Key Engineering Materials Vols. 645-646

Abstract: In the fabrication of SiC high-temperature sensors, the phosphosilicate glass film (PSG) is deposited on the SiC wafer as the intermediate layer, achieving the bonding of two SiC wafers. The ease of bonding is affected by the PSG film thickness uniformity. In this paper, the flow field distribution in the LPCVD tube is simulated under different deposition conditions,obtaining the wafer surface uniformity of gas flow distribution, which was verified by the experiment, The aim of this work is to study the impact of several deposition conditions such as the location of SiC wafers, wafer inclination and wafer spacing on the thickness uniformity of PSG film. Experimental results show that SiC wafer position in the tube and its own inclination and wafer spacing affect gas flow field on the SiC wafer surface,leading to uneven distribution of gas flow, thus affect the thickness uniformity of PSG.

Abstract: Chiral sculptured thin films (STFs) Glancing-angle deposition (GLAD) thin films are nanoengineered to meet the requirements of a variety of applications such as micro filters, sensors, and waveguides due to their unique frequency characteristics which cannot be achieved by conventional solid materials. For the design, it is necessary to understand the elastic properties of STFs. To facilitate this, we report on our newly developed advanced micro-scale vibration testing process. In the testing, specially designed micro-specimens with surface areas of tens by tens of microns are excited using a piezoelectric (PZT) actuator and the resonance frequencies are detected by a laser device in the vertical or lateral directions successfully. The anisotropy elastic modulus of STFs composed of helical nanosprings are identified on the basis of vibration testing. The thin film shows strong characteristic anisotropy that the solid one hardly can attain. The micro-scale testing technique can be extended to other materials and microstructures.

Abstract: The two-dimensional material MoS₂ has attracted a growing attention due to its potential applications in electronic devices in recent years [1,2,3], and the monolayer MoS₂ is a direct gap semiconductor with a band gap of 1.8eV [4]. In the existing studies, it has indicated that MoS₂ can get an available magnetism with doping transition metal atoms [5], and is expected to be a new generation of diluted magnetic semiconductor (DMS) [6,7]. Moreover, we found that Fe-doped MoS₂ could present a strong magnetism but a semimetal characteristic, losing its original semiconductor properties while obtaining magnetism. Therefore, it is necessary to explore some methods to make monolayer MoS₂ exhibit both magnetic and semiconductor properties. In this paper, we propose the method of N, Fe atoms co-doping to achieve this objective. The structural, electronic and magnetic properties of MoS₂ doped with transition metal Fe and VA atoms have been investigated by first principle calculations based on density functional theory. The 3×3×1 supercell of monolayer MoS₂ as a calculation model has been used. The result shows that pure MoS₂ has no magnetism, while Fe-doped MoS2 exhibits a good magnetism about 1.849μB but a semimetal characteristic. This is due to that Mo-4d, S-2p, Fe-3d states has a strong coupling around the Femi energy for the introduction of Fe atom, and the Femi energy only pass through the spin-up density of states. For the co-doping with VA atoms and Fe atoms, it is found that the magnetic moment of Fe-N, Fe-P and Fe-As co-doped MoS₂ is 0.956μB, 0.775μB, 0.782μB. Moreover, the Fe-N co-doped MoS₂ presents semiconductor characteristics, in contrast, Fe-P and Fe-As co-doped MoS₂ appear semimetal properties. It indicates that the semimetal characteristic of Fe-doped MoS₂ could change into indirect band gap semiconductor due to the introduction of N atom. The band gap is 0.2eV. Our study demonstrate that the method of Fe, N co-doping could make MoS₂ have good magnetic and also semiconductor properties at the same time.

Abstract: In this paper, two aspects in the wet glass etching, the pre-annealing of the glass and the mask process, are taken into consideration to achieve the deep and defect-free wet etching of Pyrex glass. Compared with the conventional strategies, i.e., HF solution component and mask kinds, our experiment results prove the pre-annealing is another key role to obtain theoretical isotropy character in wet etching. Besides, the high temperature pre-annealing dramatically improves the structure profiles and reduces the notching defects. Additionally, a novel multilayer mask process is proposed. With 1.5μm PR/ 100nm Au/ 100 nm Au/ 20 nm Cr mask and > 450 °C pre-annealing, > 150 μm deep and non-pin-holes Pyrex glass structures are achieved and the roughness of etched surface is lower than 1 nm.

Abstract: In this paper, we mainly analyzed the different influence of hydrophilic SiO₂ particles and hydrophobic SiO₂ particles on the properties of PI/Al₂O₃ films. PI/Al₂O₃/SiO₂(hydrophilic) films and PI/Al₂O₃/SiO₂(hydrophobic) films with 16 wt% content of Al₂O₃ and 3 wt‰, 5 wt‰, 7 wt‰ content of SiO₂ respectively were prepared by in-situ polymerization method. Firstly, the hydrophilic SiO₂ particles and hydrophobic SiO₂ particles were investigated by transmission electron microscopy (TEM), infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The results showed that the average sizes and infrared characteristic peaks of two kinds of SiO₂ particles were similar, and two kinds of SiO₂ particles were amorphous inorganic particles. Then the morphology of the composite films was characterized by scanning electron microscopy (SEM), and the mechanical properties, the breakdown field strength and corona resistant time were tested and analyzed. Results indicated that inorganic particles added to PI/Al₂O₃/SiO₂(hydrophilic) films dispersed better than that of PI/Al₂O₃/SiO₂(hydrophobic) films. When the content of SiO₂ particles was 5 wt‰, the mechanical properties and corona resistance of PI/Al₂O₃/SiO₂(hydrophilic) films were best, the tensile strength, elongation at break and corona resistance times of the films were: 132.44 MPa, 12.64%, 378 min, respectively. The breakdown field strength of PI/Al₂O₃/SiO₂(hydrophilic) films was only 211.15 kV/mm, and inferior to that of PI/Al₂O₃/SiO₂(hydrophobic) films (232.08 kV/mm).

Abstract: Bi₂Te₃ nanowires are prepared by a low-cost and facile hydrothermal method without any surfactant. The structure and morphology of the nanowires are characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscope. The influences of experimental conditions on the products are investigated. The growth mechanism is proposed based on the experimental results. This work is promising for the synthesis of Bi₂Te₃ nanowires with less impurity.

Abstract: The electronic and magnetic properties of graphene functionalized by 4f-orbital RE-metal atoms (Ce, Nd, Sm and Eu) were investigated by the density functional theory (DFT) calculations. The results of binding energy and geometry parameters showed that the hollow site, the center of a carbon hexagon, was the most stable adsorption structure. Furthermore, the PDOS results suggested that the electronic hybridization between the RE-adatoms and C atoms was mainly contributed by the 5d orbitals, whereas the 4f-orbital of the metal adatoms dominated the net magnetic moments of the systems significantly.

Abstract: Melt electrospinning is a novel technology in the field of 1D micro/nanostructure fabrication. Decreasing the diameter and promoting surface morphology of melt fiber are the key for the application of melt electrospinning technology. Heated sheath gas is introduced to build up melt electrospinning direct-write technology, and then orderly micro/nanofibers can be direct-written. The heated sheath gas provided a good way to increase the temperature of melt jet, by which solidification can be slowed. With the help of heated sheath gas, the diameter of melt fiber can be decreased. The affects of process parameters on the diameter of melt electrospinning fiber was investigated, the diameter of melt electrospinning fiber increased with the increasing of temperature of spinneret and feed rate, but decreased with the increasing of voltage and distance between spinneret and collector. Heated sheath gas is an excellent method to promote the application of melt electrospinning.

Abstract: Some typical kinds of PCD and PCBN compacts are selected to be machined by WEDM, and a series of processing tests are taken. After machining, the surface roughness of cutting section, the processing quality of cobalt-rich interface layer and the edge of superhard material layer are measured by surface profiler and 3D microscope. The results show that processing quality is affected by superhard particle size and concentration greatly, and better processing quality can be obtained after several cutting of WEDM. The minimum sharpening allowance of PCD cutting tools can be controlled within 4~15μm after WEDM, and within 10μm for PCBN BNX20, while BZN6000 needs larger follow-up workload of sharpening.