposition of supersaturated GeSn epilayer. This work presents a rapid, and low-cost method to synthesis high-density GeO₂ NW and its hierarchical structures which have the potential applications for optoelectronic communication/detection, superhydrophobic surfaces, photocatalyst, and sensing.     

Boro/carbothermal reduction co-synthesis of dual-phase high-entropy boride-carbide ceramics

Dense, dual-phase (Cr,Hf,Nb,Ta,Ti,Zr)B₂-(Cr,Hf,Nb,Ta,Ti,Zr)C ceramics were synthesized by boro/carbothermal reduction of oxides and densified by spark plasma sintering. The high-entropy carbide content was about 14.5 wt%. Grain growth was suppressed by the pinning effect of the two-phase ceramic, which resulted in average grain sizes of 2.7 ± 1.3 µm for the high-entropy boride phase and 1.6 ± 0.7 µm for the high-entropy carbide phase. Vickers hardness values increased from 25.2 ± 1.1 GPa for an indentation load of 9.81 N to 38.9 ± 2.5 GPa for an indentation load of 0.49 N due to the indentation size effect. Boro/carbothermal reduction is a facile process for the synthesis and densification of dual-phase high entropy boride-carbide ceramics with both different combinations of transition metals and different proportions of boride and carbide phases.     

Improvement of flexural bond strength of zirconia-resin cement by surface patterning using sub-nanosecond UV laser

Zirconia is a dental material that shows excellent biocompatibility and high strength in clinical applications. This study aims to evaluate the effects of ultrafast laser applications. The surface nanostructures were classified into three groups. Group 1 was generated using the burst mode, with three different distances between dots: 52 µm (Group 1a), 104 µm (Group 1b), and 156 µm (Group 1c). Group 2 was processed using the scanning mode configuration, with a set of parallel lines. Group 3 was also processed using this scanning configuration creating a set of square-shaped patterning. Group 4 was the control group. After the surface treatments, a pair of zirconia specimens was bonded end to end with resin cement. Flexural bond strength (FBS) test was applied in a universal test machine. Multiple comparisons were performed using a one-way analysis of variance and the Tukey's HSD test. All the samples that were treated with the laser showed higher FBS values than the untreated surface. Using the burst mode, preformed circular-shaped surface on an angle of 90⁰ at 52 µm distance (Group 1a) showed the highest FBS values among all groups (p < .05). Groups 2 and 3 had significantly higher values than 1b and 1c.     

Natural citric acid (lemon juice) assisted synthesis of ZnO nanostructures: Evaluation of phase composition,morphology, optical and thermal properties

In recent years, because of their excellent electrocatalytic action and applications in different fields, metal oxide nanostructures have received massive consideration from scientists. Zinc oxide nanostructures are useful materials for a range of sensing applications and possess admirable electrocatalytic properties and stability. The current research presents the natural citric acid assisted synthesis of ZnO nanostructures and their structural, optical, morphological and thermal properties. X-ray diffraction was studied for the phase assessment of as prepared (Z1) and annealed ZnO (Z2) nanostructures and the crystallite sizes of the Z1 and Z2 samples were also located in the range between 35 nm and 38 nm. FESEM and TEM experiments were carried out to explore the surface features of Z1 and Z2 samples. The polycrystalline existence of the samples is demonstrated by the hexagonal, cubic and spherical shaped ZnO nanostructures. The energy band gap of Z1 and Z2 samples was determined (3.16 eV for Z1 and 3.12 eV for Z2) from the UV spectrum. The impact of annealing treatment on the thermal stability of ZnO nanostructures was studied and the main peak was observed for the Z1 sample at ~249 °C and for the Z2 sample at ~289 °C.     

Coarse-grained β-Si3N4 powders prepared by combustion synthesis

Coarse-grained β-Si₃ N₄ powders were prepared by combustion synthesis under N₂ pressure of 6 MPa, with a low diluent content of not more than 10 wt.% and high reaction temperature of >1900°C. β-Si₃ N₄ was obtained as the major phase in the products, except for a small amount of residual Si. The addition of carbon black was effective to reduce the residual Si, but resulted in the formation of β-SiC when too much carbon black was used. The coarse-grained β-Si₃ N₄ powders consisted of β-Si₃ N₄ crystals with an average thickness of more than 10 µm, and some crystals were thicker than 20 µm. The growth mechanism of the coarse β-Si₃ N₄ crystals was discussed, associated with the particular reaction conditions in combustion synthesis.     

Controllable preparation of boron nitride microspheres and their epoxy-based thermoconductive composites

How to prepare spherical boron nitride (BN) particle with different size is an extremely challenging work. In this paper, the controllable preparation of spherical BN particle from nanospheres to microsphere was realized by changing the synthesis temperature of trimethyl borate (B(OMe)₃) and ammonia. The spherical precursor (SP) with high oxygen content was obtained first, and then it was heated under flowing ammonia atmosphere to form stable boron nitride microspheres (BNMS). The BNMS exhibits onion-like cavitation structure with a diameter of 0.8–3.4 μm. The effects of the lower reaction temperature (700–825 °C) and gas flow rate on the spherical precursor are discussed. A possible mechanism is proposed to explain the formation of precursors and the appearance of onion-like structure. It is believed that the formation of microsphere is due to the deposition and growth of BO species during the flow process of nanosphere. In addition, the effect of the addition of BNMS on the thermal conductivity of epoxy resin (EP) composites was investigated.     

Atomic structures of bamboo-type boron nitride nanotubes with cup-stacked structures

Bamboo-type boron nitride (BN) nanotubes with cup-stacked structures were produced by annealing of Fe₄N and boron particles at 1000 °C for 5 h in nitrogen atmosphere. The iron nitride particles were reduced to α-Fe. Atomic structure models and the formation mechanism were proposed from the results of high-resolution electron microscopy (HREM), image simulations and molecular mechanics calculations. The nanotube structures would be stabilized by stacking of BN cup-layers.     

Processing and properties of reactively densified TiB2-AlN-hBN conductive ceramics with tunable compositions

Reactive sintering has been widely utilized to densify ceramics. Although the technology has numerous advantages, the composition of as-sintered ceramics is hard to be adjusted unless additional products are added. In this work, a novel solid-state route based on the reactions among TiN, Al and B were proposed to consolidate TiB₂-AlN-hBN ceramics (TAB) with tunable compositions (hBN ≤ 42 vol%, AlN ≤ 45 vol%). Dense TAB with high relative density, refined grain size and homogeneous microstructure were obtained via spark plasma sintering at 1800 °C and 60 MPa. It was found that exothermic reactions between the reactants could be ignited during heating, and the order of temperature at which the combustion process occurred was just opposite to the sequence of adiabatic temperature for individual reactions. Effects of hBN and AlN amounts on the densification, microstructure, mechanical properties, machinability and electrical resistivity of as-sintered ceramics were comprehensively investigated. Compared to AlN, hBN content played a more obvious role on those properties. As hBN contents increased from 0 to 42 vol%, the flexural strength, fracture toughness, Vickers hardness and modulus of TAB continuously decreased. TAB with hBN amounts ≥ 13 vol% exhibited better machinability with surface roughness lower than 2.9 µm after machining. Nevertheless, their electrical resistivity values at room temperature fluctuated in a narrow range between 43 μΩ·cm and 83 μΩ·cm, irrelevant with the hBN amounts.     

Effects of Al particle size and nitrogen pressure on AlN combustion synthesis

This study investigates the combustion synthesis of AlN fibers using an NH₄Cl additive and reports the effects of Al particle size (3, 30, and 180 µm) and N₂ pressure (0.10, 0.25, and 0.50 MPa) on the purity and morphology of AlN fibers. The combustion temperature was directly measured during the synthesis to elucidate the formation mechanism of the AlN fibers. The phase purity and morphology of the products were studied using X-ray diffraction and scanning electron microscopy, respectively. When the particle size of Al was reduced from 180 to 3 µm, the purity of the AlN product increased significantly owing to the large reaction area, which increased the combustion temperature. Furthermore, lower N₂ pressures enhanced the formation of AlN nanofibers due to the accelerated gasification of Al. The optimum values of the particle size of Al and the N₂ pressure for the formation of high-purity AlN nanofibers were found to be 3 µm and 0.10 MPa, respectively.     

Strengthening Hf0.95Ta0.05B2 ceramic with ultrafine grains and high-density dislocations

Full densification and fine microstructures are the two key optimization targets of ceramic materials. Although fine Hf_0.95Ta_0.05B₂ powder (∼ 0.36 µm) has been synthesized, it was still difficult to obtain densified Hf_0.95Ta_0.05B₂ ceramics with ultrafine grains (< 1 µm) using conventional high temperature sintering. Increasing sintering pressure could provided higher densification driving force, but it usually negatively promoted grain growth for nanoceramics. Our strategy was to gain the fully dense Hf_0.95Ta_0.05B₂ ceramic under a high pressure at a selected temperature with retarded grain growth. In this work, fully dense Hf_0.95Ta_0.05B₂ ceramic was prepared at 1700 °C under a high pressure of 200 MPa. The limited grain growth was achieved with the average grain size of 0.6 µm. Therefore, the mechanical properties were significantly improved, including Vickers hardness (24.8 GPa) and fracture toughness (4.2 MPa.m^1/2), which were ascribed to Hall-Petch and dislocation strengthening mechanism.     

Facile synthesis of ZnO nanostructures and enhancement of their sinterability via short-time cryo-milling

Zinc oxide (ZnO), a wide band-gap semiconductor, has received a great interest due to its potential applications in various fields both as nanostructures and as sintered compacts. In this study, we report on the synthesis of the ZnO nanostructures and facilitation of their sintering for the production of fine-grained dense compacts. The facile synthesis of gram scale ZnO nanostructures was achieved by thermal decomposition of zinc acetate dihydrate (Zn(Ac)₂·2H₂O) or Zn(Ac)₂·2H₂O/graphite mixtures at 300 °C for 12 h. Thermal decomposition of Zn(Ac)₂ resulted in the formation of mostly ZnO nanoparticles with wurtzite structure along with ZnO nanorods, while the addition of graphite significantly promoted the growth of ZnO nanowires. Microstructural and phase properties of the obtained ZnO nanostructures were determined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) techniques, all of which revealed the successful synthesis of high quality ZnO nanostructures. In addition to synthesis and characterization of the ZnO nanostructures, we report on the enhancement of their sinterability by a subsequent cryogenic milling for a short duration of 5 min. As a result of the applied cryo-milling, fabrication of highly dense (96.2%) sintered compacts with fine grain sizes (572 nm) could be achieved after pressureless sintering at 1000 °C for 2 h.     

Growth of carbon nanotube arrays using nanosphere lithography and their application in field emission devices

The present study investigates the patterned growth of carbon nanotubes (CNTs) by microwave plasma assisted chemical vapor deposition (MPCVD) and their field emission (FE) properties. The nanosphere monolayers were used as a mask for deposition of ultrathin (~ 3 nm) cobalt (Co) layer by DC sputtering. Periodic arrays of Co catalyst islands were obtained after the removal of spheres. Microscopic and Raman spectroscopic studies revealed the patterned growth of multiwall CNTs on catalyst islands. The CNTs length was around 10 µm and diameter was of 40–60 nm. The field emission properties were also compared with I–V characteristics of the un-patterned CNTs grown under the same conditions. The onset fields for un-patterned and patterned samples were nearly the same, 0.64 V/µm and 0.67 V/µm, respectively for a 10 µA current.     

Properties of super heat-resistant silicon carbide fibres with in situ BN coating

An in situ BN coating was prepared on the surface of a nearly stoichiometric continuous SiC fibre with trademark Cansas-3301 (C3). The coated fibre was then subjected to continuous pyrolysis at 1800 °C, obtaining a fibre named Cansas-BN-1800 (C18). After annealing in Ar at 1500 °C for 1 h, the strength retention ratio of C3 was 49%, and that of C18 was almost unchanged. The strength decrease of the C3 fibre was mainly caused by the formation of surface defects resulting from fibre decomposition and active oxidation. However, the in situ BN coating on C18 protected the fibre from forming surface defects, resulting in high strength. Due to slight growth of the grain and purification of the grain boundary during fast heating at 1800 °C, C18 showed excellent creep resistance in the range of 1200–1500 °C.     

Fabrication of functional zirconia surfaces using a two-beam interference setup employing a picosecond laser system with 532-nm wavelength: Morphology,microstructure, and wettability

The aim of this work was to evaluate the feasibility of the fabrication of microtextures in zirconia using the direct laser interference patterning (DLIP) technique. A green ultra-short pulsed laser (532 nm, 10 ps) with a two-beam interference setup was used to produce line-like structures with a spatial period of 3 µm. For a fixed set of fluence and pulse-to-pulse overlap values (6.2 J/cm², 81%), periodic structures were successfully created for different hatch distances. The average depth of the features ranged from 0.37 µm for a hatch distance of 14.4 µm up to 0.84 µm for a hatch distance of 12.4 µm. However, a further decrease in hatch distance did not result in an increase in depth since the region of the ridges is also ablated. Scanning electron microscopy analysis showed pores formation on the laser grooves, but no sign of micro-cracking could be observed. Wettability tests showed an increase in hydrophobicity after DLIP. These results bring exciting perspectives on the fabrication of micro-textures with DLIP on zirconia surface.     

High aspect ratio sapphire micromachining by ultraviolet laser-induced plasma-assisted ablation (LIPAA)

The ultraviolet laser-induced plasma-assisted ablation performed in this article can attain deep and high-quality engravings in sapphire without necessitating volatile solutions or expensive equipment such as high-power ultrashort-pulsed lasers. The dominant mechanism of ablation is discovered to be from the direct ablation of excited sapphire surfaces and not from the plasma generated from the target material. Only an initial deposition from the target is needed to initiate the direct ablation. Note that 20-µm wide, 30-µm deep channel and hole features with a surface roughness (Sa) of .65 µm are achieved at an etching rate of .3 µm per pulse without the need for extensive cleaning. Engravings can reach up to 150-µm depths at a maximum tapering angle of 5° until the shrinking absorbent surface vanishes, and 500-µm wide 430-µm deep topside through-cutting is achieved. This study characterizes the morphology of direct laser ablation of transient absorbent sapphire surfaces. This method demonstrates the potential for the low-cost rapid engraving of high aspect ratio features in transparent sapphire substrates.     

Fabrication of surface-treated BN/ETDS composites for enhanced thermal and mechanical properties

The ceramic/polymer composites based on epoxy-terminated dimethylsiloxane (ETDS) and boron nitride (BN) were prepared for use as thermal interface materials (TIMs). 250 µm-sized BN was used as a filler to achieve high-thermal-conductivity composites. To improve the interfacial adhesion between the BN particles and the ETDS matrix, the surface of BN particles were modified with silica via the sol–gel method with tetraethyl orthosilicate (TEOS). The interfacial adhesion properties of the composites were determined by the surface free energy of the particles using a contact angle test. The surface-modified BN/ETDS composites exhibited thermal conductivities ranging from 0.2 W/m K to 3.1 W/m K, exceeding those of raw BN/ETDS composites at the same weight fractions. Agari׳s model was used to analyze the measured thermal conductivity as a function of the SiO₂-BN concentration. Moreover, the storage modulus of the BN/ETDS composites was found to increase with surface modification of the BN particles.     

In situ anisotropic NiO nanostructure growth at high temperature and under water vapor

Anisotropic growth of nanostructures from individual nickel nanoparticles was observed during in situ heating experiments in an environmental scanning electron microscope (ESEM) at 800°C under water vapor atmosphere. The morphology of nanostructures exhibited one directional growth with rates ranging below 1.8 nm/s. Energy dispersive X-ray spectroscopy and selected area electron diffraction confirmed NiO stoichiometry of the growing nanostructures. Variations of the oxygen partial pressure during ex situ annealing and in situ ESEM heating experiments elucidate that anisotropic NiO growth is energetically favored in areas where the local surface energy density is relatively high. Growth of NiO nanostructures was absent in dry air and dry nitrogen environments and required the presence of water vapor. The results of this study suggest that the manipulation of surface energy prior to exposure to water vapor at elevated temperatures can prevent unwanted oxide nanostructure growth.     

Molten salt synthesis and antioxidation property of SiC-coated mesocarbon microbeads

Mesocarbon microbead (MCMB) is a prospective candidate as the raw material for nuclear graphite. However, the poor resistance to high-temperature oxidation limits its application. Herein, a dense SiC coating was prepared by molten salt synthesis on the surface of MCMB to improve its antioxidation performance. The effect of molten salt synthesis reaction time on the phase composition, microstructure, and antioxidation performance of the SiC-coated MCMB particles was investigated. A theoretical model was established to explain the SiC coating growth rule well, in conformity with the carbon vacancy diffusion mechanism in SiC coating. The SiC coating synthesized for 7 h with the thickness of .385 µm remarkably promoted the high-temperature antioxidation property of MCMB. The kinetics analysis indicated that the SiC coating obstructed the oxygen diffusion effectively during the oxidation process. The as-fabricated SiC-coated MCMBs with good oxidation resistance show great promise for application in nuclear industry and other antioxidative fields.